The formation of hydrogen peroxide in the combustion of hydrogen at low pressures

Abstract

Hydrogen peroxide has been obtained in appreciable concentrations when a flame of hydrogen and oxygen burning at pressures of 3 to 4 cm. mercury was directed against a surface cooled to —180° C ( c . 5 to 10%), and when an oxy-hydrogen mixture was exploded by the action of a spark, in glass or aluminium vessels c . 2 x 15 cm., cooled in liquid air, the products were shown to contain up to 30% hydrogen peroxide ( w / v ), depending on the conditions. The conditions controlling the formation of hydrogen peroxide in such explosions have been investigated more fully than was done by previous authors. It was found that as the overall gas pressure was decreased the yield of peroxide passed through a maximum value. The effects were also investigated of varying the dimensions of the vessels, the gas composition, the external temperature, and other factors. A series of experiments was carried out in order to ascertain whether the hydrogen peroxide was formed in the gas phase or only on the walls. In these experiments, the ultra-violet absorption spectrum of the explosion region was studied. It was found that a considerable quantity of hydrogen peroxide was present in the gas phase, even when the walls of the reaction tube were not cooled. The results are discussed in the light of modern views regarding the combination of hydrogen and oxygen. It would appear that the peroxide is formed by two mechanisms; one of these consists of recombination of hydroxyl radicals on the cold walls, the other involves the formation of an excited HO 2 radical, and its subsequent reaction with a hydrogen molecule to give a hydrogen atom and a molecule of excited hydrogen peroxide, which then decomposes unless it is stabilized and frozen on the walls. No other explanation has been found to account for all the observed facts, and at the same time not to contradict established mechanisms for the hydrogen oxygen reaction.