REACTIVE REMOVAL OF UNSTABLE MIXED NO- CHEMICAL DIFFUSION AND REACTION FRONT PROPAGATION(CO ADLAYERS )

A lattice-gas model is developed to describe the reactive removal of a preadsorbed, mixed NO+CO adlayer covering a Pt(100) surface, via redu ction of NO with CO, and behavior of the model is analyzed. Since NO d issociation requires an adjacent empty site, the NO+CO covered surface constitutes an unstable steady state. The creation of vacancies leads NO dissociation, the reaction of CO with the O formed by dissociation , the subsequent creation of more vacancies, and thus the autocatalyti c removal of the adlayer. The high mobility of most adspecies leads to an initial ''disperse stage'' of adlayer removal, characterized by an exponential increase in the number of highly dispersed vacancies. The reafter follows a transition to a ''reaction front propagation'' stage of adlayer removal, where a chemical wave develops that propagates in to the NO+CO covered region of the surface with roughly constant veloc ity, and leaves in its wake a surface populated only by excess reactan t. We provide a suitable rate equation formulation for the initial dis perse stage, but focus on a reaction-diffusion equation analysis of re action front propagation, examining, in detail, behavior for long time s where the front is nearly planar. We emphasize that it is necessary to incorporate the coverage-dependent and tensorial nature of chemical diffusion in the mixed adlayer. Both these features reflect the inter ference on the surface diffusion of each adspecies by coadsorbed speci es. Thus, a key component of this work is the development of an approp riate treatment of chemical diffusion in mixed layers of several adspe cies. (C) 1998 American Institute of Physics.