Surface chemistry on colloidal metals: spectroscopic study of adsorption of small molecules
Colloidal solutions of monodispersed palladium in various size ranges from <10 Å to 75 Å have been prepared by metal vapour synthesis, chemical reduction of Pd(OAc)2 or by reaction of Pd(dibenzylideneacetone)2 with hydrogen or CO, in the presence of stabilizing polymers. CO is adsorbed readily onto the surface of the colloidal particles. The infrared spectrum of the adsorbed CO is size dependent. The largest palladium colloid (75 Å by TEM), which contains well formed microcrystals, shows only bridging CO (1944 cm–1). The smallest colloid (10 Å by SAXS) shows only terminal CO (2037 cm–1), and the intermediate size colloids (18 and 23 Å by TEM) show both bridging and terminal CO. The populations of bridging and terminal CO adsorption sites can be altered by modification of the method of preparation or by subsequent addition of reagents which disrupt the polymer–metal interaction.
The 75 MHz 13C NMR spectra of CO adsorbed on the palladium colloids are also size dependent. The smaller colloids (10 and 18 Å) exhibit broad lines (w1/2= 12 and 20 ppm, respectively) centred near 190 ppm. The larger colloids, which contain metallic palladium as shown by TEM and XRD, show radically different behaviour. Under 3 atm 13CO only a free CO resonance is directly observed, which exhibits a temperature-dependent linewidth characteristic of chemical exchange. A spin-saturation transfer experiment establishes the presence of a second CO site in the system, in exchange with the free CO. The second species, assigned to CO adsorbed on microcrystalline palladium, has a mean chemical shift of 800 ppm in the case of the 75 Å colloid and 700 ppm in the case of the 23 Å colloid.
These experiments are designed to investigate the surface chemistry of colloidal transition metals in organic solvents by standard spectroscopic techniques, with a view to establishing a relationship between metallic clusters and molecular clusters, and to determining the changes in chemical and physical properties which come about as cluster size increases from the molecular scale to the bulk.