The influence of solution composition on the kinetics of “reduced” CO2 electrooxidation at polycrystalline platinum

The electrooxidationof “reduced” CO₂ intermediate adsorbed at polycrystalline Pt was studied for comparison in 0.5 M H₂SO₄, 0.05 M HClO₄ and 1 M KOH. The voltammograms showed a current peak multicity in 0.05 M HClO₄ and in 1 M KOH. Current transients in 0.05 M HClO₄ showed a complex decay at E_o, between 0.6 and 0.8 V. In 1 M KOH these transients showed a minimum and a maximum which shift in the time scale with E_ad.The kinetic interpretation of the results was attempted considering a Temkin isotherm at low positive potentials. Tafel slopes obtained from the dependence of E_p vs. log υ are close to RT/F at high positive potentials. This value accounts for a chemical reaction as rds. Anion Adsorption at the polycrystalline Pt electrode would change the surface energy for Pt(OH) electroformation and hence it should influence the electrooxidation processes indirectly.     

Structural and magnetic properties of “one-dimensional” barium vanadium triselenide

BaVSe₃ has been synthesized and its crystal structure determined at 293(2)°K. The structure was solved in the hexagonal space group P6₃/mmc (D⁴_6h), with a = 6.9990(11) and c = 5.8621(13) Å. Scans (2 Θ) of a polycrystalline sample revealed that BaVSe₃ undergoes a transition to an orthorhombic unit cell (b′ 3^1/2 a, a′ a, c′ c) at 303(5)°K. Magnetic susceptibility measurements between 4 and 300°K indicate that BaVSe₃ is paramagnetic down to 41(1)°K, where magnetic ordering occurs, with a magnetic moment in the ordered phase of 0.2 μ_B per vanadium atom. The orthorhombic lattice distortion may be caused by the “freezing in” of “soft” vibrational modes.     

Activation of electrode surfaces: semiquantitative characterization of electrode surfaces modified with heteropolyanions

Several Techniques have been used successively or in combination to approach a better understanding of surfaces modified by silicotungstic heteropolyanion. Due to the strikingly high activity of these electrodes for the hydrogen evolution reaction, in particular, there exists a need to identify the chemical composition of the catalyst, to evaluate the amount of catalyst on the surface, and to study the electrochemical behaviour of the “catalyst” itself. Plasma emission spectrometry shows semi-quantitatively that the catalyst resembles the starting material closely, although it is not possible at present to give the exact chemical formula. Electron microprobe analysis (EMA) confirms the presence of tungsten, and especially silicon, on the surface, even though the concentration of this last element is very low. The absence of platinum in the starting HPA, as well as on the electrode surface, is also clearly demonstrated by EMA, which is of prime importance in showing that the “HPA catalyst” is active by itself. Combined coulometry and UV-visible spectroscopy have been used to evaluate the amount of HPA consumed during electrolysis. Cyclic voltammetry shows a surface redox couple corresponding to the catalyst, and well separated from the first redox couple of SiW₁₂O^4-₄ in 0.5 M H₂SO₄ solution. A better understanding of the activation-deactivation processes of the catalytic electrode surface ensues.     

The oxycarbonates Sr4(Fe2−xMnx)1+y(CO3)1−3yO6(1+y): nano, micro and average structural approach

The possibility to synthesize layered oxycarbonates, with nominal composition Sr₄Fe_2−xMn_xO₆CO₃ involving trivalent manganese, with 0≤x≤1.5, is reported for the first time. The structural study of Sr₄FeMnO₆CO₃ using NPD, HREM, Mössbauer and XANES, shows that this phase is closely related to n=3 member of the Ruddlesden–Popper family. It derives from the latter by replacing the middle layer of transition metal octahedra by triangular CO₃ groups, with two different “flag” and “coat hanger” configurations. The magnetic order is antiferromagnetic and fundamentally different from the magnetic behavior of Sr₄Fe₂O₆CO₃.     

The impedance behavior of the cell Au/HClO4 · 5.5 H2O/Au in the temperature range 4.2–300 K

The impedance of the cell Au/HClO₄-5.5 H₂O/Au was investigated in the frequency range 1 to 10⁵ Hz between 4.2 and 300 K. The analysis of the data enables an evaluation of important electrolyte properties such as conductivity and dielectric constant in a wide range of temperatures, predominantly in the solid state of the electrolyte HClO₄-5.5 H₂O (T_f = 228 K). The double layer capacity of the gold electrodes was also determined; it shows a qualitatively similar result compared with previous measurements. In the solid state, the ionic conductivity exhibits two distinct activation energies of 0.37 and 0.54 eV corresponding to the two phases present in HClO₄-5.5 H₂O above and below 170 K. Below 120 K the activation energy becomes very small and tends to zero around 80 K indicating possible tunneling processes in the rigid H₂O structure. At about the same temperature the dielectric constant reaches its low temperature limit with a value _∞ ≈ 11 which is considerably higher than the value of pure ice of _∞ ≈ 3.     

“Complex” versus “free radical” mechanism for the catalytic decomposition of H2O2 by ferric ions

Kinetic and spectrophotometric measurements made during the Fe³⁺ ion catalyzed decomposition of H₂O₂ have been analyzed using the computer simulation method. Improved values of the rate constants of the “complex scheme” and of the molar absorptivities ofthe intermediates were obtained: k₃/K_M = 4.94 M^?1 min^?1, k₄ = 193 M^?1 min^?1, ε_I/K_M = 52.3 M^?2 cm^?1, ε_II = 25.7 M^?1 cm^?1. The simulation revealed details of the reaction which were unavailable by other means and which explained several features of its kinetics. The total amount of O₂ evolved in the reaction using [H₂O₂] ～ 10^?2 M has been calculated and found to be nearly stoichiometric. O₂ evolution experiments in this region cannot, thus, distinguish between the “complex mechanism” predicting nearly stoichiometric evolution of O₂ and the “free radical mechanism” predicting exactly stoichiometricamounts of O₂. There are discrepancies within the “free radical scheme” with regard to the correct values of the rate constants to fit the reactions of H₂O₂ both with Fe²⁺ and Fe³⁺ ions, as well as other reactions assumed to proceed via free radicals.     

An experimental study on the “sequential order” rules in generalized two-dimensional correlation spectroscopy

Following our theoretical analysis on the “sequential order” rules in generalized two-dimensional (2D) correlation spectroscopy (H. Huang, Anal. Chem. 79 (2007) 8281–8292), an experimental study was conducted to test the “sequential order” rules using the FT-NIR data of poly(3-hydroxybutyrate) (PHB)/poly(l-lactic acid) (PLA) blends under uniaxial elongation and parallel polarization. The local sequential order concept proposed for the generalized two-dimensional (2D) correlation spectroscopy is now more clearly stated; “the intensity change at ν1 occurs predominantly before ν2” means that the starting time of the intensity change at ν1 is prior to that at ν2. It is this local sequential order which reflects the real and intuitive sequential order between two events in generalized situations. It has been found that the integrated/overall sequential order results obtained from the 2D correlation analysis may be contradictory to the intuitive local sequential order. In addition, different integrated/overall sequential orders could be obtained by selection of different sampling intervals from a certain set of experimental data, or choosing different number of the contours for the same sampling interval. These new experimental findings are a perfect reinforcement to our previous theoretical study and have further demonstrated the uncertainty of applying the “sequential order” rules in generalized 2D correlation spectroscopy.     

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.     

Concentration Dependence of Mössbauer Parameters of SnCl−3 and SnBr−3 Complex Ions

Mössbauer parameters of frozen salt solutions could be correlated with apparent degree of dissociation, γ. KSnCl₃ and KSnBr₃ were dissolved in 0.5 M H₂SO₄, whereas NH₄ SnCl₃ was dissolved in 0.5 M HClO₄. Mössbauer studies were made at different concentrations.     

Kinetics and mechanism of the chromium(III)–picolinato chelate ring opening in some chromium(III)–oxalato–picolinato complexes in acidic aqueous solutions

Two Cr^III–picolinato complexes were obtained and characterized in solution. The [Cr(C₂O₄)(pyac)₂]^– and [Cr(C₂O₄)₂(pyac)]^2– ions (pyac = picolinic acid anion) in acidic solutions undergo a reversible one-end Cr^III–picolinato chelate ring opening via Cr^III—N bond breaking. The reaction rate was determined spectrophotometrically in the 0.1–1.0 M HClO₄ range at I = 1.0 M. The observed pseudo-first order rate constant depends on [H⁺] according to the equation: k _obs = a + b[H⁺] + c/[H⁺]. A reaction mechanism, which assumes participation of the protonated and unprotonated forms of the reactants, has been proposed. The kinetic parameters a, b, c have been defined as a = k ₁, b = k ₂ Q ₁, c = k _–1/Q ₂, where k ₁, k _–1,k ₂ are rate constants for the forward and reverse processes and Q ₁, Q ₂ are the protolytic equilibrium constants in the term of the proposed mechanism. The activation parameters have been determined and discussed.     

Insights into the enhanced tolerance to carbon monoxide on model tungsten trioxide-decorated polycrystalline platinum electrode

Polycrystalline platinum decorated by WO₃ nanoparticles (WO₃/Pt_pc) is used as a model electrode to gain insights into the enhanced tolerance to carbon monoxide (CO) observed on such composite materials. Bifunctional-type reactions involving WO₃ and Pt active sites are observed, such as hydrogen spill-over or the electrooxidation of CO molecules adsorbed on Pt sites neighboring the WO₃ nanoparticles. The resulting CO_ad-free Pt sites are active for the hydrogen oxidation reaction (HOR), thereby enhancing the HOR activity for WO₃/Pt_pc electrode relatively to bare Pt_pc in 300 ppm CO/H₂ saturated HClO₄ electrolyte. However, this bifunctional effect occurs exclusively for CO molecules weakly adsorbed on Pt, i.e. only for a small fraction of the CO_ad fully covering the Pt surface.     

Intergrowth in tunnel structures: The titanates A2Ti6O13)n·A′Ti4Og

A new structural family, (A₂M₆O₁₃)_n·A′M₄O₉, was isolated and studied by means of X-ray diffraction, electron diffraction, and electron microscopy. The structure consists of an ordered intergrowth of two types of structural units: A₂Ti₆O₁₃ and hypothetical A′M₄O₉, both characterized by zigzag ribbons of, respectively, 2 × 3 and 2 × 2 edge-sharing octahedra, joined by corner sharing to form a series of open tunnels containing A and A′ cations. The monoclinic unit-cell parameters can be deduced, for an “n” term, from those of A₂Ti₆O₁₃.     

Electron transfer at tetrahedral cobalt(II), part IV: kinetics of silver(I) catalysed chlorate reduction

Summary The very slow reduction of ClO _inf3 ^su– by [CoW₁₂O₄₀]^6- is markedly catalysed by Ag¹ in aqueous HClO₄ solution at I = 1.0 M (NaClO₄). The reaction obeys the rate law: -d[reductant]/dt where a = (1.28 ± 0.10) × 10^-4M^-1s^-1 and b = (2.22 ± 0.20) × 10^-4M^-3s^-1, at T = 60.1 ± 0.1°C and [H⁺] = 0.10–1.50 M. Activation parameters have been determined for each pathway and the reaction is discussed in terms of the outer-sphere mechanism.Part III: ref. 1.     

Synthesis and characterization of [Zn(2,2′-bipy)]2(H2PO3)4: A neutral zinc phosphite cluster containing ZnO3N2 and H2PO3 units

The hydrothermal synthesis, crystal structure and some properties of a zinc phosphite with a neutral cluster, [Zn(2,2′-bipy)]₂(H₂PO₃)₄, are reported. This compound crystallizes in the triclinic system of space group P-1 (No. 2), a=8.3067(5) Å, b=8.9545(4) Å, c=10.0893(6) Å, α=95.448(2)°, β=99.7530(10)°, γ=103.461(2)°, V=712.23(7) ų, Z=1. The cluster consists of 4-membered rings formed by alternating ZnO₃N₂ square pyramids and H₂PO₃ pseudo pyramids, with two “hanging” H₂PO₃ groups attached to each of the Zn centers. The clusters are linked together by extensive multipoint hydrogen bonding involving the phosphite units to form a sheet-like structure. This compound represents the first example of zinc phosphite with P---OH bonds. An intense photoluminescence was observed from this compound upon photoexcitation at 388 nm.     

The ethanol electrooxidation reaction at rough PtRu electrodeposits: A FTIRS study

In this work, the yields of CO₂ and acetic acid are studied as a function of the roughness of PtRu electrodeposits of fixed atomic composition during the electrooxidation of ethanol in the presence of 0.1 M HClO₄. Based in the findings of a previous work, electrodeposits of three roughnesses (calculated using Scanning Tunneling Microscopy) were prepared and ethanol electrooxidation products were monitored in situ, using FTIR spectroscopy. The results confirm that the pathway producing CO₂ is favored at rougher surfaces and are interpreted on the basis of the existence of a major number of superficial defects at theses surfaces which favors the scission of C–C bonds.     

Voltammetric approach to multicomponent electrochemical systems at platinum electrode surfaces

Adsorption interactions of different systems on electrodispersed and smooth platinum electrodes were explored through anodic stripping voltammetry using a conventional flow cell technique at 25°C. A first adsorbate was formed at the platinum electrode in contact with the solution containing the adsorbable species I, and subsequently the platinum—adsorbate I system was put into contact with a solution containing the adsorbable species II, where I CO, CH₃OH and reduced CO₂, and II C₂H₄, C₂H₂ and CO, and vice versa. The anodic stripping voltammograms revealed either a total or partial displacement of adsorbate I by adsorbate II, or chemical interactions among adsorbed residues I and Il. The following decreasing platinum—adsorbate interaction sequence was established C₂H₂-2 > CO > C₂H₄ > CH₃OH ≈ CO₂.     

New effective and selective extractants for separation and determination of silver

Summary Isopropyl-N-alkylthiocarbamates have been studied as extractants for preconcentration of silver trace amounts and for separation of silver macroquantities. It has been shown that the 0.05M reagents in chloroform extract trace amounts of silver with high distribution coefficients from HNO₃, H₂SO₄, HClO₄ and HCl solutions in wide range of their concentrations. The highest selectivity is obtained when HNO₃, H₂SO₄ or HClO₄ solutions are used. Besides silver only mercury and palladium are extracted. When HC1 solutions are used, gold is also transferred into organic phase. Isopropyl-N-alkylthiocarbamates are effective for extraction of silver from its concentrated (up to 1M) solutions, particularly from HNO₃ solutions: more than 99.9% of silver is extracted even at 2-fold reagent excess. Some characteristics of reagents themselfes have been investigated, such as dissociation, protonation, distribution between organic and aqueous phases and association in organic solvents. The thion forms are found to be predominant. Silver is extracted as coordination solvated compounds Ag(HL) _n X, where X=Cl, Br, J, NO₃. Sulfur serves as a donor atom for their formation.The procedures for silver determination in geological samples by flame atomic absorption (3×10^–7%) and atomic emission (1×10^–7%) have been developed. The procedures for atomic emission determination of 23 trace elements (5×10^–8–1×10^–5%) and for electrothermal atomic absorption determination of 17 trace elements (4×10^–9–8×10^–6%) in high purity silver have also been developed.

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Neue wirksame und selektive Extraktionsmittel für die Trennung und Bestimmung von Silber

Zusammenfassung Isopropyl-N-alkylthiocarbamate wurden als Extraktionsmittel für die Anreicherung von Silberspuren und für die Abtrennung von Makromengen Silber geprüft. Es ergab sich, daß solche 0,05M, in Chloroform gelöste Reagenzien Silberspuren mit hohen Verteilungskoeffizienten aus HNO₃, H₂SO₄, HClO₄ und HCl extrahieren. Die beste Selektivität wurde mit HNO₃, H₂SO₄ oder HClO₄erzielt. Neben Silber werden nur Quecksilber und Palladium extrahiert. Verwendet man HCl, so geht auch Gold in die organische Phase. Isopropyl-N-alkylthiocarbamate eignen sich für die Extraktion von Silber aus konzentrierten (bis zu 1M) Lösungen, besonders aus HNO₃: mehr als 99,9% Silber werden mit zweifachem Reagensüberschuß extrahiert. Einige charakteristische Eigenschaften der in Rede stehenden Reagenzien wurden untersucht: z. B. die Dissoziation, die Protonation, die Verteilung zwischen organischer und wäßriger Phase und die Assoziation in organischen Lösungsmitteln. Hauptsächlich liegen diese Reagenzien in Thionform vor. Silber wird als Ag(HL) _n X extrahiert, worin X für Cl, Br, J oder NO₃ steht. Schwefel fungiert als Donatoratom für deren Bildung.Verfahren zur Silberbestimmung in geologischen Proben mit Hilfe der Flammen-Atomabsorption (3×10^–7%) bzw. der atomaren Emission (l×10^–7) wurden ausgearbeitet. Außerdem wurden auch Verfahren zur Bestimmung von 23 Spurenelementen mittels atomarer Emission (5×10^–8 bis 1×10^–5%) und von 17 Spurenelementen mittels elektrothermaler Atomabsorption (4×10^–9 bis 8×10^–6%) in hochgereinigtem Silber entwickelt.

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The paper has been presented at the Fifth European Conference on Analytical Chemistry (Euroanalysis V), Cracow, August 26–31, 1984.     

Lithium intercalation into PbNb2S5, PbNbS3, SnNb2Se5, BiVS3, SnVSe3, and PbNb2Se5 misfit layer chalcogenides

The intercalation of lithium into various misfit layer chalcogenides of two different stoichiometries was performed by using n-butyl lithium on powders. The reaction was found to proceed topochemically, and a greater expansion in the c direction and higher lithium contents were observed in the lithiated phases with “MM′₂X₅” approximate stoichiometries compared to “MM′X₃” stoichiometries. This behaviour difference is assigned to the different stacking sequence of the slices of the two sublattices formed by double layers of MX and sandwiches of M′X₂. Lattice distortions are induced during lithiation, leading to changes in the relative orientation of MS-type bilayers and to complete amorphization after long reaction times. The synthesis and partial characterization of a new misfit layer selenide of nominal composition “PbNb₂Se₅” is also reported. The value of the c-dimension (c = 37.3₇ Å) suggests a stacking sequence PbSe---NbSe₂---NbSe₂---PbSe---NbSe₂---NbSe₂, etc. This material becomes highly unstable on lithium intercalation and decomposes to its constituents after a few hours of lithiation.     

[{(NiOH)2Mo10O36(PO4)Ti2}n]: a novel chainlike trimetal heteropolyanion based on pseudo-keggin fragments

A new mixed Mo/Ni/Ti heteropoly compound [C₅H₅NH]₅ [(NiOH)₂Mo₁₀O₃₆(PO₄)Ti₂] has been hydrothermally synthesized and structurally determined by the single-crystal X-ray diffraction. Black prismatic crystals crystallize in the monoclinic system, space group P2(1)/n, a=11.2075(2), b=37.8328(5) c=13.0888(1) Å, β=101.4580(10)°, M=2276.13, V=5439.19(13) ų, Z=4. Data were collected on a Siemens SMART CCD diffractometer at 293(2) K in the range of 1.68<θ<25.09° using the ω-scan technique (λ=0.71073 Å R(F)=0.0872 for 9621 reflections). The title compound contains a trimetal heteropolyanion polymer and “trans-titanium”-bridging pseudo-Keggin fragments linked to a chain.     

Kinetics and mechanism of the reaction between dimethylformamide and chromium(VI)

The kinetics of oxidation of N,N‐dimethylformamide by chromium(VI) has been studied spectrophotometrically in aqueous perchloric acid media at 20°C. The rate showed a first‐order dependence on both [Cr(VI)] and [DMF], and increased markedly with increasing [H⁺]. The order with respect to [HClO₄] was found to lie between 1 and 2. The rate was found to be independent of ionic strength as well as of any inhibition effect of Mn(II). The formation of superoxochromium(III) ion was detected in an aerated solution of chromium(VI), DMF and HClO₄. The proposed mechanism, involving two reaction pathways, leads to the rate law, rate = K_a1 [HCrO₄⁻] [DMF] (k_I K_a2 [H⁺]²+k_II[H⁺]). The first pathway, with rate constant k_I, involves the formation of chromium(V) and a free radical. The second pathway, with rate constant k_II, involves the formation of Cr(IV), CO₂ and dimethylamine. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 409–415, 1999