In situ solid-state NMR studies of ethanol photocatalysis: characterization of surface sites and their reactivities

In situ solid-state NMR methodologies have been employed to investigate the photocatalytic oxidation of ethanol (C2H5OH) over two TiO2-based catalysts, Degussa P-25 powder and a monolayer TiO2 catalyst dispersed on porous Vycor glass. Two adsorption sites were observed for ethanol, a chemisorbed species identified as an ethoxide species, and a hydrogen-bonded species. The Ti-bound ethoxide was found to be the more photocatalytically reactive species. C-13 magic-angle spinning (MAS) experiments revealed that long-lived intermediates, including acetaldehyde (CH3CHO), acetic acid, formic acid, and acetate were observed under dry conditions and in the presence of molecular oxygen. Similar reaction intermediates form on the surfaces of both catalysts. C-13 cross-polarization MAS experiments allowed us to identify and follow the evolution of surface-bound species during W irradiation. Acetate, which forms from mobile acetic acid, appears to be bound to the non-irradiated surfaces of the powdered TiO2 catalyst, and was also observed on the surface of TiO2/PVG catalyst, and further degradation was not possible. The presence of molecular oxygen was found to be essential for photooxidation to proceed. (C) 1999 Elsevier Science B.V. All rights reserved.