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 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 H 2 O 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 σ 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 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 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.

Reduction of N2 with H2 on palladium surfaces at low temperatures

“…The reaction involves H atoms and hence gives an evidence for the H 2 dissociation. The argument agrees with the report that H atoms resulting from H 2 on a PdO (1 0 1) thin film react with the surface O, leading to the formation of H 2 O [18].…”

Atom Tunneling in Chemistry

“…[229,230] Even though this Review mainly focuses on molecular systems,w ew ill now briefly discuss atom tunneling on surfaces.H ydrogen atoms have been frequently observed to tunnel in surface processes. [231][232][233] It was shown that the motion of hydrogen on Cu(001), [234][235][236][237] Pd/Cu (111), [238,239] Ru(0001), [240] W(110), [241] and Ni(100) [242][243][244] surfaces is enhanced by tunneling at low temperatures.E ven the motion of CO on aC u(111) surface was shown by scanning tunneling microscopy (STM) to occur below 6K,w ith atemperature-independent hopping rate. [245] In the field of heterogeneous catalysis,t he CO oxidation and the dissociative H 2 Od esorption as well as the OH dissociation on various metal (111) surfaces were shown to be affected by tunneling.…”

Promotion of CO oxidation on PdO(101) by adsorbed H2O

“…The PdO(101) surface was modeled by a rectangular 4 × 1 unit cell, 172 with a corresponding 4 × 2 × 1 Monkhorst-Pack k-point mesh [21]. As 173 in our prior studies [33][34][35], the PdO(101) film was strained (a = …”