J. Phys. Chem. 1993,97, 666-672
666
Molecular and Magnesia-Supported Tetrairidium Carbonyl Clusters: Characterization by Laser Raman Spectroscopy G. Mestl,t N. D. Triantafillou,*H. Knozinger,**tand B. C. Gates*~t~**~ Institut fur Physikalische Chemie der Universitht Miinchen, Sophienstrasse 1 I , 8000 Miinchen 2, Germany, and Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware I971 6 Received: July 2, 1992; In Final Form: October 16, I992
Laser Raman spectroscopy has been used to characterize a family of iridium cluster carbonyls, including [PhdP] [ H I ~ ~ ( C O ) I[K] I ] , [HIr4(C0)11],[Ir4(CO)n], and [Ph4P]z[HIrg(C0)22] in methanol solution and in the solid state. A supported analogue of [HIr4(C0)11]- was prepared by adsorption of [Ir4(CO)12] on MgO that had been treated at 400 OC. It was identified on the basis of comparisons with the Raman spectra of the molecular clusters listed above. High-quality Raman spectra were recorded for the MgO-supported [HIr4(CO) 111containing 6 wt 76 Ir, but the Raman spectra of the MgO-supported [HIr4(C0)11]- containing only 1 wt 76 Ir are of such low quality that no clear identification of the supported species was possible.
Introduction
Raman spectroscopy has been used to characterize surfaces of many catalysts that include metals, but spectra of the metals themselves have not been 0btained.l The penetration depth of incident radiation is considerably less than the wavelength of the radiation; thus, the scattering cross section for regular Raman scattering is very The almost free electrons at the metal surface reflect the incident radiation rather than scattering it. It can be shown4 that in crystalline metal lattices, only acoustic phonons are allowed. Only surface corrugation and coupling with magnons can lead to Brillouin scattering of radiation by metal^.^ Electronic Raman scattering by metals and semiconductors is sometimes possible if the frequency of the incident radiation matches electronicinteror intraband transitions in the material.5 Metal catalysts are typically clusters or crystallites dispersed on porous, high-area metal oxide supports. These materials are difficult to characterize with Raman spectroscopy6 for the following reasons: (1) the samples are usually black or gray, and the incident light is absorbed by multiple reflection on the crystallite surfaces instead of being Raman scattered; (2) the metal loadings are usually low; (3) the clusters or crystallites of metal are nonuniform and expected not to give sharp Raman bands; (4) incident laser radiation may destroy the surface structures; ( 5 ) metal oxide supports sometimesfluorescestrongly; and (6)the surface species react with air, requiring equipment for air-free sample handling and in situ characterization. However, metal clusters in the most highly dispersed supported metal catalysts resemble molecular metal clusters, for which many excellent Raman data have been reported. Raman spectra of small metal clusters provide valuable structural information because they give evidence of metal-metal as well as metalligandvibrations.' It is thus logical toexpect that progress toward characterization of supported metal catalysts with Raman spectroscopy should develop from characterization of supported metal clusters that have nearly uniform structures.* The prospective advantages of such samples are the following: (1) the clusters may be so small as to lack metallic properties; (2) many of the samples have colors other than black or gray; (3) the t Institut fiir Physikalische Chemie der Universitlt Miinchen.
* University of Delaware.
I Present address: Department of Chemical Engineering, University of California, Davis, Davis, CA 95616.
supported metal clusters may be nearly uniform so that their spectra can be interpreted by comparison with those of molecular analogues. The literature reports only several attempts to measure Raman spectra of supported metal clusters with nearly molecular structures. Deeba et al.9 reported a spectrum for triosmium carbonyl on y-A1203,observing a band at 160 cm-l, near where an Os-Os vibration occurs for [Os3(C0)12]. The assignment of this spectrum to a (p-0)-bridged cluster has been discussed by Kn6zinger7and Alexiev et a1.I0 Choplin et a1.I1 and Theolier et a1.I2 communicated Raman data for a triosmium carbonyl and a triruthenium carbonyl on SiO2, respectively, but no spectra were shown. These reports thus give indications of metal-metal bonds in supported metal clusters, but they fall short of determining structures. The goals of this research were to measure Raman spectra of a well-defined and relatively stable supported metal carbonyl cluster, namely, [ H I ~ ~ ( C O ) Ion I ] -MgO, which has been characterized by infrared and extended X-ray absorption finestructure spectroscopies and by extraction into solution by cation metathesis." Several improvements in the Raman measurement technique have been applied: (1) to minimize contamination by air, the samples were prepared with air-exclusion techniques and sealed in NMR tubes; (2) to minimize sample heating, samples in NMR tubes were rotated and/or cooled by a laminar flow of liquid N2 over the tube and/or irradiated with chopped laser beams with various powers. Experimental Methods [Ir4(CO)I~]wasobtained from Stremand usedwithout further purification. [K] [HIr4(C0)11] and [PLP] [ H I ~ ~ ( C O ) were II] synthesizedfrom [Ir4(CO)12] and K2CO3 in methanol by literature methods.I4 [Ph4P]~[Ir~(C0)22] was prepared from [Ph4P][HIr4(CO)11] by evacuation for several hours at room temperature.Is MgO powder (EM Science) was calcined at various temperatures (300,400, and 700 "C) for 10 h under vacuum followed by 2 h in 0 2 and another 2 h under vacuum. (In what follows, the subscript on MgO refers to the calcination temperature in "C.) The surface area of M g 0 m was 70 m2/g, and that of M g 0 7 ~ was 40 m2/g. The samples were loaded in a N2-filled glovebox (MBraun, MB-1SOM;
