CO2 Methanation of Biogas over 20 wt% Ni-Mg-Al Catalyst: on the Effect of N2, CH4, and O2 on CO2 Conversion Rate

Biogas contains more than 40% CO2 that can be removed to produce high quality CH4. Recently, CH4 production from CO2 methanation has been reported in several studies. In this study, CO2 methanation of biogas was performed over a 20 wt% Ni-Mg-Al catalyst, and the effects of CO2 conversion rate and CH4 selectivity were investigated as a function of CH4, O2, H2O, and N2 compositions of the biogas. At a gas hourly space velocity (GHSV) of 30,000 h−1, the CO2 conversion rate was ~79.3% with a CH4 selectivity of 95%. In addition, the effects of the reaction temperature (200–450 ◦C), GHSV (21,000–50,000 h−1), and H2/CO2 molar ratio (3–5) on the CO2 conversion rate and CH4 selectivity over the 20 wt% Ni-Mg-Al catalyst were evaluated. The characteristics of the catalyst were analyzed using Brunauer–Emmett–Teller surface area analysis, X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy. The catalyst was stable for approximately 200 h at a GHSV of 30,000 h−1 and a reaction temperature of 350 ◦C. CO2 conversion and CH4 selectivity were maintained at 75% and 93%, respectively, and the catalyst was therefore concluded to exhibit stable activity.


Introduction
The recent years have witnessed a growing interest in the regulation of greenhouse gases and the quest for sustainable renewable energy to combat global warming. This has culminated in the demand for an efficient energy storage system (ESS) that can stabilize electric power systems with high output fluctuations. The lithium-ion battery is an ESS widely employed in various energy generation systems owing to its high energy density and efficiency; however, its short shelf life and low storage capacity limit its long-term power storage [1]. The availability of organic waste, which is a sustainable energy source, can increase with economic and population growth. Consequently, much attention has been drawn to the utilization of biogas, as it can be easily obtained from livestock (organic) waste and urban solid waste. Biogas, which typically contains 40-65 vol% CH 4 , 40-50 vol% CO 2 , and minor quantities of (the subsequently removed) N 2 , H 2 S, O 2 , and H 2 O [2], is employed as a high-concentration CH 4 fuel after more than 40% of CO 2 is removed using absorbents or amines [3]. The power-to-gas technology generates H 2 from water by employing renewable energy and produces CH 4 via the methanation of H 2 and CO 2 . Further, CO 2 methanation and reverse water gas shift (RWGS) are competing processes that occur during the production of CH 4 from CO 2 , as described by Equations (1)-(3) [4]. This has The activation energy of Ni catalysts for methanation was 93.61 kJ/mol in Ni/ZrO 2 catalysts [29], 75 kJ/mol in Ni/Al 2 O 3 catalysts [30], and 75 kJ/mol in Ni/Al hydrotalcite catalysts [31].
The present work studies the production of CH 4 by the reaction of biogas CO 2 with hydrogen over a 20wt% Ni-Mg-Al catalyst with high dispersion of Ni metal and BET (Brunauer-Emmett-Teller) value and characterizes this catalyst by several instrumental analyses. The conditions for the reaction of biogas, including reaction temperature, space velocity, and H 2 /CO 2 ratio, were varied to investigate their effects on CO 2 conversion, CH 4 yield, and selectivity. In addition, because biogas is a mixture of various gases and trace elements, the effect of the concentration of these components (N 2 , O 2 , CH 4 , and CO 2 ) on CO 2 conversion, CH 4 yield, and selectivity were studied. Based on this, the optimal reaction conditions for producing CH 4 from biogas were determined, under which a stability test of the 20 wt% Ni-Mg-Al catalyst was performed for 200 h.

Effect of Reaction Temperature on CO 2 Conversion
The dependence of CO 2 conversion and CH 4 selectivity on reaction temperature at a gas hourly space velocity (GHSV) of 30,000 h −1 and H 2 /CO 2 ratio of 4 is displayed in Figure 1. As shown in the figure, CO 2 conversion increased as the temperature increased and reached a maximum at 400 • C, whereas CH 4 selectivity and yield showed the highest values at 350 • C. This trend is probably the result of CO 2 methanation suppression above 350 • C, at which the RWGS reaction (Equation (3)) is enhanced, which resulted in the conversion of CO 2 to CO. This result agreed with that reported by Mohammad et al. [9], wherein the CO concentration increased as the RWGS reaction increased at 400 • C, whereas the methane selectivity decreased. A study by Jia et al. [29] also reported that the highest CO 2 conversion and CH 4 yield were observed at 350 • C as the temperature increased, and a further increase in temperature decreased the CO 2 conversion because of the thermodynamic equilibrium limit [32].
Catalysts 2020, 10, x FOR PEER REVIEW 3 of 16 investigate their effects on CO2 conversion, CH4 yield, and selectivity. In addition, because biogas is a mixture of various gases and trace elements, the effect of the concentration of these components (N2, O2, CH4, and CO2) on CO2 conversion, CH4 yield, and selectivity were studied. Based on this, the optimal reaction conditions for producing CH4 from biogas were determined, under which a stability test of the 20 wt% Ni-Mg-Al catalyst was performed for 200 h.

Effect of Reaction Temperature on CO2 Conversion
The dependence of CO2 conversion and CH4 selectivity on reaction temperature at a gas hourly space velocity (GHSV) of 30,000 h −1 and H2/CO2 ratio of 4 is displayed in Figure 1. As shown in the figure, CO2 conversion increased as the temperature increased and reached a maximum at 400 °C, whereas CH4 selectivity and yield showed the highest values at 350 °C. This trend is probably the result of CO2 methanation suppression above 350 °C, at which the RWGS reaction (Equation (3)) is enhanced, which resulted in the conversion of CO2 to CO. This result agreed with that reported by Mohammad et al. [9], wherein the CO concentration increased as the RWGS reaction increased at 400 °C, whereas the methane selectivity decreased. A study by Jia et al. [29] also reported that the highest CO2 conversion and CH4 yield were observed at 350 °C as the temperature increased, and a further increase in temperature decreased the CO2 conversion because of the thermodynamic equilibrium limit [32].
In addition, the activation energy obtained from the 20 wt% Ni-Mg-Al catalyst was approximately 74.2 kJ/mol, which was similar to that obtained by using Ni/Al2O3 (75 kJ/mol) in the study by Carbarino [30].  Figure 2 shows the effects of the H2/CO2 ratio at a reaction temperature of 350 °C and a GHSV of 30,000 h −1 on CO2 conversion and CH4 selectivity and yield. Figure 2a shows the results obtained for different H2/CO2 ratios by increasing the amount of H2 at a given amount of CO2 and by increasing the amount of CO2 at a given amount of H2. Figure 2b shows the effects of increasing the reactant amounts at a given H2/CO2 ratio. As shown in Figure 2a, the CO2 conversion and CH4 yield increased by approximately 15% when the H2/CO2 ratio increased from 3.5 to 5, whereas the CH4 selectivity remained almost constant. As shown in Figure 2b, the same H2/CO2 ratio led to similar CO2 conversion and CH4 selectivity and yield, regardless of the amount of reactants. These results were similar to those from a study by Rahmani [33] in which a 15% increase was observed when the H2/CO2 ratio was increased from 3 to 4, and the study by Aziz et al. [34] also reported that the concentration In addition, the activation energy obtained from the 20 wt% Ni-Mg-Al catalyst was approximately 74.2 kJ/mol, which was similar to that obtained by using Ni/Al 2 O 3 (75 kJ/mol) in the study by Carbarino [30]. Figure 2 shows the effects of the H 2 /CO 2 ratio at a reaction temperature of 350 • C and a GHSV of 30,000 h −1 on CO 2 conversion and CH 4 selectivity and yield. Figure 2a shows the results obtained for different H 2 /CO 2 ratios by increasing the amount of H 2 at a given amount of CO 2 and by increasing Catalysts 2020, 10, 1201 4 of 16 the amount of CO 2 at a given amount of H 2 . Figure 2b shows the effects of increasing the reactant amounts at a given H 2 /CO 2 ratio. As shown in Figure 2a, the CO 2 conversion and CH 4 yield increased by approximately 15% when the H 2 /CO 2 ratio increased from 3.5 to 5, whereas the CH 4 selectivity remained almost constant. As shown in Figure 2b, the same H 2 /CO 2 ratio led to similar CO 2 conversion and CH 4 selectivity and yield, regardless of the amount of reactants. These results were similar to those from a study by Rahmani [33] in which a 15% increase was observed when the H 2 /CO 2 ratio was increased from 3 to 4, and the study by Aziz et al. [34] also reported that the concentration of hydrogen affects the catalytic activity because of hydrogen adsorption onto the surface of the catalyst and the conversion to methane via hydrogenation.

Effect of H 2 /CO 2 Ratio on CO 2 Conversion
Catalysts 2020, 10, x FOR PEER REVIEW 4 of 16 of hydrogen affects the catalytic activity because of hydrogen adsorption onto the surface of the catalyst and the conversion to methane via hydrogenation.

Effect of GHSV on CO2 Conversion
The effects of increasing GHSV on CO2 conversion and CH4 selectivity and yield at a reaction temperature of 350 °C and H2/CO2 ratio of 4 are shown in Figure 3a,b, respectively. The effect of increasing the reactant flow rate on GHSV of 21,000-50,000 h −1 is shown in Figure 3a, and the effect of increasing the amount of nitrogen at a given reactant flow rate on GHSV of 9000-38,000 h −1 is shown in Figure 3b. As shown in the figure, CO2 conversion and CH4 yield/selectivity showed a general decreasing trend as the GHSV increased, and the effect for (b) was larger than that for (a). This is because an increase in GHSV shortens the time during which the reactants CO2 and H2 are in contact with the catalyst, thus reducing the amount of reactants adsorbed onto the surface of the catalyst. These results were consistent with the experimental results reported by Abate et al. [35] (a) (b)

Effect of GHSV on CO 2 Conversion
The effects of increasing GHSV on CO 2 conversion and CH 4 selectivity and yield at a reaction temperature of 350 • C and H 2 /CO 2 ratio of 4 are shown in Figure 3a,b, respectively. The effect of increasing the reactant flow rate on GHSV of 21,000-50,000 h −1 is shown in Figure 3a, and the effect of increasing the amount of nitrogen at a given reactant flow rate on GHSV of 9000-38,000 h −1 is shown in Figure 3b. As shown in the figure, CO 2 conversion and CH 4 yield/selectivity showed a general decreasing trend as the GHSV increased, and the effect for (b) was larger than that for (a). This is because an increase in GHSV shortens the time during which the reactants CO 2 and H 2 are in contact with the catalyst, thus reducing the amount of reactants adsorbed onto the surface of the catalyst. These results were consistent with the experimental results reported by Abate et al. [35]. shown in Figure 3b. As shown in the figure, CO2 conversion and CH4 yield/selectivity showed a general decreasing trend as the GHSV increased, and the effect for (b) was larger than that for (a). This is because an increase in GHSV shortens the time during which the reactants CO2 and H2 are in contact with the catalyst, thus reducing the amount of reactants adsorbed onto the surface of the catalyst. These results were consistent with the experimental results reported by Abate et al. [35] (a) (b)

Effect of Initial CH 4 Concentration on CO 2 Conversion
The effect of changing the initial CH 4 concentration to 0, 50, and 65 vol% for CO 2 conversion at a reaction temperature of 400 • C, GHSV of 30,000 h −1 , and H 2 /CO 2 ratio of 4 is shown in Figure 4. As shown in the figure, increasing the content of CH 4 in the reactant gas to 40, 50, and 65 vol% led to low CO 2 conversions of approximately 67, 64, and 54%, respectively, resulting in up to a 20% decrease in the CO 2 conversion compared to that in the absence of CH 4 . This phenomenon is attributed to the Le Chatelier principle, in which the initial CH 4 present in the reactant gas inhibits the conversion to CH 4 . These results were also reported in a simulation study by Jürgensen et al. [36], wherein CO 2 conversion decreased as the initial methane concentration increased. The effect of changing the initial CH4 concentration to 0, 50, and 65 vol% for CO2 conversion at a reaction temperature of 400 °C, GHSV of 30,000 h −1 , and H2/CO2 ratio of 4 is shown in Figure 4. As shown in the figure, increasing the content of CH4 in the reactant gas to 40, 50, and 65 vol% led to low CO2 conversions of approximately 67, 64, and 54%, respectively, resulting in up to a 20% decrease in the CO2 conversion compared to that in the absence of CH4. This phenomenon is attributed to the Le Chatelier principle, in which the initial CH4 present in the reactant gas inhibits the conversion to CH4. These results were also reported in a simulation study by Jürgensen et al. [36], wherein CO2 conversion decreased as the initial methane concentration increased.

Effect of Initial CO2 Concentration on CO2 Conversion
The effects of increasing the initial CO2 concentration on CO2 conversion and CH4 selectivity and yield at a reaction temperature of 350 °C, GHSV of 30,000 and 50,000 h −1 , and H2/CO2 ratio of 4 are presented in Figure 5. As shown in the figure, an increase of CO2 in biogas from 10 to 14 vol% at a GHSV of 30,000 h −1 resulted in a 2% increase in CO2 conversion and a 3% increase at a GHSV of 50,000 h −1 . This is probably due to the increase in reaction temperature owing to the exothermic reaction of CO2 methanation with increasing amounts of the reactants; thus, the increase in CO2 conversion is more at a GHSV of 50,000 h −1 than that at 30,000 h −1 , because more reactants are present in the former

Effect of Initial CO 2 Concentration on CO 2 Conversion
The effects of increasing the initial CO 2 concentration on CO 2 conversion and CH 4 selectivity and yield at a reaction temperature of 350 • C, GHSV of 30,000 and 50,000 h −1 , and H 2 /CO 2 ratio of 4 Catalysts 2020, 10, 1201 6 of 16 are presented in Figure 5. As shown in the figure, an increase of CO 2 in biogas from 10 to 14 vol% at a GHSV of 30,000 h −1 resulted in a 2% increase in CO 2 conversion and a 3% increase at a GHSV of 50,000 h −1 . This is probably due to the increase in reaction temperature owing to the exothermic reaction of CO 2 methanation with increasing amounts of the reactants; thus, the increase in CO 2 conversion is more at a GHSV of 50,000 h −1 than that at 30,000 h −1 , because more reactants are present in the former than in the latter.

Effect of Initial N2 Concentration on CO2 Conversion
Approximately 15% of N2 exists in landfill gas-a type of biogas-which is used as an inactive gas to prevent the deactivation of catalysts caused by the spot exothermic reaction of CO2 methanation. The effect of the presence of 0, 10, and 30% N2 in the reactant gas on CO2 conversion at a GHSV of 15,000 h −1 and H2/CO2 ratio of 4 is shown in Figure 6. When nitrogen concentrations were 10 and 30% in the reactant, the respective CO2 conversions were approximately 3 and 5% lower than that in the absence of N2. This is believed to be due to the decrease in the reactant concentration as the N2 concentration increased, leading to less heat generation, which, in turn, lowers the reaction temperature.

Effect of Initial Oxygen Concentration on CO2 Conversion
The effects of the presence of oxygen in the reactant gas on CO2 conversion at a GHSV of 30,000 h −1 , H2/CO2 ratio of 4, and reaction temperatures of 300 and 400 °C are shown in Figure 7. When 4% oxygen was present in the reactant gas, CO2 conversion and selectivity decreased by approximately 5% and 3%, respectively. This is thought to be a protected re-oxidation reaction due to the presence

Effect of Initial N 2 Concentration on CO 2 Conversion
Approximately 15% of N 2 exists in landfill gas-a type of biogas-which is used as an inactive gas to prevent the deactivation of catalysts caused by the spot exothermic reaction of CO 2 methanation. The effect of the presence of 0, 10, and 30% N 2 in the reactant gas on CO 2 conversion at a GHSV of 15,000 h −1 and H 2 /CO 2 ratio of 4 is shown in Figure 6. When nitrogen concentrations were 10 and 30% in the reactant, the respective CO 2 conversions were approximately 3 and 5% lower than that in the absence of N 2 . This is believed to be due to the decrease in the reactant concentration as the N 2 concentration increased, leading to less heat generation, which, in turn, lowers the reaction temperature.

Effect of Initial N2 Concentration on CO2 Conversion
Approximately 15% of N2 exists in landfill gas-a type of biogas-which is used as an inactive gas to prevent the deactivation of catalysts caused by the spot exothermic reaction of CO2 methanation. The effect of the presence of 0, 10, and 30% N2 in the reactant gas on CO2 conversion at a GHSV of 15,000 h −1 and H2/CO2 ratio of 4 is shown in Figure 6. When nitrogen concentrations were 10 and 30% in the reactant, the respective CO2 conversions were approximately 3 and 5% lower than that in the absence of N2. This is believed to be due to the decrease in the reactant concentration as the N2 concentration increased, leading to less heat generation, which, in turn, lowers the reaction temperature.

Effect of Initial Oxygen Concentration on CO2 Conversion
The effects of the presence of oxygen in the reactant gas on CO2 conversion at a GHSV of 30,000 h −1 , H2/CO2 ratio of 4, and reaction temperatures of 300 and 400 °C are shown in Figure 7. When 4% oxygen was present in the reactant gas, CO2 conversion and selectivity decreased by approximately 5% and 3%, respectively. This is thought to be a protected re-oxidation reaction due to the presence of oxygen in the reactant gas, which prevents the forward reaction in CH4 synthesis paths (1) and (2). According to the mechanism proposed by Lin et al. [21], CO2 is separated from the oxygen vacancies

Effect of Initial Oxygen Concentration on CO 2 Conversion
The effects of the presence of oxygen in the reactant gas on CO 2 conversion at a GHSV of 30,000 h −1 , H 2 /CO 2 ratio of 4, and reaction temperatures of 300 and 400 • C are shown in Figure 7. When 4% Catalysts 2020, 10, 1201 7 of 16 oxygen was present in the reactant gas, CO 2 conversion and selectivity decreased by approximately 5% and 3%, respectively. This is thought to be a protected re-oxidation reaction due to the presence of oxygen in the reactant gas, which prevents the forward reaction in CH 4 synthesis paths (1) and (2). According to the mechanism proposed by Lin et al. [21], CO 2 is separated from the oxygen vacancies on the Ni metal and support material, which are produced during the reduction of the catalysts, and it is reported that a decrease in the oxygen vacancies on the catalyst and support reduces the catalytic activity. Therefore, it is considered that the CO 2 conversion and selectivity decrease in the presence of oxygen because of the decrease in the amount of oxygen vacancies on the Ni catalyst and support, which prevents CO 2 from being converted to CO or carbon species, or because of the re-oxidation of CO.
Catalysts 2020, 10, x FOR PEER REVIEW 7 of 16 on the Ni metal and support material, which are produced during the reduction of the catalysts, and it is reported that a decrease in the oxygen vacancies on the catalyst and support reduces the catalytic activity. Therefore, it is considered that the CO2 conversion and selectivity decrease in the presence of oxygen because of the decrease in the amount of oxygen vacancies on the Ni catalyst and support, which prevents CO2 from being converted to CO or carbon species, or because of the re-oxidation of CO. Figure 7. Effect of O2 in reaction gas.

Stability and Activity of Catalyst Test
To evaluate the activity and stability of the catalysts, CO2 methanation was conducted at a GHSV of 30,000 h −1 , reaction temperature of 350 °C , and H2/CO2 ratio of 4 for 200 h. As shown in Figure 8, the CO2 conversion was constant at 75%, and CH4 selectivity was 93% for a reaction time of 200 h. These results confirmed the activity and stability of the 20 wt% Ni-Mg-Al catalyst for the CO2 methanation reaction. The catalytic activity was retained even upon storage at room temperature.

Catalyst Preparation
The catalyst used in this study is a 20 and 40 wt% Ni-Mg-Al catalyst (supported by Korea Institute of Energy Research, Daejeon, Korea). The catalyst was prepared on a Ni metal that exhibits high catalytic activity and CH4 selectivity. The catalyst was prepared by mixing calculated ratios of

Stability and Activity of Catalyst Test
To evaluate the activity and stability of the catalysts, CO 2 methanation was conducted at a GHSV of 30,000 h −1 , reaction temperature of 350 • C, and H 2 /CO 2 ratio of 4 for 200 h. As shown in Figure 8, the CO 2 conversion was constant at 75%, and CH 4 selectivity was 93% for a reaction time of 200 h. These results confirmed the activity and stability of the 20 wt% Ni-Mg-Al catalyst for the CO 2 methanation reaction. The catalytic activity was retained even upon storage at room temperature.
Catalysts 2020, 10, x FOR PEER REVIEW 7 of 16 on the Ni metal and support material, which are produced during the reduction of the catalysts, and it is reported that a decrease in the oxygen vacancies on the catalyst and support reduces the catalytic activity. Therefore, it is considered that the CO2 conversion and selectivity decrease in the presence of oxygen because of the decrease in the amount of oxygen vacancies on the Ni catalyst and support, which prevents CO2 from being converted to CO or carbon species, or because of the re-oxidation of CO. Figure 7. Effect of O2 in reaction gas.

Stability and Activity of Catalyst Test
To evaluate the activity and stability of the catalysts, CO2 methanation was conducted at a GHSV of 30,000 h −1 , reaction temperature of 350 °C , and H2/CO2 ratio of 4 for 200 h. As shown in Figure 8, the CO2 conversion was constant at 75%, and CH4 selectivity was 93% for a reaction time of 200 h. These results confirmed the activity and stability of the 20 wt% Ni-Mg-Al catalyst for the CO2 methanation reaction. The catalytic activity was retained even upon storage at room temperature.

Catalyst Preparation
The catalyst used in this study is a 20 and 40 wt% Ni-Mg-Al catalyst (supported by Korea Institute of Energy Research, Daejeon, Korea). The catalyst was prepared on a Ni metal that exhibits high catalytic activity and CH4 selectivity. The catalyst was prepared by mixing calculated ratios of

Catalyst Preparation
The catalyst used in this study is a 20 and 40 wt% Ni-Mg-Al catalyst (supported by Korea Institute of Energy Research, Daejeon, Korea). The catalyst was prepared on a Ni metal that exhibits high catalytic activity and CH 4 selectivity. The catalyst was prepared by mixing calculated ratios of Ni(NO 3 ) 2 6H 2 O, Al(NO 3 ) 2 9H 2 O, and Mg(NO 3 ) 2 6H 2 O solutions at 60 • C until the Ni content was 20, 40 wt%, and a precipitate was obtained after adding a precipitant to the mixture and stirring for approximately 1 h while maintaining a constant pH. The precipitated catalyst precursor was repeatedly washed with distilled water and filtered with a filter press until the pH reached approximately 7.0. The catalyst precursor was then dried in an oven at 150 • C for 12 h, and 20 and 40 wt% Ni-Mg-Al 2 O 3 catalysts were prepared through a heat treatment process under an air atmosphere at 600 • C for 4 h. Prior to their use, all catalysts were heated to the reduction temperature under a gas flow of 100 mL/min (20% H 2 , 80% N 2 ) for 2 h, and the catalysts were reduced while the temperature was maintained for 4 h. Figure 9 illustrates the variations in catalytic activity of the 40 wt% Ni-Mg-Al 2 O 3 catalysts at the reaction temperatures of 350 and 400 • C as a function of changes in the reduction temperatures to 450, 550, 700, and 800 • C. The CO 2 conversion initially showed a significant increase as the reduction temperature increased, while the CO 2 conversions at 700 and 800 • C were similar; therefore, 700 • C was used as the reduction temperature of the catalysts. Ni(NO3)2 ㆍ 6H2O, Al(NO3)2 ㆍ 9H2O, and Mg(NO3)2 ㆍ 6H2O solutions at 60 °C until the Ni content was 20, 40 wt%, and a precipitate was obtained after adding a precipitant to the mixture and stirring for approximately 1 h while maintaining a constant pH. The precipitated catalyst precursor was repeatedly washed with distilled water and filtered with a filter press until the pH reached approximately 7.0. The catalyst precursor was then dried in an oven at 150 °C for 12 h, and 20 and 40 wt% Ni-Mg-Al2O3 catalysts were prepared through a heat treatment process under an air atmosphere at 600 °C for 4 h. Prior to their use, all catalysts were heated to the reduction temperature under a gas flow of 100 mL/min (20% H2, 80% N2) for 2 h, and the catalysts were reduced while the temperature was maintained for 4 h. Figure 9 illustrates the variations in catalytic activity of the 40 wt% Ni-Mg-Al2O3 catalysts at the reaction temperatures of 350 and 400 °C as a function of changes in the reduction temperatures to 450, 550, 700, and 800 °C. The CO2 conversion initially showed a significant increase as the reduction temperature increased, while the CO2 conversions at 700 and 800 °C were similar; therefore, 700 °C was used as the reduction temperature of the catalysts.

CO2 Methanation Apparatus and Activity Test
Isothermal CO2 methanation experiments were conducted in a plug-flow system ( Figure 10) at steady state with Ni catalysts loaded into the reactor. The catalytic reactor has an outer diameter of 12.7 mm (1/2"), a thickness of 1.257 mm, a length of 209 mm, and Inconel 800 HT composed of 80% Ni, 14% Cr, and 6% Fe. During the experimentation, a mesh sieve was installed in the lower part of the reactor to support the catalyst layer, and 0.5 g of the catalyst was loaded. A back-pressure regulator (BPR) in the latter section of the reactor was used to control the reaction pressure from 1 to 9 atm. A water trap in the latter section of the BPR was used to remove the water generated from the reaction, and a high-pressure check valve was installed to prevent gas backflow. All tubes used were of SUS (Steel Use Stainless) grade with an outer diameter of Φ3.2 mm and a thickness of 0.8 t, and the products were analyzed using gas chromatography (GC). To prevent the condensation of the products inside the tube during this process, a line heater was installed and maintained above 150 °C.

CO 2 Methanation Apparatus and Activity Test
Isothermal CO 2 methanation experiments were conducted in a plug-flow system ( Figure 10) at steady state with Ni catalysts loaded into the reactor. The catalytic reactor has an outer diameter of 12.7 mm (1/2"), a thickness of 1.257 mm, a length of 209 mm, and Inconel 800 HT composed of 80% Ni, 14% Cr, and 6% Fe. During the experimentation, a mesh sieve was installed in the lower part of the reactor to support the catalyst layer, and 0.5 g of the catalyst was loaded. A back-pressure regulator (BPR) in the latter section of the reactor was used to control the reaction pressure from 1 to 9 atm. A water trap in the latter section of the BPR was used to remove the water generated from the reaction, and a high-pressure check valve was installed to prevent gas backflow. All tubes used were of SUS (Steel Use Stainless) grade with an outer diameter of Φ3.2 mm and a thickness of 0.8 t, and the products were analyzed using gas chromatography (GC). To prevent the condensation of the products inside the tube during this process, a line heater was installed and maintained above 150 • C. The reaction products were characterized using a GC (YL Instrument 6500 GC, YL Instruments Co., Anyang, Korea), SS COL 10FT 1/8" PORAPACK N (Model: 13052-U), a Phase None Matrix 45/60 Molecular Sieve 13X was used for the GC columns, and argon was used as the carrier gas. Hydrogen, methane, and carbon monoxide were analyzed with a thermal conductivity detector (TCD), whereas carbon dioxide was analyzed using a flame ionization detector (FID). Characterization in the GC oven was conducted by maintaining the temperature initially at 35 °C for 0-6 min and raising it to approximately 170 °C at a ramp rate of 15 °C/min. FID characterization was performed at 250 °C by supplying 35 mL/min of hydrogen and 300 mL/min of oxygen, while TCD characterization was carried out at 150 °C under a gas flow rate of 35 mL/min of hydrogen and 20 mL/min of Ar. CO2 conversion (XCO2), CH4 selectivity (SCH4), and CH4 yield (YCH4) were calculated according to Equations (4)-(6) [37].  Table 1.  The reaction products were characterized using a GC (YL Instrument 6500 GC, YL Instruments Co., Anyang, Korea), SS COL 10FT 1/8" PORAPACK N (Model: 13052-U), a Phase None Matrix 45/60 Molecular Sieve 13X was used for the GC columns, and argon was used as the carrier gas. Hydrogen, methane, and carbon monoxide were analyzed with a thermal conductivity detector (TCD), whereas carbon dioxide was analyzed using a flame ionization detector (FID). Characterization in the GC oven was conducted by maintaining the temperature initially at 35 • C for 0-6 min and raising it to approximately 170 • C at a ramp rate of 15 • C/min. FID characterization was performed at 250 • C by supplying 35 mL/min of hydrogen and 300 mL/min of oxygen, while TCD characterization was carried out at 150 • C under a gas flow rate of 35 mL/min of hydrogen and 20 mL/min of Ar. CO 2 conversion (X CO2 ), CH 4 selectivity (S CH4 ), and CH 4 yield (Y CH4 ) were calculated according to Equations (4)-(6) [37].
Using a reactant gas flow rate of 250 mL/min, a reaction temperature of 350 • C, GHSV of 30,000 h −1 , and an H 2 /CO 2 ratio of 4 as the basis, methanation reaction experiments were conducted by changing the experimental conditions, as listed in Table 1.

Brunauer-Emmett-Teller (BET) Measurement
The BET specific surface area, which is one of the key catalyst properties, was measured using ASAP 2020 Plus Physisorption (Center for Advanced Materials Analysis, Suwon University) (Micrometrics Instruments, Norcross, GA, USA). A known amount of catalyst sample was added to the BET measurement tube, and the moisture in the catalyst was removed through a pretreatment process under a vacuum of 10 µm Hg by heating to 250 • C at a ramp rate of 10 • C/min for 12 h, after which the weight of the catalyst sample was measured.
Measurements of the specific surface area, pore volume, and pore size of the fresh (20 and 40 wt% Ni-Mg-Al) and spent (20 wt% Ni-Mg-Al catalyst after 200 h of use) catalysts are summarized in Table 2. The BET surface area of the spent catalyst decreased in comparison with that of the fresh catalyst. In addition, an increase in the Ni content led to a decrease in the specific surface area, which is attributed to the blocking of catalyst pores with increasing Ni content, similar to the results reported by Daroughegi et al. [12] with catalysts containing greater than 20 wt% of Ni. In addition, these observations were similar to the results obtained by A. Zhao et al. [38]-the Ni metal crystal size increased in the catalysts comprising Ni loadings greater than 20 wt%. As shown in Table 3, the 20 wt% catalyst also showed a high TOF (Turnover Frequency) value. TOF values towards CO 2 were defined as the number of CO 2 molecules converted over per surface metallic Ni active site per second. High TOF value means that the catalytic activity is high. Therefore, the 20 wt% catalyst showed a higher CO 2 conversion than the 40 wt% catalyst (Tables 3 and 4). Table 2. Brunauer-Emmett-Teller (BET) surface area, pore volume, and pore size of 20 wt% Ni-Mg-Al catalyst (fresh and spent).

X-ray Diffraction (XRD) Characterization
The elemental composition of the catalysts was characterized using an XRD diffractometer (Center for Advanced Materials Analysis, Suwon University) (Thermo Fisher Scientific, ARL Equinox 3000, MA, USA). The catalyst powder samples were pretreated at 250 • C for 5 h to remove moisture, and the crystals of the catalysts were analyzed. Cu-Kα radiation was used to fix the axis of the sample, and measurements were performed at 30 mA and 40 kV over a 2θ range 10-80 • using a scanning speed of 8θ/min. The XRD analysis results of the catalytic supports and Ni catalysts before and after the reaction are shown in Figure 11. Diffraction peaks that appear at 2θ values of 37.  Figure 11 shows the XRD characterization of the catalysts before and after reduction at 700 • C and on the spent catalyst 200 h after the reaction. As shown in the figure, peaks corresponding to Ni metal were not observed in the fresh catalyst prior to the reaction, whereas NiO peaks were observed [39]. In contrast, the reduction catalyst and spent catalyst exhibited prominent Ni metal peaks at 2θ values of 51.85 • and 76.3 • , respectively [30].
Catalysts 2020, 10, x FOR PEER REVIEW 11 of 16 and measurements were performed at 30 mA and 40 kV over a 2θ range 10-80° using a scanning speed of 8θ/min. The XRD analysis results of the catalytic supports and Ni catalysts before and after the reaction are shown in Figure 11. Diffraction peaks that appear at 2θ values of 37 Figure 11 shows the XRD characterization of the catalysts before and after reduction at 700 °C and on the spent catalyst 200 h after the reaction. As shown in the figure, peaks corresponding to Ni metal were not observed in the fresh catalyst prior to the reaction, whereas NiO peaks were observed [39]. In contrast, the reduction catalyst and spent catalyst exhibited prominent Ni metal peaks at 2θ values of 51.85° and 76.3°, respectively [30].

H2-TPR Analysis
The interaction and reduction between the catalyst metal and support were characterized by H2temperature-programmed reduction (H2-TPR, AutoChem II 2920 V5.02, Micromeritics Instruments, Norcross, GA, USA). As shown in Figure 12, the H2 consumption of the catalysts appeared at 300-450 and 600-700 °C. The two peaks correspond to the reduction of the NiO particles. The first peak refers to the reduction of NiO particles that exhibit a weak interaction due to MgO, and the second peak that appears at high temperatures corresponds to the reduction of NiO. Al2O3 particles with spinel structures exhibit a strong interaction between the NiO particles and the Al2O3 support. An improved H2 consumption was observed as the Ni content of the catalysts increased.

H 2 -TPR Analysis
The interaction and reduction between the catalyst metal and support were characterized by H 2 -temperature-programmed reduction (H 2 -TPR, AutoChem II 2920 V5.02, Micromeritics Instruments, Norcross, GA, USA). As shown in Figure 12, the H 2 consumption of the catalysts appeared at 300-450 and 600-700 • C. The two peaks correspond to the reduction of the NiO particles. The first peak refers to the reduction of NiO particles that exhibit a weak interaction due to MgO, and the second peak that appears at high temperatures corresponds to the reduction of NiO. Al 2 O 3 particles with spinel structures exhibit a strong interaction between the NiO particles and the Al 2 O 3 support. An improved H 2 consumption was observed as the Ni content of the catalysts increased.

X-ray Photoelectron Spectroscopy (XPS) Characterization
To investigate the oxidation state of the catalysts, XPS (Center for Advanced Materials Analysis, Suwon University) (K-Alpha plus, Thermo Fisher Scientific, East Grinstead, UK) was conducted to measure the binding energy. XPS characterization was performed under vacuum using Al-Kα radiation on a fresh catalyst and on the spent catalyst after 200 h of reaction, without any separate sample pretreatment process ( Figure 13). The oxidation state of Ni can be determined from the binding energy (BE) of the XPS Ni2p3/2 spectrum. The Ni2p3/2 BE of NiO is 855-856 and 860.5 eV, and that of the Ni metal is 852.3-852.6 eV [40]. Also, the Ni2p1/2 BE of the Ni metal is 873-875 eV. As shown in the figure, the XPS spectrum for the BE of the fresh catalyst displayed peaks at 855.0 and 860.4 eV that are associated with NiO, and a small amount of Ni(OH)2 (peaks at 865.6 eV). The XPS profile of the spent catalyst revealed the presence of Ni metal (851.9 eV) and NiO (855 eV and 860.3 eV).

X-ray Photoelectron Spectroscopy (XPS) Characterization
To investigate the oxidation state of the catalysts, XPS (Center for Advanced Materials Analysis, Suwon University) (K-Alpha plus, Thermo Fisher Scientific, East Grinstead, UK) was conducted to measure the binding energy. XPS characterization was performed under vacuum using Al-Kα radiation on a fresh catalyst and on the spent catalyst after 200 h of reaction, without any separate sample pretreatment process ( Figure 13). The oxidation state of Ni can be determined from the binding energy (BE) of the XPS Ni2p3/2 spectrum. The Ni2p3/2 BE of NiO is 855-856 and 860.5 eV, and that of the Ni metal is 852.3-852.6 eV [40]. Also, the Ni2p1/2 BE of the Ni metal is 873-875 eV. As shown in the

X-ray Photoelectron Spectroscopy (XPS) Characterization
To investigate the oxidation state of the catalysts, XPS (Center for Advanced Materials Analysis, Suwon University) (K-Alpha plus, Thermo Fisher Scientific, East Grinstead, UK) was conducted to measure the binding energy. XPS characterization was performed under vacuum using Al-Kα radiation on a fresh catalyst and on the spent catalyst after 200 h of reaction, without any separate sample pretreatment process ( Figure 13). The oxidation state of Ni can be determined from the binding energy (BE) of the XPS Ni2p3/2 spectrum. The Ni2p3/2 BE of NiO is 855-856 and 860.5 eV, and that of the Ni metal is 852.3-852.6 eV [40]. Also, the Ni2p1/2 BE of the Ni metal is 873-875 eV. As shown in the figure, the XPS spectrum for the BE of the fresh catalyst displayed peaks at 855.0 and 860.4 eV that are associated with NiO, and a small amount of Ni(OH)2 (peaks at 865.6 eV). The XPS profile of the spent catalyst revealed the presence of Ni metal (851.9 eV) and NiO (855 eV and 860.3 eV).

Scanning Electron Microscopy (SEM)-Energy-Dispersive X-ray Spectroscopy (EDX) Characterization
SEM-EDX was used to investigate the surface morphology of the catalysts and the dispersion of Ni. (Center for Advanced Materials Analysis, Suwon University) (APREO SEM, FEI, Hillsboro, OR, USA) was used to obtain images at 20,000 × magnification of the catalyst samples prepared by removing powders and dust, drying at 120 • C for 1 h, and coating with metal (Au). SEM images of the fresh catalyst and the spent catalyst after 200 h of reaction are presented in Figure 14. As shown in the SEM images, nanoscale particles were uniformly dispersed throughout the surface. Owing to the decomposition of Mg(NO 3 ) 2 ·6H 2 O to MgO at temperatures above 600 • C, it was confirmed that spherical [41] particles were uniformly dispersed.
3.3.5. Scanning Electron Microscopy (SEM)-Energy-Dispersive X-ray Spectroscopy (EDX) Characterization SEM-EDX was used to investigate the surface morphology of the catalysts and the dispersion of Ni. (Center for Advanced Materials Analysis, Suwon University) (APREO SEM, FEI, Hillsboro, OR, USA) was used to obtain images at 20,000 × magnification of the catalyst samples prepared by removing powders and dust, drying at 120 °C for 1 h, and coating with metal (Au). SEM images of the fresh catalyst and the spent catalyst after 200 h of reaction are presented in Figure 14. As shown in the SEM images, nanoscale particles were uniformly dispersed throughout the surface. Owing to the decomposition of Mg(NO3)2·6H2O to MgO at temperatures above 600 °C, it was confirmed that spherical [41] particles were uniformly dispersed. The results of the elemental composition analysis of the catalyst using EDX and of the surface morphology characterization from the SEM image are shown in Table 5. The contents of Ni, Mg, and Al metals from the cross-section of the catalysts are summarized in Table 5 (the data is for indicative purposes only).

Conclusions
In this study, the reaction between H2 and the CO2 present in biogas was explored, and experiments on CO2 methanation for producing CH4 were conducted over a 20 wt% Ni-Mg-Al catalyst. The optimal conditions for CO2 methanation over the 20 wt% Ni-Mg-Al catalyst were determined based on the effects of the reaction temperature, GHSV, and H2/CO2 ratio on CO2 conversion. Furthermore, experiments investigating the effects of CO2, CH4, N2, and O2 concentrations (i.e., biogas composition) on CO2 conversion led to the following conclusions.
1) CO2 conversion increased as the reaction temperature increased, but decreased beyond 400 °C, and the highest values of CH4 selectivity and yield were obtained near 350 °C. This is due to the thermodynamic equilibrium limit that reduces the CO2 conversion at temperatures above 400 °C [29] and inhibits the methanation reaction above 350 °C via the RWGS reaction, which increases the production of CO and decreases the CH4 selectivity. The activation energy at this point was 72.4 kJ/mol.
2) CO2 conversion and CH4 selectivity increased as the H2/CO2 ratio increased from 3.5 to 5.0, whereas the CO2 conversion resulted in similar values as long as the H2/CO2 ratio remained the same regardless of the H2 and CO2 concentrations when the H2/CO2 ratio was varied. Increasing the GHSV The results of the elemental composition analysis of the catalyst using EDX and of the surface morphology characterization from the SEM image are shown in Table 5. The contents of Ni, Mg, and Al metals from the cross-section of the catalysts are summarized in Table 5 (the data is for indicative purposes only). Table 5. Elemental composition of fresh 20 wt% Ni-Mg-Al catalyst determined by scanning electron microscopy-energy-dispersive X-ray spectroscopy.

Conclusions
In this study, the reaction between H 2 and the CO 2 present in biogas was explored, and experiments on CO 2 methanation for producing CH 4 were conducted over a 20 wt% Ni-Mg-Al catalyst. The optimal conditions for CO 2 methanation over the 20 wt% Ni-Mg-Al catalyst were determined based on the effects of the reaction temperature, GHSV, and H 2 /CO 2 ratio on CO 2 conversion. Furthermore, experiments investigating the effects of CO 2 , CH 4 , N 2 , and O 2 concentrations (i.e., biogas composition) on CO 2 conversion led to the following conclusions.
1) CO 2 conversion increased as the reaction temperature increased, but decreased beyond 400 • C, and the highest values of CH 4 selectivity and yield were obtained near 350 • C. This is due to the thermodynamic equilibrium limit that reduces the CO 2 conversion at temperatures above 400 • C [29] and inhibits the methanation reaction above 350 • C via the RWGS reaction, which increases the production of CO and decreases the CH 4 selectivity. The activation energy at this point was 72.4 kJ/mol.
2) CO 2 conversion and CH 4 selectivity increased as the H 2 /CO 2 ratio increased from 3.5 to 5.0, whereas the CO 2 conversion resulted in similar values as long as the H 2 /CO 2 ratio remained the same regardless of the H 2 and CO 2 concentrations when the H 2 /CO 2 ratio was varied. Increasing the GHSV of the reactant gas and its N 2 concentration shortened the contact time between the reactant and catalyst and reduced the CO 2 conversion.
3) A higher initial concentration of CO 2 in the biogas led to a slightly higher CO 2 conversion, but increasing the initial CH 4 concentration to 40, 50, and 65 vol% decreased the CO 2 conversion by up to 20% compared to that in the absence of CH 4 . This trend is believed to be the result of Le Chatelier's principle, in which the conversion of CH 4 was suppressed by the initial concentration of CH 4 in the reactant gas. The selectivity was shown to be independent of CH 4 concentration and remained constant. 4) Investigation of the effect of the concentrations of N 2 and O 2 components of biogas revealed that CO 2 conversion decreased by approximately 5% in the presence of 10 vol% N 2 . In the presence of O 2 , the CO 2 conversion decreased as a result of the decrease in the number of active sites of the Ni catalyst due to oxygen, which also reduced the CH 4 selectivity because of the re-oxidation reaction.
5) The stability of the 20 wt% Ni-Mg-Al catalyst for CO 2 methanation was evaluated from experiments at 350 • C for 200 h. The catalyst is expected to exhibit stable activity, because the CO 2 conversion and CH 4 selectivity were maintained at constant values of 75 and 93%, respectively.
As mentioned above, because of its stability over 200 h and CO 2 conversion over 75%, as indicated by the activity test of the 20 wt% Ni catalyst, the prepared catalyst exhibits excellent performance. In future, we plan to (i) apply the developed strategy to power plants (which generate much CO 2 ) and food waste treatment plants (which generate much anaerobic digestion gas) to produce methane gas and (ii) carry out demonstration tests for use in natural gas grids and natural gas-powered vehicles.