6384
J . Phys. Chem. 1990, 94, 6384-6390
Electron Spin Resonance and Electron Spin Echo Modulation Study of Paramagnetic Ru Species Generated in Li-X, Na-X, K-X, and Ca-X Zeolites: Adsorption of Carbon Monoxide, Oxygen, and Water Guan-Dao Lei and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 (Received: June 26, 1989; In Final Form: March 26, 1990)
Paramagnetic ruthenium species are formed in X zeolites with different cocations after evacuation at 300 OC and subsequent adsorption of various molecules. The various species formed after activation were characterized by electron spin resonance (ESR) and electron spin echo modulation (ESEM) spectroscopies. No major difference was observed in the ESR spectra I~. of these zeolites did not after activation for the zeolites exchanged with [ R u ( N H ~ ) ~ ] or C I [~R u ( N H ~ ) ~ C I ] CActivation generate any substantial amount of paramagnetic species. However, exposure of the activated zeolite, except for RuLi-X, to CO, 02.and H20resulted in a significant increase in the spin concentration which was assigned mainly to Ru(II1) species. The increase in the spin concentration is suggested to be due to dissociation of Ru(1II) dimers in the @-cageof the zeolite structure when an adsorbate is added. Adsorption of CO and O2generated one or more Ru(lI1) adducts depending on the nature of the cocation. The number of coordinated CO molecules was assessed by infrared spectroscopy. Adsorption of D20generated Ru( 111) coordinated to three water molecules based on ESEM data; this adduct has the same coordination geometry regardless of the cocation type, but the cocation does affect the rhombic g-factor magnitudes. ESEM results also show that Ru(1II) species are situated less than 0.36 nm from an aluminum nucleus.
Introduction Ruthenium-exchanged zeolites have been reported as effective catalysts for a variety of reactions including methanation,' Fischer-Tropsch,2 and water-gas shift3 reactions. These catalytic properties of ruthenium have prompted a number of studies of the chemistry of ruthenium complexes within zeolite^.^' Zeolite cocations have been shown to have significant effects on the location and geometrf as well as the catalytic activity9of the metal ion. [Ru(NH3),]CI3 exchanged into Na-Y zeolite has been shown to undergo autoreduction by the amine ligands during dehydration. Verdonck et a1.I0 concluded that at least 80% of the ruthenium was reduced to the zerovalent metallic state on the basis of hydrogen uptake and infrared results after evacuation at 350 OC. But Gustafson et ale6showed by electron spin resonance (ESR) that some Ru(II1) still exists in Na-Y zeolite after dehydration and evacuation at 300 OC. In this study, [Ru(NH3),]C13 and [Ru(NH3),CI]CI2 were used as parent complexes to exchange Ru(II1) into X zeolite with Li+, K+, Na+, and Ca2+as major cocations. After activation at 300 OC and CO or O2adsorption Ru(l1I) adducts and dioxygen species were formed which depend on the cocation type. Adsorption of H 2 0 gave only one adduct for all cocations except with Li+ as cocation. The paramagnetic ruthenium species were characterized by ESR and electron spin echo modulation (ESEM) spectroscopies. (1) Jacobs, P.A.; Nijsand, H. H.; Uytterhoeven, J. B. Prep.-Am. Cfiem. SOC.,Diu. Per. Cfiem. 1978, 23, 469. (2) Nijs, H.: Jacobs, P. A,: Uytterhoeven, J. B. J . Chem. SOC.,Cfiem. Commun. 1979, 180. (3) Verdonck, J. J.; Jacobs, P. A.; Uytterhoeven, J. B. J . Cfiem. SOC., Chem. Commun. 1979, 18 1. (4) (a) Verdonck, J. J.; Schoonheydt, R. A.; Jacobs, P. A. J . Pfiys. Cfiem. 1981, 85, 2393. (b) Verdonck. J. J.: Schwnheydt, R. A.; Jacobs, P. A. J. Pfiys. Cfiem. 1983, 87, 683. ( 5 ) Jacobs, P. A.; Chautillon, R.; De Laet; Verdouck, J. J. fntrazeofife Chemistry; American Chemical Society: Washington, DC, 1983; p 439. (6) Gustafson. B. L.: Lin, M. J.; Lunsford, J. H. J. Pfiys. Cfiem. 1980,84, 321 I . (7) Wan, B.; Lunsford, J. H. Inorg. Cfiim. Acra 1982, 65, 29. (8) (a) Ichikawa. T.; Kevan. L. J . Am. Cfiem.SOC.1983, 105, 402. (b) Goldfarb, D.; Kevan, L. J. Pfiys. Cfiem. 1983, 105.402. (c) Narayana, M.; Kevan, L. J. Cfiem. Soc., Faraday Trans. I 1986, 82, 402. (9) (a) Lee, H.; Kevan. L. J . Phys. Cfiem. 1986, 90, 5776. (b) Lee, H.; Kevan, L. J. Pfiys. Cfiem. 1986, 90, 5781. (IO) Verdonck, J. J.; Jacobs, P.A.: Genet, M.; Pocelet, G. J . Cfiem.SOC., Faraday Trans. I 1980, 76, 403.
0022-3654/90/2094-6384$02.50/0
Experimental Section Linde 13X (Na-X) zeolite was repeatedly washed with 0.1 M sodium acetate solution to remove excess Fe3+ impurities. This zeolite was then ion-exchanged four times at 70 OC with 0.05 M solutions of K', Li+, and Ca2+ with either NO3- or CI- as the counterion. These zeolites will be referred to as Li-X, Na-X, K-X, and Ca-X. Ruthenium ion (1.5 wt %) was exchanged into these zeolites at room temperature by using [ R u ( N H ~ ) ~ C I ] C ~ ~ or [ R u ( N H ~ ) ~ ] C (Strem I ~ Chemicals, Inc.). The exchanged samples were stirred for about 24 h and filtered, and the zeolites were then washed with water and dried at room temperature. The Ru content was determined by commercial atomic absorption analysis. Samples were evacuated at room temperature and heated to 300 OC at a rate of 60 OC/h and then were evacuated for 1 h at 300 OC to a residual pressure of
