Role of catalysts in dehydrogenation of MgH ₂ nanoclusters

Abstract

A fundamental understanding of the role of catalysts in dehydrogenation of MgH2 nanoclusters is provided by carrying out firstprinciples calculations based on density functional theory. It is shown that the transition metal atoms Ti, V, Fe, and Ni not only lower desorption energies significantly but also continue to attract at least four hydrogen atoms even when the total hydrogen content of the cluster decreases. In particular, Fe is found to migrate from the surface sites to the interior sites during the dehydrogenation process, releasing more hydrogen as it diffuses. This diffusion mechanism may account for the fact that a small amount of catalysts is sufficient to improve the kinetics of MgH2, which is essential for the use of this material for hydrogen storage in fuel-cell applications.hydrogen storage ͉ transition metals ͉ diffusion ͉ catalysis T here is an increasing trend in the use of fuel cells in vehicles (1, 2) and as replacement for batteries in mobile phones and laptop computers (3, 4). In recent years, complex light metal hydrides (5, 6) have attracted considerable attention as hydrogen storage materials because of their large gravimetric density. The metal-hydrogen bonds in these materials are strong, however, and the poor kinetics and thermodynamics do not make these materials suitable for mobile applications. Therefore, the primary focus of research has been to find ways to improve the kinetic and thermodynamic behavior of these light metal hydrides by weakening the metal-hydrogen bond. Although the high thermodynamic stability [⌬H ϭ Ϫ75 kJ (mol H 2 ) Ϫ1 ] and dissociation temperature (Ϸ400°C) combined with poor kinetics impede the use of MgH 2 for hydrogen storage, it does possess some attractive features. First, it is a low-cost material because of the abundantly available magnesium. Second, it has a gravimetric density of 7.7 wt % and, unlike the alanates, all of this hydrogen can be desorbed. Third, being a binary alloy, the role of catalysts is easier to study than in the ternary alanates. The current research on MgH 2 has focused on reducing the strength of the metal-hydrogen bond by nanostructuring and/or using catalysts. It has been shown that mechanical ball milling can reduce the particle size to Ϸ10 nm where defects, large surface areas, and grain boundaries help to improve the kinetics and thermodynamics of hydrogen sorption (7). Further milling with transition metal additives such as Ti, V, Fe, Co, and Ni leads to greatly enhanced hydrogen sorption kinetics (8-10). Despite a considerable amount of experimental work, a fundamental understanding of how nanostructuring and/or catalysts improve the kinetics and thermodynamics of MgH 2 is still lacking. It is needless to emphasize that an understanding of where the catalytic atoms reside and how a small amount of catalysts can improve the thermodynamic behavior is necessary for the synthesizing of materials with optimal performance. Here, an attempt to provide such an understanding is made by carrying out first-principles calculations o...

MgH2 Dehydrogenation Thermodynamics: Nanostructuring and Transition Metal Doping

“…Simulation has also demonstrated that nanoscale (MgH 2 ) n clusters, cut from the bulk, also possess lower dehydrogenation enthalpies than the bulk. 17,18,19,20 Interestingly however, in these simulations only very small clusters (n = 1 to 4) possess lower dehydrogenation enthalpies than the bulk, larger clusters actually possess dehydrogenation enthalpies greater than that of the bulk, nanostructuring will only improve thermodynamic properties if it is possible to synthesize MgH 2 as molecular-size units, larger units are less stable than the bulk. At the nanoscale, it is well-known that cluster structures can be vastly different from the bulk, strongly modifying energetics.…”

“…Substitutional doping of nickel onto magnesium sites of the MgH 2 crystal has been demonstrated to reduce dehydrogenation thermodynamics. 18,25 Interstitial doping sites for Ni dopants are lower in energy, 54 however upon complete dehydrogenation to Mg the Ni interstitials would migrate to fill Mg vacancies. As the (MgH 2 ) n clusters present radically different structural motifs to the bulk, it is expected that the defect formation energies are also different.…”

Electronic Principles of Hydrogen Incorporation and Dynamics in Metal Hydrides

“…The obtained cohesion was larger than experimentally observed which is an indication of the covalent bonding contribution to MgH 2 [49]. Although each of the aforementioned calculations [42,44,[48][49][50][51][65][66][67][68][69][70][71][72] give some information, a complete and coherent explanation of numerous experimental findings has not been found.…”

“…Here we will briefly outline the influence of transition metal (TM) impurities on the Mg/MgH 2 system properties. Extensive experimental [3,4,[6][7][8]44,[55][56][57][58][59][60][61][62] and theoretical (computational) [21,44,[67][68][69][70][71][72]91,[93][94][95][96][97][98] investigations have been devoted to the subject, and the importance of the TM(d)-H(s) interaction for the TM catalytic properties [67,71,72,93,95,96] established. To illustrate this, the difference between the center of the TM 3d-band and H(s) level, and its position relative to the 3d-band [99], are presented in Figure 8.…”

“…However, the electronic aspects of these numerous and versatile phenomena induced in MgH 2 by various treatments have not yet been completely resolved, even though a large number of theoretical and computational investigations [42,44,[48][49][50][51][65][66][67][68][69][70][71][72] have been reported. In one such early report, the Born-Mayer type of calculations of the MgH 2 lattice energy was performed, assuming that the compound was purely ionic [19].…”