Synthesis and Characterization of Platinum-Based Fuel Cell Catalysts
Highly active Pt-based nanoparticle fuel cell catalysts were synthesized and explored by electrochemical NMR (EC-NMR). Pt/Ru catalysts were obtained by spontaneous deposition of Ru onto platinum nanoparticles. Their reactivity toward methanol oxidation maximized at ruthenium packing density 0.4--0.5, displaying twice the activity of the commercial Pt-Ru alloy catalyst. COs adsorbed on these Pt/Ru nanoparticle consisted of two domains: CO on Pt domains behaved like CO on pure Pt while CO near Ru islands had highly shifted 13C NMR resonances, weaker Korringa relaxation, and thermal diffusion at higher temperatures. The change in CO 2pi* Fermi level local density of states (Ef-LDOS) suggested Ru weakened Pt-CO bond. The Ru spontaneous deposition was further applied to carbon-supported Pt nanoparticles of 2.0 to 8.8 nm size. Their reactivity increased with increasing particle size. Optimum Ru packing density was 0.2--0.4 depending on the particle size. Formic acid fuel cell catalysts were produced by spontaneous deposition of Pd on Pt nanoparticles. The CO stripping peak showed positive shift with increasing Pd. The optimum Pd packing density was 0.79 for Pt/Pd and 1.5 for Pt/Pd/C. From the analysis of the 13C NMR data, we concluded that (1) CO chemisorbed onto the Pt/Pd catalysts existed on two domains: Pt sites and Pd sites, (2) CO on Pd underwent fast diffusion, (3) the 5sigma orbital of CO had significant contribution to CO-Pd bond, and (4) the Pd sites Ef-LDOS decreased due to Pt. Commercial Pt-Ru alloy nanoparticles and the effect of heat-treatment on them were investigated. The single Gaussian peak Pt NMR spectrum and the slow-beat in the spin-echo decay curve suggested Pt surface enrichment. The 13C spin-lattice relaxation measurements indicated that Ru decreased the metal surface and the adsorbed CO 2pi* E f-LDOS. The 220°C-treated sample had decreased E f-LDOS of surface Pt as evidenced by longer spin-lattice relaxation time. The Pt NMR spectrum of 600°C-treated sample showed evidence of subsurface layer Pt atoms. The CVs of the heat-treated samples indicated increased surface concentration of Ru. The reactivity toward methanol oxidation increased 3 times by 220°C treatment, suggesting that heat treatment can be utilized to optimize surface composition.
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University of Illinois at Urbana-Champaign