Abstract:
Ruthenium (Ru) is a 4d transition metal that in the bulk adopts hexagonal closepacked
(hcp) structure at all temperature ranges, and novel face-centered cubic (fcc)
Ru nanoparticles (NPs) have been observed to be more efficient than conventional
hcp Ru NPs larger than 3 nm. It has recently attracted much attention as a potential
application in removal of car exhausts due to high catalytic activity for CO oxidation
and preventing CO poisoning in fuel-cell system. We here report the 3-dimentational
atomic-scale structures of fcc and hcp Ru NPs using high-energy X-ray diffraction
(HEXRD), Rietveld analysis, pair distribution function (PDF), and reverse Monte
Carlo (RMC) modelling. Hard X-ray photoelectron spectroscopy (HAXPES) can
provide important information on the influence of NP size on electronic properties.
The HEXRD and HAXPES of Ru NPs were performed at BL04B2 and BL15XU at
SPring-8, the world largest third-generation (8 GeV) synchrotron radiation facility
located in Hyōgo prefecture, Japan. We observed higher stability of the lattice
distortion of fcc Ru NPs with increasing particle size. The PDF analysis results show
that the structural disordered Ru NPs at short- to intermediate-range atomic
distances. The order parameter for fcc Ru NPs decreased with increasing particle size
due to the loosely packing atomic arrangement and may explain an origin of higher
catalytic activity of fcc Ru NPs. In this study, the observed trend of increasing
catalytic activity of fcc Ru NPs was also discussed using their core-levels and valence
band electronic structures. This work was partly supported by ACCEL, Japan
Science and Technology Agency (JST) and also partly supported by Ministry of
Education, Culture, Sports, Science and Technology of Japan (OS: 15K04616).
