Reconstruction and loss of platinum catalyst during oxidation of ammonia

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 plat inum. 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 feat ures observed during these reactions. The experiments that have been conduc ted are: (1) heat treatment of platinum wires in different gaseous atmosphe res, (2) oxidation of ammonia along a platinum wire, (3) microprobe analysi s 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 oxida tion process. Through these experiments it has been shown that Pt, PtO and PtO2 are the o nly platinum-containing species in the gas phase. Rhodium is also transport ed via the gas phase by similar, simple species and this results in an incr easing Ph concentration toward the tip of the 'cauliflower curds'. A workin g hypothesis has been advanced to explain the dynamic growth of the 'caulif lowers' through etching of specific planes and depositions on edges and oth er planes. (C) 2001 Elsevier Science B.V. All rights reserved.