Photosensitised reduction of water to hydrogen using water-soluble zinc porphyrins
We have confirmed an earlier report by Kalanasundaram and Gratzel that a positively charged, water-soluble zinc porphyrin photosensitises the reduction of water to H2 with high efficiency. Using MV2+ as electron relay and EDTA as sacrificial electron donor, the quantum yield for production of ½H2 is ca. 0.6. The reaction mechanism involves reduction of MV2+ by triplet porphyrin and the porphyrin π-radical cation so produced is reduced by EDTA. The concentrations of reactants have been optimised for production of H2 and to limit destruction of the porphyrin. Under optimised conditions the turnover with respect to the porphyrin can reach 6000.
In addition, some consideration has been given to ways of improving øH2 and of increasing the fraction of sunlight that can be harvested. In this respect, Cd porphyrins may possess suitable properties.
Finally, we note that the porphyrin π-radical cation may possess the thermodynamic capacity to oxidise water to O2, but this is very much borderline.