Self-organized Ruthenium–Barium Core–Shell Nanoparticles on a Mesoporous Calcium Amide Matrix for Efficient Low-Temperature Ammonia Synthesis

Article abstract of DOI:10.1002/anie.201712398

A low-temperature ammonia synthesis process is required for on-site synthesis. Barium-doped calcium amide (Ba-Ca(NH₂)₂) enhances the efficacy of ammonia synthesis mediated by Ru and Co by 2 orders of magnitude more than that of a conventional Ru catalyst at temperatures below 300 °C. Furthermore, the presented catalysts are superior to the wüstite-based Fe catalyst, which is known as a highly active industrial catalyst at low temperatures and pressures. Nanosized Ru–Ba core–shell structures are self-organized on the Ba-Ca(NH₂)₂ support during H₂ pretreatment, and the support material is simultaneously converted into a mesoporous structure with a high surface area (>100 m² g^?1). These self-organized nanostructures account for the high catalytic performance in low-temperature ammonia synthesis.

Full text of DOI:10.1002/anie.201712398

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Accepted Article
Title: Self-organized Ruthenium-Barium Core-Shell Nanoparticles on a
Mesoporous Calcium Amide Matrix for Efficient Low-Temperature
Ammonia Synthesis
Authors: Masaaki Kitano, Yasunori Inoue, Masato Sasase, Kazuhisa
Kishida, Yasukazu Kobayashi, Kohei Nishiyama, Tomofumi
Tada, Shigeki Kawamura, Toshiharu Yokoyama, Michikazu
Hara, and Hideo Hosono
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201712398
Angew. Chem. 10.1002/ange.201712398
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10.1002/anie.201712398
Angewandte Chemie International Edition
COMMUNICATION
Self-organized Ruthenium-Barium Core-Shell Nanoparticles on a
Mesoporous Calcium Amide Matrix for Efficient Low-Temperature
Ammonia Synthesis
Masaaki Kitano+, Yasunori Inoue+, Masato Sasase, Kazuhisa Kishida, Yasukazu Kobayashi, Kohei
Nishiyama, Tomofumi Tada, Shigeki Kawamura, Toshiharu Yokoyama, Michikazu Hara*, and Hideo
Hosono*
Dedication ((optional))
site ammonia production. Low temperature NH3 synthesis has
been investigated using organometallic complexes,
Abstract: A low-temperature ammonia synthesis process is required
for on-site synthesis. Here, we report that barium-doped calcium
amide (Ba-Ca(NH2)2) significantly enhances the ammonia synthesis
activities of Ru and Co by two-orders of magnitude more than that of
a conventional Ru catalyst below 300ºC. Furthermore, the present
catalysts are superior to that of the wüstite-based Fe catalyst known
as a highly active industrial catalyst at low temperatures and
pressures. Nanosized Ru-Ba core-shell structures are self-organized
on the support during hydrogen pretreatment, and the support
material is simultaneously converted into a mesoporous structure with
a high surface area (>100 m2 g-1). These unique self-organized
nanostructures account for the high catalytic performance in low-
temperature ammonia synthesis.
photocatalysts, plasma, and electrochemical methods.[3] Although
some of these systems have succeeded in N2 reduction to NH3,
even at room temperature, the efficiency and stability are still
insufficient for industrial application. Ru-based catalysts are well
known as efficient catalysts that function under milder reaction
conditions than those used with the Fe-based catalyst process.[4]
However, Ru-based catalysts exhibit much lower activity than Fe-
based catalysts, especially at low reaction temperatures
(<370ºC).[2a] Ru-based catalysts are strongly inhibited by
hydrogen adsorption at low reaction temperatures;[5] therefore,
there have been many recent attempts to overcome the
drawbacks of Ru catalysts and realize low temperature NH3
synthesis.[6]
We have recently reported that electrides, hydrides (Ca2NH),
and calcium amides (Ca(NH2)2) significantly promote the activity
of Ru catalysts at low reaction temperatures with low activation
energy (50–60 kJ mol-1) when these materials are used as
catalyst supports.[7] These materials typically have very low work
functions (WFs; 2.3–3.5 eV), which provide electrons to Ru to
enhance the N2 dissociation. Furthermore, hydride ions (H‒ ions)
are formed in these catalysts and reversible reaction between H‒
ions and anionic electrons occurs at the Ru-support interfaces
during NH3 synthesis.[7c] This smooth anion exchangeability
suppresses hydrogen poisoning on the Ru surface. Another group
also recently reported that composite catalysts containing
transition metals (TM = Cr, Mn, Fe, Co) and hydride such as LiH
and BaH2 exhibit high catalytic activity and low activation energy
(45–65 kJ mol-1) for NH3 synthesis, in which metal-N-H species
(such as amides or imides) are formed as intermediates.[8] Thus,
metal-N-H systems are considered as effective supports or
promoter for NH3 synthesis.[9] However, further effort is required
to understand the origin of the high catalytic activity to develop a
new way to enhance the catalytic performance.
Herein, we demonstrate that the activity of Ru-loaded
Ca(NH2)2 for NH3 synthesis can be significantly enhanced by the
introduction of Ba into the Ca(NH2)2 matrix and H2 treatment,
which resulted in the highest activity ever reported for
heterogeneous catalysts. Detailed characterization reveals that
the Ru-Ba core-shell structure and the mesoporous structure of
the support are self-organized during the H2 pretreatment. Such a
unique structural change is a key factor of the high catalytic
performance.
N
itrogen (N2) activation to ammonia (NH3) is a key technology
for supporting human life because NH3 is used as a source for the
production of synthetic fertilizers, nitric acid, and nitrogen-
containing chemicals.[1] Owing to strong N≡N bond, industrial
NH3 synthesis (Haber-Bosh process) must be conducted using
promoted iron-based catalysts at high reaction temperatures
(400-500ºC).[2] Furthermore, high pressure (10-30 MPa) is
required due to thermodynamic limitations. As a result, large-
scale and robust plants are required for the Haber-Bosh process.
In the past few years, many researchers have focused on the
development of an efficient process for NH3 synthesis under
milder
reaction
conditions.
Low
temperatures
are
thermodynamically favorable for high NH3 production and small-
scale NH3 synthesis process has been in recent demand for on-
[*]
[*]
Prof. Dr. M. Kitano, Dr. M. Sasase, Dr. K. Kishida, Dr. Y. Kobayashi,
K. Nishiyama, Prof. Dr. T. Tada, Dr. S. Kawamura, Dr. T.
Yokoyama, Prof. Dr. H. Hosono
Materials Research Center for Element Strategy, Tokyo Institute of
Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503,
Japan
E-mail:hosono@msl.titech.ac.jp
Dr. Y. Inoue, Prof. Dr. M. Hara, Prof. Dr. H. Hosono
Laboratory for Materials and Structures, Tokyo Institute of
Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503,
Japan
Ba-doped Ca(NH2)2 was prepared by the reaction of Ca and
Ba metal with liquid NH3 at ‒40ºC, and the resultant solution was
heated at 100ºC for 1 h. The obtained powder was then combined
with Ru (to produce Ru/Ba-Ca(NH2)2) by heating with various Ru
precursors under H2 gas flow at 400ºC. As shown in Figure S1,
the Ru/Ba-Ca(NH2)2 catalyst prepared using ruthenium(ш)
acetylacetonate (Ru(acac)3) exhibits the highest activity for NH3
synthesis. Figure S2 shows the effect of Ba-doping on the
catalytic activity of Ru/Ba-Ca(NH2)2. A small amount of Ba-doping
E-mail: hara.m.ae@m.titech.ac.jp
Dr. K. Kishida, Dr. Y. Kobayashi, Prof. Dr. M. Hara, Prof. Dr. H.
Hosono
ACCEL, Japan Science and Technology Agency, 4-1-8 Honcho,
Kawaguchi, Saitama, 332-0012, Japan
These authors contributed equally to this work.
[+]
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201712398
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COMMUNICATION
Ru/Ba-Ca(NH2)2 is relatively resistant to
dry O2 (Figure S8). Although new catalytic
process needs to be developed, the
present catalyst has a great impact on the
study of ammonia synthesis.
The local structure of the Ru/Ba-
Ca(NH2)2 catalyst after NH3 synthesis was
evaluated using high-angle annular dark-
field scanning transmission electron
microscopy (HAADF-STEM) and energy-
dispersive X-ray spectroscopy (EDX). As
shown in Figure 2A, small-sized Ru
nanoparticles are highly dispersed and
epitaxially attached onto the support
material, which is analogous to our
previous
result
for
Ba-free
Figure 1. Temperature dependence of NH3 synthesis activity of various catalysts at (A) 0.1 MPa and (B)
0.9 MPa. The amount of metal loading for Ru/Ba-Ca(NH2)2, Cs-Ru/MgO, and Co/Ba-Ca(NH2)2 were 10,
10, and 8 wt%, respectively.
Ru/Ca(NH2)2.[7d] The size of the Ru
particles is mainly distributed in the range
of 1.5‒4.5 nm, which is much smaller
(3 at%) was effective to enhance the activity of the Ru/Ca(NH2)2
catalyst, although Ru/Ba(NH2)2 has negligible activity.
than those of Ru/Ca(NH2)2 and Ru/Ba(NH2)2 (Figures S9 and
S10). The catalytic properties of these materials are summarized
in Table 1. The difference in the NH3 synthesis rates of the tested
catalysts is much larger than that of the Ru surface area; therefore,
the high catalytic activity of Ru/Ba-Ca(NH2)2 cannot be explained
only by the number of surface Ru sites. The turnover frequency
(TOF) with Ru/Ba-Ca(NH2)2 is much superior to those of the other
Ru catalysts, which suggests that the intrinsic activity of the Ru
Figure 1 shows the NH3 synthesis activity of Ru(10
wt%)/Ba(3 at%)-Ca(NH2)2 as a function of reaction temperature at
0.1 and 0.9 MPa. The catalytic activity of Cs-Ru/MgO and an
industrial benchmark Fe catalyst with the wüstite structure (Figure
S3) are also shown in this figure. The wüstite-based catalyst is
known to be more active than the magnetite-based catalyst at low-
temperatures and low-pressures.[10] At atmospheric pressure (0.1
MPa), the activity of Cs-Ru/MgO, which is one of the most active
Ru catalysts reported to date,[5b, 11] exceeded that of the Fe-based
catalyst to a significant extent. However, this trend was reversed
below 300ºC. On the other hand, Ru/Ba-Ca(NH2)2 exhibited much
higher catalytic activity than the other catalysts, and the NH3
concentration formed above 340ºC reached thermodynamic
equilibrium. Furthermore, the catalytic activity of Ru/Ba-Ca(NH2)2
is much superior to those of other metal-amide based catalysts
(Figure S4), indicating significant promoting effect of Ba-Ca(NH2)2.
The catalytic activities of Ru/Ba-Ca(NH2)2 were increased
significantly at 0.9 MPa and the maximum NH3 synthesis rate
(60.4 mmol g-1 h-1) was recorded at 360ºC, which is ca. 6 times
higher than that of the Fe-based catalyst. Furthermore, Ru/Ba-
Ca(NH2)2 works as an efficient catalyst for NH3 synthesis below
300ºC, at which Cs-Ru/MgO exhibits negligibly low activity. For
instance, the NH3 synthesis rate (7.5 mmol g-1 h-1) of Ru/Ba-
Ca(NH2)2 at 260ºC is higher by two orders of magnitude than that
of Cs-Ru/MgO (0.072 mmol g-1 h-1). Ba-Ca(NH2)2 also effectively
promotes the activity of the Co catalyst, which exceeds the
catalytic performance of the Fe and Cs-Ru/MgO catalysts. It
should be noted that Ru and Co-loaded Ba-Ca(NH2)2 catalysts
exhibited significantly higher catalytic activity than state-of-the-art
catalysts reported to date, not only with respect to NH3 synthesis
rate, but also the NH3 yield (Table S1). These results demonstrate
that the Ba-Ca(NH2)2 support functions as an efficient promoter of
Ru and Co catalysts in low-temperature NH3 synthesis. The
activity of Cs-Ru/MgO was decreased slightly with an increase in
the reaction pressure at low temperatures (200-340ºC) (Figure
S5), which is attributed to hydrogen poisoning over the Ru catalyst.
Conventional Ru catalysts are known to be sensitive to hydrogen
adsorption in NH3 synthesis, especially at low reaction
temperatures, which results in the strong suppression of N2
dissociation over the Ru catalyst.[5] The activity of Cs-Ru/MgO is
consequently lower than that of the Fe-based catalyst at low
reaction temperatures and elevated pressure (Figure 1B). In
contrast, the activity of Ru/Ba-Ca(NH2)2 increased with the
reaction pressure, even at 300ºC (Figure S6), indicating high
resistance to hydrogen poisoning. Furthermore, the NH3 synthesis
rate of Ru/Ba-Ca(NH2)2 remained constant for 100 h (Figure S7),
and the total amount of produced NH3 reached 320 mmol (more
than 260 times that with Ba-Ca(NH2)2), demonstrating the high
durability of the catalyst. The activity of Ru/Ba-Ca(NH2)2 was
decreased when it was exposed to air before reaction, whereas
25
(A)
(B)
20
15
10
5
0
0
1
2
3
4
5
6
7
8
9
10
Particle size (nm)
(C)
(D)
(E)
5 nm
5 nm
Figure 2. (A) HAADF-STEM image and (B) corresponding particle size
distributions of Ru/Ba-Ca(NH2)2 after NH3 synthesis. (C) Enlarged image of
Ru particles on the catalyst surface. EDX elemental mapping images of (D)
Ba (yellow) and (E) Ca (green), where Ru is displayed as red.
Table 1. Catalytic properties of various catalysts with 10 wt% Ru
d
Am
[m2 g-1][a]
rNH3
TOF
[s-1]
Catalyst
[nm][a]
[mmol g-1 h-1][b]
2.7
31.8
4.4
178.5
15.2
109.5
33.9
90.9
51.7
92.7
49.0
13.4 x 10-3
157.9 x 10-3
5.3 x 10-3
17.1 x 10-3
0.3 x 10-3
0.6 x 10-3
0.4 x 10-3
0.8 x 10-3
Ru/Ba-Ca(NH2)2
Ru/Ca(NH2)2
Ru/Ba(NH2)2
Cs-Ru/MgO
23.3
5.7
14.2
5.3
0.3
9.3
5.2
0.4
9.8
[a]Average Ru particle size (d) and the surface area of Ru per Ru weight
(Am) were determined by STEM measurements (upper) and H2
chemisorption method (lower). [b]NH3 synthesis rate (rNH3); conditions:
pressure (0.9 MPa), temperature (300ºC).
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10.1002/anie.201712398
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catalyst was significantly enhanced by the Ba-Ca(NH2)2 system.
The surface area of Ru on each catalyst estimated by H2
chemisorption is much smaller than that determined by STEM
observation. This difference originates from the coverage of the
Ru surface with the support material. Among the tested catalysts,
Ru/Ba-Ca(NH2)2 has the lowest H2 chemisorption sites although
it has much smaller Ru particles observed in STEM image than
the other catalysts. It should be noted that the Ru nanoparticles
on Ba-Ca(NH2)2 are covered by a thin ca. 1 nm layer, which leads
to the clear contrast between the core and shell (Figure 2C). This
overlayer accounts for the low H2 chemisorption, while it would
possess porous structure to adsorb reactant gas molecules. EDX
elemental mapping showed that both Ba and Ca species are
distributed on the Ru particles (Figures 2D, E). The EDX line
profile analysis also revealed that Ru is located at core region
whereas the Ca and Ba signals are distributed throughout the
entire particle (Figure S11). On the other hand, the core-shell
structure was not observed for Ba-free Ru/Ca(NH2)2 and Ca-free
Ru/Ba(NH2)2 (Figures S9 and S10). Accordingly, the lattice
mismatch between Ca(NH2)2 and Ba(NH2)2 would contribute to
the formation of the overlayer on the Ru nanoparticles.
Ca(NH2)2 decreased after H2 pretreatment (Table S2), which
indicates sintering of the material during heat treatment. Notably,
the isotherm for the Ru/Ba-Ca(NH2)2 catalyst has a distinct
hysteresis loop at P/P0 = 0.5-0.9 (type IV), which indicates that a
high density of mesopores is formed in this material during H2
pretreatment (Figure 3B). The surface areas of the Ru/Ba-
Ca(NH2)2 catalysts increased steeply after H2 treatment at 300ºC,
as shown in Figure 3C. The drastic increase in the surface area
is attributed to the formation of a mesoporous structure (Figure
S13). As shown in Figure S14, Ru(acac)3 was decomposed at
200–250ºC in a H2 atmosphere. These results suggest that Ru
nanoparticles are formed on the Ba-Ca(NH2)2 support above
250ºC during H2 pretreatment, and a mesoporous structure is
subsequently produced by partial decomposition of the Ba-
Ca(NH2)2 support by the Ru catalyst. It should be noted that no
such structural change was observed for Ru/Ba(NH2)2 (Figure
S15, Table S2). Thus, we can conclude that the unique
mesoporous structure is formed only by the reaction of Ru
nanoparticles with Ca(NH2)2 during the H2 pretreatment.
Furthermore, the mesoporous structure remained unchanged
during 113 h of reaction, demonstrating the excellent stability of
the catalyst (Figure 3D).
The surface composition of the catalyst was examined using
X-ray photoelectron spectroscopy (XPS). As summarized in Table
S2, the Ba species is homogeneously distributed in the as-
prepared Ba-Ca(NH2)2 (Ba/Ca = 0.03). In contrast, the Ba/Ca ratio
of Ru/Ba-Ca(NH2)2 was much larger than 0.03, indicating that Ba
species preferentially migrate to the catalyst surface via Ru, which
leads to the formation of an overlayer on the Ru particles. X-ray
diffraction (XRD) patterns for Ca(NH2)2, Ba(NH2)2, Ba-Ca(NH2)2,
and Ru/Ca(NH2)2 after reaction (300ºC, 113 h), are shown in
Figure S12. The XRD pattern of Ba-Ca(NH2)2 was almost identical
to that of Ca(NH2)2, while weak peaks due to Ba(NH2)2 were also
observed. The diffraction peaks significantly decreased, and
Ca2NH or CaNH phases were formed after the reaction, implying
that the mixture of these crystal structures was formed after the
reaction. In addition, broad peaks due to Ru were also observed,
which indicates the formation of small-sized Ru particles.
Kinetic analyses were conducted to elucidate the reaction
mechanism for NH3 synthesis over Ru/Ba-Ca(NH2)2. Table S3
shows that the apparent activation energy for NH3 synthesis over
Ru/Ba-Ca(NH2)2 (59.4 kJ mol-1) is almost one-half that of Cs-
Ru/MgO (124.3 kJ mol-1), which provides evidence that N2
cleavage over Ru is effectively promoted by Ba-Ca(NH2)2. This
value is close to those for Ru-loaded electrides (50-60 kJ mol-1)[7a,
7c]
which implies a similar reaction mechanism, in which the
formation of N-Hx species is the rate-limiting step.[7b] The rate-
determining step (RDS) for NH3 synthesis over the Ru/Ba-
Ca(NH2)2 catalyst was studied based on the Langmuir-
Hinshelwood mechanism.[12] As shown in Figures 4 and S16, the
experimental data is well-fitted with the modeled rates, based on
the assumption that the N-Hx (x = 1–3) formation reaction is the
RDS. These results demonstrate that the RDS for NH3 synthesis
Nitrogen adsorption-desorption isotherms and Barrett-
Joyner-Halenda (BJH) pore size distributions of the catalyst are
presented in Figure 3. The isotherms for Ba-Ca(NH2)2 before and
after H2 pretreatment at 400ºC are similar in shape to type II
isotherms in IUPAC classification terms. The surface area of Ba-
(A)
(B)
N(ad) + H(ad) → NH(ad)
N2(ad) → 2N(ad)
5
4
3
2
1
0
5
4
3
2
1
0
R2 = 0.847
R2 = 0.999
0
1
2
3
4
5
0
1
2
3
4
5
Experimental reaction rate (mmol g-1 h-1)
(C)
7
6
5
4
3
2
1
0
m/z = 30, 15N2
m/z = 18, 15NH3
m/z = 17, 15NH2
m/z = 16, 15NH
0
1
2
3
4
5
6
Figure 3. (A) Nitrogen adsorption-desorption isotherms and (B) BJH pore
size distribution curves for (a) Ru/Ba-Ca(NH2)2, (b) Ba-Ca(NH2)2, and (c)
Ba-Ca(NH2)2 after heat treatment at 400ºC and 0.1 MPa for 3h under H2
gas flow. (C) Surface area of Ru/Ba-Ca(NH2)2 after heat treatment at
various temperatures under H2 gas flow. (D) Nitrogen adsorption-
desorption isotherms for (a) Ru/Ba-Ca(NH2)2 after NH3 synthesis at 300ºC
for 20 h and 113 h.
Time (h)
Figure 4. Best fit results for reaction rates over Ru/Ba-Ca(NH2)2 at 260ºC
and 0.1 MPa with respect to the rate equations derived with different RDS
(A: N2 activation, B: NH formation). (C) Reaction time profiles for NH3
synthesis from 15N2 and H2 over Ru/Ba-Ca(NH2)2 at 340ºC.
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10.1002/anie.201712398
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COMMUNICATION
over Ru/Ba-Ca(NH2)2 could be any formation steps of N-Hx
In summary, we have developed highly active and stable
catalysts containing a Ba-Ca(NH2)2 support and Ru or Co
nanoparticles for low-temperature NH3 synthesis. The catalytic
activity of Ru/Ba-Ca(NH2)2 is ca. 6 and 100 times higher than that
of the industrial benchmark Fe catalyst (at 340ºC) and Cs-
Ru/MgO (at 260ºC), respectively. Hydrogen pretreatment of the
species rather than the N2 dissociation step.[13]
The reaction orders with respect to N2, H2, and NH3 are also
listed in Table S3. Both the N2 and H2 reaction orders for Ru/Ba-
Ca(NH2)2 are close to unity, and the reaction order of NH3 is -0.92.
This observation suggests that N-Hx species populate the catalyst
surface more densely than N and H adatoms. This tendency is
completely distinct from that for a conventional Ru catalyst such
as Cs-Ru/MgO. The NH3 order (-0.35) was less negative than the
H2 order (-0.50), which indicates that the surface of Cs-Ru/MgO
is poisoned by hydrogen. Most Ru catalysts have a negative
reaction order with respect to H2 at low temperatures, whereas
Ru/Ba-Ca(NH2)2 has a positive H2 order, even at 260ºC. The
unusual reaction order for Ru/Ba-Ca(NH2)2 is attributed to the
unique core-shell structure shown in Figure 2C. Reversible
exchange between hydride ions and electrons occur at Ru-
Ca(NH2)2 interfaces, which imparts high tolerance to hydrogen
poisoning.[7d] A similar reaction is considered to occur between Ru
and Ba-Ca(NH2)2 shell. Next, the possibility of NH3 formation from
the Ba-Ca(NH2)2 support was investigated using isotopic 15N2 and
H2. As 15N2 decreased, the signals measured for m/z =18 and 17
increased with the intensity ratio in the range of 0.77-0.82 (Figure
4), where these signals were derived from 15NH3. On the other
hand, 14NH3 was not produced by the reaction, which confirms
that N atoms in Ba-Ca(NH2)2 are not used for NH3 formation, but
gas-phase N2 molecules are activated over the Ru catalyst by
electron donation from the Ba-Ca(NH2)2 support.
Density functional theory (DFT) calculations were performed
to investigate the electron-donation ability of Ba-Ca(NH2)2. Table
S4 summarizes the WFs and formation energies of NH2 defects
(ENH2def) on Ca(NH2)2 (101) and Ba(NH2)2 (101) surfaces. The
calculated WF for Ba(NH2)2 is smaller than that for Ca(NH2)2,
which indicates the superior electron donating ability of Ba(NH2)2
over Ca(NH2)2. When an NH2 unit was removed from the surface
of each amide, the WFs of Ca(NH2)2 and Ba(NH2)2 were
significantly reduced to 2.1 and 1.9 eV, respectively. Such a low
WF is attributed to the formation of anionic electrons confined at
the sites of NH2 vacancies. Similar results were observed for other
surfaces (Table S5). The calculated ENH2def for Ba(NH2)2 is much
smaller than that for Ca(NH2)2, which suggests that the formation
of anionic electrons by NH2 desorption as ammonia is much easier
for Ba(NH2)2 than for Ca(NH2)2. Thus, by combining DFT
calculations and STEM observations, low WF Ba(NH2)2-x is
formed near the Ru surface during H2 pretreatment, which
facilitates NH3 synthesis over Ru/Ba-Ca(NH2)2 via electron
donation from Ba(NH2)2-x to Ru nanoparticles. Although the WF of
anionic electrons at the Ba(NH2)2 surface is lower than that at
Ca(NH2)2, the catalytic activity of the former is much lower than
that of the latter (Table 1). No mesopore formation or epitaxial
growth of Ru nanoparticles occurred for pure Ba(NH2)2, which
could be due to the large lattice mismatch between Ru and
Ba(NH2)2.
Ru/Ba-Ca(NH2)2 catalyst provides
a mesoporous support
structure and induces the formation of Ru-Ba core-shell structures.
This unique structure is self-organized and stable for a long time.
The overlayers derived from the Ba-Ca(NH2)2 support suppress
H2 poisoning at the Ru surface at low reaction temperatures via
reversible exchange reaction between hydride ions and electrons.
In addition, the formation of low-WF Ba(NH2)2-x at the catalyst
surface promotes N2 dissociation over the Ru catalyst, which
shifts the bottleneck in NH3 synthesis from N2 dissociation to N-
Hx bond formation. The present results demonstrate that alkaline-
earth amide materials can realize the optimal potential of
transition metal catalysts for low-temperature NH3 synthesis.
Acknowledgements
This work was supported by a fund from the Accelerated
Innovation Research Initiative Turning Top Science and Ideas into
High-Impact Values (ACCEL) program of the Japan Science and
Technology Agency (JST), and the ENEOS Hydrogen Trust Fund.
We appreciate the technical assistance of M. Okunaka and S.
Fujimoto.
Keywords: Low-temperature ammonia synthesis • core-shell
structure • mesoporous • alkaline earth metal amide • ruthenium
[1]
[2]
J. M. Thomas, W. J. Thomas, Principles and practice of heterogeneous
catalysis, John Wiley & Sons, 2014.
a) H. Liu, Ammonia synthesis catalysts: innovation and practice, World
Scientific, 2013; b) T. Kandemir, M. E. Schuster, A. Senyshyn, M.
Behrens, R. Schlögl, Angew. Chem. Int. Ed. 2013, 52, 12723-12726.
a) D. V. Yandulov, R. R. Schrock, Science 2003, 301, 76-78; b) K.
Arashiba, Y. Miyake, Y. Nishibayashi, Nat. Chem. 2011, 3, 120-125; c)
K. T. Ranjit, T. K. Varadarajan, B. Viswanathan, J. Photochem.
Photobiol., A 1996, 96, 181-185; d) G. L. Wendt, J. E. Snyder, J. Am.
Chem. Soc. 1928, 50, 1288-1292; e) C. J. M. van der Ham, M. T. M.
Koper, D. G. H. Hetterscheid, Chem. Soc. Rev. 2014, 43, 5183-5191.
a) K. Aika, A. Ozaki, H. Hori, J. Catal. 1972, 27, 424-431; b) A. Ozaki,
Acc. Chem. Res. 1981, 14, 16-21; c) K. Aika, Catal. Today 2017, 286,
14-20; d) G. Prieto, F. Schüth, Angew. Chem. Int. Ed. 2015, 54, 3222-
3239; e) N. Saadatjou, A. Jafari, S. Sahebdelfar, Chem. Eng. Commun.
2015, 202, 420-448.
[3]
[4]
[5]
[6]
a) S. E. Siporin, R. J. Davis, J. Catal. 2004, 225, 359-368; b) F. Rosowski,
A. Hornung, O. Hinrichsen, D. Herein, M. Muhler, G. Ertl, Appl. Catal., A
1997, 151, 443-460.
The improvement of the catalytic activity and stability of
Ru/Ca(NH2)2 by Ba-doping has been already reported in our
previous work,[7d] in which Ru/Ba-Ca(NH2)2 prepared by using
Ru3(CO)12 was pretreated at 340ºC. The catalytic activity was not
largely improved although the catalyst stability was significantly
improved. The XPS analyses revealed that the surface Ba/Ca
ratio of the Ru/Ba-Ca(NH2)2 in the current work is much higher
than that of the previous catalyst (Table S2). The surface Ba
species mainly contribute to the overlayer formation on the Ru
nanoparticles (Figure 2), suggesting that the catalytic activity of
surface Ru sites on the current Ru/Ba-Ca(NH2)2 are effectively
enhanced by the surface Ba species rather than the case of the
previous one. In addition, the present Ru/Ba-Ca(NH2)2 has a
larger surface area (100 m2 g-1) than the previous catalyst (58 m2
g-1). The formation of Ru-Ba core-shell structure and the
mesoporous structure with a large surface area can be caused by
the H2 pretreatment of Ba-Ca(NH2)2 with Ru(acac)3 at 400ºC
(Figure S17).
a) K. Sato, K. Imamura, Y. Kawano, S. Miyahara, T. Yamamoto, S.
Matsumura, K. Nagaoka, Chem. Sci. 2017, 8, 674-679; b) R. Manabe, H.
Nakatsubo, A. Gondo, K. Murakami, S. Ogo, H. Tsuneki, M. Ikeda, A.
Ishikawa, H. Nakai, Y. Sekine, Chem. Sci. 2017, 8, 5434-5439.
a) M. Kitano, Y. Inoue, Y. Yamazaki, F. Hayashi, S. Kanbara, S. Matsuishi,
T. Yokoyama, S. W. Kim, M. Hara, H. Hosono, Nat. Chem. 2012, 4, 934-
940; b) M. Kitano, S. Kanbara, Y. Inoue, N. Kuganathan, P. V. Sushko,
T. Yokoyama, M. Hara, H. Hosono, Nat. Commun. 2015, 6, 6731; c) M.
Kitano, Y. Inoue, H. Ishikawa, K. Yamagata, T. Nakao, T. Tada, S.
Matsuishi, T. Yokoyama, M. Hara, H. Hosono, Chem. Sci. 2016, 7, 4036-
4043; d) Y. Inoue, M. Kitano, K. Kishida, H. Abe, Y. Niwa, M. Sasase, Y.
Fujita, H. Ishikawa, T. Yokoyama, M. Hara, H. Hosono, ACS Catal. 2016,
6, 7577-7584.
[7]
[8]
a) P. K. Wang, F. Chang, W. B. Gao, J. P. Guo, G. T. Wu, T. He, P. Chen,
Nat. Chem. 2017, 9, 64-70; b) W. B. Gao, P. K. Wang, J. P. Guo, F.
This article is protected by copyright. All rights reserved.

10.1002/anie.201712398
Angewandte Chemie International Edition
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Chang, T. He, Q. R. Wang, G. T. Wu, P. Chen, ACS Catal. 2017, 7, 3654-
3661.
[9]
a) P. L. Bramwell, S. Lentink, P. Ngene, P. E. de Jongh, J. Phys. Chem.
C 2016, 120, 27212-27220; b) R. Michalsky, A. M. Avram, B. A. Peterson,
P. H. Pfromm, A. A. Peterson, Chem. Sci. 2015, 6, 3965-3974; c) K. Aika,
A. Ozaki, J. Catal. 1974, 35, 61-65.
[10] H. Z. Liu, X. N. Li, Z. N. Hu, Appl. Catal., A 1996, 142, 209-222.
[11] a) K. Aika, A. Ohya, A. Ozaki, Y. Inoue, I. Yasumori, J. Catal. 1985, 92,
305-311; b) K. Aika, T. Takano, S. Murata, J. Catal. 1992, 136, 126-140.
[12] I. Langmuir, T. Faraday. Soc. 1922, 17, 0621-0654.
[13] Y. Kobayashi, M. Kitano, S. Kawamura, T. Yokoyama, H. Hosono, Catal.
Sci. Technol. 2017, 7, 47-50.
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COMMUNICATION
Entry for the Table of Contents (Please choose one layout)
Layout 1:
250
Ru-Ba core-shell nanoparticles
Masaaki Kitano,
and a mesoporous calcium amide
matrix are self-organized during
H2 treatment of Ru/Ba-Ca(NH2)2,
which imparts much higher
catalytic performance for low-
temperature ammonia synthesis
than other Ru catalysts and an
industrial Fe catalyst.
Yasunori Inoue, Masato
Sasase, Kazuhisa
Kishida, Yasukazu
Kobayashi, Kohei
Nishiyama, Tomofumi
Tada, Shigeki
Kawamura, Toshiharu
Yokoyama, Michikazu
Hara, and Hideo
200
150
Self-organizedstructure
100
50
0
Ru/Ba-Ca(NH2)2
Ba-Ca(NH2)2
0
0.2
0.4
0.6
0.8
1.0
Hosono*
Relative pressure (P/P0)
Page 1. – Page 5.
Self-organized
Ruthenium-Barium Core-
Shell Nanoparticles on a
Mesoporous Calcium
Amide Matrix for Efficient
Low-Temperature
Ammonia Synthesis
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