Angewandte
Communications
Chemie
(
57%wt. Ag in the solid), in agreement with the equimolar
but intertwined, with a high amount of shared interface area
that facilitates interaction, and where a representative fraction
of the Ag remains accessible to gas-phase reactants.
ratio of the initial solution (see the Experimental Section and
Figure S1 in the Supporting Information). X-ray diffraction
(
XRD) and HR-TEM analysis confirmed the presence of two
The evaluation of the catalytic activity of the Ag/CuO
hybrid catalyst for ethylene epoxidation was first carried out
in the absence of promoters and compared with a reference
silver-based catalyst (20% wt. Ag) supported on a-alumina
and prepared by a conventional wetness impregnation
method (Figure S7). The performance of this catalyst is
¯
crystallographic phases assigned to metallic Ag (Fm3m cubic)
and CuO (C2/c monoclinic), respectively (Figures S2, S4, S5).
To gain further insight into the distribution of the Ag
nanostructures in relation to the CuO phases, high-angle
annular dark field scanning transmission electron microscopy
(
HAADF-STEM) images were acquired, including a tomo-
similar to other unpromoted Ag/Al O3 catalysts in the
2
graphic analysis carried out by taking up to 140 STEM images
at varying angles from À70 to 708 and reconstructing a three-
dimensional model of a single Ag/CuO tube. Different
morphologies ranging from small nanoclusters to anisotropic
rods accounted for the heterogeneous variety of shapes
displayed in this Ag/CuO catalyst (Figure 1a; Figures S3–S5).
The energy-dispersive (EDX) mapping analysis of the different
layers of material compositions suggested a core–shell distri-
bution with an irregular Ag core (outlined in red in Fig-
ure 1b,c) in conjunction with multiple clusters of Ag nano-
particles of a few nanometers in size (Figure 1b, AgNPs shown
in blue). A CuO outer shell (highlighted in yellow/orange in
Figure 1c; see also Figure S6 and the Movie in the Supporting
Information) surrounds these Ag nanostructures but interest-
ingly, does not completely covering them. In summary, from
the above observations, and especially from the 3D tomo-
graphic reconstruction, a picture emerges of a complex hybrid
nanostructure in which metallic Ag and CuO are segregated
literature (see for instance Ref. [9]). The results obtained
during reaction under identical experimental conditions of
both solids (that is, the Ag/Al O reference catalyst and the
2
3
Ag/CuO system) are summarized in Table S1. It can be
observed that the Ag/CuO nanostructure greatly outperforms
the reference Ag/Al O catalyst. At 2258C, the conversion
2
3
obtained with the Ag/CuO catalyst is more than 20 times
higher, but the reaction rate, referred to the mass of Ag, is
more than 40 times higher. Furthermore, with the Ag/CuO
catalyst, selectivity values are generally over 10 points higher
at equivalent conversions and 21% yields can be achieved at
only 2258C. Furthermore, the Ag/CuO structure starts to be
active at a much lower temperature and 1008C is sufficient to
obtain measurable conversions, instead of the 1758C typically
[
18]
reported for this reaction.
As can be inferred from the preceding discussion, we
credit the enhanced performance of the Ag/CuO catalyst to
the strong interplay between both phases, with a high degree
of entanglement shown through electron microscopy images.
The presence of Cu can withdraw electrons from nearby Ag
atoms rendering the Ag more electropositive, thereby
increasing the electrophilicity of adsorbed oxygen species
[
19]
that favor the direct formation of EO.
Perhaps the most compelling evidence regarding the
singular nature of the Ag/CuO catalyst compared to standard
epoxidation catalysts can be obtained from the different
behavior against poisoning by chlorine-containing com-
pounds. To study this, we challenged both Ag/CuO and Ag/
Al O reference catalysts by co-feeding of a Cl precursor (1,2-
2
3
dichloroethane, DCE) during the ethylene epoxidation reac-
tion in the absence of ethane (ethane is generally added in
industrial practice to counterbalance the poisoning effect of
[
3,20]
Cl).
Their behavior was remarkably different, as shown in
Figure 2, where the evolution of the rates of formation of EO
and CO during the ethylene epoxidation reaction at 2008C is
2
shown.
As expected, in the absence of ethane in the feed, the Ag/
Al O catalyst is quickly poisoned. After 30 min, the reaction
2
3
rate is one tenth of the initial, and becomes negligible after 2 h
on stream (Figure 2b). In contrast, for the Ag/CuO catalyst the
EO formation rate initially increases, reaching a maximum
after about 25 min of poisoning and smoothly decreasing
thereafter (Figure 2a). Interestingly, for the Ag/CuO catalyst
the effects of Cl pretreatment are permanent, and depend
solely on the total amount of DCE fed to the reactor. If at any
time during pretreatment the DCE feed is stopped, the catalyst
remains thereafter “frozen” in that state, with constant
conversion and selectivity (Figure S8), independent of the
amount of Cl previously dosed. Moreover, the catalyst main-
Figure 1. Morphochemical analysis of the Ag/CuO catalyst: a) Selec-
tion of representative HAADF-STEM images and the corresponding
EDX mapping analyses to determine the chemical distribution of Ag
(
red) and Cu (yellow) on the catalyst; b) Snapshot of the Ag fraction in
a HAADF-STEM 3D tomography reconstruction of a Ag/CuO nano-
tube: the red figure corresponds to the bulk Ag inner core while the
blue dotted images are randomly distributed individual Ag NPs;
c) Equivalent side-view reconstruction now including the CuO envelope
(
yellow color) covering the Ag core. The presence of Ag/CuO inter-
twined areas with Ag islands emerging on the surface can be detected.
All scale bars=25 nm.
2
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Angew. Chem. Int. Ed. 2016, 55, 1 – 5
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