Highly Efficient Ru/MgO Catalyst with Surface-Enriched Basic Sites for Production of Hydrogen from Ammonia Decomposition

Article abstract of DOI:10.1002/cctc.201900306

Development of highly active and stable catalyst for decomposition of ammonia to CO_x-free hydrogen is an urgent and challenging task. Here, MgO (c-MgO) supported Ru nanoparticles derived from Ru/4MgCO₃ ? Mg(OH)₂ ? 4H₂

Full text of DOI:10.1002/cctc.201900306

Accepted Article
Title: Highly Efficient Ru/MgO Catalyst with Surface-Enriched Basic
Sites for Production of Hydrogen from Ammonia Decomposition
Authors: Ping Chen, Xiaohua Ju, Lin Liu, Xilun Zhang, Ji Feng, and
Teng He
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To be cited as: ChemCatChem 10.1002/cctc.201900306
Link to VoR: http://dx.doi.org/10.1002/cctc.201900306
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ChemCatChem
10.1002/cctc.201900306
Highly Efficient Ru/MgO Catalyst with Surface-Enriched Basic
Sites for Production of Hydrogen from Ammonia Decomposition
[
a]
[a]
[a,b]
[a,b]
[a]
[a]
Xiaohua Ju, Lin Liu,* Xilun Zhang, Ji Feng, Teng He, Ping Chen*
Dedication ((optional))
Abstract: Development of highly active and stable catalyst for
decomposition of ammonia to CO -free hydrogen is an urgent and
challenging task. Here, MgO (c-MgO) supported Ru nanoparticles
derived from Ru/4MgCO ·Mg(OH) ·4H O composite was prepared
by deposition precipitation method and investigated as catalyst for
NH decomposition. The use of 4MgCO ·Mg(OH) ·4H O as support
sustainable society.
For catalytic NH
x
3
decomposition, supported Ru-based
catalysts typically show high activity at relative low temperatures
[
3g, 3i, 4]
3
2
2
(400-500°C).
Up to now, a variety of supports, such as
[
5]
[3i]
[5b]
[6]
[3i, 7]
[3i, 8]
2
SiO ,
2
TiO ,
zeolite, Al
2 3
O
, MgO,
activated carbon,
[
3i, 3j, 9]
[10]
3
3
2
2
carbon nanotubes,
and C12A7 electrides,
have been
precursor leads to high density of basic sites and highly dispersed
Ru nanoparticles (3.8-6.0 nm) on the Ru/c-MgO catalysts. Thus, the
Ru/c-MgO catalysts show much enhanced activities and robust
stability in the long-term run (over 100 hours). Characterization
results reveal that the much enhanced dispersion of Ru
nanoparticles, high density of surface base sites and strong
interaction between Ru nanoparticles and c-MgO all benefit the high
applied for Ru-based catalysts. Experimental observations
reveal that the property of support is a critical factor for
determining the catalytic activities of the supported Ru catalysts
in NH
3
decomposition. Among the different Ru-based catalysts,
Ru/MgO catalysts have received extensive attention for its high
[
3i, 7]
stability in NH
3
decomposition.
However, the activities of
Ru/MgO catalysts obtained from different methods at relatively
low temperature (< 450°C) still need to be improved due to the
efficient Ru/c-MgO catalysts in NH
3
decomposition. Further
[
11]
improvements in activity can be achieved by modification of Ru/c-
MgO catalyst with an optimized KOH content. Therefore, it is
serious kinetic barrier.
As a basic metal oxide with suitable
basic strength, MgO possesses ability to affect the geometric
and electronic properties of supported Ru nanoparticles as a
result of strong interaction between Ru nanoparticles and MgO
support. The electron-donating ability of basic sites in MgO
results in a significant positive influence on the metal-support
interactions, and which is crucial for creation of highly active
electron-rich Ru center. However, little attention has been paid
to the influence of basic properties on the catalytic performances
3 2 2
believed that the use of 4MgCO ·Mg(OH) ·4H O as support
precursor opens opportunity to achieve highly active and stable
Ru/MgO catalysts through tuning the basic properties of catalysts.
Introduction
of Ru/MgO catalyst in NH
In this paper, we demonstrate that MgO supported Ru
nanoparticles derived from Ru/4MgCO ·Mg(OH) ·4H
composite (Ru/c-MgO) exhibits high catalytic activity and
stability for NH decomposition. The combination of Powder X-
ray diffraction (XRD), Nitrogen-adsorption/desorption,
transmission electron microscope (TEM), CO temperature-
programmed desorption (CO -TPD), CO chemisorption, and H
temperature-programmed reduction (H -TPR) techniques has
been applied to study the effect of MgO support precursor on the
physicochemical properties of Ru/c-MgO catalysts as well as
3
their catalytic activities in NH decomposition. It is proved that
Ru/c-MgO catalysts which exhibit high density of basic sites can
enhance the Ru dispersion and metal-support interaction,
3
decomposition.
Hydrogen is one of the most promising energy carriers for the
future energy systems, but the low volumetric energy density of
hydrogen (in both compressed gas and liquid form) makes it
3
2
2
O
[
1]
difficult to store. In recent years, ammonia, high gravimetric
3
3
(
2 2 2
17.7 wt.% H ), high volumetric H density (121 kg H /m in the
liquid form), high energy density (ca. 3 kWh/kg), liquid state
under mild conditions (20°C and 0.8 MPa) and billion tons of
annual production in world-wide, has attracted increasing
attention as a promising carrier for hydrogen storage and
2
2
2
2
[
2]
transportation.
NH decomposition reaction in presence of appropriate catalyst,
which is a critical step for utilization of NH as H carrier for
proton-exchange membrane fuel cells (PEMFC).
utilization of NH as a feedstock offer a promising method for
producing CO -free H , which is essential to build a more
The CO
x
-free
H
2
can be released via
3
3
2
[
3]
Thus,
3
ultimately enhancing the activity and stability in NH
3
x
2
decomposition. Moreover, the influence of KOH modification on
the activity and stability properties of Ru/c-MgO catalyst was
also investigated.
[a]
X. Ju, Dr. L. Liu, X. Zhang, J. Feng, Dr. T. He, Prof. Dr. P. Chen
Dalian National Laboratory for Clean Energy, State Key Laboratory
of Catalysis
Dalian Institute of Chemical Physics, Chinese Academy of Sciences
Dalian, 116023, China
Results and Discussion
E-mail: liulin@dicp.ac.cn
pchen@dicp.ac.cn
Catalytic performance of Ru/c-MgO-DP catalysts
[b]
X. Zhang, J. Feng
University of Chinese Academy of Sciences
Beijing, 100049,China
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ChemCatChem
10.1002/cctc.201900306
also prepared with DP method and tested for NH
For the catalysts with similar Ru content, the activities follow the
order of 5%Ru/c-MgO-DP > 5%Ru/MgO-DP > 5%Ru/Al -DP >
%Ru/SiO -DP (Figure 1b). The NH conversion of 5%Ru/c-
MgO-DP can reach 80.6% at 450°C, which is much higher than
that of the 5%Ru/MgO-DP (67.7%), 5%Ru/Al -DP (46.1%),
and 5%Ru/SiO -DP samples (17.5%). Above results clearly
show that the use of 4MgCO ·Mg(OH) ·4H O for the preparation
3
decomposition.
2 3
O
5
2
3
2 3
O
2
3
2
2
of Ru/c-MgO-DP catalyst is critical in achieving high catalytic
activity. The activity of 3%Ru/c-MgO-DP catalyst is about 75.0%
at 450°C, which is also higher than that of the 5%Ru/MgO-DP,
2 3 2
5%Ru/Al O -DP and 5%Ru/SiO -DP at the same temperature.
Remarkably, not only the 5%Ru/c-MgO-DP but also the 3%Ru/c-
MgO-DP and 1%Ru/c-MgO-DP have much higher activity than
that of the 5%Ru/Al
50°C, the NH conversion of 1%Ru/c-MgO can reach 55.0%,
while the NH conversion of 5%Ru/Al -DP and 5%Ru/SiO -DP
catalysts are only about 46.1% and 17.5%, respectively. Above
results clearly indicated that 4MgCO ·Mg(OH) ·4H O is an more
effective support precursor to prepare highly efficient catalysts
than the conventional oxide supports such as MgO, SiO and
Al
2 3 2
O -DP and 5%Ru/SiO -DP catalysts. At
4
3
3
O
2 3
2
3
2
2
2
O .
2 3
A comparison of activity of Ru/c-MgO-DP catalysts with those
of the other efficient Ru-based catalysts for NH decomposition
reported in literatures is listed in Table 1. The NH conversion of
%Ru/c-MgO-DP is superior to most of the unmodified
supported Ru catalysts in literatures. The calculation of H
3
3
5
2
formation rate of catalyst per gram was based on the weight of
the reduced Ru/c-MgO catalyst. It can be found that the
3
%Ru/c-MgO-DP and 1%Ru/c-MgO-DP catalysts also possess
much higher H formation rate than most of Ru-based catalysts
under similar reaction condition. Considering the amount of Ru
content in 1%Ru/c-MgO-DP catalysts, the H formation rate of
catalyst per gram Ru is 4.3 times that of typical efficient
3
Figure 1. NH conversion as a function of reaction temperature over Ru-based
catalysts prepared from different supports: (a) 5%Ru/c-MgO-DP (■); 5%Ru/c-
MgO-IM (□). (b) 5%Ru/c-MgO-DP (■); 3%Ru/c-MgO-DP (●); 1%Ru/c-MgO-DP
2
(
2 3 2
▲); 5%Ru/MgO-DP (▼); 5%Ru/Al O -DP (♦); 5%Ru/SiO -DP (◄).
2
5
%Ru/MgO-DP catalyst at same reaction condition.
In addition, their catalytic behavior of 3%Ru/c-MgO-DP was
Figure 1a shows the temperature dependence of NH
3
conversion for Ru/c-MgO catalysts prepared by deposition
precipitation method (DP) and impregnation method (IM). It can
be seen that the preparation methods have a great influence on
the catalytic performances of the Ru/c-MgO catalysts. The
further examined with a supply of pure ammonia at various gas
hour space velocity (GHSV) values (2,000-30,000 mL/gcat·h).
Figure 2a depicts effect of GHSV on the catalytic performance of
3
3%Ru/c-MgO-DP catalyst. It can be seen that NH conversion at
5
5
%Ru/c-MgO-DP exhibits much higher catalytic activity than the
%Ru/c-MgO-IM in the temperature ranging from 300-550°C
the reaction temperature of 300-500°C decreases remarkably
with increase of GHSV because of the increase in the residence
time of the reactants. At the temperature of 400°C, nearly
complete conversion can be obtained with a GHSV as high as
(Figure 1a), manifesting the importance of preparation method
for the formation of highly active catalysts. The 5%Ru/c-MgO-DP
readily shows apparent activity starting from 300°C, while
2,000 mL/gcat·h.
5
%Ru/c-MgO-IM displays negligible activity at temperature
below 400°C. The NH conversion of 5%Ru/c-MgO-DP is 2.8
times that of 5%Ru/c-MgO-IM at 450°C, and 5.0 times that of
%Ru/c-MgO-IM at 425°C. For the 5%Ru/c-MgO-DP sample,
The long-term stability is critical for evaluating the catalytic
performance of catalyst in NH decomposition (Figure 2b). At a
3
3
relatively high space velocity of 30,000 mL/gcat·h, the catalytic
activity of 3%Ru/c-MgO-DP was tested at 500°C. Quite
impressively, no deterioration of the catalytic activity was
observed for 110 hours. Thus, the stability of the Ru/c-MgO-DP
catalyst is firmly established as the conversion remained at
93% level almost constant over a relatively long period of time.
Here, Ru/c-MgO-DP catalyst not only gives high activity but also
gives high stability, demonstrating the great potential for the
5
only a very small amount of chlorine species (0.1 wt%) were
included in the reduced sample. However, a large amount of
chlorine (1.0 wt%) still remained in 5%Ru/c-MgO-IM even after
the sample was calcined and reduced at 550°C. The chlorine
species have negative effect for 5%Ru/c-MgO-IM catalysts, and
should partially responsible for its relatively low activity.
~
To evaluate the influence of supports on the catalytic
performances of supported Ru catalysts, Ru catalysts supported
3
application as efficient catalyst in NH decomposition reaction.
2 2 3
on the conventional oxides such as MgO, SiO , and Al O were
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ChemCatChem
10.1002/cctc.201900306
3
Table 1. NH conversion over supported Ru catalysts at 450°C (GHSV: 30,000 mL/gcat·h).
Catalyst
Method
Ru loading (wt %)
NH
3
conversion
%)
H
2
formation rate
H
2
formation rate
(mol/gRu min)
Reference
.
.
(
(mmol/gcat min)
a
Ru/Al
2
O
3
IM
10
10
31.5
34.5
43.3
30.8
27.2
23.3
28.7
41.3
56.5
67.7
55.0
75.0
80.6
11.5
11.4
14.5
10.3
9.1
7.8
9.6
13.8
18.9
22.7
18.4
25.1
0.12
0.11
0.30
0.22
0.19
0.16
0.20
0.49
0.54
0.48
2.09
0.89
[9]
[9]
[16]
[16]
[16]
[16]
[16]
[28]
[29]
This work
This work
This work
This work
a
Ru/SiO
2
IM
b
Ru/CNTs
Ru/MgO
IM
4.8
4.8
4.8
4.8
4.8
2.8
3.5
5.0
0.9
2.9
4.7
b
IM
b
Ru/TiO
Ru/Al
Ru/AC
2
IM
b
2
O
3
IM
b
IM
Ru/MgO
Ru/MgO
Ru/MgO
Ru/c-MgO
Ru/c-MgO
Ru/c-MgO
Polyol reduction
DP
DP
DP
DP
DP
27.0
0.60
[
a] Prepared by wetness incipient impregnation with water as solvent. [b] Prepared by wetness incipient impregnation with acetone as solvent.
Figure 2. (a) Effect of GHSV on the catalytic performance of 3%Ru/c-MgO-DP
catalyst for NH decomposition at different temperature: 500°C (■); 450°C (●);
00°C (▲); 350°C (▼); 300°C (♦). (b) Stability of 3%Ru/c-MgO-DP catalyst for
Figure 3. XRD patterns of as-prepared (top) and reduced (bottom)
3
5
4
%Ru/MgO-DP, 1%Ru/c-MgO-DP, 3%Ru/c-MgO-DP, and 5%Ru/c-MgO-DP.
MgCO ·Mg(OH) ·4H O, Mg(OH) and MgO were also included for
4
3
2
2
2
,
a
3
NH decomposition at 500°C.
comparison.
Structure characterization of the Ru/c-MgO-DP catalysts
The structures of the samples before and after reaction were
investigated by powder XRD. Figure 3 shows XRD patterns of
the as-prepared and reduced Ru/MgO samples prepared from
different supports. For the as-prepared Ru/MgO-DP sample,
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ChemCatChem
10.1002/cctc.201900306
only diffraction peaks corresponding to Mg(OH)
observed, indicating the conversion of MgO into Mg(OH)
the preparation process. In comparison with that of Ru/MgO-DP
sample, a remarkable difference can be observed for the as-
prepared Ru/c-MgO-DP samples. Only peaks corresponding to
2
phase can be
during
4MgCO
3
·Mg(OH)
2
·4H
2
O into MgO after reduction (Figure 3a). In
2
the XRD pattern of the reduced 5%Ru/MgO-DP sample, the
weak and broad peaks (at Bragg angles of 38.4, 42.2 and 44.0°)
diffraction features attributable to metallic Ru can be identified,
indicating the crystal size of Ru particles was big enough to be
detected by XRD. For the reduced Ru/c-MgO-DP samples, no
diffraction features attributable to metallic Ru can be observed
for the sample with a low Ru content of 1%. For the reduced
Ru/c-MgO-DP samples with a higher Ru content, besides the
diffraction lines of MgO support, small and broad peaks
corresponding to Ru metal can also be observed.
the 4MgCO
pattern of Ru/c-MgO-DP samples with different Ru contents,
indicating the structure of 4MgCO ·Mg(OH) ·4H O retained after
3 2 2
·Mg(OH) ·4H O phase can be observed in the XRD
3
2
2
preparation. For all the reduced Ru/c-MgO-DP and Ru/MgO-DP
samples, only diffraction peaks corresponding to MgO can be
observed, which reveals the decomposition of Mg(OH) and
2
Figure 4. TEM images and corresponding Ru particle size distributions of 5%Ru/MgO-DP (a and e), 1%Ru/c-MgO-DP (b and f), 3%Ru/c-MgO-DP (c and g), and
%Ru/c-MgO-DP (d and h).
5
MgO-DP and 5%Ru/c-MgO-DP are 3.8, 3.9 and 6.0 nm,
respectively, which are much smaller than that of reduced
Table 2. Properties of Ru/MgO-DP and Ru/c-MgO-DP samples.
5
%Ru/MgO-DP sample (9.8 nm). The 1%Ru/c-MgO-DP and
3%Ru/c-MgO-DP catalysts have similar metal particle size. For
the 4MgCO ·Mg(OH) ·4H O support, plenty of hydroxyl and
carbonate groups on the surface allow the Ru ions to be
Ru
Metal
Ru particle
Surface particle dispersion by CO size by CO
area (m /g) size by chemisorption (%)chemisorption
Sample
2
3
2
2
3+
TEM (nm)
(nm)
10.5
3.6
3.5
6.4
3+
5
1
3
5
%Ru/MgO-DP
%Ru/c-MgO-DP
%Ru/c-MgO-DP
%Ru/c-MgO-DP
283.6
47.7
53.2
51.0
9.8
3.8
3.9
6.0
12.8
37.1
38.7
21.1
adsorbed. In fact, almost all of the Ru ions can be adsorbed on
the surface of 4MgCO ·Mg(OH) ·4H O as evidenced by the
colorless of the solution after stirring for 1 h. The surface
3
2
2
3+
adsorbed Ru ions can be precipitated and deposited as highly
dispersed Ru precursor species (RuO
Ru(OH) ) on the surface of 4MgCO ·Mg(OH)
the release of NH
process. In the following reduction process, highly dispersed Ru
nanoparticles can be formed on the surface of c-MgO support
x
(OH)
y
, RuO
2 2
·2H O, or
The influence of support on the particle size distribution and
morphology of dispersed Ru nanoparticles was studied by TEM.
The metal particle size of the reduced catalyst is shown in the
TEM images in Figure 4. It can be seen that loading of Ru on the
x
3
2
·4H O along with
2
3
from hydrolysis of urea during the preparation
3 2 2
surface of 4MgCO ·Mg(OH) ·4H O leads to the formation of
which come from the decomposition of 4MgCO
In comparison with Ru/MgO-DP sample, the plenty of basic sites
of c-MgO support derived from 4MgCO ·Mg(OH) ·4H O can
3 2 2
·Mg(OH) ·4H O.
highly dispersed Ru nanoparticles on the support. The particle
size distribution and average diameter of Ru nanoparticles for
the reduced 5%Ru/MgO-DP and Ru/c-MgO-DP samples with
different Ru contents can be estimated by measuring around
3
2
2
enhance the interaction between Ru species and c-MgO
supports, which can induce the much enhanced dispersion of Ru
nanoparticles on the surface of c-MgO support. Therefore,
2
00 particles randomly taken from TEM images. Quantitatively,
the average metal particle sizes of 1%Ru/c-MgO-DP, 3%Ru/c-
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ChemCatChem
10.1002/cctc.201900306
loading of Ru on the 4MgCO
much enhanced dispersion of Ru in comparison with that of the
MgO support, which should account for the remarkably
increased catalytic activity of Ru/c-MgO-DP samples for NH
decomposition.
3
Ru catalyzed NH decomposition has been demonstrated to
3
·Mg(OH)
2
·4H
2
O support leads to
can be observed in the relatviely reaction temperature ranging
from 300 to 350°C. The TOF values follow the order of 5%Ru/c-
MgO-DP > 1%Ru/c-MgO-DP > 3%Ru/c-MgO-DP > 5%Ru/MgO-
DP. Above results clearly show that both the composition of the
catalysts and reaction temperature can influence the intrisic
activity of Ru/MgO-DP catalysts. It worthy of mentioning that
3
be a structural sensitive reaction, and the activity of Ru-based
catalyst can be greatly influenced by the dispersion, size and
NH
3 2 3
synthesis, a rise in TOF over promoted Ru/Al O and AC-
supported Ru catalysts has been observed with the increase of
size of Ru particles.
decomposition on supported Ru catalysts is also highly structure
sensitive, with TOFH2 values increasing by almost 2 orders of
magnitude as the particle size increases from 0.8 nm to >7
[
12]
[12a, 12b]
morphology of Ru nanoparticles.
influence of Ru dispersion on the catalytic performance of the
%Ru/MgO-DP and Ru/c-MgO-DP samples, CO chemisorption
was also applied to determine the Ru dispersion. From Table 2,
it can be seen that the use of 4MgCO ·Mg(OH) ·4H O as
To further study the
It has been found that NH
3
5
[
12c]
3
2
2
nm.
Herein, the TOFH2 values of 5%Ru/c-MgO-DP catalyst
support precursor has a significantly influence on the dispersion
degree of Ru nanoparticles. According to the CO chemisorption
results, the Ru dispersion value of 5%Ru/c-MgO-DP is about
also shows obvious decrease in comparison with 5%Ru/MgO-
DP with larger particle size, which agrees well with the previous
reported Ru/MgO catalyst systems in NH decomposition. For
3
[
12]
2
1.1%, which is larger than that of the 5%Ru/MgO-DP with
the Ru/c-MgO-DP catalysts with Ru loading of 1 wt.% and 3
wt.%, similar Ru dispersion value and Ru particle size can be
obtained. However, remarkable different TOFH2 value can be
observed for the 1%Ru/c-MgO-DP and 3%Ru/c-MgO-DP. In
comparison with 3%Ru/c-MgO-DP, much enhanced TOFH2 value
can be observed on 1%Ru/c-MgO-DP in the temperature range
or 300-500°C. For example, the 1%Ru/c-MgO-DP give much
same Ru loading (12.8%). The Ru particle size of 5%Ru/c-MgO-
DP catalysts estimated by CO chemisorption was about 6.4 nm,
which is consistent with the value obtained from TEM results
(~6.0 nm). The CO chemisorption results also show that the Ru
content has a notable impact on the Ru dispersion of Ru/c-MgO-
DP samples, and Ru/c-MgO-DP samples with low Ru content
induces a much enhanced Ru dispersion. Consistent with the
catalytic activity results, both TEM and CO chemisorption results
-
1
higher TOFH2 values of 9.4 s than that of the 3%Ru/c-MgO-DP
-
1
(3.8 s ) at 450°C. Decomposition on Ru is known to be
structure-sensitive reactions as a result of highly dependence of
3 2 2
reveal that the use of 4MgCO ·Mg(OH) ·4H O as support
precursor induces a much enhanced dispersion of Ru on the
surface of Ru/c-MgO-DP samples.
The H formation rate per gram Ru of the Ru/MgO catalysts
2
calculated to reveal the intricl catalytic activities of the Ru/MgO
catalysts studied. It can be found that the Ru/c-MgO-DP
2 3
active B5 sites on the particle size and shape. In a Ru/Al O
system, both experimental results and theoretical results show
that the number of active B5 sites is highly dependent on particle
shape and increases with particle size up to 7 nm for flat
nanoparticles. The maximum TOF (based on total exposed Ru
atoms) and number of active B5 sites occur at ∼ 7 nm for
elongated nanoparticles compared to at ∼ 1.8-3 nm for
catalysts possess higher H
that of 5%Ru/MgO-DP catalyst in the temperature range of 350-
00°C (Figure 5a). Among the Ru/c-MgO-DP catalysts with
different Ru contents, the H formation rates follow the order of
%Ru/c-MgO-DP > 3%Ru/c-MgO-DP > 5%Ru/c-MgO-DP. The
2
formation rate in comparison with
[
12c]
5
hemispherical nanoparticles.
Here, both the Ru dispersion
2
value and Ru particle size are average values estimated from
the highly dispersed Ru particles with different sizes and
irregular morphologies. Thus, the remarkable difference in the
TOFH2 values of 1%Ru/c-MgO-DP and 3%Ru/c-MgO-DP should
mainly arise from the obvious difference in the particle size
distribution and morphology of non-uniform Ru nanoparticles on
the c-MgO surface.
1
H
2
formation rate of 1%Ru/c-MgO-DP catalyst is 0.29
mol/(gRu·min), 3.9 times that of typical 5%Ru/MgO-DP catalyst
at 350°C, while the H formation rate of 1%Ru/c-MgO-DP was
.3 times that of 5%Ru/MgO-DP at 450°C. Above results clearly
2
4
show that the apparent high activity of Ru/c-MgO-DP in
comparison with that of the Ru/MgO-DP catalyst.
The textural properties of Ru/c-MgO-DP samples were studied
The H
calculated by on the H
2
formation turnover frequency (TOFH2), which can be
formation rate to the number of exposed
by
N
2
sorption. Figure6 shows
N
2
adsorption/desorption
2
isotherms of the reduced 5%Ru/MgO-DP and Ru/c-MgO-DP
samples with different Ru contents. The reduced 5%Ru/MgO-DP
exhibits type IV isotherms with H3 hysteresis as defined by the
IUPAC. The hysteresis loops in the relative pressure P/P0
between 0.5 and 1.0 indicates the presence of uniform
mesopores in the reduced 5%Ru/MgO-DP, which is consistent
with our previous report. The BET surface areas of reduced
5%Ru/MgO-DP and Ru/c-MgO-DP samples with different Ru
contents are summarized in Table 2. Table 2 shows the specific
surface area, average Ru particle size (estimated by TEM and
CO-chemisorption), and Ru dispersion of the four Ru-based
catalysts. It can be seen that the 5%Ru/MgO-DP has a high
Ru surface atoms per gram of catalyst, has been used as a
standard parameter for the evaluation of intrinsic catalytic
performance of different catalyst. Figure 5b shows that the
TOFH2 values of Ru/c-MgO-DP and Ru/MgO-DP catalysts all
increase with the increase of reaction temperature. In the
temperature range of 400-550°C. Among the different Ru/MgO-
DP catalysts with different Ru contents, the TOF values follow
the order of 1%Ru/c-MgO-DP > 5%Ru/MgO-DP > 5%Ru/c-MgO-
DP > 3%Ru/c-MgO-DP. At 500ºC, the 1%Ru/c-MgO-DP displays
a significantly TOFH2 of 9.5 s ,which is not only much higher
than that of 5%Ru/MgO-DP (6.3 s ), but also outperforms
%Ru/c-MgO-DP (3.9 s ) and 5%Ru/c-MgO-DP (4.8 s )
catalyst under similar condition. However, a much different order
-
1
-
1
-
1
-1
2
3
surface area of 283.6 m /g, which is not only larger than that of
the MgO derived 4MgCO
2
3 2
·Mg(OH) ·4H
2
O (142.6 m /g), but also
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ChemCatChem
10.1002/cctc.201900306
larger than that of the Ru/c-MgO-DP samples with different Ru
2
contents (~50 m /g). Generally a higher surface area of support
produces higher Ru dispersion. However, 5%Ru/MgO-DP which
has the highest surface area only gives a lowest Ru dispersion.
Thus, the surface areas of the catalysts may not be the key
factor in causing the difference of the Ru dispersions and
catalytic performances of 5%Ru/MgO-DP and Ru/c-MgO-DP
catalysts studied here.
Figure 6. N
2
sorption isotherms of reduced 5%Ru/MgO-DP (▼), 1%Ru/c-
MgO-DP (■), 3%Ru/c-MgO-DP (●), and 5%Ru/c-MgO-DP (▲).
2
Figure 7. CO -TPD spectra of reduced 5%Ru/MgO-DP (a), 1%Ru/c-MgO-DP
(b), 3%Ru/c-MgO-DP (c), and 5%Ru/c-MgO-DP (d).
Table 3. Basic properties of 5%Ru/MgO-DP and Ru/c-MgO-DP samples
Figure 5. H
temperature on different Ru-based catalysts: 5%Ru/MgO-DP (■);
%Ru/MgO(C)-DP (●); 3%Ru/MgO(C)-DP (▲); 5%Ru/MgO(C)-DP (▼).
2
formation rate (a) and TOFH2 (b) as a function of reaction
Surface area
Density of
basic sites
(μmol/m )
Peak
temperature
1
Quantity
2
(
m /g)
sample
3
2
(
cm /g)
(°C)
1
62.1
30.7
23.8
55.6
29.4
57.8
29.7
54.7
25.1
56.0
283.6
283.6
47.7
47.7
53.2
53.2
51.1
51.1
3.7
5
%Ru/MgO-DP
6
8.8
1
%Ru/c-MgO-
DP
159.6
48.5
161.4
56.9
162.6
49.4
27.5
54.1
24.9
45.9
22.0
49.0
6
3
%Ru/c-MgO-
DP
6
5
%Ru/c-MgO-
DP
6
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ChemCatChem
10.1002/cctc.201900306
The CO
basic property of the 5%Ru/MgO-DP and Ru/c-MgO-DP
samples. Figure 7 displays the CO desorption profiles of
%Ru/MgO-DP and Ru/c-MgO-DP with different Ru contents in
the temperature range of 100-800°C. Similar with that of the
%Ru/MgO-DP sample, the Ru/c-MgO-DP samples with
different Ru contents also show similar desorption peaks at ca.
2
-TPD measurement was conducted to examine the
sample, the onset reduction temperature is at about 60°C, and
several broad reduction peaks with peak temperatures at about
90, 130, 160, and 195°C can be observed. The presence of
several broad reduction peaks in the temperature range of 60-
200°C clearly reveals the interaction between different kinds of
Ru species with MgO support in 5%Ru/MgO-DP sample. The
onset reduction temperature of 5%Ru/c-MgO-DP is also at about
60°C, which is similar with that of the 5%Ru/MgO-DP sample.
Different from that of 5%Ru/MgO-DP sample, only a broad peak
2
5
5
160 and 650°C respectively. The density of basic sites on the
%Ru/MgO-DP and Ru/c-MgO-DP samples as calculated by the
5
integral method based on the area of the corresponding samples. at ca. 187°C can be observed for 5%Ru/c-MgO-DP sample.
As shown in Table 3, the high temperature (ca. 650°C) basic
density of Ru/c-MgO-DP samples are in the range of 45-54
Decrease of Ru loading amount from 5% to 1% resulted in a
remarkable difference in the TPR spectra of Ru/c-MgO-DP
samples. Besides a broad peak at temperature below 300°C, an
additional broad peak centered at about 460-480°C appeared for
the 3%Ru/c-MgO-DP and 1%Ru/c-MgO-DP sample. As the
Ru/c-MgO-DP sample with different Ru content were obtained
from same method, the remarkable difference in the reduction
behavior should be highly related to the differences of metal–
support interaction induced by the high density basic sites.
Some of weakly interacted Ru species with the support could be
easily reduced, while strongly interacted Ru species with the
support could only be reduced at much improved temperature.
With the decrease of Ru loading, both the low temperature (150-
2
2
μmol/m , about 5-6 times that of 5%Ru/MgO-DP (8.8 μmol/m ).
Note also that the Ru/c-MgO-DP samples give much higher
density of low temperature (ca. 160°C) basic sites than that of
the 5%Ru/MgO-DP sample. These profiles suggest that the
Ru/c-MgO-DP samples possess significantly enhanced density
of surface basic sites, which may account for the much
3
enhanced NH decomposition activity of Ru/c-MgO-DP samples.
300°C) and high temperature reduction peak (400-600°C) of
Ru/c-MgO-DP showed a tendency to move to the higher
temperatures, indicating much enhanced interaction between Ru
species and MgO support in Ru/c-MgO-DP. The TPR results
show the positive role of high density of basic sites in promoting
the strong metal-support interaction in Ru/c-MgO-DP samples,
which may also beneficial for the enhancement of catalytic
activity.
Kinetic test of Ru/MgO-DP catalysts
Figure 8. TPR profiles of (a) 5%Ru/MgO-DP, (b) c-MgO, (c) 1%Ru/c-MgO-DP,
d), 3%Ru/c-MgO-DP, and (e) 5%Ru/c-MgO-DP.
(
2
H -TPR experiments were also carried out to reveal the origin
of remarkable difference in the catalytic activities of Ru/MgO-DP
and Ru/c-MgO-DP samples. To exclude the influence of other
reaction (e.g. methanation) and desorbed species (e.g. H
CO , CH ) on the determination of H consumption, the Ru/c-
MgO-DP samples were firstly treated at 450°C for 2 h, which can
convert the 4MgCO ·Mg(OH) ·4H O support precursor into MgO
2
O,
2
4
2
3
2
2
as proved by XRD (Figure S2) and TG-DTG results (Figure S3).
Thus, the detected peaks during the TPR testing detected by
TCD detector can be attributed to the consumption of H
the reduction of Ru precursor species. For a comparison, the
TPR profile of 4MgCO ·Mg(OH) ·4H O treated at 450°C for 2 h
was also recorded. As shown in Figure 8, no H consumption
peaks can be observed from the TPR profiles of the
MgCO ·Mg(OH) ·4H O treated at 450°C. For 5%Ru/MgO-DP
2
during
Figure 9. Arrhenius plots of the supported Ru catalysts: 1%Ru/c-MgO-DP (■);
3%Ru/c-MgO-DP (●); 5%Ru/c-MgO-DP (▲); 5%Ru/MgO-DP (▼); 5%Ru/
3
2
2
Al -DP (♦); 5%Ru/SiO -DP (◄).
2
O
3
2
2
4
3
2
2
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ChemCatChem
10.1002/cctc.201900306
Based on NH
3
conversion values far away from the
equilibrium value, the apparent activation energy of different
catalysts can be calculated. Figure 9 shows Arrhenius-type plots
for the rates of NH
DP, 5%Ru/MgO-DP, 5% Ru/Al
3
decomposition reaction over x%Ru/c-MgO-
-DP, and 5%Ru/SiO -DP
O
2 3
2
catalysts. Correlating well with the catalytic activity results, a
remarkable decrease in the apparent activation energy can be
observed for 5%Ru/c-MgO-DP catalyst in comparison with that
of the Ru-based catalysts from metal oxide supports. The
apparent activation energy of 5%Ru/c-MgO-DP is 67.3 kJ/mol,
which is smaller than that of 5%Ru/MgO-DP (85.0 kJ/mol), 5%
2 3 2
Ru/Al O -DP (88.2 kJ/mol) and 5%Ru/SiO -DP (104.3 kJ/mol)
samples. For the Ru/c-MgO-DP samples with different Ru
contents, it can be seen that the Ea values of Ru/c-MgO-DP
decreased obviously with increase of Ru content, which is
[
11b]
consistent with our previous results.
For different Ru/c-MgO-
DP samples, the 5%Ru/c-MgO-DP sample gives a smallest the
apparent activation energy. Due to the complex of the supported
Ru catalysts, the catalytic performance as well as apparent
activation energy of supported Ru catalysts can be influenced by
many different factors such as the surface area of support, Ru
dispersion, Ru particle size and morphology, interaction between
Ru and support, and basic property of support. Here, for the
Ru/c-MgO-DP samples with similar surface area, the highly
dispersed Ru nanoparticles, high density of basic sites, and
strong interaction between Ru nanoparticles and c-MgO may all
responsible for the decrease of Ea values with increase of Ru
content.
Effect of KOH as modifying agent for 3%Ru/c-MgO-DP
catalyst
For NH
3
decomposition reaction, the rate-determining step is the
Figure 10. (a) NH3 conversion as a function of reaction temperature over
3
%Ru/c-MgO-DP and KOH modified 3%Ru/c-MgO-DP catalysts: 3%Ru/c-
MgO-DP (●); K-3%Ru/c-MgO-DP (K/Ru=1/3) (○); K-3%Ru/c-MgO-DP
K/Ru=1/2) (∆); K-3%Ru/c-MgO-DP (K/Ru=1/1) (). (b) Stability of K-3%Ru/c-
MgO-DP (K/Ru=1/2) catalyst for NH decomposition at 475°C.
associative desorption of nitrogen atoms from the catalyst
surface. Alkali or alkaline earth ions have been demonstrated as
efficient promoters for supported Ru or Fe catalysts due to the
(
3
[
4a, 13]
electron donation effect.
In this work, KOH was applied as
the promoter to modify the reduced 3%Ru/c-MgO-DP catalyst.
As shown in Figure 10a, KOH modification has an obvious
influence on the catalytic performance of 3%Ru/c-MgO-DP
catalyst. The NH conversions of K-3%Ru/c-MgO-DP (K/Ru=1/3)
3
and K-3%Ru/c-MgO-DP (K/Ru=1/2) show an obvious increase in
comparison with that of the pristine 3%Ru/c-MgO-DP at
Besides activity, the durability of the catalysts is another
important property in the practical applications. The catalytic
performance of the K-3%Ru/c-MgO-DP (K/Ru=1/2) catalyst was
also measured in a long term test. Even the 3%Ru/c-MgO-DP
catalyst has been fully reduced before KOH modification, the K-
3%Ru/c-MgO-DP (K/Ru=1/2) catalyst still need to be activated
3
temperature above 400°C. The NH conversion over K-3%Ru/c-
MgO-DP (K/Ru=1/2) can reach 96.6% at 475°C, which is much
higher than that of the pristine 3%Ru/c-MgO-DP (87.1%). The
through exposure to the NH
temperature. For the K-3%Ru/c-MgO-DP (K/Ru=1/2) catalyst
directly tested at 450-475°C, only a very low NH conversion of
25-30% can be obtained. However, remarkable enhancement of
activity at 450°C can be obtained when the catalyst was pre-
3
stream at optimized high
NH
75°C, which is similar with that of the K-3%Ru/c-MgO-DP
K/Ru=1/2). The KOH modification of Ru/c-MgO-DP has an
3
conversion over K-3%Ru/c-MgO-DP (K/Ru=1/3) is 95.6% at
3
4
(
optimum K/Ru molar ratio. Further increase of K/Ru molar ratio
from 1/2 to 1/1 resulted with a remarkably decrease of catalytic
activity in the temperature range from 300-500°C. The decrease
of activity of K-3%Ru/c-MgO-DP (K/Ru=1/1) should result from
the covering of Ru sites with excess of KOH species, which
have been often observed for the alkali modified Ru/CNT
3
treated in a NH flow at temperature of 500-520°C before the
activity test. In the following long isothermal test at 475°C, no
obvious activity loss is observed and approximately 95.6% of
3
NH conversion is maintained over 120 hours, demonstrating
that the remarkably high stability of the KOH modified catalyst
(Figure 10b). The activation temperature is critical for activation
of the K-3%Ru/c-MgO-DP (K/Ru=1/2) catalyst, further increase
of activation temperature to 550°C results in a much decreased
[
4a]
catalyst.
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ChemCatChem
10.1002/cctc.201900306
activity at 450°C (65.2%), which may be related with the partial
loss of K species at the high temperature.
To evaluate the effect of DP method on the structure and performance
2 3 2
of the different supports, Ru/MgO-DP, Ru/Al O and Ru/SiO with Ru
loading of wt.% prepared by DP method and tested in NH
5
3
decomposition. MgO powder was purchased from Shenyang chemical
reagent plant, China. To compare the influence of DP method and
incipient wetness impregnation on the catalytic performance, 5%Ru/c-
MgO-IM sample prepared by incipient wetness impregnation. The
Conclusions
In summary, this work demonstrates that the use of
concentration of RuCl
3
.xH
2
O was 25 mg/mL. The acetone solution was
·4H O support, and then the solvent was
added to the 4MgCO ·Mg(OH)
3
2
2
4
3 2 2
MgCO ·Mg(OH) ·4H O as support precursor can strongly affect
removed in air to obtain the air-dry sample. Subsequently, the sample
was dried at 80°C for another 2 h. After that, the sample was heated in
flowing Ar to 550°C for 2 h.
the basic properties of Ru/MgO catalysts as well as their
catalytic
Ru/4MgCO
catalysts can decompose NH
efficiency and robust stability in the long-term runs (over 100 h).
Compared to the conventional MgO support, the use of
4
increases the density of basic sites on the surface of Ru/c-MgO
catalysts. Meanwhile, the high density of basic sites also has a
significant effect on the dispersion of Ru nanoparticles and
metal-support interaction. The highly dispersed Ru nanoparticles,
high density of basic sites, and strong interaction between Ru
nanoparticles and c-MgO all benefit the excellent catalytic
performance of Ru/c-MgO catalysts in NH
work confirms the importance of basic properties of Ru/MgO
catalysts for NH decomposition, and which opens opportunity to
activities
2 2
·Mg(OH) ·4H O
for
NH
composite derived Ru/c-MgO
to produce CO -free H with high
3
decomposition.
The
3
The as-prepared 3%Ru/c-MgO-DP sample was reduced in flowing NH
at 550°C for 2 h. Then KOH promoter was loaded onto the reduced
%Ru/c-MgO-DP sample by incipient wetness impregnation from the
3
3
x
2
3
ethanol solution of KOH. The sample was dried at 80°C for overnight,
and the obtained KOH modified 3%Ru/c-MgO-DP catalyst was denoted
as K-3%Ru/c-MgO-DP.
MgCO
3
2
·Mg(OH) ·4H
2
O
as support precursor significantly
Characterization
The Ru metal loading in Ru/c-MgO-DP sample was determined by
inductively coupled plasma-atomic emission spectroscopy (ICP-AES,
Optima 2000DV, USA). XRD patterns were recorded on an X’Pert Pro
(
ą
PANAlytical) diffractometer with Cu K radiation (λ = 0.1542 nm) at 40
3
decomposition. This
kV and 40 mA. The contents of residual chlorine in the prepared catalysts
were determined on a PANAlytical Zetium X-ray fluorescent spectrometer.
3
Nitrogen
adsorption/desorption
isotherms
were
undertaken
develop highly efficient catalysts through tuning the basic
properties of catalysts.
isothermally at -196°C with a Micromeritics ASAP 2020 analyzer. The
-
3
samples were degassed at 300°C and 10 Pa for at least 8 h before the
measurements. The surface areas are determined by applying the
Brunauer-Emmett-Teller (BET) method in the relative pressure range of
0
.05-0.25. The pore volume is calculated at the relative pressure of 0.99.
CO -TPD measurements were conducted on a Micromeritics 2920
apparatus. In a typical CO -TPD experiment, the sample was activated in
a flow of He (99.999%) at a rate of 10°C/min to 400°C and kept at 400°C
for 2 h prior to the adsorption of CO at 100°C. After purging the
physically adsorbed CO at 100°C with He, the sample was heated to
900°C at a rate of 10°C/min, and the CO liberated was detected by a
chromatograph equipped with thermal conductivity detector to determine
the amount of CO desorbed from the catalysts.
Thermogravimetric Analysis-Differential Thermal Analysis (TG-DTA)
measurements were carried out on a Netzsch STA449C thermal analysis
2
system. A heating rate of 2 C/min was applied to 5 mg sample. H -TPR
was operated in a home-made apparatus. Prior to each measurement,
the sample (0.1 g) was heated from room temperature to 450°C with a
heating rate of 5°C/min under Ar flow (30 mL/min), and kept at 450°C for
Experimental Section
2
2
Reagents
Ruthenium chloride hydrate (RuCl
Tuosi Chemical Co., LTD, Mg(NO
Kemiou Chemical Reagent Co., Ltd.,
Sinopharm Chemical Reagent Co., Ltd..
3
·xH
2
O) was obtained from Shanghai
O was obtained from Tianjin
CO was obtained from
2
3
)
2
·6H
2
2
K
2
3
2
2
Preparation of supports
4
MgCO
Mg(NO
CO
5°C. The product was obtained by filtration to separate the precipitate
3
·Mg(OH)
·6H O (0.7 mol, 0.4 mol/L) aqueous solution was mixed with
3
(0.7 mol, 0.4 mol/L) aqueous solution, stir violently for 1 hour at
2 2
·4H O precursor was prepared by precipitation method.
3
)
2
2
K
1
2
from the solution, and then washed for three times with deionized water.
Finally, the product was dried at 80°C for 6 h. XRD pattern of the
1
20 min. After being cooled to room temperature, the gas flow was
switched to 5% H /Ar (30 mL/min) and reduced the sample with a
2
obtained
product
·4H
according
O (Figure S1).
well
with
the
standard
temperature ramp of 10°C/min. The TPR profiles were measured with a
gas chromatograph equipped with thermal conductivity (TCD) detector.
The reducibility of the loaded metal component was estimated based on
4
MgCO ·Mg(OH)
3
2
2
Preparation of Ru/c-MgO-DP
2
the extent of H consumption.
The Ru/c-MgO catalyst with Ru loading of 5.0 wt.% was prepared by
deposition-precipitation method (DP) according to our previous report.
The morphologic characterization was performed on a JEOL 2100X
transmission electron microscope operating at 200 kV. In order to obtain
suitable samples, the catalyst powders were dispersed in ethanol by
ultrasonic and then a drop of the solution was placed on a carbon-coated
copper grid.
The exposed Ru surface area was measured by CO pulse
chemisorption using an automated chemisorption analysis instrument
from Altamira (AMI-200). The catalyst was loaded into a 1/4 inch quartz
tube for each test. Prior to the pulse CO chemisorption measurement,
Prior to the pulse CO chemisorption measurement, the catalyst was
RuCl
3
·xH
2
O (0.5 g Ru, 0.5 g/L) aqueous solution was first mixed with
·Mg(OH) ·4H O powder. Subsequently, a designated
2
2.2 g of 4MgCO
3
2
2
amount of urea with a molar ratio of Ru to urea at 1:200 was added as
the precipitation agent. The mixture was reacted at 80°C for 8 h and
aged at room temperature for 12 h. The product was obtained by filtration
to separate the precipitate from the solution, and then washed for three
times with deionized water. Finally, the product was dried at 80°C for 6 h.
To evaluate the effect of ruthenium content on catalyst, we’ve prepared a
series of the catalysts, identified as x%Ru/c-MgO-DP, where
corresponding to the weight percent of Ru metal in the catalysts.
x
reduced at 400°C for 2 h in 10% H
temperature in He. CO pulses were introduced to the sample (250 mL of
2
/Ar, and then cooled down to room
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ChemCatChem
10.1002/cctc.201900306
pure CO) and the CO uptake was measured using a gas chromatography
equipped with TCD detector. The Ru dispersion was calculated by
assuming a CO:Ru stoichiometry of 1:1.
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NH
derived from
2 2
Ru/4MgCO ·Mg(OH) ·4H O which
3
-to-H
2
: The Ru/c-MgO catalysts
Xiaohua Ju, Lin Liu,* Xilun Zhang, Ji
Feng, Teng He, Ping Chen*
3
have high density of basic sites and
enhanced Ru dispersion show
excellent catalytic activity and stability
Page No. – Page No.
Highly Efficient Ru/MgO Catalyst with
Surface-Enriched Basic Sites for
Production of Hydrogen from
Ammonia Decomposition
3
in NH decomposition.
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