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All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

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↗
DOE OSTI2014

First-principles Study of Phenol Hydrogenation on Pt and Ni Catalysts in Aqueous Phase

The effects of aqueous phase on the reactivity of phenol hydrogenation over Pt and Ni catalysts were investigated using density functional theory based ab initio molecular dynamics (AIMD) calculations. The adsorption of phenol and the first hydrogenation steps via three carbon positions (ortho, meta and para) with respect to the phenolic OH group were studied in both vacuum and liquid phase conditions. To gain insight into how the aqueous phase affects the metal catalyst surface, increasing water environments including singly adsorbed water molecule, mono- (9 water molecules), double layers (24 water molecules), and the bulk liquid water which (52 water molecules) on the Pt(111) and the Ni(111) surfaces were modeled. Compared to the vacuum/metal interfaces, AIMD simulation results suggest that the aqueous Pt(111) and Ni(111) interfaces have a lower metal work function in the order of 0.8 - 0.9 eV, thus, making the metals in aqueous phase stronger reducing agents and poorer oxidizing agents. Phenol adsorption from the aqueous phase is found to be slightly weaker that from the vapor phase. The first hydrogenation step of phenol at the ortho position of the phenolic ring is slightly favored over the other two positions. The polarization induced by the surrounding water molecules and the solvation effect play important roles in stabilizing the transition states associated with phenol hydrogenation by lowering the barriers of 0.1 - 0.4 eV. The detailed discussion on the basis of the interfacial electrostatics from the current study is very useful to understand the nature of a broader class of metal catalyzed reactions in liquid solution phase. This work was supported by the US Department of Energy (DOE), Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences and Office of Energy Efficiency and Renewable Energy. Pacific Northwest National Laboratory (PNNL) is a multiprogram national laboratory operated for DOE by Battelle. Computing time was granted by the grand challenge of computational catalysis of the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL) and by the National Energy Research Scientific Computing Center (NERSC). EMSL is a national scientific user facility located at Pacific Northwest National Laboratory (PNNL) and sponsored by DOE’s Office of Biological and Environmental Research.

aqueous phase↗
DOE OSTI2014

Thermochemical Insight into the Reduction of CO to CH3OH with [Re(CO)]+ and [Mn(CO)]+ Complexes

To gain insight into thermodynamic barriers for reduction of CO into CH3OH, free energies for reduction of [CpRe(PPh3)(NO)(CO)]+ into CpRe(PPh3)(NO)(CH2OH) have been determined from experimental measurements. Using model complexes, the free energies for the transfer of H+, H–, and e– have been determined. A pKa of 10.6 was estimated for [CpRe(PPh3)(NO)(CHOH)]+ by measuring the pKa for the analogous [CpRe(PPh3)(NO)(CMeOH)]+. The hydride donor ability (ΔG°H–) of CpRe(PPh3)(NO)(CH2OH) was estimated to be 58.0 kcal mol–1, based on calorimetry measurements of the hydride transfer reaction between CpRe(PPh3)(NO)(CHO) and [CpRe(PPh3)(NO)(CHOMe)]+ to generate the methylated analog, CpRe(PPh3)(NO)(CH2OMe). Cyclic voltammograms recorded on CpRe(PPh3)(NO)(CMeO), CpRe(PPh3)(NO)(CH2OMe), and [CpRe(PPh3)(NO)(CHOMe)]+ displayed either a quasireversible oxidation (neutral species) or reduction (cationic species). These potentials were used as estimates for the oxidation of CpRe(PPh3)(NO)(CHO) or CpRe(PPh3)(NO)(CH2OH), or the reduction of [CpRe(PPh3)(NO)(CHOH)]+. Combination of the thermodynamic data permits construction of three-dimensional free energy landscapes under varying conditions of pH and PH2. The free energy for H2 addition (ΔG°H2) to [CpRe(PPh3)(NO)(CO)]+ (+15 kcal mol–1) was identified as the most significant thermodynamic impediment for the reduction of CO. DFT computations indicate that ΔG°H2 varies by only 4.3 kcal mol–1 across a series of [CpXRe(L)(NO)(CO)]+, while the experimental ΔG°H– values for the analogous series of CpRe(PPh3)(NO)(CHO) varies by 12.9 kcal mol–1. The small range of ΔG°H2 values is attributed to a minimal change in the C–O bond polarization upon modification of the ancillary ligands, as determined from the computed atomic charges. This work was supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. Pacific Northwest National Laboratory (PNNL) is a multiprogram national laboratory operated for DOE by Battelle.

carbon monoxide↗
DOE OSTI2015

Impact of zeolite aging in hot liquid water on activity for acid-catalyzed dehydration of alcohols

The catalytic performance of zeolite in aqueous medium depends on a multitude of factors, such as the concentration and distribution of active sites and framework integrity. Al K–edge extended X–ray absorption fine structure and 27Al MAS NMR spectroscopies in combination with DFT calculations are used to determine the distribution of tetrahedral Al sites both qualitatively and quantitatively for both parent and 48 h 160 ºC water treated HBEA catalysts. There is no evidence of Al coordination modification after aging in water. The distribution and concentration of Al T–sites, active centers for the dehydration of cyclohexanol, do not markedly impact the catalytic performance in water, because the Brønsted acidic protons are present in the form of hydrated hydronium ions and thus have very similar acid properties. The results suggest that all Brønsted acid sites are equally active in aqueous medium. The decrease of zeolite catalytic performance after water treatment is attributed to the reduced concentration of Brønsted acid sites. Increasing the stability of pore walls and decreasing the rate of Si–O–Si group hydrolysis may result in improved apparent zeolite catalytic performance in aqueous medium. Authors thank B. W. Arey (PNNL) for HIM measurements, T. Huthwelker for support during Al XAFS measurements at the Swiss Light Source (PSI, Switzerland), J. Z. Hu and S. D. Burton (PNNL) for support during NMR experiments. This work was supported by the U. S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. MD acknowledges support by the Materials Synthesis and Simulation Across Scales (MS3 Initiative) conducted under Laboratory Directed Research & Development Program at PNNL. HIM imaging and NMR experiments were performed at the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the DOE Office of Science, Office of Biological and Environmental Research, located at Pacific Northwest National Laboratory (PNNL). PNNL is a multiprogram national laboratory operated for the DOE by Battelle Memorial Institute under contract # DE-AC05-76RL0-1830

alcohol dehydration↗
DOE OSTI2015

Dehydration pathways of 1-propanol on HZSM-5 in the presence and absence of water

The Brønsted acid-catalyzed gas-phase dehydration of 1-propanol (0.075-4 kPa) was studied on zeolite H-MFI (Si/Al = 26, containing minimal amounts of extraframework Al moieties) in the absence and presence of co-fed water (0-2.5 kPa) at 413-443 K. It is shown that propene can be formed from monomeric and dimeric adsorbed 1-propanol. The stronger adsorption of 1-propanol relative to water indicates that the reduced dehydration rates in the presence of water are not a consequence of the competitive adsorption between 1-propanol and water. Instead, the deleterious effect is related to the different extents of stabilization of adsorbed intermediates and the relevant elimination/substitution transition states by water. Water stabilizes the adsorbed 1-propanol monomer significantly more than the elimination transition state, leading to a higher activation barrier and a greater entropy gain for the rate-limiting step, which eventually leads to propene. In a similar manner, an excess of 1-propanol stabilizes the adsorbed state of 1-propanol more than the elimination transition state. In comparison with the monomer-mediated pathway, adsorbed dimer and the relevant transition states for propene and ether formation are similarly, while less effectively, stabilized by intrazeolite water molecules. This work was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, and was performed in part using the Molecular Sciences Computing Facility (MSCF) in the William R. Wiley Environmental Molecular Sciences Laboratory, a DOE national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research and located and the Pacific Northwest National Laboratory (PNNL). PNNL is operated by Battelle for DOE.

1-propanol↗
DOE OSTI2016

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↗
DOE OSTI2016

CO Oxidation on Au/TiO 2 : Condition-Dependent Active Sites and Mechanistic Pathways

We present results of ab initio electronic structure and molecular dynamics simulations (AIMD), as well as microkinetic modeling of CO oxidation catalyzed by TiO2 supported Au nanocatalysts. A coverage-dependent microkinetic analysis, based on energetics obtained with density functional methods, shows that the dominant kinetic pathway, activated oxygen species, and catalytic active sites are all strongly depended on both temperature and oxygen partial pressure. A dynamic single atom catalytic (DSAC) mechanism for CO oxidation is proposed to be the dominant (but not the exclusive) pathway for this reaction under oxidative conditions below T < 400 K. This mechanism proceeds via an Au+-CO species that transfers from the Au-cluster to a surface oxygen ad-atom, where it reacts with the TiO2 support with a Mars van Krevelen mechanism to form CO2 and subsequently reintegrates back into the gold cluster to complete the catalytic cycle. At 300 = T = 600 K oxygen-bound single Au+CO sites and the perimeter sites of the Au-nanoparticle work in tandem to optimally catalyze the reaction. Above 600 K a variety of alternate pathways associated with the perimeter sites of the Au nanoparticle and/or the single atom site are found to be the most active pathways. Overall our results highlight the importance of the dynamic behavior of interfacial area between the catalysts and the support, which allows for a high degree of adaptation. This work was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences and performed at the Pacific Northwest National Laboratory (PNNL). PNNL is a multi-program national laboratory operated for Department of Energy by Battelle. Computational resources were provided at W. R. Wiley Environmental Molecular Science Laboratory (EMSL), a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research located at PNNL and the National Energy Research Scientific Computing Center (NERSC) at Lawrence Berkeley National Laboratory.

Wang, Yang-Gang↗
DOE OSTI2016

Triphosphine-Ligated Copper Hydrides for CO 2 Hydrogenation: Structure, Reactivity, and Thermodynamic Studies

The copper(I) triphosphine complex LCu(MeCN)PF 6 (L = 1,1,1-tris(diphenylphosphinomethyl)ethane), which we recently demonstrated is an active catalyst precursor for hydrogenation of CO 2 to formate, reacts with H 2 in the presence of a base to form a cationic dicopper hydride, [(LCu) 2 H]PF 6 . [(LCu) 2 H] + is also an active precursor for catalytic CO 2 hydrogenation, with equivalent activity to that of LCu(MeCN) + , and therefore may be a relevant catalytic intermediate. In this study, the thermodynamic hydricity of [(LCu) 2 H] + was determined to be 41.0 kcal/mol by measuring the equilibrium constant for this reaction using three different bases. [(LCu) 2 H] + and the previously reported dimer (LCuH) 2 can be synthesized by the reaction of LCu(MeCN) + with 0.5 and 1 equiv of KB(O i Pr) 3 H, respectively. The solid-state structure of [(LCu) 2 H] + shows threefold symmetry about a linear Cu–H–Cu axis and significant steric strain imposed by bringing two LCu + units together around the small hydride ligand. [(LCu) 2 H] + reacts stoichiometrically with CO 2 to generate the formate complex LCuO 2 CH and the solvento complex LCu(MeCN) + . The rate of the stoichiometric reaction between [(LCu) 2 H] + and CO 2 is dramatically increased in the presence of bases that coordinate strongly to the copper center, e.g. DBU and TMG. In the absence of CO 2 , the addition of a large excess of DBU to [(LCu) 2 H] + results in an equilibrium that forms LCu(DBU) + and also presumably the mononuclear hydride LCuH, which is not directly observed. Due to the significantly enhanced CO 2 reactivity of [(LCu) 2 H] + under these catalytically relevant conditions, LCuH is proposed to be the catalytically active metal hydride.ted by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences. Pacific Northwest National Laboratory (PNNL) is a multiprogram national laboratory operated for DOE by Battelle.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Selective Catalytic Reduction over Cu/SSZ-13: Linking Homo- and Heterogeneous Catalysis

Active centers in Cu/SSZ-13 selective catalytic reduction (SCR) catalysts have been recently identified as isolated Cu2+ and [CuII(OH)]+ ions. A redox reaction mechanism has also been established, where Cu-ions cycle between CuI and CuII oxidation states during SCR reaction. While the mechanism for the reduction half-cycle (CuII CuI) is reasonably well understood, that for the oxidation half-cycle (CuI CuII) remains an unsettled debate. Herein we report detailed reaction kinetics on low-temperature standard NH3-SCR, supplemented by DFT calculations, as strong evidence that the low-temperature oxidation half-cycle occurs with the participation of two isolated CuI ions, via formation of a transient [CuI(NH3)2]+-O2-[CuI(NH3)2]+ intermediate. The feasibility of this reaction mechanism is confirmed from DFT calculations, and the simulated energy barrier and rate constants are consistent with experimental findings. Significantly, the low-temperature standard SCR mechanism proposed here provides full consistency with low-temperature SCR kinetics. The authors gratefully acknowledge the US Department of Energy (DOE), Energy Efficiency and Renewable Energy, Vehicle Technologies Office for the support of this work. The research described in this paper was performed in the Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is operated for the US DOE by Battelle.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

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↗
DOE OSTI2019

Genesis and Stability of Hydronium Ions in Zeolite Channels

Zeolite-catalyzed reactions are profoundly altered by the presence of water changing the nature of the Brønsted acid site. High resolution solid-state NMR spectroscopy shows how water interacts with zeolite Brønsted acid sites converting them to hydrated hydronium ions over a wide range in temperature and thermodynamic activity of water. A signal at 9 ppm was observed at loadings of 2–9 water molecules per Brønsted acid site, and assigned to hydrated hydronium ions on the basis of chemical shift calculations and the observation of the signal after addition of HClO4. The intensity of 1H-29Si cross-polarization signal first increased and then decreased with increasing water chemical potential. This shows that the hydrogen bonds between water molecules and tetrahedrally coordinated aluminum in zeolite lattices weaken with the formation of hydronium ion-water clusters and increase the mobility of protons. Above 140 °C, however, fast proton exchange between bridging hydroxyl groups and water occur even in the presence of only one water molecule per acid site, suggesting that water acts at higher temperatures as a competitor for zeolite protons rather than as constituent of catalytically active hydronium ions. This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences. Portions of the work were performed at the William R. Wiley Environmental Molecular Science Laboratory, a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research located at Pacific Northwest National Laboratory, a multiprogram national laboratory operated for DOE by Battelle Memorial Institute. Addi-tional 1H MAS NMR spectra we acquired using a Bruker 600 MHz NMR spectrometer acquired with support from the US Department of Energy, Office of Science, office of Basic Energy Sciences (Pro-ject Number 66628).

Zeolite, Brønsted acid, water, hydronium ion, NMR ↗
DOE OSTI2019

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.↗
DOE OSTI2019

Molecular Level Understanding of the Free Energy Landscape in Early Stages of Metal-Organic Framework Nucleation

The assembly mechanism of hierarchical materials controlled by the choice of solvent and presence of spectator ions. In this paper, we use enhanced sampling molecular dynamics methods to investigate these effects on the configurational landscape of metal-linker interactions in the early stages of synthesis, using MIL-101(Cr) as a prototypical example. Microsecond-long well-tempered metadynamics (WTmetaD) uncover a complex free energy structural landscape, with distinct crystal (C) and non-crystal (NC) like configurations and their equilibrium population. In presence of ions (Na+, F-), we observe a complex effect on the crystallinity of secondary building units (SBUs), by encouraging/suppressing salt bridges between C configurations and consequently controlling the percentage of defects. Solvent effects are assessed by introducing N, N-dimethylformamide (DMF) instead of water, where SBU adducts are appreciably more stable and compact. These results shed light on how solvent nature and ionic strength impact the free energy of nucleation phenomena that ultimately control materials synthesis and defect formation. The work described in this publication was supported by the US Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences and performed at Pacific Northwest National Laboratory (PNNL). The authors acknowledge the use of the UCL Legion High Performance Computing Facility (Legion @ UCL) and associated support services in the completion of this work. This research used resources of the National Energy Research Scientific Computing Centre, a DOE Office of Science User Facility. Battelle operates PNNL for DOE under Contract DE-AC05-76RL01830.

Kollias, Loukas↗
DOE OSTI2021

ESI-MS Identification of the Cationic Phosphine-Ligated Gold Clusters Au1-Au22: Insight into the Gold-Ligand Ratio and Abundance of Larger Clusters

Triphenylphosphine (PPh3)-ligated gold clusters offer promising potential applications due to their relative ease of synthesis and usefulness in forming advanced cluster architectures. While previous studies reported cationic PPh3-ligated gold clusters with core sizes of Au1 - Au4, Au6 - Au11, and Au¬13 - Au14, there has not been definitive identification by mass spectrometry of larger clusters in the Au12 - Au25¬ range. Herein, we survey a polydisperse solution of cationic PPh3-ligated gold clusters using high mass-resolution (M/?M = 60,000) electrospray ionization mass spectrometry (ESI-MS). To improve the sensitivity and mass resolution of larger clusters for unambiguous identification, we increased the number of scan averages and reduced the range of mass collection windows to 200 m/z, thereby mitigating potential mass and ion abundance bias resulting from smaller “building block” gold clusters and other solution components present in higher abundance. In addition to the previously reported clusters, we identified several new species including Au5(PPh3)5+, Au12(PPh3)9HCl2+, Au15(PPh3)9Cl2+, Au16(PPh3)10Cl22+, Au17(PPh3)113+, Au18(PPh3)102+, Au19(PPh3)10Cl2+, Au20(PPh3)12H33+, Au21(PPh3)10Cl2+, and Au22(PPh3)10Cl22+, indicating that a full range of clusters between Au1 - Au22 may be observed in a single polydisperse solution. Considering all of the observed clusters, our findings provide evidence that the “magic number” icosahedral Au13 may be the transition point in cluster growth between smaller clusters, exhibiting a 1:1 gold-to-ligand ratio, and larger clusters, wherein subsequent gold atoms are added to the core without an equal number of accompanying ligands. Our method demonstrates that reducing the range of m/z collection windows and increasing the number of scan averages can improve instrument sensitivity for cationic gold clusters and enable a more complete survey of polydisperse solutions, thereby providing new insights to guide and validate the results of other characterization methods and theoretical calculations. This work was supported by the US Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences. MH acknowledges support from the DOE Science Undergraduate Laboratory Internship (SULI) program. HH acknowledges support from the DOE Office of Workforce Development for Teachers and Scientist (WDTS) under the Visiting Faculty Program (VFP). This work was performed using EMSL, a national scientific user facility sponsored by the DOE's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is a multiprogram national laboratory operated for DOE by Battelle.

Hewitt, Michael↗
DOE OSTI2013

Photooxidation and Photodesorption in the Photochemistry of Isobutene on TiO2(110)

The photochemistry of isobutene was examined on the rutile TiO2(110) surface as a function of the surface pretreatment condition and irradiation temperature using temperature programmed desorption (TPD) and photon stimulated desorption (PSD). Isobutene adsorbs molecularly on the clean TiO2(110) surface without detectable thermal decomposition. Preadsorption of oxygen, either as atoms or chemisorbed molecules, did not promote thermal reactions with isobutene, but instead blocked isobutene adsorption sites. Ultraviolet (UV) light irradiation of isobutene adsorbed on the clean surface led to depletion through photodesorption without significant photodecomposition. Isobutene PSD yields increased with increasing surface temperature suggesting that activated molecules sample their physisorbed potential energy surface during photodesorption. Preadsorption of oxygen promoted partial photooxidation of adsorbed isobutene to acetone, methacrolein and isobutanal. Acetone was only detected when molecular oxygen was present, indicating that O2 addition occurred across the C=C bond. In contrast, results from use of D6-isobutene indicated that coadsorption with either O adatoms or O2 molecules led to photochemical production of methacrolein (and likely isobutanal) through C-H bond cleavage on a methyl group. Irradiation an adlayer comprised of isobutene isolated from the surface by 1 ML of preadsorbed O2 showed the most photoconversion of isobutene, which suggests that photoactivation of adsorbed O2 is a key step in partial photooxidation of isobutene. Comparison of the isobutene PSD and oxidation product yields as a function of surface temperature between 20 and 120 K indicates a competition between photooxidation and photodesorption that varies with temperature. ‘Direct’ charge transfer events between isobutene and the surface, favored at higher temperature, compete with partial oxidation pathways initiated by ‘indirect’ activation of isobutene by O2, which is favored at low temperature. Access of O2 to the surface is critical to achieving desired isobutene photooxidation rates and products, and isobutene photodesorption may provide a means of regulating the isobutene surface coverage. Work reported here was supported by the U.S. Department of Energy, Office of Basic Energy Science, Division of Chemical Sciences, Geosciences, and Biosciences, and performed in the Williams R. Wiley Environmental Molecular Science Laboratory (EMSL), a Department of Energy user facility funded by the Office of Biological and Environmental Research. Pacific Northwest National Laboratory is a multiprogram national laboratory operated for the U.S. Department of Energy by the Battelle Memorial Institute under contract DEAC05-76RL01830.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2013

State of Supported Pd during Catalysis in Water

In operando X-ray absorption was used to measure the structure and chemical state of supported Pd nanoparticles with 3 -10 nm diameter in contact with H2 saturated water at 298-473 K. The Pd-Pd distances determined were consistent with the presence of subsurface hydrogen, i.e., longer than those measured by others for bare, reduced Pd particles, and within the range of distances for Pd hydrides. During the Pd-catalyzed hydrogenation of phenol, cyclohexanone, cyclohexanol or cyclohexene in the presence of water, the Pd nanoparticles exhibited a lengthening of the Pd-Pd bond that we attribute to a change in the concentration of sorbed H related to the steady state of H at the surface of the Pd particles. This steady state is established by all reactions involving H2, i.e., the sorption/desorption into the bulk, the sorption at the surface, and the reaction with adsorbed unsaturated reactants. Thus, first insight into the chemical state of Pd and the H/Pd ratio during catalysis in water is provided. The Pd particles did not change upon their exposure to water or reactants; nor did the spectra show any effect from the interaction of the Pd particles with various supports. The experimental results are consistent with ab initio molecular dynamic simulations, which indicate that Pd-water interactions are relatively weak for Pd metal and that these interactions become even weaker, when hydrogen is incorporated into the metal particles. This work was supported by the US Department of Energy (DOE), Office of Basic Energy Sciences (BES), Division of Chemical Sciences, Geosciences & Biosciences. Pacific Northwest National Laboratory is a multi-program national laboratory operated for DOE by Battelle through Contract DE-AC05-76RL01830.

hydrodeoxygenation↗
DOE OSTI2013

Pairing mechanism among ionic liquid ions in aqueous solutions. A molecular dynamics study

In this study, we carried out molecular dynamics simulations to examine the molecular mechanism for ionic liquid pair association in aqueous solutions. We chose the commonly studied imidazolium-based ionic liquid pairs. We computed potentials of mean force (PMF) for four systems—1,3-dimethlylimidazoliumchloride; 1,3-dimethlylimidazolium iodide; 1-methly-3-octylimidazolium chloride; and 1-methly-3-octylimidazolium iodide. Our PMF studies show a stronger interaction for the ion pairs of systems involving dimethlylimidazolium as the cation species compared to that of the systems containing octylimidazolium. This result indicates a decrease in ion-pair association as the cation alkyl tail length increases. We also studied the kinetics of ion-pair dissociation using different rate theories such as the Grote-Hynes and Kramer’s theories. As expected, the computed rate results significantly deviated from results obtained from transition state theory because it does not account for dynamical solvent effects. Dissociative barrier curvatures are found to be very small for the systems investigated because the transmission coefficients computed using Grote-Hynes theory and Kramer’s theory are approximately equal. Our analysis of the rotational dynamics of cations revealed that the time scales for molecular reorientation are longer for cations with longer alkyl tails. This work was supported by the U.S. Department of Energy (DOE), Office of Basic Energy Sciences (BES), Division of Chemical Sciences, Geosciences, and Biosciences. Pacific Northwest National Laboratory is a multiprogram national laboratory operated for DOE by Battelle. The calculations were carried out using computer resources provided by BES.

Annapureddy, Harsha V.↗
DOE OSTI2013

Charge and Electric Field Fluctuations in Aqueous NaCl Electrolytes

Conventional wisdom concerning crystallization assumes that when NaCl crystallizes from a supersaturated solution, the solvated ions retain their ionic character and simply relocate from their hydration spheres to their most stable positions in the crystal lattice. However, this conventional picture is at odds with observations over 200 years ago reporting the emission of long-lived light resulting from the crystallization of certain salts – appropriately referred to as crystalloluminescence. This suggests that electronic structure plays an essential role in crystallization. Strong electric field fluctuations in the gas or condensed phases can drive changes in electronic structure. We have calculated the fluctuation of charge, scalar electric potentials, and vector electric fields for concentrated aqueous NaCl electrolytes. The H2O molecules in the 1st solvation shell of the ions serve as a sink for electron density originating on Cl-. Our analysis reveals that the electric fields inside aqueous electrolytes are extremely large (up to several V/Å) and thus may alter the ground and excited electronic states in the condensed phase. Furthermore, our analysis shows that the potential and field distributions are largely independent of concentration. We find that the field component distributions to be Gaussian for the ions and non-Gaussian for the O and H sites (computed in the lab frame of reference), however, these non-Gaussian distributions are readily modeled via an orientationally averaged non-zero mean Gaussian plus a zero mean Gaussian. These calculations and analyses provide the first steps toward understanding the magnitude and fluctuations of charge, electric potentials and fields in aqueous electrolytes and what role these fields may play in driving charge redistribution/transfer during crystalloluminescence. We would like to gratefully acknowledge helpful discussions with Gregory K. Schenter. This work was supported by the U.S. Department of Energy’s (DOE) Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences program and used resources of the National Energy Research Scientific Computing Center (NERSC), which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. Pacific Northwest National Laboratory (PNNL) is operated by Battelle for the US Department of Energy.

charge redistribution↗