Zweielektroden-dünnschichtversuche zur Pb-adsorption an platin

The catalysis and inhibition of formic acid electro-oxidation that may accompany metal-submonolayer formation on polycrystalline platinum was studied quantitatively using the rotating ring-disk electrode (RRDE) technique. Under controlled flux conditions one can attain limiting current behavior for the oxidation of formic acid and thereby determine the upper limit for the optimum adatom coverage for lead and bismuth. These values are found to be 30-35% of the real area of the platinum electrode. The electro-oxidation experiments in this paper are correlated with the literature dealing with polycrystalline and single-crystal studies of hydrogen adsorption and formic acid oxidation at platinum to explain differences of the behavior of various underpotential deposited (UPD) species. The poisoning of the electro-oxidation process of formic acid requires the presence of adsorbed hydrogen or unhindered access to the first adsorbed intermediate C. OOH that is produced. During the initial stages of UPD, submonolayers of silver or copper do not deposit significantly on Pt(100), the plane that is responsible for most ofthe electro-oxidation offormic acid. Therefore, UPD silver or copper cannot inhibit poisoning of the formic acid oxidation process by decreasing hydrogen adsorption, or restricting access to C OOH via a third body mechanism. Bismuth, lead, and other UPD species with radii larger than platinum do deposit significantly on Pt(100) at low UPD coverages, and can therefore slow the poisoning of the formic acid process on this plane.Research on the oxidation of formic acid on nobIe metals and its electrocatalysis has been going on for more than a century (1-3). Interest in this process has gained considerable momentum in the past two decades due to the process's significance in the fuel cell technology and because the simplicity of the formic acid molecule provides a model for the electro-oxidation of other organic fuels. A formic acid oxidation mechanism that describes the principal experimental results involves, as the initial stage, an oxidative electrosorption step (4, 5) resulting in the weakly adsorbed carboxyl adradial, COOH, and H +. The carboxyl adradial is bonded to ~ne electrode site and may then either be electro-oxidized directly to CO2 and H + or react with other adsorbed species on the platinum surface and transform into organic poisons. These poisons inhibit further oxidation of formic acid by blocking those reaction sites required in the initial stage of the oxidation. The similarity between adsorption products produced from formic acid and methanol oxidation under potentiodynamic (4) and potentiostatic (6, 7) control, and that the initial interaction of CH3OH with an activated electrode results in the loss of the three hydrogens on the carbon atom (8), led Capon and Parsons to deduce that the chemical composition of the major poisoning species in formic acid oxidation is COH, which is strongly adsorbed at three adjacent electrode sites. A minor poisoning species, with the structure C(OH)e occupyin...

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“…6. This isotherm agrees with the isotherm reported by Schmidt and Wuthrich (35), who employed the thin layer-twin electrode technique.…”

Section: Irreversible Deposition Of Bismuth On Platinummsupporting

confidence: 91%

“…and Wuthrich(35), who employed the thin layer-twin electrode technique.Limi$ing currents for the ]ormic acid oxidation at an RDE.--According toCapon and Parsons (4), in the region where Had is thermodynamically unstable, the…”

mentioning

confidence: 99%

Adatom Coverage Dependences and Specificity Effects on the Rate of Formic Acid Electro‐oxidation at Polycrystalline Platinum

Shabrang

Mizota

Bruckenstein

1984

J. Electrochem. Soc.

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“…In the presence of Pb 2þ the onset of Pb upd overlaps to a significant extent with the oxide reduction waves. [31][32][33][34] Furthermore, Pb 2þ co-adsorption with PtO x has also been described; 31,34 recent reports of various Pt 3Àx Pb x O 4 phases 35 may stimulate further exploration of multicomponent oxide formation in the present system where the cationic species come from both solution and solid state precursors. In pH ¼ 3, 0.05 mol/l Pb 2þ electrolyte the reversible potential for lead deposition is expected to be 0.013 V more positive than that for the proton reduction.…”

Section: Resultsmentioning

confidence: 80%

Electrodeposition of Pt_100−xPb_x Metastable Alloys and Intermetallics

Hwang

Bonevich

Kim

et al. 2011

J. Electrochem. Soc.

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The electrodeposition of a series of metastable Pt-Pb alloys and intermetallic phases as well as elemental Pt is demonstrated using an acid electrolyte comprised of 0.05 mol/l Pb(ClO4)2 and/or 0.001 mol/l K2PtCl4. Pt-Pb films were deposited at various potentials relative to the reversible potential for Pb/Pb2+ (EPb/Pb2+ = −0.80 V SSE). A metastable fcc Pt-Pb solid solution is formed at potentials between −0.2 and −0.78 V SSE. A monotonic increase in the fcc lattice parameter with decreasing potential corresponds to a rise in Pb content that spans the composition range from Pt to beyond Pt3Pb. The intermetallics, PtPb, PtPb4 and elemental Pb form at more negative potentials. The films are single or multiphase depending on the growth potential and substrate. Thermal annealing leads to phase separation of the deposits into the respective equilibrium intermetallic phases whose volume fractions enable the overall film composition to be determined. At more negative potentials, between −0.79 and −0.82 V SSE, the ordered hexagonal PtPb intermetallic phase is directly formed by electrodeposition. Co-deposition of Pt100−xPbx at potentials positive of −0.8 V SSE (EPb/Pb2+) occurs by a combination of Pb underpotential deposition with overpotential Pt deposition.

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“…The horizontal shift of these curves is correlated to the deviation of ZE from Z ---F2. Pt/Pb + + and Bi/Pb + + systems.--In these systems Schmidt and WCithrich (37,90) found the formation of alloys in the underpotential range. Moreover, the sorption process on the platinum electrode is not reversible.…”

Section: Resultsmentioning

confidence: 98%

“…They cannot predict anything about the arrange-ment of the adsorbed species. The formation of multi~ atomar metal layers or multiatomar islands in the underpotential range was supposed by Schultze (49), Thirsk (75), Breiter (73,74), Wfithrich (35,37,90), and Kolb (85). The formation of such instable prenuclei would be very interesting in considering the nucleation phenomena of the compact metal at potentials which are more negative than the equilibrium potential e0,Me/Mez+.…”

Section: Structure Of Metal Filmsmentioning

confidence: 95%

The Formation of Monolayer Metal Films on Electrodes

Lorenz¹,

Hermann²,

Wüthrich³

et al. 1974

J. Electrochem. Soc.

170

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The two-and three-dimensional nucleation and the metal ion adsorption process are discussed as initial steps for electrocrystallization on the same or on a foreign substrate. The metal ion adsorption takes place in the underpotential range, which is more positive than the equilibrium potential of the nucleated metal/metal ion electrode. Thermodynamic considerations show that an overlapping of disturbing processes in the underpotential range can be characterized by a factor ZE, which is called charge-coverage coefficient or electrosorption valency. It is a function of different independent variables. The metal monolayer model is satisfied only in the simplest case for ZE :-Z (valency of metal ions in solution).The different experimental methods for determining the thermodynamics, kinetics, and structure of metal ion adsorbates are critically discussed. The most reliable thermodynamic results are obtained by the twin-electrode thinlayer technique. Kinetic data can be obtained by pulse measurements under potentiostatic-galvanostatic conditions. The structure of metal films can be determined by combining different methods including optical investigations.Results are given for different systems. In the systems Au/Ag +, Au/T1 +, and Au/Pb + + the metal monolayer model is practically valid. In the case of Cu + + ion adsorption on Au the redox reaction forming Cu + ions acts as a disturbing process. A cosorption of anions takes place in the system Ag/Pb + +. The formation of alloys in the underpotential range was observed in the systems Pt/Pb + + and Bi/Pb + + The structure of metal ion adsorbates must be in correlation with the nucleation phenomena at more negative potentials. In many systems no three-dimensional nucleation overvoltage is observed by using a quasi-steadystate experimental technique. The density of the crystal imperfections and the crystallographic orientation of the surface planes influence the metal ion adsorption process and the structure of films. The results obtained in the systems Ag (poly-and monocrystalline)/T1 + can be explained by assuming the formation of superlattices on the electrode surface.Kinetic investigations in the systems Au/Ag +, Au/T1 +, and Au/Pb § + show relatively high exchange current densities for the metal ion adsorption process. In these cases the experimental results can be explained by assuming surface diffusion of adions as the rate determining step in the adsorption process. In the system Au/Cu + +, a charge transfer step is rate determining, which probably forms precedingly Cu + ions.Crystallization phenomena play an important role in the case of cathodic metal deposition. They include different steps after the metal ion has passed through the electrochemical double layer and is still partially solvated. The final state can be described as the in-* Electrochemical Society Active Member. ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 205.208.120.206 Downloaded on 2014-12-26 to IP ) unless CC L...

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Zweielektroden-dünnschichtversuche zur Pb-adsorption an platin

Cited by 36 publications

References 17 publications

Adatom Coverage Dependences and Specificity Effects on the Rate of Formic Acid Electro‐oxidation at Polycrystalline Platinum

Adatom Coverage Dependences and Specificity Effects on the Rate of Formic Acid Electro‐oxidation at Polycrystalline Platinum

Electrodeposition of Pt_100−xPb_x Metastable Alloys and Intermetallics

The Formation of Monolayer Metal Films on Electrodes

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Zweielektroden-dünnschichtversuche zur Pb-adsorption an platin

Cited by 36 publications

References 17 publications

Adatom Coverage Dependences and Specificity Effects on the Rate of Formic Acid Electro‐oxidation at Polycrystalline Platinum

Adatom Coverage Dependences and Specificity Effects on the Rate of Formic Acid Electro‐oxidation at Polycrystalline Platinum

Electrodeposition of Pt100−xPbx Metastable Alloys and Intermetallics

The Formation of Monolayer Metal Films on Electrodes

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Product

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Electrodeposition of Pt_100−xPb_x Metastable Alloys and Intermetallics