Reconstruction and loss of platinum catalyst during oxidation of ammonia

Abstract

Platinum and its alloys are widely used as catalysts for several oxidation reactions during which they often experience an extensive reconstruction of the surface and develop excrescences that look like ‘cauliflowers’ of platinum. This work has aimed at finding a possible mechanism for the formation of the ‘cauliflowers’ and related features. The experimental basis for the work is the oxidation of ammonia on Pt and Pt/10%Rh materials and the features observed during these reactions. The experiments that have been conducted are: (1) heat treatment of platinum wires in different gaseous atmospheres, (2) oxidation of ammonia along a platinum wire, (3) microprobe analysis of the variations in the relative content of Pt and Rh in ‘cauliflowers’ and wires, and (4) an in situ mass spectrometric investigation of the oxidation process. Through these experiments it has been shown that Pt, PtO and PtO 2 are the only platinum-containing species in the gas phase. Rhodium is also transported via the gas phase by similar, simple species and this results in an increasing Rh concentration toward the tip of the ‘cauliflower curds’. A working hypothesis has been advanced to explain the dynamic growth of the ‘cauliflowers’ through etching of specific planes and depositions on edges and other planes.