Surface-catalyzed reaction

The sequence of events in a surface-catalyzed reaction comprises (1) diffusion of reactants to the surface (usually considered to be fast) (2) adsorption of the reactants on the surface (slow if activated) (3) surface diffusion of reactants to active sites (if the adsorption is mobile) (4) reaction of the adsorbed species (often rate-determining) (5) desorption of the reaction products (often slow) and (6) diffusion of the products away from the surface. Processes 1 and 6 may be rate-determining where one is dealing with a porous catalyst [197]. The situation is illustrated in Fig. XVIII-22 (see also Ref. 198 notice in the figure the variety of processes that may be present). 

Zambelli T, Wintterlin J and ErtI G 1996 Identification of the active sites of a surface-catalyzed reaction Soienoe 273 1688-90... 

Wintterlin J, Vdikening S, Janssens T V W, Zambelli T and ErtI G 1997 Atomic and macroscopic reaction rates of a surface-catalyzed reaction Soienoe 278 1931-4... 

Heterogeneous catalytic studies should also be concerned with the significance of adsorption and desorption rates and equilibria of the reactants, intermediates and products. Yang and Hougen (1950) tabulated the expressions for surface catalyzed reactions controlled by various steps. 

FIG. 1 Schematic description of the relevant steps involved in a surface catalyzed reaction within the reactive regime. The cycle starts with the empty sites of the surface (top) and is followed by interactions between reactants and the surface (right branch). Such interaction finally leads to the reaction (bottom) and desorption of the products (left branch), a process which generates new empty sites (top). 

Chapter 10 begins a more detailed treatment of heterogeneous reactors. This chapter continues the use of pseudohomogeneous models for steady-state, packed-bed reactors, but derives expressions for the reaction rate that reflect the underlying kinetics of surface-catalyzed reactions. The kinetic models are site-competition models that apply to a variety of catalytic systems, including the enzymatic reactions treated in Chapter 12. Here in Chapter 10, the example system is a solid-catalyzed gas reaction that is typical of the traditional chemical industry. A few important examples are listed here ... 

Bimolecular Reactions. Models of surface-catalyzed reactions involving two gas-phase reactants can be derived using either the equal rates method or the method of rate-controlling steps. The latter technique is algebraically simpler and serves to illustrate general principles. 

Although recognizing the existence of the concerted cyclic mechanism, it has been proposed that most preparative pyrolyses proceed as surface-catalyzed reactions.336... 

These assumptions are the basis of the simplest rational explanation of surface catalytic kinetics and models for it. The preeminent of these, formulated by Langmuir and Hinshelwood, makes the further assumption that for an overall (gas-phase) reaction, for example, A(g) +...- product(s), the rate-determining step is a surface reaction involving adsorbed species, such as A s. Despite the fact that reality is known to be more complex, the resulting rate expressions find wide use in the chemical industry, because they exhibit many of the commonly observed features of surface-catalyzed reactions. 

By combining surface-reaction rate laws with the Langmuir expressions for surface coverages, we can obtain Langmuir-Hinshelwood (LH) rate laws for surface-catalyzed reactions. Although we focus on the intrinsic kinetics of the surface-catalyzed reaction, the LH model should be set in the context of a broader kinetics scheme to appreciate the significance of this. 

The two rate laws given by equations 8.4-24 and -26 (Types I and II) are used extensively to correlate experimental data on surface-catalyzed reactions. Nevertheless, there are many surface-catalyzed reaction mechanisms which have features not covered by LH kinetics. 

The mechanisms, and hence theoretically derived rate laws, for noncatalytic heterogeneous reactions involving solids are even less well understood than those for surface-catalyzed reactions. This arises because the solid surface changes as the reaction proceeds, unlike catalytic surfaces which usually reach a steady-state behavior. The examples discussed here are illustrative. 

Gasification reactions of solids The reactions of solids with gas-phase reactants to form gaseous products are generally described in the same manner as are surface-catalyzed reactions. The reaction of carbon with water vapor is an example ... 

The rate of a surface catalyzed reaction, A2+B = C+D+E, is determined by the reaction between adsorbed A and gas phase B. Substance A2 dissociates upon adsorption. The Arrhenius equation applies to all constants. Find the temperature at which the initial rate is a maximum. 

Experimental data are shown for the rate of a surface catalyzed reaction, 2A = B. It is expected that the rates of surface reaction and diffsion to the surface both are factors. The diffusional coefficient is kd = 137.5. 

It is important to note that adsorption does not necessarily lead to a catalytic reaction but the surface catalyzed reactions always occur through adsorption. In their catalytic action, the surfaces are specific in nature. Ni and Cu surfaces are very good catalysts for hydrogenation processes. The physical nature of a surface also influences its catalytic efficiency. Those atoms, which are at the peaks, edges etc. have high residual fields and are likely to have greater adsorption capacity. Taylor (1925) postulated that the adsorption and subsequent reaction takes place preferentially on certain parts of the surface, which are called active centers. The active center may constitute a small portion only of the total surface. Moreover, all active centers where adsorption occurs are not always catalytically effective. 

MODEL STUDIES OF SURFACE CATALYZED REACTIONS CO thermal daaorption... 

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