Showing papers in "Physical Chemistry Chemical Physics in 2002"

Reaction mechanism of soot formation in flames

Abstract: Chemical reactions and physical processes responsible for the formation of polycyclic aromatic hydrocarbons and soot in hydrocarbon flames are reviewed. The discussion is focused on major elements in the present understanding of the phenomena, clarification of concepts central to the present state of the art, and a summary of new results.

A fully direct RI-HF algorithm: Implementation, optimised auxiliary basis sets, demonstration of accuracy and efficiency

Abstract: A direct HF algorithm using the resolution of identity for Coulomb and exchange integrals (RI-HF) was implemented within the program system TURBOMOLE. A variational procedure for the optimisation of auxiliary functions is presented as well as optimised auxiliary basis sets for large basis sets up to Br. The accuracy of RI-HF energies and of MP2 energies based on RI-HF wave functions is demonstrated for a large test set of molecules. Accuracy of first order properties is documented for selected cases. The size dependency of the RI errors and the efficiency of the method are investigated for closo-boranes [BnHn]2− (n = 4–12).

The emission spectrum and the radiative lifetime of Eu3+ in luminescent lanthanide complexes

Abstract: Although luminescent complexes of lanthanide ions and organic ligands have been studied intensively, relatively little attention has been paid to the natural (or ‘radiative’) lifetime of the lanthanide centered luminescent state in these systems. Here, the applicability of the well-known Judd–Ofelt theory to the emissive properties of Eu3+ complexes is investigated. Moreover, it is demonstrated experimentally that the radiative lifetime of the 5D0 excited state of Eu3+ can be calculated directly from its corrected emission spectrum, without using Judd–Ofelt theory. We also discuss briefly the possibility of finding the natural lifetimes of lanthanide ions other than Eu3+.

Excited-state hydrogen detachment and hydrogen transfer driven by repulsive 1πσ* states: A new paradigm for nonradiative decay in aromatic biomolecules

Abstract: The combined results of ab initio electronic-structure calculations and spectroscopic investigations of jet-cooled molecules and clusters provide strong evidence of a surprisingly simple and general mechanistic picture of the nonradiative decay of biomolecules such as nucleic bases and aromatic amino acids. The key role in this picture is played by excited singlet states of πσ* character, which have repulsive potential-energy functions with respect to the stretching of OH or NH bonds. The 1πσ* potential-energy functions intersect not only the bound potential-energy functions of the 1ππ* excited states, but also that of the electronic ground state. Via predissociation of the 1ππ* states and a conical intersection with the ground state, the 1πσ* states trigger an ultrafast internal-conversion process, which is essential for the photostability of biomolecules. In protic solvents, the 1πσ* states promote a hydrogen-transfer process from the chromophore to the solvent. Calculations for chromophore–water clusters have shown that a spontaneous charge-separation process takes place in the solvent shell, yielding a microsolvated hydronium cation and a microsolvated electron. These results suggest that the basic mechanisms of the complex photochemistry of biomolecules in liquid water can be revealed by experimental and theoretical investigations of relatively small chromophore–water clusters.

Raman spectroscopy of molybdenum oxides

Abstract: A special sample was prepared by controlled oxidation of MoO2, which contained MoO2, Mo4O11 and MoO3, in order to extend the knowledge about the resonance Raman effect in reduced molybdenum oxides from those close to MoO3 to those close to MoO2. This knowledge is of paramount importance because technical partial oxidation catalysts often contain intermediate Mo oxides of the Magneli type, e.g. Mo4O11, or Mo5O14. The Raman spectra of orthorhombic Mo4O11 and MoO2 have been identified in a Raman microspectroscopic image of 100 single spectra recorded of a mixture of MoO3, MoO2 and Mo4O11. A resonance Raman effect was proven to be responsible for the detection of the molybdenum oxide phases Mo4O11 and MoO2 in dilution with BN when excited at a laser wavelength of 632.8 nm by comparison with Raman microspectroscopic images of the identical sample when excited at 532 nm. The resonance Raman detection of reduced molybdenum oxide phases is discussed in the above mentioned context of their active role in catalytic partial oxidation reactions.

Formation and consumption of single-ring aromatic hydrocarbons and their precursors in premixed acetylene, ethylene and benzene flames

Abstract: Kinetic modeling is becoming a powerful tool for the quantitative description of combustion processes covering different fuels and large ranges of temperature, pressure and equivalence ratio. In the present work, a reaction mechanism which was developed initially for benzene oxidation, and included the formation of polycyclic aromatic hydrocarbons was extended and tested for the combustion of acetylene and ethylene. Thermodynamic and kinetic property data were updated. If available, data were taken from the recent literature. In addition, density functional theory as well as ab initio computations on a CBS-Q and CBS-RAD level, partially published previously, were carried out. Quantum Rice–Ramsperger–Kassel analysis was conducted in order to determine pressure-dependent rate constants of chemically activated reactions. The model was developed and tested using species concentration profiles reported in the literature from molecular beam mass-spectrometry measurements in four unidimensional laminar premixed low-pressure ethylene, acetylene and benzene flames at equivalence ratios (ϕ) of 0.75 and 1.9 (C2H4), 2.4 (C2H2) and 1.8 (C6H6). Predictive capabilities of the model were found to be at least fair and often good to excellent for the consumption of the reactants, the formation of the main combustion products as well as the formation and depletion of major intermediates including radicals. Self-combination of propargyl (C3H3) followed by ring closure and rearrangement was the dominant benzene formation pathway in both rich acetylene and ethylene flames. In addition, reaction between vinylacetylene (C4H4) and vinyl radical (C2H3) contributed to benzene formation in the ϕ = 1.9 ethylene flame. Propargyl formation and consumption pathways which involve reactions between acetylene, allene, propyne and singlet and triplet methylene were assessed. Significant overpredictions of phenoxy radicals indicate the necessity of further investigation of the pressure and temperature dependence and the product distribution of phenyl oxidation. The possible formation of benzoquinones, the ratio of the ortho and para isomers and their degradation pathways are of particular interest.

Molecular properties in solution described with a continuum solvation model

Abstract: The study of molecular systems in a condensed phase with quantum-mechanical tools introduces problems not present in analogous studies on isolated molecules. Some of these problems are presented in the introductory part to justify a strategy to elaborate a theoretical model starting from an accurate continuum description of the solvent. This is here achieved through the polarizable continuum model (PCM), which also permits explicit consideration of solvent molecules. The decision of relying on the description of a large number of properties to refine the model is then justified. The consideration of complex molecular properties of a various nature has led to the elaboration of numerous additional features not present in the basic version of PCM, some among which are here briefly presented: local field and nonequilibrium effects, specific solute–solute and solute–solvent interactions, description of condensed phase with more complex structure. The paper ends with the presentation of some results, selected among a larger set of available examples, and limited to a few properties, to show the potentialities of the approach.

Stable solvates in solution of lithium bis(trifluoromethylsulfone)imide in glymes and other aprotic solvents: Phase diagrams, crystallography and Raman spectroscopy

Abstract: Lithium bis(trifluoromethylsulfone)imide (LiTFSI), a promising electrolyte for high energy lithium batteries, forms several stable solvates having low melting points in aprotic solvents. In a previous study (D. Brouillette, G. Perron and J. E. Desnoyers, J. Solution Chem., 1998, 27, 151), it was suggested, based on thermodynamic studies, that such stable solvates may persist in solution and influence their properties. To verify this hypothesis, phase diagrams and Raman spectra have been measured for solutions of LiTFSI in acetonitrile, propylene carbonate and glymes (n(ethyleneglycol) dimethyl ether or Gn), which have the chemical structure CH3–O–(CH2–CH2–O)n–CH3 for n = 1 to 4 and 10. The relative intensities of the LiTFSI and solvent Raman bands are proportional to the concentration for systems without solvates. The systems for which stable solvates were identified in the phase diagram show important changes in the relative intensities for both the LiTFSI and the solvent Raman bands at concentrations corresponding to particular stoichiometries and support the conclusion that stable solvates are present in the solutions. The structure of the crystalline G1:LiTFSI solvate was determined by X-ray crystallography. Structures for (G2)2:LiTFSI and (G1)3:LiTFSI solvates are proposed.

On the kinetics of hydrocarbons oxidation from natural gas to kerosene and diesel fuel

Abstract: Kinetic reaction mechanisms are necessary for modeling the combustion, oxidation and ignition of commercial fuels consisting of complex mixtures of hydrocarbons. Since they are generally too complex to be considered in the models directly, simple model-fuels are preferred. These model-fuels consist in a simple mixture of hydrocarbons for which kinetic oxidation models are validated. The oxidation of a large variety of hydrocarbons was studied experimentally in a jet-stirred reactor to build the needed kinetic reaction mechanisms. These detailed kinetic reaction mechanisms were assembled to model the oxidation of commercial fuels. The capabilities of these kinetic models to simulate the oxidation of natural gas, kerosene and gas oil are presented together with needs for new kinetic measurements.

Phase behaviour of room temperature ionic liquid solutions: an unusually large co-solvent effect in (water + ethanol)

Abstract: A surprising mixed solvent effect, both in its magnitude and direction, has been found in the phase diagram of the ternary mixture of ([C4mim][PF6]+(water+ethanol)) For a molar ratio of 1∶1 of water to ethanol, the co-solvent effect in the near-critical demixing temperature can be as large as 80 K

Modulation of methylene blue photochemical properties based on adsorption at aqueous micelle interfaces

Abstract: Methylene Blue (MB+) is a sensitizer that has been used for a variety of applications including energy conversion and photodynamic therapy (PDT). Although its photochemical properties in isotropic solution are well established, its effect in vivo and in restricted reaction environments is somewhat erratic. In order to understand its photochemical behavior when it interacts with biomolecules, in particular with membranes, MB+ properties were studied in sodium dodecyl sulfate (SDS) and cetyl trimethylammonium bromide (CTAB) solutions. Because of an electrostatic attraction, SDS and MB+ form complexes, changing the properties of both the micelles and the MB+ solutions. Surface tension measurements show that the c.m.c. of SDS decreases from ∼7 mM to ∼70 μM when the MB+ concentration increases from 0 to 45 μM. Above the c.m.c., binding of MB+ in the micelle pseudo-phase causes the formation of aggregates (mostly dimers) as attested by the increase in the absorption at 580 nm and the decrease in fluorescence emission. The extent of dimer formation is dependent on the relative concentrations of MB+ and SDS. In the presence of excess of SDS, MB+ is mainly in the monomer form and at low SDS concentration dimers are favored. Such effect, which was not observed in CTAB micelles, was modeled qualitatively by considering that MB+ molecules partition to the micelle pseudo-phase which favors or disfavors dimers as a function of its volume. MB+ transient species were characterized by laser flash photolysis and NIR emission showing the presence of triplets and subsequently singlet oxygen at high SDS concentration and semi-reduced and semi-oxidized MB+ radicals at low SDS concentration. Therefore it was shown that, depending on the ground state MB+ monomer/dimer equilibrium, induced by the micelles, the photochemical properties of MB+ can be shifted from a Type II (energy transfer to oxygen forming singlet oxygen) to a Type I mechanism (electron transfer forming the semi-reduced and the semi-oxidized radicals of MB+).

Interparticle electron transfer between size-quantized CdS and TiO2 semiconductor nanoclustersDedicated to Professor Frank Wilkinson on the occasion of his retirement.

Abstract: Photoinduced electron transfer in a size-quantized CdS/TiO2 composite system has been investigated using emission and transient absorption spectroscopy. Quantum-sized CdS and TiO2 particles were synthesized in reverse micelles using di-octyl sulfosuccinate (Aerosol-OT, AOT) as the surfactant stabilizer. The particle sizes of CdS and TiO2 were controlled by varying water-to-surfactant molar ratio, wo = [H2O]/[AOT], with values of 1, 2.5, 5 and 10. The blue-shift in the absorption onset confirmed size-quantization of these semiconductor particles. Electron transfer from photoexcited CdS to TiO2 was found to depend on the particle size of TiO2, where charge transfer was observed only when TiO2 particles were sufficiently large (>12 A). Interactions with smaller size TiO2 particles (⩽10 A) with CdS instead led to enhancements in emission with an increase in quantum yield from 2.3% to 8.8%. Picosecond laser flash photolysis experiments have been carried out to elucidate the interparticle electron transfer processes in the CdS/TiO2 reverse micellar system.

Zeolite acid strength and reaction mechanisms in catalysis

Abstract: The heterogeneous catalysts of greatest practical importance are acidic zeolites, and these cannot be studied using the traditional methods of UHV surface science. As recently as 1994 there was near universal agreement that zeolites were superacids, and that many of the most important reaction mechanisms in catalysis were based upon simple carbenium ions and other exotic high-energy intermediate species. This Paper reviews a series of investigations, many of them using solid-state NMR, that led to the reclassification of zeolite acidity and a theoretical basis for understanding carbenium ion stability in these fascinating solids. Building on this better understanding of zeolite acid strength and the types of carbenium ions that are reasonable intermediates in zeolites, it was possible to elucidate many of the features of a challenging mechanistic problem: methanol to hydrocarbon catalysis.

Gibbs energy of solvation of organic ions in aqueous and dimethyl sulfoxide solutions

Abstract: The Gibbs energy of solvation of several ions in water and dimethyl sulfoxide (DMSO) solutions was obtained through the use of thermodynamic equations relating ΔGsolv* of the ion with gas phase basicity, pKa, ΔGsolv* of neutral species and the Gibbs energy of solvation of the proton. We have used the most accurate and recent values for these properties, and this report provides 56 Gibbs energy of solvation values in aqueous solution and 30 in DMSO solution. Our results support the general view that anions are much better solvated in aqueous solution than in DMSO. An important example is the hydroxide ion for which the Gibbs energy of transfer from water to DMSO is 26 kcal mol−1. The majority of anions have a Gibbs energy of transfer in the range 10 to 15 kcal mol−1. In the case of cations, DMSO has a larger solvation ability but the difference in the Gibbs energy of solvation between water and DMSO is not greater than 5 kcal mol−1. The present data can be very useful for the development of continuum solvation models.

Preparation of nanocrystalline ceria particles by sonochemical and microwave assisted heating methods

Abstract: Nanocrystalline ceria (CeO2) particles have been successfully prepared via sonochemical and microwave assisted heating routes from aqueous solutions containing (NH4)2Ce(NO3)6, hexamethylenetetramine and poly (ethylene glycol)-19000 (PEG). The products were characterized by techniques such as powder X-ray diffraction, transmission electron microscopy, high-resolution transmission electron microscopy, selected area electron diffraction, Brunauer–Emmett–Teller nitrogen adsorption and UV–Visible absorption spectroscopy. Analysis of the results showed that the products had uniform shape, narrow size distribution and displayed conspicuous quantum size effects.

Capillary condensation in heterogeneous mesoporous networks consisting of variable connectivity and pore-size correlation

Abstract: Heterogeneous three-dimensional mesoporous networks (A. J. Ramirez-Cuesta, S. Cordero, F. Rojas, R. J. Faccio and J. L. Riccardo, J. Porous Mater., 2001, 8, 61, ) constructed under the premises of the dual site–bond model have been used as probe substrates to study the effects of variable connectivity and pore-size correlation on the aspects of both hysteresis loops and primary sorption scanning curves. The shapes of the hysteresis loops obtained from sorption simulation in networks of diverse morphologies are compared and discussed. It is found that the precursor structural parameters of the Monte Carlo simulated networks together with the sorption algorithm used in this work, can lead to IUPAC types H1, H2 and H3-like hysteresis loops, depending on the values chosen for the pore-size distribution parameters and mean connectivity. Network morphology also influences greatly the mechanisms of sorption processes in poorly or highly size correlated porous substrates. Sorption results on these 3-D porous specimens help to visualize the extents of pore blocking (vapour percolation) and delayed adsorption (liquid percolation) phenomena and also to foresee the most appropriate methods to ascertain the structure of porous materials.

XPS, XANES and EXAFS investigations of CuO/ZnO/Al2O3/ZrO2 mixed oxide catalysts

Abstract: Systematic X-ray photoelectron spectroscopy (XPS), X-ray induced Auger electron spectroscopy (AES), X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) studies were undertaken to investigate the electronic structure, chemical states and local geometry of the active species in the CuO/ZnO/Al2O3/ZrO2 multicomponent mixed oxide catalysts employed in the oxidative steam reforming of methanol (OSRM) reaction for H2 production. The core level XPS and AES indicated the existence of CuO and ZnO-like species. Two kinds of zirconium species, one similar to that of ZrO2 and another with relatively higher electron density were noticed from the Zr 3d core level XPS of Zr- containing catalysts. The valence band (VB) XPS studies revealed that for Zr-containing catalysts, the Cu 3d anti-bonding orbital splits from the main VB and shifts toward lower binding energy (BE). The surface Cu/(Al + Zr) ratios were found to be close to those in the bulk while segregation of Zn at the surface was evidenced in all samples. The XANES and EXAFS results also indicated the existence of CuO and ZnO-like species, whose local environments are modified with respect to the chemical composition. The EXAFS study of the Zr-containing catalysts indicated the existence of a “Cu–O–Zr” bonding with a Cu–Zr distance in the range 3.5 to 3.9 A. The results indicated the existence of a Cu–Zr synergistic interaction in these catalysts which improved the catalytic performance in the OSRM reaction

Alkali ion migration mechanisms in silicate glasses probed by molecular dynamics simulations

Abstract: The application of the molecular dynamics computer simulation technique to the problem of elucidating alkali ion migration mechanisms in alkali silicate glasses is reviewed. Some new results are presented that help to clarify the processes and their timing. In particular, it is shown that alkali ions jump into empty sites; that is, the mechanisms owe more in character to their crystalline vacancy counterpart rather than their interstitial cousins.

The nature of proton transport in fully hydrated Nafion

Abstract: The diffusion of protons has been studied in fully hydrated Nafion® with a recently constructed non-equilibrium statistical mechanical transport model. Radial cross-sectional profiles of the effective friction and diffusion coefficients were computed in an electrolyte membrane pore with a hydration of 22.5 water molecules per sulfonic acid fixed site. Input parameters were taken from recent SAXS measurements of the hydrated membrane and electronic structure calculations of water clusters with CF3SO3H, the associated acid for the side chain termination. The calculations revealed that the effective friction coefficient increases by more than two orders of magnitude as the proton is brought from the center of the pore to within 4 A of the fixed sites. The model calculated a diffusion coefficient of 1.92 × 10−9 m2 s−1, without ‘fitting’ any parameters, for a proton moving along the pore center, in good agreement with experimental measurements. In addition, the model also identified a predominantly vehicular transport mechanism in regions of the cross section of the pore where the proton is within 12 A of the pore wall. This was distinguished from the central region of the pore (within 4 A of the center axis) where a component of the conduction is via the Grotthuss mechanism. This investigation has demonstrated the applicability of this transport model in the prediction of diffusion coefficients in fully hydrated membranes.

The preparation and characterisation of H1-e palladium films with a regular hexagonal nanostructure formed by electrochemical deposition from lyotropic liquid crystalline phases

Abstract: The hexagonal (H1) lyotropic liquid crystalline phases of C16EO8 (octaethyleneglycol monohexadecyl ether) and Brij® 56 non-ionic surfactants have been used to template the electrochemical deposition of nanostructured palladium films. The resulting H1-e palladium films were characterised by SEM, TEM and X-ray. The films contain regular hexagonal arrays of cylindrical pores separated by palladium walls with a centre to centre distance of 5.8 nm. Electrochemical studies show that these films have very high surface areas of the order of 91 m2 g−1. Studies of the hydrogen evolution reaction on these H1-e palladium films in acid show that the formation of adsorbed hydrogen can be readily distinguished because of the high surface area to volume ratio of the films (of the order of 107 cm2 cm−3). Hydrogen insertion into the palladium films is fast and the formation of both the α and β-hydride phases is observed in the voltammetry at potentials which are similar to those reported for bulk palladium. The electrodes are stable towards repeated cycling to form the β-hydride phase showing that the hydrogen insertion and concomitant lattice expansion does not destroy the H1 nanostructure.

A new, fast, semi-direct implementation of linear scaling local coupled cluster theory

Abstract: A new way to compute the external exchange matrices in the local coupled cluster (LCC) theory is presented, which eliminates the most important bottleneck of our previous linear scaling LCC methods. It is based on a decomposition of the transformed two-electron integral set involving four external indices into blocks belonging to quadruples of atoms. A new additional transformation module was developed, which generates this very compact 4-external integral set before the LCC iteration loop is entered. The length of this integral set and the computational cost for producing it scale linearly with molecular size. Using these precomputed integrals, their contraction with the amplitudes, i.e. the assembly of the external exchange matrices occurring in each LCC iteration now is performed directly in the (external) space of the projected AOs (AOs, atomic orbitals) rather than in AO basis as previously, and proceeds exceedingly fast (3 min compared to 15 h with our previous algorithm, for the largest test-molecule considered in this paper).

The spectroscopy of water vapour: Experiment, theory and applications

Abstract: The recent spectroscopy of water vapour in the ground electronic state is reviewed. Experimental advances from the microwave to the near ultraviolet spectral regions are surveyed. On the theoretical front, new approaches to the calculation of vibration–rotation energy levels are covered. Water spectroscopy finds extensive application in astronomy, atmospheric science and combustion research. An illustrative summary of these applications is presented.

Coadsorption of CO and O2 on small free gold cluster anions at cryogenic temperatures: Model complexes for catalytic CO oxidation

Abstract: The cluster ions Aun(CO)m(O2)u− (n = 2,3; m = 1, u = 1,2) are successfully synthesized via coadsorption of oxygen and carbon monoxide onto mass-selected gold clusters inside a temperature controlled ion trap at 100 K. The investigated gold clusters Au1–3− show a significant size dependence in the reactivity toward oxygen and carbon monoxide. Whereas no reaction products are detected for the monomer ion, simultaneous adsorption of CO and O2 onto the dimer leads to the formation of Au2(CO)(O2)− which is regarded as a model complex to study the mechanism of catalytic CO oxidation on gold clusters. In the case of Au3− the conditioning of the cluster by CO preadsorption is found essential to enable O2 coadsorption.

Raman spectroscopy of AMn2O4(A = Mn, Mg and Zn) spinels

Abstract: First order Raman spectra in the region 200–800 cm−1 have been collected for AMn2O4 (A = Mn, Mg and Zn) tetragonal spinels. A possible correlation between Raman phonons and AO4 and MnO6 unit vibrations is proposed. Structural changes have been analyzed following the evolution of the Raman spectra with x for the Mg1−xMnxMn2O4 solid solution; the effect of the spinel inversion has been also studied on MgMn2O4 quenched samples from high temperatures. The results, taking into account also the X-ray diffraction and electron paramagnetic resonance data, show the influence of the Jahn–Teller effect on the Raman scattering for this class of materials.

Dynamic channels of a porous coordination polymer responding to external stimuli

Abstract: The coordination polymer synthesized by the reaction between porous copper terephthalate and 4,4′-bipyridine (4,4′-bipy) has a porous structure consisting of two mutually interpenetrating three-dimensional frameworks with large cavities. It exhibits novel adsorption behavior and has a dynamic structure in which its own frameworks change in response to external stimuli such as pressure.

Oxidation, ignition and combustion of toluene: Experimental and detailed chemical kinetic modeling

Abstract: The oxidation of toluene was studied in a jet-stirred reactor at 1 atm. New experimental results were obtained over the high temperature range 1000–1375 K, and variable equivalence ratio (0.5 ⩽ φ ⩽ 1.5). Concentration profiles of reactants, stable intermediates and final products were measured by probe sampling followed by on-line and off-line GC analyses. These experiments were modeled using a detailed kinetic reaction mechanism (120 species and 920 reactions, most of them reversible). This kinetic scheme was also used to simulate the ignition of toluene–oxygen–argon mixtures and the burning velocities of toluene–air mixtures. The presently proposed mechanism has already been validated by simulating the oxidation of benzene at 0.46 to 10 atm under stirred-reactor conditions, the ignition of benzene–oxygen–argon mixtures and the combustion of benzene in flames. Sensitivity analyses and reaction path analyses, based on species rates of reaction, were used to interpret the results. The routes involved in toluene oxidation have been delineated: toluene oxidation proceeds via the formation of benzyl, by H-atom abstraction, and the formation of benzene, by H-atom displacement yielding methyl and benzene; benzyl oxidation yields benzaldehyde, that further reacts yielding phenyl whereas benzyl thermal decomposition yields acetylene and cyclopentadienyl; further reactions of cyclopentadienyl yield vinylacetylene.

OH-initiated oxidation of benzene

Abstract: The present work represents a continuation of part I of this series of papers, in which we investigated the phenol yields in the OH-initiated oxidation of benzene under conditions of low to moderate concentrations of NOx, to elevated NOx levels. The products of the OH-initiated oxidation of benzene in 700–760 Torr of N2/O2 diluent at 297 ± 4 K were investigated in 3 different photochemical reaction chambers. In situ spectroscopic techniques were employed for the detection of products, and the initial concentrations of benzene, NOx, and O2 were widely varied (by factors of 6300, 1500, and 13, respectively). In contrast to results from previous studies, a pronounced dependence of the product distribution on the NOx concentration was observed. The phenol yield decreases from approximately 50–60% in the presence of low concentrations ( 10 000 ppb) NOx concentrations. In the presence of high concentrations of NOx, the phenol yield increases with increasing O2 partial pressure. The rate constant of the reaction of hydroxycyclohexadienyl peroxyl radicals with NO was determined to be (1.7 ± 0.6) × 10−11 cm3 molecule−1 s−1. This reaction leads to the formation of E,E-2,4-hexadienedial as the main identifiable product (29 ± 16%). The reaction of the hydroxycyclohexadienyl radical with NO2 gave phenol (5.9 ± 3.4%) and E,E-2,4-hexadienedial (3.4 ± 1.9%), no other products could be identified. The residual FTIR product spectra indicate the formation of unknown nitrates or other nitrogen-containing species in high yield. The results from the present work also show that experimental studies aimed at establishing/verifying chemical mechanisms for aromatic hydrocarbons must be performed using NOx levels which are representative of those found in the atmosphere.

Interfacial structure of solid-stabilised emulsions studied by scanning electron microscopy

Abstract: Low temperature field emission scanning electron microscopy has been used to visualise the interfacial structure of emulsions stabilised solely by solid particles. Examples are given which include both oil-in-water and water-in-oil emulsions stabilised by either sub-micron silica or polystyrene latex particles, and either cyclohexane, toluene or a medium chain length triglyceride as oil. Evidence is shown of either a close-packed arrangement of particles adsorbed at the curved oil/water interface or of the formation of flocs of particles separated by particle-free regions on the surface. In emulsions stabilised by silica, the stabilising layer appears to be aggregates composed of partially fused primary particles. For emulsions containing latex, individual particles are seen to form a close-packed monolayer at the surface of drops. The findings are discussed in relation to the structure of particle monolayers at planar oil/water interfaces studied previously.