Homolytic Cleavage of Water on Magnesia Film Promoted by Interfacial Oxide-Metal Nanocomposite

TL;DR

This study uses density functional theory to reveal that ultrathin magnesia films on metal substrates enable energetically favorable homolytic water dissociation, a process difficult on bulk magnesia, with implications for water splitting technologies.

Contribution

It demonstrates for the first time that nanoscale magnesia films promote homolytic water splitting, contrasting with bulk behavior, and provides detailed energetic insights into the process.

Findings

Homolytic dissociation energy on monolayer magnesia is -1.192 eV.

Homolytic dissociation becomes less favorable as oxide thickness increases.

Activation barriers for heterolytic to homolytic transition range from 0.733 to 1.849 eV.

Abstract

The atomic scale insights into the interaction of water with oxide surface are essential for elucidating the mechanism of physiochemical processes in various scientific and practical fields, since the water is ubiquitous and coats multifarious material surface under ambient conditions. By utilizing periodic density functional theory calculations with van der Waals corrections, herein we report for the first time the energetically and thermodynamically favorable homolytic dissociative adsorption behavior of water over magnesia (001) films deposited on metal substrate. The water adsorption on pristine magnesia (001) is quite weak and the heterolytic dissociation is the only fragmentation pathway, which is highly endothermic with large activation barrier of 1.167 eV. The binding strength for the molecular and dissociative adsorption configurations of water on hybrid MgO(001) films are significantly larger than corresponding configurations on bare magnesia (001). The homolytic dissociative adsorption energy of water on monolayer oxide is calculated to be -1.192 eV, which is even larger than all the heterolytic dissociative adsorption states. With the increase of the oxide thickness, the homolytic dissociative adsorption energy decreases promptly, indicating the nanoscale of oxide film play a crucial role in homolytic splitting of water. The homolytic dissociative adsorption structure could be obtained by transformation reaction from heterolytic dissociative adsorption structure, presenting activation barriers 0.733, 1.103, 1.306, 1.571, and 1.849 eV for reactions on 1 ML to 5 ML films. It is anticipated that the results here could provide inspiring clue and versatile strategy for enhancing chemical reactivity and properties of insulating oxide toward homolytic water splitting processes involved in novel and unprecedented applications.