Evidence for sinking of small particles into substrates and implications for heterogeneous catalysis

Abstract

THE shapes and structures of small metal particles (1–10 nm in size) supported on oxide substrates and the effect of substrate-particle interactions in such systems are of substantial importance for heterogeneous catalysis. Here we present, for the first time, experimental evidence using atomic-resolution electron microscopy of gold clusters on MgO smoke particles, which shows that a small particle sitting on a substrate may be thermodynamically unstable and can 'sink' into the substrate. A simple theoretical model is proposed to explain this effect. The results may be of importance in small-particle catalyst systems. The stability of the particle with respect to sinking, and hence reactivity, can be changed by chemisorption of impurities, which will change the surface free energies. A plausible model of catalyst deactivation is that the particle sinks diffusively into the substrate in the course of time. Regeneration of the catalyst could require changing the surface free energy by chemisorption, causing the particle to float back onto the substrate surface.