Oxygen Reduction Reaction Catalysts Prepared from Acetonitrile Pyrolysis over Alumina-Supported Metal Particles

Abstract

Noble-metal-free active catalysts for the oxygen reduction reaction (ORR) in an acidic environment were prepared from the pyrolysis of acetonitrile at 900 degrees C over alumina and metal-doped alumina. This work includes analyses of the nitrogen-doped carbon preparation process, characterization of the carbon materials formed, and activity testing for the ORR. The nitrogen-containing carbon nanostructures that formed during the pyrolysis of acetonitrile could be purified by washing the product with hydrofluoric acid. A wide range of techniques were used to characterize the solid carbon products of the acetonitrile decomposition. While the samples have many similar physical properties, X-ray photoelectron spectroscopy and transmission electron microscopy showed evidence that differences in the nanostructure and surface functional groups of the samples are likely to account for observed differences in oxygen reduction activity. The most active catalysts were prepared over alumina impregnated with up to 2 wt % Fe, although the catalysts that were prepared by acetonitrile pyrolysis over alumina with no metal doping still had significant activity. In comparison to a 20 wt % platinum on Vulcan carbon catalyst, the most active samples only have an additional 100 mV overpotential. The selectivity of the catalysts for complete oxygen reduction to water followed a trend similar to activity. The best selectivity to water versus peroxide obtained was 99%, or equivalently, an n of 3.98 (i.e., 3.98 electrons transferred out of a maximum of 4 electrons per mole of oxygen that is reduced), as determined by rotating ring-disk electrode testing.