Rational Synthesis for a Noble Metal Carbide

Article abstract of DOI:10.1021/jacs.9b09219

Transition metal carbides have attractive physical and chemical properties that are much different from their parent metals. Particularly, noble metal carbides are expected to be promising materials for a variety of applications, particularly as efficient catalysts. However, noble metal carbides have rarely been obtained because carbide phases do not appear in noble metal-carbon phase diagrams and a reasonable synthesis method to make noble metal carbides has not yet been established. Here, we propose a new synthesis method for noble metal carbides and describe the first synthesis of rhodium carbide using tetracyanoethylene (TCNE). The rhodium carbide was synthesized without extreme conditions, such as the very high temperature and/or pressure typically required in conventional carbide syntheses. Moreover, we investigated the electronic structure and catalytic activity for the hydrogen evolution reaction (HER). We found that rhodium carbide has much higher catalytic activity for HER than pure Rh. Our study provides a feasible strategy to create new metal carbides to help advance the field of materials science.

Full text of DOI:10.1021/jacs.9b09219

Article
pubs.acs.org/JACS
Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Rational Synthesis for a Noble Metal Carbide
Takuo Wakisaka,† Kohei Kusada,*,† Dongshuang Wu,† Tomokazu Yamamoto,‡ Takaaki Toriyama,§
Syo Matsumura,‡,§ Hiroshi Akiba,∥ Osamu Yamamuro,∥ Kazutaka Ikeda,⊥ Toshiya Otomo,⊥
Natalia Palina,# Yanna Chen,∇ Loku S. R. Kumara,∇ Chulho Song,∇ Osami Sakata,#,∇,○ Wei Xie,¶
Michihisa Koyama,¶,☆ Yoshiki Kubota,⬡ Shogo Kawaguchi,⧫ Ryan L. Arevalo,□ Susan M. Aspera,□
Elvis F. Arguelles,□ Hiroshi Nakanishi,□ and Hiroshi Kitagawa*,†
†Division of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa-Oiwakecho, Sakyo-ku, Kyoto 606-8502, Japan
‡Department of Applied Quantum Physics and Nuclear Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395,
Japan
§The Ultramicroscopy Research Center, Kyushu University, Motooka 744, Nishi-ku, Fukuoka 819-0395, Japan
∥Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan
⊥Institute of Materials Structure Science, High Energy Accelerator Research Organization, 203-1 Shirakata, Tokai, Ibaraki 319-1106,
Japan
#Synchrotron X-ray Group, Research Center for Advanced Measurement and Characterization, National Institute for Materials
Science (NIMS), 1-1-1 Kouto, Sayo-gun, Hyogo 679-5148, Japan
∇Synchrotron X-ray Station at SPring-8, Research Network and Facility Services Division, NIMS, 1-1-1 Kouto, Sayo-gun, Hyogo
679-5148, Japan
○Department of Innovative and Engineered Materials, Tokyo Institute of Technology, 4259-J3-16, Nagatsuta, Midori, Yokohama
226-8502, Japan
¶INAMORI Frontier Research Center, Kyushu University, Motooka 744, Nishi-ku, Fukuoka 819-0395, Japan
☆Global Research Center for Environment and Energy Based on Nanomaterials Science, National Institute for Materials Science,
1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
⬡Department of Physical Science, Graduate School of Science, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan
⧫Research and Utilization Division, Japan Synchrotron Radiation Research Institute (JASRI), SPring-8, 1-1-1 Kouto, Sayo-gun,
Hyogo 679-5198, Japan
□National Institute of Technology, Akashi College, 679-3 Nishioka, Uozumi, Akashi, Hyogo 674-8501, Japan
S
* Supporting Information
ABSTRACT: Transition metal carbides have attractive
physical and chemical properties that are much different
from their parent metals. Particularly, noble metal carbides are
expected to be promising materials for a variety of
applications, particularly as efficient catalysts. However,
noble metal carbides have rarely been obtained because
carbide phases do not appear in noble metal−carbon phase
diagrams and a reasonable synthesis method to make noble
metal carbides has not yet been established. Here, we propose
a new synthesis method for noble metal carbides and describe
the first synthesis of rhodium carbide using tetracyanoethylene
(TCNE). The rhodium carbide was synthesized without extreme conditions, such as the very high temperature and/or pressure
typically required in conventional carbide syntheses. Moreover, we investigated the electronic structure and catalytic activity for
the hydrogen evolution reaction (HER). We found that rhodium carbide has much higher catalytic activity for HER than pure
Rh. Our study provides a feasible strategy to create new metal carbides to help advance the field of materials science.
INTRODUCTION
■
Transition metal carbides, such as WC,1−4 Fe3C,5,6 and
Co2C,7,8 have attracted a great amount of attention for
Received: August 31, 2019
industrial applications, such as catalysts and hard materials,
© XXXX American Chemical Society
A
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because they show fascinating physical, chemical, and
mechanical properties. In general, transition metal carbides
are synthesized by heating together a transition metal and a
carbon source, such as graphite or hydrocarbons, to promote
atomic diffusion of C atoms.9,10 Given the high temperatures
required, the products synthesized by this method tend to form
an equilibrium phase. However, it has not been reported that
noble metal carbides are obtained by just heating noble metals
with a carbon source because there is no noble-metal carbide
phase in the equilibrium phase diagrams.11 However, if noble
metal carbides existed, then some of the important properties
of the most expensive noble metal elements like their catalytic
activity would be enhanced by atomic alloying with the
cheapest element of C.
EXPERIMENTAL SECTION
■
Synthesis of Rhodium Carbide. To synthesize rhodium carbide,
we chose Rh(III) acetylacetonate, ethylene glycol (EG) and TCNE as
a precursor, a reducing agent, and an organic oxidant,14 respectively.
Rh(III) acetylacetonate (0.1 mmol, Strem Chemicals) and TCNE
(0.03 mmol, TCI) were added to EG (50 mL, Wako Chemicals), and
the solution was heated in an eggplant flask in an oil bath at 200 °C
for 6 h. After the reaction was completed, the product was separated
by centrifugation. The details of the synthesis conditions for other
samples are described in the Supporting Information (SI).
Characterization. The structure of rhodium carbide was
determined by synchrotron PXRD measurement (λ = 0.5807 Å
(Figures 1a and S1) and λ = 0.5791 Å (see below) at SPring-8
Recently, a new nonequilibrium synthesis method for solid-
solution alloy nanoparticles that are unobtainable in the bulk
states was reported.12,13 The solid-solution alloy nanoparticles
were synthesized with a quick chemical reduction method in
which constituent metal atoms were concurrently supplied by
tuning the reduction rate of the metal ions, so they could
immediately aggregate and form an alloy even if they are
immiscible in the bulk states. However, these new alloys
comprise transition metals, and although a noble metal and
carbon or nitrogen also are immiscible in the bulk states, such
noble metal carbides and nitrides have not yet been developed.
Here, we propose a new facile synthesis method to produce
noble metal carbides. We hypothesized that a noble metal
carbide would be obtained by a concurrent supply of noble
metal and C atoms. The bottom-up approach, especially for a
liquid-phase chemical reduction method, could be applicable
to finely tune the supply rate of the metal atoms. However, C
atoms cannot be supplied by a reduction process, like metal
atoms. Therefore, we focused on an organic compound to
supply C atoms following decomposition by heating it in the
reducing liquid phase. In addition, for the concurrent supply of
noble metal and C atoms, we utilized an organic oxidant. An
organic oxidant would arrest the reduction process of noble
metal ions and stop functioning as an inhibitor of the reduction
process by its decomposition. Therefore, the decomposition of
the organic oxidant would trigger the reduction of noble metal
ions as well as a supply of C atoms. This would result in a
concurrent supply of noble metal and C atoms, and thus noble
metal carbide should be obtained. Moreover, a liquid-phase
synthesis is more suitable for mass production than conven-
tional carbide syntheses that require extreme conditions, such
as a high temperature and/or pressure.
As a first demonstration for our hypothesis, we focused on
the combination of Rh and C, which is an immiscible system,11
and successfully synthesized rhodium carbide (Rh2C) by a
chemical reduction method using an organic oxidant,
tetracyanoethylene (TCNE). The obtained Rh2C was
characterized by powder X-ray diffraction (PXRD), neutron
powder diffraction (NPD) and scanning transmission electron
microscopy (STEM) coupled with electron energy-loss
spectroscopy (EELS) analysis. The electronic structure of the
Rh2C was investigated by hard X-ray photoelectron spectros-
copy (HAXPES) and density functional theory (DFT)
calculations. Moreover, we investigated catalytic activity in
the hydrogen evolution reaction (HER) of Rh2C, which
showed much higher activity than Rh at reduced loading of the
costly noble metal specie.
Figure 1. (a) The synchrotron PXRD patterns of the obtained
compound (red) and bulk fcc Rh (black). (b) The NPD pattern of
the obtaind compound (red dots) and the results of the Rietveld
refinement (the blue line is calculated pattern; the gray line is the
background; and the black line is the residual). (c) The structural
model obtained from the Rietveld refinement of the synchrotron
PXRD pattern. The red spheres are Rh atoms, and the black spheres
are C atoms. (d) The MEM electron density map for Rh2C. The
yellow surface is the electron density surface (0.65 electrons per Å3).
beamline BL02B215 and NPD measurement using a time-of-flight
high intensity total diffractometer (NOVA) installed at the Materials
and Life Science Experimental Facility (MLF) at the Japan Proton
Accelerator Research Complex (J-PARC). Rietveld refinement of the
PXRD pattern and the calculation of the electron density map with
the maximum entropy method (MEM) were performed using
ENIGMA,16 and refinement for the NPD pattern was performed
using Z-Code developed by KEK. High-angle annular dark-field
(HAADF)-/annular bright-field (ABF)-STEM observation and
STEM-EELS analysis were performed using an aberration-corrected
electron microscope JEOL JEM-ARM 200F operated at 200 kV at
The Ultramicroscopy Research Center, Kyushu University. The 3D
tomographic images were reconstructed from the HAADF-STEM tilt
image series recorded over an angle range from −70° to 65° by
simultaneous iterative reconstruction technique (SIRT)17 imple-
mented in the TEMograpy software (System in Frontier, Inc.)
Investigation of the Electronic Structure. The HAXPES
spectra were recorded at SPring-8 beamline BL15XU at the incident
photon energy of 5.95 keV. The DFT calculations were carried out
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using the Vienna ab initio simulation package (VASP)18−21 within the
generalized gradient approximation based on the Perdew−Burke−
Ernzerhof (GGA-PBE)22−25 functional with the projector augmented-
wave method (PAW).26,27 A cutoff energy of 700 eV and 9 × 9 × 9 k-
point grid were employed.
Catalyst Preparation. The Rh reference material was prepared
from Rh2C by heating Rh2C at 150 °C for 2 h under a 1 atm hydrogen
atmosphere to prepare Rh with the same shape as the Rh2C (Figure
S2). The Rh2C and Rh dispersed in 7 mL of ethanol were sonicated
for 1 h and mixed with Vulcan XC 72R carbon and 23 mL of
isopropanol. After sonicating for 4 h, the products were centrifuged
and dried. Then, 5 mg of the catalysts were suspended and sonicated
in a mixture of 300 μL water, 600 μL isopropanol and 100 μL Nafion.
Ten microliters of the catalyst ink was deposited on a grassy carbon
working electrode (5 mm diameter) (Pine Instruments).
Electrochemical Measurements. All electrochemical measure-
ments were performed at room temperature on a CHI 760E
electrochemical analyzer (CH Instruments) in a standard three-
electrode system using a gold mesh electrode and a Hg/HgO
electrode as the counter and reference electrodes, respectively. The
electrolyte was Ar-saturated 1.0 M KOH. The potential was converted
to the reversible hydrogen electrode (RHE). The polarization curves
were obtained at a scan rate of 50 mVs−1 with 85% iR compensation.
Figure 2. (a) BF-STEM image of Rh2C. (b, c) Atomic resolution in
STEM images. (b) The HAADF-STEM image along the [100] zone
axis. (c) the ABF-STEM image along the [001] zone axis and the
structural models obtained from the Rietveld refinement (insets). The
red spheres are Rh atoms, and the black spheres are C atoms. (d)
HAADF-STEM image and STEM-EELS images of (e) Rh-M, (f) C-K
,and (g) overlay of Rh and C (scale bar, 5 nm).
RESULTS AND DISCUSSION
■
Figure 1a shows synchrotron PXRD patterns of the obtained
compound and bulk Rh. The pattern of the obtained
compound is totally different from the patterns of Rh or the
Rh precursor (Figures 1a and S1). Although this PXRD pattern
has not been observed in any other Rh compound, we found
that the pattern is similar to that of cobalt carbide (Co2C).28
To investigate the crystal structure, we performed Rietveld
refinement using a hypothetical model of Rh2C with the space
group of Pmnn that refers to Co2C and obtained a good fit with
the lattice parameters of a = 3.07667(5) Å, b = 4.76708(9) Å,
and c = 4.72274(8) Å (Figure S1b and Table S1a,b). The
obtained compound was stable until 523 K (Figure S3).
Furthermore, to confirm the position of C atoms and the ratio
of Rh to C, we performed an NPD measurement and Rietveld
refinement (Figure 1b, Table S1c,d). The NPD pattern also fit
well with the same model. These results indicated that the
obtained compound was Rh2C (Figure 1c). In the structure of
Rh2C, the ABAB stacking sequence of the Rh planes can be
seen and is similar to a hexagonal close-packed structure along
the [001] zone axis (Figure S4a); a C atom is located at a
distorted octahedral site among the Rh atoms (Figure S4b),
whereas the bulk Rh adopts a face-centered cubic (fcc)
structure with the close-packed planes stacked in the ABCABC
sequence. Figures 1d and S4c,d show the electron density map
obtained using the MEM for Rh2C, suggesting that there are
bonds between the Rh and C atoms.
the [001] zone axis (Figures 2c and S6b). The ABF-STEM
image contains clear black dots that indicate heavy Rh atoms
and fuzzy black dots that imply light C atoms at the center of
hexagons formed by the Rh atoms. STEM-EELS maps of Rh-M
and C-K show that the Rh and C atoms are distributed
homogeneously over the whole particle (Figure 2d−g). In
addition, the EELS spectrum does not show an obvious N-K
edge around 401 eV, even though C-K and Rh-M3 edges were
clearly observed around at 284 and 496 eV, respectively
(Figure S7). Therefore, the obtained compound was not Rh
nitride, but Rh2C.
The electronic structures of the Rh2C were investigated by
HAXPES and a DFT calculation using the structural model
obtained from the Rietveld refinement. The valence-band
(VB) HAXPES spectra of Rh2C and Rh (Figure 3a,b,
respectively) clearly show four and three peaks, respectively,
Microscopic analysis also confirmed the model structure
obtained by the X-ray and neutron diffraction studies. The
bright-field (BF)-STEM images show that the average size of
the obtained Rh2C is 99.3 23.5 nm, and the particle consists
of small tabular nanocrystals, which was confirmed by 3D
electron tomography (Figures 2a and S5). Figure 2b is a
HAADF-STEM image viewed from the flat side of the crystal,
and it shows an ABAB stacking sequence of Rh atoms with the
(002) lattice spacing of 2.3 Å. This image and its Fourier-
transform (FT) pattern are consistent with the structural
model obtained from the Rietveld refinement corresponding to
a view along the [100] zone axis (Figure S6a). The ABF-
STEM image observed from a lateral side of the crystal and its
FT pattern correspond to the structural model viewed along
Figure 3. (a) The HAXPES spectrum of Rh2C and (b) Rh. (c) The
calculated total density of sates of Rh2C and (d) Rh. In the spectrum
of Rh2C, four main peaks are denoted, whereas in that of Rh, three
main peaks are denoted.
C
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Figure 4. (a) The PXRD patterns of the products synthesized with different amounts of TCNE. The open circles indicate the peaks of Rh2C and
the closed squares indicate the peaks of Rh. (b) Photographs of the synthesis solutions at increasing reaction times. From left to right: 5 min, 10
min, 20 min, 30 min, 40 min, 50 min, 1 h, 2 h, 3 h, 4 h, 5 h, and 6 h. (c) The TEM images of the synthesis solutions with TCNE (upper) and
without TCNE (lower) (scale bar, 50 nm). (d) The consumption percentage of the Rh precursor during the synthesis. These data were obtained by
the normalized absorbance of the peak at 324 nm in the UV−vis spectra of the solutions with TCNE (red) and without TCNE (blue) as written in
the Experimental Section. (e) The PXRD patterns of the products synthesized with the different electron acceptors. (λ = 0.5791 Å)
which correspond to the calculation results (Figure 3c,d).
According to the partial densities of states (PDOS) of Rh2C
(Figure S8), peaks 3 and 4 are formed by the hybridization of
Rh and C orbitals, which is consistent with the existence of the
bonds suggested in the MEM electron density map (Figures 1d
and S4). In addition, no energy gap at the Fermi level was
observed in Rh2C, indicating a metallic nature. The core-level
HAXPES spectra indicate that the valence state of Rh in Rh2C
is 0 (Figure S9).
in the TCNE during the synthesis process, ultraviolet−visible
(UV−vis) absorption spectroscopy was performed with a
TCNE solution (Figure S10). The absorption peak of the
TCNE at approximately 240 nm gradually disappeared,
indicating that TCNE is decomposed by heating in EG. This
suggests that C atoms are gradually supplied by the TCNE
following its decomposition during the synthesis process.
These results revealed that TCNE works as a carbon source
and is essential for the synthesis of Rh2C.
To verify our hypothesis that the decomposition of an
organic oxidant, TCNE, would trigger a concurrent supply of
noble metal and C atoms, the roles of TCNE as a carbon
source and an inhibitor of the reduction were studied. First, the
hypothesized role as a carbon source was confirmed by
decreasing the amount of TCNE for syntheses (Figure 4a). In
the synthesis of Rh2C, we used 0.03 mmol TCNE and
obtained only the Rh2C phase (Figure S1). However, in the
case without TCNE, the fcc Rh phase was obtained as usual,
and the mixture of Rh2C and Rh was synthesized using 0.01
mmol TCNE. Therefore, the amount of TCNE plays a key role
in the formation of Rh2C. In addition, to investigate the change
Second, the role of TCNE as an inhibitor of the reduction of
Rh ions was investigated. During synthesis, the color of the
solution with TCNE changed from transparent to black,
implying that the formation of nanoparticles, was slower than
that of the solution without TCNE (Figure 4b). TCNE is one
of the most well-known electron acceptors.14 Since an electron
acceptor attracts electrons and works as an oxidant, it is
expected that TCNE can work as an inhibitor of the reduction
of Rh ions in the reaction. We observed the particle growth
process in the synthesis solutions with/without TCNE by
TEM (Figure 4c). In the solution without TCNE, particles
were clearly observed starting at 10 min (Figure S11), rapidly
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grew and formed agglomerates until 20 min elapsed. In
contrast, in the solution with TCNE, small particles were
sparsely observed at 20 min, and they gradually grew for more
than 2 h. These results imply that TCNE acts as an inhibitor
for the reduction of Rh ions. The heating time dependence of
the consumption percentage of Rh precursor was estimated by
the intensity of the UV−vis absorption peak of Rh
acetylacetonate29 during the synthesis process (Figures 4d
and S12). The consumption percentage of Rh precursor in the
solution with TCNE increased at slower rate with increasing
time than that in the solution without TCNE. In the UV−vis
spectra of the TCNE solution (Figure S10), the main TCNE
peak decreased for the first 20 min, which means that the
TCNE started to decompose in the first 20 min. The start of
the nucleation observed by TEM coincides with the start of the
decomposition of TCNE observed by the UV−vis analysis.
These results suggest that TCNE acts as an inhibitor and that
its decomposition induces the reduction of Rh ions. Given that
TCNE works as a carbon source, both the supply of C atoms
and the reduction of Rh ions were accompanied by the
decomposition of TCNE, and Rh2C was formed by the
concurrent supply of Rh and C atoms. To further investigate
the correlation between the redox potential of an organic
oxidant and the formation of Rh2C, we examined other organic
oxidants with different redox potentials to synthesize Rh2C
(Figures 4e, S13, and S14). Rh2C was obtained using 7,7,8,8-
tetracyanoquinodimethane (TCNQ) or 2,3,5,6-tetrafluoro-
TCNQ (F4TCNQ) but not using 1,4-benzoquinone, chloranil,
or 1,2,4,5-tetracyanobenzene (TCNB). This tendency follows
the order of the redox potential of the organic oxidants.30
F4TCNQ, TCNE, and TCNQ have high redox potentials
(+0.60 V,31,33 + 0.29 V,31,32 and +0.22 V,31−33 respectively),
whereas chloranil, 1,4-benzoquinone, and TCNB have low
redox potentials (+0.05 V,31−33 −0.46 V,32 and −0.65 V,32
respectively). Therefore, it is concluded that novel Rh2C can
be successfully synthesized by choosing organic oxidants with
an optimum redox potential to tune the reduction of Rh for a
concurrent supply of Rh and C atoms.
Rh2C is expected to show unique properties because its
electronic and surface structures are different from those of Rh.
As a first demonstration, we investigated the catalytic activity
for an alkaline HER, which has become one of most required
reactions in the past few years.34−38 This reaction is a crucial
step in electrochemical water splitting and plays an important
role in energy conversion for the development of hydrogen-
based energy resources. The catalytic performance was
investigated with a standard three-electrode system in 1.0 M
KOH aqueous solution. We performed linear sweep
voltammetry about 20 wt % Rh2C/C (Figure 5a). The 20 wt
% Pt/C and Rh/C were also investigated as references, and Pt
is well-known as the best catalyst for HER. We compared the
activities by overpotentials and Tafel plots (Figure 5b,c). The
overpotentials at −5 mA cm−2 are 13.0 mV (Rh2C/C), 13.4
mV (Pt/C) and 31.6 mV (Rh/C) (Figure 5b). The Tafel
slopes of Rh2C/C, Pt/C and Rh/C are 74.5 mV dec−1, 73.4
mV dec−1 and 114.5 mV dec−1, respectively (Figure 5c). These
results suggest that the catalytic activity of Rh is enhanced by
alloying with C, resulting in Rh2C exhibiting a comparable
HER activity to Pt. In addition, Rh2C showed high stability
during HER and higher conductivity than Rh (Figures S15 and
S16). To clarify the mechanism of the enhancement,
theoretical studies were performed on the (001), (111), and
(021) surfaces of Rh2C, which were confirmed by microscopic
Figure 5. (a) The HER polarization curves of 20 wt % Rh2C/C (red),
Pt/C (black), and Rh/C (blue). The data were recorded in a 1.0 M
KOH electrolyte with a scan rate of 50 mV s−1. (b) The
overpotentials at −5 mA cm−1. (c) The Tafel plots and fitting lines
to obtain the Tafel slopes for 20 wt % Rh2C/C (red), Pt/C (black
overlapped with red) and Rh/C (blue). (d) The 3-state free energy
diagram for HER. The structural models show the OH* precovered
surfaces used for calculations. The blue spheres are hydrogen atoms,
and the gray spheres are oxygen atoms.
analysis (Figure S5). Among these surfaces, the C-terminated
surfaces are more stable than the Rh-terminated surfaces and
low indexed surfaces such as (100) and (010) (Figure S17a).
Although the hydrogen adsorption free energy (ΔGH*) is well-
known as a reasonable descriptor to discuss the HER activity of
a catalyst,39 the effect of hydroxyl species adsorbed on the
surface (OH*) must be considered because the HER activity
in alkaline solution is strongly affected by OH*.40−43 Given the
pH of the test conditions, the C sites of the Rh2C surfaces are
covered with OH* under the reaction potential (Figure S17c).
Therefore, we calculated the free energy of H* on the OH*
precovered C-terminated Rh2C(001), (111), and (021)
surfaces (Figure S17d). While a hydrogen atom adsorbs on
an hcp hollow site on the Rh(111) surface, a hydrogen atom
adsorbs on the bridge site of Rh atoms of the C-terminated
Rh2C(001) surface. Figure 5d shows the free energy diagram
of H* on OH* precovered surfaces. A catalyst with a free
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energy of H* close to zero is considered to show a high
activity. The diagram indicates that the C-terminated Rh2C-
(001) surface is the most active for HER. These theoretical
studies revealed that alloying with C leads the different
hydrogen adsorption sites to provide suitable adsorption
energy for HER.
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CONCLUSIONS
■
Noble metal carbides have rarely been investigated. Our results
demonstrated the first rational and facile synthesis of a noble
metal carbide that did not use special conditions, such as a
high temperature or high pressure. A concurrent supply of
metal and carbon atoms, which is crucial for the new synthesis
method for noble metal carbides presented here, was realized
by adding an effective organic oxidant to the reduction
reaction. Moreover, the catalytic activity of Rh2C for HER is
higher than that of Rh. This new synthesis strategy could be
applied to other metal carbides by choosing an optimum
organic oxidant and create novel metal carbides.
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.9b09219.
Reaction conditions for the syntheses of the samples,
measurements conditions of UV−vis absorption meas-
urements and TEM images of the synthesis condition,
calculation conditions for catalytic activities, supporting
figures, and tables (PDF)
(11) Massalski, T. B.; Okamoto, H.; Subramanian, P. R.; Kacprzak,
L. Binary Alloy Phase Diagrams; ASM International: Materials Park,
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AUTHOR INFORMATION
■
Corresponding Authors
*kusada@kuchem.kyoto-u.ac.jp
*kitagawa@kuchem.kyoto-u.ac.jp
ORCID
Osamu Yamamuro: 0000-0002-5456-8932
Loku S. R. Kumara: 0000-0001-9160-6590
Osami Sakata: 0000-0003-2626-0161
Hiroshi Nakanishi: 0000-0001-9843-1527
Hiroshi Kitagawa: 0000-0001-6955-3015
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This research was supported by the ACCEL program, Japan
Science and Technology Agency (JST), JPMJAC1501. STEM
observations were performed as part of a program conducted
by the Advanced Characterization Nanotechnology Platform
sponsored by the MEXT of the Japanese Government.
Synchrotron XRD and HAXPES measurements were carried
out at SPring-8 under proposal No. 2016B1506, No.
2017A1165, No. 2017A4910, and No. 2017B4908. The
experiment at the Materials and Life Science Experimental
Facility of the J-PARC was performed under a user program
(Proposal No. 2017A0241). The activities of the INAMORI
Frontier Research Center, Kyushu University are supported by
KYOCERA Corporation.
F
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