Langmuir 1996,II,3431-3434
343 1
Effect of COz on the Performance of Au/lMnO, and Pt/SnO, Low-Temperature CO Oxidation Catalysts Gar B. Hoflund" and Steven D. Gardnert Department of Chemical Engineering, University of Florida, Gainesville, Florida 3261 I
David R. Schryer, Billy T. Upchurch, and Erik J. Kielin NASA Langley Research Center, Hampton, Virginia 23665 Received January 19, 1995@ The performance of AulMnO, and WSnO, catalysts for lowtemperature (< 100 "C) CO oxidation under stoichiometric conditions has been examined for the case where 8%or 16% C02 is added to the feed gas. High concentrations of C02 in the reaction gas mixture adversely affect the behavior of these catalysts due to C02 retention at their surfaces. The magnitude of this detrimental effect can be reduced by adding an Fe promotor-toboth types of catalysts and optimizing various preparative and pretreatment variables.
Introduction Efforts to develop efficient low-temperature (< 100 "C) CO oxidation catalysts began over 10years ago when Stark and co-~orkersl-~ demonstrated that platinized tin oxide is effective for this function. Their primary interest was to develop a catalyst for use in closed-cycle C02 lasers4ps in order to maintain a high-power output. 0 2 formed by decomposing COa in the laser electrode region results in rapid power loss. This problem can be overcome by catalytically combining the stoichiometricamounts of CO and 0 2 formed in the discharge. Since then, other applications relating to air pollution control and use as sensor materials have become important. Many catalysts have been screened over the last 7 years6-13 in order to identify new catalysts for lowtemperature CO oxidation. A number of catalysts have been identified which may be classified as noble metals on reducible oxides (NMRO catalysts6). There are many variables involved in the preparation, pretreatment, and reaction which all significantly affect the catalytic activity and decay behavior of the different catalysts. In an effort to understand the behavior of these catalysts, characPresent address: Department of Chemical Engineering, Mississippi State University, P.O. Box 9595, Mississippi State, MS 39762.
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Abstract published in Advance A C S Abstracts, September 1, 1995. (1) Stark, D. S.;Crocker, A.; Steward, G. J. J.Phys. E: Sci. Instrum. 1983,16, 158. (2) Stark, D. S.; Harris, M. R. J.Phys. E:Sci.Instrum. 1983,16,492. (3) Stark, D. S.;Harris, M. R. J.Phys. E: Sci. Instrum. 1988,21,715. (4) Batten, C. E., Miller, I. M., Wood, G. M., Jr., Willetts, D. V., Eds. Closed-Cycle, Frequency-Stable COz Laser Technology. Proceedings of
a workshop held a t NASA Langley Research Center, Hampton, VA, June 10-12, 1986; NASA Conference Publication 2456, 1987. (5)Schryer, D. R., Hoflund, G. B., Eds. Low-Temperature COOzidation Catalysts for Long-LifeCOz Lasers. Collected papers from a n international conference held a t NASA Langley Research Center, Hampton, VA, Oct 17-19, 1989; NASA Conference Publication 3076, 1990. (6) Haruta, M.; Kobayashi, T.; Yamada, N. Chem. Lett. 1987, 405. (7) Haruta, M.; Yamada, N.; Kobayashi, T.; Iijima, S. J.Catal. 1989, 115, 301. (8)Haruta, M.; Tsubota, S.; Kobayashi, T.; Kageyama, H.; Genet, M.; Delmon, B. J. Catal. 1993, 144, 175. (9) Boccuzzi, T.; Tsubota, S.;Haruta, M. J.Electron Spectrosc. Rel. Phenom. 1993,64165,241. (10) Cunningham, D.; Tsubota, S.; Kamijo, N.; Haruta, M. Res. Chem. Interm. 1993, 19, 1. (11) Haruta, M.; Tsubota, S.; Kobayashi, T.; Ueda, A.; Sakurai, H.; Ando, M. Proc. 10th Intern. Cong.Catal. 1993, 2657. (12) Gardner, S. D.; Hoflund, G. B.; Schryer, D. R.; Schryer, J.; Upchurch, B. T.; Kielin, E. J. Langmuir 1991, 7, 2135. (13) Lin, S. D.; Bollinger, M.; Vannice, M. A. Catal. Lett. 1993, 17, 245.
terization studies using X-ray photoelectron spectroscopy (XPS),Auger electron spectroscopy (AES),and ion scattering spectroscopy(ISS)have been carried out by Hoflund and co-workers on WSnO, catalysts,14-16Au/MnO, catalyst~,~ and ' Ada-Fe2Os and AdCo304 catalysts.18 The catalytic activity data presented in the studies of these c a t a l y s t ~ ~ - *have J ~ - ~been ~ obtained using reactor feed mixtures containing only low concentrations of the reactants CO and 0 2 usually in the stoichiometric ratio4t518J9-21or 0 2 However, recently presented reaction data on WSnO, and Au/MnO, c a t a l y s t ~ l ~ , ~ ~ demonstrate that C02 retention is a primary factor contributing to the decay of their activities and that W SnO, catalysts typically are affected more severely than Au/MnO, catalysts. These results suggest that the presence of significant amounts of C02 in the feed gas may res& in poisoning of these catalysts. Since the C02 concentration level is quite large in COPlasers (typically 8-16%), it is necessary to examine the influence of high CO2 concentrations on the behavior of WSnO, and A d MnO, catalysts. CO oxidation over Pt metal occurs by a LangmuirHinshelwood mechanism between adsorbed 0 and adsorbed CO to form C02, which rapidly desorbs. Although C02 does not inhibit this reaction, CO does act as an inhibitor at low temperatures (
