RADICAL INTERMEDIATES IN THE PHOTOSYNTHETIC GENERATION OF H2O2 WITH AQUEOUS ZNO DISPERSIONS

The technique of spin trapping has been applied to a study of the photosynthesis of hydrogen peroxide in aqueous zinc oxide dispersions. In additive-free systems, the hydroxyl radical was detected whereas in systems containing either formate or oxalate, the ·CO2- radical was observed. The measurement of oxygen uptake was also accomplished on these same systems. Comparison of the number of radicals with the amount of H2O2 formed and of the quantum efficiencies determined by both electron spin resonance and oxygen uptake strongly suggest that these radicals are major participants in the mechanism of hydrogen peroxide photosynthesis. A mechanism is in fact suggested which is consistent with the observation of these radical species. It appears that additives such as formate or oxalate function by acting at least partially as hydroxyl radical scavengers. The conversion ratio (defined as the number of hydrogen peroxide molecules formed per one molecule of oxygen consumed) is introduced and is observed to be a function of initial oxygen concentration in the additive free systems. This reaction also proceeds in the presence of a surfactant. © 1979 American Chemical Society.