Electrocatalytic Reactions and Mechanisms

In early attempts to oxidize hydrocarbons electrochemically, organic solvents and corrosion-resistant electrodes (PbO, C, Pt) were used to overcome low reactant solubility and anode dissolution at extreme potentials, -I-1.8 V and up to 4.5 V (326, 327). The primary anodic reaction was usually oxygen evolution or solvent decomposition. The electrode material, nonetheless, affected the product even at the small attainable yields. Thus, toluene oxidized to traces of aldehydes on PbO2 (333), while on Pt it yielded up to 19% benzaldehyde (326). The catalytic efifect of the anode, however, on rate and selectivity was not realized. 

The goal of maximum energy generation by oxidation of carbonaceous species often thwarted detailed examination of occasional selective oxidations, such as ethylene oxidation to acetaldehyde on Pd or Au (28, 29, 370) or to ethylene oxide on Ag (330) or methanol and benzyl alcohol oxidation to formates and benzaldehyde, respectively (6-32, 54, 250, 333). Product yields were usually determined at one potential only or even galvanostatically (330), and the combined effects of potential, catalyst, reactant concentration, and cell design or mixing on reaction selectivity are unknown at present. Thus, reaction mechanisms on selective electrocatalysis are not well understood with few exceptions. For instance, ethylene oxidation on solid pal- 

Reaction (79) results in catalyst deterioration and Step (81) forms colloidal 

Propylene was similarly oxidized electrochemically to acetone and glycol in the presence of mercurous and thallium salts 334,335). It is noteworthy that increasing anodic potentials ( +1.8 V) favored cleavage of the propylene chain to yield formic and acetic acid. It is conceivable, then, that unexplored low anodic potentials or electrogenerative oxidation at 0.6-1.0 V could minimize or eliminate these undesirable side reactions. The significance of the nature of the redox couple and its concentration in solution for controlling reaction selectivity has already been emphasized earlier. 

Apart from carbonaceous reactants, inorganic species have been oxidized electrocatalytically, albeit only in exploratory studies. Thus, HjS was oxidized to sulfur on platinized porous carbon 250, 336) in a process that could be the electrochemical equivalent of the Claus process (337)  

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