Catalytic ammonia oxidation on platinum: mechanism and catalyst restructuring at high and low pressure

Abstract

Catalytic ammonia oxidation over platinum has been studied experimentally from UHV up to atmospheric pressure with polycrystalline Pt and with the Pt single crystal orientations (533), (443), (865), and (100). Density functional theory (DFT) calculations explored the reaction pathways on Pt(111) and Pt(211). It was shown, both in theory and experimentally, that ammonia is activated by adsorbed oxygen, i.e. by O_ad or by OH_ad. In situ XPS up to 1 mbar showed the existence of NH_x (x = 0,1,2,3) intermediates on Pt(533). Based on a mechanism of ammonia activation via the interaction with O_ad/OH_ad a detailed and a simplified mathematical model were formulated which reproduced the experimental data semiquantitatively. From transient experiments in vacuum performed in a transient analysis of products (TAP) reactor it was concluded that N₂O is formed by recombination of two NO_ad species and by a reaction between NO_ad and NH_x,ad (x = 0,1,2) fragments. Reaction-induced morphological changes were studied with polycrystalline Pt in the mbar range and with stepped Pt single crystals as model systems in the range 10⁻⁵–10⁻¹ mbar.