G Model
CATTOD-9820; No. of Pages9
ARTICLE IN PRESS
L. Angelo et al. / Catalysis Today xxx (2015) xxx–xxx
5
these catalysts is around 20 nm independently of the preparation
method, and in spite of different specific surface areas, same pore
3
−1
volumes were calculated (around 0.38 cm g ).
The crystalline structures of the catalysts after calcination are
presented in Fig. 1. Diffraction peaks corresponding to copper
oxide and zinc oxide are observed for both catalysts. The zirco-
nium oxide diffraction peaks are not observed suggesting that this
phase is in an amorphous or a micro-crystallite state [43]. The
◦
◦
wide peak located between a 2 of 15 and 30 is due to the sam-
ple holder glass. By comparing both methods of synthesis, same
diffractograms are obtained and also similar CuO and ZnO crys-
tallite sizes, around 10 nm (Table 1), supposing that the choice
of the synthesis does not lead to deep modifications in catalyst
structure.
The reducibility of copper species was determined by TPR exper-
iments and the results are presented in Fig. 2. For 30CuZn-ZpH, TPR
◦
profile shows one reduction peak at 152 C with a shouldered peak
◦
at 164 C suggesting two reduction steps, probably because of dif-
ferent insertions or interactions between copper and the support
[
38]. This profile can be explained by distinct copper oxide reduc-
tion steps, the first one related to the reduction of small crystalline
CuO clusters, and the second one attributed to a strong interaction
between copper ions and the support [38]. The use of micro-
fluidic continuous coprecipitation to prepare 30CuZn-ZM catalyst
modifies the reduction profile leading to two reduction peaks but
Fig. 5. Section in the reconstructed volume of a representative fragment of the
30CuZn-ZpH catalyst.
a different distribution of copper oxide depending on the region
nature.
Similar study has been performed on 30CuZn-ZM catalyst
and the results are presented in Figs. 6–9. This catalyst does
not present two regions contrary to 30CuZn-ZpH and seems to
be more homogeneous (Fig. 6 compared to Fig. 3). In fact, the
◦
◦
which are spread over a wide zone (between 140 C and 210 C)
◦
and are switched to higher temperatures (peaks at 167 C and
1
◦
85 C).This suggests that the micro-fluidic preparation method
leads to a modification of CuO-support interactions. The theoreti-
−
1
cal hydrogen consumption (4.72 mmolH2
g
) was compared to the
three oxides (CuO, ZnO and ZrO ) location is clearly less obvi-
values obtained for 30CuZn-Z catalysts and the reduction percent-
2
0
ous to distinguish than for 30CuZn-ZpH. Furthermore, the EDX
analysis of the zone 1 of Fig. 6a detailed in Fig. 7 and Fig. 8,
shows the presence of copper, zinc and zirconium regardless to
the area studied. The presence of silver on the EDX analysis is
due to the use of a silver grid. These results attest for a better
homogeneity of the catalyst 30CuZn-ZM prepared by continuous
coprecipitation. The electron tomography analysis (Fig. 9) high-
lights the presence of two CuO particles sizes: around 5.3 nm
age of CuO to Cu were determined assuming the only reduction
of CuO and a copper oxide content of 30% (respectively 98%
and 105% for 30CuZn-ZpH and 30CuZn-ZM). The catalysts present
finally similar reducibility closed to 100% confirming a total copper
reduction.
The copper surface areas calculated by N O reactive frontal
2
chromatography are given in Table 1. The highest copper sur-
2
−1
face area is observed for 30CuZn-ZM (14.5 m gcata ) which
presented the lowest BET surface area and not for 30CuZn-ZpH
(pink circle) and 2 nm (red arrow), corresponding respectively to
2
−
1
2
−
1
the previously observed sizes for the two zones, one rich in zinc
and the second rich in zirconium, for 30CuZn-ZpH. Consequently,
there seems to be the presence of two different morphologies
but which are not obvious to distinguish. It is thus difficult to
conclude if the zinc oxide and the zirconia are mixed or if a
ZnO layer covers a zirconia layer. However the assumed better
homogeneity for 30CuZn-ZM explains the highest copper surface
area.
These results obtained by TEM analysis were also used to
highlight the zirconium oxide state that could not be previously
determined by XRD. Since no crystalline lattice has been observed
on the particles attributed to zirconia, ZrO2 is considered amor-
phous in the studied samples.
(
10.5 m gcata ) whose BET surface area was of 79 m gcata , sug-
gesting that a high copper surface area is not directly correlated
with a high BET surface.
The catalyst morphology after calcination observed by TEM is
shown in Fig. 3 for 30CuZn-ZpH. This catalyst presents two different
morphologies: a first region (zone A) composed of large particles
and a second region (zone B) composed of small particles. This
highlights an inhomogeneous morphology of the sample. To bet-
ter understand this inhomogeneity, STEM-EDX analyses (Fig. 4)
in these two regions were conducted. It shows that the zone A
is rich in zinc with a high level of copper but only a few traces
of zirconium, while for the zone B a high level of both zirco-
nium and copper is observed. The same observations were made
by analyzing different areas of the sample. The copper oxide is
then present in both regions. In order to have a better understand-
ing of the copper oxide dispersion in these two regions, electron
tomography approach was used. It provides a three dimensional
representation of the studied object and the location of different
phases within the material. The reconstruction volume of a repre-
sentative fragment of 30CuZn-ZpH catalyst is presented in Fig. 5.
The zone A, rich in zinc oxide, shows that copper covers part of
the surface of large ZnO particles, evidenced by red arrows in
Fig. 5. In contrary, in the zone B, rich in zirconium oxide, copper
oxide is under the form of nanoparticles (green arrows) dispersed
3.2. Catalytic activity
The results obtained for 30CuZn-Z catalysts in CO2 hydrogena-
◦
tion reaction at 240, 260, 280, 300 and 320 C at 50 bar and with a
−
1
GHSV of 10,000 h are presented in Table 2.
Firstly the influence of reaction temperature on catalytic activ-
ity was studied. The catalytic results indicate that the conversions
of H2 and CO2 increase gradually with the reaction temper-
◦
ature until 280 C when the H2 conversions stabilize around
9% whereas the CO2 conversions keep increasing from 22% at
◦
◦
280 C to 26% to 320 C. These results are directly correlated
2
◦
zone: zone A ≈ 5.5 nm and zone B ≈ 2.4 nm. These results evidence
to a progressive decrease in methanol selectivity (at 240 C,
Please cite this article in press as: L. Angelo, et al., Catalyst synthesis by continuous coprecipitation under micro-
fluidic conditions: Application to the preparation of catalysts for methanol synthesis from CO /H , Catal. Today (2015),
2
2