Characterization of Fe-ZSM-5 catalyst for selective catalytic reduction of nitric oxide by ammonia

Article abstract of DOI:10.1006/jcat.2000.2935

XPS, H₂-TPR, ESR, and FTIR spectroscopy were used to characterize Fe-exchanged ZSM-5 (Fe/Al = 0.193), the most active catalyst known for the SCR of NO with ammonia. Fe cations were present as Fe³⁺ ions with tetrahedral coordination, along with a small amount of Fe²⁺ and aggregated Fe³⁺ ions. The Fe³⁺ ions could be partially reduced to Fe²⁺ ions by H₂ at 300°-600°C, but the oxidation was reversible when O₂ was introduced into the reduced catalyst at 500°C. NO molecules could be oxidized by O₂ to adsorbed N₂O₃, NO₂, and nitrate. The NO_x adspecies were not stable at > 300°C in helium, but the adsorbed NO and NO₂ could be observed in flowing NO + O₂/He. Ammonia molecules were adsorbed on Broensted acid and Lewis acid sites of Fe-ZSM-5 to generate NH₄⁺ ions (majority) and coordinated NH₃ (minority), respectively. The pretreatment by SO₂ + O₂ at 400°C increased the Broensted acidity of the Fe-ZSM-5 due to formation of surface sulfate species of iron, resulting in an enhancement of SCR activities by the presence of H₂O + SO₂ at > 350°C. NH₄⁺ ions, NO, and NO₂ adspecies may all play an important role in the SCR reaction on the Fe-ZSM-5 catalyst.

Full text of DOI:10.1006/jcat.2000.2935

Journal of Catalysis 194, 80–90 (2000)
doi:10.1006/jcat.2000.2935, available online at http://www.idealibrary.com on
Characterization of Fe-ZSM-5 Catalyst 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 January 6, 2000; revised May 19, 2000; accepted May 24, 2000
(SCR) with hydrocarbon or ammonia in the presence of
excess oxygen is an efficient technology for reducing their
Fe-exchanged ZSM-5 (Fe/Al = 0.193), the most active catalyst
known for theselectivecatalyticreduction (SCR)ofNOwith ammo-
nia, was characterized by X-ray photoelectron spectroscopy (XPS),
H2 temperature-programmed reduction (H2-TPR), electron spin
resonance (ESR), and FT-IR spectroscopy. XPS and ESR results
indicated that iron cations were present mainly as Fe3+ ions with
tetrahedral coordination, along with a small amount of Fe2+ and
aggregated Fe3+ ions. The Fe3+ ions could be partially reduced to
Fe2+ ions by H2 at 300–600 C, but the oxidation was reversible
when O2 was introduced into the reduced catalyst at 500 C. FT-
IR spectra showed that NO molecules could be oxidized by O2 to
adsorbed N2O3, NO2, and nitrate. The NOx adspecies were not sta-
ble at above 300 C in He, but the adsorbed NO and NO2 could be
observed in flowing NO + O2/He. NH3 molecules were adsorbed on
Brønsted acid and Lewis acid sites of Fe-ZSM-5 to generate, respec-
tively, NH+4 ions (majority) and coordinated NH3 (minority). The
NH+4 ions with three hydrogen atoms (3H structure) bonded to AlO4
tetrahedra of ZSM-5 were more stable at high temperatures (e.g.,
300–400 C) than those with two bonds and the coordinated NH3.
The pretreatment by SO2 + O2 at 400 C increased the Brønsted
acidity of the Fe-ZSM-5 due to formation of surface sulfate species
of iron. This resulted in an enhancement of SCR activities by the
presence of H2O + SO2 at high temperatures (>350 C). At 300 C,
the NH+4 ions with 3H structure were active in reacting with NO
and NO + O2, but the reaction rate with NO + O2 was much higher
than that with NO. The results indicate that NH+4 ions with 3H
structure as well as NO and NO2 adspecies play an important role
in the SCR reaction on the Fe-ZSM-5 catalyst. The role of Fe3 +
is to oxidize NO to NO2. Overexchange decreased the activity; the
c
emissions. Fe-exchanged ZSM-5 catalysts have been exten-
sively studied for abating nitrogen oxides quite recently
(1–15). Fe-ZSM-5 was first found to be active for SCR of
NO with hydrocarbon by Sato et al., but the activity was
moderate (1). More recently, Feng and Hall (2) reported an
overexchanged Fe-ZSM-5 (Fe/Al = 0.92) prepared by ion-
exchange from oxalate salt. This catalyst was much more ac-
tive than Cu-ZSM-5 and underexchanged Fe-ZSM-5 (i.e.,
ion-exchange level < 100% ) for SCR of NO with isobu-
tane as reductant. The overexchanged Fe-ZSM-5 was also
stable under hydrothermal conditions and was resistant to
sulfur dioxide. But unfortunately, these results were diffi-
cult to reproduce in the authors’ laboratory later (3). Chen
and Sachtler (4) subsequently studied the same reaction on
overexchanged Fe-ZSM-5 (Fe/Al = 1) prepared by using
anaerobic sublimation of volatile FeCl3 to H-ZSM-5. They
also reported a high activity and durability on the Fe-ZSM-
5 for SCR with hydrocarbons. Because Fe-ZSM-5 has a high
catalytic activity and a good hydrothermal stability and is
resistant to sulfur dioxide, it has attracted intense interest.
It has also been studied for SCR of NO with ammonia (5–9),
N2O decomposition (10–12), and N2O reduction with hy-
drocarbon (13) and with ammonia (14).
In our previous work (5, 6), a series of Fe-exchanged
molecular sieves (ZSM-5, mordenite, Y-zeolite, and MCM-
41) were studied for SCR ofNO with ammonia. It wasfound
that Fe-ZSM-5 and Fe-MOR showed the highest activities
among all known catalysts for the reaction. The maximum
activity was obtained on the Fe-ZSM-5 with a moderate
exchange level (Fe/Al = 0.19–0.43). For example, Fe-ZSM-
5 (Fe/Al = 0.19) showed nearly 100% NO conversions at
400–550 C under conditions with a high space velocity
(GHSV = 4.6 105 1/h). As compared with the commercial
vanadia catalyst, the Fe-ZSM-5 catalyst was 5 times more
active at 400 C and 7 times more active at 450 C, based
on the first-order rate constants (6). This catalyst also func-
tioned in a broader temperature window, produced only
N2 (rather than N2O) and H2O, and showed a substantially
lower activity for oxidation of SO2 to SO3 (5). The SCR
optimal Fe/Al ratio was 0.19–0.43.
2000 Academic Press
Key Words: selective catalytic reduction; SCR of NO with ammo-
nia; Fe-exchanged ZSM-5 catalyst; characterization of Fe-ZSM-5;
nitrogen oxides.
INTRODUCTION
The removal of nitrogen oxides, including NO, NO2, and
N2O, has received much interest in recent years because of
its environmental importance. Selective catalytic reduction
1 To whom correspondence should be addressed. E-mail: yang@
umich.edu.
80
0021-9517/00 $35.00
c
Copyright
2000 by Academic Press
All rights of reproduction in any form reserved.

Fe-ZSM-5 FOR SCR OF NO WITH NH3
81
activity of the Fe-ZSM-5 was further improved by the pres- and washed 5 times with deionized water. The obtained
ence of H2O and SO2 at high temperatures. solid was first dried at 120 C for 12 h, then calcined at 500 C
Although Fe-ZSM-5 has shown excellent performance for 6 h in air. The Fe content in the sample measured by neu-
for SCR of nitrogen oxides, only limited work has been tron activation analysis was 1.59 wt% , i.e., Fe/Al = 0.193.
performed in the characterization of this material (15–18).
From H2-TPR and NO adsorption experiments, Lobree
XPS analysis. The XPS experiment was carried out on
a Perkin–Elmer PHI 5400 ESCA system at room tempera-
ture under 10 8–10 9 Torr, using Mg K radiation. Fe 2p3/2
et al. (18) concluded that the main form of Fe in the Fe-
ZSM-5 catalysts was Fe3+ when the Fe/Al ratio was less
binding energy was calibrated relative to the carbon impu-
rity with a C1s band at 284.7 eV.
than or equal to 0.19. An increase in Fe content above this
level resulted in an increase of the concentrations of Fe2+
TPR experiment. H2-TPR was performed in a fixed-
bed quartz reactor with an inner diameter of 10 mm. The
ions and FeOx particles. For overexchanged Fe-ZSM-5, the
state of Fe is still in debate. Chen and Sachtler proposed a
temperature was controlled by an Omega (CN-2010) pro-
paired oxygen-bridged active site [(HO)Fe–O–Fe(OH)]2+
grammable temperature controller. In the experiment, 0.1 g
Fe-ZSM-5 was pretreated in a flow of He (50 ml/min, purity
in their Fe-ZSM-5 (4). However, using in situ ESR experi-
ments, Kucherov et al. (16) reported that most of the iron
99.9998% ) at 400 C for 30 min to remove adsorbed species
in the Fe-ZSM-5 existed as isolated FeO+ ions in tetrahe-
(e.g., H2O) on the catalyst. After the sample was cooled to
dral and distorted tetrahedral sites. More recently, based
room temperature in He, the reduction of the Fe-ZSM-5
on the EXAFS (extended X-ray absorption fine structure
was carried out from 30 to 700 C in a flow of 5.34% H2/N2
spectroscopy) results, Joyner and Stockenhuber (17) sug-
(27 ml/min) at 10 C/min. The consumption of H2 was mon-
gested that Fe in Fe-ZSM-5 had an average composition
itored continuously with a thermal conductivity detector.
of Fe4O4 and was in the form of nanoclusters. They also
˚
The water produced during reduction was trapped in a 5 A
concluded that the small Fe4O4 clusters were more active
(per iron atom) than iron ions for the hydrocarbon SCR
reaction.
molecular sieve column.
ESR analysis. ESR spectra were recorded on a Brucker
EMX ESR spectrometer, under the conditions of a mi-
crowave power of 6.3 mW and a modulation amplitude of
5.0 G. Before the ESR experiment, Fe-ZSM-5 samples were
first treated in various gases at different temperatures. The
samples were then purged with He for 30 min at room tem-
perature and sealed in Pyrex tubes under He atmosphere.
The sealed samples were transferred to the ESR sample
holder and the ESR spectra were recorded at 20 C.
However, the above work mainly focused on the overex-
changed Fe-ZSM-5 catalysts for hydrocarbon SCR reac-
tion. To our knowledge, no characterization studies on Fe-
ZSM-5 for the ammonia SCR reaction have been reported.
Since the mechanisms for the two SCR reactions are differ-
ent, it isnecessaryto understand the state ofFe in Fe-ZSM-5
and the adsorption behavior of reactants, e.g., NO and NH3,
on the catalyst. In this work, spectroscopic and other tech-
niques have been applied to study the Fe-ZSM-5 catalyst.
The techniques included XPS (X-ray photoelectron spec-
troscopy), H2-TPR (temperature-programmed reduction),
ESR (electron spin resonance), and FT-IR spectroscopy.
The Fe-ZSM-5 with an Fe/Al ratio of 0.193 was chosen for
the study because it showed the highest ammonia SCR ac-
tivity in our previous work (6). The oxidation state and
redox behavior of Fe as well as the interactions between
Fe-ZSM-5 and various gases, e.g., NO, NO2, NO + O2, NH3,
H2O, and SO2, were studied.
FT-IR study. Infrared spectra were recorded on a Nico-
let Impact 400 FT-IR spectrometer with a TGS detector.
Self-supporting wafers of 1.3 cm diameter were prepared
by pressing 20-mg samples and were loaded into a high-
temperature IR cell with BaF2 windows. The wafers could
be pretreated in situ in the IR cell. Unless otherwise indi-
cated, the wafers were first treated at 500 C in a flow of
He for 15 min, and then cooled to desired temperatures,
i.e., 400, 300, 200, 100, and 30 C. At each temperature, the
background spectrum was recorded in flowing He and was
subtracted from the sample spectrum that was obtained at
the same temperature. In the experiment, the IR spectra
EXPERIMENTAL
were recorded by accumulating 100 or 16 scans at a spec-
1
Catalyst preparation. Fe-ZSM-5 was prepared by using tral resolution of 4 cm
conventional ion-exchange procedure, as described in de-
.
During the FT-IR experiments, the gas mixtures (i.e.,
tail elsewhere (5). Two grams NH4-ZSM-5 (Si/Al = 10, ob- NO/He, NO + O2/He, NO2/He, NH3/He, H2O/He, and
tained from Air Products and Chemicals Inc.) was added to SO2 + O2/He) had the same concentrations as those used
200 ml of 0.05 M FeCl2 (99% , Aldrich) solution with con- in the activity measurements (5), i.e., 1000 ppm NO (when
stant stirring. The ion exchange was carried out in air at used), 1000 ppm NO2 (when used), 1000 ppm NH3 (when
room temperature, which is defferent from the procedure used), 2% O2 (when used), 5% H2O (when used), 500 ppm
that Mobil Company used, i.e., ion-exchange in inert gas SO2 (when used), and balance of He. The total gas flow rate
and at above 55 C (9). After 24 h, the mixture was filtered was 500 ml/min (ambient conditions). The premixed gases

82
LONG AND YANG
(1.01% NO/He, 0.98% NO2/He, 1.00% NH3/He, 0.99%
SO2/He, 5.0% O2/He, and 5.34% H2/N2) were supplied by
Matheson.
NO oxidation to NO2. The experiment for NO oxida-
tion to NO2 was performed in a fixed-bed, quartz flow re-
actor. A 50-mg sample was used, and the conversion at
each temperature was obtained after 1 h of steady-state
reaction. The reactant gas composition was as follows:
1000 ppm NO, 2% O2, and balance He. The total flow rate
was 500 ml/min (ambient conditions). NO concentration
was continually monitored by a chemiluminescent NO/NOx
analyzer (Thermo Electro Corp., Model 10). NO conver-
sion to NO2 was obtained by using the equation NO con-
version to NO2 = (([NOx] [NO])/[NOx]) 100% , where
NOx represents NO + NO2.
FIG. 2. H2-TPR profile of Fe-ZSM-5 catalyst.
RESULTS
ESR spectra of Fe-ZSM-5. After Fe-ZSM-5 was treated
in He at 400 C for 30 min and then cooled to room tem-
perature, a strong narrow line at g = 4.29 was observed on
the ESR spectrum (Fig. 3a), which can be assigned to Fe3+
ions in tetrahedral coordination (16). Also, a weak signal
was detected at g 2.0 originating from aggregated Fe3+
ions (not shown). The aggregated Fe3+ ions might come
from the hydrolysis of residual ferric ions in FeCl2 solu-
tion during the ion-exchange process. It is noted that the
sample was sealed in a Pyrex tube and the empty Pyrex
tube also showed a signal at g = 4.29, but the intensity
was only one fourth as strong as that of the Fe-ZSM-5.
When the catalyst was heated from 30 to 700 C in flowing
5.34% H2/N2 (similar to the above H2-TPR experiment)
and then cooled to room temperature, the Fe3+ signal at
g = 4.29 decreased significantly (Fig. 3b). The intensity of
the signal was almost the same as that of the empty Pyrex
tube. This indicates a reduction of Fe3+ ions. When the
H2-reduced sample was treated at 500 C in 5.0% O2/He
for 15 min, the signal at g = 4.29 increased again (Fig. 3c),
suggesting that O2 oxidized iron back to tetrahedral Fe3+
reversibly. The intensity of the Fe3+ signal in the O2-
treated sample was slightly stronger than that in the He-
treated sample (Fig. 3a), suggesting that more tetrahe-
dral Fe3+ ions formed. The above H2-TPR and ESR data
suggest that the redox cycle between Fe3+ and Fe2+ is
reversible.
XPS spectrum of Fe 2p3/2
. Figure 1 shows the XPS spec-
trum of Fe 2p3/2 on the fresh Fe-ZSM-5. A broad XPS band
centered at 711.7 eV was observed on the sample. This value
is close to the binding energy of 2p3/2 of iron in Fe2O3 and
FeCl3 ( 711 eV) (19), indicating that the iron in the Fe-
ZSM-5 was mainly present as a valence of +3.
H2-TPR profile of Fe-ZSM-5. Figure 2 is an H2-TPR
profile of Fe-ZSM-5 that was pretreated in He at 400 C.
A peak at 385 C and a shoulder at 462 C were observed
on the TPR profile, which can be attributed to the reduc-
tion of iron species at two different sites. Integration of
the area of the TPR peaks yielded an H2/Fe molar ratio of
0.45. This is close to the value of 0.5 required for the re-
duction of Fe3+ to Fe2+. The lower ratio of H2 consumption
to Fe suggests that some Fe2+ ions may also exist in the
Fe-ZSM-5. Taking into account the facts that pure Fe2O3
will be reduced to metallic iron by H2 at about 530 C and
the ratio of H2/Fe is 1.5 (4), it is likely that iron in the
Fe-ZSM-5 existed in the form of Fe3+ ions.
FT-IR study. The IR spectrum of Fe-ZSM-5 at room
1
temperature is shown in Fig. 4. A strong band at 3610 cm
and severalweaker bandsat 3740, 1982, 1872, and 1635cm
1
were observed. The strong band at 3610 cm 1 has been at-
tributed to OH stretching of the Brønsted acid (17, 18).
The high Al/Si ratio in the ZSM-5 (Al/Si = 1/10) resulted in
a strong Brønsted acidity. The weak band at 3740 cm 1 can
be assigned to hydroxyl stretching vibration of the Si–OH
group at crystal termination (20). The bands at 1982 and
FIG. 1. XPS spectrum of Fe 2p3/2 on Fe-ZSM-5 catalyst.

Fe-ZSM-5 FOR SCR OF NO WITH NH3
83
FIG. 3. ESR spectra of Fe-ZSM-5 catalyst at 25 C: (a) after treatment in He at 400 C for 30 min; (b) after reduction by 5.34% H2/N2 from 30 to
700 C, and then (c) reoxidation by 5.0% O2/He at 500 C for 15 min.
1872 cm 1 are due to the zeolite overtone bands (17). The of NO to NO2 and nitrate species. The NOx adspecies, with
1635 cm 1 band is likely due to adsorbed H2O.
much stronger intensities, were also observed on the NO2
The adsorption of NO, NO + O2, and NO2 on Fe-ZSM-5 adsorbed Fe-ZSM-5 (Fig. 5c), suggesting stronger adsorp-
was studied first. Figure 5 shows the IR spectra after the tion for NO2 than for NO.
catalyst was treated with these gases for 30 min followed
After the Fe-ZSM-5 that was treated with NO + O2 was
1
by purging with He for 15 min at room temperature. When heated to 100 C in He, the bands at 1682 and 1575 cm
1000 ppm NO/He was introduced to the sample, a strong either disappeared or decreased sharply, while the band at
peak was observed at 1876 cm 1. This band is close to the 1614 cm (due to NO2 adspecies) increased slightly and
1
vibration frequency of gas-phase NO (1880 cm 1) and has shifted to 1624 cm (Fig. 6b). This suggests that N2O3
1
been assigned to Fe2+ mononitrosyls [Fe2+(NO)] (15, 17, and nitrate adspecies desorbed or transformed to NO2
18). After the NO-treated sample was purged with He for adspecies. As the temperature was further increased, the
15 min, the band at 1876 cm 1 disappeared and two small 1624 cm 1 band also decreased. All of the bands vanished at
peaks were observed at 1617 and 1575 cm 1 (Fig. 5a), which 400 C (Fig. 6e). However, when the Fe-ZSM-5 was heated
can be assigned to adsorbed NO2 and nitrate species, re- in flowing 1000 ppm NO + 2% O2/He, the IR spectra were
spectively (15, 21–23). This suggests that NO molecules are different, as shown in Fig. 7. At 200 C, three bands due to
weakly adsorbed and some of them were oxidized to NO2 weakly adsorbed NO (1876 cm 1), NO2 (1624 cm 1), and
and nitrate species by the residual O2 species weakly ad- nitrate (1575 cm 1) adspecies were detected (Fig. 7a). When
1
sorbed on the catalyst. On the spectrum of NO + O2 ad- the temperature was increased to 300–400 C, the 1575 cm
sorbed on Fe-ZSM-5, three intense bands at 1682, 1614, band due to nitrate species vanished, while the bands at
and 1575 cm were detected (Fig. 5b). The new band at 1876 and 1624 cm 1 due to, respectively, NO and NO2 ad-
1
1682 cm 1 is probably due to adsorbed N2O3 species since speciesdecreased and were then split into two pairsofbands
it is close to the IR band at 1690 cm 1 for gaseous N2O3 (24). at 1906 and 1852 cm 1 as well as at 1629 and 1600 cm 1, re-
Oxygen improved NO adsorption, clearly due to oxidation spectively (Figs. 7b and 7c). This indicates that the NO and

84
LONG AND YANG
FIG. 4. IR spectrum (100 scans) of Fe-ZSM-5 catalyst at 30 C, with
FIG. 6. IR spectra (100 scans) of Fe-ZSM-5 treated in flowing
empty cell as background.
1000 ppm NO + 2% O2/He at 30 C for 30 min and then purged by He
at (a) 30, (b) 100, (c) 200, (d) 300, and (e) 400 C.
FIG. 5. IR spectra (100 scans) of Fe-ZSM-5 treated with (a) 1000 ppm
NO/He, (b) 1000 ppm NO + 2% O2/He, and (c) 1000 ppm NO2/He, for
30 min followed by purge with He for 15 min at 30 C.
FIG. 7. IR spectra (100 scans) of Fe-ZSM-5 treated in flowing
1000 ppm NO + 2% O2/He at (a) 200, (b) 300, and (c) 400 C.

Fe-ZSM-5 FOR SCR OF NO WITH NH3
85
Fe-ZSM-5. With increasing temperatures, the intensities of
1
the 3353 and 3290 cm bands increased first (at the ex-
pense of the 3050 and 2795 cm 1 bands), passing through a
maximum at 200 C, and then decreased at higher temper-
atures (Figs. 8b–8e). This suggests an NH+4 transformation
from 2H structure to 3H structure at higher temperatures.
The intensities of all other bands decreased with increasing
temperature, as shown in Figs. 8b–8e, indicating desorption
of NH3. The IR bands at 1587 and 1276 cm 1 due to coor-
dinated NH3 disappeared at 200 C, whereas those due to
NH+4 ions with 3H structure (at 3353, 3290, and 1473 cm
)
1
were still detected at 400 C in He (Fig. 8e). This result in-
dicates that NH+4 ions with 3H structure are more stable at
high temperatures than the other ammonia adspecies.
When Fe-ZSM-5 was treated with 5% H2O/He for 30 min
at room temperature, a strong broad band was observed at
1
1633 cm (Fig. 9a), indicating that H2O molecules were
adsorbed on the catalyst. However, the H2O adsorption
was not very strong. After the H2O-adsorbed catalyst was
1
heated to 300 C in flowing He, the IR band at 1633 cm
vanished. When the fresh Fe-ZSM-5 was treated at 400 C
for 30 min in flowing 500 ppm SO2 + 2% O2/He and then
purged with He for 15 min, a peak at 1363 cm 1 appeared
(Fig. 9b), suggesting the formation of sulfate species on the
FIG. 8. IR spectra (100 scans) of Fe-ZSM-5 treated in flowing
1000 ppm NH3/He at 30 C for 30 min and then purged by He at (a) 30,
(b) 100, (c) 200, (d) 300, and (e) 400 C.
NO2 adspecies were still present on the catalyst in flowing
NO + O2/He at high temperatures.
The IR spectra of ammonia adsorbed on Fe-ZSM-5 at
different temperatures are shown in Fig. 8. After the sam-
ple was treated in flowing 1000 ppm NH3/He for 30 min
and then purged with He for 15 min at 30 C, a strong
1
band at 1473 cm and weaker bands at 3353, 3290, 3050,
2795, 1705, 1587, 1276, and 2600–2900 cm 1 were observed
1
(Fig. 8a). The broad band between 2600 and 2900 cm is
attributed to physisorbed ammonia. The bands at 1705 and
1473 cm 1 are due to the symmetric and asymmetric bend-
ing vibrations, respectively, of NH+4 that is chemisorbed
on the Brønsted acid sites, while the bands at 1587 and
1
1276 cm can be assigned to asymmetric and symmetric
bending vibrations, respectively, of the N–H bonds in NH3
coordinately linked to Lewis acid sites (25). The bands at
higher wavenumbers are attributed to the N–H stretching
vibration of NH+4 ions located at the AlO4 tetrahedra of the
ZSM-5 framework. The bands at 3353 and 3290 cm 1 can be
assigned to NH+4 ions with three hydrogen atoms bonded to
three oxygen ions of AlO4 tetrahedra (3H structure), while
the bands at 3050 and 2795 cm 1 are due to the NH4+ ions
with two hydrogen atoms bonded to AlO4 tetrahedra (2H
structure) (21, 26). The above results indicate that there are
FIG. 9. IR spectra (100 scans) of Fe-ZSM-5 catalyst: (a) after 5%
H2O/He adsorption for 30 min and then purge with He for 15 min at 30 C;
(b) after treatment by SO2 + O2/He for 30 min and then purge with He for
many more Brønsted acid sites than Lewis acid sites on the 15 min at 400 C, and then (c) cooling to 30 C.

86
LONG AND YANG
1
Fe3+ sites of the Fe-ZSM-5 catalyst (27). The 1363 cm
==
band comes from asymmetric S O stretching frequencies
==
of an “organic” sulfate species which has covalent S O
double bonds (27). When the temperature was decreased
to room temperature, the intensities of the “organic” sul-
fate bands (1363 cm 1) decreased, while a new band at
1
1
1310 cm formed (Fig. 9c). The band at 1310 cm is at-
tributed to a chelating bidentate inorganic SO24 bond to
iron which has ionic SO bonds with a partial double-bond
character (27). This suggests a structural transformation
from “organic” sulfate to inorganic sulfate with decreasing
temperature.
When the H2O-adsorbed Fe-ZSM-5 was treated in flow-
1
ing 1000 ppm NH3/He, the band at 1633 cm due to ad-
sorbed H2O decreased continually; at the same time, two
new bands at 1473 and 1276 cm 1 due to NH4+ ions and co-
ordinated NH3 appeared. After the sample was treated for
30 min, the IR band due to adsorbed H2O vanished com-
pletely, while only ammonia adspecies (1704, 1587, 1473,
and 1276 cm 1) were observed (Fig. 10b). This indicates
that ammonia adsorption is stronger than H2O adsorption
on the catalyst, which can be attributed to its stronger ba-
1
sicity than H2O. Integration of the 1473 cm band area
showed an increase in Brønsted acidity by approximately
FIG. 11. IR spectra (16 scans) taken at 300 C upon passing 1000 ppm
NO over the NH3 presorbed on Fe-ZSM-5 for (a) 0, (b) 2, (c) 5, (d) 10,
and (e) 15 min.
8% as compared to the fresh catalyst (Fig. 10a). After the
Fe-ZSM-5 (that was pretreated by SO2 + O2/He at 400 C)
adsorbed NH3 for 30 min at room temperature, the sul-
1
fate band at 1310 cm shifted to 1274 cm 1. Meanwhile,
a strong band at 1473 cm 1 and two weaker bands at 1704
and 1587 cm 1 appeared (Fig. 10c). Electron transfer from
the adsorbed ammonia to the sulfate species resulted in
a decrease in the S–O vibration frequencies. The band at
1
1276 cm due to symmetric vibration of the NH3 coor-
dinated to Lewis acid sites was overlapped by the sulfate
band at 1274 cm 1. Integration of the 1473 cm 1 band area
indicated an increase in Brønsted acidity by approximately
23% as compared to the fresh Fe-ZSM-5 catalyst (Fig. 10a).
The surface sulfate species increased the Brønsted acidity
of the Fe-ZSM-5.
The reactions of ammonia adspecies with NO and
NO + O2 were also studied. Fe-ZSM-5 was first treated with
NH3/He for 30 min followed by He purge at 300 C. NO/He
was then introduced into the cell and IR spectra were
recorded as a function of time (Fig. 11). As noted above,
NH+4 ions with 3H structure (3353, 3290, and 1473 cm
)
1
were generated after Fe-ZSM-5 was treated with NH3/He,
and their IR bands did not decrease in flowing He for
30 min. After 1000 ppm NO/He was passed over the sample
FIG. 10. IR spectra (100 scans) of 1000 ppm NH3/He adsorption on
(a) fresh, (b) H2O preadsorbed, and (c) SO2 + O2 pretreated (400 C) Fe-
ZSM-5 catalyst at room temperature.

Fe-ZSM-5 FOR SCR OF NO WITH NH3
87
Fe2+ form, most iron cations in the Fe-ZSM-5 are present
as Fe3+ ions with tetrahedral coordination, besides a small
amount of Fe2+ and aggregated Fe3+ ions. Oxygen oxidized
Fe2+ to Fe3+ during calcination of the catalyst. An equilibra-
tion of Fe2+ ⇔ Fe3+ exists in the Fe-exchanged ZSM-5. Ox-
idation or reduction treatment will, respectively, increase
or decrease the ratio of Fe3+/Fe2+. The Fe3+ ions were re-
duced to Fe2+ ions in flowing H2/N2 at high temperatures.
But when the reduced catalyst was treated by 5% O2/He
at 500 C, the Fe2+ ions were oxidized back to tetrahedral
Fe3+ ions. This is different from the result reported on the
overexchanged Fe-ZSM-5 that was prepared by sublima-
tion of FeCl3 vapor into H-ZSM-5 (16). In that case, when
the overexchanged Fe-ZSM-5 was treated in flowing 1%
H2/He at 400 C, the oxidation of Fe2+ to tetrahedral Fe3+
ions by 10% O2/He was irreversible, resulting in a loss of
the ESR signal due to tetrahedral Fe3+ ions and the appear-
ance of a new broad signal due to -Fe2O3 aggregates (16).
The iron cations in the underexchanged Fe-ZSM-5 seem
to be more stable under redox treatment than those in the
overexchanged Fe-ZSM-5.
The FT-IR spectra indicate that NO, NO2, and nitrate
species were adsorbed on Fe-ZSM-5 catalyst after the sam-
ple was treated, respectively, with NO/He, NO + O2/He,
and NO2/He (Figs. 5–7). This is similar to the results ob-
tained on CuO/Al2O3 and zeolite catalysts (28–30), but
different from that obtained on vanadia-based catalysts.
Most researchers agreed that nitrogen oxides do not ad-
sorb on the fully oxidized vanadia (31, 32). NO adsorp-
tion was weak, which may be due to low Fe2+ concentra-
tion in the catalyst because NO is bonded more strongly to
Fe2+ than to Fe3+ (33, 34). The presence of O2 increased
NO adsorption, forming N2O3, NO2, and nitrate adspecies.
The NO2 and nitrate species resulted from the reaction be-
tween adsorbed O2 and gaseous NO (35). NO2 adsorption
was much stronger than NO adsorption. With increasing
temperature, the bands due to nitrogen oxide adspecies
decreased (Fig. 6). The NO2 adspecies was the dominant
species at 100–300 C. NOx TPD experiments showed that
the NO2 adspecies was not observed on the NOx adsorbed
H-ZSM-5, and its concentration increased with iron con-
tent in the Fe-ZSM-5, suggesting that the NO2 was bonded
to iron sites (36). All of the IR bands due to nitrogen oxides
adspecies vanished at 400 C in He. However, the adsorbed
NO and NO2 were still observed on the catalyst in flowing
NO + O2/He at 300–400 C (Fig. 7).
FIG. 12. IR spectra (16 scans) taken at 300 C upon passing 1000 ppm
NO + 2% O2 over the NH3 presorbed on Fe-ZSM-5 for (a) 0, (b) 1, (c) 2,
(d) 3, and (e) 5 min.
for 2–5 min, the bands attributed to NH+4 ions decreased.
1
At the same time, a new band was observed at 1875 cm
(Fig. 11c). As mentioned above, the band at 1875 cm 1 was
due to weakly adsorbed NO species on Fe2+ sites. These re-
sults indicate that the reaction between NH+4 ions and NO
occurred. With increasingtime, the NH+4 bandswere further
decreased and vanished at 15 min, while only NO adspecies
was detected (Fig. 11e). The IR spectra of the reaction be-
tween ammonia adspecies and NO + O2/He at 300 C are
shown in Fig. 12. By comparison, after NO + O2/He was
passed over the ammonia adsorbed Fe-ZSM-5 for 1 min,
the bands attributed to NH4+ ions decreased significantly.
All of the NH+4 ions bands vanished in 5 min. At the same
time, a weak peak was observed at 1875 cm 1, suggesting
formation of adsorbed NO species on Fe2+ sites. The above
results indicate that oxygen increased the reaction rate be-
tween NO and NH+4 ions. Oxygen also oxidized some fer-
rous ions to ferric ions and thus decreased the formation of
NO adspecies bonded to Fe2+ ions.
For the ammonia SCR reaction, it has been accepted
that ammonia is adsorbed on the Brønsted or Lewis acid
sites to form, respectively, NH+4 or coordinated NH3, then
gaseous or adsorbed nitrogen oxides react with them to
form N2 and H2O, although some different mechanisms
have been proposed (31, 32, 38–42). Hence surface acid-
ity of the catalyst is important for ammonia SCR reac-
tion because strong surface acidity is beneficial to ammonia
DISCUSSION
The above XPS, H2-TPR, and ESR results indicate that,
although iron cations were exchanged with NH4-ZSM-5 in

88
LONG AND YANG
adsorption. As shown in Fig. 4, many Brønsted acid sites ex- stable than NO2 adspecies at high temperatures. The NH4+
ist on the Fe-ZSM-5. When the catalyst was treated in flow- ions were active in reacting with NO + O2, and they were
ing NH3/He, NH3 molecules were adsorbed on these sites consumed completely with the introduction of 1000 ppm
to form NH+4 ions (Fig. 8). The NH4+ ions are present in two NO + 2% O2 in 5 min at 300 C. By comparison, when pass-
forms, 3H structure (three hydrogen atoms bonded to the ing 1500 ppm NO + 2.2% O2 over the ammonia-adsorbed
AlO4 tetrahedra) and 2H structure (two hydrogen atoms H-ZSM-5 at 300 C, Eng and Bartholomew (21) reported
bonded to the AlO4 tetrahedra). The 3H structure is more that NH+4 bands were still observed after 10 min. This
stable than the 2H structure at high temperatures(>100 C). suggests that Fe3+ in the Fe-ZSM-5 promoted the reac-
The NH+4 ions with 3H structure were still observed when tion between NO + O2 and NH+4 ions, which is in good
the temperature was increased to 400 C in He, indicat- agreement with our previous performance results show-
ing a strong Brønsted acidity on the Fe-ZSM-5 (Fig. 8). ing that Fe-ZSM-5 was much more active than H-ZSM-5
By comparison, ammonia adspecies (both NH+4 ions and for the SCR reaction (6). The NH+4 ions on the Fe-ZSM-5
coordinated NH3) on V2O5/TiO2 were almost removed at could also react with NO at 300 C, but the reaction rate
around 300 C (25). This may be one of the reasons that was much lower than that with NO + O2 (Figs. 11 and 12).
Fe-ZSM-5 showed much higher SCR activity than vana- The presence of O2 improved significantly the reactivity
dia catalysts. Furthermore, when Fe-ZSM-5 was treated by between NO and NH+4 ions, which is consistent with the
SO2 + O2/He, its Brønsted acidity increased. This was iden- promoting role of oxygen for the SCR reaction (6). The
tified by an increase in the intensity of the 1473 cm 1 band enhancement may be related to the formation of NO2 ad-
(due to NH+4 ions) when ammonia was introduced into the species. Our TPSR (temperature programmed surface re-
sample (Fig. 10). The increase in the Brønsted acidity is in action) also showed that the reaction rate of NO2 + NH4+
line with the improvement of the SCR activity at higher was much higher than that of NO + NH+4 on the Fe-ZSM-
temperatures by H2O and SO2, as reported in our previous 5 (36). NO2 may be an intermediate for the SCR reaction.
studies (5, 6). It is known that, when some metal oxides, The NO reduction path on the Fe-ZSM-5isprobablysimilar
such as Fe2O3, TiO2, and ZrO2, are treated by H2SO4 or to that on H-ZSM-5 (21) and Fe-exchanged TiO2-pillared
(NH4)2SO4 followed by calcination at high temperatures, or clay (46). It involves the reaction between one molecular
are treated by SO2 + O2 at high temperatures, their acidi-
ties will be increased significantly (43). After the Fe-ZSM-5
was treated at 400 C in flowing SO2 + O2/He, SO2 was oxi-
dized to SO3 by O2 and a sulfate species was formed. These
were identified, respectively, in our previous SO2 oxida-
tion experiment (5) and the above IR experiment (Fig. 9).
The sulfate ions bonded to iron show a characteristic IR
1
absorption band at near 1375 cm (27). In the case of
==
sulfate ions, S O has a covalent double bond and has a
much stronger affinity to electrons as compared with that
of a simple metal sulfate; hence, the Lewis acid strength of
metal ions becomes substantially stronger by the inductive
==
effect of S O in the complex. When a water molecule is
bonded to the Lewis acid site, the Lewis acid site becomes
a Brønsted acid site. This will increase the Brønsted acidity
of the catalysts that contain iron, in this case, Fe-ZSM-5.
Consequently, the adsorption amount of ammonia is in-
creased. This phenomenon is similar to that observed on
Fe3+-exchanged TiO2-pillared clay catalysts, except that the
increase in Brønsted acidity by SO2 + O2 for the Fe-ZSM-5
is less significant than that for the Fe-exchanged pillared
clays (44, 45). This is due to a lower iron content in the for-
mer catalyst (1.59 wt% in Fe-ZSM-5 vs 4.32–20.1 wt% in
Fe-exchanged pillared clays). The above result indicates a
promoting role of surface sulfate species for the Fe-ZSM-5
and the importance of surface acidity for ammonia SCR
reaction.
FIG. 13. Oxidation activity of NO to NO2 by O2 on H-ZSM-5 and
Fe-ZSM-5 under the conditions of 50 mg sample, 1000 ppm NO, 2% O2,
and 500 ml/min total flow rate. The empty-tube conversion under the same
The above results suggest that both NOx and NH3 could
adsorb on the Fe-ZSM-5, but formed NH+4 ions were more conditions was below 1.5% at 300–450 C.

Fe-ZSM-5 FOR SCR OF NO WITH NH3
89
NO2 and two NH+4 ions to form an active intermediate,
ACKNOWLEDGMENTS
We gratefully acknowledge Dr. John Armor of Air Products for pro-
viding the NH -ZSM-5 sample. This work was supported by the Electric
4
Power Research Institute.
which subsequently reacts with another gaseous or weakly
adsorbed NO to produce N2 and H2O. Therefore, both NH4+
ions and NO2 adspecies are important for NO reduction.
As compared to H-ZSM-5, Fe3+ in the Fe-ZSM-5 enhances
oxidation of NO to NO2 and thus increases the SCR ac-
tivity. As shown in Fig. 13, when 1000 ppm NO + 2% O2
was passed over the catalysts, only 4–8% NO conversions
to NO2 were observed on H-ZSM-5 at 300–500 C, whereas
much higher NO conversions were obtained on Fe-ZSM-
5 under the same conditions. At the same time, ZSM-5
provides Brønsted acid sites to activate ammonia. How-
ever, the replacement of protons on ZSM-5 by iron ions
will decrease its Brønsted acidity, as shown in our TPD
experiments (36). Hence, an appropriate iron content in
the Fe-ZSM-5 is favorable for a high SCR activity. This
is in good agreement with our previous SCR performance
in which the maximum NO conversion was obtained on
the Fe-ZSM-5 with 1.59–3.58 wt% iron and a further in-
crease of iron content decreased NO conversion at high
temperatures (6).
REFERENCES
1. Sato, S., Hirabayashi, H., Yahiro, H., Mizuno, N., and Iwamoto, M.,
Catal. Lett. 12, 193 (1992).
2. Feng, X., and Hall, W. K., J. Catal. 166, 368 (1997).
3. Hall, W. K., Feng, X., Dumesic, J., and Watwe, R., Catal. Lett. 52, 13
(1998).
4. Chen, H. Y., and Sachtler, W. M. H., Catal. Today 42, 73 (1998).
5. Long, R. Q., and Yang, R. T., J. Am. Chem. Soc. 121, 5595 (1999).
6. Long, R. Q., and Yang, R. T., J. Catal. 188, 332 (1999).
7. Ma, A. Z., and Grunert, W., Chem. Commun., 71 (1999).
8. Long, R. Q., Yang, R. T., and Zammit, K. D., U.S. Patent pending,
“Superior Fe-ZSM-5 Catalysts for Selective Catalytic Reduction of
NO by Ammonia,” U.S. Patent file 60/136,626.
9. Farnos, M. D., McWilliams, J. P., Sharma, S. B., Shihabi, D. S.,
Stevenson, S. A., and Vartuli, J. C., U.S. Patent 5451387, 1995.
10. Valyon, J., Millman, W. S., and Hall, W. K., Catal. Lett. 24, 215 (1994).
11. Chang, Y. F., McCarty, J. G., and Zhang, Y. L., Catal. Lett. 34, 163
(1995).
12. Rauscher, M., Kesore, K., Monnig, R., Schwieger, W., Bibler, A., and
Turek, T., Appl. Catal. A 184, 249 (1999).
13. Pophal, C., Yogo, T., Tanabe, K., and Segawa, K., Catal. Lett. 44, 271
(1997).
CONCLUSIONS
Based on the above results, it can be concluded that the
main form of Fe in the Fe-ZSM-5 (Fe/Al = 0.193) catalyst
14. Mauvezin, M., Delahay, G., Kisslich, F., Coq, B., and Kieger, S., Catal.
Lett. 62, 41 (1999).
was Fe3+ with tetrahedral coordination, along with a small 15. Chen, H. Y., Voskoboinikov, T., and Sachtler, W. M. H., J. Catal. 180,
amount of Fe2+ and aggregated Fe3+ ions. The Fe3+ ions
171 (1998).
16. Kucherov, A. V., Montreuil, C. N., Kucherova, T. N., and Shelef, M.,
Catal. Lett. 56, 173 (1998).
17. Joyner, R., and Stockenhuber, M., J. Phys. Chem. B 103, 5963 (1999).
could be partially reduced to Fe2+ ionsbyH2 at high temper-
atures (300–600 C), but the oxidation was reversible when
O2 was introduced to the reduced catalyst at 500 C. NH3
molecules were adsorbed on the Brønsted acid and Lewis
acid sites to generate, respectively, NH+4 ions (majority)
and coordinated NH3 (minority). The NH+4 ions with three
hydrogen atoms bonded to the AlO4 tetrahedra of ZSM-5
were more stable at high temperatures than those with two
18. Lobree, L. J., Hwang, I.-C., Reimer, J. A., and Bell, A. T., J. Catal. 186,
242 (1999).
19. Wagner, C. D., Riggs, W. M., Davis, L. E., Moulder, J. F., and
Mullenberg, G. E., Eds., “Handbook of X-Ray Photoelectron Spec-
troscopy.” Perkin–Elmer Corp., Eden Prairie, 1979.
20. Jacobs, P. A., and Von Ballmoos, R., J. Phys. Chem. 86, 3050 (1982).
21. Eng, J., and Bartholomew, C. H., J. Catal. 171, 27 (1997).
bonds. The pretreatment by SO2 + O2 at 400 C increased 22. Li, Y., and Armor, J. N., J. Catal. 150, 388 (1994).
23. Valyon, J., and Hall, W. K., J. Phys. Chem. 97, 1204 (1993).
24. Laane, J., and Ohlsen, J. R., Prog. Inorg. Chem. 27, 465 (1980).
25. Topsøe, N. Y., J. Catal. 128, 499 (1991).
26. Teunissen, E. H., van Santen, R. A., Jansen, A. P. J., and van
the Brønsted acidity of the Fe-ZSM-5 due to the formation
of surface sulfate species of iron. This resulted in an en-
hancement of the SCR activities by H2O + SO2 at high tem-
peratures (>350 C). NO molecules adsorbed only weakly
Duijneveldt, F. B., J. Phys. Chem. 97, 203 (1993).
on the Fe-ZSM-5, forming Fe2+ mononitrosyl species. In the 27. Yamaguchi, T., Jin, T., and Tanabe, K., J. Phys. Chem. 90, 3148 (1986).
28. Centi, G., Perathoner, S., Biglino, D., and Giamello, E., J. Catal. 152,
presence of O2, NO was oxidized to adsorbed N2O3, NO2,
and nitrate species. The adsorbed nitrogen oxides were not
stable at above 300 C in He, but the NO and NO2 adspecies
75 (1995).
29. Kato, A., Matsuda, S., Kamo, T., Nakajima, F., Kuroda, H., and Narita,
T., J. Phys. Chem. 85, 4099 (1981).
could be observed in flowing NO + O2/He. The adsorbed
30. Brandin, J. G. M., Andersson, L. A. H., and Odenbrand, C. U. I., Catal.
NH+4 ions were active in reacting with NO and NO + O2
at 300 C, but the reaction rate with NO + O2 was much
higher than that with NO. The present results indicate that
NH+4 ions as well as NO and NO2 adspecies may all play
an important role in the SCR reaction on the Fe-ZSM-5
catalyst.
Today 4, 187 (1989).
31. Bosch, H., and Janssen, F., Catal. Today 2, 369 (1988).
32. Busca, G., Lietti, L., Ramis, G., and Berti, F., Appl. Catal. B 18, 1
(1998).
33. Segawa, K. I., Chen, Y., Kubsh, J. E., Delgass, W. N., Dumesic, J. A.,
and Hall, W. K., J. Catal. 76, 112 (1982).
34. Guglielminotti, E., J. Phys. Chem. 98, 9033 (1994).

90
LONG AND YANG
35. Lobree, L. J., Hwang, I.-C., Reimer, J. A., and Bell, A. T., Catal. Lett.
63, 233 (1999).
41. Odenbrand, C. U. I., Bahamonde, A., Avila, P., and Blanco, J., Appl.
Catal. B 5, 117 (1994).
36. Long, R. Q., and Yang, R. T., unpublished results.
42. Topsøe, N.-Y., Dumesic, J. A., and Topsøe, H., J. Catal. 151, 241 (1995).
37. Janssen, F. J. J. G., van den Kerkhof, F. M. G., Bosch, H., and Ross, 43. Yamaguchi, T., Appl. Catal. 61, 1 (1990).
J. R. H., J. Phys. Chem. 91, 5921 (1987).
44. Long, R. Q., and Yang, R. T., J. Catal. 186, 254 (1999).
45. Yang, R. T., and Cichanowicz, J. E., U.S. Patent 5,415,850, “Pillared
Clays as Catalysts for Selective Catalytic Reduction of Nitrogen Ox-
ides by Ammonia,” 1995.
38. Schramlmarth, M., Wokaun, A., and Baiker, A., J. Catal. 124, 86(1990).
39. Went, G. T., Leu, L. J., Rosin, R. R., and Bell, A. T., J. Catal. 134, 492
(1992).
40. Ozkan, U. S., Cai, Y., and Kumthekar, M. W., J. Catal. 149, 390 (1994).
46. Long, R. Q., and Yang, R. T., J. Catal. 190, 22 (2000).