Strong Kinetic Isotope Effect in the Dissociative Chemisorption of H2 on a PdO(101) Thin Film

Abstract

We investigated the dissociative chemisorption and oxidation of H2 and D2 on a PdO(101) thin film using temperature-programmed desorption (TPD) experiments and density functional theory (DFT) calculations. We find that the dissociation of H2 is highly facile on PdO(101), with more than 90% of a saturated H2 layer dissociating below 100 K. Most of the dissociated hydrogen reacts with the surface to produce H2O that desorbs above 350 K during TPD. Our experimental data demonstrate that H2 dissociation on PdO(101) occurs by a precursor-mediated mechanism in which a molecularly chemisorbed H2 species acts as a necessary precursor to dissociation. The experimental data also reveal that a kinetic isotope effect strongly suppresses the dissociation of D2 on PdO(101) terraces and causes the kinetic branching to shift toward desorption of the molecular D2 precursor. DFT calculations predict that H2 binds relatively strongly on PdO(101) by forming a σ complex on a coordinatively unsaturated (cus) Pd site. Using DFT, we identified only a single pathway for H2 dissociation that generates stable products on PdO(101). In this pathway, the adsorbed H2 σ complex dissociates by transferring an H atom to a neighboring cus-O site, thereby producing an OH species and an H atom bound to a cus-Pd site. Zero-point-corrected barriers determined for this pathway fail to explain our experimental observations of facile dissociation of H2 on PdO(101) and a strong kinetic isotope effect that suppresses D2 dissociation. We present evidence that quantum mechanical tunneling dominates the dissociation of H2 on PdO(101) at low temperatures and that differences in tunneling rates are responsible for the large kinetic isotope effect that we observe experimentally.