Kinetics of the reaction of water vapour with crystalline lithium hydride

Abstract

The kinetics of the reaction of water vapour with crystalline lithium hydride to produce hydrogen, and either lithium oxide or hydroxide, has been investigated over the temperature range 0–121°C. The chemical nature of the solid product is determined solely by the amount of water vapour added in any single dose; when this is only sufficient to react with a single surface layer of lithium hydride or less, the oxide is found almost exclusively but when there is sufficient water to react with several layers, only the hydroxide is formed. On introduction, water vapour is rapidly and almost completely removed from the gas phase by the solid hydride (and products) but the hydrogen-producing process continues for several hours by reaction of sorbed water with the hydride. Diffusion of sorbed water to the reaction interface is not rate-controlling.

The kinetics of hydrogen production, irrespective of the chemical nature of the product, is adequately represented by 1//_tlog (a//_a–x)–Cx//_t=D. The parameters C, D vary with temperature, and decrease independently as the product accumulates. To account for this latter variation, it is suggested that annealing and recrystallization processes occur in the oxide layers in the time-interval between successive additions of water vapour, and that these structural changes affect the adsorptive capacity of the layers for water. A quantitative development of these concepts has been given and values for rate constants and activation energies for the hydrogen-producing reaction, and also some relative heats of adsorption of water by the oxides have been evaluated. The formation of the hydroxide that takes place when larger doses of water vapours are employed is the result of rapid reaction of unreacted sorbed water with the freshly-formed oxide, which is always the initial product of reaction.