scispace - formally typeset
Search or ask a question

Showing papers in "The Journal of Physical Chemistry in 1980"




Journal ArticleDOI
TL;DR: In this paper, the improved canonical variational theory is extended to reactions in a quantum mechanical world where internal energies of reactants are quantized, and a detailed discussion of vibrationally adiabatic models for transmission coefficients for conventional transition state theory and three versions of variational transition-state theory are also presented.
Abstract: The improved canonical variational theory, presented previously for reaction rates in a classical mechanical world, has been extended to reactions in a quantum mechanical world where internal energies of reactants are quantized. A detailed discussion of vibrationally adiabatic models for transmission coefficients for conventional transition-state theory and three versions of variational transition-state theory are also presented. The relationship of the improved canonical variational transition-state theory to the classical limit of these transmission coefficients has been shown. The improved canonical variational theory and the new quantal and classical transmission coefficients are illustrated and tested by applications to quantal collinear and three-dimensional reactions rates for several reactions. Semiclassical approximations to the quantal transmission coefficients are also examined. Applications considered are collinear H + H/sub 2/ and isotopic analogs, Cl + H/sub 2/ and isotopic analogs, and I + H/sub 2/ and three-dimensional D + H/sub 2/, Cl + HD, I + H/sub 2/, I + D/sub 2/, O + H/sub 2/, and F + H/sub 2/.

652 citations



Journal ArticleDOI
TL;DR: The model hydrodesulfurization catalysts studied in this article contained 1-30% MoO/sub 3/ on δ-gamma-alumina and were calcined at 400/sup 0/-700/sup 1 0/C for 1-32 hr and reduced at 500/sup 2 0 /C in hydrogen.
Abstract: The model hydrodesulfurization catalysts studied contained 1-30% MoO/sub 3/ on ..gamma..-alumina and were calcined at 400/sup 0/-700/sup 0/C for 1-32 hr and reduced at 500/sup 0/C in hydrogen. At low MoO/sub 3/ loadings (to approx. 8%), molybdenum in the tetrahedral lattice of alumina was the main species; at higher coverage of MoO/sub 3/, to monolayer coverage (approx. 20% MoO/sub 3/), molybdenum in the octahedral lattice of ..gamma..-alumina became more important; around monolayer concentration of MoO/sub 3/, Al/sub 2/(MoO/sub 4/)/sub 3/ started to form; and at higher molybdena concentration, bulk MoO/sub 3/ was also observed. An increase in calcination temperature and time favored formation of the tetrahedral lattice species and of Al/sub 2/(MoO/sub 4/)/sub 3/.

403 citations



Journal ArticleDOI
TL;DR: In this article, a thermodynamic theory of micelle formation and growth is presented, which makes use of the experimentally established result that the micelles have the shape of prolate spherocylinders.
Abstract: A strong increase has been measured in the size and polydispersity of sodium dodecyl sulfate (SDS) micelles on increasing the detergent concentration in solutions containing high salt (NaCl) concentration. The mean hydrodynamic radius and the polydispersity of the micelles were measured. A thermodynamic theory of micelle formation and growth is presented. This theory makes use of the experimentally established result that the micelles have the shape of prolate spherocylinders. The micellar size and polydispersity is determined at each temperature and salt concentration by 2 chemical potentials. These are the chemical potential changes associated with transferring a detergent monomer from the solvent to either the cylindrical or spherical region of the micelle. Using only these 2 chemical potentials as parameters, one can predict accurately the hydrodynamic radius, the polydispersity, and the scattered light intensity over the entire experimental range of detergent concentration, temperature, and salt concentration. 43 references.

399 citations


Journal ArticleDOI
TL;DR: In this article, the conversion of unpromoted, unsupported metallic catalysts into carbides during Fischer-Tropsch synthesis (CO:H2:He = 1:1:3, 1 atm) was studied with Mossbauer spectroscopy, X-ray diffraction, carbon content analysis, and reaction kinetic measurements.
Abstract: The conversion of unpromoted, unsupported metallic iron catalysts into carbides during Fischer-Tropsch synthesis (CO:H2:He = 1:1:3, 1 atm) was studied with Mossbauer spectroscopy, X-ray diffraction, carbon content analysis, and reaction kinetic measurements. From a comparison between experiments at different temperatures and literature data, it is concluded that both reaction conditions and the nature of the iron catalyst determine the combination of carbides that will be formed. Investigation of single-phase carbides revealed that the X-ray diffraction pattern commonly ascribed to a pseudohexagonal carbide Fe2C actually belongs to the carbide ∈′-Fe2.2C. At synthesis temperatures of 513 K and lower, unknown iron-carbon species were found, referred to as FexC. It is believed that FexC represents poorly defined structures between α-Fe and a crystallographic carbide. The behavior of metallic iron catalysts during Fischer-Tropseh synthesis at 513 K was studied in more detail as a function of time. It was found that the rate of hydrocarbon formation was initially low, passed through a maximum, and decreased thereupon, while the conversion of α-Fe into carbides started at a high rate and decreased rapidly. These results can be understood as the consequence of either a competition between bulk carbidization and hydrocarbon synthesis or a relatively slow activation of α-Fe for the formation of hydrocarbons in which bulk carbidization plays no role. Deactivation is caused by the formation of an excessive amount of inactive carbon at the surface of the catalyst.

391 citations


Journal ArticleDOI
TL;DR: In this paper, the reduction of nitro blue tetrazolium (NBT/sup 2 +/) by CO/sub 2//sup -/ and O/sub 1/2/1/s/sup -1/
Abstract: The reduction of nitro blue tetrazolium (NBT/sup 2 +/) by CO/sub 2//sup -/ and O/sub 2//sup -/ has been studied by the stopped-flow method and the pulse-radiolysis technique. Both reductants form the tetrazolinyl radical, and the rate constants are k/sub NBT/sup 2 +/ + CO/sub 2//sup -// = (6.4 +- 0.2) x 10/sup 9/ M/sup -1/ s/sup -1/ and k/sub NBT/sup 2 +/ + O/sub 2//sup -// = (5.88 +- 0.12) x 10/sup 4/ M/sup -1/ s/sup -1/. In the absence of other reactants, the tetrazolinyl radicals (NBT/sup +/.) disappear by a pH-dependent second-order disproportionation reaction to produce monoformazan (MF/sup +/) and nitro blue tetrazolium (NBT/sup 2 +/) ions. The tetrazolinyl radical (NBT/sup +/.) has an absorption maximum at 405 nm with a pH-independent molar extinction coefficient epsilon/sub 405nm/ = 15,000 +- 350 M/sup -1/ cm/sup -1/. Monoformazan (MF/sup +/) has an absorption maximum at 530 nm; its molar extinction coefficient varies from epsilon/sub 530 nm/ = 25,400 +- 1200 M/sup -1/ cm/sup -1/ at pH 9.5-11.0 to epsilon/sub 530nm/ = 12,800 +- 640 M/sup -1/ cm/sup -1/ at pH 5.7-6.7. Mechanisms for the overall reaction are discussed.

363 citations


Journal ArticleDOI
TL;DR: In this article, a combination of Raman spectroscopy and temperature-programmed reduction (TPR) was used for qualitative and quantitative structural analysis of vanadium(V) oxide supported on 7-A1203, GO2, Cr2O3, Si02, Ti02, and Zr02.
Abstract: Vanadium(V) oxide supported on 7-A1203, GO2, Cr2O3, Si02, Ti02, and Zr02 was studied by X-ray fluorescence, by X-ray diffraction, and especially by the combination of Raman spectroscopy and temperature-programmed reduction (TPR) for qualitative and quantitative structural analysis, respectively. Catalysts were prepared via ion-exchange and wet-impregnation methods. The V contents ranged from - 1 to 40 wt % V. At low surface concentrations only surface vanadate phases of two-dimensional character are observed for all carriers. According to Raman and TPR data the structure of these surface vanadate species is independent of the preparation technique. At medium and high surface concentrations, the webimpregnated samples already contain crystalline V20b At equal surface concentrations the ion-exchanged catalysts contain no V205 crystallites. An exception is Si02 on which also crystalline V206 is formed in both preparation techniques. Monolayer stability toward thermal treatment decreases in the order AZO>3 Ti02> Ce02,w hereas on heating ion-exchanged V/Si02 the crystalline V206 spreads out over the silica surface. The reducibilities of the ion-exchanged catalyats, as measured by TPR, can be used as a measure for the contact interaction between vanadia and the carrier oxides. At temperatures of 500-800 K, this interaction ranges from strong with titania to weak with silica as a carrier.

351 citations


Journal ArticleDOI
TL;DR: In this paper, the dependence of the CMC on the electrolyte concentration, the alkyl chain length, and the counterion valency has been analyzed in the cell model of micellar solutions.
Abstract: Within the cell model of micellar solutions electrostatic effects can be described on the basis of the nonlinearized Poisson-Boltzmann Equations for a spherically symmetrical system. All the components in the system contribute to the electrostatic free energy, and closed-form expressions for the chemical potentials that have been derived. These expressions have been used to calculate the dependence of the CMC on the electrolyte concentration, the alkyl chain length, and the counterion valency. The amphiphile activity decreases above the CMC, and counterion association also has been discussed. In spite of deficiencies in the model, it gives predictions that are in good agreement with experimental findings. This indicates that for inorganic cations the major part of the micelle-ion interactions is of a nonspecific nature and that specific ion-binding effects are of secondary importance for the energetics. 62 references.

Journal ArticleDOI
TL;DR: In this article, light scattering from aqueous solutions of sodium dodecyl sulfate has been measured in the presence of NaCl of different concentrations at different temperatures, and the angular dependence indicates formation of a rodlike micelle having a length of 597 angstroms.
Abstract: Light scattering from aqueous solutions of sodium dodecyl sulfate has been measured in the presence of NaCl of different concentrations at different temperatures. In NaCl solutions more concentrated than 0.45 M, surfactant micelles formed at the critical micelle concentration further associate into large micelles with increasing surfactant concentration. In 0.60 and 0.80 m NaCl at 25 C, light scattering exhibits anomalous dissymmetry attributable to the formation of trace amounts of microgel particles. In 0.80 M NaCl at 35 C, light scattering is normal and gives the micelle aggregation number as high as 1410 at 1.10 x 10/sup -2/ g cm/sup -3/. The angular dependence indicates formation of a rodlike micelle having a length of 597 angstroms. It is concluded that sodium dodecyl sulfate forms spherical micelles at low NaCl concentrations (< 0.45 M), but it associates into rodlike micelles at high NaCl concentrations when its micelle concentration is high. 41 references.

Journal ArticleDOI
TL;DR: The first small-angle X-ray scattering study of Triton X-100 at different temperatures and ionic strengths in order to measure molecular weight, shape, and hydration of this nonionic surfactant is described in this paper.
Abstract: This work describes the first small-angle X-ray scattering study of Triton X-100 at different temperatures and ionic strengths in order to measure molecular weight, shape, and hydration of this nonionic surfactant. The X-ray scattering study is performed on the detergent at different electron density contrast by varying the electron density of the solvent by addition of glycerol or thallium nitrate. This study detects, on the basis of small-angle X-ray scattering experiments and by using a different contrast between the Triton X-100 micelles and the applied solvent, the hydrophobic core when using the assumption of a 2-step density level within the molecule. Furthermore, quasielastic light scattering measurements of aqueous solutions of Triton X-100 were performed which allow measurement of the diffusion coefficient. 30 references.


Journal ArticleDOI
TL;DR: In this article, the thiocyanate dosimeter was calibrated against the Fricke system to obtain a value of 46,400 +-600 (molecules/100 eV) for the product of yield and extinction coefficient of (SCN)/sub 2/O-saturated solutions.
Abstract: A study was undertaken to calibrate the thiocyanate dosimeter most commonly used for determination of OH concentration in pulse radiolysis experiments. The dosimeter which involves 10/sup -2/ M SCN/sup -/ in N/sub 2/O-saturated solutions was calibrated against the Fricke system. The calibration led to a value of 46,400 +- 600 (molecules/100 eV) M/sup -1/ cm/sup -1/ for the product of yield and extinction coefficient of (SCN)/sub 2//sup -/ at 10/sup -2/ M SCN/sup -/. This yield is 10-15% greater than most previously quoted values. It is emphasized that yields of OH scavenging processes are significantly dependent on solute concentration and that reports of radiolysis dosimetry should specify the reference system being used in order that results may be correlated. (BLM)


Journal ArticleDOI
TL;DR: Ikuichiro Izumi, Wendell W Dunn, Keith O. Wilbourn, Fu-Ren F. Fan, and Allen J. Bard as mentioned in this paper proposed a homogeneous photocatalytic oxidation of hydrocarbons on platinized titanium dioxide powders.
Abstract: The Journal of Physical Chemistry is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Heterogeneous photocatalytic oxidation of hydrocarbons on platinized titanium dioxide powders Ikuichiro Izumi, Wendell W. Dunn, Keith O. Wilbourn, Fu-Ren F. Fan, and Allen J. Bard J. Phys. Chem., 1980, 84 (24), 3207-3210• DOI: 10.1021/j100461a015 • Publication Date (Web): 01 May 2002 Downloaded from http://pubs.acs.org on February 13, 2009





Journal ArticleDOI
TL;DR: Formation of structures of microemulsions, micelles, and vesicles as well as substrate solubilization therein are summarized and attention is focused on the utilization of micro emulsions as reaction media.
Abstract: Microemulsions, micelles, and vesicles are compared as media for membrane mimetic photochemistry. These systems solubilize, concentrate, compartmentalize, organize, and localize reactants; maintain proton and/or reactant gradients; alter quantum efficiencies; lower ionization potentials; change oxidation and reduction properties; change dissociation constants; affect vectorial electron displacements; alter photophysical pathways and rates; alter chemical pathways and rates; stabilize reactants, intermediates, and products; and separate products (charges). Formation of structures of microemulsions, micelles, and vesicles as well as substrate solubilization therein are summarized. Attention is focused on the utilization of microemulsions as reaction media. 72 references.






Journal ArticleDOI
TL;DR: In this paper, the micelle formation of two ionic surfactants (viz., sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB)) has been studied in organic solvents of various dielectric constant and intermolecular H-bonding capability (i.e., n-methylacetamide (NMA), formanide (FA), dimethyl sulfoxide (ME/sub 2/SO), and n-n'dimethylformamide (DMF)), at
Abstract: Micelle formation of 2 ionic surfactants (viz., sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB)) has been studied in organic solvents of various dielectric constant and intermolecular H-bonding capability (viz., n-methylacetamide (NMA), formanide (FA), dimethyl sulfoxide (ME/sub 2/SO), and n-n'dimethylformamide (DMF)), at different temperatures by conductance measurements. Delta HMDO and delta SMO, standard enthalpies and entropies of micelle formation, respectively, have been determined by studying the variation of the critical micelle concentration with temperature. Compensation plots have been obtained for both the surfactants in the above solvents. The value of slope of these plots are almost identical with each other and also with that obtained in aqueous medium. Micelle formation in these solvents has been explained on the basis of several factors such as dielectric constant of the medium, its intermolecular H-bonding capability, and also its bulk structuredness. 20 references.