The photocatalytic oxidation of methanol on a rutile TiO2(110) surface was studied by means of thermal desorption spectroscopy (TDS) and X-ray photoelectron spectroscopy (XPS). The combined TDS and XPS results unambiguously identify methyl formate as the product in addition to formaldehyde. By monitoring the evolution of various surface species during the photocatalytic oxidation of methanol on TiO2(110), XPS results give direct spectroscopic evidence for the formation of methyl formate as the product of photocatalytic cross-coupling of chemisorbed formaldehyde with chemisorbed methoxy species and clearly demonstrate that the photocatalytic dissociation of chemisorbed methanol to methoxy species occurs and contributes to the photocatalytic oxidation of methanol. These results not only greatly broaden and deepen the fundamental understanding of photochemistry of methanol on the TiO2 surface but also demonstrate a novel green and benign photocatalytic route for the synthesis of esters directly from alcohols or from alcohols and aldehydes.

The interaction of atomic hydrogen and H 2 O with stoichiometric and partially reduced CeO 2 (111) thin films deposited on a Cu(111) substrate was investigated by temperature programmed desorption and X-ray photoelectron spectroscopy. On stoichiometric CeO 2 (111) surface, the adsorption of atomic H(g) leads to the formation of surface hydroxyl (OH(a)) and H 2 O(a) as well as the reduction of Ce 4+ into Ce 3+ . On reduced CeO 2 (111) surfaces, the stability of OH(a) was enhanced by the presence of oxygen vacancies. Upon heating, surface hydroxyls undergo two competing reaction pathways: one is the associative desorption of OH(a) releasing H 2 O and creating oxygen vacancies (OH(a) + OH(a) → H 2 O(g) + O lattice + O vacancy ), and the other one is to produce H 2 via OH(a) + OH(a) → H 2 (g) + 2O lattice . The presence of oxygen vacancies in CeO 2 favors the reaction pathway of H 2 formation. At 115 K, reversible dissociation and molecular adsorption of H 2 O occur on stoichiometric CeO 2 (111) surface, but irreversible dissociation of H 2 O occurs on reduced CeO 2 (111) surfaces. These results deepen the fundamental understanding of the influence of oxygen vacancies on the reactivity of surface hydroxyls and water on CeO 2 surface.

The photocatalytic surface reactions of formaldehyde (HCHO) on rutile TiO 2 (110) surface were studied by means of thermal desorption spectroscopy and X-ray photoelectron spectroscopy and compared with its thermally catalytic surface reactions. Thermally catalytic surface reactions of formaldehyde on rutile TiO 2 (110) surface are dominated by the carbon−carbon bond formation reaction of HCHO adsorbed at oxygen vacancies to produce C 2 H 4 via a diolate (−OCH 2 CH 2 O−) surface intermediate. During the photocatalytic surface reactions, HCHO adsorbed at the Ti 5c sites of rutile TiO 2 (110) surface is photocatalytically oxidized to a transient formyl (HCO) species that facilely transforms to adsorbed formate (HCOO) species. HCOO species and HCHO adsorbed at oxygen vacancies undergo novel surface reactions to produce CO, C 2 H 4 , and CH 3 OH at elevated temperatures whose mechanisms were successfully identified. The healing of bridging oxygen vacancies on rutile TiO 2 (110) surface by preadsorption of water suppresses the oxygen vacancy-mediated coupling reaction of formaldehyde but does not affect its photocatalytic surface reactions. These results not only greatly deepen the fundamental understanding of photochemistry of formaldehyde on TiO 2 surface but also demonstrate novel surface reactions of organic functional groups formed by the combined photocatalytic and thermally catalytic reactions of formaldehyde on oxide surfaces.