Characterization of Oxidation and Reduction of Pt–Ru and Pt–Rh–Ru Alloys by Atom Probe Tomography and Comparison with Pt–Rh

Pt-based alloys containing Rh and Ru are effective catalysts in a range of applications, including pollution control and low-temperature fuel cells. As the Pt group metals are generally rare and expensive, minimizing the loading of them while also increasing the efficiency of catalyst materials is a continual challenge in heterogeneous catalysis. A smart method to “nanoengineer” the surface of the nanocatalyst particles would greatly aid this goal. In our study, the oxidation of a Pt–8.9 at. % Ru alloy between 773 and 973 K and the oxidation and oxidation/reduction behavior of a Pt–23.9 at. % Rh–9.7 at. % Ru alloy at 873 K for various exposure times were studied using atom probe tomography. The surface of the Pt–Ru alloy is enriched with Ru after oxidation at 773 K, whereas it is depleted in Ru at 873 K, and at 973 K. The surface oxide layer vanishes at higher temperatures, leaving behind a Pt-rich surface. In the case of the Pt–Rh–Ru alloy, oxidation initiates from the grain boundaries, forming an oxide with a stoichiometry of MO2. As the oxidation time increases, this oxide evolves into a two-phase nanostructure, involving a Rh-rich oxide phase (Rh, Ru)2O3and a Ru-rich oxide phase (Ru, Rh)O2. When this two-phase oxide is reduced in hydrogen at low temperatures, separate Rh-rich and Ru-rich nanoscale regions remain. This process could, therefore, be useful for synthesizing complex island structures on Pt–Rh–Ru nanoparticle catalysts.