Spectroscopic Properties of Colloidal Metals

The literature on colloidal metals is extensive and an adequate survey covering all aspects of their physical and chemical properties would require an interdisdpli-nary authorship of some size. This chapter reflects the predilections and limitations of the author and will concentrate on the chemical aspects of metal colloid science. Suffident physical discussion will be given to support the descriptions of the structures and spectroscopic properties of colloidal metals in the context of their chemical applications to catalysis. 

A prototypical example of a molecular probe used extensively to study the mineral adsorbent-solution interface is the ESR spin-probe, Cu2+ (Sposito, 1993), whose spectroscopic properties are sensitive to changes in coordination environment. Since water does not interfere significantly with Cu11 ESR spectra, they may be recorded in situ for colloidal suspensions. Detailed, molecular-level information about coordination and orientation of both inner- and outer-sphere Cu2+ surface complexes has resulted from ESR studies of both phyllosilicates and metal oxyhydroxides. In addition, ESR techniques have been combined with closely related spectroscopic methods, like electron-spin-echo envelope modulation (ESEEM) and electron-nuclear double resonance (ENDOR), to provide complementary information about transition metal ion behaviour at mineral surfaces (Sposito, 1993). The level of sophistication and sensitivity of these kinds of surface speciation studies is increasing continually, such that the heterogeneous colloidal particles in soils can be investigated ever more accurately. 

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