PNNL

Abstract

The competitive interaction between acetone and water for surface sites on TiO2 (110) was examined using temperature programmed desorption (TPD). Two surface pretreatment methods were employed, one involving vacuum reduction of the surface by annealing at 850 K in ultrahigh vacuum (UHV) and another involving surface oxidation with molecular oxygen. In the former case the surface possessed about 7% oxygen vacancy sites and in the latter reactive oxygen species (adatoms and molecules) were deposited on the surface as a result of oxidative filling of vacancy sites. On the reduced surface, excess water displaced all but about 20% of a saturated d6-acetone first layer to physisorbed desorption states, whereas about 40% of the first layer d6-acetone was stabilized on the oxidized surface against displacement by water through a reaction between oxygen and d6-acetone. The displacement of acetone on both surface is explained in terms of the relative desorption energies of each molecule on the clean surface and role of intermolecular repulsions in shifting their respective desorption features to lower temperatures with increasing coverage. Although first layer water desorbs from TiO2 (110) at slightly lower temperature (275 K) than submonolayer coverages of d6-acetone (340 K), intermolecular repulsions between d6-acetone molecules shift its leading edge for desorption to 170 K as the first layer is saturated In contrast, the desorption leading edge for first layer water (with or without coadsorbed d6-acetone) was at 210 K. This small difference in the onsets for d6-acetone and water desorption resulted in the majority of d6-acetone being compressed into islands by water and eventually displaced from the first layer when excess water was adsorbed. On the oxidized surface the species resulting from reaction of d6-acetone and oxygen was not influence by increasing water coverages. This species was stable on the clean surface up to 375 K (well past the first layer water TPD feature) where it decomposed mostly back to d6-acetone and atomic oxygen. These results are discussed in terms of the influence of water in inhibiting acetone photo-oxidation on TiO2 surfaces.