Heterogeneous catalysis parameters

A reason for using microkinetics in heterogeneous catalysis is to have comprehensive kinetics and a transparent reaction mechanism that wonld be useful for re or design or catalyst development. Furthermore, in the long run, the exparimental effort to develop a microkinetics scheme can be less than that for a Langmuir-Hinshelwood (LH) or powa--law scheme because of the more fundamental nature of the reaction kinetics parameters. 

D.A. Rudd, L.A. Apuvicio, J.E. Bekoske and A.A. Trevino, The Microkinetics of Heterogeneous Catalysis (1993), American Chemical Society, Washington DC]. Ideally, as many parameters as can be determined by surface science studies of adsorption and of elementary steps, as well as results from computational studies, are used as the input in a kinetic model, so that fitting of parameters, as employed in Section 7.2, can be avoided. We shall use the synthesis of ammonia as a worked example [P. Stoltze and J.K. Norskov, Phys. Rev. Lett. 55 (1985) 2502 J. Catal. 110 (1988) Ij. 

Froment, G.F., "Model Discrimination and Parameter Estimation in Heterogeneous Catalysis", AIChE. 1, 21 1041 (1975). 

Mesostructured materials with adjustable porous networks have shown a considerable potential in heterogeneous catalysis, separation processes and novel applications in optics and electronics [1], The pore diameter (typically from 2 to 30 nm), the wall thickness and the network topology (2D hexagonal or 3D cubic symmetry) are the major parameters that will dictate the range of possible applications. Therefore, detailed information about the formation mechanism of these mesostructured phases is required to achieve a fine-tuning of the structural characteristics of the final porous samples. 

On solids, the amount and strength of acid or basic sites are quite independent parameters, so both of them must be analyzed independently for a complete characterization. Additionally, several different families of acid sites may occur in the same solid surface, so their distribution must be characterized. The key to the effective utilization of microcalorimetry in heterogeneous catalysis is the judicious choice of gas-phase molecules for study. 

This type of co-catalyhc influence is well known in heterogeneous catalysis, in which for some reachons an acidic support will achvate a metal catalyst more efficiently than a neutral support In this respect, the acidic ionic liquid can be considered as a liquid acidic support for the transihon metal catalysts dissolved in it As one would expect, in those cases in which the ionic liquid acts as a co-catalyst, the nature of the ionic liquid becomes very important for the reactivity of the transihon metal complex. The opportunity to opdmize the ionic medium used, by vari-ahon of the halide salt, the Lewis acid, and the ratio of the two components forming the ionic liquid, opens up enormous potential for ophmizahon. However, the choice of these parameters may be restricted by some possible incompahbilihes with the feedstock used. Undesired side reachons caused by the Lewis acidity of the ionic Hquid or by strong interaction between the Lewis acidic ionic Hquid and, for example, some oxygen funchonalihes in the substrate have to be considered. 

Three fundamental processes can limit overall reaction rates in heterogeneous catalysis mass transfer of the reactants from the bulk liquid phase(s) to the surface of the solid catalyst, diffusion of the reactants from the catalyst surface to the active site, and the intrinsic reaction at the active site 61,62). Each of these processes depends on one or more experimental parameters, as shown in Fig. 1. 

Most standard chemical engineering tests on kinetics [see those of Car-berry (50), Smith (57), Froment and Bischoff (19), and Hill (52)], omitting such considerations, proceed directly to comprehensive treatment of the subject of parameter estimation in heterogeneous catalysis in terms of rate equations based on LHHW models for simple overall reactions, as discussed earlier. The data used consist of overall reaction velocities obtained under varying conditions of temperature, pressure, and concentrations of reacting species. There seems to be no presentation of a systematic method for initial consideration of the possible mechanisms to be modeled. Details of the methodology for discrimination and parameter estimation among models chosen have been discussed by Bart (55) from a mathematical standpoint. 

One structural parameter of potential interest in the adsorption of simple diatomic molecules, such as CO, NO and N2, is the intramolecular bondlength. Much of the motivation for studying such adsorbates is related to the way adsorption modifies the chemistry of these species as the basis for heterogeneous catalysis. Many such reactions involve scission of the intramolecular bond, and if the adsorption is of the... 

In heterogeneous catalysis, the term active site is also used extensively [7,8], The density of active sites per unit surface area of the catalyst is an important parameter in catalyst analysis and development [9], However, whereas the surface area is relatively easily determined experimentally [10], the number of active sites in heterogeneous catalysts is not easily estimated. Therefore, although both fields use turnover numbers (reactant converted per unit time per active site) to describe activity, only the enzymologists can be sure that the quantitation of this parameter is adequate. 

Heterogeneous catalysis is a surface phenomenon, therefore the overall kinetic parameters are dependent on the real exposed catalyst surface area. In the supported systems only a part of the photocatalyst is accessible to light and to substrate. Besides, the immobilized catalyst suffers from the surface deactivation since the support could enhance the recombination of photogenerated electron-hole pairs and a limitation of oxygen diffusion in the deeper layers is observed. 

A knowledge of the kinetics of the reaction at the active sites is of primary importance in determining the nature of catalytic action in heterogeneous catalysis. Information about the nature of the catalyst-substrate interaction can be obtained from the way in which the rate constants in the kinetics change on variation of such parameters as temperature, catalyst treatment, and catalyst composition. In addition, these constants are the quantities which should correlate with structural information such as that obtained by the methods of solid state physics. However, the true kinetics at the active sites is not always obtained unless certain precautions are taken, as has been pointed out in a recent volume of Advances in Catalysis (1). 

Despite the convenience of handling and separation in heterogeneous catalysis, many other parameters have a strong influence on the stereochemical outcome pressure temperature modifier purity of substrates high substrate specificity catalyst preparation, which includes type, texture, and porosity of the support dispersion impregnation reduction and pretreatment of the metal.22 Reproducibility of catalyst activities and enantioselectivities can often be a problem attributed to variations in catalyst preparations and purity of the substrate.5-22... 

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