Investigations on hydrogen spillover. Part 1.—Electrical conductivity studies on titanium dioxide

Abstract

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 spilt-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 °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.