Techniques in Electrocatalytic Studies

Understanding the activity and selectivity properties of electrocatalysts requires the characterization of catalyst surfaces, determination of adsorption characteristics, identification of surface intermediates and of all reaction products and paths, and mechanistic deliberation for complex as well as model reactions. Electrochemical and classical methods for adsorption studies are well documented in the literature (5, 7-9, 25, 24, 373. Here, we shall outline briefly some prominent electrochemical methods and some nonelectrochemical techniques that can provide new insight into electrocatalysis. Electrode kinetic parameters can be determined by potentionstatic methods using the methodology of Section II1,D,3. 

In this procedure, possible reduction of the adsorbed entities by the discharged hydrogen is ignored. It is further assumed that the hydrogen coverage is not affected by the presence of other surface species, which may not be correct, as discussed earlier. Some desorption of the adsorbed molecules may also occur during the potential variation, despite the short duration of the experiments. With some electrodes, such as Pd, in which hydrogen absorption may take place, further inaccuracies are expected. 

Because of the potential dependence of adsorption, it would be preferable to obtain the surface coverage potentiostatically rather than at constant current. In the potentiostatic technique the potential is rapidly changed to a very anodic (or cathodic) value after initial equilibration at the desired potential, concentration, and temperature (189. The resulting variation of current with time, obtained on an oscilloscope, is now a measure of 

The potentiostatic method is less ambiguous than the galvanostatic one. Its application, however, requires more sophisticated instrumentation. The rise time of the potentiostat should be fast enough to ensure rapid step change of the potential. Errors may arise from slow rise times as well as from current integration. With porous electrodes, all sites may not be under the same potential diffusion of reactant into or out of the pores may be slow compared with the potential change, which can lead to incorrect estimates of surface coverage and utilization. 

Because of the imposed potential variation, the potentiodynamic technique presents similar uncertainties as the galvanostatic method. Possible desorption of the adsorbate, owing to potential change, can complicate the results. Oxide formation in certain potential regimes may be more important in the potentiodynamic than in the galvanostatic method. Uncertainties from potential and concentration variations within porous electrocatalysts can be 

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For surface structure studies, perhaps the most popular technique has been LEED (373). Elastically diffracted electrons from a monoenergetic beam directed to a single-crystal surface reveal structural properties of the surface that may differ from those of the bulk. Some applications of LEED to electrocatalyst characterization were cited in Section IV (106,148,386). Other, less specific, but valuable surface examination techniques, such as scanning electron microscopy (SEM) and X-ray microprobe analysis, have not been used in electrocatalytic studies. They could provide information on surface changes caused by reaction, some of which may lead to catalyst deactivation (256,257). Since these techniques use an electron beam, they can be coupled with previously discussed methods (e.g. AES or XPS) to obtain a qualitative mapping of the structure and composition of a catalytic surface. 

Used to identify structures, neither of these techniques is new although only since the 1980s has it been possible to make them practical. To follow the reaction path and build up knowledge on an atomic scale that would contribute directly to the design of catalysts is not quite there the time response needs to get down to the millisecond, and that, like so much else in fundamental science, depends on federal research programs. It is a reasonable speculation to suggest that methods such as the two referred to here will be in routine use in electrocatalytic studies well before 2050. 

In principle, the choice of a specific electrochemical technique is not crucial in the study of electrocatalytic processes. However, from a practical and qualitative point of view, cyclic voltammetry is, in most cases, suited for a rapid assessment of an electrocatalytic process triggered by POMs. The interest stems from the following general behavior most POMs undergo a series of reversible one- and two-electron reductions and these reduced forms act usually as the active species, inducing an increase of the corresponding... 

The aim of this chapter is to show that the choice of a catalyst formulation leading to increase the activity and the selectivity of a given electrochemical reaction involved in a fuel cell can only be achieved when the mechanism of the electrocatalytic reaction is sufficiently understood. The elucidation of the mechanism caimot be obtained by using only electrochemical techniques (e.g. cyclic voltammetry, chronopotentiometry, chrono-amperometiy, coulo-metry, etc.), and usually needs a combination of such techniques with spectroscopic and analytical techniques. A detailed study of the reaction mechanism has thus to be carried out with spectroscopic and analytical techniques under electrochemical control. In short, the combination of electrochemical methods with other physicochemical methods cannot be disputed to determine some key reaction steps. For this purpose, it is then necessary to be able to identify the nature of adsorbed intermediates, the stractuie of adsorbed layers, the natirre of the reaction products and byproducts, etc., and to determine the amormt of these species, as a fimction of the electrode potential and experimental conditions. 

Electroactive dendrimers are defined as those that contain functional groups capable of undergoing fast electron transfer reactions [85], The combination of specific electron transfer properties of redox active probes with the unique structural properties of dendrimers offers attractive prospects of their exploitation in electrocatalytic processes of biological and industrial importance [86], Further, the interest in dendrimers containing electroactive units also relies on the fact that electrochemistry is a powerful technique to elucidate the structure and purity of dendrimers, to evaluate the degree of electronic interaction of their chemically and/or topologically equivalent or non- equivalent moieties, and also to study their endo- and exo-receptor capabilities [87],... 

The first studies by UV-visible transmission spectroscopy were carried out using an optically transparent electrode (OTE) such as indium oxide [140,141]. Unfortunately an OTE does not allow the nature and the structure of the electrode material to be changed and these play a key role in electrocatalytic processes. Only reflectance spectroscopy is able to investigate in situ, various electrode materials [142], This was effectively checked for the first time with cobalt porphyrin-doped polypyrrole films using the electroreflectance technique [106,143]. This allowed the characterization of the redox properties of the modified PPy electrode and the determination of the redox potential of the Co"VCo" couple. The catalytic effect towards the ORR was also... 

Dttring the same period in situ IR spectroscopy started its active development. Nowadays the multivarious versions of this technique are widely applied in routine electrocatalytic studies, with predominating attention to adsorption of organic species. However Bewick s IR studies of hydrogen adsorption on Pt poly and on single crystals of several platinmn metals should not be forgotten. 

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