The reaction mechanism for highly effective hydrodechlorination of: P -chlorophenol over a Pd/CNTs catalyst

Article abstract of DOI:10.1039/c6ra21213a

Carbon nanotubes (CNTs) supported Pd catalysts (1 wt% and 5 wt%) were prepared by a conventional impregnation method and tested in liquid-phase hydrodechlorination (HDC) of para-chlorophenol (p-CP) under mild conditions (313 K and atmospheric pressure) with H₂. CNTs as a catalyst support offered better performance than activated carbon (AC). The conversion of p-CP reached 72% and 82% over 5 wt% Pd/AC and 5 wt% Pd/CNTs, respectively, within 30 min. When the content of Pd was reduced to 1 wt%, the conversion of p-CP over Pd/AC and Pd/CNTs was 55% and 83%, respectively. The mechanism of this phenomenon was studied through catalyst characterization (XRD, TEM, and BET). The abovementioned results indicated that the different activities of the catalysts over different supports mainly resulted from the porous structure of the supports and nanoparticle diameters of the active metal.

Full text of DOI:10.1039/c6ra21213a

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The reaction mechanism in highly effective
hydrodechlorination of p-chlorophenol over Pd/CNTs catalyst
Cite this:
Lijuan Lana,b, Fanglin Dua, *, Chuanhai Xiac,*
Carbon nanotubes (CNTs) supported Pd catalysts (1 wt % and 5 wt %) were prepared by
conventional impregnation method and tested in liquidꢀphase hydrodechlorination (HDC) of
Received 00th July 2016,
Accepted 00th July 2016
paraꢀchlorophenol (
CNTs as a catalyst support offered better performance than activated carbon (AC). The
conversion of ꢀCP reached 72% and 82% over 5 wt % Pd/AC and 5 wt % Pd/CNTs within 30
min. When the content of Pd was reduced to 1 wt %, the conversion of ꢀCP over Pd/AC and
pꢀCP) under mild conditions (313 K and atmospheric pressure) with H2.
DOI:
p
www.rsc.org/
p
Pd/CNTs was 55% and 83%. The mechanism of this phenomenon was studied through catalyst
characterization (XRD, TEM, and BET). Above results indicated that the different activities of
the catalysts over different supports mainly resulted from the porous structure of the supports
and nanoparticles diameters of the active metal.
other carbon materials such as carbon nanofibers28ꢀ30 and
graphites,28, 29 have been received more and more attention in the
1. Introduction
Chlorophenols (CPs) have been used extensively for the synthesis
of pesticides, medicine, dye, plastic, and ethanol alterant. However,
they have been listed as priority pollutants by the US Environmental
Protection Agency (EPA) and European regulatory authorities due to
their high toxicity, persistence and low biodegradability.1 Therefore,
it is an urgent task to develop effective treatment methods for the
degradation of CPs. Now CPs is mainly being destroyed by
fields of environmental and industrial heterogeneous catalysis. Since
their discovery by Iijima in 1991, CNTS had led to growing interest
in nanocomposites, synthesis, and pharmaceutics owing to their
unique chemical structures and oneꢀdimensional macromolecules
that possess extraordinary electrical, thermal and chemical
stability.37 The unique structural features and electronic property
have attracted a lot of research work in the use of CNTs as adsorbing
3
5
7
bioremediation,2,
photochemical degradation,4,
irradiation,6,
39
materials,38,
especially as hydrogen storage materials40ꢀ42 and
oxidative degradation,8, 9 metalꢀmediated reductive dechlorination,
catalyst supports.43ꢀ45 In many heterogeneous catalytic reactions,46, 47
CNTS as catalyst supports showed much better activity and
selectivity compared with conventional supports. Singh et al.40 have
reported that transition metal atoms such as Pd, Pt, Ni, etc. could be
stabilized on sp2-hybridized carbon atoms inducing multiple bonding
of molecular hydrogen with adsorption energies intermediate
between physisorption and chemisorption.
10, 11
and catalytic hydrodechlorination (HDC).12ꢀ14 Unfortunately,
most of these methods require rigorous reaction conditions with
special equipment, and/or many of them are impractical scaleꢀup.
Among these methods mentioned above, catalytic HDC is an
environmental
friendly
method,
which
can
convert
organochlorinated pollutants into recyclable raw materials with low
energy consumption and can also prevent the generation of toxic byꢀ
products under relatively mild conditions.
Many supported catalysts used CNTs as the support exhibited
good catalytic activity in the hydrogenation reaction; however,
CNTs as the support has received little attention in liquidꢀphase
HDC. In our previous work,48ꢀ51 Pd/AC catalysts exhibited high
catalytic activity in liquidꢀphase HDC. In this paper, AC and CNTs
were chosen to study the role of the supports. Pd/CNTs and Pd/AC
catalysts were prepared and their catalytic performance was
compared in liquidꢀphase HDC of pꢀCP. In order to investigate the
influence of catalyst supports, samples of the catalysts were analyzed
by characterization (ICP, XRD, TEM, SEM, and BET).
Generally, catalytic HDC is carried out using supported metallic
catalysts such as Pt,15, 16 Ni,17, 18 Rh,19, 20 Ru,18, 21, 22 Pd.19, 23, 24
Among these supported metal catalysts, Pd catalyst is widely used
owing to its high catalytic activity, resistance to the attack of acid
and base, and relative stability under severe conditions,25, 26 Besides
active metals, supports also play an important role in the HDC
reaction. Up to now, various supports have been developed such as
19, 21, 27
31ꢀ33
34
active carbon (AC),15,
Al2O3,30,
pillared clays,26,
SiO2,30, 31ꢀ33 zeolite,35 and ZrO2,36 for catalytic HDC. Of all, AC is
regarded as a conventional catalyst support applied in industry due to 2. Experimental
its huge specific surface area and high stability which is benefit to
the HDC reaction and the recovery of metal elements. Recently,
2.1. Chemicals
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AC (20ꢀ50 mesh) used in the study was purchased from Aladdin
Industrial Corporation, Shanghai, China. The multiꢀcarbon
nanotubes (layers: 8ꢀ15; length: 0.5ꢀ5 ꢁm) was obtained from
Shandong Dazhan Nano Materials Co., Ltd. pꢀCP was purchased
from Aladdin Industrial Corporation, Shanghai, China. The other
regents, such as nitric acid, base and solvents, were analytical grade
and were supplied by Sinopharm Chemical Regent Co., Ltd.,
Shanghai, China. Deionized water was used in the reaction. The
purity of hydrogen gas used in the experiments was more than
99.99%.
Cp-CP,0 ꢀC
Conversion(%) =
pꢀCP,t ×100
Cp-CP,0
where CpꢀCP,0 (mmol Lꢀ1) was the initial concentration of pꢀCP, Cpꢀ
(mmol Lꢀ1) was the concentration of pꢀCP at time t in the
CP,t
aqueous phase.
2.4 Catalyst characterization
Powder Xꢀray diffraction (XRD) of the catalysts was recorded
with XRDꢀ7000S/L of Shimadzu with a Cu Kα radiation at 40 kV
and 30 mA. The samples were scanned at rate of 0.1o/s over the 10o
≤ 2θ ≤ 80o range with a scan time of 5 s stepꢀ1.
2.2. Catalyst Preparation
The morphology of the catalyst was characterized using
transmission electron microscopy (TEM, a JEOL Mode HEMꢀ
2011EM, Japan) at an accelerating voltage of 120 KV. The crushed
catalysts were prepared first by dispersing in ethanol, and then two
drops of the dispersed samples depositing on a copper mesh grid. At
least 300 individual Pd particles were counted for each catalyst. The
mean Pd particle sizes as a number average diameter were quoted in
this paper:
The AC which was grinded to 200 mesh and CNTs were
respectively contacted (agitation at 300 rpm) with nitric acid in oil
bath for 10 h at 353 K. Meanwhile, concentrated hydrochloric acid
was added to increase the solubility of PdCl2 in aqueous solution.
Then the mixture was diluted with deionized water, followed by
filtering, thoroughly washed with an excess of water (until PH
approached 7) and ovenꢀdried at 373 K for 12 h. The acidified AC
and CNTs were obtained.
5 g of acidified AC and CNTs were respectively added to 100 mL
vessel with a certain amount of water, sonicated in a bath for 30 min,
a Pd(Cl)2 solution with the loading of Pd was added dropwise at 303
K with constant agitation (100 rpm), placed at room temperature
over 2 h. Then formaldehyde and NaOH solution were added
dropwise to the mixture with constant agitation (100 rpm) as Pdꢀ
based catalyst was prepared by reduction with formaldehyde as a
reducing agent in the alkaline conditions. Standing over 6 h, the
mixture was filtered, washed with distilled water repeatedly to
remove chloridion in the final product, then ovenꢀdried at 373 K for
24 h. The Pd/AC and Pd/CNTs catalysts were obtained.
n
i i
d
∑
i
dn =
n
i
∑
i
The scanning electron microscopy (SEM) was conducted using a
Hitachi S4800 field emission SEM, operated at an accelerating
voltage of 20 kV; Specific BET surface area and BJH pore volume
analyses were performed using the commercial Micrometrics
ST2000B unit; N2 at 77 K served as sorbate. Before measurement,
the samples were outgassed at 433 K for 16 h.
3. Results and discussion
The Pd loading was determined by inductively coupled plasmaꢀ
optical emission spectrometry (ICPꢀOES, VistaꢀPRO, Varian Inc.).
3.1 Catalyst characterization: XRD and ICP
2.3. Catalytic procedure
XRD patterns scanned over angles (10oꢀ80o) were given in Fig. 1.
Characteristic peaks of carbon for AC and CNTs observed at 2θ
values of 26o, 43o, and 53o were respectively identified to diffraction
of (0 0 2), (1 0 0), and (0 0 4) planes.52 The peaks at 2θ values of
40o, 46o, and 68o in 5 wt % Pd/AC and 5 wt % Pd/CNTs can be
assigned to (1 1 1), (2 0 0), and (2 2 0) crystalline Pd planes which
were consistent with the PDF standard card of Pd (JCPDS 05ꢀ
0681).53 According to the XRD patterns of 5% Pd/CNTs and 5%
Pd/AC, it would be reasonable to presume that the catalyst active
component had been successfully loaded on AC and CNTs.
The HDC of pꢀCP was carried out in a 100 mL threeꢀnecked flask
with magnetic stirring at 1200 rpm. The reaction vessel was placed
in a temperatureꢀcontrolled heating water bath with a precision of ±
1 K. pꢀCP was reacted with hydrogen gas over some amount of
supported Pd catalyst in water solutions under mild conditions (313
K and atmospheric pressure). H2 flow (10 mL/min) and stirring
speed (1200 rpm) served to minimize external H2 transport
limitations. The stability of the catalysts was tested under the same
conditions. In this paper, the concentration of pꢀchlorophenol in
reaction solution was in high concentration (5.0 g/L, 400mg in 80
mL solution). Yet, the amount of Pd catalysts in reaction solution
was only 30 mg. Thus, the effect of adsorption in the absence of H2
would be negligible.
For 1 wt % Pd/CNTs catalysts, the corresponding diffraction peak
of Pd was only found at 2θ values of 40o (Fig.1a), which was
weakened in contrast to XRD pattern of 5 wt % Pd/CNTs. Similarly,
the XRD pattern of 1 wt % Pd/AC exhibited a weak peak at 2θ
values of 40o (Fig. 1b). The weakened Pd peak of XRD patterns
possibly due to the low Pd loading amounts of 1 wt % Pd/CNTs and
Pd/AC.
The composition of the reaction system was determined by HPLC
(Agilent 1200) equipped with a chromatographic column (XDBꢀ
C18: 4.6 × 150 mm, 5 ꢁm). The conversion of pꢀCP was applied to
account for the catalytic activity of the catalysts in the HDC reaction,
which is defined as the following:
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only 55% over 1 wt % Pd/AC within the same reaction time.
Moreover, the conversion of pꢀCP over 1 wt % Pd/CNTs reached
100% within 50 min, yet the conversion of pꢀCP over 1 wt % Pd/AC
was only 83%. When the content of Pd was reduced from 5 wt % to
1 wt %, the activity difference between Pd/CNTs and Pd/AC became
obvious (Fig. 3). Compared to Pd catalysts on other supports and
those reported in the literatures, it was found that the average HDC
rates over Pd/CNTs in the study were much higher than that of Pd
catalysts on other supports and those reported in the literatures
(Table S1 in Supplementary data). This indicated that Pd/CNTs
exhibited much higher catalytic activity in the HDC of pꢀCP.
100
(c)
(b)
(a)
10
20
30
40
50
60
70
80
2
θ/(°)
Fig. 1 (a). XRD patterns for (a) CNTs, (b) 1% Pd/CNTsand (c)5% Pd/CNTs
80
60
40
(c)
Pd/AC
Pd/CNTs
20
(b)
(a)
0
0
10
20
30
40
50
60
Reaction time / min
10
20
30
40
50
60
70
80
Fig. 2. Reaction profile of catalytic HDC of pꢀCP over 5 % Pd/AC and 5%
Pd/CNTs. Reaction conditions: deionized water (80 ml), pꢀCP (800 mg),
NaOH(275 mg), catalysts (10 mg), temperature(313 K), H2: 10 mL minꢀ1
2
θ/
(
°
)
Fig. 1 (b). XRD patterns for (a) AC, (b) 1%Pd/AC and (c)5% Pd/AC
In order to evaluate the actual loading amounts of Pd on CNTs
and AC, ICPꢀMS analysis of Pd/AC and Pd/CNTs were carried out,
and the actual loading amounts of Pd on AC and CNTs were 5.01%,
0.98%, 4.97%, and 1.03%, respectively. This meant that the actual
loading amounts of Pd were almost the same as the theory loading
amounts of Pd on AC and CNTs. The results demonstrated that Pd
active component had been successfully loaded on AC and CNTs.
100
80
60
40
Pd/CNTs
Pd/AC
20
3.2 Catalytic activity of Pd/AC and Pd/CNTs
0
0
10
20
30
40
50
60
To evaluate the catalytic activity of 5 wt % Pd/AC and 5 wt %
Pd/CNTs, catalytic HDC of pꢀCP was performed at 313 K under
atmospheric pressure. The activity profiles presented the conversion
of pꢀCP with reaction time over 5 wt % Pd/CNTs and 5 wt % Pd/AC
catalysts (Fig. 2). In the first 30 min, the conversion of pꢀCP reached
72% and 82% over 5 wt % Pd/AC and 5 wt % Pd/CNTs catalysts,
respectively, which meant the activity of the two catalysts were very
high. However, as the proceeding of the HDC reaction, the
conversion increased slowly and the conversion of pꢀCP over 5 wt %
Pd/AC and 5 wt % Pd/CNTs catalysts at 60 min were 96% and 95%,
respectively.
Reaction time /min
Fig. 3. Reaction profile of catalytic HDC of pꢀCP over 1% Pd/AC and 1% Pd
/CNTs. Reaction conditions: deionized water (80 ml), pꢀCP (400 mg), NaOH
(150 mg), catalysts (30 mg), temperature (313 K), H2: 10 mL minꢀ1
Subsequently, catalytic HDC of pꢀCP was repeated 3 times with
the catalyst recovered after each reaction to evaluate the stability of
5 wt % and 1 wt % Pd/CNTs (Fig. 4). As shown in Fig. 4, the
conversion of pꢀCP within 60 min over 5 wt % Pd/CNTs was 95%
for the first time, and drops to 90% in the 3rd run. Yet for 1 wt %
Pd/CNTs, the conversion of pꢀCP within 50 min 100% for the first
time, and drops to 94% in the 3rd run. It can be seen that the activity
of Pd/CNTs catalyst gradually decreased with the increase of reused
time but the damping was not obvious. These results suggested that
Pd/CNTs possessed high activity and stability and could be reused
for at least 3 times for the HDC of pꢀCP under mild conditions.
As shown in Fig. 2, the conversion rate of pꢀCP over both 5 wt %
Pd/AC and 5 wt % Pd/CNTs were high with no obvious difference.
The high activity of the two catalysts might owing to the high
loading of Pd. Thus, 1 wt % Pd/CNTs and 1 wt % Pd/AC catalysts
were chosen to investigate the activities of catalysts (Fig. 3). As
illustrated in Fig. 3, the conversion of pꢀCP was 83% over 1 wt %
Pd/CNTs in the first 30 minutes, while the conversion of pꢀCP was
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and 7 that 5 wt % and 1 wt % Pd/AC catalysts yielded metal particle
of around 1ꢀ12 nm, while 5 wt % and 1 wt% Pd/CNTs catalysts
yielded metal particle of around 1ꢀ7 nm (most of them in 3ꢀ6 nm).
Moreover, Pd/CNTs catalysts were characterized by a small average
diameter (2.60 nm for 1 wt % Pd/CNTs versus 4.30 nm for 5 wt %
Pd/CNTs) and a narrow particle size distribution (more than 70% of
the total particles numbers within 3 nm for 1 wt % catalyst, and more
than 50% of the total particles numbers within 5 nm for 5 wt %
catalysts). On the other hand, Pd/AC catalysts presented a large
average diameter (6.85 nm for 1 wt % Pd/AC versus 8.31 nm for 5
wt % Pd/AC) and an extensive particle size distribution (Table 1). In
additional, it can be seen from Table 1 that the dispersion of Pd on
CNTs for 5 wt % Pd/CNTs and 1 wt % Pd/CNTs catalysts are 48%
and 79%, while the dispersion of Pd on AC for 5 wt % Pd/AC and 1
wt % Pd/AC catalysts are 25% and 30%. This suggested the
dispersion of Pd on CNTs are much higher than that of Pd on AC.
100
80
60
40
20
0
1
2
3
Run number
Fig. 4. Repeated HDC of pꢀCP over 1% Pd/CNTs for 50 min (filled square)
(reaction condition the same as Fig. 2) and 5 % Pd/CNTs for 60 min (open
square) (reaction condition the same as Fig. 1).
The catalysts exhibited different catalytic activity in the HDC of
pꢀCP when AC and CNTs were used as catalyst supports. Why the
activity was higher with CNTs as catalyst support? Singh et al.40
have reported that Pd could be stabilized on sp2-hybridized carbon
atoms. What was the other effect of CNTs as support on catalytic
performance? In order to get more accurate evidence for the roles of
AC and CNTs, TEM, SEM, BETꢀpore analysis were performed in
the following research.
Table 1 Particle diameter and dispersion of the catalysts as determined from TEM
catalyst
dPd ( nm)
4.30
8.31
2.60
6.85
D (%)a, 55
5% Pd/CNTs
5% Pd/AC
1% Pd/CNTs
1% Pd/AC
48
25
79
30
a Calculated from dTEM, D, metal dispersion
Above results suggested that the Pd nanoparticles were not well
distributed on the AC surface due to agglomeration, yet they were
highly dispersed covering large area of the CNTs with minimal
aggregation. Generally, smaller particle size leads to larger specific
surface area, stronger adsorption function and more active sites.22, 29
In the HDC of pꢀCP, Pd/CNTs possessed a small average Pd
diameter and a narrow particle size distribution which enabled the
catalyst to keep high activity.
3.3 Catalyst characterization: TEM and SEM
The characteristics of the supports is one of the important factors
that affect the dispersion and nanoparticle sizes of Pd as catalyst
active component, which plays an important role in the HDC
reaction.22, 54 Thus, catalyst characterizations (TEM, SEM, and BET)
were introduced to analyze samples of Pd/AC and Pd/CNTs
catalysts.
Subsequently, SEM analysis of AC and CNTs were performed to
illustrate the role of AC and CNTs in the process of Pd loading (Fig.
8 and 9). As shown in Fig. 8a, AC was in a form of granular stack
with the diameter of the granules in the range of about 50 and 300
ꢁm. Fig. 8b, 8c, and 8d showed that there were so many holes (the
diameters between 0.8 and 2 ꢁm) on the surface which could explain
why AC possessed large surface area.
A
B
20
5% Pd/CNTs
5% Pd/AC
D
C
18
16
14
12
10
8
6
4
Fig. 5. TEM images of (A) 5% Pd/AC, (B) 5% Pd/CNTs, (C) 1% Pd/AC, (D) 1%
Pd/CNTs
2
0
Firstly, the fresh Pd/AC and Pd/CNTs catalysts were analyzed by
TEM. The representative TEM images were given in Fig. 5, and the
particle distribution of TEM images are provided in the Fig. 6 and 7.
As illustrated in Fig. 5b and 5d, metallic Pd nanoparticles distributed
randomly on the external wall of CNTs. It can be seen from Fig. 6
Pd diameter (nm)
Fig.6. Size distributions of Pd nanoparticles on 5% Pd/AC and CNTs.
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35
30
25
20
15
10
5
1% Pd/CNTs
1% Pd/AC
A
B
C
C
D
0
5
5
5
5
5
5
5
5
5
.
8
.
.
.
2
.
.
.
5
.
6
.
7
0
1
3
4
ꢀ
ꢀ
5
ꢀ
5
ꢀ
5
ꢀ
ꢀ
ꢀ
5
ꢀ
ꢀ
5
.
0
5
5
5
.
.
.
.
.
.
.
0
1
2
3
4
5
6
7
Pd diameter (nm)
Fig. 7. Size distributions of Pd nanoparticles on 1% Pd/AC and CNTs
The SEM images provided in Fig. 9 showed that CNTs had totally
different surface structures compared to AC. CNTs were composed
of fibrous structures with closed top and hollow inner cavity. The
pipe walls of CNTs were made up of multilayer graphene curled up
seamless with the diameter between 10 and 30 nm and length up to
micrometer. CNTs catalysts produced from conventional
impregnation method possessed a certain amount of functional sites
on the outer surface which made it easily for Pd nanoparticles to
attach on them (Fig. 9). Yet the growth of Pd nanoparticles was
limited on the curved surface which provided a small actual contact
area with the result that smaller Pd namoparticles were obtained on
CNTs catalysts compared to AC. On the contrary, Pd nanoparticles
could enter into the internal microporous of AC and attach in them.
AC owned blocky structure and it could provide enough area for Pd
nanoparticles to grow into larger particles. That is, the position of the
reaction active sites and the loading area of CNTs and AC were quite
different. Mass transfer rate is one of the important factors that affect
the HDC rate of pꢀCP. In the HDC of pꢀCP, the substrate and H2
could easily be adsorbed onto the outer surface of CNTs because of
the little mass transfer resistance. On the contrary, there was much
transfer resistance for the substrate and H2 to reach into the internal
microporous of AC.
Fig. 9. SEM images of raw material of carbon nanotubes (A) 10 um, (B) 2 um, (C)
1 um, (D) 200 nm
3.4 Catalyst characterization: BET- pore analysis
Furthermore, BETꢀpore analysis was carried out to obtain more
comprehensive information about the structure of AC and CNTs
catalysts. The total pore volume, and average pore radii of 5 wt %
Pd/AC and 5 wt % Pd/CNTs were shown in Table 2.
The low temperature nitrogen adsorptionꢀ desorption isotherms of
AC displayed in Fig. 10a was consistent with type IV (IUPAC
classification) adsorption/desorption isotherms. It was convex
upward and rose rapidly at low P/P0 area which Dubinin56 called this
phenomenon ‘micropore filling’. This illustrated that AC was
indicative of a micropore structure with an estimated 25% of the
total pores with radii < 2 nm,41 which coincided with those obtained
from Fig. 10b. At higher P/P0 (P/P0>0.4), adsorbing capacity tended
to a constant value with a hysteresis loops which meant a small
number of mesoporous existed in AC.29
Table 2 Physicochemical characteristics of the catalysts
SBET (m2/g)
Dpore(nm)a
Vpore (cm3/g)a
0.8690
Pd/AC
1356.7
3.46
Pd/CNT
217.73
20.7
1.4368
a BJH method.
A
B
Similarly, CNTs was also consistent with type IV multilayer
adsorption/desorption isotherms. However, CNTs was indicative of a
mesoporous structure. At low P/P0 area, the adsorption quantity was
little because of the lack of microporous, but the
adsorption/desorption isotherms rised rapidly at higher P/P0 area due
to capillary condensation which generated hysteresis loops. The type
of hysteresis loop of CNTs belonged to type A57 which was
characterized by cylindrical hole and uniform pore size. The result
was consistent with the structure characteristics of CNTs which
intertwined together to form mesopores. The pore size distribution in
C
D
Fig. 8. SEM images of raw material of activated carbon (A) 10 um, (B) 2 um, (C)
1 um, (D) 200 nm
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the regions of mesoꢀ (from 2 to 50 nm) and macroꢀ (> 50 nm) pores
(from a BJH analysis) were given in Fig. 10b.
Shandong Province: “the Coastal Resources and Environment team
for BlueꢀYellow Area”, National Science Foundation of China
(21377162 and 51272115) and Natural Science Foundation of
Shandong Province (ZR2012EMM001).
0.4
600
Notes and references
400
0.2
(Ⅰ)
200
a
College of Materials Science and Engineering, Qingdao University of
0.0
(Ⅰ)
0
1200
2.0
1.5
1.0
0.5
0.0
ꢀ0.5
Science and Technology, Qingdao 266042, China
b
900
600
300
0
School of Environment and Materials Engineering, Yantai University,
Yantai 264005, China
c School of Resources and Environmental Engineering, Ludong University,
Yantai 264025, China
(Ⅱ)
(
Ⅱ
)
0.0 0.2 0.4 0.6 0.8 1.0
1
10
100
Relativepressure
(
P/ P
°
)
Corresponding author:
Porediameter (nm)
(a)
(b)
* Prof. Dr. Fanglin Du, E-mail: fldu@qust.edu.cn, Tel.: +86 532 84022870
** Prof. Dr. Chuanhai Xia, E-mail: chxia_ldu@hotmail.com, Tel.: +86 535
6016605
Fig. 10 (a). N2 adsorption/desorption isotherms (at 77 K) for (I) 5% Pd/AC, (II)
5% Pd/CNT. (b) pore size distribution, labeling as above.
Pd/AC exhibited a high BET surface area (1357 m2/g, recorded in
Table 2), while the surface area of Pd/CNTs was only 217 m2/g.
Pd/CNTs showed greater adsorption capacity than Pd/AC even
though Pd/AC exhibited a higher BET surface area. Fig. 10a showed
that at p/p0 ≈ 1, the VN2 adsorbed on AC and CNTs were 600 cm3/g
and 1200 cm3/g, respectively. At present, a research hotspot of CNTs
is that it is applied as hydrogen storage material, hydrogen gas could
be well adsorbed and stored on CNTs.40ꢀ42 When CNTs was used as
catalyst support, it was beneficial to HDC reaction due to the high
concentration of hydrogen gas on it. Moreover, CNTs comprised
mainly mesopores and the average pore diameter was about 20 nm.
The absence of micropores in the CNTs led to a higher catalytic
activity due to the less mass transfer limitation contrast to AC which
had a microporous structure. The average pore diameter of AC was
about 3 nm, the presence of a large number of micropores inside AC
highly hindered the pꢀCP and phenol diffusion which resulted in a
lower activity despite the high surface area.
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View Article Online
DOI: 10.1039/C6RA21213A
The reaction mechanism in highly effective
hydrodechlorination of p-chlorophenol over
Pd/CNTs catalyst
Lijuan Lan1, 2, Fanglin Du1, *, Chuanhai Xia3, *
1College of Materials Science and Engineering, Qingdao University of Science and
Technology, Qingdao 266042, China
2School of Environment and Materials Engineering, Yantai University, Yantai 264005,
China
3School of Resources and Environmental Engineering, Ludong University, Yantai
264025, China
* Corresponding author:
Prof. Dr. Fanglin Du, E-mail: fldu@qust.edu.cn, Tel.: +86 532 84022870
Prof. Dr. Chuanhai Xia, E-mail: chxia_ldu@hotmail.com, Tel.: + 86 535 6016605
Cl
OH
Cl
OH
Cl
OH
OH
Cl
OH
OH
Cl
OH
OH
Na
OH
OH
Pd
Pd
Na
Na
Cl
OH
Na Na
Pd
OH
Pd
H2
H2
Cl
OH
OH
H
Cl
OH
Cl
Cl
H
Cl
Na
Cl
Cl
H
Na
H
Cl
Cl
H
H
H
H
H
H
Cl
H
Na
H
Pd
Cl
Pd
Cl
OH Na
Pd
Cl
OH
Pd
Na
Na
H
H
AC
CNTs
Different mass-transfer mechanism of CNTs and AC in the HDC of p-CP.