Electrocatalytic reduction of nitrate on copper electrode in alkaline solution

Article abstract of DOI:10.1016/j.electacta.2008.08.003

This work is devoted to the study of the kinetics and reaction mechanism of nitrate reduction on a copper electrode in 0.1 M NaOH, which acts as the supporting electrolyte. The experimental methods include cyclic voltammetry (CV), cronoamperometry (CA), c

Full text of DOI:10.1016/j.electacta.2008.08.003

Electrochimica Acta 54 (2009) 996–1001
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Electrochimica Acta
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Electrocatalytic reduction of nitrate on copper electrode in alkaline solution
a,b,∗,1
Gabriela Elena Badea
a
Department of Energy Engineering and Management, Faculty of Energy Engineering, University of Oradea, Universit a˘ t¸ ii 1, 410087 Oradea, Romania
b
Focus Studio, Srl, Romania
a r t i c l e i n f o
a b s t r a c t
Article history:
This work is devoted to the study of the kinetics and reaction mechanism of nitrate reduction on a copper
electrode in 0.1 M NaOH, which acts as the supporting electrolyte. The experimental methods include
cyclic voltammetry (CV), cronoamperometry (CA), controlled-potential electrolysis (CPE), and coulometry.
In CV, there are three potential regions where charge transfer reactions take place, reactions which are
Received 10 March 2008
Received in revised form 30 July 2008
Accepted 2 August 2008
Available online 9 August 2008
−
associated with NO3 and/or intermediates reduction. Two isopotential points observed in CV indicate
the existence of some competitive adsorption processes at the electrode surface.
The three charge transfer steps were also made evident in the CA, CPE and coulometry studies. The
Keywords:
Nitrate
Copper electrode
Alkaline solution
Electrocatalytic mechanism
−
correlation of the experimental results with the literature data led to the conclusion that NO3 reduction
2
−
on a copper electrode in 0.1 M NaOH has an intermediate (N2O2 ) species, which reduces to N2 at a
potential of about −1.3 V and to NH4OH at potential values lower than −1.4 V (both values are vs. SCE).
©
2008 Elsevier Ltd. All rights reserved.
1
. Introduction
(3) biological methods [13,14], which, in addition to being slow and
difficult to control, cannot be used for nitrate concentrations
−
1
The electrochemical reduction of nitrate has interesting reac-
tion applications, especially with regard to the decontamination
of surface and underground waters. The intensive agriculture [1,2]
and nuclear industries [3] are the principle sources of nitrate con-
tamination of the water. The maximum level admitted in the EU
larger than 1000 mg L to avoid poisoning the bacteria;
(4) electrochemical methods, which do not require a reduction
agent and which could be applied at large nitrate concentra-
tions, are selective and form final products based upon the
electrolyte pH, cathode material, and applied potential.
−1
−1
for nitrates in potable water is 50 mg L for adults and 15 mg L
for children. [2,4–6]. The high concentrations of nitrates in potable
water can cause serious health problems, such as liver disease, can-
cer, or “blue baby syndrome”.
For the nitrates removal from the potable water, various meth-
ods have been proposed:
Electrochemical reduction of nitrates has been investigated on
pure metal electrodes, including Pt [15,16], Pd [17,18], Cu [19–23], Ag
[23], Ni [24,25], Rh [26], Sn [27], Pb [28], binary alloys (such as CuPt
[29], CuSn [30] CuZn [31]), PdRh1.5 /Ti [32], metallic electrodes mod-
ified by upd deposition [33–35] or opd deposition [16,22,36–39],
and by adding ions with electrocatalytical properties in the elec-
trolyte, as MoO42 , WO4 , and cobalt complexes [40].
−
2−
(
1) chemical reduction with hydrogen in the presence of catalysts
7–9], but the necessity of a reduction agent and the formation
In an alkaline medium, various authors indicated nitrogen
or/and ammonium as reaction products of nitrate reduction,
depending on the electrode chemical nature and the applied poten-
tial. The nitrogen was found as main reaction product of the
nitrate reduction at some electrodes: Pt–platinized electrodes [15],
Ni–platinized electrodes [24], electrodes obtained by electrode-
posited rhodium films on titanium substrate [26] or Sn electrode
[
of toxic products, such as nitrite and ammonium, are disadvan-
tages;
(
2) physical–chemical processes (ionic change [10], electrodialysis
[
11] or inverse osmosis [12]), but the nitrates are only separated,
not destroyed;
[
27]. Nearly complete reduction of nitrates and nitrites to nitro-
gen, ammonia, or nitrous oxides in alkaline nuclear wastewater on
lead cathode, in a divided cell, was reported by Genders et al. [28].
Polatides and Kyriacou [30] studying the nitrate reduction in 0.1 M
K SO on various cathodes reported that the more efficient cath-
∗ Correspondence address: Department of Energy Engineering and Management,
Faculty of Energy Engineering, University of Oradea, Universit a˘ t¸ ii 1, 410087 Oradea,
Romania. Tel.: +40 259 408 440; fax: +40 259 408 406.
E-mail address: gbadea@uoradea.ro.
2
4
odes with regard to the nitrate conversion to nitrogen were Al and
the alloy Sn85Cu15
1
ISE member.
.
0
013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2008.08.003

G.E. Badea / Electrochimica Acta 54 (2009) 996–1001
997
The electrocatalytical behaviour of copper in reduction reac-
tions is maintained by Burke and Sharna [41] in a recent paper.
The good electrochemical activity for nitrate reduction has been
observed on electrodes made of metallic Cu [21–23,30] and elec-
trodeposited Cu [38] in alkaline solutions, and on binary electrodes
such as CuPt [29], CuPd [42] in acidic media. The nitrate reduction
at copper electrode in alkaline medium is of interest because the
system is less likely to produce oxides of nitrogen as by products
[
22,23,30,43] and because it is less corrosive when compared to
reduction in acidic medium.
In this paper, an electrocatalytical mechanism for nitrate reduc-
tion at a copper electrode in an alkaline medium is proposed on
the basis of the cyclic voltammetry, cronoamperometry, controlled
potential electrolysis, and coulometry studies.
2. Experimental
CV, CA, CPE and coulometry studies have been performed on
the nitrate reduction reaction on a copper cathode (99.99% purity)
in 0.1 M NaOH (pH 12) solution. Glass cells with three electrodes
both divided and undivided were used, for CV, CA experiments
and CPE studies, respectively. In the CPE experiment the solu-
Fig. 1. Cyclic voltammograms in function of cyclic number for system: Cu/0.1 M
NaOH + 80 mM NO3− (scan rate: ꢀ = 50 mV s−1, potential range: −700 to −1650 mV
tion was stirred. The working electrodes had areas of 0.413 and
2
1
0 cm for CV, CA and CPE measurements, respectively. The prepa-
vs. SCE). Insert: Figure with more cyclic voltammograms.
ration of the working electrode surface consisted of polishing with
000-grade emery paper, degreasing with acetone, washing with
distilled water, followed by pickling in 50% HNO3 solution for
0 s, and, finally, washing with double-distilled water. The counter-
electrodes were made from Pt sheet with bigger areas than those
of the working electrodes. The reference electrode was a saturated
calomel electrode (SCE) connected to the working electrodes by a
Luggin capillary.
3
The characteristic potentials vary during the first few cycles
and remain practically constant thereafter. The currents reach a
steady state after the 15th cycle and the next voltammograms were
recorded at this cycle.
1
In Fig. 1, the appearance of the isopotential points, P at −1.42 V in
ꢀ
the cathodic sense and P at −1.32 V in the anodic sense, can be seen.
The instrumentation consisted of a PAR Model 173 potentiostat,
a function generator, and a Radelkis X-Y recorder.
On the cathodic sense, the isopotential point P appears after the 9th
cycle, while on the anodic sense it appears after the 3rd cycle, as it
can be seen in the insert in Fig. 1. The appearance of an isopotential
point shows a strict relation between the covering degrees of the
two species and the irreversible blocking of the electrode surface
independent of potential, according to Horanyi and coworkers [37].
The adsorbed hydrogen determines a drastic current decreas-
ing in the first cycles, followed by the current increase due to
hydrogen evolution reaction over that. A distinct step for the ammo-
nium forming (C) and the limiting current increase of this can be
observed after the 9th cycle, when all subsequent voltammograms
pass through the same point, i.e. the isopotential point P. This means
that only after the 9th cycle, an adsorbed nitrate reduction inter-
mediate is in a strict relation with the adsorbed hydrogen; the two
adsorbed species react further to form the ammonium.
The solutions were prepared using chemically pure reagents
and double-distilled water. The nitrate was added as NaNO3.
The solutions were not de-aerated because the oxygen reduction
peak is separated from the nitrate reduction wave by hundreds
of millivolts. All measurements were made at room temperature
(
297 ± 1 K).
3. Results and discussion
3.1. CV and CA data
Fig. 1 shows the cyclic voltammograms of the nitrate reduction
reaction at a copper electrode in 0.1 M NaOH solution containing
−
8
0 mM NO3 as a function of the cycle number. On the cathodic
Other cyclic voltammograms were recorded as a function of
nitrate concentration and potential scan rate. The results are sum-
marized in Figs. 2 and 3.
sense of the voltammograms, three steps of charge transfer are
associated with reduction of nitrate or intermediates: an inflexion
(
A) at about −1.1 V, a distinct peak (B) at about −1.3 V, and a limiting
According to Fig. 2(a), the peak potential, Ep (B), shifts to the
negative direction with an increase in nitrate concentration, which
is in accordance with the behaviour of the irreversible charge
transfer reactions. The peak and limiting currents increase with
nitrate concentration, confirming the participation of nitrate ion
or intermediates at these charge transfer steps. Although the peak
and limiting currents vary linearly with nitrate concentration, the
straight lines do not pass through the origin. Therefore, the impli-
cated electrode processes are not under pure diffusion control.
Fig. 3 shows the effect of the potential scan rate, ꢀ, on the peak
current, ip (Fig. 3(a)), both on the peak potential, Ep, and on the
semi-peak potential, E1/2, (Fig. 3(b)), corresponding to peak B. The
dependence ip (B) − ꢀ1 (Fig. 3(a)) is not linear, confirming the
kinetic control of the process, probably by a surface reaction.
current (C) at the potentials <−1.45 V. At more negative potentials
the current increases on account of hydrogen evolution reaction.
ꢀ
ꢀ
In the reverse direction, the limiting current (C ) and the peak (B )
appear as well.
The reduction processes described by the peak and limiting cur-
rents must be the same in both the cathodic and the anodic sense,
corresponding to the potential range, because the currents change
in the same direction with an increase of the cycle number: the (B)
and (B ) peak currents decrease owing to the adsorption of nitrate-
reduction products or hydrogen, blocking the electrode surface and
slowing down the nitrate electroreduction rate; the (C) and (C )
limiting currents increase on account of accumulation of an inter-
mediate formed at the electrode surface during the (B) peak.
ꢀ
ꢀ
/2

9
98
G.E. Badea / Electrochimica Acta 54 (2009) 996–1001
−
Fig. 2. The effect of nitrate concentration on the peak potential (a) and on the peak and limiting currents (b) (from CV for system Cu/0.1 M NaOH + x mM NO3 , cycle 15,
−1
ꢀ
= 50 mV s , potential range: −700 to −1650 mV vs. SCE).
The shift to the negative direction of the (B) peak and semi-
peak potentials (Fig. 3(b)) as well as the value of difference
nitrate to nitrite because neither peak A nor the inflexion is present
in the CV obtained from the nitrite ion reduction, as shown pre-
viously [20]. At more negative potential (−1230 and −1400 mV)
appear steps of constant current, which are a consequence of some
slow rate-determining processes such as adsorption or a chemical
reaction coupled with the charge transfer reaction [46]. The step
of constant current may be attributed to the necessity of a time
for intermediate concentration at the surface to attain a sufficient
level.
(
Ep−E ) ≈ 185 mV support arguments for the irreversible char-
p/2
acter of the charge transfer reaction. For an irreversible reaction,
the peak and semi-peak potentials are correlated with the charge
transfer coefficient (˛) by the relation [44]:
RT
Ep − Ep/2 (mV) = 1.857
.
(1)
˛
nF
At 298 K, ˛n = 0.258, which speaks to the important role of the
adsorption process in the reaction kinetics and mechanism corre-
sponding to the second charge transfer step.
3.2. CPE and coulometry data
Analyzing the dependence of the current function (ipꢀ−1/2) and
of the differential ꢁEp/2/ꢁ log ꢀ on log ꢀ (Fig. 4(a and b), respec-
tively) in accordance with Nicholson–Shain schemes [45,46], peak
B corresponds to the case VIII—catalytic reaction and irreversible
charge transfer.
The controlled potential electrolysis performed at potentials
of −1230 and −1400 mV (both are vs. SCE). These potential val-
ꢀ
ꢀ
ues correspond to the peaks B , respectively C from CV showed
in Fig. 1. The used solution for CPE measurements was 20 mM
NaNO3 in 0.1 M NaOH. During the electrolysis the current–time
curves were recorded. The current–time (I–t) curves recorded
have presented steps of constant current, similar to cronoam-
perometric curves from Fig. 5, corresponding to the potentials
Previously [20], the CV for the system (copper/5 mM
NaNO + 1 mM NaOH + 31 mM NaSO4 at 0.1 M ionic strength)
3
was presented and the distinct peak A, which was analyzed
according to Nicholson–Shan schemes, showed that it corresponds
to a case IV—reversible chemical reaction followed by irreversible
charge transfer.
The typical chronoamperometric curves at different potential
steps are shown in Fig. 5. At the potential step of −1100 mV, the
current decreases exponentially with time, this corresponds to a
simple charge transfer reaction controlled by diffusion of the reac-
tant at the interface. The corresponding reaction is the reduction of
−
1230 and −1400 mV vs. SCE. Faraday’s law states that 96,480 C
are consumed per equivalent of substances electrolyzed. The
number of electrons, n, consumed per mole of electroactive sub-
stance, at a given moment, was calculated with the relation
[
46]:
Q
n =
,
(2)
96, 480m
Fig. 3. Plots ip (B), Ep (B) and Ep/2 (B) vs. ꢀ2/2 (From CV for system: Cu/0.1 M NaOH + 0.1 M NO3−, cycle 15, potential range: −700 to −1650 mV vs. SCE).

G.E. Badea / Electrochimica Acta 54 (2009) 996–1001
999
Fig. 4. Plots of the cathodic current function, ipcꢀ−1/2, (a) and the change in cathodic half-peak potential, ꢁEpc/2/ꢁlog ꢀ, (b) as functions of potential scan rate for the peak B.
−
(
System: Cu/0.1 M NaOH + 0.1 M NO3 , cycle 15).
where m is the number of moles of reactant electrolyzed and Q is
the number of coulombs consumed, which had been deduced from
I–t curve.
nitrate reduction on Cu electrode in 0.1 M K SO4 at the potential
value of −1.5 V vs. Ag/AgCl.
2
The corresponding dependence I–n for the two potentials is
shown in Fig. 6(a) and (b). By linear regression, the number of
electrons transferred at each potential step is obtained.
At a potential of −1230 mV, two potential steps can be seen,
corresponding to n = 1.62 and 5.2, respectively. This implies the
3.3. Reaction mechanism
Analyzing the experimental results and the literature data, the
following reaction mechanism may be maintained for the nitrate
reduction in an alkaline medium on a copper electrode.
−
−
scheme: NO3 → NO → (1/2)N .
The first charge transfer step corresponds to the potential
of approximate −1100 mV vs. SCE range, corresponding to the
inflexion (A) of CV from Fig. 1. According to the cronoamper-
ometric transient at −1100 mV, the corresponding reaction is a
simple charge transfer reaction controlled by diffusion of the reac-
tant at the interface. In accordance with the Nicholson–Shain
scheme [45,46], however, peak A corresponds to case IV [20], i.e.,
a reversible chemical reaction that foregoes an irreversible charge
transfer. Because this inflexion is not present on the CV obtained
2
2
At a potential of −1400 mV, n = 1.71 was obtained for the first
step and n = 8.26 for the second, which corresponds to the scheme:
−
−
NO3 → NO → NH .
2
3
After electrolysis, the solution was analyzed with Nessler reac-
tive (Ammonium Test-Aquamerck). The electrolyzed solution at a
potential of −1230 mV vs. SCE contained 3–5 mM ammonium. Con-
form to the presented anterior data, the principal reaction product
is the nitrogen. In this potential range, the nitrogen is reported as
reaction product, also, by Tucker et al. [26] at nitrate reduction
on activated rhodium electrode in alkaline solution. The solution
electrolyzed at a potential of −1400 mV vs. SCE contained ∼20 mM
ammonium. This means that all amount of nitrate initially present
in solution was reduced at ammonium. The ammonium forma-
tion on Cu, at more cathodic polarisation, was reported, also, by
Polatides and Kyriacou [30]. They found a 77.3% of NH3 formed at
−
for the NO2 ion reduction, as shown previously [20], it leads to
the assumption that the first charge transfer step corresponds to
the nitrate reduction to nitrite. The chemical reaction that precedes
the chargetransferis the reversible nitrateadsorptionon thecopper
surface.
The catalytic effect of the copper electrode for this step may
be supported by the Incipient Hydrous Oxide-Adatom Mediator
−
Fig. 5. Chronoamperometric curves obtained at different potential step for system: Cu/0.1 M NaOH + 5 mM NO3
.

1000
G.E. Badea / Electrochimica Acta 54 (2009) 996–1001
−
Fig. 6. Plots of the dependence I–n, at the controlled potential electrolysis—(a) −1230 mV and (b) −1400 mV (both vs. SCE), for system: Cu/0.1 M NaOH + 20 mM NO3 . The
number of transferred electrons for each reaction step was calculated by linear regression.
−
(
IHOAM) model [47], which describes an activated chemosorption
The NO intermediate specie, resulted in the reaction (3) is
instable and his stabilisation is made by dimerisation. The dim-
mer formation is facilitated by copper electrode, because the bond
model. According to the IHOAM model, the oxidation of the pre-
monolayer on the surface of polycrystalline metals in the presence
of water and the formation of a hydrated species has a catalytic
effect on the global reaction, due to the transformation of the
metallic ion in an active centre M* and the supplementary electron
generation.
length Cu Cu (1.32 Å) is comparable with the bond length N
N
(1.4 Å)[48]. The electrocatalytic effect of the copper electrode could
be explained by an activated chemisorption model, where the pres-
ence of semioccupied d orbitals promotes the adsorption of the
active intermediate species NO and N O
Thus, at a high current density, when the number of NO ions at
the electrode surface is large, a dissociative adsorption is produced
−
−
2−
The reduction of a dissolved oxidant, such as NO3 , involves
, as shown in Fig. 8.
2
2
−
interaction with an active surface metal atom (M*) which supplies
the electrons (ze) for reduction of nitrate to NO2 . This active metal
−
2−
atom is converted simultaneously to the incipient hydrous oxide.
The hydrous oxide will be reduced, and the mediator (M*) will be
restored, by a flow of current (iext) from the external circuit.
The electrocatalytic mechanism for the first nitrate reduction
peak (A) conforms to the IHOAM model and to case IV of the
Nicholson–Shain mechanism (irreversible charge transfer coupled
with reversible chemical reaction) may be represented by the
scheme shown in Fig. 7.
that generates the N O
specie, which is adsorbed stronger than
2
2
−
NO on the electrode surface because it forms two bonds with the
metal [48]. The adsorption process for this step is also sustained by
the low value of the charge transfer coefficient (˛ = 0.258). The reac-
tion (3) and (4) are fast, they take place practical at equilibrium. The
chemical reaction (5) is the rate-determined step, followed by the
irreversible charge transfer reaction (6) with the formation of N2.
N2 has as direct intermediates N O or H N O species [49]. The
2
2
2
2
The second charge transfer step (B peak from CV: Fig. 1), conform
to Nicholson–Shain schemes corresponds to the case VIII—catalytic
chemical reaction and irreversible charge transfer. In accordance
with controlled potential electrolysis (Fig. 6(a)) the number of
transferred electrons in this step is five and corresponds to N2 as
reaction product. On the cronoamperogram recorded at potential
of −1230 mV (Fig. 5(b)) appears a step of constant current, which
is a consequence of some slow rate-determining processes such as
adsorption or a chemical reaction coupled with the charge transfer
reaction [46]. The mechanism of the second charge transfer step
may be described by the following reactions:
thermodynamic potentials [43], in the case N O/N2 (−0.94 V vs.
2
ENH) is nearly close from the second step potential found in this
work, while in the case H N O /N (−1.52 V vs. ENH) is more neg-
2
2
2
2
ative. N O and N2 were reported as reaction products for nitrate
2
reduction at porous Pt electrode in sulphuric acid solutions by
Nashimura et al. [49] at potentials more positive than the potentials
from this work. The succession of the reactions (3)–(6) is in accor-
dance with the scheme for nitrate reduction to nitrogen shown in
the paper [50].
ꢀ
The third charge transfer step (C and C on CV from Fig. 1)
takes place in the hydrogen region, only after the formation of
the isopotential point P. In accordance with the electrolysis at the
potential of −1400 mV vs. SCE, the number of transferred electrons
is 8 (Fig. 6(b)), corresponding to the ammonium as reaction prod-
uct. The cronoamperogram recorded at this potential (Fig. 5(c))
present a step of current constant, too, i.e. a step that implies a slow
chemical reaction [46], which means that a time for a reaction inter-
mediate accumulation, on the electrode surface, is needed before
the charge transfer reaction continues. The most probable reaction
2
(NO2 )ads + 2H O + 4e ↔ 2(NO ) + 4OH−
−
−
(3)
(4)
2
ads
−
2−
2
(NO ) ↔ (N O
)ads
ads
2
2
rds
(
(
N O2
−
)
+ H O−→ (N O)
+ 2OH−
(5)
(6)
2
2
2
2
ads
ads
−
N O) + H O + 2e → N + 2OH
2
ads
2
2
The total reaction is given by summarizing the reactions (3)–(6):
−
−
2
NO2 + 5H O + 6e → N + 10OH
(7)
2
2
Fig. 8. Illustration of the electrocatalytic reduction for nitrate intermediates at cop-
Fig. 7. Scheme of electrocatalytic mechanism for the first nitrate reduction peak (A)
conforms to the IHOAM model [47].
per electrode as activated chemisorption model (I and II are the resonance structures
of the N2O2 intermediate).
2
−

G.E. Badea / Electrochimica Acta 54 (2009) 996–1001
1001
[
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mechanism for this step may be described by the reactions:
[
rds
−
+ 2OH−
(
N O )
+ 2H O−→ (H N O )
(8)
(9)
2
2
2
2
2
2
ads
ads
1
−
(
H N O ) + 10H O + 8e → 2NH OH + 12OH
2
2
2
ads
2
4
[
or
[
(
H N O ) + 8Hads → 2NH OH
(10)
2
2
2
ads
4
[
(
H N O )
2
and Hads are the two intermediate species, which
2
2
ads
[
covering degrees are in a strict relation, at the isopotential points.
At more negative potentials, the covering degree with hydrogen
increases and the reaction (10) takes place.
A general scheme for the nitrate reduction on the copper elec-
trode in alkaline solutions is shown in Fig. 9.
Thus, the nitrogen and ammonium formation are competitive
reactions on a copper electrode in alkaline solution. The nitrogen
is the predominant reaction product at low cathodic polarisation,
while the ammonium is the main reaction product at high cathodic
polarisation.
1
[
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0
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The reaction proceeds in three charge transfer steps, which cor-
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