Critical behavior associated with the loss of bistability for an atomistic model for CO oxidation on surfaces in the limit of infinite diffusion of CO, was analyzed. A 'hybrid' treatment which incorporated a lattice-gas description of the O adlayer, and tracked just the number of adsorbed CO, was used. The study elucidated fluctuation effects observed in experiments of CO oxidation on the nanoscale facets of metal-field emitter tips.