Glycerol as a probe molecule to uncover oxidation mechanism in photocatalysis

[Display omitted] ► Glycerol is a probe to assess different aspect of photocatalytic reaction rates. ► Single site photocatalysis is ruled out. ► Reactivity in the chemisorbed state leads to CC bond cleavage. ► Reactivity in the physisorbed state leads to H-abstraction. ► Photocatalytic reaction mechanism and back reactions depend on surface chemistry. Glycerol is a good probe molecule to understand what type of photocatalytic mechanism could occur at the catalyst surface, as the analysis of the produced intermediates is able to discriminate between a direct electron transfer and a radical mediated oxidation mechanism. The mechanism of transformation of glycerol is explored by an extensive analysis of formed intermediates as a function of glycerol concentration on two different commercial TiO2 powders. Over TiO2 Degussa P25 the transformation rate of glycerol shows a sharp maximum as a function of the substrate concentration. The type of product detected at low glycerol concentration (glyceraldehyde and dihydroxyacetone) changes after the sharp maximum giving mainly formaldehyde and glycolaldehyde. On Merck TiO2, characterized by a lower density and more uniform population of hydroxyls at surface sites, mainly glyceraldehyde and dihydroxyacetone are observed. Because these products are formed through an OH-like mechanism, the products observed on P25 derive from a direct electron transfer. The results are rationalized invoking the surface complex of glycerol where an inner sphere electron transfer occurs, which leads to glycerol fragmentation, and where the substrate-mediated recombination plays the major role. A simplified kinetic approach confirms the observed rate dependence on concentration. The working hypothesis is also confirmed by addition of fluoride anions that compete for surface sites and impede glycerol complexation. Without the possibility of inner sphere electron transfer, on P25 only glyceraldehyde and dihydroxyacetone are observed. Thus different catalysts differ for the possibility of surface complexation.