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

catalysts

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

Danbee Han 1, Yunji Kim 1, Hyunseung Byun 1, Wonjun Cho 2 and Youngsoon Baek 1,*

1 Department of Environmental and Energy Engineering, University of Suwon, Hwaseong-si 18323, Korea; [email protected] (D.H.); [email protected] (Y.K.); [email protected] (H.B.) 2 Unisys International R&D, Bio Friends Inc., Yuseong-gu, Daejeon 34028, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-31-220-2167

Received: 27 August 2020; Accepted: 14 October 2020; Published: 16 October 2020

Abstract: 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 1 compositions of the biogas. At a gas hourly space velocity (GHSV) of 30,000 h− , the CO2 conversion rate was ~79.3% with a CH4 selectivity of 95%. In addition, the effects of the reaction temperature 1 (200–450 ◦C), GHSV (21,000–50,000 h− ), 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 1 approximately 200 h at a GHSV of 30,000 h− 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.

Keywords: power to gas; CO2 methanation; Ni catalyst; biogas utilization; CO2 hydrogenation

1. 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% CH4, 40–50 vol% CO2, and minor quantities of (the subsequently removed) N2,H2S, O2, and H2O[2], is employed as a high-concentration CH4 fuel after more than 40% of CO2 is removed using absorbents or amines [3]. The power-to-gas technology generates H2 from water by employing renewable energy and produces CH4 via the methanation of H2 and CO2. Further, CO2 methanation and reverse water gas shift (RWGS) are competing processes that occur during the production of CH4 from CO2, as described by Equations (1)–(3) [4]. This has

Catalysts 2020, 10, 1201; doi:10.3390/catal10101201 www.mdpi.com/journal/catalysts Catalysts 2020, 10, 1201 2 of 16 become a topic of research considering the possibility of a ‘carbon-neutral fuel’ and the replacement of natural gas as well as ESSs [5].

kJ CO + 4H CH + 2H O ∆H K = 164 (1) 2 2 ↔ 4 2 298 − mol kJ CO + 3H CH + H O ∆H K = 206 (2) 2 ↔ 4 2 298 − mol kJ CO + H CO + H O ∆H K = 42.1 (3) 2 2 ↔ 2 298 mol

Biogas is initially subjected to purification to remove impurities such as H2S, H2O, and siloxanes. Subsequently, CO2 and highly concentrated CH4 are separated, so that CH4 can be utilized as fuel [6]. In addition to being used as a fuel, biogas generated by catalytic reforming is also used to produce high-value-added chemicals (e.g., methanol, acetic acid, dimethyl ether, ammonia, and Fischer–Tropsch oil), which greatly contributes to the reduction of greenhouse gas emissions [7,8]. Moreover, biogas is an affordable and suitable raw material for syngas production, even though syngas is now largely produced through the CH4 reforming of natural gas. In a recent study, Mohammad et al. [9] compared the activity of Al2O3-supported Ni, Co, Fe, and Mo catalysts during CO2 methanation. The Ni catalyst showed the highest CO2 conversion and CH4 selectivity, followed by Co, Fe, and Mo. The study by Aziz et al. [10] on the activity of mesostructured silica nanoparticle (MSN)-supported Ni, Fe, and Mo catalysts demonstrated that the Fe catalyst was active at high temperatures, whereas the Ni catalyst exhibited the highest catalytic activity at lower temperatures (<350 ◦C) [11]. Daroughegi et al. [12] conducted CO2 methanation experiments on Al2O3-supported Ni catalysts with different metal loadings, and the results revealed that the specific surface area and CO2 conversion increased with an increase in the Ni loading from 15, 20, and 25 wt%, whereas the specific surface area and CO2 conversion decreased at 33 wt%. In the study on Ni/γ-Al2O3, Cho et al. [13] reported that the highest dispersion, CH4 selectivity, and reaction rate were observed at 20 wt% Ni content when the Ni content was varied from 15 to 50 wt%. Affar et al. [9] compared the activities of the 10 wt% Ni catalysts supported on SiO2, MCN (Mesoporous carbon nitrides), and Al2O3. Among the experiments performed at 360 ◦C, the Ni-Al2O3 catalyst showed the highest CO2 conversion and CH4 selectivities of 82.9 and 97.9%, respectively. Further, the SEM (Scanning Electron Microscopy) results confirmed that the Ni particles were more uniformly dispersed in the 10 wt% Ni-Al2O3 catalyst than those in the 10 wt% MCM (Mobile Crystalline Material) and SiO2 catalysts. It has also been reported that the catalytic activity of SiO2-supported catalysts for methane production decreases in the presence of H2O. On MCM, Aziz [14] reported that the CH4 conversion and selectivity decrease because of the consumption of the carbonyl species by conversion to CO2 in the presence of water vapor, via the water gas shift reaction. Vetrivel et al. [15] and Wang et al. [16] reported that the fabrication of the catalysts by reduction on a CeO2 support generated more surface oxygen vacancies, which resulted in high CO2 conversion, CH4 selectivity, and enhanced catalyst stability. Typically, promoters are used for improving the activity of supported catalysts. For example, the promoter MgO can increase carbon resistance [17], thermal stability [18], and the dispersion of Ni/Al2O3 catalysts [19,20], affording enhanced activity [21,22]. Thus, MgO, in combination with support materials such as Al2O3 or SiO2 has been proposed as a support for methanation catalysts [23,24]. Bette et al. [25] demonstrated that the maximum CO2 conversion of 74% was obtained with a 59 wt% Ni/(Mg,Al)Ox catalyst, and the addition of MgO to Ni/SiO2 resulted in a conversion of 66.5% [14]. In addition, MgO is a basic material that absorbs carbon dioxide and reduces catalyst deactivation via sintering and carbon deposition [26]. While catalysts become inactive because of the water produced during CO2 methanation, MgO reacts with water to generate magnesium hydroxide (Mg(OH)2) and thereby mitigates catalyst deactivation [27,28]. Catalysts 2020, 10, 1201 3 of 16

The activation energy of Ni catalysts for methanation was 93.61 kJ/mol in Ni/ZrO2 catalysts [29], Catalysts 2020, 10, x FOR PEER REVIEW 3 of 16 75 kJ/mol in Ni/Al2O3 catalysts [30], and 75 kJ/mol in Ni/Al hydrotalcite catalysts [31]. The present work studies the production of CH4 by the reaction of biogas CO2 with hydrogen investigate their effects on CO2 conversion, CH4 yield, and selectivity. In addition, because biogas is over a 20wt% Ni-Mg-Al catalyst with high dispersion of Ni metal and BET (Brunauer–Emmett–Teller) a mixture of various gases and trace elements, the effect of the concentration of these components (N2, value and characterizes this catalyst by several instrumental analyses. The conditions for the reaction O2, CH4, and CO2) on CO2 conversion, CH4 yield, and selectivity were studied. Based on this, the of biogas, including reaction temperature, space velocity, and H2/CO2 ratio, were varied to investigate optimal reaction conditions for producing CH4 from biogas were determined, under which a stability theirtest of eﬀ theects 20 on wt% CO 2Niconversion,-Mg-Al catalyst CH4 yield,was performed and selectivity. for 200 In h. 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. Results2) on CO and2 conversion, Discussion CH 4 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-Al2.1. Effect of catalyst Reaction was Temperature performed on for CO 2002 C h.onversion

2. ResultsThe dependence and Discussion 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 2.1.figure, Eﬀect CO of2 Reactionconversion Temperature increased on as CO the2 Conversion 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 The dependence of CO2 conversion and CH4 selectivity on reaction temperature at a gas hourly result of CO2 methanation suppression1 above 350 °C, at which the RWGS reaction (Equation (3)) is space velocity (GHSV) of 30,000 h− and H2/CO2 ratio of 4 is displayed in Figure1. As shown in the enhanced, which resulted in the conversion of CO2 to CO. This result agreed with that reported by ﬁgure, CO2 conversion increased as the temperature increased and reached a maximum at 400 ◦C, Mohammad et al. [9], wherein the CO concentration increased as the RWGS reaction increased at whereas CH4 selectivity and yield showed the highest values at 350 ◦C. This trend is probably the 400 °C, whereas the methane selectivity decreased. A study by Jia et al. [29] also reported that the result of CO2 methanation suppression above 350 ◦C, at which the RWGS reaction (Equation (3)) highest CO2 conversion and CH4 yield were observed at 350 °C as the temperature increased, and a is enhanced, which resulted in the conversion of CO2 to CO. This result agreed with that reported byfurther Mohammad increase et in al. temperature [9], wherein decreased the CO concentration the CO2 conversion increased asbecause the RWGS of the reaction thermodynamic increased equilibrium limit [32]. at 400 ◦C, whereas the methane selectivity decreased. A study by Jia et al. [29] also reported that In addition, the activation energy obtained from the 20 wt% Ni-Mg-Al catalyst was the highest CO2 conversion and CH4 yield were observed at 350 ◦C as the temperature increased, approximately 74.2 kJ/mol, which was similar to that obtained by using Ni/Al2O3 (75 kJ/mol) in the and a further increase in temperature decreased the CO2 conversion because of the thermodynamic equilibriumstudy by Carbarino limit [32 [30].].

FigureFigure 1.1. EEffectﬀect ofof reactionreaction temperature.temperature.

2.2. EffectIn addition, of H2/CO the2 R activationatio on CO energy2 Conversion 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 Figure 2 shows the effects of the H2/CO2 ratio at a reaction temperature of 350 °C and a GHSV of Carbarino [30]. 30,000 h−1 on CO2 conversion and CH4 selectivity and yield. Figure 2a shows the results obtained for

2.2.different Effect H of2 H/CO2/CO2 ratios2 Ratio by on increasing CO2 Conversion 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 Figure2 shows the e ffects of the H2/CO2 ratio at a reaction temperature of 350 ◦C and a GHSV of 2 2 2 4 amounts 1at a given H /CO ratio. As shown in Figure 2a, the CO conversion and CH yield increased 30,000 h− on CO2 conversion and CH4 selectivity and yield. Figure2a shows the results obtained for by approximately 15% when the H2/CO2 ratio increased from 3.5 to 5, whereas the CH4 selectivity different H2/CO2 ratios by increasing the amount of H2 at a given amount of CO2 and by increasing 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

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the amount of CO2 at a given amount of H2. Figure2b shows the e ﬀects of increasing the reactant amounts at a given H2/CO2 ratio. As shown in Figure2a, the CO 2 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 Figure2b, the same H 2/CO2 ratio led to similar CO2 conversion andCatalysts CH 204 20selectivity, 10, x FOR PEER and yield,REVIEW regardless of the amount of reactants. These results were similar4 of to 16 those from a study by Rahmani [33] in which a 15% increase was observed when the H2/CO2 ratio was increasedof hydrogen from affects 3 to 4, andthe catalytic the study activity by Aziz because et al. [34 ] of also hydrogen reported adsorption that the concentration onto the surface of hydrogen of the acatalystﬀects the and catalytic the conversio activityn becauseto methane of hydrogen via hydrogenation. adsorption onto the surface of the catalyst and the conversion to methane via hydrogenation.

(a) (b)

FigureFigure 2.2. EEffectﬀect ofof HH22//COCO22 mole ratio along withwith ((aa)) COCO22 or H22 variation; (b) reactant amountamount variation.variation.

2.3. Effect of GHSV on CO Conversion 2.3. Effect of GHSV on CO22 Conversion The effects of increasing GHSV on CO conversion and CH selectivity and yield at a reaction The effects of increasing GHSV on CO22 conversion and CH44 selectivity and yield at a reaction temperature of 350 C and H /CO ratio of 4 are shown in Figure3a,b, respectively. The e ffect of temperature of 350 ◦°C and H22/CO22 ratio of 4 are shown in Figure 3a,b, respectively. The effect of 1 increasingincreasing the the reactant reactant flow flow rate rate on on GHSV GHSV of of 21,000–50,000 21,000–50,000 h− h−is1 is shown shown in in Figure Figure3a, 3a and, and the the e ff ecteffect of 1 increasingof increasing the amountthe amount of nitrogen of nitrogen at a givenat a given reactant reactant flow rate flow on rate GHSV on ofGHSV 9000–38,000 of 9000 h–38,000− is shown h−1 is in Figure3b. As shown in the figure, CO conversion and CH yield/selectivity showed a general shown in Figure 3b. As shown in the figure,2 CO2 conversion 4and CH4 yield/selectivity showed a decreasinggeneral decreasing trend as trend the GHSV as the increased, GHSV increased, and the eandffect the for effect (b) was for larger(b) was than larger that than for (a).that Thisfor (a). is becauseThis is because an increase an increase in GHSV in shortens GHSV shortens the time the during time which during the which reactants the reactants CO2 and CO H22are and in H contact2 are in withcontact the with catalyst, the catalyst, thus reducing thus reducing the amount the ofamount reactants of reactants adsorbed adsorbed onto the onto surface the of surface the catalyst. of the Thesecatalyst. results These were results consistent were consistent with the experimentalwith the experimental results reported results byreported Abate etby al. Abate [35]. et al. [35]

(a) (b)

Figure 3. Effect of increased gas hourly space velocity and N2 concentration at H2/CO2 ratio of 4 and

reaction temperature of 350 °C (a) total reactant flow rate (b) N2 flow rate (N2 concentration).

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.

(a) (b)

Figure 2. Effect of H2/CO2 mole ratio along with (a) CO2 or H2 variation; (b) reactant amount variation.

2.3. 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

Catalystscatalyst.2020 These, 10, 1201 results were consistent with the experimental results reported by Abate et al. [35]5of 16

(a) (b)

CatalystsFigure 2020 3., 10 EEffect,ff xect FOR of PEER increasedincreased REVIEW gasgas hourlyhourly spacespace velocityvelocity andand NN22 concentrationconcentration atat HH22//COCO22 ratio of 4 andand 5 of 16 reaction temperaturetemperature ofof 350350 ◦°C(C (a)) totaltotal reactantreactant flowflow raterate ((bb)N) N22 flowflow rate (N22 concentration). 2.4. Effect of Initial Concentration of Biogas Components on CO2 Conversion 2.4. Effect of Initial Concentration of Biogas Components on CO Conversion 2 2.4.1. Effect of Initial CH4 Concentration on CO2 Conversion 2.4.1. Effect of Initial CH4 Concentration on CO2 Conversion

The effect of changing the initial CH4 concentration to 0, 50, and 65 vol% for CO2 conversion at 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−1,1 and H2/CO2 ratio of 4 is shown in Figure 4. As a reaction temperature of 400 ◦C, GHSV of 30,000 h− , and H2/CO2 ratio of 4 is shown in Figure4. shown in the figure, increasing the content of CH4 in the reactant gas to 40, 50, and 65 vol% led to low As shown in the ﬁgure, increasing the content of CH4 in the reactant gas to 40, 50, and 65 vol% led to CO2 conversions of approximately 67, 64, and 54%, respectively, resulting in up to a 20% decrease in low CO2 conversions of approximately 67, 64, and 54%, respectively, resulting in up to a 20% decrease the CO2 conversion compared to that in the absence of CH4. This phenomenon is attributed to the Le in the CO2 conversion compared to that in the absence of CH4. This phenomenon is attributed to the Chatelier principle, in which the initial CH4 present in the reactant gas inhibits the conversion to CH4. Le Chatelier principle, in which the initial CH4 present in the reactant gas inhibits the conversion to These results were also reported in a simulation study by Jürgensen et al. [36], wherein CO2 CH4. These results were also reported in a simulation study by Jürgensen et al. [36], wherein CO2 conversionconversion decreased decreased as as the the initial initial methane methane concentration concentration increased. increased.

Figure 4. Eﬀect of initial CH content in the reactant gas. Figure 4. Effect of initial CH4 4 content in the reactant gas.

2.4.2. Eﬀect of Initial CO2 Concentration on CO2 Conversion 2.4.2. Effect of Initial CO2 Concentration on CO2 Conversion The eﬀects of increasing the initial CO2 concentration on CO2 conversion and CH4 selectivity The effects of increasing the initial CO2 concentration on CO2 conversion1 and CH4 selectivity and and yield at a reaction temperature of 350 ◦C, GHSV of 30,000 and 50,000 h− , and H2/CO2 ratio of 4 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 than in the latter.

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are presented in Figure5. As shown in the ﬁgure, an increase of CO 2 in biogas from 10 to 14 vol% 1 atCatalysts a GHSV 2020, of 10, 30,000 x FOR PEER h− resultedREVIEW in a 2% increase in CO2 conversion and a 3% increase at a GHSV6 of 16 1 of 50,000 h− . This is probably due to the increase in reaction temperature owing to the exothermic reactionCatalysts of2020 CO, 102, xmethanation FOR PEER REVIEW with increasing amounts of the reactants; thus, the increase in CO6 of2 16 1 1 conversion is more at a GHSV of 50,000 h− than that at 30,000 h− , because more reactants are present in the former than in the latter.

Figure 5. Effect of reactant concentration (CO2 concentration = CO2/(H2 + CO2 + N2)) at a reaction temperature of 350 °C . Figure 5. Effect of reactant concentration (CO2 concentration = CO2/(H2 + CO2 + N2)) at a reaction temperatureFigure 5. Effect of 350 ofC. reactant concentration (CO2 concentration = CO2/(H2 + CO2 + N2)) at a reaction 2.4.3. Effect of Initial N◦ 2 Concentration on CO2 Conversion temperature of 350 °C . 2.4.3.Approximately Effect of Initial N15%2 Concentration of N2 exists in on landfill CO2 Conversion gas—a type of biogas—which is used as an inactive gas2.4.3. to Effect prevent of Initial the deactivation N2 Concentration of catalysts on CO2 C causonversioned by the spot exothermic reaction of CO2 Approximately 15% of N2 exists in landfill gas—a type of biogas—which is used as an inactive gas methanation. The effect of the presence of 0, 10, and 30% N2 in the reactant gas on CO2 conversion at to preventApproximately the deactivation 15% of of catalysts N2 exists caused in landfill by the gas spot—a exothermic type of biogas reaction—which of CO is2 methanation.used as an inactiv The e a GHSV of 15,000 h−1 and H2/CO2 ratio of 4 is shown in Figure 6. When nitrogen concentrations were1 effgasect of to the prevent presence the of 0, deactivation 10, and 30% ofN2 catalystsin the reactant caus gased on by CO the2 conversion spot exothermic at a GHSV reaction of 15,000 of h CO− 2 10 and 30% in the reactant, the respective CO2 conversions were approximately 3 and 5% lower than andmethanation. H2/CO2 ratio The of effect 4 is shown of the presence in Figure of6. 0, When 10, and nitrogen 30% N concentrations2 in the reactant were gas on 10 CO and2 conversion 30% in the at that in the absence of N2. This is believed to be due to the decrease in the reactant concentration as reactant,a GHSV the of respective15,000 h−1 and CO2 Hconversions2/CO2 ratio wereof 4 is approximately shown in Figure 3 and 6. When 5% lower nitrogen than concentrations that in the absence were the N2 concentration increased, leading to less heat generation, which, in turn, lowers the reaction of10 N 2and. This 30% is in believed the reactant, to be due the torespective the decrease CO2 inconversions the reactant were concentration approximately as the 3 and N2 concentration 5% lower than temperature. increased,that in the leading absence to lessof N heat2. This generation, is believed which, to be indue turn, to the lowers decrease the reaction in the reactant temperature. concentration as the N2 concentration increased, leading to less heat generation, which, in turn, lowers the reaction temperature.

FigureFigure 6.6. EEffectﬀect ofof NN22 inin biogasbiogas atat reactionreaction temperaturetemperature ofof 350350 ◦°CC..

2.4.4. Eﬀect of Initial Oxygen Concentration on CO2 Conversion 2.4.4. Effect of Initial Oxygen Concentration on CO2 Conversion Figure 6. Effect of N2 in biogas at reaction temperature of 350 °C . The eﬀects of the presence of oxygen in the reactant gas on CO2 conversion at a GHSV of 30,000 2 1 The effects of the presence of oxygen in the reactant gas on CO conversion at a GHSV of 30,000 h− ,H2/CO2 ratio of 4, and reaction temperatures of 300 and 400 ◦C are shown in Figure7. When 4% h−2.4.4.1, H2/CO Effect2 ratio of I nitialof 4, andOxygen reaction Concentration temperatures on COof 3002 Conversion and 400 °C are shown in Figure 7. When 4% oxygen was present in the reactant gas, CO2 conversion and selectivity decreased by approximately The effects of the presence of oxygen in the reactant gas on CO2 conversion at a GHSV of 30,000 5% and 3%, respectively. This is thought to be a protected re-oxidation reaction due to the presence h−1, H2/CO2 ratio of 4, and reaction temperatures of 300 and 400 °C are shown in Figure 7. When 4% of oxygen in the reactant gas, which prevents the forward reaction in CH4 synthesis paths (1) and (2). oxygen was present in the reactant gas, CO2 conversion and selectivity decreased by approximately According to the mechanism proposed by Lin et al. [21], CO2 is separated from the oxygen vacancies 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

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oxygen was present in the reactant gas, CO2 conversion and selectivity decreased by approximately 5%Catalysts and 20 3%,20, respectively.10, x FOR PEERThis REVIEW is thought to be a protected re-oxidation reaction due to the presence7 of of 16 Catalysts 2020, 10, x FOR PEER REVIEW 7 of 16 oxygen in the reactant gas, which prevents the forward reaction in CH4 synthesis paths (1) and (2). Accordingon the Ni met to theal and mechanism support proposedmaterial, which by Lin are et al. produced [21], CO duringis separated the reduction from the of oxygen the catalysts, vacancies and on the Ni metal and support material, which are produced during2 the reduction of the catalysts, and onit is the reported Ni metal that and a decrease support material,in the oxygen which vacancies are produced on the during catalyst the and reduction support of reduces the catalysts, the catalytic and it it is reported that a decrease in the oxygen vacancies on the catalyst and support reduces the catalytic isactivity reported. Therefore, that a decrease it is considered in the oxygen that the vacancies CO2 conversion on the catalyst and selectivity and support decrease reduces in the catalyticpresence activity. Therefore, it is considered that the CO2 conversion and selectivity decrease in the presence activity.of oxygen Therefore, because itof is the considered decrease thatin the the amount CO conversion of oxygen and vacancies selectivity on the decrease Ni catalyst in the and presence support, of of oxygen because of the decrease in the amount2 of oxygen vacancies on the Ni catalyst and support, oxygenwhich prevents because CO of the2 from decrease being inconverted the amount to CO of or oxygen carbon vacancies species, or on because the Ni catalystof the re and-oxidation support, of which prevents CO2 from being converted to CO or carbon species, or because of the re-oxidation of whichCO. prevents CO from being converted to CO or carbon species, or because of the re-oxidation CO. 2 of CO.

Figure 7. Effect of O2 in reaction gas. FigureFigure 7.7. EEfﬀfectect ofof OO22 inin reactionreaction gas.gas. 2.5.2.5. StabilityStability andand ActivityActivity ofof CatalystCatalyst TestTest 2.5. Stability and Activity of Catalyst Test ToTo evaluate evaluate the the activity activity and and stability stability of of the the catalysts,catalysts, CO CO22 methanationmethanation waswas conductedconducted atat aa GHSVGHSV 2 To evaluate1−1 the activity and stability of the catalysts, CO methanation was conducted at a GHSV ofof 30,00030,000 h h− ,, reactionreaction temperature temperature of of 350 350◦C, °C and, and H 2H/CO2/CO2 ratio2 ratio of of 4 for 4 for 200 200 h. Ash. As shown shown in Figure in Figure8, the 8, of 30,000 h−1, reaction temperature of 350 °C , and H2/CO2 ratio of 4 for 200 h. As shown in Figure 8, COthe2 COconversion2 conversion was constantwas constant at 75%, at 75%, and CH and4 selectivityCH4 selectivity was 93% was for 93% a reaction for a reaction time of time 200 h.of These200 h. the CO2 conversion was constant at 75%, and CH4 selectivity was 93% for a reaction time of 200 h. resultsThese conﬁrmed results confirmed the activity the and activity stability and of stability the 20 wt% of the Ni-Mg-Al 20 wt% catalyst Ni-Mg for-Al the catalyst CO2 methanation for the CO2 These results confirmed the activity and stability of the 20 wt% Ni-Mg-Al catalyst for the CO2 reaction.methanation The catalyticreaction. activityThe catalytic was retained activity evenwas retained upon storage even atupon room storage temperature. at room temperature. methanation reaction. The catalytic activity was retained even upon storage at room temperature.

FigureFigure 8.8. StabilityStability testtest overover 2020 wt%wt% Ni-Mg-AlNi-Mg-Al catalysts.catalysts. Figure 8. Stability test over 20 wt% Ni-Mg-Al catalysts. 3. Materials and Methods 3. Materials and Methods 3.1. Catalyst Preparation 3.1. Catalyst Preparation The catalyst used in this study is a 20 and 40 wt% Ni-Mg-Al catalyst (supported by Korea 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 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 high catalytic activity and CH4 selectivity. The catalyst was prepared by mixing calculated ratios of

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3. Materials and Methods

3.1. Catalyst Preparation Catalysts 2020, 10, x FOR PEER REVIEW 8 of 16 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 Ni(NO3)2 ㆍ 6H2O, Al(NO3)2 ㆍ 9H2O, and Mg(NO3)2 ㆍ 6H2O solutions at 60 °C until the Ni content high catalytic activity and CH4 selectivity. The catalyst was prepared by mixing calculated ratios of was 20, 40 wt%, and a precipitate was obtained after adding a precipitant to the mixture and stirring Ni(NO3)26H2O, Al(NO3)29H2O, and Mg(NO3)26H2O solutions at 60 ◦C until the Ni content was for approximately 1 h while maintaining a constant pH. The precipitated catalyst precursor was 20, 40 wt%, and a precipitate was obtained after adding a precipitant to the mixture and stirring for repeatedly washed with distilled water and filtered with a filter press until the pH reached approximately 1 h while maintaining a constant pH. The precipitated catalyst precursor was repeatedly approximately 7.0. The catalyst precursor was then dried in an oven at 150 °C for 12 h, and 20 and 40 washed with distilled water and filtered with a filter press until the pH reached approximately 7.0. wt% Ni-Mg-Al2O3 catalysts were prepared through a heat treatment process under an air atmosphere The catalyst precursor was then dried in an oven at 150 ◦C for 12 h, and 20 and 40 wt% Ni-Mg-Al2O3 at 600 °C for 4 h. Prior to their use, all catalysts were heated to the reduction temperature under a gas catalysts were prepared through a heat treatment process under an air atmosphere at 600 ◦C for 4 h. flow of 100 mL/min (20% H2, 80% N2) for 2 h, and the catalysts were reduced while the temperature Prior to their use, all catalysts were heated to the reduction temperature under a gas flow of 100 mL/min was maintained for 4 h. (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 Figure9 illustrates the variations in catalytic activity of the 40 wt% Ni-Mg-Al 2O3 catalysts at the reaction temperatures of 350 and 400 °C as a function of changes in the reduction temperatures to 450, reaction temperatures of 350 and 400 ◦C as a function of changes in the reduction temperatures to 550, 700, and 800 °C. The CO2 conversion initially showed a significant increase as the reduction 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 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. was used as the reduction temperature of the catalysts.

FigureFigure 9.9. EffectEﬀect of of reduction reduction temperature temperature on on CO CO2 2conversionconversion and and CH CH4 selectivity4 selectivity over over the the40 wt% 40 wt% Ni- Ni-Mg-AlMg-Al catalyst. catalyst.

3.2. CO2 Methanation Apparatus and Activity Test 3.2. CO2 Methanation Apparatus and Activity Test Isothermal CO2 methanation experiments were conducted in a plug-ﬂow system (Figure 10) at 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 steady state with Ni catalysts loaded into the reactor. The catalytic reactor has an outer diameter of mm (1/2”), a thickness of 1.257 mm, a length of 209 mm, and Inconel 800 HT composed of 80% Ni, 14% 12.7 mm (1/2”), a thickness of 1.257 mm, a length of 209 mm, and Inconel 800 HT composed of 80% Cr, and 6% Fe. During the experimentation, a mesh sieve was installed in the lower part of the reactor Ni, 14% Cr, and 6% Fe. During the experimentation, a mesh sieve was installed in the lower part of to support the catalyst layer, and 0.5 g of the catalyst was loaded. A back-pressure regulator (BPR) the reactor to support the catalyst layer, and 0.5 g of the catalyst was loaded. A back-pressure in the latter section of the reactor was used to control the reaction pressure from 1 to 9 atm. A water regulator (BPR) in the latter section of the reactor was used to control the reaction pressure from 1 to trap in the latter section of the BPR was used to remove the water generated from the reaction, and a 9 atm. A water trap in the latter section of the BPR was used to remove the water generated from the high-pressure check valve was installed to prevent gas backﬂow. All tubes used were of SUS (Steel reaction, and a high-pressure check valve was installed to prevent gas backflow. All tubes used were Use Stainless) grade with an outer diameter of Φ3.2 mm and a thickness of 0.8 t, and the products were of SUS (Steel Use Stainless) grade with an outer diameter of Φ3.2 mm and a thickness of 0.8 t, and the analyzed using gas chromatography (GC). To prevent the condensation of the products inside the tube products were analyzed using gas chromatography (GC). To prevent the condensation of the during this process, a line heater was installed and maintained above 150 C. products inside the tube during this process, a line heater was installed and◦ maintained above 150 °C.

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FigureFigure 10. 10. SchematicSchematic of ofCO CO2 methanation.2 methanation.

TheThe reaction reaction products products were were characterized characterized using using a aGC GC (YL (YL Instrument Instrument 6500 6500 GC GC,, YL YL Instruments Instruments Co.,Co., Anyang Anyang,, Korea Korea),), SS SS COL COL 10FT 10FT 1/8” 1/8” PORAPACK PORAPACK N N (Model: (Model: 13052 13052-U),-U), a Phase a Phase None None Matrix Matrix 45/60 45/60 MolecularMolecular Sieve Sieve 13 13XX w wasas used used for for the the GC GC columns, columns, and and argon argon was was used used as as the the carrier carrier gas gas.. Hydrogen, Hydrogen, methane,methane, and and carbon carbon monoxide monoxide were were analyzed analyzed with with a thermal a thermal conductivity conductivity detector detector (TCD), (TCD), whereas whereas carboncarbon dioxide dioxide was was analyzed analyzed using using a flame a flame ionization ionization detector detector (FID). (FID). Characteriza Characterizationtion in the GC in the oven GC wasoven conducted was conducted by maintaining by maintaining the temperature the temperature initially initially at 35 at °C 35 for◦C for0–6 0–6 min min and and raising raising it it to to approximatelyapproximately 170 170 °C◦ Cat ata ramp a ramp rate rate of 15 of 15°C/min.◦C/min. FID FIDcharacterization characterization was wasperformed performed at 250 at °C 250 by◦ C supplyingby supplying 35 mL 35/min mL/ min of hydrogen of hydrogen and and 300 300mL mL/min/min of of oxygen, oxygen, while while TCD TCD characterization characterization was was carried out at 150 C under a gas flow rate of 35 mL/min of hydrogen and 20 mL/min of Ar. CO carried out at 150 °C◦ under a gas flow rate of 35 mL/min of hydrogen and 20 mL/min of Ar. CO2 2 conversion (XCO ), CH selectivity (S ), and CH yield (Y ) were calculated according to Equations conversion (XCO2),2 CH4 selectivity4 (SCH4CH4), and CH4 yield4 (YCH4CH4) were calculated according to Equations (4)(4)–(6)–(6) [37]. [37 ]. CO ( ) = ( 2 ) XCO2 % 1 CO 100 (4) − CH4 + CO2 + CO2) × X퐶푂2 (%) = (1 − ) × 100 (4) CH4 + CO + CO2) CH4 SCH4 (%) = ( CH ) 100 (5) CH44+ CO × SCH4 (%) = ( ) × 100 (5) CH4 + CO XCO2 SCH4 YCH4 (%) = × (6) XCO2 ×100SCH4 YCH4 (%) = (6) 100 1 Using a reactant gas flow rate of 250 mL/min, a reaction temperature of 350 ◦C, GHSV of 30,000 h− , andUsing an H 2a/CO reactant2 ratio gas of 4flow as the rate basis, of 25 methanation0 mL/min, a reactionreaction experimentstemperature were of 350 conducted °C, GHSV by of changing 30,000 h−the1, and experimental an H2/CO2 conditions, ratio of 4 as as listed the basis, in Table methanation1. reaction experiments were conducted by changing the experimental conditions, as listed in Table 1.

Table 1. Experimental condition for CO2 methanation reaction. Table 1. Experimental condition for CO2 methanation reaction. Items Condition Items Condition Reaction Temperature (◦C) 200–450 ReactionCH4 and COTemperature2 composition (°C ratio) (%)200 65:35,–450 50:50, 40:60 Variable GHSV (h 1) 21,000–50,000 CH4 and CO2 composition− ratio (%) 65:35, 50:50, 40:60 H2/CO2 mole ratio 3.5, 4, 4.5, 5 Variable GHSV (h−1) 21,000–50,000 O2 (cc/min) 10.5 (4%) H2/CO2 mole ratio 3.5, 4, 4.5, 5 O2 (cc/min) 10.5 (4%)

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3.3. Catalyst Characterization

3.3.1. 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 Table2. 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 Table3, the 20 wt% catalyst also showed a high TOF (Turnover Frequency) value. TOF values towards CO2 were defined as the number of CO2 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 CO2 conversion than the 40 wt% catalyst (Tables3 and4).

Table 2. Brunauer–Emmett–Teller (BET) surface area, pore volume, and pore size of 20 wt% Ni-Mg-Al catalyst (fresh and spent).

Catalyst BET(m2/g) Total Pore Volume (m3/g) Pore size(Å) Fresh 180.3 0.36 81.5 20 wt% Ni-Mg-Al spent 148.9 0.30 81.1 Fresh 152.6 0.31 82.1 40 wt% Ni-Mg-Al spent 107.8 0.32 82.4

Table 3. Ni loading and dispersion of Ni-Mg-Al catalysts and their Turnover Frequency values based

on CO2 conversion.

a 1 b Catalyst Ni Loading (wt%) Ni Dispersion(%) Ni Particle Size (nm) TOF(s− ) 20wt% Ni-Mg-Al 20 3.57 28.3 0.36 40wt% Ni-Mg-Al 40 3.68 27.5 0.17 a b Estimated from H2 chemisorption, TOF of CO2 conversion at 350 ◦C.

Table 4. CO2 conversion and selectivity of CH4 with diﬀerent Ni loadings.

Catalyst Reaction Temperature (◦C) CO2 Conversion (XCO2) CH4 Selectivity (SCH4) 350 75.0 94.2 20 wt% Ni-Mg-Al 400 75.6 93.0 350 61.2 96.4 40 wt% Ni-Mg-Al 400 72.2 95.3

3.3.2. 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, Catalysts 2020, 10, 1201 11 of 16 Catalysts 2020, 10, x FOR PEER REVIEW 11 of 16

MA, USA). The catalyst powder samples were pretreated at 250 C for 5 h to remove moisture, and the and measurements were performed at 30 mA and 40 kV over◦ a 2θ range 10–80° using a scanning crystals of the catalysts were analyzed. Cu-Kα radiation was used to fix the axis of the sample, and speed of 8θ/min. The XRD analysis results of the catalytic supports and Ni catalysts before and after measurements were performed at 30 mA and 40 kV over a 2θ range 10–80 using a scanning speed the reaction are shown in Figure 11. Diffraction peaks that appear at 2θ◦ values of 37.3°, 43.3° and of 8θ/min. The XRD analysis results of the catalytic supports and Ni catalysts before and after the 62.9° are associated with the NiO phase, whereas those at 44.55°, 51.85°, and 76.3° are associated with reaction are shown in Figure 11. Diffraction peaks that appear at 2θ values of 37.3 , 43.3 and 62.9 are Ni metal. The 2θ values of 45.86 and 66.91 correspond to MgO, whereas the diffraction◦ ◦ peaks◦ at 2θ associated with the NiO phase, whereas those at 44.55 , 51.85 , and 76.3 are associated with Ni metal. values of 37.4°, 46.07°, and 66.9° are associated with◦ the alumina◦ phase.◦ Figure 11 shows the XRD The 2θ values of 45.86 and 66.91 correspond to MgO, whereas the diffraction peaks at 2θ values of 37.4 , characterization of the catalysts before and after reduction at 700 °C and on the spent catalyst 200◦ h 46.07 , and 66.9 are associated with the alumina phase. Figure 11 shows the XRD characterization of after◦ the reaction.◦ As shown in the figure, peaks corresponding to Ni metal were not observed in the the catalysts before and after reduction at 700 C and on the spent catalyst 200 h after the reaction. fresh catalyst prior to the reaction, whereas NiO◦ peaks were observed [39]. In contrast, the reduction As shown in the figure, peaks corresponding to Ni metal were not observed in the fresh catalyst prior catalyst and spent catalyst exhibited prominent Ni metal peaks at 2θ values of 51.85° and 76.3°, to the reaction, whereas NiO peaks were observed [39]. In contrast, the reduction catalyst and spent respectively [30]. catalyst exhibited prominent Ni metal peaks at 2θ values of 51.85◦ and 76.3◦, respectively [30].

FigureFigure 11. 11.X-ray X-ray di diffractionﬀraction patterns patterns of of (a ()a fresh,) fresh, (b ()b reduced) reduced at at 700 700◦C °C in in the the fresh fresh state, state and, and (c )(c spent) spent 20 wt% Ni-Mg-Al catalysts. 20 wt% Ni-Mg-Al catalysts.

3.3.3. H2-TPR Analysis 3.3.3. H2-TPR Analysis The interaction and reduction between the catalyst metal and support were characterized by The interaction and reduction between the catalyst metal and support were characterized by H2- H2-temperature-programmed reduction (H2-TPR, AutoChem II 2920 V5.02, Micromeritics Instruments, temperature-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 Norcross, GA, USA). As shown in Figure 12, the H2 consumption of the catalysts appeared at 300– and 600–700 ◦C. The two peaks correspond to the reduction of the NiO particles. The ﬁrst peak refers to450 the and reduction 600–700 of °C. NiO The particles two peaks that correspond exhibit a weak to the interaction reduction due of the to MgO,NiO particles. and the secondThe first peak peak refers to the reduction of NiO particles that exhibit a weak interaction due to MgO, and the second that appears at high temperatures corresponds to the reduction of NiO. Al2O3 particles with spinel peak that appears at high temperatures corresponds to the reduction of NiO. Al2O3 particles with structures exhibit a strong interaction between the NiO particles and the Al2O3 support. An improved spinel structures exhibit a strong interaction between the NiO particles and the Al2O3 support. An H2 consumption was observed as the Ni content of the catalysts increased. improved H2 consumption was observed as the Ni content of the catalysts increased.

Catalysts 2020, 10, x FOR PEER REVIEW 12 of 16

Catalysts 2020, 10, x FOR PEER REVIEW 12 of 16 Catalysts 2020, 10, 1201 12 of 16

Figure 12. Hydrogen temperature-programmed desorption profiles of 20 and 40 wt% Ni-Mg-Al catalysts (supported by KIER).

Figure 12. Hydrogen temperature-programmed desorption profiles of 20 and 40 wt% Ni-Mg-Al 3.3.4. XFigure-ray P hotoelectron 12. Hydrogen S pectroscopy temperature-programmed (XPS) Characterization desorption profiles of 20 and 40 wt% Ni-Mg-Al catalysts (supported by KIER). catalysts (supported by KIER). To investigate the oxidation state of the catalysts, XPS (Center for Advanced Materials Analysis, 3.3.4. X-ray Photoelectron Spectroscopy (XPS) Characterization Suwon3.3.4. UniversityX-ray Photoelectron) (K-Alpha S pectroscopyplus, Thermo (XPS) Fisher Characterization Scientific, East Grinstead, UK) was conducted to measureTo investigate the binding the energy. oxidation XPS state characterization of the catalysts, was XPS performed (Center for underAdvanced vacuum Materials using Analysis, Al-Kα radiationSuwonTo University) oninvestigate a fresh (K-Alphacatalyst the oxidation and plus, on state Thermothe ofspent the Fisher catalysts,catalyst Scientific, after XPS 200(Center East h of Grinstead, forreaction, Advanced UK)without wasMaterials any conducted separate Analysis, to samplemeasureSuwon pretreatment theUniversity binding) energy. ( processK-Alpha XPS ( Figureplus, characterization Thermo 13). The Fisher oxidation was Scientific performed state, East ofunder Ni Grinstead vacuumcan be , determined usingUK) was Al-K conductedα fromradiation the to bindingonmeasure a fresh energy catalystthe binding(BE) and of the on energy. XPS the spentNi2p3/2 XPS catalyst characterization spectrum. after The 200 wasNi2p3/2 h of performedreaction, BE of NiO without under is 855 any vacuum–856 separate and using 860.5 sample Al eV,-Kα andpretreatmentradiation that of theon process Nia fresh metal (Figure catalyst is 852.3 13 and).–852.6 The on oxidation eVthe [40]. spent Also, state catalyst t ofhe NiNi2p1/2 after can be200 BE determined h of of the reaction, Ni metal from without the is 873 binding– any875 eV.separate energy As shown(BE)sample of in the the pretreatment XPS figure, Ni2p3 the/ 2 processXPS spectrum. spectrum (Figure The for Ni2p313) the. The BE/2 BE oxidationof ofthe NiO fresh isstate catalyst 855–856 of Ni displayed and can860.5 be determinedpeaks eV, and at that85 5 from.0 of and the the binding energy (BE) of the XPS Ni2p3/2 spectrum. The Ni2p3/2 BE of NiO is 855–856 and 860.5 eV, 8Ni60. metal4 eV that is 852.3–852.6 are associated eV [ 40with]. Also, NiO, theand Ni2p1 a small/2 BEamount of the of Ni Ni(OH) metal is2 (peaks 873–875 at eV.865 As.6 eV). shown The in XPS the profilefigure,and that of the the of XPS thespent spectrum Ni catalystmetal for is revealed852.3 the BE–852.6 of the the eV presence fresh [40]. catalyst Also, of Ni the displayedmetal Ni2p1/2 (851.9 peaksBE eV) of atthe and 855.0 N NiOi metal and (855 860.4is eV873 eVand–875 that 860.3 eV. are As shown in the figure, the XPS spectrum for the BE of the fresh catalyst displayed peaks at 855.0 and eV)associated. with NiO, and a small amount of Ni(OH)2 (peaks at 865.6 eV). The XPS profile of the spent catalyst860.4 eV revealed that are the associated presence with of Ni NiO, metal and (851.9 a small eV) andamount NiO of (855 Ni(OH) eV and2 (peaks 860.3 eV).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).

(a) (b)

FigureFigure 13. 13. XX-ray-ray photoelectron photoelectron spectra spectra of of ( (aa)) fresh fresh and and ( (bb)) spent spent 20 20 wt% wt% Ni Ni-Mg-Al-Mg-Al catalysts. catalysts.

3.3.5. Scanning Electron Microscopy(a) (SEM)–Energy-Dispersive X-ray Spectroscopy(b) (EDX)

CharacterizationFigure 13. X-ray photoelectron spectra of (a) fresh and (b) spent 20 wt% Ni-Mg-Al catalysts. 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 ×

Catalysts 2020, 10, x FOR PEER REVIEW 13 of 16

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 CatalystsNi. (Center2020, 10for, 1201 Advanced Materials Analysis, Suwon University) (APREO SEM, FEI, Hillsboro,13 ofOR 16, 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 removingthe fresh catalyst powders and and the dust, spent drying catalyst at 120after◦ C200 for h 1of h, reaction and coating are presented with metal in (Au).Figure SEM 14. As images shown of thein the fresh SEM catalyst images, and nanoscale the spent particles catalyst were after uniformly 200 h of reaction dispersed are presentedthroughout in the Figure surface. 14. AsOwing shown to inthe the decomposition SEM images, of nanoscale Mg(NO3 particles)2·6H2O to were MgO uniformly at temperatures dispersed above throughout 600 °C, theit was surface. confirmed Owing that to the decomposition of Mg(NO ) 6H O to MgO at temperatures above 600 C, it was conﬁrmed that spherical [41] particles were uniformly3 2· 2 dispersed. ◦ spherical [41] particles were uniformly dispersed.

(a) (b)

Figure 14. Scanning electron microscopy images of (a) fresh and (b) spent 20 wt% Ni-Mg-Al catalysts. Figure 14. Scanning electron microscopy images of (a) fresh and (b) spent 20 wt% Ni-Mg-Al catalysts. The results of the elemental composition analysis of the catalyst using EDX and of the surface 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 Table5. The contents of Ni, Mg, and 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 Table5 (the data is for indicative Al metals from the cross-section of the catalysts are summarized in Table 5 (the data is for indicative purposes only). purposes only). Table 5. Elemental composition of fresh 20 wt% Ni-Mg-Al catalyst determined by scanning electron microscopy–energy-dispersiveTable 5. Elemental composition X-ray of fresh spectroscopy. 20 wt% Ni-Mg-Al catalyst determined by scanning electron microscopy–energy-dispersive X-ray spectroscopy. Element C O Ni Mg Al Element C O Ni Mg Al Weight (%) 4.82 36.99 16.80 2.59 38.80 Weight (%) 4.82 36.99 16.80 2.59 38.80

4. Conclusion Conclusionss

In this this study, study, the the reaction reaction between between H2 and H the2 and CO the2 present CO2 inpresent biogas inwas biogas explored, was and explored, experiments and onexperiments CO2 methanation on CO2 methanat for producingion for CH producing4 were conducted CH4 were over conducted a 20 wt% over Ni-Mg-Al a 20 wt% catalyst. Ni-Mg The-Al optimalcatalyst. conditions The optimal for conditionsCO2 methanation for CO over2 methanation the 20 wt% over Ni-Mg-Al the 20 catalyst wt% Ni were-Mg determined-Al catalyst based were ondetermined the eﬀects based of the onreaction the effects temperature, of the GHSV, reaction and temperature, H2/CO2 ratio GHSV, on CO 2 andconversion. H2/CO2 ratio Furthermore, on CO2 experimentsconversion. investigating Furthermore, the experiments eﬀects of CO 2 investigating, CH4,N2, and the O2 concentrations effects of CO (i.e.,2, biogasCH4, N composition)2, and O2 onconcentrations CO2 conversion (i.e., ledbiogas to the composition) following conclusions. on CO2 conversion led to the following conclusions. 1) COCO22 conversion increasedincreased asas the the reaction reaction temperature temperature increased, increased but, but decreased decreased beyond beyond 400 400◦C, and°C, and the the highest highest values values of CHof CH4 selectivity4 selectivity and and yield yield were were obtained obtained near near 350 350◦ °C.C. ThisThis isis duedue to the thermodynamic equilibriumequilibrium limitlimit thatthat reducesreduces thethe COCO22 conversion at temperatures above 400 ◦°CC[ [29]29] and inhibits the methanation reaction above 350 ◦ °CC vviaia the RWGS reaction, which increases the production ofof CO CO and and decreases decreases the the CH CH4 selectivity.4 selectivity. The The activation activation energy energy at this at point this waspoint 72.4 was kJ /mol.72.4 kJ/mol.2) CO2 conversion and CH4 selectivity increased as the H2/CO2 ratio increased from 3.5 to 5.0, whereas2) CO the2 conversion CO2 conversion and CH resulted4 selectivity in similar increased values asas longthe H as2/CO the2 H ratio2/CO increased2 ratio remained from 3.5 the to same 5.0, regardlesswhereas the of CO the2 Hconversion2 and CO2 resultedconcentrations in similar when values the Has2 /longCO2 asratio the was H2/CO varied.2 ratio Increasing remained the the GHSV same ofregardless the reactant of the gas H2 andand itsCO N2 concentrations2 concentration when shortened the H2/CO the contact2 ratio was time varied. between Increasing the reactant the GHSV and catalyst and reduced the CO2 conversion. 3) A higher initial concentration of CO2 in the biogas led to a slightly higher CO2 conversion, but increasing the initial CH4 concentration to 40, 50, and 65 vol% decreased the CO2 conversion by up to 20% compared to that in the absence of CH4. This trend is believed to be the result of Le Chatelier’s principle, in which the conversion of CH4 was suppressed by the initial concentration Catalysts 2020, 10, 1201 14 of 16

of CH4 in the reactant gas. The selectivity was shown to be independent of CH4 concentration and remained constant. 4) Investigation of the eﬀect of the concentrations of N2 and O2 components of biogas revealed that CO2 conversion decreased by approximately 5% in the presence of 10 vol% N2. In the presence of O2, the CO2 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 CH4 selectivity because of the re-oxidation reaction. 5) The stability of the 20 wt% Ni-Mg-Al catalyst for CO2 methanation was evaluated from experiments at 350 ◦C for 200 h. The catalyst is expected to exhibit stable activity, because the CO2 conversion and CH4 selectivity were maintained at constant values of 75 and 93%, respectively. As mentioned above, because of its stability over 200 h and CO2 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 CO2) 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.

Author Contributions: Data curation Y.K. and H.B.; investigation and writing-original draft, D.H.; writing-review and editing Y.B. and D.H., funding acquisition W.C., validation W.C. and Y.B.; supervision Y.B. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Korean government (Ministry of Trade, Industry and Energy, 2018, Funding source No. 20182010106370). Acknowledgments: This study was conducted with the support of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Korean government (Ministry of Trade, Industry and Energy, 2018). (Funding source No. 20182010106370, Fine Dust Mitigation Clean Fuel DME Engine Demonstration Research Project). This study was conducted with the support of the Basic Science Research Capacity Enhancement Project through the Korea Basic Science Institute (National research Facilities and Equipment Center) grant funded by the Ministry of Education. (2019R1A6C1010013) Conﬂicts of Interest: The authors declare no conﬂict of interest.

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