Electrocatalytic systems

It is very difficult in view of the vast amount of experimental data to draw general conclusions that would hold for different, let alone all electrocatalytic systems. The crystallographic orientation of the surface undoubtedly has some specific influence on adsorption processes and on the electrochemical reaction rates, but this influence is rather small. It can merely be asserted that the presence of a particular surface orientation is not the decisive factor for high catalytic activity of a given electrode surface. 

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. 

Figure 4.1 Schematic of the atomic structure of the active three-phase interface between the metal particle that catalyzes the reaction, the carbon support necessary to conduct electrons, and the polymer electrolyte and solution necessary to conduct protons for electrocatalytic systems.

The reduction of protons is one of the most fundamental chemical redox reactions. Transition metal-catalyzed proton reduction was reviewed in 1992.6 The search for molecular electrocatalysts for this reaction is important for dihydrogen production, and also for the electrosynthesis of metal hydride complexes that are active intermediates in a number of electrocatalytic systems. 

Low-valent cobalt pyridine complexes, electrogenerated from CoCl2 in DMF containing pyridine and associated with a sacrificial zinc anode, are also able to activate aryl halides to form arylzinc halides.223 This electrocatalytic system has also been applied to the addition of aryl bromides containing an electron-withdrawing group onto activated alkenes224 and to the synthesis of 4-phenylquinoline derivatives from phenyl halides and 4-chloroquinoline.225 Since the use of iron as anode appeared necessary, the role of iron ions in the catalytic system remains to be elucidated. 

The organic compounds were dissolved or dispersed in an aqueous solution of the catalyst, with or without an organic cosolvent, and the net oxidations were carried out at applied potentials causing the oxidation of Ru11 to RuIV complex (0.6-0.8 V vs. SCE). It has been demonstrated that this electrocatalytic system is capable of providing a general and selective method for the oxidation of alcohols, aldehydes, cyclic ketones, and C—11 bonds adjacent to alkeneic or aromatic groups. 

Among the main goals of electrochemical research are the design, characterization and understanding of electrocatalytic systems, (1-2) both in solution and on electrode surfaces. (3.) Of particular importance are the nature and structure of reactive intermediates involved in the electrocatalytic reactions.(A) The nature of an electrocatalytic system can be quite varied and can include activation of the electrode surface by specific pretreatments (5-9) to generate active sites, deposition or adsorption of metallic adlayers (10-111 or transition metal complexes. (12-161 In addition the electrode can act as a simple electron shuttle to an active species in solution such as a metallo-porphyrin or phthalocyanine. 

The application of ultra-high vacuum surface spectroscopic methods coupled to electrochemical techniques t21-241 have provided valuable information on surface structure/reactivity correlations. These determinations, however, are performed ex-situ and thus raise important concerns as to their applicability to electrocatalytic systems, especially when very active intermediates are involved. 

An added difficulty that arises in the in-situ spectroscopic study of electrocatalytic systems in solution is that the active species will be located in the vicinity of the electrode so that the material in solution will generally represent a large background signal making the detection and identification of related species difficult. Thus, it would be ideal to be able to probe only that region proximal to the electrode surface and furthermore to be able to obtain structural information of the species involved. 

One of the emerging uses of solid-state electrocatalytic systems is in fuel cells, to convert a significant portion of the Gibbs free energy change of exothermic reactions into electricity rather than heat. The thermodynamic efficiency of such power generating schemes compares favorably with thermal power generation which is limited by Carnot-type constraints. 

Since the two-electron reduction to formic acid or CO requires a lower potential, electrolysis using a multielectron transfer catalyst in aqueous or in low-protic media can be carried out at considerably lower voltages. The simplest electrocatalytic system for CO2 reduction is an electrochemical cell that contains a working electrode, a reference electrode, a homogeneous electrocatalyst, the supporting electro-... 

The kinetic behaviour of electrochemical biosensors is most commonly characterized using the dependence of the steady-state amperometric current on the substrate concentration. This type of analysis has some limitations because it does not allow for a decoupling of the enzyme-mediator and enzyme-substrate reaction rates. The additional information required to complete the kinetic analysis can be extracted either from the potential dependence of the steady-state catalytic current or from the shift of the halfwave potential with substrate concentration [154]. Saveant and co-workers [155] have presented the theoretical analysis of an electrocatalytic system... 

As was just outlined, the presence of a solvent can dramatically affect metal-catalyzed reaction chemistry. It is well established that polar solvents can enhance reactions that have a greater degree of charge separation in the transition state than in their reactant state. The solvent acts to stabilize the transition state over the reactant state. This effectively lowers the activation barrier. Solution effects are, therefore, of critical importance to electrocatalytic systems. 

More recent work [72-81] has addressed the key theoretical aspects related to adsorption and electron-transfer process for important electrocatalytic system, such as, for example, CO adsorption and oxidation on transition metal [76] (Fig. 29). 

The HOR and HER are the most thoroughly studied electrocatalytic systems [105-114]. Excellent reviews are given by Breiter [115], Lasia [116], and Markovic [117]. Conway and Bockris first established, in 1957, a linear relation between log /0 and the metal work function 0 [118]. Parsons [60] and Gerischer [119]... 

This electrocatalytic system is of great importance [123-136] both for efficient fuel cell operation [123, 125] and for efficient electrolysis [123]. The progress in understanding oxygen electrocatalysis has been reviewed by Yeager [129], Appleby [130],... 

In recent years, and starting from electrochemical promotion studies [23], certain rules have been extracted for the selection of promoters for catalytic and electrocatalytic systems. 

The above rules can also help rationalize some of the key observations in electrocatalytic systems of great theoretical and practical importance. The example of CO oxidation on Pt in aqueous solutions is quite illustrative It is well established [117] that the activity of Pt(lll) increases dramatically in the sequence Br r HCIO4 NaOH or KOH and the oxidation ignition starts at 1.1, 0.92, and 0.65 V (versus RHE) [117]. In the latter case (0.1 M KOH), the onset of the preignition starts at 0.25 V (versus RHE), that is, in the Huppotential region. The CO oxidation proceeds via reaction between adsorbed CO and OH-in a Langmuir-Hinshelwood (L—H)-type mechanism. This implies that in alkaline solutions adsorbed OH can exist even at potentials below 0.25 V (versus RHE). 

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