Characterization of Water Dissociation on $\alpha$-Al$_{2}$O$_{3}$$(1\bar{1}02)$: Theory and Experiment

TL;DR

This study combines experimental and theoretical methods to analyze water dissociation on a specific alumina surface, revealing a low energy barrier and delocalized surface states that influence water-surface interactions.

Contribution

It provides the first detailed characterization of water interaction with the $ ext{(1}ar{1} ext{)}02$ surface of $ ext{α}$-Al$_{2}$O$_{3}$, including vibrational and energetic insights.

Findings

Water adsorbs only at a specific surface site.

The dissociation barrier is very low at 0.06 eV.

Surface deuterons can become delocalized at moderate temperatures.

Abstract

The interaction of water with $\alpha$-alumina (i.e. $\alpha$-Al$_{2}$O$_{3}$ surfaces is important in a variety of applications and a useful model for the interaction of water with environmentally abundant aluminosilicate phases. Despite its significance, studies of water interaction with $\alpha$-Al$_{2}$O$_{3}$ surfaces other than the $(0001)$ are extremely limited. Here we characterize the interaction of water (D$_{2}$O) with a well defined $\alpha$-Al$_{2}$O$_{3}$$(1\bar{1}02)$ surface in UHV both experimentally, using temperature programmed desorption and surface-specific vibrational spectroscopy, and theoretically, using periodic-slab density functional theory calculations. This combined approach makes it possible to demonstrate that water adsorption occurs only at a single well defined surface site (the so-called 1-4 configuration) and that at this site the barrier between the molecularly and dissociatively adsorbed forms is very low: 0.06 eV. A subset of OD stretch vibrations are parallel to this dissociation coordinate, and thus would be expected to be shifted to low frequencies relative to an uncoupled harmonic oscillator. To quantify this effect we solve the vibrational Schr\"odinger equation along the dissociation coordinate and find fundamental frequencies red-shifted by more than 1,500 cm$^{\text{-1}}$. Within the context of this model, at moderate temperatures, we further find that some fraction of surface deuterons are likely delocalized: dissociatively and molecularly absorbed states are no longer distinguishable.