Performance of palladium-tin bimetallic catalysts supported on activated carbon for the hydrodechlorination of 4-chlorophenol

Article abstract of DOI:10.1039/c7ra07754h

The influence of Sn on the activity and selectivity of Pd catalysts for the hydrodechlorination (HDC) of 4-chlorophenol (CP) in an aqueous phase was studied. The Pd-Sn bimetallic catalysts were prepared using the coimpregnation method and characterised using temperature-programmed reduction, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and hydrogen pulse chemisorption measurements. The results showed that when the molar ratio of Sn/Pd is less than 1/1, the addition of Sn to Pd catalysts improves the dispersibility of Pd particles and improves the catalytic activity of HDC.

Full text of DOI:10.1039/c7ra07754h

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Performance of palladium–tin bimetallic catalysts
supported on activated carbon for the
Cite this: RSC Adv., 2017, 7, 40437
hydrodechlorination of 4-chlorophenol
a
ab
*
and Deren Fang
Xu Fang
The influence of Sn on the activity and selectivity of Pd catalysts for the hydrodechlorination (HDC) of 4-
chlorophenol (CP) in an aqueous phase was studied. The Pd–Sn bimetallic catalysts were prepared using the
coimpregnation method and characterised using temperature-programmed reduction, high-resolution
transmission electron microscopy, X-ray photoelectron spectroscopy, and hydrogen pulse chemisorption
measurements. The results showed that when the molar ratio of Sn/Pd is less than 1/1, the addition of Sn to
Pd catalysts improves the dispersibility of Pd particles and improves the catalytic activity of HDC.
Received 14th July 2017
Accepted 11th August 2017
DOI: 10.1039/c7ra07754h
rsc.li/rsc-advances
were found to be highly active at removing nitrate from water,22
but the HDC of CPs using this catalyst has not been reported.
Introduction
Therefore, we prepared a series of Pd–Sn bimetallic catalysts
supported on AC, and found that they are highly active in the
Chlorophenols (CPs) are the chemical raw materials and inter-
mediate products most widely used in the manufacturing of
pesticides, leather, dyes, and disinfectants.1 However, CPs are
also persistent toxic compounds that destroy the environment
and which must be eliminated from water. In recent years,
liquid phase catalytic hydrodechlorination (HDC) has been
used to displace chlorine with hydrogen in the pollutant,
forming low- or non-toxic products under mild conditions. This
process is considered promising for the treatment of chlori-
nated organic pollutants,2 and is efficient for managing
compounds with various concentrations of chlorine. Although
HDC does not completely destroy CPs, it efficiently transforms
them into substantially less harmful species, which involves
a signicant decrease in the ecotoxicity of the effluent. HDC
reactions have been studied using a number of metals,
including Pd,3–13 Pt,14 and Rh,15 with Pd being the most common
active metal due to its considerable adsorption and dissociation
of H2.16 Catalytic support for HDC also plays a key role in both
the catalytic activity and stability of the metals, and alumina
and activated carbon (AC) are the support systems most re-
ported in the literature.17,18 Bimetallic nanoparticles (usually Pd
decorated with another metal) exhibit superior catalytic
performance compared with pure Pd catalysts due to their
enhanced tunability in chemical compositions and metal
structures,19 and include compounds such as Pd–Fe,20 Pd–Al,21
Pd–Cu,11 and Pd–In.12
Table 1 The dispersion and surface area of Pd of the catalysts
Molar ratios
of Sn/Pd
Pd dispersion
(%)
Pd surface area
Catalysts
(m2 gꢀ1
)
1% Pd
SP-0.1/1
SP-0.3/1
SP-0.5/1
SP-1/1
—
28.1
34.6
45.0
30.0
29.5
24.3
18.6
62.5
83.0
94.3
76.5
60.1
58.2
33.4
0.1/1
0.3/1
0.5/1
1/1
3/1
5/1
SP-3/1
SP-5/1
The study of new and efficient bimetallic catalysts continues
to have broad potential. For example, Pd–Sn bimetallic catalysts
aInstitute of Applied Catalysis of Yantai University, Yantai 264005, China. E-mail:
fdr@ytu.edu.cn
bLight Hydrocarbon Comprehensive Utilization Engineering Center, Yantai University,
Yantai 264005, China
Fig. 1 XPS spectra for Pd 3d of catalysts SP-0.5/1, SP-1/1 and SP-3/1.
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Experimental
Catalyst preparation
The Pd–Sn bimetallic catalysts were prepared using the
coimpregnation method, and commercial coconut shell AC
with a diameter of 0.045–0.075 mm was used as a support.
The Pd content was xed at 1 wt%, and the Sn content was
varied to obtain Sn/Pd molar ratios of 0.1/1, 0.3/1, 0.5/1, 1/1,
3/1, and 5/1; the corresponding catalysts were designated SP-
0.1/1, SP-0.3/1, SP-0.5/1, SP-1/1, SP-3/1, and SP-5/1, respec-
tively. A typical catalyst preparation method was used to
impregnate the precursors of the catalysts with the desig-
nated amount of aqueous PdCl2 and aqueous SnCl4. The
catalysts were then dried at 110 ꢁC for 2 h and calcined at
350 ꢁC for 2 h. Notably, the catalysts must be activated at 623
K for 2 h by using a ow of pure H2 at atmospheric pressure
prior to performing high-resolution transmission electron
microscopy (HRTEM), X-ray photoelectron microscopy
(XPS), H2 pulse chemisorption measurements, and the HDC
test; by contrast, the sample used to determine temperature-
programmed reduction (TPR) characterisation does not
require prereduction.
Fig. 2 XPS spectra for Sd 3d of catalysts SP-0.5/1, SP-1/1 and SP-3/1.
HDC of CPs, specically 4-CP. The results indicated that adding
an appropriate amount of Sn can improve the catalytic activity
of the Pd/AC catalyst for the HDC of CPs.
Fig. 3 TEM micrographs of reduced catalyst 1% Pd, SP-0.1/1, SP-0.3/1, SP-1/1 and SP-3/1 (1), (2) 1% Pd, (3), (4) SP-0.1/1, (5), (6) SP-0.3/1, (7), (8)
SP-1/1, (9), (10) SP-3/1, (11) SP-0.1/1 diffraction pattern, (12) SP-0.3/1 diffraction pattern, (13) SP-1/1 diffraction pattern, (14) SP-3/1 diffraction
pattern, (15) 1% Pd diffraction pattern.
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source and a high-performance Al monochromatic source. All
XPS spectra were referenced to the C 1s peak at 284.6 eV.
TPR was conducted using a TP5000 multifunction absorber.
The catalyst weight was 50 mg, the particle size was 0.045–0.075
mm, a mixture of 10% H2/90% N2 was used as the reducing gas,
and the temperature was increased from 20 to 300 ꢁC at a rate of
Catalyst characterisation
HRTEM images were obtained using a JEM-200CX electron
microscope equipped with a computer system for automatic
operation and data processing; these images revealed the
bright- and dark-eld morphologies of the catalyst. Addition-
ally, the electron diffraction spectrum and high-resolution
atomic number contrast images were obtained using high-
angle annual dark-eld imaging.
5
ꢁC minꢀ1. The total H2 consumption was detected using
a thermal conduction detector (TCD).
Finally, H2 pulse chemisorption measurements of the cata-
lysts were performed using a TP5000 multifunction absorber
XPS was performed using a Thermo Fisher Scientic ESCA-
LAB 250 spectrometer equipped with a standard Mg K-alpha
Fig. 4 (1) SP-0.1/1 PSD, (2) SP-0.3/1 PSD, (3) SP-1/1 PSD, (4) SP-3/1 PSD, (5) 1% Pd PSD.
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and the TCD. The catalyst weight was 0.10 g. Prior to the H2
The HRTEM results in Fig. 3 and 4 indicate that the size of Pd
pulse chemisorption measurements, the catalysts were acti- particles on the catalysts ranged from 2 to 5 nm. The mean
vated at 623 K for 2 h by using a ow of pure H2 at atmospheric particle size was 2.60 nm for SP-0.1/1, 2.55 nm for SP-0.3/1,
pressure. The catalysts were then degassed, also at 623 K, for 2 h 3.31 nm for SP-1/1, 4.03 nm for SP-3/1, and 2.71 nm for pure
under a ow of Ar before being cooled to 300 K. H2 (5 vol%) was Pd (1%). The gure also reveals that when the Sn/Pd molar ratio
then injected into the Ar stream owing through the sample is relatively low, Pd and Sn particles are well dispersed on the AC
bed. The injection loop was 0.5 mL. In the subsequent metal support, with a normal distribution of the active phase over the
dispersion calculations, H2 was assumed to be chemisorbed surface of the catalyst; additionally, these particles are smaller
only on the Pd composites. The surface areas and the dispersion in size compared with those of the pure Pd catalyst. When the
of Pd are listed in Table 1.
Sn content in the catalysts increases, the particle size also
gradually increases; some agglomerations can be observed in
the HRTEM results. Therefore, adding a small amount of Sn to
the Pd/AC catalyst could improve the dispersion of Pd on the
surface of the catalyst. Moreover, the Pd particles become
covered by Sn particles when the amount of Sn increases, which
leads to an increase in the grain size and lower catalytic activity.
Catalytic HDC
The HDC of 4-CP (Sigma, purity > 99.6%) was conducted under
atmospheric pressure in a mixing glass tank reactor, with
electromagnetic stirring to ensure maximum contact between
the catalyst and the reactant.11 Aer reduction, 0.2 g of the
catalyst was added to 200 mL of an aqueous mixture of 4-CP
(1000 ppm) and NaOH (NaOH/4-CP molar ratio ¼ 1.1/1). H2 was
then added to the reactor at a rate of 50 mL minꢀ1, which thus
initiated HDC. Aer 7.5 min, 1 mL of the solution was extracted
and analysed using gas chromatography with a ame ionisation
detector and polyethylene glycol capillary column.
Results and discussion
The XPS spectra in Fig. 1 illustrate that Pd 3d had two main
peaks (335.1 and 340.2 eV) in all three catalysts, which corre-
spond to the Pd 3d5/2 and Pd 3d3/2 binding energies of Pd0,
respectively.23 In Fig. 2, the peak at 485.2 eV corresponds to the
binding energy of Sn0.24,25 This means that in the catalysts, both
Pd and Sn exist in a metal state. Specically, during the
reduction process, SnO2 was reduced to the metal atom Sn and
came in close contact with Pd. A few Pd–Sn pseudoalloy parti-
cles then formed aer the reduction.26
Fig. 6 The influence of different catalysts on the conversion of 4-CP.
Fig. 5 The TPR profiles of the catalysts.
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Fig. 5 shows the TPR proles of the catalysts aer the drying approximately 460 K was obviously increased; the tempera-
process. The TPR prole of the pure Pd catalysts showed only tures of the peaks also subsequently increased. The peak
one reduction peak at 350 K.11,12 By contrast, the TPR prole of reduction to 370–400 K for the SP-0.3/1, SP-0.5/1, and SP-1/1
the pure Sn catalysts showed two reduction peaks at 442 and samples may be attributed to the reduction of PdO and
475 K, which may correspond to the reduction of Sn4+ to Sn2+ SnOx, which triggered contact and cross-distribution between
and of Sn2+ to Sn0, respectively. The TPR prole of SP-0.1/1 was the catalysts. Following additional increases in the amount of
similar to that of the pure Pd catalyst, indicating that adding Sn (SP-3/1 and SP-5/1 samples), the reduction peak of PdO
a small amount of Sn did not alter the microstructure of the Pd disappeared; the temperature of the reduction peaks gradu-
particles. It is suggested that the reduction mechanism of the ally became closer to the temperature of the peak of SnO2
Pd–Sn bimetallic catalysts may involve the reduction of PdO peak; and the number of reduction peaks decreased until they
initially, followed by the reduction of SnO2.
became a single peak (SP-5/1), which indicates that a pseu-
Here, the peak intensity of PdO was decreased when Sn doalloy was formed in the catalyst.
content was increased, whereas the peak intensity of SnO2 at
Fig. 7 The influence of the catalysts on the hydrogenation products.
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Through a comprehensive examination of the experimental hydrogenolysis with pure Pd and Pd–Sn bimetallic catalysts to
results, it can be concluded that when there is less Sn, the metal form phenol; 4-Cp was subsequently further hydrogenated
Pd and Sn particles are separately distributed on the surfaces of to form cyclohexanone. The selectivity of the products is shown
the catalysts; by contrast, when there is more Sn, some degree of in Fig. 7. In order of the selectivity of PhOH, the catalysts were
overlap between Pd and Sn particle dispersion occurs. This ranked as follows: SP-5/1 > SP-3/1 > SP-1/1 > SP-0.5/1 > pure Pd >
overlap leads to Pd–Sn aggregates, and thus the formation of SP-0.3/1 > SP-0.1/1; notably, they were ranked in reverse
a pseudoalloy.
regarding the selectivity of cyclohexanone. The tendency of the
Fig. 6 depicts the performance of the catalysts in the HDC three bimetallic catalysts to select the phenol and cyclohexa-
reaction. In order of the amount of HDC, the catalysts were none is consistent.
ranked as follows: SP-0.1/1 > SP-0.1/1 > SP-0.5/1 > pure Pd > SP-1/
With the catalytic reaction time was prolonged, the selec-
1 > SP-3/1 > SP-5/1. When the Sn/Pd molar ratios were 0.1/1 or tivity of phenol gradually decreased (Fig. 7a), whereas the
0.3/1, 4-CP was completely hydrodechlorinated within 30 min; selectivity of cyclohexanone gradually increased (Fig. 7b) for all
when the Sn/Pd molar ratios were 0.5/1, 1/1, 3/1, or 5/1, 4-CP was of the catalysts. In other words, the pure Pd monometallic
completely hydrodechlorinated within 75 min. The results show catalyst and the Pd–Sn bimetallic catalysts engaged in different
that Pd–Sn bimetallic catalysts are more active than Pd–Cu catalytic activities during the hydrogenolysis of the C–Cl bond
bimetallic catalysts and Pd–In bimetallic catalysts. The of 4-CP and C]C double bond of phenol. Moreover, as the
highest activity of Pd–Cu bimetallic catalyst completed the amount of Sn in the Pd–Sn bimetallic catalysts increases, the
hydrochlorination of 4-CP process in 60 minutes.11 Not all ratios selectivity of cyclohexanone decreases and the selectivity of
Pd–In bimetallic catalysts have completed hydrodechlorination phenol increases. It was thus concluded that Sn was dispersed
of 4-CP process with in 120 minutes.12 These results demon- around the Pd particles adequately, effectively separating the Pd
strate that adding specic amounts of Sn to the pure Pd catalyst particles and preventing them from clustering when the Sn/Pd
increased catalytic activity. Moreover, according to the results molar ratio was low. When the Sn/Pd molar ratios were high,
presented in Table 1, the catalytic activity was correlated with Sn covered the surface of the pure Pd catalyst and formed a Pd–
the dispersion of Pd across the surface of the catalysts. In order Sn pseudoalloy; this reduced the surface area of Pd. According
of the amount of dispersion and surface area covered, the to these results, Pd is the sole active centre for both reactions.
catalysts were ranked as SP-0.3/1 > SP-0.1/1 > SP-0.5/1 > pure Pd
Fig. 8 illustrates the new catalysts can be reused up to six
> SP-1/1 > SP-3/1 > SP-5/1, which is consistent with the ordering times and the results show that the conversion of 4-CP is almost
of catalyst activity. Among all of the catalysts, SP-0.3/1 had the constant.
highest Pd dispersion and most surface area covered; therefore,
SP-0.3/1 had the highest catalytic activity in the HDC of 4-CP.
Conclusion
According to the results of a gas chromatography–mass
spectrometry analysis, cyclohexanone is another product (in
addition to phenol) that emerges during the HDC of 4-CP in an
aqueous phase. This indicates that the HDC of 4-CP was
a tandem reaction, in which 4-CP was initially subjected to
A series of Pd–Sn/AC bimetallic catalysts was prepared, and
their catalytic performance regarding the HDC of 4-CP in an
aqueous phase was examined. Compared with the pure Pd/AC
monometallic catalyst, Pd catalysts with various amounts of
Fig. 8 Influence of the repeated use of catalysts on the conversion of 4-CP.
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Sn exhibited increased Pd dispersion, larger Pd surface areas, 11 D. Fang, W. Li, J. Zhao, et al., RSC Adv., 2014, 4(103), 59204–
and more catalytic activity in the HDC of 4-CP. However, Pd–Sn 59210.
bimetallic catalysts with an excessive amount of Sn inhibited 12 J. Zhao, W. Li and D. Fang, RSC Adv., 2015, 5(53), 42861–
the HDC of 4-CP. In addition, Pd–Sn bimetallic catalysts with 42868.
higher amounts of Sn were associated with less conversion of 13 X. X. Ma, Y. Liu, S. J. Liu and C. H. Xia, Appl. Catal., B, 2014,
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´
´
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and J. J. Rodrıguez, Ind. Eng. Chem. Res., 2008, 47, 3840–
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15 Y. Ren, G. Fan and C. Wang, J. Hazard. Mater., 2014, 274, 32–
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Conflicts of interest
16 H. Chen, Z. Y. Xu, H. Q. Wan, J. Z. Zheng, D. Q. Yin and
S. R. Zheng, Appl. Catal., B, 2010, 96, 307–313.
17 Z. M. de Pedro, E. Diaz, A. F. Mohedano, J. A. Casas and
J. J. Rodriguez, Appl. Catal., B, 2011, 103, 128–135.
18 Z. Jin, X. Wang, S. Wang, D. Li and G. Lu, Catal. Commun.,
2009, 10, 2027–2030.
19 J. Xu, T. White, P. Li, C. H. He, J. G. Yu, W. K. Yuan and
Y. F. Han, J. Am. Chem. Soc., 2010, 132, 10398–10406.
20 V. Nagpal, A. D. Bokare, R. C. Chikate, C. V. Rode and
K. M. Paknikar, J. Hazard. Mater., 2010, 175, 680.
21 B. Yang, S. B. Deng, G. Yu, H. Zhang, J. H. Wu and Q. F. Zhuo,
J. Hazard. Mater., 2011, 189, 76.
There are no conicts to declare.
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