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), wereexaminedas 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 degrees 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 degrees 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 50150 (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 degrees 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. (c) 2006 Elsevier B.V. All rights reserved.