Showing papers in "Physical Chemistry Chemical Physics in 1999"

The surface acidity of solid oxides and its characterization by IR spectroscopic methods. An attempt at systematization

Abstract: The bases of IR spectroscopic methods for the characterization of the surface acidity, of the Lewis type and of the Bronsted type, of solid, simple and mixed oxides are briefly reviewed. The results are described and a systematization is proposed. The surface acidity is essentially associated with the ionicity/covalency of the element–oxygen bond, mainly affected by the size and charge of the cation.

Vibrational spectroscopy of bulk and supported manganese oxides

Abstract: Manganese oxide catalysts, both bulk and supported on γ-Al2O3 and SiO2, have been studied by Raman and FTIR spectroscopies, their phase composition being determined by X-ray diffraction. The supported catalysts were prepared via pore volume impregnation from nitrate precursors, the atomic ratio of manganese to aluminium and silicon, respectively, being in the range from 0.5/100 to 18/100. The use of Raman analysis in the microscopic configuration allowed the spectra to be taken at different points of the surface and revealed the inhomogeneity of the catalyst. Besides the Raman features of the β-MnO2 and α-Mn2O3 phases, other signals were assigned to isolated Mn2+ ions accommodated in tetrahedral vacancies on the support surface and to some epitaxial layers of γ-Mn2O3 and manganese silicate, respectively. The FTIR spectra, though not as useful because of the strong bands of the support that overlap those of manganese oxides, support these findings.

Stability of oil-in-water emulsions stabilised by silica particles

Abstract: We describe the preparation and properties of oil-in-water emulsions stabilised by colloidal silica particles alone. The charge on the particles and their extent of flocculation, assessed via turbidity measurements, can be modified by pH control and addition of simple electrolytes. The stability of emulsions to both creaming and coalescence is low in the absence of electrolyte, and the effects of adding salt are dependent on the type of salt. In systems containing NaCl, emulsions are less stable once the particles are flocculated. In the presence of either LaCl3 or tetraethylammonium bromide (TEAB), emulsion stability increases dramatically for conditions where the silica particles are weakly flocculated; extensive flocculation of the particles however leads to destabilisation of the emulsions. For TEAB, relatively large emulsions of diameter around 40 µm remain very stable for up to 3 months at salt concentrations corresponding to the onset of coagulation of the colloid. Such emulsions are themselves strongly flocculated.

The prediction of inorganic crystal structures using a genetic algorithm and energy minimisation

Abstract: A genetic algorithm has been used to generate plausible crystal structures from the knowledge of only the unit cell dimensions and constituent elements. We successfully generate 38 known binary oxides and various known ternary oxides with the Perovskite, Pyrochlore and Spinel structures, from starting configurations which include no knowledge of the atomic arrangement in the unit cell. The quality of the structures is initially assessed using a cost function which is based on the bond valence model with a number of refinements. The lattice energy, based on the Born model of a solid, is minimised using a local optimiser for the more plausible candidate structures. The method has been implemented within the computational package GULP. An extensive collection of Buckingham potential parameters for use in such simulations on metal oxides is also tabulated.

Estimation of mean size and shape of small metal particles by EXAFS

Abstract: The possibility to estimate the mean size and shape of metal particles from their average coordination number, which is typically obtained from EXAFS (extended X-ray absorption fine structure) analysis, is evaluated. While the number of neighbors in the first and second coordination shell were found to be not significantly dependent on the particle shape, and thus can be used to estimate the particle size, a substantial influence of the particle shape was found to be present for the coordination numbers of higher neighboring shells. Consequently, a method using the coordination number of the first or second shell (N1 or N2) to estimate the average particle size and that of a higher shell (N3 or N4) to estimate the particle shape is presented.

Perfluorosulfonated membrane (Nafion): FT-IR study of the state of water with increasing humidity

Abstract: A detailed study of the Nafion–water system has been performed using water sorption isotherm, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and FT-IR with the aim of investigating the state of water in the Nafion membrane. The last technique in particular can yield valuable information on molecular interactions, especially those involving water molecules. The first water molecules interact with sulfonic groups, forming hydronium H3O+ and –SO3- ions, while the following water molecules aggregate with H3O+ to form counter-ion clusters with five or six water molecules. Increasing the water content promotes cluster growth and cluster aggregation with the formation of a continuous water phase, where for every four molecules of water one has two hydrogen bonds, and each of the other three only one. All the sorption, calorimetric and infrared data agree with the proposed model of two non-interacting phase systems: the water-phase and the perfluoro-phase with sulfonic groups at the interphase. The only observed effects of water on Nafion polymer are sulfonic group dissociation and partial destruction of crystallinity associated in polymer swelling.

Spin-forbidden dehydrogenation of methoxy cation: a statistical view

Abstract: A non-adiabatic version of RRKM theory is applied to predicting the microcanonical rates for different mechanisms of spin-forbidden dissociation of methoxy cation and its isotopically substituted derivatives, to formyl cation and dihydrogen. The predictions are in agreement with experimental results on this system, and in particular with the occurrence of a “direct’' mechanism for dissociation, rather than of an indirect one via hydroxymethyl cation. Abinitio computations were used throughout to provide the parameters needed to apply the non-adiabatic RRKM theory, and the success of this strategy is shown to be promising for other applications in polyatomic systems. Finally, the kinetic energy release distribution for loss of hydrogen from methoxy and hydroxymethyl cations are computed using abinitio “direct dynamics’' classical trajectories at the HF/6-31G** level, their similarity is also in agreement with experiment.

NIR FT Raman spectroscopic study of flame soot

Abstract: During combustion soot goes through different molecular structures. The Raman spectra of flame soot, collected in premixed flames at different flame heights and flame temperatures, were analysed in regard to their structural properties and in comparison with those of disordered graphite. Besides the G- and D-band of disordered graphite, a third Raman band at approximately 1188 cm-1 was observed which was tentatively assigned to mixed s–s bondings or to the C–C and C2C stretching vibration modes of polyene-like structures. The G-band position, the D-band width, and ID/IG intensity ratio describe the graphitic order in the particles. According to this result the graphitic order of the untreated soot particles decreases with decreasing flame temperature and flame height, whereas the size La of the graphitic sheets is nearly constant.

Ab initio potential-energy functions for excited state intramolecular proton transfer: a comparative study of o-hydroxybenzaldehyde, salicylic acid and 7-hydroxy-1-indanone

Abstract: Potential-energy profiles along the minimum-energy reaction path for intramolecular proton transfer in the 1ππ* excited state have been calculated for the title compounds. The CASSCF and CIS electronic-structure methods have been employed for excited-state geometry optimization. Single-point energy calculations along the reaction path have been performed using the CASPT2 and TDDFT methods. The TDDFT method has been tested against accurate CASSCF and CASPT2 data for malonaldehyde. CASPT2 yields transition energies for photon absorption and emission which are in excellent agreement with experimental data (within 0.2 eV). The CASPT2 potential energy functions exhibit, however, artifactual kinks (on a scale of a single kcal mol-1) which reflect inherent limitations of the CASSCF-based perturbation approach. TDDFT yields potential-energy functions which are essentially parallel to the CASPT2 functions and free of artifacts. Transition energies for absorption and emission are systematically overestimated, however, by about 0.5 eV in TDDFT. For all three title compounds, a barrierless 1ππ* potential-energy function is predicted. The location of the 1ππ* minimum varies from near-enol in salicylic acid to near-keto in 7-hydroxy-1-indanone.

Predicting the binding energies of H-bonded complexes: A comparative DFT study

Abstract: Comparisons with the results of coupled cluster calculations were made to assess the quality of density functionals in predicting the electronic binding energies of H-bonded complexes. A variety of different density functionals, namely B3LYP, B97-1, PBE0, HCTH, BLYP, PBE, LDA and a recently derived improvement of the HCTH functional (HCTH38), as well as the standard abinitio Hartree–Fock and second-order Moller–Plesset perturbation theory methods were applied using a triple-ζ plus double polarisation basis set. Equilibrium structures, counterpoise corrected binding energies and harmonic frequencies were calculated for the (HF)2, (HCl)2, (H2O)2, (CO)(HF), (OC)(HF), (FH)(NH3), (ClH)(NH3), (H2O)(NH3) and (H3O+)(H2O) complexes. Although the hybrid methods performed well in general, the new HCTH38 functional as a pure GGA predicted binding energies of better quality than the B3LYP functional. Bond length changes and frequency shifts were compared to MP2 results.

Mechanism of the Fenton reaction. Evidence for a new intermediate

Abstract: For the first time, the time dependence of the amount of O2 evolved in the reaction of Fe2+ with H2O2 (the Fenton reaction) was measured by following the decrease of the total [Fe3+] formed upon quenching the reaction mixture with an excess of a strongly acidic solution of Fe2+. The disappearance of Fe2+ in the reaction was followed by quenching it with o-phenanthroline and measuring the absorbance of the Fe2+-phenanthroline complex formed at λ=510 nm. At [H2O2]<10-4 mol dm-3, and quenching the reaction with Fe2+ ions, the formation of a new intermediate was observed. It was identified as the mixed valence binuclear species {FeOFe}5+. A new mechanism for the reaction has been proposed in which FeO2+ acts as the key intermediate. Several rate constants and ratios of rate constants have been determined. The existence of free radicals in the system does not seem to be compatible with the data.

Coordination and siting of Cu+ ions in ZSM-5: A combined quantum mechanics/interatomic potential function study

Abstract: Siting and coordination of Cu+ ions in zeolite ZSM-5 have been studied by a combined quantum mechanics/interatomic potential function technique. A new Cu(I)–O interaction potential has been parameterized based on abinitio data which is compatible with abinitio-parametrized shell model potentials for zeolites. Several different sites of Cu+ inside ZSM-5 have been found. The structure of the site and the coordination of the Cu+ ion depend on the T-site where the Si atom is replaced by an Al atom. If Al is at the edge of the main and sinusoidal channels the Cu+ ion prefers to occupy thc open space in the channel intersection and it is coordinated to two oxygen atoms of the AlO4 tetrahedron. The largest binding energy of Cu+ with ZSM-5 was found for Cu+ located inside a six-membered ring on the wall of the sinusoidal channel, where it can coordinate to three or four oxygen atoms of the zeolite framework. The Cu+ sites predicted are in accord with available experimental results.

Homogeneous nucleation and the Ostwald step rule

Abstract: We compare the pathways for homogeneous nucleation in a number of different systems. In most cases, the simulations show that the nucleation pathways are markedly different from what is assumed in classical nucleation theory. We find that homogeneous nucleation exhibits, at the microscopic level, features that were formulated over a century ago by Ostwald to account for the nucleation of metastable bulk phases. We observe that the structure of the precritical nuclei may differ qualitatively from that of (post)critical nuclei. Surprisingly, in the interface of the larger nuclei, traces of the structure of the smaller nuclei survive.

Pentagon adjacency as a determinant of fullerene stability

Abstract: Optimisation of geometries of all 40 fullerene isomers of C40, using methods from molecular mechanics and tight-binding to full abinitio SCF and DFT approaches, confirms minimisation of pentagon adjacency as a major factor in relative stability. The consensus predictions of 11 out of 12 methods are that the isomer of lowest total energy is the D2 cage with the smallest possible adjacency count, and that energies rise linearly with the number of adjacencies. Quantum mechanical methods predict a slope of 80–100 kJ mol-1 per adjacency. Molecular mechanics methods are outliers, with the Tersoff potential giving a different minimum and its Brenner modification a poor correlation and much smaller penalty.

An ab initio study of the photochemistry of azobenzene.

Abstract: The photochemistry under n→π* and π→π* excitation and the thermal isomerization of azobenzene are studied by a theoretical approach. Two mechanisms are considered: torsion around the N2N double bond and the inversion of one N atom. We optimize the most important geometries with CASSCF and we calculate the potential energy curves for the lowest states (4 singlets and 2 triplets) with multireference perturbation theory. Our calculations support the current view that inversion is the preferred pathway for ground state isomerization and probably also by n→π* excitation, while torsion occurs by π→π* excitation. We find that two states, 21A and 21B in C2 symmetry, are important in the π→π* photochemistry.

Structure of pure SDS and DTAB micelles in brine determined by small-angle neutron scattering (SANS)

Abstract: The geometrical structure of pure SDS and DTAB surfactant micelles in the absence of added salt as well as its dependence on the concentration of NaBr have been investigated at 40°C using small-angle neutron scattering (SANS). In contrast to previous SANS measurements on the same systems we have analysed the scattering data in the entire regime of scattering vectors that are relevant for determining the structure of the micelles. Our obtained results for pure surfactant micelles, as well as those of mixed catanionic micelles presented in a recent study, show somewhat unexpectedly that ordinary surfactant micelles are shaped as circular or elongated bilayers (tablets). Both SDS and DTAB micelles appeared to be disk-like in pure D2O and the corresponding data were best fitted with a model for (monodisperse) oblate ellipsoids of revolution with half axes a=12.0 A, b=20.3 A ([SDS]=1.0 wt.%) and a=12.4 A, b=21.6 A ([DTAB]=1.0 wt.%). The half axis b related to the disk radius increases in both cases with an increasing amount of added salt to about 23 A (SDS) and 24 A (DTAB) at [NaBr]=0.1 M and at about [NaBr]=0.2 M the SDS micelles become tablet-shaped, i.e. tri-axial ellipsoids with half axes a

Clusters of aluminium, a density functional study

Abstract: Density functional calculations have been performed to study the structure and energetics of mainly neutral Aln with n up to 153. Clusters up to n=15 have been investigated systematically, in part by simulated annealing techniques. No pronounced islands of stability are found in this range although Al7 and Al13 are of special importance. The treatment of selected larger clusters with different packings and cluster shapes leads to the following conclusions. Icosahedral packing is favoured only for n=13; around n=55, decahedral packing is most stable, whereas fcc is definitely preferred, energetically, for n>80. Among clusters of comparable size and packing those with dominantly (111) surfaces are found to be most stable. These trends are rationalized by considering the average number of next neighbours and the internal strain arising from a mismatch of bond distances. Extrapolation of the computed cluster binding energies yields a cohesive energy of bulk Al of 3.3–3.4 eV, in close agreement with experiment (3.36 eV).

CO2 hydrogenation over metal/zirconia catalysts

Abstract: Metal/zirconia catalysts have attracted considerable interest for the hydrogenation of carbon dioxide. Various preparation methods such as impregnation, co-precipitation, sol–gel synthesis, and controlled oxidation of amorphous metal alloys have been used, leading to catalysts with very different properties. The catalytic behaviour of these materials is greatly influenced by the active metal and by the interfacial contact area between the metal and zirconia. Cu and Ag catalyse mainly methanol formation, while methane is formed over, e.g., Ni, Ru and Rh. Examples for less reactive metal constituents are Pd, Rh, Pt and Au, catalysing simultaneously the formation of methanol, carbon monoxide (by reverse water–gas shift reaction) and methane. The structural and catalytic properties of the various catalysts are compared and possible reaction pathways are discussed.

FTIR-spectroscopy of molecular clusters in pulsed supersonic slit-jet expansions

Abstract: A new approach to the Fourier transform infrared (FTIR) absorption spectroscopy of molecular clusters in pulsed supersonic jets is developed to the point where it is competitive with high-sensitivity laser absorption techniques for intermediate and large molecular systems. A combination of rapid spectral acquisition and of a buffered jet chamber enables the use of intense gas pulses which cover complete interferometer scans. Applications to (N2O)n, (CH3OH)n and (HCl)n demonstrate the capabilities of this technique. Investigations of the association of bulky alcohols and of clusters within clusters illustrate some ongoing research.

Ab initio determination of the induced ring current in aromatic molecules

Abstract: The aromatic ring current shieldings (ARCS) approach is a new method to determine the strength of the induced ring current which is related to the molecular aromaticity. In the ARCS method, the strength of the induced aromatic ring current and the size of the current ring are obtained from nuclear magnetic shieldings calculated in discrete points along a line perpendicular to the molecular plane starting at the center of the molecule. The induced ring currents are used to determine the relative degree of aromaticity for benzene, furan, pyrrole, cyclohexane, cyclohexadiene, thiophen, p-benzoquinone, cyclopentadienide anion, cycloheptatrienyl cation, 1,4-dihydronaphthalene, and prophin.

Structure–functionality relationships of grafted Ti-MCM41 silicas. Spectroscopic and catalytic studies

Abstract: Detailed spectroscopic measurements have been used to elucidate the nature of Ti(IV)-centred active sites that were anchored to a mesoporous silica surface (MCM41) by two distinct routes from titanocene precursors. The catalyst prepared in dry argon (Ti-MCM41 [Ar]) is more active than that prepared in air (in the presence of water vapour) in its activity in the epoxidation of cyclohexene with tert-butylhydroperoxide (TBHP). The degree of loading of the titanium onto the silica support also influences the precise nature of the active sites. In Ti-MCM41 [Ar] samples containing less that 2 wt.% of Ti, the most abundant species are tetrahedrally bonded Ti(IV) active sites which absorb at 210–230 nm in diffuse reflectance (DR) UV–Vis spectra and exhibit an emission at 430 and 490 nm when excited with a 250 nm light. Samples of catalyst with greater than 4 wt.% loading are shown to contain TiO2-like microclusters. These species absorb at λ250 nm in the DR UV–Vis and emit very weakly in the 500–600 nm region. In the case of Ti-MCM41 [air] catalysts, which were prepared in the presence of atmospheric water at the MCM41 surface, even at the lowest Ti loading (e.g. ⩽0.5%) an incipient formation of oligomers occurs. It is proposed that dimers or very small oligomers which absorb at around 250 nm in the DR spectra are responsible for a very strong emission at around 500 nm in the photoluminescence spectra. The abundant presence of these species might well explain the lower catalytic performance of Ti-MCM41 [air] in comparison with that of Ti-MCM41 [Ar]. The anchoring of the Ti species at the surface silanol groups of MCM41 was in all cases followed by FTIR spectroscopy.

SAFT-VR modelling of the phase equilibrium of long-chain n-alkanes

Abstract: The statistical associating fluid theory for potentials of variable attractive range (SAFT-VR) has been used in the calculation of the phase equilibria for long-chain n-alkanes and their mixtures. We treat the molecules as chains formed from united-atom hard-sphere segments, with a square-well potential of variable attractive range to describe the dispersive forces. A empirical relationship derived in earlier work is used to determine the number of such segments in relation to the carbon number for each member of the homologous series. Simple linear relationships between the potential model parameters and molecular weight have been determined enabling predictions of the fluid phase equilibria of heavier n-alkane molecules for which no experimental data are available. This is illustrated by a comparison with simulation data from the literature for the coexisting densities of n-octatetracontane (C48H98). Additionally we have examined binary mixtures of methane+n-hexadecane (C16H34) and hexane+n-tetradecane (C14H30) with the SAFT-VR approach, using simple Lorentz–Berthelot combining rules to determine the unlike interaction parameters. This study will allow an extension of the SAFT-VR approach to polymeric systems.

The reactivity of chlorine atoms in aqueous solution Part II.The equilibrium SO4-+Cl-ClNsbd+SO42-

Abstract: A study of the two simultaneous equilibria, reactions (1) and (2), has been performed by pulse radiolysis in Ar-saturated aqueous solution at 25°C. The forward and reverse rate constants of equilibrium (2) were determined at 0.3 mol dm-3 ionic strength to be k2=(6.1±0.2)×108 and k-2=(2.1±0.1)×108 d mol-1 s-1. These yield the equilibrium constant K2=k2/k-2=(2.9±0.2) at 0.3 mol dm-3 ionic strength, or 1.2 corrected to zero ionic strength. As part of the study, the reactions of SO4- with S2O82-, t-BuOH and water were found to be <1.5×103, (7.8±0.2)×105 d mol-1 s-1 and (690±120) s-1, respectively. The relevance of these results to cloud chemistry is discussed.

Surface investigation on CexZr1-xO2 compounds

Abstract: The surface of CexZr1-xO2 compounds has been studied by Fourier transform infrared (FTIR) spectroscopy using adsorbed probe species. Methoxy species, from methanol dissociative adsorption, compensate the coordinative unsaturation of surface cations. From the ν(OC) vibration of methoxy species it was shown that mixed compounds are solid solutions with a surface composition equal to the nominal bulk composition. This result was confirmed by X-ray photoelectron spectroscopy (XPS). However, compared to pure compounds, an increase of exposed Ce4+ cationic sites related to capping oxygen lability was observed for mixed compounds. From the adsorption of CO and pyridine probes it was shown that the Lewis acidities of mixed compounds were intermediate between those of pure ceria and zirconia, which are weak, being slightly higher for zirconia. Using CO2 as a probe for basicity investigation of the surface, it was shown that the various carbonate species observed for mixed compounds were already observed for pure ceria or zirconia with no striking variation of the overall carbonatation. However, from a proposed interpretation of the relative importance of various species, such as monodentate or polydentate carbonates, it was inferred that surface oxygen species are more easily relaxed in ceria-rich mixed compounds than in ceria itself, and that the surface framework of cerium cations is stabilised by zirconium, the effect being the opposite in the case of the surface framework of zirconium ions.

Large scale molecular dynamics simulation of self-assembly processes in short and long chain cationic surfactants

Abstract: We report on an investigation of the structural and dynamical properties of n-nonyltrimethylammonium chloride (C9TAC) and erucyl bis[2-hydroxyethyl]methylammonium chloride (EMAC) micelles in aqueous solution. A fully atomistic description was used, and the time evolution was computed using molecular dynamics. The calculations were performed in collaboration with Silicon Graphics Inc. using the large-scale atomic/molecular massively parallel simulator (LAMMPS) code (version 5.0, CRADA Collaboration, Sandia National Laboratory, USA, 1997) on a range of massively parallel platforms. Simulations were carried out in the isothermal–isobaric (N, P, T) ensemble, and run for up to 3 ns. Simulated systems contained approximately 50 surfactant cations and chloride counterions, surrounded by 3000 water molecules. Starting from different initial configurations (spherical micelle, wormlike micelle) in the case of the C9TAC molecule, we observe shape transformations on the timescale of nanoseconds, micelle fragmentations, and surfactant–monomer exchange with the surrounding medium. Starting from a random distribution of surfactant molecules in the solution, we observe the mechanism of micelle formation at the molecular level. The mechanism of self-assembly or fragmentation of a micelle is interpreted in terms of generalised classical nucleation theory. Our results indicate that, when these systems are far from equilibrium and at high surfactant concentration, the basic aggregation–fragmentation mechanism is of Smoluchowski type (cluster–cluster coalescence and break up); closer to equilibrium and at lower surfactant concentration, this mechanism appears to follow a Becker–Doring process (stepwise addition or removal of surfactant monomers). In the case of the EMAC molecule, we have characterised two different structures (spherical and cylindrical) of the micelle, and have found that water penetration is not important. We have also studied the effect of the introduction of co-surfactant (salicylate) molecules to the EMAC system; hydrogen bonds between surfactant head groups and co-surfactant molecules were observed to play an important role in stabilising wormlike micelles.

X-ray photoelectron spectroscopy of Au/Fe2O3 catalysts

Abstract: Gold catalysts supported on iron oxide have been characterized by X-ray photoelectron spectroscopy (XPS). Depending on the method of preparation and pretreatment, different gold species have been detected on the catalyst surface. In the absence of thermal treatment, the XPS spectra of the impregnated samples show the formation of gold(III) (oxy)chloride species. No metallic gold is formed. Both metallic and oxidized gold species are instead present on the surface of the samples prepared by coprecipitation. Oxidized gold(III) species are predominant on the uncalcinated catalysts. After treatment at high temperature and/or under reaction conditions the amount of metallic gold increases. The XPS data have been correlated with the activity of the investigated catalysts in the oxidation of carbon monoxide at low temperature.

Heterogeneous conversion of NO2 on carbonaceous surfaces

Abstract: In the present study the heterogeneous conversion of NO2 on commercial soot, freshly prepared flame soot and commercial soot treated with sulfuric acid has been investigated. The experimental results indicate a complex reaction mechanism in which the soot has to be treated as a reactant leading to deactivation of active sites on the surface with increasing NO2 consumption. Mean uptake coefficients γ for the first few minutes of the reaction of ∽10-6 have been determined for a consumption of ∽1013 NO2 cm-2. The γ-values decrease to <10-8 for a consumption of ∽1015 NO2 cm-2, indicating that the influence of soot on the partitioning of NOx in the atmosphere is negligible. Nitrous acid (HONO) and nitric oxide (NO) were found to be the major gas phase products. HONO yields up to 80% are observed, clearly showing that the nature of the reaction is non-catalytic. The humidity dependence of the reaction system was studied and indicates that water is necessary for HONO formation. In the reaction of pure HONO on soot surfaces, high yields of NO were observed, indicating a consecutive process: NO2→HONO→NO. When soot was coated with sulfuric acid the yield of HONO decreased while that of NO increased. This behaviour is caused by an increasing reactivity of the intermediate HONO on the modified surface. The experimental results indicate that without any recycling mechanism HONO formation on soot surfaces is not of major importance in the atmosphere.

Photocatalytic decomposition of benzene over TiO2 in a humidified airstream

Abstract: Photocatalytic decomposition of benzene over TiO2 in the gas phase at room temperature was studied with a fixed-bed flow reactor. In a humidified airstream ([H2O]=2.2%), benzene was efficiently decomposed to CO2 and CO with the selectivities of 93 and 7%, respectively. The selectivities were almost independent of the benzene conversion, indicating that CO is not the intermediate of CO2 in the reaction. The selectivity of CO was in the range of 7–10% with varying concentration of O2, H2O, and benzene. The formation of phenol and brownish carbonaceous matter attributable to polymeric products was observed on the catalyst surface. In the absence of O2, benzene oxidation did not proceed at all, showing that O2 is essential for the reaction. The presence of H2O not only suppressed the formation of the carbon deposits on the catalyst surface, but also accelerated the decomposition of them to CO2 and CO. Diffuse reflectance IR study showed that the presence of H2O regenerated the surface hydroxyl groups of TiO2 which were consumed in the photoreaction. With increase in the benzene concentration, the benzene conversion was decreased and the amount of carbon deposits on the catalyst surface was increased.

Simulation of amphiphilic mesophases using dissipative particle dynamics

Abstract: We study a dense solution of an amphiphilic species using the dissipative particle dynamics (DPD) algorithm, focussing on the smectic mesophase. Since DPD is locally momentum-conserving, it gives at large length scales a faithful representation of the isothermal hydrodynamics of the system. Results are presented for the phase diagram of a minimal amphiphile model, consisting of rigid AB dimers in a solution of C monomers, for the coarsening dynamics of a polydomain smectic phase, and for the formation of a monodomain smectic when shear is applied.

Glycerol condensed phases Part II.A molecular dynamics study of the conformational structure and hydrogen bonding

Abstract: An analysis of the conformational properties and hydrogen bonding in the condensed phases of glycerol is reported using the same model as adopted in Part I (Phys. Chem. Chem. Phys., 1999, 1, 871). Structural properties of the liquid and glassy states are analyzed in relation to the molecular backbone conformation of the glycerol molecule. The effects of hydrogen bonding and of temperature on the conformational distribution are analyzed. The structural and dynamical properties of hydrogen bonding in glycerol are also investigated. The results are consistent with available experimental observations and clarify many important and interrelated aspects of the microscopic structure of liquid, glassy and crystalline phases of glycerol.