Bimetallic Ni-Fe Supported by Gadolinium Doped Ceria (GDC) Catalyst for CO 2 Methanation

CO 2 conversion into fuels and high value-added chemical feedstocks, such as methane


Introduction
Global carbon dioxide emissions increased by 40% from 2000 to 2019, reaching a record high of 36.3 billion tons in 2021, contributing to global warming, the greenhouse effect, and severe environmental pollution [1,2].Therefore, it is methanation reaction rate is faster, the reaction conditions are milder, and can be carried out under atmospheric pressure [5].CO2 (g) + 4H2 (g) → CH4 (g) + 2H2O (l) ΔH = −164kJ/mol [6] The hydrogenated product, CH4, is available as a fuel and chemical feedstock for further manufacturing of multi-carbon products [7].Carbon dioxide is a greenhouse gas that is abundantly found in the atmosphere.Utilizing carbon dioxide to produce methane would not only reduce the concentration of greenhouse gas in the atmosphere but would also be able to partially fulfill the energy demand [8,9].
The key to the highly effective conversion of CO2 to CH4 is the application of high-performance catalysts.Many different metals mainly in groups 8-10 such as Ni, Pd, Pt, Co, Rh, Fe, and Ru have been reported as active metals for fixed bed CO2 methanation based on CO2 conversion and CH4 selectivity as summarized below [10,11]: Activity to methanation: Ru > Rh > Ni > Fe > Co > Os > Pt > Ir > Mo > Pd > Ag > Au Selectivity for methane: Pd > Pt > Ir > Ni > Rh > Co > Fe > Ru > Mo > Ag > Au Among these catalysts, Ni-based catalysts are the most commonly CO2 methanation catalysts due to their low price, abundant reserves, high CO2 conversion, and CH4 selectivity [5,[12][13][14].The activity of Ni-based catalysts is highly dependent on the natural character of the supports, which mainly affects the ability to adsorb and activate CO2, the particle size of metal, the reducibility of metal phase, the interactions between metal and support, and the reaction mechanism [15,16].Therefore, the choosing of support material to improve the activity and long-time stability of Nibased catalysts is still a challenge [7,12].
Methanation catalysts can be used as a monometallic and multimetallic catalyst supported on various oxide, such as: TiO2, SiO2, -Al2O3, Y2O3, ZrO2, YSZ, and CeO2, and highly organized framework materials [8,12,[17][18][19].It is well-known that conventional Ni-based catalysts showed high catalytic activity at the high reaction temperature, low dispersion and reducibility, as well as nanoparticle sintering [14].To overcome this drawback, addition of second transition metal (e.g., Fe, Co) or a noble metal (e.g., Ru, Rh, Pt, Pd and Re) has been reported to enhance the catalytic activity and stability of Ni-based catalysts for methanation reaction by improving the dispersion of active metal through Ni-M (metal) alloy formation and adjusting the interaction between Ni and the support [13,14].It was also reported that NiFe alloys are active and stable catalysts for dry reforming of methane, since Fe can promote carbon gasification and significantly reduce coking through an intricate dealloying and realloying mechanism [14].
Catalysts 16NixFe/Al2O3 (x is 0, 1, 2, 4, 6, 8 prepared by incipient wetness impregnation method and applied for the production of synthetic natural gas (SNG) from CO hydrogenation in slurry-bed reactor were studied by Meng et al. [20].It was found that the introduction of iron improved the catalytic performance of low-temperature CO methanation due to the formation of Ni-Fe alloy.G. De Piano et al. investigated the effect of Fe content in Ni-Fe bimetallic catalysts supported on Ce0.8Zr0.2O2(CZ) and SiO2 (S) for CO2 methanation.Bimetallic N7.5F2.5/CZshowed high activity stability during 30 h and no absence of carbon deposition after long-term treatment as well as no evidence of Ni sintering/decoration [21].
GDC has been utilized for catalytic applications due to their properties.Several its good properties are it has mixed ionic electronic conductivity under a reducing atmosphere, a sufficient ionic conductivity between 500 and 700 °C, and a good catalytic activity in CO2 hydrogenation [22].However, few research has been done regarding to CO2 methanation over GDC as catalyst supports so far.A study of CO2 methanation using nickel supported on GDC as catalyst at atmospheric pressure varying reaction temperature (300-600 ºC) and space velocity (GHSV = 10,000 -50,000 h −1 ) was reported by Vita et al. [23].Ni/GDC catalysts with different Ni content (15-50 wt%) were synthesized by the solution combustion synthesis and the result showed the catalytic performance increased by increasing the Ni content due to enhanced metalto-support interaction, basicity and oxygen vacancies.Excellent stability was observed over 200 h of time-on-stream.Study on bimetallic catalysts for CO2 methanation was investigated over Ni and Ni-Fe catalysts supported on GDC in the temperature range 200-400 ºC [24].The result showed that both CO2 and H2 conversion decreased in the order Ni/GDC > Ni3Fe1/GDC> Ni1Fe1/GDC > Ni1Fe3/GDC.No catalytic activity was produced by Fe/GDC only.Maximum CO2 conversion (>90%) was observed at 400 ºC, with almost 100% selectivity to CH4.The superior activity of monometallic Ni/GDC and bimetallic Ni-Fe/GDC catalysts was ascribed to the presence of surface oxygen vacancies induced by the GDC support, an enhanced basicity of the Ni-rich samples, as well as to the ability of the Ni-GDC to interact with CO2.Similar study was also conducted by Frontera et al.The nickel-iron supported on GDC have been tailored for simultaneous methanation of carbon monoxide and carbon dioxide.It was found that NiFe/GDC catalyst exhibited its highest carbon conversion at 500 °C [6].
Regarding the literature review, a few researchers focused on the use of bimetallic nickel-iron supported on GDC for low-temperature CO2 methanation.There have been limited studies concerned on catalyst with high activity at low temperature and enhanced anti-sintering and anti-coking properties for CO2 methanation.Therefore, this research intends to lowtemperature of CO2 methanation.The objective of this research is to investigate the catalytic performance of Ni-Fe/GDC catalyst for lowtemperature CO2 methanation.

Catalyst Preparation
Ni-Fe/GDC catalyst was prepared by physical mixture method.Briefly, NiO was mixed by Fe2O3, and GDC as support (Ni: Fe: GDC ratio is 30: 30: 40 wt.%) under dry condition.Afterwards, the obtained solid was calcined at high temperature of 1400 ºC for 2 h.Prior the reaction, the Ni-Fe/GDC catalyst powder was reduced in situ with 10 ml/min H2 flow at various temperature in the range of 500 ºC until 700 ºC for 2 h.

Catalyst Characterization
Powder X-ray diffraction patterns were collected using a Rigaku RINT 2000 equipped with a Cu K (=1.5418Å) source and the Brag-Brentano θ-θ configuration in the 10-90° 2θ range, with 0.05° step size and 2 s acquisition time.The surface area of the catalysts was measured by N2 adsorption desorption isotherms at 77 K using a Micromeritics ASAP 2010 apparatus.Before measurement, the catalysts were degassed at 300 ºC in N2 for 5 h.The surface area was calculated by the Brunauer-Emmet-Teller (BET) method in the equilibrium pressure range 0.05 < P/P° < 0.3.H2temperature programmed reduction (H2-TPR) of the samples were performed using BEL MULTI-TASK-TPD.Analysis was carried out on 300 mg of sample, heating from 50 to 800 ºC at a heating rate of 5 °C/min with 10% H2 flow rate of 5 ml/min and holding at the final temperature for 5 h.The H2 consumption was measured by a mass spectrometer (MS).CO2-temperature programmed desorption (CO2-TPD) of the samples were performed using BEL MULTI-TASK-TPD.Analysis was carried out on 1000 mg of sample, heating from room temperature to 500 ºC at a heating rate of 5 °C/min with helium gas flow rate of 50 mL/min for 30 min.After the cleaning with He gas, the samples were cooled to 50 ºC, switched in CO2 and saturated down at 50 ºC.Then, the samples were purged again with He.The CO2 consumption of it was measured by an MS detector with heating from 50 ºC to 500 ºC.

Catalytic Activity Tests
The performance of the catalysts was evaluated for CO2 methanation.Prior to the activity tests, 1.00 g of catalyst was placed inside a stainless steel fixed bed reactor (inner diameter: 9.0 mm), with quartz wool at both ends, and reduced in situ with 10 mL/min H2 flow, increasing the temperature from room temperature up to 700 °C and isothermally kept at this temperature for 2 h.Different reduction temperatures were employed to determine the effect of reduction temperature.Afterwards, the feed mixture was flowed through the reactor.The reactions were performed in a fixed bed reactor operating at atmospheric pressure by means of gaseous mixtures of H2/CO2/Ar with different volumetric ratios in a temperature range 200-320 °C with 40 °C increments and GHSV 18,750 h −1 .Each temperature step was maintained for 60 min.
The performance of the catalysts was evaluated in terms of CO2 conversion, CH4 selectivity, CH4 production rate, and TOF.These values were calculated by the equations given as Equation ( 1), ( 2), (3), and (4), respectively.
where, X CO2 is CO2 conversion (%), CO2 in is molar amount of CO2 based on (mol) and CO2 out is CO2 molar amount of outlet (mol).
where, S CH4 is CH4 selectivity (%), N CH4 is molar amount of formed CH4 (mol/g/h) and N total is molar amount of the whole formed carbon compounds (mol/g/h).1,000,000 where CH4 pro is CH4 production rate (mmol/g/h), N CH4 is molar amount of formed CH4 (mol/g/h).Turnover frequency (TOF) for methane production is calculated as follows: MPR denotes methane production rate (mmol.gcat - .h - )), and AAS expresses amount of active sites (Ni amount on the catalyst surface) (mmol.gcat - ).The amount of active sites (Ni amount on the catalyst surface) of each catalyst was measured by CO pulse titration and H2 pulse titration at 50 ⁰C using BEL MULTI-TASK-TPD.

Product Analysis
The products were analyzed by two online gas chromatographs (Shimadzu GC-14B) consisted of two detectors, thermal conductivity detector (TCD) and flame ionization detector (FID) in series.Each of GCs have a Molecular Sieve 5A and a Porapak T columns, for analyses of gases and liquids including CO2, respectively.

Characterization of the Catalysts
The specific surface area of the catalyst samples was calculated from their respective N2 adsorption isotherms as shown in Table 1.GDC showed higher specific surface area compared to Ni-Fe/GDC.It indicated that the deposition of bimetallic Ni-Fe changed the textural properties of the GDC support as shown by BET surface areas (0.69 m 2 .g−1 ) that are lower than GDC support (13.2 m 2 .g−1 ).It is also worth noting that the specific surface area of Ni-Fe/GDC catalyst decreased in comparison with GDC support after introducing Ni and Fe species, indicating that Ni and Fe nanoparticles successfully entered the interior of GDC support and blocked the pore during the preparation [7].The specific surface area of Ni-Fe/GDC catalysts after reduced at temperature of 700 °C tend to remain stable in the range of 0.66 m 2 .g−1 .It indicated that GDC as support contributed to maintain the specific surface area and minimize the sintering effect of Ni and Fe particles in Ni-Fe/GDC at high reduction temperature.
The XRD analysis was carried out to find out the presence of the metallic states of active metals.Figure 1 shows the XRD patterns of reduced Ni, GDC and Ni-Fe/GDC catalysts.Reduced Ni catalyst exhibited the characteristic peaks of Ni at 2θ angle of 44.3°, and 51.8°, indicating that NiO was reduced to metallic Ni°.Both of GDC and Ni-Fe/GDC catalysts showed the existence of cubic fluorite structure of GDC support at 2θ = 28°, 33°, 47°, 56°, 59°, 69°, 76° and 79° as shown in Figure 1.The new diffraction peaks due to the interaction between iron and nickel in Ni-Fe/GDC catalysts were observed at 2θ = 44.35°and 51.65° can be assigned to the (111) and ( 200) of cubic phase of FeNi3 species (JCPDS card No. 65-3244).It revealed the formation of Ni-Fe alloy.Combined with the catalytic performance described in Figure 4-6, the results suggested that the formation of Ni-Fe alloy plays an important factor for CO2 methanation reaction over the Ni species that existed in the catalysts [20,25].
Metal-support interactions and metal reducibility are assessed by H2-TPR (Figure 2).Based on the H2-TPR characterizations, most of NiO can be reduced to Ni at the reduction Table 1.Textural properties of GDC support and Ni-Fe/GDC catalysts.temperature of 500 ºC.Generally, bulk NiO is reduced at about 300 ºC [7,18].The peaks at higher temperatures suggested different level of interactions between Ni species and the support [18,23,26].From Figure 2, it can be seen that the peaks at 340-695 ºC in the Ni-Fe/GDC catalysts were attributed to the reduction of dispersed NiO that strongly interacted with the GDC support.
To determine the surface basic sites, CO2-TPD measurements were performed for all reduced catalysts (Figure 3).There are three CO2 desorption regions depending on the desorption temperature, corresponding to weak basic sites (< 200 ºC), moderate basic sites (200-400 ºC) and strong basic sites (> 400 ºC), respectively [27].GDC and Ni-Fe/GDC reduced catalyst display mainly a high density of weak basic sites.As can be seen in Figure 3, Ni reduced catalyst shows weak and medium basic sites.Li et al. [2] proposed that CO2 was adsorbed by surface hydroxyl groups with weak basic sites to generate the reactive carbonate species that further reacted with dissociated H atoms to produce CH4.Whereas, basic sites promoted the formation of reactive bidentate carbonates and followed by hydrogenation to CH4.However, the strong basic sites did not provide to CO2 methanation reaction [7,21].The use of GDC support plays a pivotal role in reaction, enhancing the basicity of the Ni-Fe/GDC catalyst and improving the dissociation of carbon oxide species adsorbed on Ni sites [28].

Catalytic Activity Results of CO2 Methanation
To study the effect of feed ratios, several test runs were performed at reaction temperatures of 200 to 320 ºC with a total feed flow rate of 100 mL/min.CO2 conversion, CH4 selectivity and CH4 production rate results are displayed in Figures 4-6.It can be seen that almost all of the catalysts are not active below 200 ºC.Different reduction temperatures were chosen to determine the effect on the performance of catalysts for CO2 hydrogenation reaction at the feed gas ratio H2/CO2 = 4/1.The higher reduction temperature of Ni-Fe/GDC catalysts improved the performance of catalysts on methanation reaction.Ni-Fe/GDC catalysts produced higher CO2 conversion and CH4 selectivity compared with GDC and Ni catalyst.The best performance of the catalysts was located at the temperature of 280 ºC.The CO2 conversion and CH4 selectivity are decreased at higher reaction temperature, due to the enhancement of CO Boudouard reaction, reverse methane reforming reaction by CO2, and watergas shift reaction [18,25].This result was better than previous research using Ni-Fe/GDC catalysts that performed lower CO2 methanation activity (XCO2 < 20%) at reaction temperature under 300 ºC [6].As shown in Figure 5, methanation activity of nickel-iron supported by GDC was better than those on Ni unsupported and GC support only.GDC catalyst performed poor methanation activity at all reaction temperatures conducted.Meanwhile, Ni-Fe/GDC catalysts performed higher CH4 production rate per catalyst weight compared with GDC catalyst in all reaction temperatures carried on.It suggested that the existence of GDC minimized the sintering of nickel and iron metal particle at higher reaction temperature and contributed to the catalytic activity of Ni-Fe/GDC catalyst for methanation at higher reaction temperature.Compared with Ni catalyst, Ni-Fe/GDC catalysts also exhibited higher CH4 production rate at reaction temperature of 280 ºC.It confirms that the addition of Fe in Ni-Fe/GDC catalyst could facilitate the dispersion of Ni particles and enhance the activity of catalyst at low temperatures.CH4 production rates of Ni-Fe/GDC catalysts tend to increase by the increase of reduction temperature in the following order: 500 °C < 600 °C < 700 °C.The Ni-Fe/GDC catalyst reduced at 700 °C for 2 h produced the highest CH4 production rates of 17.6 mmol.gcat−1 .h−1 at reaction temperature of 280 ºC.It was slightly higher compared to Ni-Fe/GDC catalyst reduced at 600 °C for 2 h which produced CH4 production rates of 17.2 mmol.gcat−1 .h−1 at 280 ºC.
Ni/Al2O3 catalysts are well-known as common catalysts for methanation due to the high specific surface area of Al2O3 [18,29].Figure 6 shows the TOF for methane (methane produced per nickel site per second) production by CO2 methanation on nickel-iron supported by GDC compared with nickel supported by Al2O3, and nickel unsupported.From Figure 6, we find out that the TOF for methane production by CO2 methanation on Ni-Fe/GDC catalyst was higher than Ni/Al2O3 and Ni catalyst due to the existence of GDC support and the addition of Fe2O3.The use of GDC as supports contribute to the higher catalytic activity compared with Al2O3 due to the increase of oxygen vacancies by the addition of ceria to rare earth oxides (Gd2O3) forms a solid Ce 4+ oxide solution with CeO2 that contains mainly cations.This presence of surface oxygen vacancies enhances the surface basicity of pure CeO2 and promotes CO2 methanation [24,30].The addition   of Fe2O3 promotes CO2 hydrogenation compared with the Ni catalysts without GDC or/and Fe2O3 by electronic modification of Ni, forming Ni-Fe alloy nanoparticles [21,31,32].The highest TOF (4529.32 h −1 ) was produced by Ni-Fe/GDC catalyst at reaction temperature of 280 ºC.The best result obtained at 280 ºC for Ni-Fe/GDC catalyst is superior to previously reported Ni-Fe based catalyst as shown in Table 2 [21].
Deactivation of a metal catalyst can occur due to several factors, such as adsorption of impurities from the feed or product streams, coke deposition on the catalyst surface, oxidation of metal, metallic surface area reduction from sintering or leaching, and a drop in surface area from pore blockage [33].XRD analysis was performed on Ni-Fe/GDC spent catalyst.Metal-supported spent catalysts usually show the peaks for graphitic coke formation at 2θ = 62° and atomic coke formation at 2θ = 30°, respectively [34].From the XRD pattern in Figure 7, no such peak was found for Ni-Fe/GDC spent catalysts.

Conclusions
In summary, we have studied CO2 methanation activity over bimetallic Ni-Fe supported by GDC catalyst.Ni-Fe/GDC catalyst exhibited the highest CO2 conversion (46.5%) at 280 ºC, with almost 100% selectivity to CH4 under a GHSV of 18,750 h −1 at H2/CO2 = 4.The TOF value of Ni-Fe/GDC (4529 h -1 ) was the highest than that of Ni and common CO2 methanation catalyst, Ni/Al2O3 catalysts at 280 ºC, further displaying the outstanding low-temperature catalytic activity.

Figure 4 .
Figure 4. Catalytic performance of Ni, GDC and Ni-Fe supported by GDC at various reduction temperatures.

Figure 5 .
CH4 production rate per catalyst weight.