MgH₂ Dehydrogenation Thermodynamics: Nanostructuring and Transition Metal Doping

Abstract

Controversy currently exists as to the true effects of nanostructuring and transition-metal doping on the dehydrogenation of MgH 2 . Following extensive datamining of structurally related compounds, we present for the first time, especially for the larger clusters, new stable structures for (MgH 2 ) 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 (MgH 2 ) 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

The role of density functional theory methods in the prediction of nanostructured gas-adsorbent materials

“…1), two families of GAM which at present are being investigated also very thoroughly [70][71][72][73][74]. For the sake of focus, other important mechanisms exploited in gas storage and sequestration processes (e.g., gas binding in bulk chemical form and thermodynamic destabilization of nanocrystals and clusters [75][76][77][78][79]) and related families of materials (e.g., metal oxides and binary and ternary hydride compounds [80][81][82][83][84][85]), will not be discussed exhaustively in the present work.…”

Simulation, Modelling and Design of Hydrogen Storage Materials

“…First-principles electronic structure can be gainfully utilized for 'tailoring' such behaviour and thereby designing a suitable material for efficient storage of molecular hydrogen. For example, light metal hydrides are envisioned as novel class of materials that can show a major shift towards more favourable thermodynamics, kinetics and reversibility of the hydriding reaction by tailoring its particle size, physical confinement, and doping with suitable materials (Bhattacharya and Das, 2013;Shevlin and Guo, 2013). On the other hand, doping of nanostructured materials with suitable metal atoms can fundamentally change the nature of hydrogen bonding and hence provide a lightweight material capable of meeting the requirements of an ideal hydrogen storage material (Bhattacharya et al, 2009a).…”

“…However, the chemically strong metal-hydrogen bonding causes a significantly high hydrogen desorption enthalpy (~0.8 eV/H 2 molecule) (Shevlin and Guo, 2013), thereby resulting in an unfavourable hydrogen desorption temperature of ~300 o C (Liu et al, 2013). Due to this fact, their usage for the practical hydrogen storage application has been retarded.…”

“…This happens due to the higher diffusivity of hydrogen and higher surface to volume ratios of the nanoclusters. Eventually, a computational analysis on MgH 2 nanoclusters of different sizes (Shevlin and Guo, 2013) reported that nanostructuring of MgH 2 leads to the improvement of the dehydrogenation characteristics only for smaller size Mg n H 2n (n = 1-4) nanoclusters.…”

DFT study of the coverage-dependent chemisorption of molecular H 2 on neutral cobalt dimers

“…The adsorption of molecular hydrogen (H2) on metal clusters is of interest in several fields, among which trace-gas sensors, heterogeneous catalysis, metallurgy, photocatalysis and hydrogen storage [1][2][3][4][5][6]. The interaction of hydrogen with cobalt surfaces is of particular industrial importance due to the use of cobalt as a Fisher-Tropsch (F-T) catalyst, in which the dissociation of hydrogen is one of the critical mechanistic steps towards the subsequent hydrogenation of carbon into hydrocarbons [7][8][9][10][11][12][13].…”