Study on the Oxygen Bonding Energy to Pt by Valence Band DOS Measurement of Pt Nano-Particle Catalysts

Abstract

In order to improve the catalytic activity and durability of PEFC cathode Pt catalysts the particle size and ligand effects on the oxygen bonding energy of a Pt nano-particle catalyst were evaluated from the valence band DOS of Pt by using resonant inelastic X-ray scattering (RIXS). The RIXS measurement is suitable to probe changes of the valence band states by oxygen adsorption because it is element specific and compatible with ambient pressure. A hard X-ray whose photon energy is slightly lower than the Pt L 3 absorption edge was introduced on the sample to eliminate fluorescence component and subsequent emission by transition from the Pt valence states to the 2p core hole was detected as a function of energy transfer as shown in Fig. 1.The RIXS spectra of Pt nano-particle catalysts measured by 11.563 keV excitation are shown in Fig. 2. The mean particle diameters are evaluated from TEM images to be from 1.6 to 3.9 nm. The main peaks in the region between 2 and 4 eV can be assigned as X-ray Raman peak which comes from the bonding between platinum and oxygen. The X-ray Raman peak position decreases with increasing mean particle diameter as shown in Fig. 2.The energy transfer value of X-ray Raman peak position can be closely related to the oxygen bonding energy with Pt. Therefore Figure 2 suggests that the oxygen bonding energy to Pt decreases with the Pt particle size. Strong bonding between oxygen and Pt might reduce the catalytic activity and the durability of Pt nano-particles with diameters less than 3 nm. Alloying with transition metals also changes the bonding energy of oxygen to Pt. Based on these results, relationship between electronic structure and catalytic activity will be discussed. Acknowledgement This work was supported by New Energy and Industrial Technology Development Organization (NEDO). The RIXS experiments were approved by the Japan Atomic Energy Agency (JAEA) Proposal Review Committee (Proposal No. 2010B3575, 2011A3574 and 2013A3514).