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How to efficiently oxidize H2O to O2 (oxygen evolution reaction, OER) in photoelectrochemical cells (PEC) is a great challenge due to its complex charge transfer process, high overpotential, and corrosion. So far no OER mechanism has been fully explained atomistically with both thermodynamic and kinetics. IrO2 is the only known OER catalyst with both high catalytic activity and stability in acidic conditions. This is important because PEC experiments often operate at extreme pH conditions. In this work, we performed first-principles calculations integrated with implicit solvation at constant potentials to examine the detailed atomistic reaction mechanism of OER at the IrO2 (110) surface. We determined the surface phase diagram, explored the possible reaction pathways including kinetic barriers, and computed reaction rates based on the microkinetic models. This allowed us to resolve several long-standing puzzles about the atomistic OER mechanism. [ABSTRACT FROM AUTHOR]

We report density functional theory (M06L) calculations including Poisson-Boltzmann solvation to determine the reaction pathways and barriers for the hydrogen evolution reaction (HER) on MoS2, using both a periodic two-dimensional slab and a Mo10S21 cluster model. We find that the HER mechanism involves protonation of the electron rich molybdenum hydride site (Volmer-Heyrovsky mechanism), leading to a calculated free energy barrier of 17.9 kcal/mol, in good agreement with the barrier of 19.9 kcal/mol estimated from the experimental turnover frequency. Hydronium protonation of the hydride on the Mo site is 21.3 kcal/mol more favorable than protonation of the hydrogen on the S site because the electrons localized on the Mo-H bond are readily transferred to form dihydrogen with hydronium. We predict the Volmer-Tafel mechanism in which hydrogen atoms bound to molybdenum and sulfur sites recombine to form H2 has a barrier of 22.6 kcal/mol. Starting with hydrogen atoms on adjacent sulfur atoms, the Volmer-Tafel mechanism goes instead through the M-H + S-H pathway. In discussions of metal chalcogenide HER catalysis, the S-H bond energy has been proposed as the critical parameter. However, we find that the sulfur-hydrogen species is not an important intermediate since the free energy of this species does not play a direct role in determining the effective activation barrier. Rather we suggest that the kinetic barrier should be used as a descriptor for reactivity, rather than the equilibrium thermodynamics. This is supported by the agreement between the calculated barrier and the experimental turnover frequency. These results suggest that to design a more reactive catalyst from edge exposed MoS2, one should focus on lowering the reaction barrier between the metal hydride and a proton from the hydronium in solution. [ABSTRACT FROM AUTHOR]

A series of rhodium(III) bis(quinolinyl)benzene (bisqx) complexes was studied as candidates for the homogeneous partial oxidation of methane. Density functional theory (DFT) (M06 with Poisson continuum solvation) was used to investigate a variety of (bisqx) ligand candidates involving different functional groups to determine the impact on RhIII(bisqx)-catalyzed methane functionalization. The free energy activation barriers for methane CH activation and Rh-methyl functionalization at 298 K and 498 K were determined. DFT studies predict that the best candidate for catalytic methane functionalization is RhIII coordinated to unsubstituted bis(quinolinyl)benzene (bisq). Support is also found for the prediction that the η2-benzene coordination mode of (bisqx) ligands on Rh encourages methyl group functionalization by serving as an effective leaving group for SN2 and SR2 attack. [ABSTRACT FROM AUTHOR]

Recent experiments in our laboratory have shown that the probability of gaseous HNO3 deprotonation on the surface of water is dramatically enhanced by anions. Herein, we report a quantum chemical study of how a HNO3 molecule transfers its proton upon approaching water clusters containing or not a chloride ion. We find that HNO3 always binds to the outermost water molecules both via donating and accepting hydrogen-bonds, but the free energy barrier for subsequent proton transfer into the clusters is greatly reduced in the presence of Cl−. As the dissociation of HNO3 embedded in water clusters is barrierless, we infer that interfacial proton transfer to water is hindered by the cost of creating a cavity for NO3−. Our findings suggest that nearby anions catalyze HNO3 dissociation by preorganizing interfacial water and drawing the proton-away from the incipient [H+---NO3−] close ion-pairs generated at the interface. This catalytic mechanism would operate in the 1 mM Cl− range (1 Cl− in ∼5.5 × 104 water molecules) covered by our experiments if weakly adsorbed HNO3 were able to explore extended surface domains before desorbing or diffusing (undissociated) into bulk water. © 2012 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Differences in the extent of protonation of functional groups lying on either side of water-hydrophobe interfaces are deemed essential to enzymatic catalysis, molecular recognition, bioenergetic transduction, and atmospheric aerosol-gas exchanges. The sign and range of such differences, however, remain conjectural. Herein we report experiments showing that gaseous carboxylic acids RCOOH(g) begin to deprotonate on the surface of water significantly more acidic than that supporting the dissociation of dissolved acids RCOOH(aq). Thermodynamic analysis indicates that> 6 H2O molecules must participate in the deprotonation of RCOOH(g) on water, but quantum mechanical calculations on a model air-water interface predict that such event is hindered by a significant kinetic barrier unless OH ions are present therein. Thus, by detecting RCOO we demonstrate the presence of OH on the aerial side of on pH > 2 water exposed to RCOOH(g). Furthermore, because in similar experiments the base (Me)3N(g) is protonated only on pH < 4 water, we infer that the outer surface of water is Brønsted neutral at pH ~3 (rather than at pH 7 as bulk water), a value that matches the isoelectric point of bubbles and oil droplets in independent electrophoretic experiments. The OH densities sensed by RCOOH(g) on the aerial surface of water, however, are considerably smaller than those at the (>1 nm) deeper shear planes probed in electrophoresis, thereby implying the existence of OHgradients in the interfacial region. This fact could account for the weak OH signals detected by surface-specific spectroscopies. [ABSTRACT FROM AUTHOR]