Hydration Mechanisms of Tungsten Trioxide Revealed by Water Adsorption Isotherms and First-Principles Molecular Dynamics Simulations

Abstract

WO3 is one of the most interesting materials for photocatalysis due to its optical absorption in UV–vis, good charge carrier transport, and stability against photocorrosion. A detailed knowledge of the hydration state of WO3 is indispensable to gain more precise mechanistic insights into its photocatalytic activity. To the best of our knowledge, the hydration process of WO3 has been scarcely studied so far and only from a theoretical point of view, mostly by means of static density functional theory calculations. Here, we use manometric gas sorption experiments combined with extensive ab initio molecular dynamics simulations to investigate the hydration mechanisms of WO3 at room temperature. We demonstrate that water adsorbs molecularly in a two-step mechanism: the first water sublayer interacts with the under-coordinated surface tungsten atoms through electrostatic interactions, and, in a second step, a full layer is formed through hydrogen bonds established with the pre-adsorbed water molecules and with the under-coordinated surface oxygen atoms. The computed value of −90 kJ·mol–1 for the isosteric enthalpy of adsorption of the first water molecules is in excellent agreement with the experimental value estimated using the Clausius–Clapeyron approach from the experimental adsorption isotherms. The isosteric enthalpy of adsorption steadily decreases in absolute value when the surface coverage is increased to tend toward the enthalpy of condensation of water after three adsorption layers.

Mg-Exchanged Gismondine for Superior CO2/N2 and CO2/CH4 Separations

“…Molecular dynamics simulations with 0−4 CO 2 molecules per cell were performed to compute successive enthalpies of adsorption of CO 2 , which can be considered as isosteric adsorption enthalpies at different coverages. 51 One molecule per cell corresponds to 0.94 mmol/g for Na-GIS and 0.96 mmol/g for NaMg-GIS.…”

“…Therefore, for a deeper understanding of the heat of adsorption behavior in NaMg1-GIS, the successive adsorption enthalpies, corresponding to isosteric adsorption enthalpies at different coverages, 51 have also been computed by ab initio molecular dynamics simulations at 300 K as described in the computational details. For consistency with the experimental data, CO 2 molecules were added successively up to 4 molecules per cell in Na-GIS and 2 in NaMg-GIS.…”