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Characterization of NOx species in dehydrated and hydrated Na- and Ba-Y, FAU zeolites formed in NO2 adsorption
Journal of Electron Spectroscopy and Related Phenomena 150 (2006) 164–170
Characterization of NOx species in dehydrated and hydrated Na- and Ba-Y, FAU zeolites formed in NO2 adsorption J´anos Szanyi a,∗ , Ja Hun Kwak a , Sarah Burton b , Jose A. Rodriguez c , Charles H.F. Peden a a
Interfacial Chemistry and Engineering, Chemical Sciences Division, Pacific Northwest National Laboratory, 902 Battelle Blvd. MSIN K8-93, Richland, WA 99352, USA b High Field NMR Facility, Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, 902 Battelle Blvd. MSIN K8-93, Richland, WA 99352, USA c Department of Chemistry, Brookhaven National Laboratory, Upton, NY 11973, USA Available online 6 October 2005
Abstract Adsorbed ionic NOx species formed upon the interaction of NO2 with dehydrated or hydrated Na- and Ba-Y, FAU zeolites were characterized using FT-IR/TPD, solid state NMR, and XANES techniques. NO2 disproportionates on both dehydrated catalyst materials forming NO+ and NO3 − species. These ionic species are stabilized by their interactions with the negatively charged zeolite framework and the charge compensating cations (Na+ and Ba2+ ), respectively. Although the nature of the adsorbed NOx species formed on the two catalysts is similar, their thermal stabilities are strongly dependent on the charge compensating cations. In the presence of water in the channels of these zeolite materials new paths open for reactions between NO+ and H2 O, and NO2 and H2 O, resulting in significant changes in the adsorbed ionic species observed. These combined spectroscopic investigations afforded the understanding of the interactions between water and NO2 on these zeolite catalysts. © 2005 Elsevier B.V. All rights reserved. Keywords: NOx reduction; Adsorbed ionic NOx species; FT-IR; XANES; NMR
1. Introduction The removal of harmful gases from automotive exhaust streams is one of the major challenges facing the catalyst community. In particular, the reduction of NOx becomes increasingly challenging as lean-burn engines are gaining widespread use. Under oxygen rich operation, traditional three-way catalysts are unable to reduce NOx due, primarily, to their high hydrocarbon oxidation activity, which deprives the catalyst of an effective reducing agent. Today, new approaches are being developed that will enable us to comply with regulations which require decreasing NOx levels around the globe. Non-thermal plasma-assisted selective catalytic NOx reduction is one the new technologies that has emerged in the past several years as a potential candidate for diesel engine exhaust control. Early studies in our laboratory [1,2] have shown that alkali and alkaline earth ion exchanged Y, FAU (Y) zeolites are effective catalysts for the selective reduction of NOx when they are used in conjunction with non-thermal
∗
Corresponding author. E-mail address: [email protected] (J. Szanyi).
0368-2048/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2005.05.007
plasma. In the process the exhaust gases exiting the diesel engine are reformed in the plasma reactor, and this reformed gas stream is then passed through the zeolite catalyst where the actual NOx reduction takes place. The role of the plasma [3,4] is two-fold, i.e. oxidation of some of the hydrocarbons to more reactive partial oxygenates (e.g. aldehydes), and the complete conversion of NO into NO2 . Thus the NOx reduction process actually involves the reduction of NO2 with these partially oxidized hydrocarbons. In previous studies [5,6] we have shown that Na- and Ba-Y zeolites are the two most active catalysts for this process in the temperature range of interest (450–650 K). The NOx reduction activity of Ba-Y was especially high (>80%) in the temperature regime of 450–520 K. In order to elucidate the high activities of these materials we have initially investigated their NOx adsorption properties. In volumetric adsorption studies [5] we found that the amount of NO2 adsorbed was approximately twice as much on Ba-Y than on Na-Y. Our goal was to find some correlation between NOx reduction efficiency and NO2 adsorption properties. In our FTRIR studies we have shown [7,8] that the adsorbed species formed upon NO2 exposure are very similar over both Na- and Ba-Y zeolites. Adsorption of NO2 at room temperature results
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in the formation of ionic NOx species, NO+ and NO3 − ion pairs in the absence of water. Although the chemical nature of the adsorbed NOx species were very similar (the same NO+ , NO2 − , and NO3 − ions) on the two catalysts studied, their thermal stabilities and reactivities with water were very different [8]. The adsorbed NOx are held stronger in Ba-Y than in Na-Y that is evidenced by the higher NO2 desorption temperature for Ba-Y in comparison with Na-Y. Adsorbed water also has a dramatic effect on the adsorption of NO2 on both of these materials. We have reported [8] how the presence of water influences some of the adsorbed ionic species (in particular NO+ ). In the presence of water HONO/HNO3 formed in the reactions of adsorbed NOx species with H2 O. The rich chemistry that takes place on these catalysts during NO2 adsorption with or without the presence of water warranted further investigations into the adsorption of NO2 . In this study we applied FT-IR/TPD, 15 N solid state NMR, and XANES to further our understanding of the NO2 adsorption process. The results of these studies confirm our previously reported model on the adsorption of NO2 [7] and the effect of water on NO2 adsorption [8] on Na- and Ba-Y zeolites. 2. Experimental The preparation of the catalyst samples investigated in these studies was discussed in a previous publication [5]. Briefly, the Na-Y powder was obtained from Zeolyst, and used as received. The Ba-Y sample was prepared with aqueous Baacetate solution in the following fashion: the parent Na-Y was ion exchanged twice with the Ba-containing solution, and after each ion exchange step the sample was calcined at 773 K for 4 h. This method of sample preparation afforded the maximization of the Ba ion exchange level. The number of Ba2+ ions in this sample is about 25/unit cell, very close to the theoretically achievable maximum of 27/unit cell (at Si/Al = 2.5). The crystallinity of the catalysts was confirmed by XRD. No extra framework alumina species were detected by 27 Al NMR. 15 N solid state NMR spectra were acquired on a Varian/Chemagnetics CMX Infinity 300 MHz instrument, equipped with a Varian/Chemagnetics 7.5 mm HX MAS probe operating at a spectral frequency of 30.40651 MHz. In the in situ NMR cell 0.5 g of Na-Y or Ba-Y sample was pre-treated under vacuum (<1 × 10−7 Torr) at 773 K for 2 h in order to remove adsorbed water. After dehydration, the sample was cooled down to room temperature for NO2 adsorption. The amount of NO2 adsorbed was kept around the chemisorption capacity (∼4NO2 /supercage in Ba-Y, and ∼2NO2 /supercage in Na-Y), which was achieved by allowing equilibration with the NO2 gas for at least 1 h. The NO2 -saturated Na- and Ba-Y samples were transferred into the gas-tight rotor (7.5 mm O.D.) for solid state 15 N-NMR measurements. All 15 N NMR spectra were externally referenced to 15 N-ammonium chloride at 0 ppm. All spectra were obtained spinning at 5 kHz and using a 10 s recycle delay. The XANES experiments were carried out at the beam line U7A of the national synchrotron light source (NSLS), Brookhaven National Laboratory. The spectra were collected in the electron yield mode by using a channeltron multiplier near
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the powder sample surface. The energy resolution of the instrument was 0.3–0.4 eV. For the XANES experiments the zeolite samples were dehydrated at 700 K for 2 h prior to NO2 adsorption at room temperature. In the NO2 desorption experiments the samples were held at the indicated anneal temperature for 30 s and then cooled down to room temperature for taking the N K-edge XANES spectra. As standards, KNO2 and KNO3 were used for the identification of the nitrogen in nitrite, and nitrate species, respectively. The FT-IR measurements were carried out with a Mattson Research series spectrometer equipped with and MCT detector, operating at 4 cm−1 resolution. Each spectrum was the average of 64 scans and referenced to a background spectrum recorded with the activated zeolite sample in the IR beam. The sample was mounted onto a fine tungsten mesh, which in turn, was attached to copper heating leads on the sample holder rod. A chromel/alumel thermocouple was spot-welded to the top center of the tungsten mesh that allowed us to monitor the temperature of the resistively heated sample throughout the measurements. The IR cell itself was a 2(3/4) stainless steel cube equipped with CaF windows, and connected to a gas handling sytem, pumping station, and a mass spectrometer (UTI 100). This setup allowed us to monitor the adsorbed NOx species, and to carry out temperature programmed desorption (TPD) experiments. Prior to each IR/TPD experiments the sample was dehydrated by annealing at 773 K for 4 h in vacuum (<1 × 10−7 Torr). In order to minimize the effect of water (re-adsorption from the walls of the IR cell and the gas manifold) on the room temperature adsorption of NO2 on these zeolite samples, the system was baked out overnight. 3. Results and discussion 3.1. FT-IR 3.1.1. NO2 Adsorption on dehydrated samples Adsorbed NOx species formed upon the interaction of NO2 with Na- and Ba-Y samples were characterized first by FT-IR spectroscopy. As we have mentioned in Section 2 extra care had to be taken for the preparation of fully dehydrated zeolite samples since zeolites are able to hold water strongly in their pore structure. Annealing the samples at 773 K for extended periods of times and baking out the IR cell and the connecting gas lines ensured the preparation of almost fully dehydrated samples. IR spectra obtained upon the adsorption of NO2 on the fully dehydrated Na- and Ba-Y zeolites at room temperature are displayed in Fig. 1A and B, respectively. The IR spectra of adsorbed NOx species on these two samples are very similar in their character. There is a broad feature between 1300 and 1500 cm−1 , and a doublet band in the 2000–2200 cm−1 spectral region. As we have discussed it in great detail previously [7,8], the broad band at the lower frequency range (∼1500 cm−1 ) represents the νNO vibrations of NO3 − species, while the lower frequency portion of the doublet represents νNO vibrations of the adsorbed NO+ species (2020 cm−1 in Na-Y and 2050 cm−1 in Ba-Y). The higher frequency portion of these doublets was assigned to νNO vibrations of NO+ species in NO+ NO2 or NO+ N2 O4
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Zeo− represents the negatively charged zeolite framework that is neutralized by the cation (Mex+ ) Na+ or Ba2+ in this case. The adsorption and desorption of NO2 is reversible, as only NO2 desorbs from these fully dehydrated samples in TPD experiments carried out following NO2 saturation at room temperature [8]. Although the adsorbed NOx species are very similar in the two zeolites studied here, their thermal properties are different. NO2 desorbs in a single desorption feature at ∼370 K from Na-Y as NO+ and NO3 − recombines to NO2 . On the other hand, two desorption features are observed in the TPD spectrum of NO2 in Ba-Y [8]. The lower temperature peak is at 370 K, and the other one is at ∼470 K. The first peak represents NO2 desorption from the NO+ NO2 adducts, while the high temperature feature arises from the recombinative desorption of NO2 from NO+ and NO3 − . In both cases (Na- and Ba-Y) the IR bands representing NO+ and NO3 − lose their intensities simultaneously, and disappear at 370 and 470 K in Na- and Ba-Y, respectively. In the IR spectrum obtained after NO2 adsorption on the dehydrated Ba-Y, a small vibrational feature corresponding to νOH vibrations can be seen at 3620 cm−1 . The appearance of this peak can be regarded as an indicator of non-perfect dehydration of the zeolite material, as we will discuss it below. 3.1.2. NO2 adsorption on hydrated Na- and Ba-Y The IR spectra following NO2 adsorption on hydrated Naand Ba-Y zeolites are significantly different from those obtained from the dehydrated samples. In these experiments water was added to the fully dehydrated samples, and then NO2 was introduced into the system. The most significant difference in the IR spectra between the hydrated and dehydrated samples is the complete absence of the NO+ species in the spectra of the hydrated samples (see Fig. 1). Also, several new features in the νOH vibrational region can be seen on these hydrated samples. In the hydrated Na-Y, a new IR absorption band developed at 1270 cm−1 upon exposure to NO2 . This band represents νNO vibration of NO2 − species formed in the reaction of NO+ originally formed in reaction (1) and H2 O: NO+ + H2 O ⇔ H+ + HONO
(3)
adducts. These adducts are only formed in the presence of excess NO2 /N2 O4 . The adsorption of NO2 occurs by the disproportionation into NO+ and NO3 − and the subsequent stabilization of the thus formed ionic species in the ionic sites of the zeolites:
The same species is formed on Ba-Y, but the frequency of the νNO vibration of the NO2 − species is either too low to be seen due to the limited transmittance of this sample below 1220 cm−1 , or it overlaps with the broad NO3 − bands. Its formation, however, is indirectly evidenced by the appearance of the νOH band of HONO at 3620 cm−1 . In both materials the formation of NO+ is completely eliminated by the presence of water, while the formation of NO3 − is just as facile as it was on the fully dehydrated samples. Nitrate species can form in two different ways in these hydrated zeolites; in the disproportionation of NO2 in accordance with Eq. (1), and in the direct reaction between NO2 and H2 O:
2NO2 ⇔ NO+ + NO3 −
3NO2 + H2 O ⇔ 2HNO3 + NO
Fig. 1. IR spectra following room temperature adsorption of NO2 on dehydrated (red) and hydrated (blue) Na-Y (A) and Ba-Y (B).
(1)
Zeo− · · · Mex+ + NO+ + NO3 − ⇔ Zeo− NO+ · · · NO3 − Mex+ (2)
(4)
(The formation of NO in this reaction was substantiated by mass spectrometry analysis of the gas phase.) The thus formed HNO3 can interact with Mex+ ions in cationic positions to form
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Br¨onsted acidic OH groups and Ba-nitrate species. This process is evident from the series of IR spectra recorder with increasing amounts of NO2 (not shown). First the νOH vibrational band of HONO develops and, as the amount of NO2 introduce into the cell increases the zeolitic OH groups at 3645 and 3550 cm−1 develop. We have also observed that at high NO2 pressures in Na-Y both the 3620 cm−1 (νOH of HONO) and 1270 cm−1 (νNO of HONO) features disappear, as the zeolitic OH groups develop. This suggests that in the presence of excess NO2 , HONO can be oxidized to HNO3 due to the high oxidizing power of NO2 . This process is also accompanied by the release of an NO molecule: HONO + NO2 ⇔ HNO3 + NO
(5)
The thermal stabilities of the NOx species formed on the hydrated samples differ significantly from those we have seen for the fully dehydrated ones. As we discussed in detail elsewhere [8], on both zeolites there is a large shift in the NO2 desorption temperature to higher values. The single NO2 desorption feature seen at 370 K on Na-Y is transformed into a band centered at ∼470 K. On Ba-Y we have also shown this transformation in the high temperature feature to even higher temperatures (from 470 to 650 K), while we have seen hardly any change in the lower temperatures TPD peak [8]. The experiments, however, were carried out in somewhat different manner in that study, i.e. the samples were first saturated with NO2 followed by H2 O adsorption. The sequence of adsorption (NO2 or H2 O first) does not affect either the IR or the TPD spectra in Na-Y, since the thermal stability of NO+ NO2 in this material is low, and it is removed by simple room temperature evacuation. Therefore, the incoming water can interact only with the adsorbed NO+ species. On BaY, however, the stability of the NO+ NO2 adduct is much higher, and it is not removed by simple evacuation at room temperature. Thus, when water is introduced into the IR cell it can interact with both NO+ and NO+ NO2 adducts. We can see that water easily reacts with NO+ just as it does on Na-Y, however, the NO+ in the NO+ NO2 adduct seem to be protected from the reaction with H2 O. This is the reason why we see the lower temperature TPD feature unchanged after H2 O addition onto the NO2 -saturated Ba-Y sample. However, when we adsorb NO2 onto the hydrated Ba-Y sample the results are different. In this case we cannot form the NO+ NO2 adduct, since the NO+ that is formed in the NO2 disproportionation readily reacts with the water already present in the zeolite. This would also explain the different IR spectra seen in the two cases, which differ only in the order of NO2 and H2 O adsorption. 3.2. XANES N K-edge XANES spectra recorded from hydrated and dehydrated Ba-Y samples are displayed in Fig. 2 together with the reference spectra from KNO2 , and KNO3 . These two commonly used standards show the electronic transitions within the NO2 − and NO3 − ions. In the KNO3 standard the N Kedge has an intense resonance at ∼406.5 eV, which can be assigned to 1s → 2a2 () electronic transitions. The features in the 410–420 eV region may come from excitations into the 5a1
Fig. 2. N K-edge XANES spectra for dehydrated and hydrated Ba-Y exposed to NO2 at 300 K; also included are the spectra for the KNO2 and KNO3 standards. The N K-edge spectra were taken after exposing the sample to 5 Torr of NO2 at 300 K for 15 min.
and 5e empty orbitals of the nitrate ions [9]. The intense absorption feature in the KNO2 standard at ∼402.5 eV represents 1s → 2b1 () electronic transitions, while the broad feature between 407 and 417 eV arises from the excitations into the 7a1 and 5b2 vacant orbitals of NO2 − [9]. In the N K-edge XANES spectrum of the “dehydrated” Ba-Y absorption features characteristic of both the NO2 − and NO3 − ions can be seen. The intensity of the absorption feature arising from the nitrate ion is significantly higher than that of the nitrite ion. Beside these two absorption bands a third resonance feature is also present at an even lower photon energy of ∼399.5 eV on this dehydrated sample. In the adsorption of NO2 on a defect rich TiO2 (1 1 0) surface Rodriguez et al. [10] assigned the small absorption resonance at ∼399 eV to adsorbed atomic nitrogen. On highly reduced Ce0.8 Zr0.2 O2−x (1 1 1) a feature proposed to represent adsorbed atomic nitrogen was observed at ∼399.5 eV [11]. These adsorbed N atoms were stable on the TiO2 (1 1 0) surface up to at least 400 K, and it was observed even at 900 K on the Ce0.8 Zr0.2 O2−x (1 1 1) surface. In the adsorption of NO2 on polycrystalline metallic magnesium, atomic nitrogen was shown to give a weak absorption resonance at around 396.5 eV [12]. In our case the formation of atomic nitrogen produced by the direct dissociation of NO2 is not operational. However, as we
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have shown above in the IR results, NO+ NO3 − ion pairs form in the disproportionation of NO2 and stabilized by their interactions with the negatively charged zeolite framework and the charge compensating cations, respectively. Therefore, we assign the absorption feature at 399.5 eV to adsorbed NO+ ions. The question that remains to be answered is why we see the resonance feature at 402.5 eV in the dehydrated Ba-Y sample. This feature represents NO2 − ions, which are not observed in the IR spectra on the fully dehydrated samples. However, prior to XANES measurements the sample was annealed only at 700 K for 2 h. At this temperature there are still significant amount of water is left in the channels of the zeolite. As we have discussed above in the IR section, prolonged heating at 773 K and careful bakeout of the system was required to obtain a fully dehydrated zeolite sample. Since water was probably still present in the zeolite structure in these XANES measurements on the “dehydrated” sample, reaction (4) could take place resulting in the formation of all three ionic species (NO+ , NO3 − , and NO2 − ). According to reaction (4), NO is also produced in the reaction between H2 O and NO2 , therefore one might assign the 399.5 eV XANES feature to adsorbed NO. However, in our FT-IR studies we have never observed any adsorption of NO on these zeolite samples, not even when in the presence of pure NO. In the adsorption of NO2 on hydrated Ba-Y, only two absorption resonances are observed in the XANES spectra, the ones that represent NO3 − (406.5 eV) and NO2 − (402.5 eV). These results are in full agreement with our IR observations: the presence of water prevents the adsorption of NO+ by reaction (3). We have also mentioned in our discussion of NO2 adsorption on hydrated Na-Y that in the presence of a large excess of NO2 some of the NO2 − may be oxidized to NO3 − . This might be the reason of the different NO3 − /NO2 − XANES resonance feature intensity ratios between the “dehydrated” and hydrated samples. The XANES spectra obtained in the NO2 adsorption on dehydrated and hydrated Na-Y zeolites are very similar to those reported here for Ba-Y, and are not shown for brevity. The main difference between the spectra is the significantly reduced intensities for all the absorption features in Na-Y in comparison to those in Ba-Y. This reflects the fact that the NO2 sorptoin capacity of Ba-Y is approximately twice higher than that of Na-Y [5]. The relative intensities of the NOx species observed on Na- and Ba-Y after room temperature NO2 adsorption are compared in Fig. 3 for the dehydrated (panel A) and hydrated (panel B) samples. (The different intensity ratios of NO3 − /NO2 − observed for Ba- and Na-Y may arise from the different level of dehydration of the two samples.) 3.3. NMR Solid state 15 N NMR spectra obtained from the Na- and Ba-Y samples after 15 NO2 exposure at room temperature are displayed in Fig. 4. Adsorption of 15 NO2 onto dehydrated Na-Y results in the appearance of two peaks in the NMR spectrum at 336 and 370 ppm chemical shifts, relative to 15 N-ammonium chloride at 0 ppm. The feature at 336 ppm chemical shift is very close to that reported for nitrate ions [13]. The other peak at
Fig. 3. Relative intensities of the characteristic resonance peaks on the dehydrated (A) and hydrated (B) Na- and Ba-Y zeolites exposed to NO2 at 300 K.
higher chemical shift might be assigned to the adsorbed 15 NO+ species formed upon the disproportionation of NO2 and subsequent adsorption on the negatively charged zeolite framework. The nitrate feature in 15 NO2 -saturated, dehydrated Ba-Y is at very similar chemical shift (334 ppm) as it was seen in Na-Y. However, at around 340 ppm chemical shift a small feature can be distinguished in the Ba-Y case that was not seen in Na-Y. The position of this peak (∼340 ppm) is very close to that of the nitrate ions (334 ppm), therefore, we assign it to a nitrate ion that is adsorbed onto Ba ions that are occupying different sites in the zeolite. This altered chemical environment around the different cationic sites can explain the appearance of the two nitrate features at close proximity. On the other hand, on Na-Y we are unable to distinguish among nitrate ions co-ordinated to Na ions in different crystallographic positions. This may originate form the significantly broader NMR nitrate peak in the Na-Y case, or, more probably, from the much more homogeneous average chemical environments around the much smaller Na ions. The NMR spectra changed dramatically when water was added onto the 15 NO2 -saturated Na- and Ba-Y samples. In both cases, the NMR peak we have assigned to adsorbed 15 NO+ species completely disappeared, while feature of adsorbed 15 NO − became extremely sharp upon H O addition. There 3 2 were only very small changes in the positions of the NMR features of adsorbed nitrates, which, in part can be explained by the
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not been used in NMR studies of these reactions. The primary problem of using NMR for the investigation of NOx adsorption is the fact that neutral species of both 15 NO and 15 NO2 are invisible by NMR spectroscopy because they are paramagnetic. Therefore, we can expect to see only charged NOx species. In one of the rare 15 N NMR studies Wu and Larsen [15] reported two NMR features at −2 and 36 ppm chemical shifts (relative to neat CH3 NO2 ) after they contacted a Na-ZSM5 zeolite sample with a propane + 15 NO + O2 gas mixture at room temperature. They assigned these peaks to Na15 NO3 (−2 ppm) and CH3 O15 NO2 (36 ppm). The chemical shift difference between these two features (38 ppm) is very close to that we see for dehydrated Na-Y (34 ppm) after 15 NO2 exposure. However, in our case there is no hydrocarbon present in the system that could result in the formation of an organic nitrate species. Also, they only observe the presence of the NMR peak at 36 ppm upon the interaction of the Na-ZSM5 with the reactant gas mixture at 298 K. This temperature seems to be too low to expect any reaction between the hydrocarbon and NO + O2 (mainly since they use propane as a reductant instead of propene). In fact, when they contacted the gas mixture with the Na-ZSM5 at 423 K and then looked at the surface at room temperature with NMR, they observed only the peak at −2 ppm. We would expect to see even more of the species represented by the 36 ppm feature when the reaction is run at higher temperature. After the 423 K reaction the NMR feature at 36 ppm chemical shift is completely absent, while the peak representing NO3 − is still there with high intensity. Thus, we conclude that in their case on Na-ZSM5 at room temperature NO readily reacts with O2 to form NO2 , which then can adsorb on the catalyst as NO+ NO3 − ion pairs and propane is just present as a non-reactive spectator. We believe that the NMR feature they reported at 36 ppm can be assigned to the adsorbed, charged NOx species of NO+ . This is in agreement with our NMR results on the dehydrated zeolite samples and is confirmed by our FT-IR findings as well. 4. Conclusions Fig. 4. 15 N MAS NMR spectra of 15 NO2 on Na- and Ba-Y zeolites at 300 K. (Red spectra: dehydrated samples; blue spectra: hydrated samples.)
much sharper peaks in the hydrated samples. The broad peaks recorded from the dehydrated samples make the peak identification less accurate. As we have discussed above in the IR section, water can readily react with adsorbed NO+ in accordance with reaction (3). In the presence of a large amount of NO2 , however, HONO can be converted to HNO3 resulting in the complete absence of a 15 NO2 − NMR feature in either of the two samples. The sharpening of the nitrate NMR features is a consequence of the co-ordination of H2 O molecules around these adsorption complexes. In Ba-Y we can also observe the much better resolution of the two different nitrate peaks (335 and 340 ppm) on this hydrated sample. Isotopically labeled NOx compounds are routinely used in FT-IR spectroscopy for the identification of adsorbed surface species and reaction intermediates in NOx reduction catalysis [14]. However, 15 N-labeled NO2 , to our best knowledge, have
Using a variety of spectroscopic techniques (FT-IR, XANES, solid state NMR), we were able to identify unambiguously the processes that take place in NO2 adsorption on the basic zeolites of Na- and Ba-Y. On the fully dehydrated zeolite samples NO2 can only adsorb as NO+ NO3 − ion pairs that form in the NO2 disproportionation (Eq. (1)). The NO+ and NO3 − ionic species formed are stabilized by their interactions with the negatively charged zeolite framework, and the charge compensating cations, respectively. In TPD/FT-IR experiments we see the desorption of only NO2 from these fully dehydrated samples, and the IR features representing these two ionic species disappear simultaneously. The situation becomes more complex when NO2 is adsorbed onto hydrated zeolites or when H2 O is added to the NO2 -saturated zeolite samples. In the presence of H2 O in the zeolite channels the NO+ formed in the NO2 disproportionation can react with water to form HONO. Furthermore, direct reaction between NO2 and H2 O is facile as well (Eq. (4)), and results in the formation of HNO3 and NO. The thus formed HNO3 can participate in an ion exchange process that 15 N
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ultimately lead to the formation of Br¨onsted acidic OH groups. There is significant difference in the adsorbed species on Ba-Y depending on the order of adsorption. When H2 O is adsorbed on Ba-Y prior to NO2 adsorption the formation of the NO+ NO2 adduct is inhibited as evidenced by the complete absence of the IR features representing these species. However, in the reverse case (NO2 first, followed by H2 O adsorption) the signature of NO+ readily disappears from the IR spectrum as this species reacts with the incoming water, and under the same circumstances the NO+ NO2 adduct remains almost unchanged (reacts slowly with H2 O) as NO2 seems to protect NO+ form the reaction with water. This is not the case in Na-Y, since the thermal stability of the NO+ NO2 adduct is much lower in this material, and is completely decomposed to NO+ (ads.) and NO2 (gas) upon room temperature evacuation. Therefore, the remaining NO+ (NO2 adsorption first) or the NO+ formed in NO2 disproportionation (H2 O adsorption first) readily reacts with water to produce HONO. Acknowledgements The FT-IR and NMR experiments were carried out at the Pacific Northwest National Laboratory (PNNL). PNNL is operated for the US DOE by Battelle Memorial Institute under contract no. DE-AC067RLO1831. Funding for this work was provided by the DOE, Office of Basic Energy Sciences, Division
of Chemical Sciences. The XANES data were collected at the NSLS in Brookhaven National Laboratory; a facility supported by the Division of Materials Science, and Chemical Sciences of DOE. References [1] M.L. Balmer, R. Tonkyn, S. Yoon, A. Kolwaite, S. Barlow, G. Maupin, J.W. Hoard, SAE 1999-01-3640. [2] A.G. Panov, R.G. Tonkyn, M.L. Balmer, C.H.F. Peden, A. Malkin, J.W. Hoard, SAE 2001-01-3513. [3] C.R. MaLarnon, B.M. Penetrante, SAE 982433. [4] B.M. Penetrante, R.M. Brusasco, B.T. Merritt, W.J. Pitz, G.E. Vogtlin, M.C. Kung, H.H. Kung, C.Z. Wan, K.E. Voss, SAE 983508. [5] J.H. Kwak, J. Szanyi, C.H.F. Peden, J. Catal. 220 (2003) 291. [6] J.H. Kwak, J. Szanyi, C.H.F. Peden, Catal. Today 89 (2004) 135. [7] J. Szanyi, J.H. Kwak, R.A. Moline, C.H.F. Peden, PCCP 5 (18) (2003) 4045. [8] J. Szanyi, J.H. Kwak, C.H.F. Peden, J. Phys. Chem. B 108 (2004) 3746. [9] J.A. Rodriguez, Surf. Sci. 230 (1990) 335. [10] J.A. Rodriguez, T. Jirsak, G. Liu, J. Hrbek, J. Dvorak, A. Maiti, J. Am. Chem. Soc. 123 (2001) 9597. [11] G. Liu, J.A. Rodriguez, J. Hrbek, J. Dvorak, J. Phys. Chem. B. 105 (2001) 7762. [12] J.A. Rodriguez, T. Jirsak, S. Sambasivan, D. Fischer, A. Maiti, J. Chem. Phys. 112 (2000) 9929. [13] K.L. Anderson-Altmann, D.M. Grant, J. Phys. Chem. 97 (1993) 11096. [14] K.I. Hadjiivanov, J. Suassy, J.L. Freisz, J.C. Lavalley, Catal. Lett. 52 (1998) 103. [15] J. Wu, S.C. Larsen, Catal. Lett. 70 (2000) 43.
Characterization of NOx species in dehydrated and hydrated Na- and Ba-Y, FAU zeolites formed in NO2 adsorption J´anos Szanyi a,∗ , Ja Hun Kwak a , Sarah Burton b , Jose A. Rodriguez c , Charles H.F. Peden a a
Interfacial Chemistry and Engineering, Chemical Sciences Division, Pacific Northwest National Laboratory, 902 Battelle Blvd. MSIN K8-93, Richland, WA 99352, USA b High Field NMR Facility, Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, 902 Battelle Blvd. MSIN K8-93, Richland, WA 99352, USA c Department of Chemistry, Brookhaven National Laboratory, Upton, NY 11973, USA Available online 6 October 2005
Abstract Adsorbed ionic NOx species formed upon the interaction of NO2 with dehydrated or hydrated Na- and Ba-Y, FAU zeolites were characterized using FT-IR/TPD, solid state NMR, and XANES techniques. NO2 disproportionates on both dehydrated catalyst materials forming NO+ and NO3 − species. These ionic species are stabilized by their interactions with the negatively charged zeolite framework and the charge compensating cations (Na+ and Ba2+ ), respectively. Although the nature of the adsorbed NOx species formed on the two catalysts is similar, their thermal stabilities are strongly dependent on the charge compensating cations. In the presence of water in the channels of these zeolite materials new paths open for reactions between NO+ and H2 O, and NO2 and H2 O, resulting in significant changes in the adsorbed ionic species observed. These combined spectroscopic investigations afforded the understanding of the interactions between water and NO2 on these zeolite catalysts. © 2005 Elsevier B.V. All rights reserved. Keywords: NOx reduction; Adsorbed ionic NOx species; FT-IR; XANES; NMR
1. Introduction The removal of harmful gases from automotive exhaust streams is one of the major challenges facing the catalyst community. In particular, the reduction of NOx becomes increasingly challenging as lean-burn engines are gaining widespread use. Under oxygen rich operation, traditional three-way catalysts are unable to reduce NOx due, primarily, to their high hydrocarbon oxidation activity, which deprives the catalyst of an effective reducing agent. Today, new approaches are being developed that will enable us to comply with regulations which require decreasing NOx levels around the globe. Non-thermal plasma-assisted selective catalytic NOx reduction is one the new technologies that has emerged in the past several years as a potential candidate for diesel engine exhaust control. Early studies in our laboratory [1,2] have shown that alkali and alkaline earth ion exchanged Y, FAU (Y) zeolites are effective catalysts for the selective reduction of NOx when they are used in conjunction with non-thermal
∗
Corresponding author. E-mail address: [email protected] (J. Szanyi).
0368-2048/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2005.05.007
plasma. In the process the exhaust gases exiting the diesel engine are reformed in the plasma reactor, and this reformed gas stream is then passed through the zeolite catalyst where the actual NOx reduction takes place. The role of the plasma [3,4] is two-fold, i.e. oxidation of some of the hydrocarbons to more reactive partial oxygenates (e.g. aldehydes), and the complete conversion of NO into NO2 . Thus the NOx reduction process actually involves the reduction of NO2 with these partially oxidized hydrocarbons. In previous studies [5,6] we have shown that Na- and Ba-Y zeolites are the two most active catalysts for this process in the temperature range of interest (450–650 K). The NOx reduction activity of Ba-Y was especially high (>80%) in the temperature regime of 450–520 K. In order to elucidate the high activities of these materials we have initially investigated their NOx adsorption properties. In volumetric adsorption studies [5] we found that the amount of NO2 adsorbed was approximately twice as much on Ba-Y than on Na-Y. Our goal was to find some correlation between NOx reduction efficiency and NO2 adsorption properties. In our FTRIR studies we have shown [7,8] that the adsorbed species formed upon NO2 exposure are very similar over both Na- and Ba-Y zeolites. Adsorption of NO2 at room temperature results
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in the formation of ionic NOx species, NO+ and NO3 − ion pairs in the absence of water. Although the chemical nature of the adsorbed NOx species were very similar (the same NO+ , NO2 − , and NO3 − ions) on the two catalysts studied, their thermal stabilities and reactivities with water were very different [8]. The adsorbed NOx are held stronger in Ba-Y than in Na-Y that is evidenced by the higher NO2 desorption temperature for Ba-Y in comparison with Na-Y. Adsorbed water also has a dramatic effect on the adsorption of NO2 on both of these materials. We have reported [8] how the presence of water influences some of the adsorbed ionic species (in particular NO+ ). In the presence of water HONO/HNO3 formed in the reactions of adsorbed NOx species with H2 O. The rich chemistry that takes place on these catalysts during NO2 adsorption with or without the presence of water warranted further investigations into the adsorption of NO2 . In this study we applied FT-IR/TPD, 15 N solid state NMR, and XANES to further our understanding of the NO2 adsorption process. The results of these studies confirm our previously reported model on the adsorption of NO2 [7] and the effect of water on NO2 adsorption [8] on Na- and Ba-Y zeolites. 2. Experimental The preparation of the catalyst samples investigated in these studies was discussed in a previous publication [5]. Briefly, the Na-Y powder was obtained from Zeolyst, and used as received. The Ba-Y sample was prepared with aqueous Baacetate solution in the following fashion: the parent Na-Y was ion exchanged twice with the Ba-containing solution, and after each ion exchange step the sample was calcined at 773 K for 4 h. This method of sample preparation afforded the maximization of the Ba ion exchange level. The number of Ba2+ ions in this sample is about 25/unit cell, very close to the theoretically achievable maximum of 27/unit cell (at Si/Al = 2.5). The crystallinity of the catalysts was confirmed by XRD. No extra framework alumina species were detected by 27 Al NMR. 15 N solid state NMR spectra were acquired on a Varian/Chemagnetics CMX Infinity 300 MHz instrument, equipped with a Varian/Chemagnetics 7.5 mm HX MAS probe operating at a spectral frequency of 30.40651 MHz. In the in situ NMR cell 0.5 g of Na-Y or Ba-Y sample was pre-treated under vacuum (<1 × 10−7 Torr) at 773 K for 2 h in order to remove adsorbed water. After dehydration, the sample was cooled down to room temperature for NO2 adsorption. The amount of NO2 adsorbed was kept around the chemisorption capacity (∼4NO2 /supercage in Ba-Y, and ∼2NO2 /supercage in Na-Y), which was achieved by allowing equilibration with the NO2 gas for at least 1 h. The NO2 -saturated Na- and Ba-Y samples were transferred into the gas-tight rotor (7.5 mm O.D.) for solid state 15 N-NMR measurements. All 15 N NMR spectra were externally referenced to 15 N-ammonium chloride at 0 ppm. All spectra were obtained spinning at 5 kHz and using a 10 s recycle delay. The XANES experiments were carried out at the beam line U7A of the national synchrotron light source (NSLS), Brookhaven National Laboratory. The spectra were collected in the electron yield mode by using a channeltron multiplier near
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the powder sample surface. The energy resolution of the instrument was 0.3–0.4 eV. For the XANES experiments the zeolite samples were dehydrated at 700 K for 2 h prior to NO2 adsorption at room temperature. In the NO2 desorption experiments the samples were held at the indicated anneal temperature for 30 s and then cooled down to room temperature for taking the N K-edge XANES spectra. As standards, KNO2 and KNO3 were used for the identification of the nitrogen in nitrite, and nitrate species, respectively. The FT-IR measurements were carried out with a Mattson Research series spectrometer equipped with and MCT detector, operating at 4 cm−1 resolution. Each spectrum was the average of 64 scans and referenced to a background spectrum recorded with the activated zeolite sample in the IR beam. The sample was mounted onto a fine tungsten mesh, which in turn, was attached to copper heating leads on the sample holder rod. A chromel/alumel thermocouple was spot-welded to the top center of the tungsten mesh that allowed us to monitor the temperature of the resistively heated sample throughout the measurements. The IR cell itself was a 2(3/4) stainless steel cube equipped with CaF windows, and connected to a gas handling sytem, pumping station, and a mass spectrometer (UTI 100). This setup allowed us to monitor the adsorbed NOx species, and to carry out temperature programmed desorption (TPD) experiments. Prior to each IR/TPD experiments the sample was dehydrated by annealing at 773 K for 4 h in vacuum (<1 × 10−7 Torr). In order to minimize the effect of water (re-adsorption from the walls of the IR cell and the gas manifold) on the room temperature adsorption of NO2 on these zeolite samples, the system was baked out overnight. 3. Results and discussion 3.1. FT-IR 3.1.1. NO2 Adsorption on dehydrated samples Adsorbed NOx species formed upon the interaction of NO2 with Na- and Ba-Y samples were characterized first by FT-IR spectroscopy. As we have mentioned in Section 2 extra care had to be taken for the preparation of fully dehydrated zeolite samples since zeolites are able to hold water strongly in their pore structure. Annealing the samples at 773 K for extended periods of times and baking out the IR cell and the connecting gas lines ensured the preparation of almost fully dehydrated samples. IR spectra obtained upon the adsorption of NO2 on the fully dehydrated Na- and Ba-Y zeolites at room temperature are displayed in Fig. 1A and B, respectively. The IR spectra of adsorbed NOx species on these two samples are very similar in their character. There is a broad feature between 1300 and 1500 cm−1 , and a doublet band in the 2000–2200 cm−1 spectral region. As we have discussed it in great detail previously [7,8], the broad band at the lower frequency range (∼1500 cm−1 ) represents the νNO vibrations of NO3 − species, while the lower frequency portion of the doublet represents νNO vibrations of the adsorbed NO+ species (2020 cm−1 in Na-Y and 2050 cm−1 in Ba-Y). The higher frequency portion of these doublets was assigned to νNO vibrations of NO+ species in NO+ NO2 or NO+ N2 O4
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Zeo− represents the negatively charged zeolite framework that is neutralized by the cation (Mex+ ) Na+ or Ba2+ in this case. The adsorption and desorption of NO2 is reversible, as only NO2 desorbs from these fully dehydrated samples in TPD experiments carried out following NO2 saturation at room temperature [8]. Although the adsorbed NOx species are very similar in the two zeolites studied here, their thermal properties are different. NO2 desorbs in a single desorption feature at ∼370 K from Na-Y as NO+ and NO3 − recombines to NO2 . On the other hand, two desorption features are observed in the TPD spectrum of NO2 in Ba-Y [8]. The lower temperature peak is at 370 K, and the other one is at ∼470 K. The first peak represents NO2 desorption from the NO+ NO2 adducts, while the high temperature feature arises from the recombinative desorption of NO2 from NO+ and NO3 − . In both cases (Na- and Ba-Y) the IR bands representing NO+ and NO3 − lose their intensities simultaneously, and disappear at 370 and 470 K in Na- and Ba-Y, respectively. In the IR spectrum obtained after NO2 adsorption on the dehydrated Ba-Y, a small vibrational feature corresponding to νOH vibrations can be seen at 3620 cm−1 . The appearance of this peak can be regarded as an indicator of non-perfect dehydration of the zeolite material, as we will discuss it below. 3.1.2. NO2 adsorption on hydrated Na- and Ba-Y The IR spectra following NO2 adsorption on hydrated Naand Ba-Y zeolites are significantly different from those obtained from the dehydrated samples. In these experiments water was added to the fully dehydrated samples, and then NO2 was introduced into the system. The most significant difference in the IR spectra between the hydrated and dehydrated samples is the complete absence of the NO+ species in the spectra of the hydrated samples (see Fig. 1). Also, several new features in the νOH vibrational region can be seen on these hydrated samples. In the hydrated Na-Y, a new IR absorption band developed at 1270 cm−1 upon exposure to NO2 . This band represents νNO vibration of NO2 − species formed in the reaction of NO+ originally formed in reaction (1) and H2 O: NO+ + H2 O ⇔ H+ + HONO
(3)
adducts. These adducts are only formed in the presence of excess NO2 /N2 O4 . The adsorption of NO2 occurs by the disproportionation into NO+ and NO3 − and the subsequent stabilization of the thus formed ionic species in the ionic sites of the zeolites:
The same species is formed on Ba-Y, but the frequency of the νNO vibration of the NO2 − species is either too low to be seen due to the limited transmittance of this sample below 1220 cm−1 , or it overlaps with the broad NO3 − bands. Its formation, however, is indirectly evidenced by the appearance of the νOH band of HONO at 3620 cm−1 . In both materials the formation of NO+ is completely eliminated by the presence of water, while the formation of NO3 − is just as facile as it was on the fully dehydrated samples. Nitrate species can form in two different ways in these hydrated zeolites; in the disproportionation of NO2 in accordance with Eq. (1), and in the direct reaction between NO2 and H2 O:
2NO2 ⇔ NO+ + NO3 −
3NO2 + H2 O ⇔ 2HNO3 + NO
Fig. 1. IR spectra following room temperature adsorption of NO2 on dehydrated (red) and hydrated (blue) Na-Y (A) and Ba-Y (B).
(1)
Zeo− · · · Mex+ + NO+ + NO3 − ⇔ Zeo− NO+ · · · NO3 − Mex+ (2)
(4)
(The formation of NO in this reaction was substantiated by mass spectrometry analysis of the gas phase.) The thus formed HNO3 can interact with Mex+ ions in cationic positions to form
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Br¨onsted acidic OH groups and Ba-nitrate species. This process is evident from the series of IR spectra recorder with increasing amounts of NO2 (not shown). First the νOH vibrational band of HONO develops and, as the amount of NO2 introduce into the cell increases the zeolitic OH groups at 3645 and 3550 cm−1 develop. We have also observed that at high NO2 pressures in Na-Y both the 3620 cm−1 (νOH of HONO) and 1270 cm−1 (νNO of HONO) features disappear, as the zeolitic OH groups develop. This suggests that in the presence of excess NO2 , HONO can be oxidized to HNO3 due to the high oxidizing power of NO2 . This process is also accompanied by the release of an NO molecule: HONO + NO2 ⇔ HNO3 + NO
(5)
The thermal stabilities of the NOx species formed on the hydrated samples differ significantly from those we have seen for the fully dehydrated ones. As we discussed in detail elsewhere [8], on both zeolites there is a large shift in the NO2 desorption temperature to higher values. The single NO2 desorption feature seen at 370 K on Na-Y is transformed into a band centered at ∼470 K. On Ba-Y we have also shown this transformation in the high temperature feature to even higher temperatures (from 470 to 650 K), while we have seen hardly any change in the lower temperatures TPD peak [8]. The experiments, however, were carried out in somewhat different manner in that study, i.e. the samples were first saturated with NO2 followed by H2 O adsorption. The sequence of adsorption (NO2 or H2 O first) does not affect either the IR or the TPD spectra in Na-Y, since the thermal stability of NO+ NO2 in this material is low, and it is removed by simple room temperature evacuation. Therefore, the incoming water can interact only with the adsorbed NO+ species. On BaY, however, the stability of the NO+ NO2 adduct is much higher, and it is not removed by simple evacuation at room temperature. Thus, when water is introduced into the IR cell it can interact with both NO+ and NO+ NO2 adducts. We can see that water easily reacts with NO+ just as it does on Na-Y, however, the NO+ in the NO+ NO2 adduct seem to be protected from the reaction with H2 O. This is the reason why we see the lower temperature TPD feature unchanged after H2 O addition onto the NO2 -saturated Ba-Y sample. However, when we adsorb NO2 onto the hydrated Ba-Y sample the results are different. In this case we cannot form the NO+ NO2 adduct, since the NO+ that is formed in the NO2 disproportionation readily reacts with the water already present in the zeolite. This would also explain the different IR spectra seen in the two cases, which differ only in the order of NO2 and H2 O adsorption. 3.2. XANES N K-edge XANES spectra recorded from hydrated and dehydrated Ba-Y samples are displayed in Fig. 2 together with the reference spectra from KNO2 , and KNO3 . These two commonly used standards show the electronic transitions within the NO2 − and NO3 − ions. In the KNO3 standard the N Kedge has an intense resonance at ∼406.5 eV, which can be assigned to 1s → 2a2 () electronic transitions. The features in the 410–420 eV region may come from excitations into the 5a1
Fig. 2. N K-edge XANES spectra for dehydrated and hydrated Ba-Y exposed to NO2 at 300 K; also included are the spectra for the KNO2 and KNO3 standards. The N K-edge spectra were taken after exposing the sample to 5 Torr of NO2 at 300 K for 15 min.
and 5e empty orbitals of the nitrate ions [9]. The intense absorption feature in the KNO2 standard at ∼402.5 eV represents 1s → 2b1 () electronic transitions, while the broad feature between 407 and 417 eV arises from the excitations into the 7a1 and 5b2 vacant orbitals of NO2 − [9]. In the N K-edge XANES spectrum of the “dehydrated” Ba-Y absorption features characteristic of both the NO2 − and NO3 − ions can be seen. The intensity of the absorption feature arising from the nitrate ion is significantly higher than that of the nitrite ion. Beside these two absorption bands a third resonance feature is also present at an even lower photon energy of ∼399.5 eV on this dehydrated sample. In the adsorption of NO2 on a defect rich TiO2 (1 1 0) surface Rodriguez et al. [10] assigned the small absorption resonance at ∼399 eV to adsorbed atomic nitrogen. On highly reduced Ce0.8 Zr0.2 O2−x (1 1 1) a feature proposed to represent adsorbed atomic nitrogen was observed at ∼399.5 eV [11]. These adsorbed N atoms were stable on the TiO2 (1 1 0) surface up to at least 400 K, and it was observed even at 900 K on the Ce0.8 Zr0.2 O2−x (1 1 1) surface. In the adsorption of NO2 on polycrystalline metallic magnesium, atomic nitrogen was shown to give a weak absorption resonance at around 396.5 eV [12]. In our case the formation of atomic nitrogen produced by the direct dissociation of NO2 is not operational. However, as we
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have shown above in the IR results, NO+ NO3 − ion pairs form in the disproportionation of NO2 and stabilized by their interactions with the negatively charged zeolite framework and the charge compensating cations, respectively. Therefore, we assign the absorption feature at 399.5 eV to adsorbed NO+ ions. The question that remains to be answered is why we see the resonance feature at 402.5 eV in the dehydrated Ba-Y sample. This feature represents NO2 − ions, which are not observed in the IR spectra on the fully dehydrated samples. However, prior to XANES measurements the sample was annealed only at 700 K for 2 h. At this temperature there are still significant amount of water is left in the channels of the zeolite. As we have discussed above in the IR section, prolonged heating at 773 K and careful bakeout of the system was required to obtain a fully dehydrated zeolite sample. Since water was probably still present in the zeolite structure in these XANES measurements on the “dehydrated” sample, reaction (4) could take place resulting in the formation of all three ionic species (NO+ , NO3 − , and NO2 − ). According to reaction (4), NO is also produced in the reaction between H2 O and NO2 , therefore one might assign the 399.5 eV XANES feature to adsorbed NO. However, in our FT-IR studies we have never observed any adsorption of NO on these zeolite samples, not even when in the presence of pure NO. In the adsorption of NO2 on hydrated Ba-Y, only two absorption resonances are observed in the XANES spectra, the ones that represent NO3 − (406.5 eV) and NO2 − (402.5 eV). These results are in full agreement with our IR observations: the presence of water prevents the adsorption of NO+ by reaction (3). We have also mentioned in our discussion of NO2 adsorption on hydrated Na-Y that in the presence of a large excess of NO2 some of the NO2 − may be oxidized to NO3 − . This might be the reason of the different NO3 − /NO2 − XANES resonance feature intensity ratios between the “dehydrated” and hydrated samples. The XANES spectra obtained in the NO2 adsorption on dehydrated and hydrated Na-Y zeolites are very similar to those reported here for Ba-Y, and are not shown for brevity. The main difference between the spectra is the significantly reduced intensities for all the absorption features in Na-Y in comparison to those in Ba-Y. This reflects the fact that the NO2 sorptoin capacity of Ba-Y is approximately twice higher than that of Na-Y [5]. The relative intensities of the NOx species observed on Na- and Ba-Y after room temperature NO2 adsorption are compared in Fig. 3 for the dehydrated (panel A) and hydrated (panel B) samples. (The different intensity ratios of NO3 − /NO2 − observed for Ba- and Na-Y may arise from the different level of dehydration of the two samples.) 3.3. NMR Solid state 15 N NMR spectra obtained from the Na- and Ba-Y samples after 15 NO2 exposure at room temperature are displayed in Fig. 4. Adsorption of 15 NO2 onto dehydrated Na-Y results in the appearance of two peaks in the NMR spectrum at 336 and 370 ppm chemical shifts, relative to 15 N-ammonium chloride at 0 ppm. The feature at 336 ppm chemical shift is very close to that reported for nitrate ions [13]. The other peak at
Fig. 3. Relative intensities of the characteristic resonance peaks on the dehydrated (A) and hydrated (B) Na- and Ba-Y zeolites exposed to NO2 at 300 K.
higher chemical shift might be assigned to the adsorbed 15 NO+ species formed upon the disproportionation of NO2 and subsequent adsorption on the negatively charged zeolite framework. The nitrate feature in 15 NO2 -saturated, dehydrated Ba-Y is at very similar chemical shift (334 ppm) as it was seen in Na-Y. However, at around 340 ppm chemical shift a small feature can be distinguished in the Ba-Y case that was not seen in Na-Y. The position of this peak (∼340 ppm) is very close to that of the nitrate ions (334 ppm), therefore, we assign it to a nitrate ion that is adsorbed onto Ba ions that are occupying different sites in the zeolite. This altered chemical environment around the different cationic sites can explain the appearance of the two nitrate features at close proximity. On the other hand, on Na-Y we are unable to distinguish among nitrate ions co-ordinated to Na ions in different crystallographic positions. This may originate form the significantly broader NMR nitrate peak in the Na-Y case, or, more probably, from the much more homogeneous average chemical environments around the much smaller Na ions. The NMR spectra changed dramatically when water was added onto the 15 NO2 -saturated Na- and Ba-Y samples. In both cases, the NMR peak we have assigned to adsorbed 15 NO+ species completely disappeared, while feature of adsorbed 15 NO − became extremely sharp upon H O addition. There 3 2 were only very small changes in the positions of the NMR features of adsorbed nitrates, which, in part can be explained by the
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not been used in NMR studies of these reactions. The primary problem of using NMR for the investigation of NOx adsorption is the fact that neutral species of both 15 NO and 15 NO2 are invisible by NMR spectroscopy because they are paramagnetic. Therefore, we can expect to see only charged NOx species. In one of the rare 15 N NMR studies Wu and Larsen [15] reported two NMR features at −2 and 36 ppm chemical shifts (relative to neat CH3 NO2 ) after they contacted a Na-ZSM5 zeolite sample with a propane + 15 NO + O2 gas mixture at room temperature. They assigned these peaks to Na15 NO3 (−2 ppm) and CH3 O15 NO2 (36 ppm). The chemical shift difference between these two features (38 ppm) is very close to that we see for dehydrated Na-Y (34 ppm) after 15 NO2 exposure. However, in our case there is no hydrocarbon present in the system that could result in the formation of an organic nitrate species. Also, they only observe the presence of the NMR peak at 36 ppm upon the interaction of the Na-ZSM5 with the reactant gas mixture at 298 K. This temperature seems to be too low to expect any reaction between the hydrocarbon and NO + O2 (mainly since they use propane as a reductant instead of propene). In fact, when they contacted the gas mixture with the Na-ZSM5 at 423 K and then looked at the surface at room temperature with NMR, they observed only the peak at −2 ppm. We would expect to see even more of the species represented by the 36 ppm feature when the reaction is run at higher temperature. After the 423 K reaction the NMR feature at 36 ppm chemical shift is completely absent, while the peak representing NO3 − is still there with high intensity. Thus, we conclude that in their case on Na-ZSM5 at room temperature NO readily reacts with O2 to form NO2 , which then can adsorb on the catalyst as NO+ NO3 − ion pairs and propane is just present as a non-reactive spectator. We believe that the NMR feature they reported at 36 ppm can be assigned to the adsorbed, charged NOx species of NO+ . This is in agreement with our NMR results on the dehydrated zeolite samples and is confirmed by our FT-IR findings as well. 4. Conclusions Fig. 4. 15 N MAS NMR spectra of 15 NO2 on Na- and Ba-Y zeolites at 300 K. (Red spectra: dehydrated samples; blue spectra: hydrated samples.)
much sharper peaks in the hydrated samples. The broad peaks recorded from the dehydrated samples make the peak identification less accurate. As we have discussed above in the IR section, water can readily react with adsorbed NO+ in accordance with reaction (3). In the presence of a large amount of NO2 , however, HONO can be converted to HNO3 resulting in the complete absence of a 15 NO2 − NMR feature in either of the two samples. The sharpening of the nitrate NMR features is a consequence of the co-ordination of H2 O molecules around these adsorption complexes. In Ba-Y we can also observe the much better resolution of the two different nitrate peaks (335 and 340 ppm) on this hydrated sample. Isotopically labeled NOx compounds are routinely used in FT-IR spectroscopy for the identification of adsorbed surface species and reaction intermediates in NOx reduction catalysis [14]. However, 15 N-labeled NO2 , to our best knowledge, have
Using a variety of spectroscopic techniques (FT-IR, XANES, solid state NMR), we were able to identify unambiguously the processes that take place in NO2 adsorption on the basic zeolites of Na- and Ba-Y. On the fully dehydrated zeolite samples NO2 can only adsorb as NO+ NO3 − ion pairs that form in the NO2 disproportionation (Eq. (1)). The NO+ and NO3 − ionic species formed are stabilized by their interactions with the negatively charged zeolite framework, and the charge compensating cations, respectively. In TPD/FT-IR experiments we see the desorption of only NO2 from these fully dehydrated samples, and the IR features representing these two ionic species disappear simultaneously. The situation becomes more complex when NO2 is adsorbed onto hydrated zeolites or when H2 O is added to the NO2 -saturated zeolite samples. In the presence of H2 O in the zeolite channels the NO+ formed in the NO2 disproportionation can react with water to form HONO. Furthermore, direct reaction between NO2 and H2 O is facile as well (Eq. (4)), and results in the formation of HNO3 and NO. The thus formed HNO3 can participate in an ion exchange process that 15 N
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ultimately lead to the formation of Br¨onsted acidic OH groups. There is significant difference in the adsorbed species on Ba-Y depending on the order of adsorption. When H2 O is adsorbed on Ba-Y prior to NO2 adsorption the formation of the NO+ NO2 adduct is inhibited as evidenced by the complete absence of the IR features representing these species. However, in the reverse case (NO2 first, followed by H2 O adsorption) the signature of NO+ readily disappears from the IR spectrum as this species reacts with the incoming water, and under the same circumstances the NO+ NO2 adduct remains almost unchanged (reacts slowly with H2 O) as NO2 seems to protect NO+ form the reaction with water. This is not the case in Na-Y, since the thermal stability of the NO+ NO2 adduct is much lower in this material, and is completely decomposed to NO+ (ads.) and NO2 (gas) upon room temperature evacuation. Therefore, the remaining NO+ (NO2 adsorption first) or the NO+ formed in NO2 disproportionation (H2 O adsorption first) readily reacts with water to produce HONO. Acknowledgements The FT-IR and NMR experiments were carried out at the Pacific Northwest National Laboratory (PNNL). PNNL is operated for the US DOE by Battelle Memorial Institute under contract no. DE-AC067RLO1831. Funding for this work was provided by the DOE, Office of Basic Energy Sciences, Division
of Chemical Sciences. The XANES data were collected at the NSLS in Brookhaven National Laboratory; a facility supported by the Division of Materials Science, and Chemical Sciences of DOE. References [1] M.L. Balmer, R. Tonkyn, S. Yoon, A. Kolwaite, S. Barlow, G. Maupin, J.W. Hoard, SAE 1999-01-3640. [2] A.G. Panov, R.G. Tonkyn, M.L. Balmer, C.H.F. Peden, A. Malkin, J.W. Hoard, SAE 2001-01-3513. [3] C.R. MaLarnon, B.M. Penetrante, SAE 982433. [4] B.M. Penetrante, R.M. Brusasco, B.T. Merritt, W.J. Pitz, G.E. Vogtlin, M.C. Kung, H.H. Kung, C.Z. Wan, K.E. Voss, SAE 983508. [5] J.H. Kwak, J. Szanyi, C.H.F. Peden, J. Catal. 220 (2003) 291. [6] J.H. Kwak, J. Szanyi, C.H.F. Peden, Catal. Today 89 (2004) 135. [7] J. Szanyi, J.H. Kwak, R.A. Moline, C.H.F. Peden, PCCP 5 (18) (2003) 4045. [8] J. Szanyi, J.H. Kwak, C.H.F. Peden, J. Phys. Chem. B 108 (2004) 3746. [9] J.A. Rodriguez, Surf. Sci. 230 (1990) 335. [10] J.A. Rodriguez, T. Jirsak, G. Liu, J. Hrbek, J. Dvorak, A. Maiti, J. Am. Chem. Soc. 123 (2001) 9597. [11] G. Liu, J.A. Rodriguez, J. Hrbek, J. Dvorak, J. Phys. Chem. B. 105 (2001) 7762. [12] J.A. Rodriguez, T. Jirsak, S. Sambasivan, D. Fischer, A. Maiti, J. Chem. Phys. 112 (2000) 9929. [13] K.L. Anderson-Altmann, D.M. Grant, J. Phys. Chem. 97 (1993) 11096. [14] K.I. Hadjiivanov, J. Suassy, J.L. Freisz, J.C. Lavalley, Catal. Lett. 52 (1998) 103. [15] J. Wu, S.C. Larsen, Catal. Lett. 70 (2000) 43.