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d
n
4
r
4
n
g
|
2
Figure 4.6
(Top right) dependence of allylic alcohol selox rate upon surface PdO;
(top left) schematic of operando liquid-phase reactor; (bottom left) evo-
lution of Pd K-edge XAS of Pd/Al
2
O
3
catalyst during cinnamyl alcohol
aerobic selox; (bottom right) temporal correspondence between Pd oxi-
dation state and selox activity in cinnamyl alcohol selox.
Adapted from Refs 36 and 37 with permission from The Royal Society of
Chemistry.
.
competitive solvent effects.
70,71
Under mild reaction temperatures, pal-
ladium nanoparticles were partially oxidized and unperturbed by exposure
to sequential alcohol or oxygen pulses (Figure 4.7). Crotonaldehyde formed
immediately upon contact of crotyl alcohol with the oxide surface, but only
desorbed upon oxygen co-adsorption. Higher reaction temperatures induced
PdO reduction in response to crotyl alcohol exposure, mirroring that
observed during liquid-phase selox; however, this reduction could be fully
reversed by subsequent oxygen exposure. Such reactant-induced re-
structuring was exhibited by all palladium nanoparticles, but the magnitude
was inversely proportional to particle size.
72
These dynamic measurements
decoupled the relative reactivity of palladium oxide from metal, revealing
that PdO favoured crotyl alcohol selox to crotonaldehyde and crotonic acid,
whereas metallic palladium drove secondary decarbonylation to propene
and CO in accordance with surface science predictions.
46
Recent ambient-pressure XPS investigations of crotyl alcohol-O
2
gas
mixtures over metallic and oxidized Pd(111)
single-crystal
surfaces
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