The effect of quaternary ammonium salts on the anodic oxidation of ethanol

Abstract

A study was made of the effect of additives on the rate of deactivation of platinized platinum in the anodic oxidation of ethanol in order to determine whether additives that form a hydrophobic layer on platinum would protect against normal poisoning processes. The data were interpreted in terms of a mechanism in which the oxidation of the alcohol proceeded at different rates on the bare and inhibited surface in a reaction that was zero order with respect to alcohol and first order with respect to the areas of each type of surface, while the inhibition reaction was second order with respect to the fraction of the surface that was bare. Without additives this mechanism agreed with data at 0.9 V (measured with respect to a saturated calomel electrode) but at 0.5 V the contribution from the inhibited route was negligible and the rate became approximately independent of the area on the more heavily platinized surfaces. Tetrabutylammonium perchlorate did not influence any of the rate constants, indicating that it was not adsorbed on the platinum. Solutions containing chlorides caused marked decreases in the rates of oxidation on the bare surface, but at 0.9 V caused only small decreases in the rate of oxidation on the inhibited surface, indicating that chlorides are only weakly adsorbed on the inhibited surface. The addition of a large quaternary ion together with the chlorides increased the effect of the chloride, confirming the fact that these salts acted by ion pairing with the adsorbed chloride. None of the additives had a significant effect on the rate of poisoning of the electrode at 0.5 V. At 0.9 V neither sodium chloride nor tetrabutylammonium perchlorate affected the rate of inhibition, but the addition of tetrabutylammonium perchlorate to the sodium chloride or the use of the commercial surfactant Hyamine 3500 (a large quaternary ammonium chloride) caused significant decreases in the rate of the inhibition reaction.