Effects of Thermal Fluctuations on the Hydroxylation and Reduction of Ceria Surfaces by Molecular H2

The hydroxylation of oxide surfaces driven by molecular H-2 dissociation plays a central role in a wide range of catalytic redox reactions. The high reducibility and oxygen storage capacity of ceria (CeO2) surfaces account for its extensive use as active catalyst support in these redox reactions. By means of ab initio molecular dynamics simulations, we investigate the hydroxylation and reduction of ceria surfaces and demonstrate the so-far unrecognized effects of atomic thermal fluctuations into the mechanism and kinetics of H-2 dissociation. The reaction free-energy hyper-surface is sampled and mapped at finite temperature by combining Hubbard-U density functional theory (DFT+U), ab initio molecular dynamics, metadynamics, and umbrella sampling methods. Our molecular dynamics simulations show that the explicit inclusion of thermal fluctuations into the reaction thermodynamics alters the mechanism of H-2 dissociation, changes the nature of the rate-limiting transition state, and decreases the activation temperatures by more than 25%. The results are discussed in the context of kinetic measurements and provide novel insight into the hydroxylation and reduction steps that control the catalytic activity and selectivity of ceria surfaces.