Desorption of products

Process 2, the adsorption of the reactant(s), is often quite rapid for nonporous adsorbents, but not necessarily so it appears to be the rate-limiting step for the water-gas reaction, CO + HjO = CO2 + H2, on Cu(lll) [200]. On the other hand, process 4, the desorption of products, must always be activated at least by Q, the heat of adsorption, and is much more apt to be slow. In fact, because of this expectation, certain seemingly paradoxical situations have arisen. For example, the catalyzed exchange between hydrogen and deuterium on metal surfaces may be quite rapid at temperatures well below room temperature and under circumstances such that the rate of desorption of the product HD appeared to be so slow that the observed reaction should not have been able to occur To be more specific, the originally proposed mechanism, due to Bonhoeffer and Farkas [201], was that of Eq. XVIII-32. That is. 

The absorption of reactants (or desorption of products) in trickle-bed operation is a process step identical to that occurring in a packed-bed absorption process unaccompanied by chemical reaction in the liquid phase. The information on mass-transfer rates in such systems that is available in standard texts (N2, S6) is applicable to calculations regarding trickle beds. This information will not be reviewed in this paper, but it should be noted that it has been obtained almost exclusively for the more efficient types of packing material usually employed in absorption columns, such as rings, saddles, and spirals, and that there is an apparent lack of similar information for the particles of the shapes normally used in gas-liquid-particle operations, such as spheres and cylinders. 

Desorption of product molecules from the surface to the pores. 

A volcano plot correlates a kinetic parameter, such as the activation energy, with a thermodynamic parameter, such as the adsorption energy. The maximum in the volcano plot corresponds to the Sabatier principle maximum, where the rate of activation of reactant molecules and the desorption of product molecules balance. 

Consider the reversible reaction A B which proceeds via three elemental steps, viz. adsorption of A on an active site, reaction of adsorbed A, and desorption of product B ... 

Hougen- Watson Models for Cases where Adsorption and Desorption Processes are the Rate Limiting Steps. When surface reaction processes are very rapid, the overall conversion rate may be limited by the rate at which adsorption of reactants or desorption of products takes place. Usually only one of the many species in a reaction mixture will not be in adsorptive equilibrium. This generalization will be taken as a basis for developing the expressions for overall conversion rates that apply when adsorption or desorption processes are rate limiting. In this treatment we will assume that chemical reaction equilibrium exists between various adsorbed species on the catalyst surface, even though reaction equilibrium will not prevail in the fluid phase. 

Carbon monoxide oxidation is a relatively simple reaction, and generally its structurally insensitive nature makes it an ideal model of heterogeneous catalytic reactions. Each of the important mechanistic steps of this reaction, such as reactant adsorption and desorption, surface reaction, and desorption of products, has been studied extensively using modem surface-science techniques.17 The structure insensitivity of this reaction is illustrated in Figure 10.4. Here, carbon dioxide turnover frequencies over Rh(l 11) and Rh(100) surfaces are compared with supported Rh catalysts.3 As with CO hydrogenation on nickel, it is readily apparent that, not only does the choice of surface plane matters, but also the size of the active species.18-21 Studies of this system also indicated that, under the reaction conditions of Figure 10.4, the rhodium surface was covered with CO. This means that the reaction is limited by the desorption of carbon monoxide and the adsorption of oxygen. 

The relative simplicity of CO oxidation makes this reaction an ideal model system of a heterogeneous catalytic reaction. Each of the mechanistic steps (adsorption and desorption of the reactants, surface reaction, and desorption of products) has been probed extensively with surface science techniques, as has the interaction between O2 and CO " . These studies have provided essential information necessary for understanding the elementary processes which occur in CO oxidation. 

The simplified description presented here did not consider the processes that give rise to activation polarization, except for attributing it to sluggish electrode kinetics. A detailed discussion of the subject is outside the scope of this presentation, but processes involving absorption of reactant species, transfer of electrons across the double layer, desorption of product species, and the nature of the electrode surface can all contribute to activation polarization. 

It increased the rate, suggesting that sonication assisted in breaking dovm intermediates and/or assisted in the desorption of products. 

Surface kinetics, or what happens at the surfaces, interior or exterior of the particle. This may involve the adsorption of reactant A onto the surface, reaction on the surface, or desorption of product back into the gas stream. 

Chemisorption of reactant, wholly or in part a rearrangement of chemisorbed species on the surface to a desorbable product(s) and desorption of product or products from the surface. 

Absorption of Butadiene by Cyanocobaltate(II). Of the various substrates reduced by this catalyst system, butadiene was especially convenient for use as a model substrate in a study of mechanism, since its absorption, as well as desorption of product butenes, could be readily followed using a gas buret, and the products formed were easily analyzed by vapor phase chromatography. 

Surface effects and adsorption equilibria thus will significantly influence the course of photoelectrochemical transformations since they will effectively control the movement of reagents from the electrolyte to the photoactivated surface as well as the desorption of products (avoiding overreaction or complete mineralization). The stability and accessibility toward intermolecular reaction of photogenerated intermediates will also be controlled by the photocatalyst surface. Since diffusion and mass transfer to and from the photocatalyst surface will also depend on the solvent and catalyst pretreatment, detailed quantitative descriptions will be difficult to transfer from one experiment to another, although qualitative principles governing these events can be easily recognized. 

Throughout this section, it will be assumed that the mass transfer of reactants and products and the adsorption of reactants or desorption of products are not rate-limiting. 

Vapor-phase alkylation of benzene by ethene and propene over HY, LaY, and REHY has been studied in a tubular flow reactor. Transient data were obtained. The observed rate of reaction passes through a maximum with time, which results from build-up of product concentration in the zeolite pores coupled with catalyst deactivation. The rate decay is related to aromatic olefin ratio temperature, and olefin type. The observed rate fits a model involving desorption of product from the zeolite crystallites into the gas phase as a rate-limiting step. The activation energy for the desorption term is 16.5 heal/mole, approximately equivalent to the heat of adsorption of ethylbenzene. For low molecular weight alkylates intracrystalline diffusion limitations do not exist. 

Although other possibilities cannot as yet be absolutely ruled out, the evidence strongly indicates that in this study the desorption of product molecules from the surface (pore mouths) of the zeolite crystallites is a rate-limiting step. Further, product desorption limitations are probably also responsible for the maxima in rates previously reported (7, 8, 9) and may be a more general phenomenon for zeolite systems. Such limitations... 

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