Showing papers in "Journal of the Chemical Society, Faraday Transactions in 1991"
TL;DR: In this paper, the standard molar Gibbs free energies of hydration, ΔhydG°, of 109 (mainly inorganic) ions ranging in their charges from −3 to +4 have been compiled and interpreted in terms of a model used previously for other thermodynamic quantities.
Abstract: The standard molar Gibbs free energies of hydration, ΔhydG°, of 109 (mainly inorganic) ions ranging in their charges from –3 to +4 have been compiled and interpreted in terms of a model used previously for other thermodynamic quantities of hydration. The main contributions to ΔhydG° are the electrostatic effects, resulting in solvent immobilization, electrostriction, and dielectric saturation in a hydration shell of specified thickness, and further such effects on the water that surrounds this shell. Other effects contribute to ΔhydG° to a minor extent only.
1,574 citations
TL;DR: In this article, the ultraviolet-visible absorption spectra of colloidal particles of 52 of the metallic elements, calculated from the optical constants of the metals by means of Mie theory, are given.
Abstract: The ultraviolet–visible absorption spectra are given for 10 nm diameter colloidal particles of 52 of the metallic elements, calculated from the optical constants of the metals by means of Mie theory. For most of the elements the spectra cover the range 200–900 nm. Well resolved absorption bands are observed for colloidal Sc, Ti, V, Y, Cd, Eu, Yb, Hg and Th as well as for colloids of Cu, Ag, Au and the s-block metals. However, for the majority of the colloidal metallic elements in this size range there is only a continuous absorption in the visible range, rising to broad and poorly resolved absorption bands in the ultraviolet near 200 nm. The difference in the way that the spectra of colloidal particles of different metals change when the particle shape is varied from spherical to spheroidal is investigated systematically in the dipole approximation. This is achieved by means of contour plots of the absorbance cross-section for the particles vs. the real and imaginary parts of the dielectric function for the metals, and this method of investigation is extended also to hollow spherical particles. The results suggest that Ca, Sr, Ba, Eu, Yb, Th and possibly Sc, Ti, V and Y may merit experimental investigation as new metals for exhibiting surface-enhanced Raman scattering.
1,501 citations
TL;DR: In this paper, the reduction of CeO2 by hydrogen has been studied from 300-1200 K by several complementary techniques: temperature-programmed reduction (TPR), magnetic susceptibility measurements, Fourier transform infrared (FTIR), UV-VIS diffuse reflectance and X-ray photoelectron (XP) spectroscopy.
Abstract: The reduction of CeO2 by hydrogen has been studied from 300–1200 K by several complementary techniques: temperature-programmed reduction (TPR) and magnetic susceptibility measurements, Fourier-transform infrared (FTIR), UV–VIS diffuse reflectance and X-ray photoelectron (XP) spectroscopy. Two CeO2 samples were used with B.E.T. surface areas of 115 and 5 m2 g–1, respectively. The concentration of Ce3+ was determined in situ by measuring the magnetic susceptibility and the CeIII photoemission line. The reduction began at 473 K, irrespective of the initial surface area of the ceria. In the case of the low-surface-area sample, an intermediate reduction step was observed between 573 and 623 K, corresponding to the reduction of the surface. This intermediate step was less easily observed in the case of the high-surface-area ceria. In both cases, the reduction led to a stabilised state with the formal composition CeO1.83. Temperatures higher than 923 K were required to reduce the ceria further. The surface CeIII content determined by XPS was close to that determined by magnetic susceptibility measurements. The intensity of the 17 000 cm–1 band in the UV–VIS reflectance spectrum also varied with the degree of reduction. Finally, the evolution of the surface species observed by IR spectroscopy was in good agreement with the results from the other techniques. The IR results indicated large changes in the concentration and nature of both the hydroxyl and the polydentate carbonate species during the reduction process. The adsorption of oxygen on samples previously reduced to the composition CeO1.83 led to almost complete reoxidation at room temperature. The state of the initial B.E.T. surface did not influence the oxidation process. A slight excess adsorption of oxygen was evident on the surface. This was thermodesorbed at 380 K under vacuum.
577 citations
TL;DR: In this article, a simulation study of methane in silicalite at room temperature is presented, where adsorption isotherms and diffusion coefficients have been calculated for a range of pressures.
Abstract: Molecular simulation studies of methane in silicalite at room temperature are reported. Adsorption isotherms and diffusion coefficients have been calculated for a range of pressures. At low pressures (<20 bar) adsorption is predominantly at specific potential-energy minima in the silicalite channels while at higher pressures adsorption is determined by the total accessible channel volume. Diffusion is found to be strongly anisotropic with the fastest diffusion along the straight channels. Good agreement is obtained with experimental results for orientationally averaged diffusion coefficients. At low pressure, diffusion is well described by a pseudo-Bosanquet formula which identifies two independent contributions to diffusional resistance (from collisions with walls and from intermolecular collisions). At short times significant molecular force correlations arise due to the diffuculty of methane molecules passing each other in the channel intersections of the silicalite lattice. Some preliminary results for butane diffusion coefficients in silicaliate are also reported.
272 citations
TL;DR: In this article, the hydrothermal synthetic conversion of aluminium-rich zeolites to organo-zeolites in the presence of N,N,N-trimethyl-1-adamantammonium cations is extended to the faujasite system.
Abstract: The hydrothermal synthetic conversion of aluminium-rich zeolites to organo-zeolites in the presence of N,N,N-trimethyl-1-adamantammonium cations is now extended to the faujasite system. Rates of conversion are seen to be dramatically affected by various treatments of the FAU (faujasite) zeolites. Rates are depressed by steaming FAU zeolites, by ion-exchanging them with unreactive organic cations, or by silylating the surface of the zeolite. Surprisingly, the inhibition is removed by calcining away the organosilyl groups. For a reactive FAU zeolite (Y-62), the extent of conversion was proportional to organo-cation content, with other variables fixed; rates of conversion were also related to hydroxide ion concentration at the start of reaction.
197 citations
TL;DR: In this article, the presence of different types of acidic centres can be ascertained by proton magic-angle spinning nuclear magnetic resonance (1H MAS NMR) measurements on weakly rehydrated dealuminated zeolites and amorphous silica-aluminas.
Abstract: On weakly rehydrated dealuminated zeolites and amorphous silica–aluminas the presence of different types of acidic centres can be ascertained by proton magic-angle spinning nuclear magnetic resonance (1H MAS NMR) measurements. A 1H NMR line at ca. 6.5 ppm is caused by water adsorption on Lewis-acid sites. The shift of the 1H NMR line of Bronsted-acid sites (bridging OH groups) to lower field for hydrated samples can be interpreted quantitatively by a fast proton exchange between water molecules, bridging OH groups and hydroxonium ions.
184 citations
TL;DR: In this paper, surface vanadia species on a high-surface area silica support have been investigated by UV/visible diffuse reflectance, infrared and laser Raman spectroscopies.
Abstract: Surface vanadia species on a high-surface-area silica support have been investigated by UV/visible diffuse reflectance, infrared and laser Raman spectroscopies. The catalysts were prepared and characterized under water-free conditions, as the hydration state of the surface strongly influences the structure of the dispersed vanadia. Two types of surface species have been identified. Monomeric and oligomeric vanadia species, with a tetrahedral coordination geometry of oxygen ligands around the central vanadium ion, give rise to a narrow VO stretching absorption at 1040 cm–1. The second species is characterized by a Raman band at 920 cm–1 and a charge-transfer absorption at 28 000 cm–1, and corresponds to vanadia ribbons of limited lateral extent, with the vanadium ion situated in the centre of a square pyramid. Vibrational data from aqueous vanadate ions are inadequate as a reference for describing the vanadia species on a dehydrated surface.For vanadia supported on titania and on titania/silica mixed oxides, extended patches of a two-dimensional vanadia monolayer are known to be formed, as characterized by a charge-transfer band at 25 000 cm–1 and Raman bands at 1015, 700, 485 and 265 cm–1. In contrast, no such layer structures are generated on silica owing to the weak interaction of the immobilized species with the support. Instead, rapid formation of crystalline V2O5 is observed even at low loadings. To describe these differences, the concept of wetting is discussed.Adsorption of water onto Lewis-acidic vanadia sites creates new Bronsted centres that are more acidic than the surface silanol groups. A comparison of H2O and D2O adsorption experiments shows that water interacts with the surface vanadyl groups; the VO double bond is transformed into a geminal V(OH)2 diol in this process. This behaviour is different from that of vanadia supported on TiO2 or TiO2/SiO2 mixed oxides, where water is physisorbed to the vanadyl group with a concomitant decrease in VO bond order.
173 citations
TL;DR: In this article, a calorimetric method was proposed to evaluate the extent of hydrophilic and hydrophobic patches when both are present at the surface of quartz dust.
Abstract: Surface dehydroxylation of amorphous and crystalline silicas (quartz dust) has been investigated from the standpoint of the development of hydrophobicity upon thermal treatment. Hydrophobicity occurs when only siloxane bridges and isolated silanols (IR band at ca. 3750 cm–1) are present and is monitored by an enthalpy of adsorption of water lower than the latent heat of liquefaction (44 kJ mol–1). This calorimetric method allows the evaluation of the extent of hydrophilic and hydrophobic patches when both are present at the surface. All silicas develop hydrophobicity upon thermal treatment in vacuo, but quartz is much less easily dehydroxylated than amorphous materials. It is still mainly hydrophilic after outgassing at 1073 K, whereas pyrogenic silicas (Aerosil) become hydrophobic upon outgassing at T < 673 K. Quartz is also characterized by a few very reactive sites (q 90 kJ mol–1), absent on the amorphous specimens. Both these facts might be related to the specific quartz pathogenicity. Rehydroxylation at room temperature of dehydroxylated silicas occurs to a very limited extent. Hydrophilic patches exhibit a marked heterogeneity towards water with an enthalpy of adsorption decreasing from 90 to 44 kJ mol–1. The enthalpy of adsorption approaches 44 kJ mol–1 corresponding to the addition of multilayers of adsorbed water.
173 citations
TL;DR: In this article, molecular dynamics simulation techniques have been used to investigate the behavior of sorbed CH4 and C2H4 in zeolite ZSM-5, allowing for framework in addition to molecular motions.
Abstract: Molecular dynamics simulation techniques have been used to investigate behaviour of sorbed CH4 and C2H4 in zeolite ZSM-5. Our calculations allowed for framework in addition to molecular motions. Diffusion trajectories were obtained for the sorbed molecules. Simulated diffusion coefficients were in acceptable agreement with experiment.
157 citations
TL;DR: In this article, it was shown that the addition of certain stereochemically related diphosphonates produces a characteristic morphological change leading to disc and elliptical morphologies.
Abstract: Synthetic barite crystals, prepared under conditions chosen to mimic those encountered in off-shore oil fields, have a simple rhombic plate-like morphology in which large (001) faces are bound by smaller (210) sides. The addition of certain stereochemically related diphosphonates produces a characteristic morphological change leading to disc and elliptical morphologies. These results are shown to be consistent with a binding model in which the diphosphonate ion replaces two sulphates in the (011) surface. The implications of these results to the case of gypsum precipitation is also considered.
146 citations
TL;DR: In this paper, the bilinear calibration method was used to correct the permittivity spectra and the corrected spectra could be fitted with a single relaxation time with a small amount of Cole-Davidson behaviour.
Abstract: Dielectric relaxation measurements in the frequency range 10 MHz–10 GHz have been carried out in tert-butyl alcohol–water mixtures with various concentrations over the temperature range 273–313 K using a time-domain reflectometry (TDR) method. The bilinear calibration method as suggested by Cole has been used to correct the permittivity spectra. The corrected spectra could be fitted with a single relaxation time with a small amount of Cole–Davidson behaviour. Deviations from ideal mixing behaviour in the permittivity parameter (Iµo–Iµ∞) and relaxation time (τ) suggested the formation of a polymeric structure in tert-butyl alcohol–water mixtures. The dielectric relaxation behaviour showed the same structural changes as observed in ultrasonic relaxation. However, the maxima in excess permittivity and excess relaxation time occurred at different positions. This could not be explained by a simple model of the polymeric structure.
TL;DR: In this paper, the authors used electron microscopy and electron diffraction to determine the structure, orientation and morphology of CaCO3 crystals at the early stages of growth under compressed Langmuir monolayers of stearic acid [CH3(CH2)16CO2H] and octadecylamine [CH 3(CH 2)17NH+3] and showed that the preferred orientation of calcite can be rationalized in terms of geometric and stereochemical matching between the carboxylate headgroups and the ions in the crystal face.
Abstract: Transmission electron microscopy and electron diffraction have been used to determine the structure, orientation and morphology of CaCO3 crystals at the early stages of growth under compressed Langmuir monolayers of stearic acid [CH3(CH2)16CO2H] and octadecylamine [CH3(CH2)17NH+3]. At [Ca]≈ 9 mmol dm–3, elongated plate-like calcite single crystals of narrow particle size distribution were nucleated under fully compressed stearate monolayers. The crystals were elongated along the c axis and oriented with a {1text-decoration:overline10} face parallel to the monolayer/solution interface. Subsequent development of the crystals resulted in rhombohedral outgrowth into the bulk solution. Calcite crystals nucleated under partially compressed monolayers showed the same crystal chemical properties but had a more uniform size population. Nucleation under octadecylamine monolayers resulted in disc-shaped vaterite single crystals of narrow particle size distribution. Two types of disc were observed: (a) circular/hexagonal crystals oriented with their c axis perpendicular to the monolayer, and (b) oval-shaped crystals oriented with their a axis perpendicular to the monolayer. Changes in monolayer surface pressure did not influence these properties. The preferred orientation of calcite can be rationalized in terms of geometric and stereochemical matching between the carboxylate headgroups and the ions in the {1text-decoration:overline10} crystal face. There is no geometric match for vaterite but a stereochemical relationship exists on stearate monolayers with respect to the (00.1) face. For octadecylamine monolayers, bidentate binding of HCO–3 may be important in determining the preferential orientation of nuclei formed at the organic surface.
TL;DR: In this article, the state of adsorbed CO on a Cu electrode was studied with voltammetric and chronopotentiometric measurements, showing that the surface of the copper electrode is fully covered with adsored CO during the electro-reduction of CO2.
Abstract: Electroreduction of CO2 at Cu electrodes in aqueous electrolytes yields CH4 and C2H4, with adsorbed CO formed as an intermediate species. The state of adsorbed CO on a Cu electrode was studied with voltammetric and chronopotentiometric measurements. Adsorbed CO severely restricts hydrogen formation. During cathodic polarisation, coverage by adsorbed CO of the Cu electrode is ca. 90% in CO-saturated electrolytes. The adsorbed CO is easily desorbed by stirring the electrolyte vigorously as revealed by chronopotentiometric measurements. The chronopotentiometric measurements also confirmed that the surface of the copper electrode is fully covered with adsorbed CO during the electro-reduction of CO2, indicating that the rate-determining step for the reduction of CO2 to hydrocarbons must include a reaction which involves the adsorbed CO. The cross-sectional area of adsorbed CO was estimated to be 39 × 10–16 cm2, and the rate of desorption is also discussed using coulometric measurements reported previously.
TL;DR: The crystallization of CaCO3 under compressed Langmuir monolayers of stearic acid [CH3(CH2)16CO2H], octadecylamine [CH 3(CH 2)17NH2] and cholesterol (C27H45OH) has been studied over a range of supersaturation conditions by optical and scanning electron microscopy and X-ray diffraction as discussed by the authors.
Abstract: The crystallization of CaCO3 under compressed Langmuir monolayers of stearic acid [CH3(CH2)16CO2H], octadecylamine [CH3(CH2)17NH2], octadecanol [CH3(CH2)17OH] and cholesterol (C27H45OH) has been studied over a range of supersaturation conditions by optical and scanning electron microscopy and X-ray diffraction. At total [Ca]= 9 mmol dm–3, negatively charged stearate monolayes induced the oriented nucleation of calcite. The crystals were of two related morphological types, one of which was formed through the physical realignment of the oriented crystals at the organic surface during growth. Reducing the total [Ca] to 4.5 mmol dm–3, resulted in oriented vaterite nucleation on the stearate monolayers. Vaterite was the major product on positively charged octadecylamine films, independent of [Ca]. Unlike the vaterite crystals of stearate films, these crystals were of two distinct morphological forms which represented two specific nucleation orientations. Neutral monolayers of octadecanol inhibited crystallization and those of cholesterol gave random non-oriented calcite deposition analogous to the control experiments. Morphological analyses indicated that the crystals were oriented with their [1text-decoration:overline1.0](calcite) and [00.1] and [11.0](vaterite) axes perpendicular to the monolayer surface.
TL;DR: In this paper, a reinterpretation of given experimental results for SM(k) is given in terms of a size and charge polydisperse model and the systematic analysis of the effects of polydispersity on SM (k).
Abstract: Experimental information on the microscopic structure of charged colloidal dispersions is usually extracted from the intensity of scattered light l(k) which can be linked to the ‘measured structure factor’SM(k). A reinterpretation of given experimental results for SM(k) is given in terms of a size and charge polydisperse model and the systematic analysis of the effects of polydispersity on SM(k). In our model, the interaction between macroions is assumed to be of a Yukawa type and the polydispersity is characterised by histograms with standard deviations from 10 to 40% with up to 10 components. The partial structure factors Sαβ(k) are evaluated by solving the multicomponent Ornstein–Zernike (OZ) equations in connection with the thermodynamically self-consistent closure of Rogers–Young (RY). The accuracy of the RY approximation is demonstrated by comparing the results with simulation data on monodisperse Yukawa systems. The results for SM(k) for polydisperse systems show significant differences from results obtained by treating the systems as being monodisperse. For SM(k) a large increase is found at small k as well as a shift in the main peak. These features are discussed in terms of the fluctuation and scattering abilities of each component of the dispersion. The role played by the charge and the size polydispersity is also analysed by introducing the generalised Bhatia–Thornton structure factors. Finally, SM(k) is compared with scattering data. Quantitative agreement is found for all k values and, in particular, in the range of small k in which all one-component models are particularly inaccurate. The difference between SM(0) for a polydisperse system and the isothermal osmotic compressibility is emphasised.
TL;DR: In this article, an experimental sedimentation coefficient was used to interpret observations of settling suspensions, and the coefficient was paired with Kynch's kinematic theory to interpret sedimentation coefficients.
Abstract: Suspensions of colloidal particles execute Brownian motion and thus exhibit thermodynamic properties analogous to those of molecular systems. Hard-sphere colloidal silica suspensions undergo a disorder–order transition, i.e. freezing or crystallization, at high volume fractions. In suspensions of small particles the slow sedimentation permits the transition to occur at the bottom where the bulk of the crystalline sediment is formed by one-dimensional crystallization, as illustrated with photographs and scanning electron microscopy. X-Ray tomography measurements reveal a volume fraction discontinuity coincident with the observed crystal boundary occurring between the sediment and hindered setting region of the Kynch theory. Larger particles, however, form amorphous sediments because their rate of accumulation at the bottom exceeds the maximum crystal growth rate, in accord with classical kinetic theory. This theory is paired with Kynch's kinematic theory using an experimental sedimentation coefficient to interpret observations of settling suspensions.
TL;DR: In this paper, the effects of dehydration and the thermal stability of 12molybdo(tungsto)phosphoric acids, and the related compounds 1-vanado-11-moly bdo-tung sto-phosphorous acids, all having the Keggin structure, have been studied at different temperatures in a special infrared cell allowing experiments in a controlled atmosphere.
Abstract: The 12-molybdo(tungsto)phosphoric acids, and the related compounds 1-vanado-11-molybdo(tungsto)phosphoric acids, all having the Keggin structure, have been studied at different temperatures in a special infrared cell allowing experiments in a controlled atmosphere. The effects of dehydration and the thermal stability are discussed. The anhydrous acids are evidenced only for the two 12-molybdo- and 12-tungsto-phosphoric acids: for the vanado compounds, the results are in agreement with the release of vanadium from the host Keggin structure, when increasing the temperature and duration of the treatments in a dry atmosphere. The effect of exposure to water vapour after the thermal treatments is also discussed.
TL;DR: In this paper, partial molar heat capacities of the compounds N-acetyl glycinamide, Nacetyl-L-alaninamide, glycyl-DL-alanine and DL-alanyl glycine have been obtained.
Abstract: Partial molar heat capacities, in water at 25 °C, of the compounds N-acetylglycinamide, N-acetyl-L-alaninamide, N-acetylglycylglycinamide, N-acetylglycyl-L-alaninamide, N-acetyl-L-alanylglycinamide, N-acetyl-L-alanyl-L-alaninamide, glycyl-DL-alanine and DL-alanylglycine have been obtained. The information obtained, for the amides, is discussed in terms of group-additivity approaches, and group contributions are evaluated. The values for these differ in some instances, and particularly that for the heat capacity of the peptide group, from those obtained in an earlier study. Comparison of the parent amino acids and peptides with the amides shows that there is a marked contribution to the heat capacities of the zwitterionic compounds from electrostatic sources.Some results are presented of the partial molar volumes, also at 25 °C, of N-acetylglycyl-L-alaninamide, N-acetyl-L-alanylglycinamide and DL-alanylglycine.A correlation is evident between the heat capacity and volumetric interaction coefficients, for amino acids, peptides and N-acetylamides, obtained from the molality dependences of the apparent molar properties, and this has been rationalised using a simple electrostatic approach.
TL;DR: In this article, the electrooxidation of ascorbic acid was examined at polypyrrole electrodes doped with chloride and dodecylbenzene sulphonate ion.
Abstract: The electrooxidation of ascorbic acid is examined at polypyrrole electrodes doped with chloride and dodecylbenzene sulphonate ion. Both the kinetics of oxidation and the reproducibility of the electrochemical response are found to be enhanced at the polymer-modified electrode compared to that at an unmodified metal electrode. Kinetic data for ascorbate oxidation at the polymer-modified electrodes is evaluated and rationalised in terms of specific mechanistic sequences. The use of polypyrrole-based electrodes as active components in an amperometric chemical sensor for ascorbate is evaluated.
TL;DR: In this article, numerical integration of the differential equation for cyclic voltammetry has been performed for the case where the reverse potential is set prior to the forward peak potential, and a working curve stemming from these integrations allows the determination of the half-wave potential of species limiting the potential window.
Abstract: The numerical integration of the differential equation for cyclic voltammetry has been performed for the case where the reverse potential is set prior to the forward peak potential. A working curve stemming from these integrations allows the determination of the half-wave potential of species limiting the potential window. This technique has been applied to the measurement of formal Gibbs energies of transfer of ions across the water/1,2-dichloroethane interface. The validity of the TATB (tetraphenylarsonium tetraphenylborate) assumption is also discussed.
TL;DR: In this article, the authors have used molecular dynamics calculations to study the behavior of dense monolayers of long-chain alkyl thiol molecules, HS(CH2)14X (X = CH2-CH3, CH3-C N, and H-O-H) that form by selfassembly from solution onto a gold substrate.
Abstract: Molecular dynamics calculations have been used to study the behaviour of dense monolayers of long-chain alkyl thiol molecules, HS(CH2)14X (X =—CH2—CH3, —C N, and —O—H) that form by self-assembly from solution onto a gold substrate. The simulation systems consisted of 90 flexible chains with sulphur headgroups and the appropriate terminal groups X. Periodic boundary conditions in the plane of the surface and a surface-corrugation potential encouraged the molecules to form a triangular lattice at a fixed surface density of 21.4 A2 per chain. The interaction potentials were chosen to model monolayers of molecules on an Au(111) surface. Around room temperature, methyl-terminated chains are canted and rotating about their long axes, and there are a significant number of conformational defects at the chain ends. Monolayers formed from molecules with polar terminal groups adopt more rigid structures. For X =—OH, there is hydrogen bonding between the chain ends but the surface is disordered, with a significant fraction of ‘free’—OH groups. For X =—CN, the terminal groups are organized in a ferroelectric structure, which is incommensurate with the underlying triangular lattice of the sulphur headgroups. The simulated surface structures correlate well with deductions based on the wetting characteristics of these and related self-assembled monolayers.
TL;DR: In this paper, infrared spectroscopy has been used to analyse the gas-phase reaction products and the related adsorbed species obtained between room temperature and 400 °C from the dehydrogenation/dehydration reactions of propan-2-ol over a series of differently calcined catalysts of TiO2, ZrO2 and HfO2.
Abstract: Infrared spectroscopy has been used to analyse the gas-phase reaction products and the related adsorbed species obtained between room temperature and 400 °C from the dehydrogenation/dehydration reactions of propan-2-ol over a series of differently calcined catalysts of TiO2, ZrO2 and HfO2. The ZrO2 and HfO2 results were independent of the calcination pretreatment, and the surfaces of these oxides, like that from a TiO2 sample calcined at 800 °C, were dehydroxylated. Different results were obtained from a TiO2 sample calcined at 300 °C which had a hydroxylated surface. The acidic sites and reactivities of the surfaces of TiO2(300 °C) and TiO2(800 °C) were explored by pyridine adsorption and infrared spectroscopy. Only Lewis-acid sites were detected by pyridine.On raising the reaction temperature, in all cases the dehydrogenation reaction to give acetone occurred either before or simultaneously to the onset of the dehydration reaction to give propene. Acetone production was most pronounced over ZrO2 and HfO2 but also occurred more with TiO2(800 °C) than with TiO2(300 °C). The dehydrogenation reaction was largely quenched by pre-adsorbed pyridine on both TiO2 samples. The TiO2(300 °C) catalyst showed the presence of adsorbed propan-2-ol and 2-propoxide groups at room temperature. The dehydroxylated ZrO2, HfO2 and TiO2(800 °C) samples only showed appreciable amounts of 2-propoxide groups. In each case the 2-propoxide ions occurred in two different forms, presumably formed by adsorption on different types of sites.Both the acetone and propene products appeared as absorptions from 2-propoxide surface species decreased in intensity, so the latter are clearly reactive species. Gas-phase acetone production was followed by the chemisorption of acetone at a higher temperature. This subsequently decomposed to give surface acetate species, and finally at 400 °C to give CO2 and methane in the gas phase. Propene did not give rise to adsorbed species, or to further products in the gas phase.At the higher temperatures, above 300 °C, the reaction was always selective in favour of the dehydration reaction. However, each of the dehydroxylated catalysts showed some selectivity in favour of the dehydrogenation reaction over the earliest temperature range for alcohol decomposition, between 200 and 250 °C.A discussion is given of possible mechanistic pathways for the production of surface 2-propoxide species and the two types of products, based on the infrared-supported assumption that the different adsorbed forms of 2-propoxide [and possibly adsorbed propan-2-ol on TiO2(300 °C)] are reactive intermediates.
TL;DR: In this paper, the authors defined the electrodiffusional Peclet number and showed that it is universally of the order of unity, independent of the system's size and typical electrolyte concentration.
Abstract: This paper describes electroconvection, a non-gravitational free convection which may arise in macroscopic domains of electrolyte solution. Electroconvection originates from the interaction of a self-consistent electric field with the corresponding space charge within the limits of validity of local electroneutrality approximation. The electrodiffusional Peclet number is defined and is shown by dimensional analysis to be universally of the order of unity, independent of the system's size and typical electrolyte concentration. Electrodiffusional ionic concentration and electric potential fields, which form in the course of concentration polarization at an electrically inhomogeneous cation permselective periodic membrane, are calculated. It is shown that these fields are incompatible with mechanical equilibrium of the ionic fluid, that is electroconvection is bound to arise in this constellation. Results of numerical computations in a two-dimensional model of electroconvection flow at a conductively inhomogeneous periodic membrane are presented along with some explicit expressions for the electroconvectional flow rate, obtained through an asymptotic analysis, valid in the vicinity of electrodiffusional limiting current.
TL;DR: The effect of supports such as silica, alumina, amorphous aluminosilicate and magnesia on the activity of heteropoly acids (HPA) for reactions requiring strong and very strong acid sites has been studied as discussed by the authors.
Abstract: The effect of supports such as silica, alumina, amorphous aluminosilicate and magnesia on the activity of heteropoly acids (HPA) for reactions requiring strong and very strong acid sites has been studied. It has been found that 12-tungstophosphoric acid (HPW) mounted on alumina and silica catalyses cumene cracking at 150 °C, with conversion ca. 90%, and toluene disproportionation at temperatures as low as 250 °C. The catalytic performance increased with increasing HPA content and reached its maximum in the loading range 38–50 wt.%. With > 38 wt.% HPW on silica and > 60 wt.% HPW on γ-alumina the supported HPW catalysts showed catalytic activity for low-temperature isomerisation of hexane as did unsupported HPW. Activity for the latter reaction indicates the presence of very strong acid centres with acid strength approaching that of superacids. No activity for the reactions studied was found for magnesia-supported HPW.
TL;DR: In this paper, it has been shown that framework and non-framework cobalt species exhibit different spectroscopic properties and can be distinguished from each other, and the framework cobalt acceptance and the stability of the different structures decrease in the order CoAPO4-5 > COAPO-4-11 > CoAPE4-16 > CoAPE4-34.
Abstract: CoAPO molecular sieves of the structure types 5, 11, 16 and 34 have been investigated by means of electron absorption spectroscopy, temperature-programmed reduction and infrared spectroscopy in combination with pyridine adsorption, the purpose being to determine the amounts of framework and non-framework cobalt and to study the properties of incorporated cobalt with respect to stability, redox behaviour and associated acidic properties. It has been shown that framework and non-framework cobalt species exhibit different spectroscopic properties and can be distinguished from each other. The framework cobalt acceptance and the stability of the different structures decrease in the order CoAPO4-5 > CoAPO4-11 > CoAPO4-16 > CoAPO4-34. In stable structures such as 5 and 11, framework cobalt can perform reversible redox reactions CoII⇌ CoIII, which are accompanied by changes in the concentration of Bronsted and Lewis acid sites.
TL;DR: In this article, a direct computer search based on qualitative molecular orbital theory predicts a chiral, D2 symmetry structure for the fullerene C76 and the anticipated spectroscopic properties of this structure are in satisfactory agreement with recent experimental results.
Abstract: A direct computer search based on qualitative molecular orbital theory predicts a chiral, D2 symmetry structure for the fullerene C76. The anticipated spectroscopic properties of this structure are in satisfactory agreement with recent experimental results.
TL;DR: In this article, the electrophoresis of drops and bubbles is examined as a function of the δ-potential and several other parameters, and it is shown that drop and bubble migration is dictated by the net electrochemical stress acting along the interface.
Abstract: We have examined the electrophoresis of drops and bubbles, computing the electrophoretic mobility as a function of the ζ-potential and several other parameters. Our treatment differs from previous work in that we incorporate a more representative picture of the interface. We have found that drops and bubbles are electrophoretically distinct from particles; perhaps the most striking result obtained was that, when the diffuse layers are thin, conducting drops do not always migrate in the direction that would be anticipated from the sign of their surface charge. Thus, the ζ-potential alone is not sufficient to characterize the surface. The analysis shows the sense of the migration is dictated by the net electrochemical stress acting along the interface. For similar reasons, large inviscid spheres tend to remain stationary at modest ζ-potentials and, in contrast to rigid particles, their mobility is actually enhanced by polarization of the double layer. Further, we have uncovered conditions for which the mobility of non-conducting drops is insensitive to the interior viscosity. This ‘solidification effect’ stems in part from interfacial tension gradients associated with specific adsorption of the ionic solutes, as well as from polarization and, moreover, need not involve the presence of surface-active impurities.
TL;DR: In this article, a model of cation-anion aggregates was applied to the solutions of polyoxometalates in DMF and DMSO to explain the different behavior of the acids in water, acetonitrile and acetone.
Abstract: Solutions of Keggin-structure polyoxometalates have been studied by IR, Raman and UV spectroscopies, with various solvents and counterions. For the tetraalkylammonium compounds, cation–anion aggregates are observed. This model of cation–anion aggregates can be applied to the solutions of the acids in DMF and DMSO. The different behaviour of the acids in water, acetonitrile and acetone can be explained in terms of anion–anion interactions, similar to those existing in the solid state. The results are discussed in connection with the use of polyoxometalate solutions for preparing supported catalysts by impregnation.
TL;DR: The solvatochromic parameters π* for the polarity/polarizability, β for the hydrogen-bond accepting (electron-pair-donating) ability, and α for the ability of binary aqueous tetrahydrofuran, N,N-dimethylformamide, pyridine, dimethyl sulphoxide, formamide and formic acid have been determined spectrophotometrically at 25 °C as functions of the composition of the mixtures as discussed by the authors.
Abstract: The solvatochromic parameters π* for the polarity/polarizability, β for the hydrogen-bond accepting (electron-pair-donating) ability, and α for the hydrogen-bond-donating ability of binary aqueous tetrahydrofuran, N,N-dimethylformamide, pyridine, dimethyl sulphoxide, formamide and formic acid have been determined spectrophotometrically at 25 °C as functions of the composition of the mixtures. They were compared with the local composition of the environment of each kind of molecules in the mixtures, obtained from theoretical models. These parameters were also determined for several water-saturated alkanols and tri-n-butyl phosphate, that are of significance in liquid–liquid distribution studies of organic solutes.
TL;DR: In this article, Fourier transform infrared spectroscopy with photo-acoustic detection and chemical modification with trichlorosilane was used to study the porosity of three types of silica gel with different pore distributions.
Abstract: The distribution and reactivity of the different hydroxyl species on the surface of silica gel are studied using Fourier-transform infrared spectroscopy with photo-acoustic detection and chemical modification with trichlorosilane. The effect of the porosity is investigated using three types of silica gel with different pore distributions. The number of free and bridged hydroxyl groups on the surface of silica gel and their reactivity towards trichlorosilane is quantitatively assessed. It is concluded that smaller pore radii favour the condensation of bridged hydroxyl groups to free ones with increasing pretreatment temperature. Trichlorosilane reacts exclusively with free hydroxyl groups at high pretreatment temperatures, and partially with bridged hydroxyl groups at low pretreatment temperatures. The reactivity of the free hydroxyl groups is not affected by the pore radius, whereas the bridged hydroxyls become less reactive towards trichlorosilane as the pore radius decreases.