Electrocatalytic effect

A third way to increase both the active surface area and the number of oxygenated species at the electrode surface is to prepare alloy particles or deposits and then to dissolve the non-noble metal component. This technique, which is similar to that used to prepare Raney-type catalysts, yields very high surface area electrodes and hence some improvements in the electrocatalytic activities compared with those of pure platinum. However, it is always difficult to be sure whether the mechanism of enhancment of the activities is due to this effect or the possible presence of remaining traces of the dissolved metal. Results with PtyCr and PtSFe were encouraging, although the effect of iron is still under discussion. From studies in a recent work on the behavior of R-Fe particles for methanol electrooxidation, it was concluded that the electrocatalytic effect is due to the Fe alloyed to platinum. ... 

Electrocatalytic effects cannot be studied in depth without a detailed knowledge of the kinetics and mechanism of the electrochemical reaction being examined. In fact, diverse effects can be exerted on the reaction ... 

Apart from these macrokinehc hmitahons, two more effects exist that may influence the overaff efectrochemicaf characteristics of an electrode with disperse catalyst in the posihve or negahve direchon (1) an influence of the crystallite size itself on the intrinsic catalyhc achvity (Section 28.5.4) and (2) an influence of the catalyst substrate (Sechon 28.5.5). These two effects have great importance for the prachcaf use of disperse catalysts and for the theoretical analysis of electrocatalytic effects. 

Binary combinations of platinum and less noble metals (Pt/Sn, Pt/Re, Pt/Ru, Pt/Pd) have electrocatalytic effects upon the rate of oxidation of methanol and parent... 

J.C. Vidal, J. Espuelas, E. Garda-Ruiz, and J.R. Castillo, Amperometric cholesterol biosensors based on the electropolymerization of pyrrole and the electrocatalytic effect of Prussian-Blue layers helped with self-assembled monolayers. Talanta 64, 655 (2004). 

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

It is important to clarify that there have been, in the literature, some examples of electrochemical processes on CNT-modified electrodes on which an apparent electrocatalytic process associated to the CNTs seems to take place (that is from the edge-plane-like sites) where in fact that was not the case. An example is the apparent electrocatalytic oxidation ofhydrazine at MWNTelectrodes [64,65]. Such electrochemical behavior has been demonstrated to be a consequence of iron impurities contained in the CNTs that were responsible for the observed electrocatalytic effects (Figure 3.7). Therefore, caution is needed when reporting catalytic effects of CNTs under a given redox system and a careful comparison vdth, for instance, edge HOPG is mandatory to make sure that the CNTs are the responsible for the electrochemical enhancement. 

An improved adsorption of DNA bases has been observed at a chemically modified electrode based on a Nafion/ruthenium oxide pyrochlore (Pb2Ru2-x FhxOj-y modified GC (CME). Nafion is a polyanionic perfiuorosulfonated ionomer with selective permeability due to accumulation of large hydrophobic cations rather than small hydrophilic ones. The Nafion coating was demonstrated to improve the accumulation of DNA bases, while the ruthenium oxide pyrochlore proved to have electrocatalytic effects towards the oxidation of G and A. The inherent catalytic activity of the CME results from the Nafion-bound oxide surface being hydrated. The catalytically active centers are the hydrated surface-boimd oxy-metal groups which act as binding centers for substrates [50]. 

Two are the main factors governing the activity of materials (i) electronic factors, related to chemical composition and structure of materials influencing primarily the M-H bond strength and the reaction mechanism, and (ii) geometric factors, related to the extension of the real surface area influencing primarily the reaction rate at constant electronic factors. Only the former result in true electrocatalytic effects, whereas the latter give rise to apparent electrocatalysis. 

Another remark is the relatively mild electrocatalytic effect resulting in a shift of the wave of 20 mV and an increase of the peak currents by approx-... 

The best approach is normally an in situ determination based on voltammetry or charging curves, usually within the hydrogen adsorption region [96]. It is of course necessary to know the actual value of 0H for absolute determinations, but the method is practicable on a relative basis. The method becomes absolute only in a few cases, in particular for Pt electrodes [97] for which the catalytic activity per metal atom, which is the parameter really needed to evaluate electrocatalytic effects, can be calculated [98]. Sometimes, results are reported relative to the surface area measured on the basis of the limiting current for a redox reaction [99], but what is obtained is only the macroscopic surface in which asperities of a height higher than the diffusion layer thickness can only be accounted for. 

Coldly worked Ni annealed at different temperatures has not shown [268] any electrocatalytic effect in acid solution while a drop has been observed in alkali as annealing is carried out above a given temperature. This has been interpreted as a support to the mechanism in which Naad is the intermediate (cf. steps (6) and (7) above). However, a characterization of the state of the surface, which may differ in alkalis and in acids, is lacking. 

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... 

Ion implantation is often recommended as an efficient tool to enhance electrocatalysis either by disrupting the surface structure of the catalyst or by placing active atoms on an inactive (or less active) matrix. The latter possibility (which links this section with Section 3.3 devoted to adatoms) offers also a way to the use of extremely small amounts of active but expensive materials. In order to investigate the effect of surface damages, self-implantation or ion beam bombardment is the most appropriate approach. Implantation of Ni on Ni has led to a modest enhancement of the surface area, but not to electrocatalytic effects [279]. On the other hand, Pt bombarded with neutrons has shown an increase in the activity for hydrogen evolution [280]. However, it has been suggested that this is not related to the formation of surface defects, but rather to the effect of the radioactivity induced on the electrode and on the electrolyte. 

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]. 

The electrocatalytic effect of small crystallites is attributed to the metal-support interaction [326, 330], a well known and widely discussed topic in catalysis [334]. In particular, since precious metals have high electron work functions, electrons are injected from the support into the crystallites thus modifying considerably the electron density which becomes a function of the crystal size. The fact that the effect of different supports is not visible [335] can be explained in terms of a large Abetween different supports are very small [326]. However, in the case of carbonaceous supports, different preparations of the support may result in sizable effects because the morphology of the overlayer can be influenced [336]. 

The pigmented electrode has shown a more marked electrocatalytic effect [351]. The Tafel slope in acid solution decreases substantially with respect to pure Ni. The decrease in overpotential is substantial even with respect to Pt. SEM analyses have shown that the surface is very rough and that no apparent damage issues from a prolonged electrolysis. The considerable roughness of the surface may be the source of the observed resistance towards the deactivating action of Cd and Hg impurities in solution. 

The catalytic activity of these oxometalates is well documented [360, 361]. An inactive surface of Ti02 becomes an efficient catalyst for H2 evolution as it is derivatized with silicotungstic acid [362, 363]. However, while real electrocatalytic effects seem likely for a pigmented Ni surface in view of the lower Tafel slope observed (which can also be due to some activation of the Ni itself), these are not completely established for the surface of pure oxometalates surface area effects could be entirely responsible for the apparent activation. The real surface state of these electrodes deserves to be further investigated since these materials might fall into the category of amorphous phases. 

Since Ru02 and Ir02 are usually prepared by thermal decomposition of suitable precursors on an inert support, the morphology of the active layer is very like that of a compressed powder [486]. The surface area plays an important role since the roughness factor can be between 102 and 103. However, the low Tafel slope observed is a clear indication of electrocatalytic effects and the high surface area is the factor which extends the low Tafel slope to much higher current densities. Thus, the combination of these two factors renders these oxides very efficient electrocatalysts for H2 evolution. 

Metallurgical methods are usually needed to prepare intermetallics [535]. Therefore, an extension of the surface area cannot be expected. If there is an increase in activity this can realistically be attributed to electrocatalytic effects. Since in most cases the Tafel slope is not lower but actually higher than for Ni [53], the activity enhancement has to be related to a change in the adsorptive properties of the surface so that both the M - H surface bond strength and the degree of coverage with H are different. For instance, in the case of TiFe, a lower M - H bond strength and a lower... 

At a high cathodic potential (region II), a sharp transition is observed at the potential referred to as ET. The authors demonstrate that the sudden increase of the electrode kinetics could not be attributed to the sole electrochemical reduction of the electrode material, nor to the electrolyte reduction. They conclude that after the transition, the main electrode process is still an oxygen electrode reaction with a major change of mechanism, leading to the onset of an important electrocatalytic effect. This assertion is sustained by the analysis of ... 

From the above reasoning one could expect that the pre-deposition of small amounts of noble metals on the Ti02 surface in a form of the intermediate sub-layer, which can induce the electroactive electronic surface states in the Ti02 band gap, may enhance the electrocatalytic effect of subsequently deposited Cu particles. Actually, the photocatalytic deposition of silver particles in amount of 5xl014 atoms/cm-2, which on its own only slightly increases the electrocatalytic activity of Ti02 electrode, leads to 2-3-fold enhancement of the electrocatalytic activity of Cu particles subsequently deposited in a relatively high concentration (1016-1017 atoms/cm 2) [52],... 

Surface — In physics and chemistry, the term surface means the termination of a solid or liquid phase bordering to vacuum. As this is an almost impossible to realize case (at least the equilibrium vapor phase of the liquid or solid phase will always boarder to the condensed phase), surface means practically always the interface between two phases, i.e., two solid phases, two liquid phases, a solid phase and a liquid phase, a solid phase and a gas phase, or a liquid phase and a gas phase. Hence, the term interface should be preferred. Since - electrodes are a major field of research in - electrochemistry (interfacial electrochemistry) the study of surfaces (interfaces) with respect to their structure, effects on -> electron transfer and - ion transfer reactions, its changes in electrochemical reactions, its - electrocatalytic effects, etc. are of major importance. 

There is reason to pause here, and refer back to the electrocatalysis discussion in the introduction. Best electrocatalysis yields maximum power density from minimum platinum loading. For a cathode platinum particle to be electrocatalytically effective, it must be in electrical contact with the conducting porous electrode structure. It must also be in contact with the membrane, or a biconductor layer on the membrane surface. 

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