Physical Chemistry - American Chemical Society

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The Journal of

Physical Chemistry

0 Copyright 1995 by the American Chemical Society

VOLUME 99, NUMBER 4, JANUARY 26, 1995

LETTERS Identification of Cu Sites in ZSMJ Active in NO Decomposition Blanka Wichterlovh,* Jifi DiideCek, and Alena Vondrovh J. HeyrovsM Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, CZ-182 23 Prague 8, Czech Republic Received: August 24, 1994; In Final Form: November 21, 1994@

Using Cu+ luminescence enabled the identification of two main Cu sites present in the MFI matrix and reflected in the 480 and 540 nm emission bands. It has been found that the activity of Cu-ZSM-5 in NO decomposition is controlled by the concentration of the sites with the Cu+ emission at 540 nm and by the reducibility of these Cu sites controlled by the framework SUA1 ratio. These Cu sites are incorporated into the zeolite as (Cu2+0H)+ ions, bearing after calcination an extra framework oxygen, and exchanged in the vicinity of one framework aluminum.

Introduction A unique and stable activity in NO decomposition has been found for Cu-ZSM-5, especially for those with a high degree of exchange and overexchange of CU.'-~ Although much effort has been given to elucidate the mechanism of NO decompoiti ion,^-" there is a considerable lack of information on the identification and structure of the Cu species active in NO decomposition. Because of the high activity of Cu overexchanged ZSM-5, much attention has been given to identify the Cu species, especially in such zeolites. Nevertheless, these zeolites may also contain some nondefined Cu oxidic species. Shpiro et a1.12 employing XPS and AES suggested formation of small Cu-0 clusters in overexchanged Cu-ZSM-5 and the presence of single Cu ions in ion exchanged zeolite. Kuroda et al.13 reported oxygen-bridged Cu planar complexes in Cu mordenites. Similarly, Kucherov and Slinkinlo indicated two coordinations of Cu, square planar and square pyramidal, in both mordenites and ZSM-5according to the ESR spectra of Cu2+. Anpo et al.14 found in Cu+ emission spectra of Cu+ZSM-5 two maxima at 450 and 540 nm, which they ascribed to Cu+ monomeric and Cu+ dimeric species, respectively, analogously as attributed by Barrie et al.15for Cu ions supported @

Abstract published in Advance ACS Abstracts, January 1, 1995.

on alumina. An EXAFS study16 produced evidence for the Cu-0 distances in Cu-ZSM-5 of 2.01 and 3.13 A, the latter one occumng in the overexchanged samples. Recently, we have reported that the Cu sites in zeolites can be well characterized by the Cu+ luminescence spectral7 and that the emission bands of the individual Cu+ coordinations are in sound agreement with the characteristic shifts in the NO molecule vibration adsorbed on the corresponding divalent Cu sites.18 It has been shown17s18that the Cu-ZSM-5 zeolites contain two dominant types of Cu sites: (i) Cu ions adjacent to two framework A1 atoms (Cu+ emission at 480 nm, IR band of Cu2+-N0 at 1906 cm-', Cu2+ESR axially symmetrical and split signal with the parameters of the parallel component gll = 2.33 and All =140-160 G) and (ii) Cu ions in the vicinity of one framework A1 atom (Cu+ emission at 540 nm, IR band of Cu2+-N0 at 1895 cm-l, Cu2+ ESR signal with gll = 2.27 and All = 170-180 G). The latter species are present in Cu-ZSM-5 regardless of Cu concentration but prevail in zeolites with a higher SUA1 ratio and with the Cu content approaching and exceeding the exchange degree of 100%. This contribution reports evidence that the defined Cu sites, characterized by the Cu+ luminescence at 540 nm, are those

0022-3654/95/2099-1065$09.00/0 0 1995 American Chemical Society

Letters

1066 J. Phys. Chem., Vol. 99, No. 4, 1995 TABLE 1: Zeolite Characterization" zeolite number SUA1 CulAl NdA1 Cu (wt%) CUMO (wt%) 2.00 1.70 0.09 Cu-ZSM-5/1 22.6 0.51 1.42 0.15 2.04 Cu-ZSM-5/2 22.6 0.52 2.03 1.10 0.07 Cu-ZSM-5/3 14.1 0.33 2.03 1.os Cu-ZSM-5/4 14.1 0.33 0.03 1.48 0.05 2.23 Cu-ZSM-5/5 14.1 0.35 0.04 2.47 1.38 Cu-ZSM-5/6 14.1 0.41

Note that Cu-ZSM-5/3-5/6 contain some protons balancing the framework charge; cf. text. Cu sites active in NO decomposition. Moreover, the Cu-zeolite activity depends on the Si/Al ratio, Le., reducibility of the Cu site.

Experimental Section Na-ZSM-5 zeolites of different SUA1 ratio (composition in wt%: Si02,93.95; A1203,3.33; Na20,2.70; CaO, 10.01; H20, 10.01 and Si02, 91.84; A1203, 5.63; Na20, 0.22; CaO, 10.01; H20, 0.31) were exchanged with Cu, using CuC12 or copper acetate solution, in such a way to obtain the same concentration of Cu in zeolites Cu-ZSM-5/1-5/4 with different population of Cu sites characterized by Cu+ luminescence at 480 and 540 nm. Moreover, using different Cu loading (Cu-ZSM-5/5 and -5/6) it was possible to obtain samples with different distributions of Cu sites. The time resolved Cu+ luminescence spectra were recorded for Cu+-ZSM-5 (Cu2+ zeolite was reduced by hydrogen at conditions to obtain maximum Cu+ luminescence intensity) after the sample irradiation by a pulse excimer laser at 308 nm, on a kinetic spectrometer (Spectra Physics; for details see ref 17). The catalytic activity of Cu zeolites was tested in a throughflow reactor with an inlet NO concentration of 4000 ppm in helium and total feed of 100 mL min-', catalyst weight -200 mg, and in the temperature range 520-770 K. NO and NO2 were analyzed at the inlet and outlet of the reactor by a chemiluminescence NO/NOx analyzer (Vamet 138). No NO2 was detected in the reaction products (detection limit 5 ppm).

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Table 1 presents chemical composition of the ion-exchanged Cu-ZSM-5 zeolites and a fraction of Cu sites corresponding to the luminescence at 540 nm; for illustration the luminescence spectra of Cu-ZSM-5/1 and Cu-ZSM-5/4 are depicted in Figure 1. It is seen that Cu-ZSM-5 containing a lower number of framework AI atoms exhibits a prevailing number of the 540 nm luminescence sites; moreover, the ion exchange from copper acetate leads to a higher number of these sites compared to that one with copper chloride. This is understandable, because a higher pH of the copper acetate solution favors formation of (Cu2+-OH)+ specie^,^ which are precursors of the Cu+ sites with the 540 nm luminescence. A prevailing number of Cu+ sites with the luminescence at 540 nm for the zeolite with a higher SUA1 ratio indicates that these Cu sites are adjacent to one A1 framework atom (cf. ref 17). The catalytic activity of Cu-ZSM-5 zeolites expressed as a conversion of NO into nitrogen with a dependence on temperature is depicted in Figure 2. A typical conversion vs temperature dependence for Cu-ZSM-5 is observed (cf. refs 1, 2, and 4), indicating a change of the reaction kinetics. For a correct comparison of activity of individual Cu-ZSM-5 zeolites a temperature of 670 K (conversion not exceeding 21%), at the increasing part of the activity vs temperature curve, has been chosen. The calculated turnover frequencies (TOF) of NO per total Cu atom have been correlated with the concentration of

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Figure 1. Cut emission spectra of Cu-ZSM-5 reduced in hydrogen to a maximum intensity of Cut: (a) Cu-ZSM-5/4, (b) Cu-ZSM-5/1.

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Figure 2. Conversion of NO into nitrogen on Cu-ZSM-5 zeolites with dependence on temperature. The numbers indicate the number of corresponding zeolite; the weight of Cu in catalyst was in all cases 3.5 mg. Cu sites in the zeolite identified by the luminescence at 540 nm (see Figure 3). It is clearly seen from Figure 2 and Table 1, that even though the Cu-ZSM-5/1-5/4 zeolites contain nearly equal amounts of Cu, the conversion of NO differs dramatically; that implies different activity of Cu sites. It is not surprising as these zeolites exhibit different populations of Cu sites reflected in intensity of the Cu' luminescence bands at 480 and 540 nm (see Figure 1 and Table 1). Figure 3 shows that the activity, expressed as TOF, of the Cu-ZSM-5/1-5/6 is proportional to the number of Cu sites exhibiting luminescence at 540 nm. As has been the activity of Cu zeolites is connected with the presence of Cu+ ions and the reducibility of Cu2+ strongly

J. Phys. Chem., Vol. 99, No. 4, 1995 1067

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depends on the framework negative charge given by the SUA1 ratio (cf. ref. 17). Hence, it can be expected that the activity of the Cu-zeolite will depend on the Cu ion reducibility. This is reflected in Figure 3, where the TOF of NO per Cu atom vs concentration of Cu sites with the 540 nm luminescence splits roughly into two sets. One for Cu-zeolites with Si/A1 ratio of 14.1 and the other for that of 22.6. As expected, the Cu ions in zeolites with a lower number of framework A1 atoms will be more easily reduced to Cu+ than the Cu-zeolites with more A1 per unit cell. Therefore, these zeolites possess higher decomposition activity. All this implies that the redox mechanism (Cu2+ - Cu') operates in NO decomposition reaction, as and not that one suggested by Iwamoto et aL3 and Hall et assuming participation of the divalent Cu2+(N0)2complex as an active inte~mediate.~Moreover, the data presented here indicate that the highest activity in NO decomposition is achieved with the high silica zeolites, containing presumably more single framework A1 atoms compared to those with a higher content of framerwork Al, where pairing of A1 atoms (Si-0-A1-0-Si-0-A1-0-Si) is expected. However, this is in contrast with a review paper of Morretti," who concluded that the highest activity possess Cu-ZSM-5 zeolites of a low Si/A1 ratio. al.,536

Conclusions It can be concluded that a proportional relationship has been established between the catalytic activity of the Cu-ZSM-5 zeolites in NO decomposition and the population of the Cu sites exhibiting Cu+ luminescence at 540 nm. It should be stressed that such sites are present and are active in zeolites with the degree of exchange below as well as above 100%. Therefore, the activity of a particular zeolite is suggested to depend on the number of Cu sites, which are bonded in close vicinity of one framework A1 atom and bearing an extraframework oxygen. These results evidence that the active sites for NO decomposition are not small Cu-0 clusters, which can be also present in the

overexchanged zeolites, but the defined Cu species, chargebalanced by the negative framework, with a characteristic yellow-green Cu+ luminescence at 540 nm. But, a final conclusion cannot be made from these data, as to whether the active site is represented by some long-range Cu+* C u + dimer (analogously as suggested for Cu+ ions supported on alumina) or by a Cu+ monomer. Further investigations with respect to these active sites are in progress. However, it can be stated that the active Cu species are the sites originating from the (Cu2+-OH)+ cations present in salt solution, entering the cationic sites balanced by one framework aluminum and transforming under the zeolite dehydration probably into charged species bearing an extralattice oxygen. The occurrence of well-dispersed single A1 atoms in the framework of MFI structure, enabling formation of Cu with the emission at 540 nm, is the reason for high and stable activity of Cu-ZSM-5 compared to other structural types of Cuzeolites. The dependence of the reaction rate on the negative charge of the zeolite framework, Le., on the reducibility of the Cu active sites, evidences that the reaction of NO decomposition follows a redox mechanism.

Acknowledgment. The financial support of the Grant Agency of the Czech Republic, Project No. 203/93/1130, and of the US.-Czech Science and Technology Program, Project No. 93050, is greatly acknowledged. References and Notes (1) Iwamoto, M. In Zeolites and Related Microporous Materials: State of the Art 1994;Weitkamp, J., et al., Eds.; Elsevier Science Publishers: Amsterdam, 1994; Stud. .SUI$ Sci. Catal. 1994,84, 1395 and references therein. (2) Iwamoto, M.; Yahiio, H.; Tanda, K.; Mizuno, N.; Mine, Y.; Kagawa, S.J. Phys. Chem. 1991,95,3721. (3) Iwamoto, M.; Hamada, H. Catal. Today 1991,10,57. (4) Valyon, J.; Hall, W. K. J. Phys. Chem. 1993,97,120; Catal. Lett. 1993,19,109. (5) Hall, W. K.; Valyon, J. Catal. Lett. 1992,IS, 311. (6) Li, Y.; Hall, W. K. J. Catal. 1991,129,202. (7) Giamello, G.; Murphy, D.; Magnacca, G.; Morterra, C.; Shioya, Y.; Nomura, T.; Anpo, M. J. Catal. 1992,136,510. (8) Spoto, G.; Bordiga, S.; Scarano, D.;Zecchina, A. Catal. Lett. 1992, 13,39. (9) Shelef, M. Catal. Lett. 1992,15,305. (10) Kucherov, A. V.; Slinkin, A. A. J. Phys. Chem. 1989,93,864. (11) Moretti, G. Catal. Lett. 1994,23, 135. (12) Shpiro, E. S.; Gruenert, W.; Joyner, R. W.; Baeva, G. Catal.Lett. 1994,24, 159. (13) Kuroda, Y.; Kotani, A,; Maeda, H.; Moriwaki, H.; Morimato, T.; Nagao, M. J. Chem. Soc., Faraday Trans. 1992,88, 1583. (14) Anpo, M.; Nomura, T.; Shioya, Y .; Che, M.; Murphy, D.; Giamello, E. In Proceedings of the 10th International Congress on Catalysis, New Frontiers in Catalysis; Guczi L., et al., Eds.; Elsevier Science Publishers: Amsterdam, 1993; p 2155. (15) Barrie. J. D.: DUM. , B.:, Hollinesworth., G.:. Zink., J. I. J. Phvs. Chem. 1989,93,3958. ' (16) Hamada. H.: Matsubavashi. N.: Shimada. H.: Kintaichi. Y.: Ito. T.; Nishijima, A. Catal. Lett. i990,5, 189. (17) DEdeEek, J.; Wichterlovi, B. J. Phys. Chem. 1994,98,5721. (18) DEdeEek, J.; Sobalik, Z.; TvanXkovB, Z.; Kauckf, D.; Wichterlovfi, B., submitted to J. Phys. Chem. Y

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