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Effects of structure on the carbon dioxide methanation performance of Co-based catalysts
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Effects of structure on the carbon dioxide methanation performance of Co-based catalysts Guilin Zhou*, Tian Wu, Hongmei Xie, Xuxu Zheng Key Laboratory of Catalysis Science and Technology of Chongqing Education Commission, Department of Chemistry and Chemical Engineering, Chongqing Technology and Business University, Chongqing 400067, China
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abstract
Article history:
Mesoporous Co/KIT-6 and Co/meso-SiO2 catalysts were prepared via hydrogen reduction
Received 16 February 2013
and were subsequently used in CO2 catalytic hydrogenation to produce methane. The
Received in revised form
properties of these catalysts were investigated via low-angle X-ray diffraction (XRD), Bru-
13 May 2013
nauereEmmetteTeller (BET) analysis, and transmission electron microscopy (TEM). The
Accepted 23 May 2013
results indicate that the synthesized Co/KIT-6 and Co/meso-SiO2 catalysts have meso-
Available online xxx
porous structures with well-dispersed Co species, as well as high CO2 catalytic hydrogenation activities. The Co/KIT-6 catalyst has a large specific surface area (368.9 m2 g1) and a
Keywords:
highly ordered bicontinuous mesoporous structure. This catalyst exhibits excellent CO2
Ordered mesoporous
catalytic hydrogenation activity and methane product selectivity, which are both higher
Co-based catalyst
than those of the Co/meso-SiO2 catalyst at high reaction temperatures. The CO2 conversion
Carbon dioxide
and methane selectivity of the Co/KIT-6 catalyst at 280 C are 48.9% and 100%, respectively.
Methanation
The high dispersion of the Co species and the large specific surface area of the prepared Co-
Catalytic hydrogenation
based catalysts contribute to the high catalytic activities. In addition, the highly ordered, bicontinuous, mesoporous structure of the Co/KIT-6 catalyst improves the selectivity for the methane product. Crown Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Carbon dioxide (CO2), a compound widely existing in nature, is considered an inert gas because of its high thermodynamic stability. CO2 is regarded as a potential carbon raw material, but also as an air pollutant [1]. CO2 is a cheap, nontoxic, and abundant C-1 feedstock, and its chemical utilization is a challenge and important topic. Many processes have been used to utilize CO2 fully, including CO2 reforming of CH4 to produce synthesis gas [2], hydrogenation to produce methanol [3] and methane [4], and the synthesis of dimethyl carbonate [5], cyclic carbonate [6], and dimethyl ether [7]. The reduction of CO2 requires high-energy electron donors such as the highenergy reducing agent H2, carbon negative ions, or external
energy (light energy). CO2 reduction technology (i.e., CO2 methanation technology) was first proposed by the French chemist [8]. CO2 catalytic hydrogenation for methanation has attracted the most attention in C-1 chemical research because of its strategic significance. This catalytic hydrogenation can overcome the bottleneck, which involves CO2 immobilization and resource utilization. The social energy and chemical industry structures can also be seriously affected by CO2 chemistry. However, the direct synthesis of methane from CO2 is favorable not only for the reduction of greenhouse gas emissions, but also for the development of new carbon sources for organic compounds as well as of new ways to utilize CO2. In the last few decades, considerable effort has been made in the development of catalysts with high CO2
* Corresponding author. Tel.: þ86 23 62769076; fax: þ86 23 62769785 605. E-mail address: [email protected] (G. Zhou). 0360-3199/$ e see front matter Crown Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.05.130
Please cite this article in press as: Zhou G, et al., Effects of structure on the carbon dioxide methanation performance of Cobased catalysts, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2013.05.130
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methanation activity. Previous studies on catalysts for CO2 catalytic hydrogenation methanation mainly focused on supported Ru, Rh, Ni, and Pd catalysts. Ru, Rh, and Pd are precious metal catalysts with good CO2 catalytic hydrogenation activities [9e11]. The Ru catalyst has an excellent lowtemperature CO2 catalytic methanation activity compared with other studied precious metal catalysts, exhibiting high CO2 conversion at 400 C [12,13]. However, noble metal catalysts are expensive, requires high hydrogenation reaction temperatures (>300 C), and have low methane selectivity with high CO2 catalytic hydrogenation conversion. These properties limit the application of noble metal catalysts in the catalytic hydrogenation of CO2. Meanwhile, among the nonnoble metal catalysts, the Ni-based ones have been widely studied because of their high methanation and catalytic activities and low costs [14e16]. Previous reports showed that the Ni catalysts used in the CO2 hydrogenation reaction require high reaction temperatures; however, these catalysts have low CO2 conversion with high methane selectivity [14e16]. The Ni/MCM-41 catalyst reported by Du [17] exhibited a higher CO2 catalytic hydrogenation activity at 400 C. A highly dispersed Ni/La2O3 catalyst, which was prepared via the wet impregnation method, exhibited superior CO2 hydrogenation activity and methane selectivity at 350 C and 1.5 MPa [18]. Usually, catalytic reaction properties can be affected by the catalyst composition and structure (e.g., specific surface area, pore size distribution, pore size, and structure). The desired catalytic performances, including high CO2 catalytic hydrogenation activity and selectivity, can be achieved through the proper selection of the preparation method. The catalysts, which were prepared via conventional methods, have small specific surface areas (<50 m2 g1), low porosity, and irregular pore structures, which limit its catalytic performance. Mesoporous materials, which are characterized by large specific surface areas (>100 m2 g1), developed pore structures, and wide pore diameters, have exhibited superior catalytic properties, thus making them of great interest to researchers in the field of catalysis. For mesoporous catalysts, more focus has been given to the synthesis of mesoporous metal oxide catalysts and their catalytic properties [19e21]. The mesoporous metal catalysts used for CO2 catalytic hydrogenation have not been reported to date. Therefore, studies on the preparation of mesoporous metal catalysts and their CO2 catalytic hydrogenation performances would be conducive to the development of novel catalyst materials for CO2 immobilization and its subsequent utilization. The present work aims to develop new mesoporous metal catalysts with high activities and selectivities for CO2 catalytic hydrogenation for methane production. Therefore, the Co/ KIT-6 and Co/meso-SiO2 catalysts were prepared and characterized via low-angle X-ray power diffraction (XRD), BrunauereEmmetteTeller (BET) analysis, and transmission electron microscopy (TEM). The reducible properties of the Co/ KIT-6 and Co/meso-SiO2 catalyst precursors were investigated via hydrogen temperature-programmed reduction (H2-TPR). The catalytic performances of the prepared catalysts were also investigated through CO2 catalytic hydrogenation for methane production.
2.
Experimental
2.1.
Catalyst preparation
The ordered mesoporous Co/KIT-6 and Co/meso-SiO2 catalysts were prepared via the excess impregnation method. Co(NO3)2$6H2O (Tianjin Kermel Chemical Reagent Co., Ltd., China) was used as the metal oxide precursor, whereas the synthesized KIT-6 and commercial meso-SiO2 (mesoporous SiO2, Qingdao Ocean Chemical Reagent Co., Ltd., China) were used as the supports for the Co/KIT-6 and Co/meso-SiO2 catalysts, respectively. The large mesoporous cubic Ia3d silica, which was previously designated as KIT-6, was prepared according to the procedure described by Ryoo et al. [22]. The prepared KIT-6 and meso-SiO2 were each appended in an aqueous cobaltous nitrate solution, which was prepared by dissolving a specified amount of cobaltous nitrate in deionized water. The mixtures were then evaporated at a fixed temperature, dried at 383 K for 24 h, and calcinated in air at 773 K for 3 h. All obtained catalyst precursors contained 20 wt% of the elementary substance cobalt which obtained according to the dosage of the used precursor nitrates. The catalyst precursors were then subjected to hydrogen reduction to produce the Co/KIT-6 and Co/meso-SiO2 catalysts. These catalysts were then used for CO2 catalytic hydrogenation to produce methane.
2.2.
Catalyst characterization
2.2.1.
H2-TPR studies
Hydrogen temperature program reduction (H2-TPR) was carried out as follows: 20 mg of the support cobalt precursor material was placed in a U-type quartz tube, 5.0 vol% H2/Ar gas mixture was passed through the tube at a flow rate of 25 ml min1, and the reduction temperature was raised from room temperature to 650 C at a rate of 10 C/min. The consumption of hydrogen was measured by a thermal conductivity detector (TCD).
2.2.2.
Low-angle XRD characterization
Low-angle XRD patterns were obtained on a Rigaku D/MaxA) radiation 2500/PC diffractometer with a Cu Ka (l ¼ 1.5418 source operated at 30 kV and 40 mA. The diffraction patterns were collected from 0.5 to 5.0 at a rate of 1 /min.
2.2.3.
BET characterization
The N2 adsorptionedesorption isotherms were obtained at 77 K using a Quantachrome Autosorb-1 instrument. The samples were degassed under vacuum for 4 h at 573 K prior to the measurements. The BET method was used to calculate the specific surface areas. The pore size distributions were derived from the desorption branches of the isotherms using the BarretteJoynereHalanda (BJH) method.
2.3.
TEM characterization
Transmission electron microscopy (TEM) was performed using a Tecnai G2 Spirit microscope operating at an acceleration voltage of 120 kV. Prior to TEM measurement, samples
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were prepared by ultrasonication in ethanol. A drop of the resultant suspension was evaporated on a carbon-coated copper grid.
2.4.
Catalytic activity measurement
Catalytic tests were conducted in a tubular, continuous-flow, fixed-bed quartz reactor (6 mm i.d.) under atmospheric pressure. The quartz tube reactor containing 100 mg of the supported cobalt oxide precursors were then placed inside the tubular furnace. High-purity hydrogen was flowed downward through the reactor containing the catalyst bed, while an electronic mass flow controller (D07-7A/ZM, China) was used to control the hydrogen gas flow rate. The catalyst samples were reduced at 400 C at a 100 ml min1 hydrogen flow rate for 150 min, followed by cooling under a H2 flow to the given temperature. The temperature was measured using a thermocouple projected into the catalyst bed and was monitored ¨ GU, model 708P, by a temperature programmable controller (U China). The feed gas mixture consisting of CO2 (10.0 vol%), H2 (46.0 vol%), and Ar (44.0 vol%) was passed over the catalysts at a flow rate of 36.67 ml min1, which is equivalent to a gas mass space velocity of 22,000 ml g1 h1. The CO2 catalytic hydrogenation conversion and selectivity data have been obtained and repeated at given temperature for 120 min. The gas products were monitored using an on-line gas chromatograph (GC 6890 II) equipped with a thermal conductivity detector (TCD).
3.
Results and discussion
3.1.
H2-TPR profiles
The H2-TPR test was conducted on the prepared Co3O4/KIT-6 and Co3O4/meso-SiO2 samples to study their reducible properties and determine their optimal preparation conditions. The TPR profiles are shown in Fig. 1, where the a, b, and g H2TPR peaks for the prepared support cobalt oxide precursor
samples appear at 220, 320, and 370 C, respectively. The samples prepared using KIT-6 and meso-SiO2 as supports have similar H2-TPR signals. The a and b peak intensities and peak areas are significantly weaker than those of g. However, the KIT-6esupported sample has high-intensity a and b peaks compared with the meso-SiO2-supported sample. The occurrence of multiple reduction peaks indicates the presence of a number of reducible cobalt species in the precursor samples. Co3O4 can be reduced by hydrogen to form Co via a two-step reduction (Co3O4 / CoO / Co0). The lowtemperature hydrogen consumption peaks a and b, which are located at 220 and 320 C, respectively, can be attributed to the reduction of the well-dispersed Co3O4 to produce CoO on the surfaces of the KIT-6 and meso-SiO2 supports [23]. The results indicate that large amounts of highly dispersed Co3O4 are present on the surfaces of KIT-6 and meso-SiO2. This result can be attributed to the large specific surface area of the two supports. The a and b peaks of the Co3O4/KIT-6 sample are obviously stronger than those of the Co3O4/meso-SiO2 sample. These results indicate that the cobalt oxide species exhibits better dispersion on the surface of the KIT-6 support than on the meso-SiO2 support. Therefore, the Co species can be dispersed well on the surface of the KIT-6 support to obtain a better-dispersed Co/KIT-6 catalyst through the hydrogen reduction of Co3O4/KIT-6. The g peak can be attributed to the reduction of bulk Co3O4, which are mainly present in the pore structure of the KIT-6 and meso-SiO2 supports. CoO, which is formed via Co3O4 reduction at low temperatures, can be reduced by hydrogen to produce Co, resulting in the appearance of a TPR peak for hydrogen consumption at high reduction temperatures [23]. Therefore, the g peak can also be attributed to the reduction of CoO. As shown in Fig. 1, most cobalt oxides can be reduced by hydrogen at 400 C. The results indicate that the cobalt oxide species can be completely reduced at 400 C for 150 min to form Co species. That is, the Co/KIT-6 and Co/meso-SiO2 catalysts can be prepared via hydrogen reduction at 400 C for 150 min. This result is consistent with those of previous reports [24].
3.2. γ
Intensity / a.u.
β
α
Co O /KIT-6
Co O /meso-SiO 100
150
200
250
300 350 400 Temperature / C
450
500
550
600
Fig. 1 e Studies of reducible property for supported cobalt oxide precursor.
3
BET characterization
The porosity of the prepared Co/KIT-6 and Co/meso-SiO2 catalysts were measured via N2 sorption determination. Fig. 2 gives the N2 adsorptionedesorption isotherm and the corresponding pore size distribution. The isotherms are typical type IV, with a H1 hysteresis loop. The prepared Co/KIT-6 catalyst displays a narrow pore size distribution, narrower than that of the Co/meso-SiO2 catalyst. The pore diameters of the Co/KIT-6 and Co/meso-SiO2 catalysts were determined as approximately 6.44 and 8.06 nm, respectively, based on the isotherm desorption branches obtained using the BJH method. The results indicate that the Co/KIT-6 and Co/meso-SiO2 catalysts have mesoporous structures. The specific surface areas of the Co/KIT-6 and Co/meso-SiO2 catalysts (approximately 368.9 and 185.8 m2 g1, respectively) were calculated based on the BET method. The Co/KIT-6 catalyst has a large specific surface area, about twice as large as that of the Co/meso-SiO2 catalyst. The large specific surface area can also promote the loading Co species to disperse on the support surface, thereby
Please cite this article in press as: Zhou G, et al., Effects of structure on the carbon dioxide methanation performance of Cobased catalysts, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2013.05.130
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catalyst was not be observed at the 0.5 e5.0 range. This result indicates that the Co/meso-SiO2 catalyst has a disordered structure. Therefore, the ordered bicontinuous mesoporous structure facilitates the diffusion and transfer of the reactant and product molecules in the Co/KIT-6 catalyst, resulting in the improvement of its catalytic performance.
550 500
Quantity adsorbed(cm /g, STP)
450 400 350 300
3.4.
250
150 100 Co/meso-SiO
50
Co/KIT-6 0 0.0
0.1
0.2
0.3
0.4
0.5 P / P
0.6
0.7
0.8
0.9
1.0
Fig. 2 e N2 adsorptionedesorption isotherms and pore size distribution of Co/KIT-6 and Co/meso-SiO2 catalysts.
providing more activity centers for the CO2 catalytic hydrogenation reaction. At the same time, the wide pore diameter and narrow pore size distribution can provide favorable conditions for CO2 molecular adsorption and activation, thus leading to the high CO2 catalytic hydrogenation performance of the Co/KIT-6 catalyst.
3.3.
TEM characterization
200
TEM was performed to analyze the mesoporous structures of the prepared Co/KIT-6 and Co/meso-SiO2 catalysts as well as the dispersion of the Co species on the KIT-6 and meso-SiO2 supports. The TEM images of the prepared Co/KIT-6 and Co/ meso-SiO2 catalyst samples are shown in Fig. 4. The Co/KIT-6 catalyst displays an obvious ordered pore structure, whereas the Co/meso-SiO2 catalyst does not. A well-ordered threedimensional cubic array of mesopores is observed for the Co/ KIT-6 catalyst sample. The centers of two adjacent pores in this array are approximately 10.5 nm apart, and the pore diameter is about 5.2 nm. This result is consistent with those of the BET and low-angle XRD. Meanwhile, the pore structure
Low-angle XRD characterization
Intensity / a.u.
As previously discussed for H2-TPR, the Co/KIT-6 and Co/ meso-SiO2 catalysts can be prepared via hydrogen reduction. The low-angle XRD was used to study the ordered structure of the prepared Co/KIT-6 and Co/meso-SiO2 catalysts. The lowangle XRD of the Co/KIT-6 catalyst (Fig. 3) has a sharp intense peak at 2q ¼ 0.97 , which corresponds to the (211) plane. According to previous reports [25,26], the prepared Co/ KIT-6 catalyst has a well-ordered mesoporous structure and belongs to the Ia3d bicontinuous cubic symmetry space group. Meanwhile, the low-angle XRD peak of the Co/meso-SiO2
Co/KIT-6
Co/meso-SiO2 0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
2 theta /
Fig. 3 e Low-angle XRD patterns for the Co/KIT-6 and Co/ meso-SiO2 catalysts.
Fig. 4 e TEM graphs of Co/KIT-6 and Co/meso-SiO2 catalysts.
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of the Co/meso-SiO2 catalyst cannot be clearly observed. The TEM image does not show the presence of any significant cobalt agglomerates on the KIT-6 and meso-SiO2 surfaces, thus indicating that the Co species are well-dispersed on the KIT-6 and meso-SiO2 surfaces as well as in the mesopores. The effective dispersion of the Co species on the surface of the KIT6 and meso-SiO2 supports can be attributed to the large surface area and wide pore diameter of the KIT-6 and meso-SiO2 supports. This result is consistent with those of the H2-TPR analysis. In general, a well-dispersed Co species can provide active sites for the hydrogenation reaction.
3.5.
Catalytic activity test
The catalytic hydrogenation reaction using the prepared Co/ KIT-6 and meso-SiO2 catalysts was investigated at a hydrogenecarbon dioxide ratio (H2/CO2) of 4.6, a space velocity of 22,000 ml g1 h1, and at different reaction temperatures. The results are shown in Fig. 5a and b. Fig. 5a shows that the Co/
a
55 50 45 40
Conversion / %
35 30 25 20 15 10 Co/KIT-6 Co/meso-SiO
5 0 200
210
220
230
240
250
260
270
280
290
300
Temperature / C
b 100 95 90
Methane selectivity / %
85 80 75 70 65 60 Co/KIT-6 Co/meso-SiO
55 50 200
210
220
230
240
250
260
270
280
290
300
Temperature / C
Fig. 5 e a. The relation between the carbon dioxide conversion and reaction temperature on Co/KIT-6 and Co/ meso-SiO2 catalysts. b. The relation between the methane selectivity and reaction temperature on Co/KIT-6 and Co/ meso-SiO2 catalysts.
5
KIT-6 catalyst has a higher catalytic hydrogenation activity for CO2 than the Co/meso-SiO2 catalyst. In addition, the CO2 conversion increases with increasing reaction temperature. Meanwhile, Fig. 5b shows that the Co/KIT-6 catalyst has higher methane selectivity than the Co/meso-SiO2 catalyst at temperatures above 260 C. The CO2 conversion on the Co/KIT-6 catalyst increases from 15.5% to 48.9% when the reaction temperature increases from 200 C to 280 C. The main hydrogenation product using this catalyst is methane, and the selectivity is retained at 100%. And 51.0% CO2 conversion and 98.9% methane selectivity are observed when the reaction temperature increases to 300 C. For the Co/meso-SiO2 catalyst, the CO2 conversion and methane selectivity at 280 C is 40.0% and 94.1%, respectively. However, the carbon conversion increases to 43.5% and the methane selectivity sharply decreases to 78.7% when the reaction temperature increases to 300 C. The Co/KIT-6 catalyst shows an excellent catalytic performance compared with the Pd catalysts in a previous report [9]. CO2 can be hydrogenated to produce methane on the appropriate catalyst. The hydrogenation methanation reaction can be expressed as CO2 þ 4H2 / CH4 þ 2H2O. The reverse wateregas shift (RWGS) reaction may occur for hydrogen gas, with the reaction expressed as CO2 þ H2 / CO þ H2O [10]. The presence of CO was not detected, indicating that the RWGS reaction did not occur on the prepared Co/KIT-6 and Co/mesoSiO2 catalysts during CO2 catalytic hydrogenation. The other catalytic hydrogenation products, including methanol and other C2þ compounds, were not detected, except for methane. These results indicate that methane is the main product of the CO2 catalytic hydrogenation on the Co/KIT-6 and Co/mesoSiO2 catalysts. As the reaction temperature is increased, more reactant molecules (including CO2 and H2) can be induced to participate in the CO2 hydrogenation reaction, thereby resulting in high catalytic activity in the CO2 catalytic hydrogenation at high temperatures. The high catalytic activity of Co/KIT-6 and Co/meso-SiO2 indicates that the Co species, which was prepared via hydrogen reduction of cobalt oxides, is an important catalytic active species for CO2 catalytic hydrogenation. The TEM results indicate that the Co species were well-dispersed on the KIT-6 and meso-SiO2 support surfaces, thereby providing more active centers for the formation of active H2 and CO2 species. The BET results show that the prepared Co/KIT-6 and Co/meso-SiO2 catalysts have large specific surface areas that exceed 180 m2 g1 as well as wide pore diameters of approximately 6.44 nm. A large specific surface area can provide a large space for more active centers to adsorb and activate the reactant molecules, resulting in the formation of more active species for the CO2 catalytic hydrogenation reaction. At the same time, the wide pore diameter can provide favorable conditions for the reactant molecules to diffuse and transfer in the catalyst. More hydrogen molecules can then be activated to form active hydrogen species in the hydrogenation reaction, thereby improving the CO2 catalytic hydrogenation activity of the Co/ KIT-6 and Co/meso-SiO2 catalysts. As shown in Fig. 5, the Co/ KIT-6 catalyst has higher CO2 catalytic hydrogenation activity than the Co/meso-SiO2 catalyst. These results can be attributed to the large specific surface area and ordered mesoporous structure of the Co/KIT-6 catalyst, as indicated by the BET and TEM results.
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The low-angle XRD, BET, and TEM results indicate that the prepared Co/KIT-6 catalyst has a large specific surface area, highly dispersed Co species, wide pore diameter, narrow pore size distribution, and highly ordered mesoporous structure. The large specific surface area promotes the adsorption and activation of the reactant molecules (CO2 and hydrogen), resulting in the improvement of the CO2 catalytic hydrogenation. The prepared Co/KIT-6 catalyst has highly dispersed Co species, which can provide more active centers for the CO2 catalytic hydrogenation. The high hydrogenation selectivity can also be attributed to the highly ordered mesoporous structure and wide pore diameter of the Co/KIT-6 catalyst. This highly ordered and bicontinuous mesoporous structure and the wide pore diameter can provide superior conditions for the diffusion of the methane molecules produced and their transplantation in the Co/KIT-6 catalyst. In particular, the bicontinuous mesoporous structure can ensure that the methane molecules produced in the pores of the Co/KIT-6 catalyst are transplanted freely. The methane molecules produced by CO2 hydrogenation can be easily desorbed from the Co/KIT-6 catalyst to the gas phase to achieve high methane selectivity. On the other hand, the Co/meso-SiO2 catalyst does not have the bicontinuous mesoporous structure, thereby limiting the diffusion and transplantation of the methane molecule produced and promoting methane decomposition at high reaction temperatures. Therefore, the methane produced has low selectivity at high reaction temperatures.
4.
Conclusions
(1) The H2-TPR results indicate that the supported cobalt oxide precursor can be reduced by hydrogen to form the Co/KIT-6 and Co/meso-SiO2 catalysts. TEM results show that the formed Co species are well-dispersed on the surfaces of the KIT-6 and meso-SiO2 supports. (2) The low-angle XRD, TEM, and BET results show that the prepared Co/KIT-6 and Co/meso-SiO2 catalysts have large specific surface areas and high supported Co species dispersion. In addition, these catalysts exhibit mesoporous structures, particularly the prepared Co/KIT-6 catalyst, which has a highly ordered bicontinuous mesoporous structure. (3) The Co/KIT-6 catalyst has a higher CO2 catalytic hydrogenation activity than the Co/meso-SiO2 catalyst. The CO2 conversion can exceed 48.9%, and the methane selectivity can be retained at 100% until 280 C. The Co/KIT-6 catalyst exhibits higher methane selectivity than the Co/meso-SiO2 catalyst at high reaction temperatures; this result can be attributed to the highly ordered bicontinuous mesoporous structure of the Co/KIT-6 catalyst.
Acknowledgments The work was financially supported by Youth Doctoral Fund of Chongqing Technology and Business University (No. 1152011).
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[21] Harun T, Massimiliano C, Ferdi S. Ordered mesoporous Co3O4 as highly active catalyst for low temperature CO-oxidation. Chem Commun (Camb) 2008;34:4022e4. [22] Kleitz F, Choi SH, Ryoo R. Cubic Ia3d large mesoporous silica: synthesis and replication to platinum nanowires, carbon nanorods and carbon nanotubes. Chem Commun 2003;17:2136e46. [23] Li JL, Coville NJ. Effect of boron on the sulfur poisoning of Co/TiO2 FischereTropsch catalysts. Appl Catal A 2001;208:177e84.
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Effects of structure on the carbon dioxide methanation performance of Co-based catalysts Guilin Zhou*, Tian Wu, Hongmei Xie, Xuxu Zheng Key Laboratory of Catalysis Science and Technology of Chongqing Education Commission, Department of Chemistry and Chemical Engineering, Chongqing Technology and Business University, Chongqing 400067, China
article info
abstract
Article history:
Mesoporous Co/KIT-6 and Co/meso-SiO2 catalysts were prepared via hydrogen reduction
Received 16 February 2013
and were subsequently used in CO2 catalytic hydrogenation to produce methane. The
Received in revised form
properties of these catalysts were investigated via low-angle X-ray diffraction (XRD), Bru-
13 May 2013
nauereEmmetteTeller (BET) analysis, and transmission electron microscopy (TEM). The
Accepted 23 May 2013
results indicate that the synthesized Co/KIT-6 and Co/meso-SiO2 catalysts have meso-
Available online xxx
porous structures with well-dispersed Co species, as well as high CO2 catalytic hydrogenation activities. The Co/KIT-6 catalyst has a large specific surface area (368.9 m2 g1) and a
Keywords:
highly ordered bicontinuous mesoporous structure. This catalyst exhibits excellent CO2
Ordered mesoporous
catalytic hydrogenation activity and methane product selectivity, which are both higher
Co-based catalyst
than those of the Co/meso-SiO2 catalyst at high reaction temperatures. The CO2 conversion
Carbon dioxide
and methane selectivity of the Co/KIT-6 catalyst at 280 C are 48.9% and 100%, respectively.
Methanation
The high dispersion of the Co species and the large specific surface area of the prepared Co-
Catalytic hydrogenation
based catalysts contribute to the high catalytic activities. In addition, the highly ordered, bicontinuous, mesoporous structure of the Co/KIT-6 catalyst improves the selectivity for the methane product. Crown Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Carbon dioxide (CO2), a compound widely existing in nature, is considered an inert gas because of its high thermodynamic stability. CO2 is regarded as a potential carbon raw material, but also as an air pollutant [1]. CO2 is a cheap, nontoxic, and abundant C-1 feedstock, and its chemical utilization is a challenge and important topic. Many processes have been used to utilize CO2 fully, including CO2 reforming of CH4 to produce synthesis gas [2], hydrogenation to produce methanol [3] and methane [4], and the synthesis of dimethyl carbonate [5], cyclic carbonate [6], and dimethyl ether [7]. The reduction of CO2 requires high-energy electron donors such as the highenergy reducing agent H2, carbon negative ions, or external
energy (light energy). CO2 reduction technology (i.e., CO2 methanation technology) was first proposed by the French chemist [8]. CO2 catalytic hydrogenation for methanation has attracted the most attention in C-1 chemical research because of its strategic significance. This catalytic hydrogenation can overcome the bottleneck, which involves CO2 immobilization and resource utilization. The social energy and chemical industry structures can also be seriously affected by CO2 chemistry. However, the direct synthesis of methane from CO2 is favorable not only for the reduction of greenhouse gas emissions, but also for the development of new carbon sources for organic compounds as well as of new ways to utilize CO2. In the last few decades, considerable effort has been made in the development of catalysts with high CO2
* Corresponding author. Tel.: þ86 23 62769076; fax: þ86 23 62769785 605. E-mail address: [email protected] (G. Zhou). 0360-3199/$ e see front matter Crown Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.05.130
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methanation activity. Previous studies on catalysts for CO2 catalytic hydrogenation methanation mainly focused on supported Ru, Rh, Ni, and Pd catalysts. Ru, Rh, and Pd are precious metal catalysts with good CO2 catalytic hydrogenation activities [9e11]. The Ru catalyst has an excellent lowtemperature CO2 catalytic methanation activity compared with other studied precious metal catalysts, exhibiting high CO2 conversion at 400 C [12,13]. However, noble metal catalysts are expensive, requires high hydrogenation reaction temperatures (>300 C), and have low methane selectivity with high CO2 catalytic hydrogenation conversion. These properties limit the application of noble metal catalysts in the catalytic hydrogenation of CO2. Meanwhile, among the nonnoble metal catalysts, the Ni-based ones have been widely studied because of their high methanation and catalytic activities and low costs [14e16]. Previous reports showed that the Ni catalysts used in the CO2 hydrogenation reaction require high reaction temperatures; however, these catalysts have low CO2 conversion with high methane selectivity [14e16]. The Ni/MCM-41 catalyst reported by Du [17] exhibited a higher CO2 catalytic hydrogenation activity at 400 C. A highly dispersed Ni/La2O3 catalyst, which was prepared via the wet impregnation method, exhibited superior CO2 hydrogenation activity and methane selectivity at 350 C and 1.5 MPa [18]. Usually, catalytic reaction properties can be affected by the catalyst composition and structure (e.g., specific surface area, pore size distribution, pore size, and structure). The desired catalytic performances, including high CO2 catalytic hydrogenation activity and selectivity, can be achieved through the proper selection of the preparation method. The catalysts, which were prepared via conventional methods, have small specific surface areas (<50 m2 g1), low porosity, and irregular pore structures, which limit its catalytic performance. Mesoporous materials, which are characterized by large specific surface areas (>100 m2 g1), developed pore structures, and wide pore diameters, have exhibited superior catalytic properties, thus making them of great interest to researchers in the field of catalysis. For mesoporous catalysts, more focus has been given to the synthesis of mesoporous metal oxide catalysts and their catalytic properties [19e21]. The mesoporous metal catalysts used for CO2 catalytic hydrogenation have not been reported to date. Therefore, studies on the preparation of mesoporous metal catalysts and their CO2 catalytic hydrogenation performances would be conducive to the development of novel catalyst materials for CO2 immobilization and its subsequent utilization. The present work aims to develop new mesoporous metal catalysts with high activities and selectivities for CO2 catalytic hydrogenation for methane production. Therefore, the Co/ KIT-6 and Co/meso-SiO2 catalysts were prepared and characterized via low-angle X-ray power diffraction (XRD), BrunauereEmmetteTeller (BET) analysis, and transmission electron microscopy (TEM). The reducible properties of the Co/ KIT-6 and Co/meso-SiO2 catalyst precursors were investigated via hydrogen temperature-programmed reduction (H2-TPR). The catalytic performances of the prepared catalysts were also investigated through CO2 catalytic hydrogenation for methane production.
2.
Experimental
2.1.
Catalyst preparation
The ordered mesoporous Co/KIT-6 and Co/meso-SiO2 catalysts were prepared via the excess impregnation method. Co(NO3)2$6H2O (Tianjin Kermel Chemical Reagent Co., Ltd., China) was used as the metal oxide precursor, whereas the synthesized KIT-6 and commercial meso-SiO2 (mesoporous SiO2, Qingdao Ocean Chemical Reagent Co., Ltd., China) were used as the supports for the Co/KIT-6 and Co/meso-SiO2 catalysts, respectively. The large mesoporous cubic Ia3d silica, which was previously designated as KIT-6, was prepared according to the procedure described by Ryoo et al. [22]. The prepared KIT-6 and meso-SiO2 were each appended in an aqueous cobaltous nitrate solution, which was prepared by dissolving a specified amount of cobaltous nitrate in deionized water. The mixtures were then evaporated at a fixed temperature, dried at 383 K for 24 h, and calcinated in air at 773 K for 3 h. All obtained catalyst precursors contained 20 wt% of the elementary substance cobalt which obtained according to the dosage of the used precursor nitrates. The catalyst precursors were then subjected to hydrogen reduction to produce the Co/KIT-6 and Co/meso-SiO2 catalysts. These catalysts were then used for CO2 catalytic hydrogenation to produce methane.
2.2.
Catalyst characterization
2.2.1.
H2-TPR studies
Hydrogen temperature program reduction (H2-TPR) was carried out as follows: 20 mg of the support cobalt precursor material was placed in a U-type quartz tube, 5.0 vol% H2/Ar gas mixture was passed through the tube at a flow rate of 25 ml min1, and the reduction temperature was raised from room temperature to 650 C at a rate of 10 C/min. The consumption of hydrogen was measured by a thermal conductivity detector (TCD).
2.2.2.
Low-angle XRD characterization
Low-angle XRD patterns were obtained on a Rigaku D/MaxA) radiation 2500/PC diffractometer with a Cu Ka (l ¼ 1.5418 source operated at 30 kV and 40 mA. The diffraction patterns were collected from 0.5 to 5.0 at a rate of 1 /min.
2.2.3.
BET characterization
The N2 adsorptionedesorption isotherms were obtained at 77 K using a Quantachrome Autosorb-1 instrument. The samples were degassed under vacuum for 4 h at 573 K prior to the measurements. The BET method was used to calculate the specific surface areas. The pore size distributions were derived from the desorption branches of the isotherms using the BarretteJoynereHalanda (BJH) method.
2.3.
TEM characterization
Transmission electron microscopy (TEM) was performed using a Tecnai G2 Spirit microscope operating at an acceleration voltage of 120 kV. Prior to TEM measurement, samples
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were prepared by ultrasonication in ethanol. A drop of the resultant suspension was evaporated on a carbon-coated copper grid.
2.4.
Catalytic activity measurement
Catalytic tests were conducted in a tubular, continuous-flow, fixed-bed quartz reactor (6 mm i.d.) under atmospheric pressure. The quartz tube reactor containing 100 mg of the supported cobalt oxide precursors were then placed inside the tubular furnace. High-purity hydrogen was flowed downward through the reactor containing the catalyst bed, while an electronic mass flow controller (D07-7A/ZM, China) was used to control the hydrogen gas flow rate. The catalyst samples were reduced at 400 C at a 100 ml min1 hydrogen flow rate for 150 min, followed by cooling under a H2 flow to the given temperature. The temperature was measured using a thermocouple projected into the catalyst bed and was monitored ¨ GU, model 708P, by a temperature programmable controller (U China). The feed gas mixture consisting of CO2 (10.0 vol%), H2 (46.0 vol%), and Ar (44.0 vol%) was passed over the catalysts at a flow rate of 36.67 ml min1, which is equivalent to a gas mass space velocity of 22,000 ml g1 h1. The CO2 catalytic hydrogenation conversion and selectivity data have been obtained and repeated at given temperature for 120 min. The gas products were monitored using an on-line gas chromatograph (GC 6890 II) equipped with a thermal conductivity detector (TCD).
3.
Results and discussion
3.1.
H2-TPR profiles
The H2-TPR test was conducted on the prepared Co3O4/KIT-6 and Co3O4/meso-SiO2 samples to study their reducible properties and determine their optimal preparation conditions. The TPR profiles are shown in Fig. 1, where the a, b, and g H2TPR peaks for the prepared support cobalt oxide precursor
samples appear at 220, 320, and 370 C, respectively. The samples prepared using KIT-6 and meso-SiO2 as supports have similar H2-TPR signals. The a and b peak intensities and peak areas are significantly weaker than those of g. However, the KIT-6esupported sample has high-intensity a and b peaks compared with the meso-SiO2-supported sample. The occurrence of multiple reduction peaks indicates the presence of a number of reducible cobalt species in the precursor samples. Co3O4 can be reduced by hydrogen to form Co via a two-step reduction (Co3O4 / CoO / Co0). The lowtemperature hydrogen consumption peaks a and b, which are located at 220 and 320 C, respectively, can be attributed to the reduction of the well-dispersed Co3O4 to produce CoO on the surfaces of the KIT-6 and meso-SiO2 supports [23]. The results indicate that large amounts of highly dispersed Co3O4 are present on the surfaces of KIT-6 and meso-SiO2. This result can be attributed to the large specific surface area of the two supports. The a and b peaks of the Co3O4/KIT-6 sample are obviously stronger than those of the Co3O4/meso-SiO2 sample. These results indicate that the cobalt oxide species exhibits better dispersion on the surface of the KIT-6 support than on the meso-SiO2 support. Therefore, the Co species can be dispersed well on the surface of the KIT-6 support to obtain a better-dispersed Co/KIT-6 catalyst through the hydrogen reduction of Co3O4/KIT-6. The g peak can be attributed to the reduction of bulk Co3O4, which are mainly present in the pore structure of the KIT-6 and meso-SiO2 supports. CoO, which is formed via Co3O4 reduction at low temperatures, can be reduced by hydrogen to produce Co, resulting in the appearance of a TPR peak for hydrogen consumption at high reduction temperatures [23]. Therefore, the g peak can also be attributed to the reduction of CoO. As shown in Fig. 1, most cobalt oxides can be reduced by hydrogen at 400 C. The results indicate that the cobalt oxide species can be completely reduced at 400 C for 150 min to form Co species. That is, the Co/KIT-6 and Co/meso-SiO2 catalysts can be prepared via hydrogen reduction at 400 C for 150 min. This result is consistent with those of previous reports [24].
3.2. γ
Intensity / a.u.
β
α
Co O /KIT-6
Co O /meso-SiO 100
150
200
250
300 350 400 Temperature / C
450
500
550
600
Fig. 1 e Studies of reducible property for supported cobalt oxide precursor.
3
BET characterization
The porosity of the prepared Co/KIT-6 and Co/meso-SiO2 catalysts were measured via N2 sorption determination. Fig. 2 gives the N2 adsorptionedesorption isotherm and the corresponding pore size distribution. The isotherms are typical type IV, with a H1 hysteresis loop. The prepared Co/KIT-6 catalyst displays a narrow pore size distribution, narrower than that of the Co/meso-SiO2 catalyst. The pore diameters of the Co/KIT-6 and Co/meso-SiO2 catalysts were determined as approximately 6.44 and 8.06 nm, respectively, based on the isotherm desorption branches obtained using the BJH method. The results indicate that the Co/KIT-6 and Co/meso-SiO2 catalysts have mesoporous structures. The specific surface areas of the Co/KIT-6 and Co/meso-SiO2 catalysts (approximately 368.9 and 185.8 m2 g1, respectively) were calculated based on the BET method. The Co/KIT-6 catalyst has a large specific surface area, about twice as large as that of the Co/meso-SiO2 catalyst. The large specific surface area can also promote the loading Co species to disperse on the support surface, thereby
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catalyst was not be observed at the 0.5 e5.0 range. This result indicates that the Co/meso-SiO2 catalyst has a disordered structure. Therefore, the ordered bicontinuous mesoporous structure facilitates the diffusion and transfer of the reactant and product molecules in the Co/KIT-6 catalyst, resulting in the improvement of its catalytic performance.
550 500
Quantity adsorbed(cm /g, STP)
450 400 350 300
3.4.
250
150 100 Co/meso-SiO
50
Co/KIT-6 0 0.0
0.1
0.2
0.3
0.4
0.5 P / P
0.6
0.7
0.8
0.9
1.0
Fig. 2 e N2 adsorptionedesorption isotherms and pore size distribution of Co/KIT-6 and Co/meso-SiO2 catalysts.
providing more activity centers for the CO2 catalytic hydrogenation reaction. At the same time, the wide pore diameter and narrow pore size distribution can provide favorable conditions for CO2 molecular adsorption and activation, thus leading to the high CO2 catalytic hydrogenation performance of the Co/KIT-6 catalyst.
3.3.
TEM characterization
200
TEM was performed to analyze the mesoporous structures of the prepared Co/KIT-6 and Co/meso-SiO2 catalysts as well as the dispersion of the Co species on the KIT-6 and meso-SiO2 supports. The TEM images of the prepared Co/KIT-6 and Co/ meso-SiO2 catalyst samples are shown in Fig. 4. The Co/KIT-6 catalyst displays an obvious ordered pore structure, whereas the Co/meso-SiO2 catalyst does not. A well-ordered threedimensional cubic array of mesopores is observed for the Co/ KIT-6 catalyst sample. The centers of two adjacent pores in this array are approximately 10.5 nm apart, and the pore diameter is about 5.2 nm. This result is consistent with those of the BET and low-angle XRD. Meanwhile, the pore structure
Low-angle XRD characterization
Intensity / a.u.
As previously discussed for H2-TPR, the Co/KIT-6 and Co/ meso-SiO2 catalysts can be prepared via hydrogen reduction. The low-angle XRD was used to study the ordered structure of the prepared Co/KIT-6 and Co/meso-SiO2 catalysts. The lowangle XRD of the Co/KIT-6 catalyst (Fig. 3) has a sharp intense peak at 2q ¼ 0.97 , which corresponds to the (211) plane. According to previous reports [25,26], the prepared Co/ KIT-6 catalyst has a well-ordered mesoporous structure and belongs to the Ia3d bicontinuous cubic symmetry space group. Meanwhile, the low-angle XRD peak of the Co/meso-SiO2
Co/KIT-6
Co/meso-SiO2 0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
2 theta /
Fig. 3 e Low-angle XRD patterns for the Co/KIT-6 and Co/ meso-SiO2 catalysts.
Fig. 4 e TEM graphs of Co/KIT-6 and Co/meso-SiO2 catalysts.
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of the Co/meso-SiO2 catalyst cannot be clearly observed. The TEM image does not show the presence of any significant cobalt agglomerates on the KIT-6 and meso-SiO2 surfaces, thus indicating that the Co species are well-dispersed on the KIT-6 and meso-SiO2 surfaces as well as in the mesopores. The effective dispersion of the Co species on the surface of the KIT6 and meso-SiO2 supports can be attributed to the large surface area and wide pore diameter of the KIT-6 and meso-SiO2 supports. This result is consistent with those of the H2-TPR analysis. In general, a well-dispersed Co species can provide active sites for the hydrogenation reaction.
3.5.
Catalytic activity test
The catalytic hydrogenation reaction using the prepared Co/ KIT-6 and meso-SiO2 catalysts was investigated at a hydrogenecarbon dioxide ratio (H2/CO2) of 4.6, a space velocity of 22,000 ml g1 h1, and at different reaction temperatures. The results are shown in Fig. 5a and b. Fig. 5a shows that the Co/
a
55 50 45 40
Conversion / %
35 30 25 20 15 10 Co/KIT-6 Co/meso-SiO
5 0 200
210
220
230
240
250
260
270
280
290
300
Temperature / C
b 100 95 90
Methane selectivity / %
85 80 75 70 65 60 Co/KIT-6 Co/meso-SiO
55 50 200
210
220
230
240
250
260
270
280
290
300
Temperature / C
Fig. 5 e a. The relation between the carbon dioxide conversion and reaction temperature on Co/KIT-6 and Co/ meso-SiO2 catalysts. b. The relation between the methane selectivity and reaction temperature on Co/KIT-6 and Co/ meso-SiO2 catalysts.
5
KIT-6 catalyst has a higher catalytic hydrogenation activity for CO2 than the Co/meso-SiO2 catalyst. In addition, the CO2 conversion increases with increasing reaction temperature. Meanwhile, Fig. 5b shows that the Co/KIT-6 catalyst has higher methane selectivity than the Co/meso-SiO2 catalyst at temperatures above 260 C. The CO2 conversion on the Co/KIT-6 catalyst increases from 15.5% to 48.9% when the reaction temperature increases from 200 C to 280 C. The main hydrogenation product using this catalyst is methane, and the selectivity is retained at 100%. And 51.0% CO2 conversion and 98.9% methane selectivity are observed when the reaction temperature increases to 300 C. For the Co/meso-SiO2 catalyst, the CO2 conversion and methane selectivity at 280 C is 40.0% and 94.1%, respectively. However, the carbon conversion increases to 43.5% and the methane selectivity sharply decreases to 78.7% when the reaction temperature increases to 300 C. The Co/KIT-6 catalyst shows an excellent catalytic performance compared with the Pd catalysts in a previous report [9]. CO2 can be hydrogenated to produce methane on the appropriate catalyst. The hydrogenation methanation reaction can be expressed as CO2 þ 4H2 / CH4 þ 2H2O. The reverse wateregas shift (RWGS) reaction may occur for hydrogen gas, with the reaction expressed as CO2 þ H2 / CO þ H2O [10]. The presence of CO was not detected, indicating that the RWGS reaction did not occur on the prepared Co/KIT-6 and Co/mesoSiO2 catalysts during CO2 catalytic hydrogenation. The other catalytic hydrogenation products, including methanol and other C2þ compounds, were not detected, except for methane. These results indicate that methane is the main product of the CO2 catalytic hydrogenation on the Co/KIT-6 and Co/mesoSiO2 catalysts. As the reaction temperature is increased, more reactant molecules (including CO2 and H2) can be induced to participate in the CO2 hydrogenation reaction, thereby resulting in high catalytic activity in the CO2 catalytic hydrogenation at high temperatures. The high catalytic activity of Co/KIT-6 and Co/meso-SiO2 indicates that the Co species, which was prepared via hydrogen reduction of cobalt oxides, is an important catalytic active species for CO2 catalytic hydrogenation. The TEM results indicate that the Co species were well-dispersed on the KIT-6 and meso-SiO2 support surfaces, thereby providing more active centers for the formation of active H2 and CO2 species. The BET results show that the prepared Co/KIT-6 and Co/meso-SiO2 catalysts have large specific surface areas that exceed 180 m2 g1 as well as wide pore diameters of approximately 6.44 nm. A large specific surface area can provide a large space for more active centers to adsorb and activate the reactant molecules, resulting in the formation of more active species for the CO2 catalytic hydrogenation reaction. At the same time, the wide pore diameter can provide favorable conditions for the reactant molecules to diffuse and transfer in the catalyst. More hydrogen molecules can then be activated to form active hydrogen species in the hydrogenation reaction, thereby improving the CO2 catalytic hydrogenation activity of the Co/ KIT-6 and Co/meso-SiO2 catalysts. As shown in Fig. 5, the Co/ KIT-6 catalyst has higher CO2 catalytic hydrogenation activity than the Co/meso-SiO2 catalyst. These results can be attributed to the large specific surface area and ordered mesoporous structure of the Co/KIT-6 catalyst, as indicated by the BET and TEM results.
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The low-angle XRD, BET, and TEM results indicate that the prepared Co/KIT-6 catalyst has a large specific surface area, highly dispersed Co species, wide pore diameter, narrow pore size distribution, and highly ordered mesoporous structure. The large specific surface area promotes the adsorption and activation of the reactant molecules (CO2 and hydrogen), resulting in the improvement of the CO2 catalytic hydrogenation. The prepared Co/KIT-6 catalyst has highly dispersed Co species, which can provide more active centers for the CO2 catalytic hydrogenation. The high hydrogenation selectivity can also be attributed to the highly ordered mesoporous structure and wide pore diameter of the Co/KIT-6 catalyst. This highly ordered and bicontinuous mesoporous structure and the wide pore diameter can provide superior conditions for the diffusion of the methane molecules produced and their transplantation in the Co/KIT-6 catalyst. In particular, the bicontinuous mesoporous structure can ensure that the methane molecules produced in the pores of the Co/KIT-6 catalyst are transplanted freely. The methane molecules produced by CO2 hydrogenation can be easily desorbed from the Co/KIT-6 catalyst to the gas phase to achieve high methane selectivity. On the other hand, the Co/meso-SiO2 catalyst does not have the bicontinuous mesoporous structure, thereby limiting the diffusion and transplantation of the methane molecule produced and promoting methane decomposition at high reaction temperatures. Therefore, the methane produced has low selectivity at high reaction temperatures.
4.
Conclusions
(1) The H2-TPR results indicate that the supported cobalt oxide precursor can be reduced by hydrogen to form the Co/KIT-6 and Co/meso-SiO2 catalysts. TEM results show that the formed Co species are well-dispersed on the surfaces of the KIT-6 and meso-SiO2 supports. (2) The low-angle XRD, TEM, and BET results show that the prepared Co/KIT-6 and Co/meso-SiO2 catalysts have large specific surface areas and high supported Co species dispersion. In addition, these catalysts exhibit mesoporous structures, particularly the prepared Co/KIT-6 catalyst, which has a highly ordered bicontinuous mesoporous structure. (3) The Co/KIT-6 catalyst has a higher CO2 catalytic hydrogenation activity than the Co/meso-SiO2 catalyst. The CO2 conversion can exceed 48.9%, and the methane selectivity can be retained at 100% until 280 C. The Co/KIT-6 catalyst exhibits higher methane selectivity than the Co/meso-SiO2 catalyst at high reaction temperatures; this result can be attributed to the highly ordered bicontinuous mesoporous structure of the Co/KIT-6 catalyst.
Acknowledgments The work was financially supported by Youth Doctoral Fund of Chongqing Technology and Business University (No. 1152011).
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