Changes of Oxygen Adsorption State of Pt Nanoparticle Catalyst on the Carbon Support By the Ion Irradiation

Abstract

Pt nanoparticles (NPs) on an Ar+-irradiated glassy carbon (GC) substrate were recently found to show a higher oxygen reduction reaction (ORR) activity than those on the non-irradiated one [1]. This finding suggests that the Pt nanoparticles would be electronically modified by vacancy introduction due to the irradiation into GC. Our theoretical research revealed that vacancies in graphite would lower a d-band center of the Pt nanoparticles [2]. The downshift of d-band center of Pt 5d electrons leads to higher ORR activity because the downshift causes the downshift of the antibonding orbital level of the oxygen adsorbed states of Pt (Pt-O) closing to Fermi level and then results in weaker Pt-O bond [3]. However, it has not been revealed whether the Pt-O antibonding level of Pt NPs is downshifted by the ion irradiation into the carbon support. Therefore, we performed in situ x-ray absorption fine structure (XAFS) measurements, which can observe the unoccupied states of the Pt 5d orbital binding with the adsorbed oxygen, of Pt NPs on the irradiated GC and derived the oxygen adsorbed surface states.The GC substrates were irradiated with 380 keV Ar+ at the fluences of 7.5×1015 ions/cm2 in the Takasaki Ion Accelerators for Advanced Radiation Application (TIARA) facility of the Takasaki Advanced Radiation Research Institute. The Pt nanoparticles were then deposited by RF magnetron sputtering. The Pt deposition was also done on the non-irradiated GC substrates for comparison. The XAFS spectra were measured at BL14B1 in SPring-8. We first measured the XAFS spectra of the irradiated and non-irradiated samples reduced by 1 atm. 100 °C H2 (10%) + He (90%) gas flowing at 100 ml/min for 10min. Then the samples were exposed under mixed gas with O2 + He gas flowing at 100 ml/min at room temperature to adsorb the oxygen to Pt nanoparticles and were measured XAFS spectra. In order to observe the oxygen adsorbed surface states of Pt NPs, we derived the Dm spectra by the subtraction from the XANES spectra after the reduction from the those during the oxygen exposure.Figure 1 shows the Dμ spectra for the Pt/GC and Pt/irradiated GC together with the results of peak fitting. In the non-irradiated sample, we could fit using one peak at around 11562 eV (denoted as peak 1), whereas needed peak 1 in addition to peak 2 on the higher energy side. Since the Dμ spectra represent the change of the electronic state in Pt by oxygen adsorption, the convex peak means that new electronic states are formed at the energy position by the oxygen adsorption. The main peak (peak 1) is the antibonding level of the oxygen adsorbed Pt (Pt-O). On the other hand, the peak 2 is considered to be an oxide species that is more strongly bind to oxygen than Pt-O. It needs future study to elucidate its origin.Comparing the peak 1 of the non-irradiated and the irradiated sample, we found that the peak position for the irradiated sample is 0.7 eV lower than that for the non-irradiated one. This indicates that the Pt-O antibonding level shifts toward low energy and gets close to the Fermi level. Thus, the shift leads to easy electron filling to the antibonding level and thus to a weaker Pt-O bond. In this presentation, we will discuss the change of electronic structure of Pt nanoparticles by the ion irradiation in detail.