Surface Chemistry of Formaldehyde on Rutile TiO2(110) Surface: Photocatalysis vs Thermal-Catalysis

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.