(98g) Strong Kinetic Isotope Effect in the Dissociative Chemisorption of H2 On a Pdo(101) Thin Film
AIChE Annual Meeting
2010 Annual Meeting
Catalysis and Reaction Engineering Division
Fundamentals of Surface Reactivity
Monday, November 8, 2010 - 2:36pm to 2:57pm
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.