Adsorption of alkanes on stoichiometric and oxygen-rich RuO2(110)

Abstract

We investigated the molecular adsorption of methane, ethane, propane and n-butane on stoichiometric and oxygen-rich RuO₂(110) surfaces using temperature-programmed desorption (TPD) and dispersion-corrected density functional theory (DFT-D3) calculations. We find that each alkane adsorbs strongly on the coordinatively-unsaturated Ru (Ru_cus) atoms of s-RuO₂(110), with desorption from this state producing a well-defined TPD peak at low alkane coverage. As the coverage increases, we find that alkanes first form a compressed layer on the Ru_cus atoms and subsequently adsorb on the bridging O atoms of the surface until the monolayer saturates. DFT-D3 calculations predict that methane preferentially adsorbs on top of a Ru_cus atom and that the C₂ to C₄ alkanes preferentially adopt bidentate configurations in which each molecule aligns parallel to the Ru_cus atom row and datively bonds to neighboring Ru_cus atoms. DFT-D3 predicts binding energies that agree quantitatively with our experimental estimates for alkane σ-complexes on RuO₂(110). We find that oxygen atoms adsorbed on top of Ru_cus atoms (O_ot atoms) stabilize the adsorbed alkane complexes that bind in a given configuration, while also blocking the sites needed for σ-complex formation. This site blocking causes the coverage of the most stable, bidentate alkane complexes to decrease sharply with increasing O_ot coverage. Concurrently, we find that a new peak develops in the C₂ to C₄ alkane TPD spectra with increasing O_ot coverage, and that the desorption yield in this TPD feature passes through a maximum at O_ot coverages between ∼50% and 60%. We present evidence that the new TPD peak arises from C₂ to C₄ alkanes that adsorb in upright, monodentate configurations on stranded Ru_cus sites located within the O_ot layer.