On the role of reactant transport and (surface) alloy formation for the CO tolerance of carbon supported PtRu polymer electrolyte fuel cell catalysts

The role of atomic scale intermixing for the electrocatalytic activity of bimetallic PtRu anode catalysts in reformate operated polymer electrolyte fuel cells (PEFC) was investigated, exploiting the specific properties of colloid based catalyst synthesis for the selective preparation of alloyed and nonalloyed bimetallic catalysts. Three different carbon supported PtRu catalysts with different degrees of Pt and Ru intermixing, consisting of W carbon supported PtRu alloy particles (PtRu/C), (ii) Pt and Ru particles co-deposited on the same carbon support (Pt+Ru/C), and (iii) a mixture of carbon supported Pt and carbon supported Ru (Pt/C+Ru/C) as well as the respective monometallic Pt/C and Ru/C catalysts were prepared and characterized by electron microscopy (TEM), X-ray absorption spectroscopy, and CO stripping. Their performance as PEFC anode catalysts was evaluated by oxidation of a H-2/2%CO gas mixture (simulated reformate) under fuel cell relevant conditions (elevated temperature, continuous reaction and controlled reactant transport) in a rotating disk electrode (RDE) set-up. The CO tolerance and H-2 oxidation activity of the three catalysts is comparable and distinctly different from that of the monometallic catalysts. The results indicate significant transport of the reactants, COad and/or OHad, between Pt and Ru surface areas and particles for all three catalysts, with only subtle differences from the alloy catalyst to the physical mixture. The high activity and CO tolerance of the bimetallic catalysts, through the formation of bimetallic surfaces, is explained, e.g., by contact formation in nanoparticle agglomerates or by material transport and subsequent surface decoration/surface alloy formation during catalyst preparation, conditioning, and operation. The instability and mobility of the catalysts under these conditions closely resembles concepts in gas phase catalysis.