of ruthenium atoms in a site. If there is preferential interaction of copper with the defects, as can be inferred from the results of the present work, it would be reasonable for copper to have a different effect on the catalytic activity of ruthenium when defects are present. Consequently, the effect of copper on catalytic activity could well be different for a ruthenium powder than it is for a smooth (001) plane of ruthenium.The results of the present investigation clearly demonstrate the value of ultraviolet photoemission spectroscopy of physisorbed xenon as a probe of surfaces of interest in catalysis. Since it is sensitive to localized properties of the surface, it is a powerful probe for the detection of surface heterogeneity. It can therefore make a useful contribution in studies of the structure sensitivity of surface-catalyzed reactions.

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A coloured charge-transfer complex formed by adsorption of Fe(CN/& at the surface of Ti02 particles and electrodes upon photoexcitation injects electrons into the conduction band of this semiconductor with a quantum yield of at least 37%.

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A novel catalytic material (mesoporous TiOz films) for the cathodic detection of hydrogen peroxide in air-saturated aqueous solution is described. The immobilization of glucose oxidase (GOD) as enzyme model onto the TiOz electrode provides an efficient biosensor for the arnperometric detection of glucose at -0.15 V (vs. SCE). GOD molecules were immobilized in an inorganic laponite gel and cross-linked by glutaraldehyde. The highest sensitivity and detection limit were 3.33 mA M-' cm-' and 15 pM, respectively. In addition different procedures of enzyme immobilization including cross-linking with glutaraldehyde and physical entrapment in a functionalized polypyrrole film have been compared.