AVS2001 Session EC-MoM: Surface Processes in Electrocatalysis

Monday, October 29, 2001 9:40 AM in Room 111

Monday Morning

Start	Invited?	Item
9:40 AM	Invited	EC-MoM-1 Surface Science Studies of Model Fuel Cell Electrocatalysts N. Markovic (Lawrence Berkeley National Laboratory)
10:20 AM		EC-MoM-3 The Surface Electrochemistry of Carbon Monoxide and Methanol on Solid Electrodes and Supported Catalysts C. Korzeniewski, D. Kardash-Richardson (Texas Tech University) The electrochemical oxidation of methanol and related small molecules has been of special interest in relation to fuel cell research. With improvements in the design of fuel cells that operate on methanol and hydrogen gas, there has been a great deal of interest in the chemical steps involved in the oxidation of methanol and its by-products on metal electrodes. In recent years, we have approached the study of methanol oxidation pathways with the use of electrochemical and spectroscopic analysis techniques. Dissociative chemisorption leading to adsorbed carbon monoxide (CO) is probed with surface infrared spectroscopy, formaldehyde is quantified with fluorescence spectroscopy, and the conversion of adsorbed CO to carbon dioxide is investigated with linear sweep voltammetry and potential step techniques. This presentation will describe studies of methanol electrochemistry on Pt and Pt-Ru alloy solid electrodes and supported nanometer-scale catalysts. In situ infrared measurements between ambient and 85 °C demonstrate methanol dissociative chemisorption is thermally activated on high Ru content bulk alloys. On solid Pt and Pt-Ru electrodes, formaldehyde can be an important by-product of methanol oxidation. However, under similar conditions the oxidation of methanol on nanometer-scale catalysts of Pt and Pt-Ru appears to be more complete. Attempts to correlate the properties of nanometer-scale metal particles and methanol oxidation pathways will be discussed.
10:40 AM		EC-MoM-4 Measurement of Elevated Temperature Rate Processes Involved in Methanol Electro-Oxidation on Supported Platinum Catalysts T.H. Madden, E.M. Stuve (University of Washington) Development of fuel cell catalysts for elevated temperature applications requires electrochemical techniques able to simulate these conditions. A dual electrolyte flow cell (DEFC) technique has been developed, capable of independent half-cell measurements of methanol oxidation and poisoning adlayer coverage in supported catalyst layers over a wide range of temperatures (25-100 °C) and pressures (0-2.5 atm gauge) at ~100% catalyst utilization while maintaining potential control at all times.¹ Recent measurements indicate that increasing the temperature from 50 to 100 °C at 0.35 V_PdH results in substantial increase in methanol electro-oxidation rates with a ~70 kJ/mol activation energy yet only slight variation in the measured adlayer charge. Tafel slope and stripping charge results at 100 °C indicate the onset of significant CO electro-oxidation rates at 0.33 V_rhe. High CO₂ yields measured at 100 °C and potentials less than 0.33 V_rhe indicate a possible role of CO thermal desorption or parallel-path oxidation. However, independent measurements of CO electro-oxidation, CO thermal desorption, and parallel-path oxidation are required to elucidate the respective role of these processes in elevated temperature methanol electro-oxidation. Results of such measurements will be presented for supported Pt catalysts using the DEFC technique augmented with pulsed reactant injection. ¹ T.H. Madden and E.M. Stuve, submitted to J. Electrochem. Soc. (2001).
11:00 AM		EC-MoM-5 Ruthenium, Osmium, and Palladium Modified Platinum Elecrodes: Surface Structure and Reactivity A. Crown, A. Wieckowski (University of Illinois at Urbana-Champaign) We have used Scanning Tunneling Microscopy (STM) to examine spontaneously deposited ruthenium and osmium adlayers on the well-defined Pt(111), Pt(100), and Pt(110) electrodes.^1,2 Clearly, ruthenium and osmium are deposited as arrays of surface islands, as in the case of ruthenium deposits obtained by electrolysis.³ Using STM, we have calculated ruthenium and osmium coverage values obtained by spontaneous deposition for various deposition times. The islands formed are mainly monoatomic--only a small fraction of the islands, depending on the Pt face, display a second monolayer deposit. Using in-situ STM, two-dimensional motions (or surface rearrangements) of highly structured islands of electrodeposited ruthenium, osmium, and palladium will be studied. The surface motions of platinum surface atoms, those of adsorbed carbon monoxide generated from methanol, and oxygen-containing species will also be investigated. We will attempt to address the surface mobility of the islands at various admetal coverage values. Examination, in situ, of the growth process will further elucidate the formation process, e.g., whether the islands tend to merge when the surface is exposed to a supporting electrolyte. Electrode potential will also be adjusted to examine the surface structure changes. The reactivity of these surfaces with respect to small organic compounds will also be examined. These experiments will lead us to the development of specific surface dynamics-reactivity relationships for the field of electrochemical surface science, with regards to surface poisoning phenomena and, in general, to electrocatalysis, including fuel cell catalysis. ¹ Crown, A., Moraes, I.R.; Wieckowski, A.; J. Electroanal. Chem., 2001, 500, 333. ² Crown, A.; Wieckowski, A.; PCCP, in press. ³ S. Cramm, K. A. Friedrich, K.-P. Geyzers, U. Stimming and R. Vogel, Fresenius J. Electroanal. Chem. 1997, 358, 189-192. .