Activity of Co ion sites in ZSM-5, ferrierite, and mordenite in selective catalytic reduction of NO with methane

Article abstract of DOI:10.1006/jcat.2000.2925

The SCR of NO with methane (CH₄-SCR) in an excess of oxygen was investigated over CoH-, CoNa-, CoNaK-, CoBa- and Co_xO_y-Co-zeolites of ZSM-5, ferrierite, and mordenite structures. The exchanged Co ions exhibited activity in CH₄-SCR of NO while protons or oxide-like Co species in Co-zeolites contributed to the CH₄-SCR activity mostly by enhancement of oxidation of NO to NO₂. The sequence of Co-zeolites activity in CH₄-SCR of NO (per zeolite weight) was Co-ZSM-5 = Co-ferrierite >> Co-mordenite for the highest Co loadings at Si/Al of 12, 8.4 and 9.2. For the average Co ion site in the zeolite (in terms of turnover frequency, TOF, values) it was Co-ZSM-5 > Co-ferrierite >> Co-mordenite. From a comparison of the dependence of TOF values for NO conversion to N₂ per Co ion and of the relative concentrations of the exchanged α-, β- and γ-type Co ions on the total Co concentration, the most active Co sites were estimated. The α-type Co ions, bound to framework oxygens of the wall of the main channel for mordenite and ferrierite, were the most active. In ZSM-5, the β-type Co ions, coordinated to the deformed six-membered ring at the intersection of straight and sinusoidal channels showed the highest activity. The main factors contributing to individual Co ion activity were cation location in the inner zeolite volume, coordination to the framework and distances between Co sites.

Full text of DOI:10.1006/jcat.2000.2925

Journal of Catalysis 194, 318–329 (2000)
doi:10.1006/jcat.2000.2925, available online at http://www.idealibrary.com on
Activity of Co Ion Sites in ZSM-5, Ferrierite, and Mordenite in Selective
Catalytic Reduction of NO with Methane
Dalibor Kaucky´, Alena Vondrova´, Jirˇ´ı Deˇdecˇek, and Blanka Wichterlova´1
J. Heyrovsky´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejsˇkova 3, CZ-182 23 Prague 8, Czech Republic
Received February 7, 2000; revised May 8, 2000; accepted May 12, 2000
Such activity, stable in the time on stream, was not reported
either with these cations implanted in zeolites with frame-
works containing high concentrations of aluminium (Si/Al,
ca. 2.5) or for cobalt oxides dispersed on inorganic amor-
phous carriers. It has also been found that the CH4-SCR
activity of Co–ferrierites and Co–ZSM-5 is not linearly pro-
portional to the concentration of the Co ions (4, 5). This
indicated the presence of Co ions with different activities.
All these findings point out that siting and coordination of
the Co ions at different cationic sites of zeolites might be an
important factor affecting their catalytic activity. However,
siting and coordination of the Co ions to framework oxy-
gens and of transition metal ions generally, in pentasil ring
zeolites have not been known and have not been accessible
from the standard XRD analysis (cf. Ref. (10)).
Our approach to estimation of the cationic sites of Co
ions in mordenite, ferrierite, and ZSM-5 zeolites was based
on the determination of characteristic Vis spectra of the
Co(II) ions in these zeolites (11–13) and local framework
structures known from XRD (10). Three types of cationic
sites for the Co(II) ions (denoted as , , and ) were sug-
gested in mordenite, ferrierite, and ZSM-5 structures (see
the Discussion, Figs. 12 and 13). The distribution of the
individual Co ion types in these zeolites was determined
by quantitative analysis of the Co ion Vis spectra (11–13).
Investigations of the perturbation of the local framework
T–O–T bonds hosting the -, -, and -type Co ion sites by
monitoring antisymmetric T–O–T IR vibrations in a skele-
tal window indicated the sequence in the strength of bond-
ing of these Co ions to framework oxygens at the cationic
sites (3, 14, 15) to be Co- < Co- < Co- . Details on the
structure and location of the individual Co ions are given
in the preceding paper (16).
Selective catalytic reduction of NO with methane (CH4-SCR) in
an excess of oxygen was investigated over CoH–, CoNa–, CoNaK–,
CoBa–, and CoxOy–Co-zeolites of ZSM-5, ferrierite, and mordenite
structures. The exchanged Co ions have been shown to exhibit ac-
tivity in CH4-SCR of NO, while protons or oxide-like Co species in
Co–zeolites contribute to the CH4-SCR activity mostly by enhance-
ment of the oxidation of NO to NO2. The sequence of Co–zeolites
activity in CH -SCR of NO (per zeolite weight) was Co–ZSM-5
=
4
Co–ferrierite Co–mordenite for the highest Co loadings at Si/Al
of 12, 8.4, and 9.2, but for the average Co ion site in the zeolite
(in terms of turnover frequency, TOF, values) it was Co–ZSM-5 >
Co–ferrierite
Co–mordenite. From a comparison of the depen-
dence of TOF values for NO conversion to N2 per Co ion and of
the relative concentrations of the exchanged -, -, and -type Co
ions (determined from quantitative analysis of the characteristic
Co(II) Vis spectra) on the total Co concentration, the most active
Co sites were estimated. The -type Co ions, bound to framework
oxygens of the wall of the main channel of mordenite and ferrierite,
are the most active among the Co ions in these zeolites. On the other
hand, in ZSM-5 the -type Co ions, coordinated to the deformed
six-membered ring at the intersection of straight and sinusoidal
channels possess the highest activity. The main factors contributing
to the individual Co ions’ activity are suggested to be cation loca-
tion in the inner zeolite volume, coordination to the framework, and
c
distances between the Co ions.
2000 Academic Press
Key Words: NO; SCR of NO; Co zeolites; Co ions; Co–ZSM-5;
Co–ferrierite; Co–mordenite.
INTRODUCTION
SCR of NOx by low paraffins, including methane, in an
excess of oxygen has appeared to be a promising method
for abatement of NOx from flue exhaust gases, as produced
by combustion processes (1–3). Ion-exchanged Co–ZSM-5,
Co–ferrierite (4–6), and Ga– and In–ZSM-5 zeolites (7–9)
containing low amounts of aluminium (Si/Al molar ratio >
8) and pentasil rings in their frameworks have been shown
to exhibit high activity and selectivity to nitrogen in CH4-
SCR of NO in an acceptable temperature range, 350–500 C.
Different types of bonding of the -, -, and -type Co
ions in pentasil ring zeolites have been shown to be re-
flected also in their different reactivities with respect to
NO bonding (16) and activity in CH4-SCR of NO (17), as
demonstrated for Co–ferrierites. The most open and weakly
bound -type Co ions in ferrierite were the most active and
exhibited a high tendency to the formation of dinitrosyls
compared to the other types of Co ions.
1 To whom correspondence should be addressed. E-mail: wichterl@
jh-inst.cas.cz.
318
0021-9517/00 $35.00
c
Copyright
2000 by Academic Press
All rights of reproduction in any form reserved.

Co ION SITES IN ZSM-5, FERRIERITE, AND MORDENITE
319
CH4-SCR of NO with methane is a complex reaction, Co ion exchange for Co–FER and Co–MFI zeolites are
where an excess of oxygen in the reaction mixture is a nec- given in Refs. (12) and (13), respectively.
essary condition for the catalytic reaction performance (4,
CoH–MOR (3.55 wt% Co) was prepared from H–MOR
5, 18, 19). This observation and investigations of the CH4- of chemical composition (in wt% ) 89.4 SiO2, 8.98 Al2O3,
SCR with NO2 led to the conclusion that the oxidation of and 1.58 H2O (Si/Al = 8.5) by ion exchange with 0.1 M
NO to NO2 represents the first step of the process, followed Co(NO3)2 at RT, followed by calcination in air at 500 C and
by the reduction of NOx by methane, which is at the same repeated Co ion exchange. CoBa–MOR (0.87 wt% Co) was
time oxidized to CO2 and/or CO (4, 5, 17, 18). While oxygen prepared byion exchange ofH–MOR with 0.1M Ba(NO3)2,
is necessary to produce NO2, on the other hand, it competes calcination in oxygen at 500 C, and ion exchange with 0.1 M
with NOx for methane oxidation. Single exchanged Co ions Co(NO3)2. KCo–MOR (0.50 wt% Co) was prepared from
in ZSM-5 and ferrierite were suggested to be responsible H–MOR by ion exchange with 0.1 M Co(NO3)2 at RT, cal-
for SCR of NOx by methane, while oxidic CoxOy species cination in oxygen at 500 C, and ion exchange with 0.05 M
were considered to provide activity for oxidation of NO to KNO3. Details of the preparation of Co–MOR zeolites are
NO2 (20).
given in Ref. (11).
This paper analyzes and compares the activity and selec-
Cobalt oxide-like speciesin Co–FER were prepared from
tivity of Co ions exchanged in zeolites with MFI, FER, and the ion-exchanged CoNaK–FER/0.12 by dosing 0.01 M
MOR structures in CH4-SCR of NO. An attempt is made NaOH into a suspension of CoNaK–FER in water to keep
to determine and understand the activity of the Co ions at the pH at 10. CoxOy(OH)z species were expected to be
different cationic sites of these zeolites from the viewpoint formed and converted by calcination at a temperature of
of their coordination and bonding to framework oxygens 500 C into undefined oxide-like CoxOy species; the zeo-
as well as location in the zeolite structures. To evaluate lite was denoted as CoxOy–Co–FER. Chemical analysis of
the CH4-SCR activity of the individual Co ions at differ- Co–zeolites, after their dissolution, was done by atomic ab-
ent sites, the contribution to this reaction of the protonic sorption spectrometry and is given in Table 1. The chemical
sites and “supported” cobalt oxide-like species, in some composition of a zeolite, denoted as, e.g., CoNH4–MFI/0.5,
cases present with the Co ions in zeolites, has also been means cobalt ions exchanged into the NH4 form of a zeolite,
investigated.
Co/Al molar ratio of 0.5.
TABLE 1
Chemical Composition of Co–Zeolites on Dry Bases
EXPERIMENTAL
Catalyst Preparation
Zeolite
Co wt%
Co/Al
Si/Al
NaK–FER of chemical composition (in wt% ) 85.4 SiO2,
8.62Al2O3, 2.65Na2O, 3.24K2O, and 0.14H2O (Si/Al = 8.4),
NH4–MFI zeolite (both TOSOH Corp., Japan) of com-
position (in wt% ) 92.6 SiO2, 6.29 Al2O3, and 1.12 H2O
(Si/Al = 12.5), and Na–MFI zeolite (kindly provided by
the Institute of Oil and Hydrocarbon Gases, Slovakia)
of composition (in wt% ) 91.2 SiO2, 5.49 Al2O3, and 3.24
Na2O (Si/Al = 14.1) were used as parent zeolites. NH4–
FER was prepared from NaK–FER by ion exchange at
room temperature (RT) with 1.0 M NH4NO3; see Ref.
(16). The XRD proved the existence of a pure, crys-
talline zeolite phase. Transmission electron micrographs
showed small crystals (average size 1–2 m) of parent
zeolites.
CoNH4–FER
CoNH4–FER
CoNH4–FER
CoNH4–FER
CoNH4–FER
CoNaK–FER
CoNaK–FER
CoNaK–FER
CoNaK–FER
CoNaK–FER
CoNaK–FER
CoNaK–FER
CoNaK–FER
0.40
0.63
2.08
2.16
3.62
0.12
0.27
0.52
0.64
1.01
1.52
2.47
2.60
0.05
0.07
0.23
0.25
0.42
0.01
0.03
0.06
0.07
0.12
0.18
0.28
0.31
8.4
8.4
8.4
8.4
8.4
8.4
8.4
8.4
8.4
8.4
8.4
8.4
8.4
CoNH4–MFI
CoNH4–MFI
CoNH4–MFI
CoNH4–MFI
CoNH4–MFI
CoNa–MFI
CoNa–MFI
0.94
1.55
2.87
3.14
3.83
0.50
0.54
0.83
1.21
2.29
0.13
0.23
0.42
0.46
0.55
0.09
0.10
0.15
0.22
0.41
12.5
12.5
12.5
12.5
12.5
14.1
14.1
14.1
14.1
14.1
CoNH4–FER (0.40–3.62 wt% Co), CoNaK–FER (0.12–
2.60 wt% Co) and CoNH4–MFI (0.94–3.83 wt% Co),
CoNa–MFI (0.50–2.29 wt% Co) were prepared by Co
ion exchange of the corresponding parent zeolites with
Co(NO3)2 or Co(acetate)2 solutions at RT or 70 C; the lat- CoNa–MFI
CoNa–MFI
CoNa–MFI
ter temperature was used to obtain highly loaded samples.
The pH of the solution with a zeolite was kept at a con-
stant value of 5.5 during ion exchange by dosing with 0.01
CoK–MOR
0.50
0.87
3.55
0.06
0.10
0.35
8.5
8.5
8.5
CoBa–MOR
M HNO3. Finally, the zeolites were thoroughly washed with
CoH–MOR
distilled water and dried in air. Detailed parameters of the

´
KAUCKY ET AL.
320
Catalyst Characterization
5890 II, equipped with a fused silica column (molecular
˚
sieve, 5 A; length, 30 m; i.d., 0.53 mm; inner coating, 25 m).
Diffuse reflectance Vis spectra of Co-zeolites (dehy-
drated at 480 C) were collected by a Perkin–Elmer Lambda
19 UV–Vis–NIR spectrometer equipped with a diffuse re-
flectance attachment with an integrating sphere coated by
BaSO4. The parent zeolite, treated under the same con-
ditions as the Co–zeolites, served as a reference. The Vis
spectra of Co–MOR, Co–FER, and Co–MFI are given in
Refs. (11), (12), and (13), respectively. The relative concen-
trations of the -, -, and -type Co(II) ions in these ze-
olites were determined from the integral intensities of the
corresponding Vis spectral components and absorption co-
efficients of the individual Co ions in mordenite, ferrierite,
and ZSM-5 (for details on the spectra recording and quan-
titative analysis, see Refs. (11–13).
IR spectra of Co-zeolites (dehydrated at 480 C) in the
region of OH groups, 4000–3000 cm 1, were monitored to
determine the concentration of protonic sites. The spec-
tra were recorded on thin transparent plates (thickness
ca. 7 mg cm 2) placed in a heatable cell connected to a vac-
uum system at ambient temperature on a Nicolet Magna-
IR system 550 FT-IR spectrometer, with a MCT-B detector
cooled with liquid nitrogen. From the integral intensities
of the band of the bridging OH groups at 3610 (MFI and
MOR) and 3604 cm 1 (FER) and by using the extinction co-
efficient given in Ref. (21), the concentration of the protonic
sites relative to the total number of available exchangeable
sites was determined.
Other products, HCN, organic oxygenates, or isocyanate-
like substances, were checked for selected experiments by
an off-line gas chromatographic analyzer with a mass selec-
tive detector.
The conversion values were defined as
ci0 ci
ci0
xi (% ) =
100
[1]
for total conversion of NO and CH4; i.e., xNO and xCH
,
4
where ci0 is the concentration of the reactant (NO or CH4)
before and ci is the concentration of the reactant after the
reaction.
Conversion of NO to N2 and NO2 or CH4 to CO or CO2
was given by the equation
cj
ci0
xj (% ) =
100,
[2]
where cj means the concentration of NO2, 2xN2 or CO2, CO
in the reactor outlet.
The turnover frequency (TOF) was defined as molecules
of NO converted to N2 per second and Co atom present in
a zeolite.
RESULTS
Depending on the reaction conditions, the CH4-SCR of
NO over H–, CoH–, and CoNa–zeolites resulted in the for-
mation of N2, NO2, CO, and CO2. No N2O, organic oxy-
genates, or nitrogen-containing products were detected in
the gaseous phase.
Catalytic Experiments
SCR of NO with CH4 on Co-zeolites was carried out in
a quartz through-flow microreactor, typically with 400 mg
of the catalyst. The reactant stream, containing 900 ppm
NO, 1200 ppm CH4, 25,000 ppm O2, and the rest helium,
was kept constant at a total flow of 100 ml/min. The cor-
responding GHSV was 7500 h 1, assuming a specific cata-
lyst density of 0.5 g cm 3. The catalyst was pretreated in
a helium stream at 450 C for 1 h. Then, the reactor was
cooled down to a temperature of 200–300 C and contacted
with a flow of the reaction mixture. A catalytic test was
then carried out typically for 2 h; NO conversion reached
a constant value usually within 30 min. After the catalytic
test, the temperature of the reactor was increased (at a rate
of 5 K/min) up to the next selected temperature with the
catalyst under the stream of reactants. In a separate experi-
ment methane was oxidized by gaseous oxygen at a similar
feed composition, i.e., 1000 ppm CH4, 25,000 ppm O2, and
the rest helium at a total flow rate of 100 ml/min.
Conversion of NO in CH4-SCR over H–Zeolites
Parent H–MFI, H–FER, and H–MOR exhibited, with in-
creasing temperature, increasing conversion of NO to nitro-
gen in CH4-SCR (see Figs. 1 and 2). The activity of H–MFI
was higher compared to those of H–MOR and H–FER,
but much lower in comparison with the activity of CoH–
MFI zeolites. In the temperature range of 200–400 C, NO
was preferably oxidized to NO2, but due to the thermo-
dynamic equilibrium, the concentration of NO2 was sup-
pressed above 400 C. Methane was not completely oxidized
to carbon dioxide, and roughly half of it was converted to
carbon monoxide. Na forms of all the zeolites were com-
pletely inactive.
Conversion of NO in CH4-SCR over
Ion-Exchanged Co-Zeolites
The product composition was analyzed as follows. Con-
centrations of NO and NO2 in the inlet and outlet of the re-
Figure 3 shows the dependence of conversion of NO to
actor were monitored by a NO/NOx luminescence analyzer NO2 and N2 and conversion of CH4 to CO2 on CoH–MFI,
(VAMET-CZ). Concentrations of CH4, N2, CO, and N2O typical for all the investigated Co-zeolites. With increasing
were determined by a gas chromatograph Hewlett Packard temperature up to 300 C, the converted NO was preferably

Co ION SITES IN ZSM-5, FERRIERITE, AND MORDENITE
321
FIG. 3. Dependence of conversion of (— —) NO and (— —) CH
❒
∇
4
FIG. 1. Dependence of conversion of (— —) NO, (— —) NO to
❒
1
and of NO (—❍—) to N2 and to (— —) NO in SCR of NO with CH4
1
2
NO , (—❍—) NO to N , (— —) CH , and (— —) CH4 to CO on tem-
∇
♦
2
2
4
on temperature over CoH–MFI/0.55.
perature in SCR of NO with CH4 over H–MFI.
(Fig. 5), conversion of NO to nitrogen increased and the
maxima of the conversion vs temperature profiles were
shifted to lower temperatures. The temperature profiles
for zeolites differing in cobalt concentration indicate that
oxidized to NO2. At this temperature SCR of NO to nitro-
gen started to take place, being accompanied by the oxida-
tion of CH4 to CO2; only for Co–MOR was carbon monox-
ide, as well as CO2, observed in the products. Maximum
conversion of NO to N2 was attained at ca. 400 C, when
conversion of NO to NO2 approached zero. Above this
temperature, the conversion of NO decreased, while con-
version of methane was continuously increased, because of
its oxidation by gaseous oxygen. If the reaction tempera-
ture was again decreased below that exhibiting maximum
conversion, the original conversion values were restored,
indicating that the state of cobalt in the zeolite was pre-
served. This effect was observed for all the Co-zeolites.
With increasingconcentration ofthe Co ionsin CoH–and
CoNa–ZSM-5 (Fig. 4) and in CoH– and CoNaK–ferrierite
FIG. 4. Dependence of conversion of NO to N2 in SCR of NO with
FIG. 2. Dependence of conversion of NO to N2 on temperature
CH4 on temperature and Co/Al loading for (a) CoNa–MFI (— —) 0.41,
❒
in SCR of NO with CH4 over (—❍—) H–MFI, (— —) H–FER, and
(—❍—) 0.22, (— —) 0.15, (— —) 0.10, (— —) 0.09; (b) CoH–MFI
❒
1
∇
♦
(— —) H–MOR.
(— —) 0.55, (—❍—) 0.46, (— —) 0.42, (— —) 0.23, (— —) 0.13.
❒ 1
1
∇
♦

´
KAUCKY ET AL.
322
TABLE 2
Concentrations of the Individual Co Ions and Co Ions
in CoxOy–Co–FER and in “Parent” CoNa–FER/0.12
Zeolite
Co( )/Al Co( )/Al Co( )/Al Co/Al Co(CoxOy)/Al
CoNaK–
FER/0.12
CoxOy–Co–
FER/0.12
0.02
0.02
0.09
0.06
0.01
0.01
0.12
0.1
—
0.03
Conversion of NO in CH4-SCR over Co-Zeolites
of MFI, FER, and MOR Topologies
Figure 7 compares conversion of NO to nitrogen depend-
ing on the temperature for Co-zeolites of different topolo-
gies at low and high concentrations of the Co ions. At low
Co loadings, Co–MFI exhibited convincingly a higher ac-
tivity than Co–FER. The lowest activity was found with
Co–MOR. It is to be noted that the conversion values of
the low Co-loaded zeolites were not affected by protonic
sites, as these were not present in the CoNa-zeolites. At
high Co loadings of about Co/Al 0.4, i.e., the highest load-
ing which can be achieved for these exchanged Co-zeolites,
the activity of Co–FER and Co–MFI was comparable, but
that of Co–MOR was substantially lower. Moreover, low
activity of Co–MOR in CH4-SCR of NO above 500 C was
also manifested in ca. 30% selectivity to CO, in contrast
to Co–FER and Co–MFI, where the exclusive formation of
CO2 was observed. However, as ferrierite possessed a lower
Si/Al framework ratio, and thus a higher weight content of
cobalt than Co–MFI (see Table 1 and Fig. 7), it followed
that the Co ions in the MFI structure, even at high loadings,
exhibited the highest activity like that at the low cobalt
concentration. Thus, the CH4-SCR activity of the Co ions
FIG. 5. Dependence of conversion of NO to N2 in SCR of NO with
CH4 on temperature and Co/Al loading for (a) CoNaK–FER (— —)
❒
0.31, (—❍—) 0.28, (— —) 0.18, (— —) 0.12, (— —) 0.07, (— —)
1
∇
♦
᭿
0.06, (— —) 0.03, and (— —) 0.01; (b) CoH–FER (— —) 0.42,
᭹
᭡
❒
(—❍—) 0.25, (— —) 0.23, (— —) 0.07, (— —) 0.05.
1
∇
♦
the conversion of NO to nitrogen is not linearly propor-
tional to the Co content in zeolites. This implies the pres-
ence of Co ions with different activity. It is also seen that
the CoH-zeolites exhibit a higher conversion than CoNa-
zeolites at a similar Co concentration. This difference be-
tween CoH and CoNa forms of both zeolite structures
is higher at low Co loadings than at high loadings. The
effect is more pronounced with Co–ferrierites than with
Co–ZSM-5.
Conversion of NO in CH4-SCR of NO
over CoxOy–Co–Zeolites
To elucidate the effect of dispersed cobalt oxide-like
species on the SCR activity of Co-zeolites, possibly present
in the highly loaded zeolites, such cobalt species were pre-
pared in CoNa–ferrierite by transformation of a part of the
exchanged Co ions into CoxOy species (see Table 2). The
CoxOy–Co–FER exhibited much higher conversion of NO
to NO2 compared to CoNa–FER, but its CH4-SCR activity
FIG. 6. Dependence of conversion of NO to (—❍—) and (— —) N
∇
2
and to (— —) and (— —) NO in SCR of NO with CH on temperature
1
❒
2
4
in NO conversion to nitrogen was not different (see Fig. 6). for CoNaK–FER/0.12 and CoxOy–Co–FER /0.12 respectively.

Co ION SITES IN ZSM-5, FERRIERITE, AND MORDENITE
323
though the concentration of Co ions in the latter zeolite was
higher than that in the former (see Table 1).
Turnover Frequency (TOF) Values of the CH4-SCR
over Co Ions in Zeolites
To compare the activity of the individual Co ions in CH4-
SCR of NO to nitrogen, it was necessary to choose a tem-
perature at which the conversion value is below ca. 20%
and below the inflection point of the conversion vs tem-
perature profile, where the Arrhenius rate equation can
be assumed to be satisfied. As this can hardly be done for
the entire Co concentration range in the zeolite, we trans-
formed the dependence of NO conversion vs temperature
into an Arrhenius plot, log xN vs 1/T . We used the value
2
of xN obtained from the linear part of this dependence to
2
FIG. 7. Dependence of conversion of NO to N2 in SCR of NO
with CH4 for (— —) CoNa–MFI/0.10, (—❍—) CoNaK–FER/0.12, and
calculate the reaction rates and related TOF values. This
is illustrated for CoNa–FER/0.28 in Fig. 9. In cases where
the conversion values were out of the limit given above for
the given temperature, the extrapolated conversion values
taken from a dashed line in Fig. 9 were used.
❒
(— —) CoBa–MOR/0.10 and (— —) CoH–MFI/0.42, (— —) CoH–
1
᭿
᭹
FER/0.42, and (— —) CoH–MOR /0.35.
᭡
is very different, depending on the topology of the zeolite
matrix.
The obtained TOF valuesare plotted against Co/Alratios
for CoNaK–, CoH–FER and CoNa–, CoH–MFI zeolites in
Figs. 10 and 11, respectively. It is seen that the TOF values
do not exhibit a constant value, but depend on the Co/Al
concentration. These dependences are different for CoH
and CoNa forms of zeolites and also for FER and MFI
structures. This observation clearly evidenced that the Co
ions in the studied zeolites exhibit quite different activities,
with respect not only to different zeolite structures but also
to the Co ions present in one zeolite type.
Conversion of CH4 over H–, CoH–, and CoNa–Zeolites
Methane oxidation by oxygen on H–ZSM-5 proceeded
at temperatures above 500 C with incomplete oxidation
and roughly 30% of methane was oxidized only to car-
bon monoxide. Both the CoNa– and CoH–zeolites exhib-
ited much higher activity, resulting also in oxidation of
methane to carbon dioxide, and the conversion did not de-
pend on whether the Co-zeolites originated from the par-
ent Na- or NH4-zeolites (Fig. 8). Surprisingly, the activity of
Co–ZSM-5 was much higher than that of Co–ferrierite, al-
To analyze the activity of the individual Co ions at dif-
ferent cationic sites ( , , and ), we used the previously
determined distribution of the individual -, -, and
-
type Co ions in Co–ZSM-5 and Co–ferrierite samples (see
FIG. 8. Dependence ofconversion ofCH4 oxidation byoxygen to CO2
on temperature for (— —) CoH–MFI/0.46, (—❍—) CoNaK–MFI/0.41,
FIG. 9. Arrhenius plot of dependence of conversion of NO to N2,
❒
(— —) CoH–FER/0.42, and (— —) CoNaK–FER/0.31 and for H–MFI
log(xN ) on 1/T , over (— —) CoNa–FER/0.28 and (- - - -) its linear
᭿
᭹
❒
2
(— —) conversion of CH to (— —) CO and (— —) CO.
approximation.
1
∇
♦
4
2

´
KAUCKY ET AL.
324
Co loading strongly depends on the type of co-cation (H+
vs Na+) present in the zeolite and on the zeolite structure.
This is discussed in detail elsewhere (12, 13) where the de-
termination of the individual
, , and Co ion cationic
sites in FER and MFI structures and their population by
the Co ions was done.
With Co–ferrierites, the TOF value increases with in-
creasing Co concentration, but at Co/Al above 0.3 the pres-
ence of some CoxOy species can be a reason for decreasing
TOF values. The same trend as that of TOF vs Co/Al ex-
hibits the relative concentration of the -type Co ions. For
CoNaK–FER the decreasing TOF values with increasing
Co/Al are also in agreement with the decreasing concen-
tration of the -type Co ions. At Co/Al above 0.15, when,
besides the Co ions, protons were also exchanged into fer-
rierite, their presence might be a reason for an apparent
increase in the activity of the Co ions. Therefore, for a com-
parison of TOF values and relative concentrations of the
FIG. 10. Dependence of TOF, relative concentration of the -, -, and
-type Co ions and relative concentration of H+ ions to the total number of
exchangeable sites, on Co/Al in CoH–FER (TOF at 400 C) and CoNaK–
FER (TOF at 400 C). Bold lines indicate region taken for evaluation of
this dependence.
Refs. (13) and (12), respectively); for additional samples
the Co(II) Vis spectra (not shown in the figure) were mon-
itored and analyzed according to the procedures described
in Refs. (12) and (13). The concentration of protons in
Co-zeolites was determined from the quantitative analy-
sis of the IR bands at 3610 and 3604 cm 1, corresponding
to the bridging SiOHAl groups in ZSM-5 and ferrierite, re-
spectively, and using extinction coefficients given elsewhere
(21). With increasing Co ion exchange degree, the concen-
tration of protons in CoH–zeolites decreased. In CoNa-
zeolites at higher Co loadings, the protons, as well as the
Co ions, were also exchanged (see Figs. 10 and 11).
Figures 10 and 11 relate the TOF values in CH4-SCR of
NO to nitrogen for CoH- and CoNa-zeolites of FER and
MFI structures, the relative concentrations of the -, -, and
-type Co ions, and the concentration of the protonic sites
(relative to the total number of exchangeable sites) to the
cobalt concentration (Co/Al) in zeolites. The population of
FIG. 11. Dependence of TOF, relative concentration of the -, -, and
-type Co ions and relative concentration of H+ ions to the total number
of exchangeable sites, on Co/Al in CoH–MFI (TOF at 350 C) and CoNa–
MFI (TOF at 370 C). Bold lines indicate region taken for evaluation of
the , , and cationic sites by the Co ions with increasing this dependence.

Co ION SITES IN ZSM-5, FERRIERITE, AND MORDENITE
325
TABLE 3
lows the sequence H–MFI H–FER > H–MOR, although
H–FER and H–MOR contain many more protonic sites
(1.70 compared to 1.05 mmol/g for H–MFI). This sequence
is analogous to that one found for CH4-SCR of NO and
methane oxidation with Co-zeolites, showing the strong in-
fluence of zeolite structure on zeolite activity, even though
factors other than zeolite topology may be involved.
Contribution ofprotonicsitesto the SCR activityofCoH-
zeolites is obvious, as it follows from a comparison of the
activity of Co-zeolites given in parts (a) and (b) of Figs. 4
and 5 and in Figs. 10 and 11. For both MFI and FER zeolite
structures, the activity of CoH-zeolites is higher compared
to that of CoNa forms. As the SCR activity of protons is very
low, it implies that they contribute to this reaction mostly
via enhancement of the rate of oxidation of NO to NO2.
A similar function might be ascribed to cobalt oxide-like
species present, besides the Co ions, in highly loaded zeo-
lites. As shown in Fig. 6, the presence of cobalt oxide-like
species in the zeolite increases substantially NO conver-
sion to NO2 at a lower temperature and thus the rate of
NO2 formation. This can be expected to enhance the rate
of SCR of NO, the result of which is the same NO conver-
sion for the zeolite containing a lower concentration of the
exchanged Co ions and some CoxOy species. An increase in
the rate of the SCR reaction for Co-zeolites containing, in
addition to the Co ions, dispersed cobalt oxide species was
also reported by Stakheev et al. (20). To summarize, both
the protonic sites and cobalt-oxide-like species contribute
to CH4-SCR of NO by enhancement of the rate of NO2 for-
mation; however, their own activity in the SCR reaction is
low.
To understand the different activities of the Co zeolites
with different topologies, we have attempted to analyze the
activity of the individual Co ions in zeolites of MFI, FER,
and MOR structures. This has been enabled by knowledge
of the distribution of the exchanged Co ions among the in-
dividual cationic sites in zeolites, as obtained from quanti-
tative analysis of the characteristic Vis spectra of the Co(II)
ions. It was evidenced that the ion-exchanged Co-zeolites
up to the concentration of Co/Al 0.35, used in this study, did
not contain cobalt oxide species, which might be possibly
formed by hydrolysis of cobalt salt solutions during ion ex-
change. The presence of exclusively exchanged Co ions in
low Co-loaded zeolites follows from the quantitative anal-
ysis of the IR bands of antisymmetric T–O–T vibrations in
Turnover Frequency (TOF) in CH4-SCR of NO and Concentra-
tion of the - and -Type Co Ions in CoK–MOR/0.06 and CoBa–
MOR/0.10; Conversion of NO at 538 C at Conditions of the Cata-
lytic Test
1
Co/Al Co (wt% ) xN (% ) TOF (s
)
(% )
(% )
(% )
15
2
5
5
0.06
0.10
0.50
0.87
3.60
10.90
6.7 10
11.6 10
25
60
60
40
Note. Calculated TOF ( ) = 17.9 10 5 and TOF ( ) = 2.1 10 5 s
.
1
Low concentration of the -type Co ions is not included in the calculation.
-, -, and -type Co ions depending on the total Co con-
centration, the zeolites with low concentrations of Co ions,
where the presence of CoxOy species was excluded and
trends of TOF values on Co/Al were not substantially af-
fected by too high differences in the concentration of pro-
tons, were selected.
An analogous analysis, under the same conditions, was
done for Co–MFI zeolites. Again, a different dependence
of TOF values, as well as a different distribution of Co ions
among
, -, and -sites on Co/Al concentration, was ob-
tained for CoNa– and CoH–MFI zeolites. In contrast to
Co–FER, with both the CoH– and CoNa–MFI zeolites the
same trend in the dependence of TOF values on cobalt con-
centration was obtained for the concentration of the -type
Co ions.
Table 3 provides TOF values for CH4-SCR of NO on Co–
mordenites with different concentrations of the individual
Co ion types, determined elsewhere (11). In comparison
with the Co–FER and Co–MFI zeolites, Co–MOR cata-
lyzesthe CH4-SCR of NO at much higher temperatures(see
Fig. 7), indicating its general lower activity. Also, oxidation
of ca. 30% of methane to CO reflects low activity of the Co
ions in mordenite. By comparison of the TOF values for
Co–MOR (because of low activity of Co–MOR, at 538 C)
with different distributions of the - and -type Co ions
(low concentration of the -type cations is neglected), it
follows that the -type Co ions exhibit roughly 10 times
higher activity than the types.
DISCUSSION
It isgenerallyaccepted that the SCR ofNO with methane, the region of skeletal vibrations induced by the Co ions sit-
and paraffins generally, takes place via oxidation of NO to uated exclusively at the cationic sites (see Refs. (14) and
NO2, followed by NOx reduction by paraffins to nitrogen (15). Therefore, the reported TOF values for Co zeolites
(3–5, 18, 19). Protonic sites of H–MFI, H–FER, and H– with the cobalt concentration below Co/Al = 0.35 reflect
MOR oxidize NO to NO2 (shown for H–MFI in Fig. 1), the activity of the exchanged Co ions. As already pointed
but their activity in SCR is substantially lower compared out, prevailing importance in the analysis of the activity of
to that of Co–zeolites. This is also manifested in methane the individual Co ions has been given to the results obtained
incomplete oxidation to a mixture of carbon monoxide and with Co-zeolites containing non-transition-metal ions and
dioxide. The activity of H-zeolites in CH4-SCR of NO fol- not protons as co-cations.

´
KAUCKY ET AL.
326
FIG. 12. Suggested cationic sites of the -, -, and -type Co ions in (a) MOR, (b) FER, and (c) and (d) MFI structures (cf. Refs. 11–13, respectively);
(c) view through the straight channel and (d) view through the sinusoidal channel, straight channel indicated by dashed lines.
Figures 12 and 13 illustrate the location of the -, -,
(iii) The -type Co ions are in the so-called “boat-
and -type Co ions in MFI, FER, and MOR frameworks shaped” site of mordenite (site C, see Ref. (10)) and anal-
as analyzed and suggested elsewhere (see Refs. (13), (12), ogous framework local structures in ferrierite and ZSM-5.
and (11), respectively):
This Co site represents the most packed highly coordinated
Co ion with the highest strength of Co ion bonding to frame-
work oxygens.
(i) The -type Co ions are coordinated to the walls of
the main channel of mordenite (site E according to the
notation of Mortier, cf. Ref. (10)) and ferrierite (site B)
It isto be pointed out that the population ofthe individual
and the straight channel of the ZSM-5 structure. They are sites by the Co ions depends on the concentration of cobalt
suggested to be coordinated to framework oxygens of six- in the zeolite, on the presence of co-cations, namely, protons
member rings formed from two-folded pentasil rings. The or non-transition-metal ions, and on the zeolite structural
-type Co ions exhibit the weakest bonding to framework type. With all the zeolite structures studied here, the most
oxygens among the Co ions.
(ii) The -type Co ions are suggested to be coordinated
populated site is the site (50% –70% ), followed by the
site (10% –30% ), and then by the least populated site
to framework oxygens of the twisted eight-member rings of (0% –15% ). Therefore, to simplify the analysis of different
the mordenite cavity (site A), of the deformed six-member activities of Co-zeolites of MFI, FER, and MOR structures,
rings of the ferrierite cavity (site G), and of the deformed we have focused on the activity of the cobalt and sites.
six-member rings at the intersection of the straight and si- It should also be mentioned that all the Co ions in MFI,
nusoidal channels of the ZSM-5 structure. Perturbation of FER, and MOR zeolites are accessible to all reactants and
the framework bonds at the cationic sites by these Co products of the CH4-SCR reaction.
ions indicates medium strength of bonding to framework
oxygens.
From the analysis of the dependence of TOF values and
relative concentrations of the -, -, and -type Co ions on

Co ION SITES IN ZSM-5, FERRIERITE, AND MORDENITE
327
FIG. 13. Schematic representation of cationic sites in the channels of (a) mordenite, (b) ferrierite, and (c) ZSM-5.
cobalt content in ferrierite and ZSM-5 (Figs. 10 and 11) and of the Co ions in CH4-SCR of NO not only the expected
the TOF values for the - and -type Co ions in mordenite strength of bonding of the Co ions to the framework but
(Table 3, see also Figs. 12 and 13), it follows:
also cation location in the inner volume of the zeolite might
be important, although all the Co ions are accessible to
reactants.
(i) In ferrierite and mordenite, the most active sites are
suggested to be the -type Co ions. These ions are coordi-
nated to framework oxygens in the main channel of mor-
This implies that the -type Co ions in the MFI structure
denite and the main ten-member ring channel of ferrierite. are located at the most advantageous position with respect
The highly active -type Co ions are easily accessible to to the reactant adsorption and formation of reaction inter-
reactants and intermediates and they exhibit the weakest mediates. With MOR and FER structures, the -type Co
bonding to framework oxygens among the individual Co ions are also accessible to the CH4-SCR reactants, but as
ions (cf. Refs. (3), (11), (12), and (15));
(ii) On the other hand, in ZSM-5, surprisingly, the
they are located in the interconnected small channel of mor-
denite and inside the small eight-member ring channel of
-
type Co ions are indicated to be the most active sites. These ferrierite, they might be diffusionally hindered compared
Co sites are coordinated to oxygens in the deformed six- to the -type Co ions located in the main channels of fer-
member ring of the MFI structure (cf. Ref. (13)) like in rierite and mordenite and compared to the -type Co ions
ferrierite. But the site in the MFI structure is located just in the MFI structure.
at the intersection of the straight and sinusoidal channels, in
However, there might also be another factor affecting
contrast to the sites in ferrierite and mordenite, where it is the rate of the catalytic reaction, i.e., distances of the Co
in the eight-member ring and the interconnecting channel, ions, if the SCR reaction proceeds with the participation of
respectively. This observation points out that for the activity the two neighboring sites. It can be assumed that, at one

´
KAUCKY ET AL.
328
site, oxidation of NO to NO2 takes place, and the other site and might also be accessible to the expected intermediates
is involved in the activation of methane. The active sites of the CH4-SCR reaction, but their catalytic activity is quite
might be two Co ions, a Co ion with a proton, or a Co ion different. This finding originates from their coordination at
with a CoxOy species. While the distances between the Co different cationic sites with different strengths of Co ion
ions, and protons or CoxOy species cannot be estimated, a bonding to framework oxygens and different positions of
rough estimation of distances was done for the exchanged the exchanged cations in the inner volume of the zeolite,
Co ions between the most populated Co ion sites, i.e., the given by its topology.
- and -type Co ions in MOR, FER, and MFI structures.
(iv) The most active Co sites in ferrierite and mordenite
The distances were calculated from the XRD data of the are suggested to be the -type Co ions. These ions are coor-
“naked” MOR, FER, and MFI frameworks and under the dinated to framework oxygens in the wall of the main chan-
following geometrical simplifications. The -type Co ions nel of mordenite and the main ten-membered ring channel
are located in the plane of the rectangle of the six-member of ferrierite. They are easily accessible to reactants and in-
ring, forming the wall of the main channel of mordenite termediates and exhibit the weakest bonding to framework
and the straight ten-member ring channel of ZSM-5 and oxygens among the Co ions. The most populated -type
ferrierite, and the -type Co ions are placed in the cen- Co ions in ferrierite and mordenite, located in the eight-
ter of the twisted eight-member ring of mordenite and in member ringchannelsand interconnected narrowchannels,
the center of the deformed six-member ring of ferrierite respectively, might be diffusionally hindered. In addition,
and ZSM-5. The closest Co sites have been found with smaller Co–Co distances for the -type Co ions in ferrierite
˚
the -type Co ions in the MFI structure (ca. 4.7 A), while are suggested to contribute to much higher activities of Co–
in ferrierite the closest sites are represented by the
-
ferierites than at Co–mordenites.
˚
type Co ions (ca. 5.6 A). The distances between the other
(v) In contrast to Co–FER and Co–MOR, with Co–MFI
Co sites are much greater. Therefore, it can be suggested the most populated -type Co ions were indicated to be
that the distances between the Co active sites, differing the most active sites. These Co sites are coordinated to
substantially in the individual studied zeolites, might also oxygens in the deformed six-membered ring of the MFI
contribute to the different activities of the individual Co structure as in ferrierite, but in contrast to ferrierite they
ions and thus to the activities of Co-zeolites with different are located at an advantageous position, at the intersec-
topologies.
tion of the straight and sinusoidal channels. Moreover, the
Co–Co distances between the -type Co ions in ZSM-5 are
the shortest among the three Co ion types and those ions
located in different topologies. All these factors might con-
CONCLUSIONS
From the simultaneous analysis of the distribution of tribute to the activity of the Co ions being highest in ZSM-5
Co ions at three cationic sites, denoted as , and , of zeolite.
,
the presence of protons or sodium as co-cations, and of
cobalt oxide-like species in Co–ZSM-5, Co–ferrierite, and
Co–mordenite, and their conversion in SCR of NO with
methane, the following conclusions can be made:
It can be summarized that the combination of kinetic and
spectroscopic results allowed us to identify the most active
Co sites in the Co-zeolites of MFI, FER, and MOR struc-
tures in CH4-SCR of NO and thus to explain the different
(i) Protons and cobalt oxide-like species contribute to activities of the Co-zeolites with different topologies. This
the CH4-SCR reaction, by enhancement of the rate of oxi- showed that the location of the Co ions in the inner zeo-
dation of NO to NO2.
lite volume, their coordination, and the distances between
(ii) The effect of zeolite topology on the CH4-SCR cata- the Co ions play important roles in their catalytic activity in
lytic activity of the Co ions in zeolites is dramatic. The high- the CH4-SCR ofNO. But the degree ofcontribution ofthese
est activity, in terms of TOF values per average Co ion, individual factors to the resulting activity of the Co ions is
shows the Co ions in ZSM-5 followed by those ions in fer- still not clear and requires further study. The siting of the
rierite and with those ions in mordenite showing the lowest Co ions in the zeolite structure, highly important for their
activity. However, as Co–ferrierite (Si/Al 8.4) can accom- activity, is governed, besides the distribution of aluminium
modate more Co ions, the activity of Co–ZSM-5 (Si/Al 12.5) in the framework, by the zeolite topology providing local
and Co–ferrierite with complete Co loading per unit weight framework structures, which satisfy requirements of the Co
of the Co–zeolite are comparable. Thus, generally, because ions for their coordination.
of the possible synthesis of the ferrierite framework with
Si/Al of ca. 8 compared to ZSM-5 with a minimum Si/Al of
ca. 12, their reaction rates per unit weight of the catalyst in
ACKNOWLEDGMENTS
the CH4-SCR of NO are comparable.
The authors acknowledge financial support of the Academy of Sciences
(iii) The individual -, -, and -type Co ions in all the
studied zeolites (see Fig. 12) are accessible to the reactants and Chemicals, Inc., Pennsylvania.
of the Czech Republic under Project S4040016 and from the Air Products

Co ION SITES IN ZSM-5, FERRIERITE, AND MORDENITE
329
REFERENCES
13. Deˇdecˇek, J., Kaucky´, D., and Wichterlova´, B., Microporous Meso-
porous Mater. 35–36, 483 (2000).
1. Iwamoto, M., and Yahiro, H., Catal. Today 22, 5 (1994).
2. Shelef, M., Chem. Rev. 95, 209 (1995).
14. Sobalı´k, Z., Tvaruzˇkova´, Z., and Wichterlova´, B., J. Phys. Chem. B
102, 1077 (1998).
3. Wichterlova´, B., Deˇdecˇek, J., and Sobal´ık, Z., in “Proceedings 12th
International Zeolite Conference, 1998” (M. M. J. Treacy, B. K.
Marcus, M. E. Bisher, and J. B. Higgins, Eds.), p. 941. Materials Re-
search Society, Warrendale, PA, 1999.
15. Sobalı´k, Z., Deˇdecˇek, J., Ikonnikov, I., and Wichterlova´, B., Micro-
porous Mesoporous Mater. 21, 525 (1998).
16. Sobalı´k, Z., Deˇdecˇek, J., Kaucky´, D., Wichterlova´, B., Drozdova´, L.,
and Prins, R., J. Catal. 194, 330 (2000).
4. Li, Y. J., and Armor, J. N., J. Catal. 150, 376 (1994).
5. Li, Y. J., Slager, T. L., and Armor, J. N., J. Catal. 150, 388 (1994).
6. Li, Y. J., and Armor, J. N., Appl. Catal. B 5, L257 (1995).
7. Li, Y. J., and Armor, J. N., J. Catal. 145, 1 (1994).
8. Armor, J. N., Catal. Today 26, 147 (1995).
9. Yogo, K., Ihara, M., Terasaki, I., and Kikuchi, E., Catal. Lett. 17, 303
(1993).
10. Mortier, W. J., in “Compilation of Extraframework Sites in Zeolites.”
Butterworth, London, 1982.
17. Kaucky´, D., Deˇdecˇek, J., Vondrova´, A., Sobal´ık, Z., and Wichterlova´,
B., Collect. Czech. Chem. Commun. 63, 1781 (1998).
18. Ogura, M., Aratani, N., and Kikuchi, E., in “Progress in Zeolite Micro-
porous Materials” (H. Chon, S.-K. Ihm, and Y. S. Uh, Eds.), Elsevier,
Amsterdam 1997. Stud. Surf. Sci. Catal. 105, 1593 (1997).
19. Lobree, L. J., Aylor, A. W., Reimer, J. A., and Bell, A. T., in “Proceed-
ings, 11th International Congress on Catalysis, Baltimore, 1996 (J. W.
Hightower, W. N. Delgass, E. Iglesia, and A. T. Bell, Eds.). Elsevier,
Amsterdam, 1996. Stud. Surf. Sci. Catal. 101, 661 (1996).
20. Stakheev, A. Y., Lee, C. W., Park, S. J., and Chong, P. J., Catal. Lett. 38,
271 (1996).
11. Deˇdecˇek, J., and Wichterlova´, B., J. Phys. Chem. B 103, 1462
(1999).
12. Kaucky´, D., Deˇdecˇek, J., and Wichterlova´, B., Microporous Meso-
porous Mater. 31, 75 (1999).
21. Wichterlova´, B., Tvaruzˇkova´, Z., Sobal´ık, Z., and Sarv, P., Micro-
porous Mesoporous Mater. 24, 223 (1998).