Comparative study of carbon monoxide adsorption and oxidation of the chemisorbed species at smooth and platinized platinum electrodes

The adsorption of carbon monoxide and the anodic oxidation of the chemisorbed species were investigated at room temperature under the same experimental conditions on foils of smooth and platinized platinum for which hydrogen adsorption is very similar. Both the weakly and strongly bonded species (type II and type I species) were formed to the same extent on the two electrodes in acidic electrolytes. While the freshly platinized platinum electrode behaved like the smooth electrode with predominance of the one-site adsorption of CO, the aging of the platinized electrode led to an increase of the two-site adsorption. The oxidation rate of strongly bonded species at constant potential decreased when the extent of two-site adsorption increased. The electrochemical results are discussed in the light of recent work in the gas phase.     

Estimating the PDMS-Coated, SPME-Fibre/Water- and Fibre/Gas-Partition Coefficients of Chlorinated Ethenes by Headspace-SPME

In this work, a simple, fast and reproducible method is presented for the determination of fibre/liquid-phase and fibre/gas-phase partition coefficients of five chlorinated ethenes on a poly-(dimethylsiloxane), PDMS-coated, solid-phase microextraction fibre, by employing a headspace HS-SPME coupled with gas chromatography. The partition coefficients were estimated by a numerical method using a Level-I fugacity method coupled with parameter-estimation software. Dimensionless partition coefficients between SPME fibre and liquid as well as gas phases were obtained at temperatures of 10 °C, 25 °C and 30 °C. The partition coefficients of the fibre and the gas phase, K _fg, increase with decreasing temperature by a factor of ≈2 to 6, and they are directly proportional to the linear slope of the regression line. The same tendency is observed for the partition coefficient of the fibre and liquid phase, K _fw, in a factor ≈1.2 to 2.0. The sorption enthalpy is higher in the gas phase; therefore, the sorption onto the fibre is favoured at lower temperatures. The correlation of the log K _ow versus log K _fw and log K _oa versus log K _fg shows a linear relationship with the number of chlorine atoms in the C = C molecule. Long-term experiments resulted in sorption to Teflon surfaces and possible losses in 43 mL vials, not observed in 250 mL Boston bottles.     

Anion Transport in BaR2F8 Crystals at Elevated Temperatures

Electroconduction of BaR₂F₈ crystals (R = Y_0.9Er_0.1, Y_0.5Yb_0.5, Er_0.945Tm_0.05Ho_0.005) with the structure -BaTm₂F₈ (monoclinic syngony, spatial group C2/m) is studied at 323–1073 K. Effect of partial pyrohydrolysis on the conduction of Ba(Y, Er)₂F₈ single crystals is investigated. Anion conductivity of crystals of Ba(Y, Yb)₂F₈ and Ba(Er, Tm, Ho)₂F₈ is measured at high temperatures. To a first approximation, there is no change in the ion transport mechanism in these crystals at elevated temperatures. Charge carriers in BaR₂F₈ crystals are, most probably, fluorine vacancies, and the anion conductivity reaches 1–2 mS cm^–1 at 1073 K.     

ATR-IR studies of CO adsorption from solutions

The formation of strongly bonded carbonaceous species from simple fuels on platinum metal electrodes at open circuit at potentials in the hydrogen region is widely accepted. Attenuated total reflection (ATR)-IR spectroscopy was used in this work to investigate the adsorption of such particles. A platinum film sufficiently stable for such studies in acid solution could be obtained on a germanium reflection element with a 0.5 nm layer of chromium and a 5 nm layer of platinum. The adsorption of CO leads to a strong band at 2000 cm⁻¹ assigned to linearly bonded CO, a broad band at 1800 cm⁻¹ assigned to bridge-bonded and possibly multiple bonded CO and a weak band at 1430 cm⁻. Clearly, the reaction of methanol, formaldehyde or formic acid produces the strong CO band.     

Continuous and discontinuous solutions for optimum thrust nozzles of given length

In its first sections, the paper deals with optimum thrust nozzles of given length and exit radius for flows with swirl. The computation is based on a modification of methods familiar for flows without swirl. Rather extensive numerical results show that the swirl does not impair the specific impulse attainable at a given nozzle length. The analysis suggests that the assumption of isentropic continuous flows, on which this approach is based, may sometimes be too restrictive. A survey of plane nozzles shows, on the other hand, that discontinuities need to be admitted only if, besides the length, a rather large radius of the nozzle is prescribed. Discontinuous solutions have been thoroughly investigated by Shmyglevskiy. At least in principle, we use the same line of thought, but considerable simplifications are possible if one starts with the variational formulation of Rao. In its numerical discussion and also in some analytical details, the present paper goes beyond Shmyglevskiy's results. The problem is conveniently discussed in astate plane, which has the local state of the flow (flow direction and speed or Mach number) as independent variables. By taking into account second variations, one can determine the boundary of the region for which continuous solutions give the (local) maximum. This boundary coincides with the locus of points at which the solution in the physical plane would fold back into itself. Another limitation of the original approach emerges if one asks under which conditions the thrust can be increased by admitting along the control surface values of the entropy that are higher than those of the oncoming flow. The conditions for isentropic and nonisentropic jumps are formulated and evaluated next, and a survey of the discontinuities which satisfy conditions for isentropic and also for selected nonisentropic jumps is given. Up to this point, the analysis is concerned only with the state distribution along the control surface. Jumps of the state in the interior require the occurrence of centered compression waves. Sample computations show that, in most cases, flow fields of this character can be generated by the choice of the nozzle shape. In some cases, no nozzle contours exist which generate the optimizing state distribution along the control surface as determined by the present analysis. It would then be necessary to include from the very beginning conditions for the realizability of the flow field.     

Study of operating conditions of plasma spray torch using lowelectric power levels

The scaling-down of plasma spray torches is connected with problems of anodic arc attachment fluctuations. It Is known that thermally constricted arcs in molecular gases generate unstable anodic arc roots in the nozzle of the torch. Moreover, using a usual plasma spray torch at reduced electrical power level in an argon-hydrogen gas mixture at a pressure of about 120 mbar, the arc is strongly influenced by hydrodynamic forces. Especially, its long time behavior shown by the arc voltage course is very variable and nonreproducible. It is the purpose of this work to optimize the nozzle geometry and the gas flow rate in order to obtain stabilized operating conditions of the plasma torch for an arc current of 100 A. It will be shown that the design of the entrance region and the diameter of the anodic nozzle influence the are column motion as well as the level and the frequency of the voltage fluctuations. With a particularly modified anodic nozzle, stable and reproducible operating conditions of the plasma torch, which is used to synthesize diamond layers, will be obtained     

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.