Characterization of oxidation and reduction of a platinum–rhodium alloy by atom-probe tomography

Abstract

An active challenge in heterogeneous catalysis is to minimize the quantities of the expensive platinum group metals used without causing degradation of the overall catalytic efficiency in a chemical reaction. To achieve this goal, a thorough atomic-scale understanding of these materials under reactive conditions is required. This will enable the design and production of “nano-engineered” catalysts, optimised for cost, stability and performance. In this study, the oxidation and reduction behaviour of a Pt–Rh alloy between 873 and 1073 K was investigated by atom-probe tomography (APT). Detailed observations of the concentration profiles at the oxide/metal interfaces show that the growth of Rh 2O 3 oxide is limited by diffusion of Rh in the alloy. By varying the oxidation conditions, it was possible to calculate the activation energy for Rh diffusion in Pt–Rh as 236 ± 41 kJ/mol, together with diffusion coefficients for Rh for a range of temperatures. Reduction of the oxide phase left a thin, almost pure, layer of the most reactive (and expensive) element, Rh, on the surface of the specimen, suggesting a simple route for engineering the formation of the core–shell structure Pt–Rh nanoparticles.