Electrocatalytic hydrogenation

Abstract

Electrocatalytic hydrogenations (ECHs) of unsaturated organic compounds have been carried out since the beginning of the last century [1-3]. It has been suggested quite early that electrochemical reductions of unsaturated organic compounds in aqueous media, on cathodes of spongy transition metals, might involve reaction of the unsaturated substrate with M(H)ads [4]. ECH involves the generation of chemisorbed hydrogen, M(H)ads (M is an adsorption site), by reduction of water (Equation.44.1) and its reaction with an adsorbed organic substrate, M(Y = Z)ads (Equation 44.3) followed by desorption of the hydrogenated product, M(YH-ZH)ads (Equation 44.4). If the YH-ZH sigma bond is sufficiently weak, its hydrogenolysis may also take place (Equation 44.5). Most of the organic functional groups that have been hydrogenated by heterogeneous catalytic hydrogenation (CHhet) have also been hydrogenated by ECH [5-12]. A large variety of catalytically active materials have been used as cathode [5-18]. A number of reviews on the ECH of organic compounds have been published [5-12]. This chapter does not present an extensive review on the various organic functional groups that have been hydrogenated by ECH. It rather consists of a discussion with examples: on the mechanism of ECH, on the advantages and disadvantages of ECH when compared to CHhet and when compared to the hydrogenation of organic compounds via an electroreduction process (electronation-protonation [EP] mechanism), on the selectivity of ECH of nitro compounds and of conjugated enones, and on studies on well-defined surfaces (single crystals). Reviews have also been published on studies on well-defined surfaces [19-21].