Heterogeneous solids

That this is not always the case should be expected. In fact, if it was not for heterogeneous localization of some flow phenomena, it would be very diflicult to initiate secondary explosives, or to effect shock-induced chemical reactions in solids. Heterogeneous shear deformation in metals has also been invoked as an explanation for a reduction in shear strength in shock compression as compared to quasi-isentropic loading. We present here a brief discussion of some aspects of heterogeneous deformation in shock-loaded solids. 

Fig. 7.5. Nucleation in solids. Heterogeneous nucleotion con take place at defects like dislocations, grain boundaries, interphase interfaces and free surfaces. Homogeneous nucleation, in defect-free regions, is rare.

Surely, it is now time to reformulate the questions considered to be fundamental to shock-compression science. The questions must consider shock-compressed matter as it exists as a highly defective solid, heterogeneous in character, with significant anisotropic components and heterogeneous processes that are not in thermodynamic equilibrium. 

Example 7.6 Suppose a liquid-solid, heterogeneously catalyzed reaction is conducted in a jacketed, batch vessel. The reaction is A B. The reactants are in the liquid phase, and the catalyst is present as a slurry. The adiabatic temperature rise for complete conversion is 50 K. The reactants are charged to the vessel at 298 K. The jacket temperature is held constant at 343 K throughout the reaction. The following data were measured ... 

The Eley-Rideal mechanism for gas-solid heterogeneous catalysis envisions reaction between a molecule adsorbed on the solid surface and one that is still in the gas phase. Consider a reaction of the form... 

This equation gives (0) = 0, a maximum at =. /Km/K2, and (oo) = 0. The assumed mechanism involves a first-order surface reaction with inhibition of the reaction if a second substrate molecule is adsorbed. A similar functional form for (s) can be obtained by assuming a second-order, dual-site model. As in the case of gas-solid heterogeneous catalysis, it is not possible to verify reaction mechanisms simply by steady-state rate measurements. 

The yield of a gas-solid heterogeneous reaction depends not on the total time that molecules spend in the reactor but on the time that they spend on the catalyst surface. The contact time distribution provides a standardized measure of times spent in the absorbed state. A functional definition is provided by the following equation applicable to a first-order, heterogeneous reaction in an isothermal reactor ... 

The isolated Ru(0) nanoparticles were used as solids (heterogeneous catalyst) or re-dispersed in BMI PP6 (biphasic liquid-liquid system) for benzene hydrogenation studies at 75 °C and under 4 bar H2. As previously described for rhodium or iridium nanoparticles, these nanoparticles (heterogeneous catalysts) are efficient for the complete hydrogenation of benzene (TOP = 125 h ) under solventless conditions. Moreover, steric substituent effects of the arene influenced the reaction time and the decrease in the catalytic TOP 45, 39 and 18h for the toluene, iPr-benzene, tBu-benzene hydrogenation, respectively, finally. The hydrogenation was not total in BMI PPg, a poor TOE of 20 h at 73% of conversion is obtained in the benzene hydrogenation. 

The ultrasonic cleaning bath is clearly the most accessible source of laboratory ultrasound and has been used successfully for a variety of liquid-solid heterogeneous sonochemical studies. There are, however, several potential drawbacks to its use. There is no means of control of the acoustic intensity, which will vary from bath to bath and over the lifetime of a single cleaning bath. In addition, their acoustic frequencies are not well controlled and differ from one manufacturer to another, and reproducibility from one bath to another may therefore suffer. Reproducible positioning of the reaction flask in the bath is critical, since standing waves... 

Sample type Homogeneous or heterogeneous as gasses, liquids, slurries, solids Heterogeneous, often blends of solids... 

Some gas—solid heterogeneous reactions follow a similar rate law. Under certain conditions, the rate of hydrogenation of ethene is described by the equation... 

Refining Catalyst Most catalysts used in the refining industry are solid heterogeneous catalysts. The chemical reactions they enhance would not proceed at all, or would do so quite slowly, in the absence of the catalyst. Reactions of this nature are believed to occur due to a dramatic disruption of the existing chemical bonds of an absorbed molecule. Molecules or molecule fragments may enter into reactions much different from those which occur in uncatalyzed reactions. 

There are conceptual difficulties in the optional methods of measuring TCJ for LE. However, these difficulties are much less severe than for solid heterogeneous expls for which... 

Gas-Solid Heterogeneous Reaction Mixtures. Gas-solid heterogeneous reaction mixtures may be advantageously irradiated in annular (immersion-type) photochemical reactors. Again, the content of solid particles is limiting the size and the productivity of the reactor system. This is of particular importance when the solid support is used to specifically adsorb substrates or products of the photochemical reaction the first to enhance specificity of radical substitution reactions [20], the latter to reach better photostability and to ensure optimal purity. 

We shall find that the rate equations of gas-solid heterogeneous catalytic reactions (Chapter 3) also do not, in general, have the same form as equation 1.4. 

For gas-solid heterogeneous reactions particle size and average pore diameter will influence the reaction rate per unit mass of solid when internal diffusion is a significant factor in determining the rate. The actual mode of transport within the porous structure will depend largely on the pore diameter and the pressure within the reactor. Before developing equations which will enable us to predict reaction rates in porous solids, a brief consideration of transport in pores is pertinent. 

To complete the discussion of factors involved in the design of gas-solid heterogeneous catalytic reactors we will examine several aspects of the kinetics of chemical reactions occurring in the presence of a catalyst surface. We consider, for heuristic purposes, the equilibrium reaction ... 

The potential of IR ellipsometric spectroscopy (IRES) for investigating surface processes and reactions relevant to gas-solid heterogeneous catalysis is examined, both for single crystal and model dispersed catalytic systems. With it, structural and chemical changes can be followed over a wide range of temperature and gas pressure, allowing one to thermally stabilize intermediates for investigation, and study surface species under conditions close to those in practical catalytic reactions. 

Rock A solid heterogeneous nonliving material consisting of two or more minerals and/or other naturally occurring solid substances. 

Concepts of Modem Catalysis and Kinetics, I. Chokendorff and J. W. Niemantsverdriet, Wiley-VCH 2003, 452 pp., ISBN 3-527-30574-2. This specialized book deals only with classic gas/solid heterogeneous catalysis. It contains excellent technical explanations and has a strong mathematical and physical approach, which makes for rather heavy reading. It covers many surface reaction mechanisms and catalyst characterization techniques. 

Spectroscopy in Catalysis, J. W. Niemantsverdriet, VCH 1995, 288 pp., ISBN 3-527-28726-4. This is an excellent book on heterogeneous catalysis characterization. It is highly specialized, and aimed at advanced graduate students and researchers. It covers only classic gas/solid heterogeneous catalysis, but if you want to specialize in characterizing solid catalysts, this is the book for you. 

Catalyst inhibition is traditionally associated with biocatalytic processes, but can also apply to homogeneous and heterogeneous catalysis. Competitive inhibition is analogous to competitive adsorption in gas/solid heterogeneous catalysis, where two molecules from the gas phase compete for the same active site on the catalyst surface. A competitive inhibitor is any chemical species I which can bind to the same site as the substrate, or to another site on the enzyme (an allosteric site). The overall reaction scheme is then given by Eqs. (2.58)-(2.60), where El indicates an enzyme-inhibitor complex. 

Figure4.2 Thetwo main mechanismsin gas/solid heterogeneous catalysis a Langmuir-Hinshelwood b Eley-Rideal.

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