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

Cation binding of Li(I), Na(I) and Zn(II) to cobalt and iron sulphide clusters - electronic structure study

The binding of alkaline (Li⁺ and Na⁺) and zinc (Zn²⁺) cations to mononuclear disulphides MS₂ and to persulphides, containing an S-S bond, M(S₂), to binuclear disulphides M₂S₂ and persulphides M₂(S₂) and to cubic tetranuclear sulphides M₄S₄ where M = Fe, Co, is examined by density functional theory with the B3LYP functional, and dispersion corrections were applied. For the small-sized clusters (up to two transition metal centres), the energy gaps between different configurations were verified by CCSD(T) calculations. Persulphides M(S₂) are more stable than disulphides MS₂ as bare clusters, upon carbonyl and chloride ligand coordination and upon cation binding (Li⁺, Na⁺, Zn²⁺). The one-electron reduction of alkali cations and two-electron reduction of Zn²⁺ reverses order of stability and the planar disulphides (MS₂-reduced cation) become more stable; the energy gap disulphide to persulphide increases. In all reduced clusters, zinc ions form bonds with sulphur and with the transition metal centre (Co or Fe). Lithium cations also form bonds to cobalt or iron, but only in the M₂S₂ clusters, upon reduction. Energy barriers were calculated for the disulphide to persulphide reaction in the Zn-Co-S₂ system in the isolated clusters (gas-phase), in water, acetonitrile and 1-Cl-hexane solution. Most significant decrease in the energy barriers were obtained with less-polar solvents, acetonitrile, and particularly, 1-Cl-hexane. In M₄S₄ clusters, the cations do not reach optimal coordination to the sulphur centres. The global minima of M₂S₂ clusters are antiferromagnetic; in the reduced Zn-M₂S₂ clusters, magnetic moment is induced at zinc centres as a result of charge transfer between Zn and Co or Zn and Fe.

Catalytic formation of oxalic acid on the partially oxidised greigite Fe3S4(001) surface

Greigite (Fe₃S₄), with its ferredoxin-like 4Fe-4S redox centres, is a naturally occurring mineral capable of acting as a catalyst in the conversion of carbon dioxide (CO₂) into low molecular-weight organic acids (LMWOAs), which are of paramount significance in several soil and plant processes as well as in the chemical industry. In this paper, we report the reaction between CO₂ and water (H₂O) to form oxalic acid (H₂C₂O₄) on the partially oxidised greigite Fe₃S₄(001) surface by means of spin-polarised density functional theory calculations with on-site Coulomb corrections and long-range dispersion interactions (DFT+U-D2). We have calculated the bulk phase of Fe₃S₄ and the two reconstructed Tasker type 3 terminations of its (001) surface, whose properties are in good agreement with available experimental data. We have obtained the relevant phase diagram, showing that the Fe₃S₄(001) surface becomes 62.5% partially oxidised, by replacing S by O atoms, in the presence of water at the typical conditions of calcination [Mitchell et al. Faraday Discuss. 2021, 230, 30-51]. The adsorption and co-adsorption of the reactants on the partially oxidised Fe₃S₄(001) surface are exothermic processes. We have considered three mechanistic pathways to explain the formation of H₂C₂O₄, showing that the coupling of the C-C bond and second protonation are the elementary steps with the largest energy penalty. Our calculations suggest that the partially oxidised Fe₃S₄(001) surface is a mineral phase that can catalyse the formation of H₂C₂O₄ under favourable conditions, which has important implications for natural ecosystems and is a process that can be harnessed for the industrial manufacture of this organic acid.

Facet-Dependent Photoinduced Transformation of Cadmium Sulfide (CdS) Nanoparticles

Microbial-mediated transformation of anthropogenic Cd²⁺ controls its distribution, bioavailability, and potential risks. However, the processes readily form CdS nanoparticles (CdS-NPs), which exhibit dissolution behavior different from that of larger sized particles. Here, we investigated the effects of morphologies and facets of CdS-NPs on their photoinduced dissolution. Three CdS-NPs, CdS-sphere, CdS-rod, and CdS-sheet, and one nanosized biogenic CdS (Bio-CdS) were synthesized with different dominant facets of {101}, {100}, {001}, and {111} and thus distinct surface chemistry. As explored by HRTEM, EPR, and DFT calculations, photogenerated e^-/h⁺ pairs were more likely to generate on CdS-sheet surfaces due to higher surface energies and a narrower band gap, facilitating the formation of •OH and thereby faster dissolution (k_obs = 6.126-6.261 × 10^-2 h^-1). The wider band gaps of CdS-sphere and CdS-rod caused less formation of O₂^•- and •OH, leading to slower oxidative dissolutions (k_obs = 0.090-0.123 and 2.174-3.038 × 10^-2 h^-1, respectively). Given the similar surface energy as that of CdS-sheet, the dissolution rate of Bio-CdS was close to that of CdS-rod and CdS-sheet, which was 1.6-3.5 times faster than that of larger sized CdS, posing higher environmental risks than thought. Altogether, this work revealed the facet effects on the dissolution of CdS-NPs, manifesting a deeper understanding of metal sulfides' environmental behaviors.

Mechanistic study of hydrazine decomposition on Ir(111)

Hydrogen transport and storage technology remain one of the critical challenges of the hydrogen economy. Hydrazine (N2H4) is a carbon-free hydrogen carrier which has been widely used as fuel in the field of space exploration. We have combined experiments and computer simulations in order to gain a better understanding of the N2H4 decomposition on Ir catalyst, the most efficient catalyst for hydrazine decomposition up to date. We have identified metallic Ir rather than IrO2 as the active phase for hydrazine decomposition and carried out density functional theory (DFT) calculations to systematically investigate the changes in the electronic structure along with the catalytic decomposition mechanisms. Three catalytic mechanisms to hydrazine decomposition over Ir(111) have been found: (i) intramolecular reaction between hydrazine molecules, (ii) intramolecular reaction between co-adsorbed amino groups, and (iii) hydrazine dehydrogenation assisted by co-adsorbed amino groups. These mechanisms follow five different pathways for which transition states and intermediates have been identified. The results show that hydrazine decomposition on Ir(111) starts preferentially with an initial N-N bond scission followed by hydrazine dehydrogenation assisted by the amino group produced, eventually leading to ammonia and nitrogen production. The preference for N-N scission mechanisms was rationalized by analyzing the electronic structure. This analysis showed that upon hydrazine adsorption, the π bond between nitrogen atoms becomes weaker.

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.

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'.