Effect of Nb2O5 content on hydrogen reaction kinetics of Mg

The effect of Nb2O5 concentration on the kinetics of the magnesium hydrogen sorption reaction at 300degreesC and 250degreesC is investigated. Absorption and desorption kinetics of nanocrystalline magnesium catalyzed with 0.05, 0.1, 0.2, 0.5, and 1 mole% Nb2O5 are determined at 300 and 250degreesC. Fastest kinetics are obtaitled using 0.5 mole% Nb2O5. At 300degreesC, absorption and desorption of 7 wt.% of hydrogen are facilitated in 60 and 90 s, respectively. At 250degreesC, more than 6 wt.% are absorbed in 60 s and desorbed again in 500 s. Respective reaction rates and activation enthalpies are calculated. The activation energy for desorption varies exponentially with Nb2O5 concentration and reaches a limit of 61 kJ/mole at 1 mole%. The rate-limiting step for a particular parameter set is deduced by fitting the experimentally obtained transformed phase fraction and time data with analytical kinetics rate expressions. The results show that there is a change in the rate-limiting step with catalyst content. (C) 2003 Elsevier B.V. All rights reserved.