FTIR Study of CO and NO Adsorption and Coadsorption on Ni-ZSM-5

May 2, 2002 - Ni-ZSM-5 and Ni/SiO2 samples have been characterized by the IR spectra of adsorbed and coadsorbed NO and CO. Adsorption of CO at room ...
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FTIR Study of CO and NO Adsorption and Coadsorption on Ni-ZSM-5 and Ni/SiO2 Mihail Mihaylov and Konstantin Hadjiivanov* Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria Received December 17, 2001. In Final Form: March 28, 2002 Ni-ZSM-5 and Ni/SiO2 samples have been characterized by the IR spectra of adsorbed and coadsorbed NO and CO. Adsorption of CO at room temperature on Ni/SiO2 leads to formation of Ni2+-CO species (ν(CO) at 2192 cm-1) that are easily destroyed by evacuation. The carbonyls formed on Ni-ZSM-5 after CO adsorption are two types and characterized by bands at 2220 and 2212 cm-1. In line with this high frequency, both species are much more stable than the Ni2+-CO species on Ni/SiO2 and are highly resistant toward evacuation. Reduction of Ni-ZSM-5 with CO results in the appearance of Ni+ ions. With CO these ions form Ni+(CO)2 dicarbonyls (νs at 2136 cm-1 and νas at 2092 cm-1), which lose one of their CO ligands during evacuation and are thus converted into Ni+-CO linear species (2109 cm-1). In contrast, no Ni+ ions are produced after reduction of the Ni/SiO2 sample. NO adsorption on Ni/SiO2 results in formation of Ni2+-NO species (1870 cm-1) which are converted, under NO equilibrium pressure, into Ni2+(NO)2 dinitrosyls (νs at 1870 cm-1 and νas at 1842 cm-1). Similar species are formed on Ni-ZSM-5: the mononitrosyls are characterized by bands at 1905-1895 cm-1, whereas the bands typical of dinitrosyls are at ca. 1900 cm-1 and 1874-1862 cm-1. After evacuation only mononitrosyls are present on the sample. With the Ni/SiO2 sample NO replaces preadsorbed CO. However, the situation with Ni-ZSM-5 is quite different. Here, mixed Ni2+(CO)(NO) species are clearly detected (ν(CO) at 2147 cm-1 and ν(NO) at 1863 cm-1). In these complexes the bond between Ni2+ and the ligands is weakened and, as a result, CO is easily removed by evacuation. The reasons for the different properties of nickel cations in different matrixes are discussed. It is proposed that the low coordination number of cations in ZSM-5 is the main reason for the formation of mixed complexes.

1. Introduction Metal-exchanged ZSM-5 zeolites are among the most studied catalysts for selective catalytic reduction (SCR) of nitrogen oxides by hydrocarbons.1-7 In this connection, the high activity of the classic Cu-ZSM-5 catalysts1-3 and the resistance of Fe-ZSM-5 toward water vapor4,5 deserve to be mentioned. Some years ago6 it was reported that, over Co-ZSM-5 and Ni-ZSM-5, the reaction can be performed by methane as a reducing agent, which offers the possibility of replacing ammonia as a reductant in the SCR from stationary sources of nitrogen oxides. Because of the higher activity of Co-ZSM-5 reported in the original work,6 cobalt-containing systems have been hugely studied.8-13 However, Tang et al.7 have recently established a ca. 100% activity of Ni-ZSM-5 in the microwave* To whom correspondence may be addressed. E-mail: kih@ svr.igic.bas.bg. (1) Iwamoto, M.; Yahiro, H.; Yu-u, Y.; Shundo, S.; Mizuno, N. Shokubai 1990, 32, 430. (2) Held, W.; Ko¨nig, A.; Richter, T.; Puppe, L. Soc. Automot. Eng., Trans., Sect. 4 1990, No. 900 496, 209. (3) Shelef, M. Chem. Rev. 1995, 95, 209. (4) Feng, X.; Hall, W. K. Catal. Lett. 1996, 41, 45. (5) Chen, H. Y.; Voskoboinikov, T.; Sachtler, W. M. H. J. Catal. 1998, 180, 171. (6) Li, Y.; Armor, J. N. In Natural Gas Convertion II; Curry-Hyde, H. E., Howe, R. F., Eds.; Elsevier: Amsterdam, 1994; p 103. (7) Tang, J.; Zhang, T.; Ma, L.; Li, L.; Zhao, J.; Zheng, M.; Lin, L. Catal. Lett. 2001, 73, 193. (8) Inaba, M.; Kintaichi, Y.; Haneda, M.; Hamada, H. Catal. Lett. 1996, 39, 269. (9) Hamada, H.; Kintaichi, Y.; Inaba, M.; Tabata, M.; Yoshinari, T.; Tsucji-da, H. Catal. Today 1996, 29, 53. (10) Hadjiivanov, K.; Avreyska, V.; Tzvetkov, G.; Stefanov, P.; Chupin, C.; Mirodatos, C.; Marinova, Ts. Surf. Interface Anal. 2001, 32, 175. (11) Hadjiivanov, K.; Tsyntsarski, B.; Nikolova, T. Phys. Chem. Chem. Phys. 1999, 1, 4521. (12) Djonev, B.; Tsyntsarski, B.; Klissurski, D.; Hadjiivanov, K. J. Chem. Soc., Faraday Trans. 1997, 93, 4055.

discharge-assisted reduction of NO to N2 by CH4 in the presence of O2. This report places Ni-ZSM-5 between the most promising SCR catalysts. One of the possible reasons for the unique catalytic properties of metal-exchanged zeolites is the low coordination state of the metal cations in them. We have performed comparative studies of Cu-ZSM-5 and Cu/SiO214 and Ag-ZSM-5 and Ag/SiO215 and have established that, in the zeolite matrix, both cations are characterized by a higher number of coordinative vacancies and can thus coordinate two or three small molecules as CO. Because of the practical importance of Ni-ZSM-5, we decided to characterize the nickel ions in this sample by analyzing the IR spectra of adsorbed and coadsorbed probe molecules (CO, NO) and to compare the results obtained with analogous results on Ni/SiO2. The first step was a detailed study of CO adsorption on Ni-ZSM-5. We found16 that, at low temperature, two CO molecules can be simultaneously bonded to one Ni2+ cation in Ni-ZSM-5 and the geminal complexes are easily destroyed to linear monocarbonyls. A sample reduced in CO is characterized by the presence of Ni+ cations, which can coordinate up to three CO molecules at low temperature. The different properties of oxidized and reduced sample have been explained by the higher ionic radius of Ni+, compared to Ni2+, which allows, sterically, coordination of more small molecules to one cation. In this study we report the results of a comparative study of CO and NO adsorption and coadsorption on Ni(13) Ivanova, E.; Hadjiivanov, K.; Klissurski, D.; Bevilacqua, M.; Armaroli, T.; Busca, G. Microporous Mesoporous Mater. 2001, 46, 299. (14) Hadjiivanov, K.; Kantcheva, M.; Klissurski, D. J. Chem. Soc., Faraday Trans. 1996, 92, 4595. (15) Hadjiivanov, K.; Kno¨zinger, H. J. Phys. Chem. B 1998, 102, 10936. (16) Hadjiivanov, K.; Kno¨zinger, H.; Mihaylov, M. J. Phys. Chem. B 2002, 106, 2618.

10.1021/la015739g CCC: $22.00 © 2002 American Chemical Society Published on Web 05/02/2002

Coadsorption of CO and NO on Ni-ZSM-5 and Ni/SiO2

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ZSM-5 and Ni/SiO2 samples, in both oxidized and partially reduced forms. It is demonstrated that, here again, nickel ions in the zeolite matrix are characterized by a high number of effective coordinative vacancies. 2. Experimental Section The silica support used was a commercial Aerosil sample with a specific surface area of 336 m2 g-1. The preparation of Ni/SiO2 by a grafting procedure was described elsewhere.17 Briefly, 10 g of SiO2 was suspended in 150 mL 0.1 M Ni2+ solution obtained from Ni(NO3)2 and containing 12.5 wt % ammonia (pH 12.3). The mixture was agitated for 1 h, and then the precipitate was filtered, washed thoroughly with water, and dried. Finally, the sample was calcined for 1 h at 623 K in order to remove adsorbed ammonium ions. The Ni/SiO2 sample thus obtained contained 1.72 wt % Ni. The low nickel concentration as well as the preparation technique ensure a high dispersion of the nickel ions which facilitates direct comparison of their properties with the properties of nickel ions exchanged in zeolites. The starting H-ZSM-5 material was supplied from Degussa and had a Si/Al ratio of 26.8. The main crystal diameter along the (011) lattice direction, estimated by the X-ray diffraction (XRD) line broadening, was 50 nm. Ni-ZSM-5 was prepared by ion exchange using 0.2 M Ni2+ solution obtained from Ni(NO3)2: 2 g of the sample was suspended in a 50 mL solution of Ni2+ and stirred for 10 h at 370 K. This procedure was performed three times and then the precipitate was filtered, washed thoroughly with hot water (353 K), dried at 383 K, and calcined at 673 K. The nickel concentration in the sample was 0.4 wt %, which corresponds to an exchange degree of 22%. The IR spectra were recorded on a Nicolet Avatar-320 spectrometer with a spectral resolution of 2 cm-1 accumulating 64 scans. Self-supporting pellets were prepared from the samples and treated directly in the IR cell. The latter was connected to a vacuum-adsorption apparatus with a base pressure below 10-3 Pa. Prior to the adsorption measurements, the samples were activated by 1 h of calcination at 723-773 K and 1 h of evacuation at the same temperature. The XRD analysis was made by a TUR M62 apparatus with computer-controlled HGZ-4 goniometer and using Co KR radiation. Carbon monoxide (99.5%) and oxygen (99.6%) were supplied by Merck. Nitrogen monoxide was provided by Messer Griesheim, GmbH, and had a purity of >99.0%. Before adsorption, carbon monoxide and oxygen were passed through a liquid nitrogen trap while NO was additionally purified by fraction distillation.

3. Results 3.1. Initial Characterization of the Samples. The IR spectrum of activated silica displays, in the ν(OH) region, a sharp band with a maximum at 3742 cm-1 characterizing isolated silanol groups.18 The IR spectrum of the activated Ni/SiO2 sample is essentially the same, which provides evidence that the silanol groups have not been involved in the grafting process. The IR spectrum of the activated H-ZSM-5 mainly presents, in the ν(OH) region, two narrow bands with maxima at 3740 and 3610 cm-1, which characterize silanol groups and the acidic zeolite hydroxyls, respectively18 (Figure 1, spectrum a). The band at 3610 cm-1 is less intense in the spectrum of the Ni-ZSM-5 sample (Figure 1, spectrum b), which is evidence of replacement of some acidic protons by nickel ions. Comparing the intensities of the bands at 3610 cm-1, one can abruptly estimate the exchange degree, here ca. 30%. This is in good agreement with the data obtained by the chemical analysis. No bands assignable to Ni2+-OH species were detected. 3.2. Adsorption of CO on Ni/SiO2. Adsorption of CO on a Ni/SiO2 sample activated at 773 K results in formation (17) Hadjiivanov, K.; Mihaylov, M.; Klissurski, D.; Stefanov, P.; Abadjieva, N.; Vassileva, E.; Mintchev, L. J. Catal. 1999, 185, 314. (18) Zecchina, A.; Otero Area´n, C. Chem. Soc. Rev. 1996, 25, 187.

Figure 1. FTIR spectra of activated H-ZSM-5 (a) and Ni-ZSM-5 (b) samples.

of a band at 2192 cm-1, which decreases in intensity with the equilibrium pressure and disappears after evacuation. In agreement with data from the literature,16,17,19-34 the band at 2192 cm-1 is assigned to Ni2+-CO species. The spectra are essentially the same when the sample is reduced in CO (1 kPa pressure) at temperatures up to 573 K. CO adsorption on a sample reduced at 723 K leads to the appearance of two weak bands with maxima at 2085 and 2055 cm-1. In agreement with data from the literature,35-37 the 2085 cm-1 band is attributed to subcarbonyls of metallic nickel while the 2055 cm-1 band corresponds to linear Ni0-CO species. These results are evidence of the presence of metallic nickel on our sample; i.e., in this case the reduction of Ni2+ has proceeded directly to Ni0. 3.3. Adsorption of CO on Ni-ZSM-5. A detailed study of CO adsorption on this sample has already been reported.16 In this work we have obtained essentially the same results, and the main points will be only briefly described. The introduction of CO (2.7 kPa equilibrium pressure) to the activated Ni-ZSM-5 sample leads to the (19) Sault, A. G.; Peden, C. H. F.; Boespflug, E. P. J. Phys. Chem. 1994, 98, 1652. (20) Anderson, J. A.; Daza, L.; Fierro, J. L. G.; Rodrigo, M. T. J. Chem. Soc., Faraday Trans. 1993, 89, 3651. (21) Sheng, T. C.; Rebenstorf, B. Langmuir 1991, 7, 2659. (22) Hadjiivanov, K.; Klissurski, D.; Kantcheva, M.; Davydov, A. J. Chem. Soc., Faraday Trans. 1991, 87, 907. (23) Angell, C. L.; Schaffer, P. C. J. Phys. Chem. 1966, 70, 1413. (24) Davydov, A. IR Spectroscopy Applied to Surface Chemistry of Oxides; Nauka: Novosibirsk, 1984. (25) Harrison, P. G.; Thornton, E. W. J. Chem. Soc., Faraday Trans. 1 1978, 74, 2703. (26) Peri, J. B. J. Catal. 1984, 86, 84. (27) Peri, J. B. Discuss. Faraday Soc. 1966, 41, 121. (28) Lausch, H.; Moerke, V.; Vogt. F.; Bremer, H. Z. Anorg. Allg. Chem. 1983, 499, 213. (29) Orlova, L.; Lokhov, Yu.; Ione, K.; Davydov, A. Izv. Akad. Nauk. SSSR, Ser. Khim. 1979, 1937. (30) Ione, K. G.; Romannikov, V. N.; Davydov, A. A.; Orlova, L. B. J. Catal. 1979, 57, 126. (31) Kasai, P. H.; Bishop, R. J., Jr.; McLeod, D., Jr. J. Phys. Chem. 1978, 82, 279. (32) Bonnevoit, L.; Cai, F. X.; Che, M.; Kermarec, M.; Legendre, O.; Lepetit, C.; Olivier, D. J. Phys. Chem. 1987, 91, 5912. (33) Hadjiivanov, K.; Mihaylov, M.; Abadjieva, N.; Klissurski, D. J. Chem. Soc., Faraday Trans. 1998, 94, 3711. (34) Garbowski, E.; Primet, M.; Mathieu, M. V. In Proceedings of the VI International Zeolite Conference; Olsen, D., Bisio, A., Eds.; Butterworths: Guildford, U.K., 1985; p 396. (35) Sheppard, N.; Nguyen, T. T. In Advances in Infrared and Raman Spectroscopy; Clarke, R. J., Hester, R. E., Eds.; Heyden: London, 1978; Vol. 4, p 67. (36) Mihaylov, M.; Hadjiivanov, K.; Kno¨zinger, H. Catal. Lett. 2001, 76, 59. (37) Zaki, M. I. In Catalysts in Petroleum Refining and Petrochemical Industries; Absi-Halabi, M., et al., Eds.; Elsevier: Amsterdam, 1995; p 569.

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Figure 2. FTIR spectra of CO adsorbed on Ni-ZSM-5: equilibrium pressure of 2700 (a), 800 (b), and 200 Pa CO (c), after short evacuations (d, e), and under dynamic vacuum (f-i).

Figure 3. FTIR spectra of NO adsorbed on Ni/SiO2: equilibrium pressure of 1900 (a), 950 (b), 500 (c), 250 (d), and 130 Pa NO (e), after short evacuation (f), and under dynamic vacuum (g).

appearance of an intense band at 2212 cm-1 with a pronounced shoulder at 2220 cm-1 (Figure 2, spectrum a). Computer deconvolution also reveals the presence of a broad and weak band at 2200 cm-1 (see the inset in Figure 2). In addition, very weak bands at 2170 and 2155 cm-1 are detected. Two more low intensity bands at 2135 and 2092 cm-1 are also visible. Decrease of the equilibrium pressure and evacuation result in fast disappearance of the 2200 cm-1 feature and the bands at 2170 and 2155 cm-1 (Figure 2, spectra b-i). A decrease in intensity of the bands at 2220 and 2212 cm-1 is also observed. However, the band at 2212 cm-1 diminishes faster. The bands at 2135 and 2092 cm-1 disappear after a prolonged evacuation, and a band at 2109 cm-1 emerges instead. In agreement with data from the literature,16,17,19-34 the bands at 2220 and 2212 cm-1 are assigned to Ni2+-CO species. Here, the carbonyls are much more stable than the Ni2+-CO carbonyls detected with the Ni/SiO2 sample. This is consistent with the conception of the stability of σ-carbonyls: the higher the stretching frequency, the higher the stability. Indeed, many data implicate that, due to their high charge, Ni2+ ions form exclusively a σ-bond with CO.38 These results are also in agreement with other studies comparing Cu+ and Ag+ cations in ZSM-5 and on SiO2: it was reported14,15 that the high coordinative unsaturation of cations in ZSM-5 is the reason of formation of a stronger σ-bond between the cation and CO as compared to the case with SiO2. On the basis of low-temperature CO adsorption experiments,16 the weak feature at 2200 cm-1 is attributed to Ni2+(CO)2 species which are formed, in a negligible extent, even at room temperature. The band at 2170 cm-1 is assigned to CO H-bonded to the zeolite acidic hydroxyls.18 In principle, CO can be polarized by OH groups at low temperatures, but the high acidity of the bridged OH groups on H-ZSM-5 is the reason that a small amount of OH-CO species are detected even at ambient temperature. This assignment is supported by the red shift of part of the 3610 cm-1 band to ca. 3340 cm-1 (detected in the difference spectra) under a high CO equilibrium pressure. The band at 2155 cm-1 coincides in position with the OH-CO bands typical of Si-OH groups.18 However, the very low stability of these complexes (the shift of the Si-OH bands after low-temperature CO adsorption is -90 cm-1 39) allows rejection of this interpretation. Moreover, no shift of the silanol hydroxyls

is noticed in the spectra under CO. We suggests that the minor band at 2155 cm-1 is rather due to CO adsorbed on a negligible amount of a “NiO”-like phase on our sample presumably located at the outer zeolite surface.40,41 The weak bands at 2135 and 2092 cm-1 are assigned (see below) to Ni+(CO)2 species formed with the participation of a small fraction of Ni+ cations created during the sample activation.24,32,42-44 The corresponding linear carbonyls are characterized by the band at 2109 cm-1 seen after evacuation of the sample. These bands are not detected with a sample pretreated with a NO + O2 mixture (this mixture is known to possess a high oxidation ability), which is consistent with the above assignment. The sample was reduced by CO (723 K, 1 kPa CO pressure) and then cooled to ambient temperature in this CO atmosphere. The IR spectrum of the sample thus treated is characterized by three main bands in the carbonyl stretching region, namely, at 2212, 2136, and 2092 cm-1. The set of bands at 2136 and 2092 cm-1 is assigned to the symmetric and antisymmetric CO modes of geminal Ni+(CO)2 dicarbonyls.24,32,42-44 The intensity of the band at 2212 cm-1 is slightly reduced as compared to the same band detected on the oxidized sample, and its high-frequency shoulder at 2220 cm-1 is absent. This is in line with the fact that some of the Ni2+ ions (preferentially those forming the 2220 cm-1 carbonyls) have been reduced to Ni+. Evacuation leads to the disappearance of the bands characterizing gem-species and appearance, on their place, of a band at 2109 cm-1, assigned to linear Ni+ monocarbonyls.16,20,24,26-33,42,43 These results indicate that a ZSM-5 matrix stabilizes the Ni+ cations. As was described above, the attempts to produce Ni+ ions on the Ni/SiO2 sample failed; in the latter case the Ni2+ ions were directly reduced to metallic nickel. 3.4. Adsorption of NO on Ni/SiO2. NO adsorption (1.9 kPa equilibrium pressure) on the activated Ni/SiO2 sample results in the appearance of two bands in the IR spectrum, their maxima being at 1870 and 1842 cm-1 (Figure 3, spectrum a). Decreasing the equilibrium pressure and evacuation lead to a decrease of the intensity of both bands; however, the band at 1842 cm-1 decreases

(38) Neyman, K.; Ro¨sch, N. Chem. Phys. 1993, 177, 561. (39) Beebe, P.; Gelin, P.; Yates, J. T., Jr. Surf. Sci. 1984, 148, 526.

(40) Busca, G.; Lorenzelli, V.; Sanchez-Escribano, V. Chem. Mater. 1992, 4, 595. (41) Vesecky, S. M.; Xu, H. P.; Goodman, D. W. J. Vac. Sci. Technol., A 1994, 12, 2114. (42) Kermarec, M.; Olivier, D.; Richard, M.; Che, M.; Bozon-Verduraz, F. J. Phys. Chem. 1982, 86, 2818. (43) Kermarec, M.; Lepetit, C.; Cai, F. X.; Olivier, D. J. Chem. Soc., Faraday Trans. 1 1989, 85, 1991. (44) Garbowski, E.; Vedrine, J. C. Chem. Phys. Lett. 1977, 48, 550.

Coadsorption of CO and NO on Ni-ZSM-5 and Ni/SiO2

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Figure 4. FTIR spectra of NO adsorbed on Ni-ZSM-5: equilibrium pressure of 1330 (a), 670 (b), 330 (c), 130 (d), and 70 Pa NO (e), after short evacuation (f), and evolution of the spectra under dynamic vacuum (g, h).

Figure 5. FTIR spectra of CO and NO coadsorbed on Ni/SiO2: adsorption of CO (1.3 kPa equilibrium pressure) (a) and after subsequent admission of small doses of NO up to 250 Pa partial pressure (b-j).

faster (Figure 3, spectra b-g). At moderate coverages it is clearly seen that the band at 1870 cm-1 has a complex contour. Computer deconvolution has suggested it to consist of at least two components. It is to be noted that these bands are not removed by evacuation. Moreover, they are still present in the spectra after 373 K evacuation. The bands at 1870 and 1842 cm-1 are in the N-O stretching region and are thus generally assigned to N-O modes.45 Since the exact determination of the species involves experiments which will be described further on, a more detailed assignment of these bands will be proposed in the discussion section. 3.5. Adsorption of NO on Ni-ZSM-5. Introduction of NO (1.3 kPa equilibrium pressure) to the Ni-ZSM-5 sample results in the appearance of two groups of bands in the 2000-1800 cm-1 region (Figure 4, spectrum a). A strong band with a maximum at 1902 cm-1 and a pronounced lower frequency shoulder is recorded. In addition, two lower-intensity bands with maxima at 1874 and 1862 cm-1 are distinguished. Decreasing the equilibrium pressure down to 30 Pa leads to a decrease in intensity of all bands (Figure 4, spectra b-e). During evacuation, the bands at 1874 and 1862 cm-1 strongly decrease in intensity (the band at 1874 cm-1 faster) and almost disappear from the spectrum (Figure 4, spectra f-h). The complex band at 1902 cm-1 also displays an initial intensity decrease but then is highly resistant toward evacuation. Computer deconvolution reveals its complex character: it consists of one main peak at 1895 cm-1 and two weak features at 1906 and 1902 cm-1. The results resemble those obtained with the Ni/SiO2 sample, and here again, the exact assignment of the bands in the 1950-1800 cm-1 region will be proposed in the discussion section. Note, however, the higher stretching frequency of the nitrosyl bands as compared to the Ni/SiO2 sample, which is in agreement with the higher electrophilicity of the Ni2+ ions in Ni-ZSM-5. In addition to the nitrosyl bands, a band at 2134 cm-1, assigned to NO+,46 and a weak band at 1630 cm-1, most probably characterizing nitratospecies,45 have been recorded. 3.6. Coadsorption of CO and NO on Ni/SiO2. Introduction of CO (870 Pa equilibrium pressure) to the sample resulted in formation of the already described Ni2+-CO species characterized by a band at 2192 cm-1

(Figure 5, spectrum a). Then, the cell was connected to a reservoir containing NO, and the spectra were collected with time, i.e., after the slow diffusion of NO to the sample. It is seen (Figure 5, spectra b-f) that the CO band gradually decreases in intensity and is slightly blue shifted. Simultaneously, a band at 1870 cm-1 starts to develop in the N-O stretching region. After the CO band has decreased ca. twice in intensity, a new band at 1842 cm-1 also starts to develop (Figure 5, spectrum d). Both NO bands go on increasing even after the full disappearance of the CO band (Figure 5, spectra g-j). Subsequent evacuation results in an intensity decrease of the 1870 cm-1 band, while the 1842 cm-1 band almost disappears. These results are evidence that NO replaces CO from the sample surface. Some experiments were performed with addition of CO to the NO-Ni/SiO2 system. NO (130 Pa equilibrium pressure) was introduced into the cell which resulted in the appearance of the already described bands at 1870 and 1842 cm-1. Addition of CO (2.5 kPa equilibrium pressure) to the system did not lead to changes in the spectrum. After a short evacuation, the nitrosyl bands decreased in intensity. No carbonyls were formed after subsequent CO adsorption (2.5 kPa equilibrium pressure). Hence, the CO adsorption sites had remained blocked. 3.7. Coadsorption of CO and NO on Ni-ZSM-5. To study the coadsorption of CO and NO on Ni-ZSM-5, we first produced surface Ni2+-CO species by adsorption of CO (2 kPa equilibrium pressure). This resulted in the appearance of the band at 2212 cm-1 and its higherfrequency shoulder (Figure 6, spectrum a). Then, similarly to the experiments with Ni/SiO2, NO was allowed to diffuse to the sample. As a result, the carbonyl bands at 2220 and 2212 cm-1 started to decrease progressively in intensity (Figure 6, spectra b-o). Simultaneously, a new, very strong band at 2147 cm-1 developed at their expense. The latter had a pronounced shoulder at 2133 cm-1, which was already assigned to NO+. The NO+ species are usually produced after NO adsorption on metal-exchanged zeolites. Computer deconvolution also revealed a broad and weak feature at 2106 cm-1. Two peculiarities of the process should be noted: (i) the band at 2220 cm-1 disappears faster than that at 2212 cm-1 and (ii) the integral intensity of the band at 2147 cm-1 is higher by a factor of 1.23 than the intensity of the bands at 2220-2212 cm-1 (the 2133 cm-1 band was subtracted for these calculations). However, taking into account the reversibility of the carbonyls characterized by the band at 2147 cm-1, we estimated that the extinction coefficient of this band is ca. 1.5 times

(45) Hadjiivanov, K. Catal. Rev.-Sci. Eng. 2000, 42, 71. (46) Hadjiivanov, K.; Saussey, J.; Freysz, J.-L.; Lavalley, J.-C. Catal. Lett. 1998, 52, 103.

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Figure 6. FTIR spectra of CO and NO coadsorbed on Ni-ZSM5: adsorption of CO (2 kPa equilibrium pressure) (a) and after subsequent admission of small doses of NO up to 400 Pa partial pressure (a-o).

Figure 7. FTIR spectra of NO and CO coadsorbed on Ni-ZSM5: adsorption of NO (400 Pa equilibrium pressure) (a) and addition of CO (4 kPa partial pressure) to the system (b), decreasing of the total equilibrium pressure to 200 Pa (c), short evacuation (d), under dynamic vacuum (e), after subsequent introduction of CO (2.5 kPa equilibrium pressure) (f), and after evacuation (g).

higher than the extinction coefficient of the bands at 22202212 cm-1. Simultaneously with the described changes, three bands developed almost in parallel in the NO stretching region, their maxima being at 1905, 1897, and 1863 cm-1 (Figure 6, spectra b-o). To obtain more information about the nature of the species formed, some additional coadsorption experiments at a low initial NO pressure have been performed. NO adsorption (400 Pa equilibrium pressure) results in a spectrum analogous to that shown in Figure 4, spectrum a (Figure 7, spectrum a). Addition of CO (4 kPa equilibrium pressure) leads to the simultaneous appearance of two bands at 2146 and 1864 cm-1. At the same time the 1900 cm-1 band slightly decreases (Figure 6, spectrum b). Decrease of the equilibrium pressure and short evacuation result in a decrease in intensity of the bands at 2146 and 1864 cm-1 (Figure 7, spectra c, d) and only the bands at 2134 and 1900 cm-1 remain after a prolonged evacuation (Figure 7, spectrum e). Subsequent CO adsorption provokes the reappearance of the bands at 2146 and 1864 cm-1 and the appearance of another band at 2209 cm-1 (Figure 7, spectrum f). The latter band is assigned to Ni2+CO species, and its presence provides evidence that a fraction of the Ni2+ sites has been liberated. Subsequent evacuation restores the spectrum registered before the second admission of CO (Figure 7, spectrum g).

Mihaylov and Hadjiivanov

Figure 8. FTIR spectra of NO and CO coadsorbed on Ni-ZSM5: adsorption of NO (1.7 kPa equilibrium pressure) (a) and subsequent admission of CO (2.9 kPa partial pressure) (b).

Figure 9. FTIR spectra of CO and NO coadsorbed on reduced Ni-ZSM-5: adsorption of CO (670 Pa equilibrium pressure) (a) and after admission of a small dose of NO (b).

Some experiments have been performed with a higher (1.7 kPa) initial NO equilibrium pressure. The spectrum registered under these conditions is characterized by bands at 2133, 1902, 1874, and 1863 cm-1 (Figure 8, spectrum a). Introduction of CO (2.9 kPa partial pressure) to the NO-Ni-ZSM-5 system provokes a decrease in intensity of the bands at 1902 and 1874 cm-1 and development, at their expense, of a band at 1863 cm-1 (Figure 8, spectrum b). In the carbonyl stretching region the band at 2133 cm-1 increases in intensity and the difference spectra clearly show development of a band at 2146 cm-1. The above experiments clearly demonstrate the appearance on Ni-ZSM-5, in the presence of CO and NO simultaneously, of mixed Ni2+(CO)(NO) species, characterized by ν(CO) at 2146 cm-1 and ν(NO) at 1863 cm-1. No similar species have been detected on a Ni/SiO2 sample which is evidence of the lower ability of Ni2+ ions on silica to coordinate two adsorbate molecules simultaneously. 3.8. Coadsorption of CO and NO on Reduced NiZSM-5. Finally, we studied the coadsorption of CO and NO on a reduced Ni-ZSM-5 sample. The reduction was performed at 723 K in a CO atmosphere. Adsorption of CO (670 Pa equilibrium pressure) on this sample resulted in the appearance of Ni2+-CO species (2212 cm-1) and Ni+(CO)2 geminal complexes (2137 and 2092 cm-1) (Figure 9, spectrum a). Subsequent introduction of a small amount of NO to the system resulted in almost full disappearance of the initially recorded carbonyl bands (Figure 9, spectrum b). Strong bands at 2148, 1902, and 1864 cm-1 developed instead. The set of bands at 2148 and 1864 cm-1 was already assigned to Ni2+(CO)(NO) species. Thus, the

Coadsorption of CO and NO on Ni-ZSM-5 and Ni/SiO2

results imply that NO oxidizes Ni+ ions on Ni-ZSM-5, which hinders the possibility of formation of mixed Ni+(CO)x(NO)y species. 4. Discussion 4.1. Interpretation of the IR Bands Observed after CO Adsorption. Carbon monoxide is a probe molecule which allows selective monitoring of Ni2+, Ni+, and Ni0 on surfaces. CO forms mainly σ and electrostatic bonds with the Ni2+ ions. The formation of a σ-bond is accompanied by a decrease of the electron density of the weakly antibonding 5σ orbital of CO and as a result the C-O bond order and the C-O stretching frequency increase. The electrostatic bond (which is weaker than the σ-bond) also leads to an increase of the C-O stretching frequency because of the vibrational Stark effect. That is why the CO stretching frequency in the Ni2+-CO species is higher than the frequency of gaseous CO (2143 cm-1) and is usually observed in the 2220-2180 cm-1 region.16,17,19-34 Only with NiO and bulk mixed oxides containing nickel the carbonyls of Ni2+ are detected at lower frequencies.40,41 Since both bonds (σ and electrostatic) are relatively weak, the Ni2+-CO species are usually detected under a certain CO equilibrium pressure. Stable Ni2+-CO species have been found with nickel-exchanged zeolites only.23,24,34 Our experiments on CO adsorption of Ni-ZSM-5 revealed the existence of two kinds of Ni2+-CO species characterized by stretching C-O frequencies at 2220 and 2212 cm-1, respectively. Contrary to most observations, these species are highly resistant toward evacuation. This fact can easily be explained by the sample nature. It is well-known14,15 that metal cations in a ZSM-5 matrix are characterized by a high electrophility as compared to cations on oxide supports and even in other zeolites. This is due to the low coordination of the metal cations which also allows simultaneous adsorption of two or three small molecules on them. In agreement with this, we have reported16 that, at low temperature, CO adsorption on Ni-ZSM-5 leads to formation of Ni2+(CO)2 species. A small fraction of these species were observed in this study even at room temperature (band at 2200 cm-1). CO adsorption on Ni/SiO2 results in formation of Ni2+CO species that are definitely less stable than the respective species on Ni-ZSM-5: they are easily destructed by evacuation. This also follows from the relatively low stretching frequency (2192 cm-1) of the carbonyls which indicates formation of relatively weak σ and electrostatic binds. Here again, the results can be explained by the weaker electrophilicity of metal cations supported on silica as compared to the same cations in a ZSM-5 matrix. In line with this, no evidence of coordination of more than one CO molecule to one Ni2+ cation has been found for Ni/SiO2. Ni+ cations possess a lower charge and a higher p-electron density than Ni2+. As a result, CO is coordinated to Ni+ ions by both, σ- and π-back-bonds (here the electrostatic interaction is weak). Due to the synergistic effect between both bonds, the carbonyls formed are strong (stable up to 423 K). The formation of a π-back-bond results in occupation of the 2π* antibonding CO orbitals, and as a result, the C-O bond order decreases, which lowers the C-O stretching frequency. The exact position of the C-O band in the Ni+-CO species depends on the balance between the σ- and π-bonds. However, since the effect of the π-bond on the C-O frequency is stronger than that of the σ-bond, usually the Ni+-CO species absorb at frequencies lower that 2143 cm-1. Analysis of the available literature data16,20,24,26-33,42,43 indicates they are found in

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the 2160-2110 cm-1 region. Some authors have reported formation of dicarbonyls of Ni+ under CO equilibrium pressure.24,32,42-44 These species are characterized by νs(CO) at 2145-2131 cm-1 and νas(CO) at 2100-2081 cm-1. Interaction of our Ni-ZSM-5 sample with CO at 673 K results in reduction of part of the Ni2+ ions to Ni+. With CO these cations form Ni+-CO species absorbing at 2109 cm-1. Even at low equilibrium CO pressures the monocarbonyls are converted into dicarbonyls characterized by bands at 2137 and 2092 cm-1. This willingness of the Ni+ in our sample to form dicarbonyls can be again easily explained by the low coordination number of Ni+ in a ZSM-5 matrix. A peculiarity that will be useful in the following discussion is the much higher CO extinction coefficient of the carbonyls of Ni+ as compared to the Ni2+CO species. All attempts to produce Ni+ ions on our Ni/SiO2 sample by means of reduction with CO failed. Hence, Ni+ ions are stabilized in the ZSM-5 matrix. This can be explained by the fact that the most favorable occupation of the cationic sites in highly siliceous zeolites is that of univalent cations. With metallic nickel CO forms mainly a π-bond and, as a result, the CO stretching frequency falls well below 2143 cm-1. The interaction of CO with Ni0 depends on the metal particle size. With highly dispersed nickel CO forms a series of compounds:17,20,26,33,35-37,47 linear Ni0-CO species (band at ca. 2060-2025 cm-1); bridged carbonyls (below 1980 cm-1); Ni0(CO)x (x ) 2 or 3) subcarbonyls (ca. 21002070 cm-1) and adsorbed Ni(CO)4 (ca. 2150-2130, 21002030, and 2050-1830 cm-1), which can pass into the gas phase (band at 2157 cm-1). The carbonyl bands detected on our reduced Ni/SiO2 sample after CO adsorption are indicative of the presence of metallic nickel. In particular, the band at 2085 cm-1 is assigned to subcarbonyls and the 2055 cm-1 band, to linear carbonyls. The detailed characterization of metallic nickel is beyond the aims of this study. We would like to note, however, that no metallic nickel has been detected on the reduced Ni-ZSM-5 sample, which can be explained by the above hypothesis about the stabilization of Ni+ in a ZSM-5 matrix. 4.2. Interpretation of the IR Bands Observed after NO Adsorption. Being a ligand, NO shows some differences as compared to CO. NO is a stronger base and, when no π-back-donation occurs, the nitrosyls are stronger than the respective carbonyls. A typical example is the Cu2+ cations. They are not acidic enough to form stable carbonyls (as a rule the Cu2+-CO species are registered at a low-temperature only) but form stable nitrosyl species.48 Another peculiarity of NO is that, due to the presence of an unpaired electron, it tends to dimerize. Very often NO adsorption leads exclusively to formation of dinitrosyl species.45 Formally, they can be regarded as adsorbed N2O2, since the existence of two coordinative vacancies is not needed for their formation. The NO stretching frequency of the nitrosyl complexes of Ni2+ is usually observed near the frequency at which gaseous NO absorbs (1876 cm-1). Analysis of literature data shows that these species have been registered at 1890-1810 cm-1.17,24-26,34,45,49-52 This suggests some π-backdonation occurring presumably as a result of the σ - π bond synergism. Morrow and Moran,51 based on isotopic (47) Mohana Rao, K.; Spoto, G.; Zecchina, A. Langmuir 1989, 5, 319. (48) Hadjiivanov, K.; Dimitrov, L. Microporous Mesoporous Mater. 1999, 27, 49. (49) Atanasova, P.; Agudo, A. L. Appl. Catal. B 1995, 5, 329. (50) Ziolek, M.; Nowak, I.; Sobczak, I.; Poltorak, H. K. Stud. Surf. Sci. Catal. 2000, 130, 3047. (51) Morrow, B. A.; Moran, L. E. J. Catal. 1980, 62, 294. (52) Mariscal, R.; Navarro, R. M.; Pawelec, B.; Fierro, J. L. G. Microporous Mesoporous Mater. 2000, 34, 181.

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substitution studies, attribute a band at 1864 cm-1 (appearing after NO adsorption on Ni/SiO2) to dinitrosyls. According to these authors,51 only the symmetric mode of the species is IR active, the respective νas being inactive because of the fact that the N-N axis is parallel to the surface. Garbowski et al.34 have detected two bands at 1895 and 1855 cm-1 after NO adsorption on Ni-mordenite. These authors have assigned the band at 1895 cm-1 to linear species and the band at 1855 cm-1 to νs of geminal dinitrosyls (νas coinciding with ν(N-O) of the linear species). The existence of two bands in the N-O stretching region has been explained by Mariscal et al.52 by a heterogeneity of the Ni2+ sites. The bands observed after NO adsorption on our Ni/ SiO2 sample are with maxima at 1870 and 1842 cm-1. These results could be taken as evidence of the existence of two families of Ni2+ ions on the sample surface, which form two different types of Ni2+-NO species, as proposed by Mariscal et al.52 However, in such a case the lower frequency of the bands at 1842 cm-1 presupposes a significant π-component of the Ni2+-NO bond. As a result, the respective nitrosyls should be characterized by a high stability. This is just opposite to our experimental observations. Our results can be rationalized by the assumption of Garbowski et al.34 After NO adsorption Ni2+-NO species (1870 cm-1) are formed. The latter are converted, in NO equilibrium pressure, into Ni2+(NO)2 dinitrosyls (νs at ca. 1870 cm-1 and νas at 1842 cm-1). Analysis of literature data45 shows that νs of surface dinitrosyl species have been found in the 1940-1765 cm-1 region and νas, at 18351691 cm-1. The antisymmetric modes registered by Garbowski et al.34 and in this study are at a higher frequency than those usually reported. This fact can be explained by a relatively weak interaction between the two adsorbed NO molecules resulting in a relatively small spectral split. Indeed, most of the reported dinitrosyls concern cases when the adsorbate-adsorbate interaction is very strong and the geminal species are more stable than the linear nitrosyls. Another argument supporting the above hypothesis is that no heterogeneity of the nickel sites has been confirmed by means of CO adsorption. The results obtained after NO adsorption on Ni-ZSM-5 are in general similar to those described above for Ni/ SiO2. Here, one complex band with a maximum centered at ca. 1900 cm-1 dominates at low coverages. This band can be assigned to linear Ni2+-NO species, its complex contour arising from the heterogeneity of the Ni2+ sites as has also been observed when testing the sample with CO. The higher N-O stretching frequency, as compared to the case of Ni/SiO2, is determined by the stronger Ni2+NO bond arising from the highest electrophilicity of the Ni2+ ions in Ni-ZSM-5. Indeed, the nitrosyls on Ni-ZSM-5 are more resistant toward evacuation than are the nitrosyls formed on Ni/SiO2. At higher coverages two more bands, at 1874 and 1862 cm-1, are observed, and the band at ca. 1900 cm-1 has an enhanced intensity. By analogy with Ni/SiO2, the band at 1900 cm-1 is assigned to the νs modes of Ni2+(NO)2 geminal species, while the corresponding νas vibrations are found at 1974 and 1862 cm-1. Here again, the existence of two bands can be explained by the heterogeneity of the Ni2+ sites. The antisymmetric modes of the Ni2+(NO)2 species formed on Ni-ZSM-5 are characterized by unusual high frequencies. It is known that when the simultaneous bonding of two small molecules at one site is only determined by the low coordination number of the cation and the adsorbate-adsorbate interaction is negligible, the

Mihaylov and Hadjiivanov

geminal species are characterized by one IR mode, which is usually indicative of lower acidity of the cation.53 Typical examples are the Na+(15N2)2 and K+(15N2)2 geminal species formed after low-temperature 15N2 adsorption on Na-EMT and K-EMT, respectively.54 The Na+-15N2 linear species are characterized by a band at 2258.5 cm-1. The formation of this complex decreases the electrophility of the Na+ cation and the coordination of a second 15N2 molecule results in formation of geminal Na+(15N2)2 species where each 15N2 molecule is more weakly bonded as shown by the lower 15N-15N stretching frequency (2255 cm-1). Due to the higher ionic radius and lower electrophilicity of the K+ ions, the spectral difference between K+-15N2 and K+(15N2)2 species is even smaller (2252 and 2251.5 cm-1, respectively). The shift in the above cases can be determined as a pure local chemical shift. As shown by CO adsorption experiments, the Ni2+ ions in Ni-ZSM-5 possess at least two coordinative vacancies each. We are of the opinion that, when geminal dinitrosyls are formed on Ni-ZSM-5, the positions of the stretching frequencies of this complex are determined by both an adsorbate-adsorbate interaction and the local chemical shift. This can explain the unusually high frequency of the observed νas modes. 4.3. Interpretation of the IR Bands Observed after Coadsorption of CO and NO. Mixed carbonyl-nitrosyl complexes have been reported for different systems. Typical examples are Pd2+(CO)(NO)24 and Rh+(CO)(NO).55 However, it seems that a suitable electronic configuration is necessary for their formation. Thus, dicarbonyls14,56 and dinitrosyls57 are easily formed on Cu+ cations in Cu-ZSM5, but no mixed species have been observed. In what follows we shall show that another requirement for the formation of mixed species is, at least in some cases, a low coordination number of the complexing metal cation. There are only two reports31,34 concerning carbonylnitrosyl adsorption complexes with participation of nickel ions, both concerning Ni-mordenite. Studying CO-NO coadsorption on this sample, Kasai et al.31 registered bands at 2155 and 1867 cm-1 and attributed them to the ν(CO) and ν(NO) modes, respectively, of Ni+(CO)(NO) species. When only CO was adsorbed on the sample, the authors registered Ni2+-CO species at 2212 cm-1, Ni+-CO at 2110 cm-1, and geminal Ni+(CO)2 complexes at 2140 and 2094 cm-1. Adsorption of NO alone provoked appearance of a band at 1900 cm-1 which the authors assigned to Ni+NO species. Garbowski et al.34 have registered only carbonyls and (di)nitrosyls of Ni2+ after separate adsorption of CO and NO. Coadsorption experiments of CO-NO resulted in bands at 2145 and 1855 cm-1 assigned to mixed Ni2+(CO)(NO) species. Our experiments on CO-NO coadsorption on Ni-ZSM-5 produced bands similar to those detected by Kasai et al.31 and Garboski et al.34 on Ni-mordenite. Our results support the idea that the oxidation state of nickel in the mixed species is 2+. Dosage of NO to Ni-ZSM-5 with preliminary formed Ni2+-CO species results in gradual conversion of the Ni2+-CO band at 2212 cm-1 into a band at 2147 cm-1 (Figure 6). Since no Ni+ cations were present before NO admission and NO was not likely to reduce Ni2+ ions, we assign the 2147 cm-1 band to the C-O modes of mixed (53) Hadjiivanov, K.; Ivanova, E.; Klissurski, D. Catal. Today 2001, 76, 59. (54) Hadjiivanov, K.; Massiani, P.; Kno¨zinger, H. Phys. Chem. Chem. Phys. 1999, 1, 3831. (55) Iizuka, T.; Lunsford, J. H. J. Mol. Catal. 1980, 8, 391. (56) Spoto, G.; Zecchina, A.; Bordiga, S.; Ricchiardi, S.; Martra, G.; Leofanti, G.; Petrini, Appl. Catal. B 1994, 3, 151. (57) Spoto, G.; Bordiga, S.; Scarano, D.; Zecchina, A. Catal. Lett. 1992, 13, 39.

Coadsorption of CO and NO on Ni-ZSM-5 and Ni/SiO2

Ni2+(CO)(NO) complexes. The respective N-O modes are at 1864 cm-1. Indeed, it is evident from Figure 7 that both bands change in synchrony in the different experiments. Upon evacuation the mixed Ni2+(CO)(NO) species easily lose their CO ligand, thus being converted into stable Ni2+-NO mononitrosyls. This shows that the bond between Ni2+ and CO is significantly weakened after the insertion of a NO molecule into the carbonyl complex. This is in line with the much lower C-O stretching frequency in the mixed complexes indicating a much weaker σ-bond. Careful analysis of the spectra, however, suggests some π-back-donation from CO to Ni2+ in the mixed species. This follows from the higher C-O extinction coefficient as compare to the Ni2+-CO species. Indeed, it is well-known that formation of a π-back-bond strongly enhances the intensity of the carbonyl bands.24 Note, however, that the CO extinction coefficient in the Ni2+(CO)(NO) species remains much lower than the same coefficient in the carbonyls of Ni+. Because of the low binding energy of CO in the Ni2+(CO)(NO) species, the latter are not typical in the presence of excess of NO. In this case NO replaces CO and dinitrosyls predominate (Figure 8). At lower NO pressures linear nitrosyls are detected along with the mixed complexes. Coadsorption of CO and NO on a reduced Ni-ZSM-5 sample also results in formation of Ni2+(CO)(NO) species (Figure 9), which indicates that the Ni+ cations are easily oxidized to Ni2+ in a NO atmosphere. The experiments on coadsorption of CO and NO on our Ni/SiO2 sample evidenced replacement of preadsorbed CO by NO alone. 4.4. Comparison of the Properties of Nickel Ions in Ni-ZSM-5 and on Ni/SiO2. Finally, we shall discuss the effect of the support on the properties of nickel ions. The described results unambiguously demonstrate that the coordinative vacancies of Ni2+ ions in Ni-ZSM-5 exceed in number those of Ni2+ ions on silica by one at least. As a result, dicarbonyls, dinitrosyls, and mixed carbonylnitrosyl species are formed on Ni-ZSM-5. Due to the higher coordination number of Ni2+ cations supported on silica, they are able to form neither dicarbonyls nor mixed Ni2+(CO)(NO) species. In this case, however, formation of dinitrosyls is found. This phenomenon can be explained by the strong adsorbate-adsorbate interaction between the NO molecules. Hence, as has also been discussed above, formation of dinitrosyl does not necessarily require the existence of two coordinative vacancies and the dinitrosyl species can be regarded as adsorbed N2O2. The results obtained are in agreement with our previous observations on Cu-ZSM-5 and Cu/SiO2, on one hand,14

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and Ag-ZSM-5 and Ag/SiO2, on the other,15 where a larger number of coordinative vacancies have been found with cations in a ZSM-5 matrix. ZSM-5 is a pentasil zeolite. According to recent theoretical studies,58 Ni2+ cations in Ni-ZSM-5 are preferentially located in flat 5-rings and are low-coordinated. However, part of them can be found in 6-rings. This is a possible explanation for the heterogeneity of the Ni2+ sites observed in this study. Analysis of the literature data indicates that cations in other zeolites can also be low-coordinated, a typical example being Nimordenite.31,34 It has been suggested53 that the maximal number of small molecules that can be simultaneously coordinated to one cation in zeolites depends on the characteristics of the occupied cationic site (determined by the zeolite) and the ionic radius of the complexing cation. Since the ionic radius of Ni+ is bigger than the radius of Ni2+ cations, up to three CO molecules can be attached to one Ni+ site from Ni-ZSM-5 at low temperature.16 On the basis of this observation, one could expect formation of various Ni+(CO)x(NO)y species on reduced Ni-ZSM-5. Unfortunately, we have been unable to detect similar species because of the fact that the Ni+ cations are easily oxidized by NO. 5. Conclusions Ni2+ ions exchanged in ZSM-5 are characterized by a high number of coordinative vacancies. As a result, they can form mixed Ni2+(CO)(NO) complexes that are not produced on Ni/SiO2. Dinitrosyls are also easily formed whereas dicarbonyls are produced at low temperatures only. Neither dicarbonyls nor mixed Ni2+(CO)(NO) complexes are formed on Ni/SiO2, but dinitrosyls of Ni2+ are produced on this sample. This is explained by the strong adsorbate-adsorbate interaction in the case of NO. Ni+ cations in Ni-ZSM-5 are also characterized by a high number of coordinative vacancies which allows easy coordination of two CO molecules to one cation at ambient temperature and of three CO molecules at low temperatures. These ions are, however, easily oxidized by NO to Ni2+, which hinders isolation of their nitrosyl and carbonyl-nitrosyl complexes. Acknowledgment. K.H. is indebted to the Alexandervon-Humboldt-Foundation. LA015739G (58) Rice, M. J.; Chakraborty, A. K.; Bell, A. T. J. Catal. 2000, 194, 278.