(589a) Pruning of Catalytic Reaction Networks: A Comparison of Campbell's Degree of Rate Control with Reaction Step Resistance

Many complex catalytic reaction mechanisms start off as being “hypothetical,” i.e., they emerge from a researcher’s knowledge of the catalytic chemistry on a given catalyst coupled with intuition, or are based on a stoichiometric, or graph-theoretical, generator, but can soon become “real,” when modern quantum-chemical methods are used to predict their reaction energies and activation barriers. Whether the imagined molecular and kinetic complexity of a network is justified in reality, however, is an open question; one which we are not yet fully equipped to answer unequivocally.

The tools that are currently available for simplifying complex microkinetic models include: 1) a comparison of the energy landscape of different pathways; 2) a comparison of step reversibility, as proposed by Dumesic, and 3) Campbell’s degree of rate control (RDC). The first of these, although the most common, is mainly a qualitative tool, while the second, involves only a thermodynamic criterion, not kinetic. The most useful and rigorous tool so far is Campbell’s DRC, which involves a partial differential analysis of the relative change in the rate of overall reaction (OR) for a relative small change in the rate constant of a step, holding constant all other step rate as well as equilibrium constants, i.e., it is a numerical sensitivity analysis. With the help of the water-gas shift reaction network and other examples, we show that our alternate methodology, as a part of our Reaction Route (RR) Graph approach, which involves a comparison of reaction step “resistance,” is just as rigorous and, in fact, more revealing than Campbell’s DRC, and allows transparent pruning of complex catalytic reaction networks.