INVESTIGATIONS ON HYDROGEN SPILLOVER .1. ELECTRICAL-CONDUCTIVITY STUDIES ON TITANIUM-DIOXIDE

The influence of hydrogen spillover on the conductivity of polycrystalline platinum-containing (in comparison to platinum-free) titania samples has been investigated. A special two-component sample geometry, consisting of a Pt/TiO2 part and a TiO2 part, was used to separate the conductivity effects after exposure to hydrogen which could arise owing to changes of the platinum/titania interface and to the action of the split-over hydrogen species. It was found that the electrical resistance of Pt/TiO2 samples was decreased by treatment with hydrogen at low temperatures (room temperature to 80-degrees-C) and that, vice versa, the evacuation of hydrogen led to an increase in sample resistance. The electrical resistance of pure TiO2 was not affected by hydrogen. Using two-component (Pt/TiO2)-TiO2 samples the conductivity changed in both sample components after hydrogen adsorption. The decrease of resistance in the TiO2 component of the (Pt/TiO2)-TiO2 sample occurred with a delay due to the diffusion of the spilt-over hydrogen species from the Pt/TiO2 into the TiO2 component. The conductivity change in the TiO2 component can be correlated with the action of activated hydrogen species formed on platinum and having diffused onto the titania support in a spillover form. The spilt-over hydrogen species can be described as surface electron donors, i.e. as coexisting H atoms and H+ ions. Their chemisorption on titania corresponds to an electron transfer to the support and, thus, to an increase of the n-type conductivity.