Electrocatalysis of CO oxidation and the interfacial structure of the CO adlayer (COad) on the Pt(111) surface in 0.5 M H2SO4 were examined by using the rotating disk electrode method in combination with in situ surface X-ray diffraction measurements. The results presented here elucidate the roles played by two different forms of COad:  one which is oxidized at lower overpotentials, in the so-called preoxidation region, we characterize as a weakly adsorbed state (COad.w), and a strongly adsorbed state (COad,s) which is oxidized at higher overpotentials. The COad,w state forms at saturation coverage by adsorption at E < 0.15 V and assumes a compressed p(2 × 2) structure containing 3 CO molecules in the unit cell (0.75 CO/Pt). We propose that oxidative removal of COad,w is accompanied by simultaneous relaxation of the CO adlayer, and that the remaining COad (≈0.6 CO/Pt) assumes a new bonding state which we identify as COad,s. The COad,s state is present in a structure lacking long-range order. Despite the r...

Electrooxidation of CO and H2/CO Mixtures on Pt(111) in Acid Solutions

Potential dependence of the saturation CO coverage of Pt electrodes: The origin of the pre-peak in CO-stripping voltammograms. Part 3: Pt(poly)

Electrocatalysts by design: from the tailored surface to a commercial catalyst

The Pt(100)−CO interaction in aqueous electrolytes was examined by using rotating disk methods in combination with in-situ surface X-ray scattering (SXS) measurements. The analysis of the SXS results indicates that the topmost platinum atoms expand away from the second layer by ca. 4% when Hupd was completely displaced from Pt(100) by CO to form a saturated layer of CO. Assuming that gas-phase heats of adsorption for CO apply as well to the liquid−solid interface, we estimate that the Gibbs energy change for the displacement of Hupd by CO on Pt(100) is close to −90 kJ/mol. A Pt(100)−CO surface normal interlayer spacing of 1.4 ± 0.4 A was extracted from SXS measurements, suggesting that CO is adsorbed primarily at the 2-fold bridge-bonded sites, or possibly a mixture of bridge and atop sites. In contrast to the Pt(111)−CO system, no structures of COad with long-range order were formed on Pt(100). Two different forms of COad are formed at the Pt(100)−electrolyte interface:  the weakly adsorbed state which i...

Surface Electrochemistry of CO and H2/CO Mixtures at Pt(100) Interface: Electrode Kinetics and Interfacial Structures

The oxidation of carbon monoxide on a Pt(111) electrode surface is studied under transient and stationary reaction conditions in a 0.1 M HClO4 aqueous solution as a function of the applied electrode potential. The optical response is investigated by optical second harmonic generation (SHG) at 280 nm and infrared absorption spectroscopy (IRAS) starting at low potentials where the electrode is not reactive [e.g., 0.1 V vs the reversible hydrogen electrode (RHE)] towards more positive potentials where CO oxidation is initiated. For transient reaction conditions, in the absence of CO dissolved in solution, the oxidation starts at about 0.5 V vs. RHE and consists of a fast oxidation of about 10% of the adlayer and of a second reaction with slower kinetics involving the removal of the complete CO layer (overlayer stripping). Under steady-state conditions in CO-saturated solution the CO adlayer is stable up to 0.9 V vs RHE. At 0.63 V an overlayer phase transition is indicated by a 20% increase of the isotropic c...

Potential-dependence of CO adlayer structures on Pt(111) electrodes in acid solution: Evidence for a site selective charge transfer

The oxidation wave of CO preadsorbed at 50 mV on Pt(1 1 1), (1 0 0) and (1 1 0) electrodes in phosphate buffer solution of pH 3 was observed as a function of the sweep rate. The sweep rate dependence of the peak current and peak potential, as well as the form of the wave, were examined on the basis of the Gilman mechanism that the electron transfer from a complex consisting of CO and oxygen containing species is the rate-determining step. An electron transfer step from CO itself was excluded. The peak current and peak potential analyses and the wave simulation gave the same value for Δf, the change in the interaction energy during the formation of the activated complex from the reactants. Δf was sweep-rate and surface-structure dependent. The nature of Δf was discussed.

H. Narumi

Papers

Analysis of irreversible oxidation wave of adsorbed CO at Pt(1 1 1), Pt(1 0 0) and Pt(1 1 0) electrodes