γ-Al2O3-AlN and Effects of AlN on

Jul 22, 2010 - AlN, a substance usually used as a heat-resisting material due to its high-temperature stability and high thermal conductivity, was app...
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J. Phys. Chem. C 2010, 114, 13716–13721

Physicochemical Properties of Ni/γ-Al2O3-AlN and Effects of AlN on Catalytic Performance of Ni/γ-Al2O3-AlN in Partial Oxidation of Methane Hui-min Liu and De-hua He* InnoVatiVe Catalysis Program, Key Laboratory of Organic Optoeletronics and Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua UniVersity, Beijing 100084, China ReceiVed: April 5, 2010; ReVised Manuscript ReceiVed: July 2, 2010

AlN, a substance usually used as a heat-resisting material due to its high-temperature stability and high thermal conductivity, was applied in partial oxidation of methane for modifying γ-Al2O3 support. γ-Al2O3-AlN, prepared by mechanically mixing γ-Al2O3 and AlN, was used as support of a Ni-based catalyst. The physicochemical properties of Ni/γ-Al2O3-AlN and the effects of AlN on the catalytic performance of Ni/γ-Al2O3-AlN were studied in detail by means of X-ray diffraction, temperature-programmed reduction, Brunauer-EmmettTeller method, NH3-temperature-programmed desorption, thermogravimetric analysis, X-ray fluorescence spectrometer, and H2-chemisorption techniques. The results showed that Ni/γ-Al2O3-AlN exhibited high methane conversion and high selectivities to CO and H2. The addition of AlN, a substance with higher thermal conductivity than γ-Al2O3, leads to the higher thermal conductivity of Ni/γ-Al2O3-AlN than Ni/γ-Al2O3, and for this reason, after the same time of reaction, the Ni particles on Ni/γ-Al2O3-AlN showed a lower degree of sintering. Besides, as compared with γ-Al2O3, AlN is basic rather than acidic, that is, the addition of AlN to Ni/γ-Al2O3 made the acidic sites on Ni/γ-Al2O3-AlN reduce greatly, which finally resulted in a smaller amount of carbon deposited on Ni/γ-Al2O3-AlN. 1. Introduction With the depletion of petroleum, the effective utilization of natural gas has attracted more and more attention.1 In recent years, partial oxidation of methane (POM), which is the dominant component of natural gas, has shown great promise for the production of synthesis gas (CO + H2), which is suitable for further use in the production of oxygenated products as well as Fischer-Tropsch synthesis for the production of liquid hydrocarbons.2-4 Partial oxidation of methane is a mild exothermic reaction, which is more energy efficient, potentially more selective, and yields a more suitable H2/CO ratio for Fischer-Tropsch processes. For the reaction, different catalysts have been studied, including the Ni-based catalysts2,3,5 and the noble metal-based catalysts, such as Rh, Ru, Ir, and Pd.4,6-8 The noble metal-based catalysts present good activity; nevertheless, their high costs limit their applications. Fortunately, the Ni-based catalysts are easily available, and their catalytic performance is almost the same as that of noble metal-based catalysts, which provide them the most potential in industrial applications. Ni/Al2O3, one of the catalysts applied industrially in steam reforming of methane, also exhibited excellent catalytic activity in the partial oxidation of methane. However, at the same time, they suffer from the problem of sintering and carbon deposition at high reaction temperatures.9-11 Sintering is referred to as the phenomenon of migrations of atoms or molecules at high temperatures, which will lead to grain growth and the loss of specific surface area. Because partial oxidation of methane usually operated at high temperatures (such as 700 °C), the problem of sintering is serious. Many methods have been employed to solve the problem. Reports show that feedstocks (CH4 + O2) diluted with inert gas (N2 or Ar) allowed * To whom correspondence should be addressed. Tel/Fax: +86-1062773346. E-mail: [email protected].

the reaction to be operated at relatively lower partial pressures of the feedstocks, which could lead to a relatively small amount of heat generation and finally a lower degree of Ni sintering.12 It is also reported that diluting the catalysts with inert substances had similar effects.13 Pascaline Leroi14 employed SiC instead of Al2O3 as a catalyst support and found that the high thermal conductivity SiC could prevent the formation of hot spots on the catalyst surfaces and inhibit sintering of the catalyst; however, it suffered from Ni loss because of the weak interactions between Ni and SiC. For carbon deposition, it has been found that the acidic property of supports is an important factor that affects carbon deposition. Choudhary15,16 compared the catalytic performance of Ni-based catalysts and discovered that no obvious carbon was deposited on Ni/CaO, since CaO is a basic support. Miao et al.17 modified the Ni/Al2O3 catalyst with alkaline materials, and it was found that the ability to resist carbon deposition had been improved on the modified catalysts. Shang et al.18 studied the catalytic performance of nitrified Ni/SiC in the partial oxidation of methane, and the result was that, as compared with Ni/SiC, less carbon was deposited on the nitrified Ni/SiC. Nowadays, as a method to increase the basicity of catalysts, nitrification has been studied in detail. Ernst19 found that the nitrified zeolites showed basic properties. Zhang et al.20 have also found that there were changes in the acidic amount and acidic strength after the nitrification of HZSM-5. Guan et al.21 employed nitrified HZSM-5 to the ethylation of ethylbenzene, and it has been reported that nitrified zeolites could inhibit carbon deposition to some extent. AlN is a substance with good thermal stability and high thermal conductivity,22 and the thermal conductivity of AlN is much higher than that of γ-Al2O3. Supports with high thermal conductivity benefit heat transfer within the catalyst beds and avoid overheating on the catalyst surfaces, which finally slow down the sintering of catalysts. In addition, AlN does not show

10.1021/jp103033v  2010 American Chemical Society Published on Web 07/22/2010

Ni/γ-Al2O3-AlN in Partial Oxidation of Methane acidic properties. That is to say, modifying γ-Al2O3 with AlN could not only improve the heat conductivity properties of γ-Al2O3 but also adjust the acidic properties of γ-Al2O3. The Ni-based catalysts with γ-Al2O3-AlN as supports have not been reported in the partial oxidation of methane. In this paper, γ-Al2O3 was modified with AlN to improve the heat conductivity properties of the supports and to adjust the acidic properties of the catalysts. The physicochemical properties of Ni/γ-Al2O3AlN and the effects of AlN on the catalytic performance of Ni/ γ-Al2O3-AlN in the partial oxidation of methane were studied. 2. Experimental Section 2.1. Catalyst Preparation. γ-Al2O3 (provided by Aluminum Corporation of China limited) and AlN (provided by Toyou Aluminum Corporation) with a molar ratio of 1/2 were mechanically mixed and then calcined at 600 °C for 5 h to prepare the γ-Al2O3-AlN support. The Ni/γ-Al2O3-AlN catalyst was prepared by impregnating Ni(NO3)2 · 6H2O (analytical grade reagent, provided by Shantou Xilong chemical factory) with a γ-Al2O3-AlN support at room temperature for 10 h, then dried at 110 °C for 12 h, and calcined at 650 °C for 5 h. For comparison, Ni/γ-Al2O3 and Ni/AlN were also prepared with the same method. 2.2. Catalyst Characterization. The specific surface areas of the catalysts and the supports were measured by N2 adsorption-desorption with the Brunauer-Emmett-Teller (BET) method on a Micromeritics ASAP 2010 C analyzer. Before measurements, the catalysts were degassed at 473 K for 2 h. X-ray diffraction (XRD) was used to investigate the crystalline phases of the Ni particles and the supports. The XRD patterns were obtained with a Bruker D8 Advance X-ray Diffractometer, using Ni-filtered Cu radiation and instrumental settings of 3 Kw. The scanning was within a range of 2θ from 10° to 80° at a scanning rate of 2°/min. The crystal sizes of Ni particles on the catalysts were determined by means of the X-ray line broadening method using the Scherrer equation.23 The reduction behaviors of the catalysts were characterized by temperature-programmed reduction (TPR), and the TPR profiles were measured by a Quantachrome adsorption instrument. Before TPR, the catalysts were treated at 500 °C for 0.5 h under the flow of highly pure Ar (99.999%, 110 mL/min) to clean the surfaces of the samples and then cooled down to room temperature. In the TPR process, a sample (0.1 g) of the catalysts was heated from room temperature to 950 °C (for Ni/γ-Al2O3AlN, the temperature was kept at 950 °C for another 1 h), with a heating rate of 15 °C/min under a flow of 5% H2/Ar (110 mL/min). The gas of 5% H2/Ar was dehydrated and deoxygenated before entering the sample cell, and effluent gas from the sample cell was passed through an isopropanol-liquid nitrogen cool trap to remove water and then analyzed by a TCD detector. Thermogravimetric analysis (TGA) was performed on Mettler Toledo TGA/SDTA851e. The spent catalysts (0.020 g) were heated from room temperature to 850 °C in static air at a heating rate of 10 °C/min. In the recorded profiles, the weight loss before 400 °C was attributed to the 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 the coke amount was based on the net weight of the catalyst. Therefore, the amount of coke on catalysts could be estimated according to the following formula:

carbon deposited (g/g cat) ) (M1 - M2)/M3 × 100% That is, the amount of carbon deposition is defined as the weight of carbon deposited versus the net weight of catalysts.

J. Phys. Chem. C, Vol. 114, No. 32, 2010 13717 Here, M1 represents the weight percent of spent catalyst after the desorption of water, M2 represents the weight percent of coked catalyst after burning off the coke, and M3 represents the net weight of catalyst. The acidic properties of the catalysts were measured by NH3 temperature-programmed desorption (NH3-TPD). The NH3-TPD profiles were measured by a Quantachrome adsorption instrument. The catalysts (0.1 g) were first treated in highly pure He (99.999%, 110 mL/min) at 500 °C for 0.5 h. After that, the catalysts were saturated with flowing 1% NH3-99% He at 100 °C and then flushed with highly pure He (110 mL/min) to remove the physically adsorbed NH3. Finally, the desorption of NH3 was carried out in flowing highly pure He (110 mL/ min) from 100 to 800 °C with a heating rate of 15 °C/min. The actual contents of Ni on the catalysts were analyzed by an X-ray fluorescence spectrometer (XRF, Shimadzu XRF-1800). 2.3. Catalytic Performance. The POM reaction was conducted in a fixed-bed quartz reactor with an inner diameter of 5 mm under atmosphere. A sample (0.1 g) of the catalysts without dilution with inert substances was packed in the center of the quartz reactor. A furnace was used to heat the quartz tube reactor. A thermocouple, placed in the center of the furnace, was used to control the temperature of the furnace. Another two thermocouples, installed in the quartz tube reactor and contacted with the inlet and outlet sides of the catalyst bed, were used to measure the temperatures of the inlet and outlet sides of the bed. Before reaction, the catalyst was reduced with 20 mol % H2/Ar at 600 °C for 2 h, then the temperature was raised to the reaction temperature (700 °C), and then CH4 and O2 with a molar ratio of 2/1 were introduced into the reactor at a total flow rate of 165 mL/min. (The concentration of CH4 is 66.7 mol %, and the concentration of O2 is 33.3 mol %, and no inert gas was introduced) The effluent gas was first cooled down in a cool trap (about 0 °C) to remove water in the products, and gas products were analyzed by two online gas chromatographs (GC), both equipped with TDX-01 columns and connected with TCD detectors. The GC with Ar as the carrier gas was used to analyze the volume ratio of H2/CO in the products, and the GC with H2 as the carrier gas was used to analyze the components of CH4, CO, O2, and CO2 in the products. The relative amount of the gases in the products was calculated by the normalization method, and the data obtained by these two GC were linked by CO amount. The equations are shown as follows:

conversion of CH4 ) (Fin-CH4 - Fout-CH4) /Fin-CH4 × 100% selectivity of CO ) Fout-CO / (Fin-CH4 - Fout-CH4) × 100% selectivity of H2 ) Fout-H2 /2/ (Fin-CH4 - Fout-CH4) × 100% where F is volume flow (mL/min). 3. Results and Discussion 3.1. Texture Properties of the Supports. The XRD patterns of γ-Al2O3, AlN, and γ-Al2O3-AlN are shown in Figure 1. As compared with the standard spectra, it could be seen that the AlN was hexagonal AlN [88-2363], and the Al2O3 was γ-Al2O3 [10-0425], while on γ-Al2O3-AlN, both AlN and γ-Al2O3 existed, with each retaining its original phase. The texture properties of γ-Al2O3, AlN, and γ-Al2O3-AlN were examined by N2 adsorption-desorption isotherm measure-

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Figure 3. Pore size distributions of γ-Al2O3 and γ-Al2O3-AlN. (1) γ-Al2O3-AlN and (2) γ-Al2O3. Figure 1. XRD patterns of γ-Al2O3, AlN, and γ-Al2O3-AlN. (1) γ-Al2O3, (2) γ-Al2O3-AlN, and (3) AlN. a, Intensity × 5.

TABLE 1: Physicochemical Properties of Supports and Relevant Catalysts

catalysts

specific surface area of supports (m2/g)a

actual content of Ni (wt %)b

sizes of NiO before reductions (nm)c

Ni/γ-Al2O3 Ni/γ-Al2O3-AlN Ni/AlN

142.5 81.1 2.0

5.45 4.80 5.15

not detected not detected 27

a

Measured by BET. b Measured by XRF. c Measured by XRD.

Figure 2. Adsorption-desorption isotherms of γ-Al2O3, AlN, and γ-Al2O3-AlN. (1) AlN, (2) γ-Al2O3-AlN, and (3) γ-Al2O3.

ments. The adsorption-desorption isotherms are shown in Figure 2. For AlN, the desorption isotherm overlapped with the adsorption isotherm, which meant that there were no pores on AlN. For γ-Al2O3, it exhibited an IV type isotherm, indicating the existence of mesopores. For γ-Al2O3-AlN, it possessed a similar isotherm with γ-Al2O3; however, it should be noted that, as compared with γ-Al2O3, the desorption branch of γ-Al2O3AlN shifted to a relatively high pressure. This suggested that the pore sizes of γ-Al2O3-AlN were larger than those of γ-Al2O3. Figure 3 is the BJH pore size distributions of γ-Al2O3 and γ-Al2O3-AlN. It is clearly observed that, as compared with γ-Al2O3, the average pore size of γ-Al2O3-AlN increased, and the pore size distribution became broad on γ-Al2O3-AlN. Maybe this was because the nonporous AlN had blocked some of the small pores of γ-Al2O3. The specific surface areas of γ-Al2O3, AlN, and γ-Al2O3AlN and the sizes of Ni crystal particles over the relevant catalysts are listed in Table 1. It could be seen that the mesoporous γ-Al2O3 had the largest specific surface area of the three, whereas AlN, a substance without porous structures, had the smallest specific surface area. For γ-Al2O3-AlN, prepared by mechanically mixing γ-Al2O3 and AlN, its specific surface area showed a compromise of γ-Al2O3 and AlN. It also could

Figure 4. XRD patterns of Ni-based catalysts before reduction. (1) Ni/γ-Al2O3, (2) Ni/γ-Al2O3-AlN, and (3) Ni/AlN. a, Intensity × 5. b, Intensity × 3.

be discovered that, as expected, for Ni/γ-Al2O3 and Ni/γ-Al2O3AlN, the specific surface areas of the supports were large, and the sizes of NiO particles before reductions were small. 3.2. XRD Patterns of Ni-Based Catalysts before Reduction. The XRD patterns of Ni/γ-Al2O3, Ni/γ-Al2O3-AlN, and Ni/AlN before reduction are shown in Figure 4. It could be observed from Figure 4 (3) that the diffraction peaks for NiO were observed on Ni/AlN and the phases of AlN did not change after the loading of Ni. For Ni/γ-Al2O3 and Ni/γ-Al2O3-AlN samples, NiAl2O4 may be formed due to the incorporation of NiO phase to the γ-Al2O3 phase. From the standard spectra of γ-Al2O3 and NiAl2O4, it could be seen that the diffraction peaks for γ-Al2O3 [10-0425] (2θ at 31.97, 37.68, 45.85, and 66.85) and NiAl2O4 [78-1601] (2θ at 31.39, 36.99, 44.99, and 65.52) were close. Therefore, the diffraction peaks of γ-Al2O3, Ni/γ-

Ni/γ-Al2O3-AlN in Partial Oxidation of Methane

Figure 5. TPR profiles of Ni-based catalysts. (1) Ni/AlN, (2) Ni/γAl2O3-AlN, and (3) Ni/γ-Al2O3.

Al2O3, and Ni/γ-Al2O3-AlN from 60 to 70° are enlarged to show the difference, and the results are shown in the inset in Figure 4. It could be seen that, as compared with γ-Al2O3, the main diffraction peaks for Ni/γ-Al2O3 and Ni/γ-Al2O3-AlN shifted to a lower angle; that is, the diffraction peaks were ascribed to NiAl2O4.25 However, the diffraction peaks for γ-Al2O3 overlapped with those for NiAl2O4. For the fresh catalyst Ni/AlN, the size of NiO was 27 nm, and for the fresh catalysts Ni/γAl2O3 and Ni/γ-Al2O3-AlN, the diffraction peaks of the NiO phase were not detected, and perhaps, the NiO particles on the two catalysts were so small that they were out of the limit of XRD detection. 3.3. Reduction Behaviors of Ni/γ-Al2O3, Ni/γ-Al2O3-AlN, and Ni/AlN. Figure 5 exhibits the reduction behaviors of Ni/ γ-Al2O3, Ni/γ-Al2O3-AlN, and Ni/AlN. For Ni/AlN, there was only one broad reduction peak at about 450 °C, which was attributed to the reduction of free nickel particles on the catalyst.24 This indicates that over Ni/AlN, the interaction between Ni particles and AlN was weak. For Ni/γ-Al2O3, the major peak shifted to 850 °C, and it was attributed to the reduction of NiAl2O4.24 Besides, for Ni/γ-Al2O3, a trace amount of hydrogen consumption was observed between 400 and 650 °C, and maybe, it was attributed to the reduction of NiO that weakly interacted with γ-Al2O3. For Ni/γ-Al2O3-AlN, two reduction peaks were observed. The reduction peak at 580 °C was attributed to the reduction of NiO that weakly interacted with γ-Al2O3, and the reduction peak at 830 °C was ascribed to the reduction of NiAl2O4. As compared with Ni/γ-Al2O3, the reduction temperature of Ni/γ-Al2O3-AlN was lower. That is to say, the addition of AlN made the Ni particles more easily reduced. 3.4. XRD Patterns of Ni-Based Catalysts after Reduction. The XRD patterns of Ni/γ-Al2O3, Ni/γ-Al2O3-AlN, and Ni/AlN after reduction at 600 °C for 2 h (before reaction) are shown in Figure 6. It could be observed that, as compared with the unreduced catalysts, the crystalline phases of supports remained unchanged. The inset in Figure 6 also showed the existence of the NiAl2O4 phase on Ni/γ-Al2O3 and Ni/γ-Al2O3-AlN.25 What’s more, the diffraction peaks for Ni were observed. That is to say, the catalysts were reduced to some extent under the reduction conditions employed. The sizes of Ni particles on the reduced catalysts calculated with Scherrer equation are listed in Table 2, and it could be observed that the sizes of Ni on Ni/γ-Al2O3 and Ni/γ-Al2O3-AlN were small (7 nm), and the sizes of Ni on Ni/AlN were relatively large (35 nm).

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Figure 6. XRD patterns of Ni-based catalysts after reductions. (1) Ni/γ-Al2O3, (2) Ni/γ-Al2O3-AlN, and (3) Ni/AlN. a, Intensity × 5. b, Intensity × 2.

TABLE 2: Sizes of Ni Particles after Reactions and Amount of Carbon Deposition

catalysts Ni/γ-Al2O3 Ni/γ-Al2O3-AlN Ni/AlN a

sizes of Ni amount of carbon particles after deposition after sizes of Ni after reductions 50 h reactions 50 h reactions (nm)a (%)b (nm)a 7 7 35

16 12 74

3.1 1.1 7.2

Measured by XRD. b Measured by TGA.

Figure 7. NH3-TPD profiles of supports. (1) AlN, (2) γ-Al2O3-AlN, and (3) γ-Al2O3.

3.5. Acidic Properties of Supports. NH3-TPD was used to measure the acidic properties of these supports, and the profiles are shown in Figure 7. For γ-Al2O3, there was a broad desorption peak of NH3 at about 200 °C, indicating there were acidic sites on γ-Al2O3. On the contrary, no desorption peaks were observed for AlN, which meant that there were no acidic sites on AlN. As to γ-Al2O3-AlN, there was a desorption peak of NH3 at about 200 °C just like γ-Al2O3; however, the NH3 desorption peak from γ-Al2O3-AlN was much smaller than that from γ-Al2O3. This indicated that there were fewer acidic sites on γ-Al2O3AlN than on γ-Al2O3. 3.6. Catalytic Performance of the Three Catalysts. The catalytic performances of Ni/γ-Al2O3, Ni/AlN, and Ni/γ-Al2O3-

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Figure 8. Catalytic performance of Ni/γ-Al2O3, Ni/AlN, and Ni/γ-Al2O3-AlN. (1) Ni/AlN, (2) Ni/γ-Al2O3, and (3) Ni/γ-Al2O3-AlN.

Figure 9. Stability test of Ni/γ-Al2O3-AlN in POM (550 h TOS). (1) CH4 conversion, (2) CO selectivity, and (3) H2 selectivity.

AlN are shown as CH4 conversion and CO and H2 selectivities with time on stream (TOS) in Figure 8. It could be seen from Figure 8b that no great difference existed in CO and H2 selectivity except those on Ni/AlN, which decreased during the 50 h TOS. In Figure 8a, the initial catalytic activities of Ni/γAl2O3-AlN, Ni/γ-Al2O3, and Ni/AlN were similar, and the conversions of CH4 were 85, 81, and 79% at the initial stage of reactions, respectively. However, Ni/γ-Al2O3-AlN presented highly stable CH4 conversion for 50 h TOS, while obvious deactivation occurred over Ni/AlN, and the conversion of CH4 declined slightly during the 50 h TOS over Ni/γ-Al2O3. For Ni/γ-Al2O3 and Ni/AlN, the conversion of CH4 dropped from 81 to 79% and from 79 to 71%, respectively, within 50 h TOS, whereas the conversion of CH4 over Ni/γ-Al2O3-AlN was almost no change within 50 h TOS. Further prolonging the reaction time over Ni/γ-Al2O3-AlN to 550 h, still no obvious deactivation occurred, and CH4 conversion was maintained at about 80% from 150 to 550 h TOS (Figure 9). This is to say, Ni/γ-Al2O3AlN possessed high stability in POM. 3.7. Crystalline Structures of Catalysts after Reaction. The XRD patterns of catalysts after reactions are shown in Figure 10. It could be seen that, for the used catalysts, the phases of Ni were observed. As an exception, NiO phases could still be detected over used Ni/AlN. It is likely that the NiO particles, which were easily reduced, were easily reoxidized during POM.10 The sizes of Ni particles after 50 h TOS were calculated with the Scherrer equation and are listed in Table 2. It was clearly observed that, as compared with the sizes of Ni particles

Figure 10. XRD patterns of Ni-based catalysts after reaction (50 h TOS). (1) Ni/γ-Al2O3, (2) Ni/γ-Al2O3-AlN, and (3) Ni/AlN. a, Intensity × 5.

on the reduced catalysts (shown in Table 2), the degree of Ni particles growth on Ni/γ-Al2O3-AlN was lower than those on Ni/γ-Al2O3. For Ni/γ-Al2O3-AlN, the sizes of Ni particles grew from 7 to 12 nm, whereas for Ni/γ-Al2O3, the sizes of Ni particles grew from 7 to 16 nm. That is, the process of sintering of Ni particles slowed down on Ni/γ-Al2O3-AlN. Possibly, it is related with the conductivity properties of AlN. AlN is a substance with high thermal conductivity (the heat conduction coefficient of commercial AlN is in the range of 130-170 W/m K),22 much higher than that of γ-Al2O3 (about 10 W/m K). The addition of AlN to Ni/γ-Al2O3 quickened heat transfer between catalyst particles. The heat was exported out of the catalyst bed in time, and then, the problem of hot spots on the catalyst surfaces could be alleviated, and finally, the problem of sintering could be slowed down. Besides, the inset in Figure 10 showed that for the used Ni/ γ-Al2O3 and Ni/γ-Al2O3-AlN catalysts, the XRD diffraction peaks of 2θ at about 66.8, 45.9, and 37.7 were ascribed to Al2O3 phase. That is to say, the NiAl2O4 phase was reduced during a long time reaction. Relatively speaking, the Ni2+ confined in the crystal lattice was immune to sintering. It has been reported that the formation of NiAl2O4 could prevent Ni from sintering into large clusters.26 Therefore, it could be inferred that it was the active Ni0 that crystallized to big particles during reactions. In the XRD patterns, also, it could be seen that the used Ni/ AlN exhibited a strong diffraction peak attributed to graphic

Ni/γ-Al2O3-AlN in Partial Oxidation of Methane

J. Phys. Chem. C, Vol. 114, No. 32, 2010 13721 4. Conlusions γ-Al2O3-AlN support, which was prepared by mechanically mixing γ-Al2O3 and AlN, possessed a relatively large specific surface area, high temperature stability, and high thermal conductivity and also retained the original phases of γ-Al2O3 and AlN. As compared with γ-Al2O3, the amount of acidic sites on γ-Al2O3-AlN was reduced. The Ni particles were small and well-dispersed on Ni/γ-Al2O3-AlN, and as compared with the Ni particles on Ni/γ-Al2O3, those on Ni/γ-Al2O3-AlN were more easily reduced. Ni/γ-Al2O3-AlN exhibited high CH4 conversion and high selectivities to CO and H2 in POM and also presented relatively stable catalytic activity in 550 h TOS. The sintering and growing of Ni particles and carbon deposition over Ni/γAl2O3-AlN were greatly inhibited.

Figure 11. TGA analysis of used catalysts (50 h TOS). (1) Ni/γ-Al2O3AlN, (2) Ni/γ-Al2O3, and (3) Ni/AlN.

carbon, while no carbon was detected over used Ni/γ-Al2O3 and Ni/γ-Al2O3-AlN (the result was also confirmed by TGA). It was reported that, as compared with POM, a carbon deposition reaction required a relatively large number of active nickel sites.27 That is to say, the smaller the sizes of the Ni particles over catalysts, the stronger the ability to resist carbon deposition is. Therefore, it could be inferred that carbon was apt to deposit on the Ni/AlN catalyst on which the sizes of Ni particles were largest. 3.8. Anticarbon Deposition Properties of Ni/γ-Al2O3-AlN. TGA was carried out to investigate the anticarbon deposition properties of Ni/γ-Al2O3-AlN, Ni/γ-Al2O3, and Ni/AlN after 50 h TOS. The amount of carbon deposited over these catalysts is listed in Table 2. It could be seen that the amount of carbon deposited on Ni/γ-Al2O3 was almost three times as much as that on Ni/γ-Al2O3-AlN. Guo et al. reported that the surface acidity of the supports was another important factor that affected the resistance of Ni-based catalysts to carbon deposition except the sizes of Ni particles.28 Increasing the basicity of the catalysts can improve their CO2 adsorption ability, which could result in an increase in the chance of the reaction between deposited carbon and adsorbed CO2 and also inhibit the Boudouard reaction.29,30 No great difference existed between the sizes of Ni particles over Ni/γ-Al2O3-AlN and Ni/γ-Al2O3, so it could be speculated that it was the higher acidity of γ-Al2O3 than γ-Al2O3-AlN that lead to the higher amount of carbon deposited on Ni/γ-Al2O3. However, the amount of carbon deposited on Ni/AlN was the most of all. The reason might be that the size of Ni particles on the catalysts could influence the amount of carbon deposition. For Ni/AlN, the small specific surface area of AlN (2.0 m2/g) leads to the relatively large sizes of Ni particles. It has been reported that the larger of the Ni particles on the catalysts, the more carbon is deposited.27 Therefore, carbon was apt to deposit on the Ni/AlN catalyst. The morphology of carbon deposited over these catalysts was also investigated by TGA, as shown in Figure 11. For all of the three catalysts, there were weight losses at about 600 °C. It is reported that the weight loss before 600 °C was attributed to the burning off of active carbon, and the weight loss at temperatures higher than 600 °C was caused by inert carbon, such as carbon nanotubes.31 Therefore, it could be inferred that all of the carbon deposited over the three catalysts was inert carbon.

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