THE REDUCTION OF AQUEOUS CARBONATE TO METHANOL, PHOTOCATALYZED BY TIO2 PHTHALOCYANINE

Two titania pigments (rutile type) prepared either by a sulphate or chloride process, were coated with either Fe2+ - or Co2+ -phthalocyanine dyes (1-3% coverage) to produce efficient photocatalysts. Heterogeneous photocatalysed reduction of aqueous Na2CO3 (IM) was achieved using commercially coated TiO2 powders, irradiated with uv/visible light (mainly of wave length 254 nm) from a low-pressure mercury lamp. Band gap irradiation of titania leads to production of electrons in the conduction band of the semiconductor. It is likely that the photoproduced electrons reduce carbonate ions to CH3OH, HCHO and HCOO-. The products were determined spectrophotometrically using the conventional analytical methods. The obtained methanol yield increased linearly with increasing irradiation time after 6 h, but a graph of the relationship did not pass through the origin, indicating a slowing down at times >6h. The rutile pigments prepared by either process, and coated with Fe2+ - or Co2+-phthalocyanine, gave an optimum methanol yield at 2% surface coverage, presumably ascribed to more phthalocyanine screening the TiO2 surface more effectively and thereby reducing the actual light intensity 'seen' by the semiconductor. Changing the redox potential of the phthalocyanine dye by changing its central metal from Fe to Co affects the yields of reduction products. The order of yields: CH3OH > HCHO > HCOO- was observed which may be accounted for by the likely photocatalysed reduction of the last two products into CH3OH. © 1990 Pergamon Press plc.