Methanol electrooxidation on platinum mesh electrodes bonded to solid polymer electrolytes

Methanol electrooxidation on smooth platinum electrodes bonded to solid polymer electrolytes was studied in water and acid solution by voltammetric measurements with different scanning rates. An enhancement of the oxidation rates was observed in these systems as compared to identical platinum electrodes in contact with liquid electrolytes. This electrocatalytic effect strongly depends on the measuring conditions and on the electrode potential. The reasons for the catalytic effects at different potentials are discussed. Received: 8 January 1997 / Accepted: 1 December 1997     

Electroreduction of carbon dioxide: Part II. The mechanism of reduction in aprotic solvents

The mechanism of electroreduction of carbon dioxide in aprotic solvents on mercury, lead, tin, indium and platinum is studied using the photoemission method and the method of stationary polarization curves. When comparing the data of photoemission and “dark” (polarization) measurements it was found that in the first Tafel region of the polarization curves the rate-determining step is the transfer of the second electron to (CO₂)^.?₂ anion-radicals formed as a result of the interaction of initially generated CO^.?₂ anion-radicals with adsorbed CO₂ molecules. In the second Tafel region the rate-determining step is the transfer of the first electron to an adsorbed CO₂ molecule. The peculiarities of electroreduction of carbon dioxide in aprotic solvents can be explained provided that the effect of potential on adsorption of CO₂ and anion-radicals and the effect of repulsion of negatively charged radicals are taken into account.     

Mechanism of adsorption,electroreduction and hydrogenation of compounds with ethylenic bonds on platinum and rhodium: Part II. Comparison of the electroreduction and catalytic hydrogenation processes

The catalytic liquid-phase hydrogenation of maleic acid on platinum and rhodium has been investigated. It is shown that the rate-determining step of this process as well as of the electroreduction process of maleic acid is the interaction of the chemisorbed particle of maleic acid with the adsorbed hydrogen which is formed at the preceding rapid stage of either the dissociative adsorption of molecular hydrogen, or the electrochemical stage of hydrogen ion discharge. The rate of the process with the same degree of surface coverage with hydrogen and chemisorbed particles of maleic acid does not depend on whether the process is carried out catalytically or electrochemically, on whether maleic acid and hydrogen were preliminarily adsorbed on the surface of the electrode-catalyst or not. With due regard for the mutual influence of chemisorbed particles participating in the rate-determining stage, the main kinetic equations for the electroreduction and catalytic hydrogenation processes have been derived. The difference in the rates of electroreduction of maleic acid on platinum and rhodium, with the same degree of electrode surface coverage with reactants, is shown to be the result of differences in the adsorption heats (or bonding strength with the surface) of hydrogen and maleic acid on these two metals. Experimental procedures are described in Part I [1].     

Electrocatalytic and adsorption properties of platinum microparticles electrodeposited onto glassy carbon and into Nafion® films

A comparative investigation of electrocatalytic and adsorption properties of platinum microparticles electrodeposited onto a glassy carbon surface (Pt/GC) and within a thin Nafion® film formed on a GC electrode (Pt/Nf/GC) is described. As test reaction the methanol oxidation in sulfuric acid solutions is used. Dependences of the steady-state specific reaction rates upon potential and methanol concentration were established, as well as those of the platinum surface coverage with methanol chemisorption products upon concentration. It was shown that at higher platinum loadings (above 60 μg cm⁻²) the specific activities of Pt/GC and Pt/Nf/GC are nearly the same and close to that of smooth platinum. At such loadings the surface coverage of the platinum deposit surface with organic particles does not differ from that of smooth platinum. At very low platinum loadings in the polymeric matrix (10–30 μg cm⁻²) a considerable decrease in the adsorption of strongly chemisorbed methanol particles is observed. These deposits are characterized by a low specific activity, which may be caused by the decrease of the platinum particle’s size, leading to a decrease in the amount of weakly bound methanol particles participating in the limiting reaction step.     

A monopole mass filter with a hyperbolic V-shaped electrode

In this article we compare the classical monopole mass filter of von Zahn and the monopole mass filter with a hyperbolic V-shaped electrode. The experimental results and those of computer simulation for both mass spectrometers are presented. We show that the replacement of a conventional 90 degrees V-shaped electrode by an electrode with a hyperbolic profile substantially improves the peak shape of any given mass, and increases the mass resolution by a factor of 3-4 and the abundance sensitivity by a factor of 100. The potential of high analytical performance combined with electroforming techniques for electrode manufacture indicate future practical uses of such instruments. Copyright 1999 John Wiley & Sons, Ltd.     

A three body approach to study the structural properties of 2-n halo nuclei and the search for Efimov states

The discovery of neutron rich isotopes of the lightest elements on the neutron drip line exhibiting a halo structure has opened up new vistas in research activities. The novel structural features associated with the halo phenomena have been the subject for extensive theoretical and experimental investigations in recent times. In this talk, I propose to present a broad overview of the recent developments in this field, bringing out the striking features which show that a large number oflight nuclei near the neutron drip line are characterized by a clear separation between a ‘normal’ core nucleus and a loosely bound low density veil of neutrons. Specifically, the two neutron halos offer a natural premises, from a theoretical standpoint, to employ three body techniques for studying their detailed structural properties. A considerable part of the talk will be devoted to report and highlight the results on a number of light halo nuclei such as ¹¹Li, ¹¹Be, ¹⁹B and ²²C on which we have been carrying out investigations employing a simple but realistic three body model. These three body systems which have been termed as ‘Borromean’ (i.e while three body systems are bound, the corresponding binary subsystems on the other hand are unbound) are characterized by large spacial extension and very low separation energy of the neutron. They are, therefore, ideally suited for exploring the possibility of the existence of Efimov states in two neutron halo nuclei. We have recently carried out the three body analyses to predict the possibility of the occurrence of such states on which experimental work at various laboratories is underway.     

Fuel Cells: State-of-the-Art and Major Scientific and Engineering Problems

Basic results of research and engineering-technical work on fuel cells (mostly membrane and methanol systems), obtained mainly in the last decade, are reviewed, with attention focused on the electrocatalytic aspect. Many works on the engineering, construction, and production of fuel cells and on the mass transfer remain outside this review, as do intrinsically theoretical works on electrocatalysis. Questions of technical and economic adaptation of these to various applications are discussed.     

Shape resonances in slow electron scattering by aromatic molecules. I. Anthraquinone derivatives

A series of anthraquinone (C(14)O(2)H(8)) derivatives has been studied by means of electron capture negative ion mass spectrometry (ECNI-MS), photoelectron spectroscopy (PES), and AM1 quantum chemical calculations. Mean lifetimes of molecular negative ions M(-.) (MNI) have been measured. The mechanism of long-lived MNI formation in the epithermal energy region of incident electrons has been investigated. A simple model of a molecule (a spherical potential well with the repulsive centrifugal term) has been applied for the analysis of the energy dependence of cross sections at the first stage of the electron capture process. It has been shown that a temporary resonance of MNI at the energy approximately 0.5 eV corresponds to a shape resonance with lifetime 1-2.10(-13) s in the f-partial wave (l = 3) of the incident electron. The next resonant state of MNI at the energy approximately 1.7 eV has been associated with the electron excited Feshbach resonance (whose parent state is a triplet npi* transition). In all cases the initial electron state of the MNI relaxes into the ground state by means of a radiationless transition, and the final state of the MNI is a nuclear excited resonance with a lifetime measurable on the mass spectrometry timescale. Copyright 1999 John Wiley & Sons, Ltd.     

The mechanism of tin-promoted electrochemical oxidation of organic substances on platinum

The promoting action of tin in the binary system platinum tin has been studied. A direct correlation of polarization and adsorption measurements has been carried out using an adsorption method of applying the promoter. The method of fast potentiodynamic pulses has been used to control the amount of tin and organic species on the surface. Studies have been made with smooth and platinized electrodes on the adsorption and oxidation of formic acid and methanol. A comparison of electro-oxidation rates at constant organic species coverage has made it possible to reveal the true catalytic effect as a result of introduction of tin. This effect has been found to be independent of both the nature of the substance under oxidation and the state of the electrode surface (smooth or rough). The promoting effect of tin is observed in the reactions where the limiting stage is represented by R+OH_ads; in the reactions where the limiting stage is represented by electron transfer or else the interaction R+H_ads, the promoting effect of tin does not show up, which fact points to the selectivity of its catalytic action.     

Generalized scheme of chemisorption,electrooxidation and electroreduction of simple organic compounds on platinum group metals

A generalized scheme of successive stages for chemisorption, electrooxidation and electroreduction of simple organic compounds from methane to CO₂ on platinum group metals has been developed on the basis of experimental data. The actual pathway of electrooxidation or electroreduction of an organic compound and the yield of the reaction products depend on the ratio between the rates of individual steps which, in their turn, are determined by various factors (potential, temperature, coverage of the surface with H_ads and OH_ads, with reacting and intermediate organic particles, as well as with foreign particles). The accumulation of chemisorbed compounds on the electrode surface depends on the relationship between the rates of their formation and subsequent transformation.