Effect of coadsorbed water on the photodecomposition of acetone on TiO2(110)

The influence of coadsorbed water on the photodecomposition of acetone on TiO2 was evaluated using temperature-programmed desorption (TPD) and the rutile TiO2(110) surface as a model photocatalyst. Of the two major influences ascribed to water in the heterogeneous photocatalysis literature (promotion through OH radical supply and inhibition due to site blocking), only the negative influence of water was observed. As long as the total water + acetone coverage was maintained well below the first-layer saturation coverage (1 ML), little inhibition of acetone photodecomposition was observed. However, as the total water + acetone coverage exceeded 1 ML, acetone was preferentially displaced from the first layer to physisorbed states by water, and the extent of acetone photodecomposition was attenuated. The immediate product of acetone photodecomposition was adsorbed acetate, which occupies twice as many surface sites per molecule as acetone. Because the acetate intermediate was more stable on the TiO2(110) surface than either water or acetone (as gauged by TPD), and because it had a lower photodecomposition rate than acetone, additional surface sites were not opened up during acetone photodecomposition to allow previously displaced acetone molecules to reenter the first layer.