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Showing papers in "The Journal of Physical Chemistry in 1979"


Journal ArticleDOI
TL;DR: In this article, a general theory for polymer adsorption using a quasi-crystalline lattice model is presented, where the Bragg-Williams approximation of random mixing within each layer parallel to the surface is adopted.
Abstract: We present a general theory for polymer adsorption using a quasi-crystalline lattice model. The partition function for a mixture of polymer chains and solvent molecules near an interface is evaluated by adopting the Bragg-Williams approximation of random mixing within each layer parallel to the surface. The interaction between segments and solvent molecules is taken into account by use of the Flory-Huggins parameter x; that between segments and the interface is described in terms of the differential adsorption energy parameter xs. No approximation was made about an equal contribution of all the segments of a chain to the segment density in each layer. By differentiating the partition function with respect to the number of chains having a particular conformation an expression is obtained that gives the numbers of chains in each conformation in equilibrium. Thus also the train, loop, and tail size distribution can be computed. Calculations are carried out numerically by a modified matrix procedure as introduced by DiMarzio and Rubin. Computations for chains containing up to lo00 segments are possible. Data for the adsorbed amount r, the surface coverage 0, and the bound fraction p = O/r are given as a function of xs, the bulk solution volume fraction c#J,, and the chain length r for two x values. The results are in broad agreement with earlier theories, although typical differences occur. Close to the surface the segment density decays roughly exponentially with increasing distance from the surface, but at larger distances the decay is much slower. This is related to the fact that a considerable fraction of the adsorbed segments is present in the form of long dangling tails, even for chains as long as r = 1000. In previous theories the effect of tails was usually neglected. Yet the occurrence of tails is important for many practical applications. Our theory can be easily extended to polymer in a gap between two plates (relevant for colloidal stability) and to copolymers.

1,180 citations



Journal ArticleDOI
TL;DR: In this paper, the authors described the rate constants for thermal unimolecular reactions and recombinations at the low pressure limit, at the high pressure limit and in the intermediate falloff range.
Abstract: This paper describes the calculation of rate constants for thermal unimolecular reactions and recombinations at the low pressure limit, at the high pressure limit, and in the intermediate falloff range, as well as the calculation of specific rate constants for unimolecular rearrangements. The most uncertain factors of the theory are identified by comparison with the NO/sub 2/, ClNO, H/sub 2/O, and O/sub 3/ systems. Weak collision and centrifugal barrier effects are discussed for low pressure rate constants. Simplified adiabatic channel calculations are proposed for specific rate constants and high pressure rate constants. Reduced falloff curves are presented in factorized form with weak collision and strong collision broadening factors. Simple falloff expressions are derived. 5 figures, 44 references, 3 tables.

764 citations


Journal ArticleDOI
TL;DR: In this article, a spin trap was used to detect free-radical intermediates formed during in situ irradiation of TiO/sub 2/ and platinized polysilicon powders in aqueous solution.
Abstract: Spin trapping and ESR detection (with the traps ..cap alpha..-phenyl N-tert-butyl nitrone and ..cap alpha..-(4-pyridyl N-oxide) N-tert-butyl nitrone) were employed to detect free-radical intermediates formed during in situ irradiation of TiO/sub 2/ and platinized TiO/sub 2/ powders in aqueous solutions. Evidence for the production of hydroxyl radical (.OH) (formed in the oxidation of water) and perhydroxyl radical (HO/sub 2/.) (probably formed in a reduction step) is presented. Other spin adducts, attributed to decomposition reactions of the spin traps, were also found. The results suggest an important role for photogenerated .OH in many photocatalytic and photosynthetic processes at TiO/sub 2/ powders.

533 citations



Journal ArticleDOI
TL;DR: In this paper, a quantitative analysis of XPS intensities for supported catalysts was based on a model of cubic catalyst crystallites between sheets of support and the assumption that the relative ESCA intensities of the support and catalyst depend on the photoelectron cross section, the atomic ratio of catalyst and support, the escape depths of the electrons, the support surface area, the catalyst weight fraction, and the atomic densities of both support and catalysts.
Abstract: A quantitative analysis of XPS (ESCA) intensities for supported catalysts was based on a model of cubic catalyst crystallites between sheets of support and the assumption that the relative ESCA intensities of the support and catalyst depend on the photoelectron cross section, the atomic ratio of catalyst and support, the escape depths of the electrons, the support surface area, the catalyst weight fraction, the atomic densities of support and catalyst, and the detector efficiency. The model predicted accurately the densities of new ESCA measurements of alumina-supported rhenium oxide and available data for silica-supported platinum and fluorine-contining aluminas at low fluorine contents; low data points for alumina containing Vertical Bar3: 10Vertical Bar3< fluorine were possibly due to the formation of aluminum hydroxyfluoride and aluminum fluoride which were partly ''invisible'' to ESCA. A method for estimating crystallite size is outlined.

406 citations


Journal ArticleDOI
TL;DR: In this article, an equation for the representation of the dielectric constant of water over this range was developed, and the Debye-Hueckel limiting law parameters for osmotic and activity coefficients, enthalpies, heat capacities, volumes, compressibilities, and expansibilities were calculated.
Abstract: In preparation for work with aqueous electrolytes at above saturation pressures and at temperatures to 350/sup 0/C, an equation was developed for the representation of the dielectric constant of water over this range. With this equation and an equation of state for water, the Debye--Hueckel limiting law parameters for osmotic and activity coefficients, enthalpies, heat capacities, volumes, compressibilities, and expansibilities were calculated and are presented. 5 figures, 4 tables.

390 citations


Journal ArticleDOI
TL;DR: In this paper, the Raman and UV spectra indicated that adsorption on the alumina surface occurred in all cases via the MoO/sub 4/(-2) ion, probably due to locally increased pH.
Abstract: Catalysts containing 1, 3, and 8Vertical Bar3< molybdena on ..gamma..- or g-alumina were prepared by impregnating the support with (NH/sub 4/)/sub 6/Mo/sub 7/O/sub 24/ at pH 6 or 11, at which pH's molybdenum is in the octahedral Mo/sub 7/O/sub 24/(-6) form or the tetrahedral MoO/sub 4/(-2) form, respectively. The Raman and UV spectra indicated that adsorption on the alumina surface occurred in all cases via the MoO/sub 4/(-2) ion, probably due to locally increased pH. The removal of water during drying and calcination caused polymerization of the isolated tetrahedral molybdenum species into a two-dimensional Mo-O-Mo structure. The degree of polymerization depended on the molybdenum concentration, the heat treatment, and the pH of the original adsorption which influences the molybdenum distribution. The coordination number of the molybdenum in the polymeric structure, which was probably affected by the alumina surface hydroxyl density and distribution, could not be determined. Some bulk molybdenum trioxide was formed on all samples after calcination except for a 3Vertical Bar3< MoO/sub 3/ sample impregnated at pH 11.

359 citations





Journal ArticleDOI
TL;DR: A selective survey of the chemical kinetic literature involving flow tube measurements of elementary reaction rate constants is presented in this article, where the origins of the flow tube method, the experimental technique, the measurement of rate constants, and an analysis of the inherent errors are described.
Abstract: A selective survey was made of the chemical kinetic literature involving flow tube measurements of elementary reaction rate constants. The origins of the flow tube method, the experimental technique, the measurement of rate constants, and an analysis of the inherent errors are described. Emphasis is placed on the discussion of the strengths and limitations of the method as a source of kinetic data. 1 figure, 88 references, 2 tables.




Journal ArticleDOI
TL;DR: Experimental and simulated powder X-ray diffraction patterns obtained for ZSM-5 zeolites with silica-alumina ratios of 70:1 to Vertical Bar 3: 3,000:1 show that the pattern of as-synthesized samples are consistent with idealized orthorhombic symmetry, but that the patterns of samples converted to the ammonium or hydrogen forms exhibit a change to monoclinic symmetry as mentioned in this paper.
Abstract: Experimental and simulated powder X-ray diffraction patterns obtained for ZSM-5 zeolites with silica-alumina ratios of 70:1 to Vertical Bar3: 3,000:1 show that the pattern of as-synthesized samples are consistent with idealized orthorhombic symmetry, but that the patterns of samples converted to the ammonium or hydrogen forms exhibit a change to monoclinic symmetry. The change is a reversible, displacive transformation between two symmetry forms that causes no change in framework topology. Generally, the higher the silica-alumina ratio, the easier is the change from the parent orthorhombic to monoclinic symmetry after treatment.


Journal ArticleDOI
TL;DR: In this article, an analytical study of free radicals of high negative redox potential such as cap alpha-alcohol radicals were found to transfer electrons to colloidal silver particles stabilized by sodium dodecyl sulfate in aqueous solution.
Abstract: Organic free radicals of high negative redox potential such as ..cap alpha..-alcohol radicals were found to transfer electrons to colloidal silver particles stabilized by sodium dodecyl sulfate in aqueous solution. The colloidal particles thus became a pool of stored electrons that could reduce water to form hydrogen or react with suitable acceptors in solution. The organic radicals were produced by irradiation, using suitable scavengers for the primary radicals from the radiolysis of the aqueous solvent. The solutions initially contained silver ions at 1 x 10/sup -4/ - 2 x 10/sup -3/ M. At doses below 10/sup 5/ rd, the silver ions were completely reduced to form the colloidal catalyst. In this dose range, the corresponding hydrogen yield amounted to 1 molecule per 100 eV. It increased steeply at higher doses up to 3 molecules per 100 eV. The H/sub 2/ yield decreased with increasing dose rate and with increasing pH in alkaline solutions. It was highest at a concentration of sodium dodecyl sulfate of 1 x 10/sup -3/ M, i.e., far below the critical micelle concentration of this surfactant. Changes in the absorption spectrum of the colloid are attributed to changes in the size of the silver particles upon chargingmore » up with electrons. The competition of radical-colloid reactions with radical-radical deactivation in the bulk of solution or at the surface of the colloidal particles is also discussed. 11 figures.« less


Journal ArticleDOI
TL;DR: In this article, the authors measured the optical absorption spectra of solvated electrons in H/sub 2/O and D/sub O and found that the spectrum at a given A/A/sub max/ shows a shift of +0.05 eV in the low-energy wing, reaching 0.03 eV at the absorption maximum.
Abstract: The optical absorption spectra of solvated electrons in H/sub 2/O and D/sub 2/O have been measured at 274, 298, 340, and 380 K. All the spectra were fitted very well with the Gaussian and Lorentzian shape functions at the low- and high-energy sides of the absorption maximum, respectively, excluding the high-energy tail. The spectrum does not shift uniformly with temperature. The temperature coefficient of absorption decreases rapidly with increasing energy on the low-energy side of the absorption maximum, while it changes only slightly on the high-energy side. When the temperature increases the Lorentzian width remains constant, the Gaussian width varies proportionally to T/sup 1/2/, and the spectrum becomes more symmetrical. On going from H/sub 2/O to D/sub 2/O we found that the spectrum at a given A/A/sub max/ shows a shift of +0.05 eV in the low-energy wing. The shift decreases with increasing energy, reaching 0.03 eV at the absorption maximum. On the high-energy side of the band the shift becomes negative at h..nu.. > 2.2 eV. The shift on the low-energy side seems to be related to the difference of the zero-point energies of the inter- and intramolecular vibrations. The wavelength dependence of the temperature and isotope effects ismore » consistent with the model that different types of excitation occur on the low- and high-energy sides of the absorption band. The temperature and isotopic dependence of the low-energy side are consistent with its width being due to phonon interactions.« less

Journal ArticleDOI
TL;DR: Canonical variational transition state theory, microcanonical VAE, and Miller's unified statistical theory were used in this paper to correct two major deficiencies of the conventional transition state theories.
Abstract: Canonical variational transition state theory, microcanonical variational transition state theory, and Miller's unified statistical theory were used in an attempt to correct two major deficiencies of the conventional transition state theory. These are: (1) the necessity of extra assumptions to include quantum mechanical tunneling effects and (2) the fundamental assumption that trajectories crossing a dividing surface in phase space proceed directly to products. The accuracy of these approximate methods were tested by performing calculations for several collinear reactions of hydrogen, deuterium, chlorine, or iodine, with five isotopes of hydrogen molecules and comparison of these results with those from accurate quantitative calculations of the reaction probabilities as functions of energy and of the thermal rate constants as functions of temperature. 49 references, 28 figures, 17 tables.


Journal ArticleDOI
TL;DR: An IR spectroscopic study of the reduction of copper-exchanged Linde Na-Y zeolite with hydrogen or carbon monoxide and oxidation with oxygen or oxygen-18 showed that copper(I) and AlO(+1) are formed during reduction, and that oxidation generates copper(II) ions but does not change the aluminum species as mentioned in this paper.
Abstract: An IR spectroscopic study of the reduction of copper-exchanged Linde Na-Y zeolite with hydrogen or carbon monoxide and oxidation with oxygen or oxygen-18 showed that copper(I) and AlO(+1) are formed during reduction, and that oxidation generates copper(II) ions but does not change the aluminum species. Trigonally coordinated aluminum was not detected.

Journal ArticleDOI
TL;DR: In this paper, the temperature dependence of the ECL emission yield of Ru(bpy)?+ in acetonitrile has been determined and was found to correlate closely with the luminescence quantum yield, @e, of the lowest Rn* metal-to-ligand charge transfer (MLCT) excited state.
Abstract: The temperature dependence of the ECL emission yield, @ECL, of Ru(bpy)?+ in acetonitrile has been determined and was found to correlate closely with the luminescence quantum yield, @e, of the lowest R~(bpy),~+ dn* metal-to-ligand charge transfer (MLCT) excited state. Evidence is presented that at low temperatures (< -30 "C) the ECL emission yield becomes equal to the luminescence quantum yield implying an efficiency for excited state formation in the electron transfer reaction of Ru(bpy)?+ with Ru(bpy),+ of near 100%. High efficiencies for excited state formation are rationalized on the basis of the excited state spectroscopic properties of Ru(bpy)gP+ and the resulting implications to electron transfer theory and the ECL mechanism are discussed.

Journal ArticleDOI
TL;DR: In this article, X-ray photoelectron and ion scattering spectroscopies provided evidence that nickel does not interact with a silica support but forms nickel monoxide; that on..gamma..-alumina, low nickel contents form two nickel species which interact with the support, of which one is in tetrahedral alumina sites and difficult to reduce and the other, in octahedral sites and easy to reduce.
Abstract: Studies of supported nickel catalysts by X-ray photoelectron and ion scattering spectroscopies provided evidence that nickel does not interact with a silica support but forms nickel monoxide; that on ..gamma..-alumina, low nickel contents form two nickel species which interact with the support, of which one is in tetrahedral alumina sites and difficult to reduce and the other, in octahedral sites and easy to reduce; that at higher calcination temperatures, nickel in octahadral sites diffuses to tetrahadral sites; and that higher nickel loadings on ..gamma..-alumina, nickel monoxide forms on top of the interacting nickel species. The studies were performed with 2-33.2Vertical Bar3< nickel on ..gamma..-alumina and 4-35Vertical Bar3< nickel on silica, prepared by impregnation with nickel nitrate and calcined at 400/sup 0/-600/sup 0/C.



Journal ArticleDOI
TL;DR: In this article, the authors draw analogies between the properties of zeolites and solutions, including the accessibility of atoms, cation and proton mobility, framework atom mobility, ionizing and electrolyte properties, formation of transition metal complexes, acidity, and activity coefficients in ion exchange.
Abstract: Analogies were drawn between the properties of zeolites and of solutions, including the accessibility of atoms, cation and proton mobility, framework atom mobility, ionizing and electrolyte properties, formation of transition metal complexes, acidity, and activity coefficients in ion exchange. The properties of solutions, including properties connected with acidity (solvent, structure, and concentration effects), electrode potential, collective interactions, and solubility product, were extended to zeolites. The analysis shows that for dealuminated zeolites and oxyacids of the TO/sub n/(OH)m formula, the acid strength increases with increasing number of oxygens; that defining the activity coefficients for the ions and atoms of the zeolite framework might elucidate zeolite adsorption and catalytic activity; and that the behavior of transition metal cations in zeolites is related to electrode phenomena.


Journal ArticleDOI
TL;DR: In this article, a model for the mechanism of absorption (or release) of hydrogen by LaNi/sub 5/ is presented, which is consistent with the observed kinetics of absorption and desorption of H/sub 2/H/sub 4/ over LaNi-sub 5 /LaNi/Sub 5/.
Abstract: A model is presented for the mechanism of absorption (or release) of hydrogen by LaNi/sub 5/. The surface of LaNi/sub 5/ is extensively coated by La/sub 2/O/sub 3/. The surface oxide forms in two ways: (1) by reaction of gaseous oxygen with LaNi/sub 5/ or (2) by diffusion of dissolved oxygen to the surface where it reacts with LaNi/sub 5/ to form oxide and precipitate Ni, some of which is also at the surface. Hydrogen reaches the underlying LaNi/sub 5/ by diffusion in monatomic form along the La/sub 2/O/sub 3/--Ni interface. The dissociation of H/sub 2/ at the point of entry into the interfacial region is rate controlling for absorption. Release of hydrogen involves diffusion of monatomic hydrogen along the interface followed by recombination to form molecular hydrogen at the top of the interfacial region. The latter is rate controlling. The model is in accord with the known surface features of the intermetallic and is consistent with the observed kinetics of absorption and desorption of H/sub 2/ by LaNi/sub 5/. It is also consistent with the kinetics of hydrogenation of C/sub 2/H/sub 4/ over LaNi/sub 5/ and LaNi/sub 5/H/sub 2/ /sub 4/. In the latter, interfacial diffusion of monatomic hydrogenmore » is rate controlling. The present model differs from earlier models which regarded either the phase transformation or mass transport through macroscopic cracks of fissures as the rate-controlling step. 2 figures.« less