Activation of Colloidal PtRu Fuel Cell Catalysts via a Thermal "Conditioning Process"

The activation of platinum-ruthenium (Pt-Ru) fuel cell catalysts, which were made by supporting nanoscopic colloidal precursors on conductive carbon (Vulcan XC72) is brought about by reactive annealing ("conditioning process") at 250-300 degrees C in a particular set-up. The tetraalkylammonium (A) or triorganoaluminium (B) protecting shell is cleaved from the Pt-Ru metal surface in three mandatory steps under flowing argon, oxygen, and hydrogen (30 min each). Ex-situ TEM micrographs were used to monitor changes in particle size during this conditioning process. Additionally, X-ray absorption spectroscopic measurements on the Pt L-III- edge provided insight into the processes occurring in the metallic core of the particles. While a small increase in site is found with conditioning, the dominant structural change in both catalyst types (A and B) is a reduction in the disorder inside the metallic particles, particularly in those having organoaluminium-protected nanoparticles (B). During oxidation under flowing oxygen, platinum atoms only undergo oxidation in the bimetallic Pt-Ru catalyst type (B) while the platinum atoms in the bimetallic Pt-Ru catalyst (A) remain strictly zerovalent. This difference may be attributed to differences in the internal structure of the as prepared colloidal particles (A and B) as evidenced from Pt L-III EXAFS-data and XANES spectra.