Shevlin, SA; Guo, ZX; (2013) MgH2 Dehydrogenation Thermodynamics: Nanostructuring and Transition Metal Doping. The Journal of Physical Chemistry C , 117 (21) pp. 10883-10891. 10.1021/jp3117648.

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Abstract

Controversy currently exists as to the true effects of nanostructuring and transition-metal doping on the dehydrogenation of MgH2. Following extensive datamining of structurally related compounds, we present for the first time, especially for the larger clusters, new stable structures for (MgH2)n clusters, where n = 1 to 10. Using density functional theory and the harmonic approximation we determine the enthalpy of dehydrogenation for all of these clusters. All clusters have very different structures from the bulk, with one- to fourfold hydrogen coordinations observed, and three- to seven-fold magnesium coordinations. We find that, apart from the smallest clusters, enthalpy is larger than for the bulk. Nanostructuring does not improve dehydrogenation enthalpies. We attribute this to surface energy effects; as the (MgH2)n clusters reduce in size bulk cuts become less stable until a stabilising reconstruction occurs which strongly modifies the cluster structure. This increases the magnitude of the dehydrogenation enthalpy. Accurately determining the structures of clusters is essential in determining gas-release thermodynamics for applications. Additionally we investigate modifications of these ACS Paragon Plus Environment clusters, in particular Ni-doping. We find that Ni substitutional doping energies are substantially lower than in the bulk, and that H2 removal energies are substantially less. Nickel-doping will improve the dehydrogenation thermodynamics and kinetics of MgH2 clusters.

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