Evidence for Electron Transfer between Graphene and Non-Covalently Bound π-Systems.

Hybridizing graphene and molecules possess a high potential for developing materials for new applications. However, new methods to characterize such hybrids must be developed. Herein, the wet-chemical non-covalent functionalization of graphene with cationic π-systems is presented and the interaction between graphene and the molecules is characterized in detail. A series of tricationic benzimidazolium salts with various steric demand and counterions was synthesized, characterized and used for the fabrication of graphene hybrids. Subsequently, the doping effects were studied. The molecules are adsorbed onto graphene and studied by Raman spectroscopy, XPS as well as ToF-SIMS. The charged π-systems show a p-doping effect on the underlying graphene. Consequently, the tricationic molecules are reduced through a partial electron transfer process from graphene, a process which is accompanied by the loss of counterions. DFT calculations support this hypothesis and the strong p-doping could be confirmed in fabricated monolayer graphene/hybrid FET devices. The results are the basis to develop sensor applications, which are based on analyte/molecule interactions and effects on doping.

Transition metal attenuated mechanism for protective alumina formation from first principles.

A mechanistic perspective on the growth of protective oxides on high temperature alloys at elevated temperatures is provided. Early, defect rich transient alumina is understood to form by outwards diffusion of oxygen vacancies and electrons. The impact of transition metal (TM) ions (Sc, Ti, V, Cr, Mn, Fe, Co, Ni) on the oxygen vacancy diffusion and electron transport in α-alumina was studied by employing density functional theory. Activation energies for electron transfer E A(ET) between oxygen vacancies in pure as well as TM doped α-alumina were subject to analysis, and similarly so for the TM and charge dependent activation energy for oxygen vacancy diffusion E A(V O). E A Q (ET) were found to be ∼0.5 eV while 2 eV < E A Q (V O) < 5 eV was obtained. The higher and lower E A Q (V O) values correspond to uncharged and doubly charged V O sites, respectively. Redox processes among V O sites, addressed by a bipolaron approach, were understood to enhance V O mobility and thus to facilitate oxide growth. TM adatoms induced asymmetry in the potential energy surface for oxygen vacancy diffusion was subject to analysis. Competition for electrons between all-Al3+surrounded oxygen vacancies and vacancies adjacent to the late 3d adatoms comes out in favor of the latter. A novel take on the 3rd element effect in FeCrAl emerges from analysis of the ternary TM-TM*-Al system.

Communication: Towards catalytic nitric oxide reduction via oligomerization on boron doped graphene.

We use density functional theory to describe a novel way for metal free catalytic reduction of nitric oxide NO utilizing borondopedgraphene. The present study is based on the observation that borondopedgraphene and O-N=N-O(-) act as Lewis acid-base pair allowing the graphene surface to act as a catalyst. The process implies electron assisted N=N bond formation prior to N-O dissociation. Two N2 + O2 product channels, one of which favoring N2O formation, are envisaged as outcome of the catalytic process. Besides, we show also that the N2 + O2formation pathways are contrasted by a side reaction that brings to N3O3 (-)formation and decomposition into N2O + NO2 (-).

The bifurcation point of the oxygen reduction reaction on Au-Pd nanoalloys.

The oxygen reduction reaction is of major importance in energy conversion and storage. Controlling electrocatalytic activity and its selectivity remains a challenge of modern electrochemistry. Here, first principles calculations and analysis of experimental data unravel the mechanism of this reaction on Au-Pd nanoalloys in acid media. A mechanistic model is proposed from comparison of the electrocatalysis of oxygen and hydrogen peroxide reduction on different Au-Pd ensembles. A H2O production channel on contiguous Pd sites proceeding through intermediates different from H2O2 and OOH(σ) adsorbate is identified as the bifurcation point for the two reaction pathway alternatives to yield either H2O or H2O2. H2O2 is a leaving group, albeit reduction of H2O2 to H2O can occur by electrocatalytic HO-OH dissociation that is affected by the presence of adsorbed OOH(σ). Similarities and differences between electrochemical and direct synthesis from H2 + O2 reaction on Au-Pd nanoalloys are discussed.

Confinement dependence of electro-catalysts for hydrogen evolution from water splitting.

Density functional theory is utilized to articulate a particular generic deconstruction of the electrode/electro-catalyst assembly for the cathode process during water splitting. A computational model was designed to determine how alloying elements control the fraction of H2 released during zirconium oxidation by water relative to the amount of hydrogen picked up by the corroding alloy. This model is utilized to determine the efficiencies of transition metals decorated with hydroxide interfaces in facilitating the electro-catalytic hydrogen evolution reaction. A computational strategy is developed to select an electro-catalyst for hydrogen evolution (HE), where the choice of a transition metal catalyst is guided by the confining environment. The latter may be recast into a nominal pressure experienced by the evolving H2 molecule. We arrived at a novel perspective on the uniqueness of oxide supported atomic Pt as a HE catalyst under ambient conditions.

Single atom hot-spots at Au-Pd nanoalloys for electrocatalytic H2O2 production.

A novel strategy to direct the oxygen reduction reaction to preferentially produce H(2)O(2) is formulated and evaluated. The approach combines the inertness of Au nanoparticles toward oxidation, with the improved O(2) sticking probability of isolated transition metal "guest" atoms embedded in the Au "host". DFT modeling was employed to screen for the best alloy candidates. Modeling indicates that isolated alloying atoms of Pd, Pt, or Rh placed within the Au surface should enhance the H(2)O(2) production relative to pure Au. Consequently, Au(1-x)Pd(x) nanoalloys with variable Pd content supported on Vulcan XC-72 were prepared to investigate the predicted selectivity toward H(2)O(2) production for Au alloyed with Pd. It is demonstrated that increasing the Pd concentration to 8% leads to an increase of the electrocatalytic H(2)O(2) production selectivity up to nearly 95%, when the nanoparticles are placed in an environment compatible with that of a proton exchange membrane. Further increase of Pd content leads to a drop in H(2)O(2) selectivity, to below 10% for x = 0.5. It is proposed that the enhancement in H(2)O(2) selectivity is caused by the presence of individual surface Pd atoms surrounded by gold, whereas surface ensembles of contiguous Pd atoms support H(2)O formation. The results are discussed in the context of exergonic electrocatalytic H(2)O(2) synthesis in Polymer Electrolyte Fuel Cells for the simultaneous cogeneration of chemicals and electricity, the latter a credit to production costs.

Switching on the electrocatalytic ethene epoxidation on nanocrystalline RuO2.

Ruthenium-based oxides with rutile structure were examined regarding their properties in electrocatalytic ethene oxidation in acid media. A possible promoting effect of chloride ions toward oxirane formation was explored. Online differential electrochemical mass spectrometry combined with electrochemical polarization techniques were used to monitor the potential dependence of organic products resulting from ethene oxidation as well as the reaction solution decomposition products. Quantum chemical modeling by means of density functional theory was employed to study key reaction steps. The ethene oxidation in acid media led to CO(2), whereas oxirane was formed in the presence of 0.3 M Cl(-). In the Cl(-) promoted oxidation on RuO(2), oxirane and a small amount of CO(2) were the only detected electro-oxidation products at potentials below the onset of Cl(2) and O(2) evolution, resulting from Cl(-) and water oxidation. It is demonstrated here that the epoxidation is a surface-related electrocatalytic process that depends on the surface properties. Cl acts as the epoxidation promoter that switches off the combustion pathway toward CO(2) and enables the epoxidation reaction channel by surface reactive sites blocking. The proposed epoxidation mechanism implies binuclear (recombination) mechanism for O(2) evolution reaction on considered surfaces.

Quantum chemical modeling of propene and butene epoxidation with hydrogen peroxide.

The mechanism of hydrogen peroxide assisted epoxidation of propene, 1-butene, trans-2-butene, cis-2-butene, and isobutene was studied using density functional theory calculations. The results are rationalized in the context of the previously proposed direct pathway for epoxidation of ethene with hydrogen peroxide and compared to the indirect pathway involving Ti(IV) peroxide groups. The indirect Ti(IV) peroxide pathway displays a 57.8 kJ mol(-1) activation enthalpy for the rate limiting step [Phys. Chem. Chem. Phys. 2007, 9, 5997]. In contrast, a lowering of the activation enthalpy is observed for the direct mechanism according to 72.3 (ethene), 53.9 (1-butene), 53.5 (propene), 46.9 (trans-2-butene), 46.6 (isobutene), and 42.6 (cis-2-butene) kJ mol(-1) when the reaction takes place on a binuclear Ti(IV) dihydroxide site. These values clearly show that the direct pathway becomes the most favorable. The stability of the epoxides toward hydrolysis to the corresponding diols are also addressed. The present work clearly demonstrates the generality and efficiency of a binuclear dihydroxide site in catalyzing the epoxidation of olefins with hydrogen peroxide, thus avoiding the formation of a surface peroxide group.

Chemical-to-mechanical energy conversion in biomacromolecular machines: a plasmon and optimum control theory for directional work. 1. General considerations.

To rationalize coherence and mechanochemical aspects of proteins acting as molecular machines, a plasmon concept for dealing with protein nonequilibrium dynamics is introduced and tested with respect to thermodynamic consistency. A stochastic optimum-control theory for protein conformational diffusion is developed and the corresponding stochastic Newton's second law derived for optimum-controlled conformational diffusion in proteins. The plasmon concept is shown to be consistent with this theory, in that optical plasmons can pump entropy out of (or into) the protein, decreasing (or increasing) its conformational diffusion and, at the same time, help decrease intra- and intermolecular friction, as well as (potentially) break the symmetry of the latter. Instead, acoustic plasmons may break the spatial symmetry of a protein's "potential of mean force", thus converting it into an effective Brownian ratchet potential by applying quasistatic deformational corrections to the former. These concepts seem to be of rather general applicability and might also be useful when studying, for example, intercalation of cationic dyes into DNA duplexes, positively charged oligopeptide transduction through cell membranes, or even DNA translocation through nanopores.

NO(2) dissociation on Ag(111) revisited by theory.

NO(2) dissociation on Ag(111) is investigated with first-principles calculations. For single NO(2) molecules, a high adsorption potential energy is found to prohibit dissociation. This result is surprising as experiments indicate dissociation at low temperatures. Neither entropy effects nor irregularities in the potential energy surface can remedy the discrepancy. Instead it is proposed that collective Eley-Rideal type of reaction mechanisms can drive the dissociation.

Quantum chemical modelling of ethene epoxidation with hydrogen peroxide: role of catalytic sites.

Ethene epoxidation with hydrogen peroxide was studied on hydroxylated binuclear metal sites, using DFT calculations at the B3LYP/6-311+G(d,p) level of theory. A decrease of the activation enthalpy of approximately 100 kJ mol(-1) was observed compared to the gas phase reaction between hydrogen peroxide and ethene. It was previously shown that micro-solvation with water reduces the activation enthalpy by approximately 77 kJ mol(-1) and only the additional 24 kJ mol(-1) can be attributed to the binuclear site. Three different metal centres were tested, Ti(iv), Si(iv) and Ge(iv), in order to investigate any specific role of the metal centre on the activation enthalpy. The results clearly show that the activation enthalpy is independent on the nature of the metal centre. This emphasises the role of the hydrogen bonded network provided by the hydroxylated metal sites, on the stabilisation of the transitions state. In ref. 1 (A. Lundin, I. Panas and E. Ahlberg, J. Phys. Chem. A, 2007, 111, 9080) it was demonstrated that, at the transition state and upon micro-solvation, the hydrogen peroxide entity becomes polarized within the hydrogen bonding network, forming a negatively-charged fragment distant from the ethene molecule and a positively-charged fragment directly involved in the oxygen insertion step. The same mechanism was found to hold also for the reaction at the binuclear catalytic site, since the required hydrogen bonding is effectively provided by the hydroxylated metal centres. This mechanism is compared to the two-step pathway which employs a metal peroxide intermediate. Both reaction channels were found to be plausible in confined environments.

Determination of the Cd-bearing phases in municipal solid waste and biomass single fly ash particles using SR-microXRF spectroscopy.

By using an excitation energy of 27.0 keV, synchrotron radiation-induced micro-X-ray fluorescence (SR-microXRF) is employed to extract information regarding the composition and distribution of Cd-bearing phases in municipal solid waste (MSW) and biomass fly ashes. Significance of observation is based on statistics of totally more than 100 individual MSW and biomass fly ash particles from a fluidized bed combustion (FBC) plant. Cd concentrations in the parts-per-million range are determined. In general, although previous leaching studies have indicated Cd to be predominant in the smaller-size ash particles, in the present study Cd is more evenly distributed throughout all the particle sizes. For MSW fly ashes, results indicate the presence of Cd mainly as CdBr2 hot-spots, whereas for biomass fly ashes, which exhibit lower CdX2 concentration, a thin Cd layer on/in the particles is reported. For both ashes, Ca-containing matrixes are found to be the main Cd-bearing phases. Support for this observation is found from independent first-principles periodic density functional theory calculations. The observations are condensed into a schematic mechanism for Cd adsorption on the fly ash particles.

Quantum chemical modeling of ethene epoxidation with hydrogen peroxide: the effect of microsolvation with water.

Quantum chemical calculations were performed to study the mechanism of ethene epoxidation with hydrogen peroxide. The calculations were carried out at the B3LYP/6-311+G(d,p) level of theory. The applicability of this functional to the problem at hand, including basis set effects, was validated by CCSD(T) and CASSCF based multireference MP2 calculations. A mechanism was determined where hydrogen peroxide becomes polarized in the transition state upon binding to the ethene molecule. The distant hydroxide fragment of the attached hydrogen peroxide molecule becomes partly negatively charged, while the other part of the molecule involves a proton and becomes partly positively charged. In the absence of water an activation energy of 139.7 kJ mol(-1) was determined for the isolated H(2)O(2) + C(2)H(4) system. By microsolvating with water, the impact of a hydrogen-bonded network on the activation energy was addressed. A 43.7 kJ mol(-1) lowering of the activation energy, DeltaE(a), was observed when including up to 4 water molecules in the model. This effect results from the stabilization of the proton and hydroxide fragments in the transition state. The findings are discussed in the context of previous theoretical studies on similar systems. Effects of adding or removing a proton to mimic acidic and alkaline conditions are addressed and the limitations of the model in solvating the excess charge are discussed.

A mechanistic investigation of ethylene oxide hydrolysis to ethanediol.

The B3LYP/6-311+G(d,p) description is employed to study the heterolytic ring opening mechanisms under microsolvation conditions for ethylene oxide in acidic, neutral, and alkaline environments. In acid and alkaline media, a concerted trans S(N)2 reaction is strongly favored as compared to the corresponding cis reaction. The importance of the nucleophile, water in acidic media and hydroxide ion in alkaline media, for lowering the activation enthalpy is emphasized and activation energies of approximately 80 and approximately 60 kJ mol(-1) are obtained under acid and alkaline conditions, respectively. Under neutral conditions, the trans S(N)2 mechanism becomes inaccessible because it invokes the formation of a transient H+ and OH- pair across the 1,2-ethanediol molecule. Rather, epoxide ring opening is achieved by hydrolysis of a single water molecule. The latter mechanism displays significantly greater activation enthalpy (205 kJ mol(-1)) than those in acid and alkaline environments. This is in agreement with experiment. Product distributions of simple olefins in neutral aqueous media, as well as the detrimental impact of acid/base conditions for the selectivity of epoxidation catalysts in aqueous media, are discussed.

Theoretical investigation of linalool oxidation.

This study concerns the autoxidation of one of the most used fragrances in daily life, linalool (3,7-dimethyl-1,6-octadien-3-ol). It reacts with O2 to form hydroperoxides, which are known to be important contact allergens. Pathways for hydroperoxide formation are investigated by means of quantum mechanical electronic structure calculations. Optimized molecular geometries and harmonic vibrational frequencies are determined using density functional theory (DFT). Insight into how the addition of O2 to linalool occurs is obtained by establishing a theoretical framework and systematically investigating three smaller systems: propene, 2-methyl-2-butene, and 2-methyl-2-pentene. 2-Methyl-2-pentene was chosen as a model system and used to compare with linalool. This theoretical study characterizes the linalool-O2 biradical intermediate state, which constitutes a branching point for the further oxidation reactions pathways. Thus, the observed linalool oxidation product spectrum is discussed in terms of a direct reaction path, the ene-type mechanism, and the radical mechanism. The major hydroperoxide found in experiments is 7-hydroperoxy-3,7-dimethyl-octa-1,5-diene-3-ol, and the calculated results support this finding.

Quantum chemical modelling of the rate determining step for oxygen reduction on quinones.

Two inner-sphere electrocatalytic channels for quinone-mediated reduction of molecular oxygen to form hydrogen peroxide have been addressed by means of density functional theory. Each of the channels comprises an initial rate determining chemical step and a subsequent electrochemical reduction step by which peroxide is produced. The reduction mechanism was determined for 9,10-anthraquinone and 9,10-phenanthrenequinone and the quantum chemical results are compared with experimental results. Two distinctly different structures were determined for the critical chemical step depending on whether the catalytic site is present as HQ* or Q*-. While a superoxo species is formed on HQ*, a van der Waals (vdW) type compound is formed on Q*-. It is shown that the Gibbs energy of activation for the semiquinone/oxygen reaction is largely determined by the entropy term. The results explain the experimentally observed pH dependence of the O2 reduction rate on quinone functionalised electrodes.

Quantum chemical modeling of the reduction of quinones.

A systematic study of the redox properties of six parent quinones has been carried out using quantum chemistry methods. The reduction of the ortho (o-) and para (p-) isomers of benzoquinone and naphthoquinone, 9,10- anthraquinone and 9,10-phenantrenequinone to the corresponding hydroquinones and semiquinone radicals was investigated at the B3LYP/6-311+G(d,p) level of theory. Thermodynamic functions in the gas-phase were calculated for all the reduction reactions. Gibbs energies of reaction and standard potentials in water for the reductions were determined using the IEF-PCM model and an empirical correction to the calculations based on the limited thermodynamic data available for the quinones. Potentials were calculated both for the direct reduction to the quinols, and for the two-step reduction via the neutral semiquinones. The calculated potentials for the 2e-, 2H+ reductions were found in good agreement with experiment and to display the same trends as gas-phase enthalpies and energies, i.e., to correlate with the number of C=C double bonds, as well as on the relative position of the C=O groups. The small deviations between experiment and theoretically predicted standard potentials were found to originate from basis set incompleteness and the shortcomings in the B3LYP exchange correlation functional rather than the models used for the thermochemical calculations or description of solvation. Accurate theoretical shifts in standard potentials for the p-/o- pairs of Q <--> HQ and HQ <--> H2Q reactions are presented and compared to experiment. Reliable standard potentials and shifts for the neutral semiquinones are predicted for the first time.

Toward a realistic description of NO(x) storage in BaO: the aspect of BaCO3.

NO(x) storage over hexagonal BaCO3(110) is investigated using first-principles calculations. Special focus is put on the importance of surface decarbonation. Upon decarbonation, supported BaO quasi-molecules are formed and a small drive toward (BaO)n cluster formation is predicted. Introduction of NO2 makes the decarbonation energetically relevant, while forming NO2-BaO-NO2 units, on the decarbonated surface. With this configuration, it is possible to replace all surface carbonates with nitrites and nitrates, forming a BaCO3 supported BaNO3NO2 overlayer. Thermodynamic considerations are employed to elaborate on the thermal stability of the formed NO(x) overlayers.