c. Jpn.

Bull. Chem. Soc. Jpn., 74, 1499–1505 (2001)
e Chemical © 2001 The Chemical Society of Japan
ciety of Jan
e Chemical
Simultaneous Removal of NO and N2O over Pd-ZSM-5 Catalysts andciety of JaFT-IR Observations of their Decomposition Routes to N2nM
Simultaneous Removal of NO + N2O
Y. Izumi et al.

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Yasuo Izumi,* Tetsuro Kizaki, and Ken-ichi Aika
Department of Environmental Chemistry and Engineering, Interdisciplinary Graduate School of Science andEngineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502(Received December 1, 2000)

Simultaneous removal of nitrogen monoxide and dinitrogen oxide with propene was investigated over Pd-H-ZSM5 catalysts. Results were compared with reports of simultaneous removal of NO and N2O in the presence of O2 over CoZSM-5 and Fe-MFI. The N2O conversion was suppressed in the presence of NO, and the simultaneous removal rates
were essentially the same as the NO conversion rates over Co-ZSM-5 and Fe-MFI. In contrast, the simultaneous removal
in the presence of O2 was found to be accelerated over Pd-H-ZSM-5 and conventional Pd/SiO2 compared to the sum of
the corresponding individual removal rates of NO and N2O. An intense FT-IR peak of NO±0 ligated to the Pd cation inside the ZSM-5 cage was observed at 1868 cm−1 in the promoted simultaneous removal reactions. Under the conditions
for the individual removal of NO or N2O and the unpromoted simultaneous removal of NO + N2O, only a weak peak at
1842 cm−1 was observed. The oxygen atom that dissociates from N2O should be the key to promoted catalysis in the coexistence of NO and N2O in order to maintain the Pd cation site clear. The oxygen controls the concentration of surface
carbonaceous species to effectively decompose NO, and so as not to inhibit the NO activation.

The catalytic removal of nitrogen oxides has been intensively investigated in relation to environmental problems.1 Eliminating nitrogen monoxide usually entails the formation of dinitrogen oxide as a by-product, which also affects the environment. In this context, simultaneous catalytic removal of NO
and N2O is a simplified and improved process for environmental catalysis. Simultaneous removal of NO and N2O was reported on Co-ZSM-52 and on Fe-MFI catalysts.3 Atmospheric
oxygen and/or water often affects the rates of NO and/or N2O
catalytic removal.2,3 It is important to investigate catalysts active in the presence of oxygen and/or water vapor. They arepresent in industrial situations where there is a need for NO
and/or N2O removal.
From a scientific point of view, the reaction mechanism of
NO catalytic removal with reductants (e.g., hydrocarbons orammonia) is rather complicated. This is because the process
has the potential to form various intermediates and products;
N2, N2O, NO2, CO and CO2 are all possible products when ahydrocarbon is utilized as a reductant. In contrast, if the simultaneous removal process allows us to selectively form N2, thereaction mechanism should be relatively simple.
The Co-ZSM-5 catalyst is effective for both NO and N2O
removal. Simultaneous removal was tested on Co-ZSM-5 withmethane in the presence of O2 at 673–773 K, and the rates
were essentially the same as those for the removal of NO
alone.2 In the simultaneous removal on Fe-MFI with propane
in the presence of O2 at 523 − 773 K, the NO conversion rates
were similar to those for the individual conversion of NO,
whereas the N2O conversion rates were significantly suppressed when compared to the individual conversion of N2O.3
A common feature of these two metal-exchanged zeolites isthat the NO conversions were not affected by the presence of[BULLETIN

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N2O, whereas the N2O conversions were inhibited with thepresence of NO.
In this paper, the simultaneous removal of NO and N2O over
Pd-H-ZSM-5 catalysts is reported and compared with resultsfor Pd-Na-ZSM-5 and conventional Pd/SiO2 catalysts. Palladium is one of the components of the three-way catalyst (Rh, Pt,Pd) for automobile exhaust gas. Because palladium generally
activates hydrocarbons,4–7 propene was chosen as the reductant
for NxO (x = 1, 2) rather than ammonia.8 ZSM-5 has been
widely investigated as an NO reduction catalyst by using hydrocarbons, especially as the metal-exchanged form.2,4,7,9–11
The promoted simultaneous reactions rather than the sum of
the individual removal rates of NO or N2O are reported for PdH-ZSM-5 and Pd/SiO2. The simplified reaction mechanisms(the routes from NO and N2O to N2) were monitored by in situ
FT-IR (Fourier-transformed infrared) spectroscopy. The key
features that affect the promotion of NO removal in the presence of N2O are proposed.
Experimental
Catalyst Preparation. The Pd-H-ZSM-5 catalyst was prepared by continuously mixing 3.0 g of H-ZSM-5 and 0.10 g ofpalladium(Ⅱ) nitrate in distilled water for 24 h. The mixture wasfiltrated and washed seven times with distilled water. The powderthat was obtained was dried at 393 K for 12 h and heated in air at
673 K for 4 h. ICP (inductively coupled plasma) results demonstrated that all the Pd ions were incorporated in/on the zeolite (Pd1.5 wt%). H-ZSM-5 was prepared by the ion exchange of NaZSM-5 (SiO2/Al2O3 = 23.3, primary crystalline size 0.1–0.5 µm,
Tosoh) with aqueous ammonium nitrate at 290 K for 24 h. ICP
measurements revealed that 98% of the sodium sites were substituted with protons. The pore area of ZSM-5 is 5.3 by 5.6 Å or 5.1

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1500 Bull. Chem. Soc. Jpn., 74, No. 8 (2001)
by 5.5 Å, and the pore volume is 0.29 cm3 g−1 based on the cyclohexane adsorption measurements. A Pd-Na-ZSM-5 catalyst was
prepared from Na-ZSM-5 (Pd 1.6 wt% in dry sample) using a similar procedure. The atomic ratio of incorporated palladium atoms
to Na+ ions in Na-ZSM-5 before mixing with Pd2+ was 11%.
A conventional Pd/SiO2 catalyst was prepared by the incipient
impregnation method12,13 by using silica (Davisson No. 952) andpalladium(Ⅱ) nitrate with distilled water. The powder that was obtained was dried at 393 K for 12 h and subsequently heated in air
at 673 K for 4 h. The Pd loading was 0.5 wt%.
Kinetic Measurements. The catalytic removals of NO and/or N2O were performed in a closed circulating system (117 cm3)
over either 0.12–0.16 g of Pd-H-ZSM-5 and Pd-Na-ZSM-5 orconventional Pd/SiO2 catalysts. The flow rate of gas was 10–30
cm3 min−1. A closed circulating system connected to a vacuum
system was chosen rather than a conventional flow system in order
to correlate the reactivity to infrared spectroscopic data obtainedon a basis of vacuum cell + catalyst disk + reaction gas. Before
catalysis, the catalysts were treated in oxygen (27 kPa) for 1h, inhydrogen (27 kPa) for 1 h, and finally under vacuum for 0.5 h at
653 K. The reaction gas pressures were 0.56, 0.56, 8.1, and 0.56
kPa for NO, N2O, O2, and propene, respectively. The remainder ofthe gas necessary to obtain a pressure of 101.3 kPa was made up
with helium (balance). The combinations of reaction gases usedwere as follows: NO; NO + O2; NO + C3H6; NO + O2 + C3H6
(individual removal of NO); N2O; N2O + O2; N2O + C3H6; N2O+ O2 + C3H6 (individual removal of N2O); NO + N2O; NO +N2O + O2; NO + N2O + C3H6; and NO + N2O + O2 + C3H6 (simultaneous removal). Most of the kinetic measurements were
performed with impurity moisture levels less than 0.01 kPa. A
control experiment was done in the presence of 0.56 kPa water gas
for Pd-H-ZSM-5 and Pd/SiO2.
Molecular Sieve 5A (N2, O2, and CO) and Unibeads C (NO,N2O, and CO2) columns were utilized for GC analysis with a detector system of TCD (Shimadzu GC-8A). The reaction rates became constant within 1 h for all the reactions at 548 K. The reaction temperature was fixed, based on preliminary reactions: theformation rates of N2 and O2 reached the steady state and weremaintained for more than 24 h at 548 K. The relatively low temperature was the compromise between high enough conversion
and better quality of FT-IR data. Increasing temperature caused
an increase in the conversion, but did decrease the S/N ratio andpeak intensity of FT-IR spectra due to the larger background anddesorption of surface species at > 600 K. Although a steady state
can be realized only for zero-order reactions in a closed circulating system, the observed constant rates are nearly steady-state
rates because catalysis occurred on Pd-H-ZSM-5 at relatively low
temperature (548 K) and relatively low conversion. The (pseudo)
steady-state rates were used in this work. The gas decrease induced by sampling was taken into account in estimating the reaction rate.
FT-IR Measurements. The FT-IR spectra were measured bya Jasco, Valor Ⅲ (single beam configuration, energy resolution 1
cm−1, MCD detector), in transmission mode with an in situ quartz
glass cell (Fig. 1). The in situ cell, capped by sodium chloride
windows,14–19 was combined with a closed circulating system.
Eighty milligrams of catalysts were pressed into a disk (φ = 20mm). The temperature and ambient gas pressure for the sample
disk can be controlled in the in situ cell within the limits of 173–823 K and 0–101.3 kPa, respectively. Gas phase spectra were
measured using the in situ cell without a catalyst sample.
The sample disk was treated in the same procedure as for kinet-[BULLETIN

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Simultaneous Removal of NO + N2O

Fig. 1. In situ infrared sample cell.
ic measurements. The sample was cooled down from 653 K (pretreatment) to 548 K (reaction temperature), and the reaction gas
was introduced to the sample. The in situ spectra were measured
at 548 K. After the introduction of reaction gas, the infrared peakintensity first increased and then saturated within 1–3 h. The
spectra for the catalyst sample before the introduction of reaction
gas were subtracted from the in situ spectra. Equivalent N2 formation rates were obtained for NO/N2O reactions with powder in a
U-shaped tube in the closed circulating system as compared topressed disk catalysts in an infrared cell connected to the closed
circulating system. This fact suggests that bulk mass transfer effects to obtain the catalyst active sites were not important on theoverall limiting kinetic rate.

Results
Kinetics. Twelve kinds of NxO (x = 1, 2) removal reactions listed in the Experimental section were performed over
Pd-H-ZSM-5 and conventional Pd/SiO2 catalysts at 548 K.
The steady-state rates are summarized in Table 1. For all thereactions (individual or simultaneous reactions and in the presence or absence of O2 and propene), the rates were faster on
Pd-H-ZSM-5 than on conventional Pd/SiO2, even taking into
account the difference of Pd loading of each catalyst (1.5 and
0.5 wt%, respectively). Namely, the rates on Pd-H-ZSM-5
were more than three times faster than those observed on Pd/
SiO2 in the corresponding reaction. H-ZSM-5 (Table 1) andSiO2 were entirely inactive or showed smaller reactivity for
NO removal at 548 K. NO reduction with propane or propene
over H-ZSM-5 is reported at 673 K.5 This result demonstrates that the Pd atom is the pertinent component of the active site for NxO removal in these reaction conditions. The efficiency of Pd-H-ZSM-5 was reported for NO reduction withmethane.4,7,20 The acidity of zeolite was suggested to be an important factor in controlling the reaction rate.
For the reactions of NxO and NxO + O2 (Table 1-i and ii),
the rates of simultaneous reactions were almost the same as
those for the individual reaction rates of N2O. In the absence

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Bull. Chem. Soc. Jpn., 74, No. 8 (2001) 1501

Table 1. Steady-State N2 Formation Rates (at 1 h Passed from the Beginning of Reaction; 10−6 mol h−1 g-cat−1) from NO
and/or N2O over Pd-H-ZSM-5 and Supported Pd/SiO2 Catalysts at 548 K

Catalyst
Pd-H-ZSM-5

Pd-Na-ZSM-5
H-ZSM-5
Pd/SiO2NxO TypeNO
N2O
NO + N2O
NO + N2O + H2Ob)
NO
N2O
NO
N2O
NO
N2O
NO + N2O
NO + N2O + H2Oa)

(i)
NxO
0.7
5.1
5.0
—
0.7
2.2
0
1.6
0.2
0.5
0.6
—

(ii)
NxO + O2
0.4
5.3
5.0
—
—
—
—
—
—
—
—
—

Reaction Gas(iii)
NxO + C3H6
4.9
23
28
—
2.7
—
0
14
0.4(±0.05)
1.7(±0.1)
3.7(±0.2)—(iv)
NxO + O2 + C3H6
5.6(±0.1)
21(±1.2)34(±0.9)a)28(±0.9)4.2—
0120.5(±0.1)
2.5(±0.1)
4.0(±0.1)
3.1(±0.3)

The pressures of NO, N2O, O2, and propene were 0.56, 0.56, 8.1, and 0.56 kPa. The balance of the pressure to 101.3 kPa
was made up with helium.
a) Corresponding to 2.7% conversion (2[N2]/([NO] + 2[N2O]). b) 0.56 kPa of water was also present.

Fig. 2. The changes of concentration of reactants and products on the Pd-H-ZSM-5 catalyst for individual removal in NO + O2
+ propene (a) and for simultaneous removal in NO + N2O + O2 + propene (b). Reaction conditions are in footnote of Table 1.

of propene at 548 K, the reaction of N2O to N2 was so fast thatthe reaction of NO was negligible or the surface reaction siteof NO was blocked by the presence of N2O.
For the reaction of NxO + propene (Table 1-iii), the reactions were dramatically promoted. All the reactions were promoted by the addition of propene, demonstrating that the N2O
reduction was also promoted by propene. The promotion of
NO reduction by propene is well known. The major reaction
route of propene should be the interaction with N2O, based on
the fact that the reaction in N2O + propene was about fivetimes faster than the reaction in NO + propene over both PdH-ZSM-5 and Pd/SiO2 catalysts (Table 1). The rates of simultaneous reactions were the sum of the independent reactions ofNO or N2O for Pd-H-ZSM-5 and larger than the sum of the independent reactions for the conventional Pd/SiO2 catalyst(76(±22)% enhancement). The error of enhancement ratio
was based on the data deviations (Table 1) of individual removal rates for NO or N2O and of simultaneous removal rate

[BULLETIN

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for NO + N2O. The rate data deviations were estimated as thestandard deviations when several reaction runs were performedin the same reaction conditions. Hence, in the presence of propene at 548 K, removal reactions of both NO and N2O proceeded. The reaction of NO was not suppressed by the coexistence of N2O for these Pd catalysts.
In the presence of O2 and propene (Table 1-iv), reactions became rather faster than in the case in propene (Table 1-iii), except for the case of N2O removal over Pd-H-ZSM-5. The reaction enhancement in the presence of O2 was reported for the
NO reduction by methane/propane over the Co-ZSM-5 catalysts.2 The rates of simultaneous reactions were larger than thesum of the individual reactions for both Pd-H-ZSM-5 and Pd/
SiO2 catalysts (promotion ratio 28(±9) and 33(±12)%, respectively). The promotions of simultaneous removal contrast
to the results of simultaneous removal on Co-ZSM-5 and FeMFI with hydrocarbon in the presence of O2. The simultaneous removal rates were essentially the same as those ob-

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Fig. 3. In situ FT-IR spectra for Pd-H-ZSM-5 for 180 min at
548 K in various reaction conditions.

served for the individual removal rates of NO on Co-ZSM-5
and Fe-MFI.2,3
The effects of 0.56 kPa of water were tested for Pd-H-ZSM5 and Pd/SiO2 catalysts in NO + N2O + O2 + propene.
20% of activity loss was observed in the case of both catalysts (Table 1). The N2 formation reactions over Pd-Na-ZSM5 were always slower than those over Pd-H-ZSM-5 (Table 1).
The time courses of gas conversion on the basis of N and C
atoms during the individual removal of NO and simultaneous
removal in the presence of O2 and propene on Pd-H-ZSM-5 areillustrated in Figs. 2a and b, respectively. As the concentration(reactants and products) was based on N or C atom, the concentration of N2O (0.56 kPa) was twice as large as that of NO(0.56 kPa) at t (reaction time) = 0 in Fig. 2b. Figs. 2a and b
show the time courses during 300 and 120 min, respectively.
In Figure 2a (individual NO removal), 130% of CO2 wasformed compared to the degree of N2 formation (C and N atombasis), but CO was produced in negligible quantities. Basedon the detected products ratio at the steady state (after 60 min
in Fig. 2), the reaction stoichiometry is simplified as 2NO +C3H6 + 7/2O2 → N2 + 3CO2 + 3H2O (the CO2: N of N2 ratio
is 150%). The hydrocarbon combustion C3H6 + 9/2O2 →3CO2 + 3H2O seems not to dominate the process. In Figure
2b (promoted simultaneous removal), the NO decrease and theN2 increase were by about six times faster than in Fig. 2a. The
N2O decreased in the first 30 min, and remained (apparently)
constant later. The formation ratios of CO and CO2 to N2 were

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Simultaneous Removal of NO + N2O

similar to the case of Fig. 2a. The CO formation rates reacheda maximum in the first 30 min and became negligible in 1–2 h
in Figs. 2a and b. The reason may be that the oxygen atom adsorption on the Pd surface gradually increased and became
constant within 1–2 h of the reaction being initiated, which isconsistent with the decrease of N2O (Fig. 2b) within first 30
min.
FT-IR Observations. The in situ FT-IR measurements
were performed for Pd-H-ZSM-5 in NO, NO + propene, NO+ O2 + propene, NO + N2O + propene, and NO + N2O + O2
+ propene at 548 K. Spectra were measured at this temperature. Peaks were observed in the region of 2000–1700 cm−1
(Fig. 3). The wavenumbers of the peaks are summarized in Table 2, along with the reported values of NO adsorption.
An intense absorption peak was observed at 1880 cm−1 inNO and at 1868 cm−1 in NO + N2O + O2 + propene (Figs. 3aand e). A resolved shoulder peak also exists in NO at 1836
cm−1, but the region was not well resolved in NO + N2O + O2
+ propene. The intense peak in NO + N2O + O2 + propene
was broader than that in NO, and was not symmetric, suggesting a weak feature on the lower-energy side.
The intense peak was not detected in NO + propene, in NO+ O2 + propene, or in NO + N2O + propene (Figs. 3b– d). A
medium-intensity peak was observed at 1842 cm−1 in NO +propene. The peak intensity became weaker or the peak disappeared by the addition of O2 or N2O. These peaks were notsymmetric, suggesting an additional weak feature around 1870
cm−1.
An intense, broad peak was observed at 2269–2251 cm−1 in
NO + O2 + propene and in NO + propene at 548 K for PdNa-ZSM-5 (not shown). Under these conditions, no peak was
detected in this region for Pd-H-ZSM-5.
Two very broad peaks were also observed at 1730 and 1650
cm−1 in NO at 548 K for Pd-H-ZSM-5. The intensity was negligible, even when compared to weak peaks at 1842–1836 cm−1
(Fig. 3). The two broad peaks were also observed in NO at
548 K for conventional Pd/SiO2 catalyst. The FT-IR spectrum
for conventional Pd/SiO2 was essentially the same as those inRefs. 12 and 13.
Discussion
FT-IR Peak Assignments. The infrared absorption measurements were performed at 548 K, and three kinds of peaks(1880, 1868 and 1842–1836 cm−1) were observed (Table 2 andFig. 3). The NO adsorption peak on H-ZSM-5 was observed at1877 cm−1, which is almost the same wavenumber as for free
NO: 1876 cm−1. However, the intensity was only a few percent compared to the peak due to NO adsorption on Pd-HZSM-5. The N2O adsorption was not observed on Pd-H-ZSM5 in the detection limit of FT-IR.16 Hence, the observed peaks
should be due to adsorbed NO. It is also possible for them to
be intermediate species of the reaction route starting from NO
and terminating with N2, because the FT-IR measurements
were performed at 548 K. However, the adsorption peaks due
to NCO,21 NO2,10,11 or N218 in the region of 1920–1800 cm−1
are not reported, or the adsorbed species have already desorbed
at 548 K. The intense, broad peak at 2269–2251 cm−1 observed on Pd-Na-ZSM-5 can be assigned to NCO.21
The adsorption of NO on reduced metallic particles on a Pd/

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Bull. Chem. Soc. Jpn., 74, No. 8 (2001) 1503

Table 2. Observed Absorption Peak Wavenumbers (cm−1) in the FT-IR Spectra for Pd-H-ZSM-5- in Various Reactions at 548 K (a)and Reported Values of NO Adsorption Wavenumbers (cm−1) (b)(a)
Catalyst
Pd-H-ZSM-5

NO1880 (s),1836 (m, sh)NO + propene
1842 (m)NO + O2 + propene
1842 (w, br)

NO + N2O + propene
1842 (m, br)

NO + N2O + O2 + propene
1868 (s)2994 (w)2937 (w)2903 (w)

Blank

1876

The pressure of the gas was the same as in Table 1.
(b)
Catalyst
Pd/SiO2
Pd-ZSM-5
Cu-ZSM-5

Fe-Ya)

NO
1660–1645,
1750–1730
1881, 1836 (NO)1827, 1734 (twin (NO)2δ−),
1813 (NOδ−),1906 (NOδ+),
1964 (Cun–NOδ+)
1918, 1815 (twin NO)
1882, 1850, 1760 (NO)

Reference
12, 13102223a) Similar peak pattern was observed also for Fe-mordenite and Fe-ZSM-5.

SiO2 surface was reported at 1660–1645 and 1750–1730 cm−1
(Table 2).12,13 They were assigned as linear or bridged NO on
the Pd particles. Particle size effects on the NO stretching
wavenumber are known. The NO stretching wavenumber increases as the coverage of NO increases, due to the dipole–dipole interactions.13 However, even when the particle size effects and the dipole–dipole coupling effects are taken into account, the wavenumbers of peaks in Fig. 3 are larger by morethan 100 cm−1 compared to reported peaks on metallic Pd.
This difference demonstrates that the observed NO peaks(1880, 1868 and 1842–1836 cm−1) were ascribed to NO on
isolated Pd cations inside the zeolite cage and that broad, very
weak peaks at 1730 and 1650 cm−1 were due to NO on metallic Pd particle under our reaction conditions.
As examples of NO adsorption on metal cations inside zeolites, the absorption peak values are listed in Table 2b for PdZSM-5,10 Cu-ZSM-522 and Fe-ZSM-5, Fe-Y, and Fe-mordenite.23 The peak energy splitting of symmetric and asymmetricpeaks of twin NO species was 93–103 cm−1. Looking at the
splitting between peaks in Fig. 3, which is 26–44 cm−1, we do
not assign them to twin NO species. In fact, their intensities
did not follow the same trend in the time course of measurements of the catalytic reaction or of the gradual gas-phaseevacuation. For the mononitrosyl species, the adsorption peakenergy varied by 93–151 cm−1 for Cu-ZSM-5 and 32–122 cm−1
for Fe-Y (Fe-ZSM-5) due to the difference of charge on the nitrosyl group. Therefore, the peaks at 1880, 1868, and 1842–
1836 cm−1 can be assigned as mononitrosyl NOδ+, NO±0, andNOδ−, respectively, on the Pd cation. Rather than other species(N2O, propene, or O2) directly affecting the wavenumber of nitrosyl on Pd (change from Figs. 3a to b–d), these other species
may preferentially block adsorption sites for NOδ+ or NO±0.
The charge on the nitrosyl group is based on each wavenumber[BULLETIN

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compared to the free NO (1876 cm−1). Whether the 4d orbitalof the Pd cation back-donates charge to the NO π* orbital orwhether the filled NO σ orbital donates a charge to the unfilled
Pd 4d orbital, the νN-O stretching wavenumber will decrease or
increase, respectively, compared to the free value of 1876
cm−1.24
The peak at 1842 cm−1 appears in the presence of propene,
and can be assigned as bridging or multifold CO on the Pd instead. As no corresponding peak of terminal CO on Pd was
detected within the range 2100–1900 cm−1 and CO formationin 3 h of the reactions NO + propene + O2 or NO + N2O +propene at 548 K was negligible (Fig. 2), the possibility of the1842 cm−1-peak assignment being due to CO seems small.
The peaks observed at 1881 and 1836 cm−1 on oxidized 0.5
wt% Pd-ZSM-5 at 300 K were assigned to mononitrosyl on PdⅠ
ligated by lattice oxygen atoms and to mononitrosyl on PdⅠ,
where one of the O ligands was replaced by a water molecule,respectively.10 In the cases shown in Figs. 3b–e, it is possiblethat the water molecule of the reaction product ligated to the
Pd ion.
In the presence of O2, an additional peak at 2133–2125 cm−1
has been reported, and this was assigned as NO2+ (Cu-MFI81),22 NO2 (Pd-H-ZSM-5),10 or NO+ (Pd-H-ZSM-5).11 No
peaks were observed in this wavenumber region for Pd-HZSM-5 and Pd-Na-ZSM-5 in in situ reaction condition in thisstudy. This comparison is not contradictory because the peakat 2133 cm−1 completely desorbed at 373 K,11 whereas ourmeasurement temperature of FT-IR was 548 K.
The NxO removal was enhanced by the coexistence of NO
and N2O in the case of reactions of NO + N2O + O2 + propene on Pd-H-ZSM-5 when compared to the sum of the individual removal rates in the corresponding conditions (Table 1).
The reactions were monitored by in situ FT-IR. The intense00409

1504 Bull. Chem. Soc. Jpn., 74, No. 8 (2001)

peak of mononitrosyl NO±0 was observed with similar peak intensity to the NO adsorption (Figs. 3e and a), suggesting theimportance of mononitrosyl NO±0 in catalysis. In the absenceof N2O in the reaction mixture, only the very weak peak ofmononitrosyl NOδ− (or weak CO) was observed in NO + O2
+ propene (Fig. 3c).
In the absence of O2, the simultaneous removal rate of NO+ N2O was the sum of the individual removal rates of NO andN2O on Pd-H-ZSM-5 (Table 1). Only the peak due to mononitrosyl NOδ− (or weak CO) was observed both in NO + propene and in NO + N2O + propene (Figs. 3b and d).
Reaction Mechanism. One of the possible effects of propene is to offer carbon in the reaction route via NCO.5,21 The
activation of propene is believed to be related to the acidity ofthe zeolite.4,20 The effects of acidity were implied in the comparison of the activity between Pd-H-ZSM-5 and Pd-Na-ZSM5 (Table 1). The N2 formation rates were always higher overmore acidic Pd-H-ZSM-5 than over less acidic Pd-Na-ZSM-5.
Compared to the gas-phase propene molecules (νCH = 2991.5,
2932.7, and 2869.8 cm−1),25 three weak peaks at 2994, 2937,
and 2903 cm−1) were observed for Pd-H-ZSM-5 in NO + N2O+ O2 + propene at 548 K (Table 2). As these three peaks
should be typical vibration peaks of CH2 and CH3, surface carbonaceous species were proposed to exist during catalysis.
Surface carbonaceous species derived from propene react withadsorbed NO (or NO2 in the presence of O2),3,6 or adsorbed
CO reacts with nitrogen species, and further reacts with another NO molecule (or NO2 in the presence of O2) to form N2 +CO2 or N2O (Scheme 1-i). 5,21 The carbonaceous species canbe consumed to form CO and CO2 in the route of by-reaction(Scheme 1-iv) or can remain as surface deposits that act as an
inhibitor of catalysis. It is unclear why the NCO species was
detected on Pd-Na-ZSM-5 and not on Pd-H-ZSM-5 in NO +propene or in NO + O2 + propene at 548 K. The N2 formation
rates on the former were slower (55–75%) than those of thelatter. The NCO on the Pd of the former catalyst may poison
the site, but NCO did not adsorb on the latter catalyst in thetime scale of the FT-IR measurements (i.e. stayed on the surface during a very short time), and may convert quickly to N2.
We propose that the key to the promotion of simultaneous
removal on Pd-H-ZSM-5 is the dissociated oxygen atom of theadsorbed N2O (Scheme 1-ii). The oxygen atom separated from
N2O on the catalyst surface may play a role in controlling the

Scheme 1. Proposed reaction pathway of the promoted simultaneous catalytic removal of NO and N2O in the presence of O2 and propene.

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appropriate concentration of surface carbonaceous species tobelow the level that leads to inhibition of NO adsorption anddissociation (Scheme 1). The importance of carbonaceous
species produced from NO + hydrocarbon has been suggested
elsewhere.3,6,26,27 The effects of oxygen derived from N2O may
be similar to the effects of the oxygen derived from O2 or NO(Scheme 1),2 but the dissociation of N2–O is thermodynamically far easier than that of O–O or N–O. N2O decomposition isreported to be insensitive to the presence of O2, whereas NO
decomposition is often influenced by the presence ofO2.2,4,6,8,9,11,20,28 This general trend may suggest different active
sites for NO and N2O dissociation. An alternative interpretation is that the reactivity of NO and N2O is different with metal
ion in zeolite, and dissociated oxygen species may have different chemical natures. More reactive oxygen species dissociated from N2O were reported on Cu-ZSM-5 (Cu2+-O−)28 and on
Fe-ZSM-5 (α-oxygen).29 Rate dependence on reactant pressure should be monitored to make this effect of O of N2O morecertain. The N2O concentration was constant in Fig. 2b after
30 min of reaction, due to the steady-state balance of N2O decomposition and the formation from NO. With the major effects of dissociated oxygen derived from N2O to remove thecarbonaceous species on surface Pd cation, the NO±0 on the Pd
cation (1868 cm−1) became the major surface species (Fig. 3e).
As the concentration of this species is the balance of the reaction path rates in Scheme 1, the promotion effects were not observed in NO + N2O + propene (in the absence of O2) on PdH-ZSM-5.
The surface site of the 1868 cm−1-peak is suggested to be
responsible for the dissociation of NO (Scheme 1-i) in the promoted simultaneous removal. Aparicio et al. reported the correlation between the N2O decomposition reaction and themononitrosyl IR peak intensity, and suggested that the mononitrosyl site is the active site for Fe-ZSM-5.23 The Pd ion introduced to H-ZSM-5 (0.3 wt% of Pd) was reported to exist as
Pd2+ in the reaction condition of the NO + CH4 + O2 reaction
at 773 K, based on the X-ray absorption fine structure spectroscopy.7 The Pd ion introduced to H-ZSM-5 (0.44 wt% ofPd) existed as Pd2+ below 610 K and is suggested to be the active site for NO reduction by methane, based on the FT-IR
measurements.11 As the pertinent Pd sites for NO and/or N2O
decompositions were found to be dispersed Pd2+ inside zeolite(Pd-H-ZSM-5) and small Pd particles (Pd/SiO2) in this paper,
the reaction mechanism of NO and/or N2O decompositions
should be different.
The intensity of two very broad peaks (1730 and 1650 cm−1)
in NO at 548 K for Pd-H-ZSM-5 that could be ascribed tomononitrosyl on Pd particles was negligible. Either only asmall amount of Pd particles existed, or the surface of the Pd
metal particles was dominantly covered by oxygen (similar effects were reported in Ref. 11) such that it could not adsorb
NO in the reaction conditions used for NO and/or N2O removal. Either possibility supports Pd2+ sites inside zeolite to be
active sites in the case of Pd-H-ZSM-5 catalyst.
The promoted simultaneous removal was observed for Pd/
SiO2 but not for Pd-H-ZSM-5 in the presence of propene andabsence of O2 (Table 1-iii). One of the possible reasons of thisdifference is the different dissociation ability of N2–O bond either in the case of Pd microparticle (Pd/SiO2) or Pd ion (Pd-H-

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Bull. Chem. Soc. Jpn., 74, No. 8 (2001) 1505

ZSM-5). The steady-state formation rate of N2 + 1/2O2 from
N2O was as much as 10 times faster on Pd-H-ZSM-5 than on
Pd/SiO2 (Table 1-i), but the initial non-stoichiometric N2 formation was faster on Pd/SiO2 rather than on Pd-H-ZSM-5.
Therefore, the dissociation of N2–O bond was faster on Pd microparticle surface than on Pd ion site, but the formation anddesorption of O2 was slow on Pd microparticles. In the presence of propene, formed surface O species can be converted bycarbonaceous species derived from propene. N2–O bond dissociation (Scheme 1-ii) was fast enough on Pd/SiO2, and thepromoting effect of O(ad) (i.e. promoted simultaneous removal) for the reaction step (iv) of Scheme 1 may be observed in
the presence of propene only for Pd/SiO2.
The biggest question still remains; why simultaneous removal was promoted via the oxygen atom dissociated from
N2O only on the Pd ion and not on the Co and Fe ions.2,3 As
H-ZSM-5 was inactive for NO removal in the reaction conditions in this paper, the difference may be ascribed to the difference of valence state among metals. These metal-zeolite catalysts were prepared from Pd2+, Co2+, and Fe2+.2,3 As exchanged Co2+ and Fe2+ can be oxidized to Co3+ and Fe3+, dissociated O may be bound to Co3+ or Fe3+ and stabilized. On
the other hand, the redox of palladium is between Pd0 and Pd2+
and dissociated O may be unstable when it is bound to Pd2+.
These difference of valence states among metals may be themajor reason that promoted simultaneous removal of NO +N2O was observed only in the case of Pd catalysts. The oxygen atom dissociated from N2O may be relatively unstable on
the Pd site, and further reaction with carbonaceous species
formed from propene can occur very quickly.
Conclusions
1. Simultaneous removal of NO + N2O was promoted in O2+ propene over Pd-H-ZSM-5 and conventional Pd/SiO2 catalysts, and in propene on conventional Pd/SiO2, when comparedto the sum of the corresponding individual removal rates ofNO or N2O.
2. In the conditions that promoted simultaneous removal,
an intense peak ascribed to the NO±0 on Pd cation in a ZSM-5
cage was observed at 1868 cm−1. When NO was introduced to
Pd-ZSM-5, the mononitrosyl (NOδ+) peak was observed at1880 cm−1. In the other conditions used for individual removal or unpromoted simultaneous removal, a medium or weak
peak was observed at 1842 cm−1.
3. Based on peak intensity in FT-IR spectra, the pertinentstate of palladium for NO and/or N2O decomposition was
found to be Pd2+ for Pd-H-ZSM-5, in contrast to the two broad
peaks ascribed to surface metal particles observed for conventional Pd/SiO2.
4. The promotion effects of the oxygen atom dissociatedfrom N2O were suggested as a mechanism for keeping the Pdsite clear from extra carbonaceous species. The relative stability of the oxygen may be related to the difference between promoted simultaneous removal on the Pd site and essentially thesame rates of simultaneous removal as NO removal (N2O removal was inhibited) on the Co and Fe sites.

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This work was partially supported by a grant from Toray
Science Foundation (No. 98-3901, Y.I.).

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