Article
pubs.acs.org/JPCC
Effects of Alkaline Earth Metal Amides on Ru in Catalytic Ammonia
Decomposition
Pei Yu,†,∥ Jianping Guo,*,† Lin Liu,† Peikun Wang,†,∥ Fei Chang,†,∥ Han Wang,†,∥ Xiaohua Ju,†
and Ping Chen*,†,‡,§
†
Dalian Institute of Chemical Physics, ‡State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, and §Collaborative
Innovation Center of Chemistry for Energy Materials, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian
116023, People’s Republic of China
∥
University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China
S Supporting Information
*
ABSTRACT: The effects of alkaline earth metal amides (Mg(NH2)2, Ca(NH2)2, and
Ba(NH2)2) on Ru in catalyzing NH3 decomposition were investigated. The catalytic
activities rank in the order of Ru−Ba(NH2)2 > Ru−Ca(NH2)2 > Ru−Mg(NH2)2, among
which Ru−Ba(NH2)2 and Ru−Ca(NH2)2 catalysts have higher intrinsic activities (TOF)
and lower apparent activation energies than those of Ru−Mg(NH2)2 and Ru/MgO
catalysts, indicating that Ca(NH2)2 and Ba(NH2)2 may have different roles from those of
Mg(NH2)2 and MgO. The TPR (temperature-programmed reaction) results show that
Ca(NH2)2 or Ba(NH2)2 decomposes to N2 and H2 rather than NH3 in the presence of Ru.
Ru may promote the NHx (x = 1, 2) coupling to H2 and N2 and change the decomposition
pathways of Ca(NH2)2 and Ba(NH2)2. Kinetic analyses reveal that the Ru promoted NHx
coupling to H2 and N2 may be the rate-determining step for catalytic ammonia
decomposition. We suggest that the catalysis is very likely fulfilled via (1) Ru catalyzes the
decomposition of amides to form H2, N2, and imides through an energy more favorable
pathway and (2) imides react with NH3 to regenerate amides. The presence of Ca(NH2)2
or Ba(NH2)2 creates a NHx-rich environment, and Ru mediates the electron transfer from NHx to facilitate NHx coupling to N2
and H2.
■
INTRODUCTION
NH3 decomposition catalyzed by transition metals is one of the
thoroughly investigated reactions in heterogeneous catalysis.1
The works before the 1990s on NH3 decomposition were
conducted mainly to get insights into its reverse reaction, NH3
synthesis.2,3 Until recently, NH3 has been proposed as a
promising COx-free (x = 1, 2) hydrogen carrier because of its
abundance based on the well-established Haber−Bosch
ammonia production process, high hydrogen content (17 wt
%), high energy density (3 kWh/kg), and facile storage and
transportation.4−7 To realize the practical utilization of NH3 as
a hydrogen carrier, the development of highly active catalyst is
of great importance. To date, various kinds of transition
metals,8−10 alloys,11,12 metal carbides, and nitrides13−15 have
been evaluated, among which Ru-based catalyst is the most
active.16 Carbon8,17−19 and N-modified carbon20−23 materials
are generally believed to be better supports than the oxide
materials for Ru because they may have the ability to donate
electrons and thus can weaken the metal−N bond and facilitate
the recombinative desorption of N atoms adsorbed on Ru
surfaces, which is a rate-determining step. A recent work
showing effective electron donation from the unique inorganic
electride of [Ca24Al28O64]4+(e−)4 to Ru leading to enhanced
catalytic activity has been reported.24 It is a common practice to
add small amount of promoters, such as alkali or alkaline earth
© 2016 American Chemical Society
metal oxides and hydroxides, to enhance the catalytic activities
of transition metals.25−28 However, the promotional capabilities
and promoting mechanisms of alkali or alkaline earth metals or
compounds are still controversial. The alkalis have usually been
regarded as electronic promoters, with similar roles as the
electride. Mg and Ca are considered to be structural promoters,
whereas Ba has been proposed to be either electronic29,30 or
structural promoter.31 Yin et al. reported that the order of
promoting effect can be ranked as K > Na > Li > Ba > Ca on
the Ru/CNTs catalysts,32 whereas Zhu et al. found that the
promoting effect follows the order K > Na > Ca > Li on the
Ru/CMK-3 catalysts.33 Nagaoka reported that Ru/Pr6O11
doped with alkali metal oxides, except for Li2O, exhibited
higher NH3 conversions than bare Ru/Pr6O11. In contrast, Ru
samples doped with alkaline earth metal oxides showed lower
NH3 conversions than the bare Ru/Pr6O11.34
Recently we reported that the promoting effects of alkalis
depend strongly on their chemical forms. Li, commonly
regarded as the least promoting alkali, when in the form of
lithium amide (LiNH2), can synergize with Ru leading to
extraordinarily high catalytic activity that is even superior to
Received: December 1, 2015
Revised: January 18, 2016
Published: January 19, 2016
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Activity was measured at 25 °C interval. Ammonia conversion
data reported here were collected after 20 min time-on-stream
at the corresponding temperatures. The flow rates of 5 vol %
NH3/Ar (99.999%) and pure NH3 (99.999%) both were 30
mL/min regulated by a mass flow controller (Brooks
instrument, model 5850 E). The exhaust gas composition
including the unconverted NH3 and produced N2 was analyzed
by using an online gas chromatograph (GC-2014C, Shimadzu)
equipped with a Porapak N column and a thermal conductivity
detector.
Catalyst Characterization. ICP-OES (inductive coupled
plasma optical emission spectrometer) measurements: ICPOES (PerkinElmer ICP-OES 7300DV) was used to determine
the content of Ru in the catalyst. All of the catalysts contain ca.
5 wt % Ru.
XRD (X-ray diffraction) measurements: Phase identification
of samples was performed by using a PANalytical X’Pert
diffractometer with Cu Kα radiation at 40 kV and 40 mA. A
self-made sample cell covered with KAPTON film was used to
protect samples from air or moisture contamination.
Gaseous products analysis: The gaseous products after ballmilling RuCl3 with Mg(NH2)2, Ca(NH2)2, and Ba(NH2)2
samples were identified by a mass spectrometer (Hiden HPR20) adopting bar mode, which recorded the signals of m/z from
1 to 45 simultaneously.
BET (Brunauer, Emmett, and Teller) method: A Micromeritics ASAP 2010 automated physisorption instrument was
used to measure the N2 adsorption isotherm of self-made MgO
at liquid N2 temperature (−196 °C). The specific surface area
was determined from the linear portion of the BET plot. Before
measurement, the sample was heated to 300 °C and kept at this
temperature for 5 h. The specific surface area of MgO is ca. 350
m2/g.
HRTEM (high resolution transmission electron microscopy)
measurements: To identify the chemical state, morphology, and
particle size of Ru, HRTEM (JEM-2100) was performed on
these catalysts at 200 kV. Typically, the catalyst powder was
dispersed in tetrahydrofuran (THF) and dropped on a carboncoated copper TEM grid.
TPR (temperature-programmed-reaction) measurements:
The gaseous products from samples during the heating process
were measured by using a homemade TPR system equipped
with an online mass spectrometer (Hiden HPR-20), which
recorded the signals of H2 (m/z = 2), N2 (m/z = 28), and NH3
(m/z = 17) simultaneously. Pure Ar was used as the carrier gas
(40 mL/min). In each test, ca. 10 mg quartz fiber was loaded in
the middle of the straight quartz reactor (diameter: 0.6 cm;
length: 25 cm) to hold the catalyst powder. 30 mg of sample
was loaded and heated from room temperature to 500 °C at a
ramping rate of 5 °C/min.
Kinetic analysis: Kissinger’s approach36−38 was employed to
determine the apparent activation energy (Ea) of gas release
from the Ru−Ca(NH2)2 and Ru−Ba(NH2)2 samples. The
Kissinger equation is described as
that of K-promoted Ru/MgO for NH3 decomposition.35 The
role of Li was suggested to stabilize the NHx (x = 1, 2) species,
and Ru mediates the electron transfer facilitating the NHx
coupling to form N2 and H2. It is therefore of interest to find
whether amides of other alkali or alkaline earth metals can have
similar roles in catalytic NH3 decomposition.
In the present work, we aim to investigate the effects of
alkaline earth metal (Mg, Ca, and Ba) amides on the catalytic
behavior of Ru for NH3 decomposition. Our experimental
results show that the catalytic activities rank in the order of
Ru−Ba(NH2)2 > Ru−Ca(NH2)2 > Ru−Mg(NH2)2, among
which Ru−Ba(NH2)2 and Ru−Ca(NH2)2 catalysts have higher
intrinsic activities (TOF) and lower apparent activation
energies than those of Ru−Mg(NH2)2 and reference Ru/
MgO catalysts, indicating that Ca(NH2)2 and Ba(NH2)2 may
have different roles from those of Mg(NH2)2 and MgO.
■
EXPERIMENTAL SECTION
Catalyst Preparation. Mg(NH2)2 was synthesized by
reacting metallic Mg powder (Sigma-Aldrich, 99%) with
purified NH3 (Dalian CREDIT, 99.999%) at 300 °C for ca.
200 h. Ca(NH2)2 and Ba(NH2)2 were prepared by reacting
calcium (Alfa-Aesar, 99%, shot diameter: ca. 1 cm) and barium
(Sigma-Aldrich, 99%, shot diameter: ca. 2 cm) metals with
liquid ammonia in closed systems at room temperature for 1
day. The synthesized samples were collected for XRD (X-ray
diffraction) characterization as shown in Figure S1. Figure S1
shows no any other phases except the alkaline earth metal
amides. RuCl3·4.7H2O was dried at 210 °C for 7 h under
vacuum. TG (thermogravimetric) measurement was carried out
on postdried sample showing no obvious weight loss until 500
°C, indicating that crystal water was removed completely. The
Ru-alkaline earth metal amide composite catalysts were
prepared by ball-milling self-made alkaline earth metal amides
(1 g) with RuCl3 (114 mg) in an iron jar at 150 rpm for 3 h on
a Retsch planetary ball mill (PM 400). The desired loadings of
Ru in the catalysts were 5 mg per g catalyst. The obtained
samples were denoted as Ru−Mg(NH2)2, Ru−Ca(NH2)2, and
Ru−Ba(NH2)2, respectively. High surface area MgO was
prepared by the precipitation method. C4H6O4Mg·4H2O
(DAMAO, 99%) and H2C2O4 (Kermel, 99%) were used as
starting materials. C4H6O4Mg·4H2O and H2C2O4 were
dissolved in distilled water. The C4H6O4Mg aqueous solution
was added to H2C2O4 aqueous solution with constant stirring
for 3 h to ensure complete precipitation. After filtration, the
precipitate was washed three times and dried in an oven at 100
°C for 2 h. The dried precursor was calcined at 540 °C for 4 h
under argon flow, and then the high surface area MgO was
obtained. MgO supported Ru catalyst was prepared by the
incipient wetness impregnation of MgO with acetone solution
of RuCl3. Prior to test, the sample was reduced in H2 (30 mL/
min) at 400 °C for 2 h. All catalysts were stored in a glovebox
filled with Ar to protect samples from air or moisture
contamination.
Catalyst Testing. Ammonia decomposition reaction was
performed on a continuous-flow fixed-bed straight quartz
reactor (diameter: 0.6 cm; length: 25 cm) at atmospheric
pressure. Typically, ca. 10 mg quartz fiber was loaded in the
middle of the quartz reactor to hold the catalyst powder. 30 mg
of catalyst powder was loaded, calcined in an argon flow at 250
°C for 2 h, and tested in the temperature range of 250−500 °C
under 5 vol % NH3/Ar or 300−600 °C under pure NH3 flow.
The temperature was raised at a ramping rate of 5 °C/min.
d[ln(β /Tm 2)/d(1/Tm)] = −Ea /R
where Tm is the peak temperature at which the maximum
reaction rate is attained, β is the heating rate, Ea is the activation
energy, and R is the gas constant. The TPR technique was
employed to collect the peak temperatures of H2 at various
heating rates (5, 6, 8, and 10 °C/min). The dependency of
ln(β/Tm2) on 1/Tm was plotted, and the slope of the fitted line
was used to determine the value of Ea/R.
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■
RESULTS AND DISCUSSION
The Ru−alkaline earth metal amide composite catalysts were
prepared by ball-milling mixtures of RuCl3 and alkaline earth
metal amides (Mg(NH2)2, Ca(NH2)2, and Ba(NH2)2). The
gaseous products after ball-milling were analyzed by mass
spectrometer. NH3 and N2 were detected for ball-milled
RuCl3−Ca(NH2)2 and Ba(NH2)2 samples except for the
RuCl3−Mg(NH2)2 sample (Figure S2). The formation of N2
indicates the occurrence of redox reaction between RuCl3 and
Ca(NH2)2 (or Ba(NH2)2) during the ball-milling process, and
it is expected that the Ru3+ should be reduced simultaneously.
The solid products after ball-milling were collected for further
XRD characterizations. However, no any of the crystalline
phases related to Ru metal can be observed except for the
unreacted alkaline earth metal amides (Figure 1), which may
Figure 2. HRTEM images of (a) Ru−Mg(NH2)2, (b) Ru−Ca(NH2)2,
and (c) Ru−Ba(NH2)2 catalysts collected after heating at 500 °C
under an Ar atmosphere.
Figure 1. XRD patterns of as-prepared (a) Ru−Mg(NH2)2, (b) Ru−
Ca(NH2)2, and (c) Ru−Ba(NH2)2 samples.
due to the high dispersion of Ru particles on the Ca(NH2)2 (or
Ba(NH2)2) or unreduced RuCl3 on the Mg(NH2)2. After
calcination at 500 °C in Ar flow, samples were collected and
characterized by HRTEM. As shown in Figure 2, nanoparticles
show lattice spacing of the (100), (002), or (101) crystallographic planes of metallic Ru, and no particle with lattice
spacing corresponding to ruthenium oxide was detected in all of
these three composite catalysts, indicating that metallic Ru
formed after calcination. Statistical analyses by counting 100 Ru
nanoparticles provide the information on particle size
distributions of Ru−Mg(NH2)2, Ru−Ca(NH2)2, Ru−Ba(NH2)2, and Ru/MgO samples (Figure 3), and the mean
particle sizes and dispersions of Ru are summarized in Table 1.
The average particle sizes of Ru in all samples are all around 2−
4 nm. It has been widely recognized that both NH3 synthesis
and decomposition are structure-sensitive reactions.39 Structure
sensitivity of the reaction indicates the need for a polynuclear
cluster as an active site. B5-type site consists of an arrangement
of three Ru atoms in one layer and two further Ru in the layer
directly above this at a monatomic step on an Ru(0001) terrace
and was proposed as Ru active sites for the ammonia synthesis
process.40 Based on this model, the concentration of B5 sites
has been estimated to be the highest for Ru particles with size
around 2−4 nm.39,41 The similar particle size distributions of
Ru−alkaline earth metal amides allow the comparison of the
effects of alkaline earth metal amides on catalytic behaviors of
Ru nanoparticles.
Shown in Figure 4a is the temperature dependence of NH3
conversion over a series of Ru-based catalysts under a flow of
Figure 3. TEM images of (a) Ru−Mg(NH2)2, (b) Ru−Ca(NH2)2, (c)
Ru−Ba(NH2)2, and (d) Ru/MgO catalysts after catalytic testing at 600
°C under a pure NH3 atmosphere. Insets: Ru particle size
distributions.
pure NH3. Alkaline earth metal amides affect the catalytic
activities of Ru dramatically. The order of NH3 conversion can
be ranked as Ru−Ba(NH2)2 > Ru−Ca(NH2)2 > Ru−Mg(NH2)2 below 525 °C. Under diluted NH3 atmosphere, the
same sequence of activity is shown in Figure S3. It is worth
pointing out that the activity of Ru−Ba(NH2)2 has an obvious
drop above 350 °C under 5 vol % NH3/Ar (Figure S3) and
above 475 °C under pure NH3 atmosphere, which may be due
to the fact that Ba(NH2)2 has a relatively lower melting point of
280 °C, and the separation of metallic Ru from Ba(NH2)2 and
the aggregation of metallic Ru may occur.
The Arrhenius plots of all catalysts are compared in Figure
4b. The apparent activation energy (Ea) of Ru/MgO is 99.3 ±
8.6 kJ/mol, which is very similar to that of Ru−Mg(NH2)2
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Table 1. Properties and Activities of Ru-Based Catalysts
catalyst
Ru loadinga (wt %)
av particle sizeb (nm)
dispersionc (%)
H2 formation rated (mmolH2/gcat/min)
TOFH2 (s−1)
Ru−Mg(NH2)2
Ru−Ca(NH2)2
Ru−Ba(NH2)2
Ru/MgO
Ru/CNTse
5.0
4.6
4.4
4.7
4.8
2.6
3.2
3.7
2.7
2
50.1
41.0
35.2
48.3
21.1
1.21
4.60
8.07
4.22
5.7
0.14
0.42
1.29
0.33
1
a
Ru actual loading was determined by ICP-OES. bStatistical analyses by counting 100 Ru nanoparticles from TEM images. cRu dispersion was
calculated employing model proposed by Anderson;42 the calculation process is available in the Supporting Information. dH2 formation rate was
measured under pure NH3 flow at 400 °C. WHSV (weight hourly space velocity) = 60 000 mLNH3/gcat/h. eThe properties and activities of Ru/CNTs
are available from ref 43.
Figure 4. Temperature dependence of NH3 conversion over Ru-based
catalysts (a) and the corresponding Arrhenius plots (b). Reaction
conditions: sample loading, 30 mg; flow rate, 30 mL/min, pure NH3.
(101.4 ± 7.7 kJ/mol). Compared with Ru/MgO and Ru−
Mg(NH2)2, a significant drop in apparent activation energies
(ca. 25 kJ/mol) can be achieved for Ru−Ca(NH2)2 and Ru−
Ba(NH2)2, which are 76.7 ± 2.0 and 73.1 ± 1.7 kJ/mol,
respectively. Table 1 summarizes the TOF values calculated
from the dispersions of Ru nanoparticles. The TOF values of
Ru−Ba(NH2)2 and Ru−Ca(NH2)2 are 1.29 and 0.42 s−1, which
are 8.2 and 2 times higher than that of Ru−Mg(NH2)2 (0.14
s−1) at 400 °C, respectively. It is worth pointing out that TOF
value of Ru−Ba(NH2)2 is even superior to that of highly active
Ru/CNTs catalyst (1.1 s−1, 400 °C, WHSV = 150 000 mLNH3/
gcat/h).6 Considering the similar particle size distributions of Ru
on all of the catalysts, the different apparent activation energies
and intrinsic activities (TOF) reflect that Ru−Ca(NH2)2 and
Ru−Ba(NH2)2 may have different reaction pathway from Ru/
MgO and Ru−Mg(NH2)2 in catalytic ammonia decomposition.
To investigate the interactions of Ru and alkaline earth metal
amides, TPR measurements were carried out. As shown in
Figures 5a and 5b, the onset and peak temperatures of NH3
over Ru−Mg(NH2)2 are ca. 300 and 375 °C, respectively,
which exhibit no significant difference from those of neat
Mg(NH2)2, indicating the decomposition pathway of Mg-
Figure 5. TPR profiles of (a) ball-milled Mg(NH2)2, (b) Ru−
Mg(NH2)2, (c) ball-milled Ca(NH2)2, (d) Ru−Ca(NH2)2, (e) ballmilled Ba(NH 2 ) 2 , and (f) Ru−Ba(NH 2 ) 2 samples. Reaction
conditions: sample loading, 30 mg; Ar flow rate, 40 mL/min; ramping
rate, 5 °C/min.
(NH2)2 to release NH3 is not altered in the presence of Ru.
However, the formation of H2 and N2 above 300 °C can be
observed for Ru−Mg(NH2)2 showing the main difference from
that of neat Mg(NH2)2. The fact that the peak temperatures of
H2 and N2 coincide very well with the NH3 peak temperature
(ca. 375 °C) is interesting. It can be proposed that Mg(NH2)2
decomposes to release NH3,44 which further decomposes to
release N2 and H2 over Ru nanoparticles. Above results
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that Ru may also has a catalytic function that directly
decomposes Ba(NH2)2 to form H2, N2, and BaNH through
an energy more favorable path (R4).
evidence that there is no obvious interaction between
Mg(NH2)2 and Ru, particularly in the temperature range of
250−500 °C.
For the Ru−Ca(NH2)2 sample, however, the presence of Ru
affects the decomposition behavior of Ca(NH2)2 significantly.
As shown in Figure 5c, two NH3 peaks are observed for the
neat Ca(NH2)2, which can be ascribed to 3Ca(NH2)2 →
(CaNH)2·Ca(NH2)2 + 2NH3 and (CaNH)2·Ca(NH2)2 →
3CaNH + NH3 (R1), respectively.45 The first NH3 peak
temperature almost does not change after the addition of Ru,
whereas the second NH3 peak which is observed in the neat
Ca(NH2)2 (T = 429 °C) disappears in the Ru−Ca(NH2)2
sample (Figure 5d). Instead, H2 and N2 are observed with the
peak temperatures at 388 °C, which are 41 °C lower than the
second NH3 peak of neat Ca(NH2)2. We propose here that
direct decomposition of (CaNH)2·Ca(NH2)2 to from N2, H2,
and CaNH (R2) in the presence of Ru may occur, similar to the
case of Ru−LiNH2 that we reported previously.46 We deduce
that Ru should have a catalytic function that directly promote
the decomposition of (CaNH)2·Ca(NH2)2 to release H2 and
N2 through an energy more favorable path. The following two
reactions may represent the thermal decomposition paths,
respectively:
(CaNH)2 Ca(NH 2)2 → 3CaNH + NH3
(CaNH)2 Ca(NH 2)2 → 3CaNH +
without Ru
3
1
H 2 + N2
2
2
1
3
2Ba(NH 2)2 → 2BaNH + NH3 + ( N2 + H 2)
2
2
without Ru
(R3)
Ba(NH 2)2 → BaNH +
3
1
H 2 + N2
2
2
with Ru
(R4)
The effects of Ru on Ca(NH2)2 and Ba(NH2)2 are quite
similar to the case of Ru−LiNH2,46 in which Ru promotes the
NHx coupling and changes the decomposition pathways of
Ca(NH2)2 and Ba(NH2)2. As shown in Figure 6, the activation
(R1)
Figure 6. Kissinger’s plots of Ru−Ca(NH2)2 (red) and Ru−Ba(NH2)2
(blue) samples.
with Ru
(R2)
The possibility of an alternative pathway of self-decomposition
of (CaNH)2·Ca(NH2)2 to form CaNH and NH3 which further
decomposes to release N2 and H2 over Ru nanoparticles cannot
be excluded. However, if this path is the possible pathway, the
production rates of N2 and H2 should be expected to depend
on the formation rate of NH3 from the decomposition of
(CaNH)2·Ca(NH2)2; then the H2 peak temperature of Ru−
Ca(NH2)2 should be the same as the second NH3 peak
temperature of pure Ca(NH2)2. However, as shown in Figure
5d, H2 and N2 peak temperatures are 388 °C, which are 41 °C
lower than the second NH3 peak temperature of neat
Ca(NH2)2. Hence, the former pathway, i.e., direct formation
of H2 and N2 from (CaNH)2·Ca(NH2)2 (R2) may prevail in
this case.
The decomposition behavior of Ba(NH 2 ) 2 is more
complicated than those of Mg(NH2)2 and Ca(NH2)2. Jacobs
et al. reported that Ba(NH2)2 decomposes to Ba(NH)1−xN2/3x
(x = 0.1) and NH3 after heating to 370 °C.47 With the aid of
mass spectrometer (MS), we observed the evolution of
considerable amount of H2 and N2 besides NH3 as shown in
Figure 5e. XRD pattern shows that BaNH forms after TPR
treatment to 500 °C (Figure S4). Combined with TPR and
XRD results, reaction R3 may occur during the TPR process.
The reaction pathway of formation of N2 and H2 during the
decomposition process of neat Ba(NH2)2 is not clear yet;
however, it indicates that the NHx coupling over Ba(NH2)2 to
release N2 and H2 (R3) has a much lower kinetic barrier than
those of neat Ca(NH2)2 and Mg(NH2)2 and even than LiNH2.
For the Ru−Ba(NH2)2 sample, as shown in Figure 5f, the
evolution of NH3 was significantly reduced. Similar to the case
of Ru−Ca(NH2)2, the difference between NH3 peak temperature of neat Ba(NH2)2 and N2 or H2 peak temperature of Ru−
Ba(NH2)2 suggests that the direct route of decomposition of
Ba(NH2)2 to release N2 and H2 (R4) may prevail. We deduce
energies of reactions R2 and R4 were obtained by using
Kissinger’s method. The activation energies of reactions R2
(72.3 kJ/mol) and R4 (67.3 kJ/mol) are consistent with the
apparent activation energies of NH3 decomposition over Ru−
Ca(NH2)2 (76.7 kJ/mol, Figure 4b) and Ru−Ba(NH2)2 (73.1
kJ/mol, Figure 4b) very well, respectively, evidencing that Ru
catalyzing the decomposition of amides to form H2 and N2 may
be the rate-determining step for catalytic NH3 decomposition.
We suggest that the catalysis over Ru−Ca(NH2)2 and Ru−
Ba(NH2)2 catalysts is very likely fulfilled via two steps: (1) Ru
catalyzes the decomposition of amides to form H2, N2, and
imides, and (2) imides react with NH3 to regenerate amides.
That is to say, the combination of reaction R2 (or R4) and
reverse reaction R1 (or R3) allows the fulfillment of the
catalytic cycle of NH3 decomposition, among which Ru
catalyzing NHx coupling to N2 and H2 is the rate-determining
step for catalytic NH3 decomposition. This case is very different
from that NH3 decomposition over transition metals. On the
surfaces of transition metals, it is widely accepted that NH3
decomposition proceeds consecutive dehydrogenation followed
by recombinative desorption of N and H atoms, among which
the recombinative desorption of N atoms is the ratedetermining step.6
Although the NHx coupling over neat Ca(NH2)2 is very
difficult (Figure 5c shows minor amount of H2 and N2) because
of the large repulsive interaction between these two negatively
charged NHx groups, the presence of Ru may decrease the
negative charge of NHx groups through electron transfer
leading to a reduced kinetic barrier of NHx coupling. The
function of Ru together with the easiness of formation of N2
and H2 for neat Ba(NH2)2 (Figure 5e shows large amount of
H2 and considerable amount of N2) indicates the kinetic barrier
of NHx coupling on Ru−Ba(NH2)2 is much lower than that of
Ru−Ca(NH2)2, which may account for the higher intrinsic
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*(P.C.) E-mail [email protected]; Tel 86-411-84379905; Fax
86-411-84379583.
activity of Ru−Ba(NH2)2 for NH3 decomposition. The
presence of Ca(NH2)2 or Ba(NH2)2 creates a NHx-rich
environment and Ru mediates the electron transfer from
NHx to facilitate NHx coupling to N2 and H2. However, for the
Ru−Mg(NH2)2 composite, Mg(NH2)2 decomposes to Mg3N2
(Figure S5) rather than MgNH (unstable above 300 °C),
resulting in the less chances of interaction of Mg(NH2)2 with
Ru; thus, no significantly synergistic effect of Mg(NH2)2 on Ru
can be obtained, and the catalytic activity of Ru−Mg(NH2)2
mainly comes from metallic Ru. In that case, magnesium amide,
imide, or nitride can only function as a support.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
The authors thank the financial support from the Project of
National Science Funds for Distinguished Young Scholars
(51225206) and National Natural Science Foundation of China
(51472237 and 21473181).
■
■
CONCLUSION
In summary, the effects of alkaline earth metal amides
(Mg(NH2)2, Ca(NH2)2, and Ba(NH2)2) on Ru in catalyzing
NH3 decomposition have been evaluated. NH3 conversions
rank in the order of Ru−Ba(NH2)2 > Ru−Ca(NH2)2 > Ru−
Mg(NH2)2, among which Ru−Ba(NH2)2 and Ru−Ca(NH2)2
catalysts have higher intrinsic activities (TOF) and lower
apparent activation energies than those of Ru−Mg(NH2)2 and
reference Ru/MgO catalysts, indicating that Ca(NH2)2 and
Ba(NH2)2 may have different roles from those of Mg(NH2)2
and MgO. The TPR results show that Ca(NH2)2 and
Ba(NH2)2 decompose to N2 and H2 rather than NH3 in the
presence of Ru. Ru may promote the NHx coupling to H2 and
N2 and change the decomposition pathways of Ca(NH2)2 and
Ba(NH2)2. The facts that the activation energies of (R2) and
(R4) are consistent with the apparent activation energies of
NH3 decomposition over Ru−Ca(NH2)2 and Ru−Ba(NH2)2
very well, respectively, suggest that Ru catalyzing the
decomposition of amides to form H2 and N2 may be the
rate-determining step for catalytic NH3 decomposition. We
suggest that the catalysis over Ru−Ca(NH2)2 and Ru−
Ba(NH2)2 catalysts is very likely fulfilled via (1) Ru catalyzes
the decomposition of amides to form H2, N2, and imides
through an energy more favorable pathway and (2) imides react
with NH3 to regenerate the amides. In this context, Ba(NH2)2
and Ca(NH2)2 are neither electronic nor structural promoters.
They participate in the catalytic reaction. The presence of
Ca(NH2)2 or Ba(NH2)2 creates a NHx-rich environment, and
Ru mediates the electron transfer from NHx to facilitate NHx
coupling to release N2 and H2.
■
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ASSOCIATED CONTENT
S Supporting Information
*
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jpcc.5b11768.
XRD patterns of self-made alkaline earth metal amide
samples; MS (mass spectrum) analysis of collected
gaseous products after ball-milling RuCl3 with Ca(NH2)2; temperature dependence of NH3 conversion
over Ru-based catalysts under 5 vol % NH3/Ar; XRD
pattern of Ba(NH2)2 sample collected after TPR to 500
°C; XRD pattern of decomposition products of Mg(NH2)2 at 500 °C; Ru dispersion calculation method
(PDF)
■
REFERENCES
AUTHOR INFORMATION
Corresponding Authors
*(J.G.) E-mail [email protected]; Tel 86-411-84379583;
Fax 86-411-84379583.
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DOI: 10.1021/acs.jpcc.5b11768
J. Phys. Chem. C 2016, 120, 2822−2828
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