FeS2 Surface Research Articles

Quantifying the origin of inter-adsorbate interactions on reactive surfaces for catalyst screening and design.

The adsorption energy of reactant molecules and reaction intermediates is one of the key descriptors of catalytic activity of surfaces and is commonly used as a metric in screening materials for design of heterogeneous catalysts. The efficacy of such screening schemes depends on the accuracy of calculated adsorption energies under reaction conditions. These adsorption energies can depend strongly on interactions between adsorbed molecules in the adlayer. However, these interactions are typically not accounted for in screening procedures that use DFT-based zero-coverage adsorption energies. Identifying the physical mechanisms behind these interactions is essential to model realistic catalyst surfaces under reaction conditions and to understand the dependence of adsorption energies on reaction parameters like surface strain and composition. This article describes a method to quantitatively resolve the observed inter-adsorbate interactions into various direct adsorbate-adsorbate interactions (i.e. Coulombic and steric) and surface-mediated interactions (i.e. adsorbate-induced surface relaxation and change in electronic structure) by combining density functional theory and cluster-expansion calculations of coverage-dependent adsorption energies. The approach is implemented on a model catalyst surface of FeS2(100) reacting with H2S molecules. We find that the adsorption energy of H2S molecules can be affected by over 0.55 eV by the repulsive inter-adsorbate interactions caused primarily by the adsorbate-induced changes to the electronic structure of the FeS2 surface. These interactions also show a strong monotonic dependence on surface strain, being three times stronger on compressively strained surfaces than on surfaces under tensile strain. The large magnitude of inter-adsorbate interactions as well as their strong dependence on lattice strain demonstrate the need for using coverage-dependent adsorption energies for more accurate screening, for example for strained catalytic systems like core-shell and overlayer structures.

Comparison of Multilayer Water Adsorption on the Hydrophobic Galena (PbS) and Hydrophilic Pyrite (FeS2) Surfaces: A DFT Study

To investigate the essential difference in the water adsorption on the hydrophobic galena (PbS) and hydrophilic pyrite (FeS2) surfaces, a single, mono-, and multilayer water adsorptions on the PbS and FeS2(100) surfaces have been simulated by means of density functional theory. The calculated results show that the water molecule adsorption on the hydrophobic PbS surface is via a hydrogen bond between the S and H atom and on the hydrophilic FeS2 surface is mainly via the interaction between surface Fe and the water O atom. On the galena surface, the hydrogen bond effect of multilayer water weakens the S···H bond interaction and leads to the galena surface being hydrophobic. For the pyrite surface, the hydrogen bond effect of the multilayer water enhances the activity of the O 2p orbital and favors the interaction of the O 2p and Fe 3d eg orbital, which results in the pyrite surface being hydrophilic and undergoing a greater relaxation. These variations may have a great influence on the subsequent interfacial reactions on the galena and pyrite surfaces.

Photoelectrochemical Behavior of FeS2 in Aqueous and Nonaqeuous I−/I3− Electrolytes

The photoelectrochemical behavior of iron pyrite (FeS2) in contact with I−/I3− in acetonitrile was studied and compared to an aqueous system. This work was motivated by the proposed link between FeS2 surface chemistry and its unexpectedly low photovoltage, and the prevalence of aqueous surface chemistry across a wide potential range. The results revealed that the nonaqueous system employed is suitable for further, strictly anhydrous and anoxic electrochemical studies. However, it was also seen that FeS2/aqueous-electrolyte interactions were not responsible for limiting the observed photovoltage to well below what is expected based on the bandgap of FeS2. The influence of concentration of dissolved I2 on the observed photovoltage was investigated experimentally and compared to a model. A central component of the proposed model was that shunting was assumed to take place at physically distinct regions of the electrode and that mass-transport to and from these regions could be treated separately from mass-transport to the regions responsible for the rectifying behavior of the FeS2/liquid junction. The calculated photovoltage as a function of cathodic limiting current showed the same qualitative trend as experimental data with varying concentrations of added I2.

Adsorption of methylamine on mackinawite (FES) surfaces: A density functional theory study

We have used density functional theory calculations to investigate the interaction between methylamine (CH3NH2) and the dominant surfaces of mackinawite (FeS), where the surface and adsorption properties of mackinawite have been characterized using the DFT-D2 method of Grimme. Our calculations show that while the CH3NH2 molecule only interacts weakly with the most stable FeS(001), it adsorbs relatively strongly on the FeS(011) and FeS(100) surfaces releasing energies of 1.26 eV and 1.51 eV, respectively. Analysis of the nature of the bonding reveals that the CH3NH2 molecule interacts with the mackinawite surfaces through the lone-pair of electrons located on the N atom. The electron density built up in the bonding region between N and Fe is very much what one would expect of covalent type of bonding. We observe no significant adsorption-induced changes of the FeS surface structures, suggesting that amine capping agents would not distort the FeS nanoparticle surfaces required for active heterogeneous catalytic reactions. The vibrational frequencies and the infrared spectra of adsorbed methylamine have been calculated and assignments for vibrational modes are used to propose a kinetic model for the desorption process, yielding a simulated temperature programmed desorption with a relative desorption temperature of <140 K at the FeS(011) surface and <170 K at FeS(100) surface.

Intrinsic Vacancies and Their effect on Corrosion Reactivity at the FeS2 (100) Surface

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Controlling morphology and phase of pyrite FeS2 hierarchical particles via the combination of structure-direction and chelating agents

Pyrite FeS2 hierarchical microparticles consisting of cubic-like crystals with well defined {100} faceted cubic textures were successfully synthesized using a simple hydrothermal method with the presence of ethylenediamine (en) and polyvinylpyrrolidone (PVP). The roles of PVP and en in the synthesis process were elucidated by the investigation of morphology and crystalline phases of the products under different reaction conditions and by the analysis of atomic structures of the pyrite (100) and (111) surfaces with different termination layers. While en chelates with Fe2+ and reacts with S8 of the reaction agents to help with dispersion and nucleation as well as the role of reducing agent, PVP acts as both a phase regulator due to its reducing ability and a surface capping agent to bind the iron atoms at the FeS2 (111) surface and direct the formation of {100} facets. It was found that the oxidation state of iron ions and the species of anions in the iron reagents also played important roles in the formation of crystalline phase and morphology of the products. The pyrite hierarchical microparticles could be utilized as a new semiconductor ink for solution processing of photovoltaic devices and photodetectors as well as cathode materials for lithium batteries.

Oxidation of FeS by oxygen-bearing acidic solutions

Oxidation of FeS in oxygen-bearing acidic solutions was investigated at different temperatures (25 to 45 °C) and pH (2.75 to 3.45). The rate of the oxidative dissolution of FeS is strongly dependent on pH. The reaction order with respect to hydrogen ions has been found to be 1.03 ± 0.02 at 25 °C, and the apparent activation energy ( E a ) is 41.6 ± 10.7 kJ mol −1 at initial pH 3.00, suggesting that the FeS oxidative dissolution is controlled by the diffusion of oxidant species across a sulfur-rich layer (SRL) that undergoes chemical transformations leading to an increase in the mean number of sulfur atoms in polysulfide chains and the rearrangement of these chains. Fourier transform infrared spectroscopy and X-ray diffraction results obtained for the FeS samples reacted for 72 h at 25 °C and pH between 2.75 and 3.45 indicate the formation of goethite, of lepidocrocite, and of poorly ordered solid phases (assigned as SRL) on initial surfaces. The experimental data suggest a mechanism based on the protonation of FeS surfaces followed by oxidation of FeS by dissolved oxygen to produce Fe 2+, S 0, and S 2− n . Fe 2+ is unstable under oxidative conditions and transforms into Fe(OH) 3(s), goethite and lepidocrocite.

Calculation of electron structure by density function theory and electrochemical process of surface (100) of FeS2

The electron structure of FeS2 surface (100) was computed by DFT (density function theory) and the process of electron transfer in sulfide flotation was simulated through ab-initio calculation. The results show that the interaction between xanthate and FeS2 is controlled by the energy of valence band. The products and degree of the reaction depend on the density of state of valence band and concentration of positive hole in valence band. Interaction between xanthate and pyrite can be changed by modifying the election structure of the surface of pyrite. Xanthate is adsorbed on the surface of intrinsic pyrite. But the amount of xanthate adsorbed on the surface of the pyrite with sulfur vacancy is more than that on the surface of the intrinsic pyrite due to the higher electron and vacancy density. Xanthate is not adsorbed on the surface of pyrite with Fe vacancy because of its high Fermi energy.

Selenite reduction in Boom clay: Effect of FeS2, clay minerals and dissolved organic matter

Several experiments were set up to study Se speciation and solubility in the reducing Boom clay environment, starting from oxidized Se species which were added in oversaturation with respect to the thermodynamic solubility of reduced Se solid phases. Upon introduction of SeO32− to FeS2-containing samples, adsorption of SeO32− occurred at the FeS2 surface, and led to a reduction and precipitation of a Se0 solid phase with a solubility of 3×10−9 M (after 60 days). In the presence of humic substances, an association of Se with these humic substances was observed and the 3×10−9 M solubility limit was not reached in the same time delay. Upon introduction of SeO32− to Boom clay suspensions (equilibration up to 9 months), the initial adsorption of SeO32− on the solid phase was increased with respect to systems containing only FeS2, due to the presence of (illite) clay minerals. This competing adsorption process, and the presence of humic substances, again decreased the kinetics of reduction with respect to FeS2 samples. Also, an association of Se with Boom clay humic substances was observed, and amounted up to ∼10−7 M in some samples after 9 months equilibration.

Chemical Activity of Iron in [2Fe-2S]-Protein Centers and FeS2(100) Surfaces

Iron atoms bonded to sulfur play an important role in proteins, heterogeneous catalysts, and gas sensors. First-principles density functional calculations were used to investigate the structure and chemical activity of a unique [2Fe-2S] center in the split-Soret cytochrome c (Ssc) from Desulfovibrio desulfuricans. In agreement with a previously proposed structural model [Abreu et al., J. Biol. Inorg. Chem. 2003, 8, 360], it is found that the [2Fe-2S] cluster is located in a surface pocket of the Ssc and bonded to only three cysteines. The [2Fe-2S] center in the Ssc is nonplanar and somewhat distorted with respect to canonical [2Fe-2S] centers seen in proteins where the iron-sulfur unit is bonded to four cysteines. In the Ssc, the lack of one Fe-cysteine bond is partially compensated by the separation between the cysteines that minimizes electrostatic repulsion among these ligands. The unique structure of the [2Fe-2S] center in the Ssc makes the center more chemically active than canonical [2Fe-2S] centers in proteins, (RS)(4)[2Fe-2S] inorganic complexes, and an FeS2(100) surface. A [2Fe-2S] center in the Ssc interacts efficiently with electron acceptors (O2, NO, CO) and poorly with a Lewis base such as H2O. The interaction with molecular oxygen is so strong that eventually oxidatively destroys the [2Fe-2S] unit. The bonding energy of the ligands to the [2Fe-2S] centers and FeS2(100) surface increases following the sequence: H2O << CO < NO < O2. The higher the electron affinity of the ligand, the larger its bonding energy. A relatively large positive charge on the Fe cations in FeS2(100) makes this sulfide surface less reactive toward O2, CO, and NO than the [2Fe-2S] centers in proteins and inorganic complexes.

Oxygen adsorption on pyrite (100) surface by density functional theory

Pyrite (FeS2) bulk and (100) surface properties and the oxygen adsorption on the surface were studied by using density functional theory methods. The results show that in the formation of FeS2(100) surface, there exists a process of electron transfer from Fe dangling bond to S dangling bond. In this situation, surface Fe and S atoms have more ionic properties. Both Fe2+ and S2− have high electrochemistry reduction activity, which is the base for oxygen adsorption. From the viewpoint of adsorption energy, the parallel form oxygen adsorption is in preference. The result also shows that the state of oxygen absorbed on FeS2 surface acts as peroxides rather than O2.

XPS study of the oxidized pyrite surface

The core level spectra of pristine and oxidized mineral pyrite FeS2 surfaces were studied using synchrotron excited photoelectron spectroscopy. In addition to the bulk feature, three surface core level shifted components were identified in the sulphur 2p spectra. After measuring the spectra of the clean pristine surface, the sample was oxidized for 12 h in O2 atmosphere. The effect of this treatment of the specimen, besides causing weakening in the signal intensities, was observed as changes in the surface core level shifts (SCLS). The surface components of the oxidized pyrite spectra were shifted towards higher binding energies compared to those of the pristine sample. The reason for this phenomenon is briefly discussed.

Initial oxidation of fractured surfaces of FeS2(1 0 0) by molecular oxygen, water vapor, and air

Synchrotron-based Fe 2p, S 2p, and O 1s photoemission spectroscopy was used to study the initial stages of oxidation of UHV-fractured (1 0 0) surfaces of pyrite by molecular oxygen, water vapor, and ambient air. Molecular oxygen alone reacts measurably with pyrite surface to yield new oxidized species only for high doses of ∼10 7 L, while reaction with water vapor does not take place even at doses as high as 10 10 L. In both cases, however, monosulfide defects on fractured pyrite surfaces are eradicated by the dosing with O 2 and H 2 O. For high O 2 partial pressures, the extent of reaction with oxygen is comparable to that produced by equivalent air exposures, indicating that the presence of water is not strictly necessary for initial oxidation of pyrite(1 0 0) to occur in our experiments. The formation of several intermediate oxidation products, including sulfur oxoanions and zero-valent sulfur, are quantified by spectral decomposition as a function of dose, and the mode of their formation is discussed.

Density-functional theory of xanthate adsorption on the pyrite FeS2(100) surface

We have used the density-functional theory (DFT) method with a plane wave-pseudopotential basis to compute the structure and properties of a model xanthate molecule () and its adsorption characteristics on the (100) surface of pyrite (FeS2). Results from DFT calculations suggest that by itself does not readily adsorb on the (100) surface. However, adsorption of a xanthate–Fe–hydroxide complex species on the surface Fe site is energetically favourable, and this is consistent with experimental results which indicate that adsorption of such a molecular complex is likely to be one mechanism responsible for collector-induced surface hydrophobation of real FeS2 surfaces.

Molecular dynamics simulation of water in a contact with an iron pyrite FeS2 surface.

A strong adsorption of the water molecules to the pyrite surface is shown by a molecular dynamic simulation of the water-iron pyrite FeS2 interface. Water molecules closest to the pyrite surface are bound by an electrostatic interaction to the iron atoms in grooves running parallel to one of the crystal axes. The grooves are about two atoms wide and are directed along 010 for the (001) surface. The position of the water-surface potential minimum and the energy of adsorption were determined by optimization for a single water molecule at the interface. At room temperature and normal density there are altogether three distinguishable layers of water above the surface. One is associated with the groove: one with H bonding to the sulphur atoms comprising the ridges separating the grooves, and the third with the soft wall boundary between the absorbed water layers and bulk region of water. Simulations were also used to explore the effect of a temperature range significant for geophysical studies.

Density-functional theory studies of xanthate adsorption on the pyrite FeS2(110) and (111) surfaces

We have used the density functional theory (DFT) method with a plane wave-pseudopotential basis to compute the structure and properties of a model xanthate molecule (HOCS2−) and its adsorption characteristics on the pyrite FeS2(110) and (111) surfaces. Molecular calculations revealed that HOCS2− and CH3CH2OCS2− have similar head group electronic structure and, therefore, the use of the model xanthate is a justifiable approximation in simulations of xanthate head group attachment to FeS2 surfaces. Results from DFT calculations suggest that HOCS2− readily undergoes dissociation at the fourfold coordinated surface Fe on the (110) surface, and the bridging S of the (111) surface. These results suggest that xanthate may undergo chemisorption at defect sites on real FeS2 surfaces, which contain low-coordinate Fe sites and sites in proximity to cleaved S–S bonds.

Corrosion Inhibition and Photoactivity Behavior of N-Substituted Polycarbazole-Coated Natural Pyrite Electrode

Abstract Conductive films of poly (N-vinylcarbazole) (PVCz) and polycarbazole (PCz) were coated electrochemically on a natural iron sulfide (FeS2) surface. The corrosion, photocorrosion, and photoactivity characteristics of coated electrodes were investigated in potassium iodide/iodide (KI/I2) redox electrolyte. However, PVCz was obtained by oxidative polymerization of N-vinylcarbazole (NVCz) by ceric ammonium nitrate (CAN) and showed photoconductivity also was deposited on the natural FeS2 surface chemically from toluene/cyclohegzanone solution, and the electrode was tested for photoactivity. Electrochemical coating inhibited corrosion and photocorrosion of FeS2 causing some decrease in photoactivity, while chemical coating of thin, transparent PVCz on the surface enhanced the photoactivity and inhibited the corrosion and photocorrosion.

Reactivity of the (100) Plane of Pyrite in Oxidizing Gaseous and Aqueous Environments: Effects of Surface Imperfections

The oxidation-induced degradation of pyrite in mine tailing piles is of significant interest, since the resulting production of sulfuric acid has a severe detrimental impact on the surrounding environment. Hence, there has been much effort to understand the mechanism of pyrite oxidation. Much of the information concerned with the surface reactivity of pyrite, however, has been inferred from macroscopic observations during aqueous studies. Here, we directly investigated model FeS2(100) surfaces after exposure to well-defined oxidizing environments, with X-ray photoelectron spectroscopy, to evaluate the mechanism of pyrite oxidation at a microscopic level. Our studies showed that, in the pure H2O vapor environment, oxidation of FeS2(100) was spatially limited to nonstoichiometric or sulfur-deficient surface sites. Results further suggested that thiosulfate was a long-lived intermediate that ultimately converted to sulfate on the pyrite surface in this environment. Significant oxidation of the disulfide grou...

Surface Oxidation of Pyrite as a Function of pH

Pyrite, the most abundant metal sulfide at the surface of Earth, plays a key role in many processes such as acid mine drainage, redox cycling of metals at oxic-anoxic boundaries of lake bottom, and degradation of pollutants. The oxidation of pyrite was studied in batch experiments over a large range of pH (2.5−12), with trace oxygen. Surface analysis of the samples was performed using X-ray photoelectron spectroscopy (XPS). Speciation of the aqueous species was investigated by inductively coupled plasma atomic emission spectrometry (ICP-AES), ionic chromatography, and UV−vis spectrophotometry. The pyrite surface can drastically change with the pH, which was never at steady state and tended to reach an acidic value whatever the initial pH. For pH <4, Fe(II) and SO42- were released into solution; from XPS analyses, the pyrite surface presented O−H groups, an Fe-deficient composition Fe1-xS2, and iron(III) (hydr)oxide traces. Whatever the pH, the sulfur of the FeS2 surface was mainly under the (−I) state oxidation. When the pH increased, Fe(II) disappeared and the surface was covered with iron(III) (hydr)oxides. This overlayer did not passivate the sample against further oxidation, and a decrease in pH was still observed.

Photoemission of Adsorbed Xenon, X-ray Photoelectron Spectroscopy, and Temperature-Programmed Desorption Studies of H2O on FeS2(100)

The reaction of H2O with the (100) crystallographic plane of pyrite, FeS2, has been investigated in the vacuum environment with photoemission of adsorbed xenon (PAX), temperature-programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). TPD data indicate that H2O desorbs from FeS2(100) in a broad range of temperatures (150−300 K). XPS data suggests that the vast majority of H2O that initially adsorbs on pyrite at 79 K desorbs from pyrite during thermal annealing to 300 K. PAX, a technique that is sensitive to the short range order of a surface, has been used to elucidate the types of sites that are available on FeS2(100) for the binding of adsorbate. Within the resolution of our PAX data, the surface of FeS2(100) consists of at least two types of sites. It is proposed that these two types of sites are associated with the stoichiometric surface and defect (i.e., sulfur-deficient or anion vacancy) sites. PAX further suggests that at low adsorbate coverage, H2O predominately resides on defect sites. As the coverage of H2O is increased, defect sites become saturated and additional adsorption occurs on the less reactive stoichiometric surface.