Charcoal-supported catalyst with enhanced thermal-stability for the catalytic combustion of volatile organic compounds

Article abstract of DOI:10.1016/j.apcata.2016.04.028

In this study, charcoal powder originated from thinned wood was selected as the support material, and transition metals were used as the catalytic active phases to prepare a series of the charcoal-supported catalysts for the catalytic oxidation of volatile organic compounds. We found that the waste wood derived charcoal with poor thermal-stability could be converted into highly active and durable VOCs elimination catalyst in one-step. The addition of transition metal followed by a thermal-treatment in He (at 1000-1400 °C) significantly improved the thermal-stability of charcoal. Co gave the best promotion effect among the tested metals (Co, Ni, and Fe). The transition metals could not only facilitate the transformation of amorphous carbon structure into graphitic structure during the preparation of the support but also serve as an active phase in the VOCs oxidation reaction. When a charcoal-supported Cu-Co bimetallic catalyst was subjected to the toluene catalytic oxidation, the complete oxidation of toluene was achieved at 237 °C, which was far lower than the thermal resistance temperature of the catalyst (350 °C). Especially, in the ethyl acetate catalytic oxidation, the Cu-Co bimetallic catalyst gave an excellent activity that is slightly superior to a Pt/Al₂O₃ catalyst.

Full text of DOI:10.1016/j.apcata.2016.04.028

Applied Catalysis A: General 522 (2016) 32–39
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Charcoal-supported catalyst with enhanced thermal-stability for the
catalytic combustion of volatile organic compounds
YongTao Liaoa, Lu Jiab, RuiJie Chena, OuYun Gua, Makoto Sakuraib, Hideo Kameyamab,
Lu Zhoua, Hua Maa,∗,1, Yu Guoa,∗,1
a College of Chemical Engineering, State Key Laboratory of Materials-Oriented Chemical Engineering, NanJing Tech University, 5 Xinmofan Road, NanJing
210009, PR China
b Department of Chemical Engineering, Tokyo University of Agriculture and Technology, 24-16, Nakacho 2, Koganei-shi, Tokyo 184-8588, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
In this study, charcoal powder originated from thinned wood was selected as the support material, and
transition metals were used as the catalytic active phases to prepare a series of the charcoal-supported
catalysts for the catalytic oxidation of volatile organic compounds. We found that the waste wood derived
charcoal with poor thermal-stability could be converted into highly active and durable VOCs elimina-
tion catalyst in one-step. The addition of transition metal followed by a thermal-treatment in He (at
1000–1400 ◦C) significantly improved the thermal-stability of charcoal. Co gave the best promotion effect
among the tested metals (Co, Ni, and Fe). The transition metals could not only facilitate the transforma-
tion of amorphous carbon structure into graphitic structure during the preparation of the support but
also serve as an active phase in the VOCs oxidation reaction. When a charcoal-supported Cu-Co bimetallic
catalyst was subjected to the toluene catalytic oxidation, the complete oxidation of toluene was achieved
at 237 ◦C, which was far lower than the thermal resistance temperature of the catalyst (350 ◦C). Espe-
cially, in the ethyl acetate catalytic oxidation, the Cu-Co bimetallic catalyst gave an excellent activity that
is slightly superior to a Pt/Al2O3 catalyst.
Received 26 November 2015
Received in revised form 9 April 2016
Accepted 25 April 2016
Available online 27 April 2016
Keywords:
Charcoal
Thermal-stability
Transition metal addition
Catalytic combustion
Volatile organic compounds
© 2016 Published by Elsevier B.V.
1. Introduction
a much lower temperature, which made it a more attractive and
efficient way in practical utilities.
In recent years, volatile organic compounds (VOCs) have con-
tributed significantly to air pollution [1]. The control of their
emissions has therefore become imperative. Since the environmen-
tal legislations all over the world are imposing stringent standards
on VOCs emissions, efficient technologies for the removal of VOCs
must be developed. Nowadays, there are numerous different meth-
ods for removing VOCs such as adsorption, thermal oxidation and
catalytic oxidation. Thermal oxidation was one of the most effec-
tive means for the VOCs elimination, but it usually proceeds at high
temperatures (≥800 ◦C) and would result in the formation of some
hazardous compounds (dioxins, carbon monoxide, nitrogen oxides,
etc.) which were even more dangerous than the ones to be elimi-
nated [2]. The catalytic oxidation could achieve a similar effect at
Noble metal-based catalysts (Pt, Pd, Rh etc.) exhibit high activ-
ity and selectivity at low temperature [3–5], but they are unstable
in the presence of chloride compounds [6]. Especially with respect
to the cost and availability of most noble metals, the use of these
catalysts in commercial applications of large-scale involves eco-
nomic limitations. Transition metals such as Cu, Co, Ni, Fe, Mn,
etc. were widely investigated as the alternative to noble metals
in the VOCs catalytic oxidation [1,7–11]. Comparing with noble
metal-based catalysts, transition metal-based catalysts are gen-
erally less active but more resistant towards chloride poisoning.
Their costs are also significantly lower, which make them espe-
cially suitable for the mass production and application. The activity
of transition metals based VOCs catalysts could be enhanced via
using appropriate supports. Conventional metal oxides such as
Al2O3, CeO2, TiO2, SiO2, or mixed oxides (e.g. MnOx CeO2 [12],
Co3O4 CeO2 [13]) have been extensively studied and used as cata-
lyst support for eliminating VOCs emission. Recently, carbonaceous
support has received increased attention as an alternative to the
conventional support, due to its superiority in economic, secure and
environmental aspects. Some researchers have revealed favorable
∗
Corresponding authors.
E-mail addresses: gmahua@njtech.edu.cn (H. Ma), mguoyu@njtech.edu.cn
(Y. Guo).
1
These authors contributed equally to this work.
http://dx.doi.org/10.1016/j.apcata.2016.04.028
0926-860X/© 2016 Published by Elsevier B.V.

Y. Liao et al. / Applied Catalysis A: General 522 (2016) 32–39
33
catalytic activity attained over carbonaceous support catalysts such
as activated carbon [1,7,11,14,15] and graphitic carbon [16,17].
Zhang et al. [15] examined the effectiveness of the Pt/AC for the
oxidation of VOCs at temperatures below 200 ◦C. The activity of
Pt/AC for benzene oxidation was found to be superior to that of
Pt/alumina, in particular under humid conditions. A comparison of
alumina-supported transition metal catalysts (Co, Fe, Ni, and Cu)
with AC-supported catalysts reported by Lu et al. [1] showed that
the conversion efficiency of alumina-supported catalysts was lower
than the AC-supported catalysts.
Despite the enhancement of activity by applying carbonaceous
materials as support, there are still potential risks in practical utili-
ties. A temperature around 300 ◦C (or higher) is generally required
to completely oxidize VOCs over the AC-supported transition metal
catalysts [1,7,11]. The temperature of 300 ◦C is a potential hazard
to the thermal-stability of the AC based catalyst, because activated
carbon support in the catalysts can be oxidized by oxygen at this
temperature and results in serious catalyst degradation. Further-
more, from a practical viewpoint, the potential temperature rise
of the catalytic bed in a VOCs combustor caused by the adiabatic
temperature rise cannot be neglected since the VOCs catalytic oxi-
dation is exothermic, which might aggravate this safety hazard.
Graphitic carbons are stable under elevated temperatures, but the
synthesis of graphite with high surface area is complex and the
preparation conditions are uneconomical and severe, in particular,
the extremely high temperature (ca. 30000 ◦C) [17–20]. For exam-
ple, Zhang et al. [20] reported a preparation of graphitized porous
carbon microspheres by the spray drying technique using carbon
black nanoparticles as the primary carbon resource. The graphiti-
zation treatment was conducted at 2800 ◦C. Besides, commercially
available graphite flakes are usually of small surface area, which
makes it unsuitable to be used directly as supports for catalyst. It is
highly preferred to enhance the thermal-stability of carbonaceous
supports under mild conditions.
In this study, in order to utilize efficiently waste wood (e.g.
thinned wood), a charcoal powder derived from thinned wood was
selected as the support material, and transition metals were used as
the catalytic active phases to prepare a series of charcoal-supported
catalysts for the VOCs catalytic oxidation (model VOCs: toluene and
ethyl acetate). In order to enhance the thermal-stability of char-
coal, a thermal treatment was conducted under mild temperature
range (1000–1400 ◦C), which was far lower than the commercial
graphitization temperature of ca. 3000 ◦C [17–20]. We found that
the addition of transition metal followed by a thermal-treatment in
He (at 1000–1400 ◦C) significantly improved the catalyst thermal-
stability. Co gave the best promotion effect among the tested
transition metals (Co, Ni, and Fe). When a charcoal-supported
Cu-Co bimetallic catalyst was subjected to the toluene catalytic oxi-
dation, the complete oxidation of toluene was achieved at 237 ◦C,
which was far lower than the thermal resistance temperature of the
catalyst (350 ◦C). Especially, in the ethyl acetate catalytic oxidation,
the Cu-Co bimetallic catalyst gave a dramatic activity comparable
and even slightly superior to a Pt/Al2O3.
was maintained at room temperature for 2 h under vigorous stir-
ring. After a vacuum drying, the sample was dried in air at 120 ◦C for
12 h. The dried samples were crushed and sieved into 30–40 mesh.
In the subsequent thermal-treatment, the powder sample was
heated to different temperatures (1000–1400 ◦C) in 100 mL/min
He (at a ramping rate of 10 ◦C/min), and maintained at the same
temperature for 3 h. In this paper, these samples were denoted
as M-C(Hex) (where M = Co, Fe, Ni; He = Helium; x = the thermal-
treatment temperature). In order to decrease the preparation cost,
a pre-treatment of vacuum desorption was not conducted ahead
of the He thermal-treatment. Additionally, some thermal-treated
samples were further calcined in air at 350 ◦C for 3 h, denoted as
M-C(Hex, Air350).
After a He thermal-treatment at 1000–1400 ◦C, ca. 25–28%
weight loss was observed over the raw charcoal, which should be
attributable to the adsorbed water or oxygen on the charcoal sur-
face and/or in-volatile organic compounds remaining in the meso-
and/or micro-pores of the raw charcoal.
The Co-C(He1400, Air350) was used to prepare a catalyst sample
with the Cu addition, designated Cu/Co-C(He1400, Air350). The Co-
C(He1400, Air350) powder was immersed into an aqueous solution
of copper nitrate at room temperature for 2 h, followed by a vacuum
drying. Subsequently, the resulted sample was dried at 120 ◦C for
12 h and then calcined at 350 ◦C for 3 h in air.
As catalyst control, a graphite powder (070-01325, Wako Pure
Chemical Industries, Ltd.), an alumina powder (90928, Soekawa
Chemical Co., Ltd.) and an activated carbon powder (037-02115,
Wako Pure Chemical Industries, Ltd.) were used in this paper.
An alumina-supported Pt (or Co) catalyst was prepared by
impregnating alumina in a H2PtCl6 (or Co(NO3)2) solution. After
impregnation, the samples were dried naturally for 4 h and then
calcined at 500 ◦C for 3 h in air. Resulted catalysts were desig-
nated Pt/Al2O3 and Co/Al2O3, respectively. Co loading in Co/Al2O3
is 20.6 wt%. Pt/Al2O3 has a Pt loading of 2.2 wt% with a Pt disper-
sion of 28.5%. The Pt dispersion was determined by the CO-pulse
adsorption method, where pre-treatment conditions were set at
100 ◦C for 30 min in 70 mL/min He, and then at 400 ◦C for 30 min.
Sample reduction was conducted at 400 ◦C in 70 mL/min H2 for 1 h.
Finally, the sample was cooled in He. The Pt dispersion was calcu-
lated based on the amount of adsorbed CO and assuming adsorption
stoichiometry CO:Pt = 1:1. An activated carbon-supported Co cata-
lyst (Co/AC) was prepared by immersing the activated carbon in a
Co(NO3)2 solution at room temperature. After a vacuum drying, the
sample was dried at 120 ◦C for 12 h and then calcined at 300 ◦C for
3 h in air. Co loading in Co/AC is 19.2 wt%.
The preparation conditions of catalysts used in this paper are
summarized in Table 2.
2.2. Catalyst characterization
Powder XRD analyses of the samples were obtained using an
X-ray diffractometer (RAD-II C, Rigaku Corp.) with nickel-filtered
Cu-K␣ radiation. BET specific surface area was determined using
the nitrogen adsorption method (SA3100, Beckman Coulter, Inc.).
Metal loading was determined using an inductively coupled plasma
spectrometer (ICPS-7510, Shimadzu Corp.). Table 2 shows the
result of the ICP analysis obtained over different samples.
2. Experimental
2.1. Catalyst preparation
Thermogravimetric analyses were performed on a TGA-51 (Shi-
madzu Corp.). In this study, two methods were used to conduct
TG analysis: dynamic TG and stationary TG. In the dynamic TG
analysis, the sample was pre-treated in 50 mL/min air at room tem-
perature for 30 min and then at 120 ◦C for 1 h. Subsequently, the
sample was heated to 1000 ◦C in 50 mL/min air at a ramping rate of
10 ◦C/min, while the weight change was continuously monitored
using a thermo-balance. The sample weight relative to the stable
value after the 120 ◦C pre-treatment was used in this paper, i.e. rel-
A charcoal powder derived from thinned wood (BET surface
area = 69.4 m2/g, supplied from Japan Interaction in Science &
Technology Forum) was used as the support material, designated
C(raw). Table 1 shows the composition of charcoal ash. Raw char-
coal was added into an aqueous solution of cobalt (iron, nickel)
nitrate under ambient conditions and the weight ratio of the added
metallic element to charcoal was controlled to 20/80. The solution

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Y. Liao et al. / Applied Catalysis A: General 522 (2016) 32–39
Table 1
Composition of charcoal ash (relative to the raw charcoal weight).
Composition
wt%
K2O
SiO2
CaO
Cl
P2O5
MnO
ZnO
MgO
SO3
Fe2O3
0.021
CuO
Al2O3
0.003
Total
2.5
1.349
0.535
0.170
0.107
0.100
0.098
0.037
0.037
0.035
0.007
Table 2
Sample preparation conditions and metal loading (ICP).
Preparation conditions
Raw charcoal
ICP metal loading (wt%)
C(raw)
C(He1000)
C(He1400)
Fe-C(He1000)
Ni-C(He1000)
Co-C(He1000)
Co-C(He1400)
Co-C(He1400, Air350)
Cu/Co-C(He1400, Air350)
Co/AC
–
–
–
C(raw) → in He at 1000 ◦C for 3 h
C(raw) → in He at 1400 ◦C for 3 h
Fe impregnation (Fe/charcoal = 20/80a) → in He at 1000 ◦C for 3 h
Ni impregnation (Ni/charcoal = 20/80) → in He at 1000 ◦C for 3 h
Co impregnation (Co/charcoal = 20/80) → in He at 1000 ◦C for 3 h
Co impregnation (Co/charcoal = 20/80) → in He at 1400 ◦C for 3 h
Co-C(He1400) → in Air at 350 ◦C for 3 h
NDb
ND
Co: 33.6
Co: 35.8
Co: 36.2
Co: 32.1; Cu: 8.9
Co: 19.2
Co: 20.6
Pt: 2.2
Co-C(He1400, Air350) → Cu impregnation → in Air at 350 ◦C for 3 h
Co impregnation (Co/active carbon = 20/80) → in Air at 300 ◦C for 3 h
Co impregnation of alumina → in Air at 500 ◦C for 3 h
Co/Al2O3
Pt/Al2O3
Pt impregnation of alumina → in Air at 500 ◦C for 3 h
a
The weight ratio of the added metallic element to charcoal (or active carbon).
ND: not determined.
b
ative weight. The data of DTG (relative to temperature) was used
to determine the initial combustion temperature of C-containing
sample. A specific temperature, where the absolute value of DTG
was greater than 0.02, was considered to be the initial combustion
temperature point. As for the stationary TG analysis, the sample
was heated to 120 ◦C (10 ◦C/min) in 50 mL/min air, and maintained
for 1 h. Then, it was heated to different temperatures at a rate of
10 ◦C/min, and maintained at the same temperature for several
hours. The stable value after the 120 ◦C stage was used to calcu-
late the relative weight. A mass spectrometer (Qulee-YTP, Ulvac,
Inc.) was used to continuously monitor the TG outlet gas, which
contained m/z = 28 (CO/N2) and m/z = 44 (CO2).
2.3. VOCs catalytic oxidation
Fig. 1. Dynamic TG analyses (50 mL/min air, 10 ◦C/min) over the graphite powder
(a), the active carbon powder (b) and the raw charcoal powder (c).
In this study, toluene and ethyl acetate were used as model gases
of real VOCs. Air was flowed into a saturator that was filled with liq-
uid toluene (or ethyl acetate) (Kokusan Chemical Co., Ltd.) and then
mixed with the volatilized organic gas. The concentrations were
adjusted by controlling the saturator temperature. Before activity
test, all catalysts were crushed and sieved to 30–40 mesh. The cat-
alyst powder was loaded into a fixed bed quartz reactor (i.d. 6 mm)
to test its activity. The reaction temperature was controlled using
a K-type thermocouple placed in the catalyst bed center. The total
gas flow was set at 30 mL/min (i.e. F/W = 18000 mL/(g h)), and it
contained 400 ppm toluene in air (or 600 ppm ethyl acetate in air).
Both the inlet and outlet stream mixtures were analyzed with an
online gas chromatograph (GC-14B, Shimadzu Corp.).
Thermal treatment in an inert atmosphere was applied to
enhance the thermal-stability of the charcoal supports. Various
transition metal elements (Fe, Ni, and Co) were used as graphitiz-
ing catalysts to add into the raw charcoal during the process. Fig. 2
shows the dynamic TG analysis over the thermal-treated charcoal
supports with or without the metal addition. The He thermal-
treatment at 1000 ◦C for 3 h enhanced the thermal-stability of the
C(raw) for its initial combustion temperature was increased from
270 to 310 ◦C. The metal addition to the C(raw) followed by the ther-
mal treatment further elevated the thermal-stability of charcoal
supports. The Co addition provided the highest thermal-stability
among the tested metal elements. The presence of Co significantly
increased the initial combustion temperature from 310 to 430 ◦C
as compared to the C(He1000). Furthermore, a comparison of the
Co-C(He1000) with the Co-C(He1400) shows that higher thermal
treatment temperature provides higher thermal-stability.
3. Results and discussion
3.1. Effect of the metal addition
Fig. 1 shows the result of the dynamic TG analysis (10 ◦C/min,
in air) obtained over the C(raw), commercial graphite powder and
activated carbon. The initial combustion temperature was 595 ◦C
for the graphite, 395 ◦C for the AC, and 270 ◦C for the C(raw). The
combustion temperature of the C(raw) derived from thinned wood
is significantly lower than the working temperature of transition
metal based catalyst (ca. 300 ◦C), which makes it inappropriate to
serve as the support of VOCs oxidation catalyst. In other words,
efforts must be made to improve its thermal-stability.
Since transition metals have been proven to be active towards
VOCs oxidation reaction [1,7–11], the charcoal supports with
metal addition were directly tested in the catalytic oxidation of
toluene (Fig. 3). As reference, the reaction was also carried out
over the charcoal support thermal-treated at 1000 ◦C and the raw
carbonaceous supports (graphite, AC, C(raw)). No obvious cat-
alytic activity was observed over the C(raw), the graphite or the
AC. However, when the thermal-treated charcoal supports with
metal addition were used in the present reaction, the catalytic

Y. Liao et al. / Applied Catalysis A: General 522 (2016) 32–39
35
Fig. 2. Dynamic TG analyses (50 mL/min air, 10 ◦C/min) over different samples.
Fig. 4. Stationary TG analyses at 350 and 400 ◦C over different samples (a), and
ms-signals obtained over the Co-C(He1400) (b).
Fig. 2d. Nevertheless, it should be noted that after the He thermal-
treatment, the formation of metallic Co is inevitable due to the
reduction by carbon. During the dynamic TG analysis in air, the
increment in catalyst weight caused by the oxidation of metallic
Co will superimpose on the carbon consumption curve. As a result,
it is difficult to judge strictly and correctly the initial combustion
temperature by using merely the dynamic TG result. In addition,
considering possible experimental error, the rapid heating speed of
10 ◦C/min in the dynamic TG also aggravates this difficulty because
some slight changes may be neglected. Therefore, different from the
dynamic TG, a long-term stationary TG test is required to evaluate
strictly the catalyst thermal-stability.
Fig. 3. Toluene conversion over different charcoal catalysts with the 1000 ◦C treat-
ment. Reaction conditions: 400 ppm toluene in air, F/W = 18000 mL/(g h).
oxidation of toluene could carry on smoothly under low tem-
peratures. The Co-C(He1000) gave the highest oxidation activity,
and a 100% toluene conversion could be achieved at 300 ◦C. The
catalytic activity with respect to metal was observed to follow
a particular order: Co > Fe > Ni. Relevant literatures have inves-
tigated catalytic activity for the toluene combustion by using
different metals (Cu, Co, Fe, Ni, etc.) supported on different materi-
als. They reported respectively the activity sequences with respect
to metals as follows: Cu > Mn > Fe > V > Mo > Co > Ni > Zn (15 wt%
metal/Al2O3, the incipient wetness impregnation method) [21];
Cu > Fe > Cr > Mn > Co > Mo > Ni (5 wt% metal/Al2O3, the incipient
impregnation method) [22]; Cu > Co > Fe > Ni (1.5 wt% metal/AC, the
polyol method) [1], and Co > Cu > Ti > Fe > Ni (bulk oxide catalyst, the
precipitation method) [23]. Although toluene activity of the catalyst
will be determined remarkably by preparation conditions, such as
active metal, catalyst support, metal loading and calcination tem-
perature, Cu, Co, Mn, and Fe are considered generally to be favorable
active metal, but not Ni.
3.2. Promoted thermal-stability by the Co addition
For a practical VOCs combustor, a potential temperature rise
of the catalytic bed caused by the adiabatic temperature rise
cannot be neglected, because VOCs catalytic combustion is exother-
mic. Therefore, a VOCs oxidation catalyst must have a favorable
thermal-stability at higher temperatures than the VOCs complete
oxidation temperature. According to the preliminary experiment
results shown in Fig. 3, 350 ◦C was set as a safe upper limit of tem-
perature. In Fig. 4, a stationary TG test was carried out to evaluate
the long-term thermal-stability of the charcoal supported catalyst
at 350 ◦C or higher temperatures.
When the Co-C(He1000) was subjected to the stationary TG
test at 350 ◦C, the catalyst weight gradually declined in a very
slow speed, and ca. 2.5% weight loss was observed after aging in
air for 10 h. The disagreement of thermal resistance temperature
between Fig. 4a and d was attributed to the very slow combus-
tion rate. Therefore, it is dangerous to use the Co-C(He1000) in the
toluene oxidation, in view of the potential adiabatic temperature
rise. As an approach, the thermal treatment temperature was fur-
ther increased to 1400 ◦C to improve the thermal-stability of the
catalyst. As shown in Fig. 4a, after aging in air at 350 ◦C for 10 h,
According to the results shown above, the Co-C(He1000) seems
to be a suitable catalyst for the toluene oxidation because the
complete oxidation temperature of toluene (300 ◦C) is far lower
than the catalyst combustion temperature (430 ◦C) obtained from

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Y. Liao et al. / Applied Catalysis A: General 522 (2016) 32–39
simultaneously occurs in the thermal treatment. No other specia-
tion (e.g. oxide or carbide) attributable to cobalt was observed in
Fig. 5. In addition, an increase in the thermal treatment temperature
caused a gradual growth of Co crystal size, for example, 32.1 nm at
900 ◦C, 35.0 nm at 1000 ◦C, 37.8 nm at 1200 ◦C, 41.6 nm at 1300 ◦C,
48.3 nm at 1400 ◦C.
Generally, synthetic methods of graphite are complicated and
preparation conditions are uneconomical and severe. An extremely
high temperature of ca. 3000 ◦C is required for the graphitization
process [17–20]. Early researches conducted by Inagaki et al. [24]
indicated that at 1000–1500 ◦C transition metals (cobalt, nickel,
iron, etc.) could work as the catalysts to accelerate the graphitiza-
tion reaction during the carbonization of different polymers such as
poly(vinyl chloride) (PVC), poly(vinyl alcohol) (PVA) and poly(vinyl
pyrrolidone) (PVP). The formation of graphite crystals was sup-
posed to occur through the following steps: thermal decomposition
and carbonization of vinyl polymers below 500 ◦C, reduction of
metal oxides to metal by carbonaceous products and then catalytic
action of metals to precipitate graphite. In this study, however, we
confirmed that the existence of cobalt could also catalyze directly
the conversion of amorphous carbon structures into graphitic
structures without such decomposition-precipitation mechanism.
Metal ions on the carbon support would probably first grow into
nano-scaled particles at a temperature of several hundred-degree
celsius [25], then melt at 1000–1400 ◦C, a temperature which is
much lower than their melting point due to their small size [26]
to form eutectics [27], and finally go through which the amor-
phous carbon supports are transferred into graphitic structures. It is
worth noting that the poor thermal-stability of waste wood derived
charcoal should probably be associated with its amorphous struc-
ture, which could be crucial for the formation of eutectics and the
graphitic structure with the appearance of metallic catalyst [28].
In Fig. 5g, graphite diffraction profiles composed from a sharp
peak at 26.4◦ and a small and broad band located around 26◦ were
observed over the Co-C(He1400), revealing the coexistence of non-
graphitized part with graphite. In other words, the crystallinity
was not yet very high under our preparation conditions. The co-
existence of non-graphitized carbon could be used to explain the
first combustion peak of the Co-C(He1400) shown in Fig. 4b (when
temperature increased from 120 to 350 ◦C). In order to investigate
the oxidative gasification of char, Ganga Devi et al. [29] prepared
metal species doped chars by the pyrolysis progress of carboxy
methyl cellulose ion-exchanged with different metal ions (Cu, Ni,
Co, Fe, and Ca). It was found that the metal species in the char
existed as an oxide or elemental metal or carbide, depending on
the heat-treatment temperature (under N2). Metal additives acted
as active catalysts to catalyze the gasification of char, while their
oxides were reduced by char. Different from a generally accepted
oxygen transfer mechanism, they proposed a catalytic cycle that
involved the formation of weakly bound O atoms dissociatively
adsorbed on the surface of the catalyst, which was subsequently
transferred to the C atom. Here, we could use this gasification mech-
anism to explain the reduction of cobalt shown in Fig. 5 and the
aforementioned first combustion peak in Fig. 4b. On the other hand,
with regard to the favorable thermal-stability of Co-C(He1400) at
350 ◦C for 10 h (as is shown in Fig. 4b), it is suggested to be due
to the formation of more stable graphite catalyzed by metal addi-
tives, although we have no enough evidence yet. Relevant study is
currently in progress.
Fig. 5. XRD analyses over different samples.
no obvious weight loss was observed over the Co-C(He1400). This
result is in agreement with the result of Fig. 2 that the higher ther-
mal treatment temperature provides the higher thermal-stability.
In contrast to the stable Co-C(He1400), ca. 11.5% weight loss was
revealed over the C(He1400), indicating the importance of the Co
addition in enhancing the thermal-stability of the charcoal sup-
ported catalyst. In addition, in Fig. 4a, the AC powder did not exhibit
a favorable thermal-stability at 350 ◦C, because ca. 6% weight loss
was observed after 10 h.
Fig. 4b shows the ms-signals of CO/N2 (m/z = 28) and CO2
(m/z = 44) in the TG outlet gas obtained over the Co-C(He1400).
The m/z of 44 was associated with the formation of CO2 caused
by the carbon combustion. The m/z of 28 was assigned to the for-
mation of CO, because extra nitrogen could not form during the
carbon combustion. In Fig. 4b, two obvious CO2 peaks appeared
in the temperature rise stages from 120 to 350 ◦C and from 350
to 400 ◦C. The former peak was accompanied by a slight increase
in the catalyst weight (<1.0 wt%), which indicated that the oxi-
dation of metallic Co by oxygen and part of carbon combustion
simultaneously occurred in this region. The determination result
of ms-signal showed that ca. 12% of carbon (relative to the car-
bon weight in a fresh Co-C(He1400)) was consumed in this region.
No other mass-peak was observed during the 10 h stationary test,
which demonstrates undoubtedly that the Co-C(He1400) is stable
at 350 ◦C in air.
Fig. 5 shows the XRD results over different charcoal catalysts.
The broad diffraction line of the C(raw) indicated that carbon in
the raw charcoal existed in the form of an amorphous structure. A
small peak appeared at 26.4◦ after the 1400 ◦C thermal-treatment.
It was attributable to the formation of graphite although its inten-
sity was very weak. The transformation was originated from the
graphitizing process under high temperature treatment. When Co
was added to the raw charcoal (Co-C(He1400)), an obvious and
sharp peak was observed at 26.4◦ (Fig. 5g). Furthermore, in the
presence of the Co addition, an increase in the thermal treatment
temperature gradually intensified the 26.4◦ peak attributable to the
formation of graphite. Although their crystallinity was not yet very
high as compared with the commercial graphite (sample h), the
results illustrates that the Co addition promotes the transforma-
tion from amorphous carbon to graphite (i.e. graphitization). From
Fig. 5, diffraction lines of metallic cobalt (2␪ = 44.2, 51.5 and 75.9◦)
could also be seen on the samples with the Co addition (samples
c–g), which indicates that the reduction of cobalt oxide by carbon
Nevertheless, it should be noted that a high crystallinity is not
urgently required for the present study, because the VOCs cat-
alytic combustion reactions usually occur in the temperature region
of 100–350 ◦C. The result in Fig. 5 is considered to be valuable
because it evidences that the Co addition can decrease the graphiti-
zation temperature and provide a charcoal catalyst with moderate
thermal-stability for the VOCs catalytic combustion.

Y. Liao et al. / Applied Catalysis A: General 522 (2016) 32–39
37
Fig. 7. Stationary TG analysis coupled with a mass spectrometer over Cu/Co-
C(He1400, Air350).
Fig. 6. Toluene conversion over different catalysts. Reaction conditions: 400 ppm
toluene/air, F/W = 18000 mL/(g h).
3.3. Application of charcoal catalyst to VOCs catalytic oxidation
Fig. 6 shows the toluene catalytic oxidation over the charcoal-
supported Co catalysts. As control experiments, an alumina-
supported Pt (or Co) catalyst and an activated carbon-supported Co
catalyst were also tested. In addition, some researches on the cat-
alytic oxidation have reported the favorable activity of transition
metal oxides (e.g. Mn2O3) used as bulk catalysts without support
[30]. In our study, a bulk CoO catalyst was also used as reference.
Among all the tested catalysts, Pt/Al2O3 gave the best activ-
ity for toluene catalytic oxidation while bulk catalyst CoO without
support showed poor activity in comparison with supported cata-
lysts. The charcoal-supported Co catalysts gave a moderate activity
from 225 to 300 ◦C, and the complete oxidation of toluene could
be achieved at 300 ◦C. Comparison of the Co-C(He1000) with Co-
C(He1400) shows that the higher thermal treatment temperature
suppresses the low temperature toluene conversion, which is prob-
ably due to the Co sintering caused by the higher thermal treatment
temperature. However, when the Co-C(He1400) was calcined in air
at 350 ◦C for 3 h before the activity test (i.e. Co-C(He1400, Air350)),
the complete oxidation temperature of toluene was shifted from
300 to 287 ◦C. This result suggests that the toluene catalytic oxida-
tion is dependent not only on catalyst support or supported metal
kind but also on the redox state of supported metal. Similar to the
charcoal-supported Co catalysts, an activated carbon-supported Co
catalyst also completely oxidized toluene when the temperature
approached 300 ◦C, but its low temperature activity was lower than
the charcoal-supported Co catalyst. This result is coincident with
the results reported in the literatures [1,7,11] that a temperature
near 300 ◦C (or higher) is required to completely oxidize toluene
over the AC-supported transition metal catalysts (Cu, Co, Ni, Fe,
etc.). Although less Co loading is probably one of reasons for the
lower activity of the Co/AC than that of the charcoal-supported Co
catalyst, the complete oxidation temperature of 300 ◦C is a poten-
tial hazard because the AC is unstable under oxidative atmosphere
when temperature nears 350 ◦C (as shown in Fig. 4a).
Fig. 8. XRD analyses over Co-C(He1400) and Cu/Co-C(He1400, Air350).
ing catalyst for the present reaction as a substitute for the noble
metal-based catalysts.
Using different catalyst supports (such as alumina [32,33], sil-
ica [34]), some relevant literatures have reported higher catalytic
activity of Cu-Co bimetallic catalyst than that of individual compo-
nents for VOCs combustion [33] and CO hydrogenation to higher
alcohols [32–34]. The formation of Cu-Co spinel species on the cat-
alyst surface was reported to be the main reason for the higher
activity of Cu-Co bimetallic catalyst. In Fig. 8, the XRD analy-
sis results obtained over the Co-C(He1400) and Cu/Co-C(He1400,
Air350) are presented. When Cu was added to the Co-C(He1400,
Air350), cobalt oxides (CoO, Co3O4) could be seen over the surface
of Cu/Co-C(He1400, Air350). It should be noted that part of metallic
cobalt species (2␪ = 44.2, 51.5 and 75.90◦) still remains on the cata-
lyst surface due to the low calcination temperature [35]. According
to the literatures [32–34], diffraction peaks located at 31.1 and 44.7◦
should be associated with the Cu-Co spinel. Thus, the formation of
Cu-Co spinel is a possible interpretation for the excellent toluene
activity of the Cu/Co-C(He1400, Air350) shown in Fig. 6.
When applying the Cu/Co-C(He1400, Air350) in the catalytic
combustion of ethyl acetate (Fig. 9), the Cu-Co bimetallic catalyst
gave a dramatic activity comparable to and even slightly superior to
the Pt/Al2O3. The complete oxidation of ethyl acetate was achieved
at 212 ◦C, which was lower than that of the Pt/Al2O3 (225 ◦C). This
result suggests the better oxidation ability of the Cu-Co bimetal-
lic catalyst for removing oxygenated hydrocarbon compared to
Pt-based catalyst.
In Fig. 6 an excellent activity was observed when Cu was added
to the Co-C(He1400, Air350). The complete oxidation temperature
of toluene was decreased considerably from 287 to 237 ◦C. Fig. 7
shows the thermal-stability of the Cu/Co-C(He1400, Air350) using
the stationary TG analysis. The result is interesting that the Cu/Co-
C(He1400, Air350) has a favorable thermal-stability in air when
temperature is ≤350 ◦C. No obvious weight loss or COx formation
was observed during the 16 h stationary TG test. Since Cu can serve
as a graphitization catalyst as well [31], we believe that it also
played an important role in stabilizing the charcoal support. These
results suggest that the Cu/Co-C(He1400, Air350) is a very promis-
When
using
the
Cu/Co-C(He1400,
Air350),
the
byproduct formation and the carbon balance were also
investigated. It was found that the conversion to CO2
(=([CO2]inlet − [CO2]outlet)/(7 × [Toluene]inlet) × 100%)
was
in
agreement with the toluene conversion. That is, the catalyst
almost oxidized toluene into CO2, and no byproduct formed during

38
Y. Liao et al. / Applied Catalysis A: General 522 (2016) 32–39
tages over the commercial alumina and/or ceria-supported noble
metal catalyst, such as convenient recycling of nonrenewable metal
resources, the recovery of secondary resources from solid wastes
of thinned wood and controlling carbon dioxide emissions.
4. Conclusions
In this study, the charcoal-supported transition metal catalysts
were prepared to investigate their catalytic activity for the VOCs
catalytic oxidation. The waste wood derived charcoal with poor
thermal-stability could be converted into highly active and durable
VOCs elimination catalyst in one-step.
We found that a combination of the metal addition and the
high temperature thermal-treatment in He significantly improved
the thermal-stability of charcoal. Among the tested metals, Co
gave the best promotion effect. A charcoal-supported Co catalyst
thermal-treated in 1400 ◦C (Co-C(He1400)) exhibited a favorable
thermal-stability at 350 ◦C in air, because no weight loss or COx for-
mation was observed during the 10 h stationary TG test. Improved
thermal-stability of charcoal was associated with the promoted
transformation from amorphous carbon to graphite by the Co addi-
tion. The charcoal-supported Co catalyst thermal-treated at 1400 ◦C
gave favorable activity for the toluene oxidation as compared to
the alumina or activated carbon-supported Co catalysts. The com-
plete oxidation of toluene was achieved when the temperature
approached 300 ◦C. The addition of Cu to the Co-C(He1400, Air350)
considerably promoted the toluene oxidation. Especially, when
applying the Cu-Co bimetallic catalyst in the ethyl acetate catalytic
oxidation, it gave a dramatic activity that was slightly superior to
the Pt/Al2O3.
Fig. 9. Ethyl acetate oxidation over different catalysts. Reaction conditions: 600 ppm
ethyl acetate/air, F/W = 18000 mL/(g h).
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