Gold-Based Catalysts - Transition Metal Catalysis in Aerobic Alcohol Oxidation - page 135

traditional catalyst synthetic methods often fail to produce high metal dis-
persion on the support. One way around this limitation is the immobiliza-
tion of a preformed metallic colloidal dispersion (sol), a methodology
leading to good control of metal particle size as it reduces the influence of
the supporting material on metal dispersion. 6,11,12 Before considering the
sol immobilization protocol in detail, a summary of the principal prepar-
ation methods is presented. In the impregnation method ('incipient wet-
ness'), a solution of the metal precursor is allowed to fill the pores of the
supporting material up to incipient wetness imbibition of its volume. The
metal dispersion ultimately obtained is markedly affected by the con-
ditions of drying. Since the original procedure generally led to gold par-
ticles larger than 5 nm, which show little activity, modified impregnation
methods were developed in which the key issue was impregnation with
hydrogen tetrachloroaurate followed by an additional step to ion exchange
chlorides with the hydroxyls of a base. The resulting material was dried and
then activated by reduction with hydrogen gas, not by calcination, in a
method that is simple and more suitable for the production of effective
gold catalysts, as the liquid-phase oxidation of glucose to gluconic acid
demonstrates. 43,44
Another popular approach for preparing supported gold-based catalysts is
the deposition-precipitation (DP) method. This consists in the deposition of
the metal hydroxide or hydrated oxide on the surface of a support by grad-
ually increasing the pH of the metal precursor solution in which the support
is suspended. The precipitate may be nucleated by surface functional
groups, thus allowing the active phase to be more tightly attached to the
support. As a gold precursor, hydrogen tetrachloroaurate is the favorite
choice also for this kind of preparation. In aqueous media, hydrogen
tetrachloroaurate( III ) hydrolyzes to release various species with different
amounts of chloride, water and hydroxyls {e.g., [AuCl 4 ] , [AuCl 3 (H 2 O)],
[AuCl 3 OH] , [AuCl 2 (OH) 2 ] , [AuCl(OH) 3 ] , [Au(OH) 4 ] }, whose composition
can vary with both pH value and chloride concentration. Typical precipi-
tating agents are either sodium hydroxide (DP NaOH) or urea (DP urea).
An advance in terms of stability, reproducibility and overall catalytic
performance was reached when the immobilization of a preformed gold
colloidal dispersion (sol) on activated carbons and various oxides was set
up. 5,6 This method, in fact, presents great advantages with respect to other
procedures, mainly regarding the nanoparticles size control and metal
dispersion. The gold nanoparticles (ideal average diameter 1-10 nm) are
prepared in the presence of a stabilizing/protecting agent (polymer, sur-
factant, polar molecule) and subsequently immobilized on a supporting
material, this latter step being crucial for achieving optimum metal dis-
persion. The type and amount of the protecting agent [mainly poly(vinyl
alcohol) (PVA) and polyvinylpyrrolidone (PVP), but also polyhydroxylated
compounds such as D -glucose], the reducing agent (i.e. NaBH 4 ), pH value,
gold sol concentration and gold loading on the support deeply affect the
final catalytic performance. Also, the surface properties and morphology of

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.