The electrochemical deposition of Ru on Pt(111) electrodes has been investigated by electron diffraction, Auger spectroscopy, and cyclic voltammetry in a closed UHV transfer system. At small coverages Ru formed a monatomic commensurate layer, at higher coverage mostly small islands with a bilayer height were detected. When the Pt was almost completely covered by Ru, three-dimensional clusters developed. The island structure of Ru changed upon electrooxidation of CO, reflecting an enhanced mobility of Ru. Adsorption and electrooxidation of CO have been studied on such Ru-modified Pt(111) electrodes using cyclic voltammetry and in situ FTIR spectroscopy. Compared to the pure metals, the Ru−CO bond is weakened, the Pt−CO bond strengthened on the modified electrodes. The catalytic activity of the Ru/Pt(111) electrode toward CO adlayer oxidation is higher than that of pure Ru and a PtRu alloy (50:50). It is concluded that the electrooxidation of CO takes place preferentially at the Ru islands, while CO adsorbed on Pt migrates to them.

No abstract

No abstract

The parameters entering the kinetics for the mechanism of catalytic CO oxidation have been adapted for a Pt ( 110) surface, giving rise to a two-variable model correctly predicting bistability. Oscillations are obtained when, in addition, the adsorbate-driven 1 X2-1 Xl structural phase transition ofPt ( 110) is taken into account. Mixed-mode oscillations can be qualitatively explained by including the faceting of the surface as a fourth variable. The limitations of the model essentially stem from the fact that only ordinary differential equations have been analyzed so far neglecting spatial pattern formation. It is discussed which dynamic phenomena observed experimentally in the CO oxidation on Pt( 110) will probably not be adequately describable without taking spatial effects into account.

The experimental characterization of the current/outer potential ͑I/U͒ behavior during the electrochemical CO oxidation on Pt͑100͒, Pt͑110͒ and Pt͑111͒ is used as the first step towards a thorough investigation of the processes occurring during the electrochemical formic acid oxidation. The CO study is followed by new cyclovoltammetric results during the electrochemical formic acid oxidation on the corresponding Pt single crystals. At high concentrations of formic acid, the cyclovoltammograms revealed a splitting of the large current peak observed on the cathodic sweep into two peaks whose dependence on scan rate and reverse potential was investigated. It turned out that the presence of a sufficiently large ohmic resistance R was crucial for oscillatory instabilities. Given an appropriate resistance, all three Pt surfaces were found to exhibit current oscillations at both low and high formic acid concentrations. On Pt͑100͒ stable mixed-mode oscillations were observed. In addition, the sensitivity of the oscillations to stirring was investigated. Whereas the period-1 oscillations were found to be independent of stirring, the mixed-mode oscillations transformed into simple oscillations with stirring. The mechanism giving rise to instability and oscillations is described.

No abstract

cxperimental and simulated normalized current. The simulated normalized current i / i 0 was generated with k, = 1 . O cm s-l, a = 0.5. and D = 2.4 X IO-' cm2 s-I.IoS l~l f i u r d Spcctrioir of N02BF, in Solution. In a Schlenk flask, a 0. I M solution of NO?+BFJ-in acetonitrile was made up under an argon atniosphcrc. The Schlenk flask was connected to a flowthrough AgCl ccll (0.1-mm Teflon spacer) using a 20-gauge Tcflon tubing. Another Schlenk flask was connected to the outlet (109) See also discussion by Bard and Faulkner in ref 33. The Fortran program is availablc on rcqucst. of the cell by using a 20-gauge Teflon tubing. The system was thoroughly flushed with argon. With the cell placed directly in the beam of infrared spectrometer, the sample flask was pressurized with argon, and the sample was slowly injected (20 m L min-I) into the IR cell. At a constant flow of the sample solution, the 1R spectrum was measured every 5 s o n a Nicolet IO DX FT spectrometer with 2 cm-l resolution. Acknowledgment.A general approach is presented briefly to the mechanistic characterization of chemical oscillatory reactions, based on a ncw operational classification of simple chemical oscillators, i.e., oscillators that contain only one source of instability (autocatalysis), and categorization of their species. We use stoichiometric network analysis to classify oscillatory reactions according to their basic unstable feature and the type of their dominant negative feedback loop. The species are first categorized into those essential and nonessential for the Occurrence of oscillations. The essential species are further divided into subcategories according to thcir rolcs in the mcchanism. The suggestcd proccdurc includes operational criteria for the assignment of a chemical oscillator to onc of the defined categories of mechanisms and for the identification of the roles of the species. Altogether 25 abstract modcls and realistic mechanisms of simple oscillators have been investigated; all fit into the four defined categories of incchanisms. Thc classification and proccdurcs prescntcd briefly here arc fully devcloped in an article to appear in Ado. C'hefH. PhYJ.

The rate of catalytic CO oxidation on Pt(100) and (110) surfaces at low pressures (≤10−4 Torr) and under isothermal conditions may exhibit sustained temporal oscillations which are coupled with periodic transformations of the surface structures between reconstructed and nonreconstructed phases, the latter exhibiting higher oxygen sticking coefficients and hence higher reactivity. With Pt(100) the two surface phases exhibit a much larger difference in reactivity (=oxygen sticking coefficient) than with Pt(110), which effect accounts for the qualitative differences in the oscillatory behavior: if two of the control parameters (say pO2, T) are kept fixed, the third (pCO) may be varied with Pt(100) over a fairly wide range without leaving the oscillatory region. Minor (<1%) fluctuations of the partial pressures associated with the varying reaction rate are hence without any noticeable effect. Coupling between surface reaction and diffusion causes wave propagation of the surface phase transformations and therefore spatial self-organization, as demonstrated by scanning LEED experiments. With Pt(110), on the other hand, the oscillatory region is very narrow. In this case mass transport through the gas phase as caused by the small pressure variations associated with the reaction lead to synchronization between different parts of the surface. Computer simulations with the cellular automaton technique confirm qualitatively the experimental findings and support the conclusions reached.