Showing papers in "Journal of the Chemical Society, Faraday Transactions in 1988"
TL;DR: In this paper, the most efficient catalysts were found to be IrO2, Co3O4, RuO2 and NiCo2O4 and Mn2O3 for the fourelectron oxidation of water to O2 under photochemical conditions.
Abstract: Metal oxides, in the form of dispersed powders, have been tested as potential catalysts for the four-electron oxidation of water to O2 under photochemical conditions. The most efficient catalysts were found to be IrO2, Co3O4, RuO2, NiCo2O4, Rh2O3 and Mn2O3 and, in particular, high activity was observed with IrO2. Comparison of the oxide structure with its observed rate of O2 generation under standard conditions has allowed formulation of a few general requisites for an effective catalyst. Samples of iridium oxide deposited onto the surface of a second (inert) oxide were tested for their O2-evolving capability. The efficiency of the system depended markedly upon the nature of the support. Materials that favour formation of small deposits of iridium oxide (e.g. ZnO, MgO, TiO2) are the best supports, whilst O2 production is almost completely inhibited with acidic supports. Many metal oxides can be prepared in the form of hydrates of variable composition. These materials also function as O2-evolving catalysts, the efficiency of the process depending upon any thermal pretreatment. This finding is explained in terms of changes in structure and composition of the oxide that occur upon heating.
505 citations
TL;DR: The instantaneous shear modulus G and compactive strength Py of aggregate networks formed from silica particles with a mean diameter of 26 nm have been determined as a function of particle concentration as mentioned in this paper.
Abstract: The instantaneous shear modulus G and compactive strength Py of aggregate networks formed from silica particles with a mean diameter of 26 nm have been determined as a function of particle concentration. The data are compared with similar data obtained earlier for a range of polystyrene spheres with diameters between 60 and 960 nm and with compactive strength data obtained for polystyrene spheres at higher volume fractions by Sutherland. It is shown that clusters of submicron spheres formed by rapid aggregation become spacefilling and form a network at a critical volume fraction Φg of ca. 0.05. Above this concentration the data for Py and G suggest that aggregate networks show universal behaviour which is consistent with the scalings G∼ϕµ, dPy(ϕ)//d ln ϕ∼G(ϕ), with µ= 4 ± 0.5. This latter value for the exponent agrees well with that predicted by Ball and Brown by assuming the clusters comprising the network are fractal. For diffusion-limited cluster–cluster aggregation (DCA) they obtained a value of µ= 3.6. The data for Py imply a particle size dependence of the type Py∼am with m between –2 and –3, where a is the particle radius. More data are required to establish the precise dependence; the observed trend is, however, not inconsistent with what might be expected from a consideration of interparticle forces which implies a scaling of a–2.3. The scaling behaviour of the yield stress in shear flow and the dependence of the shear modulus on strain for non-negligible strains is also discussed.
357 citations
TL;DR: Two pure anatase preparations and one sulphate-contaminated preparation have been treated in vacuo at different temperatures, focussing the attention on the earliest stages of dehydration as well as on rehydration as discussed by the authors.
Abstract: Two pure anatase preparations and one sulphate-contaminated preparation have been treated in vacuo at different temperatures, focussing the attention on the earliest stages of dehydration as well as on rehydration. The hydrated layer is made up of hydroxyl groups and of un-dissociated water molecules coordinated at surface cationic centres. The vacuum removal of the latter component, which starts at low temperatures and is complete at some 470 K, creates strong Lewis-acidic sites that reversibly chemisorb CO at ambient temperature up to a coverage of 10–20%. This COads species is very sensitive to inductive effects produced by surface species, including CO itself, and is quite useful in revealing the dehydration/rehydration stage of the surface as well as any alteration of the distribution between OH-bearing and H2O-bearing centres, produced by the vacuum thermal treatments. The vacuum removal of OH groups, which starts at temperatures as high as 450 K, creates only a small number of Lewis acidic sites that are strong enough to chemisorb CO. Sulphate contamination, which derives from the preparative route of the anatase, severely modifies the surface properties, in that it eliminates virtually all of the surface OH centres and produces strong inductive effects through the solid that are monitored by the CO species adsorbable at the sites which, when hydrated, coordinate water with a higher adsorption heat and a higher CO stretching frequency.
256 citations
TL;DR: In this paper, the effect of experimental conditions on temperature-programmed reduction profiles has been studied and a criterion to select appropriate experimental conditions through a simple parameter that relates heating rate (β), sample size (S0), and hydrogen feed to the reactor (FC0), is given.
Abstract: The effect of experimental conditions on temperature-programmed reduction (t.p.r.) profiles has been studied. In the case of t.p.r. curves that include several reduction steps an inadequate selection of these parameters leads to distorted profiles; these distortions may produce a complete loss of resolution for the different reduction steps. A criterion to select appropriate experimental conditions through a simple parameter that relates heating rate (β), sample size (S0) and hydrogen feed to the reactor (FC0), is given. Finally, the conditions required to obtain reliable activation energies for the reduction processes from the variation in temperature of the t.p.r. maxima with β, without making any previous assumptions about the reduction kinetics, are discussed.
232 citations
TL;DR: X-ray absorption spectroscopy has been employed to clarify the environmental structure around vanadium atoms in silica-supported and γ-alumina-supported vanadium oxide catalysts as discussed by the authors.
Abstract: X-Ray absorption spectroscopy has been employed to clarify the environmental structure around vanadium atoms in silica-supported and γ-alumina-supported vanadium oxide catalysts. Catalysts containing 2.8% of vanadium by weight were prepared with NH4VO3 and VO(acac)2 as impregnation agents. X.p.s. (V 2p3/2) of the catalysts showed that the vanadium atoms in these catalysts are pentavalent. E.s.r. signals from V4+ in the reduced catalysts indicated that paramagnetic VO4–4 and (VO)2+ ions are generated by the reduction on silica and alumina, respectively. Although the dispersion of vanadium oxide in the catalysts prepared with VO(acac)2 solution was found to be higher than those prepared with NH4VO3, the XANES and EXAFS spectra do not exhibit a significant difference for the two types of the catalyst, indicating that the dominant surface species are the same at such a low level of vanadium loading. Analysis of the XANES spectra suggests that VO4 tetrahedra are the dominant species on alumina and the vanadates on silica are square pyramidal; part of the vanadium species is present as V2O5 microcrystallites. EXAFS spectra of the catalysts and their Fourier-transforms show that VO4 units are isolated on alumina, and that the majority of vanadates on silica are polymeric. These findings by EXAFS/XANES spectroscopy indicate that VO4 on silica and VO5(or VO6) on γ-alumina, as detected by e.s.r., are only a minority species on each support even at a low level of loading of vanadium.
223 citations
TL;DR: In this article, the influence of surface additions of precious metals and of metal oxide particles on the gas response behavior of tin dioxide is described, and it is found that the use of surface additives capable of either pinning the Fermi level of the tin dioxide or altering the rate of combustion would profoundly affect the gas sensor response of the material.
Abstract: It has been shown earlier (J. F. McAleer, P. T. Moseley, J. O. W. Norris and D. E. Williams, J. Chem. Soc., Faraday Trans. 1, 1987, 83, 1323) that the gas response of porous pellets of tin dioxide depends crucially on electronic surface states involving adsorbed oxygen, and on the rates of combustion reactions involving the gases to be detected. It is expected, therefore, that the use of surface additives capable of either pinning the Fermi level of the tin dioxide or altering the rate of combustion (depending on choice of material and temperature) would profoundly affect the gas sensor response of the material. This is found to be the case. Studies of the influence of surface additions of precious metals and of metal oxide particles on the gas response behaviour of tin dioxide are described. The use of precious metals at temperatures near to ambient imposes an oxygen-independent Schottky barrier on the tin dioxide surface and results in a distinct low-temperature response to carbon monoxide, probably by adsorption on the precious metal modifying the surface potential (and hence the Schottky barrier). This particular effect is expected only when the precious metal particles distributed over the oxide surface are extremely small (1–10 nm): the effect disappears if the catalyst particles are aggregated by heating. The distribution of particles of a foreign oxide (here Ag2O or ZnO) on the surface of the tin dioxide also appears to pin the surface-energy levels to those of the additives, and tin dioxide treated in this way exhibits gas-sensing properties that are modified to resemble those of the additive oxide in bulk. At higher temperatures, especially in the presence of a precious-metal catalyst, the gas is completely oxidised within a thin outer shell of the specimen and consequently the response to the gas of the measured resistance of the pellet disappears. Indeed, a response to the reaction products (H2O and CO2) is obtained.
184 citations
TL;DR: In this article, the authors measured thermomechanical properties characteristic of each non-crystallizing solute: T′g, the subzero glass transition temperature of a maximally freeze-concentrated solution, and W′g, the amount of unfrozen water (UFW) kinetically immobilized in the glass at T´g.
Abstract: Aqueous solutions of 24 small carbohydrates have been analysed by low-temperature differential scanning calorimetry (d.s.c.). The method, based on analogue derivative thermograms, measured two thermomechanical properties characteristic of each non-crystallizing solute: T′g, the sub-zero glass transition temperature of a maximally freeze-concentrated solution and W′g, the amount of unfrozen water (UFW) kinetically immobilized in the glass at T′g. For 84 low-molecular-weight polyhydroxy compounds (PHCs) analysed to date, T′g ranged from –85 °C for ethylene glycol to –13.5 °C for maltoheptaose, and increased monotonically with increasing Mw. T′g plotted vs.M–1w showed a linear correlation characteristic of an homologous family of glass-forming linear oligomers. W′g ranged from 1.9 g UFW g–1 for ethylene glycol to 0.2–0.3 g UFW g–1 for several sugars and polyols, including maltoheptaose, and decreased with increasing T′g, showing fair linear correlations for several series of homologous solutes. We describe here the use of T′g and W′g, as invariant physico-chemical properties of glass-forming solutions at sub-zero temperatures, to interpret thermomechanical behaviour of small-carbohydrate–water systems in non-equilibrium glassy and ‘rubbery’ states, define structure–activity relationships and explain and often predict functional behaviour of such PHCs in various applications.
170 citations
TL;DR: In this paper, the use of magic-angle spinning to give CRAMPS is discussed, and the relationship between proton isotropic chemical shifts and hydrogen-bond distances is explored in some detail.
Abstract: Recent trends of general interest in high-resolution NMR of solids are mentioned, and some examples briefly discussed. Stress is laid on the use of the multiple-pulse technique, particularly when combined with magic-angle spinning to give CRAMPS. Two studies of proton CRAMPS are described, one concerning hydrogen bonding in carboxylic acids and the other involving phosphonic acid derivatives. The relationship between proton isotropic chemical shifts and hydrogen-bond distances is explored in some detail. A clear correlation is found.
161 citations
TL;DR: In this article, a linear correlation between the radius of the free space available to the xenon atom and the chemical shift was shown to be a function of the number of clathrate hydrates and a clathrasil sample.
Abstract: 129 Xe NMR spectra were obtained for xenon trapped in the cages of clathrate hydrates and a clathrasil sample. The data, together with shift data for solid xenon, have yielded a linear correlation between the radius of the free space available to the xenon atom and the chemical shift. This observation cannot be rationalized by using simple binary collision theory. In order to account for the observation of anisotropic chemical shifts for xenon trapped in non-spherical environments a simple multiple-site model was developed. Several applications of 129Xe NMR spectroscopy are also presented. These include the identification of new clathrate hydrates, the use of 129Xe NMR to follow changes in site symmetry in a clathrasil and a cyclodextrin inclusion compound, and the observation of trapping sites in crystalline and amorphous solids.
156 citations
TL;DR: In this paper, the spin diffusion in 13C NMR of double 13C-labeled sodium acetate trihydrate (SAC) and in 31P NMRI of zinc(II) bis(O,O′-diethyldithiophosphate)(ZNP) has been studied under magic-angle spinning conditions.
Abstract: Spectral spin diffusion in 13C NMR of double 13C-labelled sodium acetate trihydrate (SAC), and in 31P NMR of zinc(II) bis(O,O′-diethyldithiophosphate)(ZNP) has been studied under magic-angle spinning conditions. Spin-diffusion time constants, TSD, were determined from the intensities of the spinning sidebands in experiments using rotation-synchronized DANTE pulse sequences, at several different spinning frequencies. The theory of Suter and Ernst, developed for spectral spin diffusion in single crystals, was extended to the case of polycrystalline samples rotating under magic-angle spinning conditions. We considered two mechanisms for the spin diffusion, i.e. dipolar interaction and J-coupling. The spin-diffusion time constants, TSD, were related to the zero-quantum lineshape functions in a manner similar to the theory of Suter and Ernst. The zero-quantum lineshape functions were estimated from the observed single-quantum lineshape functions. In the present studies the dependence of the experimental values for TSD on the rotational frequency vr are in good agreement with those calculated from the theory based on the dipolar interaction mechanism. The values of TSD for SAC showed a deep minimum at Δω≈ 2ωr, and a shallow minimum at Δω≈ 3ωr. This phenomenon is rotational relaxation resonance.
153 citations
TL;DR: In this paper, the ET polarity values for binary liquid mixtures, derived from the solvatochromic behavior of a pyridinium betaine, have been used to investigate the preferential solvation of the betaine by the components of 17 binary solvent mixtures.
Abstract: The ET polarity values for binary liquid mixtures, derived from the solvatochromic behaviour of a pyridinium betaine, have been used to investigate the preferential solvation of the betaine by the components of 17 binary solvent mixtures. In systems involving two organic liquids the ET values have been interpreted in terms of the solvation of the betaine by the more polar component and some agreement with previous n.m.r. studies was obtained. In aqueous–organic solvent mixtures, however, the betaine was preferentially solvated by the organic component. The findings have been related to the rates of solvolysis in mixed-solvent systems, and preferential solvation is suggested as a reason for the solvent dependence of the rates of reaction.
TL;DR: In this article, partial molar heat capacities and volumes for a homologous series of amino acids and dipeptides have been measured in aqueous 1 mol kg-1 glucose and sucrose solutions at 298.15 K using flow microcalorimetry and densimetry, respectively.
Abstract: Partial molar heat capacities and volumes for a homologous series of amino acids and dipeptides have been measured in aqueous 1 mol kg–1 glucose and sucrose solutions at 298.15 K using flow microcalorimetry and densimetry, respectively. These data have been utilized, in conjunction with the data obtained for them in water earlier, to deduce the partial molar heat capacities and volumes of transfer from water to 1 mol kg–1 aqueous glucose and sucrose solutions. A comparison of these transfer parameters with similar ones in aqueous sodium chloride and calcium chloride solutions is made. The results are explained using the cosphere overlap model and the factors governing the stability of proteins in these sugar solutions have been discussed.
TL;DR: In this paper, the adsorption and reaction of H2, CO, CO2, OH(a)+ CO(g) and CO2(a+H2(g)) have been studied in detail by infrared spectroscopy.
Abstract: The adsorption and reaction of H2, CO, CO2, OH(a)+ CO(g) and CO2(a)+ H2(g) have been studied in detail by infrared spectroscopy. Hydrogen is dissociatively adsorbed to form the OH and the Zr—H species and CO is weakly adsorbed as the molecular form. The infrared spectrum of adsorbed species of CO2 over ZrO2 shows three main bands at ca. 1550, 1310 and 1060 cm–1 which can be assigned to the bidentate carbonate species. The reaction of OH(a)+ CO(g) at 373 K gave rise to formate and bidentate carbonate species, and after having introduced hydrogen onto this system at 473 K, methoxide species appeared. When hydrogen was introduced over CO2-preadsorbed ZrO2, formate and methoxide species also appeared. It is concluded that the formation of the formate and methoxide species results from the hydrogenation of bidentate carbonate species.
TL;DR: In this article, the backward implicit (BI) method was applied to describe complex electrode reaction mechanisms involving coupled homogeneous kinetics and several kinetic species, taking place at channel electrodes, and it was shown how the transport-limited currents and current-voltage curves associated with these processes may be computed.
Abstract: It is shown how the backwards implicit (BI) method may be applied to the description, under steady-state conditions, of complex electrode reaction mechanisms involving coupled homogeneous kinetics and several kinetic species, taking place at channel electrodes. Specifically the EC, ECE, DISP1 and DISP2 mechanisms are examined, and it is shown how the transport-limited currents and current–voltage curves associated with these processes may be computed. The advantages of the method, particularly when compared with analytical approaches, are stressed.
TL;DR: In this article, a sensor for glucose formed in a one-step process is described based on a new doped polyindole film coated onto a platinum surface; a polymer-entrapped glucose oxidase electrode can be operated as an amperometric glucose sensor.
Abstract: A sensor for glucose formed in a one-step process is described based on a new doped polyindole film coated onto a platinum surface; a polymer-entrapped glucose oxidase electrode can be operated as an amperometric glucose sensor. The new glucose sensor has fast response time (25–40 s) with high storage and operational stability ( > 35 days). Electrochemical rate constants for the new enzyme electrode have been calculated using the expressions derived by Albery et al.(J. Chem. Soc., Faraday Trans. 1, 1986, 82, 1033). It has been found that unsaturated enzyme kinetics are rate-limiting for conducting polymer-coated enzyme electrodes formed in a one-step process.
TL;DR: In this article, an experimental technique for measuring coalescence times of emulsion sized oil droplets at a planar oil-water interface is described, and the authors show that there is a positive correlation between coalescence stability and the mechanical strength of protein films adsorbed at the oil/water interface.
Abstract: An experimental technique for measuring coalescence times of emulsion sized oil droplets at a planar oil–water interface is described. Distributions of coalescence times as a function of droplet size are presented for systems with n-hexadecane as the oil phase and a protein solution of β-casein, κ-casein or lysozyme as the aqueous phase (10–4 wt% protein, pH 7, ionic strength 0.1 mol dm–3, 25 °C). When the planar interface is aged for only 20 min there is no measurable difference in the distribution of coalescence times for the three proteins. However, when the planar interface is aged for 72 h there is a sizeable fraction of droplets, different for the three proteins, which does not coalesce at all, and the relative efficiencies of the adsorbed proteins in preventing coalescence lie in the order: lysozyme > κ-casein > β-casein. Time-dependent surface shear viscosities at the oil–water interface are reported for adsorbed films of the same three proteins under exactly the same experimental conditions. After 20 min the surface viscosity values for all three proteins are low ( < 1 mN m–1 s), but after 72 h their values are distinctly different: 200 mN m–1 s (lysozyme), 60 mN m–1 s (κ-casein) and < 1 mN m–1 s (β-casein). These results are consistent with the view that there is a positive correlation between coalescence stability and the mechanical strength of protein films adsorbed at the oil–water interface.
TL;DR: In this paper, the trimerization equilibrium constant of rhodamine 6 G in aqueous solution as determined by absorption spectroscopy is higher than that of the dimer previously reported.
Abstract: The trimerization equilibrium constant of rhodamine 6 G in aqueous solution as determined by absorption spectroscopy is higher than that of the dimer previously reported. Similarities observed between the absorption spectra of both aggregates are explained by exciton theory considering a sandwich structure for the aggregates. The lack of emission from the aggregates implies an important decrease in the fluorescence quantum yield of the dye, and the rate constant of the quenching produced by the dimer and trimer are calculated separately. The dominant mechanism for the quenching seems to be long-range energy transfer from the monomer to the aggregates, being affected by molecular diffusion as well as excitation energy migration between monomers.
TL;DR: In this paper, the dealumination process of a Y zeolite was studied and it was shown that it leads to the formation of silica and/or silica-alumina type phases.
Abstract: 29 Si and 27Al m.a.s.n.m.r. techniques have been used to study the dealumination process of a Y zeolite. Dealumination by steaming or SiCl4 treatment leads to the formation of silica and/or silica–alumina type phases. Extraframework aluminium shows a different type of coordination (tetra-, penta- or octa-hedral) depending on the method of dealumination.
TL;DR: In this article, a functional integral representation of the grand canonical classical statistical integral for the inhomogeneous Coulomb fluid is derived and the thermodynamic potential is obtained in a closed form if the Gaussian approximation for the fluctuations around the mean electrostatic potential is used.
Abstract: A functional integral representation of the grand canonical classical statistical integral for the inhomogeneous Coulomb fluid is derived. The charged species are confined between two interfaces, also defining the dielectric inhomogeneity in the system, bearing constant surface charges. The thermodynamic potential is obtained in a closed form if the Gaussian approximation for the fluctuations around the mean electrostatic potential is used. The formalism embodies the mean field (Poisson–Boltzmann) terms generalized by the presence of image interactions plus the correlation (fluctuation) terms, which give significant correction to the classical expressions for the force between charged interfaces. The numerical results for a counterion-only system with charged surfaces are treated in detail and compared with simulation data.
TL;DR: The chlorine atom (Cl˙) has been generated in aqueous solution by reaction of Cl˙ with SO˙-4 and H2PO˙4, obtained by metal-catalysed decomposition of the appropriate peroxides.
Abstract: The chlorine atom (Cl˙) has been generated in aqueous solution by reaction of Cl– with SO˙–4 and H2PO˙4, obtained by metal-catalysed decomposition of the appropriate peroxides. E.s.r. experiments in conjunction with a fast-flow method establish that Cl˙ is highly reactive, readily undergoing rapid addition, hydrogen-abstraction and electron-transfer reactions (k≈ 108–109 dm3 mol–1 s–1). The factors which influence the observed selectivity (energetics and polar effects) are discussed.
TL;DR: In this paper, the mesophase sequence of lyotropic liquid-crystalline phases occurring in a range of cationic surfactants with water has been determined using the penetration optical microscopy technique.
Abstract: The sequence of lyotropic liquid-crystalline phases occurring in a range of cationic surfactants with water has been determined using the ‘penetration’ optical microscopy technique. Of the order of 50 different surfactants were examined. The variation of the polar group structure includes the series —NH+3, —NMe+3, NEt+3, NPr+3 and NBu+3. In addition, the effect of altering counter-ion and alkyl-chain length are also examined. Other surfactants studied include 2-hydroxyethyl alkylammonium salts, nitroxide ring substituents, alkyl pyridinium salts and alkyl indole surfactants where the indole substituent is part of the hydrophobic chain.The mesophase sequences are similar to the patterns of behaviour already known for non-ionic and anionic surfactants, with hexagonal (H1), bicontinuous cubic (V1) and lamellar (Lα) being the most frequently observed phases (in addition to micellar solution). For long-chain surfactants, intermediate (Int) rather than V1 phases are shown to form at temperatures just above the Krafft boundary. Qualitatively, much of the phase behaviour can be analysed in terms of alkyl chain packing constraints and intermicellar forces. For bulky alkyl headgroups mesophase formation is suppressed, but the hydroxyethyl substituent causes surprisingly little change from a trimethylammonium headgroup. The behaviour of the indole surfactants suggests that the indole group resides mainly in the micelle interior.
TL;DR: In this article, high-field proton magic-angle sample-spinning (MAS) spectroscopy is used to yield high-resolution 1H spectra of smectic, nematic and hexagonal-II phase lipids, from which isotropic chemical shifts, order parameters and relaxation times can be determined.
Abstract: High-field proton magic-angle sample-spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy is shown to yield high-resolution 1H spectra of smectic, nematic and hexagonal-II phase lipids, from which isotropic chemical shifts, order parameters and relaxation times (T1, T1ρ and T2) can be determined. Such experiments are possible because of the special form of the dipolar Hamiltonian in such systems. Resolution is about the same as that obtained with sonicated systems, using conventional NMR techniques. We also show that 13C MAS NMR spectra, of both fluid and solid phases, are even better resolved, and in some cases resonances can be observed in MAS NMR spectra which are not observable in sonicated systems. For example, essentially all of the carbon atoms in cholesterol (CHOL) can be readily detected and assigned in a lecithin–CHOL bilayer, using MAS, while few can be seen in sonicated bilayers. This leads directly to the observation of cholesterol in intact biological membranes, such as human myelin, where over 50 peaks can be observed, and ca. 40 of these resonances can be assigned to specific, single-carbon-atom sites in the membrane. In addition, a number of experiments with massively deuterated lipids are reported. Combination of cross-polarization techniques with MAS, and difference spectroscopy, leads to the observation of essentially pure sterol spectra (in the presence of lipid) and pure lipid spectra (in the presence of CHOL). Analysis of chemical-shift results indicates a substantial deshielding of chain carbon atom resonances caused by the presence of CHOL, due presumably to increased trans chain segments, an effect mirrored in variable temperature spectra of human myelin, and in goldfish myelin. Taken together, these results suggest a resurgence in NMR studies of membranes may soon occur.
TL;DR: In this paper, the conversion of methanol into hydrocarbons over ZSM-5 zeolites has been studied with a special emphasis on improving the selectivity to aromatic molecules.
Abstract: The conversion of methanol into hydrocarbons over ZSM-5 zeolites has been studied with a special emphasis on improving the selectivity to aromatic hydrocarbons. The introduction of zinc or gallium cations to ZSM-5 appreciably increased the yield of or selectivity for aromatic hydrocarbons. Thus, under the reaction conditions of 700 K, methanol pressure of 40 kPa and W/F= 9.0 g h mol–1, the yields of aromatic hydrocarbons over H-ZSM-5, Ga-ZSM-5, and Zn-ZSM-5 were 40.3, 48.2 and 67.4%, respectively. Over Zn-ZSM-5 the distribution of aromatic products on a molar basis was as follows; benzene (5.8%), toluene (28.0%), xylenes + ethylbenzene (45.6%) and C9+(20.6%). The reaction conditions were optimized to obtain the highest yield of aromatic hydrocarbons over Zn-ZSM-5. The yield of aromatics increased with temperature up to 700 K, but a further temperature increase led to the decomposition of methanol to carbon oxides. The selectivity for aromatics gave a maximum of 70% at a methanol partial pressure of 22 kPa. The yield of aromatic hydrocarbons increased with the degree of cation-exchange of H-ZSM-5 with Zn-cations, indicating the essential role of zinc species in the enhancement of the selectivity to aromatics. It is concluded that the zinc species has a capability of efficiently converting alkene intermediates to aromatic hydrocarbons.
TL;DR: In this article, the rotation-vibration energies of triatomic molecules were derived from the Born-Oppenheimer potential-energy function and the refinement of the parameters in an analytical expression for the potential energy function by least squares fitting to experimental data.
Abstract: The present communication is concerned with the calculation of rotation–vibration energies for triatomic molecules directly from the Born–Oppenheimer potential-energy function and the ‘inversion’ of this process: the refinement of the parameters in an analytical expression for the potential-energy function by least-squares fitting to experimental data. Three approaches to these problems are presented. First the non-rigid-bender model (see P. Jensen and P. R. Bunker, J. Mol. Spectrosc., 1986, 118, 18, and references therein) is briefly outlined, and secondly a Morse-oscillator-based model for equilateral triangular molecules is described. The primary molecules for which the latter model is applicable are the H+3 molecular ion and its isotopes (see P. Jensen and V. Spirko, J. Mol. Spectrosc., 1986, 118, 208, and references therein). With this method, a preliminary experimental potential-energy surface for H+3 has been obtained from experimental data. In the non-rigid bender model, the focus was on obtaining an accurate description of the bending and rotation of the triatomic molecule, whereas in the model for H+3, the primary aim was to improve the description of the stretches. The experience gathered in the work with these two models has been used as the basis for the Morse-oscillator–rigid-bender internal dynamics (MORBID) Hamiltonian (P. Jensen, J. Mol. Spectrosc., in press) which is presented as the third method for calculating the rotation–vibration energies of a triatomic molecule; MORBID combines the non-rigid-bender description of the bending and rotation motion with a Morse-oscillator description of the stretching. With MORBID, rotation–vibration energies for H2O are calculated from a published ab initio potential-energy surface.
TL;DR: In this article, a thermal treatment of TiO2 powders with various surface hydroxyl densities has been conducted and their activity for photocatalytic oxygen evolution in aqueous suspensions has been investigated in the presence of various electron acceptors.
Abstract: TiO2 powders with various surface hydroxyl densities have been prepared by thermal treatment (600–1000 °C), and their activity for photocatalytic oxygen evolution in aqueous suspensions has been investigated in the presence of various electron acceptors, i.e. AgNO3, K2PtCl4 and KBrO3. The activity increased drastically with the thermal treatment. The oxygen evolution rate was found to be inversely proportional to the surface concentration of OH–, indicating an inhibitory effect of surface hydroxyl groups with respect to photocatalytic oxygen evolution. On highly hydroxylated samples the formation of surface bound peroxides appears to be favoured over oxygen generation.
TL;DR: In this paper, the self-association of Rhodamine 6G in ethanol has been studied by determining the dimerization and trimerization constants at different temperatures and the excitonic interpretation of the absorption spectra suggests a linear structure for the aggregates.
Abstract: The self-association of Rhodamine 6G in ethanol has been studied by determining the dimerization and trimerization constants at different temperatures. The excitonic interpretation of the absorption spectra suggests a linear structure for the aggregates. The nature of the bonding has been also studied.
TL;DR: In this article, water sorption as a function of activity (aH2O) by H+-, Li+-, N(CH3)4++- and Cs+-exchanged forms of a perfluorosulphonate ion exchanger (Nafion-117) and by 4 and 8% crosslinked Dowex 50 W resins has been investigated in order to derive sorption isotherms using the isopiestic technique.
Abstract: Water sorption as a function of activity (aH2O) by H+-, Li+-, N(CH3)4+- and Cs+-exchanged forms of a perfluorosulphonate ion exchanger (Nafion-117) and by 4 and 8% crosslinked Dowex 50 W resins has been investigated in order to derive sorption isotherms using the isopiestic technique. All exchangers show the sequence of water sorption as H+ > Li+ > N(CH3)4+ > Cs+. However, the H+ and Li+ forms of Nafion-117 absorb significantly larger amounts of water, while the Cs+ form sorbs much less water at all water activities compared to the corresponding ionic forms of Dowex 50W × 8, the N(CH3)+4 isotherms of the two exchangers being quite close to each other. Sorption of water by the 4 and 8% crosslinked Dowex 50W resins is found to be the same at low aH2O( Dowex 50W × 8 > Dowex 50W × 4 (any ionic form). The results are discussed in relation to the relative degrees of hydration of the (respective) ions, including the sulphonate anion, and the differences between the two types of ion exchangers.
TL;DR: In this paper, a new reaction-path model for describing reactions in polyatomic molecular systems is presented, based on the cartesian coordinates of the atoms, which leads to a simpler treatment of the interaction between the reaction coordinate and the bath.
Abstract: A new kind of reaction-path model for describing reactions in polyatomic molecular systems is presented, based on the cartesian coordinates of the atoms. Not only does this lead to a simpler treatment of the interaction between the ‘system’(i.e. the reaction coordinate) and the ‘bath’(the other degrees of freedom) than earlier reaction-path, models based on the (curvilinear) steepest-descent reaction path, but in many cases it also provides a more natural description of the dynamics. The resulting Hamiltonian has the standard form of a cartesian ‘system’ linearly coupled to a harmonic ‘bath’, the dynamics of which is treated in this paper by the basis set method of Makri and Miller (J. Chem. Phys., 1987, 86, 1451). Application to a model of H-atom transfer in malonaldehyde shows that the overall approach (both the cartesian reaction-path model and the basis-set method treatment of the dynamics) is quite successful.
TL;DR: In this article, the catalysts were characterized by chemical analyses, X.i.r.t. and F.s.r., e.g. spectroscopies, and thermogravimetry and tested in the oxidation of o-xylene to phthalic anhydride and in the ammoxidation of toluene to benzonitrile.
Abstract: V–Ti–O (rutile) catalysts have been prepared by hydrolysis of VIV–TiIV solutions and low-temperature calcination (673 K). The catalysts were characterized by chemical analyses, X.r.d., e.s.r. and F.t.i.r. spectroscopies, and thermogravimetry and tested in the oxidation of o-xylene to phthalic anhydride and in the ammoxidation of toluene to benzonitrile. Different vanadia species were identified: (i) a VIV species present in substitutional solid solution within the rutile lattice, which is stable as regards both oxidation and reduction, (ii) a VV species, chemically interacting with the rutile surface and constituting the so-called monolayer which is easily reducible to VIV, (iii) surface VO2+ ions formed by reduction during the catalytic tests from the VV monolayer species and (iv) highly disordered and poorly crystalline V2O5. The VO2+ surface ions formed during the catalytic tests are thought to be the active sites of hydrocarbon activation during both oxidation and ammoxidation. A modification of the chemical structure occurs during permanence in the reaction environment, this modification depending on the reductive power of the gas phase. In the case of the ammoxidation of toluene a change in the relative amounts of the different vanadia species occurs, while in the case of o-xylene oxidation only an higher average degree of reduction is observed, as compared to the samples before reaction. A comparison with V–Ti–O (anatase) samples suggests that the crystalline structure of TiO2 does not influence the activity of the V–Ti–O catalysts, but possibly the number of surface VO2+ ions does, and is strongly related to the method of preparation and to the activation procedure.
TL;DR: In this article, the mass-action model was applied to calculate the aggregation number n of C8E4 micelles in the presence of 0.1 mol kg−1 KCl at 25 °C.
Abstract: Calorimetric measurements have been made of the differential enthalpies of solution in water of liquid n-octyl tetraoxyethylene glycol monoether C8E4 and Triton X-100, as function of concentration, at three different temperatures. Experiments have also been carried out with C8E4 dissolved in 0.1 mol kg–1 KCl at 25 °C. Enthalpies of solution of monomers at infinite dilution and micelles at the c.m.c., respectively, were calculated and enthalpies of micelle formation have been derived. Heat-capacity changes for dissolution and for micelle formation were calculated from the temperature variation of the solution enthalpies. The aggregation number n of C8E4 micelles has been derived from the calorimetric titration experiments by applying the mass-action model to micelle formation. At 25 °C a value of n= 23 is found which is compatible with the formation of small spherical micelles with a radius equal to the length of the extended n-octyl chain. There are no indications of micellar growth from the calorimetric results as the liquid–liquid phase boundary is approached in the C8E4–water system. The presence of 0.1 mol kg–1 KCl had no significant effect on the enthalpies of solution and micelle formation of C8E4 and had no detectable influence on the aggregation number.