J. Wang et al. / Catalysis Today 149 (2010) 157–162
159
increased from 0.2 to 0.8 A. (2) The cathode is still made by Au
paste. MoVTeNbO paste was applied as anode and catalyst on to
the other surface of the BICUVOX.10 disc. At this case, 50 mg
MoVTeNbO catalyst was ground up with glycol and then pasted to
the anodic side of the membrane followed by heating at 400 8C for
2
h to eliminate the glycol. This configuration is denoted as Au/
BICUVOX.10/MoVTeNbO (Fig. 1b), and the applied current was
adjusted from 0.01 to 0.06 A. In both modes, the operating
temperature was 380 8C. The flow rate of air to the cathodic
chamber was 50 ml/min. The flow rates of propane and water
vapor to the anodic chamber were 1 and 12 ml/min, respectively,
and the mixture of propane and water vapor was diluted by helium
to keep the total flow rate 28 ml/min.
To check the effect of gaseous oxygen, the selective oxidation
reaction of propane was carried out in a conventional fixed-bed
quartz tubular reactor at atmospheric pressure at 380 8C. 2 g
MoVTeNbO catalyst was introduced into the reactor and the feed
3 8 2 2
composition was kept at C H :H O (gas):(O + He) = 1:12:15 by
adjusting the oxygen and helium flow rates simultaneously. The
gaseous oxygen was varied from 1.5 to 3 ml/min.
The blank reactor with Au/BICUVOX.10/Au in the absence of
MoVTeNbO catalyst was tested under similar conditions to that
with the MoVTeNbO catalyst. Under an imposed current from 0.2
2 2 4 2 6
to 0.8 A, only traces of CO , C H and C H were detected and the
amounts were negligible compared to those obtained in the reactor
with the MoVTeNbO catalyst.
The liquid products were collected with cold water and
analyzed by GC (Agilent 6890) equipped with FID and the gas
products were analyzed with a TCD.
3
. Results and discussion
3.1. Structures and morphologies of the membrane
Fig. 2 shows the X-ray diffraction patterns of the BICUVOX.10
powders synthesized by the EC process (Fig. 2A) and SSR method
Fig. 2B) as a function of the calcination temperature. Fig. 2A shows
(
Fig. 2. XRD patterns of BICUVOX.10 oxide powders synthesized by (A) the EC
no obvious change with temperature for BICUVOX.10 powder
synthesized by the EC method up to around 600 8C at which an
undesirable phase appears. This phase can be satisfactorily
method and (B) the SSR method at different temperatures. The main diffraction
peaks of BICUVOX.10 (*), BiVO
4
(*) and Bi
2
O
3
(ꢂ) are shown.
ascribed as BiVO
4
[23]. A further increase in temperature leads
3.2. Oxygen permeation performance of membranes derived from EC
and SSR method
to the appearance of a desirable orthorhombic BICUVOX phase at
around 650 8C. In contrast, for the BICUVOX.10 powder synthe-
sized by the SSR method, the desired crystallographic phase was
fully developed until the temperature reached 800 8C (Fig. 2B),
which is close to the melting point of the membrane. Below 800 8C,
the XRD patterns differ somewhat from those observed for the
In order to investigate the ability of BICUVOX.10 membranes
prepared using different synthetic methods to separate oxygen gas,
a series of oxygen permeation tests were performed at 400 8C on
BICUVOX.10 membranes synthesized by the EC and SSR methods,
with optimized sintering temperatures of 700 8C and 800 8C,
respectively. As shown in Fig. 4, it was apparent that the
BICUVOX.10 membrane prepared from the SSR method was more
stable than that from the EC method. This result shows that the
preparation method played an important role in the oxygen
permeation performance of the BICUVOX.10 membrane. For the
membrane from the EC method, when the applied current was
higher than 0.8 A, the Faradic efficiency decreased dramatically
and was accompanied by a significant decline in the oxygen
permeation (Fig. 4A). However, for membranes prepared with the
SSR method, it is important to note that the Faradic efficiency was
above 80% over the entire current intensity range studied. In
particular, the volume of oxygen transferred by the membrane was
strictly proportional to the current density (Fig. 4B). From the
report by Boivin and co-workers [19], it is known that, at low
current density, the oxygen molecules are dissociated in the usual
way at the triple point boundary within the gold grid, the
electrolyte and the gas phase. With increase of current density, the
number of triple points becomes too small and a partial reduction
2 3
material by the EC method, showing the absence of a Bi O phase.
This suggests that the difference in temperature at which the
orthorhombic BICUVOX structure appeared with the different
preparation methods.
SEM measurements were carried out to study the morphologies
of BICUVOX.10 membranes. Typical surface and cross-sectional
SEM images of membranes synthesized by the SSR and EC methods
with sintering at 800 8C for 2 h are shown in Fig. 2. It is clear from
the SEM images that ceramic grains with clear boundaries are
densely packed together, and the grain size of the membrane from
the EC method is much larger than that from the SSR method,
indicating that a directional grain growth took place (Fig. 3A) for
the membrane derived from the EC process. Although the
membranes appear dense from the top, there are many closed
pores in the cross-sections of the membranes prepared with both
methods. It is important to note that the closed pores in the
membrane from SSR are larger than those in the membrane from
EC. This could be due to the larger initial particle size of the powder
from the SSR method.