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Langmuir 1997, 13, 2726-2730
Characterization and CH4 Oxidation Activity of Cr/Al2O3 Catalysts Paul Worn Park† and Jeffrey S. Ledford*,‡ Department of Chemistry, The Center for Fundamental Materials Research, Michigan State University, East Lansing, Michigan 48824-1322 Received September 25, 1996. In Final Form: March 5, 1997X X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) have been used to characterize a series of Cr/Al2O3 catalysts (designated “Cry”). The information obtained from surface and bulk characterization has been correlated with the CH4 oxidation activity of the Cry catalysts. For catalysts with Cr/Al atomic ratio ) 0.013, XPS results indicated that most of the Cr was present as a highly dispersed Cr6+ surface phase. Catalysts with intermediate Cr loadings (0.027 e Cr/Al atomic ratio e 0.080) showed no XRD peak characteristic of chromium oxide; however, XPS data indicated that the Cr dispersion decreased and the concentration of Cr3+ species increased with increasing Cr content. For catalysts with high Cr loadings (Cr/Al atomic ratio g 0.107), large Cr2O3 crystallites were detected by XRD. The specific activity for CH4 oxidation increased with increasing Cr content up to Cr/Al ) 0.107. This has been attributed to an increase in the amount of Cr(III)-Cr(VI) cluster present in the catalysts. A decrease in CH4 oxidation activity observed for Cr-rich catalysts (Cr/Al ) 0.13) has been ascribed to the formation of poorly dispersed Cr2O3.
Introduction Alumina-supported chromium oxides have been studied extensively due to their potential application in emission control catalysis.1-3 Chemical and physical properties such as low-temperature efficiency for hydrocarbon4-6 and chlorinated hydrocarbon oxidation,7-10 low selectivity for Cl2 formation8,11 and high resistance to HCl poisoning12 during chlorinated hydrocarbon oxidation, NO reduction with CO13 and NH3,14 high thermal stability,6,15 and mechanical durability12 make Cr/Al2O3 catalysts suitable candidates for industrial waste treatment. Chromium oxide is also an effective promoter for copper oxide catalysts used for CO and hydrocarbon oxidation and NO reduction.13,16,17 The surface structure of alumina-supported chromium oxide catalysts depends on the metal loading, calcination † Current Address: Center for Catalysis and Surface Science, Northwestern University, Evanston, IL 60208. ‡ Current Address: Mobil Chemical Company, Films DivisionTechnical Center, 729 Pittsford-Palmyra Road, Macedon, NY 14502. X Abstract published in Advance ACS Abstracts, April 15, 1997.
(1) Kummer, J. T. Prog. Energy Combust. Sci. 1980, 6, 177. (2) Armor, J. N. Appl. Catal. B 1992, 1, 221. (3) Zwinkels, M. F. M.; Jaras, S. G.; Menon, P. G.; Griffin, T. A. Catal. Rev.sSci. Eng. 1993, 35, 319. (4) Dmuchovsky, B.; Freerks, M. C.; Zienty, F. B. J. Catal. 1965, 4, 577. (5) Kang. Y.-M.; Wan, B.-Z. Appl. Catal. A 1994, 114, 35. (6) Prasad, R.; Kennedy, L. A.; Ruckenstein, E. Combust. Sci. Technol. 1980, 22, 271. (7) Manning, M. P. Hazard. Waste 1984, 1, 41. (8) Weldon, J.; Senkan, S. M. Combust. Sci. Technol. 1986, 47, 229. (9) Kolaczkowski, S. T.; Beltran, F.; Crittenden, B. D.; Jefferies, T. M. Process Saf. Environ. Prot. 1990, 68, 49. (10) Drago, R. S.; Petrosius, S. C.; Grunewald, G. C.; Brendley, W. H., Jr. In Environmental Catalysis; Armor, J. N., Ed.; ACS Symposium Series 552; American Chemical Society: Washington, DC, 1994; Chapter 28. (11) Agarwal, S. K.; Spivey, J. J.; Butt, J. B. Appl. Catal. 1992, 82, 259. (12) Johnson, A. J.; Meyer, F. G.; Hunter, D. I.; Lombardi, E. F. Incineration of polychlorinated biphenyls using a fluidized bed incinerator. Rockwell International RFP-3271; 1981. (13) Shelef, M.; Otto, K.; Gandhi, H. J. Catal. 1968, 12, 361. (14) Wong, W. C.; Nobe, K. Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 179. (15) Simon, S.; van der Pol, A.; Reijerse, E. J.; Kentgens, A. P. M.; van Moorsel, G.-J. M. P.; de Boer, E. J. Chem. Soc., Faraday Trans. 1995, 91, 1519. (16) Dwyer, F. G. Catal. Rev. 1972, 6, 261.
S0743-7463(96)00931-6 CCC: $14.00
temperature, and extent of hydration.18-26 For low chromium loadings (e10 wt % Cr2O3/Al2O3 200 m2/g), most of the chromium is present as a highly dispersed Cr(VI) surface species. XPS studies indicate that the proportion of Cr(VI) on the alumina support decreases with increasing Cr content18,19,21 or with increasing calcination temperature.21,27 This has been attributed to the formation of Cr(III) species on the support surface.19,21,27 Ellison and co-workers28-32 studied Cr/Al2O3 catalysts using magnetic susceptibility, diffuse reflectance spectroscopy (DRS), SIMS, and ESR. They proposed that a mixed oxidation state cluster (Cr6+-O2--Cr3+-O2--Cr6+; γ-phase) forms on the alumina support even at 1 wt % Cr on 157 m2/g Al2O3. Wachs and co-workers22,33 reported monolayer coverage of chromium oxide (12 wt % CrO3/Al2O3 180 m2/ g) based on Raman, XRD, and FTIR data. This is in good agreement with the theoretical monolayer coverage calculated from the cross section of K2Cr2O734 and ion scattering spectroscopy (ISS) results.35 For high chromium loadings (>10 wt % Cr2O3/Al2O3 200 m2/g), crystal(17) Kapteijn, F.; Stegenga, S.; Dekker, N. J. J.; Bijsterbosch, J. W.; Moulijn, J. A. Catal. Today 1993, 16, 273. (18) Okamoto, Y.; Fujii, M.; Imanaka, T.; Teranishi, S. Bull. Chem. Soc. Jpn. 1976, 49, 859. (19) Jagannathan, K.; Srinivasan, A.; Rao, C. N. R. J. Catal. 1981, 69, 418. (20) Gorriz, O. F.; Corbera´n, V. C.; Fierro, J. L. G. Ind. Eng. Chem. Res. 1992, 31, 2670. (21) Rahman, A.; Mohamed, M. H.; Ahmed, M.; Aitani, A. M. Appl. Catal. 1995, 121, 203. (22) Vuurman, M. A.; Hardcastle, F. D.; Wachs, I. E. J. Mol. Catal. 1993, 84, 193. (23) Hardcastle, F. D.; Wachs, I. E. J. Mol. Catal. 1988, 46, 173. (24) Vuurman, M. A.; Wachs, I. E.; Stufkens, D. J.; Oskam, A. J. Mol. Catal. 1993, 80, 209. (25) Vuurman, M. A.; Wachs, I. E. J. Phys. Chem. 1992, 96, 5008. (26) Weckhuysen, B. M.; Schoonheydt, R. A.; Jehng, J.-M.; Wachs, I. E.; Cho, S. J.; Ryoo, R.; Kilistra, S.; Poels, E. J. Chem. Soc., Faraday Trans. 1995, 91, 3245. (27) Cimino, A.; De Angelis, B. A.; Luchetti, A.; Minelli, G. J. Catal. 1976, 45, 316. (28) Ellison, A.; Oubridge, J. O. V.; Sing, K. S. W. Trans. Faraday Soc. 1970, 66, 1004. (29) Ellison, A.; Sing, K. S. W. J. Chem. Soc., Faraday Trans. 1978, 74, 2017. (30) Ellison, A. J. Chem. Soc., Faraday Trans. 1 1984, 80, 2567. (31) Ellison, A.; Sing, K. S. W. J. Chem. Soc., Faraday Trans. 1978, 74, 2807. (32) Ellison, A. J. Chem. Soc., Faraday Trans. 1 1984, 80, 2581. (33) Turek, A. M.; Wachs, I. E.; DeCanio, E. J. Phys. Chem. 1992, 96, 5000.
© 1997 American Chemical Society
Cr/Al2O3 Catalysts
line Cr2O3 is observed using Raman spectroscopy and X-ray diffraction. Bulk Cr(III)-Al2O3 forms after high-temperature calcination (g800 °C) of crystalline Cr2O3 and the alumina support.22 Despite the importance of Cr/Al2O3 catalysts in emission control applications, the study of CH4 oxidation over Cr/ Al2O3 catalysts has been relatively limited.36,37 Little effort has been devoted to investigating systematically the relationship between the surface structure and CH4 oxidation activity of Cr/Al2O3 catalysts. Anderson et al.36 have studied CH4 oxidation over various alumina-supported transition metal oxide catalysts and reported that chromium oxide shows the highest methane oxidation activity. Kuznetsova et al.37 have reported that CH4 oxidation activity increases and levels off with increasing Cr content. They proposed that Cr(VI) is the main active component for the CH4 oxidation reaction. The present work is part of a broad study to investigate structure-activity correlations for transition metal oxidebased emission control catalysts. In this paper, X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) have been used to determine the effect of Cr loading on the chemical state and dispersion of chromium oxide phases supported on γ-alumina. The information derived from these techniques is correlated with CH4 oxidation activity to develop a more complete understanding of Cr/ Al2O3 catalysts. Experimental Section Catalyst Preparation. The Cr-modified alumina carriers were prepared by pore volume impregnation of γ-alumina (Cyanamid, surface area ) 203 m2/g, pore volume ) 0.6 mL/g) using solutions of chromium(III) nitrate (Mallinckrodt, Analytical Reagent). The alumina was finely ground (
