Electrocatalysts homogeneous

The small metal particle size, large available surface area and homogeneous dispersion of the metal nanoclusters on the supports are key factors in improving the electrocatalytic activity and the anti-polarization ability of the Pt-based catalysts for fuel cells. The alkaline EG synthesis method proved to be of universal significance for preparing different electrocatalysts of supported metal and alloy nanoparticles with high metal loadings and excellent cell performances. 

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... 

Many polypyridyl metallic complexes have been used successfully as electrocatalysts in homogeneous and heterogeneous systems and generally CO and 11 COO are the main products of reduction. Among these, the two complexes, [Re(bpy)(CO)3Cl] and [Ru(bpy)2(CO)X]" + (X = CO, Cl, H 11 2 or 1), have attracted much attention because of their characteristic... 

Catalysis is known as the science of accelerating chemical transformations. In general, various starting materials are converted to more complex molecules with versatile applications. Traditionally, catalysts are divided into homogeneous and heterogeneous catalysts, biocatalysts (enzymes), photocatalysts, and electrocatalysts, which are mainly used... 

Fig. 3. Schematic illustration of the synthesis of metal nanoparticles within dendrimer templates. The composites are prepared by mixing of the dendrimer and metal ion, and subsequent chemical reduction. These materials can be immobilized on electrode surfaces where they serve as electrocatalysts or dissolved in essentially any solvent (after appropriate end-group functionalization) as homogeneous catalysts for hydrogenation and other reactions...

Pyridinium and its substituted derivatives are effective and stable homogeneous electrocatalysts for the aqueous multiple-electron, multiple-proton reduction of... 

Seshadri G, Chao L, Bocarsly AB. A new homogeneous electrocatalyst for the reduction of carbon dioxide to methanol at low overpotential. J Electroanal Chem. 1994 372 145-50. 

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 use of these surface-immobilized electrocatalysts allows for the easy removal of the catalysts from the reaction vessel, and the use of much lower quantities of catalyst which is here highly concentrated in the reaction layer. In many cases the immobilization of the catalyst on the electron source provides its stabilization and allows an marked increase of the turnover frequency compared to the numbers found in related homogeneous systems. Taking... 

This chapter elaborates on the methods and incentives for using nanomaterials as precursors to electrocatalysts. This precursor method facilitates tailoring of precursors with controlled structures and control of the interface between two metals. By use of this method homogeneous alloys, segregated alloys, layered bimetallics, and decorated paticles are all readily accessible. The incentive for the use of this concept is that we can preprepare and thoroughly characterize the active components of electrocatalysts with the application of modern analytical techniques, including synchrotron radiation, electron microscopy. X-ray diffraction, and electrochemical examination of the surface. 

Scherson, Palenscar, Tolmachev and Stefan provide a critical review of transition metal macrocycles, in both intact and thermally activated forms, as electrocatalysts for dioxygen reduction in aqueous electrolytes. An introduction is provided to fundamental aspects of electrocatalysis, oxygen reduction, and transition metal macrocydes. Since the theoretical and experimental tools used for investigation of homogeneous and heterogeneous electrocatalysis are considerably different, these topics are given separate discussion. The influence of the electrode surface on adsorbed macrocydes, and their influence on mechanism and rates of 02 reduction is treated in detail. Issues related to pyrolyzed macrocydes are also described. 

Homogeneous catalysis by redox metals is also known for nonelectro-chemical processes. Thus, ethylene is oxidized to acetaldehyde in the Wacker process in aqueous solutions containing Pd " (504). Apart from complex formation and insertion (505), ionic oxidation and reduction may take place. It is noteworthy that palladium oxidation to form ions that act as homogeneous catalysts has been suggested as an important step in ethylene electrooxidation on solid palladium electrocatalysts 28, 29). 

Recently, electrocarbonylation of phenol with CO to DPC was achieved in MeCN solution at ambient CO pressure using a heterogeneous Pd/C [102] and a homogeneous Af-heterocyclic carbene Pd electrocatalyst, the activity of which increased with the electron-donating ability of the ligand (Eq. (12.4)) [103] ... 

Figure 2.25 provides a comparative XRD study of homogeneously alloyed Cu/Pt/C system (a) and the onion-type Pt Cu electrocatalyst (b). 

Fig. 2.25 XRD patterns of (a) homogeneously alloyed Cu/Pt nanoparticles on carbon and (b) an onion-type Pt Cu electrocatalyst (20wt.% metal loading in either case).

Takasu et al. [27] prepared a homogenized Pt-Ru/C electrocatalyst with a high-specific activity for methanol oxidation from carbon black and ethanolic solutions of Pt(NH3)2(N02)2 and RuN0(N03). The specific activity for methanol electrooxidation increased with an increase in the Pt/Ru particle size. The concept of larger particle size aiding in the activity of methanol oxidation was experimentally verified [28-33]. 

Most electrocatalytic reactions are based on a signal generated by heterogeneous electron exchange between the dissolved analyte and the electrode which has been modified as described above. However, the use of M-N4 complexes as homogeneous electrocatalysts where both the catalyst and the analyte are in solu-... 

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