Non-Thermalized Precursor-Mediated Dissociative Chemisorption at High Catalysis Temperatures

Quasi-classical trajectory calculations and vibrational-state-selected beam-surface measurements of CH$_4$ chemisorption on Ir(111) reveal a non-thermal, hot-molecule mechanism for C-H bond activation. Low-energy vibrationally excited molecules trap in the physisorption well, and react before vibrational and translational energies accommodate with the surface. Reaction probability is strongly surface-temperature dependent and arises from the pivotal role of Ir atom thermal motion. In reactive trajectories, the mean outward Ir atom displacement largely exceeds that of the transition state geometry obtained through a full geometry optimization. The study also highlights a new way for (temporary) surface defects to impact high-temperature heterogeneous catalytic reactivity. Instead of reactants diffusing to and competing for geometrically localized lower barrier sites; transient, thermally activated surface atom displacements deliver low-barrier surface reaction geometries to the physisorbed reactants.