PtO Formation Research Articles

Kinetic Monte Carlo Simulations of Electrochemical Oxidation and Reduction of Pt(111)

Kinetic Monte Carlo simulations of electrochemical oxidation and reduction are presented that match many features of the experimentally observed electrochemical and morphological response of Pt(111). Included in the simulation are all relevant microscopic transitions, including the formation of Pt-OH and Pt-O from Pt, surface diffusion of all three species, as well as an effective place exchange diffusion at high potential. A detailed description of this approach to modeling such a complex surface is also presented. Overall, it is found that many features of the Pt(111) CV, including hydroxylation, hysteresis, and surface roughening, can be correlated to events associated with n-coordinated surface species, such as the hydroxylation wave corresponding to a one-electron oxidation of 9-coordinated terrace sites. Oxidation to Pt-O species at potentials above 1.0 V are shown to correlate to the presence of growing surface roughness, and the simulations suggest the onset of Pt-O formation in steady-state cyclic voltammetry is dominated by the oxidation of 8-coordinated step edges rather than terrace sites. Implications for the stability of Pt(111) catalysts after thousands of voltammetric cycles are discussed.

Probing the correlation between Pt-support interaction and oxygen reduction reaction activity in mesoporous carbon materials modified with Pt-N active sites

Pt nanoparticles (NPs) on nitrogen-functionalized mesoporous carbon have proved to be interesting materials for oxygen reduction reaction. In this paper, we employed a new synthetic route to simultaneously deposit Pt NPs and introduce N-doping in the carbon support, by using dichloro(1,10-phenanthroline)platinum(II) or dichloro(2,2′-bipyridyl)platinum(II) as a precursor of both platinum and nitrogen. Pt NPs (average size 2.5 nm) were successfully synthesized by solid state reduction of Pt(II) under N2/H2 flow at 650 °C on a commercial mesoporous carbon. The synthesis was also carried out by employing PtCl2 or Pt(acac)2 as platinum precursors and 1,10-phenanthroline or 2,2′-bipyridyl as nitrogen dopant agents and the resulting materials were taken for comparison. XPS analysis confirmed that during the thermal treatment the ligand degrades and the nitrogen is embedded in the mesoporous carbon structure, resulting in a surface modification of the carbon support with preservation of bulk conductivity and thermal stability. Electrochemical characterization at a rotating disk electrode in O2-saturated 0.1 M HClO4 revealed a superior catalytic activity for ORR, in terms of E1/2 and mass activity, in those catalysts showing higher PtN interaction, expressed as binding energy shift of nitrogen components. Furthermore, cyclic voltammetry analysis in Ar purged 0.1 M HClO4 electrolyte showed that nitrogen doping appears to inhibit PtO formation. The influence of nitrogen incorporation in the carbon matrix on Pt NPs/support interaction was also rationalized on the basis of density functional theory simulations.

Formation of Pt Skin Layer on Ordered and Disordered Pt-Co Alloys and Corrosion Resistance in Sulfuric Acid

The formation of the Pt-enriched layer (Pt skin layer) formed at the surface of the ordered and disordered Pt-Co alloy specimens by a dealloying treatment and its corrosion resistance under potential cycling in sulfuric acid were examined to clarify the dissolution behavior of the Pt skin layer in polymer electrolyte fuel cell-operating conditions. The ordered and disordered Pt-Co specimens were obtained by heat treatment at 1073 and 1473 K, respectively. Co at the alloy surfaces dramatically dissolved in an early phase of the dealloying treatment in naturally aerated 0.5 M H2SO4 at 298 K, and Pt skin layers were formed. Pt skin layers ca. 2 monolayer in thickness were formed on the Pt-Co alloy specimens by the dealloying treatment in 0.5 M H2SO4. The cyclic voltammetry measurements of the Pt-Co specimens showed the existence of Pt skin layers after the dealloying treatment and the inhibition of the Pt oxide formation on the Pt skin layers. The Pt oxide formation for ordered Pt-Co was more suppressed than that for the disordered Pt-Co. The Pt skin layers on the Pt-Co specimens exhibited a higher corrosion resistance than pure Pt, and the dissolution of the Pt skin layer for ordered Pt-Co was more inhibited than that for disordered Pt-Co under potential cycling in the potential range of 0.6–1.4 V in 0.5 M H2SO4 (dissolution test). A thin, continuous Pt skin layer remained at the surface of the ordered Pt-Co specimen after the dissolution test. The formation of a uniform Pt skin layer seems to provide high oxidation resistance to the surface, leading to high corrosion resistance.Graphical The dissolution of Pt skin layer for the ordered Pt-Co was highly suppressed compared with that for the disordered Pt-Co and pure Pt. The suppression of Pt-O formation seems to be the main reason for the high corrosion resistance. The formation of a thin and continuous Pt skin layer is important to prevent the dissolution of Pt.

Establishing the potential dependent equilibrium oxide coverage on platinum in alkaline solution and its influence on the oxygen reduction

The oxidation process of polycrystalline platinum subjected to alkaline solution is re-examined using a combination of cyclic voltammetry and potential hold techniques in Ar, H2 and O2 purged 0.1 M KOH solution. The hydrogen oxidation reaction is used as a probe for forming a complete OH adlayer, whereas the comparison of cyclic voltammetry with potential hold techniques allows the investigation of the dynamics as well as a charge balance of the processes. Comparing the different findings in a parameterized form, we are able to discern the potential regions of OH and O adlayers as well as PtO formation at quasi steady state conditions: only low amount of OH exists up to 0.7 VRHE. Increasing the potential the OH coverage increases and a OH monolayer is formed at 0.93 VRHE, which gradually further oxidizes to Oad increasing the potential up to 1.08 V, above which PtO oxide is formed. Comparing the oxidation process to the oxygen reduction reaction it is shown that there is a clear correlation between the formation of a full OH adlayer and the inhibition of the oxygen reduction.

Electrochemical Atomic Layer Deposition (E-ALD) of Pt Nanofilms Using SLRR Cycles

This paper describes platinum nanofilm formation via the electrochemical form of atomic layer deposition (E-ALD), where the E-ALD cycles are based on surface limited redox replacement (SLRR) reaction. SLRR is where an atomic layer (AL) of a reactive (sacrificial) metal is exchanged for a more noble metal. In the present study both Cu and Pb AL were investigated as sacrificial atomic layers for replacement with both Pt(II) and Pt(IV) precursors. 25 E-ALD cycles were used to form Pt nanofilm deposits. Initial deposits contained an order of magnitude more Pt than expected, evidenced by a factor of 7 increase in surface roughness. Overly positive potentials achieved during the exchange promoted excess deposition and surface roughening. It is proposed that anionic Pt precursors adsorbed more strongly at high potentials, making them difficult to rinse from the cell. Those remaining adsorbed Pt anions are then reduced to Pto when the potential was shifted negative for deposition of the sacrificial element. The result was Pto formation at a large overpotential, which, contributed to excessive Pt deposits and roughening. Increased rinsing of the anionic Pt precursors from the cell eliminated the excess Pt deposition and roughening, resulting in the expected layer by layer growth of an ALD process.

CO electrooxidation on a polycrystalline Pt electrode: A wall-jet EQCN study

Abstract Pre-adsorbed and bulk (continuous) CO oxidation on a polycrystalline Pt electrode were examined in a wall-jet electrochemical quartz crystal nanobalance (EQCN) setup, using both differential and integral evaluation of the EQCN data, to get further insights into the kinetics and mechanism of this important fuel-cell related electrocatalytic reaction. The hydrogen underpotential adsorption–desorption features in the base cyclic voltammogram of a Pt film are accompanied by significant changes in the electrode mass due H-upd induced desorption–adsorption of anion. In the double-layer region small capacitive currents are accompanied by comparatively large reversible mass changes indicating anion adsorption/desorption (96.5 g mol−1 assigned to bisulfate). OH and oxygen electrosorption from water at potentials more positive of 1.0 V result in relatively small variations in the electrode mass (16 g mol−1 for PtOH and ca. 9 g mol−1 for PtO formation, respectively). The CO-adlayer stripping first leads to the electrode mass decrease in the “pre-peak” region, followed by a fast mass increase within the main stripping peak due to re-adsorption of bisulfate anion (91 g mol−1). A mass-transport limited current for bulk CO oxidation under continuous flow of CO-saturated electrolyte leads to negligible mass changes (0–1 g mol−1) in the PtO region, suggesting that bulk CO oxidation is mediated by electroformed PtO.

Microgravimetric and voltammetric study of Zn underpotential deposition on platinum in alkaline medium

The microgravimetric and voltammetric responses of a polycrystalline Pt electrode in 0.1 mol L −1 NaOH solution in the presence and the absence of 1.6 × 10 −4 Zn 2+, at 0.1 V s −1 were analyzed. During the positive potential sweep, the water molecules are progressively substituted by OH − ions, prior to PtO formation. The voltammetric charges obtained under the Zn ads dissolution peaks suggest that 0.7 monolayers are deposited, with each Zn ad-atom occupying one active site and transferring two electrons. The total loss of mass due to the dissolution of the Zn ad-layer was 136.6 ng cm −2 and the mass increase due to PtO formation was found to be only 12 ng cm −2 less than the theoretical ones, 157.5 and 37.4 ng cm −2, respectively, indicating that both processes are overlapped. In this way it is proposed that an adsorbed by-layer of Zn and OH − ions is formed.

Activated pulsed amperometric detection of cysteine at platinum electrodes in acidic media

AbstractAnodic voltammetric detection of sulfhydryl compounds, represented here by cysteine, at Pt electrodes in acidic media occurs concomitantly with anodic generation of surface oxide. It is speculated that the adsorbed hydroxyl species (PtOH), proposed to be the intermediate product in formation of the inert oxide (PtO), is the source of O‐atoms transferred to the sulfonic acid produced by oxidation of the sulfhydryl moiety. However, as a consequence of this electrocatalytic response mechanism, application of the traditional three‐step waveform for pulsed amperometric detection (PAD) of sulfhydryl compounds is accompanied by a large background signal resulting from PtO formation. To diminish the background signal, the traditional three‐step PAD waveform has been modified by insertion of a brief (100 ms) anodization step prior to the detection step to activate the Ptsurface by the generation of PtOH with minimal conversion to PtO. The subsequent negative step from the activation potential (Eact) to the detection detection potential (Edet < Eact) permits the PtOH to be harvested within the detection mechanism without significant background signal.

Anodic Oxidation of Ethylenediaminetetraacetic Acid on Platinum Electrode in Alkaline Medium

The anodic oxidation of ethylenediaminetetraacetic acid (EDTA) was studied in alkaline medium on a smooth platinum electrode. Bulk electrolysis indicated that stable organic intermediates (formaldehyde and glyoxal) are formed during the oxidation of EDTA and that complete oxidation to can be achieved. The proposed pathway suggests that the acetate groups in EDTA are initially oxidized, generating formaldehyde and ethylenediamine. The rest potential of EDTA (0.066 to 0.164 V vs. Hg/HgO) was observed to be higher than for other organic species. In alkaline medium, very little EDTA oxidation was found to occur on bare platinum. Limiting‐current behavior due to PtO formation was observed immediately positive of the rest potential. Tafel behavior (Tafel slope 120 mV/dec) was observed in the potential region positive of the cessation of the bulk of oxide film formation and negative of the onset of evolution. The reaction order of EDTA was determined to be ∼0.5, and that of was close to zero. The reaction mechanism consistent with the experimental data involves Temkin‐type adsorption and a first‐electron‐transfer rate‐determining step.

An improved heat-responsive electrode for the measurement of electrochemical peltier heat: The peltier heat for electrosorption and electrodesorption of oxygen on a platinized platinum electrode in sulfuric acid solution

A new plate-type electrode with high sensitivity and responsive to temperature change was made using a thick film thermistor. The electrochemical Peltier heat for the oxygen surface process at a platinized platinum electrode in 0.5 M H 2SO 4 solution was measured with this electrode by potentiodynamic and galvanostatic transient techniques. It was demonstrated that most of the Peltier heat is caused by the overpotential due to the irreversible oxygen electrode reaction. That is, a Tafel-type relations between the Peltier heat and the current was confirmed. A step-wise heat change corresponding to consecutive stages of platinum lattice occupation by OH was observed. The amount of heat evolved on PtO formation was apparently larger than that on PtOH formation. The results were compared with those obtained by the voltammetric measurement.