70
Z.-J. Zuo et al. / Catalysis Communications 34 (2013) 69–72
2
.2. Catalytic activity test
trends with reaction time. The results show that ethanol selective re-
duction occurs due to the increase in methanol selectivity because the
CO conversion shows no obvious change with reaction time.
Our group previously reported that surfactants mainly interact
The CuO–ZnO–Al
.5 L slurry reactor under continuous mechanical agitation and then
and N (N /H =4)
under atmospheric pressure for 8 h at 553 K. The temperature was
then decreased to 523 K. Syngas (H /CO=2) was introduced into
the reactor at 4.0 MPa with a flow rate of 150 mL/min. The gaseous
products were analyzed using an online gas chromatograph equipped
with a flame ionization detector to detect ethanol, methanol, DME,
and hydrocarbons as well as a thermal conductivity detector (TCD)
to detect carbon monoxide and hydrogen. The liquid products were
collected daily and analyzed offline using the gas chromatograph.
2 3
O slurry catalysts (20 g) were loaded into a
0
reduced using a gas mixture composed of H
2
2
2
2
2 3
with Cu and Zn during the preparation of the CuO–ZnO–Al O slurry
catalyst. The strength or weakness of the interaction among elements
is a key factor in catalytic performance [16,17]. In the present work,
differences in the addition sequences resulted in strong interactions
between the active metals with zinc and aluminum oxide. As such, a
significant difference in CO conversion and ethanol selectivity was
observed.
2
3.2. XPS characterization
2
.3. Catalyst characterization
The chemical state and surface components of the catalysts under
different addition sequences were characterized by XPS. The Cu 2p
XPS spectra of the catalysts before reduction and after reaction are
shown in Fig. 2. Based on the Cu 2p spectra, the Cu 2p3/2 binding
energies of all samples were approximately 932.3 eV, which could
Before catalyst characterization, the slurry catalysts were centrifuged,
extracted using petroleum ether, and then dried at room temperature.
X-ray photoelectron spectroscopy (XPS) was conducted using an
ESCALAB 250 spectrometer (VG Scientific Ltd., UK) equipped with
monochromated Al Kα (hν=1486.6 eV, 150 W).
0
+
be attributed to Cu or (and) Cu species [18–21]. The absence of sat-
ellite peaks between 940 and 945 eV indicates the absence of Cu2
ions in all of the samples, which is in accordance with our X-ray dif-
fraction results (see Fig. S2).
+
The H
2
temperature-programmed reduction (TPR) test was
performed in a fixed-bed reactor. For each TPR experiment, 50 mg
samples were loaded into the reactor and heated to 783 K at a rate of
Fig. 3 shows the X-ray-excited Auger electron spectroscopy results
1
(
0 K/min using a temperature controller in a reduction gas of H
5/95) with a flow rate of 30 mL/min. Hydrogen consumption was
recorded using a TCD.
2
/N
2
of the Cu L VV of the catalysts obtained under different addition
sequences before reduction and after reaction. A peak was observed
3
at about kinetic energy (KE) 916.1 eV for the CuZn+Al sample,
+
which indicates that the Cu species on the surface were mostly Cu .
3
. Results and discussion
By contrast, the peak of the Al+CuZn sample was widely dispersed,
i.e., the concentration of copper species on the catalyst surface was
3
.1. Catalytic activity
3
too low to detect. After reaction, the Cu L VV Auger spectra of the
CuZn+Al sample were close to that of the Al+CuZn sample before
reduction. A peak was observed at about KE 918.5 for the samples
after reaction. This result illustrates that Cu0 was the main copper
species on the catalyst surface.
2 3
The catalytic performance of CuO–ZnO–Al O slurry catalysts pre-
pared using different addition sequences is shown in Fig. 1. The two
catalysts show significant differences in CO conversion and ethanol
selectivity. The Al+CuZn catalyst had higher CO conversion but
poor ethanol selectivity. Compared with the Al+CuZn catalyst, the
CO conversion of CuZn+Al was low. However, high ethanol selectiv-
ity was observed at the initial stages of reaction, reaching approxi-
mately 40%. The ethanol selectivity then sharply decreased to 5%
after 5 d of reaction. Fig. S1 shows the methanol and DME selectiv-
ities. The methanol and ethanol selectivities exhibited opposite
The surface compositions of the catalysts before reduction and
after reaction are listed in Table 1. The Cu/Zn and Cu/Al ratios of the
CuZn+Al catalyst surface before reduction were higher than that of
the Al+CuZn catalyst surface. After reaction, the Cu/Zn and Cu/Al ra-
tios decreased, which may decrease ethanol selectivity. Therefore,
+
ethanol synthesis may require not only higher Cu on the catalyst
surface but also a higher Cu/Zn ratio.
Fig. 1. Catalytic performance of CuO–ZnO–Al
rate=150 mL/min, H /CO=2).
2 3
O catalysts by different addition sequences as time on stream (reaction condition; T=523 K, P=4.0 MPa, feed flow
2