Christopher D. Taylor

First-Principles Calculations of the Electrochemical Reactions of Water at an Immersed Ni ( 111 ) ∕ H2O Interface

Electrochemical processes occurring in aqueous solutions are critically dependent upon the interaction between the metal electrode and the solvent. In this work, density functional theory is used to calculate the potentials for which molecular water and its activation products adsorbed hydrogen and hydroxide are stable when in contact with an immersed Ni111 electrode. The adsorption geometries of water and its dissociation products are also determined as functions of potential. At zero kelvin, water activates to form a surface hydroxide overlayer at potentials anodic of �0.5 V vs a normal hydrogen electrode NHE. The cathodic activation of water to form a surface hydride occurs at potentials negative of �0.3 V NHE. There is a potential range at which both H and OH form on the surface, in agreement with inferences made from the experimental literature. The surface hydroxide/oxide phase transition occurs at 0.2 V NHE. The increased binding of oxygen to the surface at progressively anodic potentials correlates with weakening nickel-nickel interactions and the lifting of a metal atom above the surface plane. Thermodynamic extrapolations are made to ambient 300 K and elevated 600 K temperatures and Pourbaix diagrams calculated for the

Theoretical Analysis of the Nature of Hydrogen at the Electrochemical Interface Between Water and a Ni ( 111 ) Single-Crystal Electrode

The adsorption, absorption, and diffusion of hydrogen on and into metals are elementary processes that are important to a wide variety of electrochemical and corrosion processes. Ab initio gradient corrected density functional theoretical calculations were carried out in order to probe these processes for hydrogen at the aqueous/Ni(lll) interface under well-defined electrochemical conditions. The binding of hydrogen on Ni(lll) in the presence of water is considered using a fully atomistic model of the solution environment. We calculate the changes in hydrogen binding energy due to the presence of water at the interface, as well as due to changes in applied potential and surface charge. Binding energies for hydrogen at the hexagonally close-packed and octahedral sites shifted endothermically as the potential was made more anodic, indicating that reductive partial charge transfer occurs. Conversely, hydrogen binding at the tetrahedral site was found to be partially oxidizing. The calculation of vibrational modes allowed the extrapolation of ab initio results to room temperature conditions. Surface Pourbaix diagrams were constructed to predict the chemical states of hydrogen obtained over a Ni( 111) single-crystal surface as a function of pH and potential. These calculations indicate that the Tafel recombination mechanism is not active for Ni( 111) at 300 K, but rather the hydrogen evolution reaction proceeds by the Heyrovsky mechanism following a very short potential span of underpotential deposited hydrogen as one scans the surface cathodically. These results apply to an ideal (111) single-crystal surface in the absence of surface impurities and adsorbed ions (other than hydrogen).

Activity of N-coordinated multi-metal-atom active site structures for Pt-free oxygen reduction reaction catalysis: Role of *OH ligands

We report calculated oxygen reduction reaction energy pathways on multi-metal-atom structures that have previously been shown to be thermodynamically favorable. We predict that such sites have the ability to spontaneously cleave the O2 bond and then will proceed to over-bind reaction intermediates. In particular, the *OH bound state has lower energy than the final 2 H2O state at positive potentials. Contrary to traditional surface catalysts, this *OH binding does not poison the multi-metal-atom site but acts as a modifying ligand that will spontaneously form in aqueous environments leading to new active sites that have higher catalytic activities. These *OH bound structures have the highest calculated activity to date.

First-Principles Prediction of Equilibrium Potentials for Water Activation by a Series of Metals

In this paper, we describe and apply an ab initio quantum-mechanical model to examine the electrochemical dissociation of water over a number of close-packed and near-close-packed metal surfaces. We have examined a series of metals important for electrochemical energy generation and corrosion-resistant alloy development including Ni, Cu, Ru, Au, Mo, Pd, and Pt. Advances in electrochemical theory depend on a detailed understanding of the interplay between surface chemistry and electrochemical phenomena. Herein, we have used ab initio quantum-mechanical methods to examine the double-layer regions for H 2 O over these metals and compare the equilibrium potentials for the initial steps of water reduction and oxidation at the surface with known experimental quantities. We find that in its current form, the model developed herein is semiquantitative: it allows for the correct prediction of trends and the size of the double-layer regions, but in some cases it results in significant deviation with known absolute equilibrium potentials. Additional discussion is provided that outlines steps that may be taken to improve the quantitative accuracy of this model.

Atomistic Modeling of Corrosion Events at the Interface between a Metal and Its Environment

Atomistic simulation is a powerful tool for probing the structure and properties of materials and the nature of chemical reactions. Corrosion is a complex process that involves chemical reactions occurring at the interface between a material and its environment and is, therefore, highly suited to study by atomistic modeling techniques. In this paper, the complex nature of corrosion processes and mechanisms is briefly reviewed. Various atomistic methods for exploring corrosion mechanisms are then described, and recent applications in the literature surveyed. Several instances of the application of atomistic modeling to corrosion science are then reviewed in detail, including studies of the metal-water interface, the reaction of water on electrified metallic interfaces, the dissolution of metal atoms from metallic surfaces, and the role of competitive adsorption in controlling the chemical nature and structure of a metallic surface. Some perspectives are then given concerning the future of atomistic modeling in the field of corrosion science.

First-Principles Prediction of the Effects of Temperature and Solvent Selection on the Dimerization of Benzoic Acid

We introduce a procedure of quantum chemical calculations (B3P86/6-31G**) to study carboxylic acid dimerization and its correlation with temperature and properties of the solvent. Benzoic acid is chosen as a model system for studying dimerization via hydrogen bonding. Organic solvents are simulated using the self-consistent reaction field (SCRF) method with the polarized continuum model (PCM). The cyclic dimer is the most stable structure both in gas phase and solution. Dimer mono- and dihydrates could be found in the gas phase if acid molecules are in contact with water vapor. However, the formation of these hydrated conformers is very limited and cyclic dimer is the principal conformer to coexist with monomer acid in solution. Solvation of the cyclic dimer is more favorable compared to other complexes, partially due to the diminishing of hydrogen bonding capability and annihilation of dipole moments. Solvents have a strong effect on inducing dimer dissociation and this dependence is more pronounced at low dielectric constants. By accounting for selected terms in the total free energy of solvation, the solvation entropy could be incorporated to predict the dimer behavior at elevated temperatures. The temperature dependence of benzoic acid dimerization obtained by this technique is in good agreement with available experimental measurements, in which a tendency of dimer to dissociate is observed with increased temperatures. In addition, dimer breakup is more sensitive to temperature in low dielectric environments rather than in solvents with a higher dielectric constant.

A First-Principles Model for Hydrogen Uptake Promoted by Sulfur on Ni(111)

First-principles calculations have been employed to assist in the development of a three-zone model for hydrogen uptake promoted by sulfur on the (111) surface plane of Ni. The three zones suggested by previous work of Protopopoff and Marcus [J. Vac. Sci. Technol. A, 5, 944 (1987).] consist of (1) a site-blocking zone, where sulfur prevents hydrogen occupation at four adsorption sites per sulfur atom, (2) a promotion zone, where nearby sulfur raises the chemical potential of hydrogen in the six next-nearest-neighbor sites and beyond these two zones, (3) a free zone in which there is no influence of the adsorbed sulfur. The model is based on adsorption energies of hydrogen on Ni(111) as well as the sulfur―hydrogen interaction energies calculated from density functional theory. The results from the model indicate that the maximum promotion for hydrogen uptake occurs when up to 0.1 monolayer (ML) coverage of sulfur is present, and beyond this limit the site-blocking features of sulfur begin to dominate, thus reducing the available surface area down to 0 at 0.25 ML of sulfur.

Cohesive Relations for Surface Atoms in the Iron-Technetium Binary System

Iron-technetium alloys are of relevance to the development of waste forms for disposition of radioactive technetium-99 obtained from spent nuclear fuel. Corrosion of candidate waste forms is a function of the local cohesive energy (Eloc) of surface atoms. A theoretical model for calculating Eloc is developed. Density functional theory was used to construct a modified embedded atom (MEAM) potential for iron-technetium. Materials properties determined for the iron-technetium system were in good agreement with the literature. To explore the relationship between local structure and corrosion, MEAM simulations were performed on representative iron-technetium alloys and intermetallics. Technetium-rich phases have lower Eloc, suggesting that these phases will be more noble than iron-rich ones. Quantitative estimates of Eloc based on numbers of nearest neighbors alone can lead to errors up to 0.5 eV. Consequently, atomistic corrosion simulations for alloy systems should utilize physics-based models that consider not only neighbor counts, but also local compositions and atomic arrangements.

Predictions of Surface Electrochemistry of Saturated and Alkaline NH4Cl Solutions Interacting with Fe(110) from Ab Initio Calculations

Ammonium chloride (NH4Cl) precipitation can present deleterious effects on refinery surfaces when it combines with condensed water vapor to produce highly concentrated chloride and ammoniacal solutions. Herein, we have utilized density functional theory methods to compute the adsorption energies for various NHx, OHx, Cl, and H species on the lowest energy surface presented by iron. Adsorption energies are combined with thermodynamic analysis to develop phase diagrams for the various species that may dominate the surface adsorption coverage. Our results indicate that N, O, Cl, and H each possess regions of predominance on the surface Pourbaix diagram under conditions where a saturated NH4Cl solution is present. While N typically does not interfere with O adsorption except at very high anodic potentials, and is unlikely to depassivate any protective oxide films, Cl can compete with O surface stability, providing a competitive mechanism for hindering repassivation and/or accelerating the rate of metal dissol...

Changes in valence, coordination and reactivity that occur upon oxidation of fresh metal surfaces

To promote a greater understanding of the process and nature of metal passivation, we have performed a first-principles analysis of partially oxidized Ni(111) and Ni(311) surface and ultra-thin film NiO layers on Ni(111). We have adopted a bimodal theoretical strategy that considers the oxidation process using either a fixed generalized gradient approximation (GGA) functional for the description of all atoms in the system, or a perturbation approach, that perturbs the electronic structure of various Ni atoms in contact with oxygen by application of the GGA+U technique. This strategy allows us to assess the relative merits of the two approaches, and whether or not the two approaches are at variance with one another as concerns the process of metal passivation. We consider oxygen binding in the cases of isolated atomic adsorption, the development of an oxygen monolayer, and epitaxial NiO(111) monolayers and bilayers with various terminations. Selective application of GGA+U drives structural and charge-transfer processes at the interface, in particular, the octopolar reconstruction of high oxygen coverage pre-passive systems, which, in fact, template an epitaxial NiO(111)-oriented film. These outputs are observable through the development of cationic states in the nickel atoms at the interface, the emergence of a band gap in the projected density of states and in oxygen binding energies that approach the energy of oxide formation.