Theoretical design of catalysts for the heterolytic splitting of H2

Here, we briefly review recent advances in H-2 storage technologies relying on mixed proton-hydride and destabilized hydride materials. We establish a general relationship across different materials: the higher the effective H content, the higher the temperatures needed to completely desorb H-2. Nevertheless, several systems show promising thermodynamics for H-2 desorption; however, the desorption kinetics still needs to be improved by the use of appropriate catalysts. Prompted by the importance of heterolytically splitting stable dihydrogen molecules for proton-hydride technologies, we attempt to theoretically design novel H-2 transfer catalysts. We focus mainly on M4NM4H8 catalysts (M = V, Ti, Zr, Hf, and Nm = Si, C, B, N), which should be able to preserve their functionality in the strongly reducing environment of a H-2 storage system. We are able to determine the energy of H-2 detachment from these molecules, as well as the associated energy barriers. In order to optimize the properties of the catalysts, we use isoelectronic atom-by-atom substitutions, vary the valence electron count, and borrow the concept of near-surface alloys from extended solids and apply it to molecular systems. We are able to obtain control over the enthalpy and electronic barriers for H-2 detachment. Molecules with the coordinatively unsaturated > Ti=Si < unit exhibit particularly favorable thermodynamics and show unusually small electronic barriers for H-2 detachment (> 0.27 eV) and attachment (> 0.07 eV). These and homologous ZrSi frameworks may serve as novel H-2 transfer catalysts for use with emerging lightweight hydrogen storage materials holding 5.0-10.4 wt % hydrogen, such as Li2NH,Li2Mg(NH)(2), Mg2Si, and LiH/MgB2 (discharged forms). Catalytic properties are also anticipated for appropriate defects on the surfaces and crystal edges of solid Ti and Zr silicides, and for Ti=Si ad-units chemisorbed on other support materials.