Surface chemistry and photochemistry of small molecules on rutile TiO(001) and TiO(011)-(2 × 1) surfaces: The crucial roles of defects.

Surface chemistry and photochemistry of small molecules on the rutile TiO(001) and TiO(011)-(2 × 1) surfaces were studied by low energy electron diffraction, thermal desorption spectroscopy, and x-ray photoelectron spectroscopy. It was found that the TiO(001) surface mainly exhibits the defects of Ti interstitials in the near-surface region, while the TiO(011)-(2 × 1) surface mainly exhibits the defects of double-oxygen vacancies. The defect structures of TiO surfaces strongly affect their adsorption and thermal/photodesorption behaviors. On the TiO(001) surface, CHOH and HO dissociatively adsorb at the surface Ti sites near Ti interstitials; O molecularly adsorbs at the surface Ti sites adjacent to Ti interstitials, forming photoactive O species that undergoes a hole-mediated photodesorption process; CO adsorbs at the nearest surface Ti sites close to the Ti interstitials, but CO does not, and the resulting CO species is photoactive; and both CO and CO species adsorbed at the normal Ti sites are photoinactive. On the TiO(011)-(2 × 1) surface, O adsorbs only at the double-oxygen vacancy sites, and the resulting O species dissociates to form two oxygen atoms to refill in the oxygen vacancies upon heating; CO adsorbs at the double-oxygen vacancy sites, but CO does not, and the resulting CO species is photoactive; and both CO and CO species adsorbed at the surface Ti sites are photoinactive. These results broaden the fundamental understandings of the chemistry and photochemistry of TiO surfaces, and the established structure-reactivity relation of small molecules on TiO surfaces is useful in probing complex structures of TiO powder catalysts.