Langmuir 1990,6, 1743-1744
1743
Generation of Protonic Acid Sites Originating from Molecular Hydrogen on the Surface of Zirconium Oxide Promoted by Sulfate Ion and Platinum Kohki Ebitani, Hideshi Hattori,' and Kozo Tanabe Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060, Japan Received March 6, 1990.In Final Form: May 22, 1990 An infrared spectroscopic study of pyridine adsorbed on the surface of zirconium oxide promoted by both sulfate ions and platinum shows that molecular hydrogen converts into protonic acid sites on the surface with concomitant decrease in Lewis acid sites at temperatures above 423 K. It seems that the molecular hydrogen in vapor phase is dissociated on platinum to form hydrogen atoms which migrate onto the oxide and loose their electrons to convert into protonic sites.
Introduction Acid-base property is proposed t o be one of t h e important surface chemical properties of the metal oxide catalysts.' Although it is well-known that the acid-base properties of metal oxide vary with activation temperature and additives such as sulfate ion, chlorine ion, metal cation, etc., the dynamic modification of acid-base property by the interaction with a gaseous molecule such as hydrogen has been scarcely observed. The interactions between molecular hydrogen and supported metal catalysts have been studied by many investigators in terms of hydrogen spillover? and the nature of t h e spiltover hydrogen has been studied for the characteristic actions of the ~ a t a l y s t . ~One - ~ of the mechanisms of hydrogen spillover involves the heterolytic process. Hydrogen molecule dissociates into two H atoms on the metal. The H atoms spill over onto the support, where the formation of an H+and H(OHand e-)9JO or the formation of the hydroxyl groupsl1J2 occurs. Thus, the spilt-over hydrogen has a possibility to act as a protonic acid. However, the formation of the protonic acid sites originating from molecular hydrogen has not been reported yet. In the present article, we wish to report the spectroscopic evidence demonstrating that a spilt-over hydrogen forms a protonic acid site. 374979*
Experimental Section The Pt/S042--Zr02sample was prepared by impregnation of S04*--Zr02with HzPtCb aqueous solution followed by drying at (1) Tanabe, K.; Misono, M.; Ono, Y.; Hattori, H. New Solid Acids and Bases; Kodansha Elsevier: Tokyo, Amsterdam, 1989. (2) Conner, W.C., Jr.; Pajonk, G . M.; Teichner, S. J. Adu. Catal. 1986, 34,l. (3) Levy, R.;Boudart, M. Ind. Chim. Belg. 1973,38,506. Levy, R.B.; Boudart, M. J. Catal. 1974, 32,304. (4) Matauda, T.; Fuse, T.; Kikuchi, E. J. Catal. 1987,106, 38. (5) Teichner, S.J.; Mazabrand, A. R.; Pajonk, C.;Gardes, C. E. E.; Hoang-Van, c . J . Colloid Interface Sci. 1977,58,88. (6) Gnep, N. S.;Martin de Armando, M. L.; Guisnet, M. In Spillover of Adsorbed Species; Pajonk, C . M., Teichner, S. J., Eds.; Elsevier: Amsterdam, 1983; p 309. (7) Parera, J. M.; Figoli, N. S.; Jablonnski, E. L.; Sad, M. R.; Beltramini, J. N. In Catalyst Deactiuation; Delmon, B., Fromet, G . F., Eds.; Elsevier: Amsterdam, 1980; p 571. (8) Conesa, J. C.; Munuera, C.; Muioz, A,; Rives, V.; Sanz, J.; Soria, J. In ref 6, p 149. (9) Huiginga, T.; Prins, R. J . Phys. Chem. 1981, 85, 2156. (10) Herrmann, J. M.; Disdier, J.; Pichat, P. In Metal-Support and Metal-Additiue Effectsin Catalysis; Imerik, B. et al., Eds.; Elsevier: Amsterdam, 1982; p 27. Herrmann, J. M. J. Catal. 1989, 128, 43. (11) Cavanagh, R. R.; Yates, J. T., Jr. J. Catal. 1981,68,22. (12) Baumgarten, E.;Denecke, E. J. Catal. 1985,95, 296.
0743-7463190f 2406-1743$02.50/0
383 K and calcination at 873 K in air. The amounts of Pt and S were 0.5 and 1.5 wt %, respectively. For IR spectroscopy, a self-supported wafer placed in an in situ IR cell was preteated at 623 K in a hydrogen flow for 1.5 h. After cooling to room temperature, the sample was exposed to 2 Torr of pyridine at 423 K and then evacuated at 673 K. Then, hydrogen (600 Torr) was introduced into the cell, and the sample was heated stepwise at different temperatures by a 50 K increment up to 623 K. For determination of the fraction of protonic and Lewis acid sites on the surface, the integrated absorbances of the bands at 1450 cm-I (due to pyridine chemisorbed on Lewis acid sites: coordinated pyridine) and 1490 cm-I (due to both the coordinated pyridine and pyridine chemisorbed on protonic acid sites:
pyridinium i0n)13 were used with the tangent background (shaded part in Figure 1). In calculation of the fraction of protonic acid sites and Lewis acid sites, the extinction coefficientsof two forms of adsorbed pyridine measured by Hughes and White" were employed: 3.26 cm/pmol for the coordinated pyridine at 1450 cm-l, 3.40 cm/pmol for the protonated pyridine at 1490 cm-I, and 0.56 cmlpmol for the coordinated pyridine at 1490 cm-l.
Results and Discussion IR spectra of adsorbed pyridine before and after heating in the presence of hydrogen are shown in Figure 1. Before introduction of hydrogen, Lewis acid sites were predominant on the surface of Pt/S042--ZrOz (Figure la). Heating in the presence of a hydrogen molecule causes the peak intensity a t 1450 cm-' to decrease and the peak intensity a t 1490 cm-l to increase as well as the peak intensity at 1550 cm-l, which is due to pyridinium ion (Figure lb). It is obvious that the pyridine molecules adsorbed on a Lewis acid site (coordinated pyridine) are protonated to convert into t h e pyridinium ion by t h e interaction with molecular hydrogen. Thus, the IR spectral change indicates that the molecular hydrogen in vapor phase converts into a protonic site which has an acidity sufficiently strong to protonate the pyridine molecule. The changes in the fraction of protonic and Lewis acid sites to the total number of acid sites caused by exposure to hydrogen at different temperatures (423-623 K) are shown in Figure 2. The result indicates that the fraction of Lewis acid sites decreases with an increase in the fraction of protonic acid sites. The fraction of protonic acid sites in the total acid site reaches ca. 80% by heating in the presence of hydrogen at 623 K. During heating in the presence of hydrogen, the loss in the number of ad(13) Parry, E. P. J. Catal. 1963, 2,374. (14) Hughes, T. R.; White, J. M. J . Phys. Chem. 1967, 71, 2192.
0 1990 American Chemical Society
Ebitani et al.
1744 Langmuir, Vol. 6,No. 12, 1990
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423
473 523 573 623
*- e x po sur e temp era t ur e / K
H
Figure 2. Change in fraction of P-Py and L-Py caused by exposure to hydrogen at different temperatures.
1500
1400
Wave n um b e r / c m-
’
Figure 1. IR spectral change for adsorbed pyridine with heating in the presence of hydrogen: (a) before exposure to hydrogen; (b) after exposure to hydrogen at 623 K.
sorbed pyridine ws scarcely observed (the total number of adsorbed pyridine is constant). Thus, it is suggested that the number of Lewis acid sites decreases with an increase in the number of protonic acid sites. It is also indicated that the proton formation from molecular hydrogen requires high temperature. Without hydrogen or platinum, such a spectrum change was not observed on heating in the presence of molecular hydrogen. Thus, it is plausible that the change in the fraction of each acid site caused by exposure to hydrogen is due to spillover (migration) of hydrogen adsorbed on platinum to the support. The above results demonstrate that the protonic acid sites of Pt/SO1z--ZrOz originate from molecular hydrogen; a hydrogen molecule dissociates into two hydrogen atoms
on platinum, and hydrogen atom spills over (migrates) onto the surface of S04z--Zr02 and looses ita electron to convert to a protonic acid site. This protonic acid site has the possibility to modify the catalytic features such as activity, selectivity, coke formation, etc. Strong acid sites on the support seem to be required to cause the formation of a protonic acid site originating from molecular hydrogen. As reported, the presence of sulfate ions on zirconium oxide results in the formation of strong acid sites.’5J6 Actually, without sulfate ions, the formation of strong acid sites was not observed in the presence of hydrogen. In addition to the strong acid sites, what is required of the support to preserve the acid sites originating from molecular hydrogen will be the subject to study for a new type of solid acid catalysts. Registry No. Pt, 7440-06-4; ZrOz, 1314-23-4;so,*-, 14808pyridine, 110-86-1. 79-8;Hz,1333-74-0; (15) Hino, M.; Kobayashi, S.;Arata, K. J.Am. Chem. SOC.1979,101, 6439. (16)Yamaguchi,T.; Jin, T.;Tanabe, K. J.Phys. Chem. 1986,90,3148. Jin, T.; Yamaguchi, T.; Tanabe, K. Ibid. 1986,” 4794.
