Enhanced Photo-Oxidation of Formaldehyde on Highly Reduced o-TiO2(110)

Abstract

Photo-oxidation of formaldehyde is important because it is an atmospheric pollutant and a possible intermediate along the pathway for CO2 reduction to methanol. By using temperature-programmed reaction spectroscopy as the main tool, we demonstrate that the efficiency for photo-oxidation of formaldehyde to adsorbed formate on rutile TiO2(110) is strongly dependent on the degree of reduction of the surface and near-surface region. The most efficient photoreaction occurs when O adatoms are present on titania that is highly bulk reduced. These data suggest that electron–hole pairs created deep in the bulk do not play a significant role in the photo-oxidation of formaldehyde. Exposure of the material to O2 heals most bridging oxygen vacancies, creates O adatoms in the Ti five-coordinate rows, and quenches charge at the surface due to the presence of interstitial defects in the near-surface region. Under these conditions, the surface is oxidized but the bulk remains reduced. The major product of formaldehyde photo-oxidation is adsorbed formate; the highest yield is for a highly reduced sample containing O adatoms. The efficiency of formate production on a highly reduced surface is about four times smaller than on one with a similar degree of bulk reduction but containing O adatoms on the surface. On reduced surfaces (not containing O adatoms), the source of the additional O to make formate is derived from a less efficient and more complex photodecomposition pathway of formaldehyde. With increasing bulk reduction of the crystal, the photoefficiency on the reduced surface is severely diminished, suggesting that bridging O vacancies quench photo-oxidation. All photoproducts remain on the surface; only formaldehyde desorption is detected during exposure to UV light. A combination of scanning tunneling microscopy and density functional theory calculations were used to establish that formaldehyde forms a surface dioxyalkylene complex (H2CO–Oad) with oxygen adatoms. This intermediate loses hydrogen to a hole at a nearby bridging O to yield the formate photoproduct. These results are discussed in the broader context of photo-oxidation.