Proximity factors

[wittregr]

We know from BEP relationships that activation energies are correlated to heats of reaction. In a microkinetic model heats of reaction can change based on a number of factors that change the surface energies of adsorbed species. The most important of these is lateral interactions. As surface concentrations of species change, adsorption energies change, and heats of reaction change. Therefore, in kinetic models where BEP's are used to determine activation energies, changes in surface reaction energies are reflected properly in the activation energies. Reactions using a "fixed" activation energies (fixed wrt surface heats of reaction) as specified in the surf.inp, gas.inp, EAs.inp, or EAg.inp the changes in surface reaction energies are not reflected in the surface reaction activation barriers. To address this Chemin uses a proximity factor as describer by Grabow, L. C. et al. (2008) (‘Mechanism of the water gas shift reaction on Pt: First principles, experiments, and microkinetic modeling’, Journal of Physical Chemistry C, 112(12), pp. 4608–4617).

The proximity factor ranges from 0 - 1 and represents the reaction coordinate of the transition state for that reaction and functions identically to the BEP slope appling a fraction of the change in heat-of-reaction to the activation energy.

Chemkin provides for a default proximity factor in tube.inp (omega) and a flag (Iomega) to enable both proximity factors and a per reaction proximity factor file (omega.inp). The omega.inp file contains a list of reactions and associated proximity factors. If Iomega is True then, for all reactions not using BEP's, Chemkin first looks in the omerga.inp for a proximity factor and, if not found, uses the default value specified in tube.inp.