Of all of the methods available to prepare metal-supported catalysts,
impregnation is the simplest, least expensive, and most
prevalent. Impregnation can be termed wet or dry, depending on
whether the volume of impregnating solution is greater than or
equal to the pore volume of the support. Dry impregnation often
is termed incipient wetness impregnation (IWI). With wet impregnation
(WI) when pH is not controlled, the pH of the impregnating
solution can vary quite dramatically and often ends up near
the support point of zero charge (PZC), at which point no metal
precursor–support interaction occurs [1]. After impregnation, various
drying and pretreatment steps can be used to remove the
metal ligands and to reduce the metal to its catalytically active
state.
Recent progress has been made in catalyst impregnation
through fundamental studies of the adsorption process. A landmark
work is the postulation of Brunelle that the adsorption of
noble metal complexes onto common oxides supports was essen- tially coulombic in nature [2]. The hydroxyl groups that populate
oxide surfaces become protonated and thus positively charged below
a characteristic pH value or become deprotonated and thus
negatively charged above the characteristic pH value. The pH at
which the hydroxyl groups are neutral is termed the PZC. A simple
intuitive picture of this surface chemistry is depicted in Fig. 1.
Brunelle cited many instances in which oxides placed in solutions
at pH values below their PZC would adsorb such anions as hexachloroplatinate
[PtCl6]−2, whereas at pH values above their PZC
would adsorb such cations as platinum tetraammine [(NH3)4Pt]+2.
In either case, the metal complex might be considered to deposit onto the surface via strong electrostatic adsorption (SEA). SEA is a
special case of wet impregnation in which the final pH is targeted
to the pH range in which the electrostatic interaction is strongest.
This mechanism has been ascertained for the uptake of cationic
Pt ammine ([Pt(NH3)4]+2) over a variety of amorphous silicas in
previous work [3]. It also was demonstrated that catalysts prepared
via SEA had a higher dispersion of Pt than those prepared via DI
at the same metal loadings [4]. In the current study, it is desired
to examine the applicability of the SEA method to other metal ammine
complexes over a typical amorphous silica, which is the focus
of this paper, and to mesoporous silicas, which were treated in the
companion paper [5].