Site-Specific Photosimulated Reactions of O$_{2}$ on TiO$_{2}$(110)

We report the direct observation at an atomic level with high-resolution scanning tunneling microscopy of photostimulated reactions of single O$_{2}$ molecules on reduced TiO$_{2}$(110) surfaces at 50 K. Two distinct reactions of O$_{2}$ desorption and dissociation occur at different active sites of terminal Ti atoms and bridging O vacancies, respectively demonstrating the critical relation between photoreactivity and adsorption sites on TiO$_{2}$. These two reaction channels follow very different kinetics. Hole-mediated O$_{2}$ desorption is promptly and fully completed, while electron-mediated O$_{2}$ dissociation is much slower and is quenched above some critical O$_{2}$ coverage. Density functional theory calculations indicate that both coordination and charge state of an O$_{2}$ molecule chemisorbed at specific site largely determine a particular reaction pathway.