Effect of Microwave Irradiation Parameters on Co/Fe Hydrotalcite Nanocatalysts for the Total Oxidation of VOCs

Article abstract of DOI:10.1002/ejic.201801528

A series of Co/Fe hydrotalcite-like samples were prepared by the traditional co-precipitation method and treated under microwave irradiation at different temperatures, powers, and for various durations. The resulting samples have been calcined to obtain the nano-oxide catalysts. Characterization by different physicochemical techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM), temperature programmed reduction by hydrogen (H₂-TPR), and X-ray photoelectron spectrometry (XPS) was carried out. The nanocatalysts were also tested in the total oxidation of propene. Comparing both conventional and microwave synthesis, a significant improvement in the catalytic activity was found for the samples prepared with microwave treatment. A minimum of irradiation time, power, and temperature (1 min, 40 W, and 40 °C) is enough to get higher surface area, larger pore size, smaller crystallite size, and a higher amount of reducible species than those for the non-irradiated solid. All these factors are responsible for the enhancement of the catalytic activity.

Full text of DOI:10.1002/ejic.201801528

DOI: 10.1002/ejic.201801528
Full Paper
Microwave Synthesis of Oxides
Effect of Microwave Irradiation Parameters on Co/Fe
Hydrotalcite Nanocatalysts for the Total Oxidation of VOCs
[
a,b]
[a]
[a]
[b]
Cynthia Abou Serhal,
Isabelle Mallard, Christophe Poupin, Madona Labaki,
[a]
[a]
Stéphane Siffert,* and Renaud Cousin*
Abstract: A series of Co/Fe hydrotalcite-like samples were pre- were also tested in the total oxidation of propene. Comparing
pared by the traditional co-precipitation method and treated both conventional and microwave synthesis, a significant im-
under microwave irradiation at different temperatures, powers, provement in the catalytic activity was found for the samples
and for various durations. The resulting samples have been cal- prepared with microwave treatment. A minimum of irradiation
cined to obtain the nano-oxide catalysts. Characterization by time, power, and temperature (1 min, 40 W, and 40 °C) is
different physicochemical techniques including X-ray diffraction enough to get higher surface area, larger pore size, smaller crys-
(
XRD), scanning electron microscopy (SEM), temperature pro- tallite size, and a higher amount of reducible species than those
grammed reduction by hydrogen (H -TPR), and X-ray photoelec- for the non-irradiated solid. All these factors are responsible for
2
tron spectrometry (XPS) was carried out. The nanocatalysts the enhancement of the catalytic activity.
Introduction
treatment of LDHs gives homogeneous mixed metal oxide cata-
lysts, with a good thermal stability and a large surface area.
Hydrotalcites containing cobalt have been widely studied in
Volatile organic compounds (VOCs) are harmful pollutants for
human health and environment. An efficient way to decrease
the emissions of these pollutants is their total oxidation into
less toxic products. The use of catalysts allows to decrease the
temperature required for such process and also to give selec-
tively the desired products. The nanocatalysts which will be
used in our study are prepared by hydrotalcite route.
[4,5]
several catalytic applications: Fischer–Tropsch reactions,
[6]
[7–9]
aldol condensation, methane reforming,
CO hydrogen-
[10,11]
[12–14]
ation,
NO and SO removal,
and in particular VOCs
x 2
[15]
total oxidation. Indeed, Gennequin et al. have found that the
mixed oxide Co Al O exhibits high activity and good selectivity
6
2 9
[16]
in the total oxidation of toluene. Bialas et al. have also found
a high activity for the catalyst Co Al O in toluene oxidation,
Hydrotalcite-like compounds (HT) or Layered Double
Hydroxides (LDHs) are a well-known class of layered clay materi-
6
2 9
with a T50 (temperature at which 50 % of toluene is oxidized)
of 257 °C. These results are in total agreement with those ob-
[
1]
x+ n–
als, with the general formula [M(II)1-xM(III) (OH) ] (A )x/n·
x
2
yH O. Structurally, LDHs consist of two dimensional brucite like
[17]
2
tained by Thao et al., where the catalyst Co-Mg-Al containing
sheets (Mg(OH) ), in which a part of the divalent cations M(II)
2
the highest loading in cobalt presents the best activity and se-
is substituted by trivalent cations M(III). Each cation is located
in the center of an octahedron and surrounded by six hydroxyl
groups (OH). The positive charge (x ) resulting from this re-
placement, is balanced by anions (A ) (most commonly carb-
onate) and water molecules, located in interlayer domains.
These materials have attracted much attention in numerous ap-
plications such as adsorption, medicine, industry reactions, and
especially as catalyst precursors. Indeed, controlled thermal
lectivity towards styrene oxidation. Furthermore, Aguilera et
[18]
al.
have compared two series of hydrotalcite Cu-Mn-Mg-Al
+
and Co-Mn-Mg-Al towards three different VOCs: butanol, eth-
anol, and toluene. The solid giving the best catalytic perform-
ance was the catalyst containing cobalt.
n–
[
2]
Usually hydrotalcite-like compounds are obtained by the tra-
ditional co-precipitation method. However the main disadvan-
tage of this preparation is the time required during maturation
phase in order to get a well-structured and crystallized mate-
[
3]
[
19–21]
[
[
a] Université du Littoral Côte d'Opale (ULCO),
rial,
Several new synthesis methods have been developed
Unité de Chimie Environnementale et Interactions sur le Vivant
in the aim to improve hydrotalcites textural and structural prop-
(
UCEIV, E.A. 4492), MREI,
[22]
erties.
Indeed, many authors have synthesized hydrotalcites
F-59140 Dunkerque, France
E-mail: renaud.cousin@univ-littoral.fr
[
23–25]
by using the sol-gel method,
Nevertheless, expensive re-
stéphane.siffert@univ-littoral.fr
b] Lebanese University,
actants are used during this preparation. Moreover, urea hydrol-
ysis method has also been investigated in hydrotalcite like com-
Laboratory of Physical Chemistry of Materials (LCPM)/PR2N,
Faculty of Sciences,
Fanar, P.O. Box 90656, Jdeidet El Metn, Lebanon
ORCID(s) from the author(s) for this article is/are available on the WWW
under https://doi.org/10.1002/ejic.201801528.
[26–28]
pounds synthesis,
However, long period of time is re-
[
29]
[22]
quired and small surface areas are obtained. Recent stud-
ies have shown that ultrasound assisted method,
manipulation time and gives larger surface area.
[
29,30]
reduces
[
31]
However,
Eur. J. Inorg. Chem. 0000, 0–0
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comparing both ultrasound and microwave irradiation meth-
[
32]
ods, Genty et al.
ance towards toluene total oxidation with the catalyst Co Al O
2 9
have obtained the best catalytic perform-
6
treated under microwave irradiation.
Indeed, one of the most interesting method and in major
development since world war II, is microwave (MW) irradia-
[
20,33,34]
tion,
Microwaves are electromagnetic irradiation with
wavelength 1 mm to 1 m corresponding to a frequency range
of 0.3 to 300 GHz. In order to avoid any interference with the
wide range of frequencies designated for the telecommunica-
tion domain, it should be noted that, a frequency about
2
.45 GHz is only allowed to be used for microwave heating
[
35,36]
reactions.
Microwave assisted route has been widely used
in chemistry field since the 1950s. In fact, microwave irradiation
is used as a source of heating in many studies, in order to re-
duce the manipulation time and therefore to considerably save
[
34]
energy. Interestingly, microwave treatment showed a signifi-
cant improvement in hydrotalcite synthesis method.
[
37]
In our previous work,
we investigated a series of mixed
oxides, based on Co, Mg, Al, and Fe, prepared by the hydro-
talcite route using microwave irradiation, as catalysts in the re-
action of the total oxidation of the VOC: propene. The mixed
oxide Co Fe O exhibited the best catalytic activity among the
4
2 7
studied solids.
The aim of this work is to investigate the influence of micro-
wave irradiation temperature, power, and time used during the
synthesis process, on the performance of the nanocatalyst
Co Fe O towards the total oxidation of the VOC propene.
4
2 7
Results and Discussion
1
. Uncalcined Samples
X-ray diffraction patterns of all uncalcined samples are pre-
sented in Figure 1. All samples exhibited one phase correspond-
ing to the hydrotalcite phase desired. However, a small shoulder
was detected at approximately 2 θ = 33.5°, assigned to the
minor phase of cobalt hydroxide α-Co(OH) . It should be noted
2
that this appearance is not related to microwave irradiation
treatment, as it is also detected on the sample Co Fe -HT pre-
4
2
pared by the conventional method. In all samples, hydrotalcite
structure was obtained as a major phase.
However, the series of samples treated at different irradiation
temperatures (Figure 1a) shows other modifications. In fact, the
MW-80 catalyst subjected to irradiation at a temperature equal
to 80 °C has the most intense lines. However, peak intensities
decrease and new phase are detected for solids exposed to
irradiation temperatures above 80 °C. The lines detected for
MW-150 and MW-170 at approximately 2θ = 18°; 31°; and 36°
are attributed to spinels CoFe O and Co O . Similarly, by evalu-
Figure 1. X-ray diffraction patterns of the uncalcined samples treated under
microwave irradiation at different (a) heating temperatures, (b) powers, and
2
4
3 4
(
c) durations. H: Hydrotalcite phase (JCPDS No. 22–0700); h: Cobalt hydroxide
α-Co(OH) (JCPDS No. 46–0605); ●: Co (JCPDS No. 42–1467) or/and
CoFe (JCPDS No. 22–1086.
ating the influence of temperature on the hydrotalcite struc-
ture, peak intensities decrease with decreasing the irradiation
temperature from 80 to 40 °C.
2
3 4
O
2 4
O
The effect of the microwave irradiation power (Figure 1b)
does not show any modification of the structure even at the
highest power of 200 W. However, as the power of the irradia- As a result, the MW-200W sample with the highest power exhib-
tion increases, the intensity of the peaks increases respectively. its the strongest peaks. It is important to note that all samples
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subjected to microwave irradiation at any power, show more tive and negative charges present in the hydrotalcite structure.
intense lines than Co Fe -HT.
Furthermore, the increase in the c value detected for solids sub-
This can be also observed at different microwave heating mitted to microwave irradiation at high temperatures (150 and
durations. In fact, from the X-ray diffractograms in Figure 1c, 170 °C) compared to the conventional Co Fe -HT could be also
4
2
4
2
all dried solids submitted to microwave irradiation exhibit the ascribed to the decrease in the interactions between the posi-
desired hydrotalcite phase whatever is the duration of irradia- tive and negative charges due to spinels formation detected in
tion. However peak intensity decreased by reducing the dura- XRD (Figure 1a).
[
38]
tion from 60 min to 1 min. It is to be added that other study
The values of the nanocrystallite size calculated for the un-
calcined samples exposed to the microwave irradiation are pre-
sented in Table 1. This calculation was based on the most in-
tense peak, located at around 2 θ = 11.6°. The series of samples
exposed to microwave irradiation at different temperatures ex-
hibited a higher crystallite size compared to the sample pre-
pared by the conventional method. This shows a better crystalli-
zation of the solid with the use of the microwave irradiation as
shown by XRD analysis (Figure 1a). However, for MW-40 sub-
jected to microwave irradiation at a temperature of 40 °C, a
showed a destruction of the hydrotalcite phase when the sam-
ples are exposed to microwave irradiation at 90 W for more
than 1 min.
It is to be noted that an increase in peak intensity is ob-
served for all solids exposed to microwave irradiation during
the ripening phase compared to the conventional Co Fe -HT.
4
2
The cell parameters have been calculated for all the samples.
The hydrotalcite structure has the 3R rhombohedral symmetry.
The cell parameter a (cation-cation average distance in the
layer) was calculated from the line at 2 θ = 59–60°, correspond-
ing to the Miller indices (1 1 0), as a = 2 d(1 1 0), and the parame-
ter c = 3 c′ (c′ represents the thickness of a layer and an inter-
layer) was calculated from the line at 2 θ = 11.6–11.9°, corre-
sponding to the Miller indices (0 0 3), as c = 3 d(0 0 3). All calcu-
lated parameters (a, c) are presented in Table 1.
decrease of the crystallite size to 12.5 nm is observed. Indeed,
[31]
Climent et al.
were able to highlight the effect of tempera-
ture on the synthesis of Mg-Al hydrotalcite materials. It has
been shown that with increasing the temperature of microwave
irradiation during the ripening phase an increase in crystallite
size was observed, that can be correlated with the increase in
crystallinity.
Table 1. Cell parameters (a and c), crystallite size, and SBET of the uncalcined
samples.
In addition, smaller crystallites are observed with the two
other series of samples exposed to microwave irradiation for
different powers and durations respectively. In addition, an im-
provement of crystallinity degree correlated with an increase in
the crystallite size is obtained with the increase of the power
and of the duration of the microwave treatment. Similar results
2
Sample
Co Fe -HT
a [Å]
3.11
c [Å]
D [nm]
17
S
BET (m g–1)
4
2
22.65
68
MW-170
MW-150
MW-80
MW-40
3.12
3.12
3.12
3.11
22.86
22.82
22.78
22.32
17.8
17.8
18.7
12.5
41
66
75
83
[
40]
were obtained by Herrero et al. while varying the microwave
duration time.
MW-200W
MW-100W
MW-80W
MW-40W
3.12
3.11
3.11
3.11
22.69
22.40
22.32
22.19
14
77
84
82
91
The surface areas of the dried samples treated under micro-
wave irradiation are higher than that of the sample Co Fe -HT
12.9
12.7
11.9
4
2
2
–1
(
68 m g ) synthesized via the conventional method (Table 1).
However, lower values are obtained for the two samples MW-
70 and MW-150, treated under microwave above 130 °C.
Hence, by decreasing the temperature of the treatment to
0 °C, an increase in the value of the specific surface area is
MW-60min
MW-30min
MW-15min
MW-1min
3.11
3.11
3.12
3.11
23.23
22.32
22.25
22.19
15
69
74
83
91
12.5
13.9
11.9
1
8
2
–1
observed and a maximum value of 83 m g is reached for
the solid MW-40 treated at a minimum heating temperature of
No significant difference is observed between the values of
a of the different samples regardless of the treatment tempera-
ture, power, and duration. This cell parameter is quite similar
for all the samples. In fact, this parameter is related to the metal
4
0 °C.
For the second series of samples treated at different irradia-
tion powers, the specific surface areas of the hydrotalcite mate-
rials vary according to the power applied. It can be noticed that
with the decrease of the power, the specific surface of the solids
increases. Thus, the solid MW-40W has the highest specific sur-
cations which are the same (Co2 and Fe ) in all the samples.
Regarding the c parameter, it gives an indication on the elec-
trostatic forces between the layer cations and the interlayer an-
ions. The more the c value decreases, the more the basal spac-
+
3+
2
–1
face area, 91 m g .
[
39]
ing is small, indicating stronger electrostatic interactions.
By
By decreasing the irradiation time from 60 min to 1 min, an
increase in the specific surface areas of the dried samples is
observed. As a result, the solid treated for one minute has the
observing the three series, a decrease in the c parameter value
is observed when the temperature, power, and duration of the
irradiation decreases. This result indicates that an increase in
electrostatic attractions is induced between the positive charge
located in the brucite layer and the negative charge located in
the interlayer domain. By applying minimum temperature,
power, and duration of 40 °C, 40 W, and 1 min respectively, a
maximum electrostatic interaction is observed between posi-
2
–1
highest specific surface area, 91 m g . However, a long irradia-
tion time of 60 min does not appear to have an effect on the
specific surface area. Indeed, microwave irradiation causes the
increase of the specific surfaces of the dried solids even at the
lowest microwave parameters.
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2
. Calcined Samples
Table 2. Crystallite size, SBET, and temperature of 50 % of propene conversion
(
T
50) of the catalysts.
Catalyst
Co Fe -HT500
The XRD patterns of the solids Co Fe -HT500 and MW500–40W
4
2
BET (m g–1)
2
T50 (°C)
Crystallite size [nm]
10.6
S
are presented in Figure 2. The diffractograms are very similar.
After the thermal treatment at 500 °C, the hydrotalcite structure
was completely destroyed for all the samples and transformed
into mixed oxides of Co O and CoFe O corresponding to the
divalent and trivalent cations present. It should be noted that
both oxides have almost the same 2 θ values and similar cell
parameters, and thus it is hard to distinguish them by XRD. No
other crystalline phase was detected in XRD. Furthermore, no
difference between the X-ray diffractograms of the calcined sol-
ids prepared was noted, regardless of the temperature, power,
and duration of the microwave irradiation (not shown).
4
2
60
244
MW-170
MW-150
MW-80
MW-40
8.6
8.5
10.3
9.7
33
42
80
89
261
238
221
212
3
4
2 4
MW-200W
MW-100W
MW-80W
MW-40W
9.2
8.7
8.2
7.6
69
67
74
80
227
221
218
214
MW-60min
MW-30min
MW-15min
MW-1min
10
9
8.6
7.6
65
68
75
80
224
220
213
214
the conventional sample. Indeed, the increased surface area can
provide greater availability of the active phase and therefore a
possible improvement in the catalytic activity.
In order to investigate the difference in morphology of the
nanocatalyst Co Fe O prepared with and without microwave
4
2 7
irradiation, SEM analysis was performed. From SEM images of
Co Fe -HT500 and MW500–40W presented in Figure 3a and Fig-
4
2
ure 3b respectively, it can be seen that the two samples showed
a nanostructure. However, a difference in morphology between
the two nanocatalysts is noticed. In fact, Co Fe -HT500 resulting
4
2
from the conventional method is composed of an aggregation
of big and non-uniform particles. However, the calcined sample
resulting from the microwave irradiation method has sharper
grains with smaller and uniform crystallite size. It is obvious that
microwave treatment leads to a decrease in crystallite size.
Figure 2. X-ray diffraction patterns of the calcined samples, Co
MW500–40W. C: Co (JCPDS No. 42–1467) or/and CoFe
086).
4
Fe
2
-HT500 and
3
O
4
2 4
O (JCPDS No. 22–
1
The crystallite size of the oxides was calculated and pre-
sented in Table 2. This calculation was based on the most in-
tense peak, located at around 2 θ = 37°. In all cases, the average
crystallite size of the oxides formed under microwave irradia-
tion is smaller than that of the oxide obtained by the conven-
tional method. It is then concluded that microwave irradiation
gives oxides with lower crystallinity degree than those obtained
by the conventional method. It is worth to mention that a de-
crease of the crystallite size is observed when the microwave
heating temperature, power, and duration decrease. Microwave
irradiation enhances the nucleation even at the lowest heating
temperature, power, and irradiation time, giving higher number
[
41]
of small crystallite sizes.
After calcination at 500 °C, the specific areas decrease for
solids treated at temperatures above 130 °C (Table 2). These
values are even lower than the specific surface area of the cata-
lyst Co Fe -HT500. On the other hand, an increase of the spe-
4
2
cific surface areas after calcination is obtained for the two solids
treated at 80 and 40 °C.
Furthermore, calcination at 500 °C results in a decrease of
the specific surface area for all the samples treated at different
powers and durations. Despite this decrease, the oxides formed
have higher specific surface areas than the oxide issued from
Figure 3. SEM images (a) Co
mapping of Co, Fe, O in the Co
irradiation (MW500–40W).
4
Fe
2
-HT500, (b) MW500–40W and (c–f) elements
Fe nanocatalyst treated under microwave
4
2 7
O
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From these images it can be assumed a better porosity for
MW500–40W compared to Co Fe -HT500. Figure 3c to Figure 3e
4
2
represent the elements mapping of the nanocatalyst resulting
from the microwave irradiation method. Each element is uni-
formly distributed. The mapping shown in the Figure 3f
presents the obtained Co Fe O nanocatalyst treated under mi-
4
2 7
crowave irradiation and composed of Co, Fe, and O uniformly
distributed on the surface.
Texture characteristics for the two samples (Co Fe -HT500
4
2
and MW500–40W) were also determined by porosimetry. The
isotherms obtained are of type IVa (according to IUPAC), charac-
teristic of mesoporous materials (results not shown). The pore
size of Co Fe -HT500 is around 19 nm. This value increased to
4
2
2
0
6 nm for MW500–40W. Moreover, the porous volume is around
.30 and 0.26 for Co Fe -HT500 and MW500–40W respectively.
4
2
Therefore, the irradiation with MW increases the porosity ac-
cording to the modification observed by microscopy.
The aim of the H -TPR technique is to examine the reducibil-
2
ity of the species present (cobalt and iron oxides) and to study
the effect of microwave irradiation on the reducibility of the
Figure 4. Temperature-programmed reduction profiles of the Co
and MW500-T samples.
4 2
Fe -HT500
catalysts. The H -TPR profiles of the calcined samples treated
2
under microwave irradiation at different heating temperatures
are displayed in Figure 4. The maximum reduction temperatures
and the consumption of hydrogen for all samples are reported
in Table 3.
Hence, the second peak could be also attributed to the reduc-
tion of CoFe O species, also detected by XRD, according to the
following equations:
2
4
All the profiles of the three series show two peaks, which are
attributed to the two-step reduction of cobalt species, more CoFe
O
+ H
→ Co + Fe O + H O
2 3 2
(3)
(4)
(5)
(6)
2
4
2
specifically of the oxide Co O . Indeed, XRD evidenced the pres-
3
4
3
Fe O + H → 2Fe O + H O
2
[
42–44]
3
2
3
4
2
ence of this oxide. According to the literature,
the first
peak between 300 °C and 320 °C is associated with the reduc- Fe3O4 + H → 3FeO + H O
2
2
tion of Co O species into CoO, and the second reduction peak
at higher temperature (> 400 °C) with the reduction of CoO into
Co (metallic cobalt), according to the following two equations:
3
4
FeO + H → Fe + H O
2
2
For the first series of samples treated under microwave at
different heating temperature shown in Figure 4, the first reduc-
tion peak is similar in shape to that of Co Fe -HT500. It is worth
to mention that the second reduction peak decreases in inten-
sity and spreads over a larger temperature range when the mi-
Co O + H → 3CoO + H O
(1)
(2)
3
4
2
2
4
2
CoO + H → Co + H O
2
2
However, other studies have shown that the reduction of pure crowave irradiation temperature increases to 150 and 170 °C.
[
45]
Co O species shows one reduction peak.
Conversely, for the sample exposed at the lowest temperature
3
4
Table 3. Maximum reduction temperature and hydrogen consumption of each sample.
Catalyst Maximum reduction temperature (°C)
Theoretical H
Experimental H
2
consumption = 18139 μmol H
2
/gcata
st
nd
1
peak
2
peak
2
consumption (μmol H /gcata)
2
Co
4
Fe
2
-HT500
291
409
16622
MW500–170
MW500–150
MW500–80
MW500–40
301
289
286
262
498
477
409
389
8931.7
8750.3
15869
18687
MW500–200W
MW500–100W
MW500–80W
MW500–40W
310
309
300
280
450
447
438
401
17932
17950
18144
18329
MW500–60min
MW500–30min
MW500–15min
MW500–1min
298
295
286
280
445
436
410
401
17933
18295
18310
18329
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MW500–40, both peaks are shifted to lower temperatures. The
first peak is shifted significantly from 291 to 262 °C with a de-
crease in the intensity. The second one is shifted from 409 to
3
89 °C (Table 3). These peaks shifts due to microwave irradia-
[
32,46]
tion are in agreement with previous studies,
where also a
comparison between conventional and microwave synthesis
has shown some shifts in the reduction peaks. As for the second
and the third series, it can be noticed that when the power or
the duration of microwave irradiation is decreased, the reduc-
tion temperatures are shifted to lower values. In addition,
MW500–200W, MW500–100W, and MW500–80W are less reduci-
ble than the convention sample Co Fe -HT500, whereas,
4
2
MW500–40W is more reducible than Co Fe -HT500.
4
2
For the third series, MW500–60min and MW500–30min are
less reducible compared to Co Fe -HT500. However, MW500–
4
2
15min and MW500–1min have similar or higher reducibility
than Co Fe -HT500.
4
2
Moreover, a difference in the total hydrogen consumption
is also observed between catalysts treated under microwave
irradiation and the conventional sample.
For the two catalysts MW500–170 and MW500–150, a de-
crease in the H consumption is detected that indicates a lower
2
quantity or a harder reducibility of reducible species. However,
an increase in hydrogen consumption with the decrease of the
temperature to 80 °C is observed.
Furthermore, all other microwave irradiated samples present
higher H consumption than the solid prepared by the conven-
2
tional method, except for MW500–170 and MW500–150 sam-
ples. In addition, all microwave samples present experimental
H2 consumption values very close to the theoretical value. The
more open structure of these microwave irradiated samples
could explain the higher accessibility (and quantity) of the re-
ducible species.
In fact, the microwave treatment during the aqueous phase
preparation of the solid induces local heating points causing
agitation of the water molecules in interaction with the solid.
This agitation will cause morphological and structural modifica-
tions in particular the decrease of crystallinity and the increase
of the porosity and the specific surface. The more open struc-
ture observed of the microwave irradiated sample could then
explain the higher accessibility (and then quantity) of the re-
ducible species which are important for the redox reactions.
3
. Catalytic Tests and XPS Analyses
The catalytic activity of the prepared mixed oxides treated un-
der microwave irradiation was evaluated towards the total oxid-
ation of propene. The propene conversion vs. reaction tempera-
ture (light-off curve) was plotted for each catalyst (Figure 5). In
all cases, the products formed were exclusively CO and H O.
2
2
First of all, the effect of the temperature of microwave irradi-
ation was evaluated (Figure 5a). The light-off curves show a
significant improvement in the performance of the catalysts
treated under microwaves at 40 and 80 °C. By comparing the
T , a difference of 32 °C is obtained between the two catalysts
Figure 5. Propene conversion vs. reaction temperature for the samples treated
under microwave irradiation at different (a) temperatures, (b) powers, and (c)
duration times.
5
0
MW500–40 and Co Fe -HT500. From the T (°C) values listed in > MW500–80 > MW500–150 > Co Fe -HT500 > MW500–170.
4
2
50
4
2
Table 2, the order of catalytic activity is as follows: MW500–40 Moreover, according to this classification, a correlation between
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the temperature of the microwave heating and the catalytic bility of cobalt and iron oxides may be one of the reasons for
activity can be established. In fact, when the temperature of the high activity of the Co Fe catalyst. Hence, the reduction of
4
2
the microwave irradiation is equal to 170 °C, the catalytic activ- active cobalt species was enhanced by the presence of iron
ity towards the total oxidation of propene is the lowest. How- that has accelerated the reduction and therefore increased the
ever, when the temperature becomes lower than 130 °C, a sig- catalytic performance.
nificant improvement in catalytic activity is noted.
In order to understand the nature of the active species
In a second time, in order to study the microwave power present at the surface resulting from the different preparation
effect, the duration of irradiation was maintained at 1 min, and methods, XPS analyses were carried out on Co Fe -HT500 and
4
2
the power of microwave irradiation was varied between 40 and MW500–40W catalysts.
00 W. As it can be observed in Figure 5b, by increasing the
The oxygen, the cobalt, and the iron species have been stud-
2
power of the microwave irradiation, the light-off curves are ied with the photopeaks of O 1s, Co 2p, and Fe 2p respectively.
shifted towards higher temperature. As a result, the sample The binding energies of the different oxygen, cobalt, and iron
MW500–200W has the lowest catalytic activity among this se- species present in the both catalysts Co Fe -HT500 and
4
2
3
+
2+
ries of catalysts. It is then concluded that a minimum power of MW500–40W, as well as O /O and Co /Co ratios are reported
II
I
40 W (associated with a lower temperature of 40 °C) is sufficient in Table 4. Three components were observed for the photopeak
to significantly improve the catalytic oxidation of propene. Con- O 1s (Figure 6 a). The first component O corresponds to oxygen
I
2
–
[47]
sequently, we used this power to study the effect of microwave species (O ) in the network.
irradiation time.
the lowest binding energy at 529 eV. The second component
For the third series presented in Figure 5c, all samples sub- OII has a binding energy of 531 eV. This component corre-
This component is located at
[
48]
2
–
–
[49]
jected to microwave irradiation at 40 W for different durations sponds to surface oxygen (O /O ) or hydroxyl groups.
Fi-
from 1 to 60 min show better catalytic activity compared to nally, the third component OIII corresponds to the oxygen spe-
the catalyst from the conventional method (Co Fe -HT500). By cies present in carbonate and water molecules. This peak is at
4
2
comparing the effect of microwave irradiation time on catalytic a higher binding energy, 532.5 eV. Often the good activity of a
performance, it is inferred that as the time increases from 1 min catalytic oxidation is correlated with a high mobility of surface
to 60 min, the light-off curve is slightly shifted towards higher oxygen. In fact, it can be noticed that the ratio O /O is higher
II
I
temperature. However, no significant difference is obtained be- for the MW500–40W catalyst treated under microwave com-
tween the light-off curves corresponding to irradiation times of pared to the catalyst from the conventional method (Co Fe -
4
2
1
1
and 15 min. The two catalysts MW500–1min and MW500– HT500). This result can explain the higher catalytic activity ob-
5min show similar catalytic activities which are the highest tained for MW500–40W. In fact, greater mobility of surface oxy-
ones towards propene oxidation.
gen obtained for the MW500–40W prepared by microwave irra-
It is to be noted that, in each of the three studied series, the diation provides higher availability of oxygen and therefore im-
most active catalyst presents the highest surface area, the low- proves the catalytic performance.
est crystallite size, and the highest value of hydrogen consump-
The Co 2p XPS spectra of the both catalysts Co Fe -HT500
4 2
tion that indicates a highest amount of the species that are and MW500–40W showed the presence of two components
likely to be reduced. All these characteristics could be in the due to the spin–orbit coupling of the Co 2p and Co 2p1/2
3
/2
[
50]
origin of the improvement of the catalytic activity.
states with satellite peaks indicating that cobalt is present in
In fact, the redox mechanism is adopted for the oxidation mixed oxidation states.[50,51] The signal Co 2p3/2 consists of a
of hydrocarbons in the presence of transition metal catalysts, main photoemission peak located at a binding energy of 780 eV
2
+
3+
suggesting a Mars Van-Krevelen mechanism. As a result, the for Co and 779 eV for Co (Figure 6b) and two satellite peaks
search for an easily reducible oxide is always envisaged in order appearing at about 784.9 and 789.4 eV and corresponding to
3
+
[52,53]
to improve the catalytic activity of VOC oxidation and thus to the Co ions.
promote the oxidation of these pollutants. Hence, microwave located at 796 eV for Co and 794 eV for Co and a satellite
The Co 2p signal has three components
1/2
2
+
3+
irradiation leads to a greater amount of species that are likely peak corresponding to Co ions at about 803 eV. The Co3+/
2
+
2
+
3+
to be reduced, leading to an enhancement in the catalytic per- Co ratio was calculated using the relative percentage of Co
2
+
formances compared to the conventional sample.
and Co ions of the Co 2p signal. From Table 4, Co Fe -
3/2 4 2
[
37]
3+
2+
In addition, it has been shown in our previous work,
the HT500 has higher Co /Co ratio than MW500–40W. The pro-
important role of iron species. In fact, the simultaneous reduci- portion of Co ions on the surface is greater for MW500–40W
2
+
Table 4. Binding energy (BE) for O 1s, Co 2p levels, Fe 2p levels, OII/O
, and Co3+/Co2+ ratios for the Co
Fe -HT500 and MW500–40W catalysts.
I 4 2
Catalyst BE (O 1s) [eV] Surface content BE (Co 2p1/2) [eV] BE (Co 2p3/2) [eV]
Surface content
Co /Co
BE (Fe 2p) [eV]
2p3/2
Ref.
3+
Co2+
3+
2+
3+
2+
O
I
O
II
O
III
O
II/O
I
Co
Co
Co
2p1/2
Co
4
Fe
2
-HT500
530
531.5 533
0.32
0.63
794.6 796.1
794.7 796.1
779.7
779.4
780.7
780.6
1.10
0.83
711
710.8
710.5
724
724
723.7
MW500–40W
Literature
529.8 531.7 533
530.2 531.2 532.4
[
56]
57]
58]
59]
60]
[
795
796.9
779.6
781.6
[
709–710
724
[
710.3–710.6 723.7–724.1
[
710
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The iron species has been studied with photopeak Fe 2p. The
spectra of samples, shown in Figure 6c, showed the presence
of two components Fe 2p3/2 and Fe 2p1/2. The Fe 2p3/2 signal
consists of a photoemission peak at a binding energy at 710 eV
while the Fe 2p signal is at a higher binding energy at about
1
/2
[
54,55]
7
24 eV.
Conclusion
Layered double hydroxides containing cobalt and iron were
successfully synthesized using the microwave assisted method.
Furthermore, the effect of microwave heating temperature,
power, and duration has been studied. Treating the solids with
microwaves leads to an enhancement of the catalytic activity
towards propene total oxidation. The lowest temperature,
power, and irradiation time used (40 °C, 40 W, and 1 min) were
enough to achieve such an improvement due to an increase in
the specific surface area, pore size, and amount of reducible
species as well as due to a decrease in the nanosize of the
crystallites. Moreover, the XPS analyses showed a higher pro-
2
+
portion of Co ions and greater mobility of surface oxygen,
which could explain the higher activity of the MW500–40W cat-
alyst compared to Co Fe -HT500 synthesized using the conven-
4
2
tional method. Moreover, the more open structure of the micro-
wave irradiated sample (with higher pore size) could explain
the higher accessibility (and quantity) of the reducible species
important for the redox reactions and then for a high catalytic
activity.
Experimental Section
1. Synthesis Using Microwave Irradiation
A series of Co/Fe hydrotalcite-like samples was prepared by the
traditional co-precipitation method with a molar ratio M2 /M = 2.
+
3+
A nitrate aqueous solution containing the appropriate amount of
2
+
the divalent cation Co , Co(NO ) ·3 H O (CHEM-LAB, 97 %), and the
3
2
2
trivalent cation Fe3 , Fe(NO ) ·9H O (CHEM-LAB, 98 %), was added
+
3
3
2
dropwise at room temperature, into a stirred sodium carbonate
Na CO (1 M, ACROS, 99.5 %) solution. The pH was maintained fixed
2
3
at 10 using sodium hydroxide NaOH (2 M, PANREAC, 98 %) solution.
However, after complete precipitation of the mixture, a microwave
treatment was carried out in a Biotage cavity microwave oven with
a stirring mode. Then, all samples were filtered, washed with hot
deionized water until neutralization, and dried in an oven at the
same temperature for 48 h. The as-obtained precipitate was ground
–
1
–1
and calcined under an air flow (2 L h ,1 °C min ) at 500 °C for 4 h.
Three series of samples were prepared. The microwave irradiation
parameters in each series were used according to the Biotage cavity
microwave oven conditions.
The first series of samples were treated at different heating temper-
atures of 170, 150, 80, and 40 °C for various duration times of 44 sec,
3
min, 30 min, and 30 min respectively. The temperature range in
Figure 6. XPS spectra of (a) O 1s, (b) Co 2p and (c) Fe 2p for Co
and MW500–40W.
4 2
Fe -HT500
the microwave oven is between 40 °C and 250 °C. It is important
to note that a maximum temperature of 170 °C can be used in our
work. Indeed, higher temperatures could lead to pressures above
10 Pa corresponding to the limit pressure in the vials. The duration
microwave method compared to Co Fe -HT500. Thereby, the
4
2
large proportion of Co2 ions can be related to the better cata- times of 44 sec and 3 min for 170 °C and 150 °C were then chosen
+
lytic activity obtained for MW500–40W.
according to the pressure limits. The solids were named MW-(T) for
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the uncalcined and MW500-(T) for the calcined samples, where T
represents the microwave irradiation temperature.
X-ray photoelectron spectroscopy (XPS) analyses were conducted
on a Kratos Axis Ultra spectrometer with a monochromatic Al Kα
(
hν = 1486.6 eV) radiation source. The power of the source was
In order to compare the effect of microwave irradiation power, the
second series of samples was treated at a fixed optimum duration
of 1 min and the solutions were heated at the minimum tempera-
ture of 40 °C with different microwave powers. The solids were
named MW-(P)W for the uncalcined and MW500-(P)W for the cal-
cined samples (oxides), where P represents the microwave irradia-
tion power in Watt.
maintained at 150 W. The binding energy (BE) was calibrated based
on the line position of C 1s (285 eV). The simulations and the quan-
tifications of the data were carried out on the CasaXPS software.
3. Catalytic Test: The catalytic oxidation of propene was realized at
atmospheric pressure. 100 mg of catalyst were loaded into a fixed
bed U-shaped glass reactor. Before each test, a pretreatment of the
catalyst was carried out, in flowing air at 150 °C for 30 min. A total
flow rate of 100 mL min containing 6000 ppm of propene in air
balance was used during the oxidation test. The mixture of gases
The third series of samples was treated at a fixed temperature and
power (40 °C and 40 W) for various duration times. The duration
range in the microwave oven is between 60 min and 1 min. Samples
were denoted MW-(t)min and MW500-(t)min for the dried and cal-
cined samples respectively. The t represents the microwave irradia-
tion time in minute.
–1
(
propene + air) was passed through the reactor while the tempera-
–1
ture was increased from 150 to 500 °C (1 °C min ). The reactants
and the oxidation products were then analyzed using a Varian CP-
4
900 microGC.
For comparison purposes, a sample was prepared by the conven-
tional route, without microwave irradiation. The as-prepared sample
will be denoted by Co Fe -HT before calcination and by Co Fe -
Acknowledgments
4
2
4
2
HT500 after calcination.
The University of Littoral – Côte d'Opale (ULCO) and the
Lebanese University are gratefully acknowledged for financial
support. The authors would like to thank the Agence Universi-
taire de la Francophonie – Région du Moyen-Orient (AUF) and
the ULCO for research fellowship. The Interreg V project “Depo-
llutAir” (France-Wallonie-Vlaanderen) funded by the European
Union is also gratefully acknowledged for financial support.
2
. Characterization Techniques
Structural analysis of the different catalysts was carried out at room
temperature by X-ray diffraction (XRD) technique, using a Bruker D8
advance diffractometer, with a copper anode CuKα radiation (λ =
1
.5406 Å). An angular range of 4° < 2 θ < 80° was used for the
uncalcined samples, whereas a range of 20° < 2 θ < 80° was used
for the calcined samples, with a resolution of 0.02° and a count
time of 2 s per step. Identification of the crystalline phases was Keywords: Bimetallic nanocatalysts · Cobalt · Iron ·
made with the Joint Committee on Powder Diffraction Standards
JCPDS) files, using EVA software.
Nanoparticles · Hydrotalcite
(
In order to get more information on the effect of the microwave
irradiation treatment on the crystallinity of each sample, the crystal-
lite size was determined using the Debye–Scherrer equation:
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Received: December 15, 2018
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Microwave Synthesis of Oxides
C. A. Serhal, I. Mallard, C. Poupin,
M. Labaki, S. Siffert,*
R. Cousin* .......................................... 1–11
Effect of Microwave Irradiation Pa-
rameters on Co/Fe Hydrotalcite Na-
nocatalysts for the Total Oxidation
of VOCs
Co/Fe hydrotalcite-like samples have areas, sharper grains with smaller, uni-
been prepared by use of microwave form crystallite size, better reducibility
(
MW) during the synthesis. Optimiza- of the oxide species, and they lead to
tion of the temperature, duration, and high catalytic activity in the total oxid-
power of the MW was carried out. The ation of propene.
samples have higher specific surface
DOI: 10.1002/ejic.201801528
Eur. J. Inorg. Chem. 0000, 0–0
www.eurjic.org
11
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