Density functional theory and a linear Gibbs free energy relationship are employed in a theoretical investigation of catalytic properties of cobalt-graphite-nitride systems for O-2 reduction to hydrogen peroxide and water. Nitrided graphite edges, with N atoms substituting one or two CH groups, are modeled to establish some of the effects of N on edges with and without Co added. The calculations show that a bare graphite edge with one N atom, which the calculations indicate is not hydrogenated at potentials greater than 0.3 V, is not active for O-2 reduction because OOH bonds too weakly. At potentials lower than 0.3 V, for which N is hydrogenated, making it a radical center, the NH edge is not active for O-2 reduction because OOH bonds too strongly, resulting in a high overpotential for its reduction to H2O2 on this site. Over a Co site bridging two N substituting for CH on an edge, the onset formation potential for OOH(ads) is about 0.4 V for Co-0, 0.8 V for Co-II in the form of Co(OH)(2), 0.7 V for Co-II in the form H2OCo(OH)(2), and 0.7 V for Co-III as Co(OH)(3). Later steps have higher predicted reversible potentials. A water molecule bonds to each of the Co centers but most weakly in the case of H2OCo(OH)(2), which means this cobalt center is least likely to be blocked against O-2 interaction with it. All of the cobalt complexes are predicted to bond weakly, 1.5 eV and less, to the graphite edge N atoms, which means that the catalyst is not expected to be stable due to cobalt dissolution as soluble Co2+.

A series of copper complexes were tested theoretically for oxygen electro reduction, beginning with bare Cu and Cu+ centers, with and without N-bearing ligands, and ending with one, two, and three H2N–Cu–Imidazole CuI centers. Adsorption energies for reaction intermediates formed during the four one-electron reduction steps to water were calculated, using B3LYP hybrid density functional theory, and then used in a linear free energy relationship to predict the reversible potential for each step. CuI sites not poisoned by O(ads), OH(ads), and H2O were sought, and so initial screening was based on the predicted reversible potentials for O(ads) reduction to OH(ads) and OH(ads) reduction to H2O and the adsorption bond strengths of O2 and H2O. The CuI centers in H2N–Cu–Imidazole and H2N–Cu–Imidazole had the best properties in this screening. The former was used to model four-electron reduction by copper laccases, some of which have very little overpotential. On the basis of calculations on a model for 3-CuI catalytic sites composed of three H2N–Cu–Imidazole in loose association, the following conclusions were reached: (i) the reduction potential for OH bonded to the model is higher than calculated for Pt, which is consistent with the higher observed overpotentials for Pt compared to lacasses; (ii) H2O bonds weakly to the CuI centers and does not poison them; (iii) model-dependent heat losses were calculated for non-electron transfer steps and it is shown how they contribute to the overpotential for the overall four-electron reduction. Finally, it is shown that CH3S–Cu–(Imidazole)2 possesses the electron donor–acceptor properties that allow it to be an intermediate in electron transfer to the catalytic site. This study shows that loosely coordinated CuI centers present opportunities for four-electron oxygen reduction at low overpotential.

Cobalt sulfides have been known for more than 30 years to be active toward oxygen reduction, and cobalt selenides have shown less activity. In this paper, a theoretical analysis is made of the four-electron reduction reaction of oxygen to water over the mixed anion and cation (202) surface of the pentlandite structure Co9Se8, one of several selenide phases. Reversible potentials for forming adsorbed reaction intermediates in acid are predicted using adsorption energies calculated with the Vienna ab initio simulation program (VASP) and the known bulk solution values together in a linear Gibbs energy relationship. Comparison with an earlier theoretical analysis of pentlandite structure Co9S8 shows that the overpotential is predicted to be larger for the selenide by around 0.22 V. Cobalt selenide electrodes of unspecified stoichiometry were prepared chemically on glassy carbon discs, and polarization curves were measured using rotating discs. When heat-treated at 900 degrees C, the onset potential for O-2 reduction was found to be 0.5 V (normal hydrogen electrode, NHE), whereas electrodes not subject to heat-treatment were inactive. For Co3S4, onset potentials in the literature are similar to 0.8 V (NHE), consistent with a similar to 0.3 V higher measured overpotential for the selenide. The theoretical predictions for the pentlandite sulfide and selenide surfaces are in qualitative agreement.

We explore the hydrogen anode reaction chemistry at the Ni-zirconia triple phase boundary in solid oxide fuel cells by using hybrid density functional quantum chemistry calculations and cluster models. The activation energy for H spillover is calculated to be the same order of magnitude as experimental estimates at the reversible potential. Proton transport on the oxide surface is shown to be activated by strongly held hydrogen-bonded water molecules: in the absence of H2O the activation energy is calculated to be 4.98 eV and the water molecule reduces the activation energy to 0.25 eV Substitutional Y3+ (for Zr4+) is shown to slow proton diffusion when present in the zirconia surface. (c) 2006 Elsevier B.V. All rights reserved.

Quantum chemical predictions are made of reversible potentials for intermediates formed during the four-electron reduction of O-2 by copper laccase, using three loosely associated partially ligated Cu-I centers as models for the active site. Two mechanisms are studied, the platinum surface mechanism in acid electrolyte, and the proposed laccase mechanism for basic electrolyte. Both are found viable. The Cu-II-OH- reduction to H2O step has the lowest potential, making it responsible for the predicted low overpotential. Compared to laccase, the overpotential is somewhat high but completing the ligand coordination would be expected to lead to smaller predicted overpotentials. (c) 2005 The Electrochemical Society.

The optimized minimum-energy geometries of the four smallest cyclic amines—aziridine (C2H5N), azetidine (C3H7N), pyrrolidine (C4H9N), and piperidine (C5H11N)—were determined using ab initio methods and density functional calculations. The heats of formation of above-mentioned compounds were calculated using isodesmic reactions and known heats of formations of other compounds in the reactions. Proton affinities were calculated for the compounds using the same calculation methods that were used for calculation the heat of formation. The methods varied in their ability to calculate accurate ΔfH’s, with MP2 calculations being the most accurate and HF calculations the least. Most methods were able to predict the proton affinities well, typically to within 30kJ/mol (or about 5%).

Ab initio and density functional calculations were employed to investigate the energetics of the interaction of nitric oxide with the first three alkali metals (Li, Na, K). Both singlet and triplet states were explored with the use of HF and MP2 methods combined with four different basis sets (6-31G, 6-31G(d), 6-311G(d), 6-311+G(d)). Potential energy surfaces (PES) were plotted and examined for M–NO complexes in their singlet and triplet states. Energy gaps between linear and bent structures were calculated and analyzed. Structural parameters are presented for stationary points on the singlet and triplet surfaces of M + NO.