Effect of RuOxHy Island Size on PtRu Particle Aging in Methanol

The aging properties of two different commercial PtRu black catalysts, Johnson-Matthey HiSpec6000 and Tanaka TEC90110, were observed in 1M trifluoromethanesulfonic acid with 0.3 M methanol as a function of both time and potential cycling. Cu underpotential deposition, cyclic voltammetry, and x-ray absorption spectroscopy, using both the extended x-ray absorption fine structure and Ay x-ray absorption near-edge spectroscopy analysis techniques, on samples after potential cycling between 0.02 and 0.8 V, revealed the PtRu aging mechanism and offer an explanation of why the two catalysts age differently. The Tanaka catalyst had relatively large RuOxHy islands on the surface and underwent little dissolution and agglomeration after 40 cycles. This had little detrimental effect on the CO oxidation properties as seen in the CO stripping data, and therefore, this catalyst can be expected to be relatively more tolerant to poisoning in direct methanol fuel cells. The Johnson-Matthey catalyst, on the other hand, was found to have much smaller Ru islands initially, underwent more Ru dissolution/agglomeration, and showed definite signs of an increase in Ru island size and partial oxidation to RuOxHy. The CO stripping data for this catalyst show a significant increase in onset potential with aging, reflecting a change from the predominant bifunctional to ligand-effect mechanism for the CO oxidation.