Water as a Catalyst: Imaging Reactions of O2 with Partially and Fully Hydroxylated TiO2(110) Surfaces

The reactions of molecular oxygen with bridging hydroxyl groups (OHb'S) formed by H2O dissociation on bridging oxygen vacancies (V-o's) of TiO2(110) are studied at low and high OHb coverages as a function of the O-2 exposure, using scanning tunneling microscopy, temperature programmed desorption, and electron stimulated desorption techniques. In agreement with prior studies, oxygen adatoms (O-a), hydroperoxyls (HO2), and terminal hvdroxyls (OHt) are observed as intermediates of the reactions with O-2 ultimately leading to H2O as a product. Here, we show that water plays an important role in the room-temperature reactions of O-2 with both partially and fully hydroxylated TiO2(110). On partially hydroxylated surfaces, water is found to be involved in the reaction cycle that leads to the consumption of O-a and V-O sites thus resulting in a practically O-a- and V-O-free surface. In these reactions, water is observed to participate in multiple ways-as a reactant, product, and catalyst. On fully hydroxylated TiO2(110), water is found to mediate the diffusion of surface species such as OHb that would otherwise be stationary and thus brings reactants together, catalyzing the reactions with O-2. As a result, the O-a, HO2, and OHt intermediates are not observed in STM, while OHb species are available on the surface.