18374
J. Phys. Chem. B 2006, 110, 18374-18384
Oxygen Reduction Reaction Catalysts Prepared from Acetonitrile Pyrolysis over Alumina-Supported Metal Particles Paul H. Matter, Eugenia Wang, Maria Arias, Elizabeth J. Biddinger, and Umit S. Ozkan* Department of Chemical Engineering, The Ohio State UniVersity, Columbus, Ohio 43210 ReceiVed: April 9, 2006; In Final Form: June 27, 2006
Noble-metal-free active catalysts for the oxygen reduction reaction (ORR) in an acidic environment were prepared from the pyrolysis of acetonitrile at 900 °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.
Introduction Reducing the required platinum loadings in proton exchange membrane (PEM) fuel cell electrodes, particularly in the cathode, is a high priority to help initiate the commercialization of PEM fuel cells. Viable alternative electrocatalysts to platinum are materials derived from the pyrolysis of nitrogen, carbon, and iron (or cobalt) precursors. These materials could include pyrolyzed organometallic macrocycles1-12 or pyrolysis products from simpler starting materials, such as metal salts and nitrogencontaining organic molecules.13-22 Nonnoble metal catalysts have been shown to have comparable activity to platinum catalysts.14,19,21,22 However, the nature of the active site for oxygen reduction in nonnoble metal catalysts is still not well understood. It has been well-established that nitrogen, carbon, and Fe (or Co) must be present during the pyrolysis to form highly active catalysts,14,20,23,24 although less active catalysts can be prepared with only nitrogen and carbon.5,17,21,22,25-27 Proposals for the source of activity in this class of materials have ranged from active sites where metal ions are stabilized by nitrogen groups15 to sites that are formed catalytically in the presence of a metal but themselves do not contain the metal.5,28,29 Obtaining a better understanding of the active site in these nonnoble metal catalysts may lead to the development of materials that could completely eliminate the need for platinum in the cathode of PEM fuel cells. In recent publications, we have reported on the potential role of nanostructure in the activity of carbon-based nonnoble metal * Author to whom correspondence should be addressed. E-mail:
[email protected].
catalysts for the oxygen reduction reaction (ORR).21,22 During acetonitrile pyrolysis, iron particles were observed to act as catalysts for the formation of carbon nanostructures with a higher degree of edge plane exposure. This phenomenon could also be occurring in heat-treated macrocycle materials. Maldonado and Stevenson reported the formation of nitrogen-containing carbon fibers from the pyrolysis of a phenanthroline precursor.9 These fibers had significant disorder and edge plane exposure and were active for oxygen reduction in basic to neutral electrolytes. (Acidic electrolyte testing was not reported.) Therefore, it is possible that in this class of heat-treated Fe/ N/C catalysts the metal is merely acting as a catalyst for the formation of more favorable structures for the ORR. Previously, we demonstrated the possibility of purifying the carbon that forms during catalyst preparation by using a support (alumina) that is soluble in a strong acid.22 This is also a viable approach to prepare active nitrogen-containing carbon ORR catalysts free of metal. The formation of carbon nanofibers from metal particles at elevated temperatures in a hydrocarbon atmosphere is a wellestablished process.30-34 Similarly, nitrogen-doped carbon nanofibers can be prepared by chemical vapor deposition of carbon and nitrogen precursors in the presence of metal particles.20,22,35-37 The feed atmosphere, the metal composition, and the support for the metal particles can all affect the resulting nanostructure of the carbon that forms.30,33,38 Therefore, in this study we pay particularly close attention to the effect of the support (alumina or Fe- or Ni-doped alumina) used on the resulting nanostructure of the carbon that forms during acetonitrile pyrolysis. The nanostructure and physical properties of the nitrogen-doped carbon are related to ORR performance. The role of Fe as a
10.1021/jp062206d CCC: $33.50 © 2006 American Chemical Society Published on Web 08/25/2006
Oxygen Reduction Reaction Catalysts catalyst for the formation of the ORR active sites versus its role as part of the active ORR site is also discussed. Experimental Methods Catalyst Preparation. In-house-prepared alumina was used as a support for the high-temperature pyrolysis of acetonitrile to produce active catalysts. The high-purity alumina was prepared by a sol-gel technique, in which aluminum tri-secbutoxide (ATB, Aldrich, 97%,
