Non-Classical Kinetics in Hydrogen-Deuterium Exchange Reaction on Metal Catalyst
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NON-CLASSfCAL KINETIICS INHYDROGEN-DEUTEBJIUM EXCHANGE RIEACT¶ION ON METAL CATALYST C and Raoul Kopelman Department of Chemistry, University of Michigan, Ann Arbor, MI 48109
ABSTRACT Monte Carlo simulations of the A2 + B2 --> P (with P = AA, AB, and BB) reaction on various monodisperse island systems provided various models for the reaction H2 + D2 --> HD on alumina supported platinum catalysts. Anomalous reaction kinetics and reaction orders due to the effects of diffusion, reactant correlation, and ensemble character of the catalyst in low dimensional systems are observed not only at early time but also at steady state. Concentration effects and spillover studies indicate that surface diffusion and hydrogen spillover may be responsible for the anomalous behaviors. IINTRODUCTION The hydrogen-deuterium exchange reaction on a metal catalytic surface, one of the 17 well-known surface reactions, has been investigated over the past sixty years. However, the conditions employed in these investigations were very different from those employed in practical heterogeneous catalysis. For example, a single-crystal under vacuum may show very different reaction kinetic behavior than a metal catalyst dispersed on a support under gas pressures of an atmosphere or more. The anomalous kinetic behaviors were suggested by both preliminary experimental and simulation studies. Computer simulation studies indicate that the ensemble character of the metal catalyst can cause some anomalies. Monte Carlo simulations of the various elementary reactions (e.g., A + A --> Products and A + B --> Products) on monodisperse 89
islands showed anomalously high reaction orders, i.e., anomalous kinetics. Experimental studies of hydrogen-deuterium exchange reaction also show the anomalous reaction kinetics: Early time of flight measurements of the exchange reaction at low pressure on platinum island catalyst systems indicate anomalously high reaction orders.10 These interesting surface diffusion-limited kinetic behaviors are investigated here further by Monte Carlo simulations of the A 2 + B2 --> P (with P = AA, AB, and BB) reaction on
various monodisperse island systems. SIfMULATfIONS
The main task is to study the effects of diffusion, reactant correlation, and ensemble character of the catalyst in low dimensional systems. The Random Walk method was employed to simulate the surface diffusion. The particle distributions were analyzed by counting the number of nearest neighbors. Simulations were carried out on a Dec 5000 Risc computer. Two different geometries, 'one large lattice' and 'many-islands lattice' were used for 1-dimensional and 2-dimensional systems. A many-islands lattice consists of many individual islands which have the same size. There is a wall surrounding each island. When a particle walks into the wall, it will be reflected to the other direction. Thus, the diffusion process will be limited inside the island but by the landing and desorption processes, each island is connected to the others. Also, a perfect lattice where the energy o
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