Mechanism and kinetics of the selective NO reduction over Co-ZSM-5 studied by the SSITKA technique: 2. Reactivity of NOx-adsorbed species with methane

Article abstract of DOI:10.1016/j.jcat.2004.03.032

Steady-state isotopic transient kinetic analysis (SSITKA) was applied to study the reactivity of adsorbed NO_x species with methane over Co-ZSM-5 during CH₄-SCR of NO at different partial oxygen concentrations. Numerical analysis of the isotope responses after switching from ¹⁴N¹⁶O to ¹⁵N¹⁸O, from ¹⁶O₂ to ¹⁸O₂, from ¹²CH₄ to ¹³CH₄, and from C ¹⁶O₂ to C¹⁸O₂ in the feed gas was performed. This enables determination of (i) the concentration of different forms of NO_x adspecies on Co sites, (ii) their reaction rates with methane, and (iii) the concentration of surface intermediates resulting from the interaction of NO_x species with methane. The NO reduction with CH₄ in the presence of oxygen was shown to proceed by two different pathways with participation of (1) NO₂^δ+ species (formed on nanoclusters of cobalt oxide located inside the zeolite channels) and (2) NO₂^- nitrite complexes (formed on larger cobalt oxide particles located outside the zeolite channels), the reaction rate by the former being appreciably higher under the SCR reaction conditions studied. Mononitrosyl species appear not to be directly involved in the overall process. Modeling results indicate that the rate of the first route is limited by the interaction of NO₂^δ+ species and adsorbed methane, while in the second route the formation of nitrite species is a rate-determining step. Based on the obtained results, an overall reaction mechanism of the CH₄-SCR of NO in the presence of oxygen is proposed.

Full text of DOI:10.1016/j.jcat.2004.03.032

Journal of Catalysis 225 (2004) 179–189
www.elsevier.com/locate/jcat
Mechanism and kinetics of the selective NO reduction over Co-ZSM-5
studied by the SSITKA technique
2. Reactivity of NOx-adsorbed species with methane
E.M. Sadovskaya,a A.P. Suknev,a L.G. Pinaeva,a V.B. Goncharov,a B.S. Bal’zhinimaev,a
C. Chupin,b J. Pérez-Ramírez,b,1 and C. Mirodatos b,∗
a
Boreskov Institute of Catalysis, pr. Lavrentieva, 5, 630090 Novosibirsk, Russia
b
Institut de Recherches sur la Catalyse, 2 av. A. Einstein, F-69626 Villeurbanne cedex, France
Received 3 January 2004; revised 17 March 2004; accepted 22 March 2004
Available online 12 May 2004
Abstract
Steady-state isotopic transient kinetic analysis (SSITKA) was applied to study the reactivity of adsorbed NO species with methane over
x
Co-ZSM-5 during CH -SCR of NO at different partial oxygen concentrations. Numerical analysis of the isotope responses after switching
4
14 16
15 18
16
18
12
13
16
18
from
N
O to
N
O, from
O to O , from CH to CH , and from C O to C O in the feed gas was performed. This
2 2 4 4 2 2
enables determination of (i) the concentration of different forms of NO adspecies on Co sites, (ii) their reaction rates with methane, and
x
(iii) the concentration of surface intermediates resulting from the interaction of NO species with methane. The NO reduction with CH in
x
4
δ+
the presence of oxygen was shown to proceed by two different pathways with participation of (1) NO
cobalt oxide located inside the zeolite channels) and (2) NO nitrite complexes (formed on larger cobalt oxide particles located outside the
species (formed on nanoclusters of
2
−
2
zeolite channels), the reaction rate by the former being appreciably higher under the SCR reaction conditions studied. Mononitrosyl species
appear not to be directly involved in the overall process. Modeling results indicate that the rate of the first route is limited by the interaction
δ+
of NO
species and adsorbed methane, while in the second route the formation of nitrite species is a rate-determining step. Based on the
2
obtained results, an overall reaction mechanism of the CH -SCR of NO in the presence of oxygen is proposed.
4
2004 Elsevier Inc. All rights reserved.
Keywords: SSITKA; deNO ; CH -SCR; Co-ZSM-5; Kinetics; Mechanism; Cobalt species; Active sites
x
4
1. Introduction
sential to further investigate the elementary steps involved
in the reaction of NOx species with methane during deNOx
CH4-SCR in the presence of oxygen, which has been inves-
tigated in Part II.
In the still widely open research area of NO selective cat-
alytic reduction (SCR) by CH4 instead of NH3, Part I of
our investigation elaborated on a mechanistic understand-
ing of the transformations of NO with oxygen over an ion-
exchanged Co-ZSM-5 catalyst [1]. From that study, the ki-
From the mechanistic investigations reported in the lit-
erature, it is generally postulated that during SCR, surface
NOx species interact with methane to form surface inter-
mediates, which are further converted and decomposed into
the reaction products [2–14]. However, the detailed struc-
ture of these species is unclear. Many authors have spec-
ulated that nitrite–nitrate complexes are involved in the
process [2–8]. However, Wang et al. [13,14] suggested that
adsorbed NO species responsible for methane activation can-
not be ascribed unambiguously to nitrite–nitrate complexes,
and therefore denote these species as NOy. In a previous
in situ DRIFT (diffuse reflectance infrared Fourier trans-
δ+
netics and mechanism of reactive NO2 species, active in-
termediates in the SCR of NOx, as well as the number of
active sites for NO adsorption and their possible location in
the zeolite structure were elucidated. This information is es-
*
Corresponding author. Fax: +33 472 445399.
E-mail address: mirodatos@catalyse.univ-lyon1.fr (C. Mirodatos).
On academic leave from Norsk Hydro, Agri Research Centre, Pors-
1
grunn, Norway.
0021-9517/$ – see front matter
doi:10.1016/j.jcat.2004.03.032
2004 Elsevier Inc. All rights reserved.

180
E.M. Sadovskaya et al. / Journal of Catalysis 225 (2004) 179–189
form) spectroscopic investigation [15], we have observed
that NO2δ+ species were formed in addition to nitrite–nitrate
complexes after NO adsorption in the presence of oxy-
gen over Co-ZSM-5. The intensity of the band ascribed to
sessment of the different elementary steps involved enables
identification of rate-determining steps in the overall SCR
process and to evaluate the concentration of active cobalt
species.
δ+
NO2 species declined much more rapidly than the one of
nitrite–nitrate and mononitrosyl species after methane addi-
tion to a NO + O2 mixture at reaction temperature (400–
500 ◦C). Multiple NO species on isolated Co ions like dini-
trosyls were not identified under the conditions applied.
Based on this key feature, another reaction route of NO re-
2. Experimental
2.1. Catalysts
δ+
duction was proposed, involving these NO2 species as
The Co-ZSM-5 catalyst used in this study was prepared
from Na-ZSM-5 (molar SiO2/Al2O3 ratio = 37) by an
ionic exchange technique with a Co(NO3)2 solution fol-
lowed by drying and calcination at 500 ◦C [1,8]. The cobalt
content in Co-ZSM-5 was 1.8 wt% that corresponds to
1.9 × 1020 at.Co/gcat, i.e., to a degree of exchange (DE)
of 292%, by considering the total number of exchange
sites per ZSM-5 unit cell being equal to half the num-
ber of Al atoms per unit cell (one Co2+ for two H+, i.e.,
Cox/2AlxSi96−xO192,16H2O) [18]. Though no crystalline
cobalt oxide phases (CoO or Co3O4) were detected by X-
ray diffraction spectroscopy, indicating that the size of cobalt
oxide particles did not exceed 30–40 Å, it was shown from
XPS and TEM studies [18] that different cobalt forms coex-
ist in the catalyst, including: (i) Co oxides particles (CoxOy)
at the external surface of the zeolite crystals, (ii) oligonu-
clear or nanoclusters of cobalt oxide inside the zeolite pores,
formed of tetrahedral Co2+ ions agglomerated with O2−
ions, most likely located at the intersection of straight and
zig-zag channels, and (iii) isolated tetrahedral and octahe-
dral cobalt ions Co2+ located at ion-exchange sites. Sieved
0.3- to 0.5-mm particles were used in the present study, af-
ter checking that no external mass transfer (or extraparticle)
limitation occurred (no change in conversion as a function
of particle size). Occurrence of internal mass transfer limi-
tation (intracrystalline diffusion) will also be discarded, as
discussed later (see Discussion).
active intermediates able to react with activated forms of
methane. A similar conclusion on the participation of NO2
species in the reaction with methane was proposed by Boix
et al. [12].
δ+
In order to quantify the concentration and reactivity of
the various NOx adspecies potentially participating in the
SCR with CH4 under steady-state conditions, a SSITKA
(steady state isotope transient kinetic analysis) technique
has been applied. As a matter of fact, while DRIFT spec-
troscopy mostly detects “spectator” adspecies under steady-
state conditions, SSITKA enables a quantitative determina-
tion of the concentration of adsorbed active intermediate
species, as well as their rates of formation and consump-
tion [16,17]. In Part I of this work [1], the dynamics of
isotopic exchange between the NO + O2 and the catalyst un-
der adsorption/desorption equilibrium were investigated in
the absence of reducing agent. Three types of adsorbed NOx
species were distinguished on Co sites, based on their rate
of 15N exchange with gaseous NO decreasing substantially
δ+
among the following sequence: mononitrosyls ꢀ NO2
species > nitrite complexes. From the dynamics of this ex-
change, rate constants of their formation and decomposition
were determined. In addition, by analyzing the dynamics of
18O exchange between gaseous NO and the various pools
of oxygen present in the catalyst, it was proposed that the
δ+
high reactivity of NO2 species originates from its unique
interaction with oligonuclear cobalt oxidic species and hy-
droxyl groups in the zeolite channels. On the contrary, either
less reactive mononitrosyls on isolated cobalt ions or nitrite
complexes on larger cobalt oxide particles located outside
the zeolite framework or at defect interfaces would not play
a major role in the SCR process. This indicates an essential
participation of the zeolite matrix in the overall reaction.
In this paper, the dynamics of the above species have been
investigated in the presence of methane using a SSITKA
technique upon analysis of the isotopic responses 14N16O →
15N18O, 16O2 → 18O2, 12CH4 → 13CH4, and C16O2 →
C18O2. The rate constants of NOx reaction with methane
were estimated from the differences between concentrations
of NOx adspecies determined under adsorption/desorption
equilibria (i.e., in the absence of reducing agent) and under
SCR conditions. The dynamics of 13C-label transfer have
been also investigated in order to determine the pathways
of CO2 formation as well as to estimate the concentrations
of C,N-containing intermediate complexes. A detailed as-
2.2. SSITKA experiments
SSITKA experiments were carried out as follows: when
the steady state was achieved under NO + CH4 + O2 + He
flow, the gas mixture was replaced stepwise by the same one
containing isotopically labeled molecules: (i) doubly labeled
nitric oxide 15N18O containing 94.6% of 15N and 92.9% of
18O, (ii) labeled oxygen 18O2 containing 99% of 18O, and
(iii) labeled methane 13CH4 containing 98% of 13C. The cat-
alyst was loaded in a packed-bed reactor, consisting of a high
thermo-conductive tube with internal diameter of 3 mm and
length of 120 mm. In order to determine whether the reactor
behavior can be described as a plug flow, 1 vol% of argon
was added into the feed gas containing the isotope mole-
cules. Table 1 shows the experimental conditions in the var-
ious isotopic switches carried out: 14N16O → 15N18O, 16O2
→
18O2, 12CH4 → 13CH4, and C16O2 → C18O2. The last
isotopic switch was performed to analyze a possible isotope

E.M. Sadovskaya et al. / Journal of Catalysis 225 (2004) 179–189
181
Table 1
◦
Experimental conditions during the transient isotopic switches in SSITKA at 450 C and 1 bar
14 16
N
15 18
16
18
12
13
16
18
O →
N
O
O
→
O
CH
→
CH
C
O
→ C
O
2
2
2
4
4
2
W, g
0.23
120
0.1
100
0.1
120
0.1
60
F, ml/min
GHSV, 1/h
Feed, vol%
15,000
NO: 0.6
30,000
NO: 0.95
36,000
NO: 0.3
CH : 0.3–0.6
18,000
CO : 2
a
2
CH : 0.75
CH : 1.1
4
4
4
O : 0.15–3
2
O : 5
2
O : 3
2
a
He was used as the balance gas.
sities of peaks at m/e 15 and 17, respectively. Mass m/e
46 corresponds to labeled C16O18O, after correction from
the side NO2 formation. The concentration of 14N2 was cal-
culated by subtraction of the known contribution of C16O2
and C16O18O from the intensity at m/e 28 (under excess
oxygen, CO formation is negligible [18]). The respective
contributions from 14N16O and 15N2 (m/e = 30) as well as
from 16O2 and 14N18O (m/e = 32) were determined taking
into account that the total (chemical) concentration of each
compound does not change in the course of SSITKA exper-
iments. Since NO2 concentration was very low during the
experiments (less than 0.5% from that of NO), its contri-
bution into m/e 30 was neglected. In the experiments with
labeled 13CH4, the concentrations of H2O, NO, O2, 12CO2,
and 13CO2 were determined from intensities of parent peaks
at m/e 18, 30, 32, 44, and 45, respectively. The concentra-
tions of N2 and 13CH4 were calculated by subtracting the
contributions of CO2 and H2O from intensities at m/e 28
and 17, respectively, and the concentration of 12CH4 was
determined by subtracting the contribution of 13CH4 from
the intensity at m/e 15.
Fig. 1. N yield vs temperature during deNO HC-SCR over Co-ZSM-5
x
2
at different ion-exchange degrees (DE) of cobalt. Conditions: 0.1 vol%
−1
NO + 0.25 vol% CH + 5 vol% O in He; GHSV = 8000 h
;
4
2
P = 1 bar [18].
2.3. Numerical analysis of SSITKA responses
exchange of reaction products in the catalyst. The temper-
ature in these experiments was 450 ◦C, which was shown
previously [15] to correspond for overexchanged zeolites to
The model of isotope transfer carried out under steady-
state reaction proceeding in a plug-flow reactor can be writ-
ten by the system of hyperbolic differential equations,
δ+
the optimized concentration of reactive NO2 species be-
fore methane admission, therefore, to the maximum N2 yield
obtained under CH4-SCR of NO conditions. As a matter of
fact, a typical N2 yield was about 45%, as shown in Fig. 1
from [18], with 50% NO conversion and 90% N2 selectivity
and a marginal selectivity to NO2 of 10%, since N2O was
never detected under our reaction conditions. Depending on
the inlet methane and oxygen concentration, the conversion
of CH4 conversion into CO2 and H2O ranges from 30 to
60%.
The gas composition at the reactor outlet was continu-
ously analyzed with a quadrupole mass spectrometer VG
Sensorlab 200D, which can monitor 16 masses simulta-
neously. The mass spectrometric data were quantified us-
ing fragmentation patterns determined experimentally from
calibration gases. The concentrations of H218O, 14N15N,
15N16O, 15N18O, 16O18O, 18O2, C16O2, C16O18O, and
C18O2 were determined from the intensities of parent peaks
at m/e 20, 29, 31, 33, 34, 36, 44, 46, and 48, respectively,
and the concentrations of CH4 and H216O from the inten-
ꢀ
ꢂ
Ni1
Ni2
ꢁ−→
ꢁ←−
∂Ciαi
∂t
U ∂Ciαi
G
+
= −
αi
wik
−
wilαl
,
V
∂ξ
V N
k=1
l=1
Nj1
Nj2
ꢁ−→
ꢁ←−
∂αj
θj
= αj
wjk
−
wjlαl,
∂t
k=1
l=1
with initial conditions,
t = 0: αi = αi0, αj = αj0
ξ = 0: αi = αinput
.
i
The significance of the various parameters is listed in un-
der Notation. The model takes into account the change in
isotope-label concentration in the reactants and products and
surface intermediate both in time and axial coordinate along
the catalytic bed.
The various parameters were determined by the mini-
mization of root-mean-square deviation of experimental data

182
E.M. Sadovskaya et al. / Journal of Catalysis 225 (2004) 179–189
written as follows:
2NO + CH4 + O2 → N2 + CO2 + 2H2O,
CH4 + 2O2 → CO2 + 2H2O.
(1)
(2)
From the above equations, the efficiency of methane in the
SCR process can be expressed as the ratio of N2 and CO2
outlet concentrations, E(CH4) = [N2]out/[CO2]out. E(CH4)
varies between 1, which corresponds to a pure SCR process
without methane oxidation with O2 [Eq. (1)] and 0, which
corresponds to a pure methane oxidation process [Eq. (2)]
without NO reduction.
Fig. 2 shows the steady-state reaction rates of NO reduc-
tion and CH4 oxidation as a function of the oxygen content
in the reaction mixture.
Fig. 2. Reaction rates of (F) NO and (") CH and (P) hydrocarbon effi-
4
ciency during deNO HC-SCR over Co-ZSM-5 at different O concentra-
x
2
The rates of NO and CH4 conversion dramatically in-
crease in the O2 concentration range between 0–0.6 vol%.
At higher oxygen concentrations the reaction rates experi-
ence slight changes. The hydrocarbonefficiency was close to
1 at low oxygen concentration and strongly declined as the
O2 content increased, reaching the value of 0.6 for 3 vol%
of O2. The side concentration of NO2, probably associated
with the decomposition of nitrite species on cobalt oxide par-
ticles (mostly located outside the zeolite matrix [18]), will
not be considered further in this paper.
tions. Conditions: 0.6 vol% NO + 0.75 vol% CH + 0–3 vol% O in He;
4
2
◦
−1
T = 450 C; GHSV = 15,000 h ; P = 1 bar.
from calculated ones,
T
ꢃ
n
ꢁ
ꢄ
ꢅ
F(ϑ) =
αl(t, ϑ) − αexp(t) 2 dt → min,
¯
¯
l
l=1
0
where αlexp(t) and αl(t, ϑ) are the experimental and calcu-
lated dependencies of isotope fractions in the reactants and
products, respectively. The Newton algorithm of functional
minimization was used, and the differential equations were
solved by the difference method, as described in [19].
¯
3.2. Switch from 14N16O to 15N18O in
NO + CH4 + 16O2 + He
Fig. 3a shows the changes in 15N isotope fraction in NO
at the reactor outlet at different O2 concentrations in compar-
ison with the response curve of argon, added to the mixture
containing the 15N18O isotope.
3. Results
3.1. Steady-state reaction rate and hydrocarbon efficiency
A delay of 2 s is observed for Ar appearance in the gas
phase after the isotopic switch, which corresponds to the res-
idence time of inert gas in the system composed between
The overall reactions of SCR of NO with methane in the
presence of oxygen and of direct methane oxidation can be
15
18
Fig. 3. Experimental (symbols) and simulated (lines) isotopic fractions of (a) N and (b) O in NO compared to the Ar response vs time upon switching
14 16
15 18
from
N
O to
N
O in 0.6 vol% NO + 0.75 vol% CH + O + He flow (solid lines) at (e) 0.15 vol%, (") 0.6 vol%, (E) 3 vol% O , and (×) 0.6 vol%
4
2
2
NO + 3 vol% O in He. Conditions in Table 1.
2

E.M. Sadovskaya et al. / Journal of Catalysis 225 (2004) 179–189
183
15
18
14 16
Fig. 4. Isotopic fractions of (a) N in NO and N compared to the Ar response and (b) O in NO, CO , and H O vs time upon switching from
N
O to
2
2
2
15 18
N
O in 0.6 vol% NO + 0.75 vol% CH + 3 vol% O in He. Conditions in Table 1.
4
2
the switching valve and the mass spectrometer, therefore in-
cluding reactor and tubing. The stepwise shape of the Ar
response curve indicates in addition that this reactor can be
well described with a plug-flow model. The 15N concentra-
tion within the nonreacted NO at the reactor outlet is shifted
by several seconds from the Ar signal, suggesting that there
is a pool of reversibly adsorbed NO species onto the cat-
alyst, in fast exchange with the gas phase. The observed
delay corresponds to the mean residence time of the reac-
tant in this reversible pool. Similar delayed response curves
(× in Fig. 3a) were obtained in the SSITKA experiments
in the absence of methane, i.e., under adsorption/desorption
equilibrium [1]. It was shown for this case that a first pool
of nitrogen atoms in fast exchange with the gas phase was
formed of cobalt mononitrosyls, as being the most rapidly
formed and weakly bound NOx species. Then the nitrogen
also transferred to the reaction products after the SSITKA
switch.
In Fig. 4a the 15N label appears in N2 at the reactor outlet
earlier than in the unconverted NO, the isotopic fraction of
15N in N2 remaining higher than the one in NO for at least
15
10 s (αN15 > α ). Such an effect may be easily explained
NO
2
taking into account the plug–flow behavior in our reactor.
As seen below, the unconverted NO reactant is in fast and
reversible adsorption equilibrium with the catalyst forming
a large pool of reversible adspecies, as cobalt mononitro-
δ+
syls and NO2 species, delaying the label appearance at
the reactor outlet. If in the mean time, part of adsorbed NO
is transformed into a product, N2, which immediately des-
orbs into the gas phase, the labeled molecules of the product
(15N in N2) will appear almost from the beginning of the
transient switch, i.e., as soon as the isotopic label of the re-
actant penetrates the reversible pool of adsorbed reactant in
the inlet part of the catalytic bed (15N in NOads).
δ+
label was also exchanging in the pool of NO2 species and
then of nitrite complexes. The rates of isotope replacement
δ+
in mononitrosyls and NO2 species were found to be sim-
Comparison of the dynamics of labeled nitrogen and oxy-
gen transfer in Figs. 4a and b, respectively, indicates that
the 18O label appears in the unconverted NO significantly
later than 15N (approximately 5 vs 50 s, respectively) and
at a very low rate, even more slowly than in the absence of
methane (by comparing E and × in Fig. 3). This slow trans-
fer of 18O from NO reactant is also observed in CO2 at a rate
close to that observed for NO but at a still lower rate for H2O
(Fig. 4b). This can be ascribed to the secondary oxygen ex-
change between the relatively small pool of NOx species and
the large pool of exchangeable oxygen atoms belonging to
the zeolite lattice [1]. The instantaneous rate of labeled oxy-
gen transfer into the zeolite lattice can be calculated by sub-
tracting the amount of 18O atoms at the reactor outlet (sum-
ming label concentrations in NO, CO2, and H2O) from that
at the reactor inlet. This rate was slightly higher under the re-
action conditions than under adsorption/desorption equilib-
rium at the same NO and O2 concentrations (8.4 × 1017 and
7.7 × 1017 molec/(gs), respectively, at 200 s). Despite the
ilar, whereas the replacement rate in nitrite complexes was
one order of magnitude lower. This results in a very slow re-
turn of the 15N concentration in outlet NO to a steady state
corresponding to 15N-label content in inlet NO.
In the presence of methane (SCR), one can see that the
15N label appears much earlier in the unconverted NO gas
phase and its concentration increases more rapidly in com-
parison with what is observed in the absence of methane,
under adsorption/desorption equilibrium (compare E and ×
in Fig. 3a). This means that the amount of weakly bounded
NOx species participating in the fast isotope exchange with
δ+
NO (i.e., mononitrosyls and NO2 species) strongly de-
creases under reaction conditions. In addition, the very slow
relaxation at the end of the transient curves in the absence
of methane, which was attributed to nitrite complexes [1], is
not observed under SCR conditions, indicating that the con-
centration of the latter species is very low in the presence of
methane. As expected, the labeled atoms from 15N18O were

184
E.M. Sadovskaya et al. / Journal of Catalysis 225 (2004) 179–189
18
2
Fig. 6. Experimental (points) and simulated (lines) isotopic fractions of
O
in
16
18
in (!) NO and (") CO vs time upon switching from
O
to
O
2
2
18
Fig. 5. Isotopic fraction of O in NO and CO vs time upon switching
2
0.95 vol% NO + 1.1 vol% CH + 5 vol% O in He. Conditions in Table 1.
4
2
14 16
15 18
16
18
from
N
O to
N O in 1.2 vol% NO in He and from C O to C O
2 2
in 2 vol% CO in He, respectively. Conditions in Table 1.
2
3.4. Switch from 16O2 to 18O2 in
14N16O + CH4 + O2 + He mixture
When 16O2 is switched to 18O2 in the overall steady-state
NO+CH4 +O2 +He mixture, the transient concentration of
18O label within the unconverted oxygen follows strictly the
inert Ar response (as shown in Fig. 3), confirming that no
reversible oxygen dissociates on the catalyst. Fig. 6 shows
the 18O transfer from gaseous oxygen to CO2 together with
large amount of labeled oxygen transferred into the catalyst,
no 18O is transferred into gas-phase oxygen (either as 18O2
or 16O18O), which means that all the steps involving activa-
tion and interaction of O2 with NOx proceed irreversibly.
The slow transfer of labeled oxygen within the oxygen-
containing products CO2 and H2O as noted above obviously
reflects the reversible interaction between these polar mole-
cules and the zeolite pool of oxygen (Fig. 4b). As a result of
this oxygen exchange the label concentration in the reaction
products declines along the catalyst bed, but not so sharply
as that in NO.
the transfer to unconverted NO. In this experiment, the 18
transfer from gaseous oxygen to H2O was not recorded.
O
It can be seen that 18O label appears in NO with a de-
lay of about 15 s after CO2 (Fig. 6), which can result only
from isotope exchange of NO with the large pool of catalyst
oxygen atoms. In fact, a substantial part of labeled oxygen
is transferred into the catalyst as concluded from the com-
parison between the amount of 18O2 consumed with that of
18O atoms at the reactor outlet. Since participation of mole-
cular oxygen in isotopic exchange with the catalyst oxygen
is negligible (as shown above and in [1]), it is unclear at
this stage how 18O-label transfer into the catalyst proceeds
during the catalytic reaction or as a result of CO2 and H2O
re-adsorption and why 18O label does not appear into NO
immediately, but with a substantial delay. Numerical analy-
sis of 18O isotope responses was carried out to further eluci-
date these aspects (Section 3.6).
3.3. Switch from C16O2 to C18O2 in CO2 + He
In order to gain more insight into the interaction between
the reaction products and the catalyst surface, a SSITKA
switch from C16O2 to C18O2 was carried out in a 2 vol%
CO2 + He flow (see Table 1), and compared to the already
described NO switch response, as depicted in Fig. 5.
The 18O label in the CO2 gas phase appears and sta-
bilizes much faster than in the case of NO (Fig. 5). This
means that the rate of 18O exchange between the CO2 and
the catalyst is not negligible, though considerably lower than
that for NO. As a consequence, the oxygen exchange which
occurs between the reaction products and the catalyst super-
imposes with the initial (as formed) content of oxygen label
in the products. Therefore no straightforward conclusion on
the mechanism of 18O label transfer into the reaction prod-
ucts can be derived from the shape of isotope responses. To
this end, the rate of isotope exchange between the reaction
products and the catalyst should be computed, and then the
mechanism of oxygen transfer could be assessed using nu-
merical analysis of the isotope responses.
3.5. Switch from 12CH4 to 13CH4 in NO + CH4 + O2 + He
mixture
Fig. 7 shows the dynamics of 13C label transfer into CH4
and CO2 as compared with the response curve of argon.
A delay of ca. 1 s of isotope response in methane rela-
tive to that of argon was observed. Labeled 13C appeared in
CO2 simultaneously with that in CH4, but its concentration
increased considerably slower. The slight shift of methane
response relative to argon indicates a minor accumulation of
reversible methane, most likely loosely bounded within the

E.M. Sadovskaya et al. / Journal of Catalysis 225 (2004) 179–189
185
15
Scheme 1. Pathways for N-label transfer under SCR reaction conditions
∗
∗
∗
where [z- N] , [z- N] and [z- N] are the concentrations of all interme-
1
2
3
∗
∗
δ+
∗
−
, and NO
2
diate complexes allowing the conversion of NO, NO
2
adspecies into N .
2
sorption/desorption steps in the absence of reducing agent
can be expressed by means of the corresponding rate con-
13
Fig. 7. Experimental (points) and simulated (lines) isotopic fractions of
C
stants determined in [1] (wi = kiCNO, w = k−iθi). Since
−i
in CH (e) and CO (a) compared to the Ar response (!) vs time after
switching from CH to CH in 0.3 vol% NO+0.4 vol% CH +3 vol%
4
2
the dynamics of 15N label switch in NO is determined by
the formation and desorption rates of NOx species (steps
1–3 in Scheme 1), this process can be described with the
concentrations of NOx species (θi) as the only unknown pa-
rameters. These parameters were fitted to the experimental
data according to the procedure described elsewhere [19].
The calculated concentrations of the various adsorbed NOx
species obtained both under adsorption/desorption equilib-
rium in the absence of methane (from [1]) and under SCR
reaction conditions (at the same NO and O2 inlet concentra-
tions) are reported in Table 2.
12
13
4
4
4
O
in He. Conditions in Table 1.
2
zeolite matrix. In turn, the chemical interaction of methane
with the catalyst to form reacting intermediates seems to be
irreversible. The concentration of these carbon-containing
intermediates will be determined from numerical simulation.
Another important observation can be made from the dy-
namics of CO2 isotope response. In Fig. 7, it can be seen that
relatively fast increase of CO2 isotope fraction proceeds for
the initial period of time followed by pronounced decelera-
tion relative to exponential dependence. This result strongly
suggests that CO2 does not originate from a single pathway
but several routes leading to CO2 proceed with different re-
action rates.
As can be seen, the concentration of cobalt mononitrosyls
was ca. 15% lower under the reaction conditions than under
adsorption/desorption equilibrium. This corresponds to the
decrease of NO concentration in the gas phase during SCR
(length-averaged drop of this value is also close to 15%).
Therefore, adsorption–desorption equilibrium between NO
in the gas phase and mononitrosyl adspecies is kept estab-
lished under SCR conditions, and the rate of mononitrosyl
conversion due to the SCR reaction is negligible compared
to the rates of their formation and desorption. For this reason,
the mononitrosyl contribution to the total NO conversion
into N2 remains marginal and its accurate estimation is not
possible.
3.6. Simulation results
3.6.1. Nitrogen labeling
According to our previous results [1], 15N label can trans-
fer from NO into three types of adsorbed NOx species:
mononitrosyls adsorbed on cobalt ions, NO2 species ad-
sorbed on oligonuclear oxide cobalt species in the channels,
with participation of hydroxyl groups in the zeolite, and ni-
trite complexes adsorbed on larger cobalt oxide particles.
Taking into account possible participation of all these com-
plexes in N2 formation, a general scheme of 15N-label trans-
fer under reaction conditions can be considered (Scheme 1),
where [z-∗N]1, [z-∗N]2 and [z-∗N]3 are the concentrations
of all intermediate complexes resulting from the conversion
δ+
In contrast to mononitrosyls, the sharp decrease of
NO2δ+ and nitrite concentrations in the presence of methane
(SCR conditions) as compared to adsorption/desorption
equilibrium conditions exceeds considerably what could be
assigned to the ∼ 15% decrease in NO concentration in the
gas phase (Table 2). This clearly indicates the participation
δ+
δ+
of these adspecies in the reaction. Since NO2 species and
of mononitrosyls NO, adsorbed NO2 species, and nitrite
complexes (NO2−), respectively, into N2.
nitrite complexes are located on different active sites, the
δ+
SCR reaction with a participation of NO2 and nitrites ad-
This scheme accounts for an eventual isotope exchange
between mononitrosyl and nitrite complexes but excludes
the exchange of both complexes with NO2 species, be-
cause they are located on different active sites. As a matter
of fact, mononitrosyls and nitrites can be formed on isolated
ions and/or cobalt oxide particles, respectively, while for-
species is most likely to proceed via parallel routes, both
including a reaction step with methane.
δ+
δ+
Quite reliable estimations of the rates of NO2 and ni-
trite conversion were obtained from the differences between
their adsorption and desorption rates (Table 2). The sum of
these rates was found to be close to the total rate of N2 for-
mation. Then, one can conclude based on the balance ratio
δ+
mation of adsorbed NO2 requires the interface between
the nanoclusters and zeolite lattice [1,18]. The rates of ad-

186
E.M. Sadovskaya et al. / Journal of Catalysis 225 (2004) 179–189
Table 2
Estimated values of NO adsorbed species concentrations and the rates of their conversion in the SCR reaction
x
−19
Oxygen inlet
concentration,
vol%
Concentration of adsorbed NO species, [molec/g] × 10
SCR reaction rate,
x
−19
[molec/(g s)]×10
Adsorption–desorption
SCR reaction
(in the presence of CH )
∗
∗
δ+
∗
−
2
[z- NO]
[z- NO
]
[z- NO
]
(in the absence of CH )
2
4
4
δ+
∗
∗
δ+
∗
−
∗
∗
∗
−
]
2
[z- NO]
[z- NO
]
[z- NO
]
[z- NO]
[z- NO
]
[z- NO
2
2
2
0.3
0.6
3
0.64
0.64
0.64
0.85
1.05
1.90
0.11
0.15
0.53
0.61
0.56
0.55
0.3
0.8
1.2
ꢀ 0.1
ꢀ 0.1
ꢀ 0.1
0
0
0
0.006
0.016
0.020
0.002
0.006
0.008
that the rate of N2 formation via direct interaction of cobalt
mononitrosyls with methane is negligible with respect to the
other two pathways.
Table 3 shows the rates of CO2 formation by the both
routes. One can see that the reaction rates increase as the
CH4 concentration in the feed gas increases, but the ratio
r1/r2 = 2.3 does not change. This ratio is close to that be-
Numerical analysis of the dynamics of N2 isotope re-
placement also enables the estimation of the total concen-
δ+
tween the reaction rates for NO2 species and nitrite com-
δ+
plexes (0.020/0.008 = 2.5). The concentrations of adsorbed
species formed during methane conversion to CO2 ([z-∗C]1
and [z-∗C]2) were also determined (Table 3). The proper na-
ture of these adsorbed species or reacting intermediates will
be discussed later on.
tration of surface CNOxHy complexes, formed after NO2
and nitrites adspecies interaction with activated methane ad-
species (noted [z-∗N]1 and [z-∗N]2 in Scheme 1). This con-
δ+
centration is about 20% of that of NO2 species in all the
experiments (not reported in Table 2 for sake of clarity). As
shown in Fig. 4a, a good agreement was found between the
experimental and the calculated dynamic responses.
3.6.3. Oxygen labeling
From the simulation of the experiments with labeled oxy-
gen (N18O and 18O2), it was concluded that the observed rate
of 18O isotope replacement in CO2 can be described with
a two-step scheme of isotope exchange CO2 ↔ [Osur] ↔
[Obulk], [Osur] being the oxygen pool formed by the ad-
sorption/desorption active sites (cobalt oxide particles and
nanoclusters) and [Obulk] being the oxygen pool formed by
the zeolite oxygen framework. This isotope exchange is lim-
ited by the rate of label transfer from the adsorption sites
(Osur) into the zeolite bulk (Obulk). Taking into account that
18O distribution in CO2 and H2O is close to equilibrium,
it can be proposed that isotope exchange between the cata-
lyst oxygen and the H2O proceeds on the same sites as the
exchange with CO2 (most probably on hydroxyl groups of
the zeolite), and is also limited by the label transfer from
OH groups into the oxygen pool of the zeolite framework.
Then, one can roughly estimate highest possible total rate
of 18O label transfer from CO2 and H2O into the catalyst
under the reaction conditions. Numerical analysis of 18O la-
bel transfer under the reaction conditions was performed for
the experiments with both labeled 15N18O and labeled 18O2.
Numerical analysis of the experiments with labeled oxygen
showed that isotope exchange with the reaction products
cannot account for the observed acceleration of the label
transfer; i.e., there are other pathways of 18O-label trans-
3.6.2. Carbon labeling
Numerical analysis of 13C-label transfer confirms the in-
volvement of two pathways involving activated CH4 species
during the SCR of NOx. As a matter of fact, the calculated
isotope responses were found not fitting with the experimen-
tal ones in the case of one-step or consecutive scheme of
label transfer. The only satisfying agreement between the
experimental and the calculated responses was obtained in
the case of a parallel scheme of 13C-label transfer with two
pathways (Scheme 2), where [z-∗C]1 and [z-∗C]2 are the
concentrations of all intermediate complexes resulted from
CH4 conversion into CO2, with reaction rates r1 and r2, re-
spectively.
13
Scheme 2. Parallel pathways for C-label transfer under SCR reaction con-
∗
∗
ditions, where [*CH ] is reversibly adsorbed methane, [z- C] and [z- C]
4
1
2
are the concentrations of all intermediate complexes resulted from CH ir-
4
reversible conversion into CO , with reaction rates r and r , respectively
(most likely involving methoxy species OCH ).
3
2
1
2
Table 3
∗
∗
Estimated concentrations of carbon containing reacting intermediates [z- C] and [z- C] and rates of reaction into CO
Expt.
1
2
2
−19
−19
Feed composition, vol%
Surface concentration, [molec/g]×10
Reaction rate, [molec/(g s)]×10
r /r
1
2
∗
∗
NO
O
CH
[z- C]
[z- C]
0.084
0.010
0.015
r
r
2
2
4
1
2
1
1
2
3
0.3
0.3
0.3
3.0
3.0
3.0
0.3
0.4
0.6
0.016
0.022
0.026
0.019
0.025
0.031
0.008
0.011
0.014
2.4
2.3
2.2

E.M. Sadovskaya et al. / Journal of Catalysis 225 (2004) 179–189
187
port from gas-phase O2 into the catalyst surface. The same
conclusion can be drawn from the simulation results of the
experiments with labeled NO: a new pathway of the label
transfer from NO into the catalyst appeared in addition to
the exchange with the reaction products and with adsorbed
NOx complexes. Considering different schemes of 18O-label
transfer, we had in mind two facts: the absence of reverse
label transfer from the catalyst into O2 as well as the ab-
sence of isotope exchange between O2 and the catalyst under
adsorption-desorption equilibrium (i.e., in NO + O2 + He
mixture in the absence of methane). The most probable way
to explain these facts is to permit the label transfer on the
step of active sites reoxidation. As concluded in [1], the
pathway leading to NO2δ+ formation is preceded by the ox-
idation of the active sites. Then, supposing that both oxygen
atoms from these complexes participate in the reaction prod-
ucts formation and that the active sites can be reoxidized by
both O2 and NO (with the reoxidation rate being equal to
the rate of these complexes conversion) we described well
the dynamics of 18O transfer in the experiments with both
labeled NO and labeled oxygen (see Figs. 3b and 6).
Scheme 3. Two main parallel pathways allowing the CH -SCR of
4
NO into N over Co-ZSM-5. Note that CH and NO + O addition
2
4
2
along the proposed pathways are most likely to proceed via adsorbed
∗
∗
∗
species: for CH = [ CH ], [z- C] and [z- C] (Scheme 2) and for
4
4
1
2
NO + O = mononitrosyl and activated oxygen over Co nanoclusters, re-
2
spectively
located at the interface between oligonuclear cobalt oxidic
species (or nanoclusters) and hydroxyl groups of the zeolite
lattice, while nitrite species are preferably formed on large
CoxOy particles (CoO and/or Co3O4), located on the sur-
face or outside the zeolite crystals [18]. Based on all these
statements, the reaction proceeds according to the two mech-
anistic pathways 1 and 2 presented in Scheme 3.
The activity of mononitrosyl NO on Co sites with respect
to methane is negligible as follows from the equality of sum-
δ+
marized rates of NO2 and nitrite conversion and the total
4. Discussion
rate of NO reduction into N2. This has also been concluded
by other authors [4,20] where NOx (x = 2 or 3) surface
species are considered as the only reaction intermediates.
However, the fact that mononitrosyls are not able to acti-
vate methane does not enable one to exclude the possibility
that part of these species can participate in the reaction due
to their conversion into nitrite complexes, likely after mi-
gration from isolated sites to the cobalt oxide particles (see
Scheme 1).
4.1. Internal diffusion limitation analysis
Before discussing the possible mechanistic pathways
from the SSITKA results and modeling, the possibility of
intracrystalline mass-transfer effects has to be considered
since it might alter the ratio between the reaction rates via
δ+
−
different pathways (with participation of NO2 and NO2
species). From the kinetics of NOx adspecies formation pre-
viously reported in [1], the measured rate constant of NO
adsorption allows us to estimate (by smallest extreme) the
values of NO diffusion coefficient for the studied catalyst:
DNO/L2 ꢀ 50 s−1 (where L is molecule radius). Since ki-
netic radiuses of NO and CH4 molecules are close, their
diffusion coefficients are also comparable. From the mea-
sured rate of methane consumption and concentration of
NOx complexes, the effective rate constant of CH4 inter-
action with NOx can be estimated. These values are about
It can be discussed at this stage how the mechanism pro-
posed from this SSITKA investigation (Scheme 3) compares
with the mechanisms most often discussed in the literature
for deNOx reaction. Most of deNOx SCR mechanisms can
be classified into two groups: “decomposition” and “reduc-
tion” mechanisms. In the first case N2 formation results
from NO decomposition on reduced sites containing oxygen
vacancies, which, in turn, after being oxidized by the NO
decomposition, are rereduced by the hydrocarbon [21–23].
This “decomposition” scheme is totally unlikely in our case
since DRIFT and TAP transient kinetic experiments [15]
have excluded the possibility of NOx decomposition to form
N2 in the absence of methane.−It can therefore be concluded
10 s−1, which is considerably lower than DCH /L2. Hence,
any internal mass-transfer effects can be considered as neg-
ligible in the present study.
4
δ+
4.2. Main mechanistic pathways
that namely NO2 and NO2 species directly react with
CH4. This scheme corresponds to the reduction mechanism,
which includes N2 formation originated from a series of
consecutive transformations with participation of two re-
acting agents. There are four variants of such elementary
steps: (i) NO reaction with adsorbed carbon species formed
from hydrocarbon [24]; (ii) NO reaction with partially ox-
idized hydrocarbon [21,23,25]; (iii) hydrocarbon reaction
with NOx complexes formed due to NO oxidation [2,3,6,8,
21,23]; (iv) reaction of NOx complexes with carbonaceous
species (“bifunctional mechanism”) [26]. Mechanisms (i)
Two types of SSITKA experiments, investigating the
transfer of 15N from NO to N2 and the transfer of 13C from
CH4 to CO2, have independently shown that the formation of
N2 and CO2 during CH4-SCR of NO over Co-ZSM-5 pro-
ceeds via by two parallel routes. The rates of these routes
are similar, as estimated from the 15N and 13C data. N2
δ+
formation proceeds essentially via adsorbed NO2 species
and nitrite complexes (NO2−). These adspecies are located
δ+
on different active sites [1]: NO2 species are specifically

188
E.M. Sadovskaya et al. / Journal of Catalysis 225 (2004) 179–189
and (ii) can most likely be discarded, because NO instead
of CH4 forms strong adsorbed complexes in our case. Our
reaction scheme is thought to be closer to mechanism (iii)
(with gaseous CH4), or even better from mechanism (iv)
since it involves the participation of both weakly bound
methane (noted [*CH4] in Scheme 2, as reversibly adsorbed
methane), and activated complexes resulted from CH4 irre-
versible conversion into CO2 (noted [z-∗C]1 and [z-∗C]2 in
Scheme 2).
that not only do cobalt sites play a major role per se, but
they also act in combination with acid sites. The nuclearity
and specific location of the cobalt entities in close vicin-
ity of the acidic sites remains a very challenging aspect in
this deNOx catalysis. An advanced molecular description of
these most active sites located at channel intersection is pro-
posed in [18]. Indeed, turnover number of the reaction on the
nanocluster sites interacting with zeolite OH groups is about
25 times higher than that on larger cobalt oxide particles lo-
cated outside the zeolite channels. Kinetically, an influence
of the acid sites is not revealed on the step of interaction
with methane (as was shown above, the rates of this step for
NO2δ+ and nitrites are comparable), but it becomes apparent
mainly on the step of NOx complex formation. In this case
the intrinsic formation rate (or turnover frequency referred
According to our modeling results, the reaction pathway
δ+
via NO2
adspecies (pathway 1 in Scheme 3) is ∼ 2.5
faster than that occurring via nitrite complexes (pathway 2
in Scheme 3). Since the number of the active sites in path-
way 1 (oxidic Co nanoclusters in tight interaction with OH
species) is only 10% of the total amount of cobalt in the cat-
alyst, it can be inferred that the intrinsic rate per active site is
about 25 times higher in pathway 1 than in pathway 2. How-
δ+
to one active site) of NO2 species stabilized in the zeolite
channel with participation of OH groups is several orders of
magnitude higher than that of nitrite complexes [1]. A con-
finement effect which would gather on the same nanocluster
δ+
ever, if one compares the ratio between the rate of NO2
interaction with methane and their concentration with the
same ratio for nitrite complexes, it seems that the rate con-
stant of nitrite interaction expressed by this mode is at least
not lower than that for NO2δ+ species. In other words, nitrite
complexes are not less reactive with respect to methane than
NO2δ+. Comparatively the low overall rate of the reaction
proceeding via nitrite complexes (pathway 2), considering
their relatively high activity with respect to methane, can be
caused by two factors: (i) a low rate of formation and/or (ii)
a low conversion rate of C,N-containing intermediates re-
sulted from nitrite reaction with methane. It is obvious that
the first factor predominates in our experiments: the concen-
tration of nitrite complexes drops to zero under SCR con-
ditions, indicating that their reaction with methane proceeds
very fast. Accordingly, the rate-limiting step of pathway 2 is
δ+
NO2 species stabilized by OH groups, and methane ad-
species to form the intermediate complexes CNOxHy able
to react in turn with another NO adspecies to form gaseous
N2 and CO2 is likely to explain this unique intrinsic rate.
The proper nature of the activated methane intermediates
noted [z-∗C]1 and [z-∗C]2 in Scheme 2 (or CH4 in Scheme 3)
and which concentrations are reported in Table 3 remains
unclear. From IR studies reported in [18], the formation of
methoxy radicals (CH3–O) both on CoO nanoclusters and
cobalt oxide particles is most likely under SCR conditions.
These methoxy species would react with the close NOx ad-
δ+
species (NO2
or nitrite) following the above-described
SCR pathways. Eventually, they could also be oxidized di-
rectly into CO2, especially at higher temperatures when the
concentration of NOx adspecies is decreasing. This would
explain the decrease in N2 yield at high temperature as ob-
served in Fig. 1 [18].
the nitrite formation.
δ+
As for pathway 1 proceeding via NO2
intermedi-
δ+
ates, Table 2 has shown that the concentration of NO2
species decreases appreciably in comparison with that under
adsorption–desorption equilibrium, but does not fall to zero.
This concentration is raised with oxygen content in the re-
action mixture, i.e., these species are not able to completely
react with methane. It is not straightforward to unambigu-
Numerical analysis of the dynamics of 18O label transfer
provides valuable information to further unravel the reaction
δ+
mechanism. It was shown in our previous works that NO2
formation is preceded by reversible and dissociative oxy-
gen adsorption, the rate of this step being very low (about
1016 molec/(g s)) [1,27]. In principle, the rate of oxygen
ously identify the rate-limiting step in this case, which can
δ+
δ+
be tentatively assigned to the interaction of NO2
with
adsorption should limit the rate of NO2 formation under
methane. According to the literature [2–14], the interaction
of adsorbed NOx complexes with CH4 is generally assumed
to be the rate-limiting step in this reaction. According to our
results, this holds only for one route (pathway 1); neverthe-
less, since this route is dominant, total reaction rate should
be limited by the rate of methane activation. In this respect
our data do not contradict the other results in the literature.
the reaction conditions. However, according to our estima-
tions, the rate of the reaction proceeding with participation
of NO2 species exceeds the value of 1017 molec/(gs).
δ+
Thus, it seems that the formation rate of the oxidized sites
δ+
participating in NO2 production increases under the re-
action conditions. On the other hand, the experiments with
labeled oxygen show that the rate of 18O-label transfer into
the catalyst is also raised. According to numerical analysis
results, this increase is not only due to the isotope exchange
with the reaction products, but also caused by the appear-
ance of an additional pathway of the label transfer concerned
with the steps of the catalytic cycle. It can be proposed that
this acceleration of 18O-label transfer into the catalyst active
4.3. Intrinsic activity of active sites and reacting
intermediates
Concerning to the nature of the most active sites in the
SCR of NO with CH4 over Co-ZSM-5, our results indicate

E.M. Sadovskaya et al. / Journal of Catalysis 225 (2004) 179–189
189
phases is directly associated with the increase of the forma-
E(CH4) CH4 efficiency in SCR process
tion rate of the oxidized sites. But this situation can only
occur if oxygen from the oxidized sites participates in the
catalytic cycle and is transferred into the reaction products;
then, the label transport into the catalyst accompanies the re-
oxidation step. Reoxidation of the active sites by molecular
oxygen is likely to proceed simultaneously with formation of
the products, and the active sites can be reoxidized (i.e., re-
generated) by both O2 and NO. This scheme of the reaction
mechanism allowed us to simulate all the data obtained in
SSITKA experiments with both labeled NO and labeled O2.
F
N
t
Volumetric flow rate, ml/s
Avogadro number (6.023 × 1023), molec/mol
Time, s
V
Reactor volume, cm3
−→
wjk
Rate of formation of adsorbate j in reaction k,
molec/(gcat s)
Rate of decomposition of adsorbate j in reaction l,
molec/(gcat s)
←−
wjl
W
αi
αj
ξ
Catalyst weight, g
Isotopic fraction of gas-phase i species
Isotopic fraction of adsorbed j species
Dimensionless bed length
5. Conclusions
θj
Concentration of adsorbate j, molec/gcat
A steady-state isotopic transient kinetic analysis (SSITKA)
technique has been applied to investigate the reactivity of
adsorbed NOx complexes on different cobalt species in
Co-ZSM-5 with methane in the presence of oxygen. Nu-
merical analysis of the responses during isotopic switches
References
[1] E.M. Sadovskaya, A.P. Suknev, L.G. Pinaeva, V.B. Goncharov, B.S.
Bal’zhinimaev, C. Chupin, C. Mirodatos, J. Catal. 201 (2001) 159.
[2] Y. Li, T.L. Slager, J.N. Armor, J. Catal. 150 (1994) 388.
[3] A.T. Bell, Catal. Today 38 (1997) 151.
[4] A.W. Aylor, L.J. Lobree, J.A. Reimer, A.T. Bell, Stud. Surf. Sci.
Catal. 101 (1996) 661.
18
13
of 14N16O → 15N18O, 16O2 → O2, and 12CH4 → CH4
was performed. The concentrations of various adsorbed NOx
species on the catalyst surface, including mononitrosyls, ni-
δ+
[5] B.J. Adelman, T. Beutel, G.-D. Lei, W.M.H. Sachtler, J. Catal. 158
(1996) 327.
trites, and NO2 species and their relative reaction rates
with methane were determined. In addition, the concentra-
tions of surface intermediates resulting from NOx interac-
tion with methane were also estimated. The SCR of NO with
CH4 into N2 and CO2 proceeds via two differentiated path-
ways: (1) with participation of NO2δ+ species formed at the
interface between oligonuclear cobalt oxidic species and hy-
droxyl groups of the zeolite lattice and (2) with participation
of nitrite complexes on larger cobalt oxide particles located
outside the zeolite channels. Mononitrosyl species adsorbed
on isolated Co ions were found not to be directly active for
activation of methane, although they can participate in the
[6] A.D. Cowan, R. Dumpelmann, N.W. Cant, J. Catal. 151 (1995) 356.
[7] F. Witzel, G.A. Sill, W.K. Hall, J. Catal. 149 (1994) 229.
[8] V.A. Sadykov, S.A. Beloshapkin, E.A. Paukshtis, G.M. Alikina, D.I.
Kochubei, S.P. Degtyarev, N.N. Bulgakov, S.A. Veniaminov, E.V.
Netyaga, E.V. Bunina, A.N. Kharlanov, E.V. Lunina, V.V. Lunin, V.A.
Matyshak, A.Ya. Rozovskii, Polish J. Environ. Stud. 6 (1997) 21.
[9] A.D. Cowan, N.W. Cant, B.S. Haynes, P.F. Nelson, J. Catal. 176 (1998)
329.
[10] E.A. Lombardo, G.A. Sill, J.L. d’Itri, W.K. Hall, J. Catal. 173 (1998)
440.
[11] D.B. Lukyanov, J.L. d’Itri, G. Sill, W.K. Hall, Stud. Surf. Sci.
Catal. 101 (1996) 651.
[12] A. Boix, R. Mariscal, J.L.G. Fierro, Catal. Lett. 68 (2000) 169.
[13] X. Wang, H.Y. Chen, W.M.H. Sachtler, Appl. Catal. B 29 (2001) 47.
[14] X. Wang, H.Y. Chen, W.M.H. Sachtler, J. Catal. 197 (2001) 281.
[15] L.G. Pinaeva, E.M. Sadovskaya, A.P. Suknev, V.B. Goncharov, V.A.
Sadykov, B.S. Balzhinimaev, T. Decamp, C. Mirodatos, Chem. Eng.
Sci. 54 (1999) 4327.
δ+
overall reaction by interconversion to nitrite species. NO2
species and nitrite complexes have similar reactivity with re-
spect to methane. The higher reaction rate of pathway 1 is
δ+
caused by a substantially higher rate of NO2 formation
in comparison with that for nitrites in pathway 2 under the
reaction conditions studied.
[16] J. Happel, Isotopic Assessment of Heterogeneous Catalysis, Academic
Press, 1986.
[17] C. Mirodatos, Catal. Today 9 (1991) 83.
[18] C. Chupin, PhD dissertation, Lyon 1 University (2001).
[19] E.M. Sadovskaya, D.A. Bulushev, B.S. Bal’zhinimaev, Kinet. Ka-
tal. 40 (1999) 61.
Acknowledgments
[20] C. Shi, M.J. Cheng, Z.P. Qu, X.F. Yang, X.H. Bao, Appl. Catal. B 36
(2002) 173.
[21] M. Iwamoto, H. Yahiro, Catal. Today 22 (1994) 5.
[22] R. Burch, P.J. Millington, Catal. Today 29 (1996) 37.
[23] H. Hamada, Catal. Today 22 (1994) 21.
This work was carried out within the Twinning Pro-
gram between the Institut de Recherches sur la Catalyse
(Villeurbanne, France) and the Boreskov Institute of Cataly-
sis (Novosibirsk, Russia). This work was partially supported
by Russian Fund of Basic Research, Grant 00-03-22004.
[24] K.C.C. Kharas, Appl. Catal. B 2 (1993) 207.
[25] J. Vassallo, E. Miro, J. Petunchi, Appl. Catal. B 7 (1995) 65.
[26] P. Ansell, A.F. Diwell, S.E. Golunski, J.W. Hayes, R.R. Rajaram, T.J.
Truex, A.P. Walker, Appl. Catal. B 2 (1993) 81.
[27] B. Wichterlovà, J. Deˇdecˇek, Z. Sobalík, in: G. Centi, et al. (Eds.),
Catalysis by Unique Metal Ion Structures in Solid Matrices, Kluwer
Academic, Dordrecht, 2001, pp. 31–53.
Appendix A. Notation
Ci
Concentration of gas-phase i species, vol%