Electrocatalytic process

As discussed in Chapter 10, most of synthetically or energetically relevant electrochemical processes involve high kinetic constraints, represented by cathodic and/ or anodic overpotentials. Electrocatalysis allows lowering such overpotentials and obtaining effective electrosynthetic procedures. 

The encapsulation of catalytically active species into porous solids is one of the possible strategies of particular interest. Thus, Bessel and Rolison (1997b,c) compared the electrocatalytic effect of zeolite Y-encapsulated Co(salen) (salen = N,N -bis(salicylidene)ethylenediamine) on the reaction between benzyl chloride and carbon dioxide in tetrahydrofuran/hexamethylphosphoramide, with that exerted by the same complex in solution. Using a large surface area reticulated vitreous carbon electrode immersed into suspensions of Co(salen) NaY in solutions of the reagents, the effective electrocatalytic turnover is ca. 3000 times that of homogeneous Co(salen) under comparable conditions. Remarkably, coulometric experiments indicated that only 

It should be noted that electrocatalytic effects in zeolites can be influenced by 

Microheterogeneous electrocatalysis can be performed with zeolite-based catalysts using suspensions of such materials in organic solvents with low ionic strength by applying a DC voltage to suspensions of zeolite particles, as described by Rolison and Stemple (1993) for the Pd -Cu -NaY catalyzed oxidation of propene, but a wide variety of systems has been studied. 

In the case of electron transfer mediators immobilized on electrode surfaces, one can define a specific activity, S, as the quotient between catalytic peak current, /peat, and catalyst deposition charge, Cc.u (Sa = ipcn/Qcut) (Wang et al., 2004). In most systems, one can define apparent activation energy, satisfying an Arrhenius-type equation  

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Techniques are described which obtain the IR absorption spectra of species, either adsorbed or free In the electrode/electrolyte solution Interphase. Applications slanted towards topics relevant to electrocatalytic processes are discussed to Illustrate the capabilities of the methods In probing molecular structure, orientation and Interactions. 

In electrocatalysis, the major subject are redox reactions occurring on inert, nonconsumable electrodes and involving substances dissolved in the electrolyte while there is no stoichiometric involvement of the electrode material. Electrocatalytic processes and phenomena are basically studied in aqueous solutions at temperatures not exceeding 120 to 150°C. Yet electrocatalytic problems sometimes emerge as well in high-temperature systems at interfaces with solid or molten electrolytes. 

The catalytic activity of the N4 complexes depends both on the nature of the central metal ion and on the nature of the ligand and aU substituents. It was found that the metal ion is the active site where the electrocatalytic process is accomplished. During its adsorption, an oxygen molecule forms a stable complex (adduct) with the... 

As the generality of equations of type (5.2.3) should not be exaggerated (e.g. in the presence of strong adsorption, such as in electrocatalytic processes, they are no longer valid), the basic features of electrochemical kinetics will be explained by using the simple electrode reaction... 

The electrode processes of oxygen represent a further important group of electrocatalytic processes. The reduction of oxygen to water... 

So far, several examples have been given of the inhibition of electrocatalytic processes. This retardation is a result of occupation of the catalyti-cally more active sites by electroinactive components of the electrolyte, preventing interaction of the electroactive substances with these sites. The electrode process can also be inhibited by the formation of oxide layers on the surface and by the adsorption of less active intermediates and also of the products of the electrode process. 

M. Faraday was the first to observe an electrocatalytic process, in 1834, when he discovered that a new compound, ethane, is formed in the electrolysis of alkali metal acetates (this is probably the first example of electrochemical synthesis). This process was later named the Kolbe reaction, as Kolbe discovered in 1849 that this is a general phenomenon for fatty acids (except for formic acid) and their salts at higher concentrations. If these electrolytes are electrolysed with a platinum or irridium anode, oxygen evolution ceases in the potential interval between +2.1 and +2.2 V and a hydrocarbon is formed according to the equation... 

Several other polypyridyl metal complexes have been proposed as electrocatalysts for C02 reduction.100-108 For some of them HCOO- appears as the dominant product of reduction. It has been shown for instance that the complexes [Rhin(bpy)2Cl2]+ or [Rh n(bpy)2(CF3S03)2]+ catalyze the formation of HCOO- in MeCN (at —1.55 V vs. SCE) with a current efficiency of up to 80%.100,103 The electrocatalytic process occurs via the initially electrogenerated species [RhI(bpy)2]+, formed by two-electron reduction of the metal center, which is then reduced twice more (Rlr/Rn Rh°/Rh q. The source of protons is apparently the supporting electrolyte cation, Bu4N+ via the Hoffmann degradation (Equation (34)). 

Efremov B.N., Tarasievich M.R. Electrocatalysis and Electrocatalytic Processes (in Russian). In Sbornik Nauchnykh Trudov (Collection of Sci. Transactions), Kiev Naukova Dumka, 1986 44-71. 

As shown in Section 2.2.7, chemical reactions may be triggered by electrons or holes from an electrode as illustrated by SrnI substitutions (Section 2.5.6). Instead of involving the electrode directly, the reaction may be induced indirectly by means of redox catalysis, as illustrated in Scheme 2.15 for an SrnI reaction. An example is given in Figure 2.30, in which cyclic voltammetry allows one to follow the succession of events involved in this redox catalysis of an electrocatalytic process. In the absence of substrate (RX) and of nucleophile (Nu-), the redox catalysis, P, gives rise to a reversible response. A typical catalytic transformation of this wave is observed upon addition of RX, as discussed in Sections 2.2.6 and 2.3.1. The direct reduction wave of RX appears at more negative potentials, followed by the reversible wave of RH, which is the reduction product of RX (see Scheme 2.21). Upon addition of the nucleophile, the radical R is transformed into the anion radical of the substituted product, RNu -. RNu -... 

One obtains the first indication of the possible existence of an electrocatalytic process from cyclic voltammetry. 

A few terms of the electrocatalytic process still need to be defined. 

The first is the catalytic efficiency of the electrocatalytic process, which in the case of the electrochemically induced reaction is called coulombic efficiency. It is determined by the number of product molecules formed per electron consumed. In our example, the consumption of 0.02 electrons per molecule indicates a coulombic efficiency of 50 molecules produced per electron consumed. 

The driving force of the electrocatalytic process is expressed by the variation of the free energy of the overall process (which must clearly be negative), according to the relationship ... 

Furthermore, electrolysis at the first reduction peak (Ew = — 1.25 V) is completed in a few seconds, consuming about 0.05 electrons per molecule. At the end of the exhaustive reduction the cyclic voltammogram displays only the irreversible peak at —1.45 V. These data unequivocally indicate the existence of the following electrocatalytic process ... 

All these observations point to the occurrence of a 8 2 rather than an outer sphere, dissociative electron-transfer mechanism in cases where steric constraints at the carbon or metal reacting centres are not too severe. It is, however, worth examining two other mechanistic possibilities. One of these is an electrocatalytic process of the Sg -type that would involve the following reaction sequence. If, in the reaction of the electron donor (nucleophile), the bonded interactions in the transition state are vanishingly small, the alkyl radical is formed together with the oxidized form of the electron donor, D . Cage coupling (144) may then occur, if their mutual affinity is... 

Concentrating on the operation of the so-called membrane electrode assembly (MEA), E includes irreversible voltage losses due to proton conduction in the PEM and voltage losses due to transport and activation of electrocatalytic processes involved in the oxygen reduction reaction (ORR) in the cathode catalyst layer (CCL) ... 

It is important to clarify that there have been, in the literature, some examples of electrochemical processes on CNT-modified electrodes on which an apparent electrocatalytic process associated to the CNTs seems to take place (that is from the edge-plane-like sites) where in fact that was not the case. An example is the apparent electrocatalytic oxidation ofhydrazine at MWNTelectrodes [64,65]. Such electrochemical behavior has been demonstrated to be a consequence of iron impurities contained in the CNTs that were responsible for the observed electrocatalytic effects (Figure 3.7). Therefore, caution is needed when reporting catalytic effects of CNTs under a given redox system and a careful comparison vdth, for instance, edge HOPG is mandatory to make sure that the CNTs are the responsible for the electrochemical enhancement. 

The Chemical Process Engineering Research Institute (CPERI) works on bench and pilot plant hydrogen production units, SOFCs, polymer electrolyte proton conductors, and high-temperature electrocatalytic processes. 

The incorporation of redox-active organometallic units within or on the periphery of dendritic structures is an especially challenging target because such molecules ate good candidates to play a key role as multielectron transfer mediators in electrocatalytic processes of biological and industrial importance. In particular, the organometallic ferrocene moiety is an attractive redox center to integrate into dendritic structures, not only because it is electrochemically well behaved in most... 

In the mid-1960s, Dessy and coworkers [12, 13] provided an extensive survey of the anodic and cathodic reactions of transition metal organometallic species, including binary (homoleptic) carbonyls, and this provided a stimulus for many later detailed studies. Whereas the electrochemistry of heteroleptic transition metal carbonyls is covered elsewhere in this volume, that of the binary carbonyls, which is covered here, provides paradigms for the electrochemistry of their substituted counterparts. A key aspect is the generation of reactive 17-electron or 19-electron intermediates that can play key roles in the electrocatalytic processes and electron-transfer catalysis of CO substitution by other ligands. 

Electrocatalytic processes have been used for the production of useful chemicals, for instance, N2O and NH2OH from nitrates. 

The investigation of adsorption phenomena occurring on Pt and Pd electrodes is an important task for the clarification of their behavior in electrocatalytic processes. A characteristic feature for these systems is the hydrogen adsorption reflected by the shape of voltammetric curves taken under suitable conditions. This experimental approach constitutes the intersection point where the problem of perchlorate... 

The comparison of the voltammetric characteristics of PWig and P2W18 is useful to highlight the peculiarities of the former complex. Figure 21 shows in superimposition the cyclic voltammograms (CVs) of the two complexes in a pH 0.3 sulfate medium. The potential domain was selected to avoid any deleterious derivatization of the electrode surface [28]. Furthermore, such domain is the most useful for elucidation of electrocatalytic processes. Here, the voltammetric pattern is restricted to the first three waves observed for PWig in this medium. In... 

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... 

Table 13 gathers the main electrocatalytic processes triggered by POMs dissolved in solution. The electrocatalytic reductions of dioxygen, hydrogen peroxide and of several NOx including nitrite, nitric oxide, and nitrate are selected for a more detailed description. 

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