State-of-the-art and perspectives of the catalytic and [630605]

Accepted Manuscript
Title: State-of-the-art and perspectives of the catalytic and
electrocatalytic reduction of aqueous nitrates
Authors: Juan Mart ´ınez, Alfredo Ortiz, Inmaculada Ortiz
PII: S0926-3373(17)30120-0
DOI: http://dx.doi.org/doi:10.1016/j.apcatb.2017.02.016Reference: APCATB 15415
To appear in: Applied Catalysis B: Environmental
Received date: 4-11-2016
Revised date: 2-2-2017Accepted date: 5-2-2017
Please cite this article as: Juan Mart ´ınez, Alfredo Ortiz, Inmaculada Ortiz, State-of-the-
art and perspectives of the catalytic and electrocatalytic reduction of aqueous nitrates,Applied Catalysis B, Environmental http://dx.doi.org/10.1016/j.apcatb.2017.02.016
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State -of-the-art and perspectives of the catalytic and electrocatalytic
reduction of aqueous nitrates.
Juan Martínez, Alfredo Ortiz, Inmaculada Ortiz *
Department of Chemical and Biomolecular Engineering, ETSIIT, University of Cantabria, Avda. Los
Castros 46, 39005 Santander, Spain.
*corresponding author: [anonimizat] Phone: +34 942201585; Fax: +34 942201591

2

Graphical abstract

H2O
(NO 3-)
H2O

N2

3

HIGHLIGHTS

Catalytic and electrocatalytic reduction of aqueous nitrates overview

Mechanisms of Nitrate reduction to nitrogen

Catalysts, operating conditions and reactor configuration assessment

4
INDEX
1. Introduction
2. Catalytic Reduction of Nitrates
2.1 Mechanism of catalytic nitrate reduction
2.2 pH influence
2.3 Monometallic Catalysts
2.4 Bimetallic Catalysts
2.5 Continuous Reactors
2.6 Catalytic membrane systems for nitrate reduction
3. Electro catalytic reduction
3.1 Mechanism of nitrate electro catalytic reduction
3.2 Configurations of electro catalytic reduction cells
3.3 Electrode materials
4. Conclusions

Submitted to Applied Catalysis B: Environmental
February 2017

1
ABSTRACT

Keywords: Nitrate, Catalytic Reduction, Electro catalytic Reduction, Groundwater
Nitrate pollution of groundwater, which is mainly caused by the application of nitrogen –
based fertilizers in intensive agriculture, is a widespread problem all over the world and
a potential risk for public health. Reverse osmosis, ion exchange and electrodialysis are
currently used for water denitrificat ion, yielding a high ly concentrate d reject water that
requires economic and environmental costs for disposal . Nitrate reduction technologies
that are able to convert nitrate into inert nitrogen gas have appeared that are promising ,
cost effective and environmentally friendly. Among these technologies , attention has
been focused on i) the chemical reduction over mono – and bimetallic catalyst s with
hydrogen as the reduc ing agent and ii) electro catalytic reduction processes over metallic
anodes . Although selectivity values towards N 2 of greater than 90% are achieved with
both technologies, the undesired formation of ammonium as a reaction by-product is
still the main drawback preventing their implementation at larger scale s. For this reason,
the development of new catalytic and electrodic materials as well as novel reactor
configurations to avoid ammonium formation have been extensively investigated in the
last few years to increase the effectiveness and competitiveness of both technologies. In
this paper, an overview of the current state -of-the-art of both catalytic reduction and
electroreduction of nitrates is presented, highlighting their potential and their main
drawbacks along with guidelines for future research .

2
1. INTRODUCTION
In the past several centuries, demand for clean drinking water has risen significantly.
Pollution of groundwater, which represents the main drinking water source, is becoming
a global problem. Nitrate pollution of aquifers, caused mainly by th e application of
nitrogen -based fertilizers in intensive agriculture, is one of the most widespread causes
for groundwater contamination in many countries due to the rapid progress of their
agricultural and industrial activities. Nitrate can pose a health risk for humans because
the human body reduces it to nitrite, which may cause metahemoglobinemia, also
known as “blue baby syndrome ”, and transforms it into the precursor of carcinogenic
nitrous amine. For these reasons, the European Union and the United S tates of America
limit the concentration of nitrogen compounds in drinking water. For example, The
Nitrate Directive (EC, 1991) is the European legislation that sets maximum
concentration s of 50, 0.1 and 0.5 ppm for NO 3-, NO 2- and NH 4+, respectively. Even
lower limits have been recommended by The World Health Organization : 10, 0.03, and
0.4 ppm for NO 3-, NO 2- and NH 4+, respectively.
Currently , nitrate removal from drinking water is carried out mainly by several
commercially available physicochemical technologies such as electrodialysis (ED) ,
reverse osmosis (RO) or ion exchange (IE). However, concentrated nitrate brine is
produced by these physicochemical processes, requiring post-treatment of the effluents
with high associated costs. Another possibil ity to remove nitrates from water is the use
of biological denitrification, which reduces the nitrates to nitrogen using
microorganisms in a biological reactor. Although high ly concentrated nitrate waste
stream s are avoided, the possibility of bacterial co ntamination of the drinking water or
the sludge formed during the process make biological denitrification no t competitive for
nitrate removal when compared against physicochemical processes.

3
The global energy and environmental situation has led to increasing demand for green
technologies for the sustainable production of clean water that are not energy intensive
and with no environmental impact. Jensen et al. [1] provided an overview of the actual
management strategies and treatment options for nitrate and nitrite removal , including
the cost and common problems of the commercially available technologies which in
turn justify research on novel strategies that can impr ove upon the conve ntional
techniques.
Several emerging technologies capable of reduc ing NO 3- to N 2 while avoiding
producing a waste stream have been proposed in the last few years. Catalytic
hydrogenation of nitrate , studied for the first time by Vorlop et al. [2], appears as one of
the most promising technolog ies. Hörold et al. [3,4] reported in 1993 a widely
recognized reaction mechanism based on catalytic reduction over bimetallic catalysts
using a noble metal (Pd, Pt) and a transition metal (Cu, Sn, In) in the presence of
hydrogen as the reductant [5-7]. Batch reactors with bimetallic catalysts supported on
alumina powder have been mostly used to test the reaction performance [5,7-11].
Although t otal nitrate removal has been reached, the formation of undesirable ammonia
is the main drawback of the catalytic reduction . Develop ment of novel catalyst s and
configurations to improve nitrogen selectivity is the principal challenge to be solved for
the catalytic reduction of nitrates to become competitive. With this goal , many research
efforts in the last few years have focused on the use of different support materials [12-
14], reactor configurations such as catalytic membrane reactors [15-25], and the use of
zero valen t iron (ZVI) as the catalyst [26,27] .
Nitrate electroreduction has also been considered as an alternative to transform nitrates
to nitrogen gas in drinking water treatment . Some of the advantages associated with
this process are no sludge production, a small area occupied by the plant and relatively

4
low investment costs . The reaction mechanism in the electrochemical cell, as described
by Paidar et al. [28], depend s mainly o n the type of the electrocatalytic material, the pH
of the solution and the cathode potential . Nitrogen, nitrite and ammonia are obtained as
the principal products of the nitrate electroreduction [28]. The selective reduction of
nitrate to nitrogen becomes even more difficult when water with a low nitrate
concentration, typical in groundwater, is treated. To improve the process performance
and selectivity , novel electrod ic materi als and cell configurations have been widely
investigated .
This work aims to give an overview of the state-of-the-art of the most promising nitrate
reduction strategies . In particular , this paper focuse s on the study of the catalytic
reduction and the electroreduction processes due to their high potential to achieve more
sustainable nitrate removal . Fundamentals and reaction mechanisms will be explained .
The effect s of different catalysts, support materials, reactor or cell configurations and
the influe nce of the operation conditions will be analysed and compared . The positive
and negative aspects for each technology will be evaluated .

2. CATALYTIC REDUCTION OF NITRATES
2.1 Mechanism of catalytic nitrate reduction
The reaction mechanism of nitrate removal through chemical reduction over bimetallic
catalysts has been widely studied by different authors since the nineties [6,7,29 -37].
Nitrate reduction is carried out in the presence of hydrogen as a reducing agent over the
surface of a catalyst consisting of both a noble metal and a transition metal deposited on
a support. Palladium and platinum have been mainly used as the noble metals for nitrate
reduction due to their favourable hydrogen adsorption abilities. In addition, the best

5
activity and selectivity have been achieved with Cu, Sn or In as the promoter metal ,
Figure 1 .
Numerous investigations and discussions have been performed to clarify how the
reaction proceeds . A general and detailed mechanism for NO 3- and NO 2- reduction by
hydrogen as reducing agent in aqueous solutions over mono and bimetallic Pd -M
catalysts (square brackets symbolise the active surface centers) that is mostly accepted
is as follows [18],
NO 3- + 2M[]  [M2O]surf + NO 2- (1)
H2 +2Pd[]  2Pd[H] (2)
[M2O]surf + 2Pd[H]  2M[] +2Pd[] +H 2O (3)
NO 3- + Pd[H]  Pd[NO 2] +OH- (4)
Pd[NO 2] ↔ NO 2 +Pd[] (5)
2 NO 2 + 2 OH-  NO 2- + NO 3- + H 2O (6)
NO 2- + Pd[H]  Pd[NO] +OH- (7)
Pd[NO 2]+ Pd[H]  Pd[NO] + Pd[OH] (8)
Pd[NO] + Pd[]  Pd[N] + Pd[O] (9)
Pd[O] + 2 Pd[H]  3 Pd[] + H 2O (10)
Pd[N] + Pd[N]  2Pd[] +N 2 (11)
Pd[NO] +Pd[H]  Pd[N2O] +H2O (12)
Pd[N2O] + Pd[H]  Pd[] +H2O + N2 (13)
Pd[N] + Pd[H ]  Pd[NH] +Pd[] (14)
Pd[NH] + Pd[NH]  N2 + H2 + 2Pd[] (15)
Pd[NH] + Pd[H] ↔ Pd[NH 2] + Pd[] (16)
Pd[NH 2] + Pd[H]  Pd[NH 3] + Pd[] (17)
Pd[NH 3]  NH 3 + Pd [] (18)
NH 3 + H 2O  NH 4+ + OH- (19)
Pd[OH] + Pd[H]  H2O + 2Pd[] (20)

M: Cu, Sn. In
With the exception of steps (1) and (3) the same sequence of equations can be suggested
to describe the reaction mechanism over monometallic Pd catalyst.
The reaction occurs when nitrate ions and hydrogen, both adsorbed on the bimetallic
catalyst surface (Eq.1 and E q.2), get in contact (Eq.3). In a first step, bimetallic active
sites are required to initiate the reduction of nitrate to nitrite. The transition metal
promotes the reduction of nitrate in a redox process in which the promoter metal is

6
oxidized . In contrast, the role of the precious metal is t o activate hydrogen, allowing the
in situ reduction of the second metal , as shown in Figure 1 [34, 38]. No activity was
found when monometallic Pd or Pt catalyst by themselves were used to reduce nitrates
(Eq. 4) in absence of a reducible support or a promoter metal, which plays an active role
in the redox mechanism responsible for nitrate reduction.

Figur e 1. Reaction mechanism of the catalytic reduction of nitrate
.
Second, nitrite ions, which have been identified as intermediate products in the nitrate
reduction, are reduced on the surface of the noble metal to NO (ads) (Eq.7 and Eq. 8) as
the key intermediate in the generation of nitrogen and ammonium [6,29, 32,33,39 -42].
This time, the noble metal itself can reduce the nitrite ions by activated hydrogen.
Hörold et al. [3,4] reported a mechanism to generate nitrogen [32,33,39 -41] from
NO (ads) directly (E qs.9-11) and through stepwise reaction s in which N2O appear s as
intermediate in the reduction mechanism (Eq s. 12-13).
Nevertheless, the generation of ammonia , the main drawback of this technology,
depends on the NO 2- reduction path. Wärna et al. [30] proposed a reaction system for
nitrate reduction over Pd -Cu/Al 2O3 in which the generation of ammonia is supposed to
occur v ia the stepwise addition of hydrogen to the adsorbed NO with the formation of
successive hydrogenation intermediates: NH (ads) (Eq. 14) , NH 2(ads) (Eq. 16) and NH 3(ads)
(Eq. 17). Participation of the NO (ads) species in the pathway of NH 4+ production has also
been illustrated by Zhang et al. [7]. In that work , isotopically labelled nitrogen species
were used to follow nitrite hydrogenation over Pd -In/Al 2O3 catalyst. The observations
demonstrated that catalytic NO (ads) reduction can lead to two stable end -products: NH 4+

7
and N 2. More recently, Shin et al. analysed the role of NO as an intermediate in NH 4+
evolution [43]. In this report, two different pathways for the reduction of NO to NH 4+
are proposed: NO dissociation, in agreement with the above -mentioned ammonium
pathway, and NO hydrogenation , where HNO (ads), H 2NO (ads), H 2NOH (ads), NH 2(ads) and
NH 3(ads) are the successive intermediates.
The study of the influence of Pd-M ratio on the selectivity of the nitrate reduction
pathway has been addressed in different works [18, 33]. When catalysts with a low
content of the second metal are used, only few bimetallic and large monometallic
ensembles are present in the catalyst . In this case the nitrate removal activity is lower, as
fewer bimetallic ensembles are present. During the reaction course, the now larger
palladium ensembles will be provided with nitrite and therefore N -species with a slower
rate, whereas the palladium ensembles will be covered with the reductant species as
usual. This leads to a remarkable shift in the N:reductant ratio, which now is smaller
with a corresponding low selectivity. If catalysts with a high content of the second metal
are used, these cataly sts exhibit many and large bimetallic ensembles, whereas
monometallic palladium ensembles are few and small, even if the total crystallite size is
broadened to some extent in these catalysts. Now the situation is again different. Nitrate
now is reduced at a high rate and a corresponding high rate will be obtained for the
production of nitrite. As in this case the palladium ensembles are fewer and smaller, not
every nitrite molecule which is produced can directly re -adsorb at the palladium surface,
which is “overcrowded”. Therefore, the intermediate nitrite release is much higher for
these catalysts than for catalysts with a lower content of the second metal. The high
ammonium formation, which is observed for these catalysts simply results from such
palladium ensembles, which are too small to enable the pairing of two N -species, so that
in the end ammonium is formed. [8, 33]. Thus, the Pd -M ratio of the bimetallic catalyst

8
plays a crucial role in the priority pathways through the reactions that take place as well
as in the activity of NO 3- reduction and selectivity towards N 2.
With all, t he most accepted mechanism for the catalytic hydrogenation of nitrates from
water is shown in Figure 2. It can be concluded that active bimetallic sites with an
optimal Pd-M ratio are required to carry out the reduction of nitrate to nitrite and that
monometallic Pd (or Pt) sites are needed to hydrogenate nitrites to the end -products.
Nitrogen is produced through reactions (1 1, 13 and 15 ) and a t the same time, the
intermediate NH 2 is achieved via two different pathways, NO dissociation and NO
hydrogenation, to give ammonium as the final product at the reaction 19 . [7].

Figure 2. Mechanism of the catalytic nitrate reduction over bimetallic catalysts.

2.2 pH Influence
At different pH values, the metal surface will be covered with different adsorbed
species, which are hydrogen species at low pH values and hydroxide or even oxide
species at high pH values. Since nitra te needs to be adsorbed prior to its reduction, the
repulsion between the negative nitrate ions and the reduced or more negatively charged
metal surface at lower pH values leads to the observed drop in the activity at lower pH
values. This effect is marked ly different for the three promoter metals under
consideration (Cu, Sn and In). Indeed, the maximum activity has been observed at about
pH 9 for the Pd –Cu catalyst. For high pH values a drop in the activity has been

9
observed for all the catalysts. As the c atalytic reaction proceeds, the increase in OH-
concentration due to reactions (4), (7) and (19 ) increases the pH. These hydroxide
species can competitively be adsorbed on the active sites of the bimetallic catalyst,
resulting in the inhibition of nitrate and nitrite adsorption, limiting the nitrate reduction
activity and favouring the accumulation of nitrites in the aqueous phase at high pH
values [44,45]. Here, the active sites at the catalyst surface are progressively blocked by
the strongly adsorbing ox ygenated species (e.g. OH, OH−, Ox−), i.e. the bimetallic site is
oxidized. Thus, high activity of nitrate reducing catalysts is favoured at pH values, at
which the second metal or the bimetallic ensemble, respectively, is positively charged to
some extent to facilitate the adsorption of the opposite charged nitrate ion. Obviously,
the pH value has some effect on the palladium sites as well. The catalyst surface is
increasingly covered with strongly adsorbing oxygenated species (hydroxide ions) whit
the inc rease in the pH value. These hydroxide species may act as blocking barriers
disabling the pairing of N -species diffusing on the surface [33]. In a way, this effect
resembles the negative effect, which catalysts with a high ratio (M -Pd) of the second
metal have on the selectivity, resulting in less palladium active sites available, at which
only ammonium can be formed. Again, the formed nitrite ions cannot directly re -adsorb
at the palladium ensembles, which are blocked by hydroxide ions. As avoiding NH 4+
formation is still the main challenge in the catalytic hydrogenation of nitrates from
polluted waters, the pH value must be controlled during the reduction reaction to
improve the activity and selectivity of the process. An interesting application was
report ed by Pintar et al. [46,47] who found almost stoichiometric transformation of
nitrates to nitrites conducting the catalytic nitrate reduction consecutively in separate,
single -flow fixed -bed reactors. In the first reactor , that was packed with a Pd –Cu
bimetallic catalyst, nitrate ions were transformed to nitrites at pH 12.5 with selectivity

10
as high as 93%. Liquid -phase nitrite hydrogenation to nitrogen was conducted in the
second reactor unit packed with a Pd monometallic catalyst working at low pH values
of 3.7 and 4.5, respectively.
To control de value of pH , different inorganic and organic buffers have been widely
tested. For instance, the use of zwitterionic organic buffers such as 2-(N-
morpholino)ethanesulfonic acid (MES) or N-Cyclohexyl -2-aminoethanesulfonic acid
(CHES) were compared with phosphate and bicarbonate b uffers at pH values of 6 and
10, respectively . MES and CHES buffers allowed higher nitrate removal than the
phosphate and bicarbonate buffers due to the inhibitory role of the anions of the
inorganic buffers on the catalytic nitrate reduction [48,49] . However, carbon dioxide ,
formic acid [8,11,39,42,43,50 -60] or hydrochloric acid [6,36,40,45 -47,57,61 -64] have
been the most used compounds to control the pH and to keep the nitrate solution
acidified . These buffers behave different ly: pH control with HCl has been shown to be
more dependent o n the system fluid dynamic s, while CO 2 allows better control of the
pH inside the pores of the catalyst since it is better distribut ed in the reaction system
[57,63] .
OH- + CO 2  CO 32- + H 2O (21)
CO 32- + CO 2 +H 2O  2HCO 3- (22)
Operating in a continuous reactor , both the activity and selectivity significantly
increase d with CO 2 buffering instead of HCl buffering [57]. CO 2 bubbling keeps the pH
of the aqueous solution between 5 and 6 during the reaction by neutralizing OH- ions by
reactions (21) and (22 ) [32]. Although the activity and selectivity are enhanced with
lower pH, significant dissolution of the bimetallic particle s may occur with pH values
below 4.0 [41,47] . In addition, in the treatment of drinking water, lowering the pH
would require an additional step to return the pH to a neutral value .

11

2.3 Monometallic catalyst
Reaction rate, activity and selectivity towards the end-products in catalytic nitrate
reduction are significantly affected by the support , catalyst composition and structure .
On the one hand, i t has been reported that Pd or Pt monometallic catalysts are inactive
for nitrate reduction when they are supported over non -reducible supports such as Al2O3
[36,58,60,63,65] , active carbon (AC) [41,66,67] , Nb 2O5 [63], SiO 2 [36], pumice [39],
resin [62] or zeolites [68-70]. The reason for the inactivity is the lack of a promoter
metal, which plays an active role in the redox mechanism responsible for nitrate
reduction. However, monometallic Pd or Pt catalysts deposited on reducible supports
and iron -based catalysts have shown sufficient activity in water denitr ification, reaching
total nitrate conv ersion [50,71] .
The use of SnO 2 as the active support for NO 3- reduction in the absence of an added
promoter was proposed for the first time by Gavagnin et al. [71]. In this case, nitrate
reduction occurs due to the presence of tin in a low oxidation state , promoted by the
active role of Pd in SnO 2 reduction [50]. Table 1 reports the selectivity and conversion
values achieved with active Pd/SnO 2 catalysts, along with other active monometallic
catalysts reported in the literature with different buffer systems.
Table 1. Active monometallic catalyst for nitrate r eduction
Metal wt% Support Mode Co [mgNO3 -/l] X (%) SN2
(%) SNH4 (%) [NO2 -]max
(ppm) Initial rates
(ppm/min ) Activity
(mol/min
gcat ) x 10-5 pH Buffer Red. Ref.
Pd 5 Al-Pillared
Clay Semi -Batch 100 64 – 42 –
–
0.7 6f CO 2 H2 [56]
Pd 5 SnO 2 Semi -Batch 100 100 – 18.3 0
2.1
– 3.4 o-5f CO 2 H2 [50]
Pd 5 SnO 2 Semi -Batch 100 100 – 17.2 0
2.1
– 4.2 o-4.4 f Acetate H2 [50]
Pd 5 SnO 2 Semi -Batch 100 100 – 48.2 1.9
5.7
– 4.5 o-4.5 f HCl H2 [50]
Pd 5 SnO 2 Semi -Batch 100 100 – 41.7 2.8
5.26
– 4o-5f HCl H2 [71]
Pd 5 SnO 2 Semi -Batch 100 100 – 13.8 0.5
1.05
– 4o-5f CO 2 H2 [71]
Pd 5 CeO 2 Semi -Batch 100 100 ±52 ±48 –
–
– – CO 2 H2 [72]

12
Pd 1.6 CeO 2 Semi -Batch 100 25 – >50 0
–
– – CO 2 H2 [51]
Pd 1.6 CeO 2 Semi -Batch 100 100 – 80 0
–
– – No H2 [51]
Pt 0.2 CeO 2 Continuous 60 93 46.0 54.0 0
–
– – No H2 [37]
Pt 0.5 CeO 2 Continuous 60 99 45.0 55.0 0
–
– – No H2 [37]
Pt 0.5 CeO 2 Continuous 60 30 76.0 24.0 0
–
– – CO 2 H2 [37]
Pt 1 CeO 2 Continuous 60 99 0.0 100.0 0
–
– – No H2 [37]
Pt 1 F-CeO 2 Continuous 60 90 79.0 21.0 0
–
– – No H2 [37]
Pd 5 TiO 2 Semi -Batch 100 ±55 56 – 0
–
5.1 – CO 2 H2 [73]

Using carbon dioxide or acetate as buffers in a semi -batch reactor, 5 wt% Pd/SnO 2
catalyst exhibits rather good activity. Even higher activity was found using HCl [50]
despite the larger amount of ammonium that was produced. A more favourable N2
selectivity was found using CO 2 buffering with a final NH 4+ concentration of 4 ppm,
which still exceeds the established legal limits [50,71] .
The use of TiO 2 as a partially reducible support with monometallic catalyst s has also
been investigated for nitrate reduction . A 5 wt% Pd/TiO 2 catalyst was tested in a slurry
reactor with a constant feed flow-rate of carbon dioxide and hydrogen. Relatively high
activity for the removal of nitrates was achieved, which in some cases was higher than
the activity of the bimetallic Pd -Cu/Al 2O3 catalyst at the same reaction conditions [73].
The reaction is controlled by the generation of Ti3+ centres , which seem to result from
the generation of Pd β-hydride s pecies [73-75]. This type of catalyst is known for its
strong hydrogenation character, a high nitrate reduction rate that favours ammonium
evolution and, consequently, a nitrogen gas selectivity of lower than 60% [73].
CeO 2 has been tested as a partially reducible support as well. When the catalyst is
exposed to hydrogen, cerium is reduced f rom Ce4+ to Ce3+, and oxygen vacancies are
generated at the metal -support interface . In this way, the oxygen atoms present in the
nitrate ions can interact with the support, leading to their reduction over the Pd/CeO 2
catalyst , while the noble metal would maintain the ceria support in its reduced state. The

13
activit y of monometallic Pd/CeO 2 catalyst was reported for the first time by Epron et al.
[51] in 2002. Despite the high activity show n for nitrate reduction, the CeO 2 support is
not suitable for water denitr ification due to the high selectivity towards ammonium. As
shown in Table 1, when CO 2 is used to control the pH , the catalyst is deactivated
independently of the noble metal used , decreasing the conversion from 100% to 25% or
from 99 % to 30% for palladium or p latinum, respectively. This is due to the inhibition
of the oxygen vacancies caused by the presence of carbonate or bicarbonate species at
the cerium surface, resulting in the loss of activity for nitrate reduction [37,51,72] .
Fe-based monometallic catalysts have proven to be active for the catalytic reduction of
nitrates as well. The Fe3+ and Fe2+ oxidation states are easily interchangeabl e. Fe3+
present in the surface o f the catalyst is reduced to Fe2+ when it is in contact with H 2.
Iron in its Fe2+ oxidation state acts as a reducing agent and can convert nitrate to
nitrogen while recover ing its initial oxidation state, Fe3+. Figure 3 depicts the redox
mechanism . The use of m onometallic Fe catalyst supported on activated carbon was
investigated by Shukla et al. using batch and continuous reactors [76]. The overall
selectivity towards nitrogen was nearly 100% with conversion s of approximately 5 0%
in both systems. The capacity of Fe to reduce nitrates in the presence of hydrogen has
also been examined using monometallic catalysts supported on Fe -based supports. For
instance, monometallic Pd catalysts with pillared clays as a suppor t [77] or green rusts
modified with trace metals (Pt, Cu, Zn) [78,79] , which enhance the reaction rate of
nitrate reduction, showed satisfactory activity , but high ammonium production was
observed .

14

Figure 3. Mechanism of nitrate reduction over monometallic Fe catalyst. [76]

Zero valent iron (ZVI) systems are the most representative catalysts conta ining iron
studied in the literature so far . The general mechanism of ZVI in water denitr ification is
based on its adsorptive capability towards nitrates in addition to its reductive activity. In
the presence of nitrates, Fe0 is oxidized to form Fe2+/Fe3+ in a redox process in which
nitrates are reduced to form nitrite, nitrogen and ammonium [80,81] . In the proposed
pathways , direct and indirect electron mass transfer processes, which are defined in
reactions (23-28), are involved [81-84].
Fe0  Fe2+ + 2e- (23)
Fe0 + NO 3- + 2H+  Fe2+ + NO 2- + H 2O (24)
4Fe0 + NO 3- + 10H+  4Fe2+ + NH 4+ + 3H 2O (25)
5Fe0 + 2NO 3- +12H+  5Fe2+ + N 2 + 6 H 2O (26)
3Fe0 + NO 2- + 8H+  3 Fe2+ + NH 4+ + 2H 2O (27)
3Fe0 + 2NO 2- + 8H+  3 Fe2+ + N 2 + 4H 2O (28)
Low price and low toxicity , which make ZVI suitable for direct injection into aquifers
for in situ nitrate remediation, are the most remarkable advantages of this catalyst .
Furthermore , the use of nano -scale zero valent iron ( nZVI) ha s shown several benefits
compared to micro -scale ZVI, such as high reduction capacity, large specific surface
area and low requi red dosage [26,82,83,85] . The need for a reducing agent able to
reduce the Fe2+/Fe3+ species back to Fe0 to close the catalytic circle [86] and the fact
that ammonium was found to be the main specie s in the final produc t are the principal

15
disadvantages that make this option still impractical . ZVI modified by noble and
transition metals ha s been determined to enhance the selectivity to nitrogen gas
[26,85,87] .
Thus, the use of monometallic catalysts (Pd or Pt) deposited on reducible supports
(SnO 2, TiO 2, CeO 2, and pillared clays) using appropriate CO 2 buffering favours the
increase in the activity of nitrate reduction. However, these systems are not sufficiently
selective to the formation of nitrogen due to the strong tendency to hydrogenation of
these supports that promotes the generati on of ammonium. Fe -based monometallic
catalysts and ZVI systems have been reported to reach nearly 100 % N 2 selectivity but
poor nitrate conversion rates. These drawbacks have been addressed through the
implementation of bimetallic catalysts .

2.4 Bimetall ic Catalyst
Since the proposal of Vorlop and Tacke in 1989, the use of bimetallic catalyst s to
reduce nitrates dissolved in water has been the focus of numerous researchers. Many
publications have studied the reaction mechanism s to improve the activity and
selectivity of the reaction towards nitrogen gas while avoiding the production of
ammonium. For this purpose , laboratory -scale experiments with slurry reactors have
been used to study the influence of different variables that can affect the pr ocess. The
use of different metal pairs and supports ha s been shown to play a fundamental role in
the final performance. For this reason, m ultiple combination s of noble metal s (Pd, Pt,
Rh) and promo ter metal (Cu, Sn, In, Ag, Au, Ni) deposited on different supports have
been examined with the aim of achieving a good compromise between nitrate removal
and nitrogen gas production. Pd and Pt have been reported as the most active and
selective metals towards nitrogen compared to other precious metals , such as Ir, Rh, Ru

16
[3,4,56,67] . When palladium and platinum were compared to each other as the noble
metal component in bimetallic catalyst s, similar values of N 2 selectivity were found.
However, much higher activity for nitrate reduction was achieved using Pd -based
catalyst s [3,4,88 -90].
The pairs Pd:Cu, Pd:Sn and Pd: In, which have provided the best performance in
catalytic nitrate reduction, have been compared under the same conditions in different
works and clear differences were observed among them . In the results reported by Pintar
et al. using synthetic nitrate solutions to carry out the catalytic reduction, Cu-promoted
catalyst s were more active and more selective to nitrogen than the corresponding Pd :Sn
catalyst when both were supported on alumina spheres [91]. However, when nitrate
reduction with Pd :Cu and Pd :Sn s upported on Al 2O3 was investigated using natural
water from some Spanish aquifers polluted with nitrates , better results were obtained
with the Pd:Sn couple because this catalys t was less affected by water hardness and
conductivity . In addition, NH 4+ formation was more pronounced with the catalyst
containing Cu [11,53] . It has been reported that In -promoted catalysts are highly active
to abate nitrates in water with better effectiveness than Pd :Cu catalysts in terms of
selectivity and long -term stability [6,62,92,93] . However, in a different work a slightly
lower selectivity towards nitrogen than that obtained with a Pd:Sn catalyst was found
[40].
Yoshinaga et al. [41] highlighted the relevance of the structure and geometry of the
metal particles in the final mechanism of nitrate reduction. In this sense, although edge
and corner sites of Pd microcrystals possess a high ability for hydrogenation, favouring
the formation of NH 4+, nitrogen is favourably formed on the terrace sites of Pd crystals
with mild hydrogenation abilities (Figure 4) [94]. If edge and corner sites are occupied
by promo ter metal particles, the production of nitrogen gas will be enhanced [41]. Thus ,

17
it is concluded that the nitrate reduction rate and the selectivity for the final products are
significantly affected by the dispersio n of the particles on the support, the size of the
particles and the formation of agglomerates or alloys. These factors, which are strongly
dependent o n the preparation method, the metal loading and the Pd:Me ratio (Me = Cu,
In or Sn) , determine the distan ce between active sites of palladium and noble metal,
influencing the interactions between the reactants and the catalyst.

Figure 4. Proposed models of Pd and Pd -Cu particles [41]
This explains why the optimization of the metal loading in the catalysts and the Pd:Me
ratio are so important for improv ing the rate of the catalytic reduction of nitrates.
Therefore, t he adequate structural and physico -chemical characterization of the catalys ts
provides valuable information for a better understanding of the catalysts behaviour in
terms of stability, activity and selectivity. Thus, a wide range of characterisation
techniques have been used t o report useful information, e.g. i) dispersion and
composition of metal particles in the catalysts and supports are commonly described by
X-ray diffraction techniques (XRD) ; ii) t he specific surface area is calculated applying
the BET method (SBET) to N2 adsorption isotherms ; iii) t he size distribution of the
metallic particles is determined by transmission electron microscopy (TEM) ; iv) t he
metal content and elemental ratios of the catalysts are measured by X -ray fluorescence ;
v) X-ray photoelectron spectra (XPS) give information about the evolution of the
oxidation and chemical state of the active phases and it is used to determine the binding
energy of th e different element core levels; vi) the average metal crystallite diameter

18
and the metal dispersion in the reduced samples can be estimated from CO
chem isorption wit h the double isotherm method ; vii) H2 pulse chemisorption
measurements shows dispersion of palladium based on the hemispherical model and ,
viii) temperature -programmed reduction of hydrogen (H -TPR) is used to investigate the
reducibility of th e metals in the catalysts. [53, 65, 106]
The results of many publications with different metal loadings and Pd:Me ratios were
collected and are listed in Table 2. Most of the possible combinations have similar
trends. Once the maximum conversion is reached with a specific Pd:Me ratio, further
addition of the promoter metal does not improve the reaction rate or selectivity
[51,52,88,95,96] . This occurs because once the amount of promoter metal is sufficiently
large to guarantee the reduction of the total nitrate concentration adsorbed on the
palladium sites , larger amount s of the transition metal only decrease the active surface
of Pd and thu s, the reduction power of the catalyst. When Al 2O3 particles are used as the
support with a metal loading of 5 wt% Pd, optimal Pd:Cu or Pd:In ratios of 4 were
reported [3,4,52,97] . Nevertheless, different optimal metallic ratios have been published
with different catalytic systems. For instance, Fe -containing supports such as
maghemite or pillared clays required a lower Pd:Cu ratio, probably due to the role of Fe
in the reaction mechanism [56,95,98] . The influence of the metal loading is also
significant . Most authors have worked with Pd or Pt loadings between 0.5 and 5 wt%
with no agreement about the optimum composition needed to reach maxim um activity
or selectivity in water denitrification.
Table 2. Perfo rmance of alumina -based ca talysts in the catalytic reduction of nitrates

Catalyst wt% Support Co
(mg/l) X (%) SN2
(%) SNH4
(%) [NO2 -]max
(ppm Activity
(mol/min
gcat) x 10 -5 pH Buffer Red. Ref.

19
Pd:Cu 0.92:0.32 Al2O3 360 >90 93.5 3.4 – – – No H2 + Air [99]
Pd:Cu 0.92:0.32 Al2O3 100 92.7 95 1.7 – – – No H2 + Air [99]
Pt:Cu 0.75:0.25 Al2O3 62 100 5 0 1.1 6.5 H3PO 4 H2 [34]
Pd:In 9.03:2.23 Al2O3 30 100 72.1 27.9 0 – 4 CO 2 H2 [52]
Pd:Sn 4.7:1.5 Al2O3 100 75 92.8 – 0 2 5 CO 2 H2 [100]
Pd:Cu 3.9:0.98 Al2O3 100 95 78.5 21.3 1.3 – 6 CO 2 H2 [101]
Pd:Cu 1.6:0.5 Al2O3 100 – <40 60 – 13 11f No H2 [38]
Pd:Cu 5:1.25 Al2O3 100 100 82 14 – 4 6 – H2 [4]
Pd:Cu 2.4:0.86 Al2O3 100 100 87 3.9 – – 5 CO 2 H2 [63]
Pd:In 1:0.5 Al2O3 100 99.6 93.6 6.4 0 – 5 HCl H2 [36]
Pd:In 1:0.25 Al2O3 100 100 80.3 19.6 0.1 – 5 HCl H2 [36]
Pd:In 1:0.25 Al2O3 100 100 75 25 0 78.2 5 HCl H2 [93]
Pt:In 1:0.25 Al2O3 100 67.7 25.3 0.6 18 5 HCl H2 [102]
Pd:In 1:0.25 Al2O3 100 ±100 ±79 ±20 – – 5 CO 2 H2 [57]
Pd:Cu 5:1.5 Al2O3 90 100 ±76 ±20 – – 11.2 f CO 2 H2 [103]
Pd:Cu 4.7:1.7 Al2O3 200 100 97 – – 25 6.5 f No H2 [31]
Pd:Cu 5:1.25 Al2O3 124 >99.5 93.6 – 12.8 1.6 7 HCl H2 [40]
Pd:Cu 5:1.25 Al2O3 100 100 93.5 6.5 – – 5 HCl HCOOH [33]
Pd:Cu 5:1.25 Al2O3 100 100 97.7 2.3 – – 5 HCl H2 [33]
Pd:Sn 5:1.25 Al2O3 124 >99.5 97.1 – – – 5 HCl H2 [40]
Pd:Sn 5:1.25 Al2O3 100 100 93.5 6.5 – – 9 HCl H2 [33]
Pd:In 1:0.25 Al2O3:SiO 2 (25:75) 100 82.2 74.4 25.6 0 10.5 5 HCl H2 [93]
Pd:In 1:0.25 Al2O3:SiO 2 (50:50) 100 99.9 78 22 0 27 5 HCl H2 [93]
Pd:In 1:0.25 Al2O3:SiO 2 (75:25) 100 99.9 68.7 28 3.3 84 5 HCl H2 [93]
Pd:In 1:0.25 Al2O3:SiO 2 (75:25) 100 89 89.2 10.8 0 15.8 5 HCl H2 [93]
Pd:In 1:0.25 Al2O3:SiO 2 (75:25) 100 93.2 94.8 5.2 0 15.8 5 HCl H2 [93]
Pd:Cu 5:1.5 Al2O3-TiO 2 30 100 70.8 29.2 0 – 6.1 CO 2 H2 [61]
Pd:Cu 1.5:0.5 Al2O3-TiO 2 100 ±97 ±60 ±38 – – – No H2 [99]
Pd:Cu 0.75:0.36 CeO 2-Al2O3 100 97.1 89.4 8.9 – – – No H2 + Air [104]
Pd:Cu 0.75:0.37 Cr2O3-Al2O3 100 95.3 89.8 6.6 – – – No H2 + Air [104]
Pd:Cu 0.50:0.34 Mn 2O3-Al2O3 100 92.6 88.6 6.8 – – – No H2 + Air [104]
Pd:Cu 0.64:0.35 SrO-Al2O3 100 92.2 82.3 13.5 – – – No H2 + Air [104]
Pd:Cu 0.77:0.37 TiO 2-Al2O3 100 68.8 92 1.5 – – – No H2 + Air [104]
Pd:Cu 0.61:0.35 Y2O3-Al2O3 100 88.8 84.5 10.5 – – – No H2 + Air [104]

20
The catalyst preparation protocol and the salt used also have a direct effect on the final
size and distribution of the metallic particles over the support surface
[60,66,91,96,105,106] . The cataly tic behaviour in nitrate reduction experiments was
affected by metal -metal and metal -support interactions that can be modified by the
temperature of both the reduction and calcination steps [49,89,90,93] . In the same way,
the surface area of the support for bimetallic catalyst s has been known to determine the
dispersion of metal particles, which influence s the catalytic activity for nitrate
reduction. The higher the surface area of the support is, the higher the catalytic activity
for nitrate reduction is [62,107] . In this way , alumina, which is a well-known and
suitable support that provides high surface area and good stability of metallic particles,
has been extensively used in the literature as the catalyst support for nitrate reduction.
Table 2 presents a summary of the conversion values availab le in the literature for the
catalytic hydrogenation of nitrates using alumina -based catalysts. Total nitrate removal
with nitrogen selectivit y of higher than 9 0% was reached . The best result achieved was
97% of conversion of the initial 200 mg/l nitrate solution to nitrogen gas using a Pd-
Cu/Al 2O3 catalyst [31]. However, with Pd -In/Al 2O3 or Pd -Sn/Al 2O3 catalysts, lower
selectivity ( but still higher than 90%) was observed [33,36,40,93,100] . Marchesini et al.
[99] studied the combination of Al 2O3 and SiO 2 as the support for a Pd-In catalyst . The
catalysts that have their Pd active sites on Al 2O3 show higher activity to reduce nitrates
than those that have them on SiO 2, in good agreement with the results reported by
Hörold et al. [2-4]. It was observed that the presence of SiO 2 improves the selectivity to
gaseous nitrogen. Al 2O3 has a pI (isoelectric point) of 8 when the reaction medi um has a
pH of 5. At this pH , Al2O3 has a positive superficial charge that will produce a negative
counterion layer ( i.e., NO 3−, NO 2−), phenomenon that could explain the good catalytic
activity. As SiO 2 has a pI of 2, it presents a negative superficial charge producing a

21
positive counterion layer ( i.e., H3O+, NH 4+). In equilibrium, the ammonia present in this
layer could prevent the production of more ammonia. This excess positive charge leads
to the formation of N 2(g) as a final product instead of ammonia [93].
Both the conversion of NO 3- and the production of NH 4+ vary significantly with the
support chemical composition. For th is reason, Pd -Cu supported on various mixed metal
oxides (MO x = CeO 2, SrO, Mn 2O3, Y 2O3 and TiO 2) ha ve been investigated by
Constantinou et al. [104] . Metal oxides were coated over the external surface of γ-
alumina spheres before incipient wetness impregnation of bimetallic solution . CeO 2,
MnO 2, and Cr 2O3 support coatings appear to be the most active for nitrate conversi on,
whereas high selectivity towards NH 4+ and low activity for nitrate reduction were
produced with SrO and TiO 2 coatings , respectively ( Table 2) .
Due to the effect of the support on the performance of catalytic nitrate reduction, many
publications have explored the use of different supports to improve the results achieved
with alumina -based supports. In Table 3, the performance of different supports in the
catalytic nitrate reduction reported by different authors is shown. As shown , the silica
support is the most selective towards nitrogen , achieving selectivity values of up to 98%
when 90% of the initial nitrates are reduced. The main drawback of silica catalysts is
that they have a lower nitrate reduction rate than alumina [57,93,100,108] . In contrast,
higher ammonium production was achieved when titania was used as the support
despite showing better reduction rates of nitrates [49,109] . Compared with metal oxides,
carbon -based supports offer a greater versatility because of their unique surface
properties that allow easy recovery of precious metals, the ability to achieve high metal
loading a nd dispersion, stability at low pH, and the ability to control the porosity [96].
The use of carbon materials as catalyst supports for nitrate reduction has been mainly

22
studied in the literature with active carbon (AC) or carbon nanotubes (CNTs)
[13,89,90,92,96,110] .
Table 3. Summary of the influence s of the support on the catalytic reduction of nitrates

In particular, Sakamoto et al. [112] observed that the activity of nitrate reduction using a
Pd:Cu catalyst supported on AC was higher than the activities of the same Pd:Cu Catalyst wt% Support Co (mg/l) X (%) SN2 (%) SNH4 (%) pH Buffer Reductor Ref.
Pd:Cu 1:0.5 Al-Pillared Clay 100 36 – 55.3 6f CO 2 H2 [56]
Pd:In 5:2.5 Al-Pillared Clay 100 100 – 16.5 6f CO 2 H2 [56]
Pd:Sn 5:2.5 Al-Pillared Clay 100 100 – 25 6f CO 2 H2 [56]
Pd:Cu 2:1 CNT 100 100 80 20 5.5 CO 2 H2 [13]
Pt:Cu 1:0.3 CNT 100 99 29 67 5.5 CO 2 H2 [89]
Pd:Cu 3.9:0.98 Dower 1×4 (IE resin) 100 83 71 26.8 6 HCl H2 [101]
Pd:Cu 1.6:1.6 Hematite 30 96.6 69.1 30.9 6 CO 2 H2 [98]
Pd:Cu 0.5:0.5 Maghemite 132.8 >95 45 50 5.7 – H2 [95]
Pd:Cu 5:1.5 Mg/Al hydrotalcite 90 100 92.2 7.78 11.2 f CO 2 H2 [103]
Pd:Cu 1:1 MWCNT 100 64 76 23 5.5 CO 2 H2 [111]
Pd:Cu 1.9:0.6 Nb 2O5 100 100 89 3.1 5 CO 2 H2 [63]
Pd:Cu 0.75:0.25 Pumice 100 100 – 1.2 5.5 CO 2 H2 [39]
Pd:Cu 5:1.5 ZrO 2 100 100 – 27.9 ±5 CO 2 H2 [71]
Pd:Cu 5:1.5 SnO 2 100 100 – 20.7 ±5 CO 2 H2 [71]
Pd:Cu 0.5:1.2 MOR 200 99.9 51.8 17.9 – CO 2 H2 [69]
Pd:Cu 0.5:1.2 FER 200 96.2 14.1 4.9 – CO 2 H2 [69]
Pd:Cu 0.5:1.2 ZSM -5 200 95.8 8 12.1 – CO 2 H2 [69]
Pd:Cu 0.5:1.2 Y 200 97 41.2 34.1 – CO 2 H2 [69]
Pd:Cu 3:1 TiO 2 50 100 50.2 47.7 6 MES H2 [49]
Pd:Cu 4:1.5 TiO 2 100 99.9 41.6 50.5 5o-8f No H2 [109]
Pd:Cu 4:1.5 TiO 2-2%Nb 100 99.9 45.9 50.1 5o-8f No H2 [109]
Pd:Cu 4:1.5 TiO 2-7%Mg 100 99.9 45.6 53.8 5o-8f No H2 [109]
Pd:In 1:0.25 SiO 2 100 ±15 ±20 ±66. 7 5 CO 2 H2 [57]
Pd:In 1:0.25 SiO 2 100 90.8 98.6 1.4 5 HCl H2 [36]
Pd:In 1:0.25 SiO 2 100 ±95 ±97. 9 ±2.1 5 HCl H2 [57]
Pd:Sn 4.7:1.5 SiO 2 100 75 93.2 – 5 CO 2 H2 [108]
Pd:Sn 5:1.5 SiO 2 100 100 – 5.3 5 – 2.7 f HCOOH – CO 2 HCOOH [100]
Pt:In 1:0.25 SiO 2 100 27 89.3 3.7 5 HCl H2 [57]
Pd:Cu 5:0.6 AC 200 100 63 – 5.4 CO 2 H2 [45]
Pd:Cu 2:0.63 AC 200 86.1 2.4 8.4 10.5 f No H2 [112]
Pd:Cu 1:1 AC 100 95 76 21 5.5 CO 2 H2 [110]
Pd:Cu 2:1 AC 100 100 52 48 5.5 CO 2 H2 [13]
Pd:Cu 2:1 AC 100 87.8 47.5 52 4.5 o-5.5 f CO 2 H2 [96]
Pd:Cu 5:0.6 AC 200 83 21 – 5.4 o-10f No H2 [45]
Pd:In 1.8:0.18 ACF 100 80 84 – 5 HCl H2 [92]
Pt:Cu 1:0.3 AC 100 64 33 51 5.5 CO 2 H2 [89]
Rh:Cu 1:1 AC 100 84 22 77 5.5 CO 2 H2 [110]

23
catalyst supported on a metal oxide , such as TiO 2, Al 2O3 or ZrO 2, at the same operati ng
conditions. Despite the low production of NH 4+, close to 80% of the initial nitrates were
converted into non-desirable nitrite due to t he inhibition of nitrite reduction by the
formed OH-. As shown in Table 3, other authors improved on those results by using
carbon dioxide or hydrochloric acid as buffers to avoid the effect of increasing pH
[92,96] . Soares at al. reported 80% selectivity towards nitrogen with total conversion of
nitrates [13] with Pd:Cu/CNT catalyst . However, the concentration of ammonium was 5
ppm , still exceeding the limits established by European legislation for ammonium
concentration in drinking water . The use of Pd:Cu/Mg/Al hydrotalcite as the catalyst
partially solve d the problems associated with the diffusion limitations in the catalytic
reduction of nitrates. In this case, the reactant ions and reaction products are captured or
released by ionic interactions at the hydrotalcite interlayer space. Nitrate and the
produced nitrite ions are kept inside t he support , avoiding discharge to the aqueous
medium . The selectivity towards nitrogen is increased, yielding values higher than 90%
because of the higher concentration of nitrogen compounds on the surface of the
catalyst [103] . Nb 2O5 catalyst support s were tested in the reduction of nitrate and
compared with alumina. The performance of 2 wt% Pd :0.5 wt% Cu bimetallic catalyst
supported on niobia was similar to the same metallic composition supported on alumina
in terms of activity and N 2 selectivity [63]. The effect of Fe -containing supports , such as
Al-pillared clays, maghemite or haemat ite, in the hydrogenation of aqueous nitrate by a
bimetallic catalyst w as also reported [56,95,98] . A large production of undesirable
ammonium was observed for these catalysts because of the involvement of the iron in
the reaction according to the mechanism described above for the Fe-supported
monometallic catalyst.

24
After examining the results o f the use of different catalysts tested for the catalytic
reduction of nitrates in slurry reactors , it can be concluded that monometallic catalyst
with partially reducible supports such as Pd/SnO2, Pd/CeO2 , Pd/TiO 2 or Fe -based
catalyst s, due to their high formation of NH 4+, are not competitive when compared to
bimetallic catalysts. Regarding bimetallic catalysts, although the Pd-Cu pair has been
the most studied catalyst due to its good performance , Pd-Sn and Pd -In have shown
promising results in the last few years. Specifically, Pd -Sn catalysts have displayed
higher activity and selectivity than Pd -Cu. In addition, the importance of the support
composition on the final performance of the catalys ts in nitrate reduction has also been
investigated . Alumina is the most widely used support due to its high activity and
selectivity , high specific area and high stability. Although other supports have been
studied , only carbon -based support s are competitive against alumina. However, the
production of ammonium with all the tested catalysts is still too high, requiring further
study to improve on the current results.
The loss of catalyst activity or deactiva tion is still one of the main drawbacks in the
application of catalytic processes entailing many research effo rts focused on the
responsible phenomena and their control. Factors such as the deposition method, size
and distribution of the bimetallic particles on the catalyst surface are responsible of the
gradual deactivation and leaching of the metal from the support. [5] Deactivation of the
catalyst can also be explained by the non -reversible oxidation of the M –Pd couple.
Redox cycles of the Pd and Sn (or Cu) species explain this behaviour, that is, the active
sites are re -oxidized and reduced repeatedly; this non -reversible oxidation of some Pd –
Sn species has been reported as one reason for the slow deactivation observed for these
catalysts after some hours of reaction. [53] Additionally, the competitive adsorption of
different species present in water on the active sites [23 –25] is another likely cause of

25
the catalyst deactivation. The catalyst exposed to organic matter, or high calcium
content, will suffer partial deactivation hiding the active metallic sites of the catalyst;
high sulfate content in the aqueous solution and the reducing ambient of the reaction are
probably responsible for the formation of some sulphur reduced species that slowly
deactivate the palladium sites. Thus, sulfide quickly fouled the catalyst resulting in
increased NH3 production and ultimately no activity for NO 3- reduction. Besides,
Pd/CeO 2 catalysts can be po isoned by bicarbonates and carbonates. [11]
2.5 Continuous Reactors
The analysis and results of the catalytic activity and selectivity in the reduction of
aqueous nitrates previously shown have been obtained with batch or semi -batch
reactors . However, further implementation of the process will require the analysis of
continuous operation and process scale -up. Fixed bed [9,41,48,57,66] and trickle -bed
[55,67] continuous reactor s have been successfully tested in catalytic experiments to
remove nitrates from water. For instance, the behaviour of Pd-In bimetal lic alumina
catalyst in a fixed bed reactor to carry out water denitrification was studied by Mendow
et al. [57]. Continuous flow experiments were performed in a glass tubular reactor with
a 10 mm inner diameter with three grams of catalyst. During the experiment, 200 ml
min-1 of the initial solution (100 mg/l KNO 3) was continuously fed. Carbon dioxide was
compared to hydrochloric acid as the buffer system , and the best conversion and
selectivity results , 93% and 78 %, respectively , were obtained when the feed solution
was acidified with CO 2, which neutralized the OH- ions, avoiding the generation of a
pH gradient inside the reactor, when HCl was used. Moreover, i n this reactor , the
conversion of nitrates depended on the hydrogen flow rate . This fact is explained by the
higher turbulence generated by the higher flow rate, which decreas ed the mass transfer
limitations . However, as may be expected, it was found that the selectivity to N 2

26
decreased with increasing hydrogen flow rate. On the other hand, Chaplin et al. [9]
developed a packed bed reactor with a Pd-In/γ-Al2O3 catalyst where gases were
delivered to the liquid upstream of the reactor using a membrane module gas exchange r
(see Figure 5). It was similarly observed that the production of NH 4+ is highly
dependent on the inlet solution conditions.

Figure 5. Catalytic reduction of nitrate process us ing a packed bed reactor [9]

The effect of dissolved oxygen on the catalytic NO 3- reduction in continuous reactor s
was investigated for the first tim e by Constantinou et al. [104] . Dissolved oxygen
hinder s NH 4+ production , thus changing the ratio of N:H species on the catalyst surface.
This fact was later confirme d by Theologides et al. [48,113] , who concluded that the
production of ammonium could be decreased by up to three times when 20 vol % of air
is added to the gas fed to the fixed bed catalytic reactor.
To implement the technology for the treatment of real polluted waters , the influence of
natural ions present in groundwater must be considered . The effect of typical
groundwater ions w as investigated working with a three -phase continuous flow stirred

27
tank reactor with liquid phase volume of 180 ml , a liquid flow rate of 4 ml/min and an
initial nitrate concentration of 75 mg/l [48]. It was concluded that the presence of HCO 3-
and Na+ ions in the liquid stream can lead to a significant decrease of both the catalyst
activity and selectivity , whereas the presence of CO 42- and PO 43- does not significantly
affect the catalyst behaviour in catalytic water denitrification. In contrast, the presence
of Cl- slightly increases the catalysts activity. T hese result s are not in agreement with
other authors who have reported the negative effect of Cl- ions in the catalytic reduction
of nitrates where the reduction was almost completely inhibited [61].
Other interesting work has been reported on the effect of the natural water composition
on the catalytic reduc tion of nitrates in continuous reactors . The results obtained by
Pintar et al. [55] show that when drinking water is used instead of synthetic solutions
made with distilled water , the nitrate disappearance rate and the reaction selectivity
decrease appreciably due to the presence of dissolved ionic species. More recently, it
has also been reported that lower selectivity towards nitrogen is always obtained when
using natural water than when using distilled water conta ining nitrates [11,53] , while a
low concentration of dissolved organic compounds did not exhibit any appreciable
influence on the reaction. To the best of our knowledge, the lowest selectivity towards
NH 4+ achieved in catalytic nitrate reduction in continuous reactors using natural water
has been 30% with 90% nitrate conversion using Pd-Sn/Al 2O3 with a total metal loading
of 5 wt% and 2.5 wt% for palladium and tin, respectively [32,53] .
The use of packed bed or trickle -bed reactors at large scale presents some drawbacks,
one of the most important being the loss of catalyst particles due to the sweep effect of
the liquid strea m. For this reason, Hähnlein et al. proposed the use of a hollow fibre
dialyser module for the retention of the small catalyst particles in the intracapillary
volume [114] . The schem atic of the hollow fibre dialyser, filled with catalytic particles,

28
is show n in Figure 6. The nitrate -polluted stream, bubbled with hydrogen, is pumped
through the extracapilla ry volume of the hollow fibre module. The reaction occurs when
the liquid phase contacts the catalyst located insi de the hollow fibre . The catalyst was
completely retained. However , an active layer is considered to reduce the diffusional
limitations in the membrane system. Best results were reported using the 5%Pd –
l.25%Sn/A1203 catalyst , hydrogen as reductant agent and HCO 3- for buffering effect
that provided 19.3 mg NO 3-/ (g cat. *h) activity and 88% selectivity to N 2.

Figure 6. Schem atic of a hollow fibre dialyser filled with microscopic catalysts [114] .

2.6 Catalytic membrane systems for nitrate reduction
Catalytic membrane reactors, CMRs, which have been developed in the last several
decades, combine reaction and separation in a single step where one (or more) re actant
or product species is removed and separated selectively. CMR s are especially
recommended for industrial processes in which the performance is limited by the
reaction equilibrium . The selective removal of one of the reaction products through the
membrane shifts the chemical equilibrium to product formation and thus increases the
process performance . Therefore, CMRs are intensified reactors that allow for easier
catalyst optimization , smaller reactor sizes and lower energy consumption [115] .

29

a) Conventional reactors reviewed in the preceding sections, i.e., slurry or fixed bed
reactors that operate in either batch or semi -batch mode , suffer strong diffusional
limitations due to the low solubility of hydrogen in water and the limite d accessibility of
H2 to the catalyst surface. CMRs have been proposed as an alternative to overcome
previous drawbacks and to enhance the performance of the catalytic reduction of
nitrates. In this way, the membrane act s as a support for the metal catalys t and as a
gas/liquid/solid contactor , promoting the contact between reactants and catalyst. In this
sense, two configurations have been reported in the literature: (a) the interfacial
configuration, in which the gas/liquid interfa ce is located inside the catalytic zone and
(b) the flow-through configuration, in which the gas phase is first dissolved in the liquid
phase pumped through the catalytic membrane , as shown in Figure 7.

Figure 7. (a) CMR interfacial configuration and (b) CMR flow-through configuration [115]

Nitrate hydrogenation in CMR s has been mainly studied with the catalyst supported on
commercial ceramic membranes, typically alpha and gamma alumina tubes with pore
sizes between 60 and 400 nm. Ilinitich et al. compared the results obtained in the
reduction of nitrates using Pd-Cu deposited on alumina powder and a catalytic
membrane made of alumina with a pore size of 1 µm [20]. In this study, pronounced
diffusional limitations on alumina powder were observed , with the flow -through
b)

30
catalytic membrane reactors achieving the best nitrate reduction rate. These results
agree with those reported by different authors [16,22] .
Strukul et al. [23] reported a lower activity of CMRs based on the addition of Pd-Cu and
Pd-Sn catalyst s to an alumina support with an external thin laye r coating of sol-gel
deposited zirconia or titania compared to the activity obtained with powder catalyst s.
The results were attributed to the lack of control of the por e size of the membranes that
showed cracks after being used. Regardless , high nitrate conversions (99%) were
achieved when working with a high partial pressure of hydrogen that promoted
ammonium formation .
Catalytic reactor s with t ubular multilayer membr anes were investigated by different
authors [15-17,25] as hydrogen diffusor s with the goal of creating an efficient contact
between the different phases (hydrogen, aqueous nitrate solution and catalyst) , thus
improving the catalytic ni trate reduction . For instance, Daub et al. [17] proposed a
catalytic reactor with a tubular membrane with internal and external diameter s of 7 and
10 mm, respectively . These membranes were formed by different layers of α-Al2O3 with
a graded pore size and an external layer (20 µm) formed by α -Al2O3 (60 -100 nm), ZrO 2
(65 nm), TiO 2 (5 nm ) or ɤ-Al2O3 (5 nm ). The authors analysed the i nfluence of the
metal incorporation method and compared the performance of catalytic membran es
made with the m etal organic chemical v apor deposition (MOCVD) and impregnation
methods using palladium and tin as promoter metals . They concluded that this
configuration facilitates the change and adjustment of the hydrogen pressure , which
influence s the activity and selectivity . Therefore, this configuration offers a great
potential to adapt the process to the varying needs of an industrial plant with variable
water flowrate and nitrate content .

31
In the studies of water denitrification reported by Che n et al. [15,16] , Pd and Cu were
deposited using the co-impregnation method on a ceramic membrane made from 95% of
α-alumina, with a diameter, thickness and pore size of 10 mm, 1 mm and 2 µm,
respectively .

Figure 8. Schem atic of the CMR setup used by Chen et al. [16].
The schem atic of the experimental system used in this case is show n in Figure 8. Nitrate
solution ( 50 mg/l) is contact ed with hydrogen with a partial pressure equal to 0.15 MPa
for 2 hours. A nitrate conversion of 70% and a selectivity higher than 90% were reached
[15,16] . Similar results were obtained by Whebe et al. [25] more recently using a
tubular multilayer membrane of alumina where the influence of the pore size in the
outermost external layer was studied. Membranes with a smaller pore size (5 nm)
displayed bett er results than those with higher pore sizes (10 and 25 nm) , reaching 92%
nitrate conversion and a selectivi ty towards nitrogen of approximately 8 0% [25].

Figure 9. Gas–liquid–solid contac t in the porous ceramic reactor [116].

32
In the above -described tubular membrane reactors , the active layer was located on the
external surface of the membranes. However, the possibility of incorporat ing this active
layer in the lumen side of tubular membranes made of porous alumina has been
reported . In this way, direct contact is allowed between the nitrate polluted aqueous
stream s, fed to the lumen side , and the gaseous phase , fed through the shell sid e of the
reactor , as shown in Figure 9. This configuration attempts to increase the nitrate
reduction rate by decreasing the diffusional mass transfer resistance in the aqueous
phase. This procedure was used by Aran et al. to carry out nitrite reduction ex periments ,
showing comparable results to those obtained with other tubular membrane reactors in
terms of the reduction rate [116,117].
While CMR s for nitrate reduction mostly use ceramic membranes, a few references to
polymeric supports have also been reported . Ludkte et al. employed Pd-Cu/Al 2O3
catalysts (2.5 µm) supported on a microporou s polyetherimide (PEI) membrane [21].
The catalyst powder was dispersed in the precursor solution, which was used to prepare
the hollow fibre membranes following the dry-wet spinning technique. Figure 10 show s
an SE M photograph of the cross section of these catalytic membranes that contain 27
wt% of catalyst .

Figure 10. SEM image of the polymeric hollow fibre with 27 wt% of catalyst particles Pd:Cu/Al 2O3 [21].
The experiments carried out with this membrane reported a nitrogen selectivity close to
80% with no variation in pH or temperature. In addition, this study show ed that

33
ammonium production could be minimized by decreasing the contact time of the nitrate
solution with the catalytic phase. In addition , this membrane configuration showed good
stability of the metallic particles on the membrane, which is one of the most important
challenges in polymeric membranes [21].

Table 4. Summary of the results of nitrate catalytic reduction using CMR
Membrane Pore Size Catalyst Method Feed
[NO 3-] SN2 (%) XNO3 – (%) Ref.
Sheet α -Al2O3 1 µm Pd:Cu (1:0,8) Co-
impregnation 200 ppm 70.2 95 [18]
Tubular multilayer α-
Al2O3 + {ZrO 2, TiO 2 or ɤ-
Al2O3} 0,1 µm Pd:Sn (1,67:1) MOCVD 100 ppm 91 50 [17]
Tubular multilayer α-
Al2O3 {ZrO 2, TiO 2 or ɤ-
Al2O3} 0,1 µm Pd:Cu (2,5:1) Sol-Gel 50 ppm 58 100 [23]
Sheet α -Al2O3 1,5-2 µm Pd:Cu (4:1) Co-
impregnation 100 ppm 91.6 57 [16]
Tubular multilaye r 3x α-
Al2O3 + ɤ-Al2O3 5-25 nm Pd:Cu (2,5:1) Evaporation –
Crystallization 50 ppm 60-70* 92 [25]
Pd/Cu – Al2O3 particles in
PEI Hollow Fibre Pd:Cu (4,7:1) Dry-wet
spinning 100 ppm 75-80* [21]
Sheet – Polyamid e 0,4 µm Pd:Cu (3,67:1) 200 ppm 84* 45-50* [19]
* Ap proximate values obta ined from graphical results

Ilinich et al. [19] reported the use of mono – and bimetallic catalytic membranes
prepared via deposition of Pd and Cu on a flat sheet polyamide microfiltration
membrane. These membranes were employed, similar to in the previous cases, to
hydrogenate the aqueous solution of nitrates in a flow -through reactor (Figure 7b). It
was concluded that the activity of the process is controlled by the mass transfer rate of
hydrogen to the liquid phase by diffusion . This conclusion, which is in good agreement
with previous studies , shows the importance of improving the contact between the three
phases (G/L/S) while at the same time decreasing the mass transfer limitations. Table 4
collects and compar es the results obtained with membrane systems in the literature to
perform the catalytic reduction of nitrates.

34
Despite the improved mass transfer rate produced with CMR s, the results achieved with
CMR s are comparable with those achieved with the catalyst particles show n in tables 2
and 3. However, the selectivity values of the membrane systems are too low because of
the high partial pressures of hydrogen that promote NH 4+ formation used in the
experiments. The challenge in the design and performance of CMR s, as well as in other
continuous reactor configurations, is to optimize the operation variables to avoid
ammonium formation in a way that a higher selectivity of the process towards nitrogen
gas is obtained .

3. ELECTRO CATALYTIC REDUCTION
Electrochemical processes offer advantages such as eliminating the requirement for
chemicals before and after the treatment, a small area occupied by the plant, not
producing sludge and relatively low investment costs [118] . The high-energy efficiency
and environmental compatibility of electrochemical processes makes them a promising
technology for different applications. Electrodialysis , electrocoagulation and catalytic
electroreduction have been considered as possible electrochemical alternatives for
aqueous nitrate remediation . Neither electrodialysis nor electrocoagulation are capable
of reduc ing nitrate into harmless N 2; they only separate it from the polluted water,
producing high ly concentrated nitrate -containing wastewater, which , as consequence,
needs additional treatment at an additional cost. Electroreduction of nitrates has been
widely investigated as an alternative for the treatment of drinking water and wastewater .
Nitrates are mainly converted to nitrogen, nitrite and ammon ia in an electrolysis cell.
3.1 Mechanism of nitrate electrocatalytic reduction.
The reduction reaction occurs at and is catalysed by the cathod ic surface where in
addition to N 2, NO 2-, and NH 4+, other cathodic products can be involved , such as NO 2,

35
NO, N2O, NH 2OH or NH 2NH 2. While ammonium may constitute the desired final
product of the reaction when the nitrate concentration is high, as it may be further
recovered and reutilized , the challenge of the electrocatalytic denitrification of
groundwater is the same as the catalytic reduction of nitrates, which is to achieve a
100% selectivity towards N 2.
The main cathodic reactions that can occur in the electroreduction of nitrates were
reported by Paidar et al. [28]:
NO 3- + H 2O + 2e-  NO 2- + 2OH-, E0 = 0.01 V (29)
NO 3- + 3H 2O + 5e-  1/2N 2- + 6OH-, E0 = 0.26 V (30)
NO 3- + 6H 2O + 8e-  NH 3 + 9OH-, E0 = -0.12 V (31)
NO 2- + 2H 2O + 3e-  1/2N 2- + 4OH-, E0 = 0.406 V (32)
NO 2- + 5H 2O + 6e-  NH 3 + 7OH-, E0 = -0.165 V (33)
NO 2- + 4H 2O + 4e-  NH 2OH + 5OH-, E0 = -0.45 V (34)
Hydrogen evolution is the main side reaction at the cathode:
2H2O + 2e-  H2 + 2OH- (35)
The desired cathodic process is the reduction of nitrate to nitrogen by reaction (30), but
the reaction mechanism in the electroreduction of nitrates principally depends o n the
cathode material, the applied cathodic potential, the cell configuration, the pH of the
aqueous solution and the presence of other anions.
The main anodic reaction, whe n no other ions are present, is oxygen evolution:
4OH-  O2 + 2H 2O + 4e- (36)
If chloride ions, which are common in wastewater or drinking water, are present in the
aqueous solution of nitrates, hypochlorite ions can be formed due to the anodic
oxida tion of Cl- by reactions (37 -39) [119]. Hypochlorite ions are responsible for two
anodic reactions that influence the process in different way s. First , ammonium ions

36
produced at the cathode because of the nitrate reduction are oxidized and transformed to
nitrogen gas by reaction (40 ) [119,120], improving the selectivity towards nitrogen. On
the other hand, nitrite ions are oxidized back to nitrat e by reaction (41), decreasing the
efficiency of the process.
2Cl-  Cl2 + 2e- (37)
Cl2 + H 2O  HClO + H+ + Cl- (38)
HClO  OCl- + H+ (39)
2NH 3 + 2OCl-  N2 + 2HCl + 2H 2O + 2e- (40)
NO 2- + OCl-  NO 3- + Cl- (41)
The i nfluence of chloride ions on the electroreduction of nitrates using different
cathodic materials has been reported [121-125]., Li et al. [122] studied the influence of
Cl- in the overall performance of the process using Fe, Cu and Ti cathodes. In all cases,
increasing the chloride concentration decrease d the nitrate reduction rate. Szpyrkowicz
et al. reported no improvement with chloride ion s when Pd -Cu, Pd -Co or Pd -Cu-Co
sprayed on stainless steel (SS) or titania were used as cathod ic materials [120].
Nevertheless, a positive effect of the chloride concentration on nitrate electroreduction
was repor ted with tin [126], boron doped diamond ( BDD ) [119] and graphite (GF)
electrodes [124]. It can be concluded that depending on the electrode materials, the
presence of chloride ions can lead to different phenomena that influence the
electrochemical nitrate reduction rate [122,123,127-129].
3.2 Configurations of electro catalytic reduction cells
The single chamber cell (SCC) and dual-chamber cell (DCC) have been the most used
electrocatalytic cell configuration s for the electroreduction of aqueous nitrates. The
SCC reactor has a unique compartment in which both electrodes are in contact with the

37
electrolyte ; in the DCC reactor , the cathodic chamber is separated from the anodic one
by a cation exchange membrane (CEM) , as depicted in Figure 11 .

Figure 11. Mechanism of SCC and DCC in the presence of chloride ions . Adapted from [124]
Ammonium ions, which are present in the cathodic chamber in equilibrium with free
ammonia , are transported through the CEM into the anodic compartment due to the
concentration gradient . Here, ammonium ions can be converted to nitrogen gas by
reaction (40 ), that is, by reacting with hypochlorite ions that were formed from chloride
oxidation, as indicated by reactions ( 37-39) [120]. However, nitrite ions cannot reach
the anodic chamber because of the CEM, avoiding the oxidation of nitrite to nitrate.
Thus , the anodic compartment works as a “sink” for NH 4+ ions [120]. The reaction
mechanism s in both single chamber and dual chamber cells are depicted in Figure 11.
The i mprovement o f the electroreduction performance using DCC compared to SCC has
been demonstrated by Ding et al. [124] with different cathod ic materials (GF, Cu 90Ni10,
and Ti) ; graphite achiev ed the best nitrate removal performance (70%). These results
are consistent with the observations publishe d by Szpyrkowicz et al. [120] with the
sprayed Pd -Cu/SS, Pd -Co/Ti and Pd -Co-Cu/Ti cathodes.
The electrochemical nitrate reduction using conventional electrochemical cells needs a
high conductiv ity medium that adds to the difficulty of separat ing the supporting
electrolyte from the treated water. In this context, a new reactor configuration with no
supporting electrolyte was presented by Hasnat et al. [130]. In this reactor, H 2O
molecules are split into O 2 and H+ on the anode surface. The protons migrate through

38
the H+-condu cting Nafion membrane to the cathode surface to be converted into atomic
hydrogen under the applied potential. The produced hydrogen is responsible for the
chemical reduction of nitrates a nd nitrites at the cathode surface . The a node and cathode
are supported over the external surfaces of the Nafion -117 film. The electrodes are
prepared via the deposition of Pt on the both sides of the membrane. Cu is also
deposited on the cathode to fabricate an asymmetric reactor (Pt|Nafion|Pt -Cu, Figure
12). The distance between the electrode s correspond s to the membrane thickness,
which is 180 µm. The main drawback of this configuration is that ammonia was
produced on the Cu surface [130]; when other cathodic materials (Pd, Pt -Pd, Pd -Cu, Pd –
Ag) were tested [131], even higher ammonia production was achieved. To increase the
selectivity to inert N 2, new reaction conditions and cathodic materials must be explored.

Figure 12. Schem atic of the electrochemical reactor with Nafion -117 membrane as electrolyte [130].
A novel reacto r configuration in which catalyst particles are introduced into the cathodic
chamber of a dual compartment cell was recently proposed to combine the advantages
of catalytic and electrocatalytic reduction of nitrate [132,133]. As shown in Figure 13,
the protons generated in the anode migrate to the cathodic chamber through the proton
exchange membrane that acts as a reducing agent . The beneficial effects of introducing
Pd-Cu/Al 2O3 catalysts in the cath odic compartment of an SC C with graphite electrodes
were reported by Zhang et al. Both the nitrate reduction rate and the selectivity of the

39
process towards nitrogen were increased, reaching 8 0% N2 selectivity when nitrates
were completely removed. The enhancement in the performance of the process is due to
the presence of the coupled catalytic reduction of nitrate with the appropriate amount of
in situ generated hydrogen by electrolysis as a reducing agent . In the study reported by
Lan et al. [133], in which Pd -Sn/AC particles were introduced in a DCC with DSA
electrodes, 90% of the nitrates were converted to nitrogen gas when the initial
concentration was 24.6 mg NO 3-/l.

Figure 13. Schematic diagram of an electrocatalytic reactor based on Pd –Sn/AC particles [133].

3.3 Electrode materials
The performance of the electrocatalytic reduction of nitrates in terms of selectivity
towards nitrogen and n itrate reduction rate is strongly dependent o n the electrode
materials, supporting electrolyte, applied potential and pH of the medium. The
challenge remains to find suitable operating conditions and electrode materials to reduce
nitrate and simultaneously oxidize the unfavourable ammonia formed during the

40
process, obtaining inert nitrogen as the main product. The r eaction pathway in the
catalytic electroreduct ion has been investigated for many electrode materials.
For instance, both the reaction pathways and the nitrate electroreduction performance
using a Sn cathode in a dual chamber cell have been reported by Katsounaros et al.
[126,134-136]. The ammonium production path, in which hydroxylamine appears as
intermediate, was clarif ied, and the main reaction that leads to N2 was discussed in
terms of two possible reduction routes from NO . In the first route , the identification of
the reaction intermediates was no t possible ; in the second route , N2O appears as the
main intermediate [136]. Therefore, it has been proved that reduction on Sn is more
efficient at very negative potentials because both the reduction rate and the selectivity
towards nitrogen are higher at that condition . Nitrogen selectivity of 70% was achieved
with a Sn cathode in DCC when 95% of the nitrate s were reduced from an initial
concentration of 100 mg/l. The corrosion of tin appears as the most serious drawback of
these electrodes since the presence of tin in the bulk water is non -acceptable [134].
Bismuth cathodes for the electrochemical reduction of nitrates were also studied by the
same research group [137-139]. In this case, th e rate of cathodic corrosion of Bi was
significantly lower than the tin corrosion rate , and its presence in the final solution was
not a problem. In addition, the electrocatalytic activity of Bi had the best performance
when compared with other metals such as Sn, Al, In or Pb [138]. This is due to the
electrochemical reduction of nitrate on bismuth proceed ing through an autocatalytic
reaction, the rate of which is significantly higher than that of the electrochemical
reaction . Moreover, the main product s were nitrogen gas and nitrous oxide with
selectivities of 65% and 22 %, respectively, working with 0.4 M NaHCO 3/0.4 M
Na2CO 3 as the electrolyte , an initial nitrate concentration of 0.0 5 M NaNO 3 and very
negative potenti als (-2.6 V) .

41
Different electrode materials , such as graphite (GF) [124,140,141] , SiC [140] , SS
[140,142] , Ni [141] , Cu [141,143 -146], Pt [141,147,148] , Pd [149] , Pb [139,140] , Ti
[122,129] , Rh [149] conductive diamond electrode (CDE) and boron -doped diamond
(BDD) [119] , have been studied in the literature to improve the performance of the
electroreduction of nitrates. Graphit e and copper showed the highest catalytic activity
compared with the other mater ials [124,129,140,141]. Nevertheless, when Cu or GF
electrodes were used, harmful ammonium and nitrite appear as the main products of the
process in alkaline media [124] as well as in acidic media [141]. Furthermore, several
studies have investigated the n itrate electroreduction on different bimetallic, ternary
metallic or alloy electrodes to improve the activity and the gaseous nitrogen selectivity
due to the likely synergic effect between the different materials . For instance, significant
improvement in the nitrate electroreduction was obtained with Cu-Ni [150-153], Cu-Sn
[144], Cu-Rh [152,154], Cu-Zn [123,143,155] Cu-Pd [156-159], and Sn -Pd [160-163]
cathode s, all of which have improved corrosion resistance and stability relative to
monometallic Cu or Sn electrodes. In the case of the Cu-Zn cathode , ammonia was
found as the main product by Mattarozzi et al. [155]. This agrees with the results
reported by Macova et al. [143]. Even though Cu-Ni cathode s with high copper content
showed high selectivity to ammonium and lower activity than Cu -Rh cathodes , Cu-Ni
alloy is considered a promising and suitable cathode due to it s good stability under most
conditions [152]. On the other hand, although Pd cathodes have displayed practically no
activity in nitrate electroreduction [159,160], they might enhance the electrocatalytic
activity of Cu or Sn cathodes . In the case of Pd -Cu electrodes, the role of Cu is to
promote the reduction of NO 3- to NO 2-, and the role of Pd is to reduce nitrite to N 2 with
high selectivity [156]. The use of higher amounts of Pd in the electrode composition has
been shown to be beneficial towards N2 selectivity. A maximum N 2 selectivity of 70%

42
has been reported using Pd-Cu modified graphite cathode s (95 at% Pd, 5 at% Cu) in an
alkaline medium with 1 M NaOH as the electrolyte. When neutral pH was used, nitrite
and ammonia production was higher [158]. Reyter et al. reported that the higher the Pd
at% in the electrode is, the higher the nitrogen selectivity is, and the lower the overall
reduction activity is [159]. Similar results were reported by Anastasopoulos et al. with
nitrate electroreduction activity as high as 84% using Cu [157]. The role of Pd on the
electroreduction of nitrates with tin -modified Pd electrodes was found to be similar to
the explanation presented above for Pd -Cu electrodes. N2O was formed as the main
product in the reduction of nitrate ions with 84% selectivity (9 2% of nitrates reduced)
when the reduction proceeded in an electrolysis cell with 0.1 M HClO 4 as the electrolyte
and Sn -Pd on Au electrodes. In addition, hydroxylamine was formed as the main non –
volatile product , whereas low amount s of ammonia were found (<2%) [161,162]. More
recently, a similar product distribution was found with Pd -Sn film electrodes by Birdja
et al. [160].
Acceptable values of nitrogen selectivity during nitrate electroreduction are very
difficult to achieve. The selective anodic oxidation of ammonia, as explained above in
reactions (39 -43), has appeared as a promising approach in combination with the
electrocatalytic reduction of nitrates. If suitable electrode materials and operation
conditions are chosen, the oxidation of NH 4+ (and NO 2- if no CEM is included in the
electrolysis cell) can occur in a selective way towards nitrogen gas. In this way, it has
been reported that ammonia oxidation into nitrogen can be enhanced by using BDD or
Ti/PtOx -IrO 2 anodes in a single compartment cell (Table 5), increasing the nitrate
electroreduction rate and the selectivity towards gaseous nitrogen [151,164].

43

Table 5. Overview of the results published in the literature for the electrocatalytic denitrification with selective
ammonium oxidation towards nitrogen. Adapted from [163]
Anode Cathode Electrolyte Cell pH Performance Ref.
Ti/IrO2 -Pt Fe 100 mg/L NaNO 3,
0.5 g/L Na 2SO4,
different amounts
of NaCl Undivided 3 < pH < 11
Uncontrolled XNO3 – = 80% , NH 3 and
NO 2- below detection
limit [166]
IrO 2-Ta2O5,
CDE SS 21 mM NaNO 3, 0.02
M Na 2SO4 Undivided pH = 2
Uncontrolled XNO3 – = 90%. NO 2-, NH 3
by products formed [167]
Ti/IrO 2-Pt Fe, Cu, Ti 100 mg/L NaNO 3,
0.5 g/L Na 2SO4,
different amounts
of NaCl Undivided Uncontrolled Without NaCl: Fe (X NO3 – =
93%) With 0.5 g/L
NaCl:(X NO3 – = 93% and
SN2=100% ) [122]
Ti/IrO 2-
TiO 2-RuO 2 Cu/Zn 60 mg/L NaNO 3, 0.5
g/L NaCl Undivided pH = 7.8
PBS buffer XNO3 – = 94.41% ,
suppression of NH 3 (0.5
g/L NaCl added) [168]
BDD, DSA SS 1.76 mM NaNO3,
different amounts
of NaCl Undivided pH controlled XNO3 – = 100%, NH 3, NO 2-
and ClO 3- by products
formed [142]
Ti/IrO2 -Pt Cu/Zn 100 mg/L NaNO 3,
0.5 g/L Na 2SO4,
different amounts
of NaCl Undivided Uncontrolled XNO3 – = 90%. With 0.5 g/L
NaCl, Neither NO 2-, NH 3
were detected [128]
Ti/IrO2 0.1 M NaNO 3, 0.5 M
NaCl and 0.01 M
NaOH Undivided pH = 12 XNO3 – < 50% ; SN2 = 100% [146]

High pH and the presence of chloride permit the production of hypochlorite, enhancing
the oxidation of ammonia to nitrogen and the production of ammonium in the cathode
as a nitrate reduction product. Thus, selectivity of 100% to N2 was also reached by
oxidizing all the ammonia produced with a Ti/IrO 2 anode in an undivided cell with
alkaline conditions (pH = 12) and an energy consumption of 14.7 kWh kg-1 by Reyter et
al. [151]. In the research carried out by Li et al., the selectivity of the process towards
nitrogen increased to 100% (87% of nitrate conversion) by using a Ti/IrO 2-Pt anode
with a high oxidation rate of ammonium to N 2 [122]. In a similar study carried out by
the same author [128], almost complete nitrate conversion to nitrogen gas was reached
using a Cu -Zn cathode and a Ti/IrO 2-Pt anode in the presence of sodium chloride as the
supporting el ectrolyte with no pH control. The discovery of the conditions that are

44
needed to achieve high nitrate removal and high nitrogen selectivity (a basic pH, the
addition of electrolyte or the use of chloride ions to oxidize ammonium to nitrogen in
the anode ) makes electro catalytic reduction a real alternative for wastewater treatment.
However, in the case of drinking water treatment, the use of high pH or chemicals is not
a valid option because of the additional treatment needed to adjust the pH or to reduce
the concentr ation of the different species used in the electrolysis, electrolyte or the
chloride ions down to acceptable limits .

4. CONCLUSIONS
A comprehensive review of the current state-of-the-art of both catalytic and
electro catalytic reduction of nitrates is presented. These emerging and promising
technologies for nitrate removal from groundwater and wastewater have been
extensively studied because of their capacity to convert nitrates to nitrogen in a selective
way. However, co-production of ammonium is still the main drawback of these
processes despite the nitrogen selectivity reaching greater than 90%.
Monometallic catalysts prepared by using partially reducible supports have shown
lower performance in terms of activity and N 2 selectivity th an bimetallic catalyst s.
Although the Pd -Cu catalyst was the most active and selective when synthetic water
was used, it was more strongly affected by the ions present in natural waters than other
bimetallic catalysts , such as Pd -Sn or Pd -In. Although Pd-In catalyst was the most
active, Pd -Sn showed a similar reduction rate of nitrates but lower ammonium
production . Despite the good results achieved with batch reactors in laboratory
conditions , when more realistic continuous reactor and natural water conditions were
tested, the performance of the catalytic nitrate reduction showed an ammonium

45
selectivity of 30% in the best reported case. The main disadvantages of packed bed or
trickle -bed reactors, which are the most commonly used configurations to carry out
catalytic reduction are the loss of catalyst particles and the diffusional limitations .
Therefore, catalytic membrane reactor s have been identified as a sound alternative to
perform this process. Nevertheless, more research is required to propose a competitive
reactor configuration that improves the performance of the continuous reactors reported
so far .
In the electro catalytic reduction processes , total selectivity towards inert nitrogen gas
and high nitrate reduction were achieved when Ti/IrO 2 anod es were used in a single
chamber cell in which ammonium was homogeneously oxidized due to the presence of
chloride ions. However, high pH values and the addition of electrolyte and chloride ions
are required to achieve high efficacy in the process. These n eeds imply additional step s
to neutralize the pH and to reduce the concentrations of the ionic species to the levels
allowed for drinking water, making the process less competitive for drinking water
production . For this reason, the electroreduction of nit rates in water seems to be a more
realistic alternative for the treatment of wastewater.

Acknowledgements
This research work was supported by the Spanish Ministry of Economy and
Competitiveness (Projects CTQ2015 -66078 -R, CTM2014 -57833 -R).

46

47

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List of figure captions
Figure 1. Reaction mechanism of the catalytic reduction of nitrate .
Figure 2. Mechanism of the catalytic nitrate reduction over bimetallic catalysts.
Figure 14. Mechanism of nitrate reductio n over monometallic Fe catalyst [76]
Figure 15. Proposed models of Pd and Pd -Cu particles [41]
Figure 16. Catalytic reduction of nitrate process using a packed bed reactor [9]
Figure 17. Schematic of a hollow fibre dialyser filled with microscopic catalysts [114]
Figure 18. (a) CMR interfacial configuration and (b) CMR flow -through configuration
[115]
Figure 19. Schematic of the CMR setup used by Chen et al. [16]
Figure 20. Gas –liquid–solid contact in the porous ceramic reactor [116]
Figure 21. SEM image of the polymeric hollow fibre with 27 wt% of catalyst particles
Pd:Cu/Al 2O3 [21]
Figure 11. Mechanism of SCC and DCC in the presence of chloride ions. Adapted from
[124]

67
Figure 22. Schem atic of the electrochemical reactor with Nafion -117 membrane as
electrolyte [130]
Figure 13. Schematic diagram of an electrocatalytic reactor ba sed on Pd –Sn/AC
particles [133]