ECS Transactions, 61 (13) 47-53 (2014)
10.1149/06113.0047ecst ©The Electrochemical Society
Density Functional Theory Study of the Alkali Metal Cation Adsorption on Pt(111),
Pt(100), and Pt(110) Surfaces
I. Matanovića*, P. Atanassova, F. H. Garzonb, N. J. Hensonc
a
Department of Chemical and Nuclear Engineering, University of New Mexico,
Albuquerque, New Mexico 87131, USA
b
Materials Physics and Applications Division, Los Alamos National Laboratory, Los
Alamos, New Mexico 87545, USA
c
Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico
87545, USA
*e-mail: [email protected]
Abstract
We used density functional theory to study the adsorption of
hydrogen, lithium, sodium, and potassium cations on different
surfaces of platinum, namely the Pt(111), Pt(110), and Pt(100)
surfaces. It was found that at low H+ concentrations alkali metal
cations can compete with hydrogen for adsorption on all the
studied platinum surfaces leading to a site blocking effect during
the electrochemical processes involving adsorption of hydrogen in
alkaline media. The strongest site blocking effect is predicted to
occur on the Pt(111) surface as hydrogen and alkali metal cations
adsorb in the same fcc-hollow adsorption site. On the Pt(110) and
Pt(100) surface hydrogen and alkali cations adsorb on different
sites and can co-exist on the surface – the most favorable
adsorption site for hydrogen is a bridge site, while the hollow site
is favored for all the studied alkali metal cations. Based on the
calculated adsorption Gibbs free energies and the number of
available adsorption sites on different surfaces, the probability of
the site blocking effect by alkali cations on different surfaces of
platinum was determined as Pt(111)>Pt(110)>Pt(100).
Introduction
The adsorption of alkali metals on simple metallic surfaces and the effect it might have
on reactions important to fuel cell electrochemistry has recently drawn a considerable
amount of attention. It was demonstrated experimentally that the nature of the alkali
metal cation in the electrolyte affects the electro-oxidation of molecules such as ethylene
glycol (1), glycerol (2), hydrogen (3), methanol (3), and the reduction of oxygen (4).
Mechanism proposed by D. Strmcnik et al. (3) explained the interference of alkali cations
in alkaline media by considering non-covalent interactions of partially solvated alkali
cations with hydroxide adsorbed on the metal surfaces. Although the specific adsorption
of the ionic species directly onto the electrode surface could contribute to the decrease of
efficiency of the electrochemical processes, they did not discuss the effect of alkali cation
adsorption. J. N. Mills et al. were among first (5) to consider the possibility that the alkali
cations might impact the mechanism of heterogenous reactions by blocking the active
sites on the surface of the catalysts and applied first principle calculations to study the
adsorption of different alkali metal cations to fcc(111) surfaces of Ag, Au, Ni, Pt, and Pd.
Their calculations showed that at low H+ concentrations, alkali cations could compete
47
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ECS Transactions, 61 (13) 47-53 (2014)
with hydrogen for adsorption sites on the Pt(111) surface and could, thus, decrease the
efficiency of the electrochemical processes such as hydrogen oxidation reaction (HOR) in
alkaline media. It was also shown that Na+ and Cs+ ions might show the highest site
blocking effect.
In this work we use Density Functional theory (DFT) to study the adsorption of hydrogen,
lithium, sodium, and potassium on different surfaces of platinum, namely the (111), (110),
and (100) surfaces and discuss the difference that the adsorption of various alkali metal
cations might have on the adsorption of a hydrogen atom on different surfaces of
platinum. Due to its wide application in catalysis, the platinum surface and
electrochemical processes on platinum have been subjects of extensive experimental and
theoretical investigations; however, we are not aware of any work that studied the
adsorption of different alkali metal cations on different facets of platinum. We thus
believe that this work might provide important information about the differences in the
efficiency of electrochemical processes in alkaline media that involve adsorbed hydrogen
on different platinum surfaces.
Computational details
All the calculations were performed using the spin polarized generalized gradient
approximation (GGA) to density functional theory (DFT) with the Perdew-Wang (PW91)
(6-8) exchange-correlation functional. The PW91 functional was shown to perform
reasonably well in the studies of ORR activity on the platinum and platinum alloys
surfaces and some nanoparticles (9,10). The projector augmented wave method (11,12)
was used as implemented in the Vienna Ab initio Software Package (VASP) (13-16) and
reciprocal space was sampled with a k-point Monkhorst-Pack mesh (17) of the first
Brillouin zone with 9x9x1 points. The plane-wave basis cutoff energy was set to 400 eV.
Methfessel-Paxton smearing (18) of order 2 with a value of σ =0.2 was used to aid
convergence. We estimate that the energies are converged to within 0.001 eV per atom
with these criteria and choice of pseudopotential. The Pt(111) surface was represented by
2 3 x2 3 unit cell with a size of 9.70 x 9.70 Å, while Pt(110) surface was represented
by 2x3 unit cell with a size of 7.96 x 8.44 Å. To model the Pt(100) surface we used 2x2
unit cell with a size of 7.96 x 7.96 Å. In all cases the unit cells consisted of four layers of
metal atoms and a vacuum region of 20 Å. During the optimization two bottom layers of
metal atoms were held fixed in order to model the presence of a bulk region.
The cation adsorption process onto the metal surface can be written as a reaction:
* +C aq+ + e − → C *
(1)
+
where * represents the bare surface, C aq the aqueous solvated cation, and C* cation
adsorbed on the metal surface. The Gibbs free energy change for the reaction (1) was
calculated by considering following elementary processes
* +C g → C * (2)
C g+ + e − → C g (3)
C aq+ → C g+
(4)
The Gibbs free energy change for reaction (1) is obtained by summarizing energy
changes of elementary processes (2), (3), and (4):
∆ r G (ads ) = ∆ r G2 + ∆ r G3 + ∆ r G4
(5)
∆ r G (ads ) = ∆E ad + ∆ZPE − T∆S + Ei + E solv
48
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ECS Transactions, 61 (13) 47-53 (2014)
where ∆Ead and ∆ZPE are the changes in the electronic and zero point energy calculated
using DFT, T∆S is the translational entropy correction associated with reaction (2), Ei is
the experimental ionization energy for the cation in the gas phase (reaction 3), and Esolv is
the experimental solvation free energy for the cation in a 1M aqueous solution (reaction
4). It was assumed that the adsorption of cations is occurring with a single electron
transferred to the electrode to form the adsorbed species.
Results and discussion
Hydrogen adsorption
Most preferable adsorption site of hydrogen on Pt(111) surface is the fcc-hollow site (19).
We calculated the adsorption energy of hydrogen in the fcc site using the GGA-PW91
level of theory as -2.79 eV with respect to the hydrogen atom in the gas phase as the
reference. On the Pt(110) surface, hydrogen atoms can adsorb on four inequivalent
adsorption sites (Figure 1): on top of the platinum atoms, in the hollow sites, and in short
and long bridges between the two platinum atoms. The adsorption energies calculated for
the top, hollow, short bridge, and long bridge sites are -2.82 eV, -2.32 eV, -2.87 eV, and 2.50 eV, respectively. Because of the higher symmetry there are only three inequivalent
adsorption sites on the Pt(100) surface. The adsorption energy for a hydrogen atom was
calculated as -2.72 eV, -2.53 eV, and -2.91 eV in the top, hollow, and the bridge site on
the Pt(100) surface, respectively. Using equation 5 the Gibbs free energy change for the
adsorption of the hydrogen on the most preferable adsorption site on the Pt(111), Pt(110),
and Pt(100) surface was calculated as -4.59 eV, -4.65 eV, and -4.70 eV. The change in
the Gibbs free energy can be related to the equilibrium adsorption potential using the
simple relation between the reaction energy and the electrochemical potential:
∆ G (ads )
Uf = r
ne
where e denotes the number of electrons exchanged in the reaction. Using this relation
adsorption potential for H atom was calculated as +4.59 V, +4.65 and +4.70 V for the
Pt(111), Pt(110), and Pt(100) surfaces at pH=0. Taking into account that the adsorption
potential can be shifted to the scale of a normal hydrogen electrode by subtracting 4.6 V,
adsorption potentials of hydrogen on Pt(111), Pt(110), and Pt(100) are then calculated as
-10 meV, +50 meV and +100 mV indicating that the Pt(100) surface has the highest
affinity for hydrogen and the adsorption of hydrogen will occur at more positive
potentials than on the Pt(110) or Pt(111) surface.
Table 1. Adsorption energies of different cations on the Pt(111), Pt(110), and Pt(100)
surfaces and the Gibbs free energy changes for the reaction in the reaction (1). H+ is
adsorbed on fcc sites on the Pt(111) surface, and in bridging sites on Pt(100), and
Pt(110). Alkali cations are adsorbed on the fcc sites on the Pt(111) and in the hollow sites
on Pt(100), and Pt(110) surfaces (Figure 1). Adsorption energies in this table correspond
to the most favorable adsorption sites.
Pt(111)
Pt(110)
Pt(100)
H+
∆Ead/eV ∆rG/eV
-2.79f
-4.59
-2.87sb
-4.65
b
-2.91
-4.70
Li+
∆Ead/eV ∆rG/eV
-3.33f
-3.27
-3.47h
-3.40
h
-3.49
-3.43
Na+
∆Ead/eV ∆rG/eV
-2.89 f
-3.64
-2.96 h
-3.72
-2.97 h
-3.71
K+
∆Ead/eV ∆rG/eV
-3.13 f
-3.85
-3.17 h
-3.84
-3.17 h
-3.80
h – hollow, f – fcc, b – bridge, sb – short bridge, lb – long bridge
49
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ECS Transactions, 61 (13) 47-53 (2014)
Figure 1. Adsorption of hydrogen (top) and sodium (bottom) on Pt(111), Pt(110), and
Pt(100) surfaces. h – hollow, f – fcc-hollow, hc- hcp-hollow, b – bridge, sb – short
bridge, lb – long bridge
Alkali metal cation adsorption
On the Pt(111) surface alkali metal cations prefer to adsorb on the same adsorption site as
hydrogen i.e. in the fcc-hollow site. The adsorption energies on the fcc(111) site were
calculated as -3.33 eV, -2.89 eV, and -3.13 eV for lithium, sodium, and potassium,
respectively. The Gibbs free energy changes calculated for hydrogen, lithium, sodium
and potassium are -4.59 eV, -3.27 eV, -3.64 eV, and -3.81 eV at pH=0 and with a 1M
concentration of cations in the solution. Thus, at pH=0, the Gibbs free energy for
adsorption of hydrogen is significantly more negative than for any alkali metal cation.
However, at pH=14 the Gibbs free energy of H+ adsorption will shift by +826 mV to a
more positive value, i.e., to -3.76 eV which is comparable to the adsorption Gibbs free
energies of alkali cations, especially to that of potassium and sodium. Consequently, we
can conclude that in alkaline media adsorption of alkali metal cations starts to compete
with the adsorption of hydrogen. Note however, that in our model we did not include the
solvation of alkali metal cations upon adsorption on the surface and that the values
reported in Table 1 give only the upper limit for the adsorption Gibbs free energies. It is
expected that the solvation will have a little effect on the Gibbs free energy of hydrogen
adsorption (19), but will decrease the Gibbs free energy of adsorption for all alkali
cations (5) suggesting even more pronounced competition for the fcc-hollow adsorption
sites between hydrogen and alkali metal cations on the Pt(111) surface. Also, based on
the Gibbs free energy changes in Table 1, one might expect that potassium would cause
the strongest site blocking effect, while lithium would show the weakest effect. Within
the model used in this study, that conclusion is somewhat unreliable. Namely, the
solvation of the alkali cations on the surface is expected to push the adsorption Gibbs free
energies towards more negative values and as lithium has the highest solvation energy
50
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ECS Transactions, 61 (13) 47-53 (2014)
(Table 3), the solvation model is expected to change the adsorption Gibbs free energy of
lithium for the largest amount. Further studies including the solvation effect are needed in
order to gain a better understanding of the differences in the site blocking effect of
different alkali metal cations.
On the Pt(110) and Pt(100) surfaces alkali cations adsorb in the hollow sites (Figure 1)
shared between four platinum atoms and do not compete with the adsorption of hydrogen
in its most favorable, bridge adsorption site. Adsorption energies of lithium, sodium, and
potassium in the hollow site are calculated as -3.47 eV, -2.96 eV, and -3.17 eV on the
Pt(110) surface and -3.49 eV, -2.97 eV, and -3.17 eV on the Pt(100) surface (Table 1).
At pH=14 Gibbs free energy of hydrogen adsorption becomes -3.82 eV on the Pt(110)
surface and -3.88 eV on the Pt(100) surface and again becomes comparable to the Gibbs
free energies of the alkali cations. Thus, it is expected that in alkaline media, hydrogen
and alkali metal cations will co-exist on the Pt(110) and Pt(100) surfaces, as was
previously shown for lithium (19).
In order to block the adsorption of hydrogen, alkali metal cations have to adsorb in the
short bridge site on the Pt(110) surface and the bridge site on the Pt(100) surface. We
calculated the adsorption energies and Gibbs free energies for the adsorption of lithium,
sodium, and potassium in the hydrogen adsorption sites for the Pt(110) and Pt(100)
surfaces as summarized in Table 2. As can be seen, the Gibbs free energies for the
cations in the hydrogen adsorption sites are most negative for the Pt(111) surface
Table 2. Adsorption energies of different cations on the Pt(111), Pt(110), and Pt(100)
surfaces and the free energy changes for reaction (1). H+ and alkali cations adsorbed in
fcc sites on the Pt(111), in bridge sites on the Pt(100) and Pt(110) surfaces illustrating the
blocking effect.
Pt(111)
Pt(110)
Pt(100)
H+
∆Ead/eV ∆rG/eV
-2.79 f
-4.59
-2.87 sb
-4.65
-2.50 lb
-4.28
-2.91 b
-4.70
Li+
∆Ead/eV ∆rG/eV
-3.33 f
-3.27
-2.95 sb
-2.89
lb
-3.33
-3.27
-3.20 b
-3.14
Na+
∆Ead/eV ∆rG/eV
-2.89 f
-3.64
-2.54 sb
-3.30
-2.85 lb
-3.61
-2.82 b
-3.57
K+
∆Ead/eV
-3.13 f
-2.95 sb
-3.07 lb
-3.10 b
∆rG/eV
-3.81
-3.63
-3.76
-3.74
h – hollow, f – fcc, b – bridge, sb – short bridge, lb – long bridge
Table 3. Zero point energy changes, entropy changes at 300K, solvation energies at
300K and 1M concentrations, and ionization potentials used to calculate the free energy
change for cation adsorption on different Pt surfaces.
H+
Li+
Na+
K+
∆ZPE(111)
0.147
0.039
0.023
0.012
∆ZPE(110)
0.169
0.042
0.024
0.013
∆ZPE(100)
0.169
0.049
0.025
0.013
*
-T∆S
0.357
0.430
0.477
0.497
Ei *
+13.64
+5.39
+5.14
+4.34
Esolv*
-11.336
-4.985
-3.887
-3.151
*Experimental values. Ionization energies were taken from Ref (21), solvation energies
were taken from Ref (22), 300 K and 1M concentration, and entropy values from Ref
(23).
51
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ECS Transactions, 61 (13) 47-53 (2014)
confirming that the site blocking effect by alkali cations will be the most pronounced on
the (111) facet of platinum. Adsorption Gibbs free energies of alkali cations are less
negative in the short bridge sites of the Pt(110) surface than they are on the bridge sites of
Pt(100) surface, but they are more negative on long bridge sites of the Pt(110) surface
than they are in the bridge sites of the Pt(100) surface. Thus, based on these values it is
difficult to conclude whether the site blocking effect in the bridge sites will be more
pronounced on the Pt(110) or the Pt(100) surface. However, as Pt(100) has a stronger
affinity for hydrogen than the Pt(110) surface and has also more hydrogen adsorption
sites per unit area than the Pt(110) surface, we could conclude that alkali metal cation
adsorption will have the smallest effect on the efficiency of electrochemical processes on
the (100) facet of platinum. However, other mechanisms of interference such as changes
in the electronic and structural properties of the electrode are also still plausible.
Our results correlate well with the results of Markovic group which showed that reaction
activity for HOR/HER increases with the sequence Pt(111) < Pt(100) < Pt(110) in both
acidic and alkaline media (24), however, it is difficult to conclude whether the reported
activity in alkaline media is influenced by the electrolyte and further experiments which
will study the effect of the electrolyte nature on the performance of the platinum
electrodes are needed. Moreover, it is known that the surface steps are most likely
responsible for overall catalytic activity of a typical platinum electrode (25) and further
DFT studies of the cation blocking effect on surface steps will be of great interest.
Conclusions
Based on DFT calculations of adsorption energies and Gibbs free energies, it was shown
that at high pH adsorption, the Gibbs free energy of hydrogen on platinum facets
becomes comparable to the adsorption Gibbs free energies of alkali metal cations,
indicating interference of alkali metal cations in the electrochemical processes involving
adsorption of hydrogen by a site blocking mechanism. However, on the Pt(111) surface
hydrogen and alkali cations preferentially adsorb on the same adsorption site while on the
Pt(110) and Pt(100) surface hydrogen and alkali cations adsorb on two different sites and
can thus co-exist on the surfaces. This site blocking effect of alkali cations is expected to
be more pronounced on the Pt(111) surface than on the Pt(110) or Pt(100) surface. Gibbs
free energies for the adsorption of alkaline cations on the hydrogen adsorption sites are
determined to decrease in the following order: Pt(111)>Pt(100)>Pt(110), however, as the
Pt(110) surface has a smaller affinity for hydrogen and less active sites per unit cell than
Pt(100), the site blocking effect might be more pronounced on the Pt(110) surface than
on the Pt(100) surface.
Acknowledgments
I.M. wishes to thank Dr. Yu Seug Kim, Materials Synthesis and Integrated Devices
Group, Los Alamos National Laboratory for useful discussions. This work was supported
by LDRD-DR grant number 20120003DR program. Computational work was performed
using computational resources of Los Alamos National Laboratory and National Energy
Research Scientific Computing Center supported by the Office of Science of the U.S.
Department of Energy under Contract No. DE-AC02-05CH11231. Los Alamos National
Laboratory is operated by Los Alamos National Security, LLC for the National Nuclear
Security Administration of the U.S. Department of Energy under contract DE-AC5206NA25396. This paper has been designated LA-UR-14-24536.
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ECS Transactions, 61 (13) 47-53 (2014)
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