Effect of indium-modified palladium catalysts on the hydrodechlorination of 4-chlorophenol

Article abstract of DOI:10.1039/c5ra05957g

The influence of In addition on the activity and selectivity of supported Pd catalysts in the hydrodechlorination (HDC) of 4-chlorophenol (CP) was studied. The catalysts were characterized using temperature-programmed reduction, scanning electron microscopy, transmission electron microscopy, and hydrogen pulse chemisorption measurements. Bimetallic Pd-In/Al₂O₃ catalysts containing 1 wt% of Pd and with various In-Pd molar ratios were characterized by a high 4-CP conversion, which reached approximately 100% in a 2 h run. The results demonstrated that adding In to the Pd catalyst improved the dispersion of Pd particles and increased the HDC catalytic activity when the In-Pd molar ratio was less than 1/1.

Full text of DOI:10.1039/c5ra05957g

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Effect of indium-modified palladium catalysts on
the hydrodechlorination of 4-chlorophenol†
Cite this: RSC Adv., 2015, 5, 42861
*
Jinbo Zhao, Wanjun Li and Deren Fang
Received 3rd April 2015
Accepted 7th May 2015
DOI: 10.1039/c5ra05957g
www.rsc.org/advances
The influence of In addition on the activity and selectivity of supported showed a higher activity and superior selectivity than did pure
Pd catalysts in the hydrodechlorination (HDC) of 4-chlorophenol (CP) Pd catalysts, such as Pd–Bi4 and other bimetallic catalysts
was studied. The catalysts were characterized using temperature- studies such as Pd–Fe,16 and Pd–Al,17 in recent literature.
programmed reduction, scanning electron microscopy, transmission Therefore, search for new and full understanding of currently
electron microscopy, and hydrogen pulse chemisorption measure- functioning bimetallic catalysts constitute important chal-
ments. Bimetallic Pd–In/Al2O3 catalysts containing 1 wt% of Pd and lenges for researches. The Pd–In bimetallic catalysts sup-
with various In–Pd molar ratios were characterized by a high 4-CP ported on Al2O3 showed a high activity in liquid-phase
conversion, which reached approximately 100% in a 2 h run. The denitration;18,19 however, the HDC of CPs by using this cata-
results demonstrated that adding In to the Pd catalyst improved the lyst was not analyzed. We prepared a series of Pd–In bimetallic
dispersion of Pd particles and increased the HDC catalytic activity catalyst supported on Al2O3 and found that it has a high
when the In–Pd molar ratio was less than 1/1.
activity in the HDC of CPs. In this paper, we present the results
of the HDC of 4-CP on the In–Pd/Al2O3 bimetallic catalyst. The
results indicated that adding In can improve the catalytic
activity of the Pd/Al2O3 catalyst for the HDC of CPs, and the
action principle of In is discussed.
1. Introduction
Chlorophenols (CPs) have been widely used in various prod-
ucts such as disinfectants, pesticides, and personal care
formulations and can be detected in some aquatic environ-
ments.1 In recent years, numerous studies on the catalytic
hydrodechlorination (HDC) of CPs in liquid and gas phases
have been published. HDC of CPs in aqueous phase has been
studied with different metallic supported catalysts. These
studies showed that catalysts for the HDC of CPs are primarily
supported precious metals, such as Pd,2–10 Pt,11 and Rh.12
Among these metals, Pd is the most active and selective cata-
lyst.13 A variety of Pd catalysts deposited on different supports
such as active carbon (AC) and Al2O3 have been synthesized.14
It is reported that the catalytic properties of the Pd catalysts
can be improved by adjusting the particle size of the Pd
metallic phase.14 The catalysts with smaller Pd particle size
hold high surface to volume ratio and are resistant to deacti-
vation from the depositing Cl through the scission of the C–Cl
bonds.15 Some supported Pd-based bimetallic catalysts
2. Experimental section
2.1 Catalyst preparation
The Pd–In bimetallic catalysts were prepared using the
sequentially incipient wetness impregnation method, and g-
Al2O3 with a diameter of 0.045–0.075 mm was used as a
carrier. The Pd content was xed at 1 wt%, and the In content
was varied to obtain In–Pd molar ratios of 0.05/1, 0.1/1, 0.3/1,
0.5/1, 1/1, 2/1, and 3/1, and the corresponding catalysts were
designated as IP-1, IP-2, IP-3, IP-4, IP-5, IP-6, and IP-7,
respectively. A typical catalyst preparation method was used
to impregnate the precursors of the catalysts with the desig-
nated amount of aqueous PdCl2 to yield the desired Pd
content and then dried at 110 ꢀC for 2 h and calcined at 350 ꢀC
for 2 h. Aer cooling, the catalysts were impregnated with
aqueous In(NO3)3, then dried at 110 ꢀC for 2 h, and calcined at
350 ꢀC for 2 h. We also prepared a pure supported In catalyst
containing 3 wt% of In and a pure supported Pd catalyst
containing 1 wt% of Pd by using the same method and
designated them as 3% In and 1% Pd, respectively. The
catalysts were reduced before HDC was performed using
Institute of Applied Catalysis of Yantai University, Yantai 264005, China. E-mail: fdr@
ytu.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
ꢀ
10.1039/c5ra05957g
gaseous H2 at 350 C for 2 h.
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Al2O3 surface. The particle morphology of Pd and In was
determined using transmission electron microscopy (TEM) on a
JEOL JEM2000EXII microscope.
2.2 Catalyst characterization
Powder XRD patterns of the solid products were studied using a
Shimadzu XRD-6100 X-ray diffractometer (Cu K radiation)
equipped with a computer system to perform automatic oper-
ation and data processing.
XPS was performed using a Thermo Fisher Scientic
ESCALAB 250 spectrometer equipped with a standard Mg
K-alpha source and a high performance Al monochromatic
source. XPS spectra were referenced to the C1s peak at
284.6 eV.
Temperature-programmed reduction (TPR) was performed
using a TP5000 multifunction absorber. The catalyst weight
was 50 mg, the particle size was 60–80 mesh, 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 5 C minꢁ1. H2
ꢀ
consumption was detected using
detector (TCD).
a thermal conduction
H2 pulse chemisorption measurements of the catalysts
were performed using a TP5000 multifunction absorber and
the TCD. The catalyst weight was 0.10 g. Before the H2 pulse
chemisorption measurements were performed, the catalysts
were activated at 623 K for 2 h by using a ow of pure H2 at
atmospheric pressure. The catalysts were then degassed at
623 K for 2 h under owing Ar and cooled to 308 K. H2
(5 vol%) was then injected into the Ar stream owing through
the sample bed. The injection loop was 0.5 ml. In the
subsequent metal dispersion calculations, H2 was assumed
to be chemisorbed only on the Pd ensembles. The Pd
surface areas and dispersions of the catalysts are listed in
Table 1.
Scanning electron microscopy (SEM) images were obtained
using a Hitachi S-4800 FE-SEM equipped with a computer
system for performing automatic operations and data process-
ing. The dried sample was carefully transferred to a carbon tape
on the SEM template, coated using a gold coater, and analyzed
using SEM at 10 kV. Energy-dispersive X-ray (EDX) experiments
were performed using an EX-350 EDX microanalyzer equipped
with a computer system for conducting automatic operations
and data processing. EDX mapping was performed to visualize
elemental and spatial information regarding In and Pd on the
Table 1 The dispersion and surface area of Pd of the catalysts
Molar ratios
of In–Pd
Pd dispersion
(%)
Pd surface area
Catalysts
(m2 gꢁ1
)
2.3 Studies on catalytic hydrodechlorination
The HDC of 4-CP (Sigma, purity > 99.6%) was performed
at atmospheric pressure in a mixing glass tank reactor
with electromagnetic stirring to ensure maximum contact
between the catalyst and the reactant.20 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 introduced into the reactor at a rate of 50 ml
1% Pd
IP-1
IP-2
IP-3
IP-4
IP-5
IP-6
IP-7
—
23.7
30.9
41.6
37.0
29.8
28.3
21.1
12.6
52.9
69.0
92.7
82.5
66.5
63.2
47.0
28.2
0.05/1
0.1/1
0.3/1
0.5/1
1.0/1
2.0/1
3.0/1
Fig. 1 XRD patterns of the reduced catalysts with different molar ratio of In to Pd.
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Fig. 2 TEM micrographs of reduced catalyst 1% Pd, IP-2 and IP-7 (1), (2) 1% Pd, (3), (4)IP-2 (In–Pd ¼ 0.1/1), (5), (6) IP-7 (In–Pd ¼ 3/1), (7) 1% Pd
diffraction pattern, (8) IP-2 diffraction pattern, (9) IP-7 diffraction pattern, (10) 1% Pd PSD, (11) IP-2 PSD, (12) IP-7 PSD.
minꢁ1, thus initiating the HDC. Aer 7.5 min, 1 ml of the glycol capillary column. We also evaluated the activity of
solution was extracted and analyzed using gas chromatog- these catalysts in phenol hydrogenation by using the same
raphy with a ame ionization detector and a polyethylene method.
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Fig. 1 shows the XRD patterns of the catalysts. Diffraction
peaks of the g-Al2O3 support at 2q equals to 31.9, 37.6, 39.5, 45.9
and 67.1 were expected in the XRD patterns of Pd/g-Al2O3 and
Pd–In/g-Al2O3 (CAS no. 1344-28-1), but Pd and In has not
detected. Because of the low content of Pd and In in the cata-
lysts, the reection peak of which were covered by the back-
ground of g-Al2O3, and was difficult to observe.
Fig. 2 shows the results of TEM, which demonstrated
that the particle sizes of Pd on the catalysts were in the range
of 2–5 nm or that specic point was 2.71 nm for 1%
Pd, 2.26 nm for IP-2 (In–Pd ¼ 0.1/1) and 4.22 nm for IP-7 (In–
Pd ¼ 3/1). The results demonstrated that a small amount of
In addition to the Pd/g-Al2O3 catalyst could improve the
dispersion of Pd on the surface of the catalyst. The SEM/EDS
images (ESI†) show that Pd and In coexist in the same
microzones of IP-2 and IP-7; this indicates that In and Pd
were supported on Al2O3 and formed a Pd–In pseudoalloy on
the catalysts.20
3. Results and discussion
By using H2 pulse chemisorption measurements, we calcu-
lated the metal Pd dispersion and surface area.20,21 The Pd
surface areas and dispersions of the catalysts are listed in
Table 1. Compared with the 1% Pd monometallic catalyst,
bimetallic catalysts with In–Pd molar ratios of 0.05/1, 0.1/1,
0.3/1, 0.5/1, and 1/1 (IP-1, IP-2, IP-3, IP-4, and IP-5, respec-
tively) exhibited incrementally higher Pd dispersions and
surface areas. However, bimetallic catalysts with In–Pd molar
ratios of 2/1 and 3/1 (IP-6 and IP-7) exhibited lower Pd
dispersions and surface areas, indicating that adding In
changed the Pd dispersions and surface areas of the bimetallic
catalysts. Adding an appropriate amount of In increased the
Pd dispersions and surface areas of the Pd-based bimetallic
catalysts compared with those of the Pd monometallic
catalysts.
Fig. 3 The TPR profiles of the catalysts.
Fig. 5 XPS spectra forIn3d of catalysts 3% In, IP-2 and IP-7.
Fig. 4 XPS spectra for Pd3d of catalysts 1% Pd, IP-2 and IP-7.
42864 | RSC Adv., 2015, 5, 42861–42868
Fig. 6 The influence of different catalysts on the conversion of 4-CP.
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Fig. 3 shows the H2-TPR proles of the catalysts. The TPR between the PdO particle. Because the reduction of PdO must be
proles of the 1% Pd monometallic catalysts showed only one precede over In2O3 particle. In the case of 1% Pd monometallic
reduction peak at 345 K, whereas the 3% In monometallic catalyst the reduction rate of PdO is fast due to the reduced
catalysts showed two reduction peaks at 555 K and 600 K, which metal Pd can dissociative adsorb hydrogen molecular and the
may correspond to the reduction of In3+ to In2+ and In2+ to In0, hydrogen atom on the surface of Pd may migrate to the neigh-
respectively. The TPR proles of the Pd–In bimetallic catalysts boring PdO through the overow effect so as to accelerate the
were similar to those of the 1% Pd monometallic catalyst but reduction of remainder PdO in the particle. In the case of
differed from those of the 3% In monometallic catalyst. The bimetallic Pd–In/Al2O3 catalysts, the PdO particle was separated
reduction mechanism of the Pd–In bimetallic catalysts may by In2O3 particle and reduced in accordance with the order of
involve the reduction of PdO initially, followed by the reduction rst PdO then In2O3, while the reduction of In2O3 is very diffi-
of In2O3 by the hydrogen atom dissociatively adsorbed on the cult than PdO, so the reduction peak of them was shied to the
surface of Pd through the overow effect and formed close higher temperature.
contact mixed particle of Pd and In in the case of lower In–Pd
The XPS spectra of Pd 3d (Fig. 4) demonstrated that in all of
molar ratio; a few Pd–In pseudoalloy particles formed aer the the three catalysts, Pd 3d had two main peaks at 335.1 eV and
reduction in the case of higher In–Pd molar ratio, because of the 340.3 eV, which correspond to the binding energy of Pd 3d5/2
interaction between PdO and In2O3.19 When the In–Pd molar and Pd 3d3/2 of Pd0, respectively.23 This demonstrated that the
ratio was bigger than 0.1/1 the reduction peaks was shied to addition of In to Pd monometallic catalyst could not changed
the higher temperature than pure Pd monometallic catalyst, the state of the metal Pd. The XPS spectra of In 3d (Fig. 5)
this may indicate that there are some obstacles to the reduction demonstrated that in all of the three catalysts, In 3d had two
of PdO particle owing to the existence of In2O3 particle in main peaks at 444.1 eV and 451.1 eV, which correspond to the
binding energy of In 3d5/2 and In 3d3/2 of In0, respectively.24,25
Combining the results of TEM, XPS and TPR, one may come to
the conclusion that in the reduction process the In2O3 has been
reduced to the metal atom In and formed close contact mixed
particle of Pd and In in the case of lower In–Pd molar ratio; a few
Pd–In pseudoalloy particles formed aer the reduction in the
case of higher In–Pd molar ratio otherwise it cannot interpret
why the Pd particle size of sample IP-7 is bigger than the sample
1% Pd. From the H2-TPR proles it is demonstrated the indium
oxide was really reduced with PdO at the same time. Certainly,
perfect alloy or core shell structure was not formed in our
catalysts unlike at work of Deepak et al.26–28 in which the sample
was made by micelle method which can provide a large number
of similar composition of particles as well as they can guarantee
the uniform distribution of two kinds of elements in the
particle. We will further carry out research in the future in this
respect.
The HDC catalysis performance of the catalysts is shown in
Fig. 6. A great difference obviously existed in the catalytic
activity between the various catalysts. The HDC catalytic activity
of the catalysts followed the order IP-2 > IP-3 > IP-1 > IP-4 > IP-5 >
1% Pd > IP-6 > IP-7 > 3% In. When the In–Pd molar ratios were
0.05/1, 0.1/1, 0.3/1, 0.5/1, and 1/1, 4-CP was completely hydro-
dechlorinated within 75 min; however, when the In/Pd molar
ratios were 2/1 and 3/1, 4-CP was not completely hydro-
dechlorinated even within 120 min. This indicated that adding
the appropriate amount of In to modify the Pd monometallic
catalyst can improve the catalytic activity. According to the
Fig. 7 The influence of the catalysts on the hydrogenation products.
Scheme 1 The equation of the HDC of 4-CP.
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results shown in Table 1, the catalytic activity was correlated selectivity of cyclohexanone gradually increased for all the
with the Pd dispersions and surface areas of the catalysts. The catalysts (Fig. 7b). According to these results, the catalytic
Pd dispersions and surface areas of the catalysts followed the activities of the catalysts followed the order IP-2 > IP-3 > IP-1 >
order IP-2 > IP-3 > IP-1 > IP-4 > IP-5 > 1% Pd > IP-6 > IP-7, which IP-4 > IP-5 > 1% Pd > IP-6 > IP-7 for the hydrogenolysis of
is consistent with the catalytic activity order of the catalysts. the Cl–C bond; whereas the catalytic activities for the hydro-
Among all the catalysts, IP-2 had the highest Pd dispersion and genation of phenol followed the order IP-1 > IP-2 > 1% Pd > IP-
surface area; therefore, IP-2 showed the highest catalytic activity 5 > IP-6 > IP-7 in an aqueous solution (Fig. 8). This demon-
in the HDC of 4-CP. No conversion of 4-CP was observed using strated that the supported In catalyst has no catalytic activity
the 3% In monometallic catalyst. Therefore, the active center for for the hydrogenation of phenol. Therefore the catalytic active
the HDC of 4-CP was Pd.
center for both the hydrogenolysis of the Cl–C bond and the
According to the gas chromatography–mass spectrometry hydrogenation of the C]C double bond of phenol was Pd.
analysis, cyclohexanone was another product, in addition to Fig. 7b and 8 showed that the tendencies of the hydrogenation
phenol, in the HDC of 4-CP in an aqueous phase. This indicated of phenol and the selectivity of cyclohexanone were similar.
that the HDC of 4-CP was a tandem reaction in which 4-CP was This shows that the Pd monometallic catalyst and Pd–In
initially subjected to hydrogenolysis over the 1% Pd and Pd–In bimetallic catalysts differed in their catalytic activities in the
bimetallic catalysts to form phenol, and the phenol was further hydrogenolysis of the C–Cl bond of 4-CP and in the hydro-
hydrogenated to form cyclohexanone; the reaction pathway is genation of the C]C double bond of phenol. Compared with
shown in Scheme 1.20,22
that of the 1% Pd monometallic catalyst, the cyclohexane
The selectivity of the products is shown in Fig. 8. The selectivity of the Pd–In bimetallic catalysts gradually
selectivity of PhOH followed the order IP-7 > IP-6 > IP-5 > IP-4 decreased with an increase in the In content of the catalyst.
> IP-3 > 1% Pd > IP2 > IP-1, whereas the selectivity of cyclo- These results illustrated that catalysts having a high In
hexanone followed the reverse order. Variation tendencies of content (In–Pd > 0.1) can inhibit cyclohexanone formation
the selectivity of phenol gradually decreased as the reaction during the HDC of 4-CP. The TPR results showed that the Pd–
time increased for all catalysts (Fig. 7a). However, the In bimetallic catalysts had only one reduction peak, indi-
cating that Pd–In pseudoalloy formed in the catalysts in the
case of higher In–Pd molar ratio. From Fig. 4, we can conclude
that the Pd–In pseudoalloy formation in the Pd–In bimetallic
catalysts did not favor the hydrogenation of phenol to form
cyclohexanone. The Pd–In pseudoalloy formation reduced the
quantity of Pd active centers, thereby affecting the hydroge-
nation of the benzene ring of phenol. The hydrogenolysis of
the C–Cl bond of 4-CP may require fewer Pd active centers
compared with the hydrogenation of the benzene ring of
phenol.20 By combining the results of TPR and H2 pulse
chemisorption (Table 1), we concluded that metal In
dispersed around Pd particles adequately, effectively sepa-
rating the Pd particles and preventing further clustering of
the Pd particles when the In–Pd molar ratio was low. When
the In–Pd molar ratios were higher, the metal In covered the
surface of Pd and formed some amounts of the Pd–In pseu-
doalloy (Scheme 2); this reduced the surface area of Pd.
According to these results, Pd is the sole active center for both
reactions, reducing activity.
Fig. 8 The influence of different catalysts on the conversion of PhOH.
Scheme 2 Schematic diagram of atomic arrangement.
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The ability to reuse the new catalysts has been made for eight
times and the results (Fig. 9 and 10) showed that the catalyst can
be reused for many times.
4. Conclusion
A series of Pd–In/g-Al2O3 bimetallic catalysts was prepared, and
their catalytic performance in the HDC of 4-CP in an aqueous
phase was studied. Compared with the Pd monometallic cata-
lyst, Pd catalysts with the appropriate amounts of In added
exhibited increased Pd dispersions and surface areas and
considerably increased catalytic activity in the HDC of 4-CP.
However, adding an excessive amount of In to the Pd–In
bimetallic catalysts inhibited the HDC of 4-CP. In addition, with
an increase in the In content of the Pd–In bimetallic catalysts,
the conversion of phenol through hydrogenation was gradually
reduced; that is, the close contact mixed particle of Pd and In in
the Pd–In bimetallic catalysts inhibited the hydrogenation of
phenol. The stability of catalytic activity of Pd–In/g-Al2O3
bimetallic catalysts in the HDC of 4-CP is higher and can be
reused many times.
Fig. 9 The influence of reuse times of catalysts on the conversion of
4-CP.
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