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H Magic Angle Spinning NMR Evidence for Dissociative Adsorption of Hydrogen on Ag+-Exchanged A- and Y-Zeolites
Toshihide Baba,*,† Norito Komatsu,† Hidenori Sawada,† Yoshihiro Yamaguchi,† Toshiro Takahashi,‡ Hisashi Sugisawa,§ and Yoshio Ono†,| Department of Chemical Engineering, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8552, Japan, Japan Energy Analytical Research Center Co., Ltd., Toda-shi, Saitama, Japan, and Analytical Instrument Division, JEOL Ltd., 1-2 Musashino 3-Chome, Akishima-shi, Tokyo, Japan Received June 30, 1999. In Final Form: September 8, 1999 1H magic angle spinning NMR gave firm evidence for formation of acidic protons (4.0 ppm) and hydride species (-1.8 ppm), where the former exists as OH groups (acidic protons) and the latter has an interaction with three equivalent Ag atoms, existing as Ag3H when Ag+-exchanged A-zeolite is reduced with hydrogen. The formations of Ag3H and of acidic protons by the heterolytic dissociation of H2 over Ag3+ are reversible. The heterolytic dissociation of H2 was also observed over Ag-Y-zeolite, the silver hydride species being observed at -0.1 ppm.
Introduction Much attention has been drawn to the reduction behavior of silver cations and the chemistry of the silver species in zeolites such as Ag-A.1-14 Uytterhoeven and co-workers found that vacuum thermal treatment of Ag-A and Ag-Y promotes intrazeolitic autoreduction of Ag+ ion and the formation of a ncolor center.1
2(ZO-Ag+) f 1/2O2 + 2Ag0 + ZO- + Z+
(1)
where ZO- represents the zeolite lattice and Z+ a Lewis acid site. At 600 K, the degree of reduction amounts to almost 8% in fully exchanged Ag-A. Gellens et al. studied the X-ray powder diffraction (XRD) of Ag-A and found the formation of linear Ag32+ clusters.2 They also found the formation of Ag2 and Ag3 clusters in Ag-Y-zeolite.3 Kim and Seff determined the crystal * To whom correspondence may be addressed: fax, 81-3-57342878; e-mail,
[email protected]. † Tokyo Institute of Technology. ‡ Japan Energy Analytical Research Center Co. § JEOL Ltd. | Present address: National Institution for Academic Degrees, Nagatsuda-cho 4259, Midori-ku, Yokohama 226-0027, Japan. (1) Jacobs, P. A.; Uytterhoeven, J. B.; Beyer, H. K. J. Chem. Soc., Faraday Trans. 1 1979, 75, 56. (2) Gellens, L. R.; Mortier, W. J.; Schoonheydt, R. A.; Uytterhoeven, J. B. J. Phys. Chem. 1981, 85, 2783. (3) Gellens, L. R.; Mortier, W. J.; Uytterhoeven, J. B. Zeolites 1981, 1, 11. (4) Kim, Y.; Seff, K. J. Phys. Chem. 1987, 91, 668. (5) Kim, Y.; Seff, K. J. Phys. Chem. 1987, 91, 671. (6) Beyer, H.; Jacobs, P. A.; Uytterhoeven, J. B. J. Chem. Soc., Faraday Trans. 1 1976, 72, 674. (7) Jacobs, P. A.; Linart, J. P.; Nijs, H., Uytterhoeven, J. B.; Beyer, H. K. J. Chem. Soc., Faraday Trans. 1 1979, 73, 1745. (8) Jacobs, P. A.; Uytterhoeven, J. B.; Beyer. H. K. J. Chem. Soc., Faraday Trans. 1 1979, 73, 1755. (9) Ozin, G. A.; Hugues, F.; Mattar, S. M.; McIntosh, D. F. J. Phys. Chem. 1983, 87, 3445. (10) Ozin, G. A.; Hugues, F. J. Phys. Chem. 1983, 87, 94. (11) Baker, M. D.; Ozin, G. A.; Godber, J. J. Phys. Chem. 1985, 89, 305. (12) Baker, M. D.; Godber, J.; Ozin, G. A. J. Phys. Chem. 1985, 89, 2299. (13) Baba, T.; Akinaka, N.; Nomura, M.; Ono, Y. J. Chem. Soc., Chem. Commun. 1992, 339. (14) Sun, T.; Seff, K. Chem. Rev. 1994, 94, 857.
structure of Ag-A (Ag4.6Na7.4-A), which has been evacuated at 623 K by single-crystal X-ray diffraction methods and found that an Ag63+ cluster is present in each large cavity.4 The cluster is in a form of a nearly linear Ag30 molecule, each atom of which is coordinated to an Ag+ ion. The authors found also the presence of Ag63+ and Ag54+ clusters in Ag-A (Ag7.6Na4.4-A).5 Silver cations in zeolites are also reduced with hydrogen to generate protons and silver metal.9,10
ZO-Ag+ + 1/2H2 f Ag0 + ZOH
(2)
The chemistry of the reduction, however, seems more complex than that expressed by eq 2, and much effort has been devoted to elucidating the state of silver species. The formation of silver clusters has been studied with many techniques. Beyer et al. reported that the reduction of Ag-Y at low temperatures resulted in the formation of highly dispersed silver clusters containing unreduced silver and the formation of acidic OH groups.6 Highly dispersed silver metal can also be reoxidized by oxygen treatment.7,8 Kim and Seff also reported the reoxidation of highly dispersed silver metal.15 Ozin and co-workers studied the Ag-Y and Ag-A systems extensively with a diffuse reflectance spectroscopy and a far-infrared spectroscopy and suggested the presence of several Agnq+ clusters, depending on the temperatures of hydrogen reduction.9-12 They also show a reversible transformation between Ag3+ and Ag30 by treatment with hydrogen and oxygen.9 Though the physicochemical state of Ag clusters has been the subject of much attention, the sate of chemisorbed hydrogen has never been directly investigated. We have also reported that reversible interconversion occurs between Ag+ ions and metallic silver in Ag-Y as evidenced by XRD and IR measurements of CO chemisorbed on Ag+.13,16 Moreover, the catalytic activity of Ag-Y for disproportionation of ethylbenzene is reversibly enhanced by the presence of hydrogen in the gas phase.17 (15) Kim, Y.; Seff, K. J. Phys. Chem. 1978, 82, 921. (16) Baba, T.; Akinaka, N.; Nomura, M.; Ono, Y. J. Chem. Soc., Faraday Trans. 1 1993, 89, 56. (17) Baba, T.; Ono, Y. Zeolites 1987, 7, 292.
10.1021/la990849x CCC: $18.00 © 1999 American Chemical Society Published on Web 10/13/1999
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Thus, the effect vanishes when hydrogen is eliminated from the system and the catalytic activity returns to the original level by reintroducing hydrogen into the system. The catalytic activity reversibly changes with the hydrogen partial pressure. However, no enhancement of catalytic activities of H-Y by hydrogen is observed. The catalytic activity of Ag-Y in the presence of hydrogen is higher than that of H-Y. The enhancement effect of gaseous hydrogen implies the existence of a unique adsorbed state of hydrogen. The aim of the present work is to directly observe the state of chemisorbed hydrogen by means of 1H magic angle spinning (MAS) NMR. Experimental Section Na-A (SiO2/Al2O3 ) 2.0) and Na-Y (SiO2/Al2O3 ) 2.5) were obtained from Toso Co. Ltd. and Mizusawa Kagaku Co. Ltd., respectively. Ag-exchanged zeolites were prepared from Na+exchanged zeolites using a conventional ion-exchange procedure with a silver nitrate solution at room temperature. The degrees of Ag+ ion exchange of Ag-A and of Ag-Y were determined with atomic absorption analysis, being 54% and 60%, respectively. Samples for 1H MAS NMR measurements were prepared as follows; Ag+-exchanged zeolite (0.40 g) was packed in a glass tube with sidearms, each of which was connected to a glass capsule used for 1H MAS NMR measurements. The sample was heated under oxygen with a heating rate of 0.3 K min-1 from room temperature to 673 K, kept the same temperature for 3 h and then heated under vacuum at 673 K for 2 h. The Ag-A and Ag-Y were reduced under 40 kPa of hydrogen at 313 and 473 K, respectively. The consumption of hydrogen was manometrically monitored during the reduction. The evolution of water during reduction was negligible. The degree of the reduction of Ag+ ions in zeolite was estimated by assuming that hydrogen consumed was used solely to reduce Ag+ ions to Ag0 metal. After the sample was cooled to room temperature, it was transferred under hydrogen into a glass capsule to fill it completely and evenly. The neck of the capsule was then sealed, while the sample itself was maintained at 77 K. 1H MAS NMR spectra were recorded on a Chemagnetics CMXInfinity spectrometer operating at 400 MHz. To reduce 1Hbackground signals from the probe material, the DEPTH2 pulse sequence was used.18 The π/2 pulse width and the recycle delay were 2.0 ms and 20 s, respectively. The glass tube, in which the sample was sealed, was inserted into a zirconia rotor (5 mm diameter). The rotation frequency of the glass capsule was 4.0 kHz. The spectra were recorded at room temperature. The chemical shift was referenced to tetramethylsilane (TMS) with the usual conventions.
Results and Discussion The 1H MAS NMR spectrum of Ag-A was recorded in the presence of 40 kPa of hydrogen after reducing Ag-A with hydrogen (40 kPa) at 313 K for 30 min. The amount of hydrogen consumption was 3.02 × 10-4 mol/g, which is equal to 21% reduction of Ag+ (first hydrogen consumption). The partially reduced Ag-A under the above conditions will be designated hereafter as R-Ag-A. Two kinds of peaks were observed at (4.0 ( 0.1) and (-1.8 ( 0.1) ppm as shown in Figure 1 a. Since acidic protons were observed at 3.9-4.4 ppm in various zeolites,19 the peak at 4.0 ppm is attributed to acidic protons, which is generated by the reduction of Ag+ with hydrogen as shown in eq 1. Though the protons attributed to silanol groups are observed around 2 ppm,20 they were slightly observed. Four peaks were observed around -1.8 ppm. The ratios of peak intensities were 1:3:3:1 and the coupling constant (18) Cory, D. G.; Ritchey, W. M. J. Magn. Reson. 1988, 80, 128. (19) Pfeifer, H.; Freude, D.; Hunger, M. Zeolites 1985, 5, 274. (20) Pfeifer, H. NMR Basic Principles and Progress; Blumich, B., Ed.; Springer-Verlag: Berlin, 1993; Vol. 31, p 32.
Figure 1. 1H MAS NMR spectra of Ag-A recorded at 298 K in the presence or absence of hydrogen: (a) Ag-A reduced with H2 (40 kPa) at 313 K for 30 min (R-Ag-A), the spectrum recorded in the presence of 40 kPa of H2; (b) R-Ag-A evacuated at 313 K for 2 h (O-Ag-A); (c) O-Ag-A reexposed to H2 (40 kPa) at 313 K for 30 min.
was (131 ( 1) Hz, indicating that three equivalent Ag atoms and/or ions interact with a proton. Since metallic silver does not adsorb hydrogen,21 the cationic silver must be the chemisorption center for hydrogen molecules. The area of the peak at 4.0 ppm considering the area of spinning sidebands was 3.2 times larger than that of the peak at -1.8 ppm, indicating that the ratio of the amount of acidic protons to that of chemisorbed hydrogen species is 3.2. Therefore, the heterolytic dissociation of hydrogen molecule proceeds over cationic silver species, i.e., Ag3+, and the following scheme for the formation of Ag3-H and that of acidic protons is proposed as a plausible mechanism for hydrogen chemisorption.
2Ag+ + H2 f 2Ag0 + 2H+
(3)
2Ag0 + Ag+ f Ag3+
(4)
Ag3+ + H2 f Ag3H + H+
(5)
Moreover, the four peaks around -1.8 ppm as shown in Figure 1a indicate that three silver species in Ag3H have a triangle structure, since the interaction between the nucleus of three silver species and a proton is equivalent. Kim and Seff reported the formation of Ag6n+ such as Ag63+ except Ag3+ in Ag-A.4 As mentioned above, the ratio of the amount of H+ to that of Ag3-H in R-Ag-A was nearly equal to 3, suggesting that the main cation silver cluster is Ag3+ and that Ag6n+ is slightly formed under this reduction condition, though paramagnetic silver-hydride species such as Ag6-Hn cannot be detected by 1H MAS NMR. There is a possibility that Ag6-Hn is formed by the reduction of Ag-A with hydrogen at higher temperature (21) Knal, Z. In Catalysis; Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: Berlin, 1982, Vol. III, p 231.
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than 313 K. Understanding to physicochemical property of silver hydride species requires further studies. The reversible change of acidic protons and of Ag3H were demonstrated by 1H MAS NMR measurements of Ag-A in the presence or absence of H2. When R-Ag-A was evacuated at 313 K for 2 h (hereafter referred as O-Ag-A), the intensities of 1H MAS NMR peaks of acidic protons and Ag3H as shown in Figure 1b were diminished by about 87% as compared with the peaks of R-Ag-A in the presence of H2. This result indicates that both acidic protons and Ag3H are transformed into H2 and Ag+ by eliminating H2 from the system. Namely the back reactions of eqs 3-5 proceeded. When O-Ag-A was then reexposed to hydrogen (40 kPa) at 313 K for 30 min, the peaks at 4.0 ppm and -1.8 ppm reappeared as shown in Figure 1c. The intensities of these peaks were restored to those of the original peaks in Figure 1a. At this stage, the amount of hydrogen consumption was 2.86 × 10-4 mol/g, which is nearly equal to that of the first hydrogen consumption. In Ag-A partially reduced with hydrogen, the results of 1H MAS NMR measurements and the amount of hydrogen consumption give clear evidence of the reversible transformation among silver species, acidic protons, and hydrogen molecules as shown in eqs 6-8. +
0
2Ag + H2 a 2 Ag + 2H
+
2Ag0 + Ag+ a Ag3+ +
Ag3 + H2 a Ag3H + H 1H
(6) (7)
+
(8)
MAS NMR measurements showed that However, the extent of the reversibility decreased as the degree of Ag+ reduction increased. This is in conformity with the fact that the number of Ag3+ should decrease at higher degree of reduction, because of increasing number of bigger silver cationic clusters such as Agn+ (n > 3) than Ag3+ at high degree of Ag+ reduction. It has been shown that a larger metallic silver crystal is formed at the external surface of the zeolites.6,7 The dissociated adsorption of hydrogen was also observed on Ag-Y. After Ag-Y was exposed to hydrogen of 40 kPa at 373 K for 15 min, the sample was cooled to room temperature and the hydrogen pressure was adjusted to 40 kPa. The degree of Ag+ reduction was 22%. The 1H MAS NMR spectrum of the Ag-Y in the presence of hydrogen (40 kPa) was recorded at room temperature. As shown in Figure 2, three peaks were observed. Their chemical shift values were determined, after deconvolution, to be (4.7 ( 0.1), (3.9 ( 0.1), and (-0.1 ( 0.1) ppm, respectively. The peaks at 4.7 and 3.9 ppm are attributed to acidic protons in the sodalite cage and in the supercage, respectively, since these chemical shit values are in accord with those of acidic protons in H(87%)-Y. This implies that protons formed upon reduction of Ag+ ions are stabilized as bridging hydroxyl groups, as expected from eq 2. The peak at -0.1 ppm, which was not observed in H-Y, was ascribed to silver hydride species adsorbed on cationic silver clusters. It did not split as well as the case of R-AgA, suggesting that the size of silver cationic clusters (Agn+)
Figure 2. 1H MAS NMR spectrum of Ag-Y recorded at 298 K in the presence of 40 kPa of H2. Ag-Y reduced with 40 kPa of H2 at 373 K for 15 min.
Figure 3. 1H MAS NMR spectrum of Ag-Y recorded at 298 K in the presence of 14 kPa of CH4. Ag-Y exposed to 14 kPa of CH4 at 393 K for 1 h.
is bigger than that of Ag3+ in Ag-A.
Agn+ + H2 f AgnH + H+
(9)
As mentioned above, the heterolytic dissociation of H2 proceeds over silver-exchanged zeolites. This result encouraged us to expose CH4 instead of H2 to Ag-Y zeolite. The dissociated adsorption of CH4 was also observed on Ag-Y. After Ag-Y was exposed to CH4 of 14 kPa at 393 K for 1 h, the sample was cooled to room temperature, the 1H MAS NMR spectrum of the Ag-Y in the presence of CH4 (14 kPa) was recorded. As shown in Figure 3, two peaks were observed. Their chemical shifts were 0.4 and -0.1 ppm. The peak at -0.1 ppm was not observed by exposing CH4 to Na-Y and H-Y, respectively, while the peak at 0.4 ppm was observed by exposing CH4 to not only Ag-Y but also Na-Y and H-Y. Thus, the peak at -0.1 ppm was observed by only exposing CH4 to Ag-Y. This chemical shift was good agreement with that of the silver hydride species that was observed by exposing H2 to Ag-Y zeolite as mentioned above. Therefore, the peak at -0.1 ppm observed by exposing CH4 to Ag-Y can be attributed to silver hydride species adsorbed on cationic silver clusters, Agn+, which are presumably formed by the intrazeolitic autoreduction of Ag+ ion as shown in eq 1. The following scheme for the heterolytic cleavage of C-H bond of CH4 is proposed as a plausible mechanism.
Agn+ + CH4 f Ag3H + CH3+
(10)
Conclusion 1H
MAS NMR has given unequivocal evidence for the heterolytic dissociation of hydrogen and C-H bond of CH4 on Ag+-exchanged zeolites. LA990849X
