Tungsten-Doped Titanium Dioxide in the Rutile Structure: Theoretical Considerations

Tungsten-doped titanium dioxide has the potential to replace conventional carbon black in catalyst support applications. In this paper, structural and electronic properties of W-doped rutile are theoretically studied. Lattice parameters as well as W−W pairing in models for a range of doping are calculated and match well with the experimental results. W doping leads to in-plane expansion and c-axis contraction in the rutile structure. The pairing of tungsten atoms is a Peierls-type distortion, resulting in less cluttering of bands around the Fermi energy. Our computational finding of paired structures as energetically more stable is in agreement with the literature on similar systems. W-doped rutile is conducting at both high and low doping levels; the states involved near the Fermi level are predominantly W(5d)

Development of a Reactive Force Field for Iron−Oxyhydroxide Systems

We adopt a classical force field methodology, ReaxFF, which is able to reproduce chemical reactions, and train its parameters for the thermodynamics of iron oxides as well as energetics of a few iron redox reactions. Two parametrizations are developed, and their results are compared with quantum calculations or experimental measurements. In addition to training, two test cases are considered: the lattice parameters of a selected set of iron minerals, and the molecular dynamics simulation of a model for α-FeOOH (goethite)-water interaction. Reliability and limitations of the developed force fields in predicting structure and energetics are discussed

Differential Pair Distribution Function Study of the Structure of Arsenate Adsorbed on Nanocrystalline γ-Alumina

Structural information is important for understanding surface adsorption mechanisms of contaminants on metal (hydr)oxides. In this work, a novel technique was employed to study the interfacial structure of arsenate oxyanions adsorbed on γ-alumina nanoparticles, namely, differential pair distribution function (d-PDF) analysis of synchrotron X-ray total scattering. The d-PDF is the difference of properly normalized PDFs obtained for samples with and without arsenate adsorbed, otherwise identically prepared. The real space pattern contains information on atomic pair correlations between adsorbed arsenate and the atoms on γ-alumina surface (Al, O, etc.). PDF results on the arsenate adsorption sample on γ-alumina prepared at 1 mM As concentration and pH 5 revealed two peaks at 1.66 Å and 3.09 Å, corresponding to As–O and As–Al atomic pair correlations. This observation is consistent with those measured by extended X-ray absorption fine structure (EXAFS) spectroscopy, which suggests a first shell of As–O at 1.69 ± 0.01 Å with a coordination number of ∼4 and a second shell of As–Al at ∼3.13 ± 0.04 Å with a coordination number of ∼2. These results are in agreement with a bidentate binuclear coordination environment to the octahedral Al of γ-alumina as predicted by density functional theory (DFT) calculation

ATR–FTIR and Density Functional Theory Study of the Structures, Energetics, and Vibrational Spectra of Phosphate Adsorbed onto Goethite

Periodic plane-wave density functional theory (DFT) and molecular cluster hybrid molecular orbital–DFT (MO–DFT) calculations were performed on models of phosphate surface complexes on the (100), (010), (001), (101), and (210) surfaces of α-FeOOH (goethite). Binding energies of monodentate and bidentate HPO₄^2– surface complexes were compared to H₂PO₄^– outer-sphere complexes. Both the average potential energies from DFT molecular dynamics (DFT–MD) simulations and energy minimizations were used to estimate adsorption energies for each configuration. Molecular clusters were extracted from the energy-minimized structures of the periodic systems and subjected to energy reminimization and frequency analysis with MO–DFT. The modeled P–O and P---Fe distances were consistent with EXAFS data for the arsenate oxyanion that is an analog of phosphate, and the interatomic distances predicted by the clusters were similar to those of the periodic models. Calculated vibrational frequencies from these clusters were then correlated with observed infrared bands. Configurations that resulted in favorable adsorption energies were also found to produce theoretical vibrational frequencies that correlated well with experiment. The relative stability of monodentate versus bidentate configurations was a function of the goethite surface under consideration. Overall, our results show that phosphate adsorption onto goethite occurs as a variety of surface complexes depending on the habit of the mineral (i.e., surfaces present) and solution pH. Previous IR spectroscopic studies may have been difficult to interpret because the observed spectra averaged the structural properties of three or more configurations on any given sample with multiple surfaces