Surface Science 471 (2001) 151±162
www.elsevier.nl/locate/susc
Theoretical study of the adsorption of urea related species on
Pt(1 0 0) electrodes
Maite Garcõa-Hern
andez a,b, Uwe Birkenheuer a,1, Anguang Hu a,2,
sch a
Francesc Illas b,*, Notker Ro
a
b
Institut f ur Physikalische und Theoretische Chemie, Technische Universit
at M
unchen, 85747 Garching, Germany
Departament de Quõmica Fõsica & Centre Especial de Recerca en Quõmica Te
orica, Universitat de Barcelona, C/Martõ i Franqu
es 1,
08028 Barcelona, Spain
Received 14 August 2000; accepted for publication 10 October 2000
Abstract
We present a cluster model study on the two most likely urea adsorption complexes at Pt(1 0 0) electrodes. A goal of
the investigation is to determine whether urea or ureylene, a HNCONH biradical species, forms the most stable adsorption complex. Geometry optimisations of both urea species adsorbed at various surface sites have been carried out
with the parallel P A R A G A U S S program package using gradient-corrected density functionals. Scalar-relativistic allelectron calculations as well as pseudopotential calculations have been performed. For each optimised structure, vibrational frequencies have been calculated. From energy considerations, ureylene adsorption is found in both types of
calculations to be preferred over urea adsorption, consistent with the interpretation of electrochemical measurements,
even though e€ective core potential results tend to underestimate the binding energies. This assignment is supported by
the calculated CO vibrational stretching frequency of chemisorbed ureylene which is closer to the value obtained from
``in situ'' Fourier transform infrared spectroscopy in an electrochemical environment than the CO frequency calculated
for adsorbed urea. Ó 2001 Elsevier Science B.V. All rights reserved.
Keywords: Platinum; Density functional calculations; Chemisorption
1. Introduction
*
Corresponding author. Tel.: +34-93-402-1229; fax: +34-93402-1231.
E-mail address: [email protected] (F. Illas).
1
Present address: Max-Planck-Institut f
ur Physik komplexer
Systeme, N
othnitzer Strasse 38, 01187 Dresden, Germany.
2
Present address: Department of Chemistry, University of
Minnesota, Minneapolis, MN 55455 0431, USA.
There is little doubt that understanding adsorption processes at a molecular level requires full
comprehension of the interactions between molecules and solid surfaces [1±3]. This claim for understanding at a microscopic level is even more
stringent for interactions occurring at electrochemical interfaces since it is impossible to control
all parameters at a level similar to that which
can be achieved in experiments carried out under
0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 0 3 9 - 6 0 2 8 ( 0 0 ) 0 0 9 0 1 - 8
152
M. Garcõa-Hern
andez et al. / Surface Science 471 (2001) 151±162
ultra-high vacuum (UHV), conditions. A case in
point is ``in situ'' Fourier transform infrared
spectroscopy (FTIR), applied in an electrochemical environment. While this technique enables to
obtain precise experimental data about the species
that are adsorbed at the electrode, their interpretation is not simple at all and thus in situ FTIR
does not permit an univocal assignment because of
the diculty, if not the impossibility, to compare
to spectra of known species. Therefore, it is not
surprising that the interpretation of FTIR in situ
spectra is most often based on comparison to existing inorganic complexes. However, this complex-to-surface analogy does not guarantee a
correct assignment [4]. As an alternative, quantum
chemistry investigations applied to realistic surface
models can provide an adequate, yet ¯exible,
computational approach to the chemistry of such
complex systems. Computational experiments permit to verify or discard a given hypothesis and
suggest new interpretations of experimental data.
The present work is devoted to such a theoretical
study of the adsorption of urea and related species
on a single crystal Pt(1 0 0) electrode.
Several experimental studies [5±10] addressed
structural aspects of urea adsorption at platinum electrodes and reactions of such adsorption
systems. These investigations were carried out using di€erent well-de®ned single-crystal surfaces
and used a variety of experimental techniques
including voltammetry [5,6,8], radiochemistry [8],
``ex situ'' low electron energy di€raction [6,8]
(LEED), and Auger electron spectroscopy [6,8].
These data permitted to derive the dependence
of the potential on urea coverage, the potential
saturation value, and the saturated urea adlayer
surface structure. One particular feature of the
adsorption of urea in the Pt(1 0 0) surface is the
very large excess charge compared to that measured in other crystallographic surfaces. This
charge excess due to the presence of urea in the
media indicates that the adsorbate undergoes a
charge-transfer process at the electrode. The adsorption and desorption processes take place with
a net charge transfer of two electrons per adsorbed
molecule [5,6,8]. This indicates that urea adsorption may produce adsorbed ureylene according to
the following electrochemical process:
Hydrogen adsorption at the Pt(1 0 0) surface
may be an important intermediate stage of this
reaction. The adsorption and desorption voltammogram peaks recorded on Pt(1 0 0) are very sharp
and indicate that urea adsorption is reversible
[5,6,8]. On the other hand, the stable c…5 20†
LEED pattern observed on a clean Pt(1 0 0) surface changes with urea adsorption to a c…2 4†
LEED pattern [6,8]. Radiochemistry measurements [8] gave a coverage of 0:26 0:04 ML
(number of molecules per Pt(1 0 0) unit cell) which
is fully consistent with the value of 0:24 0:03 ML
obtained from the Auger electron spectroscopy
carbon signal [8]. Climent et al. reported complementary studies of the adsorption of urea on
Pt(1 0 0) [11] and Pt(1 1 1) [12] electrodes. The
voltammetric and charge displacement analysis on
Pt(1 0 0) are similar to those reported by Wieckowski et al. [5±10] under analogous conditions.
Moreover, the integrated charge was found to be
larger than the one corresponding to adsorption
and desorption of a single hydrogen atom by
50%, thus indicating the existence of additional
urea related surface processes. In situ FTIR spectra reported by Feliu et al. [11] provide additional
information on the nature and structure of the
urea species adsorbed on Pt(1 0 0). A bipolar band,
typical of potential dependent frequencies of the
vibrational modes of adsorbed species, appears
centred at 1705 cm±1 . Based on the similarity to the
urea carbonyl group in coordination compounds
[13,14] and on the fact that this peak does not
appear at potentials at which there is no adsorbed
urea, the peak at 1705 cm±1 was assigned to urea
bonded at the Pt(1 0 0) surface through the nitrogen atoms, in a bridge con®guration and with the
carbonyl group normal to the electrode surface.
Likewise, once the electrode has been in contact
with the urea solution, the appearance of this peak
M. Garcõa-Hern
andez et al. / Surface Science 471 (2001) 151±162
neither depends on the electrode potential nor on
the presence or absence of urea in the solution
where the Pt electrode is re-immersed.
The ®rst theoretical study of urea adsorbed on
Pt(1 0 0) was reported by Rikvold and Wieckowski
[7]. In this work urea adsorption on Pt(1 0 0) was
investigated within the gas-lattice model employing a Monte Carlo simulation. This simple model
suggests that urea is adsorbed along Pt±Pt bonds
occupying two adsorption sites and co-exists with
hydrogen atoms adsorbed on top of the platinum
atoms. Based on the fact that most of the surface
hydrogen atoms desorb at a potential close to that
corresponding to urea adsorption, these authors
suggest that urea adsorbs on sites formerly occupied by hydrogen atoms. Further Monte Carlo
simulations within the lattice gas-model [8±10]
considered urea coordination at the platinum
surface through the nitrogen atoms. Clearly, a
non-empirical approach appears desirable.
The present work is devoted to the analysis of
the adsorption properties of urea and related
species on Pt(100) electrodes. Binding energies,
geometrical parameters, and vibrational frequencies of urea ± (H2 N)CO(NH2 ) ± and ureylene ±
(HN)CO(NH) ± the two most likely adsorbed
species, are studied using a cluster model approach
to represent the Pt(1 0 0) surface and gradientcorrected density functionals (DFs) to account for
correlation. Relativistic e€ects due to the presence
of platinum atoms are also taken into account (for
details see below).
2. Surface cluster models
The cluster model approach to chemisorption
assumes that the properties of interest are of local
nature and, hence, restriction to a ®nite fraction of
the extended surface does not introduce serious
artefacts. This approach has long been used to
model surfaces and chemisorption phenomena and
its success and limitations are well documented [1±
3]. Reasonably large cluster models have been
employed in the present study to simulate the
various adsorption sites of urea and ureylene on
the Pt(1 0 0) surface. Adopting the Ptn …m1 ; m2 ;
153
Fig. 1. Schematic view of the cluster models used to represent
di€erent adsorption sites of the Pt(1 0 0) surface: (a) Pt14 (8,6),
(b) Pt9 (4,5), and (c) Pt9 (5,4).
m3 ; . . .† notation where n denotes the total number
of platinum atoms in the cluster and mi the number of atoms in the ith crystal layer (starting with
the top-most one at the surface), the cluster models
chosen for the various adsorption sites of Pt(1 0 0)
are Pt14 (8,6), Pt9 (5,4), and Pt9 (4,5), respectively
(see Fig. 1). These cluster models were considered
as rigid fractions of a Pt bulk crystal with the
nearest-neighbour Pt distance set to the experi [15]. The geometry optimental value of 2.77 A
misation for urea and ureylene on Pt(1 0 0) was
carried out using analytical energy gradients. C2
point group symmetry was imposed during geometry optimisation with the only additional
constraint ± beside the ®xed Pt±Pt distances ± that
the orientation of the molecular N±CO±N plane is
preserved. In this way one ensures that the adsorbate will not change site during the geometry optimisation. In all cases the calculated gas
phase geometry of urea was taken as starting
154
M. Garcõa-Hern
andez et al. / Surface Science 471 (2001) 151±162
con®guration for both urea and, after abstracting
two hydrogen atoms, for ureylene.
Four di€erent adsorption sites have been considered, the corresponding positions of the urea
skeleton are indicated schematically in Fig. 2.
These sites can be characterised as follows: (i) on
top site ± each nitrogen atom on top of a Pt atom
(Fig. 2a), (ii) hollow site ± each nitrogen atom
above a four-fold hollow site (Fig. 2b), (iii) bridgeontop site ± each nitrogen atom in a bridging position above two surface Pt atoms such that there
is one Pt atom just below the C atom (Fig. 2c), and
(iv) bridge-hollow site ± each nitrogen atom again
in a bridging position, but with a four-fold hollow
below the C atom (Fig. 2d).
3. Computational details
Recall that relativistic e€ects are important for
systems containing heavy atoms; hence they must
be included to describe the electronic structure of
the Pt surface clusters used in the present study.
Two di€erent approaches have been used for
that purpose. In a ®rst series of calculations all
electrons (AE), are treated explicitly within a scalar-relativistic variant [16] of the linear combination of Gaussian-type orbitals DF method [17]
(LCGTO-DF) as implemented in the program
P A R A G A U S S for parallel computers [18,19]. The
same computer code was used for a second series
of calculations (the ECP approach) where scalarrelativistic e€ects are introduced indirectly through
the usage of pseudopotentials [20]; in the present
study these are relativistic pseudopotentials of
the Stuttgart type [21]. In both cases, a large and
¯exible basis was employed to expand the Kohn±
Sham orbitals. For the AE Pt atoms the basis set is
(21s,17p, 12d,7f) contracted to [9s,8p,5d,2f]. The
primitive basis set is derived from a (19s,14p,10d,
5f) GTO basis [22] by adding two s, three p, two d,
and two f exponents as proposed by Ferrari et al.
[23] in their study of Pt clusters supported on zeolites. For the other atoms ¯exible GTO basis sets
were employed [24,25]. For C, N, and O, these are
(9s,5p,1d) basis sets contracted to [5s,4p,1d], and
for H the basis is (6s,1p) contracted to [4s,1p].
In the pseudopotential calculations a small core
containing 60 electrons was used for Pt; the 5s, 5p,
5d, and 6s electrons are included in the valence
shell together with a (7s,6p,5d) primitive basis
set contracted to [6s,3p,2d] [26]. The gradientcorrected BP exchange-correlation functional, a
combination of BeckeÕs exchange functional [27]
and PerdewÕs correlation functional, [28] was employed throughout. All calculations were carried
out in spin-restricted fashion, since no open-shell
con®gurations were expected among the potential
adsorption species.
4. Structure and stability of urea species on Pt(1 0 0)
First, we brie¯y describe the results obtained for
gas phase urea and compare them with available
theoretical and experimental data. Both microwave spectroscopy [29] and matrix isolation studies [30] have shown that, contrarily to organic
chemistry textbook arguments, urea is not a planar
molecule. More recently Godfrey et al. [31] combined MP2/6-311++G(d,p) calculations with all
previous experimental data to fully characterise
the structure of the urea molecule. Later, Rousseau et al. [32,33] reported a detailed assignment of
the vibrational frequencies of urea and deuterated
urea. The geometric parameters of free gas phase
urea predicted by the present DF, calculations (see
Table 1) are in excellent agreement with the MP2
results just mentioned as well as with experiment
[31]. The accuracy of the DF calculations is about
for distances, 1±2° for bond angles, and 5°
0.01 A
for the pyramidalisation angle as de®ned in Ref.
[29].
We now turn to the adsorption process. Within
the ECP approach, the equilibrium geometry of
both adsorption species, urea and ureylene, have
been determined all for adsorption sites of Pt(1 0 0)
described above. For ureylene, which is the most
interesting species from the electrochemical point
of view, the adsorption geometries were also determined by scalar-relativistic AE calculations. A
comparison of these two approaches will be presented later. The structural parameters of the adsorbed species are reported in Table 1 (for urea)
and Table 2 (for ureylene). The ®rst surprising
result, even without any reference to gas-phase
M. Garcõa-Hern
andez et al. / Surface Science 471 (2001) 151±162
155
Fig. 2. Schematic top and side view of the adsorption complexes considered (only the NCON skeleton of urea is shown): (a) ontop, (b)
hollow, (c) bridge-ontop, and (d) bridge-hollow.
156
M. Garcõa-Hern
andez et al. / Surface Science 471 (2001) 151±162
Table 1
Equilibrium structure of urea on Pt(1 0 0) from a relativistic pseudopotential description of the Pt core electrons
Site cluster
Distances
C±O
C±N
N±Hup
N±Hdown
N±N
N±Pt
N±surf a
H±Pt
H±surf a
Ontop
Pt14 (8,6)
±
±
1.207
1.459
1.040
1.030
2.463
2.205
2.200
Hollow
Pt14 (8,6)
1.235
1.386
1.017
1.024
2.343
3.853
3.399
2.899
2.420
Bridge-ontop
Pt9 (5,4)
±
±
1.212
1.457
1.029
1.029
2.423
2.946
2.685
Bridge-hollow
Pt9 (5,4)
Free urea
1.232
1.390
1.030
1.018
2.337
3.525
3.234
2.504
2.265
1.228
1.396
1.021
0.998
2.338
Angles
O±C±N
122.4
122.3
123.7
122.8
C±N±Hup
106.8
114.1
107.5
118.9
C±N±Hdown
108.6
120.6
107.5
114.2
N±C±N
115.2
102.4
112.5
114.4
O±C±N±Hup
ÿ73.1
14.2
56.6
ÿ157.3
O±C±N±Hdown
41.7
160.2
ÿ56.5
ÿ14.2
angles in degrees. A C2 symmetry constraint was imposed during the geometry optimisation.
Distances in A,
a
Distance to the top ``crystal'' plane of the cluster model.
±
±
±
±
123.1
112.8
119.2
113.7
12.8
148.9
Table 2
Equilibrium structure of the ureylene radical on Pt(1 0 0) from a relativistic pseudopotential description of the Pt core electrons
Site cluster
C±O
C±N
N±H
N±N
N±Pt
N±surf a
Ontop
Pt14 (8,6)
Distances
1.244
1.375
1.021
2.338
1.993
1.981
Hollow
Pt14 (8,6)
1.227
1.399
1.028
2.235
2.459
1.696
Bridge-ontop
Pt9 (5,4)
1.224
1.404
1.031
2.311
2.208
1.882
Bridge-hollow
Pt9 (5,4)
1.221
1.450
1.030
2.491
2.120
1.598
Angles
O±C±N
121.8
127.0
124.6
120.7
C±N±H
113.6
111.6
109.2
106.5
N±C±N
116.4
106.0
110.8
118.5
O±C±N±H
0.0
0.1
50.5
0.0
angles in degrees. A C2 symmetry constraint was imposed during the geometry optimisation.
Distances in A,
a
Distance to the top ``crystal'' plane of the cluster model.
urea, is that the relative orientation of the hydrogen atoms of the amide groups with respect to the
surface di€ers substantially among the four different sites, see Fig. 2. This is at variance with the
case of ureylene precisely because of the lack of
one hydrogen atom in each amide group (see below). For urea in the ontop and bridge-ontop sites,
the lone pairs of the nitrogen atoms point towards
Free urea
±
±
1.228
1.396
1.018
2.338
123.1
112.5
113.7
ÿ12.8
the surface, and the hydrogen atoms of each amide
group point away from the surface (see middle
columns of Fig. 2a and c), i.e. the surface bond is
mediated through the lone-pairs on the amide
nitrogen atoms. Compared to the gas phase, the
C±O distance decreases concomitantly with an
increase of the C±N and the two N±H distances.
Note also that for the bridge-ontop adsorption site
M. Garcõa-Hern
andez et al. / Surface Science 471 (2001) 151±162
the urea molecule becomes C2v symmetric, even
though this is not enforced by symmetry constraints. At the hollow and bridge-hollow sites,
apart from a slight elongation of the N±H and
C±O bonds and a shortening of the C±N bonds,
the most interesting feature is that the atoms
closest to the surface are two hydrogen atoms (see
middle columns of Fig. 2b and d). Hence, at these
two sites urea interacts with the surface directly
through one hydrogen atom per amide group
rather than through the nitrogen lone pairs. Apparently a bond con®guration is preferred which
may be regarded as a precursor for dehydrogenation of urea via hydrogen adsorption on Pt(1 0 0).
In spite of having lost two hydrogen atoms, the
geometry of adsorbed ureylene is rather reminiscent of that of gas-phase urea. In fact, in adsorbed
ureylene the C±O distance remains almost unchanged with respect to free urea as do most of the
bond angles (see Table 2). Much more interesting
is the position of the remaining hydrogen centres
relative to the urea skeleton, right column of Fig.
2. Except for the bridge-ontop site, the hydrogen
atoms lay almost perfectly in the N±(CO)±N
plane, indicating a symmetric two-fold coordination (including the lone-pair) of the nitrogen atoms
toward the Pt surface. The resulting coordination
of the ureylene nitrogen atoms can be rationalised
in terms of an almost sp3 hybridisation. Two of the
sp3 hybrid orbitals form the N±C and the N±H
bonds while the two remaining hybrids establish
the surface bonding. Hereby, the platinum surface
is meant to provide the one extra electron necessary to form the two equivalent single bonds.
Notice that the participation of the extra electron
to the surface bond does not necessarily imply a
charge donation to the adsorbate. The formation
of such a two-fold coordination is obvious for the
most stable adsorption site, the bridge-hollow site,
which in turn rationalises the energetic preference
of that site. For the hollow and ontop sites, this
two-fold coordination is not apparent, indicating a
more delocalised bonding mechanism. For the
bridge-ontop site, on the other hand, the dihedral
OCNH angle is about 50°, indicating a deviation
form planarity, at variance from the value zero for
the other cases discussed above. The corresponding structure is again compatible with a typical
157
sp3 -type hybridisation of the amide nitrogen atoms, with one lone pair of each nitrogen atom
oriented almost perpendicularly to the molecular
plane and the other one pointing directly towards
the surface. Finally, it is worth mentioning that
the surface bonding mechanism for ureylene to the
bridge-hollow site is favoured with respect to the
direct hydrogen-surface interaction encountered in
urea adsorption at that site simply because of the
need to saturate the free valences of the ureylene
radical.
Now we turn to a discussion of the relative
stability of urea and ureylene adsorbed at the
di€erent surface sites. In order to compare the
adsorption energies of the various species, a thermodynamic cycle has been used. In this way, one
avoids any explicit reference to the energy of the
gas-phase biradical and at the same time one does
not introduce any bias on the relative stability. For
adsorbed urea, the interaction energy is given by
that of the simple reaction:
CO…NH2 †2 ‡ Ptn …m1 ; m2 † ! CO…NH2 †2…ads†
…1†
As usual, a negative value indicates that products
are more stable than reactants. However, for adsorbed ureylene it is convenient to refer to the
adsorbed species plus adsorbed hydrogen. Therefore, the interaction energy is calculated as the
energy of the following reaction:
CO…NH2 †2 ‡ Ptn …m1 ; m2 †
! CO…NH2 †2…ads† ‡ 2H…ads†
…2†
Here, H…ads† designates adsorbed atomic hydrogen
although one must realise that in the electrochemical environment this adsorbed species
becomes oxidised. In order to obtain a unique
reference energy for H…ads† , the interaction energy
of atomic hydrogen was computed for di€erent
high symmetry sites of Pt(1 0 0) using the various
cluster models shown in Fig. 1. The largest interaction energy, 69.0 kcal molÿ1 for the ECP approach (72.6 kcal molÿ1 with the AE treatment),
was found for adsorption of a single H at the
bridge site of the Pt14 (8,6) cluster which falls well
into the range of about 60±80 kcal molÿ1 for hydrogen-metal binding energies as typically observed in experiment [34,35]. This value was taken
158
M. Garcõa-Hern
andez et al. / Surface Science 471 (2001) 151±162
to compute the interaction energies according to
reaction (2).
It is worth comparing the present cluster calculations to recent periodic DF calculations using
the ADF-Band code [36]. These authors report
adsorption energies for H on Pt(1 1 1) relative to
gas phase H2 . For a 0.25 coverage the adsorption
energy calculated obtained with the BP functional
is about 11 kcal molÿ1 . Using the experimental
binding energy of ÿ103.3 kcal molÿ1 for the hydrogen molecule and the results reported by Olsen
et al. [36], it is straightforward to obtain the corresponding atomic hydrogen chemisorption energy. This leads to a value of 63 kcal molÿ1 that
can be directly compared to the present cluster
model results reported above. This result corroborates the cluster estimates discussed above. Note,
however, that the slab model calculations refer to
Pt(1 1 1), use a frozen core approximation, and a
di€erent basis set whereas the present calculations
are for H adsorption on Pt(1 0 0). Therefore, only
an approximate comparison is meaningful. Given
the di€erences between both theoretical studies,
the agreement between cluster and periodic calculations is rather satisfactory.
For adsorbed urea, both hollow and bridgehollow sites are most stable (ÿ5 kcal molÿ1 ) and
energetically nearly equivalent, the former being
lower in energy by 0.7 kcal molÿ1 only (Table 3).
Adsorption in the ontop site is endothermic by
0.6 kcal molÿ1 and, ®nally, the interaction above
the bridge-ontop site is highly disfavoured by more
than 25 kcal molÿ1 . Moreover, given the wellknown limitations of cluster models when adsorption energies are to be determined [1±3], we
consider these ®rst three cases as comparable in
their binding energy (see below).
A rather di€erent situation occurs when we
consider the relative stability of adsorbed ureylene.
With an interaction energy of 27 kcal molÿ1 , the
most stable adsorption complex is at the bridgehollow site (Table 3). The adsorption process
described by reaction (2) is preferred over nondissociative adsorption of urea ± reaction (1) ± at
the bridge hollow site by about 22 kcal molÿ1 . This
is consistent with the hypothesis that upon adsorption urea looses two hydrogen atoms [11]. Interaction at the three other sites is far less favourable
than in the case of urea. This result is of special
relevance since it is in agreement with the experimental observation of a urea adlayer which is
strongly bound to the Pt electrode [11]. In fact, this
adlayer is so tightly bound to the electrode surface
that it is detected even after the electrode is removed from the urea solution and an in situ FTIR
measurement is performed after inserting the
electrode into a urea-free solution [11]. Therefore,
the bridge hollow adsorption of the urea molecule
can be regarded as a ®rst step of a more complex
adsorption mechanism in an electrochemical environment.
Next, we will address the accuracy of the
pseudopotential approach compared to scalarrelativistic AE calculations. This comparison has
been carried out for ureylene on the four active
sites described above. Overall, the adsorbate
structures arising from the more accurate AE calculations are very similar to those obtained with
the ECP approach. The changes in the geometry of
adsorbed ureylene are essentially negligible. The
only noticeable geometrical variation is found for
shorter in
the Pt±N distance; it is by 0.03±0.06 A
the AE calculations. Quite noticeable di€erences
are obtained for the adsorption energies. The AE
calculations result in an additional stabilisation of
the adsorbed species by 18±29 kcal molÿ1 compared to pseudopotential results (Table 3). This
energy di€erence is larger than usually encoun-
Table 3
Interaction energies (in kcal molÿ1 ) of urea and ureylene on a Pt(1 0 0) surface according to the reactions (1) and (2) (see text) as
obtained from relativistic pseudo potential (ECP) and AE calculationsa
Urea
Ureylene
a
ECP
ECP
AE
Ontop
Hollow
Bridge-ontop
Bridge-hollow
0.6
ÿ10.1
ÿ30.1
ÿ5.2
54.3
41.6
26.6
38.0
20.1
ÿ4.6
ÿ26.5
ÿ55.4
Negative energies imply products being more stable than reactants.
M. Garcõa-Hern
andez et al. / Surface Science 471 (2001) 151±162
tered in transition metal compounds and indicates
some limitation of the Pt pseudopotential representation (likely due to the ECP basis set). Note,
however, that from Table 3 one can see that these
energy di€erences are fairly uniform over the
whole series of adsorption sites investigated and
hence, the relative stability of the di€erent adsorption complexes is not a€ected.
Finally, we come to a discussion of the adsorption energetics as it emerges from the results
obtained with ECP and AE models. Besides the
increase of the binding energies in AE calculations,
we need to discuss two further limitations of the
computational methodology employed. Since, so
far the basis set superposition error (BSSE) has
been neglected, the calculated interaction energies
may be somewhat too large. On the other hand,
common exchange-correlation approximations
(such as the BP functional used in the present
work) are known to underestimate weak interactions of about 5 kcal molÿ1 or less as found here
for some adsorption complexes (Table 3) [37].
To estimate the basis set superposition error, we
applied the counterpoise correction [38] to urea
adsorbed in the most stable hollow adsorption
site. This correction decreases the ECP binding
energy by about 3 kcal molÿ1 , from ÿ5.2 to ÿ2.4
kcal molÿ1 . On the other hand, binding energies
obtained from the relativistic AE calculations are
expected to exhibit a larger BSSE e€ect. Indeed,
with the counterpoise correction, the interaction
energy of ureylene at the bridge-hollow site is reduced by about 7 kcal molÿ1 , while the ECP result
changes by about 2 kcal molÿ1 only. However, it is
obvious that even with the BSSE correction taken
into account, ECP binding energies are notably
underestimated compared to the corresponding
AE results, by about 10±25 kcal molÿ1 . With such
increased interaction energies (as they are expected
to derive from AE calculations even in cases where
the ECP approach yields weak interactions only),
the above caveat concerning weak interactions
does not seem pertinent in the present context.
In any case, the stability trend of the various
sites is expected to be identical for relativistic AE
and relativistic ECP results, but some caution is
required when discussing and interpreting absolute
values of the interaction energies.
159
5. Vibrational frequencies of urea and urea species
on Pt(1 0 0)
Despite its apparent simplicity and the considerable amount of theoretical and experimental
work [31±33], the vibrational spectrum of gasphase urea is still far from being completely
interpreted. Interestingly, the present DF calculations are in very good agreement with experimental data, especially for the frequencies in the range
from 1000 to 3500 cmÿ1 , corroborating some
previous assignments that had to be quali®ed as
tentative [30]. The main interest of the present
work lies in the mCO stretching frequency of the
adsorbed species because this is the only frequency
that is identi®ed in the in situ FTIR experiments.
Nevertheless, the gas phase vibrational spectrum is
of interest for comparison, too. The calculated
vibrational frequencies together with other available data are reported in Table 4. For gas-phase
urea, the accuracy of the present BP results is
noteworthy, with a mean absolute deviation of 22
cmÿ1 and maximum deviations of at most 50 cmÿ1
in the less favourable case. It is also worth pointing
out that this accuracy is signi®cantly better than
that of the MP2 method for the soft modes of urea
[31] and comparable to that of empirically corrected HF results [32].
For the adsorption complexes, full vibrational
analyses have been carried out for both adsorbed
urea and ureylene. However, since the experimental information is rather limited there is little
value in reporting the full series of data here. Instead, selected vibrational frequencies, namely for
the C±O, C±N and N±H symmetric stretching
motions, are reported in Table 5. Unfortunately,
several di€erent adsorption complexes exhibit
calculated C±O stretching frequencies near 1700
cmÿ1 . For urea, somewhat higher CO frequencies
are calculated, from about 1700±1800 cmÿ1 , than
for ureylene, where the CO frequencies of the
various cluster models range from about 1600±
1700 cmÿ1 (Table 5). Therefore, from these computed vibrational data it is impossible to infer
either the nature of the adsorbed species or to the
active site. However, for the most stable species
and adsorption site, ureylene at the bridge-hollow
site, the calculated C±O stretching frequency
160
M. Garcõa-Hern
andez et al. / Surface Science 471 (2001) 151±162
Table 4
Vibrational frequencies (in cmÿ1 ) of gas-phase urea as obtained from Becke±Perdew, BP, DF calculations compared to available
experimental dataa
Present work Assignment
BP
Experimental
Ref. [33]
ss (NH2 )
xa (NH2 )
d(CN)
sa (NH2 )
xs (NH2 )
d(CO)
x(CO)
ms (CN)
qa (NH2 )
qs (NH2 )
ma (CN)
ds (NH2 )
da (NH2 )
m(CO)
ms (NH2 )a
ms (NH2 )s
ma (NH2 )a
ma (NH2 )s
426 (A)
458 (B)
475 (A)
537 (B)
557 (B)
585 (A)
747 (B)
919 (A)
1014 (B)
1148 (A)
1370 (B)
1584 (A)
1587 (B)
1748 (A)
3466 (B)
3473 (A)
3590 (B)
3591 (A)
Ref. [32]
Ref. [30]
775
1032
227
410?
542?
578?
618?
790?
1014
1157
1394
1604
1749
1776
3434
3460
3533
3559
1394
1594
1594
1734
3440
3440
3448
3548
410
578
790
960
1014
1394
1594
1594
1734
3440
3440
3548
3548
a
The question marks indicate tentative experimental assignments. The symmetry character of the calculated modes, A or B of the C2
point group, is also given.
Table 5
Calculated pseudopotential BP values of the symmetric vibrational frequencies of urea and ureylene (in cmÿ1 ) at various sites of the
Pt(1 0 0) surfacea
Assignment
Ontop
Pt14 (8,6)
Hollow
Pt14 (8,6)
Bridge-ontop
Pt9 (5,4)
Bridge-hollow
Urea
ms (CN)
m(CO)
ms (NH2 )s
ma (NH2 )s
707
1772
3139
3362
930
1646
3391
3584
737
1736
3313
3347
933
1681
3264
3568
919
1748
3473
3591
ms (CN)
m(CO)
ms (NH2 )s
ma (NH2 )s
Ureylene
ms (CN)
m(CO)
ms (NH)s
1046
1583
3491
772
1646
3410
860
1658
3280
764 (773)
1693 (1686)
3395 (3552)
919
1748
3473
ms (CN)
m(CO)
ms (NH2 )s
Gas phase urea
Pt9 (5,4)
Assignment
a
Frequencies calculated at the all-electron level are given in parentheses. Calculated gas phase frequencies of urea and their assignment are given for comparison.
amounts to 1693 cmÿ1 , which is very close to 1705
cmÿ1 , the frequency experimentally observed by
Climent et al. [11] in in situ FTIR experiments.
Remarkably, by comparison to di€erent inorganic
complexes, this experimental group deduced precisely the same assignment.
Notice that, after urea adsorption as ureylene at
the bridge-hollow site, the C±N stretching mode
undergoes a shift to a lower frequency, consistent
with the trend observed in coordination compounds having urea as an N-bonded ligand [14].
However, the fact that the surface-complex anal-
M. Garcõa-Hern
andez et al. / Surface Science 471 (2001) 151±162
ogy permits, in this case, a correct assignment of
the coordination mode and of the observed vibrational frequency does not validate this approach
in general. It is important to re-iterate that an
adsorbate frequency assignment by comparison to
the vibrational frequencies of these adsorbates
acting as ligands in complexes can be very misleading [4]. In the present case of urea adsorbed on
Pt(1 0 0) electrodes, both frequencies obtained either from the in situ FTIR measurements or in Ptcomplexes are fortuitously close, but this is by no
means a strict rule. Actually, inspection of Table 5
reveals that variations of the order of 100 cmÿ1 are
typically encountered for the C±O stretching frequency of the two urea species as a function of
coordination to the Pt atoms of the electrode. This
clearly indicates the limitation of the surfacecomplex analogy. Rather, comparison to reasonable theoretical models of the type used in the
present work provides a more adequate framework for understanding the adsorbate±substrate
chemical bonding that occurs in these adsorption
complexes.
Finally, it is worth pointing out that the present
analysis remains largely unchanged if the computationally more accurate AE approach is employed. In fact, in Table 5 we also present a
comparison of the frequencies for the most stable
bridge-hollow site obtained either by the ECP
approach or by the scalar-relativistic AE approach. The vibrational frequencies of interest
di€er by less than 4%. Therefore, we conclude that
as far as vibrational frequencies are concerned, the
approximate treatment of the platinum core via a
relativistic pseudopotential results in a considerable saving of computational time without any
signi®cant loss of accuracy.
6. Summary and conclusions
The adsorption of urea and ureylene on a
Pt(1 0 0) single crystal electrode has been studied
using two di€erent DF based quantum chemistry
approaches (AE scalar-relativistic and pseudopotential) as well as reasonably large cluster model
representations of di€erent active sites of Pt(1 0 0).
161
The choice for urea and ureylene is related to the
fact that electrochemical experiments suggest urea
to lose two H atoms in the adsorption process
leading to adsorbed ureylene. Four di€erent adsorption sites have been modelled and the geometry, the interaction energy and the vibrational
frequencies of both adsorbed species have been
calculated. In the present study, dissociative urea
adsorption ± with two removed hydrogen atoms to
adsorbed as atomic hydrogen on Pt(1 0 0) ± is
found to be the most favourable adsorption process. Thus the present study corroborates previous
interpretations derived from electrochemical experiments [11]. The bridge-hollow site, schematically shown in Fig. 2d, turns out to be the most
stable adsorption complex of ureylene. Furthermore, calculated stretching frequencies of the C±O
vibrations of adsorbed urea and ureylene, also at
di€erent sites, exhibit similar values, all of them
rather close to the value obtained in the in situ
FTIR experiment. Hence, the calculated vibrational frequencies alone do not provide enough
information for deciding whether the adsorbed
species is urea or ureylene and even less on the
nature of the adsorption site. Yet, the cluster
model calculations permit an assignment of the
adsorbed species and the active site based on calculated interaction energies. Interestingly, for the
most stable species, adsorbed ureylene, at the most
favourable site, the bridge-hollow site, the calculated value for the C±O stretching frequency is
closest to the experimental value.
To summarise, the relative stability of the
di€erent adsorbed species and the vibrational
frequency analysis support the hypotheses by
Climent et al. [11] that urea adsorbs as ureylene,
that this species is bonded to the surface through
the two nitrogen atoms, and that adsorption occurs at the bridge-hollow site. Nevertheless, it is
worth pointing out that the hypotheses suggested
in the work of Climent et al. [11] were mostly
based in chemical intuition and on invoking the
so-called surface-complex analogy whose success is
by far not guaranteed in general [4]. Here, it was
demonstrated that ``®rst principles'' calculations
lead to the same ®nal picture, advocating that
theoretical investigations on models of adsorbates
at surfaces provide an alternative and more secure
162
M. Garcõa-Hern
andez et al. / Surface Science 471 (2001) 151±162
way to interpreting in situ electrochemical interfaces at a molecular level.
Acknowledgements
The authors are indebted to Prof. Juan Feliu
and Dr. Victor Climent for bringing the problem
of urea adsorption to their attention. M. GarcõaHernandez is grateful to the ``Generalitat de Catalunya'' for a predoctoral grant. This work has
been supported by Deutsche Forschungsgemeinschaft, Fonds der Chemischen Industrie, Spanish
``Ministerio de Educaci
on y Ciencia'', project CICyT PB98-1216-C02-01, and ``Generalitat de Catalunya'', project 1999-SGR-00040.
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