Surfaces, solid, catalysis

There is of course attenuation of the signal, as shown in Fig. 5, taken from Joyner and Roberts (28) The gas phase spectrum will also be obtained, but this usually can be separated easily from the signal of the solid. This sample cell arrangement thus permits the study of the stationary-state surface during catalysis and also its evolution in response to pulses and step functions in the gas composition. The temperature of the sample should be controlled so that the surface can be studied during temperature-programmed desorption and reaction. 

It is important to emphasize that the atomic oxygen anion-radical plays a role in catalytic oxidation occnrring on varions oxide surfaces. For instance, O reacts with methane at room temperatnre over varions metal oxides (Lee and Gralowsky 1992). On solid catalysis, Q- is more reactive toward alkanes and alkenes than other ionic oxygen species. Iwamoto and Lnnsford (1980) assumed that O is the active oxygen species oxidizing benzene to phenol on with 70% selectivity at... 

Surface-type catalysis is ordinary heterogeneous catalysis, whereby the reactions take place on the two-dimensional surface (on the outer surface and pore walls) of solid catalysts. The reaction rate is proportional to the catalyst surface area. 

The mechanism of action of some Co porphyrins has been investigated [339]. It has been suggested that Co (I) is responsible for the reduction of water so that a sort of surface redox catalysis is operative. The principle of the activity is not very different from that of composite solid materials, e.g., oxides. Various solvents have been tested, and it has been found that the catalyst is especially active in neutral solution since the redox potential of the Co(II)/Co(I) couple approaches the potential of the H+/H2 couple. 

H.P.Boehm and H.Knozinger, Nature and estimation of functional groups on solid surfaces, in Catalysis, Vol.4, Springer-Verlag, Berlin etc., 1983, pp. 39-207. 

Reactions that occur in the presence of a solid catalyst, as on a metal surface (heterogeneous catalysis) often follow zero-order kinetics. For instance, the rate of decomposition of N02(g) at high pressures on a platinum metal surface does not change if we add more NO2. This is because only the NO2 molecules on the surface can react. If the metal surface is completely covered with NO2 molecules, no additional molecules can be adsorbed until the ones already there have reacted and the products have desorbed. Thus, the rate of the... 

Having made its way to the interior surface of the porous particle molecule A is now ready for the first chemical step, adsorption on (he surface. In catalysis, adsorption is almost always chemisorption. Chemisorption results from chemical bonds between the molecule (adsorbate) and the solid surface (adsorbent). It is therefore very specific/ and receptive sites for chemisorption must exist. Physical adsorption comes from general van der Waals forces, which are physical in origin, weaker than chemisorption, and not specific. Chemisorption stops when a monolayer of adsorbed molecules is formed. It is activated with energies around lOkcal mole, is exothermic with enthalpy changes of -IS to -40kcal mole, is slowly reversible or even irreversible, and is the key step in activation of reaction intermediates. 

For example, many catalytic cycles involve the transfer of protons. Common intermediates are carbenium ions and carbanions, and the catalysts include soluble and solid acids and bases and enzymes. The catalytic cycles may be similar, whether the proton donor (or acceptor) is a soluble molecule or ion or a functional group on a surface. Similarly, catalysis proceeding via organometallic intermediates may involve soluble transition metal complexes, metalloenzymes, or metal surfaces. Catalysis by metals is, however, much more complicated than acid-base catalysis, and the analogies between soluble metal complexes and surfaces cannot yet be developed beyond a few selected examples. 

T Ando. Inorganic solid supported reagents as acids and bases. Stud Surface Sci Catalysis 90 9-20, 1994. 

It has been demonstrated that three different types of catalysis are possible for solid HPAs (211,212) (a) surface type, (6) pseudoliquid or bulk type (I), and (c) bulk type (II) catalysis (Fig. 19). In surface-type catalysis, the catalytic events occur, as for many other solid catalysts, on the outer surface and consequently, the reaction rate for acid-catalyzed reactions should be, in principle, proportional... 

The early heterogeneous catalysis study was simply based on metal powders, while highly dispersed catalyti-cally active components with high-surface solid supports has quickly become the main stream. Compared to homogeneous system, heterogeneous catalysis possesses a number of beneficial features. One of the primary aims of... 

Abstract Attention should be devoted to the measurements of the adsorption properties of catalytic surfaces when they have to work in liquid-solid heterogeneous conditions. The mutual characteristics of the surface and the liquid affect the reagent interactions with the surface sites which could be engaged with the liquid interaction and then not-available for the reagent coordination. This leads to observe effective adsorption properties that could be different from the intrinsic properties of the surface. The possibility to quantitatively determine the effective acid properties of catalytic surfaces by base adsorption is here showed. The adsorption can proceed in any type of liquid of various characteristics (apolar, polar, aprotic, protic) with dynamic (pulse liquid chromatographic method) or equilibrium (liquid recirculation chromatographic method) methods. The measurements of effective acidity allows finding more sound relations with the catalytic activity for a better comprehension of the catalyst work and for a more correct determination of the turnover numbers in liquid-solid catalysis. 

D. W. Dwight, M. E. Counts, and J. P. Wightman, Colloid and Interface Science, Vol. ni. Adsorption, Catalysis, Solid Surfaces, Wetting, Surface Tension, and Water, Academic, New York, 1976, p. 143. 

D. A. King and D. P. Woodruff, eds.. The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Elsevier, Amsterdam, 1982. 

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