Temperature-programmed desorption/surface reaction (TPD/TPSR) study of Fe-exchanged ZSM-5 for selective catalytic reduction of nitric oxide by ammonia

Article abstract of DOI:10.1006/jcat.2000.3118

NO, NO₂, and N₂O from combustion of fossil fuels have been a major source of air pollution. The abatement of these NO_x emissions is urgent due to their implication in photochemical smog, acid rain, ozone depletion, and greenhouse effects. TPD and temperature-programmed surface reaction (TPSR) were used to study Fe-exchanged ZSM-5 for SCR of NO with ammonia. TPD profiles revealed that NO_x and NH₃ adsorbed on Fe-ZSM-5. Physisorbed NO_x and NH₃ were not considerably affected by Fe content. Chemisorbed NO_x increased with increasing Fe content, while chemisorbed NH₃ decreased due to substitution of protons by Fe ions. TPSR results showed that NH₃ adsorbed species were quite active in reacting with O₂, NO, NO + O₂, and NO₂, following the reactivity rank order NO₂ ～ NO + O₂ > NO > O₂. NO_x adsorbed species were also reactive to NH₃ at high temperatures. With NO_x and NH₃ coadsorbed on Fe-ZSM-5, TPSR with gaseous He, NO₂, and NO exhibited two types of reactions for N₂ formation. One reaction near 55°C originated from decomposition of ammonium nitrate that was not affected by Fe³⁺ content. The other reaction at 170°-245°C was due to an adsorbed complex, possibly [NH₄/⁺]₂NO₂, reacting with NO₂ or NO. A possible reaction path was proposed for NO reduction involving NO₂ and [NH₄/⁺]₂NO₂ as intermediates. Since the reactivity of [NH₄/⁺]₂NO₂ to NO was higher than that to NO₂, it was deduced that [NH₄/⁺]₂NO₂ preferred to react with NO and not NO₂, both of which were present in the SCR reaction. This could be the reason for N₂ being the only product for SCR on Fe-ZSM-5.

Full text of DOI:10.1006/jcat.2000.3118

Journal of Catalysis 198, 20–28 (2001)
doi:10.1006/jcat.2000.3118, available online at http://www.idealibrary.com on
Temperature-Programmed Desorption/Surface Reaction (TPD/TPSR)
Study of Fe-Exchanged ZSM-5 for Selective Catalytic Reduction
of Nitric Oxide by Ammonia
R. Q. Long and R. T. Yang1
Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136
Received May 11, 2000; revised November 4, 2000; accepted November 4, 2000; published online February 1, 2001
emissions is increasingly urgent. An efficient technology
for reducing NOx emission from power plants is selective
catalytic reduction (SCR) with ammonia in the presence of
oxygen.
Temperature-programmed desorption (TPD) and temperature-
programmed surface reaction (TPSR) were employed to study Fe-
exchanged ZSM-5 for selective catalytic reduction (SCR) of NO
with ammonia. TPD profiles of ammonia and NOx showed that
both NOx and NH3 adsorbed on Fe–ZSM-5. Physisorbed NOx and
NH3 were not affected significantly by iron content. With increas-
ing iron content, chemisorbed NOx (mainly NO2 bonded to iron
sites) increased while chemisorbed NH3 (mainly NH+4 on Brønsted
acid sites) decreased due to substitution of protons by iron ions.
The TPSR results indicated that ammonia adsorbed species were
quite active in reacting with NO, O2, NO + O2, and NO2 (producing
H2O, N2 and/or N2O), following the reactivity rank order NO2
NO + O2 > NO > O2. NOx adsorbed species were also reactive to
NH3 at high temperatures. With NH3 and NOx coadsorbed on Fe–
ZSM-5, TPSR with gaseous He, NO, and NO2 showed two kinds
of reactions for N2 formation. One reaction near 55 C originated
from decomposition of ammonium nitrite, which was not affected
by Fe3+ content. The other reaction at higher temperatures (170–
245 C) was due to an adsorbed complex, probably [NH+4 ]2NO2,
reacting with NO or NO2. A possible reaction path was proposed
for NO reduction involving NO2 and [NH+4 ]2NO2 as intermediates.
Since the reactivity of [NH+4 ]2NO2 to NO (producing only N2 at
170 C) was higher than that to NO2 (producing both N2 and N2O
at 200 C), it is reasonable to deduce that [NH+4 ]2NO2 prefers to
react with NO and not NO2, both of which are present in the SCR
reaction. This may be the reason for N2 being the only product for
c
In the ammonia SCR reaction, catalysts with different
compositions have been investigated (1). The commercial
catalysts that are used today are V2O5 mixed with WO3
and/or MoO3 supported on TiO2. More recently, H zeolites
and ion-exchanged molecular sieves (pillared clays, ZSM-5,
mordenite, Y zeolite, etc.) have received much attention for
the SCR reaction (3–18). They were found to be more active
than vanadia catalysts. In particular, Fe-exchanged ZSM-5
(Fe/Al = 0.19–0.43) showed nearly 100% NO conversions
at 400–550 C under conditions with a high space velocity
(GHSV = 4.6 105 h 1) (14–16). This is much higher than
the commercial V2O5 + WO3/TiO2 catalyst (67% NO con-
version) under the same conditions (15). In addition, N2
was the only detectable N-containing product and no N2O
was observed in the entire temperature range (250–600 C)
for the Fe–ZSM-5 catalysts. This is different from vanadia
catalysts, on which much N2O and NO were detected when
the reaction temperature was above 450 C due to oxidation
of ammonia by oxygen (1, 2, 14).
For ammonia SCR reaction, the adsorption of reactants,
i.e., NH3 and NO, has been extensively investigated on
different catalysts. As discussed in detail in recent reviews
(1, 2), it is generally accepted that, for vanadia catalysts,
the SCR reaction follows an Eley–Rideal-type mechanism;
i.e., a strongly adsorbed ammonia species reacts with a
gaseous NO molecule to form molecular N2. Temperature-
programmed desorption (TPD) and Fourier transform
infrared (FTIR) experiments showed that NO does not
adsorb on the surface of oxidized vanadia under reaction
conditions (2). However, for molecular sieve-type catalysts,
it was reported that both NOx and NH3 could adsorb on
the catalysts (2, 3, 7, 10, 14, 19–22). On H-mordenite,
Kiovsky et al. (3) observed that NO2 was more reactive
with NH3 than NO and an increase in NO2/NO ratio
increased NOx reduction activity. They suggested that NO2
was an active intermediate for NO reduction by ammonia.
SCR on Fe–ZSM-5.
2001 Academic Press
Key Words: selective catalytic reduction; selective catalytic re-
duction of NO with ammonia; Fe-exchanged ZSM-5 catalyst;
temperature-programmed desorption; temperature-programmed
surface reaction.
INTRODUCTION
Nitrogen oxides (NO, NO2, and N2O) from combustion
of fossil fuels have been a major source of air pollution. Due
to their implication in photochemical smog, acid rain, ozone
depletion, and greenhouse effects, abatement of these NOx
1 To whom correspondence should be addressed. E-mail: yang@
umich.edu.
20
0021-9517/01 $35.00
c
Copyright
2001 by Academic Press
All rights of reproduction in any form reserved.

(TPD/TPSR) OF Fe-EXCHANGED ZSM-5
21
Over H–ZSM-5, Eng and Bartholomew (10) also detected mation could be obtained on Fe–ZSM-5 and, consequently,
NO2 formation and concluded that it played an important very high SCR activity was achieved (14), because iron ions
role in the ammonia SCR reaction. In addition, by using in ZSM-5 were much more active than iron oxide for the
IR spectroscopy, Ito et al. (19) found that both nitroso- NH3 SCR reaction (14, 15). Fe(300)–ZSM-5 was prepared
nium ion (NO+) and nitrito ion (NO2 ) were formed on by sublimation of FeCl3 vapor into H–ZSM-5 at 320 C for
Ce-exchanged mordenite. NO+ reacted with coordinated 2h, followed byhydrolyticremovalofchlorine, byfollowing
NH3 at low temperatures and NO2 reacted with NH+4 the procedure given by Chen and Sachtler (23). The cata-
ions at higher temperatures, producing N2 and H2O. In lysts obtained were first dried at 120 C in air for 12 h, then
our previous studies, NO2 was observed on Fe-exchanged calcined at 500 C for 6 h. Fe2+ in the catalyst was oxidized
TiO2-pillared clay (Fe–TiO2–PILC) (20) and Fe-exchanged to Fe3+ (24, 25). Finally, the samples obtained were pelleted
ZSM-5 catalysts (14) when introducing NO + O2/He. The and ground to 60–100 mesh. The Fe contents, measured by
reaction rate between NO2 and NH3 was much higher than neutron activation analysis, were 2.40 wt% in Fe(95)–ZSM-
that between NO and NH3. Moreover, the SCR activity on 5 and 6.69 wt% in Fe(300)–ZSM-5. For catalyst designation,
the Fe–TiO2–PILC catalyst correlated well with the rate the number in parentheses indicates the Fe-exchange level.
of formation of NO2 (20). Therefore, for molecular sieve The iron-exchange level was calculated as 3 (number of
catalysts, NO2 plays an important role in the reduction of iron ions)/(number aluminum ions) because most of the
NO by ammonia in the presence of oxygen.
iron was in the ferric form on the catalysts (21–25). FeCl3
The aim of the present study is to gain new insights (97.0% ), iron powder, and HCl solution (2.00 M) were sup-
into the mechanism of NO reduction on Fe–ZSM-5 by us- plied by Fisher.
ing TPD and temperature-programmed surface reaction
(TPSR). To obtain information on the acidity and re-
dox properties of catalyst, TPD is used to study ad-
sorption/desorption behavior of NH3 and NO + O2 on
Fe–ZSM-5 with various iron contents. Since both NOx and
NH3 can adsorb on Fe–ZSM-5, the following reactions
are studied in the TPSR experiments: (1) NH3-adsorbed
species reacting with gaseous NO, O2, NO + O2, and NO2,
(2) NOx-adsorbed species reacting with gaseous NH3, and
(3) NOx + NH3-coadsorbed species reacting with NO and
NO2. The relative reactivities between different surface
species and gaseous species are evaluated and a possible
reaction path for reducing NO is discussed.
TPD and TPSR Studies
TPD and TPSR experiments were carried out in a fixed-
bed quartz reactor with an inner diameter of 10 mm. The
temperature was controlled by an Omega programmable
temperature controller (CN-2010). Before the experiment,
100 mg sample was pretreated in He at 500 C for 1 h
to remove adsorbed H2O and other gases. As reported
by Lobree et al. (21), Fe2+ began to form at 500 C by
autoreduction of Fe3+ in Fe–ZSM-5 with Fe/Al > 0.19,
similar to that in Cu–ZSM-5 (26). However, our previous
H2-TPR and ESR results indicated that the iron was still
present mainly in the ferric form after this pretreatment
(25). After the sample was cooled to room temperature,
the He flow was switched to a flow of 1000 ppm NH3/He for
1.5 h. Mass spectra showed that the intensity of ammonia
had stabilized. The reactor was then purged with He for
another 1 h. For the TPD experiment, He was passed
EXPERIMENTAL
Preparation of Catalyst
NH4–ZSM-5 (Si/Al = 10) was obtained from Air Prod- through the reactor and ammonia TPD was performed by
ucts and Chemicals Inc. This material contained 0.07% ramping the temperature at 10 C/min to 600 C. For the
Fe2O3 impurity. H–ZSM-5 was prepared by calcining NH4– TPSR experiments, subsequent to the ammonia adsorp-
ZSM-5 at 500 C in air for 3 h. Fe(95)–ZSM-5 was obtained tion step, the He flow was switched to different flows of
by ion exchange of 2 g H–ZSM-5 with a mixed solution that 1000 ppm NO/He, 2% O2/He, 1000 ppm NO + 2% O2/He,
contained 200 ml of 0.1 M HCl and 0.73 g iron powder. Dur- or 1000 ppm NO2/He. At the same time, the reactor was
ing preparation of the catalyst, hydrochloric acid reacted ramped at 10 C/min to 600 C. A magnetic deflection-type
with iron metal to generate ferrous ions and then Fe2+ ions mass spectrometer (AERO VAC, Vacuum Technology
exchanged with H–ZSM-5. To prevent oxidation of the fer- Inc.) was used to monitor continuously the effluent from
rous ion to the ferric form, the ion exchange was performed the reactor, which contained NH3 (mass 17 minus the
under a helium flow at room temperature for 10 days. This contribution of H2O), H2O (mass 18), N2 (mass 28), NOx
method (16) is different from the procedure that was used (mass 30), O2 (mass 32), and N2O (mass 44). It is noted that
in the Mobil patent (i.e., ZSM-5 was exchanged directly an NO2 signal at mass 46 was not detected with our mass
with a solution of ferrous salts above 55 C in an inert at- spectrometer. Even when 1000 ppm NO2/He was used, the
mosphere) (17). Our method avoids hydrolysis of the Fe3+ intensity of mass 46 was still too weak to be detected (but
ions, which are often present in ferrous salts as an impurity. the mass 30 signal was strong). Therefore, we used mass 30
Hence a high ion-exchange level without iron oxide for- as the signals for NOx (NO + NO2) in this work.

22
LONG AND YANG
For NOx adsorption, the sample was first treated with He
at 500 C for 1h followed byexposure to 1000ppm NO + 2%
O2/He for 1 min, and then cooled to room temperature in
flowing NO + O2 in 1 h. The subsequent procedure was the
same as that used in the ammonia TPD and TPSR experi-
ments described above, except that 1000 ppm NH3/He was
used in the TPSR experiment.
The TPSR experiments between adsorbed NOx and
NH3 as well as the reactions of adsorbed NOx + NH3 with
gaseous NO and NO2 were also studied. In these cases, af-
ter the He-treated sample (at 500 C) was cooled to room
temperature, it was exposed to 1000 ppm NO + 1000 ppm
NH3 + 2% O2/He for 1.5 h. The reactor was then purged
with He for another 1 h. Subsequently, the sample was
heated linearly to 600 C in different flows of He, 1000 ppm
NO/He or 1000 ppm NO2/He. The effluent gases were ana-
lyzed continuously with the mass spectrometer.
The gases were obtained by blending different premixed
gases, using He as the diluent gas. The gas concentration
was calibrated by standard gas with the mass spectrome-
ter. The total gas flow rate was 500 ml/min (ambient condi-
tions). The premixed gases (1.01% NO/He, 0.98% NO2/He,
and 1.00% NH3/He) and He (99.995% ) were supplied by
Matheson without additional purification. The mass spec-
trum showed that about 0.05% H2O vapor was present in
the 1.00% NH3/He cylinder.
FIG. 1. TPD profile of NH3 from H–ZSM-5 and Fe–ZSM-5 catalysts
(number in parentheses indicates Fe-exchange level). The samples were
first treated with NH3/He at 25 C (see details in text).
RESULTS
content, the NH3 desorption peak at 390 C decreased sig-
nificantly, which was due to the fact that the Brønsted acid
protons of ZSM-5 were substituted by Fe2+ or Fe3+. Dur-
ing the ammonia TPD experiments, no oxidation products
(such as N2, N2O, or NOx) were detected.
TPD of Ammonia
The TPD profiles of ammonia on H–ZSM-5, Fe(95)–
ZSM-5, and Fe(300)–ZSM-5 are shown in Fig. 1. Ammonia
desorption was observed over a wide temperature range for
these samples. Two main peaks (both broad) were found at
160 and 390 C. Our previous FTIR spectra (25) showed
that, after Fe–ZSM-5 was treated in flowing 1000 ppm
TPD of NOx
NOx TPD experiments were also conducted on H–ZSM-
NH3/He, physisorbed NH3 (at 2600 and 2900 cm 1), co- 5, Fe(95)–ZSM-5, and Fe(300)–ZSM-5. The results are
ordinated NH3 (minority, at 1587 and 1276 cm 1), and shown in Fig. 2. Only one peak at 75 C was observed on
NH+4 ions (majority, at 3353, 3290, 3050, 2795, 1705, and H–ZSM-5, which probably came from physisorbed NOx.
1473 cm 1) were generated. With increasing temperatures, It was reported that NO could be oxidized to NO2 by O2
the IR bands due to physisorbed and coordinated NH3 dis- and then stored on H–ZSM-5 (10, 18). As shown in Fig. 2,
appeared below 300 C, whereas those due to NH+4 ions with two desorption peaks at 75 and 310 C were detected on
three hydrogen atoms bonded to three oxygen ions of AlO4 the Fe–ZSM-5 catalysts, which can be attributed to NOx
1
tetrahedra (3H structure) (at 3353, 3290, and 1473 cm
)
adsorbed at two different types of sites. Since the peak at
were still detected at 400 C in He (25). This suggests that 310 C was not observed on the H–ZSM-5, one can con-
the NH+4 ions with 3H structure were more stable at high clude that the NOx was bonded to iron sites. The intensity
temperatures than the other ammonia adsorbed species. of the peak at 310 C increased with iron content. Our pre-
Therefore, it is reasonable to assign the ammonia species vious FTIR spectra showed that N2O3 (1682 cm 1), NO2
desorbed at 160 C to physically adsorbed NH3 and coordi- (1614 cm 1), and nitrate species (1575 cm 1) were formed
nated NH3 and that desorbed at 390 C to the NH4+ ions with on the Fe–ZSM-5 that was treated with NO + O2. N2O3 was
3H structure. Further evidence for the assignments was ob- not stable and decomposed below 100 C, while NO2 was
tained by a separate TPD experiment on NH4–ZSM-5. In the dominant species at high temperatures (25). Hence, the
this experiment, no NH3 desorption peak near 160 C was peak at 75 C on Fe–ZSM-5 probably came from desorption
seen, but a peak at 390 C was observed. With increasing Fe of N2O3 and physisorbed NOx, while the peak at 310 C was

(TPD/TPSR) OF Fe-EXCHANGED ZSM-5
23
FIG. 3. TPSR profile in 1000 ppm NO/He following exposure of
Fe(95)–ZSM-5 to 1000 ppm NH3/He at room temperature for 1.5 h.
FIG. 2. TPD profile of NOx from H–ZSM-5 and Fe–ZSM-5 catalysts
(number in parentheses indicates Fe-exchange level). The samples were
first treated with NO + O2/He at 500 C (see details in text).
due to desorption of NO2 species (majority) and decompo-
sition of nitrate (minority). During the desorption of NOx,
a weak O2 desorption peak was also detected at 310 C on
the Fe–ZSM-5 catalysts, but other N-containing products
such as N2 and N2O were not observed, indicating that NO
decomposition did not occur.
TPSR of Ammonia Adsorbed Species
TPSR experiments were obtained by first saturating
Fe(95)–ZSM-5with ammonia, followed byheatingin differ-
ent flows. Figure 3 shows the results of TPSR between am-
monia adsorbed species and gaseous NO/He. NH3 desorp-
tion was significant at low temperatures (i.e., below 100 C).
At higher temperatures, the reaction between NO and NH3
adsorbed species occurred, producing N2 and H2O. The
maximum NO consumption and N2 production occurred
at about 260 C. O2 or N2O was not detected in this experi-
ment.
The TPSR results between ammonia adsorbed species
and gaseous O2 are shown in Fig. 4. Similar to the re-
action between NO and ammonia adsorbed species, NH3
desorption was dominating at low temperatures. Above
250 C, O2 oxidized ammonia adsorbed species to nitro-
gen and water. No N2O or NOx was observed in the entire
process.
FIG. 4. TPSR profile in 2% O2/He following exposure of Fe(95)–
ZSM-5 to 1000 ppm NH3/He at room temperature for 1.5 h.

24
LONG AND YANG
tween NO2 and the NH3 adsorbed species was faster than
the rate of ammonia desorption. At 200 C, the amount of
N2 formation was close to that of N2O formation. When
the temperature was increased to above 250 C, decompo-
sition of NO2 to NO became significant, as indicated by O2
formation.
TPSR of NOx Adsorbed Species with Ammonia
Since the foregoing NOx TPD profile showed that NOx
could adsorb on Fe(95)–ZSM-5, the TPSR experiment be-
tween NOx adsorbed species and ammonia was investi-
gated. In this experiment, Fe–ZSM-5 was first treated with
NO + O2/He, as indicated in the above TPD experiment.
Subsequently, 1000 ppm NH3/He was passed over the NOx
adsorbed sample and the temperature was increased. As
shown in Fig. 7, the intensity of NH3 decreased sharply
at low temperatures, while only a trace amount of N2 was
produced. Ammonia adsorption dominated under this con-
dition. Also, a small amount of NOx desorbed at low tem-
peratures, possibly being displaced by ammonia. With in-
creasing temperature, the reaction between NH3 and NOx
adsorbed species occurred, forming N2 and H2O. The rate
of N2 formation reached a maximum at 230 C. Only a trace
amount of N2O was detected at 230 C.
FIG. 5. TPSR profile in 1000 ppm NO + 2% O2/He following expo-
sure of Fe(95)–ZSM-5 to 1000 ppm NH3/He at room temperature for 1.5 h.
TPSR of NOx Adsorbed Species with Ammonia
Adsorbed Species
The TPSR results between 1000 ppm NO + 2% O2/He
mixture and presorbed NH3 on Fe–ZSM-5 are shown in
Fig. 5. In contrast to that with NO only (Fig. 3), NO was
consumed significantly at 200 C. Meanwhile, large amounts
of N2 and H2O were produced (Fig. 5). The reaction tem-
perature was lower than that with either NO or O2 alone.
Again, N2O was not observed. Since H2O is more difficult to
desorb from ZSM-5 than N2, the apparent formation tem-
perature for water was slightly higher than that for N2. As
compared with Fig. 3, more N2 and H2O were produced and
also less ammonia was desorbed at low temperatures. It is
clear that oxygen promoted the reaction between NO and
ammonia adsorbed species. In this experiment, it was diffi-
cult to detect oxygen consumption due to the high oxygen
concentration used.
Since both NOx and NH3 could adsorb on Fe–ZSM-5,
the TPSR experiments of adsorbed NOx and adsorbed NH3
The reaction between NO2 and ammonia adsorbed
species is shown in Fig. 6. After 1000 ppm NO2/He was
introduced into the reactor, the intensity of NOx decreased
immediately and sharply while much N2 was generated with
increasing temperature. This indicates that NO2 reacted
with ammonia adsorbed species and/or adsorbed on the
catalyst at low temperatures. Maximum NO2 consumption
occurred at 65 and 200 C. Correspondingly, much N2 was
formed. In addition, N2O formation was also observed at
200 C. Ammonia desorption was not detected in the entire
temperature range, suggesting that the rate of reaction be-
FIG. 6. TPSR profile in 1000 ppm NO2/He following exposure of
Fe(95)–ZSM-5 to 1000 ppm NH3/He at room temperature for 1.5 h.

(TPD/TPSR) OF Fe-EXCHANGED ZSM-5
25
FIG. 7. TPSR profile in 1000 ppm NH3/He following exposure of
Fe(95)–ZSM-5 to 1000 ppm NO + 2% O2/He at 500 C.
FIG. 8. TPSR profile in He following exposure of Fe(95)–ZSM-5 to
1000ppm NO + 1000ppm NH3 + 2% O2/He at room temperature for 1.5h.
were also studied. In this case, the mixture gas containing
1000 ppm NO + 1000 ppm NH3 + 2% O2/He was passed
through Fe(95)–ZSM-5 at room temperature for 1.5 h. As
indicated above, both NOx and NH3 adsorbed species were
present on the catalyst. The sample obtained was purged
with He for 1 h and then ramped to 600 C. The peaks for
N2 formation were observed at 55 and 245 C (Fig. 8), indi-
cating that two different reactions between NOx and NH3
adsorbed species had taken place. A small amount of N2O
was observed at 245 C. The desorption of NOx and NH3
was detected at low temperatures.
After Fe–ZSM-5 was treated with NO + NH3 + O2/He
and then purged with He, the sample was heated in flowing
1000 ppm NO/He. As shown in Fig. 9, nitrogen formation
occurred at 50 and 170 C when NO was consumed at 170 C.
N2O formation was not observed. A small amount of NH3
was desorbed at low temperatures.
Figure 10 shows the TPSR experiment of NO2 with
adsorbed NOx + ammonia. After 1000 ppm NO2/He was
passed through the catalyst that was preadsorbed with NOx
and ammonia and the temperature was increased, NO2 ad-
sorption and/or the reaction between NO2 and adsorbed
NOx + NH3 were observed. Similar to the TPSR experi-
ment of NO2 with NH3 adsorbed species (Fig. 6), N2 for-
mation occurred at 50 and 200 C by consuming NO2. Also,
N2O formation was observed at 200 C and NO2 decompo-
sition occurred at high temperatures.
FIG. 9. TPSR profile in 1000 ppm NO/He following exposure of
Fe(95)–ZSM-5 to 1000 ppm NO + 1000 ppm NH3 + 2% O2/He at room
temperature for 1.5 h.

26
LONG AND YANG
maximum N2 formation were 330 and 260 C, respectively
(Figs. 3 and 4). Also, with O2 or NO, more NH3 desorbed
at low temperatures and less N2 formation was observed.
By comparison, NO + O2 and NO2 were more active in
reacting with NH3 adsorbed species, as indicated by the
results that the reactions occurred at lower temperatures
( 200 C) and less NH3 desorption was detected (Figs. 5 and
6). With different gases, the amount of N2 product increased
in the order O2 ( 28 mol) < NO ( 66 mol) < NO + O2
( 150 mol) < NO2 ( 170 mol + 53 mol N2O). In con-
trast, the amount of desorbed NH3 decreased in the se-
quence O2 ( 120 mol) > NO ( 100 mol) > NO + O2
( 30 mol) > NO2 ( 0 mol). NO consumption and N2
formation increased significantly in the presence of O2. Be-
cause O2 could oxidize NO to NO2 and the reactivity of NH3
adsorbed species with NO2 was much higher than that with
NO, NO2 was probably an intermediate for the SCR reac-
tion. As shown in Fig. 7, NOx adsorbed species was also
reactive to NH3. Since NH3 was strongly and abundantly
adsorbed on Fe–ZSM-5, it was possible that the reaction oc-
curred between adsorbed NH3 (rather than gaseous NH3)
and NOx adsorbed species.
FIG. 10. TPSR profile in 1000 ppm NO2/He following exposure of
Fe(95)–ZSM-5 to 1000 ppm NO + 1000 ppm NH3 + 2% O2/He at room
temperature for 1.5 h.
When NH3 and NOx were coadsorbed on Fe–ZSM-5 at
room temperature, TPSR of the coadsorbed species with
He, NO/He, and NO2/He showed N2 formation near 55
and 170–245 C (Figs. 8–10). The N2 formation at two peaks
suggests two types of reaction occurred. To study the reac-
DISCUSSION
The above TPD experiments indicated that both NOx tions, two separate experiments were performed. One was
and NH3 adsorbed on the Fe–ZSM-5 catalysts, which was the TPSR between NOx adsorbed species and NH+4 ions.
in agreement with our previous FTIR results (14, 25). Ph- After saturation with NH3 at room temperature, Fe(95)–
ysisorbed and chemisorbed NOx and NH3 species were ZSM-5 was heated at 150 C in He for 30 min. NH3 TPD
formed (Figs. 1and 2). The addition ofiron to ZSM-5did not profile showed only one broad peak at 390 C, indicating
influence significantly physisorbed NOx and NH3 species. no physisorbed or coordinated NH3 on the sample. Fe–
They were related to the structure of ZSM-5 and were not ZSM-5 was then exposed to 1000 ppm NO + 2% O2/He
stable at high temperatures. In contrast, chemisorbed NH3 at room temperature for 1.5 h followed by purging with
(mainly NH+4 ) and NOx (mainly NO2) were strongly af- He for another 1 h. Subsequently, the temperature was in-
fected byFe. Asexpected, the replacement ofH + byFe2+ or creased linearly. As shown in Fig. 11, only one peak for
Fe3+ resulted in a decrease in the number of Brønsted acid N2 and N2O formation was observed at 280 C. The other
sites and thus decreased the formation of NH+4 ions. Lobree experiment was the TPSR between ammonia adsorbed
et al. (21) also reported a decrease in Brønsted acidity species and chemisorbed NOx species (mainly NO2). In
when increasing Fe content in Fe–ZSM-5. However, even this case, the NOx adsorbed Fe–ZSM-5 was treated at 70 C
when the protons in ZSM-5 were completely substituted for 30 min and then cooled to room temperature in He.
by FeCl3, quite an amount of Brønsted acid sites remained The NOx TPD experiment showed that only one desorp-
on Fe(300)–ZSM-5 after hydrolysis (27). This catalyst is tion peak appeared at 310 C, indicating that N2O3 or ph-
still very active for the SCR reaction (15). A careful anal- ysisorbed NOx was not present. Fe–ZSM-5 was then ex-
ysis using 1.00% NH3/He as standard gas indicated that posed to 1000 ppm NH3/He at room temperature for 1.5 h.
the amount of NH+4 ions was approximately 50 mol on TPSR in He showed that the reaction between NH3 ad-
Fe(95)–ZSM-5. By comparison, the chemisorbed NOx were sorbed species and chemisorbed NO2 took place at 245 C,
bonded directly to iron sites and increased with Fe content. forming N2 and H2O (Fig. 12). The above two experiments
The amount of chemisorbed NOx species (mainly NO2) was suggest that N2 formation near 55 C resulted from the re-
approximately 20 mol on Fe(95)–ZSM-5.
actions of physisorbed NH3 with N2O3 and/or physisorbed
The TPSR results indicated that NH3 adsorbed species NOx. It might not be related to the iron ions of Fe–ZSM-
was reactive with NO, O2, NO + O2, and NO2, producing 5. This conclusion was further supported by the TPSR re-
H2O, N2, and/or N2O. Among them, O2 and NO were less sult that the N2 formation near 55 C was also observed on
reactive with NH3 adsorbed species. The temperatures for H–ZSM-5 that was treated with NH3 + NO + O2 at room

(TPD/TPSR) OF Fe-EXCHANGED ZSM-5
27
temperatures, the catalyst surface was dominated by NO2
and NH+4 ions (25). TPD experiments showed that NO2
desorbed at a lower temperature than NH+4 ions (310 C vs
390 C, Figs. 1 and 2), hence it was more mobile at high
temperatures. On Fe–ZSM-5 coadsorbed with NO2 and
NH+4 , some [NH4+]x[NO2]y-type species could form, e.g.,
[NH+4 ]2[NO2], assuggested byEngand Bartholomewon H–
ZSM-5 (10). The foregoing TPD results also indicated that
the ratio of NH+4 /NO2 was approximately 2.5 for Fe(95)–
ZSM-5. [NH+4 ]2[NO2] could be formed from the diffusion
of one molecular NO2 to two neighboring NH+4 ions. Al-
ternatively, the small pore sizes in ZSM-5 facilitated the
formation of this complex without the need for surface mo-
bility. In the presence ofNO and NO2, [NH+4 ]2[NO2]reacted
with them to produce N2 and/or N2O at high temperatures
according to two reactions:
[NH+4 ]2NO2 (s) + NO(g) → 2N2 + 3H2O + 2H+,
[5]
[NH+4 ]2NO2 (s) + NO2(g) → N2 + N2O + 3H2O + 2H+.
[6]
This was identified by the above TPSR experiments. TPSR
between NO and NOx + NH3 adsorbed species showed that
a large amount of N2 ( 145 mol) formed at 170 C and
no N2O formation was observed (Fig. 9). By comparison,
FIG. 11. TPSR profile in He of Fe(95)–ZSM-5 that was treated with
1000 ppm NH3/He at room temperature for 1.5 h, heated in He at 150 C for
0.5 h, and then exposed to 1000 ppm NO + 2% O2/He at room temperature
for 1.5 h.
temperature. On the Fe–ZSM-5 surface, N2O3 and ph-
ysisorbed NOx (NO + NO2) could react with physisorbed
NH3 to form ammonium nitrite in the presence of trace
H2O impurity (which was present in our experiments and
probably in all experiments by others as well) according to
the following reactions:
N2O3 + 2NH3 + H2O → 2NH4NO2,
NO2 + NO + 2NH3 + H2O → 2NH4NO2,
[1]
[2]
2NO2 + 2NH3 + H2O → NH4NO3 + NH4NO2. [3]
It is known that ammonium nitrite is not stable and decom-
poses at 60–70 C (28). Hence N2 formation near 55 C may
be due to decomposition of ammonium nitrite:
NH4NO2 → N2 + 2H2O.
[4]
During the TPSR experiment between NO + O2 and NH3
adsorbed species, ammonium nitrite formation was rather
slow at low temperatures due to slow NOx formation.
Hence, the reaction near 55 C was not observed (Fig. 5).
The reactions at 170–245 C are more important for the
SCR mechanism because high SCR activity was usually ob-
tained at high temperatures for Fe–ZSM-5 (14, 15). At high
FIG. 12. TPSR profile in He of Fe(95)–ZSM-5 that was treated with
1000 ppm NO + 2% O2/He for 1 h, heated in He at 70 C for 0.5 h, and
then exposed to 1000 ppm NH3/He at room temperature for 1.5 h.

28
LONG AND YANG
almost the same amounts of N2 ( 71 mol) and N2O
( 63 mol) were obtained at 200 C during TPSR of NO2
with adsorbed NOx + NH3 (Fig. 10). It is noted that reaction
[6] ocurred between equimolar NO2 and NH+4 . When the
amount of NO2 was less than that of NH+4 , as in the case
of TPSR of adsorbed NOx + NH3 in He (Fig. 8), the N2O
formed might be further reduced by NH+4 according to the
reaction
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ACKNOWLEDGMENTS
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We gratefully acknowledge Dr. John Armor of Air Products for provid-
ing the NH –ZSM-5 sample. This work was supported by Electric Power
4
Research Institute.