Photosynthesis of hydrogen peroxide by crystal phosphors

Abstract

The photosynthesis of hydrogen peroxide in the presence of zinc oxide, cadmium sulphide or basic zinc carbonate, as a means of storing solar energy, is re-examined. It is shown that with due allowance for losses of incident energy by the processes of reflection and luminescence, a maximum quantum yield of 0.5 can be obtained when no nonluminous transitions are incurred. Of the two quanta required to produce one equivalent of hydrogen peroxide, one is consumed to excite an electron whilst the other is, most likely, used to release a negatively charged oxygen or water molecule from the surface of the solid catalyst. The central problem in the recovery of hydrogen peroxide from these systems lies in the efficient removal of hydroxyl free radicals which are produced in concentrations equivalent to ·OOH or H·, the precursors of hydrogen peroxide. As the expenditure of chemical potential required to take care of ·OH is greater than the gain in free energy due to the generation of H 2O 2, these processes, and others based on the same electronic processes, must be considered thermodynamically unsound.