Pd-Cu catalysts supported on anion exchange resin for the simultaneous catalytic reduction of nitrate ions and reductive dehalogenation of organochlorinated pollutants from water

Article abstract of DOI:10.1016/j.apcata.2018.11.002

The present work proposes the simultaneous removal of these classes of pollutants by a catalytic hydrotreatment processes. For this purpose, bimetallic Pd-Cu catalysts (with mass ratio Pd:Cu of 4:1) supported on macroporous strong base anion resin were prepared by different methods. The catalysts were characterized (by XRD, SEM-EDX, XPS, AAS and H₂ chemisorption) and tested in a continuous flow system. The selected catalyst preparation protocol consists in a two-step method, which implies the deposition of palladium by ion exchange and the subsequent deposition of copper by controlled reaction on the surface of the pre-reduced palladium. The effectiveness of the catalyst in the simultaneous reduction of nitrate and hydrodechlorination of 4-chlorophenol was demonstrated. By adjusting the initial pH and the flow rate of the aqueous solution, nearly complete hydrodechlorination of 4-chlorophenol can occur together with selective nitrate reduction at a conversion of 95% and a selectivity to N₂ of 92% (this value contains the contribution of all gaseous products, including the eventually formed NO_x). The bimetallic catalyst was found to remains relatively stable after 100 h of test time.

Full text of DOI:10.1016/j.apcata.2018.11.002

Accepted Manuscript
Title: Pd-Cu catalysts supported on anion exchange resin for
the simultaneous catalytic reduction of nitrate ions and
reductive dehalogenation of organochlorinated pollutants from
water
˘ ˘
Authors: Corina Bradu, Constantin Capa¸t, Florica Papa, Ligia
´
Frunza, Elena-Alina Olaru, Gregorio Crini, Nadia
´
˘
Morin-Crini, Elise Euvrard, Ioan Balint, Irina Zgura, Cornel
Munteanu
PII:
S0926-860X(18)30549-0
DOI:
Reference:
https://doi.org/10.1016/j.apcata.2018.11.002
APCATA 16871
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
5 August 2018
29 October 2018
2 November 2018
˘ ˘
Please cite this article as: Bradu C, Capa¸t C, Papa F, Frunza L, Olaru E-Alina, Crini G,
´
˘
Morin-Crini N, Euvrard E, Balint I, Zgura I, Munteanu C, Pd-Cu catalysts supported
on anion exchange resin for the simultaneous catalytic reduction of nitrate ions and
reductive dehalogenation of organochlorinated pollutants from water, Applied Catalysis
A, General (2018), https://doi.org/10.1016/j.apcata.2018.11.002
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Pd-Cu catalysts supported on anion exchange resin for the simultaneous
catalytic reduction of nitrate ions and reductive dehalogenation of
organochlorinated pollutants from water
Corina Bradua,d,*, Constantin Căpăța, Florica Papab, Ligia Frunzac, Elena-Alina
Olarua, Grégorio Crinid, Nadia Morin-Crinid, Élise Euvrardd, Ioan Balintb, Irina
Zgurăc, Cornel Munteanub
a University of Bucharest, PROTMED Research Centre, Splaiul Independenței 91-95, sect. 5,
050107 Bucharest, Romania, corina.bradu@g.unibuc.ro
b Institute of Physical Chemistry of the Romanian Academy, 060021 Bucharest, Romania
c National Institute of Material Physics,077125 Măgurele, Romania
d Université de Franche-Comté, UMR 6249 Chrono-environnement usc INRA, Faculté des
Sciences & Techniques, 25030 Besançon cedex, France
Graphical abstract

Highlights




The deposition procedure of second metal strongly affects Pd-Cu/resin performances;
A two-step catalyst synthesis protocol: ion exchange-surface reduction was selected;
-
Pd-Cu catalyst is effective on simultaneous NO3 reduction and 4-CP dechlorination;
The catalyst shows good stability for 100 hours of test time.
Abstract
The present work proposes the simultaneous removal of these classes of pollutants by a
catalytic hydrotreatment processes. For this purpose, bimetallic Pd-Cu catalysts (with mass
ratio Pd:Cu of 4:1) supported on macroporous strong base anion resin were prepared by
different methods. The catalysts were characterized (by XRD, SEM-EDX, XPS, AAS and H2
chemisorption) and tested in a continuous flow system. The selected catalyst preparation
protocol consists in a two-step method, which implies the deposition of palladium by ion
exchange and the subsequent deposition of copper by controlled reaction on the surface of the
pre-reduced palladium. The effectiveness of the catalyst in the simultaneous reduction of
nitrate and hydrodechlorination of 4-chlorophenol was demonstrated. By adjusting the initial
pH and the flow rate of the aqueous solution, nearly complete hydrodechlorination of 4-
chlorophenol can occur together with selective nitrate reduction at a conversion of 95% and a
selectivity to N2 of 92% (this value contains the contribution of all gaseous products,
including the eventually formed NOx). The bimetallic catalyst was found to remains
relatively stable after 100 hours of test time.
Keywords
Pd-Cu catalyst, anion exchange resin, nitrate reduction, hydrodechlorination, water treatment

1. Introduction
Nitrate ions and organochlorine compounds such as pesticides and their degradation
by-products are pollutants of concern found in natural waters originating mainly from
agricultural areas [1].
The nitrogen cycle is altered by anthropogenic activities, particularly by the intensive
agricultural practices, which lead to a gradual increase of nitrate ions concentration in natural
water bodies. High concentration of nitrates in surface water negatively affects aquatic
ecosystems through its toxic effect on aquatic fauna or by water eutrophication. Excess
nitrates in groundwater (frequently used as a drinking water source) can lead to enhanced
exposure of human population being responsible for specific diseases such as
methemoglobinemia and several types of cancer [2,3].
Contamination of natural waters by pesticide residues, especially organochlorine
compounds, also causes considerable concern. Despite their usefulness in agriculture,
pesticides may produce a wide range of toxic side effects representing a potential hazard to
the environment. Organochlorine compounds are often characterized by high toxicity and
persistence, being designated as priority pollutants [4]. They are readily bioaccumulated in
aquatic and terrestrial organisms, being notorious for their ability to biomagnify in the food
chain.
Therefore, significant research efforts are devoted to the development of practical
methods which are able to effectively remove these pollutants from natural waters, especially
from those used as water supplies.
A series of physico-chemical and biological methods were developed for the
elimination of nitrate ions from water. Among these, catalytic reduction of nitrate is an
attractive technique that would be suitable for water treatment [5,6]. This method was
reported for the first time by Vorlop and Take [7] and presents several advantages. One of its

major benefits is that no waste is generated during the treatment process as in the case of ion
exchange, reverse osmosis or biological processes. Nevertheless, a crucial aspect remains the
catalyst selectivity issue because the presence of nitrite and ammonium ions by-products is
highly undesirable in drinking water.
Great efforts are made in order to obtain catalyst with high activity and selectivity
towards nitrogen formation. The catalysts developed for this purpose are mainly supported
bimetallic systems based on noble metals such as Pd, Pt, Rh and Ru, promoted especially
with Cu, Sn and In [8-11]. Pd-Cu is, by far, the most studied metallic pair for this application
and offers promising prospects [12]. Generally, Pd-Cu catalysts were found to have higher
activity and/or selectivity to nitrogen than Pd-Sn or Pd-In catalysts [13,14]. However, several
studies claim that Pd-Sn catalysts perform better than the Pd-Cu ones [15,16].
The nature of the support is also important, playing a crucial role in the activity and
selectivity of the catalytic system. Various carriers have been proposed, γ-Al2O3, SiO2, TiO2,
hydrotalcite and carbonaceous materials being among the most common ones [17-21]. Ion
exchange resins have also been considered promising support candidates. There is a relatively
small number of studies dedicated to the investigation of this type of carrier.
The use of ion exchange resins as catalyst carriers for the selective reduction of nitrate
was proposed by Gašparovičova et al. [22-24]. Both cation and anion exchange resins were
studied. Although the presence of the acid sites of the cation exchange resin contributes to
increase the selectivity towards N2 of the Pd-Cu supported catalysts (e.g. from 20%, for a
resin free of acid sites, to 60%, for a resin with acid sites), Pd-Cu catalysts deposited on anion
exchange resin showed higher specific activity (e.g. 94 % selectivity to nitrogen at 61%
nitrate conversion) [22,23]. The superior performances of the catalysts supported on anion
exchange resin were attributed to the higher mobility of the anions compared to cation
exchange resin support. It was also highlighted that the selectivity to nitrogen is higher on

anion exchange resin carriers than on γ-Al2O3 [24]. Comparable results were obtained by
Neyertz et al. (2010) for the reduction of nitrate ions on Pd-Cu/anion exchange resin, at a
-
more elevated concentration, 150 ppm NO3 -N (expressed as nitrogen from nitrate) instead of
-
-
the usually employed concentration of 22.6 ppm NO3 - N (100 ppm NO3 ). More recently, N2
selectivity of 100% for complete nitrate conversion was claimed for bimetallic Pd-Cu catalyst
supported on anion exchange resin [25]. This notable performance was achieved in a batch
-
reactor with NO3 previously retained on catalyst surface by ionic exchange and then
maintained for 6 to 24 h under 6 bar H2/CO2 pressure (hydrogen partial pressure of 3 bar) at
-
moderate temperature (298 K and 333 K). The selective conversion of NO3 to N2 is
-
explained by the fast retention of the HO-, resulted in NO3 hydrogenation process, on the ion
-
exchange sites of the resin vacated by NO3 consumption during reaction progress. A 100%
selectivity to nitrogen has never been reported previously. The only drawbacks of this method
could be the demanding time or temperature and pressure requirements in the regeneration
step.
To the best of our knowledge, the catalytic liquid-phase reduction of nitrate on
bimetallic catalysts supported on ion exchange resin was so far investigated in semi-batch or
batch reactors and no studies have been published on fixed-bed reactors operating in
continuous flow.
With respect to organochlorine compounds in water, several methods like adsorption
or advanced oxidation processes have been proposed for their removal [26,27]. An alternative
approach might be their catalytic hydrodechlorination which leads to less toxic organic
compounds, thereby reducing the ecotoxicity of the treated water. Noble metals,
preponderantly Pd, on various supports have been studied for the liquid phase
hydrodechlorination of organochlorine compounds from water such as organochlorine
pesticides, chlorophenols and chloroacetic acids [11,28-30]. Complete dehalogenation of

mono- and poly-chlorinated organic compounds in aqueous solution (e.g. chloroacetic acids
and chlorophenols at 0.02 - 0.2 mM) has been achieved in the presence of palladium-based
catalysts using hydrogen under ambient temperature and pressure [29,31,32]
The studies reported in the literature dedicated to treatment of poly-contaminated
waters are scarce. Preliminary catalytic reduction tests with tri- and tetra-chloroethylene
added to a nitrate solution were performed by Centi and Perathoner [33]. They reported a
conversion of 90% for chlorinated compounds (no other information provided) and suggested
the possibility of simultaneous removal of nitrate and halogenated compounds. Several
studies investigated the influence of nitrate and other inorganic anions on the aqueous phase
reduction of organochlorine compounds with zero-valent iron or with hydrogen in the
presence of Pd supported catalysts [34-38]. Different results regarding the influence of
nitrate on the hydrodehalogenation of organic compounds on Pd/Al2O3 catalysts were
reported. It was found that, the presence of nitrate at moderate to high concentrations (0.37 –
1.29 mM) had no effect on the hydrodechlorination rate of trichloroethylene (0.031 mM)
[37]. Other authors concluded that the hydrodehalogenation of 1, 2-dibromo 3-chloropropane
(0.018 mM) was strongly affected by the presence of 0.45 mM nitrate, which lowered the
conversion of halogenated compounds with about 50 % [38]. The absence of any effect of
-
NO3 on trichloroethylene dehalogenation was explained by the lack of activity of
monometallic palladium catalyst on nitrate reduction (no nitrate conversion). In contrast, in
-
the study of hydrodehalogenation of 1, 2-dibromo 3-chloropropane, 90% of NO3 was
removed.
This work assesses the effectiveness of catalytic hydrotreatment process for
simultaneous removal of nitrate and 4-chlorophenol in aqueous solution. For this purpose,
bimetallic Pd-Cu catalysts supported on macroporous strong base anion resin were prepared
by different methods, characterized and tested using a continuous flow system. The reasons

for choosing anion exchange resin as support are: (i) to ensure a high dispersion and
-
controlled distribution of the active components; (ii) to enable the NO3 internal diffusion.
2. Experimental
2.1. Bimetallic Pd-Cu catalysts synthesis
In this work, a series of bimetallic Pd-Cu catalysts (with mass ratio Pd:Cu of 4:1)
supported on an anion exchange resin was prepared. The synthesis of bimetal-resin catalysts
was carried out via two main routes: (i) introduction of both metals by ion exchange with the
functional groups of the resin; (ii) introduction of palladium by ion exchange and subsequent
deposition of copper by controlled surface reaction. The carrier was a commercial anion
exchange resin, A-520E Purolite, having poly-(styrene di-vinyl benzene) matrix (S-DVB)
+
with quaternary ammonium functional groups (-CH2-N(CH3)3 ) (ion exchange capacity of 2.8
molc∙kg-1; ionic form: Cl-) and a macroporous texture (average pore size 50 nm), kindly
provided by Purolite Romania.
For the ion exchange route, the metal precursors were tetrachloropalladate(II) and
tetrachlorocuprate(II) complexes (Reaction 1). The precursor solutions were obtained by
dissolution of PdCl2 (p.a. – Fluka) and CuCl2∙2H2O (p.a. – AppliChem) in a HCl solution (0.1
– 8.0 mM). The copper deposition was carried out simultaneously or following palladium
deposition. The obtained catalysts are denoted as PdCu-1 and PdCu-2 respectively (Table 1).
For the simultaneous deposition method, the dried A520E resin in the Cl- ionic form (for 24
h, at 50oC) was contacted with a solution containing both metal precursors and maintained
under continuous mixing (GFL Test Tube Rotator 3025, at 15 rpm) for 0.5 – 24 h. The
reduction of metals previously retained on the resin by ion exchange was achieved using a
solution of sodium formate (1.5 M) (contact time of 0.5 h, at 50oC). Finally, the obtained
catalyst was washed with demineralized water until the complete elimination of residual Cl-

and HCOO- ions (verified through ion chromatography). In the case of successive metal
deposition by ion exchange, an intermediate reduction step with HCOONa was realised
between the palladium and the copper retention stages (Table 1).
+
+
2(S-DVB)-N(CH3)3 Cl- + [MCl4]2- → [(S-DVB)-N(CH3)3 ]2[MCl4]2- + 2Cl-
(1)
where M = Pd or Cu
The second preparation route consisted in a controlled surface reaction in which
copper ions were reduced by the hydrogen adsorbed on the pre-reduced Pd surface. (Reaction
2) [39]. Thus, after the retention of palladium by ion exchange and its reduction with sodium
formate (stages described in the precedent protocol), a Cu(NO3)2 solution ([Cu(NO3)2] = 3.0
g∙L-1, pH < 3) was contacted with the monometallic precursor for 0.5 h. at 50oC. The obtained
catalyst, denoted as PdCu-3 (Table 1), was washed with demineralized water until the
-
complete elimination of residual NO3 , Cl- and HCOO- ions (verified through ion
chromatography).
2Pd-H + Cu2+ → Pd2Cu + 2H+
(2)
The theoretical concentration of Pd and Cu in the bimetallic catalysts was 2% and
0.5%, respectively. Monometallic catalyst with 2% Pd (denominated Pd-0) was also prepared
for comparison.
2.2. Catalysts characterization
The exposed Pd surface area on resin was estimated by H2 chemisorption at 25oC
using a ChemBet-3000 Quantachrome Instrument equipped with a thermal conductivity

detector (TCD). Before the H2 chemisorption measurements, the catalysts were reduced with
H2 at 70oC for 1h then cooled to room temperature under Ar flow to remove the adsorbed H2
on the catalyst surface.
Atomic absorption spectrometry (AAS) was used to determine the metal loading on
catalysts. A Thermo Elemental, Solaar M5 atomic absorption spectrometer with flame
atomizer was used in absorption mode (measuring time = 4 s; air: acetylene flow = 1.2
L⸱min-1; bandwidth = 0.5 nm; background - D2 Quadline lamp). For the analysis, the
samples were digested in a microwave digester (Ethos Sel, Milestone) using a mixture of
hydrogen peroxide and HNO3 : HCl (3:1).
The crystalline structure of the active phase constituents was analysed with D8
Advance Bruker-AXS diffractometer using Cu Kα radiation (k = 1.54 Å). The diffraction
patterns were recorded in the 2Ɵ = 30−135° domain. The average crystallite size was
calculated from peak broadening by using a Lorentzian function to fit the peak shapes.
X-ray photoelectron spectroscopy (XPS) measurements were carried out, using a
SPECS photoelectron spectrometer with a PHOIBOS 150 analyser, to determine the chemical
state of the active components in the synthesized catalysts. The X-ray source, XR-50,
operates on an Al anode (hν = 1486.7 eV) at 300 W, 24 mA and 12.5 kV. The acquisition was
made with a pass energy of 10 eV for individual spectra and 50 eV for the extended ones.
The samples were deposited on a silicon substrate, and a flood gun system was used for
neutralization of the sample charging. The C 1s line of the support was used as standard for
internal calibration. The data were fitted with Voigt functions using SCF 2.3 software.
The distribution of the metals on the resin surface was evaluated by Scanning
Electron Microscopy coupled with Energy-dispersive X-ray spectroscopy (SEM-EDX), using
a high-resolution microscope, FEI Quanta 3D FEG model, operating at 20 kV, equipped with
an energy dispersive X-ray (EDS) spectrometer, Apollo X. The analyses were performed in

high vacuum mode, with Everhart–Thornley secondary electron (SE) detector. The
preparation of the samples was minimal and consisted in immobilizing the beads on a double-
sided carbon tape, without coating.
2.3. Experimental setup and analytical methods
The catalytic reduction tests were carried out at room temperature (21.0 ± 1.0 oC) in a
continuous flow system with a fixed bed reactor (trickle bed reactor) containing 0.4 g catalyst
(1cm3 bulk expanded volume). The aqueous solution circulates at the flow rate of 0.25 to 1.0
mL∙min-1, concurrently with the gas flow (25 mL·min-1). The initial concentrations of nitrate
and 4-chlorophenol (4-CP) were 100 mg·L-1 and 50 mg·L-1 respectively. For the preliminary
-
tests, a concentration of 300 mg·L-1 NO3 was used. The pH correction (when necessary) was
-
achieved using an adequate NaNO3/HNO3 ratio (total NO3 concentration was maintained at
100 mg·L-1). Prior steady-state establishment, the elimination of nitrate ions from solution
takes place both by ion exchange and catalytic reduction pathways as suggested in figure 1.
The same experimental setup was employed to evaluate the retention of 4-CP onto
catalyst (by adsorption or by ion-exchange). The retention tests were performed in Ar flow
(25 mL·min-1) for different pH values.
The removal of nitrate, the formation of nitrite and ammonia, and the release of the
chloride anion by the anion exchange resin were quantified through ion chromatography. The
analyses were performed on a Dionex ICS-900 apparatus equipped with a conductivity
detector and anion or cation suppressors. An AMMS 300 column was used for the anionic
species. The mobile phase was a solution of mixed sodium carbonate and sodium acid
carbonate flowing with 1 mL·min−1. For ammonia, a CMMS 300 column and a solution of

methanesulfonic acid as mobile phase were used. The selectivities to nitrite, ammonium and
molecular nitrogen were calculated by the equations below:
(eq. 1)
(eq. 2)
(eq. 3)
where
is the initial amount of nitrate (mmol) and
,
and
are the
amounts of nitrate, nitrite and ammonium at the time t (mmol). The selectivity to nitrogen
was calculated by equation (3). The SN2 value contains the contribution of all gaseous
products, which eventually are formed beside N2 (i. e. N2O or NO). This is considered
reasonable assumption since the NOx intermediates generated in the reduction of nitrate
(Reactions 3 - 9) on Pd-based catalysts (Pd, Pd-Cu, Pd-In) have been often reported to have
negligible small concentration in the gas phase [25, 40-45]. For bimetallic Cu-Pd catalysts
with atomic ratio Pd/Cu = 0.25, the selectivity to N2O in the hydrogenation of nitrate or
nitrite was significantly increased as reported by Nakamura et al. [46]. However, in the
presence of bimetallic Pd-Cu catalyst with atomic ratio Pd/Cu ≥ 1, it is probable that N2O
produced is rapidly reduced to nitrogen as suggested in [45] which could explain the low or
undetectable concentration of N2O. Regarding the nitric oxide, its presence was evidenced
exclusively in the adsorbed form [42,44]. Nevertheless, the presence in our outlet gas of other
nitrogen species beside N2 cannot be ruled out completely. So, the calculated SN2 could be
more precisely defined as selectivity to nitrogen gaseous species.
-
-
NO3 + H2 → NO2 + H2O
(3)
(4)
(5)
2 NO2 + H2 → 2 NO + 2 HO-
-
2 NO + H2 → N2O + H2O

+
2 NO + 5 H2 → 2 NH4 + 2 HO-
(6)
(7)
(8)
(9)
N2O + H2 → N2 + H2O
2 NO2 + 3 H2 → N2 + 2 H2O + 2 HO-
-
NO2 + 3 H2 → NH4 + 2 HO-
-
+
The concentrations of 4-chlorophenol and phenol (identified as the only
hydrodechlorination reaction product) were measured using a Varian ProStar liquid
chromatograph equipped with an UV-VIS detector (at λ = 275 nm) and Zorbax SB-C18
column. The mobile phase was a mixture of phosphate buffer solution (20 mM)/acetonitrile,
with a ratio of 50/50, flowing with 1.2 mL/min.
Chromatographic analyses were performed in duplicate. The conversions and
-
-
+
selecvities were calculated using mean values of NO3 , NO2 and NH4 concentration in 5
aliquots collected in 2.5 h of time interval. An exception was made for the endurance test
where the values were calculated for periods of 5 h (average calculated from 10 consecutive
aliquots). Typical investigated steady-state period was 6 h and 86 h for the endurance test.
Also, three selected experiments were performed twice in order to verify the reproducibility.
According with our calculation, the relative uncertainty lies between 0.6% and 1.3% for NO3
and 4-CP conversions and between 0.9% – 3.4% for the selectivities to nitrogen species.
-
For the hydrodechlorination of 4-CP, the carbon mass balance of the reaction was closed to >
95%. Total N mass balance cannot be calculated since gaseous nitrogen species were not
assessed.
The Pd and Cu leaching, during the experiments was evaluated through inductively
coupled plasma mass spectrometry (ICP-MS) measurements, using a Thermo Fisher
Scientific X Series 2 system II with an ESI-SC2 DX fast bundle autosampler. The analysis

reliability was assessed with certified reference materials (drinking water ERM-CA011b,
European Reference Materials) and was within 10% of the certified values.
3. Results and discussions
3.1. Catalysts preparation
It was found that immobilization of metals by ion exchange onto the resin depends on
acidity of precursor solutions. The effect of the HCl concentration on the metal retention
from precursor solution was significant in the case of copper. As an example, for Cu2+
solutions of 1.3 g·L-1, the HCl concentration was varied from 0.5 to 8.0 M. The maximum
copper retention was obtained for HCl 6.0 M (see Fig. 2). This outcome might be explained
by the interplay of two phenomena: (i) dependence of [CuCl4]2- complex formation on the
concentration of chloride ions; (ii) competition between [CuCl4]2- and Cl- for the exchange
sites of resin. Thus, low concentrations of HCl may be insufficient for the formation and
stabilization of the [CuCl4]2- complex [47], while an excessive Cl- concentration would limit
the retention of [CuCl4]2- on the ion exchange sites due to the competition between the two
anions.
The amount of Pd deposited onto resin was only slightly affected by the HCl
concentration (data not shown). However, the variation the HCl concentration from 0.1 to 6.0
M and/or the contact time between precursor solution and anionic resin (0.5 – 24 h), induced
changes in Pd radial distribution. Thus, peripheral to uniform distribution of palladium could
be obtained, as illustrated in Fig. 3. We have chosen a broad shell distribution of metal (see
Fig. 3.b) because it is a convenient compromise between an optimal use of the support and
+
minimization of mass-transport phenomena, which is an important factor in NH4 formation
mechanism [19].

It is worth mentioning that copper retention from the Cu(NO3)2 solution on the
palladium monometallic catalyst was complete by controlled surface reaction. It can be safely
assumed that the Cu deposition occurred on pre-reduced Pd surface (see catalyst
characterisation).
3.3. Preliminary tests
The catalysts prepared as afore mentioned, were subjected to preliminary tests for
nitrate reduction. In the beginning, the removal of nitrate ions takes place both by ion
exchange and catalytic reduction pathways (Fig. 1). After a period of 390 to 520 minutes,
depending on the catalyst, the steady-state was attained, and the elimination of nitrate ions
can be assumed to take place only by catalytic reduction process. The nitrate removal degree
calculated at steady-state for PdCu-1, PdCu-2 and PdCu-3 bimetallic catalysts as well as for
Pd-0 monometallic reference, is present in Table 1.
Table 1. Synthetized Pd-Cu/anionic resin catalysts and their performance in the reduction of
-
-
NO3 (Preliminary tests: NO3 initial concentration = 300 mg·L-1; solution flowrate = 1.0 mL· min-1;
H2 flow rate = 25 mL· min-1, pHi = 6.0)
It is generally accepted that copper acts as a promoter in the first reaction step
consisting in nitrate conversion to nitrite [21,48,49]. As expected, all bimetallic Pd-Cu
catalysts lead to higher nitrate conversion than the monometallic Pd catalyst (Table 1).
Comparing the bimetallic catalysts, it can be observed that the second metal deposition
procedure strongly affect catalyst performance. Higher nitrate removal was obtained for the
catalyst prepared by deposition of copper by controlled surface reaction on the pre-reduced

palladium, PdCu-3. This might be the result of a better contact between the two metals
achieved in the case of PdCu-3, as compared to catalyst in which cooper was introduced by
ion exchange, especially to PdCu-1 prepared by simultaneous deposition of cooper and
palladium.
Assuming that cooper is oxidized during redox conversion of nitrate to nitrite and
then subsequently reduced by palladium activated hydrogen (Reactions 10 and 11), a close
contact Pd-Cu is required to enable the hydrogen spill-over from noble metal to promoter
[49,50]. These results are in agreement with those obtained by Neyertz et al. (2010), who
found a higher nitrate conversion for resin supported Pd-Cu catalyst prepared by controlled
surface deposition of copper, as compared to Pd-Cu catalyst obtained by ion exchange. A
two-step method was also considered more favourable for the preparation of Pd-Cu/Al2O3
catalysts versus a single step procedure as higher selectivity to nitrogen was obtained [12].
However, Gašparovičová et al. [22] obtained a more active catalyst by simultaneous
deposition of Pd and Cu onto cation exchange resin, as compared to a successive deposition
method.
-
-
NO3 + 2Cu
Cu2O + NO2
(10)
(11)
Cu2O + 2Pd-H
2 CuPd + H2O
Based on the results obtained in terms of nitrate removal degree, PdCu-3 was selected
for further investigations.
3.4. Catalysts characterization
It is expected that selected deposition protocol ensures a good dispersion of the metal
onto support. To verify this assumption, catalysts with higher metal contents were prepared

(palladium up to 7%, keeping similar Pd-Cu ratio). The palladium crystallite dimension
calculated from the XRD diffractograms was around 7 nm for the bimetallic PdCu-3 catalyst
(2% Pd). Increasing palladium content to 4% and 7%, the crystallite average size remains
practically unchanged (Table 2). This reveals a suitable dispersion of palladium deposited
through the ion exchange method.
It should be noted that the XRD diffraction pattern of Pd-Cu catalysts showed only
the main characteristic peaks of palladium metal with face-centered-cubic structure. The
characteristic diffraction lines of copper could not be observed (Fig. 4). The low crystallinity
of the cooper phase is a pertinent explanation for the absence of XRD diffraction lines [51].
The PdCu-3 diffractogram does not provide any information regarding a possible Pd-
Cu interaction, but the calculated lattice parameter (Table 2) allowed us to assume that the
formation of a Pd-Cu alloy is possible. Thus, an average composition of the nanocrystals
calculated from XRD data according to Vegard's law:
aalloy = x·aPd +(1 − x)aCu
where a represents the lattice constant (aPd = 3.8907Å, aCu = 3.6149 Å)
and x is the fraction of the element
was found to be Pd98.8Cu1.2.
The chemisorption of H2 was used to asses Pd qualitatively and quantitatively on
monometallic and bimetallic catalysts (H2 is adsorbed selectively only by the exposed Pd
sites). The stoichiometry of the adsorption H:Pd atoms was assumed to be 1:1 [52]. The H2
chemisorption also reveals a good dispersion of palladium. The surface area of exposed
palladium for the monometallic catalyst (Pd-0) is around 2.37 m2·g-1 but decreases to 1.15
m2·g-1 for the bimetallic PdCu-3 catalyst. The explanation in exposed metal surface area is
that Pd surface became partially covered with copper phase. This result is supported by SEM-
EDX investigations. The bimetallic catalyst of choice (PdCu-3) was analysed by Scanning

Electron Microscopy and the metals were mapped on the cross section of the catalyst beads
by Energy-dispersive X-ray spectroscopy (Fig. 5). The palladium mapping shows that the
metal is generally well distributed in a broad shell, with a higher density of particles on the
outer part and no visible agglomeration as a result of the selected preparation protocol. The
copper deposited in the second stage by catalytic reduction follows the same pattern as the
first metal, confirming thus the deposition of copper on the palladium surface.
Table 2. Lattice parameter and crystallite size of palladium in the bimetallic Pd-Cu catalyst
with different metal load
The composition and chemical state of the catalytic active phase of PdCu-3 was also
evaluated by XPS. In the region of Pd3d, well separated spin-orbit components (Δ = 5.26 eV)
are present, that were also found in the spectra of our samples. The XPS spectra are shown in
Fig. 6, together with the results of their decomposition into components using Voigt
functions. The curve fitting shows that Pd3d spectra of the samples consist of two main
doublets characteristic for palladium atoms in different oxidation states. The Pd in metallic
state, with binding energy (BE) at 335.0 - 335.3 eV, was the predominant component (≈ 86
%) [53,54]. The position of the 337.0 - 337.4 eV band in the Pd3d5/2 region is indicative of
Pd2+. This BE value is intermediate between those of PdO and of PdCl2 reported in the
literature [54-56].
It is probable that palladium reduction is not complete, and a small part of the metal subsists
in the chloride form. The spectrum of, [PdCl4]2- precursor retained by ion exchange on resin
shows in Pd3d5/2 region a characteristic BE band at 337.2 eV (SI, Fig. S1). The surface
oxidation of the palladium in contact with air during drying and storage of the catalyst cannot

be fully excluded. The presence of Pd2+-Pd0 mixture might have a beneficial contribution in
the hydrodechlorination process. It was reported that both electro-deficient and zero-valent
species are required for the hydrodechlorination of the organic chlorinated compounds [32].
Cu2p3/2 binding energies are located near 932.5 eV (Fig. 7b). Although the
multivalent state of Cu complicates the discrimination between distinct Cu species, the
deconvoluted peak located at 932.1 eV is close to metallic Cu0 state but can be also attributed
to Cu+ in Cu2O (932 eV). The satellites characteristic for the 2+ oxidation state cannot be
well distinguished (XPS spectra of Cu for binding energy in the range of 920-980 eV are
presented in SI, Fig. S2). Therefore, it can be said that copper is preponderantly found in
metallic state and/or as Cu+ [20]. Unfortunately, the Auger CuLMM spectrum could not be
used to discriminate between different oxidation states of copper due to its very poor
resolution.
3.5. Catalytic tests
3.5.1. Nitrate reduction
The selected catalyst (PdCu-3) was firstly tested for nitrate reduction reaction. A first
experiment was carried out at nitrate solution flow rate of 1.0 mL·min-1 (without pH
correction) and H2 pressure of 1 atm. As can be observed from Fig. 7, the nitrate was
-
completely removed within the first 6 hours of experiment. The complete elimination of NO3
was achieved through combined ion-exchange and catalytic reduction processes. After this
-
period, NO3 concentration gradually increased reaching a plateau at 30 mg·L-1 when the ion-
exchange sites have been exhausted (as can be deduced from the drop of Cl- concentration of
effluent). The dynamic of the nitrite concentration followed a similar trend which is
-
+
consistent with the partial retention of NO2 on the resin sites. Since the NH4 was not

retained on the resin, its equilibrium concentration in the effluent was more rapidly
-
-
established compared to NO3 and NO2 . The steady-state was attained, after the complete
-
displacement of Cl- from the ion-exchange centres of the resin by NO3 , when the removal of
nitrate underwent only by catalytic reduction. After the steady-state was reached, the
conversion of nitrate was established at around 70%. As expected, the increase of contact
time by lowering the flow rate of the aqueous solution lead to augmentation in nitrate
-
conversion (Fig. 8). The conversion of NO3 was higher than 93% for a flow rate of 0.50
mL·min-1 and 100% at 0.25 mL·min-1. There was also an enhancement of selectivity towards
molecular nitrogen from 49% to 65%, due to the further reduction of nitrite intermediate. In
contrast, the undesired increase of selectivity to ammonium from 19% to 32% was observed.
Literature data reveal that the selectivity of catalytic nitrate reduction strongly
depends on pH and on hydrogen pressure, which correlates with its solubility in the aqueous
solution [39,50]. Lowering the hydrogen pressure is typically achieved by diluting H2 either
with N2 or CO2 [10,40]. The use of carbon dioxide contributes also to pH adjustment by
acting as a buffer, avoiding thus the pH increase due to hydroxide ions formation (Reactions
4,6,8 and 9). Generally, it was observed that ammonia generation is hindered at low
hydrogen pressure [57].
Thus, to limit the formation of ammonium, the hydrogen partial pressure was lowered
to 0.50 and 0.25 atm by dilution with Ar (we avoided using N2 which is a reaction product).
The nitrate solution flow rate for these catalytic tests was 0.5 mL·min-1. In these experimental
conditions, a reduction in ammonium selectivity could be observed, but the gain in nitrogen
selectivity is not significant, since the selectivity to nitrite increases as well (Table 3).
Moreover, the conversion drops with 14 to 23%, when the hydrogen pressure was reduced to

0.50 and 0.25 atm, respectively. Consequently, the reduction of hydrogen partial pressure was
not anymore considered as an effective strategy in our case.
The negative influence of high pH values on the selectivity toward N2 is well known
[6]. The pH control is classically achieved by addition of strong acids, such as HCl, by
bubbling CO2, or by using formic acid both as reductant and in situ source of CO2
[16,49,58,59]. Catalysts containing acid sites, like carboxylic group, are also proposed to
prevent the excessive increase of pH during the reaction [22,60]. According to the literature
data, it is recommended to maintain the pH in the range of 5.0 – 6.0, which is true for batch
systems [16,18,57,58]. In a continuous flow system, a gradient of pH is generated along the
catalytic bed. Thus, the experiments were started with more acidic solutions (initial pH of 2.7
- 3.1). In order to limit the number of anionic species competing for the sites of ion
exchanging resin carrier, the pH correction was realized by conveniently adjusting the
-
NaNO3/HNO3 ratio (the total NO3 concentration was kept at 100 mg·L-1). The conversion
values measured in this case are similar with those obtained for a solution with initial pH of
6.1. Moreover, the selectivity to the molecular nitrogen was higher, increasing up to 93%
(Table 3).
Table 3. Influence of H2 partial pressure and pH on catalyst performances
(For the H2 partial pressure tests, the initial pH was 6.1; For pH tests, the H2 pressure was of 1 atm;
Nitrate solution flowrate was 0.5 mL∙min-1 for all tests)
3.5.2. Hydrodechlorination of 4-chlorophenol
The 4-chlorophenol (4-CP) was chosen as a model pollutant for hydrodechlorination
reaction due to its simple molecular structure and the large number of studies reported on the
phenolic compounds. In addition, 4-CP is frequently present in the anthropic impacted sites

being a degradation product of a series of pesticides such as 2,4-dichlorophenoxiacetic acid
and pentachlorophenol [61-63].
The S-DVB polymer is a good sorbent for phenols in water and it finds some
applications in this field [64]. Moreover, phenols retention by ion exchange on the functional
groups of the resin could be expected. Indeed, retention tests performed for 4-CP (50 mg·L-1),
at different pH values show that the chlorinated phenol could be retained on the A520E resin
both by sorption on the S-DVB matrix and by ion exchange mechanism, depending on the
solution pH. As can be seen in Fig. 9, a more important retention of 4-CP accompanied by
chloride release from the resin ion exchange centres is obtained at pH of 9.8, as compared to
the pH of 5.9. It can be assumed that sorption on the S-DVB matrix is the predominant route
of 4-CP retention at pH below the pKa of the organic molecule (pKa 4-CP = 9.41), while at pH
above the pKa, retention of 4-CP occurs in an increased proportion through ion exchange.
Anyway, considering that the present experiments are performed in acidic medium (selected
condition: 3.1<pH< 7.7), the retention of 4-CP and/or phenol resulted in the
hydrodechlorination process (pKa phenol = 9.95) takes place preponderantly by sorption.
The 4-CP hydrodechlorination tests performed with PdCu-3 catalyst revealed its high
efficiency for this reaction. Complete conversion of 4-CP to phenol (the only reaction
product) was obtained for the parameters selected for the nitrate reduction process (aqueous
solution flow rate of 0.5 mL·min-1, initial pH of 3.1 and H2 pression of 1 atm). The
conversion of the 4-CP was evaluated for a time longer than 800 min, after the end of the
sorption process. The reaction advancement followed the calculation of carbon mass balance.
3.5.3. Simultaneous removal of nitrate and of 4-chlorophenol

-
Further the PdCu-3 catalyst was tested for simultaneous reduction of NO3 and
dehalogenation of 4-CP under the previously selected conditions. After the steady-state has
-
been reached, the conversion of 4-CP and that of NO3 are of 97% and 93% respectively,
while the selectivity to nitrogen of 90% is slightly smaller than for the simple nitrate
reduction process. This small decrease could be assigned to the competition between nitrite
ions and 4-CP for the active sites of monometallic palladium.
Fine tuning of the experimental parameters, aqueous solution flow rate and pH, led to
the improvement of hydrotreatment process performances Thus, for an initial pH of 2.9 and a
flow rate of 0.4 mL·min-1, the nitrate conversion reached 95% with a selectivity to nitrogen
of 93% and the conversion of 4-CP was 99% (Fig. 10 and Table 4).
3.5.4. Catalyst stability
Endurance test was performed to assess the catalyst stability. It can be observed that
after 100 hours of running, the catalyst performances, in terms of nitrate conversion and
selectivity toward nitrogen, are little affected (Table 4). The conversion diminished only with
5%, and the selectivity to N2 with less than 3%. Regarding the hydrodechlorination, the
conversion of 4-CP remains unaffected.
-
Table 4. Endurances tests ([NO3 ] = 100 mg·L-1 [4-CP] = 50 mg·L-1; pHi of 2.9; solution flow rate
of 0.4 mL·min-1; H2 pressure of 1 atm)

`
The amount of metals leached from the catalyst during the reductive treatment process
was also evaluated. The Pd and Cu concentrations were measured at different time intervals
(10 samples in total). Insignificant leaching of palladium was found, the global loss is
estimated at less than 0.02% of the amount contained by the fresh catalyst. As expected, the
copper leaching was higher, especially in the first ten hours. The maximal concentration of
copper in treated water was of 5.8 µg∙L-1. However, the amount of leached copper did not
exceed 0.3 % of the initial amount.
Furthermore, the XPS analysis revealed that there are no significant differences
between palladium spectra of fresh and spent catalyst. An additional contribution situated at
935.7eV was detected for the used catalyst as compared to the fresh sample (Fig. 6). This
value is a bit far from the BE for CuO: The corresponding species might be copper
hydroxide. The presence of Cu2+ in the used catalyst is in agreement with the role expected
for metallic copper in the nitrate reduction [50].
Conclusion
It was found that the performances of the bimetallic Pd-Cu catalyst supported on S-
DVB anion exchange resin in the selective reduction of nitrate strongly depend on the second
metal deposition procedure. The selected preparation protocol consisted in a two-step
method, implying the deposition of palladium by ion exchange and the subsequent deposition
of copper by controlled reaction on the surface of the pre-reduced palladium. This protocol
allowed a controlled deposition of metals on the support assuring: (i) a good dispersion of Pd
(confirmed by XRD and H2 chemisorption); (ii) controlled radial distribution of metals in the
resin bead (confirmed by SEM-EDX) and (iii) suitable proximity between Pd and Cu
(confirmed by SEM-EDX and H2 chemisorption). The catalyst obtained through this method,

-
PdCu-3 shows high nitrate removal degree and high selectivity in the reduction of NO3 to
gaseous nitrogen.
The effectiveness of the PdCu-3 catalyst in the simultaneous reduction of nitrate and
hydrodechlorination of 4-CP was demonstrated as well. Nearly complete hydrodechlorination
of 4-CP can be achieved simultaneously with selective nitrate reduction at a conversion of
95% and a selectivity to N2 of 92% (calculated disregarding NOx contribution), by adjusting
the initial pH and the flow rate of the aqueous solution.
The PdCu-3 catalyst was found to be relatively stable for 100 hours of running. The
conversion of nitrate decreasing slightly from 95% to 90%, while the conversion of 4-CP
seems to be unaffected. Moreover, it is noteworthy that the metals leaching was insignificant
for Pd and moderate for Cu.
Considering the promising results obtained in this work the next planned research step
will consist of testing our proof concept for real waters at laboratory scale as well as at pilot
scale.
Acknowledgements: The authors thank the Executive Agency for Higher Education,
Research, Development and Innovation Funding of Romania (UEFISCDI) for the financial
support under the PNII Project No. 100/2012 and PNIII 8PTE/2016.

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