The formation of molecular hydrogen through photolysis of water vapor in the presence of oxygen

Abstract

1. A light source was constructed for the investigation of the photolysis of water vapor. A xenon discharge arc of a pressure of about 50 mm in a thin wall quartz tubing was used. 2. Molecular hydrogen formed from water vapor in the presence of other gases was measured by employing tritium as a tracer. A vacuum system was constructed, allowing the separation of water vapor from hydrogen to better than 10−8 parts, and the counting of tritiated hydrogen in a Geiger counter. All measurements were carried out in a semi-quantitative way and it was found that the light source and the technique of measuring small amounts of free hydrogen by employing a tritium tracer can be used effectively for further studies of the reactions involving hydrogen and water. 3. Self-decomposition of tritiated water vapor with and without addition of oxygen was measured and was found to be negligible under the particular conditions of the experiments over periods of several months. 4. The photochemically induced exchange of tritium between tritiated water vapor and molecular hydrogen was studied. The quantum yield of this exchange was found to be under the conditions of the experiments of the order of one and probably slightly larger than one. 5. It was possible to demonstrate the formation of free hydrogen from the photolysis of water vapor in the presence of oxygen and to measure these amounts as a function of oxygen pressure. The steady state concentrations of H2 formed from water vapor by irradiation in the presence of oxygen under the influence of UV light were found to be smaller than expected. 6. Photochemical oxidation of H2 by O2 under the particular conditions of the experiments and at pressures of about one millimeter was found to have a quantum yield of the order of magnitude of one.