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The interaction of iron pyrite with oxygen, nitrogen and nitrogen oxides:
a first-principles study
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Marco Sacchi,* Martin C. E. Galbraith and Stephen J. Jenkins
Received 11th November 2011, Accepted 13th January 2012
DOI: 10.1039/c2cp23558g
Sulphide materials, in particular MoS2, have recently received great attention from the surface
science community due to their extraordinary catalytic properties. Interestingly, the chemical
activity of iron pyrite (FeS2) (the most common sulphide mineral on Earth), and in particular its
potential for catalytic applications, has not been investigated so thoroughly. In this study, we use
density functional theory (DFT) to investigate the surface interactions of fundamental
atmospheric components such as oxygen and nitrogen, and we have explored the adsorption and
dissociation of nitrogen monoxide (NO) and nitrogen dioxide (NO2) on the FeS2(100) surface.
Our results show that both those environmentally important NOx species chemisorb on the
surface Fe sites, while the S sites are basically unreactive for all the molecular species considered
in this study and even prevent NO2 adsorption onto one of the non-equivalent Fe–Fe bridge sites
of the (1 1)–FeS2(100) surface. From the calculated high barrier for NO and NO2 direct
dissociation on this surface, we can deduce that both nitrogen oxides species are adsorbed
molecularly on pyrite surfaces.
1. Introduction
Materials based on transition metal sulphides have recently
become popular in the surface science and catalyst communities
due in part to the extraordinary biomimetic catalytic properties
of MoS2 surfaces and MoS nanoparticles reported by Nørskov
and co-workers.1,2 In this context it is quite surprising that pyrite
(FeS2), an ubiquitous iron-containing mineral and the most
abundant sulphide material in the world has received limited
attention so far, and that the chemical properties of the pyrite
surfaces are still relatively unexplored.
As a mineral, pyrite is often found during the mining of
coal, iron and other transition metal and coinage metal
containing ores. The reaction of iron pyrite with water and
sulphates is the cause of acid mine drainage (AMD), a hugely
devastating phenomenon related to abandoned mines and
mine wastes that produces an increase in the acidity of streams
and natural aqueous environments, as well as in the concentration of dangerous heavy metals and metalloids.3
The few previous Ultra High Vacuum (UHV) surface
science studies concerning reactions on pyrite surfaces have
mostly focused on oxidation,4–6 hydration5–11 and desulphurisation.8,12 Recently, in the context of the ‘‘Iron–Sulphur
World Theory of Wächtershäuser’’,13 a limited number of
interesting studies have focused on the adsorption of simple
biomolecules (glucose and amino acids) on pyrite.14–16 Other
researchers have concentrated their attention on the electronic
properties of clean pyrite surfaces. In fact, since iron pyrite is a
semiconductor with an energy gap of B0.9 eV,17 in the past
FeS2 has been considered as a potential alternative to silicon
for fabrication of solar cells and batteries.18–21
Even though the physical properties and band structure of
pyrite are by now well characterised, and a few of its surface
reactions have been investigated, the potential catalytic activity of
pyrite remains for the most part unexplored, probably due to the
long standing opinion that surface sulphur (present in a 2 : 1 ratio
on the clean pyrite surface) would act as an inhibitor of possible
surface reactions. Molecular and atomic sulphur is known to
depress the activity of industrial catalysts when adsorbed on
metallic iron-based catalysts, but clearly such a view is of only
limited reliability when considering the reactivity of sulphide
compounds.
In the present study we explore theoretically the adsorption
of several important inorganic molecules on the FeS2(100)
surface, the most stable surface plane of pyrite. Furthermore,
as an attempt to explore the potential employability of natural
iron sulphides for environmental catalysis, we investigate the
dissociation of nitrogen oxides on this surface.
2. Theoretical method
Department of Chemistry, University of Cambridge, Lensfield Road,
Cambridge, CB2 1EW, UK. E-mail: [email protected];
Fax: +44(0)1223 762829; Tel: +44(0)1223 763519
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The method we use to calculate adsorption structures and
transition states has been described in detail in several
recent studies.22,23 and shall only be summarized briefly here.
Phys. Chem. Chem. Phys., 2012, 14, 3627–3633
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The total energy calculations reported in this study have been
performed using CASTEP, a plane wave, periodic boundary
conditions DFT code.24,25 For the present calculations we use
Vanderbilt Ultrasoft Potentials.26 A generalized gradient approximation (GGA) level of theory was included through the
Perdew–Wang ‘91(PW91) exchange and correlation functional.27
Integration over the Brillouin zone of the (1 1) FeS2 unit cell
was achieved by sampling reciprocal space over a 3 3 1
Monkhorst–Pack28 k-point mesh. The plane wave basis set
was expanded to a 340 eV energy cut-off to ensure convergence.
Six layers of Fe atoms (with associated S atoms) were found to be
sufficient to describe the FeS2 surface. We fixed the bottom three
FeS2 layers and left the three topmost layers (a total of 6 Fe and
14 S atoms in the case of the S-terminated surface used for our
adsorption studies) free to relax. Convergence with respect to
k-point sampling, kinetic energy cut-off, and slab thickness was
tested and found to be satisfactory. For instance, increasing the
energy cut-off from 340 eV to 380 eV decreased the total energy of
the bulk pyrite unit cell by about 0.001%, while increasing the
k-points sampled from 4 to 45 did not change significantly the
total energy (energy decreases by less than 0.001%). Increasing
the numbers of Fe layers in the pyrite slab from six to nine
decreased the energy per atom of less than 0.01%.
3. Results and discussion
3.1
Bulk properties
Pyrite crystallises in a cubic structure (Fig. 1), characterized by
iron atoms sitting on the same fcc substructure as Na atoms in a
rock salt crystal, and sulphur atoms positioned as dimers in the
complementary fcc sublattice equivalent to the Cl anions in
NaCl. The primitive unit cell contains four Fe atoms and eight
sulphur atoms with each Fe atom coordinated to six S atoms.
Two structural parameters, a (the unit cell edge length) and
u (defining the position of the S atoms in the unit cell and
expressed as a fraction of the parameter a), are normally used to
characterize the pyrite unit cell. Using the theoretical method
described above, we calculated a = 5.364 A and u = 0.384, close
to the experimental values (a = 5.416 A, u = 0.385).
We calculated the electronic band structure of bulk pyrite
and found it, within the limits of DFT, to be consistent with
previous theoretical and experimental studies. In particular,
we estimated a direct bandgap of 0.67 eV and an indirect
bandgap of 0.62 eV, both of them slightly smaller than the
experimentally measured bandgap (B0.9 eV).17 DFT is well
known to underestimate the magnitude of band gaps in
semiconductors and the overall accuracy of our band structure
calculation is consistent with the results of a previous similar
study performed by Cai et al. in which the authors reported a
band gap of 0.48 eV.29
3.2 Clean FeS2(100) surface terminations
The (100) plane of pyrite has been by far the most investigated
surface of this material,11,12,29–33 although theoretical studies
have also considered less stable surfaces, such as the (111) and
(110) facets.34–36 For the FeS2(100) surface, three possible surface
terminations are possible, and the relative stability of these
terminations has been recently investigated using DFT methods
by Alfonso.37 In order of decreasing stability in sulphur-lean
environments they are: the S-terminated (the only stoichiometric
termination), the S–S-terminated and the Fe-terminated
surface.37 In a sulphur-rich environment, the S–S terminated
surface (characterized by a topmost layer of sulphur dimers)
becomes the most stable. In this study, we will explore the
activity of pyrite for the reduction of nitrogen oxides in
sulphur-lean conditions, and therefore we limit our considerations to the stoichiometric (S-terminated) FeS2(100) surface. We
have nevertheless performed test calculations of the clean
FeS2(100) surface for the other non-stoichiometric terminations
(Fig. 2) with a focus on the presence of surface spin polarization.
We observed that the (1 1) Fe-terminated FeS2(100) surface
has a spin angular momentum of 3.10 mB localized on each
surface iron atom, while each atom of the S–S terminated surface
has zero net spin. Finally, the stoichiometric, S-terminated
surface has zero total spin and a negligible spin polarization on
each of the top-layer atoms. The stable FeS2(100) surface
termination under S-lean conditions is thus devoid of spin.
The clean FeS2(100) surface undergoes some relaxation
between the first and the second Fe layer, with a distance
between the surface Fe atom and the S atoms on the same
layer that remains almost identical to the bulk pyrite Fe–S
bond length (B2.2 Å), while the distance between surface
Fe and the S atom immediately below is reduced by 0.1 Å.
3.3 NO adsorption and dissociation
The adsorption of NO on the stoichiometric (S-terminated)
(1 1) FeS2(100) surface was investigated on two different
adsorption sites: the first-layer Fe atoms and S atoms. Three
different NO orientations were used as initial configurations
Fig. 1 The crystal structure of iron pyrite. Fe atoms are grey, S atoms
are yellow.
3628
Phys. Chem. Chem. Phys., 2012, 14, 3627–3633
Fig. 2 FeS2(100) surface Fe-terminated (a), S-terminated (b) and the
S–S-terminated (c).
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for calculating the most favourable adsorption geometry, with
oxygen pointing either towards or away from the surface, and
with the NO bond parallel to the surface plane. The S sites
were found to be much less reactive with respect to NO
adsorption, and only one of the three initial NO orientations
considered (with oxygen pointing up) produced a converged
stable structure, while the configurations with the O atom
binding to a S atom and with NO parallel to the surface did
not produce a stable final structure. The final optimised
geometries of adsorbed NO (NOads) are shown in Fig. 3,
while the calculated adsorption energies and some geometrical
parameters are displayed in Table 1.
The most favourable NO adsorption site and orientation by
far was found to be the one with NO atop an Fe atom, with the
N atom bonded to the surface and the N–O bond close to
vertical with respect to the surface normal. In the most stable
configuration the binding energy of NO is 1.71 eV. Adsorption
on the S sites was found to be much less energetically favourable.
Direct binding between the oxygen atom and the S site was found
to be energetically unfavourable, while the nitrogen atom of NO
can bind to a sulphur atom with the formation of a weak bond
(Eads = 0.13 eV, with a S–N bond length d(S–N) of 2.48 Å).
We calculated the density of states (DOS) for the isolated
NO molecule (NOgas) and for NO adsorbed on the most stable
adsorption site on pyrite (top of an Fe atom) which is shown
in Fig. 4. The DOS of NOgas was translated so that the
3s orbitals were aligned with the same band for NOads. The
linear projection of the DOS (pDOS) of the FeS2/NOads
system onto NOads allow us to better understand the details
of the molecule-substrate interaction.
A degree of covalent interaction is visible between the 4s
orbital of NO and the p orbitals of the sulphur dimers and
between the 5s, 1p and 2p orbitals with the Fe surface d band.
Comparing the DOS of NOgas and FeS2/NOads one can
observe a significant down-shift of the 2p and 5s orbital.
Fig. 5 shows the 2p orbital for the isolated (gas-phase)
molecule and for adsorbed NO. Strong interaction between
the adsorbate and the substrate via mixing of this orbital
causes a lowering in energy of the 2p orbital by about 2 eV.
Table 1 Adsorption energies, surface spin polarization (s) and bond
lengths for NO on FeS2(100)
Configuration
DEads (eV)
d(Fe–N)
Fe–NO
Fe–ON
Fe–(NO)
S–NO
1.71
0.57
0.28
0.13
1.72
2.04
d(Fe–O)
d(S–N)
s (h)
2.48
0.40
0.51
0.46
0.46
1.92
2.04
Fig. 4 Top graph: DOS of NO adsorbed on the most favorable
adsorption site of the stoichiometric (S-terminated) FeS2(100) (red
line) and of the clean FeS2(100) surface (black line). Middle graph:
projected DOS (pDOS) of adsorbed NO (blue line). The orbital labels
refer to the MOs of the isolated NO and assignment of pDOS peaks
(blue) indicates the new position of these orbitals upon adsorption.
Bottom graph: DOS for isolated NO molecule.
Fig. 6 shows the electron density difference (defined as Dr =
rFeS2/Noads rFeS2 rNOgas) due to the adsorption of NO on
FeS2(100) with respect to the isolated molecule and clean
surface. As one would expect, the electron density increases
between the nitrogen and the Fe atom, whereas some electron
density depletion is visible underneath the top Fe atom. Other
minor regions of electron density depletion are also visible in
the second layer substrate Fe atoms that do not form a direct
Fig. 3 Side and top view of the optimized adsorption structures of NO on the stoichiometric (S-terminated) FeS2(100). N atoms are blue and
O atoms are dark red. (a,b) NO adsorbed on a Fe atom via a N–Fe and a O–Fe bond respectively. (c) NO adsorbed horizontally on top of an
Fe atom. (d) NO weakly adsorbed on top of a S atom.
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NO bond length (d(N–O) = 1.199 Å) that is very close to the
value for the isolated molecule (d(N–O) = 1.188 Å).
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3.4 NO2 adsorption and dissociation
Fig. 5 (a) LUMO (2p) orbital of adsorbed NO compared with the
LUMO of the isolated molecule (b). The participation of the LUMO
in the bonding is clearly visible by the pronounced elongation of the
LUMO (2p) orbital towards the surface Fe atom. The involved Fe
orbital is clearly of dxz type. Purple/turquoise coloration here denotes
the wavefunction phase. The energy of the NOads band at the G point
is 1.98 eV and 1.85 eV for the two spin components.
Fig. 6 Top-view (a) and side-view (b) of the charge density difference
plot relative to NOads showing charge transfer upon adsorption of NO
on the FeS2(100) atop Fe site. Magenta contours indicate electron
density increase by 0.14 electrons/Å3; blue contours indicate electron
density decrease by 0.14 electrons/Å3.
bond with NO. On the NO internuclear axis region, the
density difference does not point to any significant change in
the strength of covalent binding of NO, as confirmed by the
We investigated the adsorption of NO2 on three different sites
of the FeS2(100) surface: two atop configurations with the
N atom oriented downwards (Fe–NO2 configuration) or with
one of the two O atoms bonded to a surface Fe atom
(Fe–ONO) respectively, and a bridge configuration with the
NO2 bonded on two neighbour Fe atoms by the two O atoms
(Fe–ONO–Fe). The adsorbed NO2 geometries are shown in
Fig. 7 while the adsorption energies and bond lengths are
reported in Table 2.
The most stable adsorption configuration was found to be
that in which the NO2 occupies a bridge position between two
non-equivalent surface Fe atoms (Fe–ONO–Fe configuration
in Table 2) with an adsorption energy of 1.49 eV. As it was
shown in the previous section, NO can occupy a single
adsorption site (either an Fe or a S atom), whereas NO2
adsorbed in a bridge site (Fe–ONO–Fe) can occupy two
adjacent Fe sites, therefore a (2 1) cell was chosen as unit
cell for our theoretical study. There are two equivalent bridge
sites within a (2 1) cell, and therefore the NO2 coverage for
adsorption on a bridge site (0.5 ML) is twice as much as for
NO2 on atop sites (0.25 ML). For the most stable geometry,
NO2 on a bridge site, we have also calculated the binding
energy for the 0.25 ML coverage and found that it is slightly
lower than for the 0.5 ML coverage (1.44 eV at 0.25 ML
compared to 1.49 eV at 0.5 ML), suggesting that a small
attractive stabilization potential exists between the two NO2
molecules in the same cell. Since every surface Fe atom is
surrounded by four other Fe atoms at the same distance, there
would be potentially four bridge sites for NO2 adsorption, but
among those four there are two non-equivalent pairs of
equivalent bridge sites: a pair of the type already considered
(shown in Fig. 7(c)) and another pair that are very unfavourable
for adsorption because a sulphur atom is located very close to the
position ideally occupied by the N atom (Fig. 8(b)). The steric
repulsion between the N atom and the surface S atom causes this
binding site to be very unfavourable for adsorption. For a (2 1)
cell it was found that this bridge site did not produce any stable
Fig. 7 Optimized adsorption structures of NO2 on FeS2(100). In this Fig. we use the following color conventions: Fe atoms are grey, S atoms are
yellow, N atoms are blue and O atoms are dark red.
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Table 2 Adsorption energies, surface spin polarization (s) and bond
lengths for NO2 on FeS2(100)
Configuration
DEads (eV)
d(Fe–N)
Fe–NO2
Fe–ONO
Fe–ONO–Fe
1.24
1.07
1.49
1.94
d(Fe–O)
y (ML)
s (h)
1.96
2.02
0.25
0.25
0.50
0.25
0.00
0.23
adsorbed NO2, while for the (1 1) cell the calculations resulted
in an only slightly positive binding energy (0.18 eV), about a
factor of eight less stable than the adjacent non-equivalent bridge
site (Fe–ONO–Fe).
We investigated the bonding of NO2 adsorbed in the most
stable adsorption configuration by analyzing the projected
density of states (pDOS) of the adsorbed molecule and comparing
it with those of the clean surface and the isolated molecule (Fig. 9).
As in the case of NO, the NO2 molecule has low energy orbitals
(3a1 and 2b2) that do not interact significantly with the surface
bands of pyrite, hence their correspondent peak position is
unshifted with respect to the isolated molecule. The change in
the DOS in the region between 12.5 eV and 9 eV is due to the
interaction of the 4a1 orbital of NO2 with the 3s bands of pyrite.
The interval between 7 eV and 0 eV entails the interaction
between the orbitals 3b2, 1b1, 5a1, 1a2, 4b2 and 6a1 with the 3s and
3p bands of sulphur and the 3d bands of iron. The energies of the
6a1 orbital of adsorbed NO2 are 0.40 eV and 0.12 eV for the
two spin components, whereas for the isolated NO2 the spin-split
orbitals are at 0.19 eV and 1.74 eV.
The bonding of NO2 on pyrite has also been analyzed by
plotting the electron density difference and looking at the
regions of charge accumulation and depletion upon adsorption
(Fig. 10). One can see that there is a significant charge accumulation between the O atoms and the Fe atoms, confirming the
presence of a strong covalent bond between NO2 and the surface,
while at the same time charge depletion is also clearly observed in
the intranuclear region of NO2(ads). The NO bonds softening is
confirmed by a slight elongation of the N–O bonds in NO2(ads).
The relative importance of electron back-donation is suggested
by the presence of clear orbital interaction between the NO2
LUMO and the dz2 orbitals of Fe (Fig. 11).
Fig. 8 (a) NO2 adsorbed on the most favorable site bridge site
(Fe–ONO–Fe) of the (1 1)–FeS2(100) cell (the distance d(N–S)
between the N atom and the underlying S atom is 3.20 Å). (b) NO2
adsorbed on the non-equivalent bridge site of (1 1)-FeS2(100) cell,
having an S atom positioned immediately adjacent to the N atom
(d(N–S) = 1.76 Å). The steric repulsion between N and S causes this
position to be energetically unfavorable.
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Fig. 9 Top graph: DOS of NO2 adsorbed on the most favorable
adsorption site of FeS2(100) (red line) and of the clean FeS2(100)
surface (black line). Middle graph: projected DOS (pDOS) of the
adsorbed NO2 (NO2,ads). Bottom graph: DOS for isolated NO2
molecule.
3.5 NO and NO2 dissociation on FeS2(100)
Before exploring the potential catalytic properties of pyrite for
reduction of nitrogen oxides, we have evaluated the most
favourable adsorption sites for atomic and molecular nitrogen
and oxygen (Table 3). The minimum energy adsorption sites
for N2 and O2 are displayed in Fig. 12. For both N2 and O2
adsorption on Fe atoms is more favourable than the adsorption
on S atoms. We also tested the adsorption for two different initial
molecular orientations: with the molecular axis parallel or
perpendicular to the surface. We found that both N2 and O2
adsorbs preferentially atop Fe atoms, with an almost vertical
orientation of the molecular axis (Fig. 12(a) and (b)).
The enthalpy of adsorption of N atom with respect to gas
phase N2 is always positive, meaning that the adsorbed
N fragments are always less thermodynamically stable than the
parent gas-phase N2 molecule. Comparing the relative stability of
the N adsorption sites it was found that the minimum energy site
is on Fe atoms. On the contrary, the O atom adsorbs quite
strongly on pyrite and, surprisingly, it was observed that an
Fig. 10 Top-view (a) and side-view (b) of the charge density difference plot relative to NO2(ads) on a bridge site, showing charge transfer
in the region between the molecule and the Fe atoms upon adsorption
of NO2. Magenta contours indicate electron density increase by 0.02
electrons/Å3; blue contours indicate electron density decrease by 0.02
electrons/Å3.
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Fig. 11 LUMO (6a1) orbital of adsorbed NO2 (top panel) compared
with the LUMO of the isolated NO2 (bottom panel). The energy of the
6a1 band of NO2,ads at the G point is 0.40 eV and 0.12 eV for the
two spin components.
Fig. 13 TS for NO dissociation on a FeS2(100) bridge site. The length
of the dissociating N–O bond is 1.88 Å and the calculated barrier
height is 2.66 eV.
Table 3 Adsorption energies, surface spin polarization (s) and bond
lengths for O2 and N2 on FeS2(100). In the table are reported the
adsorption energies for the molecular species on Fe sites only, because
S sites are non-reactive for N2 and O2
Molecule
Orientation
DEads (eV)
d(Fe–O)
O2
O2
N2
N2
Ver.
Hor.
Ver.
Hor.
0.57
0.30
0.99
–0.05
1.88
1.93
d(Fe–N)
s (h)
1.80
2.19
1.8
0.0
0.0
0.0
Fig. 14 TS for NO2 dissociation on a FeS2(100) bridge site (equivalent
to bridge site of Fig. 8(a)). The length of the dissociating N–O bond is
2.25 A and the calculated barrier height is 2 eV.
Fig. 12 Most favorable adsorption sites for molecular oxygen and
nitrogen on pyrite. (a) O2 adsorbed on an atop Fe site of FeS2(100),
the binding energy is 0.57 eV and the molecular axis is tilted 451 with
respect to the surface normal. (b) N2 adsorbed on an atop Fe atom, the
binding energy is about 1 eV and the molecule is aligned in within
5 degrees with respect to the surface normal.
O adatom is more stable on S atoms (Ea = 1.34 eV) than on
Fe atoms (Ea = 0.77 eV).
We have investigated the dissociation reactions NO - N + O
and NO2 - NO + O on the (1 1)-FeS2(100) cell by applying a
combination of Linear Synchronous Transit and Quadratic
Synchronous Transit methods (LST/QST).38 The minimum
energy reaction pathway for NO dissociating leaves N on the
atop Fe site (most favourable site) and produces O adsorbed on a
free Fe site (Fig. 13). The reaction is endothermic (+0.38 eV) and
has very high energy (2.66 eV) (higher than the binding energy
of NO) therefore it appears that NO is likely to chemisorb
molecularly on FeS2 at any surface temperature. Another reaction
path for NO dissociation was considered, with the most stable
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Phys. Chem. Chem. Phys., 2012, 14, 3627–3633
adsorption site for NO as reactant state and the most stable
surface adsorption sites for N and O as product state, i.e. the
N atom on an iron site and the O atom on a sulphur site. This TS
search however yielded a very high reaction barrier of 5.44 eV.
For NO2 dissociation we considered as a starting point NO2
adsorbed on the most stable bridge site producing NO and O
fragments on the two Fe atoms binding sites (Fig. 14). This
reaction is also slightly endothermic (+0.15 eV) and has an
energy barrier of 2 eV.
If we compare the energy requirement for the direct dissociation
of NO and NO2 on FeS2 with those recently calculated by
Chen et al.39 for NO and NO2 on Fe{111} we note that on
Fe{111} NO2 dissociate with an energy barrier of less than 0.5 eV,
while NO dissociation proceeds with an energy barrier of about
0.9 eV.
4. Conclusions
In this work we have investigated the interaction of iron pyrite
with several molecules: O2, N2, NO and NO2. We found that
most of the species adsorb relatively strongly on the FeS2(100)
surface and that for all the chemical species considered in this
study the most favourable adsorption sites are the top layer
iron atoms. We have also explored the activity of FeS2 surface
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for NO and NO2 reduction by calculating the energy barrier
for the direct dissociation of these adsorbed species. The
calculated high activation barriers for these reactions suggest
that dissociation of nitrogen oxides species on this surface is
unlikely even at high temperatures.
Acknowledgements
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We acknowledge Stephen Driver for useful discussions and
comments. MS would like to thank the Swiss National Science
Foundation for financial support of this work.
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