Correlation between kinetics and chemical bonding state of catalyst surface in catalyzed magnesium hydride

We reported on the hydrogen desorption properties, microstructure, kinetics, and chemical bonding state of catalyst surface for composites of MgH2 and 1 mol% Nb2O5 ball-milled for 0.02 h, 0.2 h, 2 h, 20 h under 1 MPa H2 atmosphere, as well as hand-mixed (HM) one. Hydrogen desorption properties were significantly improved by ball-milling with Nb2O5. Then, we estimated by Kissinger Method the activation energy (Ea) of hydrogen desorption reaction that decreased with the increase of ball-milling time. Especially, Ea of the sample ball-milled for 0.2 h was drastically decreased, compared with that of the sample ball-milled for 0.02 h. TEM observations revealed that the distribution of Nb2O5 in MgH2 was gradually improved during ball-milling. On the other hand, we confirmed by XPS that in the sample ball-milled for 0.2 h, Nb2O5-x phase(s) existed at least on the surface. It can be suggested that these deoxidized Nb2O5-x phases eventually decrease Ea as substantial catalyst rather than Nb2O5 itself.

► MgH2 and 1 mol% Nb2O5 ball-milled composites were studied.
► The kinetics of desorption was improved with the increase of milling time.
► The particle size was gradually reduced during milling.
► Nb2O5-x phase(s) were found on the surface of the samples milled for 0.2 h
► Refined particle size and reduced Nb may lead to an improvement of kinetics.