High Active Co/Mg1-xCex3+O Catalyst: Effects of Metal-Support Promoter Interactions on CO2 Reforming of CH4 Reaction

Co/Mg1−XCeXO (x = 0, 0.03, 0.07, 0.15; 1 wt% cobalt each) catalysts for the dry reforming of methane (DRM) reaction were prepared using the co-precipitation method with K2CO3 as precipitant. Characterization of the catalysts was achieved by X-ray diffraction (XRD), X-ray fluorescence spectroscopy (XRF), X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (H2-TPR), Brunauer–Emmett–Teller (BET), transmission electron microscopy (TEM), and thermal gravimetric analysis (TGA). The role of several reactant and catalyst concentrations, and reaction temperatures (700–900 °C) on the catalytic performance of the DRM reaction was measured in a tubular fixed-bed reactor under atmospheric pressure at various CH4/CO2 concentration ratios (1:1 to 2:1). Using Xray diffraction, a surface area of 19.2 m2.g−1 was exhibited by the Co/Mg0.85Ce0.15O catalyst and MgO phase (average crystallite size of 61.4 nm) was detected on the surface of the catalyst. H2 temperature programmed reaction revealed a reduction of CoO particles to metallic Co0 phase. The catalytic stability of the Co/Mg0.85Ce0.15O catalyst was achieved for 200 h on-stream at 900 °C for the 1:1 CH4:CO2 ratio with an H2/CO ratio of 1.0 and a CH4, CO2 conversions of 75% and 86%, respectively. In the present study, the conversion of CH4 was improved (75%–84%) when conducting the experiment at a lower flow of oxygen (1.25%). Finally, the deposition of carbon on the spent catalysts was analyzed using TEM and Temperature programmed oxidation-mass spectroscopy (TPOMS) following 200 h under an oxygen stream. Better anti-coking activity of the reduced catalyst was observed by both, TEM, and TPO-MS analysis. Copyright © 2021 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0).


Introduction
The massive dependence on fossil fuels has these challenges, sustainable energy solutions are necessary. The production of biogas (CH4 and CO2) by the biomass digestion in the absence of oxygen (fermented wastes) has been utilized as a fuel for the production of power and heat. Biogas has also been used for industrial feedstock as a renewable carbon source in the syngas production (CO and H2) through an environmental and economical friendly reaction [1,2]. One of the methods for this process is the dry reforming that requires intensive energy along with using CO2 as an oxidant, as shown in Eq. (1). (1) Syngas is an important feedstock that can be converted effectively to fuel (gasoline, gasoil, gasoline, dimethyl ether (DME), and methanol) using the Fischer-Tropsch synthesis [3]. However, variations in the molar ratios of H2/CO are essential to concur with the industrial syngas applications. For instance, an H2/CO ratio of 2 is mandatory for the synthesis of methanol [4]; while that for DME synthesis is 1 under a one-step process [5]. Depending on the synthesized fuel, an H2/CO ratio of 1-2 is required for the process of Fischer-Tropsch [6].
Reverse water-gas shift (RWGS) reaction (Eq. (2)) was found to influence the reaction's equilibrium during the syngas production from CH4 and CO2 (Eq. (1)) resulting in a low H2/CO ratio. (2) Other side reactions such as the decomposition of CH4 (Eq. (3)) and Boudouard reaction (disproportionation reaction) (Eq. (4)) were also associated with dry reforming. (3) Zhang et al. [7] demonstrated a direct relation between the decomposition of CH4 and CO disproportionation and the carbon deposition on the catalyst. Moreover, as the temperature of the reaction increased from 550 °C to 650 °C, the carbon deposition occurred more than the dry reforming of CH4 (DRM). Thus, the careful selection of catalyst is crucial for preventing carbon formation and improving the DRM reaction [7]. Furthermore, the deposition of carbon can be reduced and even eliminated by supporting the metal in a metal oxide with strong ba-sicity [8]. This finding results due to the chemical adsorption of the support that induces the catalyst to chemisorb CO2 in the DRM in which CO2 eventually forms CO by reacting with C (Eq. (5)). (5) Due to their availability, and inexpensive cost, the use of nickel and cobalt has been highly recommended for the reforming process. However, a main disadvantage when using nickel or cobalt is the carbon formation that eventually deactivates the catalyst [9]. Accordingly, many studies have been conducted with the aim of improving the catalytic activity and stability of the cobalt and nickel catalysts in the reforming process [10]. The addition of strong Lewis base promoters (CaO or MgO) was found to deactivate the Ni-based catalysts. This can be improved by the chemisorption of CO2 to lessen the coke deposition that forms CO by reacting with the deposited carbon [11]. A study by Hui Wang [12] revealed the dependence of the catalysts containing Co on the nature of the used support composite and showed that Co was only active when supported on Mg-Al-Ox composite support [12]. In addition, the preparation of Co-Ce/ZrO2 catalysts by Paksoy et al. [13] revealed an increased stability when adding Ce, due to the minimization of the Co oxidation during the reaction of DRM. In the same study, no significant formation of carbon on the surface of the spent catalyst was observed along with no significant metal sintering. Also, the periodic redox of the catalysts was accelerated by the eminent transfer performance of Ce and the surface oxygen storage. A study by Casanovas et al. [14] on the various promoters modified Co-based catalysts demonstrated that in comparison to the other promoters, the selectivity and the catalytic performance of Na modified Co/ZnO catalysts was higher than the unmodified catalysts. Like wise, CaO and MgO (alkaline earth metal oxides) were studied for their potential as promoters modified Ni-based catalysts by Jang et al. [15]. Study findings revealed a higher stability and hindrance of carbon deposition by the MgO modified catalyst at an elevated reaction space velocity. The prevention of carbon deposition by the catalyst was due to the strong Lewis basicity on Mg modified catalyst, resulting in an outstanding CO2 chemical adsorption, favorable for eliminating the deposition of carbon through CO disproportionation (Eq. (4)). In the same study, resistance to the deposition of carbon was observed more in the The present study aimed at developing a stable catalyst that is highly active with strong selectivity and ability to reduce the deposition of carbon on the catalyst during DRM reaction. The method of co-precipitation included the precipitant K2CO3. In the study, 1% Co from cobalt acetylacetonate was impregnated into MgO-Ce2O3 to prepare Co/Mg1-XCeXO catalysts. The catalytic stability, effect of the conversion temperature and the CO2 and CH4 concentrations on the prepared catalysts were also investigated in the DRM. In addition, improvements in the CH4 conversion when passing a stream of 1.25% O2 gas in the process were also determined.

Preparation of Catalysts
The promoter-supports Mg1-xCe 3+ xO (x= 0.00, 0.03, 0.07, 0.15) were prepared by the coprecipitation method [17] using an aqueous solution of Mg(NO3)2.6H2O (0.1M) and Ce(NO3)3.6H2O and 1M K2CO3 as precipitants. Precipitant filtering and soaking the sample in hot water was then carried out followed by drying the sample (120 °C for 12 h). Successively, the pre-calcination of the sample took place in an open furnace to discard the CO2 from the precipitant for 5 h at 500 °C. The sample was then pressed into disks (600 kg/m 2 ), followed by 20 h of calcination at 1150 °C to increase the mechanical properties and ensure an efficient interaction between the promoter (Ce2O3) and the support (MgO).

Catalysts Characterization
X-ray diffractometer (Shimadzu model XRD-6000) was utilized in this study and the crystals size was calculated using the Debye-Scherrer relationship [18]. Kratos Axis Ultra DLD system fixed with a monochromatic Al Kα (1486.6 eV) and Al and Mg X-ray sources were used to acquire the X-ray photoelectron spectroscopy (XPS) results. The operation of the X-ray gun (source of excitation) was carried out on an emission current of 20 mA combined with 15 kV voltages (Kratos Analytical Limited) [19]. Pass energy size was fixed at 100 and 40 eV. Region of interest for both the photo electron signals (O1s, Mg2p, Co2p and Ce3d) and the narrow scan were similar to each other. A Philips glass diffraction X-ray tube of broad focus at 2.7 kW was used for the radiation process.
The H2-temperature programmed reduction (TPR) analysis (Thermo Fisher -Thermo Finnegan TPDRO 1100, accompanied by a thermal conductivity detector) for the calcined samples was carried out at a 10 °C/min heating rate and a 50-1000 °C temperature range. For the provision of the reducing atmosphere, 5% H2/Ar flow was fed during the analysis. Additional TPR experiments were carried out using similar conditions at a temperature range of 50-700 °C to measure the degree of reduction of each catalyst. The degree of reduction was calculated utilizing the following equation from the integration of the peak area under the TPR profile at 700 °C and 1000 °C, respectively.
The total catalyst's surface area was measured using the Brunauer-Emmett-Teller (BET) method with nitrogen adsorption set at -196 °C. For the analysis, Thermo Fisher Scientific S.P.A (model: Surfer Analyzer, Thermo Fisher Scientific) nitrogen adsorption-desorption analyzer was adopted. An apparatus for transmission electron microscopy (TEM), Hitachi H7100 TEM with an increasing voltage (10 MV) was used to diagnose the crystal system and the catalyst's homogeneity. Mettler Toledo TG-DTA (Pt crucibles, Pt/Pt-Rh thermo-couple) with a heating range of 50 to 1000 °C was used to carry out the thermo-gravimetric analysis (TGA).

Catalytic Evaluations
Syngas production (H2/CO) for the reforming of biogas was achieved using a fixed bed stainless steel micro-reactor (i.d. Ø = 6 mm, h = 34 cm). A mass flow gas controller (SIERRA instruments) and an online gas chromatography (GC) (Agilent 6890N; G 1540N) equipped with Varian capillary columns HPPLOT/Q and HP-MOLSIV were connected to a reactor. Prior to the initiation of the process, a lowering of the catalyst to about 0.02 g was conducted by flowing 5% H2/Ar at 700 °C, and holding for 3 h. The purpose of the reduction step was to convert the (Ni 2+ , Pd 2+ , and Pt 2+ ) phase of the catalyst to the metal (Ni, Pd, and Pt) phase at the active sites of the catalysts. The temperature of the reaction was maintained and checked using the thermo-couple placed into the catalyst chamber. The calculations of the CH4 and CO2 conversions and the selectivity of H2 and CO, as well as syngas (H2/CO) ratios were based on Eqs. (6)-(10): . However, no diffraction peaks were observed for the 1% Co catalyst in all the patterns because of the minimal amount of these elements. A cubic form was recorded for the crystal system of all samples due to the cubic shaped particles in the catalyst [20]. XRD findings were supported by FE-SEM and TEM results.

Catalysts
The method of XRF was utilized for all the catalyst's components elemental analysis. Table 1 displays the cobalt percentage (slightly > 1) due to the incomplete precipitation of the metal precursors, Ce and Mg in the method of co-precipitation affecting the results to a minor extent [18].

XPS analysis
The analysis of XPS was implemented to s t u d y t h e r e d u c e d c a t a l y s t ' s (Co/Mg0.85Ce 3+ 0.15O) surface composition. An XPS examination of the surface of the catalysts % 100 in out  Figure 2b. The narrow scan of the XPS spectra for the Mg2p region of the nano-catalyst recorded one peak at 47 eV (binding energy) (Figure 2c)

Surface Area of Brunauer-Emmett-Teller (BET)
The surface area values of BET (SBET) along with the reduced catalyst supports pore properties are displayed in Table 2. A 17.6 Å pore radius, 0.064 cm 3 /g pore volume, and a 9.5 m 2 /g surface area were recorded for the Co/MgO catalyst. Nonetheless, an increase in the surface area and volume was noticed upon the addition of Ce2O3 (promoter). This elevation may be attributed to the strong interaction between the metal (Co) and support (MgO−Ce2O3). The current findings are concurrent with the findings by earlier reports [22]. However, by adding Ce2O3 at a temperature of 1150 °C, the loss of the surface area during calcination has been curbed, resulting in an increase in the catalyst surface area (   An increase in the surface area was observed upon the increase in the amount of Ce. The pore radius of all the catalysts was affected by the Ce2O3 concentration level. A pore radius of 24.3, 38.8 and 52.7 Å was recorded for the catalysts Co/Mg0.97Ce 3+ 0.03O, Co/Mg0.93Ce 3+ 0.07O and Co/Mg0.85Ce 3+ 0.015O, respectively. The pore volume slightly increased following the addition of cerium to 0.105, 0.145 and 0.153 cm 3 /g for Co/Mg0.97Ce 3+ 0.03O, Co/Mg0.93Ce 3+ 0.07O and Co/Mg0.85Ce 3+ 0.015O catalysts, respectively. Figure 3(a-d) demonstrates the TEM images of the synthesized catalysts distribution and morphology. The particle size and cubic structures were analysed by TEM to characterize crystals at approximately 40-80 nm (smallest). Regular shaped particles were observed by the catalysts Co/MgO, Co/Mg0.97Ce 3+ 0 .0 3O, Co/Mg0.93Ce 3+ 0.07O, Co/Mg0.85Ce 3+ 0.15O. Meanwhile, the cobalt metal uniformly supported the regular shape of the support and together formed a smaller particle size and resulted in a considerable homogeneity in metal dispersion. Figure 3(a-d) shows a 2D cubic texture devoted to the catalyst [23]. The pores of the catalyst were of uniform size (~18 nm) which concurred with the findings from the BET as sown in Table 2. Several particles of cobalt were loaded on the external surface of the support Mg0.85Ce 3+ 0.15O uniformly, which differed from the crystalline sites within the porous structure. The reduced homogeneity of Co particle as compared to other catalysts (due to the regulated crystal growth in the narrowly distributed channels) can explain the discrepancy in the metal particle sizes. The increase in the supported cobalt particle size was in the following sequence; Co/MgO < Co/Mg0.97Ce 3+ 0.03O < Co/Mg0.93Ce 3+ 0.07O < Co/Mg0.85Ce 3+ 0.15O, corresponding to the Scherrer equation results (Table 1).

Temperature programmed reduction (H2-TPR)
The reducibility of cerium for the reforming of cobalt catalysts was characterized by H2-TPR. Table 3 demonstrates the profiles of H2-TPR for Co/Mg1-xCe 3+ xO (where x= 0.00, 0.03, 0.07, 0.15) and Figure 4(a-d) illustrates the catalysts patterns. A reduction in the crystallite Co-O may have caused the occurrence of the Co/MgO catalyst peak at the temperature of 540 °C [24]. Figure 4(b-d) demonstrates the H2-TPR profiles for Co/Mg0.97Ce 3+ 0.03O, Co/Mg0. 93Ce 3+ 0.07O, and Co/Mg0. 85Ce 3+ 0.15O. The first peak was formed for Co/Mg0. 97Ce 3+ 0.03O, Co/Mg0. 93Ce 4+ 0.07O, and Co/Mg0. 85Ce 3+ 0.15O at temperatures of 560 °C, 553 °C, and 544 °C respectively, due to the reduction of Co-O to Co 0 . The second peak for Co/Mg0.97Ce 3+ 0.03O, Co/Mg0.93Ce 3+ 0.07O, and Co/Mg0.85Ce 3+ 0.15O at temperatures of 464 °C, 470 °C, and 473 °C respectively, corresponded to the Ce2O3 reduction on the surface of the Co/Mg1-xCe 3+ xO catalysts. Meanwhile, the third peak was formed at temperatures 710 °C, 735 °C, and 797 °C for Co/Mg0.97Ce 3+ 0.03O, Co/Mg0.93Ce 3+ 0.07O, and Co/Mg0.85Ce 3+ 0.15O, respectively, and was attributed to the reduction of Ce2O3 in the bulk of the catalysts. The reduction in the second peak's temperature as compared to the third peak was attributed to the reduction enthalpies. This may be due to the incorporation of MgO into Ce2O3 and the hindrance of sinter-ing that enhances the dispersion of Ce2O3 particles [22,25]. The other possible explanation may be due to the stronger interaction between Ce2O3 and cobalt metal [26]. The H2consumption of 281.1 µmol/g catalyst was used for the reduction of total Co-O to Co on Co/MgO. The total H2-consumption's amount of the reduced catalysts Co/Mg0.97Ce 3+ 0.03O, Co/Mg0.93Ce 3+ 0.07O, and Co/Mg0.85Ce 3+ 0.15O was recorded at 360.6, 417.4 and 611.5 µmol/g respectively, as calculated from the three peak areas indicating a possible Co−O reduction, and a partial Ce2O3 reduction. An enhancement in the reducibility of the catalysts was observed following the addition of the Ce2O3 promoter, especially the catalysts with MgO support possibly as a result of the support's acidicbasic properties. As the Ce2O3 content in the catalyst increased, it became easier to reduce the catalyst using H2, indicating a higher presence of the active sites, hence, an increase in the conversion rate of CH4 and CO2 in the catalyst Co/Mg0.85Ce 3+ 0.15O. It is clear that Mg1−xCe 3+ xO (higher basicity as compared to

Thermal analysis
The catalysts Co/MgO, Co/Mg0.97Ce 3+ 0.03O, Co/Mg0.93Ce 3+ 0.07O, and Co/Mg0.85Ce 3+ 0.15O components are illustrated by Figure 5(a-d). At first, due to the N2 gas adsorption on the compound, a slight weight increase was noted. A weight loss of 4% and 2%, was recorded for the Co/MgO and Co/Mg0.93Ce 3+ 0.07O, respectively, which may be due to the moisture removal from the Co/Mg1-xCe 3+ xO catalysts, whereas, the results for the Co/Mg0.97Ce 3+ 0.03O and Co/Mg0.85Ce 3+ 0.15O catalysts were stable despite the increase in temperature, indicating the thermal stability of the compounds. Thermal stability for the compounds was achieved at 500 °C, corresponding to the elevated melting point of Ceria (2177 °C) and Magnesia (2852 °C) that resulted in a good interaction among the components of the catalyst. The thermal analysis findings were consistent with the results by Gaddalla [27].

The role of the concentration of the reactant on conversion
The CH4 and CO2 conversion, and the selectivity (H2/CO ratio) revealed the activity of the dry reforming reaction. Upon an increase in the temperature to 900 °C, CO and H2 were observed in the outlet gas of the blank tests (reaction without catalyst) which may be attributed to the reaction of decomposition of methane (Eq. (3)). Using Mg1−xCe 3+ xO without the main catalyst (metals) resulted in lowering the CH4 conversion (32%) and CO2 (41%) with an H2/CO ratio of 0.3% indicating the possibility of a weak reaction on the promoter-support pores as presented by the results of the BET. On the contrary, when using the catalyst Co/Mg1−xCe 3+ xO, an elevation in the rate of CH4 and CO2 conversion and the H2/CO ratio was observed. Figure 6 demonstrates the effects of the reactant (CH4:CO2) ratio on the CH4, CO2 conversion, and H2/CO ratio. The reaction was conducted using two ratios of (CH4:CO2); 1:1 and 2:1. By increasing the CO2 concentration in the (CH4:CO2) ratio to 1:1, the CH4 and CO2 conversion and the H2/CO ratio were increased due to the decline in the carbon deposition on the catalyst that produced CO by reacting with the excess CO2 (Eq. 5). Besides, the role of the doped cobalt metal on the promoter-support in the catalytic reaction was imperative. It has been noted that the most CH4 (75%) and CO2 (86 %) conversion was observed by Co/Mg0.85Ce 3+ 0.15O catalyst with a 1:1 of CO2 : CH4, and a 1.0 H2/CO ratio. Nevertheless, at a 2:1 ratio, the conversion of the CO2 and CH4 gases was recorded at 77% and 68%, respectively with a 0.8 H2/CO ratio. This finding demonstrated that the best deactivate resistance of the catalyst stands at a 1:1 ratio due to the decline in carbon formation which leads to a high H2 and CO selectivity ( Figure  6). Similar findings were also acquired by the other catalysts reported previously [28].
The initial conversion trend of CH4 and CO2 concurred with the number of Co 0 active sites, indicating the significance of the degree of reduction in the Co/Mg−Ce−O based catalyst system. On the contrary, the catalysts prereduction was achieved at 700 °C, due to the DRM operating temperature in the current study (700 °C). As can be seen in the TPR results, the 700 °C reduction temperature was inadequate for the reduction of the complex CoO species in the Co/Mg1−xCexO catalysts. The degree of reduction calculations can be implemented to get a quantitative analysis of the catalyst's reducibility [29]. The estimated degree of reduction values are displayed in Table  3. An estimated value of 100% was obtained for the Co/Mg0.85Ce0.15O catalyst owing to its easier reducibility. Hence, the high Co/Mg0.85Ce0.15O catalytic activity can be attributed to the complete complex CoO species conversion into Co 0 . The degrees of reduction exhibited by the Co/Mg0.93Ce0.07O, Co/Mg0.97Ce0.03O, and Co/MgO catalysts were 92, 89, and 70%, respectively, whereas a fraction of the complex CoO species stayed in the inactive oxide form.
Positive findings in the present study may have been due to the favorable cobalt metal interaction with the promoter-support and the good Ce2O3-MgO catalyst basicity. Unfavorable results were previously reported by Laosiripojana [30] and Guo et al. [31] due to the usage of the catalyst Ni/Al2O3 which demonstrated a weak interaction between the support and Ni and low Al2O3 basicity.
H2/CO ratio and CO2 and CH4 conversion for the catalysts decreased in the following order Co/Mg0.85Ce 3+ 0.15O > Co/Mg0.93Ce 3+ 0.07O > Co/Mg0.97Ce 3+ 0.03O > Co/MgO indicating that the most efficient catalyst among the other studied catalysts was Co/Mg0.85Ce 3+ 0.15O. Findings clarified the dependency of the rate of formation of the H2 and CO gases in the DRM reaction on the amount of solid solution, MgO-Ce2O3 in the catalyst. As such, the more the amount of the solid solution MgO−Ce2O3, the more the H2 and CO gases formation rate. Hence, the role of the solid solution MgO−Ce2O3 formation is crucial in the generation of active sites for the DRM reaction. This occurs due to the capacity of the entire Ce2O3 in stabilizing both oxides. In the catalyst, reduction at 700 °C was only observed at the surface of the Ce2O3 of the solid solution, MgO−Ce2O3. The generated Ce sites remained in close vicinity with the solid solution hindering Ce sintering [32]. Table 4 illustrates the Co/Mg1−xCe 3+ x O selectivity and activity (higher than that of Co/Mg1−xCe 3+ xO) in the reaction of DRM.
Moreover, the elevation in the CH4 and CO2 conversion rate was due to the particle size involved in the activity of the reaction. Using the TEM analysis and the equation of Debye Sher-  rer's, the Co doping metal was prepared with a particle size as minuscule as nanoparticles. Hence, the crucial role of the particle size is evident in the activity of the reaction. Elevation in the reactant's selectivity (yield) and conversion may be a result of the decrease of the particles into nano-ranged sizes, along with having the highest BET surface area (19.2 m 2 /g) ( Table  2) and the highest H2-consumption in H2-TPR (611.5 µmol/g of active sites) ( Table 3).

The role of temperature on the conversion
The Co/Mg0.85Ce 3+ 0.15O catalyst selectivity and activity at a temperature range of 700-900 °C can be seen in Figure 8. Generally, an enhancement in the CH4:CO2 ratio of (1:1) was noted upon increasing the temperature from 700-900 °C which may be attributed to the strong endothermic nature of the dry reforming of methane reaction (Eq. (1)). Earlier research reported an increase in the rate of conversion at higher temperatures [33]. It is noted that an elevation in the temperature (700-900 °C) led to an increase in the CH4 conversion of Co/Mg0.85Ce 3+ 0.15O (42% to 75%) and an elevation in the CO2 conversion from 53% to 86%. However, at a temperature higher than 900 °C, no evident elevation in the CO2 and CH4 conversion rates was observed. Figure 8 illustrates the catalyst H2/CO ratio at a range of temperatures. At a temperature lower than 900 °C, the recorded sample's H2/CO ratio was <1. The lowering in the ratio of H2/CO may be due to the extra H2 in the reverse water-gas-shift reaction (RWGS), to produce CO [Eq. (2)]. At a temperature of 900 °C, the Co/Mg0.85Ce 3+ 0.15O catalyst's H2/CO ratio was recorded at 1.0, indicating a small contribution from the RWGS reaction (Eq. (2)) [34].

Stability tests
As can be seen in Figure 9, a high rate of methane and carbon dioxide diffusion was observed at a temperature of 900 °C. Initially, the adsorption of Methane on the nickel's catalyst surface took place to yield hydrogen resulting in the accumulation of carbon on the nickel's surface as can be seen below (Eqs. (11) It has been known that the carbon deposition on the cobalt metal surface curbs the catalyst's stability, counteracted by the Ce2O3 promoter availability that reactivates the catalyst by eliminating the deposited carbon. The main reason behind the reaction lasting for ≥ 200 h was the utilization of the promoter Ce2O3 in the catalyst that helped ensure stability and a strong coke resistance. Ce2O3 also ensured the removal of the carbon formed on the catalyst during the reaction of DRM. This was followed by the carbonate types formation (Ce2O2CO3), especially Ce2O3, that has the potential of  changing carbon dioxide into O and CO. Lastly, an O atom was generated with the C that was deposited on the Co metal catalyst to yield CO. Based on the results, the deposition of carbon on the catalyst decreased significantly as can be seen below (Eqs. (21) and (22)): In conclusion, the above mechanism prevents the carbon deposition on the Co/Mg0.85Ce 3+ 0.15O catalyst surface making the catalyst fit for long term usage.
3.2.5 Characterization of the catalyst postreaction.
Evaluating the coke formation on the catalyst Co/Mg0.85Ce 3+ 0.15O took place by the TPO-MS, TEM images, and BET post-reaction tests. There was no observed coke deposited on the catalyst surface as demonstrated by the TPO-MS profile ( Figure 10). The spent Co/Mg0.85Ce 3+ 0.15O catalyst's TEM analysis supported the above finding ( Figure 11). Even following 200 h of stream testing, the original catalyst structure was maintained as can be seen in the figure. Moreover, the 2D-cubic texture of the spent catalyst was maintained. A slight elevation in the spent catalyst pore size from 52.7 Å to 57.9 Å was reported. The BET analysis also demonstrated a marginal increase from 19.2 to 20.1 m 2 /g in the spent catalyst surface area. The absence of filamentous carbon on the spent catalyst concluded the negligibility of the deposition of coke.
3.2.6 Improving the catalyst's activity and stability.
The DRM reaction can be improved by conducting research at a lower flow of oxygen (1.25%). As demonstrated by Figure 12, an enhancement in the CH4 conversion (75% to 84%) following the addition of an oxidant (O2, to partially or completely synthesize methane) and utilizing the exothermicity of the reaction (to provide the required heat directly to the DRM reactant mixture) was observed [17]. No effect was seen on the ratio of H2/CO and the conversion of CO2, may be due to the reaction of CH4 with O to yield H2O and CO (Eq. (23)). Lastly, syngas was produced as a result of the reaction of steam with the deposited carbon (Eq. (24)). O2 has the potential of oxidizing the coke deposited on the catalyst (Eq. (25)), hence, reducing the deposition of carbon and improving the catalyst's lifetime.

Conclusion
The impact of several operating parameters on the reaction of DRM were studied in a fixedbed reactor utilizing Co/Mg1−XCe 3+ XO (x= 0, 0.03, 0.07, 0.15; 1 wt% cobalt each) catalysts prepared using the co-precipitation method with K2CO3 as precipitant. A high BET surface area (19.2 m 2 .g −1 ) was possessed by the Co/Mg0.85Ce 3+ 0.15O catalyst along with an easier reducibility. TEM image showed a relatively uniform cubic shape with a diameter of about 58 nm. In addition, the formation of MgO phase on the surface of the catalyst with a dimension size of 61.4 nm was detected utilizing XRD patterns. Using H2-TPR measurement, the reduction of CoO to metallic Co 0 phase was identified at different reduction temperatures dependent on the degree of metal-support interaction and the location of CoO particles on the surface or inside the channels of mesoporous Mg0.85Ce 3+ 0.15O support. In general, the catalytic performance of the 1:1 ratio of CH4:CO2 for the Co/Mg0.85Ce 3+ 0.15O catalyst for 200 h onstream at 900 °C was unchanged with CH4, CO2 conversions and H2/CO ratio of 75%, 86% and 1.0, respectively. TPO-MS and TEM images were implemented to evaluate the formation of coke on the catalyst Co/Mg0.85Ce 3+ 0.15O, postreaction tests. No deposition of coke was observed on the catalyst surface and by conducting research at a lower flow of oxygen (1.25%), the conversion of CH4 was improved from 75% to 84%.