XPS and STEM study of the interface formation between ultra-thin Ru and Ir OER catalyst layers and perylene red support whiskers

The interface formation between nano-structured perylene red (PR) whiskers and oxygen evolution reaction (OER) catalysts ruthenium and iridium has been studied systematically by XPS and STEM. The OER catalyst over-layers with thicknesses ranging from ~0.1 to ~50 nm were vapor deposited onto PR ex-situ. STEM images demonstrate that, with increasing thickness, Ru and Ir transform from amorphous clusters to crystalline nanoparticles, which agglomerate with increased over-layer thickness. XPS data show a strong interaction between Ru and PR. Ir also interacts with PR although not to the extent seen for Ru. At low coverages, the entire Ru deposit is in the reacted state while a small portion of the deposited Ir remains metallic. Ru and Ir bonding occur at the PR carbonyl sites as evidenced by the attenuation of carbonyl photoemission and the emergence of new peak assigned to C-O single bond. The curve fitting analysis and the derived stoichiometry indicates the formation of metallo-organic bonds. The co-existence of oxide bonds is also apparent

Composition and structure of the RuO2(110) surface in an O2 and CO environment: implications for the catalytic formation of CO2

The phase diagram of surface structures for the model catalyst RuO2(110) in contact with a gas environment of O2 and CO is calculated by density-functional theory and atomistic thermodynamics. Adsorption of the reactants is found to depend crucially on temperature and partial pressures in the gas phase. Assuming that a catalyst surface under steady-state operation conditions is close to a constrained thermodynamic equilibrium, we are able to rationalize a number of experimental findings on the CO oxidation over RuO2(110). We also calculated reaction pathways and energy barriers. Based on the various results the importance of phase coexistence conditions is emphasized as these will lead to an enhanced dynamics at the catalyst surface. Such conditions may actuate an additional, kinetically controlled reaction mechanism on RuO2(110).Comment: 12 pages including 8 figure files. Submitted to Phys. Rev. B. Related publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm

The surface structure of RuO2: A leed, auger and XPS study of the (110) and (100) faces

The (110) and the (100) surfaces of RuO2 show well defined LEED patterns. The surface lattice dimensions agree within 5% with the values predicted from X-ray diffraction. Both surfaces undergo a series of reversible surface reconstructions which result in well defined LEED patterns. These states come about from the loss of surface oxygen and each oxygen and each state is characterized by a well defined O Ru ratio. The final state is an overlayer of Ru metal. Furthermore, nearly equal concentrations of RuO3 are observed on all of the surfaces regardless of the structure which is present. This suggests that RuO3 plays a significant role in the stability of RuO2 surfaces

Local film thickness and photoresponse of thin anodic TiO2 films on polycrystalline titanium

Auger depth profiling was used to determine the local film thickness of a thin anodic oxide grown on a polycrystalline Ti substrate. The oxide thickness was studied as a function of substrate crystallography and final growth voltage. These results were related to local photocurrent measurements obtained using photoelectrochemical microscopy. Variations in the film thickness are too small to account for the non-uniform photocurrent response. The non-uniform photoresponse is instead attributed to variations in the defect density of the oxid

Surface properties of RuO2 + IrO2 mixed oxide electrodes

The surface properties of RuO2 + IrO2 mixed oxide layers deposited on Ti by thermal decomposition of the corresponding chlorides in aqueous solutions have been investigated by means of cyclic voltammetry and ex situ UHV (AES and XPS) techniques. The data obtained have shown that the surface is enriched with Ir and also that the bulk composition is likely to be inhomogenous. The surface composition estimated on the basis of the voltammetric curves has been found to agree excellently with the results of ex situ surface analysis. Finally, the samples have been found to "absorb" protons from the solution through a mechanism which does not interfere with the estimation of the electrochemically active surface area on the basis of the voltammetric charge

Anodic stability and electrochromism of electrodeposited ruthenium-iridium coatings on titanium

The anodic behaviour of galvanically plated ruthenium-iridium coatings on titanium was investigated. The dependence of the Tafel slopes and the stability of the coatings for the oxygen evolution reaction in sulphuric acid on the electrode composition was established. A change in the Tafel slope from 33 to 90 mV per decade on going from pure ruthenium to pure iridium was found. The presence of 40% Ir in the coating yielded 300 times higher stability in comparison with the coating of pure ruthenium. In addition, a square-wave pulsing between -0.25 and 1.25 V vs. the saturated calomel electrode produced an oxide film with electrochromic properties which were directly related to the charge enhancement factor of the grown oxide