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