Y. Li et al. / Catalysis Communications 73 (2016) 12–15
13
weight of K
lysts and the weight of CeO
cause the total mass fraction of K
determine the weight of K O and the ratio of K
The preparation procedures were as follows: (1) Fe
mixed with an aqueous solution of Mg(NO ·6H O and Ce(NO
to obtain a metastable suspension; (2) poured the suspension into a
mixed solution of (NH Mo 24·4H O and K CO slowly during stirring;
3) NH OH was used to adjust the pH value of the mixture to 8 to get a
2
O and CeO
2
in our catalysts. We prepared about 144 g cata-
was set to be 13.5, 14.5, 15.5 and 17.5 g. Be-
O and CeO was fixed, we could
O to CeO in each catalyst).
powder was
peaks arising from CeO
weight ratio of K
creasing content and grain size of CeO
of K Fe22 34, the contents in K/Ce-1.56 and K/Ce-1.40 are higher than
that in K/Ce-1.75 and K/Ce-1.12, through analysis by external standard
method based on the corresponding diffraction peaks (Supporting in-
2
become stronger and sharper with a decreasing
2
2
O to CeO
2
. Obviously, this can be attributed to the in-
[14]. While for the active phase
2
2
2
2
2
2
2
O
2
O
3
3
)
2
2
3 3 6 2
) · H O
formation). Kotarba et al. has pointed out that the addition of CeO
2
4
)
6
7
O
2
2
3
had a positive influence on the formation of active phases [15], and
this might be an important reason why K/Ce-1.75 catalyst does not pos-
(
4
heavy slurry; (4) aged the slurry for 1 h, dried at 120 °C for 4 h and cal-
cined at 250 °C for 6 h in air to produce a catalyst precursor; (5) mixed
the catalyst precursor with sodium carboxymethylcellulose, cement and
moderate amount of water to produce a paste, then extruded which to cy-
lindrical strips (Φ3 × 5 mm); and (6) the final catalysts were obtained
after the stripes were dried at 120 °C for 4 h and calcined at 850 °C for 4 h.
sess highest K
Therefore, the K
the catalysts, but also affects the amount of active phases.
2
Fe22
O
34 content (K/Ce-1.75 has the lowest CeO
2
loading).
2
content in
2
O/CeO
2
ratio not only directly decides CeO
2
3.2. H -TPR tests
H
2
-TPR profiles are given in Fig. 2. Several peaks can be observed
2
.2. Characterization of catalysts
from each curve. For instance there are two strong peaks at about 450
and 750 °C. They are corresponding to the reduction process of
Crystalline phases of the prepared catalysts were analyzed using a
3
+
2+
2+
Fe → Fe (low temperature peak) and Fe → Fe (high temperature
peak) [4,6,16]. As a whole, higher CeO content hinders the reducibility
) [10]. Furthermore, two intense hydro-
powder X-ray diffraction (XRD) analyzer. Valence states of elements
in the catalysts were studied by X-ray photoelectron spectroscopy
2
of Fe3 (Fe
+
O ) to Fe (Fe O
2+
2 3 3 4
(
XPS) measurements on a PHI Quantum-2000 instrument. Temperature
programmed reduction of H (H -TPR) tests were performed under H
Ar flow (30 ml/min; H volume fraction was 5%) in a temperature range
from 50 to 900 °C with a heating rate of 10 °C/min. Consumption
amount of H was continuously monitored by a thermal conductivity
detector in a mass spectrometer.
gen consumption bands can be observed at about 475 and 780 °C (the
circled zones in Fig. 2). They result from the reduction of surface oxygen
2
2
2
/
2
and bulk oxygen from CeO
2
(CeO
(Ce ) have similar crystal structures (CaF
Ce and Ce are easy to coexist.
2
→ Ce
2
O
3
) [13,17]. Because CeO
2
4
+
3+
(
Ce ) and Ce
2
O
3
2
-typed),
2
4
+
3+
3
+
3+
Copresence of Fe and Ce is possible on the basis of the reduction
2
+/3+
+
3+/4+
potential of the Fe
and Ce
couples. Significantly, the com-
plays a positive role in EBDH: during
2
.3. Catalytic activity tests
bined action of Fe3 and Ce
3+
EBDH reaction, α-hydrogen of ethylbenzene attacks the acid site of
The evaluation of the catalysts for EBDH was carried out using a cy-
lindrical fixed-bed reactor with the diameter and length of 14 and
50 mm, respectively. 5 ml catalysts (about 6.5 g), with particle diame-
ter of 0.71–1.18 mm, were loaded into the reactor for catalyzing EBDH
reaction at 610 °C. During the reaction process, a mixture of gas of eth-
3
+
the catalysts (Fe ) and simultaneously, β-hydrogen attacks the basic
site (Ce3 ), as shown in Fig. S2 [10,18,19], while positive charge on α-
carbon at the transition state can be stabilized by the aromatic ring.
Based on the point of view mentioned above, K/Ce-1.40 and K/Ce-1.12
catalysts may have better catalytic activity, because their relatively
+
5
ylbenzene (2.5 ml/h) and H
the reactor. Liquid hourly space velocity is 0.5/h. The end-products
containing styrene, toluene, benzene and ethylbenzene, etc.) were an-
2
O (3.5 ml/h) was continuously injected into
3
+
high CeO
2
content is beneficial for the keeping of Fe
(instead of
being reduced to Fe ) and the coexisting of Fe and Ce3+.
2
+
3+
(
alyzed by gas chromatography with the assistance of a flame ionization
detector.
3.3. XPS measurements
3
. Results and discussion
2 3 2 2
The reaction mechanism of EBDH catalyzed by Fe O –K O–CeO cat-
alysts can be divided into two types: (a) direct dehydrogenation mech-
anism and (b) oxygen transfer dehydrogenation mechanism. Schematic
expressions of these potential mechanisms are shown in Fig. S3. As
3
.1. Phase compositions of the catalysts
Fig. 1 shows the XRD spectra of various catalysts. Diffraction peaks
2
metal oxide with variable valence, CeO can introduce a certain amount
due to K
2
Fe22
O
34 and CeO
2
are detected from all the catalysts. The
of oxygen vacancies to the catalyst, thus optimizing the activity of lattice
oxygen, increasing electron transfer channels and making the dehydro-
genation reaction easier to happen [10,20]. Therefore, it is necessary to
detect the valence states of O and Ce elements in the catalysts.
Oxygen species have two spectral peaks in O 1s XPS spectrograms of
Fe–K–Ce oxide based catalysts see Fig. 3: the peak at ~530.5 eV is as-
cribed to lattice oxygen of metal oxide and that at ~532.5 eV arises
from adsorbed oxygen [17]. The Ce 3d XPS profiles in Fig. 4 are more
complicated due to mixing of Ce 4f levels with O 2p states. Following
the previous literature [17,20], the electronic transitions in Ce 3d3/2
and Ce 3d5/2 levels can be divided into several sets of features ground
as U and V lines, respectively. The U'''/V''' doublet is associated with
primary photo emission from Ce4 . The U''/V'' and U/V doublets result
from electron transfer from a filled O 2p orbital to an empty Ce 4f orbital.
The U'/V' doublet is caused by photo emission from Ce3 cations. These
evidences imply that cerium is present in the form of hybrid oxidation
+
+
states of Ce3 and Ce
+
4+
[17,20]. This coincides well with the result
obtained from catalyst TPR curves. Relatively, K/Ce-1.40 catalyst has
stronger U'''/V''' and U/V doublets, corresponding to an effective
electron transfer ability.
Fig. 1. XRD patterns of the catalysts.
Li, Yang
