Rapid, Reversible Heterolytic Cleavage of Bound H2
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Kinetic and Mechanistic Studies of Carbon-to-Metal Hydrogen Atom Transfer Involving Os-Centered Radicals: Evidence for Tunneling
We have investigated the kinetics of novel carbon-to-metal hydrogen atom transfer reactions, in which homolytic cleavage of a C-H bond is accomplished by a single metal-centered radical. Studies by means of time-resolved IR spectroscopic measurements revealed efficient hydrogen atom transfer from xanthene, 9,10-dihydroanthracene and 1,4-cyclohexadiene to Cp(CO)2Os• and (n5-iPr4C5H)(CO)2Os• radicals, formed by photoinduced homolysis of the corresponding osmium dimers. The rate constants for hydrogen abstraction from these hydrocarbons were found to be in the range 1.54 × 105 M 1 s 1 -1.73 × 107 M 1 s-1 at 25 °C. For the first time, kinetic isotope effects for carbon-to-metal hydrogen atom transfer were determined. Large primary kinetic isotope effects of 13.4 ± 1.0 and 16.6 ± 1.4 were observed for the hydrogen abstraction from xanthene to form Cp(CO)2OsH and (n5-iPr4C5H)(CO)2OsH, respectively, at 25 °C. Temperature-dependent measurements of the kinetic isotope effects over a 60 -C temperature range were carried out to obtain the difference in activation energies and the pre-exponential factor ratio. For hydrogen atom transfer from xanthene to (n5-iPr4C5H)(CO)2Os•, the (ED - EH) = 3.25 ± 0.20 kcal/mol and AH/AD = 0.056 ± 0.018 values are greater than the semi-classical limits and thus suggest a quantum mechanical tunneling mechanism. The work at BNL was carried out under contract DE-AC02-98CH10886 with the U.S. Department of Energy and supported by its Division of Chemical Sciences, Geosciences & Biosciences, Office of Basic Energy Sciences. RMB also thanks the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences for support. Pacific Northwest National Laboratory is operated by Battelle for the U.S. Department of Energy.
hydrogen atom transfer↗
Kinetic Analysis of Competitive Electrocatalytic Pathways: New Insights into Hydrogen Production with Nickel Electrocatalysts
The hydrogen production electrocatalyst Ni(PPh2NPh2)22+ (1) is capable of traversing multiple electrocatalytic pathways. When using dimethylformamidium, DMF(H)+, the mechanism of formation of H2 catalyzed by 1 changes from an ECEC to an EECC mechanism as the potential approaches the Ni(I/0) couple. Two recent electrochemical methods, current-potential analysis and foot-of-the-wave analysis (FOWA), were performed on 1 to measure the detailed chemical kinetics of the competing ECEC and EECC pathways. A sensitivity analysis was performed on the electrochemical methods using digital simulations to gain a better understanding of their strengths and limitations. Notably, chemical rate constants were significantly underestimated when not accounting for electron transfer kinetics, even when electron transfer was fast enough to afford a reversible non-catalytic wave. The EECC pathway of 1 was found to be faster than the ECEC pathway under all conditions studied. Using buffered DMF: DMF(H)+ mixtures led to an increase in the catalytic rate constant (kobs) of the EECC pathway, but kobs for the ECEC pathway did not change when using buffered acid. Further kinetic analysis of the ECEC path revealed that added base increases the rate of isomerization of the exo-protonated Ni(0) isomers to the catalytically active endo-isomers, but decreases the net rate of protonation of Ni(I). FOWA on 1 did not provide accurate rate constants due to incomplete reduction of the exo-protonated Ni(I) intermediate at the foot of the wave, but FOWA could be used to estimate the reduction potential of this previously undetected intermediate. This research was supported as part of the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. Pacific Northwest National Laboratory is operated by Battelle for the U.S. Department of Energy.
electrocatalysis↗
A Bimetallic Nickel–Gallium Complex Catalyzes CO 2 Hydrogenation via the Intermediacy of an Anionic d 10 Nickel Hydride
Large-scale CO2 hydrogenation could offer a renewable stream of industrially important C1 chemicals while reducing CO2 emissions. Critical to this opportunity is the requirement for inexpensive catalysts based on earth-abundant metals instead of precious metals. We report a nickel-gallium complex featuring a Ni(0)→Ga(III) bond that shows remarkable catalytic activity for hydrogenating CO2 to formate at ambient temperature (3150 turnovers, turnover frequency = 9700 h-1), compared with prior homogeneous Ni-centred catalysts. The Lewis acidic Ga(III) ion plays a pivotal role by stabilizing reactive catalytic intermediates, including a rare anionic d10 Ni hydride. The structure of this reactive intermediate shows a terminal Ni-H, for which the hydride donor strength rivals those of precious metal-hydrides. Collectively, our experimental and computational results demonstrate that modulating a transition metal center via a direct interaction with a Lewis acidic support can be a powerful strategy for promoting new reactivity paradigms in base-metal catalysis. The work was supported as part of the Inorganometallic Catalysis Design Center, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences under Award DE-SC0012702. R.C.C. and M.V.V. were supported by DOE Office of Science Graduate Student Research and National Science Foundation Graduate Research Fellowship programs, respectively. J.C.L., S.A.B., and A.M.A. were supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. Pacific Northwest National Laboratory is operated by Battelle for the U.S. Department of Energy.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Thermodynamic Hydricity of [FeFe]-Hydrogenases
In this Perspective, recent advances in the mechanistic understanding of [FeFe]-hydrogenases are analyzed using thermodynamic principles that have been instrumental for the rational design of state-of-the-art molecular electrocatalysts for production and oxidation of H2. After remaining elusive for many years, a terminal hydride state has now been identified in several native and artificial [FeFe]-hydrogenases. Determination of the hydricity of this crucial catalytic intermediate reveals that modification of amino acid residues in the proton transport channel changes the intrinsic thermodynamic bias of the enzyme to favor oxidation of H2 via heterolytic cleavage of the H H bond. From this vantage point, the structure and catalytic function of the hydride state and the controversial super-reduced state are considered with an eye towards key hydrogen bonding interactions between the active site and the extended protein environment. This research was supported as part of the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences. Pacific Northwest National Laboratory is operated by Battelle for DOE.
Wiedner, Eric S.↗
Molecular mechanism of CO2 and SO2 molecules binding to the air/liquid interface of 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid: A Molecular dynamics study with polarizable potential models
Molecular dynamics simulations with many-body interactions were carried out to understand the bulk and interfacial absorption of gases in 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4). A new polarizable molecular model was developed for BMIMBF4, which was found to give the correct liquid density, but also had good agreement with experiment for its surface tension and X-ray reflectivity. The potential of mean force of CO2 and SO2 were calculated across the air-BMIMBF4 interface, and the bulk free energies were calculated with the free energy perturbation method. A new polarizable model was also developed for CO2. The air-BMIMBF4 interface had enhanced BMIM density, which was mostly related to its butyl group, followed by enhanced BF4 density a few angstroms towards the liquid bulk. The density profiles were observed to exhibit oscillations between high BMIM and BF4 density, indicating the presence of surface layering induced by the interface. The potential of mean force for CO2 and SO2 showed more negative free energies in regions of enhanced BF4 density, while more positive free energies in regions of high BMIM density. Moreover, these gases showed free energy minimums at the interface, where the BMIM alkyl groups were found to be most prevalent. Our results show the importance of ionic liquid interfacial ordering for understanding gas solvation in them. This work was supported by the US Department of Energy Basic Energy Sciences' Chemical Sciences, Geosciences & Biosciences Division. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Computational study of ion distributions at the air/liquid methanol interface
Molecular dynamic simulations with polarizable potentials were performed to systematically investigate the distribution of NaCl, NaBr, NaI, and SrCl2 at the air/liquid methanol interface. The density profiles indicated that there is no substantial enhancement of anions at the interface for the NaX systems in contrast to what was observed at the air/aqueous interface. The surfactant-like shape of the larger more polarizable halide anions is compensated by the surfactant nature of methanol itself. As a result, methanol hydroxy groups strongly interacted with one side of polarizable anions, in which their induced dipole points, and methanol methyl groups were more likely to be found near the positive pole of anion induced dipoles. Furthermore, salts were found to disrupt the surface structure of methanol, reducing the observed enhancement of methyl groups at the outer edge of the air/liquid methanol interface. With the additional of salts to methanol, the computed surface potentials increased, which is in contrast to what is observed in corresponding aqueous systems, where the surface potential decreases with the addition of salts. Both of these trends have been indirectly observed with experiments. This was found to be due to the propensity of anions for the air/water interface that is not present at the air/liquid methanol interface. This work was supported by the US Department of Energy Basic Energy Sciences' Chemical Sciences, Geosciences & Biosciences Division. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.
10 SYNTHETIC FUELS↗
Structure and Dynamics of N, N-diethyl-N-methylammonium Triflate Ionic Liquid, Neat and with Water, from Molecular Dynamics Simulations
We investigated by means of molecular dynamics simulations the properties (structure, thermodynamics, ion transport, and dynamics) of the protic ionic liquid N,N-diethyl-N-methyl-ammonium triflate (dema:Tfl) and of selected aqueous mixtures of dema:Tfl. This ionic liquid, a good candidate for a water-free proton exchange membrane, is shown to exhibit high ion mobility and conductivity. For bulk melts in the temperature range of 303-453K, both liquid densities and enthalpies of vaporization are found to decrease roughly linearly with increasing temperature. The radial distribution functions reveal a significant long-range structural correlation. The ammonium cations [dema]+ are found to diffuse slightly faster than the triflate anions [Tfl]-, and both types of ions exhibit enhanced mobility at higher temperatures, leading to higher ionic conductivity of these ionic liquids. Analysis of the dynamics of ion pairing clearly points to the existence of long-lived contact ion pairs in this ionic liquid. We also examined the effects of water on the ionic properties of dema:Tfl-water mixtures. From the structural analysis it was found that water molecules tend to replace counter ions in the coordination shell and hydrogen bond to both ions, thus weakening their mutual association. As water concentration increases, water molecules start to connect with each other and eventually form a large network that percolates through the system. It is also found that water has a strong influence on the ion dynamics in the mixtures. As the concentration of water increases, both translational and rotational motion of [dema]+ and [Tfl]- are significantly enhanced. As a result, higher ionic conductivity is observed with increased hydration level. This work was supported by the US Department of Energy Basic Energy Sciences' Chemical Sciences, Geosciences & Biosciences Division. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.
08 HYDROGEN↗
Formation, characterization and reactivity of adsorbed oxygen on BaO/Pt(111)
The formation of adsorbed O (Oad) species and their reactivities in CO oxidation on BaO/Pt(111) (at two BaO coverages) were studied with temperature programmed desorption (TPD), infrared reflection absorption (IRA) and X-ray photoelectron (XP) spectroscopies. In neither of these two systems was the Pt(111) surface completely covered with BaO. On the system with lower BaO coverage (~45 % of the Pt(111) surface is covered by BaO), two different Oad species form following the adsorption of O2 at 300 K: O adsorbed on BaO-free Pt(111) sites (OPt) and at the Pt-BaO interface (Oint). On the system with higher BaO coverage (~60 % of the Pt(111) surface is covered by BaO), two types of Oint are seen at the Pt-BaO interface. The desorption of OPt from the BaO-free portion of the Pt(111) surface gives an O2 desorption peak with a maximum desorption rate at ~690 K. Migration of Oint to the Pt(111) sites and their recombinative desorption give two explosive desorption features at ~760 and ~790 K in the TPD spectrum. The reactivities of these Oad species with CO to form CO2 follow their sequence of desorption; i.e., the OPt associated with the BaO-free Pt(111) surface, which desorbs at 690 K, reacts first with CO, followed by the Oint species at the Pt-BaO interface (first the one that desorbs at ~760 K and finally the one that is bound the most strongly to the interface, and desorbs at ~790 K). This work was supported by the US Department of Energy Basic Energy Sciences' Chemical Sciences, Geosciences & Biosciences Division. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Semiempirical Self-Consistent Polarization description of bulk water, the liquid-vapor interface, and cubic ice
We have applied an efficient electronic structure approach, the semiempirical self-consistent polarization neglect of diatomic differential overlap (SCP-NDDO) method, previously parametrized to reproduce properties of water clusters by Chang, Schenter, and Garrett [J. Chem. Phys. 128, 164111 (2008)], and now implemented in the CP2K package, to model ambient liquid water at 300 K (both the bulk and the liquid-vapor interface) and cubic ice at 15 K and 250 K. The SCP-NDDO potential retains its transferability and good performance across the full range of conditions encountered in the clusters and the bulk phases of water. In particular, we obtain good results for the density, radial distribution functions, enthalpy of vaporization, self-diffusion coefficient, molecular dipole moment distribution, and hydrogen-bond populations, in comparison to experimental measurements. This work was supported by the US Department of Energy Basic Energy Sciences' Chemical Sciences, Geosciences & Biosciences Division. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Computational study of hydrocarbon adsorption in metal-organic framework Ni2(dhtp)
Molecular dynamic simulations were carried out to study the sorption, structural properties, and diffusivities of n-hexane and cyclohexane adsorbed in Ni2(dhtp). The results indicated strong interactions between the alkanes and the host material. The free energy perturbation method was employed to investigate the adsorption free energies of methane, ethane, n-butane, n-hexane and cyclohexane. For linear alkanes, the free energy lowered as the length of the carbon chain increased. Also, the adsorption of n-hexane was preferred over cyclohexane, due to its ability to rearrange its structure to maximize contacts with the host. Furthermore, due to the large pore size of Ni2(dhtp), higher loadings of alkanes did not significantly affect the alkane structure, and enhanced the free energy of adsorption for subsequent alkanes being loaded. According to our studies, Ni2(dhtp) has a very promising potential for adsorption and storage of alkanes. This work was supported by the US Department of Energy Basic Energy Sciences' Chemical Sciences, Geosciences & Biosciences Division. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.
03 NATURAL GAS↗
Understanding the Surface Potential of Water
We have resolved the apparent inconsistency in quantifying the electrochemical surface potential at the liquid-vapor interface when using explicit ab initio electronic charge density and effective atomic partial charge models of liquid water. It is shown that regions of space must be excluded for the evaluation of the electric potential from a charge distribution to be useful in the interpretation of electrochemical measurements. This is not the case for work functions, electron diffraction or electron holography measurements. We find that the resulting electric potentials from partial charge models and ab initio charge distributions are quite different except for those regions of space between the molecules where the electron density is nearly zero. We compare our computed ab initio surface potential with high-energy electron diffraction and holography measurements and find good agreement. This work was supported by the US Department of Energy Basic Energy Sciences' Chemical Sciences, Geosciences & Biosciences Division. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Catalytic decomposition of Ba(NO3)2 on Pt(111)
The decomposition of Ba(NO3)2 formed on BaO/Pt(111) (Pt(111) surface is partially covered by BaO) in the presence of CO was studied using temperature programmed desorption (TPD), infrared reflection absorption (IRA) and X-ray photoelectron (XP) spectroscopies. The exposure of BaO/Pt(111) to elevated NO2 pressure (1.0×10-4 Torr) at 450 K leads to the formation of Ba(NO3)2, chemisorbed O (OPt) and Pt-oxide-like domains. During TPD, the Ba(NO3)2 begins to thermally decompose near 490 K, releasing NO and NO2 with the maximum NOx desorption rate seen at 605 K. The OPt species formed following the exposure of BaO/Pt(111) to NO2 react with CO to release CO2 at 450 K. The consumption of OPt during CO oxidation initiates the migration of O from the Pt-oxide-like domains to the chemisorbed phase, where the CO oxidation reaction occurs. Therefore, the removal of OPt by CO leads to the reduction of oxidized Pt, and to the formation of metallic Pt(111) domains, where, subsequently, catalytic decomposition of Ba(NO3)2 can take place. The Pt-catalyzed decomposition of Ba(NO3)2 occurs readily at 450 K, a temperature much lower than the onset of the decomposition temperature of Ba(NO3)2 in the presence of oxidized Pt. This work was supported by the US Department of Energy Basic Energy Sciences' Chemical Sciences, Geosciences & Biosciences Division. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Atomistic Simulations of Perfluoro Phosphonic and Phosphinic Acid Membranes and Comparisons to Nafion
We used classical molecular dynamics (MD) simulations to investigate the nanoscale morphology and proton transport properties of perfluoro phosphonic (FPA) and phosphinic acid (FPA-I) membranes as they are being considered for use in low temperature fuel cells. We systematically investigated these properties as a function of the hydration level. The changes in nanostructure, in transport dynamics of water and hydronium ions, and in water network percolation were extracted from MD simulations and compared with Nafion. Phosphonic and phosphinic acid moieties in FPA and FPA-I, have lower acidity than sulfonic acid in Nafion, yet the diffusion of water was observed to be faster in FPA and FPA-I than in Nafion, particularly at low hydration levels. However this did not give rise to notable differences in hydronium ion diffusion and water network percolation for these membranes over Nafion. Similar observations were also reported by our group recently in a study of perfluoro-sulfonyl imide membranes carrying stronger super-acids than sulfonic acid of Nafion. These findings together suggest no strong apparent correlation between the acidity strength of the functional acid groups and the dynamics of water and hydronium ions in hydrated polymer electrolyte membranes (PEMs) with similar fluorocarbon backbones and acidic group-carrying side chains. This work was supported by the US Department of Energy Basic Energy Sciences' Chemical Sciences, Geosciences & Biosciences Division. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.
30 DIRECT ENERGY CONVERSION↗
Density functional theory study of acetaldehyde hydrodeoxygenation on MoO3
Periodic spin-polarized density functional theory calculations were performed to investigate acetaldehyde (CH3CHO) hydrodeoxygenation on the reduced molybdenum trioxide (MoO3) surface. The perfect O-terminated α-MoO3(010) surface is reduced to generate an oxygen defect site in the presence of H2. H2 dissociatively adsorbs at the surface oxygen sites forming two surface hydroxyls, which can recombine into a water molecule weakly bound at the Mo site. A terminal oxygen (Ot) defect site thus forms after water desorption. CH3CHO adsorbs at the O-deficient Mo site via either the sole O-Mo bond or the O-Mo and the C-O double bonds. The possible reaction pathways of the adsorbed CH3CHO with these two configurations were thoroughly examined using the dimer searching method. Our results show that the ideal deoxygenation of CH3CHO leading to ethylene (C2H4) on the reduced MoO3(010) surface is feasible. The adsorbed CH3CHO first dehydrogenate into CH2CHO by reacting with a neighboring terminal Ot. The hydroxyl (OtH) then hydrogenates CH2CHO into CH2CH2O to complete the hydrogen transfer cycle with an activation barrier of 1.39 eV. The direct hydrogen transfer from CH3CHO to CH2CH2O is unlikely due to the high barrier of 2.00 eV. The produced CH2CH2O readily decomposes into C2H4 that directly releases to the gas phase, and regenerates the Ot atom on the Mo site. As a result, the reduced MoO3(010) surface is reoxidized to the perfect MoO3(010) surface after CH3CHO deoxygenation. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
The Distribution of Ti3+ Surface Sites in Reduced TiO2
We describe a DFT+U study of the (110) rutile surface with oxygen vacancies (Ov). Oxygen vacancies leave behind two excess unpaired electrons per Ov the formation of two Ti3+ ions. We investigate the location of the Ti3+ ions within the first three surface layers. In total we obtained 49 unique solutions of possible Ti3+ pairs, to examine the stability of all Ti types (e.g. five-coordinated surface Ti, six-coordinated surface Ti, sub-surface sites, etc.). Our results show that sub-surface sites are preferred, but that many configurations are close in energy, up to 0.3-0.4 eV of each other. In contrast to findings in previous work, we show that sites directly adjacent to the Ov are unstable. Analysis of our results show that the two Ti3+ ions within a pair behave independent of each other, as there are little electronic interactions between the excess electrons associated with these sites. We also examined the migration of Ti3+ sites from the surface into the bulk and find the surface locations to be preferred by ~ 0.5 eV relative to the bulk. Our systematic results provide a comprehensive picture of excess electrons that indicates that they are not trapped or localized at specific sites, but are distributed across several sites due to nearly degenerate Ti3+ states. This work was supported by the US Department of Energy Basic Energy Sciences' Chemical Sciences, Geosciences & Biosciences Division. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Anion Effects on Interfacial Absorption of Gases in Ionic Liquids. A Molecular Dynamics Study
Molecular dynamics simulations with many-body interactions were carried out to systematic study the effect of anion type, tetrafluoroborate [BF4] or hexafluorophosphate [PF6], paired with the cation 1-butyl-3-methylimidazolium [bmim], on the interfacial absorption of gases in room temperature ionic liquids (RTILs). The potentials of mean force (PMF) of CO2 and H2O at 350 K were calculated across the air-liquid interfaces of [bmim][BF4] and [bmim][PF6]. We found that the PMFs for H2O exhibited no interfacial minima at both interfaces, while the corresponding PMFs for CO2 had significant free energy minima there. However, the PMFs for H2O showed a much higher interfacial free energy than in the bulk for [bmim][BF4], but only a slightly higher interfacial free energy for [bmim][PF6] than in bulk. The reason for this was due to the more hydrophilic nature of the [BF4] anion, and the fact that [BF4] was found to have little propensity for the interface. Our results show that H2O is much more likely to be found at the air-[bmim][PF6] interface than at the air-[bmim][BF4] interface. The free energies of solvation were found to be more negative for [bmim][BF4] than [bmim][PF6] for water and similar for CO2. This observation is consistent with experimental Henry’s law coefficients. Our results show that anion type, in addition to affecting the free energy of solvation into RTILs, should also significantly influence the uptake mechanism. This work was supported by the US Department of Energy Basic Energy Sciences' Chemical Sciences, Geosciences & Biosciences Division. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Hydrogen Adsorption on Ga2O3 Surface: A Combined Experimental and Computational Study
In the present work, hydrogen adsorption on the Ga2O3 surfaces was investigated using Fourier transform infrared spectroscopy (FTIR) measurements and periodic density functional theory (DFT) calculations. Both the FTIR and DFT studies suggest that H2 dissociates on the Ga2O3 surfaces, producing OH and GaH species. The FTIR bands at 3730, 3700, 3630 and 3600 cm-1 are attributed to the vibration of the OH species whereas those at 2070 and 1990 cm-1 to the GaH species. The structures of the species detected in experiments are established through a comparison with the DFT calculated stretching frequencies. The O atom of the experimentally detected OH species is believed to originate from the surface O3c atom. On the other hand, the H atom that binds the coordinately unsaturated Ga atom results in the experimentally detected GaH species. Dissociation of H2 on the perfect Ga2O3 surface, with the formation of both OH and GaH species, is endothermic and has an energy barrier of 0.90 eV. In contrast, H2 dissociation on the defective Ga2O3 surface with oxygen vacancies, which mainly produces GaH species, is exothermic, with an energy barrier of 0.61 eV. Accordingly, presence of the oxygen vacancies promotes H2 dissociation and production of GaH species on the Ga2O3 surfaces. Higher temperatures are expected to favor oxygen vacancy creation on the Ga2O3 surfaces, and thereby benefit the production of GaH species. This analysis is consistent with the FTIR results that the bands assigned to GaH species become stronger at higher temperatures. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.
08 HYDROGEN↗