Enhancement of Oxygen Reduction Reaction Activity of Pt By Tuning Its D-band Center Via Transition Metal Oxide Support Interactions

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 nanoparticles were 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. A possible contribution of lattice strain (geometric effect) to the enhanced ORR activity is also briefly discussed. Our results demonstrate that the ORR activity of Pt NPs deposited on a series of the metal oxide supports can be deliberately controlled by the choice of support and CSCNTs.