Direct reduction of nitrite to N2 on a Pt(100) electrode in alkaline media

Article abstract of DOI:10.1021/ja1092503

The selective reduction of NO₂^- to N₂ in 0.1 M NaOH was obtained at a Pt(100) electrode in a narrow but distinct potential region. This is the first report of such selectivity for this reaction on Pt(100), which is known to be the most catalytically active platinum surface toward NO₂^- reduction in alkaline media. Both ammonia and nitrous oxide are ruled out as possible reaction intermediates on the basis of online electrochemical mass spectrometry. Based on earlier work on ammonia oxidation, NH₂ adsorbates are speculated to be involved in the reaction mechanism.

Full text of DOI:10.1021/ja1092503

Published on Web 12/08/2010
Direct Reduction of Nitrite to N2 on a Pt(100) Electrode in Alkaline Media
Matteo Duca,* Mar Oroval Cucarella, Paramaconi Rodriguez, and Marc T. M. Koper*
Leiden Institute of Chemistry, Leiden UniVersity, P.O. Box 9502, 2300 RA Leiden, The Netherlands
Received October 25, 2010; E-mail: m.duca@chem.leidenuniv.nl; m.koper@chem.leidenuniv.nl
Abstract: The selective reduction of NO2- to N2 in 0.1 M NaOH
was obtained at a Pt(100) electrode in a narrow but distinct
potential region. This is the first report of such selectivity for this
reaction on Pt(100), which is known to be the most catalytically
active platinum surface toward NO2- reduction in alkaline media.
Both ammonia and nitrous oxide are ruled out as possible reaction
intermediates on the basis of online electrochemical mass
spectrometry. Based on earlier work on ammonia oxidation, NH2
adsorbates are speculated to be involved in the reaction
mechanism.
Wastewater remediation by means of electrochemical techniques
has become an active field of research.1 A specific challenge in
-
-
this area is the abatement of nitrite NO2 and nitrate NO3 via
electrochemical reduction, provided that selectivity to harmless N2
is achieved. This task is by no means trivial, as nature performs
this multielectron conversion with a multienzyme array.2-5 Here
we report the remarkable direct reduction of nitrite to N2 on a
monometallic and monocrystalline Pt surface in alkaline media. N2
-
production from NO2-/NO3 has been claimed before, although
only with multimetallic systems, such as Pd/Sn/Au,6,7 Cu/Pd,8 or
Rh on Ti9 for NO3 reduction and Sn-modified Pd, Rh, Ru, Ir
-
electrodes for HNO2 reduction.10 However, we note that only for
the Cu/Pd catalyst the N2 formation was observed in situ during
voltammetry using online mass spectrometry (see below).
Figure 1. Cyclic voltammetry during OLEMS measurements (a) and
ion current profiles for m/z ) 14 (b) and m/z ) 28 (c) in 0.1 M NaOH
containing 2 mM NaNO2. The working electrode was a Pt(100) electrode,
V ) 1 mV/s. The arrows indicate the direction of the potential sweep.
The blank voltammogram inserted in panel (a), thin line, was recorded
at V ) 50 mV/s.
Among the transition metals, Pt exhibits a special activity toward
the reduction of various nitrogen-containing molecules.3,11 Different
well-ordered monocrystalline Pt surfaces can display dramatically
different behavior toward a certain reaction, as evidenced for NH3
oxidation.3,12,13 Structure sensitivity for nitrite reduction at Pt
electrodes has been demonstrated in alkaline media,3,14,15 while in
acidic media HNO2 reduction is largely insensitive to the surface
structure.3,16 In agreement with earlier results of the Kita group,14
we find that the reactivity of the three Pt basal planes in a nitrite-
containing 0.1 M NaOH solution increases in the order Pt(111) <
Pt(110) , Pt(100) (see Figure S1). The Pt(100) shows a large
reduction peak centered at 0.35 V vs RHE, which was also
previously reported14 and ascribed to NH3 formation, as evidenced
by prolonged constant-potential electrolysis. Ye et al.14 also showed
that Pt(100) can subsequently reoxidize NH3 to N2 in the same scan
at ca. 0.6 V. Indeed, Pt(100) has a unique activity for the oxidation
of NH3 to N2.12,13,17 However, the strong influence of the
pretreatment on the electrochemistry of the Pt(100) surface in
alkaline media18 has prompted us to reinvestigate NO2- reduction
in 0.1 M NaOH at a Pt(100) electrode. Surprisingly, we observed
from 0.06 V sweeping in the positive direction (see Supporting
Information for additional details). The voltammetric profile features
two reduction peaks, one between 0.25 and 0.45 V and a second
smaller centered around 0.55 V, respectively. This latter feature
was not reported in the experiments of the Kita group.14 It is
reasonable to assume that this discrepancy stems from an improve-
ment in the quality of the Pt(100) surface. The blank voltammogram
of our electrode is also included in Figure 1: it can be compared to
the CV profiles reported in a recent study from our group.18 In
brief, the peaks at 0.48 V (positive-going) and 0.28 V (negative-
going) evidence the presence of long-range (100) terraces, along
with a minor amount of shorter (100) domains and (110) defects.
Cycling a Pt(100) electrode in alkaline media has been shown to
compromise long-range (100) order and to introduce defects;18
consequently, we stress that only freshly flame annealed electrodes
were used in the present study. Moving to the other panels of Figure
1, real-time monitoring of the conceivable gaseous products (N2,
NO, N2O) during the voltammetric scan using online mass
spectrometry (OLEMS)19 shows the formation of N2 in the restricted
potential range of the second voltammetric peak around 0.55 V
-
the highly selective direct reduction of NO2 to N2 on Pt(100), to
be reported on below. This is a significant finding as such an
observation has not been made before for a (monometallic)
electrocatalyst.
Figure 1a, shows the first voltammetric scan of a Pt(100)
electrode, in a 0.1 M NaOH + 2 mM NaNO2 solution, starting
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C O M M U N I C A T I O N S
The direct involvement of NH3 was also ruled out by an
additional experiment shown in the Supporting Information (Figure
S3). A Pt(100) electrode was immersed in a nitrite-containing 0.1
M NaOH solution, and a voltammetric profile was recorded in the
so-called hanging meniscus rotating disk configuration (HMRD)20
(to enhance transport to the electrode). Subsequently, increasing
amounts of NH3 were added: a detrimental effect on the peak related
to N2 formation was observed. In addition, the reduction peak at
0.35 V is retarded to higher potentials. Therefore, NH3 cannot be
a key participant in the formation of N2 and, moreover, its presence
could even suppress N2 formation and influence its own formation
in the first peak. The HMRD setup also allows modulation of the
rotation rate. Figure S4 shows that a change in rotation rate does
not affect the magnitude of the peak current of the feature related
with N2 production. This piece of evidence indicates that the
reaction is not under diffusion control, possibly involving only
surface-bound species. Moreover, if NH3 were an intermediate in
the N2 formation, an increase of the rotation rate would cause a
decrease in the peak current by enhancing the escape of NH3 away
from the electrode. However, this effect is not observed (Figure
S4).
Figure 2. Chronoamperometric profile during OLEMS measurements (A)
and ion current profiles for m/z ) 28 (B) in 0.1 M NaOH containing 2 mM
NaNO2. The working electrode was a Pt(100) electrode, V ) 1 mV/s. The
electrode was kept at 0.55 V for 300 s, followed by a potential step to 0.65
V. The m/z current immediately before t ) 0 is also shown.
The elucidation of the mechanistic steps leading to N2 formation
and the identification of the surface intermediates involved in this
reaction remain somewhat elusive, especially since no additional
information concerning the reaction intermediates can be inferred
from e.g. FTIR (Fourier-Transform Infrared) spectroscopy in the
external reflection configuration, due to the accumulation of gas
bubbles in the thin layer, disturbing the reflectance measurements.
However, on the basis of previous works concerning ammonia
oxidation and its adsorbates in the E ) 0.4-0.55 V range in alkaline
media,12-14,17,21,22 NH2,ads was suggested as a stable surface-
adsorbed intermediate up to ca. 0.5 V. The potential for which we
report N2 evolution corresponds to the foot of the main wave of
NH3 oxidation on Pt(100). For this surface, it has been suggested
that two NH2 combine to form hydrazine to finally oxidize to N2,
this reaction made possible by the high NH2,ads coverage.13,23 On
the other hand, the OLEMS and voltammetric data for nitrite
reduction described above show only reduction currents, and no
evidence for the ammonia oxidation, or the direct involvement of
ammonia in the N2 formation reaction, was obtained. We therefore
(see Figure 1b and c). Significantly, NO and N2O were not detected
during the nitrite reduction voltammetry.
The reduction peak associated with the detection of N2 (and its
fragment N) appears both in the positive-going and in the negative-
going scan, although it is much smaller in the latter case. This
positively parallels the lower MS current detected for N2 in the
negative-going scan. Especially the reduction in the positive-going
scan is strong evidence in favor of a direct and selective reduction
-
of NO2 to N2, as opposed to N2 formation via intermediate NH3
oxidation, as the latter reaction would have led to an oxidation
current. Ammonia is a ubiquitous product of the reduction of
nitrogen species at Pt electrodes and was reported by Ye et al. to
be the mainseven onlysproduct in the broad reduction peak
between 0.25 and 0.45 V,14,15 a conclusion that we have confirmed
with an experiment described in the Supporting Information (Figure
S2). To exclude the simultaneous formation of NH3 during the N2
formation peak at 0.55 V (a potential at which Pt(100) is capable
of oxidizing ammonia13), we designed an experiment to rule out
the occurrence of NH3 as an intermediate or a byproduct.
-
propose the following tentative reaction scheme. First NO2 is
reduced to NOads, followed by N-O bond breaking leading to an
NHx intermediate, presumably NH2,ads
:
After pretreating the electrode as described above, the Pt(100)
was immersed at E ) 0.06 V and the potential was stepped directly
to a N2 generating potential of E ) 0.55 V, kept at this value for
300 s, and next stepped to E ) 0.65 V. In this way, if any ammonia
would be formed at E ) 0.55 V, we would expect some of it to
escape into solution, and it should then be oxidized at E ) 0.65 V
as the Pt(100) electrode should oxidize NH3 to N2 at that potential,
but not reduce nitrite to N2. Figure 2 shows the results of this
experiment. During the first 300 s at 0.55 V, a small reduction
current is recorded due to nitrite reduction (Figure 2a), along with
clear evidence of continuous N2 evolution as recorded in the
OLEMS (Figure 2b). However, after the potential has been stepped
to 0.65 V, N2 evolution immediately reduces to the background
level, showing that no appreciable NH3, oxidizable at 0.65 V, was
generated during nitrite reduction at 0.55 V. Therefore, these results
offer additional evidence for the selective and direct reduction of
4H2O + NO-2 + 5e- f NH2,ads + 6OH-
(1)
Next, the NH2,ads intermediate must yield N2 in a reduction reaction:
NO2- + NH2,ads + e- f N2 + 2OH-
(2)
-
with NO2 in the solution phase, or involving NOads in an
intermediate step. Various pieces of evidence discussed in this
communication, in fact, suggest that a slow, surface-confined
reaction should be operative in the N2 formation region. Further
investigations are currently underway in our laboratory to obtain
additional insight into the intermediate species.
In conclusion, we have reported here a unique direct and selective
reduction of NO2- to N2 at a Pt(100) electrode in 0.1 M NaOH at
a potential close to 0.55 V. The combination of voltammetry and
OLEMS exclude N2O and ammonia as possible reaction intermedi-
ates. In light of previous work on NH3 oxidation in the same
electrolyte, we propose that NH2,ads plays a key mechanistic role,
-
NO2 to N2 at Pt(100). The same experiment, but with the first
potential step to 0.35 V, yields significant N2 detection at E ) 0.65
V during the second step, showing the formation of ammonia at
0.35 V (Figure S2).
-
reacting with NO2 or NOads to form N2 in a reduction reaction.
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C O M M U N I C A T I O N S
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Our work not only represents a unique example of NO2 removal
with the production of a benign species but also provides a clear
illustration of a highly specific reduction reaction operative on
Pt(100), a surface which is usually better known as a very selective
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tended to Pt nanoparticulate systems featuring (100) preferential
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for nitrite and nitrate abatement.
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Acknowledgment. We acknowledge partial financial support
from the European Commission (through FP7 Initial Training
Network “ELCAT”, Grant Agreement No. 214936-2), an Erasmus
grant awarded to M.O.C., and financial support from The Nether-
lands Organization for Scientific Research (“NWO-Middelgroot”)
for the purchase and development of the online electrochemical
mass spectrometer.
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Supporting Information Available: Experimental details, a com-
parison of the behavior of the three basal planes toward nitrite reduction,
an OLEMS measurement evidencing NH3 formation in the major peak,
and experiments probing the effect of NH3 addition and the influence
of rotation rate in HMRDE configuration. This material is available
free of charge via the Internet at http://pubs.acs.org.
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A. J. Electroanal. Chem. 2006, 588, 331–338.
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