NMR Spectroscopy on Colloidal Metals

NMR resonance of solid platinum metal. The chemical shift of 42.S MHz lies between those of platinum metal (41.5 MHz) and HaPtQt (43.0 MHz), and is interpreted as being a consequence of the submetallic nature of the 1.6 nm platinum particles. Thus the size issue for the transition from metallic properties to molecular properties is introduced. The symmetry of the band is surpriang since more than 50 % of the platinum atoms are surface atoms at this particle size and might be expected to be resolved from the resonances for the internal platinum atoms. [234, 236] The surface atoms can, in fact, be resolved after the addition of CO to the colloid, which results in the appearance of a low intensity resonance at 42.46 MHz (Fig. 6 29b), that is, up field (lower frequency) of the main band. This signal is assigned to the surface platinum atoms as the adsorption sites for CO. Displacement of CO by iodide broadens this resonance through the interaction with the quadrupolar iodide nucleus. 

The observation of NMR resonances for molecules adsorbed on small metal particles, as opposed to resonances of the metal particles themselves, also benefits from the mobility the of colloidal partides in liquids. The C NMR of CO adsorbed on colloidal palladium has been reported for several systems. [33, 113, 227] When CO is adsorbed on colloidal palladium which is known to be metallic on the basis of its TEM images and X-ray diffraction pattern, it is to be expected that the resonance for the adsorbed %0 would be greatly shifted to low field, consistent with the large negative Knight shift of the metal. 

In a liquid state high-resolution NMR study of adsorption on colloidal palladium, [33] it was found that under 3 atm. of 99% CO, the resonance which would correspond to carbon monoxide adsorbed on the 7.0 nm crystalline palladium colloid stabilized in methanol solution with PVP could not be directly 

This structural effect on the chemical shifts of adsorbed molecules is further demonstrated in the NMR spectra of CO on some palladium colloids derived from palladium vapor (see Section 6.2.2.4). If these are amorphous, as suspected, then their metallic properties (conduction bands) which give rise to the Knight shift should therefore not be as well developed. They adsorb CO in a predominantly terminal mode implying a disordered or rough surface (see Section 6.4.3.2), and thus might be expected to be less metallic in the context of this technique. In fact, the C resonance for CO adsorbed on such vapor derived 

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