Elucidating Rate-Determining Steps of Surface-Catalyzed Reactions Exhibiting Isothermal Rate Multiplicity

Microkinetic models of surface-catalyzed reactions may admit multiple steady-state solutions of species balances in the absence of both mass and heat transfer limitations. As a result, the rate(s) of reactant consumption or product formation exhibit multiple rate states, a situation commonly called isothermal rate multiplicity. Established metrics for identifying the rate-determining (or rate-limiting) step of a reaction mechanism include reversibility, sensitivity, and degree of rate control. For reactions exhibiting rate multiplicity, these metrics indicate that disparate states (branches) in plots of rate versus the reactant concentration have rates that are limited by different steps in the catalytic mechanism. During competitive adsorption, high or “ignited” rate branches are controlled by adsorption of the reacting species requiring fewer active sites than that of a second reacting species that blocks the adsorption of the former. Low or “extinguished” branches are limited by the adsorption of the latter species, requiring more sites than the former. Reversibility, sensitivity, degree of rate control, and ultimately the rate-determining step of a sequence within the region of multiplicity are determined not only by the operating condition but also by the initial state of the catalyst surface. It is shown that despite also being multivalued, sensitivities and degrees of rate control sum to unity across stable and unstable steady-state branches. Thermodynamic degrees of rate control can also be used to estimate multivalued surface coverages along all branches, thereby demonstrating consistency with previously derived mathematical relationships. Pt-catalyzed carbon monoxide oxidation case studies show that the rate-determining step changes from CO to O2 adsorption depending on whether the system resides on ignited or extinguished branches, respectively. Analogously, Pt–Pd-catalyzed methane oxidation is controlled by either O2 or CH4 adsorption depending on whether the system resides on the ignited or extinguished rate branches.