Carbon Dioxide Methanation over Nickel-Based Catalysts Supported

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Carbon dioxide methanation over Nickel-based catalysts supported on various mesoporous material Xinpeng Guo, Atsadang Traitangwong, Mingxiang Hu, Cuncun Zuo, Vissanu Meeyoo, Zhijian Peng, and Chunshan Li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03826 • Publication Date (Web): 01 Feb 2018 Downloaded from http://pubs.acs.org on February 6, 2018

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Carbon dioxide methanation over Nickel-based catalysts supported on various mesoporous material Xinpeng Guoa,b, Atsadang Traitangwongb, Mingxiang Huc, Cuncun Zuob, Vissanu Meeyoob, Zhijian Penga*, Chunshan Lib* b

a School of Engineering and Technology, China University of Geosciences, Beijing 100083, PR China Beijing Key Laboratory of Ionic Liquids Clean Process, State Key Laboratory of Multiphase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China. c State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China

Abstract Diverse supports (ZSM-5, SBA-15, MCM-41, Al2O3 and SiO2) with various mesoporous structures were introduced to fabricate nickel-based catalysts for the CO2 methanation by incipient wetness impregnation method. Ni/ZSM-5 catalyst displayed the most active catalytic properties, followed with Ni/SBA-15, Ni/Al2O3, Ni/SiO2 and Ni/MCM-41 catalysts. The excellent catalytic property of Ni/ZSM-5 catalyst was resulted from the basic property and the synergistic effect between nickel metal and support. The reactivity of the reaction intermediate monodentate formate in Ni/ZSM-5 catalyst was more active than that of bidentate formate species as identified by in situ infrared spectroscopy. The Ni/ZSM-5 catalyst performed with excellent stability and no deactivation up to 100 h. XRD, BET and TGA/DTA characterization further indicated that Ni/ZSM-5 catalyst had excellent resistance to carbon deposition and metal sintering. What is more, the kinetics of CO2 methanation over Ni/ZSM-5 catalyst was also studied. Keywords: nickel catalysts, mesoporous material, CO2 methanation, basic property, monodentate formate

*

Corresponding author: Pengzhijian. E-mail: [email protected] Chunshan Li. TeL/FaX: +86-10-82544800; E-mail: [email protected]

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1. Introduction Economic growth and global warming have caused increased levels of primary greenhouse gas carbon dioxide (CO2), which is thus attracting considerable research attention [1, 2]. Therefore, CO2 utilization and reduction in CO2 emission are being considered

[3]

. Noticeably, hydrogen gas can be obtained on large scales from many

industries, which produce an excess of hydrogen or by renewable energy such as solar energy, hydropower, and biomass

[4-7]

. Therefore, recycling of CO2 through its

hydrogenation to high added value products appears to be a very promising approach. Among many options for CO2 utilization (capture, sequestration, conversion, etc.), converting CO2 to useful CH4 is a very valuable elementary step in C1 chemistry and it is a solution for recycling of carbon resources [8-11]. Especially, CO2 methanation is a reaction that has gained fundamental research interest and has potential commercial applications [12]. A series of VIII metals such as Ni

[13-15]

, Ru

[16,18]

, Pt

[19]

, Rh

[20–22]

and Pd

[23,24]

supported on various porous materials, are usually applied in CO2 methanation. Nevertheless, the extensive application of noble metal catalysts is limited by their high cost. Ni-based catalysts have been receiving considerable attentions in the industrial field because of their comparable methanation activity and low material cost. The support has been reported to have a significant influence on the morphology, lattice phase and active sites dispersion of the catalyst. Therefore, a catalyst with different supports would have significantly different catalytic properties. Numerous catalyst supports have been conducted to investigate the effects on the catalytic properties in CO2 methanation, such as Al2O3, SBA-15, ZrO2, CeO2, MgO and zeolite [28-32]

. Presently, highly dispersed and sufficient surface area supported metal catalysts

are considered as the promising catalyst for CO2 methanation which have been the


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focus of considerable research

[25-27,49]

. To meet the requirements of industrial

production, the catalyst should possess high activity, selectivity and stability. Nickel-based catalysts with different supports exhibit variable interaction between nickel and the support (metal-support effect), which display significant effectiveness in catalytic activity and selectivity

[33-35]

. The ordered mesoporous materials as a

potential candidate provide the opportunity to produce highly dispersed Ni catalysts. ZSM-5 has been well studied so far for methane reforming with CO2, NO reduction with methane and carbon monoxide methanation

[36-38]

because of the presence of

ordered porous structure, high surface area and a moderate amount of basic sites on ZSM-5. Notably, these properties are determined in sorption and catalysis. Also, the synergistic effect of metal active sites and the ZSM-5 support are necessary for superior catalytic performance [39]. In the present study, nickel-based catalysts with various supports were used for CO2 methanation. Generally, ZSM-5 is a considerably versatile support material (ordered porous structure, high surface area) which can disperse nickel particles well to achieve a promising performance in catalytic reactions. We aimed to investigate the correlation between the catalytic activity and physicochemical properties which focus on the interpretation of the role of the support and the basicity of the catalyst. The catalytic activities of the nickel-based catalysts for CO2 methanation were examined. Moreover, for comparison catalysts supported on other mesoporous materials (SBA-15, MCM-41, Al2O3 and SiO2) were also investigated. 2. Experimental 2.1. Sample preparation Nickel-based catalysts supported on molecular sieves were synthesized by the impregnation method [32, 36]. Commercial Ni(NO3)2·6H2O (≥98.0%) was applied as a


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nickel source which was purchased from Xilong Co., Ltd. The following materials were used as catalyst support: ZSM-5(Xuyang chemical Co., Ltd.), MCM-41(Juteng Chemical Co., Ltd), Al2O3 (Xilong Co., Ltd), SiO2 (Xilong Co., Ltd), and SBA-15(Nanjing XFNANO Materials Tech Co., Ltd). Nickel-based catalysts were obtained with 10 wt% Ni loading by the wet impregnation method. First, the nickel nitrate was dissolved in deionized water at room temperature. Then the aqueous solution was added into the support material. After 24 h, the samples were dried in an oven before calcination in air by a temperature programmed (5℃/min) from room temperature to 500 °C and then maintained at 500 °C for 5 h. 2.2. Catalyst Characterization The crystalline phase of catalysts after calcination and H2 pretreatment was determined by X-ray diffraction (XRD) using a Rigaku Smart Lab X-ray powder diffractometer with Cu Kα radiation. The XRD patterns were recorded with 2θ ranged from 10o to 90o. The mean average crystallite size of metallic NiO was evaluated using the Scherrer equation (D=0.89λ/βcosθ), where λ is the X-ray wavelength (λ=0.154 nm), β is the line broadening of the nickel reflection in radians, and θ is the Bragg angle. The morphology of the catalysts was performed with a transmission electron microscope (TEM) with the Model JEOL JEM-2100 system. The samples were dispersed ultrasonically in ethanol and were deposited on a holey carbon-supported grid. The BET analysis of the catalysts was determined by N2 adsorption–desorption isotherms using a micromertics ASAP 2460 apparatus. To ensure the accuracy of the data, the samples were outgassed at 350 °C for 6 h before being subjected to N2 adsorption. The pore size distribution was calculated relying on the density function


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theory (DFT) method. Thermo-gravimetric/Differential Thermal (TGA/DTA, DTG-60H) was performed on a simultaneous Shimadzu DTA-TG analyzer, which can record TGA and DTA curves simultaneously. The catalyst samples were heated from room temperature to 700 °C at a rate of 10 °C min-1 under air flow. CO2

temperature-programmed

desorption

(CO2-TPD)

and

H2

temperature-programmed reduction (H2-TPR) were performed with an AutochemII 2920 Chemisorption Apparatus (Micrometric). The basic property of the catalyst was examined by CO2-TPD. 50 mg catalysts with the sizes between 20 and 40 mesh were fixed in a quartz tube reactor. Prior to the measurements, the catalysts were reduced under H2 stream at 500 °C for 2 h and cooled to 50 °C in He flow, followed by CO2 stream. Subsequently, the sample was heated up to 650 °C at a rate of 10 K min−1 under He. The desorbed CO2 was detected by a thermal conductivity detector (TCD). For H2-TPR, the samples were pretreated under air flow at 500 °C for 1h and cooled down to room temperature in He flow. The gas composed of 10% H2-Ar was adsorbed onto the samples for 1 h at 100 °C. The temperature was elevated to 900 °C from room temperature at a heating rate of 10 °C/min. The dispersion of surface active Ni sites of the catalyst was calculated based on the results of H2 pulse adsorption. In situ FTIR spectroscopy was used to characterize the species formed over catalysts during the reaction of CO2 methanation. The spectrum was recorded on a Nicolet 6700 spectrometer equipped with a cell. Prior to the measurements, the samples were activated at 500 °C for 1 h in hydrogen atmosphere. After cooling to 300 °C in Ar and collecting a background spectrum in flowing Ar, a reaction gas (4% CO2-16% H2-80% Ar), was fed to the sample for 1 h. The spectra were recorded by


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accumulating 50 scans with a resolution of 4 cm−1. 2.3 Catalyst evaluation The catalytic properties for CO2 methanation were investigated in a fixed-bed quartz reactor with an interior diameter of 8 mm at atmospheric pressure and in a temperature range of 200-450 °C. A thermocouple was installed in the middle of the catalyst bed to measure the temperature. 1.0 ml of the catalyst (20-40 mesh) was packed into the reactor. Prior to the evaluation of catalysts, the samples were reduced at a rate of 5 °C/min in a gaseous mixture of H2 and N2 (2:3, v/v) for 4 h at 500 °C at a flow rate of 100 ml/min-1 in the fixed-bed. The reaction gas mixture with H2/CO2 = 4 was introduced into the reactor with a total flux of 40 mL/min at gas hourly space velocity (GHSV) 2400 h-1. Further, the CO2 conversion was analyzed on line by gas chromatography (GC6890) equipped with a 2 m TDX01 column and a TCD detector. The calculation formulas of the CO2 conversion, methane selectivity and CO2 turn over frequency (TOF CO2) are described as follows: CO conversion % =

CH! selectivity % = TOF ) * =

%&

× 100

× 100

+,- ×./0 12 ×34×*567 89 ×4

(1) (2) (3)

where [CO2]in, and [CO2]out represent a molar flow rate of reactant CO2 in the inlet and outlet respectively(ml/min). [CH4] is the molar flow rate of the CH4 product in the outlet. FCO2in represent the volume flow rate of CO2 fed into the reactor (mL min−1), XCO2 is the conversion of CO2, NA is the avogadro’s constant (6.02×10^23mol−1), Vm is the molar volume of CO2 (22.4 L/mol). A denotes the number of the Ni surface active sites. 3. Results and discussion 3.1. XRD


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Fig. 1 XRD patterns of the Ni-based catalysts (a) before reduction (b) reduced at 500°C

The XRD patterns (Fig.1a) of all the catalyst before reduction show three main characteristic diffraction peaks at 37.3◦, 43.3◦, and 62.9◦, which can be indexed to (111), (200) and (220) of NiO phase, respectively. What is more, for Ni/Al2O3 catalyst distinct peaks of NiAl2O4 were observed at 37.0◦, 45.0◦ and 65.5◦. The average crystal size of NiO was calculated from the main peak marked from its indices (220) plane using Scherrer’s formula. The NiO crystallite sizes of the supported nickel catalysts were varied between 14-30 nm depending on the type of the support (table 1). Among these catalysts, the Ni/ZSM-5 catalyst displayed the smallest catalyst size of 14.32 nm. The NiO particles were observed in the following size order: Ni/ZSM-5 < Ni/SiO2 < Ni/SBA-15 < Ni/MCM-41, which revealed that the support had important influence on the size of metal particles. After reduction at 500 C, the peaks of metallic Ni were detected which was illustrated by XRD patterns, as shown in Fig.1 (b). For all the reduced catalysts, the diffraction peaks at 44.5◦, 51.85◦, and 76.37◦ were attributed to the crystalline Ni. No NiO crystallites were observed for all the catalysts by the reduction treatment. Therefore, it was expected that NiO was fully reduced to metallic state after hydrogen reduction. The XRD pattern of Ni/MCM-41 catalyst showed the diffraction peak was more intense and sharper than that of other catalysts. It indicated that the size of the metal particles of Ni/MCM-41 was larger, which was consistent with the catalyst before reduction. The intensities of the diffraction peak slightly decreased with the introduction of Ni to ZSM-5, while no peaks shifts were observed, suggesting that the addition of Ni did not disturb the ordered structure of ZSM-5, only a minor structural degradation of the ZSM support. No obvious Ni crystals were


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detected for Ni/ZSM which may be attributed to the superposition of Ni with ZSM peaks. In addition, the crystal sizes of Ni particles were so small and beyond the detection limit of XRD. 3.2. TEM images

Fig. 2 TEM image of nickel-based catalysts

The morphology of the nickel-based catalysts was characterized by transmission electron microscopy (TEM). As shown in Fig. 2, the TEM image displayed that all the catalysts had an irregular structure and had no indication of highly dispersed NiO nanoparticles. The different nickel-based catalysts consisted of metal nanoparticles with varying particle sizes. The dispersion of metal particles on the surface of ZSM-5 was relatively higher than that of other nickel-based catalysts. From the TEM image, NiO particles were obviously present on the surface of the catalysts and the average sizes of NiO nanoparticles on Ni/ZSM-5 catalyst obviously the smallest among the five catalysts. This result is consistent with the XRD analysis. 3.3. N2 adsorption-desorption Table 1 Physical properties of the Ni-based catalysts

Fig. 3 N2 adsorption–desorption isotherm of (A) Ni/Al2O3, (B) Ni/SiO2, (C) Ni/ZSM-5, (D) Ni/SBA-15, (E) Ni/MCM-41.The inset shows the pore size distribution.

Fig. 3 shows the nitrogen adsorption-desorption isotherms and corresponding pore size distribution of all catalysts. For all the mesoporous material all isotherms were typical type IV isotherms with H2 hysteresis loops according to IUPAC classification, indicating that most of the pores in the catalyst fell within the mesoporous range. For Ni/ZSM-5 catalyst the appearance of a type H2 hysteresis loop at P/Po= 0.4-0.9 indicated that one capillary condensation was caused by the mesopores inside the


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Ni/ZSM-5 catalyst

[40]

. The pore size distribution for Ni/ZSM-5 catalysts was quite

narrow and monomodal, showing concentration distribution of mesoporous at approximately 3.8 nm. Moreover, a similar isotherm was observed on the Ni/SBA-15 catalyst, while the profiles of pore size distribution corresponding to Ni/SBA-15 catalyst with larger pore size than Ni/ZSM-5 catalyst. The Ni/Al2O3 catalyst (Fig. 3A) had a wide pore size distribution with an average value of approximately 6.07 nm. Fig. 3B shows narrow and monomodal pore size distribution in the Ni/SiO2 catalyst. Meanwhile, the peak pore diameter was considerably broader than those of Ni/ZSM-5, Ni/SBA-15 catalysts as the peak width at half-maximum. The nitrogen isothermal adsorption line of Ni/MCM-41 catalyst was almost completely superimposed on the desorption ones, indicating reversible pore filling and emptying. The pore size distribution in the Ni/MCM-41 catalyst was also monomodal, showing a quite narrow peak pore diameter. As a whole, Ni/ZSM-5 catalyst had relatively uniform and smaller pore size than other catalysts. Table 1 summarizes the specific surface areas, pore volumes and mean diameter of the Ni-based catalysts. The nickel catalysts supported on various carriers were quite different. The pore volume and mean pore diameter of Ni/ZSM-5 catalyst were lower than that of other catalysts, perhaps indicating that the NiO particles were dispersed inside the pores of ZSM-5 and the catalyst had smaller metal crystallite size, which were consistent with the results of TEM and XRD. The surface area was varied from 220.19 m2/g to 768.31 m2/g, with Ni/SBA-15 catalyst showed the highest surface area. Usually, the high surface area is beneficial for the dispersion of metal in the catalysts. Thus, the surface area is not the single determining factor in NiO dispersion and the size of NiO on the support could also influence their dispersion [29].


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3.4. CO2-TPD and H2-TPR measurements

Fig. 4 H2-TPR profiles of nickel-based catalysts

Fig. 4 shows the reduction patterns of nickel-based catalysts. Typically, the catalysts showed two main reduction peaks at 300-450 °C and 450-650 °C respectively, which implied that there are two different kinds of reduction species during the reaction. The first reduction peak is basically identical to the reduction of free NiO particles to metal Ni. Therefore, free NiO particles exist on the surface of these four catalysts and the NiO species was weakly interacted with the support. The first reduction peak area of Ni/MCM-41 and Ni/SiO2 was higher than that of other catalysts, indicating that the main species of the two catalysts were free NiO. According to the literature, the free NiO species (almost no interaction with the support) did not contribute to the methanation reaction. While the NiO species with appropriate strength interaction with the support could be reduced to active metallic Ni which was beneficial to the CO2 methanation

[44]

.The second reduction peak at

high temperature revealed that there was a strong interaction between the NiO particles and the support which made them more difficult to be reduced. The reduction peaks of Ni/ZSM-5 catalyst shift to higher temperature and possess larger peak areas, indicating the stronger interaction between NiO particles and support. The reduction of the NiO species causes different amounts of defects which are related to the interaction strength with the support and provide the anchoring sites for the Ni particles

[42, 43]

. It should be noted that the Ni/Al2O3 catalyst showed only one

reduction peak at 650 °C, indicating a strong interaction between NiO particles and support. This implied that more Ni2+ ions entered in the Al2O3 structure. Moreover, the H2 pulse adsorption of Ni/ZSM-5 catalyst displayed the strongest H2 adsorption


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capacity and highest Ni dispersion (13.1%, Table 1) This is in accordance with the results of TEM.

Fig. 5 CO2-TPD profiles of nickel-based catalysts and ZSM-5 support

To further research the surface properties of the nickel-based catalysts, CO2-TPD was carried out to determine the basic strength and CO2 adsorption capacity (Fig. 5). For all the nickel-based catalysts two strong CO2 desorption peaks were observed, an obvious peak centered at about 180 °C and a broad one at 270 °C (except Ni/SiO2). The former peak was assigned to desorption of CO2 from weak basic sites, which revealed that all samples possess weak basic sites. The bare supports ZSM-5 also displayed a weak basic sites peak with the maximum at approximately 140 °C. Compared to ZSM-5 support, the adsorption peak of Ni/ZSM-5 catalyst shifted to high temperature and the quantity of basic sites was increased. Therefore, incorporation of nickel seemed to the formation of stronger basic sites. This fact may point a stronger interaction between Ni and ZSM-5 support and led to enhance basicity of Ni/ZSM-5 catalyst. The amount of weak basic sites on Ni/ZSM-5 catalyst was much larger than that of Ni/SBA-15, Ni/MCM-41and Ni/SiO2 catalyst (Table 1). The latter one of Ni/ZSM-5 and Ni/MCM-41 catalysts at higher temperature showed medium basic sites with fairly low density. And Ni/ZSM-5 catalyst had larger amount of medium basic sites than Ni/MCM-41 catalyst. For Ni/SBA-15 and Ni/SiO2 catalyst the desorption peak shifted toward higher temperature indicating the gradually strengthened basicity. CO2 adsorbed on weak and medium basic sites are both favorable to the methanation reaction while the strong basic sites on catalysts cannot participate in CO2 activation [41]. 3.5. Catalytic activity


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Fig. 6-1 CO2 conversions and CH4 yield of CO2 methanation at different temperature, H2/CO2=4:1, and GHSV = 2400 h−1

The CO2 conversion and methane selectivity for CO2 methanation reaction of nickel-based catalysts at 200-450 °C are presented in Fig. 6-1. With the increase of reaction temperature the CO2 conversion increased gradually then reached a maximal value. Subsequently, the value decreased as elevating the reaction temperature and the CH4 yield also showed similar trend with the rising of reaction temperature. CO2 methanation is a strong exothermal reaction, thus based on the thermodynamic analysis, low temperature is conducive to the reaction but kinetically disadvantage to the reaction because of the slow reaction rate. The methanation on the above nickel-based catalysts is probably kinetically controlled reaction. The Ni/ZSM-5 catalyst exhibited the best CO2 conversion with the increase of temperature from 200 °C to 450 °C. And the CO2 conversion reached about 76% at 400 °C for Ni/ZSM-5 catalyst. The high activity of Ni/ZSM-5 could be due to the existence of small size metal particles and high dispersion of the metal on the support which were on the basis of XRD, TEM and BET result. According to H2-TPR results, the NiO species which was moderate strength interaction with the support could be reduced to active metallic Ni which was beneficial to CO2 methanation reaction

[42,44]

. What is

more, the Ni/ZSM-5 catalyst exhibits a relatively large number of weak and medium basic sites which are both favorable to the methanation reaction. Thus, it could be concluded that there was a rough relationship between the catalytic activity and the support property of the catalysts. On the other hand, the Ni/MCM-41 and Ni/SiO2 catalyst exhibited inferior activities for the CH4 formation especially at low temperatures. Their low activities were partly due to the NiO species (more free NiO species and less moderate strength NiO species) which was displayed in the H2-TPR measurements. The sequence of catalytic activity is as follows: Ni/ZSM-5 >


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Ni/SBA-15 > Ni/Al2O3 > Ni/SiO2 > Ni/MCM-41. The major products were methane with a small amount of carbon monoxide in the experiments. And the selectivity of CH4 was highly reached to 99% in the low temperature region, so that the CO2 conversion and CH4 yield were almost coincident at each temperature for the nickel-based catalysts. The CH4 selectivity slightly decreased at 450°C which was mainly due to the reverse water gas shift reaction (CO2 + H2 → CO+ H2O) at high temperatures.

Fig.6-2. Relationship between the TOF value and the amount of CO2 in the nickel-based catalyst.

As shown in Fig.6-2, the relationship between TOF value and the amount of CO2 for various nickel-based catalysts displayed a linear correlation with the maximum TOF values obtained for the Ni/ZSM-5 catalyst. This further confirms the cooperative catalysis between nickel metal and the basicity property. The four nickel-based catalysts with different support show different TOF values, indicating that the synergistic effect between nickel metal and support played an important role in enhancing catalytic activities for CO2 methanation. Table 2 Test for heat and mass transfer limitation with Ni/ZSM-5 catalyst.f

CO2 methanation is a strongly exothermic reaction. In order to avoid the effect of heat release on reaction rate, the Koros-Nowak test is required. The 10% Ni/ZSM-5 and 15% Ni/ZSM-5 catalysts were prepared and ground to 40 mesh. About 0.5 g of the catalyst was introduced into the reactor tubing at a certain gas hourly space velocity (GHSV) 2400 h-1. As can be seen from the table, the TOF values of the two Ni/ZSM-5 catalysts are the same, indicating that the effect of heat productivity on the reaction rate is eliminated under the operating conditions.


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3.6. Catalytic stability

Fig. 7 Stability test result of Ni/ZSM-5 catalyst for CO2 methanation at 400 °C, H2/CO2=4:1, GHSV = 2400 h−1

Fig. 8 Characterization for the spent Ni/ZSM-5 catalyst: (a) XRD and (b) TGA/DTA

Ni/ZSM-5 catalyst was tested for the stability at 400 °C due to its relatively excellent catalytic performance in CO2 methanation. As shown in Fig. 7, the catalyst exhibited extremely good stability for CO2 conversion in 100 h, almost no obvious decrease occurred of CO2 conversion and CH4 selectivity remained at about 99%. After the stability experiment, the reaction activities of the spent Ni/ZSM-5 catalyst were subsequently tested repeatedly. Almost no obvious change was observed in the CO2 conversion over Ni/ZSM-5 catalyst at each temperature point compared to the fresh catalyst. These results indicated that the Ni/ZSM-5 catalyst presented excellent activity and stability under reaction conditions. Fig. 8(a) shows the XRD patterns of the fresh and spent catalysts，in which no carbonaceous species diffraction peak was detected. It could infer that the deposited carbon existed in the form of amorphous structure which was more reactive than crystalline carbon

[48]

. Compared to the fresh

catalyst, the XRD diffraction peak intensity of the spent catalyst increased slightly, indicating slightly partial sintering of the spent catalyst. And it also verified by the BET result, which is the weak increase in pore diameter of the spent catalyst. Thus the diffraction peak of the fresh and spent catalysts showed no significantly change, indicating that the crystalline structure of the catalysts was stable and the catalyst had good anti-sintering properties. Therefore, to further study deposited carbon, the spent catalyst was investigated by TGA/DTA which was presented in Fig. 8(b). The first weight loss less than 300 °C was attributed to the removal of physical adsorbed water


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and some absorbed CO2 elimination. Due to the oxidation of surface metal, a gradual weight increase was observed between 300 and 400 °C. Finally, a slight weight loss was detected at 400-650 °C due to the oxidation of deposited carbon to CO and/or CO2. The results confirmed that there was so few deposited carbon on the spent catalyst was found. The XRD, BET and TGA/DTA analysis of the spent catalyst revealed that the Ni/ZSM-5 catalyst was resistant to carbon deposition and metal sintering. The good stability of the catalyst could be attributed to its relatively high basicity and the synergistic effect of the nickel metal and support. 3.7. In situ FTIR analysis.

Fig. 9 In situ FTIR spectra of adsorbed gases (CO2 + H2) on Ni/ZSM5, Ni/MCM-41 catalyst at 300 °C.

The adsorption intermediates and reaction pathway for CO2 methanation have been a controversial topic in the field of heterogeneous catalysis. Two possible mechanisms for CO2 methanation have been proposed, that is, a formate or carbonate route and a CO route. The reaction mechanism is highly correlated with the property of the catalyst. The Ni/ZSM-5 and Ni/MCM-41 catalysts were selected because the two materials provided significantly different catalytic activities as shown in Fig. 6. As presented in Fig. 9, the FTIR adsorption spectra obtained at 300 °C under CO2 and H2 atmosphere for 60 min over Ni/ZSM-5 and Ni/MCM-41 catalysts. Notably, two peaks at 1340 and 1600 cm-1 were observed for the Ni/ZSM-5 catalyst. Moreover, the characteristic band of methane, which appeared at 1301 cm−1 was in good accordance with the excellent performance for Ni/ZSM-5 catalyst. In the spectra for Ni/MCM-41 catalyst a new peak at 1590 cm-1 was observed after the introduction of CO2 and H2. In this research, the two catalysts were detected with the adsorption species of formate and no adsorbed carbon monoxide was observed. For formate species over


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Ni-based catalysts, the adsorption configuration has been proposed. For Ni/ZSM-5 and Ni/MCM-41 catalysts, the band at 1340 cm-1 was ascribed to the monodentate formate species and the band at 1600 cm-1 and 1590 cm-1 were assigned to the bidentate formate species, correspondingly

[41,45]

. The formate species were derived

from the hydrogenation reaction of carbonates and further participated in the production of CH4 on Ni catalyst

[46]

. Therefore, the result suggested that CO2

methanation over Ni/ZSM-5 catalyst proceeds via the formation of formate species intermediate by the hydrogenation of carbonates species other than the CO route. As an important intermediate for the methanation, the reactivity of different formate species was studied by teams of researchers. The monodentate formate species were reported to react more quickly with hydrogen than the bidentate formate species and significantly conducive to CO2 methanation [41, 47]. As shown in Fig. 9, the Ni/ZSM-5 catalyst had one monodentate formate species at 1340 cm-1 and one bidentate formate species at 1600 cm-1. Nevertheless, the Ni/MCM-41 catalyst has only one bidentate formate species at 1590 cm-1. Therefore, based on the excellent catalytic activity of Ni/ZSM-5 catalyst we can effectively confirm that the monodentate formate was more active in the methanation reaction than the bidentate formate species. 3.8 Measurements of kinetics The kinetics research of hydrogenation of carbon dioxide to methane were conducted over a range of temperature from 250 °C to 400 °C and a range of feed rate from 20 ml/min to 80 ml/min over Ni/ZSM-5 catalyst (20-40mesh). The kinetic model was constructed based on some assumptions: a) it is no difference in the activity of the active sites scattered on the surface of the catalyst; b) the temperature gradient and concentration gradient between the gas phase and catalyst surfaces were negligible; c) both internal and external diffusion effects are eliminating and the data


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are obtained under kinetically controlled. Thus, the kinetic empirical equation could be described as: : = Kp4 = p> ?

(1)

Where K is the kinetic constants, pA, pB, represent the pressure of CO2 and H2, respectively. The a and b were the reaction order of them correspondingly, and rA is the reaction rate for CO2 methanation reaction. In addition, CO2 conversion can be described as: @=1−

BC 12

(2)

BC DEF

Where CA in, CA out refer to the concentrations of CO2 reactant in the inlet and outlet respectively (mol/L). the kinetic equation (1) can be converted as follows： GBC ×*H I

G J

= KK4= K>?

(3)

W is the mass of the catalyst, g; F is the molar flow of CO2, (mol/h). Take the natural logarithm of both sides of the eq.3 can obtain the formula 4 and a+b=n. Q

O* @ = 1 − LK4 MN P R n − 1 O* .

(4)

According to formula 4, the dynamic correlation experiment was carried out. The effects of space velocity and reaction temperature were researched. After fitting calculation, the results are shown in figure 10 and the values of n, K and R2 of the fitting equation at different reaction temperatures were obtained. The curves show good exponential correlations with R2 above 0.97. The fitting results are good, and the corresponding fitting parameters were accurate.

Figure 10. Effect of temperature on the CO2 conversion with reaction time

Based on Arrhenius equation, the pre-exponential factor (k0) and reaction activation


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

energy (Ea) were calculated. The relationship of lnK vs 1/RT is presented in Figure 11. The values of Ea and k0 are obtained using linear regression. The Ea was calculated to be 52.69 kJ·mol−1 by the average slope and according to the intercept k0=9358. The R2=0.989 indicating that the kinetic equation was reasonable for the reaction. The values of apparent activation energy for CO2 methanation reaction for reported Ni/SiO2 are 82 and 89 KJ/mol-1[50,51], for Ni/Al2O3 catalyst is 80-106 kJ/mol-1 [51,52]

which are slightly higher than the Ni/ZSM-5 catalyst. It is indicated that the

Ni/ZSM-5 catalyst had higher catalytic activity than other nickel-based catalysts.

Figure 11. Arrhenius plot of CO2 methanation over Ni/ZSM catalyst

4. Conclusions Nickel-based catalysts supported on different molecular sieves were prepared by impregnation method for CO2 methanation. The Ni/ZSM-5 catalyst exhibited the highest CO2 conversion among all the samples with 99% CH4 selectivity. The catalytic performance was closely related to the intrinsic properties of the catalyst, including basic property, size of metal particles, and synergistic effect of the nickel metal and support. The CO2 desorption behavior of Ni catalysts suggested that weak and medium basic sites are both positive to the methanation reaction and the catalytic activities were found to be correlated to the concentration of basic sites. The reduction of the NiO species causing different amounts of defects in Ni/ZSM-5 was responsible for the anchoring sites for the active Ni particles. The active intermediates of Ni/ZSM-5 and Ni/MCM-41catalyst were compared during CO2 methanation. The in situ FTIR spectra revealed that CO2 methanation should proceed via the formation of formate intermediate and that the monodentate formate was more active in the methanation reaction than the bidentate formate species. The Ni/ZSM-5 catalyst


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maintain good stability with no deactivation during the entire experiment period. Results of catalytic tests and kinetics study showed that Ni/ZSM-5 exhibited great potential in CO2 methanation because of good activity and stability. Acknowledgements This work was supported by the National Science Fund for Excellent Young Scholars (No.21422607), NSFC-NRCT joint project (No.51661145012), and Key Research Program of Frontier Sciences, CAS (NO. QYZDB-SSW-SLH022).


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References [1] Davis, S J.; Caldeira, K; Matthews, H. D. Future CO2 emissions and climate change from existing energy infrastructure. Science 2010, 329(5997), 1330-1333. [2] Younas, M.; Kong, L. L.; Bashir, M. J. K.; Nadeem, H.; Shehzad, A.; Sethupathi, S. Recent Advancements, Fundamental Challenges, and Opportunities in Catalytic Methanation of CO2. Energy Fuels 2016, 30, 8818-8831 [3] Wang, T.; Meng, X.; Li, P.; Ouyang, S.; Chang, K.; & Liu, G. Photoreduction of CO2, over the well-crystallized ordered mesoporous TiO2, with the confined space effect. Nano Energy 2014, 9, 50-60. [4] Wang, W.; Wang, S.; Ma. X.; Gong, J. Recent advances in catalytic hydrogenation of carbon dioxide. Chem. Soc. Rev. 2011, 40, 3703-3727. [5] Benson, E.; E.; Kubiak, C. P.; Sathrum, A. J. Smieja, J. M. ChemInform Abstract: Electrocatalytic and Homogeneous Approaches to Conversion of CO2, to Liquid Fuels. Chem. Soc. Rev. 2009, 38, 89-99. [6] Cheng, D.; Negreiros, F. R., Aprà, E.; & Fortunelli, A. Computational approaches to the chemical conversion of carbon dioxide. Chemsuschem 2013, 6, 944-965. [7] Razzaq, R.; Li, C.; Usman, M.; Suzuki, K.; & Zhang, S. A highly active and stable Co4N/γ-Al2O3 catalyst for CO and CO2 methanation to produce synthetic natural gas (SNG). Chem. Eng. J. 2015, 262(FEB), 1090-1098. [8] Wang, Z. Q.; Xu, Z. N.; Peng, S. Y.; Zhang, M. J.; Lu, G.; & Chen, Q. S. A High-Performance and Long-Lived Cu/SiO2 Nanocatalyst for CO2 Hydrogenation. Acs Catalysis 2015，7 [9] Zhu, H.; Razzaq, R.; Li, C.; Muhmmad, Y.; & Zhang, S. Catalytic Methanation of Carbon Dioxide by Active Oxygen Material CexZr1-xO2 Supported Ni-Co Bimetallic Nanocatalysts. AIChE J. 2013, 59, 2567-2576


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Energy & Fuels

[10] Abate, S.; Barbera, K.; Giglio, E.; Deorsola, F.; Bensaid, S.; & Perathoner, S.; Pirone, R; Centi, G. Synthesis, characterization and activity pattern of Ni-Al hydrotalcite

catalysts in CO2 methanation. Ind. Eng. Chem. Res. 2016, 55(30). [11] Younas, M.; Kong, L. L.; Bashir, M. J. K.; Nadeem, H.; Shehzad, A.; Sethupathi, S. Recent Advancements, Fundamental Challenges, and Opportunities in Catalytic Methanation of CO2. Energy Fuels 2016, 30(11). [12] Thampi, K.; R.; Kiwi, J.; Graetzel, M. Methanation and photo-methanation of carbon dioxide at room temperature and atmospheric pressure. Nature 1987, 327, 506-508. [13] Krämer, M.; Stöwe, K.; Duisberg, M.; Müller, F.; Reiser, M.; Sticher, S.; Maier, W. F. The impact of dopants on the activity and selectivity of a Ni-based methanation catalyst. App. Catal., A 2009, 369, 42-52. [14] Du, G.; Lim, S.; Yang, Y.; Wang, C.; Pfefferle, L.; Haller, G. L. Methanation of carbon dioxide on Ni-incorporated MCM-41 catalysts: The influence of catalyst pretreatment and study of steady-state reaction . J. catal. 2007, 249, 370-379. [15] Razzaq, R.; Zhu, H.; Jiang, L.; Muhammad, U.; Li, C.; Zhang, S. Catalytic methanation of CO and CO2 in coke oven gas over Ni–Co/ZrO2–CeO. Ind. Eng. Chem. Res. 2013, 52, 2247-2256. [16] Wang, F.; He, S.; Chen, H.; Wang, B.; Zheng, L.; Wei, M.;... & Duan, X. Active Site Dependent reaction mechanism over Ru/CeO2 catalyst toward CO2 methanation. J. Am. Chem. Soc. 2016, 138, 6298-6305. [17] Abe, T.; Tanizawa, M.; Watanabe, K.; Taguchi, A. CO2 methanation property of Ru nanoparticle-loaded TiO2 prepared by a polygonal barrel-sputtering method. Energy Environ. Sci. 2009, 2, 315-321. [18] Solymosi, F.; Erdöhelyi, A.; Kocsis, M. Methanation of CO2 on supported Ru


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catalysts. J. Chem. Soc., Faraday Trans. 1981, 77, 1003-1012. [19] Beaumont, S. K.; Alayoglu, S.; Specht, C.; Michalak, W. D.; Pushkarev, V. V.; Guo, J.; Somorjai, G. A. Combining in situ NEXAFS spectroscopy and CO2 methanation kinetics to study Pt and Co nanoparticle catalysts reveals key insights into the role of platinum in promoted cobalt catalysis. J. Am. Chem. Soc. 2014, 136, 9898-9901. [20] Kang, Y.; Liu, J.; & Su, S. L. study on adsorption and remove of fluorine from water by modified zeolite. Advanced Materials Research Adv. Mater. Res. 2013, 726-731, 2295-2299. [21] Aziz, M. A. A.; Jalil, A. A.; Triwahyono, S.; Ahmad, A. CO2 methanation over heterogeneous catalysts: recent progress and future prospects. Green Chemistry. 2015, 17, 2647-2663. [22] Swalus, C.; Jacquemin, M.; Poleunis, C.; Bertrand, P.; Ruiz, P. CO2 methanation on Rh/γ-Al2O3 catalyst at low temperature: “In situ” supply of hydrogen by Ni/activated carbon catalyst. App. Catal., B 2012, 125, 41-50. [23] Karelovic, A.; Ruiz, P. Improving the hydrogenation function of Pd/γ-Al2O3 catalyst by Rh/γ-Al2O3 addition in CO2 methanation at low temperature. ACS Catalysis. 2013, 3, 2799-2812. [24] Erdöhelyi, A.; Pásztor, M.; Solymosi, F. Catalytic hydrogenation of CO2 over supported palladium. J. catal. 1986, 98, 166-177. [25] Vance, C. K.; Bartholomew, C. H. Hydrogenation of carbon dioxide on group VIII metals: III, Effects of support on activity/selectivity and adsorption properties of nickel. App. Catal. 1983, 7, 169-177. [26] Yan, X.; Liu, Y; Zhao, B.; Wang, Z.; Wang, Y.; Liu, C. J. Methanation over Ni/SiO2: effect of the catalyst preparation methodologies. Int. J. Hydrogen Energy


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2013, 38, 2283-2291. [27] Singh, U. K.; & Vannice, M. A. Influence of metal-support interactions on the kinetics of liquid-phase citral hydrogenation. J. Mol. Catal. A: Chem. 2000, 163(1-2), 233-250. [28] Zhou, L.; Wang, Q.; Ma, L.; Chen, J.; Ma, J.; Zi, Z. CeO2 promoted mesoporous Ni/γ-Al2O3 catalyst and its reaction conditions for CO2 methanation. Catal. Lett. 2015, 145, 612-619. [29] Lu, B.; Yiwen, J. U.; Abe, T.; & Kawamotoa, K. Grafting Ni particles onto SBA-15, and their enhanced performance for CO2 methanation. Rsc Advances 2015. 5, 56444-56454. [30] Yamasaki, M.; Habazaki, H.; Asami, K.; Izumiya, K.; Hashimoto, K. Effect of tetragonal ZrO2 on the catalytic activity of Ni/ZrO2 catalyst prepared from amorphous Ni–Zr alloys. Catal. Commun. 2006, 7, 24-28. [31] Jwa, E.; Lee, S. B.; Lee, H. W.; Mok, Y. S. Plasma-assisted catalytic methanation of CO and CO2 over Ni-zeolite catalysts. Fuel Process. Technol. 2013, 108, 89-93. [32] Guo, M.; Lu, G. The effect of impregnation strategy on structural characters and CO2 methanation properties over MgO modified Ni/SiO2 catalysts. CataL. Commun. 2014, 54, 55-60. [33] Sokolov, S.; Kondratenko, E. V.; Pohl, M. M.; Barkschat, A.; Rodemerck, U. Stable low-temperature dry reforming of methane over mesoporous La2O3-ZrO2 supported Ni catalyst. Appl. Catal., B 2012, 113-114, 19-30. [34] Bacariza, M. D. C.; Graça, I.; Bebiano, S.; Lopes, J. M.; & Henriques, C. Magnesium as promoter of the CO2 methanation on Ni-based USY zeolites. Energy Fuels 2017, 31, 9776-9789 [35] Nichele, V.; Signoretto, M.; Menegazzo, F.; Gallo, A.; Dal Santo, V.; Cruciani, G.;


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Cerrato, G. Glycerol steam reforming for hydrogen production: Design of Ni supported catalysts . Appl. Catal., B 2012, 111, 225-232. [36] The, L. P.; Triwahyono, S.; Jalil, A. A.; Mamat, C. R.; Sidik, S. M.; Fatah, N. A.; ... Shishido, T. Nickel-promoted mesoporous ZSM-5 for carbon monoxide methanation. RSC Advances 2015, 5, 64651-64660. [37] Rahmani, F.; Haghighi, M.; Vafaeian, Y.; Estifaee, P. Hydrogen production via CO2 reforming of methane over ZrO2-Doped Ni/ZSM-5 nanostructured catalyst prepared by ultrasound assisted sequential impregnation method. J. Power Sources 2014, 272, 816-827. [38] Li, Y. J.; Battavio, P. J.; Armor, J. N. Effect of Water Vapor on the Selective Reduction of NO by Methane over Cobalt-Exchanged ZSM-5. J. Catal. 1993, 142, 561-571. [39] Teh, L. P.; Triwahyono, S.; Jalil, A. A.; Firmansyah, M. L.; Mamat, C. R.; Majid, Z. A. Fibrous silica mesoporous ZSM-5 for carbon monoxide methanation. App. Catal., A. 2016, 523, 200-208. [40] Dash, R. K.; Yushin, G.; & Gogotsi, Y. Synthesis, structure and porosity analysis of microporous and mesoporous carbon derived from zirconium carbide. Microporous Mesoporous Mater. 2005, 86, 50-57. [41] Pan, Q.; Peng, J.; Sun, T.; Wang, S.; Wang, S. Insight into the reaction route of CO2 methanation: Promotion effect of medium basic sites. Catal. Commun. 2014, 45, 74-78. [42] Wang, Y. H.; Liu, H. M.; Xu, B. Q. Durable Ni/MgO catalysts for CO2 reforming of methane: activity and metal-support interaction. J. Mol. Catal. A: Chem. 2009, 299, 44-52. [43] Liu, Z.; Xu, W.; Yao, S.; Johnson-Peck, A. C.; Zhao, F.; Michorczyk, P.; ... &


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Rodriguez, J. A. Superior performance of Ni-W-Ce mixed-metal oxide catalysts for ethanol steam reforming: Synergistic effects of W-and Ni-dopants. J. Catal. 2015, 321, 90-99. [44 ] Yan, Y.; Dai, Y.; He, H.; Yu, Y.; Yang, Y. A novel W-doped Ni-Mg mixed oxide catalyst for CO2 methanation. Appl. Catal., B. 2016, 196, 108-116. [45] Prairie, M. R.; Renken, A.; Highfield, J. G.; Thampi, K. R.; Grätzel, M. A fourier transform infrared spectroscopic study of CO2, methanation on supported ruthenium. J. Catal. 1991, 129, 130-144. [46] Park, J. N.; McFarland, E. W. A highly dispersed Pd–Mg/SiO2 catalyst active for methanation of CO2. J. Catal. 2009, 266, 92-97. [47] Pan, Q.; Peng, J.; Wang, S.; Wang, S. In situ FTIR spectroscopic study of the CO2 methanation mechanism on Ni/Ce0.5Zr0.5O2. Catal. Sci. Technol. 2014, 4, 502-509. [48] Carrero, A.; Calles, J. A.; Vizcaíno, A. J. Effect of Mg and Ca addition on coke deposition over Cu-Ni/SiO2 catalysts for ethanol steam reforming. Chem. Eng. J. 2010, 163, 395-402. [49] Vannice, M. A. The catalytic synthesis of hydrocarbons from H2 CO mixtures over the group VIII metals: I. The specific activities and product distributions of supported metals. J. Catal. 1975, 37(3), 449-461. [50] Weatherbee, G. D., & Bartholomew, C. H. Hydrogenation of CO2, on group VIII metals: II. kinetics and mechanism of CO2 hydrogenation on nickel. J. Catal. 1983, 77(2), 460-472. [51] Koschany, F., Schlereth, D., & Hinrichsen, O. On the kinetics of the methanation of carbon dioxide on coprecipitated NiAl(O)x. Appl. Catal. B. 2016, 181, 504-516. [52] Maatman, R., & Hiemstra, S. A kinetic study of the methanation of CO2 over


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nickel-alumina. J. Catal.1990, 62(2), 349-356.


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Table and Figure Captions Table 1 Physical properties of the Ni-based catalysts Table 2 Test for heat and mass transfer limitation with Ni/ZSM-5 catalyst. Fig. 1 XRD patterns of the Ni-based catalysts (a) before reduction (b) reduced at 500°C Fig. 2 TEM image of nickel-based catalysts Fig. 3 N2 adsorption–desorption isotherm of (A) Ni/Al2O3, (B) Ni/SiO2, (C) Ni/ZSM-5, (D) Ni/SBA-15, (E) Ni/MCM-41.The inset shows the pore size distribution. Fig. 4 H2-TPR profiles of nickel-based catalysts Fig. 5 CO2-TPD profiles of nickel-based catalysts and ZSM-5 support Fig. 6-1 CO2 conversions and CH4 yield of CO2 methanation at different temperature, H2/CO2=4:1, and GHSV = 2400 h−1 Fig.6-2. Relationship between the TOF value and the amount of CO2 in the nickel-based catalyst. Fig. 7 Stability test result of Ni/ZSM-5 catalyst for CO2 methanation at 400 °C, H2/CO2=4:1, GHSV = 2400 h−1 Fig. 8 Characterization for the spent Ni/ZSM-5 catalyst: (a) XRD and (b) TGA/DTA Fig. 9 In situ FTIR spectra of adsorbed gases (CO2 + H2) on Ni/ZSM5, Ni/MCM-41 catalyst at 300 °C. Figure 10. Effect of temperature on the CO2 conversion with reaction time Figure 11. Arrhenius plot of CO2 methanation over Ni/ZSM catalyst


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Table 1 Physical properties of the Ni-based catalysts SBET (m2/g)a

Catalyst

Vp (cm3/g) a

Pore size

Nio crystal

nCO2(mmo

(nm)a

size (nm)b

l g-1cat)c

DNi (%)d

TOF/10-3S-1

Ni/Al2O3

259.74

0.469

7.217

---

0.262

11.1

6.57

Ni/SiO2

220.19

0.875

15.896

16.40

0.157

11.8.

4.36

Ni/ZSM-5

299.82

0.231

3.082

14.32

0.264

13.1

7.57

Ni/SBA-15

768.31

0.989

5.150

19.52

0.213

12.6

5.92

Ni/MCM-41

673.08

0.686

4.095

30.29

0.140

8.5

3.42

Ni/ZSM-5spent

290.04

0.232

3.120

28.96

-

-

a

Obtained from N2 adsorption-desorption b Determined by Xrd (Scherrer’s equation) c Values calculated based on CO2-TPD. d Calculated according to the H2 pulse adsorption. e Calculated according to Ni dispersion and CO2 conversion of the catalysts at 250℃

Table 2 Test for heat and mass transfer limitation with Ni/ZSM-5 catalyst.f Catalysts

XCO2/%

Ni/ wt%

TOF(10-3 s-1)

10%Ni/ZSM-5 15% Ni/ZSM-5

18.9 27.1

10 15

7.59 7.56

f

The reaction was performed at 250 °C, reaction gas mixture with H2/CO2 = 4, (GHSV) 2400 h-1.

Fig. 1 XRD patterns of the Ni-based catalysts (a) before reduction (b) reduced at 500°C Ni/MCM-4

Ni/SiO2

Ni/Al2O3

Ni/ZSM-5

Ni/SBA-15

Fig. 2 TEM image of nickel-based catalysts


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Fig. 3 N2 adsorption–desorption isotherm of (A) Ni/Al2O3, (B) Ni/SiO2, (C) Ni/ZSM-5, (D) Ni/SBA-15, (E) Ni/MCM-41.The inset shows the pore size distribution.

Fig. 4 H2-TPR profiles of nickel-based catalysts


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Fig. 5 CO2-TPD profiles of nickel-based catalysts and ZSM-5 support

Fig. 6-1 CO2 conversions and CH4 yield of CO2 methanation at different temperature, H2/CO2=4:1, and GHSV = 2400 h−1

Fig.6-2. Relationship between the TOF value and the amount of CO2 in the nickel-based catalyst.


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Energy & Fuels

Fig. 7 Stability test result of Ni/ZSM-5 catalyst for CO2 methanation at 400 °C, H2/CO2=4:1, GHSV = 2400 h−1

Fig. 8 Characterization for the spent Ni/ZSM-5 catalyst: (a) XRD and (b) TGA/DTA

Fig. 9 In situ FTIR spectra of adsorbed gases (CO2 + H2) on Ni/ZSM5, Ni/MCM-41 catalyst at 300 °C.


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Figure 10. Effect of temperature on the CO2 conversion with reaction time

Figure 11. Arrhenius plot of CO2 methanation over Ni/ZSM catalyst

Table of Contents Graphics The schematic illustration of reaction process for CO2 methanation over nickel-based catalysts.

CH4(g),CO(g), H2O(g) C-formate CO

O OH

Ni Support ACS Paragon Plus Environment

Carbon Dioxide Methanation over Nickel-Based Catalysts Supported

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