Catalyst elementary processes

Farkas and Sherwood (FI, S5) have interpreted several sets of experimental data using a theoretical model in which account is taken of mass transfer across the gas-liquid interface, of mass transfer from the liquid to the catalyst particles, and of the catalytic reaction. The rates of these elementary process steps must be identical in the stationary state, and may, for the catalytic hydrogenation of a-methylstyrene, be expressed by ... 

In the application of the principle of microscopic reversibility we have to be careful. We cannot apply this concept to overall reactions. Even Eqs. (4.43) - (4.45) cannot be applied unless we know that other reaction steps (e.g., diffusional transport) are not rate controlling. In a given chemical system there are many elementary reactions going on simultaneously. Rate constants are path-dependent (which is not the case for equilibrium constants)and may be changed by catalysts. For equilibrium to be reached, all elementary processes must have equal forward and reverse rates... 

If S is consumed, the reaction will proceed until either S or A is depleted. However, when S is regenerated (i.e., S is acting as a catalyst), the reaction will proceed until A is consumed. In the latter case, the surface reaction may be viewed as a sequence of the following elementary processes (1) reactant A adsorbs to site S,... 

Mox represents the metal ion catalyst in its oxidised form (Ceexperimentally determined empirical rate law and does clearly not comprise stoichiometrically correct elementary processes. The five reactions in the model provide the means to kinetically describe the four essential stages of the BZ reaction ... 

Heterogeneous catalysis is clearly a complex phenomenon to understand at the molecular level. Any catalytic transformation occurs through a sequence of elementary steps, any one of which may be rate controlling under different conditions of gas phase composition, pressure, or temperature. Furthermore, these elementary processes occur catalytically on surfaces that are usually poorly understood, particularly for mixed oxide catalysts. Even on metallic catalysts the reaction environment may produce surface compounds such as carbides, oxides, or sulfides which greatly modify... 

Abstract Oxidation catalysis is of extreme importance in many areas of chemistry. The intrinsic mechanisms of oxidation reachons are, however, quite often understood only rather poorly, and catalyst research is mostly, if not exclusively, based on enhrely empirical approaches. In this respect, gas-phase experiments can provide a complementary approach in that they can allow the investigation of the elementary processes in oxidation catalysis step by step. By such, some general insight can be obtained, which may assist in the development of more efficient oxidahon catalysts. 

Intermediate processes of catalyzed organic reactions may involve neutral free radicals R , positive ions R+, or negative ions R as short-lived reactants. A classification of catalysts and processes from the point of view of elementary reactions between reagents and catalysts Is logically desirable but has not yet been worked out. However, there is a wealth of practice more or less completely documented, some proprietary but available at a price. The ensuing discussions are classified into kinds of catalysts and into kinds of processes. 

LUMO, the symmetry restriction may not be circumvented. In other words, the role of the catalyst is not to circumvent the symmetry restriction but to provide some of the elementary processes which avoid the reaction route of the symmetry restriction. Wood and Wise (/) demonstrated these circumstances in their study of the hydrogenation of cyclohexene over a gold film which was intimately bonded to the outer side of a Pd-Ag thimble, as described in Fig. 2. The thimble is semipermeable to hydrogen diffusion, that is, hydrogen diffuses from the Pd-Ag side to the Au side, but the diffusion from the Au side to the Pd-Ag side is negligible. Such characteristic semipermeability is responsible to the deficient ability of the Au surface for... 

If a catalytic cycle composed of several elementary processes is promoted on an isolated single site, we could make distinctions about the function of the active sites. For example, some metal complexes which are active for the isomerization reaction of olefins via alkyl intermediates are not effective catalysts for the hydrogenation reaction, and such differences in catalytic ability of the metal complexes is explained by the numbers of coordinatively unsaturated sites which are available for the reactions as described schematically in Scheme 7. 

A catalyst provides for sets of elementary processes (often ca lled elementary steps) which link reactants and products and which do not occur in the absence of the catalyst. For example, suppose the reaction... 

Some heterogeneous catalytic reactions proceed by a sequence of elementary processes certain of which occur at one set of sites while others occur at sites which are of a completely different nature. For example, some of the processes in the reforming reactions of hydrocarbons on platinum/ alumina occur at the surface of platinum, others at acidic sites on the alumina. Such catalytic reactions are said to represent bifunctional catalysis. The two types of sites are ordinarily intermixed on the same primary particles ( 1.3.2) but similar reactions may result even when the catalyst is a mixture of particles each containing but one type of site. These ideas could, of course, be extended to crea te the concept of polyfunctional catalysis. 

The question then arises as to how to explain this sharp decrease in the parameters. Boudart et al. [127,128] ascribe it to the significant decrease in the surface coverage by oxygen, but the surface coverage must depend on the parameters of the elementary processes taking place in the system the primary reason must be simply the value of the parameter. Apparently, the sharp drop in the model parameters must be attributed to the decreased number of active surface sites of the catalyst due to the formation of inactive oxides or PtC complexes [119,122]. The model must account for the catalyst deactivation [122, 125]. 

If a chemical reaction is operated in a flow reactor under fixed external conditions (temperature, partial pressures, flow rate etc.), usually also a steady-state (i.e., time-independent) rate of reaction will result. Quite frequently, however, a different response may result The rate varies more or less periodically with time. Oscillatory kinetics have been reported for quite different types of reactions, such as with the famous Belousov-Zha-botinsky reaction in homogeneous solutions (/) or with a series of electrochemical reactions (2). In heterogeneous catalysis, phenomena of this type were observed for the first time about 20 years ago by Wicke and coworkers (3, 4) with the oxidation of carbon monoxide at supported platinum catalysts, and have since then been investigated quite extensively with various reactions and catalysts (5-7). Parallel to these experimental studies, a number of mathematical models were also developed these were intended to describe the kinetics of the underlying elementary processes and their solutions revealed indeed quite often oscillatory behavior. In view of the fact that these models usually consist of a set of coupled nonlinear differential equations, this result is, however, by no means surprising, as will become evident later, and in particular it cannot be considered as a proof for the assumed underlying reaction mechanism. 

A feasible reaction scheme includes all the reactants and products, and it generally includes a variety of reaction intermediates. The validity of an elementary step in a reaction sequence is often assessed by noting the number of chemical bonds broken and formed. Elementary steps that involve the transformation of more than a few chemical bonds are usually thought to be unrealistic. However, the desire to formulate reaction schemes in terms of elementary processes taking place on the catalyst surface must be balanced with the need to express the reaction scheme in terms of kinetic parameters that are accessible to experimental measurement or theoretical prediction. This compromise between molecular detail and kinetic parameter estimation plays an important role in the formulation of reaction schemes for analyses. The description of a catalytic cycle requires that the reaction scheme contain a closed sequence of elementary steps. Accordingly, the overall stoichiometric reaction from reactants to products is described by the summation of the individual stoichiometric steps multiplied by the stoichiometric number of that step, ai. 

Che, M., From unit operations to elementary processes A molecular and multidisciplinary approach to catalyst preparation. Stud. Surf. Sci Catal. 130,115 (2000). 

The second line of research in catalysis has been aimed at determining the structure and composition of catalysts and interpreting catalysis in terms of the elementary processes occurring on the catalyst surface. Pursuit of these objectives has depended heavily on the availability of instrumentation and has advanced at a rate closely correlated with the rate of introduction of... 

Depending on the kinetics of the different elementary processes involved in the formation of the precipitate, a temperature increase might lead to an increase in crystallite size, as was observed for the crystallization of pseudoboehmite [24] or iron molybdates [25]. However, in other cases no influence of the precipitation temperature on the crystallite size of the final catalyst was reported [26], or a decrease was reported, as for the ZnO system [27],... 

Much more can be said about the magnitude of pre-exponcntial factors and activation energies of elementary processes based on statistical thermodynamics applied to collision and reaction-rate theory [2, 61], but in view of the remark above one should be cautious in their application and limit it to well-defined model reactions and catalyst surfaces. 

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