Modeling triangular titration fronts in the O-2+H-2 reaction on a catalytic Rh(111) surface

We present a model for the titration of an oxygen saturated catalytic Rh(111) surface with hydrogen. Oxygen is removed by reaction-diffusion fronts. Experimentally, these fronts have been observed to be either isotropic or triangular depending on the conditions of preparation of the oxygen layer as well as on temperature and hydrogen pressure. If we model only the surface reaction and the diffusion of hydrogen, we obtain isotropic fronts with velocities in the range of 2-3 mum s-1. These results are in line with experimental measurements for surfaces exposed to oxygen for a short period. To correctly reproduce the possible triangular shape of the titration fronts and the smaller front velocities of 0.1-1 mum s(-1) for experiments with the surface exposed to oxygen for a long time (>1 h), we have to include the formation of a subsurface oxygen-rich phase and its removal. It is assumed that the phase transition between the oxygen-rich and oxygen-free subsurface phases occurs via front propagation, and the front speed has triangular symmetry compatible with the symmetry of the crystalline bulk. By fitting parameters describing the propagation of the phase transition front, its anisotropy and its coupling to the reaction-diffusion front on the surface, we have been able to quantitatively reproduce all experimental observations presented by Schaak and Imbihl in Chem. Phys. Lett. 283, 386 (1998). (C) 2002 American Institute of Physics