Principles of the preparation methods and definitions

It is desirable to begin this chapter with some general remarks about of use of terms to describe methods of preparation, because they are sometimes used loosely and inaccurately. The methods fall into two classes (i) the support and the gold precursor are formed at the same time and (ii) the gold precursor is applied to the preformed support. The procedure for method (i) is termed coprecipitation (Section 4.2.2), and although capable of many minor variations (e.g. as in the sol-gel method, Section 4.4.1), it does not have the same wealth of alternative procedures as does the second method. These may be listed as follows. 

In impregnation the pores of the support are filled with a solution of the gold precursor. Where just the needful volume is used, the method is impregnation to incipient wetness, but sometimes an excess of solution is used and the solvent removed by evaporation, so that concentrated solution finally resides in the pores (Sections 4.2.1 and 4.3.1). In these methods, the metal dispersion ultimately obtained depends critically on the conditions of drying.1 

Other manifestations of the second class of method include deposition of colloidal gold onto a support (Section 4.3.6), and photochemical or sono-chemical activation of the precursor to encourage its interaction with the support. 

We will try to separate as far as possible the procedures of preparation from the chemical reactions that occur during these preparations. For instance in the case of impregnation to incipient wetness, both the metal ions and the counter-ions are deposited on the support, but the speciation of the metal-containing ions may vary with the pH of the solution in contact with the support, and the way in which they interact with the support will also depend on the PZC of the support. 

---