In situ vibrational study of the initial steps during urea electrochemical oxidation

Article abstract of DOI:10.1021/jp9700793

The electrochemical oxidation of urea has been studied by in situ Fourier transform infrared spectroscopy in supporting electrolytes of different pH. It has been found that the adsorbates and reaction products are strongly pH dependent. In acidic solutions, the urea is adsorbed through the nitrogen atoms. A two-proton loss is proposed for the adsorption process, related to an excess reversible charge at the end of the hydrogen adsorption/ desorption. Adsorbed CO is formed as a result of the hydrolysis of the urea molecule close to the platinum electrode. The main product detected has been CO₂. For neutral solutions, the urea molecule leads to the formation of CNO^- and [N₂O₂]^2- ions at potentials in the double-layer region of the platinum electrode. At higher potentials, formation of NO₂ and NO₃^- have been detected. In alkaline media the coordination of adsorbed urea depends on the potential. At lower potentials urea is coordinated through a single NH₂ group, and at more anodic potentials the adsorbed urea changes to an adsorption through the oxygen atom.

Full text of DOI:10.1021/jp9700793

J. Phys. Chem. B 1997, 101, 6443-6449
6443
In Situ Vibrational Study of the Initial Steps during Urea Electrochemical Oxidation
†
Aˆ ngela C. S. Bezerra, Eduardo L. de S a´ , and Francisco C. Nart*
Instituto de Qu ´ı mica de S a˜ o Carlos, C.P. 780, 13560-970 S a˜ o Carlos (SP), Brazil
X
ReceiVed: January 6, 1997; In Final Form: May 1, 1997
The electrochemical oxidation of urea has been studied by in situ Fourier transform infrared spectroscopy in
supporting electrolytes of different pH. It has been found that the adsorbates and reaction products are strongly
pH-dependent. In acidic solutions, the urea is adsorbed through the nitrogen atoms. A two-proton loss is
proposed for the adsorption process, related to an excess reversible charge at the end of the hydrogen adsorption/
desorption. Adsorbed CO is formed as a result of the hydrolysis of the urea molecule close to the platinum
electrode. The main product detected has been CO
2
. For neutral solutions, the urea molecule leads to the
-
2-
formation of CNO and [N
higher potentials, formation of NO
adsorbed urea depends on the potential. At lower potentials urea is coordinated through a single NH
and at more anodic potentials the adsorbed urea changes to an adsorption through the oxygen atom.
2 2
O ]
ions at potentials in the double-layer region of the platinum electrode. At
-
2
and NO
3
have been detected. In alkaline media the coordination of
group,
2
1
. Introduction
sharp voltammetric peak, while on the cathodic side a (1×1)
6
unreconstructed structure could be deduced.
The electrochemical oxidation of urea has been the subject
Auger spectra of the emersed electrode with a c(2×4) LEED
pattern show that the urea molecule is preserved with the CdO
and C-N bonding, but the N-H bond cleavage could not be
of some interest in the past due to the possibility of developing
portable dialysis devices. The effect of urea on glucose sensor
devices or biofuel cells has been also the reason for some
investigation of the urea oxidation. The results found for the
1
6
excluded.
2
In the case of polycrystalline platinum, the anomalous
voltammetric peak observed at -0.2 V could have its origin on
some kind of phase transition, although the polycrystalline nature
of the electrode makes it difficult to describe this phase transition
or even to detect an ordered layer as in the case of Pt(100). On
the other hand, it seems to us that a faradic charge transfer, as
oxidation of urea on platinum electrodes in phosphate-buffered
solutions (pH 7.4) have discouraged further development of such
devices. The main reason is the formation of the byproducts
together with CO2 and N2. The mass spectra (differential
electrochemical mass spectrometry, DEMS) showed the forma-
3
tion of toxic products such as NO2 and N2O, which have
5
already proposed for the voltammetric reversible peak, does
anesthetic effects. These products have been detected in a large
range of potentials, thus making it difficult to use platinum
electrodes for this purpose.
The electrochemistry of urea, however, presents some intrigu-
ing features, mainly in acidic media, which have attracted the
interest of some researchers in recent years.
not rule out phase transition effects. In alkaline media, the urea
seems to be more stable than in acidic media at low potentials.
To our knowledge, no spectroscopic identification of the
products of urea oxidation in alkaline media has been reported
so far. It is expected that the main products are N2 and CO32
-
,
but side reactions can produce some other byproducts, as in
the case of neutral media.
In acidic media, it has been observed that the contact of urea
with polycrystalline platinum electrodes at low potentials
produces an adsorbed residue whose oxidation proceeds at the
In the investigation of adsorbed urea and of the initial stages
of urea oxidation, no vibrational data have been reported so
far. Specular reflectance IR spectra can give some idea about
the type of coordination of the adsorbed urea molecule, albeit
nitrogen coordination has been anticipated by other authors.5
It is the objective of this work to explore the possible products
and adsorbates formed during the initial steps of the urea
oxidation on platinum electrodes in acidic, neutral, and alkaline
media by using in situ Fourier transform infrared spectroscopy
4
same potential range as that of adsorbed carbon monoxide.
Interestingly this process takes place only after flushing the
solutions containing urea for one containing only the supporting
electrolyte. In the presence of dissolved urea no voltammetric
oo bx si de ar vt i eo dn . peak at this potential range (∼600-900 mV) is
On the other hand, when urea is present in solution, two
additional voltammetric peaks of a reversible reaction with the
centers at -0.2 and 0.1 V Vs Ag/AgCl for polycrystalline
platinum and one unusually sharp, very reversible volammetric
,6
(FTIR) measurements to identify reaction products and inter-
mediates.
peak at -0.05 V Vs Ag/AgCl for the Pt(100) single crystal are
observed.5
2. Experimental Section
This behavior has been interpreted, mainly in the case of urea/
Pt(100), as a first-order phase transition between a monolayer
of urea in the more anodic potential range to a monolayer of
The in situ FTIR experiments were carried out using a
BOMEM DA8 infrared spectrometer, equipped with a nitrogen-
cooled mercury cadmium telluride detector. The electrochemi-
cal cell used for both cyclic voltammetry and in situ IR
spectroscopy was made of polytetrafluoroethylene and equipped
with a CaF2 prismatic window with 60° beveled faces.
5
,6
adsorbed hydrogen in the more cathodic potential range.
A
more detailed investigation using low-energy electron diffraction
(LEED) shows a c(2×4) structure on the positive side of the
A polycrystalline platinum electrode with a diameter of 10
mm was used for the spectroscopic experiments. A platinum
wire with a small sphere at the extremity was used for the
†
On leave from the Department of Chemistry of the Universidade Federal
do Paran a´ .
X
Abstract published in AdVance ACS Abstracts, July 1, 1997.
S1089-5647(97)00079-5 CCC: $14.00 © 1997 American Chemical Society

6444 J. Phys. Chem. B, Vol. 101, No. 33, 1997
Bezerra et al.
-1
bands centered at 1610 and 1490 cm . These bands have been
assigned to the CdO and asymmetric C-N2 stretching vibra-
7
tions. The fact that the bands are pointing upward in the case
of the R/R0 spectra implies that urea is consumed when the
electrode potential is changed from -0.16 V to the sample
potential at 0.25 V Vs Ag/AgCl.
The p-polarized light spectrum shows the same bands pointing
upward due to the urea loss from solution together with two
-1
bipolar bands located at 2030 and 1792 cm . These two bands
are located at the position of adsorbed CO in the a-top and
multicoordinated form at polycrystalline platinum electrodes.
The bipolar nature of the bands means that the urea molecule
decomposes in acid solution close to the electrode. Adsorbed
CO is then formed as an intermediate species due to the urea
hydrolysis. It is well known that urea hydrolyzes more easily
+
in acidic media than in neutral media. However, CO2 and ND4 ,
due to purely chemical hydrolysis, in solution have not been
detected during the time scale of the experiments. Thus, it is
very likely that the platinum surface catalyzes the urea
decomposition, with CO as an adsorbed reaction intermediate.
CO2 has been detected only for potentials above 0.45 V
originating from the electrochemical decomposition of urea.
Another possibility is that at low potentials the CO2 produced
by hydrolysis is reduced to adsorbed CO at the platinum surface.
A close comparison of the two spectra shows a shoulder
-
1
located at 1632 cm for the case of the p-polarized light only,
indicating that this shoulder can be assigned to some adsorbed
species. To separate this band from the band of dissolved urea
depletion in the thin electrolyte layer, we can carry out a
difference spectrum between the s- and p-polarized light. To
do so, we need to scale the s-polarized spectrum to the same
intensity as that of the p-polarized spectrum. Since the band
Figure 1. In situ reflection-absorption spectra of the platinum
2
electrode in a solution containing 0.1 M urea in 0.1 M DClO in D O
obtained with s-polarization (upper spectrum) and p-polarization (lower
4
spectrum): reference potential, -0.16 V; sample potential, 0.25 V;
-
1
spectral resolution, 8 cm
.
-
1
located 1490 cm has the same shape in both spectra, we can
use it to normalize the intensity of both spectra. The two spectra
are then ratioed to eliminate the upward pointing bands at 1610
electrochemical experiments. A platinum flat ring was used as
the counter electrode; Ag/AgCl was used as the reference
electrode. All potentials are referred to this electrode if not
otherwise indicated.
-
1
and 1495 cm in order to reveal the shoulder more in detail.
As can be seen in the spectra of Figure 2, a very well-defined
-
1
Solutions were prepared from Merck Suprapur or P. A.
chemicals and Millipore Milli Q water. Solutions of 0.1 M
HClO4 + 0.1 M H4N2CO, 0.1 M KF + 0.1 M H4N2CO, and
potential-dependent band between 1617 and 1629 cm can be
identified.
It has been observed that this adsorbate presents a reversible
behavior, since the spectral band disappears when the electrode
potential is made more negative. The presence of this band
can be related with the suggestion based on voltammetric
experiments that some reversibly adsorbed species at the
platinum electrode in acidic media is present when urea is
present in solution. We reproduce in Figure 3 the cyclic
voltammogram of the platinum electrode in a 0.1 M HClO4
0
.1 M KOH + 0.1 M H4N2CO were used for acidic, neutral,
and alkaline media investigations, respectively. For the spec-
troscopic experiments, deuterated reagents have been used.
The in situ FTIR spectra were obtained by stepping the
potential between the reference and sample potential progres-
sively, i.e. after each measured potential the electrode was
stepped to the reference potential. Each spectrum corresponds
to 512 scans. The reference potential was set at -0.16 V for
the acidic and neutral solutions and -0.75 V for alkaline
solutions. Spectra were computed as the reflectance ratio R/R0
solution without and with urea in solution. The result is
basically the same as that reported by Wiekowski et al.,5
showing an excess of adsorption/desorption charge in the
presence of urea. At higher urea concentrations the same
qualitative behavior is observed, but an overall decrease in the
adsorption/desorption of hydrogen can be observed in Figure
(sample/reference). This procedure results in positive bands due
to the loss of solution species and negative bands due to
formation of new species.
3
b for 0.2 M H2NCONH2 in solution. This overall decrease is
3
. Results and Discussions
related to the formation of adsorbed CO, as demonstrated by
the spectrum with p-polarized light in Figure 1. The adsorbed
CO blocks the sites for hydrogen adsorption. However, the
reversible peak located at 0.07 V Vs Ag/AgCl is observed as
well, indicating the presence of the reversibly adsorbed species.
3.1. Oxidation of Urea in Acidic Solutions. Figure 1 shows
the in situ reflection-absorption spectra of the platinum
electrode in the presence of 0.1 M urea solution in 0.1 M DClO4
in D2O using s- and p-polarized radiation. The use of s- and
p-polarized radiation allows the distinction between bands due
to adsorbed species and those of species dissolved in solution,
since s-polarized radiation does not interact with molecules
adsorbed at the electrode.
The potential-dependent band centered between 1619-1629
-
1
cm is located in the typical range for CdO stretching
vibrations. Platinum-urea complexes coordinated Via nitrogen
present a CdO band at 1715 cm , a wavenumber ca. 30 cm
higher than that of the CdO stretching vibration of free urea
-1 8
-1
The spectra obtained with s-polarized light shows the typical
bands of urea in the D2O solution pointing upward, namely the
-1
(1683 cm ). For deuterated urea, the mode with the predomi-

Electrochemical Oxidation of Urea
J. Phys. Chem. B, Vol. 101, No. 33, 1997 6445
Figure 4. Schematic representation of the most probable structure for
adsorbed urea on a polycrystalline platinum electrode in acidic solutions.
center for the CdO vibration at ca. 10-20 cm- higher than
that for the same mode of free deuterated urea. Thus, the
difference in band shifts between adsorbed urea and urea
complexes are not very different.
1
Although the band shift upon coordination observed in the
spectrum of Figure 2 is lower than that of the platinum
complexes, there is no a priori reason to believe that for
adsorbed urea the same band shift should be expected, as
discussed below.
The excess charge for the hydrogen desorption from poly-
crystalline surfaces when urea is present in solution gives a value
-
2
of 53.6 µC cm . Radiotracer determination of the urea
adsorption coverage for the same solution concentration up to
9
0
1
.35 V Vs reversible hydrogen electrode (RHE) gives 2.5 ×
-
10
-2
0
mol cm . If we consider that adsorbed urea occupies
two adsorption sites, the excess charge obtained above gives
-
10
-2
Figure 2. Spectra obtained from the ratio between s- and p-polarized
radiation for different electrode potentials. The s-polarized radiation
spectra were scaled to the same intensity as that of the p-polarized
light spectra.
2.8 × 10
mol cm if two electrons are transferred per
molecule upon urea adsorption. If two electrons are transferred
for each adsorbed molecule, it is sound to suppose that two
protons are released per urea molecule, giving an adsorbed
molecule with the formula HNCONH. In this case, the most
likely geometry for the adsorbed molecule is as shown in Figure
4
, where the urea molecule loses two protons and is adsorbed
Via two nitrogen atoms, in agreement with the spectroscopic
data.
The partial oxidation of the adsorbed urea (accompanied by
the two-proton loss) could rationalize this lower band shift
compared to that of the Pt/urea complexes. Ab initio calculation
of the vibrational spectrum of H2NCONH2 and HNCONH gives
a strong decrease of the vibrational mode where the CdO
displacement dominates. Namely, for the calculated spectrum
-
1
the neutral urea has this mode at ca. 1900 cm , while for the
-
1
urea without two protons this mode lies at ca. 1250 cm . The
theoretical value for this mode lies too high as compared to the
experimental result, but the relevant result here is that the proton
loss leads to a strong decrease in band frequency for the free
urea. It could be expected that such effect also occurs for
adsorbed urea. It is worthwhile to say that the overall intensity
decreases upon proton loss, according to the calculations.
Attempting to explain such a low band shift for the adsorbed
urea, we need to consider the electronic charge density over
the atoms in the molecule. The ab initio calculation gives a
decrease in the electronic density on the oxygen atoms from
-
0.61 to -0.49 for the free urea molecule. The partial oxidation
Figure 3. Cyclic voltammograms of the polycrystalline platinum
of the adsorbed urea thus decreases the overall charge density
of the molecule, even after the proton loss to maintain the
neutrality of molecule. The lower electronic density over the
oxygen atoms should cause a decrease in bond order, thus
producing a lower band shift as compared to the platinum
complexes.
electrode in 0.1 M HClO
4 2
(dashed lines) and containing 0.2 M H -
NCONH (full line) (charge increase upon urea introduction, 25 µC
2
-
2
-4
cm ) (a) and 5 × 10 M H
2
NCONH
2
(charge increase upon urea
-
2
-1
introduction, 53.6 µC cm ) (b) at 50 mV s
.
-
1
nance of CdO stretching vibration is located at 1610 cm .
-
1
The strong shift of the 1683 cm band upon deuteration is due
to the coupling of this vibration with NH2 deformation, as stated
by Yamaguchi et al.7 The surface-coordinated urea has the band
The observed band position corresponds to a mean coverage
of ca. 0.2, considering two adsorption sites per adsorbed
molecule. Since both lateral interactions and the electric field

6446 J. Phys. Chem. B, Vol. 101, No. 33, 1997
Bezerra et al.
Figure 6. Cyclic voltammograms obtained during a voltammetric cycle
of an electrodeposited platinum electrode in 0.1 M HClO
oxidation of 0.1 M urea in HClO aqueous solution (upper panel). On
line simultaneous detection of m/z 44 (for CO ) and 32 (for N ) during
the electrochemical oxidation of 0.1 M urea in 0.1 M HClO
4
and for the
4
2
2
4
.
Figure 5. In situ reflection-absorption spectra of the platinum
electrode in a solution containing 0.1 M urea in 0.1 M DClO in D
obtained with p-polarization at the different electrode potentials
4
2
O
-
1
indicated: reference potential, -0.16 V; spectral resolution, 8 cm
.
at the interface shift the band to higher wavenumbers, it would
be expected that the singleton frequency at zero field is even
smaller than the observed value.
The set of spectra at different potentials is presented in Figure
5. At all potentials a-top and multicoodinated adsorbed CO
are present, and the shoulder assigned to the adsorbed urea is
shifted to higher wavenumbers at more anodic potentials. At
0.45 V it is possible to observe the production of CO2,
originating from the electrochemical oxidation of the urea
molecule in solution. N2 is also produced, as can be seen by
the DEMS experiment in Figure 6, where the production of N2
follows the production of CO2.
3
.2. Oxidation of Urea in Neutral Media. The spectra of
Figure 7 show the oxidation of urea at different potentials. The
reference potential in this case has been taken in the double-
layer region, since the desorption into the thin solution layer
causes drastic changes in the pH for neutral solutions.
The main feature at low potentials (below 250 mV) is located
-
1
-
at 2161 cm and can be assigned to the CNO ion, resulting
from the oxidation of the urea molecule. For the spectra at 50,
1
-
1
50, and 250 mV another feature at about 1608 cm due to
the consumption of urea from the solution can be detected. At
50 mV the CNO- band decreases in intensity, and another
positive band located at 1418 cm can be detected. This
positive band grows at 350 mV.
2
-
1
Figure 7. In situ reflection-absorption spectra of the platinum
electrode in a solution containing 0.1 M urea in 0.1 M KF in D
obtained with p-polarization at the different electrode potentials
2
O
For more clarity the spectrum obtained at 350 mV has been
reproduced in Figure 8, together with a spectrum obtained with
s-polarized light at the same potential.
-
1
indicated: reference potential, -0.16 V; spectral resolution, 8 cm
.
The reference potential was taken in the “double-layer” region to avoid
large pH changes in the thin layer.
Practically all observed bands are present in both spectra.
-1
The band at 2161 cm in the p-polarized light spectrum is much
weaker in the s-polarized spectrum, but the band position allows
assigned to the urea molecule, which means that at the reference
potential (-0.16 V) the urea decomposes generating some
dissolved product, which is consumed at more positive poten-
tials. s-Polarized light indicates that they are due to dissolved
species.
-
the assignment of this band to dissolved OCN (see discussion
below).
The band at 1418 cm-1 is an upward pointing band, meaning
that this species is consumed at 0.35 V. This band cannot be
It is important to remark that the 1631 cm- band occurs at
1

Electrochemical Oxidation of Urea
J. Phys. Chem. B, Vol. 101, No. 33, 1997 6447
Figure 8. In situ reflection-absorption spectra of the platinum
electrode in a solution containing 0.1 M urea in 0.1 M KF in D O
2
obtained with s- and p-polarization at 0.35 V: reference potential, -0.16
-
1
V; spectral resolution, 8 cm
.
Figure 9. In situ reflection-absorption spectra of the platinum
2
electrode in a solution containing 0.1 M urea in 0.1 M KOD in D O
the same band position as the CdO stretching vibration
wavenumber of the adsorbed urea in acidic media. To
distinguish both products, we make use of the surface selection
rule. The surface selection rule asserts that s-polarized light
does not interact with adsorbed species. Because in neutral
obtained at the different electrode potentials indicated in the figure:
-
1
reference potential, -0.75 V; spectral resolution, 8 cm
.
2
-
It is very likely that the [N2O2] ion disproportionates to
-
give NO2 and NO3 , according to the overall electrochemical
-
1
media the 1631 cm band is also observed with s-polarized
light, this feature is assignable to dissolved products, in contrast
with acidic media, where this band is not observed with
s-polarized light. Using the surface selection rule, the 1418
oxidation:
2
-
-
+
[
N O ] + 3H O f NO + NO + 6H + 7e (1)
2 2 2 2 3
-
1
and 1364 cm bands are equally due to dissolved products.
-
1
The band centered at 1418 cm can be assigned to the trans
3
.3. Adsorption of Urea in Alkaline Media. In alkaline
2
-
[
1
N2O2] ion, which presents the symmetric N-O stretching at
solution the contact of urea with the electrode produces a very
small amount of adsorbed CO, as can be observed in the spectra
of Figure 9. On the other hand the consumption of urea from
solution during the step from -0.75 to -0.55 V can be clearly
detected in all spectra.
-
1
8
419 cm in the solid state. Another band in the spectral
window, above 1050 cm (the cutoff of the CaF2 prismatic
window), due to the N-N stretching at 1121 cm is expected
to be very weak and difficult to observe.
-
1
-
1
The products formed present basically two bands, one at 1631
Parallel to the consumption of urea, it is possible to observe
another interesting spectral feature, which changes with the
-
1
cm , which can be assigned to the dissolved nitrogen-
containing species by taking into account the spectral range in
which they appear.
-
1
applied potential. A strong band located at 1456 cm at
potentials below 0.5 V can be observed. This band decreases
in intensity and practically vanishes for potentials higher than
At this potential range, it is reasonable to expect N2O, NO,
-
NO2, and NO3 as oxidation products, moreover if we take into
-
0.4 V. Simultaneously another feature located between the
two bands of urea in solution can be observed for potentials
account that [N2O2]2- is the intermediate product. Using
3
DEMS, Bolzan and Iwasita detected N2O and NO2. Detection
higher than -0.4 V.
of nitrate ions is not possible by DEMS.
The band located at 1556 cm- is present only with
1
N2O presents bands at 2224 and 1285 cm-1 which are not
p-polarization, as in Figure 10, at -0.3 V. It can be concluded
observed in the spectra of Figure 7. On the other hand, the
antisymmetric stretching mode of NO2 presents one band at
-
1
that the band at 1556 cm is due to some adsorbed species.
-
1
-
1
The band at 1465 cm has not been observed with s-polarized
light as well, thus it should also be an adsorbed species.
1
1
610 cm . The symmetric stretching, which is expected at
-
1
8
318 cm , is hardly observed even in the gas phase. The
The evolution of the spectra of Figure 9 indicates that the
adsorbed species changes the allowed spectral feature, which
indicates either a change in the chemical identity of the adsorbate
or a change in symmetry without changing the chemical nature
of the adsorbed species.
closeness between the expected band for NO2 and the observed
-
1
spectral feature at 1631 cm allows a tentative assignment of
this band to dissolved NO2. Finally the band observed at 1364
-
1
cm lies very close to the expected degenerate stretching
vibration of the nitrate ion. Therefore, the products which better
fit the bands observed in the spectra of Figure 7 are NO2 and
The assignment of spectral features of adsorbed species can
be done by analyzing similar coordination compounds. A good
-
NO3 .

6448 J. Phys. Chem. B, Vol. 101, No. 33, 1997
Bezerra et al.
Figure 11. Schematic representation of the most probable structure
for adsorbed urea on platinum electrodes in alkaline solution. Structure
(
a) is for potentials below -0.40 V and structure (b) for potentials
above -0.40 V.
Figure 10. In situ reflection-absorption spectra of the platinum
electrode in a solution containing 0.1 M urea in 0.1 M KOD in D
obtained with s- and p-polarization at -0.3 V: reference potential,
2
O
-
1
-
0.75 V; spectral resolution, 8 cm
.
correlation between adsorbed ions and molecules and ligands
11,12
in inorganic complexes has been found.
In inorganic complexes both NH2 (or nitrogen) and oxygen
coordination of urea to the metallic center are known. It is likely
that these two kinds of coordination are also possible on surface.
Urea/platinum complexes present nitrogen coordination with
-
1
the symmetric CN stretching located at 1020 cm , the
-
1
unsymmetric CN stretching at 1395 cm , and CdO stretching
-
1
at 1715 cm . On the other hand urea/chromium complexes
present the oxygen-bonded urea coordinated to the metallic
center with the symmetric CN stretching at 1020 cm- and the
1
-
1
unsymmetric and the CdO stretching at ca. 1505 cm .
-
1
The feature observed at lower potentials at 1465 cm fits
relatively well with that of the unsymmetric CN stretching of
-
1
nitrogen-bonded complexes, while the 1556 cm feature is
closer to that of the CdO stretching of oxygen-bonded
complexes.
The unsymmetric CN stretching mode of a C2V molecule
displays the dipole moment displacement (the dynamic dipole
moment) perpendicular to the molecular axis. Therefore, it
would be forbidden in the case of a molecule coordinated by
two nitrogen (or NH2) groups. One possibility would be that
the urea molecule is adsorbed through only one NH2 group and
is tilted on the surface. In this case the molecule loses the C2V
symmetry. In the case of the CN vibration, the number of
spectral features remains the same, since no degenerate modes
are present for the C2V symmetry.
The important result is that the axis through the CdO bonding
for a tilted molecule is almost parallel to the surface and the
dynamic dipole moment is almost perpendicular to the surface.
In this case the mode, which was the unsymmetric stretching
vibration in the C2V symmetry, is allowed by the surface
selection rule and can be detected. On the other hand, the CdO
stretching vibration should present a very small component
perpendicular to the surface and should be hardly observable.
On the other hand, the oxygen-end-down molecule presents
again the C2V symmetry, where the CdO stretching is allowed
while the unsymmetric CN stretching is forbidden by the surface
selection rule. The symmetric CN stretching is a very weak
band for urea in solution and it should be difficult to be
Figure 12. Cyclic voltammogram at 50 mV s-1 of the polycrystalline
platinum electrode in 0.1 M NaOH (solid line) and 0.1 M NaOH
-
3
containg 6 × 10 M H
2 2
NCONH : first cycle after urea introduction
(
‚‚‚) and second cycle (---).
observed. In the present case, the D2O solvent probably
encloses this band, making its detection even more difficult.
In Figure 9, urea is adsorbed Via one of the NH2 groups at
potentials lower than -0.4 V, and for potentials higher than
-0.4 V the molecule rotates to be adsorbed Via oxygen atoms,
according to the scheme of Figure 11.
In the case of acidic media, the proposed adsorption structure
includes a charge transfer with the loss of two protons per
molecule. In the case of alkaline media, unfortunately, it is
not possible to detect charge transfer by cyclic voltammetry as
can be seen in Figure 12. Furthermore the total hydrogen
adsorption/desorption charge decreases with time, indicating that
probably adsorbed CO is formed. This indication is supported
by the small bands in the spectra of Figure 9. In conclusion, it
is difficult to assert if charge transfer takes place or not when
urea is adsorbed in alkaline media.
In the spectra of Figure 8, a weak feature can be seen at ca.
-1
1750 cm . This band is not potential-dependent, and in Figure

Electrochemical Oxidation of Urea
it appears as an upward pointing band in the spectrum with
s-polarized light. The most likely explanation for this band is
that it is due to some instrumental drift. In the spectrum of the
beam splitter, a strong band is observed at this position. Note
that the other spectral features observed are located in zones
free of the beam splitter bands.
J. Phys. Chem. B, Vol. 101, No. 33, 1997 6449
9
Acknowledgment. The Brazilian agencies FAPESP and
FINEP are gratefully acknowledged for financial support as are
CNPq and CAPES for fellowships.
References and Notes
(
1) Yao, S. J.; Wolfson, K. S., Jr.; Ahn, B. K.; Liv, C. C. Nature 1874,
4. Concluding Remarks
241, 471.
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2) Marincic, L.; Soeldner, J. S.; Giner, J.; Colton, C. K. J. Electrochem.
The products and intermediates during the electrochemical
Soc. 1979, 26, 1687-1692.
oxidation of urea are strongly pH-dependent. In acidic and
alkaline solutions the presence of CO resulting probably from
the slow hydrolysis is detected even at very low potentials.
A reversibly adsorbed urea molecule is also observed for
acidic and alkaline solutions. The adsorption geometry depends
on pH. In acidic media the urea is probably adsorbed through
the nitrogen, following a charge transfer with the loss of two
protons. In alkaline solutions the urea is first adsorbed through
one of the two NH2 groups, producing a tilted molecule on the
surface. At more positive potentials (possibly more positive
than the point of zero charge) the molecule is flipped, and the
final adsorption is an oxygen-end-down adsorbed urea molecule.
The main products observed in acidic media are CO2 and N2.
In neutral media no adsorbed molecule has been detected,
possibly due to the high reference potential used. The main
products are CNO- and [N2O2] ions in solution for lower
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3) Bolzan, A. E.; Iwasita, T. Electrochim. Acta 1988, 33 (1), 109-
1
(4) Petrii, O. A.; Vassina, S. Ya. J. Electroanal. Chem. 1993, 349,
1
97-209.
5) Rubel, M.; Rhee, C. K.; Wieckowski, A.; Rikvold, P. A. J.
Electroanal. Chem. 1991, 315, 301-306.
6) Gamboa-Aldeco, M.; Mrozek, P.; Rhee C. K.; Wieckowski, A.;
Rikvold, P. A.; Wang, Q. Surf. Sci. 1993, 297, L135-L140.
7) Yamaguchi, A.; Miyazawa, T.; Shimanouchi, T.; Mizushima, S.
Spectrochim. Acta 1957, 10, 170-178.
8) Nakamoto, K. Infrared and Raman Spectra of Inorganic and
Coodination Compounds; Wiley: New York, 1978.
9) Horanyi, G.; Inzelt, G.; Rizmayer, E. M. J. Electroanal. Chem.
1979, 98, 105-117.
10) Outka, D. A.; Madix, R. J.; Fischer, G. B.; DiMaggio, C. Langmuir
1986, 2, 406-411.
11) Nart, F. C.; Iwasita, T. J. Electroanal. Chem. 1992, 322, 289-
300.
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2-
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potentials and NO2 and NO3 when the potential is made more
positive.
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