Surface chemistry and photochemistry of small molecules on rutile TiO2(001) and TiO2(011)-(2 × 1) surfaces: The crucial roles of defects

Abstract

Surface chemistry and photochemistry of small molecules on the rutile TiO₂(001) and TiO₂(011)-(2 × 1) surfaces were studied by low energy electron diffraction, thermal desorption spectroscopy, and x-ray photoelectron spectroscopy. It was found that the TiO₂(001) surface mainly exhibits the defects of Ti interstitials in the near-surface region, while the TiO₂(011)-(2 × 1) surface mainly exhibits the defects of double-oxygen vacancies. The defect structures of TiO₂ surfaces strongly affect their adsorption and thermal/photodesorption behaviors. On the TiO₂(001) surface, CH₃OH and H₂O dissociatively adsorb at the surface Ti sites near Ti interstitials; O₂ molecularly adsorbs at the surface Ti sites adjacent to Ti interstitials, forming photoactive O₂ species that undergoes a hole-mediated photodesorption process; CO adsorbs at the nearest surface Ti sites close to the Ti interstitials, but CO₂ does not, and the resulting CO species is photoactive; and both CO and CO₂ species adsorbed at the normal Ti⁴⁺ sites are photoinactive. On the TiO₂(011)-(2 × 1) surface, O₂ adsorbs only at the double-oxygen vacancy sites, and the resulting O₂ species dissociates to form two oxygen atoms to refill in the oxygen vacancies upon heating; CO₂ adsorbs at the double-oxygen vacancy sites, but CO does not, and the resulting CO₂ species is photoactive; and both CO and CO₂ species adsorbed at the surface Ti⁴⁺ sites are photoinactive. These results broaden the fundamental understandings of the chemistry and photochemistry of TiO₂ surfaces, and the established structure-reactivity relation of small molecules on TiO₂ surfaces is useful in probing complex structures of TiO₂ powder catalysts.