Surface modified ruthenium nanoparticles: Structural investigation and surface analysis of a novel catalyst for oxygen reduction

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 nanoparticles prepared by thermolysis of Ru-3(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, X-ray diffractometry, thermogravimetry coupled with mass spectrometry, X-ray photoelectron spectroscopy, and extended X-ray absorption fine structure analysis were performed. It was found that the as-grown catalytic particles prepared without Se 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 the presence of Se, Ru-Se and Se-O bondings have additionally been found at the surface. After heat treatment at 900 C under vacuum, organic residues and ruthenium oxides could be removed. The particles have grown to a size of about 10 nm, the surfaces of which are covered by Ru-Se and Ru-SeO3 units. The as-grown and heat-treated catalysts were characterized electrochemically by cyclovoltammetry, rotating disk electrode with and without methanol in the electrolyte, and rotating ring disk electrode measurements to quantify H2O2 production. As expected from structural analysis, best results have been obtained with heat-treated, Se-modified ruthenium catalysts. It is proposed that in the heat-treated samples an interaction between Se2-, [Se-2](2-), and SeO32- decorating the surface of the ruthenium particles is responsible for the improved oxygen reduction process.