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Infrared studies of NO adsorption and co-adsorption of NO and O2 onto cerium-exchanged mordenite (CeNaMOR)
MICROPOROUS MATERIALS ELSEVIER
Microporous Materials 4 ( 1995 ) 455 465
Infrared studies of NO adsorption and co-adsorption of NO and 02 onto cerium-exchanged mordenite (CeNaMOR) E. Ito ~, Y.J. Mergler b, B.E. Nieuwenhuys b, H. van Bekkum a.., C.M. van den Bleek a Department q]" Chemical Technology and Materials" Science, Delft UniversiO' of Technology, Julianalaan 136, 2628 BL Delft, Netherlands b Gorlaeus Laboratoria, University o f Leiden, P.O. Box 9502. 2300 RA LeMen. Netherlandv
Received 9 January 1995; accepted 21 March 1995
Abstract
Infrared (IR) studies on NO adsorption and co-adsorption of NO and 02 onto cerium- and lanthanum-exchanged mordenite (CeNaMOR and LaNaMOR, respectively) were performed in order to elucidate the role of redox properties of cerium (Cem/Cd v) in the oxidation of NO to NO2, an important preliminary step in the NO reduction catalysis. NO adsorption onto both CeNaMOR and LaNaMOR leads to the formation of N 2 0 (2247 cm-~), NO + (nitrosonium ion; 2162 cm -~) and NO~- species (nitrito or/and nitrato, 1300-1500 cm-1). These are thought to arise from disproportionation of NO towards N 2 0 and N203 and the subsequent ionization of N203 towards NO + and NO2. This scheme is supported by the transient observation of molecular N203. The co-adsorption of NO and O2 onto CeNaMOR and LaNaMOR resulted in the enhanced formation of NO + and NO3 (1515, 1488 1497 and 1333 cm ~), which is accounted for by the formation of NO2 and its subsequent ionization via N 2 0 4 towards NO + and NO3. Combining IR and NO temperature-programmed desorption (TPD) data, it is proposed that the formation of NO ~ is associated with zeolite acid sites, and that NO;- (nitrato) species are coordinated to lanthanide cations. Furthermore, the NO ' and NO~- species were found to desorb more easily from CeNaMOR than from LaNaMOR. The redox properties of cerium (Cem/Ce Iv) may contribute to the easier desorption of these oxidized NO species. Kevwordv. Lanthanide-exchanged zeolite; NO + formation; Nitrosation: NO oxidation; (CeH~/CeJV)redox couple
1. Introduction
The catalytic removal of NOx from exhaust gases is currently one of the most investigated subjects a m o n g environmental catalysis studies [ 1]. In NOx reduction catalysis, transition metals, in particular d-block transition metals such as noble metals (Pt, Rh, Pd, etc.) [2], metal oxides (V20 s, CuO, Cr203, MnOx, etc.) [3] or these metals or metal ions accommodated in zeolites [4] are often applied. Actually, the interaction of d-block trans* Corresponding author. 0927-6513/95/$9.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0927-6513(95)00026-7
ition metals with NO often involves a d-electron and/or an empty d-orbital, which leads to the formation of metal nitrosyls [5]. Nitrosyl coordination can be found as N O ~+, NO (neutral) or N O 6-, with IR absorptions in the region 1900-1600cm -1 [6]. Electron transfer from the metal d-orbital to the rc*-orbital (anti-bonding) of N O weakens the N O bond (i.e. formation of NO ~ ) [5,7], and this species is supposed to be susceptible to decomposition [7 9]. On the other hand, g*-electron transfer from N O to a metal strengthens the N O bond (i.e. formation of N O 6+). This species will decompose less easily, but
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E. Ito et aL/Microporous Materials 4 (1995) 455-465
it is probably more reactive in the presence of a reductant, since NO 6+ is a more "electrophilic" reagent than NO itself towards a "nucleophilic" reductant. The high potential of d-transition metals for NO coordination often leads to effective NO decomposition or NO reduction. An intriguing aspect of the NOx reduction catalysis is that non d-block redox metals (e.g. Ga- [10] and Ce-zeolites [11,12]), non-redox metals (e.g. In- [10] and La-zeolites [12,13]) and even simple proton-type zeolites [ 14-16] can be excellent NO~ reduction catalysts. Metal oxides generally have surface acid sites, and metal-exchanged zeolites can also show Bronsted acidity in the following two ways: upon reduction of a metal ion (e.g. M "+ + r e d u c t a n t ~ M ~"-u+ + H + + oxidized products) for non d-block redox metals and by dissociation of a water ligand ( M ( H 2 0 ) " + ~ M(OH)~"-I)++H +) for non-redox multivalent cations. These acid sites are assumed to play a role in the oxidation of NO towards N O 2 (intermediate) for NOx reduction with NH 3 [14,15] or with hydrocarbons [10,11,16] under oxygen-rich conditions. Furthermore, the acid sites may interact with reductants in NOx reduction, e.g. the enhanced adsorption of NH3 and the activation of hydrocarbon reductants. We have recently reported that ceriumexchanged zeolites (mordenite and ZSM-5) are highly active and selective catalysts for NOx reduction with NH3 in the presence of oxygen [17], and the NO~ reduction catalysis of cerium-exchanged mordenite (CeNaMOR) was studied in detail in comparison with lanthanum-exchanged mordenite (LaNaMOR) and H-MOR [12]. LaNaMOR and H-MOR showed a similar high-temperature catalytic activity (400-550°C), while CeNaMOR exhibited a prominent activity in a wide temperature range (250-550°C). The specific lowtemperature activity of CeNaMOR for NO reduction was ascribed to its high NO oxidation capacity, and we proposed an NO2-intermediated reaction mechanism for CeNaMOR in the low-temperature region (< 400°C) [ 12]:
We report here IR studies of NO adsorption and co-adsorption of NO and 02 onto CeNaMOR and LaNaMOR. Cerium and lanthanum cations are both known to show Bronsted acidity by water ligand dissociation, and their coordination chemistry is very similar. Cerium differs, however, in exhibiting redox properties (Cem/CeIV), and that can possibly account for a distinguished NO oxidation capacity of CeNaMOR at low temperatures. The present IR investigation aims at contributing to understanding of the high performance of CeNaMOR in NO oxidation towards N O 2 ; a preliminary step in NOx reduction with NH3. It should be noted that NO 2 formation is reported to be important in NO reduction with hydrocarbons over Ce [11], Ga [10], In-ZSM-5 [10], H-zeolite catalysts [16], etc. as well. We will discuss our IR data while comparing with NO-TPD and NO oxidation results obtained earlier for CeNaMOR and LaNaMOR [12].
2. Experimental 2.1. Preparation of cerium- and lanthanumexchanged mordenite Mordenite (CBV-10A, SiOz/A1203=13.1, Na form) was obtained from PQ Corporation. Cerium-exchanged mordenite (CeNaMOR) was prepared by exchanging 25 g of mordenite powder in an 8.3 mM aqueous solution (1.5 1) of cerium acetate (Alfa Products, 99.9%) at 100°C for 5-7 h. For lanthanum-exchanged mordenite (LaNaMOR), 5 g of mordenite were exchanged in 1.51 of a 2 m M lanthanum trichloride (LaC13.xH20) solution at 70°C for 15-20h. Samples were then filtered, washed with deionized water and dried at 120°C. Analyses of samples by inductively coupled plasma atomic emission spectroscopy (ICP-AES) showed 58% and 57% of Na + to be exchanged stoichiometrically with Ce3+ or La 3+, respectively.
2.2. Infrared measurements NO + ~02 ---*N O 2
( 1)
NO + NO 2 + 2NH3 ---2N2 + 3H20
(2)
IR measurements were performed with a Mattson Galaxy 2020 spectrometer (DTGS detec-
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E. Ito et aL/Microporous Materials 4 (1995) 455 465
tor) under static conditions using a vacuum cell equipped with CaF2 windows. The spectra were recorded with a resolution of 2 c m - ~ for 64 scans for each spectrum. Catalyst sample powder was pressed to a thin self-supporting disk (1 ton, 1 s) and was pre-treated at 330-350°C in high vacuum (10 5-.10 6Torr) for 2.5h. The background spectra of C e N a M O R or L a N a M O R were then taken at different temperatures (100, 200, 300 and 350°C) before admission of any reactant gas, and used for subtraction from the spectra taken in the presence of reactant gases. All spectra presented here are background-subtracted. The gases employed for adsorption were NO (99.8%, Messer Griesheim), 02 (99.998%, Messer Griesheim) and NH3 (99.98%, Hoek Loos).
3.2. NO adsorption onto CeNa-MOR and LaNa-MOR
3. Results
Table 1 List of observed absorption bands in Figs. 2 5 and their assignments
3.1. Spectra of cerium- or lanthanum-exchanged mordenite before adsorption
Species
CeNa-MOR frequency (cm- 1)
LaNa-MOR frequency (cm ~)
N20 NO ~ NO 3 (nitrato)
2247 2162 1333, 1603 1418 1625, 1304,
2247 2162 1333. 1497, 1515, 1603 143(I 1625, 3500 1305, 1520, 1912 1740
The spectra of C e N a M O R and L a N a M O R taken before gas adsorption showed a broad absorption around 3500 cm -1, a sharp peak at 3610cm -1, and small peaks at 3650, 3686 and 3740 cm 1 (Fig. 1). The absorption at 3610 cm -1 is assigned to Bronsted acid sites in mordenite [18], and other small peaks at 3650, 3686 and 3740 cm 1 are attributed to Ce-OH/La-OH,
A1-OH and SiOH, respectively [18,19]. The broad band around 3500 cm 1 may indicate the presence of water, with its bending-mode absorption at 1635cm 1. In addition, the hydrogen-bonded hydroxyl species Si-OH/Ce-OH/La-OH may also contribute to the band around 3500 cm-1.
Cerium-exchanged mordenite (CeNaMOR) The observed absorption bands originated from NO are listed in Table 1. Upon NO admission (1 Tort) at room temperature, a sharp peak appeared at 2247 cm -1, and another peak at 2162 cm -1 (Fig. 2a). The peak at 2247 cm -1 can
ONO (nitrito) H20/HNO 2 N203 N204
2170 1488, 1515,
3500 1521, 1912
2.0 4 0.4-
1.5 " ~ . .
~(a)
~
/
•.~ '<
O
0.2-
(c)
""
1.0 (b)
"'
O-
0.5
4000
(a) 3500
3000
2500
2000
1500
Wavenumber / cm- l 4000
3500
3000
2500
2000
1500
Waveuumber / cm-1 Fig. 1. Spectra of (a) CeNaMOR and (b) LaNaMOR before gas adsorption.
Fig. 2. NO adsorption onto CeNaMOR. (a) NO l Torr at room temperature, (b) NO pressure increased up to 5 Tort, (c) NO 5 Torr after 10 min and (d) temperature increased up to 100"C with NO 5 Torr.
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E. Ito et al./Microporous Materials 4 (1995) 455 465
be assigned to N20 [20,21], and the absorption band at 2162 cm -a is assigned to NO + [20,22,23], as will be discussed later. By increasing the NO pressure to 5 Torr, the absorptions of N20 and NO + increased in size, while new peaks grew at 1304, 1521, 1625, 1912cm -1 with a broad one around 3500 cm 1 and a negative peak at 3610cm ~ (Fig. 2b). The peaks observed at 1304, 1521 and 1912cm 1 can be assigned to NzO3 [21,24], and the absorptions at 1625 and 3500 cm 1 probably originate from water [25] or from nitrous acid [26]; this will be discussed later in more detail. After 10 min at 5 Torr of NO, the absorptions of N20 and NO + and of H20 or HNO2 further increased, whereas the absorptions ascribed to N203 disappeared. In addition, a new broad band at 1400-1550cm -~ was observed, which can be assigned to absorptions by nitrate or nitrite ions in general [20,27], which will hereafter be referred to as " N O x " species (Fig. 2c). By raising the temperature to 100°C, the N20 absorption mostly disappeared, while the NO + band remained. Furthermore, the appearance of maxima around 1515, 1488 and 1320cm -1 were noticed in the NO2 absorption region (Fig. 2d). These can be assigned to nitrato (coordinated NO~- ion) species formed on lanthanum or cerium ions [28-30]. Meanwhile, the absorptions by water or nitrous acid became larger.
Lanthanum-exchanged mordenite (LaNaMOR) NO adsorption onto L a N a M O R provided spectra similar to those observed with CeNaMOR (Fig. 3, Table 1). This indicates that the formation of NzO, NO + and NO~ species does not require the intervention of the cerium redox couple (CenI/Cew). Note that the formation of water or nitrous acid was also observed, while the negative peak at 3610 cm -1 was much weaker than that of CeNaMOR.
0.8
0.6 0.4 <
0.2
(a) 4000
3500
3000 2500 2000 Wavenumber / cm- 1
1500
Fig. 3. NO adsorption onto LaNaMOR. (a) NO 1 Torr at room temperature, (b) NO pressure increased to 5 Torr, (c) NO 5 Torr after 10 min and (d) temperature increased up to 100°C with NO 5 Torr.
an absorption at 1418 cm-1, indicative for nitrito (oxygen-coordinated nitrite ion: ONO ) species [20], appeared immediately (Fig. 4a, Table 1). The presence of water or nitrous acid was also indicated (1625 and 3500 cm-1), and a negative peak was observed at 3610cm -1. Compared with the absorption of NO only (Fig. 2d), it is evident that the presence of oxygen greatly enhanced the formation of nitrato (NO3) and nitrito (ONO-) species and NO + (nitrosonium ion). Raising the temperature to 200°C, the NO + peak decreased, while the nitrato absorptions remained almost unchanged (Fig. 4b). Raising the temperature fur-
1.2
(c) 0.8
e~ 0.4
3.3. Co-adsorption of NO and 02 Cerium-exchanged mordenite (CeNaMOR) NO (5 Torr) and Oz (5 Torr) were consecutively admitted to CeNaMOR at 100°C. The absorption bands due to NO + (2162 cm-1), NO3 (1488 cm -1 with shoulders at 1603, 1515 and 1333 cm-1), and
4000
3500
3000 2500 2000 Wavenumber / cm-I
1500
Fig. 4. Co-adsorption of NO and 02 onto CeNaMOR. (a) NO (5Torr) and O z (5Torr) at 100°C, (b) 200°C, (c) 300°C and (d) 350°C.
E. lto et aL/Microporous Materials 4 (1995) 455 465
ther to 300'C, the NO + band and the NO;- bands substantially diminished, which was accompanied by the appearance of nitrito absorption at 1418cm ~ (Fig. 4c). At 3500C, the nitrato and nitrito peaks decreased further (Fig. 4d). Meanwhile, the absorptions assigned to water or nitrous acid became smaller at 300 and 350'C (Figs. 4c and d). Note that a negative peak observed at 3610cm i remained visible during this measurement, becoming smaller at higher temperatures. The appearance of a small broad band around 3200cm i might indicate that the hydrogen bond between zeolite O H groups and the adsorbed species originated from NO. Lanthanum-exchanged mordenite ( L a N a M O R ) The co-adsorption of NO and 02 onto L a N a M O R and C e N a M O R resulted in the formation of rather similar adsorbed species: NO + (2162cm 1), NO~ (1473, 1510, 1330 (broad) cm 1), and water or nitrous acid (Fig. 5, Table 1 ). Nevertheless, differences were found between C e N a M O R and L a N a M O R in the temperature of the formation/disappearance of these species. Comparing the spectrum of L a N a M O R (Fig. 5a) with that of CeNaMOR (Fig. 4a) upon NO/O2 admission at 100°C, the nitrato ( N O 3 ) bands were considerably smaller for L a N a M O R and, in addition, nitrito was not observed. Raising the temperature to 200:C, the nitrato absorptions were greatly enhanced, while the NO + band and the
H 2 0 / H N O 2 bands remained almost unchanged (Fig. 5b). Note that the NO + species started to disappear at 200°C on CeNaMOR (Fig. 4b). A weak broad band observed at 1740 cm- 1 might be assigned to N204 [31,32], whereas the origin of another weak band at 1850 cm- ~ remains unclear. A small broad peak was present around 3200 cm 1 as in the case of CeNaMOR, and it may indicate the presence of hydrogen-bonded species on zeolite. Increasing the temperature to 300°C, the NO + species decreased substantially, and the nitrato peaks became smaller, which was accompanied by the appearance of a shoulder attributed to nitrito species (1430cm t) (Fig. 5c). At 350'-C, the nitrato peaks further decreased, and the presence of nitrito species became more evident (Fig. 5d). In comparison with the spectrum of C e N a M O R (Fig. 4c), the effect of a temperature raise to 300 35ff~C on the N O , species was apparently smaller for LaNaMOR.
4. Discussion 4.1. Cerium- and lanthanum-exchanged mordenite
The spectra of CeNaMOR and L a N a M O R before gas absorptions were rather similar. Both spectra showed the peak at 3610 cm l, which is in good agreement with a reported IR absorption frequency of Bronsted acid sites in mordenite [18]. Since C e N a M O R and L a N a M O R were prepared by a stoichiometric exchange of Ln 3 + (lanthanide cation) for Na +, the presence of the acid sites originates from the "lanthanide acidity" due to water ligand dissociation: Ln(H20) 3*
04 __y+--- . . . . . . . . . . o ~ 40t)0
3500
3000
2500
2000
1500
Wavenumber [ cm- 1 Fig. 5. C o - a d s o r p t i o n o f N O a n d 0 2 o n t o L a N a M O R . (a) N O ( 5 T o r r j and O2 ( 5 T o r r ) at 100+'C, ( b ) 200°C, (c) 300~C and (d) 350+C.
459
--~ Ln(OH
)2+ + H +
(3)
Only a tiny peak was observed for L n ( O H ) species (3650cm 1), which should be another consequent product according to Eq. 3. Such an apparent inconsistency was already reported for La-Y [19], in which case it was attributed to the formation of oxo-bridged species: LnxO~.(OH) = [19,33]. The formation of oxo-bridged species was suggested for Ce-Y as well [34]; thus, lanthanum and cerium ions in mordenite may have also formed small cationic oxo species during the
460
E. Ito et al./Microporous Materials 4 (1995) 455-465
pretreatment. An alternative explanation for a relatively low intensity of Ln(OH-) band is the occurrence of hydrogen bonding, which would yield a shifted and broadened peak around 3500 cm- 1. Further investigations are necessary. 4.2. N O adsorption onto C e N a M O R and LaNaMOR
The admission of NO to CeNaMOR and LaNaMOR at room temperature leads in both cases to the appearance of N20, NO + and NO~species ( x = 2 or/and 3). The presence of N20 and NO~- suggests a disproportionation of NO and the formation of adsorbed NO2 species, as shown below in Eqs. 4-6. 2NO-,N20 + O(ad)
(4)
NO + O(ad) ~NOz(ad)
(5)
3NO-*N20 + NO2
(6)
Disproportionation of NO was reported over a variety of metal ions in zeolites, e.g. CaY [21], Ba-Y [35], Cu-ZSM-5 [36], Au-Y [37] and Ni-Y [38] at room temperature, and Na-Y at 77 K [21]. The observed activity order for this reaction was C a Y > N a Y > H Y [21], which may be accounted for by the enhanced polarization in the zeolite due to the presence of metal cations with a high valence state [18]. In addition, metal ions could also offer adsorption sites for NO, for instance in the form of an oxygen-coordinated hyponitrite ion (N202) 2-, which is subject to decomposition into N20 over CeO2 [39]. In addition to NO disproportionation, we suggest the following consecutive reaction to take place on CeNaMOR and LaNaMOR: NO + NO2~-N203--~NO + + N O 2
(7)
The formation of N20 3 during NO disproportionation was observed for BaY [35], NaY [21], CaY [21], AuY [37] and NiY [38]. This can be explained by an equilibrium of NO, NO2 and N203 [35], which is also supported by a recent report on the formation of N20 a upon the co-adsorption of NO and NO2 onto Na-zeolites [40]. In our spectra, three peaks indicative to N203 were observed (Figs. lb, 2b and Table 1), and
their fast disappearance with time (Fig. 2c) suggests the subsequent reaction. The observation of the absorptions at 2162 cm-1 and at 1300-1500cm -~ (NO2) should be noted as the subsequent ionization reaction. We assigned the absorption at 2162 cm-1 to NO + (nitrosonium ion), and we would like to discuss this essential assignment in the following section. 4.3. Assignment o f the peak at 2162 cm -~
Historically, there have been two interpretations for the absorption at 2120-2170cm -1 often found upon the adsorption of NO: one is NO + (the nitrosonium ion) and the other NO+ (the nitronium ion). NO + is reported to absorb in the 2100-2300cm -1 region, whereas NO~- is found in the 2200-2400cm -1 region [20,27]. Therefore, simply from the observed frequency (2120-2170 cm 1), the assignment to NO + seems more probable. Terenin and co-workers [22,23] first assigned the band to NO+ in 1959. However, in 1971, Chao and Lunsford [21] proposed to assign the band to NO+. They observed this peak upon NO 2 adsorption, and this was supposed to be a strong support for the assignment to NO+. Later, in 1989, Odenbrand et al. [15a] observed an absorption at 2160 cm -~ upon NO adsorption on H-MOR, and ascribed it to NO +, which assignment was in accordance with the proposed reaction pathway of NO reduction with NH3 over H-MOR. Nevertheless, the appearance of the peak upon NO 2 adsorption is nowadays often referred to, and it still leads many groups to assign the band to NO + . In such a complex case, the most reliable data can be obtained from isotope substitution experiments. In 1992, lwamoto et al. [8] have reported an IR isotope study for NO adsorption with 15NO and I#NO over Cu-ZSM-5. Upon I#NO adsorption, a small peak was observed at 2125cm -I besides strong absorptions of nitrosyls on the dtransition metal ion copper. Using ~SNO instead of 14NO, the small peak at 2125 cm- ~was observed to shift towards 2087 cm- 1, and they assigned the band to NO+. However, we realized that the shift from 2125 to 2087 cm -I precisely corresponds to the value calculated for NO +, whereas for linear
E. Ito et al./Microporous Materials 4 (1995) 455 465
NO2~ a shift to 2075 cm -1 would be expected (Table 2) [41]. Even a strongly bent 1 5 N O r ion with an O N O angle of 134.5 ° (NO 2 gas molecule) vibrates at 2078 cm ~. A stronger bending could theoretically lead to a vibration at 2087 cm-~, however, N O I with such an extreme bending is not realistic. Therefore, in agreement with the author [42], we concluded that the 2125 cm -~ band is due to NO + This assignment, in fact, gives a direct evidence for the ionization of N 2 0 a t o N O + and NO2 ( x = 2 or/and 3) in zeolite as presented in Eq. 4. N203 is, in fact, known to ionize towards NO + and NO2 in aqueous solution, and this reaction is particularly utilized to generate N O + in nitrosation reactions [43]. Kasai and Bishop [35] first indicated the ionization of N2Oa in zeolite, based on electron spin resonance (ESR) investigations. These authors suggested that such an ionization phenomenon effectively lowers the high electrostatic energy of the zeolite [18]. Furthermore, a schematic zeolite picture was presented, where ionized NO and NO2 (as N O + and N O 2 , respectively) are stabilized by a minus charge of a "zeolite anion" and a plus charge of an "exchanged cation" [35].
Table 2 Isotopic frequency shifts calculated for NO + and NO~ Species Adsorption Calculated isotope- Observed isotopemode shifted frequency shifted frequency (cm 1) (cm- l ) NO ~ NO~
Linear" Bent b (2~1 150' 134.3 90'
2087 2075 2087 2076 2078 2087
The original frequency observed upon 14NO adsorption at 2125 cm 1 shifted towards 2087 cm -1 upon 15NO adsorption
[81. a X Y X: a three atomic molecule: (wi/w)z=mv(l + 2miv/mix)/miy(l+2mv/mx) where w and wi are an original and isotopic-shifted frequency and where mx, my, mix and m~r are the atomic weights of X and Y and of their isotope molecules applied [41 ]. b (w%v)2 = rnxmv [mix+ 2m~v(sin ~)Z]/mixm~v[mx + 2my(sin ct)2]: :~= ~/_ X -Y-X.
461
4.4. Co-adsorption of NO and 02: the effect of the presence of oxygen Besides the elucidated absorption of NO + as discussed above, the appearance of NO + upon NO2 adsorption, as reported by Chao and Lunsford [21], remains to be explained. Our results of the co-adsorption of NO and 02 onto C e N a M O R and L a N a M O R provided some indications to answer this question. The consecutive admission of NO and 02 to C e N a M O R brought about strong absorption bands of NO +, nitrito species ( O N O - ) and nitrato species (NO3: 1333, 1488, 1515 and 1603cm-~). For nitrato species, the bands at 1488 and 1333 cm ~ are probably attributed to a monodentate coordination [28,29,44], and the shoulders found at 1510 and 1603 cm ~ might indicate the presence of a bidentate coordination as well [30,44]. The simultaneous formation of NO + and NO~ upon the co-adsorption of NO and O2 could be explained by the following consecutive reactions: 2NO + O 2 ~ N 2 0 4 ~ N O + + N O 3
{8)
This scheme also explains the observation of NO + upon N O 2 adsorption, since N 2 0 4 , present in equilibrium with N 0 2 , is known to dissociate into NO + (HNO2) and NO3 (HNO3) in an aqueous solution [31]. Given the strong electrostatic field of the zeolite, which enables a dissociation of N 2 0 3 into NO + and NO2, the ionization of N 2 0 4 (Eq. 8) is likely to take place as well. Such an ionization towards NO + and NO3 was suggested to occur on TiO2 [45,46]. Our case is complex, since two pathways that produce NO + and NO3 are conceivable. One is the formation of N 2 0 4 and the subsequent dissociation into NO + and NO3, and the other is the formation of NO + and NO2 via N 2 0 3 followed by the conversion of N O 2 t o N O 3 in the oxidizing atmosphere. With respect to the first pathway, it is known that the presence of NO and O2 at relatively low temperatures (<200°C) leads to some NOz formation, and the small N204 peak observed on E a N a M O R supports this assumption. On the other hand, the second pathway is also very likely, since we actually observed broad
462
E. Ito et al./Microporous Materials 4 (1995) 455-465
nitrato bands upon NO adsorption, which is most likely a result of conversion of nitrito to nitrato
species (Fig. 2d). With CeNaMOR, the observation of the nitrito peak ( O N O - ) upon the NO/O2 admission also supports this pathway (Fig. 4a). Our present results do not allow us to distinguish between the two possible schemes. At 300°C with CeNaMOR, most of the nitrato bands diminished and a new band appeared at 1418 cm-1, which can be assigned to nitrito species [20]. With L a N a M O R , a similar spectral change was observed, though it was less pronounced than in the case of CeNaMOR. The disappearance of nitrato bands and the simultaneous appearance of nitrito species suggests that nitrato species present at 100°C have transformed into nitrito losing oxygen at a temperature as high as 300-350°C. Another possibility for the formation of nitrito was suggested by Brandin et al. [15c], who proposed an equilibrium between NO + and O N O - , with the involvement of an oxide ion of the zeolite anion sites. However, there is no evidence for the validity of such an equilibrium at this moment. 4.5. Location o f N O + and the appearance o f absorptions at 1625 and around 3500 c m - 1
We have observed N O + (2160-2167 cm -1) on C e N a M O R and L a N a M O R , and this absorption band was also reported for H - M O R [15a]. It suggests that the formation of N O + may be related to the presence of the zeolite acid sites. Our results of N O - T P D agree with this assumption (Fig. 6 [17]). With H-MOR, there is only one NO desorption peak observed at 170°C, while L a N a M O R and C e N a M O R show the peak at 170°C in addition to another peak at 270°C. The desorption of NO at 170°C can, therefore, be linked with the presence of zeolite acid sites. This assumption was supported by a separate N O - T P D experiment with N a M O R , showing only a small peak at 170°C. Given the present IR results that NO + was the first species to desorb prior to NO:~ species upon a temperature raise, the low-temperature T P D peak at 170°C can be consistently linked with the observation of N O +. One might wonder where NO + ions could be located in the zeolite. The schematic zeolite picture
40 o
:~'\
30
/ /: \,
20
jJ'
/
'/'~
/ /
g o oz
10
//
,/
o t-'l,
100
,
, "-- . . . . . . 200 300
400
500
600
Temperature / °C Fig. 6. Temperature-programmed desorption of NO from CeNaMOR (--), LaNa-MOR ( ) or H-MOR (---). After a pretreatment, NO was adsorbed at 100°C, followed by a flush of argon. Desorption was carried out under argon at 5°C/min up to 560°C, and the product gas was analyzed by mass spectrometry.
presented by Kasai and Bishop [35] suggests that N O + is most probably associated with the framework anionic sites of the zeolite. In our case, the formation of N O + is conceivable in the following two ways: with Bronsted sites involved (Eq. 9) and with metal cationic species involved (Eq. 10). NO + N O 2 + [Ln~Oy(OH)z]" +...nZ- + H +Z-+NO+Z - + N O 2 + H + + [LnxOy(OHL-]"+...nZ (9) -,NO+Z + [LnxOy(OH)= (NO2)] ~"- ~)+...(n - 1 ) Z +H+Z -
(10)
where [LnxOy (OH)z]" + = lanthanide cationic oxo species and Z - = zeolite framework anion. According to Eq. 9, NO + forms on the zeolite framework anion site with the simultaneous relocation of an acidic proton. In this case, the H + ions may protonate metal cationic species: [Ln~Oy(OH)z] "+ together with coordination of NO~-, which leads either to the formation of water or/and coordinated hydroxide and a nitrito ligand, or to the formation of H N O 2 (nitrous acid) weakly associated with [LnxOy(OHL] "+. The absorptions observed at 3500 and 1625cm 1 can here be referred to, since these bands indicate the formation of water (3500 and 1635 cm-1) [25] or nitrous acid (cis: 3588, 1699, 1265 cm 1; trans: 3424, 1640,
1~ Ito et al./Microporous Materiab 4 (1995) 455 ~465
1261 cm ~; usually present in the mixed form) [26]. The observation of the third peak around 1260- 1265 cm -~ is expected for nitrous acid, however, it is difficult to distinguish a peak in this region, due to the strong zeolite band (1000 1250 cm ~). Eq. 10, by contrast, allows the formation of NO + without the involvement of zeolite acid sites. NO + can form on the zeolite framework anion site, which becomes available due to the decreased effective charge of metal cationic species upon a coordination of NO2. The essential difference between these two schemes (Eqs. 9 and 10) lies in the involvement of Bronsted acid sites. It should be noted that a negative peak at 3610cm -~ is observed in our spectra, which indicates the disappearance of acidic hydroxyl from the zeolite framework. We observed it together with the appearance of NO + , particularly in the case of C e N a M O R (Figs. 4 and 5); hence, NO + formation could be related with the disappearance of Bronsted sites as expected from Eq. 9. However, a quantitative correlation could not be established, probably due to a overlap of the 3610 cm 1 signal with a growing broad band at 3500cm ~.
4.6. Formation q[NO~, species and the role of metal cations Our IR data showed the two sorts of NOx species (NO + and NOx ) both on C e N a M O R and LaNaMOR, and NO + was found to desorb first upon the temperature increase. Compared to the N O - T P D results (Fig. 6), showing two desorption peaks with CeNaMOR and L a N a M O R , NO + is assumed to be related with the first NO desorption peak (170~C) and, thus, the NOx species should correspond to the second N O - T P D peak (270°C). H-MOR did not show the second N O - T P D peak, which indicates that metal cations present in zeolite may contribute to the stabilization of NOx ions by their higher coordination capacity. This increased stability of the NO~ coordination by metal cations may further lead to the enhancement of the total degree of ionization; the equilibria in Eqs. 7 and 8 could be driven more to the right. This is apparently reflected in the T P D results, where CeNaMOR and L a N a M O R exhibited
463
larger amounts of desorbed NO in total than the 100% H+-exchanged H-MOR. The pronounced ionization of nitrogen oxides by the presence of high-valent metal cations can also be understood in terms of the enhancement of the electrostatic fields of the zeolite. 4. 7. Role of redox properties of cerium
(CeUl/CeW ) in NO oxidation to NO2 In N O reduction with ammonia in the presence of oxygen, the specific low-temperature activity of CeNaMOR was assumed to originate from its higher activity in NO oxidation to NO2 (Eq. 1) in the low-temperature region. This reaction will, therefore, in some way involve the redox properties of cerium (Cem/CetV). The present IR studies performed with NO and 02 over CeNaMOR and L a N a M O R can be well related to the NO oxidation activities of the respective catalysts. As we have already discussed, NO adsorption onto C e N a M O R and L a N a M O R in both cases resulted in the formation of the same adsorbed species: N20, NO +, NO2 and NO3- indicating NO disproportionation and the subsequent ionization. Therefore, the redox properties of cerium (Cen~/Ce TM) are not essential for these consecutive reactions. Nevertheless, the following redoxinvolved NO disproportionation might have additionally taken place on CeNaMOR 2CenI+2NO+2H+~2Ce~V+N20+H20
(11)
which was proposed to account for the results of the NO-TPD of C e N a M O R [ 17]. The co-adsorption of NO and 02 and the subsequent temperature raise also resulted in the tbrmation of the same adsorbed species (NO +, N O 2 and NO3) on the two mordenite samples. However, differences were found in the formation/ desorption behavior of these adsorbed species. Upon the admission of NO and 02 at 10ffC, the formation of NOx species was more pronounced on C e N a M O R than on LaNaMOR. This suggests that N O ; is more easily formed on CeNaMOR than on LaNaMOR. However, it probably does not make an essential difference between CeNaMOR and L a N a M O R in NO oxidation carried out at 200-400 C [12], considering the
464
E. Ito et al./Microporous Materials 4 (1995) 455 465
comparable formation of NO + and NOx species on the two mordenites at 200°C (Figs. 4b and 5b). A more important difference was indicated in the temperatures required for decomposition of the NO+/NO2 species. The desorption of NO + started at 200°C on CeNaMOR, whereas NO + on LaNaMOR became smaller only at 300°C and, in addition, the NO~- species were found to be held more tightly on LaNaMOR than on CeNaMOR at 300-350°C. The easier desorption of NO + and NO2 species, which are actually the ion pairs of NO oxidation products (N203 and N204), indicates a higher turnover frequency of active sites with CeNaMOR. One could imagine the redox couple of cerium to be involved here in the following way, e.g. Cem+NO+l/202--* CelV(NO~-) ~ C e III+ NO2. Linking the present IR data with the CeNaMOR catalysis of NO reduction with NH3 in the presence of oxygen [12] is not straightforward due to the presence of the third component NH 3. However, we believe that two important observations here: the formation of the ionized NO oxidation products, NO + and NO2 (nitrito, nitrato) species, and their higher availability by easier desorption from CeNaMOR, could explain the efficient consumption of NOx species in NO reduction with NH3 in the presence of oxygen. In particular, the presence of the ionized pair NO+/NO2 observed here strongly indicates the involvement of a nitrosation-type reaction in the presence of NH 3. This is, in fact, equivalent to nitrosation of NH3 with NzO3, which is known to proceed at -85°C evolving N 2 [47].
5. Conclusion NO adsorption onto CeNaMOR and LaNaMOR leads to the disproportionation of NO towards N 2 0 and N 2 O 3 at room temperature, and N203 was found to ionize further into NO + (2160 cm-1, the nitrosonium ion) and NO~- (nitrito or/and nitrato, 1300-1500cm -1) species. The co-adsorption of NO and 02 on CeNaMOR and LaNaMOR resulted in the formation of NO + and NO 3 (nitrato). This can be accounted for by another equilibrium: 2NO +02~NzO4~-NO + _I_ NO3. It was shown that NO + is associated with
the lattice anionic sites of the zeolite and the NO~- species with exchanged cations. The difference between CeNaMOR and LaNaMOR was found in the desorption behavior of the NO + and NO;- species; they were easier to desorb from CeNaMOR than from LaNaMOR. This may contribute to the high turnover frequency of the NO oxidation reaction, and it could account for the higher activity of CeNaMOR for NO reduction with NH3 in the presence of oxygen.
Acknowledgements We acknowledge the financial support for VROM (Ministry of Housing and Environmental Affairs). We greatly appreciate Prof. Dr. Masakazu Iwamoto for his kind cooperation in the essential assignment of NO +. Dr. Marcello S. Rigutto is thanked for important suggestions.
References [1] [2] [3] [4] [5]
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Microporous Materials 4 ( 1995 ) 455 465
Infrared studies of NO adsorption and co-adsorption of NO and 02 onto cerium-exchanged mordenite (CeNaMOR) E. Ito ~, Y.J. Mergler b, B.E. Nieuwenhuys b, H. van Bekkum a.., C.M. van den Bleek a Department q]" Chemical Technology and Materials" Science, Delft UniversiO' of Technology, Julianalaan 136, 2628 BL Delft, Netherlands b Gorlaeus Laboratoria, University o f Leiden, P.O. Box 9502. 2300 RA LeMen. Netherlandv
Received 9 January 1995; accepted 21 March 1995
Abstract
Infrared (IR) studies on NO adsorption and co-adsorption of NO and 02 onto cerium- and lanthanum-exchanged mordenite (CeNaMOR and LaNaMOR, respectively) were performed in order to elucidate the role of redox properties of cerium (Cem/Cd v) in the oxidation of NO to NO2, an important preliminary step in the NO reduction catalysis. NO adsorption onto both CeNaMOR and LaNaMOR leads to the formation of N 2 0 (2247 cm-~), NO + (nitrosonium ion; 2162 cm -~) and NO~- species (nitrito or/and nitrato, 1300-1500 cm-1). These are thought to arise from disproportionation of NO towards N 2 0 and N203 and the subsequent ionization of N203 towards NO + and NO2. This scheme is supported by the transient observation of molecular N203. The co-adsorption of NO and O2 onto CeNaMOR and LaNaMOR resulted in the enhanced formation of NO + and NO3 (1515, 1488 1497 and 1333 cm ~), which is accounted for by the formation of NO2 and its subsequent ionization via N 2 0 4 towards NO + and NO3. Combining IR and NO temperature-programmed desorption (TPD) data, it is proposed that the formation of NO ~ is associated with zeolite acid sites, and that NO;- (nitrato) species are coordinated to lanthanide cations. Furthermore, the NO ' and NO~- species were found to desorb more easily from CeNaMOR than from LaNaMOR. The redox properties of cerium (Cem/Ce Iv) may contribute to the easier desorption of these oxidized NO species. Kevwordv. Lanthanide-exchanged zeolite; NO + formation; Nitrosation: NO oxidation; (CeH~/CeJV)redox couple
1. Introduction
The catalytic removal of NOx from exhaust gases is currently one of the most investigated subjects a m o n g environmental catalysis studies [ 1]. In NOx reduction catalysis, transition metals, in particular d-block transition metals such as noble metals (Pt, Rh, Pd, etc.) [2], metal oxides (V20 s, CuO, Cr203, MnOx, etc.) [3] or these metals or metal ions accommodated in zeolites [4] are often applied. Actually, the interaction of d-block trans* Corresponding author. 0927-6513/95/$9.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0927-6513(95)00026-7
ition metals with NO often involves a d-electron and/or an empty d-orbital, which leads to the formation of metal nitrosyls [5]. Nitrosyl coordination can be found as N O ~+, NO (neutral) or N O 6-, with IR absorptions in the region 1900-1600cm -1 [6]. Electron transfer from the metal d-orbital to the rc*-orbital (anti-bonding) of N O weakens the N O bond (i.e. formation of NO ~ ) [5,7], and this species is supposed to be susceptible to decomposition [7 9]. On the other hand, g*-electron transfer from N O to a metal strengthens the N O bond (i.e. formation of N O 6+). This species will decompose less easily, but
456
E. Ito et aL/Microporous Materials 4 (1995) 455-465
it is probably more reactive in the presence of a reductant, since NO 6+ is a more "electrophilic" reagent than NO itself towards a "nucleophilic" reductant. The high potential of d-transition metals for NO coordination often leads to effective NO decomposition or NO reduction. An intriguing aspect of the NOx reduction catalysis is that non d-block redox metals (e.g. Ga- [10] and Ce-zeolites [11,12]), non-redox metals (e.g. In- [10] and La-zeolites [12,13]) and even simple proton-type zeolites [ 14-16] can be excellent NO~ reduction catalysts. Metal oxides generally have surface acid sites, and metal-exchanged zeolites can also show Bronsted acidity in the following two ways: upon reduction of a metal ion (e.g. M "+ + r e d u c t a n t ~ M ~"-u+ + H + + oxidized products) for non d-block redox metals and by dissociation of a water ligand ( M ( H 2 0 ) " + ~ M(OH)~"-I)++H +) for non-redox multivalent cations. These acid sites are assumed to play a role in the oxidation of NO towards N O 2 (intermediate) for NOx reduction with NH 3 [14,15] or with hydrocarbons [10,11,16] under oxygen-rich conditions. Furthermore, the acid sites may interact with reductants in NOx reduction, e.g. the enhanced adsorption of NH3 and the activation of hydrocarbon reductants. We have recently reported that ceriumexchanged zeolites (mordenite and ZSM-5) are highly active and selective catalysts for NOx reduction with NH3 in the presence of oxygen [17], and the NO~ reduction catalysis of cerium-exchanged mordenite (CeNaMOR) was studied in detail in comparison with lanthanum-exchanged mordenite (LaNaMOR) and H-MOR [12]. LaNaMOR and H-MOR showed a similar high-temperature catalytic activity (400-550°C), while CeNaMOR exhibited a prominent activity in a wide temperature range (250-550°C). The specific lowtemperature activity of CeNaMOR for NO reduction was ascribed to its high NO oxidation capacity, and we proposed an NO2-intermediated reaction mechanism for CeNaMOR in the low-temperature region (< 400°C) [ 12]:
We report here IR studies of NO adsorption and co-adsorption of NO and 02 onto CeNaMOR and LaNaMOR. Cerium and lanthanum cations are both known to show Bronsted acidity by water ligand dissociation, and their coordination chemistry is very similar. Cerium differs, however, in exhibiting redox properties (Cem/CeIV), and that can possibly account for a distinguished NO oxidation capacity of CeNaMOR at low temperatures. The present IR investigation aims at contributing to understanding of the high performance of CeNaMOR in NO oxidation towards N O 2 ; a preliminary step in NOx reduction with NH3. It should be noted that NO 2 formation is reported to be important in NO reduction with hydrocarbons over Ce [11], Ga [10], In-ZSM-5 [10], H-zeolite catalysts [16], etc. as well. We will discuss our IR data while comparing with NO-TPD and NO oxidation results obtained earlier for CeNaMOR and LaNaMOR [12].
2. Experimental 2.1. Preparation of cerium- and lanthanumexchanged mordenite Mordenite (CBV-10A, SiOz/A1203=13.1, Na form) was obtained from PQ Corporation. Cerium-exchanged mordenite (CeNaMOR) was prepared by exchanging 25 g of mordenite powder in an 8.3 mM aqueous solution (1.5 1) of cerium acetate (Alfa Products, 99.9%) at 100°C for 5-7 h. For lanthanum-exchanged mordenite (LaNaMOR), 5 g of mordenite were exchanged in 1.51 of a 2 m M lanthanum trichloride (LaC13.xH20) solution at 70°C for 15-20h. Samples were then filtered, washed with deionized water and dried at 120°C. Analyses of samples by inductively coupled plasma atomic emission spectroscopy (ICP-AES) showed 58% and 57% of Na + to be exchanged stoichiometrically with Ce3+ or La 3+, respectively.
2.2. Infrared measurements NO + ~02 ---*N O 2
( 1)
NO + NO 2 + 2NH3 ---2N2 + 3H20
(2)
IR measurements were performed with a Mattson Galaxy 2020 spectrometer (DTGS detec-
457
E. Ito et aL/Microporous Materials 4 (1995) 455 465
tor) under static conditions using a vacuum cell equipped with CaF2 windows. The spectra were recorded with a resolution of 2 c m - ~ for 64 scans for each spectrum. Catalyst sample powder was pressed to a thin self-supporting disk (1 ton, 1 s) and was pre-treated at 330-350°C in high vacuum (10 5-.10 6Torr) for 2.5h. The background spectra of C e N a M O R or L a N a M O R were then taken at different temperatures (100, 200, 300 and 350°C) before admission of any reactant gas, and used for subtraction from the spectra taken in the presence of reactant gases. All spectra presented here are background-subtracted. The gases employed for adsorption were NO (99.8%, Messer Griesheim), 02 (99.998%, Messer Griesheim) and NH3 (99.98%, Hoek Loos).
3.2. NO adsorption onto CeNa-MOR and LaNa-MOR
3. Results
Table 1 List of observed absorption bands in Figs. 2 5 and their assignments
3.1. Spectra of cerium- or lanthanum-exchanged mordenite before adsorption
Species
CeNa-MOR frequency (cm- 1)
LaNa-MOR frequency (cm ~)
N20 NO ~ NO 3 (nitrato)
2247 2162 1333, 1603 1418 1625, 1304,
2247 2162 1333. 1497, 1515, 1603 143(I 1625, 3500 1305, 1520, 1912 1740
The spectra of C e N a M O R and L a N a M O R taken before gas adsorption showed a broad absorption around 3500 cm -1, a sharp peak at 3610cm -1, and small peaks at 3650, 3686 and 3740 cm 1 (Fig. 1). The absorption at 3610 cm -1 is assigned to Bronsted acid sites in mordenite [18], and other small peaks at 3650, 3686 and 3740 cm 1 are attributed to Ce-OH/La-OH,
A1-OH and SiOH, respectively [18,19]. The broad band around 3500 cm 1 may indicate the presence of water, with its bending-mode absorption at 1635cm 1. In addition, the hydrogen-bonded hydroxyl species Si-OH/Ce-OH/La-OH may also contribute to the band around 3500 cm-1.
Cerium-exchanged mordenite (CeNaMOR) The observed absorption bands originated from NO are listed in Table 1. Upon NO admission (1 Tort) at room temperature, a sharp peak appeared at 2247 cm -1, and another peak at 2162 cm -1 (Fig. 2a). The peak at 2247 cm -1 can
ONO (nitrito) H20/HNO 2 N203 N204
2170 1488, 1515,
3500 1521, 1912
2.0 4 0.4-
1.5 " ~ . .
~(a)
~
/
•.~ '<
O
0.2-
(c)
""
1.0 (b)
"'
O-
0.5
4000
(a) 3500
3000
2500
2000
1500
Wavenumber / cm- l 4000
3500
3000
2500
2000
1500
Waveuumber / cm-1 Fig. 1. Spectra of (a) CeNaMOR and (b) LaNaMOR before gas adsorption.
Fig. 2. NO adsorption onto CeNaMOR. (a) NO l Torr at room temperature, (b) NO pressure increased up to 5 Tort, (c) NO 5 Torr after 10 min and (d) temperature increased up to 100"C with NO 5 Torr.
458
E. Ito et al./Microporous Materials 4 (1995) 455 465
be assigned to N20 [20,21], and the absorption band at 2162 cm -a is assigned to NO + [20,22,23], as will be discussed later. By increasing the NO pressure to 5 Torr, the absorptions of N20 and NO + increased in size, while new peaks grew at 1304, 1521, 1625, 1912cm -1 with a broad one around 3500 cm 1 and a negative peak at 3610cm ~ (Fig. 2b). The peaks observed at 1304, 1521 and 1912cm 1 can be assigned to NzO3 [21,24], and the absorptions at 1625 and 3500 cm 1 probably originate from water [25] or from nitrous acid [26]; this will be discussed later in more detail. After 10 min at 5 Torr of NO, the absorptions of N20 and NO + and of H20 or HNO2 further increased, whereas the absorptions ascribed to N203 disappeared. In addition, a new broad band at 1400-1550cm -~ was observed, which can be assigned to absorptions by nitrate or nitrite ions in general [20,27], which will hereafter be referred to as " N O x " species (Fig. 2c). By raising the temperature to 100°C, the N20 absorption mostly disappeared, while the NO + band remained. Furthermore, the appearance of maxima around 1515, 1488 and 1320cm -1 were noticed in the NO2 absorption region (Fig. 2d). These can be assigned to nitrato (coordinated NO~- ion) species formed on lanthanum or cerium ions [28-30]. Meanwhile, the absorptions by water or nitrous acid became larger.
Lanthanum-exchanged mordenite (LaNaMOR) NO adsorption onto L a N a M O R provided spectra similar to those observed with CeNaMOR (Fig. 3, Table 1). This indicates that the formation of NzO, NO + and NO~ species does not require the intervention of the cerium redox couple (CenI/Cew). Note that the formation of water or nitrous acid was also observed, while the negative peak at 3610 cm -1 was much weaker than that of CeNaMOR.
0.8
0.6 0.4 <
0.2
(a) 4000
3500
3000 2500 2000 Wavenumber / cm- 1
1500
Fig. 3. NO adsorption onto LaNaMOR. (a) NO 1 Torr at room temperature, (b) NO pressure increased to 5 Torr, (c) NO 5 Torr after 10 min and (d) temperature increased up to 100°C with NO 5 Torr.
an absorption at 1418 cm-1, indicative for nitrito (oxygen-coordinated nitrite ion: ONO ) species [20], appeared immediately (Fig. 4a, Table 1). The presence of water or nitrous acid was also indicated (1625 and 3500 cm-1), and a negative peak was observed at 3610cm -1. Compared with the absorption of NO only (Fig. 2d), it is evident that the presence of oxygen greatly enhanced the formation of nitrato (NO3) and nitrito (ONO-) species and NO + (nitrosonium ion). Raising the temperature to 200°C, the NO + peak decreased, while the nitrato absorptions remained almost unchanged (Fig. 4b). Raising the temperature fur-
1.2
(c) 0.8
e~ 0.4
3.3. Co-adsorption of NO and 02 Cerium-exchanged mordenite (CeNaMOR) NO (5 Torr) and Oz (5 Torr) were consecutively admitted to CeNaMOR at 100°C. The absorption bands due to NO + (2162 cm-1), NO3 (1488 cm -1 with shoulders at 1603, 1515 and 1333 cm-1), and
4000
3500
3000 2500 2000 Wavenumber / cm-I
1500
Fig. 4. Co-adsorption of NO and 02 onto CeNaMOR. (a) NO (5Torr) and O z (5Torr) at 100°C, (b) 200°C, (c) 300°C and (d) 350°C.
E. lto et aL/Microporous Materials 4 (1995) 455 465
ther to 300'C, the NO + band and the NO;- bands substantially diminished, which was accompanied by the appearance of nitrito absorption at 1418cm ~ (Fig. 4c). At 3500C, the nitrato and nitrito peaks decreased further (Fig. 4d). Meanwhile, the absorptions assigned to water or nitrous acid became smaller at 300 and 350'C (Figs. 4c and d). Note that a negative peak observed at 3610cm i remained visible during this measurement, becoming smaller at higher temperatures. The appearance of a small broad band around 3200cm i might indicate that the hydrogen bond between zeolite O H groups and the adsorbed species originated from NO. Lanthanum-exchanged mordenite ( L a N a M O R ) The co-adsorption of NO and 02 onto L a N a M O R and C e N a M O R resulted in the formation of rather similar adsorbed species: NO + (2162cm 1), NO~ (1473, 1510, 1330 (broad) cm 1), and water or nitrous acid (Fig. 5, Table 1 ). Nevertheless, differences were found between C e N a M O R and L a N a M O R in the temperature of the formation/disappearance of these species. Comparing the spectrum of L a N a M O R (Fig. 5a) with that of CeNaMOR (Fig. 4a) upon NO/O2 admission at 100°C, the nitrato ( N O 3 ) bands were considerably smaller for L a N a M O R and, in addition, nitrito was not observed. Raising the temperature to 200:C, the nitrato absorptions were greatly enhanced, while the NO + band and the
H 2 0 / H N O 2 bands remained almost unchanged (Fig. 5b). Note that the NO + species started to disappear at 200°C on CeNaMOR (Fig. 4b). A weak broad band observed at 1740 cm- 1 might be assigned to N204 [31,32], whereas the origin of another weak band at 1850 cm- ~ remains unclear. A small broad peak was present around 3200 cm 1 as in the case of CeNaMOR, and it may indicate the presence of hydrogen-bonded species on zeolite. Increasing the temperature to 300°C, the NO + species decreased substantially, and the nitrato peaks became smaller, which was accompanied by the appearance of a shoulder attributed to nitrito species (1430cm t) (Fig. 5c). At 350'-C, the nitrato peaks further decreased, and the presence of nitrito species became more evident (Fig. 5d). In comparison with the spectrum of C e N a M O R (Fig. 4c), the effect of a temperature raise to 300 35ff~C on the N O , species was apparently smaller for LaNaMOR.
4. Discussion 4.1. Cerium- and lanthanum-exchanged mordenite
The spectra of CeNaMOR and L a N a M O R before gas absorptions were rather similar. Both spectra showed the peak at 3610 cm l, which is in good agreement with a reported IR absorption frequency of Bronsted acid sites in mordenite [18]. Since C e N a M O R and L a N a M O R were prepared by a stoichiometric exchange of Ln 3 + (lanthanide cation) for Na +, the presence of the acid sites originates from the "lanthanide acidity" due to water ligand dissociation: Ln(H20) 3*
04 __y+--- . . . . . . . . . . o ~ 40t)0
3500
3000
2500
2000
1500
Wavenumber [ cm- 1 Fig. 5. C o - a d s o r p t i o n o f N O a n d 0 2 o n t o L a N a M O R . (a) N O ( 5 T o r r j and O2 ( 5 T o r r ) at 100+'C, ( b ) 200°C, (c) 300~C and (d) 350+C.
459
--~ Ln(OH
)2+ + H +
(3)
Only a tiny peak was observed for L n ( O H ) species (3650cm 1), which should be another consequent product according to Eq. 3. Such an apparent inconsistency was already reported for La-Y [19], in which case it was attributed to the formation of oxo-bridged species: LnxO~.(OH) = [19,33]. The formation of oxo-bridged species was suggested for Ce-Y as well [34]; thus, lanthanum and cerium ions in mordenite may have also formed small cationic oxo species during the
460
E. Ito et al./Microporous Materials 4 (1995) 455-465
pretreatment. An alternative explanation for a relatively low intensity of Ln(OH-) band is the occurrence of hydrogen bonding, which would yield a shifted and broadened peak around 3500 cm- 1. Further investigations are necessary. 4.2. N O adsorption onto C e N a M O R and LaNaMOR
The admission of NO to CeNaMOR and LaNaMOR at room temperature leads in both cases to the appearance of N20, NO + and NO~species ( x = 2 or/and 3). The presence of N20 and NO~- suggests a disproportionation of NO and the formation of adsorbed NO2 species, as shown below in Eqs. 4-6. 2NO-,N20 + O(ad)
(4)
NO + O(ad) ~NOz(ad)
(5)
3NO-*N20 + NO2
(6)
Disproportionation of NO was reported over a variety of metal ions in zeolites, e.g. CaY [21], Ba-Y [35], Cu-ZSM-5 [36], Au-Y [37] and Ni-Y [38] at room temperature, and Na-Y at 77 K [21]. The observed activity order for this reaction was C a Y > N a Y > H Y [21], which may be accounted for by the enhanced polarization in the zeolite due to the presence of metal cations with a high valence state [18]. In addition, metal ions could also offer adsorption sites for NO, for instance in the form of an oxygen-coordinated hyponitrite ion (N202) 2-, which is subject to decomposition into N20 over CeO2 [39]. In addition to NO disproportionation, we suggest the following consecutive reaction to take place on CeNaMOR and LaNaMOR: NO + NO2~-N203--~NO + + N O 2
(7)
The formation of N20 3 during NO disproportionation was observed for BaY [35], NaY [21], CaY [21], AuY [37] and NiY [38]. This can be explained by an equilibrium of NO, NO2 and N203 [35], which is also supported by a recent report on the formation of N20 a upon the co-adsorption of NO and NO2 onto Na-zeolites [40]. In our spectra, three peaks indicative to N203 were observed (Figs. lb, 2b and Table 1), and
their fast disappearance with time (Fig. 2c) suggests the subsequent reaction. The observation of the absorptions at 2162 cm-1 and at 1300-1500cm -~ (NO2) should be noted as the subsequent ionization reaction. We assigned the absorption at 2162 cm-1 to NO + (nitrosonium ion), and we would like to discuss this essential assignment in the following section. 4.3. Assignment o f the peak at 2162 cm -~
Historically, there have been two interpretations for the absorption at 2120-2170cm -1 often found upon the adsorption of NO: one is NO + (the nitrosonium ion) and the other NO+ (the nitronium ion). NO + is reported to absorb in the 2100-2300cm -1 region, whereas NO~- is found in the 2200-2400cm -1 region [20,27]. Therefore, simply from the observed frequency (2120-2170 cm 1), the assignment to NO + seems more probable. Terenin and co-workers [22,23] first assigned the band to NO+ in 1959. However, in 1971, Chao and Lunsford [21] proposed to assign the band to NO+. They observed this peak upon NO 2 adsorption, and this was supposed to be a strong support for the assignment to NO+. Later, in 1989, Odenbrand et al. [15a] observed an absorption at 2160 cm -~ upon NO adsorption on H-MOR, and ascribed it to NO +, which assignment was in accordance with the proposed reaction pathway of NO reduction with NH3 over H-MOR. Nevertheless, the appearance of the peak upon NO 2 adsorption is nowadays often referred to, and it still leads many groups to assign the band to NO + . In such a complex case, the most reliable data can be obtained from isotope substitution experiments. In 1992, lwamoto et al. [8] have reported an IR isotope study for NO adsorption with 15NO and I#NO over Cu-ZSM-5. Upon I#NO adsorption, a small peak was observed at 2125cm -I besides strong absorptions of nitrosyls on the dtransition metal ion copper. Using ~SNO instead of 14NO, the small peak at 2125 cm- ~was observed to shift towards 2087 cm- 1, and they assigned the band to NO+. However, we realized that the shift from 2125 to 2087 cm -I precisely corresponds to the value calculated for NO +, whereas for linear
E. Ito et al./Microporous Materials 4 (1995) 455 465
NO2~ a shift to 2075 cm -1 would be expected (Table 2) [41]. Even a strongly bent 1 5 N O r ion with an O N O angle of 134.5 ° (NO 2 gas molecule) vibrates at 2078 cm ~. A stronger bending could theoretically lead to a vibration at 2087 cm-~, however, N O I with such an extreme bending is not realistic. Therefore, in agreement with the author [42], we concluded that the 2125 cm -~ band is due to NO + This assignment, in fact, gives a direct evidence for the ionization of N 2 0 a t o N O + and NO2 ( x = 2 or/and 3) in zeolite as presented in Eq. 4. N203 is, in fact, known to ionize towards NO + and NO2 in aqueous solution, and this reaction is particularly utilized to generate N O + in nitrosation reactions [43]. Kasai and Bishop [35] first indicated the ionization of N2Oa in zeolite, based on electron spin resonance (ESR) investigations. These authors suggested that such an ionization phenomenon effectively lowers the high electrostatic energy of the zeolite [18]. Furthermore, a schematic zeolite picture was presented, where ionized NO and NO2 (as N O + and N O 2 , respectively) are stabilized by a minus charge of a "zeolite anion" and a plus charge of an "exchanged cation" [35].
Table 2 Isotopic frequency shifts calculated for NO + and NO~ Species Adsorption Calculated isotope- Observed isotopemode shifted frequency shifted frequency (cm 1) (cm- l ) NO ~ NO~
Linear" Bent b (2~1 150' 134.3 90'
2087 2075 2087 2076 2078 2087
The original frequency observed upon 14NO adsorption at 2125 cm 1 shifted towards 2087 cm -1 upon 15NO adsorption
[81. a X Y X: a three atomic molecule: (wi/w)z=mv(l + 2miv/mix)/miy(l+2mv/mx) where w and wi are an original and isotopic-shifted frequency and where mx, my, mix and m~r are the atomic weights of X and Y and of their isotope molecules applied [41 ]. b (w%v)2 = rnxmv [mix+ 2m~v(sin ~)Z]/mixm~v[mx + 2my(sin ct)2]: :~= ~/_ X -Y-X.
461
4.4. Co-adsorption of NO and 02: the effect of the presence of oxygen Besides the elucidated absorption of NO + as discussed above, the appearance of NO + upon NO2 adsorption, as reported by Chao and Lunsford [21], remains to be explained. Our results of the co-adsorption of NO and 02 onto C e N a M O R and L a N a M O R provided some indications to answer this question. The consecutive admission of NO and 02 to C e N a M O R brought about strong absorption bands of NO +, nitrito species ( O N O - ) and nitrato species (NO3: 1333, 1488, 1515 and 1603cm-~). For nitrato species, the bands at 1488 and 1333 cm ~ are probably attributed to a monodentate coordination [28,29,44], and the shoulders found at 1510 and 1603 cm ~ might indicate the presence of a bidentate coordination as well [30,44]. The simultaneous formation of NO + and NO~ upon the co-adsorption of NO and O2 could be explained by the following consecutive reactions: 2NO + O 2 ~ N 2 0 4 ~ N O + + N O 3
{8)
This scheme also explains the observation of NO + upon N O 2 adsorption, since N 2 0 4 , present in equilibrium with N 0 2 , is known to dissociate into NO + (HNO2) and NO3 (HNO3) in an aqueous solution [31]. Given the strong electrostatic field of the zeolite, which enables a dissociation of N 2 0 3 into NO + and NO2, the ionization of N 2 0 4 (Eq. 8) is likely to take place as well. Such an ionization towards NO + and NO3 was suggested to occur on TiO2 [45,46]. Our case is complex, since two pathways that produce NO + and NO3 are conceivable. One is the formation of N 2 0 4 and the subsequent dissociation into NO + and NO3, and the other is the formation of NO + and NO2 via N 2 0 3 followed by the conversion of N O 2 t o N O 3 in the oxidizing atmosphere. With respect to the first pathway, it is known that the presence of NO and O2 at relatively low temperatures (<200°C) leads to some NOz formation, and the small N204 peak observed on E a N a M O R supports this assumption. On the other hand, the second pathway is also very likely, since we actually observed broad
462
E. Ito et al./Microporous Materials 4 (1995) 455-465
nitrato bands upon NO adsorption, which is most likely a result of conversion of nitrito to nitrato
species (Fig. 2d). With CeNaMOR, the observation of the nitrito peak ( O N O - ) upon the NO/O2 admission also supports this pathway (Fig. 4a). Our present results do not allow us to distinguish between the two possible schemes. At 300°C with CeNaMOR, most of the nitrato bands diminished and a new band appeared at 1418 cm-1, which can be assigned to nitrito species [20]. With L a N a M O R , a similar spectral change was observed, though it was less pronounced than in the case of CeNaMOR. The disappearance of nitrato bands and the simultaneous appearance of nitrito species suggests that nitrato species present at 100°C have transformed into nitrito losing oxygen at a temperature as high as 300-350°C. Another possibility for the formation of nitrito was suggested by Brandin et al. [15c], who proposed an equilibrium between NO + and O N O - , with the involvement of an oxide ion of the zeolite anion sites. However, there is no evidence for the validity of such an equilibrium at this moment. 4.5. Location o f N O + and the appearance o f absorptions at 1625 and around 3500 c m - 1
We have observed N O + (2160-2167 cm -1) on C e N a M O R and L a N a M O R , and this absorption band was also reported for H - M O R [15a]. It suggests that the formation of N O + may be related to the presence of the zeolite acid sites. Our results of N O - T P D agree with this assumption (Fig. 6 [17]). With H-MOR, there is only one NO desorption peak observed at 170°C, while L a N a M O R and C e N a M O R show the peak at 170°C in addition to another peak at 270°C. The desorption of NO at 170°C can, therefore, be linked with the presence of zeolite acid sites. This assumption was supported by a separate N O - T P D experiment with N a M O R , showing only a small peak at 170°C. Given the present IR results that NO + was the first species to desorb prior to NO:~ species upon a temperature raise, the low-temperature T P D peak at 170°C can be consistently linked with the observation of N O +. One might wonder where NO + ions could be located in the zeolite. The schematic zeolite picture
40 o
:~'\
30
/ /: \,
20
jJ'
/
'/'~
/ /
g o oz
10
//
,/
o t-'l,
100
,
, "-- . . . . . . 200 300
400
500
600
Temperature / °C Fig. 6. Temperature-programmed desorption of NO from CeNaMOR (--), LaNa-MOR ( ) or H-MOR (---). After a pretreatment, NO was adsorbed at 100°C, followed by a flush of argon. Desorption was carried out under argon at 5°C/min up to 560°C, and the product gas was analyzed by mass spectrometry.
presented by Kasai and Bishop [35] suggests that N O + is most probably associated with the framework anionic sites of the zeolite. In our case, the formation of N O + is conceivable in the following two ways: with Bronsted sites involved (Eq. 9) and with metal cationic species involved (Eq. 10). NO + N O 2 + [Ln~Oy(OH)z]" +...nZ- + H +Z-+NO+Z - + N O 2 + H + + [LnxOy(OHL-]"+...nZ (9) -,NO+Z + [LnxOy(OH)= (NO2)] ~"- ~)+...(n - 1 ) Z +H+Z -
(10)
where [LnxOy (OH)z]" + = lanthanide cationic oxo species and Z - = zeolite framework anion. According to Eq. 9, NO + forms on the zeolite framework anion site with the simultaneous relocation of an acidic proton. In this case, the H + ions may protonate metal cationic species: [Ln~Oy(OH)z] "+ together with coordination of NO~-, which leads either to the formation of water or/and coordinated hydroxide and a nitrito ligand, or to the formation of H N O 2 (nitrous acid) weakly associated with [LnxOy(OHL] "+. The absorptions observed at 3500 and 1625cm 1 can here be referred to, since these bands indicate the formation of water (3500 and 1635 cm-1) [25] or nitrous acid (cis: 3588, 1699, 1265 cm 1; trans: 3424, 1640,
1~ Ito et al./Microporous Materiab 4 (1995) 455 ~465
1261 cm ~; usually present in the mixed form) [26]. The observation of the third peak around 1260- 1265 cm -~ is expected for nitrous acid, however, it is difficult to distinguish a peak in this region, due to the strong zeolite band (1000 1250 cm ~). Eq. 10, by contrast, allows the formation of NO + without the involvement of zeolite acid sites. NO + can form on the zeolite framework anion site, which becomes available due to the decreased effective charge of metal cationic species upon a coordination of NO2. The essential difference between these two schemes (Eqs. 9 and 10) lies in the involvement of Bronsted acid sites. It should be noted that a negative peak at 3610cm -~ is observed in our spectra, which indicates the disappearance of acidic hydroxyl from the zeolite framework. We observed it together with the appearance of NO + , particularly in the case of C e N a M O R (Figs. 4 and 5); hence, NO + formation could be related with the disappearance of Bronsted sites as expected from Eq. 9. However, a quantitative correlation could not be established, probably due to a overlap of the 3610 cm 1 signal with a growing broad band at 3500cm ~.
4.6. Formation q[NO~, species and the role of metal cations Our IR data showed the two sorts of NOx species (NO + and NOx ) both on C e N a M O R and LaNaMOR, and NO + was found to desorb first upon the temperature increase. Compared to the N O - T P D results (Fig. 6), showing two desorption peaks with CeNaMOR and L a N a M O R , NO + is assumed to be related with the first NO desorption peak (170~C) and, thus, the NOx species should correspond to the second N O - T P D peak (270°C). H-MOR did not show the second N O - T P D peak, which indicates that metal cations present in zeolite may contribute to the stabilization of NOx ions by their higher coordination capacity. This increased stability of the NO~ coordination by metal cations may further lead to the enhancement of the total degree of ionization; the equilibria in Eqs. 7 and 8 could be driven more to the right. This is apparently reflected in the T P D results, where CeNaMOR and L a N a M O R exhibited
463
larger amounts of desorbed NO in total than the 100% H+-exchanged H-MOR. The pronounced ionization of nitrogen oxides by the presence of high-valent metal cations can also be understood in terms of the enhancement of the electrostatic fields of the zeolite. 4. 7. Role of redox properties of cerium
(CeUl/CeW ) in NO oxidation to NO2 In N O reduction with ammonia in the presence of oxygen, the specific low-temperature activity of CeNaMOR was assumed to originate from its higher activity in NO oxidation to NO2 (Eq. 1) in the low-temperature region. This reaction will, therefore, in some way involve the redox properties of cerium (Cem/CetV). The present IR studies performed with NO and 02 over CeNaMOR and L a N a M O R can be well related to the NO oxidation activities of the respective catalysts. As we have already discussed, NO adsorption onto C e N a M O R and L a N a M O R in both cases resulted in the formation of the same adsorbed species: N20, NO +, NO2 and NO3- indicating NO disproportionation and the subsequent ionization. Therefore, the redox properties of cerium (Cen~/Ce TM) are not essential for these consecutive reactions. Nevertheless, the following redoxinvolved NO disproportionation might have additionally taken place on CeNaMOR 2CenI+2NO+2H+~2Ce~V+N20+H20
(11)
which was proposed to account for the results of the NO-TPD of C e N a M O R [ 17]. The co-adsorption of NO and 02 and the subsequent temperature raise also resulted in the tbrmation of the same adsorbed species (NO +, N O 2 and NO3) on the two mordenite samples. However, differences were found in the formation/ desorption behavior of these adsorbed species. Upon the admission of NO and 02 at 10ffC, the formation of NOx species was more pronounced on C e N a M O R than on LaNaMOR. This suggests that N O ; is more easily formed on CeNaMOR than on LaNaMOR. However, it probably does not make an essential difference between CeNaMOR and L a N a M O R in NO oxidation carried out at 200-400 C [12], considering the
464
E. Ito et al./Microporous Materials 4 (1995) 455 465
comparable formation of NO + and NOx species on the two mordenites at 200°C (Figs. 4b and 5b). A more important difference was indicated in the temperatures required for decomposition of the NO+/NO2 species. The desorption of NO + started at 200°C on CeNaMOR, whereas NO + on LaNaMOR became smaller only at 300°C and, in addition, the NO~- species were found to be held more tightly on LaNaMOR than on CeNaMOR at 300-350°C. The easier desorption of NO + and NO2 species, which are actually the ion pairs of NO oxidation products (N203 and N204), indicates a higher turnover frequency of active sites with CeNaMOR. One could imagine the redox couple of cerium to be involved here in the following way, e.g. Cem+NO+l/202--* CelV(NO~-) ~ C e III+ NO2. Linking the present IR data with the CeNaMOR catalysis of NO reduction with NH3 in the presence of oxygen [12] is not straightforward due to the presence of the third component NH 3. However, we believe that two important observations here: the formation of the ionized NO oxidation products, NO + and NO2 (nitrito, nitrato) species, and their higher availability by easier desorption from CeNaMOR, could explain the efficient consumption of NOx species in NO reduction with NH3 in the presence of oxygen. In particular, the presence of the ionized pair NO+/NO2 observed here strongly indicates the involvement of a nitrosation-type reaction in the presence of NH 3. This is, in fact, equivalent to nitrosation of NH3 with NzO3, which is known to proceed at -85°C evolving N 2 [47].
5. Conclusion NO adsorption onto CeNaMOR and LaNaMOR leads to the disproportionation of NO towards N 2 0 and N 2 O 3 at room temperature, and N203 was found to ionize further into NO + (2160 cm-1, the nitrosonium ion) and NO~- (nitrito or/and nitrato, 1300-1500cm -1) species. The co-adsorption of NO and 02 on CeNaMOR and LaNaMOR resulted in the formation of NO + and NO 3 (nitrato). This can be accounted for by another equilibrium: 2NO +02~NzO4~-NO + _I_ NO3. It was shown that NO + is associated with
the lattice anionic sites of the zeolite and the NO~- species with exchanged cations. The difference between CeNaMOR and LaNaMOR was found in the desorption behavior of the NO + and NO;- species; they were easier to desorb from CeNaMOR than from LaNaMOR. This may contribute to the high turnover frequency of the NO oxidation reaction, and it could account for the higher activity of CeNaMOR for NO reduction with NH3 in the presence of oxygen.
Acknowledgements We acknowledge the financial support for VROM (Ministry of Housing and Environmental Affairs). We greatly appreciate Prof. Dr. Masakazu Iwamoto for his kind cooperation in the essential assignment of NO +. Dr. Marcello S. Rigutto is thanked for important suggestions.
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