Promotion Effect of Ga?Co Spinel Derived from Layered Double Hydroxides for Toluene Oxidation

Article abstract of DOI:10.1002/cctc.201800764

Considering the radius similarity of Al³⁺ (0.50 ?) and Ga³⁺ (0.62 ?), Al?Co and Ga?Co layered double hydroxides (LDH) was prepared as precursor to improve the synergetic interaction of the transition metals. The Al?Co and Ga?Co spinel catalysts were obtained by the following calcination for the toluene oxidation. The Ga?Co (E_a=79.5 kJ mol^?1) showed higher activity than the Al?Co (E_a=95.4 kJ mol^?1), and it exhibited excellent stability and resistance to carbon deposition during a 50 h reaction period at 300 °C. Characterization results indicated that the improved activity of Ga?Co spinel could be attributed to the higher reducibility (H₂-TPR), more surface Co³⁺ and adsorbed oxygen in quantity (XPS) and adsorbed toluene (Toluene-TPD) than Al?Co spinel. Compared with traditional Ga?Co binary catalyst, the spinel structure possessed more B-sites cations outermost of the surface and surface oxygen vacancies for the toluene adsorption and ring-open reaction.

Full text of DOI:10.1002/cctc.201800764

Accepted Article
Title: Promotion effect of Ga-Co spinel derived from layered double
hydroxides for toluene oxidation
Authors: Qilei Yang, Dong Wang, Chizhong Wang, Kezhi Li, Yue Peng,
and Junhua Li
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the VoR from the journal website shown below when it is published
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To be cited as: ChemCatChem 10.1002/cctc.201800764
Link to VoR: http://dx.doi.org/10.1002/cctc.201800764
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Promotion effect of Ga-Co spinel derived from layered double
hydroxides for toluene oxidation
Qilei Yang, Dong Wang, Chizhong Wang, Kezhi Li, Yue Peng*, and Junhua Li [a]
Considering the radius similarity of Al3+ (0.50 Å) and Ga3+ (0.62
Å), Al-Co and Ga-Co layered double hydroxides (LDH) was
prepared as precursor to improve the synergetic interaction of
the transition metals. The Al-Co and Ga-Co spinel catalysts
were obtained by the following calcination for the toluene
oxidation. The Ga-Co (Ea = 79.5 kJ mol−1) showed higher activity
than the Al-Co (Ea = 95.4 kJ mol−1), and it exhibited excellent
stability and resistance to carbon deposition during a 50 h
reaction period at 300 ℃. Characterization results indicated that
the improved activity of Ga-Co spinel could be attributed to the
higher reducibility (H2-TPR), more surface Co3+ and adsorbed
oxygen in quantity (XPS) and adsorbed toluene (Toluene-TPD)
than Al-Co spinel. Compared with traditional Ga-Co binary
catalyst, the spinel structure possessed more B-sites cations
outermost of the surface and surface oxygen vacancies for the
toluene adsorption and ring-open reaction.
defined by M3+/(M2+ + M3+) ratio.[8] LDH precursors can generate
variety of spinel oxides through controlled thermal
a
decomposition. Transformation of the LDH precursors into spinel
oxides is a topotactic process because the layers comprise
ordered and uniformly distributed metal cations.[9] The obtained
spinel oxides usually possess a higher surface area and metal
cations with a better dispersion capacity and good thermal
stability.[10] In this work, The BET surface areas are summarized
in Table 1, the specific surface area of Al-Co spinel and Ga-Co
spinel are 120.9 and 84.5 m2 g-1, respectively.
In Al-Co LDH, Co is the active element and Al is typically an
inert component. It would be interesting to study the effects of
replacing the inert component Al3+ with a trivalent metal (M3+),
which has catalytic activity. Considering the limitations of LDH
formations, such as those related to the valence of metal cations
and the sizes of the M2+ and M3+ radii, the candidate should
maintain a valence state that is trivalent and has a cationic
radius that is similar to Al3+ (0.50 Å). Thus, a good candidate is
suggested to be Ga in Group 13 in the periodic table.[11] A Ga-
based catalyst was also found to exhibit better catalytic
performance with respect to the destruction of volatile aromatic
pollutants in photocatalysts.[12] Alternatively, Ga-Co spinel is a
highly efficient electrochemical catalyst because of the existence
of oxygen vacancies.[13] Recently, the mixed spinel oxides that
containing Co, Ni and Ga, have different Co/Ni ratios, and
implement Ga as the trivalent metal cations derived from LDH,
demonstrate higher capacitance during the oxygen evolution
reaction.[14] In addition, the structures and properties of the
obtained spinel oxides can be modified by altering the
composition of the LDH precursors.[10] Therefore, combining Co
and Ga may be a promising strategy to improve the VOCs
oxidation.
To the best of our knowledge, there are very few reports on
the Co-Ga based catalysts for VOCs oxidation. Here, we report
on the synthesis of a Co-Ga spinel that uses LDH as a catalyst
precursor, and the catalytic oxidation of toluene in terms of
catalyst performance.
XRD patterns of the as-prepared precursors of Ga-Co LDH
and Al-Co LDH are presented in Figure S1(Supporting
Information). The corresponding patterns of the LDH with the
characteristic reflections at 2θ values of 11.8°, 23.7° and 34.6°
corresponding to (003), (006) and (012) planes of crystalline Al-
Co LDH, respectively. Furthermore, no detectable impurity
phases can be observed. Compared with the peaks of Al-Co
LDH, a slight peak down shift was observed for Ga-Co LDH. It
can be due to the radius difference between Ga3+ (0.62 Å) and
Al3+ (0.50 Å), resulting in a perturbation within the hydrotalcite
lattice.[15]
Volatile organic compounds (VOCs), derived from the prevailing
energy and transport technologies, became a serious problem
due to its short- and long-term health risks.[1] Catalytic oxidation
is one of the most effective technologies for the VOCs removal
at low temperature.[2] The reported catalysts for the VOCs
oxidation can be classified into two categories.[3] (1) Supported
noble metal catalysts usually exhibit high catalytic activity.
However, the disadvantages of these catalysts, such as limited
resources, easy poisoning and sintering, hinder their widely
industrial applications.[4] (2) Transition metal oxides usually
possessed high thermal stability, good resistant to poisoning and
low cost.[5] Co-based catalyst exhibited good catalytic activity for
toluene oxidation due to its multivalent nature and
nonstoichiometric composition. These properties favor the
adsorption and activation of VOCs and O2, even though it shows
less effective as compared to the noble metal.[6] Our previous
work reported the Al-Co spinel catalysts, derived from layered
double hydroxides (LDH), possessed excellent activity and
thermally stable for the VOCs oxidation.[7]
LDH, a large family of two-dimensional materials, can serve
as a precursor and improve the dispersion of active components
on catalyst support. The general formula is [M1-
2+Mx3+(OH)2]x+[Ax/n ]n−·mH2O, where M2+ and M3+ are metal
x
cations, respectively. An− is a counter anion and x = 0.17-0.33
[a]
Q. Yang, Dr. D. Wang, Dr. C. Wang, K. Li, Prof. Y. Peng, Prof. J. Li
State Key Joint Laboratory of Environment Simulation and Pollution
Control
School of Environment
Tsinghua University
Beijing 100084 (P.R. China)
Figure 1 illustrates the XRD patterns of Al-Co spinel, Ga-Co
spinel and Ga-Co sol-gel calcined at 500 ºC. For Al-Co spinel, all
the peaks for Al-Co spinel between the standard peaks of
E-mail: pengyue83@tsinghua.edu.cn
Supporting information for this article is given via a link at the end of
the document.
AlCo2O4,
Al2CoO4
and
Co3O4,
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10.1002/cctc.201800764
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Ga-Co sol-gel
Ga-Co spinel
Al-Co spinel
Figure 2. H2-TPR spectra (a), O2-TPD spectra (b) and Toluene-TPD
spectra (c) of Ga-Co spinel and Al-Co spine.
20
30
40
50
60
70
80
2Theta (o)
because it had a larger surface area and increased spinel
geometry, which thus significantly affected the monomer yield
via increased catalytic activity.[16]
Figure 1. XRD spectra Al-Co spinel, Ga-Co spinel and Ga-Co sol-
gel.
The redox ability of the as-obtained Ga-Co spinel and Al-Co
spinel were studied using the H2-TPR and O2-TPD techniques,
as presented in Figure 2a-b, respectively. There were two
peaks in the H2-TPR profile of each sample. The reduction
peaks at 412 °C and 682 °C are attributed to the reduction of
Co3+ to Co2+ and then Co2+ to metallic cobalt, respectively.[17]
Comparing the peaks of Al-Co spinel revealed that all peaks of
the Ga-Co spinel tended toward lower temperatures, indicating
that the Ga in the Ga-Co spinel can promote the reduction of
Co3+. The similar results were also reported by He et al.[18] who
studied the CoGa-ZnAl-LDO/γ-Al2O3 catalyst for higher alcohol
synthesis from syngas. Thus, the Ga-Co spinel exhibited better
low-temperature reducibility than the Al-Co spinel. The O2-TPD
results can further confirm this viewpoint, as it can be seen that
the O2 desorption temperature of the Ga-Co spinel (405 °C ) is
lower than that of the Al-Co spinel (435 °C ); this suggests that
Ga promotes low-temperature oxygen species adsorption in the
Ga-Co spinel. Note that better low-temperature reducibility is
beneficial to improving toluene oxidation. Quantitative analysis
of the O2-TPD profiles and the oxygen amount were respectively
performed and calculated by using the normalization method
described in Table 1. It can be seen that the oxygen amount per
unit area of Ga-Co spinel (7.5 μmol m-2) is higher than that of Al-
Co spinel (5.7 μmol m-2).
these peaks can be indexed to the spinel oxide phases (Co2AlO4,
JCPDS 38-0814; CoAl2O4, JCPDS 44-0160; Co3O4, JCPDS 74-
2120).[7b, 7c] All the peaks for Ga-Co spinel and Ga-Co sol-gel are
between the standard peaks of Ga2CoO4 and Co3O4, which were
attributed to the mixed spinel oxide phases (CoGa2O4, JCPDS
11-0698; Co3O4, JCPDS 74-2120).[14] But we cannot assign
these peaks to the specific phases of spinel, because several
spinel phases (AlCo2O4, Al2CoO4, CoGa2O4 and Co3O4) have
the relatively close XRD peak positions. Therefore, we preferred
to assign the XRD patterns to the combination or mixture of
these spinel species. The similar results were also reported in
previous works.[7b, 7c, 14] Meanwhile, the average crystallite sizes
of all samples were calculated by X-ray diffraction with the
Scherrer equation. The average crystallite sizes of Al-Co and
Ga-Co spinels are 7.8 and 8.4 nm, respectively; however, for the
Ga-Co sol-gel that does not use LDH as the catalyst precursor,
the size is 34.7 nm. Thus, it can be seen that implementing LDH
as the catalyst precursor is conducive to the formation of smaller
catalytic nanoparticles, which is essential to the improvement of
the catalytic activity and stability.
Compared with Figure S1, the peaks corresponding to LDH
disappeared, replaced by the peaks of mixed spinel oxide,
indicating that the LDH structure was completely transformed
into the mixed spinel oxide after calcination. The main reason is
that the gallium (aluminum) and cobalt ions are uniformly
distributed at the atomic level in the layers of LDH, which is
beneficial for forming the spinel structure.[9] The mixed metal
oxide spinel is our target catalysts, and it was found to exhibit
higher activity and thermal stability as compared to monophasic
metal oxides, this is
The toluene-TPD experiment was performed to further
investigate the difference of toluene adsorption capacity of Ga-
Co and Al-Co spinel, as presented in Figure 2c. Only one
toluene desorption peak at 150 °C was found for both samples.
Additionally, Ga-Co spinel exhibited a higher toluene adsorption
amount (0.043 μmol m-2) than Al-Co spinel (0.021 μmol m-2),
suggesting that the Ga-Co spinel adsorbs more toluene species
than the Al-Co spinel. As known, the first step of the toluene
Table 1. BET surface areas, surface element compositions, oxygen amount (O2-TPD), toluene adsorption amount (Toluene-TPD), toluene
conversion and apparent activation energy (Ea) of Ga-Co spinel and Al-Co spinel.
Surface element molar
ratio (XPS)
Toluene conversion (℃) and apparent
Oxygen
amount
BET surface
area (m2 g-1)
Toluene adsorption
activation energy (kJ mol-1)
Catalysts
amount (μmol m-2)
(μmol m-2)
Co3+/Co2+
0.90
Oads/Olatt
0.59
T50
305
275
T90
320
288
Ea
Al-Co spinel
Ga-Co spinel
120.9
84.5
5.7
7.5
0.021
0.043
95.4
79.5
1.47(1.45)a
0.73(0.68)a
a
the surface element molar ratio (XPS) of Ga-Co spinel after 50 h stability at 300 ℃.
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Figure 3. XPS spectra of Ga-Co spinel and Al-Co spinel.
oxidation is its adsorption on catalyst surface.[19] Thus, it can be
deduced that the Ga-Co spinel provides more adsorbed toluene
species for reaction, thereby increasing the reaction rate.
XPS is an effective surface analysis technique to study the
surface metal oxidation states and the adsorbed species over
the surface of a sample. Figure 3 shows the XPS spectra of Co
2p and O 1s for Ga-Co spinel and Al-Co spinel. The Co 2p3/2 and
Co 2p1/2 peaks were located at ∼780 and 795 eV. The Co 2p
spectra were decomposed into Co2+ and Co3+ signals with
additional satellites. The peaks at ∼780 and 795 eV were
assigned to Co2+, while the peaks at 782 and 797 eV were
assigned to Co3+.[20] The fitted O 1s spectra displayed two major
oxygen contributions centered at ∼530 and 532 eV. These
bands can be attributed to the surface lattice oxygen (Olatt
species and surface adsorbed oxygen (Oads) species.[21]
)
According to the quantitative analysis of the XPS spectra, the
surface Co3+/Co2+ and Oads/Olatt atomic ratios were calculated as
summarized in Table 1. The surface Co3+/Co2+ ratio of the Ga-
Co
spinel (1.47) is higher than that of the Al-Co spinel (0.90), and
the surface Oads/Olatt molar ratio of the Ga-Co spinel (0.73) is
higher than that of the Al-Co spinel (0.59); this proves that the
Ga-Co spinel is more abundant in surface Co3+ and Oads species
than the Al-Co spinel. According to the conclusions of Ye et
al.[6d] and Li et al.[7a], the samples with a higher proportion of
Co3+ and Oads species showed higher catalytic performance
because the adsorbed oxygen species could attack an organic
molecule and
Figure 4. Toluene conversion over Ga-Co spinel, Al-Co spinel and
Ga-Co sol-gel (a), long-term stability of Ga-Co spinel at 300 ℃ and
TG curve (insert) for Ga-Co spinel after 50 h stability (b). Reaction
conditions: 0.10 g catalyst, 1000 ppm toluene, 20% O2, balance N2,
total flow rate = 100 mL min-1, WHSV = 60000 mL g-1 h-1.
thus facilely degrade the carbon skeletons as toluene is fully
oxidized.[22]
than CO2 were detected in the catalytic system, which was
confirmed by a carbon balance of 99.5% in each run. T50 and T90
(corresponding to toluene conversion = 50 and 90%) are
summarized in Table 1.
The XPS spectra for Ga 3d (Ga-Co spinel) and Al 2p (Al-Co
spinel) are presented in Figure S2. The Ga 3d spectra were
resolved into two individual component peaks (19.6 eV and 22.4
eV); this phenomenon is attributed to the fact that Ga is in the
form of a Ga-Co spinel rather than the pure Ga2O3 (20.5 eV).[23]
In addition, the binding energy (BE) of Al 2p are shown to be
located at 73.8 eV for Al-Co spinel; this is in agreement with the
BE results of Al that have been observed in the spinel of Co-
Al.[24] These results are also consistent with the XRD results,
which show that no other monophasic impurities (Ga2O3 or Al2O3
phase) were formed in the Ga-Co or Al-Co spinel.
These results suggest that the catalytic activity increased in
the following order: Ga-Co spinel > Al-Co spinel > Ga-Co sol-gel.
It is important to note that Ga-Co spinel (T90 = 284 ℃) is much
more active than Ga-Co sol-gel (T90 = 326 ℃), meaning that
implementing LDH as the catalyst precursor facilitates toluene
conversion; this is primarily because the Ga-Co spinel
nanoparticles derived from Ga-Co LDH are smaller than the Ga-
Co catalysts employed in the citrate sol-gel method. It is well
known that a smaller catalyst nanoparticle size corresponds to a
higher number of active sites exposed on the catalyst surface,
which thus increase the catalytic activity. Additionally, Ga-Co
Toluene oxidation over Ga-Co spinel, Al-Co spinel and Ga-Co
sol-gel catalysts were measured in the range of 250360 °C.
Evaluation of the catalytic performance of all samples is
presented in Figure 4a. No carbon-containing products other
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spinel is also much more active than Al-Co spinel (T90 = 320 ℃)
in toluene oxidation.
testing. The superior low-temperature catalytic activity of the Ga-
Co spinel is associated with more surface Co3+ and Oads species.
Ga-Co spinel after 50 h at 300 ℃ was further investigated by the
XPS technology. Compared with the mole ratio of Co3+/Co2+
(1.47) and Oads/Olatt (0.73) of the fresh Ga-Co spinel, Co3+/Co2+
(1.45) and Oads/Olatt (0.68) for Ga-Co spinel after stability
decreases slightly. Meanwhile, there is nearly no change can be
obtained by comparing the XPS spectra (Co 2p, O 1s and Ga 3d)
for Ga-Co spinel and Ga-Co spinel after 50 h stability in Figure
S5. These results indicate that the Ga-Co spinel exhibited
excellent catalytic stability including excellent resistance to
sintering and carbon deposition.
As compared to the Al-Co spinel, the Ga substitution for Al in
Co-based spinel, the Al-Co spinel with three kinds spinel (Co3O4,
CoAl2O4 and Co2AlO4) transform to the Ga-Co spinel with two
kinds spinel (Co3O4 and CoGa2O4), which can promote the Co3+
species reduction, due to the Co2+ and Co3+ ions in spinel
phases can interact with neighboring groups through the Co-O
bonds polarized by Al3+ (with the formation of CoAl2O4 or
Co2AlO4 phases).[25] Meanwhile, CoGa2O4 in Ga-Co spinel
possessed abundant oxygen vacancies,[13] which further
providing adequate O species to take part in toluene oxidation.
Therefore, the better low-temperature reducibility, the easier
release of the adsorption oxygen, more abundant adsorbed
toluene species and more surface Co3+ and Oads species result
Typical references about the related cobalt based oxide
catalysts for toluene oxidation at comparable reaction condition
are shown in Table S1. Among the various cobalt metal oxide
in the superior low-temperature catalytic activity for Ga-Cospinel. catalysts (such as Co2AlO4,[25] Co3O4,[25] LaCoO3/SBA-15[28] and
To illustrate the intrinsic catalytic difference of Ga-Co spinel
and Al-Co spinel, the Ea value was calculated for a more
detailed comparison. The Arrhenius plots for toluene conversion
were shown in Figure S3. On the basis of the slope of the linear
fit of the Arrhenius scatter plots, the Ea for toluene conversion
over the Ga-Co spinel and Al-Co spinel were calculated and
summarized in Table 1. The Ea value of the Ga-Co spinel (79.5
kJ mol−1) was lower than that of the Al-Co spinel (95.4 kJ mol−1).
The surface oxygen species is most likely the cause of the
difference in Ea. It is universally acknowledged that the oxidation
of VOCs over transition metal oxide catalysts takes place as
according to the Mars-van Krevelen type redox cycle, and that
the nucleophilic attack of Olatt results in the occurrence of this
reaction.[26] Thus, the Olatt species play a vital role in the catalytic
reaction. As compared to the Al-Co spinel, the Ga-Co spinel
better facilitated O species release according to the O2-TPD,
which enhanced the Olatt mobility on the catalyst and resulted in
an improvement in the Mars-van Krevelen mechanism. The fact
that the Ga-Co spinel had the lowest Ea value confirmed that the
Ga-Co spinel exhibits the best catalytic activity for toluene
combustion. These results suggest that the Ga-Co spinel better
facilitated toluene oxidation than the Al-Co spinel.
CoMn0.5[29]), the Co-Ga spinel showed better toluene oxidation
performance. With further loading of noble metal, it can be
compared to the state-of-the-art catalysts (Pd/CoAlO[7c] and 1%
[29]
Pt/Al2O3
) for toluene oxidation. In this work, the Ga
substitution for Al in Co-based spinel promote the reducibility of
Co3+ and increase the ratio of surface Co3+ and Oads. This
strategy provides insight for the design of new catalysts for
VOCs oxidation.
In conclusion, the Ga-Co spinel exhibited better catalytic
performance than the Al-Co spinel. This was due to the
substitution of Al with Ga in the Al-Co spinel, resulting in better
low-temperature reducibility, higher surface oxygen percentage
and Co3+ concentration in the Ga-Co spinel, as well as better
adsorption/desorption capacities for the oxygen and toluene.
Furthermore, the Ga-Co spinel that employed LDH as the
catalyst precursor also demonstrated excellent stability and no
catalyst deactivation was observed during the 50 h on stream at
300 ℃.
Experimental Section
To probe the stability of Ga-Co spinel catalyst, an on-stream
reaction experiment was carried out over Ga-Co spinel for 50 h
at 300 ℃, as shown in Figure 4b. No decrease in the catalytic
activity was observed during the stability test. To better illustrate
the stability of Ga-Co spinel, it was characterized via TG, XRD
and XPS after stability test. The amount of carbon deposited on
the Ga-Co spinel after 50 h was analyzed via TG and the results
are illustrated in Figure 4b. The weight loss of the catalyst was
2.7 wt.%, and according to the O2-TPD results, this includes the
2.02 wt.% weight loss of the O species. There was little weight
reduction within the temperature range of 30-900 °C, indicating it
showed excellent resistance to carbon deposition. This excellent
resistance to carbon deposition may be partially attributed to the
high oxygen vacancies in the Ga-Co spinel, which activate the
oxygen species that consequently react with the deposited
carbon.[27] As is shown in Figure S4, the phase structure of the
Ga-Co spinel was well retained after stability test, and no extra
peaks were observed. The diffraction peaks intensities of the
Ga-Co spinel are shown to have slightly increased following
stability testing. The corresponding average crystallite size of
Ga-Co spinel increased from 8.4 nm to 9.3 nm following stability
The Ga-Co LDH and Al-Co LDH composite with Ga: Co and Al: Co molar
ratio of 1: 2 was prepared by co-precipitation method (see details in the
Supporting Information, SI). Before reaction, the prepared Ga-Co LDH
and Al-Co LDH were calcined at 500 ℃ for 3 h in muffle and the calcined
catalyst was marked as Ga-Co spinel and Al-Co spinel, respectively.
Meanwhile, in order to compare the Ga-Co spinel structure is formed
without the catalyst precursor (LDH), Ga-Co spinel was prepared
according to the traditional citrate sol-gel method to make a comparison
(SI). The resultant sample was labeled as Ga-Co sol-gel. Catalytic
evaluation and material characterization are provided in detail in the SI.
Acknowledgements
This work was supported by the National Key Research and
Development Program of China (2017YFC0211102, 2017YFC0211003
and 2016YFC0209203) and the National Science Foundation (21777081).
Conflict of interest
The authors declare no conflict of interest.
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10.1002/cctc.201800764
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7066.
Keywords: layered double hydroxides • oxidation • spinel oxides
• toluene • VOCs
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10.1002/cctc.201800764
ChemCatChem
COMMUNICATION
Entry for the Table of Contents
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COMMUNICATION
Qilei Yang, Dong Wang, Chizhong
Wang, Kezhi Li, Yue Peng*, and Junhua
Li
Page No.1 – Page No.5
Promotion effect of Ga-Co spinel
derived from layered double
hydroxides for toluene oxidation
Graphic Abstract
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