Ammonia Oxidation with O2 and NO on a VOx/Rh(111) Catalyst: A Comparison

We have studied catalytic ammonia oxidation with NO and O₂ on a VO_x/Rh(111) catalyst at θ_V < 0.5 monolayer equivalents (MLE) with photoemission electron microscopy (PEEM) at 10^–4 mbar and with spectroscopic photoemission and low-energy electron microscopy (SPELEEM) in the 10^–6 mbar range. With O₂ as oxidant, VO_x condenses into islands of macroscopic size, i.e., of diameters ranging from tens to several hundreds of microns. With NO a hole pattern in the VO_x layer develops under reaction conditions. In the NH₃ + O₂ reaction microspot-LEED (μLEED) identifies a (√3 × √3)-moiré pattern inside the VO_x islands and on the surrounding metal surface. In NH₃ oxidation with NO reaction microspot X-ray photoelectron spectroscopy (μXPS) shows the presence of nitrogen species on the bare metal surface, as well as on the VO_x layer. With NO as reactant, the interface region between the VO_x covered and bare metal surface areas is strongly broadened with about 100 μm width as compared to ≈30 μm width with O₂ as oxidant. Our data suggest that in ammonia oxidation over VO_x/Rh(111), oxidation with NO is less effective than oxidation with O₂.