Langmuir 2002, 18, 6875-6880
6875
NO and CO Adsorption on Over-Exchanged Cu-MCM-22: A FTIR Study Alberto Frache, Marcella Cadoni, Chiara Bisio, and Leonardo Marchese*,† Dipartimento di Scienze e Tecnologie Avanzate, Universita` del Piemonte Orientale “Amedeo Avogadro”, C.so Borsalino, 54, 15100, Alessandria, Italy
Artur J. S. Mascarenhas and Heloise O. Pastore*,‡ Instituto de Quı´mica, Universidade Estadual de Campinas, CP 6154, CEP 13083-970, Campinas, SP, Brazil Received March 7, 2002. In Final Form: June 6, 2002 Copper ions on over-exchanged Cu-MCM-22 were monitored by CO and NO adsorption at 300 K and NO adsorption at 77 K. Two different species of both Cu(I) and Cu(II) ions were produced by vacuum thermal treatment at 823 K. CuI-CO complexes were formed upon CO adsorption and converted to CuI(CO)2 complexes upon increasing the CO pressure. CO adsorbed on copper ions on small oxide aggregates or in oligomeric cationic compounds was also found. Mononitrosylic CuI-NO complexes (band at 1813 cm-1) were formed upon NO adsorption and completely oxidized to CuII-NO complexes (absorptions at 1899 and 1908 cm-1) at 300 K upon increasing NO pressure. Nitro, nitrate, and nitrito species were formed by reaction of NO at 300 K with oxidic surface aggregates. Low-temperature NO adsorption experiments inhibited both the Cu(I) to Cu(II) conversion and the reactivity of the aggregates and offered clear-cut evidence of the presence of two Cu(I) sites in the MCM-22 zeolite (IZA code, MWW) structure. Besides, under higher NO pressure, mononitrosylic CuI-NO complexes transform to dinitrosylic CuI-(NO)2 complexes. Dimers of NO, as free species or bound to copper ions, were also observed and formed probably by NO confinement within the small spaces of the MWW cages at very low temperatures. The peculiar reactivity of Cu-MCM-22 under NO adsorption at 300 K suggests that this material is a potential catalyst for NOX abatement.
Introduction Transition-metal-exchanged zeolites are known to be active in many catalytic reactions of environmental interest, such as nitrogen oxide abatement processes. Copper-exchanged ZSM-5, β and mordenite zeolites have been extensively investigated for their effective catalytic behavior in these reactions.1 Over-exchanged Cu-ZSM-5 catalysts have been established as very active in the decomposition of NO2,3 and N2O4 and also in SCR (selective catalytic reduction) with hydrocarbons;5 however, they present serious limitations for practical purposes,6 one of the most important being their dealumination under operation conditions. This poses stringent restrictions since the dealumination changes the copper location, coordination, and degree of clustering. Therefore, the investigation of copper-exchanged zeolites involves essentially the search for solutions to the problems that impart limitations for the technological application of these materials. In this direction, MCM-22 zeolite (IZA code, MWW) seems to have interesting structural and physical chemical * To whom correspondence should be addressed. † Tel: 00 39 0131 287435. Fax: 00 39 0131 287416. E-mail:
[email protected]. ‡ Tel: 00 55 19 37883095. Fax: 00 55 19 37883023. E-mail:
[email protected]. (1) Armor, J. N. Microporous Mesoporous Mater. 1998, 22, 451. (2) Iwamoto, M.; Yahiro, H.; Torikai, Y.; Yoshoka, T.; Mizuno, N. Chem. Lett. 1990, 1967. (3) Iwamoto, M.; Yahiro, H.; Tanda, K.; Mizuno, N.; Mine, Y.; Kagawa, S. J. Phys. Chem. 1991, 95, 3727. (4) Li, Y.; Armor, J. N. Appl. Catal., B 1992, 1, L21. (5) Traa, Y.; Burger, B.; Weitkamp, J. Microporous Mesoporous Mater. 1999, 30, 3. (6) Armor, J. N. Catal. Today 1995, 26, 99.
characteristics. It was first synthesized by Mobil Oil7 and has a peculiar structure composed of two independent channel systems, both accessed by 10-membered-ring (MR) windows (4.0 × 5.7 Å):8 a 10-MR sinusoidal twodimensional system (4.0-5.5 Å), very much like the ZSM-5 zigzag channels in terms of geometry, and a 12-MR system formed by interconnected supercages (7.1 × 7.1 × 18.2 Å3). In the as-synthesized form, MCM-22 is a layered material. In the intralayer space are found the 10-MR channels, and the layer surface is composed of pockets, that consist of half-supercages.9 The complete threedimensional structure is formed by condensation of the layers, originating complete 12-MR supercages. This zeolite has been used as an acid catalyst in many reactions,10-13 because its particular structure combines properties of large- and medium-pore zeolites13,14 and presents high hydrothermal stabilility with an unusual dealumination resistance,12 elevated surface area, and high sorption capacity. Cu-MCM-22 and Cu,Ln-MCM-22 (Ln ) lanthanumtype cation) have been initially proposed as additives (7) Rubin, M. K.; Chu, P. U.S. Patent 4,959,325, 1990. (8) Leonowicz, M. E.; Lawton, J. A.; Lawton, S. L.; Rubin, M. K. Science 1994, 264, 1910. (9) Lawton, S. L.; Leonowicz, M. E.; Partridge, R. D.; Chu, P.; Rubin, M. K. Microporous Mesoporous Mater. 1998, 23, 109. (10) Corma, A. Microporous Mesoporous Mater. 1998, 22, 343. (11) Corma, A.; Davis, M.; Forne´s, V.; Gonza´lez-Alfaro, V.; Lobo, R.; Orchille´s, A. V. J. Catal. 1997, 167, 438. (12) Wu, P.; Komatsu, T.; Yashima, T. Microporous Mesoporous Mater. 1998, 22, 343. (13) Pater, J. P. G.; Jacobs, P. A.; Martens, J. A. J. Catal. 1999, 184, 262. (14) Corma, A.; Corell, C.; Martı´nez, A.; Pe´rez-Pariente, J. Stud. Surf. Sci. Catal. 1994, 84, 859.
10.1021/la0257081 CCC: $22.00 © 2002 American Chemical Society Published on Web 08/10/2002
6876
Langmuir, Vol. 18, No. 18, 2002
to fluid cracking catalysis (FCC), allowing a decrease in the level of NOX emission during the catalyst regeneration process.16 Cu-MCM-22 has also been reported as a highly active catalyst in SCR with hydrocarbons17 and in the decomposition of nitrous oxide.18 In regard to the siting of copper ions in Cu-based zeolites, Nachtigall et al.19 carried out an intensive computational study of the location, structure, and coordination of isolated Cu+ ions in ZSM-5 by a combined quantum mechanics/ interatomic potential function technique. The authors pointed out that the triplet and singlet states of Cu+ have different coordinations to the walls of the zeolite which reflects in their different CuI-OZ binding distances. Therefore, in these two situations they are sensitive to the surroundings, either in the channels or in their intersections, and present different stabilities.20 X-ray absorption studies have shown that after the dehydration process, the oxidation state of the vast majority of copper is +1 and that its average coordination in Cu-Y at sites I*, II*, and II21 is 2.8 at an average distance of 1.99 Å, the same as found for Cu-ZSM-5.22 The coordination number of CuI in Cu-Y increases upon CO adsorption and causes migration from the hardly accessible sites I* and II* to the more open sites II. However, despite all these studies, certain details of siting and reactivity of the copper ions in Cu-based zeolites are better evaluated by spectroscopic studies of the adsorption of probe molecules such as CO and NO.19-28,30-37 Therefore, this work aims at showing the siting, coordination, oxidation and aggregation states of copper ions in over-exchanged Cu-MCM-22 through the FTIR monitoring of CO and NO adsorption. The reactivity of the copper ions in the presence of adsorbed NO is also evaluated. (15) Unverricht, S.; Hunger, M.; Ernst, S.; Karge, H. G.; Weitkamp, J. Stud. Surf. Sci. Catal. 1994, 84, 37. (16) Absil, R. P. L.; Bowes, E.; Green, G. J.; Marler, D. O.; Shihabi, D. S.; Socha, R. F. U.S. Patent 5,085,762, 1992. (17) Corma, A.; Palomares, A. E. O.; Forne´s, V. Res. Chem. Intermed. 1998, 24, 613. (18) Mascarenhas, A. J. S.; Andrade, H. M. C.; Pastore, H. O. Stud. Surf. Sci. Catal. 2001, 135, 322. (19) Nachtigall, P.; Nachtigallova, D.; Sauer, J. J. Phys. Chem. B 2000, 104, 1738. (20) Nachtigall, P.; Davidova, M.; Silhan, M.; Nachtigallova, D. Stud. Surf. Sci. Catal. 2001, 135, 177. (21) Palomino, G. T.; Bordiga, S.; Zecchina, A.; Marra, G. L.; Lamberti, C. J. Phys. Chem. B 2000, 104, 8641. (22) Lamberti, C.; Bordiga, S.; Salvalaggio, M.; Spoto, G.; Zecchina, A.; Geobaldo, F.; Vlaic, G.; Bellatreccia, M. J. Phys. Chem. B 1997, 101, 344. (23) Millar, G. J.; Canning, A.; Rose, G.; Wood, B.; Trewartha, L.; Mackinnon, I. D. R. J. Catal. 1999, 183, 169. (24) Konduru, M. V.; Chuang, S. S. C. J. Phys. Chem. B 1999, 103, 5802. (25) Ma´rquez-Alvarez, C.; McDougall, G. S.; Guerrero-Ruiz, A.; Rodrı´guez-Ramos, I. Appl. Surf. Sci. 1994, 78, 477. (26) Wichterlova´, B.; Deˇdeˇcek, J.; Sobalı´k, Z.; Vondrova´, A.; Klier, K. J. Catal. 1997, 169, 194. (27) Borgard, G. D.; Molvik, S.; Balaraman, P.; Root, T. W.; Dumesic, J. A. Langmuir 1995, 11, 2065. (28) Rakic´, V. M.; Hercigonja, R. V.; Dondur, V. T. Microporous Mesoporous Mater. 1999, 27, 27. (29) Wasowicz, T.; Prakash, A. M.; Kevan, L. Microporous Mater. 1997, 12, 107. (30) Spoto, G.; Bordiga, S.; Scarano, D.; Zecchina, A. Catal. Lett. 1992, 13, 39. (31) Jang, H. J.; Hall, W. K.; d’Itri, J. L. J. Phys. Chem. 1996, 100, 9416. (32) Konduru, M. V.; Chuang, S. S. C. J. Catal. 2000, 196, 271. (33) Centi, G.; Perathoner, S. Appl. Catal., A 1995, 132, 179. (34) Iwamoto, M.; Yahiro, H.; Mizuno, N.; Zhang, W.; Mine, Y.; Furukawa, H.; Kagawa, S. J. Phys. Chem. 1992, 96, 9360. (35) Giamello, E.; Murphy, D.; Magnacca, G.; Morterra, C.; Shioya, Y.; Nomura, T.; Anpo, M. J. Catal. 1992, 136, 510. (36) Hadjiivanov, K. I. Catal. Rev.sSci. Eng. 2000, 42, 71. (37) Marchese, L.; Bordiga, S.; Coluccia, S.; Martra, G.; Zecchina, A. J. Chem. Soc., Faraday Trans. 1993, 89, 3483.
Frache et al.
Figure 1. FTIR spectra of CO adsorbed at room temperature on Cu-MCM-22 at a 200% exchange level (curve 1, pCO ) 35 Torr; curve 13, under vacuum, 1.0 × 10-4 Torr residual pressure). Inset: comparison between samples at 200% and 100% exchange levels (spectra recorded under vacuum, 1.0 × 10-4 residual pressure).
Experimental Section Zeolite MCM-22 was prepared by hydrothermal treatment of a gel with the composition 4.44Na2O/30SiO2/Al2O3/17,76hexamethyleneimine/889H2O at 423 K and 60 rpm for 7 days.18 The obtained material was thoroughly washed, dried, and calcined at 853 K under dry oxygen. The calcined sample was first exchanged with a solution of NaNO3 (0.1 mol L-1) and subsequently exchanged with a solution of Cu(NO3)2‚3H2O. To obtain an over-exchanged sample, after this period the pH was adjusted to 7.5 with NH4OH.2 The material was washed, dried, and calcined at 773 K under oxygen. The structure was confirmed by X-ray diffraction (XRD), and the copper content was verified by ICPAES (cation exchange level of 200%). Fourier transform infrared (FTIR) experiments on pelletized samples were recorded with an ATI Mattson Research Series FTIR spectrometer at resolution of 4 cm-1 and by means of specially designed cells which were permanently connected to a vacuum line (ultimate pressure e 10-5 Torr) to make adsorptiondesorption experiments. The samples were pretreated by heating at 823 K (2 K min-1) under vacuum for 4 h. CO adsorption was performed at 300 K, while NO adsorption was performed at 300 K and 77 K.37
Results and Discussion CO Adsorption at 300 K. CO adsorption at 300 K (Figure 1, curve 1) on over-exchanged Cu-MCM-22 produced two broad bands at 2178 and 2151 cm-1 which decreased in intensity at lower CO doses and almost disappeared upon evacuation at 300 K (Figure 1, curve 13). In this situation, the formation of a new broad band centered at 2157 cm-1, that is stable under vacuum conditions, was simultaneously observed. This band is highly asymmetric in the low-wavenumber region, 21502125 cm-1, suggesting that additional species contribute to that absorption. The bands at 2178 and 2151 cm-1 are due to the symmetric and asymmetric stretching modes of dicarbonyl complexes on monovalent Cu+ ions, Cu+(CO)2, while the band at 2157 cm-1 is assigned to the stretching vibration of the monocarbonylic Cu+(CO) complexes. The evolution of the 2178 and 2151 cm-1 bands upon decreasing the CO pressure indicates that a conversion of Cu+(CO)2 into Cu+(CO) species takes place within the Cu-MCM-22 cavities/channels. Similar results were also obtained for over-exchanged Cu-ZSM-5 prepared by solid-state reaction with CuCl22 and for Cu-ZSM-5 prepared by conventional exchange procedures with copper(II) acetate23,24 in different over-exchanged levels.
NO and CO Adsorption on Over-Exchanged Cu-MCM-22
Langmuir, Vol. 18, No. 18, 2002 6877
Table 1. FTIR Absorptions of Carbonylic Complexes Formed upon CO Adsorption at 300 K over Cu-MCM-22 (200%) IR absorption/cm-1 2157 2178 2151 2150-2125
assignment CuI-CO,
monocarbonyl complexes (on isolated copper ions) CuI-(CO)2, dicarbonyl complexes (symmetric) CuI-(CO)2, dicarbonyl complexes (asymmetric) CuI-CO, monocarbonyl complexes (on small oxidic aggregates)
To explain the origin of the asymmetry of the band at 2157 cm-1 in Cu-MCM-22, it should be recalled that besides a main band at 2157 cm-1, a shoulder at 2135 cm-1 was found on Cu-ZSM-5 as well,25 and this was assigned to CO adsorbed on Cu+ ions in different coordination to the framework.26 Moreover, in the case of Cu-Y it was shown that CO adsorption on different sites results in distinct bands at ca. 2135, 2145, and 2160 cm-1.25,27,28 Although at least four different sites were proposed for the Cu-MCM-22 zeolite,29 in the experiments performed here it was not possible to observe distinct bands, but only the above-mentioned asymmetry on the low-wavenumber side of the main band at 2157 cm-1 could be seen, which might be due to the result of the overall combination of different sites. However, this behavior can also be accounted for by the total amount of copper present in the Cu-MCM-22 sample; in fact, in the spectrum of CO adsorbed on Cu-MCM-22 at a lower exchange level (100%) the asymmetry is not obvious (inset of Figure 1). This last observation is in favor of the presence of copper ions in small oxidic aggregates, either cationic oligomeric or oxidelike copper species, which was confirmed by NO adsorption at 300K (vide infra). All of the assignments to the carbonylic species formed by the adsorption of CO on the surface of Cu-MCM-22 described here are collected in Table 1. NO Adsorption at 300 K. NO adsorption at 300 K was performed from low to high pressure (Figure 2). At a low NO dose (0.05 Torr, curve 1), two main bands were observed at 1813 and 1899 cm-1, this last one with a shoulder at 1908 cm-1 (Figure 2A, curve 1). The band at 1813 cm-1 is commonly assigned to the stretching of mononitrosyl complexes on copper(I), and the band at higher frequency (1908-1899 cm-1) is in the region where the mononitrosylic complexes on copper(II) absorb.25,30,31 Upon the increase of NO pressure, the band at 1813 cm-1 of the CuI-NO complexes decreased whereas the band at a higher wavenumber related to the CuII-NO species increased, those absorbing at 1899 cm-1 being more abundant in comparison to the 1908 cm-1 ones. The experiment clearly shows not only that there is a complete conversion of CuI-NO to CuII-NO under NO pressure at 300 K but also that two distinct CuII sites are present. Noteworthily, a fraction of CuII ions is already observed right after the adsorption of very low NO doses although the majority of the copper ions are in the monovalent state. This can be the result of the vacuum treatment at 832 K, which leads to copper autoreduction.33 Simultaneous to this conversion, a family of bands in the region between 1650 and 1250 cm-1 was formed and these increased very fast only under high-pressure conditions (pressure g 5 Torr, Figure 2B, curves 15-17). These bands are related to different vibration modes of nitrate complexes on CuII ions; the structures of the most abundant ones are depicted in Chart 1. It is possible to distinguish at least five main bands: at 1291 cm-1, a band that can be tentatively assigned to nitro or bidentate nitrito species (Chart 1, structures 1 and 2);36 at 1517 and 1571 cm-1, a doublet due to chelating nitrate species (Chart 1, structure 3); and finally a strong band at 1621 cm-1, with a shoulder at 1612 cm-1, attributed to bridged nitrate species (Chart 1, structure
Figure 2. FTIR spectra of NO adsorbed on Cu-MCM-22 (200%) at room temperature, from 0.05 Torr (curve 1) to 10 Torr (curve 17), and after 45 min of contact with 10 Torr of NO (curve 18): (A) in the high-wavenumber region and (B) in the low-wavenumber region. Insets: comparison between samples at 200% and 100% exchange levels (spectra recorded after 45 min of contact with 10 Torr NO).
4).21 An interconversion between the band at 1621 cm-1 and that at 1612 cm-1 which depended upon the NO pressure was observed. To clarify the nature of the species involved in this conversion, additional detailed experiments are necessary; however, this is beyond the scope of the present work. Finally, another broad and weak band was observed in the range between 2150 and 2050 cm-1 and is assigned to NO2 adsorbed species.32 All of the assignments to the NO complexes on Cu-MCM-22, to the dimers, and to the products of NO oxidation are in Table 2. The formation of the nitrate complexes and NO2 reveals that Cu-MCM-22 has similar reactivity toward NO to that found for other Cu-exchanged zeolites. However, this system has some peculiarities. Over Cu-ZSM-5, three bands at 1827, 1812, and 1734 cm-1 related to copper(I) nitrosylic complexes are commonly observed.22,34,35 The first and the last are due to the symmetric and asymmetric stretching of dinitrosylic species, while the central one is attributed to the stretching
6878
Langmuir, Vol. 18, No. 18, 2002
Frache et al.
Table 2. FTIR Absorptions of Complexes Formed upon NO Adsorption at 300 and 77 K over Cu-MCM-22 (200%) IR absorption/cm-1
a
assignment
1801 1810 1825 and 1727 1727 1899 1908
Nitrosyl Species CuI-NO, mononitrosyl complexes on site A CuI-NO, mononitrosyl complexes on site B CuI-(NO)2, dinitrosyl complexes (symmetric and asymmetric stretching modes) CuI-(NO)2, dinitrosyl complexes (asymmetric) CuII-NO, mononitrosyl complexes on site A CuII-NO, mononitrosyl complexes on site B
1621 and 1612 1571 and 1517
Nitrate Species (Chart 1) bridged nitrate species on CuII, species 4 chelating nitrate species on CuII, species 3
1865 1787 1855 1784 1682
NO Dimers symmetric stretching of cis-N2O2 asymmetric streching of cis-N2O2 symmetric and asymmetric stretching modes of a van der Waals (vdW) dimera asymmetric dimeric compound, NONO, N-bound to Cu asymmetric dimeric compound, NONO, O-bound to Cu
1291 1902-1910 and 1710 2150-2050
Other Species nitro or bidentate nitrito species on CuII (Chart 1, species 1 and 2) physisorbed NO species NO2 adsorbed species
Denomination according to ref 39.
Chart 1
of mononitrosylic species. Other forms of NO adsorbates identified in this case are Cu2+(O-)(NO) at 1895 cm-1, Cu2+(NO) at 1910 cm-1, NO2 on Brønsted acid sites at 2124 cm-1, nitrate species in the range of 1630-1300 cm-1,24 and N2O on Cu(I) around 2240 cm-1.32,35 A distinctive feature that emerges from the comparison between Cu-MCM-22 and Cu-ZSM-5 is the absence of dinitrosylic species of Cu(I) in the experiments conducted at room temperature on Cu-MCM-22. Probably, they are not observed either because Cu(I) species are not as stable in the MWW structure as they are in the MFI one or because the rate of the oxidation of CuI in the MWW structure is too high as to avoid the observation of these bands. Experiments of NO adsorption performed at 77 K clarified this point (see below). Such fast and complete oxidation of CuI to CuII, as observed in Cu-MCM-22 under NO adsorption, has never
been observed in any other copper-containing systems. Cu-MCM-22 samples with different Cu concentrations are under investigation to shed some light on this very interesting result. We can anticipate that the reactivity depends on both the Cu loading and the pretreatment adopted for decomposing the catalyst precursor after the exchanging procedure. In particular, the reactivity decreases for samples with Cu-exchange levels lower than 150% treated in conditions similar to the ones adopted in this work. As an example, in the inset of Figure 2A it is shown that after prolonged time of contact of NO on a 100% Cu-exchanged sample some Cu(I) nitrosylic species are still present (bands of exceedingly weak intensity at around 1815 cm-1). These species are more stable (and their concentration is comparable with that in the wellknown Cu-ZSM-5) when the samples are pretreated in an inert rather than an oxidizing atmosphere. Additionally, a significant increase of the nitrate-related bands at 1650-1250 cm-1 occurred even after the conversion of CuI to CuII was complete, and this is a timedependent phenomenon (Figure 2B, curve 15). This is in favor of the existence of cationic oligomeric or oxide-like copper species in this zeolite. The assumption is even supported by the fact that the adsorption of NO over a sample with a lower exchange level, Cu-MCM-22 (100%), generated weak bands in the region of nitrate complexes (Figure 2B, inset), indicating that these species are probably formed by the interaction of NO with multicentered copper species such as the ones described, that are present in the over-exchanged sample as also shown by temperature-programmed reduction (TPR) experiments.18 NO Adsorption at 77 K. Experiments of NO adsorption at liquid nitrogen temperature were performed in order to inhibit the surface reactivity and check if in the MWW structure the formation of other nitrosylic species was possible. The experiment was run by adsorbing NO under 3.5 Torr of pressure because it was found that essentially all copper sites available were monitored under this pressure condition. Subsequently, the NO was progressively removed by lowering the pressure at around 100120 K. For reasons of clarity, the outcome of this experiment was represented in two parts, with the spectra obtained under high NO pressure (3.5 to 1.0 × 10-1 Torr, Figure
NO and CO Adsorption on Over-Exchanged Cu-MCM-22
Langmuir, Vol. 18, No. 18, 2002 6879
low temperatures.39 On the other hand, the component at 1784 cm-1 correlates with the band at 1682 cm-1, and they are assigned to the asymmetric dimeric compound NONO formed in the presence of Lewis acids.36 The bands at 1910 and at 1902 cm-1 and the shoulder at 1710 cm-1 are so far of an unknown nature; however, they must be due to a physisorbed state since they disappear in the low NO pressure regime (Figure 3B). At a pressure of 1.0 × 10-1 Torr of NO, a simpler spectrum was found (Figure 3B, curve 15) which consisted of three main bands at 1905, 1825, and 1727 cm-1. On the basis of the data obtained for Cu-ZSM-5 at 110 K,22 it was possible to assign the couple of bands at 1825 and 1727 cm-1 to the dinitrosyl complexes on copper(I). Upon further decrease of NO pressure to 1.0 × 10-3 Torr, the disappearance of the dinitrosylic species Cu+(NO)2 was observed and was clearly related to the formation of Cu+(NO) as indicated by the appearance of the band at 1801 cm-1 that is later covered by another one at 1810 cm-1 at the lowest NO doses. The clean and smooth transformation is evidenced by the clear isosbestic point formed at 1818 cm-1. Two Cu(I) sites (named site A and B in Table 2) were therefore monitored at the liquid nitrogen temperature, and these might convert to two Cu(II) sites at higher temperatures. The last spectrum collected after NO desorption (curve 26, Figure 3B) shows only the bands related to the mononitrosylic species on isolated copper(I and II) ions. No bands were observed in the regions of nitrate complexes, adsorbed NO2 or N2O, showing that the surface reactivity was completely inhibited, including the oxidation from copper(I) to copper(II) observed in the experiments performed at room temperature. Conclusions Figure 3. FTIR spectra of NO adsorbed at 77 K on Cu-MCM22 at a 200% exchange level (curve 1, 3.5 Torr NO) and desorbed at 100-120 K: (A) under high NO pressure (from 3.5 to 1.0 × 10-1 Torr, curve 15) and (B) under low NO pressure (from 1.0 × 10-1 to 1.0 × 10-3 Torr, curve 26).
3A) and low NO pressure (1.0 × 10-1 to 1.0 × 10-3 Torr, Figure 3B). The spectrum obtained upon the adsorption of the maximum NO dose (Figure 3A, curve 1) showed six main bands at 1682, 1727 (with a shoulder at ca. 1710 cm-1), 1787, 1825, 1865-1855, and 1910-1902 cm-1. Upon decreasing the NO pressure from 3.5 to 1.0 × 10-1 Torr (Figure 3A, curves 1-15), the bands at 1682, 1787, 18651855, and 1910-1902 cm-1 and the shoulder at 1710 cm-1 disappear almost completely, supporting their assignment to very weakly physisorbed or liquid-like NO. Under these conditions, N2O2 dimers can be formed within the zeolite cages and/or bound to Lewis acid sites36 and some of these species could be clearly identified. The composite nature of the band at 1787 cm-1 was defined for it shifts to 1784 cm-1 upon decreasing the NO pressure. Similarly, under these conditions the bands at 1865-1855 cm-1 become more clearly resolved. As far as the nature of the bands at 1865 and 1787 cm-1 is concerned, they show a similar behavior upon decreasing NO pressure and correspond to the symmetric and asymmetric modes of the cis-N2O2 dimer, respectively.38 Besides that, the band at 1855 cm-1 can be attributed to a special kind of cis-N2O2 dimer found in experiments with NO entrapped in a solid neon matrix and formed by a van der Waals interaction between two adjacent NO molecules at very (38) Dinerman, C. E.; Ewing, G. E. J. Phys. Chem. 1970, 53, 626.
The adsorption of NO and CO at room temperature and at 77 K has evidenced some important characteristics of the over-exchanged Cu-MCM-22 samples. CO adsorption made it possible to certify the presence of Cu(I) in the over-exchanged Cu-MCM-22 samples, by the presence of the bands related to dicarbonylic complexes, CuI-(CO)2, at 2178 and 2151 cm-1, and monocarbonylic complexes, CuI-(CO), at 2157 cm-1. The presence of oxidic phases was suggested on the basis of a high asymmetry in the region of 2150-2125 cm-1 of the band at 2157 cm-1. Upon NO adsorption, an unusually complete conversion of mononitrosylic CuI-NO, that absorbs at 1813 cm-1, to CuII-NO complexes, that absorb at 1899 and 1908 cm-1, was observed. Moreover, this surface, when treated as shown here, is capable of oxidizing NO to adsorbed nitro, nitrate, and nitrito species more extensively than the Cubased molecular sieves known before, showing that very active oxide aggregates or oligomeric cationic compounds are formed by this procedure. At least two sites for copper exchange in the MWW structure were found, as evidenced by low-temperature NO adsorption; however, further structural studies are needed for their correlation with the ones already assigned in the literature.29 Surface reactivity was deeply inhibited at 77 K, and it was possible to observe bands related to dinitrosylic species but not the ones due to nitro, nitrate, and nitrito species. On the other hand, dimers of NO, as free species or bound to copper ions, were also observed and were formed probably by NO confinement within the small spaces of the MWW cages at very low temperatures. These results suggested that over-exchanged Cu-MCM22 is a potential catalyst for NOX abatement processes, (39) Kometer, R.; Legay, F.; Legay-Sommaire, N.; Schwenter, N. J. Phys. Chem. 1994, 100, 8373.
6880
Langmuir, Vol. 18, No. 18, 2002
such as NO decomposition or selective catalytic reduction of NOX. Acknowledgment. The “Fundac¸ a˜o de Amparo a` Pesquisa do Estado de Sa˜o Paulo” (FAPESP, 98/01262-7) and the Italian “Ministero della Ricerca Scientifica e Tecnologica” (MURST, “Progetti di Rilevante Interesse Nazionale”, Cofinanziamento 2000) are acknowledged for the financial support of this work. Note Added in Proof. During the publication, the catalytic performances of these materials were monitored
Frache et al.
by Dr. B. Palella and Dr. R. Pirone (Istituto di Richerche sulla Combustione IRC-CNR, Piazzale V. Tecchio 80, 80125 Napoli, Italy). Very promising results were obtained in the decomposition of NO and N2O and in the selective catalytic reduction of NO with CO. Even more significantly, during the N2O decomposition the activity was maintained in the presence of water in the feed, and the catalyst retained a good activity after steam treatments (under He at 550 °C in the presence of 2% of water for 60 h), confirming that both the structure and the metal phase are stable under hydrothermal conditions. LA0257081
