Ockham's razor for paring microkinetic mechanisms: Electrical analogy vs.Campbell's degree of rate control

Abstract

Elucidation of the key molecular steps and pathways in an overall reaction is of central importance in developing a better understanding of catalysis. Campbell's degree of rate control (DRC) is the leading methodology currently available for identifying the germane steps and key intermediates in a catalytic mechanism. We contrast Campbell's DRC to our alternate new approach involving an analysis and comparison of the "resistance" and de Donder "affinity," that is, the driving force, of the various steps and pathways in a mechanism, in a direct analogy to electrical networks. We show that our approach is as just rigorous and more insightful than Campbell's DRC. It clearly illuminates the bottleneck steps within a pathway and allows one to readily discriminate among competing pathways. The example used for a comparison of these two methodologies is a DFT study of the water-gas shift reaction on Pt-Re catalyst published recently.

Requiem for the Rate-Determining Step in Complex Heterogeneous Catalytic Reactions?

“…Reactions 2020, 3, x FOR PEER REVIEW 2 of 11 modelling can be computationally very demanding, especially when coupled with transport phenomena [16].…”

“…Such an assumption of a rate-determining step makes the rate expressions more tractable, facilitating the kinetic modelling of mainly steady-state kinetics even at the expense of losing important mechanistic information. Moreover, in several instances, without a substantial reduction in complexity, kinetic modelling can be computationally very demanding, especially when coupled with transport phenomena [16].…”

“…The most popular is probably the degree of rate control proposed by Campbell, based on the general principles of differential sensitivity analysis to determine sensitivity of the output to one input parameter [19][20][21][22][23][24][25]. It was argued [16] that an approach analogous to electrical networks is even better suited for the identification of the key steps determining the reaction rate allowing the derivation of accurate rate expressions even for mechanisms with nonlinear steps. Such nonlinear steps are present not only in heterogeneous catalytic reactions, such as, for example, a water gas shift reaction [16], decomposition of ammonia [26,27] or methanol [28], but also in organometallic catalysis.…”

“…It was argued [16] that an approach analogous to electrical networks is even better suited for the identification of the key steps determining the reaction rate allowing the derivation of accurate rate expressions even for mechanisms with nonlinear steps. Such nonlinear steps are present not only in heterogeneous catalytic reactions, such as, for example, a water gas shift reaction [16], decomposition of ammonia [26,27] or methanol [28], but also in organometallic catalysis. In the latter case, the reaction order in catalysts deviates from unity [5].…”