Selective catalytic reduction at quasi-perfect Pt(100) domains: A universal low-temperature pathway from nitrite to N2

Article abstract of DOI:10.1021/ja203234v

The highly selective conversion of nitrite to N₂ at a quasi-perfect Pt(100) electrode in alkaline media was investigated with a particular emphasis on its structure sensitivity and its mechanism. High-quality (100) terraces are required to optimize the catalytic activity and steer the selectivity to N₂: defects of any symmetry dramatically reduce the N₂ evolution at [(100) × (110)] and [(100) × (111)] surfaces. On the other hand, nitrite reduction proves to be an additional example of the unique intrinsic ability of (100) surfaces to catalyze reactions involving bond breaking and successive bond formation. In the present case, (100) is able to reduce nitrite to NH_2,ads, which in a certain potential window combines with NO_ads to give N₂ in a Langmuir-Hinshelwood reaction. Our findings are similar to those for other processes generating N₂, from bacterial anoxic ammonia oxidation ( anammox ) to the high-temperature NO + NH₃ reaction at Pt(100) crystals under ultra-high-vacuum conditions, thus suggesting that the combination of these two nitrogen-containing species is a universal (low-temperature) pathway to N₂. The advantages of this pathway over other N₂-generating pathways are pointed out.

Full text of DOI:10.1021/ja203234v

ARTICLE
pubs.acs.org/JACS
Selective Catalytic Reduction at Quasi-Perfect Pt(100) Domains:
A Universal Low-Temperature Pathway from Nitrite to N2
Matteo Duca,*,† Marta C. Figueiredo,‡ Victor Climent,‡ Paramaconi Rodriguez,† Juan M. Feliu,‡ and
Marc T. M. Koper*,†
†Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands
‡
ꢀ
Instituto de Electroquimica, Universidad de Alicante, Apt. 99, E-Alicante, Spain
ABSTRACT: The highly selective conversion of nitrite to N2 at
a quasi-perfect Pt(100) electrode in alkaline media was inves-
tigated with a particular emphasis on its structure sensitivity and
its mechanism. High-quality (100) terraces are required to
optimize the catalytic activity and steer the selectivity to N2:
defects of any symmetry dramatically reduce the N2 evolution at
[(100) ꢁ (110)] and [(100) ꢁ (111)] surfaces. On the other
hand, nitrite reduction proves to be an additional example of the
unique intrinsic ability of (100) surfaces to catalyze reactions
involving bond breaking and successive bond formation. In the present case, (100) is able to reduce nitrite to NH2,ads, which in a
certain potential window combines with NOads to give N2 in a LangmuirꢀHinshelwood reaction. Our findings are similar to those
for other processes generating N2, from bacterial anoxic ammonia oxidation (“anammox”) to the high-temperature NO + NH3
reaction at Pt(100) crystals under ultra-high-vacuum conditions, thus suggesting that the combination of these two nitrogen-
containing species is a universal (low-temperature) pathway to N2. The advantages of this pathway over other N2-generating
pathways are pointed out.
’ INTRODUCTION
electrode materials, ranging from bioinspired moieties to syn-
thetic metal-containing molecules to polycrystalline or single-
crystal surfaces.
The global imbalances in the nitrogen cycle have been in-
vestigated by several authoritative studies in the field of the
environmental sciences,1,2 which have shown that this issue poses
a threat potentially more menacing than CO2 accumulation. The
extraction of atmospheric N2 via combustion (to NOx) or via the
HaberꢀBosch synthesis of NH3 has led to an accumulation of
nitrogen oxyanions in soils and groundwater due to acid deposi-
tion and consequent runoff (for atmospheric NOx) or overuse of
ammonium fertilizers, ultimately leading to the production of nit-
rate and nitrite by bacterial reactions. Therefore, the levels of nit-
rate and nitrite in ground- and wastewater must be kept under
close scrutiny, and novel abatement methods, superiorto thewide-
spread but energy-intensive bacterial active-sludge treatment,3ꢀ5
are urgently needed.
Among the noble metals, Pt has long been recognized for its
high activity toward the reduction of nitrogen-containing mole-
cules7 and has been studied in detail, more recently also in the
form of well-defined monocrystalline electrodes.10,19,20 Within
this field of study, the Leiden group has recently demonstrated a
unique reactivity and selectivity at Pt(100) surfaces for nitrite
reduction to N2.13 Not only does this achievement satisfy one
of the requirements of electrochemical wastewater treatment
(conversion of pollutant species into harmless dinitrogen), it also
highlights the special ability of the Pt(100) surface in reactions
involving bond breaking and bond making,23 the selective
23ꢀ28
oxidation of NH3 to N2
being one such example relevant
for this study. In this reaction, highly stabilized ammonia frag-
ments (NH2,ads) recombine to give the intermediate N2H4,
which is rapidly oxidized to N2. This information was funda-
mental in proposing a tentative mechanism for selective nitritꢀe
reduction to N2: the recombination of stable NH2,ads and NO2
at the surface was speculated,13 although the detailed mechanistic
events remained elusive.
Valuable clues to unraveling the steps leading to N2 can be
obtained by a comparison of nitrite reduction with two similar
processes belonging to completely different research fields: the
Electrochemical processes could ideally be used to treat nitrate-
laden aqueous waste, provided that the activity is optimized and
full selectivity to N2 is achieved, thus creating a human-driven
closed detour of the geochemical nitrogen cycle. Both require-
ments have spurred wide-ranging research6 addressing potentiꢀal
nitrogen-containing pollutants such as NO, NO2ꢀ, and NO3
.
In addition, the electrochemical reactivity of these simple mol-
ecules offers significant insight into fundamental mechanistic as-
pects of the redox features of the nitrogen cycle.7 From this point
of view, numerous studies7 have highlighted the complex reac-
ꢀ 8ꢀ16
tion pathways of the electrochemical reduction of NO2
and NO3
,
ꢀ 17ꢀ22 with a potentially wide range of products. This
Received: April 8, 2011
Published: May 31, 2011
lack of product selectivity has prompted an ongoing screening of
r
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so-called“anammox”(anoxic ammoniaoxidation) bacterialsewage
treatment7,29,30 and the selective catalytic reduction (SCR)31 of
NO by NH3 to N2 under ultra-high-vacuum (UHV) conditions
at a Pt(100) surface.32ꢀ35 In the former, anoxic ammonium-oxi-
dizing bacteria, naturally occurring in seawater, exploit an alter-
Table 1. List of Single-Crystal Surfaces Used in This Studya
zone
Miller indices
terrace-step notation
[011]
(39,1,1)
(29,1,1)
(23,1,1)
(15,1,1)
(11,1,1)
(7,1,1)
20(100) ꢁ (111)
15(100) ꢁ (111)
12(100) ꢁ (111)
8(100) ꢁ (111)
6(100) ꢁ (111)
4(100) ꢁ (111)
20(100) ꢁ (110)
15(100) ꢁ (110)
10(100) ꢁ (110)
7(100) ꢁ (110)
5(100) ꢁ (110)
4(100) ꢁ (110)
ꢀ
native path in the nitrogen cycle30,36 by using NO2 obtained
ꢀ
from enzymatic reduction of NO3 as an oxidizer in a further
reaction that converts NH4+ to N2. Isotope ꢀlabeling experiments29
in which nitrate was supplied as 15NO3 demonstrated that
recombination to 14NH4 occurs, giving rise to mixed-isotope
+
14N15N. These bacteria feature unique cellular components that
are able to handle and withstand toxic, unstable N2H4; incidentally,
we have noticed7 that these bacteria and Pt(100) electrodes (see
above) seem to have a similar pathway for ammonia oxidation.
SCR conversion of NO to N2 at Pt(100) at high temperature
has attracted much interest in the literature (recently reviewed by
Hu et al.31) as concerns both the experimental behavior of this
system under UHV conditions31ꢀ34 and its theoretical simu-
lation,35,37,38 the latter mainly concerned with a rationalization of
the oscillatory behavior observed during prolonged reaction.
Adsorbed NO plays a very important role: there is compelling
evidence that the hexagonal reconstruction of Pt(100) can be
lifted upon NO adsorption, creating the (1 ꢁ 1) orientation,
which is the only one active toward NO reduction to N2 because
it offers a favorable surface for the stabilization of NHx frag-
[001]
(20,1,0)
(15,1,0)
(10,1,0)
(7,1,0)
(5,1,0)
(4,1,0)
a The notation in the rightmost column is n(terrace) ꢁ step, where n is
the number of terrace atoms.
low-temperature variant of the SCR mechanism is responsible for
the high selectivity to N2. In particular, NHx,ads and NOads will be
identified as the key surface species that take part in a Lang-
muirꢀHinshelwood recombination, which is the defining step of
the overall mechanism leading to N2. Additionally, the role of
surface defects in decreasing the catalytic activity and selectivity
to N2 of the electrode will be discussed: quasi-perfect (100) do-
mains are required to maximize nitrite conversion to N2, pro-
viding useful hints for designing practical catalysts for this re-
action. From a broader perspective, we demonstrate the intimate
similarities of the “anammox-type” N2 evolution in such diverse
fields as electrocatalysis, fundamental surface science, and applied
bacterial bioengineering. We will also briefly discuss the advan-
tages of this type of N2 formation over other known mechanisms,
especially with regard to reaching the desired high selectivity.
ments.32,35,37 Although θNO > θ
in all cases, due to the higher
NHx
heat of adsorption of NOads on Pt(100), evidence of mutual
stabilization of NOads and NHx,ads was found,32,33 with the en-
suing formation of NOadsꢀNHx,ads (x = 1ꢀ3) complexes at the
periphery of the NO islands, where the N2 evolution preferen-
tially occurs.33 Isotope labeling experiments32 using 15NO and
14NH3 have indeed evidenced the formation of 14N15N, along
with single-isotope 14N14N and 15N15N. The determination of
the elementary steps leading to N2 need to take into account all
temperature-dependent dissociation processes (NO decomposi-
tion, mainly),31,34 and the accepted scheme can be summarized as
’ EXPERIMENTAL SECTION
NOads f Nads + Oads
ð1Þ
ð2Þ
ð3Þ
ð4Þ
Electrochemical Experiments. Platinum single-crystal electro-
des were oriented, cut, and polished from small (2ꢀ3 mm diameter)
single-crystal beads as reported previously.39 The different electrodes
used, including Miller indices, terrace-step notation, and crystallographic
zone, are given in Table 1.
~
~
NH3;ads f Nads + 3Hads
~
~
Nads + Nads f NN
Before every experiment, the electrode was flame annealed and
cooled in a reducing atmosphere containing H2 + Ar40 and protected
with water in equilibrium with this gas mixture to prevent contamination
before immersion in the electrochemical cell.41 The counter electrode
was a platinum spiral wire. Potentials were measured against a reversible
hydrogen electrode (RHE) connected to the cell through a Luggin cap-
illary. The electrolyte solutions were deoxygenated prior to the mea-
surements with Ar (6.0); during the measurements, the penetration of
oxygen into the cells was minimized with a continuous blanketing Ar
flow. The voltammetric experiments were carried out in two classical
three-electrode electrochemical cells, one of them containing the blank
solution (namely, 0.1 M HClO4) and a second one containing the test
solution (0.1 M NaOH + 2 mM NaNO2). The blank solution was used
to characterize the electrode surface and determine the surface order.
The electrode was rinsed and transferred to the test solution containing
nitrite anions. So-called transfer experiments14,42ꢀ44 were carried out
according to the following procedure: a saturated NO adlayer at a
Pt(100) electrode was produced following the method described by
Rodes et al.,44 in which the electrode must be immersed, at open circuit
potential, into a 0.1 M HClO4 solution containing 0.01 M NaNO2 for
30 s. Less dense adlayers can be obtained by reducing the immersion
2Hads + Oads f H2O
~
where we have “labeled” the ammonia nitrogen N in reaction 3 to
indicate explicitly that N2 may arise in principle from a recombi-
nation of two N atoms originating from any of the two reacting
species. However, Hallock et al. have remarked34 that, in view of
the clear isotopic excess of 14N15N measured in the experiments
at T < 500 K, and given that NO decomposition becomes much
slower, the decomposition of the more labile NHx fragments must
be predominant below this temperature, supplying “reducing
agents” (Nads) to NOads in the following process:
~
~
Nads + NOads f NN + Oads
ð5Þ
which then represents the actual overall reaction leading to N2
evolution.
This paper will elucidate the mechanistic origin of the unique
reactivity of (100) sites toward nitrite reduction to N2. By
combining spectroelectrochemical studies and mass spectro-
metry experiments with isotope labeling, we will show that a
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Figure 1. Cyclic voltammetry profiles for nitrite reduction at the
Pt(100) surface in 0.1 M NaOH in the absence (dashed line) and in
the presence of 2 mM NaNO2. The starting potential Estart = 0.1 V, while
the upper potential limit (Eup) is progressively increased to higher
potentials. The dotted line refers to Eup = 0.6 V, the thick line refers to
Eup = 0.8 V, and the thin lines refer to three selected intermediate
potentials. The scan rate was in all cases 50 mV/s. For a description of
the individual signals labeled in the figures, refer to the text.
Figure 2. Cyclic voltammetry profiles for nitrite reduction at the
Pt(100) surface in 0.1 M NaOH for various scan rates in the presence
of 2 mM NaNO2. The scan rate is v = 50 mV/s (thick line), v = 5 mV/s
(dashed line), and v = 200 mV/s (thin line). The starting potential Estart
=
0.1 V. The potential was first swept to 0.06 V and then reversed until
0.8 V. The inset displays the dependence of the charge of the peak at
0.55 V (R2) on the scan rate.
time or the nitrite concentration. The electrode was then rinsed and
transferred to the cell containing the working solution. NHx adsorbates
can be generated in a similar way as first described by De Vooys et al.,45
by contacting the electrode with an ammonia-containing 0.1 M NaOH
solution at constant potential (see below for more details). All the
experiments were done at room temperature.
Electrolyte solutions were prepared from concentrated perchloric acid
(Merck Suprapur), sodium hydroxide (Merck Suprapur), sodium nitrite
(Sigma-Aldrich, 99.999%), and ultrapure water (Elga Purelab Ultra or
Milli-Q Millipore, 18.2 MΩ cm). Labeled sodium nitrite was supplied by
Cambridge Isotope Laboratory (15 N, 98+%). The measurements were
performed with an EG&G 175 signal generator, an eDAQ EA161 poten-
tiostat, and an eDAQ e-corder ED401 recording system.
The solution was not stirred during the experiments, and a flow of
blanketing Ar was maintained to protect the solution from oxygen. All
OLEMS experiments were carried out at a scan rate of 1 mV/s. The
OLEMS setup does not allow a quantitative analysis of the signals.
However, if the experiments are repeated with the same PTFE tip and at
a comparable pressure (measured with a full-range pressure gauge), the
relative magnitudes of ion currents measured prove to be highly
reproducible. An internal, semiquantitative calibration can also be
carried out, and further details will be reported below when the results
are presented. The potential was controlled with a computer-controlled
IviumStat potentiostat (Ivium Technologies).
Spectroelectrochemical Experiments. Fourier-transform in-
frared spectroscopy (FTIRS) experiments were performed with a Nicolet
Magna 850 spectrometer, equipped with an MCT detector. The spectro-
electrochemical cell was provided with a prismatic CaF2 window beveled
at 60°. Spectra shown are composed of 200 or 1000 interferograms
(indicated in the figure) collected with a resolution of 8 cmꢀ1
and p-polarized light. They are presented as absorbance, according to
A = ꢀlog(R/R0), where R and R0 are the reflectances corresponding to
the single beam spectra obtained at the sample and reference potentials,
respectively.
All the spectroelectrochemical experiments were conducted at room
temperature, with a reversible hydrogen electrode (RHE) and a plat-
inum wire used as the reference and counter electrodes, respectively.
The electrochemical cell and the electrolyte solutions were prepared as
described in the previous section.
Online Electrochemical Mass Spectrometry (OLEMS). A
detailed description of the setup can be found in previous publi-
cations.14,46 In brief, the flame-annealed single-crystal electrode, pro-
tected by a droplet of ArꢀH2-saturated water, was exposed to the
working electrolyte in a hanging-meniscus configuration under potential
control, and a PTFE tip, connected to the mass spectrometer, was
positioned at approximately 10 μm from the single-crystal electrode.
’ RESULTS
Electrochemical Experiments. Figure 1 depicts the blank
cyclic voltammetry of a well-ordered Pt(100) surface in NaOH
(0.1 M) along with the voltammetric response in the presence of
nitrite anions for successively increased values of the positive
vertex potential (Eup).
Several features can be identified in the broadest voltammetric
profile recorded from 0.06 to 0.8 V at 50 mV/s; for these values of
Estart and Eup negligible faradic currents were measured at the two
extremes of the potential window. In the first positive-going
sweep, a major reduction peak at 0.4 V can be observed (R1),
followed by a broad oxidation peak at 0.55ꢀ0.75 V (O1). Upon
reversal of the scan direction at E = 0.8 V, several reduction
signals can be observed in the negative-going scan. A minor peak
at 0.63 V (R3) is followed by a more intense peak at 0.55 V (R2);
the largest signal is still the broad peak centered at 0.4 V (R1). On
the basis of previous studies,10,13 the following assignment of the
peaks can be suggested: R1 arises from direct nitrite reduction to
ammonia,10 whereas R2 is ascribed to selective nitrite reduction
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Figure 4. Peak charge of R2. The peak integration was performed with
voltammetric data recorded at v = 50 mV/s (starting potential E = 0.1 V,
2 mM NaNO2). The step density has been calculated as 1/(n ꢀ 0.5) for
(100) ꢁ (111) surfaces and 1/n for (100) ꢁ (110) surfaces. Values of n
can be found in Table 1 for the various surfaces.
species involved in O1, can enhance the ensuing reduction pro-
cess R2. Additional information on the nature of the various
peaks can be discerned from a comparison of the voltammetric
profiles at different scan rates, as shown in Figure 2.
All peaks are generally characterized by a decreased peak
current with decreasing scan rate, although this decrease is less
noticeable when the scan rate is reduced from 50 to 5 mV/s. It
should be noted that R3 disappears at the lowest scan rate, which
is in agreement with the previous report13 where it was not ob-
served at 1 mV/s. Therefore, it can be assumed that R3 is a
shoulder of the main reduction process of peak R2. We shall now
discuss the behavior of R2 and O1 in more detail. The R2 peak
potential does not shift noticeably within the range of v values
analyzed (only a 15 mV shift across a two-decade range); on the
other hand, the peak charge follows the trend shown in the inset
of Figure 2: the amount of charge measured remarkably increases
with a decrease of the scan rate (that is, with an increased
experimental time scale). The trend is best fitted by an expo-
nential decay. Our results agree with previous observations by the
Kita group,10 who noticed that the charge of R1 also increases
with a decrease in scan rate. Therefore, the overall process giving
rise to R2 must include a slow surface-confined step. On the other
hand, we interpret the increase of the peak current of O1 with the
scan rate as a consequence of a process involving a species that
can diffuse away from the electrode unless the time scale of the
potential sweep is short enough. As we mentioned above, we
identify this species with NH3 produced in the R1 region.
To probe the importance of long-range (100) facets for the
selective reduction of NO2ꢀ, we have also studied the effect of
the introduction of steps of known orientation into the (100)
structure. The voltammetric profiles of nitrite reduction of some
of these stepped surfaces are shown in parts a and b of Figure 3
for the (111) and (110) step orientation, respectively. The peak
pattern of the (100) electrode is largely conserved, but the
magnitude of all signals decreases with an increasing step density,
regardless of the orientation of the step. This effect affects all
peaks, although to different extents. The large reduction peak at
ca. 0.4 V, associated with the formation of ammonia, also does
not decrease remarkably when surfaces with long terraces are
used. The oxidation signal above 0.6 V does not shrink appre-
ciably when (111) steps are introduced unless a high step density
Figure 3. (a) Nitrite reduction profiles on selected Pt[(100) ꢁ (111)]
surfaces: thick line, (100); dashed line, (29,1,1); dotted line, (15,1,1);
thin line, (7,1,1). (b) Nitrite reduction profiles for selected Pt[(100) ꢁ
(110)] surfaces: thick line, (100); dashed line, (10,1,0); dotted line,
(7,1,0); thin line, (4,1,0). (c) Comparison between the voltammetric
response of a Pt(100) electrode cooled in an argon + hydrogen atmo-
sphere (thick line) and in ambient air (thin line). For the blank
voltammogram of a well-ordered Pt(100) electrode, refer to Figure 1.
For all three panels, v = 50 mV/s and the NaNO2 concentration is 2 mM
in 0.1 M NaOH.
to dinitrogen.13 O1 falls in the region where Ye et al.10 observed
reoxidation of ammonia, generated at R1, to dinitrogen. The
modification of the upper potential limit allows us to investigate
the relationship between the various voltammetric features, and
as depicted in Figure 1, there is a correlation between the growth
of O1 and the increase of R2. The growth of the two signals is
maximal when Eup is increased between 0.6 and 0.75 V, and at
higher potentials they both level off to an almost constant value.
Therefore, within the time scale of this specific electrochemical
experiment, the oxidation of a particular intermediate, or surface
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are able to reach the maximum catalytic activity toward N2
evolution from nitrite, and the interruption of such long-range
order with defects, even if (100) terraces are very long along the
steps, reduces the activity of the Pt surface.
The importance of high-quality (100) facets was further
corroborated by an additional experiment involving a nonopti-
mal procedure of electrode pretreatment, which will prevent the
surface from reaching an ideal (100) orientation by inducing
various types of defects.40,47ꢀ49 Using Pt(100), we performed
flame-annealing as described in the Experimental Section, fol-
lowed by cooling in ambient air rather than in a controlled
oxygen-free atmosphere. The activity toward nitrite reduction of
the electrode was then tested and compared to that of a well-
ordered (100) electrode. Figure 3c shows the voltammetric pro-
file of such “air-cooled” Pt(100). The peak related to N2 evolu-
tion has completely disappeared, along with the oxidation signal
recorded above 0.6 V. The reduction peak associated with
ammonia formation is still present, although it features a much
lower peak current and a less positive peak potential with respect
to the crystal cooled in an argon + hydrogen atmosphere. We also
note that the profile of the air-cooled crystal shows a close simi-
larity to the voltammogram previously published by Ye et al.10
Spectroelectrochemical Experiments: FTIRS of (100) and
Selected Stepped Surfaces. In situ IR spectroelectrochemistry
has been employed for the identification of possible reaction
intermediates and can be applied to single-crystal surfaces (in the
external reflection configuration). IR-active vibrations assigned to
(adsorbed) NO,43,44,50 NO2ꢀ 44 NO3ꢀ 17,21 and NHx fragments42
, ,
have previously been observed with a comparable configuration.
In the following spectra, the positive bands correspond to the
products formed during the reaction, while negative bands arise
due to the consumption of species present at the reference po-
tential. The electrodes were always brought into contact with the
nitrite solution at controlled potential (0.1 or 0.8 V) where
negligible faradic current is measured. After collection of the ref-
erence spectrum, the potential was progressively stepped to the
sample potentials: these values are reported in the corresponding
figures and captions (see below).
As an additional comment, the IR experiments were per-
formed in the thin layer configuration, and it was observed that
the gas formation—in our case N2 evolution—disturbed the
acquisition of the optical signal through the thin layer. Therefore,
no IR spectra could be acquired in the region corresponding to
gas formation, more precisely 0.5 V < E < 0.65 V. However,
selected experiments were performed with a reference potential
just outside this window to compare the electrode surface state at
the two extremes of the N2-forming region. Additionally, the
reference potential was selected as close as possible to the sample
potential to minimize the influence of the water bands on the
spectra (potential-driven modification of the water configuration
in the thin layer). This is especially important when a continuous
reaction is studied, for which reactant depletion may arise in the
thin-layer configuration. On the other hand, such a configuration
is ideally applicable to simpler cases such as the removal of an
adsorbed species from the electrode surface.
Figure 5. (a) FTIRS spectra for nitrite reduction at Pt(100) in a 0.1 M
NaOH solution containing 2 mM NaNO2. A total of 1000 interfero-
grams were collected with a resolution of 8 cmꢀ1. Thin line: reference
potential 0.1 V and sample potential 0.4 V. Thick line: reference
potential 0.4 V and sample potential 0.7 V. (b) FTIRS spectra for nitrite
reduction in 0.1 M NaOH/D2O containing 2 mM NaNO2. A total of
200 interferograms were collected with a resolution of 8 cmꢀ1. Thin line:
reference potential 0.5 V and sample potential 0.8 V. Thick line:
reference potential 0.8 V and sample potential 0.5 V.
Figure 5a displays two results of FTIR spectra for Pt(100) in
the presence of NO2ꢀ. Four bands are clearly observed in the
spectra: the signal at 1640 cmꢀ1 corresponds to water bending51
and arises from the water depletion from the thin layer; the other
bands can be attributed to the NꢀOads stretching mode
(1553 cmꢀ1),44 NꢀH bending mode (1442 cmꢀ1),42 and
is reached. The trend of the peak charge of R2 with respect to the
step density is shown in Figure 4. The introduction of steps
dramatically reduces the corresponding charge, with a noticeable
decrease even for low step densities. We conclude that only
surfaces with long-range-ordered two-dimensional (100) domains
asymmetric NO2 bending mode (1234 cmꢀ1).44,51,52 If the
ꢀ
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a
Scheme 1. Stability Potential Ranges of NHx,ads and NOads
a The full tilted square indicates a potential corresponding to FTIRS
evidence of the formation of a surface species, while the empty circle
indicates a potential corresponding to FTIRS evidence of the removal of
the species. The region of gas evolution, where no FTIRS data can be
collected, is highlighted as a shadowed rectangle. The potential axis is
not to scale.
potential is stepped from a reference starting potential of 0.1 to
0.4 V, we observe that the band assigned to NHx species is
positive, which means that NHx species are produced at 0.4 V
with a simultaneous depletion of solution-phase nitrite, a process
highlighted by the negative band at 1234 cmꢀ1. Subsequently,
when the potential is stepped from 0.4 to 0.7 V across the N2
formation region, the band at 1442 cmꢀ1 becomes negative,
indicating that NHx species are removed from the thin layer at
0.7 V with respect to 0.4 V. This process is accompanied by the
appearance of a positive band related to adsorbed NO. There-
fore, this experiment shows that various processes take place at
the Pt(100) electrode along the sequence 0.1 V f 0.4 V f 0.7 V:
first conversion of nitrite to “NꢀH” species, presumably also
involving NH3, then removal of these NꢀH species and the
appearance of surface NO.
Although the band related to adsorbed NO is unambiguous,
the interference of the neighboring water band was removed by
repeating the experiment in the presence of D2O. In this way, the
water bending signal is displaced to lower wavenumbers.53 We
were then in the condition to perform an accurate experiment
aimed at determining the potential at which this band disappears,
signaling the removal of the NO adsorbed at the surface. The
experiment in D2O adds little information about other bands, for
example, the nitrite bending, which are now masked by the D2O
signal. Figure 5b displays the results of a potential-step FTIRS
experiment in D2O. When the reference potential is chosen as
close as possible to the N2 region (0.5 V) and then stepped to the
working potential (0.8 V), a positive band corresponding to the
appearance of NOads is observed, consistent with the results dis-
played in Figure 5a. If a new reference spectrum is acquired and
the potential is then returned to 0.5 V, the NO band changes sign,
showing removal of this species. When taken together, the FTIRS
experiments in regular and heavy water allow us to highlight the
identity of the key surface species involved in nitrite reduction
(NHx,ads and NOads) and their stability potential ranges, which are
summarized in Scheme 1.
Figure 6. Spectra for nitrite reduction on Pt(100), Pt(7,1,1), and
Pt(5,1,0) in 0.1 M NaOH and 2 mM NaNO2 in H2O. A total of 200
interferograms were collected with a resolution of 8 cmꢀ1. The potential
program involved a staircase with the following steps: 0.1 V f 0.4 V f
0.7 V f 0.4 V f 0.1 V. The thick lines represent the positive-going
direction (from 0.1 to 0.7 V) and thin lines the negative-going direction
(from 0.7 to 0.1 V).
and the stability of the surface intermediates: Pt(7,1,1) and
Pt(5,1,0) were selected as model stepped surfaces. Their FTIR
spectra are shown in Figure 6.
The experiment shown in Figure 6 involves a complete sweep
between 0.1 and 0.7 V, performed by means of the following
stepwise potential program: 0.1 V f 0.4 V f 0.7 V f 0.4 V f
0.1 V. For every step, a new reference potential was acquired at
the starting E value. As an additional difference from the experi-
ment reported in Figure 5a, the number of interferograms ac-
quired is much smaller: this choice is advantageous because it
allows a faster data acquisition, but it suffers from the drawback
that vibrations related to NHx and nitrite are somewhat less in-
tense than in Figure 5, and the very intense vibration of NOads
dominates the spectra of Figure 6. As the spectra of Pt(100) are in
agreement with those of Figure 5, we concentrate on the spectra
of Pt(7,1,1) and Pt(5,1,0). The spectroelectrochemical analysis of
the Pt(7,1,1) surface, displayed in Figure 6b, reveals that no NHx
bands at 1449 cmꢀ1 are observed upon stepping the potential
from 0.1 to 0.4 V, suggesting a more limited nitrite consumption,
in agreement with the voltammetric data of Figure 2, where
Pt(7,1,1) features a much lower reduction current at R1 than
Pt(100). The second step to 0.7 V shows that the stepped surface
generates less NO than the basal plane, as demonstrated by the
weakerNObandobserved. However, the conservationofthe band
position indicates that the adsorption coordination is the same
on both Pt(7,1,1) and Pt(100), i.e., a multifold coordination on
The stability potential ranges are necessarily discontinuous
because of the stepped potential programs employed in FTIRS
experiments. Scheme 1 depicts this fact by showing the possible
stability ranges as dotted lines. Despite this fact, and the presence
of a wide potential window where FTIRS data cannot be col-
lected, we can unambiguously affirm that NHx fragments are the
dominant surface species for E < 0.4 V while NO predominates
when E > 0.7 V. Further FTIRS experiments were aimed at
determining the influence of the surface structure on the identity
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(100) domains. NO issubsequentlyremovedwhenthepotential is
stepped again to lower values, as expected.
In the case of the Pt(5,1,0) surface (having 110 steps), a small
band related to NHx species can still be observed after the first
step to 0.4 V: a partial contribution of the fairly reactive (110)
sites10,13 for the reduction of nitrite to ammonia cannot be
excluded. At 0.7 V, the NO band can be observed, along with
another band at 1357 cmꢀ1. This band can be assigned to the
asymmetric bending of adsorbed nitrite in a “nitro” configuration
(MꢀN coordination)17,54 or to solution-phase nitrate.52 We
favor the former assignment, with (110) sites most likely being
involved in the adsorption of nitrite. When the potential is step-
ped back to 0.4 V, no NO consumption is observed; on the con-
trary, NO is still present and adsorbed at the surface as indicated by
the bipolar band. The NO is only stripped from the surface at
potentials lower than 0.4 V and can only be removed in the last step
to 0.1 V. This is most likely related to the highaffinity of NO toward
step sites of (110) geometry, as reported in previous works.7
The spectroelectrochemical experiments on short-terrace
stepped surfaces shed additional light on the origin of the de-
crease in reactivity as a result of the introduction of steps on
(100) terraces: (111) steps cause a diminished conversion of nit-
rite into both NHx,ads and NOads, thus acting as inert surface do-
mains. On the other hand, (110) steps do not depress the for-
mation of NHx while displaying a high affinity toward both
Figure 7. OLEMS data for N2 evolution on the stepped surfaces
investigated in this paper. Squares refer to the (100) electrode, triangles
to surfaces featuring (111) steps, and circles to those with (110) steps.
Two peak parameters are reported. The left column contains the
normalized peak height (I*) for both the positive- and the negative-
going scans. The right column contains the normalized peak area (A*).
For both parameters a nonlinear fitting was performed using an
exponential decay function, displayed in the figure as dotted lines. All
OLEMS data were collected during potential scans with Estart = 0.06 V
and v = 1 mV/s.
ꢀ
NO2 and NO, the latter residing at the surface across the
potential region of N2 formation; therefore, this may suggest that
(110) domains are the favorite adsorption sites for NO, where it
is firmly bound and consequently excluded from further reaction
at the (100) terrace. In other words, (110) steps could be
described as self-poisoned as long as NO is bound.
OLEMS Experiments with (100) Electrodes and Selected
Stepped Surfaces. The activity and selectivity series obtained by
cyclic voltammetry was further corroborated by a study of the
influence of the step density on the MS ion current related to N2
(m/z = 28). During the measurements, m/z = 28 was the only
mass recorded to optimize the signal intensity: we should
however remark that a preliminary OLEMS experiment includ-
ing also m/z = 30 (NO, N2O) and m/z = 44 (N2O) confirmed
that for a (7,1,1) surface no gaseous product other than N2 is
formed during nitrite reduction, which can confidently be as-
sumed to be true for surfaces with larger terraces as well (our
previous studies13 on Pt(100) also excluded other gaseous side
products). Although the OLEMS setup has some intrinsic
limitations to a quantitative analysis, a semiquantitative compar-
ison of different experiments can still be carried out, and a
correction was introduced by measuring the steady-state ion
current related to hydrogen evolution, m/z = 2 at E = ꢀ0.1 V for
all electrodes at the end of the experiment, without changing the
geometry (i.e., tipꢀelectrode distance) of the system.
surface. An additional increase in the step density causes a further
decrease of the intensity of the N2 MS peak until the lowest
values are reached for (7,1,1) and (5,1,0) surfaces. In fact, the
(7,1,1) electrode evolves an amount of N2 very close to the
detection limit of the OLEMS setup in the positive-going scan
and below this limit in the negative-going scan (see Figure 7).
The trend reported in Figure 7, which can qualitatively be fitted
by an exponential decay, can be positively compared to the plot in
Figure 3, although the electrochemical analysis and the OLEMS
experiments were carried out in different conditions. Conse-
quently, it is by no means surprising that the trends of Figures 3
and 7 are not identical, the latter showing a much more abrupt
decrease of the MS peak parameters for N2 evolution even with
very small step densities. However, both techniques agree on the
limiting terrace width for a detectable N2 evolution (four atoms),
and the mass spectroscopy data corroborate the observation that
steps—regardless of their symmetry—exert a deleterious effect
on the reaction pathways leading to N2.
An additional experiment was carried out to analyze the cata-
lytic performance of one-dimensional (100) sites, aimed at deter-
mining the smallest (100) domain active toward N2 evolution. A
suitable [n(111) ꢁ (100)] single-crystal electrode was chosen, in
our case (533), having n = 10. Although a reduction current was
indeed recorded in the potential region typical of nitrite reduc-
tion at (111) sites, no N2 was simultaneously detected in the
OLEMS apparatus. Therefore, we can exclude that one-dimen-
sional (100) steps are active toward N2 evolution, which instead
requires wide two-dimensional (100) domains.
Figure 7 shows the m/z = 28 normalized peak heights (I* = I/
Inormalization) and areas (A* = A/Inormalization) for the positive-
going (from 0.1 to 0.8 V) and the negative-going half-cycles as a
function of the step density of selected surfaces from (100) down
to large, medium, and small terrace widths for both (111) and
(110) steps (see Table 1 for additional information).
The figure shows that, irrespective of the parameter employed
(peak height or area) or the scan direction analyzed, the amount
of N2 formed during the potentiodynamic cycle is strongly
dependent on the step density: for surfaces with wide terraces
such as (39,1,1) and (20,1,0), the magnitude of N2 evolution
already decreases by more than 50% with respect to the (100)
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Figure 8. Voltammetric profile of a NO-saturated Pt(100) electrode in
a 0.1 M NaOH solution containing 2 mM NaNO2: first scan, thick line;
second scan, thin line. Estart = 0.75 V, and v = 50 mV/s.
Transfer experiments with NO and NHx. Mass Spectro-
metry and Electrochemical Results. Since NO was observed
with FTIRS during nitrite reduction at Pt(100), we performed an
electrochemical “transfer” experiment to probe the influence of
the presence of a saturated NO adlayer (prepared as specified in
the Experimental Section) on N2 evolution. In this experiment
the presence or the absence of the R2 peak was considered as the
only evidence of generation of this gas at this stage. A saturated
coverage of NO of ca. 0.5 monolayer55,56 has been estimated for
Pt(100) under electrochemical conditions. The NO-covered elec-
trode was contacted with a nitrite-containing 0.1 M NaOH solu-
tion at E = 0.8 V, and a potential sweep in the negative direction
was started. The two subsequent voltammetric scans are shown
in Figure 8.
Figure 9. (a) Cyclic voltammetry during OLEMS measurements in a
0.1 M Na14NO2 solution following adsorption of 15NO at a Pt(100)
electrode. (b) Ion current profiles for m/z = 28. (c) Ion current profiles
for m/z = 29. The arrows indicate the direction of the potential sweep.
E
start = 0.8 V, and v = 1 mV/s.
The reduction profiles of the electrode are clearly different in
the first and second cycles. During the first sweep, with the elec-
trode fully covered with NO, no reduction current is detected
until 0.25 V, which can be ascribed to NOads reductive strip-
ping.44 Upon reversal of the potential scan, the broad reduction
peak R1 reappears, and in the following second cycle, R2 can be
observed again. The removal of NO from the surface is not com-
plete with the fast scan rate employed. Hence, a residual, very low
coverage of NO may still be present, but it was not found to be
deleterious for the reaction leading to N2.
A more detailed analysis of the role of adsorbed NO was
carried out using labeled Na15NO2. Our aim was to determine
the fate of adsorbed 15NO at Pt(100) during reduction of
Na14NO2 in 0.1 M NaOH. Figure 8 shows the ion current traces
for m/z = 28 (14N2) and m/z = 29 (14N15N), along with the
voltammetric profile recorded during OLEMS experiments.
The 15NO-covered electrode is not completely inhibited to-
ward N2 evolution, in contrast with the behavior observed during
standard electrochemical measurement shown in Figure 8: this
fact is ascribed to the known tendency of NOads to desorb from
the Pt(100) surface in alkaline media during long experimental
time scales.44 However, here it is important to emphasize there is
still a residual degree of electrode poisoning, evidenced by the
smaller ion current for 14N2 recorded in the first (negative-going)
half-cycle with respect to the positive-going one. Notwithstanding
this residual poisoning, N2 can be observed and a certain amount
of 14N15N is also detected, which amounts to ca. 10% of 14N2.
This fact testifies that the surface 15NO takes part in a reaction
process involving a recombination with a 14N species which must
have originated from the solution-phase nitrite. All labeled NO is
consumed in the first sweep, because no 14N15N is measured in
the positive-going scan. The m/z = 30 ion current was found to be
zero throughout the experiment, which shows that 15N15N
potentially arising from a recombination of two 15NO molecules
cannot be detected (or is below the instrumental detection limit)
A second experiment was designed to probe the role of ad-
sorbed NHx, which we detected during FTIRS experiments. It is
known24 that such a fragment can be adsorbed onto a Pt(100)
electrode from an alkaline solution of ammonia in the potential
range of NH3 oxidative adsorption (which proceeds via a dehy-
drogenation step to adsorbed NHx fragments). The potential
window of stability of NHx fragments is roughly 0.35 V < E <
0.5 V. In addition, a previous publication has shown that transfer
experiments involving NHx species are feasible.45 NHx fragments
were adsorbed from a 14NH3 solution at a constant potential (E =
0.45 V) and transferred to a second electrochemical cell contain-
ing labeled Na15NO2. Figure 10 displays the ion current traces for
m/z = 29 (14N15N) and m/z = 30 (15N2), along with the
voltammetric profile recorded during the OLEMS experiment.
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Scheme 2. Potential Ranges of Stability for NH2,ads and
NOads Obtained from a Combination of Experimental Data
from the Present Work and Literature Informationa
a The continuous line indicates the stability range for a certain molecule,
along with the reaction that leads to this molecule starting from nitrite
(see the text). The dotted line indicates, when needed, the theoretical
stability range of a surface species in the absence of nitrite in the solution.
The recombination reaction giving N2 is highlighted, along with the
simplest rate equation according to a LangmuirꢀHinshelwood model.
The potential axis is not to scale.
data concerning NOads and NHx,ads at Pt(100) electrodes in al-
kaline media allows us to obtain information about the potential
region inaccessible to FTIRS investigation, thus enabling us to
integrate and extend Scheme 1. The new scheme (Scheme 2) will
be discussed in the next paragraphs.
Scheme 2 displays NHx,ads as NH2,ads: in fact, Rosca et al.
presented evidence for the presence of NH2,ads as the dominant
intermediate during NH3 oxidation at Pt(100) in alkaline media.24
DFT calculations have shown that this ammonia fragment is char-
acterized by a larger adsorption energy at Pt (100) surfaces than
other NHx species, providing extra stabilization to NH2 on Pt-
(100) compared to other basal planes.57,58 In addition, previous
electrochemical experiments evidenced that this fragment is stable
from 0.35 to ca. 0.55 V. At 0.55 V a large oxidation signal ascribed
to bulk NH3 oxidation predominates in the voltammogram,24,27
and all NH2,ads is likely to be oxidatively removed. The first re-
duction peak of nitrite reduction (R1), located within the stability
range of NH2,ads, may therefore generate such a fragment acc-
ording to the following reaction:
Figure 10. (a) Cyclic voltammetry during OLEMS measurements in a
0.1 M Na15NO2 solution following adsorption of 14NHx at a Pt(100)
electrode. (b) Ion current profiles for m/z = 29. (c) Ion current profiles
for m/z = 30. The arrows indicate the direction of the potential sweep.
E
start = 0.45 V, and v = 1 mV/s.
During the voltammetric scan, started at E = 0.45 V, evolution
of 15N2 is observed in both cycles, with no clear poisoning effect.
However, the formation of 14N15N is detected only in the first
(positive-going) half-cycle, whereas no signal related to this mole-
cule is recorded in the negative-going sweep. Therefore, the
14NHx fragments adsorbed on the surface recombine with an-
other labeled moiety originated from the solution-phase labeled
nitrite, thus giving rise to 14N15N. It must be pointed out that no
14N2 is detected: the recombination of 14NHx fragments does not
take place or is below the sensitivity of the instrument.
4H2O + NO2ꢀ + 5eꢀ f NH2;ads + 6OHꢀ
ð6Þ
The second surface species, NOads, has been previously
studied as an adsorbate at Pt(100) in clean 0.1 M NaOH by
Rodes et al.,44 who showed that the potential window of stability
of NOads features a lower limit at E = 0.35 V. In addition, the
adsorbate is not stable during long-term experiments, showing a
tendency to desorb over time, which testifies that NO is a fairly
labile adsorbate at Pt(100) in alkaline media. Combining this
information with experimental evidence from Scheme 1, it seems
reasonable to estimate that NOads may exist throughout the N2
formation region up to 0.5 V: this is the E value where we ob-
tained clear FTIRS evidence of the removal of this species. The
conversion of solution-phase nitrite into NO will therefore be the
dominant process for potentials higher than 0.65 V:
’ DISCUSSION
The mechanistic analysis of nitrite reduction at Pt(100) is
intimately correlated with the structure sensitivity of this reac-
tion, and we shall therefore discuss the two topics alongside. The
following discussion will be mainly devoted to the mechanism of
N2 formation.
Mechanism and Structure Sensitivity of Nitrite Reduction
at Pt(100) Electrodes. FTIR evidence presented in this paper
supports the presence of two key surface species at potentials
vicinal to the potential window where N2 evolution occurs: NOads
and NHx,ads. The direct involvement of both surface species in
N2 formation was further corroborated by the OLEMS experi-
ment with labeled compounds. Scheme 1 (see above) showed
the stability potential ranges of these species. A survey of literature
H2O + eꢀ + NO2 f NOads + 2OHꢀ
ð7Þ
ꢀ
Scheme 2 shows that there is an intermediate region where the
potential ranges of stability of the two products overlap and,
consequently, interconversion of these species may occur. In
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particular, as it is known that adsorbed NO forms NH3 as the
final product during reductive stripping,7,44 it is logical to expect
that the following reaction may occur in the overlap region where
NH2,ads is stable:
upon potential inversion, will be readily available to take part in
N2 evolution as soon as some NH2,ads has formed.
Finally, we comment on the structure sensitivity of nitrite re-
duction, concentrating on the most structure-sensitive process:
R2 (R1, on the other hand, is still present when low-quality (100)
domains are produced by air-cooling after flame-annealing, so
lower quality (100) sites suffice for this reaction). The peculia-
rities of NO and NH2 adsorption at (111), (110), and (100) sites
can help with the interpretation of the structure sensitivity. NH2,
ads has already been discussed above: the (100) plane features an
outstanding stabilization of this fragment. Thus, steps of any
symmetry would simply remove productive (100) surfaces and
destabilize the NH2,ads adlayer. NO, on the other hand, is known
to be fairly strongly adsorbed at Pt(110) sites, so we can imagine
that reaction 7 is operative on this surface, too; however, NO is
possibly a strongly adsorbed intermediate and thereby prevented
from taking part in recombination (reaction 9). Indeed, the
(5,1,0) surface gave rise to clear FTIRS evidence of the presence
of NO in a broader potential region, as low as E = 0.4 V. On the
other hand, (111) sites seem the least able to perform reaction 7,
as indicated by the decreased NO FTIRS band (Figure 6).
Consequently, the (100) surface possibly offers a “golden mean”
situation, by performing reaction 7 at an adequate rate but
without stabilizing NO excessively and without achieving a
saturated NO adlayer which would poison the Pt(100) surface
(Figure 8). A combination of these factors must contribute to
determine the special behavior of Pt(100), which is the only Pt
basal plane able to feature the coexistence of the two nitrogen
moieties responsible for N2 formation. As a more general
comment, Pt(100) is an intrinsically unique surface, given its
ability to promote reactions involving bond breaking or bond
making: contrary to reactions needing low-coordination sites
(i.e., steps) to promote the formation of a certain intermediate,
the key to the high catalytic activity of Pt(100) is the extra
stabilization given to the transition state with respect to the
reactants’ configuration.23 Any surface defect acts as a hindrance
and reduces the benign effect of a pristine Pt(100) surface. In our
case, we have already mentioned that NO is likely to be only
sufficiently stable as an adlayer at Pt(100) in alkaline media, while
NH2,ads reacts to give N2 in a potential region where it is prone to
various transformations. All these factors contribute to the
efficiency of this pathway to a single product in a narrow but
distinct potential region: N2. The pathway revealed here, i.e.,
reaction 9, was not discussed in our recent review on nitrogen
cycle electrocatalysis.7 However, as mentioned in the Introduc-
tion, very similar pathways have been observed in UHV surface
science and in bacterial processes, the latter also at low (room)
temperature. All these systems have the observation of a 14N15N
product in common, with one N coming from a NHx fragment
and the other from a NO fragment. It is important to point out
the significant advantage of this pathway over the more usual
pathway for N2 formation from NO3ꢀ/NO2ꢀ/NO at low
temperature. The more typical route involves a dimerization of
NO to give a nitrous oxide intermediate (N2O), which is
subsequently reduced to N2, as observed in nitrite and nitrate
reduction on PdCu62 and PtSn63 electrodes. The classical
biological nitrogen cycle also makes use of this route. The dis-
advantage of the “N2O” pathway is that N2O is a weakly co-
ordinating molecule that easily escapes the catalytic site. There-
fore, selectivity to N2 is always far below 100%. On the other
hand, the NHx + NO route involves two strongly adsorbed
intermediates and has the potential to generate 100% N2.
NOads + 4eꢀ + 3H2O f NH2;ads + 4OHꢀ
ð8Þ
The presence of a central potential region, satisfactorily close
to the observed R2 peak potential, where the coexistence of
NOads and NH2,ads can confidently be expected, suggests that a
LangmuirꢀHinshelwood recombination may be the fundamen-
tal step leading to N2:
NOads + NH2;ads f N2 + H2O
ð9Þ
Although this reaction may not be a truly elementary step (the
nature of which cannot be concluded from experimental data
presented in this paper), this is the most likely process responsible
for N2 evolution, also in light of the similar recombination that
occurs between NO and NH3 under UHV conditions. The choice
of x = 2 for the NHx fragment is mainly supported by experi-
mental observations in previous papers (see above) concerning
NH2 stability on Pt(100) surfaces: in support of reaction 9 ab
initio and thermochemical calculations59ꢀ61 have also shown that
other recombination reactions generating N2, such as NH + NO,
proceed via a much more unfavorable energetic pathway; NH2 +
NO, on the other hand, is characterized by theformation of a fairly
stableNH2ꢀNO complex as the first step, whose existence during
the UHV NH3 + NO reaction was mentioned in the Introduction.
For the latter reason, it may be possible that the role of NOads at
Pt(100) is dual: reactant as well as stabilizer for NH2,ads
.
Scheme 2 also clearly shows that when the potential is changed
in the positive- or negative-going direction, either NH2,ads or
NOads, respectively, is already available at the surface from
previous processes and, for reaction 9 to occur, the other reactant
must be supplied in situ from an ancillary reaction of solution-
phase nitrite, in agreement with isotope labeling experiments.
This is best understood if the various processes occurring during
the voltammetric scan, shown in Figure 1 and—for the slowest
scan rate—Figure 9, are interpreted in light of Scheme 2. If the
potential is swept from left to right, after R1 the surface will be
covered with NH2,ads; two processes can in principle compete for
the removal of the adsorbed species, reaction 9 or oxidative N2
formation as observed during bulk NH3 oxidation at Pt(100):
2NH2;ads + 4OHꢀ f N2 + 4H2O + 4eꢀ
ð10Þ
Reaction 10 will dominate in the absence of coadsorbed NO
and give rise to the voltammetric peak named O1. Experimental
evidence found in this work indicates that the scan rate plays a
crucial role in steering the selectivity. At fast scan rates, reaction
10 dominates, along with oxidation of solution-phase NH3 that is
lingering in the proximity of the electrode and has not diffused
into the bulk yet. Slower scan rates, on the other hand, allow
reaction 7 to occur, providing an alternative pathway (i.e.,
reaction 9) for the removal of NH2,ads. The correlation between
voltammetric peaks O1 and R2 shown in Figure 1 can now be fully
appreciated: as the upper potential limit is extended upward, the
potential excursion will traverse an increasingly broader section
of the potential window where reaction 7 predominates. In turn,
this can cover the surface with a larger amount of NOads which,
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’ CONCLUSIONS
(16) Nakata, K.; Doi, Y.; Kubota, S.; Shimazu, K. J. Electroanal. Chem.
2010, 647, 187–193.
Nitrite conversion to N2 was studied at Pt(100) and related
[(100) ꢁ (111)] and [(100) ꢁ (110)] stepped electrodes with
in situ techniques (FTIRS, OLEMS). The insertion of increas-
ingly denser defects of any symmetry caused a rapid decrease in
the catalytic activity to N2 formation: well-ordered Pt(100) was
found to be the ideal surface for this reaction. Experimental evi-
dence supported a mechanistic scheme based on a Langmuirꢀ
Hinshelwood recombination of two surface species which ulti-
mately arise from nitrite (NOads and NHx,ads) and which can be
expected to coexist in the potential region of N2 evolution. Our
findings, highlighting the only known fully selective pathway
leading from nitrite to N2 for metals and biological systems, will
help guide the design of practical catalysts, such as nanoparticles,
with a view to applications in the field of wastewater treatment.
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’ AUTHOR INFORMATION
Corresponding Author
m.duca@chem.leidenuniv.nl; m.koper@chem.leidenuniv.nl
’ ACKNOWLEDGMENT
We acknowledge partial financial support from the European
Commission (through FP7 Initial Training Network “ELCAT”,
Grant Agreement No. 214936-2). M.T.M.K. acknowledges fi-
nancial support from The Netherlands Organization for Scien-
tific Research (NWO-Middelgroot) for the purchase and
development of the online electrochemical mass spectrometer.
J.M.F. acknowledges financial support from the Ministerio de
Ciencia e Innovaciꢀon (Spain) through Project CTQ2010-16271
(Feder).
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