Chemisorbed intermediates factor

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. 

The emphasis in this Chapter will be on the factors that determine the degree of selectivity with which the intermediate alkene is formed whereas in the previous Chapter, it lay more on the nature of the products formed, this is of lesser importance here, because under most circumstances the major product is that which arises from the addition of two hydrogen (or deuterium) atoms to the same side of the chemisorbed alkyne, i.e. Z-addition. Minor amounts of the products of -addition do occur, and the mechanism by which they are formed merits discussion. However in the case of ethyne itself, this consideration is irrelevant unless deuterium is used, so the overwhelming thrust of the work on its hydrogenation has been towards understanding the mechanism by which ethene is so selectively formed. Further significant aspects of the reaction are best discussed after a brief description of the industrial problems. 

However, several aspects should be considered in the development of new catalytic systems. Thus, in addition to an extensive knowledge of the nature of active sites, the multifunctionality (redox, acid-base, etc.) of catalysts should be finely tuned. Moreover, the stability of well-defined crystalline structures and the effect of promoters will also be important factors to be considered. But, in addition to these, the alkane feed, the reactivity of olefinic intermediates, and the stability of partial oxidation products are key aspects to take into account. In this way, modification of the catalyst surface reactivity by chemisorbed species in the oxidation of alkanes is lower than that observed for the oxidation of olefins, which can facilitate a lower volume of undesired products in the case of alkane-based processes. 

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