Microkinetic analysis

Compare the pros and cons of kinetic analysis by fitting a Langmuir-Hin-shelwood model to measured data and by microkinetic analysis. 

This study presents kinetic data obtained with a microreactor set-up both at atmospheric pressure and at high pressures up to 50 bar as a function of temperature and of the partial pressures from which power-law expressions and apparent activation energies are derived. An additional microreactor set-up equipped with a calibrated mass spectrometer was used for the isotopic exchange reaction (DER) N2 + N2 = 2 N2 and the transient kinetic experiments. The transient experiments comprised the temperature-programmed desorption (TPD) of N2 and H2. Furthermore, the interaction of N2 with Ru surfaces was monitored by means of temperature-programmed adsorption (TPA) using a dilute mixture of N2 in He. The kinetic data set is intended to serve as basis for a detailed microkinetic analysis of NH3 synthesis kinetics [10] following the concepts by Dumesic et al. [11]. 

Table 4 summarizes the rate constants kj - Aj exp(-Ej / RT) for the forward and the reverse reaction derived from our microkinetic analysis of the steady-state and transient experiments with the three catalysts, i.e. Cs-Ru/MgO, Ru/MgO, and Ru/AlaOs catalyst [24]. 

In 2001, Mirodatos et al. [89] stressed the importance of transient studies as an alternative to steady continuous reactor operations. A combination of microkinetic analysis together with transient experiments should allow the determination of the global catalytic conversion from elementary reaction steps. Prerequisite for such analysis is the correlation of experimental data with the data of a model. Compliance between the data helps to derive the reaction mechanism. 

Fastrup, B. (1994) Temperature programmed adsorption and desorption of nitrogen on iron ammonia synthesis catalysts, and consequences for the microkinetic analysis of NH3 synthesis. Top. Catal., 1, 273. 

More recently a hybrid approach to computer-assisted catalyst synthesis, based on a microkinetic analysis of the catalytic reaction, has been put forward which comprises essentially deterministic but also some non-deterministic features. For the synthesis of a catalyst with high activity and selectivity for a given reaction, the application of a microkinetic analysis has been suggested by Dumesic and co-workers [43-45]. The derivation of the microkinetics is not necessarily based on detailed kinetic experimentation but, by analogy, to similarities with other known catalytic processes. In an ideal situation, the microkinetics of a catalytic reaction are completely defined according to Dumesic and his collaborators when ... 

Dumesic and co-workers have presented various case studies in which they have applied the above concept of microkinetic analysis in order to propose new catalysts or to explain the different catalytic performance of different catalyst formulations for given reactions ... 

The authors conclude that a coordinated effort involving experimental studies and microkinetic analysis of-... 

The microkinetic analysis is certainly a scientifically interesting approach which will contribute to the identification and selection of catalytic compounds even in more complex situations as described above. One problem still to be solved is the experimental procurement and/or estimation of the parameters used in microkinetic simulations, which limits the wide applicability of the method. Providing kinetic parameters for a complex reaction network from kinetic experiments for an analogous catalyst is a time-consuming process. Despite the availability of modem experimental equipment and efficient computers, a complex reaction demands at least one man year of work [51]. The estimation of parameters by ab initio or semiempirical methods has to be considered with caution because ideal surfaces are usually assumed. 

As applied to catalysis, the microkinetic analysis of catalytic reactions is used most often. This is an instrument of an idealized description of com plex catalytic processes without consideration of the mass transfer that can affect considerably the observed kinetics of the catalytic transformations. The microkinetic analysis with the necessary consideration of the active sites balance for all types of active centers of the catalyst, even though it has several drawbacks, can provide important information about the potential influence of the very different thermodynamic factors. 

Some specific features of catalytic reactions, which are identified via the microkinetic analysis and are what make catalytic and noncatalytic reac tions qualitatively different, are discussed following. One is possible nonco incidence of the rate limiting steps (the process bottleneck ) and rate determining steps, the parameters of the latter being directly present in the expressions that describe the stationary rate of the stepwise process. 

In catalytic stepwise reactions, which involve more complex elementary transformations than scheme (4.4), the rate-determining parameters can be identified through similar considerations. Several examples of simple model schemes of catalytic transformations are given following. These schemes often are used for the microkinetic analysis of particular catalytic transformations and help to reveal the influence of various factors. 

Equations like (4.97) and (4.98) may be extremely useful, at least in the preliminary microkinetic analysis of the behavior of complex catalytic transformations with unknown elementary mechanisms The kinetic scheme of the stationary stepwise transformation in such a system is only determined using empirical coefficients Ay, while the sign or the absence of any thermodynamic forces Xj governs the system evolution along a given trajectory. 

The ammonia synthesis reaction is one of the most widely studied reactions and it has been discussed previously in this text. (See Example 1.1.1, Table 5.2.1, and Section 5.3.) In this section, results from the microkinetic analysis of ammonia synthesis over transition metal catalysts containing either iron or mthenium will be presented. 

The conditions chosen for the microkinetic analysis correspond to an industrial reactor with 2.5 cm of catalyst operating at 107 bar with a stoichiometric feed of... 

Hydrogenation is an important industrial reaction that often requires the presence of a heterogeneous catalyst to achieve commercial yields. Ethylene, C2H4, is the smallest olefin that can be used to investigate the addition of hydrogen atoms to a carbon-carbon double bond. Even though many experiments and theoretical studies have been carried out on this simple system, the reaction is still not completely understood. Microkinetic analysis provides insights into the relevant elementary steps in the catalytic cycle. 

A microkinetic analysis of ammonia synthesis over transition metals is presented in Section 7.3. Use the results of that analysis to explain how adsorbed nitrogen atoms (N ) can be the most abundant reaction intermediate on iron catalysts even though dissociative chemisorption of N2 is considered the rate-determining step. 

Ovesen, C., Clausen, B., Hammershoi, B., Steffensen, G., Askgaard, T., Chorkendorff, I., Norskov, J., Rasmussen, P., Stoltze, P., Taylor, P. (1996). A microkinetic analysis of the water-gas shift reaction under industrial conditions, /. Catalysis 158,170-180. 

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