Hydrogen storage and stability properties of Pd\u2013Pt solid-solution nanoparticles revealed via atomic and electronic structure

Loku Singgappulige Rosantha Kumara,Satoru Yoshioka,Syo Matsumura,Hiroshi Kitagawa,Osami Sakata,Hirokazu Kobayashi,Shinji Kohara,Chulho Song,Toshiaki Ina,Toshiki Yoshimoto

doi:10.1038/s41598-017-14494-7

Abstract

Bimetallic Pd1−xPtx solid-solution nanoparticles (NPs) display charging/discharging of hydrogen gas, which has relevance for fuel cell technologies; however, the constituent elements are immiscible in the bulk phase. We examined these material systems using high-energy synchrotron X-ray diffraction, X-ray absorption fine structure and hard X-ray photoelectron spectroscopy techniques. Recent studies have demonstrated the hydrogen storage properties and catalytic activities of Pd-Pt alloys; however, comprehensive details of their structural and electronic functionality at the atomic scale have yet to be reported. Three-dimensional atomic-scale structure results obtained from the pair distribution function (PDF) and reverse Monte Carlo (RMC) methods suggest the formation of a highly disordered structure with a high cavity-volume-fraction for low-Pt content NPs. The NP conduction band features, as extracted from X-ray absorption near-edge spectra at the Pd and Pt LIII-edge, suggest that the Pd conduction band is filled by Pt valence electrons. This behaviour is consistent with observations of the hydrogen storage capacity of these NPs. The broadening of the valence band width and the down-shift of the d-band centre away from the Fermi level upon Pt substitution also provided evidence for enhanced stability of the hydride (ΔH) features of the Pd1−xPtx solid-solution NPs with a Pt content of 8-21 atomic percent.

Highlights

Nanoparticles (NPs), bimetallic NPs, have attracted much attention owing to their potential applications in numerous fields of science and technology
We investigate the average crystallographic structure and local structure of Pd1−xPtx solid-solution NPs by means of high-energy X-ray diffraction coupled with atomic pair distribution function analysis and reverse Monte Carlo (RMC) modelling techniques
As we have previously reported, high-resolution transmission electron microscope (TEM) images and energy-dispersive X-ray spectroscopy (EDS) spectra of the Pd1−xPtx solid-solution NPs have revealed that Pd and Pt are homogeneously mixed at the atomic level by the process of hydrogen absorption/desorption (PHAD) at 373 K, which is a trigger for formation of Pd/Pt core/shell NPs5,11,26,27

Summary

Introduction

Nanoparticles (NPs), bimetallic NPs, have attracted much attention owing to their potential applications in numerous fields of science and technology. Density functional theory studies have shown that a Pd1−xPtx solid-solution phase is thermodynamically stable in the nanoparticle phase at 373 K, Pd and Pt are immiscible in their bulk phases[9] Such Pd1−xPtx solid-solution NPs systems could play an important role as effective catalysts[10,11,12]. Pd1−xPtx solid-solution NPs with a Pt content of 8–21 atom % possess a higher hydrogen-storage capacity than that of Pd NPs5 These nanoparticles possess a higher hydrogen-storage capacity than that of Pd/Pt core/shell NPs. According to computational investigations by Calvo and Balbuena, randomly-mixed-disordered and the ordered Pd-monolayers over a Pt system with the composition Pt3Pd7 are thermodynamically more favourable for the oxygen reduction reaction[16]. To reveal correlations between properties of the electronic structure, such as the unoccupied electronic states and their density of states (DOS), and the hydrogen storage capacity and stability of the Pd1−xPtx solid-solution NPs, we used hard X-ray photoelectron spectroscopy (HAXPES) and X-ray absorption near-edge spectroscopy (XANES)

Methods

Results

Conclusion