HYDROGEN ADSORPTION AND DIFFUSION ON A Pt(111) CLUSTER

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Surface Review and Letters, Vol. 15, No. 3 (2008) 319–327
c○ World Scientific Publishing Company
1. Introduction
HYDROGEN ADSORPTION AND DIFFUSION
ON A Pt(111) CLUSTER
J. M. MARCHETTI ∗ ,E.GONZ ÁLEZ†,‡ ,P.JASEN †,§ ,
G. BRIZUELA † and A. JUAN †,
∗ Planta Piloto de Ingeniería Química (UNS-CONICET),
Camino La Carrindanga Km. 7, 8000 Bahía Blanca, Argentina
† Departamento de Física, Universidad Nacional del Sur. Av. Alem 1253,
8000 Bahía Blanca, Argentina
‡ Departamento de Ing. Mecánica,
Universidad Tecnológica Nacional — Facultad Regional Bahía Blanca,
11 de Abril 461, 8000 Bahía Blanca, Argentina
§ Departamento de Ing. Eléctrica,
Universidad Tecnológica Nacional — Facultad Regional Bahía Blanca,
11 de Abril 461, 8000 Bahía Blanca, Argentina
cajuan@criba.edu.ar
Received 12 October 2007
The interaction of hydrogen with a platinum (111) cluster using the atom superposition and
electron delocalization–higher binding ASED-TB quantum calculation method was studied. The
metal surface was represented by a Pt cluster of seven layers. The effect of hydrogen on this
metal substrate was studied by the analysis of density of states and crystal orbital overlap
populations curves. The energy surface plots allow us to find a possible diffusion path through
the cluster from one side to the other. The Pt–Pt metal bond is weakened during H adsorption
and diffusion. The main components in the Pt–H bond are the Pt 6s (31%), 6p (26%), and
5dxz (16%) orbitals.
Keywords: Hydrogen; surface; platinum.
The interaction of hydrogen with different metals
is an important research area due to the influences
of impurities on the lattice parameters and the
electronic and mechanical properties of the solids.
Hydrogen–metal interactions are very important to
understand the mechanism of hydrogen embrittlement
and stress corrosion cracking of numerous
metal and alloys. 1–10
Platinum and its alloys are the preferred catalyst
in many industrial processes, especially for
319
hydrogenation of olefins and dehydrogenation of
paraffin’s, as well as for many others chemical
reactions. 11 Platinum has also a lot of catalytic
applications and is widely used as a car catalytic
converters and as a fuel cell electrodes. 12 Different
applications for platinum have been discovered and
recently new applications have been discovered. 13–15
Theadsorptionofatomichydrogenonanymetal
electrode or surfaces is one of the most relevant subjects
in electrochemistry, and it has been a matter
of study in recent years. 16 The overall reaction for the

March 19, 2008 14:0 01141
320 J. M. Marchetti et al.
formation of the molecular hydrogen is expressed by
the following equation 17 :
2H3O + +2e − → H2 +2H2O
2H2O+2e − → H2 +2OH
in acidic solution (1)
in neutral and alkaline solutions (2)
It is well-known that the overall reaction for
the formation of molecular hydrogen consists of two
steps. In the first step, adsorbed hydrogen is formed
from H3O + :
H3O + +M+e − → MHads +H2O (3)
H2O+M+e − → MHads +OH −
(4)
where M represents the metal surface.
In the second step, the adsorbed hydrogen is
removed from the electrode surface (M), either in a
chemical reaction (recombination or Tafel reaction):
2MHads → H2 +2M (5)
or in an electrode reaction (electrochemical deposition,
Heyrovsky’ reaction):
MHads +H3O + +e − → H2 +H2O+M (6)
MHads +H2O+e − → H2 +OH − +M (7)
After surface reactions, hydrogen diffuses to
the bulk. The interaction of hydrogen with platinum
surfaces has been deeply investigated, from an
experimental as well as from a theoretical point of
view. 18–21 Our study start with the interaction of
hydrogen with a Pt(111) surface and then with the
bulk of the metal.
2. Computational Method
The calculation of this work has been performed
using the ASED-TB method 22–25 ahybridmodification
of the Hückel method. The changes on the
extended Hückel molecular orbital method (EHMO)
was implemented with the YAeHMOP package. 21
The ASED theory is based on a physical model of
molecular and solid electronic charge density distribution
functions. 17,22–25
The ASED-MO method is semiempirical and
makes reasonable predictions of the molecular and
electronic structure. The EHMO method in its original
form is not able to optimize geometry’s correctly
as it lacks repulsive electrostatic interactions.
This deficiency can be overcome by introducing a
two-body electrostatic correction term. 23
The adiabatic total energy values were computed
as the difference between the electronic energy (E) of
the system when the impurity atom/fragment is at
finite distance within the bulk and the same energy
when the atom/fragment is far away from the solid
surface.
The hydrogen absorption energy can be expressed
as follows:
∆Eabs total = E(PtmH) − E(Ptm)
− E(H) + Erepulsion, (8)
where m is the size of the cluster (in our case, 61
metallic atoms).
The bulk Pt metal has a fcc structure with lattice
parameter a =3.92 ˚A and a nearest neighbor
distance of 2.77 ˚A. The Pt cluster has 61 metallic
atoms distributed in seven layers of seven atoms (7,
10, 10, 7, 10, 10, 7). On the surface, H atom was
located at the top site according to the literature
data. 25
To understand the interaction of hydrogen and
the cluster but also with the bulk, we used the concept
of density of states (DOS) and crystal orbital
overlap populations (COOP) curves implemented
with the program YAeHMOP. 21 The DOS curve is
a plot of the number of orbits per unit volume/per
unit energy, the COOP curve is a plot of overlap
population vs. the energy. Its integration upto the
Fermi level gives the total overlap populations of the
bonds.
A seven-layer cluster of Pt(111) has been used
in order to find the minimum energy path for a H
atom crossing from one side to the other. For this
study, simulations have been performed at different
distances outside the cluster as well as inside
in order to establish the minimum energy pathway
for the hydrogen. The first analysis starts on
the bare cluster surface. The influence of H atom
on the Pt DOS and the overlap population will
be compared with that of the clean cluster. Figure
1(a) shows the location of hydrogen on the minimum
for a top view when the hydrogen is on the
surface. Figure 1(b) shows the minimum energy location
when the hydrogen has move inside the cluster.
Figure 1(c) shows the side view of the cluster without
hydrogen.

March 19, 2008 14:0 01141
Fig. 1(a). Hydrogen at the minimum on the surface.
Fig. 1(b). Hydrogen at the minimum inside the bulk.
3. Results and Discussion
First, let us discuss the electronic structure of the
innermost fourth layer of the Pt cluster. In the DOS
of this bulk-like layer (see Fig. 2), the metal d states
form a band between −15 and −7eV. If we look
at the detailed composition of electronic states we
obtain the orbital population d 7.35 s 0.80 p 1.20 ,which
is close to d 8.54 s 0.89 p 1.18 obtained for bulk Pt. Note
that on an average any Pt atom has its s band
approximately 1/3 filled.
Figure 2 (right) is the DOS of the bare neutral
Pt(111) surface cluster model. The position of the
Fermi level shows that most of the d band is filled.
The computed electronic configurations of the surface
and bulk (three-dimensional) Pt atoms are indicated
in Table 1.
Hydrogen Adsorption and Diffusion on a Pt(111) Cluster 321
Fig. 1(c). Side view of the Pt(111) cluster.
The occupation of the valence s orbitals seems
to be slightly lower and that of the p orbitals substantially
greater than that would have been anticipated.
This is a consequence of the low energy of the
6p basis orbitals in our parameter set. The surface
layer of the cluster is negatively charged (relative to
the 4th inner layer), a consequence of the greater
number of interactions (contacts) experienced by a
bulk-like atom. 26 The width of the d band is approximately
8.0 eV for the bare Pt(111) surface and 8.4 eV
for the bulk (three-dimensional solid) thus the bulk
atom states are more spread out. These values are in
agreement with ab initio and semiempirical results
reported in the literature (Wd = 7.9eV). 27,28 The
dispersion of the s and p bands is much larger than
that of the d band, indicating the much more contracted
nature of the d orbitals.
Regarding the bonding, the COOP curves for the
Pt–Pt bulk, surface (average), and subsurface bonds
are similar. The bottom of the d band is metal–metal
bonding and the top is metal–metal antibonding (see
Fig. 3). A similar effect is seen in the s and p bands.
In the cluster, the total overlap population (OP)
for a bond in the innermost layer is 0.322. The OP
between surface Pt atoms is 0.385, and between a

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322 J. M. Marchetti et al.
Fig. 2. DOS curves for Pt(111): total (a), projected DOS on a Pt atom at the surface and on a Pt atom at the bulk
(b), projected on a Pt atom at the surface (c), and on a Pt atom at the bulk (d).
Table 1. Electron densities, overlap population (OP), charges for a seven-layer
cluster of Pt(111).
Electron Density
Atom s p d Bond OP Distance, ˚A
Ptsup 0.80 1.20 7.35 Pt–Pt 0.385 2.77
Pt bulk 0.89 1.18 8.54 Pt–Pt 0.386 2.77
Ptsup–Pt bulk Pt–Pt 0.386 2.77
Fig. 3. COOP curves for Pt(111): for Pt–Pt first neighbor with both atoms from the surface (a), Pt–Pt bond with
one atom from the surface and the other from inside the cluster (blue) (b), Pt–Pt bond with both atoms bulk (c).

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Pt atom of the first and one of the second layer is
0.384. There are fewer nearest neighbors (6) of a
surface atom, compared with the inner atoms (12),
the decrease in coordination reduces the number
of overlaps available to an atoms and this eventually
controls both the bandwidth and the total OP
value.
After analyzing the clean Pt, we introduced a
hydrogen atom into the system. There are many
studies of atomic H on a metal surface, based on
LEED or HREELS measurements. 29,30 The H atom
Hydrogen Adsorption and Diffusion on a Pt(111) Cluster 323
usually occupies a multicenter coordination site on
transition metal surfaces such as a threefold or
fourfold site. Watson et al. 31 found that the adsorption
energies of H over Pt(111) was similar for atop,
bridge, hcp hollow, and fcc hollow on a three-layer
slab of platinum using DFT calculations. A similar
result was obtained by Papoian et al. 32 and Légaré, 16
using DFT calculations based on the DACAPO software,
these authors found that H is adsorbed on
Pt(111) surface being the fcc site the most favored
over the hcp site.
Fig. 4. DOS curves for Pt(111): total (a), projected DOS on a Pt atom at the surface (solid line) and on a Pt atom
at the bulk (dotted line) (b), projected on a Pt atom at the surface (c), and on the H atom (d).
Fig. 5. COOP curves for Pt(111): for Pt–Pt first neighbor with both atoms from the surface (a), Pt–Pt bond with
one atom from the surface and the other from inside the cluster (bulk) (b), Pt–Pt bond with both atoms from the bulk
(c), Pt surface–H bond (solid line) and Pt bulk–H bond (dotted line) (d).

March 19, 2008 14:0 01141
324 J. M. Marchetti et al.
As expected, we found no significative change in
the Fermi level. The total DOS is dominated by many
bulk states and surface Pt atoms, so that the changes
are subtle. The presence of hydrogen in the vicinity
of the cluster produce changes on the DOS, this effect
isshowninFig.4.ThetotalDOScurveofPt–His
similar to the cluster without H. The effect extends
only to the Pt first neighbor to H. A small peak at the
bottomofthePtdband(−16 eV) correspond to H 1s
based state interacting with Pt 6s and Pt 6p states.
The bar on the right in the DOS plots indicates the
energy level of the H 1s orbital before interaction.
Table 2. Electron densities overlap population (OP), charges for a seven-layer Pt cluster with
H on the surface and in the bulk between layer 1 and layer 2.
Electron Density
Atom s p d Bond OP Distance, ˚A
Ptsurf 0.78 1.14 7.71 Ptsurf –Ptsurf 0.32 2.77
Ptbulk 0.81 1.13 8.36 Ptsurf–Ptbulk 0.38 2.77
H 1.046 0 0 Ptsurf–H 0.407 2.58
Ptbulk–H 0 3.31
Ptbulk 0.73 1.21 6.84 Ptbulk–Ptbulk 0.321 2.77
H 0.87 0 0 Ptbulk–H 0.211 2.58
Table 3. Porcentual contribution of the metal orbitals to the Pt–H overlap population.
6s 6px py pz dx 2 −y 2
The Pt 6s orbital population decreases from 0.897 to
0.80. The electron charge transfer from the metal to
the H atom is −0.046 e − .
Analysis of the bonding between H and the surface
reveals that the main contribution to the Pt–H
bond is from the H 1s, Pt 6s, and 6p orbitals, and
to a lesser extent the 5d orbitals. The Pt–H COOP
curve is plotted in Fig. 5.
The Pt–H interaction is bonding within the
energy window, and the bond is developed at the
expense of the Pt–Pt bond. The narrow band of
states at −16 eV is made up of mostly H states
d 2 z dxy dxz dyz
H surf–1s 31.62 26.50 0 21.22 4.39 0 0 16.47 0
H bulk–1s 32.55 0 0 36.41 0 30.94 0 0 0
Fig. 6. DOS curves for Pt(111): total (a), projected DOS on a Pt atom at the bulk (b), and on the H atom (c).

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Fig. 7. COOP curves for Pt(111): for Pt–Pt first neighbor
with both atoms bulk (a) and Pt–bulk H–bulk (b).
0
-1
-2
-3
-4
-5
-6
-7
-8
-9
-10
-11
-1 0 1
Minimun energy
Fig. 8. Energy value for a plane on yz for a constant x
on the Pt(111) cluster with H on the surface.
Z
Y
Hydrogen Adsorption and Diffusion on a Pt(111) Cluster 325
0
-1
-2
-3
-4
-5
-6
-7
-8
-9
-10
-11
-1 0 1
Minimun energy
Fig. 9. Energy value for a plane on yz for a constant x
on the Pt(111) cluster when H is in the bulk.
stabilized after adsorption. The py and d z 2,dxy, dyz
do not contribute significantly to the bonding. The
results are shown in Tables 2 and 3.
The location of H in the bulk corresponds to an
octahedral hole and is negatively charged. The H
peak in the DOS is similar to the surface case shifted
0.75 eV down below the bottom of the d band (Figs. 6
and 7).
3.1. Hydrogen diffusion
After adsorption, we have computed a minimum
energy path inside the bulk when hydrogen moves
Z
Y

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326 J. M. Marchetti et al.
Energy [eV]
-1.6
-1.8
-2
-2.2
-2.4
-2.6
-2.8
across the cluster. To perform this analysis the
seven-layer cluster has been subdivided into sublayers
and on each of them an energy surface plot was
computed. Figure 8 shows the location of hydrogen
for the surface layer and Fig. 9 shows the location of
hydrogen in the bulk.
The computed activation energy barrier for diffusion
is 0.943 eV while Watson et al. 31 have found
an adsorption energy of 0.52 eV using DFT calculations.
Figure 10 shows the diffusion energy vs. reaction
coordinate.
4. Conclusion
In this work we have studied the H adsorption on
a Pt(111) surface and its movement among a metal
cluster. We were able to understand the nature of
Pt–H surface bonding and their interactions as well
as to understand the subsequent hydrogen diffusion
through the Pt(111) cluster and the minimum energy
path possible for the H to go from one side to the
other. The computed H–Pt interaction is favorable
and the activation barrier is 0.943 eV.
Acknowledgment
The author (A. Juan) thanks the UNS-SEGCyT,
Fulbright Commission, Guggenheim Foundation and
CONICET for financial support. J. M. M would
like to thank the CONICET and the Planta Piloto
de Ingeniería Química for supporting his research.
Reaction coordinate
Fig. 10. Path of H on the Pt cluster. () minimum path.
A.J.andG.B.aremembersofCONICET.J.M.M.,
E. G. and P. J. are fellows of that institution.
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