Enhancement of the Oxygen Reduction Reaction Activity of Pt by Tuning Its d-Band Center via Transition Metal Oxide Support Interactions

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. To enable the use of transition metal oxides having low electric conductivity as supports, 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/CSCNT supports, resulting in MOx/CSCNT- and M1M2Ox/CSCNT-supported Pt NP 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 and the type of metal oxide support. A "volcano-type" dependence of the ORR activity (represented as the current density, mass activity, and specific activity at 0.9 V vs reversible hydrogen electrode (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/CSCNT catalysts. It was found that the d-band center values (ranging from −3.83 to −3.42 eV) of Pt deposited on MOx/CSCNTs and M1M2Ox/CSCNT 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 that is ca. 0.2 eV lower than that of Pt would have maximal ORR activity.