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We investigated the adsorption and oxidation of H2 on O-rich IrO2(110) using temperature programmed reaction spectroscopy (TPRS) and density functional theory (DFT) calculations. Our results show that H2 dissociation occurs efficiently on O-rich IrO2(110) at low temperature and initiates from an adsorbed H2 σ-complex on the coordinatively-unsaturated Ir atoms (Ircus). We find that on-top oxygen atoms (Oot), adsorbed on the Ircus sites, promote the desorption-limited evolution of H2O during subsequent oxidation of the adsorbed hydrogen on IrO2(110) while suppressing reaction-limited production of H2O via the recombination of bridging HO groups (HObr) (~500 to 750 K) during TPRS. The desorption-limited TPRS peak of H2O shifts from ~490 to 550 K with increasing Oot coverage, demonstrating that Oot atoms stabilize adsorbed OH and H2O species. DFT predicts that molecularly-adsorbed H2 dissociates on O-rich IrO2(110) at low temperature and that the resulting H-atoms redistribute to produce a mixture of HObr and HOot groups, with equilibrium favouring HOot groups. Our calculations further predict that subsequent H2O evolution occurs through the recombination of HObr/HOot and HOot/HOot pairs, and that these reactions represent desorption-limited pathways because the dissociative chemisorption of H2O is favoured over molecular adsorption on IrO2(110). The higher stability of HOot groups and their preferred formation causes the higher-barrier HOot/HOot recombination reaction to become the dominant pathway for H2O formation with increasing Oot coverage, consistent with the experimentally-observed upshift in the H2O TPRS peak temperature. [ABSTRACT FROM AUTHOR]

We investigated the dissociative chemisorption and oxidation of H2 and D2 on the stoichiometric IrO2(110) surface (“s-IrO2(110)”) using temperature programmed reaction spectroscopy (TPRS) and density functional theory (DFT) calculations. We find that the dissociative chemisorption of hydrogen occurs efficiently on s-IrO2(110) during adsorption at 90 K, with ~ 90% of the dissociated H2 oxidizing to H2O during TPRS and evolving in a broad feature between 400 and 800 K. We also observe small quantities of H2 desorbing in TPRS peaks at 200 and 530 K, and show that these peaks arise from the desorption of molecularly-adsorbed H2 and the recombination of atomic hydrogen, respectively. Our results demonstrate that H2 dissociation on s-IrO2(110) occurs by a precursor-mediated mechanism wherein H2 molecules adsorb strongly on coordinatively-unsaturated Ir atoms (Ircus) and the resulting σ-complexes then serve as precursors for H2 bond cleavage. Our DFT calculations predict that H2 adsorbs strongly on an atop-Ircus site, and that the H2 complex can dissociate by a facile pathway involving H-transfer to a neighboring bridging O atom (Obr) to produce an H-Ircus/HObr pair. For this pathway, we predict that the energy barrier for dissociation is ~ 65 kJ/mol lower than the binding energy of the adsorbed H2 complex. We also find that the total hydrogen uptake on s-IrO2(110) saturates at an H2 coverage of ~ 0.65 ML during adsorption at 90 K, and present evidence that this limited uptake results from a strong influence of HObr groups on H2 σ-complex formation. Finally, we used DFT to examine pathways for H2O formation on s-IrO2(110) and find that steps leading directly to H2O formation are energetically demanding and likely determine the overall rate of H2 oxidation on s-IrO2(110). [ABSTRACT FROM AUTHOR]

Advances in the fundamental understanding of alkane activation on oxide surfaces are essential for developing new catalysts that efficiently and selectively promote chemical transformations of alkanes. In this tutorial review, we discuss the current understanding of alkane activation on crystalline metal oxide surfaces, and focus mainly on summarizing our findings on alkane adsorption and C--H bond cleavage on the PdO(101) surface as determined from model ultrahigh vacuum experiments and theoretical calculations. These studies show that alkanes form strongly-bound s-complexes on PdO(101) by datively bonding with coordinatively-unsaturated Pd atoms and that these molecularly adsorbed species serve as precursors for C--H bond activation on the oxide surface. In addition to discussing the binding and properties of alkane s-complexes on PdO(101), we also summarize recent advances in kinetic models to predict alkane dissociation rates on solid surfaces. Lastly, we highlight computations which predict that the formation and facile C--H bond activation of alkane s-complexes also occurs on RuO2 and IrO2 surfaces. [ABSTRACT FROM AUTHOR]

Methane undergoes highly facile C–H bond cleavage on the stoichiometric IrO2(110)surface. From temperature-programmed reaction spectroscopy experiments, we foundthat methane molecularly adsorbed as a strongly bound s complex on IrO2(110) andthat a large fraction of the adsorbed complexes underwent C–H bond cleavage attemperatures as low as 150 kelvin (K). The initial dissociation probability of methane onIrO2(110) decreased from 80 to 20% with increasing surface temperature from175 to 300 K. We estimate that the activation energy for methane C–H bondcleavage is 9.5 kilojoule per mole (kJ/mol) lower than the binding energy of theadsorbed precursor on IrO2(110), and equal to a value of ~28.5 kJ/mol. Low-temperatureactivation may avoid unwanted side reactions in the development of catalyticprocesses to selectively convert methane to value-added products [ABSTRACT FROM AUTHOR]

We investigated the oxidation of Ir(111) by gas-phase oxygen atoms at temperatures between 500 and 625 K using temperature programmed desorption (TPD), low energy electron diffraction (LEED), low energy ion scattering spectroscopy (LEISS) and density functional theory (DFT) calculations. We find that a well-ordered surface oxide with (√ 3 × √ 3)R30° periodicity relative to Ir(111) develops prior to the formation of a rutile IrO 2 (100) layer. The IrO 2 (100) layer reaches a saturation thickness of about four oxide layers under the oxidation conditions employed, and decomposes during TPD to produce a single, sharp O 2 desorption peak at ~ 770 K. Favorable lattice matching at the oxide-metal interface is likely responsible for the preferential growth of the IrO 2 (100) facet during the initial oxidation of Ir(111), with the resulting coincidence lattice generating a clear (6 × 1) moiré pattern in LEED. Temperature programmed reaction spectroscopy (TPRS) experiments reveal that CO and H 2 O molecules bind only weakly on the IrO 2 (100) surface and LEISS measurements show that the oxide surface is highly enriched in O-atoms. These characteristics provide strong evidence that the rutile IrO 2 (100) layer is oxygen-terminated, and thus lacks reactive Ir atoms that can strongly bind molecular adsorbates. Oxygen binding energies predicted by DFT suggest that on-top O-atoms will remain adsorbed on IrO 2 (100) at temperatures up to ~ 625 K, thus supporting the conclusion that the rutile IrO 2 layer grown in our experiments is oxygen-terminated. As such, the appearance of only a single O 2 TPD peak indicates that the singly coordinate, on-top O-atoms remain stable on the IrO 2 (100) surface up to temperatures at which the oxide layer begins to thermally decompose. [ABSTRACT FROM AUTHOR]