Showing papers in "Journal of the Chemical Society, Faraday Transactions in 1977"

Kinetics of activated chemisorption. Part 2.—Theoretical models

Abstract: In activated chemisorption the plot of the reciprocal of the rate Z=(dq/dt)–1 against the time t is convex towards the Z axis at low t and concave at high t. This condition is satisfied if both the energy of activation, Et and the number of available sites, Nt decrease with the coverage q and if d2Et/dq2 < 0. Two models are considered:

Computer simulation of a gas–liquid surface. Part 1

Abstract: The gas–liquid surface of a system of Lennard-Jones (12, 6) molecules has been simulated by Monte Carlo and by Molecular Dynamic methods at temperatures which span most of the liquid range. For systems of 255 molecules the two methods lead to similar results and this agreement confirms that the density profile, as a function of height, falls monotonically from the density of the bulk liquid to that of the gas. The thickness of the surface layer is sensitive to the surface area, and appears to approach its thermodynamic limit for surface areas of 400σ2 for a system of 4080 molecules. The density profile can be represented by a hyperbolic tangent of an appropriately scaled height. The thickness of the surface is of the order of two molecular diameters at temperatures near the triple point and increases rapidly as the critical point is approached. The computed surfacetens ions agree well with those calculated by statistical perturbation theory.Monte Carlo and Molecular Dynamic simulation of a binary mixture shows clearly the adsorption of the component of higher vapour pressure; the amount absorbed agrees with that calculated from Gibbs's isotherm.

Alternative to the Elovich equation for the kinetics of adsorption of gases on solids

Abstract: The Elovich equation has long been used to describe the kinetics of adsorption of gases on solids. It is shown here that many results which have been fitted to this equation can be more satisfactorily described in terms of the simple theory of adsorption on specific sites on the surface of a solid.

Interpretation and correlation of zeolitic diffusivities obtained from nuclear magnetic resonance and sorption experiments

Abstract: The application of the n.m.r. pulsed field gradient technique to molecules adsorbed in zeolites, provides information on molecular diffusion both in the interior of the crystallites and in the inter-crystalline space. Comparison of the intra-crystalline self-diffusion coefficients obtained by this method with sorption data, reveals striking discrepancies. While the validity of the pulsed field gradient technique results can be checked in a number of experiments, the diffusion coefficients obtained from uptake measurements on zeolite crystallites of different radii exhibit an unreasonable radius dependence. It must therefore be concluded that sorption is limited by elementary processes different from intracrystalline diffusion and that, as a consequence, the diffusion coefficients obtained by sorption experiments are best regarded as apparent coefficients.Comparing, however, molecular transport parameters deduced by sorption and n.m.r. experiments more qualitatively, remarkable correlations are obtained.Since with increasing crystallite size the influence of surface effects on sorption kinetics decreases, uptake experiments with sufficiently large (i.e., natural) zeolite crystals provide diffusion coefficients comparable with those obtained by the n.m.r. pulsed field gradient technique.

Molecular dynamics computer simulation of surface properties of crystalline potassium chloride

Abstract: Computer simulation of the (100) face of crystalline potassium chloride has been achieved using the method of molecular dynamics and a lamina model of thickness 10 ionic layers. The basic cell contained 360 ions, interacting via a Tosi–Fumi potential. A solution has been obtained for the long range correction to the Coulomb potential for an ionic lamina. This differs significantly from the solution for a 3-dimensional lattice.Surface distortion, surface energy and stress have been evaluated and agree well with experiment and previous model calculations. The ratios of surface mean square amplitudes of vibration perpendicular to the surface divided by the bulk values show an approximately 25 % anharmonic enhancement over previous calculations. The layerwise velocity autocorrelation functions yield power spectra featuring a peak for the surface mono-layer at 75 cm–1 which compares favourably with the frequency of surface acoustic modes predicted by previous calculations.

Molecular beam studies of ethyl nitrite photodissociation

Abstract: The photofragment spectrum of C2H5ONO at 28 810 cm–1 has been obtained by crossing a beam of brief pulses of polarized laser light with a beam of C2H5ONO in a collision-free environment. The photodissociation products are C2H5O and NO. The angular distribution of fragment intensities peaks in a direction orthogonal to E, the electric vector of the light, indicating that the transition moment lies perpendicular to the bond that breaks. The high degree of angular anisotropy shows that the average lifetime in the upper state is short compared to a molecular rotational period, being at most 2 × 10–13 s. The distribution of fragment translational energies has also been measured, and shows that the most probable total fragment translational energy is 6000 cm–1, or about 36 % of the energy in excess of that needed to break the O—N bond. The most probable fragment total internal energy is 10 400 cm–1. The energy distribution can be matched by a simple modified impulsive model for the molecular dynamics of the photodissociation.

Dissolution and passivation of nickel. An X-ray photoelectron spectroscopic study

Abstract: The presence and nature of surface species formed on a nickel electrode after polarisation in 0.5 mol dm–3 sulphuric acid and 0.5 mol dm–3 sulphuric acid + 0.005 mol dm–3 nickel sulphate has been investigated by X-ray photoelectron spectroscopy. The film produced in the active, prepassive, passive and transpassive regions was studied. Nine different species were detected including two sulphur species. In all cases the electrode was transferred to the spectrometer under nitrogen to minimise air oxidation of the metal or film.The binding energies and intensities of the photoelectron peaks were analysed and showed the presence of NiO and Ni(OH)2. The thickness of the film in the passive and transpassive regions was calculated, assuming a continuous layer, as 10–20 A.

I.r. spectroscopic study of CO2 adsorption onto η-Al2O3

Abstract: The adsorption of CO2 at room temperature onto η–Al2O3 dehydrated at various temperatures between 25 and 800°C was studied by i.r. and microgravimetric techniques. The following surface species are formed; three σ-coordinated linear structures, two bicarbonates that can be interconverted, several “organic” carbonates and minor amounts of mono- and bi-dentate carbonates. Relative concentrations of these species depend strictly on pretreatment conditions, while the nature of the species is primarily determined by the coordination of surface cations. The overall coverage is quite small, (θ⩽ 10 %) and nearly the same as in all transition aluminas. A cooperative mechanism for CO2 adsorption is thus invoked to account for this and other features. The interaction between CO2 adsorption and catalytic activity is considered, and the probable nature of the centres responsible for exchange and equilibration reactions is suggested.

Solubility of nitric oxide in aqueous and nonaqueous solvents

Abstract: The solubility of NO in various solvents has been measured over a range of temperatures. The results are compared with the values predicted by various current theories of gas solubility.

The oxygen electrode. Part 8.—Oxygen evolution at ruthenium dioxide anodes

Abstract: The electrocatalytic behaviour of Ti-supported RuO2, prepared by thermal decomposition of RuCl3, as substrate for oxygen evolution was investigated by a variety of techniques. B.E.T. adsorption experiments showed that the RuO2 layers are highly porous with large surface area values which, however, decrease rapidly as the annealing temperature increases above ∼ 300°C. Both the charge involved in the cyclic voltammograms and the oxygen evolution rates are dependent on the true (rather than apparent) area of these electrode surfaces, and also on the pH of the solution. The results for oxygen evolution are discussed in terms of electrochemically generated unstable surface oxides whose decomposition is catalysed by protons in acid, and hydroxide ions in base. The lower reactivity of the oxide at intermediate pH value is attributed on the one hand to loss of protons by OH groups, resulting in oxygen bridging, and on the other to lack of enhanced coordination of surface ruthenium species by OH– ions, which in this pH region are present only at low activity. The charge associated with voltammetric sweeps is accounted for in terms of surface redox processes rather than bulk penetration of protons into the oxide. The need for surface area measurements as a guide to the interpretation of the electrochemical data in the case of these oxide systems is stressed.

Partial molar volumes of organic compounds in water. Part 1.—Ethers, ketones, esters and alcohols

Abstract: The partial molar volumes of a number of ethers, ketones, esters and alcohols in water at 25.0°C have been determined and related to their van der Waals volumes by one of two equations, depending on whether the molecules are spherical or cylindrical in shape. Allowance has been made for the void volume associated with each molecule and the results imply that calculated volumes must be reduced by a constant amount for each carbonyl or hydroxyl group present, owing to hydrogen bonding to water. No such reduction in the calculated volume is required for the oxygen atom of an ether. It is shown for diols that the amount of void volume per additional —CH2— group remains constant for straight-chain compounds (i.e., cylinders), whereas for spherical molecules the amount of void volume decreases with increase in the number of carbon atoms, as predicted.

Calorimetric evidence for a bidimensional phase change in the monolayer of nitrogen or argon adsorbed on graphite at 77 K

Abstract: Nitrogen adsorption on a Sterling MT 3100 graphitized carbon black is followed by isothermal microcalorimetry at 77 K. A special feature of the curve of isosteric heats is a distinct peak recorded for a coverage close to 1. Simultaneously, a sub-step is detected on the adsorption isotherm. Both effects are assigned to a bidimensional phase transition of the monolayer, changing from an hypercritical fluid state to a localized state. A similar phenomenon is observed in the case of argon adsorption: here again, a peak on the calorimetric curve and an inflexion on the isotherm are interpreted by a bidimensional fluid → solid transition; nevertheless, in the latter state, argon is not localized at graphite lattice sites.

Melting of ice in porous solids

Abstract: Differential scanning calorimetry (d.s.c.) and nuclear magnetic resonance (n.m.r.) techniques were used to study the melting of ice in porous solids.At low water contents (less than about three monolayers) no freezing or melting was observed. As the water content was increased, a single melting peak was observed as the pores filled. The amplitude of this peak reached a constant value when the pores were completely filled, and then at higher water contents a second peak was observed increasing in amplitude as more water was added. The lower melting point was characteristic of the pore water and this melting point decreased with decresing pore radius. The higher melting point (0°C) was that of the non-pore, bulk, water. The pore volume determined calorimetrically agreed with the quoted pore volume. A coefficient of 0.9 for the linear correlation between melting point depression (ΔT) and the reciprocal radius (1/r) confirmed the applicability of the Kelvin equation. It is concluded that the observed properties of water in pores are essentially bulk properties at distances of more than about 10 A from the surface, and influenced more by the Kelvin effect, i.e., capillarity, than by the surface of the silica. D.s.c. can be used as a rapid method to characterise these silicas. Two simple measurements on silica samples containing water allow an estimation of the surface area, pore volume and an average pore size to be made.

Interactions in alkane systems by depolarized Rayleigh scattering and calorimetry. Part 1.—Orientational order and condensation effects in n-hexadecane + hexane and nonane isomers

Abstract: Molecular optical anisotropies (γ2) have been obtained from depolarized Rayleigh scattering experiments on the 5 hexane isomers and 15 nonane isomers of widely varying degrees of branching. Values of γ2 have also been obtained for these molecules at high dilution in carbon tetrachloride. The latter values reflect the anisotropies of the molecular shapes of the isomers while the pure state values also contain a contribution due to correlation of the orientations of neighbouring molecules. Thus, for highly branched, isotropic isomers, the pure and dilute solution γ2 values are the same and are small. With increasing molecular anisotropy, the pure state value increases more rapidly than the dilute, the divergence reflecting the increasing correlation of molecular orientations. Values of γ2 have been found for n-hexadecane at high dilution in the hexanes and nonanes. They increase with the anisotropy of the solvent molecules and yield increasing values of the J12 parameter, which characterizes the correlation of orientations of the n-hexadecane and solvent molecules. The J12 values are given approximately by the geometric mean, (J11J22)½, of the order parameters of the pure liquids, indicating that the mixing of two liquids results in a net destruction of orientational order, i.e., ΔJ12=½(J11+J22)–J12 is positive. This change explains the variation in the heats of mixing of n-C16+ hexane systems. Molar heats of mixing have been measured at 25°C through the concentration range for n-C16 with 21 nonane isomers. With increasing nonane branching, ΔhM increases proportionally to ΔJ12. However, when nonane isomers have high degrees of steric hindrance, e.g., 3,3-diethylpentane or 2,3,3,4-tetramethylpentane, a new and large negative contribution is encountered in ΔhM. For example, ΔhM for n-C16+ 3,3-diethylpentane is S-shaped and negative at high nonane concentrations. Values of ΔvM and ΔsM also contain negative contributions for the few systems studied. The effect resembles a “condensation” of the less hindered alkane onto the large sterically hindered nonane, similar to the condensation effect found in cholesterol + lipid systems.

X-ray photoelectron spectroscopy and heterogeneous catalysis, with elimination reactions as an example

Abstract: Assuming that the EPA and EPD strength of catalytically active surface sites is the major factor responsbile for catalytic properties, XPS is shown to be a powerful tool for estimating this factor and for predicting (or interpreting) the activity and selectivity of catalysts. This is illustrated with examples taken from elimination reactions. The dehydration activity of MgHPO4 is higher than that of MgSO4, corresponding to the higher Mg 2p binding energy and hence higher EPA strength of Mg. The selectivity of oxides for production of terminal olefins from alkan-2-ols, and for dehydrogenation, is related to a high EPD strength of oxygen (low 1s binding energy). The tendency of spinels to an E2 mechanism is higher than that of alumina, in accordance with the higher Al 2p binding energy and hence higher EPA strength in alumina. In zeolites, the high EPA strength of the exchangeable cations must be considered a decisive factor in the high catalytic activity.

Role of solvent structure in solution theory

Abstract: A simple theory of aqueous non-electrolyte solutions is developed to describe the effect of solvent correlation on solute–solute interactions. It is formulated in terms of a Landau expansion of the free energy density, obtained from the Ornstein–Zernike relations, and given specific motivation through a generalised Pople model for water structure.A functional form of the solute–solute interaction of both hydrophobic and polar solutes in aqueous solutions is investigated. The theory accounts for the experimentally observed solute–solute attraction and gives a more precise meaning to empirical solute–solute potentials.

On deviations from Young's equation

Abstract: Although Young's equation, γS=γSL+γL cos θC, is one of the oldest and most-used equations of classical physics and chemistry, it is in a rather delicate position scientifically, in that it is virtually impossible to prove experimentally This is due, of course, to the uncertainties in the measurement of γS and γSL, the solid–air and solid–liquid surface tensions Recently the validity of Young's equation has been questioned as the result of a theoretical analysis of the three-phase contact region where the liquid–air surface tension, γL, may be modified by interaction with the nearby substrateThe present paper argues that Young's equation is a “macroscopic” equation which does not concern itself with the microscopic shape of the liquid surface in the vicinity of the three-phase contact region The concept of a microscopic contact angle θP is introduced θP is the angle that the free liquid surface makes with the substrate when the liquid thickness is ≪∼ 10 A, ie, microscopically near the three-phase contact line The contact angle θC given by Young's equation is shown to be the angle that the liquid surface makes with the substrate at microscopically large distances from the contact line where the modification of surface tensions by interaction is negligible (> 10 A) The connection between θP and θC is established by making only very general statements about the nature of this interaction and does not impose the unphysical restrictions on the profile shape in the neighbourhood of the three-phase contact line which lead to the conclusions of Jameson and del Cerro

Charge density effects in ion exchange. Part 1.—Heterovalent exchange equilibria

Abstract: A thermodynamic study is made on the effect of surface charge density on the sodium–cesium equilibrium in three clay minerals, Otay montmorillonite, hectorite and a series of Camp Berteau montmorillonite samples with reduced layer charge.It is shown that the standard free energy, enthalpy and entropy changes for the reversible displacement of sodium by cesium ions increase linearly with surface charge density. The opposite effect of charge density on cesium selectivity in zeolites and clay minerals is discussed in terms of electrostatic and hydration effects and the swelling properties of clays.

Theoretical prediction of phase behaviour at high temperatures and pressures for non-polar mixtures. Part 1.—Computer solution techniques and stability tests

Abstract: The classical mathematical description of critical points of binary mixtures and the computer technique used to solve the relevant equations are described. The techniques used here are applicable to any closed equation of state and one fluid model prescription.The critical points, in a given range of temperature and volume are located as the solution of two simultaneous equations: (∂p/∂v)T,x(∂2G/∂x22)T,p= 0 (∂p/∂v)T,x(∂3G/∂x32)T,p= 0 with reduced volume v and reduced temperature T as independent variables. The search procedure consists of three parts. Firstly the zeros of the first quantity are established around the perimeter of an area in the v, T plane defined by the range of v and T. Secondly the locus of the zeros of this quantity are tracked by a novel stepping procedure. The sign of the second quantity is determined at each step, a change in sign indicating that a zero and hence a critical point has just been passed. Thirdly, the critical point is accurately located by a bisection technique.Examples of the loci for mixtures are given and the critical points obtained compared with those obtained by Scott and van Konynenburg for the van der Waals equation of state.Tests for the stability of critical phases which can be applied to analytic expressions for derivatives of the free energy with respect to composition of a mixture are described.The tests are applied so that if a firm decision is made at any point between material stability, metastability and unstability the sequence of tests ends. Initially tests for mechanical stability are applied and points which are mechanically stable are subjected to further tests to determine whether the critical phase is materially stable, metastable or unstable.If (∂4G/∂x42)p,T is negative the critical phase is materially unstable otherwise a detailed study of the (∂2G/∂x22)p,T against composition diagram is needed. In the more difficult cases numerical integration of the area under the (∂2G/∂x22)p,T against composition curve has to be undertaken. The determination of stability for systems in which (∂2G/∂x22)p,T against composition diagram has multiple interacting loops is also described.

Computer simulation of the liquid–solid–vapour contact angle

Abstract: A molecular dynamics simulation procedure has been used to determine the angle of contact, θ, between a gas, a liquid and a plane solid surface. The molecules in the fluid phases interact with one another according to a Lennard-Jones (12, 6) potential and with the solid surface according to a (9, 3) potential. It is found that Young's equation γSV=γLV cos θ+γSL, where the γ's are surface tensions, is incapable of representing the observed results.

29 Si N.M.R. studies of aqueous silicate solutions

Abstract: Silicon-29 n.m.r. spectra have been obtained for a number of aqueous solutions of sodium and potassium metasilicate. The assignment of peaks to various silicate species is discussed. Linewidth and spin–lattice relaxation time measurements are reported as a function of ageing of the solutions in sealed glass tubes, and for solutions containing several paramagnetic additives (oxygen, Cr3+ and Mn2+). Silicon-29-{1H} nuclear Overhauser measurements (NOE) were also made, and it is concluded that values of T1 are markedly affected by paramagnetics, including some species leached from the glass of the tubes. The effect of using D2O as solvent is also studied.

Polarizability, proton transfer and symmetry of energy surfaces of carboxylic acid—N-base hydrogen bonds. Infrared investigations

Abstract: Carboxylic acid—N-base systems are studied by i.r. spectroscopy. The position of the proton transfer equilibrium in OH ⋯ N ⇌ O–⋯ H+N bonds can be determined by considering the i.r. bands. Thus, in the case of pure 1 : 1 mixtures, 50 % proton transfer is observed in these bonds for ΔpKa= 2.3, i.e., if the pKa of the acid is 2.3 units smaller than that of the protonated base. It is shown that a double minimum energy surface is present in these bonds. An i.r. continuum indicates that these bonds are easily polarizable. Thus, B1H ⋯ B2⇌ B–1⋯ H+B2 bonds may be easily polarizable, too, when B1≠ B2. This polarizability is greatest for bonds with 50 % proton transfer, i.e., when the weights of the proton boundary structures OH ⋯ N and O–⋯ H+N are equal. With increasing degree of asymmetry of such bonds, the polarizability does not decrease appreciably within a very considerable ΔpKa region. A major decrease is found only when the ΔpKa becomes 3.5. Even when the proton is largely located at one side of the bond, as indicated by the bands, the polarizability is weaker but can still be observed and only decreases slowly with increasing asymmetry, as indicated by the continuum. The continua show band-like structures in the region 3200–1800 cm–1 which is also explained.The value ΔpKa= 2.3 for 50 % proton transfer is only valid when systems are considered with which the N-bases have additional NH groups. When no additional NH groups are present, 50 % proton transfer is obtained at ΔpKa= 4. Hence, changes of the structure of the molecules which alter their interaction characteristics with the environment, change the symmetry of the OH ⋯ N ⇌ O–⋯ H+N bonds, too.Acid—N-base 1 : 1 mixtures in which water molecules are present are also studied. Water shifts the proton transfer equilibrium, i.e., it increases the weight of the proton boundary structure O–⋯ H+N. In the presence of four water molecules per acid base pair, the ΔpKa is reduced from 2.3 to 0.9 (N-bases with additional NH groups) or from 4 to 2 (N-bases without additional NH groups). The reasons for this environmental influence on the energy surfaces are discussed. Finally, the dissociation of the acid—N-base bonds, which occurs with increasing water content in the systems is considered.

Surface reactivity of zeolites type H-Y and Na-Y with methanol

Abstract: The surface reactions of methanol with zeolites type H-Y and Na-Y, between 20 and 350°C, were studied by infrared spectroscopy, gas liquid chromatography, adsorption isotherms and thermo-gravimetric techniques.At room temperature methanol is physically adsorbed on both zeolites. In the case of zeolite H-Y the adsorption occurs via hydrogen bonds between the OH-groups of the methanol molecules and the surface hydroxyl groups and the structural oxygen atoms of the zeolite. At 20°C, a part of the surface hydroxyl groups reacts with the methanol molecules, initiating the methoxylation of the surface, according to the scheme Si—OH + CH3OH →SiOCH3+ H2O. This esterification of the silanol groups is enhanced as the temperature increases, and reaches a maximum at ∼130°C. Above 120°C a new reaction of the surface methoxyl groups with methanol occurs, with formation of dimethylether. The production of dimethylether reaches its maximum at 210°C. Around 250°C, secondary reactions forming gaseous cracking products, predominantly butane and propene, were detected. In the temperature range studied no chemical reaction of methanol with the surface of the dehydrated Na-Y was observed. Therefore, it is concluded that the surface activity of the zeolite H-Y in dehydration of methanol is mainly due to the high Bronsted acidic character of its surface. The analysis of the methoxylation reaction in terms of surface diffusion of the methanol molecules, and of the proton exchange frequency between adsorbed methanol molecules and the zeolite surface, shows that the lifetime of the protonated CH3OH+2 molecule is the rate controlling factor of the methoxylation process.

Hydrogenation of acetylene in excess ethylene on an alumina supported palladium catalyst in a static system

Abstract: Studies have been made of the kinetics of the hydrogenation of mixtures containing 2 % of acetylene in ethylene on an alumina supported palladium catalyst in a static system. Information has been obtained about the nature of the selectivity of the reaction of acetylene. Detailed analyses of the kinetics have been made as the ratio of the pressure of acetylene to ethylene decreases and subsequently as the remaining hydrogen is used up.The poisoning action of carbon monoxide, which selectively inhibits the hydrogenation of ethylene, has also been studied.Tracer studies using carbon-13 labelled acetylene, light ethylene and hydrogen with analysis by combined gas chromatography-mass spectrometry, have shown unambiguously that ethane produced from the hydrogenation of a mixture of acetylene in ethylene comes predominantly from the ethylene.The reaction between acetylene, ethylene and deuterium gives a greater understanding of the surface processes involved and produces ethane which is mainly [2H2]ethane.It is postulated that two types of site exist on the surface, type X which hydrogenates both acetylene and ethylene and on which acetylene is adsorbed ∼2200 times more strongly than ethylene at 293 K and type Y which is easily poisoned by carbon monoxide and can hydrogenate ethylene even in the presence of acetylene.

Infrared spectra from binuclear bridging complexes of sulphate adsorbed on goethite (α-FeOOH)

Abstract: Spectra of sulphate adsorbed on synthetic goethite (α-FeOOH) show replacement of A-type (one coordinated to Fe3+) surface hydroxyls and the formation of adsorbed sulphate groups with C2v symmetry High affinity adsorption isotherms at low pH and replacement of all A-type hydroxyls (∼400 µmol g–1) by the adsorbed sulphate (200 µmol g–1) suggests that a binuclear bridging complex (ie, Fe—O—SO2—O—Fe) is formed, similar to that found for phosphate adsorption on goethiteAbsorption near 1300 cm–1 in the infrared spectra of the binuclear complex suggests that infrared criteria, recently suggested in the literature of metal sulphato complexes, for distinguishing between bridging (ie, all frequencies 1200 cm–1) sulphato complexes may need to be re-examined

Generalised Förster cycle. Thermodynamic and extrathermodynamic relationships between proton transfer, electron transfer and electronic excitation

Abstract: The thermodynamic quantities characterizing one electron reduction, protonation and electronic excitation, are mutually related in the ground and excited states by 6 thermodynamic (or approximate thermodynamic) cycles. The system of cycles is used to predict unknown values, and its validity may be extended to other compounds by means of extra thermodynamic (e.g., Hammett-type) relations. Examples of known data concerning pK and pK* values of protolytic equilibria for the oxidized (Ox) and reduced (R) species are evaluated, tabulated and discussed in a search for a correlation between the changes of pK on excitation (ΔpK*) and on reduction (ΔpKe). Both values are thermodynamically independent but a general common trend is empirically observed in several groups of systems. Some rules are derived for the excited state redox potentials and their dependence on pH, which may be useful for photochemistry.

Evolution of the acidic properties of silica—alumina gels as a function of chemical composition: infrared approach

Abstract: The adsorption of pyridine and CD3CN on silica–alumina gels covering the whole composition range (0–100 % Al2O3) has been studied by infrared spectroscopy, the amount adsorbed being measured by an electrobalance. The measurement of the maximum intensity reached by the bands of strongly adsorbed species has allowed us to sketch the evolution of the surface Bronsted sites and various types of Lewis sites as a function of the chemical composition of the adsorbant.The overall concentration of Lewis sites increases smoothly from 0 to 75 % Al2O3; the sharper increase in the high Al2O3 range is mainly due to weak sites which are revealed by a pyridine band at 1615 cm–1 while pyridine coordinated with strong Lewis sites gives a band at 1625 cm–1.The concentration of acid sites associated with the mixed silica–alumina phase corresponds to the more acidic sites observed by titration in non aqueous media. A high proportion of them are strongly acidic hydroxlys which are detected by absorption of H bond acceptors.The mixed phase develops as the Al2O3 content increases from 0 to 25 %; above 50 % Al2O3 it is progressively diluted in a mixture with other phases and is no longer observed above 85 % Al2O3.

Thermodynamics of hydrogen dissolved in palladium/rhodium and palladium/platinum alloys

Abstract: Partial thermodynamic parameters have been determined for the solution of hydrogen in Pd/Pt and Pd/Rh alloys. The partial excess entropies of absorption of hydrogen at infinite dilution decrease regularly with atom fraction platinum but not with atom fraction rhodium. This suggests that interstices are not excluded for hydrogen occupation by rhodium atoms neighbouring the interstices but are excluded by the platinum nearest neighbours. Other thermodynamic parameters for hydrogen absorption have been determined and are compared for these alloys. These results are discussed in terms of the observation that rhodium will absorb hydrogen at high pressures of hydrogen when situated within the palladium lattice whereas platinum will not.

Redox behaviour of transition metal ions in zeolites. Part 5.—Method of quantitative determination of bidisperse distributions of metal particle sizes in zeolites

Abstract: Temperature programmed reduction and oxidation techniques were used as a fast and quantitative method to determine the amount transition metal inside or outside the zeolite after a reductive treatment. The method has been applied to Ag, Cu and Ni and to different zeolites—X, Y, Z and L.For Ag and Cu zeolites it was found that small metal particles located in the zeolite cages show a low temperature maximum in the rate of oxidation. The maximum in the rate of reoxidation of the metal phase outside the zeolite is found at distinctly higher temperatures. For Ni zeolites, the determination of the bidisperse metal distribution requires first a complete reoxidation of the sample. During a subsequent reduction, two metal dispersions can be distinguished on the basis of maxima observed in the rate of reduction.

Electron spectroscopic study of nitrogen species adsorbed on copper

Abstract: The interaction of a number of nitrogen containing molecules (NO, NH3 and N2H4) with copper has been investigated by electron spectroscopy. Nitric oxide is shown to be molecularly adsorbed in a linearly bonded configuration at 85 K; this adlayer is unstable above 120 K, resulting in an exclusively oxygen adlayer at 295 K, the nitrogen being desorbed. At 85 K N(1s) peaks observed at 401 and 406 eV are associated with NOδ–(ads) and NOδ+(ads) species, the former dissociating and the latter desorbing on thermal activation of the adlayer.Interaction of nitric oxide with Cu at 295 K results in dissociative chemisorption with both oxygen and nitrogen retained at the surface. The surface concentration suggests that each nitrogen and oxygen adatom is bridge-bonded to two surface copper atoms. Estimates of absolute surface coverages are made by comparing the intensities of the N(1s) and O(1s) peaks with the Cu(2p) substrate intensity, while valence level spectroscopy (He I and II radiation) supplements core-level spectroscopy for discriminating between molecular and dissociated surface species.Ammonia dissociates at 295 K on copper while with oxidized copper the surface oxide is replaced by an imide-type surface. With hydrazine, adsorption is molecular over the temperature range 85 to 295 K, in contrast to our observations with iron.