Direct Accessing the Nanostructure of Carbon Supported Ru−Se Based Catalysts by ASAXS

Abstract

A structural model of selenium-modified ruthenium nanoparticles was derived from anomalous small-angle X-ray scattering (ASAXS) and small-angle neutron scattering (SANS) experiments and further approved and supported by TEM, XRD, and extended X-ray adsorption fine-structure (EXAFS) analysis. Carbon-supported ruthenium nanoparticles modified with selenium (RuSex/C) have been suggested as highly active and Pt-free cathode catalysts for the oxygen reduction reaction in PEM fuel cells. However, so far no definite conclusion regarding the nature of the catalytically active surface could be drawn. ASAXS and SANS experiments now revealed that the surface of Se modified Ru nanoparticles after reductive catalyst activation at elevated temperatures is not completely covered by Se. The Se rather forms patches on the Ru surface while the rest of the surface is covered by oxygen. It could be demonstrated that the different scattering contributions of the carbon black carrier and various metallic constituents of the supported nanoparticles can be well separated by means of ASAXS measurements. The structural model derived from the ASAXS energy dependencies could be confirmed by results of SANS measurements. Thus, the previously assumed core−shell model for the Se modified Ru particles was clearly disapproved.