Catalytic water dissociation by greigite Fe3S4 surfaces: density functional theory study.

A Roldan,N H De Leeuw

doi:10.1098/rspa.2016.0080

Abstract

The iron sulfide mineral greigite, Fe3S4, has shown promising capability as a hydrogenating catalyst, in particular in the reduction of carbon dioxide to produce small organic molecules under mild conditions. We employed density functional theory calculations to investigate the {001},{011} and {111} surfaces of this iron thiospinel material, as well as the production of hydrogen ad-atoms from the dissociation of water molecules on the surfaces. We systematically analysed the adsorption geometries and the electronic structure of both bare and hydroxylated surfaces. The sulfide surfaces presented a higher flexibility than the isomorphic oxide magnetite, Fe3O4, allowing perpendicular movement of the cations above or below the top atomic sulfur layer. We considered both molecular and dissociative water adsorption processes, and have shown that molecular adsorption is the predominant state on these surfaces from both a thermodynamic and kinetic point of view. We considered a second molecule of water which stabilizes the system mainly by H-bonds, although the dissociation process remains thermodynamically unfavourable. We noted, however, synergistic adsorption effects on the Fe3S4{001} owing to the presence of hydroxyl groups. We concluded that, in contrast to Fe3O4, molecular adsorption of water is clearly preferred on greigite surfaces.

Highlights

The necessity of mitigating climate change has led to policies to regulate and minimize the concentration of2016 The Authors
We considered both molecular and dissociative water adsorption processes, and have shown that molecular adsorption is the predominant state on these surfaces from both a thermodynamic and kinetic point of view
There are currently no experimental scanning tunnelling microscopy (STM) measurements in the literature, we have provided STM models in the electronic supplementary material, figure S2, which resemble those for Fe3O4 [72], and the surfaces shown by high-resolution transition electron microscopy obtained from pure greigite nanoparticles [16]

Summary

Introduction

The chemical properties of the water dissociation products (OH, H and O) are very different from those of the water molecule and may lead, for example, to the surface and bulk oxidation of many materials [17]. The electron delocalization of the DFT method leads to an underestimation of atomic charges, i.e. the DFT-derived charges are smaller than the formal oxidation states They can be used effectively in a direct comparison and to monitor changes in charges, for example as an effect of surface adsorption. We obtained an overview of the surface electronic structure by integrating the local density of states from the Fermi level to a bias by using the Tersoff–Hamann formalism [76], which is expressed as scanning tunnelling microscopy (STM) images implemented in their most basic formulation, approximating the STM tip by an infinitely small point source. We placed an H2O molecule on several non-equivalent (a)

FeA FeB

Findings

Conclusion