Sulfur Tolerant Supported Bimetallic Catalysts for Low

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Sulfur Tolerant Supported Bimetallic Catalysts for

Low Temperature Water Gas Shift Reaction

A dissertation submitted to the Division of Graduate Studies and Research of the University of

Cincinnati in partial fulfillment of the requirements of the degree of

Doctor of Philosophy (Ph.D.)

In the Department of Chemical & Environmental Engineering of the College of Engineering &

Applied Science

2019

SeongUk Yun

Committee : Dr. Vadim Guliants (Chair)

Dr. Anastasios Angelopoulos

Dr. Junhang Dong

Dr. Mingming Lu

Abstract

A series of model CuPd nanoparticles, CoMo oxide nanoparticles, different metal oxide supported Mo sulfide catalysts, and sets of different composition ratio and surface coverage of

CoMo sulfide catalysts were prepared and investigated as sulfur-tolerant WGS catalysts. For comparison, monometallic catalysts prepared by incipient wetness impregnation, as well as commercial CoMo catalysts, were also investigated. The model CoMo-S catalysts at the monolayer surface coverage employed in this research are highly promising as sulfur-tolerant

WGS catalysts displaying desirable structural, morphological, and compositional properties.

The CuPd-2 catalysts maximized the number of WGS-active Cu0 sites with the optimized ratio (2.37) of CuO/CuAl2O4, showing higher WGS activity, thermal stability, and sulfur tolerance at 250°C than any other tested Cu-based catalysts. Cu-Pd bimetallic alloy catalysts showed enhanced reducibility due to the Pd-promoting effect through hydrogen spillover and additional reducible CuO sites through Cu species diffusion from the CuO shell to Al2O3. The Mo and CoMo oxide nanoparticles were prepared by a metal colloid chemical co-reduction method by modifying the concentrations of the Mo and Co precursors during synthesis. The WGS activity of n-Mo-S and n-CoMo-S catalysts increased due to the reduction of the average particle size up to 5-Mo and

10-CoMo. The extent of sulfidation of n-Mo-S catalysts was saturated at 5-Mo and correlated with

WGS activity. 10-CoMo-S catalysts were the most active among the tested Al2O3-supported Mo- based catalysts with similar sulfur dependence to a commercial CoMo/MgO catalyst. Mo-S/ZrO2 showed the highest WGS activity in 1,000 ppm H2S-containing feed and lowest H2S dependence in H2S-free feed among ZrO2-, Al2O3-, TiO2-, CeO2-, and SiO2-supported Mo catalysts. Weak support-MoO3 interaction of ZrO2 favored a higher extent of sulfidation, correlated to the WGS activity, and stable sulfur bonding in Mo-S/ZrO2 led to low sulfur dependence. Mo5-S/ZrO2 at

II monolayer MoO3 coverage showed optimal WGS activity and extent of sulfidation, suggesting that the topmost Mo-S layer comprised WGS-active catalytic sites. CoMo-S/ZrO2 catalyst at monolayer CoMo-O coverage with Co/Mo = 0.3 catalysts was the most active WGS catalyst among all the tested catalysts in this study. This catalyst was thermally stable for at least 4 weeks of reaction test, and demonstrated low sulfur tolerance under H2S-free feed at 350°C and GHSV

35,000 h-1. Structurally, this catalyst exhibited optimized surface coverage, highly dispersed

CoMo-S species, saturated extent of sulfidation, and optimal number of active sites.

The important result of this study is that cobalt promoter facilitated the dispersion of

CoMo-S species, the formation of active surface sulfur, and the reactivity of CoMo-S, while cobalt promoter weakened the sulfur bond in CoMo-S species, leading not only to enhanced WGS activity, but also to increased sulfur dependence. However, optimal amount of Co addition could significantly reduce the active metal loading compared to the commercial CoMo catalysts, which could save a great deal of raw material cost in catalyst production. Therefore, the optimized CoMo-

S/ZrO2 catalyst is a highly active, thermally stable, chemically stable, and economically beneficial sulfur-tolerant WGS catalyst to apply in hydrogen production using biomass-derived syngas.

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Table of Contents List of Tables List of Figures

Chapter 1. Introduction ...... 1

1.1. Motivation ...... 1

1.2. Objectives ...... 5

1.3. Reference ...... 8

Chapter 2. Background and Literature review ...... 12 2.1. Hydrogen production from biomass-derived syngas ...... 12

2.1.1. The hydrogen economy ...... 12

2.1.2. Hydrogen production methods ...... 13

2.2. Water gas shift reaction and its applications ...... 17

2.2.1. Water gas shift reaction...... 17

2.2.2. Applications of the WGS reaction ...... 18

2.3. WGS Reaction Mechanism ...... 19

2.3.1. Redox Mechanism ...... 20

2.3.2. Associative Mechanism ...... 21

2.4. Current WGS catalysts for low-temperature sour shift and their limitations ...... 24

2.4.1. Conventional Cu-based and Fe-based catalysts ...... 24

2.4.2. Current sulfur-tolerant WGS reaction catalysts ...... 26

2.4.3. Sulfur-tolerant Mo sulfide-based WGS catalysts ...... 27

2.5. Novel approaches to develop sulfur-tolerant WGS catalysts ...... 31

2.5.1. Bimetallic Cu-Pd nanoparticle WGS catalysts ...... 31

2.5.2. Synthesis of Mo and CoMo nanoparticles ...... 33

2.5.3. Promoters of Mo-based catalysts ...... 34

2.5.4. Modifying supports to improve WGS activity ...... 35

2.6. References ...... 37 Chapter 3. Novel bimetallic Cu-Pd nanoparticles as sulfur-tolerant and highly active low temperature WGS catalysts ...... 49 3.1. Introduction ...... 49

3.2. Experimental methods ...... 52

3.2.1. Catalyst preparation ...... 52

3.2.2. Catalyst characterization ...... 53

3.2.3. WGS activity ...... 55

3.3. Results and discussion ...... 55

3.3.1. Morphological and structural characterization of Cu-Pd nanoparticles ...... 55

3.3.2. CuAl2O4 formation and WGS activity of Cu-Pd catalysts ...... 57

3.3.3. WGS activity of Cu-Pd nanoparticle catalysts ...... 59

3.3.4. Effect of CuO/CuAl2O4 molar ratio on WGS activity ...... 60

3.3.5. Sulfur tolerance and thermal stability of optimized Cu-Pd/Al2O3 catalyst ...... 65

3.3.6. Structural models of bimetallic Cu-Pd nanoparticles ...... 68

3.4. Conclusions ...... 71

3.5. References ...... 72 Chapter 4. Size-dependent catalytic behavior and sulfur dependence of Mo- based nanoparticles in water gas shift reaction of biomass-derived syngas ....77

4.1. Introduction ...... 77

4.2. Experimental Section ...... 81

4.2.1. Catalyst preparation ...... 81

4.2.2. TEM imaging and XRD analysis...... 82

4.2.3. Catalytic activity ...... 82

4.2.4. Surface and bulk elemental analysis ...... 82

4.3. Results and Discussion ...... 83

4.3.1. Synthesis Mo oxide and CoMo oxide nanoparticles ...... 83

4.3.2. Size effects of Mo oxide and CoMo oxide nanoparticle on WGS activity ...... 87

4.3.3. Sulfur dependence of n-Mo-S and n-CoMo-S catalysts ...... 91

4.3.4. Promoter effect on WGS activity and sulfur dependence ...... 93

4.3.5. WGS activity of Mo-S and CoMo-S catalysts under optimized reaction conditions .. 96

4.4. Conclusions ...... 99

4.5. References ...... 100 Chapter 5. Support effects on water gas shift activity and sulfur dependence of

Mo sulfide catalysts ...... 106

5.1. Introduction ...... 106

5.2. Experimental methods ...... 109

5.2.1. Catalyst preparation ...... 109

5.2.2. Catalyst characterization ...... 110

5.3. Results and discussion ...... 111

5.3.1. Elemental and structural characterization of supported Mo sulfide catalysts ...... 111

5.3.2. WGS activity of supported Mo-S species ...... 113

5.3.3. H2-TPR analysis of Mo sulfide supported on various oxides ...... 115

5.3.4. Sulfur dependence of Al2O3 and ZrO2 supported Mo-S catalysts ...... 117

5.4. Conclusion ...... 122

5.5. References ...... 123 Chapter 6. Surface coverage effects on water gas shift activity of ZrO2

Supported Mo Sulfide Catalysts ...... 128

6.1. Introduction ...... 128

VII

6.2. Experimental methods ...... 130

6.3. Results and Discussion ...... 132

6.4. Conclusions ...... 138

6.5. References ...... 139 Chapter 7. Hydrogen production over Co-promoted Mo-S water gas shift catalysts supported on ZrO2 ...... 142

7.1. Introduction ...... 142

7.2. Experimental ...... 147

7.2.1. Catalyst synthesis ...... 147

7.2.2. WGS catalytic activity ...... 148

7.2.3. Catalyst characterization ...... 148

7.3. Results and Discussion ...... 149

7.3.1. Characterization of n ML CoMo/ZrO2 catalysts...... 149

7.3.2. Characterization of Co/Mo = n CoMo-S/ZrO2 catalysts ...... 152

7.3.3. WGS activity of n ML CoMo-S/ZrO2 catalysts...... 154

7.3.4. WGS activity of Co/Mo = n CoMo-S/ZrO2 catalysts ...... 159

7.3.5. Long-term stability and H2S-dependence of CoMo-S/ZrO2 WGS catalysts ...... 170

7.4. Conclusions ...... 174

7.5. References ...... 175

Chapter 8. Recommendations for Future Research ...... 184 8.1. Recommendations for Future Research ...... 184

8.2. References ...... 187

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List of Tables

Table 2.1. Advantages and disadvantages of biomass sources for bio-fuel plants [36]...... 17 Table 2.2. Summary of recently investigated sulfur tolerant WGS reaction catalysts...... 27 Table 2.3. Summary of sulfur-tolerant Mo-based catalysts...... 28 Table 3.1. Physicochemical characteristics of Cu, Pd, and Cu-Pd catalysts...... 57

Table 4.1. Synthesis of MoOx nanoparticles by chemical reduction methods...... 80 Table 4.2. Summary of synthesis parameters for as-synthesized unsupported Mo and CoMo nanoparticles: 4.7-Mo (4.7 ± 0.7 nm), 5-Mo (5.4 ± 0.8 nm), 14-Mo (14.2 ± 3.2 nm), 23-Mo (22.6 ± 3.1 nm), 6-CoMo (6.0 ± 1.6 nm), 10-CoMo (9.7 ± 1.6 nm), 30-CoMo (31.5 ± 4.6 nm), 100- CoMo (~ 100 nm)...... 84 Table 4.3. Physicochemical characteristics of n-Mo and n-CoMo catalysts (Co wt.% and Mo wt.% were estimated by ICP-MS using as-synthesized catalyst after calcination at 500°C in air before pre-sulfidation)...... 89

Table 5.1. Mo content (wt. %), S/Mo atomic ratios, and BET surface areas of Al2O3, TiO2, SiO2,

CeO2, and ZrO2-supported Mo-S catalysts...... 111 Table 5.2. The atomic ratios of the S 2p peak area corresponding to fully sulfided Mo-S bonds and the S 2p area for Mo oxysulfide bonds (MoOxSy) of the fresh and used Mo-S/Al2O3 and Mo-S/ZrO2 catalysts estimated by XPS analysis...... 121

Table 5.3. The extent of sulfidation, normalized CO consumption rate over fresh Mo-S/Al2O3, used Mo-S/Al2O3, fresh Mo-S/ZrO2, and used Mo-S/ZrO2...... 122 Table 6.1. Mo (wt.%), atomic S/Mo ratios, BET surface areas, and surface densities (Mo 2 atoms/nm ) of Mo1-S, Mo2-S, Mo5-S, Mo10-S, and Mo15-S/ZrO2 catalysts...... 132

Table 6.2. Activation energies and TOFs for the WGS reaction over ZrO2-supported Mo1-S, Mo2-

S, Mo5-S, Mo10-S, and Mo15-S catalysts...... 138 Table 7.1. Co (wt. %), Mo (wt. %), atomic S/Mo ratios, and BET surface areas, of 0.5 ML, 1 ML,

2 ML, and 4 ML CoMo-S/ZrO2 catalysts...... 149 Table 7.2. Co (wt. %), Mo (wt. %), atomic S/Mo ratios, and BET surface areas of freshly sulfided Co/Mo = n catalysts...... 152

Table 7.3. H2 consumption during H2-TPR analysis of 0.5 ML, 1 ML, 2 ML, and 4 ML CoMo-

S/ZrO2...... 159 Table 7.4. Activation energies and TOFs of the Co/Mo = n catalysts...... 162 IX

Table 7.5. H2 consumption during H2-TPR analysis of Co/Mo = 0, 0.1, 0.3, and 1.5 CoMo-S/ZrO2 catalysts...... 166

Table 7.6. XPS results of various Co/Mo atomic ratio CoMo-S/ZrO2 (Co/Mo = 0, 0.1, 0.3, and 1.5)...... 168 Table 7.7. Atomic S/Mo ratio for fresh and used Co/Mo = n catalysts estimated by XPS analysis...... 173 Table 8.1. The price of raw material to produce catalyst [7]...... 185 Table 8.2. Estimated material cost of catalyst production over CoMo-S catalysts and commercial catalyst...... 185

Table of Figures

Figure 1.1. Trend of published papers addressing catalysts for the water gas shift reaction and water gas shift reaction with biomass ...... 4 Figure 2.1. Gasification-based energy conversion options [23]...... 15 Figure 2.2. Reaction network for the WGS reaction including both the surface redox and carboxyl associative mechanisms. The thermochemistry and kinetic barriers for all elementary steps are given in electron volts. For reactions involving bond making, the activation barriers are reported with respect to the adsorbed reactants at infinite separation from each other [51]...... 23 Figure 2.3. Sulfur chemical potentials where the corresponding binary alloy structure starts to become poisoned due to sulfur adsorption [96]...... 33 Figure 2.4. (Left) Catalytic activity as a function of sulfur exposure concentration and reaction temperature [130]. (Right) Influence of Ce-K on the catalytic activity of the Co-Mo/γ-Al2O3 catalysts [65]...... 35 Figure 3.1. TEM images of as-synthesized bimetallic (a) CuPd1, (b) CuPd2, and (c) CuPd5 nanoparticles without the alumina support before calcination at 800°C in air...... 56 Figure 3.2. XRD patterns of unsupported as-synthesized bimetallic Cu-Pd nanoparticles before calcination at 800°C in air...... 56

Figure 3.3. XRD patterns of bare γ-Al2O3, CuPd2/γ-Al2O3 and CuPd2/γ-Al2O3 calcined in air at

800°C. (+ : CuAl2O4, * : CuO)...... 58

Figure 3.4. CO conversion during WGS reaction over γ-Al2O3 supported Cu, Cu calcined at 800°C,

CuPd2, and CuPd2 calcined at 800°C (Feed: 10 vol.% CO and 20 vol.% H2O in He at GHSV = 25,000 h-1)...... 59

Figure 3.5. CO conversion during WGS reaction of γ-Al2O3 supported Cu, CuPd1, CuPd2, CuPd5,

- and Pd after 800℃ calcination (Feed: 10 vol.% CO and 20 vol.% H2O in He at GHSV = 25,000 h

1)...... 60

Figure 3.6. H2-TPR profiles of Cu-Pd catalysts after their calcination at 800°C...... 61

Figure 3.7. XPS-spectra (Cu 2p3/2 region) of Cu-Pd catalysts after calcination at 800°C in air. 2+ 2+ Dashed lines at 934.6 eV and 933.2 eV indicate Cu in CuAl2O4 and Cu in CuO, respectively...... 63

Figure 3.8. Cu0 metal surface area and CO consumption rate of Cu-Pd catalysts after 800°C calcination as a function of CuO/CuAl2O4 molar ratio...... 64

Figure 3.9. CO conversion during WGS reaction over Cu@Ni/γ-Al2O3 without calcination,

CuPd2/γ-Al2O3 after 800°C calcination, and commercial CuCrBaOx catalyst without calcination -1 (Feed: 10 vol.% CO and 20 vol.% H2O in He at GHSV = 25,000 h )...... 66

Figure 3.10. CO conversion during WGS reaction over Cu and CuPd2/γ-Al2O3 after 800°C calcination as function of time on stream (Feed: 10 vol.% CO and 20 vol.% H2O in He at GHSV = 25,000 h-1 and 250°C)...... 66

Figure 3.11. CO conversion during WGS reaction over CuPd2/γ-Al2O3 after 800°C calcination,

Cu@Ni/γ-Al2O3 catalyst (without 800°C calcination), and commercial catalyst (CuCrBaOx) without 800°C calcination in 500 ppm H2S-containing feed (Feed: 10 vol.% CO and 20 vol.% H2O in He at GHSV = 25,000 h-1 and 250°C)...... 67 Figure 3.12. CO conversion during WGS reaction over supported Cu, CuPd1, CuPd2, and CuPd5 catalysts after 800°C calcination with 500 ppm H2S-containing feed (Feed: 10 vol.% CO and 20 -1 vol.% H2O in He at GHSV = 25,000 h and 250°C)...... 68 Figure 3.13. Experimental and theoretical (Vegard’s law) lattice parameters of Cu-Pd alloys plotted as a function of Cu mole fraction in bimetallic Cu-Pd nanoparticles prior to calcination at 800°C in air...... 70

Figure 3.14. Proposed structural model for bimetallic Cu-Pd nanoparticles supported on γ-Al2O3 before (left) and after calcination at 800°C in air (right)...... 71 Figure 4.1. TEM images of as-synthesized unsupported (a) 4.7-Mo (4.7±0.7 nm), (b) 5-Mo (5.4±0.8 nm), (c) 14-Mo (14.2±3.2 nm), and (d) 23-Mo (22.6±3.1 nm)...... 84 Figure 4.2. TEM images of as-synthesized unsupported (a) 6-CoMo (6.0 ±1.6 nm), (b) 10-CoMo (9.7±1.6 nm), (c) 30-CoMo (31.5±4.6 nm), and (d) 100-CoMo...... 86 Figure 4.3. XRD patterns of as-synthesized unsupported n-Mo and n-CoMo before calcination at

500°C in air (*: (110), o: (210), v: (400), and x: (310) of MoO3)...... 87 Figure 4.4. CO reaction rate normalized by the estimated surface area of n-Mo and n-CoMo nanoparticles in supported n-Mo and n-CoMo catalysts...... 89

Figure 4.5. CO consumption rate and CO uptake over 5-, 14-, and 23-Mo-S/Al2O3 catalysts (Feed: -1 10 vol.% CO, 20 vol.% H2O, and 1,000 ppm H2S in He at GHSV = 2,500 h and 450°C)...... 91

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Figure 4.6. The CO consumption rate and atomic S/Mo ratios for 5-, 14-, and 23-Mo-S/Al2O3 -1 catalysts (Feed: 10 vol.% CO, 20 vol.% H2O, and 1,000 ppm H2S in He at GHSV = 2,500 h and 450°C)...... Error! Bookmark not defined.

Figure 4.7. CO conversion over 5-, 14-, and 23-Mo-S/Al2O3 catalysts during WGS reaction employing 1,000 ppm H2S and H2S-free feed (Feed: 10 vol.% CO and 20 vol.% H2O in He at GHSV = 2,500 h-1 and 450°C)...... 92

Figure 4.8. CO conversion over 6-, 10-, 30- and 100-CoMo-S/Al2O3 catalysts during WGS reaction employing 1,000 ppm H2S and H2S-free feed (Feed: 10 vol.% CO and 20 vol.% H2O in He at GHSV = 2,500 h-1 and 450°C)...... 93

Figure 4.9. CO conversion over 10-CoMo-S, Ni1CoMo-S, Cu1CoMo-S, Pd1CoMo-S, and

Ce1CoMo-S/Al2O3 catalysts during the WGS reaction employing 1,000 ppm H2S and H2S-free -1 conditions (Feed: 10 vol.% CO and 20 vol.% H2O in He at GHSV = 2,500 h and 450°C)...... 94

Figure 4.10. CO conversion over 10-CoMo/Al2O3 and commercial CoMo catalysts during WGS reaction employing 1,000 ppm H2S and H2S-free feed (Feed: 10 vol.% CO and 20 vol.% H2O in He at 450°C and 1.5 g of catalyst)...... 96

Figure 4.11. CO conversion over Mo-S/Al2O3 and CoMo-S/Al2O3 catalysts as a function of (a) -1 H2S concentration in feed (Feed: 10 vol.% CO and 20 vol.% H2O in He at GHSV = 2,500 h and

450°C), and (b) H2O/CO feed ratio at fixed CO concentration (Feed: 5 vol.% CO and 1,000 ppm -1 H2S in He at GHSV = 2,500 h and 450°C)...... 97 Figure 4.12. CO conversion during WGS reaction over Mo-S and CoMo-S catalysts (Feed: 10 -1 vol.% CO, 20 vol.% H2O, and 1,000 ppm H2S in He at GHSV = 2,500 h )...... 99 Figure 5.1. XRD patterns of the fresh supported Mo-S catalysts and corresponding supports. . 113

Figure 5.2. CO consumption rate over Mo-S/ZrO2, Mo-S/Al2O3, Mo-S/TiO2, Mo-S/CeO2, and Mo-

S/SiO2 catalysts during WGS reaction employing 1,000 ppm H2S and H2S-free feed (Feed: 10 vol.% -1 CO and 20 vol.% H2O in He at GHSV = 9,000 h and 450°C). Note: the feed contained 1,000 ppm

H2S during the initial 4 hours of reaction...... 115

Figure 5.3. H2-TPR profiles of (a) the supported Mo-O catalysts after calcination at 500°C in air, and (b) the supported Mo-S catalysts after pre-sulfidation...... 116

Figure 5.4. H2-TPR profiles of fresh and used Mo-S/Al2O3 and Mo-S/ZrO2 catalysts...... 118

Figure 5.5. HAADF-STEM images of (a) fresh Mo-S/Al2O3, (b) used Mo-S/Al2O3, and TEM images of (c) fresh Mo-S/Al2O3, and (d) used Mo-S/Al2O3...... 119

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Figure 5.6. XRD patterns of fresh and used Mo-S/Al2O3 and Mo-S/ZrO2 catalysts...... 120

Figure 5.7. XPS-spectra S 2p region of fresh and used Mo-S/Al2O3 and Mo-S/ZrO2 catalysts. 121

Figure 6.1. TEM images of ZrO2 supported (a) Mo2-S, (b) Mo5-S, and (c) Mo15-S...... 133

Figure 6.2. XRD patterns of ZrO2, Mo2-S/ZrO2, Mo5-S/ZrO2, and Mo15-S/ZrO2...... 134

Figure 6.3. Raman spectra of ZrO2-supported Mo1-S, Mo2-S, Mo5-S, Mo10-S, and Mo15-S catalysts...... 135 Figure 6.4. Proposed structural motifs for supported Mo-S species as a function of surface coverage on ZrO2 support...... 136

Figure 6.5. CO conversion during WGS reaction over ZrO2-supported Mo1-S, Mo2-S, Mo5-S,

Mo10-S, and Mo15-S catalysts (Feed: 10 vol.% CO, 20 vol.% H2O, and 1,000 ppm H2S in He at GHSV = 9,000 h-1 and 450°C)...... 137

Figure 6.6. Arrhenius plots of CO conversion rate for the WGS reaction observed over ZrO2- supported Mo1-S, Mo2-S, Mo5-S, Mo10-S, and Mo15-S catalysts (Feed: 10 vol.% CO, and 20 vol.% H2O, and 1,000 ppm H2S in He at CO conversion < 13%)...... 138 Figure 7.1. TEM images of fresh sulfided (a) 0.5 ML, (b) 1 ML, (c) 2 ML, and (d) 4 ML CoMo-

S/ZrO2 (white arrow: CoMo-S species, white circle: CoMo-S layer, and red circle: stacked CoMo- S layers)...... 151

Figure 7.2. XRD patterns of ZrO2, 0.5 ML, 1 ML, 2 ML, and 4 ML CoMo-S/ZrO2...... 152 Figure 7.3. TEM images of fresh sulfided (a) Co/Mo = 0 (Mo-S), (b) Co/Mo=0.1, (c) Co/Mo=0.3, and (d) Co/Mo=1.5 CoMo-S/ZrO2 catalysts (black arrow: Mo-S layer, black circle: dark spots (Mo-S species), white arrow: small dots (CoMo-S species), and white circle: CoMo-S layer). 154

Figure 7.4. CO conversion over n ML CoMo-S/ZrO2 (Table 1) in 7,000 ppm H2S containing feed -1 (50 mL/min of 10 vol. % CO and 20 vol. % H2O in helium) at 35,000 h GHSV...... 155 -1 Figure 7.5. Raman spectra for n ML CoMo-S/ZrO2 catalysts collected at 1.2 cm step size. ... 156 -1 Figure 7.6. Raman spectra for n ML CoMo-S/ZrO2 catalysts collected at 0.2 cm step size. ... 157

Figure 7.7. H2 TPR analysis of 0.5 ML, 1 ML, 2 ML, and 4 ML CoMo-S/ZrO2...... 158 Figure 7.8. CO conversion during WGS reaction over the Co/Mo = n catalysts, and commercial

CoMo catalyst (Feed: 10 vol.% CO, 20 vol.% H2O, and 7,000 ppm H2S in He at GHSV = 35,000 h-1 (Co/Mo = 0 and commercial catalysts at GHSV = 39,000 h-1))...... 160 Figure 7.9. Arrhenius plots of CO conversion rate in WGS reaction observed over Co/Mo = 0, 0.1,

0.3, and 1.5 CoMo-S/ZrO2 catalysts...... 161

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Figure 7.10. Raman spectra for the Co/Mo = n catalysts collected at 1.2 cm-1 step size...... 163 Figure 7.11. Raman spectra for the Co/Mo = n catalysts collected at 0.2 cm-1 step size...... 164

Figure 7.12. H2-TPR analysis of Co/Mo = 0, 0.1, 0.3, and 1.5 CoMo-S/ZrO2 catalysts...... 166

Figure 7.13. XPS-spectra Mo 3d region of Co/Mo = 0, 0.1, 0.3, and 1.5 CoMo-S/ZrO2 catalysts...... 167

Figure 7.14. XPS-spectra S 2p region of Co/Mo = 0, 0.1, 0.3, and 1.5 CoMo-S/ZrO2 catalysts...... 169

Figure 7.15. CO conversion during WGS reaction over Co/Mo = 0, 0.3, and 1.5 CoMo-S/ZrO2 catalysts, and commercial CoMo catalyst as a function of time on stream in 7,000 ppm H2S- -1 containing feed (50 mL/min of 10 vol. % CO and 20 vol. % H2O in helium) at 35,000 h GHSV...... 171

Figure 7.16. CO conversion over (a) n ML CoMo-S/ZrO2 and (b) Co/Mo = n catalysts during

WGS reaction employing 7,000 ppm H2S-containing and H2S-free feed (Feed: 10 vol.% CO, and -1 20 vol.% H2O in He at GHSV = 35,000 h (Co/Mo = 0 and commercial catalysts at GHSV = -1 39,000 h ) and 350°C)...... 172

Figure 7.17. WGS activity over Co/Mo = 0.3 and Co/Mo = 1.5 CoMo-S/ZrO2 during 75 hours of

WGS reaction in 7,000 ppm H2S-containing or H2S-free feed (50 mL/min of 10 vol. % CO and v0 mol. % H2O in helium)...... 173 Figure 8.1. Illustrations of proposed (a) atomic deposition method and (b) incipient impregnation method to coat ZrO2 on Al2O3...... 187

Chapter 1. Introduction

1.1. Motivation

Global attention on carbon dioxide (CO2) emissions has increased enormously in recent decades due to its relationship with the greenhouse effect, which traps the sun’s heat on Earth and leads to global warming. Global warming is one of key causes of climate change, which has disastrous environmental effects such as melting icebergs and rising sea levels. Climate change is irreversible and is largely caused by human activities. Most human activities affecting climate change originate from burning fossil fuels. Based on a report from the U.S Energy Information

Administration, 76% of CO2 emissions in 2016 originated from the combustion of fossil fuels [1].

A vast amount of research has been conducted to reduce the CO2 emissions originating from burning fossil fuels, and significant enhancements of technology have been developed. Despite all of these efforts to reduce CO emission from fossil fuels, the amount of CO2 emission from fossil fuels increased 58% in 2016 compared to the amount in 1990 [1], since the demand for fossil fuels has been growing more significantly than the advances of technology. Therefore, replacing the demand for fossil fuels with that of alternative energy sources with zero emission of CO2 has become very important.

Hydrogen is a clean energy carrier and an environmentally friendly fuel that can replace fossil fuels as a feedstock for stationary electric power plants and mobile fuel cells, with zero- emission of CO2 and a high energy density (120 MJ/kg) [2-5]. Hydrogen is also a key element for ammonia synthesis and other downstream chemical processes [6-10]. Research on hydrogen production has been motivated by recent progress in industrial fuel cell technologies [11-17].

Therefore, hydrogen could be a highly promising alternative energy resource to replace fossil fuels.

However, 96% of the hydrogen in the U.S. is produced from fossil fuels. The steam reforming of methane is a typical process to make hydrogen (40%) for further downstream chemical processes, such as ammonia synthesis and hydrotreating of petroleum. Another 38% of hydrogen is produced by partial oxidation of refinery oil, and 18% of hydrogen is produced by coal gasification [18,19]. Only less than 4% of hydrogen is produced by environmentally friendly technologies such as water-splitting. Although hydrogen is a highly promising alternative zero- emission energy resource to replace fossil fuels, challenges remain when hydrogen is produced from fossil fuels. The major challenges of fossil fuels for hydrogen production are high carbon dioxide emissions, uneven resource distribution, and the shortage of fuel reserves [20].

Furthermore, the current cost of hydrogen generated from natural gas is more expensive than natural gas production because methane steam reforming is a very energy intensive process.

Hydrogen production from biomass-derived syngas could overcome these challenges, since biomass is a carbon-neutral and naturally abundant sustainable energy resource [21-23].

Carbon-neutrality implies that plants could recycle carbon dioxide during their life cycle [6,23-

26]. Biomass could off-set the carbon dioxide released from hydrogen production plants since plants consume the carbon dioxide from the atmosphere as part of their natural growth process

(photosynthesis). Thus, hydrogen production using biomass emits low net CO2 during plants’ relatively short life cycle as compared to the cycled of coal, crude oil, and natural gas. The other benefit of biomass is its sustainable nature as the feedstock for hydrogen production, implying that it cannot be depleted like typical fossil fuel. Biomass is mostly derived from plants, which are essential to support life on this planet and will be available as a feedstock as long as they are needed to exist on this planet.

Biomass is an abundant domestic resource. In the United States, there is more available biomass for energy than that required for human food and animal feed. A recent report projects that due to the anticipated improvements in agricultural practices and plant breeding, up to 1 billion dry tons of biomass could be available for energy use annually [26]. In addition to the crops grown specifically for energy use, agriculture crop residues, forest residues, organic municipal solid waste, and animal waste can also be used a biomass feedstock. This sustainable resource can be used to produce hydrogen by gasification, which uses a controlled process involving heat, steam, and oxygen to convert biomass to syngas with low CO2 emission [27-29].

In this respect, the water gas shift (WGS) reaction has attracted significant interest in recent decades because it is the key process of hydrogen production from biomass gasification and reforming. Figure 1.1 shows that the numbers of published papers on the WGS reaction in general and the WGS reaction using biomass have increased annually over the last 20 years. However, conventional WGS catalysts are easily deactivated by sulfur-containing impurities, which are ubiquitous in biomass feedstocks. Previously reported sulfur-tolerant catalysts showed low WGS activity at economically low temperatures. Therefore, it is highly desirable to develop highly active and sulfur-tolerant WGS catalysts.

700 Catalysts for Water gas shift reaction 600 Water gas shift reaction with Biomass

500

400

300

200 Publishedpapers(counts) 100

0 1980 1990 2000 2010 2020 Year

Figure 1.1. Trend of published papers addressing catalysts for the water gas shift reaction and water gas shift reaction with biomass

Another key challenge for hydrogen production by WGS reaction of biomass gasification involves reducing the costs associated with capital equipment, operating and maintenance (O&M), and biomass feedstocks. The plants used for biomass gasification need to be localized with adequate size since the delivery and storage cost of biomass feedstocks and hydrogen have a critical impact on total cost of hydrogen production [23,26]. Intensifying the process (combining steps into fewer operations) could lower the capital cost, developing a highly stable and active catalyst could reduce the cost of O&M, and utilization of a variety of locally available biomass feedstocks could secure the low and stable cost of supplying feedstocks.

The interest in sulfur-tolerant water gas shift catalysts has increased significantly in recent years due to their economic benefits. There are three major economic advantages to using sulfur- tolerant water-gas shift catalysts for hydrogen production. First, the use of a sulfur-tolerant catalyst prolongs the useful life of WGS catalysts. The WGS catalyst cost has a critical impact on the O&M cost of hydrogen production, since the cost of active metals tripled over the last 10 years [30].

Secondly, downstream processing following the biomass-gasification requires acid gas removal (AGR) to remove sulfur components [31]. The AGR process generally requires lowering the humidity and temperature of the pre-heated raw syngas. When a WGS reactor is located downstream from the AGR process, reheating and reinjection of steam are required for the WGS process. Therefore, using sulfur-tolerant WGS catalysts can be useful to increase the efficiency of the overall hydrogen production process through its flexibility of situating the AGR process within the overall production scheme.

Lastly, most biomass contains a significant amount of sulfur (<0.7%) which is converted to H2S, SO2, COS, etc., during its gasification process [22]. Since a trace amount of sulfur compound immediately deactivates the non-sulfur tolerant catalyst, it must be removed. H2S and

SO2 are removed using limestone, dolomite, and CaO, which are economical and have broad usability. However, since COS is difficult to remove, an additional COS removal step or a COS to

H2S conversion (hydrolysis) is required in the gas cleaning step [32]. Conventional Mo-based sulfur-tolerant catalysts are well known for their use as COS hydrolysis catalysts. Therefore, using sulfur-tolerant WGS catalysts could simplify the gas cleaning step and improve the efficiency of

H2S removing process.

1.2. Objectives

This thesis research explored novel supported sulfur-tolerant catalysts for hydrogen production in the WGS reaction using syngas derived from biomass. Therefore, this thesis research focused on the following four objectives related to the development of sulfur-tolerant WGS catalysts:

(1) Demonstrate proof-of-concept for nano-sized Cu-Pd alloys with optimized Cu/Pd ratio as superior sulfur-tolerant catalysts for low-temperature WGS reaction.

(2) Elucidate the role of nano-size effects, promoter effects, and reaction conditions of CoMo nanoparticles on their activity and sulfur dependence in the WGS reaction.

(3) Study the impact of surface coverage and promoter of CoMo sulfide catalysts on WGS catalytic activity and dependence on H2S presence.

(4) Investigate WGS catalytic behavior and synergistic effects for best oxidic supports and optimized CoMo catalyst.

According to the US DOE hydrogen program, currently developed technology will produce

H2 at $3~5/kg of H2, while costs lower than $2/kg are needed for affordable technology for H2 production with near-zero emissions [33]. The objective is to develop new sulfur-resistant, chemically and thermally stable WGS catalysts using sulfur-containing syngas that meet the U.S.

DOE performance goals of 90% CO conversion, 99% selectivity, 30,000 h-1 GHSV, reaction temperature below 400°C, lifetime >5,000 hours durability, sour gas conditions (>4,000 ppm H2S), to enable the lowering of reforming costs for H2 production to < $2/kg.

The influences of the Cu/Pd ratio, CoMo nanoparticle size, oxide support type, MoO3 surface coverage, and cobalt promoter on WGS catalytic activity and sulfur dependence were examined. We studied well-defined Cu, Pd, and Cu/Pd-containing metallic nanoparticles prepared from metal colloids and supported on alumina and compared their behavior during the WGS reaction. We studied nanoscale Mo and Co/Mo WGS catalysts synthesized by chemical reduction in a liquid solvent. CoMo nanoparticle catalysts were optimized by changing particle size and phase structure in order to enhance their WGS activity while maintaining their high sulfur tolerance. Various dopants and oxide supports were explored to develop novel WGS catalysts exhibiting synergistic support-active phase interactions. We systematically investigated the surface structure of ZrO2-supported Mo catalysts by modifying their MoO3 surface coverage, and

6 elucidated their structure-activity relationships in the WGS reaction. We found the best combination of oxidic supports and CoMo-S catalysts by controlling the CoMo oxide surface coverage and Co/Mo atomic ratio.

The composition, surface coverage, type of support, and nanoscale size were studied in order to develop novel, improved WGS catalysts. The study includes contributions to advance existing knowledge through understanding the impacts of the unique nano-structure of the catalyst surface on WGS activity and on sulfur dependence. These complementary contributions lead to an optimized approach to highly active sulfur-tolerant WGS catalysts that potentially enable the reduction of process costs and enhance the efficiency of hydrogen production from biomass- derived syngas. This progress could accelerate the alternative energy transition from fossil fuel to hydrogen using biomass-derived syngas.

In Chapter 2, we provide a comprehensive literature review of previous studies of the WGS catalysts in general and sulfur-tolerant catalysts specifically. Then, we discuss our approaches to develop highly active and sulfur-tolerant WGS catalysts based on the fundamental understanding of the impact of catalyst structure on its activity and sulfur tolerance.

In Chapter 3, we report the synthesis of Cu-Pd nanoparticles supported on Al2O3 catalysts prepared by a chemical co-reduction method, and examine the effect of the Cu/Pd ratio on

0 reducibility, Cu dispersion, CuAl2O4 formation, and WGS catalytic performance.

In Chapter 4, we describe Mo and Co-Mo nanoparticles supported on Al2O3 prepared by a chemical co-reduction method, and investigate the impact of nanoparticle size on WGS activity and sulfur dependence. Experiments to find optimal reaction conditions, such as Co/Mo composition, H2S concentration, GHSV, H2O/CO ratio, and sulfidation time, are also described in this chapter.

In Chapter 5, we describe ZrO2-, Al2O3-, TiO2-, and SiO2-supported Mo sulfide catalysts prepared by an incipient wetness impregnation method. Their WGS activity and sulfur dependence in the WGS reaction were explored to identify the optimal support materials. The characteristic change of the fresh/used ZrO2- and Al2O3-supported Mo sulfide catalysts were investigated.

In Chapter 6, we study the dependence of surface MoO3 coverage on the WGS activity of

ZrO2-supported Mo catalysts. The correlation between the extent of sulfidation and WGS activity is investigated in the catalysts at various levels of MoO3 surface coverage. The optimized surface coverage and WGS activity are observed in ZrO2-supported catalysts containing Mo-S species at monolayer coverage.

In Chapter 7, we explore the best Co-promoted Mo sulfide supported on ZrO2 catalyst. The optimized catalysts were investigated by modifying the surface coverage and Co/Mo atomic ratio.

Transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, temperature-programmed reduction (TPR), and X-ray photoelectron spectroscopy (XPS) analysis were conducted to elucidate the impact of CoMo oxide surface coverage and the promoter effect of cobalt in the WGS reaction.

Finally, in Chapter 8, we provide suggestions for future research to further improve the activity of low temperature WGS catalysts in the presence of sulfur in the industrial feed conditions.

1.3. Reference

1. M. Desai, R. P. Harvey, "Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2015," Federal Register, 82, 2017, 10767

2. D. S. Reichmuth, A. E. Lutz, D. K. Manley, J. O. Keller, "Comparison of the Technical Potential for Hydrogen, Battery Electric, and Conventional Light-Duty Vehicles to Reduce Greenhouse Gas Emissions and Petroleum Consumption in the United States," International Journal of Hydrogen Energy, 38, 2013, 1200-1208

3. M. D. Paster, R. K. Ahluwalia, G. Berry, A. Elgowainy, S. Lasher, K. McKenney, M. Gardiner, "Hydrogen Storage Technology Options for Fuel Cell Vehicles: Well-to-Wheel Costs, Energy Efficiencies, and Greenhouse Gas Emissions," International Journal of Hydrogen Energy, 36, 2011, 14534-14551

4. A. Molino, G. Giordano, V. Motola, G. Fiorenza, F. Nanna, G. Braccio, "Electricity Production by Biomass Steam Gasification using a High Efficiency Technology and Low Environmental Impact," Fuel, 103, 2013, 179-192

5. W. Chen, M. Lin, T. L. Jiang, M. Chen, "Modeling and Simulation of Hydrogen Generation from High-Temperature and Low-Temperature Water Gas Shift Reactions," International Journal of Hydrogen Energy, 33, 2008, 6644-6656

6. J. Brau, M. Morandin, "Biomass-Based Hydrogen for Oil Refining: Integration and Performances of Two Gasification Concepts," International Journal of Hydrogen Energy, 39, 2014, 2531-2542

7. M. Farniaei, M. Abbasi, A. Rasoolzadeh, M. R. Rahimpour, "Enhancement of Methanol, DME and Hydrogen Production Via Employing Hydrogen Permselective Membranes in a Novel Integrated Thermally Double-Coupled Two-Membrane Reactor," Journal of Natural Gas Science and Engineering, 14, 2013, 158-173

8. R. Rauch, A. Kiennemann, & A. Sauciuc, Fischer-tropsch synthesis to biofuels (BtL process). "The Role of Catalysis for the Sustainable Production of Bio-Fuels and Bio-Chemicals," 2013, 397-443

9. T. Li, H. Wang, Y. Yang, H. Xiang, Y. Li, "Study on an Iron-Nickel Bimetallic Fischer-Tropsch Synthesis Catalyst," Fuel Processing Technology, 118, 2014, 117-124

10. S. Shao, A. Shi, C. Liu, R. Yang, W. Dong, "Hydrogen Production from Steam Reforming of Glycerol Over Ni/CeZrO Catalysts," Fuel Processing Technology, 125, 2014, 1-7

11. H. J. Alves, C. Bley Junior, R. R. Niklevicz, E. P. Frigo, M. S. Frigo, C. H. Coimbra-Araújo, "Overview of Hydrogen Production Technologies from Biogas and the Applications in Fuel Cells," International Journal of Hydrogen Energy, 38, 2013, 5215-5225

12. S. Koppatz, C. Pfeifer, R. Rauch, H. Hofbauer, T. Marquard-Moellenstedt, M. Specht, "H2 Rich Product Gas by Steam Gasification of Biomass with in situ CO2 Absorption in a Dual Fluidized Bed System of 8 MW Fuel Input," Fuel Processing Technology, 90, 2009, 914-921

13. P. Kruger, "Appropriate Technologies for Large-Scale Production of Electricity and Hydrogen Fuel," International Journal of Hydrogen Energy, 33, 2008, 5881-5886

14. H. L. Hellman, R. van den Hoed, "Characterising Fuel Cell Technology: Challenges of the Commercialisation Process," International Journal of Hydrogen Energy, 32, 2007, 305-315

15. S. Mekhilef, R. Saidur, A. Safari, "Comparative Study of Different Fuel Cell Technologies," Renewable and Sustainable Energy Reviews, 16, 2012, 981-989

16. K. Schoots, G. J. Kramer, B. C. C. van der Zwaan, "Technology Learning for Fuel Cells: An Assessment of Past and Potential Cost Reductions," Energy Policy, 38, 2010, 2887-2897

17. U. H. Jung, W. Kim, K. Y. Koo, W. L. Yoon, "Genuine Design of Compact Natural Gas Fuel Processor for 1-kWe Class Residential Proton Exchange Membrane Fuel Cell Systems," Fuel Processing Technology, 121, 2014, 32-37

18. I. Chorkendorff, J. W. Niemantsverdriet, "Heterogeneous Catalysis in Practice: Hydrogen," Concepts of Modern Catalysis and Kinetics, 2003, 301-309

19. P. Corbo, F. Migliardini, O. Veneri, "Hydrogen Fuel Cells for Road Vehicles," Hydrogen Fuel Cells for Road Vehicles, 2011,

20. DOE, Office of Fossil Energy, "Hydrogen from Coal Program," 2008, 1-85

21. V. S. Sikarwar, M. Zhao, P. Clough, J. Yao, X. Zhong, M. Z. Memon, N. Shah, E. J. Anthony, P. S. Fennell, "An Overview of Advances in Biomass Gasification," Energy and Environmental Science, 9, 2016, 2939-2977

22. A. A. Ahmad, N. A. Zawawi, F. H. Kasim, A. Inayat, A. Khasri, "Assessing the Gasification Performance of Biomass: A Review on Biomass Gasification Process Conditions, Optimization and Economic Evaluation," Renewable and Sustainable Energy Reviews, 53, 2016, 1333-1347

23. Oak Ridge National Laboratory, 2016 Billion-Ton Report: Advancing Domestic Resources for a Thriving Bioeconomy, Volume 2: Environmental Sustainability Effects of Select Scenarios from Volume 1, ORNL/TM-2016/727, 2017

24. S. Chianese, J. Loipersböck, M. Malits, R. Rauch, H. Hofbauer, A. Molino, D. Musmarra, "Hydrogen from the High Temperature Water Gas Shift Reaction with an Industrial Fe/Cr Catalyst using Biomass Gasification Tar Rich Synthesis Gas," Fuel Processing Technology, 132, 2015, 39- 48

25. A. Molino, S. Chianese, D. Musmarra, "Biomass Gasification Technology: The State of the Art Overview," Journal of Energy Chemistry, 25, 2016, 10-25

26. Oak Ridge National Laboratory, 2016 U.S. billion-ton report: Advancing domestic resources for a thriving bioeconomy. volume 1: Economic availability of feedstocks, ORNL/TM-2016/160, 2016

27. A. Tanksale, J. N. Beltramini, G. M. Lu, "A Review of Catalytic Hydrogen Production Processes from Biomass," Renewable and Sustainable Energy Reviews, 14, 2010, 166-182

28. E. Shayan, V. Zare, I. Mirzaee, "Hydrogen Production from Biomass Gasification; a Theoretical Comparison of using Different Gasification Agents," Energy Conversion and Management, 159, 2018, 30-41

29. S. K. Sansaniwal, K. Pal, M. A. Rosen, S. K. Tyagi, "Recent Advances in the Development of Biomass Gasification Technology: A Comprehensive Review," Renewable and Sustainable Energy Reviews, 72, 2017, 363-384

30. U.S. Department of the Interior, U.S. Geological Survey. (2019). Commodity statistics and information. Retrieved from https://minerals.usgs.gov/minerals/pubs/commodity/

31. K. J. Andersson, M. Skov-Skjøth Rasmussen, P. E. Højlund Nielsen, "Industrial-Scale Gas Conditioning Including Topsøe Tar Reforming and Purification Downstream Biomass Gasifiers: An Overview and Recent Examples," Fuel, 203, 2017, 1026-1030

32. C. Ratnasamy, J. Wagner, "Water Gas Shift Catalysis," Catalysis Reviews - Science and Engineering, 51, 2009, 325-440

33. National Energy Technology Laboratory, Assessment of hydrogen production with CO2 capture volume 1: Baseline state-of-the-art plants, DOE/NETL-2010/1434, 2010, United states: U.S. Department of Energy.

Chapter 2. Background and Literature review

2.1. Hydrogen production from biomass-derived syngas

2.1.1. The hydrogen economy

The hydrogen economy has experienced cycles of high expectations and insurmountable challenges. In 1995, the Department of Energy (DOE) identified hydrogen as “a critical and indispensable element of a decarbonized, sustainable energy system” that would provide secure, cost-effective, and non-polluting energy by [1]. However, these expectations of hydrogen have not yet been met. The hydrogen economy has suffered from cost and performance challenges, particularly during periods when the price of typical fossil fuels has plummeted due to global economic recession, geopolitical conflict, or technological advances involving the use of fossil fuels. However, the promise of hydrogen as an alternative energy resource to replace fossil fuel still remains high due to its zero CO2 emission. Currently, more than 1,200 energy leaders from

90 countries consider hydrogen as the lowest impact energy resource [2]. The Hydrogen Council, which consists of thirteen international corporations committed to implementing policies favoring the development of hydrogen as an energy source, recently suggested that hydrogen is the key solution to the energy transition away from fossil fuels [3].

Three factors can explain the rejuvenated interest in the hydrogen economy. First, the technology for production, distribution, and storage of hydrogen has improved significantly in recent years [4]. The DOE estimated that the cost of hydrogen production, including distribution, could approach $3.80/kg in 2015 from $4.50/kg in 2011 [5]. Second, products operated by hydrogen fuel are widely commercialized. Hydrogen fuel cell vehicles are commercially available in several countries, while 12,000 forklifts powered by hydrogen fuel cell are currently deployed in the United States [4], and 225,000 fuel cell home heating systems have been sold in Japan [6].

Third, hydrogen as an energy storage medium has attracted interest to balance long-term intermittency in electricity generation from wind and solar power [7]. Hydrogen can be a promising large-scale and long-term energy storage method compared to the proposed solutions, such as using lithium ion batteries, or sodium sulfur batteries.

2.1.2. Hydrogen production methods

Most hydrogen (~96%) is currently produced from fossil fuels: 48% from high-temperature steam reforming of natural gas, 30% from the partial oxidation of refinery oil, and 18% from coal gasification) [8,9]. The major challentes of hydrogen production using fossil fuels are high carbon dioxide emissions, uneven resource distribution, and the shortage of fuel reserves [10]. However, hydrogen production from biomass-derived synthesis gas (syngas) technologies could overcome these challenges, since the biomass is a carbon-neutral and a domestically abundant sustainable energy resource [11-13]. Biomass is a promising carbon-neutral feedstock to produce hydrogen, and is discussed in more detail below.

T-Raissi et al. provided a review of typical hydrogen production technologies by steam reforming [14]. Currently, 48% of the hydrogen in the U.S. market is produced from natural gas.

Steam reforming of methane is a typical process to obtain hydrogen for further downstream chemical processes, such as ammonia synthesis, Fisher-Tropsch synthesis of hydrocarbons, and hydrotreating of refinery oil [15-17]. However, the cost of hydrogen generated from natural gas is higher than natural gas production since methane steam reforming is an energy-intensive process and requires a high content of steam.

Although partial oxidation of oil is a possible route to hydrogen production, there remain critical drawbacks to this method. 30% of hydrogen produced today is produced by partial oxidation of refinery oil [18]. The partial oxidation reaction occurs when fuel-air mixture with

13 a low air/fuel ratio is partially combusted in a partial oxidation reaction by the following equation:

H + = CO + . s. A wide variety of feedstocks could apply to partial oxidation for hydrogen production, although more feedstock is required due to its low hydrogen yield, and carbon deposition is a critical drawback [19,20]. In addition to its low efficiency, refinery oil is unsecured energy feedstock since it is an unevenly distributed source, and the extraction cost of raw refinery oil is increasing annually.

Gasification-based methods are efficient and environmentally friendly technologies to produce hydrogen [21]. 18% of hydrogen can be produced from coal by gasification [22]. Coal gasification shows a faster reaction rate than steam reforming, requires relatively cheap feedstock, and saves energy cost by combining with electricity generating plant. Figure 2.1 shows a schematic of gasification-based energy conversion options. A gasifier can convert carbon-based feedstocks with steam and oxygen at high temperature and moderate pressure to syngas, a mixture of CO,

CO2, hydrogen, steam, hydrocarbons, and impurities. Then, the water gas shift (WGS) reaction is used to convert CO and H2O to CO2 and additional hydrogen.

Figure 2.1. Gasification-based energy conversion options [23].

However, this method has higher costs, as the solid fuel requires a high level of pre- treatment and a CO2-capture system [23,24]. The greatest challenge in hydrogen production from coal gasification is the presence of sulfur compounds, such as H2S and CoS, as impurities in the syngas of the coal gasification process, which deactivate easily the catalysts of hydrogen production and could damage further downstream processes [25] According to the US DOE hydrogen program, current catalytic gasification technologies produce H2 at $4-5/kg of H2 with a

WGS catalytic reactor, while costs lower than $2/kg are needed for affordable technology for H2 production with zero CO2 emissions [26]. The main costs of H2 production by gasification are associated with the capital cost to install the gasifier and selexonol desulfurization plant, and operation and maintenance (O&M) costs such as the costs of water supplies, initial fill of catalysts, and replacement of deactivated WGS catalysts [27].

The WGS reaction using sulfur-tolerant catalysts has advantages in its flexibility in situating the sulfur removal process, and its ability to combine both carbonyl sulfide (COS) hydrolysis and sulfur removal process into a single-unit operation [28,29] which has the potential to lead to cost reduction by simplifying the overall process. However, previously reported sulfur- tolerant Mo-based catalysts showed a low WGS activity at relevant high space velocities and low temperatures [30]. From these economic standpoints, it is highly desirable to develop highly active sulfur-tolerant WGS catalysts that display low methanation activity and strong stability.

Biomass is a promising feedstock to produce hydrogen since it is a carbon-neutral, evenly distributed, and abundant. Among the thermochemical conversion technologies, gasification is an attractive method to convert biomass feedstock into usable syngas, [10–12]. However, the efficiency of the hydrogen production process using biomass gasification (46%) is lower than that of any other hydrogen production method (56~72%) [4] (the production efficiency is defined as the ratio of the chemical energy of produced hydrogen to total energy required for hydrogen processing, including the chemical energy of feedstock) [31]. Although the sulfur content of biomass feedstock is considerably lower than that of coal, it still contains significant amounts of sulfur compounds. Therefore, it is highly desirable to develop sulfur-tolerant and highly active catalysts.

Biomass is defined as any organic material derived from plants or animals, and can be divided into four types of sources: agricultural residue, wood fibers, energy crops, and municipal waste. The advantages and disadvantages of various biomass sources are summarized in Table 2.1.

Cheah et al. reported that the H2S concentration of biomass (wood-derived) ranged from 20 to 600 ppm [32]. Yang et al. reported that the H2S content of biomass (algae) is between 0.5% and 1.5% of total dry weight [33]. Biomass derived from algae has typically higher sulfur content than

16 terrestrial biomass [34]. The sulfur content of the aerosol from co-combustion of coal and biomass was constant at 0.1 wt. %. The raw syngas from the waste (tar) of the entrained flow biomass gasification contains a similar concentration of sulfur to that in coal (>1 wt. %) [35].

Table 2.1. Advantages and disadvantages of biomass sources for bio-fuel plants [36].

Biomass Advantages Disadvantages source Agricultural Lower cost than forest residue. Existing harvest equipment and residue Renewable. storage systems are immature. (rice, corn Most of the current agricultural waste Lower economic benefit than stover, and is plowed back into the soil, thrown harvesting corn for animal feed. wheat away, or burned. straw) Wood fibers Widely used source of renewable Wood chips could become too (forestry energy. expensive to compete. and wood Current infrastructure and natural There are competing demands for wastes) resources already exist. wood fiber that may drive prices up. Use of forest waste helps decrease fire hazards associated with dead wood. Energy Requires little water or fertilizer to Enzymes needed to break down the crops grow without effort of planting. cellulose are expensive. (switch Thrives in places unsuitable for most Department of Energy and private grass and crops. investors are working diligently to miscanthus) Yields twice as much ethanol per acre solve this problem. than corn. Municipal Provides a local solution to waste Converting compostable waste to waste accumulation with a 30-50 mile radius biofuel is less efficient than recycling of its generation point. paper products. Establishing municipal waste to gasification plants improves economic welfare of local communities. 2.2. Water gas shift reaction and its applications

2.2.1. Water gas shift reaction

The water gas shift (WGS) reaction is defined as a redox-type reaction to convert water vapor and carbon monoxide into hydrogen and carbon dioxide, as follows:

CO (g) + () ⟷ () + () = −41.1 (1)

Although Italian physicist Felice Fontana had already discovered the WGS reaction in 1780, it did not obtain popularity until the industrial value of the reaction was recognized. However, from the early 20th century, the Haber-Bosch ammonia synthesis, which required a cheaper and more efficient process to produce hydrogen, and the Fisher-Tropsch reaction, which required specific ratios of carbon monoxide to hydrogen, became important triggers of systematic research into the

WGS reaction. Presently, as the hydrogen economy is gaining momentum, interest in the WGS as a promising process of hydrogen production has increased considerably.

Since the WGS reaction is reversible and mildly exothermic, WGS activity depends on temperature. The equilibrium constant for the WGS reaction can be expressed by Eq. (2) as well as a simplified Eq. (3) [37].

5693.5 49170 ln = + 1.077 ln + 5.44 × 10 T − 1.125 × 10 −

− 13.148 (2)

4577.8 = exp( − 4.33 ) (3)

Due to exothermic reversibility, the WGS reaction needs to be conducted at low temperature when pure H2 is desired. On the other hand, a high reaction temperature is required to achieve a desirable reaction rate due to kinetic limitations at low temperature. Because of these kinetic and thermodynamic limitations, a conventional WGS reactor is designed in two stages: a high-temperature shift (HTS, typically 350-450°C) and a low-temperature shift (LTS, typically ~

250°C).

2.2.2. Applications of the WGS reaction

The WGS reaction has gained renewed interest as a key process to produce pure hydrogen from gasification. The pure hydrogen produced could be used to power lightweight vehicles, 18 forklift trucks, home heating systems, and stationary electricity production plants equipped with a fuel-cell system. The United States Department of Energy (DOE) has predicted that 10% of annual energy consumption of United States could be supplied using hydrogen resources by 2030 [26].

The National Research Council has projected that hydrogen fuel could reduce gasoline consumption by 70% as early as 2050 [38].

The WGS reaction plays an important role for the gas conditioning step of feed gas to adjust the CO/H2 ratio depending on the downstream products of the refinery process, such as ammonia production, hydrocracking of petroleum, or as a fuel [39]. In addition, the raw gasifier syngas is utilized directly in a gas turbine as a fuel, and the WGS reactor can increase the hydrogen content of syngas in a gas turbine as a preliminary purification step [23].

WGS catalysts can also improve the performance of platinum catalysts in fuel cells since their stability depends on the purity of H2 in the reformate that feeds to the fuel cells. Because the hydrogen from a reformer in fuel cell contains residual CO which acts as a poison for the platinum catalyst in fuel cells, the WGS reactor can convert residual CO into usable hydrogen. The WGS reaction thus has many applications that would benefit from the optimization of its reaction conditions.

2.3. WGS Reaction Mechanism

Due to the industrial significance of hydrogen production using the WGS reaction, many researchers have investigated its reaction mechanism and developed models to describe the behavior of typical industrial catalysts (Cu- or Fe- based). Two main reaction mechanisms have been proposed: the redox mechanism, and the associative mechanism. The redox mechanism includes the successive oxidation and reduction of the surface, namely a surface oxidation H2O*

→ H2+ O* followed by a surface reduction CO* + O* → CO2 +*. In the associative mechanism,

19 intermediate species are formed on the surface from surface CO and OH, which then decompose to CO2 * and H*. These reaction mechanisms are explored in more detail below within the context of different catalysts.

2.3.1. Redox Mechanism

Ovesen et al. [40] have analyzed the kinetics of the WGS reaction over three different Cu- based catalysts under relevant experimental conditions: Cu-ZnO-Al2O3, Cu-Al2O3, and Cu-SiO2.

The elementary reaction steps are shown as follows:

1. CO(g) + * ⇔ CO* 2. H2O(g) + * ⇔ H2O* 3. H2O* + * ⇔ OH* + H* 4. OH* + * ⇔ O* + H* 5. OH* + OH* ⇔ H2O* + O* 6. CO* + O* ⇔ CO2* 7. CO2* ⇔ CO2(g) + * 8. H* + H* ⇔ H2(g) + 2* where the asterisk indicates a vacant surface site and X* means an adsorbed species. The rate- limiting step is determined by the ratio of the feed gas mixture. At low H2O/CO ratios, step 3 is rate-limiting. Step 7 is rate-limiting at high H2O/CO ratios, whereas step 4 is kinetically significant for CO2 + H2 mixtures.

The redox mechanism is the most commonly accepted mechanism for the high-temperature shift WGS reaction employing Fe-based and unpromoted Mo-S catalysts, which involves a regenerative change in the oxidation state of the catalytic metal [41-44]. In this mechanism, H2O is activated first by the abstraction of an H atom from water followed by dissociation or disproportionation of the resulting OH to afford atomic O. The CO is then oxidized by the atomic

O, forming CO2, which returns the catalytic surface back to its pre-reaction state.

Shi et al. [45] investigated the WGS reaction over the S-modified MoS2 (100) surface. The surface had Mo-edge with 37.5% S coverage and an S-edge with 50% sulfur coverage. CO was adsorbed on both Mo- and S-terminated surfaces with adsorption energies of −1.11 and −0.95 eV, respectively. H2O was adsorbed on both Mo- and S-terminated surfaces with adsorption energies of −0.81 and −0.34 eV, respectively. Subsequent dissociation of OH to H and O (redox mechanism) or association of CO with OH to form HCOO species (associative mechanism) occurred. The activation energy for OH dissociation was much lower than for the HCOO formation with CO and

OH on both terminations (Mo-edge: 1.93 vs. 3.04 eV and S-edge: 1.25 vs. 2.26 eV). This suggests that the redox mechanism was the preferred pathway for the WGS reaction, in accordance with experimental findings [46-48]. The rate-determining step was different on both surfaces. On the

Mo-terminated edge, the dissociation of OH to form surface O and H was rate-determining (Ea =

1.93 eV), whereas on the S-terminated edge, dissociation of H2O to form OH and H species (Ea

=1.70 eV) was rate-determining. The conclusions from this study are counter to the results reported for CO2 hydrogenation to methanol on the Mo6S8 cluster by Liu et al. [49].

2.3.2. Associative Mechanism

Campbell et al. [50] researched the WGS reaction in terms of an associative mechanism.

In their experimental investigation, the reaction rate was independent of partial pressure of CO and increased with the partial pressure of H2O. In addition, the surface coverages of CO and H2O were very low under their reaction conditions, which was explained by the inclusion of a hydroxyl intermediate formed from the surface dissociation of adsorbed water. Steps 4, 5, and 6 in the redox mechanism are replaced according to the following reaction steps in the associative mechanism.

The elementary reaction steps are shown below:

1. CO(g) + * ⇔ CO*

2. H2O(g) + * ⇔ H2O* 3. H2O* + * ⇔ OH* + H* 4. CO* + OH* ⇔ COOH* + * 5. COOH* + * ⇔ CO2* + H* 6. COOH* + OH* ⇔ CO2* + H2O* 7. CO2* ⇔ CO2(g) + * 8. H* + H* ⇔ H2(g) + 2*

In this mechanism, CO and H2O are adsorbed onto the surface of the metal catalyst, followed by the formation of an intermediate and desorption of H2 and CO2. In the initial step,

H2O dissociates into a metal adsorbed OH and H. The hydroxide then reacts with CO to form a carboxyl or formate intermediate which subsequently decomposes into CO2 and the metal- adsorbed H, ultimately yielding H2. While this mechanism is usually valid under low-temperature conditions, the redox mechanism which does not involve any long-lived surface intermediates is a more suitable explanation of the WGS mechanism at higher temperatures.

It is still controversial which mechanism is dominant over the Cu-based low-temperature shift WGS reaction, noble metal, and Co-promoted Mo-S catalysts. The WGS reaction cannot take place merely along the single reaction pathway and there are various possible reaction steps between adsorbed CO and surface hydroxyl groups. Density functional theory (DFT) and microkinetic modeling studies have been employed to identify the preferred reaction pathway over the model catalysts.

The thermochemistry and activation energy barriers of various elementary steps for Cu

(111) and Pt (111) surfaces are discussed by Gokhale et al. [51] in the subsequent sections and shown in Figure 2.2. The green arrow indicates the minimum energy pathway for the WGS reaction. The activation barrier for intermediate formation (0.61 eV) is significantly lower than the barrier for OH dissociation (1.76 eV), suggesting that intermediate can be formed under low- temperature WGS reaction conditions. In the detailed microkinetic modeling studies over the WGS

22 reaction [50,52-56], they suggested that the dominant reaction mechanism on Cu (100) surface is the associative mechanism. The estimated WGS reaction rate including CO oxidation by OH, followed by the COOH decomposition by OH is almost the same as the overall experimental WGS reaction rate.

Figure 2.2. Reaction network for the WGS reaction including both the surface redox and carboxyl associative mechanisms. The thermochemistry and kinetic barriers for all elementary steps are given in electron volts. For reactions involving bond making, the activation barriers are reported with respect to the adsorbed reactants at infinite separation from each other [51].

Chen et al. [57] investigated a MoS2 (100) surface for the WGS reaction to compare the redox and associative mechanisms, and these authors also investigated a third mechanism involving an active surface COOH intermediate, as proposed for the Cu (111) surface by Gokhale et al. [51]. The authors reported that COOH was a kinetically favored intermediate (Ea = 0.62 eV in CO + OH + H → COOH + H, associated mechanism); however, CO oxidation by surface O (Ea

= 0.30 eV in CO + O + 2H → CO2 + 2H) was seen to be much lower than COOH dissociation (Ea

= 1.89 eV in COOH + H → CO2 + 2H). The DFT investigation suggested that the redox mechanism

23 was the preferred reaction pathway on the Mo-edge in MoS2, and the rate-determining step was the dissociation of OH into surface O and H. Formate species (HCOO) or carboxyl (COOH)- associated mechanism on the S-edge can be possible reaction pathway in CoMo-S catalysts since the rate-limiting step differs for each catalyst and is dependent on the type of surface species bound to Mo.

Zhang et al. [58] studied the DFT calculations on the S-edge of CoMo-S WGS catalysts.

They reported that the dissociation energy of H2O (0.64eV) on the S-edge of CoMo-S was similar to the desorption energy of H2O (0.63eV), suggesting that the S-edge of CoMo-S was able to initiate the WGS reaction with H atom co-adsorption. The activation energy of the rate- determining steps via the most favorable pathway on the S-edge of CoMo-S (0.64eV, COOH- mediated mechanism) is lower than that on Mo-edge of CoMo-S (1.20eV, redox mechanism), suggesting that the WGS reaction occurs dominantly on the S-edge of CoMo-S. The reaction barriers of rate-determining steps on the S-edge of CoMo-S via the COOH-mediated associative mechanism (0.64 eV) was lower than that via the redox mechanism (0.94 eV), indicating that the

COOH-mediated associated mechanism is more favorable on the S-edge of CoMo-S.

2.4. Current WGS catalysts for low-temperature sour shift and their limitations

2.4.1. Conventional Cu-based and Fe-based catalysts

Previously, our research group investigated the WGS activity of γ-alumina-supported Cu catalysts prepared from metal colloids and by conventional support impregnation with Cu and Ni salt solutions [59]. The WGS reaction rate at 200°C for supported colloidal Cu catalysts was 2.5 times higher than that for monometallic Cu catalysts. It was also demonstrated that Cu-Ni alloy shell/Cu core nanoparticles could be synthesized as metal colloids at low temperature that displayed higher WGS activity than pure Cu catalysts without methanation behavior typical for

24 the Ni catalysts [60]. Thus, the colloidal method can offer a controlled synthesis of desired metallic nanoparticles with well-defined shape, composition, and WGS reactivity.

The major disadvantage of Cu-based catalysts is their high sensitivity to the presence of sulfur in syngas. Therefore, the WGS process utilizing Cu-based catalysts requires an expensive sulfur removal step. In addition, Cu-based catalysts are pyrophoric and increase the temperature in the WGS reactor. This temperature rise causes the Cu catalyst particles to sinter, resulting in a loss of surface area and catalytic activity [61].

Ni- and Fe-based WGS catalysts are widely used to achieve rapid chemical equilibrium in the WGS reaction [62]. However, commercial catalysts prepared by impregnation of oxide supports undergo sulfur poisoning in sulfur-containing feed, which is the primary cause of their deactivation. Previous researchers discovered that stabilization of nano-sized metallic (Ni, Fe, and

Cu) particles against sulfur poisoning by promoting with chromium is highly promising for maintaining their high activity and stability [63]. However, highly toxic and carcinogenic Cr6+ may be generated during the catalyst synthesis, the WGS reaction, and spent catalyst disposal [64].

Therefore, it is desirable to develop Cr-free sulfur-tolerant and highly active LTS WGS catalysts

[64,65].

The challenges to using Fe2O3-Cr2O3 HTS catalysts are that the catalysts are easily deactivated when H2S concentration in syngas exceeds 500 ppm [32] and the syngas must be desulfurized before reaching the HTS catalyst. Typically, the sulfur removal process requires the syngas to be cooled down in order to remove excess moisture which was introduced in the wet- scrubber process that is necessary to remove particulate matter in the incoming syngas from the gasifier [27]. Following the sulfur removal process, the syngas needs reheating and steam reinjection steps to meet HTS requirements (450°C, H2O/CO = 2) [39], which are energy intensive

25 processes required when the sulfur removal process is located before the WGS reactor. However, the use of a sulfur-tolerant catalyst can offer flexibility in when to remove the sulfur, which is attractive to reduce the capital equipment cost and O&M costs of hydrogen production.

2.4.2. Current sulfur-tolerant WGS reaction catalysts

Recently investigated sulfur-tolerant WGS reaction catalysts are summarized in Table 2.2.

The highest WGS activity of commercial Fe–Cr catalysts is reported to occur in the 400°C~500°C temperature region and above 100,000 h-1 of gas hourly space velocity (GHSV) [66], whereas 57% of CO conversion drop in 168 ppm of H2S is a significant disadvantage for application to hydrogen production using a significant amount of sulfur-containing feed. Ce-promoted Fe-Cr catalyst showed high CO conversion and thermal stability without CO conversion drop in 400 ppm H2S- containing feed [30]. Ce-promoted Fe-Cr catalysts could be promising candidates for sulfur- tolerant catalysts, but the catalysts suffered from carbon deposition and methanation side-reaction during the reaction, posing challenges for these candidates to be developed as competitive WGS reaction catalysts. Moreover, the allowance of H2S concentration (400 ppm) in feed is insufficient to operate under the sour shift reaction (sulfur-rich condition above 1,000 ppm H2S). Lanthanide sulfide catalysts were proposed to be active over the WGS reaction under 650°C~700°C up to 700

-1 ppm H2S-containing feed at GHSV = 42,000 h [67]. The CO conversion of nano-scale Eu2O2S was higher than that of the micro-scale La2O2SO4 catalyst, while the nano-scale catalyst recovered its activity faster than the micro-scale catalyst in H2S-free feed [67]. However, the CO conversion of the catalysts was decreased by 10% in 700 ppm H2S-containing feed, and lanthanide catalysts are more expensive to produce than transition metals, such as cobalt and molybdenum. Mo-based catalysts showed high WGS activity and significant increase of CO conversion in 7,400 ppm H2S- containing feed [68], which makes the Mo-based catalysts the most promising sulfur-tolerant

26 catalysts. However, it should be noted that the Mo-based catalyst required significantly low GHSV

(6,000 h-1) and showed low reaction rate compared to commercial Fe-Cr catalysts, indicating that

Mo-based catalysts require a large volume of the reactor. Therefore, it is highly desirable to enhance the WGS activity of Mo-based catalysts at GHSV ≥ 30,000 h-1.

Table 2.2. Summary of recently investigated sulfur tolerant WGS reaction catalysts.

Reaction CO conditions Conversion in H S impact on Catalyst (temperature, H S-free/H S- 2 Remark Ref 2 2 CO conversion catalysts loading, containing space velocity) feed (%)

240°C, 0.03g, 50~60% / >75% Drop 25% recovery Mo C [69] 2 125,000 h-1 10~15% (5 ppm) for 10 hours

Micron- 700°C, 0.1g, ~60% / 5~10% drop ~ 100% -1 [67] La2O2SO4 42,000 h 50~55% (70 ~700 ppm) recovery ~100% Nano- 650°C, 0.03g, 85~90% / ~10% Drop recovery for [67] Eu O S 140,000 h-1 75~80% (121 ppm) 2 2 0.25 hours Commercial 450°C, 1 g, 90~95% / 57% Drop 95% recovery [66] Fe/Cr 100,000 h-1 40%~50% (168 ppm) for 80 hours

No significant 550°C, 0.1g, 85~90% / Fe/Ce/Cr drop – [30] 60,000 h-1 85~90% (400 ppm) ~100% 400°C, 2g, 40~45% / activity CoMo-S – [68] 6,000 h-1 90~95% increase (7400 ppm) 2.4.3. Sulfur-tolerant Mo sulfide-based WGS catalysts

Table 2.3 summarizes a considerable variation in the reported activity of Mo-based sulfur- tolerant catalysts. The advantage of Mo-based WGS catalysts is their high sulfur tolerance. It has been suggested that the Mo-based catalysts even need sulfur to maintain their activity for WGS

27 systems [48]. However, conventional Mo-based WGS catalysts have suffered from their low activity at low temperatures compared to the Fe-based catalysts [70].

Table 2.3. Summary of sulfur-tolerant Mo-based catalysts.

Normalized Catalyst Temperature CO reaction rate Feed composition Ref composition and GHSV conversion at 350°C, -1 -1 µmolgcat s Co Mo / 10% CO, 20% H O in 0.5 0.5 2 180°C 90% 120 [71] Carbide He, 0 ppm

10% CO, 20% H2O, 450°C, Mo10/Al2O3 6 -1 ~ 10% 2 [46] 2,500 ppm H2S in He 1.4×10 h

Co1Mo7/ 20% CO, 40% H2O, 350°C, CeO2- -1 50~70% 2.5 [72] 43%H2, 16% CO2 3,000 h Al2O3-MgO

Co3Mo9/ 20% CO, 40% H2O, 350°C, -1 40~60% 2.0 [73] Al2O3 43%H2, 16% CO2 2,000 h

K4Ni3Mo17/ 400°C, 8% CO, 24% H2O -1 90% 3.0 [74] Al2O3 2,000 h

CoMo/ 25% CO, 15% H2O, 55% 350°C, -1 90% 7.5 [75] MgO-Al2O3 H2 in N2 5,000 h Commercial 22% CO, 66% H O, 10% 450°C, Co-Mo 2 25% 20 [70] H , 1% CO , 2,670 ppm 16,666 h-1 catalysts 2 2 Commercial 22% CO, 66% H O, 10% 450°C, Fe-Cr 2 40% 37 [70] H , 1% CO , 1,000 ppm 16,666 h-1 catalysts 2 2

K1Co4Mo16/ 2% CO, 5% H2O, 20% 350°C, -1 20% 35 [76] C H2, 1,200 ppm 70,000 h

Co4Mo8/ 30% CO, 30% H2O, 35% 350°C, -1 68% 37 [77] Al2O3_TiO2 H2, 5% CO2, 5,000 ppm 10,000 h

K10Co4Mo8/ 40% CO, 40% H2O, 15% 423°C, Al2O3- -1 50% 1.5 [78] H2, 5% CO2, 3,000 ppm 3,000 h MgAl2O4

Co4Mo24/ 32% CO, 39% H2O, 14% 400°C, -1 62% 6 [79] MgO-Al2O3 H2, 3% CO2, 830 ppm 10,000 h

132 (TiO ), Ni Mo / 33% CO, 33% H O, 352°C, 2 0.3 8 2 60~75% 97 (ZrO ), [80] metal oxides 13,000 ppm 20,000 h-1 2 33(Al2O3)

Ni0.3Mo8/ 33% CO, 33% H2O, 352°C, -1 87% 330 [81] TiO2-ZrO2 13,000 ppm 20,000 h

Co0.3Mo8/ 20% CO, 40% H2O, 350°C, -1 90% 3.5 [82] Al2O3 27.5%H2, 2.5% CO2 3,000 h

Mo0.5Pt4.7/ 10% CO, 20% H2O in 270°C, -1 ~10% 58 [83] SiO2 He, 0 ppm 150,000 h

Ni5Mo20/ 30% CO, 36% H2O, 14% 400°C, -1 60% 12 [84] TiO2 H2, 3% CO2, 450 ppm 41,000 h Note: The normalized rate at 350°C assumes the activation energies of 50 kJ/mol for all supported Mo-based catalyst regardless of their promoter or support type. The weight of loaded catalysts and GHSV assume catalyst densities of 0.3 cm3/g for metal oxide supported catalysts.

The synthesis of bimetallic Mo-based WGS catalysts has gained significant interest with the aim of improving their catalytic activity without sacrificing their sulfur resistance. Among the components, cobalt is widely chosen to increase the WGS activity and thermal stability. For example, Co-promoted bulk MoS2 catalysts exhibited more exposed active sites than unpromoted

MoS2 catalysts [47]. It was widely established that cobalt is the optimal promoter for Mo-based

WGS catalysts and Al2O3 is a promising support material to enhance the WGS activity since the

Al2O3-supported catalysts have been extensively studied over COS hydrolysis and hydrodesulfurization (HDS) reaction of refinery oil.

Modifying the support could enhance the WGS activity of Mo-based catalysts without compromising their original benefits, such as low reaction temperature, high sulfur tolerance, no side reaction, and low production cost. Laniecki et al. compared TiO2, Al2O3, and Al2O3 as a support of Ni-Mo catalysts, displaying three times higher WGS activity of TiO2-supported catalysts than that of Al2O3-supported catalyst. They suggested that modifying the support material

29 could significantly enhance the WGS activity of Mo-based catalysts, and found that TiO2(40%)-

ZrO2(60%) supported Ni-Mo catalyst was the most active among their tested catalysts.

Adding promoter onto Mo-based catalysts has been widely investigated to improve WGS activity. Alkali metal promoters are suggested to improve WGS activity by enhancing dispersion of the active phase and avoiding carbon deposition [65,85]. Ni-promoted Mo-S catalysts were reported to show enhanced WGS activity and the bimetallic synergistic effect [86-90], while Ni-

Mo catalysts possess the disadvantage of CH4 methanation and carbon deposition. Cobalt (Co) is the best known textural promoter to improve the WGS activity of Mo-based catalysts

[47,57,73,86,87,91-94]. Enhanced WGS activity and excellent thermal stability of CoMo-S/TiO2-

Al2O3 at the ratio of H2O/CO =1 have been reported, showing higher WGS activity than commercial sour gas shift catalysts (CoMo-MgO-Al2O3) [77,95].

At the beginning of WGS catalysis studies over Co-promoted Mo catalysts, cobalt was assumed to play the role to enhance dispersion of Mo-S and thermal stability. Recently, the catalytic activity, the adsorption of reactants, and reaction pathway of unpromoted Mo-S and

CoMo-S phase have been investigated for the WGS reaction over the CoMo-S catalysts by DFT calculations and experimental studies [58,73]. The CoMo-S site is the edge of MoS2 nano-slabs decorated by Co atoms, and the unpromoted Mo-S site is the MoS2 not occupied by Co. The CoMo-

S and Mo-S in CoMo-S/Al2O3 catalysts were distinguished and quantified by using CO adsorption followed by in-situ IR spectroscopy (IR/CO) at low temperature [73,88]. Chen et al. reported that the intrinsic activity of CoMo-S sites in the WGS reaction at low temperature (< 300°C) is higher than that of unpromoted Mo-S sites [73]. Recent DFT calculations over CoMo-S WGS catalysts reported that the S-edge of CoMo-S requires lower overall reaction barrier over the WGS reaction pathways to follow the COOH-associated mechanism than either the S-edge or Mo-edge of

30 unpromoted Mo-S following a redox mechanism [58]. The Co addition to Mo can reduce H2O dissociation kinetic energy barrier on the sulfur edge sites of CoMo-S compared to that of unpromoted Mo-S [57,58], which is the rate-determining step in WGS reaction.

2.5. Novel approaches to develop sulfur-tolerant WGS catalysts

2.5.1. Bimetallic Cu-Pd nanoparticle WGS catalysts

Sulfur tolerance of Cu and Ni catalysts has been traditionally introduced by ZnO addition, which is effective in trapping H2S as ZnS [7]. It is well-established that S inhibits the chemisorption of H2, CO [8], and WGS reactions [9]. A combination of electronic effects, as well as an ensemble of steric effects, have been proposed to explain the initial effects of sulfur poisoning.

On the Ni (100) surface, the experimental data for CO methanation suggested that electronic effects dominate in the sulfur-poisoning mechanism. On the other hand, for the WGS reaction on the Cu (111) surface, the decay of the reaction rate with S coverage is suggested to be the result of a site-blocking model, where adsorbed S atoms prevent H2O dissociation.

Over the long term, the deactivation of WGS catalysts in sulfur-containing feeds is attributed to physical adsorption or chemisorption of H2S at surface active sites, leading to the reduction of surface area and sulfidation of active metals resulting in a loss of catalytic activity,

[10] as H2S forms stable compounds with all transition metals. After examining the binding of S to the surfaces of several transition metals, one finds that in all cases, sulfur withdraws charge from the metal and induces a decrease in its density of states around the Fermi level [11]. The magnitude of these electronic perturbations depends on the nature of the metal. Thus, a combination of electronic and steric effects has been proposed to explain sulfur poisoning. The strategy to increase the resistance of metals to sulfur poisons may be based on the modification of the physicochemical characteristics of the metal atoms. Modification of metal atoms may be

31 achieved by the extent of alloying, by changing the metal particle size, or through changing the interactions with the support.

The major challenge for these catalysts is their sulfur poisoning through the formation of sulfur-metal bonds [15]. Theoretical DFT studies predicted that alloying Cu with Ni does not improve sulfur resistance of Cu catalysts. Recently, DFT was used to examine the sulfur tolerance of binary combinations of 10 different late transition metals (Fe, Ni, Cu, Ru, Rh, Pd, Ag, Ir, Pt, and Au) by predicting their S adsorption energy [96]. The results of previous DFT calculations are summarized in Figure 2.3. The values in bold in large square boxes show the S chemical potential where monometallic surfaces are S-poisoned. The red square boxes indicate alloys with lower S tolerance than the monometallic surfaces, while the gray square boxes designate the enhanced S- tolerant structures. Empty cells correspond to unfavorable thermodynamic data. The rows designate the substrate or the host metals, and the columns designate the metals at the surface. It has been suggested that Pd alloyed with Cu shell on Cu core is much more S-tolerant than either pure Pd or Cu. Indeed, Cu/Pd alloys have generated much interest not only as a material for H2 separation membranes but also with respect to their sulfur resistance [97]. The fcc Cu/Pd phase showed greater resistance to H2S poisoning than pure Pd. Pd- and Cu-based catalysts have also beenwidely explored as WGS catalysts [98]. However, conventional impregnation synthesis of

Cu/Pd catalysts exhibits poor control over the size, shape, and composition of the resulting bimetallic particles [99]. On the other hand, various methods of core-shell nanoparticle synthesis reported recently are promising for fine-tuning of electronic and catalytic properties of metallic nanostructures for the WGS reaction.

Figure 2.3. Sulfur chemical potentials where the corresponding binary alloy structure starts to become poisoned due to sulfur adsorption [96]. 2.5.2. Synthesis of Mo and CoMo nanoparticles

Studies modifying nanoscale structures on the catalyst surface have been undertaken in order to increase the amount of catalytic active sites. Reducing the size of MoS2 nanoparticles can increase the length of Mo-S edge, which was considered to be the location of active sites for thiophene hydrodesulfurization (HDS) [100]. The extent of Mo-S edge site exposure per unit surface area of MoS2 nanoparticles was controlled via the particle size and correlated with the

HDS activity of thiophene [100-102]. The lower stack number of MoS2 layered slabs was proposed to have more Mo-edge sites that have more exposed Mo atoms, which are proposed to be the active sites for hydrogen evolution reaction (HER) [103]. The catalytic activity in hydrogen evolution was found to correlate linearly with the number of Mo-S edge sites present in MoS2, which was controlled by the size of the MoS2 nanoparticles [104]. Topsøe suggested that brim sites of CoMo-

S in the top single layer play a key role for hydrogenation reaction [105], and that densities can be changed by modifying particle size.

Kuriki et al. controlled the MoS2 particle size by mechanical milling and obtained small

MoS2 particles displaying a high surface area and high methylnaphthalene hydrogenation activity

[106]. Moreover, Afanasiev et al. suggested that the small domain size of MoS2 particles correlated with the high surface area and high hydrogenolysis and thiophene HDS activity [107], while

Contreras et al. reported that the 10-400 nm MoS2 nanoparticles exhibited size-dependent HDS activity [108]. However, the particle size effects on the WGS activity of Mo-based nanoparticles have not been systematically investigated to the best of our knowledge.

Increasing dispersion of active sites on dense and porous metal oxides is a common method to increase the number of active sites of nanostructured catalysts [109,110]. Full dispersion of an active component can be achieved in a monolayer surface by maximizing the topmost surface area of active phase [109]. Al2O3-supported Mo-based catalysts with monolayer MoO3 surface coverage catalysts showed the highest WGS activity due to the high dispersion of MoO3 [62,111].

The highest hydrodeoxygenation (HDO) activity of anisole was observed in a Mo-O/ZrO2 catalyst with monolayer MoO3 surface coverage due to highly dispersed active Mo oxide species [112].

The modification of Mo-O dispersion on ZrO2 by controlling MoO3 loading has been shown to impact the catalytic activity of toluene ammoxidation [113]. However, the impact of MoO3 surface coverage over ZrO2 on WGS activity has rarely investigated yet.

2.5.3. Promoters of Mo-based catalysts

For the WGS reaction, cobalt is a more desirable additive component to MoS2 than nickel.

While nickel catalyzes the methanation reaction that is undesirable for the water gas shift reaction, cobalt promote Mo-based catalysts to increase the exposed active Mo-S sites and to prevent

34 thermal sintering without methanation [47]. Recently, the energy barriers of each elementary reaction step of the WGS redox mechanism on the Co-MoS2 (100) surface were found to be smaller than those determined for the pure MoS2 (100) surface by DFT calculations [57].

Figure 2.4 indicates that the Co/Mo catalysts possess high sulfur resistance in the presence of 500 ppm H2S after pre-activation under sulfur atmosphere to intentionally synthesize sulfide catalyst in H2S-containing feed. Figure 2.4 (right) suggests that Ce dopant could improve the

Co/Mo WGS catalytic activity of Co/Mo catalysts. The different CO conversion values for the two-promoter system illustrate the catalytic role of the Ce/K additive for the Co-Mo/γ-Al2O3 catalysts. This observation indicates that the CeO2-K2O components may individually play different catalytic roles, resulting in an optimum Ce/K ratio. The CO conversion over the Co-Mo-

Ce1K8 was near equilibrium at 300°C [65].

Figure 2.4. (Left) Catalytic activity as a function of sulfur exposure concentration and reaction temperature [130]. (Right) Influence of Ce-K on the catalytic activity of the Co-Mo/γ-Al2O3 catalysts [65].

2.5.4. Modifying supports to improve WGS activity

Another approach to improve sulfur tolerance of WGS catalysts relies on engineering the oxide support. Ceria-supported metallic nanoparticles were reported to be more sulfur-tolerant

35 than alumina-supported catalysts due to electron transfer from the active metal to the ceria lattice, which made active metals more electron-deficient [98]. This, in turn, weakened sulfur-metal bonds and improved sulfur tolerance of supported catalysts. Accordingly, doping ceria with high-valent dopant (+5, +7) that enhances the electron transfer from supported metal is expected to further improve the sulfur resistance of metallic catalysts.

Commercial catalysts generally employ Al2O3 or MgO as a support material to disperse an active phase consisting of a Co and Mo sulfide [114-116], but using alternative support materials could further improve catalytic activity [117]. TiO2 was suggested as a promising support for Mo- based catalysts by enhancing the reducibility and dispersion of MoO3 on TiO2 [80,118]. Sasaki and Suzuki suggested that TiO2 increased the sulfidation rate because the TiO2 surface possessed more hydroxyl groups than those of the Al2O3 and ZrO2 supports, leading to more Lewis acid sites and resulting in highly dispersed Mo sulfide species [118]. Controlling the redox properties of

CeO2-ZrO2 was reported to increase catalytic activity in iso-synthesis reaction by increasing oxygen vacancies due to the increased lattice oxygen mobility for a distortion of the oxygen sublattice [119,120]. Lian et al. reported that an optimized Mg/Al ratio enhanced Mo oxide dispersion on the surface of a support [75]. Wang et al. suggested that the mixed oxide support of

Ce-Al was promising for improving Mo dispersion and increasing the composition of the active

S4- component in the total sulfur elements [121].

36 strong MoO3-Al2O3 interaction [123-125]. Kaluza et al. reported that Mo/ZrO2 showed the most enhanced reducibility of MoO3 to MoO2 among the ZrO2-, TiO2-, Al2O3-, SiO2-, and MgO- supported Mo catalysts [126]. Shetty et al. reported that the ZrO2-supported Mo catalysts showed the weak interaction between ZrO2 and Mo species showing high catalytic activity of hydrodeoxygenation of m-cresol to toluene [127]. The MoO3 weakly interacted with support can be easily reduced to MoO2 in supported Mo-based catalysts, led to the increased formation of Mo-

S during pre-sulfidation. Therefore, it is highly desirable to study ZrO2-supported Mo catalysts over the WGS reaction due to its weak MoO3-support interaction.

The ZrO2-supported Mo-based catalyst reported to show lower catalytic activity of the

HDS reaction of benzothiophene than Al2O3 supported catalysts since the weak interaction between ZrO2 and MoO3 favored the formation of inactive Co9S8 rather than the formation of active CoMoO4 [126]. Dhanala et al. reported that metal-support interaction strongly affected the metal dispersion, metal crystallite size, and metal/metal oxide ratio, showing that Mo-Al2O3 has stronger interaction than that of Mo-ZrO2 or Mo-SiO2 [128]. Pratt et al. suggested that ZrO2 and

TiO2 had higher specific catalytic activity with highly dispersed MoS2 species due to their stronger interaction between the support and Mo sulfide [117]. Jha et al. reported that oxide support with enhanced reducibility favored cobalt dispersion and led to the exposure of more surface active metal sites and enhance WGS activity [129].

2.6. References

1. Anonymous , The 1995 Progress Report of the Secretary of Energy's Hydrogen Technical Advisory Panel, The green hydrogen report, 1995

2. World Energy Council, "World Energy Issues Monitor 2017," World Energy Issues Monitor 2017, 2017

3. Hydrogen Council, A sustainable pathway for the global energy transition.,"Hydrogen Scaling Up," Hydrogen Council.2017, 1-78

4. I. Staffell, D. Scamman, A. Velazquez Abad, P. Balcombe, P. E. Dodds, P. Ekins, N. Shah, K. R. Ward, "The Role of Hydrogen and Fuel Cells in the Global Energy System," Energy and Environmental Science, 12, 2019, 463-491

5. Office of Energy Efficiency and Renewable Energy, 3.1. hydrogen production.,"Fuel Cell Multi- Year Research, Development, and Demonstration Plan," Fuel Cell Technologies Program of Department of Energy.2015

6. P. E. Dodds, & A. Hawkes, "The Role of Hydrogen and Fuel Cells in Providing Affordable, Secure Low-Carbon Heat," H2FC SUPERGEN, London, UK, 2014

7. M. Joos, I. Staffell, "Short-Term Integration Costs of Variable Renewable Energy: Wind Curtailment and Balancing in Britain and Germany," Renewable and Sustainable Energy Reviews, 86, 2018, 45-65

8. I. Chorkendorff, J. W. Niemantsverdriet, "Heterogeneous Catalysis in Practice: Hydrogen," Concepts of Modern Catalysis and Kinetics, 2003, 301-309

9. P. Corbo, F. Migliardini, O. Veneri, "Hydrogen Fuel Cells for Road Vehicles," Springer-Verlag London, 2011, 245

10. M. Desai, R. P. Harvey, "Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2015," Federal Register, 82, 2017, 10767

11. V. S. Sikarwar, M. Zhao, P. Clough, J. Yao, X. Zhong, M. Z. Memon, N. Shah, E. J. Anthony, P. S. Fennell, "An Overview of Advances in Biomass Gasification," Energy and Environmental Science, 9, 2016, 2939-2977

12. A. A. Ahmad, N. A. Zawawi, F. H. Kasim, A. Inayat, A. Khasri, "Assessing the Gasification Performance of Biomass: A Review on Biomass Gasification Process Conditions, Optimization and Economic Evaluation," Renewable and Sustainable Energy Reviews, 53, 2016, 1333-1347

13. R. A. Efroymson, M. H. Langholtz, K. Johnson, et al., 2016 billion-ton report: Advancing domestic resources for a thriving bioeconomy, volume 2: Environmental sustainability effects of select scenarios from volume 1, ORNL/TM-2016/727, 2017

14. A. T-Raissi., Florida Solar Energy Center, Technoeconomic analysis of area II hydrogen production –Part 1, 2001

15. R. M. Navarro Yerga, M. C. Álvarez-Galván, N. Mota, J. A. VilloriadelaMano, S. M. Al- Zahrani, J. L. G. Fierro, "Catalysts for Hydrogen Production from Heavy Hydrocarbons," ChemCatChem, 3, 2011, 440-457

16. M. Farniaei, M. Abbasi, A. Rasoolzadeh, M. R. Rahimpour, "Enhancement of Methanol, DME and Hydrogen Production Via Employing Hydrogen Permselective Membranes in a Novel Integrated Thermally Double-Coupled Two-Membrane Reactor," Journal of Natural Gas Science and Engineering, 14, 2013, 158-173

17. A. L. Martínez-Salazar, J. A. Melo-Banda, M. A. Coronel-García, Pedro M. García-Vite, Iris Martínez-Salazar, J. M. Domínguez-Esquivel, "Technoeconomic Analysis of Hydrogen Production Via Hydrogen Sulfide Methane Reformation," International Journal of Hydrogen Energy, 44, 2019, 12296-12302

18. S. Ahmed, M. Krumpelt, "Hydrogen from Hydrocarbon Fuels for Fuel Cells," International Journal of Hydrogen Energy, 26, 2001, 291-301

19. Z. Ilsen Önsan, A. K. Avci, Chapter 14 - reactor design for fuel processing., In D. Shekhawat, J. J. Spivey & D. A. Berry (Eds.), "Fuel Cells: Technologies for Fuel Processing," Elsevier. Amsterdam, 2011, 451-516

20. J. G. Speight, 6 - production of syngas, synfuel, bio-oils, and biogas from coal, biomass, and opportunity fuels., In J. Oakey (Ed.), "Fuel Flexible Energy Generation," Woodhead Publishing. Boston, 2016, 145-174

21. J. Speirs, P. Balcombe, E. Johnson, J. Martin, N. Brandon, A. Hawkes, "A Greener Gas Grid: What are the Options," Energy Policy, 118, 2018, 291-297

22. C. H. Bartholomew, R. J. Farrauto, Fundamentals of industrial catalytic processes: Second edition.,"Fundamentals of Industrial Catalytic Processes: Second Edition," John Wiley & Sons. New Jersey, 2010, 906

23. G. J. Stiegel, M. Ramezan, "Hydrogen from Coal Gasification: An Economical Pathway to a Sustainable Energy Future," International Journal of Coal Geology, 65, 2006, 173-190

24. A. R. De La Osa, A. De Lucas, A. Romero, P. Casero, J. L. Valverde, P. Sánchez, "High Pressure Water Gas Shift Performance Over a Commercial Non-Sulfide CoMo Catalyst using Industrial Coal-Derived Syngas," Fuel, 97, 2012, 428-434

25. Office of Fossil Energy. , Department of Energy, Hydrogen from coal program, 2008

26. Department of Energy, "Hydrogen and Fuel Cell Technologies FY 2014 Budget Request Rollout," 25, 2013

27. National Energy Technology Laboratory. , U.S. Department of Energy, Assessment of hydrogen production with CO2 capture volume 1: Baseline state-of-the-art plants, DOE/NETL- 2010/1434, 2010

28. C. Rhodes, S. A. Riddel, J. West, B. P. Williams, G. J. Hutchings, "Low-Temperature Hydrolysis of Carbonyl Sulfide and Carbon Disulfide: A Review," Catalysis Today, 59, 2000, 443- 464

29. Z. M. George, "Kinetics of Cobalt-Molybdate-Catalyzed Reactions of SO2 with H2S and COS and the Hydrolysis of COS," Journal of Catalysis, 32, 1974, 261-271

30. G. K. Reddy, P. Boolchand, P. G. Smirniotis, "Sulfur Tolerant Metal Doped Fe/Ce Catalysts for High Temperature WGS Reaction at Low Steam to CO Ratios - XPS and Mossbauer Spectroscopic Study," Journal of Catalysis, 282, 2011, 258-269

31. National Renewable Energy Laboratory, "Controlled Hydrogen Fleet and Infrastructure Demonstration and Validation Project," CDP#13: On-Site Hydrogen Production Efficiency, 2010

32. S. Cheah, D. L. Carpenter, K. A. Magrini-Bair, "Review of Mid- to High-Temperature Sulfur Sorbents for Desulfurization of Biomass- and Coal-Derived Syngas," Energy and Fuels, 23, 2009, 5291-5307

33. C. Yang, R. Li, C. Cui, S. Liu, Q. Qiu, Y. Ding, Y. Wu, B. Zhang, "Catalytic Hydroprocessing of Microalgae-Derived Biofuels: A Review," Green Chemistry, 18, 2016, 3684-3699

34. M. Ghadiryanfar, K. A. Rosentrater, A. Keyhani, M. Omid, "A Review of Macroalgae Production, with Potential Applications in Biofuels and Bioenergy," Renewable and Sustainable Energy Reviews, 54, 2016, 473-481

35. K. J. Andersson, M. Skov-Skjøth Rasmussen, P. E. Højlund Nielsen, "Industrial-Scale Gas Conditioning Including Topsøe Tar Reforming and Purification Downstream Biomass Gasifiers: An Overview and Recent Examples," Fuel, 203, 2017, 1026-1030

36. B. M. M. Bomani, D. L. Bulzan, D. I. Centeno-Gomez, R. C. Hendricks, Biofuels as an Alternative Energy Source for Aviation-a Survey, 2009,

37. Y. Choi, H. G. Stenger, "Water Gas Shift Reaction Kinetics and Reactor Modeling for Fuel Cell Grade Hydrogen," Journal of Power Sources, 124, 2003, 432-439

38. National Research Council, A focus on hydrogen.,"Transitions to Alternative Transportation Technologies," The National Academies Press. Washington, DC, 2008, 1-126

39. C. Ratnasamy, J. Wagner, "Water Gas Shift Catalysis," Catalysis Reviews - Science and Engineering, 51, 2009, 325-440

40. C. V. Ovesen, B. S. Clausen, B. S. Hammershøi, G. Steffensen, T. Askgaard, I. Chorkendorff, J. K. Nørskov, P. B. Rasmussen, P. Stoltze, P. Taylor, "A Microkinetic Analysis of the Water-Gas Shift Reaction Under Industrial Conditions," Journal of Catalysis, 158, 1996, 170-180

41. L. S. Chen, G. L. Lü, "Study on the Effects of Cr2O3 on the Reduction Behavior of γ-Fe2O3," Journal of Materials Science, 34, 1999, 4193-4197

42. G. C. De Araújo, M. Do Carmo Rangel, "Environmental Friendly Dopant for the High- Temperature Shift Catalysts," Catalysis Today, 62, 2000, 201-207

43. D. G. Rethwisch, J. Phillips, Y. Chen, T. F. Hayden, J. A. Dumesic, "Water-Gas Shift Over Magnetite Particles Supported on Graphite: Effects of Treatments in CO/CO2 and H2/H2O Gas Mixtures," Journal of Catalysis, 91, 1985, 167-180

44. C. Rhodes, G. J. Hatchings, "Studies of the Role of the Copper Promoter in the Iron Oxide/Chromia High Temperature Water Gas Shift Catalyst," Physical Chemistry Chemical Physics, 5, 2003, 2719-2723

45. X. Shi, S. Wang, J. Hu, H. Wang, Y. Chen, Z. Qin, J. Wang, "Density Functional Theory Study on Water-Gas-Shift Reaction Over Molybdenum Disulfide," Applied Catalysis A: General, 365, 2009, 62-70

46. P. Hou, D. Meeker, H. Wise, "Kinetic Studies with a Sulfur-Tolerant Water Gas Shift Catalyst," Journal of Catalysis, 80, 1983, 280-285

47. C. R. F. Lund, "Effect of Adding Co to MoS2/Al2O3 upon the Kinetics of the Water-Gas Shift," Industrial and Engineering Chemistry Research, 35, 1996, 3067-3073

48. C. R. F. Lund, "Microkinetics of Water-Gas Shift Over Sulfided Mo/Al2O3 Catalysts," Industrial and Engineering Chemistry Research, 35, 1996, 2531-2538

49. P. Liu, Y. Choi, Y. Yang, M. G. White, "Methanol Synthesis from H2 and CO2 on a Mo6S8 Cluster: A Density Functional Study," Journal of Physical Chemistry A, 114, 2010, 3888-3895

50. C. T. Campbell, K. A. Daube, "A Surface Science Investigation of the Water-Gas Shift Reaction on Cu(111)," Journal of Catalysis, 104, 1987, 109-119

51. A. A. Gokhale, J. A. Dumesic, M. Mavrikakis, "On the Mechanism of Low-Temperature Water Gas Shift Reaction on Copper," Journal of the American Chemical Society, 130, 2008, 1402-1414

52. A. A. Gokhale, S. Kandoi, J. P. Greeley, M. Mavrikakis, J. A. Dumesic, "Molecular-Level Descriptions of Surface Chemistry in Kinetic Models using Density Functional Theory," Chemical Engineering Science, 59, 2004, 4679-4691

53. C. T. Campbell, B. E. Koel, "H2S/Cu(111): A Model Study of Sulfur Poisoning of Water-Gas Shift Catalysts," Surface Science, 183, 1987, 100-112

54. R. D. Cortright, J. A. Dumesic, "Kinetics of heterogeneous catalytic reactions: Analysis of reaction schemes," Advances in Catalysis, 46, 2001, 161-264

55. J. Nakamura, J. M. Campbell, C. T. Campbell, "Kinetics and Mechanism of the Water-Gas Shift Reaction Catalysed by the Clean and Cs-Promoted Cu(110) Surface: A Comparison with Cu(111)," Journal of the Chemical Society, Faraday Transactions, 86, 1990, 2725-2734

56. J. Yoshihara, C. T. Campbell, "Methanol Synthesis and Reverse Water-Gas Shift Kinetics Over Cu(110) Model Catalysts: Structural Sensitivity," Journal of Catalysis, 161, 1996, 776-782

57. Yan-Yan Chen, Mei Dong, Jianguo Wang, Haijun Jiao, "On the Role of a Cobalt Promoter in a Water-Gas-Shift Reaction on Co-MoS2," The Journal of Physical Chemistry C, 114, 2010, 16669-16676

58. C. Zhang, B. Liu, L. Zhao, Q. Zong, J. Gao, Y. Wang, C. Xu, "Insights into Water–gas Shift Reaction Mechanisms Over MoS2 and Co-MoS2 Catalysts: A Density Functional Study," Reaction Kinetics, Mechanisms and Catalysis, 120, 2017, 833-844

59. J. Lin, V. V. Guliants, "Hydrogen Production through Water-Gas Shift Reaction Over Supported Cu, Ni, and CuNi Nanoparticle Catalysts Prepared from Metal Colloids," ChemCatChem, 4, 2012, 1611-1621

60. J. Lin, V. V. Guliants, "Alumina-Supported Cu@Ni and Ni@Cu Core-Shell Nanoparticles: Synthesis, Characterization, and Catalytic Activity in Water-Gas-Shift Reaction," Applied Catalysis A: General, 445-446, 2012, 187-194

61. P. Kumar, R. Idem, "A Comparative Study of Copper-Promoted Water-Gas-Shift (WGS) Catalysts," Energy and Fuels, 21, 2007, 522-529

62. D. Nikolova, R. Edreva-Kardjieva, G. Gouliev, T. Grozeva,P. Tzvetkov, "The State of (K)(Ni)Mo/γ-Al2O3 Catalysts after Water-Gas Shift Reaction in the Presence of Sulfur in the Feed: XPS and EPR Study," Applied Catalysis A: General, 2006, 135-144

63. T. Huang, T. Yu, S. Jhao, "Weighting Variation of Water-Gas Shift in Steam Reforming of Methane Over Supported Ni and Ni-Cu Catalysts," Industrial and Engineering Chemistry Research, 45, 2006, 15-156

64. S. Natesakhawat, X. Wang, L. Zhang, U. S. Ozkan, "Development of Chromium-Free Iron- Based Catalysts for High-Temperature Water-Gas Shift Reaction," Journal of Molecular Catalysis A: Chemical, 260, 2006, 82-94

65. Y. Zhang, G. Zhang, Y. Zhao, X. Li, Y. Sun, Y. Xu, "Ce-K-Promoted Co-Mo/Al2O3 Catalysts for the Water Gas Shift Reaction," International Journal of Hydrogen Energy, 37, 2012, 6363- 6371

66. S. S. Hla, G. J. Duffy, L. D. Morpeth, A. Cousins, D. G. Roberts, J. H. Edwards, "Investigation of the Effect of H2S on the Performance of an Iron/Chromium-Based High-Temperature Water-

Gas Shift Catalyst using Simulated Coal-Derived Syngas," Catalysis Communications, 10, 2009, 967-970

67. S. Tan, S. N. Paglieri, D. Li, "Nano-Scale Sulfur-Tolerant Lanthanide Oxysulfide/Oxysulfate Catalysts for Water-Gas-Shift Reaction in a Novel Reactor Configuration," Catalysis Communications, 73, 2016, 16-21

68. R. G. Copperthwaite, F. M. Gottschalk, T. Sangiorgio, G. J. Hutchings, "Cobalt Chromium Oxide: A Novel Sulphur Tolerant Water-Gas Shift Catalyst," Applied Catalysis, 63, 1990, L11- L16

69. J. A. Schaidle, A. C. Lausche, L. T. Thompson, "Effects of Sulfur on Mo2C and Pt/Mo2C Catalysts: Water Gas Shift Reaction," Journal of Catalysis, 272, 2010, 235-245

70. Hla S.S., Duffy G.J., Morpeth L.D., Cousins A., Roberts D.G., Edwards J.H., "Investigation into the Performance of a Co-Mo Based Sour Shift Catalyst using Simulated Coal-Derived Syngases," International Journal of Hydrogen Energy, 36, 2011, 6638-6645

71. M. Nagai, K. Matsuda, "Low-Temperature Water-Gas Shift Reaction Over Cobalt- Molybdenum Carbide Catalyst," Journal of Catalysis, 238, 2006, 489-496

72. J. Mi, Y. Cao, J. Zhang, Y. Ma, C. Chen, D. Li, X. Lin, L. Jiang, "Effect of Ce Modification on the Structural and Catalytic Property of Co-Mo/mg(Al)O Catalyst for Water-Gas Shift Reaction," Applied Catalysis A: General, 553, 2018, 36-42

73. J. Chen, J. Zhang, J. Mi, E. D. Garcia, Y. Cao, L. Jiang, L. Oliviero, F. Maugé, "Hydrogen Production by Water-Gas Shift Reaction Over Co-Promoted MoS2/Al2O3 Catalyst: The Intrinsic Activities of Co-Promoted and Unprompted Sites," International Journal of Hydrogen Energy, 43, 2018, 7405-7410

74. A. A. Andreev, V. J. Kafedjiysky, R. M. Edreva-Kardjieva, "Active Forms for Water-Gas Shift Reaction on NiMo-Sulfide Catalysts," Applied Catalysis A: General, 179, 1999, 223-228

75. Y. Lian, H. Wang, Q. Zheng, W. Fang, Y. Yang, "Effect of mg/Al Atom Ratio of Support on Catalytic Performance of Co-Mo/MgO-Al2O3 Catalyst for Water Gas Shift Reaction," Journal of Natural Gas Chemistry, 18, 2009, 161-166

76. K. Antoniak-Jurak, P. Kowalik, M. Konkol, W. Próchniak, R. Bicki, W. Raróg-Pilecka, P. Kustrowski, J. Ryczkowski, "Sulfur Tolerant Co-Mo-K Catalysts Supported on Carbon Materials for Sour Gas Shift Process - Effect of Support Modification," Fuel Processing Technology, 144, 2016, 305-311

77. B. Liu, Q. Zong, P. P. Edwards, F. Zou, X. Du, Z. Jiang, T. Xiao, H. Almegren, "Effect of Titania Addition on the Performance of CoMo/Al2O3 Sour Water Gas Shift Catalysts Under Lean

Steam to Gas Ratio Conditions," Industrial and Engineering Chemistry Research, 51, 2012, 11674-11680

78. B. Liu, Q. Zong, X. Du, Z. Zhang, T. Xiao, H. Almegren, "Novel Sour Water Gas Shift Catalyst (SWGS) for Lean Steam to Gas Ratio Applications," Fuel Processing Technology, 134, 2015, 65- 72

79. T. Sasaki, T. Suzuki, M. Takaoka, "Reaction Selectivity to Hydrocarbons and Solid-State Carbon Over Molybdenum Sulfide-Based Shift Catalyst," Applied Catalysis A: General, 514, 2016, 83-90

80. M. Laniecki, M. Malecka-Grycz, F. Domka, "Water–gas Shift Reaction Over Sulfided Molybdenum Catalysts," Applied Catalysis A: General, 196, 2000, 293-303

81. M. Laniecki, M. Ignacik, "Water-Gas Shift Reaction Over Sulfided Molybdenum Catalysts Supported on TiO2-ZrO2 Mixed Oxides. Support Characterization and Catalytic Activity," Catalysis Today, 116, 2006, 400-407

82. J. Chen, X. Wang, Y. Yue, H. Guo, J. Mi, Y. Cao, L. Jiang, "CoMo Sulfur-Tolerant Water- Gas Shift Catalyst Derived from an Anderson-Type Heteropolyanion Precursor," Catalysis Communications, 86, 2016, 19-22

83. C. Sener, T. S. Wesley, A. C. Alba-Rubio, M. D. Kumbhalkar, S. H. Hakim, F. H. Ribeiro, J. T. Miller, J. A. Dumesic, "PtMo Bimetallic Catalysts Synthesized by Controlled Surface Reactions for Water Gas Shift," ACS Catalysis, 6, 2016, 1334-1344

84. T. Sasaki, T. Suzuki, M. Takaoka, Y. Akasaka, "Performance of a Low Temperature Reactive Sulfur-Tolerant WGS Catalyst using Industrial Coal Gasification Gas Feed," International Journal of Hydrogen Energy, 42, 2017, 2008-2017

85. D. Nikolova, R. Edreva-Kardjieva, T. Grozeva, "Water-Gas Shift Activity of K-Promoted (Ni)Mo/γ-Al2O3 Systems in Sulfur-Containing Feed," Reaction Kinetics, Mechanisms and Catalysis, 103, 2011, 71-86

86. H. Schweiger, P. Raybaud, H. Toulhoat, "Promoter Sensitive Shapes of Co(Ni)MoS Nanocatalysts in Sulfo-Reductive Conditions," Journal of Catalysis, 212, 2002, 33-38

87. E. Rodríguez-Castellón, A. Jiménez-López, D. Eliche-Quesada, "Nickel and Cobalt Promoted Tungsten and Molybdenum Sulfide Mesoporous Catalysts for Hydrodesulfurization," Fuel, 87, 2008, 1195-1206

88. A. Travert, C. Dujardin, F. Maugé, E. Veilly, S. Cristol, J. Paul, E. Payen, "CO Adsorption on CoMo and NiMo Sulfide Catalysts: A Combined IR and DFT Study," Journal of Physical Chemistry B, 110, 2006, 1261-1270

89. R. López Cordero, A. López Agudo, "Effect of Water Extraction on the Surface Properties of Mo/Al2O3 and NiMo/Al2O3 hydrotreating Catalysts," Applied Catalysis A: General, 202, 2000, 23-35

90. M. Jia, P. Afanasiev, M. Vrinat, "The Influence of Preparation Method on the Properties of NiMo Sulfide Catalysts Supported on ZrO2," Applied Catalysis A: General, 278, 2005, 213-221

91. J. Mi, Y. Cao, F. Liu, X. Lin, C. Au, J. Chen, L. Jiang, "Ammonia-Free Synthesis of Mo/CoMgAl Catalysts with Excellent Activities in Water-Gas Shift Reaction," Applied Catalysis A: General, 575, 2019, 58-64

92. B. Liu, L. Liu, Y. Chai, J. Zhao, C. Liu, "Essential Role of Promoter Co on the MoS2 Catalyst in Selective Hydrodesulfurization of FCC Gasoline," Ranliao Huaxue Xuebao/Journal of Fuel Chemistry and Technology, 46, 2018, 441-450

93. A. D. Gandubert, E. Krebs, C. Legens, D. Costa, D. Guillaume, P. Raybaud, "Optimal Promoter Edge Decoration of CoMoS Catalysts: A Combined Theoretical and Experimental Study," Catalysis Today, 130, 2008, 149-159

94. R. Prins, V. H. J. De Beer, G. A. Somorjai, "Structure and Function of the Catalyst and the Promoter in Co - Mo Hydrodesulfurization Catalysts," Catalysis Reviews, 31, 1989, 1-41

95. Renan T. Figueiredo, André L. D. Ramos, Heloysa M. C. de Andrade, J. L. G. Fierro, "Effect of Low Steam/Carbon Ratio on Water Gas Shift Reaction," Catalysis Today, 107–108, 2005, 671- 675

96. N. Inoglu, J. R. Kitchin, "Identification of Sulfur-Tolerant Bimetallic Surfaces using DFT Parametrized Models and Atomistic Thermodynamics," ACS Catalysis, 1, 2011, 399-407

97. N. Pomerantz, Y. H. Ma, "Novel Method for Producing High H2 Permeability Pd Membranes with a Thin Layer of the Sulfur Tolerant Pd/Cu Fcc Phase," Journal of Membrane Science, 370, 2011, 97-108

98. R. Watanabe, Y. Sekine, H. Takamatsu, Y. Sakamoto, S. Aramaki, M. Matsukata, E. Kikuchi, "Pt and/Or Pd Supported/Incorporated Catalyst on Perovskite-Type Oxide for Water Gas Shift Reaction," Topics in Catalysis, 53, 2010, 621-628

99. J. Kugai, E. B. Fox, C. Song, "Role of CeO2 Support for Pd-Cu Bimetallic Catalysts for Oxygen-Enhanced Water Gas Shift," Applied Catalysis A: General, 456, 2013, 204-214

100. F. Besenbacher, M. Brorson, B. S. Clausen, S. Helveg, B. Hinnemann, J. Kibsgaard, J. V. Lauritsen, P. G. Moses, J. K. Nørskov, H. Topsøe, "Recent STM, DFT and HAADF-STEM Studies of Sulfide-Based Hydrotreating Catalysts: Insight into Mechanistic, Structural and Particle Size Effects," Catalysis Today, 130, 2008, 86-96

101. P. G. Moses, B. Hinnemann, H. Topsøe, J. K. Nørskov, "The Hydrogenation and Direct Desulfurization Reaction Pathway in Thiophene Hydrodesulfurization Over MoS2 Catalysts at Realistic Conditions: A Density Functional Study," Journal of Catalysis, 248, 2007, 188-203

102. A. S. Walton, J. V. Lauritsen, H. Topsøe, F. Besenbacher, "MoS2 Nanoparticle Morphologies in Hydrodesulfurization Catalysis Studied by Scanning Tunneling Microscopy," Journal of Catalysis, 308, 2013, 306-318

103. B. Seo, G. Y. Jung, Y. J. Sa, H. Y. Jeong, J. Y. Cheon, J. H. Lee, H. Y. Kim, J. C. Kim, H. S. Shin, S. K. Kwak, et al, "Monolayer-Precision Synthesis of Molybdenum Sulfide Nanoparticles and their Nanoscale Size Effects in the Hydrogen Evolution Reaction," ACS Nano, 9, 2015, 3728- 3739

104. T. F. Jaramillo, K. P. Jørgensen, J. Bonde, J. H. Nielsen, S. Horch, I. Chorkendorff, "Identification of Active Edge Sites for Electrochemical H2 Evolution from MoS2 Nanocatalysts," Science, 317, 2007, 100-102

105. H. Topsøe, "The Role of Co-Mo-S Type Structures in Hydrotreating Catalysts," Applied Catalysis A: General, 322, 2007, 3-8

106. Y. Kuriki, K. Uchida, E. Sekreta, S. Ohshima, M. Yumura, F. Ikazaki, "Mechanical Milling of Metal Sulfide Particles and their Catalytic Activity - Hydrogenation of 1-Methylnaphthalene with Molybdenum Sulfide," Fuel Processing Technology, 59, 1999, 189-200

107. P. Afanasiev, G. Xia, G. Berhault, B. Jouguet, M. Lacroix, "Surfactant-Assisted Synthesis of Highly Dispersed Molybdenum Sulfide," Chemistry of Materials, 11, 1999, 3216-3219

108. C. Contreras, F. Isquierdo, P. Pereira-Almao, C. E. Scott, "Effect of Particle Size on the HDS Activity of Molybdenum Sulfide," Journal of Nanotechnology, 2016, 2016,

109. K. M. Choi, K. Na, G. A. Somorjai, O. M. Yaghi, "Chemical Environment Control and Enhanced Catalytic Performance of Platinum Nanoparticles Embedded in Nanocrystalline Metal- Organic Frameworks," Journal of the American Chemical Society, 137, 2015, 7810-7816

110. G. A. Somorjai, Introduction to Surface Chemistry and Catalysis, 1994,

111. M. Łaniecki, M. Małecka-Grycz, F. Domka, "Water–gas Shift Reaction Over Sulfided Molybdenum Catalysts: I. Alumina, Titania and Zirconia-Supported Catalysts," Applied Catalysis A: General, 196, 2000, 293-303

112. M. Shetty, K. Murugappan, W. H. Green, Y. Román-Leshkov, "Structural Properties and Reactivity Trends of Molybdenum Oxide Catalysts Supported on Zirconia for the Hydrodeoxygenation of Anisole," ACS Sustainable Chemistry and Engineering, 5, 2017, 5293- 5301

113. K. V. R. Chary, K. R. Reddy, G. Kishan, J. W. Niemantsverdriet, G. Mestl, "Structure and Catalytic Properties of Molybdenum Oxide Catalysts Supported on Zirconia," Journal of Catalysis, 226, 2004, 283-291

114. D. A. Solís-Casados, L. Escobar-Alarcón, T. Klimova, J. Escobar-Aguilar, E. Rodríguez- Castellón, J. A. Cecilia, C. Morales-Ramírez, "Catalytic Performance of CoMo/Al2O3-MgO-Li(X) Formulations in DBT Hydrodesulfurization," Catalysis Today, 271, 2016, 35-44

115. L. Wu, D. Jiao, J. Wang, L. Chen, F. Cao, "The Role of MgO in the Formation of Surface Active Phases of CoMo/Al2O3-MgO Catalysts for Hydrodesulfurization of Dibenzothiophene," Catalysis Communications, 11, 2009, 302-305

116. F. Trejo, M. S. Rana, J. Ancheyta, "CoMo/MgO-Al2O3 Supported Catalysts: An Alternative Approach to Prepare HDS Catalysts," Catalysis Today, 130, 2008, 327-336

117. Kerry C. Pratt, John V. Sanders, Victor Christov, "Morphology and Activity of MoS2 on various Supports: Genesis of the Active Phase," Journal of Catalysis, 124, 1990, 416-432

118. T. Sasaki, T. Suzuki, "Sulfide Molybdenum Catalysts for Water–gas Shift Reaction: Influence of the Kind of Promoters and Supports to Generate MoS2," Applied Catalysis A: General, 484, 2014, 79-83

119. M. Fernández-García, A. Martínez-Arias, A. Iglesias-Juez, C. Belver, A. B. Hungría, J. C. Conesa, J. Soria, "Structural Characteristics and Redox Behavior of CeO2-ZrO2/Al2O3 Supports," Journal of Catalysis, 194, 2000, 385-392

120. Y. Li, D. He, Q. Zhu, X. Zhang, B. Xu, "Effects of Redox Properties and Acid-Base Properties on Isosynthesis Over ZrO2-Based Catalysts," Journal of Catalysis, 221, 2004, 584-593

121. H. Wang, Z. Li, E. Wang, C. Lin, Y. Shang, G. Ding, X. Ma, S. Qin, Q. Sun, "Effect of Composite Supports on the Methanation Activity of Co-Mo-Based Sulphur-Resistant Catalysts," Journal of Natural Gas Chemistry, 21, 2012, 767-773

122. R. Candia, O. Sørensen, J. Villadsen, N. Topsøe, B. S. Clausen, H. Topsøe, "Effect of Sulfiding Temperature on Activity and Structures of Co-Mo/Al2O3 Catalysts. Ii," Bulletin Des Sociétés Chimiques Belges, 93, 1984, 763-774

123. S. M. A. M. Bouwens, F. B. M. Vanzon, M. P. Vandijk, A. M. Vanderkraan, V. H. J. Debeer, J. A. R. Vanveen, D. C. Koningsberger, "On the Structural Differences between Alumina- Supported Comos Type I and Alumina-, Silica-, and Carbon-Supported Comos Type II Phases Studied by XAFS, MES, and XPS," Journal of Catalysis, 146, 1994, 375-393

124. C. Wivel, R. Candia, B. S. Clausen, S. Mørup, H. Topsøe, "On the Catalytic Significance of a CoMoS Phase in CoMo Al2O3 Hydrodesulfurization Catalysts: Combined in Situ Mössbauer Emission Spectroscopy and Activity Studies," Journal of Catalysis, 68, 1981, 453-463

125. Y. Okamoto, K. Ochiai, M. Kawano, K. Kobayashi, T. Kubota, "Effects of Support on the Activity of Co-Mo Sulfide Model Catalysts," Applied Catalysis A: General, 226, 2002, 115-127

126. L. Kaluža, D. Gulková, Z. Vít, M. Zdražil, "Effect of Support Type on the Magnitude of Synergism and Promotion in CoMo Sulphide Hydrodesulphurisation Catalyst," Applied Catalysis A: General, 324, 2007, 30-35

127. M. Shetty, K. Murugappan, T. Prasomsri, W. H. Green, Y. Román-Leshkov, "Reactivity and Stability Investigation of Supported Molybdenum Oxide Catalysts for the Hydrodeoxygenation (HDO) of M-Cresol," Journal of Catalysis, 331, 2015, 86-97

128. V. Dhanala, S. K. Maity, D. Shee, "Roles of Supports (γ-Al2O3, SiO2, ZrO2) and Performance of Metals (Ni, Co, Mo) in Steam Reforming of Isobutanol," RSC Advances, 5, 2015, 52522-52532

129. A. Jha, Y. Lee, W. Jang, J. Shim, K. Jeon, H. Na, H. Kim, H. Roh, D. Jeong, S. G. Jeon, et al, "Effect of the Redox Properties of Support Oxide Over Cobalt-Based Catalysts in High Temperature Water-Gas Shift Reaction," Molecular Catalysis, 433, 2017, 145-152

130. T. M. Roberge, S. O. Blavo, C. Holt, P. H. Matter, J. N. Kuhn, "Effect of Molybdenum on the Sulfur-Tolerance of Cerium-Cobalt Mixed Oxide Water-Gas Shift Catalysts," Topics in Catalysis, 56, 2013, 1892-1898

Chapter 3. Novel bimetallic Cu-Pd nanoparticles as sulfur-tolerant and highly active low temperature WGS catalysts

3.1. Introduction

Hydrogen has attracted significant interest in recent decades as a fuel for electrochemical cells and internal combustion engines because of its zero carbon dioxide emissions and high energy density (120 MJ/kg)[1]. Hydrogen is typically produced together with carbon monoxide during steam reforming of natural gas [2], partial oxidation of hydrocarbons [3], and coal gasification [4].

The water gas shift (WGS) reaction is then employed to maximize hydrogen production by reacting carbon monoxide with water vapor over a catalyst. The WGS reaction is reversible and mildly exothermic; therefore, the WGS activity depends on reaction temperature. Due to thermodynamic limitations, conventional WGS reactors employ two stages: first, a high- temperature shift (HTS) typically at 350~450°C followed by a low-temperature shift (LTS) usually conducted at ~250°C [5]. Cu-based catalysts have been extensively investigated in the low- temperature WGS reaction. However, current Cu-based LTS catalysts are pyrophoric, suffer from slow kinetics at typical LTS temperatures, and are sulfur-sensitive [6].

Several Cu-based LTS catalysts have been proposed in order to improve their WGS activity and sulfur tolerance for the WGS reaction. The CO conversion at 350°C and the gas hourly space velocity (GHSV) of 4,000 h-1 over carbon-supported Cu-Ni catalysts was shown to be 50 % higher than that for monometallic Cu and Ni catalysts [7]. While Cu-Ni catalysts displayed much lower detrimental methanation activity as compared to the pure Ni catalysts, they were still highly sensitive to H2S [8]. On the other hand, Cu-Cr oxide catalysts show high activity and sulfur tolerance at H2S concentrations below ~150 ppm [9]. However, highly toxic and carcinogenic hexavalent chromium (Cr6+) compounds may be generated during catalyst synthesis, the WGS

49 reaction, and spent catalyst disposal [10]. Therefore, it is highly desirable to develop alternative,

Cr-free Cu-based LTS catalysts. The use of Cu-Fe catalysts has shown promise in overcoming the pyrophoricity of Cu-based catalysts at high temperatures. They are more active than Cu, Fe, Cu-

Zn, and Cu-Ni catalysts [11], but unfortunately require high reaction temperatures (450°C) for sufficient WGS activity and suffer from an undesirable methanation side-reaction [12].

On the other hand, metal oxide supported Pd catalysts are highly active in the WGS reaction and do not display undesirable pyrophoricity [13,14], suggesting that Pd can be a promising promoter for Cu-based low-temperature WGS catalysts [15]. Song et al. reported that CeO2- supported Cu/Pd catalysts possessed good catalytic activity in an oxygen-assisted low-temperature

WGS (OWGS) reaction [16] and showed that the Pd-promoted Cu catalysts were more active than any other combination of the noble (Pt, Au, Pd) and transition metals (Fe, Cu, Ni) that they had investigated in the OWGS reaction [17].

Overcoming sulfur poisoning through the formation of the sulfur-metal bonds is the major challenge facing the Cu-based WGS catalysts. Recently, the sulfur tolerance of binary combinations of 10 different late transition metals (Fe, Ni, Cu, Ru, Rh, Pd, Ag, Ir, Pt, and Au) was investigated by predicting the S adsorption energy on the basis of density functional theory (DFT) calculations [18]. This study suggested that Pd alloyed with Cu is more S tolerant, i.e., characterized by lower S adsorption energy, than either pure Pd or Cu. Indeed, Cu-Pd alloys have generated much interest as materials for H2 separation membranes with respect to their sulfur tolerance [19]. Additionally, the fcc Cu/Pd alloy phase showed greater tolerance to H2S poisoning than pure Pd [20]. Gonzalez et al. reported that PdCu/C was an H2S-tolerant catalyst for proton- exchange membrane fuel cells (PEMFC) at low H2S levels [21]. However, experimental studies of the Cu-Pd WGS catalysts and their sulfur tolerance have not yet been reported.

Various methods of nanoparticle synthesis reported recently appear to be promising for the fine-tuning of electronic and catalytic properties of bimetallic Cu-Pd nanoparticles for the WGS reaction. Previously, we systematically investigated γ-alumina supported Cu-Ni catalysts prepared from metal colloids and by conventional support impregnation with Cu and Ni salt solutions [22].

It was demonstrated that Cu-Ni alloy shell/Cu core and 5 ~ 20 nm sized Cu-Ni alloy nanoparticles synthesized using metal colloids displayed higher WGS activity at low temperature than conventional Cu-Ni catalysts, and the lack of methanation behavior typical of the Ni catalysts [8].

Previously, Cu/Pd nanoparticles of various sizes were synthesized by Kariuki et al. using the colloidal synthesis method, and their findings suggested that 5 ~ 10 nm Cu-Pd alloy nanoparticles are promising electrocatalysts [16]. Thus, the colloidal reduction method represents a promising synthesis method of well-dispersed Cu/Pd alloy nanoparticles with tunable compositions and controllable size for the WGS reaction.

Metallic Cu0 is well known for its WGS activity, and many studies performed to date have

0 aimed at enhancing the formation of the Cu phase [14,23-25]. Al2O3-supported Cu oxide catalysts calcined at 800°C were reported to show highly dispersed Cu0 species with partial formation of the CuAl2O4 spinel phase [26]. Yahiro et al. proposed that the spinel formation was related to

0 additional Cu surface area but offered no detailed description of how CuAl2O4 formation resulted

0 in additional Cu surface area [26]. For example, Cu can migrate onto Al2O3 surface where it is incorporated as reducible and catalytically active species into CuAl2O4. However, increased surface concentration of reducible CuO species due to CuAl2O4 formation may be offset by a lower specific surface area of CuAl2O4 as compared to CuO [27]. Thus, the extent of CuAl2O4 spinel formation needs to be optimized to maximize the surface concentration of reducible CuO species without significantly reducing the catalyst surface area. Moreover, O’Neill et al. reported that the

CuAl2O4 spinel formation in the pure CuO/Al2O3 system was temperature-sensitive, being enhanced at temperatures in excess of 750°C [28]. The Pd addition to Cu may limit the extent of

CuAl2O4 formation as compared to the CuO/Al2O3 system due to the higher melting points of Cu-

Pd alloys (1088~1419°C) [29] as compared to pure Cu (1085°C). The higher melting points of Cu-

Pd alloys indicate their increased stability[30], which may inhibit the Cu0 mobility at 800°C.

Furthermore, Subramanian and Laughlin reported that the melting points of Cu-Pd alloys increased with Pd content [29]. Therefore, the extent of CuAl2O4 spinel formation could be controlled by modifying the Cu/Pd atomic ratio.

In this study, we report Al2O3-supported bimetallic Cu-Pd catalysts investigated for the

WGS reaction in the presence of H2S. Well-dispersed bimetallic Cu-Pd nanoparticles were first fabricated through co-reduction of Cu-Pd precursor colloidal solution, followed by the impregnation of Al2O3 with these Cu-Pd nanoparticles. The Al2O3-supported bimetallic Cu-Pd nanoparticles with optimized Cu/Pd ratios displayed enhanced WGS activity and greater tolerance to H2S as compared to pure Cu catalysts. Furthermore, the results of this study suggested that the

Cu/Pd ratio of Al2O3-supported Cu-Pd catalysts could be tuned to control the extent of CuAl2O4 spinel formation to maximize the concentration of Cu0 surface active sites.

3.2. Experimental methods

3.2.1. Catalyst preparation

Cu, Pd, and Cu-Pd nanoparticles were synthesized using a liquid-phase chemical reduction route as described previously [31]. The colloidal metal nanoparticle suspension was made by mixing equal volumes of two 1 mM aqueous CTAB (surfactant and capping agent) solutions, with one solution containing hydrazine (3M) and the other one containing metal nitrate (30 mM) including 0.8 mL of 1M NaOH/10 mL of solvent. The CTAB-stabilized colloidal metal

52 nanoparticle suspension was obtained at room temperature under vigorous stirring after 2 hours.

These nanoparticle suspensions were denoted as “as-synthesized” Cu, Pd, and Cu-Pd. γ-Al2O3

(Sumitomo AKP-G15) employed as a catalytic support was added to the nanoparticle suspension followed by the sonication of the resulting slurry for 1 hour. The slurry was then filtered and dried under vacuum at 80°C. The dried powder was calcined in air at 800°C for 5 hours to remove CTAB.

The pure Pd nanoparticles were deposited at 5 wt. % relative to the γ-Al2O3 support. The deposition of both Cu and Cu-Pd nanoparticles was conducted at 10 wt. % Cu relative to the γ-Al2O3 support, while the amount of Pd deposited was 1, 2 and 5 wt. % depending on the composition of Cu-Pd nanoparticles. The bimetallic Cu-Pd/Al2O3 catalysts were denoted as CuPd1, CuPd2, and CuPd5.

The notation of Cu-Pd catalysts reflects the composition of the synthesis solution.

3.2.2. Catalyst characterization

The Cu-Pd nanoparticle structure, size distribution, and morphology were characterized by

TEM. The samples for TEM were prepared by first dispersing powdered catalysts in an ethanol/water solution and then allowing a drop of suspension to evaporate on a copper grid coated with lacey carbon film. TEM analysis was performed employing a Phillips CM20 electron microscope at a 200 kV accelerating voltage. Standard deviation of our results is used for the descriptive error range and error bar.

Crystal structures were determined by powder X-ray diffraction (XRD) using a

PANalytical X’pert diffractometer equipped with Cu Kα radiation source. The XRD data were collected in a step scan mode at 2θ = 30-70° and a step size of 0.05 °/s. The average particle size was determined from the XRD peak broadening by Scherrer’s equation, t = Kλ/β cos θ, where t is the average dimension of crystallites along the [h k l] direction; λ is the wavelength of X-ray

53 irradiation (1.5418A˚); θ is the position of the (h k l) diffraction peak; K is the Scherrer constant

(usually taken as 0.9); and β is the full width at half-maximum height.

The catalysts were characterized by X-ray photoelectron spectroscopy (XPS, Kratos Axis

Ultra XPS) and H2 temperature-programmed reduction (H2 TPR). The XPS is a surface-sensitive quantitative spectroscopic technique that measures the oxidation states and chemical compositions of heterogeneous catalysts [32]. The H2-TPR studies were performed using a tubular fixed-bed reactor equipped with an on-line Stanford Research Systems QMS 200 gas analyzer. The catalysts were heated at 5°C/min from 35°C to 580°C in H2 (10 vol.%) balanced with N2 flowing at 10 mL/min (STP). The amount of H2 consumption was estimated by the mass spectrum during TPR analysis.

The Cu0 metal surface areas and CO adsorption in these catalysts were characterized by

CO chemisorption using Micromeritics ASAP 2020 porosimeter. These results were used to determine metal dispersion and active-site concentrations, which were instrumental in elucidating the nature of the catalytic surface [33]. The N2 adsorption-desorption isotherms were measured at

77K using Micrometrics TriStar porosimeter. The pore-size distributions and surface areas were determined by the Barrett-Joyner-Halenda (BJH) [34] and Brunauer-Emmett-Teller (BET) methods [35], respectively.

Energy-dispersive X-ray spectroscopy (EDS) and Inductively Coupled Plasma Mass

Spectrometry (ICP-MS) were employed to analyze the bulk chemical composition of the catalysts.

The average elemental compositions were determined for 5 different sample locations using a 0.5

m spot size (Phillips XL30 ESEM with EDS). The Cu/Pd content in supported catalysts was determined using Agilent 7700 ICP-MS system. The samples were digested for 4 days at room

54 temperature in aqua regia (1 HNO3/3 HCl, v/v), and the supernatant was then separated by centrifugation and diluted with 2 wt. % HNO3 solution prior to the ICP-MS analysis.

3.2.3. WGS activity

The WGS activities of supported Cu/Pd catalysts were determined employing a fixed-bed tubular quartz micro-reactor (0.55 cm ID) operated at atmospheric pressure using 0.1 g of catalyst diluted with 0.2 g of quartz powder and the feed (100 mL/min) containing 10 mol. % CO and 20 mol. % H2O in helium. All catalysts were reduced under flowing 20 mol. % H2 in He at 300°C before catalytic tests. Water was injected into a flowing gas stream by a syringe pump and vaporized in the heated gas feed line before entering the reactor. A condenser filled with ice was installed at the reactor exit to remove water from the reaction products, prior to their analysis by a gas chromatography (HP-5890 II equipped with a thermal conductivity detector). The carbon balance agreed within ±5 mol. %. A sulfur-tolerance test was conducted using the above- mentioned model CO/H2O feed in He that also contained 500 ppm H2S.

3.3. Results and discussion

3.3.1. Morphological and structural characterization of Cu-Pd nanoparticles

Figure 3.1 shows the TEM images of as-synthesized bimetallic Cu-Pd nanoparticles as a function of Cu/Pd ratios, before pre-treatment and without the alumina support. These images indicated that Cu-Pd alloys consisted of 9.2, 9.1, and 8.0 nm particles on the average (CuPd1,

CuPd2, and CuPd5, respectively). The inset of each TEM image shows the size distribution of Cu-

Pd nanoparticles determined by the ImageJ size measurement function [36]. Figure 3.2 shows the

XRD patterns of as-synthesized Cu-Pd nanoparticles at different Cu/Pd ratios without the alumina support. The two dashed lines indicate a peak at 2ϴ = 43.4o (PDF 01-071-4609) corresponding to

Cu0(111), and a peak at 2ϴ = 39.1o (PDF 01-087-0637) corresponding to the Pd0(111). The main

(111) reflection of the Cu-Pd alloy was observed in the XRD pattern of the Cu-Pd nanoparticles at a value of 2ϴ located between those for the (111) reflections of pure Cu and Pd metals. The (111) reflection of the Cu-Pd nanoparticles shifted toward that of pure Pd0 with increasing Pd content.

The average crystallite sizes of 9.8, 8.9, and 8.2 nm in CuPd1, CuPd2, and CuPd5, respectively, were also estimated by the Scherrer equation (Table 3.1) [37], which agreed with the particle sizes observed by TEM confirming successful synthesis of well-dispersed 8~10 nm Cu-Pd alloy nanoparticles.

Figure 3.1. TEM images of as-synthesized bimetallic (a) CuPd1, (b) CuPd2, and (c) CuPd5 nanoparticles without the alumina support before calcination at 800°C in air.

Pd0 Cu-Pd Cu0

CuPd5

CuPd2 Intensity (a.u.) CuPd1

30 35 40 45 50 2deg.

Figure 3.2. XRD patterns of unsupported as-synthesized bimetallic Cu-Pd nanoparticles before calcination at 800°C in air.

Table 3.1. Physicochemical characteristics of Cu, Pd, and Cu-Pd catalysts. Cu CuPd1 CuPd2 CuPd5 Pd Particle sizea (nm) 20.4 ± 1.6 9.2 ± 2.6 9.1 ± 2.1 8.0 ± 2.2 7.0 ± 1.0

Particle sizeb (nm) 14.5 9.8 8.9 8.2 8.0

Cuc (wt. %) 8.97 ± 0.51 6.25 ± 0.31 5.57 ± 0.48 5.87 ± 0.58 -

Pdc (wt. %) - 0.44 ± 0.05 1.09 ± 0.01 2.16 ± 0.02 -

Cud (wt. %) 10.49 6.24 6.05 6.20 -

Pdd (wt. %) - 0.68 1.51 3.18 5.37

e CuO/CuAl2O4 1.17 1.95 2.22 4.14 -

f CuO/CuAl2O4 0.64 2.26 2.37 4.16 - BET (m2/g ) before cat 150.1 152.3 151.5 153.2 151.0 800℃ calcination BET (m2/g ) after cat 124.0 146.9 149.6 154.2 158.2 800℃ calcination CO uptake 103 ± 3 181 ± 10 207 ± 9 154 ± 17 79 ± 3 (µmol/gcat) a. Particle size was determined from TEM images b. Particle size was calculated by Scherrer equation: d = .∗. ∗ c. ESEM-EDS elemental analysis d. ICP-MS elemental analysis e. The CuO/CuAl2O4 molar ratios from XPS analysis f. The CuO/CuAl2O4 molar ratios from H2-TPR analysis 3.3.2. CuAl2O4 formation and WGS activity of Cu-Pd catalysts

Figure 3.3 shows the XRD patterns of γ-Al2O3, CuPd2/γ-Al2O3 and CuPd2/γ-Al2O3 calcined in air at 800°C. The two peaks visible at 2ϴ = 35.7°, 39°, and 48.7° in the XRD pattern of the CuPd2 catalyst before calcination at 800°C correspond to CuO(-111), (111), and (-202)

(PDF 00-041-0254), respectively. This suggests that the Cu component was partially oxidized in the aqueous medium during the Cu(-Pd) nanoparticle deposition onto γ-Al2O3 and due to the exposure to air during sample preparation for XRD analysis. Due to the high Cu content of CuPd2,

57 the diffraction peaks mainly indicate the presence of the CuO phase. The intensities of the XRD peaks corresponding to the CuO in the XRD pattern of CuPd2 (800°C) were lower than those of

CuPd2 before calcination, suggesting that crystallinity and CuO crystallite size in CuPd2 decreased after 800°C calcination. The distinct (311) and (440) reflections of CuAl2O4 at 2ϴ = 36.9° and

65.7° (PDF 00-001-1153), respectively, were observed in the XRD pattern of the CuPd2 catalyst calcined at 800°C, indicating CuAl2O4 formation during 800°C calcination.

* + * +

* CuPd2 (800oC) *

* CuPd2 Intensity (a.u.)

-Al2O3

30 40 50 60 70 2(deg.)

Figure 3.3. XRD patterns of bare γ-Al2O3, CuPd2/γ-Al2O3 and CuPd2/γ-Al2O3 calcined in air at 800°C. (+ : CuAl2O4, * : CuO).

Figure 3.4 shows the catalytic activity of γ-Al2O3 supported Cu, Cu calcined at 800°C,

CuPd2, and CuPd2 calcined at 800°C. The calcination of the CuPd2/γ-Al2O3 catalyst at 800°C resulted in a significant increase of CO conversion over the entire temperature range investigated.

This is attributed to an increase in the active area of the Cu0 phase due to Pd-facilitated Cu0

0 dispersion [38] and the presence of additional surface Cu species due to CuAl2O4 spinel formation

[26], as discussed below. The CO conversion over the pure Cu catalyst also increased after 800°C

58 calcination, but not to the same extent as that of the bimetallic Cu-Pd catalysts. The effect of calcination on the WGS activity was not significant for the pure Pd catalyst.

100 Equilibrium CuPd2 (800oC) CuPd2 80 Cu (800oC) Cu

40 COConversion (%)

0 150 200 250 300 350 400 450 o Temperature ( C)

Figure 3.4. CO conversion during WGS reaction over γ-Al2O3 supported Cu, Cu calcined at 800°C, CuPd2, and CuPd2 calcined at 800°C (Feed: 10 vol.% CO and 20 vol.% H2O in He at GHSV = 25,000 h-1). 3.3.3. WGS activity of Cu-Pd nanoparticle catalysts

The WGS activity of Al2O3-supported Cu, CuPd1, CuPd2, CuPd5, and Pd nanoparticles after 800°C calcination was determined at 125°C~450°C (Figure 3.5). Supported Cu-based nanoparticle catalysts showed a light-off temperature of 125°C. Although the light-off temperature of the Pd catalyst (250°C) was higher than that of Cu-10, the supported bimetallic Cu-Pd catalysts had almost the same light-off temperature (125°C) as the monometallic Cu catalyst, suggesting that their surfaces may be enriched in Cu. The bimetallic Cu-Pd catalysts were more active than pure Cu and Pd catalysts over the entire temperature range based on increased CO conversion. The

CuPd2 catalyst was more active than pure Cu, Pd, and other Cu-Pd catalysts over the entire temperature range. Also, their catalytic activity was higher than that previously reported by Song et al. for the Cu5Pd2/CeO2 catalyst [16]. The activity enhancement of the CuPd2 catalyst is

59 attributed to the highest CO uptake (Table 3.1), representing the largest amount of the active Cu0 phase after pre-reduction relative to other Cu-based catalysts. All of the catalysts investigated showed the absence of methanation, which is desirable for Cu-based WGS catalysts as reported previously [22].

100 Equilibrium Cu 80 CuPd1 CuPd2 CuPd5 Pd 60

40 COConversion (%) 20

0 150 200 250 300 350 400 450 o Temperature ( C)

Figure 3.5. CO conversion during WGS reaction of γ-Al2O3 supported Cu, CuPd1, CuPd2, CuPd5, - and Pd after 800℃ calcination (Feed: 10 vol.% CO and 20 vol.% H2O in He at GHSV = 25,000 h 1).

3.3.4. Effect of CuO/CuAl2O4 molar ratio on WGS activity

Figure 3.6 shows the H2-TPR profiles collected at temperatures ranging from 35°C~580°C for the Cu-Pd catalysts calcined at 800°C. Distinctly large and relatively sharp peaks at low temperatures and much broader peaks at high temperature were observed for the Cu, CuPd1,

CuPd2, and CuPd5 catalysts. Large sharp peaks with an additional shoulder (100°C~200°C) correspond to the reduction of CuO species [39]. Broad peaks at 400°C~550°C correspond to the

2+ Cu reduction in CuAl2O4 [40]. The reduction temperature of CuO (low-temperature peak) decreased from 159°C for Cu-10 to 129°C for CuPd5 with increasing Pd content (Figure 3.6). The enhanced Cu2+ reducibility may be explained by the Pd promoter effect as well as the small

60 crystallite size of the CuO phase after 800°C calcination, e.g., the XRD pattern of 800°C calcined

CuPd2 in Figure 3.3. The hydrogen spillover from Pd to the neighboring CuO has been reported to enhance the reducibility of CuO in a Cu-Pd alloy [41]. Fox et al. also reported that the CuO promotion with Pd facilitated its reduction by increasing the Cu0 dispersion through a hydrogen spillover mechanism [38]. Furthermore, Dow et al. observed enhanced reducibility of small CuO crystallites [39], while highly dispersed fine Cu0 particles were observed in CuO with poor crystallinity [26]. In this respect, the Pd addition to Cu is suggested to suppress the crystallization of CuO during 800°C calcination and enhance the reducibility of the Cu-Pd catalysts.

125 CuPd5

132 CuPd2

133 CuPd1 Consumption (a.u.) 2 H

159 Cu

100 200 300 400 500 600 o Temperature ( C)

Figure 3.6. H2-TPR profiles of Cu-Pd catalysts after their calcination at 800°C.

The CuO/CuAl2O4 molar ratio was also examined in H2-TPR studies. The relative ratios of CuO and CuAl2O4 phases were estimated from the areas of H2 consumption for the corresponding reduction peaks, normalized by the catalyst mass. Based on the results of the H2-

TPR analysis, the CuAl2O4 spinel was not reduced under the pre-reduction conditions employed in this study (at 300°C in 10 vol.% H2 in He). Therefore, the surface Cu species were present after

0 2+ pre-reduction as a mixture of Cu from reduced CuO and Cu from non-reducible CuAl2O4 spinel.

The CuO to CuAl2O4 ratios were calculated from the corresponding H2-TPR peak areas. These ratios increased with increasing Pd content: molar CuO/CuAl2O4=2.26 (CuPd1), 2.37 (CuPd2), and 4.16 (CuPd5), indicating that the formation of CuAl2O4 spinel in Cu-Pd catalysts was progressively suppressed with increasing Pd content. Therefore, the Pd addition to Cu/Al2O3 is suggested to suppress CuAl2O4 spinel formation during calcination at 800°C, while the total Cu content in these catalysts after 800°C calcination was constant (Table 3.1). The CuPd2 catalyst, which was the most active among the catalysts investigated, exhibited an optimal CuO/CuAl2O4 molar ratio of 2.37 in good agreement with the optimal CuO/CuAl2O4 ratio of 2.3 for the pure

Cu/Al2O3 catalyst reported previously [26].

The Cu 2p3/2 XPS spectra of the Cu-Pd catalysts are shown in Figure 3.7. The spectra were deconvoluted into two peaks centered at 933.2 ± 0.2 eV corresponding to Cu2+ of CuO and 934.6

2+ ± 0.4 eV corresponding to Cu of CuAl2O4, respectively [27,42,43]. The relative areas of the two peaks determined by using Peakfit 4.12 software [44] are summarized in Table 3.1. Although the

CuO/CuAl2O4 molar ratios estimated by XPS were slightly different from those obtained in H2-

TPR studies, this ratio for the most active CuPd2 catalyst was 2.22, showing good agreement with our H2-TPR result (2.37) and the previously reported optimal value (2.3).

934.6 933.2 CuPd5

CuPd2

CuPd1 Intensity (a.u.)

938 936 934 932 930 Binding energy (eV)

Figure 3.7. XPS-spectra (Cu 2p3/2 region) of Cu-Pd catalysts after calcination at 800°C in air. 2+ 2+ Dashed lines at 934.6 eV and 933.2 eV indicate Cu in CuAl2O4 and Cu in CuO, respectively.

Table 3.1 also shows the BET surface areas of CuPd/Al2O3 catalysts before and after calcination at 800°C in air. The BET surface areas of all catalysts were similar before calcination,

2 indicating that their surface areas were initially similar regardless of Pd content (150-153 m /gcat)

2 and remained similar (within 5%) after calcination at 800°C (147-154 m /gcat). These observations indicated that the CuAl2O4 spinel formed on the surface of γ-Al2O3 without adversely impacting the surface area of CuO and/or γ-Al2O3. However, the pure Cu-10 showed 20% lower BET surface area after calcination at 800°C and the lowest CuO/CuAl2O4 molar ratio of 1.17, suggesting that the extensive CuAl2O4 spinel formation at 800°C could decrease the surface area of the catalyst.

CO chemisorption was performed to determine the Cu0 surface area formed by pre- reduction at 300°C in H2, which is known as the proposed location of the WGS catalytic active

63 site. The Cu0 surface area was measured by CO chemisorption under the assumption that the Cu0 surface area was directly proportional to the amount of CO uptake [45], which is summarized in

Table 3.1. The Cu0 surface area is also shown in Figure 3.8, calculated by the following equation:

6.023 × 10 surface area (m/ ) = × × × 22414

3 where VChemisorbed CO is the volume of CO uptake (cm /gcat), Aarea is the effective area per single active metal atom (nm2/atom, 0.068 for Cu and 0.0787 for Pd), and SF indicates the stoichiometric factor (1 for CO). Standard deviation is used for the descriptive error bar.

60 9 ) Cu0 surface area cat CO consumption rate 55 ) 8 cat /g mol/s/g 2

50  7

45 6

40 surface are (m

0 5

Cu 35 4 CO consumptionCO rate ( 30 1 2 3 4 CuO/CuAl O molar ratio 2 4

Figure 3.8. Cu0 metal surface area and CO consumption rate of Cu-Pd catalysts after 800°C calcination as a function of CuO/CuAl2O4 molar ratio. Similar trends were observed for the dependence of the CO consumption rate and Cu0 surface areas on the CuO/CuAl2O4 ratios (Figure 3.8), showing a strong positive correlation between the Cu0 surface area and the WGS reaction rate. The CO consumption rate was calculated using the following formula, where Wcatalyst is the catalyst mass, is CO conversion at 450°

-1 (GHSV = 25,000 h , 10 vol.% CO, 20 vol.% H2O in He), and , is the flow rate of CO in

the feed stream (mol/s): = × ,

These results (Figure 3.8) further support the conclusions of earlier studies that Cu0 is the active surface site responsible for the WGS activity of Cu-Pd catalysts, as it is the proposed WGS active sites of Cu-Ni/Al2O3 [22], Cu-Fe/Al2O3 [46], Cu-ZnO/Al2O3 [47], and Cu-MgO/Al2O3 [48].

The volcano plots shown in Figure 3.8 suggest that CuAl2O4 formation may result in additional

0 Cu surface area for up to the molar CuO/CuAl2O4 = 2.37 (CuPd2), but excessive CuAl2O4 formation reduces Cu0 surface area and therefore the WGS activity.

3.3.5. Sulfur tolerance and thermal stability of optimized Cu-Pd/Al2O3 catalyst

Figure 3.9 shows the CO conversion of Cu@Ni/Al2O3, the commercial CuCrBaOx catalyst, and CuPd2/Al2O3 at temperatures ranging from 125°C~450°C. For comparison purpose, the

CuCrBaOx catalyst was selected as the commercial Cu-based catalyst since the commercial Cu-Cr oxide catalyst was reported to show some sulfur tolerance at low H2S concentration (~150 ppm)

[9]. The Cu@Ni catalyst (Cu core/Ni shell) was selected as the catalyst showing superior WGS activity without methanation among bimetallic Cu-Ni catalysts reported in a previous study [8].

Since the WGS activity of the Cu@Ni and commercial catalysts was found to decrease following their calcination at 800°C, these catalysts were not calcined prior to the WGS activity tests. The

CuPd2/Al2O3 catalyst was the optimized Cu-Pd catalyst identified in this study. Despite slightly lower WGS activity of the CuPd2 catalyst as compared to that of the Cu@Ni catalyst above 300°C, the Cu-Pd catalyst displayed the highest CO conversion near the target reaction temperature of

250°C. The CuPd2 catalyst was also slightly more active than the commercial catalyst above

200°C. As shown in Figure 3.10, the CuPd2 catalyst showed no deactivation for at least 12 days of WGS reaction. The activity test of the Cu catalyst was stopped after 6 days of on stream because its WGS activity already dropped by ~40 % during that time.

100

40 Equilibrium Cu@Ni CO Conversion Conversion (%)CO 20 CuCrBaOx CuPd2

0 150 200 250 300 350 400 450 o Temperature ( C)

Figure 3.9. CO conversion during WGS reaction over Cu@Ni/γ-Al2O3 without calcination, CuPd2/γ-Al2O3 after 800°C calcination, and commercial CuCrBaOx catalyst without calcination -1 (Feed: 10 vol.% CO and 20 vol.% H2O in He at GHSV = 25,000 h ).

100

CuPd2 80 Cu

40 CO (%)Conversion CO 20

0 0 2 4 6 8 10 12 Time on stream (days)

CO conversion during the WGS reaction over Cu-based catalysts in the presence of 500

ppm H2S is shown in Figure 3.11 as a function of time on stream. All catalysts showed evidence

66 of deactivation upon exposure to H2S. The Cu@Ni catalyst, which showed the lowest WGS activity at 250°C, was completely deactivated after 2 hours of exposure to 500 ppm H2S in the feed stream. In contrast to the rapid deactivation of the Cu@Ni catalyst, our best CuPd2 catalyst

(defined here as the one that showed the highest WGS activity at 250°C) was characterized by slower deactivation than the Cu@Ni catalyst. The sulfur tolerance of CuPd2 catalyst was similar to that of the commercial catalyst (CuCrBaOx), while the CuPd2 showed higher WGS activity at

250°C than the commercial CuCrBaOx catalyst. Therefore, the bimetallic Cu-Pd catalysts are indeed promising candidates as sulfur-tolerant, low-temperature, and Cr-free WGS catalysts.

However, sulfur poisoning of Cu-based catalysts was irreversible under our WGS reaction conditions, as their WGS activity was not recovered after switching off H2S. We first examined the sulfur-tolerance of the Cu-Pd catalysts synthesized in this study.

100

CuPd2 80 CuCrBaOx Cu@Ni

40 CO Conversion CO (%) 20

0 0 1 2 3 4 5 Time on stream (hours)

Figure 3.11. CO conversion during WGS reaction over CuPd2/γ-Al2O3 after 800°C calcination, Cu@Ni/γ-Al2O3 catalyst (without 800°C calcination), and commercial catalyst (CuCrBaOx) without 800°C calcination in 500 ppm H2S-containing feed (Feed: 10 vol.% CO and 20 vol.% H2O in He at GHSV = 25,000 h-1 and 250°C).

Figure 3.12 shows the WGS activity over Cu, CuPd1, CuPd2, and CuPd5 catalysts as a function of time on stream of 500 ppm H2S-containing feed of 10 vol.% CO and 20 vol.% H2O in helium at 250°C and GHSV=25,000 h-1. The Cu-Pd catalysts showed higher sulfur tolerance than pure Cu-10, which deactivated quickly during the first 3 hours of WGS reaction due to stronger sulfur adsorption on pure Cu as compared to the Cu-Pd alloy [18]. While CuPd1 lost its WGS activity after 4 hours of exposure to H2S, CuPd2 and CuPd5 deactivated more slowly over 6 hours of WGS reaction in the presence of 500 ppm H2S. The CuPd1 showed greater sulfur tolerance than

Cu, but lower than that of CuPd2 and CuPd5, which may be explained by the low Pd content of

CuPd1, which was insufficient to impart sulfur tolerance.

100

Cu 80 CuPd1 CuPd2 CuPd5 60

40 CO (%)CO Conversion 20

0 0 1 2 3 4 5 6 Time on stream (hours)

Figure 3.12. CO conversion during WGS reaction over supported Cu, CuPd1, CuPd2, and CuPd5 catalysts after 800°C calcination with 500 ppm H2S-containing feed (Feed: 10 vol.% CO and 20 -1 vol.% H2O in He at GHSV = 25,000 h and 250°C). 3.3.6. Structural models of bimetallic Cu-Pd nanoparticles

Whereas both EDS and ICP-MS elemental analyses indicated 0.5~3.2 wt.% Pd content in the Cu-Pd catalysts (Table 3.1) after 800°C calcination in air, the Pd 3d peak was not observed in the XPS spectra of Cu-Pd catalysts calcined at 800°C in air. These findings suggested that the Pd

68 component was uniformly covered by the Cu component in Cu-Pd catalysts calcined at 800°C in air, which was sufficiently thick to block the XPS Pd signal. Pd has a greater tendency to become a core component of the bimetallic Cu-Pd particles than Cu, since the surface energy of Pd (2.01

J/m2) is higher than that of Cu (1.79 J/m2) and CuO (0.74 J/m2) [49,50]. This conclusion is supported by the results of Song et al. who reported that noble metal atoms were enveloped by Cu atoms in bimetallic systems based on the results of the EXAFS analysis study [17].

Figure 3.13 shows the lattice parameters of Cu-Pd nanoparticles prior to 800°C calcination determined by XRD as a function of the overall Cu mole fraction in the CuPd1, CuPd2 and CuPd5 catalysts. The well-known Vegard’s law behavior for the Cu-Pd system is also plotted in Figure

3.13, where the lattice parameters of the Cu-Pd alloys represent the weighted averages of the lattice parameters of pure Cu and Pd [51,52]. A significant deviation was observed in Figure 3.13 between the Vegard’s law behavior and experimental results of this study suggesting that the Cu component was present both in the Cu-Pd alloy core and CuO shell.

The fraction of Cu in the CuO shell covering the Cu-Pd alloy core in these supported Cu-

Pd catalysts was estimated from the deviation between the theoretical Cu mole fractions of Cu-Pd alloys based on the Vegard’s law and the overall Cu mole fraction determined by ICP-MS elemental analysis. All Pd was assumed to be present in the Cu-Pd alloy core since Pd was the limiting component during the preparation of Cu-Pd nanoparticles. The theoretical Cu mole fractions in the Cu-Pd alloys were determined from the best line fit to Vegard’s law as the following equation: a(Å) = 3.61 Å × 1 − + 3.89 Å × , where is the Pd mole fraction in a Cu-Pd alloy, 3.61 Å is the lattice parameter of pure Cu, 3.89 Å is the lattice parameter of pure Pd, and a is the experimental lattice parameter of a Cu-Pd alloy. The predicted Cu-Pd alloy core compositions (Cu87Pd13, Cu76Pd24, and Cu58Pd42) in these catalysts were very similar to those

69 of the three consecutive stable phases (Cu87Pd13, Cu71Pd29, and Cu58Pd42) reported for the Cu-Pd phase diagram with respect to increasing Pd content [29]. For instance, the theoretical (Vegard’s law) Cu and Pd mole fractions in CuPd2 based on its experimentally measured lattice parameter are 0.76 and 0.24, respectively, while the overall Cu and Pd mole fractions in CuPd2 nanoparticles were determined by ICP-MS to be 0.87 and 0.13, respectively. Therefore, the CuPd2 nanoparticles are expected to contain 46 at.% of all Cu as a CuO shell, while the rest is present in the Cu76Pd24 alloy. This predicted alloy composition (Cu76Pd24) of the best CuPd2 catalyst reported in this study also agrees well with the theoretical predictions of Inoglu et al., that the Cu-Pd particles containing the topmost Cu layer and subsurface Cu-Pd alloy core (Cu mole fraction=0.75) were a promising sulfur-tolerant WGS catalyst on the basis of DFT calculations [18].

3.8

Vegard's law experimental

Å CuPd5

3.7 CuPd2

CuPd1 Lattice parameterLattice ( )

3.6 0.5 0.6 0.7 0.8 0.9 1.0 Cu mole fraction (x)

Figure 3.13. Experimental and theoretical (Vegard’s law) lattice parameters of Cu-Pd alloys plotted as a function of Cu mole fraction in bimetallic Cu-Pd nanoparticles prior to calcination at 800°C in air.

Based on the former assumption and our findings, the structural models of bimetallic Cu-

Pd nanoparticles are proposed in Figure 3.14. The as-synthesized Cu-Pd catalyst consisted of Cu-

Pd alloy core, while 44~51 at.% of all Cu was present as CuO shell prior to 800°C calcination.

During calcination at 800°C, Cu species in the CuO shell become mobile and migrate to the surface of the γ-Al2O3 support, and reacted with it forming some CuAl2O4. When the extent of CuAl2O4 spinel formation is optimized, additional reducible CuO sites are generated on the CuAl2O4 surface.

However, the reducible CuO sites in the CuO shell can be significantly depleted by migration of excess Cu species to form CuAl2O4 on the γ-Al2O3 support when excess CuAl2O4 spinel is formed.

Figure 3.14. Proposed structural model for bimetallic Cu-Pd nanoparticles supported on γ-Al2O3 before (left) and after calcination at 800°C in air (right). 3.4. Conclusions

Well-dispersed and uniformly sized Cu-Pd nanoparticles with tunable Cu/Pd ratios have been synthesized by a colloidal chemical reduction method and investigated as highly active and sulfur-tolerant WGS catalysts. The Pd addition enhanced the reducibility of CuO, facilitated the

0 dispersion of Cu species, and reduced the extent of CuAl2O4 spinel formation during calcination at 800°C in air. The CuPd2 catalysts exhibited the greatest density of surface active Cu0 sites corresponding to the optimal CuO/CuAl2O4 molar ratio of 2.37. The optimal CuPd2 catalyst was found to be both thermally stable and more sulfur-tolerant than the other Cu-based catalysts

71 examined in this study. It is proposed that the Cu species of the CuO shell migrate to the γ-Al2O3 support during the 800°C calcination and form CuAl2O4 spinel, generating additional reducible

CuO sites on the surface of CuAl2O4. However, the excessive formation of the CuAl2O4 spinel reduced the surface density of active Cu species in the CuO shell and decreased the total number of active sites in the Cu-Pd catalysts. These findings offer valuable insights concerning the optimization of Cu-Pd catalysts that are highly active and sulfur tolerant in a low-temperature

WGS reaction.

3.5. References

1. B. D. Solomon, V. A. Luzadis, "Renewable Energy from Forest Resources in the United States," Routledge, New York, 2008

2. R. R. Davda, J. W. Shabaker, G. W. Huber, R. D. Cortright, J. A. Dumesic, "A Review of Catalytic Issues and Process Conditions for Renewable Hydrogen and Alkanes by Aqueous-Phase Reforming of Oxygenated Hydrocarbons Over Supported Metal Catalysts," Applied Catalysis B: Environmental, 56, 2005, 171-186

3. S. Ahmed, M. Krumpelt, "Hydrogen from Hydrocarbon Fuels for Fuel Cells," International Journal of Hydrogen Energy, 26, 2001, 291-301

4. C. H. Bartholomew, R. J. Farrauto, "Fundamentals of Industrial Catalytic Processes: Second Edition," John Wiley & Sons, New Jersey, 2010

5. Y. Choi, H. G. Stenger, "Water Gas Shift Reaction Kinetics and Reactor Modeling for Fuel Cell Grade Hydrogen," Journal of Power Sources, 124, 2003, 432-439

6. D. S. Newsome, "The Water-Gas Shift Reaction," Catalysis Reviews, 21, 1980, 275-318

7. O. Arbeláez, T. R. Reina, S. Ivanova, F. Bustamante, A. L. Villa, M. A. Centeno, J. A. Odriozola, "Mono and Bimetallic Cu-Ni Structured Catalysts for the Water Gas Shift Reaction," Applied Catalysis A: General, 497, 2015, 1-9

8. J. Lin, V. V. Guliants, "Alumina-Supported Cu@Ni and Ni@Cu Core-Shell Nanoparticles: Synthesis, Characterization, and Catalytic Activity in Water-Gas-Shift Reaction," Applied Catalysis A: General, 445-446, 2012, 187-194

9. R. Prasad, P. Singh, "A Review on CO Oxidation Over Copper Chromite Catalyst," Catalysis Reviews - Science and Engineering, 54, 2012, 224-279

10. S. Natesakhawat, X. Wang, L. Zhang, U. S. Ozkan, "Development of Chromium-Free Iron- Based Catalysts for High-Temperature Water-Gas Shift Reaction," Journal of Molecular Catalysis A: Chemical, 260, 2006, 82-94

11. H. Yan, X. Qin, Y. Yin, Y. Teng, Z. Jin, C. Jia, "Promoted Cu-Fe3O4 Catalysts for Low- Temperature Water Gas Shift Reaction: Optimization of Cu Content," Applied Catalysis B: Environmental, 226, 2018, 182-193

12. M. Zhu, I. E. Wachs, "Iron-Based Catalysts for the High-Temperature Water-Gas Shift (HT- WGS) Reaction: A Review," ACS Catalysis, 6, 2016, 722-732

13. C. A. Callaghan, S. A. Vilekar, I. Fishtik, R. Datta, "Topological Analysis of Catalytic Reaction Networks: Water Gas Shift Reaction on Cu(1 1 1)," Applied Catalysis A: General, 345, 2008, 213- 232

14. X. Xu, K. Shuai, B. Xu, "Review on Copper and Palladium Based Catalysts for Methanol Steam Reforming to Produce Hydrogen," Catalysts, 7, 2017,

15. R. Watanabe, Y. Sekine, H. Takamatsu, Y. Sakamoto, S. Aramaki, M. Matsukata, E. Kikuchi, "Pt and/Or Pd Supported/Incorporated Catalyst on Perovskite-Type Oxide for Water Gas Shift Reaction," Topics in Catalysis, 53, 2010, 621-628

16. J. Kugai, E. B. Fox, C. Song, "Role of CeO2 Support for Pd-Cu Bimetallic Catalysts for Oxygen-Enhanced Water Gas Shift," Applied Catalysis A: General, 456, 2013, 204-214

17. J. Kugai, J. T. Miller, N. Guo, C. Song, "Role of Metal Components in Pd-Cu Bimetallic Catalysts Supported on CeO2 for the Oxygen-Enhanced Water Gas Shift," Applied Catalysis B: Environmental, 105, 2011, 306-316

18. N. Inoglu, J. R. Kitchin, "Identification of Sulfur-Tolerant Bimetallic Surfaces using DFT Parametrized Models and Atomistic Thermodynamics," ACS Catalysis, 1, 2011, 399-407

19. B. D. Morreale, M. V. Ciocco, B. H. Howard, R. P. Killmeyer, A. V. Cugini, R. M. Enick, "Effect of Hydrogen-Sulfide on the Hydrogen Permeance of Palladium-Copper Alloys at Elevated Temperatures," Journal of Membrane Science, 241, 2004, 219-224

20. N. Pomerantz, Y. H. Ma, "Novel Method for Producing High H2 Permeability Pd Membranes with a Thin Layer of the Sulfur Tolerant Pd/Cu Fcc Phase," Journal of Membrane Science, 370, 2011, 97-108

21. T. Lopes, V. A. Paganin, E. R. Gonzalez, "Hydrogen Sulfide Tolerance of Palladium-Copper Catalysts for PEM Fuel Cell Anode Applications," International Journal of Hydrogen Energy, 36, 2011, 13703-13707

22. J. Lin, V. V. Guliants, "Hydrogen Production through Water-Gas Shift Reaction Over Supported Cu, Ni, and CuNi Nanoparticle Catalysts Prepared from Metal Colloids," ChemCatChem, 4, 2012, 1611-1621

23. J. R. Mellor, N. J. Coville, A. C. Sofianos, R. G. Copperthwaite, "Raney Copper Catalysts for the Water-Gas Shift Reaction: I. Preparation, Activity and Stability," Applied Catalysis A: General, 164, 1997, 171-183

24. C. Ratnasamy, J. Wagner, "Water Gas Shift Catalysis," Catalysis Reviews - Science and Engineering, 51, 2009, 325-440

25. E. B. Fox, S. Velu, M. H. Engelhard, Y. Chin, J. T. Miller, J. Kropf, C. Song, "Characterization of CeO2-Supported Cu-Pd Bimetallic Catalyst for the Oxygen-Assisted Water-Gas Shift Reaction," Journal of Catalysis, 260, 2008, 358-370

26. H. Yahiro, K. Nakaya, T. Yamamoto, K. Saiki, H. Yamaura, "Effect of Calcination Temperature on the Catalytic Activity of Copper Supported on Γ-Alumina for the Water-Gas-Shift Reaction," Catalysis Communications, 7, 2006, 228-231

27. S. Velu, K. Suzuki, C. S. Gopinath, H. Yoshida, T. Hattori, "XPS, XANES and EXAFS Investigations of CuO/ZnO/Al2O3/ZrO2 Mixed Oxide Catalysts," Physical Chemistry Chemical Physics, 4, 2002, 1990-1999

28. Hugh S. t. C. O'Neill, Michael James, Wayne A. Dollase, Simon A. T. Redfern, "Temperature Dependence of the Cation Distribution in CuAl2O4 Spinel," European Journal of Mineralogy, 17, 2005, 581-586

29. P. R. Subramanian, D. E. Laughlin, "Cu-Pd (Copper-Palladium)," Journal of Phase Equilibria, 12, 1991, 231-243

30. M. D. Argyle, C. H. Bartholomew, "Heterogeneous Catalyst Deactivation and Regeneration: A Review," Catalysts, 5, 2015, 145-269

31. J. Lin, P. Biswas, V. V. Guliants, S. Misture, "Hydrogen Production by Water-Gas Shift Reaction Over Bimetallic Cu-Ni Catalysts Supported on La-Doped Mesoporous Ceria," Applied Catalysis A: General, 387, 2010, 87-94

32. G. Ertl, R. Hierl, H. Knözinger, N. Thiele, H. P. Urbach, "XPS Study of Copper Aluminate Catalysts," Applications of Surface Science, 5, 1980, 49-64

33. P. Panagiotopoulou, D. I. Kondarides, "Effects of Alkali Promotion of TiO2 on the Chemisorptive Properties and Water-Gas Shift Activity of Supported Noble Metal Catalysts," Journal of Catalysis, 267, 2009, 57-66

34. E. P. Barrett, L. G. Joyner, P. P. Halenda, "The Determination of Pore Volume and Area Distributions in Porous Substances. I. Computations from Nitrogen Isotherms," Journal of the American Chemical Society, 73, 1951, 373-380

35. S. Brunauer, P. H. Emmett, E. Teller, "Adsorption of Gases in Multimolecular Layers," Journal of the American Chemical Society, 60, 1938, 309-319

36. C. Igathinathane, L. O. Pordesimo, E. P. Columbus, W. D. Batchelor, S. R. Methuku, "Shape Identification and Particles Size Distribution from Basic Shape Parameters using ImageJ," Computers and Electronics in Agriculture, 63, 2008, 168-182

37. A. L. Patterson, "The Scherrer Formula for X-Ray Particle Size Determination," Physical Review, 56, 1939, 978-982

38. E. B. Fox, A. F. Lee, K. Wilson, C. Song, "In-Situ XPS Study on the Reducibility of Pd- Promoted Cu/CeO2 Catalysts for the Oxygen-Assisted Water-Gas-Shift Reaction," Topics in Catalysis, 49, 2008, 89-96

39. W. Dow, Y. Wang, T. Huang, "Yttria-Stabilized Zirconia Supported Copper Oxide Catalyst: I. Effect of Oxygen Vacancy of Support on Copper Oxide Reduction," Journal of Catalysis, 160, 1996, 155-170

40. F. Severino, J. L. Brito, J. Laine, J. L. G. Fierro, A. López Agudo, "Nature of Copper Active Sites in the Carbon Monoxide Oxidation on CuAl2O4 and CuCr2O4 Spinel Type Catalysts," Journal of Catalysis, 177, 1998, 82-95

41. G. X. Pei, X. Y. Liu, X. Yang, L. Zhang, A. Wang, L. Li, H. Wang, X. Wang, T. Zhang, "Performance of Cu-Alloyed Pd Single-Atom Catalyst for Semihydrogenation of Acetylene Under Simulated Front-End Conditions," ACS Catalysis, 7, 2017, 1491-1500

42. B. K. Kwak, D. S. Park, Y. S. Yun, J. Yi, "Preparation and Characterization of Nanocrystalline CuAl2O4 Spinel Catalysts by Sol-Gel Method for the Hydrogenolysis of Glycerol," Catalysis Communications, 24, 2012, 90-95

43. P. Porta, M. C. Campa, G. Fierro, M. L. Jacono, G. Minelli, G. Moretti, L. Stoppa, "CuO-ZnO- Al2O3 Mixed Oxides: Preparation, Bulk and Surface Characterization," Journal of Materials Chemistry, 3, 1993, 505-511

44. X. Tong, Z. Zhang, F. Zeng, C. Fu, P. Ma, Y. Peng, H. Lu, Y. Liang, "Recursive Wavelet Peak Detection of Analytical Signals," Chromatographia, 79, 2016, 1247-1255

45. M. H. Kim, J. R. Ebner, R. M. Friedman, M. A. Vannice, "Determination of Metal Dispersion and Surface Composition in Supported Cu-Pt Catalysts," Journal of Catalysis, 208, 2002, 381-392

46. N. Thouchprasitchai, A. Luengnaruemitchai, S. Pongstabodee, "Water-Gas Shift Reaction Over Cu-Zn, Cu-Fe, and Cu-Zn-Fe Composite-Oxide Catalysts Prepared by Urea-Nitrate Combustion," Journal of Industrial and Engineering Chemistry, 19, 2013, 1483-1492

47. K. Nishida, I. Atake, D. Li, T. Shishido, Y. Oumi, T. Sano, K. Takehira, "Effects of Noble Metal-Doping on Cu/ZnO/Al2O3 Catalysts for Water-Gas Shift Reaction. Catalyst Preparation by Adopting "Memory Effect" of Hydrotalcite," Applied Catalysis A: General, 337, 2008, 48-57

48. K. Nishida, D. Li, Y. Zhan, T. Shishido, Y. Oumi, T. Sano, K. Takehira, "Effective MgO Surface Doping of Cu/Zn/Al Oxides as Water-Gas Shift Catalysts," Applied Clay Science, 44, 2009, 211-217

49. A. Patra, J. E. Bates, J. Sun, J. P. Perdew, "Properties of Real Metallic Surfaces: Effects of Density Functional Semilocality and Van Der Waals Nonlocality," Proceedings of the National Academy of Sciences of the United States of America, 114, 2017, E9188-E9196

50. C. Gattinoni, A. Michaelides, "Atomistic Details of Oxide Surfaces and Surface Oxidation: The Example of Copper and its Oxides," Surface Science Reports, 70, 2015, 424-447

51. N. A. Al-Mufachi, S. Nayebossadri, J. D. Speight, W. Bujalski, R. Steinberger-Wilckens, D. Book, "Effects of Thin Film Pd Deposition on the Hydrogen Permeability of Pd60Cu40 wt% Alloy Membranes," Journal of Membrane Science, 493, 2015, 580-588

52. G. Bozzolo, J. Ferrante, "Lattice Parameters of fcc Binary Alloys using a New Semiempirical Method," Scripta Metallurgica Et Materiala, 26, 1992, 1275-1280

Chapter 4. Size-dependent catalytic behavior and sulfur dependence of Mo- based nanoparticles in water gas shift reaction of biomass-derived syngas

4.1. Introduction

Global attention to climate change associated with greenhouse gas emissions has led to the development of sustainable energy resources that can replace conventional fossil fuels, such as petroleum and natural gas. Hydrogen (H2) is a highly attractive energy resource because it is a zero-emission fuel that displays a high energy density [1]. However, H2 is mostly produced from fossil fuels, e.g., by the reforming of natural gas [2], partial oxidation of refinery oil [3], and coal gasification [4], all of which result in significant greenhouse gas emissions. On the other hand, biomass represents a sustainable resource to produce H2 using syngas from biomass gasification

[5], which is a carbon-neutral process [6]. During their life cycle, biomass feedstocks such as plants and algae consume atmospheric carbon dioxide, which is produced again when biomass is converted into hydrogen fuel. A recent report projects that up to 1 billion dry tons of biomass could be available annually with anticipated improvements in agricultural practices and plant breeding

[7].

The water gas shift (WGS) reaction is a key step in maximizing the production of hydrogen from biomass-derived syngas [8]. The WGS reaction is also a practical industrial reaction that accompanies a gas conditioning step [9], ammonia synthesis, and Fisher-Tropsch synthesis of hydrocarbons [10]. However, the presence of sulfur compounds in biomass-derived syngas presents a major challenge for a catalytic WGS process, since most WGS catalysts easily deactivate in the presence of sulfur-containing molecules, such as H2S. In a typical biomass gasification process (0.5~1.5 wt.% sulfur) [11], the syngas must be desulfurized before reaching the WGS catalyst because commercial low-temperature shift (LTS) catalysts, such as CuO-ZnO

77 catalysts, are easily deactivated by trace amounts of sulfur [12]. High-temperature shift (HTS)

Fe2O3-Cr2O3 catalysts are also deactivated when H2S concentration in syngas exceeds 500 ppm

[13,14].

Sulfur-dependent molybdenum-based mixed oxides have been extensively studied as so- called sour-shift catalysts to overcome the sulfur sensitivity of conventional two-stage, HTS and

LTS, WGS catalysts [15-20], including CeMo- [21], CoMo- [19], and NiMo-based catalysts [22].

Previously, NiMo/TiO2 catalysts showed higher WGS activity than commercial CoMo/Al2O3-

[23] MgO catalysts at low temperatures (200°C) and low H2O/CO atomic ratios (1.2) . K, Ce, Fe,

Mg, and Zn have been proposed as promising promoters to improve the activity of Mo-based catalysts [20,24,25]. In some studies, the presence of at least 100 ppm of H2S in the feed has been reported to be essential for maintaining the WGS activity of the sour-shift catalysts [26]. Despite extensive research conducted to date on sour-shift Mo-based catalysts, they are still less active than the conventional HTS catalysts [27]. Therefore, further improving the WGS activity of sulfur- dependent Mo-based catalysts is critical for the development of a commercial WGS process to produce hydrogen from biomass syngas.

In recent decades, the preparation of catalytic materials with nanoscale dimensions has emerged as a powerful methodology to improve catalytic activity by increasing the number of surface active sites. Mo sulfide (Mo-S) moieties present in Mo-based catalysts have been proposed as the surface active sites for the WGS reaction on the basis of DFT calculations and microkinetic studies [22,26,28,29]. The Mo-S sites form on the surface of Mo oxide nanoparticles during sulfidation [30], while the surface area of Mo oxide nanoparticles is inversely proportional to the particle size.

Previously, Kuriki et al. controlled the MoS2 particle size by mechanical milling and obtained small MoS2 particles displaying high surface areas and enhanced methylnaphthalene hydrogenation activity [31]. Moreover, Afanasiev et al. suggested that the small domain size of

MoS2 particles correlated with high surface area and high hydrogenolysis and hydrodesulfurization

(HDS) activity [32], while Contreras et al. reported that the 10~400 nm MoS2 nanoparticles exhibited size-dependent HDS activity [33]. The extent of Mo-S edge site exposure per unit surface area of MoS2 nanoparticles was controlled via the particle size and correlated with the activity in thiophene HDS [34-36]. The catalytic activity in hydrogen evolution was found to correlate linearly with the number of Mo-S edge sites present in MoS2, which was controlled by the size of the MoS2 nanoparticles [37]. Although a few studies of Mo-based nanoparticles with different particle sizes in electrochemical and HDS reactions have been reported to date, the particle size effects on the WGS activity and sulfur tolerance of Mo-based nanoparticles have not been systematically investigated.

Mo oxide nanoparticles can be prepared by several methods, such as mechanical milling, exfoliation, chemical vapor deposition, and etching [31,36,38-42]. However, chemical reduction methods are most commonly used in lab-scale research owing to their simplicity [43]. These methods enable good control of particle size and morphology by varying the concentrations of precursors, the type of surfactant, and the precursor reduction rate via a judicious choice of reducing agents, pH, temperature, and pressure. A number of chemical reduction methods have been reported to yield Mo oxide nanoparticles of different sizes, and their results are summarized in Table 4.1.

Table 4.1. Synthesis of MoOx nanoparticles by chemical reduction methods.

Precursor// Reducing // Size Material Temperature Remark Reference Solvent Capping (nm)

MoS -CdS// (NH ) MoS // oleic acid // Chen et al., 2 4 2 4 300°C 6~11 Bimetal One-pot Oleylamine 1-octadecene (2015) [44] Jeong et (Mo, Ti, Zr, Chloride // Sulfidation Oleylamine 300°C 40 al., (2012) Hf, V,Nb)-S Oleylamine with CS 2 2 [45] 2-layer (Mo, Bimetal Jeong et Chloride // Ti, Zr, Hf, Oleylamine 300°C 120 Sulfidation al., (2011) Oleylamine [46] V,Nb)-S2 with CS2 Sulfidation Wang et Nitrate // CdS CTAB 120°C 5 with S al., (2009) Octadecyl-amine powder [47] Using pre- Bokhimi et MoS // (NH ) Mo O // sulfide 2 4 6 7 24 Hyro-thermal 225°C 50 al., (2001) Hydro-thermal Water precursor [48] (thiourea) Hydroxyl- Afanasiev MoS // (NH ) MoS // Hydrazine// 2 4 2 4 100°C 30 amine et al., Hydro-thermal Water CTAC sulfate (1999) [32] Noble Me- Yuwen et MoS powder// Metal-loaded Noble MoS 2 30°C 2 al., (2014) 2 BuLi-Hexane Impregnation metal nanosheet [41] Zhang et Sulfidation Metal-S Acetate// Paraffin Oleic acid 220°C 60 al., (2009) 2 with H S 2 [49] MoS2 // Solvo- (NH4)2MoS4 // N2H4OH// 25 ~ Sulfidation Zong et al., 200°C [50] Thermal methanol PVP 90 with H2S (2009) NaBH MoO // (NH ) Mo O // 5 ~ 4 Ayi et al., 3 4 6 7 24 NaBH 180°C reducing Hydro-thermal Ionic liquid 4 100 (2015) [51] agent

In this study, we synthesized well-dispersed and uniformly sized Mo oxide and CoMo oxide nanoparticles by modifying the precursor concentration in a chemical reduction method. The size dependence of the WGS activity, the extent of sulfidation, and sulfur-dependence were examined for these Mo-S and CoMo-S catalysts. The effects of promoters (Cu, Ni, Pd, and Ce) on the WGS activity of CoMo-S catalysts was investigated, as well as the effect of reaction conditions for the Mo-S and CoMo-S catalysts.

4.2. Experimental Section

4.2.1. Catalyst preparation

Deionized (DI) water was purged overnight with N2 to remove dissolved O2. Appropriate amounts of Mo and Co precursors (1~40 mM ammonium heptamolybdate and cobalt nitrate solutions) were added to aqueous CTAB (1 ~ 30 mM) to synthesize Mo oxide and CoMo oxide nanoparticles (Table 4.2). 0.6 µl of 1M NaOH (aq.) per 1 ml of total solution was added under thorough mixing in N2 at room temperature to maintain pH 11~12 . Then, 3 ml of 1~3 M NaBH4 solution was injected into the reaction mixture under N2. After 30 min, when the solution color turned black indicating the formation of nanoparticles, excess ethanol and DI water were added.

The resulting particles were separated by centrifugation and rinsed twice by excess ethanol and DI water. After incipient wetness impregnation of the alumina supports with colloidal suspensions of as-synthesized nanoparticles, the supported catalysts were dried overnight at 80°C in air. These catalysts were denoted as-synthesized n-Mo or n-CoMo (n indicates average particle size in nm).

Ni-, Cu-, Pd- and Ce-promoted CoMo catalysts were prepared by using 10-CoMo catalyst as an unpromoted catalyst. Appropriate quantities of promoter metal nitrate solutions were added to 3 g of unpromoted 10-CoMo/Al2O3 by the incipient wetness impregnation method. The catalysts thus prepared were denoted as MxCoMo/Al2O3 (x, indicates wt.% relative to Al2O3, and M indicates promoter metal).

A commercial CoMo catalyst (Clariant Co.) was also evaluated, which contained 10.0 wt.%

MoO3, 4.0 wt.% CoO, 18.0 wt.% MgO, and 68.0 wt.% Al2O3. The pelletized commercial catalyst was crushed and sieved to obtain 100 ~ 500 µm particles.

4.2.2. TEM imaging and XRD analysis

The catalyst samples for transmission electron microscopy (TEM) were prepared by first dispersing powdered catalysts in ethanol and then allowing a drop of suspension to evaporate on a copper grid coated with lacey carbon films. TEM was performed employing a Phillips CM20 electron microscope at a 200-kV accelerating voltage. Standard deviation of the particle size is used for the descriptive error range. The crystalline phases were identified by powder X-ray diffraction (XRD) using a PANalytical X’pert diffractometer equipped with Cu Kα radiation source. The XRD data were collected in a step scan mode at a step size of 0.05°/s, collecting time of 1 s/step, 40 A filament current, and 45 kV accelerating voltage.

4.2.3. Catalytic activity

The catalytic activity of WGS catalysts was determined employing a fixed-bed tubular stainless-steel reactor (0.65 cm ID) operated at atmospheric pressure using 0.5 ~ 1.5 g catalysts to achieve a desirable gas hourly space velocity (GHSV). The feed typically contained 10 vol.% CO and 20 vol.% H2O in helium. The total feed flow rate was 50 mL/min. All catalysts were calcined at 500°C in air for 5 hours and then underwent pre-sulfidation in 1 mol.% H2S in H2 at 450°C for

18 hours prior to the catalytic tests. The pre-sulfided catalysts were denoted n-Mo-S and n-CoMo-

S catalysts. Water was injected into a flowing gas stream by a syringe pump and vaporized in a heated gas feed line before entering the reactor. A condenser filled with ice was installed at the reactor exit to collect water. The product gas mixture was analyzed by a gas chromatograph, HP-

5890 II, equipped with a thermal conductivity detector. The carbon balance agreed within ±5 mol.%.

4.2.4. Surface and bulk elemental analysis

CO chemisorption was performed on a Micromeritics ASAP2020 instrument. The sample was first reduced by H2 at 450°C for 120 min, followed by evacuation at 450°C for 30 min. The sample was then cooled to 35°C under vacuum for 30 min prior to CO chemisorption. The N2 adsorption-desorption isotherms were measured at 77 K using Micrometrics TriStar porosimeter.

The pore size distributions and surface areas were determined by the Barrett-Joyner-Halenda (BJH)

[52] and Brunauer-Emmett-Teller (BET) methods [53], respectively. The inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7700 ICP-MS) was employed to analyze the metal content in supported catalysts. The samples were digested for 4 days at room temperature in aqua regia (1 HNO3/3 HCl, v/v), then the supernatant was separated by centrifugation, and diluted with

2 wt.% HNO3 (aq.) prior to the ICP-MS analysis.

The XPS spectra of supported catalysts were collected using a Kratos AXIS Ultra X-ray photoelectron spectrometer (XPS) equipped with a monochromatized Al Kα X-ray source operated at 12 kV and 10 mA. The XPS spectra for the specific C 1s, O 1s, Mo 3d, Al 2p, and S 2p regions were collected. The charging effect was corrected based on the C 1s binding energy of 284.5 eV.

The background subtraction, normalization, and peak fitting of the data were performed using the

Peakfit software [54].

4.3. Results and Discussion

4.3.1. Synthesis Mo oxide and CoMo oxide nanoparticles

The specific synthesis parameters for unsupported Mo oxide and CoMo oxide nanoparticles are summarized in Table 4.2 [51]. Well-dispersed Mo oxide nanoparticles were obtained by modifying the precursor concentration. Figure 4.1(a) shows the TEM image of the

4.7-Mo sample suggesting the presence of well-dispersed 4.7 nm nanoparticles prepared by reducing a sulfur-containing Mo precursor (ammonium tetrathiomolybdate) with hydrazine. 5, 14,

83 and 23 nm sized Mo nanoparticles prepared by modifying the concentration of ammonium heptamolybdate reduced by NaBH4 are shown in Figure 4.1(b-d), respectively.

Table 4.2. Summary of synthesis parameters for as-synthesized unsupported Mo and CoMo nanoparticles: 4.7-Mo (4.7 ± 0.7 nm), 5-Mo (5.4 ± 0.8 nm), 14-Mo (14.2 ± 3.2 nm), 23-Mo (22.6 ± 3.1 nm), 6-CoMo (6.0 ± 1.6 nm), 10-CoMo (9.7 ± 1.6 nm), 30-CoMo (31.5 ± 4.6 nm), 100- CoMo (~ 100 nm). NaBH Co precursor Mo precursor CTAB Conc. 4 Conc. 1 mM 3 M 4.7-Mo 0 1 mM (sulfide) (hydrazine) 5-Mo 0 1 mM 1 mM 1 M 14-Mo 0 10 mM 30 mM 1 M 23-Mo 0 30 mM 30 mM 3 M 6-CoMo 1.5 mM 1 mM 10 mM 1 M 10-CoMo 5 mM 3 mM 10 mM 1 M 30-CoMo 8 mM 5 mM 30 mM 1 M 100-CoMo 24 mM 18 mM 30 mM 3 M

Figure 4.1. TEM images of as-synthesized unsupported (a) 4.7-Mo (4.7±0.7 nm), (b) 5-Mo (5.4±0.8 nm), (c) 14-Mo (14.2±3.2 nm), and (d) 23-Mo (22.6±3.1 nm).

4.7-Mo nanoparticles were synthesized using ammonium tetrathiomolybdate precursor and hydrazine under reflux. Although this synthesis method resulted in well-dispersed fine Mo nanoparticles, their yield was relatively low (<40%) as well as extremely sensitive to pH and temperature, leading to limited synthesis reproducibility. The nanoparticle yield was determined by the following equation:

(%) = × 100

where Wtheoretical is the Mo mass based on the quantity of Mo precursor used in synthesis, and Wexperimental is the Mo mass based on the results of ICP-MS elemental analysis for supported catalysts before pre-sulfidation. Furthermore, Mo precursors were only reduced at high temperature (> 100°C) when hydrazine or H2 gas were employed as reducing agents. Although large Mo oxide nanoparticles (> 100 nm) were obtained by hydrothermal synthesis (> 180°C, hydrazine), it was difficult to control the particle size in synthesis. In the case of a strong reducing agent, such as NaBH4, Mo-based nanoparticles were reduced quickly at room temperature regardless of solution pH employed. Accordingly, Mo-based nanoparticles were synthesized in high yield (>85%) at room temperature using ammonium heptamolybdate and NaBH4.

Well-dispersed 6, 10, 30, and 100 nm sized CoMo oxide nanoparticles synthesized at different precursor concentrations are shown in Figure 4.2(a-d), respectively, while the 6-CoMo sample displayed somewhat aggregated as well as rod-shaped particles. The size distributions of

Mo and CoMo oxide nanoparticles are shown in the inset of each TEM image, which were determined by the ImageJ software [55].

Figure 4.2. TEM images of as-synthesized unsupported (a) 6-CoMo (6.0 ±1.6 nm), (b) 10-CoMo (9.7±1.6 nm), (c) 30-CoMo (31.5±4.6 nm), and (d) 100-CoMo.

Figure 4.3 shows the XRD patterns of unsupported n-Mo and n-CoMo oxide nanoparticles prior to calcination at 500°C in air. 5-Mo, 14-Mo, and 6-CoMo nanoparticles showed the absence of X-ray reflections due to their relatively small particle size and structural disorder. The distinct

o peak at 2ϴ = 23.8 corresponding to the (110) reflection of MoO3 (PDF 00-005-0508) was observed in larger nanoparticles (23-Mo, 10-, 30-, and 100-CoMo). Another peak at 2ϴ = 27.4° corresponding to MoO3 (210) (PDF 00-005-0508) was observed in the XRD patterns of 10-, 30-, and 100-CoMo nanoparticles, where the intensity of the peaks increased with increasing particle size. Other peaks of MoO3 at 2ϴ = 25.6° (400) and 29.5° (310) (PDF 00-005-0508) were only observed in the 100-CoMo sample. These X-ray reflections indicated the orthorhombic MoO3 phase with different crystal aspect ratios for as-synthesized unsupported Mo oxide nanoparticles

[56,57].

Figure 4.3. XRD patterns of as-synthesized unsupported n-Mo and n-CoMo before calcination at 500°C in air (*: (110), o: (210), v: (400), and x: (310) of MoO3). 4.3.2. Size effects of Mo oxide and CoMo oxide nanoparticle on WGS activity

Figure 4.4 shows the CO reaction rates normalized by the estimated surface area of n-Mo and n-CoMo catalysts as a function of average particle size. The CO reaction rate was calculated by the following equation: =

where rCO is the CO initial reaction rates per one gram of catalysts (µmol/g-cat/s) and

2 SAnanoparticles is the geometric surface area of nanoparticles per one gram of catalysts (m /g-cat) calculated using the average particle size (Figure 1 and 2). The normalized CO reaction rates increased with particle size in the n-CoMo and n-Mo catalysts. In order words, the small particles inhibit the WGS reaction on the surface of nanoparticles. The small nanoparticles bind CO too strong to be active as compared to the large particle size. Furthermore, the actual height of the 5-

Mo and 6-CoMo nanoparticles could be less than 1.5 nm since the aspect ratio (width to height) of the Mo-based nanoparticles is 3.8, which was determined from the XRD peak of 100-CoMo by

Scherrer’s equation. The interaction between the nanoparticles and support in the small particles

(~1.5 nm) is significantly stronger than that in the large particles, resulting in the low reaction rate.

The standard deviation of the results is used for the descriptive error bar.

80 n-Mo-S

/s) n-CoMo-S 60 sulfide 2 mol/m

 40

20 Reaction rate(

0 0 20 40 60 80 100 Particle size (nm)

Figure 4.4. CO reaction rate normalized by the estimated surface area of n-Mo and n-CoMo nanoparticles in supported n-Mo and n-CoMo catalysts. Figure 4.5 shows the CO consumption rates and amounts of CO uptake normalized to moles of Mo for n-Mo-S catalysts as a function of average particle size. The CO consumption rates were determined for GHSV of 2,500 h-1 and at 450°C after 4 hours of WGS reaction employing the feed containing 1,000 ppm H2S. The CO consumption rate was calculated by the following equation:

× = ÷ where FCO is the feed flow rate of CO (mol/s), WMo is the Mo mass based on ICP-MS analysis of supported catalysts before pre-sulfidation (Table 4.3), and AWMo is the atomic mass of Mo. Higher

CO consumption rates were observed for smaller n-Mo-S nanoparticle catalysts. The CO uptake for the n-Mo-S catalysts is also summarized in Table 4.3. Standard deviation is used for the 88 descriptive error range. The 5-Mo-S catalyst showed higher CO uptake than the 14-Mo-S and 23-

Mo-S catalysts, indicating that smaller n-Mo-S particles were able to adsorb more CO which corresponded to a higher CO consumption rate. However, the correlation between the CO consumption rate and the amount of CO uptake was weak for the 5-Mo-S catalyst.

Figure 4.5. CO consumption rate and CO uptake over 5-, 14-, and 23-Mo-S/Al2O3 catalysts (Feed: -1 10 vol.% CO, 20 vol.% H2O, and 1,000 ppm H2S in He at GHSV = 2,500 h and 450°C).

Table 4.3. Physicochemical characteristics of n-Mo and n-CoMo catalysts (Co wt.% and Mo wt.% were estimated by ICP-MS using as-synthesized catalyst after calcination at 500°C in air before pre-sulfidation).

Co (wt.%) Mo (wt.%) CO-uptake (molCO/molMo) 5-Mo 0 9.2 ± 0.9 0.081 ± 0.004 14-Mo 0 9.5 ± 1.0 0.046 ± 0.003 23-Mo 0 8.7 ± 0.31 0.037 ± 0.002 6-CoMo 5.4 ± 0.7 3.9 ± 0.4 0.049 ± 0.002 10-CoMo 4.8 ± 0.3 4.4 ± 0.1 0.059 ± 0.003 30-CoMo 4.5 ± 0.2 4.6 ± 0.3 0.030 ± 0.002 100-CoMo 5.0 ± 0.5 4.2 ± 0.6 0.015 ± 0.001

The CO consumption rates (per mole of Mo) are shown in Figure 4.5 as a function of the extent of sulfidation for the n-Mo-S catalysts. The extent of sulfidation expressed by the S/Mo

89 atomic ratio was determined from the relative areas of the S 2p and Mo 3d regions in the XPS spectra. The S/Mo ratios of 5-Mo-S (0.70), 14-Mo-S (0.63), and 23-Mo-S (0.45) catalysts are shown in Figure 4.5. The S/Mo ratios increased with a decrease in the Mo particle size, showing a strong correlation with the CO consumption rate and suggesting that Mo-S sites are responsible for WGS activity of the Mo-based catalysts in agreement with previous observations

[15,26,28,29,58].

However, the 11 % increase of the S/Mo atomic ratio of the Mo-S catalysts (i.e., 0.63 for

14-Mo-S vs. 0.70 for 5-Mo-S) was much smaller than the 100 % increase in the surface area of these Mo oxide nanoparticles (per mole of Mo) estimated from their surface area to volume ratios

2 (i.e., 3.07 for 14-Mo-S vs. 6.14 cm /molMo for 5-Mo-S). These differences may be explained by the increased density of surface MoOx sites with decreasing Mo particle size that strongly interacted with Al2O3 support. [59,60] The strong MoOx-Al2O3 interaction in these catalysts was previously reported to inhibit the sulfidation of surface Mo-O species [61,62].

Figure 4.7 shows the WGS behavior of the n-Mo-S catalysts as a function of time on stream.

All n-Mo-S catalysts partially deactivated during 72 hours on stream in the absence of a textural promoter, such as Co, which creates more active sites by modifying the texture of the catalyst surface without being catalytically active itself. After 26 hours on stream, the 5-Mo-S catalyst displayed 15% and 35% higher CO conversion than the 14- and 23-Mo-S catalysts, respectively.

The decrease of CO conversion in the case of the 5-Mo-S catalyst was 5~10% lower than for the larger Mo-S nanoparticles under H2S-free conditions, indicating greater WGS activity and lower sulfur dependence of the small Mo-S nanoparticles during the first 54 hours of WGS reaction. The

5- and 14-Mo-S catalysts showed similar catalytic performance after 54 hours of WGS reaction under H2S-free conditions, while the 5-Mo-S catalyst showed higher activity and lower sulfur dependence as compared to the 14-Mo-S catalyst up to 54 hours of WGS reaction.

Figure 4.8 shows the WGS activity of the n-CoMo-S catalysts. The 10-CoMo-S catalyst showed higher CO conversion than other n-CoMo-S catalysts. Although all n-CoMo-S catalysts were activated for 18 hours under the same conditions, the 30-CoMo-S and 100-CoMo-S catalysts required an additional two and four hours, respectively, for a complete activation than smaller n-

CoMo-S catalysts. The 10-CoMo-S and 6-CoMo catalysts showed similar CO conversion after 4 hours of WGS reaction, which was 10~15% higher than that for the larger n-CoMo-S catalysts in

1,000 ppm H2S-containing feed. The CO conversion of the 100-CoMo-S catalyst decreased by 7% in H2S-free feed, while the CO conversion of the 6-CoMo-S, 10-CoMo-S, and 30-CoMo-S catalysts decreased by 25%, 15%, and 28%, respectively. All n-CoMo-S catalysts recovered their activity within two hours of the WGS reaction when 1,000 ppm H2S was re-introduced in the feed stream. The 100-CoMo-S catalyst initially underwent slow activation in the presence of 1,000 ppm

H2S, and then slowly deactivated when H2S was shut off, which may be explained by a solid-state sulfur diffusion from the core to the surface region. The 10-CoMo-S catalyst showed the highest

WGS activity among all the catalysts in the presence of H2S and second lowest sulfur dependence of WGS activity, i.e., the drop of CO conversion under H2S-free conditions. Therefore, the 10-

CoMo-S catalyst was selected as an optimal catalyst for further studies of its WGS behavior under different reaction conditions.

The WGS activity of the 10-CoMo-S catalyst promoted by Ni, Cu, Pd, and Ce is shown in

Figure 4.9. Supported Ni1CoMo-S catalyst showed 5% higher CO conversion than unpromoted

10-CoMo-S catalyst. The Ni1CoMo-S catalyst exhibited an equilibrium CO conversion in the presence of 1,000 ppm H2S, indicating that the Ni promoter enhanced the WGS activity of the 10-

CoMo-S catalyst. The Ni1CoMo-S catalyst experienced a similar decrease of CO conversion under

H2S-free conditions to that of the unpromoted catalyst, suggesting that the Ni promoter does not affect sulfur dependence. No methanation activity was observed over this promoted catalyst during

28 hours of WGS reaction.

Figure 4.9. CO conversion over 10-CoMo-S, Ni1CoMo-S, Cu1CoMo-S, Pd1CoMo-S, and Ce1CoMo-S/Al2O3 catalysts during the WGS reaction employing 1,000 ppm H2S and H2S-free -1 conditions (Feed: 10 vol.% CO and 20 vol.% H2O in He at GHSV = 2,500 h and 450°C).

The Cu1CoMo-S/Al2O3 and Pd1CoMo-S/Al2O3 catalysts showed 15% lower CO conversion than the original catalyst during the initial activity test in H2S-containing feed (0~6 hours), suggesting that Cu and Pd inhibited the WGS activity of the unpromoted 10-CoMo-S catalyst. Most Cu and Pd was assumed to be present on the surface of the promoted catalyst since the calcination temperature of the promoted catalysts (500°C) was insufficiently high to promote their migration into the catalyst bulk [63]. Subsequently, the exposed Cu and Pd species could be converted to metal sulfides during the pre-sulfidation process [64]. Both Cu-S and Pd-S species were inactive in the WGS reaction, corresponding to typical deactivated species formed due to sulfur poisoning as reported previously [64]. Thus, the active sites of the 10-CoMo-S/Al2O3 catalyst can be blocked by the surface Cu-S or Pd-S species. The Cu- and Pd-promoted catalysts also deactivated faster than other promoted catalysts under H2S-free conditions. Moreover, the Cu- promoted catalyst did not recover its activity to the same extent as the Pd-promoted catalyst in

H2S-containing feed.

The Ce-promoted catalyst showed the lowest WGS activity among all tested catalysts.

Based on the sulfur-metal bond energy [65], the Ce-S bonds (524 kJ/mol) are much stronger than the Mo-S (355 kJ/mol) or Co-S (288 kJ/mol) bonds. Accordingly, sulfur is expected to bond preferentially to Ce during the sulfidation process. Most Ce was also assumed to be present on the surface of the base catalyst for the same reason as that determined during our study of the Cu or

Pd promoters. The Ce-S species not only blocked the active sites of the 10-CoMo-S catalyst but also affected the catalytically important Mo-S bonds due to the strong sulfur affinity of Ce.

Figure 4.10 compares the commercial CoMo and 10-CoMo-S catalysts under the same

WGS reaction conditions. Both the 10-CoMo-S and commercial CoMo catalyst maintained their

WGS activity during 56 hours of WGS reaction. The 10-CoMo-S catalyst showed 13~15% higher

CO conversion than the commercial catalyst in 1,000 ppm H2S-containing feed. The CO conversion of the 10-CoMo-S catalyst was reduced by 27% (during 6~24 hours) and 15% (during

44~51 hours) in H2S-free feed, while that of the commercial catalyst decreased by 20% (during

6~24 hours) and 13% (during 44~51 hours) in the H2S-free feed. The 10-CoMo-S catalyst was more active than the commercial catalyst and exhibited similar sulfur dependence.

Figure 4.11 (a) shows the CO conversion over the Mo-S and CoMo-S catalysts as a function of H2S concentration in the feed. The CO conversion over the Mo-S catalyst in the H2S- free feed was higher than that of the CoMo-S catalyst. The CO conversion over the CoMo-S catalyst increased with H2S concentration up to 5,000 ppm, decreased by 15% at 7,500 ppm, and then remained unchanged at 7,500 ~ 10,000 ppm H2S. The CO conversion over the Mo-S catalyst gradually increased by ~20% with H2S concentration in the 0~10,000 ppm range. Both the Mo-S and CoMo-S catalysts showed similar CO conversion at very high H2S concentration of 10,000 ppm.

It should be noted that the typical sulfur content of syngas derived by biomass gasification lies in the 1,000 ~ 15,000 ppm range [11]. Previously, 100 ppm was suggested as the minimum

H2S concentration in the feed required to maintain the WGS activity of Mo-based catalysts [66].

Therefore, 1,000 ppm was selected as the realistic low H2S concentration for a biomass-derived syngas feed.

Figure 4.11. CO conversion over Mo-S/Al2O3 and CoMo-S/Al2O3 catalysts as a function of (a) -1 H2S concentration in feed (Feed: 10 vol.% CO and 20 vol.% H2O in He at GHSV = 2,500 h and 450°C), and (b) H2O/CO feed ratio at fixed CO concentration (Feed: 5 vol.% CO and 1,000 ppm -1 H2S in He at GHSV = 2,500 h and 450°C).

Figure 4.11 (b) displays the CO conversion over the Mo-S and CoMo-S catalysts as a function of the H2O/CO feed ratio for CO concentration fixed at 5 vol.%. Both catalysts showed higher activity at H2O/CO = 2 than H2O/CO = 1. The CO conversion over the CoMo-S catalyst 97 increased significantly when the H2O composition in feed increased from 5 vol.% to 10 vol.%, then gradually increased up to 40 vol.%. On the other hand, the CO conversion over the Mo-S catalyst declined above H2O/CO = 4. Liu et al. reported that high WGS activity and thermal stability at H2O/CO=1 were observed in their Mo-based catalysts promoted with TiO2 [67], but methanation and coke deposition may also occur at such a low H2O/CO ratio. On the economic grounds of steam cost, H2O/CO = 2 was selected as the optimal H2O/CO ratio in our study.

The WGS activity of the Mo and CoMo-S catalysts at 200°C~550°C is shown in Figure

4.12. Both Mo-S and CoMo-S catalysts showed a light-off temperature of 200°C, while the CoMo-

S catalyst showed higher WGS activity over the entire temperature range than the Mo-S catalyst and reached the equilibrium conversion above 500°C. The CO conversion of the Mo-S catalyst decreased above 500°C, which may be explained by a decrease of the BET surface area of the Mo-

S catalyst from 133 m2/g to 63 m2/g after the temperature-profile test (Figure 4.12), while the BET surface area of the CoMo-S catalyst was unchanged. Therefore, the deactivation of the Mo-S catalysts could be attributed to thermal sintering of the Mo-S species, which may be prevented by adding cobalt as a textural promoter.

Figure 4.12. CO conversion during WGS reaction over Mo-S and CoMo-S catalysts (Feed: 10 -1 vol.% CO, 20 vol.% H2O, and 1,000 ppm H2S in He at GHSV = 2,500 h ). 4.4. Conclusions

Mo and CoMo oxide nanoparticles displaying narrow size distributions were obtained by tuning the concentrations of the Mo and Co precursors during synthesis. The WGS activity of n-

Mo-S and n-CoMo-S catalysts increased by reducing the average particle size down to 5 nm (5-

Mo) and 10 nm (10-CoMo). However, the improvement of the WGS activity of the n-CoMo-S nanoparticles as their size decreased was minor as compared to the n-Mo-S catalysts. The 5-Mo-S catalyst showed the highest extent of sulfidation, which strongly correlated with the WGS activity.

5-Mo-S and 10-CoMo-S catalysts were the most active and least sulfur-dependent among all Mo- based catalysts investigated in this study. The 10-CoMo-S catalyst showed higher CO conversion and similar sulfur dependence as compared to the commercial CoMo catalyst. The Ni promoter enhanced the WGS activity of the 10-CoMo-S catalyst, while Cu, Pd, and Ce additives inhibited

WGS activity. The feed composition corresponding to 10 vol.% CO, 20 vol.% H2O, and 1,000 ppm H2S concentration in He, and reaction temperature of 450°C were indicated as optimal for the

WGS reaction over the Al2O3 supported Mo-S and CoMo-S catalysts of this study. Thus, the 10-

CoMo-S/Al2O3 was demonstrated to be a promising sulfur-dependent catalyst for a low- temperature WGS reaction.

4.5. References

1. B. D. Solomon, V. A. Luzadis, "Renewable Energy from Forest Resources in the United States," Routledge, New York, 2008

3. S. Ahmed, M. Krumpelt, "Hydrogen from Hydrocarbon Fuels for Fuel Cells," International Journal of Hydrogen Energy, 26, 2001, 291-301

4. C. H. Bartholomew, R. J. Farrauto, "Fundamentals of Industrial Catalytic Processes: Second Edition," John Wiley & Sons, New Jersey, 2010

5. E. Shayan, V. Zare, I. Mirzaee, "Hydrogen Production from Biomass Gasification; a Theoretical Comparison of using Different Gasification Agents," Energy Conversion and Management, 159, 2018, 30-41

6. G. Guan, M. Kaewpanha, X. Hao, A. Abudula, "Catalytic Steam Reforming of Biomass Tar: Prospects and Challenges," Renewable and Sustainable Energy Reviews, 58, 2016, 450-461

7. M. H. Langholtz, B. J. Stokes, L. M. Eaton. , Oak Ridge National Laboratory, 2016 U.S. billion- ton report: Advancing domestic resources for a thriving bioeconomy. volume 1: Economic availability of feedstocks. ORNL/TM-2016/160, 2016. Oak Ridge, TN:

8. A. A. Ahmad, N. A. Zawawi, F. H. Kasim, A. Inayat, A. Khasri, "Assessing the Gasification Performance of Biomass: A Review on Biomass Gasification Process Conditions, Optimization and Economic Evaluation," Renewable and Sustainable Energy Reviews, 53, 2016, 1333-1347

9. K. J. Andersson, M. Skov-Skjøth Rasmussen, P. E. Højlund Nielsen, "Industrial-Scale Gas Conditioning Including Topsøe Tar Reforming and Purification Downstream Biomass Gasifiers: An Overview and Recent Examples," Fuel, 203, 2017, 1026-1030

10. J. Li, Y. Yin, X. Zhang, J. Liu, R. Yan, "Hydrogen-Rich Gas Production by Steam Gasification of Palm Oil Wastes Over Supported Tri-Metallic Catalyst," International Journal of Hydrogen Energy, 34, 2009, 9108-9115

100

11. W. De Jong (Ed.). "Biomass Composition, Properties, and Characterization," John Wiley & Sons, Inc., 2014

12. C. Ratnasamy, J. Wagner, "Water Gas Shift Catalysis," Catalysis Reviews - Science and Engineering, 51, 2009, 325-440

13. S. Cheah, D. L. Carpenter, K. A. Magrini-Bair, "Review of Mid- to High-Temperature Sulfur Sorbents for Desulfurization of Biomass- and Coal-Derived Syngas," Energy and Fuels, 23, 2009, 5291-5307

14. S. Chianese, J. Loipersböck, M. Malits, R. Rauch, H. Hofbauer, A. Molino, D. Musmarra, "Hydrogen from the High Temperature Water Gas Shift Reaction with an Industrial Fe/Cr Catalyst using Biomass Gasification Tar Rich Synthesis Gas," Fuel Processing Technology, 132, 2015, 39- 48

15. Yan-Yan Chen, Mei Dong, Jianguo Wang, Haijun Jiao, "On the Role of a Cobalt Promoter in a Water-Gas-Shift Reaction on Co-MoS2," The Journal of Physical Chemistry C, 114, 2010, 16669-16676

16. Kerry C. Pratt, John V. Sanders, Victor Christov, "Morphology and Activity of MoS2 on various Supports: Genesis of the Active Phase," Journal of Catalysis, 124, 1990, 416-432

17. A. A. Andreev, V. J. Kafedjiysky, R. M. Edreva-Kardjieva, "Active Forms for Water-Gas Shift Reaction on NiMo-Sulfide Catalysts," Applied Catalysis A: General, 179, 1999, 223-228

18. M. Laniecki, M. Malecka-Grycz, F. Domka, "Water–gas Shift Reaction Over Sulfided Molybdenum Catalysts," Applied Catalysis A: General, 196, 2000, 293-303

19. C. R. F. Lund, "Effect of Adding Co to MoS2/Al2O3 upon the Kinetics of the Water-Gas Shift," Industrial and Engineering Chemistry Research, 35, 1996, 3067-3073

20. Y. Zhang, G. Zhang, Y. Zhao, X. Li, Y. Sun, Y. Xu, "Ce-K-Promoted Co-Mo/Al2O3 Catalysts for the Water Gas Shift Reaction," International Journal of Hydrogen Energy, 37, 2012, 6363- 6371

21. T. M. Roberge, S. O. Blavo, C. Holt, P. H. Matter, J. N. Kuhn, "Effect of Molybdenum on the Sulfur-Tolerance of Cerium-Cobalt Mixed Oxide Water-Gas Shift Catalysts," Topics in Catalysis, 56, 2013, 1892-1898

22. Y. Chen, M. Dong, J. Wang, H. Jiao, "Mechanisms and Energies of Water Gas Shift Reaction on Fe-, Co-, and Ni-Promoted MoS2 Catalysts," Journal of Physical Chemistry C, 116, 2012, 25368-25375

101

23. T. Sasaki, T. Suzuki, Y. Akasaka, M. Takaoka, "Generation Efficiency Improvement of IGCC with CO2capture by the Application of the Low Temperature Reactive Shift Catalyst," Energy, 118, 2017, 60-67

24. J. Mi, J. Zhang, Y. Cao, C. Chen, X. Lin, J. Chen, L. Jiang, "Sulfur Resistant WGS Catalyst for Hydrogen Production Based on CoMo Supported by Nb Modified MgAl Mixed Oxide," International Journal of Hydrogen Energy, 42, 2017, 29935-29943

25. T. Sasaki, T. Suzuki, "Sulfide Molybdenum Catalysts for Water–gas Shift Reaction: Influence of the Kind of Promoters and Supports to Generate MoS2," Applied Catalysis A: General, 484, 2014, 79-83

26. P. Hou, D. Meeker, H. Wise, "Kinetic Studies with a Sulfur-Tolerant Water Gas Shift Catalyst," Journal of Catalysis, 80, 1983, 280-285

27. G. K. Reddy, P. Boolchand, P. G. Smirniotis, "Sulfur Tolerant Metal Doped Fe/Ce Catalysts for High Temperature WGS Reaction at Low Steam to CO Ratios - XPS and Mossbauer Spectroscopic Study," Journal of Catalysis, 282, 2011, 258-269

28. C. R. F. Lund, "Microkinetics of Water-Gas Shift Over Sulfided Mo/Al2O3 Catalysts," Industrial and Engineering Chemistry Research, 35, 1996, 2531-2538

29. D. Nikolova, R. Edreva-Kardjieva, T. Grozeva, "Water-Gas Shift Activity of K-Promoted (Ni)Mo/Γ-Al2O3 Systems in Sulfur-Containing Feed," Reaction Kinetics, Mechanisms and Catalysis, 103, 2011, 71-86

30. A. Travert, C. Dujardin, F. Maugé, E. Veilly, S. Cristol, J. Paul, E. Payen, "CO Adsorption on CoMo and NiMo Sulfide Catalysts: A Combined IR and DFT Study," Journal of Physical Chemistry B, 110, 2006, 1261-1270

31. Y. Kuriki, K. Uchida, E. Sekreta, S. Ohshima, M. Yumura, F. Ikazaki, "Mechanical Milling of Metal Sulfide Particles and their Catalytic Activity - Hydrogenation of 1-Methylnaphthalene with Molybdenum Sulfide," Fuel Processing Technology, 59, 1999, 189-200

32. P. Afanasiev, G. Xia, G. Berhault, B. Jouguet, M. Lacroix, "Surfactant-Assisted Synthesis of Highly Dispersed Molybdenum Sulfide," Chemistry of Materials, 11, 1999, 3216-3219

33. C. Contreras, F. Isquierdo, P. Pereira-Almao, C. E. Scott, "Effect of Particle Size on the HDS Activity of Molybdenum Sulfide," Journal of Nanotechnology, 2016, 2016,

34. P. G. Moses, B. Hinnemann, H. Topsøe, J. K. Nørskov, "The Hydrogenation and Direct Desulfurization Reaction Pathway in Thiophene Hydrodesulfurization Over MoS2 Catalysts at Realistic Conditions: A Density Functional Study," Journal of Catalysis, 248, 2007, 188-203

102

35. F. Besenbacher, M. Brorson, B. S. Clausen, S. Helveg, B. Hinnemann, J. Kibsgaard, J. V. Lauritsen, P. G. Moses, J. K. Nørskov, H. Topsøe, "Recent STM, DFT and HAADF-STEM Studies of Sulfide-Based Hydrotreating Catalysts: Insight into Mechanistic, Structural and Particle Size Effects," Catalysis Today, 130, 2008, 86-96

36. A. S. Walton, J. V. Lauritsen, H. Topsøe, F. Besenbacher, "MoS2 Nanoparticle Morphologies in Hydrodesulfurization Catalysis Studied by Scanning Tunneling Microscopy," Journal of Catalysis, 308, 2013, 306-318

37. T. F. Jaramillo, K. P. Jørgensen, J. Bonde, J. H. Nielsen, S. Horch, I. Chorkendorff, "Identification of Active Edge Sites for Electrochemical H2 Evolution from MoS2 Nanocatalysts," Science, 317, 2007, 100-102

38. S. Butun, S. Tongay, K. Aydin, "Enhanced Light Emission from Large-Area Monolayer MoS2Using Plasmonic Nanodisc Arrays," Nano Letters, 15, 2015, 2700-2704

39. L. Zhou, B. He, Y. Yang, Y. He, "Facile Approach to Surface Functionalized MoS2 Nanosheets," RSC Advances, 4, 2014, 32570-32578

40. A. Alrasheed, J. M. Gorham, B. C. Tran Khac, F. Alsaffar, F. W. Delrio, K. -. Chung, M. R. Amer, "Surface Properties of Laser-Treated Molybdenum Disulfide Nanosheets for Optoelectronic Applications," ACS Applied Materials and Interfaces, 10, 2018, 18104-18112

41. L. Yuwen, F. Xu, B. Xue, Z. Luo, Q. Zhang, B. Bao, S. Su, L. Weng, W. Huang, L. Wang, "General Synthesis of Noble Metal (Au, Ag, Pd, Pt) Nanocrystal Modified MoS2 Nanosheets and the Enhanced Catalytic Activity of Pd-MoS2 for Methanol Oxidation," Nanoscale, 6, 2014, 5762- 5769

42. C. L. Stender, T. W. Odom, "Chemical Nanofabrication: A General Route to Surface-Patterned and Free-Standing Transition Metal Chalcogenide Nanostructures," Journal of Materials Chemistry, 17, 2007, 1866-1869

43. G. Suriati, M. Mariatti, A. Azizan, "Synthesis of Silver Nanoparticles by Chemical Reduction Method: Effect of Reducing Agent and Surfactant Concentration," International Journal of Automotive and Mechanical Engineering, 10, 2014, 1920-1927

44. Junze Chen, Xue-Jun Wu, Lisha Yin, Bing Li, Xun Hong, Zhanxi Fan, Bo Chen, Can Xue, Hua Zhang, "One-Pot Synthesis of CdS Nanocrystals Hybridized with Single-Layer Transition- Metal Dichalcogenide Nanosheets for Efficient Photocatalytic Hydrogen Evolution," Angewandte Chemie International Edition, 54, 2015, 1210-1214

45. S. Jeong, D. Yoo, J. -. Jang, M. Kim, J. Cheon, "Well-Defined Colloidal 2-D Layered Transition-Metal Chalcogenide Nanocrystals Via Generalized Synthetic Protocols," Journal of the American Chemical Society, 134, 2012, 18233-18236

103

46. S. Jeong, J. H. Han, J. Jang, J. Seo, J. Kim, J. Cheon, "Transformative Two-Dimensional Layered Nanocrystals," Journal of the American Chemical Society, 133, 2011, 14500-14503

47. A. Zhang, Q. Ma, M. Lu, G. Zhou, C. Li, Z. Wang, "Nanocrystalline Metal Chalcogenides obtained Open to Air: Synthesis, Morphology, Mechanism, and Optical Properties," Journal of Physical Chemistry C, 113, 2009, 15492-15496

48. X. Bokhimi, J. A. Toledo, J. Navarrete, X. C. Sun, M. Portilla, "Thermal Evolution in Air and Argon of Nanocrystalline MoS2 Synthesized Under Hydrothermal Conditions," International Journal of Hydrogen Energy, 26, 2001, 1271-1277

49. L. Zhang, J. M. Millet, U. S. Ozkan, "Deactivation Characteristics of Fe-Al-Cu Water-Gas Shift Catalysts in the Presence of H2S," Journal of Molecular Catalysis A: Chemical, 309, 2009, 63-70

50. X. Zong, Y. Na, F. Wen, G. Ma, J. Yang, D. Wang, Y. Ma, M. Wang, L. Sun, C. Li, "Visible Light Driven H2 Production in Molecular Systems Employing Colloidal MoS2 Nanoparticles as Catalyst," Chemical Communications, 2009, 4536-4538

51. A. A. Ayi, C. A. Anyama, V. Khare, "On the Synthesis of Molybdenum Nanoparticles Under Reducing Conditions in Ionic Liquids," Journal of Materials, 372716, 2015,

52. S. Brunauer, P. H. Emmett, E. Teller, "Adsorption of Gases in Multimolecular Layers," Journal of the American Chemical Society, 60, 1938, 309-319

53. E. P. Barrett, L. G. Joyner, P. P. Halenda, "The Determination of Pore Volume and Area Distributions in Porous Substances. I. Computations from Nitrogen Isotherms," Journal of the American Chemical Society, 73, 1951, 373-380

54. X. Tong, Z. Zhang, F. Zeng, C. Fu, P. Ma, Y. Peng, H. Lu, Y. Liang, "Recursive Wavelet Peak Detection of Analytical Signals," Chromatographia, 79, 2016, 1247-1255

55. C. Igathinathane, L. O. Pordesimo, E. P. Columbus, W. D. Batchelor, S. R. Methuku, "Shape Identification and Particles Size Distribution from Basic Shape Parameters using ImageJ," Computers and Electronics in Agriculture, 63, 2008, 168-182

56. J. B. Wagner, S. B. Abd Hamid, D. Othman, O. Timpe, S. Knobl, D. Niemeyer, D. S. Su, R. Schlögl, "Nanostructuring of Binary Molybdenum Oxide Catalysts for Propene Oxidation," Journal of Catalysis, 225, 2004, 78-85

57. A. Chithambararaj, N. S. Sanjini, S. Velmathi, A. Chandra Bose, "Preparation of H-MoO3 and a-MoO3 Nanocrystals: Comparative Study on Photocatalytic Degradation of Methylene Blue Under Visible Light Irradiation," Physical Chemistry Chemical Physics, 15, 2013, 14761-14769

104

58. J. Chen, J. Zhang, J. Mi, E. D. Garcia, Y. Cao, L. Jiang, L. Oliviero, F. Maugé, "Hydrogen Production by Water-Gas Shift Reaction Over Co-Promoted MoS2/Al2O3 Catalyst: The Intrinsic Activities of Co-Promoted and Unprompted Sites," International Journal of Hydrogen Energy, 43, 2018, 7405-7410

59. M. Shekhar, J. Wang, W. Lee, W. D. Williams, S. M. Kim, E. A. Stach, J. T. Miller, W. N. Delgass, F. H. Ribeiro, "Size and Support Effects for the Water-Gas Shift Catalysis Over Gold Nanoparticles Supported on Model Al2O3 and TiO2," Journal of the American Chemical Society, 134, 2012, 4700-4708

60. S. Rangarajan, M. Mavrikakis, "On the Preferred Active Sites of Promoted MoS2 for Hydrodesulfurization with Minimal Organonitrogen Inhibition," ACS Catalysis, 7, 2017, 501-509

61. S. M. A. M. Bouwens, F. B. M. Vanzon, M. P. Vandijk, A. M. Vanderkraan, V. H. J. Debeer, J. A. R. Vanveen, D. C. Koningsberger, "On the Structural Differences between Alumina- Supported CoMoS Type I and Alumina-, Silica-, and Carbon-Supported CoMoS Type II Phases Studied by XAFS, MES, and XPS," Journal of Catalysis, 146, 1994, 375-393

62. R. Candia, O. Sørensen, J. Villadsen, N. Topsøe, B. S. Clausen, H. Topsøe, "Effect of Sulfiding Temperature on Activity and Structures of Co-Mo/Al2O3 Catalysts. Ii," Bulletin Des Sociétés Chimiques Belges, 93, 1984, 763-774

63. M. D. Argyle, C. H. Bartholomew, "Heterogeneous Catalyst Deactivation and Regeneration: A Review," Catalysts, 5, 2015, 145-269

64. N. Inoglu, J. R. Kitchin, "Identification of Sulfur-Tolerant Bimetallic Surfaces using DFT Parametrized Models and Atomistic Thermodynamics," ACS Catalysis, 1, 2011, 399-407

65. Y. Luo (Ed.). "Comprehensive Handbook of Chemical Bond Energies," CRC Press, . Boca Ranton, FL, 2007

66. A. R. De La Osa, A. De Lucas, J. L. Valverde, A. Romero, I. Monteagudo, P. Sánchez, "Performance of a Sulfur-Resistant Commercial WGS Catalyst Employing Industrial Coal- Derived Syngas Feed," International Journal of Hydrogen Energy, 36, 2011, 44-51

67. B. Liu, Q. Zong, P. P. Edwards, F. Zou, X. Du, Z. Jiang, T. Xiao, H. Almegren, "Effect of Titania Addition on the Performance of CoMo/Al2O3 Sour Water Gas Shift Catalysts Under Lean Steam to Gas Ratio Conditions," Industrial and Engineering Chemistry Research, 51, 2012, 11674-11680

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Chapter 5. Support effects on water gas shift activity and sulfur dependence of

Mo sulfide catalysts

5.1. Introduction

Increased carbon dioxide emissions have raised awareness of global climate change caused by human activity, mostly resulting from burning fossil fuels, and have led to the development of alternative, environmentally friendly energy sources. Hydrogen is the most promising alternative energy source since it produces no CO2 emissions and has a high energy per mass [1]. However, most hydrogen is currently produced from fossil resources, such as coal and natural gas [2,3], which also generates CO2 and causes the depletion of these resources. On the other hand, hydrogen production from biomass could overcome these problems of fossil fuel use since biomass is a carbon neutral and sustainable resource [2,4-6]. Biomass can be converted into combustible gaseous products in the presence of air, steam, oxygen, CO2, or a mixture of these using a thermochemical conversion process, such as gasification and pyrolysis [7]. The main gaseous products are mixtures of hydrocarbons, carbon monoxide, and hydrogen. The syngas can be processed with catalysts and chemicals to produce other biofuels, chemical products, and pure hydrogen through downstream applications [5,8,9].

The water gas shift (WGS) reaction is the most important reaction to produce hydrogen using biomass-derived syngas [10]. The WGS reaction plays an important role for the gas conditioning step of biomass-derived syngas to adjust the CO/H2 ratio for a desired downstream application, such as ammonia production, Fischer-Tropsch synthesis of hydrocarbons, and making pure hydrogen for fuel cells [11]. A conventional two-stage WGS reactor consists of a high- temperature shift (HTS) and a low-temperature shift (LTS) stages due to thermodynamic and kinetic limitations [12]. The major challenge for WGS catalysts employed with biomass-derived

106 syngas is sulfur poisoning of these catalysts [7]. Although the sulfur content of biomass-derived syngas is usually lower than that of syngas from coal gasification [13], the sulfur content (up to

1.5% of total dry weight) is still significant to poison the catalysts used for the WGS reaction and other downstream applications [14]. Conventional HTS Fe2O3–Cr2O3 catalysts typically lose ~50% of their activity in the presence of ~500 ppm H2S [15], while conventional LTS Cu–ZnO–Al2O3 catalysts deactivate completely in ~10 ppm H2S [16]. In a typical two-stage WGS reactor, a reheating and steam reinjection steps, which are energy-intensive processes, are necessary, since the sulfur removal process must be located upstream from the WGS reactor [10]. However, the use of a sulfur-dependent catalyst could save the required energy to reheat and reinject steam by removing sulfur downstream from the WGS reactor.

Molybdenum-based catalysts have been extensively studied as sulfur-dependent WGS catalysts because of their high sulfur tolerance [11] and even sulfur dependence, since these catalysts require at least 100 ppm of H2S to maintain their WGS activity [17]. However, it is desirable to reduce sulfur dependence of Mo-based catalysts used in the WGS reaction of biomass syngas, since biomass is characterized by a wide range of sulfur content as mentioned above [7].

Moreover, the Mo-based catalysts display relatively low activity in the WGS reaction as compared to the conventional HTS catalysts [18]. Therefore, the WGS activity and sulfur dependence of Mo- based catalysts need to be further optimized in order to produce hydrogen from biomass-derived syngas.

Industrial Mo-based catalysts generally employ Al2O3 or MgO as supports to disperse an active phase consisting of a Co and Mo sulfide [19-21], while the use of other supports could further improve the WGS activity [22]. Two major support effects may contribute to enhancing the WGS activity: (1) the support can provide active site for the reaction, e.g., the surface acid sites

107 of Al2O3 [23] and oxygen stored in the CeO2 lattice [24], and (2) enhance the dispersion of the active phase by modifying the active species – support interactions.

TiO2 was suggested as a promising support for Mo-based catalysts to promote their WGS activity by enhancing the reducibility and dispersion of MoO3 on TiO2 [25,26]. Sasaki and Suzuki suggested that TiO2 increased the sulfidation rate because the TiO2 surface possessed more hydroxyl groups than Al2O3 and ZrO2 supports, leading to a greater density of Lewis acid sites and resulting in highly dispersed Mo sulfide species [26]. Controlling the redox properties of CeO2-

ZrO2 was reported to increase the catalytic activity in the hydrogenation of CO due to increased lattice oxygen mobility after incorporation of Ce4+ into the zirconia lattice [27,28].

Prior work has also sought to discover the most effective support in improving the activity of Mo-based catalysts. Lian et al. reported that the dispersion of Mo oxide in CoMo/MgO-Al2O3 catalysts could be enhanced by modifying the Mg/Al ratio [29]. Wang et al. investigated Ce-Al,

Mg-Al, Ti-Al, and Zr-Al mixed oxides and found Ce-Al mixed oxide to be a promising support which improved Mo dispersion and the methanation activity of CoMo-S catalysts [30]. Kaluza et al. reported that the ZrO2-supported Mo catalyst showed the lowest temperature of MoO3 reduction and the weakest MoO3-support interaction among the Al2O3, SiO2, TiO2, and ZrO2 supported Mo catalysts, which were not correlated with the catalytic activity in benzothiophene hydrodesulfurization (HDS) [31]. Dhanala et al. suggested that the interaction between metals (Co,

Ni, Mo) and supports (Al2O3, SiO2, ZrO2) could affect the metal dispersion, metal crystallite size, and their steam reforming activity, showing that these metals had the strongest interaction with

Al2O3 followed by SiO2 and ZrO2 [32]. It was suggested by Pratt et al. that MoS2 formed up to five conformal layers on the surface of Al2O3 and SiO2, while MoS2 formed islands with increasing

Mo loadings on the surface of ZrO2 and TiO2 after sulfidation of MoO3 catalysts [22]. The

108 thiophene HDS activity of Mo catalysts supported on ZrO2 and TiO2 was higher than that of Mo catalysts on Al2O3 and SiO2 supports [22]. Jha et al. reported that the reducibility of supported Co catalysts enhanced the cobalt dispersion and led to the increased density of surface active metal sites [33]. The Co/CeO2 catalyst showed the highest WGS activity among CeO2, ZrO2, TiO2, and

Al2O3 supported Co catalysts [33]. Whereas various metal oxides have been proposed as promising supports for a number of catalytic reactions, the catalytic behavior and sulfur dependence of ZrO2- supported Mo-S catalysts in the WGS reaction have not been investigated yet.

In this study, we investigated Mo-S catalysts deposited on various oxide supports by incipient wetness impregnation and explored their catalytic activity and H2S dependence in the

WGS reaction. The H2-TPR analysis of supported Mo catalysts was performed to characterize Mo-

O and Mo-S reducibility and their interactions with oxide supports. We compared the TEM images,

XRD patterns, and H2-TPR profiles of the fresh and used Mo catalysts deposited on Al2O3 and

ZrO2 and demonstrated that ZrO2 is a better support for Mo-based WGS catalysts than Al2O3.

5.2. Experimental methods

5.2.1. Catalyst preparation

Supported molybdenum catalysts were prepared by incipient wetness impregnation of the support materials with an aqueous solution of ammonium molybdate (Fisher Scientific) used as a precursor. Crushed and sieved 100 ~ 500 µm particles of TiO2, CeO2, Al2O3, SiO2, and ZrO2 were employed. TiO2 and CeO2 were purchased from Sigma Aldrich, γ-Al2O3 was provided by

Sumitomo Chemical, while SiO2 and monoclinic ZrO2 were provided by Saint-Gobain North

America. Appropriate amounts of the molybdenum precursor were dissolved in warm (30°C) deionized water under mild stirring. After incipient impregnation of the supports with solutions of molybdenum precursor and support materials, the supported catalysts were dried overnight at 80°C.

109

All catalysts were then calcined at 500°C under air atmosphere for 5 hours. The catalysts thus prepared were denoted as supported Mo-O catalysts.

5.2.2. Catalyst characterization

The activity of supported Mo catalysts was determined employing a fixed-bed tubular stainless steel reactor (0.65 cm ID) operated at atmospheric pressure and using 0.3 ~ 1.5 g of catalysts. The feed typically contained 10 vol.% CO and 20 vol.% H2O balanced with helium. The total feed flow rate was 50 ~ 100 mL/min. 1,000 ppm H2S was periodically added to this feed during WGS reaction conducted as a function of time on stream. All catalysts were activated in 1 mol.% H2S in H2 at 450°C for 18 hours as a pre-sulfidation step prior to WGS activity tests. The catalysts after pre-sulfidation were denoted as supported Mo-S catalysts. Water was injected into a flowing gas stream by a syringe pump and vaporized in a heated gas feed line before entering the reactor. A condenser filled with ice was installed at the reactor exit to collect water. The gas mixture was analyzed by a gas chromatograph (HP-5890 II) equipped with a thermal conductivity detector. The carbon balance agreed within ±5 mol.%.

CO chemisorption studies were performed using a Micromeritics ASAP2020 instrument.

The catalysts sample were first reduced by H2 at 450°C for 120 min, followed by evacuation at

450°C for 30 min. The samples were then cooled to 35°C under vacuum for 30 min followed by

CO chemisorption. The energy dispersive X-ray spectroscopy (EDS) was also employed to analyze the composition of the catalytic particles while performing scanning electron microscopy analysis

(Phillips XL30 ESEM with EDS). The inductively coupled plasma mass spectrometer (ICP-MS) was employed to analyze the composition of the catalysts using an Agilent 7700 ICP-MS system.

The H2-temperature programmed reduction (TPR) studies were performed using a Stanford

Research Systems QMS 200 gas analyzer. The catalysts were heated at 5°C /min from 35°C to

110

580°C in 20 vol.% H2 in argon flowing at 10 mL/min (STP). The BET surface area was measured at 77 K using a Micrometrics TriStar porosimeter.

High-resolution transmission electron microscopy (HR-TEM) and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) imaging were performed on a

FEI Tecnai F20 instrument. The samples were prepared for TEM by depositing powdered samples on lacey carbon film-coated copper grids.

5.3. Results and discussion

5.3.1. Elemental and structural characterization of supported Mo sulfide catalysts

The elemental compositions (EDS or ICP-MS) are shown in the first row of Table 5.1 demonstrating that the target weight loading (10 wt.%) was achieved for Al2O3, TiO2, and ZrO2 supported Mo-S catalysts. Some differences between the target and experimental weight loadings were found for SiO2 and CeO2 supported Mo-S catalysts, which could be attributed to the spectral overlap between Mo and S signals in the EDS analysis. Mo-S/SiO2, Mo-S/ZrO2, and Mo-S/CeO2 displayed lower CO uptake than the Mo-S/Al2O3 and Mo-S/TiO2 catalysts (Table 5.1). The large amount of CO uptake observed for the Mo-S/TiO2 and Mo-S/Al2O3 catalysts mostly resulted from

CO adsorption on the Mo edge sites as well as pure TiO2 or Al2O3 supports. Galhenage et al. reported that CO adsorption on MoS2/TiO2 occurred preferentially at Mo edge sites, while the

35 interfacial MoS2/TiO2 sites adsorbed very little CO . Travert et al. demonstrated that the IR bands of CO adsorbed on Al2O3 and the Mo edge sites were significantly more prominent than those of

36,37 CO adsorbed on the S edge sites . Therefore, the amount of CO uptake in Mo-S/TiO2 and Mo-

S/Al2O3 catalysts did not strongly correlate with the surface density of Mo-S species which are the proposed active sites for the WGS reaction.

Table 5.1. Mo content (wt. %), S/Mo atomic ratios, and BET surface areas of Al2O3, TiO2, SiO2, CeO2, and ZrO2-supported Mo-S catalysts. 111

Mo-S/Al2O3 Mo-S/TiO2 Mo-S/SiO2 Mo-S/CeO2 Mo-S/ZrO2

Mo (wt.%) 10.6 ± 1.1a 11.8 ± 1.8b 12.0 ± 1.3b 12.2 ± 3.2b 9.6 ± 1.1a

S/Moc 0.67 0.43 0.16 0.24 0.84 BET 142.3 ± 0.2 116.5 ± 0.5 219.9 ± 1.1 10.6 ± 0.1 50.1 ± 0.4 (m2/gcat) CO uptake 144.1 ± 11.0 190.0 ± 5.2 58.9 ± 2.0 19.3 ± 2.8 44.1 ± 1.8 (µmol/gcat) a. Mo wt.% analyzed by ICP-MS b. Mo wt.% estimated by EDS c. Atomic S/Mo ratios determined by the H2-TPR analysis

Figure 5.1 shows the XRD patterns of the Mo-S/ZrO2, Mo-S/Al2O3, Mo-S/TiO2, and Mo-

S/CeO2 catalysts and corresponding supports. The peaks of MoO2 visible at 2ϴ = 26.0° and 36.8°

(PDF 00-032-0671) were observed for the Mo-S/TiO2 catalyst. The peak visible at 2ϴ = 26.0° corresponding to MoO2 was observed in the XRD patterns of Mo-S/ZrO2 and Mo-S/CeO2. Minor peaks of 2ϴ = 27.3° and 37.3° (PDF 00-005-0508) corresponding to MoO3 were observed for the

Mo-S/Al2O3 catalyst. The XRD peaks of Mo-O phases in the supported Mo-S catalysts were relatively small due to relatively low Mo content. The intensity of XRD peaks corresponding to the oxide supports decreased in the supported Mo-S catalysts, suggesting that the Mo-O phases covered the support surface and partially blocked their X-rays. Broad peaks of Mo3S4 at 2ϴ = 27.7°

(PDF 01-079-9609) and MoS2 at 2ϴ = 29.0° (PDF 00-006-0097) were also observed in the XRD patterns of Mo-S/SiO2.

112

MoO3 MoO3

MoO2

Mo-S/ZrO2

Mo-S/Al2O3 Intensity (a.u.) Intensity (a.u.) ZrO2

Al2O3

20 30 40 50 60 20 30 40 50 60 2deg.) 2 (deg.)

MoO2

MoO2 Mo-S/CeO2 MoO2

Mo-S/TiO2 Intensity (a.u.) Intensity (a.u.)

TiO2 CeO2

20 30 40 50 60 20 30 40 50 2deg. 2deg.

Figure 5.1. XRD patterns of the fresh supported Mo-S catalysts and corresponding supports. 5.3.2. WGS activity of supported Mo-S species

The WGS activity of the supported Mo-S catalysts was determined at a gas hourly space

-1 velocity (GHSV) of 9,000 h for 72 hours in both H2S-containing and H2S-free feed streams, as

113 shown in Figure 5.2. The CO consumption rate was calculated using the following formula, where

Wcatalyst is the weight of the catalyst, is CO conversion at 450°C, and FCo, feed is the molar flow rate of CO in the feed stream:

= × ,

The Mo-S/ZrO2 catalyst showed the highest CO consumption rate, greatest thermal stability, and the lowest H2S dependence among all supported Mo catalysts investigated during 72 hours of WGS reaction. The Mo-S/TiO2 catalyst showed the second highest initial CO consumption rate which decreased by 20% during 54~72 hours of WGS reaction in a H2S-free feed stream. The Mo-S/Al2O3 catalyst showed the third highest initial WGS activity and second highest WGS activity after 72 hours of reaction. This catalyst showed higher thermal stability than the Mo-S/TiO2 catalyst during the first 54 hours of WGS reaction, while the Mo-S/Al2O3 catalyst was more H2S-dependent than Mo-S/TiO2, showing a 15% decrease of CO consumption rate during 54~72 hours of WGS reaction. The Mo-S/CeO2 experienced the most significant decrease of CO consumption rate during the first 5 hours in the 1,000 ppm H2S containing feed stream and its activity increased only by 10 % during 5~25 hours of WGS reaction under H2S-free conditions.

The oxygen vacancies in CeO2 could be associated with the active sites for WGS reaction under

H2S-free conditions [34,35]. However, Ce-S species formed in the 1,000 ppm H2S-containing feed stream, which prevented the formation of oxygen vacancies in CeO2. The Mo-S/SiO2 catalyst showed the lowest CO consumption rate as compared to the other supported catalysts, indicating that SiO2 is unsuitable as a support for Mo-based catalysts despite its large surface area. The CO consumption rate over all supported Mo-S catalysts gradually decreased with time on stream. The

WGS activity test for the Mo-S/CeO2 and Mo-S/SiO2 catalysts was stopped after 35 and 52 hours

114 of WGS reaction, respectively, since their WGS activity decreased more significantly as compared to other supported Mo-S catalysts.

Figure 5.2. CO consumption rate over Mo-S/ZrO2, Mo-S/Al2O3, Mo-S/TiO2, Mo-S/CeO2, and Mo- S/SiO2 catalysts during WGS reaction employing 1,000 ppm H2S and H2S-free feed (Feed: 10 vol.% -1 CO and 20 vol.% H2O in He at GHSV = 9,000 h and 450°C). Note: the feed contained 1,000 ppm H2S during the initial 4 hours of reaction.

5.3.3. H2-TPR analysis of Mo sulfide supported on various oxides

H2-TPR profiles of fresh supported Mo-O catalysts after calcination at 500°C in air and before pre-sulfidation are shown in Figure 5.3a. The two distinct H2 consumption peak regions were observed for all catalysts. The low-temperature region (281~339°C) was attributed to the reduction of MoO3 to MoO2 [36], while the shoulder peak at 343°C in the H2-TPR profile of the

Mo-O/ZrO2 may be assigned to the reduction of ZrMo2O8 observed in the Raman spectrum of Mo-

O/ZrO2 [37]. The high-temperature region (350~520°C) was associated with the reduction of the polymeric Mo oxide phase that strongly interacted with oxide supports [36]. The reduction peak in the low-temperature region shifted toward lower temperatures, indicating the enhanced MoO3 reducibility. The trend of MoO3 reducibility in supported Mo catalysts followed the order: Mo-

115

O/ZrO2 > Mo-O/Al2O3 > Mo-O/TiO2 > Mo-O/SiO2 > Mo-O/CeO2 in agreement with previous reports [31,38]. The enhanced reducibility of MoO3 to MoO2 could increase the density of the surface MoO2 sites, which are intermediates for the sulfidation of MoO3 [39]. Therefore, the density of surface Mo-S species increased due to the enhanced the reducibility of MoO3 to MoO2, while the enhanced reducibility of MoO3 to MoO2 in the supported Mo-O catalysts can be attributed to the weakened MoO3-support interactions.

Figure 5.3. H2-TPR profiles of (a) the supported Mo-O catalysts after calcination at 500°C in air, and (b) the supported Mo-S catalysts after pre-sulfidation.

Figure 5.3b shows H2-TPR profiles of the supported Mo-S catalysts after pre-sulfidation.

The following regions were observed for most Mo-S catalysts: a low-temperature region

(203~215°C) corresponding to the reduction of weakly bonded surface S atoms in Mo-S species, proposed as the active sites [40-42]; a medium-temperature region (253~345°C) corresponding to the reduction of MoO3 to MoO2 which remained as Mo-O species after pre-sulfidation [36]; and

116 lastly, the high temperature peaks (361~509°C) corresponding to the reduction to a polymeric Mo-

O phase.

The extent of sulfidation was expressed by the atomic S/Mo ratios (Table 5.1). The S/Mo atomic ratio was determined as the ratio of the area of the low-temperature H2 TPR peak

(203~215°C) corresponding to the reduction of surface Mo-S bonds to the total peak areas

(203~509°C) corresponding to the reduction of all Mo species. The S/Mo ratios in supported Mo-

S catalysts trended in the following order: Mo-S/ZrO2 (0.84) > Mo-S/Al2O3 (0.67) > Mo-S/TiO2

(0.43) > Mo-S/SiO2 (0.24) > Mo-S/CeO2 (0.16). This result agreed with the trend of MoO3 reducibility and correlated with the CO consumption rate observed after 72 hours of WGS reaction

(Figure 5.2) and strongly suggested that the Mo-S species are responsible for the WGS activity of supported Mo-S catalysts [43-46].

5.3.4. Sulfur dependence of Al2O3 and ZrO2 supported Mo-S catalysts

The H2-TPR profiles of fresh and used Mo-S/Al2O3 after 72 hours of WGS reaction, as well as fresh and used Mo-S/ZrO2 after 72 hours of WGS reaction are shown in Figure 5.4. The reduction temperature of Mo-S species in the Mo-S/Al2O3 catalyst increased slightly from 203°C to 214°C after 72 hours of WGS reaction. The increased reduction temperature of Mo-S species could be attributed to the increased ordering of the Mo-S species and the onset of crystallization of a Mo-S phase [47] in good agreement with TEM observations and XRD patterns of the used

Mo-S/Al2O3 catalyst shown in Figures 5 and 6, respectively. The Mo-S species associated with large MoS2 slabs were previously reported to show low catalytic activity [48]. The atomic S/Mo ratio of the Mo-S/ZrO2 catalyst decreased somewhat after 72 hours of WGS reaction from 0.84 to

0.75, while the reduction peak at 346°C corresponding to the inactive polymeric Mo-O phase was observed for the used Mo-S/ZrO2 catalyst. This suggested that some Mo-S species in the Mo-

117

S/ZrO2 catalyst were converted into inactive Mo-O species which resulted in decreased WGS activity.

203 Mo-S/Al2O3

253 320

214 Used Mo/Al2O3

263

207 Mo-S/ZrO2

Consumption (a.u.) 279 2 H 215 Used Mo/ZrO2

275 346

100 200 300 400 500 600 o Temperature ( C)

Figure 5.4. H2-TPR profiles of fresh and used Mo-S/Al2O3 and Mo-S/ZrO2 catalysts.

Figure 5.5a and 5b show HAADF-STEM images of the fresh Mo-S/Al2O3 and used Mo-

S/Al2O3 catalyst after 72 hours of WGS reaction. The HAADF-STEM images revealed the presence of well-dispersed 0.8 ± 0.2 nm bright spots in the fresh Mo-S/Al2O3 catalyst (Figure 5.5a) and the 2.3 ± 0.5 nm bright spots in used Mo-S/Al2O3 catalyst (Figure 5.5b). The bright spots in the HAADF-STEM images of Mo-S/Al2O3 catalyst are attributed to the higher contrast of the heavier Mo-S entities as compared to lighter Al2O3. This suggests the aggregation of Mo-S species in Mo-S/Al2O3 during 72 hours of WGS reaction consistent with the increased reduction temperature of the Mo-S species in the H2-TPR profile of used Mo-S/Al2O3. Although the

HAADF-STEM image of Mo-S/ZrO2 was analyzed as well, the presence of MoS2 phase was not detected because of similar STEM contrast of Mo-S species and ZrO2 support. While the fresh 118

Mo-S/Al2O3 catalyst showed no ordering or crystallization of Mo-S species (Figure 5.5c), 6.8 Å lattice fringes corresponding to MoS2 slabs [49,50] were observed in the TEM image of the used

Mo-S/Al2O3 catalyst after 72 hours of WGS reaction (Figure 5.5d, white circles).

Figure 5.5. HAADF-STEM images of (a) fresh Mo-S/Al2O3, (b) used Mo-S/Al2O3, and TEM images of (c) fresh Mo-S/Al2O3, and (d) used Mo-S/Al2O3.

The XRD patterns of fresh and used Mo-S/Al2O3 and Mo-S/ZrO2 catalysts were analyzed to detect aggregation and crystallization of Mo-S species after 72 hours of WGS reaction. A distinct XRD peak at 2ϴ = 33.0° corresponding to MoS2(101) (PDF 00-017-0744) in Figure 5.6 was only observed for the used Mo-S/Al2O3 catalyst. This observation was in good agreement with the TEM observations and suggested that the crystallization of Mo-S species in the used Mo-

S/Al2O3 catalyst took place during 72 hours of WGS reaction. However, no XRD reflections or lattice fringes of MoS2 slabs were observed for the used Mo-S/ZrO2, suggesting that the Mo-S species in Mo-S/ZrO2 were relatively stable as compared to those in the Mo-S/Al2O3 catalyst. 119

MoS2

Used Mo/Al2O3

Mo-S/Al2O3 Intensity (a.u.)

Used Mo/ZrO2

Mo-S/ZrO2

20 25 30 35 40 45 2deg.

Figure 5.6. XRD patterns of fresh and used Mo-S/Al2O3 and Mo-S/ZrO2 catalysts.

Furthermore, the S 2p XPS spectra of fresh and used Mo-S/Al2O3 and Mo-S/ZrO2 catalysts are shown in Figure 5.7, while the atomic ratios for the S 2p peak area corresponding to fully sulfided Mo-S bonds and the S 2p peak area for Mo oxysulfide bonds (MoOxSy) are also summarized in the Table 5.2. The proportion of the fully sulfided Mo-S bonds in the used Mo-

S/Al2O3 catalyst increased substantially from 1.56 to 4.09 after 72 hours of WGS reaction. This is attributed to the aggregation and crystallization of Mo-S species in used Mo-S/Al2O3 catalyst after

72 hours of WGS reaction, whereas those in the used Mo-S/ZrO2 catalyst remained essentially unchanged. This observation suggested that Mo-S species in the Mo-S/ZrO2 catalyst were stable during 72 hours of WGS reaction regardless of H2S presence in the feed. This stability of Mo-S species in the Mo-S/ZrO2 catalyst could explain its weaker H2S dependence as compared to the

Mo-S/Al2O3 catalyst.

120

2- Mo-S/Al2O3 S S 2p 2- S 2- SO4 2

Used Mo-S/Al O S2- 2 3 S 2- 2- 2 SO4

2- Mo-S/ZrO2 S S 2- Intensity (a.u.) 2

Used Mo-S/ZrO 2 S2- SO 2- 4 2- S2

174 172 170 168 166 164 162 160 158 Binding energy (eV)

Figure 5.7. XPS-spectra S 2p region of fresh and used Mo-S/Al2O3 and Mo-S/ZrO2 catalysts.

Table 5.2. The atomic ratios of the S 2p peak area corresponding to fully sulfided Mo-S bonds and the S 2p area for Mo oxysulfide bonds (MoOxSy) of the fresh and used Mo-S/Al2O3 and Mo-S/ZrO2 catalysts estimated by XPS analysis.

Fresh Used Fresh Used

Mo-S/Al2O3 Mo-S/Al2O3 Mo-S/ZrO2 Mo-S/ZrO2 1.565 4.09 1.93 1.98

Table 5.3 summarizes the CO consumption rate normalized by the moles of Mo-S species in the fresh and used Mo-S catalysts. The normalized CO consumption rate was calculated by the following equation: Normalized CO consumption rate = × ÷× / where FCO is the feed flow rate of CO (mol/h), WMo is Mo mass in the supported catalysts based on ICP-MS analysis of supported catalysts before pre-sulfidation (Table 5.3), AWMo is the atomic

121 mass of Mo, and RS/Mo is the atomic S/Mo ratio determined from the H2-TPR peak areas as described above (Figure 5.4).

Table 5.3. The extent of sulfidation, normalized CO consumption rate over fresh Mo-S/Al2O3, used Mo-S/Al2O3, fresh Mo-S/ZrO2, and used Mo-S/ZrO2.

Fresh Mo-S/ Used Mo-S/ Fresh Mo-S/ Used Mo-S/ Al2O3 Al2O3 ZrO2 ZrO2 S/Mo 0.67 0.63 0.84 0.75 Normalized CO 21.7 ± 1.1 17.3 ± 0.9 23.0 ± 1.2 22.6 ± 1.1 consumption rate (h-1)

The decrease of the S/Mo ratio for the Mo-S/Al2O3 catalyst from 0.67 to 0.63 during 72 hours of WGS reaction was less significant as compared to that of Mo-S/ZrO2 (from 0.84 to 0.75).

The normalized CO consumption rate decreased by 20% for the Mo-S/Al2O3 catalyst during 72 hours of WGS reaction (from 21.7 h-1 to 17.3 h-1), whereas the normalized CO consumption rate

-1 -1 over the Mo-S/ZrO2 catalyst remained essentially unchanged (23.0 h vs. 22.6 h ). These observations suggested that the partial loss of activity of the Mo-S/Al2O3 catalyst resulted mostly from the decreased catalytic activity of Mo-S species due to their aggregation and crystallization during 72 hours of WGS reaction, while the Mo-S species present in the Mo-S/ZrO2 catalyst maintained their catalytic activity.

5.4. Conclusion

ZrO2, Al2O3, TiO2, CeO2, and SiO2 supported Mo catalysts were prepared by the incipient wetness impregnation and investigated with respect to their catalytic activity and sulfur dependence in the WGS reaction. The highest WGS activity in the 1,000 ppm H2S-containing feed and the weakest dependence on H2S presence in the feed among all supported Mo-S catalysts investigated were observed for the Mo-S/ZrO2 catalyst. The weak MoO3-support interactions are

4+ expected to enhance the MoO3 reducibility in supported Mo-O catalysts, producing surface Mo -

122

O bonds during pre-sulfidation. The extent of sulfidation of the supported Mo-S catalysts was found to increase with the density of surface Mo4+-O sites as these sites were previously suggested as the intermediates during MoO3 sulfidation. Thus, the highest S/Mo atomic ratio was observed for the Mo-S/ZrO2 catalyst due to its weakest MoO3-support interaction among all investigated supports. The S/Mo atomic ratios correlated with the CO conversion and suggested that the surface

Mo-S species were the active sites for the WGS reaction over the supported Mo-S catalysts. The relatively stable Mo-S species in the Mo-S/ZrO2 catalyst could contribute to its weak H2S- dependence during 72 hours of WGS reaction. On the other hand, the Mo-S/Al2O3 catalyst experienced the aggregation and crystallization of Mo-S species during 72 hours of reaction, resulting in a partial loss of its WGS activity. Weak MoO3-support interactions in the calcined Mo-

O/ZrO2 system and the relative stability of Mo-S species in the Mo-S/ZrO2 catalyst resulted in its greater catalytic activity and weaker H2S dependence as compared to other supported Mo-S catalysts investigated in this study. Therefore, ZrO2 was indicated by this study as the most promising support for the sulfur-dependent Mo-S catalysts for the WGS reaction.

5.5. References

1. D. S. Reichmuth, A. E. Lutz, D. K. Manley, J. O. Keller, "Comparison of the Technical Potential for Hydrogen, Battery Electric, and Conventional Light-Duty Vehicles to Reduce Greenhouse Gas Emissions and Petroleum Consumption in the United States," International Journal of Hydrogen Energy, 38, 2013, 1200-1208

2. G. Guan, M. Kaewpanha, X. Hao, A. Abudula, "Catalytic Steam Reforming of Biomass Tar: Prospects and Challenges," Renewable and Sustainable Energy Reviews, 58, 2016, 450-461

3. R. R. Davda, J. W. Shabaker, G. W. Huber, R. D. Cortright, J. A. Dumesic, "A Review of Catalytic Issues and Process Conditions for Renewable Hydrogen and Alkanes by Aqueous-Phase Reforming of Oxygenated Hydrocarbons Over Supported Metal Catalysts," Applied Catalysis B: Environmental, 56, 2005, 171-186

4. G. J. Stiegel, M. Ramezan, "Hydrogen from Coal Gasification: An Economical Pathway to a Sustainable Energy Future," International Journal of Coal Geology, 65, 2006, 173-190

123

5. A. A. Ahmad, N. A. Zawawi, F. H. Kasim, A. Inayat, A. Khasri, "Assessing the Gasification Performance of Biomass: A Review on Biomass Gasification Process Conditions, Optimization and Economic Evaluation," Renewable and Sustainable Energy Reviews, 53, 2016, 1333-1347

6. C. Yang, R. Li, C. Cui, S. Liu, Q. Qiu, Y. Ding, Y. Wu, B. Zhang, "Catalytic Hydroprocessing of Microalgae-Derived Biofuels: A Review," Green Chemistry, 18, 2016, 3684-3699

7. W. De Jong (Ed.). "Biomass Composition, Properties, and Characterization," John Wiley & Sons, Inc., 2014

8. A. Molino, S. Chianese, D. Musmarra, "Biomass Gasification Technology: The State of the Art Overview," Journal of Energy Chemistry, 25, 2016, 10-25

9. S. Chianese, J. Loipersböck, M. Malits, R. Rauch, H. Hofbauer, A. Molino, D. Musmarra, "Hydrogen from the High Temperature Water Gas Shift Reaction with an Industrial Fe/Cr Catalyst using Biomass Gasification Tar Rich Synthesis Gas," Fuel Processing Technology, 132, 2015, 39- 48

10. K. J. Andersson, M. Skov-Skjøth Rasmussen, P. E. Højlund Nielsen, "Industrial-Scale Gas Conditioning Including Topsøe Tar Reforming and Purification Downstream Biomass Gasifiers: An Overview and Recent Examples," Fuel, 203, 2017, 1026-1030

11. C. Ratnasamy, J. Wagner, "Water Gas Shift Catalysis," Catalysis Reviews - Science and Engineering, 51, 2009, 325-440

12. Y. Choi, H. G. Stenger, "Water Gas Shift Reaction Kinetics and Reactor Modeling for Fuel Cell Grade Hydrogen," Journal of Power Sources, 124, 2003, 432-439

13. M. D. Kaufman Rechulski, T. J. Schildhauer, S. M. A. Biollaz, C. Ludwig, "Sulfur Containing Organic Compounds in the Raw Producer Gas of Wood and Grass Gasification," Fuel, 128, 2014, 330-339

14. S. K. Sansaniwal, K. Pal, M. A. Rosen, S. K. Tyagi, "Recent Advances in the Development of Biomass Gasification Technology: A Comprehensive Review," Renewable and Sustainable Energy Reviews, 72, 2017, 363-384

15. S. Cheah, D. L. Carpenter, K. A. Magrini-Bair, "Review of Mid- to High-Temperature Sulfur Sorbents for Desulfurization of Biomass- and Coal-Derived Syngas," Energy and Fuels, 23, 2009, 5291-5307

16. D. S. Newsome, "The Water-Gas Shift Reaction," Catalysis Reviews, 21, 1980, 275-318

17. R. Hakkarainen, T. Salmi, R. L. Keiski, "Water-Gas Shift Reaction on a Cobalt-Molybdenum Oxide Catalyst," Applied Catalysis A, General, 99, 1993, 195-215

124

18. G. K. Reddy, P. Boolchand, P. G. Smirniotis, "Sulfur Tolerant Metal Doped Fe/Ce Catalysts for High Temperature WGS Reaction at Low Steam to CO Ratios - XPS and Mossbauer Spectroscopic Study," Journal of Catalysis, 282, 2011, 258-269

19. D. A. Solís-Casados, L. Escobar-Alarcón, T. Klimova, J. Escobar-Aguilar, E. Rodríguez- Castellón, J. A. Cecilia, C. Morales-Ramírez, "Catalytic Performance of CoMo/Al2O3-MgO-Li(X) Formulations in DBT Hydrodesulfurization," Catalysis Today, 271, 2016, 35-44

20. L. Wu, D. Jiao, J. Wang, L. Chen, F. Cao, "The Role of MgO in the Formation of Surface Active Phases of CoMo/Al2O3-MgO Catalysts for Hydrodesulfurization of Dibenzothiophene," Catalysis Communications, 11, 2009, 302-305

21. F. Trejo, M. S. Rana, J. Ancheyta, "CoMo/MgO-Al2O3 Supported Catalysts: An Alternative Approach to Prepare HDS Catalysts," Catalysis Today, 130, 2008, 327-336

22. Kerry C. Pratt, John V. Sanders, Victor Christov, "Morphology and Activity of MoS2 on various Supports: Genesis of the Active Phase," Journal of Catalysis, 124, 1990, 416-432

23. F. Trejo, M. S. Rana, J. Ancheyta, A. Rueda, "Hydrotreating Catalysts on Different Supports and its Acid-Base Properties," Fuel, 100, 2012, 163-172

24. T. R. Reina, S. Ivanova, M. A. Centeno, J. A. Odriozola, "Catalytic Screening of Au/CeO2- MOx/Al2O3 Catalysts (M = La, Ni, Cu, Fe, Cr, Y) in the CO-PrOx Reaction," International Journal of Hydrogen Energy, 40, 2015, 1782-1788

25. M. Laniecki, M. Malecka-Grycz, F. Domka, "Water–gas Shift Reaction Over Sulfided Molybdenum Catalysts," Applied Catalysis A: General, 196, 2000, 293-303

26. T. Sasaki, T. Suzuki, "Sulfide Molybdenum Catalysts for Water–gas Shift Reaction: Influence of the Kind of Promoters and Supports to Generate MoS2," Applied Catalysis A: General, 484, 2014, 79-83

27. M. Fernández-García, A. Martínez-Arias, A. Iglesias-Juez, C. Belver, A. B. Hungría, J. C. Conesa, J. Soria, "Structural Characteristics and Redox Behavior of CeO2-ZrO2/Al2O3 Supports," Journal of Catalysis, 194, 2000, 385-392

28. Y. Li, D. He, Q. Zhu, X. Zhang, B. Xu, "Effects of Redox Properties and Acid-Base Properties on Isosynthesis Over ZrO2-Based Catalysts," Journal of Catalysis, 221, 2004, 584-593

29. Y. Lian, H. Wang, Q. Zheng, W. Fang, Y. Yang, "Effect of mg/Al Atom Ratio of Support on Catalytic Performance of Co-Mo/MgO-Al2O3 Catalyst for Water Gas Shift Reaction," Journal of Natural Gas Chemistry, 18, 2009, 161-166

125

30. H. Wang, Z. Li, E. Wang, C. Lin, Y. Shang, G. Ding, X. Ma, S. Qin, Q. Sun, "Effect of Composite Supports on the Methanation Activity of Co-Mo-Based Sulphur-Resistant Catalysts," Journal of Natural Gas Chemistry, 21, 2012, 767-773

31. L. Kaluža, D. Gulková, Z. Vít, M. Zdražil, "Effect of Support Type on the Magnitude of Synergism and Promotion in CoMo Sulphide Hydrodesulphurisation Catalyst," Applied Catalysis A: General, 324, 2007, 30-35

32. V. Dhanala, S. K. Maity, D. Shee, "Roles of Supports (Γ-Al2O3, SiO2, ZrO2) and Performance of Metals (Ni, Co, Mo) in Steam Reforming of Isobutanol," RSC Advances, 5, 2015, 52522-52532

33. A. Jha, Y. Lee, W. Jang, J. Shim, K. Jeon, H. Na, H. Kim, H. Roh, D. Jeong, S. G. Jeon, et al, "Effect of the Redox Properties of Support Oxide Over Cobalt-Based Catalysts in High Temperature Water-Gas Shift Reaction," Molecular Catalysis, 433, 2017, 145-152

34. J. Mi, Y. Cao, J. Zhang, Y. Ma, C. Chen, D. Li, X. Lin, L. Jiang, "Effect of Ce Modification on the Structural and Catalytic Property of Co-Mo/mg(Al)O Catalyst for Water-Gas Shift Reaction," Applied Catalysis A: General, 553, 2018, 36-42

35. Ludmila P. C. Silva, Luís E. Terra, Ana C. S. L. S. Coutinho, Fabio B. Passos, "Sour Water– gas Shift Reaction Over Pt/CeZrO2 Catalysts," Journal of Catalysis, 341, 2016, 1-12

36. H. Hu, W. Wang, Z. Liu, B. Wang, Z. Li, X. Ma, "Sulfur-Resistant CO Methanation to CH4 Over MoS2/ZrO2 Catalysts: Support Size Effect on Morphology and Performance of Mo Species," Catalysis Letters, 148, 2018, 2585-2595

37. Z. Li, Y. Tian, J. He, B. Wang, X. Ma, "High CO Methanation Activity on Zirconia-Supported Molybdenum Sulfide Catalyst," Journal of Energy Chemistry, 23, 2014, 625-632

38. M. Shetty, K. Murugappan, T. Prasomsri, W. H. Green, Y. Román-Leshkov, "Reactivity and Stability Investigation of Supported Molybdenum Oxide Catalysts for the Hydrodeoxygenation (HDO) of M-Cresol," Journal of Catalysis, 331, 2015, 86-97

39. Th Weber, J. C. Muijsers, J. H. M. C. Van Wolput, C. P. J. Verhagen, J. W. Niemantsverdriet, "Basic Reaction Steps in the Sulfidation of Crystalline MoO3 to MoS2, as Studied by X-Ray Photoelectron and Infrared Emission Spectroscopy," Journal of Physical Chemistry, 100, 1996, 14144-14150

40. B. Yoosuk, D. Tumnantong, P. Prasassarakich, "Unsupported MoS2 and CoMoS2 Catalysts for Hydrodeoxygenation of Phenol," Chemical Engineering Science, 79, 2012, 1-7

41. B. Yoosuk, J. H. Kim, C. Song, C. Ngamcharussrivichai, P. Prasassarakich, "Highly Active MoS2, CoMoS2 and NiMoS2 Unsupported Catalysts Prepared by Hydrothermal Synthesis for Hydrodesulfurization of 4,6-Dimethyldibenzothiophene," Catalysis Today, 130, 2008, 14-23

126

42. P. Afanasiev, "On the Interpretation of Temperature Programmed Reduction Patterns of Transition Metals Sulphides," Applied Catalysis A: General, 303, 2006, 110-115

43. C. R. F. Lund, "Microkinetics of Water-Gas Shift Over Sulfided Mo/Al2O3 Catalysts," Industrial and Engineering Chemistry Research, 35, 1996, 2531-2538

44. Y. Chen, M. Dong, J. Wang, H. Jiao, "Mechanisms and Energies of Water Gas Shift Reaction on Fe-, Co-, and Ni-Promoted MoS2 Catalysts," Journal of Physical Chemistry C, 116, 2012, 25368-25375

45. P. Hou, D. Meeker, H. Wise, "Kinetic Studies with a Sulfur-Tolerant Water Gas Shift Catalyst," Journal of Catalysis, 80, 1983, 280-285

46. D. Nikolova, R. Edreva-Kardjieva, T. Grozeva, "Water-Gas Shift Activity of K-Promoted (Ni)Mo/Γ-Al2O3 Systems in Sulfur-Containing Feed," Reaction Kinetics, Mechanisms and Catalysis, 103, 2011, 71-86

47. A. Tavasoli, R. M. Kiai, A. Karimi, "Effects of Particle Size on the Catalytic Performance of MWCNTs Supported Alkalized MoS2catalysts Promoted by Ni and Co in Higher Alcohols Synthesis," Canadian Journal of Chemical Engineering, 94, 2016, 1495-1503

48. B. Baubet, E. Devers, A. Hugon, E. Leclerc, P. Afanasiev, "The Influence of MoS2 Slab 2D Morphology and Edge State on the Properties of Alumina-Supported Molybdenum Sulfide Catalysts," Applied Catalysis A: General, 487, 2014, 72-81

49. T. S. Sahu, S. Mitra, "Exfoliated MoS2 Sheets and Reduced Graphene Oxide-an Excellent and Fast Anode for Sodium-Ion Battery," Scientific Reports, 5, 2015,

50. M. Ramos, G. Berhault, D. A. Ferrer, B. Torres, R. R. Chianelli, "HRTEM and Molecular Modeling of the MoS2-Co9S8 Interface: Understanding the Promotion Effect in Bulk HDS Catalysts," Catalysis Science and Technology, 2, 2012, 164-178

51. E. J. M. Hensen, V. H. J. De Beer, J. A. R. Van Veen, R. A. Van Santen, "A Refinement on the Notion of Type I and II (Co)MoS Phases in Hydrotreating Catalysts," Catalysis Letters, 84, 2002, 59-67

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Chapter 6. Surface coverage effects on water gas shift activity of ZrO2

Supported Mo Sulfide Catalysts

6.1. Introduction

The potential of hydrogen as an energy resource has attracted significant interest in recent decades because of its high energy content (120 MJ/kg) and the lack of combustion products that contribute to climate change [1]. More than 90% of hydrogen is generated from fossil fuels, mainly by steam reforming of natural gas, partial oxidation of refinery oil, and coal gasification [2], which result in CO2 emissions that pose a threat of irreversible environmental changes. Alternatively, biomass gasification followed by the water gas shift (WGS) reaction is a highly promising method to produce pure hydrogen because biomass is a carbon-neutral resource [3].

The WGS reaction has gained renewed interest as a key process to produce pure hydrogen from syngas [4]. Because the WGS reaction is exothermic, the equilibrium is shifted to the reactants with increasing reaction temperature. Therefore, it is desirable to conduct the WGS reaction at low temperature, but due to slow kinetics at low temperatures, two-stage WGS reactors are typically employed, consisting of a high-temperature shift unit (HTS, 350-450℃) and a low- temperature shift unit (LTS, ~250℃) [4]. However, conventional WGS catalysts are easily deactivated by sulfur-containing impurities, which are ubiquitous in biomass feedstocks [5].

Molybdenum-based catalysts are well-known as so-called sour-shift catalysts to overcome the limitations of conventional two-stage WGS process. However, previously reported sulfur-tolerant

Mo-based catalysts showed low WGS activity at relevant high space velocities and low temperatures as compared to the HTS WGS catalysts [6]. Therefore, it is highly desirable to develop highly active Mo-based WGS catalysts to produce pure hydrogen from biomass syngas.

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Increasing the density of surface active sites in heterogeneous catalysts is the most direct method to improve catalytic activity [7]. A common method to increase the number of active sites of nanostructured catalysts is enhancing the dispersion of an active phase on high surface area supports, such as dense and porous metal oxides [8]. The interaction between an active phase and catalytic support can stabilize the active phase on the support surface and enhance the reactivity of the active sites, resulting in improvement of the catalytic activity [9].

The excellent thermal and chemical stability of bulk ZrO2 has led to increased interest in

ZrO2 as a catalyst support [10]. The oxidizable and reducible sites on the surface of ZrO2 render it highly amenable to studies of catalytic reactions. Research in hydrotreating reactions suggested enhanced reducibility of MoO3/ZrO2 due to weak active phase-support interactions [11]. Hu et al. reported that the small ZrO2 particles enhanced the CO methanation activity of MoS2/ZrO2 catalysts [12], while Garg et al. reported that mesoporous ZrO2/SiO2-supported Mo-based catalysts exhibited higher cyclohexene hydrogenation activity than ZrO2-free catalysts [13]. However,

ZrO2-supported Mo sulfide catalysts have not been widely studied in the WGS reaction except that they were previously reported to show lower WGS activity than TiO2-supported catalysts [14].

Sasaki et al. compared ZrO2, TiO2, and Al2O3 as supports of Mo-based catalysts, and found that

TiO2-supported Mo catalysts showed the highest extent of sulfidation and CO conversion [15].

Al2O3-supported Mo-based catalysts at monolayer MoO3 surface coverage showed the highest WGS activity [14,16]. The highest activity in anisole hydrodeoxygenation (HDO) was observed for the Mo-O/ZrO2 catalyst at monolayer MoO3 surface coverage [17]. Chary et al. reported that the activity and selectivity in the toluene ammoxidation to benzonitrile over the

MoO3/ZrO2 catalyst increased with coverage up to theoretical MoO3 monolayer on the ZrO2 [18].

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However, the MoO3 surface coverage effects for ZrO2-supported Mo sulfide catalysts in the WGS reaction have not been reported yet.

In this study, we synthesized ZrO2-supported Mo sulfide WGS catalysts at different MoO3 surface coverage by the incipient wetness impregnation. The surface structures, WGS activity, and the extent of sulfidation were examined in order to find the optimal MoO3 surface coverage. Based on our findings, we associated the topmost Mo-S species with the active sites for the WGS reaction and proposed the surface Mo-S structure models as a function of MoO3 surface coverage on ZrO2.

6.2. Experimental methods

ZrO2-supported Mo catalysts were prepared by the incipient wetness impregnation of the

ZrO2 support with a solution of ammonium molybdate (Fisher Scientific) used as a Mo precursor.

Crushed and sieved 100 ~ 500 µm particles of ZrO2 were employed, which was supplied by Saint-

Gobain North America. An appropriate amount of the molybdenum precursor was dissolved in warm DI water (30°C) under moderate stirring. After incipient impregnation of ZrO2 with the Mo precursor, supported catalysts were dried overnight at 80°C. The MoO3 surface coverage of the catalysts was controlled by the Mo weight loadings during the catalyst synthesis by incipient wetness impregnation. All catalysts were calcined at 500°C in air for 5 hours and denoted Mox-

O/ZrO2 (x indicates wt.% relative to ZrO2).

The catalytic activity of these Mo catalysts was examined in a fixed-bed tubular stainless- steel reactor (0.65 cm ID) operated at atmospheric pressure charged with 0.3~1.0 g of catalyst. The feed gas mixture typically contained 10 vol.% CO and 20 vol.% H2O in helium flowing at 35 ~ 85 mL/min. Pre-sulfidation was performed in flowing 1 mol.% H2S in H2 at 450°C for 18 hours prior to WGS activity tests. The catalysts obtained after pre-sulfidation were denoted as Mox-S/ZrO2.

Water was injected into a flowing CO/He stream by a syringe pump and vaporized in a heated gas

130 feed line before entering the reactor. A condenser filled with ice was installed at the reactor exit to collect water prior to product analysis. The gas mixture was analyzed by a HP-5890 II gas chromatograph equipped with a thermal conductivity detector. The carbon balances agreed within

± 5 mol.%.

The inductively coupled plasma mass spectrometer (ICP-MS) was employed to analyze the catalyst composition using an Agilent 7700 ICP-MS system. Catalyst samples were digested for 4 days at room temperature in aqua regia (1 HNO3/3 HCl, v/v), and the supernatant was diluted with

2 wt.% HNO3 solution prior to ICP-MS analysis. The XPS spectra of the catalysts were collected using a Kratos AXIS Ultra X-ray photoelectron spectrometer with a monochromatized Al Kα X- ray source operated at 12 kV and 10 mA. The XPS spectra for specific C 1s, O 1s, Mo 3d, S2p, and Zr 3d or Al 2p regions were collected. Prior to the analysis of XPS data, the charging effect was corrected for based on the C 1s binding energy of 284.5 eV.

High resolution transmission electron microscopy (HR-TEM) imaging was performed on a FEI Tecnai F20. TEM samples were prepared by depositing powdered catalysts on a copper grid coated with lacey carbon film. Crystal structures were determined by powder X-ray diffraction

(XRD) using a PANalytical X’pert diffractometer equipped with a Cu Kα radiation source. The

XRD data were collected in a step scan mode at 2ϴ = 20 - 50° with a step size of 0.05°/s. The

Raman spectra of the powdered catalysts were collected using a Horiba T64000 Raman spectrometer (488 nm excitation from a Coherent FreD 90C Ar+ laser with a spot size of ~2 µm and 10 mW power). The N2 adsorption-desorption isotherms were measured at 77 K using a

Micrometrics TriStar porosimeter.

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6.3. Results and Discussion

The Mo content determined by ICP-MS is shown in the first row of Table 6.1, confirming that the target Mo content (x wt.%) was achieved for the ZrO2-supported catalysts. The extent of sulfidation expressed by the atomic S/Mo ratios in the Mox-S/ZrO2 catalysts was determined on the basis of the S 2p and Mo 3d regions in the XPS spectra and normalized by the area of the Zr

3d region. The BET surface areas (Table 6.1) indicated that all Mox-S/ZrO2 catalysts possessed similar surface areas regardless of the Mo content. The Mo surface density is also listed in Table

6.1, calculated by the following equation:

Surface density (atoms/nm2) = () × . × () × where Aw (Mo) is the atomic mass of molybdenum, m(Mo) is the mass of Mo in 1 g of catalyst, and SA (nm2/g-cat) is the BET surface area.

Table 6.1. Mo (wt.%), atomic S/Mo ratios, BET surface areas, and surface densities (Mo 2 atoms/nm ) of Mo1-S, Mo2-S, Mo5-S, Mo10-S, and Mo15-S/ZrO2 catalysts.

Mo1-S Mo2-S Mo5-S Mo10-S Mo15-S Moa 0.8 ± 0.0 1.8 ± 0.1 4.9 ± 0.2 9.5 ± 0.4 14.3 ± 0.6 (wt.%) S/Mob 0.77 1.07 1.54 1.27 1.18 BET 49.5 ± 0.2 51.2 ± 0.4 50.5 ± 0.3 50.2 ± 0.3 48.7 ± 0.3 (m2/g) Surface Mo 1.0 2.4 6.5 12.6 18.9 (atoms/nm2) a. Mo wt.% analyzed by ICP-MS b. S/Mo determined by the XPS analysis *Monolayer coverage: 6.5 Mo atoms/nm2 [34]

Figure 6.1 shows the TEM images of the Mox-S/ZrO2 catalyst corresponding to different

MoO3 surface coverages in Mo sulfide catalysts supported on ZrO2. The surface densities of Mo2-

S, Mo5-S, and Mo15-S catalysts correspond to the theoretical sub-monolayer, monolayer, and

132 multilayer coverages, respectively. The small dark dots (white circle) and large bright spots (white arrow) in the TEM image of Mo2-S catalyst (Figure 6.1a) indicate the Mo-S species and monoclinic ZrO2 phase, respectively. The dark color is attributed to the heavier elemental contrast of Mo-S species than lighter ZrO2. The inset of Figure 6.1a illustrates the presence of 2.4 Å and

3.7 Å lattice fringes, which correspond to the (210) and (110) d-spacings of ZrO2 [19,20]. The limited extent of the Mo-S component and the presence of distinct ZrO2 lattice fringes suggested that the Mo-S species only partially cover the support ZrO2 because the Mo content in the Mo2-S catalyst was insufficient to coat the entire ZrO2 surface. The proportion of dark spots (Mo-S species enclosed by white circles) in the TEM image of the Mo5-S catalyst (Figure 6.1b) increased substantially as compared to the Mo2-S catalyst (Figure 6.1a). Moreover, the distinct lattice fringes of ZrO2 are no longer visible in Figure 6.1b, suggesting that the Mo-S species in the Mo5-S catalyst were able to fully cover ZrO2 surface at a theoretical monolayer coverage. 3.4 Å lattice fringes corresponding to the MoO2(101) [21] are visible at the center of the Mo15-S catalyst particle (the inset of Figure 6.1c), and 6.2 Å and 6.8 Å fringes corresponding to MoS2 layers [22,23] were detected at the edge of the Mo15-S catalyst particle (Figure 6.1c), suggesting that Mo15-S catalyst consisted of Mo-O core and Mo-S shell.

Figure 6.1. TEM images of ZrO2 supported (a) Mo2-S, (b) Mo5-S, and (c) Mo15-S.

133

Figure 6.2 shows the XRD patterns of the Mo2-S/ZrO2, Mo5-S/ZrO2, Mo15-S/ZrO2, and

o o o ZrO2 catalysts. The peaks visible at 2ϴ = 28.1 , 31.5 , and 34.2 (PDF 00-036-0420) corresponding to the (-111), (111), and (002) reflections of ZrO2 were observed for all ZrO2-supported catalysts.

o o Distinct peaks of MoO2 at 2ϴ = 26.0 (-111) and 37.0 (-211) (PDF 00-032-0671) were observed in the XRD pattern of the Mo15-S/ZrO2 catalyst, but these peaks were absent in the Mo2-S and

Mo5-S catalysts (at sub-monolayer and monolayer Mo coverage, respectively), indicating the formation of MoO2 at higher Mo loadings in good agreement with their Raman spectra.

MoO2 MoO3 MoO2 Mo15-S

Mo5-S

Intensity (a.u.) Mo2-S

ZrO2

25 30 35 40 45 2(deg.)

Figure 6.2. XRD patterns of ZrO2, Mo2-S/ZrO2, Mo5-S/ZrO2, and Mo15-S/ZrO2.

Figure 6.3 shows the Raman spectra of the Mox-S/ZrO2 catalysts. The peaks at 350, 385,

-1 480, 630, and 647 cm corresponding to the ZrO2 phase were present in the Raman spectra of all catalysts [24]. The peaks at 380 and 400 cm-1 corresponding to the Mo=S stretch [25,26] were detected for most catalysts (except Mo1-S), while their intensities increased with Mo loading until reaching the monolayer coverage (Mo5-S). The Raman peak at 750 cm-1 in the Mo10-S and Mo15-

S catalysts can be attributed to the Zr-O-Mo stretch [12]. The bands at 820 and 996 cm-1 corresponding to Mo-O-Mo and Mo=O stretches [27,28] were observed in the multilayer Mo 134 catalysts (Mo10-S and Mo15-S), which were absent in the Mo-S catalysts at sub-monolayer Mo coverage. The Raman spectra further suggested that the topmost surface layer of multilayer Mo catalysts (Mo10- and 15-S) contained Mo=S sulfide, while the subsurface layers in these catalysts consisted of Mo oxide and/or Zr-O-Mo species, which are not expected to be catalytically active in the WGS reaction.

Mo-O-Mo Mo=S 820 400 Zr-O-Mo Mo15-S 750 Mo=O 996

Mo10-S

Mo5-S Intensity (a.u.) Mo2-S

Mo1-S

400 600 800 1000 -1 Wavenumber (cm )

Figure 6.3. Raman spectra of ZrO2-supported Mo1-S, Mo2-S, Mo5-S, Mo10-S, and Mo15-S catalysts.

The proposed structural motifs in these Mo-S catalysts are shown in Figure 6.4 based on the TEM, XRD, XPS, and Raman findings. The Mo content above the monolayer coverage (Mo10-

S and Mo15-S) may result in different nanoscale structures of Mo-S moieties on the ZrO2 support from those found at sub-monolayer coverage (Mo1-S and Mo2-S). The Mo-S species proposed to contain the active sites for the WGS reaction were only formed on the surface monolayer of Mo oxide during the pre-sulfidation process. The Mo oxide species in the subsurface layer may be organized into relatively inactive MoO3 or ZrMo2O8 phases [29,30] after calcination at 500°C in air (red circles in Figure 6.4). The S/Mo atomic ratios (Mo2-S: 1, Mo5-S: 1.6, and Mo15-S: 1) of

135 the proposed structural models (Figure 6.4) correspond to those determined by the XPS analysis

(Table 6.1; 1.07, 1.54, and 1.18, respectively).

Figure 6.4. Proposed structural motifs for supported Mo-S species as a function of surface coverage on ZrO2 support.

Figure 6.5 shows the CO conversion over the Mo-S/ZrO2 catalysts as a function of Mo

-1 content at 450°C and 9,000 h GHSV in a feed containing 1,000 ppm H2S, 10 vol. % CO and 20 vol. % H2O in helium. The CO conversion over the Mo-S/ZrO2 catalysts increased up to 5 wt.%

Mo content, which matched the theoretical MoO3 monolayer coverage, while the CO conversion remained unchanged at higher Mo content (10 and 15 wt.%) corresponding to multilayer coverage.

Based on the atomic S/Mo ratios shown in Table 6.1, the extent of sulfidation reached a maximum at 5 wt.% Mo, suggesting a saturated, i.e., fully sulfided surface Mo-S monolayer. This saturation behavior for the extent of sulfidation further correlated with the WGS activity (i.e., the CO conversion) of these catalysts shown in Figure 6.5. These observations suggested that the optimal catalytic activity for the Mo-S species is observed at the theoretical monolayer coverage. Our findings suggested that the most active Mo sulfide species form the topmost surface layer as a Mo-

S shell, while a catalytically inert subsurface of Mo oxide and mixed Zr-Mo oxide was present in multilayer Mo catalysts.

136

100

CO Conversion (%)CO Equilibrium 20 Mox-S/ZrO2

0 2 4 6 8 10 12 14 16 Mo wt.%

Figure 6.5. CO conversion during WGS reaction over ZrO2-supported Mo1-S, Mo2-S, Mo5-S, Mo10-S, and Mo15-S catalysts (Feed: 10 vol.% CO, 20 vol.% H2O, and 1,000 ppm H2S in He at GHSV = 9,000 h-1 and 450°C).

Figure 6.6 shows the Arrhenius plots for the WGS reaction over the Mox-S/ZrO2 catalysts conducted at low CO conversion (< 13%) to determine the initial reaction rates and minimize mass-transfer effects. Table 6.2 shows the activation energies and turnover frequencies (TOF) of the Mox-S/ZrO2 catalysts. The Arrhenius plots showed good linearity and led to the values of activation energy (Ea) that were in good agreement with previous reports (40 ~ 86 kJ/mol) [31-33].

The TOF is defined as the number of reacted CO molecules per unit time normalized by the number of catalytically active sites, which is a common metric for comparing the intrinsic catalytic activity of different catalysts. Two assumptions were made in defining the number of catalytically active sites in the Mox-S/ZrO2 catalysts. First, the Mo-S species were assumed to be the only active sites in the WGS reaction. Secondly, the number of active Mo-S species was assumed to be equal to the number of surface Mo sites in a Mo oxide monolayer before pre-sulfidation. The TOF over the

Mox-S/ZrO2 catalysts increased with Mo content up to the theoretical monolayer Mo coverage and was similar for the monolayer and multilayer catalysts.

137

-12 Mo1-S Mo2-S -13 Mo5-S

]) Mo10-S -1 s Mo15-S -1 -14 mol 2 cat -15 ln (k ln (k [m -16

-17 0.0014 0.0016 0.0018 0.0020 0.0022 0.0024 1/T (1/K)

Table 6.2. Activation energies and TOFs for the WGS reaction over ZrO2-supported Mo1-S, Mo2- S, Mo5-S, Mo10-S, and Mo15-S catalysts.

Mo1-S Mo2-S Mo5-S Mo10-S Mo15-S

Ea (kJ/mol) 72.2 52.7 46.6 47.8 40.9

TOF (s-1) 5.5×10-6 4.5×10-4 2.6×10-3 2.7×10-3 2.5×10-3 6.4. Conclusions

We synthesized ZrO2-supported Mo-S catalysts at different MoO3 surface coverage using the incipient wetness impregnation methodology. The TEM, Raman, and XRD confirmed the formation of a topmost Mo-S layer and Mo-O sub-surface in ZrO2-supported Mo-S catalysts above theoretical monolayer coverage. The CO conversion over ZrO2-supported Mo-S catalysts increased with Mo content up to the monolayer coverage, while the WGS activity correlated with the extent of Mo sulfidation. The Mo-S species in the topmost layer were proposed to be the active sites for the WGS reaction over the Mo-S/ZrO2 catalysts. These findings suggested that the Mo-

138

S/ZrO2 catalysts containing the theoretical MoO3 monolayer coverage are highly promising for the

WGS reaction conducted in the presence of sulfur-containing syngas impurities.

6.5. References

1. B. D. Solomon, V. A. Luzadis, "Renewable Energy from Forest Resources in the United States," Routledge, New York, 2008

3. E. Shayan, V. Zare, I. Mirzaee, "Hydrogen Production from Biomass Gasification; a Theoretical Comparison of using Different Gasification Agents," Energy Conversion and Management, 159, 2018, 30-41

4. C. Ratnasamy, J. Wagner, "Water Gas Shift Catalysis," Catalysis Reviews - Science and Engineering, 51, 2009, 325-440

5. W. De Jong (Ed.). "Biomass Composition, Properties, and Characterization," John Wiley & Sons, Inc., 2014

6. G. K. Reddy, P. Boolchand, P. G. Smirniotis, "Sulfur Tolerant Metal Doped Fe/Ce Catalysts for High Temperature WGS Reaction at Low Steam to CO Ratios - XPS and Mossbauer Spectroscopic Study," Journal of Catalysis, 282, 2011, 258-269

7. J. K. Nørskov, T. Bligaard, B. Hvolbæk, F. Abild-Pedersen, I. Chorkendorff, C. H. Christensen, "The Nature of the Active Site in Heterogeneous Metal Catalysis," Chemical Society Reviews, 37, 2008, 2163-2171

8. G. A. Somorjai, Y. Li, "Introduction to Surface Chemistry and Catalysis," (2nd ed.) John Wiley & Sons, Inc., New Jersey, 2010

9. K. M. Choi, K. Na, G. A. Somorjai, O. M. Yaghi, "Chemical Environment Control and Enhanced Catalytic Performance of Platinum Nanoparticles Embedded in Nanocrystalline Metal- Organic Frameworks," Journal of the American Chemical Society, 137, 2015, 7810-7816

10. D. Devaiah, L. H. Reddy, S. Park, B. M. Reddy, "Ceria–zirconia Mixed Oxides: Synthetic Methods and Applications," Catalysis Reviews - Science and Engineering, 60, 2018, 177-277

11. L. Kaluža, D. Gulková, Z. Vít, M. Zdražil, "Effect of Support Type on the Magnitude of Synergism and Promotion in CoMo Sulphide Hydrodesulphurisation Catalyst," Applied Catalysis A: General, 324, 2007, 30-35

139

12. H. Hu, W. Wang, Z. Liu, B. Wang, Z. Li, X. Ma, "Sulfur-Resistant CO Methanation to CH4 Over MoS2/ZrO2 Catalysts: Support Size Effect on Morphology and Performance of Mo Species," Catalysis Letters, 148, 2018, 2585-2595

13. S. Garg, K. Soni, G. M. Kumaran, M. Kumar, J. K. Gupta, L. D. Sharma, G. M. Dhar, "Effect of Zr-SBA-15 Support on Catalytic Functionalities of Mo, CoMo, NiMo Hydrotreating Catalysts," Catalysis Today, 130, 2008, 302-308

14. M. Łaniecki, M. Małecka-Grycz, F. Domka, "Water–gas Shift Reaction Over Sulfided Molybdenum Catalysts: I. Alumina, Titania and Zirconia-Supported Catalysts," Applied Catalysis A: General, 196, 2000, 293-303

15. T. Sasaki, T. Suzuki, "Sulfide Molybdenum Catalysts for Water–gas Shift Reaction: Influence of the Kind of Promoters and Supports to Generate MoS2," Applied Catalysis A: General, 484, 2014, 79-83

16. D. Nikolova, R. Edreva-Kardjieva, G. Gouliev, T. Grozeva, P. Tzvetkov, "The State of (K)(Ni)Mo/Γ-Al2O3 Catalysts After Water-Gas Shift Reaction in the Presence of Sulfur in the Feed: XPS and EPR Study," Applied Catalysis A: General, 297, 2006, 135-144

17. M. Shetty, K. Murugappan, W. H. Green, Y. Román-Leshkov, "Structural Properties and Reactivity Trends of Molybdenum Oxide Catalysts Supported on Zirconia for the Hydrodeoxygenation of Anisole," ACS Sustainable Chemistry and Engineering, 5, 2017, 5293- 5301

18. K. V. R. Chary, K. R. Reddy, G. Kishan, J. W. Niemantsverdriet, G. Mestl, "Structure and Catalytic Properties of Molybdenum Oxide Catalysts Supported on Zirconia," Journal of Catalysis, 226, 2004, 283-291

19. Y. Lin, F. Liang, "Synthesis of a ZrO2/Carbon Aerogel Composite with Tetragonal ZrO2 Structures Assisted by the Formation of Phenol Formaldehyde Resin," CrystEngComm, 17, 2015, 678-685

20. W. Li, H. Huang, H. Li, W. Zhang, H. Liu, "Facile Synthesis of Pure Monoclinic and Tetragonal Zirconia Nanoparticles and their Phase Effects on the Behavior of Supported Molybdena Catalysts for Methanol-Selective Oxidation," Langmuir, 24, 2008, 8358-8366

21. S. Hu, F. Yin, E. Uchaker, W. Chen, M. Zhang, J. Zhou, Y. Qi, G. Cao, "Facile and Green Preparation for the Formation of MoO2-GO Composites as Anode Material for Lithium-Ion Batteries," Journal of Physical Chemistry C, 118, 2014, 24890-24897

22. T. S. Sahu, S. Mitra, "Exfoliated MoS2 Sheets and Reduced Graphene Oxide-an Excellent and Fast Anode for Sodium-Ion Battery," Scientific Reports, 5, 2015,

140

23. M. Ramos, G. Berhault, D. A. Ferrer, B. Torres, R. R. Chianelli, "HRTEM and Molecular Modeling of the MoS2-Co9S8 Interface: Understanding the Promotion Effect in Bulk HDS Catalysts," Catalysis Science and Technology, 2, 2012, 164-178

24. M. Jia, P. Afanasiev, M. Vrinat, "The Influence of Preparation Method on the Properties of NiMo Sulfide Catalysts Supported on ZrO2," Applied Catalysis A: General, 278, 2005, 213-221

25. K. Zhou, F. Withers, Y. Cao, S. Hu, G. Yu, C. Casiraghi, "Raman Modes of MoS2 used as Fingerprint of Van Der Waals Interactions in 2-D Crystal-Based Heterostructures," ACS Nano, 8, 2014, 9914-9924

26. H. Li, Q. Zhang, C. C. R. Yap, B. K. Tay, T. H. T. Edwin, A. Olivier, D. Baillargeat, "From Bulk to Monolayer MoS2: Evolution of Raman Scattering," Advanced Functional Materials, 22, 2012, 1385-1390

27. C. Liu, W. Wang, Y. Xu, Z. Li, B. Wang, X. Ma, "Effect of Zirconia Morphology on Sulfur- Resistant Methanation Performance of MoO3/ZrO2 Catalyst," Applied Surface Science, 441, 2018, 482-490

28. S. Samantaray, G. Hota, B. G. Mishra, "Physicochemical Characterization and Catalytic Applications of MoO3-ZrO2 Composite Oxides Towards One Pot Synthesis of Amidoalkyl Naphthols," Catalysis Communications, 12, 2011, 1255-1259

29. Z. Li, Y. Tian, J. He, B. Wang, X. Ma, "High CO Methanation Activity on Zirconia-Supported Molybdenum Sulfide Catalyst," Journal of Energy Chemistry, 23, 2014, 625-632

30. E. J. Liang, Y. Liang, Y. Zhao, J. Liu, Y. Jiang, "Low-Frequency Phonon Modes and Negative Thermal Expansion in A(MO4)2 (A = Zr, Hf and M = W, Mo) by Raman and Terahertz Time- Domain Spectroscopy," Journal of Physical Chemistry A, 112, 2008, 12582-12587

31. D. Ke, Y. Tao, Y. Li, X. Zhao, L. Zhang, J. Wang, S. Han, "Kinetics Study on Hydrolytic Dehydrogenation of Alkaline Sodium Borohydride Catalyzed by Mo-Modified Co-B Nanoparticles," International Journal of Hydrogen Energy, 40, 2015, 7308-7317

32. S. S. Hla, G. J. Duffy, L. D. Morpeth, A. Cousins, D. G. Roberts, J. H. Edwards, "Investigation into the Performance of a Co-Mo Based Sour Shift Catalyst using Simulated Coal-Derived Syngases," International Journal of Hydrogen Energy, 36, 2011, 6638-6645

33. P. Afanasiev, "On the Interpretation of Temperature Programmed Reduction Patterns of Transition Metals Sulphides," Applied Catalysis A: General, 303, 2006, 110-115

34. P. Cheung, H. Liu, E. Iglesia, "Kinetics and Mechanism of Dimethyl Ether Oxidation to Formaldehyde on Supported Molybdenum Oxide Domains," Journal of Physical Chemistry B, 108, 2004, 18650-18658

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Chapter 7. Hydrogen production over Co-promoted Mo-S water gas shift catalysts supported on ZrO2

7.1. Introduction

Hydrogen is a clean energy carrier and an environmentally friendly fuel that can be used as feedstock for stationary electric power plants and mobile fuel cells with zero CO2 emission and high energy density (120 MJ/kg) [1-4]. Most hydrogen (~96%) is currently produced from fossil fuel feedstocks: 48% from high-temperature steam reforming of natural gas, 30% from partial oxidation of refinery oil, and 18% from coal gasification [5,6]. The major problems of using fossil fuels for hydrogen production are high CO2 emissions, the uneven distribution of reserves, and the shortage of fuel reserves [7]. However, hydrogen production from biomass-derived syngas technologies could overcome these problems, since biomass is a carbon-neutral and domestically abundant sustainable energy resource [8-10].

Biomass is a carbon-neutral resource because it is able to recycle carbon dioxide over the course of its life cycle. Plants consume carbon dioxide in photosynthesis from the atmosphere as part of their natural growth process. Thus, the net CO2 emissions of hydrogen production using biomass-derived syngas is low during the short-term including the its life cycle since the life cycle of biomass is relatively short as compared to the cycle of coal, crude oil, and natural gas [10-14].

In the United States, there is more available biomass as an energy source than is required for human food and animal feed. A recent report projects that due to the anticipated improvements in agricultural practices and plant breeding, up to 1 billion dry tons of biomass could be available for energy use annually [14]. In addition to the crops grown specifically for energy use, the agricultural crop residues, forest residues, organic municipal solid waste, and animal waste can be used as a biomass feedstock. This sustainable resource can be used to produce hydrogen by gasification,

142 which uses a controlled process involving heat, steam, and oxygen to convert biomass to syngas without combustion [15-17]. Several biomass gasification pilots of varying sizes have been built and operated for bio-fuel, providing a promising alternative for hydrogen production [18]. The

U.S. Department of Energy anticipates that biomass gasification plants could be deployed in the near-term timeframe [10,14].

The water gas shift (WGS) reaction is a key operation in maximizing the production of hydrogen from biomass-derived syngas [8,9]. As the WGS reaction is reversible and mildly exothermic, the WGS activity depends on reaction temperature (CO (g) + () ⟷ () +

() [ = −41.1 /]) [19,20]. Typical WGS reactors consist of a high-temperature shift (HTS, 350~450°C) and a low-temperature shift (LTS, ~250°C) due to the thermodynamic limitation of the reaction [19-21]. However, it is quite challenging to apply typical WGS catalysts in hydrogen production using biomass-derived syngas, since biomass feedstocks contain a considerable amount of sulfur [22].

The main challenge of using Fe2O3-Cr2O3 HTS catalysts is the easy deactivation of the catalyst when the H2S concentration in the syngas exceeds 500 ppm. [23] When this occurs, the syngas must be desulfurized before reaching the HTS catalyst. Typical the sulfur removal processes require cooling down the syngas to remove excess moisture that was introduced in the wet-scrubber process necessary in order to remove particulate matter in the incoming syngas from the gasifier [24]. After the sulfur removal process, the syngas undergoes reheating and steam reinjection steps to meet the HTS requirement (450°C, H2O/CO = 2) [20]. These are energy- intensive processes necessary when the sulfur-removal process is located upstream from the WGS reactor. However, the use of a sulfur-tolerant catalyst can offer flexibility in when to remove the sulfur, which is attractive to reduce the capital equipment cost, as well as the operation and

143 maintenance cost of hydrogen production. On the other hands, the commercially available Cu–

ZnO–Al2O3 (LTS) catalysts deactivate completely in the presence of trace levels of H2S (~10 ppm) in the feed [25].

The molybdenum (Mo)-based catalysts have gained significant interest as a sulfur-tolerant

WGS catalysts. However, Mo-based catalysts are the sulfur dependent catalyst, required at least

100 ppm of sulfur content to exhibit WGS catalytic activity [20,26,27]. Currently, no catalyst can maintain WGS activity over the range of sulfur content in biomass (0 ~ 15,000 ppm, in dry basis)

[22]. WGS activity of Mo-based catalysts on the sulfur presence in syngas has not widely investigated, since the Mo-based catalysts was mainly employed in coal-derived syngas or tar- derived syngas, which contained at least 4,000 ppm of sulfur compounds [28-30]. Moreover, the

WGS activity of Mo-based catalysts is relatively low as compared to conventional HTS and LTS

WGS catalysts (Fe- and Cu-based) [20,31]. Therefore, improvement of WGS activity in Mo-based catalysts are required to compete the conventional two-stage WGS catalysts.

Alkali metal promoters have been suggested to improve WGS activity by enhancing dispersion of the active phase and avoiding carbon deposition [32,33]. Nickel (Ni)-promoted Mo-

S catalysts have been reported to show enhanced WGS activity and the bimetallic synergistic effect

[34-38], whereas the CH4 methanation and carbon deposition could be typical disadvantages to apply Ni-Mo catalysts in hydrogen production. Cobalt (Co) is the best additive to improve the

WGS activity of Mo-based catalysts as increasing the active sites [34,35,39-45]. CoMo-S/TiO2-

Al2O3 catalysts showed higher WGS activity than commercial sour shift catalysts (CoMo-MgO-

Al2O3) at H2O/CO=1 [46,47].

The catalytic activity, the adsorption of reactants, and reaction pathway of unpromoted

Mo-S and CoMo-S phase have been investigated for the WGS reaction over the CoMo-S catalysts

144 by DFT calculations and experimental studies [42,48]. The CoMo-S sites and Mo-S sites in the

CoMo-S/Al2O3 catalysts were distinguished and quantified using CO adsorption followed by in- situ IR spectroscopy (IR/CO) at low temperature [36,42]. Chen et al. reported that the intrinsic activities of CoMo-S sites in the WGS reaction at low temperature (< 300°C) are higher than those of unpromoted Mo-S sites [42]. Recent DFT calculations over CoMo-S WGS catalysts reported that the overall reaction barrier of the COOH-associated mechanism on the S edge of Co-MoS2 was lower than that of the redox mechanism on the S edge of MoS2 [48]. The Co addition to Mo can reduce the height of the H2O dissociation energy (H2O → OH + H) on the sulfur edge sites of

Co-MoS2 compared to that of unpromoted Mo-S [43,48], which is one of the rate-determining steps in the WGS reaction.

Candia et al. suggested that the CoMo-S phases pre-sulfided above 500°C were strongly interacted with Al2O3 and less active over hydrodesulfurization (HDS) reaction than the catalysts pre-sulfided below 400°C [49]. Although it is unclear how the MoO3-support interactions influence the catalytic activity, the catalysts with weak MoO3-Al2O3 interaction are reported to be more active over HDS reaction, hydrogenation, and the WGS reaction than the catalysts with strong MoO3-Al2O3 interaction [50-52]. Kaluza et al. reported that Mo/ZrO2 showed the most enhanced reducibility of MoO3 to MoO2 among the ZrO2-, TiO2-, Al2O3-, SiO2-, and MgO- supported Mo catalysts [53]. Shetty et al. reported that the ZrO2-supported Mo catalysts showed the weak interaction between ZrO2 and Mo species showing high catalytic activity of hydrodeoxygenation of m-cresol to toluene [54]. The MoO3 weakly interacted with support can be easily reduced to MoO2 in supported Mo-based catalysts, led to the increased formation of Mo-S during pre-sulfidation. Therefore, it is highly desirable to study ZrO2-supported Mo catalysts over the WGS reaction due to its weak MoO3-support interaction.

145

Intensive studies of modifying the nanoscale structures on the surface of the catalyst have been investigated as another approach to increase the density of surface catalytic active sites.

Reducing the size of MoS2 nanoparticles can increase the density of the Mo-S edge sites, which is considered to possess active sites for HDS reactions of thiophene [55]. The lower stack number of

MoS2 layered slabs was proposed to have more Mo-edge sites with more exposed Mo atoms, which are assumed to be the active sites for the hydrogen evolution reaction (HER) of water [56]. Topsøe suggested that the edge sites of CoMo-S in the top single layer play a key role in hydrogenation reaction of dibenzothiophene [57]. High hydrodeoxygenation (HDO) of anisole activity was observed over the Mo-O/ZrO2 catalysts containing a theoretical monolayer MoO3 surface coverage due to the presence of highly dispersed active Mo oxide species [58]. Al2O3-supported Mo-based catalysts at monolayer MoO3 surface coverage showed the highest WGS activity [59,60]. The modified dispersion of Mo-O species on ZrO2 at different MoO3 loading has been reported by

Chary et al. [61]. However, the investigation the effect of CoMo-O surface coverage on ZrO2 support have not been widely studied in WGS reaction.

We investigated two series of ZrO2 supported CoMo-S catalysts. In the first series, we investigated the influence of CoMo-O surface coverage on a ZrO2 support on the WGS activity,

CoMo-S/ZrO2 catalysts with the CoMo-O surface coverage ranging from 0.5 to 4 layers were prepared by controlling Co and Mo content. In the second series, the effects of Co/Mo ratios at theoretical monolayer coverage were investigated. The long-term stability and H2S-dependence of

CoMo-S/ZrO2 catalysts as a function of CoMo-O surface coverage and Co/Mo atomic ratio were also investigated employing in H2S containing feed. TEM, XRD, Raman, TPR, and XPS analysis were performed to characterize both two series of CoMo-S/ZrO2 catalysts.

146

7.2. Experimental

7.2.1. Catalyst synthesis

Supported CoMo catalysts were prepared by incipient wetness impregnation of the ZrO2 support with a solution of ammonium molybdate tetrahydrate (Fisher Scientific) and cobalt nitrate

(Alfa Aesar). Crushed and sieved 100 ~ 500 µm particles of ZrO2 were employed, which were supplied by Saint-Gobain. Appropriate amounts of the molybdenum precursor and cobalt precursor were dissolved in warm (30°C) deionized (DI) water under moderate stirring. After incipient impregnation with aqueous solutions of the Co and Mo precursor, the supported catalysts were dried overnight at 80°C. The dried catalysts were then calcined at 500°C in air for 5 hours.

The catalysts thus prepared were denoted as CoMo-O/ZrO2 (CoMo oxide catalyst). All catalysts were pre-activated in flowing 1 vol. % H2S (balanced with H2) at 450°C for 18 hours before catalytic tests. The pre-sulfided catalysts were denoted as CoMo-S/ZrO2. The first series of catalysts were prepared at 1.25, 2.5, 5, and 10 wt.% Mo relative to the ZrO2 support, while the Co weight loading was same as Mo. The catalysts were denoted as n-ML catalysts, where n stands for the number of theoretical CoMo-O surface layers (0.5, 1, 2, and 4.0 layers, respectively). The

CoMo-O surface coverage of the catalysts ranging from 0.5 to 4 layers was controlled by the Co and Mo weight loadings during the catalyst synthesis by incipient wetness impregnation. The second series of catalysts were prepared at 5, 4, 1.95, and 2.5 wt.% Mo relative to the ZrO2 support, while the amount of Co was 0, 0.3, 0.6, and 2.5 wt.% depending on the Co/Mo atomic ratio to yield a theoretical monolayer coverage in all cases. This series of catalysts was denoted as Co/Mo

= n catalysts, where n stands for the Co/Mo atomic ratio. The commercially available sour gas shift catalyst (ShiftMax 820) was provided by Clariant. The commercial catalyst contained 10.0

147 wt.% MoO3, 4.0 wt.% CoO, 18.0 wt.% MgO, and 68.0 wt.% Al2O3. The pelletized commercial catalyst was crushed and sieved to select 100 ~ 500 µm sized particles.

7.2.2. WGS catalytic activity

The catalytic activity of these WGS catalysts was determined by employing a fixed-bed tubular stainless reactor (0.65 cm ID) operated at atmospheric pressure. The feed gas mixture typically contained 10 vol. % CO, 20 vol. % H2O, and 0 ~ 0.7 vol. % H2S in helium. Water was injected into a flowing feed gas mixture by a syringe pump and vaporized in a heated gas feed line before entering the reactor. A condenser filled with ice was installed at the reactor exit to collect water. The product gas mixture was analyzed by an HP-5890 II gas chromatograph equipped with a thermal conductivity detector (TCD). The carbon balances agreed within ±5 mol. %.

7.2.3. Catalyst characterization

High resolution transmission electron microscopy (HR-TEM) imaging was performed using a FEI Tecnai G2 F20. The crystal structures were determined by powder X-ray diffraction

(XRD) using a PANalytical X’pert diffractometer equipped with Cu Kα radiation source. The

Brunauer-Emmett-Teller (BET) surface area was measured at 77 K using a Micrometrics TriStar porosimeter [62,63].

The inductively coupled plasma mass spectrometry (ICP-MS) was employed to analyze the metal composition of the catalysts using an Agilent 7700 ICP-MS system. The Raman spectra of dry powdered catalyst samples in ambient air were collected using a Horiba T64000 Raman spectrometer (488 nm excitation from a coherent FreD 90C Ar+ laser with a spot size of ~ 2 µm).

The hydrogen temperature programmed reduction (H2-TPR) studies were performed using a

Stanford Research Systems QMS 200 gas analyzer. The catalyst sample was heated at 5°C/min from room temperature to 580°C in 20 vol. % H2 balanced with argon flowing at 10 mL/min (STP).

148

The XPS spectra of the catalysts were collected using a Kratos AXIS Ultra X-ray photoelectron spectrometer for the specific C 1s, O 1s, Co 2p, Mo 3d, Zr 3d, and S 2p regions. The background subtraction, normalization, and peak fitting of the data were performed using the Peakfit software

[64].

7.3. Results and Discussion

7.3.1. Characterization of n ML CoMo/ZrO2 catalysts

Table 7.1 summarizes the Co and Mo content (wt.%), BET surface areas, and atomic S/Mo ratios. The catalysts were denoted as n ML catalyst, where n stands for the number of surface

CoMo-O layers estimated by assuming that 6.5 atoms/nm2 is the theoretical monolayer CoMo-O surface coverage [65]. The surface density was calculated by the following equation:

Surface density (atoms/nm) = () × . × () × where Aw(Metal) is the atomic weight of the corresponding metal, m(Metal) is the weight of active metal per 1 g of supported catalyst, and SA (nm2/g-cat) is the BET surface area of the support.

Table 7.1. Co (wt. %), Mo (wt. %), atomic S/Mo ratios, and BET surface areas, of 0.5 ML, 1 ML, 2 ML, and 4 ML CoMo-S/ZrO2 catalysts. 0.5 ML 1 ML 2 ML 4 ML Coa (wt. %) 1.1 2.4 4.7 9.7 Moa (wt. %) 0.9 2.5 4.9 9.1 BET (m2/g) 50.3 48.2 52.5 40.3 S/Mob 1.1 1.7 1.7 1.8 a. Co and Mo wt. % analyzed by ICP-MS b. S/Mo determined by XPS analysis

Figure 7.1 shows the TEM images of n ML CoMo-S catalysts. The 0.5 ML catalyst (Figure

7.1a) showed distinct 2.8 Å and 3.7 Å fringes, which correspond to the (-111) and (110) planes of

ZrO2, where small dark dots (white arrow) suggest the presence of CoMo-S, suggesting that

CoMo-O species did not completely cover ZrO2 surface in freshly sulfided catalysts. The inset of

149

Figure 7.1b demonstrated the less distinct lattice fringes of ZrO2 and a limited number of dark dots

(white arrow) of 1 ML catalyst than those of 0.5 ML catalyst, while the blurry edge layers (white circle) appeared on the surface of ZrO2 support. This suggested that the CoMo-S species in 1 ML catalysts covered the ZrO2 support more than the CoMo-S species in 0.5 ML catalyst and formed

CoMo-S layer on the ZrO2 support at the monolayer coverage. The 6.2 ~ 6.8 Å interlayer spacings of surface layers (red circle) corresponding to those of the MoS2(002) planes [56,66] were observed in the TEM image of 2 ML catalyst (Figure 7.1c), indicating that the formation of CoMo-

S layers occurred above the theoretical monolayer coverage. Much thicker CoMo-S multilayers were evident in Figure 7.1d, indicating the increased ordering of a conformally layered CoMo-S phases in the 4 ML catalyst. The presence of CoMo-S multilayers in the TEM image (Figure 7.1d) is in good agreement with the XRD peak of CoMo2S4 observed for the 4 ML catalyst in Figure 7.2.

150

Figure 7.1. TEM images of fresh sulfided (a) 0.5 ML, (b) 1 ML, (c) 2 ML, and (d) 4 ML CoMo- S/ZrO2 (white arrow: CoMo-S species, white circle: CoMo-S layer, and red circle: stacked CoMo- S layers).

Figure 7.2 shows the XRD patterns of the n ML CoMo-S/ZrO2 catalysts and ZrO2 support.

The peaks visible at 2ϴ = 28.1°, 31.5°, and 34.2° (PDF 00-036-0420) corresponding to the (-111),

(111), and (002) reflections of ZrO2 were observed for all ZrO2-supported catalysts. A distinct

(200) peak of CoMo2S4 at 2ϴ = 30.3° (PDF 00-023-0192) was observed in the XRD pattern of the

4 ML catalyst, indicating the formation of CoMo-S multilayer with a high degree of order. The

o o weak peaks of MoO3 at 2ϴ = 27.3 (021) (PDF 00-005-0508) and CoMoO4 at 2ϴ = 32.3 (-222)

(PDF 00-025-1434) were observed in the XRD pattern of the 4 ML catalyst, but the peaks were absent in the XRD patterns of other n ML CoMo-S catalysts.

151

2 4 2 MoS MoO CoMoO 4 ML CoMo-S

2 ML CoMo-S

1 ML CoMo-S

Intensity Intensity (a.u.) 0.5 ML CoMo-S

ZrO2

10 20 30 40 2 

Figure 7.2. XRD patterns of ZrO2, 0.5 ML, 1 ML, 2 ML, and 4 ML CoMo-S/ZrO2.

7.3.2. Characterization of Co/Mo = n CoMo-S/ZrO2 catalysts

The weight loading of Co and Mo determined by ICP-MS confirmed that the Co and Mo content met the theoretical monolayer coverage. The catalysts were named Co/Mo = n catalysts for convenience, where n stands for the atomic Co/Mo ratio. The BET surface areas (Table 7.2) indicated that Co/Mo = n catalysts had similar surface areas after catalyst preparation regardless of atomic Co/Mo ratio. The extent of sulfidation of Co/Mo = 0 catalysts was lowest among the catalysts, and almost same S/Mo ratio in the catalysts above Co/Mo = 0.1.

Table 7.2. Co (wt. %), Mo (wt. %), atomic S/Mo ratios, and BET surface areas of freshly sulfided Co/Mo = n catalysts. Co/Mo = Co/Mo = Co/Mo = Mo-S 0.1 0.3 1.5 Coa (wt. %) - 0.3 0.7 2.4 Moa (wt. %) 4.8 3.8 2.8 2.5 BET (m2/g) 49.6 47.8 46.5 48.2 S/Mob 1.5 1.8 1.8 1.7 a. Co and Mo wt. % analyzed by ICP-MS b. S/Mo determined by XPS analysis

152

Figure 7.3 shows the TEM images of Co/Mo = n catalysts at monolayer coverage. The inset of Figure 7.3a (Co/Mo = 0 (Mo-S) catalyst) showed dark spots (black circles) indicating Mo-

S species and blurry edge layers (black arrows) corresponding to Mo-S layer on the ZrO2 support.

Small dark dots (white arrows) corresponding to CoMo-S species and blurry edge layers (white circles) corresponding to the CoMo-S layer on the ZrO2 support were observed in TEM images of all CoMo-S catalysts (Co/Mo = 0.1, 0.3, and 1.5). The dark dots in the CoMo-S catalysts (Figure

7.3b) were smaller than the dark dots in Mo-S catalyst (Figure 7.3a), suggesting the formation of small-sized CoMo-S species in the CoMo-S catalysts as compared to the Mo-S species in the Mo-

S catalyst. This is attributed to the Co promoter effect to enhance the dispersion of Mo-S species after Co addition [67]. The each inset of TEM images (Figure 7.3b, c, d) showed the more distinct lattice fringe of ZrO2 than that of Mo-S catalyst, while less distinct than that of 0.5 ML catalyst

(Figure 7.1a). In addition, the lattice fringe of CoMo-S multilayer was not observed in the TEM images of CoMo = n catalysts, suggesting that CoMo-S species partially covered ZrO2 surface at monolayer coverage.

153

Figure 7.3. TEM images of fresh sulfided (a) Co/Mo = 0 (Mo-S), (b) Co/Mo=0.1, (c) Co/Mo=0.3, and (d) Co/Mo=1.5 CoMo-S/ZrO2 catalysts (black arrow: Mo-S layer, black circle: dark spots (Mo-S species), white arrow: small dots (CoMo-S species), and white circle: CoMo-S layer).

No significant Co or Mo crystal structures were observed in the XRD patterns of all the

Co/Mo = n catalysts, whereas All Co/Mo = n catalysts showed only monoclinic ZrO2 characteristic

XRD peaks (not shown here). This suggests that the Co/Mo = n catalysts have the CoMo-S species at theoretical monolayer coverage without any significant crystallized phase, which should be observed in multilayer CoMo-S catalysts.

7.3.3. WGS activity of n ML CoMo-S/ZrO2 catalysts

Figure 7.4 shows the WGS activity over n ML CoMo-S catalysts at 150 ~ 450°C and GHSV

-1 = 35,000 h in 7,000 ppm H2S-containing feed of 10 vol. % CO and 20 vol. % H2O in helium. The

154

1 ML, 2 ML, and 4 ML CoMo-S catalysts showed similar CO conversion below 200°C. Most catalysts showed similar WGS activity at 450°C, except for the 4 ML catalyst. The lowest CO conversion at 450°C for the 4 ML catalyst could be explained by the lowest BET surface area of this catalyst among all n ML catalysts. The CO conversion over the 1 ML catalyst was higher than that of other catalysts at 225~400°C. This suggests that the CO conversion of n ML CoMo catalysts increased with the surface coverage up to theoretical monolayer coverage.

100 Equlibrium Curve 0.5 ML CoMo-S 80 1 ML CoMo-S 2 ML CoMo-S 4 ML CoMo-S 60

40 COConversion (%) 20

-1 GHSV 35,000 h , 7,000 ppm H2S 0 150 200 250 300 350 400 450 o Temperature ( C)

Figure 7.4. CO conversion during WGS reaction over n ML CoMo-S/ZrO2 catalysts (Feed: 10 -1 vol.% CO, 20 vol.% H2O, and 7,000 ppm H2S in He at GHSV = 35,000 h ).

Figure 7.5 shows the Raman spectra of the n ML CoMo-S/ZrO2 catalysts. The peaks at 378 and 402 cm-1 corresponding to the Mo=S stretching vibration were present in all spectra [68]. The intensity of the Mo=S stretch in the n ML CoMo-S catalysts was proportional to the number of

CoMo-S layers, indicating the formation of CoMo-S species was increased and crystallized. On the other hand, the lower intensity of Mo=S stretch in 0.5 ML and 1 ML catalysts indicates the poor crystallinity of the CoMo-S species and the formation of highly dispersed CoMo-S species in the catalysts at (sub)monolayer coverage as compared to that in the multilayer CoMo-S catalysts.

155

The broad peak at 750 cm-1 and strong peak at 996 cm-1 present in 2 ML and 4 ML catalysts can be attributed to Zr-O-Mo and Mo=O species, respectively [38], which were absent in other CoMo catalysts at (sub)monolayer CoMo-O coverage. The strong peaks at 816, 872, and 930 cm-1 corresponding to CoMoO4 were observed in the 4 ML CoMo-S catalyst [69,70], while only a broad

CoMo oxide peak was observed in other CoMo-S catalysts. The intensity of the peaks corresponding to ZrO2 decreased with the increasing number of CoMo-S layers, while the intensity of the peaks corresponding to CoMo-S and CoMo-O species increased, suggesting that the CoMo-

S species in n ML CoMo-S catalysts covered the ZrO2 support. The Raman spectra further suggested that the subsurface CoMo-O was present as the MoO3, Zr-O-Mo species, or CoMoO4 in multilayer CoMo-S catalyst, which are not expected to be active for WGS reaction [20].

MoS 488 nm 2 930

CoMoO4 4 ML CoMo-S x 8 816 MoO3 872 996

2 ML CoMo-S x 2 Zr-O-Mo 750 996

CoMoO4 930 1 ML CoMo-S Intensity (a.u.) Intensity

Co3O4 672 0.5 ML CoMo-S

x 4 ZrO2

200 400 600 800 1000 1200 -1 Wave number (cm )

-1 Figure 7.5. Raman spectra for n ML CoMo-S/ZrO2 catalysts collected at 1.2 cm step size.

Figure 7.6 shows the Raman spectra of n ML CoMo-S/ZrO2 catalysts collected for a step

-1 size of 0.2 cm in order to study in-plane vibration (E2g) and out-of-plane vibration (A1g) of the

CoMo-S layer as a function of n in n ML catalysts. The van der Waals forces between the

156 neighboring CoMo-S layers could influence the strain of the out-of-plane vibration mode (A1g)

[68]. The E2g mode corresponds to the in-plane vibration, that could be influenced by the incorporation of the promoters [68]. Stacked Mo-S layers were reported to show the blue shift of

A1g [68,71-73]. Figure 7.6 shows that the position of the A1g mode shifted toward higher wavenumber with increasing CoMo-S layers, indicating that the Mo=S vibration in the thicker

CoMo-S layers was subject to greater van der Waals forces than that in the (sub)monolayer CoMo-

-1 S. The E2g mode of n ML CoMo-S/ZrO2 catalysts was almost unchanged (379.0~379.8 cm ), suggesting similar in-plane strain due to the similar Co/Mo composition in the CoMo-S layer.

-1 Figure 7.6. Raman spectra for n ML CoMo-S/ZrO2 catalysts collected at 0.2 cm step size.

157

Figure 7.7 shows the H2-TPR profiles of n ML CoMo-S/ZrO2 catalysts after pre-sulfidation.

The low-temperature region (160~195ºC) corresponds to the reduction of weakly bonded surface sulfur atoms in CoMo-S species, the intermediate-temperature region (217~232ºC) corresponds to the reduction of weakly bonded surface sulfur atoms in Mo-S species [74-76], and the high- temperature region (275~293ºC) corresponds to the reduction of the residual MoO3 to MoO2 [77].

The trend of reduction temperature of Mo-S in supported CoMo catalysts follows the order: 4 ML

(218ºC) ≤ 2 ML (217ºC) < 1 ML (227ºC) < 0.5 ML (232ºC), indicating that the reducibility of weakly bonded sulfur in Mo-S was enhanced by increasing the number of CoMo-S layer. This result agreed with the trend of the reduction temperature of CoMo-S species (4 ML: 160ºC < 2

ML: 172ºC < 1 ML: 184ºC < 0.5 ML: 195ºC) and correlated with the extent of sulfidation (S/Mo ratio, 4 ML: 1.8 > 2 ML: 1.7 ≥ 1 ML: 1.7 > 0.5 ML: 1.1) from XPS analysis (Table 7.1).

218 4 ML CoMo-S

160 284

217 2 ML CoMo-S

172 275

227 1 ML CoMo-S

184

Consumption/ A.u. 277 2 H 0.5 ML CoMo-S 232

293 195

100 200 300 400 500 600 o Temperature ( C)

Figure 7.7. H2 TPR analysis of 0.5 ML, 1 ML, 2 ML, and 4 ML CoMo-S/ZrO2.

158

The amount of surface sulfur that is weakly bonded to Mo species and CoMo species was estimated by the H2 consumption for the corresponding reduction peaks and summarized in Table

7.3. The largest amount of H2 consumption in Mo-S species and CoMo-S species was observed in the 1 ML CoMo-S catalyst, agreed with the highest WGS activity of 1 ML CoMo-S catalyst among the n ML CoMo-S catalysts. The amount of weakly bonded sulfur in 4 ML CoMo-S catalyst decreased by 25% as compared to that of 1 ML CoMo-S catalyst. This could be attributed that the

BET surface area of 4 ML catalyst was 20 % lower than that of 1 ML catalyst, while their extent of sulfidation (atomic S/Mo ratio) estimated by XPS are similar.

Table 7.3. H2 consumption during H2-TPR analysis of 0.5 ML, 1 ML, 2 ML, and 4 ML CoMo- S/ZrO2. 0.5 ML 1 ML 2 ML 4 ML CoMo-S 7 14 17 11 (µmol/g-cat) Mo-S 25 84 64 60 (µmol/g-cat) MoO 2 22 33 18 13 (µmol/g-cat) 7.3.4. WGS activity of Co/Mo = n CoMo-S/ZrO2 catalysts

Figure 7.8 shows the WGS activity over Co/Mo = n catalysts and commercial CoMo

-1 catalyst at 150 ~ 450°C and GHSV = 35,000 h in 7,000 ppm H2S-containing feed of 10 vol. %

CO and 20 vol. % H2O in helium. All catalysts showed a light-off temperature of 150°C. The

Co/Mo = 1.5 and 0.3 catalysts showed higher CO conversion at 350 ~ 450°C than the other catalysts that was close to the equilibrium conversion. The Co/Mo = 0.1, 0.3, and 1.5 catalysts showed higher CO conversion below 350°C than unpromoted Mo-S catalyst. This indicated that

Co addition to Mo-S catalysts enhanced the WGS activity, and, moreover, the enhancement of CO conversion is manifested in the low-temperature range (250 ~ 350°C). The CO conversion of

Co/Mo = n catalysts at 350°C increased with Co content up to Co/Mo = 0.3 and then plateaued. It

159 should be noted that cobalt-rich CoMo-S/ZrO2 catalyst (Co/Mo = 1.5) was investigated with respect to the H2S dependence of the CoMo-S catalytic system since the Co/Mo = 0.3 catalyst was already highly active in the low-temperatures and almost reached equilibrium conversion at 400°C.

100 Equilibrium Curve Co/Mo = 0 (Mo-S) 80 Co/Mo = 0.1 Co/Mo = 0.3 Co/Mo = 1.5 60 Commercial catalyst

40 CO Conversion CO (%) 20

-1 GHSV 35,000 h , 7,000 ppm H2S 0 150 200 250 300 350 400 450 o Temperature ( C)

Figure 7.8. CO conversion during WGS reaction over the Co/Mo = n catalysts, and commercial CoMo catalyst (Feed: 10 vol.% CO, 20 vol.% H2O, and 7,000 ppm H2S in He at GHSV = 35,000 h-1 (Co/Mo = 0 and commercial catalysts at GHSV = 39,000 h-1)).

Figure 7.9 shows the Arrhenius plot of CoMo-S/ZrO2 (Co/Mo = n), which was determined below 13% CO conversion in the 50 mL/min feed containing the 7,000 ppm H2S, 10 vol. % CO and 20 vol. % H2O in helium in order to minimize the effect of mass and heat transfer. Our preliminary estimates of the Thiele modulus suggested that the tested CoMo/ZrO2 catalysts were not expected to suffer from intraparticle mass-transfer effects for particles sizes up to 1.5 mm. The

Arrhenius plot showed good linearity in the processed plots (Figure 7.9), and exhibited good agreement with previously reported results of Mo-based WGS catalysts (40 ~ 86 kJ/mol)

[76,78,79].

160

1 ML Mo-S -13 1 ML Co/Mo = 0.1 1 ML Co/Mo = 0.3 1 ML Co/Mo = 1.5 ]) -1 s

-1 -14 mol 2 cat

-15 ln (k ln [m (k

-16

0.0019 0.0020 0.0021 0.0022 0.0023 0.0024 1/T [1/K]

Figure 7.9. Arrhenius plots of CO conversion rate in WGS reaction observed over Co/Mo = 0, 0.1, 0.3, and 1.5 CoMo-S/ZrO2 catalysts.

Table 7.4 summarizes the activation energies (Ea) and turnover frequencies (TOF) of

Co/Mo = n CoMo-S/ZrO2 catalysts at 250°C. The TOFs of CoMo-S catalysts were higher than that of unpromoted Mo-S (Table 7.4), suggesting that Co addition could enhance the WGS activity at 250°C, corresponding to the reported trend of the enhanced intrinsic activity of CoMo-S in

Al2O3 supported CoMo catalysts at low temperature [42]. The enhanced WGS activity of in CoMo-

S catalysts after Co addition could be attributed to the lowered the H2O dissociation energy barrier on S edge sites of CoMo-S species as compared to the unpromoted Mo-S species [43,48,80]. This is beneficial for WGS reaction since the H2O dissociation (H2O → OH + H) is the rate determining step of overall WGS reaction. Moreover, the COOH-mediate associative mechanism has been proposed for WGS reaction of the CoMo-S [43,48], which could be a suitable mechanism for the

CoMo-S species at 250°C since the intermediates can be alive longer on the surface than high temperature reaction. The overall reaction barrier of the COOH-associated mechanism on the S edge of CoMo-S was lower than that of redox mechanism [48]. This suggested that CoMo-S

161 species can be more active than Mo-S species at low temperature since the associative mechanism was more dominant at low temperature region. Therefore, the formation of CoMo-S species in the catalysts is very important to explain the enhancement of CoMo-S catalysts compared to the Mo-

S catalyst.

The TOF is defined as the number of reacted molecules per unit time normalized by the number of catalytically active sites:

Turnover frequency = ×

Mo-S and CoMo-S species were assumed to be the active sites in the WGS reaction. The number of active sites was assumed to be equal to the number of surface Mo sites in a CoMo-S monolayer on ZrO2 support before pre-sulfidation. The number of reacted molecules was determined at WGS reaction at 250°C and the number of catalytic active sites was determined using the BET surface area of support and the surface density of Mo at theoretical monolayer coverage.

Table 7.4. Activation energies and TOFs of the Co/Mo = n catalysts.

Co/Mo = Co/Mo = Co/Mo = Mo-S 0.1 0.3 1.5

Ea (kJ/mol) 41.3 44.2 47.4 49.7

TOF (s-1) 0.0029 0.0057 0.0047 0.0043

Figure 7.10 shows the Raman spectra of the Co/Mo = n catalysts. The peaks at 378 and

402 cm-1 corresponding to the Mo=S stretch were present in all catalysts [68]. The intensity of the

Mo=S stretch of the CoMo-S catalysts was lower than that of the unpromoted Mo-S catalyst, which could be attributed to the poor crystallinity of CoMo-S. This suggested that the CoMo-S species is highly dispersed on ZrO2 support in CoMo-S catalysts as compared to the Mo-S species in unpromoted Mo-S catalysts. The presence of highly dispersed CoMo-S species agreed with the

162 small dark dots in TEM images proposed to be associated with the small-sized CoMo-S species

(Figure 7.3b). The less intense peaks corresponding to the monoclinic ZrO2 [38] were observed in the Raman spectra of the Co/Mo = n catalysts suggesting that the CoMo-S species covered the

-1 surface of ZrO2 support. The weak peak at 996 cm corresponding to the Mo=O stretch was also observed in the spectra of the CoMo-S catalysts (Co/Mo = 0.1, 0.3, and 1.5). The weak shoulders

-1 at 872 and 930 cm corresponding to the CoMoO4 were observed at higher Co/Mo ratios (Co/Mo

= 0.3 and 1.5) [69,70]. However, the peak intensity of Mo or CoMo oxide are not as strong as that of CoMo-S multilayers in the CoMo-S/ZrO2 catalysts. These weak Raman spectra features observed in the spectra of Co/Mo=n catalysts suggested that the surface CoMo-O coverage in these catalysts was essentially CoMo-S monolayer without significant crystallization of Co/Mo oxide phase.

488 nm MoS2

CoMoO4 930 Co/Mo = 1.5

MoO3 996Co/Mo = 0.3

996 Co/Mo = 0.1 Intensity (a.u.) Intensity

Co/Mo = 0 (only Mo) 996 x 4.0 ZrO2

200 400 600 800 1000 1200 -1 Wave number (cm )

Figure 7.10. Raman spectra for the Co/Mo = n catalysts collected at 1.2 cm-1 step size.

163

Figure 7.11 shows the Raman spectra of the Co/Mo = n catalysts collected at 0.2 cm-2 step

-1 size. All Co/Mo = n catalysts showed the similar position of the A1g mode at 400.6~401.4 cm , corresponding to the reported vibration mode of the monolayer of MoS2 [71-73], suggesting

Co/Mo = n catalysts have a Mo-S monolayer. The E2g mode of Mo=S stretch in Co/Mo = n catalysts was shifted from 378.2 cm-1 (Mo-S) to 379.0~379.2 cm-1 (CoMo-S), which could be attributed to the contracted CoMo-S lattice after Co introduction to Mo-S lattice [71]. The deconvoluted shoulder peaks at 374.6~375.0 cm-1 were assigned to the vibration mode of the Co-S species [81].

The intensity of the Co-S band in Co/Mo = 1.5 was stronger than that of Co/Mo = 0.1 and 0.3, suggesting the formation of Co-S species in cobalt-rich CoMo-S catalyst. The weak Raman band at 371.0~371.2 cm-1 could be associated with the Mo-O bending of residual Mo-O phase [82].

Figure 7.11. Raman spectra for the Co/Mo = n catalysts collected at 0.2 cm-1 step size.

164

Figure 7.12 shows the H2-TPR profiles of Co/Mo = n catalysts. The low-temperature region

(172~184ºC) corresponding to the reduction of weakly bonded surface sulfur in the CoMo-S species was observed in CoMo-S catalysts [74,75], while such low-temperature reduction peak was not observed for the Mo-S catalyst. All Co/Mo = n catalysts showed reduction peaks in the intermediate-temperature region (206~227ºC) corresponding to the reduction of weakly bonded surface sulfur in the unpromoted Mo-S species [74-76]. The reduction temperature of CoMo-S species was lower than that of Mo-S species, indicating that the surface sulfur is more weakly bonded to CoMo than Mo [83]. This finding agreed with the lower sulfur bonding energy in CoMo-

S species than in Mo-S species [84]. The H2 consumption in the CoMo-S reduction process increased with Co addition, reaching a plateau at Co/Mo = 0.3 (Table 7.5). The H2 consumption during the Mo-S reduction of the Co/Mo = 1.5 catalyst was greater than that of other catalysts, which could be attributed to the sulfur reduction in the Co-S species, which was not observed for the Co/Mo = 0.1 and 0.3 catalysts.

165

227 Co/Mo = 1.5

184 277

Co/Mo = 0.3 214 182

288

214 Co/Mo = 0.1

172 Consumption A.u./

2 285 H

206 Co/Mo = 0 (Mo-S/ZrO2)

278

100 200 300 400 500 600 o Temperature ( C)

Figure 7.12. H2-TPR analysis of Co/Mo = 0, 0.1, 0.3, and 1.5 CoMo-S/ZrO2 catalysts.

Table 7.5. H2 consumption during H2-TPR analysis of Co/Mo = 0, 0.1, 0.3, and 1.5 CoMo-S/ZrO2 catalysts. Mo-S Co/Mo = 0.1 Co/Mo = 0.3 Co/Mo = 1.5 CoMo-S - 6 13 14 (µmol/g-cat) Mo-S 79 36 36 84 (µmol/g-cat) MoO 2 15 14 12 33 (µmol/g-cat)

The XPS analysis was performed to investigate the Co promoter effect on the WGS reaction over Co/Mo = n catalysts (Figure 7.13). The binding energy (BE) position and the relative areas of corresponding XPS peaks are summarized in Table 7.6. The Mo 3d doublet at 232.2 ± 0.3 eV and 235.4 ± 0.3 eV was observed for the CoMo-S catalysts (Co/Mo=0.1, 0.3, and 0.5), which was not observed for the Mo-S catalyst (Co/Mo = 0). This suggested that the Mo doublet was associated to the CoMo-S species. The relative area of Mo doublet corresponding to CoMo-S

166 species increased with Co content up to Co/Mo = 0.3 and plateaued at Co/Mo = 1.5 (Table 7.6), suggesting that the formation of CoMo-S species was saturated at Co/Mo = 0.3 and excess Co could be present as Co-S species or CoSO4.

IV MoV Mo Co/Mo = 1.5 CoMo-S

S 2s

V IV Co/Mo = 0.3 CoMo-S Mo Mo

S 2s

V MoIV Co/Mo = 0.1 CoMo-S Mo Intensity (a.u.)Intensity

S 2s

IV Co/Mo = 0, Mo-S/ZrO Mo 2 MoV

S 2s

238 236 234 232 230 228 226 224 Binding energy (eV)

Figure 7.13. XPS-spectra Mo 3d region of Co/Mo = 0, 0.1, 0.3, and 1.5 CoMo-S/ZrO2 catalysts.

The characteristic BE at 228.6 ± 0.2 eV and 231.8 ± 0.2 eV corresponding to Mo4+ (sulfide)

4+ in Co/Mo = n catalysts were observed in all Co/Mo = n catalysts. The BE of Mo (MoS2) was almost unchanged regardless of Co content, suggesting Co was not incorporated into MoS2. The doublets at 230.0 ± 0.5 eV and 233.3 ± 0.4 eV corresponding to the Mo5+ (oxysulfide) are shown in Figure 7.13 and Table 7.6. The atomic Mo5+/Mo4+ ratios of CoMo-S catalysts (Co/Mo = 1 :

0.51, Co/Mo = 0.0: 0.43, and Co/Mo = 1.5: 0.40) were higher than that of Mo-S catalyst (0.27), suggesting Co addition to Mo-S catalysts facilitated the formation of oxysulfide species, which were widely proposed as active sites of Mo-based catalysts [20,40]. The separate peak at 226.1 ±

0.2 eV was observed in all the Co/Mo = n catalysts as well, which is the characteristic BE of S 2s.

167

Table 7.6. XPS results of various Co/Mo atomic ratio CoMo-S/ZrO2 (Co/Mo = 0, 0.1, 0.3, and 1.5). Samples Mo-S Co/Mo = 0.1 Co/Mo = 0.3 Co/Mo = 1.5 BE BE BE BE Area. % Area. % Area. % Area. % (eV) (eV) (eV) (eV) Mo IV 228.7 79% 228.9 53% 228.8 46% 228.3 47% (area %) Mo V 229.5 21% 230.0 27% 230.5 20% 229.7 19% (area %) CoMo-S 0% 231.8 21% 232.5 34% 232.1 34% (area %) S2- 161.5 81% 161.7 59% 161.6 54% 161.3 45% (area %) S 2- 2 162.4 19% 163.7 24% 163.5 22% 163.3 12% (area %) SO 2- 4 0% 168.5 17% 168.0 23% 168.1 41% (area %)

The S 2p XPS spectra of the Co/Mo = n catalysts are shown in Figure 7.14. The doublet at

161.5 ± 0.2 eV and 162.7 ± 0.2 eV were associated with S2- (sulfide) [85]. The BE of S2- in the

Co/Mo=n catalysts was almost unchanged (161.3 ~ 161.7 eV) regardless of Co content, suggesting

Co addition was not influenced the S2- (sulfide). The doublets at 163.1 ± 0.7 eV and 164.3 ± 0.7

2- eV were observed in all Co/Mo = n catalysts [86], which may be attributed to the S2 in oxysulfide

2- and CoMo-S species. The BE of S2 in the CoMo-S catalysts experienced a considerable shift (0.9

~ 1.3 eV) as compared to that of Mo-S catalyst (162.4 eV). This suggests that Co addition mostly influenced the oxysulfide and CoMo-S species.

168

2- S2- Co/Mo = 1.5 SO4 2- S2

2- S Co/Mo = 0.3 2- S 2- SO4 2

S2- Co/Mo = 0.1 2- Intensity (a.u.) Intensity SO 2- 4 S2

Co/Mo = 0, Mo-S/ZrO S2- 2

2- S2

174 172 170 168 166 164 162 160 158 Binding energy (eV)

Figure 7.14. XPS-spectra S 2p region of Co/Mo = 0, 0.1, 0.3, and 1.5 CoMo-S/ZrO2 catalysts.

2- The BE of sulfate (SO4 ) is identified at 168.0 eV ~ 168.5 eV in the CoMo-S catalysts,

2- which agreed with the characteristic BE of sulfate in previous reports [85,87,88]. The sulfate (SO4 ) species may originate from the of CoSO4 in which the contents of sulfate species increased with

2- Co addition. The relative area ratio of sulfate (SO4 ) in the CoMo-S catalysts increased with Co content (Table 7.6), whereas no sulfate peaks are observed in the Mo-S catalyst. The formation of sulfate could be attributed to the separated Co species which did not incorporate the MoS2. Some wter can be produced during sulfidation of MoO3, and the generated water may react with H2S and

Co species to form CoSO4 [89].

As the previously reported studies, the formation of CoS2, Co9S8, CoSO4, and Mo(SO4)2 were observed in XRD, XPS, and Raman analysis of CoMo-S/Al2O3 catalysts above Co/Mo > 0.5 in Al2O3 supported CoMo-S catalysts [44,90]. Therefore, when the Co content in CoMo-S/ZrO2

169 catalysts exceeds the Co/Mo > 0.3, the excess cobalt atoms may form a Co9S8 or CoS2 instead of forming CoMo-S species in CoMo-S/ZrO2 catalysts. The Co-S species are not responsible for

WGS activity [20,91], accounting for the saturated WGS activity above Co/Mo > 0.3 catalysts at low temperature.

7.3.5. Long-term stability and H2S-dependence of CoMo-S/ZrO2 WGS catalysts

Long-term stability over the WGS activity test was performed in H2S-containing feed

(Figure 7.15). The Co/Mo = 0.3 catalyst showed stable activity over four weeks of WGS reaction in a 7,000 ppm H2S-containing feed stream. The CO conversion of the Co/Mo = 1.5 catalyst was stable for first 12 days of WGS reaction, then begun to slowly decrease. The commercial CoMo catalyst showed stable WGS activity during 12 days of WGS reaction, decreased only 3 % of CO conversion. The Co/Mo = 0 (Mo-S) catalyst gradually lost its WGS activity from the beginning of

WGS reaction. These WGS activity results suggest that the Co/Mo = 0.3 catalyst at monolayer coverage was the most active among the catalysts investigated in this study and maintained their activity at 350°C in H2S-containing feed.

170

Figure 7.15. CO conversion during WGS reaction over Co/Mo = n catalysts, and commercial CoMo catalyst (Feed: 10 vol.% CO, 20 vol.% H2O, and 7,000 ppm H2S in He at GHSV = 35,000 h-1 and 350°C).

Figure 7.16 shows the CO conversion over (a) n ML and (b) Co/Mo = n catalysts in both

H2S-containing and H2S-free feed. These tests were conducted after the temperature-profile tests of these catalysts shown in Figure 7.4 and Figure 7.8. The CO conversion over the 1 ML CoMo-S catalyst was reduced by 40% with the absence of H2S in the feed (7 ~ 28 hours), while the CO conversion of the 0.5 ML, 2 ML, and 4 ML CoMo-S/ZrO2 catalysts decreased by 25%, 8%, and

18%, respectively. The WGS activity of 2 ML and 4 ML CoMo-S catalysts decreased slowly as compared to that of 0.5 ML and 1 ML CoMo-S catalysts in H2S-free feed. This could be attributed to the sulfur diffusion from the sub-surface to the surface layer during the WGS reaction in the

H2S-free feed since the S atoms are reported to exhibit significant interlayer mobility in multilayer catalysts as compared to monolayer catalysts [92].

171

Figure 7.16. CO conversion over (a) n ML CoMo-S/ZrO2 and (b) Co/Mo = n catalysts during WGS reaction employing 7,000 ppm H2S-containing and H2S-free feed (Feed: 10 vol.% CO, and -1 20 vol.% H2O in He at GHSV = 35,000 h (Co/Mo = 0 and commercial catalysts at GHSV = -1 39,000 h ) and 350°C).

The Co/Mo = 0.3 and 1.5 catalysts showed 8~26 % higher CO conversion than the Co/Mo

= 0, Co/Mo = 0.1, and commercial sour gas shift catalysts in 7,000 ppm H2S-containing feed. The decrease of CO conversion over the CoMo-S catalysts during 7~ 28 hours of WGS reaction in

H2S-free feed stream followed the order: Co/Mo = 1.5 (40% decrease) > Co/Mo = 0.3 (14%) ≥

Co/Mo = 0.1 (10%) > Co/Mo = 0 (6%). This indicated that the H2S-dependence of the catalysts increased with Co content over the WGS reaction at 350°C. The increased H2S dependence of

CoMo-S catalyst can be explained by the weakened sulfur bond in CoMo-S species after Co addition to Mo-S.

Figure 7.17 shows the CO conversion over the Co/Mo = 0.3 and Co/Mo = 1.5 catalysts during 75 hours of two cycles WGS reaction in 7,000 ppm H2S-containing or H2S-free feed. The

CO conversion over Co/Mo=1.5 catalysts in H2S-free feed was more significantly decreased than that over the Co/Mo = 0.3 catalyst, although both catalysts recovered most of their WGS activity within two hours of WGS reaction in 7,000 ppm H2S-containing feed. Table 7.7 summarizes the atomic S/Mo ratio estimated by XPS analysis over fresh and used catalysts. The used catalysts 172 were collected after WGS reaction in H2S-free feed. The atomic S/Mo ratio of Co/Mo = 0.3 catalyst decreased by 28 % during the WGS reaction in H2S-free feed, whereas that of Co/Mo=1.5 catalysts decreased by 48 %, suggesting that the decrease of CO conversion in H2S-free feed mostly resulted from the loss of sulfur in the catalysts. The lost sulfur was mainly associated with the weakened sulfur bond in CoMo-S species since the decrease of CO conversion over the unpromoted Mo-S catalyst was minor as compared to the CoMo-S catalysts.

100

40 H2S 7,000 ppm CO Conversion (%) CO H2S-free H2S-free 20 Co/Mo = 0.3 Co/Mo = 1.5 Equilibrium 350 oC, GHSV 35,000 h-1 0 0 20 40 60 80 Time (hours)

Figure 7.17. CO conversion over the Co/Mo = 0.3 and Co/Mo = 1.5 catalysts during 75 hours of WGS reaction employing 7,000 ppm H2S-containing and H2S-free feed (Feed: 10 vol.% CO, and -1 20 vol.% H2O in He at GHSV = 35,000 h and 350°C). Table 7.7. Atomic S/Mo ratio for fresh and used Co/Mo = n catalysts estimated by XPS analysis.

Fresh Used Fresh Used Co/Mo = 0.3 Co/Mo = 0.3 Co/Mo = 1.5 Co/Mo = 1.5 S/Mo 1.80 1.29 1.66 0.87

As we discussed in TPR and sulfur-dependence test, the sulfur bond of CoMo-S species could be weakened with Co addition. The CO conversion of Co/Mo = 1.5 catalysts decreased by

7 % after two cycle of WGS reaction in H2S containing and H2S-free feed, while the CO conversion of Co/Mo=0.3 catalyst was completely recovered. In addition to H2S-dependence of CoMo-S

173 species, this further suggested that the CoMo-S species above Co/Mo>0.3 were easy to lose sulfur and difficult to recover completely their lost sulfur in the H2S containing feed. This could explain that CO conversion of Co/Mo = 1.5 catalysts decreased by 10 % during 12 days of reaction in

7,000 ppm H2S containing feed. However, the sulfur bond of CoMo-S species in the Co/Mo=0.3 catalyst was stable to maintain their WGS activity for 4 weeks of reaction. This finding allows us to conclude that the Co/Mo = 0.3 is optimal atomic Co/Mo ratio. The Co/Mo=0.3 catalyst at monolayer coverage was not only the most active among the CoMo-S/ZrO2 catalysts investigated in this study, but also most stable for WGS reaction in H2S containing feed at 350°C.

7.4. Conclusions

One series of CoMo catalysts supported on ZrO2 were synthesized with different CoMo-O surface coverage ranging from 0.5 to 4 ML controlled by modifying the content of Co and Mo.

Highly dispersed CoMo-S species at monolayer coverage and increased ordering of CoMo-S layers at multilayer coverage were observed in TEM, XRD, and Raman spectroscopy of n ML

CoMo-S catalysts. The WGS activity of the n ML catalysts increased with the surface coverage up to monolayer and then decreased at 4 ML catalyst. The atomic S/Mo ratio and the H2 reduction process of the fresh n ML catalysts were investigated by XPS and H2-TPR, respectively, in order to examine the extent of sulfidation and the amount of weakly bonded surface sulfur, which correlated with the WGS activity of the catalysts. As a result, the WGS activity was optimized for n ML CoMo-S/ZrO2 catalyst and the CO conversion of the CoMo-S catalyst at monolayer coverage was found to reach almost the equilibrium conversion at 400°C and 35,000 h-1 GHSV.

Another series of CoMo-S/ZrO2 catalysts were prepared by tuning atomic Co/Mo ratio, while their surface coverages were maintained as a monolayer. Highly dispersed Mo-S species and CoMo-S species were observed for the Co/Mo = n catalysts, confirming the formation of

174

CoMo-S monolayer. The Co addition to the Mo-S facilitated the formation of CoMo-S species on

ZrO2 support, which increased with Co addition up to Co/Mo = 0.3. The excess Co above Co/Mo

= 0.3 in the CoMo-S catalysts was present as Co-S species and CoSO4. The CO conversion over

Co/Mo = n catalysts at 350°C and 35,000 h-1 GHSV also increased with Co content up to Co/Mo

= 0.3 and then plateaued at Co/Mo=1.5, suggesting the improvement of WGS activity was mostly attributed to the increased formation of CoMo-S species. However, the CO conversion over the

CoMo-S catalysts with excess Co content was highly dependent on H2S presence in the feed, which could result from the weak sulfur bond in the CoMo-S species. Therefore, the CoMo-S catalyst at the optimal Co/Mo = 0.3 containing CoMo-S monolayer is the most active (above 85% CO conversion) and stable (at least 4 weeks) WGS catalyst at 350°C and 35,000 h-1 GHSV in 7,000 ppm H2S-containing feed.

7.5. References

2. M. D. Paster, R. K. Ahluwalia, G. Berry, A. Elgowainy, S. Lasher, K. McKenney, M. Gardiner, "Hydrogen Storage Technology Options for Fuel Cell Vehicles: Well-to-Wheel Costs, Energy Efficiencies, and Greenhouse Gas Emissions," International Journal of Hydrogen Energy, 36, 2011, 14534-14551

3. A. Molino, G. Giordano, V. Motola, G. Fiorenza, F. Nanna, G. Braccio, "Electricity Production by Biomass Steam Gasification using a High Efficiency Technology and Low Environmental Impact," Fuel, 103, 2013, 179-192

4. W. Chen, M. Lin, T. L. Jiang, M. Chen, "Modeling and Simulation of Hydrogen Generation from High-Temperature and Low-Temperature Water Gas Shift Reactions," International Journal of Hydrogen Energy, 33, 2008, 6644-6656

5. I. Chorkendorff, J. W. Niemantsverdriet, "Heterogeneous Catalysis in Practice: Hydrogen," Concepts of Modern Catalysis and Kinetics, 2003, 301-309

175

6. P. Corbo, F. Migliardini, O. Veneri, "Hydrogen Fuel Cells for Road Vehicles," Springer-Verlag London, 2011, 245

7. Office of Fossil Energy. , Department of Energy, Hydrogen from coal program, 2008

8. V. S. Sikarwar, M. Zhao, P. Clough, J. Yao, X. Zhong, M. Z. Memon, N. Shah, E. J. Anthony, P. S. Fennell, "An Overview of Advances in Biomass Gasification," Energy and Environmental Science, 9, 2016, 2939-2977

9. A. A. Ahmad, N. A. Zawawi, F. H. Kasim, A. Inayat, A. Khasri, "Assessing the Gasification Performance of Biomass: A Review on Biomass Gasification Process Conditions, Optimization and Economic Evaluation," Renewable and Sustainable Energy Reviews, 53, 2016, 1333-1347

10. R. A. Efroymson, M. H. Langholtz, K. Johnson, et al. , 2016 billion-ton report: Advancing domestic resources for a thriving bioeconomy, volume 2: Environmental sustainability effects of select scenarios from volume 1, No. ORNL/TM-2016/727, 2017

11. S. Chianese, J. Loipersböck, M. Malits, R. Rauch, H. Hofbauer, A. Molino, D. Musmarra, "Hydrogen from the High Temperature Water Gas Shift Reaction with an Industrial Fe/Cr Catalyst using Biomass Gasification Tar Rich Synthesis Gas," Fuel Processing Technology, 132, 2015, 39- 48

12. J. Brau, M. Morandin, "Biomass-Based Hydrogen for Oil Refining: Integration and Performances of Two Gasification Concepts," International Journal of Hydrogen Energy, 39, 2014, 2531-2542

13. A. Molino, S. Chianese, D. Musmarra, "Biomass Gasification Technology: The State of the Art Overview," Journal of Energy Chemistry, 25, 2016, 10-25

14. M. H. Langholtz, B. J. Stokes, L. M. Eaton. , Oak Ridge National Laboratory, 2016 U.S. billion-ton report: Advancing domestic resources for a thriving bioeconomy. volume 1: Economic availability of feedstocks. ORNL/TM-2016/160, 2016

15. A. Tanksale, J. N. Beltramini, G. M. Lu, "A Review of Catalytic Hydrogen Production Processes from Biomass," Renewable and Sustainable Energy Reviews, 14, 2010, 166-182

16. E. Shayan, V. Zare, I. Mirzaee, "Hydrogen Production from Biomass Gasification; a Theoretical Comparison of using Different Gasification Agents," Energy Conversion and Management, 159, 2018, 30-41

17. S. K. Sansaniwal, K. Pal, M. A. Rosen, S. K. Tyagi, "Recent Advances in the Development of Biomass Gasification Technology: A Comprehensive Review," Renewable and Sustainable Energy Reviews, 72, 2017, 363-384

176

18. T.S. Pytlar, "Status of existing biomass gasification and pyrolysis facilities in north america," 18th Annual North American Waste-to-Energy Conference, NAWTEC18, 2010, 141-154

19. Y. Choi, H. G. Stenger, "Water Gas Shift Reaction Kinetics and Reactor Modeling for Fuel Cell Grade Hydrogen," Journal of Power Sources, 124, 2003, 432-439

20. C. Ratnasamy, J. Wagner, "Water Gas Shift Catalysis," Catalysis Reviews - Science and Engineering, 51, 2009, 325-440

21. M. Zhu, I. E. Wachs, "Iron-Based Catalysts for the High-Temperature Water-Gas Shift (HT- WGS) Reaction: A Review," ACS Catalysis, 6, 2016, 722-732

22. C. Yang, R. Li, C. Cui, S. Liu, Q. Qiu, Y. Ding, Y. Wu, B. Zhang, "Catalytic Hydroprocessing of Microalgae-Derived Biofuels: A Review," Green Chemistry, 18, 2016, 3684-3699

23. S. Cheah, D. L. Carpenter, K. A. Magrini-Bair, "Review of Mid- to High-Temperature Sulfur Sorbents for Desulfurization of Biomass- and Coal-Derived Syngas," Energy and Fuels, 23, 2009, 5291-5307

24. National Energy Technology Laboratory. , U.S. Department of Energy, Assessment of hydrogen production with CO2 capture volume 1: Baseline state-of-the-art plants, DOE/NETL- 2010/1434, 2010

25. D. S. Newsome, "The Water-Gas Shift Reaction," Catalysis Reviews, 21, 1980, 275-318

26. C. H. Bartholomew, R. J. Farrauto, "Fundamentals of Industrial Catalytic Processes: Second Edition," John Wiley & Sons, New Jersey, 2010

27. M. D. Argyle, C. H. Bartholomew, "Heterogeneous Catalyst Deactivation and Regeneration: A Review," Catalysts, 5, 2015, 145-269

28. G. Guan, M. Kaewpanha, X. Hao, A. Abudula, "Catalytic Steam Reforming of Biomass Tar: Prospects and Challenges," Renewable and Sustainable Energy Reviews, 58, 2016, 450-461

29. J. G. Speight, 6 - production of syngas, synfuel, bio-oils, and biogas from coal, biomass, and opportunity fuels., In J. Oakey (Ed.), "Fuel Flexible Energy Generation," Woodhead Publishing, Boston, 2016, 145-174

30. K. J. Andersson, M. Skov-Skjøth Rasmussen, P. E. Højlund Nielsen, "Industrial-Scale Gas Conditioning Including Topsøe Tar Reforming and Purification Downstream Biomass Gasifiers: An Overview and Recent Examples," Fuel, 203, 2017, 1026-1030

31. P. Hou, D. Meeker, H. Wise, "Kinetic Studies with a Sulfur-Tolerant Water Gas Shift Catalyst," Journal of Catalysis, 80, 1983, 280-285

177

32. Y. Zhang, G. Zhang, Y. Zhao, X. Li, Y. Sun, Y. Xu, "Ce-K-Promoted Co-Mo/Al2O3 Catalysts for the Water Gas Shift Reaction," International Journal of Hydrogen Energy, 37, 2012, 6363- 6371

33. D. Nikolova, R. Edreva-Kardjieva, T. Grozeva, "Water-Gas Shift Activity of K-Promoted (Ni)Mo/γ-Al2O3 Systems in Sulfur-Containing Feed," Reaction Kinetics, Mechanisms and Catalysis, 103, 2011, 71-86

34. H. Schweiger, P. Raybaud, H. Toulhoat, "Promoter Sensitive Shapes of Co(Ni)MoS Nanocatalysts in Sulfo-Reductive Conditions," Journal of Catalysis, 212, 2002, 33-38

35. E. Rodríguez-Castellón, A. Jiménez-López, D. Eliche-Quesada, "Nickel and Cobalt Promoted Tungsten and Molybdenum Sulfide Mesoporous Catalysts for Hydrodesulfurization," Fuel, 87, 2008, 1195-1206

36. A. Travert, C. Dujardin, F. Maugé, E. Veilly, S. Cristol, J. Paul, E. Payen, "CO Adsorption on CoMo and NiMo Sulfide Catalysts: A Combined IR and DFT Study," Journal of Physical Chemistry B, 110, 2006, 1261-1270

37. R. López Cordero, A. López Agudo, "Effect of Water Extraction on the Surface Properties of Mo/Al2O3 and NiMo/Al2O3 Hydrotreating Catalysts," Applied Catalysis A: General, 202, 2000, 23-35

38. M. Jia, P. Afanasiev, M. Vrinat, "The Influence of Preparation Method on the Properties of NiMo Sulfide Catalysts Supported on ZrO2," Applied Catalysis A: General, 278, 2005, 213-221

39. J. Mi, Y. Cao, F. Liu, X. Lin, C. Au, J. Chen, L. Jiang, "Ammonia-Free Synthesis of Mo/CoMgAl Catalysts with Excellent Activities in Water-Gas Shift Reaction," Applied Catalysis A: General, 575, 2019, 58-64

40. C. R. F. Lund, "Effect of Adding Co to MoS2/Al2O3 upon the Kinetics of the Water-Gas Shift," Industrial and Engineering Chemistry Research, 35, 1996, 3067-3073

41. B. Liu, L. Liu, Y. Chai, J. Zhao, C. Liu, "Essential Role of Promoter Co on the MoS2 Catalyst in Selective Hydrodesulfurization of FCC Gasoline," Ranliao Huaxue Xuebao/Journal of Fuel Chemistry and Technology, 46, 2018, 441-450

42. J. Chen, J. Zhang, J. Mi, E. D. Garcia, Y. Cao, L. Jiang, L. Oliviero, F. Maugé, "Hydrogen Production by Water-Gas Shift Reaction Over Co-Promoted MoS2/Al2O3 Catalyst: The Intrinsic Activities of Co-Promoted and Unprompted Sites," International Journal of Hydrogen Energy, 43, 2018, 7405-7410

43. Yan-Yan Chen, Mei Dong, Jianguo Wang, Haijun Jiao, "On the Role of a Cobalt Promoter in a Water-Gas-Shift Reaction on Co-MoS2," The Journal of Physical Chemistry C, 114, 2010, 16669-16676

178

44. A. D. Gandubert, E. Krebs, C. Legens, D. Costa, D. Guillaume, P. Raybaud, "Optimal Promoter Edge Decoration of CoMoS Catalysts: A Combined Theoretical and Experimental Study," Catalysis Today, 130, 2008, 149-159

45. R. Prins, V. H. J. De Beer, G. A. Somorjai, "Structure and Function of the Catalyst and the Promoter in Co - Mo Hydrodesulfurization Catalysts," Catalysis Reviews, 31, 1989, 1-41

46. Renan T. Figueiredo, André L. D. Ramos, Heloysa M. C. de Andrade, J. L. G. Fierro, "Effect of Low Steam/Carbon Ratio on Water Gas Shift Reaction," Catalysis Today, 107–108, 2005, 671- 675

47. B. Liu, Q. Zong, P. P. Edwards, F. Zou, X. Du, Z. Jiang, T. Xiao, H. Almegren, "Effect of Titania Addition on the Performance of CoMo/Al2O3 Sour Water Gas Shift Catalysts Under Lean Steam to Gas Ratio Conditions," Industrial and Engineering Chemistry Research, 51, 2012, 11674-11680

48. C. Zhang, B. Liu, L. Zhao, Q. Zong, J. Gao, Y. Wang, C. Xu, "Insights into Water–gas Shift Reaction Mechanisms Over MoS2 and Co-MoS2 Catalysts: A Density Functional Study," Reaction Kinetics, Mechanisms and Catalysis, 120, 2017, 833-844

49. R. Candia, O. Sørensen, J. Villadsen, N. Topsøe, B. S. Clausen, H. Topsøe, "Effect of Sulfiding Temperature on Activity and Structures of Co-Mo/Al2O3 Catalysts. Ii," Bulletin Des Sociétés Chimiques Belges, 93, 1984, 763-774

50. S. M. A. M. Bouwens, F. B. M. Vanzon, M. P. Vandijk, A. M. Vanderkraan, V. H. J. Debeer, J. A. R. Vanveen, D. C. Koningsberger, "On the Structural Differences between Alumina- Supported CoMoS Type I and Alumina-, Silica-, and Carbon-Supported CoMoS Type II Phases Studied by XAFS, MES, and XPS," Journal of Catalysis, 146, 1994, 375-393

51. C. Wivel, R. Candia, B. S. Clausen, S. Mørup, H. Topsøe, "On the Catalytic Significance of a CoMoS Phase in CoMo Al2O3 Hydrodesulfurization Catalysts: Combined in Situ Mössbauer Emission Spectroscopy and Activity Studies," Journal of Catalysis, 68, 1981, 453-463

52. Y. Okamoto, K. Ochiai, M. Kawano, K. Kobayashi, T. Kubota, "Effects of Support on the Activity of Co-Mo Sulfide Model Catalysts," Applied Catalysis A: General, 226, 2002, 115-127

53. L. Kaluža, D. Gulková, Z. Vít, M. Zdražil, "Effect of Support Type on the Magnitude of Synergism and Promotion in CoMo Sulphide Hydrodesulphurisation Catalyst," Applied Catalysis A: General, 324, 2007, 30-35

54. M. Shetty, K. Murugappan, T. Prasomsri, W. H. Green, Y. Román-Leshkov, "Reactivity and Stability Investigation of Supported Molybdenum Oxide Catalysts for the Hydrodeoxygenation (HDO) of M-Cresol," Journal of Catalysis, 331, 2015, 86-97

179

55. F. Besenbacher, M. Brorson, B. S. Clausen, S. Helveg, B. Hinnemann, J. Kibsgaard, J. V. Lauritsen, P. G. Moses, J. K. Nørskov, H. Topsøe, "Recent STM, DFT and HAADF-STEM Studies of Sulfide-Based Hydrotreating Catalysts: Insight into Mechanistic, Structural and Particle Size Effects," Catalysis Today, 130, 2008, 86-96

56. B. Seo, G. Y. Jung, Y. J. Sa, H. Y. Jeong, J. Y. Cheon, J. H. Lee, H. Y. Kim, J. C. Kim, H. S. Shin, S. K. Kwak, et al, "Monolayer-Precision Synthesis of Molybdenum Sulfide Nanoparticles and their Nanoscale Size Effects in the Hydrogen Evolution Reaction," ACS Nano, 9, 2015, 3728- 3739

57. H. Topsøe, "The Role of Co-Mo-S Type Structures in Hydrotreating Catalysts," Applied Catalysis A: General, 322, 2007, 3-8

58. M. Shetty, K. Murugappan, W. H. Green, Y. Román-Leshkov, "Structural Properties and Reactivity Trends of Molybdenum Oxide Catalysts Supported on Zirconia for the Hydrodeoxygenation of Anisole," ACS Sustainable Chemistry and Engineering, 5, 2017, 5293- 5301

59. D. Nikolova, R. Edreva-Kardjieva, G. Gouliev, T. Grozeva, P. Tzvetkov, "The State of (K)(Ni)Mo/γ-Al2O3 Catalysts After Water-Gas Shift Reaction in the Presence of Sulfur in the Feed: XPS and EPR Study," Applied Catalysis A: General, 297, 2006, 135-144

60. M. Łaniecki, M. Małecka-Grycz, F. Domka, "Water–gas Shift Reaction Over Sulfided Molybdenum Catalysts: I. Alumina, Titania and Zirconia-Supported Catalysts," Applied Catalysis A: General, 196, 2000, 293-303

61. K. V. R. Chary, K. R. Reddy, G. Kishan, J. W. Niemantsverdriet, G. Mestl, "Structure and Catalytic Properties of Molybdenum Oxide Catalysts Supported on Zirconia," Journal of Catalysis, 226, 2004, 283-291

62. S. Brunauer, P. H. Emmett, E. Teller, "Adsorption of Gases in Multimolecular Layers," Journal of the American Chemical Society, 60, 1938, 309-319

63. E. P. Barrett, L. G. Joyner, P. P. Halenda, "The Determination of Pore Volume and Area Distributions in Porous Substances. I. Computations from Nitrogen Isotherms," Journal of the American Chemical Society, 73, 1951, 373-380

64. X. Tong, Z. Zhang, F. Zeng, C. Fu, P. Ma, Y. Peng, H. Lu, Y. Liang, "Recursive Wavelet Peak Detection of Analytical Signals," Chromatographia, 79, 2016, 1247-1255

65. P. Cheung, H. Liu, E. Iglesia, "Kinetics and Mechanism of Dimethyl Ether Oxidation to Formaldehyde on Supported Molybdenum Oxide Domains," Journal of Physical Chemistry B, 108, 2004, 18650-18658

180

66. T. S. Sahu, S. Mitra, "Exfoliated MoS2 Sheets and Reduced Graphene Oxide-an Excellent and Fast Anode for Sodium-Ion Battery," Scientific Reports, 5, 2015,

67. X. Zhang, Q. Zhang, Y. Sun, J. Guo, "Hybrid Catalyst of MoS2-CoMo2S4 on Graphene for Robust Electrochemical Hydrogen Evolution," Fuel, 184, 2016, 559-564

68. K. Zhou, F. Withers, Y. Cao, S. Hu, G. Yu, C. Casiraghi, "Raman Modes of MoS2 used as Fingerprint of Van Der Waals Interactions in 2-D Crystal-Based Heterostructures," ACS Nano, 8, 2014, 9914-9924

69. T. Ono, N. Ogata, Y. Miyaryo, "Characteristic Features of Raman Band Shifts of Scheelite- Type Molybdate Catalysts Exchanged with the 18O Tracer Via Redox Reactions," Journal of Catalysis, 161, 1996, 78-86

70. B. Wang, Y. Yao, M. Jiang, Z. Li, X. Ma, S. Qin, Q. Sun, "Effect of Cobalt and its Adding Sequence on the Catalytic Performance of MoO3/Al2O3 Toward Sulfur-Resistant Methanation," Journal of Energy Chemistry, 23, 2014, 35-42

71. W. M. Parkin, A. Balan, L. Liang, P. M. Das, M. Lamparski, C. H. Naylor, J. A. Rodríguez- Manzo, A. T. C. Johnson, V. Meunier, M. Drndic, "Raman Shifts in Electron-Irradiated Monolayer MoS2," ACS Nano, 10, 2016, 4134-4142

72. M. Buscema, G. A. Steele, H. S. J. van der Zant, A. Castellanos-Gomez, "The Effect of the Substrate on the Raman and Photoluminescence Emission of Single-Layer MoS2," Nano Research, 7, 2014, 1-11

73. H. Li, Q. Zhang, C. C. R. Yap, B. K. Tay, T. H. T. Edwin, A. Olivier, D. Baillargeat, "From Bulk to Monolayer MoS2: Evolution of Raman Scattering," Advanced Functional Materials, 22, 2012, 1385-1390

74. B. Yoosuk, D. Tumnantong, P. Prasassarakich, "Unsupported MoS2 and CoMoS2 Catalysts for Hydrodeoxygenation of Phenol," Chemical Engineering Science, 79, 2012, 1-7

75. B. Yoosuk, J. H. Kim, C. Song, C. Ngamcharussrivichai, P. Prasassarakich, "Highly Active MoS2, CoMoS2 and NiMoS2 Unsupported Catalysts Prepared by Hydrothermal Synthesis for Hydrodesulfurization of 4,6-Dimethyldibenzothiophene," Catalysis Today, 130, 2008, 14-23

76. P. Afanasiev, "On the Interpretation of Temperature Programmed Reduction Patterns of Transition Metals Sulphides," Applied Catalysis A: General, 303, 2006, 110-115

77. H. Hu, W. Wang, Z. Liu, B. Wang, Z. Li, X. Ma, "Sulfur-Resistant CO Methanation to CH4 Over MoS2/ZrO2 Catalysts: Support Size Effect on Morphology and Performance of Mo Species," Catalysis Letters, 148, 2018, 2585-2595

181

78. S. S. Hla, G. J. Duffy, L. D. Morpeth, A. Cousins, D. G. Roberts, J. H. Edwards, "Investigation into the Performance of a Co-Mo Based Sour Shift Catalyst using Simulated Coal-Derived Syngases," International Journal of Hydrogen Energy, 36, 2011, 6638-6645

79. D. Ke, Y. Tao, Y. Li, X. Zhao, L. Zhang, J. Wang, S. Han, "Kinetics Study on Hydrolytic Dehydrogenation of Alkaline Sodium Borohydride Catalyzed by Mo-Modified Co-B Nanoparticles," International Journal of Hydrogen Energy, 40, 2015, 7308-7317

80. X. Shi, S. Wang, J. Hu, H. Wang, Y. Chen, Z. Qin, J. Wang, "Density Functional Theory Study on Water-Gas-Shift Reaction Over Molybdenum Disulfide," Applied Catalysis A: General, 365, 2009, 62-70

81. S. G. Lyapin, A. N. Utyuzh, A. E. Petrova, A. P. Novikov, T. A. Lograsso, S. M. Stishov, "Raman Studies of nearly Half-Metallic Ferromagnetic CoS2," Journal of Physics Condensed Matter, 26, 2014,

82. S. L. González-Cortés, T. Xiao, T. Lin, M. L. H. Green, "Influence of Double Promotion on HDS Catalysts Prepared by Urea-Matrix Combustion Synthesis," Applied Catalysis A: General, 302, 2006, 264-273

83. L. P. Nielsen, S. V. Christensen, H. Topsøe, B. S. Clausen, "Changes in Metal-Sulfur Bond Energy in Promoted and Unpromoted Molybdenum Catalysts," Catalysis Letters, 67, 2000, 81-85

84. L. S. Byskov, B. Hammer, J. K. Norskov, B. S. Clausen, H. Topsoøe, "Sulfur Bonding in MoS2 and Co-Mo-S Structures," Catalysis Letters, 47, 1997, 177-182

85. R. Bose, Z. Jin, S. Shin, S. Kim, S. Lee, Y. -. Min, "Co-Catalytic Effects of CoS2 on the Activity of the MoS2 Catalyst for Electrochemical Hydrogen Evolution," Langmuir, 33, 2017, 5628-5635

86. P. Li, Y. Chen, C. Zhang, B. Huang, X. Liu, T. Liu, Z. Jiang, C. Li, "Highly Selective Hydrodesulfurization of Gasoline on Unsupported Co-Mo Sulfide Catalysts: Effect of MoS2 Morphology," Applied Catalysis A: General, 533, 2017, 99-108

87. I. Alstrup, I. Chorkendorff, R. Candia, B. S. Clausen, H. Topsøe, "A Combined X-Ray Photoelectron and Mössbauer Emission Spectroscopy Study of the State of Cobalt in Sulfided, Supported, and Unsupported CoMo Catalysts," Journal of Catalysis, 77, 1982, 397-409

88. L. Hao, J. Yu, X. Xu, L. Yang, Z. Xing, Y. Dai, Y. Sun, J. Zou, "Nitrogen-Doped MoS2/Carbon as Highly Oxygen-Permeable and Stable Catalysts for Oxygen Reduction Reaction in Microbial Fuel Cells," Journal of Power Sources, 339, 2017, 68-79

89. L. R. Pahalagedara, A. S. Poyraz, W. Song, C. Kuo, M. N. Pahalagedara, Y. Meng, S. L. Suib, "Low Temperature Desulfurization of H2S: High Sorption Capacities by Mesoporous Cobalt Oxide Via Increased H2S Diffusion," Chemistry of Materials, 26, 2014, 6613-6621

182

90. X. Gao, Q. Xin, "Effect of Cobalt Oxide on Surface Structure of Alumina Supported Molybdena Catalysts Studied by in Situ Raman Spectroscopy," Catalysis Letters, 18, 1993, 409- 418

91. J. R. Mellor, R. G. Copperthwaite, N. J. Coville, "The Selective Influence of Sulfur on the Performance of Novel Cobalt-Based Water-Gas Shift Catalysts," Applied Catalysis A: General, 164, 1997, 69-79

92. H. Komsa, A.V. Krasheninnikov, "Native defects in bulk and monolayer MoS2 from first principles," Physical Review B, 91, 2015, 125304

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Chapter 8. Recommendations for Future Research

8.1. Recommendations for Future Research

As this dissertation demonstrated, ZrO2-supported CoMo-S catalysts showed higher WGS activity and less sulfur dependence than other Mo-based catalysts investigated. However, the model ZrO2 support has a lower BET surface area and higher bulk density than Al2O3 or MgO, which were widely employed in the commercial CoMo catalysts as support materials. The CoMo-

S/ZrO2 catalysts need larger amount of catalyst than the commercial CoMo catalyst to fill the volume of the commercial WGS reactor, offsetting the high WGS activity of CoMo-S/ZrO2.

Department of Energy has conducted a comprehensive economic analysis for hydrogen production using the commercial CoMo catalyst in coal gasification [1]. Although the report did not provide detail information about WGS reactor, we could obtain valuable hints about the required volume of WGS catalysts for initial fill and the operating conditions which were designed to apply the commercial CoMo catalysts. Based on their steam table and initial capital cost analysis of WGS reactor, the GHSV of commercial WGS reactor was 4,000~10,000 h-1, target CO conversion was ~80%, and the initial cost of catalyst to fill the WGS reactor was c.a. 3.1 M USD and the annual operation cost of replacing the deactivated WGS catalysts were c.a. 700 k USD. As

-1 we discussed in Ch 7, our best CoMo-S/ZrO2 catalyst showed >85% CO conversion at 35,000 h

GHSV, which were superior to the commercial catalysts. Therefore, we could employ our best catalyst in their cost analysis for WGS catalysts.

Reducing metal contents in the catalysts is critical to saving the cost of WGS catalyst since the active metals such as cobalt and molybdenum cost 20 ~ 300 times more than support materials

(Table 8.1). As we discussed in Chapters 5~7, the best CoMo-S/ZrO2 catalysts contain four times lower cobalt content and 50 % lower molybdenum content than commercial CoMo catalyst, while

184 the best catalyst showed higher activity and less sulfur-dependence at 350℃ and 35,000 h-1 GHSV.

Preliminary material cost estimates of catalyst production summarize in Table 8.2, indicating that the material cost for the production of the best Co/Mo=0.3 catalyst reduces only 15 % of the material cost as compared to the commercial catalysts when the cost was normalized by catalyst bulk volume. However, the material cost of the best catalyst could reduce to 50% of commercial

CoMo catalyst when the surface area of the catalysts was increased to that of Al2O3, while maintaining the WGS activity of ZrO2 supported CoMo-S catalysts. In short, we could save 1.5 M

USD of initial fill cost and 350 k USD of annual replacement cost at most when the catalyst was enhanced. Therefore, it is highly desirable to increase the surface area of the ZrO2 support, while maintaining the WGS activity and sulfur-dependence of ZrO2-supported CoMo-S catalysts in future research.

Table 8.1. The price of raw material to produce catalyst [7].

Name Price ($/kg) Remark Cobalt 84 Active metal Molybdenum 27 Active metal Alumina 0.3 Support material Magnesium 0.3 Support material Zircon 1.2 Support material

Table 8.2. The estimated material cost of catalyst production over CoMo-S catalysts and commercial catalyst. Co Mo Support Total Total Catalyst (wt.%) (wt.%) (wt.%) ($/kg.cat) ($/ft3.cat) Commercial 3.2 6.7 18/68 4.7 86 Co/Mo = 0.1 0.3 3.6 94.3 2.3 64

Co/Mo = 0.3 0.7 2.7 95.2 2.4 65

Co/Mo = 1.5 2.2 2.3 93.6 3.6 99 Mo only 0 4.7 93.0 2.4 65

185

We suggest coating ZrO2 on a base material which has a large surface area using promising methods, such as atomic layer deposition (ALD) [2] or incipient wetness impregnation [3]. Al2O3 would be a promising candidate for base metal oxide since typical Al2O3 has a surface area that is four times larger than that of ZrO2, while being half as expensive of material to purchase. Figure

8.1 shows the illustrations of the proposed methods to coat ZrO2 on Al2O3. ALD method is one of the thin-film fabrication techniques using chemical vapor deposition. A pulse of the precursor was introduced to react on the surface of base material at a time slowly one by one, and the thickness of layers was controlled by cycles of the deposition process. The advantages of ALD are controllable thickness and high purity of deposited material, whereas high purity requirement for substrate, slow deposition, and high cost are limitations of ALD method to coat. On the other hand, incipient wetness impregnation is one of the simplest and cheapest method to coat, while relatively low purity of deposited material, low reproducibility, and difficult to control thickness are limitations as compared to the ALD method. Faro et al. reported that 60~80% of the theoretical

2 ZrO2 monolayer (7.4 Zr atoms per nm ) on Al2O3 was prepared by the incipient wetness method using zirconium acetylacetonate and benzene [3]. ZrO2 was reported to deposit successfully on

Al2O3 using an atomic layer deposition method [4,5]. Onn et al. reported that CeO2-ALD on Al2O3- supported Pd catalyst showed improved WGS activity by maintaining a large surface area of Al2O3 and high catalytic activity of CeO2 [6].

186

Figure 8.1. Illustrations of proposed (a) atomic deposition method and (b) incipient impregnation method to coat ZrO2 on Al2O3.

8.2. References

1. National Energy Technology Laboratory, U.S. Department of Energy, Assessment of hydrogen production with CO2 capture volume 1: Baseline state-of-the-art plants, DOE/NETL-2010/1434, 2010

2. T. M. Onn, S. Zhang, L. Arroyo-Ramirez, Y. Chung, G. W. Graham, X. Pan, R. J. Gorte, "Improved Thermal Stability and Methane-Oxidation Activity of Pd/Al2O3 Catalysts by Atomic Layer Deposition of ZrO2," ACS Catalysis, 5, 2015, 5696-5701

3. A. C. Faro Jr., K. R. Souza, V. L. D. L. Camorim, M. B. Cardoso, "Zirconia-Alumina Mixing in Alumina-Supported Zirconia Prepared by Impregnation with Solutions of Zirconium Acetylacetonate," Physical Chemistry Chemical Physics, 5, 2003, 1932-1940

4. Y. Hu, H. Jiang, K. M. Lau, Q. Li, "Chemical Vapor Deposited Monolayer MoS2 Top-Gate MOSFET with Atomic-Layer-Deposited ZrO2 as Gate Dielectric," Semiconductor Science and Technology, 33, 2018

5. M. Tsai, P. Cheng, M. Lee, H. Lin, M. Chen, "Divergent Dielectric Characteristics in Cascaded High-K Gate Stacks with Reverse Gradient Bandgap Structures," Journal of Physics D: Applied Physics, 49, 2016

6. T. M. Onn, S. Dai, J. Chen, X. Pan, G. W. Graham, R. J. Gorte, "High-Surface Area Ceria- Zirconia Films Prepared by Atomic Layer Deposition," Catalysis Letters, 147, 2017, 1464-1470

7. U.S. Department of the Interior, U.S. Geological Survey. (2019). Commodity statistics and information. Retrieved June 21, 2018, from https://minerals.usgs.gov/minerals/pubs/commodity/

187

APPENDIX

A. Schematic diagrams of a continuous WGS activity test

The WGS activities of the catalysts were determined to employ a fixed-bed tubular quartz micro-reactor (0.55 cm ID) operated at atmospheric pressure using the feed containing 10 mol. %

CO and 20 mol. % H2O in helium. Water was injected into a flowing gas stream by a syringe pump and vaporized in the heated gas feed line before entering the reactor. A condenser filled with ice was installed at the reactor exit to remove water from the reaction products, prior to their analysis by a gas chromatography (HP-5890 II equipped with a thermal conductivity detector). The carbon balance agreed within ±5 mol. %. A sulfur-dependence test was conducted using the above- mentioned model CO/H2O feed in He that also contained H2S.

188

B. Schematic flow diagrams to synthesize the catalysts

B.1. Cu-Pd nanoparticles

B.2. CoMo nanoparticles

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B.3. Incipient Impregnation Method of supported Mo catalyts

C. Experimental Procedure of Characterization

C.1. Hydrogen temperature-programmed reduction

The H2-TPR studies were performed using a tubular fixed-bed reactor equipped with an on-line Stanford Research Systems QMS 200 gas analyzer. The catalysts were heated at 5°C/min from 35°C to 580°C in H2 (10 vol.%) balanced with Ar flowing at 10 mL/min (STP). The amount of H2 consumption was estimated by the mass spectrum during TPR analysis.

190

C.2. Transmission Electron Microscopy

Low-resolution TEM analysis was performed employing a Phillips CM20 electron microscope at a 200 kV accelerating voltage. The samples for low-resolution TEM were prepared by first dispersing nanoparticles in an ethanol/water solution and then allowing a drop of this suspension to evaporate on a copper grid coated with lacey carbon film. High-resolution transmission electron microscopy (HR-TEM) and scanning transmission electron microscopy

(STEM) imaging were performed on a FEI Tecnai F20 instrument equipped with a high angle annular dark field (HAADF) detector. The samples were prepared for HR-TEM by first dispersing nanoparticles in isopropanol and then allowing a drop of this suspension to evaporate on a lacey carbon film-coated gold grid.

191

C.3. Scanning Electron Microscopy & Energy Dispersive X-ray Specctroscopy

The energy dispersive X-ray spectroscopy (EDS) was also employed to analyze the composition of the catalytic particles while performing scanning electron microscopy analysis

(Phillips XL30 ESEM with EDS).

C.4. X-ray Photoelectron Spectroscopy

The XPS spectra of the catalysts were collected using a Kratos AXIS Ultra X-ray photoelectron spectrometer with a monochromatized Al Kα X-ray source operated at 12 kV and

10 mA. The XPS spectra for specific C 1s, O 1s, Cu 2p, Pd 3d, Mo 3d, S2p, and Zr 3d or Al 2p

192 regions were collected. Prior to the analysis of XPS data, the charging effect was corrected for based on the C 1s binding energy of 284.5 eV.

C.5. X-ray Diffraction Crystallography

Crystal structures were determined by powder X-ray diffraction (XRD) using a

(usually taken as 0.9); and β is the full width at half-maximum height.

193

C.6. Raman Spectroscopy

Raman spectra of the powder samples were collected for catalyst samples in a dried state in ambient air conditions using a Horiba T64000 Raman spectrometer (488 nm excitation from a

Coherent FreD 90C Ar+ laser with a spot size of ~ 2 µm). The beam power on the sample was 10 mW. Raman spectra of the catalysts collected for a step size of 1.2 cm-1 or 0.2 cm-1. The calibration of Raman spectroscopy was conducted prior to the analysis using a silicon chip. Each Raman spectrum was collected for 30 seconds to obtain an optimal signal to noise ratio.

194

C.7. CO chemisorption

CO chemisorption studies were performed using a Micromeritics ASAP2020 instrument.

The catalysts sample was first reduced by H2 at 450°C for 120 min, followed by evacuation at

450°C for 30 min. The samples were then cooled to 35°C under vacuum for 30 min followed by

CO chemisorption.

C.8. BET surface area

The N2 adsorption-desorption isotherms were measured at 77 K using a Micrometrics

TriStar porosimeter. The pore size distributions and surface areas were determined by the Barrett-

Joyner-Halenda (BJH) and Brunauer-Emmett-Teller (BET) methods.

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C.9. ICP-MS Analysis

The metal contents in supported catalysts were determined using Agilent 7700 ICP-MS system. The samples were digested for 4 days at room temperature in aqua regia (1 HNO3/3 HCl, v/v), and the supernatant was then separated by centrifugation and diluted with 2 wt. % HNO3 solution prior to the ICP-MS analysis.

D. EDS analysis of the fresh and used Co/Mo = 0.3 catalysts

Zr La 2.042 Mo La 2.293 S Ka 2.307 1000 Fresh catalyst Used catatlyst for 4 weeks reaction 800

600

400 Intensity (counts)

200

0 2.0 2.1 2.2 2.3 2.4 2.5 Energy (keV)

The EDS analysis of the fresh and used Co/Mo = 0.3 catalysts after 4 weeks of WGS reaction in 7,000 ppm H2S containing feed to determine that the used catalyst contains significantly

196 more sulfur than that in the fresh catalyst. The EDS spectra of the used catalyst were almost unchanged as compared to the fresh catalysts, suggesting the content of the residual sulfur in the catalysts is minor during 4 weeks of WGS reaction despite the considerable overlap of Mo and S

EDS spectra (2.293 eV and 2.307 eV, respectively).

197

Bibliography

Education

Ph.D. in Chemical Engineering, (2012-2019), University of Cincinnati, OH

M.S. in Chemical Engineering, (2008~2010), Seoul National University, Seoul, Korea

B.S. in Chemical Engineering, (2002~2008), Seoul National University, Seoul, Korea

Professional Experience

Research Assistant, (2012-2019), University of Cincinnati, OH

Project Engineer, (2010-2011), Samsung Engineering Co. Ltd., Seoul, Korea

Research Assistant, (2008-2010), Seoul National University, Seoul, Korea

Peer-reviewed Publications during Ph.D. Study

SeongUk Yun, Vadim Guliants, “Surface Coverage Effectes on Water gas Shift Activity of

ZrO2 Supported Mo Sulfide Catalysts”, Catalysis Communications, 132, 105810, 2019.

SeongUk Yun, Vadim Guliants, “Support effects on water gas shift activity and sulfur dependence of Mo sulfide catalysts”, Energy & Fuels, in press.

SeongUk Yun, Vadim Guliants, “Hydrogen production over Co-promoted Mo-S water gas shift catalysts supported on ZrO2”, Applied Catalysis A: General, in revision.

SeongUk Yun, Vadim Guliants, “Novel bimetallic Cu-Pd nanoparticles as sulfur-tolerant and highly active low temperature WGS catalysts”, Industrial & Engineering Chemistry Research, in revision.

SeongUk Yun, Vadim Guliants, “Size-dependent catalytic behavior and sulfur dependence of

Mo-based nanoparticles in water-gas-shift reaction of biomass-derived syngas“, Energy

Technology, submitted.

198

Conference Presentation

SeongUk Yun, Guliants Vadim, 254th ACS National Meeting, Washington, DC, August 20-

24, 2017, “Modifying surface coverage to improve WGS activity and sulfur-dependence of ZrO2 supported Mo catalysts”

SeongUk Yun, Guliants Vadim, 2015 AIChE Annual Meeting, Salt Lake City, UT, November

9-11, 2015, “Bimetallic Cu/Pd Nanoparticles as Low Temperature Sulfur-Tolerant WGS Catalyst”

SeongUk Yun, Guliants Vadim, 248th ACS National Meeting, San Francisco, CA, August 10-

14, 2014, “Novel bimetallic Cu-Pd catalysts for low temperature WGS reaction”

199