NH3-SCR over Cu/SAPO-34 catalysts with various acid contents and low Cu loading

Article abstract of DOI:10.1039/c3cy00453h

Four Cu/SAPO-34 catalysts are used to research the effect of their acid content on their NH₃-SCR activity, by changing the proportion of Si in the catalyst. Though the catalysts exhibit similar Cu loading performed by an ion-exchange method, th

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ARTICLE TYPE
The NH -SCR over Cu/SAPO-34 catalysts with various acid contents at
3
low Cu loading
a
a
a,b
c
Tie Yu, Jun Wang, Meiqing Shen,* and Wei Li
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
The four Cu/SAPO-34 catalysts are used to research the
structure ensure the superior SCR activities of the aged Cu-SSZ-
effect of acid contents on the NH -SCR activity through
changing the Si amounts. Though the catalysts contain the
13 catalysts, while the Cu/ZSM-5 and Cu/beta showed decreased
NO conversions due to the structure collapse by water in
3
9
6
hydrothermal treatment . Meanwhile, Fickel also studied the
small-pore Cu/CHA catalysts, which also showed higher NO
conversions and excellent hydrothermal stability than medium-
similar Cu loading by ion-exchange method, they perform
o
5
5
6
6
7
0
5
0
5
0
1
1
2
0
5
0
different NO conversions during 120-600 C. The NH -TPD
3
results show that the Si amounts can adjust the acid contents
of SAPO-34. During the low temperature range, the kinetic
tests prove that the various acid contents could not affect the
apparent activation energy (Ea) of Selective Catalytic
1
0,11
pore zeolites. Our previous works
about the Cu/CHA
2
+
catalysts had proved that the Cu species were the active sites for
NH -SCR over Cu/SAPO-34 at low temperature, and
3
hydrothermal treatments benefited the improvement of NO
Reduction by NH (NH -SCR) over Cu/SAPO-34. But it is
3
3
2+
conversion due to the increments of Cu species. Furthermore, it
found that the acid densities are related to the NO
conversions at low temperature. During the high temperature
range, the increment of acid contents in Cu/SAPO-34
catalysts inhibits the NH conversions in the NH Oxidation
is known that the Si amounts can affect the acid contents of
SAPO-34 and the acidity played an important role in NH -SCR
3
10
over Cu/SAPO-34 . However, there have not been evident
3
3
conclusions about the relation between the acid contents and the
activities at the whole temperature range, and the influence of
acid contents on the SCR mechanism is not verified.
results. And NH3 Oxidation and NH -SCR are competing
3
reactions and the former one can decrease the NO
conversions. Finally, the influence of acid contents on the NH3
Oxidation is also investigated by the kinetic tests.
In this work, the main aim is to further investigate the effect of
acid contents on the SCR activities and NH3 oxidation over
Cu/SAPO-34 catalysts. Firstly, four SAPO-34 supports with
various Si contents are synthesized and the Cu/SAPO-34 catalysts
are prepared using ion-exchange method with equivalent Cu
loading by precise control of the process. The XRD, SEM are
used to check the chabazite structure of catalysts. Secondly, the
NH -TPD and NH storage tests are performed to analyze the
1
Introduction
2
3
3
4
4
5
0
5
0
5
NOx is one of the most hazardous greenhouse gases, which can
1
cause human lung disease, photochemical smog and acid rain .
Recently, more and more strict regulations are implemented to
control the NOx emission from the stationary source and mobile
3
3
various acidities of zeolites. In addition, the kinetic tests and the
source, especially exhaust gas of vehicle. Since the NH -SCR
3
NH Oxidation experiments are used to explore the influence of
3
technology was firstly applied in 1970s, it had become one of the
most effective methods to decrease the NOx emission. The NH3-
acid contents on the SCR activities at different temperature
ranges and the competition of the NH -SCR/NH Oxidation over
3
3
SCR technology utilizes the NH as reductant to remove NOx as
3
2
-4
75 Cu/SAPO-34. Finally, the relationships between the acidities and
the SCR activities/NH3 oxidation are conducted for different
temperature ranges.
following: 4 NH +4 NO+ O =4 N +6 H O
.
3
2
2
2
The SCR zeolites catalysts have been widely applied for their
higher NO conversions and wider temperature windows than the
2
,5
oxide catalysts and noble metal catalysts
.
Recently,
2
Experimental
Cu/chabazite (CHA) catalysts have become another focus in SCR
research due to its excellent hydrothermal stability and N2
2.1 The preparation of SAPO-34 and Cu/SAPO-34
6
selectivity . SAPO-34 and SSZ-13, small pore sized molecular
8
0
The SAPO-34 was synthesized with the mole composition of 0.2
Morpholine (MA): 0.1 Al O : 0.1 P O : (0.02-0.15) SiO : 6.5
sieves with CHA structure, are built from infinite TO tetrahedral
4
2
3
2
5
2
units (T = Si, Al for SSZ-13 and Si, Al, and P for SAPO-34) with
7
8
H O by hydrothermal method. The sources of Si, P, and Al were
2
silica sol, 85% phosphoric acid and pseudoboehmite, respectively.
Firstly, the phosphoric acid and the pseudoboehmite were mixed
the same topology . Kwak reported that the Cu-SSZ-13
presented the higher NO conversion and the less byproducts
(
NO /N O) concentrations than Cu/ZSM-5 and Cu/beta.
2 2
8
5
with H O, and the mixture was fiercely stirred for 1 h. Then the
2
Moreover, their further research found that the stable CHA
silica sol and the MA were added and blended well. Thirdly, the
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homogeneous mixture was sealed in autoclave and heated at 200 55 beam.
o
C for 24 h. After crystallization, the as-synthesized sample was
Temperature Programmed Desorption by NH3 (NH -TPD)
3
o
obtained through centrifuging, washing and drying at 100 C for
h. Finally, the samples were calcined in air (300 ml/min) from
experiments were performed to evaluate the acid contents over
samples. Prior to the experiments, the catalysts were pretreated at
6
o
o
o
o
o
o
5
0
5
0
30 C to 650 C under the rate of 3 C/min and stayed at 650 C
500 C for 30 min in 5% O /N , and then cooled to 80 C in N .
2 2 2
for 6 h. The compositions of SAPO-34 were tested by x-ray 60 NH adsorption was in 500 ppm NH /N until the outlet NH
3
3
2
3
2
fluorescence spectrometer (XRF) in Table 1 and the molecular
sieves were denoted as Six, where x represented additive ratios of
the silica sol.
concentration was stable. Then, the catalysts were purged with N
o
to remove any weakly absorbed NH at 80 C. Finally, the
3
o
o
catalysts were heated from 80 C to 550 C at a ramping rate of
o
1
1
2
The preparation of Cu/SAPO-34 contained two steps by ion-
10 C /min.
exchange method. Firstly, the NH -SAPO-34 was obtained by 65 The NH /NO oxidation were performed in the equivalent reactor,
4
3
o
exchanging HSAPO-34 in ammonium nitrate solution at 80 C
the ways of packing and the pretreatment with SCR tests. The
for 3 h. Then the NH -SAPO-34 was stirred with copper sulfate
inlets contained: 500 ppm NH /NO and 5% O /N . The gas flow
4
3
2
2
solution to obtain Cu/SAPO-34. The exchange times for various
supports were precisely controlled to get equivalent Cu loading.
and the volume hourly space velocity in all experiments were
-
1
controlled at 500 ml/min and 300,000 h , respectively. The NH
3
o
o
After each ion-exchange processes, the slurry was filtered, 70 oxidation was performed from 150 C to 550 C, while the NO
o
o
o
washed and the solid was dried at 90-100 C for 16 h in oven.
oxidation was performed from 120 C to 600 C.
o
And then the dried Cu/SAPO-34 was calcined at 550 C for 4 h in
The Electron Paramagnetic Resonance (EPR) spectra were
recorded on a Bruker ESP320 spectrometer. The Bruker
ESP320E software and the special Bruker program were used for
75 data analysis. The EPR spectra were recorded at room
temperature and atmospheric pressure. In order to observe the
muffle furnace. The Cu loading over catalysts were tested by ICP
and the results were shown in Table 1. The Cu/SAPO-34 was
named as Cu/Six for short.
2
.2 The SCR activities
2
+
hyperfine structure of Cu species, dehydration of the samples
was carried out prior to EPR experiments. The Cu/SAPO-34
The SCR activity was tested in a quartz reactor (20 mm inner
diameter) using 0.1 g sample (60-80 mesh) mixed with 0.9 g
quartz (60-80 mesh) at atmospheric pressure. The catalyst was
sealed in the tube with quartz wool. The temperature was
controlled by a type K thermocouple inserted into the center of
the power catalyst. The Fourier Transform Infrared (FTIR)
spectrometer (MKS-2030) equipped with a 5.11 m gas cell was
used to measure the concentration of NO, NO , N O and NH .
-
4
o
catalysts were vacuumed at 4.0╳10 Pa for 1 h at 120 C.
2
3
3
5
0
5
80
2.4 The Kinetic Measurements
The NH -SCR kinetic tests were performed in a thin quartz tube
3
using 25 mg catalyst and 125 mg quartz sand. A relative small
particles sizes (80-100 mesh) and volume hourly space velocity
-
1
(3,600,000 h ) ensured the elimination of internal and external
2
2
3
The gas flow rates in all experiments were controlled at 500 85 diffusion respectively. Prior to the kinetic experiments, the
o
ml/min. Prior to the experiment, the catalysts were heated up to
samples were pretreated in 5% O /N at 500 C. The inlets
2 2
o
o
5
00 C and kept at 500 C for 10 min under 5% O in N . Steady-
consisted of 500 ppm NO, 500 ppm NH , 5% O with N as the
3 2 2
2
2
state activity tests were performed for all catalysts using a feed
gas composition of 500 ppm NO, 500 ppm NH and 5% O . In
balance. The kinetic steady-state measurements were obtained
from 150 C to 225 C. By assuming plug flow reactor and free of
o
o
3
2
addition, the SCR tests with 3% H O and 6% CO were also 90 diffusion limitations, the NH -SCR reaction rates can be
2
2
3
performed and the results were shown in the Fig .S6. The tested
temperature was from 120 C to 600 C. The NO conversion was
calculated as following equation:
calculated from the NO conversion by:
x
-1
o
o
rate[mol NO
x
 gcata  s ]
-1
X
NOx[%]  FNOx[L(NOx)  min ]
-1
=
[mol NO  gcata  s ]
x
NOinlet -NOoutlet
-1
-1
4
0
(1)
NO conversion [%]=
 100 [%]
mcata  60[s  min ]  22.4[L  mol ]
NOoutlet
(2)
2
.3 The characterization of zeolites
XNOx = NO Conversion, [%];
x
-
1
The XRD patterns were performed using X’Pert Pro
diffractometer operating at 40 kV and 40 mA with nickel-filtered
Cu Kα radiation (λ=1.5418 Å) in the range 5°< 2θ < 50° with a
9
5
FNOx = Flow Rate of NO , [L
·min ]
x
(NOx)
The NH Oxidation kinetic experiments were conducted under
the same condition with the NH -SCR. The inlets consisted of
00 ppm NH , 5% O with N as the balance. The kinetic steady-
3 2 2
state measurements were obtained from 410 C to 490 C.
3
3
2
4
5
step size of 0.02°. The BET surface area (m /g) was determined
5
o
o
from the liner portion of the BET plot by measuring the N2
isotherm of the samples at 77 K using F-Sorb 3400 volumetric
adsorption/desorption apparatus. Prior to the measurement, the
100 3 Results and Discussion
o
zeolites were degassed at 150 C under vacuum for 3 h.
3
.1 The characterization of zeolites
5
0
Scanning Electron Microscopy (SEM) image of the samples was
measured on a HITACHI S4800 field emission microscope. The
samples were pasted on a sample holder using a carbon tape and
then covered with Pt film to become conductive. SEM images
were taken at magnifications of 2,000 with the 3 kV electron
The composition of SAPO-34 is shown in Table 1. It is seen that
the four molecular sieves only show obvious difference of Si
amounts which can be adjusted by the additive amount of silica
05 sol in the materials. In addition, the four Cu/SAPO-34 catalysts
1
2
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present the similar Cu loading due to precise process control.
The XRD results
synthesized by the substitution of P atoms in AlPOs molecular
1
7
sieves by Si . If the mole ratio of the (Si + P)/Al is not bigger
than 1, Si atom substitutes P and forms the Si (4OAl) species. If
the ratio of (Si + P)/Al is bigger than 1, the substitution of
adjacent P and Al by Si atoms occurs and the Si islands appear as
The XRD profiles in Fig. 1 present that the four SAPO-34
1
2,13
supports all have the typical CHA structure
. In addition, the
3
5
5
peak intensities deducting the background are compared in Fig.
S1 (supplementary information) and they increase with the
increment of Si contents, which is mainly due to the higher
1
2,17
the coordination structure of Si (nOAl, n=0-3) . And this
structure defects can inhibit the growth of molecular sieves.
Furthermore, the (Si + P)/Al values for the four supports are
listed in Table S1 (the supplementary information). It is seen that
the (Si + P)/Al value increases with the increment of Si amount in
1
4,15
crystallinity for Si1.4 and Si1.0
.
40
Table 1 the composition of SAPO-34 supports and Cu/SAPO-34 catalysts
2
9
The
catalysts
The materials
ratios *
The
molecular
sieve *
Si0.08Al0.45
Si0.13Al0.44
Si0.15Al0.43
Si0.18Al0.42
The
crystal
size (µm)
8-10
4-6
4-6
The Cu
loading
(wt%)
0.89
0.89
0.90
SAPO-34. Due to this law, the Si NMR spectra of Si0.2 and
Si1.4 are conducted in Fig. S7. The Si islands form in Si1.4 due
to its highest (Si + P)/Al value and the Si0.2 almost presents no Si
a
b
Cu/Si0.2
Cu/Si0.6
Cu/Si1.0
Cu/Si1.4
1:1:0.2
1:1:0.6
1:1:1
45 islands peaks for its lowest (Si + P)/Al value. Similarly, it is
conducted that the proportion of Si islands in four SAPO-34
molecular sieves improves with the raise of (Si + P)/Al value.
Therefore, the increment of Si amounts may cause the reduction
of crystal sizes due to the more Si islands. Meanwhile, the same
SEM images of Cu/SAPO-34 and corresponding supports also
present that the loading of Cu dose not influence the structure of
zeolites.
1:1:1.4
2-4
0.94
a
1
0
* : the mole ratio of Al
2
O
3
:P
2
O
5
:SiO
were omitted as Si
y z 2
P O , (x + y +z )=1.
2
.
b
*
: the P ratios in Si
x
Al
y
P
z
O
2
x y
Al for abbreviation.
During the formula of Si
x
Al
50
Fig. 1 the XRD profiles of SAPO-34 supports
The BET results
1
5
The BET results in Table 2 illustrate that the composition of
SAPO-34 can affect their specific surface area (SSAs). The Si0.2
has smaller SSA than others, which is due to its lower
crystallinity. Besides, the SAPO-34 supports show the equivalent
SSAs with that of the corresponding Cu/SAPO-34, which reflects
that the loading of Cu dose not change the SSA of SAPO-34
supports.
2
0
55
Table 2 the specific surface area (SSA) of samples
The catalysts
The SSA of supports
The SSA of catalysts
2
2
(m /g)
(m /g)
Cu/Si0.2
Cu/Si0.6
Cu/Si1.0
Cu/Si1.4
447.6
590.9
635.9
550.6
447.1
598.4
634.5
547.1
Fig. 2 the SEM images of SAPO-34 supports and Cu/SAPO-34 catalysts
2
5
The SEM results
The SEM results in Fig. 2 show that the four SAPO-34 supports
3.2 The NH -TPD results
3
1
2,13
have the representative cubic structure of CHA
, which
NH -TPD is used to research the acidities of zeolites and Fig. 3(a)
3
consists with the XRD results. In addition, the four SAPO-34
supports present a decreasing crystal sizes in Table 1 when the Si
contents increase. The decline of crystal sizes may be due to a
60
shows that all the SAPO-34 supports contain three NH3
desorption peaks, named as A, B and C. Our previous works have
reported that the peak (A) arises at the lower temperature related
to the weak Brønsted acid sites at surface hydroxyls. The two
3
0
1
6
small number of Si islands . The SAPO-34 is considered to be
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peaks (B and C) at the higher temperature are possibly assigned
to the structural Brønsted acid sites referred to moderate and
strong acidity. In addition, compared to the support the difference
of acidic ascription over Cu/SAPO-34 samples is the peak A and
C. The peak A of Cu/SAPO-34 samples represents the adsorbed
5
0
5
0
5
NH molecules at weak Brønsted acid sites and weak Lewis acid
3
sites related to Cu species and the peak C stands for the adsorbed
NH molecules both at strong Brønsted acid sites as SAPO-34
3
supports and new Lewis acid sites created by the Cu
1
0,18,19
1
1
2
2
species.
. Moreover, the desorbed NH contents in Table 3
3
illustrate that the increment of Si contents in SAPO-34 can
improve their acid contents. The acidity of HSAPO-34 originates
from the proton for compensating the unbalanced electronic
charges due to Si incorporation into the neutral framework of
3
5
Fig. 3 the NH
3
-TPD profiles over (a) SAPO-34 molecular sieves and (b)
1
2,17
AlPOs molecular sieves
. Nevertheless, the acid contents do
o
Cu/SAPO-34 at 80 C. The conditions were: 500 ppm NH
until the outlet NH concentration was stable. The NH -TPD experiment
was ramped from 80 C to 550 C at a rate of 10 C /min.
3.3 The SCR activities test
3
was adsorbed
not scale linearly with the Si amounts, which also illustrate the
existence of Si islands. It is known that the Si islands increase
with the increment of Si amounts in the four samples..
Furthermore, the structure in Si islands (-Si-O-Si-) could not
generate acid sites and the acidity of various Si structures
presents the following sequence: Si (1OAl) ˃Si (2OAl) ˃Si
3
3
o
o
o
40
The acidity and the loaded Cu species are the main reasons which
give Cu/SAPO-34 the excellent SCR performance. Considering
the objective of this research, the four Cu/SAPO-34 catalysts
comprise the same low Cu loading to emphasize the role of acid
(3OAl) ˃ Si (4OAl) ˃Si (0OAl). Consequently, the desorption
temperature of peak C moves to higher temperature with the
increment of Si amount, which is due to the Si (nOAl, n=0-3) 45 contents in NH -SCR activities. Fig.4a reveals that the acid
3
1
7
shows the stronger acidity than Si (4OAl) in Si0.2 .
contents obviously affect the SCR activities of Cu/SAPO-34 with
different Si amounts. Cu/Si1.4 sample performs the best NO
conversion, while the Cu/Si0.2 shows the inferior NO conversion.
In addition, the Cu/Si0.6 and Cu/Si1.0 have switched SCR
activities during middle temperature and high temperature range.
The detailed reasons are related to the acid contents discussed
later.
3
Table 3 the adsorbed NH contents over SAPO-34 and Cu/SAPO-34
samples
Support
The adsorbed
NH
Sample
The adsorbed
NH
5
5
6
6
0
5
0
5
3
3
contents(mmol/g)
contents(mmol/g)
Si0.2
Si0.6
Si1.0
Si1.4
0.197
0.270
0.298
0.335
Cu/Si0.2
Cu/Si0.6
Cu/Si1.0
Cu/Si1.4
0.075
0.195
0.202
0.243
The acid contents also influence the side-products of SCR
reaction in Fig. 4b. Especially, the Cu/Si0.2 produces 33 ppm
o
N O at 295 C. It is reported that the N O formation was due to
2
2
the decomposition of NH NO , and the adsorbed NH species
could interact with the formed nitrates strongly and this
ammonia-nitrates species inhibit the decomposition of formed
nitrates for its low acid content . So the highest N O production
for Cu/Si0.2 sample is due to its lowest adsorbed NH contents
for its least acid sites. Other catalysts show excellent N2
selectivities and the concentrations of NO and N O are less than
4
3
3
Fig. 3(b) depicts that the Cu/SAPO-34 also contains three kinds
of acid sites. In addition, the sums of adsorbed NH3 over
Cu/SAPO-34 catalysts in Table 3 are less than that of
corresponding supports, which is mainly due to the replacement
3
0
20
2
3
1
0
of acid sites (Si-OH-Al) by Cu species .
2
2
1
0 ppm. The NH3 conversion in Fig. 4c presents the similar
sequence with the NO conversion in Fig. 4a, and the NH3
conversion increases to 100% with the increment of temperature.
4
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Fig. 5 the kinetic results of NH
3
-SCR over Cu/SAPO-34 catalysts at low
30
temperature. The conditions were: the inlets consisted of 500 ppm NO,
00 ppm NH , 5% O with N as the balance. The kinetic steady-state
measurements were obtained from 150 C to 225 C.
5
3
2
2
o
o
The relationship between the acid densities and the activities
To further investigate the influence of the acid contents to the
3
4
4
5
0
5
SCR activities, the following experiments about the relation
o
between NH adsorption sum and NO conversion rates at 250 C
3
were studied and the results are summarized in Fig. 6. Prior to the
tests, the catalysts are pretreated in 5% O /N and then the
2
2
(
NO+O ) are cut in. When the reaction of (NO+O ) reaches
2
2
equilibrium, the NH is cut in till stable. Consequently, the stored
3
NH contents can be calculated by subtracting NH contents of
3
3
outlets from that of inlets. The results in Fig. 6a present the rates
of NO conversions increase with the increment of stored NH3
contents gradually. There are clearly two different steps during
the process. It is considered that the NO should be oxidized to
nitrate/nitrite to participate in the SCR reaction over
Fig. 4 the results of SCR activities over various Cu/SAPO-34 catalysts. (a)
the NO conversion; (b) the concentrations of NO 、N O; (c) the NH
conversion. The feed contains 500 ppm NH , 500 ppm NO, 5% O , N
the balance. The flow rate and the volume hourly space velocity in all
2
2
3
5
3
2
2
is
-
1
experiment were controlled at 500 ml/min and 300,000 h
2
,21,22
Cu/zeolites
. Firstly, when the (NO+O ) attains equilibrium,
2
3
.4 The SCR activities differences during low temperature
some nitrate/nitrite species have formed over Cu species. The
range
NH just cut in can react with the formed nitrate/nitrite instantly
3
1
1
2
2
0
5
0
5
The NH -SCR kinetic tests at low temperature
3
50 due to the enough Cu sites for the SCR reaction. When the
The SCR results in Fig. 4 reveal completely different NO
conversions and Table 1 shows that the main difference among
the four Cu/SAPO-34 catalysts with the equivalent Cu loading is
the various acid contents. In order to explore the roles of acid
nitrate/nitrite species are decreasing and the adsorbed NH sum is
3
increasing, the rates of NO conversions slow down till steady
state for the coverage of most Cu species by adsorbed NH3
species. In addition, the Fig. 6a presents that the four catalysts
55 perform the different rates of NO conversions, which reflects that
contents, the NH -SCR kinetic experiments are performed and
3
the results are shown in Fig. 5. Firstly, the NO rates of four
catalysts show the same order as the SCR activities in Fig. 4,
which explains the differences of NO conversions over various
catalysts. Furthermore, the four parallel lines in Fig. 5 reveal that
the catalysts have the same apparent activation energy (Ea),
the adsorbed NH sum may be related to their various activities.
3
After the SCR reaction reaches equilibrium, the adsorbed NH3
contents are calculated and the results are shown in Fig. 6b. Since
the Si contents affect both the SSAs and the acid contents of
6
0
SAPO-34, the acid densities are used to express the various NH3
contents over Cu/SAPO-34 catalysts. It is seen that the rates of
NO conversions at low temperature are proportion to the acid
density of catalysts. Moreover, it is worth to note that the result in
Fig. 6 is conducted under the same conditions of SCR
performance, which stresses the role of NH3 adsorption over
which proves that the acid contents do not influence the NH3-
SCR mechanism over Cu/SAPO-34 catalysts.
In addition, the EPR are used to identify the properties of Cu
2
+
species and the four catalysts contain the same Cu species due
1
1
to the same hyperfine structure in Fig. S3 . The results in Fig. S4
65
2
+
(the supplementary information) expose that the Cu species are
Cu/SAPO-34. So the NH -SCR activity of Cu/SAPO-34 at low
3
the active sites for NH -SCR reaction.
3
temperature is close related with its acid density.
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Page 6 of 8
Fig. 7 the NH
The conditions were: the inlets consisted of 500 ppm NO, 500 ppm NH
% O with N as the balance. The kinetic steady-state measurements
3
-SCR results under high GHSV (gas hourly space velocity).
30
3
,
5
2
2
o
o
were obtained from 410 C to 490 C.
The NH Oxidation reaction results
3
It is known that the NH Oxidation and NH -SCR are competing
3
3
2
3
4
4
5
0
5
relation during SCR process at high temperature . And the NO
oxidations over Cu/SAPO-34 present very low NO conversions,
which illustrates that the NO oxidation could hardly happen (Fig.
2
S in supplementary information). So the NH Oxidation is one
3
of the decisive reasons which cause the decline of the NO
conversion at high temperature. The NH oxidation results in Fig.
3
8
a show that the acid contents influence the NH conversions
3
over different catalysts. The Cu/Si0.2 performs the highest NH3
3
Fig. 6 (a) the relationship between the NH storage and the rate of NO
o
conversion, while the Cu/Si1.4 shows the lowest NH conversion.
3
conversion on Cu/SAPO-34 samples at 250 C. Firstly, the inlets of NO
500 ppm, O 5% (vol) attained equilibrium, then the NH 500 ppm were
It is seen that the increment of acid contents could inhibit the NH3
oxidation at high temperature. Fig. 8b and 8c expose the side-
products of NH oxidation and the concentrations of NO, NO
3 2
5
0
2
3
cut-in and reach steady. The flow rate is 500 ml/min and the temperature
o
kept at 250 C. (b) The relationship between the acid density and the rate
o
of NO conversion at 250 C. The experiments conditions: After the SCR
and N O. The appearance of NO at high temperature could
2
o
equilibrium was attained on samples at 250 C, all of the inlets were
directly decline the NO conversion for SCR activities.
Furthermore, Cu/Si0.6 generates less NO than Cu/Si1.0, which
1
cutoff except N
2 3
. Then the NH -TPD was performed to estimate the
adsorbed NH sum. The flow rate is 500 ml/min and the tests were
3
o
50 may explain the higher NO rates for the former one than the later
in Fig. 7. It is considered that the NH oxidation contained two
performed at 250 C. The acid density is the adsorbed NH
3
sum to the
SSA ratios for each sample.
3
steps: the NH was oxidized to NO and the formed NO reacted
3
3
.5 The SCR activities differences during high temperature
23
with NH3 as standard NH -SCR reaction . For Cu/Si0.6 and
3
1
2
2
5
0
5
The NH -SCR kinetic tests at high temperature
The Cu/SAPO-34 catalysts perform the apparently different NO 55 rate than the later one, so the more NO content is released over
conversions at high temperature in Fig. 4a. In order to identify the
possible reasons, the kinetic tests were performed firstly. The
kinetic results in Fig. 7 present four parallel lines of NO rates as a
function of 1000/T, which illustrates that the same SCR
mechanism exists over various catalysts. Furthermore, the results 60 conversions in NH oxidation may be affected by the acid
explain the higher NO rates are responsible for the higher NO
conversions in Fig. 4a. Furthermore, the different Cu species in
Cu/Si1.0 samples, the former one presents higher NO conversion
3
Cu/Si1.0 than that over Cu/Si0.6. In addition, the NO and N O
2 2
concentrations in Fig. 8c are much lower than that of NO, but
they display the same sequence as byproduct of NO
concentrations, which means that the N selectivities and NH
2
3
3
2
4
contents in catalysts .
the samples are also studied and assigned by H -TPR in Fig. S8,
2
and the results in Fig. S9 reveal that the Cu species are not the
main reason causing the difference of SCR activity in this
research.
6
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Page 7 of 8
Catalysis Science & Technology
2
3
3
4
4
5
0
5
0
5
Combining the above data, it is concluded that the SCR activities
at high temperature are mainly affected by the competing of NH3
oxidation for NH consumption. The NH -TPD results in Fig. 3
present that only strong acid sites can adsorb NH3 at high
temperature, and the adsorbed NH3 amounts at different
temperature are listed in Table S1 (shown in the supplementary
3
3
information). It is seen that the adsorbed NH amounts at high
3
temperature increase with the raise of Si contents. Furthermore,
the DRIFT results about the different NH behaviour in NH -SCR
3
3
and NH Oxidation at high temperature are conducted in Fig. S5
3
(supplementary information). The results illustrate that the NH
species adsorbed on Cu species (896 cm , 840 cm ) were firstly
3
-
1
-1
consumed, while the adsorbed NH species over Bronsted acid
3
-
1
-1
-1
-1
-1
sites (3626 cm , 3601 cm , 3332 cm , 3286 cm , 1460 cm )
present different activities in the two reactions. It is concluded
that the more strong acid sites is beneficial to NH -SCR but not to
3
4,25
2
NH Oxidation on the basis of four catalysts
. For Cu/Si1.0
3
sample, though it show lower NO rates for SCR reaction than
Cu/Si0.6 in Fig. 7, the later one presents higher NH rates for
3
NH3 oxidation in Fig. 9. Consequently, the Cu/Si1.0 sample
performs better SCR activity than Cu/Si0.6 at high temperature.
Moreover, more studies about the effect and the further
mechanism researches about the NH3 oxidation need to been
done in future.
Fig. 8 the NH
conversion; (b) the NO concentration; (c) the NO
The conditions were: the inlets consisted of 500 ppm NH
as the balance. The flow rate and the volume hourly space velocity in all
3
Oxidation results over Cu/SAPO-34 catalysts. (a) the NH
/N O concentration .
, 5% O with N
2
3
4
. Conclusions
2
2
5
3
2
5
0
The four Cu/SAPO-34 catalysts only with various acid contents
are used to research the relation between the acid properties and
SCR activities. The acid contents of SAPO-34 can be adjusted by
the Si amounts and the increment of acid contents improves the
NO conversions of Cu/SAPO-34 catalysts.
The acid contents do not affect the apparent activation energy of
Cu/SAPO-34 catalysts at low temperature. Meanwhile, the acid
density reveals the proportional relation with NO conversion at
low temperature.
-
1
experiment were controlled at 500 ml/min and 300,000 h .
The NH Oxidation kinetic tests
3
It is seen that the NH Oxidation affect the SCR activities at high
3
1
0
temperature. So the kinetic NH3 Oxidation experiments are
conducted to further investigate the influence of acid properties to
55
the SCR activities. Fig. 9 presents the kinetic results of NH3
o
oxidations during 410-490 C, during which the NH conversions
3
are less than 20%. It is seen that the four lines show the similar
slopes, which means that the four Cu/SAPO-34 catalysts have the
At high temperature, NH -SCR and NH3 Oxidation are
competing relation and the activities of former are mainly
influenced by the later one. The acid contents also do not affect
3
1
5
60
same Ea value for NH oxidation. And the acid contents do not
3
influence the NH oxidation mechanism. Moreover, the NH rates
3
3
the activation energy of NH Oxidation, but the increment of acid
3
in Fig. 9 present the same sequence of NH conversions in Fig. 8a,
3
content dose not benefit the NH conversion and influences side-
3
which further explain the difference of NH conversions.
3
products concentrations. The increment of acid contents can
65
inhibit NH oxidation at high temperature.
3
Acknowledgement
The authors would like to acknowledge GM Global Research &
Development (RD-11-368 GB945-NV-HY) for the financial
support of this project. This work was also supported by the
National High Technology Research and Development Program
of China (863 Program, 2011AA03A405).
70
Notes and references
a
Key Laboratory for Green Chemical Technology of State Education
Ministry, School of Chemical Engineering & Technology, Tianjin
University, Tianjin 300072, China. Tel./ Fax.: +86 22 27892301; E-mail:
mqshen@tju.edu.cn
State Key Laboratory of Engines, Tianjin University, Tianjin 300072,
China
2
0
75
Fig. 9 the NH
high temperature. The conditions were: the inlets consisted of 500 ppm
NH , 5% O with N as the balance. The kinetic steady-state
measurements were obtained from 410 C to 490 C.
3
Oxidation kinetic results over Cu/SAPO-34 catalysts at
b
3
2
2
o
o
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DOI: 10.1039/C3CY00453H
Catalysis Science & Technology
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c
General Motors Global Research and Development, Chemical Sciences
and Materials Systems Lab, 3500 Mound Road, Warren, Ml 48090, USA
Electronic Supplementary Information (ESI) available: [Further details
are given in Figure S1-S7 and Table S1.]. See DOI: 10.1039/b000000x/
†
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