Microkinetics

Microkinetics is the detailed, mechanism-based modeling of chemical kinetics at the microscopic level, typically on catalytic surfaces or in confined environments. It represents a reaction system as a network of elementary steps—such as adsorption, surface reactions, desorption, and sometimes diffusion—each with its own rate constant. The approach is especially prominent in heterogeneous catalysis and electrochemistry, where the sequence and competition of elementary processes determine overall rates and selectivity.

In a microkinetic model, surface species are described by dynamic variables that follow a set of ordinary differential equations expressing production and consumption rates for every adsorbed intermediate. Rate constants are usually obtained from transition state theory or fitted to experimental data, and the elementary steps are treated according to common formulations such as Langmuir-Hinshelwood or Eley-Rideal mechanisms. Depending on the approach, the model may apply the steady-state approximation for surface intermediates to derive an overall reaction rate. Outputs typically include turnover frequencies, surface coverages, and predicted selectivities as functions of temperature, pressure, and feed composition.

Microkinetic modeling is used to interpret experiments, compare catalyst materials, and guide catalyst design by identifying rate- and selectivity-controlling steps. It complements simpler macrokinetic models and can be extended with stochastic simulations, such as kinetic Monte Carlo, to capture spatial effects and correlations neglected by mean-field rate equations. Limitations include dependence on the chosen network of elementary steps, the potentially large number of parameters, and assumptions about surface homogeneity and well-mixedness.

Overall, microkinetics provides a bridge between microscopic reaction mechanisms and macroscopic observables, enabling more accurate predictions of catalytic performance under realistic conditions.