(98g) Strong Kinetic Isotope Effect in the Dissociative Chemisorption of H2 On a Pdo(101) Thin Film

In the present study, we examined the dissociative chemisorption of H₂ and D₂ with PdO(101) using a combination of temperature-programmed reaction spectroscopy (TPRS) experiments and density functional theory (DFT) calculations. We find that H₂ dissociation is highly facile on PdO(101) at temperatures below 100 K, with more than 90% of the initially adsorbed H₂ dissociating. Most of the dissociated hydrogen reacts with the surface to produce H₂O that desorbs above 350 K during TPRS. The experimental data provides strong evidence that H₂ dissociative chemisorption occurs by a precursor-mediated mechanism on PdO(101) wherein molecularly chemisorbed H₂ acts as the precursor to dissociation. The TPRS data also reveals that a strong kinetic isotope effect influences the rate of H₂ (D₂) dissociation on PdO(101) at low temperature. For D₂, we find that molecular desorption is strongly favored over dissociation on PdO(101) terraces. DFT calculations predict that H₂ binds relatively strongly on PdO(101) by forming R-complexes on coordinatively unsaturated (cus) Pd sites. Using DFT, we identified only a single pathway for H₂ dissociation that generates stable products on PdO(101). However, 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 which suppresses D₂ dissociation. By applying tunneling corrections to DFT-derived rate coefficients, we obtain evidence that quantum-mechanical tunneling dominates the dissociation of H₂ on PdO(101) at low temperature, and that differences in tunneling rates are responsible for the large kinetic isotope effect that we observe experimentally.