The influence of pH on reaction pathways for O2 reduction on the Au(100) face

Oxygen reduction on the Au(100) face was studied by the rotating disk-ring electrode technique in solutions of anions which adsorb strongly on gold (HSO₄⁻/SO₄²⁻ and/or OH⁻) over the entire pH range. The specific adsorption of OH⁻ anions, which is a pH dependent process, is found to play the key role in determining the reaction pathways. In the absence of OH⁻ adsorption, for pHs below 6, the reduction of O₂ begins as a 2e-process. Due to the increase in local pH during O₂ reduction, the reaction pathway turns into a 4e-reduction at a certain potential depending on the pH of the solution. For pHs higher than 6, O₂ reduction begins as a 4e-process in the potential region where specifically adsorbed OH⁻ anions are present.     

Defective structure, oxygen mobility, oxygen storage capacity, and redox properties of RE-based (RE = Ce, Pr) solid solutions

Defective structures, surface textures, oxygen mobility, oxygen storage capacity (OSC), and redox properties of RE_0.6Zr_0.4O₂ and of RE_0.6Zr_0.4−xY_xO₂ (RE=Ce, Pr; x=0, 0.05) solid solutions have been investigated using X-ray diffraction (XRD), temperature-programmed desorption (TPD), temperature-programmed reduction (TPR), O₂−H₂ and O₂−CO titration, ¹⁸O/¹⁶O isotope exchange, CO pulsing reaction, and X-ray photoelectron spectroscopy (XPS) techniques. The effects of doping noble metal onto RE_0.6Zr_0.4−xY_xO₂ on oxygen mobility and surface oxygen activities have also been studied. Based on the experimental outcomes, we conclude that: (i) a Pr-based solid solution has better redox behavior than a Ce-based one; (ii) incorporation of yttrium ions in the lattices of CZ and PZ solid solutions could result in an enhancement in oxygen vacancy concentration, Ce⁴⁺/Ce³⁺ and Pr⁴⁺/Pr³⁺ redox properties, lattice oxygen mobility, and oxygen storage capacity; and (iii) doping the noble metal (Rh, Pt, and Pd) onto RE-based solid solution has positive effect on the properties concerned in this work.     

Effect of anions and cations on the aqueous phase catalytic hydrogenation of C=C bonds

The influence of solution composition (nature of anions and cations, pH) in the aqueous phase catalytic hydrogenation of C=C bonds (maleic acid) has been investigated.

In weakly acidic solutions (of pH 3 and 6) the influence of the commonly used anions (ClO₄⁻, HSO₄⁻/SO₄²⁻, Cl⁻) is less important than that obtained in solutions of pH 0.3, found in previous works [Electrochim. Acta 45 (2000) 4299]. This difference can be ascribed to a weaker adsorption of the anion on platinum, as well as, to the different nature of the competitively adsorbed maleic acid species (molecular form or anionic adsorption) depending on the solution pH. In the presence of (hydrogeno)phosphates the lowest activities are obtained implying that these anions are the most strongly adsorbed on platinum.

An important promotion of platinum activity in the presence of cations has also been found. This effect depends on the size of the cation and can be attributed to the modification of the work function and, consequently, of the adsorption properties of platinum.     

Radiotracer study of the underpotential formation of a silver sulfide monolayer on silver electrodes in alkaline medium

The formation of Ag₂S monolayer has been studied in 0.1 mol dm⁻³ NaOH supporting electrolyte by a radiotracer method in the potential range from −400 to +200 mV (on rhe scale) using ³⁵S labelled Na₂S. Comparison of the results obtained with the radiotracer method and cyclic voltammetric measurements leads to the conclusion that the formation of the Ag₂S monolayer takes place at least in two separate steps in two different potential ranges.     

Poly(ethylene oxide)–LiX rubbery electrolytes with X–X disulfonamide anions having two or three ether groups in their structures

This work deals with poly(ethylene oxide), PEO–MX (M=Li, K and Cs) amorphous electrolytes with ⁻X–X⁻, [CF₃SO₂NCH₂(CH₂OCH₂)₂CH₂NSO₂CF₃]²⁻ (EDSA) and [CF₃SO₂NCH₂CH₂(CH₂OCH₂)₃CH₂CH₂NSO₂CF₃]²⁻ (TTSA) disulfonamide anions. These dianions have X⁻ end-groups identical to anions [CF₃SO₂N(CH₂)₂OCH₃]⁻ (MESA) and [CF₃SO₂N(CH₂)₃OCH₃]⁻ (MPSA), one of which (MPSA) was reported to yield chelate-like associated species (presumably LiX₂⁻ triplets) at concentrations above EO/Li=20 in PEO. This feature of LiMPSA, evidenced through glass transition temperature (T_g) measurements, does not apply to Li₂EDSA and Li₂TTSA. Though none of these lithium salts form crystalline intermediate compounds with PEO, the limit of solubility of LiMESA (EO/Li=16) does not allow a clarification of this point for this salt. At lower concentrations, however, a conductivity comparison with the potassium and caesium salts shows that the apparent degree of dissociation (=C_Li+/C_Li) of LiMESA is comparable to that of LiMPSA. As opposed to both these salts and to some extent to Li₂EDSA, a much greater dissociation takes place for Li₂TTSA, the anion of which contains an inner, third ether group in its structure.     

A kinetic study of the formation of 12-molybdosilicate and 12-molybdogermanate in aqueous solutions by ion transfer voltammetry with the nitrobenzene-water interface

A novel electrochemical approach was developed for the kinetic study of the formation of heteropolyanions. The method (dual pulse amperometry, DPA) is based on the detections of currents due to the transfers of polyanions at the nitrobenzene-water interface. In this study, DPA was applied to the kinetic study of the formation of two Keggin anions, viz., [SiMo₁₂O₄₀]⁴⁻ and [GeMo₁₂O₄₀]⁴⁻. Prior to the kinetic study, cyclic voltammetric measurements were performed to confirm that the Keggin anion and its lacunary anion ([H₃SiMo₁₁O₃₉]⁵⁻ or [H₃GeMO₁₁O₃₉]⁵⁻) coexist at equilibrium under certain conditions. In DPA, double voltage pulses of different amplitudes were alternately applied to the interface to follow the concentrations of both the Keggin and the lacunary anions. The concentration-time profiles for the polyanions could be elucidated by the two-step consecutive reactions mechanism. The lacunary anion was then found to be the intermediate of the Keggin anion.     

Spin localization for NO adsorption on surface O atoms of metal oxides

The adsorption of NO on the oxygen site of several metal oxide surfaces is discussed. It is shown that the strength of the interaction and the variation of the bond lengths are not always correlated to the electron transfer from NO to the surface atoms. In cases of irreducible metal oxides, NO₂²⁻ may be strongly adsorbed. The formation of NO₂⁻ on reducible metal oxide is difficult unless terminal oxygen is present on the surface. Then, the reduction of the surface by transferring the unpaired electron from the NO to the surface appears in DFT calculations (VASP code).     

The influence of halide ions at submonolayer levels on the formation of oxide layer and electrodissolution of copper in neutral solutions

The formation of submonolayers of copper (I) halide preceding anodic oxide layer growth and copper electrodissolution has a remarkable influence on these two processes. Chloride electroadsorption on copper at the submonolayer level takes place as two successive stages at potentials 0.7 V lower than the reversible potential of the Cu/CuCl redox couple. Similar results are obtained for iodide, bromide and fluoride ions although the driving force for electroadsorption decreases in the order I⁻ > Br⁻ > Cl⁻ > F⁻. The relative contributions of OH⁻ and halide ion electroadsorption can be modified through the solution composition, applied potential and electroadsorption time. The increase surface coverage by the electroadsorbate diminishes the amount of the passive oxide layer and strongly increases the metal electrodissolution rate, particularly when the amount of passive oxide layer becomes smaller than that required to form a compact monolayer.     

Differential capacitance of the platinum/aqueous-solution interphase

The differential capacitance of polycrystalline platinum in aqueous H₂SO₄ and NaOH solutions has been investigated as a function of bias potential. A capacitance minimum is observed at 0·56 V in H₂SO₄ and − 0·34 V in NaOH solution. There is strong evidence for the irreversible adsorption of oxygen/OH⁻ ions and oxide-film formation. Addition of n-butylamine hinders the adsorption of oxygen/OH⁻ ions and it is itself strongly adsorbed.     

On the nature of the adsorbed species and mechanism of adsorption of In(III) from iodide solutions at the dme

The reduction of In(III) in solutions containing 0.05, 0.1 and 0.25 M of NaI and with an ionic strength of 1.00 maintained with sodium perchlorate, has been studied by means of interfacial admittance measurements at different frequencies and dc potentials of the dropping mercury electrode.

Our results demonstrate that the process takes place with specific adsorption of electroactive species. The adsorption parameters and their dependence with potential have been determined. From the analysis of our results, together with those obtained in a previous study at higher concentrations of I⁻, it can be deduced than InI⁺₂ is the species preferentially adsorbed, this adsorption taking place via a bridge-like mechanism, through I⁻ ions previously adsorbed.     

Effects of oxygen concentration on the electrical properties of ZnO films

In this paper, electrical characteristics by various oxygen content in ZnO films were studied. To control the oxygen content of ZnO films, post-thermal annealing was performed in N₂ and air ambient, led to improve crystallinity and optical properties of ZnO films. The oxygen concentration was measured by Auger electron spectroscopy. The ZnO films having the deficiency of oxygen showed the electron concentrations between 10²¹ and mid 6 × 10¹⁷ cm⁻³ and resistivity at 10⁻³–10⁻¹ Ω cm. On the other hand, when the oxygen concentration of the ZnO films was up to the stoichiometry with Zn, the ZnO films showed low electron concentration at −10¹⁷ cm⁻³ and resistivity at 10 Ω cm.     

Reversible electrochemical insertion of anions in poly-p-phenylene from aqueous electrolytes

Poly-p-phenylene (PPP) was synthesized from benzene according to the Kovacic method. Electrodes were made from this electronic insulator by cold- or hot-pressing of the loose, brown powder, under the addition of 7.5 wt. % soot (Corax L^®, Degussa AG). The electrochemical insertion and removal of anions HSO⁻₄, ClO⁻₄ in this material in aqueous solutions of the corresponding acids was investigated by slow cyclic voltammetry.

Initially, only a surface layer of about 0.1 mm thickness takes part in the electrochemical processes, which are reversible. A maximum concentration of anions in the solid of [(−C₆H₄−)⁺₇ A⁻] is attainable. The maximum degree of insertion is equal to 0.14. The insertion potential U_I shifts strongly into the negative direction with increasing concentration c of the acid. A linear U_I/c relationship is observed as in the case of graphite, where the intercalation potential is more positive by 20–200 mV for the same electrolyte. The round trip current efficiency for the insertion/removal cycle increases with increasing acid concentration attaining 100% in 14 M H₂SO₄ or 11.3 M CHlO₄. For a given concentration, increases in the same order as with graphite (H₂SO₄ < HClO₄ < HBF₄), being somewhat lower for a given electrolyte composition. From anodic current limitation (j_lim = 5–10 mA cm⁻²), a diffusion coefficient of about D = 2 × 10⁻⁷ cm² s⁻¹ is derived for the transport of anions in the bulk of PPP. The striking similarity of our results to former findings with graphite is thoroughly discussed. Some general conclusions are derived thereof.     

Solvent extraction, separation of uranium (VI) with crown ether

A solvent extraction separation of uranium with a new crown hydroxamic acid 5, 14-N, N′-hydroxyphenyl-4, 15-dioxo-1,5,14,18-tetraaza hexacosane (NHDTAHA) in the presence of cerium, thorium and lanthanides is described. The uranium is extracted with chloroform solution of NHDTAHA and the extract is directly used for GS-AAS measurements. The detection limit is 0.01 ppm with a sensitivity of 20 ng/0.005 absorbance of uranium. The extraction constants of uranium crown hydroxamic acid complexes are determined. The selectivity factors (K_uranyl/Kⁿ⁺_M) for uranium crown hyrdoxamate evaluated by comparing the K_uranyl with the stability constants for competing metal cations (Kⁿ⁺_M) and anions (Kⁿ_A⁻) and were found to be remarkably large. Uranium is preconcentrated and also determined spectrophotometrically. The molar absorptivity is 1.0×10⁴ l⁻¹ mol⁻¹ per cm at 390 nm and system obeys Beer’s law in the range 2.0–30 ppm of uranium. Uranium has been determined in standard and environmental samples.     

Formation of singlet molecular oxygen associated with the formation of superoxide radicals in aqueous suspensions of TiO2 photocatalysts

The production and decay of singlet molecular oxygen (¹O₂) in TiO₂ photocatalysis were investigated by monitoring its phosphorescence under various reaction conditions. First, the effects of additives such as KBr, KSCN, KI, H₂O₂, and ethanol on the amount of ¹O₂ produced by photo excitation of P25 TiO₂ were measured. The same additives were employed to investigate the effect on the amount of O₂⁻ produced. Comparison between the effects on ¹O₂ and O₂⁻ suggested that ¹O₂ is formed by the electron transfer mechanism, the reduction of molecular oxygens to O₂⁻ by photogenerated electrons and the subsequent oxidation of O₂⁻ to ¹O₂ by photogenerated holes. The formation of ¹O₂ decreased at pH < 5 and pH > 11, indicating that the intermediate O₂⁻ is stabilized at the terminal OH site of the TiO₂ surface in the pH range of 5 < pH < 11. Eighteen commercially available TiO₂ photocatalysts were compared on the formation of ¹O₂ and O₂⁻ in an aqueous suspension system. The formation of ¹O₂ was increased with decreasing size of TiO₂ particles, indicating that a large specific surface area causes a higher possibility of reduction producing O₂⁻ and then a large amount of ¹O₂ is formed. The difference in the crystal phase (rutile and anatase) did not affect the formation of ¹O₂.     

Adsorption of methane/ethane mixtures on dry coal at elevated pressure

The binary adsorption characteristics of methane and ethane on dry coal to 40 atm pressure have been calculated from pure-component isotherms. In some coal seams, pressures exceeding 40 atm have been recorded and the methane sampled from the virgin coal often shows a few percent of ethane. The binary adsorption characteristics were calculated by employing the ideal adsorbed solution theory of Myers and Prausnitz, and experimentally-determined (Type I) pure gas isotherms at 0, 30 and 50 °C. The coal used in this investigation was high-volatile ‘A’ bituminous (hvab) from the Pennsylvania Pittsburgh seam. Gas nonideality was accounted for by replacing pressure with fugacity. Adsorption of methane on dry coal is purely physical; the isosteric heat of adsorption does not exceed 2.4 kcal/mol^* at 30 °C on the above coal. Isobars on the resulting binary equilibrium diagram exhibited an unexpected phenomenon of intersecting each other which might be attributable to the above nonideality considerations. The region of a few percent of ethane, which is of practical importance from the viewpoint of coal seams, was expanded and reduced to an equation: V(CH₄) = −21.52 + 7.18(VF) + 16.88(VF)² −0.395(P) − 0.00661(P)² + 0.824(T) − 0.00030(T)² + 0.928(VF)(P) − 0.858(VF)(T). V(Total) = 25.9 − 23.6(VF) + 0.655(P) − 0.00875(P)² − 0.795(T) + 0.743(VF)(T) where V(CH₄) and V(Total) = cm³(STP)CH₄ and total gas respectively adsorbed per g dry coal; VF = vol. fraction of methane as analysed at 1 atm (0.94 VF 1.0); P = seam pressure, atm (0 P 40); T=seam temperature, °C(−10 T 50).     

In situ IR and pulse reaction studies on the active oxygen species over SrF2/Nd2O3 catalyst for oxidative coupling of methane

Pulse reaction method and in situ IR spectroscopy were used to characterize the active oxygen species for oxidative coupling of methane (OCM) over SrF₂/Nd₂O₃ catalyst. It was found that OCM activity of the catalyst was very low in the absence of gas phase oxygen, which indicated that lattice oxygen species contributed little to the yield of C₂ hydrocarbons. IR band of superoxide species (O₂⁻) was detected on the O₂-preadsorbed SrF₂/Nd₂O₃. The substitution of ¹⁸O₂ isotope for ¹⁶O₂ caused the IR band of O₂⁻ at 1128 cm⁻¹ to shift to lower wavenumbers (1094 and 1062 cm⁻¹), consistent with the assignment of the spectra to the O₂⁻ species. A good correlation between the rate of disappearance of surface O₂⁻ and the rate of formation of gas phase C₂H₄ was observed upon interaction of CH₄ with O₂-preadsorbed catalyst at 700 °C. The O₂⁻ species was also observed on the catalyst under working condition. These results suggest that O₂⁻ species is the active oxygen species for OCM reaction on SrF₂/Nd₂O₃ catalyst.     

FTIR study of the adsorption of single pollutants and mixtures of pollutants onto titanium dioxide in water: oxalic and salicylic acids

The use of ATR–FTIR to probe the adsorption of oxalic and salicylic acids, and of mixtures of both, onto TiO₂ (Degussa P-25) demonstrates the potential of the technique to characterise the evolution of the catalyst with time, including surface poisoning. Under equilibrium dark conditions, three surface species are formed by oxalate, and two by salicylate. Their stabilities, described by conditional Langmuir-type equilibria involving the dissociative, electroneutral adsorption of the acids H₂L, are at pH 3.7, 2.4×10⁶, 3.0×10⁴ and 3.0×10³ mol⁻¹ dm³ for oxalic acid, and 2.9×10⁵ and 9.1×10³ mol⁻¹ dm³ for salicylic acid. The nature of the species is discussed in terms of their spectral features. The displacement of each acid from the surface by addition of the other one was followed also by ATR–FTIR. The results demonstrate that oxalate displaces totally chemisorbed salicylate, whereas salicylate displaces oxalate only partially. These results are explained by assuming competitive chemisorption onto two different surface sites, plus oxalate adsorption onto a site that exhibits negligible affinity for salicylate. Irreversible adsorption by partially oxidised products in the course of photocatalytic processes can therefore be assessed by ATR–FTIR.     

过氧化氢溶液中痕量离子吸附分离用氨基功能化SBA-15的制备及性能

通过后接枝法制备了氨基功能化SBA-15介孔氧化硅(S-N),用于吸附过氧化氢溶液中的金属和阴离子及有机杂质制高纯过氧化氢.研究了接枝量对吸附剂结构和性能的影响并发现S-N中的氮含量随接枝剂用量增加而增加,最高为1.83％.S-N保留了高度有序的六方孔道结构,但比表面积和孔体积均随接枝量增加而下降.在过氧化氢溶液中,各...     

Polypyrrole materials doped with weakly coordinating anions: influence of substituents and the fate of the doping anion during the overoxidation process

The influence of weakly coordinating anions with different shapes and substituents has been studied to get the overoxidation resistance limit of the material, ORL. The anions utilized are derivatives of [Co(C₂B₉H₁₁)₂]⁻, [B₁₂H₁₂]²⁻ and [B₁₂H₁₁NH₃]⁻. The following tendencies have been established (1) boron cluster monoanions are to date the anions that offer the highest stability to overoxidation of PPy doped materials (2) the ORL stability of the material can not be attributed only to the shape of the cluster (3) monoanionic clusters are far superior than dianionic to get an ORL rise (4) cluster charge density reduction results in ORL rise as has been observed in [Co(C₂B₉H₁₁)₂]⁻ after incorporation of electron-withdrawing substituents with no electron back-donation (5) globular, rigid and large monoanions are less suitable for enhanced ORL values than elongated and non-rigid species (6) adequate anion's substitution produce a rise in the ORL of the material, thus polyether side-arms are beneficial with [Co(C₂B₉H₁₁)₂]⁻, whereas, T-shaped methylaryl groups are appealing in [B₁₂H₁₁NH₃]⁻ based materials, respectively, (7) substituents on the anions usually imply higher difficulty in the materials' growth. The high boron contents in these materials has permitted to learn on the fate of the doping anions during the overoxidation process. There is a built-up of the concentration of the doping anion in the electrolyte near surface area, whereas, a depletion is observed in the nearest inner layers.     

Photoredox processes in the Cr(VI)–Cr(III)–oxalate system and their environmental relevance

Irradiation of the [Cr(C₂O₄)₃]³⁻ complex or the chromate(VI)–oxalate mixture, or the ternary system composed of Cr(III), Cr(VI) and oxalate, leads to chromium photoreductions in consequence of the ligand to metal charge transfer (LMCT) excitations induced by artificial solar radiation. In the case of the Cr(III) complex, the photoreduction involves the innersphere electron transfer leading to the formation of the Cr(II) species and the C₂O₄⁻ radicals. On reacting with molecular oxygen, Cr(II) is oxidised to Cr(III) catalysing thereby the oxalate substitution reaction. Moreover, under specific conditions, Cr(II) can be also oxidised to Cr(VI). Chromate(VI) is not photoreducible, but in the presence of oxalate, or other sacrificial electron donor, the outersphere photoinduced electron transfer (PET) produces Cr(V) species and the C₂O₄⁻ radicals. This initiates a series of thermal reactions leading to the formation of Cr(III) and oxidized oxalate (CO₂). In the system composed of [Cr(C₂O₄)₃]³⁻ and chromate(VI), the acidic medium and anoxic conditions favour the Cr(VI) photoreduction, whereas alkaline oxygenated solutions assist the Cr(VI) photoproduction. When an approximately neutral solution equilibrated with the ambient air is irradiated intermittently, Cr(VI) is consumed and/or produced, accordingly to the time sequence of exposure and dark periods. The oscillations of Cr(VI) concentrations are accompanied by continuous oxidation of oxalate, playing the role of the sacrificial electron donor. The effects of solution pH, molecular oxygen, concentrations of reagents and cations on the reaction rates were investigated. The results of this paper revealed that the Cr(III)/Cr(VI) system under environmental conditions behaves as the photocatalytic one catalysing the oxidation of oxalate or other organic matter by molecular oxygen, contributing thereby to the abatement of pollution.