Studies of Nickel/Samarium-Doped Ceria for Catalytic Partial Oxidation of Methane and Effect of Oxygen Vacancy
catalysts
Article Studies of Nickel/Samarium-Doped Ceria for Catalytic Partial Oxidation of Methane and Effect of Oxygen Vacancy
Andrew C. Chien 1,2,* , Nicole J. Ye 1 , Chao-Wei Huang 3 and I-Hsiang Tseng 1
1 Department of Chemical Engineering, Feng Chia University, Taichung 40724, Taiwan; [email protected] (N.J.Y.); [email protected] (I.-H.T.) 2 Green Energy Development Center, Feng Chia University, Taichung 40724, Taiwan 3 Department of Chemical and Materials Engineering, National Kaohsiung University of Science and Technology, Kaohsiung 80778, Taiwan; [email protected] * Correspondence: [email protected]; Tel.: +886-424-517-250 (ext. 3691); Fax: +886-424-510-890
Abstract: We investigated the performance of nickel/samarium-doped ceria (Ni/SDC) nanocatalysts on the catalytic partial oxidation of methane (CPOM). Studies of temperature-programmed surface reaction and reduction reveal that catalytic activity is determined by a synergistic effect produced by Ni metals and metal-support interaction. Catalytic activity was more dependent on the Ni content below 600 ◦C, while there is not much difference for all catalysts at high temperatures. The catalyst ◦ exhibiting high activities toward syngas production (i.e., a CH4 conversion >90% at 700 C) requires a medium Ni-SDC interaction with an Sm/Ce ratio of about 1/9 to 2/8. This is accounted for by optimum oxygen vacancies and adequate ion diffusivity in the SDCs which, as reported, also display the highest ion conductivity for fuel cell applications.
Keywords: Ni/SDC; microwave; syngas; metal-support; partial oxidation of methane; fuel cell
Citation: Chien, A.C.; Ye, N.J.; Huang, C.-W.; Tseng, I-H. Studies of Nickel/Samarium-Doped Ceria for Catalytic Partial Oxidation of 1. Introduction Methane and Effect of Oxygen Catalytic partial oxidation of methane (CPOM) is an important process to convert Vacancy. Catalysts 2021, 11, 731. natural gas into high value-added products. In industry, chemicals such as methanol https://doi.org/10.3390/catal11060731 are produced using syngas (carbon monoxide and hydrogen, CO and H2) from highly endothermic steam reforming reactions [1,2]. Compared with the steam reforming of Academic Editor: Sónia Carabineiro methane (CH4), CPOM is a preferred technology to produce H2 or synthesis gas due to its fast kinetics and exothermic nature. Moreover, CPOM produces synthesis gas with a Received: 6 May 2021 stoichiometry of H2/CO of about two, which can be directly used for methanol or Fischer– Accepted: 10 June 2021 Tropsch synthesis [3]. On the other hand, many studies have investigated the CPOM Published: 14 June 2021 in solid oxide fuel cells (SOFCs) when CH4 is used as a fuel [4]. Direct use of CH4 in fuel cells provides a niche for producing electric power and syngas at the same time [5]. Publisher’s Note: MDPI stays neutral Nevertheless, the development of effective catalysts for the CPOM in fuel cells and the with regard to jurisdictional claims in synthesis of chemicals remains challenging [4,6]. published maps and institutional affil- Supported nickel (Ni) catalysts are widely applied in CPOM due to their low cost iations. and high effectiveness compared to noble metals [2,7,8]. However, the application of the Ni catalyst to CPOM is limited due to the deactivation of Ni nanoparticles by sintering and coking, i.e., carbon deposition [9,10]. Nickel is also an active component in the state- of-the-art anode of SOFCs, i.e., nickel/yttrium stabilized zirconia (Ni/YSZ). Exposure Copyright: © 2021 by the authors. of the anode to CH4 leads to encapsulation of the Ni site by carbon, with subsequent Licensee MDPI, Basel, Switzerland. cell degradation [9,11,12]. In practice, CPOM encounters not only coking on the active This article is an open access article site but also a competitive reaction, complete oxidation of CH to CO , which is favored distributed under the terms and 4 2 thermodynamically. Several reaction mechanisms of CPOM have been investigated [2,4,13], conditions of the Creative Commons Attribution (CC BY) license (https:// and two of them are proposed. The first is a direct mechanism where CH4 and O2 react creativecommons.org/licenses/by/ on the catalyst surface to yield syngas directly. The second is a combustion-reforming 4.0/). mechanism where CH4 and O2 proceeds in the complete oxidation first, followed by
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reforming of excess CH4 by CO2 or steam to syngas. Moreover, it is generally accepted that a metallic site catalyzes the partial oxidation of methane; by contrast, complete oxidation takes place on metal oxides [14]. As mentioned above, efficient Ni catalysts for CPOM require control of active sites over electronic and structural properties, which depend on the active phase as well as the support. Particle size is found to be critical since Ni nanoparticles are more active at lower temperatures, whereas larger Ni particles seem to be more stable with a lower deactivation rate [15]. The effect of the support on the performance of the Ni-based catalysts is also studied [16]. Alumina (Al2O3) and doped ceria (CeO2) have received attention due to a higher dispersion of the Ni sites on the alumina surface and the presence of oxygen vacancies in the ceria lattice, respectively [17]. Ceria is known to play a promoting role in transitional metals to improve the activities of small metal clusters [18,19]. Excellent catalytic performance of these metal catalysts is attributed to the mechanism of oxygen transfer from ceria to metal, which is strongly dependent on morphology and size of ceria particles, as well as on metal-support interaction [20–22]. For example, a remarkable lattice relaxation (i.e., an increase in the lattice constant), corresponding to facile oxygen mobility, can be observed in ceria nanoparticles compared with the bulk one. Otherwise, doping with a lower valent cation alters the densities of oxygen vacancies in ceria [23], which may increase oxygen adsorption and storage capacity to facilitate the removal of carbonaceous species once they have been deposited. Combinations of doped ceria and a transition metal, e.g., Ni, have been proven to increase CPOM activity [19,24]. As commonly occurs in catalysis, the activity and stability of metal catalysts depends not only on size and surface structure but also on the intricate properties of the metal-support interaction. CeO2 nanoparticles are therefore preferred compared to bulk materials, as nanoscale ceria tend to have more contact with the metal to liberate and afford more oxygen vacancies, thus exhibiting high oxidation activity [18,25,26]. Further, concentrations of the dopant in ceria not only affect the localized environment but also determine the number of oxygen vacancies [27]. Nonetheless, relevant reports on the effect of doping ratios in ceria are scarce, and most of them are limited to the discussion of physical properties, such as oxygen ion conductivity, in these materials [28–31]. It is understandable that a rational doping level is vital when employing doped ceria as electrolytes in SOFCs, since the doping level correlates with ion conductivity [32]. Although increasing doping levels gives rise to more vacancies, a high level does not necessarily yield a higher conductivity. Above a critical threshold, ion conductivity would drop off due to the limited diffusion of vacancies. Recently, we synthesized nanoscale Ni/SDC catalysts via microwave-assisted meth- ods and reported the effect of different methods and preparation parameters on CPOM activity [15]. These catalysts contained Ni and SDC, both in nanoscale, and catalyzed a nearly complete oxidation of methane to syngas at 700 ◦C. In the present study, we manipulated the contents of Ni loading and Sm/Ce ratios to investigate their effect on catalytic performance. Our results showed that reducibility and Ni-SDC interaction play crucial roles in the catalytic activity, and they are ascribed to changes in Ni particle size and oxygen vacancies. The best CPOM activity can be obtained by optimizing the x value in doped ceria, MxCe1−xO2−δ, (M: trivalent cation), and Ni loadings. These findings provide information on Ni/SDC catalysts for use in other oxidation reactions and allow for better design of other doped ceria catalysts.
2. Results 2.1. Effect of Ni Content on POM Activity Figure1 presents the conversion of methane versus temperature on the catalyst with var- ious Ni contents. The results show that the co-microwave synthesized NiO/Sm0.1Ce0.9O1.95 catalyst exhibits high catalytic activity for the oxidation of methane. All catalysts reached a conversion of ~80% above 600 ◦C. Among them, the SDC catalyst containing 8–12 wt.% Ni gave a higher activity. Examining the effect of temperature on the oxidation activity of Catalysts 2021, 11, x FOR PEER REVIEW 3 of 12
NiO/Sm0.1Ce0.9O1.95 catalyst exhibits high catalytic activity for the oxidation of methane. All catalysts reached a conversion of ~80% above 600 °C. Among them, the SDC catalyst containing 8–12 wt.% Ni gave a higher activity. Examining the effect of temperature on the oxidation activity of CH4, the conversion increased with increasing temperature. Nev- ertheless, the catalytic activity was more dependent on the content of nickel at low tem- peratures (<600 °C), while there is not much difference for all catalysts at high tempera- tures. Figure 2 presents the performance of different catalysts in terms of product selectivity (H2, CO, and CO2) as a function of temperature. The results show that the temperature below 550 °C favored total oxidation of methane on all catalysts, with CO2 as the main product and a CH4 conversion of ~40–60%. As the temperature is elevated up to 600 °C, syngas (CO and H2) was produced, as the catalytic partial oxidation of methane (CPOM) becomes dominant. Comparing the catalysts on CPOM activity and the light-off temper- ature, the one with 10 wt.% nickel was the most active at 600 °C, exhibiting a conversion of ~90%, as well as a selectivity of 40% and 33% for H2 and CO, respectively (Supplemen- tary Materials Figure S1, shown with product yield). However, the product distribution Catalysts 2021, 11, 731 3 of 12 revealed that the ratio of H2 to CO produced on all catalysts decreased with increasing temperature. The ratio of H2/CO was close to 1 at 700–800 °C, implying that H2-consuming reactions other than the CPOM reaction were probably involved. The H2 might be reacted byCH lattice4, the conversionoxygen in increasedthese SDC with nanocatalysts, increasing temperature. which easily Nevertheless, adsorb oxygen the catalyticgas due to their activity was more dependent on the content of nickel at low temperatures (<600 ◦C), while nanoscale nature, since much condensed water was observed in downstream tubings. there is not much difference for all catalysts at high temperatures.
100
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60 8 wt.% 8wt% 10 wt.% 10wt% 40 12wt% 12 wt.% 18wt%
20wt% 18 wt.% Conversion% / 20 20 wt.%
500 600 700 800 Temperature / ℃
FigureFigure 1.1. TheThe CH CH4 conversion4 conversion versus versus temperature temperature on the on Sm the0.1Ce Sm0.90.1OCe1.950.9catalystsO1.95 catalysts with various with Nivarious Ni −1 contentscontents inin thethe gas gas stream, stream, CH CH4/O4/O2 (10/5,2 (10/5, vol.%), vol.%) in, Ar,in Ar at, a at total a total flow flow rate ofrate 30 mLof 3 min0 mL .min−1.
Figure2 presents the performance of different catalysts in terms of product selectivity (a) (b) (c) (H20.50, CO, and CO2) as a function of temperature. The results show that the temperature ◦ 8 wt.% Ni 0.45 10 wt.% Ni 0.50 12 wt.% Ni 0.45 below 550 C favored total oxidation of methane on all catalysts,0.45 with CO2 as the main prod- 0.40 ◦ uct0.40 and a CH4 conversion of ~40–60%. As the temperature is elevated0.40 up to 600 C, syngas 0.35 H _10 0.35 (CO0.35 and H ) was produced, as the catalytic partial oxidation2 of methane (CPOM) H2 becomes_12 2 H2_8 CO_10 0.30 0.30 CO_12 0.30 CO_8 CO2_10 CO _12 dominant. Comparing CO _8 the catalysts on CPOM activity and the light-off temperature,2 the
Selectivity 2 Selectivity Selectivity 0.25 0.25 0.25 ◦ one with 10 wt.% nickel was the most active at 600 C, exhibiting0.20 a conversion of ~90%, as 0.20 well0.20 as a selectivity of 40% and 33% for H and CO, respectively0.15 (Supplementary Materials 500 550 600 650 700 750 800 500 550 2600 650 700 750 800 500 550 600 650 700 750 800 Figure S1, shownTemperature with / ℃ product yield).Temperature However, / the℃ product distributionTemperature revealed / ℃ that (d) (e) the ratio of H2 to CO produced on all catalysts decreased with increasing temperature. 0.8 0.55 ◦ The ratio of18 H wt.%/CO Ni was close to 1 at 700–80020 wt.% H2_20 C,Ni implying that H -consuming reactions 0.7 2 H2_18 0.50 2 CO_20 CO_18 ─■─ H other0.6 than the CPOM reaction were0.45 probably CO involved.2_20 The H might be2 reacted by lattice CO2_18 2 0.5 0.40 oxygen in these SDC nanocatalysts, which easily adsorb oxygen gas● due to their nanoscale 0.4 0.35 ─ ─ CO
nature,0.3 since much condensed water0.30 was observed in downstream tubings. Selectivity Selectivity ─▲─ CO2 0.2Figure3 shows the temperature-programmed0.25 reduction profiles of the SDC catalyst 0.20 0.1 ◦ with different nickel contents.0.15 The H2-TPR result in the range of 25–500 C provides 500 550 600 650 700 750 800 500 550 600 650 700 750 800 informationTemperature on the supported / ℃ nickel clustersTemperature regarding / ℃ crystal structure and reduction ◦ property because CeO2 is generally characterized by reduction peaks above 500 C[33]. The TPR profiles show that the catalyst with 8–12 wt.% Ni produces two reduction peaks at low and high temperatures (LT and HT), respectively, while the one with 18–20 wt.% Ni gives only one peak centered at a low temperature ~200 ◦C. The LT peak was attributed to the reduction of adsorbed oxygen on the SDC and surface NiO sites, whereas the HT resulted from the reduction in internal NiO clusters or the NiO phase interacting with the SDC [7]. The presence of the HT peak was evidence that the reduction in internal NiO sites is a relatively facile process, or that there is a weak interaction between NiO and SDC for a catalyst with low Ni loadings. Specifically, the one with 10 wt. % of Ni displayed the lowest temperature of the HT reduction peak, ~298 ◦C, implying an easy reduction ability and potentially high activity. On the contrary, the catalyst with high Ni loading tends to cause the agglomeration of nickel species and form large metal crystals (Supplementary Materials Figure S2), which usually take more time and higher temperature for reduction, as shown with a broad H2-TPR profile in Figure3. Catalysts 2021, 11, x FOR PEER REVIEW 3 of 12
NiO/Sm0.1Ce0.9O1.95 catalyst exhibits high catalytic activity for the oxidation of methane. All catalysts reached a conversion of ~80% above 600 °C. Among them, the SDC catalyst containing 8–12 wt.% Ni gave a higher activity. Examining the effect of temperature on the oxidation activity of CH4, the conversion increased with increasing temperature. Nev- ertheless, the catalytic activity was more dependent on the content of nickel at low tem- peratures (<600 °C), while there is not much difference for all catalysts at high tempera- tures. Figure 2 presents the performance of different catalysts in terms of product selectivity (H2, CO, and CO2) as a function of temperature. The results show that the temperature below 550 °C favored total oxidation of methane on all catalysts, with CO2 as the main product and a CH4 conversion of ~40–60%. As the temperature is elevated up to 600 °C, syngas (CO and H2) was produced, as the catalytic partial oxidation of methane (CPOM) becomes dominant. Comparing the catalysts on CPOM activity and the light-off temper- ature, the one with 10 wt.% nickel was the most active at 600 °C, exhibiting a conversion of ~90%, as well as a selectivity of 40% and 33% for H2 and CO, respectively (Supplemen- tary Materials Figure S1, shown with product yield). However, the product distribution revealed that the ratio of H2 to CO produced on all catalysts decreased with increasing temperature. The ratio of H2/CO was close to 1 at 700–800 °C, implying that H2-consuming reactions other than the CPOM reaction were probably involved. The H2 might be reacted by lattice oxygen in these SDC nanocatalysts, which easily adsorb oxygen gas due to their nanoscale nature, since much condensed water was observed in downstream tubings.
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60 8 wt.% 8wt% 10 wt.% 10wt% 40 12wt% 12 wt.% 18wt%
20wt% 18 wt.% Conversion% / 20 20 wt.%
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500 600 700 800 Temperature / ℃
Catalysts 2021, 11, 731 4 of 12 Figure 2. The product selectivity versus temperature on the Sm0.1Ce0.9O1.95 catalysts with various Ni Figure 1. The CH4 conversion versus temperature on the Sm0.1Ce0.9O1.95 catalysts with various Ni contents of (a) 8 wt.%, (b) 10 wt.%, (c) 12 wt.%, (d) 18 wt.%, and (e) 20 wt.% in the gas stream, CH4/O2 contents in the gas stream, CH4/O2 (10/5, vol.%), in Ar, at a total flow rate of 30 mL min−1. (10/5, vol.%), in Ar, at a total flow rate of 30 mL min−1.
(a) (b) (c) 0.50Figure 3 shows the temperature-programmed reduction profiles of the SDC catalyst 8 wt.% Ni 0.45 10 wt.% Ni 0.50 12 wt.% Ni 0.45 with different nickel contents. The H2-TPR result in the range0.45 of 25–500 °C provides infor- 0.40 0.40 0.40 mation on the supported nickel0.35 clusters regarding crystal structure and reduction prop- H _10 0.35 0.35 2 H2_12 H2_8 CO_10 erty because CeO2 is generally0.30 characterized by reduction0.30 peaks above 500 CO_12 °C [33]. The 0.30 CO_8 CO2_10 CO _12 CO _8 2
Selectivity 2 Selectivity Selectivity 0.25 0.25 TPR0.25 profiles show that the catalyst with 8–12 wt.% Ni produces two reduction peaks at 0.20 0.20 low 0.20and high temperatures (LT and HT), respectively, while0.15 the one with 18–20 wt.% Ni 500 550 600 650 700 750 800 500 550 600 650 700 750 800 500 550 600 650 700 750 800 gives only oneTemperature peak /centered ℃ at a lowTemperature temperature / ℃ ~200 °C. TheTemperature LT peak / ℃ was attributed to (thed) reduction of adsorbed (e) oxygen on the SDC and surface NiO sites, whereas the HT 0.8 0.55 18 wt.% Ni 20 wt.% H2_20 Ni resulte0.7 d from the reduction H2_18 in 0.50internal NiO clusters or the NiO phase interacting with the CO_20 CO_18 ─■─ H 0.6 0.45 CO2_20 2 SDC [7]. The presence CO2_18 of the HT peak was evidence that the reduction in internal NiO 0.5 0.40 ─●─ CO sites 0.4is a relatively facile process0.35, or that there is a weak interaction between NiO and SDC
0.3 0.30 Selectivity for a catalyst with low Ni loadings.Selectivity Specifically, the one with ─10▲ ─wt. CO 2% of Ni displayed the 0.2 0.25 0.20 lowest0.1 temperature of the HT reduction peak, ~298 °C, implying an easy reduction ability 0.15 500 550 600 650 700 750 800 500 550 600 650 700 750 800 and potentiallyTemperature high / ℃activity. On the Temperaturecontrary, / the℃ catalyst with high Ni loading tends to cause the agglomeration of nickel species and form large metal crystals (Supplementary MaterialsFigure 2. The Figure product S2), selectivity which usually versus temperature take more on time the Sm and0.1Ce higher0.9O1.95 temperaturecatalysts with variousfor reduction, Ni contents of (a) 8 wt.%, (b) 10 wt.%, (c) 12 wt.%, (d) 18 wt.%, and (e) 20 wt.% in the gas stream, as shown with a broad H2-TPR profile in Figure 3. −1 CH4/O2 (10/5, vol.%), in Ar, at a total flow rate of 30 mL min .
Figure 3. The H2-TPR profiles of the Sm0.1Ce0.9O1.95 catalysts with different Ni contents (Note: × 5 Figure 3. The H2-TPR profiles of the Sm0.1Ce0.9O1.95 catalysts with different Ni contents (Note: × 5 meansmeans original intensityintensity multiplied multiplied by by 5). 5). 2.2. Effect of Sm/Ce Ratio on POM Activity 2.2. EffectTo investigate of Sm/Ce effectsRatio on of POM Sm doping Activity levels on CPOM activity, the SDC catalysts with differentTo investigate Sm/Ce ratios effects were of synthesized Sm doping and levels compared on CPOM by temperature-programmedactivity, the SDC catalysts with differentsurface reaction Sm/Ce (TPSR).ratios were Figure synthesized4 presents MS and profiles compared of eluting by temperature gaseous products-programmed from sur- methane oxidation on the Ni/SDC catalyst during the ramping of the temperature to face ◦reaction (TPSR). Figure 4 presents MS profiles of eluting gaseous products from me- thane600 C oxidation. The TPSR on result the Ni/SDC shows that catalyst the catalysts during with the Sm/Ce ramping ratios of the of 1/9, temperature 2/8, and 3/7 to 600 °C. all provoked syngas with a burst production near 500 ◦C, followed by a progressive drop The TPSR result shows that the catalysts with Sm/Ce ratios of 1/9, 2/8, and 3/7 all provoked syngas with a burst production near 500 °C, followed by a progressive drop in activity. A relatively stable activity of syngas production was then obtained at 600 °C on the catalyst
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in activity. A relatively stable activity of syngas production was then obtained at 600 ◦C withon the the Sm/Ce catalyst ratio withs of the 1/9 Sm/Ce and 2/8, ratios while of the 1/9 one and with 2/8, a ratio while of the 3/7 onewas with subject a ratio to deac- of 3/7 tivationwas subject (note: tohere deactivation, this means (note: relatively here, stable this means because relatively of a transient stable because reaction). of aBy transient con- trast,reaction). the catalyst By contrast, of nearly the CeO catalyst2, i.e., the of nearlyone with CeO low2, i.e., Sm thedoping one withor Sm/Ce low Sm= 0.03/0.97, doping or catalyzedSm/Ce the = 0.03/0.97, complete catalyzedoxidation theof methane complete to oxidation CO2. of methane to CO2.
(a) Sm0.03Ce0.97O1.985 800 5E-8 H2 H2O 600 CO CO2 Temp. 400
Intensity/ a.u. 200 Temperature℃ /
0 0 20 40 60 80 Time / min Sm Ce O (b) 0.1 0.9 1.95 800
5E-8 H2 H2O 600 CO CO2
Temp. 400
200
Intensity/ a.u. Temperature℃ /
0 0 20 40 60 80 Time / min Sm Ce O (c) 0.2 0.8 1.9 800
5E-8 H2 H2O 600 CO CO2
Temp. 400
Intensity/ a.u. 200 Temperature℃ /
0 0 20 40 60 80 Time / min (d) Sm0.3Ce0.7O1.85 800
5E-8 H2 H2O 600 CO CO2
Temp. 400
Intensity/ a.u. 200 Temperature℃ /
0 0 20 40 60 80 Time / min
FigureFigure 4. MS 4. MS profiles profiles of eluting of eluting gas gasproducts products from from methane methane oxidation oxidation on the on theco-m co-microwaveicrowave syn- synthe- thesized 10 wt.% Ni/SDC catalyst prepared with different Sm/Ce ratios. (a) 0.03/0.97, (b) 1/9, (c) sized 10 wt.% Ni/SDC catalyst prepared with different Sm/Ce ratios. (a) 0.03/0.97, (b) 1/9, (c) 2/8, 2/8, and (d) 3/7. and (d) 3/7.
Figure 5 compares H2-TPR profiles of the Ni/SDC catalyst with different Sm/Ce ra- tios. The H2-TPR results show that three kinds of reduction zone (labeled as α, β, and γ)
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Figure5 compares H 2-TPR profiles of the Ni/SDC catalyst with different Sm/Ce ratios. Thecan Hbe2-TPR observed. results The show low that-temperature three kinds of zone reduction (α) resulted zone (labeled from as theα, βreduction, and γ) can in be adsorbed observed.oxygen in The SDC low-temperature oxygen vacancies. zone (Theα) resulted α zone from merged the reduction into zone in (β) adsorbed around oxygen 288 °C as the ◦ inSm/Ce SDC oxygenratio increased. vacancies. The The γα zone,zone mergeda right intoshoulder zone (ofβ) β around peak extending 288 C as the to Sm/Cehigh tempera- ratioture, increased.was attributed The γ tozone, thea reduction right shoulder in surface of β peak NiO extending sites and to those high temperature, weakly interacting waswith attributed the SDC to[7,3 the4]. reduction The shoulder in surface γ peak NiO was sites shaved and those down weakly with interacting an increasing with Sm/Ce the SDC [7,34]. The shoulder γ peak was shaved down with an increasing Sm/Ce ratio. ratio. Higher contents of Sm in the CeO2 raised the reduction temperature of NiO, while Higher contents of Sm in the CeO2 raised the reduction temperature of NiO, while the the interaction between NiO and SDC is weakened. By contrast, low doping of Sm in the interaction between NiO and SDC is weakened. By contrast, low doping of Sm in the CeO2 decreasedCeO2 decreased the reduction the reduction temperature temperature of NiO, whereas of NiO, the whereas interaction the interaction between NiO between and NiO SDCand SDC remained remain intense.ed intense. The catalyst The catalyst exhibiting exhibiting optimum optimum interaction interaction between between NiO and NiO and CeO22 waswas determineddetermined to to be be the the one one with with an Sm/Ce an Sm/Ce ratio ratio = 2/8. = 2/8.
Bulk NiO NiO-SDC Reduction Interaction 263.9℃
Sm/Ce = 0.3/9.7 534.5℃ 219.5℃
287.9℃ 496.5℃
Sm/Ce = 1/9 288.2℃
Sm/Ce = 2/8 *3
337.4℃ consumption a.u. /
2 459.5℃ H
Sm/Ce = 3/7 *3
100 200 300 400 500 600 Temperature / ℃
Figure 5. The H2-TPR profiles of the co-microwave synthesized 10 wt.% Ni/SDC catalyst prepared Figure 5. The H2-TPR profiles of the co-microwave synthesized 10 wt.% Ni/SDC catalyst prepared with differentdifferent Sm/Ce Sm/Ce ratios ratios (Note: (Note: * 3 * means 3 means original original intensity intensity multiplied multiplied by 3). by 3).
2.3. CrystallineCrystalline Structure Structure and and Morphology Morphology Figure6 examines the indexed powder XRD patterns for Ni/SDC (Sm xCe − O −δ, Figure 6 examines the indexed powder XRD patterns for Ni/SDC (Sm1 xxCe2 1−xO2−δ, x = x = 0.03, 0.1, 0.2, 0.3) nanoparticles. All major Bragg peaks ((111), (200), (220), (311), (222), 0.03, 0.1, 0.2, 0.3) nanoparticles. All major Bragg peaks ((111), (200), (220), (311), (222), (400), (331), (420), and (422)) corresponded to the crystallographic plane, having a face- centered(400), (331), cubic (420) fluorite, and (fcc) (422) structure) corresponded [27,29]. The peakto the belonging crystallographic to a secondary plane phase,, having such a face- centered cubic fluorite (fcc) structure [27,29]. The peak belonging to a secondary phase, as Sm2O3, was absent, indicating that there is no impurity with a single-phase formation 3+ ofsuch doped as Sm SDC2O and3, was the absent, incorporation indicating of Sm thations there into is CeO no2 impuritylattice sites with (Supplementary a single-phase for- Materialsmation of Figure doped S3). SDC As and shown the incorporation in Figure6, the of intensity Sm3+ ions of theinto most CeO intense2 lattice Bragg sites (Supple- diffractionmentary Materials peak (111), Figure shifted towardsS3). As theshown higher in angleFigure side, 6, the which intensity is attributable of the tomost the intense latticeBragg contractiondiffraction for peak the doping(111) shifted of Sm ions towards in CeO the2. Additionally, higher angle the side, SDC which peaks became is attributable broader with a decreased crystallinity as the amount of Sm increased. The decreased to the lattice contraction for the doping of Sm ions in CeO2. Additionally, the SDC peaks crystallinity was brought about probably by volume expansion of the CeO unit cell due to became broader with a decreased crystallinity as the amount of 2Sm increased. The de- an increasing level in Sm doping, since the ionic radius of Sm ion (0.108 nm) is larger than creased crystallinity was brought about probably by volume expansion of the CeO2 unit that of Ce ion (0.097 nm) [28]. Incorporating Sm into CeO2 resulted in the creation of oxygen cell due to an increasing level in Sm doping, since the ionic radius of Sm ion (0.108 nm) is vacancies and an increase in lattice parameters relative to pure CeO2, as calculated by Debyelarger Scherrer’sthan that formula.of Ce ion On (0.097 the other nm) hand,[28]. Incorporating the diffraction peaks Sm into of NiO CeO were2 resulted observed in the cre- onlyation on of the oxygen sample vacancies with Sm/Ce and = 1/9an increase and 2/8. Thein lattice presence parameters of NiO crystal relative phase to on pure these CeO2, as samplescalculated implied by Debye that there Scherrer likely’ existss formula. an optimum On the crystallite other hand, NiO the size diffraction due to interaction peaks of NiO were observed only on the sample with Sm/Ce = 1/9 and 2/8. The presence of NiO crystal phase on these samples implied that there likely exists an optimum crystallite NiO size due to interaction with the SDC. Therefore, these catalysts gave better CPOM activity, as presented in the previous section.
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with the SDC. Therefore, these catalysts gave better CPOM activity, as presented in the previous section.
(a) 10wt% Ni - Sm0.03Ce0.97O1.985
CeO2
NiO
(111) (111)
(200)
(220)
(311)
(222) (400) (331) (420) (422) Intensity (a.u.) Intensity
(b) 10 wt% Ni - Sm0.1Ce0.9O CeO1.952 CeO2 NiO NiO
20 30 40 50 60 70 80 90
2(degrees) Intensity (a.u.) Intensity
(c) CeO 10 wt% Ni - Sm0.2Ce0.8O1.92
CeO2 NiO NiO
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2(degrees) Intensity (a.u.) Intensity
(d) 10 wt% Ni - Sm0.3Ce0.7O CeO1.852 CeO2 NiO NiO
20 30 40 50 60 70 80 90
2(degrees) Intensity (a.u.) Intensity
CeO2
NiO
20 30 40 50 60 70 80 90 2(degrees)
Figure 6.6. TheThe XRD XRD patterns of the synthesized Ni/SDCNi/SDC catalystcatalyst withwith differentdifferent Sm/Ce Sm/Ce ratios, ratios, (a) (0.03/0.97,a) 0.03/0.97, (b) ( b1/9,) 1/9, (c) ( c2/8,) 2/8, and and (d ()d 3/7) 3/7..
TheThe crystal crystal particle particle size size and morphologyand morphology of the as-synthesized of the as-synthesized samples were samples analyzed were ana- by scanning and transmission electron microscopy (SEM and TEM), as shown in Figure7 . lyzed by scanning and transmission electron microscopy (SEM and TEM), as shown in It was clearly seen from the images that the particles on all catalysts were very small crystallitesFigure 7. It below was clearly 20 nm, whileseen from some the crystallites images aggregatedthat the particles together. on The all particlecatalysts size were very becamesmall crystallites larger with below an increasing 20 nm Sm/Ce, while ratio,some as crystallites examined carefullyaggregate byd TEM together. (shown The in particle thesize inset became of Figure larger7). Thewith relationship an increasing of particle Sm/Ce size ratio, versus as examined Sm/Ce ratio carefully agreed well by withTEM (shown thatin the of theinset XRD of analysis.Figure 7). Nevertheless, The relationship it is challenging of particle to distinguish size versus NiO Sm/Ce from SDC,ratio since agreed well therewith that is a uniform of the XRD distribution analysis. of Nevertheless, particles and crystalit is challenging phases. The to analysisdistinguish of XRD NiO and from SDC, EMsince results there both is a suggesteduniform distribution that the NiO of species particles were and in nanoscale crystal phases. below 5 The nm oranalysis partly of XRD non-crystalline.and EM results These both results suggested need tothat be examinedthe NiO furtherspecies by were high-resolution in nanoscale TEM. below 5 nm or partly non-crystalline. These results need to be examined further by high-resolution TEM.
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(a) (b) Sm0.03Ce0.97O1.985 Sm0.1Ce0.9O1.95
200 nm 200 nm 50 nm
(c) (d) Sm0.2Ce0.8O1.9 Sm0.3Ce0.7O1.85
200 nm 200 nm 50 nm 50 nm
Figure 7. SEM images of the as-synthesized Ni/SDC catalyst with different Sm/Ce ratios, (a) Figure 7. SEM images of the as-synthesized Ni/SDC catalyst with different Sm/Ce ratios, 0.03/0.97, (b) 1/9, (c) 2/8, and (d) 3/7. (a) 0.03/0.97, (b) 1/9, (c) 2/8, and (d) 3/7.
3.3. DiscussionDiscussion TheThe experimentalexperimental results results above above demonstrated demonstrated that that the the catalytic catalytic activity activity of of Ni/SDC Ni/SDC in in methanemethane oxidationoxidation waswas greatlygreatly affectedaffected by by the the content content of of NiO NiO and and doping doping levels levels of of Sm Sm in in thethe CeOCeO22.. TheThe effecteffect ofof NiONiO contentcontent onon thethe activity activity was was mainly mainly attributed attributed toto the the particle particle sizesize of of NiONiO andand itsits interactioninteraction with with SDC SDC at at lowlow temperatures,temperatures, i.e., i.e., below below 600 600◦ °C.C. IncreasingIncreasing thethe NiONiO contentcontent ledled toto aa lowerlower CPOMCPOM activity,activity, both both for for the the conversion conversion of of CH CH4 4and andits its selectivityselectivity to to syngas. syngas. TheThe HH22-TPR-TPR cancan yieldyield aa behaviorbehavior ofof bulkbulk reduction,reduction, whichwhich isis usuallyusually indicatedindicated by by the the greatest greatest reduction reduction temperature, temperature T, maxTmax,, in in a a TPRTPR profile.profile. As As the the particle particle size size becomesbecomes bigger,bigger, thethe TTmaxmax shiftsshifts to to higher higher temperatures. temperatures. A Alarge large peak peak also also reflects reflects more more H2 Hused2 used for forthe the reduction reduction of a of bigger a bigger catalyst. catalyst. The The H2- HTPR2-TPR result result shows shows that thatthe catalyst the catalyst con- containingtaining high high amounts amounts of ofNiO NiO displays displays a abroad broad peak peak extending extending in the temperaturetemperature rangerange investigated,investigated, indicating indicating that that more more H H22consumption consumption and and a ahigh high strengthstrength ininreduction reduction power power areare needed. needed. The The requirement requirement for for high high reduction reduction power power reflected reflected that that an intimatean intimate interaction interac- betweention between NiO andNiO SDC and orSDC a larger or a larger NiO crystalliteNiO crystallite exists. exists. Large La NiOrge particles NiO particles usually usually took moretook timemore intime the in complete the complete reduction reduction of metal of metal clusters clusters underlying underlying the inside. the inside. Compared Com- withpared other with catalysts, other catalysts, the 12 wt.%the 12 Ni wt.% catalyst Ni catalyst has a reduction has a reduction peak at peak higher at higher temperatures, temper- whichatures was, which speculated was speculated to result to from result a reduction from a reduction in surface in NiOsurface sites NiO and sites those and weakly those interactingweakly interacting with the SDC.with the Relative SDC. to Relative 10 wt.% to Ni 10 one, wt.% the Ni 12 one, wt.% the Ni 12 catalyst wt.% Ni exhibits catalyst more ex- surfacehibits more sites surface that interact sites that with interact SDC, thus with requiring SDC, thus a requiring higher reduction a higher temperature.reduction temper- The 10–12ature. wt.% The Ni10– loading12 wt.% on Ni SDC loading seems on to SDC be optimal seems forto POMbe opti reaction.mal for AsPOM a result, reaction. catalytic As a activityresult, catalytic was lower activity for the was catalyst lower for with the a catalyst higher Niwith content a higher at lowNi content temperatures, at low temper- while thereatures, is notwhile much there difference is not much for alldifference catalysts for at all high catalysts temperatures. at high temperatures. OnOn thethe otherother hand,hand, the the effect effect of of the the Sm/Ce Sm/Ce ratioratio onon activityactivity resultedresulted fromfromthe thedensity density ofof oxygenoxygen vacancies and and the the interaction interaction between between Ni Ni and and SDC SDC due due to the toir their crystalline crystalline char- characteristics [24]. The catalyst with a suitable doping ratio of Sm/Ce, i.e., 1/9–2/8, acteristics [24]. The catalyst with a suitable doping ratio of Sm/Ce,◦ i.e., 1/9–2/8, produced produced syngas steadily after the temperature was ramped to 600 C. The H2-TPR result syngas steadily after the temperature was ramped to 600 °C. The H2-TPR result evidenced evidenced that increasing the Sm/Ce ratio weakened the interaction between NiO and that increasing the Sm/Ce ratio weakened the interaction between NiO and SDC, whereas SDC, whereas a further increase seemed to trigger complex interactions between these two a further increase seemed to trigger complex interactions between these two species. The species. The XRD characterization also demonstrated that the lattice structure was altered; XRD characterization also demonstrated that the lattice structure was altered; a trend that
Catalysts 2021, 11, x FOR PEER REVIEW 9 of 12
is consistent with that observed in the TPR profiles. The doping ratio of Sm/Ce, ~1/9 to Catalysts 2021, 11, 731 9 of 12 2/8, made a catalyst with an optimum concentration of oxygen vacancies and adequate space for the ion diffusion, which accounts for the high activity toward syngas production. The effect of different Sm doping was also investigated by Raman analysis, as shown in a trend that is consistent with that observed in the TPR profiles. The doping ratio of Sm/Ce, −1 Figure~1/9 to 2/8,8. Oxygen made a vacancies catalyst with can an optimumbe evaluated concentration by an absorption of oxygen vacancies at 564 andcm , which is at- tributedadequate spaceto vibration for the ion of diffusion, Ce–O around which accounts Ce center for thenearby high activity [29,35 toward,36]. We syngas observed no such vibrationproduction. on The pure effect CeO of different2 or Sm Sm2O3 doping (Supplementary was also investigated Materials by,Raman Figure analysis, S4). In fact, the SDC −1 withas shown 20 mol.% in Figure of8 Sm. Oxygen doping vacancies is also can reported be evaluated to possess by an absorption the highest at 564 ion cm conductivity, in the applicationwhich is attributed of fuel to vibrationcells [37 of,3 Ce–O8]. Despite around Ceits centerdifferent nearby catalytic [29,35,36 ].activity, We observed the appearance of no such vibration on pure CeO2 or Sm2O3 (Supplementary Materials Figure S4). In fact, the NiOSDC withphase 20 mol.%in the ofXRD Sm doping pattern is alsowas reported probably to possess also ascribed the highest by ion the conductivity crystalline in nature of SDC incurredthe application by different of fuel cells leve [37,38ls]. of Despite Sm doping. its different Coincidentally, catalytic activity, the the appearanceNiO phase of was observed onlyNiO phaseon two in the Ni/SDC XRD pattern catalysts was probably with high also activity ascribed byfor the syngas crystalline production. nature of SDC Whether altering theincurred Sm/Ce by differentratio brings levels offorth Sm doping.crystallization Coincidentally, of NiO the NiOin the phase synthesis was observed environment only , meaning on two Ni/SDC catalysts with high activity for syngas production. Whether altering the thatSm/Ce they ratio both brings produce forth crystallization a synergistic of NiO effect in the responsible synthesis environment, for high meaningactivity that, requires further studythey both. produce a synergistic effect responsible for high activity, requires further study.
439.9
. a.u
563.2
Intensity/ Intensity/ 1145.6 x = 0
Intensitya.u. / x = 3 x = 10 x = 20 x = 30 300 600 900 1200 RamanRaman Shift Shift/ / cm-1-1
Figure 8. Raman Spectra of the as-synthesized 10 wt.% Ni/SmxCe1−xO2−δ catalyst. Figure 8. Raman Spectra of the as-synthesized 10 wt.% Ni/SmxCe1−xO2−δ catalyst. 4. Materials and Methods 44.1.. Materials Catalyst Preparation and Methods 4.1. CatalystThe nickel Preparation oxide/samarium-doped cerium oxide (NiO/SDC) was synthesized by a microwave-assisted method [24,39]. In a typical preparation, the precursors of nickel (II) nitrateThe hexahydrate nickel oxide/samarium (Ni(NO3)2·6H2O),- ceriumdoped (III) cerium nitrate hexahydrateoxide (NiO/SDC) (Ce(NO3 )was3·6H 2synthesizedO), by a microwaveand samarium(III)-assisted nitrate method hexahydrate [24,39 (Sm(NO]. In a typical3)3·6H2O) preparation, (All from Sigma the Aldrich,precursors St. of nickel (II) Louis, MO, USA)were mixed and dissolved together in ethanol. The mixture solution nitrate hexahydrate (Ni(NO3)2∙6H2O), cerium (III) nitrate hexahydrate (Ce(NO3)3·6H2O), was controlled at pH value ~14 by dropwise addition of sodium hydroxide (NaOH) and andplaced samarium(III) inside a conventional nitrate household hexahydrate microwave. (Sm(NO The synthesis3)3·6H2O) started (All as from the mixture Sigma Aldrich, St. Louis,was irradiated MO, US withA)were microwave mixed strength and dissolved ~133 W in together an on/off in cycle ethanol. (10s on The and mixture 20s off) solution was controlledfor 10 min. The at pH solid value powder ~14 precipitated by dropwise from addition the irradiated of sodium solution hydroxide was cleaned (NaOH) with and placed insidedistilled a water conventional and rinsed household with ethanol. microwave. The obtained sampleThe synthesis was then driedstarted and as calcined the mixture was ir- at 600 ◦C for 4 h. The product (NiO/SDC) had a yield of about 80 mol.% (+/− 5%) based radiatedon cerium. with Thecatalysts microwave containing strength different ~133 Ni W contents in an (8, on/off 10, 18, cycle 20 wt.% (10s based on on and the 20s off) for 10 min.NiO/SDC The solid weight) powder and varying precipitated Sm/Ce molar from ratios the irradiated (0.03/0.97, 1/9,solution 2/8 and was 3/7) cleaned were with distilled waterprepared and and rinsed specified. with All ethanol. catalysts wereThe obt keptained dried andsample ground was in then the mortar dried prior and to calcined at 600 °Creaction for 4 testsh. The and product characterizations. (NiO/SDC) had a yield of about 80 mol.% (+/− 5%) based on cerium. The4.2. Measurementcatalysts containing of Catalytic Activity different Ni contents (8, 10, 18, 20 wt.% based on the NiO/SDC weight)The measurementand varying of catalyticSm/Ce activitymolar wasratios conducted (0.03/0.97 in a fixed-bed, 1/9, 2/8 differential and 3/7) reactorwere prepared and specified.under atmospheric All catalysts pressure, were as described kept dried previously and ground [15]. The catalystin the ofmortar Ce0.1Sm prior0.9O1.95 to reaction tests andwith characterizations. different content of nickel was reduced in an H2 environment (2 vol.% in argon gas
4.2. Measurement of Catalytic Activity The measurement of catalytic activity was conducted in a fixed-bed differential reac- tor under atmospheric pressure, as described previously [15]. The catalyst of Ce0.1Sm0.9O1.95 with different content of nickel was reduced in an H2 environment (2 vol.% in argon gas
Catalysts 2021, 11, 731 10 of 12
◦ flow) at 500 C for 1 h and then exposed to CH4 (10 vol.%) and O2 (5 vol.%) in Ar (Total flow −1 rate: 30 mL min ). The catalytic activity was studied in CH4/O2 gas stream by ramping temperature from 500 to 800 ◦C at a rate of 10 ◦C min−1. The effect of the Sm/Ce ratio on the activity was investigated by a temperature-programmed surface reaction from 25 to 600 ◦C without further reduction of the catalyst. The effluent gas species were monitored by an online mass spectrometer (Hiden Analytical, HPR 20). Mass/electron (m/e) ratio in the MS were selected for H2 (2), CH4 (15), H2O (18), CO (28), O2 (32), Ar (39) and CO2 (44). The conversion and selectivity values were taken after 20 min of reaction at each temperature. The CH4 conversion (%) was calculated by [(CCH4,in − CCH4,out)/CCH4,in] × 100%, and the product selectivity was obtained by [CH2,out/(CH2,out + CCO,out + CCO2,out)] × 100%, with H2 as an example.
4.3. Characterization
H2 temperature-programmed reduction (H2-TPR) experiments were performed to study reduction behaviors of the catalyst. The TPR experiments were conducted with ~100 mg of the catalyst loaded in a stainless steel tube reactor. The catalyst was first ◦ −1 pretreated at 100 C in a gas mixture with 10 vol.% H2 in Ar (total flow rate: 20 mL min ) and heated at a ramping rate of 5 ◦C min−1 from 25 to 600 ◦C. Reduction temperature and consumption of H2 was analyzed by the mass spectrometer. A Bruker D8 Discover diffractometer (Billerica, MA, USA) was used to obtain in- formation of phase composition and crystallinity on the catalyst in the angular range 20–90◦, with a scan speed of 2.5◦ min−1. The particle size and surface structure of the as-synthesized NiO/SDC catalyst was examined by a scanning electro n microscope (SEM; JEOL JSM-7800F, Boston, Massachusetts, USA) equipped with an energy-dispersive X-ray spectrometer (EDX, Boston, MA, USA), as well as a transmission electron microscope (TEM; JEM-2010 high resolution). Raman spectra of all the samples were recorded using a Raman imaging microscope (AvaSpec-ULS TEC) with a green laser (532 nm) excitation light source.
5. Conclusions The Ni/SDC catalysts with varying NiO contents and Sm/Ce ratios were studied for catalytic partial oxidation of methane (CPOM). Increasing NiO content from 8 to 20 wt.% decreased the CPOM activity below 600 ◦C. The catalyst with a high NiO content catalyzed the complete oxidation of CH4 at low temperatures, probably due to the presence of the oxide phase with low reducibility, as evidenced by the H2-TPR results. The insufficient reduction was attributed to larger NiO crystals and their interaction with SDC. Nonetheless, the effects of NiO content became less severe with high reducing power at higher tempera- tures. On the other hand, doping Sm into ceria led to changes in the lattice parameters and the generation of oxygen vacancies, which provide a means to tailor the activity of ceria catalysts. We have shown that varying Sm/Ce ratios shifts the reaction from a complete to a partial oxidation of methane as the Sm/Ce ratio increases, whereas further increasing the ratio reverts this mechanism. The Sm content in the CeO2 affected CPOM activity by altering the reducibility of NiO on the one hand, and the interaction between NiO and SDC on the other hand. The best CPOM activity has to be optimized by considering these two factors. The information provided in this study allows property control of these microwave-synthesized Ni/SDC nanoparticles, and makes them potential materials in applications such as air control, oxygen storage, and energy.
Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/catal11060731/s1, Figure S1: Product yield of on the SDC catalysts with various Ni contents, Figure S2: XRD pattern on (a) 10 wt. % Ni/SDC and (b) 15 wt.% Ni/SDC catalyst, Figure S3: Experimental Ce3d and Sm 3d XPS spectra on 10 wt.% Ni/SDC (Sm/Ce=1/9), Figure S4: Raman analysis on the commercial CeO2 and as-synthesized SDC. Catalysts 2021, 11, 731 11 of 12
Author Contributions: Conceptualization, A.C.C., N.J.Y.; methodology, A.C.C., N.J.Y.; software, N.J.Y.; validation, formal analysis and investigation, A.C.C., N.J.Y.; writing—original draft prepara- tion, A.C.C.; writing—review and editing, C.-W.H., I.-H.T. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Ministry of Science and Technology in Taiwan under contract MOST 109-2221-E-035-021-MY3 and MOST 109-2622-E-035-019. Data Availability Statement: Data is contained within the article or supplementary material. Acknowledgments: The authors would like to acknowledge the administrative support from Feng Chia University. Conflicts of Interest: The authors declare no conflict of any personal circumstances or interest that may be perceived as inappropriately influencing the representation or interpretation of reported research results. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
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