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Activity and Durability of PtRuP Catalysts and Their Atomic Structures

PtRuP catalysts were synthesized by electroless plating method and chelate ligand was added to the synthetic solution to narrow down difference of reduction potentials in Pt and Ru ions. XRD and XAFS analyses revealed that the addition of the ligand promoted alloying of Pt and Ru atoms in the PtRuP catalyst. Methanol oxidation activity and durability were improved by the well-alloyed PtRuP catalyst, which supported bi-functional mechanism. INTRODUCTION Based on bi-functional mechanism, an explanation for improvement of CO tolerance in Pt catalyst by addition of Ru proposed by Watanabe, Pt and Ru atoms in the PtRu catalyst should be well alloyed and Ru atom should be in close vicinity to Pt one at surface of the catalyst. However, Pt core/Ru shell structure is likely to be formed in wet syntheses such as alcohol reduction and electroless plating, because Pt ion is preferentially reduced due to difference of standard reduction potentials in Pt and Ru ions. In PtRuP catalyst synthesized by the alcohol reduction method, the highest methanol oxidation activity was achieved with composition of Pt78Ru22. The composition largely deviates from Pt50Ru50 commonly known as the best composition for the PtRu catalyst. This deviation is coming from the difference of the standard reduction potentials. Bulk composition of Pt in the catalyst should be higher than 50 at.% in order to make surface composition of the catalyst into 50 to 50, because Pt enriched core is formed by the preferential reduction of Pt ion in the alcohol reduction method. In this study, PtRuP catalysts were synthesized by electroless plating method. Chelate ligand was added in the synthetic solution to narrow down the difference of the reduction potentials. Effects of the ligand addition on methanol oxidation activity and on durability of the catalysts are reported in terms of atomic structures of the PtRuP catalysts. EXPERIMENTALS PtRuP catalysts were synthesized by electroless plating method by using NaPH2O2 as a reducing agent as well as source of P. H2PtCl6, RuCl3 and NaPH2O2 were dissolved in ion exchanged water and PtRuP catalyst was deposited on carbon support at 358 K. Chelate ligand was added in the synthetic solution to narrow down the difference of the reduction potentials. Methanol oxidation activity of the catalysts was evaluated by linear sweep volammetry (LSV). Durability of the catalysts was tested by repeated cyclic voltammetry (0.2-1.1 V vs. NHE). Atomic structures of the PtRuP catalysts were analyzed by XRD and by X-ray absorption fine structure (XAFS) of Pt-LIII and Ru-K edges. RESULTS AND DISCUSSION LSV measurements showed that methanol oxidation activity was improved in PtRuP catalyst synthesized with chelate ligand. Figure 1 shows XRD patterns of PtRuP and commercialized Pt catalysts. Diffraction angle from (111) plane of PtRuP catalyst synthesized without chelate ligand is almost same as that of Pt catalyst. On the other hand, the diffraction shifted toward higher angle in PtRuP catalyst synthesized with chelate ligand. This sift toward higher diffraction angle suggests that alloying of Pt and Ru atoms in the PtRuP catalyst was advanced, because lattice spacing of Pt decreases by alloying with Ru which has smaller atomic radius than that of Pt. Results of XAFS analysis are summarized in Table 1. Here, indexes of PRu and PPt (PRu=NRu-Pt/(NRu-Ru+NRu-Pt), NRu-Pt is Pt coordination number and NRu-Ru is Ru coordination one viewing from Ru atom, PPt is viewing from Pt atom) were introduced to compare frequency of neighboring Pt atom to Ru one. PRu and PPt of PtRuP catalyst synthesized with chelate ligand are larger than those of PtRuP catalyst synthesized without ligand, which indicates alloying of Pt and Ru atoms was advanced in the PtRuP catalyst synthesized with chelate ligand. Figure 2 shows durability of PtRuP and commercialized PtRu catalysts. It was found that addition of chelate ligand improved durability of PtRuP catalyst. Compositional analysis of the repeated CV tested solution showed that dissolution amount of Ru was smaller in PtRuP catalyst synthesized with chelate ligand, which also arises from well alloyed structure of the PtRuP catalyst synthesized with chelate ligand. In conclusion, addition of chelate ligand narrowed down difference of reduction potentials of Pt and Ru ions and well alloyed PtRuP catalyst was synthesized by electroless plating method. Methanol oxidation activity and durability were improved by the well-alloyed PtRuP catalyst. REFERENCES (1) M. Watanabe and S. Motoo, J. Electroanal. Chem. Interfacial Electrochem., 60, 267 (1975). (2) Daimon and Y. Kurobe, Catalysis Today, 111, 182 (2006). Fig.1 XRD Patterns of Catalysts. 34 36 38 40 42 44 2θ (deg.) In te ns ity (a .u .) Pt (111) PtRuP with chelate ligand PtRuP without chelate ligand