Electrochemical diffusion potential in the gas phase

Potentiometric based electrochemical measurement of diffusion potential at a junction between two flowing flame plasma gases is described. A flame electrochemical cell was constructed using a specially designed burner, which supports two individual flames, each fed by separate premixed methane/oxygen/nitrogen streams. The two flames were in intimate contact, creating a flowing fluid gaseous junction. By aspirating metal salt solutions into these premixed feed gases, the concentration gradient at the interface between the two flames may be controlled. A measurable electrochemical diffusion potential was formed at this junction, the magnitude of which was dependent on the concentration ratio of charged species with different mobilities. In our flame electrolyte, the dominant charged species were atomic or molecular cations and electrons, which have a difference in mobilities of approximately three orders of magnitude. A two-electrode system, in conjunction with a high impedance electrometer was used to measure the potential difference across the flame electrochemical cell. The measured potential difference was analysed using theory developed for the liquid junction potentials by the Henderson equation.     

Laser absorption spectroscopy diagnostics of nitrogen-containing radicals in low-pressure hydrocarbon flames doped with nitrogen oxides

Absolute concentration profiles of NH2 and HNO have been measured in low-pressure methane/air flat flames doped with small amounts of NO and N2O. Addition of a small amount of nitrogen oxides does not alter significantly the flame speeds, temperature profiles and other parameters of the relatively well-understood methane/air flames. Intracavity laser absorption spectroscopy (ICLAS) and cavity ring-down spectroscopy (CRDS) are high-sensitivity techniques used to measure absolute concentrations of minor species in flames. In this work ICLAS is used to monitor NH2 and HNO, whereas CRDS is used for temperature measurements using OH spectra in the UV range. The (090)-(000) and (080)-(000) bands of the A2A1-X2B1 electronic transition of NH2 and (100)-(000) and (011)-(000) bands of the A1A"-X1A' transition of HNO are used. Methane flames of different equivalence ratios are used. NH2 and HNO are observed in the flame as well as in the zone surrounding the flame, closer to the walls of the low-pressure chamber where the burner is located. An absorption originating from the species in this zone can affect substantially the results of line-of-sight experiments. A slow flow of nitrogen through the optical window holders was added in order to separate the spectra of HNO originating from the central flame zone. Calculations based on the commonly used GRI-Mech chemical mechanism predict two maxima in the HNO concentration profile in the NO doped flames. The first is located in the vicinity of the burner, and the second is closer to the luminescence flame zone. We were able to observe the first maximum, and its measured location agrees well with prediction. On the other hand, GRI-Mech strongly underpredicts the observed absolute concentration of HNO in this maximum. The measured absolute concentrations of NH2 are in reasonable agreement with the GRI-Mech predictions.     

Interaction of alkali metals with perylene-3,4,9,10-tetracarboxylic-dianhydride thin films studied by IR spectroscopy

In order to clarify the doping behavior of different alkali metals in perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA), Fourier transform infrared spectra of PTCDA thin films doped with sodium, potassium, and cesium were measured and compared. Furthermore the vibrational properties were calculated using density-functional theory and these calculated vibrational frequencies were assigned to the experimental IR modes of the thin films.     

Electrochemical diffusion potential in a flame plasma: theory and experiment

A flame doped with an appropriate additive to produce positive ions and free electrons is a quasineutral, weak, continuum plasma. When bounded by a metallic burner upstream and a metal plate downstream, the two electrodes and flame plasma can be viewed as a gas-phase electrochemical cell. When the ion (and electron) density varies continuously along the flame axis, an expression for the diffusion potential can be derived in terms of the concentration gradient. The familiar logarithmic dependence on the ion concentration is obtained. A plasma sheath develops at the metal plate electrode; it sustains a potential difference which can be modeled by a Boltzmann distribution of the electrons in the sheath. Since the plate has to be cooled in practice, the average sheath temperature is less than the flame temperature because the sheath occurs inside the thermal boundary layer which covers the plate electrode. Inevitably, the reduced sheath temperature affects the sheath voltage. Experimental measurements of the “cell” voltage are made for the two cases of a positive concentration gradient using a sodium plasma, and a negative gradient by doping the flame with methane. As predicted theoretically, the cell voltages have opposite signs. However, the magnitude of the cell voltage seems to depend significantly on the sheath temperature which appears to decrease steadily with increasing distance downstream from the burner. It is also possible that the measured cell voltages involve unknown surface contact potentials. When compared with solution concentration cells, gas-phase flame systems exhibit both similarities and differences.     

Gas phase electrochemical detection of single latex particles

In this study we describe zero current potentiometric measurements in a gaseous flame electrolyte, for the detection of single latex particles. Combustion of polystyrene latex particles when added to a premixed hydrogen/oxygen/nitrogen flame, results in an increase in charged species relative to the surrounding hydrogen flame. As a consequence of this increase in ionic concentration over background, short-lived potential difference transients were measured between two platinum indicator electrodes placed in a two-compartment flame electrochemical cell (described in Electrochem. Commun., 2001, 3, 675-681). The frequency of the transient events was dependent on the number density of latex particles in solution. It is proposed that each short-lived transient event corresponds to the combustion of single latex particles in a flame. A potential difference maximum of 0.56 V when 3.0 microm diameter particles were added to the flame was measured. Also it was shown that it is possible to detect 0.3 microm diameter latex particles using the same technique. It is postulated that the physical basis of the potential difference is due to the establishment of diffusion/junction potential due to the increase in ionisation from polystyrene combustion at the surface of one indicator electrode. This methodology may be applied to the detection of particulates composed of ionisable species (organic or inorganic) in gaseous environment such as bacteria, viruses, pollen grains and dust.     

Interaction Between C60 Fullerene and Alkali Metals Demonstrating Superconductivity

A discret summation method [1] has been used to calculate the van der Waals dispersion interactions between an alkali metal atom and individual C₆₀ fullerene molecules, as well as between an intercalated alkali metal atom and the face-centred-cubic lattice of solid fullerite. It is known [2, 3] that the conductivities observed in the doped C₆₀ films vary considerably for different alkali atoms. Our interest was to investigate any correlation between these conductivities and the long range potential field behaviour. In the present study, we have obtained interaction potential curves of the C₆₀ fullerene molecule including within its cage, as well the potential field topography within the overall C₆₀ fullerite solid unit cell. We have found that for all intercalated alkali metals, except Cs, there are voids within the unit-cell of fullerite where the dopant experiences attractive interactions. Whereas on the other hand, inside the fullerene cage only Li and Na experience attractive forces. Importantly, it has been shown that the localization of the crystallographic sites [4] of doped alkali metals in fullerite coincide with the potential energy minima of long range van der Waals forces.     

Sensing mechanism of non-equilibrium solid-electrolyte-based chemical sensors

Solid electrolytes can be used in several different types of chemical sensors. A common approach is to use the equilibrium potential generated across a solid electrolyte given by the Nernst equation as the sensing signal. However, in some cases, stable electrode materials are not available to establish equilibrium potentials, so non-equilibrium approaches are necessary. The sensing signal generated by such sensors is often described by the mixed potential theory, in which a pair of electrochemical reactions establishes a steady state at the electrode, such that the electrons produced by an oxidation reaction are consumed by a reduction reaction. The rates of both reactions depend on several factors, such as electron exchange, active area, and gas phase diffusion, so establishment of the steady-state potential is complex and alternative explanations have been proposed. This paper will review and discuss the mechanisms proposed to explain the sensor response of non-equilibrium-based electrochemical sensors.     

An investigation of some experimental parameters in atomic fluorescence spectrophotometry

Atomic fluorescence in flames is measured by an adaptation of a commercially available flame spectrophotometer. A study is reported of the effect of background radiation and source scattering on 3 flames, air-propane, air-hydrogen and air-acetylene, and of the effects of variation of fuel gas pressure, zone of measurement in the flame, analysing monochromator slit-width and wavelength of measurement. The air-propane flame appears to offer several advantages. The atomic fluorescence of 10 metals is described; those of Co, Fe and Mn have not been previously reported. Excitation of spectra is achieved by means of an a.c. xenon arc lamp or individual discharge lamps.     

Catalytic Activity of Alkali Metal Cations for the Chemical Oxygen Reduction Reaction in a Biphasic Liquid System Probed by Scanning Electrochemical Microscopy

Chemical reduction of dioxygen in organic solvents for the production of reactive oxygen species or the concomitant oxidation of organic substrates can be enhanced by the separation of products and educts in biphasic liquid systems. Here, the coupled electron and ion transfer processes is studied as well as reagent fluxes across the liquid|liquid interface for the chemical reduction of dioxygen by decamethylferrocene (DMFc) in a dichloroethane-based organic electrolyte forming an interface with an aqueous electrolyte containing alkali metal ions. This interface is stabilized at the orifice of a pipette, across which a Galvani potential difference is externally applied and precisely adjusted to enforce the transfer of different alkali metal ions from the aqueous to the organic electrolyte. The oxygen reduction is followed by H₂O₂ detection in the aqueous phase close to the interface by a microelectrode of a scanning electrochemical microscope (SECM). The results prove a strong catalytic effect of hydrated alkali metal ions on the formation rate of H₂O₂, which varies systematically with the acidity of the transferred alkali metal ions in the organic phase.     

Emission spectra of nitrous oxide supported acetylene flames at atmospheric pressure

The emission spectra of a premixed flame of acetylene supported by nitrous oxide have been recorded under different fuel-gas mixture conditions. The emission spectra in these flames of a series of metals, for which it is difficult to obtain a significant population of ground state atoms for atomic absorption spectroscopy in more conventional flames, have also been studied. The red secondary zone which is present in the fuel-rich flames shows emission attributable to long-lived CN and NH species which form a strongly reducing atmosphere to inhibit refractory oxide formation from elements such as molybdenum, titanium and aluminium introduced into the flame. An attempt has also been made to explain some of the reactions which may occur between the flame species above the primary reaction zone.     

Formulation of Ionic‐Liquid Electrolyte To Expand the Voltage Window of Supercapacitors

An effective method to expand the operating potential window (OPW) of electrochemical capacitors based on formulating the ionic‐liquid (IL) electrolytes is reported. Using model electrochemical cells based on two identical onion‐like carbon (OLC) electrodes and two different IL electrolytes and their mixtures, it was shown that the asymmetric behavior of the electrolyte cation and anion toward the two electrodes limits the OPW of the cell and therefore its energy density. Also, a general solution to this problem is proposed by formulating the IL electrolyte mixtures to balance the capacitance of electrodes in a symmetric supercapacitor.     

An experimental and theoretical evaluation of the nitrous oxide-acetylene flame as an atomization cell for flame spectroscopy

The formation of free atoms from aerosols of metal-containing solutions introduced into nitrous oxide-acetylene flames is examined by: (a) inference from well identified reactions and equilibria prevailing in cooler flames; (b) calculations employing a thermodynamic flame model; and (c) experimental observation of relative free-atom number densities in the flames as a function of stoichiometry. The calculated partial pressures of the major natural flame species and some of the spectroscopically observed minor species are presented as a function of the flow ratio of nitrous oxide to acetylene (p). Predicted relative number densities of Na, Mg, Cu, Fe, Li, Be, Al, W, Ti and Si as a function of p are compared with measured free-atom absorbances in an argon-shielded flame. These comparisons were completed for various heights above the burner tip. The data reported show that: (a) the degree of metal atomization in the nitrous oxide-acetylene flame can be adequately described by the equilibrium state; (b) in general, when solute vaporization is complete, there exists a value of ρ at which atomization is complete for metals that form monoxides with dissociation energies less than ~ 6.5 eV; and (c) certain metals may form carbon-containing compounds in the interconal zone.     

Dynamic electrochemistry in flame plasma electrolyte

Chemistry in flames: Dynamic electrochemistry in the gas phase is described by considering the ionized medium of a flame as an electrolyte (see picture). This study opens up the possibility of accessing redox reactions that are outside the potential limits set by the solvent in conventional liquid-phase electrochemistry.     

利用激光诱导炽光研究高压环境下液体燃料层流扩散火焰碳烟分布的二维图像

研究了在高压环境下压力对液体燃料的碳烟的层流扩散火焰碳烟生成的影响。利用激光诱导炽光和消光法相结合的方法,获得了层流扩散火焰的碳烟分布二维图像,测量和分析了正庚烷的层流扩散火焰的碳烟体积分数生成随压力变化的规律。然后,引入特别设计的"滴入式火焰"装置,该设计为两种以上液体混合燃料的层流扩散火焰碳烟生成的测量提供了保障。最后,定量地分析和对比了饱和环状分子结构(环己烷和环己醇)和直链分子结构(正己烷和1-己醇)的液体燃油的层流扩散火焰的碳烟生成趋势,结果表明,环状分子结构燃油的碳烟生成倾向要强于它们对应的线性分子结构的燃油。     

Absolute analysis by flame atomic absorption spectroscopy: Present status and some problems

Recent progress and main problems encountered in the theoretical interpretation of some stages in the spectrochemical analysis by flame AAS with a slot burner and a sharp-line source are reviewed. The effect of “narrowing” of the aerosol stream as compared with the gas stream above the flame front was established theoretically and confirmed experimentally. The resulting theory describing the analyte distribution across the flame permitted to explain many features of these flames, in particular, the effect of sensitivity enhancement in the presence of an excess of matrix. A simple method is proposed for the determination of atomic diffusion coefficients.The results of calculations of the composition and temperatures of flames employed in analytical practice, obtained for a wide range of the fuel—oxidant ratio, were used to determine the capabilities of these flames as to the dissociation of monoxides. Practically total dissociation of almost all elements of the Periodic Table was proved theoretically and confirmed experimentally to occur in the nitrous oxide—acetylene flame. The formation of low-volatile lithium and tin carbides in the presence of carbon was established. This effect accounts for “anomalies” in the behavior of these elements in low-temperature flames.The line shifts Δν_s in flames were measured by interferometric scans of line profiles from a hollow-cathode lamp and flame. The existence of a theoretical relationship between Δν_s and the Lorentz line width Δν_L was confirmed. Calculations of line absorption were generalized to take into account the shift and hyperfine structure of the lines. Systematic errors in these calculations do not exceed 10%.A discussion is given of the major difficulties facing absolute measurements based on this analytical technique.     

Transition metal-doped carbon sphere as enhanced catalysts for oxygen reduction

Herein, carbon sphere (CS-T) were successfully prepared by pyrolyzation melamine formaldehyde resin. And then different transition metals (Fe, Co, Ni) were doped on carbon sphere (CS-M-900). The scanning electron microscopes and elemental mappings prove that the transition metal particles are uniformly doped on the carbon sphere. Meanwhile, the X-ray photoelectron spectrum prove that the transition metals are zero-valence. Furthermore, the electrochemical testings showed the CS-Co-900 had more positive onset potential (0.93 V), half-wave potential (0.84 V), and peak potential (0.81 V). Furthermore, the CS-Co-900 had lower charge transfer resistance (44 Ω) and smaller Tafel slope (65 mV/dec). Most importantly, the oxygen reduction reaction on the CS-Co-900 turned out to be a four electron procedure with excellent methanol tolerance. The improved electrochemical properties towards oxygen reduction reactions of carbon sphere via cobalt doping suggested a design strategy towards future high-performance electrochemical devices.     

Two-component polymer films of palladium and fullerene with covalently linked crown ether voids: effect of cation binding on the redox behavior

Redox active films have been generated via electrochemical reduction in a solution containing palladium(II) acetate and fulleropyrrolidine with covalently linked crown ethers, viz., benzo-15-crown-5 and benzo-18-crown-6. In these films, fullerene moieties are covalently bonded to palladium atoms to form a polymeric network. Films show ability to coordinate alkali metal cations from the solution. Therefore, in solutions containing salts of alkali metal cations, benzo-15-crown-5-C₆₀/Pd and benzo-18-crown-6-C₆₀/Pd films are doped with cations coordinated by crown ether moiety and anions of supporting electrolyte which enter the film to balance positive charge. These films are electrochemically active in the negative potential range due to the reduction of the fullerene moiety. Reduction of the polymer is accompanied by the transport of supporting electrolyte ions between solution and solid phase. In solution containing alkali metal salts, the process of film reduction is accompanied by the transport of anions from the film to the solution. In the presence of tetra(alkyl)ammonium salts, transport of cations from the solution to the film takes place during the polymer reduction.     

Some analytical properties of low pressure flames

An investigation has been made into the analytical behaviour of low pressure nitrous oxide-acetylene and oxy-acetylene names, between pressures of ca. 15–300 and 10–100 torr, respectively, for the purposes of atomic emission spectroscopy. Samples were introduced into the flame on a loop of tungsten wire, and the resulting emission pulses measured in the space immediately beyond the edge of the primary reaction zone. Attention was largely concentrated on metals which are difficult to determine in conventional atmospheric flames. Absolute detection limits for Sr, Al, Cr, Cu, Ti and V lie in the range 10⁻⁹–10⁻¹⁰ g. The relative atomisation efficiences of the flames used were also measured, and were found to increase with decreasing pressure; an effect attributed to the highly reducing properties of a low pressure flame, and one which was felt to account for the high analytical sensitivity obtained in these studies.