Interface, catalytic 1128 Subject

A net flow of electrons occurs across the metal/solution interface in a normal electrode reaction. The term electrocatalysis is applied to working electrodes that deliver large current densities for a given reaction at a fixed overpotential. A different, though indirectly related, effect is that in which catalytic events occur in a chemical reaction at the gas/solid interface, as they do in heterogeneous catalysis, though the arrangement is such that the interface is subject to a variation in potential and the rate depends upon it... 

Work in the area of simultaneous heat and mass transfer has centered on the solution of equations such as 1—18 for cases where the stmcture and properties of a soHd phase must also be considered, as in drying (qv) or adsorption (qv), or where a chemical reaction takes place. Drying simulation (45—47) and drying of foods (48,49) have been particularly active subjects. In the adsorption area the separation of multicomponent fluid mixtures is influenced by comparative rates of diffusion and by interface temperatures (50,51). In the area of reactor studies there has been much interest in monolithic and honeycomb catalytic reactions (52,53) (see Exhaust control, industrial). Eor these kinds of appHcations psychrometric charts for systems other than air—water would be useful. The constmction of such has been considered (54). 

Although adsorption exists as a subject of scientific investigation independent of its role in heterogeneous catalysis, it requires particular attention here because of its central role in heterogeneous catalysis. Most or all catalytic reactions involve the adsorption of at least one of the reactants. Many terms related to adsorption have already been defined in Appendix II, Part I, 1.1. These include surface, interface, area of surface or interface, and specific surface area. Appendix II, Part I, recommends A or S and a or s as symbols for area and specific area, respectively. As and as may be used to avoid confusion with Helmholtz energy A or entropy S where necessary. 

The pressed-salt method has attained wide application in studies of the infrared spectra of solids. In this method the solid sample is mixed with a powdered halide salt such as KI or KBr and the mixture is pressed into a disk at high pressures 53-55). This method reduces scattering because solid-gas interfaces are replaced by solid-salt interfaces. When this method is used,-the particle size of the solid is not of critical importance and most ordinary silica or alumina catalysts can be used without the necessity of any particle-size separation. Although it is simple experimentally, the pressed-salt method will probably never attain a major importance in catalytic work, because once the sample is embedded in the salt, it cannot be subjected to further treatment. 

The present chapter is concerned only with catalysis at the solid/liquid interface and will not deal with microheterogeneous catalysis by enzymes, micelles and polyelectrolytes even though the resulting kinetics are closely similar [4], Moreover, little reference will be made to catalytic processes involving gases as these have been the subject of Vols. 19-21 of this series, nor to catalytic polymerisations which have been treated in Vols. 14, 14A, and 15. 

In catalytic slurry reactors the locale of the reaction is the catalyst surface. Hence, in addition to the mass transfer resistance at the gas-liquid interface a further transport resistance may occur at the boundary layer around the catalyst particle. This is characterized by the solid-liquid mass transfer coefficient, kg, which has been the subject of many theoretical and experimental studies. Brief reviews are given by Shah (82). In general, the liquid-solid mass transfer coefficient is correlated by expressions like... 

Metal- and proton-exchanged zeolites have been recently attracted much attention because of their selective catalytic activity to efficiently reduce nitrogen monoxide (NO) by hydrocarbon in an 02-rich atmosphere [1]. The formation of nitrogen dioxide (NO2) from NO and O2 has been suggested as an important step in the selective reduction [2, 3] NO2 is one of rare stable paramagnetic gaseous molecules and has been subjected to electron spin resonance (ESR) studies [4-7]. The ESR parameters and their relation/to the electronic structure have been well established [4] and NO2 can be used as a "spin probe" for the study of molecular dynamics at the gas-solid interface by ESR. 

Lambert reviews the role of alkali additives on metal films and nanoparticles in electrochemical and chemical behavior modihcations. Metal-support interactions is the subject of the chapter by Arico and coauthors for applications in low temperature fuel cell electrocatalysts, and Haruta and Tsubota look at the structure and size effect of supported noble metal catalysts in low temperature CO oxidation. Promotion of catalytic activity and the importance of spillover are discussed by Vayenas and coworkers in a very interesting chapter, followed by Verykios s examination of support effects and catalytic performance of nanoparticles. In situ infrared spectroscopy studies of platinum group metals at the electrode-electrolyte interface are reviewed by Sun. Watanabe discusses the design of electrocatalysts for fuel cells, and Coq and Figueras address the question of particle size and support effects on catalytic properties of metallic and bimetallic catalysts. 

The creation of two types of chemisorbed oxygen on Pt surfaces interfaced with YSZ and subject to NEMCA conditions is also manifest clearly by temperature-programmed-desorption (TPD) [26] as shown in Fig. 10. The strongly bonded backspillover oxygen species (peak desorption temperature Tp=750-780 K) displaces the normal chemisorption state of atomic oxygen obtained via gas phase adsorption (Tp=740 K) to a significantly more weakly bonded state (Tp=680 K). The pronounced rate enhancement in NEMCA studies of catalytic... 

Kim et al. [41] reported that a significant deactivation was observed when cellulase was subjected to shear and/or exposed to air-liquid interface, which was thought to be far more severe and extensive than shear effect alone. By using sufficient additives (surfactants and non-catalytic proteins, e.g., BSA), cellulase deactivation can be, to some extent, prevented, and cellulase can be stabilized. This might be because the addition of non-ionic surfactants could reduce the contact of enzymes with the air-liquid interface because of the surface... 

In this study, we describe the multiple catalytic effects of the water pools and their interface in the reversed micelles, mainly based on our recent studies. One subject of our... 

T 0 bring to closure the plethora of possible examples on this subject we cite two more. Each illustrates a specific domain of intrinsic practical importance in which the acid-base properties of microporous solids play a dominant role in determining a broad range of applications. The examples examine the gas/solid and liquid/solid interfaces, respectively. For the gas/solid interface, the interdependence of acid-base properties and microporous structure is revealed by the catalytic properties of zeolites, especially by their shape-selective properties. It is impossible to review all the literature in this area, and only a few recent views are summarized. For the liquid/solid interface, we focus on the role of acid-base chemistry in interlayers of clay minerals. 

In recent years new types of ion exchangers, the so-called macro-reticular resins, have become commercially available. They are based on a highly cross-linked matrix (which is resistant to swelling) but contain wide rigid pores. Their catalytic action has not so far been the subject of kinetic studies, but it seems improbable that the two-phase model would apply to them [39]. It is more likely that this catalysis would involve true surface reaction which may bear some analogies to reactions which are thought to occur at the interface of immiscible liquid solvents [40]. 

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