Effect of preparation method on the performance of porous RuOx/Co3O4 catalysts for 1, 2-dichloroethane oxidation

Article abstract of DOI:10.1016/j.apcata.2021.118300

Porous Co₃O₄ with high yield was synthesized through calcination from plate-like Co(OH)(OCH₃). Then, a series of Co₃O₄ supported RuO_x catalysts were prepared using three different loading methods, including homogeneous precipitation (HP), precipitation-deposition (DP), and wet impregnation (IM). Their physicochemical properties were investigated by various characterization techniques, and their performance for catalytic oxidation of 1, 2-dichloroethane (1, 2-DCE) was also evaluated. RuO_x/Co₃O₄-HP possessed the most outstanding low-temperature reducibility, more oxygen vacancies, and surface active oxygen, correspondingly exhibited a better catalytic activity and lower selectivity of polychlorinated by-products. Moreover, RuO_x/Co₃O₄-HP also presented an excellent performance for other types of hydrocarbons abatement. The stability test showed that the RuO_x/Co₃O₄-HP showed highly stable activity for 1, 2-DCE oxidation, and the beneficial effect of water was further confirmed.

Full text of DOI:10.1016/j.apcata.2021.118300

Applied Catalysis A, General 624 (2021) 118300
Contents lists available at ScienceDirect
Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
Effect of preparation method on the performance of porous RuO /Co O
3 4
x
catalysts for 1, 2-dichloroethane oxidation
Wei Deng, Ziye Jia, Biao Gao, Simin Zhu, Dongqi Liu*, Limin Guo*
School of Environmental Science and Engineering, Huazhong University of Science and Technology, Wuhan, 430074, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Porous Co O4 with high yield was synthesized through calcination from plate-like Co(OH)(OCH ). Then, a series
3
3
Catalytic oxidation
CVOCs
of Co
3
O
4
supported RuO
x
catalysts were prepared using three different loading methods, including homogeneous
precipitation (HP), precipitation-deposition (DP), and wet impregnation (IM). Their physicochemical properties
were investigated by various characterization techniques, and their performance for catalytic oxidation of 1, 2-
1
, 2-dichloroethane
RuO /Co
x
3 4
O
dichloroethane (1, 2-DCE) was also evaluated. RuO
reducibility, more oxygen vacancies, and surface active oxygen, correspondingly exhibited a better catalytic
activity and lower selectivity of polychlorinated by-products. Moreover, RuO /Co -HP also presented an
excellent performance for other types of hydrocarbons abatement. The stability test showed that the RuO
Co -HP showed highly stable activity for 1, 2-DCE oxidation, and the beneficial effect of water was further
confirmed.
x 3 4
/Co O -HP possessed the most outstanding low-temperature
x
3 4
O
x
/
3 4
O
1
. Introduction
The removal of volatile organic compounds (VOCs) has been an ur-
such as the widely concerned CeO
CrO [14,15], and Co based catalysts.
CeO presented high activity for CVOCs combustion but was easy to
deactivate due to the inorganic Cl species deposit on active sites [16].
For MnO -based catalysts, MnO was activated into MnOCl that was
regarded as the main active site for catalytic oxidation of CVOCs.
However, it could be further chlorinated into the inactive MnCl under
harsh reaction conditions [17,18]. Although VO and CrO based cata-
2 x x
[9], MnO [10,11], VO [12,13],
x
3 4
O
2
gent task worldwide because they are the main substances for envi-
ronmental problems [1]. In recent years, more attention has been paid to
chlorinated VOCs (CVOCs) due to their more toxicity and lower
bio-degradability [2]. Catalytic oxidation is regarded as one of the most
promising ways to eliminate or reduce the CVOCs emissions because of
its high efficiency, low operating temperature, and more economically
x
x
x
x
x
x
lysts showed extremely high activity for CVOCs oxidation, their high
toxicity and easy loss severely constrained their practical application
[
3]. However, the development of catalysts with high activity, durability
and selectivity to acceptable environmental products including CO
O, and HCl is challenged [4]. Two different kinds of catalysts, such as
2
,
3 4
[14,19]. As a typical non-noble metal oxide, Co O had been demon-
H
2
strated to be one of the most efficient catalysts for the total oxidation of
hydrocarbons and CO due to its strong redox ability and abundant active
oxygen species [20,21], meanwhile, which also showed high activity for
CVOCs combustion and noticeable resistance to Cl-poisoning at high
supported noble metal and non-noble metal oxides, were widely re-
ported [5,6]. Among them, supported noble metals such as Pt, Pd, and
Rh often presented high initial activity, but they were susceptible to Cl
poisoning deactivation and easy to persuade the formation of more toxic
polychlorinated by-products, even dioxin [7]. However, it was note-
worthy that Ru as an active component in the industrial process for the
◦
temperature (>300 C) [22,23]. However, the high chlorination activity
3 4
and the slow oxidation-reduction cycle of Co O were tended to produce
more toxic polychlorinated by-products and deactivate at
low-temperature [24–26]. Thus, the enhancement of the stability at
low-temperature and the inhibition of possible chlorination was critical
for cobalt-based catalysts and some work had been reported. Lao et al.
Cl
2
production (Deacon Reaction: HCl + O
2
→Cl
2
2
+ H O) was considered
to be one of the most promising noble metals for catalytic combustion of
CVOCs due to good chemical stability and the less formation of poly-
chlorinated by-products [8]. By comparison, non-noble metal oxides
possessed comparable activity besides low-cost and earth-abundance,
3 4 x
[27] found that the Co O supported RuO catalysts showed high ac-
tivity for catalytic oxidation of chlorinated aromatics, which was
*
Corresponding authors.
E-mail addresses: liudongqi@hust.edu.cn (D. Liu), lmguo@hust.edu.cn (L. Guo).
https://doi.org/10.1016/j.apcata.2021.118300
Received 13 May 2021; Received in revised form 30 July 2021; Accepted 31 July 2021
Available online 12 August 2021
0
926-860X/© 2021 Elsevier B.V. All rights reserved.

W. Deng et al.
Applied Catalysis A, General 624 (2021) 118300
◦
associated with the generation of high active Ru-O-Co bonds and in-
crease in the surface active oxygen species due to the replacement of
180 C for 24 h. The plate-like precursor (Fig. S1) was collected by
◦
filtration and dried at 60 C in an oven for 12 h. Finally, the as-obtained
3
+
4+
◦
Co by Ru . Cao et al. [28] reported that the synthesized core-shell
Co @ZSM-5 catalysts using a microwave hydrothermal method pre-
sented an excellent CO selectivity and low by-products yield for
dichloromethane combustion. Xu et al. [29] also found that 90 % con-
light pink powder was calcined at 400 C for 4 h in an air atmosphere at
◦
ꢀ 1
3
O
4
a rate of 2 C min to produce porous Co
3 4
O .
2
The RuO /Co catalysts were prepared by wet impregnation (IM),
x
3 4
O
deposition precipitation using NaOH solution (2.0 wt. %) as precipitator
(DP), and homogenous precipitation with urea hydrolysis (HP),
respectively. The theoretical loading of Ru was 1.0 wt. % and the actual
Ru loadings were listed in Table 1. The detailed procedures were as
following:
version was observed on rare earth metal doping Co
3
O
4
(CoCeYO
x
) at
◦
2
56 C for catalytic combustion of chlorobenzene, which also essentially
reduced the generation of dichlorobenzene owing to the promoting of
redox ability. However, the development of Co-based catalysts with
preferable performances, such as better activity, durability and selec-
tivity, for CVOCs oxidation is still desired. Moreover, the versatility of
Co-based catalysts should be noticed because the organic pollutants in
realistic industrial emissions normally consisted of several pollutants.
Up to now, synergistic catalytic combustion purification of
multi-component pollutants containing non-chlorinated hydrocarbons
and CVOCs was not focused. The effects of non-chlorinated hydrocar-
bons on the behavior of CVOCs combustion, such as the selectivity of
HCl and polychlorinated by-products, were rarely reported [30].
(1) Wet impregnation method
3
2.0 mL RuCl (5.0 g/L Ru based, the same below) aqueous solution
was added to 1.0 g Co
3
O
4
and homogeneously mixed, then dried in an
◦
◦
oven at 60 C for 12 h followed by calcined at 400 C for 4 h under static
air in a muffle furnace. The prepared catalysts were denoted as RuO
Co -IM.
x
/
3 4
O
In this paper, porous Co
through a calcination-induced phase transformation of plate-like Co
OH)(OCH ) prepared by a solvothermal method using methanol as
solvent, and then the supported RuO (1.0 wt. %) catalysts were pre-
pared by three different methods. Several technologies were conducted
to study the physicochemical properties of porous Co and its sup-
ported RuO catalysts. The effect of preparation methods on catalytic
3
O
4
as the support was firstly synthesized
(2) Deposition precipitation method
1.0 g Co and 2.0 mL RuCl aqueous solution was dispersed into
(
3
3
O
4
3
x
100 mL deionized water under stirring, then NaOH aqueous solution
(2.0 wt.%) was added slowly to adjust the pH value to about 9.0. After
being aged at room temperature for 2 h, the formed solid was filtered
and washed with deionized water until pH = 7.0, then dried in an oven
3 4
O
x
◦ ◦
at 60 C for 12 h followed by calcined at 400 C for 4 h under static air in
performance, including activity and polychlorinated by-products selec-
tivity for 1, 2-dichloroethane oxidation, was comprehensively investi-
a muffle furnace. The prepared catalysts were denoted as RuO
DP.
x 3 4
/Co O -
gated. For the preferred RuO
x
catalyst, the stability and the influence of
. Furthermore,
water vapor were further evaluated compared with Co
3 4
O
catalytic oxidation of different pollutants and their multi-component
pollutants with 1, 2-dichloroethane was also synergistically studied.
(3) Homogenous precipitation method
1
3 4 3
.0 g Co O , 0.5 g urea, and 2.0 mL RuCl aqueous solution were
2
. Experimental section
added to the 100 mL deionized water under stirring, then the mixture
◦
was stirred for another 4 h in an oil bath at 80 C. After cooling down to
2
.1. Preparation of porous Co
3
O
4
and its supported RuO
x
catalysts
room temperature, the precipitate was filtered and washed, dried in an
◦
◦
oven at 60 C for 12 h and then calcined at 400 C for 4 h under static air
in a muffle furnace. The prepared catalysts were denoted as RuO
Co -HP.
The porous Co
cursor prepared by a solvothermal method using methanol as solvent
31,32]. In detail, appropriate amounts of Co(CH COO) O were
⋅4H
added to 75 mL methanol solution under stirring with a molar concen-
3
O
4
was thermally transferred from its plate-like pre-
x
/
3 4
O
[
3
2
2
2
.2. Characterizations
The phase and structure of the catalysts were determined by using X-
ꢀ 1
tration of 0.1 M⋅L . After the Co(CH
dissolved, the solution was transferred to an autoclave and then kept at
3
COO)
2
⋅4H
2
O was completely
Table 1
Physicochemical properties of the prepared catalysts and their catalytic performance for 1, 2-DCE oxidation.
H
2
-TPR
XPS (atom.)
For 1, 2-DCE oxidation
a
b
Ru
(wt.
S
BET
2
PD
R
e
H2 uptake
Ea (kJ
f
O / (O
Ru
n+
/
Co
2+
/
T
T90( C)
o
Rate
g
( mol
μ
Samples
(m
c
d
L
β
α
50
o
220
ꢀ 1
ꢀ
1
(nm)
(nm)
(mmol g
ꢀ
1
)
mol
ꢀ 1
)
+Oβ)
(Ru +
0
2+
(Co +Co3+
)
( C)
min
m
-2
)
%
)
g
)
n+
Ru
)
Co
RuO
Co
HP
RuO
Co
DP
3
O
4
0
19.1
16.4
15.0
17.3
20
18
8.082
8.093
7.33
7.31
–
0.228
0.278
0
0.38
0.43
322
266
375
304
0.05
0.55
x
/
0.92
0.89
0.90
60.4
0.76
3
O
4
-
4
-
4
-
x
/
16.1
18.0
19.1
21.4
19
19
8.091
8.088
7.25
7.23
64.4
96.0
0.255
0.244
0.74
0.72
0.42
0.40
274
280
313
315
0.36
0.22
3
O
x
RuO /
Co
3
O
IM
a
b
c
d
e
f
Ru weight contents measured by ICP-AES.
Specific surface area.
BJH adsorption average pore diameter.
Crystalline size calculated from the Scherrer’s equation, applied to the (311) reflection of Co
3 4
O .
Lattice parameter of Co
3 4
O .
2
Activation energy via the Kissinger Equation: Ln(β/T max) = K+(Ea/R)(1/Tmax) [36].
g
◦
Reaction rate was estimated based on the mol number of transformed reactant per square meter per minute at 220 C.
2

W. Deng et al.
Applied Catalysis A, General 624 (2021) 118300
ray diffraction (XRD) and Raman spectra. The morphology and ele-
3. Results and discussions
ments’ distribution was observed using scanning electron microscopy
(
FE-SEM), scanning transmission electron microscopy (FE-S/TEM),
3.1. Characterization of catalysts
transmission electron microscopy (HR-TEM), and EDS mapping. The
surface component and oxidation state were analyzed by X-ray photo-
XRD patterns of the as-synthesized precursor are shown in Fig. S1,
whose diffraction peaks could be indexed to a structure of Co(OH)
electron spectra (XPS). N
2
adsorption-desorption experiments were
carried out to obtain the specific surface area, pore size, and pore vol-
ume of the catalysts. The redox and acidic properties of catalysts were
(OCH
cation of acetate ligands driven by heating in the presence of methanol
reaction of Co(CH COO) O → Co(OH)(OCH ) + CH CO CH
+ CH
O occurred during the solvothermal process and plate-like Co(OH)
OCH ) formed [32]. Moreover, the yield of the products was close to
100 % (as the residue solutions were colorless). As evidenced in Fig. S1
and Fig. S2, XRD patterns indicated that a spinel structure of Co
(JCPDS No. 42-1467) was obtained after calcination [33]. After loading
RuO species, the crystal phase of Co was not obviously affected. The
phases corresponding to RuO
3
) according to Zhu’s report [31]. As described, a typical esterifi-
studied by H
2
-TPR and NH
3
-TPD, respectively. More details could be
3
2
3
3
3
2
3
+
found in our previous report [24].
H
2
(
3
2
.3. Activity test
3 4
O
Catalytic activities of catalysts for VOCs oxidation were tested in a
quartz tube reactor with an inner diameter of 4 mm at atmospheric
pressure. Feed stream was prepared by delivering liquid VOCs (1, 2-DCE
or toluene) with a syringe pump (KDS 100, USA) and propane standard
gas (metered by a mass flow controller) at a given concentration into a
x
3 4
O
x
species were not detected due to their
◦
high dispersion (Fig. 1A). However, the diffraction peak at 36.9 shifted
to the lower angle (Fig. 1A, inset), which was an indication of the doping
of Ru3
+/4+
species into the octahedron centers of the Co
O
spinel
+/4+
(0.68 and 0.62 Å) was larger
dry gas mixture composed of 10 % O
2
and N
2
balance. The injection part
3
4
3
was electrically heated to ensure complete evaporation of liquid 1, 2-
structure because the ionic ratio of Ru
than Co3 (0.61 Å) and led to the lattice expansion [34]. Meanwhile, the
+
DCE or toluene, and 100 mg of catalyst (40–60 mesh) was used
ꢀ 1
ꢀ 1
h ). The effects of GHSV on 1, 2-DCE con-
(
GHSV was 60,000 mL g
lattice parameter of Co
3
O
4
increased from 8.082 Å to 8.088–8.093 Å
(Table 1), which further confirmed the incorporation
3+/4+
3 4
into the lattice of Co O . Additionally, the crystalline size of
version were investigated by changing the loading amount of RuO
x
/
after loading RuO
x
ꢀ 1
Co
3
O
4
-HP under the gas flow rate of 100 mL min . 3 vol. % water vapor
of Ru
(
when used) was added by a bubbling method. The reactants and
all the samples was about 19 nm based on Scherrer equation applied for
◦
products were analyzed online by gas chromatography (GC, FULI,
GC9790II, China) equipped with FID, TCD, and nickel converting
equipment. The conversion was measured by the average value of three
sampling analyses, and the carbon balance was up to 96 % or higher. Cl2
was detected by a chemical titration method as described in our previous
work [24]. Both external and internal diffusion limitations were
excluded by varying the particle size of catalysts and W/F (the ratio of
catalyst weight to the total flow rate). More details could be found in
Supplementary Materials. Usually, the mentioned mass transfer resis-
(311) at the 2θ angle of 36.9 (Table 1). Fig. 1.B further showed Raman
spectra of Co
3
O
4
and its supported RuO
x
samples. Five Raman-activated
modes could be detected on the Co
3 4
O
. In particular, the main band at
ꢀ 1
7
about 678 cm corresponded to the A1g species in the O
scopic symmetry from octahedral sites (CoO ) and the band at about 190
cm was assigned to the F2 g symmetry from tetrahedral sites (CoO ).
Meanwhile, the band located at about 477 cm was assigned to E
h
spectro-
6
ꢀ 1
1
4
ꢀ 1
g
-
1
ꢀ 1
symmetry and the bands at 521 cm accompanied with 613 cm were
2
x
assigned to the F2 g symmetry [35]. With the adding of RuO , no new
ꢀ 1
tance was eliminated with a linear velocity higher than 7.0 cm⋅s when
the catalyst particle size was diminished to 40–60 mesh [13].
bands appeared, but the intensity of the bands decreased and the posi-
tion shifted to the low wavenumbers (Fig. 1B, inset), indicating that Ru
species incorporated into the lattice of Co
consistent with XRD results.
3
O
4
. All the results were highly
2
.4. Temperature programmed surface reaction (TPSR)
The actual Ru content of all supported RuO
x
samples determined by
The temperature-programmed surface reaction was conducted: the
ICP-AES was approximately 0.90 wt. % (Table 1), which indicated that
Ru species could be effectively loaded by the adopted three methods.
The textural properties, such as the specific surface area, average pore
feeding containing 1000 ppm 1, 2-DCE and 10 % O
2
/Ar continuously
◦
flowed through 100 mg catalyst, was heated from 100 to 400 C at the
◦
ꢀ 1
heating rate was 2 C min in a temperature controlled-reactor. 1, 2-
DCE and products were analyzed online by mass spectrometer appa-
ratus (Hiden HPR-20EGA, England).
size, and pore size distribution, were measured by N
desorption as shown in Fig. 2 and Table 1. All the samples presented a
type IV sorption isotherm with an H -type hysteresis loop in the relative
pressure range of 0.75–1.0, which confirmed the existence of mesopores
Fig. 2A). Meanwhile, the pore size distribution calculated from the
adsorption isotherms branch based on the BJH model indicated that the
2
adsorption-
2
(
3 4 x
Fig. 1. XRD patterns (A) and Raman spectra (B) of Co O and its supported RuO catalysts.
3

W. Deng et al.
Applied Catalysis A, General 624 (2021) 118300
2
Fig. 2. N sorption isotherms (A) and pore size distributions (B) for the as-prepared samples.
x 3 4 1 2 3 x 3 4 1 2 3
Fig. 3. SEM (SE, secondary electron), TEM (DF, dark field) and TEM (BF, bright field) images of RuO /Co O -HP (A , A and A ), RuO /Co O -DP (B , B and B ) and
RuO /Co -IM (C , C and C ) obtained from FE-S/TEM system.
x
O
3 4
1
2
3
4

W. Deng et al.
Applied Catalysis A, General 624 (2021) 118300
mesopores’ size was central at about 30 nm (Fig. 2B). Specific surface
step from TPR profiles obtained by tuning heating rates (Fig. 5B and
areas of Co
3
O
4
, RuO
x
/Co
3
O
4
-HP, RuO
x
/Co
3
O
4
-DP, and RuO
x
/Co
3
O
4
-IM
Fig. S4) [36]. The RuO
x
/Co
3
O
4
-HP undoubtedly presented the lowest
2
ꢀ 1
ꢀ 1
were 19.0, 16.4, 16.1, and 18.0 m
RuO
of RuO
SEM images displayed that the as-synthesized Co(OH)(OCH
cursors were composed of the plate-like nano-sheets (Fig. S1). After
calcination, the porous Co was obtained due to the decomposition of
organics. With the loading of RuO species, the morphology of all the
samples was still well maintained (Fig. S2), which was also well reflected
by the results of N sorption isotherms. Furthermore, SEM (SE, sec-
ondary electron), TEM (DF, dark field), and TEM (BF, bright field) im-
ages of porous RuO /Co were obtained from the FE-S/TEM system to
g
, respectively. The loading of
value of 60.4 kJ mol , which implied the strongest low-temperature
x
species slightly reduced the specific surface area due to the block
species into the pores.
reducibility. Moreover, the overall H
listed in Table 1. The loading of RuO
decrease of H consumption, which could be ascribed to the higher
relative atomic mass of Ru or the presence of metallic state Ru.
Fig. S5 presented NH -TPD profiles of the synthesized catalysts to
2
consumption was calculated and
x
x
species resulted in a slight
3
) pre-
2
3
O
4
3
x
determine their acidic properties. Considerable acid sites could be
◦
detected on Co
3
O
4
. Two desorption peaks distributed before 250 C and
◦
2
centered at about 400 C appeared, which corresponded to the weak and
x
moderate strong acid sites [40]. After the RuO species was loaded, the
x
3
O
4
shape of desorption profiles almost remained, but the peak area
decreased, which corresponded to the decreasing of acid sites due to the
directly investigate the morphology and porous structure of the syn-
thesized catalysts (Fig. 3). As shown in Fig. 3, the pores with a wide size
distribution were observed clearly on the synthesized samples. HRTEM
further confirmed that the existence of these pores and their wide dis-
+/3+
coverage of RuO
x
species on Co2
or oxygen vacancies as Lewis acid
sites [24,27], which were beneficial for the rupture of C-Cl bonds and
the adsorption/activation of CVOCs [41–43].
tribution (Fig. 4). However, it was difficult to find the RuO
all the porous RuO /Co samples by TEM images because of their high
dispersion or incorporation into the lattice of Co . Moreover, STEM
x
species over
3 4
Fig. 6 showed Co 2p, Ru 3p, and O 1s XPS spectra of Co O and its
x
3
O
4
x
supported RuO catalysts. Co 2p XPS spectra could be fitted to the four
3
+
2+
3
O
4
peaks associated with Co and Co , in detail, the peaks with binding
elemental mapping images in Fig. S3 revealed that the Co, O, and Ru
were highly dispersed.
energies in the range of 779.6–780.1 eV and a 2p3/2-2p1/2 splitting of
15.1 eV could be assigned to the octahedral Co3 , and the peaks with
+
The reducibility of Co
3
O
4
and its supported RuO
x
samples were
binding energy in the range of 780.9–781.4 eV and a 2p3/2-2p1/2 split-
ting of 15.5 eV corresponded to the tetrahedral Co2 (listed in Table 1).
+
evaluated by H -TPR and the results are shown in Fig. 5. A typical two-
2
step reduction was observed on Co
3
O
4
, which could be attributed to the
Additionally, the satellite peaks located at 3.6–6.5 eV (Sa1) or 9.4 eV
reduction of Co3 to Co (centered at 306 C) and Co to metallic Co
+
2+
◦
2+
(Sa2) higher than 2p3/2 could be assigned to Co and Co respectively,
2+
3+
◦
(
centered at 380 C), respectively [37]. With the addition of RuO
x
, all
-HP
presented the best reducibility (the lowest top temperature peak was at
and the third satellite peaks (Sa
3
) located at 803.3–804.2 eV (Fig. 6A)
[44]. The ratio of Co2 /(Co +Co ) was estimated (Table 1) and the
+
2+
3+
x 3 4
reduction peaks shifted to the lower temperatures and RuO /Co O
2
+
increased as the
results indicated that the content of surface Co
◦
4+
3+
3+
3+
4+
about 152 C), which was attributed to that the Ru replaced the Co
in the Co spinel structure, and two reduction processes involved:
loading of RuO
x
due to the reduction of Co by Ru or the Ru
O
4
occupied Co3 site resulting in another one Co ion formation [45].
+ 2+
3
4
+
3+
3+
◦
◦
Ru -Co
3
O
4
→ Ru -CoO and Ru -CoO → Ru -Co [27]. In addition, an
Among all the supported RuO
x
samples, RuO
x
/Co
3
O
4
-HP possessed the
◦
2+
additional minor reduction peak below 100 C was observed on the
highest Co , meaning more oxygen vacancies formed. Moreover, the
lower shift in the binding energy of Co further confirmed the occurrence
of an electron-transferring. The spectra of Ru 3p could be deconvoluted
to three peaks at about 465.5, 463.5, and 462.0 eV, which were assigned
supported RuO
nano-clusters [38,39], which was easily reduced to metallic Ru due to
the high activity for H dissociation and thus could accelerate the
reduction rate of RuO /Co [27]. The activation energy via the Kis-
x x
samples. It could be assigned to the reduction of RuO
2
to Ru6 , Ru , and Ru , respectively (Fig. 6B) [46]. Meanwhile, the
+ 4+
◦
x
3
O
4
2
n+
n+
◦
singer Equation (Ln (β/T max) = K+ (Ea/R) (1/Tmax), where Tmax is the
top temperature for the peak, K is a constant, Ea is the activation energy,
and R is the gas constant) was calculated based on the first reduction
estimated ratio of Ru / (Ru +Ru ) was listed in Table 1 and RuO
Co -HP presented the highest value. For the spectra of O 1s, two
deconvoluted peaks in the range of 528.9–530.0 eV and 531.0–531.4 eV
x
/-
3 4
O
Fig. 4. HRTEM images of porous Co
3
O
4
(A), RuO
x
/Co
O
3 4
-HP (B), RuO
/Co
x 3
O
4
-DP (C) and RuO
x
3
/Co O
4
-IM (D).
5

W. Deng et al.
Applied Catalysis A, General 624 (2021) 118300
2 x
Fig. 5. H -TPR profiles of the synthesized catalysts (A) and activation energy via the Kissinger Equation based on the first reduction step for supported RuO
samples (B).
Fig. 6. XPS spectra of Co 2p (A), Ru 3p (B) and O 1s (C) for Co
3
O
4
x
and its supported RuO catalysts.
◦
could be determined (Fig. 6C), which was assigned to lattice oxygen
temperature (about 200 C), and the maximum concentration also
species (O
oxygen species such as O
) based on the peak area indicated that more active oxygen
α
) and surface oxygen species (O
β
) including highly active
decreased from 60 ppm for Co
eration of trichloroethane was usually considered to be related to
adsorbed Cl species and Cl , and TPSR results confirmed the effluence of
Cl via the oxidation of adsorbed Cl species and formed at a higher
temperature over Co (Fig. S6). The XPS and ToF-SIMS results showed
3 4
O to about 35 ppm or lower. The gen-
ꢀ
ꢀ
2
or O [47]. The estimated ratio of O
β
/
(
O
α
+O
β
2
species generated over supported RuO
presented the highest ratio (Table 1).
x
catalysts, and RuO
x
/Co
3
O
4
-HP
2
3 4
O
that the Cl species easily deposited on the catalysts’ surface during
CVOCs oxidation and especially was captured by oxygen vacancy sites
3
3 4 x
.2. Catalytic performance of porous Co O and its supported RuO
[
50,51], which accounted for the formation of polychlorinated
catalysts
by-products as evidenced by previous reports [27,52]. The formation of
trichloroethane could be through oxy-chlorination of DCE with adsor-
Fig. 7A presented the light-off curves of 1, 2-DCE oxidation over
synthesized catalysts, and the corresponding T50/T90 (temperature for
bed Cl species or chlorination of DCE with Cl
2
. However, the formation
◦
of Cl
2
over Co
3
O
4
only occurred above 300 C, which indicated that the
5
0 % and 90 % conversion) was listed in Table 1. Considerable activity
◦
adsorbed Cl species was primarily responsible for the formation of tri-
chloroethane (namely oxy-chlorination pathway). With the addition of
was detected on Co
3
O
4
with the T50 and T90 were 322 and 375 C,
respectively. However, an obvious deactivation occurred in the range of
00ꢀ 230 ◦C, which was consistent with most of reports about CVOCs
RuO species, the removal of adsorbed Cl species accelerated in the form
2
x
of Cl
2
◦
(the lower Cl
2
generating temperature, which reduced to about
3 4 x
oxidation on Co O [24]. With the addition of RuO species, the light-off
2
60 C). Thus, the formation of trichloroethane largely decreased.
curves of 1, 2-DCE oxidation shifted to lower temperatures, and the T90
◦
Meanwhile, the Cl2 selectivity was determined by a chemical titration
method and disclosed in Fig. 7B. The formation of Cl shifted to a lower
of RuO
x
/Co
3
O
4
-HP was only 304 C, which was much lower than that of
the most reported values [11,48,49]. Meanwhile, the formation of tri-
chloroethane as the main polychlorinated by-products was unavoidable
2
◦
◦
temperature from 300 C for Co
3
O
4
to about 260 C for RuO
x
/Co
3
O
O
4
4
catalysts. Moreover, the maximum selectivity of Cl
2
(50.4 %) over Co
3
3 4
for oxidation of 1, 2-DCE over Co O and followed a classical electro-
◦
◦
was detected at 400 C, while increased to about 61.0 % at 340 C over
philic substitution mechanism. Over which, the maximum concentration
◦
x 2
supported RuO catalysts. Additionally, the curves of CO yield showed
was 60 ppm at 320 C and then declined with the rise of reaction tem-
in Fig. 7B indicated that 1, 2-DCE was almost wholly oxidized into CO
2
,
perature due to the further oxidation of trichloroethane. After loading
and only a small amount of CO formed (<20 ppm) at a low-temperature
x
RuO species, the formation of trichloroethane occurred at a lower
6

W. Deng et al.
Applied Catalysis A, General 624 (2021) 118300
Fig. 7. Light-off curves of 1, 2-DCE (A) conversion and trichloroethane concentration versus reaction temperature, CO
2
yield and Cl
2
selectivity for 1, 2-DCE
/Co -HP for
ꢀ 1
ꢀ 1
oxidation (B) (1000 ppm 1, 2-DCE, GHSV = 60,000 mL⋅g h ), effects of 1, 2-DCE concentration (C) and GHSV (D) on the performance of RuO
, 2-DCE oxidation. Gas composition: 10 %O /90 %N (v/v).
x
3 4
O
1
2
2
range, which was also confirmed by TPRS (Fig. S6). Furthermore, the
synthesized promising RuO /Co -HP also showed high activity for
concentration of 1, 2-DCE. Additionally, it could be found that the
conversion of 1, 2-DCE over RuO /Co -HP reduced as the GHSV
x
3
O
4
x
3 4
O
different pollutants oxidation, such as propane, toluene, dichloro-
increased (Fig. 7D). The T50 and T90 of 1, 2-DCE oxidation at GHSV = 60,
◦
–1 –1
◦
methane, and chlorobenzene, with T90 could reach at before 310
C
000 mL g
h
increased by 42 and 38 C compared with that of at GHSV
ꢀ 1
ꢀ 1
–1 –1
under high GHSV of 60,000 mL g
The catalytic efficiency of Co
h
(Fig. S7).
= 15,000 mL g
h . These results were expected, the residence time of
3
O
4
and its supported RuO
x
catalysts for
1, 2-DCE molecules on active sites reduced as the GHSV increased,
which led to the decrease in the conversion of 1, 2-DCE. Totally, 1,
2-DCE was adsorbed/activated on acidic sites, then oxidized by oxygen
1
, 2-DCE oxidation was further compared based on the reaction rate at
◦
2
20 C, and the results were listed in Table 1. The apparent reaction rate
ꢀ
1
-2
over Co
3
O
4
was about 0.05
μ
mol min
species, especially for the sample of
3 4
O -HP with excellent low-temperature reducibility. In detail,
m , which exponentially
2 2 2
species and finally to mainly form CO , H O, HCl, and Cl . The deposited
increased with the addition of RuO
RuO /Co
the reaction rate over RuO
more than tenfold to Co
x
Cl species were the main reason for the formation of polychlorinated
by-products. After loading the RuOx species, more active oxygen species
were generated, accompanied by enhanced reducibility, thus presenting
higher activity. Meanwhile, the formation of trichloroethane was largely
x
ꢀ
1
-2
x
/Co
3
O
4
-HP was 0.55 mol min m , which
μ
3
O
4
, and expectedly higher than 10 %Cr/TiO
2
ꢀ 1
-2
◦
ꢀ 1 ꢀ 1
h )
(
0.051
μ
mol min
m
at 225 C under GHSV of 40,000 mL⋅ g
2
retarded and the formation of Cl shifted to lower temperatures due to
ꢀ
1
-2
◦
[
48] and CrO /MnCo
x
3
O
x
(0.330
ꢀ 1
h ) [53]. When combined with the XPS results, it could
μ
mol min
m
at 225 C with GHSV =
the high activity for Deacon Reaction [24,27].
ꢀ
1
3
6,000 mL g
As is well known, the real exhaust gas usually consisted of various
kinds of VOCs, and the presence of different pollutants might have a
significant impact on the performance of catalysts for targeted VOCs
oxidation. Thus, it is meaningful to evaluate the catalystsʹ performance,
including the activity and the behavior of by-products formation, to
control multiple pollutants synergistically. Fig. 8 showed catalytic per-
be found that the reaction rate was positively correlated with the ratio of
/(O ), which indicated that the surface active oxygen species was
+O
a significant factor for the oxidation of 1, 2-DCE over porous RuO /-
Co [27]. Sequentially, the effects of inlet 1, 2-DCE concentration and
GHSV on the conversion of 1, 2-DCE over RuO /Co -HP were further
O
β
α
β
x
3 4
O
x
3 4
O
investigated. It was clear that the conversion curves shifted to higher
temperature with the increase of 1, 2-DCE concentration (Fig. S8), which
could be attributed to the deficit of the active sites for 1, 2-DCE acti-
x 3 4
formance of RuO /Co O -HP for synergistic catalytic oxidation of
toluene or propane and 1, 2-DCE. The high activity for single toluene
oxidation was observed, and the conversion of toluene could reach
◦
vation. Based on the reaction rate presented in Fig. 7C, RuO
x
/Co
3
O
4
-HP
about 100 % at 220 C. With the addition of 1, 2-DCE into the reaction
catalyst presented a similar reaction rate at a low-temperature range
system (Two contaminants: 500 ppm 1, 2-DCE and 500 ppm toluene),
the conversion profile for toluene shifted to a higher temperature due to
the deposit of Cl species from 1, 2-DCE oxidation on active sites.
Meanwhile, the conversion of 1, 2-DCE was unstable, and the conversion
◦
(
below 260 C), which did not depend on the concentration of 1, 2-DCE,
suggesting that desorption of the products might be slow and was the
◦
rate-determining step. When the temperature was higher than 260 C,
◦
Cl
2
was generated according to the results of TPSR and chemical titra-
linear relationship with the
profile shifted to the higher temperature before 250 C, which could be
tion. The reaction rates showed
a
ascribed to the competitive adsorption or oxidation between two
7

W. Deng et al.
Applied Catalysis A, General 624 (2021) 118300
Fig. 8. Catalytic performance of RuO
/Co
x 3
O
4
-HP for catalytic oxidation of toluene (A) or propane (B) and the mixture with 1, 2-DCE. (A) One contaminant: 1000
ppm toluene or 1000 ppm 1, 2-DCE; Two contaminants: 500 ppm 1, 2-DCE and 500 ppm toluene; (B) One contaminant: 1000 ppm propane or 1000 ppm 1, 2-DCE;
for balance. GHSV = 60,000 mL⋅gꢀ
1
h ; catalyst amount: 100 mg.
ꢀ 1
Two contaminants: 500 ppm 1, 2-DCE and 500 ppm propane; 10 % O
2
and N
2
molecules. Conversely, the conversion of 1, 2-DCE was accelerated at a
occurred, and the formation of trichloroethane was also inhibited
◦
high-temperature range (>260 C), which could be ascribed to the
(Fig. S9). Generally, H
2
O molecules adsorbed on defect sites such as
2
+
+
promotion of Cl
2
formation via Deacon Reaction or the increase in
cluster/nanoparticles
Co
and oxygen vacancies, thus dissociated into the OH- and H .
mobility of active oxygen formed on the RuO
x
However, these defect sites were also considered to be the active sites for
adsorbing and dissociating gaseous oxygen and 1, 2-DCE. So, the
competitive adsorption was responsible for the decline in activity. On
during the reaction [54]. Correspondingly, trichloroethane was not
detected for catalytic oxidation of two contaminants because of the
+
presence of toluene as the hydrogen source. Meanwhile, the RuO
x
/-
the other hand, the formed H could promote the removal of Cl species
Co -HP displayed a similar catalytic behavior for catalytic oxidation
3
O
4
through the formation of HCl [54,58], which lead the low trichloro-
of propane and 1, 2-DCE (Fig. 8 B). There still no trichloroethane was
formed during the whole process for two contaminants oxidation. It
should be noted that when propane and 1, 2-DCE co-feed, the conversion
of 1, 2-DCE was much higher than propane at low-temperature range. As
reported, the activation of C–H bonds was the issue key for propane
oxidation, while the activation of C-Cl bands was prior for the CVOCs
ethane concentration and with the fact that Deacon Reaction over Co
3
O
4
◦
just accelerated at a temperature higher than 300 C (TPSR, Fig. S6).
After the water was turned off, the 1, 2-DCE conversion recovered and
the selectivity to trichloroethane increased (Fig. S9). For supported
RuO
Co
x
catalysts, similar with Co
3 4 x 3 4 x
O , both RuO /Co O -HP and RuO /-
3
O
4
-IM also presented a slight decrease in 1, 2-DCE conversion in the
[
55]. The bond energy of the C–H bond in the propane molecule (401.3
initial time, from about 95 % to 87 % and 93 % to 83 %, respectively,
and then remained stable within the next 240 min. The rapid deposition
of Cl species on active sites led to the initial deactivation, which was
ꢀ 1
kJ mol ) [56] is higher than that of the C-Cl bond in the 1, 2-DCE
ꢀ 1
molecule (345.1 kJ mol ) [57], thus the 1, 2-DCE might be easier to
activate than propane. Totally, the synthesized RuO /Co -HP showed
x
3
O
4
2
confirmed by the fact that the H -TPR results of used catalysts indicated
high activity for propane or toluene, 1, 2-DCE, and their mixture
oxidation, which was considered to be a promising catalyst for catalytic
oxidation of actual VOCs emissions.
that the reduction peaks shifted to a higher temperature (Fig. S10).
However, with the addition of water vapor, a slight increase in 1, 2-DCE
3 4
conversion was observed, which was completely different from Co O .
The stability of Co
3
O
4
, RuO
x
/Co
3
O
4
-HP, and RuO
x
/Co
3
O
4
-IM was
This phenomenon was also found in the Van den Brink’s work [59],
comparatively investigated on the stream containing 1000 ppm 1, 2-
which was ascribed to the increase in mobility of active oxygen formed
◦
DCE, 10 % O
2
and a balance of N
2
at 300 C at GHSV = 60,000 mL
on the RuO
x
cluster or nanoparticles during the reaction (H
2
-TPR also
ꢀ
1
ꢀ 1
g
h under dry and wet conditions (Fig. 9). Co
3
O
4
presented a stable
confirmed the existence of RuO
x
cluster) [27]. When the water vapor
conversion of 1, 2-DCE except for a slight decline in the initial 30 min
and maintained at about 40 % within 240 min. After 3 vol. % water was
introduced, a visible decrease from 40 % to 30 % in 1, 2-DCE conversion
turned off, the conversion of 1, 2-DCE returned to about 87 % and 83 %,
respectively. Additionally, the behavior of trichloroethane formation
3 4
including selectivity and generation time was different from Co O . For
◦
Fig. 9. (A) Catalytic durability under dry and humid conditions over Co
3
O
4
, RuO
x
/Co
3
O
4
-IM and RuO
x
/Co
3
O
4
-HP at 300 C; (B) Long time stability of Co
3
O
4
and
◦
ꢀ 1
RuO
x
/Co
O
3 4
-HP for 1, 2-DCE oxidation at 320 C under dry condition. Gas composition: 1000 ppm 1, 2-DCE, 10 % O
2
and N
2
for balance. GHSV = 60,000 mL⋅g
ꢀ
1
h
; catalyst amount: 100 mg.
8

W. Deng et al.
Applied Catalysis A, General 624 (2021) 118300
example, the trichloroethane selectivity was far lower than Co
3
O
4
Declaration of Competing Interest
(
Fig. S9), forming trichloroethane over RuO
Co
izations of the used catalysts by XRD and N
x
/Co
3
O
4
-IM and RuO
x
/-
3
O
4
-HP just occurred after about 90 min and 120 min. Character-
isotherms indicated that
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
2
there were no significant modifications in lattice phase, crystalline size,
specific surface area, and pore size distribution, indicating that both the
crystal structure and bulk structure were stable under harsh conditions
Acknowledgements
(
(
Fig. S11). However, XPS results indicated that the deposit Cl species
2.35–2.45 atom. %, Table S2) was still observed on the used supported
This work was sponsored by the National Key Research and Devel-
opment Program of China (2017YFE0127400), the Natural Science
Foundation of Hubei Province (2019CFA070) and Program for Huaz-
hong University of Science and Technology (HUST) Academic Frontier
Youth Team (2018QYTD03). The authors thank the Analysis and Testing
Center of Huazhong University of Science and Technology for analytical
support.
RuO
x
catalysts but lower than that of the used Co
3
O
4
(2.93 %), which
was ascribed to the high activity of RuO
x
based catalysts for Deacon
Reaction. Additionally, the ratio of Co2 to (Co +Co ) and O
+
2+
3+
/ (O
β α
+
β 3 4
O ) also decreased, especially for Co O , which confirmed that the
x
addition of RuO species was favorable for maintaining the oxygen va-
cancies (Fig. S12 and Table S2). Fig. 9B further displayed the prolonged
◦
stability (50 h) of Co
3
O
4
and RuO
x
/Co
3
O
4
-HP at 320 C, carrying out by
feeding the stream containing 1000 ppm 1, 2-DCE and 10 % O
2
with a
, the conversion of 1,
-DCE slightly declined in the initial 60 min and then maintained at
Appendix A. Supplementary data
ꢀ
1
ꢀ 1
space velocity of 60,000 mL⋅ g
2
h . For Co
3 4
O
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.apcata.2021.118300.
about 45 % within 50 h. Meanwhile, the selectivity to trichloroethane
was up to 18 %, and the accumulated Cl species were accounted for the
formation of these polychlorinated by-products. For RuO
x 3 4
/Co O -HP,
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