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Table 2 Catalytic activities of the di(octyl)phosphinate capped ZnO and stearate-
capped Cu nanoparticles for methanol production, at different proportions, by
weight, of ZnO/Cu
ripening prevents the formation of a high concentration of the
critical ZnO/Cu interface.
The organometallic approach to nanoparticle preparation
provides a simple and versatile means of preparing fine struc-
tured colloidal catalysts with different compositions, sizes and
geometries to conventional systems, whilst avoiding excess
free ligand/surfactant in solution. The approach can provide
strongly-bound stabilising ligands whilst retaining surface activity.
The promising initial activity observed for CO2 reduction is
encouraging, and should motivate further studies of a broad
range of related systems and reactions.
Catalyst systema
ZnO : Cu (w/w)a
Activitya/mmol gÀ1 hÀ1
Ternary referencea
35 : 65
50 : 50
65 : 35
75 : 25
65 : 35
65 : 35
7371
8584
20 356
6942
1742
3578
2
2
2
3
4
2 = zinc oxide with di(octyl)phosphinate and copper with stearate capping
ligands. 3 = zinc oxide and copper both with di(octyl)phosphinate capping
ligands. 4 = zinc oxide with stearate and copper with di(octyl)phosphinate
a
capping ligands. See Table 1.
Notes and references
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Fig. 1 LHS: TEM image of particles isolated from catalyst 2 (Table 2, entry 3),
comprising an initial loading of ZnO : Cu of 65 : 35 (w/w). The highlighted areas
illustrate the possible formation of nanostructures between the ZnO and copper
particles (Fig. S18, ESI†). RHS (top) shows a HR-TEM image. RHS (bottom) shows a
photograph of the red solution obtained after the reaction with no catalyst
precipitation observed.
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discrete nanostructures. The only limited particle agglomeration
(Fig. 1 and Fig. S18, ESI†) may be partly related to sample
preparation effects. The HR-TEM image shows zinc oxide nano-
particles, in intimate contact with Cu2O particles (as identified
by lattice spacings); the oxidation of the active copper metal
likely occurred through air exposure on the TEM grid. The TEM
image shows distinctive nanostructures (marked) which appear
to combine one or two pyramidal ZnO particles with a small,
equiaxiated copper (oxide) particle (the speciation is inferred
from the HR-TEM analysis). Schimpf et al., reported the forma-
tion of related, though distinct, ‘bow-tie’ nanostructures from
ZnO/Cu catalysts used for syn-gas to methanol catalysts. The
apparent presence of related nano-structures in both reactions
may point to common catalyst species.8a
Other permutations of the ligand systems were less success-
ful. Using di(octyl)phosphinate capping ligands for both zinc
oxide and copper nanoparticles reduced the catalytic activity to
only B8% of the best system (Table 2, entry 5). Furthermore,
using a catalyst formed by adding stearate-capped zinc oxide to
di(octyl)phosphinate-stabilised Cu also reduced activity to just
B17% of the best system (Table 2, entry 6). These findings are
consistent with the need to form an interface between the ZnO
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phosphinate), the copper cannot be deposited on the ZnO
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surface, conversely, when the ZnO is unstable (with stearate),
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c
11076 Chem. Commun., 2013, 49, 11074--11076
This journal is The Royal Society of Chemistry 2013