Surface reconstruction on noble-metal catalysts during oxidation of ammonia

Surface reconstructions on Pd, Pd/5 wt.% Ni, Ag, Ir, Pt/10 wt.% Ir, Pt/20 wt.% Ir, and Au wire-shaped specimens have been studied after ammonia oxidation at 900 °C and atmospheric pressure. The tested specimens were examined by powder X-ray diffraction, electron microprobe analysis, and scanning electron microscopy. Extensive surface reconstructions, with numerous pits, well developed facets, and “cauliflower”-like excrescences were, except for Au, observed on all tested specimens. Neglecting Au, the tested catalyst materials were active for oxidation of NH3 to NO, however, the selectivity and effective lifetime of these catalysts are strongly composition dependent. Au did not catalyze ammonia oxidation and its surface reconstructions are negligible. Solid oxides of NiO, PdO and IrO2 were identified on the surfaces of Pd-Ni, Pd, and Ir specimens, respectively. The results are discussed in relation to findings for Pt-Rh alloy catalysts, for which surface reconstruction is attributed to chemical-vapor-transport reactions driven by local temperature gradients between hotspots and colder regions on the catalyst surface.