Metal-support interaction-modulated catalytic activity of Ru nanoparticles on Sm2O3for efficient ammonia decomposition

Article abstract of DOI:10.1039/d1cy00080b

Development of efficient catalysts is important for production of CO_x-free hydrogen from ammonia decomposition. Herein, a highly active catalyst with uniform Ru nanoparticles dispersed on Sm₂O₃nanorods is developedviaa facile precipitation method. Under a relatively high weight hourly space velocity of 30?000 mL g^?1h^?1, Ru/Sm₂O₃can catalyze ammonia decomposition reaction with a H₂formation rate as high as 25.9 mmol g_cat^?1min^?1at 450 °C, which is superior to most of conventional oxide-supported Ru catalysts. Moreover, we find that the activity of Ru/Sm₂O₃is highly dependent on the preparation method. The Ru/Sm₂O₃catalyst prepared by precipitation method exhibits much higher activity than the Ru/Sm₂O₃analogues by impregnation and solid milling methods. Characterization results demonstrate that the superior catalytic performance of Ru/Sm₂O₃achieved from precipitation method should originate from much enhanced interaction between Ru nanoparticles and Sm₂O₃support. These findings offer promise to explore Ru/Sm₂O₃catalyst as a new kind of efficient catalyst for ammonia decomposition and to improve the catalytic performance by modulating the metal-support interaction of the catalyst.

Full text of DOI:10.1039/d1cy00080b

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Metal–support interaction-modulated catalytic
activity of Ru nanoparticles on Sm2O3 for efficient
ammonia decomposition†
Cite this: Catal. Sci. Technol., 2021,
11, 2915
a
Xilun Zhang,ab Lin Liu,
Teng He
Ji Feng,ab Xiaohua Ju,a Jiemin Wang,ac
*
*
a
a
and Ping Chen
Development of efficient catalysts is important for production of COx-free hydrogen from ammonia
decomposition. Herein, a highly active catalyst with uniform Ru nanoparticles dispersed on Sm2O3
nanorods is developed via a facile precipitation method. Under a relatively high weight hourly space
velocity of 30000 mL g−1 h−1, Ru/Sm2O3 can catalyze ammonia decomposition reaction with a H2
formation rate as high as 25.9 mmol gcat min−1 at 450 °C, which is superior to most of conventional
−1
oxide-supported Ru catalysts. Moreover, we find that the activity of Ru/Sm2O3 is highly dependent on the
preparation method. The Ru/Sm2O3 catalyst prepared by precipitation method exhibits much higher activity
than the Ru/Sm2O3 analogues by impregnation and solid milling methods. Characterization results
demonstrate that the superior catalytic performance of Ru/Sm2O3 achieved from precipitation method
should originate from much enhanced interaction between Ru nanoparticles and Sm2O3 support. These
findings offer promise to explore Ru/Sm2O3 catalyst as a new kind of efficient catalyst for ammonia
decomposition and to improve the catalytic performance by modulating the metal–support interaction of
the catalyst.
Received 15th January 2021,
Accepted 11th February 2021
DOI: 10.1039/d1cy00080b
rsc.li/catalysis
Ru-based catalysts are highly active for the decomposition
of NH3.10,11 During the past years, Ru nanoparticles (NPs)
Introduction
Due to its abundance, renewability and zero-emission,
hydrogen has been given extensive attention as an energy
carrier. However, the storage of hydrogen is still a great
challenge in the scientific community.1,2 Ammonia, a basic
chemical for production of fertilizers, has been considered as
one of the most promising carriers for hydrogen. In this
regard, NH3 decomposition is expected to be a simple and
efficient approach for generation of high-purity COx-free H2
from NH3.3,4 To date, a variety of supported metal catalysts
(e.g. Ru, Fe, Co, Ni) have been extensively studied for NH3
decomposition reaction.5–9 However, decomposition of NH3
in the presence of a catalyst is still greatly hampered by the
sluggish kinetics. Therefore, designing an efficient catalyst
that performs well at relatively low temperatures is urgently
needed for the effective and economic production of H2 from
NH3.
loaded on high surface area supports have been widely
investigated in NH3 decomposition. Among them, oxide-
supported Ru catalysts have been explored and display high
activity and stability for NH3 decomposition. Many kinds of
oxides, including SiO2, TiO2, Al2O3, MgO, La2O3, CeO2, ZrO2
and barium hexaaluminate, have been used as supports for
Ru-based catalysts.5,12–17 To expand the scope of Ru-based
catalysts, developing new efficient oxide-supported Ru
catalysts and understanding the relationship between the
activity and structure of these catalysts are needed.
In oxide-supported metal catalysts, the function of oxide
supports is not only to disperse and stabilize metal particles
but also to generate interface phenomena through metal–
oxide interaction. Controlling metal–support interaction has
been recognized as an efficient strategy to tune the surface
geometries and electronic structure of supported metal
particles and further enhance the catalytic performances of
supported metal catalysts.18,19 Discovering support material
which can effectively enhance Ru dispersion and metal–oxide
interaction is a major concern for construction of efficient
Ru-based catalysts. In recent years, rare earth oxides such as
Pr6O11, La2O3 and CeO2 have attracted increasing interests as
superior supports for Ru-based NH3 decomposition
catalysts.14,20–29 As a typical rare earth oxide, samarium oxide
a Dalian National Laboratory for Clean Energy, State Key Laboratory of Catalysis,
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023,
China. E-mail: liulin@dicp.ac.cn, pchen@dicp.ac.cn
b University of Chinese Academy of Sciences, Beijing 100049, China
c Zhang Dayu School of Chemistry, Dalian University of Technology, Dalian
116024, China
† Electronic supplementary information (ESI) available: Fig. S1–S6. See DOI:
10.1039/d1cy00080b
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(Sm2O3) has been applied as a catalyst for oxidative coupling
of methane,30,31 oxidation of ethane and ethylene,32 CO
hydrogenation reaction33 and dehydration of alcohols.34
Because of its intrinsic features of chemical stability, high
melting point and low volatility, Sm2O3 has also been applied
as a support, promoter, stabilizer or dopant in a variety of
Au-, Ru-, Pt- and Rh-based catalysts.35–38 However, to date,
little attention has been paid to the utilization of Sm2O3 as a
catalyst support for Ru-based catalysts in NH3
decomposition.
In this paper, we explored Sm2O3 as a support and
developed an efficient Ru/Sm2O3 catalyst by deposition of
uniform Ru NPs on Sm2O3 through a precipitation method.
The catalyst has been characterized and evaluated in NH3
decomposition reaction. The Ru/Sm2O3 catalyst exhibits high
activity and stability for the decomposition of NH3 at
relatively low temperatures (400–450 °C). The effect of metal–
support interaction on the activity of the Ru/Sm2O3 catalyst
was also studied in detail. By synthesizing Ru/Sm2O3
analogues consisting of nearly identical Ru NPs and Sm2O3
nanorods via impregnation and solid milling methods, we
find that the catalytic activity of the Ru/Sm2O3 catalysts
differs substantially, which manifests the crucial role of the
interaction between Ru and Sm2O3 in regulating the activity
of the Ru/Sm2O3 catalysts. These findings can inspire the
development of new efficient oxide-supported Ru catalysts for
NH3 decomposition by modulating the metal–support
interaction of the catalysts.
was also prepared with the same procedure as that of the Ru/
Sm2O3-p sample. The obtained sample was reduced in a 5%
H2/Ar stream at 500 °C for 2 h, and the reduced sample was
denoted as 4% Ru/SBA-15-p.
Ru/Sm2O3 catalyst with a Ru mass loading of 4% was also
prepared using an impregnation method. In
a typical
synthesis, 0.35 g of Sm(OH)3 was impregnated with an
appropriate amount of ethanol solution of RuO2 NPs. The
mixture was sonicated for 30 min. The suspension was
stirred at 25 °C until the ethanol was evaporated. The
obtained sample was reduced in a 5% H2/Ar stream at 500 °C
for 2 h, and the reduced sample was denoted as 4% Ru/
Sm2O3-i.
A 4% Ru/Sm2O3 reference catalyst was also prepared by a
solid milling method with the use of Sm(OH)3 and RuO2 NPs.
An appropriate amount of RuO2 and Sm(OH)3 was milled in
a mortar by hand for 10 min. The obtained sample was
reduced in a 5% H2/Ar stream at 500 °C for 2 h, and the
reduced sample was denoted as 4% Ru/Sm2O3-m.
Characterization
Powder X-ray diffraction (XRD) patterns of the catalysts were
measured on an X'Pert Pro (PANAlytical) diffractometer with
Cu Kα radiation, operating at 40 kV and 40 mA.
Nitrogen sorption was performed at −196 °C using a
Micromeritics ASAP 2020 analyser after evacuation at 250 °C
for 2 h. The specific surface area of the sample was
calculated by the Brunauer–Emmett–Teller (BET) method.
The actual Ru loadings of the catalysts were determined
by inductively coupled plasma optical emission spectroscopy
(ICP-OES, Optima 7300DV, Perkin-Elmer, USA).
Experimental
Preparation of the Ru/Sm2O3 catalysts from different
methods
Transmission electron microscopy (TEM) images were
taken on a JEOL 2100X microscope.
Sm(OH)3 nanorods were synthesized by a hydrothermal
method according to the previous literature.39
H2 temperature-programmed reduction (H2-TPR) profiles
RuO2 NPs were prepared using a precipitation method. In
a typical synthesis, 0.06 g of KOH was dissolved in 30 mL of
water. Then, 10 mL of RuCl3 solution (0.015 M) was slowly
added to the solution under vigorous stirring. The resulting
mixture was kept at room temperature for 2 h under stirring.
The obtained product was filtered and washed with excess
water and ethanol. After that, the product was dried in an
oven at 80 °C for 8 h.
Ru/Sm2O3-p catalysts with Ru mass loadings of 1–4 wt%
were prepared by precipitation of aqueous solutions of RuCl3
with KOH solution. Briefly, 0.35 g of Sm(OH)3 was dispersed
into 30 mL of water containing 0.06 g of KOH. After stirring
for 30 min, 10 mL of RuCl3 solution (0.015 M) was slowly
added to the above solution under vigorous stirring. After
stirring at room temperature for 2 h, the precipitates were
filtered, washed with water, and finally dried at 80 °C for 8 h.
The obtained sample was reduced in a 5% H2/Ar stream at
500 °C for 2 h, and the reduced sample was denoted as 1–4%
Ru/Sm2O3-p.
of the catalysts were collected on a fixed-bed reactor
equipped with a gas chromatograph. About 50 mg of the
catalyst was loaded into a tubular quartz reactor. The analysis
was performed in a 5% H2/Ar stream (30 mL min−1), ramping
the temperature from room temperature to 800 °C at a
heating rate of 10 °C min−1.
The Ru dispersion of the catalyst was determined by CO
chemisorption on an AutoChem II 2920 instrument
(Micromeritics). About 100 mg of the catalyst was loaded and
reduced with 10% H2/Ar (30 mL min−1) at 400 °C for 1 h, and
then it was purged with He (30 mL min−1) for another 1 h.
After the sample was cooled to 50 °C in He, a high-purity CO
pulse was introduced. The CO uptake was measured using a
gas chromatograph equipped with a TCD detector. The Ru
dispersion was calculated by assuming
stoichiometry of 1 : 1.
a
CO : Ru
An X-ray photoelectron spectroscopy (XPS) study was
performed on an ESCALAB MK-II spectrometer using
monochromatic Al Kα radiation. Carbonaceous C 1s line
(284.8 eV) was used as the reference to calibrate the binding
energy.
For comparison, a sample, in which Ru NPs dispersed on
an inert SBA-15 support with a Ru mass loading of 4 wt%,
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Catalytic performance measurement
The catalytic activity for the decomposition of ammonia was
studied in a fixed-bed quartz reactor under atmospheric
pressure. Prior to the reaction, the dried catalysts (catalyst: 50
mg, 20–40 mesh) were reduced in an NH3 flow at 500 °C for
2 h. The gas reactant was pure NH3 with a flow rate of 25 mL
min−1, corresponding to a weight hourly space velocity
(WHSV) of 30 000 mL gcat h−1. The reaction temperature
−1
was set in the range of 300–500 °C. Product gas composition
was analysed online using a GC-7890 gas chromatograph
equipped with a TCD detector. The conversion of NH3 (CNH
)
3
was calculated by the following equation:
A
− A
NH3;in
NH3;in
NH3;out
CNH ð%Þ ¼
3
A
where ANH ,in and ANH ,out are the total amount of NH3 in the
3
3
inlet and outlet, respectively. The H2 formation rate was
calculated from the NH3 conversion (CNH ) as shown below:
Fig. 1 Schematic illustration of (a) the preparation of the RuO2 NPs;
deposition of the Ru NPs on the Sm2O3 support by the different
methods: (b) precipitation method, (c) impregnation method and (d)
solid milling method.
3
À
Á
WHSV × CNH × 1:5
H2 formation rate mmol gcat min−1
¼
−1
3
24:45 × 60
where WHSV is the weight hourly space velocity (30 000 mL
gcat h−1), CNH is the conversion of NH3, and 24.45 mL
−1
3
the catalyst precursors are nearly identical with that of
Sm(OH)3 support precursor (JCPDS 01-083-2036), indicating
that the introduction of RuO2 NPs does not cause any
structural change in Sm(OH)3.
mmol−1 is the molar volume of the gas at 25 °C and 1 atm.
TOFH was calculated from the H2 formation rate divided
by the exposed surface Ru atoms on the catalysts, shown as
follows:
2
The three catalyst precursors were reduced at 500 °C to
obtain the Ru/Sm2O3 catalysts, which were denoted as 4%
Ru/Sm2O3-p, 4% Ru/Sm2O3-i and 4% Ru/Sm2O3-m,
respectively. During the reduction process, the Sm(OH)3
precursor was decomposed into Sm2O3 and the RuO2 NPs
were reduced to Ru NPs. The surface areas of 4% Ru/Sm2O3-p
and 4% Ru/Sm2O3-i are 38.4 and 43.8 m2 g−1, respectively,
which are larger than that of the pristine Sm2O3 support
H2 formation rate × 10−3
¼
60 × Ru content × DRu=Ruat
À
Á
TOFH s−1
2
where DRu is the Ru dispersion of the catalyst determined by
−1
CO chemisorption and H2 formation rate (mmol gcat
min−1), and Ruat is the molar mass of Ru (101.07 g mol−1).
(25.8 m2 g−1
, Table 1). Corresponding N2 adsorption–
Results and discussion
desorption isotherms are shown in Fig. S5.† The increase in
surface area may be attributed to the reconstruction of
support in the solution during the preparation processes.
The structures of the different 4% Ru/Sm2O3 catalysts were
studied by XRD. As shown in Fig. 2, characteristic diffraction
peaks at 2θ of 28.3°, 32.8°, 47.0° and 55.8° can be observed
for all the 4% Ru/Sm2O3 samples, which match well with the
diffraction peaks from the (222), (400), (440) and (622) lattice
planes of cubic Sm2O3 (JCPDS 00-015-0813). The XRD
patterns of the different 4% Ru/Sm2O3 samples show similar
diffraction peaks to that of the Sm2O3 support. No observable
diffraction peaks corresponding to the Ru species can be
detected, indicating the high dispersion of Ru NPs in the
three 4% Ru/Sm2O3 samples.
Structural characterization of the Ru/Sm2O3 catalysts
In this study, RuO2 NPs were firstly synthesized using a
precipitation method (Fig. 1a). Fig. S1a and b† show the TEM
images of the RuO2 NPs at different scales. The spherical and
uniform-sized RuO2 NPs with an average particle size of 1.9
0.2 nm can be facilely obtained (Fig. S1c†). The high
resolution TEM image shows that the RuO2 NPs are in the
partially crystalline state (Fig. S1a†), which agrees well with
the XRD result (Fig. S2†). Here, well-crystallized Sm(OH)3
nanorods with lengths of 100–600 nm and widths of 30–60
nm (Fig. S3†) were used as support precursors to disperse
and support RuO2 NPs. Three preparation methods
including precipitation, impregnation and solid milling were
applied to disperse RuO2 NPs onto the Sm(OH)3 support
precursor (Fig. 1b–d). The actual Ru mass loadings of the
catalysts were determined by ICP-OES analysis (Table 1).
The structures of the catalyst precursors were characterized
by XRD. As shown in Fig. S4,† the characteristic peaks of all
The morphologies and sizes of the Ru NPs in the 4% Ru/
Sm2O3 catalysts prepared by different methods were
characterized by TEM. As shown in Fig. 3, the Sm2O3
supports in the three 4% Ru/Sm2O3 samples possess fine
crystallinity. TEM images reveal that an inter-planar space of
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Table 1 Physical properties of the Sm2O3 and Ru/Sm2O3 samples
Ru
loadinga area
(wt%)
(m2 g−1
Surface TEM results
CO chemisorption results
Ru dispersionc (%) Particle sizec (nm)
Sample
)
Particle sizeb (nm) Ru dispersionb (%) CO uptake (μmol g−1
)
4% Ru/Sm2O3-p 3.8
4% Ru/Sm2O3-i 3.7
4% Ru/Sm2O3-m 3.5
38.4
43.8
28.5
25.8
3.0
2.9
3.3
—
37.3
38.3
34.6
—
127.4
145.0
132.8
—
32.2
36.6
33.5
—
3.5
3.1
3.3
—
Sm2O3
—
a
b
c
Determined by ICP-OES analysis. Determined by TEM study by the equation of Borodziński and Bonarowska.40 Determined by CO
chemisorption, assuming a CO : Ru stoichiometry of 1 : 1.
0.32 nm, corresponding to the lattice fringe of the (222) facet
of Sm2O3, can be clearly identified for all the three samples.
As the Z-contrast of the Ru atoms is smaller than that of Sm
in the Sm2O3 support, it is challenging to directly distinguish
the dispersion and geometry of the Ru NPs from the more
condensed Sm2O3 support, especially when the particle size
of the Ru NPs is relatively small. As a result, only the Ru NPs
on the edge of the Sm2O3 support can be identified in the
TEM images. By counting ca. 100 Ru particles on the edge of
Sm2O3 in different regions, as shown in the TEM images
(Fig. 3d–f), statistical results of the size distributions of the
Ru NPs of the three Ru/Sm2O3 catalysts were obtained
(Fig. 3g–i). The average particle sizes of 4% Ru/Sm2O3-p, 4%
Ru/Sm2O3-i and 4% Ru/Sm2O3-m are estimated to be ca. 3.0,
2.9, and 3.3 nm, respectively, revealing similar particle sizes
of Ru in the three Ru/Sm2O3 samples. In addition, the Ru
NPs in Ru/Sm2O3-p shows a lower crystallinity by comparing
the lattice finger of the Ru NPs (insets of Fig. 3a–c). No
obvious agglomeration of Ru NPs into large particles could
be observed, showing that Sm2O3 is capable of preserving a
of the three Ru/Sm2O3 samples are similar based on the CO
pulse chemisorption results. Accordingly, the Ru particle
sizes of 4% Ru/Sm2O3-p, 4% Ru/Sm2O3-i and 4% Ru/Sm2O3-m
estimated by CO pulse chemisorption are determined to
be about 3.5, 3.1 and 3.3 nm, respectively, agreeing well with
the TEM results.
Catalytic performance for ammonia decomposition
The catalytic activities of 4% Ru/Sm2O3-p, 4% Ru/Sm2O3-i
and 4% Ru/Sm2O3-m in NH3 decomposition were evaluated.
As shown in Fig. 4a, the Sm2O3 support is inactive in NH3
decomposition. When the Ru NPs are anchored on the
surface of Sm2O3, remarkably high catalytic activity can be
obtained. Compared with the 4% Ru/Sm2O3-i and 4% Ru/
Sm2O3-m samples, 4% Ru/Sm2O3-p exhibits significantly
enhanced activity under identical conditions (Fig. 4a). In
detail, the NH3 decomposition over 4% Ru/Sm2O3-p starts at
300 °C and reaches ca. 100% conversion at 500 °C. By
contrast, obvious activity can only be detected at
temperatures above 375 °C over 4% Ru/Sm2O3-i and 4% Ru/
Sm2O3-m. At 450 °C, the NH3 conversion over 4% Ru/Sm2O3-
p is 84.6%, while the NH3 conversions over 4% Ru/Sm2O3-i
and 4% Ru/Sm2O3-m are 21.5% and 15.7% at 450 °C,
respectively. At 400 °C, the NH3 conversion over 4% Ru/
Sm2O3-p is 7.5 and 11.1 times higher than that over 4% Ru/
Sm2O3-i and 4% Ru/Sm2O3-m, respectively. These data
demonstrate the superior catalytic performance of the Ru/
Sm2O3-p catalyst.
highly dispersed geometry of Ru NPs.
A CO pulse
chemisorption method was also used to estimate the Ru
dispersion and the average particle size of the different 4%
Ru/Sm2O3 samples. As shown in Table 1, the Ru dispersions
Due to the same composition of the different 4% Ru/
Sm2O3 samples, comparison of the TOF value on each
exposed Ru atom is useful to evaluate the intrinsic catalytic
nature of Ru NPs in the different catalysts. It is also realized
that the size of Ru NPs plays a crucial role in determining
the catalytic performance of supported Ru-based catalysts in
NH3 decomposition. Many previous studies suggest that the
TOFH value varies with the sizes and shapes of Ru NPs.41–44
2
Our TEM results reveal that the average Ru particle sizes of
the three 4% Ru/Sm2O3 samples are similar (Fig. 3), but their
TOFH values vary greatly under identical reaction conditions
2
(Fig. 4b). The 4% Ru/Sm2O3-m sample, which has an average
size of 3.3 nm, shows a smallest TOFH value of 0.7 s−1 at 450
2
°C among the three samples. For 4% Ru/Sm2O3-p, the
Fig. 2 XRD patterns of the Sm2O3 support and 4% Ru/Sm2O3-m, 4%
Ru/Sm2O3-i and 4% Ru/Sm2O3-p catalysts.
average particle size is 3.0 nm. However, a markedly
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Fig. 3 Representative HRTEM and TEM images, and particle size distributions of 4% Ru/Sm2O3-p (a, d and g), 4% Ru/Sm2O3-i (b, e and h), and 4%
Ru/Sm2O3-m (c, f and i).
increased TOFH value of 3.6 s−1 at 450 °C can be observed,
min−1, which outperforms most of promoter-free Ru-based
catalysts under similar reaction conditions. It is worth to
note that the H2 formation rate per unit of Ru over the 1%
2
which is 5.2 and 4.3 times higher than that of the 4% Ru/
Sm2O3-m and 4% Ru/Sm2O3-i samples, respectively. The
superior catalytic performance of Ru/Sm2O3-p in NH3
decomposition is further justified by comparison of the
apparent activation energies (Ea) of the different catalysts.
Correlating well with the activity results, obvious differences
in the Ea values can be observed for the different 4% Ru/
Sm2O3 samples. As shown in Fig. 4c, the Arrhenius plots
reveal that the Ea value of the 4% Ru/Sm2O3-p catalyst is 85.1
kJ mol−1, which is lower than that of the 4% Ru/Sm2O3-i (96.0
kJ mol−1) and 4% Ru/Sm2O3-m (100.2 kJ mol−1) catalysts.
The activities of Ru/Sm2O3-p with different Ru mass
loadings were also investigated, and the activity increases
with the increase of the Ru loading from 1 to 4 wt% (Fig.
S6a†). Moreover, similar Ea values can be observed for the
Ru/Sm2O3-p samples with Ru mass loadings in the range of
1–4 wt% (Fig. S6b†), indicating the similar structure of active
sites in the Ru/Sm2O3-p samples with different Ru mass
loadings.
−1
Ru/Sm2O3-p catalyst at 450 °C is as high as 1338 mmol gRu
min−1, which is even higher than that of the highly active
potassium promoted K–Ru/CNT and K–Ru/MgO-DP catalysts.
In addition to the catalytic activity, stability is another crucial
parameter for evaluating the catalytic performance of a
catalyst. Here, the 4% Ru/Sm2O3-p catalyst was selected for
the stability test at 475 °C. Fig. 4d shows the catalytic
behaviour of the 4% Ru/Sm2O3-p catalyst as a function of
time on stream. The ammonia conversion remains nearly
constant during a test period of 60 h. With its superior
activity and stability, the Ru/Sm2O3-p catalyst has a practical
potential in NH3 decomposition for H2 production.
Metal–support interactions of the different 4% Ru/Sm2O3
catalysts
NH3 decomposition over Ru-based catalysts has been
recognized to be a structure sensitive reaction. Currently, the
B5 sites on the surface of Ru nanoparticles have been widely
accepted as the active sites of Ru-based catalysts for NH3
decomposition.43,45,46 It is well known that the surface
structure and chemical state of Ru NPs determine the
catalytic performance of supported Ru catalysts in NH3
The specific H2 formation rate per unit Ru or mass of
catalyst is a meaningful index to compare the activities of
various Ru-based catalysts. We thus collected, calculated and
compared the activity data of some representative supported
Ru catalysts. As shown in Table 2, the H2 formation rate of
−1
the 4% Ru/Sm2O3-p catalyst at 450 °C is 25.9 mmol gcat
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Fig. 4 Catalytic performance of the Ru/Sm2O3 catalysts for ammonia decomposition. (a) Temperature-dependent NH3 conversion and H2
formation rate. (b) Turnover frequency (TOFH ). (c) Arrhenius plots of the 4% Ru/Sm2O3-p, 4% Ru/Sm2O3-i and 4% Ru/Sm2O3-m catalysts. (d) Long-
2
term stability test of the 4% Ru/Sm2O3-p catalyst at 475 °C. WHSV = 30 000 mL gcat−1 h−1
.
Table 2 NH3 decomposition activities of various Ru-based catalysts at 450 °C
WHSV
Ru loading
(wt%)
NH3 conversion
(%)
H2 formation rate
(mmol gcat−1 min−1
H2 formation rate
(mmol gRu−1 min−1
−1
Catalyst
(mL gcat h−1
)
)
)
Ref.
Ru/SiO2
Ru/CNTs
K–Ru/CNTs
Ru/MgO–CNTs
Ru/Al2O3
Ru/AC
Ru/CaAlOx-w
Ru/BHA
Ru/MgO-DP
K–Ru/MgO-DP
Ru/c-MgO
K–Ru/Mg2Al-LDO
Ru-K/CaO
30 000
30 000
30 000
60 000
30 000
30 000
30 000
30 000
30 000
36 000
30 000
30 000
9000
30 000
22 000
18 000
30 000
30 000
30 000
30 000
30 000
10
36.4
43.3
97.3
30
23.3
28.7
20
11.2
13.3
29.9
18.4
7.1
112
277
622
379
149
183
175
470
495
914
526
285
296
376
2249
221
1338
916
683
179
137
47
5
5
48
5
5
49
16
15
15
50
51
52
53
26
14
4.8
4.8
4.85
4.8
4.8
3.5
2.74
3.5
3.5
4.7
4.6
2.8
1.96
1.0
4.8
1.0
2.0
3.8
3.7
3.5
8.8
6.1
42
12.9
17.3
32.1
24.7
13.1
8.3
56.5
87.0
80.6
42.7
90
Ru/Rb-Y zeolite
Ru/CeO2
24
7.3
100
58.2
43.6
59.7
84.6
21.5
15.7
22.5
10.7
13.4
18.3
25.9
6.6
Ru/La2O3-700-i
Ru/Sm2O3-p
Ru/Sm2O3-p
Ru/Sm2O3-p
Ru/Sm2O3-i
Ru/Sm2O3-m
This work
This work
This work
This work
This work
4.9
decomposition. Generally, many factors such as dispersion,
size and morphology of Ru NPs, interaction between Ru
NPs and support, and surface properties of support can
influence the surface structure and chemical state of Ru
NPs, leading to the remarkable difference in the catalytic
activity.
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We first examined the chemical state of the Ru NPs on
Sm2O3 by XPS. Here, a 4% Ru/SBA-15-p catalyst, in which
Ru NPs dispersed on an inert SBA-15 support, was used as
a reference for XPS characterization. Due to the overlapping
of the Ru 3d3/2 and C 1s peaks at ca. 284.8 eV, the Ru state
is analysed by the Ru 3d5/2 and 3p3/2 peaks. The Ru 3d5/2
peak of 4% Ru/SBA-15-p is found to be centred at 280.0 eV,
indicating the dominant metallic state of the Ru NPs
(Fig. 5a).54–56 The Ru 3d5/2 peak of 4% Ru/Sm2O3-p, on the
other hand, shifts to a higher value of 280.4 eV. At the same
time, the binding energy of the Ru 3p5/2 peaks of 4% Ru/
Sm2O3-p (461.9 eV) is also higher than that of 4% Ru/SiO2
(461.2 eV) (Fig. 5b). These results demonstrate that the
chemical state of the Ru NPs in 4% Ru/Sm2O3-p shows an
obvious difference from that of the Ru NPs in 4% Ru/SBA-
15-p. We infer that the Ru NPs in 4% Ru/Sm2O3-p has
stronger interaction with Sm2O3 than that of the Ru NPs in
4% Ru/SBA-15-p. As widely acknowledged, metal–support
interactions in oxide-supported metal catalysts play a critical
role in regulating the activity of catalysts.18,19
Here, we are also curious about the origin of the different
performances of the three Ru/Sm2O3 catalysts prepared using
the same Sm(OH)3 support and RuO2 NP precursor but via
different preparation methods. TEM observation (Fig. 3) and
CO chemisorption results (Table 1) confirm that the Ru NPs
in the three 4% Ru/Sm2O3 analogues give similar particle
sizes (3.1–3.5 nm) and dispersions (32.2–36.6%). Thus, the
particle size effect can be preliminary excluded. Considering
the similar particle size of the Ru NPs in the three 4% Ru/
Sm2O3 samples, we propose that the extent of interaction
between Ru NPs and Sm2O3 varies greatly under the
preparation conditions applied.
To probe the metal–support interaction of the Ru/Sm2O3
catalysts obtained from the different preparation methods,
H2-TPR was conducted (Fig. 6). For RuO2 NPs, there are two
broad peaks centred at around 127 and 213 °C,
corresponding to a two-step reduction process from the RuO2
NPs to the metallic Ru NPs.57–59 For the 4% Ru/Sm2O3-m
sample obtained from the physical mixture of Ru NPs and
Sm(OH)3, five broad peaks centred at 123, 167, 240, 382 and
457 °C can be observed. Based on the H2-TPR profiles of the
RuO2 NPs and Sm(OH)3, we can reasonably conclude that the
reduction peaks at temperatures below 320 °C may be
attributed to the reduction of the RuO2 NPs, while the
reduction peaks at temperatures above 320 °C may be
attributed to the reduction of surface oxygen on the
Sm(OH)3/Sm2O3 support. Here, the presence of the three
reduction peaks at temperatures below 320 °C indicates that
the RuO2 NPs exist in different states. The RuO2 NPs
interacting weakly with Sm(OH)3 can be reduced at lower
temperatures, while other RuO2 NPs that strongly interact
with the support can only be reduced at higher temperatures.
Similar with 4% Ru/Sm2O3-m, the majority of the RuO2 NPs
in 4% Ru/Sm2O3-i can be reduced at temperatures below 320
°C. The onset reduction temperature of the 4% Ru/Sm2O3-i
sample is about 53 °C. For the 4% Ru/Sm2O3-p sample, four
Fig. 5 Ru 3d (a) and 3p (b) core level XPS spectra of the 4% Ru/
Fig. 6 TPR profiles of RuO2 NPs, 4% Ru/Sm2O3-m, 4% Ru/Sm2O3-i,
Sm2O3-p and 4% Ru/SBA-15-p catalysts.
4% Ru/Sm2O3-p and Sm(OH)3.
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main reduction peaks centred at 161, 273, 355, and 430 °C were
observed. The first two reduction peaks around 161 and 273 °C
should be associated with the reduction of the RuO2 NPs. The
two reduction peaks centred at around 355 and 430 °C may be
assigned to the reduction of surface oxygen species. In
comparison with that of the 4% Ru/Sm2O3-i and 4% Ru/Sm2O3-
m samples, the reduction of the RuO2 NPs in the 4% Ru/
Sm2O3-p sample shifts towards higher temperatures, while the
reduction of the support shifts to lower temperatures,
providing strong evidence for the much enhanced interaction
between the RuO2 NPs and Sm(OH)3 support. We suggest that
the enhanced metal–support interaction in 4% Ru/Sm2O3-p via
the precipitation method could allow Sm2O3 to exert stronger
influence on modulating the electronic structure of Ru NPs. As
a result, the 4% Ru/Sm2O3-p sample shows the highest catalytic
activity among the three 4% Ru/Sm2O3 analogues.
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Conclusions
In summary, we demonstrate that Ru NPs on Sm2O3 catalysts
show superior activity and excellent stability for NH3
decomposition. The activity of Ru/Sm2O3 with identical
composition shows high dependence on the preparation
method. The Ru/Sm2O3 catalyst obtained from a precipitation
method exhibits much higher activity than the analogues
from impregnation and solid milling methods. These
findings offer promise to explore Ru/Sm2O3 catalyst as a new
kind of superior catalyst for NH3 decomposition.
Furthermore, this work also highlights the important role of
metal–support interaction in enhancing the activity of Ru/
Sm2O3 catalysts in NH3 decomposition and provides
opportunity to improve the catalytic performance of catalysts
by delicately modulating metal–support interactions of oxide-
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Conflicts of interest
23 S. Podila, H. Driss, S. F. Zaman, Y. A. Alhamed, A. A.
AlZahrani, M. A. Daous and L. A. Petrov, J. Mol. Catal. A:
Chem., 2016, 414, 130–139.
There are no conflicts to declare.
24 Y. Xun, X. He, H. Yan, Z. Gao, Z. Jin and C. Jia, J. Rare
Earths, 2017, 35, 15–23.
Acknowledgements
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This work is financially supported by the Strategic Priority
Research Program of the Chinese Academy of Sciences
(XDA21010208), the Dalian National Laboratory for Clean
Energy (DNL) (DICP DCLS201701) and the K. C. Wong
Education Foundation (GJTD-2018-06).
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