Desorption limiting

A summary of aniline N-methylation mechanistic features on Cui xZnxFe204 ferrospinel catalysts is given in Figure 27. It was possible, due to in-situ IR studies, to observe a dissociative adsorption and possible orientation of reactants on the catalyst surface, their conversion to product at low temperatures, and desorption-limited kinetics, all under conditions that are close to the reaction conditions. Although Cu is the active center for the aniline A-methylation reaction, and IR studies reveal that Zn acts as the main methyl species source. 

Figure 4.39. Sabatier desorption-limitation of the dissociation rate-determined kinetics.

Three obvious models which could describe the observed reaction rate are (a) concentration equilibrium between all parts of the intracrystalline pore structure and the exterior gas phase (reaction rate limiting), (b) equilibrium between the gas phase and the surface of the zeolite crystallites but diffusional limitations within the intracrystalline pore structure, and (c) concentration uniformity within the intracrystalline pore structure but a large difference from equilibrium at the interface between the zeolite crystal (pore mouth) and the gas phase (product desorption limitation). Combinations of the above may occur, and all models must include catalyst deactivation. 

For the model involving a desorption limitation (model c) a component mole balance is written over the gas phase and crystallite phase for product D (A + B - D). These are, respectively ... 

Figure 1. Rate of benzene alkylation by ethene over HY at 529°K and C C2 = 1.9. Line represents simulation of data by desorption limitation model...

System Model. The equilibrium model (model a) did not properly represent the observed rate curve because the predicted peak maximum, using this model, always occurred at least an order of magnitude earlier in time than was actually observed when measured values for all parameters were substituted into the equilibrium model. Thus a mass transfer influence—e.g.j intracrystalline diffusional limitations or product desorption limitations—must be invoked to explain the data. The diffusional limitations model might fit the data qualitatively as Tan and Fuller (6) show for their system. However, this model contains three fitting constants and should be applied only when there is sufficient evidence of diffusional limitations. 

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... 

Figure 6. Simulation of ethylbenzene ethylation over SK-600 at 577°K and C8 C2 = 0.2 by product desorption limitation model...

The temperature programmed desorption profile for the adsorption of butadiene in place of cis-2-butene is shown in Fig. 1, curve c. Two sets of products are observed. The product below 210°C is unreacted butadiene, and the products above 210°C are carbon dioxide and water. The similarity in the evolution of the combustion products of butene and butadiene is an indication that their combustion proceeds via similar reaction mechanisms. The similarity in the desorption of butadiene suggests that in butene adsorption, butadiene desorption is desorption limited. Indeed, that both butene and butadiene adsorb on the same type of sites has been confirmed by sequential adsorption experiments. The results are shown in Table III. It was found that if the C4 hydrocarbons are adsorbed sequentially without thermal desorption between adsorptions, the amounts of the final desorption products are the same as those in experiments where only the first hydrocarbon... 

For desorption-limited reactions, both ad an4 are very large compared with which is small... 

We now propose a mechanism for the hydrodemethylation of toluene. We assume that toluene is adsorbed on the surface and then reacts with hydrogen in the gas phase to produce benzene adsorbed on the surface and methane in the gas phase. Benzene is then desorbed from the surface. Since approximately 75% of all heterogeneous reaction mechanisms are surface-reaction-limited rather than adsorption- or desorption-limited, we begin by assuming the reaction between adsorbed toluene and gaseous hydrogen to be reaction-rate-limited. Symbolically, this mechanism and associated rate laws for each elementary step are ... 

These, together with the observation that acetone and propene were always evolved at the same temperature suggest that acetone and propene are formed from a common intermediate on the different surfaces, the formation or decomposition of which is the rate limiting step. The evolution of water, however, was at the same temperature as the desorption of adsorbed water. Thus the process is desorption limited. 

Regime II Desorption Limited Dechlorination Events in the Monolayer 155 K 280K. 

Another detailed and recent description of the diffusion processes for molecules released from polymer nanofibers is based on the desorption-limited theory of admixture release.In this case, the release is mainly driven by desorption, or dissolution, from nanopores on the surface of the polymer nanostructures. To describe this mechanism, one firstly defines a so-called nanoporosity factor, Np, as the ratio of the initial amount of drug admixture on the nanofiber surface, nisur/O), and the total initial amount of drug loaded in the nanofibers, mp(0) [i.e. the drug molecules on the surface plus those in the... 

Two semicircles appear in the case of hydrogen adsorption or desorption limitations coupled with evolution only (without absorption or diffusion limitations), which are related to two time constants, charge transfer and adsorption R g or desorption impedance... 

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