Modeling Hydrogen Atom Transfer Rate Constants in Heterogeneous Catalysis

Hydrogen atom transfer (HAT) is an important elementary step involved in many reactions in heterogeneous catalysis. Thus, it is important for computational catalysis studies to incorporate accurate HAT rate constant calculations into microkinetic models to develop accurate predictions and molecular-level interpretations of turnover rates, apparent activation energies, and reaction orders. Such insights are needed to understand and design catalysts for various chemical transformations. Rate constants in heterogeneous catalysis are commonly modeled using transition-state theory; however, many heterogeneous HAT reactions exhibit hallmarks of nuclear quantum effects such as hydrogen atom tunneling via kinetic isotope effects. Herein, we highlight different theoretical approaches to model HAT kinetics in heterogeneous catalysis using classical, semiclassical, and quantum-mechanical treatments of the transferring hydrogen atom. We analyze different rate constant modeling approaches starting from a vibronically diabatic representation of the reactant and product quantum states in a donor–acceptor reaction. Within this theoretical framework, the appropriate rate constant model depends on the nature of the coupling between electrons, the transferring hydrogen atom nucleus, and the surrounding classical nuclei in the system. This Perspective highlights differences between the vibronically adiabatic and nonadiabatic HAT rate constant formulations, as well as computational approaches based on density functional theory (DFT) that can be used to classify the quantum-mechanical nature of HAT. Periodic DFT calculations can be used to calculate rate constant input quantities such as activation energies and tunneling probabilities. These quantities are crucial for accurate calculations of rate constants and the construction of microkinetic models for catalytic systems where HAT exhibits nuclear quantum effects such as tunneling and reactive transitions involving excited vibronic states.