Counterintuitively, the catalytic activity of the fcc Ru NPs increases with increasing particle size, despite the decrease in surface area 6.Īccording to a review of the structures of Ru NPs smaller than 5 nm, two approaches have been pursued. For NPs larger than 3 nm, the newly obtained fcc Ru NPs are more reactive than conventional hcp Ru NPs 6. Moreover, the catalytic activity of Ru NPs, supported on γ-Al 2O 3 for CO oxidation, is dependent on the fcc and hcp structure and size 6. The structure and crystal size are controlled by adjusting the composition of mixtures of the Ru precursor and reducing agent 6. However, the dependence of properties of the fcc Ru on lattice parameters was analyzed by ab initio calculations 7, although naturally occurring bulk Ru forms only a hexagonal close-packed (hcp) structure. This phenomenon was reported in 2013 for face-centered-cubic (fcc) type Ru nanoparticles (NPs) obtained by chemical reduction 6. Ruthenium (Ru) has recently attracted much attention as a catalyst for the oxidation of CO because of its high catalytic activity 1, 2, 3, 4, 5. Enhancement of the CO oxidation activity in fcc Ru NPs may be caused by an increase in imperfections due to lattice distortions of close-packed planes and static atomic displacements. In addition, the fcc Ru NPs had larger atomic displacements than hcp Ru NPs for diameters ranging from 2.2 to 5.4 nm. In contrast, fcc Ru NPs larger than 3.5 nm, which had a larger domain surface area, lattice distortion and larger atomic displacement, exhibited higher catalytic activity than that of hcp Ru NPs of the same size. In comparing fcc Ru NPs with hcp Ru NPs, we found that the hcp Ru NPs of approximately 2 nm, which had a smaller domain surface area and smaller atomic displacement, showed a higher catalytic activity than that of fcc Ru NPs of the same size. However, for face-centered cubic (fcc) Ru NPs, the CO oxidation activity became stronger with decreasing domain surface area. For hexagonal close-packed (hcp) Ru NPs, the CO oxidation activity decreased with decreasing domain surface area. To reveal the origin of the CO oxidation activity of Ruthenium nanoparticles (Ru NPs), we structurally characterized Ru NPs through Rietveld refinement analysis of high-energy X-ray diffraction data.
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