Heterogeneous catalysis, intermediates

Diffusivity and tortuosity affect resistance to diffusion caused by collision with other molecules (bulk diffusion) or by collision with the walls of the pore (Knudsen diffusion). Actual diffusivity in common porous catalysts is intermediate between the two types. Measurements and correlations of diffusivities of both types are Known. Diffusion is expressed per unit cross section and unit thickness of the pellet. Diffusion rate through the pellet then depends on the porosity d and a tortuosity faclor 1 that accounts for increased resistance of crooked and varied-diameter pores. Effective diffusion coefficient is D ff = Empirical porosities range from 0.3 to 0.7, tortuosities from 2 to 7. In the absence of other information, Satterfield Heterogeneous Catalysis in Practice, McGraw-HiU, 1991) recommends taking d = 0.5 and T = 4. In this area, clearly, precision is not a feature. 

Of these, the most extensive use is to identify adsorbed molecules and molecular intermediates on metal single-crystal surfaces. On these well-defined surfaces, a wealth of information can be gained about adlayers, including the nature of the surface chemical bond, molecular structural determination and geometrical orientation, evidence for surface-site specificity, and lateral (adsorbate-adsorbate) interactions. Adsorption and reaction processes in model studies relevant to heterogeneous catalysis, materials science, electrochemistry, and microelectronics device failure and fabrication have been studied by this technique. 

Adsorbed Atomic Species as Intermediates in Heterogeneous Catalysis Carl Wagner... 

C. Wagner, Adsorbed Atomic Species as Intermediates in Heterogeneous Catalysis, in Adv. Catal., (1970), pp. 323-381. 

The intermediate 43 can be isolated. If a mixture of H2 and D2 is used, the product contains only dideuterated and nondeuterated compounds no monodeuterated products are found, indicating that (unlike the case of heterogeneous catalysis) H2 or D2 has been added to one alkene molecule and that no exchange takes place. ° Although conversion of 44 to the products takes place in two steps,the addition of H2 to the double bond is syn. 

Before deriving the rate equations, we first need to think about the dimensions of the rates. As heterogeneous catalysis involves reactants and products in the three-dimensional space of gases or liquids, but with intermediates on a two-dimensional surface we cannot simply use concentrations as in the case of uncatalyzed reactions. Our choice throughout this book will be to express the macroscopic rate of a catalytic reaction in moles per unit of time. In addition, we will use the microscopic concept of turnover frequency, defined as the number of molecules converted per active site and per unit of time. The macroscopic rate can be seen as a characteristic activity per weight or per volume unit of catalyst in all its complexity with regard to shape, composition, etc., whereas the turnover frequency is a measure of the intrinsic activity of a catalytic site. 

During the catalytic cycle, surface intermediates include both the starting compounds and the surface metal atoms. This working site is a kind of supramolecule that has organometallic character, and, one hopes, the rules of the organometallic chemistry can be valid for this supramolecule. The synthesis of molecular models of these supramolecules makes it possible to study the elementary steps of the heterogeneous catalysis at a molecular level. Besides similarities there are, of course, also differences between the reactivity of a molecular species in solution and an immobilized species. For example, bimo-lecular pathways on surfaces are usually prohibited. 

Characterization is a central aspect of catalyst development [1,2], The elucidation of the structures, compositions, and chemical properties of both the solids used in heterogeneous catalysis and the adsorbates and intermediates present on the surfaces of the catalysts during reaction is vital for a better understanding of the relationship between catalyst properties and catalytic performance. This knowledge is essential to develop more active, selective, and durable catalysts, and also to optimize reaction conditions. 

The Intermediate Compound Theory in Homogeneous and Heterogeneous Catalysis. 51... 

It is in the very nature of the catalytic process that the intermediate compound formed between catalyst and reactant is of extreme lability therefore not many cases are on record where the isolation by chemical means, or identification by physical methods, of intermediate compounds has been achieved concomitant with the evidence that these compounds are true intermediaries and not products of side reactions or artifacts. The formation of ethyl sulfuric acid in ether formation, catalyzed by HjSO , and of alkyl phosphates in olefin polymerization, catalyzed by liquid phosphoric acid, are examples of established intermediate compound formation in homogeneous catalysis. With regard to heterogeneous catalysis, where catalyst and reactant are not in the same... 

Enzymes are colloids and the reactions catalyzed by them are classified accordingly as microheterogeneous. As in the case of inorganic or organic heterogeneous catalysis, it is assumed that an intermediate compound is formed between enzyme and substrate.10 ... 

Mass spectrometric investigation of the pyrolysis of tetraborane by Stafford et al." gave evidence for the intermediate B4H8. As in similar work under their experimental conditions the influence of heterogeneous catalysis might be dominant. 

Cascade catalysis without recovery of intermediate products may require more than two steps, involving enzymatic, homogeneous, and heterogeneous catalysis. Several examples of this approach have been given [72, 73] one of the most representative consists of a four-step conversion of glucoside into aminodeoxysugar without intermediate product recovery. 

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