Automobile exhaust catalysis at the atomic scale: atom-probe investigations on platinum alloys

A dedicated three-dimensional atom-probe has been fitted with an in situ high pressure/temperature reaction cell. This was used for studying the interaction of gases with Pt-alloy surfaces typical of those employed in automotive exhaust catalysis. The reactions of nitric oxide (NO) and oxygen (O-2) on Pt and Pt-17.4 at.% Rh FIM specimens were explored, revealing a range of surface restructuring and segregation effects, which depend sensitively on catalyst composition, orientation, temperature and gas chemistry. Specifically, we observed that exposure of Pt-Rh to either NO or O-2 at 422-473 K and 10 mbar induced Rh surface segregation uniformly over the surface. However, on raising the temperature to 573 K or higher, the Pt-Rh{111} surface remained Rh-enriched but the Pt-Rh{011} surface became severely Rh-depleted. We attribute this observation to the possible diffusion of Rh species across the specimen apex, indicating that segregation occurs in directions both normal and parallel to the surface in Pt-Rh catalysts. Copyright (C) 2007 John Wiley & Sons, Ltd.