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SiO2 catalyst
JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGY Volume 37, Issue 5, October 2009 Online English edition of the Chinese language journal Cite this article as: J Fuel Chem Technol, 2009, 37(5), 578582
RESEARCH PAPER
Effect of ZrO2 promoter on the catalytic activity for CO methanation and adsorption performance of the Ni/SiO2 catalyst WU Rui-fang, ZHANG Yin, WANG Yong-zhao, GAO Chun-guang, ZHAO Yong-xiang* School of Chemistry and Chemical Engineering, Engineering Research Center of Ministry of Education for Fine Chemicals, Shanxi University, Taiyuan 030006, China
Abstract: The catalytic activities for CO methanation and adsorption performances of the Ni/SiO2 and Ni/ZrO2-SiO2 catalysts were investigated by a continuous flowing microreactor apparatus and in-situ diffuse reflectance Fourier transform infrared spectroscopy. The results showed that CO was completely converted at 200qC over the Ni/ZrO2-SiO2 catalyst under the reaction condition of 1% of CO, GHSV of 5000 h–1, and the atmospheric pressure. However, the CO conversion was only 35% over the Ni/SiO2 catalyst under the same reaction condition, and CO was not completely converted until 270qC, which suggested that the catalytic activity of the Ni/ZrO2-SiO2 catalyst increased with the addition of ZrO2 promoter. Meanwhile, the addition of ZrO2 promoter enhanced the CO adsorption capacity of the Ni/ZrO2-SiO2 catalyst. In the presence of H2, a large number of bridged carbonyl hydrides were formed over the Ni/ZrO2-SiO2 catalyst at lower temperatures, resulting in high catalytic activity for CO methanation. In CO methanation, the breaking of C—O bond over catalysts was by multihydrogen carbonyl hydride rather than by direct breaking. Key words: Ni/SiO2 catalyst; ZrO2 promoter; CO methanation; adsorption
CO methanation is an important reaction involved in ammonia synthesis, hydrogen production in petrochemical industry, energy development, and so on[1,2]. In recent years, with the rapid growth of the demand for natural gas and the need of energy saving and environmental protection, the synthesis of the substitute natural gas by methanation from coke oven gas has attracted a great deal of attention in academia and industry and has become an important direction for the development of coal chemical industry[3,4]. At present, nickel-based catalysts are commonly used in the methanation of a small amount of CO in hydrogen-rich gases. The commonly used supports include D- or J-Al2O3, kaolin, and calcium aluminate cement, and the promoters are alkali metal Na, alkaline-earth metal Mg, rare earth metals, and so on. ZrO2 possesses good chemical stability, redox properties, and acid-base properties; thus, it has been widely used as a promoter. Li et al[5] found that the addition of ZrO2 promoter into the CuZnAlO catalyst could increase methanol conversion and the yield and selectivity of H2 in steam-reforming reaction of methanol, resulting in good catalytic activity. Zhou et al[6] found that the addition of ZrO2
promoter could inhibit the deactivation of Co/SiO2 catalyst and improve its stability for F-T synthesis. Nickel-based catalyst supported on SiO2 aerogel that was promoted with ZrO2 was studied less in CO methanation. In this article, the Ni/SiO2 and Ni/ZrO2-SiO2 catalysts were prepared by the incipient wetness impregnation. Their catalytic activities for the methanation of a small amount of CO in hydrogen-rich gases were investigated by a continuous flowing microreactor apparatus, aiming at investigating the effect of ZrO2 promoter on the catalytic activity of the Ni/SiO2 catalyst. Meanwhile, CO adsorption and methanation over the catalysts were traced with in-situ diffuse reflectance Fourier transform infrared spectroscopy (in-situ DRIFTS) to obtain the information concerning intermediates, and then, the possible reasons for the effect of ZrO2 promoter on the catalytic activity of the Ni/SiO2 catalyst were given and the mechanism of CO methanation was revealed.
1 1.1
Experimental Catalyst preparation
Received: 06-Jan-2009; Revised: 20-Apr-2009 * Corresponding author. Tel: +86-351-7011587, Fax: +86-351-7011688, E-mail: [email protected] Foundation item: Supported by the Scientific and Technological Project of Shanxi Province (20080321017) and the National High Technology Research and Development Program of China (863 Program, 2005AA001050). Copyright 2009, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.
WU Rui-fang et al. / Journal of Fuel Chemistry and Technology, 2009, 37(5): 578582
The SiO2 aerogel support promoted with ZrO2 was prepared according to our previous work[7]. The preparation procedure is as follows: the desired amount of zirconyl nitrate dehydrate (ZrO(NO3)2·2H2O) was dissolved in anhydrous C2H5OH at room temperature. Then, acetic acid, water, and tetraethoxysilicane (TEOS) were added under constant stirring. The above mixture was dried in a stainless-steel tank by supercritical drying, and the 7%ZrO2-SiO2 aerogel support was obtained and calcined at 400qC for 3 h. The SiO2 aerogel support was prepared by the same process without adding ZrO(NO3)2·2H2O. The 30w%Ni/SiO2 and 30%Ni/7%ZrO2-SiO2 catalysts were prepared by impregnating the above supports with an aqueous solution of Ni(NO3)2·6H2O with stirring and placing overnight, followed by drying, calcining, and reducing with H2. 1.2
was first swept by N2 at room temperature for 30 min to remove the air, then reduced at 400qC for 1 h with the flowing H2 under atmospheric pressure, and finally cooled to room temperature (12qC). The background spectrum was recorded with the flowing H2. The samples were then treated with CO or the feed gas consisting of 1% of CO and 99% of H2 through the cell, with a flow rate of 15 mL/min and a heating rate of 10qC/min. The spectra were recorded at required temperature. The DRIFT spectra of the adsorbed species on the catalyst surface were obtained by subtracting the background spectra. A total of 128 scans were collected at 4 cmí1 resolution.
2
Results and discussion
2.1 Effect of reaction temperature on the activities of catalysts
Activity test of catalyst
The measurements of catalytic activity for CO methanation were carried out with a continuous flowing microreactor apparatus under atmospheric pressure. Approximately 0.1 g of catalyst was used for each run. The feed gas consisted of 1% of CO and 99% of H2 flowed through fully dried silica gel, and 0.5 nm molecular sieve was used to remove moisture before it was placed in the reaction tube. The reaction temperature ranged between 100 and 300qC. Concentration of CO before and after reaction was analyzed with GC-930 type gas chromatograph equipped with a 3-m column packed with carbon molecular sieve, a methanator, and a flame ionization detector (FID) under the condition of GHSV of 5000 h–1. Prior to all catalytic experiments, the catalysts were pretreated in flowing H2 at 400qC for 1 h to yield clean surface and then cooled in the presence of flowing H2. The products are methane and water only, so the activities of the catalysts were evaluated by CO conversion and determined by the following equation:
x
(1
S cco S co
The effect of reaction temperature on the activities of catalysts is shown in Fig.1. As seen from Fig.1, the CO methanation occurred at the initial temperature of 120qC on the both catalysts. In the range 120–200qC, CO conversion showed an evident upward trend with increasing temperature, but at the same temperature, CO conversion over the Ni/ZrO2-SiO2 catalyst was higher than that of over the Ni/Si catalyst. CO could be completely converted to CH4 over the Ni/ZrO2-SiO2 catalyst at 200qC, but CO conversion was only 35% under the same reaction conditions over the Ni/SiO2 catalyst, over which CO was not completely converted until 270qC. Apparently, the addition of ZrO2 promoter improved remarkably the catalytic activity of the Ni/SiO2 catalyst for CO methanation. 2.2 In-situ DRIFT spectra of CO and H2 adsorbed on the Ni/SiO2 catalyst
) u 100%
where x denoted CO conversion rate, S'CO denoted the peak area of the remaining CO after reaction, and SCO denoted the peak area of CO in feed gas. 1.3
In-situ DRIFTS characterization
In-situ DRIFT spectra were recorded using TENSOR 27 FTIR spectrometer (Bruker) equipped with a HVC-DRP-3-type DRIFT cell, ATC-024-2-type temperature-controlled instrument, and an MCT detector. A certain amount of catalyst was finely ground with an agate mortar and pestle and placed into the DRIFT cell where it
Fig. 1
Effect of reaction temperature on the activities of catalysts
Conditions: 1% of CO in H2; GHSV of 5000 h–1; an atmospheric pressure : Ni/ZrO2-SiO2; : Ni/SiO2
WU Rui-fang et al. / Journal of Fuel Chemistry and Technology, 2009, 37(5): 578582
Fig. 2
In-situ DRIFTS of CO adsorbed on the Ni/SiO2 catalyst at
Fig. 3
In-situ DRIFTS of CO and H2 co-adsorbed on the Ni/SiO2
different temperatures
catalyst at different temperatures
Figures 2 and 3 show the DRIFT spectra of CO adsorption or coadsorption of CO and H2 at different temperatures over the Ni/SiO2 catalyst, respectively. As can be seen from Fig. 2, for pure CO adsorption, the bands at 2174 and 2118 cm–1 observed at room temperature were assigned to gaseous CO[8]. The band at 2057 cm–1 and the weak band at 1920 cm–1 could be assigned to CO adsorbed on metal Ni in linear form and bridge form, respectively[9,10]. In the range 12–100qC, the peak intensity of linear CO became stronger gradually, but the peak intensity of bridged CO did not change with increasing temperature, indicating that increase in temperature was favorable for the formation of linear CO. At 200qC, the peak intensity of linear CO decreased significantly and the bands appeared at 2359 and 2341 cm–1, which could be assigned to gaseous CO2. The CO2 formation could originate from the dissociation of CO (CO disproportionation) accompanying with linear CO desorption[11]. With increasing temperature further, the peak intensity of bridged CO became stronger gradually until the maximum at 300qC, but the peak intensity of linear CO also became stronger gradually, and its position shifted to higher wave number at 300qC, reading as low as 2070 cm–1. This shift originated from a new band at higher wave number overlapping with the band of residual linear CO with increasing temperature. The appearance of this new band suggested that there could be newly formed adsorbed species or adsorption sites at higher temperatures[12]. For the coadsorption of CO and H2 (Fig. 3), the bands of gaseous CO also appeared at 2174 and 2118 cm–1 at room temperature, but their intensities were very weak due to smaller volume fraction of CO in the feed gas. Compared with the linear CO and bridged CO from Fig. 2, two new bands observed at 2048 and 1909 cm–1 shifted to lower wave number, which could be attributed to the formation of linear and bridged carbonyl hydrides
electron-donating property of H enhanced the electron-feedback capacity of Ni to CO, and thus, the bond of Ni-C became stronger and the bond of C—O became weaker, making ȞCO shift to lower wave number. With increasing temperature, the band at 1909 cm–1 did not shift but became stronger gradually until 100qC and weaker beyond 100qC. Meanwhile, the band at 2048 cm–1 became weaker first and then stronger until the maximum at 200qC, and its position shifted to 2060 cm–1 with increasing temperature. This shift suggested that there could be newly formed adsorbed species, which could be assigned to the carbonyl hydride originated from the adsorbed species at 2070 cm–1 in Fig. 2. At 300qC, the band at 1909 cm–1 became weaker, and the band at 2060 cm–1 became weaker significantly and its position shifted to 2040 cm–1. This shift suggested that the carbonyl hydride could be converted to the
(
,
)[11–13],
The
) on multihydrogen carbonyl hydride ( linear adsorption sites[13]. Thus, with increasing temperature, the more the number of dissociative H, the stronger the electron-donating capacity of H, leading to the enhancement of the electron-feedback capacity of Ni to adsorbed CO, and thus, the bond of Ni-C became stronger and the bond of C—O became weaker, resulting in the shift of the band to lower wave number. In addition, a weak band at 3016 cm–1 was observed, which could be assigned to gaseous CH4[14], and a stronger band appeared at 3728 cm–1, suggesting the formation of water. Apparently, the carbonyl hydride was the key intermediate product in the process of the breaking of C—O bond. 2.3 In-situ DRIFT spectra of CO and H2 adsorbed on the Ni/ZrO2-SiO2 catalyst The DRIFT spectra of CO adsorption at different temperatures over the Ni/ZrO2-SiO2 catalyst are shown in Fig. 4.
WU Rui-fang et al. / Journal of Fuel Chemistry and Technology, 2009, 37(5): 578582
Fig. 4
In-situ DRIFTS of CO adsorbed on the Ni/ZrO2-SiO2 catalyst at different temperatures A: 12–100qC
Fig. 5
In-situ DRIFTS of CO and H2 coadsorbed on the
Ni/ZrO2-SiO2 catalyst at different temperatures
In Fig. 4, the bands of gaseous CO, linear CO, and bridged CO are observed at 2174 and 2118, 2056, and 1915 cm–1 after CO was introduced at room temperature, respectively. Compared with the Ni/SiO2 catalyst, the peak intensity of linear CO and bridged CO increased obviously over the Ni/ZrO2-SiO2 catalyst, indicating the adsorption capacity of these catalysts for CO was enhanced remarkably due to the addition of ZrO2 promoter. In the range 12–100qC, the peak intensity of linear CO was several times stronger than that of over the Ni/SiO2 catalyst at the same temperature. Above 100qC, the changes of the bands over the Ni/ZrO2-SiO2 catalyst had the same trend as those over the Ni/SiO2 catalyst with further increasing the temperature. For the coadsorption of CO and H2 (Fig. 5), two new bands assigned to linear and bridged carbonyl hydrides were also observed at 2046 and 1908 cm–1 at room temperature, respectively. Compared with the Ni/SiO2 catalyst, the peak intensity of the bridged carbonyl hydride species increased significantly, but the peak intensity of the linear carbonyl hydride species decreased slightly over the Ni/ZrO2-SiO2 catalyst. Apparently, for the coadsorption of CO and H2, the addition of ZrO2 promoter was favorable for the formation of
B: 200–300qC
the bridged carbonyl hydride species. At 100qC, the band at 2046 cm–1 shifted to 2060 cm–1, and the intensity of the peak at 1908 cm–1 became stronger obviously, indicating the formation of a large number of the bridged carbonyl hydrides on the catalyst surface. At 200qC, the intensity of the peak at 2060 cm–1 became stronger remarkably and reached the maximum, but the band at 1908 cm-1 became weaker obviously. Meanwhile, the band of gaseous CH4 appeared at 3016 cm–1. As the temperature increased further, the changes of the bands over the Ni/ZrO2-SiO2 catalyst had the same trend as those over the Ni/SiO2 catalyst. The results suggested that the addition of ZrO2 promoter led to the formation of a larger number of the bridged carbonyl hydrides over the Ni/ZrO2-SiO2 catalyst at lower temperatures. From the DRIFT spectra of coadsorption of CO and H2 over the two catalysts, it was found that CO was first converted to carbonyl hydride in the presence of the dissociative H and then converted to the multihydrogen carbonyl hydride, resulting in the weakening and breaking of C—O bond. , In CO methanation, the weakening and breaking of C—O bond over those catalysts were by multihydrogen carbonyl hydride rather than by direct breaking. The carbonyl hydride intermediate mechanism was proposed for the following reasons: (1) The direct disproportionation of CO occurred at higher temperature (200qC) than that (120qC) for the methanation, (2) The linear and bridged carbonyl hydrides were formed at room temperature and the condition required for the direct breaking of the C—O bond did not exist, (3) The absence of CO2 in the methanation indicated that the direct disproportionation of CO did not occur on the catalyst surface, and (4) The changes of the linear and bridged carbonyl hydrides in peak intensity were in accord with that for CH4 formation. The activity of adsorbed CO was related directly to the proportion of the d-ʌ feedback bond originated from the
WU Rui-fang et al. / Journal of Fuel Chemistry and Technology, 2009, 37(5): 578582
interaction between CO and the metal M. The larger the proportion of d-ʌ feedback bond, the stronger the electron-feedback capacity of the metal M to adsorbed CO, and thus, the M-C bond became stronger and the C—O bond became weaker, making CO activated more easily. Hence, the activity of adsorbed CO on the catalyst surface was as follows: bridged CO > linear CO > twin CO. Thus, the more the bridged carbonyl hydrides formed on the catalyst surface, the larger the probability of the breaking of C—O bond, leading to the formation of more surface carbon, which could be hydrogenated to CH4. Compared with the Ni/SiO2 catalyst, more bridged carbonyl hydrides were formed over the Ni/ZrO2-SiO2 catalyst at lower temperatures, which may be contributed to higher catalytic activity for CO methanation.
3
Conclusions
Progress, 2007, 26(9): 1269–1273. [3] Li H C. The device of hydrogen and artificial substitute natural gas
co-produced
from
coke
oven
gas:
CN,
ZL
200620033245.5[P], 2007-05-02. [4] Tao P W, Wang X D. Production of compressed natural gas from coke oven gas. Coal Chemical Industry, 2007, 35(3): 11–14. [5] Li Y F, Dong X F, Lin W M. Effects of ZrO2-promoter on catalytic performance of CuZnAlO catalysts for production of hydrogen by steam reforming of methanol. Int J Hydrogen Energy, 2004, 29(15): 1617–1621. [6] Zhou W, Fang K G, Chen J G, Sun Y H. Effect of ZrO2 on the stability of Co/SiO2 catalyst for Fischer-Tropsch synthesis. Journal of Fuel Chemistry and Technology, 2006, 34(4): 461–465. [7] Gao C G, Zhao Y X, Liu D S. Liquid phase hydrogenation of maleic anhydride over nickel catalyst supported on ZrO2-SiO2
CO was completely converted at 270qC over the Ni/SiO2 catalyst under the reaction condition of CO of 1%, GHSV of 5000 h–1, and atmospheric pressure. The addition of ZrO2 promoter improved the catalytic activity of the Ni/ZrO2-SiO2 catalyst significantly, over which CO was completely converted at 200qC under the same reaction condition. In CO methanation, the breaking of C—O bond over the Ni/SiO2 and Ni/ZrO2-SiO2 catalysts was by multihydrogen carbonyl hydride rather than by direct breaking. The addition of ZrO2 promoter remarkably enhanced the adsorption capacity of the Ni/ZrO2-SiO2 catalyst for CO, and in the presence of H2, more bridged carbonyl hydrides were formed at lower temperatures over the Ni/ZrO2-SiO2 catalyst, which may be contributed to higher catalytic activity for CO methanation.
References
composite aerogel. Catal Lett, 2007, 118(1/2): 50–54. [8] Li Z H, Song Y, Wang B W, Ma X B, Xu G H. In situ IR study on adsorption of CO on supported Pd catalyst. Natural Gas Chemical Industry, 2002, 27(6): 25–28. [9] Hu C W, Chen Y Q, LI P, Min H, Chen Y, Tian A M. On the interaction of CO and H2 with Ni-based catalyst. Journal of Molecular Cata1ysis (China), 1995, 9 (6): 435–444. [10] Chen S Y, Sun Y H, Ding Y J, Zhou R X, Luo M F. Adsorption and Catalysis. Henan: Henan Science Technology Press, 2001. [11] Chen W M, Ding Y J, Wang T, Jiang D H, Li X M, Luo H Y. Effect of H2 on adsorption and desorption behavior of CO over Rh-Mn-Li-Ti/SiO2 catalyst. Chinese Journal of Catalysis, 2005, 26(12): 1099–1103. [12] Song D C, Li J L, Cai Q. In situ diffuse reflectance FTIR study of CO adsorbed on a cobalt catalyst supported by silica with different pore sizes. J Phys Chem C, 2007, 111(51): 18970–18977.
[1] Kustov A L, Frey A M, Larsen K E, Johannessen T, Nørskov J
[13] Chen Y Q, Gong M C, Cao Z, Zhou J L, Chen Y, Xin Q, Guo
K, Christensen C H. CO methanation over supported
X X. Study of Rh-Co bimetal catalysts: VI Surface carbonyl
bimetallic Ni-Fe catalysts: From computational studies
hydrides and their dynamic behavior on Rh2Co2/Al2O3.
towards catalyst optimization. Appl Catal A, 2007, 320(8): 98–104. [2] Zhang C. Research progress of methanation of carbon monoxide and carbon dioxide. Chemical Industry Engineering
Chinese Journal of Catalysis, 1998, 19(1): 9–13. [14] Jiang Q. Mechanism of CO2 methanation over Ru/ZrO2 catalyst. Journal of Fuel Chemistry and Technology, 2001, 29(1): 87–90.
RESEARCH PAPER
Effect of ZrO2 promoter on the catalytic activity for CO methanation and adsorption performance of the Ni/SiO2 catalyst WU Rui-fang, ZHANG Yin, WANG Yong-zhao, GAO Chun-guang, ZHAO Yong-xiang* School of Chemistry and Chemical Engineering, Engineering Research Center of Ministry of Education for Fine Chemicals, Shanxi University, Taiyuan 030006, China
Abstract: The catalytic activities for CO methanation and adsorption performances of the Ni/SiO2 and Ni/ZrO2-SiO2 catalysts were investigated by a continuous flowing microreactor apparatus and in-situ diffuse reflectance Fourier transform infrared spectroscopy. The results showed that CO was completely converted at 200qC over the Ni/ZrO2-SiO2 catalyst under the reaction condition of 1% of CO, GHSV of 5000 h–1, and the atmospheric pressure. However, the CO conversion was only 35% over the Ni/SiO2 catalyst under the same reaction condition, and CO was not completely converted until 270qC, which suggested that the catalytic activity of the Ni/ZrO2-SiO2 catalyst increased with the addition of ZrO2 promoter. Meanwhile, the addition of ZrO2 promoter enhanced the CO adsorption capacity of the Ni/ZrO2-SiO2 catalyst. In the presence of H2, a large number of bridged carbonyl hydrides were formed over the Ni/ZrO2-SiO2 catalyst at lower temperatures, resulting in high catalytic activity for CO methanation. In CO methanation, the breaking of C—O bond over catalysts was by multihydrogen carbonyl hydride rather than by direct breaking. Key words: Ni/SiO2 catalyst; ZrO2 promoter; CO methanation; adsorption
CO methanation is an important reaction involved in ammonia synthesis, hydrogen production in petrochemical industry, energy development, and so on[1,2]. In recent years, with the rapid growth of the demand for natural gas and the need of energy saving and environmental protection, the synthesis of the substitute natural gas by methanation from coke oven gas has attracted a great deal of attention in academia and industry and has become an important direction for the development of coal chemical industry[3,4]. At present, nickel-based catalysts are commonly used in the methanation of a small amount of CO in hydrogen-rich gases. The commonly used supports include D- or J-Al2O3, kaolin, and calcium aluminate cement, and the promoters are alkali metal Na, alkaline-earth metal Mg, rare earth metals, and so on. ZrO2 possesses good chemical stability, redox properties, and acid-base properties; thus, it has been widely used as a promoter. Li et al[5] found that the addition of ZrO2 promoter into the CuZnAlO catalyst could increase methanol conversion and the yield and selectivity of H2 in steam-reforming reaction of methanol, resulting in good catalytic activity. Zhou et al[6] found that the addition of ZrO2
promoter could inhibit the deactivation of Co/SiO2 catalyst and improve its stability for F-T synthesis. Nickel-based catalyst supported on SiO2 aerogel that was promoted with ZrO2 was studied less in CO methanation. In this article, the Ni/SiO2 and Ni/ZrO2-SiO2 catalysts were prepared by the incipient wetness impregnation. Their catalytic activities for the methanation of a small amount of CO in hydrogen-rich gases were investigated by a continuous flowing microreactor apparatus, aiming at investigating the effect of ZrO2 promoter on the catalytic activity of the Ni/SiO2 catalyst. Meanwhile, CO adsorption and methanation over the catalysts were traced with in-situ diffuse reflectance Fourier transform infrared spectroscopy (in-situ DRIFTS) to obtain the information concerning intermediates, and then, the possible reasons for the effect of ZrO2 promoter on the catalytic activity of the Ni/SiO2 catalyst were given and the mechanism of CO methanation was revealed.
1 1.1
Experimental Catalyst preparation
Received: 06-Jan-2009; Revised: 20-Apr-2009 * Corresponding author. Tel: +86-351-7011587, Fax: +86-351-7011688, E-mail: [email protected] Foundation item: Supported by the Scientific and Technological Project of Shanxi Province (20080321017) and the National High Technology Research and Development Program of China (863 Program, 2005AA001050). Copyright 2009, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.
WU Rui-fang et al. / Journal of Fuel Chemistry and Technology, 2009, 37(5): 578582
The SiO2 aerogel support promoted with ZrO2 was prepared according to our previous work[7]. The preparation procedure is as follows: the desired amount of zirconyl nitrate dehydrate (ZrO(NO3)2·2H2O) was dissolved in anhydrous C2H5OH at room temperature. Then, acetic acid, water, and tetraethoxysilicane (TEOS) were added under constant stirring. The above mixture was dried in a stainless-steel tank by supercritical drying, and the 7%ZrO2-SiO2 aerogel support was obtained and calcined at 400qC for 3 h. The SiO2 aerogel support was prepared by the same process without adding ZrO(NO3)2·2H2O. The 30w%Ni/SiO2 and 30%Ni/7%ZrO2-SiO2 catalysts were prepared by impregnating the above supports with an aqueous solution of Ni(NO3)2·6H2O with stirring and placing overnight, followed by drying, calcining, and reducing with H2. 1.2
was first swept by N2 at room temperature for 30 min to remove the air, then reduced at 400qC for 1 h with the flowing H2 under atmospheric pressure, and finally cooled to room temperature (12qC). The background spectrum was recorded with the flowing H2. The samples were then treated with CO or the feed gas consisting of 1% of CO and 99% of H2 through the cell, with a flow rate of 15 mL/min and a heating rate of 10qC/min. The spectra were recorded at required temperature. The DRIFT spectra of the adsorbed species on the catalyst surface were obtained by subtracting the background spectra. A total of 128 scans were collected at 4 cmí1 resolution.
2
Results and discussion
2.1 Effect of reaction temperature on the activities of catalysts
Activity test of catalyst
The measurements of catalytic activity for CO methanation were carried out with a continuous flowing microreactor apparatus under atmospheric pressure. Approximately 0.1 g of catalyst was used for each run. The feed gas consisted of 1% of CO and 99% of H2 flowed through fully dried silica gel, and 0.5 nm molecular sieve was used to remove moisture before it was placed in the reaction tube. The reaction temperature ranged between 100 and 300qC. Concentration of CO before and after reaction was analyzed with GC-930 type gas chromatograph equipped with a 3-m column packed with carbon molecular sieve, a methanator, and a flame ionization detector (FID) under the condition of GHSV of 5000 h–1. Prior to all catalytic experiments, the catalysts were pretreated in flowing H2 at 400qC for 1 h to yield clean surface and then cooled in the presence of flowing H2. The products are methane and water only, so the activities of the catalysts were evaluated by CO conversion and determined by the following equation:
x
(1
S cco S co
The effect of reaction temperature on the activities of catalysts is shown in Fig.1. As seen from Fig.1, the CO methanation occurred at the initial temperature of 120qC on the both catalysts. In the range 120–200qC, CO conversion showed an evident upward trend with increasing temperature, but at the same temperature, CO conversion over the Ni/ZrO2-SiO2 catalyst was higher than that of over the Ni/Si catalyst. CO could be completely converted to CH4 over the Ni/ZrO2-SiO2 catalyst at 200qC, but CO conversion was only 35% under the same reaction conditions over the Ni/SiO2 catalyst, over which CO was not completely converted until 270qC. Apparently, the addition of ZrO2 promoter improved remarkably the catalytic activity of the Ni/SiO2 catalyst for CO methanation. 2.2 In-situ DRIFT spectra of CO and H2 adsorbed on the Ni/SiO2 catalyst
) u 100%
where x denoted CO conversion rate, S'CO denoted the peak area of the remaining CO after reaction, and SCO denoted the peak area of CO in feed gas. 1.3
In-situ DRIFTS characterization
In-situ DRIFT spectra were recorded using TENSOR 27 FTIR spectrometer (Bruker) equipped with a HVC-DRP-3-type DRIFT cell, ATC-024-2-type temperature-controlled instrument, and an MCT detector. A certain amount of catalyst was finely ground with an agate mortar and pestle and placed into the DRIFT cell where it
Fig. 1
Effect of reaction temperature on the activities of catalysts
Conditions: 1% of CO in H2; GHSV of 5000 h–1; an atmospheric pressure : Ni/ZrO2-SiO2; : Ni/SiO2
WU Rui-fang et al. / Journal of Fuel Chemistry and Technology, 2009, 37(5): 578582
Fig. 2
In-situ DRIFTS of CO adsorbed on the Ni/SiO2 catalyst at
Fig. 3
In-situ DRIFTS of CO and H2 co-adsorbed on the Ni/SiO2
different temperatures
catalyst at different temperatures
Figures 2 and 3 show the DRIFT spectra of CO adsorption or coadsorption of CO and H2 at different temperatures over the Ni/SiO2 catalyst, respectively. As can be seen from Fig. 2, for pure CO adsorption, the bands at 2174 and 2118 cm–1 observed at room temperature were assigned to gaseous CO[8]. The band at 2057 cm–1 and the weak band at 1920 cm–1 could be assigned to CO adsorbed on metal Ni in linear form and bridge form, respectively[9,10]. In the range 12–100qC, the peak intensity of linear CO became stronger gradually, but the peak intensity of bridged CO did not change with increasing temperature, indicating that increase in temperature was favorable for the formation of linear CO. At 200qC, the peak intensity of linear CO decreased significantly and the bands appeared at 2359 and 2341 cm–1, which could be assigned to gaseous CO2. The CO2 formation could originate from the dissociation of CO (CO disproportionation) accompanying with linear CO desorption[11]. With increasing temperature further, the peak intensity of bridged CO became stronger gradually until the maximum at 300qC, but the peak intensity of linear CO also became stronger gradually, and its position shifted to higher wave number at 300qC, reading as low as 2070 cm–1. This shift originated from a new band at higher wave number overlapping with the band of residual linear CO with increasing temperature. The appearance of this new band suggested that there could be newly formed adsorbed species or adsorption sites at higher temperatures[12]. For the coadsorption of CO and H2 (Fig. 3), the bands of gaseous CO also appeared at 2174 and 2118 cm–1 at room temperature, but their intensities were very weak due to smaller volume fraction of CO in the feed gas. Compared with the linear CO and bridged CO from Fig. 2, two new bands observed at 2048 and 1909 cm–1 shifted to lower wave number, which could be attributed to the formation of linear and bridged carbonyl hydrides
electron-donating property of H enhanced the electron-feedback capacity of Ni to CO, and thus, the bond of Ni-C became stronger and the bond of C—O became weaker, making ȞCO shift to lower wave number. With increasing temperature, the band at 1909 cm–1 did not shift but became stronger gradually until 100qC and weaker beyond 100qC. Meanwhile, the band at 2048 cm–1 became weaker first and then stronger until the maximum at 200qC, and its position shifted to 2060 cm–1 with increasing temperature. This shift suggested that there could be newly formed adsorbed species, which could be assigned to the carbonyl hydride originated from the adsorbed species at 2070 cm–1 in Fig. 2. At 300qC, the band at 1909 cm–1 became weaker, and the band at 2060 cm–1 became weaker significantly and its position shifted to 2040 cm–1. This shift suggested that the carbonyl hydride could be converted to the
(
,
)[11–13],
The
) on multihydrogen carbonyl hydride ( linear adsorption sites[13]. Thus, with increasing temperature, the more the number of dissociative H, the stronger the electron-donating capacity of H, leading to the enhancement of the electron-feedback capacity of Ni to adsorbed CO, and thus, the bond of Ni-C became stronger and the bond of C—O became weaker, resulting in the shift of the band to lower wave number. In addition, a weak band at 3016 cm–1 was observed, which could be assigned to gaseous CH4[14], and a stronger band appeared at 3728 cm–1, suggesting the formation of water. Apparently, the carbonyl hydride was the key intermediate product in the process of the breaking of C—O bond. 2.3 In-situ DRIFT spectra of CO and H2 adsorbed on the Ni/ZrO2-SiO2 catalyst The DRIFT spectra of CO adsorption at different temperatures over the Ni/ZrO2-SiO2 catalyst are shown in Fig. 4.
WU Rui-fang et al. / Journal of Fuel Chemistry and Technology, 2009, 37(5): 578582
Fig. 4
In-situ DRIFTS of CO adsorbed on the Ni/ZrO2-SiO2 catalyst at different temperatures A: 12–100qC
Fig. 5
In-situ DRIFTS of CO and H2 coadsorbed on the
Ni/ZrO2-SiO2 catalyst at different temperatures
In Fig. 4, the bands of gaseous CO, linear CO, and bridged CO are observed at 2174 and 2118, 2056, and 1915 cm–1 after CO was introduced at room temperature, respectively. Compared with the Ni/SiO2 catalyst, the peak intensity of linear CO and bridged CO increased obviously over the Ni/ZrO2-SiO2 catalyst, indicating the adsorption capacity of these catalysts for CO was enhanced remarkably due to the addition of ZrO2 promoter. In the range 12–100qC, the peak intensity of linear CO was several times stronger than that of over the Ni/SiO2 catalyst at the same temperature. Above 100qC, the changes of the bands over the Ni/ZrO2-SiO2 catalyst had the same trend as those over the Ni/SiO2 catalyst with further increasing the temperature. For the coadsorption of CO and H2 (Fig. 5), two new bands assigned to linear and bridged carbonyl hydrides were also observed at 2046 and 1908 cm–1 at room temperature, respectively. Compared with the Ni/SiO2 catalyst, the peak intensity of the bridged carbonyl hydride species increased significantly, but the peak intensity of the linear carbonyl hydride species decreased slightly over the Ni/ZrO2-SiO2 catalyst. Apparently, for the coadsorption of CO and H2, the addition of ZrO2 promoter was favorable for the formation of
B: 200–300qC
the bridged carbonyl hydride species. At 100qC, the band at 2046 cm–1 shifted to 2060 cm–1, and the intensity of the peak at 1908 cm–1 became stronger obviously, indicating the formation of a large number of the bridged carbonyl hydrides on the catalyst surface. At 200qC, the intensity of the peak at 2060 cm–1 became stronger remarkably and reached the maximum, but the band at 1908 cm-1 became weaker obviously. Meanwhile, the band of gaseous CH4 appeared at 3016 cm–1. As the temperature increased further, the changes of the bands over the Ni/ZrO2-SiO2 catalyst had the same trend as those over the Ni/SiO2 catalyst. The results suggested that the addition of ZrO2 promoter led to the formation of a larger number of the bridged carbonyl hydrides over the Ni/ZrO2-SiO2 catalyst at lower temperatures. From the DRIFT spectra of coadsorption of CO and H2 over the two catalysts, it was found that CO was first converted to carbonyl hydride in the presence of the dissociative H and then converted to the multihydrogen carbonyl hydride, resulting in the weakening and breaking of C—O bond. , In CO methanation, the weakening and breaking of C—O bond over those catalysts were by multihydrogen carbonyl hydride rather than by direct breaking. The carbonyl hydride intermediate mechanism was proposed for the following reasons: (1) The direct disproportionation of CO occurred at higher temperature (200qC) than that (120qC) for the methanation, (2) The linear and bridged carbonyl hydrides were formed at room temperature and the condition required for the direct breaking of the C—O bond did not exist, (3) The absence of CO2 in the methanation indicated that the direct disproportionation of CO did not occur on the catalyst surface, and (4) The changes of the linear and bridged carbonyl hydrides in peak intensity were in accord with that for CH4 formation. The activity of adsorbed CO was related directly to the proportion of the d-ʌ feedback bond originated from the
WU Rui-fang et al. / Journal of Fuel Chemistry and Technology, 2009, 37(5): 578582
interaction between CO and the metal M. The larger the proportion of d-ʌ feedback bond, the stronger the electron-feedback capacity of the metal M to adsorbed CO, and thus, the M-C bond became stronger and the C—O bond became weaker, making CO activated more easily. Hence, the activity of adsorbed CO on the catalyst surface was as follows: bridged CO > linear CO > twin CO. Thus, the more the bridged carbonyl hydrides formed on the catalyst surface, the larger the probability of the breaking of C—O bond, leading to the formation of more surface carbon, which could be hydrogenated to CH4. Compared with the Ni/SiO2 catalyst, more bridged carbonyl hydrides were formed over the Ni/ZrO2-SiO2 catalyst at lower temperatures, which may be contributed to higher catalytic activity for CO methanation.
3
Conclusions
Progress, 2007, 26(9): 1269–1273. [3] Li H C. The device of hydrogen and artificial substitute natural gas
co-produced
from
coke
oven
gas:
CN,
ZL
200620033245.5[P], 2007-05-02. [4] Tao P W, Wang X D. Production of compressed natural gas from coke oven gas. Coal Chemical Industry, 2007, 35(3): 11–14. [5] Li Y F, Dong X F, Lin W M. Effects of ZrO2-promoter on catalytic performance of CuZnAlO catalysts for production of hydrogen by steam reforming of methanol. Int J Hydrogen Energy, 2004, 29(15): 1617–1621. [6] Zhou W, Fang K G, Chen J G, Sun Y H. Effect of ZrO2 on the stability of Co/SiO2 catalyst for Fischer-Tropsch synthesis. Journal of Fuel Chemistry and Technology, 2006, 34(4): 461–465. [7] Gao C G, Zhao Y X, Liu D S. Liquid phase hydrogenation of maleic anhydride over nickel catalyst supported on ZrO2-SiO2
CO was completely converted at 270qC over the Ni/SiO2 catalyst under the reaction condition of CO of 1%, GHSV of 5000 h–1, and atmospheric pressure. The addition of ZrO2 promoter improved the catalytic activity of the Ni/ZrO2-SiO2 catalyst significantly, over which CO was completely converted at 200qC under the same reaction condition. In CO methanation, the breaking of C—O bond over the Ni/SiO2 and Ni/ZrO2-SiO2 catalysts was by multihydrogen carbonyl hydride rather than by direct breaking. The addition of ZrO2 promoter remarkably enhanced the adsorption capacity of the Ni/ZrO2-SiO2 catalyst for CO, and in the presence of H2, more bridged carbonyl hydrides were formed at lower temperatures over the Ni/ZrO2-SiO2 catalyst, which may be contributed to higher catalytic activity for CO methanation.
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