Intrinsic kinetics, catalytic materials

For testing and optimizing catalysts, the temperature region just below that where pore diffusion starts to limit the intrinsic kinetics provides a desirable working point (unless equilibrium or selectivity considerations demand working at lower temperatures). In principle, we would like the rate to be as high as possible while also using the entire catalyst efficiently. For fast reactions such as oxidation we may have to accept that only the outside of the particles is used. Consequently, we may decide to use a nonporous or monolithic catalyst, or particles with the catalytic material only on the outside. 

By definition, the turnover frequency is expressed per number of active sites. So, catalytic samples that differ only in the amount active sites must exhibit the same values of turnover frequency. If not, heat and mass transfer phenomena are present. Specifically, the correct measurement of intrinsic kinetic data in heterogeneous catalysis is difficult due to the effect of heat and mass transfer, especially inside the pores of high specific-area materials. The turnover frequency reveals these phenomena. In other words, in the case of supported... 

Catalysis enhances the sustainability of today s world. Raw material utilization, energy efficiency, elimination of hazardous synthesis routes, pollution abatement and so on are a few examples of issues that are typically addressed by catalysis and, hence, contribute to safer, cleaner, more reliable, and more economical chemical processes (1). Catalyst development and improvement require an elaborate testing of candidate catalytic materials. Not only the physical properties, such as porosity, crystallinity, and surface composition are investigated but also and even more importantly, the functional properties such as the kinetics need to be determined. The observed kinetics is constituted by a sequence of different steps. When investigating a reaction mechanism in detail, operating conditions are selected at which the physical transport phenomena are not rate limiting. At such operating conditions, the observed kinetics are entirely determined by the chemical adsorption and reaction phenomena and, hence, they correspond to so-called intrinsic kinetics (2). 

Yet there exist cases in which so-called irreducible transport phenomena are encountered (19-21). Especially with the development of enhanced catalytically active materials, such situations are becoming more likely. For an unambiguous determination of the reaction kinetics, care should in such a case be taken that the deviations from the intrinsic kinetics regime and/or the ideal flow pattern in the reactor can be modeled as adequately as possible and, hence, that the determination of the reaction kinetics is the least compromised. 

A rate equation in terms of the local composition of reacting fluid in contact with the surface of the cata-lytically active material may be called the "intrinsic rate equation, the coefficients in this equation are "intrinsic rate coefficients. The local concentrations of reactants and products at the catalytic surface in general cannot be observed and have to be inferred from the observable composition at the boundary of a larger system, the observed rate of reaction and the kinetics... 

Hie selection of a solid catalyst for a given reaction is to a large extent still empirical and based on prior experience or analogy. However, there are now many aspects of this complex situation that are quite well understood. For example we know how the true chemical kinetics, which are an intrinsic property of the catalyst, and all the many aspects of transport of material and heat around the catalytic particles, interact. In other words, the physical characteristics around the catalyst system and their effects on catalyst performance are well known today. The chemist searching for new and better catalysts should always consider these physical factors, for they can be brought under control, and often in this way definite gains can usually be made both in activity and in selectivity. Further, this knowledge enables us to avoid... 

Intensification of catalytic processes involves innovative engineering of the MSR and the simultaneous development of the catalytically active material. The catalyst design should be closely integrated with the reactor design. Intrinsic reaction kinetics, mass and heat transfer, and energy supply or removal must all be considered to obtain a high selectivity and yield of the target product. 

This chapter proceeds with a general discussion of the overall catalytic cycle and Sabatier s principle in order to illustrate the comparison of relative kinetic and thermodynamic steps in the overall cycle. This is followed by a fundamental discussion of the intrinsic surface chemistry and the application of transition state theory to the description of the surface reactivity. We discuss the important problem of the pressure and material gap in relating intrinsic rates with overall catalytic behavior and then describe the influence of the tatic reaction environment including promoters, cluster size, support, defects, ensemble, coadsorption and stereochemistry. Lastly, we discuss the transient changes to the surface structure as well as intermediates and their influence on catalytic performance. 

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