5409
J. Phys. Chem. 1991, 95, 5409-5414
Osmium Carbonyl Clusters In Basic Zeolite Y: Characterlzation by Extended X-ray Absorption Fine Structure Spectroscopy S. D. Maloney,' P.-L.Zhou,+ M. J. Kelley,' and B. C. Gates*" Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716, and E. I. du Pont de Nemours and Company, Engineering Technology Laboratory, Experimental Station, Wilmington, Delaware 19898 (Received: August 17, 1990; In Final Form: February 3, 1991) Catalysts for CO hydrogenation were prepared by treatment in CO + H2 of samples prepared by introduction of [H2Os(CO),] from the vapor phase into the pores of zeolite Y that had been made basic by treatment with NaN,. The catalyst, before and after use in a flow reactor, was characterized with extended X-ray absorption fine structure (EXAFS) spectroscopy. The Os Llll edge EXAFS results indicate nearly the same osmium structures prior to and following catalysis; the average Os-Os coordination number is 2, consistent with the presence of triosmium clusters in the zeolite cages, and the O s 4 and O s 4 coordination numbers (3.6) indicate terminal carbonyl ligands. The results demonstrate the presence of molecular osmium carbonyl clusters in the zeolite cages; these species are suggested to be predominantly triosmium carbonyl anions.
Introduction Zeolite cages are microscopic phases in which novel structures can be synthesized and isolated. Structures in zeolite cages offer prospects as materialswith new properties, such as semiconductors1 and catalysts? Metal structures in mlites have been incorporated in the inorganic synthesis to give shapeselectivemetal-containing zeolites?~~A shipin-a-bottle synthesis was used to prepare iron phthalocyanine complexes trapped in the supercages of faujasitic zeolites; these are oxidation cataly~ts.~Metal carbonyl clusters have been synthesized in zeolite Y by introduction of metals by ion exchange followed by carbonylation6and by introduction of volatile metal carbonyls followed by condensation reactions with CO.' Zeolite-supported osmium' and rhodium*catalysts made by the latter method from [H20s(CO),] and [Rh(C0)2(aca~)], respectively, have unusual selectivities for hydrogenation of CO, and the predominant species have been inferred'ss to be metal carbonyl clusters that, once formed in the zeolite cages, are too large to migrate through the apertures, consequently becoming stabilized by entrapment in the cages. The zeolite-supported catalysts made from [H20s(CO),] have been characterized by infrared spectroscopy and suggested to incorporate small osmium carbonyl cluster anions.' The goal of the research described here was to use extended X-ray absorption fine structure (EXAFS) spectroscopy to understand better the structures of the osmium species in these samples; the samples were made with high osmium contents (about 10 wt %) to give good signal-to-noise ratios.
[H20s(CO),] was prepared by reduction of [OS~(CO),~] (Strem) with Na in liquid ammonia followed by protonation of the resulting Na2[Os(CO),] with phosphoric acid.12 The zeolite was brought in contact with vapors of [H,Os(CO),] at room temperature. The contacting lasted 24 h, after which the solid was evacuated for an hour to remove volatile excess [H20s(CO),] and treated in CO H2 (1:l molar) at 300 OC and 1 atm. The solid sample was analyzed for Os with X-ray fluorescence spectroscopy and found to contain 10.6 wt IOs prior to use as a catalyst and 10.1 wt 9% Os following use; these results are indistinguishable within experimental error. Infared Spectroscopy. Samples of the Os carbonylcontaining zeolite, prior to and following use as a CO hydrogenation catalyst, were characterized with diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS). The cell was constructed with vacuum-tight fittings and loaded in the glovebox. Details of the cell construction are given in a thesis.', X-ray Absorption Spectroscopy. Samples of the CO hydrogenation catalyst, one prior to use and another following use at low conversions (
