First principles reactive flux theory for surface reactions revealing multiple channels and recrossing dynamics.

Heterogeneous reactions typically consist of multiple elementary steps whose rate coefficients are of fundamental importance in elucidating the reaction mechanisms and establising micro-kinetic models. Transition state theory (TST) for calculating surface reaction rate coefficients often relies solely on the harmonic approximation of adsorbent vibrations and neglects recrossing dynamics. Here, we combine metadynamics with a more general reactive flux approach to calculate rate coefficients of surface reactions, overcoming these limitations of TST. We apply this approach to a textbook surface reaction, CO oxidation on Pt(111), for which rate constants have been measured, using a full-dimensional neural network potential energy surface constructed from first-principles data. A multi-dimensional free-energy surface is obtained by incorporating three collective variables, yielding rate coefficients for both CO oxidation and the competing CO desorption that agree with experimental data. Our results reveal dynamic recrossing in both reactions, which however arises from distinct physical mechanisms. This approach represents a general framework for calculating rate coefficients of elementary surface processes from first-principles, which is vital for developing predictive kinetic models for heterogenous catalysis.