Phase heterogeneous

Dehydration of 1-pentanol or 2-pentanol to the corresponding olefins has been accompHshed, in high purity and yields, by vapor-phase heterogeneous catalyzed processes using a variety of catalysts including neutral gamma —Al Og catalyst doped with an alkah metal (23), zinc aluminate (24,25), hthiated clays (26), Ca2(P0 2 montmorillonite clays (28). Dehydration of 2-methyl-1-butanol occurs over zinc aluminate catalyst at... 

The cathode reaction involves reduction of silver oxide to metallic silver [7440-22-4J. The reaction is a two-phase, heterogeneous reaction producing a substantially constant voltage during discharge. Some manganese dioxide may be added to the cathode, as in the case of mercury oxide cells. 

Selectivity among butylene isomers also occurs in vapor-phase heterogeneous catalysis, at least in the case of dehydrogenation of butenes to butadiene, where maximum yields can be obtained by employing slightly different conditions for each isomer (18). In practice, mixtures of isomers are used and an average set of conditions is employed. 

Our interest is in solution kinetics, so we will concern ourselves only with homogeneous reactions, which take place in a single phase. Heterogeneous reactions take place, at least in part, at interfaces between phases.) Further, we will mainly work with closed systems, those in which matter is neither gained nor lost during the period of observation. 

A homogeneous reaction occurs in one phase. Heterogeneous kinetics have been studied much less. 

Vaz, W. L. C. and Melo, E. (2001) Fluorescence spectroscopic studies on phase heterogeneity in lipid bilayer membranes./. Fluoresc., 11, 255-271. 

McMurray, P. H. and J. C. Wilson, Droplet Phase (Heterogeneous) and Gas Phase (Homogeneous) Contributions to Secondary Ambient Aerosol Formation As Functions of Relative Humidity, Atmos. Environ.. 

For gas phase heterogeneous catalytic reactions, the continuous-flow integral catalytic reactors with packed catalyst bed have been exclusively used [61-91]. Continuous or short pulsed-radiation (milliseconds) was applied in catalytic studies (see Sect. 10.3.2). To avoid the creation of temperature gradients in the catalyst bed, a single-mode radiation system can be recommended. A typical example of the most advanced laboratory-scale microwave, continuous single-mode catalytic reactor has been described by Roussy et al. [79] and is shown in Figs. 10.4 and... 

In such an apparatus, a chemical reaction takes place with a conversion of compound A into the products B and C. Typically, a sharp pulse of component A is fed into the column. During the passage through the column, compound A is converted into the products B and C and the amount of component A decreases. Because of their different retention times, the products B and C are concomitantly separated from each other and component A. Due to the removal of the products from the reaction zone, chemical equilibrium is never reached and the reaction will ideally proceed until the total conversion of the compound A. The reaction may take place in the stationary and/or the mobile phase. Heterogeneous reactions maybe either catalyzed by the packed adsorbent or by an additional catalyst, which is mixed with the adsorbent. 

The science of catalysis may be divided into two major parts catalysts that function in the same phase as reactants and products (homogeneous) and those that function in a different phase (heterogeneous). Both share many of the same goals and challenges identify the components that make up the actual catalyst, understand mechanisms and optimize activity with respect to rate and selectivity. 

Homogeneous (same phase) Heterogeneous (different phase)... 

Exchange of components between phases (heterogeneous reactions)... 

Transition-metal substituted or modified zeolites are currently receiving increasing attention as gas-phase heterogeneous partial oxidation catalysts, because they offer the... 

Electrochemical aspects of liquid phase heterogeneous transformations [61-69] have also to be mentioned. In these cases, either the solid phase is a catalyst or the solid phase is a reaction partner. At least two coupled redox partners are present. The catalytic reduction of nitrate with molecular hydrogen in acidic aqueous phase at a solid catalyst... 

Keywords. Oxygen absorption. Three-phase mass transfer. Dispersed phase. Heterogeneous model. Fermentation, Bulk oxygen concentration... 

The spent fuel matrix is a ceramic material with a fascinating chemical composition and a large degree of phase heterogeneity. The physical state and chemical composition of spent fuel largely depends on the bum-up of the fuel once it is taken out of the reactor. In Fig. 6 we indicate the dependence of the chemical composition on the bum-up for a series of PWR fuels. However, the fact that remains constant is that U02 constitutes the major component of spent fuel, ranging within a total of 95-98% in weight (see Fig. 7). 

The oxidation of propene is at present the most extensively studied gas phase heterogeneous oxidation process. The selective production of acrolein over cuprous oxide has been known for a very long time. However, the discovery of bismuth molybdates as highly active and selective catalysts for the oxidation to acrolein, and particularly the ammoxidation to acrylonitrile, has opened a new era in oxidation catalysis. 

In Chapter 3 we described the structure of interfaces and in the previous section we described their thermodynamic properties. In the following, we will discuss the kinetics of interfaces. However, kinetic effects due to interface energies (eg., Ostwald ripening) are treated in Chapter 12 on phase transformations, whereas Chapter 14 is devoted to the influence of elasticity on the kinetics. As such, we will concentrate here on the basic kinetics of interface reactions. Stationary, immobile phase boundaries in solids (e.g., A/B, A/AX, AX/AY, etc.) may be compared to two-phase heterogeneous systems of which one phase is a liquid. Their kinetics have been extensively studied in electrochemistry and we shall make use of the concepts developed in that subject. For electrodes in dynamic equilibrium, we know that charged atomic particles are continuously crossing the boundary in both directions. This transfer is thermally activated. At the stationary equilibrium boundary, the opposite fluxes of both electrons and ions are necessarily equal. Figure 10-7 shows this situation schematically for two different crystals bounded by the (b) interface. This was already presented in Section 4.5 and we continue that preliminary discussion now in more detail. 

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