Post on 19-Feb-2017
Nisha Verma08.07.2014
Surface Chemistry of RuO2 (Late Transition
Metal Oxide)
IntroductionPreparation (RuO2)Structure of RuO2 (110) and RuO2
(100), O-rich and Ru-rich RuO2 (110)Adsorption of CO2 and H2O on RuO2
(110) CO oxidation on RuO2 (110) Summary and Outlook
Outline
Late transition metals: Ru, Rh, Ir, Pd, and PtApplications: Catalytic combustion of natural gas Fuel cell catalysis Selective oxidation of organic compounds Promote oxidation reactions: exhaust gas remediation
AutomobilesPower plants
Introduction
Are these metals really metal?
Practical application: Oxidation catalysis Metal + O2 Metal Oxide Layer
(catalyst)Metal oxide formation occurs while the
catalyst is operating in a realistic gaseous environment
Controlled conditions are essential to grow metal oxides
Developing an atomic-level understanding of the growth and surface chemical properties of late transition metal oxides
Introduction
Initially, adsorption of O2 takes place via chemisorption
Continued oxygen adsorption transforms the oxide layer to a multilayer bulk oxide
Effect of temperature:Moderate Temperature: Formation of ultrathin
films (1-2 nm)High Temperature: Thicker, disordered clusters
Characterization techniques: SXRD, HPXPS, TPD
Introduction
Surface Chemistry of RuO2(110)
UHV Experiment: Dose 5 × 106 to 107 L of O2
Pressure: <10−6 TorrTemperature: 550 - 700 K
Between 550 and 700 K: average thickness of 1.6 nm without further oxidizing
Above 700 K: thicker RuO2 layers grow
Preparation of RuO2(110) on Ru(0001)
Bulk RuO2: rutile structure, comprised of RuO6 units
Stoichiometric RuO2 (110) surface (s-RuO2 (110)): exposes bridging oxygen atoms (Obr), 3-fold coordinated oxygen atoms (O-3f), and 5-fold coordinated Ru cations or Ru-cus
Ru-cus “coordinatively unsaturated”: oxide surface cleaves oxygen−metal bonds and generates surface metal and oxygen atoms with coordination vacancies
Structure of RuO2(110) Surface
Structure of RuO2(100) SurfaceUHV by exposing the Ru(101̅0) surface
to NO2 or large doses of O2 at surface temperatures above about 600 K
Small quantities of RuO2(110) and RuO2(101) also form on Ru(101̅0), but the RuO2(100) facet grows in significantly higher quantities
The Ru-cus and Obr atoms are bonded to one another
Oot /On-top or cus-O
Adsorb directly on top of cus-Ru atoms
Under UHV, saturation coverage of Oot atoms obtained by exposing s-RuO2 (110) between 5 and 10 L of O2 at 300 K
Expected to be unstable
O-Rich RuO2(110) Surface
Recombinative adsorption of Oot observed 400 & 500 K
Desorption peak at 1040 K: Decomposition of the oxides
Ru-Rich RuO2(110) SurfaceCreated by removing Obr atoms from the
surface, thus introducing point vacancies
Leaves a 2-fold under-coordinated Rubr atoms exposed at the surface
Thermodynamically stable under sufficiently reducing conditions
Oxygen sites with higher basicity transfer charge to CO2 and produce more stable surface CO3 δ− species
CO2 physisorbs on RuO2(110) at 85 K but a small amount also transforms to a carboxylate species (CO2
δ−)
Reversible Adsorption of CO2 on RuO2 (110)
Ru-cus sites are more electron-rich than the bulk Ru atoms and can transfer charge to CO2 to produce the carboxylate species CO2
δ− and CO2:CO2δ−
The CO3 δ− species decomposes and evolves CO2 above about 315 K, thus restoring the s-RuO2 (110) surface to its original state
Adsorption found to be more facile in O-rich RuO2 than s-RuO2
Reversible Adsorption of CO2 on RuO2 (110)
H2O chemisorbs relatively strongly on the stoichiometric RuO2(110) surface and remains in molecular form on defect-free surfaces
At low coverages, H2O chemisorbs on the Ru-cus sites and gives rise to a desorption peak between 350 and 425 K
Interacts with O-br sites as well
H2O populates a second layer state which desorbs at 190 K during TPD, followed by a multilayer which desorbs at ∼160 K
Reversible Adsorption of H2O on RuO2 (110)
CO oxidation on RuO2(110)
TPD spectra of (A) CO and (B) CO2 obtained from the stoichiometric (solid lines) and mildly reduced (dashed lines) RuO2(110) surfaces after adsorbing saturation coverages of CO
Model representations of (A) COcus species adsorbed on the s-RuO2(110) surface, and (B) the mildly reduced RuO2(110) surface saturated with CObr species
RuO2 (110) is highly active in promoting oxidative transformations of adsorbed species.
Detailed studies reveal that both cus-Ru and cus-O surface sites are needed to achieve this high reactivity.
Cus-Ru sites strongly bind adsorbed reactants and activate their internal bonds, while the cus-O sites act as strong H-atom acceptors and promote the dehydrogenation of adsorbed species.
CO2 adsorption results in the deactivation of the RuO2 (110) catalyst (catalyst poisoning).
Adsorption/oxidation of cyclic hydrocarbons on RuO2 should be investigated.
Summary and Outlook
Weaver, J. F., Chem. Rev. 2013, 113, 4164−4215Lobo A., Conrad H., Surf. Sci. 2003, 523, 279–286Zheng, G.; Altman, E. I. Surf. Sci. 2000, 462, 151-155Weaver, J. F.; Kan, H. H.; Shumbera, R. B. J. Phys.: Condens.
Matter 2008, 20, 184-191
References