Reconstruction of platinum-rhodium catalysts during oxidation of ammonia

The oxidation of ammonia to nitrogen monoxide is catalyzed by platinum-rhodium alloys. During the first few hours of operation the originally smooth surfaces on Pt-Rh catalysts with Rh contents of 0-13 and 100 wt.% are heavily reconstructed. The process starts on the grain boundaries, but soon spreads to cover the entire surface with “cauliflower”-like excrescences. Only slight reconstructions on the grain boundaries are observed for Pt/20 wt.% Rh and Pt/30 wt.% Rh catalysts. The present paper describes studies of oxidation of ammonia along Pt-Rh catalyst wires, including mass spectrometric surveys of the gaseous species formed. Scanning electron microscopy, electron microprobe, and X-ray powder diffraction characterizations were made of the catalyst wires before and after reaction. The amount and appearance of the reconstructions depend strongly on the Rh content of the catalyst. Marked concentration gradients in platinum-to-rhodium content are observed between wires and “cauliflowers” as well as within the “cauliflowers”. A qualitative account for the formation of the “cauliflowers” and related features is advanced using concepts from chemical vapor-transport reactions. PtO2 and/or RhO2 act as transporting molecules and the processes are driven by local temperature gradients between hotspots and colder regions on the catalyst surface. The ultimate net effect of the involved atomic transfer processes is to establish an approximately equiatomic (Pt50Rh50) composition at all catalyst surfaces.