A new strategy to improve catalytic activity for chlorinated volatile organic compounds oxidation over cobalt oxide: Introduction of strontium carbonate

Article abstract of DOI:10.1016/j.jics.2021.100116

Co₃O₄–SrCO₃ catalysts with various Sr/Co ratios were synthesized by the coprecipitation method, and their properties were tuned by adjusting the Sr/Co molar ratio. Furthermore, the catalytic combustion of vinyl chloride (VC) was used to evaluate the catalytic activity of the Co₃O₄–SrCO₃ catalysts. The physicochemical properties of the catalysts were studied by X-ray diffraction (XRD), infrared spectroscopy (IR), N₂ sorption, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), H₂ temperature-programmed reduction (H₂-TPR) and VC temperature-programmed desorption (VC-TPD). The results showed that the Co₃O₄–SrCO₃ catalysts exhibited composite phases of Co₃O₄ and SrCO₃ and the presence of interactions between them. As a result, the crystallization of the Co₃O₄ phase for the Co₃O₄–SrCO₃ catalysts was restrained, and the state of Co on the catalyst surface was adjusted. Furthermore, the reducibility and VC adsorption capacity of the Co₃O₄–SrCO₃ catalysts with Sr/Co molar ratios of 0.2 and 0.4 were enhanced compared with those of the Co₃O₄ catalyst. Otherwise, catalyst SrCo-0.4 exhibited excellent catalytic performance, accompanied by the highest reaction rate and the lowest apparent activation energy. More importantly, the optimized SrCO₃–Co₃O₄ catalyst showed superior catalytic performance compared with other transition metal oxides in previous literature. These results brought a new idea for promoting the activity of transition metal catalysts for the deep oxidation of chlorinated volatile organic compounds (CVOCs) by introducing alkaline-earth metal salts.

Full text of DOI:10.1016/j.jics.2021.100116

Journal of the Indian Chemical Society 98 (2021) 100116
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Journal of the Indian Chemical Society
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A new strategy to improve catalytic activity for chlorinated volatile organic
compounds oxidation over cobalt oxide: Introduction of
strontium carbonate
a
a
a
a
a
,
**
a
a
Hao Liu , Kai Shen , Hailin Zhao , Yongjun Jiang , Yanglong Guo
, Yun Guo , Li Wang ,
a,b,*
Wangcheng Zhan
a
Key Laboratory for Advanced Materials and Research Institute of Industrial Catalysis, School of Chemistry & Molecular Engineering, East China University of Science and
Technology, Shanghai, 200237, PR China
b
Frontiers Science Center for Materiobiology and Dynamic Chemistry, East China University of Science and Technology, Shanghai, 200237, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Catalytic combustion
Vinyl chloride
Cobalt oxides
Alkaline-earth metal salts
Co O –SrCO catalysts with various Sr/Co ratios were synthesized by the coprecipitation method, and their
3
4
3
properties were tuned by adjusting the Sr/Co molar ratio. Furthermore, the catalytic combustion of vinyl chloride
VC) was used to evaluate the catalytic activity of the Co –SrCO catalysts. The physicochemical properties of
the catalysts were studied by X-ray diffraction (XRD), infrared spectroscopy (IR), N sorption, scanning electron
microscopy (SEM), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), H
temperature-programmed reduction (H -TPR) and VC temperature-programmed desorption (VC-TPD). The results
showed that the Co –SrCO catalysts exhibited composite phases of Co and SrCO and the presence of
interactions between them. As a result, the crystallization of the Co phase for the Co –SrCO catalysts was
restrained, and the state of Co on the catalyst surface was adjusted. Furthermore, the reducibility and VC
adsorption capacity of the Co –SrCO catalysts with Sr/Co molar ratios of 0.2 and 0.4 were enhanced compared
with those of the Co catalyst. Otherwise, catalyst SrCo-0.4 exhibited excellent catalytic performance,
accompanied by the highest reaction rate and the lowest apparent activation energy. More importantly, the
optimized SrCO –Co catalyst showed superior catalytic performance compared with other transition metal
(
3
O
4
3
2
2
2
O
3 4
3
O
3 4
3
3
O
4
3
O
4
3
O
3 4
3
3 4
O
3
3 4
O
oxides in previous literature. These results brought a new idea for promoting the activity of transition metal
catalysts for the deep oxidation of chlorinated volatile organic compounds (CVOCs) by introducing alkaline-earth
metal salts.
1. Introduction
catalysts, transition metal oxides and molecular sieve catalysts have been
applied for the oxidation of CVOCs emissions. Among these catalysts,
Following the expeditious expansion of the chemical industry, the
transition metal oxides have received increasing attention owing to their
emission of volatile organic compounds (VOCs) has caused severe threats
to the security of human health and the atmosphere. Thus, it is urgent to
develop efficient methods to eliminate the emission of VOCs. As one class
of VOCs, chlorinated volatile organic compounds (CVOCs) have attracted
sufficient attention from researchers due to their high toxicity and high
emission. Catalytic combustion is an efficient technology to eliminate
CVOCs emissions because of the advantages of desirable high efficiency
and no secondary pollution [1–6]. Until now, noble metal-based
inexpensive materials, simple preparation and high stability [7–15]. For
3
example, the LaMnO catalyst shows excellent catalytic performance for
the oxidation of vinyl chloride (VC) due to its stable structure, strong
reducibility of Mn4 and redox cycling between Mn and Mn [16,17].
þ
4þ
3þ
In addition, CeO -based catalysts also exhibit significantly high activity
2
for CVOCs oxidation due to excellent redox properties and abundant
oxygen vacancies on their surface [18,19].
3 4
Compared to other transition metal oxides, Co O -based catalysts
*
Corresponding author. Key Laboratory for Advanced Materials and Research Institute of Industrial Catalysis, School of Chemistry & Molecular Engineering, East
China University of Science and Technology, Shanghai, 200237, PR China.
*
* Corresponding author.
E-mail addresses: ylguo@ecust.edu.cn (Y. Guo), zhanwc@ecust.edu.cn (W. Zhan).
https://doi.org/10.1016/j.jics.2021.100116
Received 31 March 2021; Received in revised form 21 July 2021; Accepted 22 July 2021
0019-4522/© 2021 Indian Chemical Society. Published by Elsevier B.V. All rights reserved.

H. Liu et al.
Journal of the Indian Chemical Society 98 (2021) 100116
show superior catalytic activity and stability for VC oxidation. VC first
collection angles of 59 and 200 mrad, respectively. The distribution of
elements on the surface was recorded by energy-dispersive X-ray spec-
troscopy (EDS) with 4 in-column Super-X detectors. The FT-IR spectra of
2þ
–
adsorbs on Co sites by forming a
π
-complex between C C and the
–
2
þ
surface Co ions (acting as Lewis acid sites) [20,21]. Subsequently, the
active oxygen species on the surface attack the adsorption intermediate
and oxidize hydrocarbon species to carbon dioxide and water. Moreover,
the Cl species occupy the oxygen vacancy on the surface and desorb at
high temperature. Finally, the formed oxygen vacancy is replenished by
oxygen in the air. Based on this reaction mechanism, optimizing the
preparation method and doping other transition metals are common
3 3 4
the SrCO –Co O composite catalyst were recorded on a Nicolet Nexus
670 spectrometer. Before scanning, the sample was mixed with KBr
powder. The states of the surface elements were analysed by X-ray
photoelectron spectroscopy (XPS). The instrument used was a Thermo-
Fisher ESCALAB 250Xi electron spectrometer equipped with Al K
α
(1486.6 eV) radiation. Before the experiments, all the samples were
ꢀ
strategies to improve the catalytic performance of Co
et al. [22] prepared Co oxide catalysts by different synthetic routes
and found that the Co -TP catalyst (template method) showed a high
catalytic activity due to its enhanced oxygen mobility and abundant
adsorbed oxygen species generated from surface defects of the Co -TP
catalyst. Wang et al. [23] adopted a metal-doping method to prepare a
RuO /Co catalyst and found that the great activity could be attributed
to the excellent reducibility due to abundant oxygen vacancies origi-
O
3 4
catalysts. Yuan
baked in a drying oven at 100 C.
O
3 4
The reducibility of catalysts was tested by temperature-programmed
reduction of H (H -TPR) technology. The experiment was operated on
2 2
a PX200 apparatus with a thermal conductivity detector (TCD). Each
3
O
4
ꢀ
O
3 4
sample (50 mg) was pretreated in N
after cooling to 25 C, the gas was switched to 40 mL/min H
2
flow for 60 min at 200 C. Then,
ꢀ
2
/Ar mixed
ꢀ
ꢀ
ꢀ
x
O
3 4
gas, and the sample was heated from 25 C to 500 C with a step of 10 C/
min.
4
þ
nating from Ru
searchers have made substantial achievements in improving the catalytic
performance of Co -based catalysts for VC deep oxidation, the devel-
opment of novel strategies capable of enhancing the catalytic perfor-
mance of Co -based catalysts is still highly desirable.
Herein, this is the first study to introduce strontium carbonate into a
oxide catalyst, aiming to improve its catalytic performance for the
deep oxidation of vinyl chloride. The corresponding SrCO –Co com-
species entering the Co
O
3 4
lattice. Although re-
The VC adsorption capacity of the catalysts was detected by
temperature-programmed desorption of vinyl chloride (VC-TPD) exper-
iments. The instrument used was a Ruimin 2060 gas chromatograph with
3 4
O
a thermal conductivity detector (TCD). The catalyst (20 mg) was pre-
ꢀ
O
3 4
treated in a N
cooling to 25 C, vinyl chloride/N
introduced for 30 min. After saturation adsorption, N
2
flow of 40 mL/min for 60 min at 200 C. Then, after
ꢀ
2
mixed gas (1000 ppm VC) was
gas (60 mL/min)
Co
O
3 4
2
3
O
3 4
was introduced and kept for 60 min to remove physically adsorbed vinyl
chloride on the catalyst surface. Finally, the catalyst was heated from 25
posite catalysts were synthesized by the coprecipitation method, and the
ratio of Sr/Co was tuned. The physicochemical properties of the catalysts
were studied by various characterizations and their catalytic perfor-
mances for the deep oxidation of VC were evaluated. Compared with the
ꢀ
ꢀ
ꢀ
C to 500 C at a heating rate of 10 C/min, and outlet gas was detected
by TCD.
3 4 3 3 4
Co O catalyst, the SrCO –Co O catalysts showed better catalytic per-
formances and selectivity for the deep oxidation of VC. Our findings
2.3. Activity measurements
provide a novel strategy for enhancing the catalytic performance of
The catalytic combustion of vinyl chloride (VC) was evaluated in a
Co
3 4
O -based catalysts in VC oxidation by combining with metal salts
fixed-bed quartz reactor at atmospheric pressure. 0.24 g of the catalyst
was used in each experiment. All catalysts were sieved to 40–60 mesh
before experiments. The reaction feed gas was composed of dry air (114
mL/min) and 2 vol % vinyl chloride (6 mL/min). The WHSV of the cat-
instead of metal oxides.
2. Experimental
ꢁ1
ꢁ1
alytic tests was kept in 30,000 mL g
h
. The reaction temperature was
2
.1. Preparation of SrCO –Co O catalysts
3 3 4
ꢀ
ꢀ
ꢀ
increased from 100 C to 400 C with a step size of 20 C, and at each
typical temperature point, the reaction conditions were stabilised for 40
min. The composition of effluent gas was analysed online continuously
every 4 min by a Ruimin 2060 gas chromatograph. The vinyl chloride
SrCO
3
–Co
3
O
4
composite catalysts were prepared through a copreci-
2
and Co(NO) ⋅H O
pitation method. Typically, stoichiometric Sr(NO)
2
2
were first added to 100 mL of deionized water, and then 50 mL of
ammonium oxalate solution was added dropwise until the colour of the
mixed solution became clear. After the resulting solution was aged for 8 h
at room temperature, the precipitate was collected by filtration, washed
with deionized water, dried at 60 C for 12 h, and then calcined at 500 C
for 3 h in air. The obtained catalysts were marked as SrCo-x, in which x
presented molar ratios of Sr and Co of 0.2, 0.4 and 1, respectively. The
(
VC) conversion was determined by the following equation:
½
VCꢂ ꢁ ½VCꢂ
in
out
XVCð%Þ ¼
ꢃ 100
ꢀ
ꢀ
½VCꢂin
where ½VCꢂ and ½VCꢂ denote the concentration of vinyl chloride in the
in
out
inlet and off-gas, respectively. The selectivity of polychlorinated
byproducts (SPB) over the samples was determined by the equation:
3 3 4
SrCO and Co O samples were also synthesized by the same method as
the reference catalysts.
ð½CH
2
Cl
2
ꢂ þ ½C
2
H
2
Cl
2
ꢂ þ ½C
2
H
4
Cl
2
ꢂÞ ꢃ 2 þ ½C
2
H
3
Cl
3
ꢂ ꢃ 3 þ ½CCl ꢂ ꢃ 4
4
S
PB
¼
2
.2. Characterization of catalysts
½
VCꢂ ꢃ R
T
in
All the SrCO
diffraction (XRD) technology on a Bruker AXS D8 Focus diffractometer
with Cu K radiation (40 kV and 40 mA) to confirm their crystal struc-
ture. Standard database cards (JCPDS) were used to identify the phases of
the catalysts. N adsorption and desorption measurements were pro-
cessed to detect the specific surface area. The experiment was conducted
at 77 K on a Micromeritics ASAP 2020 M apparatus. The specific surface
area (SSA) of the catalyst was calculated by the Brunauer-Emmett-Teller
3 3 4
–Co O composite catalysts were analysed using X-ray
where SPB denotes the selectivity of the total chlorine byproducts;
CH2Cl2ꢂ, ½C2H2Cl2ꢂ, ½C2H4Cl2ꢂ, ½C2H3Cl3ꢂ and ½CCl4ꢂ denote the concen-
trations of CH Cl , C Cl , C Cl , C Cl and CCl in the outlet gas,
½
α
2
2
H
2 2
2
2
H
4
2
2
H
3
3
4
respectively; ½VCꢂ denotes the concentration of vinyl chloride in the
in
2
inlet gas; and RT denotes the conversion of vinyl chloride at a specific
temperature.
The reaction rates of VC oxidation over the SrCo catalysts were
calculated using the following equation:
(
4
BET) method. The SEM images were obtained using a Nova Nano SEM
50 microscope operated at 15 kV. The morphology of the catalysts was
ðVVC ꢄ 22:4Þ ꢃ X
T
Rate ¼
detected by scanning transmission electron microscopy (STEM) with a
ThermoFisher Talos F200X and high angle annular dark-field (HAADF)-
STEM with a convergence semi-angle of 11 mrad and inner and outer
m
where VVC denotes the volume of VC in the inlet gas; XT denotes the
2

H. Liu et al.
Journal of the Indian Chemical Society 98 (2021) 100116
Fig. 1. (a) VC conversion curves over the SrCO
3
–Co
3
O
4
composite catalysts with different molar ratios of Sr/Co. (b) Concentrations of chlorinated byproducts as a
function of temperature over the (b) Co
SrCo-0.4 catalysts. The feed gas consisted of 1000 ppm VC and air balance, and the WHSV was 30,000 mL g
3 4 3 4
O and (c) SrCo-0.4 catalysts.(d)The selectivity to polychlorinated byproducts as a function of temperature over the Co O and
ꢁ1
ꢁ1
h
.
introduced into the Co
3 4
O catalyst, the catalytic performance of the
Table 1
SrCO –Co composite catalysts was greatly improved. Specifically,
3
3 4
O
Comparison of catalytic activity of the SrCo-0.4 catalyst with those in the ref-
erences for VC oxidation.
according to the T50, their catalytic performance decreased in the
following order: SrCo-0.4 > SrCo-0.2 > SrCo-1. Nearly 100% VC con-
Catalysts
VC
WHSV
Conversion
temperature
( C)
References
ꢀ
version was obtained at 280 C, and 50% VC conversion was obtained at
(mL⋅g 1⋅hꢁ1
ꢁ
)
concentration
ꢀ
2
32 C over the SrCo-0.4 catalyst. As shown in Fig. S1, only CO
2
product
ꢀ
(ppm)
was detected over the SrCo-0.4 catalyst during VC oxidation reaction.
Furthermore, as shown in Table 1, it is noted that compared with other
transition metal catalysts in previous literature, the SrCO –Co O com-
3 3 4
Co
3
O
4
-TP
1000
1000
1000
1000
1000
30,000
12,000
48,000
15,000
30,000
240 (T50
67 (T90
)
)
)
)
)
)
)
)
)
)
[22]
[24]
[25]
[26]
[27]
2
Co(0.7)
CeO
Co
cubic
Co -AP
249 (T50
281 (T90
308 (T50
340 (T90
251 (T50
posite catalysts had a higher catalytic activity for VC [22,24–27], which
was comparable to that of costly precious metal catalysts (Table 1).
On the other hand, for the SrCo-0.2 and SrCo-1 catalysts, a slight
decrease in VC activity appeared in the temperature ranges of 300–360
x
3
O
4
-
3
O
4
2
75 (T90
ꢀ
and 320–380 C, respectively (Fig. 1a). This abnormal degradation of VC
RuCoO
Al
hms
SrCo-0.4
x
/
-
310 (T50
345 (T90
2
O
3
conversion can be attributed to the accumulation of hydrochloric acid on
the catalyst surface and the subsequent destruction of active surface sites
1000
30,000
232 (T50
)
)
This work
[
28]. In contrast, no deactivation occurred during the reaction process
2
56 (T90
over the SrCo-0.4 catalyst. However, as shown in Fig. S2a, a slight
deactivation was observed on the SrCo-0.4 catalyst during the two cycle
tests for VC oxidation. XRD results (Fig. S2b) showed that the structure of
conversion of VC at a specific temperature, and m denotes the mass of
catalyst. For the calculation of the apparent activation energy, the con-
version rate was controlled below 15% to ensure no mass/energy transfer
limitation.
SrCO
phase of SrCO
2
that SrCO reacted with HCl to produce SrCl .
3
in the SrCo-0.4 catalyst was destroyed after two cycle tests. The
3
disappeared and the phase of SrCl appeared, indicating
2
3
The production of polychlorinated byproducts in the process of
3. Results
CVOCs oxidation was inevitable [29–32]. Thus, the distribution of pol-
ychlorinated byproducts generated over the Co
in the reaction process was investigated (Fig. 1b–d). For the Co
catalyst, five byproducts were detected, namely, CH Cl , C Cl
Cl , CCl and CH Cl , and C Cl and C Cl were dominant. As
proposed in previous works, the production of polychlorinated byprod-
ucts over the Co catalyst generally followed the ionic chlorination
3 4
O and SrCo-0.4 catalysts
3
.1. Activity and stability
3 4
O
2
2
2
H
2
2
,
Fig. 1a shows the catalytic activity of the SrCO
3
–Co
3
O
4
composite
oxide
oxide exhibited high
C
2
H
4
2
4
3
3
2
H
4
2
2
H
3
3
catalysts for the catalytic oxidation of vinyl chloride. The SrCO
was inactive for VC oxidation. In contrast, the Co
3
O
3 4
3 4
O
catalytic activity for VC oxidation, over which the T50 (the temperature
needed for 50% conversion) was 264 C. After strontium carbonate was
pathway during the VC oxidation process. Electrophilic reactions easily
occur on Co-based catalysts (or other transition metal catalysts) due to
ꢀ
3

H. Liu et al.
Journal of the Indian Chemical Society 98 (2021) 100116
Fig. 2. SEM images of the (a–b) Co
3
4 3
O , (c–d) SrCo-0.2, (e–f) SrCo-0.4, (g–h) SrCo-1 and (i–j) SrCO catalysts.
4

H. Liu et al.
Journal of the Indian Chemical Society 98 (2021) 100116
Table 2
increased after the introduction of SrCO
Fig. 3a shows the XRD patterns of the SrCO –Co
3 4
lysts with various ratios of Sr/Co. For the Co O catalyst, there were six
3
.
Physicochemical properties and redox ability of the SrCO
catalysts.
3
–Co
3
O
4
composite
3 4
O composite cata-
3
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Catalysts
S
(
BET
2
XPS data
H
g)
2
consumption (mmol/
diffraction peaks at 19.0 , 31.3 , 36.9 , 44.9 , 59.3 and 65.4 , assigned
to a typical spinel structure (JCPDS no. 74–1656) [35]. For the SrCO
catalyst, the diffraction peaks completely matched the standard card of
SrCO (JCPDS no. 05–0418), indicating that SrCO was successfully
synthesized [36]. For the SrCo-0.2, SrCo-0.4 and SrCo-1 composite cat-
m ⋅g )a
ꢁ1
b
3
Co2
Co
þ
3þ
3þ
2þ
2þ
3þ
BE of
BE of
Co
(eV)
/
Co
→
Co
Co
→ Co
Co
Co
/
2
þ
3þ
0
Co
eV)
3
3
2
þ
(
SrCO
SrCo-1
SrCo-0.4
SrCo-0.2
3
8.0
–
–
–
–
–
–
alysts, compared with the Co
3
O
4
catalyst, the intensity of the charac-
ꢀ
25.1
40.8
53.4
63.5
781.3
781.4
781.3
781.2
779.7
779.8
779.7
779.6
0.62
0.90
0.68
0.55
3.94
3.58
3.81
4.22
12.52
12.93
12.75
12.35
0.59
0.81
0.67
0.46
terized diffraction peak at 36.9 gradually decreased, and several peaks
ꢀ
at approximately 25.3 appeared and were enhanced with increasing
SrCO
SrCo composite catalysts, and the crystallinity of each oxide decreased by
mixing with them. In other words, the introduction of SrCO could
restrain the crystallization of Co , and vice versa.
To further determine the structure of the SrCO –Co
FTIR spectra of all the catalysts were recorded at 400–3000 cm and are
3 3 4 3
content. In conclusion, the Co O and SrCO phases existed in the
Co O
3 4
a
Measured by the BET method.
3
b
Calculated from H
Co content.
2
-TPR profiles after deconvolution and normalization using
3 4
O
3
3 4
O catalysts, the
ꢁ1
the chlorination of metal atoms into metal chlorides (such as Co-Cl) [33,
4]. In other words, the formation of a large number of polychlorinated
alkane byproducts was attributed to the chlorination of partial active
sites on the Co surface. However, only four byproducts were detected
over the SrCo-0.4 catalyst, and their amounts dramatically decreased
compared with those over the Co catalyst (Fig. 1c). As shown in
Fig. 1d, the selectivity of the SrCo-0.4 catalyst towards the poly-
chlorinated byproducts was much lower than that of the cobalt oxide
catalyst, indicating that the introduction of strontium carbonate could
restrain the formation of chlorinated byproducts. To reveal the detailed
mechanism of their different VC oxidation behaviour, the structural and
shown in Fig. 3b. For the Co
3
O
4
catalyst, two typical adsorption peaks
3
ꢁ1
2þ
appeared at 660 and 559 cm , assigned to the vibrations of Co -O
3þ
(
octahedral site) and Co -O (tetrahedral site) [37]. For the SrCO
3
3 4
O
catalyst, there was one strong adsorption peak at 1457 and another weak
ꢁ1
2ꢁ
adsorption peak at 859 cm , assigned to the bending vibration of CO3
in SrCO [38]. For the SrCo-0.2, SrCo-0.4 and SrCo-1 catalysts, the
3 4
O
3
adsorption peaks of both the Co
3
O
4
and SrCO
3
phases appeared at 1457,
ꢁ1
8
59, 660 and 559 cm . Furthermore, the intensity of peaks corre-
sponding to Co
3 4 3
O decreased with increasing SrCO content, while that
2ꢁ
corresponding to the CO bending vibration increased at the same time.
3
These FTIR results confirmed that the crystallinity of both Co
SrCO in the SrCO –Co catalysts was lower than that in pure Co
and SrCO according to the XRD results. To further verify the structure of
the SrCO –Co catalysts, the distribution of elements and composition
of the SrCO –Co catalysts were also analysed by EDS mapping. The
results shown in Fig. 4 confirmed that Co and SrCO phases were
present for the SrCO –Co catalysts. In conclusion, the SrCO –Co
catalysts contained both Co and SrCO phases, while their crystal-
linity decreased due to variable interactions.
3 4
O and
physicochemical characteristics of the SrCO
3
–Co
O
3 4
composite samples
3
3
O
3 4
3 4
O
were examined by various characterization methods.
3
3
3 4
O
3
3
.2. Catalyst characterization
3
3 4
O
3
O
4
3
.2.1. Structural and compositional analysis
3
3
O
4
3
3 4
O
The morphology of the SrCO
3
–Co
O
3 4
composite samples was deter-
3
O
4
3
mined by scanning electron microscopy (SEM). The Co
showed slender rod-shaped particles of approximately 5 m, as shown in
Fig. 2a and b. When SrCO was introduced (Fig. 2c and d), slender
sticklike particles mainly existed for the SrCo-0.2 catalyst. With
increasing SrCO content, new small octahedral particles assigned to the
SrCO phase appeared (Fig. 2e–h). Moreover, the number of slender
sticklike particles assigned to Co decreased. As shown in Fig. 2i and j,
small particles with various diameters between 1–3 m were mainly
observed for pure SrCO Therefore, it was concluded that the
morphology of the SrCO –Co catalysts was dependent on their
3 4
O catalyst
μ
3
3.2.2. XPS
The electronic states of catalyst surface atoms usually have a crucial
3
effect on their catalytic performance [21,39,40]. XPS technology was
conducted to detect the change in the valence states of Sr and Co on the
catalyst surface. Fig. 5a shows Co 2p3/2 XPS spectra for the
3
3 4
O
μ
3 3 4
SrCO –Co O catalysts with different molar ratios of Sr/Co. The spectra
3þ
2þ
3
.
were deconvoluted and fitted to six peaks, representing Co , Co , and
two satellite peaks as marked. Additionally, the binding energy of Co
3þ
3
3 4
O
composition. The specific surface area of all catalysts was detected and is
gradually shifted from 779.6 eV (Co
3
O
4
) to 779.8 eV (SrCo-0.4), and the
) to 781.4
2
binding energy of Co2 gradually shifted from 781.2 eV (Co
þ
3 4
shown in Table 2. Pure Co O had a high specific surface area of 63.5 m
3
O
4
ꢁ
1
2
ꢁ1
g
, while the SrCO
3
catalyst had a low specific surface area of 8 m g .
eV (SrCo-0.4), indicating that the electron density of Co slightly
increased following the introduction of Sr [41]. The binding energy of Co
The specific surface area of the SrCO
3
2
–Co
g
3
O
4
catalysts gradually
2
ꢁ1
ꢁ1
decreased from 63.5 m
g
to 25.1 m
as the SrCO content
3
Fig. 3. (a) XRD patterns and (b) FTIR spectra of the SrCO
3
3 4
–Co O composite catalysts with different molar ratios of Sr/Co.
5

H. Liu et al.
Journal of the Indian Chemical Society 98 (2021) 100116
3 3 4
Fig. 4. EDS mapping images of the SrCO –Co O composite catalysts with different molar ratios of Sr/Co.
for the SrCo-1 catalyst was lower than that of the SrCo-0.4 catalyst, which
might be explained by reducing the interaction between them based on
3.2.3. H
To reveal the redox properties of the SrCO
technologies were operated in the temperature range of 100–500 C. As
shown in Fig. 6a, no reduction peaks appeared over the pure SrCO
2
-TPR and VC-TPD
3
3 4 2
–Co O catalysts, H -TPR
the excessive proportion of Sr/Co. As a result, the ratio of Co2þ/Co on
3þ
ꢀ
the SrCo-x catalyst surface increased after the introduction of SrCO
3
and
3
reached the maximum value for the SrCo-0.4 catalyst due to its strongest
interaction between Sr and Co-based on the optimized composition. On
the other hand, the signal strength of Sr 3d for the SrCo-x catalysts
decreased with increasing Co content (Fig. 5b). Moreover, the binding
catalyst, while the pure Co
3
O
4
catalyst showed two overlapping reduc-
ꢀ
ꢀ
tion peaks at 298 C and 369 C, which were ascribed to the reduction of
3þ
2þ
2þ
0
Co to Co and Co to Co , respectively [25]. After SrCO
3
was added,
ꢀ
the two reduction peaks shifted towards lower temperatures of 279
C
ꢀ
energy of Sr 3d5/2 shifted from 133.4 eV (SrCO
42,43]. In conclusion, the strong interaction between SrCO
with electron transfer from Sr to Co was proved, which was beneficial to
3
) to 133.6 eV (SrCo-0.2)
and 352 C for the SrCo-0.2 catalyst. For the SrCo-0.4 catalyst, two
ꢀ
[
3
3
and Co O
4
reduction peaks further moved towards low temperatures of 260 C and
ꢀ
330 C. However, for the SrCo-1 catalyst, two reduction peaks moved to
2þ
3ꢁδ
ꢀ
ꢀ
producing a lower valence state of Co, such as more Co or Co
[44].
high temperatures of 289 C and 350 C. On the other hand, the area of
6

H. Liu et al.
Journal of the Indian Chemical Society 98 (2021) 100116
3 3 4
Fig. 5. XPS spectra of (a) Co 2p and (b) Sr 3d for the SrCO –Co O composite catalysts with different molar ratios of Sr/Co.
2 3 3 4
Fig. 6. (a) H -TPR and (b) VC-TPD profiles of the SrCO –Co O composite catalysts with different molar ratios of Sr/Co.
reduction peaks decreased with increasing SrCO
3
content (Table 2),
3 3 4
SrCO –Co O catalysts by tuning the crystallinity and electronic state of
which was ascribed to the decrease in Co content. It was noted that,
compared with the area of reduction peaks, the reduction temperature
was the decisive factor affecting the reducibility of catalysts [8,18,22].
Co on the surface of the catalyst.
4
. Discussion
Therefore, the reducibility of the SrCO
following order: SrCo-0.4 > SrCo-0.2 > SrCo-1 > Co
SrCO could enhance the reducibility of the Co catalyst, even if it had
no redox property. The highest reducibility of the SrCo-0.4 catalyst might
be ascribed to a strong interaction between SrCO and Co based on
the optimized composition, as shown in the XPS results. Furthermore, the
actual H consumption of the SrCO –Co catalysts was carefully
calculated after deconvolution and normalization using Co content. In
3
3
–Co O
4
catalysts declined in the
3 4
O . Interestingly,
The VC adsorption capacity and redox ability play decisive roles in VC
3
3 4
O
oxidation [21]. VC adsorption was the first step of the oxidation reaction,
facilitating the subsequent dechlorination reaction. The redox ability was
responsible for the oxidation of the intermediate species generated after
dechlorination. Therefore, enhancing the capability for VC adsorption
3
3 4
O
2
3
O
3 4
and the reducibility for Co
performance for VC oxidation.
SrCO –Co composite catalysts with different ratios of Sr/Co were
synthesized by the coprecipitation method. XRD, FTIR and SEM results
revealed that the SrCO –Co catalysts were composed of SrCO and
Co phases, and the presence of SrCO could restrain the crystalliza-
tion of Co , possibly due to intimate contact between the two com-
ponents. Furthermore, XPS results showed that electron transfer from Sr
to Co was present on the SrCO –Co catalysts, generating electron-rich
surface. To further explore the impact
on the chemical properties of the SrCO –Co com-
-TPR and VC-TPD technologies were used to determine
the redox properties and VC adsorption capacity of the samples,
respectively. H -TPR results indicated that SrCO could promote the
reducibility of the Co catalyst, and the SrCo-0.4 catalyst showed the
strongest reducibility among the SrCo catalysts. Moreover, the intro-
duction of SrCO could enhance the VC adsorption ability of the SrCo-0.2
3 4
O is a key issue to improve its catalytic
2þ
3þ
conclusion, the Co /Co ratio decreased in the following order:
3
3 4
O
3 4
SrCo-0.4 (0.81) > SrCo-0.2 (0.67) > SrCo-1 (0.59) > Co O (0.46), in
according with the XPS results.
In addition to redox properties, the adsorption capacity of the catalyst
for vinyl chloride (VC) also plays a critical role in VC oxidation. Hence, a
VC-TPD experiment was implemented to determine the VC adsorption
3
O
3 4
3
O
3 4
3
3 4
O
capacity of the SrCO
at 141, 352 and 403 C for the Co
3
–Co
3
O
4
samples. Three overlapping peaks appeared
3
3 4
O
ꢀ
O
3 4
catalyst (Fig. 6b). After the addition
2þ
3ꢁδ
Co (Co and Co ) on the Co
of adding SrCO
posite samples, H
3 4
O
ꢀ
of SrCO
3
, the VC desorption peak at 141 C disappeared for the SrCo-0.2
3
3
3 4
O
ꢀ
catalyst, and the area of the peak at 403 C significantly increased. For
2
the SrCo-0.4 catalyst, the desorption curve was similar to that for the
ꢀ
SrCo-0.2 catalyst, except for the larger peak area at 403 C. However, as
2
3
ꢀ
the SrCO
3
content further increased, only one peak at 352 C appeared
3 4
O
ꢀ
for the SrCo-1 sample, and the desorption peak at 403 C disappeared.
The total peak area decreased in the following order: SrCo-0.4 (2.41) >
3
SrCo-0.2 (1.76) > Co
3 4
O (1.32) > SrCo-1 (0.97). The low amount of VC
and SrCo-0.4 catalysts.
In summary, for the SrCo-0.2 and SrCo-0.4 catalysts, the incorpora-
3
tion of SrCO promoted the reducibility and VC adsorption capacity of
the catalysts by reducing the crystallinity and tuning the state of Co on
the surface of the catalyst, resulting in significant promotion of the cat-
a
alytic activity for vinyl chloride oxidation. The E for VC oxidation
desorption for the SrCo-1 catalyst was ascribed to the lower Co content of
the catalyst. Therefore, the VC adsorption ability of the SrCO
samples decreased in the following order: SrCo-0.4 > SrCo-0.2 > Co
SrCo-1 > SrCO . Although the SrCO
vinyl chloride, it could enhance the VC adsorption capacity of the
3
3
–Co O
4
3
O
4
>
3
3
catalyst was unable to adsorb
7

H. Liu et al.
Journal of the Indian Chemical Society 98 (2021) 100116
Declaration of competing interest
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.
Acknowledgments
This study was financially supported by the National Key Research
and Development Program of China (2016YFC0204300), the National
Natural Science Foundation of China (21922602, 22076047, 21976057),
the Shanghai Science and Technology Innovation Action Plan
(
20dz1204200) and Fundamental Research Funds for the Central
Universities.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://do
i.org/10.1016/j.jics.2021.100116.
3 3 4
Fig. 7. Arrhenius plots for VC oxidation over the SrCO –Co O composite cat-
alysts with different molar ratios of Sr/Co.
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