J . Phys. Chem. 1994,98, 5 180-5 182
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Catalytic Behavior of Palladium-Zeolite in Oxidation of Methane to Carbon Monoxide and Hydrogen Hiroshige Matsumoto Department of Chemistry, Faculty of Liberal Arts, Nagasaki University, Bunkyomachi 1-14, Nagasaki 852, Japan Received: December 20, 1993; In Final Form: March 29, 1994”
Partial oxidation of methane to synthesis gas was investigated in order to demonstrate the catalytic behavior of Pd-Y zeolite. The Pd-Y catalyst activated by a reduction-reoxidation treatment showed excellent activity and selectivity. I t was elucidated by detailed analysis of the experimental results that small clusters of Pd species formed on the zeolite could function as catalytic centers via the reversible redox mechanism between Pd and PdO.
introduction During the past decade the catalytic conversion of methane into more valuable synthesis gas (CO + H2) has been the subject of numerous studies. Among a number of methods, the partial oxidation under oxygen-deficient conditions has recently taken the limelight because of the simplicity and high selectivity of reaction. Green and co-workers reported, in their detailed studies of this process over mixed oxides1and over a variety of supported transition metals,2 that the virtual equilibrium yield of CO and H2 was achieved and that high selectivities were attributed to a sequence of complete oxidation of CH4 to C02 and HzO followed by the reforming of residual CHI by C02 and/or H20. A similar scheme was also proposed in an elegant work by Lunsford and co-workers from the viewpoints of the oxidation state and phase composition of the Ni-Al203 catalyst.3 On the other hand, Schmidt- and co-workers proposed from an engineering perspective in detail that C O and H2 were not the final but the primary products in this process on Pt-coated monoliths and related catalysts. Otsuka et a1.’ also observed the direct generation of synthesis gas over Ce02 catalyst. In the recent years, it has already been reported elsewhere839 that Cu-Y zeolite was considerably activated by a consecutive reduction-reoxidation (R-O) treatment. In the present study it is demonstrated that Pd-Y zeolite with R-0 treatment shows appreciable activity and good selectivity in the partial oxidation of CH4 into C O and HZ under oxygen-deficient conditions.
Experimental Section The Pd-Y sample (7 wt % Pd) was prepared by the ion exchange of Linde LZY-52 molecular sieves with Pd(NH3)dClz solution. The catalytic reactions were carried out under atmospheric pressure in a flow system.9 Basically, it consists of a gas feeding manifold with mass-flow controllers and a quartz reactor, having an internal diameter of 5 mm, in a computer-interfaced infrared image furnace. The temperature in the present work was nominal values measured by a platinum-rhodium thermocouple located at the center of the catalyst bed, ignoring the effect of ‘hot spots”l0 on the catalyst. In all cases reactant gases were fed at 50 mL min-1 over 65 mg of catalyst dispersed in 1000 mg of inactive quartz powder. Analyses of reactants/products mixtures were performed continuously by a computer-interfaced mass spectrometer (Alvac MSQ- 150) with a multichannel programmer and periodically by a gas chromatograph with Polapak Q and Molecular Sieve 5A columns. Details of the EXAFS instrument and data analysis were described elsewhere.9 The Pd-Y samples for the EXAFS examination were pressed into thin wafers and ~~~
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Abstract published in Advance ACS Abstructs, May 1, 1994.
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Figure 1. Catalytic activity and selectivity of Pd-Y zeolite activated by R-O treatment in CH4 oxidation (feed mixture, CH4/02/He = 2/1/ 13).
treated similarly as in the catalytic tests in the in situ cell prior to the measurements a t room temperature. The activation of Pd-Y zeolite was performed in the reactor by R-0 treatment.8~9 The treatment consisted of three consecutive steps, Le., calcination with pure 02, reduction with 25% H2 in He, and reoxidation with 25% 0 2 in H e at 725 K for 60 min.
Results and Discussion The Pd-Y catalyst with R-O treatment shows excellent catalytic behaviors in the oxidation of CHI into CO and H2 under oxygen-deficient conditions. To reduce the effect of “hot spots”I0 on the catalyst, the reactions were performed using a reactant diluted with helium. Figure 1 demonstrates the results obtained with the reactant mixture of CH4/02/He = 2/ 1/ 13 at a flow rate of 50 mL min-1. After the reactions started the system was allowed to attain steady-state conditions within 20 min. Under the conditions tested in this study, the reaction products were 0 1994 American Chemical Society
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The Journal of Physical Chemistry, Vol. 98, No. 20, 1994 5181
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Figure 2. Transient profiles of products in the redox cycle with 10%CHI and 5% 02 in helium.
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CO, Hz, COZ, and H20, accompanied by negligible quantities (CO.1%) of ethane/ethylene and formaldehyde. The steady-state conversion of CH4 increased with reaction temperature and attained 98% a t 1000 K, whereas 0 2 was consumed almost completely above 700 K (Figure 1A). The selectivities into CO and H2 increased sharply with temperature and attained about 95% at 1000 K (Figure 1B). At low temperatures, however, high 0 2 consumption and low selectivities of C O and H2 formations indicate that the complete oxidation into COZand H20 is the predominant reaction in this system. The ratio of main products, on the other hand, remained constant, Hz/CO = 2, at all temperatures tested, as shown in Figure 1C. In order to obtain insight regarding the reaction mechanism, a transient experiment was performed; Le., each reactant of CH4 or 0 2 was alternately introduced onto the catalyst. The result is shown in Figure 2. When the stream of 5% 0 2 in H e flowing onto the catalyst was suddenly changed to 10% CH4in H e at the same flow rate (50 mL m i d ) and at the same temperature (800 K), immediate formations of large portions of CO and H2 were observed. Appreciable amounts of C02 and H 2 0 were also produced a t the initial stage of this period. Upon the subsequent introduction of Oz/He to the reactor in place of CH4/He stream, neither CO nor H2 was produced, but CO2 and HzO were observed as the main reaction products. When the oxidized catalyst was again exposed to the CH4/He stream, the formation of each product was recognized in a similar manner to the first period. It is indicated, therefore, that in the absence of gaseous oxygen C O and H2 are directly produced by the reaction between CH4 in the gas phase and oxygen species on the oxidized surface. In the oxidizing mode, on the other hand, hydrocarbon species on the reduced surface (probably CH, species on Pd11-13) reacts with 0 2 in the gas phase to produce C02 and HzO. The oxidation states of the surface during the transient experiment were qualitatively investigated by extended X-ray absorption fine structure (EXAFS) spectra. The EXAFS measurements of the catalyst were performed in the in situ cell after each treatment as in the transient experiment; i.e., the spectra were measured after the oxidation with 5% O2in H e and after the reduction with 10% CH4 in H e at 800 K for 60 min. Fourier transforms14Js to real space of the obtained EXAFS spectra,
oxidizing (A), reducing (B), and operating (C) conditions, where R is the distance from Pd atoms and @ ( R ) is the radial distribution function? @(R),are shown in Figure 3A,B, where each peak is slightly replaced from true distance because of the phase shift. The main peak for the zeolite under the oxidizing condition in the Oz/He stream (Figure 3A) is observed a t 2.04 A (after correction of the phase shift), which agrees well with the Pd-0 distance in PdO crsytals. The zeolite under the reducing condition in the CH4 H e stream (Figure 3B) depicts the main peak a t 2.75 corresponding to the Pd-Pd bond in metal crystals. In order to obtain the exact structural information of Pd species in the zeolite, furthermore, inverse Fourier transformationslkl6 were performed for the main peaks in Figure 3, A and B, and compared with the calculated values from Pd-0 and Pd-Pd scatterings, respectively. The inverse Fourier transforms for the main peak in Figure 3A well reproduces the calculated values (Figure 4A), indicating that the Pd component of the zeolite in the Oz/He stream is present as PdO. In the CHd/He stream, meanwhile, the PdO species converts into Pd metal, since the inverse Fourier transforms for the main peak of Figure 3B reproduces completely the calculated values for the Pd-Pd bond in metal (Figure 4B). In the simulation shown in Figure 4B, the coordination number of Pd metal atoms formed in the zeolite is estimated to be 6-7, which is, however, distinctly smaller than 12 in infinite crystals. The discrepancy between these values is mainly attributed to the difference in the size of Pd specimen; i.e, the coordination number of Pd atoms in small particles is smaller than in large crystals because of high proportion of surface atoms. Applying the theory by Greegor and Lytle16 to the present system and assuming spherical particles of Pd metal, the average size of Pd particles in this catalyst is roughly 7-10 A. The average coordination number remains virtually constant through several redox cycles by the CH4/He and Oz/He streams during the transient experiment. The radial distribution function for the catalyst under the operating condition is shown in Figure 3C. After the treatment with reactant mixture of CH4/OZ/He= 2/1/ 14 a t 800 K for 60 min, the zeolite shows two main peaks at 2.04 and 2.75 A. These
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5182 The Journal of Physical Chemistry, Vol. 98, No. 20, 1994
is virtually present in the gas phaseunder the operating conditions (Figure 1A). Further detailed investigations,such as comparisons of the rates and activation energies in both of the direct and indirect routes, are required to explain the total mechanism of the partial oxidation of CH4 into CO and H2 over the activated Pd-Y zeolite.
Conclusion The Pd-Y zeolite catalyst with R-O treatment showed excellent activity and selectivity in the oxidation of CH4 into CO and H2 under oxygen-deficient conditions. Two possible pathways of the formations of CO and H2 were recognized; Le., one is the direct reaction between the surface oxygen and CH4 in the gas phase on the oxidized surface, and the other is the reforming of residual CH4 with primary-formed CO2 and/or H2O on the reduced surface. Analysis of EXAFS spectra of the catalyst revealed that two kinds of palladium species (small clusters of Pd and PdO) coexisted in the zeolite lattice under the operating conditions and that they could function as the catalytic centers via the reversible redox mechanism. k I A-1 Figure 4. Comparisons of the inverse Fourier transforms of main peaks (curves) in Figure 3A,B and the best-fit values (circles) calculated by Pd-O (A) and Pd-Pd (B) scatterings, respectively, where K is the photoelectron wave vector and X ( K )is the EXAFS oscillation^.^
values correspond to the Pd-O bond in oxide and the Pd-Pd bond in metal, resptively. Under the operating conditions, therefore, both of the oxidized and reduced states of Pd specimen exist together on the catalyst surface. It might be elucidated from these points of view that in the oxidation of methane under the oxygen-deficient conditions small clusters of Pd species in the zeolite function as the catalytic centers, via the reversible redox mechanism. The exact mechanism of the reaction is still obscure. On the reduced surface, however, the perfect oxidation products of C02 and H2O were produced exclusively in the presence of 0 2 in the gas phase (Figure 2). Furthermore, it was actually recognized that CO and H2 were produced at a considerable rate in the reforming reaction of CHI with C02 or HzO over the reduced Pd-Y catalyst above 700 K. Two kinds of Pd species (Pd and PdO clusters) are considered to coexist in the zeolite under the operating conditions, as shown in Figure 3C. Therefore, there must be another possible route that the primary-produced C02 and H2O react with the residual CH4 to produce CO and Hz, according to a widely accepted mechanism.1-3J7-20 In an oxygendepleted environment as in the present work, however, CO and H2 are probably produced mainly by the direct reaction between CHI in the gas phase and surface oxygen species, since no oxygen
Acknowledgment. This study was partially supported by a Grant-in Aid for Scientific Research (05650785) from the Ministry of Education, Science and Culture of Japan. The author thanks Dr. Shuji Tanabe (Nagasaki University) for his helpful assistance in EXAFS experiments. References and Notes (1) Ashcroft, A. T.; Cheetham, A. K.; Foord, J. S.;Green, M. L. H.; Grey, C. P.; Murrell, A. J.; Vernon, P. D. F. Nuture 1990, 344, 319. (2) Vernon, P. D. F.; Green, M. L. H.; Cheetham, A. K.; Ashcroft, A. T. coral. Lett 1990, 6, 181. (3) Dissanayake, D.; Rosynek, M. P.; Kharas, K. C. C.; Lunsford, J. H. J . Curd. 1991, 132, 117. (4) Hickman, D. A,; Schmidt, L. D. J . Card. 1992, 138, 267. (5) Schwartz, S. B.; Schmidt, L. D. J. Phys. Chem. 1986,90, 6194. (6) Heisenberg, D.; Schmidt, L. D. J . Cutul. 1987, 104, 441. (7) Otsuka, K.; Ushiyama, T.; Yamanaka, I. Chem. Lerr. 1993, 1517. (8) Matsumoto, H.; Tanabe, S.J. Chem. Soc., Chem. Commun. 1989, 875. (9) Matsumoto, H.; Tanabe, S.J. Phys. Chem. 1990, 94, 4207. (10) Dissayanake, D.; Rosynek, M. P.; Lunsford, J. H. J. Phys. Chem. 1993, 97, 3644. (11) Solmosi, F.; Kiss, J.; Rovesz, K. J. Phys. Chem. 1990, 94, 2224. (12) Solmosi, F.; Kiss, J.; Rovesz, K. J . Chem. Phys. 1991, 94, 8510. (13) Solmosi, F.; Revesz, K. Surf. Sci. 1993, 280, 38. (14) Stem, E. A. Phys. Reu. B 1974,10, 3027. (15) Tohji, K.; Udagawa, Y.; Mizushima, T.; Ueno, A. J . Phys. Chem. 1985, 89, 5671. (16) Greegor, R. B.; Lytle, F. W. J . Curd. 1980, 63,476. (17) Prettre, M.; Eichner, C.; Perrin, M. Trans. Furuduy Soc. 1946,43, 335. (18) Gadalla, A. M.; Sommer, M. E. Chem. Eng. Sci. 1989,44, 2825. (19) Blanks, R. F.; Wittrig, T. S.; Peterson, D. A. Chem. Eng. Sci. 1990, 45, 2407. (20) Kunimori, K.; Umeda, S.;Nakamura, J.; Uchijima, T. Bull. Chem. Soc. Jpn. 1992, 65, 2562.
