Surface Science 583 (2005) 100–106
www.elsevier.com/locate/susc
Surface segregation in palladium based alloys
from density-functional calculations
O.M. Løvvik
*
University of Oslo, Center for Materials Science and Nanotechnology, P.O. Box 1126 Blindern, N-0318 Oslo, Norway
Received 17 December 2004; accepted for publication 16 March 2005
Available online 7 April 2005
Abstract
The surface segregation in Pd based alloys has been investigated by density-functional band-structure calculations.
Twelve different metals were substituted in Pd(1 1 1) slabs at a level of 5%: Ag, Au, Cd, Cu, Fe, Mn, Ni, Pb, Pt, Rh, Ru,
and Sn. The segregation energy (the difference in calculated total free energy between surface sites and bulk-like sites)
was calculated for each alloy, and the results were in very good agreement with experimental data where available. Particularly, we predict an oscillatory depth profile for Cu and Ni, similar to what has been found experimentally for the
PdNi(1 0 0) surface. There are more or less pronounced correlations between the segregation energy and the relaxed
position at the surface of the substituted atom, the metal radius of the substituted atom, and the experimental surface
energy of the metal. It is proposed that the segregation energy is an indirect measure of the stability of Pd based hydrogen permeable membranes.
2005 Elsevier B.V. All rights reserved.
Keywords: Density functional calculations; Surface segregation; Palladium; Alloys
1. Introduction
Palladium is a metal with important existing
and potential applications both as a catalyst in
heterogeneous catalysis and in membranes for
hydrogen separation. Alloying may be used to substantially alter the catalytic properties of palla-
*
Tel.: +47 22 840 689; fax: +47 22 840 651.
E-mail address: o.m.lovvik@fys.uio.no
dium surfaces, and a detailed knowledge of the
atomic arrangement and electronic structure of alloy surfaces could aid in the search for better catalysts [1–4]. Surface segregation is a central
concept when such properties are to be described
and understood.
Thin palladium based membranes have excellent
hydrogen permeability and exhibit very high hydrogen selectivity [5,6]. The most common alloys for
hydrogen selective membranes are Pd77Ag23 and
Pd60Cu40, which both have enhanced mechanical
0039-6028/$ - see front matter 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.susc.2005.03.028
O.M. Løvvik / Surface Science 583 (2005) 100–106
and permeation properties compared to pure Pd
membranes [5,7]. There is, however, still a need to
improve the membranes; their long-term mechanical stability is for instance not yet acceptable for
the thinnest membranes. The atomistic understanding of the mechanisms that are involved in the rupturing and cracking of membranes is very limited,
and a deeper understanding could facilitate improved materials, treatments, and designs of future
hydrogen permeable membranes. One phenomenon that may be of importance for the mechanical
stability of thin metal membranes is surface segregation, including segregation to the surface of grain
boundaries. If the thermodynamic driving force towards surface segregation is large, this may lead to
excessive solid-state diffusion and hence possibly an
increased density of faults and defects in the crystal
structure. Furthermore, bulk order may be broken
at grain boundaries and interfaces if segregation is
strongly promoted, leading to local disorder and
weakened bonds between grains. Thus, the tendency of strong surface segregation may be a measure of the mechanical instability of the membrane.
Experimental studies of surface segregation
have been performed for most of the alloys in this
paper. It is for instance well-known that silver surface segregates in PdAg alloys (95% of the (1 1 1)
surface of Pd67Ag33 is silver at 720 K) [8], and that
gold [9,10] and copper [11] surface segregate in Pd
alloys. It has further been shown that Pd surface
segregates when alloyed with Fe [12,13], Ni [14–
18], Pt [19,20], Rh [21], and Ru [22]. These results
are valid for pure surfaces, and may be changed in
the presence of adsorbents. Since oxygen adsorbs
more strongly on palladium than on gold, for instance, palladium surface segregates in PdAu alloys with adsorbed oxygen [10]. The orientation
of the surface has also been shown to play a role.
The (1 1 0) surface of PdNi shows, for instance, a
larger tendency of Pd surface segregation than
the (1 1 1) surface [17]. PdNi and PdAu alloys have
shown oscillations in the atomic layer composition
near the surface [17,9], similar to what has previously been found in Pt alloys [23,24].
Most previous theoretical studies of surface segregation are based on atomic potential methods
like the (modified) embedded atom method [25],
semi-empirical N-body potentials [10], quantum
101
approximate methods [26], and tight-binding
methods [27,28]. So far, very few studies have used
ab initio methods to calculate segregation in binary Pd alloys; one such study showed surface segregation of Co in PdCo alloys [29], and another
showed surface segregation of Ag in PdAg alloys
[30]. A density-functional study concluded that
Mn overlayers on Pd are metastable [31], implying
that Pd probably surface segregates in PdMn alloys. For PdCd and PdSn there are no data on surface segregation in the literature.
This paper aims to calculate the tendency of the
12 metals mentioned above to segregate either to
the surface or away from the surface when substituted in a palladium slab. This has been done by
comparing the total energy of different substitution configurations after relaxation of internal
forces in the slab, using density functional theory
(DFT) at the generalized gradient approximation
(GGA) level. The induced local distortions of the
lattice due to such substitutions in the form of
atomic displacements from the relaxed positions
of the pure Pd slab have also been investigated. Finally the correspondence between the segregation
energy and the metal radii (half the interatomic
distance within the metals) and the surface energies of the substituted atoms is investigated.
2. Computational method
The calculations were carried out using the
Vienna Ab-initio Simulation Package (VASP)
[32,33], in which the electronic band structure is
calculated from density functional theory using a
plane-wave basis sets. The projector augmented
wave (PAW) scheme [34,35] was used to represent
the wavefunctions, and the plane-wave energy cutoff was 500 eV. The k-spacing was less than
0.02 Å 1 in reciprocal space, and the convergence
criterion for the self-consistent electronic calculations was that the energy difference between two
successive iterations be less than 10 5 eV. The tetrahedron method with Blöchl corrections was used
to smear out partial occupancies. The overall convergence of the total free energy due to the mentioned numerical parameters was around 1 meV.
In addition comes the unknown uncertainty from
102
O.M. Løvvik / Surface Science 583 (2005) 100–106
the exchange-correlation potential, in our case the
PW91 gradient-corrected functional [36].
Slabs were constructed by inserting vacuum regions of 1 nm in the z-direction. Fig. 1 shows the
geometry of one such slab, with an outline of the
2 · 2 surface unit cell being used. Shown are also
the three substitution sites investigated for the
metals. We used five-layer slabs, since this was
found to yield acceptable convergence with respect
to surface energy and segregation energy. This
means that the impurity level is 5% in these models, and that the minimum distance between each
impurity is 559 pm. The optimized bulk cell
parameters were kept fixed in the slab plane, and
the start geometry for the relaxation was the relaxed geometry of a pure Pd slab of the same size.
The residual minimization method with direct
inversion in iterative subspace implementation of
the quasi-Newton method was used for the force
minimization, keeping the bottom layer fixed to
mimic bulk continuation. The tolerance for
remaining ionic forces was 0.03 eV/Å. A single
high-accuracy calculation was performed after
reaching this criterion, in order to get reliable total
energies.
3. Results and discussion
The tendency of an impurity to segregate to or
from a surface may be expressed by the segregation
energy Esegr. This is defined as the difference in
calculated uncorrected total free energy between a
surface (or near surface) site and a bulk-like site
(in our case in the middle of the slab; site ‘‘3’’) for
each substitution metal. This means that positive
segregation energy implies a tendency to move into
the bulk, while a negative segregation energy suggests surface segregation. Fig. 2 summarizes the
calculated Esegr for the upper two layers of the 12
metals in this study. The distribution of segregation
energies at the surface is quite broad; seven of the
atoms have positive segregation energy at the surface, while five have negative segregation energy.
The largest positive energy is found for Ru and
Mn, while Pb has a particularly high negative
energy. It is interesting to note that Cu and Ag,
which improve the mechanical properties of Pd
when added, both have relatively low segregation
energies. This could mean that a low segregation
energy is beneficial for the mechanical properties
of thin alloy membranes.
The correspondence with the calculated segregation energy and what has been observed experimentally is very good. It is predicted that Ag, Au,
60
MnRu
Fe
40
Esegr (kJ/mol)
Rh
20
0
-20
-40
Ni
Cu Pt
Ag
Cd Au
Sn
-60
-80
Pb
1
2
3
Layer
Fig. 1. The five-layer slab used for the calculations. The
substitution sites ‘‘1’’ to ‘‘3’’ are marked—only one of them is
substituted at a time. The 2 · 2 surface unit cell has been
outlined. Vacuum regions of 1 nm separate the slabs in the zdirection.
Fig. 2. The segregation energy Esegr in kJ/mol per unit cell at
the three sites shown in Fig. 1 for the various substituted atoms.
Esegr is defined as the total free energy of a slab with a
substituted atom in the specified layer relative to that of a slab
with the same atom substituted in layer 3.
O.M. Løvvik / Surface Science 583 (2005) 100–106
The thermodynamic driving force for segregation, as discussed in the previous paragraphs, is
one possible important factor governing mechanical stability of a thin alloy membrane, since it induces solid-state diffusion and disorder at the
surfaces, thereby possibly creating more lattice defects and weaker bonds between grains in the
structure. Another possible factor is local distortions around the substituted atoms; large displacements could lead to defects that additionally
weaken the lattice. So how does the local geometry
around the substituted atoms change upon relaxation? Because of symmetry, the lateral position of
all atoms was unchanged, and any changes happened in the z-direction. Fig. 3 shows how the displacement in the z-direction dz of the substituted
atom relative to the equilibrium position of the replaced Pd atom varies with substitution site for the
different impurity atoms. We first note a clear negative correlation between Esegr and dz; the atoms
with largest negative Esegr (like Pb) have the largest
displacement out of the surface, and vice versa.
The displacements are not very large, but they
may be sufficient to decrease the mechanical stability of alloy membranes, since this inevitably increases disorder at grain boundaries. Experimental
results on this are scarce in the literature, but a
SEM study measured the Pd atoms to be 25 pm
higher than Ag in a Pd67Ag33(1 0 0) surface containing 95% silver [8]. The calculated displacement
50
Pb
40
Cd
dz (pm)
Cd, Pb, and Sn surface segregate in Pd, in agreement with what has been seen for Ag [8] and Au
[9,10]. Similarly, most of the atoms that are predicted not to surface segregate (having positive
segregation energies), are supported by experimental findings; Pd surface segregation has been observed in Fe [13], Ni [17,18], Pt [19,20], Rh [21],
and Ru [22]. There is one exception, however; Cu
has been observed to surface segregate in Pd–Cu
nanoparticles on a SiO2 support [11], while atomistic simulations predict Pd to surface segregate
in Cu [26]. A corrected effective medium study
found depletion of Cu at the surface at low Cu
concentrations [37]. We shall see that these conflicts may to a certain extent be explained by an
oscillatory depth profile.
The segregation energy varies monotonically
when going from layers 3 to 1 for all the atoms
except two: Cu and Ni. Additional calculations
were performed with the number of layers increased to seven, and the result remained the same
for both Cu and Ni—the most stable substitution
site is that of the second layer. This is consistent
with the oscillatory depth profile observed in
PdNi(1 0 0) surfaces; while a bulk concentration
of 50% Ni decreased to 22% in the surface layer,
the concentration was 72% in the second layer
[17]. A similar trend is expected to appear in PdCu
as well, based on the present results. This could
explain why different studies gave different results,
particularly if it turns out that the depth profile
depends on the surface orientation. It is furthermore known that there is an oscillation of the
Au content near the surface of Au3Pd(1 0 0) [9],
but this is not seen in our data. In order to check
this more thoroughly, new calculations should be
performed with systematic variation of the Au
content in several layers, but this is beyond the
current scope. There is not enough experimental
data available to see whether the strength of Esegr
generally correlates with the concentration of the
impurity at the surface. This is not necessarily
the case, since there may be effective repulsions
between the impurities at the surface due to electronic and/or geometric effects. But we do expect
a stronger and more rapid segregation taking
place at lower temperatures when the absolute
value of Esegr is larger.
103
30
Ag
20
Sn
Au
10
Pt
0
Cu
-10
Ru
-20
MnFe
Rh
Ni
1
2
3
Layer
Fig. 3. The displacement in the z-direction dz of the substituted
atom relative to the relaxed position of the replaced Pd atom in
pm.
104
O.M. Løvvik / Surface Science 583 (2005) 100–106
Esegr (kJ/mol)
of Ag is in this work out of the surface, that is the
opposite direction of what could be expected from
the experimental results. This is probably because
of the different surface compositions and/or
geometries.
It is interesting to know whether the surface
segregation is primarily a geometric or electronic
effect. To check this, we have in Fig. 4 plotted
the connection between Esegr and the metal radius
of the impurity atoms (as a measure of the geometric effect—do large atoms move towards the surface?) and the experimental surface energy Esurf
[38] (as a measure of the electronic effect—how
easily are various surfaces formed compared to
Pd?). There is a visible correlation in both cases.
One general trend is that larger atoms tend to segregate more strongly to the surface (Fig. 4(a)).
This correlation is not strong enough, however,
to be used to predict segregation direction. For
example, Cu, Mn, and Rh have larger metal radii
than Pd, but they are not predicted to surface segregate. Similarly there is evidently a positive correlation between Esegr and Esurf (Fig. 4(b)): the lower
the surface energy, the more likely we find surface
segregation. But also here the predictive power is
limited. Cu and Mn have, for instance, lower Esurf
than Pd, but are still predicted to surface segregate. Thus, surface segregation is governed by a
combination of geometric and electronic effects.
60
60
40
40
20
20
0
0
-20
-20
-40
-40
-60
(a)
(b)
-80
120
140
Covalent radius (pm)
-60
-80
1
2
3
Esurf (J/m2)
Fig. 4. The correlation between Esegr and the metal radius (half
the interatomic distance in the metals) of the substituted atoms
(a) and the experimental surface energy (b). The symbols
correspond to the same atoms as in Figs. 2 and 3, while the Pd
values are marked by open circles.
The catalytic activity of these materials may depend strongly on the alloying. It is, e.g., known
that catalytic hydrogenation may be improved by
adding Ni to Pd despite the surfaces are dominated
by Pd [14,39,16,17]; this may be explained by electronic modification of Pd by surrounding Ni
atoms. We should expect a similar effect in PdCu
alloys based on the predicted oscillatory composition of the near surface layers.
We have seen that the segregation energy is correlated with both the difference in radius and the
difference in surface energy between the two metals
of the palladium based alloys of this study. But
how is it related to the mechanical stability of
membranes based on such alloys? There does not
exist any systematic investigation of the mechanical stability of Pd based alloy membranes that
could give a quantitative answer to this question.
It is known, however, that PdAg and PdCu alloys
exhibit the best mechanical stability among these
alloys. Interestingly, Ag has the lowest negative
segregation energy in this study, while Cu has
the next lowest positive segregation energy, closely
following Pt. This gives strength to our starting
hypothesis that the segregation energy may be a
measure of the mechanical instability of the membrane.
This may seem surprising, since real membranes
have all different surfaces, not only the low-index
ones. Since surface segregation depends on the orientation of the surface, it is possible that both segregation and desegregation is found in real
membranes. We should expect, however, that this
is true only for the alloys with lowest segregation
energy; if the size mismatch and/or surface energy
difference is too large, there will probably be surface segregation for most or all surface orientations. Moreover, orientational dependence of
surface segregation has only been reported for
the alloys with lowest segregation energy. This
means that an important factor for achieving
mechanical stability could be that both species of
an alloy is found at its surfaces.
The ‘‘surfaces’’ of importance for mechanical
stability may be both the surfaces facing the vacuum (or the gas phase) and the grain surfaces facing the grain boundaries. It is likely that cracking
and rupturing of thin metal membranes take place
O.M. Løvvik / Surface Science 583 (2005) 100–106
along grain boundaries or in their near vicinity.
The segregation of metal atoms at the grain surfaces is not necessarily the same as the segregation
at surfaces towards vacuum, since impurities only
located in the grain boundaries may bond more
strongly to one of the metals of the alloy. This
means that a more realistic study of the importance of segregation for the mechanical stability
of membranes must include the effect of adsorbents. Such investigations are in preparation.
4. Conclusions
The surface segregation in Pd0.95M0.05(1 1 1)
slabs (M = Ag, Au, Cd, Cu, Fe, Mn, Ni, Pb, Pt,
Rh, Ru, and Sn) has been calculated using a density-functional band-structure formalism at the
GGA level. The lowest absolute value of the segregation energy (which also may be a sign of
mechanical stability) was found for Pt, Cu, Ni,
and Ag. This is identified as a probable measure
of the stability of alloy membranes. The displacement of the impurity from the equilibrium position
of the replaced Pd atom is negatively correlated
with the segregation energy: The larger negative
segregation energy (and thus surface segregation),
the larger displacement of the substituted atom out
of the surface. There is also a negative correlation
between the segregation energy and the size of the
substituted atoms (measured by their covalent radii) and a positive correlation between the segregation energy and the experimental surface energy of
the metals. Both geometric and electronic effects
are needed to explain the segregation.
Acknowledgements
Economic support from the Norwegian Research Council through the NANOMAT program
and generous grants of computational resources
via the NOTUR project are acknowledged.
References
[1] G.A. Somorjai, Introduction to Surface Chemistry and
Catalysis, Wiley and Sons, New York, 1994.
105
[2] J.H. Sinfelt, Bimetallic Catalysts: Discoveries, Concepts
and Application, Wiley and Sons, New York, 1983.
[3] F.B.I. Chorkendorff, S.B. Clausen, B. Hammer, A.M.
Molebroek, J. Nørskov, I. Stensgaard, Science 279 (1998)
1913.
[4] P. Liu, J.K. Nørskov, Phys. Chem. Chem. Phys. 3 (2001)
3814.
[5] S. Uemiya, Sep. Purif. Meth. 28 (1999) 51.
[6] R. Bredesen, K. Jordal, O. Bolland, Chem. Eng. Proc. 43
(2004) 1129.
[7] S.N. Paglieri, J.D. Way, Sep. Purif. Meth. 31 (2002) 1.
[8] P.T. Wouda, M. Schmid, B.E. Nieuwenhuys, P. Varga,
Surf. Sci. 417 (1998) 292.
[9] J. Kuntze, S. Speller, W. Heiland, P. Deurinck, C.
Creemers, A. Atrei, U. Bardi, Phys. Rev. B 60 (1999) 9010.
[10] Z. Kaszkur, Phys. Chem. Chem. Phys. 6 (2004) 193.
[11] S. Lambert, B. Heinrichs, A. Brasseur, A. Rulmont, J.P.
Pirard, Appl. Catal. A—General 270 (2004) 201.
[12] J.C. Bertolini, J.L. Rousset, P. Miegge, J. Massardier, B.
Tardy, Y. Samson, B.C. Khanra, C. Creemers, Surf. Sci.
281 (1993) 102.
[13] C. Creemers, Surf. Sci. 360 (1996) 10.
[14] J.C. Bertolini, P. Miegge, P. Hermann, J.L. Rousset, B.
Tardy, Surf. Sci. 331–333 (1995) 651.
[15] G.N. Derry, C.B. McVey, P.J. Rous, Surf. Sci. 326 (1995)
59.
[16] A.C. Michel, L. Lianos, J.L. Rousset, P. Delichere, N.S.
Prakash, J. Massardier, Y. Jugnet, J. Bertolini, Surf. Sci.
416 (1998) 288.
[17] S. Helfensteyn, J. Luyten, L. Feyaerts, C. Creemers, Appl.
Surf. Sci. 212 (2003) 844.
[18] G.N. Derry, R. Wan, Surf. Sci. 566 (2004) 862.
[19] D. Radosavkic, N. Barrett, R. Belkhou, N. Marsot, C.
Guillot, Surf. Sci. 516 (2002) 56.
[20] A.M. Venezia, Catal. Today 77 (2003) 359.
[21] M.A. Newton, L. Jyoti, A.J. Dent, S. Diaz-Moreno, S.G.
Fiddy, J. Evans, ChemPhysChem 5 (2004) 1056.
[22] I.G. Batirev, A.N. Karavanov, J.A. Leiro, M. Heinonen,
J. Juhanoja, Surf. Interf. Anal. 23 (1995) 50.
[23] Y. Gauthier, R. Baudoing, J. Jupille, Phys. Rev. B 40
(1989) 1500.
[24] Y. Gauthier, R. Baudoingsavois, J.J.W.M. Rosink, M.
Sotto, Surf. Sci. 297 (1993) 193.
[25] C. Creemers, P. Deurinck, S. Helfensteyn, J. Luyten, Appl.
Surf. Sci. 219 (2003) 11.
[26] G. Bozzolo, J.E. Garces, R.D. Noebe, P. Abel, H.O.
Mosca, Progr. Surf. Sci. 73 (2003) 79.
[27] C. Massen, T.V. Mortimer-Jones, R.L. Johnston, J. Chem.
Soc.—Dalton Trans. 23 (2002) 4375.
[28] L.D. Lloyd, R.L. Johnston, S. Salhi, N.T. Wilson, J.
Mater. Chem. 14 (2004) 1691.
[29] A. Saul, M. Weissmann, Phys. Rev. B 60 (1999) 4982.
[30] O.M. Løvvik, R.A. Olsen, J. Chem. Phys. 118 (2003) 3268.
[31] W. Olovsson, E. Holmstrom, A. Sandell, I.A. Abrikosov,
Phys. Rev. B 68 (2003) 045411.
[32] G. Kresse, J. Hafner, Phys. Rev. B 47 (1993) 558.
[33] G. Kresse, J. Furthmuller, Phys. Rev. B 54 (1996) 11169.
106
O.M. Løvvik / Surface Science 583 (2005) 100–106
[34] G. Kresse, D. Joubert, Phys. Rev. B 59 (1999) 1758.
[35] P. Blöchl, Phys. Rev. B 50 (1994) 17953.
[36] J.P. Perdew, J.A. Chevary, S.H. Vosko, K.A. Jackson,
M.R. Pederson, D.J. Singh, C. Fiolhais, Phys. Rev. B 46
(1992) 6671.
[37] L. Yang, Phil. Mag. A 80 (2000) 1879.
[38] F.R. de Boer, et al., Cohesion in Metals: Transition Metal
Alloys, North-Holland, Amsterdam, 1988.
[39] P. Miegge, J. Rousset, B. Tardy, J. Massardier, J. Bertolini,
J. Catal. 149 (1994) 404.