Catalytic hydrogen waves, effect

Finally, attention has been drawn to currents of hydrogen evolution which in the presence of some substances occurs at more positive potentials than in their absence. Such substances catalyze hydrogen evolution and result in high currents which are denoted catalytic hydrogen waves. Such waves are observed in the presence either of platinum group metals (57,58), where the catalysis is attributed to clusters of metals deposited on mercury or of compounds which possess acid-base properties. Catalytic effects of the latter type in solutions of simple buffers have been observed for low molecular weight compounds (59,60), as well as for proteins (61,62). Similar catalytic effects in ammoniacal cobalt (III)-solutions (63) found utilization in Brdicka reaction (64-66), used in cancer diagnosis. 

Even from the time of the early work by Brdi6ka [100] it has been known that addition of cysteine or proteins to a solution of a divalent cobalt salt leads to a shift of the reduction wave of Co (II) towards positive potentials and to the appearance, following the reduction of the cobalt, of a hump-shaped catalytic hydrogen wave however, the nature and mechanism of the processes causing these effects have not yet been finally established. It is known that both the above-mentioned phenomena are connected with the formation of... 

The shift in the catalytic hydrogen wave to more positive potentials amounts to about 60 mV with tenfold increase in the ionic strength (Line 3, Fig. 22). Consequentlythecatalytically active complex which mdergoes the electrochemical reaction and from which the cobalt is not reduced [9, 104], is a monoanion it can be assumed that this complex, in addition to Co (II), contains three cysteine anions. It is not impossible, however, that the borate ion enters into the composition of the complexes. It was not possible to trace the effect of ionic strength on the slope of the catalytic wave owing to the proximity of the cobalt reduction wave. 

The catalytic activity of various amines was foimd to be different the strongest catalysts are quinine, ephedrine, pyridine and its homologs, and cysteine. Aniline, N-methylbenzylamine, and cyclo-hexylamine showed practically no catalytic activity [111]. The catalytic activity is not related to the pK values of the amines and is to some extent specific. It is interesting that those catalysts which facilitate transfer of protons to nitro alcohols also facilitate the electrochemical reduction of hydrogen ions, i.e., give rise to catalytic hydrogen waves. The parallelism between these effects goes even... 

Electroreduction of nitro compounds is of considerable importance for electroorganic synthesis. Interesting catalytic effects were reported for the reduction of aromatic nitro compounds on Pt. Figure 11 shows that Ph, Tl, and Bi adlayers shift the half-wave potential positively by 100 to 300 mV. The catalytic effect was attributed to a change in the mechanism of the reduction of the nitro group from a catalytic hydrogenation on bare Pt to an electron-transfer mechanism on Pt/Mad, that is, a direct electron exchange between the nitro compound and the adatom-covered electrode surface, namely. 

The mechanism of catalysis and stability is still uncertain. The observed catalytic effect observed can be due to the availability of an increased surface area using a dispersion of catalytic microparticles. It is very difficult to estimate the surface area in these polymer/particle systems because well-defined hydrogen adsorption waves (integration of... 

Most of the organic catalytically active substances studied so far exert a catal5ttic effect upon hydrogen evolution. There are two main types of catalytically active substances to the first type belong substances which cause catalytic waves in unbuffered acid or buffered solutions (sometimes even at pH 11). The second class is represented by substances showing catalytic waves in the presence of some heavy metals, e.g. cobalt or nickel. 

Figure 3 compares the oxidation of hydrogen peroxide at carbon paste and mixed paste containing iron phthalocyanin at pH 5.5. The electro-catalytic response of the modified electrode was reflected by the appearance of an oxidation wave with a potential peak of +0.59 V vs SCE. Under the same conditions, no corresponding oxidation wave was observed at the conventional CPE. The explanation of this catalytic effect is probably due to an interaction between iron and hydrogen peroxide. A characteristic shift of about 60 mV per pH unit was observed between pH 3 and 9. The pH of 5.5 was chosen to obtain a well-defined peak. Several types of buffers have been tested, but no particular influence on the catalytic activity of iron phthalocyanin was observed. The modified electrode employed in this... 

It should be noted that only the inhibiting effect on hydrogen-ion reduction is observed in the case of relatively high tribenzylamine concentrations [92 ]. The absence of the catalytic wave is attributed to inhibition of the catalytic process by the film of adsorbed substance. 

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