Effective liquid phase hydrodechlorination of diclofenac catalysed by Pd/CeO2

Article abstract of DOI:10.1039/c4ra16674d

Palladium catalysts supported on Al₂O₃, activated carbon (AC), SiO₂ and CeO₂ were prepared using the impregnation and deposition-precipitation methods. The liquid phase catalytic hydrodechlorination of diclofenac on the catalysts was investigated, and the toxicity of the original and treated diclofenac solutions was evaluated using Daphnia magna. Characterization results indicated that the Pd catalyst supported on CeO₂ had a higher Pd dispersion than those supported on Al₂O₃, AC and SiO₂. The binding energy of Pd 3d_5/2 in Pd/CeO₂ was higher than Pd/Al₂O₃ with a similar Pd loading amount. Additionally, for Pd/CeO₂ prepared by the deposition-precipitation method the binding energy of Pd 3d_5/2 slightly decreased with the Pd loading amount. As for catalytic diclofenac reduction, Pd/SiO₂ exhibited a nearly negligible catalytic activity, whereas diclofenac concentration decreased by 100, 86, and 29% within 50 min of reaction on Pd/CeO₂, Pd/Al₂O₃, and Pd/AC, respectively, indicative of a catalytic activity order of Pd/CeO₂ > Pd/Al₂O₃ > Pd/AC > Pd/SiO₂. The hydrodechlorination of diclofenac on Pd/CeO₂ could be well described using the Langmuir-Hinshelwood model. Diclofenac hydrodechlorination processed via a combined stepwise and concerted pathway, and increasing Pd loading amount in Pd/CeO₂ favoured the concerted pathway. In comparison with original diclofenac, catalytic hydrodechlorination of diclofenac led to markedly decreased toxicity to Daphnia magna.

Full text of DOI:10.1039/c4ra16674d

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Effective liquid phase hydrodechlorination of
diclofenac catalysed by Pd/CeO2†
Cite this: RSC Adv., 2015, 5, 18702
*
*
Ke Wu, Xiaojun Qian, Liangyan Chen, Zhaoyi Xu, Shourong Zheng
and Dongqiang Zhu
Palladium catalysts supported on Al2O3, activated carbon (AC), SiO2 and CeO2 were prepared using the
impregnation and deposition–precipitation methods. The liquid phase catalytic hydrodechlorination of
diclofenac on the catalysts was investigated, and the toxicity of the original and treated diclofenac
solutions was evaluated using Daphnia magna. Characterization results indicated that the Pd catalyst
supported on CeO2 had a higher Pd dispersion than those supported on Al2O3, AC and SiO2. The binding
energy of Pd 3d5/2 in Pd/CeO2 was higher than Pd/Al2O3 with a similar Pd loading amount. Additionally,
for Pd/CeO2 prepared by the deposition–precipitation method the binding energy of Pd 3d5/2 slightly
decreased with the Pd loading amount. As for catalytic diclofenac reduction, Pd/SiO2 exhibited a nearly
negligible catalytic activity, whereas diclofenac concentration decreased by 100, 86, and 29% within 50
min of reaction on Pd/CeO2, Pd/Al2O3, and Pd/AC, respectively, indicative of a catalytic activity order of
Pd/CeO2 > Pd/Al2O3 > Pd/AC > Pd/SiO2. The hydrodechlorination of diclofenac on Pd/CeO2 could be
well described using the Langmuir–Hinshelwood model. Diclofenac hydrodechlorination processed via a
combined stepwise and concerted pathway, and increasing Pd loading amount in Pd/CeO2 favoured the
concerted pathway. In comparison with original diclofenac, catalytic hydrodechlorination of diclofenac
led to markedly decreased toxicity to Daphnia magna.
Received 19th December 2014
Accepted 6th February 2015
DOI: 10.1039/c4ra16674d
www.rsc.org/advances
Different treatment methods have been tested for the
removal of diclofenac in water. For example, carbonaceous
1. Introduction
sorbents were applied for adsorptive removal of aqueous
diclofenac.7,8 To achieve complete degradation, advanced
oxidation processes (AOPs) were adopted.9–13 Notably, liquid
phase catalytic hydrodechlorination (HDC) is an inexpensive
and effective method for the reductive removal of chlorinated
organic pollutants. For example, a diversity of chlorinated
pollutants could be completely dechlorinated by HDC, in which
supported Pd catalysts were usually used due to their high
catalytic activities.14–17 Recently, De Corte et al.18,19 studied the
catalytic hydrogenation of diclofenac on biogenic Pd and Pd/Au
catalysts, and found that Pd/Au exhibited a higher catalytic
activity than that of Pd. It should be pointed out that dispersion
of nanosized metal on appropriate supports could effectively
stabilize metal particle and avoid particle aggregation, resulting
in enhanced catalytic activity. To our best knowledge, the liquid
phase catalytic hydrodechlorination of diclofenac on supported
noble metal has not been systematically studied thus far.
Additionally, the catalytic HDC is considered to be an effective
detoxication method, while the potential toxicity of the dech-
lorinated product from diclofenac remains unknown.
The occurrence of organic micropollutants like active pharma-
ceutical ingredients (APIs) in the aquatic environment has been
receiving increasing attention in recent years.1 Due to insuffi-
cient biodegradation in sewage systems, APIs may eventually
enter the aquatic environment. So far, the concentration of
these pharmaceuticals in environmental matrices keeps rising
from ng lꢀ1 to mg lꢀ1 levels.2,3 Among the relevant APIs, diclo-
fenac is widely used as an analgesic and anti-inammatory drug
mainly in oral and dermal applications.1 Accordingly, it was
detected in various water samples including wastewater treat-
ment plant (WWTP) effluents and surface water. Toxicity tests
indicated that diclofenac was harmful to the liver, kidney and
gills of rainbow trout.4 Furthermore, diclofenac was found to be
capable of inducing harmful effects similar to other APIs such
as ibuprofen, naproxen, and acetylsalicylic acid.5,6 Hence, it is
highly desirable to develop effective methods to eliminate
diclofenac pollution prior to its discharge into the aquatic
environments.
State Key Laboratory of Pollution Control and Resource Reuse, Jiangsu Key Laboratory
of Vehicle Emissions Control, School of the Environment, Nanjing University, Nanjing
210093, China. E-mail: chenly@nju.edu.cn; srzheng@nju.edu.cn; Fax: +86-25-
89680367; +86-25-89680596; Tel: +86-25-89680367; +86-25-89680373
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra16674d
In the present study, Pd catalysts supported on Al2O3, acti-
vated carbon (AC), SiO2 and CeO2 were prepared by the
impregnation and deposition–precipitation methods. The
activities of the catalysts for the HDC of diclofenac were
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compared, and the residual toxicity was evaluated using on a PHI5000 VersaProbe equipped with a monochromatized Al
Daphnia magna. The results showed that complete dechlorina- Ka excitation source (hv ¼ 1486.6 eV) (ULVAC-PHI, Japan) to
tion and substantially decreased toxicity of diclofenac could be determine the valence state of the elements. The C 1s peak
achieved upon catalytic HDC on Pd/CeO2.
(284.6 eV) was used for the calibration of binding energy.
The Pd particle size and Pd dispersion of the catalyst was
determined using the CO chemisorption method. Briey, about
100 mg of catalyst was charged into a U-shaped quartz tube,
which was activated in a H2 stream (40 ml minꢀ1)ꢁat 300 ꢁC for 1
h. Aer purging by Ar gas (30 ml minꢀ1) at 300 C for 1 h, the
catalyst was cooled down to room temperature. The CO chem-
isorption was then carried out using the pulse titration model.
The CO contents in the pulses were determined using a thermal
conductivity detector (TCD). The Pd dispersions and Pd particle
sizes of the catalysts were calculated assuming average chemi-
sorption stoichiometry of CO/Pd surface ¼ 1.21,22
2. Experimental
2.1. Catalyst preparation
The catalyst support CeO2 was prepared by the precipitation
method. Briey, 0.5 M NH3$H2O was added slowly to 250 ml of
0.5 M Ce(NO3)3 solution till pH 10. The mixture was further
stirred at room temperature for 1 h. Aer repeated washing with
distilled water, and drying at 105 ꢁC for 6 h, the resultant CeO2
colloid was then calcined at 500 ꢁC for 4 h. Other supports, such
as Al2O3, SiO2 and active carbon (AC), were purchased from
Shanghai Chem. Co. (Shanghai, China). Prior to catalyst prep-
aration, Al2O3 and SiO2 were also calcined at 500 ꢁC, and AC was
dried at 105 ꢁC for 4 h. All treated supports were ground to pass
through a 400-mesh sieve (<37 mm) to avoid potential mass
transfer limitations.20
Surface zeta potentials of the catalysts were measured using
a Zeta Potential Analyzer (Zeta PALS, Brookhaven Instruments
Co.). Typically, 10 mg of the samples were dispersed in 100 ml of
deionized water and the suspension pH was adjusted to 9.0 with
0.1 M NaOH.
To test the inuence of support on catalyst activity, sup-
ported Pd catalysts with a Pd loading amount of about 2.0 wt%
but different supports (e.g. CeO2, Al2O3, SiO2, and AC) were
prepared by the deposition–precipitation method. For
comparison purposes, Pd catalyst supported on CeO2 was
prepared by the impregnation method. The Pd catalysts sup-
ported on CeO2 with varied Pd loading amounts were prepared
by the deposition–precipitation method. For the deposition–
precipitation method, calcined CeO2 support was suspended in
25 ml of distilled water, to which a desired amount of PdCl2
solution was added under stirring. The pH was adjusted to 10.5
using 1.0 M Na2CO3 solution. The resulting mixture was further
stirred for 1 h, followed by washing with distilled water, drying
at 105 ꢁC overnight, calcining at 300 ꢁC for 4 h, and reducing by
2.3. Liquid phase catalytic HDC of diclofenac
The liquid phase catalytic HDC of diclofenac was conducted at
25 ꢂ 0.5 ꢁC under atmospheric pressure of hydrogen in a 250 ml
of four-necked ask reactor with a sample port, pH-stat, H2 inlet
and outlet. Due to the low solubility of diclofenac in water at
acidic and neutral pH, the pH of diclofenac solution was
adjusted to pH 9.0 using 0.1 M NaOH solution prior to the HDC
reaction. Briey, 20 mg of catalyst was added into 200 ml of
diclofenac solution with initial concentration varied from 0.06
to 0.24 mM. The suspension was rst purging with a N2 ow (50
ml minꢀ1) under stirring (1400 rpm) for 30 min. The HDC
process started by switching the N2 ow to a H2 ow (250 ml
minꢀ1), and samples were taken at selected time intervals. The
catalyst particles in the sample were removed by fast ltration,
and the concentrations of diclofenac and potential products in
the ltrate were analyzed by a high performance liquid chro-
matography (HPLC). The mobile phase consisted of solution A
(0.01% formic acid) and B (methanol). The HPLC analysis was
operated at a total ow rate of 300 ml minꢀ1 using a mobile
phase consisting of 50% A and 50% B for 2 min, followed by
changing to 5% A and 95% B within 31 min, 50% A and 50% B
within 2 min, and holding for 5 min to re-equilibrate the
ꢁ
H2 at 300 C for 2 h. The resultant catalyst is referred to as dp-
Pd(X)/CeO2, where X is the Pd loading amount (wt%). As for the
impregnation method, CeO2 support was impregnated by a
PdCl2 solution under stirring, followed by drying at 105 ꢁC
overnight, calcining at 300 ꢁC for 4 h and reducing by H2 at 300
ꢁC for 2 h. The catalyst is denoted as im-Pd(X)/CeO2, where X is
the Pd loading amount (wt%).
2.2. Catalyst characterization
The Pd contents of the catalysts were determined by inductive column. The retention times were 27.7, 31.8 and 32.9 min for
coupled plasma emission spectrometer (ICP) (J-A1100, Jarrell- diclofenac, 2-(2-chloroanilino) phenylacetate (2-(2-Cl)-APA) and
Ash, USA). The powder X-ray diffraction (XRD) patterns were 2-anilinophenylacetic acid (2-APA), respectively. Standard
collected in a range of 20–80ꢁ with a Rigaku D/max-RA powder curves with six concentration levels ranging from 0.01 to 0.20
diffraction meter using Cu Ka radiation (Rigaku, Tokyo, Japan). mM were used for quantication of diclofenac and 2-APA.
Transmission electron microscopy (TEM) images of the samples Because commercial 2-(2-Cl)-APA was not available, its aqueous
were obtained on a JEM-2100 transmission electron microscope phase concentrations were estimated from its peak area of the
(JEM-2100, JEOL, Japan). The specic surface areas of the chromatograms based on the standard curves of 2-APA, by
catalysts were determined by N2 adsorption–desorption on a which reaction data with satisfactory mass balance (errors
Micromeritics ASAP 2020 instrument (Micromeritics Instru- within 5%) were obtained. The catalyst activity was evaluated
ꢁ
mentCo., Norcross, GA, USA) at ꢀ196 C, and were calculated using the initial activity dened as the specic removal rate of
according to the Brunauer–Emmett–Teller (BET) method. The diclofenac within the initial 6 min. Preliminary experiments
X-ray photoelectron spectroscopy (XPS) analysis was performed indicated that under the experimental conditions mass transfer
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limitations were absent, and the experimental repeatability of found to be 6.2, 9.6 and 9.2 nm, respectively, markedly larger
the initial activities was within 5% (results presented in Fig. 1S than that of dp-Pd(1.7)/CeO2. Notably, at an identical Pd loading
and 2S, ESI†).
amount dp-Pd(1.7)/CeO2 had a Pd dispersion of 86.0%, much
higher than that of im-Pd(1.8)/CeO2 (21.6%). It was previously
observed that the catalyst prepared by the deposition–precipi-
tation method had smaller metal particle size and higher
dispersion than that prepared by the impregnation method.25,26
For the dp-Pd/CeO2 catalysts, increasing Pd loading from 0.45 to
2.6 wt% led to a decrease of Pd dispersion from 89.4% to 65.8%,
reecting the gradual aggregation of Pd particles. Accordingly,
the Pd particle sizes of dp-Pd(1.7)/CeO2 and im-Pd(1.8)/CeO2
were calculated to be 1.31 and 5.20 nm, respectively. Addition-
ally, a monotonic increase of Pd particle size from 1.16 to 1.71
nm in the dp-Pd/CeO2 catalyst was identied when the Pd
loading amount increased from 0.45 to 2.6 wt%.
The compositions and oxidation states of supercial Pd
metals in the catalysts were further characterized using XPS.
The XPS spectra of the catalysts are compiled in Fig. 1 and the
resultant parameters are listed in Table 2. For the dp-Pd/CeO2
catalysts, the concentration of surface Pd atom increased with
Pd loading amount. Given an identical Pd loading amount, Pd
atomic concentration of dp-Pd(1.7)/CeO2 was 5.2%, much
higher than that of Pd(1.9)/Al2O3, conrming a higher Pd
dispersion of the former. The binding energy of the Pd core line
3d5/2 of dp-Pd(1.9)/Al2O3 was centered at 334.6 eV, indicative of
the presence of metallic Pd.27 In contrast, a much higher
binding energy was observed at 337.4 eV on dp-Pd(1.7)/CeO2,
assigned to cationic palladium species. The presence of cationic
Pd species in the catalysts could be attributed to the electron
transfer from metallic Pd to CeO2 support due to a strong metal-
support interaction.28 Additionally, the binding energies of Pd
3d5/2 in the dp-Pd/CeO2 catalysts with varied Pd loading
amounts were higher than that in Pd(1.9)/Al2O3, further veri-
fying the existence of cationic Pd species. However, increasing
Pd loading amount from 0.45 to 2.6 wt% led to a decrease in the
binding energy of Pd 3d5/2 from 337.2 to 336.8 eV. The mono-
2.4. Toxicity evaluation of original and treated diclofenac
solution
The toxicity of original and treated diclofenac solution by HDC
on dp-Pd(1.7)/CeO2 for 2 h was evaluated by acute immobiliza-
tion test of Daphnia magna Straus. Young daphnids from
monoclonal bulk cultures, aged less than 24 h were collected
and maintained in dechlorinated tap water at 20 ꢂ 1 ꢁC, under
16 hour light and 8 hour dark cycle. The tests were conducted in
accordance with the OECD guideline 202.23 The diclofenac
solution with an initial concentration of 0.63 mM was hydro-
dechlorinated on dp-Pd(1.7)/CeO2 for 2 h. Then the original and
treated diclofenac solution was separately collected as the
initial solution, and diluted into 6 concentrations containing
various proportions of the initial solution, i.e., 5%, 10%, 17.5%,
25%, 37.5% and 50%, as test liquid. Following a static design, 5
daphnids were randomly displaced into one glass vessel con-
taining 50 ml of test solution with 3 replicates. In addition to
the treatment series, one dilution-water control series was also
run using dechlorinated tap water with pH 6.8. Aer a 48 h
exposure period, immobilised individuals were counted in each
vessel for calculating EC50 with 95% condence limits by
prohibit-analysis using SPSS soware (IBM Corporation/http://
www.spss.com). The tests were performed under the same
conditions as the culture maintenance, and the daphnids were
not fed during the whole test period. The control organisms
performed well in the acute immobilisation tests, and no
inhibited daphnids were observed in the control treatments.
3. Results and discussion
3.1. Catalyst characterization
The XRD patterns of Pd catalysts with different supports are tonically decreased binding energy with Pd loading amount
compared in Fig. 3S, ESI.† Besides diffraction peaks from mainly resulted from the attenuated metal-support interaction
supports, new diffraction peaks with 2q at 40.2, 46.7 and 67.8ꢁ due to Pd particle aggregation.29
were observed on Pd catalysts supported on AC, Al2O3, and SiO2,
characteristic of typical metallic Pd with face centered cubic
(fcc) crystallographic structure. In contrast, the Pd/CeO2 cata-
3.2. Liquid phase catalytic HDC of diclofenac
lysts with varied Pd loading amounts gave nearly identical XRD The catalytic HDC of diclofenac over Pd catalysts supported on
patterns with strong diffraction peaks at 28.5, 33.0, 47.3, 56.2 different supports is presented in Fig. 2. Aer reaction for 50
and 78ꢁ, assigned to a typical cubic CeO2 phase with a uorite min, diclofenac conversion was almost negligible on Pd/SiO2,
structure.24 The absence of diffraction peaks characteristic of whereas diclofenac was removed by 29% for Pd(1.8)/AC, 86% for
metallic Pd was likely due to very high Pd dispersions (small Pd Pd(1.9)/Al2O3 and nearly 100% for dp-Pd(1.7)/CeO2, indicative of
particle sizes) in the Pd/CeO2 catalysts.
an activity order of Pd(1.7)/CeO2 > Pd(1.9)/Al2O3 > Pd(1.8)/AC >
The transmission electron microscopy (TEM) images of Pd Pd(1.8)/SiO2. The support dependent catalytic activity was
catalysts supported on CeO2 are shown in Fig. 4S, ESI.† The previously observed in the liquid phase catalytic HDC of bro-
particle size of CeO2 support varied from 5 to 15 nm, whereas Pd mated and chloroacetic acids, which was correlated to the
particles in the catalysts were almost invisible, conrming the surface charge of the support.14,30 Accordingly, the zeta poten-
high Pd dispersions and small Pd particle sizes of the catalysts. tials of Pd catalysts supported on CeO2, Al2O3, AC and SiO2 at
The average Pd particle sizes and Pd dispersions of the catalysts pH 9.0 were determined to be ꢀ33.2, ꢀ30.6, ꢀ55.1, and ꢀ62.7
could be further evaluated using the CO chemisorptions mV, respectively. Because diclofenac presented in an anionic
method and the results are presented in Table 1. The sizes of Pd form under our experimental conditions, much weaker expul-
particles of Pd(1.9)/Al2O3, Pd(1.8)/SiO2 and Pd(1.8)/AC were sive interaction between reactant and catalyst could be expected
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Table 1 Structural properties of the Pd/CeO2 catalysts
Pd loading amount
(wt%)
Pd dispersion
(%)
Pd particle
size (nm)
BET surface area
Catalysts
(m2 gꢀ1
)
im-Pd(1.8)/CeO2
dp-Pd(0.45)/CeO2
dp-Pd(0.9)/CeO2
dp-Pd(1.7)/CeO2
dp-Pd(2.6)/CeO2
1.80
0.45
0.90
1.70
2.60
21.6
89.4
87.2
86.0
65.8
5.20
1.16
1.29
1.31
1.71
66
48
47
45
45
on dp-Pd(1.7)/CeO2 and Pd(1.9)/Al2O3 than on Pd(1.8)/AC and
Pd(1.8)/SiO2. Furthermore, cationic Pd species were identied
on dp-Pd(1.7)/CeO2, which may facilitate the direct adsorption
of anionic diclofenac onto the surface of Pd particles due to the
attractive interaction, and thus enhance the catalytic activity.
The structural property and catalytic activity of supported
catalyst may differ with the preparation methods, and impreg-
nation and deposition–precipitation methods are the widely
used ones. In this study, two Pd/CeO2 catalysts with nearly
identical Pd loadings were respectively prepared by the
impregnation method and deposition–precipitation method,
and the HDC of diclofenac on the catalysts is compared in
Fig. 3. Aer 30 min of reaction, diclofenac decreased by 98% for
dp-Pd(1.7)/CeO2 and 76% for im-Pd(1.8)/CeO2, indicative of a
markedly higher catalytic activity of the former. Such a higher
catalytic activity of dp-Pd(1.7)/CeO2 could be attributed to its
much higher Pd dispersion and smaller Pd particle, as indi-
cated by CO chemisorption (see Table 1).
Fig. 1 XPS spectra of the catalysts in the Pd 3d region.
Table 2 Parameters of XPS spectra of the catalysts
Surface Pd atomic
For a heterogeneous reaction, adsorption of the reactant
onto catalyst surface is a prerequisite step. To verify the inu-
ence of reactant adsorption, the catalytic HDC of diclofenac
with varied initial concentrations was conducted, and the
results are presented in Fig. 4a. Increasing diclofenac concen-
tration from 0.06 to 0.24 mM led to increased initial activity
from 3.9 to 11.6 mM g Catꢀ1 hꢀ1, implying an adsorption
controlled catalysis mechanism. The mechanism could be
Catalysts
Pd 3d5/2 (eV)
concentration (%)
Pd(1.9)/Al2O3
334.60
337.20
337.16
337.12
336.82
2.1
1.5
2.8
5.2
9.6
dp-Pd(0.45)/CeO2
dp-Pd(0.9)/CeO2
dp-Pd(1.7)/CeO2
dp-Pd(2.6)/CeO2
Fig. 2 Catalytic HDC of diclofenac over Pd catalysts supported on
SiO2, AC, Al2O3 and CeO2. Reaction conditions: pH 9.0. Catalyst
Fig. 3 Catalytic HDC of diclofenac over im-Pd(1.8)/CeO2 and dp-
dosage: 0.10 g lꢀ1
.
Pd(1.7)/CeO2. Reaction conditions: pH 9.0. Catalyst dosage: 0.10 g lꢀ1
.
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For the HDC of diclofenac, exposed metallic Pd atoms are
considered to be the catalytic active sites. Hence, the catalytic
activity of dp-Pd/CeO2 expectedly depends on Pd content. The
HDC of diclofenac on dp-Pd/CeO2 catalysts with varied Pd
loading amounts is presented in Fig. 5a. Increasing Pd loading
amount from 0.45 to 2.6 wt% resulted in an increase of initial
reaction rate from 3.5 to 10.5 mM g Catꢀ1 hꢀ1. Such an
enhanced catalytic activity could be ascribed to the increase of
exposed Pd sites with Pd loading, as reected by CO chemi-
sorption. A deep understanding of the relationship between the
activity of exposed Pd site and the structure of Pd particle could
be obtained by comparing the turnover frequency (TOF) values
of exposed Pd atoms in the catalysts. The TOF was dened as
the HDC rate of diclofenac per exposed Pd atom within the
initial 6 min, in which the numbers of exposed Pd sites in the
catalysts were determined using the CO chemisorption method.
The results are shown in Fig. 5b. Within the tested particle size
range, the TOF value decreased rst and then kept nearly
Fig. 4 (a) Catalytic hydrodechlorination of diclofenac with varied
initial concentrations, and (b) linear plot of 1/r0 versus 1/C0. Line
represents the fitting curve using the Langmuir–Hinshelwood model.
Reaction conditions: catalyst: dp-Pd(1.7)/CeO2. pH 9.0. Catalyst
dosage: 0.10 g lꢀ1
.
further testied by tting the experimental data to the Lang-
muir–Hinshelwood model:31
bC0
r0 ¼ kqs ¼ k
(1)
(2)
1 þ bC0
1
1
1
þ
¼
r0 kbC0
k
where r0 is the initial reaction rate, C0 is the initial reactant
concentration, qs is the surface coverage of diclofenac on cata-
lyst surface, k is the reaction rate constant, and b is the
adsorption constant.
The dependence of r0 on C0 plotted as 1/C0 versus 1/r0 is
shown in Fig. 4b. A linear plot of 1/C0 versus 1/r0 with R2 above
0.99 indicated that the catalytic HDC of diclofenac on dp-
Pd(1.7)/CeO2 could be well described using the Langmuir–
Hinshelwood model, conrming the adsorption controlled
catalysis mechanism.
Fig. 5 (a) The catalytic hydrodechlorination of diclofenac on dp-Pd/
CeO2 catalysts with varied Pd loading amounts, and (b) Influence of Pd
particle size on TOF. Reaction conditions: pH 9.0. Catalyst dosage:
0.10 g lꢀ1
.
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constant with Pd particle size. It was previously reported that in
the liquid phase catalytic hydrodechlorination the activation of
C–Cl bond and H2 was the crucial steps, in which cationic Pd
species favored the activation of C–Cl bond, and large Pd
particle enhanced H2 activation by forming b-PdH phase.32 For
the dp-Pd/CeO2 catalysts, increasing Pd loading amount resul-
ted in larger Pd particles and less cationized Pd species. Hence,
the declined activity of exposed Pd atom with Pd loading
amount indicated that in comparison with H2 activation the
activation of C–Cl bond likely played a more important role in
the HDC of diclofenac.
In the catalytic HDC of diclofenac, only partially and
completely dechlorinated products were identied. Notably,
further hydrogenation of dechlorinated products was previously
observed in the HDC of 2,4-dichlorophenol, which was closely
related to the used catalysts.33–35 To clarify if further hydroge-
nation of dechlorinated product could proceed, the catalytic
hydrogenation of 2-anilinophenylacetic acid was conducted.
The results showed that aer reaction for 2 h the concentration
of 2-anilinophenylacetic acid kept nearly constant, and other
products were not determined by HPLC (see Fig. 5S, ESI†).
Hence, the catalytic HDC of diclofenac to 2-anilinophenylacetic
acid may process via stepwise or/and concerted pathways,
which was described in Scheme 1.
Fig. 6 Catalytic HDC of diclofenac on dp-Pd(1.7)/CeO2. Lines repre-
sent the fitting curves. Reaction conditions: pH 9.0. Catalyst dosage:
0.10 g lꢀ1
.
reecting a combined stepwise and concerted pathway for HDC
of diclofenac. Given identical Pd loading amounts, the k1, k2
and k3 values of dp-Pd(1.7)/CeO2 were higher than those of
Pd(1.9)/Al2O3, conrming a higher catalytic HDC activity of the
former. Additionally, the k2/k1 ratio of dp-Pd(1.7)/CeO2 was 4.3,
much higher than that of Pd(1.9)/Al2O3 (k2/k1 ¼ 3.0). The higher
k2/k1 ratio of dp-Pd(1.7)/CeO2 was likely due to the presence of
cationized Pd species, favoring the simultaneous adsorption
and activation of C–Cl bonds in diclofenac. Furthermore,
increasing Pd loading amount from 0.45 to 2.6 wt% resulted in a
marked increase of k2/k1 ratio from 2.0 to 4.7. It should be
emphasized that for dp-Pd/CeO2 increasing Pd loading amount
led to Pd particle growth, as well as slightly weakened metal-
support interaction and attenuated cationization of Pd
particle. Hence, decreased k2/k1 ratio with Pd loading amount
could be expected. Notably, Pd particle sizes of the catalysts
varied from 1.16 to 1.71 nm, comparable with the molecular
dimension of diclofenac (1.06 nm ꢃ 1.09 nm ꢃ 0.85 nm, see
Fig. 6S, ESI†). Hence, the observed increase of k2/k1 ratio with Pd
loading amount is possibly because large Pd particles could
provide enough planar Pd space, facilitating simultaneous
access to supercial Pd sites by two C–Cl bonds from diclofenac,
and thus favoring the dechlorination via the concerted
pathway.37,38
The potential dechlorination mechanism could be veried
by tting the kinetic data using following equations:36
CDF ¼ C0DF exp(ꢀ(k1 + k2)t)
(3)
k1CD0 F
C2-ð2-ClÞ-APA
¼
ðexpð ꢀ ðk1 þ k2ÞtÞ ꢀ expðꢀk3tÞÞ (4)
k3 ꢀ k1 ꢀ k2
ꢀ
k1k3CD0 F
ꢀ1
C2-APA
¼
ðexpð ꢀ ðk1 þ k2ÞtÞ ꢀ 1Þ
k3 ꢀ k1 ꢀ k2 k1 þ k2
ꢁ
1
þ
ðexpðꢀk3tÞ ꢀ 1Þ
k3
k2CD0 F
ꢀ
ðexpð ꢀ ðk1 þ k2ÞtÞ ꢀ 1Þ
(5)
k1 þ k2
where C0DF is the initial diclofenac concentration, CDF, C2-(2-Cl)-APA
and C2-APA are the concentrations of diclofenac, 2-(2-chlor-
oanilino) phenylacetate (2-(2-Cl)-APA) and 2-anilinophenylacetic
acid (2-APA) at reaction time t, k1, k2 and k3 are the rate constants
of diclofenac to 2-(2-Cl)-APA, diclofenac to 2-APA, and 2-(2-Cl)-APA
to 2-APA, respectively.
The catalytic HDC of diclofenac on dp-Pd(1.7)/CeO2 is pre-
sented Fig. 6, and the tting parameters are summarized in
Table 3. It was clear that in the HDC of diclofenac the inter-
mediate product 2-(2-Cl)-APA was detected, conrming the
stepwise pathway. Additionally, the tting results gave
substantially high rate constants (k2) for the concerted pathway,
Table 3 Fitting rate constants of different catalysts for the catalytic
HDC of DF
Catalysts
k1
k2
k3
k2/k1
R2
Pd(1.9)/Al2O3
0.009
0.01
0.017
0.025
0.028
0.027
0.02
0.045
0.108
0.132
0.030
0.03
0.034
0.049
0.053
3.0
2.0
2.7
4.3
4.7
0.996
0.998
0.998
0.997
0.995
dp-Pd(0.45)/CeO2
dp-Pd(0.9)/CeO2
dp-Pd(1.7)/CeO2
dp-Pd(2.6)/CeO2
Scheme 1 Proposed dechlorination pathways of diclofenac.
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To test the catalyst stability, dp-Pd(1.7)/CeO2 was sub-
jected to 5 consecutive reaction cycles, and the results are
presented in Fig. 7. Aer 5 cycles, the catalytic activity of
dp-Pd(1.7)/CeO2 decreased by 75%, indicative a gradual
deactivation.
3.3. Toxicity evaluation of original and treated diclofenac
solution
The inhibition rate of daphnids as a function of test solution
concentration is presented in Fig. 8. The observed inhibition
rate of the daphnids increased with diclofenac concentration,
reecting that daphnids were suitable species for the acute
toxicity test of diclofenac. The 48 h-EC50 value of diclofenac on
test daphnids was calculated to be 21.5% (in % dilution, 95%
condence limits), corresponding to a diclofenac concentration
of 0.13 mM. For diclofenac solution treated by catalytic HDC on
dp-Pd(1.7)/CeO2, the inhibition rates of the tested daphnids also
increased with the proportions of the treated solution. However,
the inhibition rates of the treated solutions with dilution ratios
of 25% and 37.5% were found to be about 20% and 60%,
respectively, indicative of markedly lower toxicity of the treated
solution as compared with the original diclofenac solution at
the same dilution ratios (independent t-test, p < 0.05). Accord-
ingly, the 48 h-EC50 value of the treated solution on Daphnia
magna was 35.1% (in % dilution, 95% condence limits), much
higher than that of original diclofenac solution.
The composition of the treated diclofenac solution was
further analyzed by high performance liquid chromatograph.
Upon HDC on dp-Pd(1.7)/CeO2 for 2 h, the residual diclofenac
concentration was below the detection limitation, and the
concentrations of 2-(2-Cl)-APA and 2-APA were found to be 0.01
and 0.59 mM, respectively. Hence, the markedly lower toxic
effects of the treated solution on Daphnia magna could be
attributed to the effective conversion of diclofenac into less
toxic 2-APA by the catalytic HDC.
Fig. 8 Inhibition rate of daphnids as a function of (A) original diclo-
fenac solution, and (-) treated diclofenac solution by
hydrodechlorination.
4. Conclusions
In the present study, Pd catalysts supported on AC, SiO2, Al2O3
and CeO2 were prepared and the HDC of diclofenac on the
catalysts was investigated. Compared with Pd catalysts with
other supports, dp-Pd(1.7)/CeO2 catalyst has a higher Pd
dispersion and more cationized Pd species due to the existence
of stronger metal support interaction. Accordingly, much
higher catalytic activity is shown on dp-Pd(1.7)/CeO2 for the
catalytic HDC of diclofenac. Increasing Pd loading amount of
dp-Pd/CeO2 catalyst enhances the reductive removal of diclofe-
nac, while the TOF value rst decreases and then kept nearly
constant with Pd loading amount. Additionally, the catalytic
HDC of diclofenac follows the Langmuir–Hinshelwood model,
reecting an adsorption controlled reaction mechanism. The
HDC of diclofenac processes via a combined stepwise and
concerted pathway, and increasing Pd loading amount favors
the concerted pathway. Toxicity test indicates that in compar-
ison with original solution HDC of diclofenac results in mark-
edly decreased toxicity. Findings in the present study highlight
promising potentials of using HDC as an effective detoxication
method for the abatement of diclofenac pollution in water.
Acknowledgements
The nancial support from National Key Basic Research
Program of China (2014CB441103) and the National Natural
Science Foundation of China (no. 21277066, 21225729 and
21237002) is gratefully acknowledged. We are indebted to the
Modern Analytical Center, Nanjing University for the catalyst
characterization.
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cycles.
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