Generation and electrochemical nanogravimetric response of the third anodic hydrogen peak on a platinum electrode in sulfuric acid media

Platinum electrodes have been investigated in sulfuric acid solutions in the hydrogen adsorption–desorption region by electrochemical quartz crystal nanobalance (EQCN). It was found that a well-developed peak (the so-called third peak) between the two main peaks appeared when, following the cycling in the oxide region, the electrode was kept at potentials just more positive than the potential of hydrogen evolution under the same conditions. The extent of this third peak and its ratio to oxidation peaks of the strongly and weakly adsorbed hydrogen depend on the waiting time at potentials mentioned above as well as on the scan rate. Similarly to the other two peaks, the simultaneous EQCN response shows a slight mass increase which can be assigned to adsorption of HSO₄ ^? ions at the platinum surface. Because the third peak appears only after a potential excursion in the oxide region, it is related to the formation of specific surface sites on which hydrogen can be adsorbed with an energy which falls between the energies of the weakly and strongly bound hydrogen. The waiting time effect indicates that this adsorption is a slow process, and it is the very reason that it cannot be observed during the second cycle. The scan rate dependence can be elucidated by the transformation of this type of adsorbed hydrogen to the other two forms.     

Effect of Cs+ ions on the electrochemical nanogravimetric response of platinum electrode in acid media

Platinum electrodes have been investigated in sulfuric acid solutions in the presence and absence of Cs⁺ ions by electrochemical quartz crystal nanobalance (EQCN). An unusual potential dependence of the quartz crystal frequency response has been observed in the presence of Cs⁺ ions. The frequency decrease is more pronounced in the region of the underpotential deposition of hydrogen, and the frequency decrease in the double layer region diminishes as the concentration ratio of Cs⁺ and H⁺ ions increases. After immersion in Cs₂SO₄ solutions the frequency change was higher than that expected taking into account the density and viscosity. The effects observed can be explained by the specific adsorption of Cs⁺ ions on the Pt surface, which competes with the hydrogen adsorption. At more positive potentials than the potential of zero charge (pzc) a desorption of the Cs⁺ ions starts. In this potential region both Cs⁺ and HSO₄^? ions are adsorbed at the platinum surface. In the double layer region the mass change caused by the desorption of Cs⁺ ions and the starting adsorption of sulfate ions compensates each other.     

Study of the surface mass changes during the redox transformations of copper(II) phthalocyanine-tetrasulfonic acid on gold in acidic media

Journal of Solid State Electrochemistry - An electrochemical quartz crystal nanobalance (EQCN) has been used to study the deposition (adsorption) and dissolution (desorption) during the redox...     

Electrochemical quartz crystal nanobalance study of the adsorption/displacement phenomena of proteins and lipids on Pt

The electrochemical quartz crystal nanobalance (EQCN) was used to measure the adsorption behavior of a series of lipids (stearate, oleate, linoleate, and gamma-linolenate) on a Pt surface from a phosphate buffer pH 7.0 solution at 295 K and to investigate their adsorption/displacement behavior with the proteins, beta-lactoglobulin and alpha-lactalbumin, which are known to cause fouling during milk processing. The EQCN technique and the complementary technique of cyclic voltammetry measured simultaneously provided information on the efficiency of solubilization of the proteins by these lipids. Excellent agreement was obtained for the surface concentration of adsorbed lipid from the surface charge density from cyclic voltammetry measurements and the change in mass from the EQCN frequency measurements. The Gibbs energy of adsorption showed the lipids to have a strong affinity for the platinum surface. Addition of protein to a preadsorbed lipid layer showed alpha-lactalbumin to be able to coadsorb with the lipids, while beta-lactoglobulin was able to desorb some of the unsaturated lipids but appeared to coadsorb with the saturated lipid, stearate. Addition of lipid to a preadsorbed protein layer showed the unsaturated lipids to be able to displace some of the protein. A comparison of the desorption ability of the lipids showed stearate to be very inefficient at removing protein, while the other three lipids were able to remove each of the proteins, with the order of efficiency for protein desorption being oleate > linoleate > gamma-linolenate.     

Anion—surface interactions: Part I. Perchlorate decomposition and sulphate adsorption hysteresis studied by voltammetry

Cyclic voltammetry has been used to investigate the behaviour of sulphate and perchlorate anions adsorbed on Ir{111} and polycrystalline rhodium. In 0.1 M HClO₄, perchlorate anions decompose on polycrystalline rhodium to give a surface intermediate which inhibits hydrogen adsorption. This is in agreement with a previous study by Wieckowski and coworkers, who showed that the surface intermediate was, in fact, adsorbed chloride. The voltammetry of the same electrode in sulphate containing electrolytes depends critically on the concentration of sulphate and the potential sweep rate. We conclude that hydrogen adsorption can only occur after significant desorption of sulphate. Hysteresis in the adsorption and desorption of sulphate results in an asymmetric hydrogen adsorption region with respect to the potential axis. Ir{111} is shown also to cause decomposition of perchlorate anions, although, contrary to the case of rhodium, the process is very much a chemical one in which electric currents, arising from perchlorate decomposition, were not detected. In sulphuric acid, a strong interaction between anions and the Ir{111} surface is observed, which precludes oxide formation at potentials less than 1 V (Pd---H), behaviour characteristic of a Pt{111} surface in the same electrolyte.     

Nickel-underpotential deposition on Pt(1 1 0) in sulphate-containing media

Nickel-underpotential deposition (UPD) on platinum electrodes has been studied in sulphuric media using cyclic voltammetry. The process seems structure-related: among the surfaces we did study, only (2 × 1)-reconstructed Pt(1 1 0) surface yields nickel-UPD. At low pH values, nickel-UPD peaks overlap with the hydrogen adsorption/desorption region. At higher pH (2–3) the overlapping is less severe: nickel coverage increases and can be estimated. Nickel-UPD on platinum (1 1 0) is a slow and irreversible process. The co-adsorption of sulphate or hydrogen-sulphate anions is probable. Nickel-UPD on Pt(1 1 0) occurs even in the presence of CO. Nickel submonolayer on Pt(1 1 0) exhibit remarkable activity towards CO-oxidation.     

葡萄糖、半乳糖和乙醇恒电流氧化过程电位振荡的EQCM研究

采用电化学石英晶体微天平(EQCM)研究了0.5 mol•L^-1 NaOH水溶液中铂电极上葡萄糖、半乳糖和乙醇恒电流氧化过程中伴随的电位振荡行为. 两个糖体系的电位振荡过程伴随EQCM频率的同步振荡响应, 而乙醇体系中相应的频率响应却非常小；三个体系振荡过程的同步动态电阻响应均很小, 表明振荡过程频率响应主要为质量效应. 虽然葡萄糖和半乳糖结构相似, 电位和频率振荡的幅度相当, 但频率波数和周期明显不同, 表明电位振荡行为对两者呈现良好的分子识别能力. 本文也讨论了相关振荡机理和NaOH浓度效应及碱性介质中铂电极电化学过程, 提出了所形成的铂氧化物主要是PtO₂-3H₂O_ad以及两糖体系振荡过程中糖酸根阴离子伴随着高/低电位在铂电极上吸/脱附的新观点.     

Analysis of the electrochemical quartz crystal microbalance response during oxidation of carbon oxides adsorption products on platinum group metals and alloys

The analysis of the electrochemical quartz crystal microbalance (EQCM) signal is presented for the case of oxidation of carbon oxides adsorption products on Pt, Rh, and their alloys. It is demonstrated that the EQCM response can be roughly approximated by the mass balance involving adsorption/desorption of various species (carbon oxides, oxygen, anions, and water molecules) and metal dissolution. The results obtained by the EQCM are in good agreement with the electrochemical data and confirm the domination of CO radicals among the products of CO₂ reduction and CO adsorption on Pt-rich electrodes. In the case of Rh-rich electrodes, the existence of additional species (CHO or COH), more reduced than CO, is suggested.     

On the mechanism of electrochemical dispersion of platinum under the action of alternating current

The behavior of platinum electrodes in alkali solutions under the action of pulse alternating current was studied. A mechanism of electrochemical dispersion of platinum was proposed. An important step of the mechanism is the discharge and cathodic insertion of alkali metal cations into the crystalline lattice of platinum followed by the decomposition of intermetallic compound Pt _m Na upon the interaction with water. Dispersion is facilitated by the presence of an oxide film on the platinum surface formed in the anodic period of pulses, a periodical alternation of electrode polarity resulting in the electrochemical injection of vacancies from the metal bulk to the surface, and hydrogen and oxygen formation in the corresponding periods.     

Oxidation of glucose at electrodeposited platinum electrodes

The oxidation of glucose at low concentrations was studied at electrodeposited platinum electrodes in 0.1 M HClO₄ using an electrochemical quartz crystal microbalance (EQCM). Experiments were performed over the whole potential range and then over selected regions of potential to investigate the processes giving rise to specific features in the cyclic voltammetry and mass response. In the region of potential where adsorbed hydrogen is present, EQCM experiments reveal the presence of adsorbate derived from glucose through the removal of features associated with the presence of adsorbed anions in the background electrolyte. Cycling over this potential region leads to a decrease in the mass of the electrode as the coverage of poisoning species increases. In the double-layer region of potential an increase in mass is seen as the adsorbate is oxidized and removed from the electrode surface, and is suggested to be a result of the replacement of adsorbed residues (formed from progressive oxidation of glucose) by both anions and fresh reactant. Restriction of the electrode potential to values above 0.2 V (SCE) prevents poison formation and mass responses indicate the presence of adsorbed glucose between 0.2 V and 0.6 V. Mass responses accompanying voltammetry and injection experiments also indicate that glucose or species derived from it can adsorb on an oxidized platinum surface.     

甲醇在欠电位沉积Ru修饰Pt电极上的催化氧化

采用欠电位沉积(upd)方法在Pt 表面沉积亚单层的Ru制备出upd-Ru/Pt 电极. 通过欠电位沉积前后电极在0.5 mol·L-1 H2SO4溶液中循环伏安图-152 - 128 mV(vs Ag/AgCl)电位范围内对氢区的数值积分确定Pt表面Ru 的覆盖度. 用电化学方法测试了甲醇在upd-Ru/Pt电极上的催化氧化, 并讨论分析了欠电位沉积电位和Ru的表面覆盖度对甲醇氧化的影响. 结果表明, Ru能够欠电位沉积到Pt表面. Pt表面欠电位沉积少量的Ru 即能大大促进甲醇的氧化.只要控制upd-Ru的沉积量, upd-Ru原子就能大大促进甲醇氧化而与沉积电位无关. Ru原子对甲醇氧化的促进作用与Ru和Pt是否形成合金无关, 而取决于Ru 在Pt表面的百分含量.     

Revealing structural effects, part II: the influence of molecular structure on the adsorption of butanol isomers on platinum

The nature of the adsorbates formed when butanol isomers interact with platinum electrodes in a perchloric acid medium was investigated by the application of on-line differential electrochemical mass spectrometry (DEMS) and cyclic voltammetry. In this way, the reactivity of the residues remaining on the electrode surface after a flow-cell experiment was established for the different molecules. It was found that the four isomers form strongly adsorbed species on the electrode, which undergo both electro-oxidation and electroreduction, depending on the potential applied at the electrode. Oxidative stripping of the adsorbates produces CO2 as the only oxidation product, whereas propane and the corresponding butane isomer are obtained on platinum in the hydrogen adsorption potential region. The yields of these hydrocarbons were found to depend strongly on the nature of the butanol isomer and on the adsorption potential. According to these results, it can be concluded that fragmentation of the butanol isomers occurs during adsorption and reduction reactions. C4 alkene and acetyl species are proposed as the adsorbed intermediates in all cases.     

Catalytically active platinum blacks prepared by magnetron sputtering in vacuum and their using in fuel cells with solid polymer electrolyte

Highly disperse platinum film were vacuum-plasma-deposited onto titanium foil and gas-diffusion layers. The platinum deposits have complicated structure. By measuring hydrogen desorption peaks, the catalysts’ active specific surface area was determined and the roughness factor calculated. The electrochemical activity of the electrodes on gas-diffusion layers in the oxygen reduction and hydrogen oxidation reactions was determined. It was shown that the catalysts’ specific activity depends on the platinum content and the Nafion-ionomer additive. The high-activity electrodes were tested in Membrane Electrode Assemblies of low-temperature fuel cells.     

Effect of Electrode Material on Electrodeposition of Tungsten Oxide

Influence exerted by the nature of an electrode-substrate on the electrochemical deposition of tungsten oxides from a metastable acid solution of isopolytungstate was studied. As substrates for obtaining tungsten oxide deposits served metallic electrodes made of gold and platinum, films of mixed indium-tin oxide on glass (ITO-electrodes) and also glassy carbon electrodes and glassy carbon electrodes coated with films of conducting polymers: polyaniline, polypyrrole, and poly-3,4-ethylenedioxythiophene. It was shown that the nature of a substrate noticeably affects the electrochemical properties of tungsten oxide deposits. These differences are attributed to the adsorption of hydrogen on platinum in the range of the deposition potentials of tungsten oxide and to the chemical interaction of polytungstate ions with the thiophene sulfur of the polymer.

     

Dissolution and adsorption of hydrogen at smooth Pd wires at potentials of the alpha phase. Influence of electrolyte

The dissolution and adsorption of hydrogen atoms was determined as a function of potential in the region of the alpha phase of a smooth Pd wire in 0.5 M H₂SO₄, 0.5 M KOH, and 0.5 M HCl by a pulse technique. Specifically adsorbable chloride ions were found to decrease the hydrogen coverage at constant potential as on platinum. In contrast to platinum hydroxyl ions did not lead to an increase of the hydrogen bond strength. The isotherms of hydrogen adsorption can be approximated by a Temkin type relationship in a range of medium coverage. The logarithm of the bulk concentration of dissolved hydrogen atoms directly below the surface was a linear function of potential between about 0.25 V and 0.1 V, interpretable in terms of the non-ideal behaviour of the atoms in the first layer below the surface.     

The raman spectroscopy of the ferricyanide/ferrocyanide system at gold, β-palladium hydride and platinum electrodes

The surface enhanced Raman spectroscopy (SERS) of ferricyanide and ferrocyanide anions at gold electrodes shows that both species are adsorbed at the interface. The variation of the spectra with potential and the nature of the cations of the support electrolyte shows that anions and cations are co-adsorbed and that the structure of the anions is markedly perturbed compared to that of the solution phase species. It is also shown that SERS of these ions at palladium electrodes can be readily generated by converting the surface to the β-palladium hydride. In this case the spectra are more simple but both anions are again adsorbed. Adsorption is also shown to take place at smooth platinum electrodes, measurements in this case being carried out by potential modulated surface unenhanced Raman spectroscopy (SUERS) but with resonance Raman enhancement of the scattering.The spectroscopic results show that electron transfer in the ferrocyanide/ferricyanide system cannot be considered to be a purely outer sphere reaction. Results indicate that the smallest change in structure of the adsorbed species takes place in supporting electrolytes containing caesium ions; these solutions give the largest value of the standard rate constant for the reaction.     

Die Koordinationschemie oxidischer Grenzflächen und ihre Auswirkung auf die Auflösungskinetik oxidischer Festphasen in wäßrigen Lösungen

Chemical processes at the hydrous oxide-solution interface — proton transfer, specific adsorption of cations and anions — can be interpreted in terms of a surface coordination model and quantified in terms of mass law equlibrium constants. With the help of this model the kinetics of dissolution of solid oxide phases can be formulated in a unifying way: In the presence of specifically adsorbable (surface-coordinating) anions the dissolution rateR depends critically on the relative concentration of surface complexesR=k[H⁺]θ _A, whereθ _A is the relative surface coverage with an anionic surface complex. The coordination model is also used to interpret the adsorption of anionic inhibitiors.     

Electrochemistry of room temperature protic ionic liquids

Eighteen protic ionic liquids containing different combinations of cations and anions, hydrophobicity, viscosity, and conductivity have been synthesized and their physicochemical properties determined. In one series, the diethanolammonium cations were combined with acetate, formate, hydrogen sulfate, chloride, sulfamate, and mesylate anions. In the second series, acetate and formate anions were combined with amine bases, triethylamine, diethylamine, triethanolamine, di-n-propylamine, and di-n-butylamine. The electrochemical characteristics of the eight protic ionic liquids that are liquid at room temperature (RTPILs) have been determined using cyclic, microelectrode, and rotating disk electrode voltammetries. Potential windows of the RTPILs have been compared at glassy carbon, platinum, gold, and boron-doped diamond electrodes and generally found to be the largest in the case of glassy carbon. The voltammetry of IUPAC recommended potential scale reference systems, ferrocene/ferrocenium and cobaltocenium/cobaltocene, have been evaluated and found to be ideal in the case of the less viscous RTPILs but involve adsorption in the highly viscous ones. Other properties such as diffusion coefficients, ionic conductivity, and double layer capacitance also have been measured. The influence of water on the potential windows, viscosity, and diffusion has been studied systematically by deliberate addition of water to the dried ionic liquids. The survey highlights the problems with voltammetric studies in highly viscous room temperature protic ionic liquids and also suggests the way forward with respect to their possible industrial use.     

Vibrational spectroscopy on platinum single-crystal electrodes: Part I. In-situ infrared spectroscopic studies of the adsorption and oxidation of CO on Pt (111) in sulphuric acid

Platinum single-crystal electrodes of 5 mm diameter were prepared for in situ infrared spectroscopic measurements by melting platinum wires. The linear potential sweep voltammograms of hydrogen adsorption/desorption on Pt (111), (110) and (100) in 0.5 M sulphuric acid are in excellent agreement with those observed on smaller platinum single-crystal surfaces.The adsorption and oxidation of CO on Pt (111) in 0.5 M sulphuric acid was studied by in situ polarization modulated infrared reflection absorption spectroscopy. The effects of the initial adsorption potential and surface reconstruction on the nature and oxidation mechanism of the adsorbed CO layer are reported.     

Cyclic voltammetric behavior of Pd–Pt–Rh ternary alloys

The electrochemical behavior of Pd–Pt–Rh alloys has been investigated using cyclic voltammetry (CV). The alloys were prepared by electrochemical codeposition as limited volume electrodes (less than 1 m in thickness). The morphology of the alloy surface and bulk compositions were examined by the SEM/EDAX method. Surface oxides generation (oxygen adsorption) and oxides reduction (oxygen desorption) currents together with hydrogen adsorption and hydrogen absorption signals can be distinguished on CV curves. During potential cycling through the full hydrogen–oxygen potential range Rh and Pd are preferentially dissolved, which is reflected in a dramatic transformation in the voltammogram shape. The composition changes involve not only the surface but also some atomic layers beneath the surface.