Convenient preparation of Pd/RGO catalyst for the efficient hydrodechlorination of various chlorophenols

Article abstract of DOI:10.1039/c5nj02349a

Uniformly dispersed Pd nanoparticles supported on reduced graphene oxide (RGO) have been conveniently synthesized by a facile one-pot approach through the co-reduction of sodium tetrachloropalladate(ii) and graphene oxide (GO) using NaBH₄ as the reducing agent. The catalytic activity of Pd/RGO was investigated for aqueous hydrodechlorination (HDC) of chlorophenols at room temperature using hydrogen balloon pressure without any additives. Results indicated that Pd/RGO exhibited an excellent activity with a reaction rate of 280.0 min^-1 g_pd^-1 for the hydrodechlorination (HDC) of 4-CP. Furthermore, the Pd nanoparticles supported on RGO can be easily recovered and reused for four times without any obvious leaching and loss of activity.

Full text of DOI:10.1039/c5nj02349a

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Convenient Preparation of Pd/RGO Catalyst for the Efficient
Hydrodechlorination of Various Chlorophenols
Weidong Jiang a,*, Zhen Xiang a, Bin Xua, Xiaojing Lib, Fuan Liu a and Guangyin Fanb, **
Received 00th January 20xx,
Accepted 00th January 20xx
Uniformly dispersed Pd nanoparticles supported on reduced graphene oxide (RGO) has been conveniently synthesized by a
facile one-pot approach through the co-reduction of sodium tetrachloropalladate(II) and graphene oxide (GO) using NaBH4
as reducing agent. The catalytic activity of Pd/RGO was investigated for aqueous hydrodechlorination (HDC) of
chlorophenols at room temperature and hydrogen balloon pressure without any additives. Results indicated that Pd/RGO
exhibited excellent activity with a reaction rate of 280.0 min-1·gpd-1 for the hydrodechlorination (HDC) of 4-CP. Furthermore,
the Pd nanoparticles supported on RGO is easily recoverable and can be reused four times without obvious leaching and
loss of activity.
DOI: 10.1039/x0xx00000x
www.rsc.org/
Metals, China. Graphite powder and sodium borohydride (NaBH4)
were obtained from Aladdin Industrial Inc., China. Other solvents
were analytical grade and used as received.
1. Introduction
Chlorophenols (CPs) have been widely used as pesticides,
fungicides, herbicides, dyes, solvents, disinfectants and wood
Graphene oxide (GO) was prepared according to the improved
Hummer’s method. 13 Typically, graphite powder (3.0 g) and KMnO4
(18.0 g) were transferred into a 500 mL round-bottom-flask. And
then the mixture of concentrated H2SO4/H3PO4 (360:40 mL) was
slowly dropped into the flask under vigorous stirring. The mixture
was heated to 50 oC and stirred for another 24 h and then 400 mL of
ice containing 3 ml of 30% H2O2 were poured into the flask. The
resulting suspension was centrifuged and washed with deionized
water, HCl (36% wt.%), and absolute ethanol several times. The
obtained solid as the aim product was dried under vacuum at room
temperature for 12h.
preservatives in industry and agriculture.1, However, the CPs
2
residues would have adverse effect on the environment since CPs
can hardly be biodegraded or self-cleaned in the environment.
Nowadays, varied techniques have been adopted to dispose the
residual CPs, among which catalytic hydrodechlorination (HDC) has
been demonstrated as a cost-effective and environment-friendly
technology.3 The HDC of CPs by various heterogeneous catalysts
containing noble metals (e.g., Pd,1,3-6 Rh,2,7-11 Pt 2,7 ) has attracted
tremendous interest due to their high reactivity.
Catalyst support plays a critical role in enhancing the activity of
noble metals due to the improved dispersion of metal nanoparticles
(NPs). Pd NPs deposited on conventional carriers, i.e., active
carbon,12 Al2O3 2,8 and pillared clays,7,9 have been tested as catalysts
for CPs degradation. Recently, graphene has been identified as an
Pd/RGO catalyst was synthesized using a one-pot co-reduction
described as follows. GO (150 mg) was dispersed in 50 mL of
deionized water under ultrasonication, giving a uniform suspension
in which a 2.88 mL of Na2PdCl4 with a Pd concentration of
1.61mg/mL was slowly added with continuously stirring. Afterwards,
a 10mL aqueous solution of NaBH4 (150mg) was added dropwise
11
excellent stabilizer due to its excellent tolerance to HCl and high
o
ability to improve the metal NPs dispersion. Although the Pd/RGO
had good reactivity towards the 4-CP HDC, the Pd/RGO preparation
was tedious and the reduction conditions of GO was rigorous to
some extent. In the current work, a facile one-pot synthetic method
was developed to prepare Pd/RGO NPs via a co-reduction of sodium
tetrachloropalladate(II) and GO by sodium borohydride. The as-
prepared Pd/RGO catalyst was found to exhibit high activity and
stability in the HDC of 4-CP under moderate conditions.
into the mixture at 0 C. The resulting black solid was obtained by
centrifugation and washed with water and absolute ethanol several
times. Finally, the black solid was dried under vacuum at 60 °C for
12 h, yielding the aim Pd/ RGO catalyst.
2.2 Activity tests
The catalytic activity of Pd/RGO towards the degradation of CPs was
evaluated at 25 oC and hydrogen balloon pressure. Typically, 5.0 mg
of Pd/RGO was added into a 25 mL two-necked round-bottom flask
equiped with a hydrogen balloon. Then 5.0 mL of 4-CP aqueous
solution with a 4-CP concentration of 2.5 g/L was transferred into
the flask under continuous stirring. The catalyst concentration was
kept at 1.0 g/L. The reaction temperature was controlled by a water
bath. The flask was purged with pure hydorgen several times to
remove the air. The reaction time was set at zero, when the
temperature of reactant solution was stable. During the course of
the reaction, samples were withdrawn from the flask periodically by
a syringe and analyzed by GC (Agilent 7890A) with a FID detector
2. Experimental Section
2.1 Reagents and materials
Na2PdCl4·6H2O were purchased from Kunming Institute of Precious
a.School of Chemical and Pharmaceutical Engineering, Sichuan University of
Science & Engineering, Zigong 643000, China. E-mail:jwdxb@163.com
b.College of chemistry and Chemical Industry, China West Normal University,
Nanchong 637009, China. E-mail:scufgy@163.com
* Corresponding author. ** Co-corresponding author.
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and PEG-20M capillary column (30 m×0.25 mm, 0.25 μm film). attributed to the O-H stretching vibration arising from hydroxyl
Nitrogen acted as carrier gas.
groups. The absorption bands associated with the C=O stretching
are at 1730 cm−1, and the C–O stretching at 1040 cm– 1 and the C=C
peak at 1610 cm−1 from unoxidized sp2 C–C bonds are detected. The
adsorption at 1230 cm−1 and 1040 cm−1 are attributed to the C=O
vibrations and C–O groups situated at the edges of GO. This testifies
the oxygen–containing groups on the GO surface, providing reactive
anchoring sites for Pd ions and inhibiting the aggregation of Pd NPs
during the reduction process. GO is successfully reduced in aqueous
phase using NaBH4, which is provided an evidence from the
decreased intensity of the peaks corresponding to the oxygen-
containing groups. Accordinally, based on the above-mentioned
characterizations we believe that the the desired Pd/RGO catalyst
was obtained with the convenient co-reduction process.
3. Results and Discussion
3.1 Characterizations of the Pd/ RGO catalyst
TEM image (Fig.1a) of Pd/RGO with a Pd loading of 3.0 wt.%
demonstrates that Pd NPs are uniformly dispersed on the surface of
RGO sheets, which is probably assigned to the interaction between
the Pd ions and oxygen-containing groups of GO, inhibiting the
aggregation of Pd NPs during the reduction process. As shown in Fig.
1b, however, Pd/RGO NPs begun to aggregate after the fifth run.
Strong diffraction peak at 2θ=26.4◦ of graphite shown in the insert
of Fig. 1c was found to disappear, and a new peak at 2θ=10.0◦
corresponding to the plane of GO appears. It was clearly vestified
that the successful oxidation of graphite to GO. In addition, the
disappearance of the characteristic peaks of GO suggests the
efficient reduction of GO. The two new peaks at the position of
25.4◦ and 43.1◦ are attributed to the (002) and (100) reflections of
RGO, respectively. Those new peaks of Pd/RGO at 2θ=39.1◦, 45.4◦,
66.1◦, which correspond to the (111), (200), and (220) planes of Pd
(JCPDS card No. 87-0637), demonstrate that Pd NPs are indeed
loaded on RGO surface through the one-pot co-reduction
procedure.
Fig. 2 XPS scans of Pd/RGO (a); XPS spectra of C1s peaks of GO (b),
RGO(c) and Pd 3d peaks of Pd/RGO (d).
10
Pd/RGO
Fig. 1 TEM images of Pd/RGO (a) and Pd/RGO after five runs (b) as well as
XRD patterns of GO, RGO and Pd/RGO (c). The insert of Fig. 1c
displays the XRD pattern of graphite.
8
3440
O-H
C-O-H
1410
C-O-H
1230
6
4
2
0
GO
1730
1610 1040
C=O
C-O-C
C=C
3420
O-H
To determine the surface compositions and the electronic states
of each element on the surface of Pd/RGO, the XPS elemental
survey scans of Pd/RGO was firstly conducted (Fig. 2). The results
obtained show the existence of Pd, oxygen, and carbon on the RGO
surface. The C1s spectrum could be deconvoluted into four peaks
which attribute the C–C, C–OH, C–O–C and HO–C=O, respectively.
The declined peak intensities of oxygen-containing groups on RGO
surface suggest the successful reduction of GO (Fig.2b and 2c). The
binding energy of Pd3d5/2 level in Pd/RGO is 335.8 eV, which is
higher than that (= 335.0 eV) of the standard zero-valent state of Pd.
The increase in binding energy of Pd(0) results from the electron
transformation from palladium to RGO due to the strong interaction
between Pd and RGO.14 The peak at 337.9 eV is assigned to the
oxidation state of Pd species, 15 and to some extent the presence of
oxidation state of Pd facilitates the HDC of 4-CP as reported in Dr.
Seoane’ work. 16
4000 3500 3000 2500 2000 1500 1000 500
-1
Wavenumber/cm
Fig. 3 Distinguishabe FT-IR adsorptions of GO and RGO-supported Pd NPs.
3.2 Effect of varied solvents on the HDC of 4ꢀchlorophenol
In this case, effects of varied solvents, including water, methanol,
ethanol, cyclohexane and 1,4-dioxane, on the catalytic preformance
of Pd/RGO were firstly evaluated for the HDC of 4-CP (Fig.4). It can
be seen that Pd/RGO displayed distinguishable reactivity in various
solvents. Specifically, 4-CP could be completely converted to phenol
in 90 min when water acted as green solvent. Meanwhile, the
reactivity of Pd/RGO in protic solvents and aprotic solvents were
obviously decreased. The initial HDC rates of 4-CP over Pd/RGO in
different solvents exhibited a marked dependence on the reaction
In fact, those facts mentioned above were also verified by the
changed FT-IR adsorptions of the surface oxygen-containing
functional groups on GO and RGO-supported Pd NPs (Fig. 3).
Namely, for GO the broad characteristic band at 3420 cm−1 is
2
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media and decreased in the order: water > methanol > ethanol > 4-CP was catalyzed by Pd/RGO, which was probably ascribed to the
cyclohexane>1,4-dioxane. strong interaction between Pd and RGO as well as the presence of
With respect to product distributions, the main product was electron-deficient Pd on RGO surface.
phenol and only trace amount of cyclohexanone (<0.1%) was
detected. This fact was probably attributed to the low
concentration of catalyst (1.0 g/L) and high initial concentration of
4-CP (2.5 g/L) as well as low reaction temperature (25 oC).17
− E a / RT
k obs = Ae
……………… (1)
0.0
-0.4
-0.8
-1.2
-1.6
-2.0
-2.4
-2.0
298K
303K
308K
313K
-2.2
-2.4
-2.6
-2.8
-3.0
-3.2
-3.4
-3.6
(a)
(b)
Additionally, the low reaction temperature also contributed to the
17
low selectivity to cyclohexanone,
which was confirmed by
observing the increase of cyclohexanone selectivity to 0.9 % when
the reaction was conducted at elevating temperature of 40 oC
(Fig.S1). Furthermore, the product distributions did not vary
obviously regardless of the solvents. The solvent with high dielectric
constant and low molar volume is beneficial to the HDC reaction.
On the one hand, the solvent with higher dielectric constant
provides stronger ionic forces to stabilize the electropositive
arenium intermediate. On the other hand, the solvent with low
molar volume means that more solvent molecular available to
interact with the charged reaction intermediate, which obviously
accelerates the HDC reaction of 4-CP. Thereupon, the highest
catalytic activity of Pd/RGO for the HDC of 4-CP in water was
ascribed to the properties of water. In comparison with the other
four organic media, the highest activity of Pd/RGO for the HDC of 4-
CP in water was due to the well-organized structure of water
originated from the formation of an H-bonding between the
dissolving ions and water with the highest/lowest values of
dielectric constant and molar volume. 18
0.00320 0.00325 0.00330 0.00335
1/T(K-1)
0
10
20
30
40
50
60
Time(min)
Fig. 5 Effect of temperature on the reactivity of Pd/RGO for the HDC of 4-CP.
Dependant of lnk upon 1/T based on the rearranged Arrhenius equation is
shown in Fig. 5(b).
The stability and recyclability of Pd/RGO for the HDC of 4-CP was
investigated with successive runs under the operated conditions.
After each run, the solid catalyst was separated from the reactor by
centrifugation and the supernatant liquid was removed. The black
solid was thoroughly washed three times with ethanol, dried in
vacuum, and reused for next run. As shown in Fig.6, the catalyst is
quite stable and no loss of activity for the HDC of 4-CP was detected
after the catalyst was reused four times. Howbeit, the activity of
Pd/RGO begun to decrease after the fifth run. Pd leching test was
firstly conducted by ICP and the results indicate that the Pd leaching
is negligible (0.4 ppm). In addition, XRD investigation shows that the
crystalline structure of the recovered catalyst is mostly retained
after five catalytic cycles (Fig.S2). The suface property of the
recycled catalyst analyzed by XPS (Fig.S3) shows that the binding
energy of Pd(0) in the recovered catalyst was shifted to 334.5 eV,
which was probaly due to the inhibition of the electron
transformation from palladium to RGO, resulting in the decrease of
the interaction between Pd and RGO. TEM image of the recycled
catalyst indicates the aggregation of Pd NPs during the recycling
process (Fig. 1b). In addtion, the element mappings (Fig. S4) of
metallic Pd on Pd/RGO catalyst provide available and visual
evidence for the aggregation and loss of Pd after five runs even if
those corresponding SEM images have no observed changes.
Therefore, the derease of interaction between Pd and RGO and the
increasing of Pd particle size were respondibale for the loss of
catalytic activity in the fifth runs. Further work should be done to
improve the reusability of Pd/RGO catalyst for practical application.
water
methanol
ethanol
cyclohexane
1,4-dioxane
(a)
100
80
60
40
20
0
0
20
40
Time(min)
60
80
100
Fig. 4 Effect of different solvents on the conversion of 4-CP.
3.3 Kinetics and recycling test of catalyst
As illustrated in Fig.5, the kinetic study showed that the conversion
of 4-CP increased with the elevation of reaction temperature in the
range from 298 to 313 K. Meantime, there is no noticeable change
towards the product distributions regardless of the temperature
variation. Rate constants (kobs) are determined from the slope of
the linear part of each plot in Fig.5(a). The activation energy (Ea) is
calculated using the Arrhenius equation (Eq. 1). Consequently, the
activation energy is estimated to be 61.5 kJ/mol based on the slope
of the lnk-(1/T) plot shown in Fig.5(b). The kinetic reaction rate of
4-CP HDC catalyzed by Pd/RGO was compared with the reported
results. The catalyst Pd/Al2O3 reported by Diaz et al.2 showed a
kinetic reaction rate of 3.33 min-1·g-1 and Pd/pillared clays reported
by Molina et al.19 exhibited a reaction rate of 7.6 min-1·g-1,
respectively. Increasing reaction rate to 13.2 min-1·g-1 was detected
by Liu et al.17 with a Pd/dendritic mesoporous silica nanospheres.
100
80
60
40
20
0
1
2
3
4
5
Recycling times
Noteworthyly, in our case the highest reaction rate of 280.0 min-1
was observed at the same reaction temperature when the HDC of
·
g-
Fig. 6 Recyclability survey of Pd/RGO for the HDC of 4-CP.
1
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1
2
H. Rong, S. Cai, Z. Niu, Y. Li, ACS Catal, 2013, 3, 1560-1563.
E. Díaz, J. A. Casas, Á. F. Mohedano, L. Calvo, M. A. Gilarranz,
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3.4 Reactivity of the Pd/RGO for the HDC of various CPs
3
4
5
6
7
8
9
To evaluate the activity of the as-prepared Pd catalyst for the HDC
of different substrates, the catalytic performance of Pd/RGO was
screened for the HDC of other five kinds of chlorophenols (Table 1).
The observations indicate that the reaction time for the complete
conversion of monochlorophenols increases in the order of 3-CP <
2-CP< 4-CP, which probably results from the distinguishable
electronic and steric hindrance of these monochlorine-substituted
substrates. Additionally, dichlorophenols (2,4-DCP and 2,6-DCP) and
trichlorophenol (2,4,6-TCP) can be completely HDC to produce
phenol as reaction time was prolonged. In brief, the HDC rates of
CPs decrease in the following order: monochlorophenols>
dichlorophenols>trichorophenol, which was consistent with a
previous study.20 The increased reaction time with the increase of
substituted substrates was probably assigned to the poisoned
active sites of Pd/RGO catalyst because of the higher content of HCl
generated during the reaction proccess.
M. Munoz, Z. M. de Pedro, J. A. Casas, J. J. Rodríguez, Appl.
Catal. A: Gen., 2014, 488, 78-85.
A. H. Pizarro, V. M. Monsalvo, C. B. Molina, A. F. Mohedano,
J. J. Rodriguez, Chem. Eng. J., 2015, 273: 363-370.
10 J. A. Baeza, L. Calvo, M. A. Gilarranz, J. J. Rodriguez, Chem.
Eng. J., 2014, 240, 271-280.
11 G. Y. Fan, Y. L. Ren, W. D. Jiang, C. Y. Wang, B. Xu, F. A. Liu,
Catal. Commun., 2014, 52, 22-25.
12 L. Calvo, M. A. Gilarranz, J. A. Casas, A. F. Mohedano, J.
J.Rodríguez, Appl. Catal. B, 2006, 67, 68-76.
Table 1 HDC of different CPs catalyzed by Pd/RGO
13 D. C. Marcano, D. V. Kosynkin, J. M. Berlin, A. Sinitskii, Z. Sun,
A. Slesarev, L. B. Alemany, W. Lu, J. M.Tour, ACS Nano, 2010,
4, 4806-4814.
14 W. J. Shen, M. Okumura, Y. Matsumura, M. Haruta, Appl.
Catal. A, 2001, 213, 225-232.
15 R, Gopinath, N. Seshu Babu, J. Vinod Kumar, N. Lingaiah, P.
S.Sai Prasad, Catal. Lett., 2008, 120, 312-319.
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Catal., 2002, 209, 279-288.
17 Y. S. Liu, Z. P. Dong, X. L. Li, X. D. Le, W. Zhang, J. T. Ma, RSC
Advances, 2015, 5, 20716-20723.
Substrates
2-CP
Product
Phenol
Phenol
Phenol
Phenol
Phenol
Conv.(%)
100
Time(min)
70
3-CP
100
60
2,4-DCP
2,6-DCP
2,4,6-TCP
100
120
100
150
18 S. Gómez-Quero, F. Cárdenas-Lizana, M. A. Keane, AlChE J.,
2010, 56, 756-767.
100
200
19 C. B. Molina, A. H. Pizarro, J. A. Casas, J. J. Rodriguez, Appl.
Catal. B, 2014, 148, 330-338.
20 Y. L. Ren, G. Y. Fan, W. D. Jiang, B. Xu, F. A. Liu, RSC
Advances, 2014, 4, 25440-25446.
Reaction conditions: temperature: 25 oC, pressure: hydrogen
balloon pressure, metal concentration: 1.0 g/L, 4-CP
concentration: 2.5 g/L(total volume 5.0 mL).
Conclusions
In summary, Pd/RGO synthesized by a facile one-pot method
exhibited excellent reactivity for the HDC of various
chlorophenols. Moreover, the as-prepared Pd/RGO could be
reused at least four times without significant deactivation.
Especially, in environment-friendly water completed
converison of 4-chlorophenol was achieved by the Pd/RGO
catalyst in 90min under moderate conditions. This advanced
composite material provides a kind of effective catalyst with
great promise for catalytic HDC of CPs in practical application.
What is more, a novel preparation stratage concerning a one-
pot co-reduction will attract researchers more attentions.
Acknowledgements
Gratefully thanks the financial support by the Applied Basic
Research Program of Science and Technology Department of
Sichuan Province (2014JY0107) and the National Natural
Science Foundation of China (21207109).
Notes and references
4
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Graphical Abstract
Convenient Preparation of Pd/RGO Catalyst for the Efficient
Hydrodechlorination of Various Chlorophenols
Weidong Jiang, Zhen Xiang, Bin Xu, Xiaojing Li, Fuan Liu and Guangyin Fan
Highly active Pd/RGO (RGO, reduced graphene oxide) was easily prepared by one pot co-reduction of
Pd ions and GO using NaBH4. The as-prepared catalyst exhibited excellent reactivity towards the
degradation of chlorophenols at mild conditions without additives.
water
100
OH
methanol
ethanol
80
cyclohexane
1,4-dioxane
60
OH
40
20
0
0
20
40
60
80
100
Time(min)
H2
Cl
Pd-RGO