Dynamics of ultrathin V-oxide layers on Rh(111) in catalytic oxidation of ammonia and CO

Abstract

Catalytic oxidation of ammonia and CO has been studied in the 10⁻⁴ mbar range using a catalyst prepared by depositing ultra-thin vanadium oxide layers on Rh(111) (θ_V ≈ 0.2 MLE). Using photoemission electron microscopy (PEEM) as a spatially resolving method, we observe that upon heating in an atmosphere of NH₃ and O₂ the spatial homogeneity of the VO_x layer is removed at 800 K and a pattern consisting of macroscopic stripes develops; at elevated temperatures this pattern transforms into a pattern of circular VO_x islands. Under reaction conditions the neighboring VO_x islands become attracted by each other and coalesce. Similar processes of pattern formation and island coalescence are observed in catalytic CO oxidation. Reoxidation of the reduced VO_x catalyst proceeds via surface diffusion of oxygen adsorbed onto Rh(111). A pattern consisting of macroscopic circular VO_x islands can also be obtained by heating a Rh(111)/VO_x catalyst in pure O₂.