Ammonia Oxidation with O₂ and NO on a VO_<i>x</i>/Rh(111) Catalyst: A Comparison

We have studied catalytic ammonia oxidation with NO and O2 on a VOx/Rh(111) catalyst at θV < 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, VOx 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 VOx layer develops under reaction conditions. In the NH3 + O2 reaction microspot-LEED (μLEED) identifies a (√3 × √3)-moiré pattern inside the VOx 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 VOx layer. With NO as reactant, the interface region between the VOx 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 VOx/Rh(111), oxidation with NO is less effective than oxidation with O2.