Promotion electrochemical

Atmospheric corrosion is electrochemical ia nature and depends on the flow of current between anodic and cathodic areas. The resulting attack is generally localized to particular features of the metallurgical stmcture. Features that contribute to differences ia potential iaclude the iatermetaUic particles and the electrode potentials of the matrix. The electrode potentials of some soHd solutions and iatermetaUic particles are shown ia Table 26. Iron and sUicon impurities ia commercially pure aluminum form iatermetaUic coastitueat particles that are cathodic to alumiaum. Because the oxide film over these coastitueats may be weak, they can promote electrochemical attack of the surrounding aluminum matrix. The superior resistance to corrosion of high purity aluminum is attributed to the small number of these constituents. 

The reader must have already identified some of the basic concepts which play a key role in understanding the electrochemical activation of heterogeneous catalysis catalysis, electrocatalysis, promotion, electrochemical promotion, spillover, backspillover. It is therefore quite important to define these terms unambiguously so that their meaning is clearly determined throughout this book. 

Chapter 11 analyzes the recently discovered mechanistic equivalence of electrochemical promotion and metal-support interactions on ionic and mixed conducting supports containing Zr02, Ce02 or Ti02. The analysis focuses on the functional identity and operational differences of promotion, electrochemical promotion and metal support interactions. 

Figure 2.3. Catalysis (0), classical promotion ( ), electrochemical promotion ( , ) and electrochemical promotion of a classically promoted (sodium doped) ( , ) Rh catalyst deposited on YSZ during NO reduction by CO in presence of gaseous 02.14 The Figure shows the temperature dependence of the catalytic rates and turnover frequencies of C02 (a) and N2 (b) formation under open-circuit (o.c.) conditions and upon application (via a potentiostat) of catalyst potential values, UWr, of+1 and -IV. Reprinted with permission from Elsevier Science.

D. Tsiplakides, J. Nicole, C.G. Vayenas, and C. Comninellis, Work function and catalytic activity measurements of an Ir02 film deposited on YSZ subjected to in situ electrochemical promotion,/. Electrochem. Soc. 145(3), 905-908 (1998). 

The crucial task remains of examining to what extent it can also describe the effect of promotion, electrochemical or classical, on catalytic reaction kinetics. More specifically we will examine to what extent it can predict the four main types of global r vs O dependence and all the associated local and global electrochemical and chemical promotional rules. 

INTERRELATION OF PROMOTION, ELECTROCHEMICAL PROMOTION AND METAL-SUPPORT INTERACTIONS THE DOUBLE-LAYER MODEL OF CATALYSIS... 

Promotion, electrochemical promotion and metal-support interactions are three, at a first glance, independent phenomena which can affect catalyst activity and selectivity in a dramatic manner. In Chapter 5 we established the (functional) similarities and (operational) differences of promotion and electrochemical promotion. In this chapter we established again the functional similarities and only operational differences of electrochemical promotion and metal-support interactions on ionic and mixed conducting supports. It is therefore clear that promotion, electrochemical promotion and metal-support interactions on ion-conducting and mixed-conducting supports are three different facets of the same phenomenon. They are all three linked via the phenomenon of spillover-backspillover. And they are all three due to the same underlying cause The interaction of adsorbed reactants and intermediates with an effective double layer formed by promoting species at the metal/gas interface (Fig. 11.2). 

Consequently the proven functional identity of classical promotion, electrochemical promotion and metal-support interactions should not lead the reader to pessimistic conclusions regarding the practical usefulness of electrochemical promotion. Operational differences exist between the three phenomena and it is very difficult to imagine how one can use metal-support interactions with conventional supports to promote an electrophilic reaction or how one can use classical promotion to generate the strongest electronegative promoter, O2, on a catalyst surface. Furthermore there is no reason to expect that a metal-support-interaction-promoted catalyst is at its best electrochemically promoted state. Thus the experimental problem of inducing electrochemical promotion on fully-dispersed catalysts remains an important one, as discussed in the next Chapter. 

Having discussed the functional equivalence of classical promotion, electrochemical promotion and metal-support interactions on 02 -conducting and mixed electronic-ionic conducting supports, it is useful to also address and systematize their operational differences. This is attempted in Figure 11.15 The main operational difference is the promoter lifetime, Tpr, on the catalyst surface (Fig. 11.15). 

Promotion, electrochemical promotion and metal-support interactions are different facets of the same phenomenon, i.e. catalytic reaction in presence of a double layer, which for the case of electrochemical promotion is in situ controllable. 

Promotion, Electrochemical Promotion, and Metal-Support Interactions... 

In an optical micrograph of a commercially available nitinol stent s surface examined prior to implantation, surface craters can readily be discerned. These large surface defects are on the order of 1 to 10 p.m and are probably formed secondary to surface heating during laser cutting. As mentioned above, these defects link the macro and micro scales because crevices promote electrochemical corrosion as well as mechanical instability, each of which is linked to the other. Once implanted, as the nitinol is stressed and bent, the region around the pits experiences tremendous, disproportionate strain. It is here that the titanium oxide layer can fracture and expose the underlying surface to corrosion (9). 

Fig. 17.9. Schemes showing the application of enzymes to promote electrochemical reactions (from Ref. 36 with permission).

Dibenzyl ditellurium was obtained in 70% yield by alkylation of the electrochemically generated ditelluride dianion with benzyl chloride in acetonitrile3. The ultrasonically promoted electrochemical reduction of tellurium powder was performed in H-type cells with the compartments separated by glass frits. Acetonitrile served as solvent and tetrabutylammonium tetrafluoroborate or hexafluorophosphate as the supporting electrolyte. At potentials beyond -1.1 V the dark-red ditelluride dianion is formed in the cathode and in the central compartment3. 

Refs. [i] StoukidesM, Vayenas CG (1981) JCatal70 137 [ii] Vayenas CG, Bebelis S, Ladas S (1990) Nature 343 625 [iii] Lambert RM, Williams F, Palermo A, Tikhov MS (2000) Topics Catal 13 91 [iv] Fdti G, Bol-zonella I, Comninellis C (2003) Electrochemical promotion of catalysis. In Vayenas CG, Conway BE, White RE (eds) Modern aspects of electrochemistry - electrochemical promotion of catalysis, vol. 36. Kluwer/Plenum, New York [v] Vayenas CG, Bebelis S, Pliangos C, Brosda S, Tsiplakides D (2001) Electrochemical activation of catalysis promotion, electrochemical promotion and metal-support interactions. Kluwer/Plenum, New York... 

Oxiranes can be prepared by electrochemical oxidation. " Regioselective w-epoxidation of polyisoprenoids will take place with excellent yields on sodium bromide-promoted electrochemical oxidation in neutral or basic medium. " " This has now been described as a general method. " " Hexafluoropropylene oxiranes have been produced by electrochemical means. " The deoxygenation of dioxetane to oxirane with triphenylphosphine has been described (Eq, 50). ... 

Part V addresses the critical issues of particle size effects, alloying, spillover, classic and electrochemical promotion, and metal-support interactions on the catalytic and electrocatalytic performance of nanoparticles. Recent experimental evidence is presented on the functional similarities and operational differences of promotion, electrochemical promotion, and metal-support interactions. 

In this chapter we review some of the key phenomenological aspects of promotion, electrochemical promotion, and metal-support interactions, underline their kinetic similarities and common fundamental origin on the basis of surface spectroscopic and theoretical investigations, including the new concept of the absolute potential of supports, and discuss some key experiments that prove their identical nature, i.e., catalysis in presence of a controllable double layer at the catalyst-gas interface. 

The goal of this chapter is to summarize and systematize the phenomenology of the three phenomena, i.e., classical promotion, electrochemical promotion, and metal-support interactions, present their striking similarities and some common rules that govern them, and demonstrate their intimate link and common molecular mechanism. 

Consequently, the proven functional identity of classical promotion, electrochemical promotion, and metal-support interactions should not lead the reader to... 

C.G. Vayenas, S. Bebelis, C. Pliangos, S. Brosda, D. Tsiplakides, Electrochemical Activation of Catalysis Promotion, Electrochemical Promotion and Metal-Support Interactions, Kluwer/Plenum Press, New York (2002). 

Electrochemical Activation of Catalysis Promotion, Electrochemical Promotion, and Metal-Support Interaction, Kluwer/ Plenum, New York. 

In fact, the equimolar Ir02 - Ti02 catalyst mixture can only be marginally promoted electrochemically, while its open-circuit activity is about ten times higher than that of pure lr02. This is the same level of rate enhancement as is achieved with pure Ir02 catalyst when it is... 

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