Improved Γ-alumina-supported Pd and Rh catalysts for hydrodechlorination of chlorophenols

Article abstract of DOI:10.1016/j.apcata.2014.09.035

The activity of monometallic (Pd or Rh) and bimetallic (iron-bearing) catalysts has been tested in the HDC of 4-chlorophenol (4-CP) in aqueous phase. The catalysts were prepared by a simple procedure consisting of incipient wetness impregnation followed by calcination and reduction at different temperatures. This allowed controlling the size, dispersion and oxidation state of the metallic nanoparticles. Increasing the temperature of calcination led to a higher dispersion of the active phase (from 21 to 46% by increasing the calcination temperature from 200 to 400 °C) and a higher ratio of zerovalent to electrodefficient palladium (M⁰/Mⁿ⁺values from 1.15 to 2.17) in the case of the Pd-catalysts. Those calcined in air at 400 °C and reduced with H₂at 350 °C yielded the highest values of the pseudo-first-order rate constant of 4-CP dechlorination (16.2 L h^?1g_cat^?1). The introduction of iron reduced the metal dispersion of both Pd and Rh but the catalysts maintained their dechlorination activity and showed magnetic properties useful for further separation from the liquid phase.

Full text of DOI:10.1016/j.apcata.2014.09.035

Accepted Manuscript
Title: Improved ␥-alumina-supported Pd and Rh catalysts for
hydrodechlorination of chlorophenols
Author: Macarena Munoz Zahara M. de Pedro Jose A. Casas
Juan J. Rodriguez
PII:
S0926-860X(14)00586-9
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2014.09.035
APCATA 15016
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
29-5-2014
10-9-2014
22-9-2014
Please cite this article as: M. Munoz, Z.M. de Pedro, J.A. Casas, J.J.
Rodriguez, Improved rmgamma-alumina-supported Pd and Rh catalysts
for hydrodechlorination of chlorophenols, Applied Catalysis A, General (2014),
http://dx.doi.org/10.1016/j.apcata.2014.09.035
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Highlights
• Increasing the calcination temperature improves both the dechlorination and hydrogenation rates
of Pd/γ-Al2O3.
• The activity of Rh/γ-Al2O3 is not significantly affected by calcination and reduction
temperatures.
• The incorporation of Fe onto the catalysts allows developing magnetic properties.
• The monometallic and bimetallic catalysts showed a high stability with negligible Pd or Rh
leaching.
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Improved γ-alumina-supported Pd and Rh catalysts
for hydrodechlorination of chlorophenols
Macarena Munoz*, Zahara M. de Pedro, Jose A. Casas and Juan J. Rodriguez
Ingenieria Quimica, Universidad Autonoma de Madrid, Crta. Colmenar km 15, 28049 Madrid, Spain
*Corresponding author phone: +34 91 497 3991; fax: +34 91497 3516; e-mail:
macarena.munnoz@uam.es
Keywords: Catalytic hydrodechlorination; 4-chlorophenol; Pd and Rh catalysts; magnetic catalysts.
Abstract
The activity of monometallic (Pd or Rh) and bimetallic (iron-bearing) catalysts has been tested in
the HDC of 4-chlorophenol (4-CP) in aqueous phase. The catalysts were prepared by a simple
procedure consisting of incipient wetness impregnation followed by calcination and reduction at
different temperatures. This allowed controlling the size, dispersion and oxidation state of the
metallic nanoparticles. Increasing the temperature of calcination led to a higher dispersion of the
active phase (from 21 to 46% by increasing the calcination temperature from 200 to 400 ºC) and a
higher ratio of zero-valent to electrodefficient palladium (M0/Mn+ values from 1.15 to 2.17) in the
case of the Pd-catalysts. Those calcined in air at 400 ºC and reduced with H2 at 350 ºC yielded the
highest values of the pseudo-first order rate constant of 4-CP dechlorination (16.2 L·h-1·gcat-1). The
introduction of iron reduced the metal dispersion of both Pd and Rh but the catalysts maintained
their dechlorination activity and showed magnetic properties useful for further separation from the
liquid phase.
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1. Introduction
Catalytic hydrodechlorination (HDC) is a promising technology for the treatment of wastewater
containing organic chlorinated pollutants. So far, it has been successfully applied to different types of
chlorinated compounds such as chlorophenols (CPs), chlorobenzenes or polychlorobiphenyls [1-4]. The
process is based on the reaction of the organochlorinated compound with hydrogen to produce hydrogen
chloride and the corresponding chlorine-free byproduct. This transformation is commonly associated to
a significant reduction of the ecotoxicity. Though there are a number of processes potentially available
for the treatment of chlorinated organic pollutants like incineration, adsorption, biological treatment or
advanced oxidation [5-8], HDC represents an environmentally friendly alternative with important
advantages. In contrast to the aforementioned techniques, HDC can be operated under ambient-like (or
close) temperature and pressure, shows a low sensitivity to pollutants concentration, is not affected by
the toxicity of the chlorinated compounds and does not lead to more toxic by-products [9, 10] as occur
upon advanced oxidation [8, 11-13]. However, HDC does not achieve the complete degradation of the
organic pollutants and thus it is regarded mostly as a detoxifying pretreatment prior to biological or
other oxidation treatments [2, 14].
A number of catalysts based on noble and transition metals (Pd, Pt, Rh Cu and Ni) have been tested in
aqueous-phase hydrodechlorination, being those based on palladium and rhodium the most active [1-3].
Regarding the support, carbon [10, 15] and alumina [3, 16] have attracted the main attention although
some other, like pillared clays, have also been studied [17, 18]. Activated carbon is an interesting
catalyst support due to its high porosity and surface area, but alumina-supported catalysts have shown
higher activity and stability so that they have been postulated as a better option by some authors. Yuan
and Keane (2004) [19] investigated hydrodechlorination of chlorophenols over Pd/C and Pd/Al2O3
catalysts, concluding that Pd/Al2O3 is less susceptible to deactivation whereas Pd/C undergoes an
appreciable loss of activity due to a marked decrease of surface area and to Pd leaching. The high
adsorption capacity of activated carbon can lead to a decrease of the catalyst surface area upon micro-
and mesoporous blockage [20-22].
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A number of authors have concluded that HDC is a structure sensitive reaction [23-25]. Thus, the
catalyst performance has shown to be affected by the size and electronic structure of the metal particles.
In general, it is accepted that a higher surface-to-volume ratio, associated to a lower nanoparticle size of
the active phase, increases the catalytic activity [23, 25]. Other important factor influencing the catalytic
activity is the oxidation state of the metal nanoparticles, which has been linked by some authors to the
own particle size [25, 26]. Therefore, the design of catalysts with controlled nanoparticle size and
oxidation state has gained large attention in the literature in the last few years [25, 27, 28]. However, the
synthesis of nanoparticles usually involves complex and expensive procedures. Therefore, it is of
interest the possibility of using simple conventional procedures capable of yielding catalysts with
metallic nanoparticles of controlled size and oxidation state. In this sense, the calcination and reduction
steps are two important milestones in the preparation of supported metallic catalysts and the temperature
is a critical variable in both cases since it plays a significant role on the metal-support interaction,
affecting to the nature of the species as well as the dispersion of the active phase. Nevertheless, so far
there are only few studies that analyze the effect of those issues on the properties of HDC catalysts and
they have been mainly related to activated carbon-supported catalysts [29, 30].
In addition to the fact that HDC is structure sensitive and the catalysts have to be prepared paying
special attention to the metal particle size and oxidation state, it is also important to consider the
separation from the aqueous phase when used for the abatement of water pollutants. In this context, the
preparation of bimetallic catalysts containing iron for the sake of developing magnetic properties
without impairing their activity represents an interesting challenge. This attempt has been scarcely
studied in the literature so far [31], being the main objectives of iron inclusion to reduce the cost of the
final catalyst and improve its activity and stability [32-34].
The aim of this work is to prepare palladium and rhodium γ-Al2O3-supported catalysts with controlled
size and dispersion of the active phase as well as the oxidation state of the nanoparticles by simply
acting on the calcination and reduction temperature. The inclusion of iron with the purpose of
developing magnetic properties in the catalyst allowing an easy separation is also studied. The activity
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and selectivity of the catalysts were tested in the hydrodechlorination of 4-chlorophenol (4-CP) in
aqueous phase.
2. Materials and methods
2.1.Preparation and characterization of the catalysts
Monometallic (Pd/γ-Al2O3; Rh/γ-Al2O3; Fe/γ-Al2O3) and bimetallic (Pd-Fe/γ-Al2O3; Rh-Fe/γ-Al2O3)
catalysts were prepared by incipient wetness impregnation of powdered γ-Al2O3 supplied by Merck
(Germany). The palladium, rhodium and iron precursors were aqueous solutions of PdCl2, RhCl3 and
Fe(NO3)3·9H2O, respectively. In the case of the bimetallic catalysts, a co-impregnation of both metals,
palladium or rhodium and iron, in a single path was carried out. The Pd and Rh loads were adjusted to a
nominal 1% (w/w), whereas the Fe load was fixed at 4% (w/w). After impregnation, all the solids were
left for 2 h at room temperature, dried for 12 h at 60 ºC and finally calcined at different temperatures,
within the range of 200 to 400 ºC in air atmosphere for 4 h [31]. After calcination, the catalyst were
reduced in H2 flow (50 N mL·min-1) for 2 h. Influence of reduction temperature within the range of 150
to 350 ºC was evaluated for the Fe/γ-Al2O3 catalyst [31], whereas the catalysts containing precious
metals were reduced at 350 ºC.
The porous structure of the fresh and used catalysts was characterized from nitrogen
adsorption-desorption at -196 ºC using a Micromeritics Tristar 3020 apparatus. The samples were
previously outgassed overnight at 150 ºC to a residual pressure of <10-3 Torr. Bulk rhodium, palladium
and iron contents were determined by inductively coupled plasma (ICP/MS Elan 6000 Perkin-Elmer
Sciex) after digestion of the samples with HNO3/HCl mixture. Elemental analyses of the fresh and used
catalysts were carried out in a LECO CHNS-932 Elemental Analyzer. The chlorine content of the
catalysts was determined by total reflection X-ray fluorescence (TXRF), using a TXRF spectrometer
8030c. Surface iron, palladium and rhodium species were analyzed by X-ray Photoelectron
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Spectrometer with a Thermo Scientific K-Alpha apparatus equipped with a Kα X-ray excitation source,
1486.68 eV. The software “XPS-Peak” was used for spectrograms deconvolution. The surface metal
content was calculated from the area of the spectral peaks. Dispersion and metal particle size of the
catalysts were determined from CO chemisorption at room temperature in a Micromeritics Chemisorb
2750 automated system equipped with Chemisoft TPx software. Stoichiometry of the CO adsorption
over the Pd and Rh atoms was assumed to be 1:1 [35, 36]. Table 1 summarizes the catalysts prepared as
well as their measured bulk metal content and BET surface area.
2.2.Experimental procedure
Hydrodechlorination of 4-CP was carried out in aqueous phase at starting concentrations of 200 mg·L-
1. HDC runs were performed in a glass slurry-type reactor (500 mL) where hydrogen was continuously
fed at 50 N mL·min-1. A catalyst concentration of 2 g·L-1 and 30 ºC temperature were always used. The
existence of mass transfer limitations was discarded after some preliminary experiments where the
effect of the catalyst particle size (50-200 μm) and stirring velocity (400-900 rpm) were checked. The
initial rate values were independent of particle size below 150 μm and of the stirring velocity within the
range of 600 to 900 rpm. Thus, to avoid internal and external mass transfer limitations the catalyst was
used in powdered form (dp <100 μm) and the stirring velocity at 700 rpm. Each run was carried out by
triplicate being the standard deviation less than 5% in all cases.
Preliminary blank experiments were conducted which confirmed the very low adsorption of 4-CP by
all the catalysts prepared in this work (less than 5% reduction of the initial concentration).
2.3.Analytical methods
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The progress of the HDC process was followed by periodically withdrawing and analyzing liquid
samples from the reactor. The catalyst was separated by filtration using a PTFE filter (pore size 0.45
μm). 4-CP, phenol and cyclohexanone were analyzed by GC/FID (430-GC, Varian) using a 30 m length
and 0.25 mm i.d. capillary column (CP-Wax 52 CB, Varian) and nitrogen as carrier gas. Chloride was
quantified by ion chromatography (Metrohm 790 Personal IC). A Metrosep A supp 5-250 column
(25 cm length, 4 mm internal diameter) was used as stationary phase and a 3.2 mM Na2CO3 aqueous
solution as the mobile phase. Total Organic Carbon (TOC) was measured using a TOC analyzer
(Shimadzu, mod. TOC, VSCH).
3. Results and discussion
3.1.Catalytic hydrodechlorination of 4-CP with Pd/γ-Al2O3 and Rh/γ-Al2O3 catalysts
Figure 1 shows the evolution of 4-CP conversion and the selectivity to reaction products upon
reaction time in the HDC of 4-CP (100 mg·L-1 aqueous solution) with the monometallic Pd/γ-Al2O3 and
Rh/γ-Al2O3 catalysts prepared at 200 ºC and 350 ºC calcination and reduction temperature, respectively.
With the Pd catalyst, almost 90% dechlorination was achieved in 5 min reaction time whereas with the
Rh catalyst only around 50% was reached at that time. However, both catalysts allowed complete
dechlorination after 30 min reaction time. The Pd/γ-Al2O3 catalyst gave rise to phenol (Ph) as the major
reaction product. The selectivity toward phenol decreased with time as increasing amounts of
cyclohexanone (C-one) were formed. These results are consistent with a consecutive reaction pathway
where 4-CP reacts with hydrogen to produce phenol, which is further hydrogenated to cyclohexanone.
In the case of the Rh/γ-Al2O3 catalyst, 4-CP was converted into phenol within the first minutes of
reaction and it was rapidly hydrogenated to produce cyclohexanone, which was further hydrogenated to
cyclohexanol (C-ol). At the end of the experiment (3h), the selectivity toward cyclohexanol was almost
100%. A high selectivity toward this compound is desirable from an environmental point of view since
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it is, by far, the less ecotoxic compound among the HDC byproducts [3, 37, 38]. The carbon and
chlorine balances, based on the identified species, were closed at 95% and 100 %, respectively. Due to
the HCl formation during HDC of 4-CP the pH of the reaction medium decreased from the initial value
(pH0 = 6) to ≈3.5. Based on these results the reaction schemes of Figure 2 are proposed. Those schemes
differ from the previously reported by Diaz et al. (2008) [3] with commercial Pd/γ-Al2O3 and Rh/γ-
Al2O3 catalysts under the same operating conditions. They proposed a series-parallel pathway where 4-
CP was converted into phenol and cyclohexanone but this last was also produced by phenol
hydrogenation as well as cyclohexanol. However and most important, in that study complete
dechlorination of 4-CP was not achieved even after 3 h reaction time and cyclohexanol was formed only
in low amounts. Molina et al. (2014) [18] have recently reported results more similar to our current ones
working with Pd and Rh supported on pillared clays.
From the reaction schemes of Figure 2, the following kinetic model is proposed, based on simple
pseudo first order rate equations, where the H2 (high excess) and catalyst concentration (always 2 g·L-1)
are included in the kinetic constants.
(1)
(2)
(3)
(4)
Table 2 reports the values of the apparent rate constants obtained by fitting the models to the
experimental results using Scientist 3.0 software. In all cases a frankly good fit of the experimental data
to the proposed model was observed. Looking at the values of kH1, it can be concluded that the Pd
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catalyst shows a higher hydrodechlorination activity, in contrast with the results reported by Diaz et al.
(2008) [3] with commercial Pd/Al2O3 and Rh/Al2O3 catalysts where close similar activities were
observed for both metals. Molina et al., (2014) [18] found that Rh was more active than Pd working
with pillared clays-supported catalyst under similar conditions. Nevertheless, they explained that fact by
the higher dispersion values of the Rh catalyst whereas Pd yielded a higher specific activity. The values
of kH2 in Table 2 reveal a significantly higher activity of the Rh/γ-Al2O3 catalyst in the hydrogenation of
phenol, giving rise to a much higher selectivity to cyclohexanone in the initial stages of the reaction.
Finally, cyclohexanone was further hydrogenated to cyclohexanol with the Rh catalyst but that rate
constant (kH3) was fairly lower than kH1 and kH2.
3.2.Influence of calcination temperature
Figures 3 and 4 show the evolution of 4-CP and the reaction products upon HDC with the Pd and Rh
catalysts, respectively, prepared at two different calcination temperatures (200 ºC and 400 ºC). As can
be seen, increasing the calcination temperature increases the activity of the Pd catalyst for
hydrodechlorination as well as for further hydrogenation of the phenol resulting from that first step.
Nevertheless, in the case of Rh no significant changes were observed. The values of the apparent rate
constants collected in Table 3 for the Pd and Rh catalysts prepared at different calcination temperatures
confirm the above conclusions on the effect of that variable.
In contrast to the significant influence of the calcination temperature on the catalyst selectivity of
Pd/γ-Al2O3, the reduction temperature (varied in the range of 150 to 350 ºC) showed a negligible effect.
Thus, we focused our attention on the characterization and performance of the catalysts obtained
varying the calcination temperature.
To explain the results obtained, the catalysts were characterized in terms of metal dispersion, mean
metal particle size (both from CO chemisorption) and the ratio of zerovalent to electrodeficient metal
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species (M0/Mn+) by X-ray photoelectron spectroscopy (XPS). Table 4 summarizes the results obtained.
The core-level spectra of Pd 3d and Rh 3d are given in Figures 5 and 6, respectively. The deconvolution
of those XPS profiles shows the existence of zerovalent and electrodeficient species in both catalysts.
The zerovalent (Pd0) and electrodeficient (Pdn+) bands were centered at binding energy values of 335
and 337.5 eV, respectively [34, 39, 40]. A number of authors [18, 25, 41] established that both zero-
valent (Pd0) and electrodeficient (Pdn+) palladium species are required for hydrodechlorination of
organochlorinated compounds. Increasing the calcination temperature from 200 to 400 ºC led to a
significant increase of the Pd0/Pdn+ ratio from 1.15 to 2.17 (Table 4). Thus, the presence of higher
proportion of Pd0 seems to improve the catalytic activity. A higher calcination temperature led also to a
significant increase of the metal dispersion and a reduction of the mean size of Pd particles, as reported
in Table 4. This fact seems to be also associated with the higher activity of the catalyst calcined at 400
ºC, which allowed a faster dechlorination and a higher selectivity towards the most hydrogenated
reaction product, cyclohexanone. Baeza et al., (2012) [25] demonstrated that there is a relationship
between the Pd0/Pdn+ ratio and the mean particle size, showing the smallest particles (2.7-4.2 nm) the
highest Pd0/Pdn+ ratios (2.86-1.49). They also reported that decreasing the particle size of Pd leads to an
increase of catalytic activity in the aqueous-phase HDC of 4-CP. According to several studies [42-44],
the activation of Pd/Al2O3 under an O2 atmosphere is due to reconstruction. Palladium particles
reconstruct by breaking large particles into both PdO dispersed on Al2O3 and large Pd particles with
PdO on the metal surface. In this sense, Zou and Gonzalez (1992) [45] also found that pre-treatment of a
Pd/SiO2 catalyst in oxygen at temperatures in the range of 100-300 ºC led to higher dispersion of around
40%. Similarly, Cordi and Falconer (1996) [46] established that heat treatment under oxygen of
Pd/Al2O3 catalyst at 500 ºC prior to the reduction step in H2 redispersed Pd on the surface.
On the other hand, although decreasing Pd particle size increases the catalytic activity due to higher
surface area, the larger particles obtained at low calcination temperatures show a more active surface
looking at the intrinsic activity (expressed as turnover frequency, TOF) (Table 3). These results are in
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good agreement with those reported by Baeza et al. (2012) [25], who also obtained the highest activity
values with the smallest nanoparticles but the largest ones exhibited higher activity per unit of metal
surface. The higher catalytic activity showed by the largest nanoparticles can be related to a
combination of different factors. According to Karpinski et al. (1996) [47], the higher activity of large
particles compared to small ones can be ascribed to geometric factors since the hydrodechlorination
reaction requires relatively large ensembles, the number of which increases with increasing particle size.
Other authors claimed the importance of electronic factors and postulate that large electron-rich
particles may assist in the cleavage of C-Cl bonds through electrons transfer to the carbon atom, which
will bear a partial positive charge owing to the electronegative character of the chlorine atom [48, 49].
Alternatively, the differential solubility of hydrogen in large and small palladium particles could also
play an important role since large palladium particles adsorb hydrogen to a greater extent than smaller
ones [50, 51]. Finally, the better resistance of larger Pd particles to inhibition/poisoning by chlorine
should also be considered [25, 52].
The deconvolution of the core-level spectra of Rh 3d also shows the presence of zerovalent (Rh0) and
electrodeficient (Rhn+) rhodium species, centered at binding energy values of 307.7 and 308.9 eV,
respectively. However, the Rh0/Rhn+ ratio remained practically unchanged with the calcination
temperature and the same can be said for the metal dispersion and mean particle size. These results are
consistent with the observed non-significant effect of calcination temperature on the activity of the Rh
catalyst.
3.3.Magnetic catalysts
The development of magnetic properties on the catalysts can be of interest for the sake of an easier
separation from the liquid phase. Thus, we have checked that possibility upon the introduction of iron.
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In a first approach a monometallic Fe/γ-Al2O3 catalyst prepared at a calcination temperature of 400 ºC
was tested in the HDC of 4-CP. This catalyst did not show any significant dechlorination activity.
Golubina et al. (2006) [32] and Choi et al. (2008) [53] arrived to the same conclusion with other iron-
based catalysts. Thus, the introduction of iron on the Pd and Rh catalysts tested so far is not expected to
improve their activity but only developing magnetic properties useful for further separation from the
reaction medium. In that sense, the reduction step must be critical and, then, different temperatures
within 150-350 ºC have been tested for that step of the preparation of bimetallic Pd-Fe and Rh-Fe
catalysts.
The development of magnetic properties was observed in the catalysts prepared at the highest
reduction temperature (350 ºC). Calcination was accomplished always at 400 ºC. Figure 7 shows the Fe
3p core level XPS spectra of the bimetallic Pd-Fe/γ-Al2O3 catalysts prepared at three different reduction
temperatures. The bands corresponding to Fe (III) and Fe (II) are centered at binding energy values
around 55.6 and 53.8 eV, respectively [54]. In the catalyst reduced at 150 ºC only the presence of Fe
(III) was detected while increasing the reduction temperature led to higher amounts of Fe (II) in the
catalyst. In fact, the Fe (II):Fe (III) ratio calculated for the catalyst reduced at 350 ºC was 0.45, similar
to the theoretical stoichiometric ratio of magnetite [54].
The results obtained in the hydrodechlorination of 4-CP with the catalysts with and without Fe,
reduced at 350 ºC and calcined at 400 ºC are summarized in Table 5. As can be seen, the addition of
iron in both the Pd- and Rh increased the dechlorination rate (kH1 value) but decreased the rates of
further hydrogenation (kH2 and kH3), especially in the case of the Pd catalyst, where a significant
decrease of the selectivity to cyclohexanone was observed. Thus, in spite of the fact that the Fe/γ-Al2O3
catalyst did not show any significant dechlorination activity, a synergistic effect seems to occur upon the
introduction of Fe on the Pd and Rh catalysts. This increase of hydrodechlorination activity can be
related to the combination of Fe and Pd or Rh, since the Pd-Fe interface has been reported to enhance
hydrogen spill over [34].
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Data on the characterization of the catalysts by XPS and CO chemisorption are included in Table 6.
The addition of Fe led apparently to a lower dispersion of the active phase in both the Pd and Rh
catalysts. However, the metal dispersion has been evaluated from CO chemisorption and the decreased
CO uptake associated to the introduction of iron can be due to surface encapsulation of Pd and Rh by
large Fe aggregates, thus reducing the availability of Pd and Rh sites. That could explain the lower
hydrogenation activity of the Fe-bearing catalysts, in agreement with previous results in the literature
[34, 55]. The decreased CO uptake of the Fe-bearing catalysts may be due also to the formation of Pd-
Fe and Rh-Fe alloys [56]. Witonska et al. (2014) [57] also found a significant decrease on CO
chemisorption when iron was incorporated onto monometallic Pd/Al2O3 catalysts. The dispersion
decreased from 23% to 3% in 5%Pd/Al2O3 and 5%Pd-20%Fe/Al2O3 catalysts, respectively.
Accordingly, they observed a decrease of the hydrogenation activity from phenol to cyclohexanone
although they also found a higher dechlorination activity. This fact was explained by the overlapping
effects of active phase dispersion, surface structure and chemical composition.
On the other hand, the ratio of zerovalent to electrodeficient metallic species increased somewhat in
the Pd catalyst upon the introduction of iron and that may partially explain its higher
hydrodechlorination activity. Nevertheless, the opposite was observed in the Rh catalyst. The values of
the M0/Mn+ ratio, obtained from the XPS spectra, can be affected by the abovementioned encapsulation
of Pd and Rh by Fe aggregates and the formation of Pd-Fe and Rh-Fe alloys, thus reducing the
conclusive value of those data.
Elemental analyses confirmed that the deposition of carbon-containing species on the surface of the
catalysts was almost negligible (<0.5%). The presence of chlorine over the catalysts was also discarded
by TXRF analyses. The values of the BET surface area as well as those of the bulk metal content are
summarized in Table 7. As observed, the BET surface area of the catalysts remained unchanged after
use (see Table 1). Regarding the metal content of the catalysts, it has to be pointed out that there was no
significant palladium or rhodium loss during the 3h-HDC experiments whereas iron leaching reached
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around 10% in all cases (see values of Table 1). In this sense, further research is needed to improve the
anchorage of Fe on the catalysts surface trying to reduce Fe leaching. Golubina et al., (2006) [32]
showed that during HDC iron oxides react with HCl giving rise to FeCl3, whose formation is
thermodynamically favored respect to PdCl2. In this sense, they concluded that the stability of bimetallic
Pd-Fe catalysts is higher than that of the monometallic Pd ones since the poisoning effect of HCl on Pd
active sites would be substantially reduced upon removal of chlorine as FeCl3. They also explained the
better performance of the bimetallic Pd-Fe catalysts by the stabilization of small metal particles as well
as the formation of alloys, which could change the geometry of the active sites and thus modify the
activity of the catalyst. Finally, it has to be considered as well that HCl can also interact with the γ-
Al2O3 support, thus preventing the poisoning of Pd active sites [58].
These authors showed that during HDC iron oxides react with HCl giving rise to FeCl3, whose
formation is thermodynamically favored respect to PdCl2. In this sense, they concluded that the stability
of bimetallic Pd-Fe catalysts is higher than that of the monometallic Pd ones since the poisoning effect
of HCl on Pd active sites would be substantially reduced by elimination of chlorine as FeCl3. They also
supported the better performance of bimetallic Pd-Fe catalysts with regard to the Pd monometallic ones
by the stabilization of small metal particles as well as the formation of alloys, which could change the
geometry of the active sites and thus, modify the activity of the catalyst
With regard to the magnetic properties of the Fe-bearing catalysts, it has to be highlighted that those
reduced at 350 ºC were easily separated from the reaction medium by applying an external magnetic
field.
4. Conclusions
The temperature of calcination showed a significant effect on the activity of own-prepared Pd/γ-Al2O3
catalyst in the HDC of 4-CP. Increasing that temperature within the 200-400 ºC range showed to
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increase the dechlorination activity as well as that of further hydrogenation of the resulting phenol. This
can be explained by the increase of the metallic dispersion (with reduction of the mean metal particle
size) and the M0/Mn+ ratio. However, the larger particles formed at the lowest calcination temperature
showed a more active surface looking at the activity per unit surface area. On the opposite, the
calcination temperature did not affect to the activity of the Rh catalysts. These lasts showed a lower
dechlorination activity but were more active for further hydrogenation, leading to cyclohexanol as the
final reaction product with almost 100 % selectivity after 3h. Both, the Pd and Rh catalysts allowed
complete dechlorination of 4-CP in short reaction time (< 15 min) under ambient-like conditions.
The introduction of iron on the Pd and Rh catalysts allows developing strong magnetic properties
upon reduction at 350 ºC without impairing their activity, which was even improved for the
dechlorination step. Those magnetic properties are useful for easier separation of the catalyst from the
liquid phase after use. It has to be also highlighted that the catalysts showed a high stability with
negligible Pd leaching. The γ-Al2O3 support as well as Fe can react with HCl, preventing the poisoning
of Pd.
Acknowledgments
This research has been supported by the Spanish MICINN through the project CTQ2008-03988 and
CTQ2010-14807 and by the CM through the project S-2009/AMB-1588.
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Page 19 of 35

Figure captions
Figure 1. Time-evolution of 4-chlorophenol conversion and selectivity to reaction products in the HDC
of 4-CP with the Pd (a) and Rh (b) catalysts.
Figure 2. Reaction pathways for 4-CP HDC with the Pd (a) and Rh (b) catalysts.
Figure 3. HDC of 4-CP with the Pd/γ-Al2O3 catalyst prepared at 200 ºC (a) and 400 ºC (b) calcination
temperature. Experimental (symbols) and predicted (solid lines) values.
Figure 4. HDC of 4-CP with the Rh/γ-Al2O3 catalyst prepared at 200 ºC (a) and 400 ºC (b) calcination
temperature. Experimental (symbols) and predicted (solid lines) values.
Figure 5. Deconvolution of the Pd 3d core level spectra of Pd/γ-Al2O3 catalysts prepared at 200 ºC (a)
and 400 ºC (b) calcination temperature.
Figure 6. Deconvolution of the Rh 3d core level spectra of Rh/γ-Al2O3 catalysts prepared at 200 ºC (a)
and 400 ºC (b) calcination temperature.
Figure 7. Deconvolution of the Fe 3p core level spectra of Pd-Fe/γ-Al2O3 catalysts reduced at 150 ºC
(a), 250 ºC (b) and 350 ºC (c). The insets show photographs of the catalysts separated by a magnet.
21
Page 20 of 35

Table 1
Table 1. Characterization of the own-prepared catalysts used in this work.
Catalysts
Preparation conditions
Bulk metal content (wt SBET
%)
Pd
(m2·g-1)
Calcination
Reduction
temperature (ºC)
350
Rh
--
Fe
--
temperature (ºC)
Monometallic
200
300
400
200
300
400
200
300
400
1.1
--
129
129
131
128
130
131
135
135
136
137
136
128
129
Pd/γ-Al2O3
Rh/γ-Al2O3
Fe/γ-Al2O3
350
350
1.0
--
--
--
4.1
150
250
350
350
350
Bimetallic
400
400
1.0
--
--
4.1
4.0
Pd-Fe/γ-Al2O3
Rh-Fe/γ-Al2O3
0.9
22
Page 21 of 35

Table 2
Table 2. Values of the apparent rate constants (L·h-1·gcat-1) of equations 1-4.
Catalyst
kH1
10.6
3.8
kH2
0.03
4.9
kH3
--
r2
0.999
0.999
Pd/γ-Al2O3
Rh/γ-Al2O3
0.6
23
Page 22 of 35

Table 3
Table 3. Values of the apparent rate constants (L·h-1·gcat-1) and TOF (min-1) for the catalysts prepared at
different calcination temperatures.
Catalysts
Pd/γ-Al2O3
Rh/γ-Al2O3
Calcination
TOF
kH1
kH2
kH3
r2
temperature (ºC)
200
300
400
200
300
400
10.6
12.6
16.2
3.8
0.03
0.06
0.09
2.5
--
--
0.999
0.997
0.998
0.999
0.992
0.989
6.7
5.1
4.6
1.5
1.5
1.4
--
0.6
0.5
0.5
3.8
4.8
3.7
4.1
24
Page 23 of 35

Table 4
Table 4. Dispersion values, mean metal particle size and M0/Mn+ ratio of the catalysts prepared at
different calcination temperatures.
Calcination
Catalyst
Dispersion (%) d (nm) M0/Mn+
temperature (ºC)
200
300
400
200
300
400
20.9
32.5
46.2
34.3
34.9
39.9
5.4
3.5
2.4
3.2
3.1
2.8
1.15
1.45
2.17
1.22
1.25
1.26
Pd/γ-Al2O3
Rh/γ-Al2O3
25
Page 24 of 35

Table 5
Table 5. Results from the HDC of 4-CP and values of the apparent rate constants (L·h-1·gcat-1) with the
catalysts reduced at 350 ºC, with and without Fe.
Catalyst
X4-CP
SPh
SC-one
SC-ol
kH1
kH2
kH3
r2
100
100
100
100
55.2
84.9
0.0
44.7
15.1
2.06
0.1
0.0
0.0
16.2 0.09
23.6 0.04
--
--
0.998
0.999
0.989
0.989
Pd/γ-Al2O3
Pd-Fe/γ-Al2O3
Rh/γ-Al2O3
97.9
99.9
3.7
6.3
4.1
2.4
0.5
0.6
0.0
Rh-Fe/γ-Al2O3
26
Page 25 of 35

Table 6
Table 6. Dispersion values and M0/Mn+ ratio of the catalysts reduced at 350 ºC, with and without Fe.
Dispersion
Catalyst
M0/Mn+
(%)
46.2
8.3
2.17
2.38
1.32
1.03
Pd/γ-Al2O3
Pd-Fe/γ-Al2O3
Rh/γ-Al2O3
39.9
14.5
Rh-Fe/γ-Al2O3
27
Page 26 of 35

Table 7
Table 7. Metal content and BET surface area of the used catalysts.
Bulk metal content
Catalyst
(% w/w)
Rh
SBET (m2·g-1)
Pd
1.0
1.0
--
Fe
--
--
--
130
126
129
127
Pd/γ-Al2O3
Pd-Fe/γ-Al2O3
Rh/γ-Al2O3
3.7
--
1.0
0.9
--
3.6
Rh-Fe/γ-Al2O3
28
Page 27 of 35

*Graphical Abstract (for review)
Graphical abstract
Pd
Rh
-Al2O3
Pd/-Al2O3
Pd/-Al2O3
Rh/-Al2O3
0,9
0,8
0,7
0,6
0,5
0
Rh/-Al2O3
/ M
n +
M
5
4
200
250
200 ºC
300 ºC
400 ºC
3
C
300
a
l
c
i
n
350
a
t
i
o
n
400
2
t
e
m
p
e
Calcination temperature
r
a
t
u
r
e
(
؛
C
)
Page 28 of 35

Figure 1 revised
Figure 1
100
80
60
40
20
100
80
60
40
20
0
a)
b)
4-CP
Ph
4-CP
Ph
C-one
C-one
C-ol
0
0
30
60
90
120
150
180
30
60
90
120
150
180
time (min)
Page 29 of 35