Adsorption of Acetonitrile on Platinum and its Effects on Oxygen Reduction Reaction in Acidic Aqueous Solutions-Combined Theoretical and Experimental Study

Combined theoretical and experimental study of acetonitrile (AcN) adsorption on platinum was performed and its effects on the kinetics of oxygen reduction reaction in HClO4 and H2SO4 solutions were examined. Using periodic density functional theory calculations, it was shown that AcN molecule can interact with Pt surface either through the unsaturated pi electron system or via lone electron pair of nitrogen atom. In both cases, adsorption energy decreases upon increasing coverage, while the modification of electronic structure of Pt surface is localized to the adsorption site. By combining the results of the DFT calculations with the results of blank cyclic voltammetry and rotating disk electrode voltammetry in O-2-saturated solutions, it was concluded that the effects of AcN on Pt surface chemistry and ORR kinetics are primarily steric in nature. Resulting measured ORR activities of polycrystalline platinum in the presence of AcN are due to the combination of (i) suppression of (bi)sulfate adsorption (in H2SO4 solution), (ii) suppression of surface oxidation (in both H2SO4 an HClO4 solution), and (iii) site blockage by adsorbed AcN (or products of its electrochemical transformations).