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

Abstract

The photocatalytic surface reactions of formaldehyde (HCHO) on rutile TiO2(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 TiO2(110) surface are dominated by the carbon–carbon bond formation reaction of HCHO adsorbed at oxygen vacancies to produce C2H4 via a diolate (−OCH2CH2O−) surface intermediate. During the photocatalytic surface reactions, HCHO adsorbed at the Ti5c sites of rutile TiO2(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, C2H4, and CH3OH at elevated temperatures whose mechanisms were successfully identified. The healing of bridging oxygen vacancies on rutile TiO2(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 TiO2 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.