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

We investigated the dissociative chemisorption and oxidation of H-2 and D-2 on a PdO(101) thin film using temperature-programmed desorption (TPD) experiments and density functional theory (DFT) calculations. We find that the dissociation of H-2 is highly facile on PdO(101), with more than 90% of a saturated H-2 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 H-2 dissociation on PdO(101) occurs by a precursor-mediated mechanism in which a molecularly chemisorbed H-2 species acts as a necessary precursor to dissociation. The experimental data also reveal that a kinetic isotope effect strongly suppresses the dissociation of D-2 on PdO(101) terraces and causes the kinetic branching to shift toward desorption of the molecular D-2 precursor. DFT calculations predict that H-2 binds relatively strongly on PdO(101) by forming a sigma complex on a coordinatively unsaturated (cus) Pd site. Using DFT, we identified only a single pathway for H-2 dissociation that generates stable products on PdO(101). In this pathway, the adsorbed H-2 sigma 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 H-2 on PdO(101) and a strong kinetic isotope effect that suppresses D-2 dissociation. We present evidence that quantum mechanical tunneling dominates the dissociation of H-2 on PdO(101) at low temperatures and that differences in tunneling rates are responsible for the large kinetic isotope effect that we observe experimentally.