The “degree of rate control” (DRC) is a mathematical concept for analysing multi-step reaction mechanisms that has proven very useful in catalysis research.1 It identifies the “rate-controlling transition states and intermediates” (i.e., those whose DRCs are large in magnitude). Even in mechanisms with over 30 intermediates and transition states, these are generally just a few distinct chemical species whose energies, if they could be independently changed, would achieve a faster net reaction rate to the product of interest. For example, when there is a single “rate-determining step”, the DRC for its transition state (TS) is 1, which means (by definition) that if this TS’s energy could be decreased by kBT (where kB is Boltzmann’s constant and T is temperature), the net rate would increase by a factor of e. Since the (relative) energies of these key adsorbed intermediates and transition states can be adjusted by modifying the catalyst or solvent, or even a reactant’s molecular structure, the DRC values provide important ideas for catalyst improvement. The species with large DRCs are also the ones whose energetics must be most accurately measured or calculated to achieve an accurate kinetic model for any reaction mechanism. A tutorial on DRC analysis is presented here. Applications of DRC analysis include: clarifying reaction kinetics, using kinetic isotope effects to test the validity of microkinetic models, improving the accuracy of computational models, improving reaction conditions, improving choice of oxidant in selective oxidation and high-throughput computational screening of catalyst materials. Since DRC values can be determined experimentally, a full microkinetic model is not required to take advantage of DRC analysis.

Degree of rate control: Powerful tool for catalysis research

2019 Fall National Meeting

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