An Unexpected Fluctuating Reactivity for Odd and Even Carbon Numbers in the TiO2-Based Photocatalytic Decarboxylation of C2-C6 Dicarboxylic Acids.

The degradation behaviours of five straight-chain dicarboxylic acids (from ethanedioic acid to hexanedioic acid) were compared in aqueous TiO2-based photocatalysis. When all other conditions were identical, the degradation rates were found to fluctuate regularly with the parity of the number of carbon atoms. Dicarboxylic acids with an even number of carbon atoms (e-DAs) always degraded more slowly than those acids with an odd number of carbon atoms (o-DAs). This unusual fluctuation in the reactivity for the degradation of dicarboxylic acids by TiO2-based photocatalysis is very closely related to the different pre-coordination modes of the acids with the photocatalyst. Attenuated total reflection FTIR (ATR-FTIR) of e-DAs labelled with 13C showed that both carboxyl groups of the acid coordinate to TiO2 through bidentate chelating forms. In contrast, only one carboxyl group of the o-DAs coordinated to TiO2 in a bidentate chelating manner, whereas the other formed a monodentate binding linkage. The bidentate chelating form with bilateral symmetric coordination did not favour degradation. Isotope-labelling experiments were performed with 18O2 to observe the different ways in which incorporated oxygen entered the initial decarboxylated products of e- and o-DAs. For the degradation of butanedioic acid, (45.9±0.5) % of the oxygen in the formed propanedioic acid came from H2O, whereas for pentanedioic acid, (97.4±0.2) % of the oxygen in the formed butanedioic acid came from H2O. Our results demonstrate that in TiO2-based photocatalysis, the reactivity of active species, such as .OH/hvb+, is far from non-selective and that the attacks of these active species on organic substrates are significantly affected by the coordination patterns of the substrates on the TiO2 surface. [ABSTRACT FROM AUTHOR]