摘要:

AbstractThe kinetics of solid/solid interfaces is of major importance for the course of heterogeneous reactions in the solid state. Solid/solid interfaces are normally sources of elastic stress and thus may interact with outer surfaces. In addition, they can offer high diffusivity paths and thus represent two‐dimensional reaction media. Essential kinetic situations are defined and basic formal considerations are summarized. Corresponding experimental examples are presented in order to illustrate kinetic phenomena at static and moving boundaries. This includes questions of exchange fluxes, boundary‐controlled solid state reactions, morphology, nonlinear phenomena and reactions in boundar

ISSN:0005-9021    

DOI:10.1002/bbpc.19981020202

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractThe structures of zirconium alkoxides Zr(OR)4, with R=n‐propoxide and n‐butoxide, and of the corresponding mixed zirconium‐titanium alkoxides, used as precursors in the sol‐gel process, were investigated by EXAFS spectroscopy at the Zr K‐edge in solution of their parent alcohols.In all investigated systems the Zr‐atoms are surrounded by six O‐atoms. Additionally, a Zr‐Zr distance was found in the pure zirconium alkoxides, indicating the existence of dimers or oligomers. In the mixture of zirconium‐ and titanium alkoxides the hetero‐association is preferred, since a Zr‐Ti dista

ISSN:0005-9021    

DOI:10.1002/bbpc.19981020203

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractThe complex dielectric permittivity of ionic plastic crystal KNO2was measured in the frequency range 20‐106Hz and the temperature range 22‐300 K. A small step‐like anomaly of the static dielectric permittivity appeared at the rhombohedral (phase II) to monoclinic (phase III) transition (Ttr2= 264. 1 K). In phase III, the static dielectric permittivity decreased gradually on cooling over a wide temperature range 50‐264 K. These results indicate that the orientation of the NO−2ion is still disordered at high temperatures in phase III and becomes ordered gradually with decreasing temperature. The dielectric dispersion associated with this motion occurred in the temperature range 70‐150 K. From these data and the previously reported15N NMR data, we propose a model of the motion of NO2−in phase III in which the anion undergoes 180°‐flip motion about the axes perpendicular to the molecular C2axis in an asymmetric double

ISSN:0005-9021    

DOI:10.1002/bbpc.19981020204

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractThe temperature dependences of the self‐diffusion coefficient and the spin‐lattice relaxation time have been studied by NMR measurements performed on a good glass former LiCl‐6H2O. The spin‐lattice relaxation time can be separated into two relaxation processes, i.e., inter‐ and intramolecular processes. We have related these relaxation processes to the translational diffusion which corresponds to the primary or α‐relaxation and to the local motions which are related to the β‐relaxation in glass forming systems. The results throw some light to the evolution with temperature of these relaxation processes in the LiCl‐6H2O system over the normal liquid, supercooled liqui

ISSN:0005-9021    

DOI:10.1002/bbpc.19981020205

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractIn this work we use the well‐known bivariate trap model with random topography in order to study the effects of surface heterogeneities on surface diffusion. Utilizing Monte Carlo simulations we characterize how the tracer, jump and chemical diffusion coefficients depend on temperature and concentration of deep traps, θ. In order to calculate the chemical diffusion coefficient we apply the fluctuation and the Kubo‐Green methods. Both methods are in good agreement at large values of θ only. The discrepancies are discussed and explained. The effects of heterogeneities are largely pronounced at low total coverage and low temperatures, where most of the adatoms are trapped by the deep traps. At high coverages the mobility of adatoms adsorbed on shallow traps dominate the diffusion beh

ISSN:0005-9021    

DOI:10.1002/bbpc.19981020206

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractPeriodic bands of crystals (Liesegang rings) are formed when a pair of soluble salts interdiffuses and reacts to form an insoluble salt. A classic example is the banded precipitation of silver chromate produced by the interdiffusion of aqueous silver nitrate and potassium chromate: 2AgNO3(aq)+K2CrO4(aq) = Ag2CrO4(s)+2KNO3(aq). This process is studied by using Taylor dispersion to measure interdiffusion coefficients, including cross‐coefficients, for aqueous AgNO3+KNO3and K2CrO4+KNO3solutions found on opposite sides of AgNO3/K2CrO4diffusion boundaries. Silver chromate readily supersaturates to 5 times its solubility. This information together with the measured diffusion coefficients and previously reported rates of precipitation are used to develop a diffusion‐precipitation model for the formation of silver chromate Liesegang rings. Numerical integration of the equations employing a Gaussian distribution of critical supersaturation values shows that a sharp supersaturation limit is not essential for periodic precipitation, and secondary band structure is predic

ISSN:0005-9021    

DOI:10.1002/bbpc.19981020207

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractThe adsorption isotherm and the differential heat of adsorption of ammonia in zeolite NaZSM5 (Si/Al=32) were measured over a wide range of equilibrium pressure at the temperature 303 K. The obtained data can consistently be interpreted to be due to the formation ofs‐meric (s= 1 − 6) ammonia/sodium cation complexes residing in the intersections of the straight and zig‐zag channels of the zeolite structure. The energies of formation and the potential energies of thes‐mers are r

ISSN:0005-9021    

DOI:10.1002/bbpc.19981020208

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractRaman and infrared spectra of aqueous CdSO4and (NH4)2SO4solutions have been recorded over a broad concentration (0. 040 to 3. 51 mol/kg) and temperature range (‐10 to 109°C). Whereas thev1SO2‐4band profile is symmetrical in (NH4)2SO4solutions, in CdSO4solutions a shoulder appears on the high frequency side at 989 cm−1,which increases in intensity with increasing concentration and temperature. The relative molar scattering coefficient of thev1SO42−v band is the same for all forms of sulfate in (NH4)2SO4and CdSO4solutions and is independent of temperature up to 109°C. The high frequency shoulder is attributed to the formation of a 1:1 inner‐sphere complex [Cd2+(OH2)5OSO32−Thev3SO42−antisymmetric stretching mode, normally forbidden in the isotropic scattering, contains a contribution in concentrated CdSO4solutions. The bending modes,v2SO42−andv4SO42−, also forbidden in the isotropic scattering, show contributions at 472 and 640 cm−1,well apart from the frequencies normally observed in the anisotropic spectrum in (NH4)2SO4solution. A polarized band at 240 cm−1has been assigned to the Cd2+‐OSO32−ligand vibration. Further spectroscopic evidence for contact ion pair formation at 25°C is provided by i.r. spectroscopy. Higher associates or anionic complexes are not required to interpret the spectroscopic data, but it has been established that at room temperature the favourable complex is an outer‐sphere complex whereas above 109°C the inner‐sphere complex is becoming the favourable one (inner‐sphere complex formation is entropically driven). The Raman spectroscopic results confirm the stepwise reaction mechanism of sulfato‐complex formation in aqueous CdSO4solution.The degree of association has been measured as a function of concentration and temperature. At concentrations above 1.0 mol · kg−1the concentration quotient, QAequal to QU,has a relatively high and constant value of 0.14±0.02. The thermodynamic association constant, enthalpy and entropy for the outer‐sphere/inner‐sphere complex formation have been estimated to be:KII= 3.3±0.3, ΔH° =(6

ISSN:0005-9021    

DOI:10.1002/bbpc.19981020209

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractThe quantum cluster equilibrium (QCE) theory is presented for liquid ammonia. The cluster equilibria that dictate phase composition are determined by the rigorous techniques of quantum statistical thermodynamics in the canonical ensemble, based on the ab initio partition function. The characteristic features of the supra‐molecular clusters which comprise the QCE model are discussed in terms of binding energies, geometries, cooperativity and charge transfer. Many possible equilibrium structures of ammonia are strongly disfavored for enthalpic and/or entropic reasons and do not show up in the QCE cluster populations. The numerical accuracy of the resulting QCE model is demonstrated by comparison with experimental data for thermodynamic properties of ammonia such as Clausius‐Clapeyron pressure dependence and specific h

ISSN:0005-9021    

DOI:10.1002/bbpc.19981020210

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractTemperature dependent NMR chemical shifts for hydrogen and nitrogen, NMR quadrupole coupling constants for deuterium and nitrogen as well as all vibrational frequencies for liquid ammonia are calculated by the ab initio quantum cluster equilibrium (QCE) method and compared with experimental measurements. All properties were obtained from equilibrium populations of ammonia clusters calculated between the melting (196 K) and the boiling (239 K) point. The calculated vibrational frequencies and chemical shifts are in good agreement with experimental NMR and Raman measurements. The theoretical quadrupole coupling constants for liquid ammonia lie midway between the available experimental data for the solid and the gas phase.

ISSN:0005-9021    

DOI:10.1002/bbpc.19981020211

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY