Reactivity Descriptors for Borohydride Interaction with Metal Surfaces

First-principles density functional theory calculations were performed to study the adsorption of borohydride (BH4–) on close-packed transition-metal surfaces, M(111) (M = Au, Pt, Ir, Os, Ag, Pd, Rh, Ru). A correlation between the relative adsorption energies of BH4ad and the d-band center of the metals is established. In terms of the adsorbate configuration, both molecular (BH4ad) and dissociated (BH4-y,ad + yHad, y = 1, ...,3) structures are possible regardless of the adsorption energy value. On Os, Rh, and Ru surfaces, molecular (i.e., undissociative) adsorption is preferred despite the strong surface binding energy of BH4ad. Orbital-specific analysis of the bonding, points to the role of the dzz and dyz states of the surface metal atoms in determining the final BH4ad configuration on all metals. However, in the presence of H2O molecules, the preference for strong molecular adsorption may be lost because of BH4ad–H2Oad interaction. Using the coadsorption on Os(111) of BH4ad and H2Oad with and without the presence of OHad (generated either by electrosorption of OH– or dissociative water adsorption), the origins of the adsorbate–adsorbate and adsorbate–metal interactions are discussed. Electronic factors to predict the BH4ad conformation on metal catalysts in water environment are proposed.