Acetonitrile Adsorption on Pt Single-Crystal Electrodes and Its Effect on Oxygen Reduction Reaction in Acidic and Alkaline Aqueous Solutions

The adsorption and reactivity of acetonitrile (CH3CN) have been studied on Pt(111) and Pt(100) electrodes in 0.1 M HClO4 and 0.1 M NaOH solutions with CH3CN concentrations ranging from 10–3 to 1 M. Cyclic voltammetry results show that these processes are structure sensitive and that the hydrogen adsorption/desorption region is partially blocked on Pt(111) in acidic solutions while the inhibition is almost complete on Pt(100) in both acidic and alkaline media. However, for Pt(111), hydrogen adsorption is practically unaltered in the 0.1 M NaOH electrolyte. In situ infrared measurements and density functional theory calculations suggest that rehybridized adsorbed acetonitrile reacts with adsorbed hydroxyl species at high potentials forming a hydroxylated adsorbed species. The latter is bonded to the Pt surface by electrodonation and can be reduced to an intermediate in which the double C–N bond is tilted with respect to the metal surface. Lastly, oxygen reduction reaction (ORR) has been investigated by using the hanging meniscus rotating disk electrode configuration. Limiting current densities decrease more drastically for Pt(100) than for Pt(111) as acetonitrile concentration is increased because of the higher acetonitrile coverage for the former one. The onset potential for ORR is shifted to less positive values in acidic media because of a blocking effect of acetonitrile. In alkaline media, the onset potential for Pt(111) is slightly more positive for low concentrations of acetonitrile because oxide formation, which hinders oxygen reduction for the more positive potentials, is inhibited due to the presence of adsorbed acetonitrile species at low coverages. This work has been financially supported by MCINN-FEDER (Spain) through project CTQ2016-76221-P. V.B.-M. thankfully acknowledges MINECO, the award of a predoctoral grant (BES-2014-068176, project CTQ2013-44803-P) and a student stay grant (EEBB-I-16-11656).