Heterogeneous catalysis is usually accomplished with materials in nanoparticulate form, and in this paper I will briefly review recent approaches to understanding catalytic processes and materials using surface science methods. This approach involves the fabrication of ultra-nanoparticulate model catalysts using MVD (metal vapour deposition) and MOCVD (metal-organic chemical vapour deposition) to form particles in the size range 1-10 nm. which can be imaged at very high spatial resolution (and in some cases at atomic resolution) using STM (scanning tunnelling microscopy). I will mainly focus on a system which we have used for anaerobic photocatalytic hydrogen production, namely Pd/TiO₂ powdered catalysts, and will describe a surprisingly strong dependence of the rate upon loading of the metal. Using well-defined materials fabricated in UHV, we have resolved a number of important catalytic phenomena using STM and other techniques, including:-

The Mechanism of Oxidation and Reduction of Titania. We will show that the main species involved in these processes is interstitial Ti³⁺ , which diffuses into the bulk after oxygen loss during high temperature thermal treatment, and which returns to the surface in an oxidising environment.

Spillover and Oxygen Storage. We have identified oxygen spillover from Pd nanoparticles supported on single crystal TiO₂ as a ring of material extending about 3nm. from the edge of the nanoparticles onto the support surface. Growth of TiO₂ layers around the Pd nanoparticles occurs due to the conversion of Ti³⁺ on the support to Ti⁴⁺, by transport of interstitial Ti³⁺ from the bulk to the surface at temperatures > 300ºC. This is determined by high temperature STM.

SMSI. The strong metal-support interaction is resolved at atomic resolution as an ‘alloyed’ surface layer of Pd:Ti. The Ti is oxidised , but is probably only in the +2 state (by XPS).

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