Direct observation of chemisorption induced changes in concentration profile in Pd–Au alloy nanosystems via in situX-ray powder diffraction

Abstract

Pd–Au nanocrystalline systems supported on silica were exposed to different gas environments and studied by in-situ XRD. Subtle changes were detected in the XRD patterns when after heating in dry air and flushing with argon a short pulse of H₂ was injected into the stream of argon. These changes were interpreted in terms of an inversion of concentration profile in Pd–Au particles from Pd segregation induced by oxygen chemisorption to Au segregation for the clean surface. Full atomistic models of the alloy nanoparticles for sizes of up to 5 nm were computed using Sutton–Chen potentials, configurational minimization and molecular dynamics. Thus it was possible to simulate the inversion of the concentration profile and its effect on the XRD pattern. The results of the simulation allowed qualitative analysis of the experimental XRD evolution. A specially designed in situ camera together with a new method of process monitoring and analysis allowed direct structural observation of the dynamics of change of the concentration profile within the nanoparticles. It is demonstrated that XRD, when aided with atomistic simulations, can be suitable to follow in situ evolution of the surface structure of nanopowders.