Deactivation of a Pd/AC catalyst in the hydrodechlorination of chlorinated herbicides

Article abstract of DOI:10.1016/j.cattod.2014.03.052

This work analyses the stability of a palladium on activated carbon (Pd/AC) catalyst in the ambient-like conditions hydrodechlorination (HDC) of the organochlorinated herbicides 4-chloro-2-methylphenoxyacetic acid (MCPA) and 2,4-dichlorophenoxyacetic acid (2,4-D) as well as 2,4-dichlorophenol (2,4-DCP), a precursor in the synthesis of the second. Continuous long term experiments (100 h time on stream) were performed at mild operating conditions (30 °C, 1 atm). The composition of the reaction effluents was analyzed and their ecotoxicity (Microtox) measured. In all cases, a significant decrease of ecotoxicity was observed due to the high dechlorination achieved. The Pd/AC catalyst maintained a constant activity along the HDC of 2,4-DCP, while it suffered an important deactivation in the HDC of 2,4-D and MCPA. From characterization of the fresh and used catalyst the adsorption/deposition of reaction byproducts on the active sites can be recognized as the main cause of deactivation. The use of activated carbon as support reduces the negative effect that the released chloride ions usually provoke on other HDC catalysts.

Full text of DOI:10.1016/j.cattod.2014.03.052

G Model
CATTOD-9002; No. of Pages6
ARTICLE IN PRESS
Catalysis Today xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Deactivation of a Pd/AC catalyst in the hydrodechlorination of
chlorinated herbicides
E. Diaz, A.F. Mohedano, J.A. Casas, L. Calvo, M.A. Gilarranz, J.J. Rodriguez∗
Ingeniería Química, Facultad de Ciencias, Universidad Autónoma de Madrid, Crta. Colmenar km 15, 28049 Madrid, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
This work analyses the stability of
a palladium on activated carbon (Pd/AC) catalyst in
Received 10 January 2014
Received in revised form 6 March 2014
Accepted 18 March 2014
Available online xxx
the ambient-like conditions hydrodechlorination (HDC) of the organochlorinated herbicides 4-
chloro-2-methylphenoxyacetic acid (MCPA) and 2,4-dichlorophenoxyacetic acid (2,4-D) as well as
2,4-dichlorophenol (2,4-DCP), a precursor in the synthesis of the second. Continuous long term exper-
iments (100 h time on stream) were performed at mild operating conditions (30 ◦C, 1 atm). The
composition of the reaction effluents was analyzed and their ecotoxicity (Microtox) measured. In all
cases, a significant decrease of ecotoxicity was observed due to the high dechlorination achieved. The
Pd/AC catalyst maintained a constant activity along the HDC of 2,4-DCP, while it suffered an important
deactivation in the HDC of 2,4-D and MCPA. From characterization of the fresh and used catalyst the
adsorption/deposition of reaction byproducts on the active sites can be recognized as the main cause of
deactivation. The use of activated carbon as support reduces the negative effect that the released chloride
ions usually provoke on other HDC catalysts.
Keywords:
Catalytic hydrodechlorination
Pd/AC
2,4-Dichlorophenol
2,4-D
MCPA
Deactivation
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
and environmentally friendly solution [1,15]. Reductive techniques
have been used for the conversion of organochlorinated pollut-
Chlorophenoxy
herbicides,
like
4-chloro-2-
ants to less toxic compounds, thus providing a detoxifying step
prior to biological oxidation. Some recent works have reported
promising results on the use of electrocatalytic hydrogenolysis
with Pd-modified cathodes, although further research is needed
to achieve higher degradation efficiency at lower cost [16,17]. Cat-
alytic hydrodechlorination (HDC) has demonstrated its capability
to deal with aliphatic and aromatic organochlorinated pollutants,
like chloroethlylenes, chlorobenzenes or chlorophenols or even
more complex molecules as alachor, diuron or clopyralid in water
using catalysts based on precious metals as active phase [18–24].
This process can operate under mild conditions (temperature and
pressure) and is suitable within a wide concentration range. Pt, Rh,
Ru and specially Pd have shown a high activity in that reaction and
regarding the support, most of the hydrodechlorination studies in
the literature have used alumina or activated carbon (AC). The sta-
bility of those catalysts is a crucial issue due to the high cost of
precious metals. However, the catalysts developed so far for HDC
usually have shown a short life due to rapid deactivation [19,25,26].
Different studies have associated that loss of activity to chloride
poisoning, sintering, leaching of the active phase or accumulation
of chlorine species on the catalyst surface [27–29]. Recent works
on HDC of trichloroethylene and 4-chlorophenol in water with Pd
catalysts have established a higher resistance to deactivation of the
catalysts supported on AC compared to those on alumina [19,26].
methylphenoxyacetic acid (MCPA) and 2,4-dichlorophenoxyacetic
acid (2,4-D), are commonly used for the control of plant growth.
On the other hand, 2,4-dichlorophenol (2,4-DCP) is used primarily
as an intermediate in the preparation of the second one. These
species have been identified in water bodies where they can enter
by different ways [1–3]. Concentrations of pesticides from 10
to 100 mg/L have been found in aqueous wastes from pesticides
containers cleaning and agricultural industries and within a
wider range (1 to 1000 mg/L) in the wastewaters from pesticides
manufacturing plants [4]. Low concentrations of these hazardous
pollutants have been also detected in municipal wastewater
treatment plants [5].
The high toxicity and low biodegrability of these compounds
have caused that they are considered chemicals of main concern
for the environment [6,7]. Different techniques have been used
for the removal of these organochlorinated pollutants, includ-
ing adsorption [8–11] and advanced oxidation processes (AOPs)
[12–14]. The combination of advanced oxidation processes with
biological treatment has been claimed as a promising cost-effective
∗
Corresponding author. Tel.: +34 914974048; fax: +34 914973516.
E-mail address: juanjo.rodriguez@uam.es (J.J. Rodriguez).
http://dx.doi.org/10.1016/j.cattod.2014.03.052
0920-5861/© 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: E. Diaz, et al., Deactivation of a Pd/AC catalyst in the hydrodechlorination of chlorinated herbicides,
Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.03.052

G Model
CATTOD-9002; No. of Pages6
ARTICLE IN PRESS
2
E. Diaz et al. / Catalysis Today xxx (2014) xxx–xxx
The better performance of Pd/AC catalyst could be associated to its
high resistance to the chloride released during the reaction. The
activated carbon interacts strongly with that chloride, protecting
the metallic active phase.
Ecotoxicity measurements were carried out by a bioassay fol-
lowing the standard Microtox test procedure (ISO 11348-3, 1998),
based on the decrease of light emission by the marine bacteria Vib-
rio fischeri (Photobacterium phosphoreum), using a Microtox M500
Analyzer (Azur Environmental), in order to determine of EC50 val-
ues of the reaction compounds.
The aim of this work is to study the HDC of three chlorinated
compounds 2,4-DCP, 2,4-D and MCPA with an own-made Pd/AC
catalyst, focusing the attention on its stability. In a previous study
[20], this catalyst showed a very good performance for detoxifica-
tion of chlorophenols-bearing synthetic wastewater through HDC,
yielding a high activity with 4-chlorophenol, which was maintained
upon long term experiments (100 h on stream) in a continuous
stirred tank reactor. The objective now is to learn on the stability of
the catalyst when dealing with more complex organochlorinated
molecules. The causes of deactivation are analyzed in depth from
the characterization of the fresh and used catalysts.
2.4. Catalyst characterization
Several analytic techniques were used to characterize the fresh
and used catalyst BET surface area, micropore volume and nar-
row mesopore volume (2–8 nm) were calculated from the 77 K
N2 adsorption–desorption isotherms obtained in a Micromeritics
apparatus (Tristar 3020). Approximately 0.15 g of sample were
used in each test. Samples were previously degassed for 12–16 h
at 150 ◦C and a residual pressure at 10−3 bar in a Micromeritics
sample degass system (VacPrep 061). The elemental composition
of the catalysts was analyzed by a CHNS analyzer (LECO CHNS-
932). Thermogravimetric analyses were performed in a Mettler
SDTA851e model TGA apparatus under 50 N mL/min continuous air
flow. A weighted amount (approaching always 20 mg) of sample
was heated at 2 ◦C/min up to 110 ◦C, temperature that was main-
tained for 1 h in order to remove the humidity of the samples.
Subsequently, the samples were heated at 2 ◦C/min up to 800 ◦C.
The bulk palladium content was determined by inductively cou-
pled plasma–mass spectroscopy (ICP–MS) in a Perkin-Elmer model
Elan 6000 Sciex apparatus equipped with an autosampler (Perkin-
Elmer model AS 91). The samples were previously digested for
15 min in a microwave oven, using a mixture of several acids at
250 ◦C.
The catalyst surface was analyzed by X-ray photoelectron spec-
troscopy (XPS) with a VG Escalab 200R spectrometer equipped with
a hemispherical electron analyzer (pass energy of 20 eV) and a Mg
KR (hꢀ = 1254.6 eV) X-ray source, powered at 120 W.
Metal dispersion on the catalysts was determined from CO
chemisorption at room temperature in a Micromeritics Chemisorb
2750 automated system equipped with ChemiSoft TPx software.
Before chemisorption measurements, the samples were reduced in
H2 flow at 150 ◦C for 2 h and then cooled down to ambient tem-
perature in He flow. The stoichiometry of CO adsorption on Pd was
assumed to be 1:1 [32].
2. Experimental
2.1. Chemicals
Aqueous solutions of 2,4-DCP (>99.0% purity, Aldrich), 2,4-D,
and MCPA (>98.0% purity, Aldrich) 0.352, 0.352 and 0.705 mM,
respectively, were always used. Those concentrations are equiv-
alent to 25 mg/L of organic chlorine. All the chemicals used were of
analytical grade and supplied by Aldrich.
2.2. Catalyst preparation
The 0.5 wt.% palladium catalyst (Pd/AC) was prepared in our lab
by incipient wetness impregnation of an activated carbon supplied
by Merck (BET surface area ≈950 m2/g; bulk density ≈ 0.5 g/cm3;
particle size ≈ 1.5 mm). The impregnation solution consisted in
PdCl2 (Sigma-Aldrich) dissolved in 0.1 N HCl. The volume of the
impregnating solution exceeded by 30% the total pore volume of
the activated carbon. Impregnation was followed by drying at room
temperature for 2 h and overnight at 60 ◦C. Then, calcination at
200 ◦C in air atmosphere and reduction at 150 ◦C under continuous
H2 flow, were carried out.
2.3. Hydrodechlorination experiments
Two replicates of each hydrodechlorination run were carried
out in a continuous basket stirred tank reactor (Carberry Spinning
Catalyst Basket) from Autoclave Engineers, provided with tem-
perature, pressure and gas flow control. The aqueous solution of
each organochlorinated reactant was fed to the reactor at 4 mL/min
and H2 was continuously passed at 50 N mL/min. A catalyst load-
ing of 2.95 g/L was always used, so that the space-time was fixed
at 2.95 kgcat h/molCl. All the experiments were performed at 30 ◦C,
atmospheric pressure and 600 rpm stirring velocity. The progress
of the reaction was followed by analyzing periodically samples
of the exit liquid stream taken with a fraction collector (FC203B-
Gilson) along the 100 h on stream of the experiments. The data
reproducibility was better than 5%. The existence of internal and
external mass transfer limitations is discarded in our experimental
conditions, as demonstrated in previous studies [30,31].
The organic compounds in the effluent from HDC of 2,4-DCP
were analyzed by GC with a flame ionization detector (GC 3900 Var-
ian) using a 30 m long × 0.25 mm i.d. capillary column (CP-Wax 52
CB). 2,4-D, MCPA and the aromatic reaction byproducts were quan-
tified by HPLC (Varian Prostar 325) with a UV detector using a C18
as stationary phase (Valco Microsorb-MW 100-5 C18) at 280 nm
and a mixture of acetonitrile and acid water (acetic acid 0.1%) as
mobile phase at 0.5 mL/min. Analysis of chloride was performed
by ionic chromatography (Metrohm 790 Personal IC). The pH was
measured with a pH meter (CRISON).
3. Results and discussion
3.1. HDC of 2,4-DCP, 2,4-D and MCPA
Fig. 1 shows the results from the long-term HDC experiments.
Adsorption onto the activated carbon lead to C and Cl unbalances
during the early stages before the steady state was reached. After
that, those balances were always closed in more than 93 and 95%,
respectively. Fig. 2 shows the reaction pathways deduced in each
case from the identified reaction products. The HDC of 2,4-DCP led
to the formation of 2-chlorophenol (2-CP) and phenol (Ph) in addi-
tion to HCl. Previous works have reported that HDC of 2,4-DCP
proceeds via 2-CP as the sole partially dechlorinated byproduct
due to the occurrence of steric hindrance, where the Cl atom in
ortho position is less susceptible to attack [33–35]. Although phenol
was the most hydrogenated byproduct identified, others previous
works have demonstrated that HDC of 2,4-DCP, under different
operating conditions can produce more hydrogenated byproducts,
like cyclohexanone and cyclohexanol [36,37].
The reaction products identified from 2,4-D HDC were 2-
chlorophenoxyacetic acid (2-CPA) and phenoxyacetic acid (PA).
Yamanaka et al. [38] studied the HDC of 2,4-D with several Pd
catalysts and concluded that the formation of PA upon successive
Please cite this article in press as: E. Diaz, et al., Deactivation of a Pd/AC catalyst in the hydrodechlorination of chlorinated herbicides,
Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.03.052

G Model
CATTOD-9002; No. of Pages6
ARTICLE IN PRESS
E. Diaz et al. / Catalysis Today xxx (2014) xxx–xxx
3
OH
OH
OH
2,4-DCP
2-CP
Ph
0.35
0.30
0.25
0.20
0.15
0.10
0.05
Cl
Cl
Cl
(a)
O
O
O
OH
OH
OH
0.20
0.15
0.10
0.05
O
O
O
Cl
Cl
Cl
(b)
2,4-D
2-CPA
PA
O
O
0.6
0.5
0.4
0.3
0.2
0.1
OH
OH
O
O
CH3
CH3
Cl
MCPA
80
MPA
100
(c)
20
40
60
Time on stream (h)
Fig. 2. HDC pathways for (a) 2,4-DCP, (b) 2,4-D and (c) MCPA.
Fig. 1. Exit concentrations upon time on stream from HDC of 2,4-DCP (a), 2,4-D (b)
and MCPA (c) with a Pd/AC catalyst (30 ◦C, 1 atm, 2.95 kgcat h/molCl).
the literature range is fairly wide in some cases. The highest EC50
values correspond in all cases to the non-chlorinated byproducts.
reactions through 2-CPA and 4-chlorophenoxyacetic acid (4-CPA).
According to the products selectivity, these authors suggested
that the HDC of 2,4-D occurred faster at para than ortho position
and, consequently, 4-CPA is not always detected. The same reac-
tion pathway has been described for the electrocatalytic reductive
dechlorination of 2,4-D [17,39,40].
3.2. Catalyst stability
Catalyst deactivation represents an important drawback for
potential application of HDC. Among the different causes of deacti-
vation of Pd catalysts poisoning by HCl is the most widely accepted
[19,41,42]. Nevertheless, so far there is no a general agreement
In the HDC of MCPA, 2-methylphenoxyacetic acid (MPA) was
the only reaction product detected in addition to HCl and no further
hydrogenation was observed under the operating conditions used.
The efficiency of HDC for the abatement of ecotoxicity has
been evaluated from EC50 values of the starting compounds and
the reaction products determined by the Microtox test. Table 1
summarizes the results obtained as well as the range of values
reported in the literature by other authors. The byproducts from
HDC showed significantly higher EC50 values than those of the
corresponding starting compounds, thus indicating the reduction
of ecotoxicity achieved upon HDC. The experimental EC50 values
determined in this work are in general in reasonable agreement
with those reported in the literature although it is also true that
Table 1
EC50 values obtained by the Microtox test and reported in literature.
This work
EC50 (mg/L)
Other authors
EC50 (mg/L) range
Compound
Reference
2,4-DCP
2-CP
Ph
2,4-D
2-CPA
PAA
MCPA
MPA
3
22
24
130
148
300
75
1–6
[7,36,54,55]
[36,55,56]
[36,57]
[7,58]
–
[59]
1–34
15–42
20–60
–
75
12–75
300
[7,60–62]
[60]
150
Please cite this article in press as: E. Diaz, et al., Deactivation of a Pd/AC catalyst in the hydrodechlorination of chlorinated herbicides,
Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.03.052

G Model
CATTOD-9002; No. of Pages6
ARTICLE IN PRESS
4
E. Diaz et al. / Catalysis Today xxx (2014) xxx–xxx
and whereas several authors suggest that the HCl released during
the HDC reaction does not have any significant effect [43,44] some
other state that it causes an important loss of activity even at low
concentrations [16,26,41].
100
90
80
70
60
50
40
30
20
10
0
The HDC of 2,4-DCP, 2,4-D and MCPA was carried out at the same
concentration of organic chloride (C0 = 25 mg/L). The initial rate of
HDC ((−rHDC), mmolCl/kgcat h), defined as the rate of Cl removal
once the pseudo-steady state was reached (≈20 h time on stream),
was determined from the concentration of Cl− in the effluent. Val-
ues of 289, 200 and 181 mmolCl/kgcat h were obtained with 2,4-DCP,
2,4-D and MCPA, respectively. Some authors have analyzed the
dependence of HDC activity on the number and position of chlorine
atoms and the presence of some other substituents in the aromatic
ring. Nevertheless, there is no general consensus. Benitez and Del
Angel [45] using a Pd/C catalyst established the following order
of reactivity: chlorophenol > chlorotoluene > dichlorobenzene, for
HDC water–methanol medium, suggesting that the rate of HDC of
those molecules is increased by the presence of electron donor
groups such as CH3 and OH. Janiak and Okal [25] found the
following sequence: 3-chlorotoluene < 1,3-dichlorobenzene < 3-
chloroanisole < 1-chloro-3-fluorobenzene < 3-chloroaniline, stabi-
lizing a correlation with the electronegativity of the heteroatom
bound to the chlorobenzene ring. Regarding the number of chlo-
rine atoms of the molecule, Keane [42] observed that the HDC
rate of several chlorophenols decreased as the number of chlorine
atoms increased. A similar conclusion was obtained by Xia et al.
[46] with a wider diversity of polychlorophenols: monochlorophe-
nol > dichlorophenol > trichlorophenols > pentachlorophenol. This
conclusion is consistent with the reduction of the electron density
of the aromatic ring associated to the presence of Cl substituents
[47].
The performance of the Pd/AC catalyst upon 100 h on stream in
the HDC of 2,4-DCP, 2,4-D and MCPA in terms of reactant conver-
sion is shown in Fig. 3. In the case of 2,4-DCP, the catalyst showed a
fairly stable behaviour with only slight changes upon the 100 h on
stream (X20 h ≈ 85%; X95 h ≈ 80%). The concentration of the chlori-
nated intermediate, 2-CP, remained also practically constant along
the experiment. This stability of Pd on activated carbon catalysts in
the HDC of chlorophenols has been previously reported [20,26,36].
However, in the case of 2,4-D and MCPA a progressive decrease
of conversion upon time on stream can be observed, indicative
of an important deactivation of the catalyst. In the former case,
the exit concentration of 2,4-D and 2-CPA increased and that of
PA decreased up to around 50 h on stream and then the catalytic
activity remained practically stable yielding an almost constant 50%
conversion of 2,4-D. A quite similar behaviour showing a constant
residual activity was observed by Ordon˜ez et al., [19], in the HDC
of trichloroethylene with a commercial Pd/AC catalyst. The experi-
ment with MCPA showed a progressive deactivation of the catalyst
2,4-DCP
2,4-D
MCPA
20
30
40
50
60
70
80
90
100
Time on stream (h)
Fig. 3. Conversion of 2,4-DCP, 2,4-D and MCPA upon time on stream.
upon the whole time on stream tested which becomes less pro-
nounced at the end of the experiment where the MCPA conversion
trends to stabilize at a fairly low value around 20%. Thus, the Pd/AC
catalyst suffered a severe deactivation in this case.
The following well-known equation has been used to describe
the rate of deactivation:
da
−
= kd × a − a
(1)
(
)
∞
dt
where kd is the deactivation kinetic constant (h−1) and a is the
catalyst activity (dimensionless, from 0 to 1) defined as the ratio
between the values of reaction rate (2,4-D or MCPA disappearance)
at time t and the initial. From the mass balance in a continuous
stirred tank reactor, the following expression is obtained:
(−r)t
(−r)0
Xt
a =
=
(2)
X0
where X0 and Xt correspond to the conversion of the starting com-
pound at the initial and t time, respectively. Integration of Eq. (1)
with limits t = 0; a = 1; t = t; a = a; (corresponding t = 0 to the time
when the pseudo-steady state in the effluent was reached) leads
to:
Xt
×t
d
X0
a =
= e−k
× 1 − a + a
(
(3)
(
)
)
∞
∞
Fitting of the experimental data to Eq. (3) was accomplished
using non-linear regression (Origin 6.1). The results are depicted in
Fig. 4 and the values of the deactivation kinetic constant and the
Fig. 4. Fitting of the experimental results from long term experiments of 2,4-D (a) and MCPA (b) HDC to the deactivation model.
Please cite this article in press as: E. Diaz, et al., Deactivation of a Pd/AC catalyst in the hydrodechlorination of chlorinated herbicides,
Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.03.052

G Model
CATTOD-9002; No. of Pages6
ARTICLE IN PRESS
E. Diaz et al. / Catalysis Today xxx (2014) xxx–xxx
5
Table 2
Values of the deactivation kinetic constant and residual activity of Pd/AC catalyst in the HDC of 2,4-D and MCPA.
Catalyst
Pd/AC
Starting compound
kd (h−1
)
a
ꢁꢂ2
r2
∞
2,4-D
MCPA
0.0533 0.0071
0.0170 0.0017
0.7721 0.0097
0.1142 0.0535
0.0005
0.0008
0.918
0.982
Table 3
Characterization of the fresh and used Pd/AC catalyst.
Catalyst
Fresh
Used in 2,4-DCP HDC
Used in 2,4-D HDC
Used in MCPA HDC
ABET (m2/g)
1025
175
360
96
119
79
93
81
Aext (m2/g)
Vmicropores (cm3/g)
Vmesopores (cm3/g)
Dispersion (%)
C (%)
Pdbulk (wt.%)
PdXPS (wt.%)
ClXPS (wt.%)
0.35
0.040
32
88
0.48
0.09
0.030
26
79
0.32
0.02
0.027
18
<0.01
0.024
8
78
82
0.22
0.80
8.6
0.20
0.98
4.1
2.55
1.9
1.41
6.7
residual activity are collected in Table 2. The deactivation rate con-
stant is significantly higher for HDC of 2,4-D, where deactivation
takes place within a shorter time giving rise to a much higher resid-
ual activity than in the case of MCPA. With this second, the value of
residual activity indicates that the catalyst undergoes almost com-
plete deactivation. The results obtained suggest that poisoning by
the chloride ions released upon HDC reactions must not be the only
cause of deactivation of the Pd/AC catalyst. The high stability shown
in the HDC of 2,4-DCP suggests a strong interaction of the activated
carbonsupportwiththechlorideionsdueto itsadsorptioncapacity.
This prevents the potential poisoning effect on the metallic active
phase [26]. On the other hand, the deactivation observed in the HDC
of 2,4-D and MCPA, severe in the second case, can be associated with
the nature of the starting compounds and the reactions products
which can be strongly adsorbed or deposited on the metal surface
restricting the access of the reacting species [48]. Albers et al. [49]
in their study on the poisoning and deactivation of Pd catalysts used
mainly in hydrogenation reactions, revealed the easy formation of
a stable CH3–Pd complex, provoking catalyst deactivation.
experiments, the dispersion values of the used catalysts must be
conditioned by the deposition of species onto the catalyst surface.
This fact must be also affecting to the measured bulk Pd content
of the used catalyst since the leaching of Pd was lower than 5% in
all the experiments. That leaching occurred during the early stages
of the HDC process and cannot be considered a significant contri-
bution to catalyst deactivation in aqueous HDC reaction [19,20].
This aspect should be remarked because the catalyst is mainly con-
stituted by Pd particles lower than 4 mm, which are considered
more susceptible to leaching by the HCl produced [18,51–53]. The
Pd content of the fresh catalyst determined by XPS is much higher
than the bulk Pd content, indicating that the metal is mainly dis-
tributed in the outermost surface. Thus, the HDC reaction must
be occurring predominantly in the external or non-microporous
surface of the catalyst. A remarkable decrease of that surface was
observed in the used catalyst, more pronounced upon HDC of 2,4-D
and MCPA. This is also the case of XPS Pd content. The accumulation
of organic species on the catalyst surface can explain those findings.
A very important increase of the surface Cl content was detected by
XPS in the used catalyst, supporting the abovementioned adsorp-
tion/deposition of organic compounds, which must correspond in
great part to organochlorinated species.
To learn more in depth on the deactivation, different techniques
were used for the characterization of the fresh and used catalyst.
The results are summarized in Table 3.
The Pd/AC catalyst presents a basically microporous structure
with a contribution of mesoporosity which represents 17% of the
BET surface area and a fairly small fraction of the pore volume.
After 100 h on stream a dramatic decrease of the surface area
was observed, affecting mainly to the microporosity. This can be
attributed to partial blockage of the entrance of micropores by
species adsorbed or deposited on the catalyst surface. The higher
decrease of BET area observed in the cases of 2,4-D and MCPA can
be explained by the higher molecular size of the starting com-
pounds and reaction byproducts. Moreover, the slightly lower pH
of the reaction media would favour the adsorption of those species
[41,50]. The accumulation of organic substances on the catalyst
upon HDC runs was also checked by thermogravimetric analysis
(TGA). A significant loss of weight occurred around 186 ◦C for all
the used catalysts, being more important for the catalysts used in
the HDC of 2,4-D and MCPA.
In a previous work it was found by STEM that the fresh Pd/AC
catalyst presents a narrow distribution of the Pd particles (2–6 nm)
and a mean particle size of 3.7 nm corresponding with a dispersion
of 30% [20]. This dispersion values is in good agreement with the
reported now in Table 3 as obtained by CO chemisorption. As can
be seen, the used catalyst shows a loss of dispersion which is much
more pronounced after HDC of MCPA and 2,4-D, consistently with
the respective loss of activity. Since agglomeration of the metal par-
ticles appears unlikely under the operating conditions of the HDC
4. Conclusions
The HDC of 2,4-DCP, 2,4-D and MCPA was carried out efficiently
with an own-made Pd/AC catalyst at mild operating conditions,
leading to a significant reduction of ecotoxicity. The catalyst
showeda highresistanceto poisoningby thechloridereleasedupon
the HDC reaction due to the strong interaction of activated carbon
with it. Neither structural changes nor leaching of Pd were signifi-
cant causes of deactivation. The more plausible explanation for the
loss of activity appears the accumulation of organic species (mainly
organochlorinated) on the catalyst surface. The stronger interac-
tion of phenoxyacetic compounds compared to phenolic ones can
explain the more severe deactivation accompanying HDC of the
chlorophenoxy herbicides 2,4-D and MCPA in contrast with 2,4-
DCP.
Acknowledgements
We gratefully appreciate financial support from the Conse-
jería de Educación of the CM through the project REMTAVARES
S-2009/AMB-1588 and the Spanish MICINN through the projects
CTQ2008-03988 and CTQ2010-14807.
Please cite this article in press as: E. Diaz, et al., Deactivation of a Pd/AC catalyst in the hydrodechlorination of chlorinated herbicides,
Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.03.052

G Model
CATTOD-9002; No. of Pages6
ARTICLE IN PRESS
6
E. Diaz et al. / Catalysis Today xxx (2014) xxx–xxx
References
[29] Y.I. Meytal-Matatov, M. Sheintuch, Ind. Chem. Eng. Res. 37 (1998) 309–326.
[30] E. Diaz, J.A. Casas, A.F. Mohedano, L. Calvo, M.A. Gilarranz, J.J. Rodriguez, Ind.
Eng. Chem. Res. 47 (2008) 3840–3846.
[31] E. Diaz, J.A. Casas, A.F. Mohedano, L. Calvo, M.A. Gilarranz, J.J. Rodriguez, Ind.
Eng. Chem. Res. 48 (2009) 3351–3358.
[32] S.H. Ali, J.G. Goodwin Jr., J. Catal. 176 (1998) 3–13.
[33] G. Yuan, M.A. Keane, Ind. Eng. Chem. Res. 46 (2007) 705–715.
[34] S. Gomez-Quero, F. Cardenas-Lizana, M.A. Keane, Ind. Eng. Chem. Res. 47 (2008)
6841–6853.
[35] E. Diaz, J.A. Casas, A.F. Mohedano, J.J. Rodriguez, Av. Cienc. Ing. 2 (2011) 23–33.
[36] L. Calvo, A.F. Mohedano, J.A. Casas, M.A. Gilarranz, J.J. Rodriguez, Carbon 42
(2004) 1377–1381.
[1] I. Oller, S. Malato, J.A. Sánchez-Pérez, Sci. Total Environ. 409 (2011) 4141–4166.
[2] M. Köck-Schulmeyer, M. Villagrasa, M. López de Alda, R. Céspedes-Sánchez, F.
Ventura, D. Barceló, Sci. Total Environ. 458–460 (2013) 466–476.
[3] D.C. Yoder, B.K. Corwin, T.C. Mueller, W.E. Hart, C.R. Mote, J.B. Wills, Trans. ASAE
44 (2013) 877–890.
[4] S. Malato, J. Blanco, C.A. Estrada, E.R. Bandala, Degradacion de plaguicidas, in:
M.A. Blesa (Ed.), Eliminacion de contaminantes por fotocatalisis heterogenea,
Digital Grafic, La Plata, Argentina, 2001, pp. 269–281.
[5] D. Barceló, M.C. Hennion, Trace Determination of Pesticides and their Degra-
dation Products in Water, Elsevier, Amsterdam, 1997.
[6] M. Tobajas, A.M. Polo, S. Sanchis, A.F. Mohedano, J.J. Rodriguez, J. Biotechnol.
150 (2010) S257–S258.
[37] S. Gomez-Quero, F. Cardenas-Lizana, M.A. Keane, Chem. Eng. J. 166 (2011)
1044–1051.
[38] I. Yamanaka, K. Nishikawa, S. Takenaka, K. Otsuka, Complete hydrodechlorinat-
ion of chloro-aromatics catalyzed by Pd/TiO2 with H2, in: A.O. Yamashita (Ed.),
Science and Technology in Catalysis, Elsevier Science, Amsterdam, 2003.
[39] I.G. Casella, M. Contursi, Electrochim. Acta 52 (2007) 7028–7034.
[40] K. Zhu, C. Sun, H. Chen, S.A. Baig, T. Sheng, X. Xu, Chem. Eng. J. 223 (2013)
192–199.
[7] A.M. Polo, M. Tobajas, S. Sanchis, A.F. Mohedano, J.J. Rodriguez, Biodegradation
22 (2011) 751–761.
[8] I. Vergili, H. Barlas, Desalination 249 (2009) 1107–1114.
[9] V. Addoridio, S. Esposito, G. Sannino, J. Agric. Food Chem. 58 (2010) 5011–5016.
[10] F. Sannino, D. Pirozzi, A. Aronne, E. Fanelli, R. Spaccini, A. Yousuf, P. Pernice,
Environ. Sci. Technol. 44 (2010) 9476–9481.
[41] G. Yuan, M.A. Keane, Appl. Catal., B: Environ. 52 (2004) 301–314.
[42] M.A. Keane, J. Chem. Technol. Biotechnol. 80 (2005) 1211–1222.
[43] G.V. Lowry, M. Reinhard, Environ. Sci. Technol. 34 (2000) 3217–3223.
[44] C.G. Schreier, M. Reinhard, Chemosphere 31 (1995) 3475–3487.
[45] J.L. Benitez, G. Del Angel, Ind. Eng. Chem. Res. 50 (2011) 2678–2682.
[46] C.H. Xia, Y. Liu, S.W. Zhou, C.Y. Yang, S.J. Liu, J. Xu, J.B. Yu, J.P. Chen, X.M. Liang,
J. Hazard. Mater. 169 (2009) 1029–1033.
[47] E.-J. Shin, M.A. Keane, Chem. Eng. Sci. 54 (1999) 1109–1120.
[48] M. Besson, P. Gallezot, Catal. Today 81 (2003) 547–559.
[49] P. Albers, J. Pietsch, S.F. Parker, J. Mol. Catal. A: Chem. 173 (2001) 275–286.
[50] O. Hamdaoui, E. Naffrechoux, J. Hazard. Mater. 147 (2007) 401–411.
[51] M.A. Aramendia, V. Borau, I.M. Garcia, C. Jimenez, F. Lafont, A. Marinas, J.M.
Marinas, F.J. Urbano, J. Catal. 187 (1999) 392–399.
[52] L.M. Gomez-Sainero, X.L. Seoane, J.L.G. Fierro, A. Arcoya, J. Catal. 209 (2002)
279–288.
[53] J.A. Baeza, L. Calvo, M.A. Gilarranz, A.F. Mohedano, J.A. Casas, J.J. Rodriguez, J.
Catal. 293 (2012) 85–93.
[54] S.J. Ren, P.D. Frymier, Chemosphere 58 (2005) 543–550.
[55] K.P. Tsai, C.Y. Chen, Environ. Toxicol. Chem. 26 (2007) 1931–1939.
[56] Y.G. Lee, S.H. Hwang, S.D. Kim, Arch. Environ. Contam. Toxicol. 50 (2006)
213–219.
[11] A. Aronne, F. Sannino, S.R. Bonavolontà, E. Fanelli, A. Mingione, P. Pernice, R.
Spaccini, D. Pirozzi, Environ. Sci. Technol. 46 (2012) 1755–1763.
[12] D. Vione, S. Khanra, R. Das, C. Minero, V. Maurino, M. Brigante, G. Mailhot, Water
Res. 44 (2010) 6053–6062.
[13] M. Kubota, M. Hayashi, H. Matsuda, H. Serizawa, J. Mater. Cycles Waste Manage.
14 (2012) 132–138.
[14] V.J.P. Vilar, F.C. Moreira, A.C.C. Ferreira, M.A. Sousa, C. GonC¸ alves, M.F. Alpen-
durada, R.A.R. Boaventura, Water Res. 46 (2012) 4599–4613.
[15] S. Sanchis, A.M. Polo, M. Tobajas, J.J. Rodriguez, A.F. Mohedano, Chemosphere
93 (2013) 115–122.
[16] T. Zhou, Y.Z. Li, T.T. Lim, Sep. Purif. Technol. 76 (2013) 206–214.
[17] Y.H. Xu, Q.Q. Cai, H.X. Ma, Y. He, H. Zhang, C.A. Ma, Electrochim. Acta 96 (2013)
90–96.
[18] M.A. Keane, ChemCatChem 3 (2011) 800–821.
[19] S. Ordonez, B.P. Vivas, F.V. Diez, Appl. Catal., B: Environ. 95 (2010) 288–296.
[20] E. Diaz, A.F. Mohedano, J.A. Casas, L. Calvo, M.A. Gilarranz, J.J. Rodriguez, Appl.
Catal., B: Environ. 106 (2011) 469–475.
[21] M. Munoz, Z.M. de Pedro, J.A. Casas, J.J. Rodriguez, Water Res. 47 (2013)
3070–3080.
[22] L. Calvo, M.A. Gilarranz, J.A. Casas, A.F. Mohedano, J.J. Rodriguez, Appl. Catal., B:
Environ. 78 (2008) 259–266.
[57] C. Curtis, A. Lima, S.J. Lozano, G.D. Veith, ASTM STP 766 (1982) 170–178.
[58] R. Zona, S. Solar, Radiat. Phys. Chem. 66 (2003) 137–143.
[59] K.L.E. Kaiser, V.S. Palabrica, Water Pollut. Res. J. Can. 26 (1991) 361–431.
[60] A. Bojanowska-Czajka, P. Drzewicz, C. Kozyra, G. Nalecz-Jawecki, J. Saw-
icki, B. Szostek, M. Trojanowicz, Ecotoxicol. Environ. Saf. 65 (2006)
265–277.
[61] C. Vismara, A. Garavaglia, Bull. Environ. Contam. Toxicol. 58 (1997) 582–588.
[62] R. Lopez-Roldan, L. Kazlauskaite, J. Ribo, M.C. Riva, S. González, J.L. Cortina, Sci.
Total Environ. 440 (2012) 307–313.
[23] M. Al Bahri, L. Calvo, A.M. Polo, M.A. Gilarranz, A.F. Mohedano, J.J. Rodriguez,
Chemosphere 91 (2013) 1317–1323.
[24] L. Teevs, K.D. Vorlop, U. Prusse, Catal. Commun. 14 (2011) 96–100.
[25] T. Janiak, J. Okal, Appl. Catal., B: Environ. 92 (2009) 384–392.
[26] Z.M. de Pedro, E. Diaz, A.F. Mohedano, J.A. Casas, J.J. Rodriguez, Appl. Catal., B:
Environ. 103 (2011) 128–135.
[27] G. Yuan, M.A. Keane, Catal. Today 88 (2003) 27–36.
[28] N.C. Concibido, T. Okuda, W. Nishijima, M. Okada, Appl. Catal., B: Environ. 71
(2007) 64–69.
Please cite this article in press as: E. Diaz, et al., Deactivation of a Pd/AC catalyst in the hydrodechlorination of chlorinated herbicides,
Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.03.052