Infrared Spectroscopy of CO on Colloidal Metals

The infrared spectrum of CO on a platinum organosol was reported in 1986 by Lewis et al. in their investigation of the nature of some purportedly homogeneous organoplatinum olefin hydrosilylation catalysts. [60] The hitherto unsuspected presence of colloidal platinum in the calatytic solutions was confirmed by TEM, and carbon monoxide was found to adsorb onto the colloidal metal (mean particle size of 23 nm) in methylene chloride, giving rise to infrared absorptions at 2050 cm and 1880 cm . TTiese bands were assigned to linear and doubly bridging CO. 

Partide size and structure can have a pronounced effect on the infrared spectra of adsorbed CO. A series of palladium colloids of differing sizes provided the basis for an analyds of this effect. [34, 113, 227] The PVP staUliz palladium colloids were prepared by the reduction of palladium acetate in methanol solution or by the reductive decomposition of bis(dibenzylideneacetone) palladium in the presence of PVP (see Section 6.2). The resulting collmdal palladium partides 

The infrared spectrum of CO adsorbed on a vapor derived 1.0 nm platinum colloid stabilized by iro-butylaluminoxane in methylqrclohexane showed only a linear CO stretch at 2035 cm. This particle size is witliin the range for the larger molecular platinum carbonyl duster anions, and the colloidal platinum can in fact be hydrolysed to mixtures of [Pt9(CO)ig] and [Pti2(CO)24] after CO adsorption. [229] 

The infrared spectra of CO on colloidal platinum, palladium, and ruthenium stabilized with either cellulose acetate or nitrocellulose have been reported. [23] Although the spectra show an interesting size dependence in the case of ruthenium in nitrocellulose, interpretation is difficult since the spectra differ markedly from those reported for CO on supported ruthenium. For the platinum sols, a preference for linear CO coordination is observed, as is the case for the supported metal, while for palladium the bridging mode is preferred exdusively. 

These examples illustrate the amenability of colloidal metals to investigation by a spectroscopic method common to both molecular and surface chemistry. 

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