Macromolecules, vol. 2: Synthesis and materials, Hans-Georg Elias, Plenum Publishing Corporation, New York, 1977. 1131 + LXXIV pages. price: $39.50

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Alkyl transfer steps in the catalytic alkylation of benzene, toluene, and cyclohexane

The rate of the catalytic redistribution of isotopic carbon in binary mixtures of benzene and toluene, benzene and ethylbenzene, toluene and xylene, toluene and ethylbenzene, cyclohexane and methylcyclohexane was investigated over supported Pt, Ir, Ru, and Au. The influence of the following experimental variables was assessed: hydrocarbon partial pressure ratios, 3 x 10-3 to 30; temperature, 185 to 400 [deg]C; catalyst support, A12O3, MgO; catalyst acidity and basicity, and method of catalyst preparation. The results are discussed in the framework of processes of competitive and reactive chemisorption of hydrocarbon mixtures. The dependence of the reaction rate upon the hydrocarbon partial pressure ratio is considered in detail. Displacement effects were found when xylene isomers were present simultaneously. The insight that the carbon redistribution reaction may provide in investigations on the thermodynamic and kinetic aspects of alkyl transfer steps at catalytic surfaces is pointed out.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/34170/1/0000458.pd

Equilibrium hydrogen transfer between benzene and C6 hydrocarbons over supported metal catalysts

A kinetic method is presented for the study of the hydrocarbon-surface interaction taking place during the catalytic conversion of hydrocarbons. The method involves the study of the rate of redistribution of an isotopic tracer between hydrocarbon molecules in thermodynamic equilibrium with the catalyst surface. A general discussion of the approach is given; its potentialities and advantages in studying the individual steps of hydrocarbon reactions are analyzed. These ideas are applied to the study of the rate of hydrogen transfer between cyclohexane and other C6 hydrocarbons on the one side and benzene on the other over Pt, Pd, Ir, Rh, and Ru supported on Al2O3 and SjO2. The investigation covered the effect on the reaction rate of the ratio of C6 hydrocarbon to benzene and of metal particle size at 117 [deg]C.With few exceptions the transfer rate increased upon increasing the ratio of cyclohexane to benzene and upon addition of molecular H2 to the reactants. For the cyclohexane-benzene combination the relative activitv among the metals investigated was Pt > Pd > Ir > Ru > Rh. In the range of Pt crystallite size studied (12-2000 A) the hydrogen surface coverage was influenced by the particle size, but the reaction rate constant per unit of metal surface area was little dependent upon crystallite size. Reactive surface efficiencies were in the range 10-4 to 10-7. Experiments on the hydrogen transfer between benzene and other C6 hydrocarbons produced the following sequence of reaction efficiency: cyclohexene > cyclohexane > methyl-cyclopentane >n-hexane, 2,3-dimethylbutane. A surface equilibrium reaction and a rate-controlling step consistent with the experimental results are postulated. The nature of the reactive metal surface, particularly the role of the adsorbed hydrogen, is pointed out. The conclusions are analyzed in the framework of the present understanding of metal catalysis.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/32823/1/0000197.pd

Surface models in heterogeneous catalysis : The synthesis of ammonia and the conversion of carbon monoxide

The establishment of chemical equilibria between gas and solid catalysts during the synthesis of NH3 and the conversion of CO is considered. Suitable solid-gas equilibrium reactions are discussed. With the correct choice of the rate-controlling step, the equilibrium surface models are able to reproduce the experimental expressions for the rates of reaction. These models represent a drastic departure from the concept of a fixed, heterogeneous surface that has been extensively used in the past for the interpretation of the experimental results on the above reactions. The two approaches, however, complement each other and the validity of each one may well be justified under different experimental conditions. Thus, a model may be considered the extension of the other by modification of some of the parameters of the system.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/33331/1/0000728.pd

Surface reactivity of supported gold : II. Hydrogen transfer between benzene and cyclohexane

The rate of redistribution of 14C between benzene and cyclohexane has been investigated over supported Au catalysts, in the temperature range 200 to 250 [deg]C, partial pressures of ~0.2 atm, and total pressure 1 atm. The catalyst variables studied were the Au concentration, 0.3 to 5 wt %, and the catalyst support, Al2O3, MgO. Two methods of catalyst preparation were employed. In one, Au particles were formed by low temperature (~100 [deg]C) chemical reduction, while in the second method by thermal decomposition of the Au salt at higher temperatures. Diameter of the Au particles ranged between 40 to >1600 A depending upon the type of support and preparative method. From the experimental results the reaction rate constant and the analytical relation between the ratio cyclohexane/benzene and reaction rate were determined. Rate constants of the order of (1 to 10) x 10-6 [mole/g(cat)sec] were calculated. The analytical relation between the cyclohexane/ benzene ratio and the reaction rate was found to change for values of the ratio [approximate]3.The extent of the available metal surface per weight of metal was found to influence the reaction rate constant. This intrinsic surface effect was detected under conditions such that the molecular mean free path of reactants and products was large in relation to the diameter of the metal particle. It is suggested that whenever particle size and molecular free path are of the same order of magnitude, surface effects do not lend to easy detection.The general picture of the reactivity of surfaces of finely dispersed Au toward hydrogen and oxygen is compared.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/32705/1/0000072.pd

Sintering of Zinc Oxide

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Supported Au---Pt catalysts : Characterization and hydrogen transfer activity between benzene and cyclohexane

Two series of Au---Pt catalysts supported on SiO2 were prepared by the incipient wetness method. In one series the total amount of metal (Au + Pt) was kept constant at about 0.7 wt% (Series A), while in a second series the amount of Pt was kept constant at about 0.7 wt% (Series B). In the latter the total amount of metal ranged between 1.03 to 6.13 wt%. The two series were characterized by H2 and O2 chemisorption, wide-angle X-ray scattering, and electron spectroscopy for chemical analysis. There was indication of the presence of mixed Au---Pt particles as well as of particles containing Au only. In depth compositional profile of Series B showed homogeneous composition extending to at least about 12 A below the surface, while in Series A compositional inhomogeneities were found. The rate of the carbon-14 equilibration between benzene and cychohexane and the rate of equilibration between H2 and D2 were measured on both catalyst series in a flow system at a total pressure of 1 atm. For the carbon-14 equilibration reaction the temperature was 205 [deg]C, contact time in the range of 10 to 30 sec. The influence of the ratio cyclohexane to benzene on the reaction rate was recorded. For the deuterium equilibration the temperature was -78 [deg]C and a ratio of pD2/pH2 = 1 was employed. The rate of the carbon-14 equilibration was found to be independent of surface composition of the Au---Pt particles for Series A, while for Series B the rate increased upon addition of Au to Pt. Differences in rate up to about one order of magnitude were observed between catalysts from Series A and B possessing a similar overall Au/Pt ratio. No difference in the rate of deuterium equilibration was found between Series A and Series B catalysts. The kinetic behavior of Series B is discussed, and suggestions of the nature of the Au---Pt interaction, responsible for the enhancement of the reactivity of Pt, are advanced. The concept of reaction sensitivity to surface structure is viewed as a dynamic factor which includes the position of the reaction steady state and the corresponding degree of reaction conversion. Consequently, a reaction may be sensitive or insensitive to the surface structure depending upon its conversion.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/23599/1/0000561.pd

Catalytic oxygen transfer between ethylene and ethylene oxide

The transfer of oxygen between ethylene and ethylene oxide has been studied at surfaces of Ag, Au, (VO2)x, and bismuth molybdate. The study was carried out by following the rate of redistribution of isotopic carbon between ethylene and ethylene oxide. The conditions employed in the experiments were: temperature 22-305 [deg]C, partial pressure ratios of ethylene oxide to ethylene 0.1-50, contact time 1-10 sec. Silver was supported on high- and low-area Al2O3 zeolite and complexed with poly(4-vinyl-pyridine). Au was supported on Al2O3 and MgO. The dependence of the rate of isotopic redistribution upon the ratio of ethylene oxide to ethylene was taken as an indication of the process of reactive and competitive chemisorptions of ethylene and ethylene oxide. Information on the nature of the adsorption, in terms of number of surface centers per adsorbate molecule, was derived. From this point of view, the catalysts tested may be divided into two groups. In one group, single-center adsorption of ethylene predominated, while in the second the adsorption of ethylene included two surface centers per molecule. Thus, on the first group, ethylene fragmentation should not be favored. These catalysts, therefore, should prove more selective to ethylene oxide in the net reaction of ethylene oxidation with molecular O2. These predictions are fully confirmed by direct experimentation.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/33511/1/0000009.pd

A note on a statistical-mechanical treatment of activation-limited surface diffusion

A brief review is given of a class of simple statistical-mechanical models for surface diffusion, applicable to the limiting case where diffusional “hopping” is controlled by the thermal activation rate of the adsorbed particle. A theoretical result of Reyes, for the pre-exponential or “frequency” factor, is discussed and compared briefly to experiment. Приводится краткий обзор класса простых статистическо-механических моделей поверхностной диффузии, примени-мых в предельных случаях, когда диффузионные “прыжки” контролируются скоростью термической активации адсорбированных частиц. Теоретические результаты Рейса отно-сительно предэкспоненциального множителя или “частот-ного” фактора обсуждаются и сравниваются с экспериментальными значениями.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43572/1/11144_2005_Article_BF02075122.pd

The distribution of reaction rates and activation energies on catalytic surfaces: Exchange reaction between gaseous benzene and benzene adsorbed on platinum

A mathematical derivation for the calculation of the distribution function of rate constants and activation energies for an isotopic exchange reaction between chemical species adsorbed on a solid surface and in the surrounding gas phase is presented. Starting from experimental kinetic isotherms, giving the rate of exchange as a function of time at various temperatures, it is shown how it is possible to derive the distribution functions of the rate constants and activation energies. The technique is applied to the exchange reaction between benzene molecules adsorbed on Pt black and molecules present in the gas phase. Experimental results on the rate of this exchange, obtained by means of C-14 labeled benzene, have been collected at 40 [deg], 60 [deg], 80 [deg], and 100 [deg] C, at constant surface coverage. The range of activation energies for the exchange reaction was found to be from about 10 to 35 kcal/mole, while the pre-exponential factor varied from about 107 to 1026 min-1. Additional observations on the chemisorption of benzene on Pt black are presented. The effects of various pretreatments of the Pt surface and of aging of the chemisorbed benzene on the exchange rate are discussed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/32196/1/0000254.pd