Al2O3 Catalysts

Energy & Fuels 2009, 23, 1925–1930

1925

Partial Oxidation of Methane to Syngas over Ni/Al2O3 Catalysts Prepared by a Modified Sol-Gel Method Yueqin Song, Huimin Liu, Shuqiang Liu, and Dehua He* InnoVatiVe Catalysis Program, Key Laboratory of Organic Optoelectronics & Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua UniVersity, Beijing 100084, China ReceiVed NoVember 1, 2008. ReVised Manuscript ReceiVed January 20, 2009

Ni/Al2O3 catalysts were prepared by the impregnation method (IM), sol-gel method (SG), and modified sol-gel method (MSG). The effects of the preparation methods on the catalytic performances in the partial oxidation of methane (POM) to syngas were investigated. The catalysts were characterized by N2-adsopriton, X-ray diffraction (XRD), temperature-programmed reduction (TPR), thermal gravimetric analysis (TGA), and temperature-programmed oxidation (TPO) in order to investigate the textural properties, crystalline structures, reduction properties, and carbon deposition behaviors. The results revealed that the modified SG method improved the specific surface area and pore volume. The catalytic performance of the Ni/Al2O3 catalysts was improved by the MSG method. Moreover, the structural stability in the reaction and the resulting resistance to carbon deposition was significantly enhanced by the MSG method. It was proposed that the high resistance to carbon deposition of catalysts resulting from the MSG method was related to the inhibition of Ni particles growth in the reaction of POM and the formation of the active amorphous carbon species.

Partial oxidation of methane (POM) to syngas has attracted much attention 1-4 because POM has several advantages compared with the steam reforming of methane to syngas, such as mild exothermicity, high conversion and high selectivity, very short residence time, and the desired H2/CO mole ratio (about 2) that is suitable for the synthesis of methanol or hydrocarbons. Large numbers of studies have been done for developing efficient catalysts and processes in POM, and many articles have reviewed in detail the recent progress of the catalyst development and reaction mechanism in POM.1,4 Ni-based catalysts have been considered to have the high activity for converting methane to syngas and require a low cost. Supports and preparation methods of catalysts have a great influence on the catalytic activity and stability for Ni-based catalysts.1,4 Al2O3, especially γ-Al2O3, was generally used as a support in Ni-based catalysts due to its large specific surface area, high thermal stability, and easy accessibility. However, γ-Al2O3 shows an acidity which could promote the formation of carbon. The addition of alkali-metal and rare-earth-metal oxides to Ni/Al2O3 could inhibit carbon deposition.5-12 The role of alkali-metal

oxide should be related to weakening the acidity of catalyst.5 The suppression of carbon deposition over rare-earth-promoted Ni/Al2O3 was related to the oxidation properties and oxygen storage capacity of the rare earth.7 However, Pechimuthu et al.6,12 considered that the reduction of the deposit carbon on the promoted catalysts with alkali metal or rare-earth metal may be related to the partial coverage of surface nickel by the promoters. Besides, the addition of zirconia promoter to Ni/ Al2O3 also enhanced the high activity and stability in POM, and the desired catalytic performance of Ni/ZrO2-Al2O3 in POM was ascribed to the highly dispersed small Ni particles.13 In addition, Ni and NiB loaded on Ca decorated Al2O3 support showed high resistance to carbon deposition in POM.14 Apparently, the addition of suitable promoter to Ni/Al2O3 could improve the catalytic performance in POM. In fact, the addition of promoters increased the complexity of the catalyst preparation. In the absence of promoters, Ni/Al2O3 catalysts prepared by the W/O microemulsion method and sol-gel method also possessed the excellent resistance to carbon deposition.15-17 However, the sol-gel method could not prohibit the sintering of catalysts in POM in the long reaction time. Little work has

* To whom correspondence should be addressed. Phone/fax: +86-1062773346. E-mail: [email protected], [email protected]. (1) Enger, B. C.; Lodeng, R.; Holmen, A. Appl. Catal., A 2008, 346, 1–27. (2) Requies, J.; Barrio, V. L.; Cambra, J. F. Fuel 2008, 87, 3223–3231. (3) Lin, Y. B.; Pillai, M. R.; Bierschenk, D. M. Catal. Lett. 2008, 124, 1–6. (4) Choudhary, T. V.; Choudhary, V. R. Angew. Chem., Int. Ed. 2008, 1828–1847. (5) Miao, Q.; Xiong, G. X.; Sheng, S. S.; Cui, W.; Xu, L.; Guo, X. X. Appl. Catal., A 1997, 154, 17–27. (6) Pechimuthu, N. A.; Pant, K. K.; Dhingra, S. C.; Bhalla, R. Ind. Eng. Chem. Res. 2006, 45, 7435–7443. (7) Silva, F. A.; Martinez, D. S.; Ruiz, J. A. C.; Mattos, L. V.; Hori, C. E.; Noronha, F. B. Appl. Catal., A 2008, 335, 145–152. (8) Choudhary, V. R.; Rajput, A. M.; Mamman, A. S. J. Catal. 1998, 178, 576–585.

(9) Zhu, T.; Flytzani-Stephanopoulos, M. Appl. Catal., A 2001, 208, 403–417. (10) Lu, Y.; Liu, Y.; Shen, S. J. Catal. 1998, 177, 386–388. (11) Ma, D.; Me, D. j.; Li, X.; Gong, M. Ch.; Chen, Y. Q. J. Rare Earths 2006, 24, 451–455. (12) Nandini, A.; Pant, K. K.; Dhingra, S. C. Appl. Catal., A 2005, 290, 166–174. (13) Li, H. S.; Wang, J. F. Chem. Eng. Sci. 2004, 59, 4861–4867. (14) Chen, L.; Lu, Y.; Hong, Q.; Lin, J.; Dautzenberg, F. M. Appl. Catal., A 2005, 292, 295–304. (15) Xu, S.; Zhao, R.; Wang, X. L. Fuel Proc. Technol. 2004, 86, 123– 133. (16) Castro Luna, A. E.; Iriarte, M. E. Appl. Catal., A 2008, 343, 10– 15. (17) Zhang, Y. H.; Xiong, G. X.; Sheng, S. S.; Yang, W. S. Catal. Today 2000, 63, 517–522.

1. Introduction

10.1021/ef800954a CCC: $40.75  2009 American Chemical Society Published on Web 03/17/2009

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been performed to study the effect of modification of sol-gel without the promoter on the catalytic performances of Ni/Al2O3 in POM. The objective of the present investigation is to improve the resistance to carbon deposition of Ni/Al2O3 catalysts in POM, which were prepared by a modified sol-gel method with glycerol. For the sake of comparison, we also prepared Ni/Al2O3 catalysts by the conventional impregnation method and nonmodified sol-gel method. 2. Experimental Section 2.1. Preparation of Catalysts. The Ni/Al2O3 catalysts were prepared by the sol-gel method (SG), modified sol-gel method (MSG), and conventional impregnation method (IM). The procedures of the SG and MSG methods were as follows. First, pseudoboehmite (supplied by Condea Corp., Germany) was added into Ni(NO3)2 solution with certain Ni concentration under vigorous stirring to form a slurry. Subsequently, diluted nitric acid (5 wt %) was dropped slowly into the slurry while strongly stirring to form the sol. The formed sol was continuously stirred for 2 h to obtain the uniform dispersion of Ni. Then, the sol was divided into two equal parts. One part was directly dried at 60 °C for 36 h to form a gel, named the SG method. Glycerol was added into another part of the sol while vigorously stirring, and the weight of glycerol was four times that of pseudoboehmite. Then, the sol containing glycerol was dried as the above procedure to form gel, named the MSG method. The Ni/Al2O3-IM sample was prepared by impregnating γ-Al2O3 (provided by Jiangyan Chemical Co.) with Ni(NO3)2 solution. For every method, two different Ni content samples were prepared (i.e., 2%Ni and 14 wt %Ni), and six samples were obtained. The samples containing 2% Ni were calcined at 700 °C for 5 h, and the samples containing 14% Ni were calcined at 850 °C for 10 h. The higher calcination temperature for the latter was employed in order to check the formation of the spinel-structured species in the case of the high Ni content. The obtained catalysts are denoted as n%Ni/Al2O3-SG, n%Ni/Al2O3-MSG, and n%Ni/ Al2O3-IM, in which n represents Ni weight percent. 2.2. Catalytic Testing. The reaction of POM was conducted in a continuous flow, fixed-bed quartz reactor (i.d. ) 5 mm) at 700 °C, GHSV ) 1.0 × 105 mL/(gcat · h) and atmospheric pressure. The load of catalyst (30-40 mesh) was 0.1 g. Before reaction, the catalyst was pretreated in a flow of 10% H2/Ar mixture (20 mL/ min) at 700 °C for 2 h. The molar ratio of CH4 to O2 in feed gas was 2, and the feed gas was not diluted by inert gas. The reaction products passed through a cool trap and were online analyzed by GC (SHIMADZU-8A and HP1490) with a TDX-01 column, connected with a TCD. 2.3. Characterization of Catalysts. X-ray Diffraction (XRD). XRD was carried out on a Bruker D8 advance X-ray diffractometer, using Ni-filtered Cu KR1 radiation at room temperature and instrumental settings of 40 kV and 40 mA. The scanning was within a range of 2θ from 5° to 80° at a scanning rate of 2°/min. H2-Temperature-Programmed Reduction (H2-TPR). H2-TPR was carried out on homemade equipment. The sample of 0.1 g was first pretreated in a flow of Ar at 200 °C for 0.5 h and then cooled to room temperature. Subsequently, the sample was again heated in a flowing 5% H2/Ar mixture (30 mL/min) from ambient temperature to 850 °C at a heating rate of 10 °C/min. Thermal GraVimetric Analysis (TGA). TGA was performed on a Mettler Toledo TGA/SDTA851e instrument. The spent catalysts (0.020 g) were heated from room temperature to 850 °C in flowing air stream at a heating rate of 10 °C /min. The flow rate of air was 30 mL/min. In the recorded profiles, the weight loss before 400 °C was attributed to desorption of water. The decrease of weight from 400 to 850 °C was caused by burning off the coke on the surface of catalysts. The calculation of coke amount was based on the net weight of catalyst or the weight of fresh catalyst. Therefore, the

Figure 1. XRD patterns of different Ni/Al2O3 catalysts (a-c represent 2%Ni/Al2O3-IM, 2%Ni/Al2O3-MSG, and 2%Ni/Al2O3-SG; d-f represent 14%Ni/Al2O3-SG, 14%Ni/Al2O3-MSG, and 14%Ni/ Al2O3-IM; (9) γ-Al2O3, (b) NiAl2O4).

amount of coke on catalysts could be estimated according to the following formula

coke amount(mg/gcat) ) (M1-M2)/M2 × 1000 Here, M1 represents the weight percent of spent catalyst after desorption of water and M2 represents the weight percent of coked catalyst after burning off the coke. N2 Adsorption Isotherms. N2 adsorption isotherms were measured by nitrogen adsorption-adsorption at -196 °C using an ASAP2010 gas adsorption analyzer (Micromeritics Corp.). Each sample was degassed at 250 °C for 5 h before the measurement. The total surface area was calculated according to the BET isothermal equation.

3. Results and Discussion 3.1. Physicochemical Properties of Ni/Al2O3. The XRD patterns of the prepared catalysts are shown in Figure 1. Generally, it is difficult to distinguish the diffraction peak of NiAl2O4 and γ-Al2O3 in XRD patterns. However, an appreciable difference in the diffraction peaks of the two substances exists. The diffraction peak in γ-Al2O3 at 37.2° is broad and weak, while the peak in NiAl2O4 spinel is relatively narrow and sharp. NiAl2O4 spinel phase appeared in the catalysts with 14% Ni. This indicates that the spinel structure in the NiO-Al2O3 system was formed after calcination at high temperature. The BET specific surface areas (SBET) of the different samples are summarized in Table 1. It could be seen that the different preparation methods of the catalysts had an obvious effect on the textural structure of the catalysts. The SBET and pore volume of the Ni/Al2O3-MSG was larger than that of the others. Moreover, with increasing calcination temperature to 850 °C, SBET of all samples significantly decreased, but the SBET of 14%Ni/Al2O3-MSG sample was 129 m2/g, still higher than that of the others. Moreover, the pore volume of 14%Ni/ Al2O3-MSG was much larger than that of the others. This showed that the modified sol-gel method with glycerol could increase the SBET and pore volume of the catalyst. Glycerol could enter the spacing of sol and complex Al3+ in sol. Moreover, glycerol had a large amount of hydroxyl groups, and the hydrogen bond between glycerol and the hydroxyl groups of Al sol could be formed, which could reduce the sintering of alumina.18 Additionally, the glycerol could be removed during the calcinations of the catalysts, and as a result, the space in pores would increase. These two factors might be mainly (18) Niu, G. X.; He, J. M.; Chen, X. Y.; Liu, Y.; Bian, M. Y.; He, A. D. Chin. J. Catal. 1999, 20, 535–540.

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Table 1. Specific Surface Area and Coke Amount of Samples 2%Ni/Al2O3 sample MSG SG IM a

14%Ni/Al2O3

14%Ni/Al2O3

SBET (m2/g)

SBET (m2/g)

Vtotal (m3/g)

SaBET (m2/g)

Vatotal (m3/g)

coke amount (%)

197.6 167.8 132.9

129.1 92.7 90.7

0.574 0.196 0.312

119.6 69.6 73.7

0.527 0.167 0.243

4.2 7.1 21.2

Represents spent catalyst.

Figure 2. TPR profiles of different Ni/Al2O3 catalysts (A-C represent 2%Ni/Al2O3-IM, 2%Ni/Al2O3-MSG, and 2%Ni/Al2O3-SG; D-F represent 14%Ni/Al2O3-IM, 14%Ni/Al2O3-MSG, and 14%Ni/ Al2O3-SG).

responsible for the increase in the specific surface area and pore volume of Ni/Al2O3 catalysts prepared by modified sol-gel with glycerol. The reduction property of the catalysts was characterized by H2-TPR technology, and the results are given in Figure 2. The reduction peak areas represent the reduction degree of the samples. For the samples with low Ni content, the reduction peak areas of different samples decreased in the order 2%Ni/ Al2O3-IM > 2%Ni/Al2O3-MSG > 2%Ni/Al2O3-SG. It was obvious that the reducibility of the sample prepared by the IM method was the largest and by the SG method the smallest. Additionally, it is noted that three peaks appeared in the TPR curve of the 2%Ni/Al2O3-IM sample, centered at about 400, 600, and 800 °C, corresponding to free NiO and NiO that weakly interact with Al2O3 and NiAl2O4,19 respectively. However, only one reduction peak appeared in the TPR profiles of the 2%Ni/ Al2O3-SG and 2%Ni/Al2O3-MSG samples at temperatures (19) Hou, Z. Y.; Yokota, O.; Tanaka, T.; Yashima, T. Catal. Lett. 2003, 89, 121–127.

higher than 800 °C, suggesting that the Ni in 2%Ni/Al2O3-SG and 2%Ni/Al2O3-MSG may exist in the form of NiAl2O4. The reduction behaviors of different catalysts indicated that the preparation methods of the catalysts had an effect on the Ni form in Ni/Al2O3, and the SG method promoted the uniform dispersion of Ni in Al2O3. Similarly, 14%Ni/Al2O3-IM also showed three reduction peaks in the TPR curve, centered at 400, 700, and 810 °C, respectively, while in TPR curves of the 14%Ni/Al2O3-SG and 14%Ni/Al2O3-MSG samples only one reduction peak was observed at about 815 °C. This further confirmed that the sol-gel method was favorable to enhance the uniformity of Ni in Al2O3 and Ni species in Ni/Al2O3-SG and Ni/Al2O3-MSG existed mainly in the form of NiAl2O4 spinel. 3.2. Catalytic Performances of Ni/Al2O3 in POM. The catalytic performances of 2%Ni/Al2O3 and 14%Ni/Al2O3 catalysts in POM are presented in Figures 3 and 4, respectively. It is evident that the 2%Ni/Al2O3 catalysts prepared by different methods showed fairly different catalytic activities (Figure 3). Methane conversion and the selectivities of H2 and CO over 2%Ni/Al2O3-SG were 68%, 70%, and 88% within 6 h of time on stream (TOS), respectively, which were lower than the corresponding values over 2%Ni/Al2O3-MSG (81% of methane conversion, above 96.8% of H2 selectivity, and 91% of CO selectivity). Since Ni0 was generally considered as the catalytically active site, the higher reducibility of 2%Ni/Al2O3-MSG than that of 2%Ni/Al2O3-SG should be mainly responsible for the higher catalytic activity in POM (in Figure 3). However, the case was completely different for 2%Ni/Al2O3-IM. The conversion of CH4 was only 74.8%, and CO selectivity was 89.1% at TOS ) 0.33 h, which were lower than the values over 2%Ni/Al2O3-MSG. With the prolongation of the reaction time, both methane conversion and CO selectivity obviously increased and approached values over 2%Ni/Al2O3-MSG at TOS ) 6 h. This suggests that an inducing period for 2%Ni/Al2O3-IM exists. The catalytic activity of 2%Ni/Al2O3-IM and 2%Ni/ Al2O3-MSG was almost the same, although the reducibility of the former was higher than that of the latter. Maybe, only a small amount of Ni0 could complete the reaction of POM. The above results indicated that the modified sol-gel method could improve the catalytic activity of 2%Ni/ Al2O3 in POM. The catalytic performances of the catalysts with 14% Ni content in POM are shown in Figure 4. The initial conversions of CH4 and selectivity for H2 or CO (TOS ) 0.33 h) over the three different catalysts were very close to each other except that the CO selectivity over 14%Ni/Al2O3-IM was a bit lower. The activities of the catalysts with 14% Ni content were close to the values over the catalysts with 2% Ni content. Moreover, the conversion of CH4 and selectivity for CO over all three catalysts exhibited a very slight decrease, and the decrement on 14%Ni/Al2O3-IM seemed a bit larger within the 100 h of duration. This demonstrated that the serious deactivation of the catalysts prepared in this study did not occur within 100 h of TOS. However, it could not be inferred that the three catalysts possessed the same reaction stability in a longer reaction period. For the sake of further clarifying the reaction stability of the

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Figure 3. Comparison in reaction performances of 2%Ni/Al2O3 in POM. Reaction conditions: 700 °C, ambient pressure, 105 h-1 of GHSV.

Figure 4. Comparison in reaction performances of 14%Ni/Al2O3 in POM. Reaction conditions: 700 °C, ambient pressure, 1.0 × 105 h-1 of GHSV.

three catalysts, the physicochemical properties of the spent catalysts after 100 h of TOS were further characterized by XRD, TG, and TPO technologies. Figure 5 gives the XRD patterns of the fresh reduced catalysts and the spent catalysts. Small diffraction peaks of metallic Ni appeared for every fresh reduced catalyst. Compared with the

fresh reduced catalysts, the diffraction peaks of metallic Ni on the corresponding spent catalysts were significantly strengthened. Moreover, the diffraction peaks of metallic Ni for the spent 14%Ni/Al2O3-IM and 14%Ni/Al2O3-SG catalysts were stronger and sharper than that for the spent 14%Ni/Al2O3-MSG catalyst. This showed that the three catalysts were further

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Figure 6. TG profiles of the spent 14%Ni/Al2O3 catalysts.

Figure 5. XRD patterns of the reduced fresh and spent 14%Ni/Al2O3 (a-f represent 14%Ni/Al2O3-IM, 14%Ni/Al2O3-MSG, and 14%Ni/ Al2O3-SG: (*) graphitic carbon, (9) NiAl2O4, (b) Ni0).

reduced during the reaction to form a large amount of Ni0, and the reduction degree was higher for 14%Ni/Al2O3-IM and 14%Ni/Al2O3-SG. Moreover, the Ni0 particles on the spent catalysts further grew with reaction time. The Ni0 particles on the spent 14%Ni/Al2O3-IM and 14%Ni/Al2O3-SG catalysts were larger than that on the spent 14%Ni/Al2O3-MSG. Therefore, it could be inferred that the thermal stability of the catalysts in reaction ambience could be improved by the modified sol-gel method. The SBET of the fresh and spent catalysts after 100 h of TOS was also compared, as listed in Table 1, from which it could be seen that the SBET of the three catalysts decreased to different extents after the reaction. The SBET decreased to 69.6 m2/g from 92.7 m2/g for 14%Ni/Al2O3-SG, to 73.7 m2/g from 90.7 m2/g for 14%Ni/Al2O3-IM, and to 119.6 m2/g from 129.1 m2/g for 14%Ni/Al2O3-MSG. Obviously, the decrement of 14%Ni/ Al2O3-MSG in SBET was the smallest, only 7.4%, and the decrement of 14%Ni/Al2O3-SG was the largest, 24.9%. In addition, the pore volume of the three catalysts after 100 h of TOS decreased obviously. The decrement of pore volume of 14%Ni/Al2O3-MSG, 14%Ni/Al2O3-SG, and 14%Ni/ Al2O3-IM was 8.2%, 14.8%, and 22.1%, respectively. The results showed that the MSG method could stabilize the structure of the catalyst in the POM reaction. The decrease in the specific surface area and pore volume may be related to the structure destruction of the catalyst, as evidenced by XRD in Figure 5 and the blocking of pores by numerous carbonaceous deposit confirmed by TG in Table 1. The amount of coke on the spent catalysts was measured by TG technology, and the results are shown in Table 1. It could be clearly seen from Table 1 that after 100 h of TOS, the amount of coke deposit on 14%Ni/Al2O3-MSG was only 4.2%, much lower than the values on 14%Ni/Al2O3-SG (11.1%) and 14%Ni/Al2O3-IM (21.2%). Apparently, the MSG method improved the resistance to carbon deposition. Generally, the stronger interaction between Ni and Al2O3 support, the formation of NiAl2O4 spinel, and small Ni particles were favorable to inhibit coke formation.17,20 First, Ni in 14%Ni/Al2O3-SG and 14%Ni/Al2O3-IM was in the form of NiAl2O4 spinel as evidenced by XRD in Figure 1 and TPR in Figure 2, while (20) Zhang, Z. L.; Verykios, X. E.; MacDonald, S. M. J. Phys. Chem. 1996, 100, 744–754.

Figure 7. TPO profiles of the spent 14%Ni/Al2O3 catalysts (a, b, and c represent 14%Ni/Al2O3-IM, 14%Ni/Al2O3-SG, and 14%Ni/ Al2O3-MSG, respectively).

partial Ni in 14%Ni/Al2O3-IM existed in the form of free NiO. Accordingly, the high resistance to carbon deposition for the former was associated to the NiAl2O4 spinel structure of the catalysts. In addition, NiAl2O4 in 14%Ni/Al2O3-IM and 14%Ni/ Al2O3-SG was further reduced in the reaction, and the formed Ni0 grew up. The large Ni0 particles were another reason for the formation of a large amount of coke.15 Although Ni in 14%Ni/Al2O3-MSG was mainly in the form of the spinel structure, similar to 14%Ni/Al2O3-SG, the latter showed better structure stability and resistance to carbon deposition than the former. Not only were the carbon deposition amounts over the different spent catalysts different, but also the carbon species may be different. The carbon species was measured indirectly by TG (Figure 6) and TPO (Figure 7). From Figure 6, it could be seen that the final temperatures of weight loss in TG over the different catalysts were different. The temperature for 14%Ni/Al2O3-IM was 836 °C, which was about 100 °C higher than that for the spent 14%Ni/Al2O3-SG and 14%Ni/ Al2O3-MSG catalysts, although the initial combustion temperatures of the coke on the three catalysts were close to each other. The difference in combustion temperature of the coke might represent the different carbon species. In addition, the profiles of TPO of the different spent catalysts showed a distinct difference. The peak areas in TPO represented the relative amount of the deposited coke. The combustion peak areas of different catalysts increased in the order 14%Ni/Al2O3-IM > 14%Ni/Al2O3-SG >14%Ni/Al2O-MSG. This is completely consistent with the results from TG measurements. In addition, two combustion peaks in TPO curves for each spent catalyst appeared. The two combustion peaks of 14%Ni/Al2O3-MSG were centered at ca. 460 and 630 °C, which were slightly lower

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than those of 14%Ni/Al2O3-SG. As for 14%Ni/Al2O3-IM, the low-temperature peak was still centered at 460 °C, while the high-temperature peak was centered at 760 °C, which was much higher than those of 14%Ni/Al2O3-SG and 14%Ni/ Al2O3-MSG. Moreover, the high-temperature peak area for 14%Ni/Al2O3-IM was much larger than that for the other catalysts. The carbon deposit burned off at high temperature (>600 °C) and low temperature (