Effective catalytic hydrodechlorination of chlorophenoxyacetic acids over Pd/graphitic carbon nitride

Article abstract of DOI:10.1039/c5ra07913f

Catalytic hydrodechlorination (HDC) of chlorophenoxyacetic acids was performed over Pd/graphitic carbon nitride (Pd/g-C₃N₄) catalysts in the present work. A series of Pd/g-C₃N₄ catalysts were prepared by a deposition-precipitation method, and characterized by X-ray diffraction (XRD), N₂ adsorption-desorption, transmission electron microscopy (TEM), CO chemisorption and X-ray photoelectron spectroscopy (XPS). The Pd/g-C₃N₄ catalysts showed excellent activity to convert dichlorophenoxyacetic acid (2,4-D) into phenoxyacetic acid (PA) and the catalytic activity was correlated with the ratio of Pd²⁺/(Pd⁰ + Pd²⁺) and Pd particle size. In addition, HDC of 2,4-D over Pd/g-C₃N₄ catalyst followed the Langmuir-Hinshelwood model, indicating an adsorption-controlled mechanism. Other chlorophenoxyacetic acids, such as 2-chlorophenoxyacetic acid (2-CPA), 4-chlorophenoxyacetic acid (4-CPA) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) could also be completely dechlorinated to PA over Pd/g-C₃N₄ catalysts.

Full text of DOI:10.1039/c5ra07913f

View Article Online
View Journal
RSC Advances
This article can be cited before page numbers have been issued, to do this please use: F. jiang, W. tan, H.
Chen, L. tan and J. liu, RSC Adv., 2015, DOI: 10.1039/C5RA07913F.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. This Accepted Manuscript will be replaced by the edited,
formatted and paginated article as soon as this is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/advances

Page 1 of 33
RSC Advances
View Article Online
DOI: 10.1039/C5RA07913F
Effective catalytic hydrodechlorination of chlorophenoxyacetic acids over
Pd/graphitic carbon nitride
a
a
a
a
b
Fang Jiang , Wenhui Tan , Huan Chen *, Ling Tan , Jingliang Liu
a Key Laboratory of Jiangsu Province for Chemical Pollution Control and Resources Reuse,
School of Environmental and Biological Engineering, Nanjing University of Science and
Technology, Nanjing 210094, China.
b School of Biochemical Environmental and Engineering, Nanjing XiaoZhuang University,
Nanjing 211171, China.
*
Corresponding author. Tel: +86ꢀ25ꢀ84303209; Fax: +86ꢀ25ꢀ84315352.
Eꢀmail: hchen404@njust.edu.cn (H. Chen)
1

RSC Advances
Page 2 of 33
View Article Online
DOI: 10.1039/C5RA07913F
Abstract
Catalytic hydrodechlorination (HDC) of chlorophenoxyacetic acids was performed
over Pd/graphitic carbon nitride (Pd/gꢀC ) catalysts in the present work. A series of
Pd/gꢀC N catalysts were prepared by depositionꢀprecipitation method, and
3
N
4
3
4
characterized by Xꢀray diffraction (XRD), N adsorptionꢀdesorption, transmission
2
electron microscopy (TEM), CO chemisorption and Xꢀray photoelectron spectroscopy
(XPS). The Pd/gꢀC
3
N
4
catalysts showed excellent activity to convert
dichlorophenoxyacetic acid (2,4ꢀD) into phenoxyacetic acid (PA) and the catalytic
2+
0
2+
activity was correlated with the ratio of Pd /(Pd +Pd ) and Pd particle size. In
addition, HDC of 2,4ꢀD over Pd/gꢀC N catalyst followed LangmuirꢀHinshelwood
3
4
model, indicating an adsorptionꢀcontrolled mechanism. Other chlorophenoxyacetic
acids, such as 2ꢀchlorophenoxyacetic acid (2ꢀCPA), 4ꢀchlorophenoxyacetic acid
(4ꢀCPA) and 2,4,5ꢀTrichlorophenoxyacetic acid (2,4,5ꢀT) could also be completely
dechlorinated to PA over Pd/gꢀC N catalyst.
3
4
Keywords: Catalytic hydrodechlorination (HDC); chlorophenoxyacetic acid;
2+
0
2+
Pd/gꢀC
3
N
4
catalyst; Pd /(Pd +Pd ) ratio; Pd particle size
2

Page 3 of 33
RSC Advances
View Article Online
DOI: 10.1039/C5RA07913F
1
. Introduction
Chlorophenoxyacetic acids, including 4ꢀchlorophenoxyacetic acid (4ꢀCPA),
,4ꢀdichlorophenoxyacetic acid (2,4ꢀD) and 2,4,5ꢀtrichlorophenoxyacetic acid
2
1
ꢀ3
(
2,4,5ꢀT), are widely used as pesticides to control harmful plant pests. Relatively
high levels of these components are found in surface and ground waters for their
intensive use in agriculture. The consequent contamination caused by the residues has
4
,5
attracted extensive attentions due to its high toxicity and carcinogenicity. In this
6
case, many techniques have been developed to remove them, such as adsorption,
7
8,9
10
advanced oxidation, photocatalysis
and reductive dechlorination techniques.
Zeroꢀvalent iron (nZVI) reduction has been proved to be an effective technology for
11,12
the conversion of chlorophenoxyacetic acid to less toxic compounds.
However,
2
+
subsequent treatment processes are required for the removal of Fe formed in the
reaction. Some developments were made by using an electrochemical approach for
1
3ꢀ15
15
the dechlorination of chlorophenoxy herbicides.
electrochemical reductive dechlorination process of 2,4ꢀD and found that
ꢀchlorophenoxyacetic acid (2ꢀCPA) and phenoxyacetic acid (PA) were the main
Sun et al. reported an effective
2
16
dechlorinated products. Recently, Zhu et al. investigated a combination of nZVI
reduction with electrochemical method for enhanced catalytic hydrodechlorination of
2
,4ꢀD.
An alternative approach for the removal of chlorophenoxyacetic acids is liquid
phase catalytic hydrodechlorination (HDC), which is one of the most effective and
environmentally friendly methods for the abatement of many organochlorinated
17ꢀ19
20,21
pollutions, such as chlorophenols,
chlorobenzenes
and halogenated
22,23
24
hydrocarbons,
etc. More recently, Diaz et al. explored the HDC process of 2,4ꢀD
and found that 2,4ꢀD could be dechlorinated to 2ꢀCPA and PA. Besides of this, there
3

RSC Advances
Page 4 of 33
View Article Online
DOI: 10.1039/C5RA07913F
are no far investigations on HDC of chlorophenoxyacetic acids and the reaction
mechanism is also scarce.
Previous literatures on HDC of organochlorinated pollutions have demonstrated
2
5,26
the important role of Pd active species on catalytic efficiency.
The dependence of
HDC activity on Pd particle size was investigated, while no general consensus has
27
been achieved. GómezꢀQuero et al. investigated the effect of metal particle size on
the HDC of 2,4ꢀdichlorophenol over Pd/Al catalysts and found that a higher
2
O
3
activity was achieved over smaller Pd particles. Nevertheless, a volcanoꢀtype
dependence of catalytic activity on Pd particle size was observed in HDC of
2
8
chloroacetic acid over Pd/ZrO catalysts. Apart from Pd particle size, Pd electronic
2
structure is another key factor which markedly affects the catalytic activity.
29
GómezꢀSainero et al. studied the catalytic HDC of CCl
4
over Pd/AC and found both
0
n+
Pd and Pd species had effect on HDC efficiency. Similar conclusions were obtained
25
in HDC of 2,4ꢀdichlorophenol over mesoporous carbon supported Pd catalysts.
3
0
31
Besides of Pd supported catalysts, bimetallic catalysts, such as PdꢀFe, PdꢀRh,
32
PdꢀCu, were approved to be efficient catalysts to treat many organochlorinated
pollutions.
Carbon nitride is a fascinating material which has attracted worldwide attention
33,34
for its excellent textural properties and extreme chemical and thermal stability.
We
have recently shown that the carbon nitride material was an effective support for Pd
dispersion due to its nitrogen functionalities, which could serve as active sites to
35
anchor Pd particle, and the Pd/graphitic carbon nitride catalysts (Pd/gꢀC
3 4
N )
prepared by photoꢀdeposition method could be used in catalytic hydrodechlorination
3
6
37
of 2,4ꢀdichlorophenol. Similarly, Feng et al. prepared an effective palladium
catalyst supported on carbonꢀnitrogen composites for aqueousꢀphase hydrogenation of
4

Page 5 of 33
RSC Advances
View Article Online
DOI: 10.1039/C5RA07913F
phenol.
In order to get further insight about the effect of Pd properties on HDC of
chlorophenoxyacetic acids, we used depositionꢀprecipitation method to prepare a
series of Pd/gꢀC N catalysts, and these catalysts were tested in HDC of 2ꢀCPA,
3
4
4
ꢀCPA, 2,4ꢀD and 2,4,5ꢀT in this work. Results showed that chlorophenoxyacetic
acids could be completely dechlorinated to PA, revealing the potential of the liquid
phase catalytic HDC method to remove chlorophenoxyacetic acids.
2
. Experimental
2
.1. Catalyst preparation
Graphitic carbon nitride (gꢀC N ) was synthesized by directly thermal
3
4
o
33,36
polymerization of cyanamide (CNꢀNH
was ground to pass through a 400ꢀmesh sieve. Supported Pd catalysts were prepared
using depositionꢀprecipitation method. Typically, 0.5 g of sieved gꢀC was
dispersed in 50 mL PdCl solution and then the pH of the solution was adjusted with 1
2
) at 550 C for 4 h.
3 4
The obtained gꢀC N
3
N
4
2
ꢀ1
M NaOH. After stirring for 1 h, a certain amount of NaBH in 0.2 mol L NaOH
4
solution was added dropwise and stirred for another 1 h. The obtained catalysts were
filtrated and washed with distilled water several times till no chloride ion was
o
detected. The resultant catalysts dried at 70 C are denoted as Pd(x)/gꢀC
3
N
4
, where x
is the Pd loading amount (wt %). A series of catalysts with different parameters were
summarized in Table 1. In order to gain catalyst 1, 2, 3 and 4, we added different
4 2
amounts of NaBH , while the amount of PdCl solution and the pH during
depositionꢀprecipitation process was the same for these catalysts. Besides, the only
difference among catalysts 3, 5 and 6 was pH values during precipitation. In
comparison with catalyst 3, 7, 8 and 9, different amounts of PdCl solution were
2
added to obtain catalysts with different Pd loading. Prior to activity evaluation, all
5

RSC Advances
Page 6 of 33
View Article Online
DOI: 10.1039/C5RA07913F
catalysts were ground to pass through a 400ꢀmesh sieve (<37 ꢁm) to make sure
38
negligible intraparticular diffusion.
.2. Catalyst characterization
The Pd content was determined by ICPꢀMS (OPTIMA7000DV, Perkin Elmer,
2
US). Before measurement, the catalysts were digested by mixed acid digestion.
Xꢀray diffraction (XRD) patterns of the catalysts were recorded on a Rigaku
D/maxꢀRA powder diffractionꢀmeter. The wavelength of the beam was 0.154 nm. The
o
o
XRD patterns were scanned in the 2θ range of 10ꢀ80 with step width 0.05 . Plots of
the 2θ data were generated from the EVA software developed by Bruker.
BrunauerꢀEmmettꢀTeller (BET) specific surface areas of the support and
2
catalysts were obtained from the N adsorptionꢀdesorption isotherms, measured at 77
K using Micromeritics ASAP 2020. Before measurement, the samples were outgassed
o
at 250 C for 4 h.
The morphologies of the catalysts were observed on a transmission electron
microscope (JEMꢀ2100, JEOL, Japan). The average Pd particle size could be
39
quantified using equation:
3
i
2
ds = n d /
∑nidi
(1)
∑
i
where n is the number of Pd particles with diameter of d , the total number (∑n )
i
i
i
is larger than 100.
Xꢀray photoelectron spectroscopy (XPS) was conducted on a PHI5000 Versa
Probe equipped with a monochromatized Al Kα excitation source (hv= 1486.6 eV)
(ULVACꢀPHI, Japan).
Pd dispersion of the catalysts was determined by CO chemisorption measurement.
ꢀ1
o
2
00 mg of reduced catalyst was flushed by Ar flow (30 mL min ) at 200 C for 1 h.
After the temperature of the catalyst was cooled down, the CO chemisorption was
6

Page 7 of 33
RSC Advances
View Article Online
DOI: 10.1039/C5RA07913F
conducted with pulses of 0.4 ml. The Pd dispersion was calculated on the assumption
40
of adsorption stoichiometry of Pd/CO =1.
2
.3. Hydrodechlorination experiments
The liquid phase catalytic hydrodechlorination experiments were conducted in a
2
50 mL threeꢀnecked flask reactor with vigorous stirring (1200 rpm) at atmospheric
o
pressure. The reaction temperature was maintained at 25 C by a thermostatic water
ꢀ1
bath. Typically, 40.0 mg of catalyst was added to 200 mL of 0.48 mmol·L reactant
solution with the pH prejusted to 11.0 by 1 M NaOH. After purged with a N (60
2
ꢀ1
ꢀ1
mL·min ) for 30 min, H was switched into the reactor (60 mL·min ). The samples
2
were taken periodically, filtrated with 0.45 ꢁm membrane and neutralized with 1 M
HCl. The concentrations of the reactant and products were quantified by HPLC with a
UV/Visible Detector (Waters 2489) at 285 nm using a 4.6×250 mm C18 column. A
mixture of methanol and 5% acetic acid (60:40, V:V) was selected as mobile phase.
The effect of mass transfer limitation under the reaction condition was evaluated.
HDC of 2,4ꢀD over catalyst 3 with different catalyst dosages was compared and the
results were presented in Fig. 1S (see Supporting information). Increasing catalyst
dosage enhanced the HDC efficiency; while the dosage normalized initial activities
41
remained nearly constant, indicating the absence of mass transfer limitation. The
initial activity was calculated by the removal rate of 2,4ꢀD within initial 3 min.
3
. Results and discussion
3
.1. Catalyst properties
Textural properties of support and catalysts are summarized in Table 1. The Pd
loading slightly varied around the theoretical value 2% for catalysts prepared with
different amount of NaBH and pH (catalyst 1 to 6), suggesting that the preparation
4
condition had no significant influence on Pd loading. Additionally, the BET surface
7

RSC Advances
Page 8 of 33
View Article Online
DOI: 10.1039/C5RA07913F
3 4
areas (SBET) of catalyst 1 to 9 were smaller than gꢀC N material, indicating the
loading of Pd resulted in a slight drop in SBET
.
The XRD patterns of gꢀC and Pd/gꢀC
3
N
4
3
N
4
catalysts are compiled in Fig. 2S.
o
o
Clearly, gꢀC N has two distinct peaks at 13.1 and 27.7 , which are indexed as (100)
3
4
and (002) diffraction, respectively (JCPDS 87ꢀ1526). The intensities of the two peaks
slightly decreased for catalyst 1 to 9, attributed to the introduction of Pd. However,
there was no diffraction peaks assigned to metallic or oxide Pd in all catalysts,
implying of a high Pd dispersion.
3
3
.2. Liquid phase catalytic HDC of 2,4ꢀD
.2.1. Effect of the catalyst preparation condition
Liquid phase catalytic HDC of 2,4ꢀD over catalyst 1 to 6 are summarized in Fig
. For all catalysts, 2ꢀCPA and 4ꢀCPA were the partially dechlorinated products and
1
PA was the ultimate dechlorinated product. The total amount of 2,4ꢀD, 2ꢀCPA, 4ꢀCPA
and PA during the procedure was approximately equaled to the initial amount of 2,4ꢀD,
reflecting of the absence of other dechlorinated product. The catalytic HDC of
organochlorinated pollutions may be completed via stepwise or/and concerted
pathways. As reported for HDC pathway of 2,4ꢀdichlorophenol, the substituted Cl can
be removed by two stepwise routes with 2ꢀchlorophenol or 4ꢀchlorophenol as reaction
intermediates or by the simultaneous removal of both substituted Cl; both of the two
42
routes produced phenol as the final HDC product. A possible reaction pathway for
HDC of 2,4ꢀD was depicted in Scheme 1. H was activated by Pd and then reacted
2
with 2,4ꢀD, consequently 2ꢀCPA, 4ꢀCPA and PA were formed by the breaking of the
CꢀCl bond. Afterwards, the produced 2ꢀCPA and 4ꢀCPA could react with active H to
PA.
Notably, the concentration of 2ꢀCPA was much higher than that of 4ꢀCPA in all
8

Page 9 of 33
RSC Advances
View Article Online
DOI: 10.1039/C5RA07913F
HDC processes over these catalysts in Fig. 1, indicating that the paraꢀposition Cl atom
was more susceptible to be eliminated than orthoꢀposition Cl as a result of steric
hindrance. The role of steric hindrance has also been reported in HDC of
15,16
2
,4ꢀdichlorophenol, as well as electrocatalytic dechlorination of 2,4ꢀD.
Among the Pd/gꢀC N catalysts prepared with different amount of NaBH
4
3
4
(catalyst 1 to 4), catalyst 3 possessed the highest catalytic activity. After reaction for
9
0 min, 0.48 mmol 2,4ꢀD was completely dechlorinated to PA. The catalytic
efficiency varied in the order catalyst 3 > catalyst 2 > catalyst 4 > catalyst 1. In
comparison with Pd/gꢀC N prepared in different pH condition (catalyst 3, 5 and 6),
3
4
the catalytic efficiency followed the order catalyst 3 > catalyst 5 > catalyst 6,
suggesting that Pd/gꢀC prepared in high pH solution favored the HDC process.
Considering the reason causing the difference in catalytic activity among these
Pd/gꢀC catalysts, we should know that the catalytic activities are relevant to the
3
N
4
3
N
4
properties of Pd, such as Pd particle size and Pd species. To obtain the morphology
and dispersion of Pd particles, TEM and CO chemisorptions measurements were
investigated. The TEM images of catalyst 1 to 6 are displayed in Fig. 2, with uniform
Pd particles clearly identified in all catalysts. The average sizes of the Pd particles
calculated by equation (1) are listed in Table 2. The Pd dispersions measured by CO
chemisorption are also listed in Table 2. They were found to be 44.20%, 48.13%,
4
6.31%, 45.66%, 45.68% and 48.52% for catalyst 1 to 6, respectively. The
corresponding Pd particle sizes calculated from CO chemisorption was consistent
with the results of TEM. Both TEM and CO chemisorption results indicated that these
catalysts exhibited very similar Pd dispersion and Pd particle size, thus the Pd particle
size was not a major factor causing the differences in HDC activities among these
catalysts. Worth noting is Pd dispersion on these catalysts was relevantly high. The
9

RSC Advances
Page 10 of 33
View Article Online
DOI: 10.1039/C5RA07913F
interaction between the pyridine nitrogen groups and the Pd nanoparticles enabled
high Pd dispersion on the gꢀC support. A previous work has also shown
wellꢀdispersed Pd nanoparticles on NH functionalized carbon nanofibers, which
attributed to the similar interaction between the pyridinic nitrogen and Pd
3
N
4
3
43
nanoparticles.
XPS was performed to understand the surface chemical states of Pd species on
Pd/gꢀC , and the results were illustrated in Fig. 3. The binding energy of Pd 3d5/2
around 335.2 eV and 337.2 eV could be attributed to the presence of Pd and Pd
3
N
4
0
2+
2+
species, respectively. Upon NaBH reduction treatment, Pd is partially retained due
4
2
+
2+
to the stabilization of Pd by interaction of Pd to Nꢀcontaining groups, such as
4
4,45
pyridine nitrogen groups which existed on the support surface.
The intensity of
increased during catalyst
preparation (catalyst 1 to 4), indicating weakened interaction. The ratios of
2
+
Pd species decreased when the amount of NaBH
4
2
+
0
2+
Pd /(Pd +Pd ) in these catalysts are summarized in Table 2. It can be clearly seen
2
+
0
2+
that the ratio of Pd /(Pd +Pd ) decreased from 0.71 (catalyst 1) to 0.39 (catalyst 4),
2
+
reflecting of decreased Pd content. For samples prepared in different pH, the ratios
2
+
0
2+
of Pd /(Pd +Pd ) followed the order catalyst 3 (pH 10) > catalyst 5 (pH 9) > catalyst
2
+
6
(pH 8), implying that higher Pd content existed in the catalyst prepared in higher
3
6
pH condition. As reported, the isoelectric point of gꢀC N was around 2.0. The zeta
3
4
potential of gꢀC N decreased as elevating the solution pH, caused strengthening of
3
4
2
+
2+
the interaction between Pd and negative gꢀC
retained during the NaBH reduction process at high pH solution.
More information was gained by the comparison of turnover frequency (TOF)
3 4
N . As a result, more Pd species were
4
2
+
0
2+
versus ratios of Pd /(Pd +Pd ) as shown in Fig. 4. As shown in Fig. 4, the initial
activity and TOF increased with the ratio up to 0.5, and then progressively decreased.
10

Page 11 of 33
RSC Advances
View Article Online
DOI: 10.1039/C5RA07913F
0
During the process, molecular H
2
could chemisorb and activated on the surface of Pd ,
2
+
whereas 2,4ꢀD tended to chemisorb onto Pd by slightly electrostatic attraction for
0
2+
the presence of chloride anion in 2,4ꢀD. Thus, both Pd and Pd could affect the
catalytic activity of Pd/gꢀC N . Similar result has been depicted by GómezꢀSainero et
3
4
29
al. on liquidꢀphase hydrodechlorination of CCl . It was demonstrated that the HDC
4
0
n+
reaction rate was related to the amount of Pd and Pd , and the maximum reaction
n+
0
rate achieved when [Pd ]/[Pd ]=1. Here, we observed that a maximum reaction rate
2+
0
2+
could be obtained in the catalyst 3 with Pd /(Pd +Pd )=0.5, which demonstrated that
0
2+
both Pd and Pd species play an important and complementary role on HDC reaction.
4
6
A dual mechanism was also proposed by Ordóñez et al. to explain the HDC of
2+
tetrachloroethylene (TTCE) over supported Pd catalysts. They found that Pd could
form a complex with TTCE, and then the complex was attacked by hydrogen
0
activated by Pd . A reaction scheme similar to previous reports can be proposed:
2
+
2
,4ꢀD can form a complex with the Pd , and this complex will be attacked by the
hydrogen to accomplish the ClꢀH exchange, which is accomplished by the reduction
2
+
0
46
of the Pd to Pd (as speculated in HDC of TTCE ). The XPS spectra of used
2+
catalyst 3 shown in Fig. 3S confirmed the reaction scheme. As seen, some Pd
0
species were reduced to Pd during the HDC process, causing the increasing amount
0
of Pd .
3
.2.2. Effect of Pd loading amount
Pd(x)/gꢀC catalysts with different Pd loading amounts were employed for
3
N
4
HDC of 2,4ꢀD. As shown in Fig. 5, the catalytic efficiency enhanced with Pd loading
amount ranging from 0.46% to 4.11%. To obtain a further understanding of the
relationship between catalytic activity and Pd properties, the Pd particle size and
species of Pd(x)/gꢀC N catalysts were analyzed by TEM, CO chemisorption and XPS
3
4
11

RSC Advances
Page 12 of 33
View Article Online
DOI: 10.1039/C5RA07913F
measurement. TEM images of catalyst 7, 8 and 9 are given in Fig. 6, where increasing
Pd loading amount resulted in rising Pd particle size. As listed in Table 2, the average
Pd particle sizes calculated by equation (1) were 2.07, 2.36 and 4.22 nm for catalyst 7,
8
and 9, respectively. The Pd particle sizes calculated from CO chemisorption were
also listed in Table 2, with analogous results of TEM. The enlarged Pd particles were
47,48
caused by the aggregation of the Pd particles with high loading.
Pd species on
catalyst 7, 8 and 9 were investigated by XPS measurement and the results were
0
2+
displayed in Fig. 4S. The Pd 3d5/2 peaks assigned to Pd (around 335.2 eV) and Pd
(around 337.2 eV) species were appeared for all catalysts. An apparent decrease of
2
+
0
2+
2+
0
2+
Pd /(Pd +Pd ) was observed when raising the Pd loading. The lower Pd /(Pd +Pd )
ratio in high Pd loading reflected the weakened interaction between Pd and gꢀC
3
N
4
support, induced by larger Pd particle size on the catalyst surface. The variation trend
of metal support interaction with Pd loading was also observed in other catalysts, such
4
9
28
as Pd supported on boronꢀdoped mesoporous carbon and Pd/ZrO .
2
Further insight was acquired by investigating the TOF versus Pd particle size and
Pd species. Fig. 7a demonstrated a steep enhancement of TOF with Pd particle size
increased from 1.90 to 2.54 nm, while further increase of Pd particle size caused a
gentle increase in TOF, suggesting that the reaction is structure sensitive. This effect
of structure sensitive has been found in other hydrodechlorination reactions.
5
0
Aramendía et al. studied the liquidꢀphase HDC of chlorobenzene over Pdꢀsupported
catalysts; they found the reaction is structureꢀsensitive and the increase in dispersion
5
1
leads to a decrease in TOF. Díaz et al. studied the Pdꢀcatalyzed aqueousꢀphase
trichloroethylene hydrodechlorination and found the reaction is strongly structure
sensitive, being the largest Pd particles the most active. They considered the formation
of palladium hydrides in the catalyst was especially important to the catalytic activity.
12

Page 13 of 33
RSC Advances
View Article Online
DOI: 10.1039/C5RA07913F
2
+
0
2+
Notably, TOF versus the ratio of Pd /(Pd +Pd ) exhibited in Fig. 7b differed
from the result of Fig. 4 (the dashed line). It indicated that the variation of TOF was
related to both Pd particle size and Pd species. As mentioned above, the activation of
H and CꢀCl bond was believed to be two crucial steps during the HDC process of
2
2
+
2
,4ꢀD. Small Pd particles with high Pd content facilitated the activation of CꢀCl
bond, while catalyst with large Pd particles would boost H activation due to the
had the most appropriate
, confirming
2
5
0
3 4
formation of βꢀPdH . Hence, although the Pd(2.09)/gꢀC N
2
+
0
2+
ratio of Pd /(Pd +Pd ), the TOF was lower than that of Pd(4.11)/gꢀC
3 4
N
the important role of Pd particle size on catalytic activity.
3
.2.3. Effect of 2,4ꢀD adsorption
Adsorption of reactants was considered to be a critical effect on HDC of many
52
28
chlorinated organics, such as 2,4ꢀdichlorophenol and chloroacetic acids . In order
to clarify the influence of adsorption on catalytic activity, HDC of 2,4ꢀD with
different initial concentration was examined and the initial catalytic activity was
shown in Fig 8a. With the increase of 2,4ꢀD concentration from 0.23 to 2.71 mM, the
ꢀ1 ꢀ1
initial catalytic activity increased from 21.21 to 65.66 mM gcat h , while further
increase of 2,4ꢀD concentration caused little enhancement in initial catalytic activity.
23,53
This phenomenon can be described by the LangmuirꢀHinshelwood model:
dC
dt
bC
r0 = −
= kθs = k
(2)
(3)
1+ bC
1
1 1
1
=
+
r
kb C0
k
0
0 s
where r is the initial catalytic activity at concentration C, θ is the 2,4ꢀD
coverage on the catalyst surface, k is the apparent reaction rate constant and b is the
equilibrium constant for 2,4ꢀD adsorption on the catalyst.
0
The plot of 1/r with 1/C is further summarized in Fig 8b. A good linear
13

RSC Advances
Page 14 of 33
View Article Online
DOI: 10.1039/C5RA07913F
2
relationship (R >0.97) suggested that the HDC of 2,4ꢀD could be well described by
LangmuirꢀHinshelwood model and the reaction was controlled by 2,4ꢀD adsorption
54
on the catalyst surface. Such phenomena was also be observed by Wu et al. ; they
found the liquid phase hydrodechlorination of diclofenac over Pd/CeO could be well
2
described using the LangmuirꢀHinshelwood model, indicating the adsorption of
reactor was crucial to catalytic efficiency.
3
.3. HDC of other chlorophenoxyacetic acid
The liquid phase catalytic HDC of 2ꢀCPA, 4ꢀCPA and 2,4,5ꢀT over catalyst 3 is
compared in Fig. 9. For all of the chlorophenoxyacetic acids, the completely
dechlorinated product was PA. In comparison of monochlorophenoxyacetic acids,
2
4
ꢀCPA could be completely dechlorinated within 40 min, while the dechlorination of
ꢀCPA was accomplished within 20 min. The catalytic activities were summarized in
Table 3. The steric hindrance caused the difference between the catalytic activities for
ꢀCPA and 4ꢀCPA, so that the paraꢀposition Cl atom was more susceptible to be
2
eliminated than orthoꢀposition Cl. The selectivities of dechlorinated products in HDC
of chlorophenoxyacetic acids were summarized in Table 3. The selectivity could be
5
5
defined as:
Ct,production
S =
×100%
(4)
C0,reactant − Ct,reactant
where C0, reactant is the concentration of reactant at the beginning of the
hydrodechlorination process, Ct,reactant is the concentration of reactant at time t, Ct,
production is the concentration of production at time t.
It was found that the selectivity of 2ꢀCPA was much higher than that of 4ꢀCPA in
HDC of 2,4ꢀD at reaction time of 10 min and 30 min. The higher dechlorinated rate of
4
ꢀCPA and the higer selectivity of 2ꢀCPA in HDC of 2,4ꢀD could be attributed to
14

Page 15 of 33
RSC Advances
View Article Online
DOI: 10.1039/C5RA07913F
steric effect. Cl on the orthoꢀposition of 2,4ꢀD was less susceptible to be attacked than
Cl on the paraꢀposition. The possible dechlorination pathway of 2,4,5ꢀT is presented
in Scheme 1. 2,4ꢀD, 2,5ꢀD, 2ꢀCPA, 4ꢀCPA and PA were detected as the dechlorinated
products during the HDC process of 2,4,5ꢀT, while 4,5ꢀD and 5ꢀCPA were not
detected. The selectivity of each product in HDC of 2,4,5ꢀT at the reaction time of 10
and 30 min was listed in Table 3. As can be seen, the selectivity of 2,4ꢀD and 2ꢀCPA
was much higher than that of 2,5ꢀD and 4ꢀCPA, indicating that 2,4ꢀD and 2ꢀCPA were
the main partially dechlorinated products.
4
. Conclusions
In the present study, Pd catalysts supported on gꢀC N were prepared and the
3
4
liquid phase catalytic hydrodechlorination of chlorophenoxyacetic acids was
investigated. Chlorophenoxyacetic acids, including 2ꢀCPA, 4ꢀCPA, 2,4ꢀD and 2,4,5ꢀT,
2
+
0
could be completely dechlorinated to PA. Both Pd and Pd are the active sites in
2
+
0
2+
HDC process, and the catalytic activity is related to the ratio of Pd /(Pd +Pd ). A
2
+
0
2+
volcanoꢀshape dependence of initial activity and TOF on Pd /(Pd +Pd ) was found,
2+
0
2+
and catalyst 3 with Pd /(Pd +Pd )=0.5 had a maximum activity. Apart from Pd
species, the HDC of 2,4ꢀD was also related to the Pd particle size, wherein large Pd
particles favored HDC process. Moreover, the HDC of 2,4ꢀD over catalyst 3 followed
LangmuirꢀHinshelwood model, reflecting an adsorptionꢀcontrolled mechanism.
Findings in this work clearly demonstrate the potential of liquid phase catalytic HDC
to effectively remove chlorophenoxyacetic acids.
Acknowledgements
The financial supports from the Natural Science Foundation of China (No.
5
1208257, 51178223 and 51478223) and the natural science foundation of Jiangsu
15

RSC Advances
Page 16 of 33
View Article Online
DOI: 10.1039/C5RA07913F
Province (BK2012405), China Postdoctoral Science Foundation (No. 2013M541677),
the Jiangsu Planned Projects for Postdoctoral Research Funds (1202007B) and the
Fundamental Research Funds for the Central University (No. 30920140112010) are
gratefully acknowledged.
References
1
2
3
4
B. Boye, M. Dieng, E. Brillas, Environ. Sci. Technol., 2002, 36, 3030.
J. Peller, O. Wiest, P. V. Kamat, J. Phys. Chem. A, 2004, 108, 10925.
Y. R. Wang, W. Chu, Water Res. 2011, 45, 3883.
M. V. Shankar, S. Anandan, N. Venkatachalam, B. Arabindoo, V. Murugesan,
Chemosphere, 2006, 63, 1014.
5
F. Sannino, P. Pernice, L. Minieri, G. A. Camandona, A. Aronne, D. Pirozzi, ACS
Appl. Mater. Interfaces, 2015, 7, 256.
6
7
8
D. M. Han, W. P. Jia, H. D. Liang, J. Environ. Sci., 2010, 22, 237ꢀ241.
B. Boye, M. M. Dieng, E. Brillas, Environ. Sci. Technol., 2002, 36, 3030.
Y. H. Tang, S. L. Luo, Y. R. Teng, C. B. Liu, X. L. Xu, X. L. Zhang, L. Chen, J.
Hazard Mater., 2012, 241ꢀ242, 323.
9
1
K. T. Ranjit, I. Willner, S. H. Bossmann, A. M. Braun, Environ. Sci. Technol., 2001,
3
5, 1544.
0 A. I. Tsyganok, K. Otsuka, Appl. Catal., B: Environ., 1999, 22, 15.
1 A. S. Fjordbøge, A. Baun, T. Vastrup, P. Kjeldsen, Chemosphere, 2013, 90, 627.
1
1
1
1
2 H. Y. Zhou, J. Han, S. A. Baig, X. H. Xu, J. Hazard Mater., 2011, 198, 7.
3 Y. H. Xu, Q. Q. Cai, H. X. Ma, Y. He, H. Zhang, Electrochim. Acta, 2013, 96, 90.
4 K. R. Zhu, S. A. Baig, J. Xu, T. T. Sheng, X. H. Xu, Electrochim. Acta 2012, 69,
3
89.
1
5 C. Sun, S. A. Baig, Z. Lou, J. Zhu, Z. X. Wang, X. Li, J. H. Wu, Y. F. Zhang, X. H.
16

Page 17 of 33
RSC Advances
View Article Online
DOI: 10.1039/C5RA07913F
Xu, Appl. Catal., B: Environ., 2014, 158ꢀ159, 38.
1
1
6 K. R. Zhu, C. Sun, H. Chen, S. A. Baig, T. T. Sheng, X. H. Xu, Chem. Eng. J.,
013, 223, 192.
7 C. B. Molina, A. H. Pizarro, J. A. Casas, J. J. Rodriguez, Appl. Catal., B: Environ.,
014, 148ꢀ149, 330.
2
2
1
1
8 G. Yuan, M. A. Keane, J. Catal. 2004, 225, 510.
9 E. Diaz, A. F. Mohedano, J. A. Casas, L. Calvo, M. A. Gilarranz, J. J. Rodriguez,
Appl. Catal., B: Environ., 2011, 106, 469.
2
2
0 N. Seshu Babu, N. Lingaiah, P. S. Sai Prasad, Appl. Catal., B: Environ., 2012, 111,
3
09.
1 J. X Chen, T. Guo, K. Lia, L. M. Sun, Catal. Sci. Technol., 2015, DOI:
0.1039/C5CY00044K.
1
2
2
2 K. Mackenzie, H. Frenzel, F. D. Kopinke, Appl. Catal., B: Environ., 2006, 63, 161.
3 Z. M. de Pedro, J. A. Casas, L. M. GomezꢀSainero, J. J. Rodriguez, Appl. Catal., B:
Environ., 2010, 98, 79.
2
2
2
2
2
4 E. Diaz, A. F. Mohedano, J. A. Casas, L. Calvo, M. A. Gilarranz, J. J. Rodriguez,
Catal. Today, 2015, 241, 86.
5 Y. Shao, Z. Y. Xu, H. Q. Wan, Y. Q. Wan, H. Chen, S. R. Zheng, D. Q. Zhu, Catal.
Commun. 2011, 12, 1405.
6 N. Seshu Babu , N. Lingaiah , R. Gopinath , P. Siva Sankar Reddy, P. S. Sai Prasad,
J. Phys. Chem. C, 2007, 111, 6447.
7 S. GómezꢀQuero, F. CárdenasꢀLizana, M. A. Keane, Ind. Eng. Chem. Res., 2008,
4
7, 6841ꢀ6853.
8 J. Zhou, Y. X. Han, W. J. Wang, Z. Y. Xu, H. Q. Wan, D. Q. Yin, S. R. Zheng, D. Q.
Zhu, Appl. Catal., B: Environ., 2013, 134ꢀ135, 222.
17

RSC Advances
Page 18 of 33
View Article Online
DOI: 10.1039/C5RA07913F
2
3
3
3
3
3
9 L. M. GómezꢀSainero, X. L. Seoane, J. L. G. Fierro, A. Arcoya, J. Catal., 2002, 209,
79.
2
0 I. A. Witońska, M. J. Walock, M. Binczarski, M. Lesiak, A. V. Stanishevsky, S.
Karski, Mol. Catal. A: Chem., 2014, 393, 248.
1 J. A. Baezaa, L. Calvo, J.J. Rodriguez, E. CarbóꢀArgibay, J. Rivas, M. A. Gilarranz,
Appl. Catal. B: Environ., 2015, 168ꢀ169, 283.
2 P. Benito, M. Gregori, S. Andreoli, G. Fornasari, F. Ospitali, S. Millefanti, M. Sol
Avila, T. F. Garetto, S. Albonetti, Catal. Today, 2015, 246, 108.
3 X. C. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J. M. Carlsson, K.
Domen, M. Antonietti, Nat. Mater., 2008, 8, 76.
4 J. Zhang, X. Chen, K. Takanabe, K. Maeda, K. Domen, J. D. Epping, X. Fu, M.
Antonietti, X. Wang, Angew. Chem. Int. Ed., 2010, 49, 441.
3
3
3
3
5 P. Zhang, F. Jiang, H. Chen, Chem. Eng. J., 2013, 234, 195.
6 W. H. Tan, T. Li, F. Jiang, H. Chen, J. L. Liu, China Environ. Sci., 34, 2014, 3099.
7 G. Feng, P. Chen, H. Lou, Catal. Sci. Technol., 2015, 5, 2300.
8 M. A. Aramendía, V. Boráu, I. M. García, C. Jiménez, F. Lafont, A. Marinas, J. M.
Marinas, F. J. Urbano, Mol. Catal. A: Chem., 2002, 184, 237.
3
4
9 G. Yuan, M. A. Keane, Appl. Catal. B: Environ., 2004, 52, 301.
0 Z. M. de Pedro, E. Diaz, A. F. Mohedano, J. A. Casas, J. J. Rodriguez, Appl. Catal.,
B: Environ., 2011, 103, 128.
4
4
4
1 G. Yuan, M. A. Keane, Chem. Eng. Sci., 2003, 58, 257.
2 S. GómezꢀQuero , F. CárdenasꢀLizana, M. A. Keane, J. Catal., 2013, 303, 41.
3 Q. Liu, Z. M. Cui, Z. Ma, S. W. Bian, W. G. Song, J. Phys. Chem. C 2008, 112,
1199.
4
4 A. Drelinkiewicz, W. Stanuch, A. Knapik, A. Ghanem, R. Kosydar, A. Bukowska,
18

Page 19 of 33
RSC Advances
View Article Online
DOI: 10.1039/C5RA07913F
W. Bukowski, J. Mol. Catal., A: Chem., 2009, 300, 8.
4
4
5 H. Chen, P. Zhang, W. H. Tan, F. Jiang, R. Tang, RSC Adv., 2014, 4, 38743.
6 S. Ordóñez, E. Díaz, R. F. Bueres, E. AsedegbegaꢀNieto, H. Sastre. J. Catal., 2010,
72, 158.
2
4
4
4
5
7 H. Chen, Z. Y. Xu, H. Q. Wan, J. Z. Zheng, D. Q. Yin, S. R. Zheng, Appl. Catal., B:
Environ., 2010, 96, 307.
8 J. Zhou, K. Wu, W. J. Wang, Y. X. Han, Z. Y. Xu, H. Q. Wan, S. R. Zheng, D. Q.
Zhu, Appl. Catal., B: Environ., 2015, 162, 85.
9 J. Zhou, K. Wu, W. J. Wang, Z. Y. Xu, H. Q. Wan, S. R. Zheng, Appl. Catal., A:
Gen., 2014, 470, 336.
0 M. Aramendía, V. Boráu, I. García, C. Jiménez, F. Lafont, A. Marinas, J. Marinas,
F. Urbano, J. Catal., 1999, 187, 392.
5
5
5
5
1 E. Díaz, L. Faba, S. Ordóñez, Appl. Catal., B: Environ., 2011, 104, 415.
2 G. Yuan, M. A. Keane, Ind. Eng. Chem. Res., 2007, 46, 705.
3 A. Pintar, J. Batista, J. Levec, T. Kajiuchi, Appl. Catal., B: Environ., 1996, 11, 81.
4 K. Wu,
RSC Adv., 2015, 5, 18702.
5 I. Witońska, A. Królak, S. Karski, J. Mol. Catal., A: Chem., 2010, 331, 21.
X. J. Qian,
L. Y. Chen,
Z. Y. Xu,
S. R. Zheng, D. Q. Zhu,
5
19

RSC Advances
Page 20 of 33
View Article Online
DOI: 10.1039/C5RA07913F
Figure captions:
Figure 1: Liquid phase catalytic HDC of 2,4ꢀD over catalyst 1, 2, 3, 4, 5 and 6. (■)
,4ꢀD, (▲) 2ꢀCPA, (○) 4ꢀCPA, (□) PA and (×) total amount.
2
Figure 2: TEM images of catalyst 1, 2, 3, 4, 5 and 6.
Figure 3: XPS profiles of Pd 3d spectra on catalyst 1, 2, 3, 4, 5 and 6. Black line
refers to the experimental data; Red line refers to the fitting curve; Blue line
0
2+
refers to Pd ; Green line refers to Pd .
Figure 4: Initial activity and TOF of the Pd/gꢀC
for HDC of 2,4ꢀD.
2
+
0
2+
3
4
N catalysts versus Pd /(Pd +Pd )
Figure 5: Liquid phase catalytic HDC of 2,4ꢀD over catalyst 3, 7, 8 and 9.
Figure 6: TEM images of catalyst 7, 8 and 9.
Figure 7: TOF of catalyst 3, 7, 8 and 9 versus (a) Pd particle size and (b)
2
+
0
2+
Pd /(Pd +Pd ) for liquid phase catalytic HDC of 2,4ꢀD. The dashed line in
b) was the result in Fig. 4.
Figure 8: (a) Initial activity of catalyst 3 as a function of initial 2,4ꢀD concentration
and (b) linear plot of 1/r versus 1/C.
Figure 9: Liquid phase catalytic HDC of 2ꢀCPA, 4ꢀCPA and 2,4,5ꢀT over catalyst 3.
(
0
20

Page 21 of 33
RSC Advances
View Article Online
DOI: 10.1039/C5RA07913F
Tables and Figures
Table 1 Textural property of g-C
Serial
3
N
4
and the Pd/g-C
3
N
4
catalysts.
a
b
NaBH :Pd
(mol/mol)
-
4
2
-1 d
Catalyst
pH
c
S
BET (m g )
number
-
g-C
3
N
4
-
57.35
56.42
55.87
49.15
49.22
30.65
35.40
54.68
49.51
40.80
1
2
3
4
5
6
7
8
9
Pd(2.09)/g-C N
Pd(1.94)/g-C N
10
10
10
10
9
0.5:1
1:1
3:1
10:1
3:1
3:1
3:1
3:1
3:1
3
4
4
4
4
4
4
4
4
4
3
Pd(1.96)/g-C
Pd(2.07)/g-C
Pd(1.98)/g-C
Pd(1.93)/g-C
Pd(0.46)/g-C
Pd(0.98)/g-C
Pd(4.11)/g-C
3
3
3
3
3
3
3
N
N
N
N
N
N
N
8
10
10
10
a
b
c
d
the Pd content were determined by ICP;
the pH value during catalyst preparation;
the molar ratio of NaBH
4
and Pd during catalyst preparation;
determined by N adsorption-desorption using the Brunauer-Emmett-Teller
2
(BET) method.

RSC Advances
Page 22 of 33
View Article Online
DOI: 10.1039/C5RA07913F
Table 2 Pd properties and parameters of XPS spectra of catalysts 1 to 9.
Average Pd
Pd
-1 a dispersion
Binding
energy (eV) Pd /(Pd +Pd )
CO uptake
µmol g )
diameter
2+
0
2+
Catalyst
(
b
c(nm)
(%)
d
0
2+
d1
d2
Pd
Pd
1
2
3
4
5
6
7
8
86.8
87.7
85.3
88.8
85.0
88.0
25.5
45.1
110.3
44.20
48.13
46.31
45.66
45.68
48.52
59.00
48.98
28.55
2.54 2.51 335.2 337.3
2.33 2.40 335.2 337.1
2.43 2.57 335.5 337.2
2.46 2.47 335.1 337.0
2.46 2.49 335.2 337.2
2.31 2.69 335.2 337.2
1.90 2.07 335.0 337.0
2.29 2.36 335.2 337.0
3.93 4.22 335.3 337.1
0.71
0.64
0.50
0.39
0.42
0.41
0.64
0.55
0.46
9
a, b, c
calculated from CO chemisorption;
calculated from TEM.
d

Page 23 of 33
RSC Advances
View Article Online
DOI: 10.1039/C5RA07913F
Table 3 Initial activity and selectivity for HDC of chlorophenoxyacetic acids over
catalyst 3.
Initial
S
2,4-D (%)
S
2,5-D (%)
S
2-CPA (%)
S
4-CPA (%)
SPA (%)
activity
mM
(
-1
-1
t
10
t
30
t
10
t
30
t
10
t
30
t
10
t
30
t
10
t
30
g
cat h )
61.78
80.56
40.08
9.64
2
4
-CPA
-CPA
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
100
100
100
100
2
,4-D
49.62 31.28 1.47
0.52 54.13 70.23
0.006 56.14 70.42
2
,4,5-T
23.32 12.95 12.59 6.63 14.38 13.92
-

RSC Advances
Page 24 of 33
View Article Online
DOI: 10.1039/C5RA07913F
Scheme 1. Dechlorination pathways of the liquid phase catalytic HDC of 2-CPA,
-CPA, 2,4-D and 2,4,5-T.
4
+
H
2-CPA
H
+
+
2H
O
2
,4-D
O
PA
+
H
HO
H
+
Cl
4-CPA
H
H
H
H2
H
H
Pd
Pd
C
N
H
2
-CPA
PA
PA
4
-CPA
2
4
-CPA
-CPA
PA
2,4-D
2
2
,4-D
,5-D
2-CPA
4-CPA
2,4,5-T
PA

Page 25 of 33
RSC Advances
View Article Online
DOI: 10.1039/C5RA07913F
Figure 1:
0
0
0
0
0
.5
.4
.3
.2
.1
0
0.5
(1)
(2)
0.4
0.3
0.2
0.1
0
0
20
40
60
80
100
120
0
20
40
60
80
100
120
Time (min)
Time (min)
0.5
0.4
0.3
0.2
0.1
0
0.5
0.4
0.3
0.2
0.1
0
(3)
(4)
0
20
40
60
80
100
120
0
20
40
60
80
100
120
Time (min)
Time (min)
0
0
0
0
0
.5
.4
.3
.2
.1
0
0.5
0.4
0.3
0.2
0.1
0
(5)
(6)
0
20
40
60
80
100
120
0
20
40
60
80
100
120
Time (min)
Time (min)

RSC Advances
Page 26 of 33
View Article Online
DOI: 10.1039/C5RA07913F
Figure 2:
Pd
Pd
Pd

Page 27 of 33
RSC Advances
View Article Online
DOI: 10.1039/C5RA07913F
Figure 3:
Experimentaldata
(1)
Fitting curve
Pd0
Pd2+
(2)
(3)
(4)
(5)
(6)
3
32
334
336
338
340
342
344
346
Binding energy (eV)

RSC Advances
Page 28 of 33
View Article Online
DOI: 10.1039/C5RA07913F
Figure 4:
Catalyst 3
Catalyst 5
Catalyst 6
Catalyst 4
Catalyst 2
Catalyst 1
2
+
2+
0
Pd /(Pd +Pd )

Page 29 of 33
RSC Advances
View Article Online
DOI: 10.1039/C5RA07913F
Figure 5:
0.5
0.4
0.3
0.2
0.1
0
■
□
:
:
7-Pd(0.46)/g-C N
3
4
4
4
8-
3-
9-
Pd(0.98)/g-C N
3
▲:
Pd(2.09)/g-C N
3
△
:
Pd(4.11)/g-C N
3
4
0
20
40
60
80
100
120
Time (min)