In situ FT-IR study of Rh-Al-MCM-41 catalyst for the selective catalytic reduction of nitric oxide with propylene in the presence of excess oxygen

Article abstract of DOI:10.1021/jp984525d

Rh-exchanged Al-MCM-41 is studied for selective catalytic reduction (SCR) of NO by C₃H₆ in the presence of excess oxygen. It shows a high activity in converting NO to N₂ and N₂O at low temperatures. In situ FT-IR studies indicate that Rh-NO⁺ species (1910-1898 cm^-1) is formed on the Rh-Al-MCM-41 catalyst in flowing NO/He, NO + O₂/He and NO + C₃H₆ + O₂/He at 100-350?°C. This species is quite active in reacting with propylene and/or propylene adspecies (e.g., ??-C₃H₅, polyene, etc.) at 250?°C in the presence/ absence of oxygen, leading to the formation of the isocyanate species (Rh-NCO, at 2174 cm^-1), CO, and CO₂. Rh-NCO is also detected under reaction conditions. A possible reaction pathway for reduction of NO by C₃H₆ is proposed. In the SCR reaction, Rh-NO⁺ and propylene adspecies react to generate the Rh-NCO species, then Rh-NCO reacts with O₂, NO, and NO₂ to produce N₂, N₂O, and CO₂. Rh-NO⁺ and Rh-NCO species are two main intermediates for the SCR reaction on Rh-Al-MCM-41 catalyst. ? 1999 American Chemical Society.

Full text of DOI:10.1021/jp984525d

2232
J. Phys. Chem. B 1999, 103, 2232-2238
In Situ FT-IR Study of Rh-Al-MCM-41 Catalyst for the Selective Catalytic Reduction of
Nitric Oxide with Propylene in the Presence of Excess Oxygen
R. Q. Long and R. T. Yang*
Department of Chemical Engineering, UniVersity of Michigan, Ann Arbor, Michigan 48109
ReceiVed: NoVember 23, 1998; In Final Form: January 21, 1999
Rh-exchanged Al-MCM-41 is studied for selective catalytic reduction (SCR) of NO by C3H6 in the presence
of excess oxygen. It shows a high activity in converting NO to N2 and N2O at low temperatures. In situ
FT-IR studies indicate that Rh-NO+ species (1910-1898 cm-1) is formed on the Rh-Al-MCM-41 catalyst
in flowing NO/He, NO + O2/He and NO + C3H6 + O2/He at 100-350 °C. This species is quite active in
reacting with propylene and/or propylene adspecies (e.g., π-C3H5, polyene, etc.) at 250 °C in the presence/
absence of oxygen, leading to the formation of the isocyanate species (Rh-NCO, at 2174 cm-1), CO, and
CO2. Rh-NCO is also detected under reaction conditions. A possible reaction pathway for reduction of NO
by C3H6 is proposed. In the SCR reaction, Rh-NO+ and propylene adspecies react to generate the Rh-NCO
species, then Rh-NCO reacts with O2, NO, and NO2 to produce N2, N2O, and CO2. Rh-NO+ and Rh-NCO
species are two main intermediates for the SCR reaction on Rh-Al-MCM-41 catalyst.
Introduction
propylene in the presence of excess oxygen. The mechanism
was studied by focusing on the surface adspecies of the catalyst
by in situ FT-IR spectroscopy under reaction conditions. MCM-
41 was chosen for this study because it has high thermal
stability, high BET surface area, and large pore volume. It has
already attracted considerable interests in recent years. It has
been studied as catalysts, support, and sorbents.13-16 In our
previous study, the platinum-doped MCM-41 catalyst showed
higher specific activity than Pt/Al2O3 for the SCR reaction.9
The present study shows that the Rh-exchanged Al-MCM-41
catalyst is more active and selective for N2 than the Pt-doped
MCM-41 catalyst. It is also shown that N2 and N2O originate
mainly from the reaction between Rh-NO+ and propylene
adspecies.
Removal of nitrogen oxides (NOx, x ) 1, 2) from exhaust
gases has been a challenging problem in recent years. Selective
catalytic reduction (SCR) of NOx by hydrocarbons in the
presence of excess oxygen has been extensively studied.1,2
Supported Pt-group metals have been reported to be active at
lower temperatures and are stable in the presence of water vapor
and sulfur dioxide.1-9 Platinum, iridium, palladium, rhodium,
and ruthenium supported on Al2O3, TiO2, SiO2, ZrO2, and
ZSM-5 have been studied.3-8 More recently, MCM-41 as a
support and ion-exchanged MCM-41 have been studied in our
laboratory9 for the SCR reaction. Both ZSM-5 and MCM-41
have channel-type pores; however, the pores are much bigger
in the MCM-41 catalysts, i.e., 0.5-0.6 nm in ZSM-5 vs 3-4
nm in MCM-41. Hence, the mass transfer resistance is consider-
ably lower in the MCM-41 catalysts. Among various noble
metals doped on Al2O3, Pt was reported to be the most active
and resistant to H2O and SO2, but it produces a substantial
amount of N2O. By comparison, Rh/Al2O3 has the highest
product selectivity for N2.5 These different behaviors may be
related to the different characteristics of the two metals and thus
two different reaction pathways in the SCR reaction. For Pt-
supported catalysts, e.g., Pt/Al2O3 and Pt/ZSM-5, the reaction
mechanism has been studied by TAP (temporal analysis of
products) and FT-IR techniques.8,10 It has been generally
accepted that the reaction path for reduction of nitric oxide
involves a two-step process in which the NO molecules are
decomposed to N and O atoms on the reduced platinum sites,
then the N atom combines another N atom or an NO molecule
to produce N2 or N2O. The oxidized Pt sites are regenerated by
reduction with hydrocarbons (e.g., C3H6).8,10,11 However, few
studies on SCR on Rh-supported catalysts have been reported
and the mechanism for NO reduction on Rh catalysts is still
unclear.2,7,12
Experimental Section
Catalyst Preparation and Characterization. Al-MCM-41
(Si/Al ) 10) was synthesized according to the procedure given
by Borade and Clearfield.17 Fumed silica (99.8%, Aldrich),
tetramethylammonium hydroxide pentahydrate (TMAOH, 97%,
Aldrich), 25 wt % cetyltrimethylammonium chloride (CTMACl)
in water (Aldrich), Al[C2H5CH(CH3)O]3 (97%, Aldrich), and
NaOH (98.1%, Fisher) were used as source materials for
preparing Al-MCM-41. Solution A was prepared by dissolving
1.325 g of TMAOH in 100 mL of deionized water and then
adding 5 g of fumed silica. Solution B was obtained by
dissolving 0.72 g of NaOH in deionized water and adding 25
mL of CTMACl followed by adding 2.19 mL of Al[C2H5CH-
(CH3)O]3 at room temperature. The two solutions were stirred
for 10-15 min, then solution A was added to solution B. The
reaction mixture had the following chemical composition:
1SiO2-0.05Al2O3-0.23CTMACl-0.11Na2O-
0.089TMAOH-125H2O
In this work, we first report the activities and product
selectivities of Rh-exchanged Al-MCM-41 for SCR of NO by
After being stirred for 15 min, the mixture was transferred into
a 250 mL three-neck flask and was then heated at 100 °C for
48 h. After filtration, the solid was washed, dried, and calcined
* Corresponding author. Telephone: (734) 936-0771. Fax: (734) 763-
0459. E-mail: yang@umich.edu.
10.1021/jp984525d CCC: $18.00 © 1999 American Chemical Society
Published on Web 03/05/1999

Rh-Al-MCM-41 Catalyst
J. Phys. Chem. B, Vol. 103, No. 12, 1999 2233
TABLE 1: Catalytic Performance of Rh-Al-MCM-41
Catalyst for SCR Reactiona
NO
conversion
(%)
C3H6
conversion
(%)
selectivity (%)
temp (°C)
N2
N2O
200
250
275
300
325
350
400
5.6
34.0
68.2
44.9
33.5
23.5
15.8
62.5
60.3
69.0
66.0
65.0
67.0
78.0
37.5
39.7
31.0
34.0
35.0
33.0
22.0
2.4
43.1
100
100
100
100
100
a Reaction conditions: 0.1 g of catalyst, [NO] ) [C3H6] ) 1000
ppm, [O2] ) 2%, He ) the balance, and total flow rate ) 250 mL/
min.
He (99.9998%) for 30 min and then cooled to desired temper-
atures, i.e., 350, 300, 250, 200, 100 °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. Thus, the IR absorption features that origi-
nated from the structural vibrations of the catalyst were
eliminated from the sample spectra. Unless otherwise stated, a
standard pretreatment procedure at 350 °C was performed before
gas adsorption. The procedure consisted of oxidizing the sample
in flowing O2 for 10 min followed by purging with He for 15
min, then reducing the sample by H2 for 10 min, and finally
flushing in He for 15 min. In the experiment, the IR spectra
were recorded by accumulating 100 or 8 scans at a spectral
resolution of 4 cm-1. The gas mixtures (i.e., NO/He, C3H6/He,
NO + O2/He, C3H6 + O2/He, NO + C3H6 + O2/He, etc.) had
the same concentrations as that used in the activity measure-
ments, i.e., 1000 ppm of NO (when used), 1000 ppm of C3H6
(when used), 2% O2 (when used), and the balance of He. The
total gas flow rate was 250 mL/min.
Figure 1. XRD pattern of Al-MCM-41 sample.
at 560 °C for 10 h in a flow of air (150 mL/min). The XRD
pattern of Al-MCM-41 (Figure 1) was consistent with that
reported previously for the Al-MCM-41 molecular sieve,13,17
and all XRD peaks could be indexed on a hexagonal lattice
with d100 ) 3.9 nm.
Rh-exchanged Al-MCM-41 was prepared by using a con-
ventional ion exchange procedure. An amount of 1 g of Al-
MCM-41 sample was added to 200 mL of 10-3 M Rh(NO3)3
solution at 70 °C with constant stirring. The pH value of the
solution was adjusted to 6 with NaOH solution in order to
maximize the rhodium ion-exchange capacity. A low pH is not
favorable for ion exchange because of competition from H+
and the fact that Rh exists as Rh3+. At high pH Rh would
precipitate as Rh(OH)3. The exchange process was carried out
for 6 h and repeated three times. After that, the mixture was
filtered and washed five times with deionized water. The
obtained solid sample was first dried at 120 °C in air for 12 h,
then heated at 400 °C for 6 h in a flow of 5.34% H2/N2. The
rhodium content in the Rh-Al-MCM-41 sample was analyzed
by neutron activation analysis and was 3.14% (i.e., 61.7% ion
exchange). The Rh dispersion was determined by CO chemi-
sorption9 and was 93%. The BET surface area, pore volume,
and average pore size of the Rh-Al-MCM-41 sample mea-
sured by N2 adsorption at -196 °C with a Micromeritics ASAP
2010 micropore size analyzer were 952 m2/g, 1.22 cm3/g, and
4.5 nm, respectively.
Catalytic Performance Measurement. The SCR activity
measurement was carried out in a fixed-bed quartz reactor.9A
0.1 g sample, as particles of 60-100 mesh, was used in this
work without any pretreatment. The activity was measured after
reaching a “steady state.” The typical reactant gas composition
was as follows: 1000 ppm of NO, 1000 ppm of C3H6, 2% O2,
and the balance He. The total flow rate was 250 mL/min
(ambient conditions). The premixed gases (1.01% NO in He
and 1.00% C3H6 in He) were supplied by Matheson Company.
The NOx concentration was continuously monitored by a
chemiluminescent NO/NOx analyzer (Thermo Electro Corpora-
tion, model 10). The other effluent gases were analyzed by a
gas chromatograph (Shimadzu, 14A) at 50 °C with a 5A
molecular sieve column for O2, N2, and CO, and a Porapak Q
column for CO2, N2O, and C3H6.
Results and Discussion
Catalytic Performance for SCR Reaction. For pure Al-
MCM-41, no NO conversion to N2 or N2O was obtained at
200-400 °C under the reaction conditions (0.1 g sample, 1000
ppm of NO, 1000 ppm of C3H6, 2% O2, and 250 mL/min of
total flow rate). Whereas, as shown in Table 1, Rh-Al-MCM-
41 was active for the SCR reaction. With increasing temperature,
the NO conversion increased first, passing through a maximum,
then decreased at higher temperatures. The maximum NO
conversion appeared at the temperature at which C3H6 conver-
sion reached 100%. At high temperatures, the decrease in NO
conversion was due to the combustion of C3H6 by O2. Carbon
dioxide was the only product (besides water) of propylene
oxidation. The nitrogen balance was above 95% in this work.
The product selectivity for N2 was between 60% and 78%. The
maximum NO conversion on the Rh-Al-MCM-41 catalyst was
slightly higher than that on the Pt/MCM-41 catalyst (68.2% at
275 °C vs 63.6% at 250 °C)9 under the same reaction conditions.
The former also had much higher product selectivities for N2
than the latter, which is in agreement with the previous result
that Rh-doped catalysts have higher N2 selectivities than Pt
catalysts.2 Since Al-MCM-41 was inactive in the SCR reaction,
it is clear that rhodium acted as active sites for NO reduction
on the Rh-Al-MCM-41 catalyst. The high activity on the
catalyst may be attributable to the high rhodium dispersion
(93%, obtained by CO chemisorption). It is known that ion
exchange can be used to prepare highly dispersed Rh in NaY
zeolite.18
FT-IR Study. Infrared spectra were recorded on a Nicolet
Impact 400 FT-IR spectrometer with a TGS detector. Self-
supporting wafers of 1.3 cm diameter were prepared by pressing
10 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. The wafers were first treated at 400 °C in a flow of
IR Spectra of NO and NO2 Adsorption on Rh-Al-MCM-
41 Catalyst. The adsorption of NO and NO2 was studied at

2234 J. Phys. Chem. B, Vol. 103, No. 12, 1999
Long and Yang
Figure 3. IR spectra (100 scans) of Rh-Al-MCM-41 in a flow of
1000 ppm of NO + 2% O2/He at (a) 100, (b) 200, (c) 250, (d) 300,
and (e) 350 °C.
Figure 2. IR spectra of (a) 1000 ppm of NO adsorbed on Rh-Al-
MCM-41 pretreated by O2, (b) 1000 ppm of NO adsorbed on Rh-
Al-MCM-41 pretreated by H2, (c) 1000 ppm of NO + 2% O2 adsorbed
on Rh-Al-MCM-41 pretreated by H2, and (d) 1000 ppm of NO2
adsorbed on Rh-Al-MCM-41 pretreated by H2. The spectra (100
scans) were collected after the gases were passed over the sample for
10 min at 250 °C.
reduced catalyst. In addition, two weak bands were seen at 1776
and 1644 cm-1. The 1776 cm-1 band can be assigned to Rh-
NO-.20-23 The NO- species with a weakened N-O bond would
be easier to dissociate. It is known that NO molecules could be
decomposed to N and O atoms on reduced Rh sites and thus
oxidize the Rh sites.21 While the assignment of the band at 1644
cm-1 is complicated, Xin et al.10 and Tanaka et al.23 assigned
the band around 1657 cm-1 on Pt catalyst to ν(ONO), but the
wavenumbers of nitrito complexes generally fall in the range
1485-1400 and 1110-1050 cm-1, as shown by Nakamoto.22
Therefore, this band could also be due to Rh-NO- at different
sites on the Rh catalyst, as suggested by Srinivas et al.20 The
IR spectrum of NO + O2/He adsorbed on Rh-Al-MCM-41
was similar to that of NO adsorbed on the oxidized sample;
i.e., Rh-NO+ (1904 cm-1), Rh-NO2 (1634 cm-1), and a
bidentate nitrato species (1541 cm-1) were observed (Figure
2c). When Rh-Al-MCM-41 was treated in a flow of NO2/
He, Rh-NO+ (1910 cm-1), Rh-NO2 (1627 and 1602 cm-1),
and a bidentate nitrato species (1554 cm-1) were formed, with
Rh-NO2 as the dominant species (Figure 2d). The formation
of Rh-NO+ indicates that NO2 molecules were partly decom-
posed to NO molecules on Rh sites.
250 °C on both oxidized and reduced Rh-Al-MCM-41 by FT-
IR spectroscopy. The oxidized Rh-Al-MCM-41 was obtained
by calcining the sample at 350 °C in a flow of O2 for 10 min
followed by purging with He for 15 min. After the sample was
exposed to a flowing NO/He for 10 min at 250 °C, three IR
bands were observed at 1900, 1634, and 1534 cm-1 (Figure
2a). The band at around 1900 cm-1 was always observed on
NO-adsorbed Rh catalysts. It is attributable to NO adsorbed on
the partially oxidized Rh sites, i.e., Rh-NO+,7,19-21 which is
generated by the donation of the unpaired electron from the 2p
π* antibonding orbital of the NO molecule to the 4d orbital of
rhodium. This results in an increase in the strength of N-O
bond, and hence, the NO molecule associated with Rh+ sites is
more difficult to decompose to N and O atoms than the free
NO molecule. The band at 1534 cm-1 is due to the ν(NdO)
vibration of the bidentate nitrato species:19
IR Spectra of Adsorbed NO + O2 at Different Temper-
atures. Figure 3 shows a series of spectra of Rh-Al-MCM-
41 in flowing NO + O2/He at different temperatures. After the
sample was treated by NO + O2/He at 100 °C for 10 min, Rh-
NO+ (1910 cm-1), Rh-NO2 (1634 cm-1), and bidentate nitrato
species (1543 cm-1) were formed (Figure 3a). In addition, a
small peak was observed at 1315 cm-1. This peak was also
seen on the NO2-adsorbed Rh-Al-MCM-41 but not observed
on the NO-adsorbed sample that was pretreated by H2. This
This species was also observed on the NO adsorbed Rh/SiO2
catalyst by Srinivas et al.20 The weaker band at 1634 cm-1 may
be assigned to NO2 adsorbed on Rh sites.7,20 NO was also
adsorbed on a reduced sample. Rh-Al-MCM-41 was reduced
by H2 at 350 °C for 10 min followed by purging with He for
15 min. NO was then adsorbed at 250 °C for 10 min. The Rh-
NO+ band was stronger and appeared at a lower wavenumber
(1895 cm-1) compared to that on the oxidized sample (Figure
2b), indicating that more Rh+ sites were produced on the
-
peak was most likely due to adsorbed NO2 species.22 This
species disappeared at 200 °C (Figure 3b). With an increase
in temperature, the Rh-NO+ band (1910 cm-1) grew to a

Rh-Al-MCM-41 Catalyst
J. Phys. Chem. B, Vol. 103, No. 12, 1999 2235
TABLE 2: Assignments of IR Bands for the Reaction C3H6
+ O2 on Rh-Al-MCM-41
band (cm-1
)
assignment
ref
3084
2984
2956
2924
2363, 2335
2103, 2046
1675
νas(dCH2), C3H6
24, 25
24, 25
24, 25
24, 25
20
20, 21
24-26
21
νs(dCH2), C3H6
νas(-CH3), C3H6
νs(-CH3), C3H6
CO2
ν(CO), CO adsorbed on different Rh sites
νas(CdO), acrolein
polyene species
1594
1490-1510
π-allyl complex (π-C3H5)
surface carboxylate species
allylic species
21
21
26
1430
1372
Figure 4. IR spectra (100 scans) of Rh-Al-MCM-41 in a flow of
1000 ppm of C3H6 + 2% O2/He at (a) 100, (b) 200, (c) 250, (d) 300,
and (e) 350 °C.
maximum intensity at 250 °C and then declined at higher
temperatures. Rh-NO+ was the dominant species at tempera-
tures below 300 °C. By comparison, increasing the temperature
from 100 to 350 °C resulted in an increase in the bidentate
nitrato species (1543 cm-1) but a decrease in the Rh-NO2
species (1634 cm-1) (Figure 3). The bidentate nitrato species
became the dominant adspecies at 350 °C owing to oxidation
of rhodium. Because NO molecules are easily decomposed to
N and O atoms, thus oxidizing the prereduced Rh sites to form
[Rh(O2)-], other NO molecules can adsorb on the oxidized Rh
sites [Rh(O2)-] to form RhO2NO, i.e., bidentate nitrato species.
At high temperatures, most of the Rh surface was covered by
oxygen atoms, and many Rh sites were oxidized to [Rh(O2)-]
sites, so bidentate nitrato became the dominant species.
IR Spectra of Rh-Al-MCM-41 in a Flow of C3H6 + O2/
He. When Rh-Al-MCM-41 was exposed to flowing C3H6 +
O2/He at 100 °C, a series of IR bands were observed (Figure
4a). The weak peaks between 3084 and 2924 cm-1 resulted from
asymmetric or symmetric C-H stretching vibrations of dCH2
and -CH3 groups of gaseous or weakly adsorbed C3H6.24,25
They disappeared after the sample was purged by He for 15
min. The stronger bands at 1675 and 1430 cm-1 can be assigned
to acrolein and carboxylate adspecies, respectively.21,24-26 The
appearance of the band at 1594 cm-1 indicates the formation
of polyene species.21 The shoulders at 1490 and 1372 cm-1 are
due to the π-allyl complex (π-C3H5) and allylic species,
respectively.21,26 The assignments of these bands are summarized
in Table 2. These results indicate that oxidation of C3H6 took
place on the Rh-Al-MCM-41 catalyst at 100 °C. With an
increase in temperature, the intensity of the carboxylate species
(1430 cm-1) grew to a maximum at 250 °C and then declined,
and it disappeared completely at 350 °C. The other adspecies,
i.e., allylic, π-C3H5, polyene, and acrolein, decreased with an
increase in temperature and vanished at 300 °C. Moreover, four
Figure 5. IR spectra (eight scans) taken at 250 °C upon passing 1000
ppm of C3H6/He over the NO + O2 presorbed on Rh-Al-MCM-41
for (a) 0, (b) 15, (c) 30, and (d) 60 s.
new bands at 2363, 2335, 2103, and 2046 cm-1 appeared at
200 °C (Figure 4b). The bands at 2363 and 2335 cm-1 can be
assigned to gaseous or weakly adsorbed CO2 species, while the
other two bands are attributed to carbon monoxide adsorbed
linearly on two different rhodium sites.20,21 Increasing temper-
ature resulted in an increase of CO2 bands but a decrease of
CO adspecies bands. The oxidation reaction of propylene has
been intensively studied on various catalysts, and the mechanism
is understood.21 The above results suggest that the reaction route
between C3H6 and O2 on the Rh-Al-MCM-41 catalyst is in
agreement with the previous mechanism; i.e., propylene mol-
ecules are first adsorbed on the active sites (Rh sites) to produce
allylic and π-C3H5 species, which can further dehydrogenate
to form a polyene species or react with oxygen species to
produce acrolein and carboxylate species. They are finally
oxidized to CO and CO2 by oxygen.
IR Spectra of Reaction Between C3H6 and NOx Adspecies.
Rh-Al-MCM-41 was first treated with NO + O2/He followed
by He purge at 250 °C. C3H6/He was then introduced, and the
IR spectra were recorded as a function of time (Figure 5). As
noted above, Rh-NO+ (1898 cm-1), Rh-NO2 (1629 cm-1),

2236 J. Phys. Chem. B, Vol. 103, No. 12, 1999
Long and Yang
Figure 7. IR spectra (100 scans) of Rh-Al-MCM-41 in a flow of
1000 ppm of NO + 1000 ppm of C3H6 + 2% O2/He at (a) 100, (b)
200, (c) 250, (d) 300, and (e) 350 °C.
Figure 6. IR spectra (eight scans) taken at 250 °C upon passing 1000
ppm of C3H6 + 2% O2/He over the NO + O2 presorbed on Rh-Al-
MCM-41 for (a) 0, (b) 15, (c) 30, (d) 60, and (e) 300 s.
and bidentate nitrato species (1536 cm-1) were formed after
Rh-Al-MCM-41 was treated with NO + O2/He (Figure 5a),
and their IR bands did not decrease in flowing He for 5 min.
After C3H6/He was passed over the sample for 15 s, the bands
due to Rh-NO+ (1898 cm-1) and bidentate nitrato species (1536
cm-1) declined rapidly, while CO2 (2363 and 2335 cm-1) and
Rh-Al-MCM-41, there was a gradual decrease in the Rh-
NO+ band (1898 cm-1) and the bidentate nitrato band (1536
cm-1) and simultaneous formation of CO2 (2363 and 2335
cm-1). Meanwhile, Rh-NCO (2174 cm-1) and Rh-CO (2103
and 2047 cm-1) were progressively formed. On the other hand,
some features that were assigned to acrolein (1670 cm-1),
polyene (1594 cm-1), π-C3H5 (1492 cm-1), and carboxylate
Rh-CO (2025 cm-1 20,21
species were formed (Figure 5b). In
)
species (1427 cm-1 21,24-26
also appeared. After 5 min, only a
)
addition, a new weak band at 2174 cm-1 was also observed,
suggesting the formation of an isocyanate complex (Rh-
NCO).7,20,21,27-29 The Rh-NCO species was detected by many
researchers when investigating the interaction of NO and CO
on supported rhodium catalysts. For example, Hecker and
Bell27,28 studied the formation of -NCO by in situ IR
spectroscopy on Rh/SiO2. They assigned the band at 2300 cm-1
to Si-NCO and that at 2190-2170 cm-1 to Rh-NCO on the
basis of a comparison with the spectra of isocyanate complex
over transition metals. The Rh-NCO species at 2172 cm-1 was
also identified by an isotope exchange experiment.29 The
decrease of the IR bands due to Rh-NO+ and bidentate nitrato
species as well as the formation of CO2, Rh-CO, and Rh-
NCO species clearly indicated that C3H6 reacted with these
nitrogen oxides adspecies. Decrease of the 1629 cm-1 band was
not apparent. This is probably due to the fact that the product
H2O, resulting from oxidation of propylene, also has an IR band
near 1629 cm-1. The bands at 1629 and 1536 cm-1 vanished
in 30 s (Figure 5c). After the sample was treated in a flow of
C3H6/He for 60 s, the Rh-NO+ and Rh-NCO bands also
disappeared and the CO adspecies became dominant on the
surface, with a linear CO band at 2004 cm-1 and a bridged CO
trace of Rh-NO+ and Rh-NCO were detected, and the other
IR features were similar to that of the fresh Rh-Al-MCM-41
catalyst exposed in flowing C3H6 + O2/He at 250 °C (as shown
in Figure 4c). These results indicate that C3H6 could also reduce
the nitrogen oxides adspecies in the presence of excess oxygen,
but the disappearance of nitrogen oxides adspecies required a
longer time (>5 min vs 1 min) compared with that in the
absence of oxygen (compare Figures 5 and 6). This is attribut-
able to the competitive consumption of C3H6 by O2. Besides,
CO2, not CO, was the main product of propylene oxidation in
the reaction between C3H6 + O2 and nitrogen oxides adspecies.
IR Spectra of Rh-Al-MCM-41 in a Flow of NO + C3H6
+ O2/He. To identify the species present on the catalyst under
reaction conditions, IR spectra were recorded in a flow of NO
+ C3H6 + O2/He when Rh-Al-MCM-41 was heated from 100
to 350 °C. As shown in Figure 7a, a series of IR bands were
observed at 3084-2924, 1899, 1730, 1675, 1635, 1600, 1507,
1428, and 1373 cm-1 at 100 °C. These bands were also detected
on the sample in flowing NO/He, NO +O2/He, and C3H6 +O2/
He (Figures 2-4). As indicated above, the weak bands between
3084 and 2924 cm-1 are due to the C-H stretching vibration
of C3H6; the bands at 1899 and 1730 cm-1 are attributable to
Rh-NO+ and Rh-NO- species, respectively. The shoulders
at 1675 and 1600 cm-1 and the weak bands at 1507, 1428, and
1373 cm-1 can be assigned to acrolein, polyene, π-C3H5,
carboxylate, and allylic species, respectively.20-26 The band at
band at 1830 cm-1 20
.
IR Spectra of Reaction Between C3H6 + O2 and Adsorbed
NOx. Figure 6 shows the IR spectra observed during the reaction
between C3H6 + O2/He and nitrogen oxides adspecies at 250
°C. As C3H6 + O2/He was passed over the NO + O2 adsorbed

Rh-Al-MCM-41 Catalyst
J. Phys. Chem. B, Vol. 103, No. 12, 1999 2237
1635 cm-1, assigned to Rh-NO2 species above, was probably
also due to H2O here because oxidation of C3H6 took place at
100 °C. When the temperature was raised to 200 °C, the IR
bands attributed to Rh-NO+ (1899 cm-1), π-C3H5 (1507 cm-1),
and carboxylate (1428 cm-1) increased (Figure 7b). Moreover,
two new peaks at 2174 and 1777 cm-1 appeared, which can be
assigned to Rh-NCO and Rh-NO- species, respec-
tively.20-22,27-29 The formation of Rh-NCO indicates that the
reaction between NO and C3H6 occurred at 200 °C. At 250 °C,
the IR bands due to CO2 at 2363 and 2335 cm-1 were detected.
In addition, a very small peak at 2241 cm-1 was also observed,
suggesting the formation of gaseous N2O.20 The IR bands of
these adspecies were changed with an increase in temperature.
Rh-NCO and carboxylate species grew to a maximum at 250
°C and then decreased at higher temperatures. The maximum
of Rh-NO+ species appeared at 300 °C. At 350 °C, besides
CO2, Rh-NO+, and Rh-NO2, a new band at 1541 cm-1 was
detected, which is attributable to the bidentate nitrato species.19,20
It is noted that propylene was consumed by oxygen and nitric
oxide at this temperature. No adsorbed or gaseous CO species
was observed during the reaction at 100-350 °C.
Reaction Mechanism of NO Reduction by C3H6 in the
Presence of O2. As indicated above, when NO was introduced
to the oxidized Rh-Al-MCM-41 or NO + O2 was passed over
the reduced sample at 250 °C, Rh-NO+, bidentate nitrato
species, and a small amount of Rh-NO2 were observed. Rh-
NO+ was the dominant species on the surface. It was also
observed on Rh-Al-MCM-41 in the presence of NO + C3H6
+ O2/He at 100-350 °C (Figure 7). The nitrogen oxides
adspecies were quite reactive toward C3H6 at 250 °C in the
absence or presence of excess oxygen, leading to the formation
of Rh-NCO, CO, and CO2 (Figures 5 and 6). However, under
reaction conditions, the bidentate nitrato species was not detected
until C3H6 was totally consumed at 350 °C (Figure 7). It cannot
be a reaction intermediate in the SCR reaction. It is also unclear
if Rh-NO2 species existed on the catalyst under reaction
conditions because of its overlap with the H2O band. Consider-
ing that the concentration of Rh-NO2 was always much lower
than that of Rh-NO+ on the NO-adsorbed Rh-Al-MCM-41
catalyst (Figures 2 and 3), its contribution to the production of
N2 and N2O would be small compared with that by Rh-NO+,
even if it is formed under reaction conditions. Therefore, Rh-
NO+ may be the main primary intermediate species for the
reduction of NO by C3H6.
with propylene took place at above 200 °C, as identified by the
formation of Rh-NCO, N2O, and CO2 species. Since the
propylene adspecies (polyene, π-C3H5, allylic species, etc.) are
strong reductants, they can also reduce the nitrogen oxides
adspecies at high temperatures. On the basis of the above results,
a possible mechanism for the reduction of NO by C3H6 in the
presence of excess O2 on the Rh-Al-MCM-41 catalyst is
present as follows:
Rh+ + NO T Rh-NO+
C3H6(g) f C3H6(a) (+[O]) f -C3H5, π-C3H5, polyene, ...
(1)
(2)
Rh-NO+ + C3H6(a), -C3H5, π-C3H5, polyene, ... f
... f Rh-NCO (3)
Rh-NCO + 1/2O2 f Rh + 1/2N2 + CO2
Rh-NCO + NO f Rh + N2 + CO2
(4)
(5)
NO + 1/2O2 f NO2
(6)
(7)
Rh-NCO + NO2 f Rh + N2O + CO2
Rh + 1/4O2 f Rh+ + 1/2O2-
(8)
The reaction begins with the adsorption of NO molecules on
the partially oxidized Rh+ sites to form Rh-NO+ (reaction 1)
and the adsorption of C3H6 on the catalyst to form propylene
adspecies, such as allylic species, π-C3H5, polyene, etc. (reaction
2). The Rh-NO+ and the adjacent propylene adspecies form
the Rh-NCO species (reaction 3). The Rh-NCO then reacts
with O2, NO, and NO2 to produce N2, N2O, and CO2 (reactions
4-7). At the same time, Rh is oxidized back to Rh+ ions
(reaction 8), and thus, a catalytic cycle for the SCR reaction is
accomplished. Besides the major reaction path, some N2 and
N2O may also come from NO dissociation because a small
amount of Rh-NO- species was also observed under reaction
conditions (Figure 7). It is known that reduced Rh metal is active
for the decomposition of NO molecules.21 The above reaction
mechanism on the Rh-Al-MCM-41 catalyst is different from
that on Pt-doped catalysts8,10,11 as well as Cu-ZSM-5,1 Co-
ZSM-5,31 and Mn-ZSM-5 catalysts.32
In addition to the SCR reaction, the propylene adspecies can
also be oxidized by oxygen. The reaction path is as follows:
When C3H6 and C3H6 + O2 reacted with the nitrogen oxides
adspecies on the Rh-Al-MCM-41 catalyst, Rh-NCO was
produced (Figures 5 and 6). This species was also observed on
the catalyst under reaction conditions (Figure 7). In the study
of the NO + CO reaction, Rh-NCO was detected on the
rhodium-doped catalysts and attracted considerable interest.19
It was considered to be formed from the reaction between CO
and Rh-N resulting from the dissociation of NO on the reduced
Rh sites. However, in the SCR reaction, we did not detect any
gaseous or adsorbed CO species on Rh-Al-MCM-41 (Figure
7). Hence, the Rh-NCO species was most probably formed
from reduction of Rh-NO+ by C3H6. The Rh-NCO species
was reported to be active in reacting with O2 and NO to form
N2 and N2O on the Rh/Al2O3 catalyst.30 Rh-NCO may also be
another intermediate during the SCR reaction.
-C3H5, π-C3H5 (+[O]) f acrolein (+[O]) f
carboxylate (+[O]) f CO2 (9)
-C3H5, π-C3H5 (+[O]) f polyene (+[O]) f CO2 (10)
The oxidation reaction competes with the SCR reaction for the
consumption of propylene.
Conclusions
Rh-Al-MCM-41 was active for the reduction of NO by
C3H6 in the presence of excess oxygen. The Rh-NO+ species
was observed by in situ FT-IR spectroscopy on the catalyst in
flowing NO/He, NO + O2/He, and NO + C3H6 + O2/He. It
could react with propylene and/or propylene adspecies (e.g.,
π-C3H5, polyene, etc.) at 250 °C in the presence or absence of
oxygen. During the SCR reaction, an isocyanate species (Rh-
NCO) was also detected. A main reaction path for the reduction
As NO + C3H6 +O2/He was passed over Rh-Al-MCM-41
at 100 °C, besides Rh-NO+ and Rh-NO-, acrolein, polyene,
π-C3H5, carboxylate, and allylic species were formed (Figure
7). This suggests that partial oxidization of C3H6 took place at
100 °C. The reactions between the nitrogen oxides adspecies

2238 J. Phys. Chem. B, Vol. 103, No. 12, 1999
Long and Yang
of NO by C3H6 was proposed. In this path, Rh-NO+ and
propylene adspecies first form a Rh-NCO species, then the
Rh-NCO species reacts with O2, NO, and NO2 to produce N2,
N2O, and CO2.
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Acknowledgment. This work was funded by the U.S.
Department of Energy, DE-FG22-96PC96206.
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