Process Intensiﬁcation of Methane Production via Catalytic Hydrogenation in the Presence of Ni-CeO 2 /Cr 2 O 3 Using a Micro-Channel Reactor

: A slight amount of Cr 2 O 3 segregation in 40 wt% NiO/Ce 0.5 Cr 0.5 O 2 was presented at the surface. The best catalytic performance towards the reaction was achieved at 74% of CO 2 conversion and 100% CH 4 selectivity at 310 ◦ C, the reactant (H 2 /CO 2 ) feed molar ratio was 4, and the WHSV was 56,500 ml N · h − 1 · g − 1cat . The mechanistic pathway was proposed through carbonates and formates as a mediator during CO 2 and H 2 interaction. Activation energy was estimated at 4.85 kJ/mol, when the orders of the reaction were ranging from 0.33 to 1.07 for n th -order, and 0.40 to 0.53 for m th -order. assembling by laser welding.


Introduction
CO 2 is one of the greenhouse gases (GHGs), well-known for causing the climate change and global warming. This study proposes CO 2 conversion to methane via CO 2 hydrogenation or the so-called methanation process. The reaction generally occurs at moderately a high temperature ranging from 250 to 400 • C [1] associated with the reverse water gas shift (RWGS) reaction [2]. Ni-doped high porous supports were extensively studied for such a process during the past decades. Ni/SiO 2 prepared using metal-organic framework (MOF) derivatives were studied by Ye et al., (2021) [3]. It was reported that the nickel phyllosilicate, formed by bonding between Ni and silicate enhanced the stability of the catalyst towards the reaction. The catalyst offered 77.2% of CO 2 conversion and 100% CH 4 selectivity; at 310 • C, under 20 bar, within 52 h of reaction time (T 50 = 270 • C). Atzori et al., (2019) [4] synthesized 25% Ni/CeO 2 -ZrO 2 (3:1 ratio of CeO 2 /ZrO 2 ) using the soft-template method which was found to provide even better catalytic performance compared with the Ni/SiO 2 . The catalyst provided 85% CO 2 conversion and 100% CH 4 selectivity at a relatively lower operating temperature of 300 • C, after running for 6 h (T 50 = 260 • C). From this study, it was suggested that CO 2 adsorption occurred on CeO 2 /ZrO 2, whereas gaseous H 2 reacted with the active Ni species. The effect of Ni content and the choice of supports in the catalyst system were studied by Gac et al., (2019) [5], where the level of Ni varied from 10 to 40% by weight on various supports, i.e., CeO 2 , ZrO 2 , and Al 2 O 3 . It was reported that the optimal Ni loading was between 20 and 30% at T 50 = 250 • C, whereas the activity of the catalyst system, when using a catalyst increases the desired products by reducing their reaction's activation energies, while also hindering the unwanted side reactions. These actions lead to the elimination of thermodynamic limitation, and less complication in a post-reaction-separation unit due to its narrower product distribution.
The methanation catalysts generally consist of two parts: (1) transition/noble metal, i.e., Ni, Fe, Co, Ru or Pd, acts as an active site [2]; and (2) (mixed) metal oxide, i.e., Al 2 O 3 , SiO 2 , ZrO 2 , CeO 2 , La 2 O 3 , MgO, TiO 2 , carbon materials and zeolites, act as a support [12,13]. CeO 2 was chosen as a support in the present work because of its outstanding oxygen storage capacity and its ability to promote the active metal dispersion [14]. Besides, the oxygen vacancy of CeO 2 can be tailored to needs by substituting the smaller transition metal ion (compared with Ce) into it such as Zr 4+ or Cr 3+ [16]. The high oxygen vacancy material is well-known to improve its acidic/basic catalytic properties for CO/CO 2 adsorption capability [15,17]. Several previous studies reported that CeO 2 /Cr 2 O 3 established higher oxygen vacancy than CeO 2 /ZrO 2 [18,19]. Vannice et al. (1977) [2] compared the activities of group VIII metals that were dispersed on Al 2 O 3 . The result showed that the order of the activity ranked from high to low as Ru >> Fe > Ni > Co > Rh > Pd > Pt > Ir. Although Ru is superior, its cost is high. Fe or Ni therefore received greater attention due to its reasonable price and sufficient catalytic activity. Nie et al. (2017) [20] studied Ni content by varying percentage of Ni, which was doped on Ce 0.9 Zr 0.1 O 2 , from 10 to 40%. It was found that CO 2 conversion reached its maximum at 95% when Ni level was 40%. According to our previous work [21], Ni doped on Ce x Cr 1-x O 2 of which the stoichiometric ratio (x) was 0.5, is one of the superior catalyst supports for simultaneously maximizing CO 2 conversion and CH 4 selectivity. In this work, 40% Ni-CeO 2 /Cr 2 O 3 was prepared using the one-pot hydrolysis method. The catalyst was wash-coated inside a stainless-steel micro-channel reactor. Physicochemical properties of the catalyst were investigated using an X-ray diffractometer (XRD), hydrogen temperature-programmed reduction (H 2 -TPR), and Fourier-transform infrared spectroscopy (FTIR). The effect of operating parameters on the catalytic performance was investigated. The mechanism of the gas-solid reaction and kinetics were also predicted.   was detected in the catalyst's diffractogram, implying that Cr was substituted into the Ce lattice with none or little segregation of Cr to the catalyst surface. This can be due to the fact that the ionic radius of 3D transition metal is smaller than the cerium ions radius (0.615 Å for Cr 3+ versus 1.01 Å for Ce 4+ [16]). The CeO 2 peaks shown in the catalyst's diffractogram were slightly shifted to a higher degree, compared with the pure CeO 2 which were positioned at 28.54, 33.12, 47.34, 56.20, 58.92, and 69.30 (JCPDS card No. 03-065-5923). This was due to the substitution of a smaller Cr ion into the CeO 2 lattice, creating a larger channel radius in the lattice and high oxygen mobility [22] while the fluorite structure was remained. The NiO peak in the catalyst was seen to be broader than that in pure NiO (a), indicating that the NiO in the catalyst system has a smaller particle size, resulting in good dispersion of Ni on the CeO 2 /Cr 2 O 3 . This result agreed with TPR results in the later section.

H2-TPR
The catalyst reducibility was studied by the temperature programmed reduction (TPR) technique using 5% H2/Ar. In Figure 2, it can be seen that for pure NiO (a), two distinct peaks were observed and centered at 380 °C (peak α) and 500 °C (peak β), attributing to a reduction in Ni 2+ ions to metallic Ni at the surface and bulk oxygen, respectively. This result agreed with the work reported by other researchers [23]. The TPR spectrum of the catalyst (b) showed three reduction peaks. The first peak (peak α) was due to reduction in NiO, which occurred at 310 °C, and lower than the reduction temperature of pure NiO. This indicates good dispersion of NiO on the CeO2/Cr2O3 [24], which resulted from the smaller particle size of Ni at the surface. The strong bonding interaction between NiO and the support (CeO2/Cr2O3) was suggested, as the peak β of the catalyst was found at a higher temperature compared with that of pure NiO [24]. The peak γ of the catalyst was found at a temperature higher than 900 °C. This peak is the result of NiCr2O4 spinel [25] or CeCrO3 perovskite [26] which were formed via NiO + Cr2O3 → NiCr2O4 or 2Ce0.5Cr0.5O2+δ → CeCrO3 + (0.5+δ)O2, indicating that a slight amount of Cr2O3 presented at the surface; however, it was not sufficient to be detected by XRD.

H 2 -TPR
The catalyst reducibility was studied by the temperature programmed reduction (TPR) technique using 5% H 2 /Ar. In Figure 2, it can be seen that for pure NiO (a), two distinct peaks were observed and centered at 380 • C (peak α) and 500 • C (peak β), attributing to a reduction in Ni 2+ ions to metallic Ni at the surface and bulk oxygen, respectively. This result agreed with the work reported by other researchers [23]. The TPR spectrum of the catalyst (b) showed three reduction peaks. The first peak (peak α) was due to reduction in NiO, which occurred at 310 • C, and lower than the reduction temperature of pure NiO. This indicates good dispersion of NiO on the CeO 2 /Cr 2 O 3 [24], which resulted from the smaller particle size of Ni at the surface. The strong bonding interaction between NiO and the support (CeO 2 /Cr 2 O 3 ) was suggested, as the peak β of the catalyst was found at a higher temperature compared with that of pure NiO [24]. The peak γ of the catalyst was found at a temperature higher than 900 • C. This peak is the result of NiCr 2 O 4 spinel [25] or CeCrO 3 perovskite [26] which were formed via NiO + Cr 2 O 3 → NiCr 2 O 4 or 2Ce 0.5 Cr 0.5 O 2+δ → CeCrO 3 + (0.5 + δ)O 2 , indicating that a slight amount of Cr 2 O 3 presented at the surface; however, it was not sufficient to be detected by XRD.

SEM Morphology before and after Catalyst Coating
The SEM images of the substrates were observed as shown in Figure Figure 3(c2) demonstrate the substrate's surface after the cleansing and annealing process. The surface appeared smoother after the cleansing process. After the annealing process, images of the grains could be seen. This was suspected to be a layer of mixed oxides, formed by the oxidation process of oxygen and the metal components in the stainless steel such as Ni, Cr and Fe. The mixed metal oxides can enhance the surface roughness and establish better chemical adhesion of the surface with the coating catalyst. Figure 4 illustrates the morphologies of the coated surface after the wash-coat process. The thickness of catalyst was determined at 144 µm. The catalyst layer was seen as uniformly scattered and homogeneous along the channel. Figure 4(b2) demonstrates micro-sized particles with different distribution of macro-pore. Where α, β, and γ are weak, moderate and strong peak interaction temperature, respectively.

SEM Morphology Before and After Catalyst Coating
The SEM images of the substrates were observed as shown in Figure 3: where (a1) and (a2) represent the surface before the cleansing process; (b1) and (b2) are for the surface after the cleansing process; and (c1) and (c2) demonstrate the substrate's surface after the cleansing and annealing process. The surface appeared smoother after the cleansing process. After the annealing process, images of the grains could be seen. This was suspected to be a layer of mixed oxides, formed by the oxidation process of oxygen and the metal components in the stainless steel such as Ni, Cr and Fe. The mixed metal oxides can enhance the surface roughness and establish better chemical adhesion of the surface with the coating catalyst. Figure 4 illustrates the morphologies of the coated surface after the wash-coat process. The thickness of catalyst was determined at 144 µm. The catalyst layer was seen as uniformly scattered and homogeneous along the channel. Figure 4(b2) demonstrates micro-sized particles with different distribution of macro-pore.   Photograph and surface morphology of the micro-channel plates: (a1) ×200 and (a2) ×1000 before cleansing; (b1) ×200 and (b2) ×1000 after cleansing, with 20% citric acid; (c1) ×70 and (c2) ×50,000 after annealing at 800 °C.

FTIR Spectroscopy before and after Exposure to Reaction
FTIR spectroscopy was applied to identify the adsorbed functional species on the catalyst when being exposed in the gaseous reactant stream, CO 2 + H 2 , at 270 • C. Figure 5 demonstrates the adsorbed species of the pre-(top) and post-exposure (bottom) catalyst. The deepest strong band at 3413-3421 cm −1 is ascribed to the O-H stretching vibration in OH groups, due to from adsorbed water and/or H-chemisorbed atom on metal. The bridged bidentate carbonate species was banded at 1630 cm −1 , 1320 cm −1 , and 1018 cm −1 , assigned to the asymmetric, symmetric CO 3 -stretching, and OCO-bending (ν as (CO 3 ), ν s (CO 3 ), and δ(OCO), respectively, as a vibration mode), which appeared in both pre-and post-exposure catalysts. While the carbonate species of the fresh catalyst were likely to be the result of an uncompleted combustion process during the calcination (in static air at 500 • C for 12 h at 10 • C·min −1 of heating rate), those in the spent catalyst were suspected to indicate the CO 2 adsorption.
Importantly, the bands located at 1383 and 1470 cm −1 , which are assigned to the CH-bending and asymmetric CO 2 -stretching (δ(CH) and ν as (COO) as a vibration mode), are attributed to formate species, according to chelating bidentate.
post-exposure catalysts. While the carbonate species of the fresh catalyst were likely to be the result of an uncompleted combustion process during the calcination (in static air at 500 °C for 12 h at 10 °C·min −1 of heating rate), those in the spent catalyst were suspected to indicate the CO2 adsorption.
Importantly, the bands located at 1383 and 1470 cm −1 , which are assigned to the CHbending and asymmetric CO2-stretching (δ(CH) and νas(COO) as a vibration mode), are attributed to formate species, according to chelating bidentate. In addition, the bending vibration of NiO bonding was indicated at a wavelength ranging from 495 to 540 cm −1 [27]. These results indicated that the carbonates and formates are the intermediates for the methanation via CO2 hydrogenation. The mechanism of CO2 hydrogenation in the presence of Ni-CeO2/Cr2O3 is proposed as schematically shown below in Figure 6: In addition, the bending vibration of NiO bonding was indicated at a wavelength ranging from 495 to 540 cm −1 [27]. These results indicated that the carbonates and formates are the intermediates for the methanation via CO 2 hydrogenation. The mechanism of CO 2 hydrogenation in the presence of Ni-CeO 2 /Cr 2 O 3 is proposed as schematically shown below in Figure 6:    Figure 7, it can be seen that CO2 conversion was increased when the amount of hydrogen was increased (or ratio of H2 to CO2 was changed from 2:1 to 4:1) (Figure 7a), indicating that the CO2 was excess in the first case (Figure 7a). CH4 selectivity was found to increase when the percentage of H2 in the feed reactant increased (Figure 7b), meaning that the H2 is the limiting agent when the H2 to CO2 ratio was 2 to 1 (Figure 7a [30], which stated that the stoichiometric ratio of H2/CO2 should be equaled 4.0 due to the basicity property on   Figure 7, it can be seen that CO 2 conversion was increased when the amount of hydrogen was increased (or ratio of H 2 to CO 2 was changed from 2:1 to 4:1) (Figure 7a), indicating that the CO 2 was excess in the first case (Figure 7a). CH 4 selectivity was found to increase when the percentage of H 2 in the feed reactant increased (Figure 7b), meaning that the H 2 is the limiting agent when the H 2 to CO 2 ratio was 2 to 1 (Figure 7a) (the  [29], where the increasing ratio of H 2 /CO 2 would cause an increase in CO 2 conversion, as the reaction was promoted by a high concentration of dissociated hydrogen on the catalyst surface [29][30][31]. The results agreed with work reported by Daroughegi et al. (2017) [30], which stated that the stoichiometric ratio of H 2 /CO 2 should be equaled 4.0 due to the basicity property on the nickel's surface species. This means that four molecules of H 2 are required to suppress CO 2 adsorption on active of the catalyst since the H 2 molecule can be entirely adsorbed on the catalyst's surface, which simultaneously has much more hydrogenate on the carbonate species.
In addition, it can be seen that in the reverse water gas shift (RWGS) reaction, (CO 2 + H 2 ↔ CO + H 2 O) was more promoted at the lower ratio of H 2 /CO 2 , which resulted in a decrease in CH 4 selectivity. By temperature range, the conversions were found to increase during the operating temperature ranging from 250 to 310 • C. Decreases in conversions and product selectivity were due to the occurring of RWGS reaction which is thermodynamically favorable at a higher temperature. The optimal temperature was determined to be at 310 • C for both ratios, providing CO 2 conversion and CH 4 selectivity at 18.57% and 95.06% for (2:1 ratio) and at 73.75% and 100% for (4:1 ratio), respectively. This result agreed with the H 2 -TPR result which reported peak α centered at 310 • C.   Table 1 compares catalytic performance of the catalyst from this work tested in a micro-channel reactor, with other recent catalysts developed tested in a packed-bed reactor by other researchers. The 40wt% Ni-CeO2/Cr2O3 catalyst in a micro-channel reactor was found to possess the best performance at a lower operating temperature, even at the higher WHSV.   Table 1 compares catalytic performance of the catalyst from this work tested in a micro-channel reactor, with other recent catalysts developed tested in a packed-bed reactor by other researchers. The 40 wt% Ni-CeO 2 /Cr 2 O 3 catalyst in a micro-channel reactor was found to possess the best performance at a lower operating temperature, even at the higher WHSV.

Kinetic Measurement
The kinetic parameters of CO 2 methanation in the presence of 40 wt% Ni-CeO 2 /Cr 2 O 3 was studied in a micro-channel reactor at temperature ranging from 250 to 280 • C. The parameters calculation was performed using the POLYMATH 6.10 Software. The temperature range was selected based on the minimal side reactions and its initial reaction-controlled regime. The ratio of H 2 to CO 2 was varied at 1:1, 1:2, and 4:1. The power-law rate expression, as described in Equation (4), was applied to fit the rate of CO 2 methanation. A simple rate equation was considered due to the fact that the rate of the reaction changes with the change in concentration of each reactant, whereas the concentration changes on the catalyst's surface was ignored. Therefore, the overall reaction rate for the irreversible reaction is focused only on the reactants: where −r CO 2 and −r H 2 are the reaction rate in mol·L −1 ·s −1 , [CO 2 ] and [H 2 ] in mol·L −1 are molar concentration of each species, k is the reaction rate constant with dependence on the temperature, n and m are the reaction order with respect to CO 2 and H 2 . The rate constant, which describes the temperature dependency of the reaction rate, is assumed to obey the Arrhenius relationship according the following equation: The negative apparent activation energy (E app ), achieved from the Arrhenius plots as illustrated in Figure 8, was determined at 4.85 kJ·mol −1 . The orders of the reaction with respect to H 2 and CO 2 were estimated at 0.33 to 1.07 for n th -order and 0.40 to 0.53 for m th -order. The apparent activation energy was benchmarked with other previous works as shown in Table 2.

Pre-Treatment of Micro-Channel Reactor
A micro-channel reactor was fabricated from two micro-structured stainless steel (316 L) plates, laser welded together by the Thai German Institute (TGI) (Chonburi, Thailand). Each plate accommodated 14 channels with dimensions of 25 mm in length, 0.37 mm in width, and 0.30 µm in depth. The gaseous inlet was assumed to be uniformly distributed amongst the 14 slots by 3 physical dots, locating at the corners of an isosceles triangle. The steps of micro-channel pre-and post-treatment was prepared as illustrated in Figure 9, where (a) is the specimens before cleansing, (b) is after cleansing with citric acid, and (c) is after annealing at 800 • C. During the cleansing process, the plates were cleaned by immersing in citric acid (20% by volume) in an ultrasonic bath for 5 h (step 1) before being rinsed by deionized water (step 2). In order to generate grains of mixed oxides on the surface, the micro-channel plates were then annealed at 800 • C for 10 h with 1 • C·min −1 of heating rate risen from room temperature (step 3). Finally, the two plates were laser-welded together after completing the catalyst coating process, described in the next section.

Catalyst Powder Preparation
Ni-CeO 2 /Cr 2 O 3 was synthesized using the one-pot hydrolysis method described elsewhere [20]. The percentage of Ni was 40% by weight with a ratio of CeO 2 to Cr 2 O 3 at 1 to 1. The stoichiometric amount of the relevant nitrate precursors: (Ni(NO 3 ) 2 .6H 2 O (CARLO ERBA, ≥99.0%); Ce(NO 3 ) 3 ·6H 2 O (ALDRICH, ≥99.0%); Cr(NO 3 ) 2 ·6H 2 O (ACROS, ≥99.0%): were dissolved together in 100 mL demineralized water. The mixture was then heated to 80 • C while stirred continuously. Ammonium carbonate (NH 4 ) 2 CO 3 solution (1 M) was added the drop-wise into the previous mixture to maintain the solution pH at 8.8 to 9.0, while the mixed solution was heated to 120 • C, to evaporate water. The residual was afterwards calcined in static air at 500 • C for 12 h at 10 • C·min −1 heating rate. The resulting catalyst powder was ground and sieved to achieve the particle size of 35 µm or smaller.
acid, and (c) is after annealing at 800 °C. During the cleansing process, the plates were cleaned by immersing in citric acid (20% by volume) in an ultrasonic bath for 5 h (step 1) before being rinsed by deionized water (step 2). In order to generate grains of mixed oxides on the surface, the micro-channel plates were then annealed at 800 °C for 10 h with 1 °C·min −1 of heating rate risen from room temperature (step 3). Finally, the two plates were laser-welded together after completing the catalyst coating process, described in the next section.

Catalyst Powder Preparation
Ni-CeO2/Cr2O3 was synthesized using the one-pot hydrolysis method described elsewhere [20]. The percentage of Ni was 40% by weight with a ratio of CeO2 to Cr2O3 at 1 to 1. The stoichiometric amount of the relevant nitrate precursors: (Ni(NO3)2.6H2O (CARLO ERBA, ≥99.0%); Ce(NO3)3·6H2O (ALDRICH, ≥99.0%); Cr(NO3)2·6H2O (ACROS, ≥99.0%): were dissolved together in 100 mL demineralized water. The mixture was then heated to 80 °C while stirred continuously. Ammonium carbonate (NH4)2CO3 solution (1 M) was added the drop-wise into the previous mixture to maintain the solution pH at 8.8 to 9.0, while the mixed solution was heated to 120 °C, to evaporate water. The residual was afterwards calcined in static air at 500 °C for 12 h at 10 °C·min −1 heating rate. The resulting catalyst powder was ground and sieved to achieve the particle size of 35 µm or smaller.

Catalyst Suspended Solution Preparation
This method was adapted from a procedure previously described elsewhere [50]. The mass ratio of Ni-CeO 2 /Cr 2 O 3 :H 2 O:PVA:CH 3 COOH was fixed at 10:84:5:1 in this study. Polyvinyl alcohol (PVA) solution, as a binder, was prepared beforehand by dissolving in deionized water under vigorous stirring at 65 • C for 2 h before being left overnight without stirring. The sieved catalyst was dispersed into the binder solution under a vigorous stirring condition. Acetic acid (CH 3 COOH, 17 M) was added into the solution while being heated to reach the temperature of 65 • C where it remained for 2 h. After the well-mixed solution was naturally cooled down to room temperature, the stirring was continued for 3 days.

Wash-Coat
The prepared catalyst suspension was wash-coated onto the surface of the microstructured substrates. The plates were left at room temperature for 4 h before they were dried in an oven at 120 • C overnight. The excess polymer binder was eliminated by a calcination process at 500 • C for 3 h. Approximately 32 mg of the catalyst were successfully coated on the surface of each plate after two times of coating.

Micro-Channel Reactor Fabrication
Two of the coated plates were laser-weld together. Figure 10 illustrates the configuration of the micro-channel reactor. The adhesion between the catalyst and surface of the substrates were tested using a drop test [20]. Both ends of the micro-channel reactor were connected with 1/8-inch stainless steel tubes by welding. To ensure there was no leakage, the following was performed: (i) one end of the reactor was clogged with a plug, (ii) inert gas was compressed through the reactor at 5 bar, (iii) the reactor was immersed into water, and (iv) a leakage can be regarded if air bubbles are noticed.
Two of the coated plates were laser-weld together. Figure 10 illustrates the configuration of the micro-channel reactor. The adhesion between the catalyst and surface of the substrates were tested using a drop test [20]. Both ends of the micro-channel reactor were connected with 1/8-inch stainless steel tubes by welding. To ensure there was no leakage, the following was performed: (i) one end of the reactor was clogged with a plug, (ii) inert gas was compressed through the reactor at 5 bar, (iii) the reactor was immersed into water, and (iv) a leakage can be regarded if air bubbles are noticed.

Characterizations
An X-ray diffractometer (Bruker AXS, Berlin, Germany, Diffractometer D8 equipment using CuKα (λ = 1.5418 Å) as a radiation source at 40 kV and 30 mA with a scanning speed of 0.02 • s −1 in 2θ ranges from 20 • to 70 • was used to identify the phase structure of the prepared catalyst. The Joint Committee on Powder Diffraction Standards (JCPDSs) cards were referenced with the result. The in situ H 2 temperature-programmed reduction (H 2 -TPR) technique was applied to investigate the reducibility of the catalyst. For this test, 0.5 g of the catalyst was charged into the quartz tube reactor (i.d. = 10 mm) before being pre-treated in 100 ml N ·min −1 of 10 vol% O 2 /Ar at 700 • C for 1 h, in order to eliminate any possible impurities on the catalyst surface. After it cooled down to room temperature, 5 vol% of H 2 /Ar was introduced into the packed-bed reactor at 100 ml N ·min −1 while the temperature was risen at 5 • C.min −1 from room temperature to 950 • C. H 2 consumption was monitored by a real-time quadrupole mass spectrometer (MS, Omnistar GSD 320, Pfeiffer Vacuum, Asslar, Germany) in the SEM-MID mode. The surface morphology of the material's surface was examined using a scanning electron microscope (SEM, SU-8230 Hitachi, Hitachi High-Tech, Minato-ku Tokyo, Japan) with an accelerating voltage of 15 kV. The sample was immobilized by mounting onto a metal stud with double sided carbon tape then gold coated to improve conductivity. The in situ Fourier-transform infrared spectroscopy (FTIR Perkin Elmer, Spectrum 2000 spectrometer, Perkin Elmer, Massachusetts, United States) equipped with a wavenumber resolution of 1 cm −1 in the range between 400 and 4000 cm −1 was applied to define the adsorbed species on the surface of 40 wt% Ni-CeO 2 /Cr 2 O 3 during CO 2 + H 2 exposure at 270 • C.

Catalytic Reaction on a Micro Channel Stainless Reactor
The experiments were carried out in the lab-prepared micro channel reactor under atmospheric pressure. The weight of the catalyst coated onto the reactor was measured at 63.7 mg. The coated catalyst was firstly in situ activated at 500 • C (testified by H 2 -TPR) in pure H 2 (99.995%, BIG) flowing through the system at 100 ml N .min −1 for 2 h, before being cooled down to the selected operating temperature, which varied at 250, 270, 290, 310, 350, to 400 • C. Ar was then purged through the catalyst bed to remove any possible excess H 2 and unwanted physisorption. A flow of the mixed gaseous reactant, H 2 and CO 2 , at each selected ratio of 2:1 and 4:1, was switched into the reactor at 60 ml N ·min −1 of the total flow rate, giving 56,500 ml N ·h −1 ·g −1 cat of WSHV. The effluent gas stream was automatically sampled every 7 min and analyzed online through GC-TCD chromatography (Shimadzu GC-2014ATF). The CO 2 conversion (X CO2 ) and CH 4 selectivity (S CH4 ) were calculated using the below equations: CH 4 selectivity, S CH 4 [%] = F CH 4 F CH 4 + F CO × 100 (4) where F in CO 2 and F out CO 2 represent the volume flow rate of CO 2 in the feed stream and outlet stream, F CH 4 and F CO denote the volume flow rate of product gas stream.

Conclusions
A total of 40 wt% of nickel-based catalyst on Ce 0.5 Cr 0.5 O 2 support was synthesized using the one-pot hydrolysis method. The best operating condition was experimented and estimated to be showing the best catalytic performance at the operating temperature of 310 • C, WHSV of 56,500 ml N ·h −1 ·g −1 cat , and feed ratio of H 2 to CO 2 at 4 to 1. The influence of the variation parameter on the catalytic activity CO 2 methanation was explained by the kinetics and the operatory conditions. The mechanistic of CO 2 methanation reaction was comprehensively revealed by FTIR spectroscopy, the main mechanism over the 40 wt% NiO/Ce 0.5 Cr 0.5 O 2 catalyst did not require CO but the reaction intermediates were replaced by carbonates and formates. The elementary reaction pathway occurred through hydrogenation on carbonates species, while formates species continued to form, and further, methoxy species. This mechanism implied that the CO 2 adsorption was occupied on a strong bonding of nickel oxide active site (on Ce 0.5 Cr 0.5 O 2 support) (testified by H 2 -TPR). The activation energy of such a catalyst for CO 2 methanation in a micro-channel reactor was calculated at 4.85 kJ/mol, which was much (10 to 30 times) lower than other catalysts tested in packed-bed reactors.

Conflicts of Interest:
The authors declare no conflict of interest.