DFT study of the interaction of a single palladium atom with γ-alumina surfaces: The role of hydroxylation

Abstract

Considering the crucial role of the γ-alumina solid phase in heterogeneous catalysis as a support of numerous active phases, a revised and improved atomistic description of γ-alumina surfaces was mandatory to furnish new highlights in the field γ-alumina supported catalysts. Two important scientific challenges in heterogeneous catalysis have recently been taken up by modern Density Functional Theory (DFT) simulations. The first challenge described in this paper is to show that DFT calculations combined with simple thermodynamic model provide an elegant way of determining the stable chemical species at the γ-alumina surface (such as hydroxyls or Lewis sites) as a function of reaction conditions. The (100) and (110) surfaces exposed mainly by the γ-alumina nanocrystallites exhibit two distinct behaviors regarding their hydroxylation states. The (110) surface maintains a high degree of hydroxyl coverage even at high temperature, whereas the (100) surface is dehydrated at low temperature. This first important step being achieved, a second challenge in heterogeneous catalysis is the interaction of the active phase with γ-alumina. In the second part of this paper, we present the adsorption of a single palladium atoms (Pd1) on the (100) and (110) γ-alumina surfaces. By determining the potential energy surface of Pd on γ-alumina, the relationship between structure and metal-oxide interaction energy at the interface is depicted. Furthermore, new insights are provided on the chemisorption and diffusion processes of Pd on the two surfaces. The adsorption energy and the hopping rate of Pd are strongly reduced when the hydroxyl coverage increases such as found on the (110) surface. As a consequence, the surface hydroxylation appears as a key parameter for understanding the active phase/support interaction and enables the interpretation of available experimental data. Copyright © 2006, Institut français du pétrole.