Fiechter, S.; Dorbandt, I.; Bogdanoff, P.; Zehl, G.; Schulenburg, H.; Tributsch, H.; Bron, M.; Radnik, J.; Fieber-Erdmann, M.: Surface modified ruthenium nanoparticles: structural investigation and surface analysis of a novel catalyst for oxygen reduction. The Journal of Physical Chemistry C 111 (2007), p. 477-487
10.1021/jp0618431
Abstract:
ABSTRACT Ruthenium catalysts modified by selenium are of interest as a methanol insensitive oxygen reduction catalyst in a Polymer Electrolyte Membrane Fuel Cell for mobile application. To elucidate the structural and chemical features of unsupported and carbon supported ruthenium nano-particles prepared by thermolysis of Ru3(CO)12 in an organic solvent with and without the presence of dissolved selenium different bulk and surface sensitive methods such as Transmission Electron Microscopy TEM, X-Ray Diffractometry XRD, Thermogravimetry coupled with Mass Spectrometry TG-MS, X-ray Photo-electron Spectroscopy XPS and Extended X-ray Absorption Fine Structure EXAFS analysis were performed. It was found that the as grown catalytic particles prepared without selenium and handled under ambient conditions are distinguished by a ruthenium core of 4 nm size, the surface of which is covered by an amorphous ruthenium oxide/hydroxide and metal organic residues from the process of synthesis. In presence of selenium Ru-Se and Se-O bondings has additionally been found at the surface. After heat treatment under vacuum organic residues could be removed. The particles can now be described by a metallic core of < 10 nm - grown by coalescence due to thermal treatment - the surface of which exhibits Ru-O, Se-O and Ru-Se bondings. While a pure ruthenium catalyst totally oxidises within several weeks the selenium containing counterpart becomes stabilised against corrosion for years. Beside the protecting property it is suggested that selenium also leads to a concentration of ruthenium d-electrons close to the top of the Fermi level facilitating four-electron transfer to reduce oxygen at the cathode side of a fuel cell.