A molecular approach for unraveling surface phase transitions: Sulfation of BaO as a model NOx trap

Article abstract of DOI:10.1002/chem.201103950

SO₃-induced surface reconstruction: The SO₃ molecule as a multidentate ligand induces remarkable surface reconstruction phenomena on alkaline earth oxide surface. By using ab initio computations, adsorption properties are derived to elucidate the thermodynamics of the SO₃-BaO system. Copyright

Full text of DOI:10.1002/chem.201103950

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DOI: 10.1002/chem.201103950
A Molecular Approach for Unraveling Surface Phase Transitions:
Sulfation of BaO as a Model NOx Trap
Nikola Rankovic,*[a, c] Cꢀline Chizallet,*[b] Andrꢀ Nicolle,[a] and Patrick Da Costa[c]
Understanding and modeling the way oxides interact with
their environment is of crucial interest for various domains
of chemistry, for example, geoscience,[1] material science,[2]
and catalysis.[3] Despite the great attention that basic solids
have received in the past years,[4] important questions about
their reactivity still remain open.[5] The basicity and the
chemical behavior of the alkaline earth oxides is governed
by the chemistry and the local environment of surface oxy-
gens and cations.[6] Among them, BaO stands out as a poten-
tial candidate for industrial depollution applications for its
remarkable affinity to adsorb amphiphilic molecules.[7,8]
More precisely, BaO is a prominent storage material for
commercial-vehicle NOx abatement systems[9,10] and for fuel-
cell applications.[11] A challenge for its large-scale implemen-
tation is represented by sulfur poisoning, leading to BaSO4
formation. Thus, understanding the way BaO interacts with
SO3 is of great interest for environmental chemistry, given
the presence of sulfur compounds in fossil fuels and the
harmful effects that SO3 has on health, the environment,
and infrastructures.[12] First-principle modeling provides
a powerful framework for describing and eventually control-
ling chemical reactions.[13,3] However, no modeling study
fully assessing SO3 adsorption thermodynamics or the effect
of adsorption-site topology on BaO has been previously re-
ported. One of the first studies in the field by Pacchioni
et al.[14] targeted SO2 adsorption on MgO and CaO. Chemi-
sorption was found to take place by anchoring the sulfur
atom to a surface oxygen atom. Density functional theory
(DFT) computations by Schneider et al.[15,16] showed that
both SO2 and SO3 adsorb as Lewis acids on a basic O5C site
on BaOACTHUNTRGEN(NUG 100) to form pyramidal-like sulfite (SO2) and tetra-
hedral-like sulfate (SO3) surface species. Similarly, Karlsen
et al.[17] performed ab inito calculations that confirmed an
increasing affinity toward SOx adsorption with increasing al-
kaline earth oxide basicity. Although they provide a consis-
tent picture of SOx chemistry on alkaline earth oxides, the
existing studies only deal with low coverages, never assess-
ing the effect of potential repulsive lateral interactions, cru-
cial to draw a picture of the NOx trap in realistic operating
conditions.
In the present work we use periodic DFT calculations and
ab initio molecular dynamics (MD) at the generalized gradi-
ent approximation (GGA) level and energies corrected
from Hartree–Fock exchange. We have investigated BaSO4
formation through SO3 adsorption on ideal BaOACTHNUTRGENUG(N 100) ter-
race surfaces and on two types of surface irregularities:
monoatomic steps and kinks. Remarkable surface recon-
struction phenomena were observed at high SO3 coverage.
The affinity of the (100) surface for SO3 surprisingly appears
to be lower than that of bulk BaO. Based on DFT results,
a thermodynamic model was conceived, completing the
image of BaO particle evolution in the presence of SO3. In
a broader sense, SO3 can be regarded as a multidentate
ligand in which all three O atoms together with the S atom
represent coordination sites. Its reactivity is a consequence
of the affinity of O atoms to coordinate a maximum number
of Ba atoms and the oxophilic S atom to create a tetrahedral
sulfate-like environment.
[a] Dr. N. Rankovic, Dr. A. Nicolle
IFP Energies nouvelles
Site topology is expected to strongly influence the adsorp-
tion energy.[18] Surface defects (structures detailed in the
1 et 4 avenue de Bois-Prꢀau
92852 Rueil-Malmaison (France)
Fax : (+33)1-4752-7068
Supporting Information) exhibit
a stronger affinity to
adsorb SO3 in comparison to the ideal (100) terraces. The
molecule adsorbs over a tetracoordinate step surface oxygen
atom to form a tetrahedral environment for the sulfur atom.
Two SO3 oxygen atoms fit in the crease of the step to coor-
dinate step (Ba4C) and lower-surface (Ba5C) sites (Figure 1),
leading to a remarkable stability of the adsorbed state with
the adsorption energy DadsU of ꢀ513 kJmolꢀ1. The presence
of a second SO3 molecule per supercell leads to saturation
of the O4C sites. Lateral interactions of two adjacent SO3
molecules on the step lower the adsorption energy for the
second molecule to ꢀ425 kJmolꢀ1. A subsequent MD on the
saturated step does not alter the results obtained for a static
optimization. Surface sulfate is slightly more stable
E-mail: nikola.rankovic@ifpen.fr
[b] Dr. C. Chizallet
IFP Energies nouvelles
Rond-point de lꢁꢀchangeur de Solaize
BP 3, 69360 Solaize (France)
Fax : (+33)4-3770-2066
E-mail: celine.chizallet@ifpen.fr
[c] Dr. N. Rankovic, Prof. P. Da Costa
Institut Jean Le Rond dꢁAlembert
UPMC Paris 6, CNRS UMR 7190
2 place de la gare de ceinture
78210 Saint-Cyr-lꢁEcole (France)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201103950.
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Figure 1. SO3 differential adsorption energies and optimized geometries on various BaO surfaces as a function of the fractional coverage. Enlarged struc-
tures of the insets are available in Supporting Information.
(ꢀ531 kJmolꢀ1) over an intrinsically more reactive kink O3C
site.
50 kJmolꢀ1 higher compared to the adsorption on bare sur-
face.
When adsorbed over a (100) terrace at low coverage
With an increase in fractional coverage, a former (100)
terrace undergoes radical structural transformations. When
four SO3 molecules are co-adsorbed per cell (0.5 ML), the
initial geometry optimization yields a stable structure in
which SO3 molecules form a square around a barium ion
that is extracted from the surface (Figure 2). A subsequent
MD simulation leads to the final reconstructed state with an
adsorption energy of ꢀ423 kJmolꢀ1 per SO3 molecule for
the 0.25 to 0.5 ML transition. In this most stable configura-
tion, three Ba atoms are extracted and two S atoms have
diffused at the surface level. These are the beginnings of
a surface versus bulk sulfation transition.
MD calculations were repeated for higher fractional cov-
erages of 0.75 and 1 ML. Interestingly, for the stoichiometric
SO3 coverage of 1 ML, all attempts to simulate a (100) sur-
face entirely covered with SO3 were unsuccessful, leading to
a desorption of one or more SO3 molecules from the sur-
face. We overcame the steric hindrance by constructing the
upper layers as a result of an epitaxial growth of bulk
(0.125 ML;
ML=monolayer),
the
most
stable
(ꢀ484 kJmolꢀ1) adsorption mode corresponds to a sulfate-
like species in which the electrophilic S atom sits above an
O
5C center. Additional interactions with Ba5C orient the mol-
ecule in such a way that two out of three O atoms are slight-
ly rotated away from the Ba5C sites. Our findings are in line
with literature data on SO3 adsorption over O5C.[15,17]
The rationale for exploring higher SO3 surface coverages
is based on the high adsorption energy obtained at
0.125 ML. Once on the surface, we observe that SO3 can
penetrate and restructure its environment to form amor-
phous bulk BaSO4. We gradually increased the number of
adsorbed molecules to explore geometries and energies for
various fractional coverages (0.25, 0.5, 0.75 and 1 ML).
When more than one molecule adsorbs simultaneously, it is
possible to imagine several co-adsorption structures. For the
case of two molecules co-adsorbed on the supercell
(0.25 ML), we conclude that the most stable conformation is
the one in which two SO3 molecules are adsorbed onto two
neighboring O5C sites (Figure 1). The (100) terrace is not re-
constructed yet, and the adsorption energy of ꢀ434 kJmolꢀ1
per SO3 molecule leading from 0.125 to 0.25 ML is about
BaSO4 on the top of the BaOACTHNUTRGNE(UNG 100) surface (Figure 2). Ad-
sorption energies given in Figure 1 suggest that from a ther-
modynamic point of view, adsorbing SO3 onto surface terra-
ces will lead to radical surface reconstruction and the
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Surface Chemistry
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ities (kinks and steps) react at the earliest stage. Typically, at
PSO =10ꢀ10 bar, kinks and steps (0.5 ML) are poisoned at
3
temperatures lower than 1350 K. A further increase in SO3
partial pressure (or a decrease in temperature at constant
PSO ) leads to restructuring of (100) terraces. However, at
3
this stage, bulk BaSO4 is thermodynamically more stable
than bulk BaO, meaning that the SO3 ligand prefers encap-
sulated (concave) bulk environments rather than flat surfa-
ces. Thus, the reconstructed (100) surface is only a metasta-
ble intermediate leading to bulk BaSO4. Step saturation is
observed as the final stage of sulfation. In the operating con-
ditions of typical fossil-fuel combustion (10 ppm S, 623–
773 K), BaSO4 is more stable than BaO, leading to the deac-
tivation of the storage material. Thermal desulfation is pro-
posed as one of the regeneration techniques requiring high
temperatures (>873 K).[19] It first leads to a desorption of
SO3 from saturated steps. Subsequently, a decomposition of
bulk BaSO4 to BaO and SO3 takes place. Surface irregulari-
ties, however, are more resistant to SO3 desorption and
remain sulfated after bulk BaSO4 has decomposed to BaO.
Because of the great stability of bulk BaSO4 compared to
bulk BaO, the role of the (100) terraces is to channel SO3 to
the core of the particle and back to the atmosphere. Our
calculations suggest that the oxide–sulfate reconstruction is
initiated by the terraces rather than the defects (which do
not induce any reconstruction). Therefore, a metastable
state in which (100) surfaces are sulfated with BaO as bulk
Figure 2. Side view of the geometries obtained after SO3 adsorption on
BaOACHTUNGTRENNUNG(100) at 0.5 ML (upper) and 1 ML (lower). Left: initial geometries,
middle: first geometry optimization, right: quenched geometries after
MD.
BaOACHTUNGTRENNUNG(100)–SO3 system will tend to stabilize at stoichiometric
coverage of qSO =1 ML. In other words, all intermediary
3
coverages are metastable.
To illustrate and compare relative stabilities of different
surface and bulk states, we built a phase diagram for the
SO3–BaO–BaSO4 system (Figure 3). A critical discussion of
the temperature/pressure conditions at which phase transi-
tions occur is given in Supporting Information. This ap-
proach reveals that, upon exposure to SO3, surface irregular-
phase only has physical meaning when increasing PSO . For
3
the reverse route, SO3 desorption follows the species ther-
modynamic stabilities. This dynamic behavior is depicted in
Figure 3. Left: BaO surface and bulk evolution as a function of SO3 partial pressure and temperature and a comparison with standard operating condi-
tions of a fossil-fuel exhaust. Experimental thermal desulfation conditions (T> 873 K, PSO <10ꢀ5 bar) are illustrated for comparison. Right: Evolutive
3
model of a BaO particle during sulfation (filled arrows, PSO increase/T decrease) and desulfation (white arrows, PSO decrease/T increase).
3
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N. Rankovic, C. Chizallet et al.
Figure 3 (right), representing an evolution of a BaO particle
during sulfation and desulfation.
Acknowledgements
The authors are grateful to Dr. P. Raybaud (IFPEN) for help in perform-
ing Phonon calculations. This work was performed using HPC resources
from IFPEN and GENCI-CINES (Grant 2011-ifp6134).
In summary, we demonstrate a strong affinity of BaO to
adsorb SO3 leading to remarkable surface reconstructions.
We propose a thermodynamic model describing the evolu-
tion of a BaO particle into BaSO4 through hysteresis of
a metastable intermediary. The phase transition is suggested
to occur via terraces and not via defects, even if the latter
intrinsically interact more strongly with SO3. These findings
are important for understanding the way SO3 poisons the
adsorbing material and developing more resistant catalyst
formulations.
Keywords: alkaline earth oxides
·
barium
·
density
functional calculations · SO3 adsorption · surface chemistry
[1] G. E. Brown, V. E. Henrich, W. H. Casey, D. L. Clark, C. Eggleston,
A. Felmy, D. W. Goodman, M. Grꢅtzel, G. Maciel, M. I. McCarthy,
K. H. Nealson, D. A. Sverjensky, M. F. Toney, J. M. Zachara, Chem.
Rev. 1999, 99, 77.
[2] R. P. Stoffel, C. Wessel, M.-W. Lumey, R. Dronskowski, Angew.
Chem. 2010, 122, 5370; Angew. Chem. Int. Ed. 2010, 49, 5242.
[3] J. K. Norskov, T. Bligaard, J. Rossmeisl, C. H. Christensen, Nat.
Chem. 2009, 1, 37.
[4] A. Corma, S. Iborra, Adv. Catal. 2006, 49, 239.
[5] H. Lauron-Pernot, Catal. Rev. Sci. Eng. 2006, 48, 315.
[6] M. Che, A. J. Tench, Adv. Catal. 1982, 31, 77.
[7] C. H. Kim, G. Qi, K. Dahlberg, W. Li, Science 2010, 327, 1624.
[8] B. M. Weiss, K. B. Caldwell, E. Iglesia, J. Phys. Chem. C 2011, 115,
6561.
[9] S. Roy, A. Baiker, Chem. Rev. 2009, 109, 4054.
[10] P. Granger, V. I. Parvulescu, Chem. Rev. 2011, 111, 3155.
[11] L. Yang, Y. Choi, W. Qin, H. Chen, K. Blinn, M. Liu, P. Liu, J. Bai,
T. Tyson, M. Liu, Nat. Commun. 2011, 2, 357.
[12] W. W. Kellogg, R. D. Cadle, E. R. Allen, A. L. Lazrus, E. A. Martell,
Science 1972, 175, 587.
[13] M. Salciccioli, M. Stamatakis, S. Caratzoulas, D. G. Vlachos, Chem.
Eng. Sci. 2011, 66, 4319.
[14] G. Pacchioni, J. M. Ricart, F. Illas, J. Am. Chem. Soc. 1994, 116,
10152.
[15] W. F. Schneider, J. Phys. Chem. B 2004, 108, 273.
[16] W. F. Schneider, J. Li, K. C. Hass, J. Phys. Chem. B 2001, 105, 6972.
[17] E. J. Karlsen, M. A. Nygren, L. G. M. Pettersson, J. Phys. Chem. B
2003, 107, 7795.
Computational Methods
Periodic DFT calculations were performed in the framework of the
GGA-PW91 functional,[20] as implemented in VASP 4.6 package.[21,22]
The interaction between core and valence electrons was described by the
projector augmented waves (PAW) approach,[23] with a cutoff energy of
400 eV. The convergence criterion for the electronic self-consistent field
(SCF) loop was set to 5ꢃ10ꢀ6 eV and geometry optimizations performed
until the convergence on forces (0.02 eVꢄꢀ1) was reached. A dipolar cor-
rection along the direction perpendicular to the slab was applied. After
performing the initial structural relaxation, velocity-scaled molecular dy-
namics were performed to efficiently investigate surface reconstruction.
(see Supporting Information for details). Reaction energies calculated
from geometries optimized at the GGA level were rectified at constant
geometry by employing the range-separated hybrid Heyd–Scuseria–Ern-
zehof (HSE06)[24, 25] exchange-correlation functional, implemented in
VASP 5.2.
We were interested in two types of values: differential adsorption energy
[DadsU, Eq. (1)], reflecting an incremental adsorption of SO3, and mean
adsorption energy [DmadesanU, Eq. (2)], relative to adsorption of n molecules
on bare surface. USO , UBaO and UBaO+nSO are the energies of SO3, BaO,
3
3
and the adsorbed states.
[18] C. Chizallet, G. Costentin, M. Che, F. Delbecq, P. Sautet, J. Phys.
Chem. B 2006, 110, 15878.
[19] D. H. Kim, J. Szanyi, J. H. Kwak, T. Szailer, J. Hanson, C. M. Wang,
C. H. F. Peden, J. Phys. Chem. B 2006, 110, 10441.
[20] J. P. Perdew, Y. Wang, Phys. Rev. B 1992, 45, 13244.
[21] G. Kresse, J. Hafner, Phys. Rev. B 1994, 49, 14251.
[22] G. Kresse, J. Furthmꢆller, Comput. Mater. Sci. 1996, 6, 15.
[23] G. Kresse, D. Joubert, Phys. Rev. B 1999, 59, 1758.
[24] J. Heyd, G. E. Scuseria, M. Ernzerhof, J. Chem. Phys. 2003, 118,
8207; Corrigendum: J. Heyd, G. E. Scuseria, M. Ernzerhof, J. Chem.
Phys. 2006, 124, 219906.
DadsU ¼ U
ꢀ U
ꢀ U
ð1Þ
ð2Þ
BaOþnSO3
BaOþðnꢀ1ÞSO3
SO3
U
ꢀ UBaO ꢀ nU
BaOþnSO3
SO3
DmadesanU ¼
n
Thermodynamic stability domains were drawn as a function of T and PSO
3
by calculating the free energy of sulfation, taking into account the chemi-
cal potential variations of SO3 (modeled as an ideal gas) and of the con-
densed phases (according to approximations given in Supporting Infor-
mation).
[25] J. Heyd, G. E. Scuseria, J. Chem. Phys. 2004, 121, 1187.
Received: December 16, 2011
Published online: && &&, 0000
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Surface Chemistry
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Surface Chemistry
N. Rankovic,* C. Chizallet,* A. Nicolle,
P. Da Costa .................... &&&& — &&&&
A Molecular Approach for Unraveling
Surface Phase Transitions: Sulfation
of BaO as a Model NOx Trap
SO3-induced surface reconstruction:
The SO3 molecule as a multidentate
ligand induces remarkable surface
reconstruction phenomena on alkaline
earth oxide surface. Using ab initio
computations, adsorption properties
are derived to elucidate the thermody-
namics of the SO3–BaO system.
Chem. Eur. J. 2012, 00, 0 – 0
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