Structures and Properties of As(OH)₃ Adsorption Complexes on Hydrated Mackinawite (FeS) Surfaces: A DFT-D2 Study

Reactive mineral–water interfaces exert control on the bioavailability of contaminant arsenic species in natural aqueous systems. However, the ability to accurately predict As surface complexation is limited by the lack of molecular-level understanding of As–water–mineral interactions. In the present study, we report the structures and properties of the adsorption complexes of arsenous acid (As­(OH)₃) on hydrated mackinawite (FeS) surfaces, obtained from density functional theory (DFT) calculations. The fundamental aspects of the adsorption, including the registries of the adsorption complexes, adsorption energies, and structural parameters are presented. The FeS surfaces are shown to be stabilized by hydration, as is perhaps to be expected because the adsorbed water molecules stabilize the low-coordinated surface atoms. As­(OH)₃ adsorbs weakly at the water–FeS(001) interface through a network of hydrogen-bonded interactions with water molecules on the surface, with the lowest-energy structure calculated to be an As–up outer-sphere complex. Compared to the water–FeS(001) interface, stronger adsorption was calculated for As­(OH)₃ on the water–FeS(011) and water–FeS(111) interfaces, characterized by strong hybridization between the S-<i>p</i> and O-<i>p</i> states of As­(OH)₃ and the surface Fe-<i>d</i> states. The As­(OH)₃ molecule displayed a variety of chemisorption geometries on the water–FeS(011) and water–FeS(111) interfaces, where the most stable configuration at the water–FeS(011) interface is a bidentate Fe–AsO–Fe complex, but on the water–FeS(111) interface, a monodentate Fe–O–Fe complex was found. Detailed information regarding the adsorption mechanisms has been obtained via projected density of states (PDOS) and electron density difference iso-surface analyses and vibrational frequency assignments of the adsorbed As­(OH)₃ molecule

Early Oxidation Processes on the Greigite Fe₃S₄(001) Surface by Water: A Density Functional Theory Study

Greigite (Fe₃S₄), the sulfide counterpart of the spinel-structured oxide material magnetite (Fe₃O₄), is a mineral widely identified in anoxic aquatic environments and certain soils, which can be oxidized, thereby producing extremely acid solutions of sulfur-rich wastewaters, so-called acid mine drainage (AMD) or acid rock drainage (ARD). Here we report a computational study of the partial replacement of sulfur (forming H₂S) by oxygen (from H₂O) in the Fe₃S₄(001) surface, derived from density functional theory calculations with on-site Coulomb approach and long-range dispersion corrections (DFT+<i>U</i>–D2). We have proposed three pathways for the oxidation of the surface as a function of H₂O coverage and pH. Different pathways give different intermediates, some of which are followed by a solid-state diffusion of the O atom. Low levels of H₂O coverage, and especially basic conditions, seem to be essential, leading to the most favorable energetic landscape for the oxidation of the Fe₃S₄(001) surface. We have derived the thermodynamic and kinetic profile for each mechanism and plotted the concentration of H₂S and protons in aqueous solution and thermodynamic equilibrium with the stoichiometric and partially oxidized Fe₃S₄(001) surface as a function of the temperature. Changes in the calculated vibrational frequencies of the adsorbed intermediates are used as a means to characterize their transformation. We have taken into account statistical entropies for H₂S and H₂O and other experimental parameters, showing that this mineral may well be among those responsible for the generation of AMD

Ni Deposition on Yttria-Stabilized ZrO₂(111) Surfaces: A Density Functional Theory Study

Nickel particles supported on yttria-stabilized zirconia (YSZ) play a significant role in the performance of solid oxide fuel cells (SOFC). We have investigated both pristine and doped ZrO₂ surfaces using spin polarized density functional theory (DFT) and also considering long-range dispersion forces. We have systematically studied Ni deposition on the bare ZrO₂(111) surface and on surfaces with two concentrations of Y, all at both high and low oxygen chemical potential. Among the several independent sites explored, the Ni adsorption preference is as follows: YSZ(111) without oxygen vacancy > YSZ(111) with oxygen vacancy > stoichiometric ZrO₂(111). For each surface, the adsorption site is similar: over the top oxygen. The evaluation of the geometric and electronic structure shows a mixing of Ni orbitals with surface atom orbitals. We have also investigated the influence of the yttrium atom on the Ni adsorption by considering up to 52 different configurations, which showed that Ni tends to adsorb away from the yttrium atom for any YSZ(111) surface, leading to a mixed electronic structure with enhanced charge transfer

CuO Surfaces and CO₂ Activation: A Dispersion-Corrected DFT+<i>U</i> Study

We have used computational methodology based on the density functional theory to describe both copper­(I) and copper­(II) oxides, followed by the investigation of a number of different low index CuO surfaces. Different magnetic orderings of all the surfaces were studied, and reconstructions of the polar surfaces are proposed. A detailed discussion on stabilities, electronic structure, and magnetic properties is presented. CuO(111) and CuO­(111) were found to have the lowest surface energies, and their planes dominate in the calculated Wulff morphology of the CuO crystal. We next investigated the adsorption of CO₂ on the three most exposed CuO surfaces, viz., (111), (111), and (011), by exploring various adsorption sites and configurations. We show that the CO₂ molecule is activated on the CuO surfaces, with an adsorption energy of −93 kJ/mol on the (011) surface, showing exothermic adsorption, while (111) and (111) surfaces show comparatively weak adsorption. The activation of the CO₂ molecule is characterized by large structural transformations and significant charge transfer, i.e., forming a negatively charged bent CO₂^–δ species with elongated C–O bonds, which is further confirmed by vibrational analyses showing considerable red shift in the frequencies as a result of the activation

How to go beyond C₁ products with electrochemical reduction of CO₂

The electrochemical reduction of CO2to produce fuels and value-added organic chemicals is of great potential, providing a mechanism to convert and store renewable energy within a carbon-neutral energy circle. Currently the majority of studies report C1products such as carbon monoxide and formate as the major CO2reduction products. A particularly challenging goal within CO2electrochemical reduction is the pursuit of multi-carbon (C2+) products which have been proposed to enable a more economically viable value chain. This review summaries recent development across electro-, photoelectro- and bioelectro-catalyst developments. It also explores the role of device design and operating conditions in enabling C-C bond generation

Active Nature of Primary Amines during Thermal Decomposition of Nickel Dithiocarbamates to Nickel Sulfide Nanoparticles

Although [Ni­(S₂CNBuⁱ₂)₂] is stable at high temperatures in a range of solvents, solvothermal decomposition occurs at 145 °C in oleylamine to give pure NiS nanoparticles, while in <i>n</i>-hexylamine at 120 °C a mixture of Ni₃S₄ (polydymite) and NiS results. A combined experimental and theoretical study gives mechanistic insight into the decomposition process and can be used to account for the observed differences. Upon dissolution in the primary amine, octahedral <i>trans-</i>[Ni­(S₂CNBuⁱ₂)₂(RNH₂)₂] result as shown by <i>in situ</i> XANES and EXAFS and confirmed by DFT calculations. Heating to 90–100 °C leads to changes consistent with the formation of amide-exchange products, [Ni­(S₂CNBuⁱ₂)­{S₂CN­(H)­R}] and/or [Ni­{S₂CN­(H)­R}₂]. DFT modeling shows that exchange occurs via nucleophilic attack of the primary amine at the backbone carbon of the dithiocarbamate ligand(s). With hexylamine, amide-exchange is facile and significant amounts of [Ni­{S₂CN­(H)­Hex}₂] are formed prior to decomposition, but with oleylamine, exchange is slower and [Ni­(S₂CNBuⁱ₂)­{S₂CN­(H)­Oleyl}] is the active reaction component. The primary amine dithiocarbamate complexes decompose rapidly at ca. 100 °C to afford nickel sulfides, even in the absence of primary amine, as shown from thermal decomposition studies of [Ni­{S₂CN­(H)­Hex}₂]. DFT modeling of [Ni­{S₂CN­(H)­R}₂] shows that proton migration from nitrogen to sulfur leads to formation of a dithiocarbimate (S₂CNR) which loses isothiocyanate (RNCS) to give dimeric nickel thiolate complexes [Ni­{S₂CN­(H)­R}­(μ-SH)]₂. These intermediates can either lose dithiocarbamate(s) or extrude further isothiocyanate to afford (probably amine-stabilized) nickel thiolate building blocks, which aggregate to give the observed nickel sulfide nanoparticles. Decomposition of the single or double amide-exchange products can be differentiated, and thus it is the different rates of amide-exchange that account primarily for the formation of the observed nanoparticulate nickel sulfides