Author's personal copy
Chemical Physics Letters 541 (2012) 32–38
Contents lists available at SciVerse ScienceDirect
Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
The effect of coadsorbed water on the stability, configuration and interconversion
of formyl (HCO) and hydroxymethylidyne (COH) on platinum (1 1 1)
Líney Árnadóttir a,⇑, Eric M. Stuve b, Hannes Jónsson c
a
b
c
School of Chemical, Biological and Environmental Engineering, 103 Gleeson Hall, Oregon State University, Corvallis, OR 97331-2702, USA
Department of Chemical Engineering, University of Washington, P.O. Box 351750, Seattle, WA 98195-1750, USA
´k, Iceland
Faculty of Science, VR-II, University of Iceland, 107 Reykjavı
a r t i c l e
i n f o
Article history:
Received 13 February 2012
In final form 8 May 2012
Available online 23 May 2012
a b s t r a c t
Two forms of the methanol electro-oxidation intermediate with stoichiometry C:H:O, COH (hydroxymethylidyne) and HCO (formyl), on Pt (1 1 1) with and without coadsorbed water were studied using
density functional theory calculations. The structure, adsorption energy and stability with respect to dissociation were calculated. Both HC@O and C–OH were stable on clean Pt (1 1 1) and with a single coadsorbed water molecule, while only the HCO configuration was stable in the presence of a whole water
layer. The vibrational modes of HC@O on a bridge site showed no mode around 1700 cm!1 characteristic
of C@O stretch making it hard to distinguish it from C–OH.
! 2012 Elsevier B.V. All rights reserved.
1. Introduction
Methanol oxidation and its reaction intermediates have been
studied intensively as part of an effort to find a good catalyst and
operating conditions for direct methanol fuel cells and methanol
synthesis. Various studies of methanol oxidation on Pt and methanol production on Ni (1 1 1) have suggested a reaction intermediate
of C:H:O stoichiometry [1–14]. Charge transfer measurements
show that the intermediate requires three electrons to oxidize,
hence the C:H:O stoichiometry. Little else is known about the intermediate, however, as it is thought to be reactive, and hence transient in nature, and present in small surface concentrations
during methanol oxidation. Measurements with infrared spectroscopy revealed the presence of an intermediate, but due to low
surface concentrations and limitations in the measurable spectrum,
these results have not provided conclusive structural information of
the intermediate. Experimental evidence for the existence of
hydroxymethylidyne (C–OH) or formyl (HC@O) on platinum has
been presented [15,16]. Both species have also been studied previously with density functional theory (DFT) calculations [3–5,17].
Hydroxymethylidyne chemisorbs through the carbon atom onto a
fcc hollow site on Pt (1 1 1) [6,17]. Theoretical studies do not agree
on the preferred adsorption site for formyl on clean Pt (1 1 1). Formyl binds through the carbon atom to either an atop site [17] or a
bridge site [3,5]. Most theoretical studies investigated adsorption
on clean Pt (1 1 1), but in a DFT study by Okamoto et al. [17] of a
⇑ Corresponding author.
E-mail addresses: [email protected] (L. Árnadóttir), stuve@u.
washington.edu (E.M. Stuve), [email protected] (H. Jónsson).
0009-2614/$ - see front matter ! 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cplett.2012.05.024
Pt (1 1 1)/water interface, hydroxymethylidyne (COH) was found
to be more stable than formyl (HCO) by 0.87 eV.
2. Computational method
All calculations were done with the Vienna Ab-initio Simulation
Package (VASP), a plane-wave implementation of DFT with the
PW91 functional [18–22]. Interaction between ions and electrons
are described by ultra-soft Vanderbilt pseudopotentials (US-PP)
[23,24]. A cut-off energy of 396 eV (29 Ry) was used for all the calculations, and the Brillouin zone was sampled using a 2 " 2 " 1
Monkhorst–Pack k-point mesh. The calculations were considered
to have converged when the maximum forces on all of the relaxed
atoms were less than 0.05 eV/Å for nudged elastic band (NEB) and
full water layer calculations, and less than 0.005 eV/Å for all other
systems. The minimum energy paths (MEP) were determined by
the climbing image, nudged elastic band method (CI-NEB) [25,26].
The Pt (1 1 1) surface consisted of 36 atoms in a rectangular cell,
12 atoms per layer in three layers. The surface was taken from a
previously relaxed Pt-bulk and relaxed again keeping only the bottom layer fixed. The calculated lattice constant for platinum was
found to be 3.98 Å (compared to 3.924 Å found experimentally
[27]). The spacing in the z-direction between the slab and its periodic images was 15 Å. Only the adsorbates were allowed to relax
during calculations. Frequencies of normal vibrational modes were
calculated from the eigenvalues of a Hessian matrix with displacements of 0.01 and 0.005 Å. For the HCO on bridge configuration
both step sizes gave the same result, but the atop configuration
exhibited an 8% difference at the lower frequencies (<600 cm!1)
for 0.005 vs. 0.01 Å displacement steps. Vibrational frequencies
Author's personal copy
33
L. Árnadóttir et al. / Chemical Physics Letters 541 (2012) 32–38
listed in Table 2 are calculated with an 0.01 Å displacement. The
adsorption energy Eads is defined as the difference between the total energy of the adsorbate on the surface Esys and a reference state
of CO and 1/2 H2 in the gas phase at an infinite distance from the
surface and the clean surface Esurf. As an example, consider the
adsorption energy of COH on a platinum surface site (S) formed
by the overall chemical reaction
1
S þ CO þ H2 ! S—COH ECOH;ads ¼ ES—COH ! E0
2
ð1Þ
0
where E is the reference energy, given by
1
Eo ¼ Esurf þ ECO þ EH2 :
2
ð2Þ
This reference state allows for direct comparison between the
adsorption energy of the COH and HCO species. The quantity Eads
defined in Eq. (1) is an energy of state of COH relative to the CO
and H2 in the gas phase. The adsorption energy of a molecule
Ea.m, Eq. (3), is often defined as the energy difference between
the adsorbed species and the same species away from the surface.
Ea:m ¼ Esys ! ðEC:H:O þ Esurf Þ:
ð3Þ
The adsorption energy calculated in this manner is also listed in
Tables 1 and 3. Interaction with coadsorbed water generally increases the adsorption energy. This increase in adsorption energy,
herein referred to as DEhyd, is defined as the difference in energy
between a hydrated complex on the surface and that of the constituent adsorbed molecules in their minima configuration with no
interactions among the molecules. As an example, consider water
coadsorption with HCO, for which the reaction and the hydration
energy DEhyd are given by:
H2 Oads þ HCOads ! HCO ! HOHads
DEhyd ¼ EHCO!HOHads ! ðEH2 O;ads þ EHCO;ads Þ:
ð4Þ
3. Results
3.1. The C:H:O intermediate on Pt (1 1 1)
Table 1 shows the two different structures at their lowest energy adsorption sites: an atop site for HCO and a fcc site for COH,
and lists different adsorption sites, adsorption energies and distances from the Pt surface plane for the two configurations. The
lowest energy adsorption site for COH, at a fcc hollow site, has significanlty lower energy than the most stable HCO configuration at
an atop site. These adsorption configurations are in good agreement with previous calculations of hydroxymethylidyne adsorption on Pt (1 1 1) [6,17], as listed in Table 1. The calculations by
Greeley et al. [6] are for a slightly smaller system, 1/9 coverage
vs. 1/12 which could explain the slight difference in adsorption energy. Previous theoretical studies do not agree on the preferred
adsorption site for formyl on clean Pt (1 1 1), however. Formyl
can bind to the Pt surface through the carbon atom to either an
atop site [17] or a bridge site [3,5]. In this Letter the atop site
was found to be slightly lower in energy and therefore more stable,
increasing the Monkhorst–Pack k-point mesh in steps to
(6 " 6 " 1) did not change that finding. A stronger bonding energy
was expected for COH than for HCO due to the surface–adsorbate
interaction that occurs through a triply bonded carbon in COH in
a hollow site vs. a singly bonded carbon for HCO at an atop site.
Experimental evidence for the C:H:O species includes infrared
(IR) spectroscopy [1,9,12], isotope studies [8] and charge balance
measurements [13,28]. Spectroscopic studies for the C:H:O species
are difficult to interpret, as the signal for the C–H stretching mode
is weak, and the O–H mode of COH is hard to distinguish from the
OH stretching frequencies of water. Table 2 lists vibrational frequencies for HCO and COH determined both experimentally and
theoretically.
No reference for the COH species was found in the literature so
the vibrational frequencies for the COH component of methyl alcohol (H3COH) from the NIST database [29] are listed for comparison.
The experimental data for HCO are all for gas phase, but a down-shift
of about 100 cm!1 can be expected upon adsorption [12]. The calculated frequencies match the experimental data reasonably well.
In Table 2 calculated vibrational frequencies are listed for HCO
and COH adsorbed on various sites on Pt (1 1 1). In general one
would expect a C@O double bond stretch for HCO around
1750 cm!1 and a C–O single bond stretch for C–OH around
1250 cm!1. When HCO is adsorbed on a bridge site the CO end of
the molecule is tilted towards the surface; the oxygen atom is
0.73 Å closer to the surface relative to the atop site and close
enough to interact with the surface. Although the adsorption
Table 1
Adsorption sites and energies for HCO and COH on Pt (1 1 1). O–Pt and C–Pt are the respective distances of oxygen and carbon from the Pt (1 1 1) plane. Oxygen is shown in red,
carbon in blue and hydrogen in black.
Species site
HCO top
HCO bridge
COH fcc
COH hcp
!Eads (eV)
!Ea.m. (eV)
!Ea.m. [6] (eV)
1.34
2.40
2.36
1.27
2.33
1.87
4.70
4.45
1.75
4.64
2.70
2.01
1.97
1.57
2.53
1.20
2.53
1.20
O–Pt (Å)
C–Pt (Å)
Top view
Side view
Author's personal copy
34
L. Árnadóttir et al. / Chemical Physics Letters 541 (2012) 32–38
Table 2
Vibrational frequencies for HCO and COH. The frequencies are listed in units of wavenumbers, cm!1. Only frequencies higher than 250 cm!1 are listed. Adsorption sites are listed
for calculations, while surface or gas phase is listed for experimental results. No experimental reference for the COH species in gas phase was found, so the vibrational frequencies
for the COH component of methyl alcohol (H3COH) from the NIST database [29] are listed for comparison. For HCO on bridge from reference [4] d(HCO) refers to bending, in-plane
(CH), and w(HCO) to wagging, out-of-plane (CH). The experimental data for HCO are all in gas-phase, but a down-shift of about 100 cm!1 can be expected upon adsorption [12].
(m = stretch, d = bend, and w = wag).
Species
Ads. site
m(OH)
m(CO)
d(COH)
m(C–Pt)
m(C–Pt)
m(C–Pt)
Notes
COH
COH
COH [6]
COH [12]
COH [41]
H3COH [29]
fcc
hcp
fcc
Surface
Surface
Gas
3635
3633
3704
1098
1093
1100
540
523
523
446
447
503
357
352
385
3681
1271
1268
1292
1270
1425
1345
1320
1033
This study
This study
GGA-PW91
IR
IR
IR
m(CH)
m(CO)
d(HCO)
w(HCO)
m(C–Pt)
m(C–Pt)
Notes
HCO
HCO
HCO
HCO
HCO
HCO
HCO
Bridge
Top
Top
Bridge
Gas
Bridge
Top
2964
2858
2692
3103
2596
2991
2801
1243
1717
1785
1184
1380
1249
1710
1135
1147
959
1205
1066
1158
1173
748
795
804
824
529
524
491
600
367
375
265
461
747
832
529
492
336
254
This study
This study
GGA-PW91
B3LYP
IR
This study
This study
[6]
[4]
[29] state B
under water bilayer
under water bilayer
energy is higher for the atop site than for the bridge site, the interaction can be seen in the vibrational frequency. For HCO on a bridge
site a vibration frequency around 1250 cm!1 for C@O is found, as
opposed to a 1750 cm!1 mode, suggesting a carbon–oxygen single
bond instead of a double bond. This is important, as the absence
of a 1750 cm!1 mode has been interpreted in favor of a COH vs.
an HCO intermediate on the basis of infrared spectroscopy [12].
Greeley et al. [6] also calculated HREELS intensities for some of
the vibrational frequencies and estimated their relative intensities.
The relative intensity of the m(C–Pt) mode at 491 cm!1 was 0.96,
with much smaller relative intensities for the w(HCO) and m(CH)
modes: 0.14 for the 804 cm!1 peak and 0.01 for the 2692 cm!1
peak, respectively. Comparing these intensity estimates with the
vibrational frequency assignments in Table 2 suggests that the
HCO wagging and CH stretching peaks will be very weak in a
HREELS spectrum (for dipole scattering), thereby confirming the
difficulty in identifying these peaks with IR spectroscopy.
3.2. COH/HCO with a single water molecule
Adding just one water molecule to the C:H:O structure adds substantial complexity to the system: various configurations of the two
C:H:O structures and a single water molecule were calculated, but
only three were found to be stable. Table 3 lists energies of adsorption and hydration, bond lengths in the adsorbed complexes and
depictions of the adsorbate configuration. (Note that, in the table
and henceforth, a water molecule is denoted as ‘w’ for simplicity.
Its location in the formula indicates the type of hydrogen bond.
For example, for w–HCO the water molecule is a hydrogen bond
acceptor: the oxygen of the water interacts with the hydrogen of
HCO. Conversely, for HCO–w water is a hydrogen bond donor: the
hydrogen of water interacts with the oxygen of HCO).
For all species studied, hydration of C:H:O leads to changes in
adsorbate configuration, as seen by comparison of Tables 3 and
1. First, a single water molecule (not shown) adsorbs parallel to
the surface on an atop site [30]. In the w-HCO complex the HOH
plane of water tilts away from the surface by approximately 20".
The HCO species rotates within the HCO plane about an axis
through the carbon atom, maintaining its alignment along [1 1 1],
such that the oxygen atom moves away, and hydrogen atom moves
toward, the surface. The result is a complex with a slightly bent
hydrogen bond wherein water acts as a donor molecule.
In the HCO–w (top/bridge) complex water moves and reorients a
substantial amount from its monomer configuration. The molecule
moves from the atop site to an approximate bridge site. The
molecular plane turns nearly perpendicular to the surface (about
25" away from perpendicular) with one hydrogen atom positioned
near the surface approximately over a threefold hollow site and the
other hydrogen positioned away from the surface. The HCO species
rotates from along the [0 0 1] direction to just 5" away from [1 1 1].
The result is a linear hydrogen bond, wherein water acts as an
acceptor molecule somewhat distant from the surface.
In the COH–w complex water shifts to an approximate hollow
site, and its molecular plane rotates substantially away from the surface. As in the HCO–w complex one hydrogen is positioned toward
the surface and one is positioned away from the surface. The COH
species retains its adsorption site (hollow) and azimuthal alignment
of the C–H bond (along [0 0 1]). The result is an approximately linear
hydrogen bond, wherein water acts as an acceptor molecule substantially distant from the surface, more so than in either of the
two previous cases.
Coadsorbed water stabilizes all of the C:H:O structures studied.
Hydrogen bonding and changes in the adsorption configuration of
the hydrated species bring about the increased stability. The
energy change of hydration DEhyd accounts for both hydrogen
bonding and changes in adsorption energy, as defined in Eq. (4).
For w–HCO the energy change of hydration is !0.24 eV as listed
in Table 3, or a 0.24 V increase in stability. This value is approximately the same as a hydrogen bond in water [31], suggesting that
the hydrogen bond is the primary source of the increased stability.
Conversely, the HCO–w complex shows essentially no hydration-induced stability, as indicated by its hydration energy of
0.04 eV. The increase in adsorption energy relative to HCO–w, from
!1.34 to !1.60 eV, is nearly accounted for by the adsorption energy of !0.30 eV for water. The small (positive) hydration energy
for the HCO–w complex could result from an absent hydrogen
bond or a hydrogen bond strength that offsets the loss of binding
energy of either water or HCO to the surface. The configuration
shown in Table 3 clearly shows evidence for a hydrogen bond, so
the latter explanation, mutually offsetting hydrogen bond and loss
of binding energies, holds.
It is interesting to note that HCO in both hydrated forms has
nearly the same configuration, one that is similar to non-hydrated
HCO at its atop site. The main difference between the two hydrated
forms of HCO is that water is close to its monomer configuration in
w–HCO, while substantially altered from its monomer configuration in HCO–w. It is straightforward to infer that water bonds more
strongly to the surface in w–HCO, and more weakly in HCO–w. This
observation agrees with the mechanism of mutually offsetting
hydrogen bond and loss of binding energies discussed above.
Author's personal copy
35
L. Árnadóttir et al. / Chemical Physics Letters 541 (2012) 32–38
Table 3
Adsorption configurations, sites and energies of C:H:O coadsorbed with a water molecule, denoted w, on Pt (1 1 1). The single water hydration energy of C:H:O is listed as DEhyd
(see Eq. (4)). For all of these calculations the adsorption energy of a water monomer was taken as 0.3 eV, as found in [30].
Species Site
w–HCO top/top
HCO–w top/bridge
COH–w fcc/top
!Eads (eV)
!Ea.m. (eV)
!DEhyd (eV)
1.88
3.29
0.24
1.60
3.01
!0.04
2.57
5.94
0.45
O–Pt (Å)
C–Pt (Å)
Ow–Pt (Å)
Ow–H–O (Å)
2.88
1.98
2.19
2.56a
2.69
2.03
3.18
2.05
2.52
1.24
3.23
1.41
Top view
Side view
a
Ow–C–O/Å.
3.3. COH/HCO and water bilayer
Although both COH and HCO are more stable coadsorbed with a
single water molecule than on their own, the same is not true
when these two configurations are inserted into a full bilayer of
water.
studies suggested an ice-like water structure
pffiffiffi pEarlier
ffiffiffi
ð 3 " 3Þ ! R30' on Pt (1 1 1) [15,32], but a rotated two-dimensional water layer was reported on Pt (1 1 1) by Glebov et al. [33].
More recently, He scattering, low energy electron diffraction
(LEED) and scanning
microscopyp
(STM)
found
pffiffiffiffiffiffi tunneling
pffiffiffiffiffiffi
ffiffiffiffiffiffi p
ffiffiffiffiffiffi monolayer
'
structures of ð 37 " 37Þ ! R25:3' and
pffiffiffiffiffiffið 39 "
pffiffiffiffiffi39
ffi Þ ! R16:1 at
different temperatures [34–38]. The 37 and 39 structures require substantially larger simulations cells: 26 water molecules
over 37 platinum atoms for the former and 28 waters over 39 platinum
for the latter. Here, we use the ‘ice-like’ H-down
pffiffiffi atoms
pffiffiffi
ð 3 " 3Þ ! R30' bilayer with eight water molecules, which, in
our estimation, makes a suitable model of the water bilayer. The
adsorbate molecule sits inside the hexagonal ring formed by the
waterlayer.
The calculations show that in a waterlayer COH will dissociate
to form COads and Hsol. CO adsorbs at a fcc hollow site and the
hydrogen atom goes into the water bilayer, traveling through via
Grotthuss-hopping diffusion (Figure 1). Even when one water molecule was removed from the bilayer to make more room for the
COH structure, it still dissociated upon relaxation. These results
somewhat contradict the findings of Okamoto et al. [17], who
found COH to be more stable than HCO in the presence of water.
Dissociation of COH to form a solvated proton only occurs when
many water molecules are around the COH admolecule, and
although Okamoto’s et al., calculations include bulk-like water,
the hydrogen atoms on the surface might be reducing the solvation
of the adsorbate and therefore preventing dissociation. Okamoto’s
et al., results are therefore more comparable with our calculations
of C:H:O single water interactions, where we also find the COH–
water to be more stable than the HCO-water configuration.
The HCO structure, on the other hand, is most stable on a bridge
site; the water molecule closest to the adsorbed structure shifts up
by about 1.4 Å to make room for the adsorbed species. The hydrogen bonds of the shifted water molecules in the bilayer are elongated from 1.8 to 2.2 Å. In these calculations the water bilayer
was first minimized and the HCO structure then added to the surface and the system minimized again. Several cycles of molecular
Figure 1. On the left, COH, which has dissociated into CO and H. The hydrogen
initially belonging to the COH structure is identified by lighter edges. Note that the
proton has started to diffuse away. On the right, HCO on a bridge site under a water
bilayer.
Author's personal copy
36
L. Árnadóttir et al. / Chemical Physics Letters 541 (2012) 32–38
dynamics calculation at 25 K for 1 ps followed by relaxations of all
adsorbed molecules were run to look for alternative minima.
According to these calculations it is energetically more favorable
for the bilayer to bend upwards to make room for the HCO species
than for any water molecule to break out of the network to bond to
the HCO molecule.
Under a water bilayer the HCO structure is most stable on a
bridge site, and the higher vibrational frequencies of that structure
were found to be 2991, 1249, 1158 and 747 cm!1, similar to the
case of HCO adsorbed alone on a bridge site, and with no frequency
around 1750 cm!1. This further suggests that under aqueous conditions the absence of a vibrational frequency around 1750 cm!1
cannot distinguish between COH and HCO on the surface; the instability of COH under a water bilayer further suggests that HCO will
be the only species of H:C:O stoichiometry in aqueous conditions.
The dissociation of COH in a water bilayer was surprising considering its stability without water and at lower water coverage.
A CI-NEB calculation was used to study the dissociation of COH,
with water present both as reactant and product in more detail.
The reaction under consideration is
COH þ H2 O ! CO þ H2 O þ H:
ð5Þ
Snapshots from the reaction path of reaction (5) are shown in
Figure 2.
The energy difference between reactants and products here is
very small, only about 0.015 eV, and the activation barrier is only
0.02 eV. Under these conditions it is hardly reasonable to consider
the reactants, COH and H2O, and the products; COads, Hads, and
H2O; as distinct configurations, despite the accumulated movement of the atoms between the two states of 1.75 Å. With more
water a hydrogen atom can structurally diffuse through the water
layer away from the COH molecule, leaving CO on the surface. This
result strongly suggests that, in the presence of a water bilayer,
only HCO, CO, and H are stable on Pt (1 1 1).
3.4. Interconversion of COH and HCO
The interconversion between COH and HCO was studied on Pt
(1 1 1). On the clean surface, interconversion between the two
forms was found to go through COads and Hads intermediates. The
activation barrier for going from HCO to COH was 0.33 eV, but a
deep valley (>1 eV) was found between the two configurations.
The valley represents bridge-bonded COads and atop-bonded Hads
on the surface. The energy difference out of the valley to form
COH was 1.5 eV. The reaction steps and activation barriers (in units
of eV) for the forward and reverse directions are given by
0:33
1:50
1:33
1:02
HCO ! COads þ Hads ! COH:
Figure 2. Snapshots from the COH + H2O? CO + H2O + H reaction path: top-view
and side views. Starting from a COH–H2O configuration, the hydrogen is easily
transferred from COH to the surface and back. The barrier for this reaction path is
insignificant ((0.02 Å), but the accumulated distance is 1.75 Å.
ð6Þ
Figure 3 shows the MEP and snapshots of the reaction pathway
from two angles.
Interconversion between COH and HCO through a stable CO
intermediate is unlikely since the configuration of COads + Hads is
significantly lower in energy than either COH or HCO. The energy
barrier going out of the CO + H well is 1.5 eV on the COH side
Figure 3. Interconversion from HCO to COH on Pt (1 1 1). Interconversion proceeds through a stable COads+Hads intermediate. On the y-axis is energy in eV and the x-axis is
reaction coordinates in Å. The images are snapshots, top and side views, are taken along the MEP. The images correspond to the initial and final configurations and the three
saddle points along the MEP.
Author's personal copy
L. Árnadóttir et al. / Chemical Physics Letters 541 (2012) 32–38
37
Since the COH configuration was found to be unstable in the
presence of a water bilayer, no interconversion calculations were
preformed for this system.
4. Summary
Figure 4. MEP path for the interconversion between HCO and COH including a
single water molecule. The images are snapshots of the molecular configuration at
initial, saddle and final point.
and 1.33 eV on the HCO side. Neither COH nor HCO is therefore
likely to be formed from COads + Hads. The reaction path is complicated, however; more images are needed for better convergence of
the dissociation barriers. It is clear, though, that the HCO ! COH
interconversion will not be facile on a clean Pt (1 1 1) surface.
3.5. Interconversion between COH and HCO on a Pt (1 1 1) surface
including a water molecule
As discussed earlier the adsorption energies of COH and HCO are
significantly enhanced by coadsorption of water. Without any
water the adsorption energy difference between adsorbed COH
and HCO is (0.5 eV, but with coadsorbed water this difference
increases to about 0.7 eV in favor of the COH structure. Figure 4
shows a MEP for the interconversion from H2O–HCO to COH–OH2,
with an energy barrier of 0.64 eV. The flat areas at the beginning
and end represent subtle readjustment of the water molecule
around the adsorbate, points (1–3) and (7–10). These readjustments of the water molecule around the adsorbate cause only small
changes in energy due to weak interaction with the adsorbate.
The large energy difference between COH–w and HCO–w
(0.98 eV) makes the interconversion from COH–w, which is lower
in energy, to HCO–w improbable. Starting from HCO to COH, on
the other hand, looks promising at first glance. The rather high
energy barrier of 0.64 eV makes even this interconversion from
HCO–w to COH–w difficult, though more likely, than the other
way around.
Table 4
Summary of results for HCO and COH adsorption energies, activation energies of
interconversion and interpretation for adsorption on the clean surface, with one
water molecule, and within a bilayer on Pt (1 1 1).
!Eads (eV)
Act. energies (eV)
Clean
With one H2O
HCOtop
HCObridge
COHfcc
More stable
1.34
1.27
1.87
COH
1.88/1.60
HCO ? CO + H
CO + H ? HCO
COH ? CO + H
CO + H ? COH
HCO ? COH
COH ? HCO
Interconversion
0.33
1.33
1.02
1.50
No
2.57
COH–w
0.64
0.98
Unlikely
Bilayer
Dissociates
HCO
Key results are summarized in Table 4. The stability of HCO and
COH on Pt (1 1 1) is strongly affected by coadsorbed water. On a
clean surface and low coverage of water (1/12 ML) the COH structure is more stable than the HCO structure. In the presence of a
water bilayer, on the other hand, HCO is the only stable form, as
the COH configuration dissociates to COads + H; the hydrogen structurally diffuses into the water bilayer even during minimization.
The energy difference between COHads + H2O and COads + H2O + Hads
is very small, and the activation barrier between the two is insignificant. This small energy difference and barrier suggest that the COH
structure can rapidly go back and forth between COads + H2O + Hads
and COHads + H2O; thus, the two structures are virtually indistinguishable. Increasing the amount of water to a full water bilayer
provides a way for the hydrogen to structurally diffuse through
the bilayer network and away from the COH hence forming COads
and hydrogen. That makes the reverse reaction to form COH from
CO and H unlikely at higher water coverage.
All the results for low water coverage suggest a more stable
COH configuration. Although both configurations, HCO and COH,
have been suggested under aqueous conditions, experimental verification is difficult. The vibrational frequencies for the H–C stretch
of HCO have low intensity, and the O–H stretching signal for COH
can be hard to distinguish from the water signal. The CO stretching
signals for the two molecules are expected to be distinguishable,
however. DFT calculation of the vibrational frequencies of COH
on a fcc hollow site and HCO on an atop site show a large difference
in the CO frequency for the two species, 1250 vs. 1750 cm!1, representing a carbon–oxygen single bond and double bond, respectively. For HCO on a bridge site, on the other hand, only the
frequency of a carbon–oxygen single bond is observed both on a
clean surface and under a water bilayer. This makes distinguishing
between bridge HCO and COH even harder. Under a water bilayer
the bridge bond HCO structure is most stable and the carbon–oxygen vibrational frequency for this structure is 1249 cm!1 indicating
a carbon–oxygen single bond. This suggests that, under aqueous
conditions, the absence of a carbon–oxygen double bond vibrational frequency cannot be used to distinguish between COH and
bridge HCO on the surface. The instability of COH under a water bilayer further suggests that a carbon–oxygen single bond vibration
frequency measured under aqueous conditions indicates bridgebonded HCO rather then COH.
The results presented here show how important it can be to
include a layer of water molecules in calculations of surface processes that occur in an aqueous environment, as has been pointed
out previously [39]. The presence of the water layer, however,
makes the interpretation of calculated energetics difficult because
the energy of the hydrogen bonded network is large and small
changes in its structure to accommodate changes in the surface species can significantly affect the energy differences. Furthermore, the
search for optimal configuration of the water molecules is a
challenging global optimization problem. When proton disorder is
included in the water bilayer, a number of different sites are formed
as has been pointed out in the context of water admolecule diffusion on the ice Ih (0 0 0 1) surface [40]. Because of the long range
electrostatic interactions, no two sites are then the same. The proper simulation of the aqueous environment at the surface in the presence of adsorbed species is a major challenge that requires both
further development of methodology and large computational
effort.
Author's personal copy
38
L. Árnadóttir et al. / Chemical Physics Letters 541 (2012) 32–38
Acknowledgments
This research was supported by the Office of Naval Research, the
National Science Foundation and The Icelandic Research Fund. A
portion of the research was performed as part of an EMSL Scientific
Grand Challenge project at the W.R. Wiley Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored
by the US Department of Energy’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. PNNL is operated for the Department of Energy by Battelle.
References
[1] B. Beden, S. Junato, J.M. Leger, C. Lamy, J. Electroanal. Chem. 238 (1987) 323.
[2] B. Beden, J.M. Leger, C. Lamy, Modern Aspects of Electrochemistry, vol. 22,
Plenum Press, New York, pp. 97–264.
[3] S.K. Desai, M. Neurock, K. Kourtakis, J. Phys. Chem. B 106 (2002) 2559.
[4] J. Gomes, J. Gomes, J. Electroanal. Chem. 483 (2000) 108.
[5] J. Greeley, M. Mavrikakis, J. Am. Chem. Soc. 124 (2002) 7193.
[6] J. Greeley, M. Mavrikakis, J. Am. Chem. Soc. 126 (2004) 3910.
[7] A. Hamnett, Catal. Today 38 (1997) 445.
[8] T. Iwasita, E. Santos, W. Vielstich, J. Electroanal. Chem. 229 (1987) 367.
[9] S. Junato, B. Beden, F. Hahn, J.M. Leger, C. Lamy, J. Electroanal. Chem. 237
(1987) 119.
[10] J. Kua, W.A. Goddard, J. Am. Chem. Soc. 121 (1999) 10928.
[11] J.M. Leger, J. Appl. Electrochem. 31 (2001) 767.
[12] T. Iwasita, F. Nart, J. Electroanal. Chem. 317 (1991) 291.
[13] S. Sriramulu, T.D. Jarvi, E.M. Stuve, J. Electroanal. Chem. 467 (1999) 132.
[14] I.N. Remediakis, F. Abild-Pedersen, J.K. Norskov, J. Phys. Chem. B 108 (2004)
14535.
[15] M.A. Henderson, Surf. Sci. Rep. 46 (2002) 5.
[16] M.I. Lopes, I. Fonseca, P. Olivi, B. Beden, F. Hahn, J.M. Leger, C. Lamy, J.
Electroanal. Chem. 346 (1993) 415.
[17] Y. Okamoto, O. Sugino, Y. Mochizuki, T. Ikeshoji, Y. Morikawa, Chem. Phys. Lett.
377 (2003) 236.
[18] G. Kresse, J. Hafner, Phys. Rev. B 47 (1993) 558.
[19] G. Kresse, J. Hafner, Phys. Rev. B 49 (1994) 14251.
[20] G. Kresse, J. Furthmuller, Comput. Mater. Sci. 6 (1996) 15.
[21] G. Kresse, J. Furthmuller, Phys. Rev. B 54 (1996) 11169.
[22] J.P. Perdew, Y. Wang, Phys. Rev. B 45 (1992) 13244.
[23] D. Vanderbilt, Phys. Rev. B 41 (1990) 7892.
[24] G. Kresse, J. Hafner, J. Phys. Condens. Matter 6 (1994) 8245.
[25] G. Henkelman, B.P. Uberuaga, H. Jónsson, J. Chem. Phys. 113 (2000) 9901.
[26] G. Henkelman, H. Jónsson, J. Chem. Phys. 113 (2000) 9978.
[27] D. Lide (Ed.), Handbook of Chemistry and Physics, 78th ed., CRC Press, Inc.,
New York, 1997–1998.
[28] S. Wilhelm, T.. Iwasita, W. Vielstich, J. Electroanal. Chem. 238 (1987)
383.
[29] P. Linstrom, W. Mallard (Eds.), NIST Chemistry WebBook, NIST Standard
Rerence Database Number 69, National Institute of Standards and Technology,
Gaithersburg MD, 2005, p. 20899. <http: webbook.nist.gov>.
[30] L. Árnadóttir, H. Jónsson, E. Stuve, Surf. Sci. 602 (2010) 1978.
[31] S.J. Suresh, V.M. Naik, J. Chem. Phys. 113 (2000) 9727.
[32] P. Thiel, T.E. Madey, Surf. Sci. Rep. 7 (1987) 211.
[33] A. Glebov, A.P. Graham, A. Menzel, J.P. Toennies, J. Chem. Phys. 106 (1997) 9382.
[34] P.J. Feibelman, N.C. Bartelt, S. Nie, K. Thuermer, J. Chem. Phys. 133 (2010).
[35] A. Hodgson, S. Haq, Surf. Sci. Rep. 64 (2009) 381.
[36] S. Standop, A. Redinger, M. Morgenstern, T. Michely, C. Busse, Phys. Rev. B 82
(2010) 161412.
[37] G. Zimbitas, S. Haq, A. Hodgson, J. Chem. Phys. 123 (2005).
[38] S. Haq, J. Harnett, A. Hodgson, Surf. Sci. 505 (2002) 171.
[39] T.R. Mattsson, S.J. Paddison, Surf. Sci. 544 (2003) L697.
[40] E.R. Batista, H. Jónsson, Comput. Mater. Sci. 20 (2001) 325.
[41] R.J. Nichols, A. Bewick, Electrochim. Acta 33 (1988) 1691.