Redox couples hydrogen evolution

In aqueous solutions at OCP in the absence of redox couples, hydrogen evolution is... 

Fig. 17.10 Combination of two photocatalytic reactions via a quinone compound. Compartment I (for hydrogen evolution) and compartment II (for oxygen production) are connected through a redox couple of DDQ and DDHQ dissolved in an oil phase.

Deposition of a small amount of noble metals such as Cu, Pt, and Au increases the kinetics of redox reactions on silicon electrodes as shown in Fig. 6.3. Deposition of equivalent of 1 to 10 monolayers of Pt on silicon surface results in a shift of about 0.2V of the onset potential for hydrogen evolution to the positive direction. Because the flatband potential does not change with the Pt deposition, the enhanced hydrogen reaction kinetics is due to the catalytic effect of the deposited metal. The energy levels of the deposited metal grains are considered to lie in the middle of the band gap and communicate favorably to the surface states both energetically and spatially. The photovoltage of n-Si coated with sparsely scattered Pt islands has been found to increase proportionally to the inaease in the potential of the redox couple. Noble metal islands effectively collect photoelectrons and thus prevent the oxidation of the silicon surface by the photoelectrons. 

In the presence of redox couples, the relative contribution and mechanism of hydrogen evolution may change, the detail of which can be found in Ref. 3. 

From an electrochemical point of view this type of catalytic reduction can be conceived as two coupled electrode processes. In the case of this hydrogenation reaction, the catalytic and electrochemical reduction differ from each other only in the means for the achievement of the reduction, e.g. molecular hydrogen, redox systems, or pure electrochemical means. Similar to heterogeneous catalytic processes, electrochemical reactions tend to occur as a sequence of very simple steps. For example, hydrogen evolution occurs as two steps, with two alternatives for the second step, corresponding to two reaction routes ... 

In the present author s view, the Gaussian distribution function based on the solvent fluctuation model, which is developed for a simple redox couple, is used too often even when the basic assumption is not valid. For example, this type of distribution function is often drawn for the hydrogen evolution reaction where the oxidized state is H+ and reduced state is H2.105 Certainly the nature of the solvation is completely different between H+ and H2. Moreover, when one considers the kinetics of the hydrogen evolution reaction, one should consider not the energy level of H+/H2 but that of H+/H(a) as Gurney did. 

Rieke, P.C. and Armstrong, N.R. (1984) Light-assisted, aqueous redox reactions at chlorogallium phthalocyanine thin-film photoconductors dependence of the photopotential on the formal potential of the redox couple and evidence for photoassisted hydrogen evolution. I. Am. Chem. Soc., 106, 47-50. 

Fermi level can reach in the semiconductor is the flat band potential Vfb which, in the case shown, is lower with respect to the H /Ha redox couple. This means that hydrogen evolution cannot take place at the metal electrode even at the highest irradiation intensity. For hydrogen evolution to occur, a positive bias must be applied to the semiconductor electrode as shown in Fig. 3d. This bias, which is usually provided by an external voltage source, should also account for the necessary cathodic and anodic ( 7a) overvoltages in order to sustain the current flow. This situation represents a condition which is frequently met with visible absorbing semiconductor metal oxides photochemically stable in aqueous environment, like WO3 orFejOa. 

In this review, after a brief overview of the structural and electronic properties of metal adlayers, there are six sections describing catalytic effects on redox couples, oxidation of organic molecules, carbon monoxide, organic electrosynthesis reactions, hydrogen evolution, oxygen reduction, and metal electrodeposition. Outside the scope of this review are other UPD processes that play a role in determining the catalytic properties of electrode surfaces such as the UPD of H and OH. 

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