Some specific potentiodynamic behaviour of certain aldehydes and monosaccharides - Some simple and some specific aldehydes

Three distinct potentiodynamic features have been scanned on cyclic voltammograms in the presence of simple (formaldehyde, acetaldehyde), and some specific (cinnam aldehyde, croton aldehyde, furfural and benzaldehyde) aldehydes: (a) anodic oxidation of aldehyde group proceeds as a direct heterogeneous reaction with weakly or reversibly adsorbed OH-adatomic group, or in overall, as an electrode kinetics similar to Volmer-Heyrovsky mechanism for hydrogen evolution (R-CHO + 2M-OH double right arrow R-COOH + H2O + 2M + 2e), which is practically the same as the Lamy mechanism; (b) as the result, the oxide growth upon Pt, Pd, Au electrode substrates becomes prevailingly or partially prevented and reflects in the proportionally reduced charge capacity for its desorption peak, and (c) the consequence of the latter represents the unusual, but sharply pronounced reverse current jump into anodic range with peaks of repeated aldehyde oxidation during reversal potential scans towards hydrogen evolving limits. Aldehyde reduction, except on Au and Hg, occurs as the heterogeneous catalytic reaction with adsorbed H-adatoms and appears competitive with hydrogen evolution, and while being for about two order of magnitude faster than H-adatom recombination, at lower current densities (about 10 mA cm(-2)) yields rather high current efficiencies (above 90%). Oxygen evolution proceeds along with aldehyde oxidation and only at rather high positive potentials prevails in both surface oxide formation and thereby reduced rate in carboxylic acid production. At extremely positive anodic potentials on Au electrode, there appears a distinct peak of carboxylic group oxidation with release of CO2, preceding oxygen evolution, or occurring along with the latter. (C) 2000 Elsevier Science Ltd. All rights reserved.