Effect of Mg-Nb oxides addition on hydrogen sorption in MgH(2)

H(2) absorption and desorption reactions in MgH(2) promoted by ball-milling with 1 mol% MgNb(2)O(6), Mg(4)Nb(2)O(9) and Mg(3)Nb(6)O(11) have been investigated. MgH(2) was milled with the bare oxides for 12 h under a high purity Ar atmosphere. Absorption and desorption reactions in the ball-milled samples were studied by in situ X-ray diffraction (XRD) in isothermal conditions with Anton Paar XRK 900 reaction chamber. XRD patterns for absorption were recorded at 573K under hydrogen pressure of 0.9 MPa and for desorption at 623 K in vacuum. Experimental data were analysed according to the Rietveld method. Ball-milled samples showed the presence of a mixture of beta and gamma allotropes of MgH(2), with significantly broadened diffraction peaks due to reduced crystallite size and strain, together with bare additives. The presence of Mg-Nb oxides significantly accelerates the hydrogen absorption and desorption processes. The amount of hydrogen absorbed in the presence of Mg-Nb-O phases is lower than the maximum stoichiometric capacity, because of the presence of a non-reactive MgO layer on the surface of the powders or at the grain boundaries. Experimental results are discussed on the basis of thermodynamic and kinetic arguments. (C) 2011 Elsevier B.V. All rights reserved.