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(typically
5m
2
g
1
), hence Pd/CeO
2
typically exhibits poor selox behaviour
owing to the resultant
B
low nanoparticle dispersions that
favour (self-
poisoning) metallic Pd.
93,95,96
Fabrication of hierarchical, porous ceria supports is possible through
the evaporation-induced self-assembly protocol (EISA),
97
permitting highly
tuneable surface areas between 100 and 250 m
2
g
1
.
88,90,98,99
Increasing the
ceria surface area and internal porosity confers similar benefits to those
observed for silica-supported palladium catalysts due to an enhanced PdO
surface density and electronic perturbation via Pd-Ce bonding,
96
which
is known to improve activity and selectivity during CO oxidation.
100-103
Although still
d
n
4
r
4
n
g
|
2
in its infancy, ceria remains a promising support
for
palladium-catalysed allylic alcohol selox.
77
The popularity of titania as a reducible metal oxide support for Pd-based
oxidation and photocatalysis has led to several elegant porous analogues.
Careful choice of the titania precursor, surfactant and synthesis conditions
permits extensive control over the support architecture and palladium-
support interactions.
104-109
PdO nanoparticles impregnated on high surface
area, porous titania by Mitsudo and co-workers demonstrated excellent
activity and selectivity (87%) in benzyl alcohol selox.
110
Mori and co-workers also explored the use of hydroxyapatite
[Ca
10
(PO
4
)
6
(OH)
2
] as a catalyst support,
111,112
comprising Ca
21
ions
surrounded by PO
4
3
tetrahedra. This structure facilitates isomorphic
substitution of calcium for transition metal cations such as Pd, thereby
generating stable monomeric phosphate complexes with an inherent
proclivity for aerobic selox.
111
Recent computational and in situ XAFS studies
showed that palladium crystallites exposing high-energy facets (110)
supported on hydroxyapatite promote O
2
dissociation in the solventless
selox of
DL
-sec-phenethyl alcohol, benzyl alcohol and cinnamyl alcohol
(Figure 4.10).
113
.
Figure 4.10
(Left) crystal structure of hydroxyapatite and (right) proposed reaction
mechanism for the oxidation of alcohols over Pd-HAP systems.
Adapted from Ref. 113 with permission from The Royal Society of
Chemistry.
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