Influence of ligand structures on methanol electro-oxidation by mixed catalysts based on platinum and organic metal complexes for DMFC

In order to investigate the influence of mixing organic metal complexes with various metal coordination structure on Pt/C catalyst for methanol oxidation reaction (MOR) in acidic media, N,N′-mono-8-quinolyl-o-phenylenediamine (mqph), N,N′-bis(anthranilidene)ethylenediamine (anthen) and N,N′-bis(salicylidene)ethylenediamine (salen), coordinating to Co(II) and Ni(II), were examined as the model compounds. M(mqph), M(anthen) and M(salen) have MN3, MN4 and MN2O2 moiety as the catalytic active sites, respectively. Pt–M(complex)/C mixed catalysts were prepared by depositing the mixture of Pt tetra-ammine chloride and each organic metal complex with various mixing ratio (w/o) on graphite powder and then heat-treating at 600 °C in Ar atmosphere. The Pt–M(complex)/C samples were put on a glassy carbon disk electrode, and tested for electrochemical MOR in 1 mol dm−3 CH3OH + 0.05 mol dm−3 H2SO4 aqueous solutions at 25 °C. The mixed catalysts, especially Pt–Ni(mqph)/C and Pt–Co(mqph)/C, were found to enhance the MOR and exhibited higher catalytic activities than Pt/C, although each organic metal complex solely showed no catalytic activity. The catalytic ability was enhanced by mixing up to 50/50 (Pt–M(complex)) mixing ratio for the Co(mqph) and up to 80/20 mixing ratio for the Ni(mqph). Larger amounts of M(complex) resulted in a decrease in the catalytic activities. These results indicate that the organic metal complexes, especially M(mqph), promote effectively the electrochemical MOR on Pt/C catalyst, the degree of which depends strongly on the ligand structure. X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectra (XAS) for the Pt–M(complex)/C mixed catalysts showed that the metal coordination structures of organic metal complexes are partially retained on the graphite powder even after the severe heat-treatment at 600 °C in Ar atmosphere. Ab initio calculations for the organic metal complexes exhibited that the Ni metal site of the Ni(mqph) interact with OH− group more strongly than those of the other organic metal complexes. This fact suggests that the Pt–M(mqph)/C electro-oxidizes a byproduct CO absorbed on Pt by “bifunctional effect” in a similar way as Pt–Ru alloy catalysts do in promoting the effective electrochemical MOR.

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