Electrodes catalytic

Similar electrodes may be used for the cathodic hydrogenation of aromatic or olefinic systems (Danger and Dandi, 1963, 1964), and again the cell may be used as a battery if the anode reaction is the ionization of hydrogen. Typical substrates are ethylene and benzene which certainly will not undergo direct reduction at the potentials observed at the working electrode (approximately 0-0 V versus N.H.E.) so that it must be presumed that at these catalytic electrodes the mechanism involves adsorbed hydrogen radicals. 

In redox reactions, the electrode is not inert in the full meaning of the term. It serves not only to feed current through the electrolyte but also acts as a catalyst (as a catalytic electrode ) determining the rates and special features of electrochemical reactions occurring at its surface. 

The rate of an electrochemical reaction depends, not only on given system parameters (composition of the catalyst and electrolyte, temperature, state of the catalytic electrode surface) but also on electrode potential. The latter parameter has no analog in heterogeneous catalytic gas-phase reactions. Thus, in a given system, the potential can be varied by a few tenths of a volt, while as a result, the reaction rate will change by several orders of magnitude. 

In electrocatalysis, in contrast to electrochemical kinetics, the rate of an electrochemical reaction is examined at constant external control parameters so as to reveal the influence of the catalytic electrode (its nature, its surface state) on the rate constants in the kinetic equations. 

The scientific literature abounds in attempted correlations between the catalytic activities, of a series of catalytic electrode metals and some set of bulk properties, of these metals. Such correlations would help in understanding the essence of catalytic action and will enable a conscious selection of the most efficient catalysts for given electrochemical reactions. 

Almost always, foreign species not involved in a given electrochemical reaction are present on the surface of catalytic electrodes. In some cases these species can have a strong or even decisive effect on reaction rate. They may arrive by chance, or they can be consciously introduced into the electrocatalytic system to accelerate (promoters) or retard (inhibitors) a particular electrochemical reaction relative to others. 

The search for new, more highly active and less expensive materials for catalytic electrodes and the attempts at reducing the loading of expensive platinum catalysts has led to numerous studies in the area of binary and multicomponent metal systems. These included various metal alloys as well as mixed microdeposits containing several... 

To the contrary, mnlticomponent nonmetallic systems such as mixed oxides often provide the possibility for a smooth or discontinuous variation of electrophysical parameters, and thns for some adjustment of their catalytic properties. In a number of cases, one can do without expensive platinum catalysts, instead using nonmetallic catalysts. Serious research into the properties of nonmetallic catalytic electrodes was initiated in the 1960s in connection with broader efforts to realize various kinds of fuel cells. 

Polyaniline (PANI) was investigated as electrocatalyst for the oxygen reduction reaction in the acidic and neutral solutions. Galvanostatic discharge tests and cyclic voltammetry of catalytic electrodes based on polyaniline in oxygen-saturated electrolytes indicate that polyaniline catalyzes two-electron reduction of molecular oxygen to H2O2 and HO2". 

One approach to potential control in UHV lies in chemical poising, roughly analogous to pinning an inert but catalytic electrode at a given potential by immersing it in a solution containing controlled concentrations of both members of a redox couple. To apply this approach in UHV we supply both members of the redox couple as species adsorbed, in controlled quantities, from the gas phase. We then allow equilibration to occur. 

Adsorption of acetic acid on Pt(lll) surface was studied the surface concentration data were correlated with voltammetric profiles of the Pt(lll) electrode in perchloric acid electrolyte containing 0.5 mM of CHoCOOH. It is concluded that acetic acid adsorption is associative and occurs without a significant charge transfer across the interface. Instead, the recorded currents are due to adsorption/desorption processes of hydrogen, processes which are much better resolved on Pt(lll) than on polycrystalline platinum. A classification of adsorption processes on catalytic electrodes and atmospheric methods of preparation of single crystal electrodes are discussed. 

Electrochemical Adsorption at Catalytic Electrodes. A classification of adsorption processes at catalytic electrodes, such as platinum or rhodium, first proposed by Horanyi (24) and further developed by Wieckowski (21,25,26), categorizes adsorption processes into three fundamental groups ... 

A catalytic electrode for the reduction of aqueous bicarbonate (C03H ) leading to a formate (F1C02 ) has been... 

Lawrence, R. J., and Wood, L. D. Method of making solid polymer electrolyte catalytic electrodes and electrode made thereby. U.S. Patent 4,272,353,1981. Fedkiw, P. S., and Her, W. H. An impregnation-reduction method to prepare electrodes on Naifon SPE. Journal of the Electrochemical Society 1989 136 899-900. Aldebert, P, Novel-Cattin, R, Pineri, M., Millet, P, Doumain, G., and Durand, R. Preparation and characterization of SPE composites for electrolyzers and fuel cells. Solid State Ionics 1989 35 3-9. 

For the catalytic electrode mechanism, the total surface concentration of R plus O is conserved throughout the voltammetric experiment. As a consequence, the position and width of the net response are constant over entire range of values of the parameter e. Figure 2.35 shows that the net peak current increases without limit with e. This means that the maximal catalytic effect in particular experiment is obtained at lowest frequencies. Figure 2.36 illustrates the effect of the chemical reaction on the shape of the response. For log(e) < -3, the response is identical as for the simple reversible reaction (curves 1 in Fig. 2.36). Due to the effect of the chemical reaction which consumes the O species and produces the R form, the reverse component decreases and the forward component enhances correspondingly (curves 2 in Fig. 2.36). When the response is controlled exclusively by the rate of the chemical reaction, both components of the response are sigmoidal curves separated by 2i sw on the potential axes. As shown by the inset of Fig. 2.36, it is important to note that the net currents are bell-shaped curves for any observed kinetics of the chemical reaction, with readily measurable peak current and potentials, which is of practical importance in electroanalytical methods based on this electrode mecharusm. 

The first methanol-fed PEM EC working with an AEM was conceived by Hunger in 1960 [15,45]. This system contained an AEM with porous catalytic electrodes pressed on both sides and led to relatively poor electrical performance (1 mA cm at 0.25 Vat room temperature with methanol and air as the reactants). Since this first attempt, many studies have been carried out to develop alkaline membranes. 

Not just any electrode will do. Catalytic electrodes are essential because they speed up the reactions so that the process creates a sizable current. A similar situation occurs in biology, where catalysts known as enzymes increase the rate of vital biochemical reactions that would oth-... 

It was (and perhaps is) a general belief in the literature that perfect voltammo-grams reflecting H adsorption on the catalytic electrodes can be obtained in HCIO4 solution. 

There were no substantial differences between the magnitudes of the photovoltages on the different substrate electrodes despite the 1,4-V range in their vacuum work functions (Fig. 7). The slight decrease in Voc on Pt substrates was caused by the enhanced rate of recombination at this highly catalytic electrode... 

Diagnostic plots for heterogeneous catalytic electrode reactions at the RRDE have many features in common with those for simple parallel reactions [178]. This type of analysis is important in the investigation of the oxygen electrode reaction where non-electrochemical surface processes can occur. 

Crucible-type oxygen sensor with catalytic electrode. In this case, the solid electrolyte is non-porous and the sensor current 1=0. 

Crucible-type oxygen sensor with non-catalytic electrode. A non-catalytic electrode (e. g. Au) is thought to delay the reaction rate in the following reaction... 

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