Active site electrocatalysts

They have an exceedingly high specific activity per active site the turnover number y is as high as 10 to 10 s in certain enzyme reactions, while at ordinary electrocatalysts having a number of reaction sites on the order of 10 cm , yhas a value of about 1 s at a current density of lOmA/cm. Thus, the specific catalytic activity of tfie active sites of enzymes is many orders of magnitude fiigher tfian tfiat of all other known catalysts for electrochemical (and also chemical) processes. 

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. 

Another important aspect of electrocatalysis is the study of dispersed high specific area and supported, both metal and non-metal, electrocatalysts. A high degree of dispersion brings about enhancement of the catalytic activity because of the specific area and energetics of active sites [140] and decrease of susceptibility of poisoning because of the improved ratio of catalyst area to impurities in solution. 

It has already been mentioned that one of most used forms of Ni is Raney Ni which is obtained from Ni-Al or Ni-Zn alloys by leaching A1 or Zn in alkaline solution. However, the properties of the resulting electrocatalyst appear to depend on the nature of the precursor [135], Methods of application of the alloys are various [135]. A particularly convenient one is the so-called LPPS (low pressure plasma spray) [146]. Raney Ni prepared in this way has shown that lower Ihfel slopes can be obtained, thus suggesting a real electrocatalytic effect (Fig. 11). On such highly porous Ni it is possible that the proportion of particularly active sites (at the edges and peaks of crystallites [262] increases considerably. However, the effect of temperature on the Tafel slope is more than anomalous [248] suggesting indeed some temperature-induced surface modifications. In fact, recrystallization phenomena are observed which can be minimized by means of small additions of Ti, Mo or Zr. The... 

The example considered is the redox polymer, [Os(bpy)2(PVP)ioCl]Cl, where PVP is poly(4-vinylpyridine) and 10 signifies the ratio of pyridine monomer units to metal centers. Figure 5.66 illustrates the structure of this metallopolymer. As discussed previously in Chapter 4, thin films of this material on electrode surfaces can be prepared by solvent evaporation or spin-coating. The voltammetric properties of the polymer-modified electrodes made by using this material are well-defined and are consistent with electrochemically reversible processes [90,91]. The redox properties of these polymers are based on the presence of the pendent redox-active groups, typically those associated with the Os(n/m) couple, since the polymer backbone is not redox-active. In sensing applications, the redox-active site, the osmium complex in this present example, acts as a mediator between a redox-active substrate in solution and the electrode. In this way, such redox-active layers can be used as electrocatalysts, thus giving them widespread use in biosensors. 

The foregoing discussion serves to show that disordered carbon structures are oxidized more readily than well-ordered graphite planes and that dislocations and active sites provide nucleation points for attack of the carbon crystallite. Another factor that must be considered is that dispersed electrocatalysts, such as platinum, on the carbon surface are not benign. The electrocatalysts interact with the carbon causing local oxidation or corrosion, i.e., the platinum catalyzes the corrosion of the carbon itself. In the presence of oxygen, which is the condition under which the electrocatalyst will operate, reduction intermediates from the oxygen (e.g., HOj) can have an accelerated corrosion effect. 

Two types of porous electrodes can be considered two- and three-phase systems, where the latter is the special case of a triphasic interface in fuel cells, where the gas, liquid, and solid coexist. In the former, the liquid reactant is dissolved in the electrolyte and transported to the active sites of the electrocatalyst. In each case, we can consider uniform, parallel, cylindrical, or conical pores that are topped at the bottom by the metal substrate and at the top by the electrolyte [19,20],... 

Electrochemical processes often involve gas evolution reactions. Formation and evolution of bubbles are most important in the case of the electrocatalyst since they can block the active sites from further reactions. 

The nucleation process starts at defined active centers of the surface called the nucleation sites. Even in the case of an electrolyte supersaturated with gas the activation of the nucleation site is required. It depends not only on the nature of the gas and the electrolyte but also on the interfacial tension and number of substrate neighbors on the electrocatalyst. In summary, the existence of a gas-electrolyte interface besides the electrolyte-solid interface is required. Some authors explained this process as nucleate boiling [73] and others [74] as the super saturation of the electrolyte with the gas, developing different equations for the current density. Another important factor to consider is the number of surface active sites available for the bubble formation and the geometry of the bubble. Moreover, the surface roughness is certainly a factor to be considered for the stability of the nucleation site. The ageing of the nucleation site also has to be considered since it results in the loss of activity after a long time. 

A typical example includes the yttria-stabilized-zirconia-based high-temperature potentiometric oxygen sensor which is widely used in automotive applications. Platinum thick films are applied, forming both the cathode and anode of the sensor. The thick electrode has a porous structure which provides a larger electrode surface area compared to non-porous structures. For current measurement, a porous electrode is desirable since it leads to a larger current output. If the metallic film serves as the electrocatalyst, a porous structure is also desirable, for it provides more catalytic active sites. On the other hand, electrodes formed by the thick-film technique do not have an exact, identical... 

The activity of electrocatalysts can further be enhanced by special sites on the surface and step, kink and edge sites, lattice vacancies, grain boundaries and dislocations have all been suggested to have a beneficial effect. This may be because they lead to sites with different free energies of adsorption or because they create unusual spacings or arrangements of potential adsorption sites. 

As far as the kinetics and mechanistic aspects of oxygen reduction on these non-noble metal electrocatalysts are concerned, it has been shown that these catalysts may reduce O2 to water with an apparent number of electrons transferred, n, that may reach values very close to 4. This is especially true for Fe-based electrocatalysts made either from Fe-N4 chelates or from cheaper Fe salt precursors. It seems also that the Fe-N2/C catalytic site, which is the most active site in catalysts obtained after a pyrolysis temperature > 800°C, is characterized by a low release of peroxide. Co-based catalysts release, on average, more peroxide than the corresponding Fe-based materials. Studies that were undertaken to decouple the direct 4-electron reduction of oxygen to water from the successive 2 X 2-electron reduction indicate that the direct 4-electron reduction path may be important for these catalysts. This result is in agreement with the quantum theoretical approach of Anderson and Sidik about a model of the pyrolyzed... 

Ginovska-Pangovska B, Dutta A, Reback ML, Linehan JC, Shaw WJ (2014) Beyond the active site the impact of the outer coordination sphere on electrocatalysts for hydrogen production and oxidation. Acc Chem Res 47(8) 2621-2630. doi 10.1021/ar5001742... 

Regarding the electrocatalyst, the similar concepts about the catalytic activity can be defined in the similar ways as Eqns (3.1)—(3.4). Actually, electrocatalysts are a specific form of catalysts that function at electrode surfaces or may be the electrode surface itself. An electrocatalyst can be heterogeneous such as a platinum surface or nanoparticles, or homogeneous like a coordination complex or enzyme. However, in this book, we are only focused on the heterogeneous electrocatalysts. The role of electrocatalyst is to assist in transferring electrons between the electrode catalytic active sites and reactants, and/or facilitates an intermediate chemical transformation. One important difference between catalytic chemical reaction and electrocatalytic reaction is that the electrode potential of the electrocatalyst can also assist in the reaction. By changing the potential of the electrocatalyst, which is attached onto the electrode surface, the electrocatalytic activity can be enhanced or depressed significantly. 

The electrocatalytic activity of an electrocatalyst can also be described using the same concept of TOP as shown in Eqn (3.1). However, the electrocatalytic reaction involves the electron transfer from the catalyst to the reactant, the reaction rate or TOP is better being expressed as the electron transfer rate at one catalytic active site of the catalyst ... 

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