Temperature- and Catalyst-Dependent Transformation from Reaction Rate-Limited to Diffusion Rate-Limited Hydrogenation of Mg with Nb₂O₅ Catalyst

Abstract

The hydrogen consumption behavior during the hydrogenation of magnesium (Mg) with niobium oxide (Nb₂O₅) catalyst was analyzed via the volumetric Sieverts method, and categorized into two types of consumption kinetic processes: increasing consumption rate with temperature at lower temperatures and almost constant consumption rate at higher temperatures. The consumption rates were measured as the hydrogen absorption rates of Mg, and were analyzed based on a first-order reaction for two types of absorption processes, reaction rate-limited and diffusion rate-limited. The apparent activation energies in the hydrogen absorption rate increased with temperature at low temperatures, and were 49.5–53 kJ/mol for Mg with 5, 10, and 20 mass% Nb₂O₅ catalyst, which were lower than the activation energy for pure Mg. The apparent activation energies in the absorption rate were almost constant at higher temperatures, ranging from 0–9.6 kJ/mol. The transformation of the absorption reaction process from a reaction rate-limited to diffusion rate-limited process with an extremely lower apparent activation energy is similar to that observed during the combustion of charcoal.

Fig. 7 Scheme of (a) reaction rate limited (RRL) and (b) diffusion rate limited (DRL).