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At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

A Cobalt Hydride Catalyst for the Hydrogenation of CO2: Pathways for Catalysis and Deactivation

The complex Co(dmpe)₂H catalyzes the hydrogenation of CO₂ at one atm and 21 ºC with significant improvement in turnover frequency relative to previously reported second and third row transition metal complexes. New studies are presented to elucidate the catalytic mechanism as well as pathways for catalyst deactivation. The catalytic rate was optimized through the choice of the base to match the pK a of the [Co(dmpe)₂(H)₂]⁺ intermediate. By using a strong enough base, the catalytic rate has a zero order dependence on the concentration of base and pressure of hydrogen, and a first order dependence on the pressure of CO₂. However, upon exceeding CO₂:H₂ ratios of greater than one, the catalytically inactive species [(μ-dmpe)(Co(dmpe)₂)₂]²⁺ and [Co(dmpe)₂CO]⁺ are observed. Research by M.S.J., M.T.M., A.M.A., and J.C.L. was supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. Research by M.L.H. and E.B.H. 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. Pacific Northwest National Laboratory (PNNL) is a multiprogram national laboratory operated for the DOE by Battelle.

catalysis↗
DOE OSTI2015

Effect of Oxygen Defects on the Catalytic Performance of VOx/CeO2 Catalysts for Oxidative Dehydrogenation of Methanol

In this work, CeO2 nanocubes with controlled particle size and dominating (100) facets are synthesized as supports for VOx catalysts. Combined TEM, SEM, XRD, and Raman study reveals that the oxygen vacancy density of CeO2 supports can be tuned by tailoring the particle sizes without altering the dominating facets, where smaller particle sizes result in larger oxygen vacancy densities. At the same vanadium coverage, the VOx catalysts supported on small-sized CeO2 supports with higher oxygen defect densities exhibit promoted redox property and lower activation energy for methoxyl group decomposition, as evidenced by H2-TPR and methanol TPD study. These results further confirm that the presence of oxygen vacancies plays an important role in promoting the activity of VOx species in methanol oxidation. We gratefully acknowledge financial support from the U.S. Department of Energy (DOE), Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences. Part of this work was conducted in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by DOE’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is a multiprogram national laboratory operated for the DOE by Battelle.

(100)↗
DOE OSTI2015

Ground and excited states of the [Fe(H2O)6]2+ and [Fe(H2O)6]3+ clusters: Insight into the electronic structure of the [Fe(H2O)6]2+ – [Fe(H2O)6]3+ complex

We report the ground and low lying electronically excited states of the [Fe(H2O)6]2+ and [Fe(H2O)6]3+ clusters using multi-configuration electronic structure theory. In particular, we have constructed the Potential Energy Curves (PECs) with respect to the iron-oxygen distance when removing all water ligands at the same time from the cluster minima and established their correlation to the long range dissociation channels. Due to the fact that both the second and third ionization potentials of iron are larger than the one for water, the ground state products asymptotically correlate with dissociation channels that are repulsive in nature at large separations as they contain at least one H2O+ fragment and a positive metal center. The most stable equilibrium structures emanate – via intersections and/or avoided crossings – from the channels consisting of the lowest electronic states of Fe2+(5D; 3d6) or Fe3+(6S; 3d5) and six neutral water molecules. Upon hydration, the ground state of Fe2+(H2O)6 is a triply (5Tg) degenerate one with the doubly (5Eg) degenerate state lying slightly higher in energy. Similarly, Fe3+(H2O)6 has a ground state of 6Ag symmetry under Th symmetry. We furthermore examine a multitude of electronically excited states of many possible spin multiplicities, and report the optimized geometries for several selected states. The PECs for those cases are characterized by a high density of states. Focusing on the ground and the first few excited states of the [Fe(H2O)6]2+ and [Fe(H2O)6]3+ clusters, we studied their mutual interaction in the gas phase. We obtained the optimal geometries of the Fe2+(H2O)6 – Fe3+(H2O)6 gas phase complex for different Fe–Fe distances. For distances shorter than 6.0 Å, the water molecules in the respective first solvation shells located between the two metal centers were found to interact via weak hydrogen bonds. We examined a total of ten electronic states for this complex, including those corresponding to the electron transfer (ET) from the ferrous to the ferric ion. The ET process is discussed and a possible path via a quasi-symmetric transition state is suggested. 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. Pacific Northwest National Laboratory (PNNL) is a multiprogram national laboratory operated for DOE by Battelle. This research used resources of the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

Miliordos, Evangelos↗
DOE OSTI2015

Modeling the Kinetics of Deactivation of Catalysts during the Upgrading of Bio-Oil

The fouling of catalysts for the upgrading of bio-oils appears to be very different from the fouling of catalysts for the hydroprocessing of petroleum-derived streams. There are two reasons for the differences: a) bio-oil contains polarizable components and phases that can stabilize reaction intermediates exhibiting charge separation and b) bio-oil components contain functional groups that contain O, notably carbonyls (>C=O). Aldol condensation of carbonyls affords very different pathways for the production of oligomeric, refractory deposits than does dehydrogenation/polymerization of petroleum-derived hydrocarbons. Colloquially, we refer to the bio-oil derived deposits as “gunk” to discriminate them from coke, the carbonaceous deposits encountered in petroleum refining. Classical gelation, appears to be a suitable model for the “gunking” reaction. Our work has helped explain the temperature range at which bio-oil should be pre-processed (“stabilized”) to confer longer lifetimes on the catalysts used for more severe processing. Stochastic modeling (kinetic Monte Carlo simulations) appears suitable to capture the rates of oligomerization of bio-oil. This work was supported by the US Department of Energy, Office of Energy Efficiency and Renewable Energy, Bioenergy Technologies Office. Pacific Northwest National Laboratory (PNNL) is a multiprogram national laboratory operated for DOE by Battelle.

bio-oil upgrading↗
DOE OSTI2013

Structural and Spectroscopic Characterization of 17- and 18-Electron Piano-Stool Complexes of Chromium. Thermochemical Analyses of Weak Cr–H Bonds

The 17-electron radical CpCr(CO)2(IMe)• (IMe = 1,3-dimethylimidazol-2-ylidene) was synthesized by the reaction of IMe with [CpCr(CO)3]2, and characterized by single crystal X-ray diffraction and by EPR, IR and variable temperature 1H NMR spectroscopy. The metal-centered radical is monomeric under all conditions and exhibits Curie paramagnetic behavior in solution. An electrochemically reversible reduction to 18-electron CpCr(CO)2(IMe)- takes place at E½ = -1.89(1) V vs Cp2Fe+•/0 in MeCN, and was accomplished chemically with KC8 in THF. The salts K+(18-crown-6)[CpCr(CO)2(IMe)]- • ½THF and K+[CpCr(CO)2(IMe)]- • ¾THF were crystallographically characterized. Monomeric ion pairs are found in the former, whereas the latter has a polymeric structure due to a network of K∙∙∙O(CO) interactions. Protonation of K+(18-crown-6)[CpCr(CO)2(IMe)]- • ½THF gives the hydride CpCr(CO)2(IMe)H, which could not be isolated, but was characterized in solution; a pKa of 27.2(4) was determined in MeCN. A thermochemical analysis provides the Cr-H bond dissociation free energy (BDFE) for CpCr(CO)2(IMe)H in MeCN solution as 47.3(6) kcal mol-1. This value is exceptionally low for a transition metal hydride, and implies that the reaction 2 [Cr-H] → 2 [Cr•] + H2 is exergonic (ΔG = -9.0(8) kcal mol-1). This analysis explains the experimental observation that generated solutions of the hydride produce CpCr(CO)2(IMe)• (typically on the timescale of days). By contrast, CpCr(CO)2(PCy3)H has a higher Cr-H BDFE (52.9(4) kcal mol-1), is more stable with respect to H2 loss, and is isolable. We thank the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences for support. M.L.H. carried out the crystallographic studies and 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. The EPR studies were performed at the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research located at PNNL. Pacific Northwest National Laboratory is a multiprogram national laboratory operated by Battelle for the U.S. Department of Energy.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

A Cobalt-based Catalyst for CO2 Hydrogenation Under Ambient Conditions

Due to the continually rising levels of CO2 in the atmosphere, research into conversion of CO2 into fuels using carbon-neutral energy is currently an important topic in catalysis. Recent research on molecular catalysts has led to improved rates of CO2 conversion to formate, but unfortunately the resulting catalysts are based on precious metals such as iridium, ruthenium and rhodium and require high temperatures and high pressures for catalytic reactivity. Using established thermodynamic properties, a cobalt-based catalyst system has been designed for the catalytic production of formate from CO2 and H2, even at room temperature and one atmosphere of pressure. Using Co(dmpe)2H (dmpe is bis(dimethylphosphino)ethane) as a catalyst in tetrahydrofuran, room temperature turnover frequencies of 3,400 h-1 at 1 atm of 1:1 CO2:H2 and 74,000 h-1 at 20 atm were obtained. These results highlight the value of basic thermodynamic properties in the rational design of catalysts. 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.

abundant metals↗
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 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 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 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 OSTI2012

Importance of Diffusion in Methanol Photochemistry on TiO2(110)

The photoactivity of methanol on the rutile TiO2(110) surface is shown to depend on the ability of methanol to diffuse on the surface and find sites active for its thermal dissociation to methoxy. Temperature programmed desorption (TPD) results show that the extent of methanol photodecomposition to formaldehyde is negligible on the clean TiO2(110) surface at 100 K due to a scarcity of sites that can convert (photoinactive) methanol to (photoactive) methoxy. The extent of photoactivity at 100 K significantly increases when methanol is coadsorbed with oxygen, however only those molecules able to adsorb near (next to) a coadsorbed oxygen species are active. Preannealing coadsorbed methanol and oxygen to above 200 K prior to UV irradiation results in a significant increase in photoactivity. Scanning tunneling microscopy (STM) images clearly show that the advent of increased photoactivity in TPD correlates with the onset of methanol diffusion along the surface’s Ti4+ rows at ~200 K. These results demonstrate that optimizing thermal processes (such as diffusion or proton transfer reactions) can be critical to maximizing photocatalytic reactivity on TiO2 surfaces. 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 under contract DEAC05-76RL01830. The research was performed using EMSL, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory.

diffusion↗
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

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 OSTI2014

Computational Studies of [Bmim][PF6]/n-Alcohol Interfaces with Many-Body Potentials

In this paper, we present the results from molecular-dynamics simulations of the equilibrium properties of liquid/liquid interfaces of room temperature ionic liquid [bmim][PF6] and simple alcohols (i.e., methanol, 1-butanol, and 1-hexanol) at room temperature. Polarizable potential models are employed to describe the interactions among species. Results from our simulations show stable interfaces between the ionic liquid and n-alcohols, and we found that the interfacial widths decrease from methanol to 1-butanol systems, and then increase for 1-hexanol interfaces. Angular distribution analysis reveals that the interface induces a strong orientational order of [bmim] and n-alcohol molecules near the interface, with [bmim] extending its butyl group into the alcohol phase while the alcohol has the OH group pointing into the ion liquid region, which is consistent with the recent sum-frequency-generation experiments. We found the interface to have a significant influence on the dynamics of ionic liquids and n-alcohols. The orientational autocorrelation functions illustrate that [bmim] rotate more freely near the interface than in the bulk, while the rotation of n-alcohol is hindered at the interface. Additionally, the time scale associated with the diffusion along the interfacial direction is found to be faster for [bmim] but slowed down for n-alcohols approaching the interface. We also calculate the dipole moment of n-alcohols as a function of the distance normal to the interface. We found that, even though methanol and 1-butanol have different dipole moments in bulk phase, they reach a similar value at the interface. This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences. Pacific Northwest National Laboratory is a multiprogram national laboratory operated for the Department of Energy by Battelle. The calculations were carried out using computer resources provided by the Office of Basic Energy Sciences.

Chang, Tsun-Mei↗
DOE OSTI2013

Comparison of the Photodesorption Activities of cis-Butene, trans-Butene and Isobutene on the Rutile TiO 2 (110) Surface

The chemical and photochemical properties of three butene molecules (cis-butene, trans-butene and isobutene) were explored on the clean rutile TiO 2 (110) surface using temperature programmed desorption (TPD) and photon simulated desorption (PSD). At the low coverage limit, trans-butene was the most strongly bound butene on the TiO 2 (110) surface, desorbing at ~ 210 K, however increased intermolecular repulsions between trans-butene molecules at higher coverage diminished its binding. Both cis-butene and isobutene saturated the first layer on TiO 2 (110) at a coverage of ~0.50 ML in a single TPD feature at 184 and 192 K, respectively. In contrast, the maximum coverage that trans-butene could achieve in its 210 K peak was ~1/3 ML, with higher coverages resulting a low temperature desorption at ~137 K. Coverages of these molecules above 0.50 ML resulted in population of second layer and multilayer states. The instability of trans-butene at a coverage of 0.5 ML on the surface was linked to the inversion center in its symmetry. In the absence of coadsorbed oxygen, the primary photochemical pathway of each butene molecule on TiO 2 (110) was photodesorption. The photoactivities of these molecules on TiO 2 (110) at an initial coverage of 0.50 ML followed the trend: isobutene > cis-butene > trans-butene. In contrast, the photoactivities of low coverages of cis-butene and trans-butene exceeded those measured at 0.50 ML. These data suggest that intermolecular interactions (repulsions) play a significant role in diminishing the photoactivities of weakly bound molecules on TiO 2 photocatalysts. 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.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Experimental and Theoretical Study of Molecular Response of Amine Bases in Organic Solvents

Reorientational correlation times of various amine bases (viz., pyridine, 2,6-lutidene, 2,2,6,6-tetramethylpiperidine) and organic solvents (dichloromethane, toluene) were determined by solution-state NMR relaxation time measurements and compared with predictions from molecular dynamics (MD) simulations. The bases and solvents are reagents in complex reactions involving Frustrated Lewis Pairs (FLP), which display remarkable catalytic activity in metal-free H2 scission. The comparison of measured and simulated correlation times is a key test of the ability of recent MD and quantum electronic structure calculations to elucidate the mechanism of FLP activity. Correla- tion times were found to be in the range 1.4-3.4 ps (NMR) and 1.23-5.28 ps (MD) for the amines, and 0.9-2.3 ps (NMR) and 0.2-1.7 ps (MD) for the solvent molecules. This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences. Pacic Northwest National Laboratory (PNNL) is a multiprogram national laboratory operated for DOE by Battelle.

Frustrated Lewis pairs↗