Resolving catalytic phenomena with scanning tunnelling microscopy

Abstract

Scanning tunnelling microscopy (STM) has proved to be an invaluable tool for investigating surface reactions and catalysis at the atomic scale on model catalysts. We show that good models of nanoparticulate catalyst surfaces can now be fabricated and imaged by the use of surface science methodology. In this paper the application of STM to some particular problems in catalysis is addressed. Specifically, these are: (i) Sintering. It is shown that sintering is a complex process, and, at least for separated nanoparticles anchored to a support, it occurs in a surface-mediated Ostwald ripening manner. That is, a reduction in particle number density and an increase in average particle size occurs by loss of atoms from the edge of nanoparticles, which then diffuse across the support to another particle. The net effect of such diffusion is that big particles get bigger at the expense of small ones, which eventually disappear. (ii) Spillover. By imaging individual nanoparticles, at elevated temperature, in the presence of gas phase oxygen, spillover of oxygen to the support is seen to occur directly. It happens because the dissociation probability of oxygen on Pd is much higher than that on titania, and spillover occurs by reaction with reduced titanium cations to grow new layers of titania around the metal nanoparticle, eventually totally encapsulating it. (iii) The so-called ‘strong metal–support interaction’ (SMSI). By the use of atomically-resolving STM this is shown to be due to the formation of an alloy-like mixed layer of Pd and Ti, which results in a surface of much reduced reactivity.