Hydrogen Production by Partial Oxidation Reforming of Methane Over Ni Catalysts Supported on High and Low Surface Area Alumina and Zirconia

processes

Article Hydrogen Production by Partial Oxidation Reforming of Methane over Ni Catalysts Supported on High and Low Surface Area Alumina and Zirconia

Anis Fakeeha 1,2, Ahmed A. Ibrahim 1, Hesham Aljuraywi 1, Yazeed Alqahtani 1, Ahmad Alkhodair 1, Suliman Alswaidan 1, Ahmed E. Abasaeed 1 , Samsudeen O. Kasim 1 , Soﬁu Mahmud 1 and Ahmed S. Al-Fatesh 1,*

1 Chemical Engineering Department, College of Engineering, King Saud University, P.O. Box 800, Riyadh 11421, Saudi Arabia; [email protected] (A.F.); [email protected] (A.A.I.); [email protected] (H.A.); [email protected] (Y.A.); [email protected] (A.A.); [email protected] (S.A.); [email protected] (A.E.A.); [email protected] (S.O.K.); mahmudsoﬁ[email protected] (S.M.) 2 King Abdullah City for Atomic and Renewable Energy, (K.A. CARE) Energy Research and Innovation Center at Riyadh, Riyadh 11421, Saudi Arabia * Correspondence: [email protected]; Tel.: +966-11-46-76859

Received: 7 March 2020; Accepted: 9 April 2020; Published: 25 April 2020

Abstract: The catalytic activity of the partial oxidation reforming reaction for hydrogen production over 10% Ni supported on high and low surface area alumina and zirconia was investigated. The reforming reactions, under atmospheric pressure, were performed with a feed molar ratio of CH4/O2 = 2.0. The reaction temperature was set to 450–650 ◦C. The catalytic activity, stability, and carbon formation were determined via TGA, TPO, Raman, and H2 yield. The catalysts were calcined at 600 and 800 ◦C. The catalysts were prepared via the wet-impregnation method. Various characterizations were conducted using BET, XRD, TPR, TGA, TPD, TPO, and Raman. The highest methane conversion (90%) and hydrogen yield (72%) were obtained at a 650 ◦C reaction temperature using Ni-Al-H-600, which also showed the highest stability for the ranges of the reaction temperatures investigated. Indeed, the time-on-stream for 7 h of the Ni-Al-H-600 catalyst displayed high activity and a stable profile when the reaction temperature was set to 650 ◦C.

Keywords: Al2O3; calcination; partial oxidation; reforming of methane; supported nickel; ZrO2

1. Introduction Today’s global energy is chiefly concentrated on power production via fossil fuels. To reduce greenhouse gas emissions, major efforts have been carried out to increase the use of renewable energy. However, the difference between renewable energy supply and energy demands necessitates further utilization of fossil fuels to supply excess energy. Fossil petroleum, commonly called natural gas, is currently employed to provide the major portion of energy requirements. The conversion of natural gas into valuable chemical products has been the goal of several studies for some decades [1–5]. The most common processes employed to achieve such products include the transformation of methane into syngas [6,7]. Methane, an ozone depleting gas, is the main component present in natural gas. Clean energy application technologies have received much consideration in recent years because of their low emissions and high efficiencies [8–10]. The use of hydrogen as fuel for fuel cells has become the target of many investigators, as hydrogen is highly efficient, nonpolluting, and sustainable [11,12]. Owing to the great demand for H2 and CO, for the petrochemical industry

Processes 2020, 8, 499; doi:10.3390/pr8050499 www.mdpi.com/journal/processes Processes 2020, 8, 499 2 of 21

(through the Fischer–Tropsch process to generate methanol and ammonia), hydrocarbon production, and petroleum reﬁning processes (hydrotreating and hydrocracking), their production by the reforming of natural gas has increased signiﬁcantly. Although the main way to obtain syngas from methane on an industrial scale is from steam reforming, as presented in Equation (1), feasible alternative methods, e.g., CO2 reforming shown in Equation (2) and the partial oxidation of methane (POM) as in Equation (3), have been investigated [13–15].

kJ CH + H O = 3H + CO ∆H = 206 (1) 4 2 2 298K mol kJ CH + CO = 2H + 2CO ∆H = 216 (2) 4 2 2 298K mol 1 kJ CH + O = 2H + CO ∆H K = 36 (3) 4 2 2 2 298 − mol

Both CO2 reforming and steam reforming processes are highly endothermic and need huge energy inputs. On the other hand, the POM route has the advantage of being an exothermic process, which has a lower energy cost. POM also provides a ratio of H2/CO = 2, which permits a straightforward syngas application for methanol or Fischer–Tropsch processes [4,14,16]. Though costly metals, like Rh, Ru, and Pt, have been employed successfully for the reactions (1–3) in terms of activity and selectivity [17–19], the high price and limited availability of the noble metals confine their applications. Instead, nickel and other transition metal-based catalysts have been stated to be active for these reactions (1–3). Nickel is preferred due to its availability, low cost, and high initial catalytic activity. Thereafter, extensive research has been performed on nickel supported over various oxides, for instance Al2O3 [20,21]. The prevalence of deactivation and sintering of active metals necessitates the development of Ni-based catalysts with improved activity and stability. The partial oxidation method has the benefit of giving high conversion of methane with superb selectivity for hydrogen using relatively high space velocities [22]. Although rapid reaction kinetics and satisfactory thermodynamics are available in the literature, partial oxidation has not been commercialized yet due to challenges that include small reduction in CO selectivity caused by over oxidation that results in a local temperature rise at the catalyst surface leading to catalyst deactivation owing to its sintering and carbon formation [23]. A literature survey indicates that substantial research efforts have been dedicated to developing appropriate catalysts via numerous supports and different catalyst pretreatment methods that will increase the H2 and CO yields. The choice of a support plays an important role in the ultimate stability of the catalysts. Dissanayake et al. investigated the partial oxidation of CH4 using 25% Ni supported on alumina [24]. The result showed that complete conversion could be obtained if the reaction temperature was above 700 ◦C. The results also revealed that the catalyst bed may be subdivided into three zones with different catalytic activity aspects. Zhang et al. studied the partial oxidation of CH4 using textural promoted alumina supported on Ni (Ni/CeO2-ZrO2/γ-Al2O3)[25]. The results indicated the effects of different preparation processes. On the other hand, Sajjadi and Haghighi examined Ni/Al2O3 by means of different preparation techniques, namely sol-gel, impregnation, and hybrid sol-gel plasma [26]. They found that each preparation technique possessed advantageous properties over the others, such as good dispersion, stability, coke resistance, etc. Usually, Al2O3 is utilized as a support for Ni-based catalysts during reforming reactions. However, several problems are related with Al2O3 support; for instance, carbon deposition readily increases the catalyst deactivation, damaging of the active phase, and formation of inactive spinel phases, such as nickel aluminate (NiAl2O4)[27]. ZrO2 is used to improve CO2 adsorption and carbon gasification [8]. Dong et al. performed the relative investigation of partial oxidation of CH4 over Ni/ZrO2, Ni/CeO2, and Ni/Ce–ZrO2 catalysts [28]. They found that over Ni/ZrO2, CH4 and O2 were activated on the surface of metallic Ni, and then adsorbed carbon reacted with adsorbed O2 to generate CO, which designed the leading path for the partial oxidation of CH4. Processes 2020, 8, 499 3 of 21

Ouaguenouni et al. investigated the preparation and catalytic activity of nickel-manganese oxide catalysts in the reaction of partial oxidation of methane [29]. In their results they found that the formation of different spinels depends on the calcination temperature. The spinel NiMn2O4 was observed to be active in the POM reaction. The catalysts calcined at 900 ◦C develop NiMn2O4 and give a higher CH4 conversion, resulting from good Ni dispersion. Alternatively, Moral et al. studied the POM using Co as the active metal supported on a mixture of MgO and Al2O3 [30]. The authors performed the reaction at 800 ◦C and studied the effects of Co and MgO loadings and the calcination temperatures. Their catalysts gave CH4 conversions of 91% using a calcination temperature of 500 ◦C and 20 wt.% Co and 63% MgO loadings. Kaddeche et al. examined the partial oxidation of methane on co-precipitated Ni-Mg/Al catalysts modified with copper or iron [4]. They investigated the effect of catalyst composition and pretreatment conditions of these catalysts at a 750 ◦C reaction temperature. The catalysts displayed very high activity and selectivity that are dependent on the conditions of preparation of the catalysts. Their results showed increased activity and selectivity with decreasing calcination temperature and increasing Ni and Al contents in the catalysts’ composition. A study on Ca-decorated Al2O3-supported Ni and NiB catalysts, showed that the calcination temperature could significantly affect the catalyst activity [31]. A higher reduction temperature was required to ensure the complete reduction of NiO on the catalysts calcined at higher temperatures; CH4 conversion and syngas selectivity decreased when catalysts were calcined at 800 ◦C. Thus, in light of the above, it is vital to evaluate the performance of Ni catalysts with regard to supports such as alumina and zirconia by using different calcination temperatures and various operating conditions including reaction temperature and time-on-stream. Attention has been focused on the influence of catalyst performance on reactants (CH4 and O2) conversion and H2 yield products in the partial oxidation reforming process. Characterizations of the spent and fresh catalysts were also performed to understand the catalytic behavior, i.e., activity and stability.

2. Experiment

2.1. Catalyst Preparation The preparation of the desired catalysts was performed using the wet impregnation method. In the preparation of 10% nickel supported on Al2O3 and zirconia catalyst, 0.9 g of the support (α-Al2O3, γ-Al2O3, α-ZrO2, or γ-ZrO2) were impregnated with a solution having 0.495 g of (Ni(NO3)2.6H2O) in 0.03 L of deionized H2O. The catalyst was subjected to drying at 120 ◦C and calcination either at 600 or 800 ◦C for two hours. The catalyst was activated inside the reactor at 800 ◦C by passing hydrogen at a rate of 40 mL/min for 2 h followed by 20 min of N2 at a rate of 30 mL/min. The synthesized catalysts and their corresponding designations are given in Table1.

Table 1. Properties of the used catalysts.

Pore Pore Total H Calcination Surface 2 Catalyst/Support Designation Volume Diameter Consumption Temperature ( C) Area (m2/g) ◦ (cm3/g) (nm) (µmol/g)

α-Al2O3 Al-L 2.5 10% Ni/α-Al2O3 Ni-Al-L-600 600 4.3 0.02 14.4 3058.9 10% Ni/α-Al2O3 Ni-Al-L-800 800 2.9 0.01 14.9 2637.3 γ-Al2O3 Al-H 260 10% Ni/γ-Al2O3 Ni-Al-H-600 600 175.9 0.61 12.1 3729.8 10% Ni/γ-Al2O3 Ni-Al-H-800 800 146.4 0.54 13.3 4472.9 α-ZrO2 Zr-L 22.6 10% Ni/α-ZrO2 Ni-Zr-L-600 600 21.5 0.16 32.0 2349.1 10% Ni/α-ZrO2 Ni-Zr-L-800 800 15.1 0.11 33.7 3116.7 γ-ZrO2 Zr-H 325 10% Ni/γ-ZrO2 Ni-Zr-H-600 600 26.7 0.16 22.5 3442.9 10% Ni/γ-ZrO2 Ni-Zr-H-800 800 6.6 0.05 34.9 3407.5 Processes 2020, 8, 499 4 of 21

2.2. Catalytic Reaction The partial oxidation reaction of methane was done at atmospheric pressure in a 0.91 cm diameter and 0.30 m long stainless steel-tube fixed-bed micro reactor (the reactor is from PID Eng. & Tech Micro activity Reference Company) using 0.1 g of the catalyst. The reaction temperature was measured by a thermocouple placed in the center of the catalyst bed. The volume ratio of the feed gases (methane/oxygen/nitrogen) was 6:3:4. The total flow rate of the feed was 32.5 mL/min (i.e., 325 mL/min/gcat). The investigation was performed at reaction temperatures of 450, 500, 550, 600, and 650 ◦C. The effluents were analyzed using an online gas chromatograph (GC-2014 SHIMADZU) equipped with a thermal conductivity detector. The conversions of CH4 and H2 yield were all determined. The methane conversion and hydrogen yield were computed as:

CH4,in CH4,out CH4 conversion (%) = − 100% (4) CH4,in ×

moles o f H2produced H Yield (%) = 100% (5) 2 2 CH × × 4,in 2.3. Catalyst Characterization Both fresh and spent catalysts were characterized by several techniques.

2.3.1. N2 Physisorption

N2 adsorption/desorption isotherms were used to ﬁnd the textural characteristics of the catalysts, calculated at 197 C with a Micromeritics Tristar II 3020 porosity and surface area analyzer. In each − ◦ test, 200–300 mg of catalyst was taken. The samples were degassed at 300 ◦C for 3 h to remove undesired adsorbed gases, organics, and water vapor. The BET technique was employed to determine the surface areas.

2.3.2. XRD X-ray diﬀraction (XRD) measurements for fresh catalysts were carried out. The XRD was performed using Rigaku (Miniﬂex), with Kα-Cu radiation at 40 kV and 40 mA. A 2θ range of 10–85◦ and a scanning step of 0.02◦ were used.

2.3.3. TPR The activation behavior of the catalysts was investigated via an AutoChem-II Micromeritics device. For each analysis, 0.07 g of the sample were pre-treated with Ar (30 mL/ min). The samples were cooled to ambient temperature before starting the analyses. Then, the sample temperature was raised from 25 to 1000 ◦C in an automatic furnace at 1 atm. During temperature ramping, a H2/Ar mixture with a volume ratio 10:90 and a ﬂow rate of 40 mL/min was applied, while the heating rate was kept constant at 10 K/min. The outlet gases were monitored by a thermal conductivity detector (TCD) to analyze the H2 consumption with respect to temperature.

2.3.4. TGA The extent of the total deposited C on the used catalysts was determined using Shimadzu thermo-gravimetric analysis (TGA) in air. From the used catalysts, 0.10–0.15 g were heated at the rate of 20 ◦C/min from 25 to 1000 ◦C while recording the mass loss.

2.3.5. CO2-TPD

The Micromeritics Autochem II apparatus was used to perform the CO2 temperature-programmed desorption (TPD). First, 5 mg of catalyst were reduced at 600 ◦C for 60 min under an He ﬂow (30 mL/min) Processes 2020, 8, 499 5 of 21

and then cooled to 50 ◦C. The CO2 flow was continued for 1 h, and the sample was then flushed with He to remove any physisorbed CO2. The peaks of desorption were recorded while temperature was changed by 10 ◦C/min. The CO2 concentration in the effluent stream was computed using a thermal conductivity detector, and the areas under the peaks provided the amount of desorbed CO2 during TPD.

2.3.6. TPO Temperature program oxidation was performed in an oxidative atmosphere to determine the type of carbon deposited on the surface of the catalyst via Micromeritics AutoChem II. The analysis was executed up to 800 ◦C under 40 mL/min of 10% O2/He. The used catalyst was pre-treated in the presence of Ar at 150 ◦C for 30 min and afterward cooled to room temperature.

2.3.7. Raman Spectroscopy Raman spectra were carried out via an NMR-4500 Laser Raman Spectrometer. A wave length with excitation beam of 532 nm was used. An objective lens with 20 magniﬁcation was used to measure × the spectra. A six mW beam power and an exposure time of 0.05 h were used. The Raman shift of the 1 spectra was computed in the range 1000–3000 cm− . The proﬁles were handled by Spectra Manager Ver.2 software.

3. Results and Discussion As a control experiment, we tested an empty reactor without catalysts under the same feed ratio and various temperatures (500–650 ◦C). The CH4 conversions registered during the test were 0.0% in the 500–650 ◦C range. Therefore, the control experiment denoted negligible interaction of the reactor tube to the catalytic activity. Figure1 exhibits the catalyst N 2 adsorption/desorption isotherms, calcined at 600 ◦C. The catalyst N2 isotherms fall under the type-IV classification. The isotherm is characterized by materials that are mesoporous (2–50 nm), in accordance with the International Union of Pure and Applied Chemistry (IUPAC) classification. The Ni-Al-H-600 sample has an H2-type hysteresis loop, indicating an ink bottle type pore (mouth is narrower and back broader) whereas the Ni-Al-L-600 sample has an H3-type hysteresis, confirming non-limited adsorption. Ni-Zr-H-600 and Ni-Zr-L-600 samples have H1 hysteresis loops indicating narrower pore size distribution. It is an indication of changes in the pore patterns with different supports. Interestingly, when the calcination temperature in Figure2 increases up to 800 ◦C, the pore pattern of Ni-Al-H-800 changes from H2 to H1, indicating a change of pore pattern from ink bottle to cylindrical. The same pore pattern change from H3 to H1 is seen in of Ni-Al-L-800. Ni-Zr-H-800 and Ni-Zr-L-800 samples have the same patterns as those of the low temperature (600 ◦C) calcined samples. The distinctive physical structure of surface area, pore volume, and pore size are shown in Table1. The addition of 10% nickel decreases the surface area and displays a relatively substantial effect on the specific surface area of the Ni-Zr-H-600 and Ni-Zr-H-800 catalysts. Increasing calcination temperature from 600 to 800 ◦C decreases the surface area and pore volume and increases the pore diameter; thus resulting in a sequence of catalysts bearing diverse inventories of hydroxyl groups. The average pore diameter of all catalysts is around 12–35 nm, which is in accord with the mesoporous characteristics of the samples. ProcessesProcesses 20202020, ,88, ,x 499 FOR PEER REVIEW 66 of of 23 21

100 120

Ni-Al-H-600 Ni-Zr-H-600 100 80

cm³/g STP cm³/g

(

( 80 60

60 40

Quantity Adsorbed Quantity Quantity Adsorbed Quantity 20

20 0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Relative Pressure (P/Po) Relative Pressure (P/Po)

14 120

Ni-Zr-L-600 12 Ni-Al-L-600 100

cm³/g STP cm³/g

10 ( ( 80 8 60 6 40 4

Quantity Adsorbed Quantity Quantity Adsorbed Quantity 20 2

0 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Relative Pressure (P/Po) Relative Pressure (P/Po)

FigureFigure 1. 1. NitrogenNitrogen adsorption adsorption/desorption/desorption isotherms isotherms of of fresh fresh catalysts catalysts calcined calcined at at 600 600 °◦C.C.

Figure3 shows the XRD spectra of catalysts pre-reduced at 600 ◦C for 1 h, then calcined at 600 and 800 ◦C. In the present study, the mesoporous alumina and zirconia were prepared by an impregnation method. Figure3A represents the XRD pattern of Ni-Zr-L-600 and Ni-Zr-L-800. This figure shows the diffraction peaks at 2θ values of 24.7, 29.5, 32.5, 34.0, 41.0, and 50.5, which can be ascribed to the planes of (022), (111), (040), (041), (113), and (114) of L-ZrO2 (ICSD 01-081-0610). The two catalysts have similar spectra, but the higher calcined catalyst presents somewhat higher intensities, which means crystallinity is improved. The XRD patterns of Ni-Zr-H-600 and Ni-Zr-H-800 (Figure3B) exhibit diffraction peaks at 2θ values of 29.5, 34.7, 50.5, 60.0, 75.1, and 85.0, which were assigned to the planes of (111), (023),(114), (115), (065), and (191) of H-ZrO2 (PDF Index 00-035-1398). In this case, the profiles are slightly different. The Ni-Zr-H-800 catalyst gives two more peaks at 2θ = 24.5 and 27.5, assigned to orthorhombic zirconia (ICSD 071964). These peaks promoted the further reduction of the surface area in comparison to less calcined samples.

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400 10 360

320 Ni-Al-H-800 8 Ni-Al-L-800 280

cm³/g STP cm³/g

(

cm³/g STP cm³/g 240 ( 6 200

160 4 120

Quantity Adsorbed

80 Quantity Adsorbed 2 40

0.0 0.2 0.4 0.6 0.8 1.0 0 0.0 0.2 0.4 0.6 0.8 1.0 Relative Pressure (P/Po) Relative Pressure (P/Po)

100

40 Ni-Zr-H-800 80 Ni-Zr-L-800

cm³/g STP cm³/g

(

( 60

20 40

Quantity Adsorbed 20

Quantity Adsorbed Quantity

0 0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Relative Pressure (P/Po) Relative Pressure (P/Po)

FigureFigure 2.2. Nitrogen adsorption//desorptiondesorption isothermsisotherms ofof freshfresh catalystscatalysts calcinedcalcined atat 800800 ◦°C.C.

TheFigure XRD 3 shows patterns the of XRD Ni-Al-L-600 spectra of and catalysts Ni-Al-L-800 pre-reduced (Figure at3 C)600 display °C for 1 di h,ff thenraction calcined peaks at at 600 2θ valuesand 800 of 12.1,°C. In 26.2, the 35.0, present 37.2, 41.0, study, 53.1, the 57.5, mesoporous 63.3, and 68.0, alumina which and were zirconia ascribed were to the prepared planes of by (101), an (103),impregnation (022), (122), method. (303), Figure (007), (330),3A represents (027), and the (141) XRD of pattern L-Al2O of3 (PDFNi-Zr Index-L-600 00-020-077). and Ni-Zr-L The-800 e. ffThisect offigure calcination shows the in these diffraction samples peaks is negligible at 2θ values and of the 24.7, profiles 29.5, are 32.5, almost 34.0, identical.41.0, and 50.5, The XRDwhich patterns can be ofascribed Ni-Al-H-800 to the andplanes Ni-Al-H-600 of (022), (111), (Figure (040),3D) (041), display (113), di ff andraction (114) peaks of L- atZrO 2θ2 values(ICSD 01 of- 19.7,081-0610). 32.5, 37.2,The 45.0,two 60.5,catalysts 65.5, have 68.0, similarand 85.0, spectra which, butwere the ascribed higher to calcined the planes catalyst of (111), presents (220), (311),somewhat (400), higher (511), (531),intensities, and (444) which of H-Almeans2O crystallinity3 (PDF Index is 00-010-0339). improved. The In these XRD catalysts,patterns theof Ni patterns-Zr-H-600 are and the same,Ni-Zr but-H- the800 Ni-Al-H-800(Figure 3B) exhibit has higher diffraction intensity, peaks which at induced2θ values further of 29.5, reduction 34.7, 50.5, of the60.0, surface 75.1, and area 85.0, and higherwhich porewere diameter assigned dueto the to theplanes higher of (111), calcination (023),(114), temperature. (115), (065), It can and be (191) inferred of H from-ZrO2 the (PDF XRD Index results 00- that035- with1398). increasing In this case, calcination the profiles temperature, are slightly the different. high surface The areaNi-Zr zirconia-H-800 samplecatalyst achievesgives two a highmore degree peaks ofat crystallinity2θ = 24.5 and whereas 27.5, assigned the low to surface orthorhombic area zirconia zirconi samplea (ICSD achieves 071964). a highThese intensity peaks promoted of the major the plane.further In reduction terms of theof the high surface surface area area in aluminacomparison support, to less all calcined peaks achieve samples. higher intensities at higher calcinationThe XRD temperatures. patterns of Ni-Al-L-600 and Ni-Al-L-800 (Figure 3C) display diffraction peaks at 2θ valuesFigure of 12.1,4 illustrates 26.2, 35.0, the 37.2, TPR 41.0, of 53.1, all catalysts. 57.5, 63.3, The and TPR 68.0, results which ofwere Ni-Zr-L-600, ascribed to Ni-Zr-L-800,the planes of Ni-Al-H-800,(101), (103), (022), and (122), Ni-Al-L-800 (303), (007), catalysts (330), show (027), that and the (141) peaks of L lie-Al in2O3 wide (PDFranges Index 00 of-020 temperature,-077). The whicheffect of represent calcination the in characteristic these samples reduction is negligible of stoichiometric and the profiles nickel are oxide almost [31 ].identical. Figure4 AThe the XRD Zr

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supported catalysts present two peaks. For instance, Ni-Zr-H-800 has peaks (maxima) at 295 ◦C and 460 ◦C, Ni-Zr-L-800 has peaks (maxima) at 314 ◦C and 462 ◦C, Ni-Zr-H-600 has peaks (maxima) at 300 ◦C and 486 ◦C and Ni-Zr-L-600 has peaks (maxima) at 482 ◦C and 640 ◦C. Low temperature (<500 ◦C) H2-TPR peaks are for reduction of free NiO and it is found that Ni-Zr-L-800 and Ni-Zr-H-800 Processes 2020, 8, x FOR PEER REVIEW 8 of 23 catalysts calcined at high temperatures have higher amounts of free NiO compared to the low calcined temperaturepatterns of Ni samples.-Al-H-800 Intermediate and Ni-Al- temperatureH-600 (Figure (< 7003D) ◦displayC) peaks diffraction indicate moderatepeaks at 2 interactionθ values of of 19.7, NiO with32.5, the37.2, support. 45.0, 60.5, In 65.5, the Zr 68.0, supported and 85.0 catalysts,, which were the ascribed lower calcination to the planes temperature of (111), (220), promotes (311), higher(400), interaction with the support. Figure4B of Al supported catalysts presents single peaks, which appear (511), (531), and (444) of H-Al2O3 (PDF Index 00-010-0339). In these catalysts, the patterns are the atsame, moderate but the or Ni high-Al ranges-H-800 ofhas temperature, higher intensity, except which in the induced Ni-Al-L-600 further catalyst, reduction which of the has surface a prominent area peakand higher (maxima) pore at diameter 360 ◦C. Generally,due to the higher the alumina calcination supported temperature. catalysts It providecan be inferred higher from metal the support XRD interaction.results that Itwith can increasing be said that calcination low temperature temperature, calcined the high alumina surface supported area zirconia samples sample have achieve reductions a peakshigh degree in the of relatively crystallinity lower whereas temperature the low regions surface thanarea zirconia do the high sample temperature achieves a calcinedhigh intensity samples. of Ni-Al-L-600the major plane. has reductionIn terms of peaks the high of free surface NiO area whereas alumina Ni-Al-L-800 support, hasall peaks a reduction achieve peak higher of NiOintensities tightly interactedat higher calcination with its support. temperatures.

Ni-Zr-L-800

Intensity (counts)

Ni-Zr-L-600

10 20 30 40 50 60 70 80 90 2q()

Ni-Zr-H-800

Intensity (counts)

Ni-Zr-H-600

10 20 30 40 50 60 70 80 90 2q()

Figure 3. Cont.

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Ni-Al-L-800

Intensity (counts)

Ni-Al-L-600

10 20 30 40 50 60 70 80 90

2θ(°)

Ni-Al-H-800

Intensity (counts) Ni-Al-H-600

10 20 30 40 50 60 70 80 90 2q() FigureFigure 3. 3.X-ray X-ray di diffractionﬀraction patterns patterns for for ((A)A) Ni-ZrNi-Zr –L–L ((B)B) Ni Ni-Zr-H-Zr-H (C) (C) Ni Ni-Al-L-Al-L (D) (D Ni) Ni-Al-H-Al-H catalysts catalysts calcined at 600 and 800 ◦C. calcined at 600 and 800 °C.

InFigure this study, 4 illustrates CO2-TPD the TPR was of used all catalysts. to examine The the TPR basicity results of of the Ni catalysts,-Zr-L-600, as Ni shown-Zr-L-800, in Figure Ni-Al5-. TheH-800, low surfaceand Ni- areaAl-L ZrO-8002 catalystssupport (Figureshow that5A) provides the peaks three lie inpeaks wide at calcinationranges of temperature, temperatures which of 600 andrepresent 800 ◦C. the Two characteristic peaks in the reduction low temperature of stoichiometric region, annickel intense oxide peak [31]. at Figure 100 ◦ C4A and the a Zr low supported intensity broadcatalysts peak present at 260–300 two ◦peaks.C, and For a small instance, peak Ni in-Zr the-H high-800 temperature has peaks (maxima region at) at 560–600 295 °C ◦andC. On460 the °C, other Ni- hand,Zr-L-800 the has proﬁle peaks of the(maxima high) surface at 314 ° areaC and ZrO 4622 (Figure°C, Ni-5ZrA)-H shows-600 has three peaks peaks (maxima for both) at 300 calcination °C and temperatures486 °C and Ni (600-Zr-L and-600 800has ◦peaksC). Two (maxima peaks) inat 482 the ° lowC and temperature 640 °C. Low region temperature at 100 ◦(<500C, with °C) lowH2-TPR and highpeaks intensity are for peaks reduction for 600 of and free 800 NiO◦C and calcination it is found temperatures, that Ni-Zr respectively,-L-800 and andNi-Zr a- distortedH-800 catalyst peak ats 300–320calcined◦ C. at There high istemperature a high intensitys have and higher broad amounts peak at ofabout free 600 NiO◦C. compared It is fundamentally to the low considered calcined thattemperature the peaks samples. at lower Intermediate temperature temperature regions can (<700 be ascribed °C) peaks to indicate the weak moderate basic sites interaction resulting of NiO from thewith physical the support. adsorption In the of Zr CO supported2 [32]. The catalysts, peak at the the lower high temperaturecalcination temperature associates to promotes the strong higher basic sitesinteraction and is accredited with the support. to the chemisorption Figure 4B of Al of supported CO2. As acatalysts result of present the numbers single of peak basics, sites, which the appear peaks have diﬀerent sizes [33]. High temperature calcined samples have wide distribution of basic sites whereas the low temperature calcined samples have a concentration of the majority weak basic sites. Processes 2020, 8, 499 10 of 21

Figure5B, displays the CO 2-TPD of the catalysts supported by the alumina. The profile shows four peaks for both calcinations. For the low surface area catalysts, Ni-Al-L-800 and Ni-Al-L-600, the peak sizes are very small, indicating low concentrations of basic sites. The peaks for the high surface area catalysts, Ni-Al-H-600 and Ni-Al-H-800, generated prominent, high intensity peaks, denoting significantProcesses contents 2020, 8, x of FOR basics PEER REVIEW sites. The peaks that appeared at lower than 150 ◦C are ascribed10 of 23 to weak basic sites, while the peaks at 250–400 ◦C are associated with medium basic sites, and the peaks at at moderate or high ranges of temperature, except in the Ni-Al-L-600 catalyst, which has a prominent 650–800peak◦C are(maxima credited) at 360 to ° strongC. Generally, basic the sites. alumina It is supported commonly catalysts agreed provide that basichigher sitesmetal on support the catalyst surfaceinteraction. can enhance It can the be adsorption said that low andtemperature the dissociation calcined alumina of CO supported2, which samples decreases have the reduction formation of carbon depositionpeaks in the relatively to a large lower magnitude temperature on regions active than metal do the surface high temperature and, hence, calcined efficiently samples. hinders Ni- the deactivationAl-L-600 of thehas catalysts.reduction peaks The COof free2-TPD NiO resultswhereas justify Ni-Al- andL-800 support has a reduction the supremacy peak of NiO of tightly Ni-Al-H-600. interacted with its support.

FigureFigure 4. H2 4.TPR H2 TPR proﬁles profiles (A ()A Zr-supported) Zr-supported catalysts, catalysts, (B ()B Al)- Al-supportedsupported catalysts. catalysts.

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FigureFigure 5.5. TPD proﬁlesprofiles of (A)) freshfresh zirconiazirconia supported-supported- and (B) fresh alumina-supportedalumina-supported catalysts catalysts calcinedcalcined atat 600600 andand 800800 ◦°C.

Figures6–9 show the catalytic activity of 10% Ni catalysts supported on high and low surface Figures 6–9 show the catalytic activity of 10% Ni catalysts supported on high and low surface area alumina and zirconia separately and calcined either at 600 or 800 ◦C. All the ﬁgures indicate CH4 area alumina and zirconia separately and calcined either at 600 or 800 °C. All the figures indicate CH4 conversion and hydrogen yield proﬁles that increase with the increase of the reaction temperature. conversion and hydrogen yield profiles that increase with the increase of the reaction temperature. ThisThis isis inin conformityconformity withwith the fact that the reaction is exothermic ( (heatheat is is generated generated)) and and,, therefore, therefore,

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according to Le Chatelier’s principle, increasing the temperature increases the products [34]. Figure according6 shows the to LeCH Chatelier’s4 conversions principle, for catalysts increasing calcined the temperature at 600 °C. The increases Ni-Al the-H- products600 catalyst [34 ].illustrates Figure6 showsbetter themethane CH4 conversions conversion for than catalysts the other calcined catalysts; at 600 the◦C. Theconversion Ni-Al-H-600 reached catalyst 90% illustrates at 650 °C. better The methaneperformance conversion Ni-Al- thanL-600 the is other lower catalysts; than that the conversion of the corresponding reached 90% Ni at- 650Al-H◦C.-600 The by performance 10%–34%, Ni-Al-L-600depending on is lower the reaction than that temperature. of the corresponding On the other Ni-Al-H-600 hand, the by 10%–34%,Ni-Zr-L-600 depending provides on better the reactionconversion temperature. than that of On Ni the-Zr- otherH-600 hand,or Ni- theAl-L Ni-Zr-L-600-600 for all reaction provides temperatures. better conversion Figure than 7 exhibits that of Ni-Zr-H-600CH4 conversions or Ni-Al-L-600 for catalysts for allcalcined reaction at temperatures. 800 °C. In this Figure case 7t heexhibits Ni-Al CH-H-4800conversions catalyst offers for catalysts better calcinedconversion at 800 than◦C. the In other this case catalysts the Ni-Al-H-800 with 87% CH catalyst4 conversion. offers better The performance conversion than of Ni the-Al other-L-800 catalysts is less withthan 87% the CH corresponding4 conversion. The Ni-Al performance-H-800 by of 5 Ni-Al-L-800%–19%, depending is less than on the the corresponding reaction temperature. Ni-Al-H-800 byAlternatively, 5%–19%, depending Ni-Zr-L-800 on provid the reactiones a slightly temperature. higher conversion Alternatively, than Ni-Zr-L-800 that of Ni-Zr provides-H-600 for a slightlymost of higherthe reaction conversion temperatures. than that From of Ni-Zr-H-600 Figures 6 and for 7 it most can be of deduced the reaction that temperatures. the calcination From at 600 Figures °C gives6 anda better7 it canconversion be deduced than that that the at 800 calcination °C. The athigh 600 surface◦C gives alumina a better support conversion performs than better that at than 800 the◦C. Thelow highsurface surface alumina alumina whereas support the performsopposite betteris true than for thethe lowzirconia surface support aluminas. Figure whereas 8 shows the opposite the % ishydrogen true for theyield zirconia of catalysts supports. calcined Figure at 8600 shows °C. Ni the-Al %- hydrogenH-600 has yieldthe highest of catalysts yield calcined of 72%, atwhile 600 Ni◦C.- Ni-Al-H-600Zr-L-600 generates has the the highest lowest yieldyield of 46% 72%, at while 650 °C. Ni-Zr-L-600 The hydrogen generates yield for the the lowest Ni-Zr yield-H-600 of catalyst 46% at 650is greater◦C. The than hydrogen that of yieldNi-Al for-L- the600. Ni-Zr-H-600 The yield profile catalyst of isdifferent greater thancatalysts that is of not Ni-Al-L-600. in-line with The that yield of profilethe conversion. of different This catalysts is attributed is not in-lineto the drop with thatin the of selectivity the conversion. of the This catalysts is attributed calcined to at the 600 drop °C. inFigure the selectivity9 displays ofthe the % hydrogen catalysts calcinedyield when at 600the ◦calcinationC. Figure9 wasdisplays done theat 800 % hydrogen°C. It indicates yield that when Ni- theAl-H calcination-800 assumes was the done highest at 800 yield◦C. Itof indicates 66%, while that Ni Ni-Al-H-800-Zr-L-800 produces assumes the the lowest highest yield yield of of55% 66%, at while650 °C. Ni-Zr-L-800 The yield producesfor the Ni the-Al lowest-L-800 yieldcatalyst of 55%is lower at 650 than◦C. that The yieldof Ni- forZr- theH-800 Ni-Al-L-800 except when catalyst the isreaction lower thantemperature that of Ni-Zr-H-800exceeds 620 ° exceptC. It can when be concluded, the reaction from temperature the perspective exceeds of 620hydrogen◦C. It can yield, be concluded,that the higher from surface the perspective area alumina of hydrogen calcined yield, at 800 that °C the(Ni- higherAl-H-800) surface is preferable area alumina for the calcined current at 800process◦C (Ni-Al-H-800) in the reaction is temperature preferable for range the currentof this study. process Table in the 2 reactionshows the temperature comparison range of catalytic of this study.performance Table2 ofshows the present the comparison work and ofthose catalytic in the performance literature; the of table the present indicates work the andsuitability those inof thethe literature;adopted techniqu the tablee indicatesin this work. the suitability of the adopted technique in this work.

100 Ni-Al-L-600 90 Ni-Zr-H-600 Ni-Al-H-600 80 Ni-Zr-H-600

Conversion % 50

CH 40

450 500 550 600 650 Temperature (C)

Figure 6. 6.Catalytic Catalytic activity activity of catalysts of catalysts calcined calcined at 600 ◦ atC: CH 6004 conversion°C: CH4 conversion against reaction against temperature reaction (450–650temperature◦C). (450–650 °C).

ProcessesProcesses2020 2020, ,8 8,, 499 x FOR PEER REVIEW 1314 of of 21 23 Processes 2020, 8, x FOR PEER REVIEW 14 of 23 100 100 90 90 Ni-Al-L-800 Ni-Al-L-800 80 Ni-Zr-H-800 80 Ni-Zr-H-800 Ni-Al-H-800 Ni-Al-H-800 70 Ni-Zr-L-800 70 Ni-Zr-L-800 60 60

Conversion % 50

CH 40 40 30 30 20 20 450 500 550 600 650 450 500 Temperature550 (C) 600 650 Temperature (C)

Figure 7. Catalytic activity of catalysts calcined at 800 °C: CH4 conversion against reaction Figure 7. Catalytic activity of catalysts calcined at 800 ◦C: CH4 conversion against reaction temperature Figure 7. Catalytic activity of catalysts calcined at 800 °C: CH4 conversion against reaction (450–650temperatureC). (450–650 °C). temperature◦ (450–650 °C). 80 80 Ni-Al-L-600 Ni-Al-L-600 Ni-Zr-H-600 70 Ni-Zr-H-600 Ni-Al-H-600 70 Ni-Al-H-600 Ni-Zr-L-600 Ni-Zr-L-600 60 60

50 50

- Yield %

H 2 40

H 40

30 30

20 20 450 500 550 600 650 450 500 Temperature550 (C) 600 650 Temperature (C) Figure 8. Catalytic activity of catalysts calcined at 600 ◦C: H2 yield against reaction temperature Figure 8. Catalytic activity of catalysts calcined at 600 °C: H2 yield against reaction temperature (450– (450–650 C). Figure650 °C). 8. ◦Catalytic activity of catalysts calcined at 600 °C: H2 yield against reaction temperature (450– 650 °C).

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80 Ni-Al-L-800 Ni-Zr-H-800 70 Ni-Al-H-800 Ni-Zr-L-800 60

- Yield %

H 40

450 500 550 600 650 ° Temperature ( C)

FigureFigure 9.9. Catalytic activity activity of of catalysts catalysts calcined calcined at at800 800 °C:◦ HC:2 Hyield2 yield against against reaction reaction temperature temperature (450– (450–650650 °C). ◦C).

Table 2. Comparison of catalytic performance of partial oxidation of methane.

Space Velocity Reaction % CH4 Catalyst Weight (g) CH4:O2 Reference (mL/g/h) Temperature (◦C) Conversion 25% Ni/Al O +TiO +CaO 0.05 1.78:1.00 6 104 650 86 [24] 2 3 2 × 10% Ni/Ce Zr O -Al O 0.5 2.00:1.00 4 104 650 67.8 [25] 0.7 0.3 2 2 3 × La NiZrO 0.01 2.00:1.00 300 104 750 40 [35] 2 6 × 8% Ni/CeO -ZrO -Al O 0.15 2.00:1.00 20 104 650 88.5 [36] 2 2 2 3 × 6% Ni/SiO 0.1 2.00:1.00 6 104 600 85 [37] 2 × Ni-Al-H-600 0.1 2.00:1.00 1.95 104 650 90 This work ×

Figure 10A shows the weight loss of the used catalysts, calcined at 600 ◦C, and operated for 5 h. The proﬁle shows the nonexistence of desorption of strongly adsorbed water and oxidation of volatile organic compounds since no weight loss below 650 ◦C was detected. The weight loss observed over 650 ◦C was alike for all catalysts and attributed to the oxidation of graphitic carbon species [38,39]. For the Ni-Al-L-600 and Ni-Al-H-600 catalysts, the weight loss was about 8.6% and 6.0%, respectively, justifying the diﬀerent conversions obtained with these two catalysts. While for the Ni-Zr-L-600 and Ni-Zr-H-600 catalysts, the weight loss was about 2.3% and 5.5%, respectively. Figure 10B shows the weight loss of the used catalysts, calcined at 800 ◦C, and operated for 5 h. The weight losses are quite low: Ni-Al-H-800, Ni-Al-L-800, Ni-Zr-L-800, and Ni-Zr-H-800 give weight losses of approximately 7.0%, 0.9%, 5.2%, and 1.0%, respectively.

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105

100

95 A

85 Ni-Al-H-600 Weight loss(%) Ni-Zr-L-600 80 Ni-Al-L-600 Ni-Zr-H-600 75

70 200 400 600 800 1000 Temperature (°C)

105

100

95 B

90 Ni-Al-H-800 Ni-Zr-L-800 85 Ni-Al-L-800

Weight loss (%) Ni-Zr-H-800 80

70 200 400 600 800 1000 Temperature (C)

FigureFigure 10. 10.TGA TGA shapesshapes ofof thethe spentspent calcinedcalcined atat ((AA)) 600600 ◦°C and ( B)) 800 800 °◦C,C, operated operated for for 5 5 h, h, during during the the reactionreaction at at 450–650 450–650◦ °C.C.

FigureFigure 11 11 exhibits exhibits thethe CH CH44 conversionconversion and H2--yieldyield profiles profiles of of the the Ni Ni-Al-H-600-Al-H-600 catalyst catalyst for for didifferentfferent reaction reaction temperatures temperatures for for aboutabout 77 hh time-on-stream.time-on-stream. Figure 11A11A shows shows methane methane conversion conversion againstagainst time-on-streamtime-on-stream for different different reaction reaction temperatures. temperatures. The The best best CH CH4 conversion4 conversion of 90% of 90%and the and

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Processes 2020, 8, x FOR PEER REVIEW 18 of 23 the highest stability is attained using a 650 ◦C reaction temperature, while 52% CH4 conversion is recordedhighest for stability a 500 ◦ C is reaction attained temperature. using a 650 The°C reaction conversion temperature, enhances while with the 52% increase CH4 conversion of the reaction is temperature,recorded for since a 500 rates °C reaction of reaction tempe increaserature. The with conversion increase ofenhances temperature. with the The increase 650 ◦ ofC givesthe reaction not only excellenttemperature, activity since but alsorates excellent of reaction stability. increase Therefore, with increase the of time-on-stream temperature. The analysis 650 °C indicates gives not thatonly the excellent activity but also excellent stability. Therefore, the time-on-stream analysis indicates that the 650 ◦C reaction temperature results in the best performance of the of Ni-Al-H-600 catalyst. Figure 11B 650 °C reaction temperature results in the best performance of the of Ni-Al-H-600 catalyst. Figure 11B shows hydrogen yield against time-on-stream for diﬀerent reaction temperatures. The highest H2 yield shows hydrogen yield against time-on-stream for different reaction temperatures. The highest H2 of 72% and the best stability is obtained at a 650 ◦C reaction temperature, while the lowest H2 yield of yield of 72% and the best stability is obtained at a 650 °C reaction temperature, while the lowest H2 28% is registered for 500 C. yield of 28% is registered◦ for 500 °C.

Figure 11. (A) Conversions of CH4 and (B) H2 yield during a 7 h stability test of the Ni-Al-H-600 Figure 11. (A) Conversions of CH4 and (B)H2 yield during a 7 h stability test of the Ni-Al-H-600 catalyst; activation 750 °C; reaction temperatures (500–650 °C), F/W = 19500 mL/g-cat. catalyst; activation 750 ◦C; reaction temperatures (500–650 ◦C), F/W = 19,500 mL/g-cat.

Processes 2020, 8, x FOR PEER REVIEW 19 of 23 Processes 2020, 8, 499 17 of 21 Figure 12 shows the TPO profiles of the spent Ni-Al-H-600 catalyst operated for 5 h at different reactionFigure temperatures. 12 shows the The TPO figure profiles shows of the peaks spent of Ni-Al-H-600 CO2 desorption catalyst at operated lower temperatures for 5 h at different for all reaction temperatures corresponding to atomic and amorphous carbonaceous species. At the high reaction temperatures. The figure shows peaks of CO2 desorption at lower temperatures for all reaction temperaturesreaction temperatures corresponding of 600 to and atomic 650 and°C, there amorphous is an additional carbonaceous peak species. corresponding At the high to graphitic reaction carbonaceous species. The negative peaks around at around 250–300 °C are ascribed to the oxidation temperatures of 600 and 650 ◦C, there is an additional peak corresponding to graphitic carbonaceous of the metallic nickel and its intensity is diminished with the increase of reaction temperature. species. The negative peaks around at around 250–300 ◦C are ascribed to the oxidation of the metallic nickelTherefore and for its intensity the same is catalyst diminished, increasing with the the increase reaction of temperature reaction temperature. above 550 Therefore °C induces for the the formation of graphitic carbon, whereas the different oxidation regions result from the different same catalyst, increasing the reaction temperature above 550 ◦C induces the formation of graphitic carbon,location whereass of the thecarbon different on the oxidation surface regionsof the catalyst. result from Carbon the different deposition locations is an ofessential the carbon feature on the of surfacereforming of thereaction catalyst.s. The Carbon different deposition carbon is structure an essentials are feature customarily of reforming assessed reactions. by Raman The differentanalysis. carbonFigure structures13 depicts arethe customarily Raman spectra assessed of spent by Raman Ni-Al- analysis.H-600 catalysts Figure 13acquired depicts for the 7 Raman h using spectra reaction of temperatures of 500, 550, 600, and 650 °C. Two comparable intensity peaks appeared at 1475 cm−1 and spent Ni-Al-H-600 catalysts acquired for 7 h using reaction temperatures of 500, 550, 600, and 650 ◦C. 1535 cm−1, corresponding to the D band, assigned to sp31 hybridized amorphous1 carbon, and G band, Two comparable intensity peaks appeared at 1475 cm− and 1535 cm− , corresponding to the D band, assignedspecified toby sp3 the hybridized presence of amorphous graphitized carbon, carbon, and respectively. G band, specified The D byband the presenceand G band of graphitized are typical carbon,bands of respectively. regular-structure The D bandd carbon and G that band form are typical on the bands surface of regular-structured of Ni-Al-H-600 carbon during that the form partial on thereforming surface reaction. of Ni-Al-H-600 The results during of theRaman partial analysis reforming confirm reaction. the formation The results of of different Raman analysistypes of confirmcarbon, theparticularly formation for of high different reaction types temperatures of carbon, particularly, as is illustrated for high in reactionthe TPO temperatures,analysis. Figure as 14 is illustratedillustrates inthe the TGA TPO analysis analysis. of the Figure Ni- Al14- Hillustrates-600 catalyst the TGAoperated analysis at different of the Ni-Al-H-600reaction temp catalysteratures operated that lasted at differentfor 5 h. The reaction result temperatures shows 4% weight that lasted loss of for the 5 h. mass The resultdue to shows gasification 4% weight of the loss gas of formed the mass on due the catalyst when the reaction temperature was 500 °C, while 8% weight loss was determined using a to gasification of the gas formed on the catalyst when the reaction temperature was 500 ◦C, while 8% reaction temperature of 650 °C. As the reaction temperature was increased, the production of carbon weight loss was determined using a reaction temperature of 650 ◦C. As the reaction temperature was grew slightly as a result of increased decomposition of CH4. increased, the production of carbon grew slightly as a result of increased decomposition of CH4.

6500C

6000C

5500C

0 TCD signal (a.u.)TCD 500 C

0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 Temperature (C)

Figure 12. TPO proﬁles of the Ni-Al-H-600 catalyst after 7 h reaction temperature operated at diﬀerent Figure 12. TPO profiles of the Ni-Al-H-600 catalyst after 7 h reaction temperature operated at different values (500, 550, 600, and 650 C). values (500, 550, 600, and 650 ◦°C).

Processes 2020, 8, x FOR PEER REVIEW 20 of 23 Processes 2020, 8, 499 18 of 21 Processes 2020, 8, x FOR PEER REVIEW 20 of 23 D band G band D band G band 650 C 650 C

600 C 600 C

550 C

Intensity (a.u.) 550 C

Intensity (a.u.)

500 C 500 C 1400 1500 1600 -1 1400 Raman shift1500 (cm ) 1600 -1 Figure 13. Raman spectra of the Ni-Al-HRaman-600 catalyst shift obtained (cm at different) reaction temperatures (500, 550, 600, and 650 °C). FigureFigure 13.13. Raman spectra of the Ni Ni-Al-H-600-Al-H-600 catalyst obtained at different diﬀerent reaction temperatures (500,

550,550, 600,600, andand 650650 ◦°C).C).

Figure 14. TGA analysis of the Ni-Al-H-600 catalyst obtained at diﬀerent reaction temperatures for 7 h. Figure 14. TGA analysis of the Ni-Al-H-600 catalyst obtained at different reaction temperatures for 7 4. Conclusionsh. Figure 14. TGA analysis of the Ni-Al-H-600 catalyst obtained at different reaction temperatures for 7 Dih. ﬀerent calcined 10% Ni catalysts supported on both high and low Al O and on both high and 4. Conclusions 2 3 low ZO2 were tested for partial oxidation reforming of methane. The catalysts were calcined at 600 and4. ConclusionsDifferent 800 ◦C. The calcined highest 10% CH Ni4 conversion catalysts supported of 90% and on H both2 yield high of 72%and werelow Al obtained2O3 and usingon both Ni-Al-H-600. high and low ZO2 were tested for partial oxidation reforming of methane. The catalysts were calcined at 600 Different calcined 10% Ni catalysts supported on both high and low Al2O3 and on both high and low ZO2 were tested for partial oxidation reforming of methane. The catalysts were calcined at 600

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The Ni-Al-H-600 catalyst retained the maximum surface area and the biggest pore volume. The study of the TGA indicated low amounts of carbon deposition except for the Ni-Al-L-600 catalyst. The TPO and Raman analyses of the catalysts denoted the presence of diﬀerent types of carbon. The best activity and stability of performance of the Ni-Al-H-600 catalyst over diﬀerent reaction temperatures (500–650 ◦C) was attained when the reaction was conducted at 650 ◦C for the period of 7 h.

Author Contributions: Experiment and Writing—original draft preparation A.F., A.A.I., A.S.A.-F., and S.O.K.; XRD analysis S.M., Preparation of catalyst, A.S.A.-F., H.A., Y.A., A.A., and S.A., Writing—review and editing, A.E.A. and A.F. All authors have read and agreed to the published version of the manuscript. Funding: The work is supported by the Deanship of Scientific Research programs of King Saud University via project No. RGP-119. Acknowledgments: The KSU authors would like to extend their sincere appreciation to the Deanship of Scientific Research at the King Saud University for its funding for this research group project # RGP-119. Conflicts of Interest: The authors declare no conflict of interest.

References

1. He, S.; Castello, D.; Krishnamurthy, K.R.; Al-Fatesh, A.S.; Winkelman, J.G.M.; Seshan, K.; Fakeeha, A.H.; Kersten, S.R.A.; Heeres, H.J. Kinetics of long chain n-paraﬃn dehydrogenation over a commercial

Pt-Sn-K-Mg/γ-Al2O3 catalyst: Model studies using n-dodecane. Appl. Catal. A Gen. 2019, 579, 130–140. [CrossRef] 2. Vermeiren, W.J.M.; Blomsma, E.; Jacobs, P.A. Catalytic and thermodynamic approach of the oxyreforming reaction of methane. Catal. Today 1992, 13, 427–436. [CrossRef] 3. Gillessen, B.; Heinrichs, H.; Hake, J.F.; Allelein, H.J. Natural gas as a bridge to sustainability: Infrastructure expansion regarding energy security and system transition. Appl. Energy 2019, 251, 113377. [CrossRef] 4. Kaddeche, D.; Djaidja, A.; Barama, A. Partial oxidation of methane on co-precipitated Ni–Mg/Al catalysts modiﬁed with copper or iron. Int. J. Hydrog. Energy 2017, 42, 15002–15009. [CrossRef] 5. Pruksawan, S.; Kitiyanan, B.; Ziﬀ, R.M. Partial oxidation of methane on a nickel catalyst: Kinetic Monte-Carlo simulation study. Chem. Eng. Sci. 2016, 147, 128–136. [CrossRef]

6. Pantaleo, G.; Parola, V.L.; Deganello, F.; Singha, R.K.; Bal, R.; Venezia, A.M. Ni/CeO2 catalysts for methane partial oxidation: Synthesis driven structural and catalytic eﬀects. Appl. Catal. B Environ. 2016, 189, 233–241. [CrossRef]

7. Gao, H.; Zhou, L.; Luo, X.; Liang, Z. Optimized process conﬁguration for CO2 recovery from crude synthesis gas via a rectisol wash process. Int. J. Greenh. Gas Control 2018, 79, 83–90. [CrossRef]

8. Zhang, L.; Zhang, Q.; Liu, Y.; Zhang, Y. Dry reforming of methane over Ni/MgO-Al2O3 catalysts prepared by two-step hydrothermal method. Appl. Surf. Sci. 2016, 389, 25–33. [CrossRef] 9. Al-Fatesh, A.S.; Abu-Dahrieh, J.K.; Atia, H.; Armbruster, U.; Ibrahim, A.A.; Khan, W.U.; Abasaeed, A.E.;

Fakeeha, A.H. Eﬀect of pre-treatment and calcination temperature on Al2O3-ZrO2 supported Ni-Co catalysts for dry reforming of methane. Int. J. Hydrog. Energy 2019, 44, 21546–21558. [CrossRef]

10. Yang, W.; Fan, A.; Yao, H.; Liu, W. Effect of reduced pressures on the combustion efficiency of lean H2/air flames in a micro cavity-combustor. Int. J. Hydrog. Energy 2016, 41, 15354–15361. [CrossRef] 11. Gan, Y.; Luo, Y.; Wang, M.; Shi, Y.; Yan, Y. Effect of alternating electric fields on the behaviour of small-scale laminar diffusion flames. Appl. Therm. Eng. 2015, 89, 306–315. [CrossRef] 12. Radfarnia, H.R.; Iliuta, M.C. Hydrogen production by sorption-enhanced steam methane reforming process using CaO-Zr/Ni bifunctional sorbent–catalyst. Chem. Eng. Process. Process Intensif. 2014, 86, 96–103. [CrossRef] 13. Kaczmarek, D.; Atakan, B.; Kasper, T. Investigation of the partial oxidation of methane/n-heptane-mixtures and the interaction of methane and n-heptane under ultra-rich conditions. Combust. Flame 2019, 205, 345–357. [CrossRef] 14. Melchiori, T.; Di Felice, L.; Mota, N.; Navarro, R.M.; Fierro, J.L.G.; Annaland, M.V.S.; Gallucci, F.

Methane partial oxidation over a LaCr0.85Ru0.15O3 catalyst: Characterization, activity tests and kinetic modeling. Appl. Catal. A Gen. 2014, 486, 239–249. [CrossRef] Processes 2020, 8, 499 20 of 21

15. Costa, D.S.; Gomes, R.S.; Rodella, C.B.; da Silva, R.B.; Fréty, R.; Teixeira Neto, É.; Brandão, S.T. Study of nickel, lanthanum and niobium-based catalysts applied in the partial oxidation of methane. Catal. Today 2018. [CrossRef]

16. Wang, F.; Li, W.-Z.; Lin, J.-D.; Chen, Z.-Q.; Wang, Y. Crucial support eﬀect on the durability of Pt/MgAl2O4 for partial oxidation of methane to syngas. Appl. Catal. B Environ. 2018, 231, 292–298. [CrossRef] 17. Hou, Z.; Chen, P.; Fang, H.; Zheng, X.; Yashima, T. Production of synthesis gas via methane reforming with

CO2 on noble metals and small amount of noble-(Rh-) promoted Ni catalysts. Int. J. Hydrog. Energy 2006, 31, 555–561. [CrossRef]

18. Fasolini, A.; Abate, S.; Barbera, D.; Centi, G.; Basile, F. Pure H2 production by methane oxy-reforming over Rh-Mg-Al hydrotalcite-derived catalysts coupled with a Pd membrane. Appl. Catal. A Gen. 2019, 581, 91–102. [CrossRef] 19. Araújo, J.C.S.; Oton, L.F.; Bessa, B.; Neto, A.B.S.; Oliveira, A.C.; Lang, R.; Otubo, L.; Bueno, J.M.C. The role

of Pt loading on La2O3-Al2O3 support for methane conversion reactions via partial oxidation and steam reforming. Fuel 2019, 254, 115681. [CrossRef]

20. Zhao, A.; Ying, W.; Zhang, H.; Ma, H.; Fang, D. Ni–Al2O3 catalysts prepared by solution combustion method for syngas methanation. Catal. Commun. 2012, 17, 34–38. [CrossRef]

21. Song, Y.; Liu, H.; Liu, S.; He, D. Partial Oxidation of Methane to Syngas over Ni/Al2O3 Catalysts Prepared by a Modiﬁed Sol Gel Method. Energy Fuels 2009, 23, 1925–1930. [CrossRef] − 22. Beretta, A.; Bruno, T.; Groppi, G.; Tavazzi, I.; Forzatti, P. Conditioning of Rh/α-Al2O3 catalysts for H2 production via CH4 partial oxidation at high space velocity. Appl. Catal. B Environ. 2007, 70, 515–524. [CrossRef]

23. Jing, Q.; Zheng, X.J.E. Combined catalytic partial oxidation and CO2 reforming of methane over ZrO2-modiﬁed Ni/SiO2 catalysts using ﬂuidized-bed reactor. Energy 2006, 31, 2184–2192. [CrossRef] 24. Dissanayake, D.; Rosynek, M.P.; Kharas, K.C.C.; Lunsford, J.H.L. Partial oxidation of methane to carbon

monoxide and hydrogen over a Ni/Al2O3 catalyst. J. Catal. 1991, 132, 117–127. [CrossRef] 25. Zhang, Q.; Shen, M.; Wen, J.; Wang, J.; Fei, Y. Partial oxidation of methane on Ni/CeO2-ZrO2/γ- Al2O3 prepared using diﬀerent processes. J. Rare Earths 2008, 26, 347–351. [CrossRef] 26. Sajjadi, S.M.; Haghighi, M. Impregnation vs. sol-gel and sol-gel-plasma dispersion of nickel nanoparticles

over Al2O3 employed in combined dry reforming and partial oxidation of greenhouse gases to syngas. Int. J. Hydrog. Energy 2018, 43, 15014–15029. [CrossRef] 27. Wu, P.; Li, X.; Ji, S.; Lang, B.; Habimana, F.; Li, C. Steam reforming of methane to hydrogen over Ni-based metal monolith catalysts. Catal. Today 2009, 146, 82–86. [CrossRef] 28. Dong, W.-S.; Jun, K.-W.; Roh, H.-S.; Liu, Z.-W.; Park, S.-E. Comparative study on partial oxidation of methane

over Ni/ZrO2, Ni/CeO2 and Ni/Ce–ZrO2, catalysts. Catal. Lett. 2002, 78, 215–222. [CrossRef] 29. Hadj-Sadok Ouaguenouni, M.; Benadda, A.; Kiennemann, A.; Barama, A. Preparation and catalytic activity of nickel–manganese oxide catalysts in the reaction of partial oxidation of methane. Comptes Rendus Chim. 2009, 12, 740–747. [CrossRef] 30. Moral, A.; Reyero, I.; Llorca, J.; Bimbela, F.; Gandía, L.M. Partial oxidation of methane to syngas using Co/Mg and Co/Mg-Al oxide supported catalysts. Catal. Today 2019, 333, 259–267. [CrossRef] 31. Chen, L.; Lu, Y.; Hong, Q.; Lin, J.; Dautzenberg, F.M. Catalytic partial oxidation of methane to syngas over Ca-decorated-Al2O3-supported Ni and NiB catalysts. Appl. Catal. A Gen. 2005, 292, 295–304. [CrossRef] 32. Wierzbicki, D.; Baran, R.; D˛ebek,R.; Motak, M.; Gálvez, M.E.; Grzybek, T.; Da Costa, P.; Glatzel, P.Examination

of the inﬂuence of La promotion on Ni state in hydrotalcite-derived catalysts under CO2 methanation reaction conditions: Operando X-ray absorption and emission spectroscopy investigation. Appl. Catal. B Environ. 2018, 232, 409–419. [CrossRef] 33. Li, D.; Li, R.; Lu, M.; Lin, X.; Zhan, Y.; Jiang, L. Carbon dioxide reforming of methane over Ru catalysts supported on Mg-Al oxides: A highly dispersed and stable Ru/Mg(Al)O catalyst. Appl. Catal. B Environ. 2017, 200, 566–577. [CrossRef] 34. Chatelier, H.L.L. Sur un énoncé général des lois des équilibres chimiques. Comptes Rendus Chim. 1884, 99, 786–789. 35. Ma, Y.; Ma, Y.; Chen, Y.; Ma, S.; Li, Q.; Hu, X.; Wang, Z.; Buckley, C.E.; Dong, D. Highly stable nanoﬁbrous

La2NiZrO6 catalysts for fast methane partial oxidation. Fuel 2020, 265, 116861. [CrossRef] Processes 2020, 8, 499 21 of 21

36. Dajiang, M.; Yaoqiang, C.; Junbo, Z.; Zhenling, W.; Di, M.; Maochu, G. Catalytic Partial Oxidation of Methane

over Ni/CeO2-ZrO2-Al2O3. J. Rare Earths 2007, 25, 311–315. [CrossRef] 37. Emamdoust, A.; La Parola, V.; Pantaleo, G.; Testa, M.L.; Shayesteh, S.F.; Venezia, A.M. Partial oxidation of

methane over SiO2 supported Ni and NiCe catalysts. J. Energy Chem. 2020, 47, 1–9. [CrossRef] 38. Padilla, R.; Benito, M.; Rodríguez, L.; Serrano, A.; Muñoz, G.; Daza, L. Nickel and cobalt as active phase on supported zirconia catalysts for bio-ethanol reforming: Inﬂuence of the reaction mechanism on catalysts performance. Int. J. Hydrog. Energy 2010, 35, 8921–8928. [CrossRef] 39. Ding, M.; Tu, J.; Zhang, Q.; Wang, M.; Tsubaki, N.; Wang, T.; Ma, L. Enhancement of methanation of bio-syngas over CeO2-modiﬁed Ni/Al2O3 catalysts. Biomass Bioenergy 2016, 85, 12–17. [CrossRef]

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