Monte Carlo study of CO oxidation on an anisotropic surface

It is well established that anisotropy does not affect the critical behavior of a system in thermodynamic equilibrium that undergoes a second-order phase transition. We study here an anisotropic kinetic model for heterogeneous catalysis that mimics the oxidation of CO on the (110) surface of a transition metal like Pd. We have mapped out the phase diagram of possible steady states for various anisotropic reaction and absorption processes as a function of the ${\mathrm{O}}_2$/CO adsorption rate and the CO diffusion rate. The phase diagram depends upon the amount of anisotropy and exhibits both a first-order and a second-order phase transition for a reactive to an unreactive steady state. We have examined the critical behavior at the second-order phase transition with finite-size scaling and have found that this model belongs to the directed percolation universality class irrespective of the anisotropy.