The Importance of Accounting for Excited States in Ab Initio Modeling of Photocatalytic Mechanism: Insights from N2 Reduction on a Ru–TiO2 Cluster

Abstract

Photocatalytic nitrogen reduction is a sustainable alternative to the Haber–Bosch process, allowing ammonia production under mild conditions using light as the driving force for nitrogen fixation. Despite several experimental studies on photocatalytic ammonia synthesis, first-principles mechanistic studies are lacking, and only the ground state is typically considered. Herein, a multiscale model of photocatalytic ammonia synthesis is build, which is ab initio, explicitly accounts for excited states, and feeds into a coupled microkinetic model. Time-dependent density functional theory is employed to study the electronic properties of a Ru3–(TiO2)6 cluster, its adsorption capabilities, and the reaction kinetics of ammonia synthesis in the ground state and the first excited state. The reaction parameters are then cast into a microkinetic model, which shows that the catalyst is active and elucidates the effect of varying reaction conditions on performance. Crucially, it is shown that without accounting for the excited states, the kinetics of the reaction are erroneously described. The reaction proceeds via a dissociative pathway, with N≡N bond cleavage as the rate-determining step. The apparent activation barrier in the excited state is decreased from 1.98 to 1.84 eV, increasing the reaction rate. This computational approach is transferable to other photocatalytic reactions.