One-pot synthesis of Cu/SAPO-34 with hierarchical pore using cupric citrate as a copper source for excellent NH3-SCR of NO performance

Article abstract of DOI:10.1002/cctc.202000818

It is still a serious challenge to develop a simple synthesized method of SAPO-34 sieve modified by Cu with high NH₃-SCR performance. The Cu/SAPO-34 samples with hierarchical structure were synthesized for the first time using cupric citrate as copper source by one-pot hydrothermal method which showed excellent NH₃-SCR performance for NO removal. Among them, the 0.15Cu/SAPO-34 catalyst with a Cu/Al ratio of 0.15 exhibited the widest temperature operating window (160–500 °C) and better hydrothermal stability. The physicochemical properties of 0.15Cu/SAPO-34 was measured to understand the impacts of the Cu species on the NH₃-SCR of NO activity by various characterization methods including XRD, NH₃-TPD, N₂ adsorption-desorption, XPS, EPR, UV-vis, NH₃-oxidation, NMR and in situ DRIFTS. The results reveal that the good activity is assigned to the isolated Cu species located at the site (I) in SAPO-34, and the co-existence of surface Cu²⁺ and Cu⁺ species with almost equal amounts. In addition, 0.15Cu/SAPO-34 also possesses suitable acidic sites which are conducive to catalytic activity. The results of in situ DRIFTS suggest that the NH₃-SCR reaction follows Eley-Rideal mechanism over 0.15Cu/SAPO-34.

Full text of DOI:10.1002/cctc.202000818

The European Society Journal for Catalysis
Supported by
Accepted Article
Title: One-pot synthesis of Cu/SAPO-34 with hierarchical pore using
cupric citrate as a copper source for excellent NH3-SCR of NO
performance
Authors: Peiqiang Wang, Zhibin Li, Xiaotong Wang, Yongming Tong,
Fulong Yuan, and Yujun Zhu
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To be cited as: ChemCatChem 10.1002/cctc.202000818
Link to VoR: https://doi.org/10.1002/cctc.202000818
01/2020

10.1002/cctc.202000818
ChemCatChem
FULL PAPER
One-pot synthesis of Cu/SAPO-34 with hierarchical pore using
cupric citrate as a copper source for excellent NH3-SCR of NO
performance
Peiqiang Wang†, Zhibin Li†, Xiaotong Wang, Yongming Tong, Fulong Yuan*, Yujun Zhu*
† Both authors contributed equally to this work
Mr. P Wang, Dr. Z Li, Ms. X Wang, Mr. Y Tong, Prof. F Yuan (Email: yuanfulong@hlju.edu.cn), Prof. Y Zhu (Email: yujunzhu@hlju.edu.cn)
Key Laboratory of Functional Inorganic Material Chemistry (Heilongjiang University), Ministry of Education, School of Chemistry and Materials, Heilongjiang
University, Harbin 150080, PR China
Supporting information for this article is given via a link at the end of the document.
Abstract: It is still a serious challenge to develop a simple
synthesized method of SAPO-34 sieve modified by Cu with high
NH3-SCR performance. The Cu/SAPO-34 samples with hierarchical
structure were synthesized for the first time using cupric citrate as
copper source by one-pot hydrothermal method which showed
excellent NH3-SCR performance for NO removal. Among them, the
0.15Cu/SAPO-34 catalyst with a Cu/Al ratio of 0.15 exhibited the
widest temperature operating window (160-500 oC) and better
hydrothermal stability. The physicochemical properties of
0.15Cu/SAPO-34 was measured to understand the impacts of the
Cu species on the NH3-SCR of NO activity by various
characterization methods including XRD, NH3-TPD, N2 adsorption-
desorption, XPS, EPR, UV-vis, NH3-oxidation, NMR and in situ
DRIFTS. The results reveal that the good activity is assigned to the
isolated Cu species located at the site (I) in SAPO-34, and the co-
existence of surface Cu2+ and Cu+ species with almost equal
amounts. In addition, 0.15Cu/SAPO-34 also possesses suitable
acidic sites which are conducive to catalytic activity. The results of in
situ DRIFTS suggest that the NH3-SCR reaction follows Eley-Rideal
mechanism over 0.15Cu/SAPO-34.
go through hydrothermal, washing, calcination, ammonia
exchange, metal exchange, calcination and multiple drying in
order.[7] Additionally, the repeated ion exchange processes,
drying and calcination processes increase significantly the
complexity of the synthesis process. Ren et al. creatively used
the inexpensive Cu-tetraethylenepentamine (Cu-TEPA) as the
organic structure-directing agent (OSDA) instead of the
expensive N,N,N-trimethyladamant ammonium hydroxide
(TMAdaOH) by one step method to synthesize the Cu/SSZ-13
zeolite with good NH3-SCR activity.[8] However, the shortcomings
of this method, such as high copper content and low
hydrothermal stability, limit its further application.[9] The catalytic
performance and hydrothermal stability were improved by
optimizing the synthesis method, adjusting the Si/Al ratio, and
using the “reverse” ion exchange method to post-process the
catalyst.[10] Soon after the one-step method for synthesizing
Cu/SSZ-13 with Cu-TEPA as the OSDA was proposed, and
Corma’s group applied this method to synthesize Cu/SAPO-
34.[6b,7] However, the NO conversion over the obtained
Cu/SAPO-34 was low, which may be caused by the existence of
a great quantity extra-framework copper species. To solve this
problem, the synthesis strategy was optimized, and another
template agent besides TEPA was introduced, and by adjusting
the ratio of the two template agents, the copper loading can be
precisely controlled.[11] After hydrothermal treatment at 750 oC,
the catalysts retained most of their original catalytic performance
in the same temperature operating window of 250-450 oC.
In order to further increase mass transfer and shorten the
reactant transport path, the hierarchical pore structure was
introduced into the zeolite catalyst.[12] Liu et al. prepared a kind
of Cu/SAPO-34 with meso-microporous with Cu-TEPA and
DIEPA as the co-OSDA by the one-pot route, the synthetized
Cu/SAPO-34 showed an excellent NH3-SCR performance in the
operating temperature window of 150-425 oC, but the
hydrothermal stability was not discussed.[13] In addition to Cu-
TEPA, there are many copper complexes suitable for one-step
method as the copper source, but some conditions need to be
met, for example, the complex molecule is easily available, the
size of the complex molecule cannot exceed the size of the CHA
cage,[8] and need stably exist in the synthetic sol, etc. In general,
it is still a serious challenge to develop a simple synthesized
method of Cu modified SAPO-34 with good NH3-SCR
performance, high hydrothermal stability and resistance of SO2.
In this paper, a series of Cu/SAPO-34 with different Cu/Al ratios
were successfully synthesized firstly by using hard template
method and controlling the different Cu/Al ratios. It was noticed
Introduction
Nitrogen oxide (NOx) is one of the major pollutants in air
pollution, which is a problem that human beings need to solve
urgently. Not only can NOx cause a variety of environmental
problems such as photochemical smog, but it can also pose
human health to a huge threat.[1] Currently, the selective catalytic
reduction of NOx with ammonia (NH3-SCR) is considered to be
the most effective method to convert NOx into N2 and H2O.[2]
The catalysts suitable for NH3-SCR reaction can be roughly
divided into three types: noble metal, metal oxides and zeolite.[3]
By contrast, as a porous material with a large surface area, rich
acidic sites and excellent shape-selective ability, zeolite is found
to be more suitable for NH3-SCR reaction.[4]
In the recent few years, transition metal and noble metal (Cu,
Fe, Ce, Mn, Ag, Pt)-modified zeolites (ZSM-5, Beta, LTA),[5]
especially Cu modified small-pore zeolites (SAPO-34, SSZ-
13),[6] have become more and more popular research topics.
Compared with other catalysts, copper modified SAPO-34, SSZ-
13 exhibit wider activity temperature window, excellent N2
selectivity, more outstanding hydrothermal stability, and superior
H2O and SO2 resistance. Normally, the synthesis process of
metal-exchanged zeolite involves multiple steps, which needs to
1
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10.1002/cctc.202000818
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that the catalysts exhibit good NH3-SCR performance, but only
0.15Cu/SAPO-34 with the Cu/Al ratio of 0.15 shows the CHA
structure of SAPO-34 according to the XRD results. Moreover,
0.15Cu/SAPO-34 was also observed superior hydrothermal
stability and resistance to SO2 and H2O.
hydrothermal aging of the catalyst on activity. The
0.15Cu/SAPO-34 was hydrothermal treated at low temperature
(80 oC, 10% H2O, 48 h) and high temperature (700 oC, 10%
H2O, 12 h), respectively. As exhibited in Figure 1C, the NO
conversion shows obvious decrease in low temperature region
of 100-180 oC over the 0.15Cu/SAPO-34 treated at low
temperature, and the activity window becomes slightly narrow
(175-475 oC) compared with the fresh catalyst. More
interestingly, as demonstrated in Figure 1D, the 0.15Cu/SAPO-
34 catalyst treated at high temperature displays the similar
catalytic performance to the treatment at low temperature. These
results suggest that the 0.15Cu/SAPO-34 can exhibit superior
hydrothermal stability. The deactivation of Cu/SAPO-34 after
hydrothermal treatment can be attributed to two aspects. One is
the destruction of the CHA structure is derived from the
hydrolysis of the Si-O-Al bonds in the zeolite framework. And
another is the migration and polymerization of active Cu species.
While isolated Cu2+ is considered to be the main active species
for NH3-SCR reaction, and a large number of Cu2+ species are
more likely to polymerize during hydrothermal treatment.
However, some researchers found that the presence of
moderate amount of Cu+ can promote the low temperature NH3-
SCR reaction, attributed to enhancing the redox cycle between
Cu2+ and Cu+.[16] In this work, the ratio of Cu2+/Cu+ calculated by
the XPS results below is 1.2 in the 0.15Cu/SAPO-34. This higher
Cu+/(Cu2+ + Cu+) ratio is favor of both the good SCR activity and
the possibility for the excellent hydrothermal stability.
Results and Discussion
In this work, a series of hierarchical xCu/SAPO-34 which was
obtained by using different ratios of Cu/Al (x=0.05, 0.10, 0.15,
0.20) was synthetized by facile one-pot hydrothermal process.
The XRD patterns (Figure S1) reveal that only 0.15Cu/SAPO-34
with the Cu/Al ratio of 0.15 shows the pure CHA topology and
other samples present the mixed phases of SAPO-34 and
SAPO-5, moreover, the 0.15Cu/SAPO-34 catalyst also exhibits
the best NH3-SCR of NO performance (Figure S2). Therefore,
the relationship between structure and activity over
0.15Cu/SAPO-34 was studied in detail through following various
characterization and activity tests.
The NO conversion is given in Figure 1A over 0.15Cu/SAPO-34
under different gas hourly space velocity (GHSV). An obvious
decline in the NO conversion is observed with the increase in
GHSV when the temperature of the NH3-SCR reaction is below
200 oC. The activity temperature window (>90% NO conversion)
o
can be maintained in the range of 180-500 C under the GHSV
of 80,000 h-1. Moreover, the NO conversion only exhibits slight
decrease at high temperature when the GHSV is increased to
120,000 h-1. It is interesting that the operating temperature
window can still be maintained in the range of 200-450 oC when
the GHSV reaches 240,000 h-1. According to above results, it
can be summarized that the NO conversion decreases slightly
with the increase of GHSV at low and high temperature,
indicating the excellent NH3-SCR performance over
0.15Cu/SAPO-34 even at higher GHSV condition. The activity of
catalyst prepared by ion-exchange method is also provided to
distinguish the difference between the two methods and the
result is shown in Figure (S3). Under the GHSV is 40000 h-1, the
catalyst prepared by one-pot method shows comparable
catalytic performance to the catalyst prepared by ion-exchange
method.
In short, using CaCO3 as the hard template and cupric citrate as
the copper source, we have successfully prepared a NH3-SCR
deNOX catalyst with high NO conversion, excellent hydrothermal
stability and tolerance of H2O and SO2.
In order to simulate the realistic NH3-SCR reaction condition,[14]
the effects of H2O or/and SO2 on the activity were tested. Figure
1B shows the H2O or/and SO2 resistance over the
0.15Cu/SAPO-34 at 200 oC under the GHSV of 40,000 h−1. In
the beginning, the activity remains stable for about 4 h in the
absence of H2O and SO2. After that, 5% H2O is added into the
feed, it is clear seen that NO conversion still maintains at 100%.
After 8 h, 100 ppm of SO2 is injected into the reactor, NO
conversion is observed to be a slight decrease. While the H2O
and SO2 are cut off, the NO conversion restores to 100% rapidly.
It is interesting that the 0.15Cu/SAPO-34 catalyst can still
maintain excellent catalytic performance in the presence of 10%
H2O. While the NO conversion reduces to 95% with the SO2
concentration increases to 200 ppm. Although the NO
conversion cannot restore completely after cutting off H2O and
SO2, it can still be maintained at about 98%. These results
demonstrate that the 0.15Cu/SAPO-34 exhibits superior NH3-
SCR performance on resistance of H2O and SO2.
Figure 1. NH3-SCR activity under different conditions of GHSV (A), SO2 and
H2O (B) and hydrothermal aging at low (C) and high (D) temperature over
0.15Cu/SAPO-34.
Figure 2 shows the XRD patterns of pure SAPO-34 and
0.15Cu/SAPO-34. Obviously, both SAPO-34 and 0.15Cu/SAPO-
34 present the diffraction peaks at 2θ of about 9.5, 14.0, 16.1,
17.8, 20.7, 25.0 and 30.7 o, belonging to the CHA topological
structure,[12] which indicates that the introduction of Cu ions has
no effect on the formation of CHA topological structure for
The presence of water vapor in the environment is
unavoidable.[15] It is necessary to investigate the influence of the
2
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0.15Cu/SAPO-34. However, the crystallinity of 0.15Cu/SAPO-34
is observed an obvious decrease compared with the SAPO-34.
Moreover, the diffraction peaks for 0.15Cu/SAPO-34 shift to low
angle compared with pure SAPO-34 slightly. The above
phenomena may be due to the crystal distortion caused by the
introduction of Cu ions. In addition, no obvious diffraction peaks
of CaCO3 and CuO are observed from the XRD spectrum,[17]
which suggests that the CaCO3 should be removed thoroughly
and the content of the CuOx species is very low or highly
dispersed in SAPO-34 evenly.
and a hysteresis loop can be observed at high relative pressure
(0.6 < P/P0 < 0.9). As listed in Figure 3A, the BET surface area
(SBET) of 0.15Cu/SAPO-34 is calculated to 513 m2·g-1, the
surface area of mesopores is 27 m2·g-1, while the micropores
volume (Vmicro) and mesopores volume (Vmeso) calculated are
0.19 cm3·g-1 and 0.08 cm3·g-1, respectively. The results of N2
adsorption-desorption
measurements
demonstrate
that
micropores and mesopores coexist in the 0.15Cu/SAPO-34
sample. Many mesopores with the pore diameter of about 6-30
nm can be observed from the result of BJH pore diameter
distribution besides the existence of micropores. The SEM
image of 0.15Cu/SAPO-34 also confirms the results displayed in
inserted Figure 3B. It can be observed that the 0.15Cu/SAPO-34
exhibits the cubic morphology with many mesopores created by
the elimination of CaCO3. Thus, the above consequences reveal
that the 0.15Cu/SAPO-34 sample possesses hierarchical pores
structure.
The N2 adsorption-desorption isotherms, pore diameter
distribution and SEM image of 0.15Cu/SAPO-34 are shown in
Figure 3. As can be seen from Figure 3A, the N2 adsorption
amount increases rapidly at low relative pressure (P/P0 < 0.1)
and a hysteresis loop can be observed at high relative pressure
(0.6 < P/P0 < 0.9). As listed in Figure 3A, the BET surface area
(SBET) of 0.15Cu/SAPO-34 is calculated to 513 m2·g-1, the
surface area of mesopores is 27 m2·g-1, while the micropores
volume (Vmicro) and mesopores volume (Vmeso) calculated are
0.19 cm3·g-1 and 0.08 cm3·g-1, respectively. The results of N2
Figure 2. XRD patterns of SAPO-34 (black) and 0.15 Cu/SAPO-34 (blue).
adsorption-desorption
measurements
demonstrate
that
micropores and mesopores coexist in the 0.15Cu/SAPO-34
sample. Many mesopores with the pore diameter of about 6-30
nm can be observed from the result of BJH pore diameter
distribution besides the existence of micropores. The SEM
image of 0.15Cu/SAPO-34 also confirms the results displayed in
inserted Figure 3B. It can be observed that the 0.15Cu/SAPO-34
exhibits the cubic morphology with many mesopores created by
the elimination of CaCO3. Thus, the above consequences reveal
that the 0.15Cu/SAPO-34 sample possesses hierarchical pores
structure.
The valence states of different copper species on the catalyst
surface are monitored with XPS and the results are shown in
Figure 4A. The Cu2p spectrum can be deconvoluted into five
peaks. According to literatures,[18] the two peaks belonging to
Cu2p3/2 is assigned to Cu+ species at binding energy (BE) of 933
eV and Cu2+ species at BE of 935.4 eV. The satellite peak is 10
eV higher than the Cu2p3/2 peak, which is the essential
characteristic of the Cu2+ species. The peak of Cu2p1/2 at the
high BE can be fitted into two peaks by deconvolution. The peak
at 952.8 eV is assigned to Cu+, while the peak at 955.4 eV is
ascribed to Cu2+. The peaks at high BE of 935.4 and 955.4 eV
correspond to the isolated Cu2+ species coordinated to the
framework oxygen atoms of SAPO-34 zeolite, while the peaks at
933 and 952.8 eV are ascribed to the isolated Cu+ species.[6a, 19]
The Cu2+/Cu+ ratio is analyzed in terms of Gaussian
deconvolution based on the peak area, and it is 1.2 for the
0.15Cu/SAPO-34 catalyst. The XPS results present that the
0.15Cu/SAPO-34 catalyst possesses a moderate Cu2+ content
(4.1%) and a suitable content of Cu+, which is in favor of the
redox cycle of copper species during the NH3-SCR process.
Schneider et al.[20] confirmed that Cu+ and Cu2+ are propitious to
NH3-SCR reaction by XAFS and DFT calculation, the NO and O2
are more inclined to adsorbed on Cu+, at the same time Cu+ is
Figure 3. N2 adsorption-desorption isotherms (A) and the pore diameter
distribution and the SEM image (B) of 0.15Cu/SAPO-34.
The N2 adsorption-desorption isotherms, pore diameter
distribution and SEM image of 0.15Cu/SAPO-34 are shown in
Figure 3. As can be seen from Figure 3A, the N2 adsorption
amount increases rapidly at low relative pressure (P/P0 < 0.1)
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oxidized to Cu2+, while the Cu2+ can be deoxidize to Cu+ after
NH3 is adsorbed.
electrons, and the value of g// (2.388) corresponds the isolated
Cu2+ species with axial symmetry. Zamadics et al.[22] discovered
that the EPR signal was accordant with the isolated Cu2+ ions
octahedrally coordinated to three framework oxygen atoms and
three water molecules which located in Site (I). Normally, the
Site (I) is regarded as the easier approachable site to the
adsorbates (NH3 and NO) than others. The isolated Cu2+ species
located this position may be main reason leading to the excellent
NH3-SCR performance of the 0.15Cu/SAPO-34.
For understanding the Cu species and its coordination nature in
SAPO-34, UV-vis spectrum was tested for 0.15Cu/SAPO-34
(Figure 4C). Two obvious bands (225 and 787 nm) and two
weak bands (456 and 496 nm) can be observed from the UV-vis
spectrum. As reported in the literatures,[23] the band at 225 nm
can be attributed to the charge transition between O of SAPO-34
and Cu+/Cu2+, while the signal of CuO particles can be found in
the broad band at about 787 nm which is ascribed to the
electron d-d transitions of Cu2+ in distorted octahedral
surrounding by oxygen. The signal of the charge transition
bands of O-Cu-O and Cu-O-Cu complexes can be found at 457
and 499 nm,[24] respectively. These bands confirm that Cu2+, Cu+
and CuO co-exist in 0.15Cu/SAPO-34 again.
The amount and strength of acid sites are two essential
elements for NH3-SCR catalysts. In order to understand the
relationship between acidity and activity, NH3-TPD was
measured over SAPO-34 and 0.15Cu/SAPO-34. For
0.5Cu/SAPO-34, the peaks at different temperature are
assigned to the NH3 desorption on the acid sites of different
o
strength according to literatures.[6c,25] The strong peak at 150 C
and the shoulder peak at 250 oC represent the adsorbed NH3
molecules on weak Brønsted acid sites of SAPO-34 and weak
Lewis acid sites related to Cu species. The peak at 450 oC is
ascribed to the residual structural Brønsted acid sites of SAPO-
34 and moderate Lewis acid sites related to Cu species. The
o
peak at 510 C stands for the adsorbed NH3 molecules on both
strong Brønsted acid sites of SAPO-34 and new Lewis acid sites
created by Cu species. For SAPO-34, the ascription of the
peaks at 150 and 250 oC are similar with 0.15Cu/SAPO-34.
However, the peak at 480 oC is much smaller compared with
literatures.[25c,d] The reason may be attributed to the different
amount and pathway of Si atoms replacing Al atoms and P
atoms in the formation of SAPO-34 derived from different
materials, feeding sequence, template agents etc. The
calculated total acid amounts are listed in Figure 5A. Compared
with the SAPO-34, 0.15Cu/SAPO-34 exhibits fewer acid sites at
low temperature but more amount acid sites at high temperature
which may be the main reason contributed to its good NH3-SCR
performance. The acidity on the catalyst surface is known to be
the vital factor for the adsorption of NH3, which is closely related
to the deNOx performance. The appropriate amount and
strength of acid sites are more conducive to the reaction,
enhancing the catalytic performance to a certain degree. In
addition, the NH3 oxidation test was performed in order to study
the oxidation capacity and the relationship between NH3
conversion and reaction temperature over 0.15Cu/SAPO-34. As
given in Figure 5B, the conversion of NH3 is very low at low
Figure 4. Cu 2p XPS (A), EPR spectrum (B) and UV-vis curve (C) of
0.15Cu/SAPO-34.
EPR spectrum is an efficient technology to discriminate the
location environment of Cu2+ in the catalyst. For all the Cu
species, like Cu2+, Cu+ and CuO etc., only the isolated Cu2+
species can produce EPR signals, and other Cu species is
silent. Cu2+ species can be located at four sites in Cu/SAPO-34
framework and the Cu2+ species in different sites shows various
EPR parameters. As displayed in Figure 4B, the EPR signal
exhibits at g// (2.388) and A// (125 G) for 0.15Cu/SAPO-34, which
indicates that all Cu2+ species exits in the same coordination
environment (Site I), located at the junction of the D6R and the
CHA cage in the catalyst.[5c,6a] According to literature,[21]
hyperfine splitting coincident with the interaction of unpaired
o
temperature below 250 C, however, it increases rapidly above
250 oC and completely consumes at 450 oC. Combined with the
activity (Figure 1A), it is suggested that the decrease in NO
conversion at high temperature should be ascribed to the
excessive oxidation of NH3.[26]
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3240-3350, 3580-3700 cm-1 after the 0.15Cu/SAPO-34 sample
is exposed to NH3 at 200 oC. Moreover, the intensity of those
bands increases with the adsorption time. According to
literatures,[30] the bands at about 1188 and 1260 cm-1 are
assigned to asymmetric vibration of N-H bands in NH3 species
adsorbed on the Lewis acid sites, and the bands at about 3245,
3280 and 3341ꢀcm-1 are ascribed to symmetric and asymmetric
+
bending vibrations of the coordinated NH4 . Meanwhile, two
obvious negative bands at 3600 and 3617 cm-1 can be attributed
to NH3 adsorbed on the bridged Al-(OH)-Si Brønsted acid sites,
and their intensities progressively increase with time.[23a] The
above results of NH3 adsorption illuminate that there are
abundant Lewis acid sites and Brønsted acid sites on the
surface of 0.15Cu/SAPO-34 catalyst. These acid sites provide
the favorable conditions for NH3 adsorption and activation which
are more conducive to NH3-SCR reaction.
Figure 5. NH3-TPD profile (A), NH3 oxidation (B), 29Si MAS NMR spectrum (C)
and 27Al MAS NMR spectrum (D) of 0.15Cu/SAPO-34.
Solid-state NMR spectra tests were used to comprehend the
coordination type of Si and Al species in the zeolite framework.
The 29Si MAS NMR measurement result of 0.15Cu/SAPO-34 is
shown in Figure 5C. Many kinds of Si species can be observed
from the deconvolution results of 29Si solid-state NMR spectrum.
The peaks at -110, -104, -99, -95 and -91 ppm are assigned to
the Si4Si, Si1Al3Si, Si2Al2Si, Si3Al1Si and Si4Al species,
respectively. While the peaks at -80 and -86 ppm can be
attributed the structure defects, and the peak at about -115 ppm
belongs to the Si islands.[27] The Si species percentage was
calculated according to the deconvolution results listed in Table
S1. By comparing the amount of different silicon species in
0.15Cu/SAPO-34 and SAPO-34, two conspicuous changes can
be observed that the amount of the Si species defects increases
and the content of the Si4Al species decreases for
0.15Cu/SAPO-34. Due to the large radius of copper ion, the
introduction of copper ion leads to the lattice distortion of
0.15Cu/SAPO-34, resulting in the formation of more defects.
Meanwhile, the introduced Cu ion occupies a part of -OH can
bring about the decrease in Si4Al and acidity, which is consistent
with the NH3-TPD results. As displayed in Figure 5D and S4, for
27Al MAS NMR spectra of the two samples, the signals at 42.8,
10 and -10 ppm are assigned to tetrahedral, pentahedral and
octahedral coordinated Al atoms, respectively.[6a,28] Interestingly,
a large amount of octahedral coordinated Al atoms can be
obtained in the samples, which indicates that a portion of Al may
not be involved in the formation of the zeolite framework or
dealumination occurs when removing CaCO3 with HCl.
Compared with SAPO-34, the amount of octahedral coordinated
Al species in 0.15Cu/SAPO-34 increases a lot, which may be
due to the fact that the addition of Cu ion leads to the change in
the coordination mode and ability of the framework Al.[29] By
comparing the changes of Si and Al species between SAPO-34
and 0.15Cu/SAPO-34, it is found that the introduction of copper
has a significant effect on the amount of strong and weak acid
sites.
Figure 6. In situ DRIFTS of NH3 adsorption (A) and reaction between NO+O2
and pre-adsorbed NH3 (B) on 0.15Cu/SAPO-34 at 200 oC.
The NO+O2 were introduced into the system after the NH3
adsorption purged by N2 for 30 min to investigate the reaction
between pre-adsorbed NH3 and the NO+O2. As shown in Figure
6B, the intensities of all the NH3 adsorption bands only exhibit a
slightly reduction after purged by N2. When NO+O2 is injected,
the peaks of all ammonia species decrease sharply. Only 3
minutes later, almost all the peaks belong to the NH3 adsorption
disappear, which reveals that whether the NH3 adsorbed on the
Brønsted or Lewis acid sites can participate the reaction with
gaseous NO + O2 immediately.
NH3 adsorption was recorded at different time by in situ DRIFTS
over 0.15Cu/SAPO-34 and the results are exhibited in Figure
6A. Several bands can be observed in the region of 1100-1300,
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10.1002/cctc.202000818
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Conclusion
By using cupric citrate as the copper source, TEA as the organic
structure-directing agent, and CaCO3 as the hard template to
build mesopore, we have successfully synthesized copper
modified SAPO-34 zeolite (0.15Cu/SAPO-34) with micro-
mesoporous hierarchical pore by facile one-step hydrothermal
method. 0.15Cu/SAPO-34 shows the superior NH3-SCR
performance for NO removal with the wide activity temperature
window of 165-500 oC, even after hydrothermal treatment with
10% H2O at low (80 oC) or high (700 oC) temperature. Moreover,
the catalyst also shows good NH3-SCR activity under high
GHSV and noticeable resistance to H2O and SO2. Combined the
XPS results, the suitable Cu+ and Cu2+ species on the catalyst is
closely related to the NH3-SCR activity. In addition, we propose
that the NH3-SCR of NO mainly follows the E-R mechanism over
0.15Cu/SAPO-34. Compared with our previous work, this
method can reduce the time required for synthesis while
maintaining the wide activity temperature window. In summary,
this is an efficient and economic method to synthesize the
Cu/SAPO-34 with excellent deNOx perfromance.
Experimental Section
xCu/SAPO-34 (x=0.05, 0.10, 0.15, 0.20) zeolites with hierarchical pore in
which x represents the theorical Cu/Al ratio, were prepared by one-pot
hydrothermal synthesis method with cupric citrate and CaCO3 as the
copper source and hard template, respectively. The synthesis steps of
xCu/SAPO-34 are as follows: (1) 3.55 g pseudoboehmite (71 wt% Al2O3,
Kermel Company) and 30 mL distilled water were added into beakers
and stirred at room temperature. (2) After 2 h, 3.25 mL H3PO4 (85 wt%,
Kermel company) was added to the solution and stirred 2-3 h. (3) 1.5 g of
SiO2 as the silicon source was introduced into the mixture under stirring
and continually stirred for 2 h. (4) 7.5 mL triethylamine (TEA) as the
OSDA was slowly introduced into the solution for 2 h. (5) With the Cu/Al
ratios were set as 0.05, 0.10, 0.15 and 0.20, cupric citrate as the copper
source was put into the gel and then stirred overnight. (6) In the interest
of producing the hierarchical pore in the catalysts, 2 g CaCO3 was
introduced into the mixture and 5 mL distilled water and 5 mL ethanol
were injected to promote the dispersion of CaCO3. (7) The final solution
was transferred into 50 mL autoclaves and heated at 200 oC for 48 h.
Then the product was washed by water to remove the raw material and
0.2 mol/L HCl solution was used to remove the CaCO3. (8) The catalysts
Figure 7. In situ DRIFTS of NO adsorption (A) and reaction between NH3 and
pre-adsorbed NO+O2 (B) on 0.15Cu/SAPO-34 at 200 oC.
To research the adsorption of NO+O2 at different time over
0.15Cu/SAPO-34, the in situ DRIFTS tests were carried on. As
displayed in Figure 7A, NO+O2 adsorbed on the surface of
0.15Cu/SAPO-34 can engender many kinds of nitrate species,
such as monodentate nitrate, bridged nitrate, bidentate nitrate,
etc.[18a] The negative band at 1200 cm-1 can be assigned to
nitrite which is generated by NO + O2 adsorbed on the surface of
0.15Cu/SAPO-34.[12] While the bands between 1500 and 1700
cm-1 are quite complicated, most of the bands cannot stable
existence on the catalyst surface. But the intensity of the band at
1624 cm-1 increases with the extension of contact time with
NO+O2, belonging to the adsorbed NO2 species formed by the
reaction NO and O2.[31] On the whole, it is not easy for NOx to
absorb on the 0.15Cu/SAPO-34 catalyst. Even if the signals are
amplified 10 times, the intensities of all these bands are too
weak to discern easily. The results of the reaction between pre-
adsorbed NO+O2 and NH3 are shown in Figure 7B. As NH3 is
introduced, the adsorbed NOx species disappears and the
characteristic peaks of adsorbed NH3 species appear
immediately. We can draw a conclusion that the generation of
NO2 can promote the “Fast SCR” reaction to some extent
although the amount of pre-adsorbed NO+O2 on the catalyst
surface is far fewer.[32]
o
were dried at 100 C to remove the water and calcined at 600 oC for 5 h
to remove OSDA. SAPO-34 was also synthesized according to the above
description except without adding cupric citrate.
The powder X-ray diffraction (XRD) patterns were measured from 5-50 o
on Bruker D8 Advance X-ray diffract meter system in the radiation of Cu
Kα (λ =0.15418 nm) at 40 mA and the step size of 0.02 o·s−1. Scanning
electron microscope (SEM) images were measured on a Hitachi S-4800
microscope at 20 kV. BET surface area and pore characterization were
measured on Micrometritics ASAP 2020 Plus by N2 adsorption-
desorption at the temperature of -196 oC. Before testing, the sample
o
(0.100 g) was pretreated in the vacuum condition with heating at 200 C
for 5 h. The amount of Cu was obtained on inductive coupled plasma
optical emission spectrometer (ICP-OES) (NexION 350D). The X-ray
photoelectron spectroscopy (XPS) was performed by the Kratos-AXIS
ULTRA DLD with the Al Kα radiation source. The binding energy (BE)
To sum up, whether it is pre-adsorbed NH3 or NO+O2, the
reaction rate occurs on 0.15Cu/SAPO-34 is very rapid. Base on
the much weaker NOx adsorption and massive adsorbed NH3
species, we think that the NH3-SCR reaction may mainly follow
the Eley-Rideal (E-R) mechanism over 0.15Cu/SAPO-34.
was referred through the
C 1s (BE = 284.6 eV). The electron
paramagnetic resonance (EPR) spectrum was measured on the Bruker
EPR A200 instrument.
6
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10.1002/cctc.202000818
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27Al and 29Si nuclear magnetic resonance (NMR) measurements were
carried on the Bruker Avance III 400 spectrometer.27 Al chemical shift and
29Si chemical shift were referred to Al(NO3)3 of 1 mol/L and Kaolin,
respectively. And the 27Al NMR and 29Si NMR data were recorded with a
Keywords: one-pot hydrothermal method • Cu/SAPO-34 •
selective catalytic reduction of NO with NH3 • cupric citrate • high
hydrothermal stability
spinning rate of 12 kHz and
6 kHz, respectively. Temperature
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by
the
expressions
(1)
and
(2):
[NO]in − [NO]out
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× 100%
(1)
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Entry for the Table of Contents
Cu/SAPO-34 with excellent NH3-SCR performance was synthesized for the first time using cupric citrate as a copper source in a one-
step method.
9
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