Microkinetic modeling corrections

This has in turn been related to the relative stability of the OMME compared to the ethylene reactant and the epoxide product [11]. It has been argued that the relative instability of the OMME intermediate on Ag compared to Group VIII metals is the main origin of the unique activity of Ag as an effective epoxidation catalyst. Whether this simple interpretation is correct remains to be seen and will require considerable further investigations. In our current studies, we propose to shed light on the competitive partial oxidation and total oxidation channels with ab initio derived microkinetic modeling [61]. 

FIGURE 8.11 Predictions of ethylene oxide (EO) selectivity on different bimetallic Ag,4M, catalysts from the microkinetic model. Two pre-exponential factors were adjusted to correctly capture the experimental EO selectivity on pure Ag. 

Two important quantities that are often evaluated from a microkinetic model are the reaction order with respect to each reactant and the apparent activation energy. Both quantities can be estimated from experiments using flow reactors [53,54], which makes them valuable parameters in validating and fine-tuning a model. Reaction order data are also some of the best indicators of the kinetics of the mechanism, and agreanent with experimental orders is a good indication that the model is capturing the correct kinetics. 

The role of kinetic and reactor modeling is crucial in the continued advancement of these catalysts as they are optimized for specific applications. We have described different mechanisms for SCR for feed compositions spanning the standard to fast to NO2 types. Convergence to the correct mechanisms is essential if predictive mechanistic-based kinetic models are to be developed. To date the kinetic models have been of the global variety. While these are usefiil for reactor optimization, microkinetic models are needed to guide rational catalyst design and the discovery of new catalyst formulations. 

In the previous examples we only considered electronic energy changes and approximated the entropy as all or nothing. In essence, we assumed that gas-phase species have 100% of their standard state entropy and surface species possess no entropy at all. These assumptions can certainly be improved and in order to construct thermodynamically consistent microkinetic models this is not just optional, but absolutely necessary. Entropy and enthalpy corrections for surface species can be calculated using statistical thermodynamics from knowledge of the vibrational frequencies, and the translational and rotational degrees of freedom (DOF). In contrast to gas-phase molecules, adsorbates cannot freely rotate and move across the surface, but the translational and rotational DOF are frustrated within the potential energy well imposed by the surface. In the harmonic limit the frustrated translational and rotational DOF can conveniently be described as vibrational modes, which in turn means that any surface adsorbate will have iN vibrational DOFs that are all treated equally. 

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