T. D. Gladysheva

The use of galvanic displacement in synthesizing Pt(Cu) catalysts with the core-shell structure

The formation of a Pt(Cu) bimetallic catalyst on the carbon support by galvanic displacement of copper electrodeposits with platinum (PtCl62− as the displacing agent) is systematically studied. Composition, structure, and electrocatalytic properties of samples corresponding to different stages of copper displacement are analyzed. For substantially long displacement times, the formation of stable Pt(Cu)st particles with the atomic ratio Pt: Cu ≈ 7: 3 is observed. The Pt(Cu)st/C electrodes are shown to be close to the Pt/C electrode as regards the adsorption of hydrogen and copper atoms and the specific activity in methanol oxidation (with 0.5 M H2SO4 as the supporting electrolyte). Such electrocatalytic behavior of Pt(Cu)st particles makes it possible to infer the formation of the “core(Pt, Cu)-shell(Pt)” structure, as confirmed by the XPS data.

Experimental check-up of the relationship between transients of current and open circuit potential for strong adsorption of neutral species and ions on a hydrogen electrode

Abstract Transients of current and open circuit potential are measured during carbon monoxide adsorption on Pt/Pt, Rh/Pt, and Ir/Pt electrodes from sulfuric acid and hydrochloric acid solutions, as well as during the adsorption of iodine and iodide ions on Pd/Pt from sulfuric acid solutions. The integral values of transients are shown to agree with those expected theoretically. The possibility of estimating open circuit potential transients based on the data on current transients is analyzed. A combination of transient measurements with other adsorption methods makes it possible to show that in a number of systems the electrode surface is recharged as a result of formation of adsorbed monolayers and to reveal differences in the nature of adlayers formed in solutions of I2 and I−.

Peculiarities of the Pt(Cu)/C catalyst formation by galvanic displacement of copper in H2PtCl4 solutions

Specific features of the formation of a Pt(Cu) catalyst by the galvanic displacement of electroplated copper (carbon support) in a PtCl42− solution are considered. The composition, the structure, and electrochemical properties of Pt(Cu) deposits in different stages of displacement are studied by a complex of methods (SEM, TEM, XPS, voltammetry, etc.). The gradual formation of a stable “core(Pt, Cu)-shell(Pt)” structure with the average atomic ratio Pt : Cu ≈ 3 : 1 is observed. The results for PtCl42− are compared with the analogous results for PtCl62− published earlier. Particularly, the reasons for the differences in the steady state potentials established on the Pt(Cu)st/C electrode in the presence of PtCl42− and PtCl62− are discussed.

Electrocatalytic activity of platinum-polyaniline and palladium-polyaniline systems obtained by cycling the electrode potential

Steady-state current densities of electrooxidation of CH3OH, HCOOH, and CO at the Pt-PAN-GC electrodes (where PAN and GC stand for polyaniline and glassy carbon, respectively) and those of electrooxidation of HCOOH at the Pd-PAN-GC electrodes are measured (per cm2 of the true metal-catalyst surface area). The found higher activity of Pt and Pd particles incorporated in PAN, as compared with Pt/Pt and Pd/Pt, is attributed to interaction between metal-catalyst particles and the polymeric matrix. The activation effect is the most pronounced for the HCOOH electrooxidation at Pd-PAN-GC. The data concerning hydrogen evolution testify in favor of a decrease in exchange currents of this reaction upon going from Pt to Pt-PAN-GC electrodes

Electrochemical Behavior of Platinum-Modified Polyaniline Films in Sulfuric Acid Solutions of Carbon Monoxide

On the Pt–PAN–GC and PAN–Pt electrodes in H2SO4solutions that are first saturated with CO and then rid of it, during anodic scans of Er, oxidation peaks are observed at 0.85–0.95 V, which disappear only after repeatedly cycling Erin the interval 0.65 to 1.1 V. The effect is attributed to the capability of a PAN film to sorb considerable CO amounts. The sorption interaction is of a chemical nature. The electrooxidation of CO in the film is closely related to the PAN oxidation, which results in its destruction.

Determining magnitude of the carbon monoxide adsorption on electrodes of platinum metals

The possibility of using current transients and open-circuit potential for correctly calculating a double-layer correction for charges spent for electrooxidation of adsorbed carbon monoxide is considered. On the basis of electrochemical measurements of adsorption of CO on electrolytic deposits of Pt and Rh in solutions of H2SO4, H2SO4 + HCl, and HCl, an estimate of the magnitude of adsorption of CO is performed with allowance made for a correct double-layer correction. The obtained values are compared with similar literature data for smooth Pt and Rh, and possible reasons for the established differences are discussed.

Variations in the total charge and the open-circuit potential during the CO adsorption on a platinum electrode in chloride solutions

Transients of the current and open-circuit potential, which occur during adsorption of CO on a Pt/Pt electrode in 1 M HCl and 0.5 M H2SO4+ 0.1 M HCl solutions, are measured. The potential dependences of the total charge in the presence and the absence of a monolayer of COadsare constructed. At potentials of the double-layer region (up to the CO oxidation potentials), the integral current transients quantitatively conform to the earlier-developed theoretical notions on the total-charge variations in the case of strong adsorption of neutral species on a hydrogen electrode. The CO chemisorption makes the potential of zero total charge much more positive and leads to the surface recharge. The possibility of a quantitative description of shifts in the open-circuit potential during the CO adsorption is analyzed.

Concurrent oxygen reduction and methanol oxidation in sulfuric acid solutions on platinized platinum electrodes

Steady-state polarization curves are compared in solutions of 0.5 M H2SO4 + O2 (saturated), 0.5 M H2SO4 + (0.005–0.1) M CH3OH, and 0.5 M H2SO4 + (0.005–0.1) M CH3OH + O2 (saturated) on a Pt/Pt electrode. A considerable difference is found between the currents in mixed solutions and those expected based on the principle of additivity of currents in CH3OH and O2 individual solutions. The surface coverages with the CH3OH and O2 adsorption products are determined in the potential range of 0.2–0.9 V (RHE). Open-circuit potentials are measured in mixed solutions. The obtained results suggest that the direct heterogeneous interaction between methanol and oxygen occurs alongside with faradaic reactions. This is assumed to lead to a decrease in methanol electrooxidation currents at E ≥ 0.8 V and their increase at E ≤ 0.65 V.

Electroreduction of carbon monoxide on palladium electrodeposits in solutions containing copper ions

It is shown that the electroreduction of CO proceeds on electrolytic deposits of palladium (edPd) in 0.5 M H2SO4 + (1–5) mM CuSO4 + CO(sat) solutions at the potentials more positive than the Cu2+/Cu equilibrium potential. Among the CO reduction products, methanol and formaldehyde are identified. The current efficiency with respect to CH3OH exceeds 75% on edPd formed in 1% PdCl2+0.5 M H2SO4 solutions. In addition, Cu+ ions, which probably form complexes with CO, are detected in the solution and are assumed to play the role of intermediate species in the mediator catalysis along with copper adatoms.

Adsorption peculiarities of carbon monoxide on iridium electrodeposits

The transients of current and open-circuit potential observed at CO adsorption on Ir/Pt electrodes in solutions of sulfuric and hydrochloric acids are shown to adequately agree with those theoretically expected. From voltammetric curves of electrooxidation of chemisorbed CO, the values of its adsorption are estimated by making a “proper double-layer correction.” It is concluded that CO is preferentially adsorbed in the bridge form. During CO adsorption from sulfuric acid solutions on an electrode with a preliminarily accumulated monolayer of copper adatoms (Cuad), small negative current are observed and Cuad are displaced by ∼50%. To explain these results, it is assumed that CO is reduced by copper adatoms.