Metal films, electrocatalytic effects

Photoelectrolysis, metal film electrocatalytic effects, 38 77-78 Photoelectron... 

XVI. Metal Film Electrocatalytic Effects In Photoelectrolysis Processes... 

Details about preparation and characterization of dispersed microcrystals can be found in review chapters [322] and will not be dealt with here. All investigations indicate that the properties of microcrystals differ considerably from those of bulk metals (and from those of adatoms and thin films as well) [328], and that they can also be influenced by the nature and texture of the support. In particular, micro-deposits of precious metals on various inert supports (Ti, Ta, Zr, Nb, glassy carbon etc.) exhibit enhanced electrocatalytic effects as evaluated per metal atom, while the mechanism of H2 evolution remains the same [329], and the enhancement increases as the crystallite size decreases [326, 331] (Fig. 17). However, while this is the case with Rh, Pt, Os and Ir, Pd shows only an insignificant increase, whereas for Ru even a drastic decrease is observed [315, 332]. Thus, the effect of crystal size on the catalytic activity appears to depend on the nature of the catalyst (without any relation with the crystal structure group) [330]. 

A classical approach to improving the sensitivity of ECP-based sensors consists of the immobilization of catalytic moieties, such as metallic particles [136-142], Prussian blue [143], ferrocene [144], metalloporphyrins [145-148], and oxometalates [149-152], within ECP films. Compared to polymers without a catalyst, these materials exhibited more pronounced electrocatalytic effects and were suitable for the detection of numerous molecules of biological interest, e.g., carbohydrates [136, 141], catecholamines [144], and NO [145,150-152]. 

The enhancement of selectivity and sensitivity of ECP-based gas sensors could also be achieved with the functionalization of ECP films by catalytic species. So, metal particles such as Pt [138, 185] and Pd [139], heteropolyanions [149, 186-190], metallopor-phyrins [148,191], metallophthalocyanines [192], and others [193, 194] have been immobilized in various ECP films. Most of these electroactive materials showed electrocatalytic effects on the oxidation or reduction of dissolved O2 [139,148,187,188,190-194], CO [138], CO2 [193], and H2 [185], which could be used for the development of electrochemical sensors. 

Catalyst films used in electrochemical promotion (NEMCA) studies are usually prepared by using commercial metal pastes. Unfluxed pastes should be used, as fluxes may introduce unwanted side reactions or block electrocatalytic and catalytic sites. This action may obscure or even totally inhibit the electrochemical promotion effect. 

The intermittent plasma-assisted vacuum deposition technique has been found to introduce the effective electrocatalytic activity and stability for CO2 reduction into metal phthalocyanine thin films formed on a glassy carbon. The films properties are significantly influenced by the chemical state of the Aim. It has been suggested that the electrode process is determined by the surface chemical reaction involving adsorbed H and/or H+ and a carbon containing intermediate ". ... 

The film thickness using a PTh matrix is much smaller than that for the other two studied matrices. In addition, only two platinization cycles are required to disperse die optimum amount of metal in this matrix. In die case of PAM and PPy a much high number of potentio namic sweeps are required to obtain die most favourable film thickness and a hi er number of platinization cycles are necessary to obtain an adequate amount of dispersed platinum. This effect can be ascribed to the different degree of porosity for these polymers and, particularly, to a hindrance of die ion exchange proems responsible for the Pt(lV) inclusion in the matrix. The time of immersion of these electrodes in a Pb(n) solution has only a slight influence on their behavior, with being all efficient catalysts of die HCOOH oxidation Electrodes form by die PTh polymeric matrix, on the other hand, show higher current densities, and die electrocatalytic activity is more stable towards successive potentiodynamic cycles. 

In any case, it is clear that, for its own nature, sol-gel material is suitable for the formation of many different composite materials [207] inorganic and organic polymers, as well as many different kinds of nano-objects possessing effective electrocatalytic properties, can find stable inclusion into a host matrix possessing all the characteristics previously listed. The most simple approach consists of adding the filler to the siliceous matrix after sol-gel formation. However, composite materials can also be obtained by synthesizing the sol-gel film in the presence of the precursor of the filler, e.g., a monomer [227] or a metal complex [228]. The synthesis of the composite material is finalized in a subsequent step, e.g. by a chemical oxidation or reduction, leading to polymer chains or metal nanoparticles, respectively, included inside the sol-gel matrix. 

In nanoparticle electrocatalysis, the area that Michael entered just some time ago in Munich, he and his coworkers rationalized the sensitivity of electrocatalytic processes to the stmcture of nanoparticles and interfaces. Studies of catalytic effects of metal oxide support materials revealed intriguing electronic structure effects on thin films of Pt, metal oxides, and graphene. In the realm of nanoparticle dissolution and degradation modeling, Michael s group has developed a comprehensive theory of Pt mass balance in catalyst layers. This theory relates surface tension, surface oxidation state, and dissolution kinetics of Pt. 

Mechanisms of electrochemical reactions of different systems, including transition metal complexes, were examined with a special attention paid to double layer effects and problems of generation and decay of intermediates which arise in such reactions. Electrodes modified with thin films of transition metal hexacyanoferrates and conducting polymers were investigated, also solid state electrochemistry in the absence of external supporting electrolyte were developed. Charge propagation rate in such mixed-valent solid systems and their electrocatalytic properties were studied. 

Electrodes modified with MPc are active catalysts for a large variety of electrochemical reactions, and can be obtained by simple adsorption of MPc s on graphite or carbon, or by coating the electrodes with a film of polymerized MPc s [4]. The electronic structures of MPcs are known to have a strong effect on their catalytic (and electrocatalytic, of course) activity. The redox potential of each MPc will depend on the nature of the metal, and can be modulated by the introduction of substituents in the phthalocyanine ring. The MPc electrodes most used for remediation are those with Ni(II), and therefore our discussion will be centered on Ni(ll) Pc s. The development of suitable sensors for the detection of organic contaminants will also be considered here. 

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