Electrochemical reduction of nitrate to ammonia at modified gold electrodes

Article abstract of DOI:10.1016/j.electacta.2003.11.025

The electrocatalytic activity of Au electrodes modified with thiourea (TU), urea (U), iodide ions (I^-) and sulfide ions (S^2-) towards reduction of NO₃^- to NH₃ have been tested. Cyclic voltammetry (CV) of the TU/Au electrode in 0.5M NaNO₃ (N₂-saturated, pH 12.5) revealed an increase of the reduction peak of NO₃^- at ca. +0.75V versus reversible hydrogen electrode (RHE) more than at the bare (i.e., unmodified) Au electrode. On the other hand, the intensity of this peak is more depressed at I^-/Au, S^2-/Au and U/Au electrodes. The Tafel slope is almost the same for the two reactions, i.e., the hydrogen evolution reaction (HER) and the nitrate reduction reaction suggesting a common rate-determining step (r.d.s.) for both reactions. Galvanostatic electrolysis of the basic nitrate solution resulted in three major reduction products i.e., NH₃, NO₂^- and H₂. The TU/Au electrode showed the highest current efficiency, Q, of the NH₃ production (about 85%), which is about 6.5 times higher than that of the NO₂^- production. While the I ^-/Au and S^2-/Au showed a sharp increase in the current efficiency for the hydrogen evolution reaction to about 43 and 47%, respectively, with a concurrent depression of the total current efficiency of the nitrate reduction.

Full text of DOI:10.1016/j.electacta.2003.11.025

Electrochimica Acta 49 (2004) 1639–1645
Electrochemical reduction of nitrate to ammonia at
modified gold electrodes
Mohamed S. El-Deab∗
Department of Chemistry, Faculty of Science, Cairo University, Cairo, Egypt
Received 22 July 2003; received in revised form 9 October 2003; accepted 25 November 2003
Abstract
The electrocatalytic activity of Au electrodes modified with thiourea (TU), urea (U), iodide ions (I−) and sulfide ions (S2−) towards reduction
−
of NO3 to NH3 have been tested. Cyclic voltammetry (CV) of the TU/Au electrode in 0.5 M NaNO3 (N2-saturated, pH 12.5) revealed an
increase of the reduction peak of NO3− at ca. +0.75 V versus reversible hydrogen electrode (RHE) more than at the bare (i.e., unmodified) Au
electrode. On the other hand, the intensity of this peak is more depressed at I−/Au, S2−/Au and U/Au electrodes. The Tafel slope is almost the
same for the two reactions, i.e., the hydrogen evolution reaction (HER) and the nitrate reduction reaction suggesting a common rate-determining
step −(r.d.s.) for both reactions. Galvanostatic electrolysis of the basic nitrate solution resulted in three major reduction products i.e., NH3,
NO2 and H2. The TU/Au e−lectrode showed the highest current efficiency, Q, of the NH3 production (about 85%), which is about 6.5 times
higher than that of the NO2 production. While the I−/Au and S2−/Au showed a sharp increase in the current efficiency for the hydrogen
evolution reaction to about 43 and 47%, respectively, with a concurrent depression of the total current efficiency of the nitrate reduction.
© 2003 Elsevier Ltd. All rights reserved.
Keywords: Waste water treatment; Electrocatalysis; Thiourea; Au electrodes; Ammonia
1. Introduction
The under potential deposition (upd) of alkali metal
cations e.g., Na or Cs, at Au electrodes, was found to
have a significant effect on the electrochemical reduction
of nitrate ions [22]. Three major reduction products were
formed upon the galvanostatic electrolysis of the alkaline
Nitrate ions in waste, from fertilisers, iron, steel, elec-
tronic, amunitions and nuclear fuel processes, constitute a
major source of water pollution [1,2]. The advantage of re-
moving nitrate ions from the waste water is quite important
for the following reasons: firstly, due to its high toxicity, it
may cause methemoglobinemia [3]. Secondly, nitrate can be
used as a direct, abundant and inexpensive source for the
manufacturing of ammonia, which is of potential importance
as a nitrogen source in fertilisers.
In this respect, the reduction of nitrate wastes receives an
increasing attention [4–10]. The electrochemical methods
are superior due to the use of clean reagents i.e., electrons
[11]. Different electrode materials have been suggested for
this purpose [12–15]. The products of the electrochemical re-
duction of nitrate are critically dependent on the nature of the
electrode material [16–21] as well as the pH of the medium
[22]. These products were shown to contain H2, NO2−, NH3,
N2 and other oxygen-containing nitrogen species.
−
nitrate solution [22], i.e. H2, NO2 and−NH3. The se-
lective electrochemical reduction of NO3 to NH3 was
found to increase with pH, reaching a maximum current
efficiency of about 60% at pH ≥ 12. Moreover, the upd
of Ge [23] was found to enhance the reduction of nitrate
and nitrite at Pt, Pd and Pt + Pd electrodes leading to the
formation of hydroxyl amine as an additional reduction
product.
Among the many different strategies used for the mod-
ification of electrode surface, formation of self-assembled
monolayers (SAMs) of organoslufur compounds on a gold
electrode has received much attention in recent years. This
modification process is known to enhance the catalytic
properties of many metallic electrodes, like Au, towards
several electrochemical reactions [24–34]. For instance,
ethanethiol was found to decrease the surface activity of
the Cd(upd)/Au electrode towards the electrochemical re-
duction of nitrate ions [35].
∗
Tel.: +202-567-6603; fax: +202-572-7556.
E-mail address: msaada68@yahoo.com (M.S. El-Deab).
0013-4686/$ – see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2003.11.025

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M.S. El-Deab / Electrochimica Acta 49 (2004) 1639–1645
This study addresses the use of modified Au electrodes as
lationships. Doubly distilled water was used to prepare the
electrolytes using analytical grade reagents. The pH of the
solutions was adjusted by addition of appropriate amounts
of NaOH. Tafel relationships were measured galvanostat-
ically using a variable dc current power supply. At each
applied current, the corresponding cathodic polarisation of
the working electrode was monitored at a memory oscillo-
scope as a function of time and the observed sudden rise of
the electrode potential (first few millivolts observed within
<100 ␮s) was subtracted from the total value to eliminate
the iR drop from the total polarisation of the working elec-
trode. The current density was calculated on the basis of the
geometric surface area of the Au electrodes. For each gal-
possible candidate cathodes for more efficient electrochem-
ical conversion of nitrate to ammonia. Thiourea (TU), urea
(U), sulfide ions (S2−) and iodide ions (I−) are used as Au
electrode modifiers in this study.
2. Experimental
For the cyclic voltammetric (CV) measurements, a con-
ventional three-electrode Pyrex glass cell was used. A sep-
aration membrane divides the cell into two compartments:
one contains the counter electrode and the other contains the
working electrode, while the reference electrode was con-
nected to the working electrode compartment through a Lug-
gin capillary, filled with the test solution, positioned near the
working electrode surface. Au electrodes (Ø = 1.6 mm hav-
ing an exposed geometric surface area of 2.01×10−2 cm2),
spiral Pt wire and an Ag/AgCl/KCl (sat.) were served as
the working, counter and reference electrodes, respectively.
The Au electrodes were polished with aqueous slurries of
successively finer alumina powder (down to 0.06 ␮m) and
were sonicated for 10 min in doubly distilled water. The
Au electrodes were then electrochemically pre-treated in
0.05 M H2SO4 solution by repeating the potential scan in
the range of −0.2 to 1.5 V versus Ag/AgCl/KCl (sat.) at
0.1 V s−1 for 10 min or until the CV characteristic for a
clean Au electrode was obtained. The surface modifica-
tions of the Au electrodes was carried out by immersing
the clean Au electrodes into aqueous solution of 10−5 M
thiourea, 10−5 M NaI, 10−5 M Na2S or 10−3 M urea for
10 min. This time is shown to be quite sufficient to prepare
a complete monolayer of the modifier on the Au electrodes
[36]. The electrodes were then washed with distilled water
several times. The modified Au electrodes will be referred
hereafter as TU/Au, I−/Au, S2−/Au and U/Au, respectively.
Electrolyte solutions were deaerated by bubbling N2 gas
for 30 min prior to the electrochemical measurements.
For the galvanostatic electrolysis experiments, an Au
sheet (99.99%) having a geometric surface area of 2.1 cm2
was used as the working electrode. While the counter and
the reference electrodes were an Au screen of 80 mesh
(1.0 cm × 4.0 cm) and a Pt sheet (1.0 cm × 0.5 cm), respec-
tively. The electrolysis cell consists of two compartments,
one containing the working as well as the reference elec-
trode, while the other compartment includes the counter
electrode. The working electrode compartment was mod-
ified by fitting of a volume-graduated cylinder to collect
the evolved hydrogen gas at the working electrode during
the galvanostatic electrolysis of the solution. Further jus-
tifications of the cell are given elsewhere [22]. Hydrogen
gas was bubbled into the solution to attain the reversible
hydrogen electrode (RHE) conditions of the Pt reference
electrode (having an equilibrium potential of −0.934 V ver-
sus Ag/AgCl/KCl (sat.) at pH 2.5) and to ensure deaeration
of the test solution during the measurements of the Tafel re-
vanostatic experiment, a current density of 2.85 mA cm−2
∼
i.e., the overpotential of the working electrode was
−0.30 0.02 V versus RHE (cf. Fig. 5) was applied for
=
3–6 h. The measured volume of the evolved hydrogen gas
(VH ,r) is corrected to the corresponding volume under
2
STP conditions, VH (STP), using the equation: VH (STP)
=
2
2
VH ,r{(1 − 25/273)(1 − PH O,r/760)}, where VH ,r and
2
2
2
PH O,r are the volume of the hydrogen gas collected and
2
the water vapour pressure, mmHg, at room temperature
(25 ◦C). Evaporating the test solution and trapping of the
NH3 into HCl solution determined the concentrat−ion of the
produced ammonia and the concentration of NO2 was de-
termined by using Chloramine-T [37]. The corresponding
currents were calculated in each case using Faraday’s law.
All potentials in the text and the figures will be referred
to the RHE. All glassware was cleaned in boiling H2SO4
+ HNO3 mixture to remove any organic contaminants.
3. Results and discussions
3.1. Cyclic voltammetry
Fig. 1 shows the cyclic voltammetric response of the ni-
trate reduction reaction at (b) bare (i.e., unmodified) Au and
(c) TU/Au electrodes in N2-saturated 0.5 M NaNO3 solu-
tion at pH 12.5, at a potential scan rate of 0.1 V s−1. Curve
a represents the blank CV response of the bare Au elec-
trode in a nitrate-free solution, i.e., N2-saturated 0.25 M
Na2SO4 solution at pH 12.5. This figure shows the existence
of a reduction peak at ca. +0.75 V versus RHE in case of
the N2-saturated 0.5 M NaNO3 solution at the bare Au and
TU/Au electrodes (curves b and c). While this peak was not
observed at the bare Au electrode in the blank solution (curve
a). Consequently, this peak can be attributed to the electro-
chemical reduction of the nitrate anion. A significant feature
of the TU/Au electrode (curve c) is the relatively high peak
current more than the bare Au (curve b). Thus, it can be con-
cluded that the nitrate reduction reaction is more favoured
at the TU/Au electrode than at the unmodified Au electrode.
Figs. 2–4 show the CVs obtained at U/Au, S2−/Au
and I−/Au electrodes, respectively, in 0.5 M NaNO3
(N2-saturated, pH 12.5) (curve b in each figure) at a poten-

M.S. El-Deab / Electrochimica Acta 49 (2004) 1639–1645
1641
a
b
–2
0.1 mA cm
a
b
no. of CV cycles increase
c
c
0.2 mA cm–2
–0.5
0
0.5
1
–0.5
0
0.5
1
E
/ (V vs RHE)
E / (V vs RHE)
Fig. 3. CVs for the electrochemical reduction of 0.5 M NaNO3 solution
(N2-saturated, pH 12.5) at (a) bare Au and (b) S2−/Au electrodes. Curve
c represents the CV response of the S2−/Au electrode in N2-saturated
0.25 M Na2SO4 (as a blank solution) at pH 12.5. Potential scan rate:
Fig. 1. CVs for the electrochemical reduction of 0.5 M NaNO3 solution
(N2-saturated, pH 12.5) at (b) bare Au and (c) TU/Au electrodes. Curve
a represents the CV response of the bare Au electrode in N2-saturated
0.25 M Na2SO4 (as a blank solution) at pH 12.5. Potential scan rate:
0.1 V s−1
.
0.1 V s−1
.
lesser extents at these electrodes, compared to the unmodi-
fied Au electrode. It is important to notice that, a consider-
able increase of the peak current intensity (at ca. +0.75 V
versus RHE) is observed upon the successive cycling of the
potential. This behaviour could be attributed to the partial
desorption of the adsorbed species (U, S2−, I− in Figs. 2–4,
tial scan rate of 0.1 V s−1. Curve a in each figure represents
the CV response obtained at the unmodified Au electrode
in the same solution for comparison. Analysis of these fig-
ures reveals the occurrence of the nitrate reduction at ca.
+0.75 V versus RHE with different intensities. This reflects
the feasibility of the electrochemical reduction of nitrate at
a
a
b
0.1 mA cm–2
b
no. of CV cycles increase
–2
no. of CV cycles increase
0.1 mA cm
c
c
0
0.5
1
0
0.5
1
E / (V vs RHE)
E / (V vs RHE)
Fig. 2. CVs for the electrochemical reduction of 0.5 M NaNO3 solution
(N2-saturated, pH 12.5) at (a) bare Au and (b) U/Au electrodes. Curve c
represents the CV response of the U/Au electrode in N2-saturated 0.25 M
Fig. 4. CVs for the electrochemical reduction of 0.5 M NaNO3 solution
(N2-saturated, pH 12.5) at (a) bare Au and (b) I−/Au electrodes. Curve c
represents the CV response of the I−/Au electrode in N2-saturated 0.25 M
Na2SO4 (as a blank solution) at pH 12.5. Potential scan rate: 0.1 V s−1
.
Na2SO4 (as a blank solution) at pH 12.5. Potential scan rate: 0.1 V s−1
.

1642
M.S. El-Deab / Electrochimica Acta 49 (2004) 1639–1645
respectively) upon the successive potential cycling. The
origin of the peaks appeared at ca. +0.3 V versus RHE in
the case of the U/Au and I−/Au electrodes, in Figs. 2 and
4, respectively, can be accounted for the reductive desorp-
tion of the adsorbed U molecules and I− ions, respectively.
In the case of S2−/Au electrodes multiple reduction peaks
were observed in the potential region from −0.2 to +0.1 V
versus RHE. Similar multiple reductive desorption peaks
for alkyl thiolates bound to Au substrates were observed
[38–41]. The origin of those peaks is attributed to the re-
ductive desorption of the S or S-containing species from
different crystal faces of Au on the polycrystalline Au sur-
face having different bond strengths [38–41]. A set of CV
measurements was performed to confirm the assignment of
the origin of those reduction peaks in a nitrate-free solution
i.e., 0.25 M Na2SO4 (N2-saturated, pH 12.5) (the data are
shown as curves c in Figs. 2–4). Similar peaks were ob-
served at +0.3, +0.3 and +0.1 V versus RHE for the U/Au,
I−/Au and S2−/Au electrodes, respectively. This confirms
the assignment of those peaks to the desorption process of
the adsorbed surface species in each case. Upon the potential
cycling, a remarkable decrease in the intensity of those des-
orption peaks around +0.3 and +0.1 V was observed with a
concurrent increase in the nitrate reduction peak at +0.75 V
versus RHE (curves b in Figs. 2–4). Consequently, it could
be concluded that the process of desorption increases the
number of the free Au sites available for the electrochemical
reduction reaction of nitrate. This conclusion is consistent
with that previously reported by Vago et al. [42]. They re-
ported that a significant hindrance of some electrochemical
reactions like the oxygen reduction reaction was observed at
Au electrodes modified with a SAM of octadecylmercaptan,
while holding the electrode at −0.8 V versus SHE resulted
in an enhancement of the electrochemical behaviour of the
electrode due to the desorption of the thiol molecules.
–600
–400
–200
0
10–4
10–3
10–2
i / A cm–2
Fig. 5. Tafel relationships for the HER from 0.25 M Na2SO4 (white
symbols, ∗) and for the nitrate reduction reaction from 0.5 M NaNO3
(black symbols, +) at: (∗, +) bare (unmodified) Au; (᭺, ᭹) TU/Au; (ᮀ,
᭿) U/Au; (᭛, ᭜) I−/Au and (ꢀ, ᭡) S2−/Au electrodes at pH of 12.5
and T = 25 ◦C.
undergo a recombination reaction leading to the evolution
of hydrogen gas according to:
H(ads) + H(ads) → H2(gas)
(2)
While in the case of the nitrate solution, the above reac-
tions take place concurrently with the electrochemical re-
duction of nitrate ions. The latter reaction may be proceed
according to the following proposed mechanism:
−
−
NO3(sol) ⇔ NO3(ads)
NO3(ads) + H(ads) + e− → NO2(ads) + OH−
,
(in equilibrium)
(3)
(4)
−
−
3.2. Tafel relationships
where the subscript (sol) re−fers to the solution-phase ni-
trate ions. The produced NO2 (ads) species may be desorbed
away from the electrode surface to the solution phase or it
may be further reduced in several elementary steps giving
rise to the formation of ammonia according to:
Fig. 5 shows the Tafel relationships for the hydrogen evo-
lution reaction (HER) and the nitrate reduction reaction from
0.25 M Na2SO4 and 0.5 M NaNO3 (pH 12.5), respectively,
at Au electrodes with different modifications. Inspection of
this figure reflects two important features: (i) the cathodic
polarisation required to sustain a specific current density is
lower in the case of nitrate solution than that required in
the case of sulfate solution and (ii) identical slopes of ca.
−120 mV per decade were obtained for the Tafel relation-
ships measured in the nitrate solution and the sulfate solu-
tion. This value of the Tafel slope suggests a one-electron
transfer rate-determining step (r.d.s.) for both reactions [43]
at which the discharge of water molecules takes place ac-
cording to:
−
NO2(ads) + 5H(ads) + e− → NH3 + 2OH−
(5)
Thus, it is presumable that the stabilisation of the pro-
duced H(ads) (generated in Eq. (1) above) is a key point which
determine the current efficiency of the reduction products
of the nitrate solution. In other words, Au electrodes with
proper modifications which can stabilise H(ads) give rise to
greater yields of ammonia. Likewise, improper modification
of the Au electrodes may result in low yields of ammonia. It
has been shown that the yield of the produced ammonia as a
result of the photocatalytic reduction of nitrate and nitrite at
metal–TiO2 electrodes is critically dependent on the nature,
amount and method of metal loading [44]. For instance no-
ble metals which can stabilise H(ads) (e.g. Ru) exhibit high
H2O + e− → H(ads) + OH−, (r.d.s.)
(1)
where the subscript (ads) refers to adsorbed species. In the
case of the sulfate solution, the produced H(ads) species may

M.S. El-Deab / Electrochimica Acta 49 (2004) 1639–1645
1643
activity towards the reduction of nitrate and nitrite [44]. Con-
sistently, hydrogen storage alloys were efficiently used as
cathodes for the electrochemical reduction of nitrate to am-
monia [45]. This type of cathodes could release the absorbed
hydrogen atoms and act as a hydrogen source, leading to a
high yield of ammonia. Consequently, it is worthwhile to
elucidate the effects of the different modifications of the Au
electrodes on the preferential electrochemical reduction of
nitrate ions to ammonia.
electrodes with different modifications, namely: unmodified
Au, TU/Au, I−/Au, S2−/Au and U/Au electrodes. It is im-
portant to note that there is an upper limit and lower limit
of the sum of the current efficiencies. The actual value of
the sum is effectively 100% with negligible errors, for the
first four electrodes. For the U/Au electrode, the sum is
about 84% suggesting the formation of other unaccounted
oxygen-containing nitrogen species (e.g. NH2OH and some
nitrogen oxides [47]).
Inspection of this table reveals the following significant
points, in comparison to the bare Au electrode:
3.3. Current efficiency
1. At the TU/Au electrode, a significant increase of Q(NH3)
is observed at the expenses of Q(NO2−), With a ratio of
Q(NH3)/Q(NO−2 ) > 6.5.
In a trial to examine the effect of the surface modification
of the Au electrodes on the yield of ammonia, a galvanostatic
electrolysis process was performed by applying a current
density of 2.85 mA cm−2 in the electrolysis cell containing
0.5 M NaNO3 at pH of 12.5 for a time ranging from 3 to
6 h. This current density was chosen due to several reasons:
2. At the I−/Au and S2−/Au electrodes, a sharp increase of
Q(H2) to about 50% of the total coulombs with a con-
current decrease of (Q(NO−2 )+Q(NH3)) was observed.
3. At the U/Au electrode, Q(H2) and Q(NO2−) are not sig-
nificantly changed, but Q(NH3) is more depressed, with
a ratio of Q(NH3)/Q(NO−2 ) < 1. Also, at this electrode,
the overall efficiency for nitrate reduction is lower than
100%.
(a) The production rate of the HER is not severe in most
cases.
(b) To obtain a detectable concentration of ammonia as a
reduction product in a reasonable electrolysis time.
(c) The corresponding cathodic polarisation in most cases
is around −0.3 V versus RHE, which allows for the co-
existence of the upd of Na and adsorbed I− on the Au
surface [22,46].
(d) The electrolysis was performed for 3 h for the cases at
which a profuse generation of hydrogen gas occurs at
the cathode (i.e., at the I−/Au and S2−/Au electrodes).
These results point out to a significant role of the surface
modification of the Au electrode in the selectivity of the
elect−rochemical reduction of the basic nitrate solution to H2,
NO2 or NH3 [35,48,49].
The observed behaviour of the modified Au electrodes
can be interpreted in the light of the specific adsorptivity of
I−, S2− and TU onto the Au electrode as follows:
The current efficiency, Q, is defined as the fraction of the
total charge, Qtotal, involved in a specific reaction and is
given by:
1. In the case of I−/Au electrode, the presence of the specifi-
cally adsorbed iodide ions at the Au surface [46] resulted
in some electrostatic repulsive interaction with the nega-
tively charged nitrate ions in the vicinity of the Au elec-
trode surface. This leads to a shift of the equilibrium (rep-
resented by Eq. (3)) towards the left and thus a significant
decrease of the overall performance of the Au electrode
towards the nitrate reduction (i.e. Q(NO−2 ) + Q(NH3))
is ob−served as a result of the decrease of the adsorbed
NO3 species at the Au electrode surface.
2. Also, the sulfide ions are known to have a strong ten-
dency to adsorb at Au surface [50]. Thus, the behaviour
of the S2−/Au electrode could be explained in a simi-
lar manner to that of the I−/Au electrode. Furthermore,
the enhancement of the HER by S2− ions is observed at
many electrodes [51–55]. For example, H2S was found
to accelerate the HER at Cr and Fe electrodes in acidic
media [52,55] and at Au electrodes surface in neutral
and alkaline media [51]. Also Pyrite electrodes (prepared
from iron rust + H2S) showed an excellent catalytic ac-
tivity towards the HER in 3.5% NaCl solution [53]. These
results are consistent with the observations of Table 1
regarding the enhancement of the HER at S2−/Au elec-
trodes demonstrated in the high current efficiency for this
reaction (ca. 50%).
Qr
Q =
(6)
Qtotal
where Qr−is the number of coulombs consumed to produce
H2, NO2 or NH3 (in the present case). The values of Qr
were calculated using Faraday’s law.
The listed values in Table 1 represent the average val-
ues of the measured quantities with an average experimen-
tal error of about 1% in the analytical techniques for each
measurement. These measurements were performed at Au
Table 1
−
Variation of the overall current efficiency, Q, for the HER, NO2 and
NH3 formation at Au electrodes modified with different adsorbed species
−
Electrode
Electrolysis
time (h)
Q(H2)
(%)
Q(NO2
(%)
)
Q(NH3)
(%)
Bare Au
TU/Au
I−/Au
S2−/Au
U/Au
6
6
3
3
6
0
4
43
47
1
42
13
10
6
58
85
50
48
37
46
Galvanostatic electrolysis of 0.5 M NaNO3 (pH 12.5) using a constant
current density of 2.85 mA cm−2 was used. The values of Q represent the
average values for the measured quantities.

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M.S. El-Deab / Electrochimica Acta 49 (2004) 1639–1645
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3. On the other hand, in the case of TU/Au electrode, the
adsorbed TU molecules (having neutral charge at this
pH) did not retard the approach of the nitrate ions to the
electrode surface. Those molecules are believed to en-
hance the preferential reduction of nitrate ions to ammo-
nia. This enhancement may arise from: (i)−the enrichment
of the Au surface by the adsorbed NO3 ions, leading
to the highest observed catalytic efficiency towards the
ammonia production and (ii) TU is known to inhibit the
HER at different cathodes [56,57]. Therefore, the rate of
the recombination reaction given by Eq. (2) is believed
to be depressed leading to enriching the Au surface by
H(ads). These two reasons may account for the observed
high current efficiency for the electrochemical reduction
of nitrate to ammonia at TU/Au electrode.
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−
The electrochemical reduction of NO3 to NH3 has been
investigated at Au electrodes modified with TU, U, I− and
S2− in 0.5 M NaNO3 (pH 12.5). A reduction peak was ob-
served at ca. +0.75 V versus RHE, the intensity of which
depends on the nature of the surface adsorbed species. The
TU/Au electrode showed the highest peak current for the
nitrate anion reduction. It also supports the highest cur-
rent efficiency for NH3 production (about 85%) in alkaline
NaNO3 solution than the U/Au, I−/Au or S2−/Au electrodes.
The highest current efficiency for the HER was observed at
I−/Au and S2−/Au electrodes (about 50%). The U/Au elec-
trode showed a current efficiency for the ammonia produc-
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