Photocatalytic degradation of aqueous hydroxy-butandioic acid (malic acid)in contact with powdered and supported titania in water

Malic acid, a model molecule for partially oxidized products found either i n biomass or in various degradative processes, could totally be mineralized by photocatalytic degradation. Laboratory experiments enabled one to deter mine the identity of 15 intermediates, logically presented in four parallel degradation pathways. The major one is initiated by a decarboxylation ('ph oto-Kolbe') reaction in the alpha-position with respect to the OH group. Pa rallel experiments have been performed in a slurry pilot solar photoreactor at PSA (Almeria, Spain), on a large scale (2501). The same main intermedia te products and the same kinetic laws were observed. TiO2 has been used eit her in slurries or supported on several rigid inert plates. Deposition of t itania on glass and quartz was carried out by a dip coating procedure and t he deposition on stainless steel by an electrophoretic deposition process. The resulting materials have been exhaustively characterized by XPS, SEM/ED X, XRD and UV-vis absorption spectroscopy. Their photocatalytic activity pa ttern as a function of the nature of the support followed the decreasing or der: TiO2/quartz > TiO2/steel = TiO2/glass much greater than photolysis. Th is decline in activity has been correlated with the presence of cationic im purities (Si4+, Na+ for glass and Cr3+, Fe3+ for stainless steel) in the ti tania layer as a consequence of the thermal treatment necessary to improve the cohesion of the titania layer and its adhesion onto the support. (C) 19 99 Elsevier Science B.V. All rights reserved.