Scanning tunneling microscopy and density functional theory studies show that oxygen adatoms (${\mathrm{O}}_a$), produced during ${\mathrm{O}}_2$ exposure of reduced ${\mathrm{TiO}}_2(110)$ surfaces, alter the water dissociation and recombination chemistry through two distinctive pathways. Depending on whether ${\mathrm{H}}_2\mathrm{O}$ and ${\mathrm{O}}_a$ are on the same or adjacent ${\mathrm{Ti}}^{4+}$ rows, ${\mathrm{O}}_a$ facilitates ${\mathrm{H}}_2\mathrm{O}$ dissociation and proton transfer to form a terminal hydroxyl pair, positioned along or across the ${\mathrm{Ti}}^{4+}$ row, respectively. The latter process has not been reported previously, and it starts from a “pseudodissociated” state of water. In both pathways, the reverse H transfer results in ${\mathrm{H}}_2\mathrm{O}$ reformation and O scrambling, as manifested by an apparent along- or across-row motion of ${\mathrm{O}}_a$’s.

• Received 6 November 2008

DOI:https://doi.org/10.1103/PhysRevLett.102.096102