Electrocatalysis intermediates

It has been often stressed that low eoordinated atoms (defeets, steps, and kink sites) play an important role in surfaee ehemistry. The existenee of dangling bonds makes steps and kinks espeeially reaetive, favoring the adsorption of intermediate species on these sites. Moreover, smdies of single-crystal surfaces with a eomplex geometry have been demonstrated very valuable to link the gap between fundamental studies of the basal planes [Pt( 111), Pt( 100), and Pt(l 10)] and applied studies of nanoparticle eatalysts and polycrystalline materials. In this context, it is relevant to mention results obtained with adatom-modified Pt stepped surfaces, prior to discussing the effect of adatom modification on electrocatalysis. 

Broadband Sum Frequency Generation Studies of Surface Intermediates Involved in Fuel Cell Electrocatalysis... 

Heinen M, Jusys Z, Behm RJ. 2009. Reaction pathways analysis and reaction intermediate detection via simultaneous differential electrochemical mass spectrometry (DBMS) and attenuated total reflection Bourier transform infrared spectroscopy (ATR-BTIRS). In Vielstich W, Gasteiger HA, Yokokawa H, eds. Handbook of Buel Cells. Volume 5 Advances in Electrocatalysis. Chichester John Wiley Sons, Ltd., in press. 

As mentioned in Section 5.1, adsorption of components of the electrolysed solution plays an essential role in electrode processes. Adsorption of reagents or products or of the intermediates of the electrode reaction or other components of the solution that do not participate directly in the electrode reaction can sometimes lead to acceleration of the electrode reaction or to a change in its mechanism. This phenomenon is termed electrocatalysis. It is typical of electrocatalytic electrode reactions that they depend strongly on the electrode material, on the composition of the electrode-solution interphase, and, in the case of single-crystal electrodes, on the crystallographic index of the face in contact with the solution. 

The electrode reaction of an organic substance that does not occur through electrocatalysis begins with the acceptance of a single electron (for reduction) or the loss of an electron (for oxidation). However, the substance need not react in the form predominating in solution, but, for example, in a protonated form. The radical formed can further accept or lose another electron or can react with the solvent, with the base electrolyte (this term is used here rather than the term indifferent electrolyte) or with another molecule of the electroactive substance or a radical product. These processes include substitution, addition, elimination, or dimerization reactions. In the reactions of the intermediates in an anodic process, the reaction partner is usually nucleophilic in nature, while the intermediate in a cathodic process reacts with an electrophilic partner. 

Electric field, gradient, 30 127 Electrocatalysis, 17 351-418, 40 87-168 see also Chemisorbed intermediates Elec-troorganic synthesis Fuel cells activation, by external field, 17 409-410 by radiation, 17 410-411 by ultrasonic irradiation, 17 411 activity-potential of comparison, 17 381-385... 

The concept of electrocatalysis and its relation to chemical surface bonding of reactive intermediates is closely related to that of heterogeneous catalysis. Following the previous section, simple Gibbs energy curves can illustrate the essential ideas of how adsorption of intermediates and their associated Gibbs energy affect the rate of an inner-sphere reaction. 

Radicals, 1139, 1147, 1193, 1416 adsorbed, in electrocatalysis, 1275 determination by rotating disk electrode, 1140 intermediate, in methanol oxidation, 1270 Radiation... 

Cells can be made in which the cathode-anode distance is only 10-3 cm. Such cells have the advantage that the total impurity present is veiy small and may not be enough to cover more than 0.1% of the electrode surface if they were all adsorbed. Thus, suppose the impurity concentration were 10-6 mol liter-1 or 10-9 mol cc 1 or 10 12 mol in the cell Because an electrode surface can cany (at most) about 10-9 mol cm-2, the maximum fraction of the surface covered with impurity molecules is 0.1%. Does work with thin-layer cells eliminate the inpurity problem in electrode kinetics It improves it. However, active sites on catalysts may occupy less than 0.1% of an electrode and preferentially attract newly arriving impurities, so that even thin-layer cells may not entirely avoid the impurity difficulty,32 particularly if the electrode reaction concerned (as with most) involves adsorbed intermediates and electrocatalysis. 

Electrocatalysis has just been described. One important feature of an electrocata -lyst is that it goes through the electrodic reaction unchanged. Its sole function is to act as an electron source or sink and as a surface for the adsorption of any intermediates involved in the reaction. Or, if one prefers to think in terms of the crystalline lattice that constitutes the solid electrocatalyst, it is clear that the lattice neither disintegr ates by its constituent particles walking off into solution nor grows by particles from the solution adding onto the lattice permanently. The surface of the electrocatalyst is a stable frontier it neither advances nor recedes. 

This reaction is about the simplest that involves intermediate radicals (adsorbed H atoms waiting to combine to form H2). A study of potential energy diagrams such as that described below can be used to comprehend why a change in the electrode potential changes the reaction rate, and thus to understand the basis of electrocatalysis. 

Two years ago, Advances in Catalysis featured a chapter on chemisorbed intermediates in electrocatalysis. In this issue we follow up with a chapter by Wendt, Rausch, and Borucinski, Advances in Applied Electrocatalysis. The successful commercial application of electrocatalysis requires a detailed, fundamental knowledge of the many catalytic phenomena such as adsorption, diffusion, and superimposition of catalyst micro- and nanostructure on the special requirements of electrode behavior. Considerable understanding of the status and limitations of electrolysis, fuel cells, and electro-organic syntheses has been obtained and provides a sound basis for future developments. 

In electrode reactions of the type H+/H2, 02/H20, and probably many organic redox systems, the electrode surface may be involved by virtue of the presence of adsorption sites where intermediates in the reaction mechanism, e.g. atomic hydrogen, are located. Generally, the reaction rate is higher at metals with a larger adsorptive capacity. This is a particular form of electrocatalysis, which is a subject of still-growing interest. 

Hydrogen evolution is the only reaction for which a complete theory of electrocatalysis has been developed [33]. The reason is that the reaction proceeds through a limited number of steps with possibly only one type of intermediate. The theory predicts that the electrocatalytic activity depends on the heat of adsorption of the intermediate on the electrode surface in a way giving rise to the well known volcano curve. The prediction has been verified experimentally [54] (Fig. 2) and the volcano curve remains the main predictive basis on which the catalytic activity is discussed [41, 55],... 

If correlations do exist for simple metals, predictions are much more difficult for composite materials. On the other hand, cathode activation has two aims (i) to replace active but expensive materials with cheaper ones, and (ii) to enhance the activity of cheaper materials so as to approach or even surpass that of the more expensive catalysts. In the case of pure metals there is little hope to find a new material satisfying the above requirements since in the volcano curve each metal has a fixed position which cannot be changed. Therefore, activation of pure metals can only be achieved by modifying its structure so as to enhance the surface area (which has nothing to do with electrocatalysis in a strict sense), and possibly to influence the mechanism and the energetic state of the intermediate in the wanted direction. This includes the preparation of rough surfaces but also of dispersed catalysts. Examples will be discussed later. 

As the particle size decreases, the ratio between the number of atoms at the surface to those in the bulk increases with a parallel decrease in the average coordination number for the metal atom, which is also expected to be a factor of electrocatalysis. It has been calculated for Pt that the minimum size of a crystallite (cluster) for all atoms to be on the surface is 4 nm, corresponding to a specific surface area of 280 m2g-1 [322] (note that this is larger than the critical particle size where absorption of H atoms disappears on Pd) [333]. It is also interesting that dispersed catalysts can in turn influence the electronic properties of the support so that an interesting combination of sites with varied properties can result [330]. At low catalyst loadings, spillover of intermediates is also possible. 

One of the drawbacks of DMFCs is the relatively slow rate of the anodic oxidation of methanol even on highly active platinum electrodes. It was shown that the Pt-Ru system is much more catalytically active than pure platinum (pure ruthenium is inert towards this reaction) (-> platinum-ruthenium -> electrocatalysis). The so-called bifunctional mechanism is broadly accepted to describe this synergistic effect, according to which organic species are chemisorbed predominantly on platinum centers while ruthenium centers more readily adsorb species OH, required for the oxidation of the organic intermediates. Usually the composition of such alloys is Pto.sRuo.s and the metal loading is 2-4 mg cm-2. 

If the heterogeneous charge transfer for the generation of the reactive intermediates is complicated by some surface character of the process it may be possible to circumvent the problem by using electrocatalysis. During electrocatalysis (Savdant, 1980) charge transfer at the electrode involves a catalyst redox couple, O/R, in a thermodynamically reversible reaction (4). 

The electrochemical techniques do not differ significantly with respect to time resolution. Pseudo first order rate constants ranging from about 0.1 to 10 S can be measured by techniques which monitor the response of the intermediate and LSV and electrocatalysis can give estimates of rate constants as high as 10 s . In the opinion of the author, the factors of most importance to be considered in selecting a measurement method of the first style are (i) the selectivity of the response, (//) the ease of obtaining reliable data, and (ill) the kinetic or thermodynamic information content of the data. Another factor of utmost importance to the non-specialist is (iv) the availability of instrumentation. 

The interaction of CO with the solid surface produces several physical and chemical effects on the vibrational properties of the adsorbed species. The adsorption of CO can be envisaged as a two-dimensional condensation, leading to lateral coupling between adsorbed molecules. The vibrational properties of adsorbed CO can thus be used to monitor the effects of other interface properties, such as surface defects, two-dimensional phase transitions [45] and co-adsorption. Finally, CO is formed as an intermediate or poison during the oxidation of several organic molecules at electrodes, thus constituting one of the subjects of interest in electrocatalysis. 

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