The Role of Surface Hydroxyls in the Mobility of Carboxylates on Surfaces: Dynamics of Acetate on Anatase TiO2(101)

The dynamics of reactive intermediates are important in catalysis for understanding transient species, which can drive reactivity and the transport of species to reaction centers. In particular, the interplay between surface-bound carboxylic acids and carboxylates is important for numerous chemical transformations, including CO2 hydrogenation and ketonization. Here, we investigate the dynamics of adsorbed, dissociated acetic acid to provide insight into the formation of reaction intermediates using scanning tunneling microscopy experiments and density functional theory calculations. We demonstrate the concomitant diffusion of bidentate acetate and a bridging hydroxyl on anatase TiO2(101) and provide evidence for the transient formation of molecular, monodentate acetic acid. The diffusion rate is strongly dependent on the position of the bridging hydroxyl and the presence of adjacent bidentate acetate(s). A facile three-step diffusion process is proposed consisting of a bidentate acetate reaction with a bridging hydroxyl to form monodentate acetic acid, rotation of monodentate acetic acid, and dissociation of monodentate acetic acid to reform bidentate acetate and a hydroxyl. This study clearly demonstrates that the dynamics of bidentate acetate could be important in the formation of monodentate species, which are proposed to be crucial for selective ketonization.