Branimir N. Grgur

Methanol electrooxidation on supported Pt and PtRu catalysts in acid and alkaline solutions

The kinetics of methanol oxidation on supported 47.5 wt.% Pt and 54 wt.% PtRu (with nominal Pt:Ru ratios of 2:3) catalysts are measured in 0.5 M H2SO4 and 0.1 NaOH at 295 and 333 K using thin-film rotating disk electrode (RDE) method. It was found that the activity of Pt and PtRu for methanol oxidation is a strong function of pH of solution and temperature. The kinetics are much higher in alkaline than in acid solution; at 333 K, a factor of 30 for Pt and a factor of 20 for Pt2Ru3 at 0.5 V. The pH effect is attributed to the pH competitive adsorption of oxygenated species with anions from supporting electrolytes. The activity of Pt and Pt2Ru3 catalysts at 333 K is higher (a factor of 5) than at 295 K. Irrespective of pH, only negligible differences in the kinetics are observed between Pt and on high Ru content Pt alloys, presumably owing to a slow rate of methanol dehydrogenation on the Ru-rich surface and insufficient number of Pt sites required for dissociative chemisorption of methanol.

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

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...

Investigation of Enhanced CO Tolerance in Proton Exchange Membrane Fuel Cells by Carbon Supported PtMo Alloy Catalyst

E-TEK, Incorporated, Natick, Massachusetts 01760, USAWe report a two- to threefold enhancement of CO tolerance in a proton exchange membrane (PEM) fuel cell, exhibited by carbonsupported nanocrystalline PtMo/C as compared to the current state of the art PtRu/C electrocatalysts. The bulk of these nanocrys-tals were comprised of Pt alloyed with Mo in the ratio 8.7:1.3 as shown by both X-ray diffraction and in situ extended X-ray ab sorp-tion fine structure measurements. Rotating disk electrode measurements and cyclic voltammetry in a PEM fuel cell indicate theonset of CO oxidation at potentials as low as 0.1 V. Further, the oxidation of CO exhibits two distinct peaks, indicating redox b ehav-ior involving oxyhydroxides of Mo. This is supported by in situ X-ray absorption near edge structure measurements at the Mo Kedge.© 1999 The Electrochemical Society. S1099-0062(98)08-029-8. All rights reserved.Manuscript submitted August 10, 1998; revised manuscript received September 28, 1998. Available electronically October 30, 1998.

Electrooxidation of H2, CO and H2/CO mixtures on a well-characterized Pt–Re bulk alloy electrode and comparison with other Pt binary alloys

We report new results for the electrooxidation of H 2 , CO and H 2 /CO mixtures on a Pt 75 Re 25 bulk alloy using surface preparation in a UHV chamber and characterization by X-ray photoelectron spectroscopy (XPS). Sputter-cleaned but not-annealed surfaces having the bulk composition were used. This alloy surface has unique properties for CO electrooxidation: finite current densities below 0.2 V (rhe), a positive reaction order in CO partial pressure of +0.5, but an unusually high slope in the log i vs E curve, about 220 mV per decade. Anodic stripping of the irreversibly adsorbed CO on this surface also occurs at an unusually low potential, initiating at about 0.15 V, but only about 1/3 of the total amount of CO on the surface is oxidized at 0.15-0.4 V, the remainder being oxidized at 0.6-0.8 V. The polarization curves for the oxidation of H 2 /CO mixtures containing 0.1-2% CO are, however, very Pt-like. It appears that the low-potential oxidation of both dissolved CO and adsorbed CO on this surface occurs exclusively at Re sites, so that with the H 2 /CO mixture the Pt sites remain fully covered (and thus deactivated) by CO ads .

The influence of polypyrrole films on the corrosion behavior of iron in acid sulfate solutions

The corrosion behavior of mild steel covered by electrodeposited polypyrrole films was investigated by electrochemical impedance spectroscopy. Electrodeposition of polypyrrole was performed from an aqueous solution of pyrrole and oxalic acid at a constant current density of 1 mA/cm2 and yields strongly adherent and smooth polymer layers. The non-linear least squares (NLS) fitting was applied to the impedance spectra to estimate the parameters of the proposed equivalent circuit, based on a physical model for the corrosion of mild steel protected by polypyrrole films in acid sulfate solutions.

Corrosion protection of mild steel by polypyrrole coatings in acid sulfate solutions

The corrosion behaviour of mild steel covered by the electrodeposited polypyrrole films was investigated by electrochemical impedance spectroscopy. Electrodeposition of polypyrrole was performed from aqueous solution of pyrrole and oxalic acid at a constant current density of 1 mA cm−2 and yields strongly adherent and smooth polymer layers. The impedance spectra were treated through the nonlinear least squares (NLS) fitting to estimate the parameters of the proposed equivalent circuit, based on physical model for the corrosion of mild steel protected by polypyrrole films in acid sulfate solutions.

Surface electrochemistry of CO on Pt(110)-(1 × 2) and Pt(110)-(1 × 1) surfaces

A new type of flame-annealing method has been developed for the preparation of surfaces having either fcc(110)-(1 × 1) or fcc(110)-(1 × 2) symmetry. In situ X-ray scattering studies show that both the Pt(110)-(1 × 2) and Pt(110)-(1 × 1) surfaces are stable in the potential region between 0 and 1.0 V. Adsorption of CO on the (1 × 2) surface in solution does not induce the (1 × 2)→(1 × 1) transition that is observed in UHV upon adsorption of CO. Nominally identical saturation coverages of hydrogen (Hupd) and COad are observed on the two surfaces, implying that two Hupd atoms or two COad molecules are adsorbed per unit cell of the reconstructed (1 × 2) surface. A significant increase in the interatomic spacing between the first and second layer of the reconstructed (1 × 2) surface occurs upon hydrogen adsorption to full coverage. Occupation of subsurface sites for Hupd are inferred from this result. The (1 × 2) surface is found to have a much higher catalytic activity for the electro-oxidation of CO; the anodic oxidation of either COad or dissolved COgas occurs at ≈ 0.3 V lower potential on the (1 × 2) than the (1 × 1) surface.

The determination of kinetics parameters of the hydrogen evolution on TiNi alloys by ac impedance

The kinetics and mechanism of hydrogen evolution at TiNi alloys electrode has been studied by means of steady-state potentiostatic polarization and ac impedance spectrum measurements. Complex-plane spectrum plots are derived for the h.e.r. at one of the most inactive, typical hydridic, TiNi, and one of the most active, typical electrocatalytic, TiNi3, intermetallic alloy compounds at various potentials. The rate constants of the forward and backward reactions of the corresponding steps were estimated by a nonlinear fitting method. It has been found that the overall reaction proceeds through a coupled Volmer-Heyrovsky mechanism. The electrocatalytic activity decreases in the order TiNi2 > TiNi3 > TiNi4 > Ti2Ni > TiNi > Ti3Ni > TiNi0.7.

Oxygen Reduction Reaction on Pt and Pt-Bimetallic Electrodes Covered by CO Mechanism of the Air Bleed Effect with Reformate

The oxygen reduction reaction (ORR) has been studied on CO-covered Pt(lll), Pt(110), Pt-poly, Pt-Ru, Pt 3 Sn(111), and Pt 3 Sn(110) electrodes in 0.5 M H 2 SO 4 solution at temperatures varying between 298 and 333 K using the rotating ring-disk electrode (RRDE) method. The RRDE measurements showed that even during O 2 reduction a substantial amount (40% of saturation) of CO still remains on all surfaces. Only a very small fraction of consumed O 2 is used for CO oxidation, while the major fraction reacts via the competing ORR, producing H 2 O 2 . The H 2 O 2 yield is strongly dependent on the fractional coverage by adsorbed CO, the surface structure, and the composition of the electrode. While at Θ C O > 0.5 monolayer (ML) the H 2 O 2 yield is very high (depending on the structure/nature of the electrode ca. 60-100%), when Θ C O falls to its steady-state value of 0.4 ML, the H 2 O 2 yield is almost the same as on the respective CO-free surfaces. Although the production of peroxide on some electrodes is rather low at 333 K (e.g., 5% on PtRu and 10% on Pt-poly), the practical consequences of the H 2 O 2 production can be significant; e.g., H 2 O 2 may cause an enhanced long-term degradation of the membrane.

Bi adsorption on Pt(111) in perchloric acid solution: A rotating ring–disk electrode and XPS study

The surface electrochemistry of reversibly and irreversibly adsorbed bismuth (Biad) was studied on Pt(111) electrodes in 0.1 M HClO4. The valence state of irreversibly adsorbed Bi (Biir) was determined by ex situ X-ray photoelectron spectroscopy (XPS). It was shown that Biir does not change its valence state during potential cycling and is adsorbed in its metallic (i.e., zero-valent) state. Underpotential deposition (UPD) of Bi (Biupd) onto Pt(111) was studied using the rotating ring–disk electrode (RRDE) technique for measurements of the Bi3+ ion specific flux. The total amount of Bi deposited at underpotential (ΘBi,upd) was determined by integration of the ion specific flux, and found to be ≈0.16 ML (1 ML≡1 Bi/1 Pt). The UPD Bi is assumed to deposit at bare Pt sites not occupied by the irreversibly adsorbed Bi. The difference between ΘBi,upd and the maximum coverage of a close-packed monolayer of fully discharged Bi adatoms (ΘBiad=0.56 ML) is taken to equal the coverage by Biir, or ≈1/3 ML. The charge under the reversible peak in the Pt(111)/Biir voltammetry (Q=160 μC cm−2) for ΘBi,ir∽1/3 ML is ascribed to enhanced adsorption of OH on Pt sites adjacent to Bi due to a change of the local potential of zero charge (p.z.c.) induced by Bi. Contrasting kinetic effects of Biir were observed on the hydrogen and carbon monoxide oxidation reactions. These effects are discussed in terms of the known role of OHad in these reactions on the Pt(111) surface.