Local bonding and atomic environments in Ni-catalyzed complex hydrides

Abstract

The local bonding and atomic environments in the Ni-catalyzed destabilized systemLiBH4/MgH2 and the quaternary borohydride-amide phaseLi3BN2H8,were studied by x-ray absorption spectroscopy. In both cases the Ni catalyst was introduced asNiCl2 and a qualitative comparison of the Ni K-edge near-edge structure suggests theNi2+ is reducedto primarily Ni0 after ball milling. The extended fine structure of the Ni K edge indicates that the Ni is coordinatedby ∼3 boron atoms with an interatomic distance of approximately 2.1 Å and∼11 Ni atoms in a split shell at around 2.5 and 2.8 Å. These results, and the lack of long-range order,suggest that the Ni is present as a disordered nanocluster with a local structure similar to that ofNi3B. In the fullyhydrogenated phase of LiBH4/MgH2 a small amount Mg2NiHx was also present. Surface calculations performed using densityfunctional theory suggest that the lowest kinetic barrier forH2 chemisorptionoccurs on the Ni3B(100) surface.