Microkinetic approach

We did not extensively discuss the consequences of lateral interactions of surface species adsorbed in adsorption overlayers. They lead to changes in the effective activation energies mainly because of consequences to the interaction energies in coadsorbed pretransition states. At lower temperatures, it can also lead to surface overlayer pattern formation due to phase separation. Such effects cannot be captured by mean-field statistical methods such as the microkinetics approaches but require treatment by dynamic Monte Carlo techniques as discussed in [25]. 

To compete with the empirical models (Temkin and improved expressions) for the best fit to experimental data cannot be the prime objective of the microkinetic approaches. Rather, they are means of checking whether our knowledge and understanding of the elementary steps correspond to the reality of catalysts under industrial synthesis conditions. 

An interesting further development in describing the kinetics of heterogeneously catalyzed reactions is the so-called microkinetics approach, whereby independent information about adsorbed species from temperature programmed desorption and spectroscopic studies are used to predetermine rate and equilibrium constants of elementary processes, thus enabling the prediction of the overall rate. Especially for metal catalyzed reactions this gives good results [9]. More information about reaction kinetics related to catalysis can be found in Refs. [10-13]. 

The wider utilization of microkinetic models is somewhat retarded by the vast amount of information needed about interactions of chemical intermediates with complex, heterogeneous catalysts. The microkinetic approach has been applied to numerous diverse chemistries including cracking, hydrogenation, hydrogenolyis, hydrogenation, oxidation reactions and ammonia synthesis to name a few. 

NONUNIFORM SURFACE MODELING CAPABILITIES A MICROKINETIC APPROACH... 

Smith et al., 2010) classifies the reaction kinetic models in microkinetic approach and the empirical method. 

Commonly, the design of a reactor requires the prediction of the rate of reaction. Two different approaches have been used to develop suitable kinetic models for the WGS reaction. The first is based on microkinetics by taking into account the elementary steps from the adsorption of the chemical species to the reaction and the product desorption the second is based on the macrokinetics that are empirical models in which the rate of reaction depends proportionally on the concentration of reactants and products and exponentially on temperature (typically expressed using the Arrhenius equation). The microkinetics approach is more complex, in particular from a mathematical and computational point of view, but it offers the possibility to better model the surface coverage and the enthalpy of the reaction (i.e., the temperature increase on the catalyst surface). Two different mechanisms for the WGS reaction are proposed in the scientific literamre the redox mechanism and the associative mechanism. 

There is now a great deal of interest in utilizing the microkinetic approach in modeling rates of catalytic reactions despite the lack so r of reliable rate constants of elementary reactions on different catalytic materials. However, the alternative approaches diat provide a simple means of understanding, explaining and predicting the kinetic behavior of complex heterogeneous catalytic reactions continue to be invaluable. The main approximations that are conventionally used to simpUfy the detailed kinetics are [1] ... 

The microkinetic approach integrates a variety of different data sources to estimate rates and surface coverages. Difficulties may arise when the surface structure changes with time and/or gas composition. With TAP the surface coverage can be directly manipulated, and dynamic models of surface coverage/kinetic relationships can be investigated. 

Macrokinetic models are stiU widely used for reactor design because they can be derived rather time- and cost-saving with an optimized set of experimental measurements (Deutschmann, 2011b). Even though the models are only valid in the limited range of conditions in which the kinetic data are derived for, they often fit their purpose in particular if the rate expressions reflect the molecular processes in some way and the rate-determining step is adequately taken into account. On the other hand, the microkinetic approach attempts to describe reactions using their most fundamental set... 

Rochoux, M., Guo, Y., Schuurman, Y., and Farrusseng, D. (2015) Determination of oxygen adsorption-desorption rates and diffusion rate coefficients in perovskites at different oxygen partial pressures by a microkinetic approach. Phys. Chem. Chem. Phys., 17,1469-1481. 

Steady-state performances of SCR-NH3 reactors can be correctly described using this microkinetic approach derived from the reaction mechanism outlined previously, as shown in Figure 21.1(a) [50],... 

A special case related to parameter estimation is processing of complex multicomponent mixtures. Single event microkinetics approach or SEMK (Fig. 11.23) considers chemical transformations of reactive moieties, rather than individual molecules, thus reducing a very high number of elementary steps to a limited number of reaction families, as well as to a limited number of kinetic parameters. 

The important difference between the microkinetics approach and the kinetic Monte Carlo simulation is that in the former diffusion is not explicitly included. Reaction probabilities are again based on the Eyring transition state rate expression. Its benefit is a substantial reduction in computational time length. Similar as in the kinetic Monte Carlo method, production rates as a function of reaction condition can be computed. These kinetic data can be correlated with changes in surface composition of the adsorbed reactant and intermediate overlayer. Also, rates of reaction intermediate production or removal can be deduced. 

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