Pd supported on boron-doped mesoporous carbon as highly active catalyst for liquid phase catalytic hydrodechlorination of 2,4-dichlorophenol

Article abstract of DOI:10.1016/j.apcata.2013.11.005

Palladium catalysts supported on both ordered mesoporous carbon (CMK-3) and boron-doped mesoporous carbon (B-CMK-3) were synthesized via the complexing-reduction method. These catalysts were characterized using X-ray diffraction, N₂ adsorption-desorption, transmission electron microscopy, and X-ray photoelectron spectroscopy, and their catalytic performance was examined for the liquid phase catalytic hydrodechlorination (HDC) of 2,4-dichlorophenol. Characterization results showed that for B-CMK-3 boron was introduced into the framework of the mesoporous carbon. Pd supported on B-CMK-3 had a smaller average Pd particle size and higher Pd ²⁺/Pd⁰ ratio than that on CMK-3, although B-CMK-3 had slightly lower surface area and pore volume than CMK-3. For Pd/B-CMK-3, increasing Pd loading led to an increase in Pd particle size and a decrease in Pd²⁺/Pd⁰ ratio. The liquid phase catalytic HDC of 2,4-dichlorophenol over Pd/B-CMK-3 followed the Langmuir-Hinshelwood model, and the catalytic reaction proceeded in both stepwise and concerted pathways. The initial reaction rates of Pd(2.7)/B-CMK-3 and Pd(2.6)/CMK-3 were 0.608 and 0.207 M gCat^-1 h^-1, respectively, reflecting a much higher catalytic activity of Pd/B-CMK-3 than that of Pd/CMK-3. For Pd/B-CMK-3, increasing Pd loading from 1.6 to 2.7 wt.% led to an increase in the initial rate from 0.260 to 0.608 M gCat^-1 h^-1, but further increase of the loading to 3.9 wt.% resulted in a slight decrease in the catalytic activity.

Full text of DOI:10.1016/j.apcata.2013.11.005

Applied Catalysis A: General 470 (2014) 336–343
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Pd supported on boron-doped mesoporous carbon as highly active
catalyst for liquid phase catalytic hydrodechlorination of
2
,4-dichlorophenol
∗
Juan Zhou, Ke Wu, Wenjuan Wang, Zhaoyi Xu, Haiqin Wan, Shourong Zheng
State Key Laboratory of Pollution Control and Resource Reuse, Jiangsu Key Laboratory of Vehicle Emissions Control, School of the Environment, Nanjing
University, Nanjing 210023, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 July 2013
Received in revised form 1 November 2013
Accepted 3 November 2013
Available online 10 November 2013
Palladium catalysts supported on both ordered mesoporous carbon (CMK-3) and boron-doped meso-
porous carbon (B-CMK-3) were synthesized via the complexing–reduction method. These catalysts were
characterized using X-ray diffraction, N2 adsorption–desorption, transmission electron microscopy, and
X-ray photoelectron spectroscopy, and their catalytic performance was examined for the liquid phase
catalytic hydrodechlorination (HDC) of 2,4-dichlorophenol. Characterization results showed that for B-
CMK-3 boron was introduced into the framework of the mesoporous carbon. Pd supported on B-CMK-3
Keywords:
2
+
0
had a smaller average Pd particle size and higher Pd /Pd ratio than that on CMK-3, although B-CMK-3
had slightly lower surface area and pore volume than CMK-3. For Pd/B-CMK-3, increasing Pd loading led
Mesoporous carbon supported Pd catalyst
B-doping
Liquid phase catalytic hydrodechlorination
2
+
0
to an increase in Pd particle size and a decrease in Pd /Pd ratio. The liquid phase catalytic HDC of 2,4-
dichlorophenol over Pd/B-CMK-3 followed the Langmuir–Hinshelwood model, and the catalytic reaction
proceeded in both stepwise and concerted pathways. The initial reaction rates of Pd(2.7)/B-CMK-3 and
2
,4-Dichlorophenol
−1
−1
, respectively, reflecting a much higher catalytic activ-
Pd(2.6)/CMK-3 were 0.608 and 0.207 M gCat
h
ity of Pd/B-CMK-3 than that of Pd/CMK-3. For Pd/B-CMK-3, increasing Pd loading from 1.6 to 2.7 wt.% led
−1
−1
h
to an increase in the initial rate from 0.260 to 0.608 M gCat
.9 wt.% resulted in a slight decrease in the catalytic activity.
, but further increase of the loading to
2013 Elsevier B.V. All rights reserved.
3
©
1
. Introduction
Chlorophenols (CPs) are important chemical raw materials and
deactivation caused by chlorine adsorption on noble metal surface
6], and carbonaceous materials are commonly used as supports.
For example, Jin et al. [7] reported a supported Pd catalyst on acti-
vated carbon (AC) being highly active for the liquid phase HDC of
4-CP at low temperature. Pedro et al. [8] evaluated the catalytic
[
intermediates commonly used in the production of leather, dyes,
pesticide and tragacanth [1,2]. Due to their high toxicity, these
chemicals have been classified as priority pollutants by the US Envi-
ronmental Protection Agency [3]. However, the widespread use
of CPs causes discharge of CPs into water and soil environments,
resulting in severe environmental pollution. It is thus compelling
to develop effective treatment methods for the removal of CPs from
water prior to their discharge to the environment.
activity and stability of Pd/Al O3 and Pd/AC catalysts for the liq-
2
uid phase catalytic HDC of 4-chlorophenol, and observed a higher
stability of Pd/AC than that of Pd/Al O . Work by Shao et al. [9]
2
3
indicated that Pd catalyst supported on ordered mesoporous car-
bon exhibited a much higher catalytic activity for the liquid phase
HDC of 2,4-DCP than that supported on activated carbon.
A variety of treatment techniques, including incineration, wet
oxidation and photocatalytic degradation, have been explored to
eliminate CPs in water. Alternatively, a cleaner and more efficient
method-catalytic hydrodechlorination (HDC), could be used to
remove CPs from aqueous solutions using supported noble metals
as catalysts under mild reaction conditions [4,5]. Liquid phase HDC
of CPs is generally conducted at alkaline pH to suppress the catalyst
Superficial metal supported on carbonaceous materials tends
to aggregate on support surface due to the weak metal–support
interaction [10]. Modification of carbonaceous support has been
attempted to improve the catalytic performance. Introduction of
surface functionalities by chemical oxidation, acid/base and plasma
treatment has shown improved metal dispersion on support sur-
face [11–13]. Alternatively, doping carbonaceous materials with
heteroatoms (e.g. N, B, or P) has been proven to be an effective
approach to tailor their surface properties [14–16] for enhanced
catalytic performance of the supported catalyst. For example, Liu
et al. [17] reported a substantial enhancement in both activity and
∗ Corresponding author. Tel.: +86 25 89680373; fax: +86 25 89680596.
E-mail address: srzheng@nju.edu.cn (S. Zheng).
0
926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2013.11.005

J. Zhou et al. / Applied Catalysis A: General 470 (2014) 336–343
337
further stirring for 30 min, 20 ml of 0.1 mol l 1 NaBH4 solution
was slowly added into the suspension under stirring within 1 h.
Supported Pd catalysts with varied loading amounts were obtained
by filtration, followed by washing with distilled water and drying
−
stability of a Pd catalyst supported on carbon nanofiber with N-
doping. In another study, Wang et al. [18] found Pd and Rh catalysts
supported on N-doped mesoporous carbon having higher activities
in the oxidation of benzyl alcohol than other carbon supports.
Boron, an element with similar atomic dimension to carbon,
is expected to fit well into the carbon matrix during B-doping
of carbonaceous support. In addition, as boron has three valence
electrons, it serves as an electron acceptor upon entering carbon
lattice, causing the electronic structure modified and graphitization
of boron-doped carbon enhanced [19–22]. Boron doping in carbon
supports was also responsible for enhanced metal–support inter-
action between superficial metal clusters and substitutional boron
defects [23,24], which may eventually affect the physicochemical
properties of superficial metal particles (e.g. their dispersion and
composition). Hence, it is hypothesized that Pd catalyst supported
on B-doped ordered mesoporous carbon would display unique
catalytic performance. However, thus far few studies have been
performed on the catalytic behavior of this type catalyst for the
catalytic HDC of CPs.
◦
at 70 C under vacuum for 15 h. The resultant catalyst was referred
to as Pd(x)/CMK-3 or Pd(x)/B-CMK-3, where x is the Pd loading
amount. To avoid possible intraparticle diffusion, the catalysts
were ground to pass through a 400-mesh sieve (<37 ␮m) prior to
activity tests [29].
2.2. Catalyst characterization
X-ray diffraction (XRD) measurements were carried out on a
Rigaku D/max-RA powder diffraction-meter with a Cu K˛ radia-
tion (Rigaku, Tokyo, Japan). The Pd content in the catalyst was
determined using an inductive coupled plasma emission spectrom-
eter (ICP) (J-A1100, Jarrell-Ash, USA). TEM images were collected
on a JEM-200CX electron microscope (JEOL Co., Tokyo, Japan).
Nitrogen adsorption–desorption isotherms were conducted on a
Micromeritics ASAP 2020 instrument (Micromeritics Instrument
In the present study, we prepared Pd catalysts supported on
ordered mesoporous carbon (CMK-3) and boron-doped ordered
mesoporous carbon (B-CMK-3), and investigated the liquid phase
catalytic HDC of 2,4-DCP over the catalysts. The results showed that
Pd/B-CMK-3 had a higher Pd dispersion and Pd2 content, giving
rise to a higher catalytic HDC activity than Pd/CMK-3.
◦
Co., Norcross, GA, USA) at −196 C. The X-ray photoelectron spec-
troscopy (XPS) was conducted on a PHI5000 VersaProbe equipped
with a monochromatized Al K˛ excitation source (hv = 1486.6 eV)
ULVAC-PHI, Japan). The C 1s peak (284.6 eV) was used for the cal-
+
(
ibration of binding energy.
2.3. Liquid phase catalytic HDC of 2,4-DCP
2
. Experimental
Liquid phase catalytic HDC of 2,4-DCP was carried out using a
batch reaction approach under atmospheric pressure of hydrogen.
A 250 ml of four-necked flask equipped with a sample port, a pH-
2
.1. Catalyst preparation
Mesoporous carbon CMK-3 and boron-doped CMK-3 were pre-
stat, a H /N2 inlet and outlet was used as the reactor. The reaction
2
pared using ordered mesoporous silica SBA-15 as the hard template
20,25,26]. For the preparation of SBA-15 template, 8.0 g of triblock
copolymer Pluronic P123 (EO20PO70EO20, Aldrich) was added to a
◦
temperature was stabilized at 20 ± 0.5 C with a water-bath (SDC-6,
[
Scientz Co., China). Briefly, 50 mg of catalyst was added to 200 ml of
−
1
−1
3.0 mmol l 2,4-DCP solution with pH pre-adjusted to 12. The reac-
tor was purged with a N2 flow (50 ml min ) under vigorous stirring
1400 rpm) for 30 min, and then the N flow was switched to a H2
flow (250 ml min ) to initiate the reaction. Samples were taken at
selected time intervals and catalyst particles were removed by fast
filtration. Concentrations of reactant and products in the filtrate
were measured using a high-performance liquid chromatography
with an ultraviolet (UV) detector at 270 nm using a 4.6 × 150 mm
HC-C18 column (Agilent). The mobile phase consisted of 60%
solution containing 240 ml HCl (2 mol l ) and 60 ml distilled water.
This mixed solution was subjected to continuous stirring at 40 C
for 4 h. Then, 18 g of tetraethoxysilane (TEOS, 98%, Shanghai Chem-
ical Co.) was slowly added. The resultant mixture was transferred
to four Teflon-lined autoclaves and heated at 100 C for 48 h. The
mesoporous silica was obtained by filtration, followed by washing
with distilled water, drying at 80 C and calcining at 550 C in air
for 6 h to remove the surfactant [27].
To prepare CMK-3, 2 g of SBA-15 was first mixed with a solution
containing 2.5 g of sucrose, 8 g of distilled water and 0.15 ml of con-
centrated sulfuric acid. The mixture was heated in an oven at 100 C
for 6 h and then 160 C for another 6 h. The resultant solid material
was ground into powders and combined with a solution containing
1
ric acid, followed by repetitive thermal treatment described above.
Then, the material was carbonized for 5 h at 900 C in nitrogen
−
1
◦
(
2
−
1
◦
◦
◦
CH CN and 40% water (v/v). Prior to liquid chromatography analy-
3
−
1
◦
sis, the basic filtrate was neutralized using 1.0 mol l HCl. Catalyst
activity was evaluated using the initial activity calculated based on
the first-order rate law at 2,4-DCP conversion below 25%. To test
the data reproducibility, two separate runs of the HDC of 2.4-DCP
on Pd(2.7)/B-CMK-3 were conducted, and the results indicated a
high data reproducibility (see Fig. 1S, supporting information).
◦
.65 g of sucrose, 5.3 g of water and 0.1 ml of concentrated sulfu-
◦
atmosphere. The SBA-15 templates was removed using 7.0 wt.% of
HF solution at room temperature, followed by washing with water
and ethanol (1:1, v/v) solution under stirring. Boron-doped meso-
porous carbon was prepared according to the method described
for CMK-3, except using an infiltration solution consisting of 2.5 g
of sucrose, 8 g of water, 0.113 g of boric acid, and 0.15 ml of concen-
trated sulfuric acid. The resulting boron-doped mesoporous carbon
was denoted as B-CMK-3.
3. Results and discussion
3.1. Catalyst characterization
The low-angle XRD patterns of SBA-15, CMK-3 and B-CMK-3
are presented in Fig. 1a. The XRD pattern of SBA-15 displayed three
◦
◦
◦
well-resolved diffraction peaks at 0.90 , 1.55 and 1.8 respectively
characteristics of (1 0 0), (1 1 0) and (2 0 0) diffractions, indicative of
the ordered mesoporous structure with p6mm hexagonal symme-
try [30]. XRD patterns of CMK-3 and B-CMK-3 also showed strong
(1 0 0) diffraction, but much weak diffractions at (1 1 0) and (2 0 0).
Compared with SBA-15, the diffraction peaks of both materials
shifted to higher angles due to the structural shrinkage of SBA-
15 during the replication process. The wide-angle XRD patterns of
Supported Pd catalysts were prepared via complexing PdCl2
with EDTA followed by NaBH4 reduction [28]. Briefly, a desired
−
1
amount of PdCl2 and 0.1 mol l
EDTA solution was added to
◦
distilled water and heated at 60 C for 40 min under vigorously
stirring to dissolve PdCl . After cooling to room temperature, the
2
−
1
solution pH was adjusted to 9.5 with 0.1 mol l NaOH solution.
Then, 1 g of CMK-3 or B-CMK-3 was added to the solution. After

3
38
J. Zhou et al. / Applied Catalysis A: General 470 (2014) 336–343
Fig. 2. (a) N2 adsorption–desorption isotherms and (b) pore size distributions of
SBA-15, CMK-3 and B-CMK-3. Filled symbols denote adsorption branch and open
symbols denote desorption branch.
Fig. 1. XRD patterns of SBA-15, CMK-3 and B-CMK-3.
TEM images of the catalysts are illustrated in Fig. 3. For all sam-
ples tested, ordered mesopore arrangements of the supports were
visible, and small Pd particles were clearly identified. The average
surface-area-weighted diameters of Pd particles supported on the
supports can be evaluated using Eq. (1) [33]:
CMK-3 and B-CMK-3 are shown in Fig. 1b. For CMK-3 and B-CMK-3,
◦
◦
two diffraction peaks were observed around 23 and 44 char-
acteristic of (0 0 2) and (1 0 1) diffractions from graphitic carbon
structure [31,32].
Nitrogen adsorption–desorption isotherms of the samples are
presented in Fig. 2a, with calculated parameters summarized in
Table 1. Typical capillary condensation was observed within a rela-
tive pressure range of 0.6–0.8 for SBA-15, and 0.40–0.65 for CMK-3
and B-CMK-3, suggesting the presence of mesopores in the sam-
ples. The BET surface area and pore volume of B-CMK-3 were
ꢀ
nidi3
nidi2
ds = ꢀ
(1)
where n is the number of Pd particles counted having diameter of
i
ꢀ
di, and the total number of particles counted ( ni) is larger than
150.
2
−1 −1
3
7
95.7 m g
and 0.83 cm g , respectively, slightly lower than
those of CMK-3. The pore size distributions of the samples were
calculated using the Barrett–Joyner–Halenda (BJH) method and are
presented in Fig. 2b. The most probable pore diameter of B-CMK-
The average diameters of Pd particles thus calculated are listed
in Table 2. For Pd(2.7)/B-CMK-3, the average size of Pd particles
was 2.5 nm, much smaller than that of Pd(2.6)/CMK-3 (4.1 nm), sug-
gesting that B-doping facilitates the dispersion of Pd particles over
mesoporous carbon. The better dispersion of Pd in Pd(2.7)/B-CMK-3
3
was identical to that of CMK-3, but much smaller than that of
SBA-15.
Table 1
Properties of SBA-15, CMK-3, and B-CMK-3.
BET surface area (m2
g
−1
)
Mesoporous volume (cm3
g
−1
)
Pore volume (cm3
g
−1
)
Pore diameter (nm)
Samples
SBA-15
CMK-3
B-CMK-3
574.3
929.2
795.7
0.77
0.88
0.74
0.80
0.93
0.83
6.5
3.7
3.8

J. Zhou et al. / Applied Catalysis A: General 470 (2014) 336–343
339
Fig. 3. TEM images of Pd catalysts supported on CMK-3 and B-CMK-3.
could be caused by the stronger metal–support interaction between
superficial Pd particles and substitutional boron defects. It was fur-
ther noticed that the average size of Pd particles increased with
increasing Pd loading, as indicated by the calculated Pd particles
on Pd(1.6)/B-CMK-3 (1.8 nm) and Pd(3.9)/B-CMK-3 (4.2 nm).
The structural properties of the supports were further inves-
tigated by XPS and their spectra are shown in Fig. 4. The carbon
For all samples tested, the Pd 3d5/2 peak could be deconvoluted
into two peaks with binding energies around 336.0 and 337.2 eV,
0
2+
assigned to Pd and Pd respectively [34,35]. Besides metallic Pd
0
(Pd ), Pd(2.6)/CMK-3 and Pd(2.7)/B-CMK-3 also contained signifi-
cant amounts of cationic Pd species (Pd2+). The calculated Pd /Pd
2+
0
ratio was 1.13 for Pd(2.7)/B-CMK-3, much higher than that for
Pd(2.6)/CMK-3 (0.75). Such a higher Pd2+ content might result
from the stronger metal–support interaction in the former cata-
lyst having substitutional boron defects. The strong metal–support
interaction was further verified by the XPS spectra of the Pd/B-
CMK-3 catalysts with varied Pd loadings (results presented in
Fig. 4d). Increasing Pd loading from 1.6 to 3.9 wt.% led to a decrease
in Pd2+/Pd ratio from 1.17 to 0.82, reflecting that the cationization
of Pd particle as well as the metal–support interaction is attenuated
with Pd particle size [36].
1
s signals of CMK-3 and B-CMK-3 were markedly broadened.
Upon deconvolution, three peaks were identified at 284.6, 286 and
89 eV, which can be assigned to lattice carbon, carbon atoms singly
2
and doubly coordinated with oxygen atoms (C O, C O), respec-
tively [31]. The XPS spectra of boron 1s signal of B-CMK-3 (see
Fig. 4b) revealed two peaks at 191.5 and 187.5 eV, corresponding
to B O and B C bonding, respectively [20].
The XPS spectra of Pd(2.6)/CMK-3 and Pd(2.7)/B-CMK-3 in
the Pd 3d region are depicted in Fig. 4c. In comparison with
Pd(2.6)/CMK-3, the Pd 3d5/2 peak in Pd(2.7)/B-CMK-3 shifted to
higher binding energy, implying the coexistence of different Pd
species in the catalysts. The XPS profiles of the samples were again
deconvoluted and the resulting parameters are listed in Table 2.
0
3
3
.2. Liquid phase catalytic HDC of 2,4-DCP
.2.1. Effect of boron-doping
Possible mass transfer limitation in the reaction system was
tested prior to catalyst activity measurement. Initial catalytic
activities of Pd(2.7)/B-CMK-3 with varied catalyst dosages were
determined and the results are presented in Fig. 2S, supporting
information. Although the reaction rate increased with catalyst
dosage, initial activities remained nearly identical, suggesting the
absence of mass transfer limitation under our experimental condi-
tions [37].
The catalytic HDC of 2,4-DCP over Pd(2.6)/CMK-3 and Pd(2.7)/B-
CMK-3 is compared in Fig. 5. Experimental results showed
that 2-chlorophenol was the only intermediate product iden-
tified in the reaction system, and cyclohexanone, a further
Table 2
Parameters of XPS spectra and Pd particle size of the catalysts.
Catalyst
Pd contenta
wt.%)
Pd particle
size (nm)
Pd2+/Pd0
b
(
Pd(2.6)/CMK-3
2.6
1.6
2.7
3.9
4.1
1.8
2.5
4.2
0.75
1.17
1.13
0.82
Pd(1.6)/B-CMK-3
Pd(2.7)/B-CMK-3
Pd(3.9)/B-CMK-3
a
Determined by ICP.
Calculated from TEM.
b

3
40
J. Zhou et al. / Applied Catalysis A: General 470 (2014) 336–343
Fig. 4. XPS profiles of (a) CMK-3 and B-CMK-3 in the C 1s region, (b) in the B 1s region, (c) Pd(2.6)/CMK-3 and Pd(2.7)/B-CMK-3, and (d) B-CMK-3 supported Pd catalysts.
hydrogenation product from phenol, was not detected. The absence
of 4-chlorophenol can be explained in terms of the steric hin-
drance effect, by which the hydrogenation of ortho-substituted Cl
is substantially suppressed in comparison with para-substituted
Cl [6,38]. Catalytic activities of the two catalysts, which had simi-
lar Pd loadings, were found to be closely related to the supports.
After reaction for 30 min, approximately 96.8% of 2,4-DCP was
reduced on Pd(2.7)/B-CMK-3, while only 59.3% of 2,4-DCP was
removed on Pd(2.6)/CMK-3, reflecting a much higher catalytic
activity of Pd(2.7)/B-CMK-3 than that of Pd(2.6)/CMK-3. As indi-
cated by TEM and XPS analysis, boron doping results in an enhanced
metal–support interaction, by which Pd particles are effectively dis-
persed and exhibit strong Pd cationization. At similar Pd contents,
the higher Pd dispersion (small Pd particle size) reflects more avail-
able Pd sites, and thus a higher catalytic activity was observed for
Pd(2.7)/B-CMK-3. Additionally, Pd2 in superficial Pd particle acts
as the active site for the activation of C Cl bond via abstraction of
nucleophilic chloride anion, facilitating the HDC of 2,4-DCP. Hence,
the smaller Pd particle size and higher Pd2 content in Pd(2.7)/B-
CMK-3 account for its higher catalytic activity.
the initial concentration of 2,4-DCP, indicating that the reaction
rate was positively related to the surface concentration of 2,4-DCP
adsorbed on the catalyst. The experimental data were further
fitted to the Langmuir–Hinshelwood model [39]:
bC0
r0 = kꢀs = k
(2)
1
+ bC0
1
r0
1
kbC0
1
=
+ k
(3)
where r is the initial reaction rate, C is the initial concentration of
0
0
2,4-DCP, ꢀ is the surface coverage of 2,4-DCP adsorption, k is the
s
reaction rate constant, and b is the adsorption constant for 2,4-DCP.
The plot of 1/r0 versus 1/C0 is presented in Fig. 6b. A good linear
relationship between 1/r0 and 1/C0 with R2 higher than 0.99 was
+
observed, confirming that the catalytic HDC of 2,4-DCP well follows
the Langmuir–Hinshelwood model.
+
3.2.3. Effect of Pd loading amount
The influence of Pd loading on the catalytic HDC of 2,4-DCP
is presented in Fig. 7. The initial catalytic activity increased from
−
1
−1
when Pd loading increased from 1.6 to
3.2.2. Effect of initial 2,4-DCP concentration
0.260 to 0.608 M gCat
h
In a heterogeneous catalysis reaction, reactant adsorption
2.7 wt.%, whereas further increasing Pd loading to 3.9 wt.% resulted
in slightly decreased catalytic activity to 0.517 M gCat
−
1
−1
h . The
on catalyst surface is considered as a prerequisite step and
the reaction rate is correlated to the concentration of reactant
adsorbed on catalyst surface, which can be well described by the
Langmuir–Hinshelwood model. To verify the potential reaction
mechanism, catalytic HDC of 2,4-DCP with varied initial concen-
trations was conducted (results presented in Fig. 6). Obviously,
the initial catalytic activity of Pd(2.7)/B-CMK-3 increased with
varied catalytic activity with Pd loading was likely related to the
structural properties of superficial Pd particles of the catalysts.
The influence of the properties of Pd particle on the catalytic
activity can be further verified from the dependency of the initial
catalytic activity normalized by Pd loading amount (results pre-
sented in Fig. 7b) on Pd particle size. Assuming exposed Pd sites

J. Zhou et al. / Applied Catalysis A: General 470 (2014) 336–343
341
Fig. 5. The catalytic HDC of 2,4-DCP over (a) Pd(2.6)/CMK-3 and (b) Pd(2.7)/B-CMK-
Fig. 6. (a) The catalytic HDC of 2,4-DCP at varied initial 2,4-DCP concentrations and
−1
3. Reaction conditions: pH 12. Catalyst dosage: 0.25 g l , 2,4-DCP concentration:
.0 mM.
(
b) linear plot of 1/r0 versus 1/C0. Reaction conditions: pH 12. Catalyst: Pd(2.7)/B-
3
−1
CMK-3. Catalyst dosage: 0.25 g l . Line represents the fitting curve using the
Langmuir–Hinshelwood model.
3
.2.4. Reaction kinetics
As depicted in Scheme 1, the liquid phase catalytic HDC of
,4-DCP could be completed through stepwise or/and concerted
in catalyst have identical catalytic activities, a decrease in the nor-
malized initial activity with Pd loading would be caused simply
by increasing Pd particle size. However, the observed correlation
between normalized initial catalytic activity and Pd particle size
followed a volcano-type relationship (see Fig. 7b), suggesting that
the activities of the exposed Pd sites differed with the structure of
superficial Pd particle.
2
mechanisms [9]. Assuming that all reactions involved follow the
pseudo-first-order kinetics, the reaction rate equations involved in
the HDC of 2,4-DCP can be described as follows:
dC2,4-DCP
R2,4-DCP
=
= −(k + k )C
(4)
(5)
(6)
1
2
2,4-DCP
dt
For catalytic HDC of 2,4-DCP on supported Pd catalyst, activa-
tion of both C Cl bond and H is believed to be the two crucial
2
steps [40,41], in which Pd2 and metallic Pd are corresponding
active sites. Therefore, a higher Pd2 content in the catalyst would
achieve a higher catalytic activity because of enhanced activation of
+
dC2-CP
R2-CP
=
= k C
− k C
1
2,4-DCP
3
2-CP
+
dt
C
Cl bond in 2,4-DCP, while a larger Pd particle would have larger
capability for H2 activation as a result of favorable formation of
-PdH phase and enhanced H2 solubility [42,43]. Compared with
dCPhenol
RPhenol
=
= k C
+ k C
2
2,4-DCP
3 2-CP
dt
␤
Accordingly, the rate constants could be calculated according to
Eqs. (7)–(9):
Pd(1.6)/B-CMK-3, Pd(2.7)/B-CMK-3 displayed a higher normalized
initial activity due to its larger Pd particle, causing enhanced H2
activation. In parallel, despite of a lower Pd2+ content, the normal-
ized initial activity of Pd(3.9)/B-CMK-3 was only slightly lower than
that of Pd(1.6)/B-CMK-3, confirming the critical role of Pd particle
size on catalyst activity.
0
C2,4-DCP = C2
exp(−(k1 + k2)t)
(7)
,4-DCP
k1
0
C2-CP = C
(exp(−(k + k )t) − exp(−k )t)
2
(8)
2,4-DCP
1
2
3
k − k − k
3
1

3
42
J. Zhou et al. / Applied Catalysis A: General 470 (2014) 336–343
Table 3
Fitting rate constants of the catalytic HDC of 2,4-DCP on Pd(2.6)/CMK-3 and Pd(2.7)/B-CMK-3.
k1 (min 1
−
)
k2 (min
−1
)
k3 (min
−1
)
k1 + k2 (min
−1
)
k2/k1
Catalyst
Pd(2.6)/CMK-3
Pd(2.7)/B-CMK-3
0.027
0.115
0.0085
0.121
0.0197
0.052
0.036
0.236
0.31
1.05
ꢁ
ꢁ
k k
− 1
reaction time t, k , k2 and k3 are the rate constants of 2,4-DCP to
0
1
3
1
CPhenol = C
(exp(−(k + k )t) − 1)
2
2,4-DCP
1
2
k − k − k
k + k
2-CP, 2,4-DCP to phenol, and 2-CP to phenol, respectively.
Listed in Table 3 are the rate constants evaluated by fitting
experimental data to Eqs. (7)–(9). The calculated conversion rates
3
1
2
1
ꢂ
ꢂ
1
k2
+k (exp(−k )t − 1)
−
(exp(−(k + k )t) − 1)
(9)
3
1
2
3
k1 + k
2
of 2,4-DCP (k + k ) over Pd(2.7)/B-CMK-3 and Pd(2.6)/CMK-3 were
1
2
where C20
is the initial 2,4-DCP concentration, C2
and CPhenol are the concentrations of 2,4-DCP, 2-CP and phenol at
, C2-CP
−1
0.236 and 0.036 min respectively, in line with the much higher
catalytic activity observed for Pd(2.7)/B-CMK-3. For both Pd(2.7)/B-
CMK-3 and Pd(2.6)/CMK-3, as k1 and k2 are substantially high,
,4-DCP
,4-DCP
2
,4-DCP reduction is believed to proceed concurrently in both step-
wise and concerted pathways. Additionally, k /k ratios were found
2
1
to be 0.31 for Pd(2.6)/CMK-3 and 1.05 for Pd(2.7)/B-CMK-3, indica-
tive of an increasing importance of the concerted dechlorination
pathway upon boron doping. Notably, the ratios of Pd2+/Pd in
Pd(2.6)/CMK-3 and Pd(2.7)/B-CMK-3 are 0.75 and 1.13, respec-
tively. The higher Pd2+ content in Pd(2.7)/B-CMK-3 favors the
simultaneous activation of C-Cl bonds in ortho- and para-positions
to the hydroxy group of 2,4-DCP, giving rise to an overwhelming
concerted dechlorination pathway for the catalytic HDC of 2,4-DCP
on Pd(2.7)/B-CMK-3.
0
4. Conclusions
In the present study, Pd catalysts supported on CMK-3 and
B-CMK-3 were prepared and the liquid phase catalytic HDC of
,4-DCP over the catalysts was studied. The HDC of 2,4-DCP over
2
B-CMK-3 followed an adsorption–controlled reaction mechanism.
At a similar Pd loading, Pd/B-CMK-3 exhibited a much higher cat-
alytic activity than Pd/CMK-3, which could be ascribed to its higher
2+
Pd content and smaller average Pd particle size. A volcano type
relationship was identified between the catalytic activity of Pd/B-
CMK-3 and Pd particle size. For both Pd/B-CMK-3 and Pd/CMK-3
catalysts, hydrochlorination of 2,4-DCP proceeded in both step-
wise and concerted pathways. However, the concerted pathway
was more pronounced for Pd/B-CMK-3, which could be explained
by its higher Pd2+ content and thus simultaneous activation of the
two C-Cl bonds of 2,4-DCP. The present study suggested that B-
doping of carbonaceous support is a feasible and effective approach
to improve the catalytic performance of supported Pd catalysts for
hydrodeclorination of 2,4-DCP.
Acknowledgements
The financial support from the Natural Science Foundation
of China (no. 21277066 and 21107043), Special Foundation for
the Doctor-subject of China (no. 20100091120016) is gratefully
acknowledged. We are indebted to the Modern Analytical Center,
Nanjing University for the catalyst characterization.
Appendix A. Supplementary data
Fig. 7. (a) The catalytic HDC of 2,4-DCP on the Pd/B-CMK-3 catalysts and (b) depend-
ence of normalized initial activity on Pd particle size. Reaction conditions: pH 12.
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.apcata.
−1
Catalyst dosage: 0.25 g l
.
2
013.11.005.
References
[
[
1] G.Q. Chen, S. Guan, G.M. Zeng, X.D. Li, A.W. Chen, C. Shang, Y. Zhou, H.K. Li, J.M.
He, Applied Microbiology Biotechnology 97 (2013) 3149–3157.
2] R. Andreozzi, I.D. Somma, R. Marotta, G. Pinto, A. Pollio, D. Spasiano, Water
Research 45 (2011) 2038–2048.
Scheme 1. Reaction pathways of the liquid phase catalytic HDC of 2,4-DCP.

J. Zhou et al. / Applied Catalysis A: General 470 (2014) 336–343
343
[
[
3] U. S. EPA, Appendix A to 40 CFR, Part 423-126 Priority Pollutants, 2012.
4] H.D.M. Roy, C.M. Wai, T. Yuan, J.K. Kim, W.D. Marshall, Applied Catalysis A:
General 271 (2004) 137–143.
[24] C.K. Acharya, C.H. Turner, Journal of Physical Chemistry
14804–14812.
[25] H.I. Lee, J.H. Kim, D.J. You, J.E. Lee, J.M. Kim, W.S. Ahn, C. Pak, S.H. Joo, H. Chang,
Advanced Materials 20 (2008) 757–762.
[26] Z.K. Huan, E.M. Zong, D. Wei, H.Q. Wan, S.R. Zheng, Z.Y. Xu, Environmental
Science 33 (2012) 3479–3483.
[27] D.Y. Zhao, J.L. Feng, Q.S. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D.
Stucky, Science 279 (1998) 548–552.
[28] Y. Zhu, Y.G. Kang, Z.Q. Zou, Q. Zhou, J.W. Zheng, B.J. Zia, H. Yang, Electrochem-
istry Communications 10 (2008) 802–805.
[29] G. Yuan, M.A. Keane, Chemical Engineering Science 58 (2003) 257–267.
[30] F.L. Liu, Z.Y. Xu, H.Q. Wan, Y.Q. Wan, S.R. Zheng, D.Q. Zhu, Environmental Toxi-
cology and Chemistry 20 (2011) 793–800.
C
111 (2007)
[
[
[
[
[
5] C.H. Xia, J. Xu, W.Z. Wu, X.M. Liang, Catalysis Communications 5 (2004)
3
83–386.
6] G. Yuan, M.A. Keane, Industrial & Engineering Chemistry Research 46 (2007)
05–715.
7] Z.H. Jin, X.Y. Wang, S.T. Wang, D. Li, G.Z. Lu, Catalysis Communications 10 (2009)
027–2030.
8] Z.M. Pedro, E. Diaz, A.F. Mohedano, J.A. Casas, J.J. Rodriguez, Applied Catalysis
B: Environmental 103 (2011) 128–135.
9] Y. Shao, Z.Y. Xu, H.Q. Wan, Y.Q. Wan, H. Chen, S.R. Zheng, D.Q. Zhu, Catalysis
Communications 12 (2011) 1405–1409.
7
2
[
[
[
[
[
[
[
[
[
10] C.E. Chan-Thaw, A. Villa, P. Katekomol, D.S. Su, A. Thomas, L. Prati, Nano Letters
0 (2010) 537–541.
11] D. Tasis, N. Tagmatarchis, A. Bianco, M. Prato, Chemistry Reviews 106 (2006)
105–1136.
12] L. Calvo, M.A. Gilarranz, J.A. Casas, A.F. Mohedano, J.J. Rodrıguez, Applied Catal-
ysis B: Environmental 67 (2006) 68–76.
13] E. Diaz, A.F. Mohedano, J.A. Casas, L. Calvo, M.A. Gilarranz, J.J. Rodriguez, Applied
Catalysis B: Environmental 106 (2011) 469–475.
14] K. Chizari, I. Janowska, M. Houlle, I. Florea, O. Ersen, T. Romero, P. Bernhardt,
M.J. Ledoux, C. Pham-Huu, Applied Catalysis A: General 380 (2010) 72–80.
15] X.C. Zhao, A.Q. Wang, J.W. Yan, G.Q. Sun, L.X. Sun, T. Zhang, Chemistry Materials
[31] S.L. Ding, S.J. Zheng, M.J. Xie, L.M. Peng, X.F. Guo, W.P. Ding, Microporous and
Mesoporous Materials 142 (2011) 609–613.
[32] Y.D. Xia, R. Mokaya, Advanced Materials 16 (2004) 1553–1558.
[33] G. Yuan, M.A. Keane, Applied Catalysis B: Environmental 52 (2004) 301–314.
[34] G.X. Yang, Y. Chen, Y.M. Zhou, Y.W. Tang, T.H. Lu, Electrochemistry Communi-
cations 12 (2010) 492–495.
[35] G.J. Zhang, Y.N. Wang, X. Wang, Y. Chen, Y.M. Zhou, Y.W. Tang, L.D. Lu, J.C. Bao,
T.H. Lu, Applied Catalysis B: Environmental 102 (2011) 614–619.
[36] J. Zhou, Y.X. Han, W.J. Wang, Z.Y. Xu, H.Q. Wan, D.Q. Yin, S.R. Zheng, D.Q. Zhu,
Applied Catalysis B: Environmental 134–135 (2013) 222–230.
[37] O.M. Ilinitch, F.P. Cuperus, L.V. Nosova, E.N. Gribov, Catalysis Today 56 (2000)
137–145.
[38] C.H. Xia, Y. Liu, J. Xu, J.B. Yu, W. Qin, X.M. Liang, Catalysis Communications 10
(2009) 456–458.
[39] Z.M. Pedro, J.A. Casas, L.M. Gomez-Sainero, J.J. Rodriguez, Applied Catalysis B:
Environmental 98 (2010) 79–85.
[40] L.M. Gomez-Sainero, X.L. Seoane, J.L.G. Fierro, A. Arcoya, Journal of Catalysis
209 (2002) 279–288.
1
1
2
2 (2010) 5463–5473.
16] D. Hulicova-Jurcakova, A.M. Puziy, O.L. Poddubnaya, F. Suarez-Garcia, J.M.D.
Tascon, G.Q. Lu, Journal of American Chemistry Society 131 (2009) 5026–5027.
17] Q. Liu, Z.M. Cui, Z. Ma, S.W. Bian, W.G. Song, Journal of Physical Chemistry C 12
(
2008) 1199–1203.
18] B. Wang, M. Lin, T.P. Ang, J. Chang, Y.H. Yang, A. Borgna, Catalysis Communica-
tions 25 (2012) 96–101.
[
[
19] L.R. Radovic, M. Karra, K. Skokova, P.A. Thrower, Carbon 36 (1998) 1841–1854.
20] D.W. Wang, F. Li, Z.G. Chen, G.Q. Lu, H.M. Cheng, Chemistry Materials 20 (2008)
[41] S. Omar, J. Palomar, L.M. Gomez-Sainero, M.A. Alvarez-Montero, M. Martin-
Martinez, J.J. Rodriguez, Journal of Physical Chemistry
14180–14192.
C 115 (2011)
7
195–7200.
[
[
[
21] X.X. Wu, L.R. Radovic, Journal of Physical Chemistry A 108 (2004) 9180–9187.
22] D.H. Zhong, H. Sano, Y. Uchiyama, K. Kobayashi, Carbon 38 (2000) 1199–1206.
[42] M.A. Aramendia, V. Borau, I.M. Garcia, C. Jimenez, J.M. Marinas, F.J. Urbano,
Applied Catalysis B: Environmental 20 (1999) 101–110.
[43] M.A. Aramendia, V. Borau, I.M. Garcia, C. Jimenez, J.M. Marinas, F.J. Urbano,
Journal of Catalysis 187 (1999) 392–399.
23] C.K. Acharya, C.H. Turner, Journal of Physical Chemistry
7706–17710.
B
110 (2006)
1