Activity of γ-Al2O3-based Mn, Cu, and Co oxide nanocatalysts for selective catalytic reduction of nitric oxide with ammonia

Article abstract of DOI:10.3906/kim-1605-50

Our studies on the selective catalytic reduction of NO (SCR-deNO) properties of M/γ-Al₂O₃ (M = Mn, Co, Cu) nanocatalysts are presented. All catalysts were prepared by homogeneous deposition precipitation using urea as the precursor f

Full text of DOI:10.3906/kim-1605-50

Turk J Chem
2017) 41: 272 – 281
Turkish Journal of Chemistry
(
http://journals.tubitak.gov.tr/chem/
¨
˙
⃝
c TUBITAK
Research Article
doi:10.3906/kim-1605-50
Activity of γ-Al O -based Mn, Cu, and Co oxide nanocatalysts for selective
2
3
catalytic reduction of nitric oxide with ammonia
1
,∗
2
3
Parvaneh NAKHOSTIN PANAHI , G ´e rard DELAHAY , Seyed Mahdi MOUSAVI
1
Department of Chemistry, Faculty of Science, University of Zanjan, Zanjan, Iran
2
Charles Gerhardt Institute, UMR 5253 CNRS/UM2/ENSCM/UM1, Advanced Materials for Catalysis
and Health Group, Higher National School of Chemistry of Montpellier, Montpellier, France
3
Faculty of Chemistry, University of Kashan, Kashan, Iran
Received: 23.05.2016
•
Accepted/Published Online: 11.10.2016
•
Final Version: 19.04.2017
Abstract: Our studies on the selective catalytic reduction of NO (SCR-deNO) properties of M/γ -Al
Co, Cu) nanocatalysts are presented. All catalysts were prepared by homogeneous deposition precipitation using urea as
the precursor for the precipitating agent. The SCR activity followed the order Mn/γ -Al > Cu/γ -Al > Co/γ -
Al . The nanocatalysts were characterized with respect to their texture (N -BET), particle size (TEM), reducibility
-TPR), and acidity (NH -TPD). The TEM analysis revealed that the metal species have superior dispersion with less
agglomeration and sintering on γ -Al support. The H -TPR results confirmed that the Mn/γ -Al nanocatalyst
contains various oxidation states of manganese, which is useful for the catalyst to maintain the DeNO activities. The
NH -TPD studies indicated that the addition of transition metal can significantly increase the surface acidity and
Mn/γ -Al showed the most adsorbed sites of NH . Characterization results indicated that the acidity and the redox
properties of the catalyst play important roles in the final catalytic activity in the SCR-NO process.
2 3
O (M = Mn,
2
O
3
2 3
O
2
O
3
2
(
H
2
3
2
O
3
2
2 3
O
3
2
O
3
3
3 2 3
Key words: NO, NH -SCR, transition metals, γ -Al O , nanocatalyst
1. Introduction
Nitrogen oxides (NOx = NO + NO2) are among the main atmospheric pollutants. They are reported to
contribute to a variety of environmental problems including acid rain and acidification of aquatic systems, ground
level ozone (smog), ozone depletion, visibility degradation and greenhouse effects1,2 Increasingly stringent limits
for exhaust emissions, particularly for nitrogen oxides from lean-burn combustion such as diesel engines, have
driven many researchers to look for suitable methods. The selective catalytic reduction (SCR) of NO with
ammonia as reductant is the most common method to catalytically reduce NO in flue gases from stationary
sources. A number of catalysts consisting of various transition metals on different supporters have been studied
for the SCR of NO reaction. Transition metals such as Cu,3 Co,4 Fe,5 and Mn6,7 have been reported to
exhibit high activity. γ -Al2 O3 has been extensively used as a support in many catalyst formulations, mainly
due to its low cost, particular texture, and good thermal stability8 Torikai et al.9 studied the performance of
alumina catalysts in NH3 -SCR reactions. They reported that the activity improves greatly with the loading of
copper and also the addition of copper results in lowering the active temperature region, the higher maximum
activity, and the enhancement of the reaction rate. Hamada et al.10 also investigated the SCR behavior of
∗
Correspondence: panahi@znu.ac.ir
272

NAKHOSTIN PANAHI et al./Turk J Chem
metal-alumina catalysts and concluded that these catalysts show excellent activity at low temperatures and
under high space velocity conditions.
Despite the number of investigations carried out on γ -Al2 O3 -supported transition metal catalysts, there
is, to the best of our knowledge, no available study that compares the activity of different transition metals
supported on γ -Al2 O3 . Therefore, the goal of this work was the comparison of three catalyst activities
(
Co/γ -Al2 O3 , Cu/γ -Al2 O3 , and Mn/γ -Al2 O3) under a common experimental set up and understanding the
effect of metal characteristics on NO conversion. Hence, in the present work, metals of cobalt, copper, and
manganese were supported on γ -Al2 O3 by deposition precipitation method and studied for the SCR of NO by
ammonia. The effects of transition metals’ modification on the microstructure and physiochemical properties
were systematically investigated by BET, NH3 -TPD, H2 -TPR, and TEM in combination with the activity
evaluation of NO catalytic removal.
2
2
. Results and discussion
.1. Characterization of catalysts
2.1.1. Analysis of the metal species particle sizes by TEM
The TEM images of the γ -Al2 O3 and M/γ -Al2 O3 nanocatalysts are shown in Figures 1a–1d. TEM analysis
detected the presence of metal species particles on the γ -Al2 O3 support. Dark spots are mainly attributed to
metal species. For all the nanocatalysts, we can observe the presence of metal species nanoparticles dispersed
homogeneously. The coprecipitation method is definitely beneficial for the homogeneous dispersion of metal
species deposited at high contents11 The TEM images also confirmed the nanoscale size of the M/γ -Al2 O3
catalysts (<100 nm).
2.1.2. BET surface area
The BET surface area, pore volume, and pore size of the γ -Al2 O3 and synthesized M/γ -Al2 O3 nanocatalysts
are summarized in Table 1. The BET surface areas and the pore volume of γ -Al2 O3 slightly decreased after
metal loading, suggesting that the introduction of metal does not obviously affect the textural properties of the
support. However, this slight decrease of the BET surface area can be attributed to the partial pore blockage by
the metal species.12 Note, however, that an apparent reduction of the BET surface area may also be caused by
the increasing density of the catalysts due to the loading of the support with metal13 The mean pore diameter
has increased with M/γ -Al2 O3 catalysts in comparison to γ -Al2 O3 . This might be due to dissociation of
some bonds in γ -Al2 O3 because of the urea alkaline environment and consequently the pore diameter is high
for M/γ -Al2 O3 catalysts.
2 3 2 3
Table 1. BET surface area, pore volume, and average pore diameter of γ -Al O and M/γ -Al O (M = Cu, Mn, Co)
nanocatalysts.
Catalyst
BET surface area (m /g)
γ−Al2O3 Cu/γ-Al2O3 Mn/γ-Al2O3 Co/γ-Al2O3
2
137
Total pore volume (cm /g) 0.246
124
127
127
3
0.237
5.8
0.252
6.1
0.261
6.2
˚
Average pore diameter (A) 5.6
The overall results by BET analysis of the M/γ -Al2 O3 nanocatalysts indicated that the introduction of
metal species has little effect on the textural properties of γ -Al2 O3 .
2
73

NAKHOSTIN PANAHI et al./Turk J Chem
2 3 2 3 2 3 2 3
Figure 1. TEM images of (a) γ -Al O , (b) Cu/γ -Al O , (c) Mn/γ -Al O , (d) Co/γ -Al O .
2.1.3. XRD
The X-ray diffraction patterns of γ -Al2 O3 and M/γ -Al2 O3 nanocatalysts are shown in Figure 2. All diffrac-
tion peaks corresponding to the γ -Al2 O3 structure could be clearly observed for all M/γ -Al2 O3 catalysts,
274

NAKHOSTIN PANAHI et al./Turk J Chem
suggesting that neither the metal loading nor the calcination process significantly affected the γ -Al2 O3 struc-
ture. There were no other peaks in XRD patterns of M/γ -Al2 O3 nanocatalysts, demonstrating that metal
species (e.g., oxides, cations) were well dispersed throughout the γ -Al2 O3 structure.
2 3 2 3
Figure 2. XRD patterns of γ -Al O and M/γ -Al O nanocatalysts.
2.1.4. Temperature-programmed reduction in H2 (H2 -TPR)
The reduction property of M/γ -Al2 O3 nanocatalysts was investigated using H2 -TPR and the results are
◦
presented in Figure 3 and Table 2. The Cu/γ -Al2 O3 catalyst showed one reduction peak around 220 C.
According to results from the literature,14 this peak can be assigned to the reduction of isolated Cu2+ ions to
Cu+ and possibly also the reduction of nanosized CuO. Three distinct reduction peaks were observed for the
Mn/γ -Al2 O3 catalyst. According to previous studies,1
5−17
the low temperature peak at 310 C is attributed
◦
to the reduction of highly dispersed and easily reducible MnO2 species, the broad middle temperature peak at
◦
4
30 C is due to the reduction of Mn2 O3 /Mn3 O4 or the bulk MnOx phase, and the high temperature peak
◦
with less intensity at 500 C is due to the reduction of Mn3 O4 /MnO. The reduction process of manganese
1
8
oxides takes place in the following order: MnO2 → Mn2 O3 → Mn3 O4 → MnO
Further reduction of
◦
MnO to Mn metal is impossible below 800 C due to its large negative value of reduction potential, which was
reported in many studies.1
9−21
2
75

NAKHOSTIN PANAHI et al./Turk J Chem
2 2 3
Figure 3. H -TPR profiles of M/γ -Al O (M = Cu, Mn, Co) nanocatalysts.
Table 2. H
2 2 3 2 3
consumption during H -TPR and acidity obtained from NH -TPD in M/γ -Al O (M = Cu, Mn, Co)
nanocatalysts.
Catalyst
γ−Al2O3 Cu/γ-Al2O3 Mn/γ-Al2O3 Co/γ-Al2O3
Total amount H2 consumed (µmol/g)
Acidity (µmol/g)
-
432
289
453
339
355
247
151
◦
The Co/γ -Al2 O3 catalyst showed one main reduction peak at 650 C, which can be ascribed to the
reduction of isolated Co2+ ions22 In addition to the main peak, a small peak at about 450 C was also observed.
◦
This peak corresponds to reduction of cobalt oxo species, such as CoO or Co3 O4 .23
A comparison of TPR profiles of M/γ -Al2 O3 nanocatalysts shows that the Mn/γ -Al2 O3 catalyst
contains various oxidation states of manganese, due to the existence of several reduction peaks. The reduction
◦
temperature of copper species in the Cu/γ -Al2 O3 catalyst (220 C) was found to be lower than that of cobalt
◦
species in the Co/γ -Al2 O3 catalyst (650 C), indicating higher reducibility of copper species in Cu/γ -Al2 O3 .
The copper species are more easily reducible than cobalt species. The main reduction peak of Co/γ -Al2 O3 is
at a higher temperature in comparison to Mn/γ -Al2 O3 and Cu/γ -Al2 O3 , implying that the redox activity of
the Co/γ -Al2 O3 catalyst is low. The decrease in the redox property leads to the low activity.
Meanwhile, according to Table 2, the amount of H2 consumed of Mn/γ -Al2 O3 (453 µmol/g) is greater
than that of Cu/γ -Al2 O3 (432 µmol/g) and Co/γ -Al2 O3 (355 µmol/g), implying that the reducing potential
of Mn/γ -Al2 O3 is higher than that of Cu/γ -Al2 O3 and Co/γ -Al2 O3 .
2.1.5. NH3 temperature-programmed desorption (TPD) study
To elucidate the role of catalyst acidity, NH3 -TPD was performed on the M/γ -Al2 O3 catalysts and the
◦
γ -Al2 O3 substrate (Figure 4). γ -Al2 O3 displayed a broad peak at 100–380 C. This peak is ascribed to
the desorption of weakly bound NH3 , which arises from the physisorbed NH3 or ammonium species and
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NAKHOSTIN PANAHI et al./Turk J Chem
also corresponds to the medium-strength acid sites, which is due to the desorption of NH3 on the Lewis
or strong Brønsted acid sites24 The NH3 -TPD profile of urea-treated γ -Al2 O3 is similar to γ -Al2 O3 and
it shows that urea does not affect the acidity of alumina. In the case of the M/γ -Al2 O3 catalysts, the
broadened desorption peak and enhanced chemisorbed NH3 amount suggest that the addition of metal species
has remarkably enhanced the concentration and acidity of acid sites21 Indeed, the addition and high dispersion
of metal species increase the acid sites distinctly because metal species generate new Lewis acid sites for NH3
◦
chemisorption. Additionally, a new distinct NH3 desorption peak centered at 440 C emerged for Mn/γ -Al2 O3 ,
which should be due to the strong Lewis acid sites originating from the high dispersion of the manganese oxide
phase24 The total amount of adsorbed ammonia, which is determined from the area under the TPD curve, is
shown in Table 2. For all of the catalysts, the acid site density has increased upon loading with the metal,
indicating that new acid sites have been created by metal species. The Mn-containing catalyst exhibited the
highest overall density of acid sites (339 µmol/g) among all the catalysts. According to the TPD profiles,
the introduction of manganese influences the formation of strong acidic sites in this catalyst. The presence of
acid sites is considered to favor NO conversion due to the preferential adsorption of NH3 on these sites, thus
initiating the reaction.25 Accordingly, one would expect a correlation of the SCR activity with the amount of
total acidity (Lewis and Brønsted acidity) and acidic strength (Tmax of ammonia desorption)26
3 2 3 2 3
Figure 4. NH -TPD profiles of γ -Al O and M/γ -Al O (M = Cu, Mn, Co) nanocatalysts.
2
2
.2. Catalytic performance
.2.1. Activity of M/ γ -Al2 O3 nanocatalysts
The M/γ -Al2 O3 nanocatalysts were tested as catalysts for the SCR of NO with NH3 . N2 and N2 O were the
only detected N-containing products in the NH3 -SCR process. Figure 5 presents the results of the catalytic
studies. The N2 O yield is not addressed here due to its minor value. Moreover, N2 selectivity was always above
90% for all tested catalysts. As is shown in Figure 5, the NO conversion increases with increasing temperature
◦
and reaches nearly 85% at 300 C for the Mn/γ -Al2 O3 nanocatalyst.
2
77

NAKHOSTIN PANAHI et al./Turk J Chem
2 3 3
Figure 5. Catalytic performance of M/γ -Al O (M = Cu, Mn, Co) nanocatalysts in the NH -SCR process.
2.2.2. Correlation between physicochemical properties and catalytic performance
Based on the above results, an attempt to correlate the catalytic performance of metal-promoted γ -Al2 O3
nanocatalysts in NO catalytic reduction with the physicochemical property of these catalysts was made. Ac-
cording to Figure 5, the Mn/γ -Al2 O3 catalyst showed better activity and the Co/γ -Al2 O3 catalyst exhibited
the lowest catalytic activity. The TPR results revealed that manganese oxide undergoes the consecutive re-
duction of MnO2 → Mn2 O3 → Mn3 O4 → MnO. The high activity of the Mn/γ -Al2 O3 catalyst can be
attributed to its ability to form variable oxidation states of manganese (MnO2 , Mn2 O3 , Mn3 O4 , and MnO)
and its oxygen storage capacity,27 which is in agreement with the study by Pavani et al28 Under NH3 -SCR
conditions with an excess of O2 , manganese oxides can convert into each other. This works like an electron
pump (between NO and NH3 molecules), enabling a redox cycle to be completed. In order to initiate and
continue such a cycle, variable oxidation states of metal seem to be necessary29
It is remarkable that Cu/γ -Al2 O3 exhibited higher activity than Co/γ -Al2 O3 in the studied tempera-
ture range, as shown in Figure 5. This might be related to facile reduction of the copper species as supported
by the TPR profiles (see Figure 3)13 The copper species are more easily reducible than cobalt species. In order
to understand the relationship between the number of reducible metal species and the catalytic activity, the
peak areas of the TPR profiles were integrated (shown in Table 2). The Mn/γ -Al2 O3 catalyst showed the
most quantitative H2 consumption, suggesting that this catalyst has the most quantitative available reducibil-
ity metal species. The Co/γ -Al2 O3 catalyst, which exhibited the lowest catalytic activity, showed the least
quantitative H2 consumption.
The chemisorption of NH3 on acid sites is an important factor for NH3 -SCR reaction. According to
NH3 -TPD results, the modification of γ -Al2 O3 with transition metals caused an increase in the Lewis acid
sites. This effect can be related to the electron acceptor behavior of the transition metals, which give rise to
additional Lewis acid centers30 The NH3 -TPD results of catalysts coincide with their catalytic activity. The
acid site density of M/γ -Al2 O3 catalysts (Table 2) matches the changing trend of the NO conversion very well
(
Figure 5), which means that the more NH3 molecules the sample adsorbs, the higher the NO conversion of the
catalyst is. The Mn-containing catalyst with the highest overall density of acid sites showed the highest NO
278

NAKHOSTIN PANAHI et al./Turk J Chem
conversion. The Cu/γ -Al2 O3 catalyst also has higher acid site density than the Co/γ -Al2 O3 catalyst, as a
result of which Cu/γ -Al2 O3 exhibited higher catalytic activity than Co/γ -Al2 O3 . Suprun et al.31 announced
that the activity increases with the acid site density for mesoporous materials. Another reason for the highest
efficiency of the Mn/γ -Al2 O3 nanocatalyst could be the availability of more and stronger NH3 adsorption sites,
which are deduced to correlate with the catalytically active sites. The results of NH3 -TPD can also explain
the low NO conversion exhibited by the Co/γ -Al2 O3 catalyst. With investigation of the reports on the topic,
it can be concluded that Mn/γ -Al2 O3 prepared by the homogeneous deposition precipitation (HDP) method
has better low temperature activity than some manganese supporting other materials for SCR of NO in the
literature26,32
In conclusion, cobalt, copper, and manganese oxide nanocatalysts supported on γ -Al2 O3 were prepared
by the HDP method and activity of these catalysts was investigated in NH3 -SCR of the NO process. The
experimental results showed that Mn/γ -Al2 O3 has the best catalytic activity, which was above 60% at the
◦
temperature range between 200 and 300 C. The nanocatalysts were characterized by BET, TEM, H2 -TPR,
and NH3 -TPD. The TEM images indicated that metal species as nanoparticles with homogeneous dispersion
were present on the γ -Al2 O3 support. The NH3 -TPD analysis indicated that introduction of transition metals
generated additional strong Lewis centers for ammonia adsorption with M/γ -Al2 O3 catalysts. From both
the NH3 -TPD and H2 -TPR results, it was concluded that the Mn/γ -Al2 O3 nanocatalyst offered the most
and strongest adsorbed sites of NH3 species and variable oxidation states of manganese; consequently, it had
the best catalytic activity among the catalysts. Furthermore, the Co/γ -Al2 O3 catalyst exhibited the lowest
activity because the redox activity of this catalyst is low and also it has fewer acid sites.
3
3
. Experimental
.1. Nanocatalyst preparation
According to a previous study,33 the preparation method of HDP using urea enables the even deposition of
metal species. Therefore, metal-based alumina catalysts were prepared by HDP using urea as the precursor for
the precipitating agent. Cu(NO3)2 .3H2 O, Co(NO3)2 .6H2 O, and Mn(NO3)2 .4H2 O were used as precursors.
Typically, an appropriate amount of metal nitrate was dissolved in distilled water, and 1 g of γ -Al2 O3 support
was added to the solution with stirring. The amount of metal in the solution corresponds to metal loading of
5
wt.% on γ -Al2 O3 . To this mixture solution, urea was added under vigorous stirring, and the temperature
◦
was gradually increased to 95 C and maintained for 5 h until the hydroxide precipitate was completed. The
resulting mixture was aged for 14 h at room temperature and then was filtered, followed by several washings with
distilled water to attain a neutral pH. The resulting precipitate was dried in an oven for 12 h and subsequently
calcined at 550 C for 4 h in air.34
◦
3.2. Nanocatalyst characterization
The nature, reducibility, and amount of metal species were estimated by H2 -TPR. The experiments were carried
out with a Micromeritics 2910 apparatus using H2 /Ar (3/97, v/v) gas at a total flowrate of 15 cm3 min
−1
◦
◦
−1
and by heating the samples from 50 to 900 C (10 C min ). In each case, 0.051 g of the catalyst was
◦
◦
previously activated at 500 C for 30 min under air, and then cooled to 50 C under 20% O2 in He. TPR
with H2 /Ar (3/97, v/v) was then started and the thermal conductivity detector continuously monitored the
H2 consumption.
2
79

NAKHOSTIN PANAHI et al./Turk J Chem
The acidity of the catalysts was measured by NH3 -TPD technique. Before NH3 adsorption, the samples
◦
◦
were pretreated at 500 C for 30 min in air and then cooled to 100 C under 20% O2 in He, followed by
ammonia adsorption with 5% NH3 /He mixture (flowrate: 40 cm3 min ) at 100 C for 30 min. Subsequently,
−1
◦
the sample was subjected to He flow (flowrate: 25 cm3 min ) for 30 min at 100 C to remove physically
−1
◦
◦
adsorbed ammonia. Ammonia desorption was carried out by raising the temperature to 550 C with a heating
−1
◦
rate of 10 C min
.
The textural properties of the catalysts were obtained from nitrogen adsorption–desorption isotherms
◦
measured at –196 C with a Micromeritics ASAP 2000 Analyzer. Before the nitrogen adsorption measurement,
◦
−5
the samples were outgassed at 250 C until a static vacuum of 3 × 10
bar was reached. The BET method
was used to calculate the specific surface area, whereas the pore size and volume were estimated using the
T-plot method.
TEM images of the catalysts were obtained to give an indication of the metal species’ size and their
dispersion on the support. The samples were ground with a mortar, and then about 1 mg of solid was dispersed
in ethanol (about 3 mL) by sonication. A drop of the dispersion was then deposited on a copper grid covered
with a Formvar carbon film.
X-ray diffraction (XRD, D 500 Siemens diffractometer, Cu Kαradiation) was utilized to determine the
crystal phase and dispersion of the metal species on the γ -Al2 O3 support. The powdered samples were pressed
◦
◦
−1
onto suitable holders and scanned within the 2? range of 5 to 75 with a scanning rate of 0.016 s .
3.3. Catalytic conversion of NO with NH3
The de-NO activity measurements of prepared nanocatalysts were carried out at atmospheric pressure in a fixed
bed reactor. In each run, a measured amount of powdered catalyst (0.2 g) was spread between quartz wool in
the reactor, and then the reactor was placed inside in a furnace that is electrically heated. Before the catalytic
tests, the catalysts were heated up 200 C and kept at this temperature for 1 h in flowing Ar (100 cm3 min
◦
−1
)
in order to eliminate possible compounds adsorbed on the catalyst surface. Subsequently, a gaseous reactant
feed consisting of 1000 ppm NO, 1000 ppm NH3 , and 5 vol.% O2 in Ar as the carrier gas was directed over
the catalyst. The overall gas flowrate was 150 cm3 min
−1
(GHSV = 12,000 h ) and the experiments were
−1
◦
performed at 200–300 C. Effluent gases after reaching a steady state were analyzed by gas chromatography
(
Shimadzu model 2010 Plus, TCD detector) equipped with a Molecular Sieve 5A column to separate N2 (as the
selective product) and N2 O (as the nonselective product). The catalytic activity for NO removal was evaluated
by the extent of NO conversion into N2 using the following equation:
[
N2]out
NO Conversion to N2% =
× 100
[
NO]in
The subscripts in and out indicate the inlet and outlet concentrations at steady state, respectively.
Acknowledgment
The authors would like to acknowledge the financial support from the University of Tabriz, University of Zanjan,
and the Iranian Nanotechnology Initiative.
280

NAKHOSTIN PANAHI et al./Turk J Chem
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