John A. Keith

Water oxidation on pure and doped hematite (0001) surfaces: prediction of Co and Ni as effective dopants for electrocatalysis.

In photoelectrochemical cells, sunlight may be converted into chemical energy by splitting water into hydrogen and oxygen molecules. Hematite (α-Fe(2)O(3)) is a promising photoanode material for the water oxidation component of this process. Numerous research groups have attempted to improve hematites photocatalytic efficiency despite a lack of foundational knowledge regarding its surface reaction kinetics. To elucidate detailed reaction mechanisms and energetics, we performed periodic density functional theory + U calculations for the water oxidation reaction on the fully hydroxylated hematite (0001) surface. We investigate two different concentrations of surface reactive sites. Our best model involves calculating water oxidation mechanisms on a pure (1×1) hydroxylated hematite slab (corresponding to 1/3 ML of reactive sites) with an additional overlayer of water molecules to model solvation effects. This yields an overpotential of 0.77 V, a value only slightly above the 0.5-0.6 V experimental range. To explore whether doped hematite can exhibit an even lower overpotential, we consider cation doping by substitution of Fe by Ti, Mn, Co, Ni, or Si and F anion doping by replacing O on the fully hydroxylated surface. The reaction energetics on pure or doped hematite surfaces are described using a volcano plot. The relative stabilities of holes on the active O anions are identified as the underlying cause for trends in energetics predicted for different dopants. We show that moderately charged O anions give rise to smaller overpotentials. Co- or Ni-doped hematite surfaces give the most thermodynamically favored reaction pathway (lowest minimum overpotential) among all dopants considered. Very recent measurements (Electrochim. Acta 2012, 59, 121-127) reported improved reactivity with Ni doping, further validating our predictions.

Theoretical investigations of the oxygen reduction reaction on Pt(111).

Computational modeling can provide important insights into chemical reactions in both applied and fundamental fields of research. One of the most critical processes needed in practical renewable energy sources is the oxygen reduction reaction (ORR). Besides being the key process in combustion and corrosion, the ORR has an elusive mechanism that may proceed in a number of complicated reaction steps in electrochemical fuel cells. Indeed, the mechanism of the ORR on highly studied Pt(111) electrodes has been the subject of interest and debate for decades. Herein, we first outline the theory behind these types of simulations and then show how to use these quantum mechanical approaches and approximations to create a realistic model. After reviewing the performance of these methods in studying the binding of molecular oxygen to Pt(111), we then outline our own results in elucidating the ORR and its dependence on environmental parameters, such as solvent, thermodynamic energies, and the presence of an external electrode potential. This approach can, in principle, be applied to other equally complicated investigations of other surfaces or electrochemical reactions.

Theoretical Elucidation of the Competitive Electro-oxidation Mechanisms of Formic Acid on Pt(111)

The mechanisms of formic acid (HCOOH) oxidation on Pt(111) under electrochemical conditions have been studied using density functional theory and then compared with the analogous gas-phase reaction. Results show that HCOOH oxidation under a water-covered surface behaves substantially differently than in the gas phase or using a solvation model involving only a few water molecules. Using these models, we evaluated the detailed reaction process, including energies and geometric structures of intermediates and transition states under the influence of different solvation models and electrode potentials. Our calculations indicate that this potential-dependent electrochemical oxidation proceeds via a multipath mechanism (involving both the adsorbed HCOOH and HCOO intermediates), a result succinctly rationalizing conflicting experimental observations. Moreover, this study highlights how subtle changes in electrochemical reaction environments can influence (electro)catalysis.

Elucidation of the Selectivity of Proton-Dependent Electrocatalytic CO2 Reduction by fac-Re(bpy)(CO)3Cl

A complete mechanism for the proton-dependent electrocatalytic reduction of CO2 to CO by fac-Re(bpy)(CO)3Cl that is consistent with experimental observations has been developed using first principles quantum chemistry. Calculated one-electron reduction potentials, nonaqueous pKas, reaction free energies, and reaction barrier heights provide deep insight into the complex mechanism for CO2 reduction as well as the origin of selectivity for this catalyst. Protonation and then reduction of a metastable Re-CO2 intermediate anion precedes Brønsted-acid-catalyzed C-O cleavage and then rapid release of CO at negative applied potentials. Conceptually understanding the mechanism of this rapid catalytic process provides a useful blueprint for future work in artificial photosynthesis.

Theoretical insights into pyridinium-based photoelectrocatalytic reduction of CO2.

The role of pyridinium cations in electrochemistry has been believed known for decades, and their radical forms have been proposed as key intermediates in modern photoelectrocatalytic CO(2) reduction processes. Using first-principles density functional theory and continuum solvation models, we have calculated acidity constants for pyridinium cations and their corresponding pyridinyl radicals, as well as their electrochemical redox potentials. Contrary to previous assumptions, our results show that these species can be ruled out as active participants in homogeneous electrochemistry. A comparison of calculated acidities and redox potentials indicates that pyridinium cations behave differently than previously thought, and that the electrode surface plays a critical (but still unknown) role in pyridinium reduction. This work substantially alters the mechanistic view of pyridinium-catalyzed photoelectrochemical CO(2) reduction.

The Electronic States of Rhenium Bipyridyl Electrocatalysts for CO2 Reduction as Revealed by X‐ray Absorption Spectroscopy and Computational Quantum Chemistry

Industrial processes and fossil fuel combustion produce carbon dioxide (CO2) unsustainably on the gigaton scale. Addressing this pressing issue has led to rapidly growing efforts to catalytically reduce CO2 to liquid fuels. [1] Recycling CO2 is a profoundly challenging problem that requires fundamental insights to guide advancements. Information regarding CO2 transformations abound, [1,2] but no industrialscale process has capably reduced CO2 to liquid fuels. Of the systems that electrocatalytically reduce CO2, the [Re(bpy)(CO)3Cl] family of compounds (bpy= 2,2’-bipyridine) is one of the most robust and well-characterized systems known to date. This system converts CO2 into carbon monoxide (CO) with high rates and efficiencies; it suffers, however, from large overpotentials that are believed to arise from accessing the highly reduced, formally Re I state in [Re(bpy)(CO)3] . This state has long been proposed as the active state of the electrocatalyst. Apart from this assumption, there is little known about the electronic structure of the catalyst in its reduced (active) state and its subsequent interaction with CO2. We recently reported stopped-flow kinetics studies showing the relative selectivities of the [Re(bpy-tBu)(CO)3] anion reacting with with CO2 and proton sources. These studies revealed that reaction rates of the anion were about 35 times faster with CO2 than with weak acid. [3b] The bpy ligand was proposed to play a non-innocent role by storing charge and preventing a doubly occupied dz2 orbital at the Re center, which would be needed to form a metal hydride. Indeed, Xray diffraction (XRD) studies of both [Re(bpy)(CO)3] and [Re(bpy-tBu)(CO)3] show the bpy ligands exhibit bond length alternation and short Cpy Cpy bonds (1.370(15) , for bpy-tBu), indicating significant electron density on these ligands. The short inter-ring bonds suggest a doubly reduced bpy ligand, more representative of a Re(bpy ) state rather than a Re(bpy ) or Re (bpy) state. The redox activities of bpy ligands as well as other non-innocent ligands have been extensively studied. To fully confirm that the non-innocence of bpy contributes to this unique catalysis, we employed experimental spectroscopy and theoretical quantum chemistry to characterize this catalyst family. We compared the halide starting materials, [Re(bpy)(CO)3Cl] (1) and [Re(bpy-tBu)(CO)3Cl] (2), the one-electron reduced dimer [{Re(bpy)(CO)3}2] (3), the twoelectron reduced anions [K([18]crown-6)][Re(bpy)(CO)3] (4) and [K([18]crown-6)][Re(bpy-tBu)(CO)3] (5), the commercially available standards, [Re(CO)5Cl] (6) and [Re2(CO)10] (7), and a synthesized Re I standard, [K([18]crown-6)] [Re(CO)5] (8). IR spectroscopy of the stretching frequencies of the carbonyl ligands characterizes the electronic states of these complexes. X-ray absorption spectroscopy (XAS) at the Re L3 absorption edge using the strong “white-line” resonance arising from 2p!5d transitions probes the Re5d unoccupied states. Kohn–Sham density functional theory (KS-DFT) calculations provide a first-principles description of electronic structures. Lastly, extended X-ray absorption fine structure (EXAFS) studies of frozen THF solutions of 1, 2, 4, and 5 confirm the monomeric nature of the catalysts and rule out solvent coordination to the Re centers in solution. Compounds 1–5 were prepared according to literature procedures. [K([18]crown-6)][Re(CO)5] (8) was prepared by the reduction of [Re2(CO)10] (7) in tetrahydrofuran (THF) by excess KC8 (potassium intercalated graphite) in the presence of [18]crown-6 (see the Supporting Information). The IR stretching frequencies of complexes 1–7 have been reported previously; however, we obtained frequencies for complexes 1–7 and the newly synthesized complex 8 under the same conditions for fair comparison (Table 1). The oneelectron reduction of the formally Re chloride species 2 results in formation of the one-electron reduced monomer, [*] Dr. E. E. Benson, M. D. Sampson, Dr. K. A. Grice, Dr. J. M. Smieja, J. D. Froehlich, Prof. Dr. C. P. Kubiak Department of Chemistry and Biochemistry, University of California, San Diego 9500 Gilman Drive,Code 0358, La Jolla, CA 92093-0358 (USA) E-mail: ckubiak@ucsd.edu

Electrochemical reactivities of pyridinium in solution: consequences for CO2 reduction mechanisms

One of the most promising CO2 reduction processes presently known suffers from a lack of fundamental understanding of its reaction mechanism. Using first principles quantum chemistry, we report thermodynamical energies of various pyridine-derived intermediates as well as barrier heights for key homogeneous reaction mechanisms. From this work, we predict that the actual form of the co-catalyst involved in pyridinium-based CO2 reduction is not the long-proposed pyridinyl radical in solution, but is more probably a surface-bound dihydropyridine species.

Experimental and Computational Study of a Direct O2-Coupled Wacker Oxidation: Water Dependence in the Absence of Cu Salts

The kinetics of the Pd[(-)-sparteine]Cl(2) catalyzed oxidation of decene using oxygen as the sole oxidant have been studied in the absence of copper salts and high [Cl(-)]. Saturation kinetics are observed for [decene] as well as a third order dependence on [water]. A mechanism is proposed involving the dissociation of two chlorides and rate-limiting formation of a three-water hydrogen bridged network and subsequent oxypalladation as supported by computational studies.

Oxygen transport in perovskite-type solid oxide fuel cell materials: insights from quantum mechanics.

CONSPECTUS: Global advances in industrialization are precipitating increasingly rapid consumption of fossil fuel resources and heightened levels of atmospheric CO2. World sustainability requires viable sources of renewable energy and its efficient use. First-principles quantum mechanics (QM) studies can help guide developments in energy technologies by characterizing complex material properties and predicting reaction mechanisms at the atomic scale. QM can provide unbiased, qualitative guidelines for experimentally tailoring materials for energy applications. This Account primarily reviews our recent QM studies of electrode materials for solid oxide fuel cells (SOFCs), a promising technology for clean, efficient power generation. SOFCs presently must operate at very high temperatures to allow transport of oxygen ions and electrons through solid-state electrolytes and electrodes. High temperatures, however, engender slow startup times and accelerate material degradation. SOFC technologies need cathode and anode materials that function well at lower temperatures, which have been realized with mixed ion-electron conductor (MIEC) materials. Unfortunately, the complexity of MIECs has inhibited the rational tailoring of improved SOFC materials. Here, we gather theoretically obtained insights into oxygen ion conductivity in two classes of perovskite-type materials for SOFC applications: the conventional La1-xSrxMO3 family (M = Cr, Mn, Fe, Co) and the new, promising class of Sr2Fe2-xMoxO6 materials. Using density functional theory + U (DFT+U) with U-J values obtained from ab initio theory, we have characterized the accompanying electronic structures for the two processes that govern ionic diffusion in these materials: (i) oxygen vacancy formation and (ii) vacancy-mediated oxygen migration. We show how the corresponding macroscopic oxygen diffusion coefficient can be accurately obtained in terms of microscopic quantities calculated with first-principles QM. We find that the oxygen vacancy formation energy is a robust descriptor for evaluating oxide ion transport properties. We also find it has a direct relationship with (i) the transition metal-oxygen bond strength and (ii) the extent to which electrons left behind by the departing oxygen delocalize onto the oxygen sublattice. Design principles from our QM results may guide further development of perovskite-based MIEC materials for SOFC applications.

Quantum Chemical Benchmarking, Validation, and Prediction of Acidity Constants for Substituted Pyridinium Ions and Pyridinyl Radicals

Sensibly modeling (photo)electrocatalytic reactions involving proton and electron transfer with computational quantum chemistry requires accurate descriptions of protonated, deprotonated, and radical species in solution. Procedures to do this are generally nontrivial, especially in cases that involve radical anions that are unstable in the gas phase. Recently, pyridinium and the corresponding reduced neutral radical have been postulated as key catalysts in the reduction of CO2 to methanol. To assess practical methodologies to describe the acid/base chemistry of these species, we employed density functional theory (DFT) in tandem with implicit solvation models to calculate acidity constants for 22 substituted pyridinium cations and their corresponding pyridinyl radicals in water solvent. We first benchmarked our calculations against experimental pyridinium deprotonation energies in both gas and aqueous phases. DFT with hybrid exchange-correlation functionals provide chemical accuracy for gas-phase data and allow absolute prediction of experimental pKas with unsigned errors under 1 pKa unit. The accuracy of this economical pKa calculation approach was further verified by benchmarking against highly accurate (but very expensive) CCSD(T)-F12 calculations. We compare the relative importance and sensitivity of these energies to selection of solvation model, solvation energy definitions, implicit solvation cavity definition, basis sets, electron densities, model geometries, and mixed implicit/explicit models. After determining the most accurate model to reproduce experimentally-known pKas from first principles, we apply the same approach to predict pKas for radical pyridinyl species that have been proposed relevant under electrochemical conditions. This work provides considerable insight into the pitfalls using continuum solvation models, particularly when used for radical species.