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.