Reactive removal of unstable mixed $NO+CO$ adlayers: Chemical diffusion and reaction front propagation

A lattice-gas model is developed to describe the reactive removal of a preadsorbed, mixed $NO+CO$ adlayer covering a Pt(100) surface, via reduction of NO with CO, and behavior of the model is analyzed. Since NO dissociation 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 autocatalytic 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. Thereafter follows a transition to a “reaction front propagation” stage of adlayer removal, where a chemical wave develops that propagates into the $NO+CO$ covered region of the surface with roughly constant velocity, and leaves in its wake a surface populated only by excess reactant. We provide a suitable rate equation formulation for the initial disperse stage, but focus on a reaction-diffusion equation analysis of reaction front propagation, examining, in detail, behavior for long times 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 interference on the surface diffusion of each adspecies by coadsorbed species. Thus, a key component of this work is the development of an appropriate treatment of chemical diffusion in mixed layers of several adspecies.