Chemisorbed intermediates electrode kinetics

The principal aims of this review are to indicate the role of chemisorbed intermediates in a number of well-known electrocatalytic reactions and how their behavior at electrode surfaces can be experimentally deduced by electrochemical and physicochemical means. Principally, the electrolytic gas evolution reactions will be covered thus, the extensive work on the important reaction of O2 reduction, which has been reviewed recently in other literature, will not be covered. Emphasis will be placed on methods for characterization of the adsorption behavior of the intermediates that are the kinetically involved species in the main pathway of the respective reactions, rather than strongly adsorbed by-products that may, in some cases, importantly inhibit the overall reaction. The latter species are, of course, also important as they can determine, in such cases, the rate of the overall reaction and its kinetic features, even though they are not directly involved in product formation. 

The involvement of chemisorbed intermediates in electrocatalytic reactions is manifested in various and complementary ways which may be summarized as follows (i) in the value of the Tafel slope dK/d In i related to the mechanism of the reaction and the rate-determining step (ii) in the value of reaction order of the process (iii) in the pseudocapacitance behavior of the electrode interface (see below), for a given reaction (iv) in the frequency-response behavior in ac impedance spectroscopy (see below) (v) in the response of the reaction to pulse and linear perturbations or in its spontaneous relaxation after polarization (see below) (vi) in certain suitable cases, also to the optical reflectivity behavior, for example, in reflection IR spectroscopy or ellipso-metry (applicable only for processes or conditions where bubble formation is avoided). It should be emphasized that, for any full mechanistic understanding of an electrode process, a number of the above factors should be evaluated complementarily, especially (i), (ii), and (iii) with determination, from (iii), whether the steady-state coverage by the kinetically involved intermediate is small or large. Unfortunately, in many mechanistic works in the literature, the required complementary information has not usually been evaluated, especially (iii) with 6(V) information, so conclusions remained ambiguous. 

In electrocatalysis, notable cases of formation of strongly bound species that are not, however, the kinetically involved intermediates in the main reaction pathway arise in the electrochemical oxidations of HCOOH, HCHO, and CH3OH at Pt anodes for those reagents, a self-poisoning intermediate, variably identified as chemisorbed CO, in bridged or linear double bonding to the electrode, or the species- C—OH, is involved (43) this species is not a principal kinetically involved intermediate in, for example, HCOOH oxidation, which proceeds at unpoisoned sites by the mechanism discussed in Section V,B,3. 

Evidently, however, another species arises in a side, self-poisoning, reaction and extensively covers the surface, inhibiting the progress of the above main reaction in the sequence of steps shown (89-91) In situ IR spectroscopy shows that this species is principally chemisorbed CO, bridged or linearly bonded to surface metal atoms. Its behavior is similar to that observed with CO directly chemisorbed at a Pt electrode from the gas phase. However, the mechanism of its catalytic formation from HCOOH is unclear. It is well known that CO can be formed from HCOOH by dehydration, but such conditions do not obtain at a Pt electrode in excess liquid water. Hence a catalytic pathway for adsorbed CO formation has to be considered. The species C=0 or C—OH are not to be regarded as the kinetically involved intermediates in the main reaction sequence (Section IV). Because the poisoning species seems to be formed in the presence of coadsorbed, H steps such as... 

---