Highly effective direct decomposition of nitric oxide by microwave catalysis over BaMeO3 (Me=Mn, Co, Fe) mixed oxides at low temperature under excess oxygen

Article abstract of DOI:10.1002/cctc.201500966

The direct catalytic decomposition of NO with high activity and N₂ selectivity is a great challenge at low temperature under excess oxygen. Herein, we report the NO decomposition by microwave catalysis over BaMeO₃ (Me=Mn, Co, Fe) mixed oxides for the first time at low temperature under excess oxygen, for which the BaCoO₃ catalyst has an outstanding activity with a high NO conversion and N₂ selectivity up to 99.8 % and 99.9 %, respectively, even at 250 °C. Comparatively, the best NO conversion is 93.7 % for BaMnO₃ and only 64.1 % for BaFeO₃ under microwave irradiation. H₂ temperature-programmed reduction, O₂ temperature-programmed desorption, and the microwave-absorbing properties of the mixed oxides were characterized to illustrate possible reasons that cause such clear differences in the catalytic performance. Importantly, the apparent activation energies for BaMnO₃, BaCoO₃, and BaFeO₃ are as low as 33.4, 13.7, and 46.7 kJ mol^-1, respectively, which suggests a significant microwave catalytic effect. NO limits: NO decomposition is reported by microwave catalysis over BaMeO₃ (Me=Mn, Co, Fe) mixed oxides for the first time at low temperature under excess oxygen, for which the BaCoO₃ catalyst has an outstanding activity with a high NO conversion and N₂ selectivity up to 99.8 and 99.9 %, respectively, even at 250 °C.

Full text of DOI:10.1002/cctc.201500966

DOI: 10.1002/cctc.201500966
Full Papers
Highly Effective Direct Decomposition of Nitric Oxide by
Microwave Catalysis over BaMeO (Me=Mn, Co, Fe) Mixed
3
Oxides at Low Temperature under Excess Oxygen
Wentao Xu, Jinjun Cai, Jicheng Zhou,* Yingpiao Ou, Wei Long, Zhimin You, and
[a]
Yushang Luo
The direct catalytic decomposition of NO with high activity
and N2 selectivity is a great challenge at low temperature
under excess oxygen. Herein, we report the NO decomposition
for BaFeO under microwave irradiation. H temperature-pro-
3 2
grammed reduction, O temperature-programmed desorption,
2
and the microwave-absorbing properties of the mixed oxides
were characterized to illustrate possible reasons that cause
such clear differences in the catalytic performance. Importantly,
the apparent activation energies for BaMnO , BaCoO , and
by microwave catalysis over BaMeO (Me=Mn, Co, Fe) mixed
3
oxides for the first time at low temperature under excess
oxygen, for which the BaCoO catalyst has an outstanding ac-
3
3
3
À1
tivity with a high NO conversion and N2 selectivity up to
BaFeO are as low as 33.4, 13.7, and 46.7 kJmol , respectively,
3
9
9.8% and 99.9%, respectively, even at 2508C. Comparatively,
which suggests a significant microwave catalytic effect.
the best NO conversion is 93.7% for BaMnO and only 64.1%
3
Introduction
[
19]
Nitrogen oxides (NO ), emitted from various fixed and mobile
temperatures up to 5008C in the CRM. However, the direct
x
sources, are considered as one of the most hazardous pollu-
tants because of their drastic consequences on ecosystems,
decomposition of NO over perovskite-type oxides is inhibited
strongly by oxygen, which leads to a drastic decrease in NO
[1,2]
[2,9]
the atmosphere, and human health.
The direct decomposi-
conversion.
Thus, the direct catalytic decomposition of NO
tion of NO into N and O offers the most attractive and ideal
into N and O with high activity and N selectivity at low tem-
2
2
2
2
2
route to remove NO with respect to economic and environ-
mental considerations as the products of the reaction are envi-
ronmentally friendly and it does not involve the addition of
perature under excess oxygen is still a great challenge.
Recently, microwave-enhanced catalytic reactions have
aroused tremendous attention as they show a drastic accelera-
tion of the catalytic reaction rate and selectivity under mild
[3,4]
toxic reducing agents, such as NH3.
Several catalysts have
[
20–28]
been investigated for the direct decomposition of NO in the
conventional reaction mode (CRM), which include noble
conditions.
Furthermore, some chemical transformations
that are impossible in the CRM can be performed successfully
[
5–7]
[8–12]
[29–31]
metals,
metal oxides,
and metal-ion-exchanged zeo-
under the microwave reaction mode.
This significant mi-
[
13–16]
lites.
Unfortunately, Cu-ZSM-5 (one of the most active cat-
crowave effect may provide a new route to address the follow-
ing two main challenges for NO decomposition over perov-
skite-type oxides at a high reaction temperature with strong
oxygen inhibition. There are several studies in which the mi-
crowave technique is applied to accelerate the reaction rate
alysts for the direct decomposition of NO) has a low hydrother-
mal stability and will be deactivated at temperatures higher
than 6008C, and noble-metal catalysts are expensive and inhib-
[17,18]
ited strongly by oxygen.
Therefore, composite mixed
[
32–34]
oxides, which include perovskite-type oxides, are recognized
as promising candidates for direct NO decomposition because
for NO removal by a microwave-discharge method
and
[
35–39]
microwave-assisted reduction method.
However, to date,
[
2,9,18,19]
they are cost effective and stable even above 10008C.
Perovskite-type oxides usually display good activity for NO de-
little attention has been paid to the microwave direct catalytic
decomposition of NO. Tang et al. reported direct NO decompo-
sition over the Fe/ZSM-5 catalyst in the microwave-heating
[3,18,19]
composition at high temperatures in the CRM.
For exam-
[
40]
ple, Ba La Mn Mg O , one of the most active perovskite-
mode with 70% NO conversion. Previously, our group re-
ported the microwave direct decomposition of NO over physi-
0.8
0.2
0.8
0.2
3
type oxides, displays catalytic activity for NO decomposition at
[
41]
cally mixed MeO -Cu-ZSM-5 (Me=Mn, Ni) catalysts
and
x
[
42]
BaMn1 Mg O3 mixed oxides
at a low temperature under
Àx
x
[
a] W. Xu, Dr. J. Cai, Prof. J. Zhou, Y. Ou, W. Long, Z. You, Y. Luo
Key Laboratory of Green Catalysis and
Chemical Reaction Engineering of Hunan Province
School of Chemical Engineering
excess oxygen, in which the NO conversion reached 94.3% for
MnO -Cu-ZSM-5 at 3008C, 92.3% for Ni O -Cu-ZSM-5 at 3508C,
2
2
3
and 99.9% for BaMn Mg O at 2508C. Although the applica-
0.9
0.1
3
Xiangtan University
Xiangtan 411105 (P.R. China)
Fax: (+86)731-58298173
E-mail: zhoujicheng@sohu.com
tion of microwave irradiation to speed up chemical reactions
continues to grow, the intrinsic nature of microwave irradiation
on chemical reactions has still not been illuminated. In particu-
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Full Papers
lar, there is still considerable controversy over the origin of the
rate acceleration, whether it is caused by thermal microwave
[21,22,29–31]
effects or nonthermal microwave effects.
Few investi-
gations have focused on the decrease of the apparent activa-
[43–45]
tion energy under microwave irradiation.
the activation energy for the direct catalytic decomposition of
NO over MeO -Cu-ZSM-5 (Me=Mn, Ni) and BaMn Mg O cata-
We found that
x
1Àx
x
3
lysts can be decreased significantly through microwave
[
41,42]
catalysis.
The choice or development of outstanding catalysts for mi-
[
40,46,47]
crowave catalytic reactions is of particular importance,
in
view that catalysts selected or designed for their microwave-
absorbing properties may speed up reactions drastically and
even facilitate some chemical transformations that are impossi-
[
47]
ble under the CRM. However, no exact knowledge can pro-
vide a reference to select or develop a superb catalyst for mi-
3 3 3 3
Figure 1. XRD patterns of BaMnO , BaCoO , and BaFeO catalysts (* BaCO
and * BaMeO3).
[40,46]
crowave chemistry at the current stage.
Little attention
has been paid to the direct decomposition of NO through mi-
crowave catalysis, particularly over perovskite-type oxides cata-
lysts. Previously, we found that BaMn1 Mg O mixed oxides
Àx
x
3
have a good catalytic performance for the microwave direct
[42]
decomposition of NO. To the best of our knowledge, there
are no reports on BaMeO (Me=Mn, Co, Fe) mixed oxide cata-
3
lysts for the direct decomposition of NO in the microwave cat-
alytic reaction mode (MCRM), and the influence of Me sites on
the microwave-absorbing properties and catalytic per-
formance. Here, we report direct NO decomposition through
microwave catalysis over BaMeO3 (Me=Mn, Co, Fe) mixed
oxides at low temperature under excess oxygen. The objective
of this study is to evaluate the potential of the application of
this new method as an effective NO emission control strategy
x
and then to illustrate the influence of Me sites on the micro-
wave-absorbing properties and catalytic performance under
microwave irradiation.
2 3 3 3
Figure 2. H -TPR profiles of BaMnO , BaCoO , and BaFeO catalysts.
er peak at 3558C before the main reduction peak at 4938C in-
4
+
Results
dicates the existence of Mn in the structure of BaMnO ,
3
4+
3+
which is attributed to the reduction of Mn into Mn and fi-
Catalyst characterization
2+
nally into Mn . For the BaCoO mixed oxide, only one broad
3
The XRD patterns of BaMeO (Me=Mn, Co, Fe) are shown in
reduction peak between 377 and 3958C can be observed. This
3
Figure 1. All the BaMeO (Me=Mn, Co, Fe) samples contain the
phenomenon could be ascribed to the simultaneous reduction
3
4
+
3+
3+
2+ [51]
diffraction peaks of the BaMeO (Me=Mn, Co, Fe) phase, and
of Co into Co and Co into Co . The temperature at
3
the main diffraction peaks for all these samples can be as-
the peak maximum (Tmax) reflects the reducibility, and peak
[
52]
signed to the BaCO phase, which indicates that BaMeO (Me=
areas represent the total amount of H consumed. The Tmax
3
3
2
Mn, Co, Fe) samples are mixed oxides. The thermal decomposi-
of BaCoO of 3878C is much lower than that of BaMnO of
4938C. Moreover, the total amount of H2 consumed for
3
3
[48]
tion temperature of BaCO3 is above 8008C.
Therefore,
BaMeO (Me=Mn, Co, Fe) samples calcined at 7008C in our
BaCoO is larger than that for BaMnO . For the BaFeO mixed
3
3
3
3
present study generated mainly the BaCO phase, which re-
oxide, two reduction peaks at 616 and 6938C can be observed.
3
[49]
3+
2+
sembles the La2 Ba NiO (x=0.6–1.2) catalysts.
The former is associated with the reduction of Fe into Fe ,
Àx
x
4
2+
H temperature-programmed reduction (TPR) measurements
2
and the latter is considered to be the reduction of Fe into
0
[51]
were performed to investigate the relative reducibility of the
Fe . The Tmax of BaMnO is much lower than that of BaFeO .
3 3
BaMeO (Me=Mn, Co, Fe) mixed oxides, and the results are
Moreover, the total amount of H consumed for BaMnO is sig-
2 3
3
listed in Figure 2. For the BaMnO mixed oxide, only one main
nificantly larger than that for BaFeO . Therefore, these results
3
3
reduction peak at 4938C and a shoulder peak at 3558C can be
suggest that the relative reducibility of these three samples in-
observed. It has been reported extensively that the complete
creases in the order of BaCoO >BaMnO >BaFeO .
3
3
3
4+
3+
2+
0
reduction of Mn , Mn , or Mn into Mn does not take place
We also performed O temperature-programmed desorption
2
[50]
over Mn-based perovskite-type oxides. However, this should-
(TPD) experiments for BaMeO (Me=Mn, Co, Fe), and the re-
3
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Figure 3. O
2
-TPD profiles of BaMnO
3
, BaCoO
3
, and BaFeO
3
catalysts.
Figure 4. Microwave-heating behavior of BaMnO , BaCoO , and BaFeO cata-
3
3
3
lysts in the MCRM at a microwave power of 150 W.
sults are shown in Figure 3. Generally, there are three types of
[
49,52,53]
desorbed oxygen species over perovskite-type oxides.
bed temperature increased with irradiation time, and a stable
temperature was then maintained (Figure 4). The stable bed
temperature was maintained at ~5038C for BaMnO3 and
The peak that appeared below 3008C could be assigned to the
desorption of ordinary chemically adsorbed and/or the weak-
est physically adsorbed molecular oxygen on the surface (de-
4938C for BaCoO under microwave irradiation for ꢀ30 min at
3
noted as a-O ). The peak at 300–7008C could be attributed to
a microwave power of 150 W. In contrast, the stable tempera-
2
desorption of oxygen adsorbed chemically on oxygen vacan-
ture of the bed was maintained at only 668C for BaFeO under
3
cies (denoted as b-O ). The peak at higher than 7008C could
be attributed to the lattice oxygen associated with the redox
the same microwave power and irradiation time. Clearly, the
microwave-adsorbing ability of BaMnO and BaCoO is much
2
3
3
processes of Me ions (denoted as g-O ). Notably, BaCO does
better than that of BaFeO . The bed temperature of the
2
3
3
[48,54]
not decompose to produce CO below 9008C.
Moreover,
BaCoO sample is slightly higher than that of BaMnO in the in-
2
3
3
no O desorption was observed for BaCO in O -TPD previous-
itial 5 min. Although the stable temperature of the BaMnO3
2
3
2
[55]
ly. Only one main desorption peak appeared for BaMnO3,
one main desorption peak with a small shoulder was seen for
BaCoO mixed oxide, and two desorption peaks for BaFeO are
sample is slightly higher than that of BaCoO , the microwave-
3
adsorbing ability of these two catalysts is still almost similar.
Therefore, BaMnO3 and BaCoO3 are excellent microwave-ab-
3
3
apparent (Figure 3). The temperature of the desorption peak
sorbing materials, whereas BaFeO is a poor one. In the follow-
3
related to g-O is 869, 846, and 8338C for BaMnO , BaCoO ,
ing experiments for the NO decomposition tests, the catalyst-
bed temperature is maintained at a specific reaction tempera-
ture by regulating the microwave input power.
2
3
3
and BaFeO , respectively. Moreover, a desorption peak related
3
to b-O is observed for the BaFeO mixed oxide. The desorp-
2
3
tion temperature of BaMnO3 and BaCoO3 are much higher
than that of BaFeO , even though the total amount of desor-
3
Effects of reaction temperature in the MCRM and CRM
bed oxygen for BaMnO3 and BaCoO3 is larger than that of
BaFeO . Therefore, the oxygen desorption capability increases
The NO conversions as a function of reaction temperatures
over BaMeO3 (Me=Mn, Co, Fe) catalysts in the MCRM are
shown in Figure 5. It is surprising that even at 1008C (a very
low reaction temperature for NO decomposition), the NO con-
version is 43.2% for BaMnO , 38.4% for BaCoO , and 39.0% for
3
in the order of BaFeO >BaCoO >BaMnO (if we consider the
3
3
3
desorption temperature and total amount of desorbed
oxygen). However, the BaMeO (Me=Mn, Co, Fe) mixed oxides
3
reported here are not proper perovskite-type oxides according
3
3
to the XRD results of BaMeO (Me=Mn, Co, Fe; Figure 1).
BaFeO in the MCRM. The NO conversion over BaMnO and
3 3
3
BaCoO catalysts increases initially and then decreases in the
3
reaction temperature range of 100–4008C. Importantly, the
Effects of reaction parameters under different reactor
systems
best NO conversion is up to 93.7% for BaMnO at 3008C and
3
[
40]
99.9% for BaCoO at 2508C. Comparatively, Tang et al. ap-
3
plied Fe/ZSM-5 as a catalyst for the direct decomposition of
NO into N and O in the microwave-heating mode and ach-
Microwave-heating behavior of BaMeO (Me=Mn, Co, Fe)
catalysts in the MCRM
3
2
2
ieved the highest NO conversion of only 70%. To the best of
[
42]
The desired temperature of the catalyst bed in the microwave
catalytic reactor is highly dependent on the microwave-absorb-
ing properties of the catalysts themselves. Here, the micro-
our knowledge, except for our previous work, there is no ex-
perimental study on the direct decomposition of NO with such
a high NO conversion at such a low temperature under excess
oxygen by microwave catalysis reported to date. Notably, the
wave-heating behavior of BaMeO (Me=Mn, Co, Fe) was stud-
3
ied at a microwave power of 150 W in the MCRM. The catalyst-
preparation route for BaMn Mg O catalysts in our previous
x 1Àx 3
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Figure 5. Effects of reaction temperature on NO conversion over BaMnO
3
,
Figure 6. Effects of reaction temperature on NO conversion (X) and N yield
2
BaCoO , and BaFeO catalysts in the MCRM (reaction conditions: molar frac-
3
3
(Y) over BaMnO , BaCoO , and BaFeO catalysts in the CRM (reaction condi-
3
3
3
À3
À3
tion of O
balance).
2
, 5%; molar fraction of NO, 0.1%; W/F=1 gscm ; and N
2
as the
tions: molar fraction of O , 5%; molar fraction of NO, 0.1%; W/F=1 gscm ;
2
and N as the balance).
2
work and BaMnO mixed oxide in this work differs in the start-
formance of BaMnO and BaCoO for the direct decomposition
3 3
3
ing barium salts. However, the best NO conversion for BaFeO3
of NO drastically. Comparatively, the BaFeO mixed oxide in the
3
is only 64.1% at 2508C (BaFeO is a poor microwave-absorbing
MCRM exhibits only a certain increase. Although the catalytic
3
material, which can be seen from Figure 4, and its catalyst-bed
temperature can only increase to 2508C in the MCRM at the
maximum microwave power of 1000 W). Therefore, BaMnO3
and BaCoO3 catalysts display much a better catalytic per-
formance for the microwave direct catalytic decomposition of
NO than BaFeO . Clearly, BaMnO and BaCoO are outstanding
activity of BaFeO at 5508C is higher than that of BaMnO ,
BaMnO3 still displays a higher catalytic performance than
3
3
BaFeO in the whole temperature range of 450–6508C. More-
3
over, BaMeO (Me=Mn, Co, Fe) mixed oxides in the CRM ex-
3
hibit a similar catalytic activity for the direct decomposition of
NO. Therefore, the catalytic performance of BaMeO (Me=Mn,
3
3
3
3
microwave catalysts for the direct decomposition of NO,
Co, Fe) mixed oxides in the CRM increases slightly in the order
whereas BaFeO is probably not suitable for this application.
of BaCoO >BaMnO >BaFeO .
3
3
3
3
The catalytic performance of BaCoO3 is better than that of
BaMnO as the reaction temperature was 150–3008C. In partic-
3
Effects of oxygen concentration in the MCRM and CRM
ular, the best NO conversion of BaCoO and BaMnO is much
3
3
higher than that of BaFeO . Therefore, the catalytic activity of
To further illustrate the impressively high catalytic performance
3
BaMeO (Me=Mn, Co, Fe) mixed oxides in the MCRM increases
of BaMeO (Me=Mn, Co, Fe) mixed oxides, the direct decom-
3
3
in the order of BaCoO >BaMnO >BaFeO .
position of NO into N and O over BaMeO (Me=Mn, Co, Fe)
3
3
3
2
2
3
To demonstrate microwave effects to speed up the direct
decomposition of NO drastically, we also studied the catalyst
mixed oxides in the MCRM was performed in the presence of
excess oxygen, and the results are shown in Figure 7. Surpris-
ingly, the oxygen concentration does not influence NO conver-
performance of BaMeO (Me=Mn, Co, Fe) in the CRM under
3
identical conditions of reactor sizes, catalyst beds, reactant
compositions, and total feed rate of the reactant gases, and
the results are shown in Figure 6. Clearly, N2 formation over
these three catalysts starts at 4508C, and the NO conversion at
sion and N selectivity in the MCRM. Importantly, the NO con-
2
version and N selectivity were up to 99.9% and 99.9%, re-
2
spectively, for BaCoO3 even with the coexistence of 10%
oxygen. Moreover, the N selectivity for BaCoO , BaMnO , and
2
3
3
4
508C is only 18.0, 30.0, and 24.0% for BaMnO , BaCoO , and
BaFeO in the MCRM was more than 99.8, 94.4, and 92.4%, re-
3
3
3
BaFeO , respectively. The highest NO conversion and N yield
spectively. The high selectivity suggested that almost all of the
NO transforms directly into N and O , and undesired reactions
3
2
are, respectively, only 45.4% and 28.7% for BaMnO , 46.8%
3
2
2
and 29.2% for BaCoO , and 37.7% and 24.9% for BaFeO at
between NO and O to form NO and the transformation of
2 2
3
3
6
508C. Previous reports have shown that the BaCO phase can
NO to N O hardly take place in the MCRM.
3
2
play a crucial role to improve the activity of perovskite-type
Normally, NO decomposition over perovskite-type oxides is
inhibited strongly by the coexistence of oxygen. We also stud-
ied the effect of the oxygen concentration on the catalytic ac-
[49,53]
oxides for the direct decomposition of NO.
The BaMnO3
mixed oxide in this study exhibits a slightly higher catalytic ac-
tivity than the corresponding BaMnO perovskite-type oxides
tivity of BaMeO (Me=Mn, Co, Fe) mixed oxides in the CRM
3
3
[
19]
in the literature, probably because BaMnO mixed oxide con-
under identical conditions, and the results are shown in
3
tains BaCO as the main phase (Figure 1). However, BaMnO3
Figure 7. Surprisingly, BaMeO (Me=Mn, Co, Fe) mixed oxides
3
3
and BaCoO mixed oxides still show a surprisingly high catalyt-
in the CRM exhibit an extremely low catalytic activity in the ab-
sence of oxygen, and the NO conversion increases significantly
with the oxygen concentration. This promoting effect of
3
ic activity at low temperatures in the MCRM (Figure 5). Clearly,
the microwave catalytic effect improves the catalytic per-
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(
Me=Mn, Co, Fe) mixed oxides that contain the BaCO phase
3
in our case.
The oxygen concentration does not influence the catalytic
performance of BaMeO (Me=Mn, Co, Fe) mixed oxides in the
3
decomposition of NO in the MCRM. This phenomenon is in ac-
[
41,42]
cordance with our previous studies.
The microwave selec-
tive effect under microwave irradiation may account for this
phenomenon. It is acknowledged widely that polar molecules
[
20,47]
can be activated by the microwave electromagnetic field.
[
39,41,42]
[32,38]
As reported previously by us
and others,
NO mole-
cules adsorbed on the active sites rather than O molecules in
2
the gas phase can be activated effectively by the microwave
electromagnetic field. Although both O and NO molecules can
2
be adsorbed physically on the active sites, only NO molecules
with a physical adsorption in the MCRM can be changed into
chemical adsorption and further translate into N and O . At
2
2
the same time, O molecules cannot be activated at such a low
2
reaction temperature because of the microwave selective
effect, and they are inactive to react with NO molecules to
form NO . Therefore, BaMeO (Me=Mn, Co, Fe) mixed oxides
2
3
in the MCRM can still display an outstanding resistance to
excess oxygen.
Effects of water vapor in the feed in the MCRM
It has been reported extensively that water vapor in the feed
will inhibit the catalytic performance of perovskite-type oxide
[
2,59]
catalysts in the direct decomposition of NO in the CRM.
We
investigated the effect of water vapor on NO conversion and
N selectivity over BaCoO mixed oxide (one of the most active
Figure 7. Effects of oxygen concentrations on a) NO conversion and b) N
2
se-
2
3
lectivity over BaMnO , BaCoO , and BaFeO catalysts in the MCRM and CRM
3
3
3
catalysts of the three mixed oxides in this study) in the MCRM.
(
reaction conditions: reaction temperature, 2508C in the MCRM and 6508C
The NO conversion and N selectivity display slight decreases
À3
2
in the CRM, respectively; molar fraction of NO, 0.1%; W/F=1 gscm ; and
in the presence of 5% H O (Figure 8). However, the BaCoO3
N
2
as the balance).
2
mixed oxide catalyst still exhibits impressively high catalytic ac-
tivity under wet conditions. Clearly, the BaCoO mixed oxide
3
catalyst in the MCRM displays a superior resistance to water
vapor. Previously, we found that BaMn Mg O displayed ex-
oxygen on the NO decomposition was also reported in previ-
0.9
0.1
3
[42]
[
56–58]
ous studies.
We speculate that the NO generated during
cellent resistance to water vapor in the MCRM. In the CRM,
2
the reaction may be an intermediate species that plays an im-
the decreasing activity of a catalyst with the addition of H O is
2
[49,56,58]
portant role in the direct decomposition of NO.
In the
presence of oxygen, the generation of NO is favored. Howev-
2
er, the surface of the catalyst is maintained in a highly oxidized
state, which would be favorable for donor-type NO adsorption
+
(
NO!NO ), and the paring of nitrogen atoms could take place
+
[56,58]
between NO and NO .
Comparatively, almost no NO was
2
2
formed in the absence of oxygen, and hence the NO decom-
position reaction hardly proceeds. At the same time, the ap-
pearance of the BaCO phase, as the NO storage component,
3
2
is beneficial to improve the activity by adsorbing the inter-
mediate NO species, and thus BaCO plays an important role
2
3
[
49]
to accelerate the catalytic recycling of NO decomposition. In
our cases, all three BaMeO (Me=Mn, Co, Fe) mixed oxides
3
contain the main phase of BaCO deduced from the XRD re-
3
sults (Figure 1). The N selectivity decreases sharply with the in-
2
crease of oxygen concentration (Figure 7), which suggests an
3
Figure 8. Effect of water vapor on the NO conversion over BaCoO in the
increase of the generation of NO . Therefore, the NO conver-
2
MCRM (reaction conditions: molar fraction of H O, 5%; molar fraction of O ,
2
2
À3
sion increases with the oxygen concentration over BaMeO3
5%; molar fraction of NO, 0.1%; W/F=1 gscm ; and N
2
as the balance).
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because of the competitive adsorption of H O and NO mole-
three catalysts. Comparatively, in the CRM, there is no differ-
2
[2,38]
cules for the active sites.
However, H O is a strongly polar
ence. The stable bed temperatures of the BaMnO and BaCoO3
2
3
molecule and has a good microwave-absorbing ability. There-
catalysts are much higher than that of BaFeO (Figure 4). The
3
fore, H O molecules can interact with the catalyst in the micro-
ranking of catalytic performance is in accordance with the mi-
crowave-absorbing properties, and BaMnO and BaCoO dis-
2
wave field as they are adsorbed on the catalyst, which results
3
3
in a strong coupling and then the desorption of H O molecules
play a much better performance than BaFeO in the MCRM.
2
3
from the adsorption sites, or weakening the adsorption of H O
Generally, a better microwave-absorbing catalyst can absorb
more microwaves. Therefore, the microwave-absorbing proper-
ties may account for the completely different catalytic per-
2
[38,40]
molecules on the catalytic surface.
Moreover, in the
MCRM, it is possible to convert NO to the intermediate species
[
40]
of NO2, which plays an important role in the direct decom-
formance in the MCRM, even though the BaMeO (Me=Mn,
3
[
49,56,58]
position of NO.
Therefore, the BaCoO3 catalyst in the
Co, Fe) mixed oxides exhibit very similar catalytic activities in
the CRM.
MCRM can still display an outstanding catalytic performance in
the presence of water.
At the same time, the microwave-adsorbing properties of
BaCoO and BaMnO are similar. However, the BaCoO catalyst
3
3
3
displays a better catalytic performance than BaMnO in the
3
Discussion
MCRM, and the best NO conversion of 99.9% for the BaCoO3
The above results indicate that the catalytic activity of BaMeO3
catalyst is much higher than that of the BaMnO catalyst of
3
(
Me=Mn, Co, Fe) mixed oxides in the MCRM and CRM for the
93.7% in the MCRM. This different catalytic activity in the
MCRM can be explained by the TPR and TPD results. The TPR
and TPD results indicate that the reducibility and oxygen de-
sorption capability of BaCoO are better than those of BaMnO .
direct catalytic decomposition of NO increase in the order
BaCoO >BaMnO >BaFeO . Moreover, the three BaMeO3
3
3
3
(
Me=Mn, Co, Fe) mixed oxides exhibit an extremely similar
3
3
catalytic activity in the CRM, whereas they exhibit completely
different catalytic performances in the MCRM. Importantly,
BaMnO and BaCoO catalysts in the MCRM display a much
Interestingly, the BaCoO catalyst displays a slightly higher ac-
3
tivity than BaMnO in the CRM. However, the poor microwave-
3
absorbing ability is the largest drawback for the BaFeO cata-
3
3
3
better catalytic performance than BaFeO . Especially, the cata-
lyst. Therefore, BaFeO3 shows the lowest activity compared
with BaCoO and BaMnO in the MCRM.
3
lytic performance of BaCoO in the MCRM is much better than
3
3
3
that of BaMnO3.
To further elucidate the microwave catalytic effect on the
decomposition of NO into N and O , we computed the appar-
To analyze detailed factors that cause this different catalytic
performance under these two different reactor systems, we
first looked into some possible factors from the H -TPR and O -
2
2
ent activation energies (E ’) of these BaMeO (Me=Mn, Co, Fe)
a
3
catalysts for the decomposition reactions of NO. The calculated
2
2
TPD results. The TPR results indicated that the relative reduci-
E ’ for BaMnO , BaCoO , and BaFeO in the CRM is 194.5, 153.7,
and 197.5 kJmol , respectively (Table 1). However, the corre-
a
3
3
3
À1
bility increases in the order BaCoO >BaMnO >BaFeO . It is re-
3
3
3
ported that the b- and g-O desorption peaks are interesting
sponding E ’ values for BaMnO , BaCoO , and BaFeO catalysts
2
a
3
3
3
for the direct decomposition of NO and they are necessary to
in the MCRM decrease to as low as 33.4, 13.7, and
[49,53]
À1
[62]
improve the activity of NO decomposition.
Clearly, the
46.7 kJmol , respectively. Similarly, Stiegman et al.
also
oxygen desorption ability seems to be the dominant parame-
ter for the catalytic performance in the process of NO decom-
found that the apparent activation energy decreased from
À1
118.4 kJmol
under conventional convective heating to
[
49,53]
À1
position.
The catalytic activity of BaMeO (Me=Mn, Co, Fe)
38.5 kJmol under microwave irradiation for the carbon–
carbon dioxide (Boudouard) reaction. This high efficiency for
the direct decomposition of NO into N and O over these cat-
3
mixed oxides in the CRM agrees well with the TPR and TPD re-
sults, and these three catalysts show a similar catalytic activity
in the CRM. However, they have completely different catalytic
performances in the MCRM. Therefore, the different catalytic
performances in the MCRM and CRM cannot be explained
simply by the TPR and TPD results.
2
2
alysts in the MCRM at such a low temperature is mainly be-
cause of the significantly low E ’ values, which shows the excel-
a
We looked into some other possible factors from the micro-
wave-absorbing properties. The microwave-heating behavior
of these catalysts in the MCRM indicates that BaMnO3 and
BaCoO3 have excellent microwave-absorbing properties,
a
Table 1. E ’ values for the direct catalytic decomposition of NO.
Mode/temperature conditions
Catalyst
E
a
’
Reference
À1 [a]
[kJmol ]
whereas that of BaFeO is poor. The TPR and TPD results do
direct decomposition of NO
CRM, 579–733 K
MCRM, 200–2508C
MCRM, 200–2508C
MCRM, 200–2508C
CRM, 600–6508C
no catalyst
Cu-ZSM-5
364
123
33.4
13.7
46.7
194.5
153.7
197.5
[60]
[16]
3
not account for such completely different catalytic activities, in
which BaMnO and BaCoO are more active than BaFeO in the
BaMnO
3
this work
this work
this work
this work
this work
this work
3
3
3
BaCoO
BaFeO3
BaMnO
3
MCRM, whereas they are similar in the CRM. The only explana-
tion of these results is probably from the differences in their
microwave-absorbing properties. In the MCRM, different mate-
rials have different microwave-absorbing properties, which
lead to a difference in the amount of microwave energy ab-
sorbed. Namely, the role of microwaves is not the same in the
3
CRM, 600–6508C
CRM, 600–6508C
BaCoO
BaFeO
3
3
[
a] E
a
’ values were calculated by the Arrhenius equation; The first-order
[61]
with respect to NO was used to calculate kinetic data.
ChemCatChem 2016, 8, 417 – 425
www.chemcatchem.org
422
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Full Papers
À1
lent microwave catalytic effect of these materials under micro-
BaFeO decrease to as low as 33.4, 13.7, and 46.7 kJmol , re-
3
[
41,42,44,45]
wave irradiation.
Moreover, the E ’ value for BaCoO is
spectively, which suggests a significant microwave catalytic
effect. The whole catalytic activity in the microwave catalytic
reaction mode is much higher than that in the CRM. Further-
more, microwave irradiation shows a microwave selective
a
3
much lower than that of BaMnO and BaFeO in the MCRM.
3
3
This is in accordance with the fact that BaCoO has the best
3
catalytic activity for the decomposition of NO in the MCRM.
Importantly, the ranking of the catalytic performance of these
effect for these BaMeO (Me=Mn, Co, Fe) catalysts, and the
3
three BaMeO (Me=Mn, Co, Fe) catalysts in the MCRM is also
catalytic activity of NO decomposition under microwave irradi-
ation is hardly influenced by the oxygen concentration.
3
in accordance with the E ’ values in the MCRM.
a
Most scientists today will agree that the energy of the mi-
crowave photon is too low to cleave molecular bonds direct-
[29]
Experimental Section
ly. However, the microwave electromagnetic field affects the
target, which is not an intact chemical bond but the process of
the destruction of old bonds or the formation of new bonds
Preparation of catalysts
[
63]
The BaMeO
(Me=Mn, Co, Fe) mixed oxides were prepared by
3
and potentially accelerates its passage through activation as
some chemical bonds could be weakened greatly in the pro-
a sol–gel method. Notably, the preparation of BaMnO mixed oxide
3
here and BaMn Mg O catalysts in our previous work is different
x
1Àx
3
[63]
[29]
cess to form a new bond. Interestingly, Kappe et al. report-
ed the direct interaction of the microwave electromagnetic
field with specific molecules, intermediates, or even transition
in the barium salt, for which the barium salt used for the latter is
[42]
Ba(Ac)2.
In this work, Ba(NO3)2 (AR grade) and Mn(NO3)2 (AR
grade) or Co(NO3)2 (AR grade) or Fe(NO3)3 (AR grade) were dis-
solved firstly in deionized water according to the stoichiometric
amounts of BaMeO3 (Me=Mn, Co, Fe) mixed oxides. Afterward,
glycol and citric acid (the molar amounts of complexing species
were 1.25 times of total metal cations) were added into the solu-
tions. Subsequently, the solutions were stirred at 808C until amor-
phous gels were formed. The obtained gels were dried at 1208C
overnight, calcined in air at 4008C for 2 h, and finally calcined at
[38]
states in the reaction medium. Similarly, Wang et al. reported
that the direct interaction of a microwave electromagnetic
field with a microwave-absorbing catalyst could cause a strong
coupling in the catalyst, and also the coupling between the
microwave electromagnetic field and NO2 molecules may
weaken the NOÀO bond and promote its activation. The ther-
mal effect of this reaction (direct catalytic decomposition of
NO) can be neglected because N and O molecules absorb mi-
7
008C for 4 h.
2
2
crowaves weakly and the NO concentration in the reactant gas
is extremely low with a molar fraction of only 0.1%, which
helps to illustrate the microwave catalytic effect in this reac-
tion. Therefore, we can infer the fundamental mechanisms of
decreasing apparent activation energies on the direct decom-
position of NO under microwave irradiation as follows: First,
the catalyst can be activated effectively by the microwave elec-
tromagnetic field through the absorption of microwave
Catalyst characterization
Powder XRD patterns were recorded by using a Rigaku D/max-II/
2500 X-ray powder diffractometer (CuK radiation).
a
H -TPR was conducted by using a FINESORB-3010 apparatus
2
equipped with a thermal conductivity detector (TCD). Catalyst sam-
ples of 100 mg were placed in a quartz tube, purged under pure
Ar with a flow of 10 mLmin at 2008C for 30 min, and cooled to
À1
energy; second, NO rather than O in the gas phase can be ac-
RT. The TPR profiles were recorded through passing a flow of 10%
2
À1
H in Ar (10 mLmin ) with heating from RT to 9008C at a rate of
tivated by the microwave electromagnetic field because micro-
2
À1
[32,38,39,41,42]
108Cmin .
waves can activate polar molecules.
The intrinsic
reason for the decreasing activation energies under microwave
irradiation is an interesting open issue, which requires further
study. Further investigations are in progress in our laboratory
to elucidate the nature of the microwave effect on heteroge-
neous catalytic reactions.
O -TPD was conducted by using a TP-5076 apparatus equipped
2
with a TCD. Catalyst samples of 100 mg were placed in a quartz
À1
tube, purged under pure O
with a flow of 10 mLmin at 5008C
2
for 60 min, and cooled to RT. The TPD profiles were recorded
À1
through passing a flow of He (30 mLmin ) with heating from RT
À1
to 9008C at a rate of 108Cmin .
Conclusions
Activity evaluation
An attractive and environmental friendly process for NO emis-
x
Microwave reactor system
sion control has been developed. Direct NO decomposition
through microwave catalysis is impressively efficient in NO
[
41,42]
We designed a new microwave catalytic reactor system
to
conversion and N selectivity with values of up to 99.8% and
study the microwave effects in the gas–solid catalytic reactions.
The experimental apparatus is presented in Figure 9. The reactor
was composed of a microwave generator system and a catalytic re-
action system. A 2.45 GHz microwave generator provided the mi-
crowave energy, and the microwave power was adjustable contin-
uously in the range of 0–1000 W. The microwave magnetron
source was connected to the resonant cavity by a rectangular
waveguide. The catalytic activity tests were performed by using
a quartz tube (540 mm in length and i.d.10 mm) at the center of
resonant cavity. The reliable monitoring of reaction temperature is
2
9
9.9%, respectively, for BaCoO with 10% oxygen and a low
3
temperature of 2508C. Moreover, BaCoO shows a superior re-
3
sistance to water vapor. The best NO conversion is 93.7% for
BaMnO3 and only 64.1% for BaFeO . Synergistic effects be-
3
tween good microwave absorption, the best reducibility, and
good oxygen desorption account for the best performance of
BaCoO in the microwave catalytic reaction mode. Importantly,
3
the apparent activation energies of BaMnO , BaCoO , and
3
3
ChemCatChem 2016, 8, 417 – 425
www.chemcatchem.org
423
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
Acknowledgements
We greatly appreciate financial support from the National Natu-
ral Science Foundation of China (No.20976147, 21506184), Sci-
ence and Technology Foundation of Hunan Province, China
(
(
No.2014K4048), China Postdoctoral Science Foundation
No.2015M572258), and PhD Startup Foundation of Xiangtan
University (No.15QDZ13). We also sincerely appreciate the
support by Synotherm Corporation for microwave experimental
apparatus.
Figure 9. Schematic diagram of the microwave catalytic reactor system.
Keywords: barium
chemistry · nitrogen oxides
·
cobalt
·
manganese
· microwave
1
7
1
. NO; 2. O
. Valve; 8. Premixer; 9. Quartz reactor; 10. Fixed bed; 11. Thermal insulation;
2. Resonant cavity; 13. Microwave generator; 14. On-line NO analyzer.
2 2
; 3. N ; 4. Storage tank; 5. Metering pump; 6. Mass-flow meter;
x
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425
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