Co-adsorption of propene and nitrogen oxides on Cu-ZSM-5: An FTIR study

BENVIRONMENTAL ELSEVIER

Applied Catalysis B: Environmental

7

( 1995) 79-93

Co-adsorption of propene and nitrogen oxides on Cu-ZSM-5: An FTIR study T.E. Hoost *, K.A. Laframboise, Ford Motor

Company,

Chemical

Engineering

Department.

Mt48121,

Received 20 February

K. Otto

MD 3179 SRL, P.O. Box 2053, Dearborn,

USA

1995; revised I3 July 1995; accepted

I3 July 1995

Abstract Cu-ZSM-5 catalysts

have generated

substantial

automotive

interest.

This work deals with interac-

oxides and hydrocarbons on these materials from 25 to 225°C using infrared spectroscopy. Co-adsorption of NO with propene on CuHZSMS produced only marginal spectral differences compared to the state of preadsorbed propene on the catalyst, indicating minimal interactions between the adsorbates. Oxygen addition to NO plus propene on CuHZSMS resulted in appreciable NO oxidation. Propene adsorption decreased considerably, apparently due to a strong affinity of NO, for the hydrocarbon. The spectra also suggest additional carbonylation and carboxylation of adsorbed hydrocarbons due to NOz. In the presence of NO? and hydrocarbons on CuHZSM5, a band appeared at 2590 cm ‘. This band did not emerge during similar adsorption on HZSMS. The co-adsorption experiments indicate the following order for extent of interactions between the oxidants and propene on CuHZSM5: NO < O? < NO?. tions between

nitrogen

Copper: ZSM-5 zeolite: Propene adsorption; Infrared spectroscopy

Kewwds:

Nitric oxide adsorption:

Nitrogen dioxide adsorption:

1. Introduction Cu-ZSM-5 is very effective in the selective catalytic reduction of NO, by hydrocarbons in the presence of excess oxygen (SCR-HC) [ 11. The mechanism and the nature of the active intermediates of the reaction are still under investigation. Infrared studies of adsorption of nitrogen oxides have revealed several surface species. Copper adsorbs nitrosonium (NO+ ) , nitrosyl (NO- ) , and dinitrosyl species [(NO);] [2,3]. The zeolite can accommodate large amounts of adsorbed nitrogen dioxide [ 41, including a surface nitronium species (NO: ) which has been * Corresponding 0926.3373/95/$09.50 9S013373(95)00181-6

author. Tel.

( + l-3 I3 ) 2482332. fax. ( + I-3 13) 5942963, E-mail [email protected].

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7 (1995) 79-93

correlated with Bronsted acidity [ 51. However, these studies do not reflect how adsorbed nitrogen oxides may interact with adsorbed hydrocarbons during SCRHC. Recent infrared studies have begun to address interactions between NO, and hydrocarbons. Ukisu et al. [6-81 produced isocyanate species (-N=C=O) on CuCs/A1203, Cu/Al,O,, and A1,03 catalysts by evacuation of co-adsorbed NO, oxygen, and propene at room temperature, followed by heating at 300°C. They also tentatively assigned infrared bands to adsorbed cyanide (CN-) and isocyanide species (R-NC; R-N=C=M; R-NC-M). Isocyanate species, which could adsorb on both alumina and copper, were surmised to be critical reaction intermediates. Ukisu et al. proposed a mechanism in which the reaction between isocyanates and NO to form dinitrogen is a key step in NO, reduction [ 71. Hayes et al. [9] also used infrared spectroscopy to observe aliphatic cyanide species (nitriles) stored on the catalyst after SCR-HC over Cu-ZSM-5. In helium, these nitriles were quite stable upon heating (to 475°C). Ex situ exposure to oxygen at 350°C produced dinitrogen. The authors speculate that the nitrile produced in the ‘storage’ process is an important intermediate in the SCR-HC. Infrared work by Misono and co-workers [ lo-121 suggests intermediacy of organic nitro (-ONO) and nitrite ( -N02) surface species during SCR-HC on CeZSM-5, Pt/SiO,, and Si02 catalysts. For Ce-ZSM-5 [ lo], bands at 1658 and 1558 cm-’ were assigned to organic nitro and nitrite compounds brought about by interactions between NOz and propene. The exact structure of these compounds was not specified. They also detected isocyanate species. A linear correlation was found between nitrogen formation and the abundance of the nitro compound. However, correlation of activity with both nitrite and isocyanate species was poor. After separately measuring the reactivity of the surface species in NO2 and oxygen, the authors suggested that the reaction between organic nitro compounds and NOz is the primary route for dinitrogen formation during SCR-HC on Ce-ZSM-5. For Pt/SiO, [ 121, they proposed that nitrogen pairing resulted primarily from reactions between organic carbonyl groups and NO*, as well as reactions between organic nitro and nitrite groups and oxygen. No significant dinitrogen formation was observed for Pt-free SiOz. Even though no isocyanate species were detected on Ptl Si02, the authors refrained from excluding the possible conversion of these intermediate organic compounds to isocyanates before dinitrogen formation. Bell et al. [ 131 recently reported in-situ FTIR measurements for steady-state SCR-HC over Cu-ZSM-5. Their work indicates that surface species observed at low temperature may not be representative of the ones they observed at high temperature. Bands at 2580 and 2290 cm- ’were attributed to an oxidized nitrogencontaining species on copper, (N,O.,),/Cu, and NJCu [ 141, respectively. These bands were detected only at approximately 400°C. The authors offered the possibility that (N,O,),/Cu formed upon NO oxidation may be a reaction intermediate for SCR-HC on Cu-ZSM-5. The precise role of this species could not be resolved. Our aim is to further elucidate interactions between nitrogen oxides and propene

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on Cu-ZSM-5 using infrared spectroscopy. We caution at the onset that while infrared spectroscopy is a useful means of studying adsorbed species, the technique allows only an indirect view of these species and their adsorption sites, and, consequently, band assignments tend to be provisional.

2. Experimental

2.1. Catalyst preparation and characterization The Cu-ZSM-5 catalyst (CuHZSM5) was prepared by copper ion exchange of HZSM-5. The acid form of the HZSM-5 zeolite (HZSMS) , having a Si02/A1,0:, ratio of 80, was obtained from PQ. BET analysis yielded a zeolite surface area of 460 m2/g. Copper was exchanged from a 0.05 M aqueous solution of cupric acetate (99.9%, Johnson Matthey) in three successive exchanges with water rinses between the exchanges. The pH of the exchange solution was initially about 5.5 and did not change noticeably upon ion exchange. The ion-exchanged sample was calcined in static air at 400°C for 12 h to remove organic residuals and to insure that Cu*’ was the predominant state of the copper. The copper content was analyzed by inductively coupled plasma (ICP) and found to be 1.78 wt.-%. This corresponds to a nominal exchange of 147% copper as cupric ions. The ‘over-exchange’ process has been postulated to arise from the formation of CuOH+ moieties [ 3,151. The CuO/A1,03 catalyst (2 wt.-% Cu) [ 161 was prepared by impregnating defumed y-alumina (Degussa Alumina C, 100 m2/g) with a solution of copper nitrate (reagent grade, J.T. Baker Analytical). Sample drying at 120°C was followed by calcination in static air at 400°C overnight. For the non-exchanged CuO/Al,O,, the nitrate precursor may result in cleaner precursor decomposition than the acetate which was used to facilitate ion-exchange on the zeolite. 2.2. Infrared spectroscopy Adsorption experiments were performed in situ using a high-vacuum IR cell (base pressure of 5. lop8 Torr) equipped with CaF, windows. Catalyst powder samples were pressed into a gold wire mesh at 2000 lb/in2. A chromel-alumel thermocouple was spot-welded to the center of the grid. The sample was fixed onto a transposable sample holder which allowed resistive heating of the specimen. Spectra were recorded using a Mattson Cygnus 100 FTIR spectrometer multiplexed to a computer [ 171. The spectral resolution was 4 cm- I. The in-situ pretreatment and the adsorption experiments were performed under static conditions, and the following materials were used. Oxygen (99.998%, Matheson) and propene (99.6%, Matheson) were used without further purification. NO (99.0%, Matheson) was purified by multiple freezing distillation between two glass flasks, equipped with cold fingers, using liquid nitrogen as the cooling agent

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[ 181. NO2was prepared using purified NO and adding excess oxygen. Equilibration at room temperature for 0.5 h was sufficient for complete conversion of NO to NO*, as evidenced by the lack of N203 formation (dark blue color for the liquid) upon cooling below - 20°C [ 191. Excess oxygen was boiled off by trap-to-trap distillation using liquid nitrogen as the coolant. In-situ catalyst pretreatment was as follows. The pre-calcined sample was heated (ca. 75”C/min) to 400°C in vacuum and evacuated at 400°C to about 1 ~Torr followed by treated in 100 Tot-r of oxygen for 25 min at temperature. The sample was then cooled to the adsorption temperature in 100 Torr oxygen and evacuated at temperature for 1 h. At this point, a background interferogram of the evacuated, calcined sample was taken at the adsorption temperature. Note that the same pretreatment of the catalyst sample was always repeated before experimentation at subsequent adsorption temperatures. The reported spectra for separate NO and hydrocarbon adsorption are difference spectra measured at the specified adsorption temperature using the corresponding interferogram of the evacuated, calcined sample as a background reference. For NO adsorption, the sample interferogram was obtained after exposing the sample to 15 Torr of stagnant NO for 15 min. For hydrocarbon adsorption, the sample was first exposed to 3 Torr of static propene for 10 min at the specified adsorption temperature. The hydrocarbon was then evacuated for 30 min at temperature to about 1 ~Torr before the sample interferogram was obtained. This interferogram was then used as a background for subsequent experiments consisting of individually exposing the propene/catalyst system to 15 Torr NO, 3 Torr oxygen, or 3 Torr NO*, or to a mixture of 15 Torr NO and 3 Torr O2 by adding O2 to the NOexposed, propene-adsorbed catalyst. The sample interferograms were obtained after 15 min at each new pressure. Contributions from IR-active gas-phase compounds were not computer subtracted when spectra were recorded in the presence of gas phase species. The co-adsorption experiments were limited to temperatures below 225°C since at higher temperatures ( > 300°C) the batch reaction tended toward completion which only reduced the opportunity to observe interactions between adsorbed reactant and intermediate species.

3. Results 3.1. Nitric oxide adsorption on CuHZSMS NO adsorption on CuHZSM5 was performed from room temperature to 300°C (Fig. 1). The band assignments are summarized in Table 1. Bands at 1823 and 1734 cm-‘, attributed to a dinitrosyl species at 25”C, were not observed upon adsorption at 150°C. The band at 18 11 cm - i, assigned to the mononitrosyl species, was less intense for adsorption at 150°C compared to 25°C. Starting with the most

T.E. Hoost et al. /Applied Cafalysis B: Environmental 7 (1995) 79-93

83

I 15 TorrNO -1

2200

2000

1800

1600

Wavenumbers Fig. 1. NO adsorption Table 1 Tentative band assignments

1400

1200

(cm-‘)

on CuHZSMS at varying temperatures.

for surface species formed during adsorption

See Section

2 for details

of nitrogen oxides on CuHZSMS

Band (cm-‘)

Assignment

References

2224 2133 1904 1898, 1975, 1854 1823 1811 1744 1734 1622-1611 1576

adsorbed N,O adsorbed NO; NO+ on CL?+ gaseous NO (NO); on Cu+, symm. NO- on Cut adsorbed N,04 (NO); on Cu+, asymm. NO;

[I51 [51 [21 ~271 121 [21 1281 121 (281 [281

NO,

facile species, the order of peak intensity attenuation resulting from adsorption at increased temperatures was (NO);, NO-, NO+, NO;, and NO,. 3.2.

Propene pre-adsorption

Fig. 2 shows spectra for propene adsorption on CuHZSMS at different temperatures. Band assignments are shown in Table 2. The region above 3200 cm- ’has

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T.E. Hoost et al. /Applied Catalysis B: Environmental 7 (1995) 79-93

+

00 00 00 9.F zk

00 20

Wavenumbers

+

00

a

00

(cm-‘)

Fig. 2. Propene on CuHZSM5 after sample evacuation.

See Section

2 for details.

been omitted because after the initial adsorption of 3 Torr propene, the highfrequency OH bands were extinguished. The spectra show the typical envelopes due to adsorbed propene in the 3000-2800 and 1600-l 300 cm- ’ranges [ 201. For adsorption at 150 and 225°C these features began to attenuate sharply because less propene adsorbed at the higher temperatures. A comparison to propene adsorption on HZSMS and CuO/A1,03 (Fig. 3) indicates that propene adsorbed primarily on the zeolite. Some negative peaks (e.g., 2023, 1904 cm-‘) appear in some of the spectra (Fig. 2). They are most likely due to shifts in the zeolite T-O-T peak overtones at these frequencies; these slight shifts create rolling peaks in ratioed data. They can become more pronounced with increased zeolite dealumination due to prolonged heat treatments, and can be minimized by minimizing the time between any background and data collection in the ratioed spectra of the zeolite. The negative 1625 cm- ’ zeolite OH peak also indicates that the samples were not at complete equilibrium. Some water formation was noted ( vlr 3450-3400 cm-‘) upon propene pre-adsorption on the zeolite-based samples, even though the deformation mode (about 1630 cm-‘) is not evident in Fig. 2 and Fig. 3. However, the water band did not change during the subsequent co-adsorption experiments, except at the adsorption temperature of 225°C where a slight attenuation was noted due to gradual water desorption at high temperature.

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Table 2 Approximate frequencies and tentative functional group assignment for bands observed upon adsorption of propene on CuHZSM5 Bands (cm-‘)

Possible assignments

References

3080 2984 2963-2956 2934 2922-2890 2865-2837 2158 1682 1630 1625-1618 1613-1545 1610 1530-1520 1510-1507 1470-1467 1445-1420 1440-1420 1388-1380 1385-1346

v,,( =CH,),

[201 [201 [201 [201 L201 [201 [211 [2W91 [20,301 [201 [20,301 [201 [20,301 [201 [201 L20291 [20,301 [20,301 [20,29,30]

3.3.

Co-adsorption

removed after evacuation

v,(=CH,) v.s(-CH,) v,,(-CH,-) v,(-CH,) v,(-CHj) v,(CO) v,,(C=O) S(H,O) v,(C=C) v.,( carboxylate

of acetate ion)

v,,(C=O) v ( monodentate carbonate ion ) v,,( carboxylate of formate ion) “(carboxylate ion) L-CH,) v,(carboxylate v,(carboxylate

in acetate ion) in formate ion)

&(-CH,)

of propene

and nitrogen oxides

CuZSM5 After adsorption and evacuation of propene on CuHZSMS (Fig. 2), NO was subsequently co-adsorbed on the catalyst to study its interaction with pre-adsorbed propene. As mentioned in Section 2, the reported spectra are difference spectra with respect to propene adsorption on CuHZSMS (Fig. 2) ; that is, the propeneadsorbed and evacuated state of each catalyst sample was used as the background to create each corresponding spectrum in Fig. 4. Fig. 4 shows that NO adsorption drastically decreased due to pre-adsorbed propene on CuHZSMS compared to NO on hydrocarbon-free CuHZSMS (cf. Fig. 1) .At 25°C (Fig. 4Al) ,NO attenuated the hydrocarbon envelope in the 3000-2800 cm- ’range. At the higher temperature of 150°C (Fig. 4Bl), both the 1902 cm-’ band and the 3000-2800 cm- ’envelope were less intense. Thus, it appears that at temperatures up to 150°C NO partly displaced adsorbed hydrocarbon. At even higher temperature (225°C Fig. 4Cl) , NO reacted with adsorbed propene, as indicated by the apparent formation of isocyanate species (2295 cm- ‘, 2204 cm- ’) [ 6-8, lo]. To simulate lean-NO, conditions, we subsequently added oxygen to the system of propene and NO on CuHZSMS. As before, the resulting spectra (Fig. 4A2, B2, C2) are difference spectra based on propene pre-adsorption on CuHZSM5 (cf. Fig. 2). At 25°C (Fig. 4A2), NO2 readily formed and resulted in the weakly adsorbed N204 dimer ( 1744 cm- ’) , adsorbed NO, ( 1622 cm- ’>, and NOj ( 1262 cm- ’) . In addition, bands at 19 12 and 1844 cm- ’ indicate increased NO adsorp-

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Propene

on:

A. CuO/A120,

at 25’C B. CuO/A120, a$225’C C. HZSMS at 25 C

0000 00 ?? ?le ??

e00

Wavenumbers

Fig. 3. Propane on CuO/A1,03

00

\6

a00

(cm-‘)

and HZSMS after sample evacuation.

See Section

2 for details.

tion. A possible assignment for the band at 2204 cm-’ might be the isocyanate. Earlier workers [ 6-8,101 have reported bands in this region and assigned them to this species. The low feature at 2 150 cm - ’most likely denotes CO formation [ 211. Methyl fragments were either displaced by or reacted with NOz, as shown by the attenuation of the 3000-2800 cm- ’ envelope [ 201. The band at 169 1 cm- ’ may signify the formation of organic carbonyls [ 201. Finally, a small band appeared at 2597 cm-‘. This band was previously assigned to (N,O,),/Cu in Cu-ZSM-5 by Belletal. [13] For these artificial lean-NO, conditions, increasing the temperature to 150°C (Fig. 4B2) decreased the adsorption of N,O,, NO;, and NO;, as shown by the attenuation of their respective bands. At 225°C (Fig. 4C2), adsorption of N20, was even less. Bands due to adsorbed isocyanate (2295 cm-‘) and CO (2128 cm-’ ) were manifest, indicating interaction between nitrogen oxides with adsorbed hydrocarbon. The lower hydrocarbon intensities suggest that the extent of reaction was greater than for co-adsorbed NO and propene on CuHZSM5 at 225°C (see Fig. 4C 1) It should also be noted that even at 225°C there was a relatively large pool of adsorbed NO; and NO, in the catalyst, indicating that NOZ could successfully compete with oxygen for adsorption sites and for subsequent interaction with the abundance of adsorbed hydrocarbons on the zeolite. Evidently, CuO/

.

T.E. Hoost et al. /Applied Catalysis B: Environmental 7 (1995) 79-93

I

87

I

Propene/CuHZSMS: Al. +NO at 25C Bl. +NO at 150C Cl. +NO at 225C A2. +NO+Oz, 25C 82. +NO+O,, 150C C2. +NO+O,. 225C

.

Cl.

-Bl =A1

-

-+I

5% 00 9c!?e00

20 00

Wavenumbers

\t00

3 00

(cm-‘)

Fig. 4. Addition of 15 Torr NO followed by 3 Torr of oxygen to propene pre-adsorbed temperatures. See Section 2 for details.

on CuHZSM5

at varying

A1203 also produced copious amounts of NO;! (Fig. 6). However, the pool of adsorbed hydrocarbons with which this NO2 could react was much smaller than on the zeolite. We also investigated the separate interactions of oxygen or NO, with propene pre-adsorbed on CuHZSMS (Fig. 5). Fig. 5A shows the difference spectrum of 3 Torr of oxygen on propene/CuHZSMS at 25°C (cf. Fig. 2). Oxygen did not appreciably interact with adsorbed propene at this temperature. At 225°C (Fig. 5B), the intensity of the 3000-2800 cm-r range decreased. A relatively strong band appeared at 2156 cm-‘, attributable to adsorbed CO. Thus, at higher temperatures, oxygen reacts with adsorbed hydrocarbon to form CO. Bands at 1670 and 1574 cm-’ signify asymmetric stretching frequencies due to adsorbed organic carbonyls and carboxylates, which are likely intermediates in the reaction. A comparison with Fig. 4Cl shows that at 225°C the intensity due to the hydrocarbon envelope between 3000 and 2800 cm-’ decreased less upon exposure of propene/CuHZSMS to oxygen versus NO. However, oxygen more readily reacted to form partially oxidized surface species. Compared to oxygen and NO, NO2 was a more effective oxidizer of propene pre-adsorbed on CuHZSM5, even at 25°C (Fig. 5C). Upon NO2 addition at 25°C

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T.E. Hoost et al. /Applied Catalysis B: Environmental 7 (1995) 79-93

3% 00 9? 00 zk00

20 00

Wavenumbers

,tJ00

\I 00

(cm-‘)

Fig. 5. Separate additions of 3 Torr of oxygen and 3 Torr of NO, to propene pre-adsorbed Section 2 for details.

on CuHZSM5.

See

strong bands due to adsorbed NO* and NO, species emerged ( 1741, 1612, 1570, 1261 cm- ‘). Bands due to methyl groups (3000-2800 cm- i) attenuated sharply. Bands due to oxygenated species appeared ( 1669, ( 1570)) 1357 cm- ’) but no CO formation was evident. It should be noted that upon NO2 addition the band at 2590 [ 131 also appeared, even at 25°C. At cm-’ tentatively attributed to (N,O,,),/Cu higher temperature (225°C Fig. 5D), less NO2 adsorbed but bands due to methyl groups (3000-2800 cm-‘) attenuated more sharply. Oxygenated species still adsorbed ( 1693, 1570 cm-‘) but now, detectable amounts of isocyanate and CO (2276 and 2127 cm-‘, respectively) were also formed. Finally, the band at 2590 cm-’ was very small upon NO2 addition at 225°C. Compared to the combined addition of NO and oxygen (Fig. 4)) NO2 addition had a more notable effect upon the attenuation of methyl bands (3000-2800 cm-‘) and upon the formation of oxygenated species, both at 25 and 225°C. Since not all NO reacted after the combined addition of NO and oxygen, these differences may be related to differences in NOz partial pressures. We also separately considered the combined adsorption of propene, NO and oxygen on supported CuO. Fig. 6Al shows no appreciable interaction of NO with propene pre-adsorbed on CuO/A1,03. The bands at 1920 and 1899 cm-’ are

T.E. Hoost et al. /Applied

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7 (1995) 79-93

89

+NO at 225C

??00 fP00 e00

20 00

Wavenumbers

\! 00

a00

(cm-‘)

Fig. 6. Addition of 15 Tom of NO followed by 3 Torr of oxygen to propene pre-adsorbed Section 2 for details.

on CuO/AI,O,.

See

attributable to adsorbed NO. Bands at 1574 and 1539 cm-’ may be due to NO; formation on the alumina support. At 225°C (Fig. 6B 1), bands due to methyl groups (3000-2800 cm- ’) attenuated far less than was observed on CuHZSM5. On CuO/ Al,O,, the formation of surface oxygenates could be observed ( 1599 cm- ’) . Isocyanate and CO production were evident as well. Simulating lean-NO, conditions by subsequent addition of oxygen at 25°C (Fig. 6A2) resulted in substantial formation of adsorbed NO* and NO, species (1751, 1621, 1537, 1312, and 1261 cm-‘). The band at 1893 cm-’ signifies NO adsorption on CuO. Oxygen addition also sharply attenuated the methyl bands (3000-2800 cm- ’) . Essentially all the observed methyl groups have reacted. A notable amount of isocyanate resulted. At 225°C (Fig. 6B2), adsorption of NO, N02, and NO; species was less than was observed at 25°C. Again, the envelope due to adsorbed methyl groups was entirely removed. Some product formation may have been obscured by the formation of nitrogen oxides (below 1750 cm-‘).

HZSM.5 Fig. 7 relates the adsorption of nitrogen oxides on propene/HZSMS at 25°C. NO adsorption on propene/HZSMS (Fig. 7Al) only marginally affected adsorbed methyl groups (3000-2800 cm- ’) . No adsorption of nitrogen oxides was observed in the absence of copper.

90

T.E. Hoost et al. /Applied Catalysis B: Environmental 7 (1995) 79-93

Qoo 9? 00 zk00

20 00

Wavenumbers

+00

\I 00

(cm-‘)

Fig. 7. (A) Addition of 15 Torr of NO followed by 3 Torr of oxygen and (B) separate addition of 3 Torr NO2 to propene pre-adsorbed on HZSMS. See Section 2 for details.

Subsequent addition of oxygen caused strong adsorption of NOz and NO; species. However, not all available NO reacted ( 1875 cm- ’) . Peak intensities due to methyl groups (3000-2800 cm- ‘) attenuated sharply. At the same time, some bands attributable to carboxylate stretching or methyl deformation appeared ( 1420, 1357 cm- ‘). The broad halo at 2148 cm-’ may indicate adsorbed CO and isocyanate species, especially since the nitronium ion (2133 cm- ‘) is not easily formed in the presence of reductants. The spectrum for NO* addition to preadsorbed propene on HZSMS (Fig. 7B) resembles that of co-adsorbed oxygen, NO, and propene (Fig. 7A2) but the peak intensities were lower and an organic carbonyl stretch at 1662 cm- ’ became more pronounced.

4. Discussion 4. I. Interactions between propene and nitric oxide NO adsorption on CuHZSMS was considerably less in the presence of propene (Fig. 4 vs. Fig. 1) . Hydrocarbon adsorption on Cu-ZSM-5 is strong and results in

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91

almost full coverage of the adsorption sites of the zeolite [ 22-241. Parrillo et al. [ 231 have shown that propene interacts strongly with Cu at the Al sites, in a nearly one-to-one stoichiometry at room temperature. Their infrared work with pyridine furthermore suggests that the Cu sites are neither Bronsted nor Lewis acid sites. Thus strongly pre-adsorbed propene impeded the formation of adsorbed NO:, NO+, NO-, and (NO); (Fig. 4). Only a modest amount of NO ( 1902, 1844 cm- ’) adsorbed. Only a small quantity of hydrocarbons reacted or was displaced upon NO addition. NO adsorption on propene/HZSMS was even less, evidently because there was no copper on which any NO could be accommodated. On Cu/A1203 (Fig. 6), hydrocarbon adsorption was weaker, resulting in slightly more nitrosyl adsorption than observed on the zeolitic materials. It should be noted that hydrocarbon reaction or displacement as measured by the intensity of the methyl envelope (3000-2800 cm-i) was small, indicating that during SCR-HC the direct reaction between NO and hydrocarbon may be relatively small. This is in agreement with reaction studies that have shown lower NO conversion for the NO-propene reaction than for the O,-NO-propene reaction over Cu-ZSM-5 [ 1,25,26].

4.2. The role of nitrogen dioxide Addition of oxygen to NO-propene/CuHZSMS resulted in considerable adsorption of N,04 (1744 cm-‘), NO, (1622 cm-‘), and NO; (1570 cm-‘) due to NO2 formation (Fig. 4). Thus we can compare the reactivity of NO* as an oxidant to that of NO and oxygen. NO1 interaction with propene was greater than was observed for NO, as evidenced by the sharp attenuation of the methyl bands ( 30002800 cm- ’) upon oxygen addition (Fig. 4A2-C2 cf. 4A 1-C 1) . NO2 interaction with propene was also greater than was observed for oxygen and propene coadsorption at both 25 and 225°C (Fig. 5A, B) . Both NO,-propene and O,--propene interactions resulted in the formation of oxygenated species ( 1667-1691, 1574 cm- ’) . However, whereas the NO* oxidant resulted in some evidence for isocyanate formation at 25 and 225’C, oxygen resulted in a significant amount of CO at 225°C. It should be noted that the O,-NO-propene system contained 1.5 Torr of oxygen in excess of what would be required for complete oxidation of NO to NO;?. Thus, oxygen in the absence of NO:! formed more completely oxidized carbonbased products, including CO. However, in the presence of both NO2 and oxygen, the hydrocarbon reacted preferentially with NO*. This reaction resulted in isocyanates, possibly (N,O,)JCu, and oxygenated surface species (Fig. 5C, D) . For 02-NO-propene on CuHZSM5 (Fig. 4C2), CO formation was not as great as would be expected from O,--propene co-adsorption. Therefore, at 225°C the NO,-propene reaction proceeded faster than the O,-propene reaction. The greater ease of the NO,-propene reaction was also evident at 25°C. Thus, Fig. 4A2 shows an appreciable decrease in adsorbed hydrocarbon and the formation of isocyanate

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and oxygenated species; however, for O,-propene, Fig. 5A does not show notable spectral changes. To examine which sites may be important during SCR-HC, we compared 02NO-propene co-adsorption on CuHZSMS, HZSMS, and Cu/A1203. First, Cu/ A&O3 was by far the best producer of adsorbed NO, ( 1612 cm- ’) . This was because the alumina was relatively hydrocarbon-free, whereas the zeolite would be mostly covered with hydrocarbon. Thus NO* availability is not the sole criterium for profuse interaction. A comparison of CuHZSMS and HZSMS shows that HZSMS formed less carboxylated surface species and did not manifest the band at 2590 cm- ‘. In fact, the band at 2590 cm- ’ required the simultaneous presence of copper, ZSM-5 support, hydrocarbon, and NO*, and it was manifest both at high and low temperatures. No bands due to organic nitro compounds [6-81 were apparent for the catalysts of our study. It is possible that they may have been obscured by other bands due to adsorbed hydrocarbons or nitrogen oxides. In summary, this work addressed some of the interactions between nitrogen oxides and hydrocarbons on prototypical Cu-ZSM-5 catalysts for lean-burn engines. Co-adsorption of propene, NO, and oxygen resulted in substantial NOz formation. This NO2 interacted more strongly with adsorbed hydrocarbons than did NO or oxygen. This may suggest that on Cu-ZSM-5, the NO2 oxidant effectively competes with oxygen for the surface, resulting in improved NO, reduction by adsorbed hydrocarbons. Consequently, the role of NO2 appears to be crucial.

5. Concluding

remarks

Co-adsorption of NO with propene on CuHZSM5 produced only marginal spectral differences, indicating minimal interactions between the adsorbates. Oxygen addition to NO and propene on CuHZSM5 resulted in appreciable NO oxidation. Upon oxygen addition, propene adsorption decreased considerably, apparently due to a strong affinity of NO2 for the hydrocarbon. The spectra also suggest some carbonylation and carboxylation of adsorbed hydrocarbon due to NO*. In the presence of NO2 and hydrocarbons on CuHZSMS, a band appeared at 2590 cm-‘, possibly due to (N,O,) ,/Cu [ 131. This band did not emerge during similar adsorption on HZSMS. The co-adsorption experiments suggest the following order for interaction between the oxidants and propene on CuHZSM5: NO < O2 < NO*.

Acknowledgements We thank C.N. Montreuil and H.W. Jen for providing the catalysts. We also appreciate the helpful comments of M. Shelef, H.W. Jen and K.M. Adams on the manuscript.

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