Reaction mechanism of selective catalytic reduction of NO with NH3 over Fe-ZSM-5 catalyst

Article abstract of DOI:10.1006/jcat.2002.3528

Fe-exchanged ZSM-5 has been found previously to be much more active than commercial vanadia-based catalysts for selective catalytic reduction (SCR) of NO to N₂ with NH₃. The NO reduction mechanism is studied in this work using combined in situ FTIR (to observe surface-adsorbed species) and online mass spectrometry (to analyze reaction products). NH₃ adsorbs quickly on Fe-ZSM-5 to generate NH₄⁺ ions, and the catalyst is quite active in oxidizing NO to NO₂ by O₂. The temperature-programmed surface reaction (TPSR) from 100 to 400°C shows that the reactivities of NH₄⁺ with NO and NO₂ decrease in the order NO + NO₂ (1:1 ratio, producing N₂) > NO₂ (producing N₂ + N₂O) ?3NO (producing N₂), with the same total NO_x concentration for the three cases. This trend is also observed in their reactivities at 200°C. It is concluded that both NO and NO₂ are involved in the reaction with NH₄⁺ ions to form N₂. Also, the reactivity of NH₄⁺ ions with NO + O₂ on Fe-ZSM-5 is much higher than that on H-ZSM-5, but the two catalysts show the same activity for the reaction between NH₄⁺ and NO + NO₂. This suggests that NO_x reduction probably takes place on the Bronsted acid sites and that the role of Fe³⁺ irons is to oxidize NO to NO₂ by O₂. A possible reaction mechanism for NO reduction involves the reaction between one NO₂ molecule and two neighboring NH₄⁺ ions to form an active intermediate NO₂(NH₄⁺)₂, which subsequently reacts with another NO to produce N₂ and H₂O. The intermediate has been detected by FTIR with a band near 1602 cm^-1.

Full text of DOI:10.1006/jcat.2002.3528

Journal of Catalysis 207, 224–231 (2002)
doi:10.1006/jcat.2002.3528, available online at http://www.idealibrary.com on
Reaction Mechanism of Selective Catalytic Reduction
of NO with NH3 over Fe-ZSM-5 Catalyst
R. Q. Long and R. T. Yang1
Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136
Received June 28, 2001; revised January 15, 2002; accepted January 15, 2002
This reaction implies 1 : 1 stoichiometry for NH3 and NO.
Many materials, such as mixed oxides and molecular sieves,
have been found to be active for the ammonia SCR reaction
(1). The commercial catalysts used today are V2O5-doped
TiO2, using WO3 and/or MoO3 as promoters. Although the
vanadia-based catalysts have been commercialized, prob-
lems still remain, e.g., high activity for oxidation of SO2 to
SO3 and toxicity of vanadia. Hence, there are continuing
efforts to develop new catalysts.
Fe-exchanged ZSM-5 has been found previously to be much more
active than commercial vanadia-based catalysts for selective cata-
lytic reduction (SCR) of NO to N2 with NH3. The NO reduction me-
chanism is studied in this work using combined in situ FTIR (to
observe surface-adsorbed species) and online mass spectrometry
(to analyze reaction products). NH3 adsorbs quickly on Fe-ZSM-5
to generate NH+4 ions, and the catalyst is quite active in oxidizing
NO to NO2 by O2. The temperature-programmed surface reaction
(TPSR) from 100 to 400◦C shows that the reactivities of NH+4 with
NO and NO2 decrease in the order NO + NO2 (1 : 1 ratio, producing
N2) > NO2 (producing N2 + N2O) ꢀ NO (producing N2), with the
same total NOx concentration for the three cases. This trend is
also observed in their reactivities at 200◦C. It is concluded that
both NO and NO2 are involved in the reaction with NH+4 ions to
form N2. Also, the reactivity of NH+ ions with NO + O2 on Fe-
ZSM-5 is much higher than that on4H-ZSM-5, but the two cata-
lysts show the same activity for the reaction between NH+4 and
NO + NO2. This suggests that NOx reduction probably takes place
on the Brønsted acid sites and that the role of Fe3+ irons is to
oxidize NO to NO2 by O2. A possible reaction mechanism for NO
reduction involves the reaction between one NO2 molecule and two
neighboring NH+4 ions to form an active intermediate NO2(NH4+)2,
which subsequently reacts with another NO to produce N2 and
H2O. The intermediate has been detected by FTIR with a band near
c
Recently, H-zeolites and transition-metal ion-exchanged
molecular sieves have received much attention as catalysts
for the SCR reaction (2–13). They were reported to be
more active than the vanadia catalysts. In particular, we
found that Fe-ZSM-5 showed the highest activity among
all known SCR catalysts. Compared with the commercial
vanadia catalyst, the Fe-ZSM-5 catalyst was five times more
active at 400◦C and seven times more active at 450◦C, based
on the first-order rate constants (9). N2 was the only de-
tectable N-containing product, and no N2O was observed
in the entire temperature range (250–600◦C) for Fe-ZSM-5
(8, 9). This is different from vanadia catalysts, with which
N2O was detected at high temperatures due to oxidation
of ammonia by oxygen (1). In addition, we found that the
presence of H2O + SO2 enhanced the SCR activity of Fe-
ZSM-5 at above 400◦C (9). We have attributed the enhance-
ment to an increase in surface acidity and a decrease in NH3
oxidation activity due to the formation of sulfate species
from SO2 oxidation (10–12). A stronger surface acidity was
beneficial for decreasing ammonia oxidation (to NO) by
oxygen(acompetitivereactionfortheSCRreaction). How-
ever, unfortunately, Sachtler and co-workers (13) misread
our results and mistakenly reported that water (not SO2)
increased the acidity.
1602 cm−1
.
ꢁ 2002 Elsevier Science (USA)
Key Words: selective catalytic reduction; SCR of NO with am-
monia; Fe-ZSM-5 catalyst; mechanism; NO2.
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
(SCR) of NO with ammonia is the most efficient technology
for abatement of NO from power plant exhaust gases. The
main reaction of SCR is
It was reported that both NOx and NH3 could adsorb
onto the surface over ion-exchanged molecular sieves (2, 6,
8, 10, 11, 13–17). It is known that NH3 molecules react with
the protons of H-form zeolites to generate ammonium ions.
NH+4 ions with two or three hydrogen atoms bonded to the
AlO4 tetrahedraofZSM-5wereobservedonH-ZSM-5(15)
and Fe-ZSM-5 (11) and they were stable at high tempera-
tures (≤450◦C). When NO + O2 reacted with Fe-ZSM-5,
4NH3 + 4NO + O2 = 4N2 + 6H2O.
[1]
1 To whom correspondence should be addressed. E-mail: yang@umich.
edu.
224
0021-9517/02 $35.00
c
ꢁ 2002 Elsevier Science (USA)
All rights reserved.

SELECTIVE CATALYTIC REDUCTION
225
adsorbed species of NO, NO2, and nitrate (NO−3 ) were de- mass spectrometer is used to analyze the reaction products
tected by IR, with NO2 as the dominating species (11, 13, from the IR cell. A direct correlation between the reac-
16, 17). The NO was assigned to that adsorbed on Fe2+ sites, tion rate of adsorbed species and the N2 formation rate is
while NO2 and NO−3 species were adsorbed on Fe3+ sites. obtained. Since NO, NO2, and NH+4 species may play an
Our previous work showed that the NOx -adsorbed species important role in the SCR reaction, the reactivities of NH4+
were not stable in a flow of He at above 300◦C, but adsorbed ions with NO, NO2, and NO/NO2 (1 : 1) with the same total
NO and NO2 could be observed in a flow of NO + O2 at NOx concentration were studied. The results provide direct
200–400◦C (11). The NH4+ ions were reactive with NO and evidence that both NO and NO2 participate in NOx reduc-
NO + O2, but the reaction rate with NO + O2 was much tion and that NO2(NH+4 )2 is a key intermediate. Finally, a
higher than that with NO alone (11, 18). In addition, the reaction mechanism for the SCR reaction on Fe-ZSM-5 is
NO2 adsorbed species was also observed on Fe-exchanged proposed.
TiO2-pillared clay when NO+O2 was introduced. The rate
of reaction between NO2 and NH3 was much higher than
that between NO and NH3 (10). Also, the SCR activity cor-
EXPERIMENTAL
related well with the formation rate of NO2 (10, 11). These
Fe-ZSM-5 was prepared using a conventional aque-
results suggest that NO, NO2, and NH+4 species may play
ous ion-exchange technique. Two grams of NH4-ZSM-5
an important role in the SCR reaction on the Fe-exchanged
molecular sieves. The SCR reaction on Fe-ZSM-5 probably
(Si/Al ≈ 10) was added to 200 ml of 0.05 M FeCl2 solu-
tion with constant stirring in air. After 24 h, the mixture
takes place according to the reaction (11, 18)
was filtered and washed with deionized water. The obtained
sample was then dried at 120◦C overnight and calcined at
2NH+4 + NO2 + NO → 2N2 + 3H2O + 2H+.
[2]
500◦C for 6 h in air. Most Fe2+ ions in the catalyst were
oxidized to Fe3+ ions (11). The Fe content, measured by
This is consistent with reaction [1] with an NH3/NO ratio of neutron activation analysis, was 1.4 wt% in the Fe-ZSM-5,
1 : 1. In fact, NO2 has been considered as an intermediate i.e., at the 50% ion-exchange level. H-ZSM-5 was obtained
for the ammonia SCR reaction on H-form zeolites. Kiovsky by calcining NH4-ZSM-5 at 500◦C for 4 h. The NH4-ZSM-5
et al. (2) observed that, on H-MOR, NO2 was more reactive was supplied by Alsi-Penta Zeolithe Gmbh (Germany).
than NO with NH3, and an increase in the NO2/NO ratio FeCl2 · 4H2O (99%) was obtained from Aldrich.
from 2 to 12 enhanced NOx reduction activity significantly.
Infrared spectra were recorded on a Nicolet Impact
Kiovsky et al. suggested that NO2 was an active intermedi- 400 FTIR spectrometer with a TGS detector. Self-
ate for NO reduction by ammonia. The difference in SCR supporting wafers of 1.3-cm diameter were prepared by
activity for zeolites was due to their different rates of NO pressing 20-mg samples and were loaded into a high-
oxidation to NO2 (2). However, Brandin et al. (4) observed temperature IR cell with BaF2 windows. The wafers were
that, an equimolar mixture of NO and NO2 yielded a higher first treated at 400◦C in a flow of He for 30 min to remove
reaction rate on H-MOR, than either NO or NO2 yielded adsorbed H2O and other gases. Our previous H2 TPR and
in the feed. In addition, they proposed that NO+, gener- ESR results indicated that the iron was still present mainly
ated from NO oxidation by O2, was the intermediate for in the ferric form after this pretreatment (11). The samples
the SCR reaction. The NO+ could also result from the oxi- were then cooled to desired temperatures, i.e., 350, 300, 250,
dation of NO by NO2 when NO + NO2 was used. But based 200, 150, and 100◦C. At each temperature, the background
on their negative correlation between SCR activity and the spectrum was recorded in a flow of He and subtracted from
rate of NO oxidation to NO2, they concluded that an ideal the sample spectrum that was obtained at the same tempe-
SCR catalyst would be one that could not oxidize NO to rature. NH3 adsorption was performed by treating the
NO2 (4). Eng and Bartholomew (15) also detected NO2 wafers with flowing 1.07% NH3/He (500 ml/min) for 30 min
formation over H-ZSM-5, and concluded that it played an and then purging with He for another 30 min. Subsequent
important role in the ammonia SCR reaction.
to the ammonia adsorption step, the He flow was switched
Although these studies provided insights into the mech- to different flows containing 1000 ppm NO, 1000 ppm NO2,
anism of NO reduction over molecular sieves, clear evi- 500 ppm NO + 500 ppm NO2, and 1000 ppm NO + 2% O2.
dence supporting NO2 and/or NO+ as intermediates, espe- With increasing temperature (at 10◦C/min) and/or time, IR
cially on Fe-ZSM-5, was not obtained. There is still debate spectra were recorded by accumulating 32 scans at a spec-
on the mechanism of SCR on H-form zeolites (2, 4, 15). tral resolution of 4 cm−1. At the same time, the effluent
The aim of the present study is to gain new insights into gases from the IR cell were monitored continuously with a
the mechanism of NO reduction on Fe-ZSM-5 by com- magnetic-deflection-type mass spectrometer (AERO VAC,
bining in situ FTIR and online mass spectrometer anal- Vacuum Technology Inc.). NH3 (m/e = 17 minus the contri-
yses. The in situ FTIR is used to observe the changes of bution of H2O), H2O (m/e = 18), N2 (m/e = 28), and N2O
surface-adsorbed species during the reactions. The online (m/e = 44) were recorded.

226
LONG AND YANG
The gases were obtained by blending different premixed
gases, using He as the diluent gas. The total gas flow rate
was 500 ml/min (ambient conditions). The premixed gases
(1.00% NO/He, 0.98% NO2/He, and 1.07% NH3/He), O2
(99.5%), and He (99.995%) were supplied by Matheson
without additional purification.
RESULTS
TPSR of NH+4 on Fe-ZSM-5. The reactivities of NH4+
ions with NO, NO2, and NO + NO2 were studied first using
temperature-programmed surface reaction (TPSR). When
Fe-ZSM-5 was treated with 1.07% NH3/He at 100◦C, a
strong IR band at 1461-cm−1 and a very weak band at
1612 cm−1 were observed (Fig. 1A). The 1461-cm−1 band is
assigned to NH+4 ions that are chemisorbed on the Brønsted
acid sites, whereas the weak band at 1612 cm−1 is due to
NH3 coordinately linked to Lewis acid sites (11, 19). It
can be seen from the spectrum that there are many more
Brønsted acid sites than Lewis acid sites on the Fe-ZSM-5.
Also, fourotherIRbandswereobservedat3353, 3290, 3052,
and 2800 cm−1 (not shown). The first two bands can be as-
signed to NH+4 ions with three hydrogen atoms bonded to
three oxygen ions of AlO4 tetrahedra (3H structure). The
other two bands are due to NH+4 ions with two hydrogen
atoms bonded to AlO4 tetrahedra (2H structure) (11, 15,
20). The NH+4 ions with the 3H structure are more stable
than those with the 2H structure (11). Since NH+4 ions are
the dominating ammonia species at high temperatures and
coordinated NH3 is not stable above 200◦C when the am-
monia SCR reaction takes place (11, 18), we will focus on
the reactivities of NH4+ ions with NOx . When 1000 ppm NO
was passed over the sample, the above IR bands decreased
with increasing temperature (10◦C/min) (Fig. 1A). Mean-
while, NH3 desorption was detected by the mass spectrome-
ter (Fig. 1B). The NO adsorbed species which was observed
at 1876 cm−1 in our previous work (11) when NO was in-
FIG. 2. (A) IR spectra and (B) product gas analysis of NH3-adsorbed
Fe-ZSM-5 heated in a flow of 1000 ppm NO2/He at 10◦C/min.
troduced into the fresh Fe-ZSM-5, was not seen, probably
due to the coverage of the surface by NH3. At above 320◦C,
the reaction between NO and NH+4 ions occurred, produc-
ing a trace amount of N2 and H2O. N2O formation was not
detected (Fig. 1B), which suggests that NH+4 ions are less
active when reacting with NO at low temperatures. This is
consistent with our previous catalytic performance result
that NO reduction activities by NH3 were very low (3–5%)
at 350 and 400◦C in the absence of O2 (9).
The TPSR results of NH+4 with NO2 are shown in
Fig. 2. When 1000 ppm NO2 was introduced onto the NH3-
adsorbedFe-ZSM-5andthetemperaturewasincreased, the
NH+4 band at 1461 cm−1 decreased. At 200◦C, three new
bands appeared at 1622, 1602, and 1568 cm−1 (Fig. 2A).
The bands at 1622 and 1568 cm−1 are attributed to ad-
sorbed NO2 and nitrate species, respectively (11, 13, 16,
17, 21). The 1602 cm−1 band is due to another type of NO2
species. It was present when both NO2 adsorbed species
and NH+4 ions were on the surface; i.e., it appeared at
200◦C after NO2 formation and then vanished at 250◦C
upon the consumption of NH+4 . This band may be due to
an intermediate for the reaction between NO2 and NH4+
ions. When NO2 molecules react with NH+4 ions, electrons
transfer from the N atoms of NH+4 to NO2. This enhances
the electron density of NO2 molecules and weakens the
N–O bonds. Consequently, the NO2 band shifts to lower
wavenumbers (e.g., 1602 cm−1). Since the pores in ZSM-5
are very small (0.52 × 0.57 nm), it is possible that the NO2
is associated directly with NH+4 ions. We assign this inter-
mediate to NO2(NH+4 )y, where y is the molar ratio of NH+
to NO2 and changes with time/temperature. As shown i4n
Fig. 2, the transfer of electrons from NH+4 to NO2 also weak-
ens the N–H bonds; thus, the NH+4 band at 1461 cm−1 shifts
to lower wavenumbers and becomes +broader. When the
temperature was above 250◦C, the NH4 band disappeared,
and only NO2 and nitrate bands were seen on the surface.
In the process, a large amount of N2, N2O, and H2O was
FIG. 1. (A) IR spectra and (B) product gas analysis of NH3-adsorbed
Fe-ZSM-5 heated in a flow of 1000 ppm NO/He at 10◦C/min.

SELECTIVE CATALYTIC REDUCTION
227
FIG. 3. (A) IR spectra and (B) product gas analysis of NH3-adsorbed
Fe-ZSM-5 heated in a flow of 500 ppm NO + 500 ppm NO2/He at
10◦C/min.
produced (Fig. 2B). Almost no NH3 desorption was de-
tected by the mass spectrometer (Fig. 2B). These results
indicate that NH+4 ions reacted with NO2, producing N2,
N2O, and H2O, according to the reaction
FIG. 4. IR spectra taken at 200◦C upon passing 1000 ppm NO/He
over NH3-adsorbed Fe-ZSM-5 for 0–16 min.
the mass spectrometer. The NH+4 band vanished in 14 min,
and the catalyst surface was dominated by the adsorbed
NO2 and nitrate species.
2NH+4 + 2NO2 → N2 + N2O + 3H2O + 2H+.
[3]
As shown in Fig. 3, when 500 ppm NO + 500 ppm NO2
were passed over the NH3-treated Fe-ZSM-5, the NH4+
band also decreased significantly with increasing temper-
ature, and new bands due to NO2 and nitrate species ap-
peared at 1624 and 1570 cm−1, respectively. At the same
time, a very large amount of N2 and H2O was formed. No
NH3 desorption or N2O formation was observed. The re-
action takes place according to reaction [2]. By comparing
the intensity of the NH+4 band at 200◦C in Fig. 3A with
that of the corresponding bands in Figs. 1A and 2A, it can
be seen that the reactivity of NH+4 ions with NO + NO2 is
higher than that with NO or with NO2 alone, although they
have the same NOx concentration. This conclusion can also
be obtained from the temperatures for N2 formation, e.g.,
100–220◦C for NO + NO2, 100–260◦C for NO2, and above
320◦C for NO.
The reactivity of NH+4 ions with NO + NO2 is shown
in Fig. 6. When 500 ppm NO + 500 ppm NO2 were intro-
duced into the IR cell at 200◦C, the band at 1439 cm−1 de-
creased quickly. Meanwhile, a large amount of N2 and H2O
was formed. Also, the NO2-adsorbed species were seen at
1624cm−1 in1min. After7min, theNH+4 banddisappeared,
and new IR bands due to Fe2+(NO) (weak, at 1876 cm−1
)
Reactivities of NH4+ on Fe-ZSM-5 at 200◦C. The IR
spectra of the reaction between NH+4 ions and NO at 200◦C
are shown in Fig. 4. After 1000 ppm NO was passed over
the NH3-treated Fe-ZSM-5 for 16 min, the IR band due to
NH+4 ions decreased slightly (approximately by 10%). The
outlet gas analysis showed that almost no N2, N2O, or H2O
formation was observed by the mass spectrometer. This fur-
ther proves that NO is quite inactive in reacting with NH4+
ions.
By comparison, when 1000 ppm NO2 was passed over the
sample, the NH+4 band decreased significantly (Fig. 5). The
IR bands due to NO2 (1624 cm−1), nitrate (1573 cm−1), and
the intermediate NO2(NH+4 )y (1605 cm−1) appeared after
2 min. Also, N2, N2O, and H2O formation was detected by
FIG. 5. IR spectra taken at 200◦C upon passing 1000 ppm NO2/He
over NH3-adsorbed Fe-ZSM-5 for 0–14 min.

228
LONG AND YANG
FIG. 8. IR spectra taken at 200◦C upon passing 1000 ppm NO + 2%
FIG. 6. IR spectra taken at 200◦C upon passing 500 ppm NO +
O2/He over NH3-adsorbed H-ZSM-5 for 0–20 min.
500 ppm NO2/He over NH3-adsorbed Fe-ZSM-5 for 0–7 min.
products were N2 and H2O. New IR bands were seen at
1876 and 1618 cm−1 after 4 min, indicating formation of
NO- and NO2 adsorbed species, respectively.
(11, 16, 17) and nitrate (weak, at 1575 cm−1) species were
observed. The present results further verify that the reac-
tivities with NH+4 ions decrease in the following sequence:
(NO + NO2) > NO2 > NO at 200◦C.
Reactivities of NH+4 on H-ZSM-5 at 200◦C. The reac-
tivities of NH+4 ions with NO + O2 and NO + NO2 on H-
ZSM-5 were also studied. After the +H-ZSM-5 was treated
with 1.07% NH3 at 200◦C, the NH4 band was also seen
at 1443 cm−1 (Fig. 8). When 1000 ppm NO + 2% O2 were
passed over the sample, the intensity of the NH+4 band de-
creased only by ca. 12% in 20 min. The formation of N2,
N2O, or H2O was not detected by mass spectrometry. It is
clear that NO + O2 is much less active on H-ZSM-5 than
that on Fe-ZSM-5 when reacting with NH+4 ions.
The IR spectra of the reaction between NH+4 ions and
NO + O2 are shown in Fig. 7. After 1000 ppm NO + 2% O2
were passed over the ammonia-adsorbed Fe-ZSM-5, the
band attributed to NH+4 ions decreased significantly and
vanished in 11 min. It is clear that oxygen increases the
reaction rate between NO and NH+4 ions. The reaction
By comparison, after 500 ppm NO + 500 ppm NO2 were
passed over the NH3-adsorbed H-ZSM-5, the NH+4 band
decreased quickly (Fig. 9), suggesting that the NH+4 ions on
H-ZSM-5 are quite reactive with NO + NO2. At the same
time, a large amount of N2 and H2O was detected in the
outlet gas. The NH+4 band disappeared in 7 min.
Figure 10 compares the decreases in the relative peak
areas of the NH+4 band at around 1443 cm−1 on H-ZSM-5
and Fe-ZSM-5 in different flowing gases at 200◦C. It can
be seen clearly that the reactivity of NH+4 ions with NO +
O2 on Fe-ZSM-5 is much higher than that on H-ZSM-5
(Fig. 10A) but they are almost the same in a flow of NO +
NO2 (Fig. 10B). Also, NO + NO2 is more reactive than
NO + O2 with NH+4 . These results support the conclusion
that the role of Fe3+ ions on Fe-ZSM-5 is to oxidize NO to
NO2 in the SCR reaction.
IR spectra of Fe-ZSM-5 in a flow of NO+NH3 +O2. To
identify the species present on the catalyst under reaction
conditions, IR spectra were recorded when Fe-ZSM-5 was
FIG. 7. IR spectra taken at 200◦C upon passing 1000 ppm NO +
2% O2/He over NH3-adsorbed Fe-ZSM-5 for 0–11 min.

SELECTIVE CATALYTIC REDUCTION
229
FIG. 9. IR spectra taken at 200◦C upon passing 500 ppm NO +
500 ppm NO2/He over NH3-adsorbed H-ZSM-5 for 0–7 min.
FIG. 11. IR spectra of Fe-ZSM-5 in a flow of 1000 ppm NO +
1000 ppm NH3 + 2% O2/He at 100–400◦C.
100–400◦C under similar reaction conditions with a GHSV
of 4.6 × 105 h−1 (9).
heated from 100 to 400◦C in a flow of 1000 ppm NO +
1000 ppm NH3 +2% O2 at steady state. As shown in Fig. 11,
the band due to NH+4 ions was observed at 1454 cm−1 at
100◦C. Raising the temperature resulted in a decrease in
the intensity of the NH+4 band. The NH+4 ions were still
observed at 400◦C. In the temperature range studied, the
IR bands due to NOx adsorbed species were not detected.
This suggests that the NO2 consumption rate (by NH3) was
much higher than the NO2 formation rate from NO + O2
under the reaction conditions. The observation of a strong
NH+4 band also indicates that NH3 was present in excess.
The formation of N2 and H2O was observed with the mass
spectrometer. Our previous catalytic performance results
also showed that 0–99% NO conversions were obtained at
DISCUSSION
Fe-ZSM-5hasbeenwidelystudiedascatalystforboththe
ammonia SCR reaction and the hydrocarbon SCR reaction.
Our previous results showed that, although iron cations
were exchanged with NH4-ZSM-5 in the Fe2+ form, most
iron cations in the Fe-ZSM-5 were present as Fe3+ ions
with tetrahedral coordination, with only a small amount
of both Fe2+ and aggregated Fe3+ ions (11). As expected,
the replacement of NH+4 (or H+) by iron cations resulted
in a decrease in the number of Brønsted acid sites. How-
ever, even when all of the protons were replaced by FeCl2+
in FeCl3, a substantial number (ca. 40%) of Brønsted acid
sites remained on the overexchanged Fe-ZSM-5 after hy-
drolysis (16). These protons provide sites for ammonia ad-
sorption, generating NH+4 ions. The foregoing data of TPSR
and activity at 200◦C showed that the reactivity of NH4+
ions with NO was very low below 320◦C (Figs. 1 and 4).
However, the addition of O2 increased the activity signif-
icantly (Fig. 7), which is in good agreement with the SCR
activity on Fe-ZSM-5 (9). The improvement is related to
the formation of NO2 from oxidation of NO by O2, be-
cause Fe-ZSM-5 is highly active in oxidizing NO to NO2
(11) and NO2 is much more reactive than NO with NH4+
ions (Figs. 1, 2, 4, and 5). Although NO2 is highly ac-
tive in reacting with NH+4 ions, the formation of N2 and
N2O is not consistent with our previous SCR result that
N2 was the only N-containing product for NO reduction
(8, 9). This suggests that NO2 is not the only reactant to
FIG. 10. Changes in peak areas of IR band near 1443 cm−1 (due to
NH+4 ions) on H-ZSM-5 and Fe-ZSM-5 at 200◦C in a flow of (A) 1000 ppm
NO + 2% O2/He and (B) 500 ppm NO + 500 ppm NO2/He.

230
LONG AND YANG
react with NH3 during the SCR reaction. Moreover, the due to a faster reaction of NO2(NH+4 )2 with NO than with
previous results indicate that the reactivity of NH+4 with NO2 (18).
NO+NO2 was much higher than that with NO2 alone with
According to the above results, we proposed a simplified
the same total NOx concentration (Figs. 3 and 6), produc- mechanism for a NO reaction with ammonia on Fe-ZSM-5,
ing only N2. All of these results support strongly that NO as shown in Fig. 12.
is also involved in the reaction with NH+4 ions for the SCR
reaction.
During the SCR reaction, gaseous NH3 molecules are
adsorbed quickly onto the Brønsted acid sites to form NH4+
At200◦C, thereactivityofNH+ ionswithNO + O2 onFe- ions, and NO molecules are oxidized to NO2 on Fe3+ sites
ZSM-5 was much higher than th4at on H-ZSM-5 (Fig. 10A), by O2. This is a slow reaction. Then one molecule of NO2
which is consistent with their respective SCR activities (9). diffuses to two adjacent NH+4 ions to form the active com-
The addition of iron ions to ZSM-5 increased the activities plex, NO2(NH+)2. The small pore size in ZSM-5 facilitates
dramatically. The increase is related to the increase in NO the formation 4of this complex. The active complex subse-
oxidation to NO2. In our previous work (11), we found quently reacts with one molecule of NO to produce N2 and
that H-ZSM-5 showed very low activity for oxidizing NO to H2O, thus completing the catalytic cycle. The total reaction
NO2. When iron ions were exchanged to H-ZSM-5, the NO is the same as reaction [1]. This reaction scheme is similar to
conversiontoNO2 wasincreasedsignificantlyfrom4–6%to that on H-ZSM-5 (15) and Fe-exchanged TiO2-pillared clay
20–30%, under the conditions of 300–400◦C, 1000 ppm NO, (10), but different from that on V2O5-based catalysts, where
2% O2, and GHSV = 4.6 × 105 h−1. The formation of NO2 N2 comes from the reaction between ammonia-adsorbed
promotes the reaction between NH+4 ions and NO. In fact, species and gaseous NO (1, 22–27). On Fe-ZSM-5, the oxi-
the reactivities of NH+4 with NO + NO2 were almost the dation of NO to NO2 is probably the rate-determining step
same on the H-ZSM-5 and Fe-ZSM-5 catalysts (Fig. 10B). It for the SCR reaction. The oxidation of NO should be near
seems that Fe3+ ions on Fe-ZSM-5 did not participate in the first order with respect to NO. It follows that the overall
NOx reduction reaction. The reaction probably took place reaction should also be first order, as verified in our kinetic
on the Brønsted acid sites, because NH+4 ions are difficult studies (28). Also, the coverage of surface by NH3 under the
to move at 200◦C. The role of Fe3+ ions is to oxidize NO to reaction conditions is in agreement with the reactions be-
NO2.
ing zero-order with respect to NH3 (28). H-ZSM-5 showed
The previously described results showed that both NOx strong surface acidity but very low activity in the oxidation
adsorbed species and NH+4 ions were observed on Fe- of NO to NO2; its SCR activity was therefore very low. Af-
ZSM-5 when NO + O2 and NH3 were introduced sepa- ter being ion exchanged with iron, the iron ions increased
rately. However, under our reaction conditions, i.e., with the oxidation rate of NO to NO2 significantly (11); hence,
NO, O2, and NH3 at high temperatures, the surface of Fe- a much higher SCR activity was expected on Fe-ZSM-5
ZSM-5 was covered mainly by NH+4 ions. NOx adsorbed (9). The active complex NO2(NH4+)2 could also react with
species were not observed (Fig. 11), which suggests that the NO2 to produce N2 and N2O, e.g., when NO2 was the only
reaction of NH+ ions with NO2 +NO was much faster than reactant (Figs. 2 and 5). However, in the presence of NO,
that of NO2 for4mation from NO + O2. As soon as NO2 was this intermediate prefers to react with NO rather than with
generated, it was consumed by NH+4 ions, so that the steady- NO2, because the reactivity of NO2(NH4+)2 with NO is
state concentration of NO2 was below the detection limit of much higher than that with NO2 according to our previ-
our IR spectrometer. In addition, the observation of NH+4 ous TPSR results (18). This explains why N2 was the only
ions also suggests that the SCR reaction took place with N-containing product for the SCR reaction on Fe-ZSM-5.
excess NH+4 , except at high temperatures when NH3 con- It is noted that, although the nitrate species with an IR band
version reached 100%. In the reaction between NH4+ and near 1573 cm−1 was also observed on the fresh Fe-ZSM-5
NO2, an intermediate NO2(NH+4 )y was observed with an IR when NO + O2 was introduced (11), it might not be an in-
bandnear1602cm−1 (Figs. 2and5). IntheSCRreaction, the termediate for the SCR reaction. This is because the nitrate
steady-state NH+4 /NO2 ratio is much larger than 1 because species comes from oxidation/disproportionation of NO2,
of the excess NH4+ ions as compared to NO2 (Fig. 11). Since but NO2 is consumed very quickly by NH+4 ions during the
the reaction between NO2(NH+4 )y and NO needs only two SCR reaction. It is difficult to further convert NO2 to nitrate
NH+4 ions (reaction [2]), the excessive NH4+ ions remain on species in the presence of NH3. On the contrary, if nitrate
the catalyst and do not participate in NOx reduction. The species were formed, it would react with NH+4 or NH3 to
NO2(NH+4 )y will actually be NO2(NH+4 )2 in the SCR reac- generate ammonium nitrate; then NH4NO3 would decom-
tion. The transfer of electrons from NH+ to NO2 weakens pose quickly to N2O and H2O, according to the reaction
both N–H and N–O bonds. This is an act4ivation process for
NH+4 ions and NO2. The intermediate NO2(NH+)2 was not
NH4NO3 → N2O + 2H2O.
[4]
observed during the SCR reaction (Fig. 11) and t4he reaction
between NH+4 ions and NO + NO2 (Figs. 3 and 6), probably It is well known that reaction [4] is the reaction by which

SELECTIVE CATALYTIC REDUCTION
231
ACKNOWLEDGMENT
This work was supported by the NSF under Grant CTS-0095909.
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Based on the above results, it can be concluded that (1)
the reactivities of NH+4 ions (preadsorbed on Fe-ZSM-5)
with NO and NO2 decrease in the following order: NO +
NO2 > NO2 ꢀ NO; (2) the reactivity of NH+ ions with
NO + O2 on Fe-ZSM-5 is much higher than4that on H-
ZSM-5, but the two catalysts show the same activity for the
reaction between NH4+ and NO + NO2; (3) the SCR re-
action needs two kinds of sites: the Brønsted acid sites for
ammonia adsorption and the metal ion sites (i.e., Fe3+ ions)
for NO oxidation to NO2; (4) a possible reaction mecha-
nism is proposed for NO reduction involving NO2 and
NO2(NH+4 )2 as intermediates, and these two intermediates
havebeendetectedbyinsituFTIR;(5)NOx reductiontakes
place on the Brønsted acid sites.