Catalytic oxidation of maleic and oxalic acids under potential control of platinum catalysts

The oxidation of maleic and oxalic acids in diluted aqueous solutions and with platinum catalysts under potential control was studied with the purpose of defining the influence of potential on the catalytic activity. This control was achieved either by an external device or was spontaneously established in the presence of the reactants. The effect of the composition and of the pH was also investigated. Oxalic acid can be oxidized in mild experimental conditions (T = 333 K, P-O2 less than or equal to 1 bar) and at potential values of the catalyst comprised between 0.7<E<1.8 V/RHE with a maximum catalytic activity at 1.3 V/RHE. The catalytic oxidation of this compound under external control of catalyst potential occurs following the same mechanism as the electrocatalytic oxidation. Oxalic acid is weakly adsorbed and its oxidation is inhibited by strongly adsorbed anions. Maleic acid needs more severe experimental condition to be oxidized (T = 383 K, P-O2 = 1-5 bars) and catalyst potentials in the range of 0.4 less than or equal to E<1.1 V/RHE. In the same potential range an active adsorbed species was detected. The catalytic oxidation of maleic acid follows the same mechanism with and without external control of catalyst potential which should be different from the mechanism of the electrocatalytic oxidation. (C) 1999 Elsevier Science B.V. All rights reserved.