Electrocatalyst selectivity

An extremely selective electrocatalyst for C02 reduction to CO in water has recently been found by Beley et a/.135 A rather simple Ni complex of 1,4,8,11-tetraazacyclotetradecane, [Ni(II)-cyclam],... 

For electrogenerative processing it is often necessary to use a different, more selective catalyst than that used in fuel cells. Although the extensive screening of various electrocatalysts for fuel cells (3-13 can provide useful guidelines for choosing selective electrocatalysts, the specificity of such catalysts is not well characterized, with few exceptions (28, 29, 31, 54. ... 

Beley, M., Colhn, J.-P, Ruppert, R., and Sauvage, J.-P. 1984. Nickel(II)-cyclam An extremely selective electrocatalyst for reduction of COj in water. Journal ofthe Chemical Society Chemical Communications 19, 1315-1316. 

Not only is the value of jQ important in electrocatalysis but also the experimental Tafel slope at the operating electrode potential. As expected in an electrocatalytic process, this complex heterogeneous reaction exhibits at least one intermediate (reactant or product) adsorbed species. Therefore, a single or simple Tafel slope for the entire process is not expected, but rather surface coverage and electrolyte composition potential dependent Tafel slopes within the whole potential domain are expected. Instead of calculating the most proper academic Tafel slope, the experimental current vs. potential curve is required for the selected electrocatalysts [4,6]. 

Another selective electrocatalyst Co phthalocyanines-poly-4-vinylpiridine (PVP) has been used to modified electrodes in this case the method of... 

Beley M, Collin JP, Ruppert R, Sauvage JP (1984) Nickel(II)-cyclam an extremely selective electrocatalyst for reduction of CO2 in water. J Chem Soc Chem Commun 1315—1316... 

Table 3.1 lists some of the anodic reactions which have been studied so far in small cogenerative solid oxide fuel cells. A more detailed recent review has been written by Stoukides46 One simple and interesting rule which has emerged from these studies is that the selection of the anodic electrocatalyst for a selective electrocatalytic oxidation can be based on the heterogeneous catalytic literature for the corresponding selective catalytic oxidation. Thus the selectivity of Pt and Pt-Rh alloy electrocatalysts for the anodic NH3 oxidation to NO turns out to be comparable (>95%) with the... 

Silver films and Ag-CaO-SragOg cermets were chosen as the anodic electrocatalysts because of their high electrical conductivity, which is necessary for electrocatalytic operation, and also because of their high (>95%) selectivity to Cg hydrocarbons at very low (<2%) CH conversions [9]. 

Figure 7. Effect of methane conversion on C2 selectivity for some of the best state-of-the-art OCM catalysts (A, based on ref 4), the simulated chromatographic reactor of Aris and coworkers (A, ref. 10) and the present work. ( ) Ag electrocatalyst, single pass (O) Ag electrocatalyst with recycle and trapping (0) Sr/LagOg catalyst, single pass ( ) Sr/La20g catalyst with recycle and trapping. Open symbols, batch operation filled symbols, continuous-flow steady-state operation.

Nickel oxide anodes are another example for a relatively simple oxide electrocatalyst used rather widely in the oxidation of organic substances (alcohols, amines, etc.) in alkaline solutions at relatively low anodic potentials (about +0.6 V RHE). These processes, which occur at an oxidized nickel surface, are rather highly selective. As an example, we mention the industrial oxidation of diacetone-L-sorbose to the corresponding acid in vitamin C synthesis. This reaction occurs at nickel oxide electrodes with chemical yields close to 100%. 

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. 

A more interesting situation is found when the homogeneous redox reaction is combined with a chemical reaction between the electrocatalyst and the substrate. In this case, the catalytic process is called chemical catalysis. 3 This mechanism is depicted in Scheme 2 for reduction. The coupling of the electron transfer and the chemical reaction takes place via an inner-sphere mechanism and involves the formation of a catalyst-substrate [MC-S] complex. Here the selectivity of the mechanism is determined by the chemical step. Metal complexes are ideal candidates... 

The selective and almost quantitative electrocatalytic formation of CO is obtained when [Ru(bpy)2(CO)2]2+ is used as electrocatalyst in aqueous acetonitrile (20 80) medium. In the absence of added water, formation of the [Ru(bpy)(CO)2]ra polymer is in competition with that of the hydride derivative [Ru(bpy)(CO)H]+, the latter being the active catalyst as demonstrated in a separate experiment using an authentic sample of this hydrido-complex.92 In those conditions HCOO" and H2 are also formed besides CO. 

Furthermore, the utilization of preformed films of polypyrrole functionalized by suitable monomeric ruthenium complexes allows the circumvention of problems due to the moderate stability of these complexes to aerial oxidation when free in solution. A similar CO/HCOO-selectivity with regards to the substitution of the V-pyrrole-bpy ligand by an electron-with-drawing group is retained in those composite materials.98 The related osmium-based redox-active polymer [Os°(bpy)(CO)2] was prepared, and is also an excellent electrocatalyst for the reduction of C02 in aqueous media.99 However, the selectivity toward CO vs. HCOO- production is lower. 

Run(Hedta)(NO+)]° and [Fen(Hedta)(NO )] have been shown to be effective electrocatalysts for the reduction of N02 in acidic aqueous media, to yield N20, N2, NH3OH+, or NH4 339,340 An element of selectivity is available by control of pH and applied potential. Steps involved in the typical six-electron reduction of nitrite to ammonia catalyzed by [Run(Hedta)(NO+)]° are summarized in Equations (67)-(69). The mechanisms by which nitrite is reduced appeared to be similar to those identified for Fe-porphyrin331 and Ru or Os-polypyridyl complexes.337... 

Techniques for attaching such ruthenium electrocatalysts to the electrode surface, and thereby realizing some of the advantages of the modified electrode devices, have been developed.512-521 The electrocatalytic activity of these films have been evaluated and some preparative scale experiments performed. The modified electrodes are active and selective catalysts for oxidation of alcohols.5 6-521 However, the kinetics of the catalysis is markedly slower with films compared to bulk solution. This is a consequence of the slowness of the access to highest oxidation states of the complex and of the chemical reactions coupled with the electron transfer in films. In compensation, the stability of catalysts is dramatically improved in films, especially with complexes sensitive to bpy ligand loss like [Ru(bpy)2(0)2]2 + 51, 519 521... 

The electrochemical rate constants for hydrogen peroxide reduction have been found to be dependent on the amount of Prussian blue deposited, confirming that H202 penetrates the films, and the inner layers of the polycrystal take part in the catalysis. For 4-6 nmol cm 2 of Prussian blue the electrochemical rate constant exceeds 0.01cm s-1 [12], which corresponds to the bi-molecular rate constant of kcat = 3 X 103 L mol 1s 1 [114], The rate constant of hydrogen peroxide reduction by ferrocyanide catalyzed by enzyme peroxidase was 2 X 104 L mol 1 s 1 [116]. Thus, the activity of the natural enzyme peroxidase is of a similar order of magnitude as the catalytic activity of our Prussian blue-based electrocatalyst. Due to the high catalytic activity and selectivity, which are comparable with biocatalysis, we were able to denote the specially deposited Prussian blue as an artificial peroxidase [114, 117]. 

The transfer of redox equivalents can be achieved by an electrocatalyst (mediator) or a modified electrode. Indirect electrolysis can lead to a better selectivity due to the specific interaction of the mediator with the substrate. However, low turnovers and the need to separate the mediator from the product are possible disadvantages, as mentioned above. The nickel hydroxide electrode [195,196] is fairly free from these disadvantages. The following mechanism for the oxidation at the nickel hydroxide electrode has been proposed in the literature [195]. 

---