Surface termination effects on metal atom adsorption on α-alumina

Abstract

To examine the effect of chemical environment on metal-support bonding, the low coverage adsorption and reaction of late transition metal (Pt, Pd, Au, and Ag) atoms on a hydrogen-free and fully hydroxylated α-alumina (0 0 0 1) surface are compared using supercell models and density function theory. These two surface terminations represent the limits of a completely dehydrated (“dry”) and fully hydroxylated (“gibbsite-like”) surface. Metal atoms exhibit similar site preferences and the same relative binding energies on the two, in the order Pt > Pd > Ag > Au. Pt, Pd, and Au bind covalently atop single O atoms, and this binding is facilitated on the dry surface by the availability of charge-accepting Lewis acidic Al sites. Ag has the lowest ionization potential of the group, binds electrostatically to both surfaces and preferentially at three-fold O sites, and as a consequence of charge delocalization exhibits greater coverage dependence in its binding. A distinguishing characteristic of the fully hydroxylated surface is the ability to exchange hydrogen between support and metal: transfer from support to metal is highly endothermic for Pd, Ag, and Au but slightly exothermic for Pt, suggesting that such transfer may be important both in Pt-alumina binding and reactivity.