Effect of O₂, CO, and NO on Surface Segregation in a Rh_0.5Pd_0.5Bulk Crystal and Comparison to Rh_0.5Pd_0.5Nanoparticles^†

We present an in situ study of the interaction of a bimetallic Rh0.5Pd0.5 bulk crystal with O-2, CO, and NO using ambient pressure X-ray photoelectron spectroscopy (APXPS) and compare it to results for 15 nm nanoparticles with the same overall composition. The bulk crystal surface has less Rh present under both oxidizing and reducing conditions than the surface of nanoparticles under identical conditions. Segregation and oxidation/reduction proceeds faster and at lower temperature for nanoparticles than for the bulk crystal. The near surface of the Rh0.5Pd0.5 bulk crystal after high temperature vacuum annealing is ca. 9% Rh measured by APXPS. Heating in 0.1 Torr O-2 to 350 degrees C increases the Rh surface composition to ca. 40%. The surface can then be reduced by heating in H-2 at 150 degrees C, leading to a chemically reduced surface with 30% Rh. Titration of CO by gas-phase O-2 from this Rh-rich surface proceeds at a much lower pressure than that on the Rh-deficient starting surface.