Effect of adsorbed uracil and its derivatives on the rate of oxygen reduction on platinum in acid electrolytes

The rate of oxygen reduction in 0.5 M H2SO4 has been controlled by adsorbing uracil and its alkyl derivatives on a Pt electrode. Addition of an ethyl group to C(5), or methyl groups to the C(5) and C(6) positions of uracil, enhances the reaction rate. 5-Ethyluracil and 5,6-dimethyluracil have such an effect, however, only when their concentrations in the electrolyte are ≤0.1 mM. Substitution of the hydrogen on N(3) with CH3, or exocyclic oxygens with OCH3 groups results in the inhibition of the reaction; 1,3-dimethyluracil and 2,4-dimethoxypyrimidine (0.1 mM) are two systems that cause inhibition. Interaction between the surface Pt atoms and the N(3) and O sites on the organic molecule, especially in the presence of CH3 or C2H5 groups on the C(5) and/or C(6) centers, are essential for the enhancement. Results of rotating disk electrode experiments suggest that at low overpotentials, 5-ethyluracil and 5,6-dimethyluracil increase the exchange current to produce higher reaction rates.