Evaluation of Co/SSZ-13 Zeolite Catalysts Prepared by Solid-Phase Reaction for NO-SCR by Methane

Article abstract of DOI:10.1002/open.202000239

Co/SSZ-13 zeolites were prepared by heating the finely dispersed mixture of NH₄-SSZ-13 and different cobalt salts up to 550 °C. Investigations by thermogravimetry – differential scanning calorimetry – mass spectrometry provided new insight into

Full text of DOI:10.1002/open.202000239

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Evaluation of Co/SSZ-13 Zeolite Catalysts Prepared by
Solid-Phase Reaction for NO-SCR by Methane
[a]
[b]
[b]
[b]
[a, c]
Rania Charrad, Hanna E. Solt,* József Valyon, László Trif, Faouzi Ayari,
[a, d]
[e]
[b]
Mourad Mhamdi,
Jenő Hancsók, and Ferenc Lónyi
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Co/SSZ-13 zeolites were prepared by heating the finely
dispersed mixture of NH -SSZ-13 and different cobalt salts up to
vapor or gas products, readily leaving the zeolite pores, and
cobalt ions in lattice positions suggesting that solid-state ion-
exchange is the prevailing process. The obtained catalysts are
4
550°C. Investigations by thermogravimetry – differential scan-
ning calorimetry – mass spectrometry provided new insight
into details of the solid-state reaction. Formation of Co carrying
hydrate melt or volatile species was shown to proceed from
chloride, nitrate, or acetylacetonate Co precursor salts upon
of good activity and N selectivity in the CH /NO-SCR reaction.
2 4
The thermal treatment of acetate or formate salts give solid
intermediates that are unable to get in contact and react with
the cations in the zeolite micropores. These catalysts contain
mainly Co-oxide clusters located on the outer surface of the
zeolite crystallites and have poor catalytic performance.
thermal treatment. This phase change allows the transport of
+
the Co species into the zeolite pores. The reaction of the NH
4
+
or H zeolite cations and the mobile Co precursors generates
[
2–5]
1
. Introduction
shown to exhibit good activity and selectivity;
however, their
thermal and hydrothermal stability was not satisfactory for
[6,7]
Selective catalytic reduction of nitrogen oxides to N2 with
methane (CH /NO-SCR) has attracted much attention and has
been described as a potential method for removing NO from
the exhaust of engines, fueled by natural gas, such as lean-burn
gas engines in cogeneration (CHP) systems. Cobalt-exchanged
zeolites, such as Co-ZSM-5 and Co-mordenite zeolites were
long-term practical application. The objective of the present
work was to prepare better catalysts by introducing active Co-
species into thermally and hydrothermally extremely stable
4
x
[8]
SSZ-13 zeolite.
[1]
The amount of cations, exchangeable into zeolites by
aqueous-phase ion exchange (APIE) is limited by the thermody-
namic equilibrium of the exchange process. Even at low
concentrations the Co ions form ion pairs and bulky aqua
complex that does not favor the formation of Co-rich zeolite
2
+
[a] R. Charrad, Dr. F. Ayari, Dr. M. Mhamdi
Laboratoire de Chimie des Matériaux et Catalyse, Faculté des Sciences de
Tunis
[9–11]
forms.
Université Tunis El Manar
Tunis (Tunisie)
[b] Dr. H. E. Solt, Dr. J. Valyon, L. Trif, Dr. F. Lónyi
Institute of Materials and Environmental Chemistry
Research Centre for Natural Sciences
Magyar tudósok körútja 2
In the solid-phase reaction, often referred to as solid-state
ion exchange (SSIE), the temperature of finely dispersed mixture
of zeolite and cobalt salt is increased to induce the reaction of
the components. Using this method cobalt can be introduced
into the zeolite in a single step. Some experiments show that
1117 Budapest (Hungary)
E-mail: solt.hanna@ttk.hu
[9,11–13]
high degree of ion exchange can be reached.
The
[
c] Dr. F. Ayari
exchange level is attributed to the non-equilibrium nature of
the process due to the volatile side product of the exchange
reaction. For instance, if chloride salt is mixed with the acidic
form of the zeolite (H-zeolite) HCl is formed that leaves the solid
Faculté de Pharmacie de Monastir, Laboratoire de développement
chimique, galénique et pharmacologique des médicaments
Université de Monastir
Rue Avicenne
5000 Monastir (Tunisie)
[9]
as gas. Further advantage of the SSIE method is that, unlike to
[d] Dr. M. Mhamdi
the APIE method, no transition-metal-containing waste solution
is emitted. It should be noted, however, that the metal
incorporation by the zeolite material is often not complete and/
or not all of the Co introduced occupy ion-exchange positions
Institut Supérieur des Technologies Médicales de Tunis
Université Tunis El Manar
Tunis (Tunisie)
[e] J. Hancsók
Institute of Chemical and Process Engineering
University of Pannonia
Egyetem utca 10
[9,12]
in the zeolite.
A variety of Co salts, such as chloride, nitrate,
acetate, etc. were suggested as potential precursors for
8200 Veszprém (Hungary)
[9,12,14]
SSIE;
however, the chemistry and the detailed processes
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/open.202000239
are usually not well established, which makes the success of the
ion-exchange process unpredictable.
In this work the influence of the cobalt salt anion on the
mechanism of the SSIE process and the cobalt species formed
during the SSIE was investigated in details and was related to
©
2020 The Authors. Published by Wiley-VCH GmbH. This is an open access
article under the terms of the Creative Commons Attribution Non-Com-
mercial NoDerivs License, which permits use and distribution in any med-
ium, provided the original work is properly cited, the use is non-commercial
and no modifications or adaptations are made.
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the catalytic properties of the so received Co/SSZ-13 prepara-
about 300–550°C), respectively (cf. Figure 1A, a and Figure S1).
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tions in the CH /NO-SCR reaction. The hydrothermal stability of
The weight loss in the latter high temperature range (2.01 wt%)
corresponds to 1.1 mmol/gcat. NH evolution, which is in good
4
the most promising catalysts was also tested.
3
+
agreement with the measured NH4 ion-exchange capacity
(
0.95 mmol NH /g ) of the NH -SSZ-13 sample.
3 cat. 4
2. Results
2
.1. Results of the TG-DTG-DSC-MS Measurements
2.1.2. CoCl ·6H O/NH -SSZ-13
2 2 4
The experimental conditions of the TG-DTG-DSC-MS measure-
ments were chosen to follow the synthesis conditions of the
Co/SSZ-13 catalysts prepared by solid phase reactions. Note
The DTG curve and the corresponding MS results obtained on
the CoCl ·6H O/NH -SSZ-13 mixture (Figure 1A, b and Fig-
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ure S2) were similar than those measured on the parent NH4-
SSZ-13 sample (Figure 1A, a and Figure S1), except that the loss
of crystalline water from the CoCl ·6H O salt resulted in
that the catalysts obtained from the NH -SSZ-13/Co-salt
4
mixtures by this preparation method were designated as Co(X)/
SSZ-13, where X stands for the anion of the cobalt precursor
salt, namely Cl for chloride, N for nitrate, Acac for acetyl
acetonate, A for acetate, and F for formate (see the details in
the Experimental section).
2
2
additional DTG peaks at 128 and 158°C (Figure 1A, b;
corresponding endothermic peaks appear at 122 and 156°C on
the DSC curve shown in Figure 1B). However, it is important to
note that CoCl ·6H O salt melts below 90°C without any weight
2
2
The DTG curves derived from the thermogravimetric (TG)
analysis and the corresponding DSC curves measured for the
parent NH -SSZ-13 and the different NH -SSZ-13/Co-salt mix-
loss and the salt forms hydrate melt, i.e., a saturated solution of
[15,16]
the hydration water.
The solution can easily penetrate into
the zeolite pores. The amount of NH3 evolved during the
thermal treatment of CoCl ·6H O/NH -SSZ-13 mixture in the
4
4
tures are shown in Figure 1A and Figure 1B, respectively. The
characteristic mass spectrometric (MS) traces of desorption and/
or decomposition products collected in situ during the TG
experiments are presented in Figures S1–S6.
2
2
4
high temperature range attained about half of that obtained for
the parent zeolite (cf. Figure S1 and S2), suggesting that
+
significant fraction of the NH4 cations was exchanged by
cobalt cations during the SSIE process given by Eq. 1:
þ
À
2þ
À
2
2
.1.1. NH -SSZ-13
2NH4 Z þ CoCl ! Co Z þ 2NH Cl
(1)
4
2
4
À
The parent NH -SSZ-13 sample presents two DTG peaks, which
(where Z represents a negatively charged segment of the
4
can be assigned to the weight loss due to water desorption
zeolite). The product of the SSIE process must be NH Cl, which
4
from the zeolite (peak between 30–300°C) and ammonia
then can leave the system by sublimation into the carrier gas
+
released during the decomposition of NH cations compensat-
over about 340°C (melting point: 338°C, boiling point: 520°C).
4
ing zeolite framework charge (low intensity peak between
Although NH Cl could not be detected by MS, possibly because
4
Figure 1. Curves of(A) DTG and (B) DSC of materials (a) NH
4 4 2 2 3 2 2
-SSZ-13, and (b-f) physical mixture of NH -SSZ-13 and (b) CoCl ·6H O, (c) Co(NO ) ·6H O, (d) Co
(C
5
H
7
O
2
)
2
, (e) Co(CH COO) ·4H O and (f) Co(HCOO) ·2H O.
3
2
2
2
2
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of its condensation on the lower temperature parts of the TG-
DSC system, its formation is clearly supported by the weight
loss (2.95 wt%) in the high temperature range (300–550°C) that
the 100–250°C temperature range. The formation of acetylace-
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À
tone from Co(C H O ) requires a hydrogen transfer (C H O
+
5
7
2 2
5
7
2
+
H !C H O ). Therefore, the ion-exchange process can be
5
8
2
is higher than that observed for the parent NH -SSZ-13 sample
rationalized as shown by Eq. 3:
4
(2.01 wt%) (cf. Figure 1A, a and b).
þ
À
2þ
À
2
2
NH4 Z þ CoðC H O Þ ! Co Z þ 2 C H O þ 2NH
(3)
5
7
2
5
8
2
3
+
2
.1.3. Co(NO ) ·6H O/NH -SSZ-13
3 2 2 4
The decomposition of NH4 -cations not consumed in the
ion-exchange process resulted in an ammonia evolution peak in
the temperature range of 350–550°C (Figure S4).
The DTG curve measured on the Co(NO ) ·6H O/NH -SSZ-13
3
2
2
4
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mixture shows a peak consisting of at least three components
in the temperature range of 30–150°C, which can be attributed
to water desorbing from the zeolite and loss of crystalline water
from the hexahydrate salt (Figure 1A, c and Figure S3). Note
2.1.5. Co(CH COO) ·4H O/NH -SSZ-13
3
2
2
4
[
15]
that Co(NO ) ·6H O melts at 55–56°C, whereas full dehydra-
The DTG curve measured on the Co(CH COO) ·4H O/NH -SSZ-
3 2 2 4
3
2
2
[17]
tion of the hydrate melt occurs only over about 110°C.
13 mixture presents a peak in the 30–200°C temperature range,
Formation of hydrate melt was indicated by the small
endothermic peak discernible at about 43°C on the correspond-
ing DSC curve (Figure 1B, c). The melt of the precursor salt
penetrates into the zeolite pores allowing, thereby, the SSIE to
proceed. The weight loss between about 150–300°C (Figure 1A,
c) was caused by the formation of gas phase products, mainly
N O and water, while smaller amount of NO and NO also
which is due to the water desorption from the zeolite and loss
of some crystalline water from the acetate salt (Figure 1A, e and
Figure S5). The weight loss over about 200°C (peaks at 227,
276, and 336°C) are mostly due to the evolution of acetic acid,
which is accompanied by the formation of acetone, CO and
water over about 300°C (Figure S5). Former studies on the
thermal decomposition of Co(CH COO) ·4H O revealed that the
2
2
3
2
2
appeared in the gas phase (Figure S3). Formation of N O and
tetrahydrate salt melts and then the crystal water is quickly
2
[22]
H O as main products is typical for the decomposition of
evaporated in the 90–140°C temperature range. Melting and
2
[18,19]
NH NO ,
which is the expected product of the SSIE process
the release of crystal water are strongly overlapping
4
3
[15]
with Co-nitrate:
processes. The appearance of a relatively sharp peak at 115°C
on the DTG curve (at 110°C on the DSC curve) are in
accordance with this notion (Figure 1, e). It follows that the
melt hydrate is quickly transformed to solid Co(CH COO) , that
þ
À
2þ
À
2
2
NH4 Z þ CoðNO Þ ! Co Z þ 2NH NO
(2)
3
2
4
3
3
2
Irreversible decomposition of NH NO to N O and H O
hardly can move into the zeolite pores. It was also shown that
formation of intermediate compounds, such as, basic Co-
acetate (possibly Co (CH COO) OH) and Co-acetoacetate (Co
4
3
2
2
[
18]
readily proceeds at 230–260°C, which is accordance with the
characteristic temperature range observed here. Interestingly,
3
3
5
no NH evolution can be observed in the high temperature
(CH COCH COO) ) via hydrolysis and decomposition reactions of
3 2 2
the dehydrated and/or partially dehydrated salts readily occurs
3
+
range over about 350°C, which suggest that those NH
4
compensating cations, not consumed in the ion-exchange
process (Eq. 2), were consumed in the reactions with the
decomposition products of NH NO (i.e. with N O, NO, NO )
between about 200–300°C. The decomposition is accompanied
[
22]
by release of gas phase acetic acid and/or water.
The
decomposition of the bulky Co compound intermediates
4
3
2
2
forming N2.
generally starts over about 300°C and finally results in the
formation of CoO, while different decomposition products, such
as acetic acid, acetone, carbon monoxide and carbon dioxide
are released into the gas phase. The MS-traces observed over
2
.1.4. Co(C H O ) /NH -SSZ-13
5 7 2 2 4
2
00°C, shown in Figure S5, clearly suggest that above processes
also prevail during the heating of the Co(CH COO) ·4H O/NH -
3 2 2 4
Heating of the Co(C H O ) /NH -SSZ-13 mixture resulted in a
5
7
2 2
4
DTG curve with two intense, somewhat overlapping peaks in
SSZ-13 mixture. Thus, the SSIE process must be strongly
hindered by the transformation of the precursor salt to bulky
intermediate Co-compounds and, finally, to CoO.
the temperature ranges of 30–150°C and about 100–250°C
(Figure 1A, d), which can be attributed to the desorption of
water from the zeolite and to the formation of acetylacetone
C H O ), respectively (Figure S4). Formation of ammonia is also
(
5
8
2
clearly discernible in this temperature range. It should be noted,
that partial volatilization of Co(C H O ) occurs before melting
2.1.6. Co(HCOO) ·2H O/NH -SSZ-13
2
2
4
5
7
2 2
20]
[
(
160°C) already between 55–130°C
and its evaporation is
The DTG curve measured on the Co(HCOO) ·2H O/NH -SSZ-13
2 2 4
nearly complete at 160°C without any noticeable
mixture shows two main peaks at around 90°C and 150°C
(Figure 1A, f), which can be assigned to water desorbing from
the zeolite and to the loss of crystalline water from the Co-
[21]
decomposition.
Thus, the easy volatilization allows the
penetration of the Co-precursor salt into the zeolite pores. The
appearance of acetylacetone and ammonia in the gas phase
clearly suggests that the Co ion exchange readily proceeds in
[15,23]
precursor salt (Figure S6).
Note that we could not detect
either HCOOH (or its decomposition products) or NH , expected
3
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to be formed at low temperature. The appearance of a sharp
Co(A)/SSZ-13 and Co(F)-SSZ-13 as new crystalline phase (Fig-
ure 2). Note that the comparison of the XRPD patterns taken
before and after catalytic test in the presence of 5.5 vol% water
vapor in the input gas stream suggest that the crystallinity of
the samples was not affected during the catalytic runs (Fig-
ure S7).
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peak at 283°C (Figure 1A and 1B, f) indicates a fast decom-
position process, in which CO, CO , and water were released
2
into the gas phase (Figure S6). Considering the characteristic
peak temperature and the products formed, this process can be
identified as the thermal decomposition of the dehydrated Co
[23,24]
(
HCOO) precursor salt
giving Co/CoO as final products.
2
These results suggest, that Co(HCOO) in its dehydrated form
2
could not move into the zeolite pores in an amount, to obtain
significant degree of SSIE.
2.2.2. Composition and Texture
1
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1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
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Cobalt contents and textural properties of the Co/SSZ-13
catalysts, prepared using different Co-salts are summarized in
Table 1. The chemical analysis suggested that about 10 to 30%
of the cobalt was lost during the SSIE process. Similar loss of
2
.2. Physico-Chemical Properties of the Co/SSZ-13 Catalysts
[12]
2
.2.1. Crystallinity
cobalt was reported by a former study. It was explained that
a fraction of Co sublimated out from the mixture of the solids in
the form of volatile Co compound during the SSIE process. The
The XRPD patterns of the parent NH -SSZ-13 zeolite and the
4
Co/SSZ-13 catalysts are compared in Figure 2. Results suggest
that the crystallinity of the samples was preserved during
introduction of cobalt regardless of the used cobalt salt. The
cobalt to framework aluminum atomic ratio (Co/Al ) in the
F
obtained catalysts was between 0.31 and 0.41.
Introduction of cobalt by solid-phase reaction was found to
cause only relatively small (7–20%) decrease of the specific
surface area (SBET and S ) and the total pore volume (V ) relative
(
311) reflection of Co O was detectable only in the patterns of
3 4
μ
p
to the corresponding property of the parent NH -SSZ-13 zeolite
4
(Table 1). The used catalysts showed nearly the same textural
properties as the fresh samples suggesting that no significant
structural damage occurred during the catalytic run. XRPD
analysis confirmed that the crystallinity of the catalysts was
retained during the solid-phase reaction and the catalytic tests
2
À 1
(
vide supra). The 40–50 m g difference between the BET
surface area and the microporous surface area corresponds to
the surface area of the mesopores. Notice that similar surface
area characterizes the mesopores of the parent NH -SSZ-13
4
sample and the catalyst preparations.
2
.3. Characterization of Co-species Formed During SSIE
Process
2
.3.1. H -TPR
2
Figure 2. XRPD patterns of the (a) parent NH
4
-SSZ-13, and the fresh (b) Co
Peaks of H -TPR patterns of Co-zeolites can be assigned to
(
Cl)/SSZ-13, (c) Co(N)/SSZ-13, (d) Co(Acac)/SSZ-13, (e) Co(A)/SSZ-13, and (f)
2
[5,12,25–29]
Co(F)/SSZ-13 catalysts.
reduction of specific Co species.
It is generally agreed
that Co-oxide species located on the outer surface of the zeolite
crystallites and CoO species, including polynuclear Co-oxo-
x
Table 1. Composition and texture.
cations, dispersed within zeolite pores give reduction peak(s) in
a
F
c
d
e
Sample
Co/Al
Co
wt %
S
BET
2
S
μ
V
p
the temperature ranges of about 100–400°C and 400–600°C,
b
À 1
2
À 1
3
À 1
[m g
]
[m g
]
[cm g
]
2+
+
respectively. Isolated Co
and [CoÀ OH] cations in ion-
NH
4
-SSZ-13
–
0.41
0.38
–
696
659
0.335
exchange positions can be reduced only over about 600°C. It
Co(Cl)/SSZ-13
Co(N)/SSZ-13
Co(Acac)/SSZ-13 0.38
Co(A)/SSZ-13
Co(F)/SSZ-13
2.56
2.42
2.41
1.94
1.98
550 (581) 510 (527) 0.283 (0.302)
621 (602) 573 (546) 0.326 (0.313)
623 (558) 571 (507) 0.317 (0.302)
642 (611) 594 (567) 0.324 (0.313)
649 (622) 605 (570) 0.326 (0.314)
was shown that the strength of cation-to-framework interaction
[25,28]
strongly affects the reducibility of the cations.
The temper-
0.31
0.31
ature of a H -TPR peak that corresponds to the reduction of a
2
cationic Co species depends on the strength of stabilization.
2
9
[
a] The Al
F
content was determined by Si MAS NMR (Figure S8). [b]
The H -TPR curves obtained for Co(A)/SSZ-13 and Co(F)/SSZ-13
2
Determined by AAS analysis of air dry samples. [c] In parenthesis SBET is
suggest that these samples contain predominantly cobalt
species reducible below 500°C. These species must be cobalt
oxides (Figure 3, d and e). The most intense reduction peaks
around 370°C indicate that majority of cobalt oxides formed
given after catalytic test. [d] Microporous surface area, S =S – Smeso. Smeso
μ
BET
was determined by the t-plot method. In parenthesis S
determined after catalytic test. [e] Total pore volume. The V
parenthesis, was determined after catalytic test.
μ
is given,
p
given in
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À 1
the 12500–30000 cm region in the UV-Vis spectrum, whereas
the Co-oxides give no signal in this spectral range.
The UV-Vis spectrum of the Co(Cl)/SSZ-13 sample (Figure 4,
a), containing cobalt mainly in ion-exchange positions (as
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
[28,31]
revealed by H -TPR) presents several component bands in the
2
À 1
1
2500–30000 cm region attributable to d-d transitions of Co
(
II) ions. Detailed assignment of the component bands in similar
[
31]
Co/SSZ-13 samples was carried out by Mlekodaj et al.. The
assigned component bands are indicated in Figure 4. The
component bands attributed to cobalt ions located in 8-
member rings (8MR, τ sites; 15000; 16100; 17000; 20500 cm ),
regular 6-member rings (6MR, σ sites, 24500; 27000 cm ) and
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
À 1
À 1
À 1
hexagonal prisms (ω sites, 18500; 21500 cm ) can be clearly
distinguished in the spectra of the Co(Cl)/SSZ-13 samples
(Figure 4, a). The spectrum measured on the used Co(Cl)/SSZ-13
catalyst indicate a slight redistribution of cobalt ions (Figure 4,
a, dashed line). In contrast, no absorption bands in the 12500–
À 1
2
7000 cm spectral range are discernible in the spectra of the
Figure 3. H
2
-TPR curves of fresh (a) Co(Cl)/SSZ-13, (b) Co(N)/SSZ-13, (c) Co
(Acac)/SSZ-13, (d) Co(A)/SSZ-13, and (e) Co(F)/SSZ-13 catalysts.
Co(F)/SSZ-13 catalyst (Figure 4, b), which was shown to contain
predominantly Co-oxides located on the outer surface of the
zeolite crystallites. These results are in full agreement with the
results of the H -TPR measurements (vide supra).
2
are located on the outer surface of the zeolite crystallites. The
H/Co atomic ratio, calculated from the hydrogen consumption
and the cobalt content of the sample, was 2.14 for Co(A)/SSZ-
2.4. FTIR Spectroscopy
1
2
3, and 2.51 for Co(F)/SSZ-13. Both values are between 2.0 and
.67 which would correspond to the full reduction of CoO and
2.4.1. The νOH region
Co O , respectively. This infers that the majority of cobalt in
3
4
these samples contains CoO and Co O in different proportions.
IR spectra of H-SSZ-13 and the Co/SSZ-13 samples in the νOH
3
4
À 1
These results are in agreement with the XRPD measurements,
which confirmed the presence of crystalline Co-oxide in Co(A)
and Co(F)/SSZ-13 samples (Figure 2).
In contrast, the Co(Cl)/SSZ-13, Co(N)/SSZ-13, and Co(Acac)/
SSZ-13 samples contain only hard-to-reduce cobalt species
giving reduction peaks well above 600°C (Figure 3, a–c), which
can be attributed to the reduction of isolated lattice cobalt
cations of the zeolite, such as Co and [CoÀ OH] . The full
reduction of these ionic cobalt species requires two hydrogen
atoms per Co atom. The obtained H/Co ratios are far below 2.0
indicating that 43.0%, 33.5%, and 74.5% of the total cobalt
content in the Co(Cl)/SSZ-13, Co(N)/SSZ-13, and Co(Acac)/SSZ-
region are shown in Figure 5A. The band at 3736 cm
corresponds to isolated silanol (SiÀ OH) groups located on the
[
32]
external zeolite surface,
whereas the band at around
À 1
3665 cm stems from OH-groups attached to extra-framework
2
+
+
1
3 samples, respectively, are not reducible below and at 850°C.
Extremely hard-to-reduce Co species were often observed in
Co-zeolites.
[12,25–27,29]
2+
The low reducibility suggests that the Co
ions are in lattice locations, where they are tightly surrounded
by oxide ions of the zeolite framework and/or the lack of near
neighbor cobalt atoms that could participate in energetically
[26,27,30]
favored formation of metal-metal bonds.
2
.3.2. UV-Vis DRS
Results of the H -TPR measurement suggested that, depending
2
on the used Co precursor salt, the obtained Co/SSZ-13 sample
contains either isolated cobalt cations or cobalt oxides as
dominant species (vide supra). The lattice cobalt cations in
dehydrated state were shown to give characteristic bands in
Figure 4. UV-Vis DRS spectra of (a) Co(Cl)/SSZ-13 and (b) Co(F)/SSZ-13
catalysts before (solid lines) and after (dashed lines) CH /NO-SCR catalytic
4
run. Spectra were collected in He flow at 550°C.
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1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
Figure 5. FTIR spectra of (A) the ν(OH) region for samples (a) H-SSZ-13, (b) Co(Cl)/SSZ-13, (c) Co(N)/SSZ-13, (d) Co(Acac)/SSZ-13, (e) Co(A)/SSZ-13, and (f) Co(F)/
SSZ-13; and (B) difference spectra of the ν(CO) region after contacting the corresponding samples with CO at 5 mbar CO pressure at room temperature for
3
0 min (solid lines) and then after 5 min evacuation (dashed lines). Before measurements pellets were activated by evacuation at 500°C for 1 h. All the spectra
were recorded at room temperature. The spectra of adsorbed CO were obtained as difference of the spectra collected before and after CO adsorption
[
33]
aluminum species. The intensity of this latter band is similar
before (Figure 5A, a) and after cobalt introduction (Figure 5A,
b–f), confirming that the ion-exchange procedure did not cause
noticeable dealumination of the zeolite framework. The pair of
spectra obtained for the Co/SSZ-13 samples (Figure 5B, b–f).
This band exists only in the presence of CO gas in the IR cell
and quickly disappears upon evacuation of the cell at room
temperature (Figure 5B, dashed lines). Because this is the only
band obtained from the adsorption of CO over H-SSZ-13 zeolite
any other band appearing in the spectrum of the catalysts must
arise from CO bound to Co species.
À 1
bands at 3611 and 3585 cm (Figure 5A, a) shows the presence
of two families of Brønsted acid bridging OH-groups (SiÀ OHÀ Al)
[33]
characteristic for H-SSZ-13 zeolites. The higher and the lower
frequency bands belong to Brønsted acid hydroxyl groups in
the 8MR and the 6MR, respectively. The lower intensity of latter
bands in the spectra of the Co(Cl)/SSZ-13, Co(N)/SSZ-13, and Co
The highest intensity spectral feature of Co-bound carbonyls
À 1
consists of a main band at 2207 cm and three well discernible
component bands appearing as shoulders on its high
À 1
À 1
(
Acac)/SSZ-13 samples (Figure 5A, b–d) suggests that substitu-
tion of both types of Brønsted acid protons occurred to a
significant extent by ionic Co species during the SSIE process. In
(2215 cm ) and low frequency side (2194 and 2171 cm ),
respectively (Figure 5B, b-f). Similar strong carbonyl band have
À 1
been observed in the 2204–2208 cm frequency range on
À 1
contrast, these component bands (at 3611 and 3585 cm )
different Co-zeolites, however, the assignment is still a matter
[
5,35–40]
mostly retained their intensities in the Co(A) and Co(F)/SSZ-13
samples (Figure 5A, e and f), indicating that the Co ion-
exchange hardly proceeded in these latter samples.
of debate.
Some research groups attributed this band to
3
+
[35,38]
3+
Co À CO species,
form carbonyls
attached to Co in exchangeable lattice cation positions. The
whereas others argued that Co does not
and this band should be assigned to CO
[
5,37]
2
+
À 1
corresponding band at 2207 cm has the highest intensity in
2
.4.2. Spectra of Adsorbed CO
all of our Co/SSZ-13 samples. Since the SSIE was carried out in
2
+
inert atmosphere, the oxidation of Co -ions in the Co(II)
3
+
The weak Lewis base CO can interact with Lewis acid sites, like
Co ions. Brønsted acid sites usually adsorb CO in detectable
precursor salts to Co
is not expected as it requires the
2
+
presence of oxygen and high temperature (>450°C) to occur in
[
41]
amount only at low temperature, e.g., at or below À 100°C or at
significant extent.
Therefore, we assign the band at
À 1
2+
elevated pressure. Due to the polarization of CO during these
2207 cm to carbonyl species formed on Co lattice cations of
zeolite SSZ-13.
The band at 2194 cm was assigned either to CO bond to
Co in oxide-like species
interactions, the CÀ O stretching frequency is shifted from that
À 1
[34]
À 1
of the free molecule (2143 cm ) to a characteristic frequency,
2
+
[5,37]
2+
which makes CO to an ideal molecule to probe electrostatic
fields around Lewis and Brønsted acid sites by means of IR
spectrometry.
As expected, the adsorption of CO did not affect the ν(OH)
bands (not shown), which is in line with former observation
that acidic OH-groups cannot retain CO in detectable amount
or to Co lattice cations located in
[39,42]
relatively narrow zeolite cages.
Since the results of the UV-
Vis DRS and H -TPR measurements indicated the presence of
2
2
+
hard-to-reduce Co species as dominating form of Co in the
catalysts prepared with chloride, nitrate, or acetylacetonate
precursor, the most intense component bands, both at 2207
[35]
À 1
2+
at room temperature.
The very weak ν(CO) band at
and 2194 cm (Figure 5B) were assigned to Co À CO carbonyl
À 1
2
226 cm (Figure 5B, a) can be attributed to CO bound to
species. Two similar overlapping component bands were
obtained from CO adsorption over Co-mordenite catalysts.
They were attributed to carbonyls of Co ions located in the
+
[39]
Lewis acid extra-framework aluminum species, such as [AlO]
species.
[32,36]
2+
Similar band appears also in the corresponding
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main channels (the higher frequency component band) and in
the side pockets (the lower frequency component band) of the
Note however, that the corresponding Co-oxo-cations represent
only a minor fraction of Co-species beside the most abundant
Co species.
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
[42]
2+
mordenite structure, respectively. Szanyi et al. also observed
a similar pair of CO bands studying Fe-SSZ-13 samples and
A band, not totally disappearing upon evacuation of the IR
2
+
À 1
attributed them to carbonyls formed on Fe ions located in
the 8MR (the higher frequency component band) and in the
cell, is discernible at around 2170 cm (Figure 5B, b-f). A similar
[43]
carbonyl band was obtained by Busca et al.
from the
6
MR (the lower frequency component band) of the chabazite
adsorption of CO over CoAl O spinel. The band was attributed
2
4
3
+
structure, respectively. Based on these results, we assign the
to CO adsorbed on octahedral Al sites in spinel structure. In
our catalysts minor amount of CoAl O might have been formed
À 1
2+
bands at 2207 and 2194 cm to carbonyls formed on Co
2
4
species located at two different cationic positions of the zeolite
structure, namely in the 8MR and the 6MR, respectively. Note
in the high-temperature solid-phase reaction of extra frame-
work Al species and cobalt salt.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
À 1
that the high frequency component band at 2207 cm was
The results of the CO adsorption experiments confirm that,
more resistant to evacuation than the low frequency band at
in line with H -TPR and UV-Vis DRS results, the Co/SSZ-13
2
À 1
2
194 cm (Figure 5B, b-d, dashed lines). This observation is in
catalysts, prepared with chloride, nitrate, or acetylacetonate Co-
[39,42]
2+
line with earlier findings
assignment.
and clearly supports our above
precursors contain mainly lattice Co
cations. In contrast,
samples obtained by SSIE using acetate or formate do not
contain similar Co species in significant amount.
À 1
The weak carbonyl band at 2215 cm (Figure 5B) can be
[39,40]
attributed to carbonyl species formed on Co-oxo-cations.
2
+
2+
[39]
3+
Both Co ions in [Co(II)À OÀ Co(II)] species and Co ions in
+
n+
[40]
[
Co(III)O] and polynuclear [Co O ] species were suggested
2.4.3. NO-SCR Activity
x
y
as possible adsorption sites. Since our catalyst preparations
were made in inert atmosphere, the involvement of [Co
Using inert quartz wool in the reactor, NO was converted to
2
+
(
II)À OÀ Co(II)] species seems to be the most likely. Three weak
NO2 in the whole temperature range (300–700°C) at a
À 1
bands appeared in the 2120–2020 cm region of the spectra of
Co(Cl)/SSZ-13, Co(N)/SSZ-13, and Co(Acac)/SSZ-13 samples
being in contact with CO at 5 mbar CO pressure at room
temperature. The bands are assignable to Co (CO) and Co
CO) species (bands at (2114, 2040 and 2093 cm )
maximum conversion level of about 6% (Figure S10). The lack
of N formation suggests that the NO-SCR reaction does not
2
proceed without a catalyst. Very similar conversion curves were
obtained over the catalysts prepared using the chloride, nitrate,
or acetonylacetonate salt of cobalt (Figure 6 A–C). Above 450°C
the reaction proceeded very selectively towards the formation
of N . Only negligible amount of NO could be detected in the
+
+
2
À 1 [39,40]
(
(Fig-
3
ure S9). These bands indirectly affirm the presence of
2
+
[
CoÀ OÀ Co] species in these samples, since CO can reduce
2
2
2
+
+
[39,40]
Co to Co at low temperature only in these oxo-cations.
product mixture. In contrast, significantly higher NO formation
2
Figure 6. Conversion of (A–E) NO in the NO-SCR reaction over (A) Co(Cl)/SSZ-13, (B) Co(N)/SSZ-13, (C) Co(Acac)/SSZ-13, (D) Co(A)/SSZ-13, and (E) Co(F)/SSZ-13.
À 1
Section (F) shows the conversion of CH
4
in these reactions. The reactant flow was 4000 ppm NO/4000 ppm CH
4
/2% O
2
/He, the GHSV was 30,000 h . Before
reaction, the catalysts were treated in situ in He flow at 700°C for 1 h. For catalytic activity reported as turnover frequency see Table S1.
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could be observed over the Co(A)/SSZ-13 and Co(F)/SSZ-13
CH þ 2 O ¼ CO þ 2 H O
(4)
(5)
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
4
2
2
2
catalysts at temperatures below 500°C, whereas the reaction
become selective for N formation only over about 600°C
2 NO þ CH þ O ¼ N þ CO þ 2 H O
2
4
2
2
2
2
(Figure 6 D–E). Table S1 reports the catalytic activity of the
samples as turnover frequencies (TOF).
Since the combustion exhausts contain water vapor in high
concentration, tests were carried out to study the effect of
steam on the catalytic activity and hydrothermal stability of the
best NO-SCR catalysts. Alternately dry and wet reactant mixture
was fed on the catalyst and the activity was monitored
The more pronounced effect of water on the methane
combustion reaction is shown by the CH -selectivity index (α),
4
which indicate the fraction of converted CH , used to reduce
4
[44]
nitric oxide to nitrogen (Eq. 6).
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
(Figure 7).
0:5 � ½NOꢀ
0
� XNO!N2
a ð%Þ ¼
� 100
(
6)
In the presence of water vapor both NO and CH4
½CH
4
ꢀ
0
� XCH
4
conversions decreased relative to those obtained with dry
reactant over all the catalysts. However, methane conversion
decreased to a substantially higher extent than the conversion
where [NO] and [CH ] stands for the concentrations of NO and
0
4 0
CH in the reactor input gas, X
for the conversion of NO to
4
NO!N2
of NO to N . It follows that the presence of water vapor
N (mol %), and X for the total conversion of CH (mol%). The
2 CH4 4
2
suppressed mainly the consumption of methane in the
undesired methane combustion reaction (Eq. 4), whereas the
constant 0.5 in Eq. 6 comes from the stoichiometric ratio of
methane to nitric oxide in Eq. 5. The higher α indices observed
under wet reaction conditions imply that in the presence of
water mainly the methane consumption in the undesired
combustion side reaction was suppressed (Table 2). It is worth
to note that the effect of water was reversible and the catalysts
showed stable activity during the applied 12–13 cycles.
main reaction leading to N formation (Eq. 5) was less affected.
2
3
. Discussion
3
.1. The SSIE Process and the Nature of Co Species Formed
The SSIE method involves heating finely powdered mixture of a
zeolite, being preferably in H or NH4 cationic form, and a
metal salt to make the metal ion to take over the charge
balancing function of lattice cations.
variety of Co salts were used to introduce cobalt cations into
zeolites by SSIE with more or less success;
underlying reasons behind the success or failure of getting Co-
zeolite with or without Co
understood. The objective of the present study was to better
understand the chemistry of the SSIE process with different
+
+
[
10,13]
In former studies a
[9,11,12,14,45]
however, the
2
+
lattice cations were not fully
+
salts of cobalt to get Co/SSZ-13 catalyst. The NH4 form of SSZ-
1
3 zeolite was used to hinder the formation of acid product
from the anion of the salt that could damage of the zeolite
structure. The SSIE with cobalt chloride, nitrate, and acetylacet-
onate resulted in Co/SSZ-13 zeolite containing cobalt ions
predominantly in ion-exchange positions. In contrast, the use of
acetate or especially formate salt led mostly to formation of Co-
2 4
Table 2. Conversion of NO to N and the CH -selectivity index (α) under
dry and wet reaction conditions
Figure 7. Effect of water on the CH
4
/NO-SCR activity of (A) Co(Cl)/SSZ-13, (B)
Co(N)/SSZ-13, and (C) Co(Acac)/SSZ-13. Dry reaction mixture contained
3000 ppm NO, 4000 ppm CH , and 2% O in He, whereas wet reaction
Sample
NO to N
%
dry
2
conversion,
wet
CH
4
selectivity index (α),
4
2
[a]
a
%
2
mixture contained additional 5.5 vol% H O on the expense of balance
À 1
dry
wet
helium. Reaction was carried out at 550°C and at GHSV of 15,000 h
alternating dry and wet feed. Bars show the conversion of NO to N
2
(green
Co(Cl)/SSZ-13
Co(N)/SSZ-13
Co(Acac)/SSZ-13
70.2
68.4
76.5
60.6
54.7
55.4
28.0
27.6
32.4
40.0
39.5
47.3
bars) and the conversion of CH (red bars) measured using dry reaction
4
mixture (full bars, odd cycle numbers) and wet reaction mixture (grafted
bars, even cycle numbers). The dry and wet reaction conditions were
maintained for 40–40 min.
[a] Average values calculated from the data shown in Figure 7.
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oxide, clustering on the outer surface of the zeolite crystallites.
Obviously, the result of the reaction depends on a number of
factors, such as, the melting temperature and vapor pressure of
the salt, the way of salt decomposition, and the diffusivity of
3.2. Relations Between the CH /NO-SCR Activity and the
Co-Species Formed by SSIE
4
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
Former studies showed that the zeolite structure and composi-
tion and the conditions of catalyst preparation determine the
catalytic activity and selectivity of Co-zeolites in the CH /NO -
[9,11,12,26,41,46]
the Co carrying species in the zeolite pores.
Our results show that the SSIE readily proceeds, if the Co
precursor salt can reach the exchangeable lattice cations, react
with them inside the zeolite crystallites and the product(s)
formed from the anion of the salt can leave the pores as vapor
or gas. The hexahydrate of Co-chloride and nitrate melts at
relatively low temperature (<100°C) forming a saturated
solution of the Co-compound in the hydration water of the
crystal. The solution readily seeps into the zeolite micropores.
4
x
[3,5,6,26,27,47,48]
SCR reaction.
Present study shows that the Co salt
used in SSIE process also affects the properties of the formed
Co sites and thus the deNO behavior of the catalyst.
x
2
+
The opinions about the role of the Co ions in the CH4/
NO -SCR reaction are not fully consistent.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
[
4–6,26,39,47,49,50]
There are
x
ample of evidence that the N -forming reaction proceeds on
2
2
+
Co ions in ion-exchange positions of zeolite. Catalytic results
shown in the present work are in agreement with this notion.
The catalyst samples containing Co ions in lattice position as
dominant Co-species showed relatively good activity and
+
The exchange reaction of the chloride precursor and the NH4
2
+
lattice cations results in the formation of Co-zeolite and NH Cl.
4
The NH Cl does not decompose, but leaves the pores by
4
sublimation/evaporation at temperatures above about 340°C.
The product of the reaction with nitrate salt are Co-zeolite and
NH NO , which readily decomposes in the 150–300°C temper-
selectivity towards N formation (Figure 6, A–C).
2
In contrast, the catalyst samples, containing mainly Co-oxide
clusters mainly on the outer surface of the zeolite crystallites,
show activity in the oxidation of NO to NO . The NO was the
4
3
ature range. The leaving products are mainly N O and H O
2
2
2
2
gases. Interestingly, in former studies the SSIE was found not to
dominating product up to about 500°C (Figure 6, D,E). These
results are in line with former observations that oxide like
species inside the zeolite channels or Co-oxide clusters formed
outside the zeolite crystallites brings about the oxidation of NO
to NO2 that is important intermediate of the NO-SCR
[
12,45]
proceed well using nitrate salt and H-form zeolite.
This
might be due to the formation of HNO during the SSIE process
3
,
which is probably an unfavorable leaving product.
Using Co-acetylacetonate the SSIE proceeds easily because
[3,26,49]
this salt volatilizes at low temperature (between 55–130°C). The
process.
Interestingly, as reaction temperature was in-
vapor moves easily into the zeolite pores. Acetylaceton and NH
creased, increasing N formation was observed over the Co(A)/
3
2
2
+
appears in the 100–250°C temperature range as leaving
product indicating proton transfer from the zeolite to the
acetylacetonate ligand during the ion exchange process.
SSZ-13 or Co(F)/SSZ-13 catalysts, containing Co ions in very
low concentration. Nitrogen was selectively formed over about
600°C. The NO is absent from the product mixture, if the rate
2
Minor amount of ionic cobalt species got in zeolite lattice
positions from acetate salt. Significant amount of Co-oxide
species was formed on the outer surface of the zeolite
crystallites. The relatively low degree of ion-exchange can be
due to simultaneous melting and dehydration of the tetrahy-
drate salt and/or to the transformation of the (partially)
dehydrated precursor salt to bulky intermediates, most prob-
ably to Co (CH COO) OH) and Co(CH COCH COO) . The trans-
of the NO -consuming N -forming reaction is higher than the
2
2
[3]
rate of the NO to NO oxidation reaction. At low concentration
of Co sites this situation probably prevails only in the high
2
2
+
temperature range (over about 600°C), where thermodynamics
2
+
allows anyhow lower NO concentrations. In the lack of Co
2
sites nitrogen formation may still occur in the reaction of NO2
and CH over Brønsted acid sites as it was demonstrated by
4
[51]
Lukyanov et al.. Nevertheless, Brønsted acid sites are much
3
3
5
3
2
2
2
+
port of these bulky intermediate products, as well as the
dehydrated precursor salt into the zeolite pores is strongly
hindered. Their decomposition readily occurs between about
less active than Co sites. At high temperature (>600°C) the
reaction over the acid sites can also contribute to the N -
2
formation.
2
00–300°C and produces Co-oxide species on the outer surface
of the zeolite crystallites.
The SSIE process with the formate salt also resulted in minor 4. Conclusions
cobalt exchange and excessive formation of Co-oxide on the
outer surface of the zeolite crystallites. The virtual absence of
HCOOH and NH3 in the reaction effluent suggests that no
noticeable Co ion-exchange occurred. The Co(HCOO) ·2H O
The type of the Co precursor salt used for the preparation of
Co/SSZ-13 zeolite catalyst by solid-state reaction strongly affects
the structure of the introduced Co-species and, thus, the activity
2
2
precursor salt becomes quickly dehydrated at around 150°C.
The decomposition of the salt at about 280°C, leaving Co/CoO
clusters on the outer surface of the zeolite crystallites. In the
catalyst treated at high temperature (700°C) in inert gas,
clusters of Co O were detected by H -TPR.
and selectivity of the catalyst in the CH /NO-SCR reaction. The
4
formation of Co carrying hydrate melt or volatile species was
shown to proceed from chloride, nitrate, or acetylacetonate Co
precursor salts upon thermal treatment. This phase change
allows the transport of the Co species into the zeolite pores.
3
4
2
+
+
The reaction of the H or NH4 zeolite cations and the mobile
Co precursors generates vapor or gas products, readily leaving
the zeolite pores, and cobalt ions in lattice positions. The
obtained catalysts are of good activity and N selectivity in the
2
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CH /NO-SCR reaction. The thermally effected transformation of
composition than that used for catalyst preparation were filled in
4
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
an alumina crucible of 100 μL volume and were heated from 25°C
acetate or formate salts give solid intermediates that are unable
to get in contact and react with the cations in the zeolite
micropores. The catalyst contains mainly Co-oxide clusters
located on the outer surface of the zeolite crystallites. These
up to 550 C at a heating rate of 20 C/min, then were kept at
°
°
550°C for one hour. The measurements were carried out in a
3
À 1
90 cm min flow of high purity (99.9999%) helium. The obtained
differential thermogravimetric (DTG) and differential scanning
calorimetric (DSC) curves were baseline corrected and further
processed by the software of the system (Calisto Processing, ver.
catalysts, show high activity in the NO reaction by O to get
2
NO . Over latter catalysts the NO-SCR reaction becomes
2
2
.06). The analysis of the evolved gases and volatiles were
selective for N only at high temperatures (>600°C), where the
2
performed using a Pfeiffer Vacuum Omni Star™ gas analysis system
(MS-EGA) connected to the TG-DSC unit. The MS was operated in
electron impact mode. The temperature of the gas splitter and
transfer line to the mass spectrometer was set to 220 C. The
measurement was carried out in SEM Bargraph Cycles acquisition
mode, where the m/z range of 5–190 was continuously scanned at
a speed of 20 ms/amu.
NO formation becomes thermodynamically limited. Under such
2
conditions the N -forming reaction can consume the full
2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
amount of the forming NO intermediate. At high temperatures
°
2
the N -forming reaction between NO and CH on the Brønsted
2
2
4
acid sites may also contribute to the SCR reaction.
The cobalt and aluminum contents of the catalysts were deter-
mined by atomic absorption spectroscopy (AAS, Pro-Varian AA-20
spectrometer).
Experimental Section
Catalyst Preparation
The X-ray powder diffraction (XRPD) measurements were carried
out by Philips PW 1870 diffractometer using monochromatic CuKα
radiation (λ=0.15418 nm). The applied generator settings were
Na-SSZ-13 zeolite was prepared using the synthesis method
[
52]
described by Gao et al.. A synthesis gel was prepared having a
composition of 10 SDA: 10 NaOH: 4 Al O : 100 SiO : 2200 H O (Si/
4
0 kV and 35 mA, whereas the scanning rate was 0.02°/sec in the
2
3
2
2
2θ region of 3–50°.
Al=12.5). The structure directing agent (SDA) was trimethyl
adamantyl ammonium hydroxide (TMAda-OH, Sachem ZeoGen
Nitrogen adsorption/desorption isotherms were determined at
À 196°C using an automated volumetric nitrogen adsorption
apparatus (Surfer, Thermo Fisher Scientific) to characterize the
textural properties of the samples. Samples were pre-treated under
dynamic vacuum at 120°C for 1 h and then at 250°C for 2 h prior
to the adsorption measurements. The specific surface area was
2825). The used sodium hydroxide, aluminum hydroxide and fumed
silica (particle size 7 nm) powder was obtained from Aldrich. A
Teflon-lined steel autoclave, containing the synthesis gel, was kept
at 160°C for 9 days under autogenous pressure without stirring the
gel. The as-synthetized zeolite product was washed with distilled
water until neutral pH was reached, filtered and dried at 110°C for
determined using the BET method (S ). The t-plot method of
BET
15 h. The SDA was removed from the zeolite channels by
Lippens and de Boer was used to get the micropore volume (Vμ)
and the surface area of mesopores (Smeso). The surface area of
calcination in air at 550°C for 12 h. The thus obtained Na-form SSZ-
+
13 was transformed into NH
-form by repeating ion exchange
micropores (S ) was obtained as the difference of S and Smeso.
4
μ
BET
3
with 1 M NH Cl solution (25 cm per g zeolite) at 70°C three times.
4
Pore volumes (V ) were calculated from the N adsorption capacity
p
2
The product was washed to chloride free, filtered, and dried at
at the relative pressure of 0.95.
1
10°C. The ammonium ion exchange capacity of the thus obtained
The reducibility of Co species was characterized by temperature-
programmed reduction with hydrogen (H
parent NH -SSZ-13 was 0.95 mmol/g
4
cat.
À TPR). Sample (~70 mg,
2
Co/SSZ-13 catalyst samples were prepared using CoCl ·6H O, Co
particle size: 0.25–0.5 mm) was placed into a quartz microreactor
2
2
(
(
NO ) ·6H O (Merck), Co(C H O ) , Co(CH COO) ·4H O, and Co
tube (I.D.=4 mm) and was pre-treated in situ in a helium flow
3
2
2
5
7
2 2
3
2
2
3
À 1
À 1
HCOO) ·2H O (Sigma-Aldrich) as Co precursor salt. The cobalt
(30 cm min , 10°Cmin ) at 700°C for 1 h. After cooling in He to
2
2
3
À 1
content of the final catalyst preparation was intended to be
3.1 wt%, which corresponds to a cobalt to framework aluminum
room temperature, sample was contacted with a 30 cm min flow
of 10% H /N and heated from 25°C up to 850°C at a rate of
2
2
À 1
atomic ratio (Co/Al
) of 0.5 and to 100% ion-exchange level
10°Cmin . The effluent gas was passed through a dry-ice trap and
a thermal conductivity detector (TCD). The hydrogen consumption
was determined from the integrated area under the TPR curve
using a calibration value obtained by the reduction of known
amount of CuO reference material at the same parameter settings.
F
2
+
assuming Co charge compensating cations. The amount of cobalt
+
salt added to NH4 -SSZ-13 zeolite was determined accordingly. The
salt and the zeolite were thoroughly homogenized in an agate
mortar for 15 min. The obtained physical mixture was heated up in
À 1
helium flow at a heating rate of 2°Cmin to 550°C and was kept
The Co-species in the catalysts were characterized by UV-Vis diffuse
reflectance spectroscopy (UV-Vis DRS) using a Scientific Evolution
at this temperature for 12 h. It was noticed that volatile cobalt
derivatives were formed that evaporated from the mixture. As a
result, the final cobalt content of the catalyst preparations became
less than 3 wt% (Table 1). The catalyst samples are designated as
Co(Cl)/SSZ-13, Co(N)/SSZ-13, Co(Acac)/SSZ-13, Co(A)/SSZ-13, and Co
3
00 UV/Vis spectrophotometer (Thermo Scientific) equipped with a
high temperature reaction chamber and a Praying Mantis diffuse
reflectance accessory. The samples were pre-treated in situ in
3
À 1
30 cm min He flow at 550°C for 1 h. The spectra were collected
(F)/SSZ-13, where the letter(s) within parenthesis refers to the anion
in He flow at 550
°
C in order to ensure the dehydrated state of the
of the used cobalt salt, namely, to chloride (Cl), nitrate (N),
acetylacetonate (Acac), acetate (A), and formate (F).
samples. BaSO4 (Alfa-Aesar, Puratronic, 99,998%) was used as
reference material. The reflectance values were converted to
Kubelka-Munk function, F(R ), using Eq. 7, where R means the
1
1
reflectance from a semi-infinite layer:
Characterization
The SSIE process was studied by thermogravimetry-differential
scanning calorimetry-mass spectrometry (TG-DSC-MS) technique
using a Setaram Labsys Evo (Lyon, France) thermal analysis system.
2
ð1 À R Þ
1
FðR Þ ¼
(7)
1
2
R
1
Finely dispersed mixtures of Co-salt and NH -SSZ-13 with the same
4
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Catalysts were characterized also by infrared spectroscopy. Spectra
were recorded by type Nicolet 6700 Fourier-Transform Infrared
Spectrometer (FTIR). Self-supporting pellets were prepared and pre-
treated in situ in the IR cell at 500°C under dynamic vacuum
could be detected throughout the catalytic tests, the N O
concentration was omitted from Eq 10.
2
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
À 6
(10 mbar) for 1 h and let to cool to room temperature in vacuum.
Acknowledgements
The cobalt species were characterized by the infrared spectra of
adsorbed CO. Spectra were collected at room temperature (64
À 1
scans, 2 cm resolution).
This work was supported by the National Research, Development
and Innovation Office of Hungary (grant numbers OTKA PD-
1
15898 and 2018-2.1.11-TÉT-SI-2018-00011). The financial support
Catalytic Tests
provided by the European Union and the State of Hungary, co-
financed by the European Regional Development Fund in the
framework of the project No. VEKOP-2.3.2-16-2017-00013, and the
support of the project of the Economic Development and
Innovation Operative Program of Hungary, GINOP-2.3.2-15-2016-
00053 are kindly acknowledged.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
The catalyst sample (100 mg, particle size: 0.25–0.50 mm) was
placed into a quartz tube reactor (I.D.=4 mm) and was pre-treated
3
À 1
in a He flow (30 cm min ) at 700°C for 1 h. The catalytic activity
was determined in the temperature range of 300–700°C at a gas
À 1
hourly space velocity (GHSV) of 30,000 h (space time 0.12 s). The
reactant gas mixture contained 4000 ppm NO, 4000 ppm CH , and
4
2
% O in He (balance). The hydrothermal stability of the catalysts
2
À 1
was studied at 550°C at GHSV of 15,000 h by changing the input
reactant gas mixture to a wet reactant mixture containing 5.5 vol%
Conflict of Interest
H O. The input change was repeated in 40 min intervals. The wet
2
gas was made by feeding water directly into the hot zone of the
quartz reactor above the catalyst bed using a micro metering
syringe pump (Infusens® 5188.21 A, MTA KUTESZ). When water was
added to the reaction mixture, the total flow rate was maintained
constant by decreasing the flow of the He balance and thus
maintaining the partial pressures of the reactants constant.
The authors declare no conflict of interest.
Keywords: NO-SCR
·
solid-state
ion
exchange
·
thermochemistry · hydrothermal stability · zeolite SSZ-13
The effluent gas of the catalytic reactor was continuously analyzed
by on-line mass spectrometer (MS, VG ProLab, Fisher Scientific) by
monitoring the following characteristic masses (m/z): 4 (for He), 12
[
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NOꢀ À ½NOꢀ
(8)
(9)
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XNO
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NOꢀ
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