Identification of active sites available for hydrogen evolution of Single-Atom Ni1/TiO2 catalysts

Using density functional theory calculations, we identify the active sites for hydrogen evolution of single-atom Ni supported on TiO2, Ni1/TiO2, catalysts (SACs). Strong binding to 2-coordinated O atoms (O2c) inhibits the dimerization of Ni atoms on the TiO2 (1 0 1) surface. Nudged elastic band calculations show that hydrogen evolution reactions can occur on SA Ni and its surrounding O atoms, which are believed to be the active sites. Free energy changes of hydrogen adsorption reveal that the hydrogen evolution activity of Ni1/TiO2 is roughly comparable to that of Pt, especially after H passivation. A significant electron transfer from SA Ni to O2c weakens the Ni-H bond and the O-H bond in the reaction. Free energy prediction and reaction simulation are consistent in identifying active sites and processes of hydrogen evolution, both of which can be combined for the rational design of SACs. Our work also suggests that the strategy of SACs based on metal oxides could potentially open the way to the design of HER catalysts using transition metals instead of noble metals.