Contributions of Abundant Hydroxyl Groups to Extraordinarily High Photocatalytic Activity of Amorphous Titania for CO2 Reduction

Despite severe charge recombination occurring within the bulk lattice, sol-gel-derived amorphous TiO2 particles with abundant OH groups (81.6 mg/g) and high surface area (274 m(2)/g) were for the first time demonstrated to exhibit respectively 8-14 and 9 times higher photocatalytic activity for CO2 reduction than their thermally derived anatase crystals and the commercial P25 powder. Moreover, the high density of the OH groups (12.45/nm(2)) enabled the amorphous oxide to exhibit higher specific surface reactivity than the crystals. The OH groups not only converted CO2 molecules into bonded bicarbonate/carbonate species to improve CO2 chemisorption but also trapped holes to form Ti-O-O-Ti species when the OH density was higher than a threshold value of 8.74/nm(2), which synergistically promoted interfacial charge transfer. Bidentate carbonate and center dot CO2- were two active species that were able to underwent CO32- -> Ti-OOCH2 -> Ti-O-CH3 -> CH4 and center dot CO2- -> CO22- -> Ti-COOH -> CO sequences on the hydroxylated surface to produce CH4 and CO products, respectively. High coverage of the chemisorbed carbonate species selected for CO2 reduction rather than H-2 evolution to proceed. Moreover, it led with CH4 as the major product. Oxygen vacancies were the major active sites on the anatase crystals. Their influences on the surface transformations were also characterized to comprehensively understand the surface-controlled activity and selectivity.