Preparation of nickel nanoparticles and their catalytic activity in the ...

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Miguel García and Caribay Urbina. Centro de Microscopia Electrónica, Facultad de Ciencias, Universidad Central de Venezuela, Caracas. 1040, Venezuela.
Preparation of nickel nanoparticles and their catalytic activity in the cracking of methane Juan Carlos De Jesúsa兲 and Ismael González PDVSA-INTEVEP, Los Teques AP 76343, Venezuela

Miguel García and Caribay Urbina Centro de Microscopia Electrónica, Facultad de Ciencias, Universidad Central de Venezuela, Caracas 1040, Venezuela

共Received 30 July 2007; accepted 28 January 2008; published 1 July 2008兲 Metallic nickel nanoparticles are excellent catalysts for the cracking of methane into carbon nanotubes and hydrogen. Decomposition of tetrahydrate nickel acetate proceeds readily in inert or hydrocarbon streams to produce metallic nickel agglomerates 共average size of 10– 80 nm兲 at around 350 ° C. In the present study, this parent salt is thermally treated in methane streams in situ in a thermogravimetric analyzer 共TGA兲, and weight changes corresponding to the carbon buildup in the metallic nickel particles are analyzed to provide some insight into the nickel-catalyzed cracking process. C to Ni atomic ratios 共C / Ni兲 estimated directly from TGA data provided a systematic approach to study the catalytic activity of the nickel nanoparticles. Methane cracking starts at temperatures as low as 400 ° C and continues efficiently until approximately 600 ° C. Between 600 and 660 ° C, methane decomposition momentarily breaks off, while presumably the catalytic system undergoes a self-reorganization. Cracking resumes at 660 ° C and continues slowly up to 950 ° C. The amount of carbon deposited in the 600– 660 ° C interval shows a linear dependence with methane concentrations, with C / Ni ratios ranging from 6 to 31. Transmission-electron microscopy images of the different C / Ni residues collected at 660 ° C show, that during cracking, narrower carbon nanotubes are produced at elevated methane concentrations, suggesting dispersion of nickel nanoparticles. © 2008 American Vacuum Society. 关DOI: 10.1116/1.2885212兴

I. INTRODUCTION The existing profusion of natural-gas sources is one of the strategic motivations in the progressing interest to explore processes for efficient methane-to-hydrogen conversion. Direct cracking 共or pyrolysis兲 of methane into its constituent elements is being considered as an environmentally attractive approach to the production of hydrogen from natural gas 共NG兲.1–4 The direct thermal decomposition of methane is an endothermic reaction, thermodynamically favored at elevated temperatures: CH4 → C + 2H2,

⌬Hⴰ = 75.6 kJ/mol.

Whatever the inlet methane concentration may be, at atmospheric pressure and temperatures higher than 600 ° C, methane conversions higher than 60% can be achieved. Due to the large energy of the C–H bond 共440 kJ/ mol兲, higher temperatures are required 共above 1200 ° C兲 to activate this hydrocarbon.5 This process directly produces hydrogen free of CO / CO2, in comparison to conventional hydrogenproduction processes, e.g., steam methane reforming and coal gasification, which require additional steps such as water gas shift and CO2 removal by adsorption.4,6 Clean hydrogen produced in this way could be used without further purification in fuel cells due to the high efficiency of the hydrogen-to-electricity conversion and the lack of emissions a兲

Author to whom correspondence should be addressed; electronic mail: [email protected]

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of pollutant gases. Carbon formed as by-product during hydrocarbon decomposition can be used as a functional material, such as fibers, graphite, carbon black, composites, and electrodes. Cracking of hydrocarbons progresses at reasonable yields and at operating temperatures lower than 1000 ° C only when using metal catalysts. It is well known that organized carbon nanostructures 共nanofilaments or nanotubes兲 are produced concurrently with hydrogen evolution. Due to their extraordinary physical and chemical properties, these ordered carbonaceous structures are seen as promising materials for challenging applications. For example, they possess great strength, chemical purity, and are chemically inert, which allows them to be ideally used as supports in various catalytic processes, especially for selective hydrogenation.7–9 Methane is the main component in NG. Nickel catalysts are one of the most important classes of heterogeneous catalysts due to their widespread use in a variety of processes, such as methanation,10–12 partial oxidation,13,14 and steam reforming.15,16 Therefore, catalytic decomposition of methane on Ni catalysts is one of the most extensively studied systems. A large amount of work has been devoted mainly to high-loaded nickel catalysts on suitable carriers such as ceramic oxides17–20 and amorphous carbon.21 It was found that active nickel particles detach from their support and travel to the tip of the carbon structures while the cracking reaction develops. It is therefore logical to assume that the size of the metal particle controls the diameter of the carbon nanotubes and that smaller nickel nanoparticles will conduct the reac-

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©2008 American Vacuum Society

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tion more efficiently before deactivation by encapsulation or agglomeration of the active phase takes place. Quantitative separation of nickel for purification of the carbon nanotubes is feasible by mild acid treatments,22 but complete removal of support often requires more complex and destructive treatments. Therefore, despite the fact that few studies have been reported, methane cracking in unsupported nickel nanoparticles presents an interesting alternative to conventional supported systems. In previous studies, we have reported that direct thermal decomposition of nickel acetate on an inert 共He and N2兲 gas atmosphere produces nickel metal nanoparticles23,24 with sizes ranging between 20 and 130 nm.24 In the present work, we show that the generation of nickel nanoparticles inside a thermogravimetric analyzer coupled to a quadrupole mass spectrometer allows the investigation of the catalytic decomposition of methane into carbon nanotubes. With the type of catalysts explored so far for this process, the challenge is to provide a high yield of carbon per mass unit of the catalyst. The C to Ni atomic ratios 共NiCX兲 estimated directly from thermogravimetric analysis 共TGA兲 data during in situ catalytic decomposition of methane may provide a systematic approach to assess the catalytic activity of nickel nanoparticles and eventually to compare performances between supported and unsupported systems. II. EXPERIMENT A. Materials

Commercially available tetrahydrate nickel acetate 关Ni共CH3COO兲2 · 4H2O兴 was used directly as the initial precursor for the generation of nickel nanoparticles. Methane 共99.9 wt % 兲 and argon 共99.9 wt % 兲 were used as received.

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FIG. 1. Cracking of methane catalyzed with nickel nanoparticles generated in situ from the thermal decomposition of nickel acetate tetrahydrate investigated by TGA, DTA, and QMS 共evolution of hydrogen monitored by m / z = 2兲. Methane concentration of 51% in argon, total gas flow of 70 ml/ min, and heating ramp 20 ° C / min.

respectively, because detailed examination of other ions ruled out the production of additional by-products. The area of H2 obtained for each representative peak was transformed to moles using a conversion factor, which was determined from a previous calibration of the mass spectrometer. C. Transmission-electron microscopy studies

Morphology analysis of carbon-nickel residues was carried out in a Jeol Jem 1220 transmission-electron microscope 共TEM兲 working at a 100 kV accelerating voltage. TEM specimens were prepared by ultrasonic dispersion of the slightly grounded samples in ethanol, and then a drop of the suspension was applied to collodion-graphite-coated copper grids.

B. Temperature-programed methane reaction and evolved gas analysis

III. RESULTS AND DISCUSSION

TGAs were carried out on a Netzsch STA 409 PC Luxx instrument. For each experiment, about 10 mg of nickel acetate was loaded into a 5 ml alumina crucible to allow extra room to keep nickel nanoparticles and carbon nanotubes inside the weighting area during the treatments. The initial gas mixture 共CH4:Ar兲 was supplied at all times at a constant flow rate of 70 ml/ min and CH4 concentrations were regulated accurately employing mass-flow controllers existing in a Netzsch PulseTA gas mixer. The heating rate was 20 ° C / min for all the measurements and a temperature range of 20– 900 ° C was chosen to probe the temperature-programed reaction in a concentration of 51% methane in argon. The temperature range was selected to be between room temperature and 600– 630 ° C, to examine the behavior of the system during a concentration-sensitive plateau revealed during the in situ catalytic transformation of methane reported in this study. Evolved gas analysis by quadrupole mass spectrometry 共QMS兲 with a Pfeiffer Omnistar model GSD301 coupled to the TGA was conducted to probe online the composition of the gas exhaust during the experiments. The mass numbers 共m / z兲 2 and 15 were used for monitoring H2 and CH4,

Thermal decomposition of transition-metal carboxylates occur in a self-reductive atmosphere, directly producing metallic particles.25–28 In a previous work,23 we demonstrated the feasibility of employing tetrahydrate nickel acetate to obtain nickel particles by decomposing this parent salt in an inert atmosphere at 350– 400 ° C. Upon heating, pure tetrahydrate nickel acetate first releases water at ⬃120 ° C, producing intermediate basic nickel acetate. On continued heating, the subsequent decomposition of the intermediate at ⬃350 ° C in inert atmosphere leads directly to the formation of finely divided metallic nickel particles with an estimated 4%–5% of carbonaceous residues.23 In the present investigation, we took advantage of essentially the same procedure. Nickel-acetate samples were first thermally decomposed to metallic nickel inside a commercial thermogravimetric analyzer, and then tested in situ in streams of methane for catalytic cracking. Figure 1 shows the weight changes of nickel acetate when heated in a stream of 51% of methane in argon at a heating rate of 20 ° C / min up to 950 ° C. Superimposed in the TGA data are the derivative of the weight curve or derivative ther-

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FIG. 2. Metal nickel particles obtained from the thermal decomposition at a heating ramp of 20 ° C / min of pure Ni共C2H3O2兲2 · 4H2O up to 400 ° C in a 50 ml/ min flow of 共a兲 Ar and 共b兲 CH4.

mal analysis 共DTA兲 and the H2+ signal collected online. Quadrupole mass spectrometry confirmed that the catalytic reaction was clean and free of by-products and that only hydrogen evolved to the gas phase. Carbon bundled to the nickel particles was obtained during the course of the decomposition reactions. The weight losses observed below 400 ° C during the decomposition of nickel acetate in methane are quite similar to the ones reported during the thermal decomposition of the pure salt in inert atmospheres. This was explained in detail in our previous work.23 Briefly, weight losses depicted in the DTA curve at about 110, 350, and 385 ° C are associated with dehydration, major decomposition of the acetate group, and the final reduction of an oxidic nickel intermediate, respectively. Figure 2 shows TEM images of the residue collected after thermal decomposition of nickel acetate at 400 ° C in a flow of pure argon 关Fig. 2共a兲兴 and in a flow of 51% of methane in argon 关Fig. 2共b兲兴. Nickel particles from the decomposition of nickel acetate shown in our TEM images seem to be larger JVST A - Vacuum, Surfaces, and Films

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than 100 nm, but we assume that this is due to insufficient dispersion during the sample preparation phase and/or magnetic agglomeration.29 Therefore, detailed size distribution and morphology of nickel ex-acetate metallic particles were not attempted from the TEM results presented in Fig. 2. However, it is worth mentioning that incipient formation of carbon nanotubes is obvious only at 400 ° C when nickel acetate is decomposed in methane, as shown in the insets in Fig. 2共b兲. Thermocatalytic decomposition of methane firmly impacts the weight gains seen in Fig. 1 beyond 400 and up to 950 ° C. Previous analysis of TEM images confirmed that methane activation may start at temperatures as low as 400 ° C, but cracking rate significantly improves with temperature, increasing the carbon deposition rate and, consequently, the weight gain in the course of the thermogravimetric tests. Methane decomposition is clearly enhanced in the 400 ° C to approximately 600 ° C temperature interval, but perhaps the key observation of our investigation is that methane decomposition momentarily breaks off as indicated by a constant-weight step observed in the TGA curve between approximately 600 and 660 ° C. Unpredictably, catalytic cracking resumed at 660 ° C and continued very slowly up to 950 ° C, where we assume that complete deactivation of the particles was approaching. Examination of the H2+ trace confirms that the observed weight gains are directly related to methane decomposition. Quantitative analysis of the peak areas reveals exactly the stoichiometry expected for the observance of the reaction of methane cracking. Therefore, weight gains due to accumulated carbon can be used to calculate exactly the amount of methane decomposed at any time during the course of the TGA experiences. Corresponding carbon-to-nickel atomic ratios 共conveniently represented in this work as NiCX兲 calculated from TGA measurements as reported here are better figures than per-gram estimations often used to quantify carbon-storage capacities in heterogeneous-supported systems because these TGA measurements are independent of the relative mass of the supports. We discovered that the amount of carbon accumulated when the reaction reaches the constant-weight zone of “temporary deactivation” was clearly sensitive to the initial concentration of the hydrocarbon. The results for such investigation for fixed methane-concentration values of 23%, 31%, 51%, and 100% 共balance argon兲 are depicted in Fig. 3. Below 400 ° C, all four TGA curves agree very well, indicating that the thermal decomposition of nickel acetate is not very sensitive to variations in methane concentration. After 400 ° C, as discussed before, carbon accumulation increases steadily and the concentration-sensitive plateau fully develops in all four experiments. NiCX values attained in each of the plateaus increase with the concentration of methane. While the cracking at 21% of methane produced a relatively low carbon yield of 6, it can be readily seen that the residue obtained at 660 ° C in pure methane showed an improved NiCX value of 30.

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FIG. 3. Cracking of methane catalyzed with nickel nanoparticles generated in situ from the thermal decomposition of nickel acetate tetrahydrate investigated by TGA. Each sample was heated linearly from room temperature to 650 ° C at 20 ° C / min, varying methane concentration in argon as indicated in corresponding traces. Total gas flow of 70 ml/ min.

All four experiments depicted in Fig. 3 were previously programed to stop at exactly 650 ° C. A close inspection of the constant-weight zones shows a discernible drift in the formation of the plateaus to higher temperatures when methane concentration is increased. The reason for this behavior is unknown. Figure 4 plots C / Ni atomic ratios calculated from accumulated carbon in the plateaus versus methane concentration. A change in slope is clearly visible when the methane concentration reaches 51%. The data can be conveniently fitted by two lines, as depicted in Fig. 4. Comparison of the curve slopes in the two regimes show that after 51%, the cracking reaction diminishes its rate by a factor of 3 and, as

FIG. 4. Relationship between atomic C / Ni ratio calculated from TGA for the residues collected at 650 ° C and methane concentration. Linear regressions of the curves are indicated in the figure. J. Vac. Sci. Technol. A, Vol. 26, No. 4, Jul/Aug 2008

FIG. 5. TEM images of carbon nanotubes obtained in the region of the temporary deactivation of the cracking reaction at 650 ° C, varying methane concentrations in argon: 共a兲 23%, 共b兲 31%, 共c兲 51%, and 共d兲 100%.

will be discussed later, this most likely corresponds to the disaggregation of large nickel particles of about 100 nm into smaller particles of about 30– 40 nm. Interpolation of the data for the faster process indicates that the onset for methane cracking within our experimental conditions is about 13% of methane. We have checked this experimentally, finding that methane is not decomposed below this concentration threshold, indicating that a critical surface concentration of hydrocarbon species in equilibrium with the gas phase is needed to trigger the catalytic cracking. Figure 5 compares TEM images of the four NiCX residues collected during the temporal deactivation step discussed previously during analysis of Fig. 3. Generally speaking, the pictures depict areas where the best images were accessible in each case, but carbon whiskers of at least two very dissimilar average diameters are clearly visible. The nickel particles 共darker spots兲 were mostly located at the tips of carbon structures where an inner channel is clearly visible. Therefore, we assume from our low-resolution TEM images that we are obtaining multiwalled carbon nanotubes. However, very often other carbon structures such as noncapped nanotubes and amorphous features are visible as well by close examination of Fig. 5. The diameters of the nickel particles were of about the same size as the growing nanotubes. Clearly, the size of the particles involved in the growth process are smaller than the ones observed for the parent metallic particles before the onset of the cracking reaction 共see Fig. 2兲. Additionally, a close examination of the TEM images presented in Fig. 5 shows small dark stains along the axis of

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FIG. 6. Diameter distribution for carbon nanotubes obtained in the region of temporary deactivation at 650 ° C, varying methane concentrations in argon: 共a兲 31 wt %, 共b兲 51 wt %, and 共c兲 100 wt %.

the growing nanotubes, presumably formed by leftovers of the nickel particles situated on the tip of the nanotubes. The presumption that nickel particles might be molten at reaction conditions during nanotube growth has been debated extensively in the literature.30 The melting temperature for bulk nickel is about 1400 ° C. The premature liquefaction of nickel nanoparticles below 650 ° C proposed during catalytic cracking of methane and other hydrocarbons clearly implies a complex equilibrium between solubility, diffusion, and precipitation of carbon in the metal phase.31 The importance of supersaturated carbon concentration and its distribution in catalytic particles for carbon-nanotube nucleation has been recently acknowledged.32 Thermodynamics33 and mechanisms for carbon filament growth34 are also discussed elsewhere. We believe that the carbon-nickel contact lowers both the melting point and the surface tension of the particles and that this interaction could be a reasonable explanation for liquefaction and the features that are left behind when the nickel particles are moving freely at operating conditions. Figure 6 shows the size distribution of the NiC10, NiC21, and NiC30 nickel-carbon residues collected during the temporary deactivation of the catalytic reaction at methane concentrations of 31%, 51%, and 100%, respectively. Nanotubediameter distribution evolves from a mononodal 60 to 70 nm average value 关Figs. 6共a兲 and 6共b兲兴 to a binodal distribution with the appearance of thinner structures of approximately 30– 40 nm 关Fig. 6共c兲兴. Ermakova et al.35 stated that during JVST A - Vacuum, Surfaces, and Films

the reaction of methane decomposition, the active nickel crystallite sizes stabilized at 30– 40 nm. Small-diameter carbon-nanotube formation is triggered in the present study at methane concentrations beyond 51%, in line with the change of slope detected in Fig. 4. Our results confirm that nickel particles progressively underwent active evolution to thermodynamically favorable sizes while the cracking reaction temporarily breaks off in the 600– 660 ° C temperature interval. Nickel catalysts might therefore be considered as self-organizing systems during the reaction of methane decomposition.36 By an appropriate selection of experimental conditions 共methane concentration and, eventually, total pressure兲 this discovery may help in tailoring highly reactive nickel particles in the 30 nm range. IV. CONCLUSIONS Thermal decomposition of nickel acetate provides a reliable and facile method to synthesize small nickel-particle agglomerates in the 100 nm size range. These are realistic model phases to investigate the nickel-catalyzed decomposition of methane and 共eventually兲 other hydrocarbons. When these investigations are carried out in situ inside a commercial thermobalance coupled to a quadrupole mass spectrometer, the processed results lead to accurate C / Ni atomic ratios that produce a straightforward methodology for the assessment of some insights into the cracking of methane.

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Thermocatalytic decomposition of methane streams at atmospheric pressure showed an interesting step between 600 and 650 ° C, in which the cracking reaction temporarily halted, but 共after a short induction period兲 carbon accumulation is restored steadily up to 950 ° C. During the course of this constant-weight process, nickel aggregates larger than 100 nm gradually disperses into smaller 30– 40 nm particles and multiwalled carbon nanotubes grow from a monomodal to a bimodal diameter distribution. Carbon-to-nickel atomic ratios for residues of the cracking process collected at 650 ° C show a measurable dependence with the concentration of methane in the reaction stream. Additional research is in progress to evaluate the role of other metals to promote nickel activity during catalytic cracking of methane. N. Muradov, Int. J. Hydrogen Energy 18, 211 共1993兲. M. Steinberg, Int. J. Hydrogen Energy 24, 771 共1999兲. 3 B. Gaudernack and S. Lynum, Proceedings of the 11th World Hydrogen Energy Conference, Stuttgart, German, 1996 共unpublished兲, p. 511. 4 N. Muradov, Z. Chen, and F. Smith, Int. J. Hydrogen Energy 30, 1149 共2005兲. 5 P. Ammendola, R. Chirone, L. Lisi, G. Ruoppolo, and G. Russo, J. Mol. Catal. A: Chem. 266, 31 共2007兲. 6 S. Takenaka, H. Ogihara, I. Yamanaka, and K. Otsuka, Appl. Catal., A 217, 101 共2001兲. 7 R. T. K. Baker, Carbon 27, 315 共1989兲. 8 N. M. Rodriguez, J. Mater. Res. 8, 3233 共1993兲. 9 K. P. de Jong and J. W. Geus, Catal. Rev. - Sci. Eng. 42, 481 共2000兲. 10 M. Agnelli, H. M. Swaan, C. Marquez-Alvarez, G. A. Martin, and C. Mirodatos, J. Catal. 175, 117 共1998兲. 11 A. E. Aksoylu and Z. Onsan, Appl. Catal., A 164, 1 共1997兲. 12 I. Alstrup, J. Catal. 151, 216 共1995兲. 13 R. Jin, Y. Chen, W. Li, W. Cui, Y. Ji, C. Yu, and Y. Jiang, Appl. Catal., A 201, 71 共2000兲. 14 Z. Liu, K. Jun, H. Roh, S. Baek, and S. Park, J. Mol. Catal. A: Chem. 1 2

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189, 283 共2002兲. H. S. Bengaard, J. K. Norskov, J. Sehested, B. S. Clauser, L. P. Nielsen, A. M. Molenbroek, and J. R. Rostrup-Nielsen, J. Catal. 209, 365 共2002兲. 16 T. Borowiecki, A. Golebiowski, and B. Stasinska, Appl. Catal., A 153, 141 共1997兲. 17 T. Zhang and M. D. Amiridis, Appl. Catal., A 167, 161 共1998兲. 18 V. R. Choudhary, S. Benerjee, and A. M. Rajput, J. Catal. 198, 136 共2001兲. 19 S. G. Zavarukhin and G. G. Kuvshinov, Appl. Catal., A 272, 219 共2004兲. 20 I. Suelves, M. J. Lázaro, R. Moliner, B. M. Corbella, and J. M. Palacios, J. Heterocycl. Chem. 30, 1555 共2005兲. 21 T. V. Reshetenko, L. B. Avdeeva, Z. R. Ismagilov, A. L. Chuvilin, and V. B. Fenelonov, Catal. Today 102–103, 115 共2005兲. 22 K. Otsuka, H. Ogilhara, and S. Takenaka, Carbon 41, 223 共2003兲. 23 J. C. De Jesús, I. Gonzalez, A. Quevedo, and T. Puerta, J. Mol. Catal. A: Chem. 228, 283 共2005兲. 24 I. Gonzalez, J. Martinez, G. Jorge, J. C. De Jesus, and C. Urbina, Proceedings of the Ninth Inter American Congress of Electron Microscopy, Havana, Cuba, 2005 共unpublished兲, Paper No. 1499. 25 M. A. Mohamed, S. A. Halawy, and M. M. Ebrahim, J. Anal. Appl. Pyrolysis 27, 109 共1993兲. 26 M. Afzal, P. K. Butt, and H. Ahmad, J. Therm. Anal. 37, 1015 共1991兲. 27 G. A. M. Hussein, A. K. H. Nohman, and K. M. A. Attyia, J. Therm. Anal. 42, 1155 共1994兲. 28 A. K. Galwey, S. G. McKee, and T. R. B. Mitchell, React. Solids 6, 173 共1988兲. 29 Z. Wang, P. Xiao, and N. He, Carbon 44, 3277 共2006兲. 30 A. Gorbunov, O. Jost, W. Pompe, and A. Graff, Appl. Surf. Sci. 197–198, 563 共2002兲. 31 R. T. Yang, P. J. Goethel, J. M. Schwartz, and C. R. F. Lund, J. Catal. 122, 206 共1990兲. 32 F. Ding and K. Bolton, Nanotechnology 17, 543 共2006兲. 33 J. W. Snoeck, G. F. Froment, and M. Fowles, J. Catal. 169, 240 共1997兲. 34 R. T. Yang and J. P. Chen, J. Catal. 115, 52 共1989兲. 35 M. Ermakova, D. Y. Ermakov, L. M. Plyasova, and G. C. Kuvshinov, Catal. Lett. 62, 3 共1999兲. 36 Y. Li, B. Zhang, X. Xie, J. Liu, Y. Xu, and W. Shen, J. Catal. 238, 412 共2006兲. 15