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 (OH b ’s) formed by H 2 O dissociation on bridging oxygen vacancies (V O ’s) of TiO 2 (110) are studied at low and high OH b 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 (HO 2 ), and terminal hydroxyls (OH t ) are observed as intermediates of the reactions with O 2 ultimately leading to H 2 O 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 TiO 2 (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 TiO 2 (110), water is found to mediate the diffusion of surface species such as OH b that would otherwise be stationary and thus brings reactants together, catalyzing the reactions with O 2 . As a result, the O a, HO 2, and OH t intermediates are not observed in STM, while OH b species are available on the surface.