Enhanced catalytic hydrodechlorination of 2,4-dichlorophenol over Pd catalysts supported on nitrogen-doped graphene

Article abstract of DOI:10.1039/c5ra17946g

Pd catalysts supported on graphene and N-doped graphene (GN-1, GN-2 and GN-3) with varied N-doping amounts were prepared using the deposition-precipitation method, and liquid phase catalytic hydrodechlorination (HDC) of 2,4-dichlorophenol (2,4-DCP) was investigated over these catalysts. The catalysts were characterized by X-ray diffraction, elementary analysis, N₂ adsorption-desorption isotherms, transmission electron microscopy, and X-ray photoelectron spectroscopy. Characterization results showed that graphene could be successfully doped by N using the heat treatment method with melamine as precursor, and N doping amounts were determined to be 5.7, 8.6 and 11.3% for GN-1, GN-2 and GN-3, respectively. Additionally, Pd²⁺/Pd⁰ ratios and Pd dispersions in the Pd/GN catalysts were much higher than those in Pd/graphene. For a similar Pd loading, the Pd dispersion of Pd/GN first increased and then decreased with the increase of N-doping amount, and the highest Pd dispersion was observed on Pd(2.9)/GN-2. Accordingly, GN supported Pd catalysts exhibited much higher catalytic activities than Pd/graphene, the catalytic reaction first increased and then decreased slightly in activity with the increase of nitrogen doping amount, and the highest activity was identified on Pd(2.9)/GN-2. Moreover, the dechlorination of 2,4-DCP over supported Pd catalysts proceeded via both a stepwise and concerted pathway, and the concerted pathway became predominant upon N doping.

Full text of DOI:10.1039/c5ra17946g

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Enhanced catalytic hydrodechlorination of 2,4-
dichlorophenol over Pd catalysts supported on
nitrogen-doped graphene†
Cite this: RSC Adv., 2015, 5, 91363
a
a
b
b
*
Juan Zhou, Quanyuan Chen, Yuxiang Han and Shourong Zheng
Pd catalysts supported on graphene and N-doped graphene (GN-1, GN-2 and GN-3) with varied N-doping
amounts were prepared using the deposition–precipitation method, and liquid phase catalytic
hydrodechlorination (HDC) of 2,4-dichlorophenol (2,4-DCP) was investigated over these catalysts. The
catalysts were characterized by X-ray diffraction, elementary analysis, N2 adsorption–desorption
isotherms, transmission electron microscopy, and X-ray photoelectron spectroscopy. Characterization
results showed that graphene could be successfully doped by N using the heat treatment method with
melamine as precursor, and N doping amounts were determined to be 5.7, 8.6 and 11.3% for GN-1, GN-
2 and GN-3, respectively. Additionally, Pd2+/Pd0 ratios and Pd dispersions in the Pd/GN catalysts were
much higher than those in Pd/graphene. For a similar Pd loading, the Pd dispersion of Pd/GN first
increased and then decreased with the increase of N-doping amount, and the highest Pd dispersion was
observed on Pd(2.9)/GN-2. Accordingly, GN supported Pd catalysts exhibited much higher catalytic
activities than Pd/graphene, the catalytic reaction first increased and then decreased slightly in activity
with the increase of nitrogen doping amount, and the highest activity was identified on Pd(2.9)/GN-2.
Moreover, the dechlorination of 2,4-DCP over supported Pd catalysts proceeded via both a stepwise and
concerted pathway, and the concerted pathway became predominant upon N doping.
Received 3rd September 2015
Accepted 20th October 2015
DOI: 10.1039/c5ra17946g
www.rsc.org/advances
surface areas and strong resistance to acid, alkalis and chlorine
poisoning during the HDC reaction.12 For example, Pedro et al.13
1. Introduction
compared the HDC of 4-chlorophenol over Pd/Al2O3 and Pd/AC
and concluded that the AC supported catalyst was less suscep-
tible to the chloride poisoning during the reaction. Diaz et al.14
also found that Pd/AC catalyst maintained a constant activity in
the HDC of 2,4-DCP.
Chlorophenols are widely used chemical raw materials and
intermediate products in the manufacture of pesticides, dyes,
leather and disinfectants.1–3 These chemicals have very strong
toxicity and low biodegradability. The discharge of chlor-
ophenols due to industrial production and human activities
could cause severe environmental pollution. Hence, effective
treatment techniques are highly demanded to eliminate chlor-
ophenol pollution in water.
At present, a variety of techniques are available to treat
chlorophenols in water, such as advanced oxidation process,4,5
incineration,6 biological methods7,8 etc. Recently, the liquid
phase catalytic hydrodechlorination (HDC) method is consid-
ered as a promising method for the treatment of chlorinated
organic pollutants.9–11 Supported noble metal catalysts are
usually employed in the HDC reactions, and carbon materials
are the most commonly used supports because of their high
For the liquid phase catalytic HDC of 2,4-DCP, smaller Pd
particles usually intrinsically higher specic activities.15 More-
over, strong metal–support interaction between Pd particles
and the carrier can form electron-decient Pd active sites to
favor the HDC reaction.16 Carbon materials normally have weak
interactions with the supported noble metal due to their strong
hydrophobicity, resulting in aggregated metal particles on the
support, and easy leaching during the liquid phase reaction.17,18
Hence, many researchers attempted to modify carbonaceous
supports to enhance the noble metal dispersion/stability as well
as the catalytic activities. Heteroatom doping (e.g. N, B, or P) has
been recognized as an effective method to modify the carbon
surface and introduce chemically reactive sites for anchoring
metal nanoparticles19–22 on carbonaceous supports. Nitrogen is
one of the mostly used elements to improve the catalytic activity
and stability. For example, Liu et al.23 found that N-doped
mesoporous carbon supported Pt catalyst with uniform Pt
dispersion exhibited a higher catalytic activity for p-nitrophenol
reduction than mesoporous carbon supported Pt catalyst.
aSchool of the Environment, Donghua University, Shanghai 201620, China
bState Key Laboratory of Pollution Control and Resource Reuse, Jiangsu Key Laboratory
of Vehicle Emissions Control, School of the Environment, Nanjing University, Nanjing
210023, China. E-mail: srzheng@nju.edu.cn; Fax: +86-25-89680596; Tel: +86-25-
89680373
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra17946g
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 91363–91371 | 91363

View Article Online
RSC Advances
Paper
Chizari et al.24 investigated the liquid-phase hydrogenation of washed thoroughly with distilled water, followed by drying at
ꢁ
cinnamaldehyde over nitrogen-doped carbon nanotubes sup- 105 C for 6 h and nally reducing under a H2 stream (40 ml
ported Pd catalysts, and concluded that the introduction of minꢀ1) at 300 ꢁC for 2 h. The resulting catalysts are referred to as
nitrogen atoms into the carbon matrix effectively modied the Pd(x)/graphene or Pd(x)/GN, where x is the Pd loading amount
chemical properties of the support, which substantially (wt%). All catalysts were ground to pass through a 400-mesh
enhanced the hydrogenation activities. In parallel, our previous sieve (<37 mm) to avoid the potential intraparticle diffusion
study found that Pd catalysts supported on B-doped meso- before catalytic test.33
porous carbon had a higher HDC activity than that on meso-
porous carbon.25
2.2. Catalyst characterization
As a new carbon material graphene consisted of monoatomic
Surface areas of the samples were measured using the Bru-
layer structure with sp2 hybridization, and processed excellent
nauer–Emmett–Teller (BET) method on a Micromeritics ASAP
electrical conductivity, high chemical and thermal stability, and
2020 (Micromeritics Instrument Co., Norcross, GA) instrument.
large surface area, which could be used as a common catalyst
The samples were degassed under N2 ow at 100 ꢁC for 4 h prior
support for the liquid phase catalytic HDC operating under very
to analyze at ꢀ196 ꢁC (77 K). Powder X-ray diffraction (XRD)
strong alkaline conditions.26–28 Up to now, however, graphene
patterns of the samples were obtained using a Rigaku D/max-RA
and doped graphene were mostly applied in electrochemical
powder diffraction-meter (Rigaku, Tokyo, Japan) equipped with
elds,29–31 and few studies on HDC of chlorinated phenols had
a Cu Ka radiation. Elemental analysis was conducted on an
been conducted over catalysts supported on graphene or
elemental analyzer (CHN-O-Rapid, Germany). The Pd contents
heteroatoms doped graphene.
in the catalysts were determined on an inductive coupled
In the present study, we prepared a series of Pd catalysts
plasma emission spectrometer (ICP) (J-A1100, Jarrell-Ash, USA).
supported on graphene and N-doping graphene with varied
Transmission electron microscopy (TEM) images of the samples
nitrogen and Pd contents, and investigated the liquid phase
were conducted on a JEM-200CX electron microscope (JEOL Co.,
catalytic HDC of 2,4-DCP over these catalysts. The results
Tokyo, Japan). X-ray photoelectron spectroscopy (XPS) was
showed that Pd catalysts supported on N-doping graphene had
performed on
a PHI-5000 Versa Probe using a mono-
smaller Pd particle sizes and higher Pd2+ concentrations,
exhibiting enhanced catalytic activities for the liquid phase
catalytic HDC of 2,4-DCP.
chromatized Al Ka excitation source (hn ¼ 1486.6 eV) (ULVAC-
PHI, Japan). The C 1s peak (284.6 eV) was used for the calibra-
tion of binding energy.
2. Experimental
2.3. Liquid phase catalytic HDC of 2,4-DCP
2.1. Catalyst preparation
The liquid phase catalytic HDC reactions of 2,4-DCP were
Graphite oxide (BET surface area of 4.5 m2 gꢀ1) was purchased carried out in a 250 ml of four-necked ask reactor with
from Xianfeng Nanomaterials company (Nanjing, China). a sample port, pH-stat, H2/N2 inlet and outlet under atmo-
Nitrogen-doped graphene (GN) were prepared using melamine spheric pressure of hydrogen/nitrogen. The reaction tempera-
as the N-containing precursor.32 Briey, 0.6 g of graphite oxide ture was stabilized at 25 ꢂ 0.5 ꢁC with a water-bath (SDC-6,
was suspended in 2 ml of alcohol solution containing a certain Scientz Co., China). Briey, 50 mg of catalyst was suspended
amount of melamine and stirred at room temperature for 5 h, in 200 ml of 3.0 mM 2,4-DCP solution with pH pre-adjusted to
then the mixture was dried by slowing vaporizing alcohol at 80 12 using 1.0 M NaOH aqueous solution. The mixture was
ꢁC. The resulting sample was placed in a tubular oven, which purged with a N2 ow (50 ml minꢀ1) for 30 min, and then the N2
was heated to 950 ꢁC in an inert N2 atmosphere with a ramping ow was switched to a H2 ow (250 ml minꢀ1) under continu-
rate of 10 ꢁC minꢀ1, and hold at 950 ꢁC for 30 min. Aer cooling ously vigorous stirring (1400 rpm). Samples were taken at
down to room temperature, the material was washed with selected time intervals and the catalyst particles were separated
boiling water several times to remove excess melamine by fast ltration. The concentrations of the reactant, interme-
decomposition products. Samples with varied N contents were diate and product in the ltrate were determined by a high-
recorded as GN-1, GN-2 and GN-3, respectively. The N-doping performance liquid chromatography with an ultraviolet (UV)
contents determined using an elemental analyzer (CHN-O- detector at 270 nm using a 4.6 ꢃ 150 mm HC-C18 column
Rapid, Germany) were 5.7, 8.6 and 11.3% for GN-1, GN-2 and (Agilent) as stationary phase and a mixture of acetonitrile and
GN-3, respectively. Graphene was prepared according to the water (60 : 40, v/v) as mobile phase. Prior to liquid chromatog-
same process without adding melamine.
raphy analysis, the ltrate was neutralized using 1.0 M HCl. The
Supported Pd catalysts with different supports (graphene initial activity of the catalyst was calculated based on the rst-
and GN) and varied Pd loading amounts were prepared by the order rate law at the conversion of 2,4-DCP below 25%. Two
deposition–precipitation method. Briey, the support was sus- separate runs of the HDC of 2,4-DCP on Pd(2.8)/GN-3 were
pended in PdCl2 solution and heated at 45 ꢁC for 2 h under conducted, and the results showed raw data reproducibility that
vigorously stirring. Then, 1.0 M Na2CO3 solution was added was better than ꢂ7% (see Fig. 3S, ESI†). To test the possible
dropwise to the suspension to adjust the solution pH to 10.5. mass transfer limitation in the reaction system, the HDC of 2,4-
Aer stirring for 1 h, supported Pd catalysts with different DCP catalyzed by Pd(2.8)/GN-3 with varied catalyst dosages was
supports and Pd loading amounts were collected by ltration, carried out (results presented in Fig. 4S, ESI†). The catalyst
91364 | RSC Adv., 2015, 5, 91363–91371
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
Table 1 Properties of graphene and GN supported Pd catalysts
dosage normalized activities remained nearly constant for the
HDC of 2,4-DCP, indicative of the absence of mass transfer
limitation under our experimental conditions.34 The fractional
conversion of 2,4-DCP (X2,4-DCP) is dened as
BET surface N-doping
area (m2 gꢀ1) amounta (%) (wt%)
Pd contentb Pd particle
Catalyst
sizesc (nm)
Pd(2.7)/graphene 120.5
0
5.7
8.6
11.3
11.3
11.3
11.3
2.7
2.9
2.9
2.8
0.51
1.47
4.1
3.9
3.5
3
3.2
1.7
2.5
3.9
X2,4-DCP ¼ (C2,4-DCP,0 ꢀ C2,4-DCP)/C2,4-DCP,0
(1)
Pd(2.9)/GN-1
Pd(2.9)/GN-2
Pd(2.8)/GN-3
Pd(0.51)/GN-3
Pd(1.47)/GN-3
Pd(4.1)/GN-3
103.5
92.6
85.5
86.7
90.9
82.4
and selectivity, with respect to 2-CP (S2-CP), is calculated from
S2-CP ¼ C2-CP/(C2,4-DCP,0 ꢀ C2,4-DCP
)
(2)
a
Determined by elementary analysis. b Determined by ICP. c Calculated
from TEM.
3. Results and discussion
3.1. Catalyst characterization
The XRD patterns of graphite, Pd(2.7)/graphene, Pd(2.9)/GN-1,
Pd(2.9)/GN-2 and Pd(2.8)/GN-3 are shown in Fig. 1. For all
supported catalysts, diffraction peaks assigned to Pd or PdO
were not identied, and thus the diffraction peaks in the spectra
can be only attributed to the supports, e.g. graphene, GN-1, GN-
2 and GN-3. For graphite, diffraction peaks were observed at
26.5ꢁ, 44ꢁ and 54ꢁ, assigned to hexagonal crystalline graphitic
carbon structure.31 As for graphene, the diffraction peaks
characteristic of graphite disappeared, and a broad and much
low diffraction peak was identied around 23.4ꢁ, reecting that
the interlayer spacing increased and laminated graphene
structure was formed by weakening the van der Waals interac-
tions between graphitic layers.35 The XRD patterns of GN-1, GN-
2 and GN-3 were similar to graphene, while the peak intensity
was stronger, might be due to the interlayer spacing was
shrinked when nitrogen source was added.
The XRD patterns of Pd/GN-3 catalysts with varied Pd
loading amounts are compiled in Fig. 1S, ESI.† The diffraction
peaks characteristic of metallic Pd was only observed around
40.2ꢁ in the catalyst with the highest Pd loading (Pd(4.1)/GN-3).
For other Pd catalysts, diffraction peaks attributed to PdO or Pd
was not identied, likely due to the high Pd dispersions in the
catalysts.
nitrogen contents of Pd(2.9)/GN-1, Pd(2.9)/GN-2 and Pd(2.8)/
GN-3 were 5.7, 8.6 and 11.3%, respectively. In comparison
with Pd/graphene, N doping led to decreased surface area, and
the surface areas of Pd/GN reduced slightly with the increase of
N-doping amount, probably because nitrogen doping sup-
pressed the expanding of graphite lamella, as also indicated by
the XRD results.
The surface properties of graphene and GN were further
studied using XPS, and the results are shown in Fig. 2S.† The
carbon 1s signals of graphene and GN-3 were found to be
centered at 284.6 eV, and the broadened carbon peak could be
deconvoluted into three peaks around 284.6, 286 and 289 eV,
ascribed to lattice carbon, carbon atoms singly and doubly
coordinated with oxygen atoms (C–O, C]O), respectively.36 The
N 1s XPS spectra of GN-1, GN-2 and GN-3 are displayed in
Fig. 2a. All samples gave two peaks around 398.6 and 401 eV,
assigned to the nitrogen species in the form of pyridinic and
graphitic type.37 The pyridinic N species were bonded with two
carbon atoms locating on the edges of graphite planes, and
graphitic nitrogen species were associated with three carbon
atoms in the graphite plane.38 Notably, the content of pyridinic-
N increased with N-doping amount in the catalysts.
The binding energy of Pd in the 3d region for Pd(2.7)/
graphene, Pd(2.9)/GN-1, Pd(2.9)/GN-2 and Pd(2.8)/GN-3 is
compared in Fig. 2b. The Pd 3d peaks of the catalysts differed in
the peak width, implying the coexistence of different Pd species.
Hence, the XPS peaks were also deconvoluted and the tting
parameters are listed in Table 2. The Pd2+/Pd0 ratios of Pd(2.9)/
GN-1, Pd(2.9)/GN-2 and Pd(2.8)/GN-3 were determined to be
0.65, 1.12 and 0.69, respectively, much higher than that of
Pd(2.7)/graphene (0.39). Notably, N doping led to the formation
of surface N containing species, which could serve as the
anchoring sites for Pd particles. Hence, such a higher Pd2+/Pd0
ratio in Pd/GN is likely due to a strong metal–support interac-
tion between Pd particles and N-containing anchoring sites
from GN support, resulting in the transfer of electrons from
metallic Pd to support, and forming electron decient Pd
particles as well as a high Pd dispersion. Similarly, Chetty et al.39
found that nitrogen species in multiwalled carbon nanotubes
acted as the anchoring sites for the deposition of Pt Ru particles.
In addition, XPS results showed that the Pd2+/Pd0 ratio rst
increased then decreased with N doping, and the maximum
Some structural properties of the catalysts, e.g. BET surface
areas and nitrogen contents of GN are shown in Table 1. The
Fig.
1 XRD patterns of graphite, Pd(2.7)/graphene, Pd(2.9)/GN-1,
Pd(2.9)/GN-2 and Pd(2.8)/GN-3.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 91363–91371 | 91365

View Article Online
RSC Advances
Paper
Table 2 Parameters of XPS spectra of Pd catalysts supported on
graphene and GN
Catalysts
Pd2+/Pd0
N/C
Pd(2.7)/graphene
Pd(2.9)/GN-1
Pd(2.9)/GN-2
Pd(2.8)/GN-3
Pd(0.51)/GN-3
Pd(1.47)/GN-3
Pd(4.1)/GN-3
0.39
0.65
1.12
0.69
0.79
0.74
0.63
0
0.046
0.055
0.087
0.089
0.09
0.088
GN-3 catalysts with varied Pd loadings were also analyzed using
XPS spectra. The tting lines are presented in Fig. 2c and the
resulting parameters are listed in Table 2. For the Pd/GN-3
catalysts, the peak area in Pd 3d region increased with Pd
loading amount. Additionally, increasing Pd loading from 0.71
to 4.1 wt% resulted in a decrease in the Pd2+/Pd0 ratio from 0.79
to 0.63, reecting an attenuated interaction between Pd particle
and the support with Pd loading amount.
The TEM images and histograms of Pd particle size distri-
butions of graphene and GN supported catalysts are presented
in Fig. 3. For all the catalysts, akes-like graphene sheets
decorated with ne Pd particles were clearly observed. The
average Pd particle sizes of the catalysts were further calculated
using eqn (1):16
.
X
X
ds ¼
nidi3
nidi2
(3)
where ni is the number of Pd particles counted with diameter of di,
and the total counted number of Pd particles is larger than 300.
The distributions of Pd particles varied from the supports
and Pd loading amounts (see Fig. 3b). For Pd(2.9)/GN-1, Pd(2.9)/
GN-2 and Pd(2.8)/GN-3, the average Pd particle sizes were 3.5,
3.0 and 3.2 nm, respectively, smaller than that of Pd(2.7)/
graphene (3.9 nm), reecting that N-doping improved the
dispersion of Pd particles. For Pd/GN catalysts, Pd particle size
rst decreased and then increased slightly when N doping
content increased from 5.7 to 11.3%, conrming a positive
impact of the metal–support interaction on Pd dispersion.
Notably, Pd(2.9)/GN-2 had smaller Pd particles than Pd(2.8)/GN-
3, implying that overabound nitrogen species (pyridinic or/and
graphitic type) on the support surface might inhibit the
dispersion of Pd particles. For the Pd/GN-3 catalysts with varied
Pd loading amounts, Pd particle sizes were 1.7, 2.5, 3.2 and 4.1
nm for Pd(0.71)/GN-3, Pd(1.47)/GN-3, Pd(2.8)/GN-3 and Pd(4.1)/
GN-3, respectively, suggesting that Pd particles tended to
aggregate on the support surface when Pd loading increased.
3.2. Liquid phase catalytic HDC of 2,4-DCP
Fig. 2 XPS profiles of (a) GN in the N 1s region, (b) Pd(2.7)/graphene,
Pd(2.9)/GN-1, Pd(2.9)/GN-2 and Pd(2.8)/GN-3 in Pd 3d region, and (c)
GN-3 supported Pd catalysts in the Pd 3d region.
3.2.1. Effect of N-doping. The catalytic HDC of 2,4-DCP
over Pd(2.8)/GN-3 with varied catalyst dosages was carried out to
check the potential mass transfer limitation, and the results are
shown in Fig. 4S, ESI.† The HDC efficiency increased with
value was observed on Pd(2.9)/GN-2(1.12), implying that the Pd/ catalyst dosage, while the initial activities remained nearly
GN catalyst had the highest metal–support interaction when N- identical, conrming the absence of mass transfer limitation in
doping amount was 8.6%. The chemical states of Pd in the Pd/ the reaction system.40
91366 | RSC Adv., 2015, 5, 91363–91371
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
The catalytic HDC curves of 2,4-DCP over Pd(2.7)/graphene
and Pd(2.8)/GN-3 are presented in Fig. 5S.† For the catalytic
HDC of 2,4-DCP, only 2-chlorophenol (2-CP) was identied as
the partially dechlorinated product. Further hydrogenation
products from phenol did not appear throughout the reaction
process. The absence of 4-chlorophenol could be ascribed to the
steric hindrance, by which para-substituted Cl was easier to be
attacked by the active H.41 For Pd(2.7)/graphene, 2,4-DCP was
removed by 77% within 120 min and 2-CP still remained in the
reaction system, while complete removal of 2,4-DCP was ach-
ieved within 50 min for Pd(2.8)/GN-3, and 2-CP disappeared
aer reaction for 120 min. The catalytic HDC of 2,4-DCP over
Pd/graphene and Pd/GN catalysts with varied N doping
amounts are further compared in Fig. 4a. Notably, the catalytic
activity was closely related to the supports, and all the Pd/GN
catalysts exhibited much higher activities than Pd(2.7)/
graphene. As evidenced by TEM and XPS analysis, GN sup-
ported Pd catalysts had smaller Pd particle sizes and higher
Pd2+ contents, due to the presence of effective anchoring sites in
N doping graphene. At an identical Pd loading amount, smaller
Pd particle in supported catalyst may provide more exposed Pd
sites. Additionally, the presence of Pd2+ facilitated the activation
of C–Cl bonds,42 giving rise to much higher catalytic activities of
Pd(2.9)/GN-1, Pd(2.9)/GN-2 and Pd(2.8)/GN-3 than that of
Pd(2.7)/graphene. In addition, though the reaction rate of Pd/
GN-2 is similar to the Pd catalysts reported previously under
the same reaction conditions,25 the catalysts preparation in this
paper is much simple and practicable.
The role of N doping was also veried by the inuence of N
doping amount on the catalytic activity, and the results are
compiled in Fig. 4b. The initial activities rst increased and
then decreased slightly with the increase of N-doping amount,
for example, theꢀi1nitial activities were 259.8, 654.3, 820.5 and
781.1 mM gcat
hꢀ1 for Pd(2.7)/graphene, Pd(2.9)/GN-1,
Pd(2.9)/GN-2 and Pd(2.8)/GN-3, respectively, exhibiting an
activity order of Pd(2.9)/GN-2 > Pd(2.8)/GN-3 > Pd(2.9)/GN-1 >
Pd(2.7)/graphene, suggesting that the catalytic activity for HDC
of 2,4-DCP had a positive relationship with the metal–support
interaction and Pd dispersion. In addition, this result showed
that superuous nitrogen species on the support surface are not
favorable to the catalyst activity, similar results were also ob-
tained by Chen et al.43
The dependence of the HDC pathway on the N doping
amounts can be demonstrated from a consideration of 2-CP
selectivity (S2-CP) as a function of 2,4-DCP consumption (X2,4-DCP);
the corresponding data for different nitrogen amounts are
presented in Fig. 4c. At lower N doping amounts, the activity/
selectivity proles for Pd(2.7)/graphene, Pd(2.9)/GN-1 and
Pd(2.9)/GN-2 overlapped, which suggests that for catalysts with
lower nitrogen concentration (<8.6%), the reaction pathway is
independent of N-doping. However, the S2-CP vs. X2,4-DCP proles
for Pd(2.9)/GN-2 and Pd(2.8)/GN-3 do not coincide, indicating
that higher N doping amount can impact on the reaction
network. The lower values of S2-CP at a given X2,4-DCP for Pd(2.8)/
GN-3 may be taken into account based on the abstraction of the
nucleophilic chloride anion in the 2-chlorophenolate species by
Pd2+ as well as nitrogen species on the catalyst.
Fig. 3 (a) TEM images and (b) histograms of Pd particle size distribu-
tions of the catalysts.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 91363–91371 | 91367

View Article Online
RSC Advances
Paper
3.2.2. Effect of Pd loading amount. The catalytic activity of
Pd/GN-3 may strongly depend on its Pd loading amount, and
the catalytic HDC of 2,4-DCP over Pd/GN-3 with varied Pd
loading amounts is presented in Fig. 5. When Pd loading
increased from 0.71 to 1.47 wt%, the normalized initial catalytic
activity increased from 226.0 to 285.6, while further increasing
Pd loading level to 4.1 wt% led to decreased catalytic activity.
Characterization results from TEM and XPS analysis showed
that the Pd/GN-3 catalyst with a higher Pd loading had larger Pd
particle size and lower Pd2+/Pd0 ratio. It was previously
concluded that C–Cl bond activation and H2 activation were two
crucial steps in the liquid phase HDC reaction,44 H2 can be
chemisorbed and dissociated on Pd0 to form active H, while the
C–Cl bonds of 2,4-DCP can be activated dissociatively on Pdn+
sites by abstraction of the nucleophilic chloride anion (Clꢀ),
thus Pd2+ and Pd0 were respective active sites. Hence, smaller
Pd particles with higher Pd2+ content favored C–Cl bond acti-
vation, while larger Pd particles with higher Pd0 content facili-
tated H2 activation by enhancing H2 solubility and forming b-
PdH phase.45 Accordingly, for the Pd/GN-3 catalyst increasing
Pd loading led to enhanced H2 activation, but to weakened C–Cl
bond activation for the catalytic HDC of 2,4-DCP, thus giving
Fig. 4 (a) Catalytic HDC of 2,4-DCP over Pd(2.7)/graphene, Pd(2.9)/
GN-1, Pd(2.9)/GN-2 and Pd(2.8)/GN-3. Reaction conditions: pH 12.
Catalyst dosage: 0.25 g lꢀ1, 2,4-DCP concentration: 3.0 mM. (b) Initial
activities of Pd catalysts supported on graphene and GN. (c) 2-CP
selectivity (S2-CP), as a function of 2,4-DCP conversion (X2,4-DCP) for
reaction over Pd catalysts supported on graphene and GN.
Fig. 5 (a) The catalytic HDC of 2,4-DCP on GN-3 supported catalysts
with varied Pd loading amounts. (b) Dependence of normalized initial
activity on Pd particle size. Reaction conditions: pH 12. Catalyst
dosage: 0.25 g lꢀ1
.
91368 | RSC Adv., 2015, 5, 91363–91371
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
a volcano-type dependence of catalytic activity on Pd loading
amount.
The plot of 1/r0 as a function of 1/C0 is presented in Fig. 6b. A
good liner relationship between 1/r0 and 1/C0 was observed with
3.2.3. Effect of initial 2,4-DCP concentration. For a hetero- R2 higher than 0.98, reecting that the HDC of 2,4-DCP obeyed
geneous catalytic reaction, catalysis is expected to occur on the Langmuir–Hinshelwood model and the surface adsorption
catalyst surface, and thus the adsorption of reactant is believed was the rate-controlling step.47
to be a prerequisite step. To gain the insight into the reaction
3.2.4. Reaction kinetics. The catalytic HDC of poly-
mechanism, catalytic HDC of 2,4-DCP with varied initial chlorinated organic pollutants could be achieved through the
concentrations on Pd(2.8)/GN-3 was conducted and the results stepwise or/and concerted pathway,48 and the possible dechlo-
are shown in Fig. 6a. The initial activity increased gradually with rination pathways are shown in Scheme 1. The reaction rate
the initial concentration, suggesting that the reaction efficiency equations for the liquid phase catalytic HDC of 2,4-DCP could
was positively correlated with 2,4-DCP concentration absorbed be depicted as follows:
on the catalyst surface. The experimental data were further
dC2;4-DCP
tted using the Langmuir–Hinshelwood model:46
R2;4-DCP
¼
¼ ꢀðk1 þ k2ÞC2;4-DCP
¼ k1C2;4-DCP ꢀ k3C2-CP
¼ k2C2;4-DCP þ k3C2-CP
(6)
(7)
(8)
dt
bC0
r0 ¼ kqs ¼ k
1 þ bC0
(4)
(5)
dC2-CP
R2-CP
¼
dt
1
1
1
¼
þ
dCphenol
r0 kbC0
k
Rphenol
¼
dt
where r0 is the initial reaction rate, C0 is the initial concentra-
tion of 2,4-DCP, qs is the coverage of 2,4-DCP on the catalyst
surface, k is the reaction rate constant, and b is the adsorption
constant.
Accordingly, the rate constants could be calculated accord-
ing to eqn (9)–(11):
C2,4-DCP ¼ C02,4-DCP exp(ꢀ(k1 + k2)t)
(9)
k1
C2-CP ¼ C0
ðexpð ꢀ ðk1 þ k2ÞtÞ ꢀ expðꢀk3ÞtÞ
2;4-DCPk3 ꢀ k1 ꢀ k2
(10)
ꢀ
ꢀ
k1k3
ꢀ1
Cphenol ¼ C20;4-DCP
ðexpð ꢀ ðk1 þ k2ÞtÞ ꢀ 1Þ
k3 ꢀ k1 ꢀ k2 k1 þ k2
ꢁ
ꢁ
1
k2
þ
ðexpðꢀk3Þt ꢀ 1Þ
ꢀ
ðexpð ꢀ ðk1 þ k2ÞtÞ ꢀ 1Þ (11)
k3
k1 þ k2
where C02,4-DCP is the initial 2,4-DCP concentration, C2,4-DCP
,
C2-CP and Cphenol are the concentrations of 2,4-DCP, 2-CP and
phenol at reaction time t, k1, k2 and k3 are the rate constants of
2,4-DCP to 2-CP, 2,4-DCP to phenol, and 2-CP to phenol,
respectively.
The rate constants were obtained by integrating and data
tting, and the tting parameters are listed in Table 3. All the
tting correlation coefficients are very high, and the rate
constants are greater than zero, indicating that the HDC of 2,4-
DCP was completed via both stepwise and concerted pathways.
The reaction rates of 2,4-DCP over Pd(2.7)/graphene, Pd(2.9)/
GN-1, Pd(2.9)/GN-2 and Pd(2.8)/GN-3 were 0.0224, 0.0873,
0.2012 and 0.1666 minꢀ1 respectively, in agreement with the
activity order of Pd(2.9)/GN-2 > Pd(2.8)/GN-3 > Pd(2.9)/GN-1 >
Fig. 6 (a) The catalytic HDC of 2,4-DCP at varied initial 2,4-DCP
concentrations. (b) Linear plot of 1/r0 versus 1/C0. Reaction conditions:
pH 12. Catalyst: Pd(2.8)/GN-3. Catalyst dosage: 0.25 g lꢀ1. Line Scheme 1 Reaction pathways of the liquid phase catalytic HDC of 2,4-
represents the fitting curve using the Langmuir–Hinshelwood model. DCP.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 91363–91371 | 91369

View Article Online
RSC Advances
Paper
Table 3 Fitting rate constants of the catalytic HDC of 2,4-DCP over Pd
catalysts supported on graphene and GN
via both stepwise and concerted pathways, and the concerted
pathway became more pronounced with the increase of N-
doping amount in the catalyst support. Furthermore, for the
Pd/GN-3 catalysts a volcano type dependency of catalytic activity
on Pd particle size was observed. The present ndings highlight
the superior role of N doping in the enhancement of the activity
of Pd/graphene for the liquid phase catalytic HDC of chlori-
nated organic pollutants.
k1
k2
k3
k1 + k2
Catalyst
(minꢀ1
)
(minꢀ1
)
(minꢀ1
)
(minꢀ1
)
k2/k1
Pd(2.7)/graphene
Pd(2.9)/GN-1
Pd(2.9)/GN-2
Pd(2.8)/GN-3
0.0166
0.0514
0.1
0.0058
0.0359
0.1012
0.0976
0.0038
0.0117
0.0263
0.0304
0.0224
0.0873
0.2012
0.1666
0.35
0.70
1.01
1.41
0.069
Acknowledgements
The nancial support from National Key Basic Research
Program of China (2014CB441103), the National Natural
Science Foundation of China (no. 21277066, 21407074 and
51508080), and the National Natural Science Foundation of
Jiangsu (BK20140598) is gratefully acknowledged. We are
indebted to the Modern Analytical Center, Donghua University
for the catalyst characterization.
References
1 G. Q. Chen, S. Guan, G. M. Zeng, X. D. Li, A. W. Chen,
C. Shang, Y. Zhou, H. K. Li and J. M. He, Appl. Microbiol.
Biotechnol., 2013, 97, 3149–3157.
2 Y. Q. Peng and G. H. Song, Green Chem., 2002, 4, 349–351.
3 R. Andreozzi, I. D. Somma, R. Marotta, G. Pinto, A. Pollio and
D. Spasiano, Water Res., 2011, 45, 2038–2048.
4 P. T. Marc, G. M. Veronica and A. B. Miguel, Appl. Catal., B,
2004, 47, 219–246.
5 S. Dhanus and H. Kiyoshi, J. Hazard. Mater., 2010, 183, 490–
496.
6 R. Weber, T. Sakurai and H. Hagenmaier, Appl. Catal., B,
1999, 20, 249–256.
7 M. Henry, A. Farrell and Q. Brid, J. Environ. Sci. Health, Part A,
2006, 41, 763–777.
8 J. H. Kim and S. T. Lee, Process Biochem., 2002, 37, 1367–
1373.
9 J. Zhou, K. Wu, W. J. Wang, Y. X. Han, Z. Y. Xu, H. Q. Wan,
S. R. Zheng and D. Q. Zhu, Appl. Catal., B, 2015, 162, 85–92.
10 C. Menini, C. Park, E. J. Shin, G. Tavoularis and M. A. Keane,
Catal. Today, 2000, 62, 355–366.
11 C. B. Molina, A. H. Pizarro, J. A. Casas and J. J. Rodriguez,
Appl. Catal., B, 2014, 148–149, 330–338.
Fig. 7 Catalytic HDC of 2,4-dichlorophenol on Pd(2.8)/GN-3 over five
cycles.
Pd(2.7)/graphene. Additionally, k2/k1 ratios were 0.35, 0.7, 1.01
and 1.41 for Pd(2.7)/graphene, Pd(2.9)/GN-1, Pd(2.9)/GN-2 and
Pd(2.8)/GN-3 respectively, clearly indicating that the concerted
dechlorination pathway became predominant with the increase
of N-doping amount in the catalyst. The higher contents of
cationic Pd and nitrogen species may facilitate the simulta-
neous activation of ortho- and para-substitutional groups,
favoring enhanced dechlorination by the concerted pathway.
To verify possible catalyst deactivation, catalyst reuse was
conducted without re-activation and the results are compiled in
Fig. 7. For Pd(2.8)/GN-3, gradual deactivation was observed with
catalyst recycle, and aer 5 cycles the catalytic activity decreased
by 23%.
12 S. Ordonez, B. P. Vivas and F. V. Diez, Appl. Catal., B, 2010,
95, 288–296.
4. Conclusions
In the present study, a series of Pd catalysts supported on gra- 13 Z. M. Pedro, E. Diaz, A. F. Mohedano, J. A. Casas and
phene and GN with varied nitrogen doping amounts were J. J. Rodriguez, Appl. Catal., B, 2011, 103, 128–135.
prepared and the liquid phase catalytic HDC of 2,4-DCP was 14 E. Diaz, A. F. Mohedano, J. A. Casas, L. Calvo, M. A. Gilarranz
investigated. At similar Pd loadings, the catalysts with N-doped and J. J. Rodriguez, Catal. Today, 2015, 241, 86–91.
graphene as the supports exhibited much higher catalytic 15 S. Gomez-Sainero, F. Cardenas-Lizana and M. A. Keane, Ind.
activities than Pd/graphene due to their higher Pd2+ contents
Eng. Chem. Res., 2008, 47, 6841–6853.
´
and Pd dispersions. Additionally, increasing N doping amount 16 G. Yuan and M. A. Keane, Appl. Catal., B, 2004, 52, 301–314.
led to rst increased and then decreased catalytic activity of Pd/ 17 Y. Shao, Z. Y. Xu, H. Q. Wan, Y. Q. Wan, H. Chen, S. R. Zheng
GN. The catalytic HDC of 2,4-DCP followed the Langmuir–
and D. Q. Zhu, Catal. Commun., 2011, 12, 1405–1409.
Hinshelwood model, reecting an adsorption controlled reac- 18 C. E. Chan-Thaw, A. Villa, P. Katekomol, D. S. Su, A. Thomas
tion mechanism. For all catalysts, HDC of 2,4-DCP proceeded
and L. Prati, Nano Lett., 2010, 10, 537–541.
91370 | RSC Adv., 2015, 5, 91363–91371
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
19 S. H. Liu, M. T. Wu, Y. H. Lai, C. C. Chiang, N. Yu and 35 P. Wu, Y. Y. Huang, L. Q. Zhou, Y. B. Wang, Y. K. Bu and
S. B. Liu, J. Mater. Chem., 2011, 21, 12489–12496. J. N. Yao, Electrochim. Acta, 2015, 152, 68–74.
20 E. Rangel and E. Sansores, Int. J. Hydrogen Energy, 2014, 39, 36 S. L. Ding, S. J. Zheng, M. J. Xie, L. M. Peng, X. F. Guo and
6558–6566. W. P. Ding, Microporous Mesoporous Mater., 2011, 142, 609–613.
21 V. B. Parambhath, R. Nagar and S. Ramaprabhu, Langmuir, 37 X. W. Wang, G. Z. Sun, P. Routh, D. H. Kim, W. Huang and
2012, 28, 7826–7833. P. Chen, Chem. Soc. Rev., 2014, 43, 7067–7098.
22 Y. A. Lv, Y. H. Cui, Y. Z. Xiang, J. G. Wang and X. N. Li, 38 L. S. Zhang, X. Q. Liang, W. G. Song and Z. Y. Wu, Phys.
Comput. Mater. Sci., 2010, 48, 621–625. Chem. Chem. Phys., 2010, 12, 12055–12059.
23 Z. Y. Liu, C. L. Zhang, L. Luo, Z. Chang and X. M. Sun, J. 39 R. Chetty, S. Kundu, W. Xia, M. Bron, W. Schuhmann,
Mater. Chem., 2012, 22, 12149–12154.
V. Chirila, W. Brandl, T. Reinecke and M. Muhler,
24 K. Chizari, I. Janowska, M. Houlle, I. Florea, O. Ersen,
Electrochim. Acta, 2009, 54, 4208–4215.
T. Romero, P. Bernhardt, M. J. Ledoux and C. Pham-Huu, 40 O. M. Ilinitch, F. P. Cuperus, L. V. Nosova and E. N. Gribov,
Appl. Catal., A, 2010, 380, 72–80. Catal. Today, 2000, 56, 137–145.
25 J. Zhou, K. Wu, W. J. Wang, Z. Y. Xu, H. Q. Wan and 41 C. H. Xia, Y. Liu, J. Xu, J. B. Yu, W. Qin and X. M. Liang, Catal.
S. R. Zheng, Appl. Catal., A, 2014, 470, 336–343. Commun., 2009, 10, 456–458.
26 L. F. Dong, R. R. S. Gari, Z. Li, M. M. Craig and S. F. Hou, 42 S. Arulmozhiraja and M. Morita, J. Phys. Chem. A, 2004, 108,
Carbon, 2010, 48, 781–787. 3499–3508.
27 L. H. Tang, Y. Wang, Y. M. Li, H. B. Feng, J. Lu and J. H. Li, 43 H. Chen, P. Zhang, W. H. Tan, F. Jiang and R. Tang, RSC
Adv. Funct. Mater., 2009, 19, 2782–2789. Adv., 2014, 4, 38743–38749.
28 M. J. Lopez, I. Cabria and J. A. Alonso, J. Phys. Chem. C, 2014, 44 L. M. Gomez-Sainero, X. L. Seoane, J. L. G. Fierro and
118, 5081–5090. A. Arcoya, J. Catal., 2002, 209, 279–288.
29 D. Chen, H. B. Feng and J. H. Li, Chem. Rev., 2012, 112, 6027– 45 M. A. Aramendia, V. Borau, I. M. Garcia, C. Jimenez,
´
6053.
J. M. Marinas and F. J. Urbano, Appl. Catal., B, 1999, 20,
101–110.
30 Q. Z. Sun and S. Kim, Electrochim. Acta, 2015, 153, 566–573.
31 Y. M. Li, L. H. Tang and J. H. Li, Electrochem. Commun., 2009, 46 A. Pintar, J. Batista, J. Levec and T. Kajiuchi, Appl. Catal., B,
11, 846–849. 1996, 11, 81–98.
32 D. Hulicova-Jurcakova, M. Seredych, G. Q. Lu and 47 X. F. Xue, K. Hanna, M. Abdelmoula and N. S. Deng, Appl.
T. J. Bandosz, Adv. Funct. Mater., 2009, 19, 438–447. Catal., B, 2009, 89, 432–440.
33 G. Yuan and M. A. Keane, Chem. Eng. Sci., 2003, 58, 257–267. 48 J. Zhou, Y. X. Han, W. J. Wang, Z. Y. Xu, H. Q. Wan, D. Q. Yin,
34 G. Yuan and M. A. Keane, Ind. Eng. Chem. Res., 2007, 46, 705–
S. R. Zheng and D. Q. Zhu, Appl. Catal., B, 2013, 134–135,
222–230.
715.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 91363–91371 | 91371