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

Mapping surface morphology and phase evolution of iron sulfide nanoparticles

The size effect on the thermodynamic phase diagram of FexSy nanoparticles.

Iron Sulfide Materials: Catalysts for Electrochemical Hydrogen Evolution

The chemical challenge of economically splitting water into molecular hydrogen and oxygen requires continuous development of more efficient, less-toxic, and cheaper catalyst materials. This review article highlights the potential of iron sulfide-based nanomaterials as electrocatalysts for water-splitting and predominantly as catalysts for the hydrogen evolution reaction (HER). Besides new synthetic techniques leading to phase-pure iron sulfide nano objects and thin-films, the article reviews three new material classes: (a) FeS2-TiO2 hybrid structures; (b) iron sulfide-2D carbon support composites; and (c) metal-doped (e.g., cobalt and nickel) iron sulfide materials. In recent years, immense progress has been made in the development of these materials, which exhibit enormous potential as hydrogen evolution catalysts and may represent a genuine alternative to more traditional, noble metal-based catalysts. First developments in this comparably new research area are summarized in this article and discussed together with theoretical studies on hydrogen evolution reactions involving iron sulfide electrocatalysts.

A density functional theory study of the hydrogenation and reduction of the thio-spinel Fe3S4{111} surface

The mineral greigite, Fe3S4, shows promising electro-reduction activity, especially towards carbon dioxide conversion to small organic molecules. We have employed density functional theory calculations with correction for the long-range dispersion forces to investigate the behavior of hydrogen on the greigite{111} surface. We have studied the adsorption, diffusion, surface reduction and associative (i.e. Volmer-Tafel mechanism) and molecular desorption of hydrogen as a function of its coverage. We found that (i) the H ad-atoms adsorb on S sites far from metallic centres in the topmost surface layer; (ii) the reduction of greigite by hydrogen is energetically unfavorable at any surface coverage; and (iii) molecular hydrogen evolution has a transition state at ∼0.5 eV above the energy of the reactants on Fe3S4{111}, which is very similar to the barrier found experimentally on Pt{111}. We have also determined the electrode potential under room conditions at which the H2 evolution reaction becomes energetically barrierless.

A kinetic model of water adsorption, clustering and dissociation on the Fe3S4{001} surface

The interaction of water with catalyst surfaces is a common process which requires investigation. Here, we have employed density functional theory calculations to investigate the adsorption of up to ten water molecules on the {001} surface of greigite (Fe₃S₄), which owing to its redox properties, is of increasing interest as a catalyst, e.g. in electro-catalysis. We have systematically analyzed and characterized the modes of water adsorption on the surface, where we have considered both molecular and dissociative adsorption processes. The calculations show that molecular adsorption is the predominant state on these surfaces, from both a thermodynamic and kinetic point of view. We have explored the molecular dispersion on the surface under different coverages and found that the orientation of the molecule, and therefore the surface dipole, depends on the number of adsorbed molecules. The interactions between the water molecules become stronger with an increasing number of water molecules, following an exponential decay which tends to the interaction energy found in bulk water. We have also shown the evolution of the infra-red signals as a function of water coverage relating to the H-bond networks formed on the surface. Next we have included these results in a classical micro-kinetic model, which introduced the effects of temperature in the simulations, thus helping us to derive the water cluster size on the greigite surface as a function of the initial conditions of pressure, temperature and external potential. The kinetic model concluded that water molecules agglomerate in clusters instead of wetting the surface, which agrees with the low hydrophilicity of Fe₃S₄. Clusters consisting of four water molecules was shown to be the most stable cluster under a wide range of temperatures and external potential.

Reactivity of CO2 on the surfaces of magnetite (Fe3O4), greigite (Fe3S4) and mackinawite (FeS)

The growing environmental, industrial and commercial interests in understanding the processes of carbon dioxide (CO2) capture and conversion have led us to simulate, by means of density functional theory calculations, the application of different iron oxide and sulfide minerals to capture, activate and catalytically dissociate this molecule. We have chosen the {001} and {111} surfaces of the spinel-structured magnetite (Fe3O4) and its isostructural sulfide counterpart greigite (Fe3S4), which are both materials with the Fe cations in the 2+/3+ mixed valence state, as well as mackinawite (tetragonal FeS), in which all iron ions are in the ferrous oxidation state. This selection of iron-bearing compounds provides us with understanding of the effect of the composition, stoichiometry, structure and oxidation state on the catalytic activation of CO2 The largest adsorption energies are released for the interaction with the Fe3O4 surfaces, which also corresponds to the biggest conformational changes of the CO2 molecule. Our results suggest that the Fe3S4 surfaces are unable to activate the CO2 molecule, while a major charge transfer takes place on FeS{111}, effectively activating the CO2 molecule. The thermodynamic and kinetic profiles for the catalytic dissociation of CO2 into CO and O show that this process is feasible only on the FeS{111} surface. The findings reported here show that these minerals show promise for future CO2 capture and conversion technologies, ensuring a sustainable future for society.This article is part of a discussion meeting issue 'Providing sustainable catalytic solutions for a rapidly changing world'.

Pressure-induced structural and spin transitions of Fe3S4

Greigite (Fe₃S₄), isostructural with Fe₃O₄ has recently attracted great scientific interests from material science to geology due to its complicated structure and electronic and magnetic configurations. Here, an investigation into the structural, magnetic and electronic properties of Fe₃S₄ under high pressure has been conducted by first-principle calculations based on density functional theory. The results show that a first-order phase transition of Fe₃S₄ would occur from the inverse spinel (SP) structure to the Cr₃S₄-type (CS) structure at 3.4 GPa, accompanied by a collapse of 9.7% in the volume, a redistribution of iron cations, and a half-metal to metal transition. In the CS-Fe₃S₄, Fe²⁺ located at octahedral environment firstly undergoes a transition from high-spin (HS) state to low-spin (LS) state at 8.5 GPa and Fe³⁺ subsequently does at 17 GPa. The Equation of State for different phases of Fe₃S₄ are also determined. Our results not only give some clues to explore novel materials by utilizing Fe₃S₄ but also shed light on the fundamental information of Fe₃O₄, as well as those of other SP-AB₂X₄ compounds.

Periodic DFT+U investigation of the bulk and surface properties of marcasite (FeS2)

Marcasite FeS₂ and its surfaces properties have been investigated by Hubbard-corrected density functional theory (DFT+U) calculations.

Surface and shape modification of mackinawite (FeS) nanocrystals by cysteine adsorption: a first-principles DFT-D2 study

The surface and shape modulation of mackinawite (FeS) nanoparticles by amino acid cysteine adsorption is investigated using a first-principles density functional theory calculations, corrected for dispersion-interactions (DFT-D2).