Nitrate and nitrite electrocatalytic reduction on Rh-modified pyrolytic graphite electrodes

Article abstract of DOI:10.1016/j.electacta.2007.03.072

The electrochemical reduction of nitrate and nitrite anions was investigated on Rh-modified pyrolytic graphite electrodes prepared by potentiostatic electrodeposition with the use of a double-pulse technique. Several important parameters (pH, temperature and composition of electrolytic medium, electrolysis potential) were varied. Only ammonia and nitrite ions were detected among NO₃^- reduction products, while NO₂^- ions are straightforwardly reduced to ammonia. The experimental data (CV measurements and the results of electrolyses) clearly show that Rh/graphite electrodes are much more efficient for NO₂^- reduction than for that of NO₃^- at room temperature which was confirmed by the determination of rate constants of corresponding reactions and the activation energy of NO₂^- reduction. The influence of carboxylate anions and tetraalkylammonium cations on the electrocatalytic activity of Rh/graphite electrodes was investigated and the inhibiting effect of HCOO^- anions on hydrogen evolution reaction and nitrate reduction was demonstrated.

Full text of DOI:10.1016/j.electacta.2007.03.072

Electrochimica Acta 52 (2007) 6237–6247
Nitrate and nitrite electrocatalytic reduction on Rh-modified
pyrolytic graphite electrodes
Oleg Bryleva,b, Mathieu Sarrazina,b, Lionel Roue , Daniel Belanger
b
a,∗
´
´
a De´partement de Chimie, Universite´ du Que´bec a` Montre´al, C.P. 8888, Succursale Centre-Ville, Montre´al, Que´bec, Canada H3C 3P8
b INRS-Energie, Mate´riaux et Te´le´communications, 1650 Blvd Lionel Boulet, C.P. 1020, Varennes, Que´bec, Canada J3X 1S2
Received 15 November 2006; received in revised form 20 March 2007; accepted 31 March 2007
Available online 5 April 2007
Abstract
The electrochemical reduction of nitrate and nitrite anions was investigated on Rh-modified pyrolytic graphite electrodes prepared by potentio-
static electrodeposition with the use of a double-pulse technique. Several important parameters (pH, temperature and composition of electrolytic
−
−
medium, electrolysis potential) were varied. Only ammonia and nitrite ions were detected among NO3 reduction products, while NO2 ions are
straightforwardly reduced to ammonia. The experimental data (CV measurements and the results of electrolyses) clearly show that Rh/graphite
electrodes are much more efficient for NO2− reduction than for that of NO3− at room temperature which was confirmed by the determination of rate
−
constants of corresponding reactions and the activation energy of NO2 reduction. The influence of carboxylate anions and tetraalkylammonium
cations on the electrocatalytic activity of Rh/graphite electrodes was investigated and the inhibiting effect of HCOO− anions on hydrogen evolution
reaction and nitrate reduction was demonstrated.
© 2007 Elsevier Ltd. All rights reserved.
Keywords: Rhodium; Double-pulse electrodeposition; Nitrate reduction; Nitrite reduction; Electrocatalysis
1. Introduction
surface [18] and electrocatalytic reduction on solid electrodes
[19–21]). Concerning the last case, numerous materials have
been tested including Cu [22], Pt [23], Pd [24], Ru [20], Ir [25]
and Rh [26–28]. The last one demonstrates the highest activity
for NO3− reduction among transition and coinage metals [20].
In our previous works, we have reported the properties of
Rh-modified pyrolytic graphite electrodes obtained by poten-
tiostatic electrodeposition and their catalytic activity towards
nitrate reduction in neutral medium [29,30]. It was shown that
the highest specific catalytic activity can be achieved using
the double-pulse electrodeposition technique, which is based
on the application of a short nucleation potential pulse (typi-
cally <200 ms) and a subsequent growth pulse at much more
positive potential [31]. The application of nucleation potential
pulse allows to obtain an enhanced surface coverage in compar-
ison with usual potentiostatic deposition and to decrease both
the mean Rh particle size from 70 to 40 nm and the surface-
to-volume ratio (4.0 × 106 cm2 cm−3), which is beneficial for
catalytic applications [30]. Then, the optimal conditions (i.e.
the potential and duration of both pulses) providing the high-
est values of specific catalytic activity were determined [30].
Potentiostatic and galvanostatic electrolyses were performed
Intensive use of fertilizers in agriculture and nitrates in some
industrial sectors causes a severe nitrate pollution of water
sources and industrial sites [1]. Such a contamination can result
in a high risk for public health causing bladder and ovary cancers
[2–5]. Various methods such as biological, physicochemical and
chemical have been proposed so far for the removal of nitrates
from potable water and wastewaters [6–11]. However, they pos-
sess various disadvantages, such as a low NO3 destruction
rate, the formation of toxic byproducts and the production of
secondary brine wastes [12].
The electrochemical methods of nitrate removal have been
intensively st−udied for the last three decades [13–16]. Several
ways of NO3 electrocatalytic reduction are known (for exam-
ple, the addition of electrocatalytically active ions to the treated
solution [17], immobilization of the catalyst at the cathode
−
∗
Corresponding author. Tel.: +1 514 987 3000x3909; fax: +1 514 987 4054.
E-mail addresses: brylev@inorg.chem.msu.ru (O. Brylev),
roue@emt.inrs.ca (L. Roue´), belanger.daniel@uqam.ca (D. Be´langer).
0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2007.03.072

6238
O. Brylev et al. / Electrochimica Acta 52 (2007) 6237–6247
at Rh-modified graphite electrodes to determine the qualitative
and quantitative composition of nitrate electrochemical redu−c-
NH3 and H2 were detected [30]. It was shown that the Rh elec-
trodeposition parameters (i.e. growth pulse potential, duration
of nucleation and growth pulses) did not influence the compo-
sition of formed products, indicating the absence of preferential
Rh crystallographic orientation [30].
It is known that different factors such as pH and temper-
ature of electrolytic medium, electrolysis parameters (applied
current or potential) can affect the products of nitrate electro-
catalytic reduction [26]. The influence of co-existing species
in electrolytic medium is also crucial, especially for industrial
applications [32–34]. Accordingly, the nitrate electroreduction
is an extremely complex process that can be partially described
by the reactions listed below (E0 = potential versus normal
hydrogen electrode (NHE)):
The reaction products of nitrate reduction on Rh electrode in
3 M HNO3 + 0.5 M H2SO4 were followed by differential elec-
trochemical mass spectroscopy and in situ FTIR measurements
[26]. It has been proposed that the nitrate reduction starts with
partial N–O bond dissociation and partial N–H bond forma-
tion generating NO and N2O. At potentials lower than 0.2 V
versus reversible hydrogen electrode (RHE) the reaction pro-
ceeds forming dissolved NH4+. For potentials lower than 0 V,
the reduction continues via a multiple pathway reaction lead-
ing to the production of N2, NH2OH and N2H2. The catalytic
activity of Rh electrode was also investigated in acidic 0.1 M
nitratesolution[20]. Nogaseousproductswerefound, indicating
that only NH2OH and NH4+ were formed. Hayden and cowork-
ers have shown that the prolonged activation of Rh surface in
0.3 M KNO3 + 0.2 M KOH + 2 M KCl aqueous solution at 70 ◦C
resulted in the decrease in the activation energy of nitrate reduc-
tion and the preferential formation of N2 [38]. The activated
Rh electrode demonstrated a later onset of HER with respect to
the non-activated ones, thus facilitating the nitrogen formation.
However, no details concerning the changes in surface composi-
tion and morphology were reported to explain such a result [38].
In the present study, we report the detailed analysis of nitrate
and nitrite reduction on Rh/graphite electrodes investigated by
cyclingvoltammetry(CV), galvanostaticandpotentiostaticelec-
trolyses performed under variable operating conditions (i.e. pH,
temperature and composition of the electrolytic solution) with
the use of various analytical techniques.
tion products in neutral medium. In all the cases, only NO2
,
• In acid medium [35]:
NO3− + 2H+ + 2e− → NO2− + H2O, E0 = 0.83 V (1)
NO3− + 10H+ + 8e− → NH4+ + 3H2O, E0 = 0.87 V
(2)
In alkaline medium the same reactions can be written as fol-
lows [35]:
NO3− + H2O + 2e− → NO2− + 2OH−, E0 = 0.01 V
2. Experimental
(3)
2.1. Preparation of Rh-modified pyrolytic graphite
electrodes
NO2− + H2O + e− → NO + 2OH−, E0 = 0.46 V
(4)
The Rh electrodeposition on pyrolytic graphite substrate was
performed at room temperature under quiescent conditions in a
three-electrode, one-compartment electrochemical cell consist-
ing of pyrolytic graphite (working electrode), Pt mesh (counter
electrode) and Ag/AgCl (3 M KCl) as a reference electrode. An
aqueous solution containing 10 mM Na3RhCl6 was used as a
rhodium source. In the case of double-pulse electrodeposition,
a short potential nucleation pulse of −800 mV versus Ag/AgCl
was applied for 20 or 100 ms, followed by a growth pulse of
different duration. A more detailed description of Rh/graphite
electrode preparation can be found elsewhere [29,30]. Further
in the text, all the nucleation and growth pulse potentials will be
referred to the Ag/AgCl electrode.
NO3− + 6H2O + 8e− → NH3 + 9OH−, E0 = −0.12 V
(5)
The hydrogen evolution reaction (HER) competes with the
above-mentioned reactions:
• In acid medium:
2H+ + 2e− → H2, E0 = 0 V
• In alkaline medium:
(6)
2H2O + 2e− → H2 + 2OH−, E0 = −0.83 V
(7)
Although the HER (Eqs. (6)–(7)) occurs at potentials more neg-
ative than those of nitrate and nitrite−reductions (Eqs. (1)–(5)),
2.2. Investigation of NO3− and NO2− reduction on
Rh/graphite electrodes
−
the slow kinetics of NO3 and NO2 reductions may result in
the partial charge consumption for gaseous hydrogen formation
[36]. It should be noted that the hydrogen adsorption blocks the
one of NO3− at the electrode surface hindering the nitrate reduc-
tion [19]. Concerning the adsorption of different species on Rh
electrode, the value of potential of zero charge (PZC) should be
taken into account. In 0.01N Na2SO4 (pH 2.5) the PZC is 0.02 V
versus NHE and linearly decreases to −0.52 V at pH 12.7 [37].
The electrochemical reduction of NOx− (x = 2 or 3) was stud-
ied employing the electrochemical cell configuration used for Rh
electrodeposition. All the aqueous solutions were prepared using
ultrapure water (18 Mꢀ, obtained from a Sybron/Barnstead
Nanopure II system). Firstly, Rh/graphite electrodes were tested
by cycling voltammetry for the HER in 1 M NaCl solution and
then subsequently in the presence of NO3− and NO2− anions, so

O. Brylev et al. / Electrochimica Acta 52 (2007) 6237–6247
6239
the contributions from hydrogen evolution and nitrite or nitrate
reduction can be qualitatively split up. The solutions were not
preliminary deaerated and this fact did not alter the shape of
obtained CV curves (data not shown) as it has been observed
before[38]. ThepotentiallimitsfortheCVcharacterizationwere
from −0.7 to −1.5 V versus SCE. We noted that upon cycling
the current slightly increased in the NOx− reduction region and
stabilized after the fifth cycle. All charge and current densities
are quoted in terms of apparent geometrical area of the substrate
graphite electrode, unless otherwise stated.
• current efficiency (or faradaic yield) CE(i) (%) for the forma-
tion of product i defined as (ni × mi × F) × 100/q, where ni
is the number of electrons involved in the reaction giving a
product i (i = NH3, NO2−, N2O and H2), mi is the quantity
of the formed species i (mol), q is the total electrical charge
consumed in course of the electrolysis and F is the Faraday
constant (96,485 C mol−1);
• selectivity S(j) (%) of nitrate reduction to a nitrogen-
containingproductjdefinedas(nj × mj × F) × 100/(q − qH ),
2
where qH is the charge consumed for hydrogen formation.
2
−
In order to establish and to quantify the products of NOx
reduction on Rh/graphite electrodes, the electrolyses were
performed in a two-compartment cell either in potentiostatic
or galvanostatic mode under mechanical stirring. A detailed
scheme of the experimental setup is given elsewhere [30].
The compartments of the cell are separated by a Nafion®
117 membrane. The cathodic compartment contains a working
Rh/graphite electrode and a reference electrode immersed in
nitrite or nitrate-containing solutions having various pH values.
As reference electrodes, either SCE (pH < 8) or Hg/HgO (pH
14) mounted in a Luggin capillary were used. The anodic com-
partment includes a Pt grid (counter electrode) plunged into 1 M
Na2SO4 or 1 M NaOH, when the initial pH value of the cathodic
compartment solution is inferior to 8 or equal to 14, respectively.
In this case, all the solutions were purged with argon for 30 min.
A Solartron 1470 potentiostat/galvanostat interfaced with a PC
and the Corrware 2 and CorrView 2 software (Scribner Asso-
ciates) was employed for the electrochemical experiments. For
the galvanostatic electrolyses the current was normalized to the
Rh deposit surface area measured by well-known procedures
[39], and the current density was adjusted to 4 mA cm−2. Further
in the text, all the potentials recorded in course of CV measure-
ments and electrolysis experiments will be referred to SCE.
Solution samples were regularly taken for the quantitative
determination of NO3−, NO2−, NH2OH and NH4+/NH3(aq).
The NO3− concentration was evaluated by UV-vis spectroscopy
(Hewlett Packard 8452A) by measuring the absorbance at
220 nm [40], while NH2OH was quantified by the absorbance at
710 nm [41]. The content of nitrite anions was evaluated from
the absorbance at 544 nm, due to the diazonium complex formed
by reaction involving NO2−, sulfanilamide (Aldrich) and N-
(1-naphtyl)-ethylenediamine dihydrochloride (98%, Aldrich)
[40]. NH4+/NH3 concentration in solution was evaluated by
Nessler’s classic method described elsewhere [40]. Gas sam-
ples were regularly taken during the electrolysis and injected
3. Results and discussion
3.1. Nitrate and nitrite reduction on Rh/graphite electrodes
CV curves recorded for the same Rh/graphite electrode in
1 M NaCl, 1 M NaCl + 0.1 M NaNO3 and 1 M NaCl + 0.1 M
NaNO2 aqueous solutions are represented in Fig. 1. Concern-
ing a 1 M NaCl solution, a slight increase in current density
at −0.75 V for the sweep in the negative potential direction is
due to the formation of Hads layer as it has been previously
reported for alkaline solution [43] and the hydrogen evolution
begins at approximately −1.0 V. In the case of 1 M NaCl + 0.1 M
NaNO3 solution, the onset of the increase in current density
occu−rs at more positive potentials pointing to the beginning of
NO3 reduction whose peak maximum is observed at −0.90 V
as previously reported [30] (Fig. 1, dotted curve). Concerning
the nitrite containing solution, an ill-defined peak or a shoulder
−
on the cathodic wave corresponding to the NO2 reduction is
observed at −1.25 V (Fig. 1, dashed curve). At more negative
potentials, the following increase in current density is related to
the competitive HER occurring at Rh surface.
To compare the catalytic efficiency of Rh/graphite electrode
for nitrite and nitrate reduction, the difference between the cor-
responding current values and 1 M NaCl at the same potential
(e.g. at −1.25 and −0.90 V) might be taken and it is higher for
˚
into a gas chromatograph (Varian 3000, molecular sieve 5 A and
200 cm × 0.3 cm). Calibrating curves were recorded using gas
mixtures with known composition (Praxair) to quantify the for-
mation of H2, N2 and N2O [42]. Then the Rh/graphite electrode
performance for nitrate reduction was quantitatively estimated
using the following criteria:
• nitrate destruction degree (%) corresponding to
the duration
t
of the electrolysis and defined −as
Fig. 1. CV curves (fifth cycle) of Rh/graphite electrode (no nucleation pulse,
growth pulse potential of −352 mV, growth pulse duration of 480 s) in 1 M NaCl,
1 M NaCl + 0.1 M NaNO3 and 1 M NaCl + 0.1 M NaNO2 solutions. Scan rate
([NO3−]0 − [NO3−]t) × 100/[NO3−]0, where [NO3
]
0
and [NO3−]t are the NO3 molar concentrations at the
beginning and at the instant t of the electrolysis, respectively;
−
20 mV s−1
.

6240
O. Brylev et al. / Electrochimica Acta 52 (2007) 6237–6247
the 1 M NaCl + 0.1 M NaNO2 solution (20.7 and 12.9 mA cm−2
,
respectively). In addition, for nitrite-containing solution a signif-
icant increase in current density is observed from −0.9 V on the
cathodic scan with respect to 1 M NaCl and 1 M NaCl + 0.1 M
NaNO3 (Fig. 1). These facts point to a higher catalytic activity
of the Rh/graphite electrodes towards nitrite reduction than for
the nitrate one.
−
The CV results allow a mechanism for the NOx reduc-
tion on Rh/graphite electrodes to be suggested. It is known
that on a Pt electrode a−competitive adsorption between H2O
(resulting in Hads), NO3 and its reduction intermediate prod-
ucts takes place [19]. Hence, a similar behavior might be also
found in the case of Rh/graphite electrodes. However, the PZC
for Rh is more negative than for Pt suggesting a stronger anion
−
adsorption [37]. When Hads and NO3
begin to be formed
ads
at −0.75 V, a large part of the surface is covered by adsorbed
−
hydrogen since H is more easily adsorbed [19]. Then the NO3
reduction starts at −0.75 V and a competition between H and
NO3− takes place to occupy free adsorption sites. At more neg-
ative potentials when H is more strongly adsorbed (as all the
measurements were performed below the PZC of Rh), it occu-
pies a more significant fraction of liberated sites thus blocking
nitrate adsorption. This results in a decrease of the current den-
sity (from −0.9 to −1.04 V) since the nitrate reduction (even to
nitrite) requires more electrons than the formation of Hads (Eqs.
(1) and (3)). At more negative potentials, the electrochemical
NO3− reduction might be suppressed because the current densi-
ties for 1 M NaCl and 1 M NaCl + 0.1 M NaNO3 are the same and
only gaseous hydrogen is electrochemically produced. Concern-
ing 1 M NaCl + 0.1 M NaNO2 solution, the onsets of nitrite and
nitrate adsorption occur at the same potential but nitrite anions
are more strongly adsorbed on Rh surface, thus from −0.9 to
−1.4 V the current density values are higher (Fig. 1) implying
that more electrons are necessary for nitrite reduction (e.g. nitrite
Fig. 2. Arrhenius plots for nitrite reduction current density at −0.89 (ꢀ) and
−1.04 V (᭹) vs. SCE on a Rh/graphite electrode (nucleation pulse of 20 ms,
growth pulse potential of −232 mV, growth pulse duration of 120 s) in 1 M
NaCl + 1 M NaNO2 solution.
the ideal gas constant (8.314 J mol−1 K−1) and T is the absolute
temperature.
−
Concerning the NO3 reduction on Rh/graphite electrodes,
at −0.89 and −1.04 V, ꢁEa was found to be equal to 32 kJ mol−1
in the same temperature range [30]. The competitive HER does
not influence the activation energy of the nitrate reduction reac-
tion significantly in this potential range as it has been already
−
observed [38]. But in the case of NO2 reduction, one can
observe two straight lines for the above-mentioned potential
values (Fig. 2). Above 45 ◦C, the activation energy increases
from 18 to 70 kJ mol−1, while ꢁEa of nitr−ate reduction remains
32 kJ mol−1 [30]. The j value for NO2 reduction increases
−
∞
reduction to ammonia requires six electrons) and the NO2
from 2.5 × 104 (T < 45 ◦C) to 2 × 1013 mA cm−2 (T > 45 ◦C),
reduction rate is higher. At potentials more negative than −1.4 V
the current densities are the same for all the three solutions. This
might mean that only H2 is electrochemically generated in this
potential range and since nitrate and nitrite anions are reduced
during electrolyses at this potential, NO3− and NO2− would be
chemically reduced by hydrogen in stat−u nascendi.
which corresponds to a general trend of simultaneous enhance-
ment of j and ꢁEa [38]. It has been reported that activation
∞
energies greater than 80 kJ mol−1 are considered typical of
chemical reaction steps, such as bond breaking, whereas ꢁEa
values lower than 20 kJ mol−1 point to diffusion controlled reac-
tion [44]. Hence, indeed the observed temperature dependence
is likely to be associated with the rate-determining step of
NO2− reduction reaction at Rh surface. The measurements were
performed for the Rh/graphite electrodes obtained under differ-
ent conditions (i.e. nucleation and growth pulse potentials and
durations), and similar results are obtained. To the best of our
knowledge, there is no report of the temperature dependence of
nitrite reduction on Rh in any medium. Lower ꢁEa values at
room temperature confirm that Rh is a more efficient catalyst
for nitrite reduction than for the nitrate one.
The activation energy (ꢁEa) of NOx reduction, might give
a hint about the reaction mechanism. In order to determine ꢁEa
−
value for NO2 reduction reaction, the current densities at the
potentials of −0.89 and −1.04 V were measured as a function
of temperature in the range from 3 to 70 ◦C. At −0.89 V, the
impact of the competitive reaction (i.e. HER) is negligible, while
at −1.04 V it is more important (Fig. 1, dashed curve). For both
cases, the variation of current density versus reciprocal absolute
temperature is represented in Fig. 2 and ꢁEa is calculated using
the following equation:
ꢀ
ꢁ
ꢁEa
RT
3.2. Kinetics of nitrate reduction on Rh/graphite electrodes
j = j exp
−
(8)
∞
According to above-mentioned issues, in neutral medium the
NO2− reduction on the investigated electrodes occurs faster than
that of NO3−. Hence, one can suppose that in this case the
where j is the current density at a given potential, j the pre-
exponential term (i.e. j limit when T tends to the infinity), R
∞

O. Brylev et al. / Electrochimica Acta 52 (2007) 6237–6247
6241
Fig. 4. Variation of the nitrate concentration with time during the galvanos-
tatic electrolysis of 1 M NaCl + 1 M NaNO3 solution on a Rh/graphite electrode
(nucleation pulse of 100 ms, growth pulse potential of −232 mV, growth pulse
duration of 60 s). Electrolysis current density is 4 mA cm−2 (normalized to the
Rh deposit surface area).
Fig. 3. Variation of the concentration of (ꢀ) nitrate, (᭹) nitrite and (ꢁ) ammo-
nia with time during the galvanostatic electrolysis of 1 M NaCl + 1 M NaNO3
solution on a Rh/graphite electrode (nucleation pulse of 100 ms, growth pulse
potentialof−232 mV, growthpulsedurationof60 s). Electrolysiscurrentdensity
is 4 mA cm−2 (normalized to the Rh deposit surface area).
when t = 0, [NO2−] = 0):
rate-determining step of nitrate reduction is their transformation
into nitrite ions. In order to quantitatively prove this statement,
prolonged galvanostatic electrolyses of 1 M NaCl + 1 M NaNO3
solution were performed, and the rate constants of NO3− reduc-
tion steps were determined.
Fig. 3showsthevariationsofnitrate, nitriteionsandammonia
concentrations with the electrolysis duration. The concentration
of NO3− diminishes progressively, while that of NO2− increases
until 42 h and then decreases. A similar shape of such curves was
reported for Pb [14], Cu [45] and bronze [46], which indicate
that the nitrate reduction on Rh/graphite electrodes also occurs
via a consecutive-reaction mechanism [47]. It can be described
as follows:
e−k t − e−k t
1
2
−
[NO2−]t = k1[NO3
]
0
(12)
k2 − k1
Taking account of previously obtained k1 value for 1 M
NaCl + 1 M NaNO3 solution (Fig. 4), the fitting of the exper-
imental data with Eq. (12) is shown in Fig. 5. The k2 value is
2( 0.1) × 10−5 s−1, which is eight times higher than k1 for the
reduction of nitrate to nitrite. This confirms a good ability of
rhodium for nitrite reduction and validates the CV results and
−
the determination of ꢁEa values. In contrast, the NO3 reduc-
tion on a Sn85Cu15 electrode in 0.1 M K2SO4 + 0.05 M KNO3
k
1
k
2
NO3−−→ NO2−−→ NH3
Thus, the corresponding differential equations are:
−
d[NO3
]
−
= −k1[NO3
]
(9)
dt
−
d[NO2
]
−
= k1[NO3−] − k2[NO2
]
(10)
dt
The solution of Eq. (9) is:
[NO3−]t = [NO3−]0e−k t
(11)
1
Thus, the nitrate concentration follows the first-order expo-
nential decay and in logarithmic coordinates its variation with
electrolysis time defines a straight line, whose slope allows
k1 = 2.6( 0.1) × 10−6 s−1 to be calculated (Fig. 4). The effect of
pH value on the nitrate destruction rate will be discussed below.
Eq. (10) is a linear differential equation, whose solution can
be written as follows taking account of the initial conditions (i.e.
Fig. 5. Variation of the nitrite concentration with time during the galvanos-
tatic electrolysis of 1 M NaCl + 1 M NaNO3 solution on a Rh/graphite electrode
(nucleation pulse of 100 ms, growth pulse potential of −232 mV, growth pulse
duration of 60 s). Electrolysis current density is 4 mA cm−2 (normalized to the
Rh deposit surface area).

6242
O. Brylev et al. / Electrochimica Acta 52 (2007) 6237–6247
solution is characterized by a k1 value which is about forty times
higher than k2 [46].
3.3. Influence of pH, applied potential and temperature on
the nature of NO3− reduction products
In course of the electrolysis of nitrate-containing solution,
the pH value increases according to Eqs. (3)–(5) and (7) and,
as mentioned above, it can significantly alter the composition
of nitrate reduction products. Due to this reason buffer solu-
tions were employed to maintain a constant pH throughout the
electrolysis duration. The electrolyses were performed in 1 M
NaNO3 within the following pH ranges, ensured by indicated
buffer solutions (simultaneously used as support electrolytes):
(a) 3.7–5.0: acetate buffer (0.5 M CH3COOH + 0.1 M
CH3COONa);
(b) 5.5–7.2: phosphate buffer (0.5 M KH2PO4 + 0.1 M
K2HPO4);
(c) >7: 1 M NaCl (no buffer);
(d) 14: 1 M NaOH (no buffer).
The anion concentration in buffer solutions did not exceed
0.6 M because higher concentrations can interfere in the nitrate
reduction due to the competitive anionic adsorption at the sur-
face of electrodes. In the cases of acetate and phosphate buffers,
pH values during the prolonged electrolyses were maintained
by periodic dropwise addition of 5 M HCl. During all the gal-
vanostatic electrolyses, the potential progressively decreased
and stabilized at −1.4 V after 2–3 h.
The variations of CE(H2), S(NH3) and S(NO2−) are rep-
resented as a function of electrolysis duration in Fig. 6a–c,
respectively. As expected, the decrease in the pH of electrolytic
solution promotes the hydrogen formation, and thus higher
CE(H2) values are obtained (Fig. 6a). At the end of the elec-
trolysis (i.e. after 12 h), CE(H2) is 41.0% for pH < 5, while in
1 M NaOH it is decreased down to 8.4% (Fig. 6a). As the hydro-
gen in statu nascendi promotes the ammonia formation, S(NH3)
should increase with the decreasing pH which can be clearly
seen in Fig. 6b. It has been already observed that highly reactive
hydrogen dissolved in hydrogen storage alloys could participate
in the nitrate reduction process and ammonia was found to be a
−
major reduction product [48]. Hence, the variations of S(NO2
)
follow a reverse trend (Fig. 6c).
In addition, it is known that nitrite anions are not stable in acid
medium and can undergo the following transformations [49]:
3HNO2 → 2NO + HNO3 + H2O
2HNO2 → NO + NO2 + H2O
3NO2 + H2O → NO + 2HNO3
(13)
(14)
(15)
Fig. 6. (a) Current efficiency (CE) of H2 formation on a Rh/graphite electrode
(nucleation pulse duration of 100 ms, growth pulse potential of −232 mV, growth
pulse duration of 60 s) in 1 M NaNO3 solutions as a function of pH value and
galvanostatic electrolysis duration. Electrolysis current density is −4 mA cm−2
(normalized to the Rh deposit surface area). (b) Selectivity of NO3 reduction
to NH3 on Rh/graphite electrode (nucleation pulse of 100 ms, growth pulse
potential of −232 mV, growth pulse duration of 60 s) in 1 M NaNO3 solution
as a function of pH value and galvanostatic electrolysis duration. Electrol-
ysis current density is 4 mA cm−2 (normalized to the Rh deposit surface area). (c)
−
−
Selectivity of NO3 reduction to NO2 on Rh/graphite electrode (nucleation
pulse of 100 ms, growth pulse potential of −232 mV, growth pulse duration
of 60 s) in 1 M NaNO3 solution as a function of pH value and galvanostatic
electrolysis duration. Electrolysis current density is 4 mA cm−2 (normalized to
the Rh deposit surface area).

O. Brylev et al. / Electrochimica Acta 52 (2007) 6237–6247
6243
This fact justifies low S(NO2−) values at pH < 5 (Fig. 6c). It
was also proved that desorbed NO was transformed into N2O in
acid medium [50]. That is why the formation of N2O, with the
currentefficiencyof4.1%, wasdetectedattheendoftheelectrol-
ysis performed in acetate buffer solution. Moreover, in this case
the sum of faradaic yields for all the detected products was not
equal to 100% which can be due to the chemical NO2− transfor-
mation into NO (non-detectable with the analytical techniques
used in this work) or into NO3− (Eqs. (13)–(15)).
The variations of nitrate destruction degree for various
electrolytic media are represented in Fig. 7. Along with pH
increase, the k1 value varies from 1.2 × 10−6 (acetate buffer)
to 4.0 × 10−6 s−1 (1 M NaOH) which confirms a negative influ-
ence of hydrogen evolution on the nitrate removal. Moreover, at
low pH values the accuracy of k1 calculation is poorer. This fact
can be explained by the detected N2O formation in acid medium
and hence, a different reduction mechanism and other kinetics
equations are involved.
CE(H2) for the electrolysis of 1 M NaCl + 1 M NaNO3 is 9.9%
but in 1 M NaCl + 1 M NaNO2 it drops down to 1.8%. These
observations confirm again a better adsorption of nitrite ions
−
in comparison with NO3 on Rh-modified graphite electrodes
which results in a better catalytic efficiency of NO2− reduction
and lower CE(H2). Concerning the products of NO2− reduction
on Rh/graphite electrodes, only ammonia is detected and for the
most negative electrolysis potential (i.e. −1.3 V), hydrogen for-
mation with a current efficiency of about 6% is observed after
the passage of 162 C cm−2
.
It was previously reported that a prolonged Rh surface act−i-
vation at 70 ◦C in a solution containing OH−, Cl− and NO3
ions allows ꢁEa values to be progressively decreased from 47
to 20 kJ mol−1 [38]. That is why the effect of temperature on
the nitrate reduction was studied for Rh/graphite electrodes.
Concerning the galvanostatic electrolyses performed in 1 M
NaCl + 1 M NaNO3 solution for approximately 3 h, the increase
in temperature from 25 to 70 ◦C causes the decrease in CE(H2)
−
from 41.2 to 33.7% accompanied by the increase of CE(NO2
)
As mentioned above, the selectivity of nitrate electrochem-
ical reduction strongly depends on the applied potential. In
order to clarify this issue, potentiostatic electrolyses were per-
formed in 1 M NaCl + 1 M NaNO3 solution within the potential
range where HER and/or nitrate reduction occur (Fig. 1), i.e.
between −0.85 and −1.45 V. As expected from the shape of
CV curve (Fig. 1), the charge passed at a given electrolysis
duration increases with decreasing potential. In general for com-
parable consumed charge values, CE(H2) and S(NH3) increase
when the applied potential becomes more negative. At −0.85 V,
while CE(NH3) remains constant. However, no products besides
those usually observed (e.g. H2, NH3 and NO2−) were detected.
Thus, bearing in mind a high efficiency of Rh/graphite electrodes
for nitrite reduction one could suppose that an electrolysis per-
formed in 1 M NaCl + 1 M NaNO2 solution would lead to the
complete transformation of nitrite to NH4+/NH3(aq). Indeed,
potentiostatic electrolyses performed in the range from −0.8 to
−1.3 V for 2 h, led to a nearly complete transformation of nitrite
to ammonia irrespectively of the electrolysis medium tempera-
ture. Finally, one can infer that the increase in temperature does
not make a significant effect on the selectivity of nitrate reduc-
tion. Perhaps, a longer duration of Rh surface activation and/or
some additives to the electrolysis medium would be required
to achieve a sufficient Rh surface activation resulting in N2
formation as earlier reported [38].
−
no hydrogen is detected and the activity towards NO3 reduc-
tion is rather low. Hence, the adsorbed hydrogen inhibits the
nitrate adsorption as stated above. When the electrolyses were
carried in 1 M NaCl + 1 M NaNO2 at the same potential (i.e.
−0.85 V), it was found that the consumed charge is more impor-
tant than in the case of nitrate-containing solution. At more
negative potentials (−1.3 V) after the passage of 30 C cm−2
,
3.4. Effect of additives to the electrolytic medium on the
selectivity of NO3− reduction
The desirable dinitrogen formation in course of nitrate reduc-
tion on Rh electrodes has been already reported and explained
by the fact that the prolonged Rh surface activation at elevated
temperatures allowed the HER onset to be shifted towards more
negative potentials [38]. Indeed, the absence of hydrogen in
−
statu nascendi stops the chemical NO3 reduction to ammo-
nia. Another way of inhibiting the HER is the use of various
additives, which are adsorbed onto the electrode surface, thus
blocking hydrogen adsorption. In fact, it was shown that the
presence of bulky hydrophobic cations such as tetrabutylammo-
nium (C4H9)4N+ hindered the HER during the electrochemical
synthesis of ethylene glycol [51]. It was also observed that for-
mateanionsHCOO− areadsorbedatRhsurfaceandsuppressthe
HER [52]. Hence, in the present study the electrochemical mea-
−
surements were performed on Rh/graphite electrodes in NO3
Fig. 7. Nitrate destruction degree as a function of time and electrolytic solution
pH during the galvanostatic electrolysis of 1 M NaNO3 solution on a Rh/graphite
electrode (nucleation pulse of 100 ms, growth pulse potential of −232 mV,
growth pulse duration of 60 s). Electrolysis current density is 4 mA cm−2 (nor-
malized to the Rh deposit surface area).
and NO2−-containing solutions in the presence of various ions
(tetraalkylammonium cations (CH3)4N+, (C2H5)4N+ and car-
boxylate anions, namely acetate CH3COO−, tartrate C4H4O6
and formate HCOO−).
2−

6244
O. Brylev et al. / Electrochimica Acta 52 (2007) 6237–6247
Fig. 8. CV curves (fifth cycle) of Rh/graphite electrode (nucleation pulse of
100 ms, growth pulse potential of −232 mV, growth pulse duration of 60 s) in
2−
1 M NaNO3 solutions containing 0.1 M NaX (X = Cl−, HCOO−, C4H4O6
,
CH3COO−). Current density is normalized to the Rh deposit surface area. Scan
rate 20 mV s−1
.
CV curves recorded in 1 M NaNO3 solutions containing
0.1 M NaX (X = Cl−, HCOO−, C4H4O62−, CH3COO−) are
shown in Fig. 8. The current density values at the most negative
potential (i.e. −1.5 V) are almost the same for all the anions
except HCOO− for which they are about 15% lower. More-
over, only in the presence of formate ions the nitrate reduction
peak is not observed. These results point out a stronger HCOO−
adsorption on the investigated electrodes comparing with that of
C4H4O62−, CH3COO−, H3O+ and NO3−. Regarding the nitrite
reduction, the addition of carboxylate anions alter neither the
shape of CV curves nor the current density values (not shown).
It should be noted that for all the investigated carboxylate
anions the oxygen atoms are responsible for their adsorption at
Fig. 9. (a) CV curves (fifth cycle) of Rh/graphite electrode (nucleation pulse
of 100 ms, growth pulse potential of −232 mV, growth pulse duration of 60 s)
in 1 M NaCl + 0.1 M NaNO3 solutions containing x M HCOONa (x = 0.1, 0.01,
0.001). Current density is normalized to the Rh deposit surface area. Scan rate
20 mV s−1. (b) CV curves (fifth cycle) of Rh/graphite electrode (nucleation pulse
of 100 ms, growth pulse potential of −232 mV, growth pulse duration of 60 s)
in 1 M NaCl solutions containing x M HCOONa (x = 0.1, 0.01, 0.001). Current
−
the metallic surface [53]. A lower intensity of NO3 reduction
peak for tartrate comparing with that for acetate can be due to a
more favorable tartrate adsorption onto Rh/graphite electrodes
which is explained by the presence of two –COO and two –OH
groups. The absence of the cathodic wave at about −1.05 V sug-
gests that the formate ions are much more efficiently adsorbed
onto Rh/graphite electrode than the above-mentioned anions
blocking the nitrate adsorption. Moreover at the beginning of
an electrolysis for the same carboxylate anion concentration,
the pH value is the lowest for HCOO−-containing solutions and
a priori one could expect a higher activity for HER, which is
not the case. In fact in the hydrogen evolution region, interac-
tions between adsorbed hydrogen and adsorbed formate may
lead either to bridge-bonded or to linear bonded CO according
to the following reactions [52,54]:
density is normalized to the Rh deposit surface area. Scan rate 20 mV s−1
.
readily adsorbed onto negatively charged Rh electrode surface
blocking the nitrate and hydrogen adsorption in the potential
region where their reduction takes place.
−
To study the effect of formate anions on NO3 reduction
in more details, CV curves were recorded in 1 M NaCl + 1 M
NaNO3 and 1 M NaCl solutions containing different amounts
of formate anions. A progressive increase in HCOO− concen-
−
tration results in a decrease of the NO3 reduction peak and in
its final disappearance, which confirms the HCOO− effective-
ness for blocking the NO3− adsorption and reduction (Fig. 9a).
Concerning the 1 M NaCl solution (Fig. 9b), the decrease in the
current density within HER potential range clearly manifests
that HCOO− ions inhibit hydrogen adsorption as previously
shown [52]. The differences between current densities in dif-
ferent solutions could be used as a comparison criterion and are
Rh–COOH + Rh–H → Rh2C O + H2O
Rh–COOH + Rh–H → Rh–CO + Rh–H2O
(16)
(17)
Both forms of CO can interconvert to each other and in this case
a positively charged carbon atom is bonded to the Rh surface.
Presumably this is not the case for the other carboxylate anions
investigated in this work and that is why the formate anions are
−
summarized in Table 1. In the case of NOx presence, the cur-
rent values are corrected with respect to 1 M NaCl solution. One

O. Brylev et al. / Electrochimica Acta 52 (2007) 6237–6247
6245
Table 1
Influence of formate anions addition on the current density values (fifth cycle)
for Rh/graphite electrode (nucleation pulse of 100 ms, growth pulse potential of
−232 mV, growth pulse duration of 60 s) in different solutions
HCOO−
Solution composition
concentration (M)
1 M NaCla
1 M NaCl + 0.1 M
NaNO3
1 M NaCl + 0.1 M
NaNO2
b
a
0
10−3
2.6
2.2
0.6
0.3
0.9
1.3
The current density is normalized to the Rh deposit surface area.
a
At −1.25 V vs. Ag/AgCl.
b
At −0.9 V vs. Ag/AgCl.
can infer that the effectiveness of NO2−−reduction increases with
formate addition contrary to the NO3 one. This fact confirms
again a more favorable adsorption of NO2− on Rh with respect
to that of NO3−, so the addition of HCOO− does not hinder the
NO2− adsorption and reduction.
−
To investigate the selectivity of NO3 reduction in the pres-
ence of formate anions, the galvanostatic electrolyses were
performed for Rh/graphite electrodes in 1 M NaNO3 + x M
HCOONa (x = 0, 0.001, 0.1) solutions. The variation of CE(H2),
S(NH3) and S(NO2−) are shown in Fig. 10a–c, respectively. The
following conclusions can be derived from these data:
(i) it can be clearly seen that CE(H2) diminishes with the
increase in HCOO− concentration and the electrolysis
duration which confirms CV results (Fig. 10a);
(ii) in the absence of formate anions at the initial stages of
the electrolysis, S(NH3) decreases following the CE(H2)
variation but then increases because of the easy reduction
−
of accumulated NO2 to NH3 (Fig. 10b). In the pres-
ence of HCOO−, S(NH3) increases during the electrolysis
though the decrease in CE(H2) would favor the ammonia
formation. This is explained by the easier adsorption of
nitrite ions at Rh/graphite electrodes where they are read-
ily reduced to NH3. It is also confirmed by the increase in
S(NH3) with HCOO− concentration (Fig. 10b);
(iii) in the absence of HCOO−, S(NO2−) dependence (repre-
sented in Fig. 10c) passes through a maximum as it has be−en
observed before [30]. In the presence of HCOO−, S(NO2
)
decreases as formed NO2− ions are efficiently adsorbed at
Rh surface and rapidly reduced to NH3, as stated above.
Decrease in S(−NO2−) with increasing c(HCOO−) is due to
a poorer NO3 adsorption and hence, slow nitrite forma-
tion.
in 1 M NaNO3 solutions as a function of HCOO− concentration and galvanos-
taticelectrolysisduration. Electrolysiscurrentdensityis4 mA cm−2 (normalize−d
Fig. 10. (a) Current efficiency (CE) of H2 formation on a Rh/graphite electrode
(nucleation pulse duration of 100 ms, growth pulse potential of −232 mV,
growth pulse duration of 60 s) in 1 M NaNO3 solutions as a function of HCOO−
concentration and galvanostatic electrolysis duration. Electrolysis current
density is 4 mA cm−2 (normalized to the Rh deposit surface area). (b) Selectivity
(S) of NO3 reduction to NH3 on a Rh/graphite electrode (nucleation pulse
duration of 100 ms, growth pulse potential −232 mV, growth pulse duration 60 s)
−
to the Rh deposit surface area). (c) Selectivity (S) of NO3 reduction to NO2
on a Rh/graphite electrode (nucleation pulse duration of 100 ms, growth pulse
potential of −232 mV, growth pulse duration of 60 s) in 1 M NaNO3 solutions
as a function of HCOO− concentration and galvanostatic electrolysis duration.
Electrolysis current density is 4 mA cm−2 (normalized to the Rh deposit surface
area).
−

6246
O. Brylev et al. / Electrochimica Acta 52 (2007) 6237–6247
Concerning the influence of tetraalkylammonium cations on
Ourmainconclusionsaboutthereductionofnitrateandnitrite
ions on Rh/graphite electrodes can be summarized as follows:
the activity of Rh/graphite electrodes towards nitrate reduc-
tion, CV measurements (not shown) revealed that the addition
of 0.1 M (CH3)4N+ to 1 M NaNO3 solution does not change
the shape of CV curve but the addition of 0.1 M (C2H5)4N+
results in decreasing the intensity of NO3− reduction peak. The
galvanostatic electrolyses performed for the same series of solu-
tions showed that the addition of (CH3)4N+ does not affect the
selectivity of NO3− reduction while the addition of (C2H5)4N+
results in increasing CE(H2). However, a proper choice of poten-
tiostatic electrolysis parameters results in diminishing CE(H2).
A 24 h electrolysis was performed at −1.1 V on a Rh/graphite
electrode in 1 M NaNO3 solution with and without the addi-
tion of 0.1 M (CH3)4N+. These conditions allow CE(H2) to be
decreased from 6.9 to 0% and hence, CE(NO2−) to be increased
from 26.7 to 57.0%. It means that (CH3)4N+ ions hinder the
hydrogen adsorption at Rh surface whic−h results in the decrease
of CE(NH3) and accumulation of NO2 anions. Nevertheless,
the addition of tetraalkylammonium cations does not permit to
achieve the dinitrogen formation on Rh/graphite electrodes.
(i) All our experimental results (CV measurements, calcula-
tion of reaction rate constants and activation energy, effect
of anion addition) demonstrate unambiguously that at room
temperat−ure Rh/graphite electrodes possess a high affinity
for NO2 anions thus facilitating their adsorption. These
electrodes are much more efficient for nitrite reduction than
for that of nitrate. Ammonia proves to be the only product
of nitrite reduction.
(ii) The rate of nitrate destruction increases along with pH
value. Indeed, in acid medium CE(H2) is higher and mainly
−
+
chemical reduction of NO3 to NH4 occurs. In alkaline
medium, the adsorption of NO3− is favored, hence they are
−
electrochemically reduced to NO2
.
(iii) The electrolysis performed at elevated temperatures does
not allow the selectivity of nitrate reduction to dinitrogen
to be improved.
(iv) The inhibiting influence of formate anions on HER and
nitrate reduction was demonstrated. Other carboxylate
anions and tetraalkylammonium cations do not signif-
icantly influence the nitrate reduction and HER on
Rh/graphite electrodes.
4. Conclusions and summary
−
The data presented above clearly demonstrate that NO2
ions are more strongly adsorbed on Rh/graphite electrodes than
NO3−. During the−initial stages of the electrolysis of a nitrate
solution, the NO2 concentration increases but remains low.
Whe−n a certain nitrite concentration is reached (ca. 0.1 M),
NO2 ions occupy a significant part of electrode surface (the
decrease in nitrate concentration promotes this process) and are
readily reduced−to NH3. Afterwards the selectivity for the for-
Acknowledgements
´
The authors are grateful to NanoQuebec, Natural Sciences
and Engineering Research Council of Canada (NSERC) and
ENPAR Technologies Inc. (Guelph, ON, Canada) for the finan-
cial support.
−
mation of NO2 and hence, NO2 concentration in solution
become lower (Figs. 5 and 10c).
References
Finally, one can suppose that the reaction mechanism
involves nitrite ions and adsorbed hydrogen resulting in the
ammoniaformationandthedecreaseinCE(H2). Themechanism
involving a chemical reaction between adsorbed hydrogen and
adsorbed nitrite ion has been already proposed for mesoporous
palladium in alkaline medium [55]. On the other hand, a very
tinyfractionofN2O(0.2%)wasdetectedaftera96 helectrolysis.
This allows us to represent schematically the nitrate reduction
mechanism on Rh-modified graphite electrodes (Scheme 1).
Surely, t−here are some intermediate species formed between
NO2(ads) and NH3 [20] but their quantitative determination
requires more complicated analytical methods than those used
in our investigation [20,26].
[1] World Health Organization, Rolling Revision of the WHO Guidelines for
Drinking-Waters Quality, Nitrates and Nitrites in Drinking-Waters, World
Health Organization, 2004.
[2] A. Kapoor, T. Viraraghavan, J. Environ. Eng. 123 (1997) 371.
[3] G.C.C. Yang, H.-L. Lee, Water Res. 39 (2005) 884.
[4] P.E. Hartman, Environ. Mutagen. 5 (1983) 111.
[5] P. Fraser, C. Chivers, Sci. Total Environ. 18 (1981) 103.
[6] B.H. Gu, Y.K. Ku, P.M. Jardine, Environ. Sci. Technol. 38 (2004) 3184.
[7] R. Molinari, P. Argurio, L. Romeo, Desalination 138 (2001) 271.
[8] A. Elmidaoui, F. Elhannouni, M.A. Menkouchi Sahli, L. Chay, H. Elab-
bassi, M. Hafsi, D. Largeteau, Desalination 136 (2001) 325.
[9] S¸. Aslan, A. Tu¨rkmann, Process Biochem. 40 (2005) 935.
[10] B.U. Bae, C.H. Kim, Y.I. Kim, Water Sci. Technol. 49 (2004) 413.
[11] S. Kerkeni, E. Lamy-Pitara, J. Barbier, Catal. Today 75 (2002) 35.
[12] X. Zhang, J. Wang, Z. Wang, S. Wang, Synth. Met. 155 (2005) 95.
[13] M. Paidar, I. Rousar, K. Bouzek, J. Appl. Electrochem. 29 (1999) 611.
[14] J. O’M. Bockris, J. Kim, J. Appl. Electrochem. 27 (1997) 623.
[15] J. Gootzen, P. Peeters, J. Dukers, L. Lefferts, W. Visscher, J. van Veen, J.
Electroanal. Chem. 434 (1997) 171.
[16] L.J.J. Janseen, M.M.J. Pieterse, E. Barendrecht, Electrochim. Acta 22
(1977) 27.
[17] Y. Xiang, D. Zhou, J.F. Rushling, J. Electroanal. Chem. 424 (1997) 1.
[18] N. Chebotareva, T. Hyokong, J. Appl. Electrochem. 27 (1997) 975.
[19] O.A. Petrii, T.Ya. Safonova, J. Electroanal. Chem. 331 (1992) 897.
[20] G.E. Dima, A.C.A. de Vooys, M.T.M. Koper, J. Electroanal. Chem. 554/555
(2003) 15.
[21] M.T. de Groot, M.T.M. Kooper, J. Electroanal. Chem. 562 (2004) 81.
Scheme 1. Nitrate reduction mechanism on Rh/graphite electrodes.

O. Brylev et al. / Electrochimica Acta 52 (2007) 6237–6247
6247
[22] A.C.A. de Vooys, R.A. Santenvan, J.A.R. van Veen, J. Mol. Catal. A 154
(2000) 203.
[23] G. Hora´nyi, E.M. Rizmayer, J. Electroanal. Chem. 143 (1983) 323.
[24] J.F.E. Gootzen, L. Lefferts, J.A.R. van Veen, Appl. Catal. A 188 (1999)
127.
[39] G. Jerkiewicz, J.J. Borodzinski, Langmuir 9 (1993) 2202.
[40] A.E. Greenberg, L.S. Clesceri, A.D. Eaton, M.A.H. Franson (Eds.),
Standard Methods for the Examination of Water and Waste Water,
18th ed., American Public Health Association, Washington, DC, 1992,
p. 4.
[25] D. De, J.D. Englehardt, E.E. Kalu, J. Electrochem. Soc. 147 (2000) 4224.
[26] M.C.P.M. da Cunha, J.P.I. De Souza, F.C. Nart, Langmuir 16 (2000) 771.
[27] K.J. Reddy, J. Lin, Water Res. 34 (2000) 995.
[28] J.W. Peel, K.J. Reddy, B.P. Sullivan, J.M. Bowen, Water Res. 37 (2003)
2512.
[41] D.S. Frear, R.C. Burell, Anal. Chem. 27 (1955) 1664.
[42] R.N. Dietz, Anal. Chem. 40 (1968) 1576.
[43] P.N. Bartlett, J. Marwan, Micropor. Mesopor. Mater. 62 (2003) 73.
[44] J.L. Ginner, P.J.J. Alvarez, S.L. Smith, M.M. Scherer, Environ. Eng. Sci.
21 (2004) 219 (and references therein).
[29] O. Brylev, L. Roue´, D. Be´langer, J. Electroanal. Chem. 581 (2005) 22.
[30] O. Brylev, M. Sarrazin, D. Be´langer, L. Roue´, Appl. Catal. B 64 (2006)
243.
[31] M. Ueda, H. Dietz, A. Anders, H. Kneppe, A. Meixner, W. Plieth, Elec-
trochim. Acta 48 (2002) 377.
[32] T.Ya. Safonova, O.A. Petrii, J. Electroanal. Chem. 448 (1998) 211.
[33] T. Omori, M.S. El-Deab, M. Osawa, J. Electroanal. Chem. 470 (1999) 46.
[34] S.J. Hsieh, A.A. Gewirth, Langmuir 16 (2000) 9501.
[35] A.J. Bard, R. Parsons, J. Jordan (Eds.), Standard Potentials in Aqueous
Solutions, Marcel Dekker, New York, 1985.
[36] D. Coleman, R.E. White, D.T. Hobbs, J. Electrochem. Soc. 142 (1995)
1152.
[37] E. Lazarova, Elektrokhimiya 18 (1982) 1654 (in Russian).
[38] Ph.M. Tucker, M.J. Waite, B.E. Hayden, J. Appl. Electrochem. 34 (2004)
781.
[45] K. Bouzek, M. Paidar, A. Sadlikova, H. Bergmann, J. Appl. Electrochem.
31 (2001) 1185.
[46] C. Polatides, G. Kyriacou, J. Appl. Electrochem. 35 (2005) 421.
[47] G.P. Sakellaropoulos, AlChE J. 25 (1979) 781.
[48] Ch. Lu, Sh. Lu, W. Qiu, Q. Liu, Electrochim. Acta 44 (1999) 2193.
[49] Q. Cai, W. Zhang, Z. Yang, Anal. Sci. 17 (2001) i917.
[50] A.C.A. de Vooys, G.L. Beltramo, B. van Riet, J.A.R. van Veen, M.T.M.
Koper, Electrochim. Acta 49 (2004) 1307.
[51] N.L. Weinberg, D.J. Mazur, J. Appl. Electrochem. 21 (1991) 895.
[52] F. Hahn, B. Beden, C. Lamy, J. Electroanal. Chem. 204 (1986) 315.
[53] J.M. Philips, F.M. Leibsle, A.J. Holder, T. Keith, Surf. Sci. 545 (2003) 1.
[54] B.B. Damaskin, O.A. Petrii, V.V. Batrakov, Adsorption of Organic Com-
pounds on Electrodes, Plenum Press, New York, NY, 1971.
[55] G. Denuault, Cl. Milhano, D. Pletcher, Phys. Chem. Chem. Phys. 7 (2005)
3545.