Mechanism of interaction of molecular hydrogen with adsorbed oxygen on platinum electrodes under open circuit conditions

Transients of open-circuit potential observed at the reaction of hydrogen molecules with oxygen preliminarily adsorbed (O_ads) on the smooth polycrystalline (pc Pt) and platinized platinum (Pt/Pt) electrodes are measured under conditions of controlled stirring of solution (0.5 M H₂ SO₄). The dependence of the surface coverage with O_ads(θ_O) on the potential in the cause of the potential decay on pc Pt are determined. It is found that for Pt/Pt, the reaction kinetics is largely determined by diffusion of H₂. For pc Pt in the range of high θ_O, the Eley-Ridiel mechanism is realized. For medium θ_O, the regions where the reaction obeys the mechanisms of Eley-Ridiel, “conjugated reactions”, and diffusion control are observed to overlap (even at the most intense stirring possible). The rate of H₂ reaction with O_ads is substantially higher compared with analogous reactions of CO, HCOOH, and CH₃OH.     

Transients of the Open-Circuit Potential Observed during the Interaction of Formic Acid with Preliminarily Adsorbed Oxygen on a Platinized Platinum Electrode

Transients of the open-circuit potential, which are observed during the interaction of formic acid with preliminarily adsorbed oxygen (O_ads) on a Pt/Pt electrode in 0.5 M H₂SO₄, are measured. It is established, by means of the method of cathodic potentiodynamic pulses, that the slowest interaction of formic acid with O_ads occurs in the region of large coverages of the electrode surface by oxygen (θ_O ∼1–0.8). A presumption is put forward that the process rate in this region is defined by a direct reaction of O_ads with molecules of formic acid from the bulk solution. It is shown that the interaction of formic acid with O_ads in the region of intermediate coverages (θ_O ∼ 0.8–0.2) proceeds via a mechanism of “conjugated reactions.” Transients of the open-circuit potential for formic acid are compared to transients for carbon monoxide obtained in analogous conditions. The substantially shorter overall time of potential decay in the case of CO (at the same concentrations) is caused by a faster reaction of CO with adsorbed oxygen in the region of large θ_O. The difference is explained by assuming that the HCOOH adsorption as opposed to CO bears a dissociative character.__________Translated from Elektrokhimiya, Vol. 41, No. 8, 2005, pp. 936–942.Original Russian Text Copyright © 2005 by Manzhos, Maksimov, Podlovchenko.     

Electrochemical Condensation of Methylviologen on Specifically‐adsorbed Anion Layers

The adsorption of methylviologen dications (MV²⁺) on single‐crystalline Au electrodes in both H₂SO₄ and HClO₄ was examined. MV²⁺ strongly interacted with sulfate and bisulfate anions adsorbed on the Au(111) electrode surface in 0.05 M H₂SO₄ under a controlled potential of 1.25 V vs. the reversible hydrogen electrode (RHE). A characteristic non‐Faradaic current was observed at 1.10 V vs. RHE. When adsorption of MV²⁺ was carried out in 0.1 M HClO₄, the electrochemical response of MV²⁺ was less than that obtained in H₂SO₄. The results show that the formation of a highly ordered sulfate/bisulfate adlayer plays an important role in the formation of condensed MV²⁺ layers. Examination of polycrystalline Au and Au(100) electrodes revealed a poor electrochemical response due to the surface roughness of the Au substrate, but the electrochemical detection was applicable to polycrystalline Au electrodes. A systematic investigation of the structural dependency of viologen derivatives showed that molecular size is important for electrostatic interactions with a highly ordered sulfate/bisulfate adlayer. The findings of the present study demonstrate successful detection of MV²⁺ at a concentration of ≤1 pM with a non‐Faradaic current.     

Transients of the Open-Circuit Potential during Carbon Monoxide Interaction with Oxygen Preliminarily Adsorbed on Smooth and Platinized Platinum Electrodes

Variations of potential E in time , observed during the carbon monoxide interaction with preliminarily-adsorbed oxygen O_ads on smooth and platinized platinum electrodes under open-circuit conditions (supporting electrolyte 0.5 M H₂SO₄), are measured. The potential decay rate on smooth Pt is more than ten times that on Pt/Pt; there are some differences in the transients as well. The obtained data suggest that CO interacts with O_ads on smooth Pt and Pt/Pt via different mechanisms. Two models for the process on smooth platinum are considered. In one model, the interaction of O_ads with CO from solution is accepted as the rate-determining step; in the other, the interaction of O_ads with CO_ads. A comparison of theoretical E vs. dependences with experimental data using the MathCad program suggests that CO interacts with O_ads via both mechanisms.     

Kinetics and mechanism of interaction between methanol and adsorbed oxygen on a smooth polycrystalline platinum electrode: Transients of the open-circuit potential

The method of transients of the open-circuit potential, combined with cathodic potentiodynamic pulses, is used for studying the methanol interaction with preliminarily adsorbed oxygen (O_ads) in 0.5 M H₂SO₄. It is established that, for the larger part of the time period required for a full reduction of a monolayer of O_ads in solutions of methanol on polycrystalline platinum, the process occurs at large coverages of the surface by O_ads (?_O). In the region of medium coverages, transients of the open-circuit potential are accurately described by the equation that corresponds to the mechanism of “conjugated reactions.” The mechanism of the methanol interaction with O_ads happens to be close to that for formic acid, which is explained by a dissociative character of these HCO compounds. Kinetic parameters characterizing the methanol reaction with O_ads in the region of large and medium coverages are determined and analyzed.     

The comparative study of CO2 + Hads reaction on platinum electrode in H2O and D2O

The observed isotopic effect in CO₂ + H_ads reaction showed that at 0.05 V the rate determining step is

formation whereas at 0.2 V the reorientation of adsorbed intermediate:

is probably the slowest step of reaction. The oxidation of adsorbed product is slower in D₂SO₄ than H₂SO₄ solution like the surface oxidation of platinum. The rate determining step of COOH_ads oxidation is a reaction with OH_ads.     

Anion-exchange separation of Pt and Pd using perchloric and hydrochloric acid solutions

On Biorad Ag-1X8 anion-exchange resin (200–400 mesh), Pd and Pt may be separated from one another by elution with 0.2M HClO₄, and 5M HClO₄, respectively. If present, Au may be retained by making the elutriants 0.003M in HCl. Alternatively, reduction by H₂SO₃ enables elution of Pt²⁺ with 6M HCl before recovery of Pd²⁺ with 0.2M HClO₄·Ir⁴⁺ is reduced to Ir³⁺ by H₂SO₃ and may be eluted ahead of Pt²⁺ by 2M HCl.     

The solubility of ozone in aqueous solutions of sulfuric, phosphoric, and perchloric acids

The solubility of ozone in pure water and aqueous solutions of sulfuric, phosphoric, and perchloric acids was determined at 20°C. An increase in the concentration of H₃PO₄ and HClO₄ (to 14.8 and 9.5 M, respectively) caused a monotonic decrease in the solubility of ozone. The solubility of ozone in sulfuric acid was minimum at a 12 M concentration; the solubility then increased and, in 17.9 M H₂SO₄, reached almost the same value as in pure water. The ratio between the concentrations of O₃ in solution and the gas phase was 0.276 in pure water, 0.122 in 12 M H₂SO₄, and 0.265 in 17.9 M H₂SO₄. The results obtained are compared with the available literature data.     

Electrolyte Effects on the Electrocatalytic Performance of Iridium-Based Nanoparticles for Oxygen Evolution in Rotating Disc Electrodes

Proton exchange membrane water electrolysers are very promising renewable energy conversion devices that produce hydrogen from sustainable feedstocks. These devices are mainly limited by the sluggish kinetics of the oxygen evolution reaction (OER). Ir-based nanoparticles are both reasonably active and stable for the OER in acidic media. The electrolyte composition and the pH may play a crucial role in electrocatalysis, yet they have been widely overlooked for the OER. Herein, we present a study on the effects of the composition and concentration of the electrolyte on commercial Ir black nanoparticles using concentrations of 0.05 M, 0.1 M and 0.5 M of both sulphuric and perchloric acid. The results show an important effect of the electrolyte composition on the catalytic performance of the Ir nanoparticles. The concentration of H₂SO₄ interferes on the oxidation of Ir and decreases the catalytic performance of the catalyst. HClO₄ does not show strong interferences in the electrochemistry of Ir. Higher catalytic performances are observed in HClO₄ electrolytes in comparison to H₂SO₄ with little effect of the concentration of HClO₄.     

Potential oscillations in the course of galvanostatic oxidation of hydrogen at platinum electrode in the presence of electrosorbing cations

An assumption has been made that in the presence of electrosorbing cations potential oscillations should be observed in the course of galvanostatic oxidation of hydrogen on platinum electrode. The reality of this assumption has been proved in the presence of Cd²⁺, Cu²⁺, Sn²⁺, Bi³⁺, Ag⁺ ions using different base electrolytes (H₂SO₄, HCl, HClO₄). Some features connected with these oscillation phenomena were considered.     

Oxidation of Am(III) and stability of Am(IV) and Am(VI) in mineral acid solutions

The oxidation of americium in HNO₃, H₂SO₄ and HClO₄ solutions by a mixture of potassium persulfate with silver salt in the presence of potassium phosphotungstate has been investigated. The influence of acid and its concentration, of (NH₄)₂S₂O₃, K₁₀P₂W₁₇O₆₁ and silver salt on Am(III) oxidation rate, yield and stability of Am(IV) and Am(VI), has been studied. The complexation of Am(III), Am(IV) and Am(VI) with phosphotungstate ions has been investigated. It has been established that Am(III) and Am(IV) form ML₂ complexes and their apparent stability constants have been estimated. The oxidation mechanism is discussed. A method for preparing of Am(IV) in 0.1–6M HNO₃, O.1–3M H₂SO₄, 0.1–1M HClO₄ solutions is proposed. The oxidation of Am(III) to Am(IV) by KBrO₃ and K₂Cr₂O₇ in HNO₃, H₂SO₄, HClO₄ solutions in the presence of K₁₀P₂W₁₇O₆₁ has been investigated.     

Solvent extraction of hafnium(IV)

^{175, 181}Hafnium(IV) was extracted by HDBP in 2-ethylhexanol from 1–10M solutions of HClO₄, HCl and HNO₃, and 1–8M H₂SO₄. As with low polar organic phase diluents, the acidity dependence of the distribution ratio of Hf, D, passes through a minimum for HClO₄, HCl, and H₂SO₄ whereas only an increase of D can be observed with increasing HNO₃ concentration. From the slope analysis the following complexes were found to be extracted (HDBP=HA): HfA₄ at <4M HClO₄ and <5M HCl, lg K_extr=9, HfX₄(HA)₄ (X=ClO ₄ ⁻ , Cl⁻ or NO ₃ ⁻ ) at >5M HClO₄, >7M HCl and 1–10M HNO₃, Hf(SO₄)A₂(HA)_3–4 at <3M H₂SO₄, and Hf(SO₄)₂ (HA)₄ at >6M H₂SO₄. Coextraction of sulphate with hafnium from H₂SO₄ solutions was evidenced in experiments with macro concentrations of Hf(IV) and³⁵SO ₄ ²⁻ . Part XX: Coll. Czech. Chem. Commun., 40 (1975) 3617.     

A kinetic study of electron transfer from L-Ascorbic acid to sodium perborate and potassium peroxy disulphate in aqueous acid and micellar media

Kinetics of oxidation of L-ascorbic acid (H₂A) by sodium perborate (SPB) and peroxy disulphate (PDS) have been investigated in aqueous acid and micellar media. Reaction kinetics indicated first-order dependence on both |oxidant| and |H₂A|. Increase in ionic strength (μ) increased reaction rate only in H₂SO₄ media. Rate of SPB oxidation of H₂A has been accelerated by acidity in HNO₃ and HCl media while a decreasing trend is observed in HClO₄ and H₂SO₄ media. The results are interpreted by various theories of acidity functions. Reaction rate is enhanced by the addition of added |H₂O₂| indicating a H₂O₂ coordinated boron species to be active in the present system. In the absence of micelle, increase in |acid| altered the PDS(SINGLEBOND)H₂A reaction rate marginally (a very small positive effect with HClO₄ and negative effect with H₂SO₄). Most plausible mechanisms have been proposed on the basis of experimental results. Activation parameters evaluated for specific kinetic constants are in accord with outer sphere electron transfer mechanism. In SPB(SINGLEBOND)H₂A system, addition of anionic micelle (Sodium lauryl sulfate) increased the rate, stabilizing the cationic species in the transition state in all the acid media. Although rate of PDS oxidation of H₂A was catalyzed by TX and inhibited by SDS at critical micellar concentration (CMC) increase in |acid| (both HClO₄ and H₂SO₄) beyond 9.6 × 10⁻⁴ M decreased the rate of oxidation. This trend was explained due to the repulsive interaction of coanion, HA⁻, and negatively charged micellar species. © 1996 John Wiley & Sons, Inc.     

Reduction of the photoexcited uranyl ion by water

Photooxidation of water by the uranyl ion was studied. Solutions of uranyl in 0.01–4.0M H₂SO₄, HClO₄, or 0.1–1.0M Na₂SO₄ and NaClO₄ containing “lacunary” heteropolytungstate (HPT) K₁₀P₂W₁₇O₆₁ or K₈SiW₁₁O₃₉ were irradiated with a nitrogen laser, a mercury or xenon lamp, or visible light. Spectrophotometric analysis showed that the irradiation results in the accumulation of U^IV. Simultaneously the formation of H₂O₂ proceeds. The quantum yield Φ of the reaction increases as the concentration of the acid or salt increases. For aerated solutions of 1M H₂SO₄ or 1M HClO₄, irradiation by light with λ=337.1 Φ is close to (1.5–2)·10⁻³. The irradiation of solutions with pH −4 for many days leads to an almost quantitative transformation of UO₂ ²⁺ into U^IV. When the irradiation was carried out in the absence of HPA, U^IV was not detected, although hydrogen peroxide was observed in the solution. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya. No. 2, pp. 282–287, February, 2000.     

Kinetics and mechanism of “reduced CO2” electrooxidation on electrodispersed platinum in different acid solutions

The electrooxidation of “reduced CO₂” electroadsorbates on electrodispersed platinum electrodes has been investigated in 0.05 M HClO₄, 1 M HClO₄, 0.5 M H₂SO₄ and 1 M H₃PO₄ at 25° C through voltammetry and potential step techniques. The overall reaction comprises three distinguishable processes, namely a first order triggering process, and two second order surface processes. The latter are influenced remarkably by the solution composition (anions). The second order reaction mechanism involves two distinguishable “ reduced CO₂” electroadsorbates which react independently with the OH species formed from H₂O electrooxidation on the bare platinum sites as the reaction proceeds. An interaction term has to be included in the rate equations to account for the experimental results. The mechanistic interpretation accounts for the values of the number of electrons per site ranging from 1 to 2.     

Aquation of K2 TcBr6 in dilute acid solutions

The photochemical aquation of K₂ TcBr₆ in 2M HRr, 1M HClO₄ and 1M H₂SO₄ has been studied. The absorption spectra of the various Tc(IV) species were measured after the electrophoretic separation. The spectrophotometric changes and the yield of each species as a function of the UV irradiation time were determined. After 25 hours of irradiation of HBr and HClO₄ solutions the main species were the cationic ones (80%) but in H₂SO₄ solution the neutral species reached a yield of 90%. The oxidation of Tc(IV) species to TcO ₄ ^– proceeded more rapidly and extensively in HClO₄ than in HBr and H₂SO₄.     

An extraordinary electrocatalytic reduction of oxygen on gold nanoparticles-electrodeposited gold electrodes

The cathodic reduction of oxygen has been investigated at a gold nanoparticles-electrodeposited gold electrode in 0.5 M H₂SO₄ solution. Two well-defined reduction peaks were observed at +50 and −250 mV vs. Ag/AgCl/KCl (sat.). Those two peaks indicated a 2-step 4-electron reduction pathway of O₂ in this strong acidic medium. The former peak was ascribable to the 2-electron reduction of O₂ to H₂O₂, while the latter was assigned to the reduction of H₂O₂ to H₂O. The observed electrocatalysis for the reduction of O₂ is attributable to the extraordinary catalytic activity of the gold nanoparticles over the bulk gold electrode, at which the 2-electron reduction peak of O₂ to H₂O₂ was observed at −200 mV.     

Substoichiometric Extraction of Mercury by Diethyldithiocarbamic Acid

The substoichiometric extraction of Hg²⁺ using diethyldithiocarbamic acid and ²⁰³Hg tracer was studied. Chloroform was employed to remove the complexes from the aqueous media which were 0.5M H₂SO₄ or 1 M HClO₄ and 0 to 5 M NaCL. - Systems containing Cl^? allowed extraction of Hg²⁺ for all DDC/Hg molar ratios, the extracted complexes being HgCl(DDC) and Hg(DDC)₂. Their exchange constant was determined. - In the absence of Cl^?, no extraction could be effected in either system if the DDC/Hg molar ratio was < 1; the H₂SO₄ system remained clear, whereas a precipitate of HgClO₄(DDC) formed in the HClO₄ system. For molar ratios > 1, the extraction of Hg²⁺ increased linearly with the addition of DDC, the extracted complex being Hg(DDC)₂.     

On the DAPS Capability for Testing the Chemisorbed Species Location

The DAPS can properly indicate the surface layer of platinum atoms that is mostly affected in the course of H₂, O₂ adsorption, H₂+O_ads and NO+H_ads interactions on the Pt(100)-(1×1) single crystal surface.