Elementary surface reaction steps comparison

Computational chemistry has reached a level in which adsorption, dissociation and formation of new bonds can be described with reasonable accuracy. Consequently trends in reactivity patterns can be very well predicted nowadays. Such theoretical studies have had a strong impact in the field of heterogeneous catalysis, particularly because many experimental data are available for comparison from surface science studies (e.g. heats of adsorption, adsorption geometries, vibrational frequencies, activation energies of elementary reaction steps) to validate theoretical predictions. 

Computational catalysis can make substantial contributions to these issues because it allows for a comparison of the rates of elementary reaction steps proposed for various mechanistic reaction paths. By use of computations, it is also possible to relate surface structure with the relative stabilities of various reaction intermediates and transition states. 

Methane dissociation requires a reduced metal surface, but at elevated temperatures oxides of the active species may be reduced by direct interaction with methane or from the reaction with H, Hg, C or CO. The comparison of elementary reaction steps on Pt and Rh illustrates that a key factor to produce hydrogen as a primary product is a high activation energy barrier to the formation of OH. A catalytic material and support which does not easily form or stabilise OH species is therefore desirable. Another essential property for the formation of Hg and CO as primary products is a low surface coverage of intermediates, such that the probability of O-H, OH-H and CO-O interactions is reduced. ... 

In order to evaluate the isomerization rate constant (via the second channel), a kinetics scheme containing all the four elementary steps, forward and backward, has been constructed. Their rate constants were calculated from the AS and A for each elementary step. Together with the thermochemistry of the reactant, product, and all the intermediates on the surface, the scheme could be computer modeled to evaluate the product yields, and then the final rate constant for the reactant —> product reaction. Figure 6.36 shows a comparison between the results of the experiment and the calculations. The difference is about a factor of 3 that corresponds to a disagreement of approximately 2.5 kcal/mol. 

The Most Abundant Surface Intermediate (MASI) Approximation Catalytic transformations may include the formation of many intermediates on the catalyst surface, which are difficult to identify. In these cases, it is impossible to formulate a kinetic model based on all elementary steps. Often, one of the intermediates adsorbs much more strongly in comparison to the other surface species, thus occupying nearly all active sites. This intermediate is called the most abundant surface intermediate masi [24]. For a simple monomolecular reaction, Aj A2, the situation can be illustrated with the following scheme ... 

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