Computational study of the interactions of small molecules with the surfaces of iron-bearing minerals

This thesis presents a comprehensive computational study of the bulk and surface properties of two major iron-bearing minerals: hematite (α-Fe2O3) and mackinawite (tetragonal FeS), and subsequently unravels the interactions of a number of environmentally important molecules with the low-Miller index surfaces of these iron-bearing minerals using a state-of-the-art methodology based on the density functional theory (DFT) techniques. First, we have used the Hubbard corrected DFT (GGA+U) calculations to unravel the interactions of a single benzene molecule with the (0001) and (01 2) surfaces of α-Fe2O3 under vacuum conditions. α-Fe2O3 is correctly described as a charge-transfer insulator, in agreement with the spectroscopic evidence when the optimized value for U = 5 eV is employed. The benzene molecule is shown to interact relatively more strongly with the (01 2) surface via cation-π interactions between the π-electrons of benzene ring and the surface Fe d-orbitals than with the (0001) where van der Waals interactions are found to play important role in stabilizing the molecule at the surface. In the second part of this thesis, DFT calculations with a correction for van der Waals interactions (DFT-D2 scheme of Grimme) have been used to simulate the bulk properties, surface structures and reactivity of layered mackinawite (FeS). We demonstrate that the inclusion of van der Waals dispersive interaction sensibly improves the prediction of interlayer separation distance in FeS, in good agreement with experimental data. The effect of interstitial impurity atoms in the interlayer sites on the structure and properties of FeS is also investigated, and it is found that these contribute considerably to the mechanical stability of the FeS structure. From the geometry optimization of the low-Miller index surfaces of FeS, we have shown the (001) surface terminated by sulfur atoms is by far the most energetically stable surface of FeS. The calculated surface energies are used successfully to reproduce the observed crystal morphology of FeS. As an extension to the surface studies, we have used the DFT-D2 method to model the adsorption mechanism of arsenious acid (As(OH)3), methylamine (CH3NH2) and nitrogen oxides (NO and NO2) molecules on the low-Miller index FeS surfaces under vacuum conditions. The As(OH)3 molecule is demonstrated to preferentially form bidentate adsorption complexes on FeS surfaces via two O‒Fe bonds. The calculated long As−Fe and As−S interatomic distances (> 3 Å) clearly suggest interactions via outer sphere surface complexes with respect to the As atom, in agreement with the experimental observations. The growth modifying properties of methylamine, the capping agent used in the synthesis of FeS, are modelled by surface adsorption. The strength of the interaction of CH3NH2 on the different FeS surfaces is shown to increase in the order: (001) < (011) < (100) < (111) and an analysis of the nature of bonding reveals that the CH3NH2 molecule interacts preferentially with the surface Fe d-orbitals via the lone-pair of electrons located on the N atom. Our simulated temperature programmed desorption process shows that methylamine is stable up to about 180 K on the most reactive (111) surface, which is comparable to the experimental desorption temperatures predicted at metallic surfaces. Finally, the catalytic properties of FeS as a nanocatalyst for the adsorption, activation and decomposition of environmentally important NOx gases have been explored, where we consider the nature of binding of the NOx species to the FeS surfaces and their dissociation reaction mechanisms

Adsorption and Desulfurization Mechanism of Thiophene on Layered FeS(001), (011), and (111) Surfaces: A Dispersion-Corrected Density Functional Theory Study

Layered transition-metal chalcogenides have emerged as a fascinating new class of materials for catalysis. Here, we present periodic density functional theory (DFT) calculations of the adsorption of thiophene and the direct desulfurization reaction pathways on the (001), (011), and (111) surfaces of layered FeS. The fundamental aspects of the thiophene adsorption, including the initial adsorption geometries, adsorption energies, structural parameters, and electronic properties, are presented. From the calculated adsorption energies, we show that the flat adsorption geometries, wherein the thiophene molecule forms multiple π-bonds with the FeS surfaces, are energetically more favorable than the upright adsorption geometries, with the strength of adsorption decreasing in the order FeS(111) > FeS(011) > FeS(001). The adsorption of the thiophene onto the reactive (011) and (111) surfaces is shown to be characterized by charge transfer from the interacting Fe d-band to the π-system of the thiophene molecule, which causes changes of the intramolecular structure including loss of aromaticity and elongation of the C–S bonds. The thermodynamic and kinetic analysis of the elementary steps involved in the direct desulfurization of thiophene on the reactive FeS surfaces is also presented. Direct desulfurization of thiophene occurs preferentially on the (111) surface, as reflected by the overall exothermic reaction energy calculated for the process (ER = −0.15 eV), with an activation energy of 1.58 eV

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-p and O-p states of As(OH)3 and the surface Fe-d 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

Activating the FeS (001) Surface for CO₂ Adsorption and Reduction through the Formation of Sulfur Vacancies: A DFT-D3 Study

As a promising material for heterogeneous catalytic applications, layered iron (II) mono‐ sulfide (FeS) contains active edges and an inert basal (001) plane. Activating the basal (001) plane could improve the catalytic performance of the FeS material towards CO2 activation and reduction reactions. Herein, we report dispersion‐corrected density functional theory (DFT‐D3) calculations of the adsorption of CO2 and the elementary steps involved in its reduction through the reverse water‐gas shift reaction on a defective FeS (001) surface containing sulfur vacancies. The exposed Fe sites resulting from the creation of sulfur vacancies are shown to act as highly active sites for CO2 activation and reduction. Based on the calculated adsorption energies, we show that the CO2 mole‐ cules will outcompete H2O and H2 molecules for the exposed active Fe sites if all three molecules are present on or near the surface. The CO2 molecule is found to weakly physisorb (−0.20 eV) com‐ pared to the sulfur‐deficient (001) surface where it adsorbs much strongly, releasing adsorption en‐ ergy of −1.78 and −1.83 eV at the defective FeS (001) surface containing a single and double sulfur vacancy, respectively. The CO2 molecule gained significant charge from the interacting surface Fe ions at the defective surface upon adsorption, which resulted in activation of the C–O bonds con‐ firmed via vibrational frequency analyses. The reaction and activation energy barriers of the ele‐ mentary steps involved in the CO2 hydrogenation reactions to form CO and H2O species are also unraveled

First-principles DFT insights into the structural, elastic, and optoelectronic properties of α and β-ZnP2: implications for photovoltaic applications

Binary II–V semiconductors are highly optically active materials, possess high intrinsic mechanical and chemical durability, and have electronic properties ideal for optoelectronic applications. Among them, zinc diphosphide (ZnP2) is a promising earth-abundant absorber material for solar energy conversion. We have investigated the structural, mechanical, and optoelectronic properties of both the tetragonal (α) and monoclinic (β) phases of ZnP2 using standard, Hubbard-corrected and screened hybrid density functional theory methods. Through the analysis of bond character, band gap nature, and absorption spectra, we show that there exist two polymorphs of the β phase (denoted as β1 and β2) with distinct differences in the photovoltaic potential. While β1 exhibits the characteristics of metallic compounds, β2 is a semiconductor with predicted thin-film photovoltaic absorbing efficiency of almost 10%. The α phase is anticipated to be an indirect gap material with a calculated efficiency limited to only 1%. We have also analysed and gained insights into the electron localization function, projected density of states and projected crystal orbital Hamilton populations for the analogue bonds between the α and β-ZnP2. In light of these calculations, a number of previous discrepancies have been solved and a solid ground for future employment of zinc diphosphides in photovoltaics has been established

Structural and Optical Properties of ZnO Thin Films Prepared by Molecular Precursor and Sol–Gel Methods

Zinc oxide (ZnO) is a versatile and inexpensive semiconductor with a wide direct band gap that has applicability in several scientific and technological fields. In this work, we report the synthesis of ZnO thin films via two simple and low-cost synthesis routes, i.e., the molecular precursor method (MPM) and the sol–gel method, which were deposited successfully on microscope glass substrates. The films were characterized for their structural and optical properties. X-ray diffraction (XRD) characterization showed that the ZnO films were highly c-axis (0 0 2) oriented, which is of interest for piezoelectric applications. The surface roughness derived from atomic force microscopy (AFM) analysis indicates that films prepared via MPM were relatively rough with an average roughness (Ra) of 2.73 nm compared to those prepared via the sol–gel method (Ra = 1.55 nm). Thin films prepared via MPM were more transparent than those prepared via the sol–gel method. The optical band gap of ZnO thin films obtained via the sol–gel method was 3.25 eV, which falls within the range found by other authors. However, there was a broadening of the optical band gap (3.75 eV) in thin films derived from MPM

Ab initio investigation of O₂ adsorption on Ca-doped LaMnO₃ cathodes in solid oxide fuel cells

We present a Hubbard-corrected density functional theory (DFT+U) study of the adsorption and reduction reactions of oxygen on the pure and 25% Ca-doped LaMnO3 (LCM25) {100} and {110} surfaces. The effect of oxygen vacancies on the adsorption characteristics and energetics has also been investigated. Our results show that the O2 adsorption/reduction process occurs through the formation of superoxide and peroxide intermediates, with the Mn sites found to be generally more active than the La sites. The LCM25{110} surface is found to be more efficient for O2 reduction than the LCM25{100} surface due to its stronger adsorption of O2, with the superoxide and peroxide intermediates shown to be energetically more favorable at the Mn sites than at the Ca sites. Moreover, oxygen vacancy defect sites on both the {100} and {110} surfaces are shown to be more efficient for O2 reduction, as reflected in the higher adsorption energies calculated on the defective surfaces compared to the perfect surfaces. We show from Löwdin population analysis that the O2 adsorption on the pure and 25% Ca-doped LaMnO3 surfaces is characterized by charge transfer from the interacting surface species into the adsorbed oxygen πg orbital, which results in weakening of the O-O bonds and its subsequent reduction. The elongated O-O bonds were confirmed via vibrational frequency analysis

Adsorption of hydrazine on the perfect and defective copper (111) surface: A dispersion-corrected DFT study

We have investigated the adsorption of hydrazine (N2H4) on perfect and defect-containing copper (111) surfaces by first-principles calculations. The long-range interactions are included in the geometry optimization through the application of the generalised gradient approximation with dispersion correction, DFT-D2 in the method of Grimme. We have studied three types of defects at the surfaces: monoatomic steps, Cu-adatoms and vacancies, where our calculations show that the adsorption energy increases as the coordination of the adsorption sites decreases. The ideal (111) is the most stable surface with the weakest adsorption of hydrazine, whilst the stepped (111) surface is the least stable and hence more reactive surface with the highest adsorption energy, where the hydrazine bridges across the step edge. We found that inclusion of the dispersion correction results in significant enhancement of molecule–substrate binding, thereby increasing the adsorption energy. This strong adsorption results in a bridging adsorption geometry for hydrazine, with a rotation around the Nsingle bondN bond where the torsional angle changes from a gauche towards an eclipsed conformer to help the molecule to bridge through both nitrogen atoms, in agreement with experimental evidence. The core-level binding shifts for the N(1 s) states upon N2H4 adsorption have been calculated at DFT level to provide further insight into the N2H4 adsorption process on the copper surfaces

Identification of Photoexcited Electron Relaxation in a Cobalt Phosphide Modified Carbon Nitride Photocatalyst

Transition metal phosphides have been recognized as efficient co-catalysts to boost the activity of semiconductor photocatalysts. However, a rigorous and quantitative understanding is still to be developed about how transition metal phosphides influence photoexcited electron dynamics. Here, we present a nanosecond time-resolved transient absorption spectroscopy (TAS) study of the photoexcited electron dynamics in carbon nitrides (g-C3N4) before and after Co and/or P modifications. Our spectroscopic study showed that Co or P lowered the initial electron density, whereas they promoted the photoexcited electron relaxation of g-C3N4, with their half-life times (t50%) of 2.5 and 1.8 ns, respectively. The formation of a CoP co-catalyst compound promoted the electron relaxation (t50%=2.8 ns) without significantly lowering the charge separation efficiency. Density functional theory (DFT) calculations were undertaken to explore the underlying fundamental reasons and they further predicted that CoP, compared to Co or P modification, better facilitates photoexcited electron transfer from g-C3N4 to reactants

A DFT plus U investigation of hydrogen adsorption on the LaFeO3(010) surface

The ABO3 perovskite lanthanum ferrite (LaFeO3) is a technologically important electrode material for nickel–metal hydride batteries, energy storage and catalysis. However, the electrochemical hydrogen adsorption mechanism on LaFeO3 surfaces remains under debate. In the present study, we have employed spin-polarized density functional theory calculations, with the Hubbard U correction (DFT+U), to unravel the adsorption mechanism of H2 on the LaFeO3(010) surface. We show from our calculated adsorption energies that the preferred site for H2 adsorption is the Fe–O bridge site, with an adsorption energy of −1.18 eV (including the zero point energy), which resulted in the formation of FeOH and FeH surface species. H2 adsorption at the surface oxygen resulted in the formation of a water molecule, which leaves the surface to create an oxygen vacancy. The H2 molecule is found to interact weakly with the Fe and La sites, where it is only physisorbed. The electronic structures of the surface–adsorption systems are discussed via projected density of state and Löwdin population analyses. The implications of the calculated adsorption strengths and structures are discussed in terms of the improved design of nickel–metal hydride (Ni–MH) battery prototypes based on LaFeO3