Dissolution and adsorption of hydrogen at smooth Pd wires at potentials of the alpha phase in sulfuric acid solution

After stepping the electrode potential rapidly down from 0.7 V, hydrogen atoms were allowed to diffuse into Pd wires for 10 or 1 s at constant potential in the region of the alpha phase. Subsequently the anodic current due to hydrogen removal at 0.7 V was recorded by the oscilloscope. The current was found to decay after a certain time according to a law characteristic for the diffusion of hydrogen from the interior of the Pd wire to the surface as rate-controlling process. The bulk concentration _SC_H of hydrogen atoms directly below the surface was computed on the basis of the diffusion process. The amount Q_H of hydrogen adsorbed on the surface was determined as the difference between the integral charge Q from the experimental current-time curves and the integral charge Q′ computed for the diffusion process within a suitable interval of time. The dependence of the bulk concentration _SC_H and of the hydrogen coverage Q_H upon potential were established and discussed.     

Influence of chemisorbed carbon monoxide on the oxidation of molecular hydrogen at smooth platinum in sulfuric acid solution

Potentiostatic current—potential curves of hydrogen oxidation were measured at 0.5 mV s^?1 on two smooth platinum electrodes of different shape in the presence of various coverages with chemisorbed carbon monoxide in 0.5 M H₂SO₄, stirred with molecular hydrogen at 1 cm³ s^?1. Carbon monoxide coverages up to 0.6 have a small influence. The hydrogen oxidation remains controlled by convective diffusion of molecular hydrogen. Above coverages of 0.6, a kinetic step becomes increasingly predominant. The effect of chemisorbed carbon monoxide on the H₂ oxidation is similar to that on hydrogen adsorption, studied previously. The correlation between the rate of the kinetic step and the free energy of hydrogen adsorption in the Temkin model of the surface is established and discussed.     

Cyclic voltammetry and electrochemical quartz crystal microbalance studies of a rhodized platinum electrode in sulfuric acid solution

The adsorption properties of a rhodized platinum electrode in sulfuric acid were studied through simultaneous cyclic voltammetry (CV) and electrochemical quartz crystal microbalance (EQCM) measurements. The processes occurring at the Rh surface during the potential scan between 0.04 and 1.3 V were analyzed in terms of the changes in mass and charge. The apparent molar masses of the species adsorbed on Rh in different potential regions were determined from the mass–charge correlation. The results obtained suggest that the desorption of hydrogen UPD initially occurs in conjunction with the incorporation of mass due to the adsorption of bisulfate/sulfate ions between 0.04 V and 0.12 V; the apparent molar mass (M ₁) in this potential region is consistent with that of bisulfate ion, assuming that one sulfate species occupies the site originally occupied by five H atoms. Between 0.12 V and 0.20 V, the apparent molar mass, M ₂, is less than M ₁. This result may be associated with a structural rearrangement of the adsorbed sulfate species. The apparent molar mass, M ₃, was determined in the potential region of 0.20–0.56 V; the value obtained for M ₃ suggests that the adsorbed species are hydrated bisulfate ions (HSO₄⁻·4H₂O). Additionally, it was found that Rh oxides formed at the surface between 0.56 V and 1.3 V could not be directly identified from the correlation between charge and mass.     

The potential dependence and kinetics of HCOOH adsorption on rhodium electrodes

The formic acid adsorption on an electrochemically prepared rhodium electrode has been studied by the radiochemical method. Electrochemical properties of the rhodium electrode surface in 0.5 M H₂SO₄ have been investigated by cyclic voltammetry. It has been shown that starting from E=0.20 V the rate of HCOOH adsorption is markedly potential dependent being practically independent of the electrode potential up to E=0.20 V. It seems that the HCOOH adsorption process may be explained on the basis of the two-sites kinetics model. The data obtained for HCOOH adsorption on a rhodium electrode have been compared with those for a platinum electrode reported previously.     

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.     

The adsorption of carbon monoxide on a platinum electrode

The adsorption of CO from 0.5 M H₂SO₄ solution on platinum has been studied using CO labelled with C-14. The adsorption of CO on Pt occurs in the potential range of hydrogen adsorption as well as in the double layer region. In the whole potential range the rate of adsorption follows first order kinetics. From the surface concentrations and charges for oxidation of adsorbed species it follows that the product of chemisorption consists at least of two kinds of species. One of them is the COOH radical probably formed by the reaction of CO with water.     

Photocurrent kinetics at electron emission from metal to electrolyte solution: Part V. Ionization rate constants of hydrogen and deuterium atoms adsorbed on mercury

Absolute ionization rate constant values of hydrogen and deuterium atoms adsorbed on mercury were measured using the method of pulse photoelectronic emission from metal into solution. In accordance with the Tafel law, these constants decrease from 2.5×10⁷s^?1 to 9×10⁵s^?1 (Table 1) in a 1 M solution of KCl in the range ?0.25 to ?0.5 V SCE. The transfer coefficient is 0.33±0.03 and the isotope ratio about 2.5. Owing to specific anion adsorption, rate constants increase as their concentration increases and KBr is added to the solution. In 0.05 M solutions of HCl and H₂SO₄, transfer coefficients are 0.30±0.05. From a comparison of measured values, with the hydrogen ion discharge rate constants found by extrapolation of experimental values into the potential range mentioned (taking into account the transfer coefficient change), the change of the Gibbs free energy in the reaction H₃O⁺+e_Me^?→H_ads+H₂O was calculated and found to be 0.87–0.99 eV at the potential of the normal hydrogen electrode. Adsorption energy of the hydrogen atom from the gas phase on a mercury electrode is 1.55±0.10 eV.The volt-ampere dependence of the hydrogen ion discharge current in the range ?0.25 to ?2.25 V corresponding to the current change by 18 orders of magnitude, agrees with the theoretically determined values (maximum deviation in the current is less than a factor of 3) for the medium reorganization energy E_r=1.75 eV. Despite constancy of the transfer coefficients of the elementary stages, in the range ?0.5 V (SCE), the effective transfer coefficient of the total hydrogen evolution processes increases from 0.5 to 1.0, as the ionization rate of the adsorbed hydrogen atoms becomes greater than their electrochemical desorption rate.     

Surface characterization of Pt electrodes using underpotential deposition of H and Cu: Part I. Pt(100)

This paper is the first in a series describing the in situ surface characterization of platinum electrodes using H and Cu deposited at underpotentials. The surface of a Pt(100) electrode pretreated by simple flame annealing and quenching in aqueous sulfuric acid is shown to contain a high concentration of defects such as vacancies and self-adsorbed Pt atoms. Adsorbed hydrogen is more strongly bound at these defects than on a uniform Pt(100) surface. Potential cycling in 1 M HCl produces a higher concentration of defects, while oxide formation and reduction in 0.5 M H₂SO₄ has the opposite effect. The nature of (100)-like sites at a polycrystalline platinum electrode is also discussed.     

Electrochemical properties of sulfur adsorbed on gold electrodes

The electrochemical properties of sulfur adsorbed on gold electrodes were studied in 10^?5M solutions of S^2? in 1 M NaOH. In general, ∵_S is less than a monolayer. At E=0.05 V only, a monolayer will be formed after long times. The sulfur layer is stable in the potential range between ?0.6 and +0.4 V. At lower potentials, sulfur can be desorbed cathodically (charge Q_red), but at higher potentials, where layers of gold oxide are formed, the sulfur is oxidized anodically (charge Q_ox). From the ratio Q_red·6/Q_ox=γ, the electrosorption valency γ=?2 is obtained. This means, that the sulfide ions are almost completely discharged during adsorption. The same layer can be formed by adsorption from polysulfide solutions, which can be explained by a break of the sulfur bond and adsorption of single sulfur atoms. The double layer capacity decreases during adsorption of sulfur indicating the formation of an insulating sulfur layer with a dielectric constant of about 2. The anodic adsorption of sulfide ions is limited by diffusion only. For longer polarisation times, the coverage is independent of time, i.e. place exchange reactions between Au and S can be excluded. The cathodic desorption as well as the anodic oxidation of the adsorbed sulfur are potential dependent charge transfer processes, as can be concluded from potentiodynamic measurements with various sweep rates.     

Influence of chemisorbed carbon monoxide on hydrogen evolution at smooth platinum electrodes in sulfuric acid solution

Cathodic current—potential curves were measured potentiostatically on two smooth platinum electrodes of different shape in 0.5 M H₂SO₄ at room temperature in the presence of chemisorbed carbon monoxide. Coverages of carbon monoxide between 0 and 0.6 lead to a relatively small decrease of the current density at constant hydrogen overvoltage. Mass transport processes remain largely rate-controlling. A rapid decrease of current follows, with a further increase of the carbon monoxide coverage. A transition from mass transport to kinetic control takes place. The rapid decrease is followed by a slower decrease at coverages above 0.85. This leveling-off results from hydrogen evolution on top of the chemisorbed layer of carbon monoxide. The retarding effect of chemisorbed carbon monoxide on hydrogen evolution is compared with the similar effect on hydrogen dissolution for the same electrodes.     

Kinetics and mechanism of glucose electrooxidation on different electrode-catalysts: Part I. Adsorption and oxidation on platinum

The processes of adsorption and electrooxidation of glucose on a smooth platinum electrode have been investigated in a wide range of pH values. It is found that glucose adsorption are platinum is accompanied by dehydrogenation of adsorbed molecules. The θ_R vs. E_r dependence represents a bell-shaped curve with unequal sides and with a maximum at E_r = 0.2 V at 0 < pH < 12 or at E_r = 0.4 when pH > 12. The kinetics of adsorption is described by the Roginsky-Zel'dovich equation, and the dependence of the steady-state coverage on the glucose bulk concentrations by the Temkin isotherm.It is shown that in the case of glucose adsorption on platinum Q^dehyd._H > Q_H, i.e. when glucose is brought into contact with a platinum electrode, the catalytic decomposition of glucose molecules occurs in addition to the formation of strongly chemisorbed particles. The transient current at E_r < 1.0 V is a current due to the ionization of hydrogen formed during adsorption with dehydrogenation of glucose and its catalytic decomposition. The glucose electrooxidation rate under steady-state conditions at E_r < 0.7 V is determined by the interaction of the chemisorbed carbon-containing particle with OH_ads. The slow step of glucose electrooxidation in the potential range 1.0 < E_r < 1.5 V is the interaction of glucose molecules from the solution bulk with the surface platinum oxide, the latter undergoing a quick electrochemical regeneration thereafter.The basic regularities and mechanism of glucose electrooxidation on platinum are shown to be analogous to those obtained earlier for such elementary organic fuels as formaldehyde and formic acid.     

Structurally heterogeneous electrode films of polyacrylamide and acrylamide/vinylpyridine copolymeric gels: Mediation of ascorbic acid oxidation

The surface chemistry, i.e. adsorption and surface phase formation, on Rh, Ir, Pt and Au single-crystal electrodes in 0.5 M H₂SO₄ has been studied in the potential range between 0 and 4 V vs. a standard hydrogen electrode by X-ray photoelectron spectroscopy and cyclic voltammetry. Both methods gave complementary results which were interpreted as potential-dependent changes from physisorption of water to adsorption of hydroxyl groups, hydroxide film formation, and eventual growth of thick adlayers of oxyhydroxide (Rh, Ir and Au) or hydroxide (Pt) with increasing applied voltage.     

Comparative study of adsorption and oxidation of formic acid and methanol on platinized electrodes in acidic solution

The results of a radio-and electrochemical study of the adsorption of HCOOH and CH₃OH and the oxidation of their chemisorption products on platinized electrode in 0.5 M H₂SO₄ have been presented and some remarks on the mechanism of the electrode processes have been given.It has been concluded that the rate determining step in adsorption of both fuels should be considered as charge transfer process. No heterogeneity of the platinum surface has been detected. It is supposed that the strongly bound intermediate is a composition of C_xOOH and CO radicals coupled by hydrogen bonding. Water molecules adsorbed on the electrode surface are most likely the surface oxidant involved in the oxidation of the chemisorbed species.     

Electrosorption of β-naphthol on graphite

Isotherms for the adsorption of β-naphthol from a buffered aqueous solution of 0.5 M K₂SO₄ onto graphite were detemrined over a range of potential of 1.27 V. The adsorbent was a packed bed of ?100 + 120 mesh graphite powder. Sufficient surface area was available to calculate accurately the amount adsorbed by measuring spectrophotometrically the change in adsorbate concentration in the bulk solution.At all potentials, a Langmuir adsorption isotherm, modified for the displacement of solvent molecules, was followed up to 60–65% of monolayer coverage. The ratio of projected areas of β-naphthol and water molecules was consistent with the experimentally derived number of solvent molecules displaced, six. The largest amount of adsorption observed, 2.5×10^?10 mol cm^?2, agreed with the calculated monolayer coverage of β-naphthol molecules lying in flat orientation on the graphite surface. Adsorption increased at more positive potentials. Over the range of potential investigated, the adsorbability constant increased sixfold. Desorption was only partially reversible.     

William B. Guenther,Chemical Equilibrium. A Practical Introduction for the Physical and Life Sciences,Plenum Press,New York-London (1975), 248 pp., price U.S. $ 23.40

The adsorption and related interfacial behavior of uracil at a mercury electrode/electrolyte solution interface has been studied by differential capacitance and maximum bubble pressure methods in 0.5 M NaF plus 0.01 M Na₂HPO₄ buffer pH 8.0. At concentrations below 24 mM uracil is adsorbed in a flat orientation on the electrode surface and occupies an area of 63 Ų. At higher concentrations and at potentials close to ?0.5 V the adsorbed uracil undergoes a reorientation and adopts a perpendicular stance on the electrode surface where it occupies an area of 39 Ų. In this perpendicular stance uracil undergoes a strong intermolecular stacking interaction with its neighbors similar to that observed between adjacent pyrimidines in nucleic acids.     

Enhancement of the electrochemical reduction of oxygen at platinum by nickel underpotential deposition

Polycrystalline Pt electrode was modified by underpotential deposition (upd) of nickel. The modification was performed by potential cycling in phosphate buffer pH 7. 0 containing NiSO₄, in which hydrogen and nickel upd processes were well separated. The maximum Ni upd coverage was found to be 0.3. Oxygen reduction was studied at bare and nickel upd-modified Pt. It was found that the reaction rate increased with increasing Ni upd coverage. At θ(Ni)=0.3, the current density was a factor of 2 higher compared to bare Pt (at the potential of 0.85 V). The capacitance of the electrode interface was determined in potential-relaxation experiments following interruption of the polarization current. It was found that the pseudocapacitance owing to a coverage by the adsorbed reaction intermediates was higher on the Ni-modified Pt surface than on bare Pt, which resulted in higher reaction rate. The influence of Ni adatoms on the surface coverage by the reaction intermediates was attributed to the inhibition of OH adsorption on Pt by OH ligands attached on neighboring Ni atoms.     

Transition‐Metal–Boron Intermetallics with Strong Interatomic d–sp Orbital Hybridization for High‐Performance Electrocatalysis

A theoretical and experimental study gives insights into the nature of the metal–boron electronic interaction in boron‐bearing intermetallics and its effects on surface hydrogen adsorption and hydrogen‐evolving catalytic activity. Strong hybridization between the d orbitals of transition metal (T_M) and the sp orbitals of boron exists in a family of fifteen T_M–boron intermatallics (T_M:B=1:1), and hydrogen atoms adsorb more weakly to the metal‐terminated intermetallic surfaces than to the corresponding pure metal surfaces. This modulation of electronic structure makes several intermetallics (e.g., PdB, RuB, ReB) prospective, efficient hydrogen‐evolving materials with catalytic activity close to Pt. A general reaction pathway towards the synthesis of such T_MB intermetallics is provided; a class of seven phase‐pure T_MB intermetallics, containing V, Nb, Ta, Cr, Mo, W, and Ru, are thus synthesized. RuB is a high‐performing, non‐platinum electrocatalyst for the hydrogen evolution reaction.     

氢原子在Be(0001)表面吸附的密度泛函理论研究

采用第一性原理的密度泛函理论研究单个氢原子和多个氢原子在Be(0001)表面吸附性质.给出了氢吸附Be(0001)薄膜表面的原子结构、吸附能、饱和度、功函数、偶极修正等特性参数.同时也讨论了相关吸附性质与氢原子覆盖度(0.06-1.33ML)的关系.计算结果表明:氢原子的吸附位置与覆盖度之间有强烈的依赖关系,覆盖度低于0.67ML时,氢原子能量上易于占据fcc或hcp的中空位置;覆盖度为0.78ML时,中空位与桥位为氢原子的最佳吸附位;覆盖度在0.89到1.00ML时,桥位是氢原子吸附能量最有利的位置;以上覆盖度中Be(0001)表面最外层铍原子的结构均没有发生明显变化.当覆盖度为1.11-1.33ML,高覆盖度下Be(0001)表面的最外层铍原子部分发生膨胀,近邻氢原子渗入到铍表面次层,氢原子易于占据在hcp和桥位.吸附结构中的氢原子比氢分子中的原子稳定.当覆盖度大1.33ML时,计算结果没有发现相对于氢分子更稳定的吸氢结构.同时从分析偶极修正和氢原子吸附垂直高度随覆盖度的变化关系判断氢覆盖度为1.33ML时,在Be(0001)表面吸附达到饱和.     

Infrared spectra of carbon monoxide adsorbed on a smooth gold electrode Part I. EMIRS spectra in acid and alkaline solutions

Electrochemically modulated infrared spectroscopy (EMIRS) was applied to the study of adsorption of CO on gold in 0.5 M sulfuric acid and 1 M sodium hydroxide solutions. A CO stretch band was observed with a peak intensity of ca. 0.2 % between 1850 and 2000 cm⁻¹ in 1 M NaOH, while a very weak band was detected between 1950 and 2050 cm⁻¹ in 0.5 M H₂SO₄. The bands were assigned to linear CO species adsorbed on the gold surface. In 1 M NaOH, electrooxidation of the strongly adsorbed CO species, which was detected by EMIRS, starts from ca. 0.5 V (RHE) with a sharp voltammetric current peak at 1.0 V at 50 mV/s, while electrooxidation of the bulk CO starts from ca. 0 V in the absence of the strongly adsorbed CO species on Au. The strongly adsorbed CO species acts as a poison for the electrooxidation of CO in the lower potential region.