Electrocatalytic reduction of nitrite on a polycrystalline rhodium electrode

Article abstract of DOI:10.1016/j.jcat.2010.07.013

This paper addresses the mechanism of the electrochemical reduction of HNO₂ and NO2- on polycrystalline rhodium. Intermediates and/or reaction products were detected by means of various (combined) techniques: rotating ring-disk voltammetry (for NH₂OH detection), online electrochemical mass spectrometry (for volatile products) and transfer experiments (for NO_ads). In acidic media, HNO₂ depletion due to homogeneous-phase reactions generates dissolved NO: the latter species can be adsorbed at Rh and is reduced to N₂O when 0.3 < E < 0.5 V (vs. RHE), while HNO₂ is reduced in a diffusion-limited wave to mainly NH₃ at potentials preceding hydrogen evolution. In alkaline media, the predominant product for the reduction of NO2- when E < 0.2 V is still NH₃, which can poison the electrode via dehydrogenation to NH_x,ads species. For more positive potentials, reduction still occurs via NO_ads and stops at NH₂OH for E > 0.3 V. The behavior of Rh is compared to Pt and explained in terms of general properties of these metals. A mechanistic scheme including NO, HNO₂ and NO2- is discussed.

Full text of DOI:10.1016/j.jcat.2010.07.013

Journal of Catalysis 275 (2010) 61–69
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Electrocatalytic reduction of nitrite on a polycrystalline rhodium electrode
a,*
a
b,c
c
a
M. Duca , B. van der Klugt , M.A. Hasnat , M. Machida , M.T.M. Koper
a
Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands
Department of Chemistry, Graduate School of Physical Sciences, Shahajalal University of Science and Technology, Sylhet 3114, Bangladesh
Department of Nano Science and Technology, Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555, Japan
b
c
a r t i c l e i n f o
a b s t r a c t
ꢀ
Article history:
Received 27 May 2010
Revised 9 July 2010
Accepted 13 July 2010
Available online 23 August 2010
This paper addresses the mechanism of the electrochemical reduction of HNO2 and NO2 on polycrystal-
line rhodium. Intermediates and/or reaction products were detected by means of various (combined)
techniques: rotating ring-disk voltammetry (for NH
trometry (for volatile products) and transfer experiments (for NOads). In acidic media, HNO
due to homogeneous-phase reactions generates dissolved NO: the latter species can be adsorbed at Rh
and is reduced to N O when 0.3 < E < 0.5 V (vs. RHE), while HNO is reduced in a diffusion-limited wave
to mainly NH
2
OH detection), online electrochemical mass spec-
2
depletion
2
2
Keywords:
Nitrite
Nitrous acid
Rhodium
On-line mass spectrometry
Rotating-disk electrode
3
at potentials preceding hydrogen evolution. In alkaline media, the predominant product
ꢀ
for the reduction of NO2 when E < 0.2 V is still NH
3
, which can poison the electrode via dehydrogenation
OH for
to NHx,ads species. For more positive potentials, reduction still occurs via NOads and stops at NH
2
E > 0.3 V. The behavior of Rh is compared to Pt and explained in terms of general properties of these met-
ꢀ
als. A mechanistic scheme including NO, HNO
2
and NO
is discussed.
Ó 2010 Elsevier Inc. All rights reserved.
2
1
. Introduction
Being the most expensive noble metal, as catalyst for practical
purposes rhodium should be used as a dispersed phase on a sub-
strate [16–18], as a coating on a supporting material [19] or in
mixed metal cathodes with Pd and Pt [20]. For example, in a study
dealing with the electrochemical abatement of nitrates in alkaline
media [19], Rh was electrodeposited on Ti foils and subsequently
electrochemically activated. The morphological changes thus
achieved ensured an enhancement of nitrate reduction over hydro-
Among the inorganic molecules involved in the nitrogen cycle,
nitrite and nitrate are regarded with particular interest, and their
levels in groundwater are kept under close scrutiny [1] given their
potential toxicity [2]. These anions are targeted by various waste-
water remediation techniques [3–7], with (electro)catalytic pro-
cesses [8] still needing further improvement to avoid undesired
byproducts such as N
2
O or NH
3
.
2
gen evolution with a remarkably high selectivity to N , albeit under
In fundamental research, nitrite is also an interesting species,
being central to the so-called electrochemical nitrogen cycle [9]
as intermediate of NO3 reduction [10] or NO oxidation [11]. In
conditions of extensive continuous potential cycling. From a more
fundamental point of view, the electrochemistry of nitrogen-con-
taining molecules on (single-crystal and polycrystalline) rhodium
electrodes has been investigated in various studies [9]. Significant
to this work is that rhodium is the most active noble- or coinage-
ꢀ
aqueous solutions, this anion displays pH-dependent homoge-
neous-phase equilibria, such as the acid–base equilibrium
ꢀ
ꢀ
HNO
2
=NO2 (pK
a
= 3.16 [12], 3.37 [13]). Therefore, the main species
metal for the electroreduction of NO3 ions in acidic media [10] and
ꢀ
changes with the pH, NO2 predominating in neutral/alkaline pH
that in neutral media nitrite reduction on rhodium is faster than
ꢀ
1
media, while HNO
2
is the dominant species at pH < 2. This fact is
nitrate reduction [16]. Tafel slope values close to 120 mV dec
highly relevant from an electrochemical point of view, as shown
by the completely different response of a polycrystalline platinum
indicated that, similarly to Pt, the first electron transfer (to give
ꢀ
NO ) is the rate-determining step (r.d.s.) [10]. However, no evi-
2
electrode in the two pH regions [14]. Besides, HNO
2
is involved in
dence for the formation of gaseous products (NO, N
2 2
O, N ) was
acid-catalyzed decomposition reactions [9,14,15] which generate,
among others, NO. NO is known to be reactive at metal electrodes
found [10], pointing to the formation of soluble products only
+
þ
3
4
(NH OH and NH ).
ꢀ
[
9], and its influence on Pt electrochemistry in HNO
2
solutions of
Only a few works directly dealing with HNO
2
=NO2 reduction on
low pH was reported recently [14].
Rh have thus far been published, covering the acidic [20–22] and the
ꢀ
neutral/alkaline pH range [16,20]. The relevance of HNO
2
= NO2
reduction for the nitrogen-cycle electrocatalysis on Rh lies in the fact
that, for the reduction of nitrate, the reaction steps following the
*
Corresponding author.
E-mail address: m.duca@chem.leidenuniv.nl (M. Duca).
ꢀ
ꢀ
first electron transfer ðNO3 ! NO Þ will more likely determine the
2
0
021-9517/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2010.07.013

6
2
M. Duca et al. / Journal of Catalysis 275 (2010) 61–69
selectivity of the overall nitrate reduction. With regard to the deter-
mination of the intermediates of nitrous acid reduction, the compar-
ison of single-crystal (basal planes) Pt and Rh electrodes was carried
out by Rodes et al. [21] with combined electrochemical and spectro-
scopic techniques (Fourier-Transform Infrared Spectroscopy, FTIRS).
Cyclic voltammetry proved that the onset of the reduction process,
ascribed to the reduction of both adsorbed species and solution
eter Copenhagen pH meter. No noticeable pH change was observed
during the experiments.
An Autolab PGSTAT20 (bi-)potentiostat was used throughout this
work. Transfer and stationary experiments were carried out with Rh
electrodes of various shapes (flag or bead), all at least 99.9% purity
(various suppliers). The Rh flag electrode was flame-annealed and
quenched in water saturated with an Ar atmosphere [10]. For all
electrochemical experiments involving stationary electrodes, the
counter was a platinum flag. A Reversible Hydrogen Electrode
(RHE) was employed as reference electrode for all experiments,
and all potentials reported in this paper are relative to the RHE.
Rotating disk gold electrodes (diameter 5 mm) were used in a
Pine Instrument Ring-Disk Teflon tip equipped with a Pt ring; rotat-
ing disk (RDE) and rotating ring-disk (RRDE) experiments were per-
formed with a Pine Instrument motor generator (MSR rotator). The
experimental collection efficiency was 0.26 ± 0.03 as calculated
2
HNO , occurred at a less positive potential than on Pt single-crystals,
with various minor waves preceding the main wave (located in the
window 0.04–0.17 V vs. RHE). This observation agrees with the de-
creased reactivity toward nitrous acid reduction for a PtRh 90:10 al-
loy with respect to bare Pt, as reported by da Cunha et al. [22]. NO
adlayers were formed upon contact with the acidic HNO solution
2
and these were stripped from the electrode during a negative-going
sweep. The peaks recorded in this experiment paralleled those of the
2
continuous reduction, suggesting that bulk HNO reduction can only
ꢀ4
ꢀ3
6
take place if the adsorbate is stripped. Additionally, a peculiar
behavior of two basal planes was reported [21]: clean Rh (1 0 0)
could not be completely recovered after adsorbate stripping, point-
ing to the presence of a very resilient adsorbate species or to the poi-
soning by partially reduced adsorbates; on the other hand Rh (1 1 0)
did not exhibit any vibrational signal typical of NOad in the FTIR spec-
tra. Rodes et al. speculated that the latter phenomenon could be ex-
plained by the coexistence of molecular and dissociated (into Nad
and Oad) NO. UHV surface-science studies reviewed by Brown and
King [23] support the validity of this hypothesis for low-coverage
NOad, and de Vooys et al. [24] have speculated on the occurrence
of NO decomposition on Rh electrodes. Literature on product
distribution during nitrite/nitrous acid reduction on Rh is scarce:
da Cunha et al. [22] showed that, for the PtRh 90:10 alloy, NO and
with the reversible couple [Fe(CN) ] /[Fe(CN) ] . A different
6
cleaning procedure was used for the RRDE tip, as described in a pre-
vious publication [14]. RRDE experiments involving Rh were carried
out by depositing the noble metal on gold from a solution of its chlo-
ride salt. Theprocedure for constant-potential Rh depositionon Au is
based upon the work of Shimazu et al. [26], with the following mod-
ification: the deposition was stopped when a pre-determined
amount of charge was measured. This ensured that the Rh deposit
3
still showed a mirror-like appearance. The 1 mM RhCl solution
was blanketed with Ar during the deposition. This solution was recy-
cled as the deposition efficiency (i.e. time needed to achieve the
deposition charge) improved with aging. This is a known feature of
deposition baths containing noble metal chlorides and has been re-
ported in studies dealing with electroless deposition [27]. The thin
foreign metal layer on Au was removed at the end of each experi-
ment by thorough polishing with alumina suspension (in sequence:
N
2
O are evolved simultaneously before N–H coupling takes place
at potentials immediately preceding hydrogen evolution, with the
+
þ
formation of NH
3
OH or NH . A Rh/Pt cathode combined to a Nafion
5–1–0.3 lm). Control experiments were performed with a polycrys-
4
þ
membrane and a Pt anode also displays a high selectivity to NH ,
talline Rh disk (diameter 6 mm) driven by a Motomatic motor: no
difference was noticed with respect to the rhodium-on-gold disk
used in combination with the Pine motor.
4
which increases with increasing pH. A predominant formation of
NH
16] for neutral media.
In this paper, we report new findings concerning the nitrite/ni-
3
on graphite-supported Rh has been reported by Brylev et al.
[
Adsorbate studies were performed using the following transfer
procedure [28]: the flag electrode was immersed in a nitrite-con-
taining solution (0.8 mM) of a chosen electrolyte under potential
control for 120 s, rinsed and then protected with a droplet of water
during the transfer to another cell containing clean electrolyte.
Adsorbates thus formed are known to undergo the transfer un-
scathed [24]. The nitrite concentration chosen for the adsorption
step equals the one used in stationary continuous reduction exper-
iments (0.8 mM).
On-line electrochemical mass spectrometry (OLEMS) [29] mea-
surements were performed on an EvoLution mass spectrometer
system (European Spectrometry Systems Ltd.). The system consists
of a Prisma QMS200 (Pfeiffer), brought to vacuum with a TMH-
trous acid reduction on Rh as a function of pH, both with respect to
the reaction mechanism and the product distribution, as deter-
mined by means of rotating ring-disk electrodes (RRDE) and on-
line electrochemical mass spectrometry (OLEMS). Similar to Pt
electrodes, in acidic media the reactivity is dominated by NO, gen-
ꢀ
erated by decomposition of HNO
2
; on the other hand, NO2 directly
takes part in the reduction pathway in alkaline media. In both
cases, reduction of the N(III) species leads to ammonia.
2
. Experimental
0
71P turbo molecular pump (60 l/s, Pfeiffer) and a Duo 2.5 rotary
3
All glassware was cleaned following a standard procedure [25]
to remove all traces of organic contaminations. Electrolyte solu-
tions were prepared with Suprapur (Merck) or comparably pure
vane pump (2.5 m /h, Pfeiffer). During measurements, the pressure
ꢀ6
inside the MS was 1–5 ꢁ 10 mbar. Pretreatment procedures and
details were published in a previous paper [14]. For OLEMS exper-
iments, an Rh bead type electrode was used.
(
Sigma–Aldrich) electrolytes and Millipore MilliQ water (resistiv-
ity > 18.2 M cm); prior to the experiments, oxygen was removed
X
by bubbling argon (purity grade 6.0) through the solution for at
least 15 min (acidic solution) or longer for alkaline solutions. Argon
blanketing was used to protect the solutions during the experi-
3. Results
3.1. Acidic media
2
ments. NaNO (99.999%, Sigma–Aldrich) was used as received
and stored in a desiccator under vacuum; contact with air was kept
as short as possible. In the pH range where nitrous acid decompo-
sition takes place, concentrations reported in this paper are meant
as nominal (initial); no real-time monitoring of the concentration
was carried out. Phosphate buffer solutions (ionic strength 0.1 M)
were prepared for the experiments in the range 2 < pH < 3; the
pH was checked before and after the experiments with a Radiom-
In the (mildly) acidic pH range, it is more appropriate to refer to
nitrous acid, as this is the predominant species [9].
3.1.1. Nitrous acid reduction and adsorbate stripping at stationary Rh
electrodes
Nitrous acid/nitrite reduction was first investigated at station-
2 4
ary Rh electrodes in (mildly) acidic media (0.5 M H SO and

M. Duca et al. / Journal of Catalysis 275 (2010) 61–69
63
0
4
.1 M phosphate buffer, pH 3). HClO was not used in this work be-
ꢀ
cause it is known that Rh catalyzes the decomposition of ClO an-
ions to Cl [30]. A transfer experiment was performed to assess the
4
ꢀ
formation of adsorbed species during nitrite reduction in 0.5 M
H SO , and the voltammetric profile of the adsorbate stripping is
2 4
shown in Fig. 1. A broad reduction peak can be seen, followed by
two other minor signals. It must be noted that a single sweep down
to 0.05 V is not sufficient to recover the blank voltammogram com-
pletely; in particular, the minor ‘‘shoulder” signal at higher poten-
tials is shifted and its charge is diminished. This phenomenon is
likely due to the ‘‘irreversible poisoning” of Rh (1 0 0) facets [21]
mentioned in Section 1. The identification of the adsorbate as NO
is supported by the dependence of the stripping peak potential
on the stripping scan rate in 0.1 M H
2 4
SO . This plot, shown in the
inset of Fig. 1, is equivalent to a ‘‘Tafel plot”, and yields a slope of
ꢀ
1
7
7 ± 4 mV dec , reasonably close to the value published for NO
in the same electrolyte [24]. The stripping charge (in 0.1 M
SO ) corrected for hydrogen adsorption [28] corresponds to a
Fig. 2. Reduction profile of 50
bold line) compared to a rotating Pt electrode (thin line),
Inset: Levich plot at E = 0.07 V.
l
M NaNO
2
in 0.5 M H
2
SO
4
at a rotating Rh electrode
v = 10 mV/s, x = 900 rpm.
H
2
4
(
NO coverage of 0.62 ML.
An increase in pH, which will minimize the decomposition of
nitrous acid, has no major effect on the formation of adsorbed
NO from a (mildly) acidic nitrite solution. The estimated NO cover-
age and the stripping charge are largely independent of the pH of
the nitrite solution; moreover, also the position of the stripping
peak potential does not change with the pH on the RHE scale, thus
showing a trivial ‘‘Nernstian” dependence on the pH.
2
formation of N O [14]. A broad reduction wave dominates the vol-
tammogram for both metals, with a slightly more positive onset
potential on Pt than on Rh; nevertheless, Rh reaches a larger cur-
rent, both for the peak-shaped signal in the negative-going sweep
and for the plateau-like region in the positive-going sweep. Lastly,
the current of the return sweep is larger (in absolute value) than
that of the forward sweep for E > 0.20 V, suggesting that the activ-
ity on Rh increases after an excursion to more negative potentials.
The dependence of the current on the rotation rate was studied
at a potential in the plateau-like region; the Levich equation [32] is
verified at E = 0.07 V, giving a straight line with zero intercept (in-
set of Fig. 2). This indicates a purely diffusion-controlled reaction,
whose n value is equal to 5.3 ± 0.1; it seems reasonable to assume
3.1.2. Nitrous acid reduction at RRDE
Evidence for diffusion limitations in the reduction of nitrous
acid can be found in the literature [14,31]; therefore, additional
experiments with rotating Rh electrodes were carried out to study
the reduction of nitrous acid under controlled diffusion conditions.
Moreover, the RRDE technique allows the detection of those solu-
ble reaction products that are electroactive at the Pt ring; in acidic
þ
+
that the major reaction product is NH4 (6 electrons) and that the
media, the detectable product is mainly NH
3
OH [14,31].
uncertainty on the value of n could arise from the overestimation
of the nitrous acid concentration, as already mentioned in a previ-
ous paper [14]. Moreover, the comparison of the plateau-like re-
gions of the voltammograms of the two metals (recorded under
The voltammetric profile for the reduction of 50
.5 M H SO is reported in Fig. 2, along with the cyclic voltammetry
of a Pt disk electrode in the same solution. A lower concentration
with respect to the experiment described in Section 3.1.1) mini-
2
lM NaNO in
0
2
4
(
the same conditions) shows a ca. 3/2 ratio between the currents
mizes the influence of the decomposition of nitrous acid [14]; in
addition, the cyclic voltammogram was recorded immediately
+
3
of Rh and Pt, respectively. As the four-electron reaction to NH OH
was demonstrated at a Pt electrode [14], this observation would
support a 6-electron reaction on Rh. Lastly, RDE experiments at a
mildly acidic pH (2.7) in the micromolar nitrite concentration
range, yielded a similar voltammetric profile; the number of elec-
trons calculated from the Levich plot was higher i.e. 5.5 ± 0.1.
after the addition of NaNO
grams shows that Rh does not display the first wave
0.3 < E < 0.5 V) typical of Pt, which has been associated with the
2
. The comparison of the two voltammo-
(
RRDE experiments with a Pt ring were aimed at detecting the
+
formation of NH
3
OH ; this molecule is easily oxidized on a Pt elec-
trode in acidic media [33]. The nitrite concentration was in the
millimolar range (0.8 mM). The comparison of the cyclic voltam-
mograms for the Pt ring electrode with the disk at open circuit
potential and the disk kept at constant potential in the diffusion-
limited potential region (Edisk = 0.07 V) is shown in Fig. 3. The
two voltammetric profiles only differ in the 0–0.2 V and in the
1
1
.1–1.5 V ranges, the voltammograms being identical in the 0.2–
.1 V range. Both at lower potentials and at higher potentials, only
a shielding of the ring current is observed when Edisk = 0.07 V, with
no change in features; the loop expected in the anodic current at
E > 1 V when hydroxylamine is formed in a nitrite-containing solu-
tion [14,31] is absent. This result supports the identification of the
reaction product of the reduction of HNO
2
on Rh at low potentials
þ
as NH . In addition (see inset in Fig. 3), another experiment was
4
carried out to detect the potential range of HNO
as previously reported for Pt [14]. The ring current at constant
potential was measured (E = 0.08 V) as an indication of HNO con-
sumption [14], while the disk potential was scanned. Two regions,
2
consumption,
Fig. 1. Voltammetric profile for the first two cycles of adsorbate stripping (bold line,
first cycle ; dashed line, second cycle) on a polycrystalline Rh electrode in 0.1 M
H
2
SO
4
, v = 20 mV/s (blank CV: thin line). Inset: Tafel slope for adsorbate stripping in
2
the same electrolyte.

6
4
M. Duca et al. / Journal of Catalysis 275 (2010) 61–69
Fig. 3. Profile of the ring current during a shielding experiment (
0 mV/s) in 0.5 M H SO + 0.8 mM NaNO . Bold line depicts Iring with Edisk = 0.07 V,
and thin line with disk at open circuit potential (o.c.p.). Inset: comparison between
the current recorded at a Rh disk (Idisk, bold line) during potential scan (
200 rpm, = 10 mV/s) and a Pt ring (Iring, Ering = 0.08 V, thin line) in the same
solution as the main figure.
x = 900 rpm, v =
1
2
4
2
x
=
1
v
characterized by different slopes of the ring current, can be ob-
served: in the range 0.2 < E < 0.45 V the ring current decreases
more slowly than in the 0.0–0.2 V region, where the direct con-
Fig. 4. Cyclic voltammetry during OLEMS measurements (A) and ion current
profiles for m/z = 30 (B) and m/z = 44 (C) in 0.5 M H SO containing 0.8 mM NaNO
2
4
2
.
2
sumption of HNO is more massive. A clear difference is also visible
The working electrode was a polycrystalline Rh bead electrode,
arrows indicate the direction of the potential sweep.
v = 1 mV/s. The
between the negative-going and the positive-going sweep, with a
steeper slope in the former; this discrepancy between the two
sweeps, also observed for Pt [14], would suggest that a larger
amount of HNO
going half of the cycle.
2
is involved in the reaction during the negative-
Fig. 8 (vide infra, OLEMS at pH 13), although the intensity of the
m/z 15 signal is far weaker at pH 3 than at pH 13. Moreover, in
the pH 3 buffer we also observed a less massive formation of gas-
eous molecules; this is directly caused by the decreased magnitude
3.1.3. On-line electrochemical mass spectrometry (OLEMS)
OLEMS experiments were carried out in order to follow the for-
of nitrous acid decomposition, both due to a decreased prevalence
ꢀ
mation of gaseous products, such as N
and N O (m/z = 44), as previously described for platinum elec-
trodes [14]. The results displayed in Fig. 4 refer to the reduction
of nitrite (0.8 mM) in 0.5 M H SO . Starting from 0.6 V, the increase
2
(m/z = 28), NO (m/z = 30)
of HNO
2
(NO
2
roughly accounts for 1/3 of the N(III) species at this
itself. Because
pH) and to a more sluggish decomposition of HNO
2
2
of intrinsic limitations of the current OLEMS setup, described in
previous publications [29,36], a quantitative comparison of the
gaseous products measured in two different experiments cannot
be obtained. However, Table 1 compares the results of a semi-
quantitative internal calibration based on the currents of hydrogen
2
4
in m/z 44 can clearly be seen, with a region of maximum intensity
ranging from 0.35 to 0.45 V both in the forward and in the negative
direction. The signal for N O rapidly decreases for E < 0.32 and
2
reaches zero for potentials lower than 0.2 V, that is, in correspon-
dence to the onset of the broad reduction wave. In addition, the po-
2 4
evolution (m/z 2) measured in 0.5 M H SO and the pH 3 buffer at
the same potential (E = 0.06 V). The Table clearly shows that
although the (baseline-corrected) current for hydrogen evolution
is in the same order of magnitude for both pH values, the normal-
tential range of N
of ring current (see Fig. 3), as discussed in the previous section, and
the formation of N O in the negative-going scan exceeds that of the
2
O formation coincides with that of slow decrease
2
ized N O current is 10 times higher for the more acidic electrolyte.
2
positive-going scan. The analysis of m/z 30, on the other hand,
shows unambiguously that NO is present in the solution and is
3.2. Alkaline media
2
consumed during the formation of N O: in fact, the NO current sig-
nal decreases monotonically during the negative-going potential
sweep, until a constant, non-zero value is reached for potentials
lower than 0.20 V. It must be noted that NO is also a fragmentation
3.2.1. Nitrite reduction at RRDE
The reduction of nitrite (50 lM) at a rotating Rh disk electrode in
0.1 M NaOH is shown in Fig. 5 (inset). The shape of the current signal
approaches a sigmoidal curve, with a broad peak feature at 0.15 V in
the negative-going (forward) scan and a plateau-like region around
0.1 V in the positive-going (backward) scan. The onset potential is
approximately at 0.4 V, and at the contact potential (0.5 or 0.45 V)
a small reduction current is already recorded. This profile can be
compared to that recorded on polycrystalline Pt electrodes in the
same electrolyte, published elsewhere [14], which instead featured
the presence of a narrow peak (both in the positive- and in the neg-
ative-going scan) centered at 0.2 V, as can be also seen in the Pt ring
profile reported in Fig. 5 (see below). The peculiar feature of Rh,
therefore, is the presence of a remarkable reduction activity at
potentials where Pt is inhibited, probably by the competition be-
tween nitrite reduction and hydrogen adsorption [37].
product of N
2
O; here, the correction for this interference was not
carried out, and the additional contribution to m/z 30 due to N
2
O
fragmentation is clearly visible in the range 0.35–0.45 V, causing
þ
a change in the slope of the current signal. The detection of NH4
would be particularly interesting to support the detection of this
product by RRDE as described earlier, but it is extremely difficult
in this pH range, this ion being largely undissociated and therefore
non-volatile. As a consequence, the OLEMS experiment was re-
peated at pH 3: under these conditions, detection of a fragment
of NH
3
(m/z 15, which is not affected by interferences from H
2
O
[
34]), was possible. This fragment is reported to be equivalent to
1
3
0% of the main m/z 17 species [35]. The formation of NH takes
place for E < 0.20 V. The current profiles are identical to those of

M. Duca et al. / Journal of Catalysis 275 (2010) 61–69
65
Table 1
2 2 4
Comparison of the normalized NO and N O ion currents for a 0.5 M H SO and a 0.1 M
phosphate buffer solution. The first column reports the baseline-corrected ion current
during H evolution, measured in the two electrolytes. These values were used to
2
normalize the NO and N
2
O currents.
Electrolyte
Baseline-corrected ion
current for m/z 2, value
at E = 0.06 V (vs. RHE)/A
Normalized
current for
m/z 30 (NO)
Normalized
current for
m/z 44 (N O)
2
3.20 ꢁ 10ꢀ
2.00 ꢁ 10ꢀ
12
0.17
4.03
0.28
0
0
.1 M phosphate
buffer, pH 3
12
.5 M H
2
SO
4
2.25
Fig. 6. Comparison of the voltammetric profiles of a stationary Rh electrode in
0.1 M NaOH (blank, thin line), with the addition of 0.8 mM NaNO
2
(bold line) or
1
.6 mM NH OH (dotted line). = 20 mV/s. Inset: Tafel slope analysis for a Rh
2
v
electrode immersed in a 0.1 M NaOH + 0.8 mM NaNO
2
solution. Potential program:
steps of 10 mV/min.
of the ring current signal, with respect to the situation at Edisk
o.c.p., can already be appreciated at disk potentials as low as
.3 V, which corresponds to the Tafel slope region discussed in
=
0
the previous section. An increased oxidation current in the poten-
tial region 1 < Ering < 1.5 V is evident, along with a shielding of the
peak-shaped reduction signal at Ering = 0.19 V. This pattern was al-
ready reported for nitrite reduction on Pt and was then ascribed to
Fig. 5. Profile of the ring current during a shielding experiment (
= 10 mV/s) in 0.1 M NaOH + 0.8 mM NaNO . Iring is shown for various Edisk values.
Inset: current recorded at a Rh disk during potential scan ( = 900 rpm, = 10 mV/
s) in 0.1 M NaOH, 50 M NaNO
x = 900 rpm,
v
2
the formation of NH
OH [14]. A decrease in the disk potential to
.2 V causes the appearance of an oxidation peak centered at
.69 V, while no change takes place in the higher range of ring
is achieved on the verge of
2
x
v
0
0
l
2
.
potentials. A full selectivity to NH
3
The influence of parameters such as reactant concentration and
rotation rate on the current was investigated, although a prelimin-
ary comment must be made. When recording the current at a con-
stant potential in the pseudo-plateau region (see Fig. 5, inset), I
decreased slowly but continuously with time. The current decrease
over a fixed amount of time was found to be dependent on the
rotation rate, with a larger increase for faster rotation rates. Con-
stant-potential measurements were therefore carried out as fast
as possible. The pseudo-limiting current measured at E = 0.07 V
hydrogen evolution (0.07 V), as at that disk potential the ammonia
oxidation peak is the only oxidation feature in the ring voltamme-
try. The shielding of the nitrite reduction current at the ring in-
creases with decreasing potential, indicating that the
consumption of nitrite at the disk increases. We can hence con-
ꢀ
clude from these RRDE data, that the reduction of NO2 proceeds
to give NH OH at low overpotential, but to NH at more negative
2
3
potentials, with an intermediate potential region with the forma-
tion of both products.
and
x
= 900 rpm increases with an increase in concentration, giv-
ꢀ
ing an apparent reaction order of 0.65 in the NO2 concentration
3.2.2. Nitrite reduction at stationary rhodium electrodes
Further experiments were carried out at stationary Rh elec-
trodes. The current profile recorded at a stationary Rh electrode
range 25–125 lM, a reaction order lower than 1 being often indic-
ative of the presence of a step involving an adsorbed (poisoning)
species. The current measured at a constant potential (E = 0.07 V)
2
immersed in a 0.1 M NaOH solution containing 0.8 mM NaNO is
in the micromolar concentration range (75
in the range 200–2000 rpm, whereas for higher
becomes independent of . Rotation rates were changed randomly
l
M) is a function of
x
shown in Fig. 6. A single peak centered at 0.09 V can be observed;
its peak current was found to be dependent on the square root of
the scan rate. A Tafel slope analysis was carried out by measuring
x
, I progressively
x
to avoid the interfering influence of the aforementioned poisoning.
The influence of rotation rate was also studied with linear sweep
the current during steady-state reduction of 0.8 mM NaNO
2
in
0
.1 M NaOH; the starting potential was E = 0.4 V and E was stepped
voltammetry, which showed that I is a function of
lower than E = 0.20 V.
x for potentials
down by 10 mV per minute. Care was taken to avoid the interfer-
ence of products at the counter-electrode, which was isolated in
a separate compartment equipped with a glass frit. The Tafel slope
Rotating ring-disk experiments are particularly important for
the detection of soluble reduction products electroactive on the
ꢀ1
was equal to 123 mV dec , with a linear region extending from
E = 0.35 V to E = 0.20 V (inset of Fig. 6). The influence of diffusive
processes at the stationary electrode may be ruled out as the log-
arithmic plots of the various voltammograms recorded at a RDE
Pt ring: NH
dation current around E = 0.65 V for NH
9,38,39] and, for hydroxylamine, above ca. E = 1 V both in acidic
2
OH and NH
3
. These species are characterized by an oxi-
3
in alkaline media
[
ꢀ
ꢀ
1
media and alkaline media [9] . The oxidation of NO2 on the Pt ring
is highly inhibited in alkaline media, presumably due to the forma-
tion of the oxide layer [9,40], and so, contrary to acidic media, it
does not interfere with other oxidative processes. The evolution
of the cyclic voltammetry profile at the ring electrode as a function
of the applied potential at the Rh disk is shown in Fig. 5. A change
reproducibly yield a slope of 120 mV dec in the same potential
region. This information suggests that the rate-determining step
of the process is the first electron transfer, which in our case would
correspond to the formation of NO, as in
H
2
O þ NOꢀ2
þ eꢀ ! NOads þ 2OHꢀ
ð1Þ
ðadsÞ

6
6
M. Duca et al. / Journal of Catalysis 275 (2010) 61–69
ping charge for a saturated NO adlayer (formed in acidic nitrite
solutions) is much larger than the charge measured during strip-
ping of the adsorbed species generated in an alkaline NaNO solu-
2
tion. Nevertheless, in both cases, the peak of NO reduction is
clearly visible, centered at E = 0.2 V. The voltammetric profile for
the adsorbates formed in the alkaline NaNO solution also displays
2
a reduction current in the region preceding the main stripping
peak (i.e. at E > 0.3 V), perhaps due to the reduction of adsorbed
ꢀ
NO ions.
2
ꢀ
Considering that NH
2
OH is a product of NO2 reduction in the
range 0.2 < E < 0.3 V, we also investigated, by means of a simple
cyclic voltammetry experiment, the reduction of a 1.6 mM NH OH
solution, prepared by dissolving (NH OH) SO in the alkaline
2
3
2
4
working solution, at a Rh electrode. The reduction profile displays
a broad peak with an onset potential of E = 0.25 V (dotted line in
Fig. 6), which is more positive than the potential at which NH
3
starts to be produced, according to RRDE data (E = 0.2 V).
Fig. 7. Voltammetric profile for the stripping of adsorbed NO in 0.1 M NaOH (bold
line, first cycle) on Rh electrode, compared to the stripping (in the same
electrolyte) of the adsorbate generated in 0.1 M NaOH + 0.8 mM NaNO (dotted
line) and the blank voltammogram (thin line). = 20 mV/s.
a
2
v
3
.2.3. On-line electrochemical mass spectrometry (OLEMS)
OLEMS experiments were carried out in order to obtain addi-
tional evidence for the formation of NH during nitrite reduction
3
in 0.1 M NaOH. Fig. 8 shows the MS ion current for the ammonia
fragment having m/z = 15, along with the signal for hydrogen for-
mation. The comparison to the latter is important to test the occur-
rence of the formation of ammonia due to catalytic hydrogenation
of nitrite on rhodium by hydrogen, which was earlier mentioned
by Brylev et al. [16]. The data in Fig. 8 clearly demonstrate that ni-
trite is reduced to ammonia on rhodium for E < 0.16 V, whereas
hydrogen evolution only commences for E < 0.07 V. Obviously,
hydrogen generated in situ will eventually reduce nitrite ions to
ammonia, and the catalytic hydrogenation will probably be active
as an alternative pathway at such low potentials. It must be noted
3
that the detection of NH shows a certain delay with respect to the
voltammetric current profile, which reaches its maximum value al-
ready at E = 0.22 V during the negative-going scan. This may be due
to the difficulty of detecting a minor fragment of a highly water
soluble (and thus repelled by the membrane of the OLEMS tip)
molecule such as NH
to 0.22 V must be related to the formation of surface species or
short-lived intermediates (see previous sections), such as NH OH:
during the OLEMS experiment, zero ion current was recorded for
the fragments of) NO, N O, N and NH OH.
3
. The current branch stretching from 0.4 V
2
(
2
2
2
4
. Discussion
Fig. 8. Cyclic voltammetry during OLEMS measurements (A) and ion current
profiles for m/z = 2 (B) and m/z = 15 (C) in 0.1 M NaOH containing 0.8 mM NaNO
2
.
The working electrode was a polycrystalline Rh bead electrode,
arrow indicates the beginning of the potential sweep.
v = 1 mV/s. The
4.1. Acidic media
The reduction of nitrous acid/nitrite at rhodium electrodes in
acidic media can be discussed by dividing the potential region into
two sections characterized by different main products: a high-po-
The NO formed is likely to occur as adsorbate, and for this reason a
characterization study of the stripping of an NO layer in alkaline
media was carried out. The NO adlayer was generated by contacting
2
tential region (0.35–0.55 V), where N O formation is detected, and
the potential range immediately preceding hydrogen evolution
þ
(
E < 0.15 V), where NH4 formation predominates.
2 4
the electrode with a nitrite-containing 0.5 M H SO solution. The
stripping voltammogram of this NO adsorbate in 0.1 M NaOH fea-
tures a single peak (Fig. 7). The dependence of the peak potential
2
4.1.1. N O evolution and NO adlayer
ꢀ1
(Epeak), dEpeak/dlogv, was equal to 90 mV dec , somewhat higher
In a previous paper dealing with nitrite reduction at Pt elec-
trodes [14], we reported the presence of a high-potential first-
wave signal in acidic media, corresponding to the formation of
than the corresponding slope in acidic media (comparable to what
reported by de Vooys et al. [24]).
Similar experiments were carried out in order to ascertain the
state of the NO adsorbate on Rh at the beginning of the reduction
of nitrite in alkaline media by keeping the electrode at E = 0.45 V in
the alkaline nitrite solution for 120 s and then transferring it to
clean alkaline electrolyte. The stripping voltammogram was then
recorded and compared to the stripping of a full NO adlayer (as
generated in acidic solution) in the 0.1 M NaOH solution. The strip-
N
2
O as measured during an OLEMS experiment. In the present
study, the formation of N O (in the same potential region as plat-
2
inum) is observed, although not accompanied by a similar pre-
wave. This fact may arise from a combination of causes. In the first
place, the presence of anions or a partially oxidized surface
(although we tend to exclude the latter case for our experiments)
2
could mask the current of N O formation. The presence of anions

M. Duca et al. / Journal of Catalysis 275 (2010) 61–69
67
ꢀ
in the potential range of N
lower PZC than platinum (reported in the literature as low as
.17 V in a pH 2.5 solution [41]), thus inducing a stronger anion
adsorption. Secondly, de Vooys et al. [24] showed that N O forma-
tion on rhodium differs mechanistically from the same reaction on
platinum, as evidenced by the difference in the Tafel slope for NO
2
O formation is certain: rhodium has a
be paralleled with the reduction of NO3 at noble metal electrodes
in acidic media: for the latter process, the removal of the first oxy-
ꢀ
ꢀ
0
gen atom (NO3 to NO ) is the rate-determining step [10]. There-
2
2
fore, it is not surprising that, as we observed in the present study
ꢀ
in acidic media and at micromolar NO2 concentrations, the direct
2
reaction of HNO is a fast, diffusion-controlled process. The differ-
reduction to N
2
O on the two metals. The value reported for rho-
ent product selectivities of the two metals reflect a difference in
the ability of Pt and Rh to adsorb NO in a dissociative fashion: Rh
displays a higher tendency to form atomic N and O species upon
exposure to NO than Pt [23,43]. Since the breaking of an N–O bond
ꢀ1
dium, much higher than 120 mV dec , was tentatively assigned
to the presence of a rate-determining chemical step (the formation
of (NO) ) preceding the first electron transfer. This suggestion can
2
be correlated with the larger NO coverage observed on Rh than on
Pt and with the metal–nitrogen interaction, which is stronger for
Rh than for Pt, as mentioned by Petrii and Safonova [37] in relation
to nitrate reduction, and confirmed by density functional theory
calculations of Zeng et al. [42] showing that the adsorption energy
of NO on Rh (1 1 1) is the highest among the transition metals. We
may thus hypothesize that the formation of a NO dimer, according
to the scheme
2 3
discriminates the formation of NH OH from NH , our findings are
further evidence of the different intrinsic properties of Rh and Pt.
4.2. Alkaline media
The low selectivity towards nitrogen oxides and the high selec-
ꢀ
ꢀ
3
3
tivity towards NH during NO2 (or NO ) reduction at a Rh elec-
trode in alkaline media is in agreement with previous reports of
various rhodium-containing systems [16,20,22]. Our results indi-
cate that the electrode, when immersed in a nitrite-containing
alkaline solution at the starting potential (0.45 V or 0.5 V), is cov-
NOads þ NOaq ¼ ðNOÞ2;ads
ð2Þ
involves the weakening of the existing metal–nitrogen bond upon
reaction with the second NO molecule coming from the solution
phase. A strong Rh–N bond would disfavor this reaction, thus ren-
dering the NO dimer formation slow enough to become the rate-
ꢀ
ered by adsorbed NO2 and NO, the latter generated by reduction
of the former. No NO was detected in the MS setup during the
ꢀ
reduction of NO , and therefore this intermediate is never des-
2
orbed, being reduced to more hydrogenated compounds directly
2
determining step of the entire N O evolution process. The effect
on the Rh surface. Since NO does not desorb, the formation of
of the pH on the formation of gaseous molecules is evident from
the trend of the normalized ion currents shown in Table 1: the
decomposition of nitrous acid is slower at higher pH.
When exposed to a solution containing NO, rhodium readily
forms robust adlayers, with NO possibly being partially dissociated
ꢀ
N
2
O is not observed [24]. The presence of adsorbed NO2 on Rh
around 0.4 V is not surprising, given the PZC of polycrystalline Rh
in alkaline media (0.23 V at pH 12.7 [41]). Hydroxide ions will be
ꢀ
the only competing species for the adsorption of NO2 ions. The
strong NO adsorption can be ascribed to the favorable interaction
between the Rh surface and NO, as explained in the previous sec-
tions. The presence of adsorbed species would also explain the
presence of excess reduction charge during adsorbate stripping
[
23] into atomic species which are more resilient to reductive
stripping than molecular NO. The presence of such species, for
example, would explain the fact that, contrary to platinum, a single
excursion into the region of NO reduction is not sufficient to re-
cover the profile for clean Rh. In addition, the higher affinity of rho-
dium toward the chemisorption of NO is shown by the dependence
of the NO coverage on the pH of the nitrite-containing generating
solution: for Pt, a significant decrease in hNO is observed when
the adlayer is generated in a nitrite-containing pH 3 buffer [14],
whereas for Rh in alkaline media the NO coverage is still substan-
tial compared to acidic media (see Section 3).
(
Fig. 7). The value of the Tafel slope recorded at the beginning of
ꢀ
the NO2 reduction process supports the hypothesis that the first
electron transfer (to form NO) is rate-determining during the
reduction process, at least in the potential range corresponding
2
to NH OH formation.
The product analysis performed with RRDE and OLEMS is cru-
cial for the understanding of the overall reaction mechanism. As
discussed in the previous sections, Rh is able to break the N–O
The tendency to form N
2
O during nitrous acid reduction at a Rh
ꢀ
bond and hydrogenate NO2 completely to NH
3
. RRDE and OLEMS
electrode is in agreement with previous observations by Brylev
et al. [16], who first recognized the remarkable decrease in the cur-
rent efficiency to nitrite in mildly acidic solutions, due to a parallel
show that ammonia formation starts to take place only for
E 6 0.20 V, similar to the behavior in acidic media, where the for-
mation of nitrogen oxides dominates for higher potentials. How-
ever, in the case of alkaline media RRDE experiments prove that
a molecule oxidizable at a Pt ring is generated at E = 0.3 V from
2
pathway involving HNO decomposition, NO generation and elec-
trochemical N O formation. A recent paper by Hasnat et al. [20]
2
also shows that the selectivity to ammonia during nitrite reduction
at Pt/Rh cathode decreases with decreasing pH, along with in-
creased formation of gaseous side products.
ꢀ
the reduction of NO2 at the Rh disk. This observation agrees with
2
the electrochemical behavior of NH OH, for which the onset poten-
tial of reduction on Rh in alkaline media is approximately 0.25 V. In
ꢀ
the NH
2
OH forming region, in addition, the shielding of the NO2
þ
4
.1.2. NH4 formation
reduction current at the ring (Fig. 5) is much lower than for lower
potentials, evidencing a lower rate of consumption of nitrite at the
disk.
þ
The formation of NH4 is peculiar to nitrous acid reduction in
acidic media at a Rh electrode; Pt, on the other hand, reduces
+
HNO
the two metals is informative and allows a broader discussion on
the similarities of HNO reduction with other processes involving
nitrogen-containing molecules. At the starting potential of Fig. 3
RDE), both Rh and Pt are covered with an NO adlayer: during
2
to NH
3
OH [14]. A comparative analysis of the behavior of
3
The details of the process leading to NH in the low-potential
region are somewhat elusive, given the overlapping effect of the
electrode poisoning. However, the data may suggest that a kineti-
cally mixed process is active, with a contribution of the diffusion of
2
ꢀ
0.5
(
NO ions to the electrode as highlighted by the I
p
vs.
v
depen-
2
the negative-going sweep, the reactivity of this adsorbed molecule
dominates, and the current will reflect the reductive removal of
dency and by the influence of the rotation rate on the current mea-
sured in the ammonia-formation reaction. In addition, the
observed reaction order, minor than 1, may be ascribed to the
involvement of an adsorbed intermediate, whose nature cannot
be further detailed without the aid of spectroscopic techniques.
NO, and the reduction of HNO
ping this process. Once the NO layer has been removed, the ‘‘clean”
metal is able to perform the direct reduction of HNO , which is a
diffusion-limited process for both Rh and Pt. This observation can
2
– if it ever takes place – is overlap-
2
3
The low-potential formation of NH , and its similarity to the

6
8
M. Duca et al. / Journal of Catalysis 275 (2010) 61–69
pH- and potential-dependent reaction mechanism as shown in
pH
Scheme 1. In acidic media, there is a competition between
0
1
2
3
4
13
ꢀ
−
−
HNO
=NO2 direct reduction and N
2
O formation via NO generated
HNO
2
NO2
NO2
2
in the aqueous phase by the acid-catalyzed decomposition of
HNO . NH is the major product at potentials close to hydrogen
+
H
−
NO2 ads
2
3
rds
evolution, whereas N
alkaline media, NO2 is the only reducible species present in the
O predominates between 0.3 and 0.5 V. In
2
ꢀ
NOads
NOaq
NOads
solution: its reduction yields NH
the Rh electrode by forming NHx,ads species. The formation of the
intermediates NOads and NH OH was also observed. The mechanis-
3
for E < 0.20 V, which may poison
2
NH OH
0
0
.3-
.2-
2
2
N O
tic information and the pH-dependent product distribution ob-
tained in the present study corroborate the results of previous
ꢀ
works on electrochemical reduction of NO on Rh: after the rate-
NH
3
3
ꢀ
ꢀ
ꢀ
+
4
determining step NO3 ! NO , the reduction of NO , which is
NH
2
2
heavily influenced by pH, will play the role of selectivity-determin-
ing step, with a higher selectivity to gaseous products in acidic
media.
0
-
NHx,ads
poison on Rh
E
Acknowledgments
ꢀ
Scheme 1. Mechanistic pathways for NO2 =HNO
electrode as a function of pH and E.
2
reduction at a polycrystalline Rh
M.D. and M.T.M.K. acknowledge partial financial support from
the European Commission (through FP7 Initial Training Network
‘
‘ELCAT”, Grant Agreement No. 214936-2), as well as financial sup-
port from the Netherlands Organization for Scientific Research
‘‘NWO-Middelgroot”) for the purchase and development of the
(
comparable process in acidic media, may be discussed in terms of
general properties of rhodium, in particular its propensity to break
online electrochemical mass spectrometer.
3
the N–O bond [42,43]. NH formation is not inhibited by hydrogen
evolution, and the two processes overlap as shown by the OLEMS
data; this is in stark contrast with Pt, where the observed inhibi-
tion in the reduction of various nitrogen-containing molecules at
Pt in acidic [10,44] and alkaline media [14,37]. A simple explana-
tion of this fact would ascribe it to the stronger affinity of a Rh sur-
face towards N than H [43].
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5
. Conclusions
ꢀ
The reduction of HNO
2
=NO2 at polycrystalline Rh electrodes is
[
[
influenced by the electrode potential and the pH of the solution.
The results of the present study can be summarized in a general

M. Duca et al. / Journal of Catalysis 275 (2010) 61–69
69
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