Atypical Adsorption of Hydrogen on Native Ultrathin Aluminum Oxide Films

Abstract

We report on an unusual hydrogen adsorption mode on native ultrathin aluminum-oxide films. Based on systematic density-functional theory (DFT) calculations on a model oxide film of varying thickness—i.e., α–Al2O3(001) supported on Al(111)—we show that on ultrathin films, H preferentially adsorbs on surface Al ions instead of O ions, as one would expect from chemical intuition. The so-adsorbed H is in hydride form, and for ultrathin films, it gets the electron charge from Al atoms at the oxide/metal interface. In contrast, for thicker films, this electron transfer ceases, and the charge instead predominantly comes from the surface O ions, making H adsorbed at Al sites inferior to H adsorbed at O sites. We further show that H adsorbed at Al sites can lead to H2 formation, which we propose to be connected to the experimentally observed hydrogen evolution in pits formed during the pitting corrosion of aluminum, where the protective oxide film has considerably degraded and is very thin.

Keywords: DFT, ultrathin films, charge transfer, hydride, hydrogen evolution