Catalytic effects of metal submonolayers formed by faradaic adsorption on the anodic oxidation of ascorbic acid at a platinum electrode

The catalytic effects of metal ions on the anodic oxidation of ascorbic acid on a Pt electrode in 1 M HClO₄ were studied by linear sweep voltammetry. The anodic peak due to a two-electron oxidation of ascorbic acid shifts to the negative potential side on the addition of Bi³⁺. This indicates the accelerating effect of Bi³⁺ on the oxidation of ascorbic acid. The presence of other metal ions, such as Pb²⁺, Hg²⁺, Tl⁺, Ag⁺ and Sb³⁺, also exerts similar effects. These metal ions were adsorbed on a Pt electrode at underpotentials and the adsorbed metals (denoted as M_ad) still remain on the electrode surface until the electrode potential goes up to and beyond the peak potential of the oxidation of ascorbic acid. On the other hand, metal ions forming no adsorbed layer on Pt, such as Co²⁺, Zn²⁺, Fe³⁺ and Ni²⁺, exhibit no catalytic effect. These facts suggest that the presence of a M_ad on Pt is essential for the promotion of the anodic oxidation of ascorbic acid. However, there is a difference in the catalytic action among the M_ad, for example, Cu_ad, Cd_ad, In_ad, Sn_ad and Mo_ad display no catalytic action.The catalytic activity depends on the degree of surface coverage by the M_ad. The maximal effect of the M_ad is attained in the submonolayer region. The effects of metal ions were discussed on the basis that the M_ad plays its major role in the removal of the adsorbed ascorbic acid occupying active sites on the electrode surface, and provides effective sites for the activation of adjacent water molecules. Furthermore, from the ¹³C NMR spectra for the oxidation products, the adsorbed water on the M_ad appears to function by promoting the subsequent hydration steps, following the electron-transfer step of ascorbic acid.     

522—Catalytic oxydation of biological components on platinum electrodes modified by adsorbed metals: Anodic oxidation of glucose

The catalytic oxidation of glucose on Pt electrodes modified by adsorbed metals was studied in 1 M HClO₄ by linear sweep voltammetry. The adsorbed metals (denoted as M_ad, such as Bi_ad and Pb_ad) formed on Pt in the potential region more positive than the reversible potential of an M⁼⁺/M^o couple, lead to a marked increase in the anodic c?urrent of glucose by about one order of magnitude. The catalytic activity depends on the surface coverage by the M_ad. The strongly adsorbed species of lactone type, which are responsible for blocking the successive oxidation, are formed on the electrode surface in the anodic processes of glucose on a bare Pt electrode. The formation of such poisonous species is accelerated in the presence of adsorbed hydrogen on Pt. The effects of M_ad were discussed on the basis that M_ad plays its major role on the Pt electrode surface in removal of the adsorbed hydrogen which initiates the formation of the poisonous species.     

Effect of Anionic Electrolytes and Precursor Concentrations on the Electrodeposited Pt Structures

Electrodeposition of functional metal surfaces has received great attention because of their useful applications. Recently, interesting electrodeposition behavior of Pt at −0.8 V (vs. Ag/AgCl) was reported, where underpotential deposited H (H_upd) layers played a unique role in the electrodeposition. Here, we report the effect of anionic electrolytes and precursor concentrations on the electrochemical deposition behavior of Pt. Depending on these two experimental parameters, two distinct Pt structures, monolayer Pt films and Pt spheres, were electrodeposited at −0.8 V. In addition to the underpotential deposited H (H_upd) layers formed at −0.8 V, the adsorption of Cl⁻ also plays a significant role in determining the electrodeposited Pt structures. When the PtCl₄²⁻ concentration was low and the Cl⁻ concentration was high enough for the adsorption of PtCl₄²⁻ to be blocked by the H_upd and Cl⁻ layers, monolayer Pt films were electrodeposited. Otherwise, further electrodeposition of Pt spheres over the monolayer Pt films occurred. The effect of other halide ion adsorption and the controlled growth of Pt spheres during the Pt electrodeposition were also investigated. The electrochemical deposition behavior of Pt demonstrated in this work provides insight into the fabrication of functional Pt surfaces.     

Catalytic oxidation of biological components on platinum electrodes modified by adsorbed metals: Anodic oxidation of glucose

The catalytic oxidation of glucose on Pt electrodes modified by adsorbed metals was studied in 1 M HClO₄ by linear sweep voltammetry. The adsorbed metals (denoted as M_ad, such as Bi_ad, and Pb_ad) formed on Pt in the potential region more positive than the reversible potential of an M_z+/M⁰ couple, lead to a marked increase in the anodic current of glucose by about one order of magnitude. The catalytic activity depends on the surface coverage by the M_ad. The strongly adsorbed species of lactone type, which are responsible for blocking the successive oxidation, are formed on the electrode surface in the anodic processes of glucose on a bare Pt electrode. The formation of such poisonous species is accelerated in the presence of adsorbed hydrogen on Pt. The effects of M_ad were discussed on the basis that M_ad plays its major role on the Pt electrode surface in removal of the adsorbed hydrogen which initiates the formation of the poisonous species.     

TiO2−CeOx−Pt Hollow Nanosphere Catalyst for Low-Temperature CO Oxidation

TiO₂−CeO₂−Pt hollow nanospheres (1 wt-% Pt) are realized using a liquid-phase strategy using NaCl as a template. The NaCl template is first coated with TiO₂ and thereafter with CeO₂ via the hydrolyzation of TiCl(OiPr)₃ and Ce(OiPr)₄ as suitable alkoxides. Finally, the NaCl template is removed by washing with water. The resulting @TiO₂−CeO₂ hollow nanospheres (□: inner cavity) exhibit an outer diameter of 140–180 nm, a wall thickness of 30–40 nm, an inner cavity of 80–100 nm, a specific surface area of 210 m²/g, a pore volume and area of 0.08 cm³/g and 191 m²/g, mainly with micropores ≥5 Å and ≤14 Å. The hollow nanosphere support is impregnated with Pt nanoparticles, using two different methods – a wet-chemical deposition (Pt(ac)₂, acetone, 25 °C) and a supercritical fluid reactive deposition (SFRD) process ([Pt(COD)Me₂], supercritical CO₂, 80 °C, 15.6 MPa) resulting in an uniform size distribution with Pt nanoparticles 2.5±0.1 nm (TiO₂−CeO₂−Pt_WCD) and 2.3±0.1 nm (TiO₂−CeO₂−Pt_SFRD) in size. The catalytic properties of the TiO₂−CeO₂−Pt hollow nanospheres are evaluated for CO oxidation between 50 and 500 °C. A promising catalytic activity and stable light-out/light-off temperatures are observed especially for the TiO₂−CeO₂−Pt_SFRD sample, indicating the suitability of hollow nanospheres as high-porosity catalyst material.     

Semi-micro ion chromatography of iodide in seawater

Large sample volume injections including both on-column analyte focusing and on-column matrix elimination techniques were examined for semi-micro ion chromatography of trace iodide (I⁻) in seawater. A semi-microcolumn (35×1 mm I.D.) packed with styrene–divinylbenzene copolymer with high anion-exchange capacity and a mobile phase of 0.03 M NaClO₄+0.5 M NaCl+5 mM sodium phosphate buffer, pH 6.0, was used. Iodide in seawater was effectively concentrated on the column by both electrostatic and hydrophobic interactions and was eluted without peak broadening. ClO₄⁻ (NaClO₄) in the mobile phase was effective for the elution of iodide and Cl⁻ (NaCl) for both the concentration of iodide (I⁻) with hydrophobicity and the removal of interference by the major anions. An excess of major anions in seawater did not disturb the detection of iodide at UV 226 nm. The relative standard deviations for successive injections of 5 and 1 μg/l I⁻ (2 ml of 35‰ artificial seawater) were 1.5 and 5.8% (n=5, each), respectively. The slope of calibration curve (by peak area) using the semi-microcolumn was ∼2.8-times higher than that for a conventional column with the same resin (150×4.6 mm I.D.) The present method had a detection limit of 0.2 μg/l I⁻ for 2 ml of 35‰ artificial seawater and was successfully applied to seawater samples.     

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.     

Kinetics of the Cu(I)/Cu(Hg) electrode reaction in acetonitrile solutions of chloride ions

The electrode reaction Cu(I)/Cu(Hg) in complex chloride solutions with acetonitrile as solvent has been investigated at the equilibrium potential by the faradaic impedance method. The ionic strength was 0.1 M with tetraethylammonium perchlorate as supporting electrolyte, and the temperature was 25°C. Differential capacity data for mercury have been determined separately, and other double-layer data have been obtained from electrocapillary measurements. From the results of the kinetic measurements at [Cu⁺]<1 mM, corresponding to potentials E-E_ecm<−120 mV, it is concluded that the solvated Cu⁺ as well as the complexes CuCl and CuCl₂⁻ take part in the charge transfer. The rate constant increases considerably on formation of CuCl. This increase can be correlated to a simultaneous, pronounced decrease in the inner-sphere solvation. The differential capacity for mercury displays a high peak at E-E_ecm∼−100 mV. It is suggested that the peak could be an adsorption-desorption peak for (C₂H₅)₄N⁺.     

The nature of cis amine Pd(II) and antitumor cis amine Pt(II) complexes in aqueous solutions

Upon addition of base the complex enPd(OH₂)₂²⁺ rapidly dimerizes to form a binuclear dihydroxo bridged dimer with no significant mononuclear hydroxo complexes at 10 mM concentration. The corresponding Pt(II) complex titrates two equivs base with pK_a 5.8 and 7.6 to give a dihydroxo complex which is stable indefinitely. Addition of only one equiv of base to yield near neutral solutions slowly produces the Pt(II) dihydroxo bridged dimer. The hydroxo groups of dienPtOH⁺, dienPdOH⁺ and enPt(OH⁻)₂ are effective nucleophiles for ester hydrolysis and their reactivities are in accord with a suggested dependence on basicity. The antitumor complexes of cis dichloro Pt(II) amines are converted to aquo and hydroxo complexes of Pt(II) amines. Due to the low ambient chloride ion concentration within a cell, aquo complexes are present in significant amounts and are most likely to be the reactive species. The reactivity of cis Pt(II) amine antitumor complexes is reduced by Cl⁻, OH⁻ and dihydroxy bridged dimer substituents, the formation of which is time dependent.     

Anodic overoxidation of poly-p-phenylene in aqueous electrolytes

Poly-p-phenylene (PPP), synthesized chemically by the Kovacič method, was mixed with 7.5% carbon black and used as a pressed thin-layer electrode (100 μm) on Pt mesh. Its overoxidation as an anode in aqueous acid electrolytes (8–18 M H₂SO₄, 8–11.3 M HClO₄, 8 M HBF₄) was investigated systematically.Slow cyclic voltammetry reveals that the reversible redox peak 1, leading to an insertion compound up to 1 F per mole (C₆H₄)₆, is following by a large second peak 2 at more positive potentials, corresponding to about 15 F with respect to the unit mentioned above. Peak separation increases with increasing acid concentration due to a negative shift of the insertion potential (peak 1) by 60 mV mol⁻¹ dm³, but normal Nernst behaviour is observed in the case of peak 2. The strong decay of the current efficiency for reversible cycling of PPP at concentrations below 10 M can be understood readily from these findings. The overoxidation starts with the formation of the polymer radical cation, which is oxidized further to the quinone, then to the product of ring opening and finally to the product of cleavage of the polymer chain. The final product seems to be a derivative of maleic acid. The p-quinone path is preferred over the o-quinone path, quite contrary to the case with graphite. The practical consequences with respect to battery application are discussed in detail.     

Methanol Oxidation at Platinum Coated Black Titania Nanotubes and Titanium Felt Electrodes

Optimized Pt-based methanol oxidation reaction (MOR) anodes are essential for commercial direct methanol fuel cells (DMFCs) and methanol electrolyzers for hydrogen production. High surface area Ti supports are known to increase Pt catalytic activity and utilization. Pt has been deposited on black titania nanotubes (bTNTs), Ti felts and, for comparison, Ti foils by a galvanic deposition process, whereby Pt(IV) from a chloroplatinate solution is spontaneously reduced to metallic Pt (at 65 °C) onto chemically reduced (by CaH₂) TNTs (resulting in bTNTs), chemically etched (HCl + NaF) Ti felts and grinded Ti foils. All Pt/Ti-based electrodes prepared by this method showed enhanced intrinsic catalytic activity towards MOR when compared to Pt and other Pt/Ti-based catalysts. The very high/high mass specific activity of Pt/bTNTs (ca 700 mA mg_Pt⁻¹ at the voltammetric peak of 5 mV s⁻¹ in 0.5 M MeOH) and of Pt/Ti-felt (ca 60 mA mg_Pt⁻¹, accordingly) make these electrodes good candidates for MOR anodes and/or reactive Gas Diffusion Layer Electrodes (GDLEs) in DMFCs and/or methanol electrolysis cells.     

The importance of concentration effects at the electrode surface in anodic stripping voltammetric measurements of complexation of metal ions at natural water concentrations

The influence of the ligand/metal ion concentration ratio on the shape, peak current and peak potential of curves obtained by anodic stripping voltammetry (a.s.v.) at the hanging mercury drop electrode is described, particularly with respect to the use of a.s.v. for speciation of metal ions at very low concentrations as is often found in natural waters. The lead(II)/triethylenetetramine system is used as a model of a fully labile reversible system. It is shown that the total metal ion concentration at the electrode surface (C^o_M) during the stripping step may be much larger (30–300 times in typical conditions) than that in the bulk solution (C_M), the exact value depending on the deposition time t_d. Consequently, changes in the peak characteristics are observed when the ligand/metal concentration ratio in the bulk of the solution, C_L/C_M, is less than 1000. Semi-empirical equations, experimentally tested, are given, which enable C^o_M/C_M to be estimated for a specified solution and a.s.v. conditions, which correct for the “surface concentration effect” when a.s.v. is used to measure complexation, and which describe the influence of the parameters such as stirring efficiency, radius of the mercury drop and C_L/C_M. The implications of the results are discussed for determinations of total metal ion in complex media, of speciation based on peak-potential shifts or stripping voltammetric curves, and of complexation capacity.     

Catalytic oxidation of biological components on platinum electrodes modified by adsorbed metals: Anodic oxidation of catecholamines

The catalytic oxidation of catecholamines on Pt electrodes modified by adsorbed metals (denoted as M_ad) was studied in 1 M HClO₄) by linear sweep voltammetry. The anodic peaks of four catecholamines (adrenaline, noradrenaline, iso proterenol and DOPA) shift to the negative potential side in the presence of M_ad, such as Bi_ad and Pb_ad. The anodic oxidation of catecholamines proceeds without the production of poisonous species on a bare Pt electrode. The catalytic activity depends on the surface coverage by M_ad (denoted as θ_M). The maximal effect of M_ad is attained at θ_M = 0.5. The presence of M_ad promotes the formation of the final product through irreversible hydroxylation following the electron transfer. From these results it was suggested that in the catalytic processes M_ad plays the major role in the provision of effective sites to activate water molecules which take part in the subsequent hydroxylation step.     

Ion exchange chromatography coupled to inductively coupled plasma mass spectrometry for the study of Pt electro-dissolution

There is an ongoing debate regarding the mechanism of Pt electrochemical dissolution. However, only off-line methods have so far been used, where separation of Pt species is performed separately from their detection. In this study, ion exchange chromatography coupled to inductively coupled plasma mass spectrometry was used for the first time to separate and detect Pt species generated by the electro-dissolution of a Pt electrode in 0.5 M aqueous H₂SO₄ solution. Because these species are either neutral or cationic, they were converted to chloro-complexes using 0.1 M KCl to enable their separation by anion exchange chromatography. Chloro-aqua complexes were observed in addition to the two predominant species, namely PtCl₄²⁻ and PtCl₆²⁻. A good linear relationship was observed between the sum of peak areas for all complexes of a given Pt oxidation state and the Pt concentration, with a detection limit of 0.1 μg L⁻¹ being reached for Pt(II) and Pt(IV). Application of this speciation analysis method to real samples generated by potential cycling using cyclic voltammetry (CV) revealed that, in general, at least 80% of Pt was present as Pt(II), irrespectively of the cyclic potential range or of temperature (up to 60 °C). Still, quantitative spike recovery was achieved after adding known amounts of Pt(II) or Pt(IV) to a sample prepared by CV, which demonstrated that no significant species inter-conversion took place.     

Electro Catalytic Properties of α, β, γ, ϵ ‐ MnO2 and γ ‐ MnOOH Nanoparticles: Role of Polymorphs on Enzyme Free H2O2 Sensing

Polymorphs of Manganese di oxide (MnO₂) such as alpha (α), beta (β), gamma (γ), epsilon (ϵ), and MnOOH type materials were prepared via hydrothermal approach under different conditions. The samples were characterized by XRD, FESEM, FT‐IR, Raman and BET analysis. Cyclic voltammetry (CV) analysis confirm that α ‐ MnO₂ shows better electro‐catalytic ability. Amperometry sensing of hydrogen peroxide (H₂O₂) was carried out by varying applied potential value with the polymorphs of MnO₂. Compared with the other phases of MnO₂, α ‐ MnO₂ shows high linear range up to 20μM. The calculated sensitivity value for H₂O₂ sensing of different phases is in the order of α ‐ MnO₂, β ‐ MnO₂, ϵ ‐ MnO₂, γ ‐ MnO_2, MnOOH and found to be 0.094 mA μM⁻¹ cm⁻² > 0.072 mA μM⁻¹ cm⁻² > 0.07 mA μM⁻¹ cm⁻² > 0.03 mA μM⁻¹ cm⁻² > 0.01 mA μM⁻¹ cm⁻² respectively. All the characterization results reveal that crystalline phase plays a vital role in electrochemical behavior rather than crystalline size, morphology, surface charge, surface area.     

The configuration of transition metal coordination compounds containing the anion of hexahydro-1,3,5-triazine-1,3,5-triol

The configuration of Ni(IV), Fe(IV), Mn(IV) and V(IV) complexes of the type M^I₂[M^IVL₂] where M^I=Li, Na, K, Rb and Cs and the ligand L is the anion C₃H₆N₃O³⁻₃ of hexahydro-1,3,5-triazine-1,3,5-triol has been obtained by studying the infrared spectra of the complexes in the region 4000-50 cm⁻¹. Vibrational assignments for the observed bands of the complex ions have been made assuming the molecular symmetry D_3d. The assignments have been based on the observed isotopic frequency shifts due to the substitutions H/D, ¹⁴N/¹⁵N, ¹²C/¹³C, ⁵⁸Ni/⁶²Ni and ⁵⁴Fe/⁵⁷Fe and on the assignment of the bands in the infrared spectrum of hexahydro-1,3,5-triazine-1,3,5-triol, C₃H₆N₃ (OH)₃.     

Oxide‐Nanotrap‐Anchored Platinum Nanoparticles with High Activity and Sintering Resistance by Area‐Selective Atomic Layer Deposition

An area‐selective atomic layer deposition (AS‐ALD) method is described to construct oxide nanotraps to anchor Pt nanoparticles (NPs) on Al₂O₃ supports. The as‐synthesized catalysts have exhibited outstanding room‐temperature CO oxidation activity, with a significantly lowered apparent activation energy (ca. 22.17 kJ mol⁻¹) that is half that of pure Pt catalyst with the same loading. Furthermore, the structure shows excellent sintering resistance with the high catalytic activity retention up to 600 °C calcination. The key feature of the oxide nanotraps lies in its ability to anchor Pt NPs via strong metal–oxide interactions while still leaving active metal facets exposed. Our reported method for forming such oxide structure with nanotraps shows great potential for the simultaneous enhancement of thermal stability and activity of precious metal NPs.     

Electrochemical behavior of 2-thiouracil on silver electrodes

The reactions of 2-thiouracil with a silver electrode during anodic deposition and cathodic stripping were investigated using a rotating silver disk electrode and linear potential scan voltammetry.The formation of two different insoluble silver salts with 2-thiouracil and its dimerization product (2-thiouracil disulfide) and the dependence of the stripping peak current on the concentration of 2-thiouracil and the potential range are discussed.It has been found that these sparingly soluble silver compounds of 2-thiouracil formed during anodic deposition can be stripped by scanning the potential in a negative direction. The influence of parameters on the shape and the height of the stripping peaks has been determined.For concentrations of 2-thiouracil less than 6×10⁻⁶ mol dm⁻³, one stripping peak was observed. Two stripping peaks are observed in the intermediate concentration range 1×10⁻⁵–5×10⁻⁵ mol dm⁻³.A random stripping process with repulsive lateral interactions in the film was observed at concentrations of 2-thiouracil ∼ 10⁻⁷ mol dm⁻³.Experimental data were found to fit a Frumkin isotherm with a Temkin coefficient, g, of 5.     

Pulsed amperometric detection of sulfur compounds: Part II. Dependence of response on adsorption time

A mathematical model is developed for the response of PAD and is applied to data from the study of I_p as a function of t_ads for evaluation of the adsorption rate constants, and the maximum molar surface coverage for thiourea at a Pt electrode. The results are, respectively: k₁ = 4.1 × 10⁴ M⁻¹ s⁻¹, k₋₁ = 1.9 s⁻¹, and Γ₀ = 1.3 × 10⁻¹⁰ mol cm⁻². The calculated adsorption equilibrium constant (k₁/k₋₁) is 2.1 × 10⁴ M⁻¹, compared to 4.9 × 10⁴ M⁻¹ calculated from the plot of 1/I_p vs. 1/c^b for c^b > 1.0 × 10⁻⁴ M and t_ads = 8500 ms. Analytical calibration procedures are examined; linear plots of 1/I_p vs 1/c^b cannot be expected for cases of mixed transport-isotherm control of detector response.