Journal of Membrane Science 453 (2014) 525–531
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Journal of Membrane Science
journal homepage: www.elsevier.com/locate/memsci
First-principles calculations on sulfur interacting with ternary
Pd–Ag-transition metal alloy membrane alloys
O.M. Løvvik n, T.A. Peters, R. Bredesen
SINTEF Materials and Chemistry, P.O. Box 124 Blindern, N-0314 Oslo, Norway
art ic l e i nf o
a b s t r a c t
Article history:
Received 4 October 2013
Received in revised form
14 November 2013
Accepted 20 November 2013
Available online 28 November 2013
Pd-alloy membranes are promising for use in fossil-fueled power plants with integrated carbon capture,
but for some applications this requires resistance to performance degradation in the presence of sulfur
containing gases like H2S. By use of atomistic modeling based on density functional theory, the role of
transition metals (TMs) in ternary Pd–Ag–TM alloys was in the current article used to identify new Pdalloys with potentially less performance degradation in contact with H2S. A number of slab models were
created, giving a wide and representative span of compositions of the two upper atomic layers of the Pdalloys. All TMs in the 4th, 5th and 6th rows of the periodic table were included, as well as some of the
poor metals. By comparing energies of pure alloy slabs, the tendency of different TMs to segregate to the
surface in vacuum was quantified; this turned out to depend only weakly on the presence of silver. In
order for the TM to be of any benefit for the surface properties of the membrane, the segregation energy
towards the bulk should not be too large. The free energy of different compositions was then calculated
for two systems: a clean surface in a gas mixture of H2 containing a fraction of 20×10–6 H2S, and a slab
with adsorbed sulfur in pure H2 atmosphere. This was used to calculate a typical release temperature for
sulfur from the surface and a maximal H2S concentration before the membrane surface can be expected
to be blocked by adsorbed S. When using a tentative threshold for the segregation energy (0.2 eV) and
the release temperature (600 1C), we arrived at the following list of potential TM additives to sulfurresistant Pd–Ag membranes: Cu, Zn, Ga, Cd, In, Sn, Pt, Au, Hg, Tl, Pb, and Bi.
& 2013 Elsevier B.V. All rights reserved.
Keywords:
Membrane
Pd-alloy
H2S
Atomistic modeling
1. Introduction
Palladium (Pd) membranes are promising in a range of technologies where hydrogen separation is involved, due to extremely
high selectivity of hydrogen gas (H2) diffusion through Pd-based
alloys and due to very high H2 fluxes. One such technology is
fossil-fuel power plants with integrated carbon capture, where
resistance to H2S is a particularly important – H2S from the fossil
fuel may lead to severely reduced performance and in some cases
breakdown of the membrane. Membranes consisting of the fcc
phase of Pd–Cu and Pd–Au alloys exhibit apparently good resistance to bulk sulfidation, but they still exhibit a sharp decrease of
the H2 flux upon H2S exposure even when bulk sulfidation is not
thermodynamically expected [1]. In the latter case, a significant
flux decrease has been observed for H2S concentrations as low as a
fraction of 2×10–6 for 2 mm thick membranes at 450 1C. The H2 flux
penalty due to H2S is significant, illustrated by the large reduction
in permeability compared to that of the more conventional Pd77
Ag23 in the absence of H2S. The ideal Pd-based alloy for such
n
Corresponding author.
E-mail address: ole.martin.lovvik@sintef.no (O.M. Løvvik).
0376-7388/$ - see front matter & 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.memsci.2013.11.035
membranes would thus display both sulfide tolerance and a
reasonable H2 permeability in the presence of H2S. Such a solution
could lead to economically viable applications of Pd-based membranes in integrated gasification combined cycle power plants.
Density functional theory (DFT) calculations have shown to be
an important tool for predicting and understanding Pd-based
membrane properties. Previous modeling studies on sulfur tolerance based on atomic-scale density functional theory (DFT)
calculations have focused on pure metals or binary alloys [2–5],
comparing the surface reactivity between various alloys and gases.
Much of the work has been dedicated to increasing the hydrogen
permeability of Pd–Cu alloys, which exhibit quite good sulfidation
tolerance but have low hydrogen permeability [6–11]. It was
shown that sulfur forms strong bonds with the surfaces and favors
hollow sites in the case of densely packed surfaces of Pd, Cu, Ag,
Au as well as some of their alloys [2–4]. The bonds were strongest
on Pd, followed by Cu, Au and Ag. It was further shown that
dissociative adsorption of H2S is exothermic on a number of (111)
transition metal surfaces (pure Ag, Au, Cu, Ir, Ni, Pd, Pt), but with
rather high kinetic barriers on the Ag(111) and Au(111) surfaces
[3,12,13]. A few studies also investigated formation of sulfides,
which is an additional hindrance to sulfur-tolerant membranes.
DFT calculated formation energies of various binary sulfides have
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O.M. Løvvik et al. / Journal of Membrane Science 453 (2014) 525–531
been used to assess the tendency of different Pd-based alloys to
form bulk sulfide phases which cannot be easily removed [8],
while Pd4S-similar clusters at alloy slabs were seen as a precursor
to sulfide formation and thus membrane collapse [4].
Our approach in this study is to investigate the adsorption
strength of sulfur on the surface of various ternary Pd–Ag-based
alloys. The aim is to enhance sulfur tolerance of Pd3Ag while
maintaining the high hydrogen permeability of this alloy. We have
thus used theoretical modeling of ternary Pd–Ag–TM alloys with the
aim to improve their sulfur tolerance. This should provide theoretical
support for the choice of transition metals improving the sulfur
resistance of Pd–Ag-based membranes. We have first investigated
the segregation behavior of various ternary Pd-based alloy materials
with and without sulfur adsorbed. This was needed to properly
calculate the strength of sulfur adsorption on alloy surfaces, which is
a measure of how easily active sites are blocked from hydrogen
adsorption and penetration. The adsorption strength was quantified
by calculating the temperature at which adsorbed S will be released
to S in the form of H2S in the gas phase, given a certain S surface
coverage. A higher release temperature means that sulfur adsorbs
more strongly on the surface, and may be trapped there at the
working temperatures of the membrane. The main hypothesis of this
work was that the sulfur adsorption energy thus can serve to rank
and exclude elements from the pool of potential additives.
It is important to be aware that other effects may also govern
the sulfur tolerance of Pd-based membranes. One is the formation
of Pd4S in the surface region, which is not possible to remove for
certain membrane materials [4]. This means that, even if a
compound cannot be excluded by the present study, it does not
necessarily mean that it will be a good material for Pd membranes; the main aim of the present work is to exclude nonpromising alloy elements through the screening procedure.
2. Methodology
Atomistic simulations of Pd-based materials for membrane
applications were performed applying the generalized gradient
approximation (GGA) of density functional theory (DFT). The
calculations were performed utilizing the Vienna ab initio Simulation Package (VASP) [14,15] which employs plane waves as basis
functions and has implemented the projector augmented wave
method [16] to represent the core regions. The Perdew–Burke–
Ernzerhof (PBE) functional [17] was used to express the exchange
correlation contributions. All numerical parameters were thoroughly checked for convergence, and most reported energies are
accurate to within approximately 1 meV per unit cell with respect
to these parameters. The cut-off energy of the plane wave expansion was 410 eV and the density of k points in the reciprocal space
numerical integrations was higher than 25 points/Å 1 in all
directions. Spin polarization was allowed. When a vacuum layer
was inserted to generate a slab geometry, only the Γ point was
used in that direction. Temperature was included by using free
energies of the gases from standard tables. It was not included
explicitly in the slab calculations, since the solid state entropy
contributions are normally much smaller than those of the gases.
An appropriate representation of realistic membrane materials
using periodic models with a limited size requires some care. The
surface properties of these materials have been the primary focus
of this study, and slab models were used to represent alloy
surfaces of various compositions. The densely packed (111) surface
of fcc unit cells was used in all the calculations, since this has
previously been demonstrated to give quite reliable results for the
catalytic properties of alloy surfaces [18]. In addition, it has been
found that this is also the preferred orientation in magnetron
sputtered films [10]. A surface unit cell of 2 2 was used; this
contains four metal atoms, which are sufficient to provide a
relatively large range of surface compositions. The thickness of
the unit cell was four or five layers, which yield 16 or 20 atoms in
the unit cell. One of the sides (the ‘bottom’ layer) was fixed to the
bulk crystal structure, representing continuation of the structure
into the bulk. Due to the three-dimensional periodicity used in
VASP, a vacuum layer was inserted between the slabs to create the
surfaces. The thickness of this layer was at least 10 Å, which was
enough to avoid interaction between the periodic slabs. Fig. 1
shows an example of a four-layer slab with the composition
Pd11Ag3TM2, where TM¼ Au. The layers of this particular slab have
the compositions Pd3Au–Pd2AgAu–Pd3Ag–Pd3Ag; this is given
the name 301-211 (see Table 1 for definition of these names).
A number of such different slab compositions were investigated, in
order to probe the effect of different surface compositions, and to
calculate the tendency of segregation to and from the surface layer.
The different slab compositions are summarized in Table 1.
Slabs with slightly different TM content (with stoichiometry
Pd12Ag3TM) were checked, and the results were very similar to
those presented here. It thus seems that the conclusions are not
very sensitive to the choice of unit cell composition. Note that the
Fig. 1. An example of a slab model with four layers having the stoichiometry
Pd11Ag3Au2. The unit cell consists of four metal atoms per layer, and the atoms are
represented as gray (Pd), green (Ag), and yellow (Au) balls. This model is
designated 301-211 (see Table 1). (For interpretation of the references to color in
this figure legend, the reader is referred to the web version of this article.)
Table 1
The different models used for the Pd11Ag3TM2 (upper 12) and Pd19TM (lower 3)
compositions. The model names are generated by listing the number of atoms of
the different types (Pd, Ag, or other transition metal (TM)) for the uppermost two
layers. In the case of Pd11Ag3TM2, the 4th (bottom) layer always consisted of 3 Pd
atoms and one Ag atom, and the 3rd layer contained the remaining atoms to
achieve the stoichiometry. For Pd19TM, the 4th and 5th (bottom) layers always
consisted of Pd4, while the 3rd layer contained the remaining atoms.
Model name
400-211
301-310
310-211
301-211
220-301
211-310
202-310
211-211
121-400
112-310
121-301
022-400
400-400
400-301
301-400
1st layer
2nd layer
Pd
Ag
TM
Pd
Ag
TM
4
3
3
3
2
2
2
2
1
1
1
0
4
4
3
0
0
1
0
2
1
0
1
2
1
2
2
0
0
0
0
1
0
1
0
1
2
1
1
2
1
2
0
0
1
2
3
2
2
3
3
3
2
4
3
3
4
4
3
4
1
1
1
1
0
1
1
1
0
1
0
0
0
0
0
1
0
1
1
1
0
0
1
0
0
1
0
0
1
0
O.M. Løvvik et al. / Journal of Membrane Science 453 (2014) 525–531
stoichiometry of the slab models does not necessarily reflect the
overall bulk composition of the alloy; there will always be
segregation effects, and the slab compositions only represent the
situation that enough TM is present to decorate up to 50% of the
upper atomic layer. How an alloy stabilizes the surface composition during operation depends on a number of factors like kinetics,
defects, and impurities, and to discuss this is far beyond the scope
of this work.
3. Results and discussion
We would like to know how addition of sulfur affects the
segregation behavior of the various alloys in our study. This has
relevance for the local atomic arrangements and thus for the
surface reactivity. It can also influence on the mechanical stability
of membranes, since large segregation energies can potentially
lead to weakening of the membrane caused by diffusion and phase
separation. In order to clarify the effect of sulfur additions, we first
calculate segregation of the slab models in vacuum, then with
sulfur adsorbed at the surface.
3.1. Segregation of transition metals in binary Pd–TM slabs
As a reference for the ternary slab calculations, computations
involving only binary Pd–TM slabs in vacuum were first performed. This is an extension of the work in [19]. These slabs
consisted of five layers and a surface unit cell of 2 2. The
stoichiometry of these slabs was Pd19TM; thus only one transition
metal was included in an otherwise pure Pd slab. The segregation
tendency of TM in Pd was assessed by the segregation energies
Esegn, which were defined as
Eseg1 ¼ Eð301400Þ–Eð400400Þ
ð1aÞ
Eseg2 ¼ Eð400301Þ–Eð400400Þ
ð1bÞ
Here E is the total electronic energy as calculated by VASP, and the
numbers refer to the model names defined in Table 1. Thus, Eseg1
(Eseg2) quantifies the tendency to move from the 3rd or middle
layer of the slab to the surface (subsurface) layer. If we assume that
the middle layer of the slab is representative of the bulk, this
means that a positive segregation energy implies the tendency
of a TM atom to move away from the surface or subsurface layer,
towards the bulk. A negative segregation energy means segregation towards the surface or subsurface layer. The calculated
segregation energies have been plotted in Fig. 2.
We see that there is a similar trend across the periodic table for
the 3d, 4d, and 5d transition metals. Eseg1 first increases with
increasing atomic number, then it decreases. This must be a tradeoff between the surface energy of the metals and the size of the
atoms. A large atom gains more energy when going to the surface
simply due to geometry; on the other hand, the surface energy of a
metal decreases approximately with the cube of the atomic size
[20]. Segregation to the uppermost layer is seen to be beneficial
only for the largest atoms – to the right of Zn, Pd, and Pt in the
periodic table. It is not surprising that the balance between size
and surface energy appears approximately at the same column in
the periodic table, since the relationship in [20] does not depend
on the row. This is consistent with experimental findings; e.g. in
Pd–Ag alloys it is seen that the outermost atomic layer consists of
pure Ag in vacuum, see e.g. [21].
The segregation to the subsurface layer follows a quite similar
trend, first increasing and then decreasing as we move to the right
in the periodic table. In this case, however, Eseg2 is negative or only
slightly positive in all the cases, which means that segregation
towards the subsurface layer is thermodynamically much more
favorable than towards the surface layer. Here this can be
explained by the geometric effect being much more important
than the surface energy, since atoms in the subsurface layer do not
contribute directly to the surface energy. Only when the curve of
Eseg1 is below that of Eseg2, segregation to the surface layer can be
expected; this is the case to the right of Pd and Pt for the 4d and 5d
metals only. This means that an oscillatory behavior of the TM
content can be expected when plotted against the layer number.
This has been seen previously for similar systems, both experimentally and theoretically (see, e.g. [22] and references therein).
It is now interesting to see whether the presence of Ag
influences the segregation tendency of TM additives. Since the
ternary models only contained four layers in order to save
computational resources, a segregation energy similar to that
defined in Eq. (1) cannot be defined in this case. Instead, we
now use the difference in energy between layers 1 and 2 as our
segregation energy; Eseg12 ¼Eseg1 Eseg2 for the binary models. This
provides the tendency to move from layer 2 to 1, and does not give
direct information about segregation from the bulk to the surface
layer; we use it here in order to compare directly the binary and
ternary alloys. In the case of the ternary models, this was
calculated as
Eseg12 ¼ Eð211310Þ–Eð310211Þ
Fig. 2. The calculated segregation energies Eseg1 (a) and Eseg2 (b) of TM in the
binary Pd19TM models along the columns of the periodic table. The energies are
defined in Eq. (1). A negative (positive) segregation implies segregation away from
(towards) the bulk.
527
ð2Þ
A positive (negative) Eseg12 implies a tendency to segregate from
(to) the uppermost surface layer, relative to the subsurface layer.
This has been plotted for the binary and ternary models in Fig. 3.
It is encouraging to see how closely the curves follow each
other for the binary and ternary models. This means that the
presence of Ag does not mean much for the qualitative surface
segregation behavior of the TM atoms, despite increased segregation energies for the most positive energies. It also means that the
four-layer models are of similar quality as the models with five
layers for the study of atomistic segregation of these surfaces. An
important conclusion from these observations is that a surface
which only has been into contact with vacuum will primarily
consist of pure Pd, except for the elements with a negative Eseg12;
La, Ag, Cd, Sn, Au, Hg, Tl, Pb and Bi. Due to the similarity with
binary membranes, we can safely assume that the same surface
segregation will be seen relative to the bulk as relative to layer 2.
We know that this can be reversed by contact with e.g. hydrogen
gas [23], but this has not been studied for the present systems yet.
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O.M. Løvvik et al. / Journal of Membrane Science 453 (2014) 525–531
Table 2
The adsorption energy Eads of S as defined in Eq. (3) for the different Pd11Ag3TM2
models studied (they are defined in Table 1), with TM¼ Cu. Only 8 of the models
have been investigated, since they constitute a representative selection of surface
layers and adsorption sites. In two of the cases two different adsorption sites were
examined, with a different selection of nearest neighbor surface sites. For the
remaining part of this study, the most stable adsorption energy is selected as the
stability of the specific composition. For sulfur on a ternary Pd11Ag3Cu2 surface, this
is 1.45 eV, achieved with the 202-310 surface model.
Model
Surface neighbors
Eads (eV)
400-211
301-211
Pd3
Pd2TM
Pd3
PdTM2
Pd2TM
Pd3
Pd2TM
PdTM2
PdAgTM
AgTM2
1.24
1.33
1.45
1.45
1.43
1.40
1.37
1.38
1.12
1.24
202-310
310-211
211-211
112-310
121-301
022-400
Fig. 3. The calculated segregation energy Eseg12 of TM in the ternary Pd11Ag3TM2
(a) and the binary Pd19TM (b) models along the columns of the periodic table. The
energy is defined in Eq. (2). A positive (negative) segregation implies segregation
away from (towards) the surface layer.
effect is significantly weaker than direct bonding to a surface
atom, we expect that this has not influenced our conclusions.
We first present the calculated adsorption energies Eads, which
are computed according to the formula
Eads ¼ EðSlab þ adsorbed SÞ þ EðH2 ðgasÞÞ–EðSlabÞ–EðH2 SðgasÞÞ
Fig. 4. Sulfur adsorption on top of the Pd11Ag3Au2 model. The coverage is 0.25 ML,
and the adsorption site is the threefold hollow site. The atoms are represented as
gray (Pd), green (Ag), yellow (Au), and olive (S) balls. The surface termination is
(111). (For interpretation of the references to color in this figure legend, the reader
is referred to the web version of this article.)
3.2. Adsorption of sulfur on the alloy surfaces
We now turn to the adsorption of sulfur on the alloy surfaces.
In this study a sulfur coverage of 0.25 monolayers (ML) was used,
which is one S atom per unit cell. This has been depicted in Fig. 4
for the Pd11Ag3Au2 model from Fig. 1. The sulfur atom was placed
at the threefold hollow site before relaxation, following previous
knowledge about sulfur adsorption on Pd [2]. In most of the cases
the relaxation procedure did not change this significantly, except
in situations where the initial site was between surface atoms with
very different affinity towards S. As an example, S moved towards
the Pd–Pd bridge position when starting from a site with two Pd
neighbors (with a strong affinity towards S) and one Ag neighbor
(with weaker S affinity).
For models with more than one element at the surface, there is
also more than one inequivalent threefold site within the surface
unit cell. We have therefore calculated the adsorption energy of
most inequivalent threefold sites, except those which obviously
display significantly weaker S adsorption than that of the Pd3
threefold site. We have in all cases used the site with fcc stacking,
which means that the metal atom directly below is in the 3rd
layer, and not the one with hcp stacking, which has a metal atom
directly below in the 2nd layer. This means that we may have
neglected some situations where S binds more strongly to an atom
in the 2nd layer at the hcp site. However, since it is likely that this
ð3Þ
That is, the adsorption energy is defined as the relative binding
strength of sulfur at the surface with hydrogen existing as H2 gas,
compared to sulfur released in the gas phase as H2S. The slab
models being compared are the same with and without adsorbed
sulfur. The energies of H2 and H2S have been calculated by placing
the molecules in a large periodic unit cell with sufficient vacuum
between the molecules to avoid interaction between the periodic
images. Competing adsorption of hydrogen was not considered,
since tests have shown that this does not influence the results
significantly [4]. Table 2 presents Eads for the models containing Cu
as an example.
We see that the adsorption energies range from 1.45 to
1.12 eV for the 10 different adsorption models that were studied.
The most stable adsorption energy ( 1.45 eV) is selected as ‘the’
adsorption energy in what follows, since the most stable adsorption is most relevant for the temperature where S in the adsorbed
state is released to the gas phase as H2S (assuming full surface
coverage of S).
We note that the most stable adsorption on Pd–Ag–Cu can be
found at the 202-310 model. This should be compared to the
energy of the pure models, where model 400-211 with the pure Pd
surface is most stable (this can be read out of Fig. 3, since the
segregation energy of Cu is positive). One could argue that only the
most stable pure surface (400-211) should be considered, since
solid state diffusion of Cu from subsurface to the surface layer
would be necessary to obtain a surface similar to that of models
202-310 (with 50% Pd and 50% Cu) from the surface of model 400211 (with 100% Pd). However, it is established that such diffusion
can happen at temperatures relevant for membrane operation.
Also, it is likely that this happens already during activation of the
membrane, where the presence of oxygen and/or hydrogen can
reverse the segregation of the uppermost atom layers. Previous
research on supported films [24–28] has shown that membranes
must be activated by exposure to air or other oxidizing atmospheres at a temperature in the range of 300–400 1C in order to
obtain values for the hydrogen permeability representative for
bulk diffusion.
The adsorption energies calculated along the lines above have
been presented in Fig. 5 for the ternary models, compared with
binary Pd3Ag and pure Pd. We first note that there is a trend across
O.M. Løvvik et al. / Journal of Membrane Science 453 (2014) 525–531
529
Fig. 5. The most stable adsorption energy of sulfur Eads as calculated by Eq. (3) and
Table 2. Each point corresponds to surfaces of the ternary compound Pd–Ag–TM,
except for Pd (pure Pd) and Ag (Pd–Ag).
the periodic table, albeit not as clear as for the segregation
behavior; the adsorption energy is first decreasing, and then
increasing when the atom number increases within a row. The
trend is consistent with the trend for transition metal–sulfur
binding energies (defined as the cohesive energy per metal–sulfur
bond) across the periodic table [29]. We also see that, compared to
the adsorption energy on pure Pd, most of the ternary models
exhibit a significantly stronger adsorption (more negative adsorption energy) of S. This indicates a very strong binding of sulfur on
these compounds, and may be used to exclude many of the metals
as good additives for sulfur resistance – if strong sulfur adsorption
automatically designates blocking of surface sites for hydrogen
adsorption. The most favorable additives in this respect can be
found in almost the same regions where surface segregation is
favorable; to the right of Ni, Rh, and Pt in the periodic table.
Even if the information in Fig. 5 can already be used to indicate
tendencies of sulfur adsorption on ternary membrane materials, it
is more useful to quantify the effect of stronger adsorption
energies in terms of the lowest release temperature Trel at which
adsorbed sulfur on the 111 surface will react with H2 to form
gaseous H2S and S thus is released from the surface. We have done
this by using literature data for the Gibbs free energy of the gas
species H2 and H2S (ΔGH2 ðgÞ and ΔGH2 SðgÞ ) for comparison, and
neglecting the entropy and temperature dependence of the
adsorption strength (which should be very small compared to
that of the gases). This has been included in a comparison of the
surface free energy of the surface with adsorbed sulfur Gsurf(Ads)
with the surface free energy of the pure slab Gsurf(Slab):
Gsurf ðAdsÞ–Gsurf ðSlabÞ ¼ ½Eads þ ΔGH2 ðgÞ –ΔGH2 SðgÞ –RT lnðpH2 S =pH2 Þ=A
ð4Þ
The surface unit cell area is A, and the partial pressures pH2 and
pH2 S of H2 and H2S are the only remaining parameters; we assume
that pH2 S =pH2 ¼ 20 10 6 in the following. The release temperature Trel for the adsorbed S species can now be found by solving
the equation
Gsurf ðAdsÞ–Gsurf ðSlabÞ ¼ 0
ð5Þ
That is, when the surface free energy of the surface with adsorbate
is higher (less stable) than that of the pure surface (or more
relevant, vacant surface adsorption sites corresponding to a locally
pure surface), S will combine with H2 in the gas phase to form H2S.
A second order polynomial parameterization of the experimental
Fig. 6. The highest release temperature of sulfur adsorption Trel (a) and the
maximal H2S concentration (b), both calculated from Eq. (5). The calculations have
assumed that pH2 S =pH2 ¼ 20 10 6 in (a) and T ¼ 450 1C in (b). Each point
corresponds to slab calculations of the ternary compound Pd–Ag–TM, except for
Pd (pure Pd) and Ag (Pd–Ag). The requirement for selecting a potential membrane
material for further study has been indicated by dotted lines.
free energies was used in order to solve Eq. (5) analytically.
Alternatively, Eq. (5) can be used to quantify the H2S pressure at
which the surface will be covered with S at a given temperature.
We have used T ¼450 1C for such calculations. The results are
displayed in Fig. 6.
We can now draw some conclusions from our results. First of
all, we need to define a threshold for Trel reflecting typical
operating conditions, which can be used as a criterion for potential
additives. We have chosen to use the requirement Trel o600 1C
when pH2 S =pH2 ¼ 20 10 6 , based on experience with such systems and the available literature. This is equivalent with requiring
that the maximal H2S concentration pH2 S =pH2 4 0:3 10 6 when
T¼ 450 1C. With this requirement, we can conclude that Mn, Cu,
Zn, Ga, Cd, In, Sn, Pt, Au, Hg, Tl, Pb, and Bi are potential additives.
Following our starting hypothesis, this means we cannot exclude
them from further study, since their sulfur adsorption strength is
rather low. Another way of stating this is that the abovementioned additives will not lead to significant blocking of
hydrogen adsorption sites by sulfur, which means that the hydrogen permeability will not be drastically reduced compared to pure
Pd3Ag when H2S is present. This evaluation does not take into
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O.M. Løvvik et al. / Journal of Membrane Science 453 (2014) 525–531
account other important issues like toxicity and volatility, which
obviously will reduce the above list of potential additives. Among
the remaining promising additives in this respect are Cu and Au,
which are already known to be beneficial additives in binary Pd
alloys.
It is important to be aware of the relatively large uncertainty
connected with DFT calculations, primarily because the exchangecorrelation term is not represented exactly (we used the PBE-GGA).
The calculated energies can usually be trusted within a range of
0.1 eV; this corresponds to a change of around 60 1C in the release
temperature and a factor of around 5 in the maximal H2S pressure.
This should not be confused with the numerical accuracy of our
calculations, which is in the order of 1 meV. Furthermore, only one
surface termination has been studied (the close-packed (111) surface),
and other surfaces could exhibit higher sulfur tolerance for some of
the excluded ternary compounds. Even if these are less abundant
surfaces than the (111) surface (which is most stable for most fcc
metals), they may exist in large enough quantities to allow for
efficient transport through the surface despite the (111) surface being
blocked by strongly adsorbed S. Also, we have only used one value of
pH2 S =pH2 in Fig. 6(a), 20 10–6. If this is reduced (increased) by an
order of magnitude, the release temperature decreases (increases) by
around 100 1C from a level of around 500 1C. Accordingly, if the
temperature in Fig. 6(b) is increased by 100 1C, the maximal H2S
concentration increases by around an order of magnitude. Finally, it
may be reasonable to open for alloys with slightly higher release
temperatures than the intuitive threshold, since a certain exchange of
adsorbates can also be expected below the release temperature. If we
assume that all this adds up to choosing 800 1C as our threshold
temperature (corresponding to a maximal H2S concentration of
1.4 10 9), we should also include Fe, Co, and Y among the potential
additives.
How does that compare to experiments? A recent study of
ternary Pd–Ag–TM membranes compared how rapidly the hydrogen flux dropped and recovered for membranes with TM¼Au, Cu,
Mo, and Y [30]. According to our hypothesis, Au and Cu should
perform well in this respect, Y should be intermediate, while the
Pd–Ag–Mo membrane should be completely blocked by sulfur due
to the strong binding energy between Mo and S. This is true for Au
and Cu, which both lead to enhanced H2 flux recovery rates from
H2S contamination compared to pure Pd–Ag membranes. However, it turns out that the Pd–Ag–Mo membrane did not perform
any worse than the others; contrary, it exhibited faster H2 flux
recovery from H2S contamination in relative terms than all the
other membranes in the study (Fig. 3 in Ref. [30]). This means that
the sulfur adsorption energy cannot be the only descriptor for
sulfur inhibition. We believe that a rationalization may be found in
Fig. 3: the segregation energy of Mo towards the bulk is among the
highest of all the TMs, indicating that Mo will not be available for
sulfur adsorption at the surface. This is consistent with the
findings in Ref. [30], where XPS depth profiles demonstrated that
the surface region of Pd–Ag–Mo is indeed depleted of Mo (Fig. 10d
in Ref. [30]). Thus, despite the strong sulfur affinity, Mo cannot be
excluded as additive to Pd–Ag membranes because of the strong
segregation tendency towards bulk. An updated hypothesis based
on the findings above then reads like this: an additive is promising
for further study (it cannot be excluded because of sulfur inhibition) when the sulfur adsorption strength is low (leading to a
reasonable release temperature) or the tendency to segregate
towards the bulk is strong.
Where does this leave our study? As we have already
remarked, there is a strong correlation between the TM–sulfur
binding energy and the segregation tendency of TM in Pd–Ag. This
is unfortunate for our conclusions, because it places most of the
elements in one of two categories: either the TM–S bonding is
weak or the TM segregation tendency is strongly towards the bulk.
The only exception is La, which exhibits both a very high
calculated release temperature and clear tendency to segregate
towards the surface.
We may nevertheless learn important lessons from the present
results. It is instructive to note that pure Pd and Pd–Ag are among
the systems with lowest release temperature in this study, even if
it is well-known that they exhibit poor sulfur resistance. The main
reason for this is the detrimental formation of sulfides like Pd4S,
which leads to loss of membrane material and eventual rupture
and breakdown. This was exemplified by a recent experimental
study, which showed that Au is among the most promising
additives for improving sulfur tolerance compared to pure Pd
and the Pd–Cu alloy system [31,32]. The most important role of Au
is then to hinder irreparable formation of Pd4S in the surface layer,
since this is much less pronounced for Pd–Cu and Pd–Au alloys
than for pure Pd and Pd–Ag alloys. This is consistent with previous
atomistic calculations which concluded that formation of Pd4S was
thermodynamically feasible for most pure Pd and binary Pd–Ag
surfaces, while Pd–Cu and Pd–Au did not display such behavior
[33].
But in order to limit formation of Pd4S in the surface region, the
additive should be present in this region. Additives which are
strongly segregated towards the bulk may be harmless when it
comes to sulfur inhibition, but then they may also be useless for
hindering formation of Pd4S. Thus, when the long term stability of
membranes is also taken into account, we have to require that the
segregation tendency of the TM to the bulk is not too strong, which
is the opposite of the previous conclusion! If this is coupled with
our updated hypothesis above, we end up with the following
description of a potential TM additive to Pd–Ag: it should be
present in the surface region (i.e. not too strong segregation
towards the bulk) and it should not bind sulfur too strongly (i.e.
not too high release temperature of S).
In effect we return to almost the same conclusions as our original
hypothesis, due to the already mentioned correspondence between S
adsorption strength and segregation tendency. The remaining question is where to place the threshold for ‘allowed’ segregation
energies: How large segregation energy Eseg12 designates ‘too strong’
segregation? To require Eseg12 o0 (segregation towards surface) is
clearly too strict, since temperature and entropy would lead to finite
population of TM at the surface even with Eseg12 40. Also, as
mentioned before, we can only expect our DFT calculations to be
reliable within 0.1 eV. A reasonable requirement could thus be
Eseg12 o0.2 eV. This combined with the requirement Trel o600 1C
leads to the following potential additives: Cu, Zn, Ga, Cd, In, Sn, Pt,
Au, Hg, Tl, Pb, and Bi. If the requirement is lifted to Eseg12 o0.4 eV
and Trel o800 1C, Co and Y can also be included among the
potential ones.
The present study did not investigate sulfide formation. As
discussed above, the irreparable formation of Pd4S at the surface
region may be most detrimental for the performance of Pd-based
hydrogen membranes, leading to failure of the membrane. This
study should thus be complemented by a screening of the most
promising materials with respect to sulfide formation in order to
reach a more conclusive recommendation. This may also include
the formation of Ag sulfides, Cu sulfides and mixed metal sulfides.
Such a study is under way [34]. Furthermore, the added transition
metals may change the diffusivity and solubility of hydrogen in the
bulk alloy, even at rather low concentration – this effect is
presently under investigation in our laboratory [10,30]. Another
possible effect of adding a third element to PdAg based membranes is to change the kinetics of dissociative hydrogen adsorption at the alloy surface, as well as the reverse desorption reaction
on the permeate side. Finally, the interplay with other adsorbents
like H2, H2O, or CO2, could shed further light on how to design
optimized materials for particular membrane applications.
O.M. Løvvik et al. / Journal of Membrane Science 453 (2014) 525–531
4. Conclusions
This modeling study was performed to identify potential
transition metal (TM) additives that can be used to improve sulfur
tolerance of ternary Pd–Ag–TM membrane alloys. It was found
that the atomic segregation of TMs played an important role in the
stability of adsorbed sulfur on the alloy surface. Without the
presence of sulfur, only a few elements were found to segregate
to the surface layer of Pd or Pd–Ag. This was reversed for many of
the additives when sulfur was adsorbed on the surface of ternary
Pd–Ag–TM compounds. The reason for this effect was due to
formation of very stable sulfur adsorbates, which correlate with
bulk sulfides with large formation energies. The tendency to sulfur
adsorption was quantified by the free energy of adsorption, which
was used to calculate a ‘release’ temperature; this was defined as
the temperature at which sulfur adsorption is not energetically
favored in a gas mixture of H2 containing a fraction of 20×10–6
H2S. It was also used to calculate the ‘maximal’ H2S concentration
at 450 1C, which is the level of H2S allowed before large parts of
the surface is blocked by adsorbed S. However, a recent experimental study has indicated that too strong segregation of the TM
towards the bulk led to surfaces very similar to those of pure Pd–
Ag membranes. To accommodate this information, our updated
requirement for a potential additive was that the release temperature be lower than 600 1C and the segregation energy towards
bulk be lower than 0.2 eV. This led to the following list of potential
additives: Cu, Zn, Ga, Cd, In, Sn, Pt, Au, Hg, Tl, Pb, and Bi. Other
criteria like low toxicity and volatility will further eliminate some
of those additives. A follow-up study will quantify the tendency of
these additives to form Pd4S at the surface, which is believed to be
the other important show-stopper for Pd-based membranes in
contact with H2S.
Acknowledgments
Support from the Research Council of Norway through the
program RCN-CLIMIT (Project no.: 215666/E20) and the European
Union through the EU-7FP CACHET-II Project, (Contract no.:
241342) (http://www.cachet2.eu/) is gratefully acknowledged.
We also gratefully acknowledge a grant of computational
resources from the Notur consortium. Discussions with R.A. Olsen
and S.M. Opalka are highly appreciated.
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