6 research outputs found
Early Oxidation Processes on the Greigite Fe<sub>3</sub>S<sub>4</sub>(001) Surface by Water: A Density Functional Theory Study
Greigite
(Fe<sub>3</sub>S<sub>4</sub>), the sulfide counterpart
of the spinel-structured oxide material magnetite (Fe<sub>3</sub>O<sub>4</sub>), 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 H<sub>2</sub>S)
by oxygen (from H<sub>2</sub>O) in the Fe<sub>3</sub>S<sub>4</sub>(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 H<sub>2</sub>O 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 H<sub>2</sub>O coverage, and especially basic conditions, seem
to be essential, leading to the most favorable energetic landscape
for the oxidation of the Fe<sub>3</sub>S<sub>4</sub>(001) surface.
We have derived the thermodynamic and kinetic profile for each mechanism
and plotted the concentration of H<sub>2</sub>S and protons in aqueous
solution and thermodynamic equilibrium with the stoichiometric and
partially oxidized Fe<sub>3</sub>S<sub>4</sub>(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 H<sub>2</sub>S and H<sub>2</sub>O and other experimental parameters,
showing that this mineral may well be among those responsible for
the generation of AMD
Structures and Properties of As(OH)<sub>3</sub> 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 (AsÂ(OH)<sub>3</sub>)
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. AsÂ(OH)<sub>3</sub> 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 AsÂ(OH)<sub>3</sub> 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 AsÂ(OH)<sub>3</sub> and the surface
Fe-<i>d</i> states. The AsÂ(OH)<sub>3</sub> 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 AsÂ(OH)<sub>3</sub> molecule
Ni Deposition on Yttria-Stabilized ZrO<sub>2</sub>(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 ZrO<sub>2</sub> surfaces using spin polarized density functional theory (DFT) and
also considering long-range dispersion forces. We have systematically
studied Ni deposition on the bare ZrO<sub>2</sub>(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 ZrO<sub>2</sub>(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 CO<sub>2</sub> Activation: A Dispersion-Corrected DFT+<i>U</i> Study
We have used computational methodology
based on the density functional
theory to describe both copperÂ(I) and copperÂ(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 CuOÂ(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 CO<sub>2</sub> on the three most exposed CuO surfaces, viz., (111),
(111), and (011), by exploring various adsorption
sites and configurations. We show that the CO<sub>2</sub> 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 CO<sub>2</sub> molecule is characterized
by large structural transformations and significant charge transfer,
i.e., forming a negatively charged bent CO<sub>2</sub><sup>–δ</sup> 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 C<sub>1</sub> products with electrochemical reduction of CO<sub>2</sub>
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 [NiÂ(S<sub>2</sub>CNBu<sup>i</sup><sub>2</sub>)<sub>2</sub>] 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 Ni<sub>3</sub>S<sub>4</sub> (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>[NiÂ(S<sub>2</sub>CNBu<sup>i</sup><sub>2</sub>)<sub>2</sub>(RNH<sub>2</sub>)<sub>2</sub>] 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, [NiÂ(S<sub>2</sub>CNBu<sup>i</sup><sub>2</sub>)Â{S<sub>2</sub>CNÂ(H)ÂR}] and/or [NiÂ{S<sub>2</sub>CNÂ(H)ÂR}<sub>2</sub>]. 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 [NiÂ{S<sub>2</sub>CNÂ(H)ÂHex}<sub>2</sub>] are formed prior to decomposition, but with oleylamine, exchange
is slower and [NiÂ(S<sub>2</sub>CNBu<sup>i</sup><sub>2</sub>)Â{S<sub>2</sub>CNÂ(H)ÂOleyl}] 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 [NiÂ{S<sub>2</sub>CNÂ(H)ÂHex}<sub>2</sub>]. DFT modeling of [NiÂ{S<sub>2</sub>CNÂ(H)ÂR}<sub>2</sub>] shows
that proton migration from nitrogen to sulfur leads to formation of
a dithiocarbimate (S<sub>2</sub>Cî—»NR) which loses isothiocyanate
(RNCS) to give dimeric nickel thiolate complexes [NiÂ{S<sub>2</sub>CNÂ(H)ÂR}Â(μ-SH)]<sub>2</sub>. 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