Showing papers in "Journal of the Chemical Society, Faraday Transactions in 1989"

Formation of hydrocarbons in the electrochemical reduction of carbon dioxide at a copper electrode in aqueous solution

Abstract: Electroreduction of CO2 at Cu in aqueous inorganic electrolytes was studied by means of voltammetric, coulometric and chronopotentiometric measurements. CO, CH4, C2H4, EtOH and PrnOH are produced at ambient temperatures. Formation of CO predominates at less negative potentials (more positive than –1.2 V vs. NHE); hydrocarbons and alcohols are favourably produced below –1.3 V vs. NHE, where the Faradaic efficiency of CO drops. CO, formed as an intermediate from CO2, is adsorbed on the Cu electrode, interfering with cathodic hydrogen formation. The adsorption strength of CO on Cu is very weak as compared with that on Pt. Adsorbed CO is reduced to Hydrocarbons and alcohols at more negative potentials. The product distribution from CO2 depends strongly upon the electrolytes employed. Formation of C2H4 and alcohols is favoured in KCl, K2SO4, KClO4 and dilute HCO–3 solutions, whereas CH4 is preferentially produced in relatively concentrated HCO–3 and phosphate solutions. The product selectivity depends upon availability of hydrogen or protons on the surface, which is controlled by pH at the electrode. The pH at the electrode is greatly affected by the electrolyte, since OH– is released in the electrode reactions. The production of hydrocarbons and alcohols is discussed in comparison with the mechanism of the Fishcher–Tropsch reaction.

Carbon monoxide and carbon dioxide adsorption on cerium oxide studied by Fourier-transform infrared spectroscopy. Part 1.—Formation of carbonate species on dehydroxylated CeO2, at room temperature

Abstract: The adsorption of CO and CO2 on cerium oxide has been studied by Fourier-transform infrared spectroscopy (F.t.i.r.). For CO adsorption at room temperature, in addition to linearly adsorbed CO (2177 and 2156 cm–1), two kinds of carbonate (unidentate: 854, 1062, 1348 and 1454 cm–1 and bidentate: 854, 1028, 1286 and 1562 cm–1) and inorganic carboxylate (1310 and 15⊙0 cm–1) species were identified spectroscopically. As for CO2 adsorption, apart from weak bands at 1728, 1396, 1219, and 1132 cm–1 attributed to bridged carbonate species, bands due to unidentate carbonate, bidentate carbonate and inorganic carboxylate species, similar to those aising from CO adsorption, were observed. Except for the linearly adsorbed-CO, all species arising from CO and CO2 are stable at room temperature in vacuo. The desorption of these species at elevated temperatures shows that the order of thermal stability is bridged carbonate

Adsorption of carbon monoxide and carbon dioxide on cerium oxide studied by Fourier-transform infrared spectroscopy. Part 2.—Formation of formate species on partially reduced CeO2 at room temperature

Abstract: The adsorption of CO on partially reduced CeO2[CeO2(573-H) and CeO2(673-H), CeO2 reduced in H2 at 573 and 673 K for 1 h, respectively] has been studied by Fourier-transform infrared spectroscopy (F.t.i.r.). The observation of formate species (771, 1329, 1369, 1558, 1587, 2852 and 2945 cm–1) formed by the reaction of CO with the surface of CeO2(673-H) at room temperature is reported. Two weak bands at 2796 and 2706 cm–1, tentatively attributed to formyl species, were also detected at room temperature when the bands of the formate species no longer appeared to grow. No band was observed when CO was dosed on CeO2(573-H) or CeO2(673-H2O)(CeO2 hydrated at 673 K for 1 h) at room temperature and 373 K. Two sharp bands at 3685 and 3643 cm–1, due to isolated hydroxyl groups, and a broad band centred at 3421 cm–1 were generated on CeO2(673-H) through H2 reduction. Among these only the OH groups giving the 3685 cm–1 band manifest activity to CO at room temperature. A mechanism involving a formyl intermediate was proposed to explain the formation of formate species on partially reduced CeO2. The partial reduction of CeO2 causes pronounced inhibition of the formation of carbonate-like species from CO at room temperature. This is consistent with the conclusions reported in Part 1 of this work.

Effects of electric fields on the rheology of non-aqueous concentrated suspensions

Abstract: The electrorheological response of hydrated poly(methacrylate) particles in a chlorinated hydrocarbon has been characterized as a function of field strength, shear rate, volume fraction and temperature. Viscosities of electrified suspensions are increased up to three orders of magnitude over the unelectrified suspension at low shear rates but decrease to the value of the unelectrified suspension at high shear rates. The observed response has been analysed by considering the fundamental forces acting on the particles and it is found that, at a given volume fraction, all of the dependencies of relative viscosity on shear rate, field strength and temperature can be collapsed into a single function of the Mason number [ηc/2Iµ0Iµc(βE)2]. The results have been further analysed in terms of a Bingham Plastic constitutive equation from which a critical Mason number naturally arises characterizing the transition from polarization to hydrodynamic control of suspension structure.

Theoretical and experimental study of the fractal nature of the structure of casein gels

Abstract: Casein gels consist of particulate disordered structures, formed by aggregation of casein micelles. The aggregation leads to flocs of a fractal nature, which form a gel as soon as the occupy the total volume. A model was developed to describe gel formation and structure usnig the concept of fractal geometry. Parameters derived from this model were compared with experimental results concerning the structure and related properties.The model was in good agreement with the dependence of the permeability or rheological properties, such as the storage modulus of the gels, on the volume fraction of casein, and with the dependence of the turbidity of the gel on wavelength.It is shown that acid casein gels can be described very well as a collection of fractal clusters with a fractal dimensionality, D, of 2.3. Rennet-induced casein gels appeared to lose their fractal behaviour due to microsyneresis. Consequently, the model was not fully valid for the latter gels.

Photocatalytic oxidation and adsorption of methylene blue on thin films of near-ultraviolet-illuminated TiO2

Abstract: Aqueous solutions of methylene blue are totally mineralized when recirculated over thin films of titanium dioxide illuminated with near-u.v. light. The rate of destruction obeys first-order kinetics with reasonable precision but the apparent first-order rate constant, k′, decreases with increasing initial concentration of solute. 1 dm3 of 10 µmol dm–3 methylene blue solution illuminated with a 20 W lamp, is decreased to 5 µmol dm–3 in 11.8 min. Sunlight from a 1 m2 parabolic trough is capable of destroying the methylene blue at 6.4 times this rate. The decrease in k′ values with increasing concentration is consistent with curves calculated using the integrated form of the Langmuir expression. The adsorption parameter determined in the analysis of the kinetics of the photocatalytic data agreed with the adsorption affinity parameter determined using the classical Langmuir adsorption isotherm for the dark equilibrium data; this indicated the key role played by adsorption in photocatalytic oxidation with titanium dioxide. The maximum quantum yield for methylene blue destruction at high flow rates with a 10 µmol dm–3 initial concentration was calculated to be 0.0092.

A phenomenological theory of long-range hydrophobic attraction forces based on a square-gradient variational approach

Abstract: Through recent surface force measurements it has been convincingly demonstrated that strong and amazingly long-range, attractive interaction forces act between hydrophobic surfaces immersed in water. Upon separating two such surfaces from molecular contact a vapour/gas cavity normally forms. This is not the case, however, when gradually diminishing the surface separation. The hydrophobic attraction forces have been recorded in this latter, metastable regime.A mean-field theory based on a square-gradient assumption is presented in this paper which is shown to account reasonably well for the surface forces found experimentally for two cylindrically shaped, hydrophobic surfaces interacting in water. The order parameter/ variational approach taken is closely related conceptually to earlier theories of repulsive hydration forces by Marcelja et al. and Cevc et al. The present theory implies that rather minor, hydrogen-bond-propagated molecular ordering effects, in the contact layers of water molecules next to the hydrophobic surfaces and in the core of the thin water film, give rise to the attraction observed. However, it does not fully address the intriguing question as to how it comes about that the hydrophobic attraction forces extend over such a wide range as 70–90 nm. It merely points in the direction that surface-induced structural changes in the core of the this water film (so far not captured by molecular dynamics simulations) which demand minimal free-energy expense may generate an interaction of a long-range nature.

Shape-selective adsorption of aromatic molecules from water by tetramethylammonium-smectite

Abstract: The adsorption of aromatic compounds by smectite exchanged with tetramethylammonium (TMA) has been studied. Aromatic compounds adsorbed by TMA–smectite are assumed to adopt a tilted orientation in a face-to-face arrangment with the TMA tetrahedra. The sorptive characteristics of TMA–smectite were influenced strongly by the presence of water. The dry TMA–smectite showed little selectivity in the uptake of benzen, toluene and xylene. In the presence of water, TMA–smectite showed a high degree of selectivity based on molecular size/shape, resulting in high uptake of benzene and progressively lower uptake of larger aromatic molecules. This selectivity appeared to result from the shrinkage of interlamellar cavities by water.

General isotherm equation for adsorption of surfactants at solid/liquid interfaces. Part 1. Theoretical

Abstract: Based on the two-step adsorption mechanism (first step : the surface-active species are adsorbed through the interactions between the surface-active species and the solid surface; second step: through the hydrophobic interaction between the adsorbed surface-active species) and the mass-action treatment, the following general adsorption isotherm equation is derived: Γ=Γ∞k1C([graphic omitted] +k2Cn–1)//1 +k1C(1 +k2Cn–1) where Γ is the amount of surfactant adsorbed at concentration C, Γ∞ is the limiting adsorption at high concentrations, k1 and k2 are the equilibrium constants involved in the first and second step, respectively, and n is the aggregation number of surface hydrophobic aggregates or hemimicelles. The equation can interpret various types of surfactant adsorption isotherms (including L-, and LS-types) published in the literature.

Diaphragm cell for high-temperature diffusion measurements. Tracer Diffusion coefficients for water to 363 K

Abstract: A modified diaphragm cell for diffusion measurements at high temperatures is described. It has been used to obtain tracer diffusion coefficients for 3HHO and H218O in water from 298 to 363 K. Where comparisons are available, the results are in good agreement with literature data. The data have been converted to self-diffusion coefficients of water and combined with existing data to provide a series of equations covering the temperature range 242–498 K.

Estimation of group dipole moments from surface potential measurements on Langmuir monolayers

Abstract: Pressure–area isotherms and surface potential data are presented for octadecyl methyl sulphoxide (OMS) and (+)-octadecyl p-tolyl sulphoxide (OTS). The surface potential measurements indicate very clearly that both compounds are anchored at the water surface by the SO group and that the plateau in the pressure–area isotherm of OTS is the results of significant molecular orientation. The Demachak and Fort model for relating group dipole moments to the surface potential of floating Langmuir monolayers is reviewed and found to be applicable to a number of compounds. However, the values deduced by these authors for the local permittivities are not appropriate for compounds with long aliphatic chains. By drawing on previously published work a new set of values has been deduced which seems to be more applicable to such compounds and to the sulphoxides investigated here.

Computer simulations of fluids in zeolites X and Y

Abstract: We report grand canonical ensemble Monte Carlo simulations of xenon and methane adsorbed in zeolites X and Y. The thermodynamic quantities sampled include the configurational energy per molecule, average occupancy, one-body density distribution function and the distribution of occupancies in the unit cell. From these quantities and the bulk properties of the fluids, adsorption isotherms and isosteric heats of adsorption can be generated and compared with experimental results. Good agreement has been found where such comparisons can be made.

Spectroscopic studies of the solvation of amides with N—H groups. Part 1.—The carbonyl group

Abstract: Infrared shifts of the CO stretch (νco) band, and n.m.r. shifts for the 13CO carbon have been studied for formamide, acetamide, N-methyl formamide and N-methyl acetamide for dilute solutions in a range of pure and mixed solvents. The results are compared with those previously reported for N,N-dimethylamides in the same systems. There are good linear relationships between Δν(13C) and νco for the pure solvent systems, provided allowance is made for the presence of two types of solvate for methanol. For mixed methanol-aprotic solvents (B) the low-frequency (νco) component for pure methanol was lost as the concentration of B was increased. The high-frequency band initially gained intensity, but this was ultimately replaced by a third band characteristic of the amide in pure B. These results suggest that the CO group forms both one and two hydrogen bonds in methanol. Aqueous solutions have a single νco band close to that for the disolvate in methanol. As [B] was increased, this gave way to a band close to that for the mono-solvate, which again was steadily replaced by the non-hydrogen-bonded form. Hence it is concluded that for all the amides, the di-hydrogen-bonded species dominates in water. Reasons for the different behaviour in methanol and water are discussed.In all cases, as well as the gain and loss of i.r. bands, those assigned to the hydrogen-bonded units shifted considerably as [B] increased. For aqueous systems these shifts are assigned to changes in secondary solvation.We have looked for specific differences between the results for the dimethyl derivatives and the present compounds, in the expectation that N—H solvation would have a discernible affect on CO solvation. There are small differences, but these are not systematic on going from RCONMe2via RCONHMe to RCONH2, and it is concluded that cooperativity effects involving CO and N—H solvation are small compared with those for water and alcohols. The 13C resonance (13CO) shifted systematically for mixed protic–aprotic solvent systems. Using the i.r.–n.m.r. correlation and the intensity changes for the νCO bands, reasonable predictions of these n.m.r. shifts were obtained.Marked changes in the amide II band were also observed, but these are less readily interpreted. Studies of the overtone infrared spectra for aqueous solutions reveal the presence of (OH)free and (NH)free bands, showing that the N—H groups are not fully hydrogen-bonded in water despite the effect of two bonds to the carbonyl group.

Quantitative analysis of calcium carbonate polymorphs by infrared spectroscopy

Abstract: The assessment of the polymorphic composition of calcium carbonate precipitates is important for understanding the mechanism of their formation at conditions of environmental or technological interest. A simple and accurate method is presented, based on infrared spectroscopy, for the quantitative analysis of calcium carbonate polymorphs. Standard curves have been constructed from the spectra of binary mixtures of pure components. For calcite–aragonite mixtures, the content in aragonite has been obtained from the ratio of the areas under the peaks at 1080 and 876 cm–1. For the aragonite–vaterite mixtures the aragonite content has been taken from the ratio of the areas under the peaks at 1785 and 873 cm–1. The vaterite content in vaterite–calcite mixtures has been obtained from the ratio of the areas under 745 and 876 cm–1. Test mixtures made of pure synthetic components gave results in excellent agreement with their true composition. The minimum content of the polymorph detected was 25 %. The standard curves obtained have been used to estimate the content of three calcium carbonate polymorphs, in precipitates formed by spontaneous precipitation in calcium carbonate supersaturated solutions.

Interaction of transition-metal acetylacetonates with γ-Al2O3 surfaces

Abstract: Virtually all acetylacetonate (acac) complexes [the only exceptions known so far are Ru(acac)3 and Rh(acac)3] react with coordinatively unsaturated (c.u.s.) Al3+ sites of γ-alumina surfaces. As far as the surface OH groups are concerned, there appears to be a correlation between the acid/base sensitivity of an acac complex and its reactivity towards these groups, i.e acac complexes which are unstable in the presence of OH– react with the basic OH groups, and those which are sensitive to H+ react (to a certain extent) with the acidic ones. High metal loadings cannot be achieved through controlled adsorption at room temperature, however; the reactivity of acidic OH groups is very low, and neutral OH groups are not involved at all. On the other hand, the M(acac)n/Al2O3 interactions are sufficiently well behaved (i) to enable the quantitative determination of c.u.s. Al3+ sites and basic OH groups and (ii) to follow surface modifications as a result of pretreatments(s).

Direct conversion of methane into methanol

Abstract: The direct oxidation of methane into methanol has been studied in a flow microreactor The effects, on the selectivity to methanol, of the type of reactor, temperature, pressure, residence time, oxygen content, gas mixing and dilution, have been investigated The results show that at relatively low pressures the use of metallic reactors and/or catalysts leads to the formation of complete oxidation products However, at higher pressures none of the experimental parameters studied has a significant influence on the selectivity to methanol, which in all cases is ca 40 %, much lower than claimed in recent publications Investigations of the stability of methanol at high temperatures and pressures in the presence of oxygen show that loss of methanol through further oxidation is unimportant except in the presence of metals such as Cu In the presence of methane and oxygen some methanol is destroyed, and this is attributed to reactions involving methyl radicals produced during the activation of methane by oxygen However, this loss of methanol is insufficient to account for the low overall selectivity observed It is concluded that the detailed design of the reactor may be of critical importance in obtaining high methanol yields

Energetics of molecular interactions in binary mixtures of non-electrolytes containing a salt

Abstract: Molar excess enthalpy data have been measured at 308.15 K for pyridine (saturated with anhydrous cupric chloride) A (s)–aniline, O-toluidine (OT), formamide(FD) or N,N-dimethyl-formamide(NND)(B) mixtures, and have been utilized to study the energetics of molecular interactions in them. The results show that compared to aniline or (NND), OT or (FD) coordinates strongly with the central copper atom in (py2 CuCl2) A(s) to yield (B py CuCl2) molecular entities.

Molecular self-diffusion of methane in zeolite ZSM-5 by quasi-elastic neutron scattering and nuclear magnetic resonance pulsed field gradient technique

Abstract: The translational mobility of methane sorbed on an NaZSM-5 sample of proven crystallinity has been studied by quasi-elastic neutron scattering (QENS) and an n.m.r. pulsed field gradient technique (n.m.r. PFGT) at different loadings and temperatures. In both methods, long-range self-diffusion is detected. However, owing to the different timescales of the two experimental methods applied, the mean diffusion paths followed are of different magnitude: in QENS, molecular translation is measured up to 6nm; in n.m.r. PFGT, the mean molecular displacements amount to some µm. Nevertheless, since the r.m.s. molecular displacements followed by both methods considerably exceed the distances between the pore intersections of the ZSM-5 channel network, in both techniques the translational self-diffusion coefficient of guest molecules inside the zeolite pores is detected. For the intracrystalline self-diffusion coefficient, D, as well as for the activation energy of self-diffusion, Ea, the values determined by the two independent methods agree very well. In the temperature region 200–250 K, the intracrystalline self-diffusion coefficients of methane in ZSM-5 are found to be of the order of 10–5–10–4 cm2 s–1, with only slight concentration and temperature dependence. The activation energy determined by both methods amounts to 4–5 kJ mol–1. Further agreement between QENS and n.m.r. PFGT is obtained by comparing the mean molecular jump lengths, which by both techniques are found to be ca. 1 nm for light hydrocarbons in ZSM-5, slightly decreasing with increasing loading.

Acid-base equilibria in aqueous micellar solutions. Part 1. - 'Simple' weak acids and bases

Abstract: The acid–base equilibria of a number of phenols, amines and carboxylic acids in aqueous micellar solutions and organic solvent–water mixtures have been examined. For the majority of the molecules investigated, the differences between the pKa values in pure water and the apparent pKa values when the molecules reside within micellar interfacial microenvironments can primarily be ascribed to the differenes between the mean intrinsic solvent properties of the interfacial and bulk phases, with an additional contribution from the electrostatic micellar surface potential in the case of the charged aqueous micellar solutions.

Simultaneous alternating current impedance/electron spin resonance study of electrochemical doping in polypyrrole

Abstract: The electrochemical doping of polypyrrole has been studied using simultaneous a.c. impedance and in-situ electrochemical e.s.r. measurements. A quantitative estimate of the polypyrrole conductivity as a function of doping level has been obtained and the corresponding change in the polaron concentration measured by in-situ electrochemical e.s.r. It was found that at low doping levels there was a good correlation between these two properties. It is suggested that the polaron may be responsible for the conduction process under such conditions. At higher doping levels conduction would appear to involve the bipolaron.

Orientational and spin–orbital dependence of interatomic forces

Abstract: The representation of weak interactions of open-shell atoms as effective anisotropic potentials is discussed. Examples, mainly from recent molecular-beam studies of collisions of magnetically orientated atoms in P states, prove that the spherically averaged component of the interactions follows systematics established for closed-shell van der Waals forces. Data are being collected to unravel similar systematic trends for the anisotropic component of the interactions.

Glass/rubber transitions and heat capacities of binary sugar blends

Abstract: The pairs of sugars glucose–fructose and fructose–sucrose have been comelted and quenched to form binary glasses. Glass-to-rubber transition temperatures Tg and heat capacities of the glasses and the crystalline sugars have been measured by computer-aided differential scanning calorimetry. Tg was found to vary smoothly with glass composition and the (constant pressure) heat capacities C of the glasses and rubbers varied monotonically with Tg. It is suggested that C is a better and experimentally more convenient indicator of Tg and the glassy state than is the inferred viscosity in terms of which Tg is usually defined. The Tg data for mixtures extrapolate smoothly to a Tg of fructose of 286 K which differs substantially from the value (373 K) previously reported. C values for the crystalline sugars agree with literature data.

s-cis and s-trans Conformers of formic, thioformic and dithioformic acids. An ab initio study

Abstract: Ab initio SCF-MO calculations have been carried out for formic, thioformic and dithioformic acids using the 6–31G* basis set. Fully optimized geometries, atomic charges, relative stabilities and harmonic force fields for s-cis and s-trans conformers of these molecules have been determined and the effects of oxygen-by-sulphur substitution analysed. A realistic description of the molecular charge distribution can be reached by introducing a quantum-mechanical correction to the Mulliken atomic charges, derived from the ‘charge’-‘charge flux’-‘overlap’(CCFO) model. Unlike reported theoretical results, the present ab initio calculations yield relative stabilities of the thioformic acid conformers in agreement with experiment [s-cis(thiol) > s-trans(thiol) > s-cis(thione) > s-trans(thione)]. The success of these ab initio calculations should be partially ascribed to the inclusion of polarization functions on all non-hydrogen atoms.

Synergy between copper and zinc oxide during methanol synthesis. Transfer of activating species

Abstract: The nature of the synergy between Cu and ZnO has been examined using temperature-programmed desorption and reaction techniques. Adsorption of CO–CO2–H2 on Cu/SiO2 and on ZnO/SiO2 results in the formation of surface intermediates which are distinguishable by different activation energies for their dissociation into CO2 and H2. A mixture of the two catalysts does not simply exhibit the characteristics of both components. After adsorption of the gas-feed on a physical mixture at 325 K, CO2 and H2 are desorbed separately. It is proposed that, during the dissociation of the species on the Cu surface, hydrogen can spill acoross the silica, causing the zinc oxide to become hydrided and hydroxylated. Further, if there is intimate contact between the phases, it could be envisaged that the hydroxyl species contribute to the creation of bidentate Cu-methanoates from CO2 and even CO. This would explain the observation that when CO–H2(at 101 325 Pa) is passed over the catalysts during programmed heating, a sample prepared by coprecipitation is the only one to generate methanol.

A B.E.T.-like three sorption stage isotherm

Abstract: The statistical B.E.T.-treatment of multimolecular sorption is extended to include three distinct sorption stages, viz.(I) the strongly sorbed monolayer, (II) the following h–1 layers, much less strongly sorbed, and (III) the remaining layers, up to infinity, of ‘pure liquid’ characteristics. The new three sorption stages (t.s.s.) isotherm embodies the classical two-parameter B.E.T. equation [stages 1(h= 1)–III] as well as the three-parameter Guggenheim–Anderson–De Boer (G.A.B.) equation [stages I–II (h=∞)].The t.s.s. isotherm, which necessarily uses a fourth parameter, retains the mathematical form of the older isotherms with two correction functions determined by the new constant h(extension of stages I and II), functions which become important only at high relative activities of the sorbate. The sorption stage III is evidenced by experimental data in this region : here the G.A.B. isotherm demands that the inverse of the mass sorbed per unit mass of sorbent should be linear in p/p°, while experimentally a deviation downwards is found, which is explained by the t.s.s. isotherm.The four parameters, determined by versatile, easy to apply, graphical methods, show a certain correlation with the type of interactions characterizing the experimental systems. With longer ranged interactions the extent of the sorption stage II increases from h= 8–15 (gas/solids) to h= 15–20 (water/biopolymers) and to h= 20–25 (water/electrolytes–polyelectrolytes). The applicability range of the t.s.s. isotherm extends up to at least p/p°= 0.95.

General isotherm equation for adsorption of surfactants at solid/liquid interfaces. Part 2. Applications

Abstract: This work describes a number of results of surfactant adsorption (including non-ionic, cationic and anionic surfactants on various adsorbents) illustrating the applications of the general isotherm equation proposed in part 1. The results show that the calculated curves from the general isotherm equation can represent the experiment data very well. Based on the calculation of the thermodynamics of adsorption, it is evident that the principal contribution to the ΔG°hm of negative value is the large negative value is the large positive value of ΔS°hm, whereas ΔH°hm is positive. Therefore, similar to the micellization in bulk solution, the second step of surfactant adsorption (see part 1) is an entropy-driven process. For S-type and LS-type isotherms, two approaches and corresponding equations for calculation of h.m.c. have been proposed respectively, and the calculated h.m.c.s. by both approaches agree with the empirical value very well.

Atomistic simulation of the surfaces of oxides

Abstract: Recent simulation studies of the structure, energetics, dynamics and impurity segregation properties of oxide surfaces are reviewed. Results are included for MgO, CaO, SrO, Li2O, c-ZrO2, SrTiO3, Al2O3, Fe2O3 and La2CuO4.

Reversible volume change of microparticles in an electric field

Abstract: Microparticles of the cross-linked sodium salt of poly(acrylate) with a diameter of 150–300 µm undergo reversible shrinkage when a d.c. electric field is applied. The shrinkage is rapid, and 90% volume change is reached within 50 s by applying an electric field of 46 V cm–1(0.3 mA). The rate of volume change is proportional to the d.c. current . From the contractile behaviour and pH measurements the contraction is explained in terms of ion transport of counter-microions in the electric field (electrokinetic phenomena).

Mechanism and solvent dependence of the solvent-catalysed pseudo-intramolecular proton transfer of 7-hydroxyquinoline in the first electronically excited singlet state and in the ground state of its tautomer

Abstract: Picosecond time-resolved fluorescence of solutions of 7-hydroxyquinoline in neat alcohols has revealed two different excited-state tautomerization processes There is a fast process which appears as the only one in the case of solutions in cyclohexane containing a small amount of alcohol This fast process occurs within electronically excited 1:2 solute–alcohol complexes The slower process is associated with tautomerization in excited solute–alcohol complexes which contain, at least initially, more than two solvent molecules The size of the alcohol molecule determines the relative importance of the two processes in the phototautomerizationThe reverse double proton transfer process of the tautomer in its ground state has been observed by transient absorption spectroscopy The groundstate tautomer decays monoexponentially with a rate constant depending both on the nature of the alcohol and on its concentration in the solution In the limit of low alcohol concentrations, the rate constant correlates with the proton donor strength of the alcohol The rate constant is temperature-dependent, and has a deuterium isotope effect which is temperature-independent A previously suggested explanation in terms of a two-step process also applies here The first step involves thermally activated solvent reorganization to achieve a proper conformation of a solute–alcohol complex The second step is considered to be a thermally unassisted proton-tunnelling process in the complex with the proper conformation

The formation of a well defined rhodium dicarbonyl in highly dealuminated rhodium-exchanged zeolite Y by interaction with CO

Abstract: I.r. spectra with unusually sharp (f.w.h.m.⩽ 5 cm–1) carbonyl bands and their corresponding 13CO satellites follow from the interaction of natural CO with both reduced and oxidized rhodium (1 wt %) introduced into dealuminated zeolite Y (US-Ex). Isotopic exchange with 13C16O, 12C18O, and 13C18O confirms the proposed formation of a well defined dicarbonyl compound with C2v symmetry and an angle of ca. 106° between the two CO groups. The formation of the dicarbonyl with reduced Rh/US-Ex at 300 K is accompanied by a loss of intensity of the hydroxyl bands of the zeolite, showing the participation of the hydroxyl groups in the formation of RhI from small rhodium clusters. The additional intensity increase of the dicarbonyl bands at 423 K proceeds without an intensity loss of the hydroxyl bands and indicates the formation of RhI by another process, presumably by CO dissociation. In addition to the carbonyl-stretching bands, combination bands are detected, which allow the estimation of other fundamental bands of the RhI(CO)2 system masked by strong bands of the zeolite framework. During the formation of the Rh dicarbonyl there are no significant changes in the far-i.r. region (400–50 cm–1) to be assinged to cation motions within the zeolite framework.