CO 2 Hydrogenation to Synthetic Natural Gas over Ni, Fe and Co–Based CeO 2 –Cr 2 O 3 CO 2 Hydrogenation to Synthetic Natural Gas over Ni, Fe and Co–based CeO 2 –Cr 2 O 3

: CO 2 methanation was studied over monometallic catalyst, i.e., Ni, Fe and Co; on CeO 2 Cr 2 O 3 support. The catalysts were prepared using one-pot hydrolysis of mixed metal nitrates and ammonium carbonate. Physicochemical properties of the pre- and post-exposure catalysts were characterized by X-Ray Powder Diffraction (XRD), Hydrogen Temperature Programmed Reduction (H 2 -TPR), and Field Emission Scanning Electron Microscope (FE-SEM). The screening of three dopants over CeO 2 -Cr 2 O 3 for CO 2 methanation was conducted in a milli-packed bed reactor. Ni-based catalyst was proven to be the most effective catalyst among all. Thus, a group of NiO/CeO 2 Cr 2 O 3 catalysts with Ni loading was investigated further. 40 % NiO/CeO 2 -Cr 2 O 3 exhibited the highest CO 2 conversion of 97.67% and CH 4 selectivity of 100% at 290 ◦ C. The catalytic stability of NiO/CeO 2 -Cr 2 O 3 was tested towards the CO 2 methanation reaction over 50 h of time-on-stream experiment, showing a good stability in term of catalytic activity. Abstract: CO 2 methanation was studied over monometallic catalyst, i.e., Ni, Fe and Co; on CeO 2 Cr 2 O 3 support. The catalysts were prepared using one-pot hydrolysis of mixed metal nitrates and ammonium carbonate. Physicochemical properties of the pre- and post-exposure catalysts were characterized by X-Ray Powder Diffraction (XRD), Hydrogen Temperature Programmed Reduction (H 2 -TPR), and Field Emission Scanning Electron Microscope (FE-SEM). The screening of three dopants over CeO 2 -Cr 2 O 3 for CO 2 methanation was conducted in a milli-packed bed reactor. Ni-based catalyst was proven to be the most effective catalyst among all. Thus, a group of NiO/CeO 2 -Cr 2 O 3 catalysts with Ni loading was investigated further. 40 % NiO/CeO 2 -Cr 2 O 3 exhibited the highest CO 2 conversion of 97.67% and CH 4 selectivity of 100% at 290 °C. The catalytic stability of NiO/CeO 2 Cr 2 O 3 was tested towards the CO 2 methanation reaction over 50 h of time-on-stream experiment, showing a good stability in term of catalytic activity. Author Contributions: Conceptualization, U.W.H. and V.T.; methodology, V.T.; software, C.K.; validation, S.A., M.H.; formal analysis, S.A.; investigation, C.K.; resources, N.L. and U.W.H.; data curation, N.L.; writing—original draft preparation, V.T. and C.K.; writing—review and editing, M.H.; visualization, N.L.; supervision, N.L. and U.W.H.; project administration, V.T.; funding


Introduction
Global warming has caused several serious impacts on the environment in recent years. Increasing CO 2 emission is anthropogenic in origin and is the main cause of global warming. Nowadays, many studies focused on two strategies to reduce atmospheric CO 2 concentration; through carbon capture and CO 2 conversion to biofuels [1,2]. The captured CO 2 can be utilized and converted into fuels and chemicals via chemical processes such as dry reforming of methane for synthesis gas production, or CO 2 hydrogenation to CH 4 , methanol or higher alcohols [3]. CO 2 methanation is one of the promising processes which involves carbon recycle from abundant CO 2 . Methane, as a product of CO 2 hydrogenation, is considered versatile and flexible as it can be injected directly into existing natural gas pipelines, or utilized as a raw material for chemical production [4]. This CO 2 hydrogenation can be looked at as Power-to-Gas process (PtG) by its means to store (and transport) energy in the form of natural gas [5]. The process refers to a conversion of renewable electricity to a gaseous energy carrier via two pathways: (1) H 2 production by water electrolysis, where wind or solar energy technologies could be integrated; and (2) H 2 conversion to CH 4 , by methanation reaction with external CO 2 capture [6]. CO 2 methanation was firstly discovered and proposed as the Sabatier reaction: CO 2

Introduction
Global warming has caused several serious impacts on the environment in recent years. Increasing CO2 emission is anthropogenic in origin and is the main cause of global warming. Nowadays, many studies focused on two strategies to reduce atmospheric CO2 concentration; through carbon capture and CO2 conversion to biofuels [1,2]. The captured CO2 can be utilized and converted into fuels and chemicals via chemical processes such as dry reforming of methane for synthesis gas production, or CO2 hydrogenation to CH4, methanol or higher alcohols [3]. CO2 methanation is one of the promising processes which involves carbon recycle from abundant CO2. Methane, as a product of CO2 hydrogenation, is considered versatile and flexible as it can be injected directly into existing natural gas pipelines, or utilized as a raw material for chemical production [4]. This CO2 hydrogenation can be looked at as Power-to-Gas process (PtG) by its means to store (and transport) energy in the form of natural gas [5]. The process refers to a conversion of renewable electricity to a gaseous energy carrier via two pathways: (1) H2 production by water electrolysis, where wind or solar energy technologies could be integrated; and (2) H2 conversion to CH4, by methanation reaction with external CO2 capture [6]. CO2 methanation was firstly discovered and proposed as the Sabatier reaction: CO2 + 4H2 ↔ CH4 + 2H2O, ∆Hr 298 = −164.8 kJ·mol −1 [7]. Although the reaction is highly exothermic and thermodynamically 2 of 12 limitations due to the high stability of CO 2 . Furthermore, heat accumulation from the reaction generally causes severe hotspots in the reactor, due to the heat transfer limitation within the process, leading to the catalyst deactivation and shortened catalyst lifespan [9]. Moreover, low operating temperature is favorable for CO disproportionation reaction (2CO www.mdpi.com/journal/catalysts capture and CO2 conversion to biofuels [1,2]. The captured erted into fuels and chemicals via chemical processes such for synthesis gas production, or CO2 hydrogenation to CH4, 3]. CO2 methanation is one of the promising processes which abundant CO2. Methane, as a product of CO2 hydrogenation, exible as it can be injected directly into existing natural gas material for chemical production [4]. This CO2 hydrogenar-to-Gas process (PtG) by its means to store (and transport) gas [5]. The process refers to a conversion of renewable elecrrier via two pathways: (1) H2 production by water electrolrgy technologies could be integrated; and (2) H2 conversion tion with external CO2 capture [6]. CO2 methanation was ed as the Sabatier reaction: CO2 + 4H2 ↔ CH4 + 2H2O, ∆Hr 298 h the reaction is highly exothermic and thermodynamically CO 2 + C, ∆H r 298 = −172.4 kJ·mol −1 ), resulting in unwanted coke deposition. In order to obtain the highest possible methane yield, it is necessary to invent a catalyst which enhances the reaction's activity, withstands sintering and counters the coking phenomenon. Various active metals (such as Ni, Fe, Co, Ru, Rh, and Pd) have been used as an active site while metal oxides (such as CeO 2 , La 2 O 3 , MgO, γ-Al 2 O 3 , SiO 2 , TiO 2 , and ZrO 2 ) have been useful as a support in a catalyst system for the CO 2 methanation reaction [3,[10][11][12][13]. Amongst these materials, CeO 2 is so far found to be the most interesting support due to its high oxygen storage capacity (OSC) and its ability to disperse the active site [14]. In addition, CeO 2 could promote the interaction between support and metal active component, such that the growth and dispersion of the metal active particles can be well distributed and controlled throughout the surface of the support, leading to the higher CO 2 conversion [15]. The number oxygen vacancy can be tailor-made by substituting smaller transition metal ions (e.g., chromium ions) into CeO 2 . The higher number of lattice oxygen can combust coke deposits and reduce the chance of sintering [13,[16][17][18]. According to previous research, Ni-, Fe-, Co doped on CeO 2 have shown relatively high activities for CO 2 methanation and possessed high stability when tested for 15 to 50 h reaction times [19][20][21][22][23].
In this work, Ni-, Fe-and Co-based CeO 2 /Cr 2 O 3 were prepared using the onepot hydrolysis method. The level of metal loading, operating temperature, reduction temperature and other relevant variables were observed as all of these parameters are wellknown to influence the catalytic performance of the catalysts [24,25]. The physicochemical properties of the synthesized catalysts were examined, comparing pre-and post-exposure by X-Ray Diffractometer (XRD), Hydrogen Temperature-Programmed Reduction (H 2 -TPR), and Field Emission Scanning Electron Microscopes (FE-SEM). The catalyzation of CO 2 methanation was conducted in a milli-packed bed reactor under atmospheric pressure where the operating temperature was varied from 200 to 350 • C. The reduction temperatures of 500 and 700 • C were chosen (via H 2 -TPR) for comparison purposes. WSHV was fixed at 27,624 mL·h −1 ·g cat −1 , and the stoichiometric reactants ratio was kept at 4 for all the experiments.

Catalyst Powder-Formed Preparation
Forty percent (by weight) x/CeO 2 -Cr 2 O 3 (where x = Ni, Fe, and Co) catalysts were synthesized by a single step preparation using (NH 4 ) 2 CO 3 (PANREAC, 30% NH 3 ) as a hydrolysis agent, the details of which are outlined in [3]. The relevant nitrate precursors Ni(NO 3 Louis, MO, USA, ≥99.0%), and Cr(NO 3 ) 2 ·6H 2 O (ACROS, Merelbeke, Belgium, ≥99.0%) were dissolved in 50 mL distilled water where the ratio of active metal (Ni, Fe, and Co) to support (1 to 1 of CeO 2 /Cr 2 O 3 ) was fixed at 40 to 60 by weight. Two molar (NH 4 ) 2 CO 3 solution was gradually dropped into the nitrate solutions until the pH reached 8.8-9.0. The mixture was continuously stirred while heated to 80 • C for 3 h. The solution's temperature was then raised again to 120 • C to evaporate water and a dark blue gel was slowly obtained. The resulting material was then calcined in moving air at 500 • C with 10 • C/min of heating for 24 h before the black powder of the catalyst was achieved. The catalyst powder was then pressed, crushed, and sieved to gain its uniform particle size ranging from 75 to 180 µm in order to avoid pressure drop that could occur across the catalyst bed.
The optimal reduction temperature of the catalyst was screened using an in situ H 2 -TPR technique which was carried out in our lab-scale conventional packed-bed reactor, connected to a Quadrupole Mass Spectrometer (PFEIFFER, MS, Omnistar GSD 320, HAKUTO) operated in a SEM-MID mode. A total of 0.5 g of the catalyst sample was packed in a quartz tube reactor (i.d. = 10 mm) and pre-treated in 10% O 2 /Ar at 500 • C for 1 h, followed by Ar purging to clean the catalyst's surface from any possible absorbed impurities. After the system reached ambient temperature, 5 % H 2 /Ar was introduced through the catalyst's bed with a total flowrate of 100 mL·min −1 while the temperature was elevated to 950 • C at 5 • C/min. Surface morphology and micro-structure of the catalysts, both pre-and post-exposure, were investigated using a Field Emission Scanning Electron Microscopes (FE-SEM, SU-8230 Hitachi, Japan) with an accelerating voltage of 15 kV.

CO 2 Methanation Activity in a Packed-Bed Reactor
CO 2 methanation was performed in a tubular packed-bed reactor under atmospheric pressure. A total of 0.2 g of catalyst was placed between two layers of quartz wool in the middle of the reactor (i.d. = 4 mm). The catalyst was reduced in 100 mL·min −1 of pure H 2 for 2 h at the achieved reduction temperature (from H 2 -TPR where NiO reduced to metallic Ni at 500 • C while Co 2 O 3 and Fe 3 O 4 reduced to metallic Co and Fe at 850 • C) from the prior reaction. Next, the process was cooled down to the desired operating temperature, varying at 200, 210, 230, 250, 270, 290, 310, and 350 • C. Ar was purged in between to remove any excess H 2 . The mixture of gaseous reactant, CO 2 :H 2 :Ar at a ratio of 1:4:5 by volume, was injected through the catalyst's bed. Total flow rate was set at 90 mL·min −1 , giving WSHV at 27,624 mL·h −1 ·g cat −1 . Moisture was condensed as a by-product using a cooler oil bath at the bottom of the reactor. After the process approached equilibrium, the dried gas products were automatically analyzed using gas chromatography coupled with a TCD detector (Shimadzu GC-2014ATF) every 7 min for 1 h. CO 2 conversion (X CO2 ), CH 4 selectivity (S CH4 ), and CH 4 yield (Y CH4 ) were calculated using the following formulas: F in CO 2 and F out CO 2 represent volumetric flow rate of CO 2 in the feed stream and outlet stream, respectively, whereas F CH 4 and F CO denote the volumetric flow rate of the product gas stream, CH 4 and CO, respectively.

Results and Discussion
XRD patterns of all the fresh catalysts (calcined in moving air at 500 • C) were achieved as shown in Figure 1. CeO 2 -Cr 2 O 3 ( ), as major crystals, were found in all samples and appeared to possess fluorite cubic structure [26], having two-theta position peaks at 28 [27,28]. Thus, the Co 4+ ion was able to compete with Cr 3+ in becoming embedded into the CeO 2 lattice, creating CeCoO 3 perovskite [27,29], as it can be seen in Figure 1c. This phenomenon could cause a decay in the catalyst's catalytic performance due to the loss of active sites, in this case, Co 3  XRD patterns of all the fresh catalysts (calcined in moving air at 500 °C) were achieved as shown in Figure 1. CeO2-Cr2O3 (■), as major crystals, were found in all samples and appeared to possess fluorite cubic structure [26], having two-theta position peaks at 28.57, 33.12, 47.49, 56.38, 58 [27,28]. Thus, the Co 4+ ion was able to compete with Cr 3+ in becoming embedded into the CeO2 lattice, creating CeCoO3 perovskite [27,29], as it can be seen in Figure 1c. This phenomenon could cause a decay in the catalyst's catalytic performance due to the loss of active sites, in this case, Co3O4. In addition, the average crystallite size of the NiO, Fe2O3, and Co3O4 on CeO2-Cr2O3 were calculated using the Scherrer's equation at 14.56, 25.03, and 25.60 nm, respectively. The smaller active site could perhaps accommodate reactants better, rendering the chance of higher catalytic performance.
.   [30]. The first reduction peak of CeO 2 -Cr 2 O 3 (b) appeared at 480 • C, where Cr 6+ ions were reduced to Cr 3+ ions. The reduction peak at 565 • C and >950 • C corresponded to the reduction of CeO 2 -Cr 2 O 3 at surface and bulk oxygen, respectively [30][31][32][33]. Substitution of Cr 2 O 3 into CeO 2 was reported to enhance oxygen vacancy of the catalyst system [16,18,34], in which its H 2 consumption was proven to be significantly higher than the pure CeO 2 .
Catalysts 2021, 11, 1159 5 of 12 sociated with 1) the reduction of Cr ions to Cr ions, 2) the reduction of Co ions to Co ions and the reduction of CeO2-Cr2O3 (and/or CeCoO3 perovskite) with surface oxygen, and 3) the reduction of Co 2+ ions to metallic Co, and the reduction of CeO2-Cr2O3 (and/or CeCoO3 perovskite) with bulk oxygen [39]. The catalyst's oxygen deficiency and number of active sites were interpreted from the hydrogen consumption, which was compared amongst all the catalysts and ordered as: NiO/CeO2-Cr2O3 > Fe2O3/CeO2-Cr2O3 > Co2O3/CeO2-Cr2O3. Three distinct peaks at 325, 675, and 940 • C were found for NiO/CeO 2 -Cr 2 O 3 catalyst (c). The first two peaks were identical to the reduction of Ni 3+ to Ni 2+ and to the reduction of Ni 2+ to metallic nickel, respectively [34][35][36]. Some reduction of Cr 6+ ions to Cr 3+ ions could be combined in the first peak, whereas the second and the third peaks represented the reduction of the Cr 2 O 3 incorporated within the CeO 2 structure at the surface and bulk level, respectively. For Fe 2 O 3 /CeO 2 -Cr 2 O 3 (d), the first peak appeared at 395 • C, representing the reduction of Cr 6+ ions to Cr 3+ ions, whereas its second and third peaks at 505 and 940 • C were attributed to the reduction of CeO 2-Cr 2 O 3 at the surface and bulk levels, respectively. The two reduction peaks observed at 505 • C and between 700 to 950 • C also represented the reduction of Fe 2 O 3 to Fe 3 O 4 and reduction of Fe 3 O 4 →FeO→metallic Fe, respectively [37,38]. Co 2 O 3 /CeO 2 -Cr 2 O 3 (e) was detected at 445, 700, and >950 • C, and associated with (1) the reduction of Cr 6+ ions to Cr 3+ ions, (2) the reduction of Co 3+ ions to Co 2+ ions and the reduction of CeO 2-Cr 2 O 3 (and/or CeCoO 3 perovskite) with surface oxygen, and (3) the reduction of Co 2+ ions to metallic Co, and the reduction of CeO 2-Cr 2 O 3 (and/or CeCoO 3 perovskite) with bulk oxygen [39]. The catalyst's oxygen deficiency and number of active sites were interpreted from the hydrogen consumption, which was compared amongst all the catalysts and ordered as: NiO/CeO 2 -Cr 2 O 3 > Fe 2 O 3 /CeO 2 -Cr 2 O 3 > Co 2 O 3 /CeO 2 -Cr 2 O 3 .

Choice of the Monometallic
Catalytic performance, in terms of CO 2 conversion (Figure 3 (left)) and CH 4 selectivity (Figure 3 (right)), of all the prepared catalysts was determined at various operating temperatures, ranging from 200 to 350 • C. CO 2 conversion tended to increase with increasing temperature for all catalysts. Amongst all the selected metals, Ni was proven as the best monometallic active site for CeO 2 -Cr 2 O 3 , considering CO 2 conversion, which was much higher than other metals (Fe and Co) starting at 260 • C. The highest CO 2 conversion over Ni/CeO 2 -Cr 2 O 3 was achieved at 290 • C, giving CO 2 conversion of 90.19%. However, CO 2 conversion decreased when the temperature was higher than 330 • C, due to its thermodynamic limitation [3,40,41]. In terms of CH 4 selectivity, Ni also showed the best performance by giving complete selectivity at 100% during all temperatures (from 200 to 360 • C), followed by Fe which offered 94% of CH 4 selectivity at its equilibrium at 290 • C. On the other hand, catalytic performance of Co as the monometallic dopant was incomparable to that of the other two, as it gave no reaction at low temperature (below 260 • C) and reached its maximum at 24% of CH 4 selectivity at 270 • C. The CH 4 selectivity was decreased at temperatures higher than 270 • C. This was due to the formation of CO as an unwanted product from the reverse water-gas shift reaction [19,42].

Choice of the Monometallic
Catalytic performance, in terms of CO2 conversion (Figure 3 (left)) and CH4 selectivity (Figure 3 (right)), of all the prepared catalysts was determined at various operating temperatures, ranging from 200 to 350 °C. CO2 conversion tended to increase with increasing temperature for all catalysts. Amongst all the selected metals, Ni was proven as the best monometallic active site for CeO2-Cr2O3, considering CO2 conversion, which was much higher than other metals (Fe and Co) starting at 260 °C. The highest CO2 conversion over Ni/CeO2-Cr2O3 was achieved at 290 °C, giving CO2 conversion of 90.19%. However, CO2 conversion decreased when the temperature was higher than 330 °C, due to its thermodynamic limitation [3,40,41]. In terms of CH4 selectivity, Ni also showed the best performance by giving complete selectivity at 100% during all temperatures (from 200 to 360 °C), followed by Fe which offered 94% of CH4 selectivity at its equilibrium at 290 °C. On the other hand, catalytic performance of Co as the monometallic dopant was incomparable to that of the other two, as it gave no reaction at low temperature (below 260 °C) and reached its maximum at 24% of CH4 selectivity at 270 °C. The CH4 selectivity was decreased at temperatures higher than 270 °C. This was due to the formation of CO as an unwanted product from the reverse water-gas shift reaction [19,42].

Effect of Metal Content on Catalytic Performance
The influence of metal content, doped on CeO2-Cr2O3, towards CO2 methanation was investigated over Ni-/CeO2-Cr2O3, where the Ni level was varied at 10, 20, 30, 40 and 50% by weight. Figure 4 showed that the percentage of all the selected Ni contents exhibited the same trend, where CO2 conversion was increased with increasing temperature and increasing amount of Ni content. The nickel content represented the amount of the active

Effect of Metal Content on Catalytic Performance
The influence of metal content, doped on CeO 2 -Cr 2 O 3 , towards CO 2 methanation was investigated over Ni-/CeO 2 -Cr 2 O 3 , where the Ni level was varied at 10, 20, 30, 40 and 50% by weight. Figure 4 showed that the percentage of all the selected Ni contents exhibited the same trend, where CO 2 conversion was increased with increasing temperature and increasing amount of Ni content. The nickel content represented the amount of the active site for CO 2 methanation reaction, thus, the higher level of Ni was unsurprisingly improved the efficiency of the reaction [41,[43][44][45]. However, excess Ni loading could cause other problems, i.e., pore blockage, coagulation and obstruction of nano-channels [46][47][48]. For this reason, there was only a small difference in CO 2 conversion, between using 40% and 50% Ni loading.

Effect of Reduction Temperature on Catalytic Performance
Two different reduction temperatures, at 500 and 700 • C, were selected for this study. Figure 5 presents relationship between CO 2 conversion and reduction temperature of the catalyst at different operating temperatures. The results showed that the catalyst which reduced at 500 • C gave the highest CO 2 conversion for all of the temperature ranges, compared to the one reduced at 700 • C. In addition, the decrease in CO 2 conversion at the higher reduction temperature (700 • C) could also be the effect of the catalyst's sintering, resulting in a lower number of active sites [49]. site for CO2 methanation reaction, thus, the higher level of Ni was unsurprisingly improved the efficiency of the reaction [41,[43][44][45]. However, excess Ni loading could cause other problems, i.e., pore blockage, coagulation and obstruction of nano-channels [46][47][48]. For this reason, there was only a small difference in CO2 conversion, between using 40% and 50% Ni loading.

Effect of Reduction Temperature on Catalytic Performance
Two different reduction temperatures, at 500 and 700 °C, were selected for this study. Figure 5 presents relationship between CO2 conversion and reduction temperature of the catalyst at different operating temperatures. The results showed that the catalyst which reduced at 500 °C gave the highest CO2 conversion for all of the temperature ranges, compared to the one reduced at 700 °C. In addition, the decrease in CO2 conversion at the higher reduction temperature (700 °C) could also be the effect of the catalyst's sintering, resulting in a lower number of active sites [49].

Catalytic Stability
The catalytic stability of the NiO/CeO2-Cr2O3 was measured in term of CO2 conversion and CH4 selectivity, illustrated in Figure 6. Approximately 97% of CO2 conversion and 100% of CH4 selectivity were achieved and maintained during 50 h of reaction time. XRD and SEM techniques were utilized for pre-and post-exposure characterization. XRD patterns of fresh NiO/CeO2-Cr2O3 catalyst was compared with the post-exposure one after the stability test, shown in Figure 7. Although both look quite similar, a decrease in fullwidth half-maximum (FWHM) was clearly noticed, indicating catalyst sintering. Compared to pre-exposure, post-exposure crystallite size was found to have increased from 11 to 13 nm, whereas particle size was doubled from 313 to 612 nm. However, the sign of sintering or deactivation was not clearly observed in TOS experiment. This could be due to the fact that the rate of reaction is rapid, to the point that the catalyst surface area becomes relatively less significant. No NiO peak was found on the diffraction pattern in either the pre-or post-exposure, indicating that the catalyst was fully reduced as peaks However, surface morphology of the pre-and post-exposure catalyst were found to be different, as shown in Figure 8. It can be seen that the particle size of the catalyst became larger after reaction due to its agglomeration, in an attempt to reduce its surface free energy.

Catalytic Stability
The catalytic stability of the NiO/CeO 2 -Cr 2 O 3 was measured in term of CO 2 conversion and CH 4 selectivity, illustrated in Figure 6. Approximately 97% of CO 2 conversion and 100% of CH 4 selectivity were achieved and maintained during 50 h of reaction time. XRD and SEM techniques were utilized for pre-and post-exposure characterization. XRD patterns of fresh NiO/CeO 2 -Cr 2 O 3 catalyst was compared with the post-exposure one after the stability test, shown in Figure 7. Although both look quite similar, a decrease in full-width half-maximum (FWHM) was clearly noticed, indicating catalyst sintering. Compared to pre-exposure, post-exposure crystallite size was found to have increased from 11 to 13 nm, whereas particle size was doubled from 313 to 612 nm. However, the sign of sintering or deactivation was not clearly observed in TOS experiment. This could be due to the fact that the rate of reaction is rapid, to the point that the catalyst surface area becomes relatively less significant. No NiO peak was found on the diffraction pattern in either the pre-or post-exposure, indicating that the catalyst was fully reduced as peaks appeared at 44.508, 51.847, and 76.372 (JCPDS No. 00-004-0850). However, surface morphology of the pre-and post-exposure catalyst were found to be different, as shown in Figure 8. It can be seen that the particle size of the catalyst became larger after reaction due to its agglomeration, in an attempt to reduce its surface free energy.       The catalyst performance was compared between this work and other works that researched other catalysts (i.e., 10Ni/CeO2 [42], 10Ni/CeO2-ZrO2 (CZ) [19], 15Ni/CZ, 15Ni-3Fe/CZ, 15Ni-3Co/CZ [42], 40Ni/CZ [3], 15Ni-2Ce/Al2O3 [50], 5Ni/CZ [51], 20Ni/Al2O3 [52], and 40Ni-5Ce/Al2O3 [47]); as shown in Figure 9. Ni/CeO2-Cr2O3 catalyst can be deemed as a superior catalyst due to its high catalytic activity (YCH4 > 95% zone) at low operating temperatures. The catalyst performance was compared between this work and other works that researched other catalysts (i.e., 10Ni/CeO 2 [42], 10Ni/CeO 2 -ZrO 2 (CZ) [19], 15Ni/CZ, 15Ni-3Fe/CZ, 15Ni-3Co/CZ [42], 40Ni/CZ [3], 15Ni-2Ce/Al 2 O 3 [50], 5Ni/CZ [51], 20Ni/Al 2 O 3 [52], and 40Ni-5Ce/Al 2 O 3 [47]); as shown in Figure 9. Ni/CeO 2 -Cr 2 O 3 catalyst can be deemed as a superior catalyst due to its high catalytic activity (Y CH4 > 95% zone) at low operating temperatures.

Conclusions
The screening of monometallic catalysts (i.e., Ni, Fe and Co) doped on CeO2-Cr2O3 support was studied in a milli-packed bed reactor. All the catalysts were prepared using one-pot hydrolysis. Ni was proven to be the most effective dopant. The amount of Ni loading was found optimal at 40% by weight, giving CO2 conversion of 98.7% and CH4 selectivity of 100% at a relatively low temperature of 290 °C. At temperatures of 200 to 350 °C, the reaction was kinetically driven by the higher operating temperature. However, thermodynamic limitation took place at temperatures higher than 350 °C where a drop in catalytic performance was observed. The catalyst was also stable during 50 h time on stream experiment. Ni-CeO2/Cr2O3 was proven to be one of the highest potential catalysts for the CO2 hydrogenation process of CH4 production.

Conclusions
The screening of monometallic catalysts (i.e., Ni, Fe and Co) doped on CeO 2 -Cr 2 O 3 support was studied in a milli-packed bed reactor. All the catalysts were prepared using one-pot hydrolysis. Ni was proven to be the most effective dopant. The amount of Ni loading was found optimal at 40% by weight, giving CO 2 conversion of 98.7% and CH 4 selectivity of 100% at a relatively low temperature of 290 • C. At temperatures of 200 to 350 • C, the reaction was kinetically driven by the higher operating temperature. However, thermodynamic limitation took place at temperatures higher than 350 • C where a drop in catalytic performance was observed. The catalyst was also stable during 50 h time on stream experiment. Ni-CeO 2 /Cr 2 O 3 was proven to be one of the highest potential catalysts for the CO 2 hydrogenation process of CH 4 production.