Electrocatalysis redox

In the field of catalysis, Au NPs appear to be particularly important as an efficient catalyst in organic reactions it can offer the most favourable combination of activity and selectivity in various catalytic reactions such as electrocatalysis, redox catalysis, carbon-carbon bond formation, and photocatalytic reactions. Moreover, recent literature reported that the Au NPs-graphene composites exhibit unprecedented catalytic activity for CO oxidation, reduction of nitro-aniline and Suzuki-Miyaura coupling reaction of chlorobenzene with arylboronic acid. 

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 idea that the cathode potential with respect to ]lt(H20)/Pt-0Hads determines the value of the pre-exponential factor in the ORR rate expression was inspired by a comment by Andy Gewirth (Urbana) in his talk in Leiden, pointing to the value of Pourbaix diagrams for understanding ORR electrocatalysis. Indeed, the information on these ORR-mediating and facilitating M/M-OH surface redox systems is to be found in Pourbaix s Atlas. 

As we demonstrate in this chapter, enzymes can be extremely active electrocatalysts at ambient temperatures and mild pH, and have significantly higher reaction selectivity than precious metals. The main disadvantage in applying redox enzymes for electrocatalysis arises from their large size, which means that the catalytic active site density is low. Enzymes also have a relatively short hfetime (usually not more than a few months), making them more suited to disposable applications. 

Figure 17.4 Cartoon representation of strategies for studying and exploiting enzymes on electrodes that have been used in electrocatalysis for fuel cells, (a) Attachment or physisorption of an enzyme on an electrode such that redox centers in the protein are in direct electronic contact with the surface, (b) Specific attachment of an enzyme to an electrode modified with a substrate, cofactor, or analog that contacts the protein close to a redox center. Examples include attachment of the modifier via a conductive linker, (c) Entrapment of an enzyme within a polymer containing redox mediator molecules that transfer electrons to/from centers in the protein, (d) Attachment of an enzyme onto carbon nanotubes prepared on an electrode, giving a large surface area conducting network with direct electron transfer to each enzyme molecule.

Anson FC, Ni CL, Saveant JM. 1985. Electrocatalysis at redox polymer electrodes with separation of the catalytic and charge propagation roles. Reduction of dioxygen to hydrogen peroxide as catalyzed by cobalt(II) tetrakis(4-A-methylpyridyl)porphyrin. J Am Chem Soc 107 3442. 

The thing to be noted here is that the ° values of the 02/ 02" and 02" H202 redox couples are -0.35 and 0.68 V vs Ag/AgCl at pH 7.4 and thus the SODs, for example, Cu, Zn-SOD (Cu (I/II)) with ° = 65mV can mediate both the oxidation of 02 to 02 and the reduction of 02" to H202. Such a bi-directional electromediation (electrocatalysis) by the SOD/SAM electrode is essentially based on the inherent specificity of the SOD enzyme which catalyzes the dismutation of 02 to 02 and H202 via a redox cycle of their metal complex moiety (Scheme 3). 

Figure 6.7 illustrates the voltammetric response of the third-generation SOD-based 02 biosensors with Cu, Zn-SOD confined onto cystein-modified Au electrode as an example. The presence of 02" in solution essentially increases both the cathodic and anodic peak currents of the SOD compared with its absence [150], Such a redox response was not observed at the bare Au or cysteine-modified Au electrodes in the presence of 02". The observed increase in the anodic and cathodic current response of the Cu, Zn-SOD/cysteine-modified Au electrode in the presence of 02 can be considered to result from the oxidation and reduction of 02, respectively, which are effectively mediated by the SOD confined on the electrode as shown in Scheme 3. Such a bi-directional electromediation (electrocatalysis) by the SOD/cysteine-modified Au electrode is essentially based on the inherent specificity of SOD for the dismutation of 02", i.e. SOD catalyzes both the reduction of 02 to H202 and the oxidation to 02 via a redox cycle of its Cu (II/I) complex moiety as well as the direct electron transfer of SOD realized at the cysteine-modified Au electrode. Thus, this coupling between the electrode and enzyme reactions of SOD could facilitate the development of the third-generation biosensor for 02". ...

Studies of protein film electrocatalysis have also been illuminating. For example, succinate dehydrogenase displays an unusual optimal potential for activity. The enzyme contains four redox sites a flavin, a... 

Fe 2S], a [4Fe-4S] and a [3Fe-4S] center. The enzyme catalyzes the reversible redox conversion of succinate to fumarate. Voltammetry of the enzyme on PGE electrodes in the presence of fumarate shows a catalytic wave for the reduction of fumarate to succinate (much more current than could be accounted for by the stoichiometric reduction of the protein active sites). Typical catalytic waves have a sigmoidal shape at a rotating disk electrode, but in the case of succinate dehydrogenase the catalytic wave shows a definite peak. This window of optimal potential for electrocatalysis seems to be a consequence of having multiple redox sites within the enzyme. Similar results were obtained with DMSO reductase, which contains a Mo-bis(pterin) active site and four [4Fe 4S] centers. 

Fig. 18b.9. Example cychc voltammograms due to (a) multi-electron transfer redox reaction two-step reduction of methyl viologen MV2++e = MV++e = MV. (b) ferrocene confined as covalently attached surface-modified electroactive species—peaks show no diffusion tail, (c) follow-up chemical reaction A and C are electroactive, C is produced from B through irreversible chemical conversion of B, and (d) electrocatalysis of hydrogen peroxide decomposition by phosphomolybdic acid adsorbed on a graphite electrode.

The term direct electrochemistry of proteins means the possibility to detect the direct exchange of electrons between the active site(s) of a protein and a (metallic or inert material) electrode without the help of redox mediators, which might favour an indirect interaction between the electrode and the protein (see the discussion on Electrocatalysis in Chapter 2, Section 1.4.4). This aspect of electrochemistry is not yet as widely explored as it deserves, but the relevant results are now analysed in a rather comprehensive fashion.1 ... 

We will discuss here applications of polyelectrolyte-modified electrodes, with particular emphasis on layer-by-layer self-assembled redox polyelectrolyte multilayers. The method offers a series of advantages over traditional technologies to construct integrated electrochemical devices with technological applications in biosensors, electrochromic, electrocatalysis, corrosion prevention, nanofiltration, fuel-cell membranes, and so on. 

In addition, such redox-active organometallic dendrimers are interesting materials with which to modify electrode surfaces. Applications of these dendrimer modified electrodes in the fields of amperometric and potentiometric biosensors, molecular recognition, as well as in electrocatalysis and photoelectrochemistry, clearly represent interesting areas of future research. 

The transfer of single electrons is observed on the current-potential dependence as a sequence of steps. This shows that clusters behave as redox reactants. There are many applications of gold clusters in various fields, such as preparation of new materials, electronics, heterogeneous catalysis and electrocatalysis, biosensors and others. 

The observation of the electrocatalysis in Fig. 1 suggests that the Ru(bpy)3 and guanine couples have similar redox potentials. Based on the kinetics of oxidation by a series of substituted Ru(bpy)3 complexes, we predicted that the redox potential of guanine was 1.1 V (all potentials versus Ag/AgCl) [17]. Later, equilibrium titrations performed by Steenken using known one-electron oxidants showed that the potential was 1.07 V at pH 7 [39], which also implied that the guanine deprotonates in our reaction. The issue of guanine deprotonation will be discussed in depth below. 

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. 

In this particular use of modified electrodes, i.e. electrocatalysis, the immobilized redox couple acts as an electron transfer mediator cycling between the reactive (catalyst) state and its non-catalytic state, as shown schematically in Figure 1. 

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