The rate of reaction for oxidation of CO over (210) and (111) single-crystal surfaces of platinum has been studied as a function of reactant pressures (PO2,PCO) and sample temperature (T), both experimentally and by computer simulation. Experimental results on both surfaces show regions with a steady high rate of reaction followed by a nonsteady transition region and, at high CO pressures, a region with low reactivity caused by CO poisoning of the surface. At constant sample temperature, the transition region can be narrow and depends critically on the ratio of the gas phase concentration of reactants (PCO/PO2). The temperature dependences of the experimental data indicate that the critical ratio and the details for the occurrence of CO poisoning are strongly affected by surface processes such as adsorption, desorption, and diffusion ordering and reconstruction phenomena. A computer simulation model of the Langmuir–Hinshelwood surface reaction as developed by Ziff et al. was used for the simulation of the reaction under flow conditions. The initial fair agreement between this model and the experiment can be significantly improved if processes such as adsorption, desorption, and diffusion are taken into account in an extended simulation model which in turn provides an insight into the kinetics of adsorbate poisoning and the effect of adsorbate-induced processes on the reaction.

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__ _ _ _~~it is t r a n h r m c d into a bulk xilver sulride. Multilayer adw'ption of sulfur can be assumed as a precursor statc during and after exposure at low temperature. During exposure a t room temperature or during heating t h e low temperature state t o room temperature the physisorbed sulfur layer is responsible for the supply or sulfur. The changes in the FEM pattern on heating above 453 K are induced by the transport of silver ions. A chemisorbed slate seems to be stable after the decomposition of Ag2S abovc 723 K. These observations differ from those on Pt a n d W but are in agreement with the generally observed higher reactivity of silver towards sulfur. Helpful discussions Hith Dr. D. Cocke, Mr. 0. Frank and Dr. W. A. Schmidt and finaiicial support by the Deutsche Forschungsgemeinschaft and the Scnat fur Wirtschaft of West-Berlin (ERP-Fonds) are gratefully acknowledged. K. Hayek expresses his appreciation for a fcllouship of the Max-Planck-Gesellschaft.

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