Chemistry Reference
In-Depth Information
supercritical phase. The low toxicity and prevalence of CO
2
also makes it
a potential green medium for catalyst preparation.
The preparation of single and mixed metal oxides for use as catalysts
supports, or as active catalysts themselves, has been achieved using
supercritical CO
2
anti-solvent processes, rapid expansion of supercritical
solutions and precipitation using supercritical H
2
O. These preparation
techniques are most comparable with the well known co-precipitation
technique. SAS preparations and precipitation using SC-H
2
O have been
shown to produce exceptionally well mixed phases, which are not as
easily accessed from conventional coprecipitation. SC-H
2
O preparations
provide a single pot process to make metal oxide samples that cannot be
achieved with SAS, without an additional calcination step. However, it
must be noted that temperatures frequently used in precipitation
with SC-H
2
O are as high as those required for calcination steps for
conventional and SAS precipitation processes. Supercritical deposition of
metal salts onto a preformed support can be considered analogous with
deposition precipitation. The term impregnation occasionally used in
publications involving this technique causes confusion as it is quite
different to wet or incipient wetness techniques common to catalyst
preparation. The supercritical process has the advantage that it is more
readily able to deposit active species into porous structures.
The preparation of catalysts using supercritical fluid techniques is still
a relatively new area and many studies are still required to scope out the
full potential for catalyst preparation. Precursor salts used may well be
more expensive than those used in conventional catalyst preparation
techniques, but in many cases the supercritical fluid approach can be
considered a greener alternative, and it provides the option to recycle
reactants relatively easily. Perhaps most importantly supercritical fluid
methods can produce novel materials, which may not be accessible
through other preparation techniques and this is an important concept
for making new materials in the process of catalyst discovery.
References
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