Abstract
The enhancement of the oxygen reduction reaction (ORR) activity of platinum nanoparticles (Pt NPs) using transition metal oxide (MOx, M = Ti, Nb, Ta, W, Y and Zr) supports has been examined. In order to enable the use of transition metal oxides having low electric conductivity as supports, the Pt NPs were formed on thin transition metal oxides formed on conducting cup-stacked carbon nanotubes (CSCNTs). Metal oxide composites (M1M2Ox) prepared from two types of transition metal (M1M2: TiNb, NbTa and TaW) precursors were also used as supports. Pt NPs were photodeposited on MOx/CSCNTs and M1M2Ox/CSCNTs supports, resulting in MOx/CSCNTs and M1M2Ox/CSCNTs-supported Pt NPs catalysts (abbreviated as Pt/MOx/CSCNTs and Pt/M1M2Ox/CSCNTs). Their ORR activities in 0.1 M HClO4 aqueous solution were found to significantly depend on the atomic ratio of M1 and M2 in M1M2Ox as well as the type of metal oxide support. A “volcano-type” dependence of the ORR activity (represented as current density, mass activity and specific activity at 0.9 V vs. RHE) on the Pt d-band center, relative to the Fermi level, was obtained in a series of the Pt/MOx/CSCNTs and Pt/M1M2Ox/CSCNTs catalysts. It was found that the d-band center values (ranging from -3.83 to -3.42 eV) of the Pt deposited on MOx/CSCNTs and M1M2Ox/CSCNTs supports were lower than that (-3.39 eV) of the reference Pt/carbon black (CB), and that the Pt/TiNbOx (Ti:Nb = 1:6.6 in atomic ratio)/CSCNTs with a d-band center of -3.59 eV exhibited the maximum ORR activity, in agreement with the theoretical expectation that an ORR catalyst having a d-band center which is ca. 0.2 eV lower than that of Pt would have maximal ORR activity.
Published Version
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