Spotlight on Large Surface Copper Cluster Role of Cu-SAPO-34 Catalyst in Standard NH3-SCR Performances

Article abstract of DOI:10.1002/cctc.201902036

In the present study, the catalytic role and behavior of different copper cluster configurations during NH₃ SCR, NO and NH₃ oxidation reactions will be studied in detail. For this purpose, two preparation methods were performed. The physico-chemical and surface properties were characterized by different techniques, such as XRD, ICP, N₂ ads/des, HR-TEM, XPS, NMR, ex-situ/in-situ DRIFT, NH₃ TPD and H₂ TPR, in order to evaluate the impact of copper incorporation method on redox active sites, and, as a consequence, to better understand the mechanism of the NH₃ SCR reaction.

Full text of DOI:10.1002/cctc.201902036

Accepted Article
Title: Spotlight on large surface copper cluster role of Cu-SAPO-34
catalyst in Standard NH3-SCR performances
Authors: Guillaume Pétaud, François Gaillard, Melaz Tayakout, Sonia
Gil, and Anne Giroir-Fendler
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To be cited as: ChemCatChem 10.1002/cctc.201902036
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ChemCatChem
10.1002/cctc.201902036
FULL PAPER - ACCEPTED ARTICLE
Spotlight on large surface copper cluster role of Cu-SAPO-34
catalyst in Standard NH
3
-SCR performances
b
b
c
a
a
Guillaume Pétaud , François Gaillard , Melaz Tayakout , Sonia Gil* , Anne Giroir-Fendler*
Abstract: In the present study, the catalytic role and behavior of
different copper cluster configurations during NH SCR, NO and NH
cages sized of 8, 6 or 4-membered ring openings (8MR, 6MR,
4MR) with a theoretical 3.8 Å aperture for the widest [6]. Following
different substitution mechanisms (SM2 and SM3), the
silicoaluminophosphate (SAPO-34) zeolite is formed during sol-
gel hydrothermal synthesis by the substitution of one P if SM2,
and one Al atoms too if SM3, within the ALPO-34 framework by
Si atoms [7a,b]. Then, framework bridged Si-OH-Al are typically
generated on the synthesized zeolite material, giving rise to the
formation of Brønsted acid sites and compensate for the negative
charge brought by Si atoms. These mentioned sites will then acts
3
3
oxidation reactions will be studied in detail. For this purpose, two
preparation methods were performed. The physico-chemical and
surface properties were characterized by different techniques, such
as XRD, ICP, N
NH TPD and H
incorporation method on redox active sites, and, as a consequence,
2
ads/des, HR-TEM, XPS, NMR, ex-situ/in-situ DRIFT,
3
2
TPR, in order to evaluate the impact of copper
3
to better understand the mechanism of the NH SCR reaction.
3
as NH storage sites, which is a key property during SCR process,
supplying NH
3
to the redox active centers during standard SCR,
Introduction
as have been shown by in-situ DRIFT studies [8]. Moreover, the
Lewis acid sites formed by copper species in Cu-SAPO-34
catalyst are considered as the most efficient when they are
located within the lattice framework in ionic exchanged positions
near 6MR openings [9a,b,c,d]. Usually, the most conventional
method for the incorporation of cations inside CHA structure
zeolites is the aqueous ionic exchange method, compared to
other processes as impregnation or solid state ionic exchange
By the combustion of fossil fuels, the produced nitrogen oxides
NO ) such as nitric oxide (NO), nitrogen dioxide (NO ) and nitrous
oxide (N O) represent one of the main atmospheric pollutants that
the transportation and industrial economies must face from
several years now, in order to respect the current and future
regulations [1a,b]. NOx are responsible for greenhouse effect,
(
x
2
2
2
where N O represent a Global Warming Potential (GWP) of 298
(
SSIE). Still, most of Cu-SAPO-34 catalysts contain
a
times higher than the one of CO
2
[2], ozone (O
3
) production at
2+
combination of all the suggested species as isolated Cu and
Cu cations, but also [Cu-O-Cu] dimers, [Cu-OH] species and
other CuO
tropospheric levels and acid rain [ . Current and future regulations
for gas emissions spotlight the required energy consumption and
pollution emission turmoil, which calls for more and more efficient
solutions. In that way, lean-burn exhaust after treatment
technologies, one of the most studied and used system, provides
high NOx conversion efficiency as the years goes by and
innovation flourished [4]. For NOx abatement in particular, the
3]
+
2+
+
[10a,b,c]
x
clusters
the different proportions of which will
greatly depend on the selected preparation method. Moreover,
the wide literature reporting the nature of these copper active sites
during NH
is still under debates
3
SCR and the different occurring mechanisms on them
[5c,11a,b,c,d]
. For more convenience, the
following Table 1 listed the different active sites mentioned during
this study which can be part of Cu-SAPO-34 catalysts.
selective catalytic reduction using ammonia (NH
3
-SCR) is one of
most effective and economic technologies on vehicles [ .For NH
4]
3
-
Despite all, the usual metal incorporation techniques as
impregnation, one pot synthesis or ionic exchange protocol,
usually place copper species in both positions, on the surface in
oxide cluster configuration and/or inside the zeolite lattice. Thus,
surface copper species can easily represent a non-negligible
fraction of the total metal amount. Despite this, their roles played
in SCR reactions are less understood than the Cu2+ exchanged
sites and remain controversial [12a,b]. It is generally accepted that
CuO surface clusters combined with exchanged cations are more
SCR reaction, the copper- and iron-exchanged zeolites with a
chabazite (CHA) structure, which feature small pore sizes, are
one of the most used catalysts, due to their high SCR activities,
[5a,b,c]
N
2
selectivity and hydrothermal resistance
. The molecular
sieve SAPO-34, the catalyst support, is constituted of different
[a]
Prof. A. Giroir-Fendler, Dr. S. Gil,
Université de Lyon, Université Claude Bernard Lyon 1
IRCELYON, CNRS, UMR 5256
active for catalyzing nonselective NH
reactions, which negatively affect the N
reduction [13a,b]. In order to optimize the SCR performance and
stability of Cu-SAPO-34 catalysts for NH SCR, it is of great
3
oxidation than for SCR
2
selectivity during NOx
2
avenue Albert Einstein, Villeurbanne F-69622, France
E-mail: anne.giroir-fendler@ircelyon.univ-lyon1.fr
sonia.gil@ircelyon.univ-lyon1.fr
3
[
b]
c]
Dr. G. Pétaud, Dr. F. Gaillard
Université de Lyon, Université Claude Bernard Lyon 1
IRCELYON, CNRS, UMR 5256
interest to control the oxidation state and structure of those
widespread active sites. Therefore during our previous research
[14],
3 3
the study of NH SCR and NH oxidation behaviors of a
2
avenue Albert Einstein, Villeurbanne F-69622, France
[
Prof. M. Tayakout
Université de Lyon, Université Claude Bernard Lyon 1
LAGEPP, CNRS, UMR 5007
catalyst obtained through specific one pot synthesis route was
initiated. By controlling the synthesis method, two distinct
catalysts presenting distinct redox site configurations were
confronted for preliminary active sites investigations, where
catalytic performances obtained thanks to the one pot catalyst
43 Bd du 11 Novembre 1918, Villeurbanne F-69622, France
Supporting information for this article is given via a link at the end of
the document
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were compared with a conventional catalyst, into which copper
was incorporated by impregnation method.
sites following different catalytic performance tests were
conducted in order to validate the observations and hypothesis
made on both catalysts and further understand their particularity.
Table 1. Active sites denomination possibly encountered within Cu-SAPO-
3
Physico-chemical and redox catalysts characterizations.
4 catalysts with corresponding insert. [
7a,10a,b,c,15]
Species
example
Relative
position
External
(support)
(S1)
To begin with, chemical and textural properties by ICP and
N adsorption/desorption were measured. Table 2 shows the
2
chemical compositions of synthesized catalysts, which evidenced
the close elemental composition between the SAPO-34 supports
HT and IE catalysts) with similar copper content (around
.5%wt.) and close (Si+P)/Al molar ratio (~0.95). Moreover, the
final Si/Al molar ratio Table 2 is also in good accordance with the
theoretical one where a ratio of 0.15 was targeted. These results
also suggest, regarding the close support chemical composition
between the H-SAPO-34 support and the IE catalyst, that the
incorporation method by ionic exchange had no important impact
on the elemental composition of final catalyst and keep the initial
composition of the support. This observation could be
corroborated with the theoretical and experimental (measured by
OES-ICP) compositions. Indeed, by considering a dehydrated
Denomination
Weak[a]
Brønsted acid
sites
T-OH (T = Si,
Al or P)
Internal
(
1
Strong[a]
Brønsted acid
sites
(support)
(S2 and
S2a for the
strongest)
T-OH-T (ex :
Si-OH-Al)
Nonframework
Al / Copper
species
Weak[a] Lewis
acid sites
External
(S7) / (S8)
Strong Lewis
acid sites:
Exchanged
Internal
(6MR /
8MR)
ZCu2+, ZCu+
copper cations
(S3/S4)
External
Nano/Micro
copper oxide
surface cluster
(
support
surface)
S5/S6)
x
ZCuO (x≤1)
SAPO-34 unit cell formed with TEAOH as templating agent, three
charges would be created by hexagonal unit cell [7a,17], which will
72 (36T, 72O) [18]
give rise to a theoretical unit cell of H Si Al18P O .
3 3 15
(
This theoretical composition was very close to the estimated
compositions obtained through OES ICP results Table 2 with
[
3
a] from the point of view of NH adsorption strength through investigated
[
16a,16b,16c,16d]
temperature
3 15.1 3 15,5 3 14,8 72
H Si2,6Al18.3P O72, H Si2,3Al18,2P O72 and H Si2,7Al18,5P O
In the present study, the catalytic role and behavior of
different copper cluster configurations during NH SCR, NO and
NH oxidation reactions will be studied in detail. For this purpose,
two preparation methods (one-pot hydrothermal and ionic
exchanged) were performed. The physico-chemical and surface
properties were characterized by different techniques, such as
corresponding to raw H-SAPO-34 support, IE and HT catalysts,
respectively. Moreover, if the catalysts and support SSA assigned
in Table 2 are typical values encountered for chabazite zeolites
3
3
[13b,19],
similar microporous volume was also observed between
raw support and catalysts, which could mean that the microporous
framework is not modified after copper incorporation for both
methods.
XRD, ICP, N
DRIFT (NH , CO), NH
impact of copper incorporation method on redox active sites, and,
as consequence, to better understand the mechanism
responsible of the obtained NH SCR performances. In that
2
ads/des, HR-TEM, XPS, NMR, ex-situ/in-situ
3
3
TPD and H TPR, in order to evaluate the
2
Table 2. Elemental composition by OES ICP of the Cu-SAPO-34 –IE, HT
catalysts and H-SAPO-34 with adsorption/desorption nitrogen analysis.
a
3
ICP data (wt.%)[a]
respect, and regarding the difficulties to selectively incorporate
only exchanged active phases, the study seeks to further improve
the role of surface species in DeNOx process and their control.
Micro-
SSA[b]
porous
Si/Al
Sample
(Si+P)/Al
m².g-1)
volume [b]
(
Cu Si Al
P
(mol.
ratio)
(
mol. ratio)
3
-1
(
cm .g )
Results and Discussion
H-SAPO-34
0
2.5 17 16 0.14
0.96
0.96
0.94
573
577
590
0.20
0.20
0.17
Cu-SAPO 34-IE 1.5 2.7 21 20 0.13
Cu-SAPO 34-HT 1.2 3.0 20 18 0.15
In order to fully understand the impact of the different
preparation methods on the active site configurations and explain
the different catalytic behaviors, four approaches were followed.
The first one consists in characterizing both support and catalysts
physico-chemical properties and surface morphology of copper
sites. Then, the Brønsted acid sites environment of catalysts were
studied to further considerations of the Brønsted and Lewis sites
interactions. The third approach consisted in analyzing different
probe molecules interactions with the catalysts active sites,
followed by TPD and DRIFT experiments, in order to characterize
the active sites behaviors as a function of the temperature with
[
[
a]
Copper amount determined by ICP measurements,
b]
Specific surface area (SSA) and total microporous volume determined by
nitrogen adsorption/desorption experiments
Moreover, structure and phase identification by XRD and
HR-TEM were conducted on both catalysts. Figure 1 depicts the
XRD patterns of H-SAPO-34 zeolite, Cu-SAPO-34 –IE and Cu-
SAPO-34-HT catalysts. All support and catalysts presented
exhibited XRD characteristic peaks at 2θ = 9.5, 12.9, 16.3, 17.7,
21.4, 24.9, 25.9, 30.6 and 31° in good agreement with the patterns
of SAPO-34 CHA-based zeolite reported in literature (PDF-00-
3
the NH key specie. Finally, specific investigations on redox active
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0
47-0429) [20a,b]. These results show that the chabazite zeolite
diameter (SMD) of 3.5 nm, which was estimated by counting
around 300 particles on the TEM images. The corresponding
particles size distribution of surface copper cluster for Cu-SAPO-
34-IE catalyst is represented in Figure 2 (c). Additional images
are available in Supplementary Information Figure S3 with
different scale for clarity purposes.
structure was maintained after the copper introduction for both
preparation methods. XRD diffractograms can also provide
information about the presence of copper clusters at the surface,
if they are large enough to be detected by XRD, regarding the
characteristic peaks of CuO phase centred at 2θ = 35,29° and
3
8,49° (PDF-04-007-0518) [18]. As expected, these signature
Figure 2. HR-TEM micrographs of Cu-SAPO-34-IE (a) and –HT (b) catalysts
with corresponding particles size distribution of the observed Cu-SAPO-34-IE
peaks associated to CuO phase were observed with the Cu-
SAPO-34-HT catalyst, which indicates the presence of relatively
large surface clusters and homogenously distributed regarding
the corresponding low peak intensities. By comparison, no signal
from the CuO phase have been detected for the ionically
exchanged catalyst, informing on the absence or at least the
presence of small sized and well dispersed surface copper
a)
clusters [21]
.
b)
Figure 1. XRD patterns of H-SAPO-34, Cu-SAPO 34-IE and -HT catalysts with
the corresponding zoom at 2θ = 35° and 40° (CuO phase identification area).
Additionally, the high resolution TEM images provide
information about the surface composition, metal dispersion and
particle size distribution of copper clusters Figure 2. Here, the
impact of the metal incorporation method was clearly visible.
According to micrograph displayed Figure 2, the catalyst
synthesized by one-pot hydrothermal method, Cu-SAPO-34-HT,
presented large copper oxide clusters (Cu O) as shown by the
2
EDX point-scanning measurements (within white dotted square in
Cu-SAPO-34-HT Figure 2d), with an average particles size about
≈
1μm. Moreover, the support surface of Cu-SAPO-34-HT did not
display particular presence of copper through EDX scans (not
shown here) observation (inside white dotted circle in Cu-SAPO-
34-HT Figure 2b), supporting the main copper configuration in
large cluster form. On the contrary, well dispersed nanosized
copper clusters were observed for the catalyst synthesized by
ionic exchange method, as visible in Figure 2a. Considering this
figure, some copper clusters, with a lattice fringe of 0.23 and 0.21
nm, were identified in Cu-SAPO-34-IE catalyst Figure 2a by their
characteristics lattice interplanar d-space values, corresponding
to the (200) crystal plane of CuO and (111) crystal plane of cuprite
clusters (c) and EDX scan of the Cu-SAPO-34-HT micro cluster (d).
Furthermore, surface site characterisations performed by
ex-situ DRIFTS are presented in Figure 3. This technique was
complementary to 28Si, 27Al and 31P MAS NMR analysis, where
the fine configuration of the Cu-SAPO-34-HT catalyst building
O, respectively [22a,b,c]. This catalyst presented a Sauter mean
Cu
2
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Vibrations
units was compared with those of the support and Cu-SAPO-34-
IE (Figure S4, Supporting Information). Ex-situ DRIFTS
measurements were carried out to investigate the presence of
copper cations in exchanged position but also to compare the
surface Brønsted acid sites presented on both catalysts. Surface
extra framework Si-OH, P-OH and intracristalline bridged Al-OH-
Molecule
-1
Compound
[Ref]
(cm )
3730
External Si-OH
External P-OH
3671
[20a,b]
3625,3600
Bridged Si-(OH)-Al
3
333
283
3
NH molecules
[20a,b,18]
3
NH +
4
adsorbed species
-1
Si were observed at 3730, 3671 and 3625-3600 cm , respectively
+
[
23]
NH3
3182
1620
1460
Cu -NH
3
[27]
[28]
.
Similar profiles were obtained for both support and catalysts
-1
NH
3
on Lewis acid sites
in the region of 4000-3000 cm , which corresponds to the
Brønsted vibration sites region [24], suggesting similar surface site
v
as. NH +
4
on Brønsted acid sites
[20a,27]
configurations between the different samples.
v
as. framework vibration (type1 and
type 2) perturbed by copper
cations
[
11a,22a,29a
891, 844
,
b]
Mono carbonyl complex [Cu(CO)]+
2154
near 8MR
Mono carbonyl complex [Cu(CO)]+
near 6MR
[30a,b,c]
CO
2
136
150
2
Multi carbonyl complex [Cu(CO)
2
]+
After surface investigation, the catalysts redox properties
were characterized through XPS and H -TPR measurements.
2
XPS measurements of Cu-SAPO 34-IE and -HT catalysts were
carried out in order to evaluate the chemical state of copper
species on both catalysts and corresponding spectra are
presented in Figure 4. The binding energy of the Cu 2p transition
peak will provide information about the oxidation state of Cu
species at the catalyst surface. For both catalysts, the Cu 2p3/2
spectra in Figure 4 displays two peaks around binding energies
of 934 eV and 932 eV and shake-up satellites signal at 942 eV
and 962 eV [31]. The binding energies at 934.8 eV and 934.6 eV
for Cu-SAPO-34-IE and Cu-SAPO-34-HT catalysts, respectively,
were attributed to the Cu2+ species [32]. This assumption was
confirmed by the corresponding shake-up satellites peaks, which
are clearly associated to Cu2+ species in the literature [33]. In
addition, the binding energies at 932.8 eV and 931.7 eV for Cu-
SAPO-34-IE and Cu-SAPO-34-HT catalysts, respectively, could
+
0
be attributed to the Cu and Cu . However, for catalysts containing
low amounts of copper, this assumption is still controversial in the
literature, where the differentiation between Cu and Cu° is limited
Figure 3. Ex-Situ DRIFT profiles under He at room temperature for H-SAPO 34
support (a), Cu-SAPO-34-IE (b) and -HT catalysts (c) after oxidative
pretreatment. With an insert corresponding to the zoomed area of Cu-SAPO-
+
by the collection of Auger electron peaks at lower binding energy
-
1
[34a,b]
34-IE profile between 1000 and 800cm .
(916 eV)
. Indeed, some authors tried to identify the CuO
species through the satellite peaks in the region of 940-948 eV
without taking into consideration the Cu 2p3/2 asymmetric profile
information and so, the possible presence of different oxidation
All the different stretching vibrations are referenced in Table
for more convenience. Moreover, the vibrations at 891 and 844
cm assigned to copper cations interfering with T–O–T structure
state [
12b,35a,b]
. On the contrary, others authors
[28,36]
consider the
3
-1
deconvolution of Cu 2p3/2 spectra in three main contributions
centered at 935.8, 934.6 and 932.6 eV, which are assigned to
(
as Si–O–Si or Si–Al–Si) for type 1 in the former and type 2
perturbation in the latter [
13a,25a,b]
were exclusively observed for
Cu2+ species coordinated to framework oxygen atoms, CuO
+
Cu-SAPO-34-IE catalyst Figure 3b. As expected, H-SAPO-34
support Figure 3a. and Cu-SAPO-34-HT catalyst Figure 3c. did
not exhibit these vibrations. These results suggest that only in the
case of Cu-SAPO-34-IE catalyst synthesized by ionic exchange
method, the copper cations are clearly localised in exchanged
species and Cu(I) species as isolated Cu ions and Cu
2
O,
respectively [37]. For this raison, the peaks centred at 932.8 eV or
931.7 eV were here denoted as Cuδ+ species, corresponding to
copper species surrounded by surface oxides without net
distinction between Cu0 and Cu species. Thus, by Cu 2p3/2
+
positions [8,26a,b]
.
2+
spectra deconvolution, the estimated proportions of Cu species
presented on Cu-SAPO-34-IE catalyst (~30%) was slightly higher
than on Cu-SAPO-34-HT catalyst (~ 20%). This difference is
directly observable notifying the Cu2 peak shape difference,
Table 3. Referenced stretching vibrations of exploited probe molecules (NH
and CO)
3
+
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where only a small shoulder appears at 934.8 eV on the profile
Figure 4, while a distinct profile is observable for Cu-SAPO-34-IE.
However, the Cuδ+ species share (~80%) on both Cu-SAPO-34
catalysts remains higher than the (II) copper oxidation state, a
proportion potentially over-estimated as the analysis condition
could favour the reduction of Cu(II) specie in Cu(I). Compared
with the principal CuO phases identified through Cu-SAPO-34-HT
by XRD measurements, copper oxides phases with Cu(I)
oxidation state were not particularly identified, highlighting their
better distribution on the catalyst surface as it was characterized
with Cu-SAPO-34-IE nano-clusters.
copper configuration, which could be attributed to the single step
reduction in Cu° of the large surface CuO clusters previously
x
observed by HR-TEM [18,38c]. This sharp peak located near the
reduction region of the nanosized copper cluster CuO but shifted
towards lower temperature, further confirms the presence of CuO
clusters on both catalysts but in different configurations.
These results lead to conclude that the preparation methods had
an important influence on redox properties of catalysts. Indeed,
species in Cu (II) valence state, in Cu-SAPO-34-IE catalyst,
presented easily reducible nanosized copper cluster at its surface,
x
and on the contrary, only micro CuO species were presented on
Therefore, these XPS results confirmed that the Cu2+ and
Cu+ species were present on both catalysts, which is in
agreement with the previous XRD and TEM results.
Cu-SAPO-34-HT catalyst surface, without any main detection of
isolated Cu2+ cations. Knowing that, the reduction capability on
the whole temperature range (200°C to 500°C) for Cu-SAPO-34-
IE, and more specifically the temperature reduction centred at
280°C of Cu-SAPO-34-HT redox sites, will consequently impact
the catalytic activity of both catalysts, which will be demonstrated
with the following tests performance.
Figure 4. XPS spectra Cu2p3/2 of Cu-SAPO-34-IE and -HT catalysts.
2
H -TPR measurements shown in Figure 5 (from room
temperature to 500°C) were performed to characterize the
reducibility of catalysts, which will allow to compare the impact of
preparation method in redox properties. The peaks attribution
remains under debates because of the different copper species
coexistence, which make peaks deconvolution really delicate
For this reasons, TPR profiles were interpreted on a
qualitative basis. While the Cu-SAPO-34-HT catalyst showed a
single reduction state with a quite symmetrical peak between
Figure 5. H
2
-TPR results of Cu-SAPO-34-IE and –HT catalysts. Reduction feed:
-
1
2
vol.% H in Ar, total flow of 40 ml.min , heating ramp from 30 °C to 500 °C
2
with a heating up rate of 10 °C/min.
[38a,b,c].
3
Surface active sites characterization by NH -TPD and in-situ
3
DRIFTS spectroscopy using NH and CO probe molecules.
2
2
50°C and 340°C well synchronized with a H O desorption peak,
Temperature-programmed desorption of ammonia was
performed to determine the NH3 affinity with H-SAPO-34 support,
Cu-SAPO-34-IE and -HT active sites. During adsorption steps at
the Cu-SAPO-34-IE catalyst presented a broad peak from 200°C
to 500°C, demonstrating the presence of different copper site
configurations [38c,39]. For Cu-SAPO-34-IE catalyst, the broad
shoulder peak defined between 160°C and 250°C is delimited in
one first consumption zone, which could be attributed to reduction
of the isolated Cu as Cu + ½ H
the following reduction zone between 250°C and 340°C, which
represents an important H consumption centered at 315°C, could
be related to the single step reduction of the Cu-SAPO-34-IE
1
50°C, the gas feed was composed of 1000 ppm NH
3
/He, 8% O
2
and H O
2
and 4% H O in order to better represent the NH
2
3
competitive adsorption on the catalyst surface and the
corresponding hydration copper sites [26a,41]. The corresponding
2
+
2+
+
+ [21,38b]
2
→ Cu + H
. Otherwise,
NH
HT and -IE catalysts are represented below, Figure 6. All the
support and catalysts presented similar NH desorption profiles
3
desorption profiles from H-SAPO-34 support, Cu-SAPO-34-
2
3
2 2
dispersed nanosized copper oxides (CuO + H → Cu° + H O)
presented the catalyst surface [40a]. Finally, at higher temperature,
one last reduction step associated to the reduction of native Cu
with two main desorption regions before 400°C (denoted as low
temperature (LT) and medium temperature (MT)). A third region
at higher temperature above 400°C was also detected for the
ionically exchanged catalyst, spotted as high temperature peak
+
2
+
0
and the previously reduced Cu to Cu could be considered ( <
00°C, not shown) as Cu+ + ½ H
→ Cu°+ H+
[40a,b,c]
. By
5
2
(
HT). The corresponding acid sites assignations in temperature
comparison, Cu-SAPO-34-HT catalyst showed a sharp centered
peak around 280 °C, demonstrating the presence of one main
are assigned in Table 4 below with S1, S7 assigned to weak
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Brønsted acid site surface hydroxyl on SAPO-34 as T-OH (with T
be enlighten, where the support H-SAPO-34 and the
hydrothermal catalyst display close desorption curve shapes.
=
Si, Al and P) and weak copper Lewis acid sites, for the LT signal
S2, S8 representing bridged Brønsted sites and moderately
[
18,26a].
[
21]
strong copper sites, for the MT peak
and S2a, S3, S4
Regarding the main sites difference between both samples,
where large copper oxide clusters are specifically present on Cu-
SAPO-34-HT, it could be firstly assumed that these large clusters
had substituted a non-negligible part of MT sites from the support.
In other words, the presence of large copper oxides clusters on
Cu-SAPO-34-HT catalyst could inhibit a small fraction of the
support Brønsted acid sites by substitution of Lewis sites of similar
strength. However, even with this obstruction, the desorption
ability of Cu-SAPO-34-HT catalyst remains close to the support
corresponding to the strongest Brønsted acid sites and the
strongest exchanged copper cations (HT) [24,42]. This description
is further assigned Table 1 for more convenience.
3 3
Table 4. Desorbed NH quantities according to NH -TPD measurements of H-
SAPO-34 support, Cu-SAPO-34-HT and Cu-SAPO-34–IE catalysts with related
median peak temperature[a]
H-SAPO-34
Cu-SAPO-34-HT
Cu-SAPO-34-IE
LT MT HT
Peak categoryc
3
capacity, according to the total amount of desorbed NH displayed
LT
MT
HT
LT
MT
HT
Desorbed NH
3
Table 4, a similar desorption trend in temperature, with a
maximum desorption intensity centered at 349°C. This important
MT peak, consequence of the bridged Brønsted acid sites and
moderately strong Lewis acid sites desorption, represents the
principal proportion of ammonia storage capacity of Cu-SAPO-34-
HT catalyst and support. Compared to H-SAPO-34 support, these
TPD results showed that the exchanged catalyst presented a
mmol g-1
(
0.162 0.398
-
0.113 0.317
0.430
-
0.210 0.276 0.09
catalyst)
Total desorbed
NH
(mmol g-1
3
0.560
0.576
catalyst)
Temperature
3
higher capacity to desorb NH at low temperature, with a shifted
263 349
-
267
349
-
255
323
421
(°C)
LT and MT peak at lower temperature. The overall stronger
desorption at low temperature of Cu-SAPO-34-IE catalyst seems
to be due to the weak copper and Si, Al or P Lewis acid sites,
which are a consequence of the copper incorporation method and
[
a]
The LT, MT and HT peaks respectively correspond to the weak, medium
and strong acid sites, and are estimated from the corresponding area of the
deconvoluted peaks.
the subsequent formation of weak Brønsted sites (as P-OH) [43a,b]
.
These peaks were observed and quantified by
deconvolution with Gaussian from the different profiles reported
On the other hand at elevated temperature, Cu-SAPO-34-IE
Figure 6. Regarding the insert Figure 6, the profile differences can
catalyst particularly presented a HT peak, not observed in the
support desorption profile.
3
Figure 6. NH -TPD profiles with the corresponding deconvoluted peaks of H-SAPO-34 support, Cu-SAPO-34-HT and –IE catalysts with an insert at upper left corner
representing the 3 corresponding TPD profiles stacked. Inlet gas composition: adsorption of 1000 ppm NH ; 8% O ; 4% H O and He as a balance at 150°C with a
3 2 2
total Flow of 40 mL/min.
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Indeed, for the exchanged catalyst around 420°C, ammonia
desorbed from the strongest acid sites of the catalyst, which could
be attributed to the presence of strong Brønsted acid sites as T-
Si also showed a fast NH
3
adsorption on these sites. Moreover,
these results demonstrated that Cu-SAPO-34-HT exhibited only
few ammonia interactions with the Lewis acid sites corresponding
to the transition metal during the adsorption process. Indeed, the
OH-T but also and mainly to the Cu2+ within the support [21,41]
.
These remarkable differences in profile shape and desorption
regions between both Cu-SAPO-34-HT and Cu-SAPO-34-IE
directly demonstrated that the preparation method had an
important impact on the acid sites nature and their relative
proportion for ammonia storage capacity. Particularly, ammonia
desorbed from Cu-SAPO-34-HT strong sites around 350°C could
NH
3
adsorption seems quite low on these sites, according to the
adsorbed on
slight detected band at 1620 cm-1 assigned to NH
3
Lewis acid sites. This last conclusion is also in accordance with
the lack of signal at 891 cm-1 and 844 cm , where no
manifestation of stretching vibration from possible interaction
between Cu2+ active sites, interfering in exchanged position, and
-1
desorbed from bulk to the reducible surface copper species (H
2
-
3
adsorbed NH were detected. These results are in agreement with
TPR results), and consequently improve the redox capability. For
Cu-SAPO-34-IE, the desorption profile is more complex to
the previous Ex-situ DRIFTS measurements, Figure 3, in which
the absence of Cu2 ions in exchanged position was probed.
+
correlate with the performances knowing that the stored NH
3
in
3
According to Cu-SAPO-34-IE, the NH adsorption behavior was
the support will more easily interact with the exchanged redox
centers due to their proximity within the zeolite structure. But it
can be specified that for the exchanged catalyst, the important LT
storage capacity will strongly impact the performances at this
quite similar to the hydrothermal catalyst profiles Figure 7 (G),
however, the adsorption on Brønsted acid sites was slower within
the first minutes from 5 to 10 min, according to the bands
evolution in the 4000 – 3000 cm-1 region. In the other hand,
temperature [18,27]. In addition, the NH
were carried out to identify the acidic surface sites on both
catalysts and characterize their affinity with NH in temperature
Thus, the ammonia was firstly adsorbed on catalyst surface and
the DRIFTS spectra were collected during 30 min of NH
exposure, Figure 7 (A and G). Then, the NH desorption profiles
were collected from 35 °C to 500 °C, Figure 7 (B-F and H-L). Note
that the catalyst surface was purged in He at room temperature in
order to remove the weakest physisorbed molecules.
-DRIFTS measurements
3
3
copper site interactions with NH were more visible, where the
bands at 3182 cm-1 and 1620 cm-1 attributed to NH
adsorbed on
3
+
3
.
Cu sites and on Lewis acid sites, respectively were also
3
observed. In that way, the NH adsorption on exchanged copper
cations interfering with the T-O-T structure was faster and the
saturation was reached within the first minutes, according to the
3
3
bands at 891 cm-1 and 844 cm
-1
[26b]
.After NH
a temperature-programmed desorption was performed in order to
evaluate the NH adsorption strength and acidic surface sites
3
adsorption at 35°C,
3
First, two regions can be distinguished, in the range of 3700 and
presented on both catalysts. The DRIFTS spectra collected at
different desorption temperatures, from 35 °C to 500 °C, are
presented in Figure 7 (B-F) and (H-L) for Cu-SAPO-34-HT and
Cu-SAPO-34-IE, respectively. All the N-H and the O-H stretching
mode vibrations remain appreciable for both catalysts at 100°C.
In respect to the DRIFTS spectra associated to Cu-SAPO-34-HT,
Figure 7 (B-F), the overall intensities of N-H and O-H stretching
vibration from 4000 cm-1 to 3000 cm-1 decreased with the
temperature increase from 35°C to 300°C. Note that the first
Brønsted acid sites to desorb were the external Si-OH, following
-1
-1
3500 cm and between 3400 to 3000 cm , which are assigned
to the O-H and the N-H stretching mode vibrations, respectively
[
8]
where the latter represent the occupation of Brønsted acid sites
by NH3 [29b]. In the region of O-H stretching vibrations, the bands
-1
-1
-1
at 3730 cm , 3671 cm and 3625-3600 cm are assigned to the
NH adsorption onto extraframework Si-OH, P-OH and bridged
3
Al-OH-Si respectively, as previously observed during Ex-situ
experiments (Figure 3).
In addition, in the region of N-H stretching vibrations, the bands
at 3333 cm-1 and 3283 cm are accordingly associated to the NH
-1
the vibration band at 3730 cm , which disappeared at 100°C. This
could indicate that only a small proportion of the total stored NH
3
-1
3
molecules and NH +
groups, the coordinated ammonia species on
Brønsted acid sites [23,24,29b]. A band at 3182 cm-1 in this region
could also be observed, attributed to the adsorption of NH
sites [44]. Moreover, a secondary region between 1700-700 cm
with four main bands at 1620, 1460, 891 and 844 cm-1 was also
observed, which can be respectively assigned to NH adsorbed
4
was desorbed from weak Brønsted acid sites at this temperature.
Nevertheless, the strongest desorption was observed between
300°C and 400°C and from the Brønsted groups, with an overall
decreasing of the bands corresponding to the O-H and N-H
+
3
on Cu
-1
-1
3
vibration bands within 4000 – 3000 cm . At higher temperature,
only a small amount of NH seems to desorb from the Brønsted
on Lewis acid sites, such as exchanged copper cations and
3
external Al 39], to NH +
[
ammonium species resulting from
acid sites, Figure 7 (E and F). This same desorption behavior from
100°C to 500°C for Cu-SAPO-34-HT catalyst was also observed
during NH -TPD measurements Figure 6, where a first desorption
3
ascribed to the weak Brønsted adsorption sites was found
between 150°C and 300°C with a centered peak at 267°C.
Moreover, the second and main desorption peak was observed at
higher temperature centred at 350°C, which is in agreement to the
4
ammonia adsorbed on Brønsted acid sites such as Si-OH, P-OH
and Al-OH-Si [24,44]. Finally, the last pair of stretching vibrations
could be attributed to the Lewis acid sites, and more particularly,
to the T-O-T internal framework vibrations perturbed by copper
ions with adsorbed NH [45].
3
Regarding Cu-SAPO-34-HT, the evolution of the external
Si-OH, P-OH and bulk Al-OH-Si Brønsted acid sites bands
showed a fast ammonia adsorption within the first minutes,
reaching a maximum of absorbance value after 5 min, according
to the bands intensity at 30 min saturation. A similar evolution
3
In-situ NH -DRIFT measurements at 300°C and 400°C, Figure 7
(D-E). For the referenced stretching vibrations, the profile
-1
-1
remains generally flat after 400°C from 4000 cm to 700 cm .
profile on the same period was found for the bands assigned to
NH +
and NH
+
groups. Moreover, in the 1700-700 cm-1 region,
4
3
the 1460 cm band assigned to NH +
-1
ammonium species
4
bounded to Brønsted acid sites such as Si-OH, P-OH and Al-OH-
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3 3
Figure 7. Cu-SAPO-34-HT (a) and Cu-SAPO-34-IE (g) DRIFTS spectra recovered during NH adsorption for 30 min to 1000 ppm NH /He, after 30 min He flush,
-
1
followed by desorption from room temperature to 500°C (b-f) and (h-l) under He. Total Flow= 40 mL.min .
Concerning Cu-SAPO-34-IE catalyst, the desorption from
external Brønsted acid sites did not occur in the same manner as
only a slight intensity decrease was observed, which was
associated to the desorption of molecularly adsorbed NH
3
and
-
+
Cu-SAPO-34-HT. The band relative to external Si-OH at 3730 cm
NH
4
between 100°C and 200°C. As observed through the DRIFT
1
was the first to vanish at 100°C, but at the same time, the
profiles (J-K), the most important Brønsted acid sites desorption
occurs between 300°C and 400°C, according to the strong
external P-OH band identified at 3671 cm-1 strongly decreased,
and was completely removed at 200°C. Moreover, the band
intensity relative to Al-OH-Si Brønsted acid sites was strongly
decreased between 200 °C and 300 °C, and completely
disappeared at 400 °C. This desorption trend corresponding to O-
H sites was also observed through the intensities of the band
intensity decrease of the bands relative to N-H vibration. The band
at 1460 cm-1 relative to NH
+
adsorbed on Brønsted acid sites
4
globally reflects this Brønsted desorption dynamic from 35°C to
400°C. Moreover, the NH
3
desorption trend from the copper sites,
+
-1
2+
such as Cu observed at 3182 cm , and the exchanged Cu
and NH +
sites at 3333 cm and 3283 cm , where
-1
-1
assigned to 891 cm and 844 cm , exhibited different desorption
-1
-1
ascribed to NH
3
4
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behaviors. The surface NH
3
– Cu+ species followed the same
zeolite framework. These two bands are associated to the Cu+
species near of 6MR positions and Cu+ cations near of 8MR,
respectively [25b,47]. Indeed, a chemical vibration shift between
35°C and 200°C was firstly observed, indicating the conversion of
Cu -(CO) to Cu -CO species, which is associated to the Cu
2
adsorbates interaction modification within the catalyst cavities.
trend comparing to that of O-H Brønsted acid sites bands between
3
4
5°C to 300°C with a strong desorption between 300°C and
00°C. At low temperature, the NH desorption from Lewis sites
3
+
+
+
was strong and almost complete at 300°C, according to the band
-1
centered at 1630 cm . The same evolution was observed at 891
cm-1 and 844 cm , where the ammonia adsorbed on exchanged
cations strongly desorbs between 35°C and 300°C. Finally, at
higher temperature between 400°C and 500°C, the desorption
was not completed on some Brønsted acid sites and exchanged
-1
-1
Moreover, during the desorption process, the band at 2154 cm
decreased while the band at 2136 cm-1 increased in temperature,
which could indicates the migration of some copper species in a
more confined position, such as isolated Cu into 6MR positions
+
2
+
Cu , according to the relative N-H stretching bands and the
vibrations due to perturbed T-O-T structure, Figure 7 (K-L). A
similar desorption profile was observed during NH
3
-TPD, Figure
6. Indeed, the first peak centered around 255 °C was observed
at lower temperature than HT catalyst (267°C), which could be
associated to the better participation of copper sites, such as
+
2+
surface Cu and exchanged Cu , and to the desorption from
weak external P-OH Brønsted sites [30a]. Moreover at higher
temperature, the results presented in Figure 7 (J-K) profiles
associated to Al-OH-Si relative bands, show an intense
desorption between 300°C and 400°C, coming from a large
desorption of ammonia on Brønsted acid sites. Finally, the last
desorption peak at high temperature around 420°C could be
associated to the desorption from the strongest exchanged
cations but also some remaining strong Brønsted acid site, Figure
7
(L) [30a]. This could indicate that the number of strong acid sites
was higher on this catalyst than on the Cu-SAPO-34-HT catalyst,
in agreement to the NH -TPD results at high temperature.
3
In addition, CO adsorption and desorption DRIFTS
+
measurements were carried out in order to discriminate the Cu
active species as a function of temperature for both catalysts.
2
+
Indeed, Cu ions are not able to form stable carbonyls during CO
adsorption due to their electrostatic interaction [30b]. Nevertheless,
Cu+ species strongly interact with CO to form mono or multi
+
]+, which are
carbonyl complexes such as [Cu(CO)] and [Cu(CO)
2
respectively assigned at 2136 cm- (site near 6MR cages), 2154
cm-1 (site near 8MR cages) for the former and 2150 cm-1 for the
latter [30c,46a,b]. The CO-DRIFT spectra, Figure 8, displayed the
desorption profile from 35 °C to 500 °C for (A) Cu-SAPO-34-IE
and (B) Cu-SAPO-34-HT from 2200 to 2100 cm-1 in the Cu-CO
stretching vibration range. This desorption process was
monitored right after the CO-adsorption and He purge step.
Concerning Cu-SAPO-34-IE catalyst, two major bands at 2150
1
[25b,46b,47].
Figure 8. DRIFTS spectra recovering the evolution of desorbed species after
previous CO-adsorption (1000 ppm CO/He) and He flush as a function of
temperature from 35°C to 500°C for Cu-SAPO-34-IE (a) and Cu-SAPO-34-HT
-1
-1
cm and 2175 cm are spotted following the CO-adsorption at
-
1
(
b) catalysts; Total Flow= 40 mL·min He.
3
5 °C, Figure 8 (A). The first one could be attributed to the CO
+
adsorption on Cu ions in a coordinated position with the O atoms
on the SAPO-34 framework (Cu -(CO)
the stretching vibrations associated to the gaseous CO
+
2
) and the second one, to
Therefore, Cu-SAPO-34-IE catalyst seems to beneficiate
[
46b]
. Note
+
the migration of Cu species into the 8MR exchanged positions
and, then into 6MR positions at high temperature (between 200°C
and 400°C). However, the reduction possibilities of Cu2+ cations
within the lattice cannot be completely ruled out under these
conditions and required careful precautions during interpretations.
Correspondingly, regarding the Cu-SAPO-34-HT catalyst,
that for both catalysts, the purging step under 40 ml/min He
seems to not be sufficient to completely vanish the gaseous CO
as visible at 2175 cm-1 at 35°C and some residual gaseous CO
are still detected within the DRIFT cell atmosphere for both
catalyst. The intensity of these two 2150 cm-1 and 2175 cm-1
bands decreased during the desorption process, which suggest
the gradual decrease of CO coverage and the removal of gaseous
CO. Next, two new bands at 2136 and 2154 cm- appeared at
+
Figure 8 (B), the isolated Cu species were identified only after
increasing the temperature to 200 °C, further confirming the
1
+
absence of initially present isolated Cu cations at the catalyst top
2
00 °C, which are associated to the formation of the above
surface. Next, the same two bands at 2136 cm-1 and 2154 cm
-1
+
mentioned carbonyl complex, and more specifically, to the Cu -
CO species coordinate to the O atoms on two different sites of the
were found during the heating process. These results confirm the
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similar capacity regarding Cu-SAPO-4-IE, to display a Cu
+
species migration to the support cavities (8MR and 6MR
positions) from 200 °C to 400°C [46b]. However, the low intensities
of these stretching vibration signals for –HT catalyst seems to
indicate a less pronounced Cu+ species migration capacity
comparing to the IE-catalyst. On the other hand, since the
exchanged Cu2+ cations presence was ruled out for Cu-SAPO-34-
HT, this observation adequately correspond to a specie migration
process. Such exchanged copper species are, indeed, NH3-SCR
active for NOx abatement and play a role in the SCR reaction
mechanism [11b,c]
.
Catalytic performances: NH
reactions.
3
3
-SCR, NO and NH oxidation
In Figure 9 are displayed the NH
3
-SCR performances in
standard condition (1) of Cu-SAPO-34 –IE and Cu-SAPO-34-HT
catalysts, with in insert, the corresponding production of by-
products. Regarding Cu-SAPO-34-IE, the catalyst presented a
stable DeNOx activity over the whole temperature range,
according to the previously characterized redox active sites
Figure 9. NH
reaction. Cu-SAPO-34-IE catalyst (full symbol), Cu-SAPO-34-HT catalyst
empty symbol) with in insert the corresponding NO and N O concentrations.
3 3
and NO conversion vs. temperature for standard NH -SCR
(
2
2
3
properties and the strong NH affinity in temperature between
Brønsted acid sites and copper species (Figure 7 (A-F)). These
Regarding to the specifically performed NH
oxidation reactions (2,3,4) in the temperature range of 200 and
25°C displayed in Figure 10 (A,C) and (B,D), respectively, the
NH and NO conversion of Cu-SAPO-34-HT do not exceed the
3
and NO
performances are in accordance with previous studies based on
4
similar exchanged catalysts [
28,39,42]
. On the other hand, Cu-
3
SAPO-34-HT catalyst behavior showed three main zones.
First of all, almost no activity is detected at 200°C, according
conversion of Cu-SAPO-34-IE catalyst for both oxidation
reactions. This observation can indicate that these side oxidation
reactions had a low impact in the NO conversion during the
standard SCR reaction of Cu-SAPO-34-HT catalyst and could
2
+
to the lack of active sites at this temperature, as exchanged Cu ,
where the catalytic reduction of NOx via NH start only at higher
3
temperatures. Next, the NO conversion increases quite
exponentially with the temperature, from 200°C to 425°C. Here
during this temperature increase, the important desorption around
have
performances, especially between 300°C and 400°C. In contrast,
NH oxidation (Figure 10 (A,C)) becomes prevalent at higher
temperature than 425°C for Cu-SAPO-34-HT, with simultaneous
production of N and NO, which could explain the clear NO
a
stronger impact on Cu-SAPO-34-IE catalytic
3
350°C, mainly from Brønsted acid sites characterized by TPD
(
Figure 6), is consistent with the modification of the reaction
2
regime within the same temperature window, Figure 9. The strong
desorption from the bulk support to the active surface cluster
conversion decreases during SCR as shown in Fig 9. Such
behavior seems to be exclusive to Cu-SAPO-34-HT catalyst.
However, the NO production for Cu-SAPO-34-IE catalyst started
at lower temperature (at 350°C) compared with Cu-SAPO-34-HT
(
Figure 6) could explain the better SCR performances reached for
this catalyst in this temperature range. Simultaneously, the
formation of additional active sites, by the migration of some
copper cations from the surface to exchanged position following
the similar temperature window (Figure 8), is in agreement with
the observed overall enhancement of the redox Cu-SAPO-34-HT
performance displayed between 200°C and 400°C Figure 8 (B).
Following these characteristics, at 425°C, Cu-SAPO-34-HT
catalyst displays a NO conversion even higher than the one of
Cu-SAPO-34-IE, demonstrating a higher SCR activity behavior
and the complementary effect of the different properties assigned
earlier. However, at higher temperatures (> 425°C), Cu-SAPO-
(
400°C). This NO production decreases after 425°C, where the
catalyst showed an increased N selectivity (Figure 10 (A)), which
could be related to the higher selectivity of NH oxidation to N2.
2
3
This catalytic behavior can also be associated to the
different redox copper species. Regarding the respective NO and
NO
2
conversion of both catalysts during NO oxidation (Figure 10
production was higher for the catalyst synthesized
(
B,D)), the NO
2
by ionic exchange than for Cu-SAPO-34-HT in the whole
temperature range. Typically, Cu-SAPO-34-IE seems to be more
2+
susceptible to form Cu-moieties (such as [Cu-O-Cu] species),
34-HT displayed a sharp decrease in activity for NO abatement
which combined with extra lattice oxygen (ELO), could be active
for NO reaction [3]. Therefore, the presence of copper species
initially in exchange positions, which were close enough to the
zeolite framework to employ such oxygen atoms, allows a high
activity. However, the large copper clusters on the surface of Cu-
SAPO-34-HT catalyst seems to be less active for this oxidation
reaction, regarding their extra lattice position [3,48a,b,c]. Thus, the
higher NO oxidation behavior of Cu-SAPO-34-IE could be
associated to the higher proportion of Cu cations active centers
close to support ELO. An observation also in accordance with the
comparing to Cu-SAPO-34-IE catalyst.
It is worth noting that the low generation of by-product NO
2
and N
range (insert Figure 9), demonstrates a good N
both catalysts. As it has been described in the literature, the
catalytic conversion of NO and NH in the 250 - 425 °C range is
not only due to the simultaneous consumption of NH and NO via
standard NH -SCR but also by NO and NH oxidation.
2
O, following Equation (4, 5) during the whole temperature
2
selectivity from
3
3
3
3
NH
3
oxidation behavior at low temperature of formed copper
dimer through monomers assembly [ . On the other hand, these
49]
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observations during NO oxidation reaction also suggested that
copper active centers of Cu-SAPO-34-HT catalyst were almost
not active for this reaction until 400°C.
Figure 10. NH
C) catalysts during NH
and - Cu-SAPO-34-HT (D) catalysts during NO oxidation reactions.
3
conversion
,
N
2
selectivity, N
2
O and NO
2
concentrations vs. temperature for Cu-SAPO-34-IE (A) and Cu-SAPO-34-HT
(
3
oxidation reactions. And NO conversion
,
N
2
O and NO concentrations vs. temperature for Cu-SAPO-34-IE (B)
2
3
NH in the reduction operation (1) better than in parasitic oxidation
3
Actually, regarding the two different NO and NH oxidation
(2-3).
behaviors between 200°C and 425°C, Figure 10 (B, D), the
characterized micro-sized clusters, which are the main redox
active centers presented on Cu-SAPO-34-HT, do not seem to be
implicated in both oxidation reactions as much as the redox
centers of Cu-SAPO-34-IE catalyst. Moreover, corresponding to
Cu-SAPO-34-HT, the parasitic oxidation of ammonia increase of
reaction regime at higher temperature than Cu-SAPO-34-IE
Therefore, the behavior of both catalysts could be explained
regarding their respective copper redox sites and their interaction
with ammonia in temperature. These results finally demonstrated
the important influence of preparation method and the active site
configurations on catalytic performances.
catalyst, Figure 10 (A,C). Which is in agreement with the difficulty Conclusions
to form ammonia oxidant copper dimers for this catalyst initially
deprive of exchanged species, compared with Cu-SAPO-34-IE.
At higher temperature, the NH oxidation will be performed by
surface copper clusters, which the production of NO can be
Knowing the increasing interest in more simple, fast and
effective catalyst synthesis, the different active sites configuration
implementation through different preparation procedure as the
one-pot method, required better understanding about the different
redox active species activities. By different catalyst preparation
routes, the role of several copper species and more precisely of
surface copper cluster were pointed out and further investigated
comparing two distinct catalysts with close chemical support
composition SAPO-34. By local observations through HR-TEM
instruments and phase identification with XRD, the presence of
3
related [
HT NH
13b,19]
and copper monomers . Regarding Cu-SAPO-34-
conversion and NO concentration profiles compared with
[49]
3
the exchanged catalyst, the oxidation process on micro sized
copper cluster is activated at higher temperature than on
nanosized copper clusters and monomers. This phenomenon will
give benefit to the NO reduction due to the direct application of
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different configuration of copper sites was revealed, displaying
micrometric surface cluster on Cu-SAPO-4-HT and nanosized
Therefore, for the support preparation, the templating agent is
primarily mixed with aluminium isopropoxide (Al[OCH(CH
3
)
2
]
3
CAS 555-
1-7) for 90 min, after that the proper amount of silica precursor (SiO CAS
12945-52-5) is stirred with the initial suspension during 30 min until
PO CAS 7664-38-2) is
3
1
2
2
CuO and Cu O clusters on the top surface of Cu-SAPO-34-IE
catalyst. The incorporation of copper cations was shown through
Ex-situ Drift analysis in a first time, where, through the different
synthesis, the surface Brønsted acid sites remain similar between
catalysts at room temperature. In addition, the Cu2+ exchanged
cations were not observed within the Cu-SAPO-34-HT catalyst
structure, indicating the particular copper configuration in surface
homogeneity. Next, phosphoric acid (85wt% H
3
4
poured with water in the mixture under stirring for 30 min. Finally, the
generated mixture is sealed in an autoclave with Teflon chamber and
placed in a furnace at 200°C for 96 hours. Once the hydrothermal
treatment is over, the autoclave is temperature quenched and the white
as-synthesized sample is wash-centrifuged with distilled water until pH 7
and finally dried at 100°C for 6 hours. The chabazite structure H-SAPO-34
is obtained after calcination of the dried powder in dynamic flow of
2
cluster form. This specificity was further confirmed with H -TPR
results also where only one sharp signal was observed,
corresponding to the reduction of CuO species to Cu°. In
comparison, Cu-SAPO-34-IE catalyst displayed a large signal
through the whole temperature range indicating about the
reduction of different copper species. Finally, probe molecules as
-1
synthetic air (Q = 4.5 L.h ) firstly with a 1°C/min ramp and a 2 hours step
at 200°C, followed with a 0.5°C ramp and a second step for 6 hours at
5
50°C (ATD-ATG experiment displaying template suppression available in
Supporting Information Figure S1) [51].
NH
where NH
3
and CO were employed for further active site investigations,
-TPD profiles were used for the acid sites strength
After calcination, the copper species are incorporated in H-SAPO-
34 by conventional liquid ion exchange (LIE) method [21]. Firstly, the zeolite
3
appreciation. Indeed, TPD measurements informed about the
similar storage capacity of both catalyst and support, and about
the strong desorption capacity of the Cu-SAPO-34-HT at medium
temperature, displaying a similar profile shape with the support H-
SAPO-34. These results were extensively corroborated with in-
situ DRIFT measurements, where CO probe molecule allowed the
demonstration of a copper active sites migration into exchanged
position for both catalysts. The combinations of these phenomena
enhance the SCR performances, as observed, of the one pot
catalyst, particularly between 200°C and 400°C. These different
conclusions were correlated with the SCR measured
is stirred in a 0,1 M ammonium nitrate solution (NH
4 3
NO CAS 6484-52-2)
at 80°C for 3h and the formed NH -SAPO-34 material is then filtered,
4
washed to pH 7 and centrifuged before drying at 100°C for one night. The
second step consists in preparing a 0,01M copper (II) acetate solution and
4
adding the NH -SAPO-34 zeolite once the aqueous solution is heated at
80°C. After 30 min, several filtration cycles were performed. The final Cu-
SAPO-34-IE catalyst was then washed, dried and calcined at 550°C in air
for 6h. The Cu-SAPO-34-HT catalyst was synthesized by only one step,
where the same different zeolite precursors were added after the first
formation of a modified template constituted of the templating agent mixed
2 3 2
with the copper precursor (acetate copper salt - Cu(CO CH ) CAS 142-
7
1-2). The following method can be find described elsewhere in previous
study [14].
performances for both catalysts, and further investigated by NH
3
and NO oxidation reactions. The reaction regime modification
displayed by Cu-SAPO-34-HT corresponded with the active site
Characterization
3
characterizations, where the delayed NH oxidation seemed to
The catalyst crystalline structures were characterized by (XRD) X-
ray Diffraction at ambient atmosphere employing Bruker D8
benefit the NO reduction process while the surface nano cluster
of Cu-SAPO-34-IE enhanced this parasitic reaction at medium
temperature. Different active site configurations which their
a
diffractometer (CuKα radiation at 0,154184 nm) equipped with a Ni filter
and 1-D fast multistrip detector (LynxEye, 192 channels on 2.95◦).
Parameters of 2θ steps of 0.02◦ from 4◦ to 40◦ for a total acquisition time
of 32 min were used for the diffractograms collection. Phase identification
was performed using the ICDD-PDF4+ database and using the Diffrac.Eva
software (Bruker). High Resolution Transmission Electron Microscopy
3
impacts on NH SCR process will be further studied through a
multi-site kinetic model, in order to access and evaluate kinetic
parameters inherent to the implemented active site configuration,
from Brønsted acid sites to copper species.
(
HR-TEM) combined with an Energy-Dispersive X-ray spectroscopy (EDX),
was employed in order to examine the local microstructure of our different
samples. These examinations were achieved with JEOL 2010
a
microscope. Its acceleration voltage was 200 kV with a LaB6 emission
filament. Before measurements, a dispersion of catalyst crushed in ethanol
was deposited onto holey carbon-covered TEM grids. Elemental
composition in Si, Al, P and Cu were obtained by Inductively Coupled
Plasma Optical Emission Spectroscopy (OES ICP) technique.
Measurements were made with an Activa instrument from Horiba Jobin
Yvon. Powders were completely dissolved with a mixture of lithium tetra-
borate and 20% HCl at 1100°C in Pt-Au crucible for Si determination. For
Cu, P and Al determination, samples were attacked with a mixture of
Experimental Section
Cu-SAPO-34 preparation methods.
Two different Cu-SAPO-34 catalysts were prepared with a similar
copper amount (1.5%wt. targeted) following distinct synthesis routes: by
one-pot hydrothermal method, which will be denoted “Cu-SAPO-34-HT”
according to our method described elsewhere [14] and a second one by
conventional ionic exchange, which will be denoted “Cu-SAPO-34-IE”.
Both zeolite supports were formed with a theoretical molar gel composition
sulphuric acid (H
2 4 3
SO ), nitric acid (HNO ) and hydrofluoric acid (HF) at
250-300°C. The X-ray Photoelectron Spectroscopy measurements (XPS)
were performed with an Axis Ultra DLD spectrometer from Kratos
Analytical using a monochromatic Al-K X-ray source with an energy of 20
eV. Tests were performed in ultra-high vacuum (10-8 – 10-10 Torr) on the
as-synthesized samples. Spectra of Si 2p, P 2p, Al 2p, Cu 2p, O 1s, C 1s
and N 1s levels were recorded at 90° take off. The binding energy of the
C1s intense peak corresponding to C-C bonds was taken as a reference
for energy calibration and fixed at 283.9 eV. The signal intensities were
measured using integrated areas under the detected peak of the spectrum,
where the Cu2+ and Cuδ+ signal peaks were associated to a binding energy
close to 934 eV and 931 eV, respectively. The experimental precision of
of 1SiO
2 2 3 2 5 2
: 6.7 Al O 6.2 P O : 161.3 H O and 7.7 of TEAOH (35 wt%
2 5 4
(C H ) N(OH); CAS 77-98-5). The copper proportion was adopted in order
to under-exchange the Cu-SAPO-34-IE catalyst and maximize the loading
of active cations within the lattice through the synthesis [50]. Following the
purpose of the study, a similar copper proportion was incorporate during
Cu-SAPO-34-HT hydrothermal synthesis, thus comparing two different
catalysts with similar copper content.
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ChemCatChem
10.1002/cctc.201902036
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the XPS semi-quantitative measurements was close to 10%. Following the
analysis operating conditions conducted at ambient temperature, the main
reductive impact on copper state could concerns the X ray beam, for XPS
measurements and electron beam for TEM analysis, on the smallest sites
as nano-scaled copper oxide clusters, potentially decreasing the Cu(II)
proportion in consequence. A limited reduction obtained through the
acquisition of stable profile during measurements.
the possible and considered global reactions were respectively the STD
SCR, which the respective turnover frequencies during standard SCR
reaction (1) were estimated for additional information as 7.53.10-4
9.74.10-
,
4
and 2.23.10-3 s-1 respectively at 200, 220 and 240°C with Cu-SAPO-34-
HT catalyst. And about 6.78.10 , 7.36.10 and 7.82.10-3 s-1 respectively
at 200, 220 and 240°C for Cu-SAPO-34-IE. But also NH and NO oxidation,
O formation and Fast SCR which the stoichiometries are shown below:
-3
-3
3
N
2
The 27Al, 29Si and 31P Magic Angle Spinning Nuclear Magnetic
Resonance (MAS NMR) measurements were performed on Bruker
AVANCE III 500 WB spectrometer at resonance frequencies of 99, 130
and 202 MHz, respectively with a sample spinning rates of 10 kHz in a
commercial 2.5 mm MAS probe at room temperature. The Specific Surface
4 NH
+ 4 NO + O
→ 4 N
→ 2 N +6 H
→ 4 NO +6 H
+6 H
O
(1)
(2)
(3)
(4)
(5)
(6)
3
3
3
3
2
2
2
4
4
4
NH
NH
NH
+ 3 O
2
2
2
2
O
+ 5 O
2
O
Areas (SSA) and micropore volumes were determined using
a
Micrometrics ASAP 2020 using nitrogen as adsorbate at -196°C. SSA and
the microporous volume were calculated following the t-plot method
following the Harkins and Jura thickness model equation [52]. Prior to
measurements, the samples were preheated at 300°C in vacuum during
2 2 2
+ 2 NO + 2 NO → 4 N +6 H O
2 2
NO + ½ O ↔ NO
4
h and measurements started at 10-8 relative pressure (p/p°). The
corresponding catalysts and support N isotherms can be observed in
Supporting Information Figure S2. The reducibility of copper species was
estimated by Temperature Programmed Reduction with H (H -TPR)
experiments. The samples were first pre-treated (100 mg of samples)
under 20% vol. oxygen balanced in N during 30 min at 500°C. Then, the
sample was cooled down to room temperature in O /N flow and flushed
with argon. The profiles were recorded under a flow of 2% vol. H in Ar (40
2
3 2 2 2
4 NH + 4 NO + 3 O → 4 N O +6 H O
2
2
3 2
NH , NO conversion and N selectivity were defined as follows:
2
2
2
2
[NH ]in- �NH
out
-1
-1
ꢀ
ml·min ) from 20 to 500 °C at a rate of 10 °C·min . Temperature
3 3
CNH3
=
x 100
x 100
(7)
[
NH3]in
Programmed Desorption measurements were obtained by using ammonia
3
as a molecular probe (NH -TPD). Samples (ca. 0.035g) were loaded into
a U-shaped quartz tube and pre-treated under oxidative atmosphere for
-
1
in
3
[NO] - �NO ꢀ
out
3
0 min at 500°C with a heating ramp of 10°C.min . After 30 min flush of
CNO
=
(8)
-1
[NO3]in
He at 35°C, a total flow of 40 ml.min of 1000 ppm NH
3
, 8% O
2
and 4%
2
H O was injected at 150 °C until saturation. The catalysts were then
exposed to He for 30 min to remove all the physically adsorbed species
before starting the desorption temperature program, which consisted of a
ramp of 2 ºC · min-1 until 500ºC. Desorbed ammonia was analysed by
Fourier Transform Infrared (FTIR) spectrometer (NICOLET) equipped with
a heated 2m gas cell and a DTGS detector.
Where, C
concentrations of specie A (ppm) in the inlet and outlet respectively.
is the conversions rate of specie A, [A]in and [A]out the
A
out out
+2 x [N O]
2
[
NO
]
2
Catalytic Tests.
selectivity N2=(1-[
x 100)
(9)
ꢀ
in-[NOx]out- �NH
ꢀ
out
NOx]in+ �NH3
3
The standard SCR performances were studied by using a u-shaped
quartz reactor (6 mm internal diameter) containing 35 mg of crushed
catalyst deposited on a quartz frit. A thermocouple was inserted near to
the catalytic bed to monitor the temperature and the reactor was placed
inside a temperature-controlled furnace. Before testing, fresh catalyst
surface was cleaned at 200 °C in flowing He. In order to avoid water
condensation and risks of ammonium nitrate precipitation, the system
temperature limit is fixed at a minimum of 200 °C and all the gas lines are
heated. All reactants gases were introduced by flow controllers and water
vapour fraction was separately supplied by a thermostated saturator,
reaching an estimated total Gas Hourly Space Velocity (GHSV) of 142 000
By assuming a continuous stirred reactor free of diffusion limitations at low
temperature conversion, the turnover frequency is estimated through the
reaction rate and the quantity of active species in mol. Cu as:
in
NOx] - [NOx]
out
x Q x (Vm)-1x(1000) x(60)
-1
-1
[
TOF=
nCu active
-1
h . The effluent NH3, NO, NO
monitored by Fourier Transform Infrared (FTIR) spectrometer
NICOLET) equipped with a heated 2 m gas cell and a DTGS detector.
Outlet nitrogen was followed with an in-line micro gas phase
chromatography (SRA % GC-R3000). The Standard NH -SCR (STD SCR)
tests performances were conducted with an inlet gas composition of 1000
ppm NH , 1000 ppm NO, 8% O , 4% H O, He as a balance and a total
Flow = 10 L/h. For separated NH and NO oxidation experiments, the inlet
gas compositions were 1000/0 ppm NH /NO or 0/1000 ppm NH /NO, 8%
4% H O with He as a balance. Each experiment was conducted 3 times
2 2
and N O gas concentrations were
-1
-1
With Q the volumetric flow (L.min ), Vm = 24.05 L.mol the molar volume
in normal condition of temperature (20°C) and pressure (1 atm), nCu active
the mol of acive species for the concerned reaction.
a
(
3
3
2
2
Surface active site characterizations
3
3
3
O
,
2
in a row for stability and repeatability accordance. Moreover, external
diffusion limitations were ruled out during the catalytic tests as it was
investigated through varying the sample load and volumetric flow under
several measurement conditions (Supporting Information Table S2).
Corresponding to studied Cu-SAPO-34 catalysts and above conditions,
Diffuse Reflectance Infrared Fourier Transform spectroscopy
measurements (DRIFT) were performed, using
a FTIR (NICOLET)
equipped with a praying mantis accessory (HARRICK high temperature
-1
test chamber) and a MCT detector at a resolution of 1 cm . 32 scans were
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ChemCatChem
10.1002/cctc.201902036
FULL PAPER - ACCEPTED ARTICLE
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ChemCatChem
10.1002/cctc.201902036
FULL PAPER - ACCEPTED ARTICLE
FULL PAPER
b
b
Cu-SAPO-34
exchanged copper cations with
surface micro cluster copper
catalyst
free
of
Guillaume Pétaud , François Gaillard ,
c
a
Melaz Tayakout , Sonia Gil* , Anne
Giroir-Fendler*a
exhibited particular selective catalytic
reduction with ammonia in standard
conditions due to combination of
ammonia desorption, surface cluster
activation and active sites creation
through temperature
Page No. – Page No.
Spotlight on large surface copper
cluster role of Cu-SAPO-34 catalyst
in Standard NH -SCR performances
3
This article is protected by copyright. All rights reserved.