Kinetic investigation of the effect of milling time on the hydrogen sorption reaction of magnesium catalyzed with different Nb2O5 contents

The effect of milling time and Nb2O5 content on the magnesium hydrogen sorption reaction is investigated in this paper. The absorption and desorption kinetics of magnesium at 300 degrees C, with 0.05, 0.1, 0.2, 0.5, 1 mol% Nb2O5 contents and after 2, 5, 10, 20, 50, 100 h milling were measured. Reaction kinetics can be enhanced up to a certain limit by increasing the milling time or increasing the Nb2O5 content. The kinetic rate constants for different milling times and Nb2O5 contents were calculated and compared. Furthermore the rate-limiting step of the absorption and the desorption reactions was determined by fitting the respective kinetic equations. While the absorption is diffusion controlled, the rate-limiting step for desorption changes, with increasing of milling time or Nb2O5 content, from chemisorption to interface velocity of the transformed phase, demonstrating the effectiveness of the Nb2O5 catalyst. (c) 2005 Elsevier B.V. All rights reserved.