Elucidation of the Surprising Role of NO in N2O Decomposition over FeZSM-5

Article abstract of DOI:10.1023/A:1026189905517

The decomposition of N₂O is strongly promoted by NO over steam-activated FeZSM-5. The promoting motinc, effect of NO is catalytic, and in addition to NO₂, O₂ is formed much more extensively at lower temperatures than in the absence of NO. The promotion effect only requires low NO concentrations in the feed, with no significant improvements at molar NO/N₂O feed ratios higher than 0.25. No inhibition by NO was identified even at a molar NO/N₂O feed ratio of 10, suggesting different sites for NO adsorption and oxygen deposition by N₂O. The latter sites seem to be remote from each other. Transient experiments using in situ FT-IR/MS and Multitrack over FeZSM-5 further elucidate the mechanism of NO promotion. The release of oxygen from the catalyst surface during direct N ₂O decomposition is a rate-determining step due to the slow oxygen recombination, which is favored by high reaction temperatures. NO addition promotes this oxygen desorption, acting as an oxygen transfer agent, probably via NO₂ species. Adsorbed NO may facilitate the migration of atomic oxygen to enhance their recombination. Less than 0.9% of Fe seems to participate in this promotion. A model is proposed to explain the phenomena observed in NO-assisted N₂O decomposition, including NO₂ decomposition.

Full text of DOI:10.1023/A:1026189905517

Kinetics and Catalysis, Vol. 44, No. 5, 2003, pp. 639–647. From Kinetika i Kataliz, Vol. 44, No. 5, 2003, pp. 700–708.
Original English Text Copyright © 2003 by Pérez-Ramírez, Mul, Kapteijn, Moulijn.
CATALYTIC REACTION
MECHANISMS
Elucidation of the Surprising Role of NO
1
in N O Decomposition over FeZSM-5
2
J. Pérez-Ramírez*, G. Mul**, F. Kapteijn**, and J. A. Moulijn**
Hydrocarbon Processes and Catalysis, Norsk Hydro, Research Center, P.O. Box 2560, N-3907, Porsgrunn, Norway
*
*
*Reactor and Catalysis Engineering, Delft University of Technology, Julianalaan 136, 2628 BL, Delft, The Netherlands
e-mail: javier.perez.ramirez@hydro.com
Received October 9, 2002
Abstract—The decomposition of N O is strongly promoted by NO over steam-activated FeZSM-5. The pro-
2
moting effect of NO is catalytic, and in addition to NO , é is formed much more extensively at lower temper-
2
2
atures than in the absence of NO. The promotion effect only requires low NO concentrations in the feed, with
no significant improvements at molar NO/N O feed ratios higher than 0.25. No inhibition by NO was identified
2
even at a molar NO/N O feed ratio of 10, suggesting different sites for NO adsorption and oxygen deposition
2
by N O. The latter sites seem to be remote from each other. Transient experiments using in situ FT-IR/MS and
2
Multitrack over FeZSM-5 further elucidate the mechanism of NO promotion. The release of oxygen from the
catalyst surface during direct N O decomposition is a rate-determining step due to the slow oxygen recombina-
2
tion, which is favored by high reaction temperatures. NO addition promotes this oxygen desorption, acting as
an oxygen transfer agent, probably via NO species. Adsorbed NO may facilitate the migration of atomic oxy-
2
gen to enhance their recombination. Less than 0.9% of Fe seems to participate in this promotion. A model is
proposed to explain the phenomena observed in NO-assisted N O decomposition, including NO decomposi-
2
2
tion.
1
INTRODUCTION
authors proposed a nitrite/nitrate redox cycle (Eqs. (IV),
V)) by reaction with N O, yielding O and N . How-
ever, no experimental evidence was provided to support
this:
(
2
2
2
An intriguing general feature of FeZSM-5 catalysts
in direct N é decomposition is that NO significantly
2
enhances the activity, while the opposite effect is usu-
ally observed for other catalytic systems, e.g., based on
transition (Cu, Co) and noble metals (Ru, Rh) [1–5].
This peculiar behavior of FeZSM-5 makes it very
–
3
–
2
N O + NO
N + O + NO ,
(IV)
(V)
2
2
2
–
2
–
N O + NO
N + NO .
2 3
2
attractive to use in applications where both N O and
NO are present, such as in tail-gas of nitric acid plants
2
The present work further investigates the mecha-
O decomposition over steam-
[
6, 7]. The positive effect of NO on the N O conversion nism of NO-assisted N
2
2
was first reported by some of us in 1996, using a activated FeZSM-5. Activity data on the effect of the
FeZSM-5 prepared by ion exchange with Fe(II) sulfate molar NO/N O feed ratio and reaction temperature on
2
[
2]. At that time it was proposed that NO in the gas- the catalytic performance is presented. Based on flow
phase scavenged adsorbed oxygen (deposited by N O and pulse experiments, in combination with in situ
2
during the oxidation of active sites, Eq. (I)), leading to FTIR/MS, steady-state and transient phenomena
the formation of NO and regeneration of the active site regarding the chemistry of N O, NO, and NO over
2
2
2
(
Eq. (II)) [3]:
FeZSM-5 are investigated, and possible catalytic cycles
in NO-assisted N O decomposition are proposed.
N O + ᮀ
N + Oᮀ ,
(I)
2
2
2
NO + Oᮀ
NO + ᮀ .
(II)
2
EXPERIMENTAL
Reduction of an oxidized site by NO as represented
in Eq. (II) should lead to a stoichiometric process in
Isomorphously substituted FeZSM-5 was synthe-
sized hydrothermally using TPAOH as the template [9,
0]. The molar ratios between the components were
H O/Si = 45, TPAOH/Si = 0.3, Si/Al = 36, and Si/Fe =
which NO and N O react to form N and NO :
2
2
2
1
N O + NO
N + NO .
(III)
2
2
2
2
Recently, Sang and Lund [8] further discussed the 152. The as-synthesized sample was calcined in air at
mechanism of NO-assisted N O decomposition over a 823 K for 10 h and converted into the H-form by three
2
FeZSM-5 catalyst prepared by sublimation. These consecutive exchanges with an ammonium nitrate solu-
tion (0.1 M) for 12 h and subsequent air calcination at
823 K for 5 h. Finally, the catalyst was treated in flow-
1
This article was submitted by the authors in English.
0
023-1584/03/4405-0639$25.00 © 2003 MAIK “Nauka /Interperiodica”

6
40
PÉREZ-RAMÍREZ et al.
X
ground of the sample at the reaction temperature under
N O
2
He. Scan length minimization was used, leading to a
complete spectrum collection time of 7.7 s. The infra-
1
0
0
0
0
.0
.8
.6
.4
red measurements were performed using a 3 vol % N O
2
in He mixture and a 5 vol % NO in He mixture (in both
cases 30 ml/min). The purity of all the gases was
>99.985%. The transients in gas-phase composition
were followed using a quadrupole mass spectrometer
1
2
3
(
Pfeiffer Thermostar) connected on line to the outlet of
the in situ infrared cell via a capillary.
A detailed description of the Multitrack (multiple
time resolved analysis of catalytic kinetics) system is
presented elsewhere [13]. Two different gases can be
dosed to the reactor by means of high-speed pulse
.2
1
7
valves, yielding pulses of 10 molecules within 100 µs.
The reactor with the catalyst sample is located in a high
vacuum system, and during pulsing the peak pressure
remains below 3 Pa. The catalyst bed (100 mg, pellet
size 125–200 µm) is packed between two layers of inert
SiC particles. At the reactor exit, the reaction products
are analyzed by a quadrupole mass spectrometer posi-
tioned in-line with the reactor axis. As the signal-to-
noise ratio in this system is excellent, single pulses are
sufficient to obtain good peak signals. This is an impor-
tant aspect, as transient phenomena may remain unob-
served when several pulse responses have to be aver-
aged. Before the Multitrack measurements, the cata-
lysts were evacuated in situ at 723 K for 3 h. The
0
5
50
650
750
850
T, K
Fig. 1. N O ( X
FeZSM-5 in different feed mixtures: (1) 1.5 mbar N O in
He and (2) 1.5 mbar N O + 0.4 mbar NO in He, (3) 1.5 mbar
N O + 0.4 mbar NO + 20 mbar O in He; m/v(N O) =
) conversion vs. temperature over ex-
N O
2
2
2
2
2
2
2
0
5
8
.65 × 10 g s/mol; P = 1 bar.
ing steam at ambient pressure (water partial pressure of
mechanistic studies of direct N O decomposition were
3
00 mbar and 30 ml/min of N flow) at 873 K for 5 h,
2
2
carried out by continuously pulsing pure N O
yielding a sample denoted as ex-FeZSM-5.
2
(
~160 nmol per pulse) with intervals of 2 s (“cycle
Activity measurements were carried out in a six-
time”) at different temperatures in the range 623–973 K
flow reactor system [11], using 50 mg of catalyst (125–
and recording the masses m/e: 28 (N ), 32 (O ), and
2
2
5
2
00 µm) and a space time of 8.65 × 10 g s/mol. The
4
4 (N O). The effect of NO on the N O decomposition
2
2
space time is defined as the ratio m/v(N O) , where m
2
0
was studied in dual-pulse experiments in which pulses
is the catalyst mass and v(N O) the molar flow of N O
2
0
2
of N O (pure, ~160 nmol per pulse) and NO (15 vol %
2
at the reactor inlet. The feed conditions used were
NO in Ar, ~24 nmol NO per pulse) are sequentially fed
to the reactor with a time interval of 1 s between the
pulses and a cycle time of 2 s. Isothermal transient des-
1
0
.5 mbar N O, 0–15 mbar NO, 0–0.2 mbar NO , and
2
2
−20 mbar O with He as the balance gas. Before reac-
2
tion, the catalysts were pretreated in 1.5 mbar N O in
2
orption effects after stopping N O or NO pulsing were
2
He at 723 K for 1 h and cooled in that gas flow to the
followed at different temperatures, ranging from 698 to
initial reaction temperature. N O, N , and O were ana-
2
2
2
923 K. In these experiments, the masses m/e 30 (NO)
lyzed with a GC equipped with a thermal conductivity
and 46 (NO ) were additionally analyzed. The purity of
2
detector. NO, NO , and NO concentrations were deter-
2
x
the gases was >99.985%.
mined with a chemiluminescence NO analyzer.
x
The FT-IR (Fourier transform infrared) measure-
RESULTS
ments were performed using an in situ infrared cell with
CaF windows [12]. The catalyst was pressed in the
Activity experiments. The N O decomposition
2
2
2
form of a self-supporting wafer (~70 mg/cm ) at a pres- activity of ex-FeZSM-5 (Si/Al = 32.6, 0.66 wt % Fe) in
2
sure of 3 ton/cm . Gas flows were set by mass-flow con- different feed mixtures is presented in Fig. 1. In a
trollers, and composition programming was carried out N O/He feed, the catalysts show substantial N O con-
2
2
using a four-way valve, which was configured to select version only above 700 K. The addition of NO
two separate gas feeds. Pretreatment of the wafer con- (0.4 mbar; NO/N O = 0.27) enhances the reaction rate
2
sisted of drying at 573 K in He for 1 h. Spectra were considerably. The N O conversion curve is shifted to
2
recorded using a Nicolet Magna infrared 860 spectrom- about 100 K lower temperatures. The presence of oxy-
eter equipped with a nitrogen cooled MCT detector and gen (20 mbar O ) in the feed (N O + NO + O in He)
2
2
2
a rapid scan interferometer. Transient experiments were hardly affects the catalyst activity. Figure 2 shows the
–
1
performed at a resolution of 4 cm and using coaddi- influence of the molar NO/N O ratio in the feed on the
2
tion of 48 scans. Spectra were recorded against a back- N O conversion over ex-FeZSM-5 at reaction tempera-
2
KINETICS AND CATALYSIS Vol. 44 No. 5 2003

ELUCIDATION OF THE SURPRISING ROLE OF NO
641
X
PNOx, mbar
N O
2
1.0
0.8
0.6
0.4
0.2
0
5
0
.4
4
3
0
0
0
.3
.2
.1
NO
1
2
3
2
1
0
0.5
1.0
1.5
2.0
10.0
NO/N O
NO2
2
0
5
Fig. 2. N O ( X
) conversion vs. the molar NO/N O feed
2
2
N O
2
50
600
650
700
750
800
T, K
ratio over ex-FeZSM-5 at different temperatures: (1) 623,
2) 648, (3) 673, (4) 698, and (5) 723 K. Partial N O pres-
(
2
sure was fixed at 1.5 mbar and partial NO pressure varied
Fig. 3. Partial pressure of (1) NO, (2) NO , and (3) NO vs.
2
x
5
from 0 to 15 mbar; m/v(N O) = 8.65 × 10 g s/mol; P =
temperature over ex-FeZSM-5. Conditions: 1.5 mbar N2O +
2
0
5
~
1
0.4 mbar NO in He; m/v(N O) = 8.65 × 10 g s/mol; P =
1
bar.
2 0
bar. Equilibrium composition of NO and NO (Eq. (VI))
2
are represented by dashed lines.
tures ranging from 623 to 723 K. N O conversion is dra-
2
matically improved at low NO partial pressures
N O with NO to NO and N (Eq. (III)) cannot solely
2
2
2
(NO/N O < 0.25). From this value up to NO/NO = 10,
2
2
explain the promotion effect observed in Fig. 1. Besides
little effect on N O conversion is noticed.
2
inducing the formation of N O, the presence of NO in
2
The NO, NO , and NO profiles provide further
2
x
the NO -containing feed also significantly enhances the
2
information on the effect of NO on the catalytic perfor-
production of O , closing the oxygen mass balance
2
mance. In a (N + NO)/He feed, the formation of NO
2
2
(
long dashed line 3 in Fig. 4). Further information on
over ex-FeZSM-5 increases as a function of reaction
temperature (Fig. 3), reaching a maximum at 650–675 K,
the relatively low amount of NO formed compared to
2
i.e., around the inflection point of the N O decomposi-
2
tion activity curve (Fig. 1). Above this temperature,
NO (O ), mbar
NO formation decreases before completely disappear-
2
2
2
1
.5
ing at 775 K. The total NO level is constant in the tem-
x
perature range investigated, indicating that NO is not
x
converted to N or N O. The formation of NO is
2
2
2
1
.0
0
.4
beyond the thermodynamic equilibrium of Eq. (VI)
(
gray area in Fig. 3), suggesting that the reaction between
2
0
.6
1
.0
N O and NO (Eq. (III)) proceeds faster than Eq. (VI):
0
.2
2NO + O2
2NO2 .
(VI)
3
2
1
The equilibrium composition of NO and NO was
2
calculated assuming an é availability corresponding
0.5
2
without
with the amount if N O conversion would be 100%
2
NO
(
PO2 = 0.75 mbar). These theoretical partial pressure
profiles of NO and NO are displayed in Fig. 3 by the
2
dashed lines. The occurrence of the N O and NO reac-
2
0
0.5
1.0
1.5
N O, mbar
tion should be attributed to the catalytic performance of
2
ex-FeZSM-5, since no NO formation was observed
2
Fig. 4. NO (1) and O (2) formed vs. N O converted at
00 K over ex-FeZSM-5 at different partial NO pressures
mbar) in the feed. Crosses ( ) represent the NO + O
measured experimentally and the diagonal (long) dashed
line the mass balance for oxygen. The numbers refer to the
NO/N O ratio in the feed. Conditions: 1.5 mbar N O + (0–
2
2
2
over an inert material (SiC) in the same feed composi-
tion (not shown).
7
(
2
2
In Fig. 4 the amount of NO produced and O pro-
2
2
duced is plotted vs. the amount of N O reacted to N at
2
2
7
00 K for different inlet partial NO pressures. These
2
2
5
1
mbar NO) in He; m/v(N O) = 8.65 × 10 g s/mol; P = 1 bar.
2 0
results clearly show that the stoichiometric reaction of
KINETICS AND CATALYSIS Vol. 44 No. 5 2003

6
42
PNOx, mbar
PÉREZ-RAMÍREZ et al.
In situ FT-IR/MS experiments. The transients
occurring when NO adsorbed on ex-FeZSM-5 is
0
.25
.20
.15
.10
exposed to N O or vice versa were measured at 523 K.
2
The gas-phase composition was changed from 5 vol %
3
NO in He to 3 vol % N O in He and back to 5 vol % NO
2
0
0
0
0
in He. Upon the introduction of N O in the cell at
NO
2
5
23 K, bands assigned to gas-phase N O appear at
2
–
1
around 2225 cm in the infrared spectra presented in
Fig. 6. A continuous decrease of the bands assigned to
adsorbed NO can be observed. After an induction
period of 30 s, a new band develops around 1632 cm ,
which continuously grows in intensity and can be
assigned to adsorbed NO on Fe(II) [14–16]. The corre-
sponding gas-phase product evolution at 523 K is
NO
–
1
NO2
2
2
.05
0
1
shown in Fig. 7 (region I). A time delay is observed
NO2
between the time of switching the gas from NO to N O
2
and the formation of N , in agreement with the infrared
2
5
50
600
650
700
750
800
T, K
observations. The formation of NO was not detected at
2
this temperature. Upon switching the gas-phase com-
position from N O back to NO, a rapid transition from
Fig. 5. Partial pressure of (1) NO, (2) NO , and (3) NO vs.
2
2
x
–
1
the 1635 cm to the original bands at around 1886 and
temperature over ex-FeZSM-5. Feed composition: 0.2 mbar
5
–1
NO in He; m/v(N O) = 8.65 × 10 g s/mol; P = 1 bar; equi-
1874 cm were observed in Fig. 8. At the same time,
2
2
0
librium compositions of NO and NO (Eq. (VI)) are repre-
2
the MS analysis shows that NO is formed immediately
2
sented by dashed lines.
(
Fig. 7, region II), as well as some N . The N produced
2 2
is related to the limited time that both NO and N O are
2
present in the infrared cell.
the amount of N O decomposed can be related to the
2
decomposition reaction of NO to NO and O . By feed-
2
2
Multitrack experiments. Multitrack was used to
further investigate the formation of O during N O
ing a mixture of NO in He over ex-FeZSM-5, the NO
2
2
2
2
decomposition and the influence of NO in the N O con-
decomposition rate was analyzed. The result for PNO2
=
2
version over ex-FeZSM-5. A typical Multitrack profile
0
.2 mbar is shown in Fig. 5. NO is converted into NO
2
during direct N O decomposition at 823 K over this cat-
2
and O over the catalyst in the temperature range where
2
alyst after 100 pulses is presented in Fig. 9. The N O
2
the promotion effect of NO occurs. Equilibrium is and N signals are very similar in shape. The O
2
2
response signal of the N O pulse deviates from the N O
reached above 700 K, which indicates that NO decom-
2
2
2
and N signal, being much broader. In order to discrim-
position and NO oxidation by NO (Eq. (VI)) occur
2
2
inate if the shape of the oxygen is due to a mechanistic
fairly fast. This result suggests that NO decomposition
2
feature of N O decomposition or simply due to nonre-
2
might contribute to the formation of O . Production of
2
active adsorption–desorption phenomena of O along
2
NO is hardly observed over inert material (SiC, not the catalyst bed, molecular oxygen was pulsed. In this
shown).
case, the pulse is very similar in shape to that of N and
2
N O(gas)
2
Fe-NO2
0
.1 Fe-NO
886 1874
1
632
t = 1.6 min
1
Delay (30 s)
N O in, t = 0
2
2
400
2200
2000
1800
1600ν, cm–1
Fig. 6. Transient infrared spectra of NO adsorbed on ex-FeZSM-5 at 523 K upon switching a flow (30 ml/min) of 5 vol % NO in
He to a flow (30 ml/min) of 3 vol % N O in He. Spectra were recorded against a background of the catalyst at 523 K in He.
2
KINETICS AND CATALYSIS Vol. 44 No. 5 2003

ELUCIDATION OF THE SURPRISING ROLE OF NO
II
NO
643
I
NO
N O
2
Delay
~30 s)
Instantaneous
(
N + NO
2
2
N2
NO2
100
0
50
150
200
250
300
t, s
Fig. 7. Transient in gas composition upon switching a flow (30 ml/min) of 5 vol % NO in He to a flow (30 ml/min) of 3 vol % N O
2
in He at t = 0 (region I) and back to a flow (30 ml/min) of 5 vol % NO in He at t = 250 s (region II).
Fe-NO
N O(gas phase) 0.1
2
Fe-NO2
t = 0.5 min
NO in, t = 0
2
400
2200
2000
1800
1600 ν, cm–1
Fig. 8. Transient infrared spectra of NO adsorbed on ex-FeZSM-5 at 523 K upon switching a flow (30 ml/min) of 3 vol % N O in
2
2
He back to a flow (30 ml/min) of 5 vol % NO in He. Spectra were recorded against a background of the catalyst at 523 K in He.
N O (not shown), confirming the validity of the first N O−NO cycles than in the experiment with only N O.
2
2
2
hypothesis. The delay of the oxygen signal was further Stopping NO pulsing results in a transition back to the
investigated by pulsing N O at different reaction tem- original catalyst behavior in six cycles (12 s), with a
2
peratures. These results are shown in Fig. 10. In the very broad O response (Fig. 11, profiles (3, 4)). At
2
range 573–673 K, a very broad oxygen signal is lower temperatures, the promotion effect lasts longer.
observed. At 723 K, the oxygen signal becomes more At 698 K, the promotion vanishes 20 cycles (40 s) after
pronounced. Around 873 K, the signal significantly the NO pulsing is stopped. This indicates that adsorbed
sharpens, indicating that the rate of desorption of oxy- species formed at the time of the NO pulse are involved
gen from the catalyst surface is significantly enhanced. in the enhanced O formation and their stability towards
2
desorption determines the duration of the effect. The
To study the effect of NO on O formation in N O
2
2
sharp oxygen signal is rapidly recovered after four
decomposition, dual-pulse experiments were carried
pulses of NO (Fig. 11, profiles (5, 6)).
out at different temperatures, in which N O and NO
2
were pulsed at 0.1 and 1 s, respectively, repeated in
During the dual-pulse experiments at these temper-
cycles of 2 s. Pulsing N O (at 0.1 s) over ex-FeZSM-5 at atures, no NO or NO signals were observed at the time
2
2
7
73 K leads to broad oxygen signal (Fig. 11, profile (1)), of the N O pulse, which indicates that NO does not
2
indicating a relatively slow desorption of O . Alternate block N O decomposition sites. NO formation was
2
2
2
pulsing of N O and NO for 10 cycles leads to a remark- observed but only at the time of the NO pulse (Fig. 11,
2
able effect on the O response at the time of the N O profile (7)). The MS technique did not allow quantifica-
2
2
pulse. The O response sharpens (Fig. 11, profile (2)), tion of the NO desorption. At high temperatures (e.g.,
2
2
indicating a significantly faster O2 desorption in 973 K), the effect of NO on the oxygen desorption
KINETICS AND CATALYSIS Vol. 44 No. 5 2003

6
44
PÉREZ-RAMÍREZ et al.
N2
9
8
7
7
6
6
5
73 K
73 K
73 K
23 K
73 K
23 K
73 K
O2
N O
2
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 t, s
Fig. 10. O profiles from Multitrack during continuous
0
0.5
1.0
1.5
2.0
t, s
2
pulsing of pure N O over ex-FeZSM-5 at different temper-
2
atures. Cycle time, 2 s.
Fig. 9. Typical Multitrack profiles of N O, N , and O dur-
2
2
2
ing pulsing of pure N O over ex-FeZSM-5 at 823 K.
2
Fe-zeolites can then be understood by a slow migration
of adsorbed oxygen atoms. This rate of desorption is
higher at higher temperatures (Fig. 10) and is attributed
to a higher oxygen mobility and recombination. Once
adsorbed atomic O species meet each other, desorption
behavior over ex-FeZSM-5 is negligible, since the O2
profiles obtained by pulsing N O only, or N O and NO
2
2
sequentially, are very similar (not shown). It should be
noted that at such high temperature N O conversion
2
takes place rapidly, since no O adsorption was
2
over the catalysts was 100% both in the absence and
presence of NO under flow conditions.
observed for FeZSM-5 [1, 18].
DISCUSSION
As already indicated in the Introduction, the benefi-
cial effect of NO on the activity of FeZSM-5 has been
attributed by Kapteijn et al. [2, 3] to scavenging of
adsorbed oxygen (é), regenerating the active site
7
N O on
2
NO on
(
Eq. (II)). The present experimental results show that
6
scavenging of adsorbed O by NO is not the only mech-
5
anism of promotion and that NO also enhances O for-
2
mation. Multitrack experiments show that oxygen des-
orption from the catalyst surface is a relatively slow
step in the reaction mechanism. In view of the slow
oxygen desorption (Figs. 9, 10), the recombination of
two adsorbed oxygen atoms (located on two different
4
3
sites) appears the most likely explanation for O forma-
2
2
1
tion (Eq. (VII)), rather than the reaction of N O with an
2
oxidized site (Eq. (VIII)):
2
Oᮀ
O + 2ᮀ ,
(VII)
2
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
N O + Oᮀ
N + O + ᮀ .
(VIII)
2
2
2
t, s
The dynamic nature of the adsorbed oxygen in the
Fig. 11. O2 profiles measured by Multitrack during catalytic
decomposition of N O over ex-FeZSM-5 at 773 K using a
zeolite was studied by Valyon et al. [17], showing that
2
cycle time of 2 s: (1) pulsing N O (at 0.1 s) and NO
2
1
2
8
1
8
adsorbed O species (deposited by N O ) in Fe–Mor
(at 1.1 s) after ten cycles; (2–4) pulsing N2O only (at 0.1 s)
two, three, and six cycles after switching off the NO valve;
can exchange with the lattice oxygen of the zeolite in
the vicinity of Fe, so the oxygen must have a certain
mobility to achieve this. The slow oxygen desorption in
(
5, 6) pulsing N O (at 0.1 s) and NO (at 1.1 s) two and four
2
cycles after switching on the NO valve, respectively;
(7) pulsing NO.
KINETICS AND CATALYSIS Vol. 44 No. 5 2003

ELUCIDATION OF THE SURPRISING ROLE OF NO
645
The rate of oxygen desorption from the catalyst sur-
In view of the results presented in this paper, various
face is greatly enhanced by NO addition. Indeed, a sig- pathway(s) are needed to explain the catalytic effect of
nificant increase was found in the amount of oxygen NO on N O decomposition. The different experimental
2
formed if NO was added to the N O-containing feed observations are depicted in the scheme. The transitions
2
(
Fig. 11), and only a relatively small amount of NO is of the different structures considered (A–D) are also
needed to dramatically increase the N O decomposition considered in the reaction mechanism. Initially, N O
2
2
rate (Fig. 2). Although during this process NO is reacts on a vacant site, yielding N and leaving an oxi-
2
2
formed, even beyond the thermodynamic equilibrium dized site (reaction (I')). On the catalyst, a substantial
between NO, O , and NO according to Eq. (VI) (see amount of adsorbed NO is present, as observed by
2
2
Fig. 3), the enhanced oxygen production strongly sug- in situ FT-IR. Due to the absence of inhibition by NO in
gests a catalytic effect of NO. If Eq. (II) were the only the activity tests and the fact that no NO signal appears
promotion route induced by the addition of NO, a pro- at the time of the N O pulse in the Multitrack experi-
2
gressive increase in conversion upon increasing the ments, it is concluded that the NO adsorption and N O
2
inlet NO partial pressure would be expected. This is not decomposition do not compete for the same site, so
the case: the promotion occurs already at relatively low both processes occur at different Fe-species (light and
substoichiometric amounts of NO, indicating the cata- dark solid gray areas). Adsorption of N O (like Oᮀ)
2
lytic nature, and reaches a limiting value at increasing and NO in two open coordinations at the same iron site
molar NO/N O feed ratios up to 10. This suggests the seems hardly probable, since dinitrosyls were not iden-
2
involvement of NO adsorption and that the sites where tified by infrared studies in this system [Oᮀ]. As previ-
NO is adsorbed are not in competition with N O ously discussed, the production of NO shown in reac-
2
2
decomposition sites. Competitive adsorption would tion (II') is certainly not the major mode of promotion,
have resulted in inhibition, especially at high partial although it cannot be completely excluded in the reac-
NO pressures. From the observation that at 698 K only tion network. In Fe-zeolite catalysts, N O activation
2
four NO pulses are needed to restore the promotion occurs next to an adsorbed *NO molecule. At these
effect, from the pulse size and the amount of catalyst temperatures (550–700 K), the oxidized site (O) subse-
used, and assuming that all NO adsorbed at Fe sites, it quently oxidizes adsorbed NO to adsorbed NO (reac-
2
can be calculated that less than 0.9% of the Fe present tion (III')), yielding structure C in the scheme. Subse-
is involved in the promotion. This suggests that a very quently, NO desorption is induced by adsorption of
2
low fraction of Fe is active in the reaction.
NO (reaction (IV')), as was observed from the Multi-
track experiments by the presence of a NO signal at the
To explain the enhancement of the é desorption
2
2
time of the NO pulse. This closes the catalytic cycle in
rate at the time of the N O pulse, an adsorbed species
2
the conversion of N O and NO to N and NO at lower
formed at the time of the NO pulse needs to be
involved. The amount of adsorbed NO will be reduced
at high temperatures, while oxygen desorption at high
temperatures already proceeds fast. This adsorption
involvement is supported by the slower decay of the
NO promotion at lower temperatures in the dual-pulse
Multitrack experiments after NO pulsing is stopped.
Sang and Lund [8] used the interconversion of nitrates
and nitrites (Eqs. (IV), (V) in Introduction) to explain
2
2
2
temperatures. This is in excellent agreement with the
transient FT-IR/MS experiments, where NO formation
2
is observed upon switching from N O to NO.
2
Different options can be proposed to explain the
enhanced oxygen formation. Again N O activation
2
occurs next to an adsorbed NO molecule, yielding
structure B in the scheme. Subsequently, a second N O
2
molecule reacts with the site, yielding N and O (reac-
2
2
the enhancement of N O conversion by NO. However,
2
tion (V')), and structure A is regenerated. The enhanced
oxygen desorption from the active center has been
ascribed to a reduced stability of adsorbed oxygen,
induced by either electronic or steric effects of the NO
in the infrared analysis of ex-FeZSM-5, nitrate bands
have never been observed but only adsorbed NO and
NO species (Figs. 6, 8).
2
The Multitrack experiments clearly show that oxy- adsorbed on neighboring oxidized sites. A more plausi-
gen desorption is triggered at the time of the N O pulse ble explanation for the enhanced oxygen desorption is
2
and not at the time of the NO pulse. NO formation is reaction (VI'). The increased N O decomposition is
2
2
observed at the time of the NO pulse, indicating dis- simply explained by the recombination of oxygen
placement of adsorbed NO by NO as confirmed by present in adsorbed NO and oxygen species deposited
2
2
transient in situ FT-IR/MS experiments. In the Multi- by N O on a neighboring site. Alternatively, O forma-
2
2
track formation of O , NO is decoupled: NO is mainly tion from NO , via reaction (VI'), requires competition
2
2
2
2
released due to displacement by NO and probably by between N O and NO to oxidize a vacant ᮀ-site. If
2
2
thermal desorption, while O is formed during the N O N O is a more efficient oxidizer of these sites, NO lev-
2
2
2
2
pulses. If NO decomposition, as presented in Fig. 5, els beyond the thermodynamic equilibrium of Eq. (VI)
2
contributed significantly to O2 formation, an O2 are indeed feasible. Thus, the adsorbed NO serves to
response would have been expected at the time of the accommodate temporarily the deposited oxygen from
NO pulse, rather than at the time of the N O pulse only. the N O, freeing the neighboring site for deposition of
2
2
This excludes the possibility that Eq. (III) is the only a second oxygen. This would imply the presence of
reason for the promotion effect.
remote sites for the N O decomposition. Consequently,
2
KINETICS AND CATALYSIS Vol. 44 No. 5 2003

6
46
PÉREZ-RAMÍREZ et al.
Fe-sites in the catalyst
O
N
O O
N N
Active Fe-sites for N O decomposition
2
A
B
ZSM-5
O O
N O N N
No.
Elementary step
Transition
O
I'
N O +
N + O
A
B
2
2
ZSM-5
II'
NO + O
NO +
B
B
C
A
C
A
2
O
N
O O O
N N
III'
IV'
NOads + O
NO + NO2ads
NO2ads +
NO + NO
2
ads
C
D
ZSM-5
O O O
V'
N O + O
N + O +
B
A
2
2
2
O
VI'
NO + O
NOads + O +
NOads + O
D
C
A
B
2
N O N N
VII'
NO +
2
ZSM-5
Scheme. Pathways in NO-assisted N O decomposition over ex-FeZSM-5.
2
the migration of oxygen atoms to recombine to molec- participate in this promotion, indicating the low density
ular oxygen constitutes the rate determining process in of active sites. Additionally, adsorbed NO may facili-
tate the migration of oxygen through NO intermedi-
N O decomposition. Taking this further, it could be
speculated that the presence of NO on the catalyst sur-
face enhances the mobility of adsorbed oxygen via an
2
2
ates, enhancing the change of recombination to O2.
intermediate NO species. This mechanism leads to
2
ACKNOWLEDGMENTS
faster oxygen recombination and thus to accelerated O2
desorption. The transfer of the oxygen atom from one
NO to the other is well possible, as previously observed
B. van der Linden and S. Mollá-Romano are grate-
fully acknowledged for assistance in the FT-IR/MS and
the Multitrack measurements.
1
5
18
for NO and N O step changes over Cu–ZSM-5 and
Cu–Y [17, 19]. Of course, O formation by two adja-
2
cent adsorbed NO groups cannot be ruled out.
2
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2
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