Catalytic oxidation of toluene over a porous Co3O4-supported ruthenium catalyst

Article abstract of DOI:10.1039/c5ra07072d

Porous Co₃O₄-MOF and Ru/Co₃O₄-MOF were prepared through a metal-organic frameworks (MOFs)-templated method, and the bulk materials resulted from precipitation method were prepared for comparison. All catalysts were tested for the catalytic oxidation of toluene in the temperature range of 150-300°C, and Ru/Co₃O₄-MOF showed apparently higher catalytic activity and CO₂ selectivity than other materials. The influence of water vapour on the catalytic oxidation of toluene was studied at 235°C and 245°C, and it could be found that the inhibition effect weakened with the increase of temperature. Additionally, the test of 12 h on-stream reaction was conducted to explore the stability of Ru/Co₃O₄-MOF.

Full text of DOI:10.1039/c5ra07072d

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Catalytic oxidation of toluene over a porous
Co3O4-supported ruthenium catalyst†
Cite this: RSC Adv., 2015, 5, 52066
a
a
Xiaolong Liu,a Jian Wang,ab Junlin Zeng,ac Xue Wang and Tingyu Zhu
*
Porous Co3O4-MOF and Ru/Co3O4-MOF were prepared through
a metal–organic frameworks
(MOFs)-templated method, and the bulk materials resulted from precipitation method were prepared for
comparison. All catalysts were tested for the catalytic oxidation of toluene in the temperature range of
150–300 ꢀC, and Ru/Co3O4-MOF showed apparently higher catalytic activity and CO2 selectivity than
other materials. The influence of water vapour on the catalytic oxidation of toluene was studied at
235 ꢀC and 245 ꢀC, and it could be found that the inhibition effect weakened with the increase of
temperature. Additionally, the test of 12 h on-stream reaction was conducted to explore the stability of
Ru/Co3O4-MOF.
Received 19th April 2015
Accepted 8th June 2015
DOI: 10.1039/c5ra07072d
www.rsc.org/advances
Ruthenium catalysts have received considerable importance
in CO oxidation,38,39 deacon process,40,41 and catalytic oxidation
of VOCs.42–44 Nevertheless, ruthenium-based materials catalyzed
1. Introduction
Volatile organic compounds (VOCs) emitted from industrial
processes and transportation activities are considered as great
contributors to air pollution and dangerous to human health.1–3
The most common approaches for VOCs removal include
thermal incineration, catalytic oxidation, biochemical methods,
adsorption strategies, and other techniques.4–9 Due to its
advantages of high efficiency, low temperature, and high
selectivity to harmless products, catalytic oxidation is regarded
as a promising pathway to reduce VOCs emissions to meet the
standards of air pollution control regulations.10,11 In recent
years, various catalysts have been used for the complete oxida-
tion of VOCs, such as noble metals,11–15 transition metal
oxides,16–20 and perovskite-type oxides.21–25 Supported platinum
and palladium materials are well established as efficient cata-
lysts for complete oxidation of VOCs,26–30 whereas their wide
applications are limited due to the high costs. Transition metal
oxides have been increasingly studied as the catalysts for
oxidation of VOCs in single or mixed metal oxides forma-
tions,31–33 usually showing lower catalytic activities than noble
metals. Recently, they have been used as supports or additives
for platinum, palladium, and gold-based catalysts in many
cases.34–37
VOCs oxidations are far less investigated compared to other
noble metals. In the reports on Ru promoted oxidation of VOCs,
ethyl acetate,45 propane,46–48 toluene,42,49 and chlorobenzene50–53
could be totally oxidized within the temperature range of
175–275 ꢀC. Noteworthy, Mitsui et al. compared the catalytic
performance of CeO2 supported noble metals in oxidation of
ethyl acetate, such as Ru, Pt, Pd, and Rh, respectively, and found
that Ru containing catalyst exhibited the highest catalytic
activity.54
Spinel cobalt oxide (Co3O4) has been well established as an
intriguing material in the complete oxidation of VOCs, and its
outstanding catalytic activity was mostly attributed to the
mobile oxygen within the Co3O4 structure.55–58 In the eld of
catalytic oxidation of VOCs, it is generally concluded that the
efficiency of the catalyst is greatly concerned with the oxygen
vacancy, redox ability, and some morphological factors such as
surface area and porous structure.59–62 Accordingly, various
nanocasting strategies were developed to modify the structures
of the catalysts, and Co3O4 with different nanostructures,
such as nanoowers, nanorods, and nanotubes, have been
prepared.58,63
In recent years, metal–organic frameworks (MOFs) have
emerged as promising sacricial templates for fabricating
hierarchically porous metal oxides through solid-state ther-
molysis method.64–67 As compared to other templates, MOFs
exhibit the advantages of high porosity, large surface area, long-
range ordering structure, and easily removable properties under
controlled calcinations conditions. Through MOFs-templated
method, various metal oxides have been prepared and
employed in different research elds, such as spindle-like
mesoporous a-Fe2O3 as lithium-ion battery anode materials64
aBeijing Engineering Research Center of Process Pollution Control, National
Engineering Laboratory for Hydrometallurgical Cleaner Production Technology,
Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190,
China. E-mail: tyzhu@ipe.ac.cn; Fax: +86-10-82544821; Tel: +86-10-82544821
bGraduate University of Chinese Academy of Sciences, Beijing 100049, China
cSchool of Chemistry and Chemical Engineering, Guizhou University, Guiyang 550003,
Guizhou, China
† Electronic supplementary information (ESI) available: The BET surface areas
and pore diameters of the catalysts (Table S1), XPS data (Table S2), and the
catalytic evaluation platform (Fig. S1). See DOI: 10.1039/c5ra07072d
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and MnxCo3ꢁxO4 nanocages as DeNOx catalysts.67 Most recently, were taken on a Tecnai G2 F20 S-TWIN eld emission trans-
Kuang et al. have prepared novel porous Co3O4 nanocubes, mission electron microscope operated at 200 kV. X-ray photo-
which exhibit high specic surface area of 120.9 m2 gꢁ1
showing great efficiency in gas sensing applications.68
,
electron spectroscopy (XPS) measurements were conducted on a
Thermo escalab 250Xi X-ray using Al Ka source.
To the best of our knowledge, porous Co3O4 supported Ru
catalyst for complete oxidation of VOCs has not been reported.
Therefore, it is necessary to investigate the preparation and
application of porous Co3O4 supported Ru catalysts in oxidation
of VOCs, such as toluene. Herein, porous Co3O4 was prepared
using MOF-templated strategy, and its supported Ru material
was obtained through impregnation method. Bulk Co3O4 and
its corresponding Ru catalyst were also prepared for compar-
ison. Tests were conducted to explore their catalytic activities in
complete oxidation of toluene. The catalysts were characterized
by BET, XRD, TEM, and XPS to study their physicochemical
properties, which correspond with their catalytic activities.
2.3. Catalytic evaluation
Catalytic activities of the catalysts were evaluated in a contin-
uous ow xed-bed quartz microreactor (i.d. ¼ 4 mm) from
ꢀ
100 to 300 C with 100 mg of catalyst (80–100 mesh). Toluene
was introduced into reactant ow by Ar through a saturator, and
its concentration in the total ow of the reactant mixture
(1000 ppm toluene + 20% O2 + Ar (balance)) was calibrated by
GC through a by-pass (Fig. S1†). The total ow ratio of the
reactant mixture was 100 mL minꢁ1 with the weight hourly
space velocity (WHSV) of 60 000 mL gꢁ1 hꢁ1. Before the test, the
sample was pretreated at 250 ꢀC in a 20% O2/Ar stream for 0.5 h.
Aer the pretreatment, the reactant gas containing toluene was
introduced to the reactor. Analysis of the reactants and products
were performed on-line with a gas chromatography (GC 2010
Plus, Shimadzu) equipped with a ame ionization detector
(FID) and the concentration of CO2 in the outlet gas was
detected by another FID with a methanizer (MTN) furnace for
converting CO2 to CH4.
2. Experimental section
2.1. Catalyst preparation
Co-MOF (ZIF-67) and porous Co3O4 were synthesized following
the procedure described in the literature with some modica-
tions.68 In a typical preparation, a 12 mL aqueous solution of
Co(NO3)2$6H2O (1.75 g, 6 mmol) was added to a 80 mL aqueous
solution of 2-methylimidazole (21.30 g, 260 mmol), and the
mixture was stirred for 6 h at room temperature. The resulting
purple precipitates were collected by centrifugation, and
washed with water for 3 times and alcohol for 3 times, respec-
3. Results and discussion
3.1. Catalyst characterization
From the XRD patterns of the as-prepared Co3O4 samples in
Fig. 1, it can be seen that these materials exhibited 31.4ꢀ, 36.9ꢀ,
38.2ꢀ, 44.8ꢀ, 59.4ꢀ, and 65.3ꢀ (2q) peaks, which are attributable
to standard Co3O4 sample (JCPDS PDF 43-1003). Any impurity
phases such as Co, CoO, Co2O3, and Co(OH)2 were not observed.
It should be noted that all diffraction peaks of the
MOFs-templated samples are apparently broaden in width and
weaken in intensity, indicating that the as-prepared materials
are composed of abundant nanosized crystallites. The XRD
patterns of Ru/Co3O4-MOF showed no obvious difference from
those of Co3O4-MOF, illustrating that Ru addition did not
signicantly change the formation of the nanostructure of
Co3O4-MOF.
The N2 adsorption–desorption isotherms and BJH pore size
distribution were performed to evaluate the specic surface
area and the porosity of all the catalysts. As shown in Fig. 2,
Co3O4-MOF and Ru/Co3O4-MOF presented typical type IV
adsorption isotherm with a H3-type hysteresis loop at a relative
pressure range of 0.8–1.0, conrming the presence of a meso-
porous structure. The specic surface area and pore size
distribution of Ru/Co3O4-MOF were similar to those of
ꢀ
tively. The obtained Co-MOF (ZIF-67) was dried at 80 C over-
night. The as-prepared Co-MOF (ZIF-67) precursor was loaded
in a ceramic crucible and then calcined in a muffle furnace at a
ramp of 5 ꢀC minꢁ1 from 30 ꢀC to 325 ꢀC and kept at this
temperature for 3 h, giving a black powder, which was denoted
as Co3O4-MOF.
Supported 1 wt% Ru/Co3O4-MOF catalyst was prepared by
impregnation method. The as-prepared Co3O4-MOF support
was impregnated with an aqueous solution containing an
ꢀ
appropriate amount of Ru(NO)(NO3)3, and then dried at 80 C
ꢀ
for 6 h and calcined at 325 C for 3 h.
Bulk Co3O4–B sample was prepared using precipitation
method with Co(NO3)2$6H2O and Na2CO3 as reagents.58 The
resulting precipitates were dried at 80 ꢀC for 6 h and calcined at
325 ꢀC for 3 h, affording bulk Co3O4–B. Impregnation of
Co3O4–B with an aqueous solution containing an approꢀpriate
amount of Ru(NO)(NO3)3 followed by calcination at 325 C for
3 h gave 1 wt% Ru/Co3O4–B.
2.2. Catalyst characterization
Physicochemical properties of the catalysts were characterized Co3O4-MOF (Fig. 2 and Table S1†). Obviously, MOFs-templated
by means of techniques. X-ray diffraction (XRD) patterns of the samples showed higher specic surface areas than the
samples were recorded on a powder diffractometer (Rigaku precipitation-resulted samples (Table S1†).
D/Max-RA) using Cu Ka radiation (40 kV and 120 mA). The
Fig. 3 showed the TEM images of Co3O4-MOF and Ru/Co3O4-
porous texture was characterized by N2 adsorption at 77 K in an MOF samples. Slightly distorted nanocube was observed,
automatic surface area and porosity analyzer (Autosorb iQ, retaining the initial morphology of the Co-MOFs precursors in
Quantachrome). Transmission electron microscopy (TEM) and some extent and conrming the successful preparation of
high resolution transmission electron microscopy (HR-TEM) expected materials. With the incorporation of Ru species, no
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Fig. 3 TEM and HRTEM images of the catalysts: (a and b) Co3O4-MOF,
(c and d) Ru/Co3O4-MOF.
Fig. 1 XRD patterns of the catalysts.
MOF is 2.0, higher than that of Ru/Co3O4–B (1.36). Ru/Co
surface atomic ratios of Ru/Co3O4-MOF and Ru/Co3O4–B were
0.038 and 0.013, revealing that Ru/Co3O4-MOF exhibits a higher
Ru dispersion (Table S2†).
3.2. Catalytic studies
The catalytic efficiencies of the catalysts for the oxidation of
toluene at a WHSV of 60 000 mL gꢁ1 hꢁ1 are shown in Fig. 5. It
can be easily seen that Co3O4-MOF showed better catalytic
activity than Co3O4–B, illustrating that the porous nano-
structure facilitates the complete oxidation of toluene. Among
the as-prepared catalysts, Ru/Co3O4-MOF gave the lowest T90 of
238 ꢀC, which is much lower than those of other catalysts, and
its catalytic activity was comparable to reported 1 wt%
Pd-loaded catalysts with the T90 temperature range of 212–
258 ꢀC in the oxidation of toluene.69,70 Noteworthy, Co3O4-MOF
Fig.
2 N2 adsorption–desorption isotherms and BJH pore size
distributions of the catalysts.
apparent change of the nanostructure was observed for
Ru/Co3O4-MOF. As shown in the high-resolution TEM images,
0.28 nm lattice spacing of Co3O4 (2 2 0) as well as 0.46 nm (1 1 1)
lattice spacing were observed. For Co3O4, the (1 1 1) planes were
considered to be inactive crystal planes since they contain only
Co2+, which makes no contributions in the catalytic oxidation.37
Ru/RuOx nanoparticles were not observed in our HRTEM
characterizations.
In order to analyze the chemical states of the Ru species, XPS
characterizations for the Ru-containing samples were con-
ducted, and the XPS spectra of Ru 3d were presented in Fig. 4. It
can be seen that the samples exhibited three Ru 3d5/2 peaks at
280.8, 281.9, and 282.8 eV, which could be assigned to Ru0,
Ru4+, and Ru6+, respectively.53 Apparently, the oxidic Ru species
dominate in both catalysts. The ratio of Ru4+/Ru6+ for Ru/Co3O4-
Fig. 4 XPS spectra of Ru 3d for Ru/Co3O4-MOF and Ru/Co3O4–B.
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ꢀ
ꢀ
showed a lower T90 (264 C) than that of Ru/Co3O4–B (286 C),
demonstrating that the porous support itself exhibits great
catalytic activity possibly due to its unique nanostructural
properties which are favorable for the adsorption and diffusion
of the toluene molecules.
CO2 selectivity represents an important performance of a
catalyst for catalytic oxidation of VOCs. Accordingly, herein CO2
selectivity was examined for both Ru/Co3O4-MOF and
Co3O4-MOF materials, and the results were illustrated in Fig. 6.
Using Ru/Co3O4-MOF as the catalyst, CO2 and H2O were the
only products, and CO2 selectivity was above 99.9%, revealing
that toluene could be completely oxidized over this sample. For
comparison, Co3O4-MOF catalyst was also employed under the
same conditions. Along with the increase of temperature from
200 ꢀC to 275 ꢀC, small amount CO was observed. The CO2
selectivity rised from 91% to 100%, and no other organic
species were observed.
Water vapor exists in the VOCs-contained ue gases with a
percentage level. Hence, we conducted the catalytic oxidation of
toluene in presence of 1.5 vol% water vapor to examine the
effect of water vapor on the catalytic performance. The most
effective material of Ru/Co3O4-MOF was utilized as the catalyst.
As shown in Fig. 7, when the temperature was raised to 235 ꢀC,
the conversion decreased from 86% to 79% within the 1st hour,
possibly due to the actual higher temperature on the surface of
the catalyst resulted from the unbalanced heat release. Then,
the conversion smoothly increased from 79% to 84% in the next
2 h. Addition of water vapor led to a sharp decrease of toluene
conversion by ca. 30%. Interestingly, when water vapor was cut
off, the catalyst showed apparent higher catalytic efficiency than
before. It was tentatively proposed that water facilitates the
dispersion of Ru/RuO2, whereas water occupies the active sites
of the catalyst leading to the inhibition effect. When abundant
water was introduced to the system, the inhibition effect played
a major role. When the water vapor was cut off, higher
Fig. 6 CO2 selectivity at different temperatures over Ru/CO3O4-MOF
and CO3O4-MOF catalysts under the conditions of toluene concen-
tration ¼ 1000 ppm, toluene/O2 molar ratio ¼ 1/200, and WHSV ¼
60 000 mL gꢁ1 hꢁ1
.
dispersion of Ru species led to an apparent promotion effect.
For the reaction conducted at 245 ꢀC, the introduction of water
vapor barely inuence the catalytic activity of Ru/Co3O4-MOF.
To evaluate the catalytic stability, on-stream toluene oxida-
ꢀ
ꢀ
tion experiment was performed at 230 C and 240 C, respec-
tively. The results are shown in Fig. 8. When the temperature
was 230 ꢀC, the toluene conversion decreased slowly within the
1st hour. Then, the conversion showed a slightly increasing
tendency, revealing that the catalytic activity became higher
with time extension. When the reaction temperature was raised
ꢀ
to 240 C, the toluene conversion sharply increased to 95.6%.
Over time, an equivalent conversion was obtained, and no
Fig.
7 Effect of water vapor on toluene conversion at different
Fig. 5 Toluene conversion as a function of reaction temperature over temperatures over Ru/Co3O4-MOF catalyst under the conditions of
the catalysts under the conditions of toluene concentration ¼ 1000 toluene concentration ¼ 1000 ppm, toluene/O2 molar ratio ¼ 1/200,
ppm, toluene/O2 molar ratio ¼ 1/200, and WHSV ¼ 60 000 mL gꢁ1 hꢁ1
.
water vapor concentration ¼ 1.5 vol%, and WHSV ¼ 60 000 mL gꢁ1 hꢁ1
.
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