Kinetics of the 2A+B2→2AB reaction complicated by spatial constraints for adsorption, formation of islands, and steps

We present Monte Carlo simulations of the steady-state kinetics of the 2A + B-2 -->2AB reaction, occurring via the Langmuir-Hinshelwood (LH) mechanism on perfect and stepped surfaces. The calculations are carried out with a realistic ratio between the rates of the elementary steps (the LH step is rapid compared with A and B-2 adsorption and slow compared with A diffusion). The main novel factor introduced into the model is that B-2 adsorption is considered to occur on next-nearest-neighbour (nnn) sites (location of B particles in nearest-neighbour sites is prohibited). This constraint, mimicking O-2 adsorption on Pt(111), changes qualitatively the reaction regimes compared with those predicted by the conventional reaction scheme. In particular, the model does not predict complete poisoning of the surface by B even if the relative B pressure is high and B diffusion is negligible. On the other hand, the model easily predicts formation of B islands if the Mn B-B lateral interaction is attractive. The size of islands is found to be very sensitive with respect to the details of the LH step. Simulating the reaction on a stepped surface, we consider that the step sites are much more reactive compared with terrace sites and that the B, sticking coefficient for steps is much higher than for terraces. In this case, the model predicts gradients in the distribution of B particles on the surface. Such non-uniform distributions are, however, generated only in a limited range of the model-parameter values. Dramatic changes in the distribution of B particles due to formation of B islands or B diffusion towards steps are demonstrated to result in nearly negligible changes in the apparent reaction kinetics. (C) 1998 Elsevier Science B.V. All rights reserved.