The performance and mechanism for the catalytic oxidation of dibromomethane (CH2Br2) over Co3O4/TiO2 catalysts

Article abstract of DOI:10.1039/c6ra00372a

Brominated hydrocarbons are a typical pollutant in exhaust gas from the synthesis process of Purified Terephthalic Acid (PTA), and may cause various environmental problems once emitted into the atmosphere. Dibromomethane (DBM) was employed as the model compound in this study, and a series of Co₃O₄/TiO₂ catalysts with various Co contents were prepared for the catalytic oxidation of DBM. The prepared catalysts were characterized by XRD, BET, SEM, TEM, XPS, H₂-TPR and NH₃-TPD. Among the prepared catalysts, CoTi-5 (5 wt% Co/TiO₂) showed the highest catalytic activity, with T₉₀ at about 346 °C, which was mainly attributed to the enrichment of well-dispersed Co₃O₄ and the high surface Co³⁺/Co²⁺ ratio, as it could provide more surface active sites and active oxygen species. The kinetic study showed that the reaction order of DBM was pseudo first-order and the reaction order of oxygen was approximately zero-order. A plausible DBM reaction mechanism over Co₃O₄/TiO₂ catalysts was also proposed based on the results of in situ FTIR and the analysis of gas products by GC-MS. The reaction process started with the adsorption on surface oxygen vacancies, breakage of C-Br bonds and partial dissociation of C-H bonds with the formation of intermediate species, and then the intermediate species were further oxidized to form CO and CO₂.

Full text of DOI:10.1039/c6ra00372a

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The performance and mechanism for the catalytic
oxidation of dibromomethane (CH Br ) over
2
2
Cite this: RSC Adv., 2016, 6, 31181
Co O /TiO catalysts†
3
4
2
Jian Mei, Songjian Zhao, Haomiao Xu, Zan Qu and Naiqiang Yan*
Brominated hydrocarbons are a typical pollutant in exhaust gas from the synthesis process of Purified
Terephthalic Acid (PTA), and may cause various environmental problems once emitted into the
atmosphere. Dibromomethane (DBM) was employed as the model compound in this study, and a series
of Co
3
O
4
/TiO
2
catalysts with various Co contents were prepared for the catalytic oxidation of DBM. The
-TPR and NH -TPD. Among the
prepared catalysts were characterized by XRD, BET, SEM, TEM, XPS, H
2
3
prepared catalysts, CoTi-5 (5 wt% Co/TiO
2
) showed the highest catalytic activity, with T90 at about 346
3+
ꢀ
3 4
C, which was mainly attributed to the enrichment of well-dispersed Co O and the high surface Co /
2+
Co ratio, as it could provide more surface active sites and active oxygen species. The kinetic study
showed that the reaction order of DBM was pseudo first-order and the reaction order of oxygen was
3 4 2
approximately zero-order. A plausible DBM reaction mechanism over Co O /TiO catalysts was also
Received 6th January 2016
Accepted 15th March 2016
proposed based on the results of in situ FTIR and the analysis of gas products by GC-MS. The reaction
process started with the adsorption on surface oxygen vacancies, breakage of C–Br bonds and partial
dissociation of C–H bonds with the formation of intermediate species, and then the intermediate species
DOI: 10.1039/c6ra00372a
www.rsc.org/advances
2
were further oxidized to form CO and CO .
this technology is the available of high activity and high stability
catalysts on the removal of brominated hydrocarbon.
1
. Introduction
In recent years, China has become the largest producer and
At present, various catalytic materials have been used as
consumer of Puried Terephthalic Acid (PTA) in the world. With catalysts for the catalytic oxidation of brominated hydrocarbon,
4
,5
the expansion of the PTA production scale, concerns have been including noble metal and transition metal oxide catalysts.
1
raised about atmospheric emission from PTA exhaust gas, and Although noble mental catalysts exhibit excellent activities, the
protests against such projects by several large groups have taken high cost and low thermal stability restrain their wide applica-
place in China in the past years. Typically, the untreated PTA tions on industries. Hence, further efforts are demanded to
exhaust gas is a mixture of various organic components, develop the cheap catalysts with high activity and high stability.
including carbon monoxide, acetic acid methyl ester, p-xylene As a typical transition metal oxide, cobalt oxides have been widely
(
PX) and brominated hydrocarbon, which can cause great studied for low temperature catalytic oxidation. For example,
2
6
threats to the environment and human health once emitted.
Rivas et al. studied the different synthesis routes of nano-
, and found that the nanocrystalline Co
Up to now, the treatment methods of PTA exhaust gas mainly crystalline Co
O
3 4
3 4
O
include activated carbon adsorption, the thermal oxidation prepared by precipitation method decomposed 1,2-dichloro-
method and catalytic oxidation. Catalytic oxidation is consid- ethane at the lowest temperature, no appreciable deactivation
ered to be the most promising technology for removal of PTA with time on stream. As a halogenated hydrocarbon, brominated
exhaust gas due to its low consumption of energy, high effi- hydrocarbon may be similar to chlorinated hydrocarbon in the
ciency and its excellent selectivity towards the formation of the certain properties. Therefore, Co
O
may be a good candidate for
less harmless products. However, the presence of brominated the catalytic oxidation of brominated hydrocarbon.
hydrocarbon easily causes the catalysts bromine poisoning, and
In this study, dibromomethane (DBM) chosen as a model
then the catalysts become deactivation. Thus, the key issue of brominated hydrocarbon was evaluated in a xed-bed ow
reactor. A series of Co /TiO catalysts with various Co
3
4
3
3
O
4
2
contents were prepared by a conventional impregnation
method. The physical and chemical properties of the catalysts,
as well as DBM conversion, were investigated. The product
distribution and catalyst stability were studied, and kinetic
parameters of DBM oxidation reaction were also calculated.
School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800
Dong Chuan Road, Shanghai, 200240, PR China. E-mail: nqyan@sjtu.edu.cn; Fax: +86
2
†
1
1 54745591; Tel: +86 21 54745591
Electronic supplementary information (ESI) available. See DOI:
0.1039/c6ra00372a
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Furthermore, based on the results of in situ FTIR and the mode with a 20 min plateau at each temperature investigated.
analysis of gas products by GC-MS, a plausible DBM reaction The inlet and outlet gases were analyzed by a gas chromato-
mechanism over Co O /TiO catalysts was proposed.
graph equipped with a ame ionization detector. The removal
efficiency of DBM was dened as:
3
4
2
2
. Experimental section
C
0
ꢁ C
h ¼
ꢂ 100%
(1)
C
0
2
.1 Catalyst preparation
A series of Co /TiO catalysts with various Co contents (1, 2.5,
, 10 and 25 wt%, calculated by the weight percentage of Co/
where C
0
and C were the inlet and outlet DBM concentrations,
3
O
4
2
respectively. The gas products were detected by gas chroma-
tography and mass spectrometry (GCMS-QP2010, Shimadzu).
The nal products CO and CO2 were measured using a gas
chromatograph (GC-14B, Shimadzu) equipped with a FID
detector with methane conversion oven. Br
absorbed in a KI solution rstly. Then, Br concentration was
5
TiO ) were prepared by a conventional impregnation method.
Commercial P25 TiO (Degussa; 80% anatase and 20% rutile
phases) was used as a support. In a typical preparation,
Co(NO
2
2
2
and HBr were
3
)
3
$6H
2
O used as the Co
3
O
4
precursor was dissolved in
$6H
2
ultrapure water, then P25 TiO
2
was added into Co(NO
3
)
3
2
O
determined by the titration with sodium thiosulfate using
starch solution as an indicator. The concentration of bromide
ions in the bubbled solution was determined by using an ion
chromatography. The selectivity of CO, CO , Br and HBr was
solution and stirred for 4 h. All the water was evaporated using
a rotary evaporation apparatus. The obtained powers were dried
at 60 C overnight and calcined in air at 500 C for 3 h. The
samples were marked for CoTi-1, CoTi-2.5, CoTi-5, CoTi-10 and
CoTi-25, respectively.
ꢀ
ꢀ
2
2
dened as, respectively:
½
COꢃ
S
CO
¼
¼
ꢂ 100%
(2)
(3)
(4)
2
.2 Catalyst characterization
½C
T
ꢃ
The X-ray diffraction (XRD) measurements were recorded on
a Shimadzu XRD-6100, and the data were collected in the 2q
range 10–80 with a scanning velocity of 10 min . The BET
½
CO
2
ꢃ
SCO2
ꢂ 100%
ꢀ
ꢀ
ꢁ1
½
C
T
ꢃ
(Brunauer–Emmett–Teller) surface areas of the samples were
½
½
Br
2
ꢃ
ꢃ
ꢀ
determined using N
2
adsorption at ꢁ196 C and using a quartz
S
Br2
¼
ꢂ 100%
C
Br
tube (Quantachrome 2200e). The morphology and microstruc-
ture of the samples were analyzed by scanning electron micro-
scope (SEM) (JSM-7800F) and transmission electron microscopy
½
HBrꢃ
Br
S
HBr
¼
ꢂ 100%
(5)
½
C
ꢃ
(TEM) (JEOL-2100F). The samples were dispersed in ethanol
where [CO] and [CO
aer the reaction, [C
oxidized DBM during the reaction, which could be calculated by
eqn (6):
2
] were the amount of CO and CO
] was the amount of carbon atom (ppm) in
2
(ppm)
with strong sonication before analysis. The H -TPR experiments
2
were investigated on the chemisorption analyzer (2920,
AutoChem II, Micromeritics). Prior to the measurement, the
T
ꢀ
samples were pretreated in Ar stream at 300 C for 2 h. Aer
ꢀ
ꢀ
cooling down to 50 C, the samples were heated to 1000 C with
ꢀ
ꢁ1
[C
T
] ¼ ([CH ꢁ [CH
Br
2
]
2
0
Br
2
])
2
(6)
2 3
temperature ramp of 10 C min in 10% H /Ar. The NH -TPD
experiments were also investigated on the chemisorption
analyzer (2920, AutoChem II, Micromeritics). Prior to the
where [CH
2
Br
2
]
0
and [CH Br ] were the amount of DBM (ppm)
2
2
in the gas stream before and aer the reaction, respectively.
Br ] and [HBr] were the amount of Br and HBr (ppm) aer the
measurement, the 200 mg samples were pretreated in He
[
2
2
ꢀ
stream at 400 C for 2 h. Then NH adsorption was performed at
3
reaction, [CBr] was the amount of bromine atom (ppm) in
oxidized DBM during the reaction, which could be calculated by
eqn (7):
ꢀ
5
0 C with 10% NH
3
/He for 1 h. Next, the samples were purged
3
in He stream for 30 min to remove physically adsorbed NH ,
ꢀ
ꢁ1
ꢀ
and then heated to 600 C with temperature ramp of 10
C
ꢁ
1
min with a 50 mL min He ow. The X-ray photoelectron
spectroscopy (XPS) results were recorded with an AXIS UItra
DLD (Shimadzu-Kratos) spectrometer with Al Ka as the excita-
tion source, and the C 1s line at 284.8 eV was used as a reference
for the binding energy calibration.
[
C
Br] ¼ ([CH
2
Br
2
]
0
ꢁ [CH
2
Br
2
]) ꢂ 2
(7)
2.4 Kinetic study
2
.3 Catalytic activity evaluation
The external diffusion effect was investigated by varying the
ꢁ
1
The performances of the catalysts for DBM oxidation were speed of feed gas while keeping a constant GHSV (60 000 h ).
tested in a xed-bed quartz tube reactor (i.d 6 mm) containing Similarly, the internal diffusion effect was checked by changing
130 mg catalyst. The typical reactant gas stream was composed the particle size of the catalysts (as shown in Fig. S2†). The
of 500 ppm DBM, 10% O and N as the balance gas in 150 mL results showed that the catalytic reaction was operated without
2
2
ꢁ
1
min , corresponding to a gas hourly space velocity (GHSV) of diffusional limitation below 10% conversion of DBM. The
0 000 h . The reaction was run from 150 to 450 C in a step kinetic experiments were carried out under differential
ꢁ1
ꢀ
6
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condition (the maximum conversion was controlled below CoTi-2.5 > CoTi-10 > CoTi-25 > CoTi-1 > TiO
2
. Pure TiO
2
ꢀ
10%). And the reaction rate was calculated as follows:
exhibited the lowest activity with T50 > 450 C. However, with the
presence of Co O species on TiO , the activities of catalysts
3
4
2
dx
x
ꢁ
r ¼ F ꢂ
z F ꢂ
(8)
were remarkably improved and the conversion curves shied
into lower temperature. Moreover, it was found that Co /TiO
dW
w
3
O
4
2
where ꢁr (mol gꢁ
1
s
ꢁ1
) was the reaction rate, F (mol s ) was the
inlet ow rate, x (%) was the conversion of DBM, and W was the
ꢁ1
catalysts exhibited a rising conversion with the increase of Co
content from 0% to 5%. CoTi-5 showed the highest activity, and
catalyst weight (g).
ꢀ
the T90 value was about 346 C. However, further increasing the
3 4 2
Co content to 25%, Co O /TiO catalysts showed a declined
catalytic activity. This result revealed that there were not enough
active sites on the surface of the catalyst when the Co content
was below 5%, and the catalytic activity dropped. Nevertheless,
2
.5 In situ FTIR experiments
In situ FTIR spectra were recorded on a Fourier transform
infrared spectrometer (FTIR, Nicolet 6700) equipped with
a smart collector and an MCT detector cooled by liquid N . The
2
diffuse reectance FTIR measurements were carried out in
a high-temperature cell with ZnSe windows. Mass ow
controllers and a sample temperature controller were used to
simulate the real reaction conditions. In situ FTIR spectra were
when the Co content was above 5%, the Co
3 4
O particles
dispersed on the surface began to agglomerate, and the Co
3 4
O
particles on the surface overlapped and the number of active
sites was reduced, which decreased the catalytic activity.
ꢁ1
recorded by accumulating 100 scans at a resolution of 4 cm
.
3.2 Catalyst characterization
Prior to each experiment, all the samples were pretreated with
3 4 2
Fig. 2 shows the XRD patterns of Co O /TiO catalysts. For all
ꢀ
10% O
2
/N
2
at 400 C for 2 h. Spectra of the clean catalyst surface
the catalysts, the pure Co phase identied was spinel-structured
Co O (PDF#43-1003). The other observed diffraction peaks
were collected and used as the background at different
3
4
temperature. Then, a 500 ppm DBM/10% O
introduced into the IR cell at 50 C for 1 h. Subsequently, the
2
/N
2
stream was
were attributed to TiO , which were in the form of anatase and
2
ꢀ
rutile. No representative diffraction peaks of Co
observed when Co content was less than 5%. The Co
3
O
O
3 4
4
were
crys-
catalyst was treated in owing 10% O /N at different temper-
2
2
ature. In situ FTIR spectra were collected at different tempera-
ture where a steady was reached.
talline might be formed in CoTi-5 and some small characteristic
peaks were observed, which might be due to its small size or
7
well dispersed state. Only when the Co content exceeded 10%,
3 4
the remarkable XRD peaks representing Co O could be
observed in the patterns and increasing Co content enhanced
3
3
. Results and discussion
.1 Catalytic performance for DBM oxidation
Fig. 1 shows the activity of Co /TiO
catalysts for DBM Co
oxidation as a function of temperature. It is convenient to
compare the catalytic activities of these catalysts by using the pore diameter of Co
reaction temperatures T50 and T90 (corresponding to the DBM into TiO , the specic surface area began to decrease, owing to
support. Moreover, when the
the intensity of the diffraction peaks of the spinel-structured
3
O
4
2
3 4
O .
Table 1 displays the specic surface area, pore volume and
/TiO catalysts. Aer the loading of Co
3
O
4
2
2
conversion ¼ 50 and 90%). The results indicated that the a clogging or covering of the TiO
2
conversion of DBM decreased in the following order, CoTi-5 > Co content increased from 1 to 5%, the specic surface area
gradually increased. However, with further increasing the Co
Fig. 1 DBM conversion over Co
3
O
4
/TiO
2
catalysts with various Co
and N balance;
contents; gas composition: 500 ppm DBM, 10% O
2
2
ꢁ1
GHSV ¼ 60 000 h
.
3 4 2
Fig. 2 XRD patterns of Co O /TiO catalysts with various Co contents.
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Table 1 Specific surface area, pore volume and pore diameter of Co
3 4 2
O /TiO catalysts was performed. As shown in Fig. 3, the
Co
3
O
4
/TiO
2
catalysts
TEM images in Fig. 3(a–e) exhibited evenly distributed Co O /
3
4
TiO2 nanoparticles with diameters of 30–40 nm. The high-
resolution transmission electron microscopy (HRTEM) image
of CoTi-25 (Fig. 3(f)) further conrmed the crystalline structure
Specic surface
Pore volume
Pore diameter
(nm)
2
ꢁ1
3
ꢁ1
Samples
TiO
CoTi-1
CoTi-2.5
CoTi-5
area (m g
)
(cm g
)
2
53.233
34.635
36.999
40.32
32.433
29.473
0.13
23.277
25.34
24.516
24.785
24.871
23.277
of Co
O
/TiO
. As seen from the gure, the interplanar spacing
(220) and TiO
101), respectively, indicating that cobalt was loaded on the
surface of TiO and existed in the form of Co , consistent with
the XRD results.
3
4
2
0.304
0.314
0.362
0.275
0.202
of 0.28 nm and 0.35 nm corresponded to Co
(
3
O
4
2
8,9
3 4
O
CoTi-10
CoTi-25
2
XPS is a good tool to investigate the surface element
composition, element oxidation sate, and adsorbed species of
10
a material. Fig. 4 shows the Co 2p and O 1s spectra of Co
3
O
4
/
content to 25%, the specic surface area began to decrease.
CoTi-5 had the biggest specic surface area among all the
TiO catalysts. As shown in Fig. 4(a), for all the Co /TiO
catalysts with various Co contents, there were two main peaks
2
3
O
4
2
3 4 2
Co O /TiO catalysts, and it might have more active sites for
corresponding to Co 2p3/2 and Co 2p1/2 at 779.9–781.3 and
catalytic reaction.
795.1–797.2 eV, respectively, with satellite peaks. By performing
To obtain the microscopic morphology information of the
catalysts, transmission electron microscope (TEM) analysis of
peak-tting deconvolution, the main Co 2p3/2 peak could be
separated into two peaks, the lower binding energy was belong
Fig. 3 TEM images of (a) CoTi-1, (b) CoTi-2.5, (c) CoTi-5, (d) CoTi-10 and (e) CoTi-25 and HRTEM image of (f) CoTi-25.
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2
+
3+
had been demonstrated to be Co [Co ]
2
O
4
, and the more
3
+
2+
surface Co /Co ratio could lead to more anionic defects. This
had been shown to bring excess surface oxygen and promote gas
13
molecular adsorption.
As shown in Fig. 4(b), one can see an asymmetrical O 1s XPS
peak of the catalysts that could be resolved into two compo-
nents at BE ¼ 529.7–530.3 and 530.4–532.1 eV. The former was
due to the surface lattice oxygen (Olat), whereas the latter was
14–16
due to the surface adsorbed oxygen (Oads).
It was also found
from Table 2 that the surface Oads/Olat ratio decreased in the
order of CoTi-5 > CoTi-2.5 > CoTi-10 > CoTi-25 > CoTi-1. The
formation of oxygen adsorbed species was due to the presence
of surface oxygen vacancies on Co
implied that there might be the coexistence of Co and Co
ions on Co /TiO catalysts. Such a deduction was supported
3 4 2
O /TiO catalysts, which
3
+
2+
3
O
4
2
by the Co 2p3/2 XPS results of the catalysts. In general, surface
adsorbed oxygen is more active than lattice oxygen due to its
high mobility, and it plays an important role in catalytic
11
oxidation reaction. Hence, more surface adsorbed oxygen will
lead to the stronger oxidization ability, which is consistent with
our DBM oxidation results.
Fig. 5 shows the H
shown in the gure, the H
CoTi-25 exhibited two reduction peaks, which were related to
2
-TPR proles of Co
3 4 2
O /TiO catalysts. As
2
-TPR proles of CoTi-1, CoTi-10 and
3
+
2+
2+
the reduction of Co to Co at lower temperature and Co to
0
7
Co at higher temperature, respectively. Whereas the H -TPR
2
proles of CoTi-2.5 and CoTi-5 exhibited three reduction
ꢀ
ꢀ
peaks between 300 C and 500 C, the rst peak was attributed
to the reduction process of some surface oxygen, and the
second and third peaks were attributed to the reduction of
3
+
2+
2+
0
17
Co to Co and Co to Co , respectively. It could also be
observed that these reduction peaks shied to lower temper-
ꢀ
ature with an increase in the Co content. The peak at 434 C in
3 4 2
Fig. 4 XPS spectra of (a) Co 2p and (b) O 1s for Co O /TiO catalysts
with various Co contents.
CoTi-1 could be related to the reduction peak of isolated Co,
ꢀ
and the peak at 640 C could be attributed to the strong
1
8
interaction between Co and Ti. When Co content was higher
3
+
to Co , while the higher binding energy could be ascribed to
ꢀ
than 2.5%, two reduction peaks appeared at about 419 C and
2
+
11
2+
Co . A spectral feature that could distinguish between Co
3
+
2+
and Co was a satellite for Co peak, because photoemitted
electrons interacted with core vacancies and valence electrons
2
+
3+ 12
of Co but not with those of Co . As shown in Table 2, the
3
+
2+
surface Co /Co ratio increased from 0.23 to 1.23 as Co
3
+
content from 1 to 5%, indicating an increase of Co percentage
on the catalyst surface. However, the surface Co /Co ratio
decreased as further increasing Co content, which illustrated
3
+
2+
3
+
2+
3 4
that excessive Co O loading decreased the surface Co /Co
ratio. The cation distribution of the spinel-structured Co
3 4
O
Table 2 XPS parameters of Co
O
3 4
2
/TiO catalysts
3
+
2+
Samples
Co /Co
Oads/Olat
CoTi-1
CoTi-2.5
CoTi-5
CoTi-10
CoTi-25
0.23
0.69
1.32
1.11
0.30
0.13
0.38
0.76
0.23
0.22
Fig. 5
2 3 4 2
H -TPR profiles of Co O /TiO catalysts with various Co
contents.
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Fig. 8 DBM conversion as a function of temperature over CoTi-5 for
ꢁ1
Fig. 6 NH
contents.
3
-TPD profiles of Co
O
3 4
/TiO
2
catalysts with various Co first to fifth run (GHSV ¼ 60 000 h ).
Surface acidities of the catalysts are found to have signi-
cantly inuence on VOCs oxidation as they provide sites for the
chemisorption of VOCs molecules. Therefore, surface acidities
Table 3 Products in outlet at each evaluated temperature over CoTi-5
ꢀ
20
Temperature ( C)
Detected substances
150
200
250
300
350
400
450
CH
CH
CH
CH
CH
2
2
2
2
2
Br
Br
Br
Br
Br
2
2
2
2
2
, CH
, CH
, CO, CO
, CO, CO
, CO, CO
3
Br, CO, HBr, H
2
O
3
Br, CO, CO , HBr, H O
2
2
2
2
2
, HBr, H
, HBr, H
, HBr, Br , H O
2 2
2
2
O
O
CO, CO
CO, CO
2
, HBr, Br
, HBr, Br
2
, H
, H
2
O
O
2
2
2
ꢀ
3 4
91 C were observed, which could be assigned to Co O
4
crystalline. As further increasing the Co content, these
reduction peaks slightly shied to higher temperature, which
1
9
might be due to the sintering. CoTi-5 showed the best
reducibility, which probably resulted in its high catalytic
performance for DBM oxidation.
2 2
Fig. 7 DBM conversion and selectivity for CO, CO , Br and HBr as Fig. 9 DBM conversion as a function of temperature over CoTi-5 in
ꢁ1
a function of temperature over CoTi-5.
various (a) O
2
and (b) DBM concentrations (GHSV ¼ 60 000 h ).
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of the catalysts are measured by NH
3
-TPD, and the results are seen, the main products of DBM conversion were CO, CO
2
and
ꢀ
shown in Fig. 6. For all the Co O /TiO catalysts, a broad HBr at a low temperature range (below 300 C). However, with
3
4
2
ꢀ
ꢀ
shoulder peak appeared in the range of 120 C and 150 C, the increase of temperature, the amount of CO was gradually
which could be the weak acid sites. Moreover, the amount of the improved and its selectivity was exceeded 50% at 335 C due to
2
ꢀ
acid sites decreased in order of CoTi-5 > CoTi-2.5 > CoTi-10 > the further oxidation of CO. When the temperature was above
ꢀ
CoTi-25 > CoTi-1. CoTi-5 had the most acid sites, which could be 350 C, Br
benecial for the catalytic oxidation of DBM.
2
was generated, which might be attributed to the
21
Deacon reaction (4HBr + O
increase of temperature, the selectivity for Br
improved, and its selectivity got to 31% at 450 C. Therefore, the
main products of DBM conversion were CO, CO , HBr and Br at
a high temperature range (above 350 C). However, the deep
2
¼ 2Br
2 2
+ 2H O). With the further
2
ꢀ
was also further
3.3 Analysis of products
2
2
ꢀ
It should be noticed that for brominated hydrocarbon oxida-
tion, high conversion is not the only criterion for good catalysts,
and selectivity of the products is more important. The ideal
decomposition products of brominated hydrocarbon are CO2
oxidation (the oxidation of CO to CO ) still needed to be further
improved.
2
and HBr or Br . Thus, the products in outlet at each evaluated
temperature over CoTi-5 were analyzed by GC-MS and the
2
3.4 Catalyst stability
results were listed in Table 3. As seen in Table 3, the byproduct For commercial application, the stability of the catalyst is an
ꢀ
CH
3
Br was emerged at low temperature (below 200 C) and it important factor. The stability of the catalyst in the reaction was
was disappeared with the increase of temperature. CO, HBr and tested by reusing the catalyst in consecutive oxidation recycles
H
2
O were always existed at each evaluated temperature, CO
2
and by performing long catalytic runs at constant oxidation
ꢀ
was generated above 200 C and Br
2
was generated above 350 temperature. As shown in Fig. 8, CoTi-5 was submitted to
successive oxidation cycles. In this case, CoTi-5 also maintained
ꢀ
ꢀ
C. Up to 400 C, CH Br was completely disappeared.
2
2
Fig. 7 shows the DBM conversion and selectivity for CO, CO2, its catalytic activity well aer ve oxidation cycles. The evolution
ꢀ
Br
2
and HBr as a function of temperature over CoTi-5. As can be of DBM conversion with time on stream at 300 C over CoTi-5 is
Fig. 10 Dependence of the DBM reaction rate on the (a and b) oxygen partial pressure and (c and d) DBM partial pressure over CoTi-5.
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presented in Fig. S3.† CoTi-5 could maintain its catalytic activity 3.6 In situ FTIR experiments
well aer 600 min of reaction. These results showed that CoTi-5
The kinetic results showed an important feature of the catalytic
reaction, which was the apparent reaction order of oxygen or
DBM was between zero- and rst-order. However, kinetic
method tended to be a general method, which still could not
explain the reaction mechanism directly. For example, the
had a good stability for DBM oxidation.
3.5 Kinetic study
Fig. 9 shows DBM conversion as a function of temperature over understanding in regard to intermediate products of the cata-
CoTi-5 under various oxygen and inlet DBM concentrations. As lytic reaction needed more direct tools. Hence, the in situ FTIR
shown in Fig. 9(a), the conversion of DBM increased with the method was used to study the mechanism on the interaction
increase of oxygen concentration, and the observed T changed between DBM molecule and Co
from 313 C to 278 C (corresponding to the oxygen concen-
O
/TiO
In situ FTIR spectra collected at different temperature over
trations of 2% and 20%, respectively). In Fig. 9(b), it was found CoTi-5 are shown in Fig. 11(a) and (b). Aer exposed to the
that the conversion of DBM decreased with the increase of DBM mixture of 500 ppm DBM, 10% O and N balance, the bands at
3072, 2992, 1600 and 1201 cm were observed. The bands at
catalysts.
5
0
3
4
2
ꢀ
ꢀ
2
2
ꢀ
ꢀ
ꢁ1
concentration. Observed T50 changed from 287 C to 314
(
C
ꢁ
1
corresponding to the DBM concentrations of 500 ppm and 3072, 2992 and 1201 cm were attributed to antisymmetric
3000 ppm, respectively). These data illustrated that both the
oxygen and the DBM concentrations might be not very sensitive
parameters. To gain further insight in this question, kinetic
studies were carried out by investigating the effect of oxygen and
inlet DBM concentrations on the reaction rate (ꢁr) as shown in
Fig. 10. The conversions were controlled below 10% to obtain
initial reaction rate for all conditions. It was found that all the
data could be explained by the power law equation as follows:
m
n
ꢁ
r ¼ k CO2
C
DBM
(9)
ꢁ
3
where k was the kinetic constant, CO2 (mol N m ) was the O
2
ꢁ
3
concentration, CDBM (mol N m ) was the inlet DBM concen-
tration, m and n were the apparent reaction orders of oxygen
and DBM, respectively. The values of m and n were obtained
from the slope of each line in Fig. 10(b) and (d).
As shown in Fig. 10(a), the reaction rate grew nonlinearly
with the increase of oxygen concentration in the studied
temperature range. In Fig. 10(c), a similar tendency could also
be seen, but the growth of the reaction rate seemed to be more
rapidly with the increase of DBM concentration. Really,
compared with the values of n, the values of m were lower
(shown in Table 4). Particularly, the reaction order of DBM was
pseudo rst-order, which indicated that the adsorption of DBM
on the catalyst surface was difficult. However, strong adsorption
caused apparent zero-order for reaction, because all the active
sites on the catalyst surface were occupied. For oxygen as
a reactant, the reaction order was closed to zero-order. It could
be supposed that if the intermediates could directly react with
gas-phase oxygen, the reaction order of oxygen would be
approximate rst-order, thus indicating that DBM molecular
might not directly react with gas-phase oxygen. Moreover, on
the basis of Arrhenius equation, the apparent activation energy
a
(E ) was also obtained.
Table 4 Kinetic parameters for DBM oxidation over CoTi-5
Kinetic parameter
453 K
463 K
473 K
483 K
m
n
0.4
0.93
23.4
0.36
0.82
0.3
0.74
0.28
0.59
ꢁ1
E
a
(kJ mol
)
Fig. 11 In situ FTIR spectra of DBM oxidation over CoTi-5 (a, b) as
a function of temperature.
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3 4 2
Fig. 12 A plausible reaction mechanism for DBM oxidation over Co O /TiO catalysts.
stretching, symmetric stretching and wagging of methylene, The reaction pathways mainly consist of two sequential cycles:
respectively, which were the approximated mode of DBM formation of formate species and oxidation of formate species
ꢀ
22,23
molecule on the surface of CoTi-5 at 50 C.
2
With the increase to CO and CO . The reaction steps were described as follows: (1)
of temperature, the intensity of these bands decreased and the adsorption of DBM molecule on surface oxygen vacancies of
ꢀ
disappeared above 100 C, suggesting either desorption or the catalyst. (2) The adsorbed DBM were attacked by the surface
ꢁ
1
consumption of the DBM molecules. The band at 1600 cm
adjacent nucleophilic oxygen species. (3) The two bromine
24
was assigned to H O absorbed on the catalyst surface, which atoms were substituted by oxygen atoms, generating absorbed
2
30
gradually disappeared with the increase of temperature. formaldehyde (hemiacetal species). (4) The adsorption of gas-
Meanwhile, the disappearance of DBM was accompanied by phase oxygen on the surface oxygen vacancies to replenish the
appearance of the new bands of 2960, 2935, 2873, 2835, 1560, consumed oxygen. (5) The Cannizzaro reaction between absor-
ꢁ
1
1
380, 1365 and 1215 cm . Among them, the bands at 2960, bed formaldehyde and hemiacetal species formed the adsorbed
ꢁ
1
ꢁ1
2
873, 1560, 1380 and 1365 cm (including 2881 cm devel- formate species and methoxy species. (6) The adsorbed formate
ꢀ
25,26
oped at 150 C) were assigned to adsorbed formate species.
species and methoxy species were further attacked by surface
While the other bands at 2935 and 2835 cm were assigned to active oxygen species to form CO and CO . The Br species
methyl antisymmetric stretching and symmetric stretching of adsorbed active sites could be removed in the form of HBr via
ꢁ1
2
27,28
ꢁ1
absorbed methoxy groups.
The band at 1215 cm
was the reaction of surface of hydroxyl groups and the dissociatively
developed at 250 C, which was assigned to bicarbonate. All adsorbed Br, or in the form of Br at high temperature via the
ꢀ
29
2
these bands developed with the oxidation of the absorbed DBM, Deacon reaction (4HBr + O
which meant that the formate groups and methoxy groups were
2
2 2
¼ 2Br + 2H O).
the main intermediate species generated on the surface of CoTi-
5
. With the further increase of temperature, the intensity of the 4. Conclusions
ꢀ
ꢀ
bands decreased distinctly at 300 C and disappeared at 350 C.
It indicated that CoTi-5 could degrade intermediate products to
A series of Co O /TiO catalysts with various Co contents
3
4
2
prepared by a conventional impregnation method were char-
acterized by XRD, BET, SEM, TEM, XPS, H -TPR and NH -TPD,
and used in catalytic oxidation of DBM, some new information
was obtained in this study. Firstly, Co /TiO catalysts were
CO and CO at high temperature. Then the adsorbed formate
2
2
3
species and methoxy species were hard to be observed above
ꢀ
3
00 C. Moreover, the desorption of CO, CO
2
and HBr as nal
3
O
4
2
products was supposed to be very quick, which was evident that
no bands assigned to these species could be observed.
Based on the results of in situ FTIR and reported mecha-
found to be very active for DBM oxidation, which had not been
reported before. The highest catalytic activity with T90 at about
ꢀ
3
46 C was obtained over CoTi-5, which might be attributed to
24,30,31
nisms of CH
2
Cl
2
oxidation over various catalysts,
a plau-
3
+
the enrichment of well dispread Co O and high surface Co /
3
4
sible reaction mechanism could be proposed for catalytic
oxidation of DBM over Co O /TiO catalysts, as shown in Fig. 12.
2
+
Co ratio, as it could provide more surface active sites and
active oxygen species. Secondly, the kinetic study showed that
3
4
2
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the reaction order of DBM was pseudo rst-order and the 11 H. Hu, S. Cai, H. Li, L. Huang, L. Shi and D. Zhang, ACS
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5
formate species and methoxy species were detected by in situ
429.
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3
O
4
2
dissociation of C–H bonds and the oxidation of CO to CO were
1767–1770.
2
started, and nally the reaction recycle was nished.
16 J. Fierro and L. G. Tejuca, Appl. Surf. Sci., 1987, 27, 453–457.
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