We report the first discovery of pure face-centered-cubic (fcc) Ru nanoparticles. Although the fcc structure does not exist in the bulk Ru phase diagram, fcc Ru was obtained at room temperature because of the nanosize effect. We succeeded in separately synthesizing uniformly sized nanoparticles of both fcc and hcp Ru having diameters of 2-5.5 nm by simple chemical reduction methods with different metal precursors. The prepared fcc and hcp nanoparticles were both supported on γ-Al2O3, and their catalytic activities in CO oxidation were investigated and found to depend on their structure and size.
Pd(x)Ru(1-x) solid solution alloy nanoparticles were successfully synthesized over the whole composition range through a chemical reduction method, although Ru and Pd are immiscible at the atomic level in the bulk state. From the XRD measurement, it was found that the dominant structure of Pd(x)Ru(1-x) changes from fcc to hcp with increasing Ru content. The structures of Pd(x)Ru(1-x) nanoparticles in the Pd composition range of 30-70% consisted of both solid solution fcc and hcp structures, and both phases coexist in a single particle. In addition, the reaction of hydrogen with the Pd(x)Ru(1-x) nanoparticles changed from exothermic to endothermic as the Ru content increased. Furthermore, the prepared Pd(x)Ru(1-x) nanoparticles demonstrated enhanced CO-oxidizing catalytic activity; Pd0.5Ru0.5 nanoparticles exhibit the highest catalytic activity. This activity is much higher than that of the practically used CO-oxidizing catalyst Ru and that of the neighboring Rh, between Ru and Pd.
The platinum-group
metals (PGMs) are six neighboring elements in
the periodic table of the elements. Each PGM can efficiently promote
unique reactions, and therefore, alloying PGMs would create ideal
catalysts for complex or multistep reactions that involve several
reactants and intermediates. Thus, high-entropy-alloy (HEA) nanoparticles
(NPs) of all six PGMs (denoted as PGM-HEA) having a great
variety of adsorption sites on their surfaces could be ideal candidates
to catalyze complex reactions. Here, we report for the first time PGM-HEA and demonstrate that PGM-HEA efficiently
promotes the ethanol oxidation reaction (EOR) with complex 12-electron/12-proton
transfer processes. PGM-HEA shows 2.5 (3.2), 6.1 (9.7),
and 12.8 (3.4) times higher activity than the commercial Pd/C, Pd
black and Pt/C catalysts in terms of intrinsic (mass) activity, respectively.
Remarkably, it records more than 1.5 times higher mass activity than
the most active catalyst to date. Our findings pave the way for promoting
complex or multistep reactions that are seldom realized by mono- or
bimetallic catalysts.
The compositional space of high-entropy-alloy
nanoparticles (HEA
NPs) significantly expands the diversity of the materials library.
Every atom in HEA NPs has a different elemental coordination environment,
which requires knowledge of the local electronic structure at an atomic
level. However, such structure has not been disclosed experimentally
or theoretically. We synthesized HEA NPs composed of all eight noble-metal-group
elements (NM-HEA) for the first time. Their electronic structure was
revealed by hard X-ray photoelectron spectroscopy and density function
theory calculations with NP models. The NM-HEA NPs have a lower degeneracy
in energy level compared with the monometallic NPs, which is a common
feature of HEA NPs. The local density of states (LDOS) of every surface
atom was first revealed. Some atoms of the same constituent element
in HEA NPs have different LDOS profiles, whereas atoms of other elements
have similar LDOS profiles. In other words, one atom in HEA loses
its elemental identity and it may be possible to create an ideal LDOS
by adjusting the neighboring atoms. The tendency of the electronic
structure change was shown by supervised learning. The NM-HEA NPs
showed 10.8-times higher intrinsic activity for hydrogen evolution
reaction than commercial Pt/C, which is one of the best catalysts.
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