This paper presents the correlation between the d-band center of Pt nanoclusters and the catalytic activity for an oxygen reduction reaction (ORR). The size-dependent d-band center was determined for Pt nanoclusters deposited on glassy carbon substrates by photoelectron spectroscopy using synchrotron radiation light. It is found that the d-band center moves toward the Fermi level (E F) with decreasing cluster size, as predicted by the first-principles calculations using linear combinations of pseudoatomic orbitals and norm-conserved pseudopotentials. The ORR activity of Pt nanoclusters measured by linear sweep voltammetry with the rotating disk electrode method reveals that the activity is diminished with decreasing cluster size and with moving the d-band center toward the E F. This is consistent with the d-band model claiming that the closer the d-band center is to E F the larger the oxygen adsorption energy is.

Site-dependent redox potentials of the Pt dissolution reaction, Pt → Pt2+(aq) + 2e−, from Pt particles with and without carbon supports were calculated by a first principles method. The calculation result showed a clear site-dependence in which the redox potentials for the Pt atoms at edges are lower than those for Pt atoms at flat surfaces. This site-dependence is roughly correlated with the d-band center of the dissolving Pt atom, and Pt atoms with higher d-band centers dissolve more easily to the solution. The effects of perfect and defected graphenes on the redox potential are less than 0.1 V. For the platinum atom attaching to the carbons, a slightly negative effect (lowering the redox potential) is obtained, because the bond strength between Pt particles and carbons is enhanced by the defect creation after the dissolution reaction.

04.09.12 KB. Accepted version ok to add spiral, Elsevier says ok while mandate is not enforced

A great film with special effects: An epitaxial Pt thin film with a thickness of 1.5 nm on a TaB2(0001) substrate (see high‐angle annular dark‐field STEM image) was tested as an electrocatalyst for the oxygen reduction reaction. Its specific activity was twice that observed for polycrystalline Pt or Pt(111). Analysis and DFT calculations showed that the Pt layer is alloyed with Ta, the electronic effect of which on Pt leads to the enhanced activity.

The effect of the size of a platinum particle on its electronic structure was measured by soft X-ray photoemission spectroscopy and calculated by a first principle calculation. The measured particle size dependence of d-band center, which is considered as a good indicator of the electronic state related with the catalytic activity for oxygen reduction reaction, showed the shift of the d-band center to lower in energy, i.e. further away from the Fermi level, for smaller particle less than about 2nm. This trend did not agreed with the theoretical calculation result.