Physics Letters A 378 (2014) 659–666

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Physics Letters A

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Adsorption and oxidation of sulfur dioxide on the yttria-stabilizedzirconia surface: ab initio atomistic thermodynamics study

Xingli Chu a, Zhansheng Lu a, Zongxian Yang a,∗, Dongwei Ma b, Yanxing Zhang a,Shasha Li a, Puyuan Gao a

a College of Physics and Electronic Engineering, Henan Normal University, Xinxiang, Henan 453007, Chinab School of Physics, Anyang Normal University, Anyang, Henan 455000, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 8 October 2013Received in revised form 17 December 2013Accepted 31 December 2013Available online 3 January 2014Communicated by R. Wu

Keywords:Atomistic thermodynamics methodInterfaceSolid oxide fuel cellSulfur dioxideOxygen-enriched YSZSulfur poisoning

The interaction of SO2 with the yttria-stabilized zirconia (YSZ) (111) and oxygen-enriched YSZ (111)(YSZ+O) surface is investigated using the first-principles method based density functional theory (DFT).It is found that SO2 is adsorbed either as a molecule or forms SO2−

3 species with new S–O bonds to a

surface oxygen on the YSZ (111) surface. In addition, there exist other species, e.g., SO3 and SO2−4 on

the very active YSZ+O (111) surface. Using the ab initio atomistic thermodynamics method, we present adetailed analysis on the stability of the SO2–YSZ/YSZ+O system as a function of the ambient conditions.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

In solid oxide fuel cells (SOFC), sulfur-containing compoundsare the most commonly encountered contaminants in readily avail-able fuels and are difficult to be completely removed efficientlyand economically. Even with small amount, they severely degradethe performance of SOFC due to the adsorption of sulfur on theanodes, which blocks the active sites for desirable reactions [1,2].Thus, the removal of the adsorbed sulfur from the anodes is a crit-ical step toward sulfur tolerance, which has been extensively stud-ied via experimental and computational approaches [3–8]. There-into, a promising way for removing the adsorbed sulfur is theoxidation of sulfur by O2 and H2O to form SO2 [9]. However, theformed SO2 is also one of the most dangerous environment pol-lutants and a very corrosive molecule, which still needs to becleaned before being emitted to the air [1,10,11]. So, it is highlydesirable to analyze the reaction of SO2 on the anode surfaces andfurther convert SO2 to other sulfur-containing species of lower risk(such as Sn species, etc.) [12,13].

On the other hand, YSZ (yttria-stabilized zirconia; 8 mol%Y2O3), owing to its high ionic conductivity and good thermal resis-tance, is commonly used as part of the anode in the current SOFCs.Therefore, knowledge of the interaction, adsorption, and oxidation

* Corresponding author.E-mail address: yzx@henannu.edu.cn (Z. Yang).

0375-9601/$ – see front matter © 2014 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.physleta.2013.12.037

processes of the SO2 molecule on the YSZ surface might help for adeeper understanding and even destructing or removing the sulfur-poisoning of the SOFC anodes. However, the direct experimentalevidence on identifying different adsorption states of SO2 moleculeon the YSZ surface is limited due to the difficulty in carrying outin situ experiments [1]. In this paper, we use the first-principlesmethod based on density functional theory (DFT) to study thesulfur oxide species (SOx) on the YSZ and oxygen-enriched YSZsurface (denoted as YSZ+O in the following). The YSZ+O surfacehas been found to be very active for the oxidation of H2 and CH4and the dissociation of O2 [14–18]. For example, Alexandr Gorskiet al. [19] combined theoretical and experimental studies of H2and CO oxidation over YSZ surface and found that the CO oxi-dation on the YSZ+O surface leads to direct CO2 formation viaEley–Rideal (ER) reaction, and in the case of H2, the surface willget hydroxylated with the direct formation of H2O. Under surfaceoxygen depleting operating conditions, the resulting YSZ surface isvirtually inert toward CO and H2 oxidation. However, to the best ofour knowledge, no direct experimental and theoretical studies onthe interaction of SO2 molecule with the YSZ and YSZ+O surfacesare available.

Furthermore, the interaction mechanisms between SO2 andsurfaces may be influenced by temperature and partial pressurein the surrounding environment [11,20,21]. For example, Sayagoet al. [22] have proposed that the impinging SO2 molecules re-act strongly with the oxygen atoms on the TiO2 surface to form

660 X. Chu et al. / Physics Letters A 378 (2014) 659–666

Fig. 1. (Color online.) Slab models: (a) for the YSZ (111) surface; (b) for the YSZ+O (111) surface. The bottom three atomic layers (in the dashed rectangular box) are fixedat their bulk positions in the calculations. The slabs are repeated in the z-direction (vertical) and separated by vacuum gaps of 15 Å. The green, gray and red balls representthe Zr, Y, and O atoms, respectively.

mainly sulfite and sulfate species, and higher exposures lead to theformation of an adsorbed SO2 multilayer. Luo et al. [23,24] havefound that, under oxidizing conditions, the sulfate is stable to ap-proximately 1073 K, above which the sulfate decomposes into SO2and O2 on ceria. When the sulfate is reduced in H2, some H2S isformed along with the Ce2O2S. Therefore, at this end, we will con-struct the phase diagrams for the adsorption and transformationof SO2 on the YSZ/YSZ+O (111) surfaces using the ab initio atom-istic thermodynamics method to provide a link between the DFTresults and environmental effects.

The article is organized as following. The description of themodel and the theoretical method employed are given in Section 2.Results and discussion are given in Section 3, and the conclusionis summarized in Section 4.

2. Model and computational details

2.1. Description of the model systems

The free SO2 molecule is simulated in a 10 × 10 × 10 Å3 cubicbox with the Monkhorst–Pack [25] k-point grid of 8 × 8 × 8 for theBrillouin zone integration. For the isolated SO2 molecule, we ob-tain an optimized S–O bond length of 1.45 Å and an O–S–O bondangle of 119.4◦ , which are in good agreement with the experimen-tal values [26] of 1.43 Å and 119.3◦ .

To obtain a representative slab for YSZ, firstly, we modelthe ZrO2 (111) surface as a slab with three O–Zr–O triple lay-ers, and then substitute two zirconium atoms with two yttriumatoms and remove one oxygen atom from the pure zirconia cellto get the stoichiometric cell of YSZ (111) as shown in Fig. 1(a).In the early researches on YSZ, ab initio calculations of G. Stapperet al. [27] identified the dopant-vacancy aggregation mechanismand confirmed the model of the displacement pattern around thedefects, namely, a next-nearest-neighbor attraction between va-cancy and yttrium, which is consistent with the experimentalscattering data [28]. We also test the configuration with both Yatoms in the surface layer and that with both Y atoms in the sec-ond multilayer. Total energy calculations reveal that the modelsystem used in this paper, namely, the configuration with oneY atom in the outmost surface multilayer and the other in thesecond multilayer, is by 0.347 and 0.192 eV more favorable thanthose with both Y atoms on the surface or in the second mul-tilayer. The effect of oxygen vacancy distribution is also assessedas well. The subsurface vacancy is by 0.322 eV more favorable

than the outmost surface vacancy and by 0.185 eV more favor-able than the vacancy in the upper layer of the second multilayer.Our results are in good agreement with model used in the refer-ences [27–29]. In order to generate the oxygen-enriched surface,an additional oxygen atom is introduced to occupy the intrinsicsubsurface oxygen vacancy of YSZ (Fig. 1(b)). Under practical con-ditions, the formation of the YSZ+O surface depends on the actualSOFC fuel gas operation and polarization conditions in the anodepolarization measurement [19]. Furthermore, a considerable num-ber of researches have been done on the configuration simulationfor the YSZ and the molecule adsorption onto the YSZ (111) andYSZ+O (111) surfaces [14–16,18,19,29].

For the adsorption, a SO2 molecule is placed on one side ofthe relaxed slabs. One O–Zr–O triple layer at the bottom is fixedand the ions in the remaining layers, as well as the adsorbates, areallowed to fully relax.

2.2. Computational methods

The DFT plane wave calculations are performed using theVienna ab initio simulation package (VASP) [30–32] with the pro-jector augmented wave method (PAW) [33,34] and the Perdew–Burke–Ernzerhof (PBE) [35] functional. YSZ and YSZ+O surfaces aremodeled using the supercell approach, where periodic boundaryconditions are applied so that it is repeated periodically through-out the three-dimensional space. All the calculations of the YSZand YSZ+O slab models are carried out using the Brillouin zonesampling with a (2 × 2 × 1) Monkhorst–Pack k-points grid and acutoff energy of 408 eV. The slabs are repeated in the z-directionand separated by 15 Å vacuum gaps. The structures are optimizeduntil the force on each atom is less than 0.02 eV/Å.

The adsorption energies are calculated according to the for-mula:

Eads = −[E(SO2/support) − E(support) − E(SO2)

](1)

where E(SO2/support) and E(support) are the calculated ener-gies of the support with and without adsorbate, respectively, andE(SO2) is the energy of a free SO2. All three types of energy arederived from optimized calculations using the same periodic boxdimensions. A positive adsorption energy corresponds to a stableadsorption structure.

The electronic structure and bonding features are analyzed bymeans of the density of states (DOS), the electron localization func-tions (ELF) [36,37] and the charge density difference (�ρ) induced

X. Chu et al. / Physics Letters A 378 (2014) 659–666 661

Table 1Selected data for the optimized SOx structures obtained from SO2 adsorption on the (111) surfaces of YSZ and YSZ+O. “S–O” refers to the intramolecular S–O bonds of SO2;“S–Osurf” represents the distance between the S and the nearest surface O; “O–S–O” means intramolecular O–S–O bond angle of SO2; q(SO2) is the net Bader-type charge onthe adsorbed SO2 entity; Eads corresponds to the adsorption energy. Calculated gas-phase values are also given.

Species Structure Surface S–O (Å) S–Osurf (Å) O–S–O (◦) q(SO2) Eads (eV)

SO2(g) 1.45 119.4SO3(g) 1.42 120.0SO2 V-I YSZ 1.46/1.49 2.35 114.3 −0.18 0.44SO2 O-I YSZ+O 1.44/1.46 3.84 119.2 −0.02 1.23

SO2−3 V-II YSZ 1.53/1.51 1.66 109.3 +0.22 1.38

SO2−3 O-II YSZ+O 1.52/1.50 1.55 114.1 +0.72 1.60

SO3 O-III YSZ+O 1.43/1.43 1.44 121.0 +1.96 2.96

SO2−4 O-IV YSZ+O 1.44/1.43 1.59/1.63 121.6 +2.05 3.34

SO2−4 O-V YSZ+O 1.50/1.43 1.48/1.61 116.2 +2.10 5.04

by the adsorption. The ELF can serve as a useful tool for the char-acterization of the bonding between adsorbate and surfaces andprovides a measure of bond order. The Bader charge analysis [38]is used to assign charges to atoms and fragments.

The �ρ maps describe the electron density rearrangement re-sulting from the adsorption of SO2, and are calculated from

�ρ(r) = ρ(SO2/support) − ρ(support) − ρ(SO2), (2)

here it is necessary to use the same atomic positions for all thethree terms, namely, those from the optimized SO2/support sys-tem.

To take into account the environmental effects, the Gibbs freeenergy G(T , P ) of the whole system is calculated as a functionof temperature (T ) and partial pressure (P ) from the DFT resultsusing the ab initio atomistic thermodynamics method, as done else-where [20,21,39–41]. For an interfacial reaction as shown in thefollowing formula,

surface1 + gas1 → surface2 (3)

where surface1 represents a reactant (YSZ or YSZ+O) in the solidphase, gas1 represents a reactant in the gas phase (SO2), andsurface2 represents a product in the solid phase with adsorbate(SO2/YSZ or SO2/YSZ+O), the variation of Gibbs free energy can beexpressed as [4,21,42,43]:

�G(T , P ) = EDFTsurface2 − EDFT

gas1 − EDFTsurface1 + F vib,ad(T )

− �Hgas1(T , P 0) + T �Sgas1 − �Hgas1

(T 0, P 0)

− kB T ln(

Pgas1/P 0). (4)

The first three terms are the DFT results, which represent Gibbsfree energy at zero temperature in vacuum or Helmholtz free en-ergy at zero temperature. The fourth term refers to the contribu-tion of vibrational free energy within the harmonic approximationfor n fundamental modes (with frequencies of ωi ) arising from theeffect of the surface and can be expressed as [41]:

F vib,ad(T ) = F vibsurface2(T ) − F vib

surface1(T ), (5)

F vib(T ) =n∑

i=1

F vib(T ,ωi)

=n∑

i=1

[1

2h̄ωi + kB T ln

(1 − exp

(−h̄ωi

kB T

))], (6)

where each term is obtained from detailed vibrational analysis ofthe various surface species, diagonalizing the complete dynamicmatrix while leaving the substrate fixed.

The fifth and sixth terms of Eq. (4) correspond to the contri-bution of gas enthalpy and entropy under atmospheric pressure(P 0 = 1 atm), respectively, and the seventh term is the contribu-tion of gas enthalpy at the reference temperature T 0 = 298.15 K,

which can be looked up in a thermodynamic database [42,44]. Thelast term means a pressure-dependent contribution [4,21,41,42],where K B is the Boltzmann constant.

Accordingly, we can construct the phase diagram to identify thesurface atomic structures at a certain temperature and/or partialpressure. It should be kept in mind that this method only gives usa rough statistical result to link the DFT result with the real envi-ronment. As to how the phase changes take place, it is a matter ofmolecule dynamics. The phase diagram could not give more detailson this topic [21].

3. Results and discussion

Various possible adsorption sites of a single SO2 molecule onthe (111) surfaces of YSZ and YSZ+O are explored, where threeadsorption directions (with the S-moiety up, down, or parallel tothe surface, respectively) at every site are investigated. Several dif-ferent surface species (e.g., the SO2 and SO3-like structures) areobserved on the both surfaces, while some additional species (SO3and SO2−

4 ) are found on the YSZ+O surface. The different speciesare identified by analyzing the connectivity in the geometries, aswell as the ELF-maps, �ρ-maps, and Bader charges. Adsorptionstructures, energies, and charge transfer to/from SO2 are reportedin Table 1.

3.1. SO2 molecular adsorption

As shown in Fig. 2, two stable molecular adsorption states areobtained, i.e., the V-I on the YSZ (111) surface and O-I on theYSZ+O (111) surface, respectively. For the both configurations, theinternal molecular structures of the adsorbed SO2 hardly changewith respect to the bond lengths and bond angle of a free neu-tral SO2 molecule (Table 1), although the SO2 in the V-I structureloses more electrons (0.18) than does the SO2 in the O-I (0.02).From Fig. 2, it is found that, for both configurations, one of the Oatoms in the adsorbed SO2 points towards to the substrate, form-ing a new Y–O ionic bond of 2.47 Å on the YSZ (111) surface and2.56 Å on the YSZ+O (111) surface, respectively. However, the lat-ter has a much larger adsorption energy than the former (1.23 eVvs. 0.44 eV). What is the possible reason for this? Considering thatboth structures have the similar adsorbed SO2 fragments, we care-fully examine the relaxation of substrate. From Fig. 2(b), the Oion indicated by the arrow shifts up from the subsurface to thesurface. This rather large structural relaxation should be responsi-ble for more exothermicity for the O-I configuration. Furthermore,from Fig. 3(a), the projected DOS (PDOS) for the adsorbed SO2 inthe O-I structure are quite similar to that of the free neutral SO2.However, for the V-I structure, the 4a1 (highest occupied molecu-lar orbital) peak is quite different from that of the O-I structure,which is due to the more electrons (∼ 0.18) gain in V-I as com-pared with those in O-I (Table 1).

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Fig. 2. (Color online.) Optimized structure for the molecularly adsorbed SO2; (a) structure V-I on the YSZ (111) surface; (b) structure O-I on the YSZ+O (111) surface. Hereand in the following, the main figure is a side view and the small inset is a top view.

Fig. 3. The (P)DOS for the SO2 entity before and after adsorption. (a) The free SO2

molecule (dashed dot line), as well as the molecularly adsorbed SO2 in the structureV-I (solid line), and the structure O-I (dashed line); (b) SO2−

3 species in structure

V-II (solid line) and structure O-II (dashed line); (c) SO2−4 species in structure O-IV

(solid line) and structure O-V (dashed line).

3.2. SO3-like structures

The principal interaction is the chemisorption of SO2 at oxygenanions. In our calculations, three types of SO3-like structures areobtained with the adsorption energies of 1.38, 1.60 and 2.96 eV,which are labeled with the V-II, O-II and O-III for the SO2 adsorp-tion on the (111) surfaces of YSZ and YSZ+O, respectively. All theadsorption energies for the SOx species are calculated using Eq. (1)with respect to the gas phase SO2. More details about the resultingstructures are shown in Figs. 4, 5(a) and Table 1.

From the Bader charge analysis shown in Table 1, we find thatthe adsorbed SO2 lose 0.22 and 0.72 electrons in the V-II and O-II

structures, respectively. These electrons mainly originate from sul-fur and primarily go to the surface O ion to which the SO2 binds.The whole SO3-like fragment has a charge of −1.7e (on the YSZsurface) or −1.2e (on the YSZ+O surface), indicating the formationof surface sulfite (SO2−

3 ). In the two SO2−3 adsorption species (V-II

and O-II), the adsorbed SO2 species bind to one surface O ion, withthe S–Osurf bond lengths of 1.66 and 1.55 Å, respectively. The in-tramolecular S–O bonds of the original SO2 are thereby lengthenedby 0.09 and 0.07 Å and the O–S–O angle decreases by ∼10◦ . Thesurface O ions in the V-II and O-II structures are pulled out fromsurfaces by ∼ 0.17 and 0.49 Å, respectively. Similar bonding struc-tures and adsorption energies have been found previously for theSO2/MO (M is alkaline or rare earth metals, Mg, Ca, Sr, Ba or Ce)systems [45–52]. For example, the DFT+U calculations of Lu etal. [52] reported the S–O bonds of 1.51–1.52 Å, a S–Osurf bond of1.65–1.70 Å, and the decreasing of the O–S–O angle by about 10◦for the SO2 adsorption on the ceria surfaces. Fig. 3(b) shows thePDOS of the adsorbed SO2 fragments in the two SO2−

3 -like struc-tures. When adsorbed to the surface, the 4a1 states of the adsorbedSO2 fragments are broadened as compared with those of the freeSO2 and shift up toward (or cross, e.g. those in the O-II structure)the Fermi energy due to the bonding with the Osurf and the loseof electrons. Due to the more electrons depletion of the adsorbedSO2 in the O-II structure, the 4a1 states of the SO2 fragment shiftup and cross the Fermi energy and becomes partially unoccupied.

Furthermore, structurally different from the SO2−3 -like struc-

tures, the direct chemisorption of SO2 on the YSZ+O surface formsa trigonal planar SO3 (sulfur trioxide, labeled with O-III) moleculewith a larger adsorption energy of 2.96 eV. From the Bader chargeanalysis, the whole SO3 complex has a charge of −0.04e. About 2.1electrons (mainly from S) are transferred from SO2 to the YSZ+Osurface, indicating that sulfur changes its oxidation state from +IVin SO2 to +VI in SO3 (cf. Table 1). For the adsorption species ofmolecular SO3, the surface O ion in the O-III structure is pulledout from the surface by ∼ 0.95 Å, and the three S–O bond lengthsand the O–S–O angle are ∼ 1.43 Å and ∼ 120◦ , which are in ac-cordance with the experimental values of 1.42 Å and 120◦ for thefree SO3 [49,53]. The PDOS peaks of the SO3 fragment (dashedline) and the free SO3 (solid line) are quite similar as shownin Fig. 6. The corresponding ELF and �ρ maps in the planes con-taining the atoms of SO2 and the Osurf ion bound to the SO2molecule are also calculated and shown in Figs. 5(b) and 5(c). TheELF map indicates that the new S–Osurf bond is covalent and thatall three S–O bonds are of equal bond order. The �ρ map revealsstrong polarization in SO3 with charge accumulations along thenew S–Osurf bond, in line with the Bader charge analysis.

The formed SO3 species can desorb from the surface and thedesorption energy is defined as

X. Chu et al. / Physics Letters A 378 (2014) 659–666 663

Fig. 4. (Color online.) Optimized structures of (a) V-II on the YSZ (111) surface; (b) O-II on the YSZ+O (111) surface.

Fig. 5. (Color online.) (a) Optimized structure of O-III; (b) contour map of the electron localization function (ELF) in the interval 0 to 1 with increments of 0.1; (c) �ρ contourmaps in the interval −1 to 1 e/Å3 with increments of 0.008 e/Å3. Here and in the following, solid lines indicate electron excess, dashed lines electron deficiency.

Fig. 6. The (P)DOS for the free SO3 molecule (solid line) and the formed SO3

molecule in structure O-III (dashed line) on the YSZ+O (111) surface.

Ede = ESO3/YSZ − EYSZ − ESO3 , (7)

where ESO3 is the calculated total energy for the ground state ofan optimized SO3 molecule in gas phase, and EYSZ and ESO3/YSZ arethe total energies of the YSZ systems without and with the SO3species. Positive energies correspond to endothermic processes.

The small desorption energy with 0.035 eV indicates the veryweak interactions between the formed SO3 species and the sub-strate. The formed SO3 species may desorb very easily from thesurface, resulting in an oxygen vacancy on the outmost surfaceof YSZ+O substrate. However as known, the oxygen ion transportmechanism in YSZ is a discrete hoping process in which the oxygen

Fig. 7. (Color online.) Schematic potential energy profiles for the migration of oxygenvacancies between the surface and subsurface.

ion vacancies can be filled by oxygen ions migrating through thecrystal lattice of YSZ. The subsurface O ion will migrate to the oxy-gen vacancy on the outmost surface with a small barrier of 0.8 eV,leaving behind the oxygen vacancy on the subsurface, which formsthe YSZ model described by Shishkin and Ziegler, and is adopt inthis study (as shown in Fig. 7).

3.3. SO4-like structures

Two different SO4-like structures are found for the adsorptionof SO2 on the YSZ+O (111) surface, but not on the YSZ (111) sur-face, which are assigned as O-IV for the bidentate structure (Fig. 8)and O-V for the monodentate sulfate structure (Fig. 9(a)), respec-tively. The SO2 molecule in the O-IV bidentate sulfate structure isattached to the two surface O ions via the S atom with an ad-sorption energy of 3.34 eV and two new S–Osurf bonds of 1.58and 1.63 Å, respectively (cf. Table 1). The two O ions are pulled out

664 X. Chu et al. / Physics Letters A 378 (2014) 659–666

Fig. 8. (Color online.) Optimized structure of O-IV on the YSZ+O (111) surface.

from the surface by about 0.46 Å. The intramolecular S–O bondsin the “SO2 entity” almost retain their gas-phase equilibrium dis-tances of 1.43 and 1.44 Å.

Structure O-V is the most stable SO2 adsorption configurationwith an adsorption energy of 5.04 eV (cf. Table 1). In this struc-ture, one surface O ion is lifted about 1.65 Å, and the subsurfaceO ion filling the intrinsic vacancy site of YSZ is pulled out to thesurface site of the former, which are indicated by the arrows andcircle. Both of them are attached to the adsorbed SO2 to form themonodentate sulfate (as shown in Fig. 9(a)). As expected, it is theadditional oxygen atom filling the vacancy site that makes the YSZmore active for molecule oxidation, in agreement with the previ-ous theoretical and experimental results [14–16,18,29].

For the two SO4-like structures, the Bader charge analysisshows that about 2.1 electrons (mainly from S) are transferredfrom SO2 to the YSZ+O surface (cf. Table 1), indicating that sulfurchanges its oxidation state from +IV in SO2 to +VI in the sulfate.The net Bader charges of both sulfates are ∼ 1.7 e. The PDOS ofboth structures (solid line for O-IV and dashed line for O-V) aregiven in Fig. 3(c). Similar to the results for the SO2−

3 -like species,the broadening of the 4a1 peak of SO2 is mainly due to the for-mation of the new covalent S–Osurf bonds between SO2 and theYSZ+O surface, leading to the formation of sulfate. The ELF mapscalculated in the same planes (Fig. 9(b)) indicate that the newS–Osurf bonds are covalent and all the four S–O bonds are of asimilar bond order. �ρ maps of the O-V structure calculated in aplane containing the SO2 atoms and in another plane (almost per-pendicular to the former) containing the two new S–Osurf bonds(Fig. 9(c)) show a large loss of electrons from the S atom and gainof electrons along the two new S–Osurf bonds.

Considering the influence of different coverage for the adsorp-tion and transformation of SO2 on the surfaces, the most stableconfiguration of SO2 adsorption (O-V) on the YSZ+O (111) surfaceis optimized using a larger (4 × 4) supercell. Our test calculationsshow that the adsorption configuration is converged with the cur-rent (2 × 2) supercell.

3.4. Phase diagrams of the SO2/YSZ

For the reaction of SO2 with the YSZ (111) and YSZ+O (111)surfaces, the phase diagram boundaries are determined by Eq. (4)with �G(T , P ) = 0, which are used to analyze the mutual trans-formation between different sulfur oxide species (SOx) in the am-bient conditions. According to thermodynamics theory, the re-action would take place when the Gibbs free energy decreases(�G(T , P ) < 0). The phase diagram boundaries for the systems

Fig. 9. (Color online.) (a) Optimized structure of O-V. (b) The corresponding ELFmaps. (c) The corresponding �ρ maps. ELF and �ρ are projected on two orthog-onal planes containing S and either both Osurf atoms or both oxygen atoms of theoriginal SO2 molecule.

of SO2–YSZ (111) and SO2–YSZ+O (111) are shown in Figs. 10(a)and 10(b), respectively. At certain SO2 partial pressures (e.g., line Iin Fig. 10(a)), the SO2 molecules on the YSZ (111) surface are ox-idized and form SO3-like structure with increasing temperature.SO2 molecules would be adsorbed on the surface when the tem-perature increases.

On the YSZ+O (111) surface (Fig. 10(b)), it is evident thatat lower SO2 partial pressure (< 10−6 atm, i.e., 1 ppm), the ad-sorbed SO2 molecules can be oxidized and form SO4-like structureat lower temperature (< 550 K, e.g., line II). At higher SO2 par-tial pressure (e.g., line III), SO2 molecules are oxidized and formSO3-like structure, and finally at even higher SO2 partial pressure

X. Chu et al. / Physics Letters A 378 (2014) 659–666 665

Fig. 10. The phase diagrams for the species formed from the SO2 adsorption on:(a) the YSZ (111) and (b) the YSZ+O (111) surfaces. The full curves are the phaseboundaries. The dotted lines from I to IV indicate conditions under some certainSO2 partial pressures, and the dotted line V indicates conditions under the sametemperature.

(e.g., line IV), the SO3-like structure is formed initially at lowertemperature (< 600 K) and then surface SO2 molecule adsorptionwill be found with increasing temperature. On the other hand, atcertain temperatures (e.g., line V), when partial pressure is too low,there would be no interaction between SO2 molecules and the sur-face; then SO2 will be oxidized and form SO4-like and SO3-likestructures when SO2 partial pressure increases; if the SO2 par-tial pressure is high enough, SO2 molecules would be adsorbed onthe surface. Moreover from Figs. 10(a) and 10(b), it is found thatat the same temperature (e.g., 800 K), the SO2 molecules on theYSZ+O (111) surface can be oxidized at lower SO2 partial pres-sure (log(P/P 0): 1.43 vs. 29.4), indicating that the YSZ+O surfaceis more active and susceptible to sulfur-poison than the YSZ sur-face.

In all, we conclude that the formation of oxidized species andtheir transformation depend strongly on the ambient conditions,such as temperature and SO2 partial pressure. Similar results havebeen found for the adsorption of SO2 on other metal oxides sur-face [22,45–48,50,52]. For example, by means of synchrotron ra-diation photoemission spectroscopy, Sayago et al. [22] found thatthe SO2 molecule adsorbs at 120 K on the TiO2 (110) surface form-ing SOx species. The saturation is reached at around 6 L, and SO2multilayer formation is found for exposures higher than 250 L.

Furthermore, due to its characteristics of withstanding theharsh thermal, mechanical, chemical environments found in auto-motive exhausts, and (at elevated temperatures) high oxygen ionconductivity, in addition to solid oxide fuel cells (SOFCs) [1,54],

YSZ is widely used in thermal barrier coatings [55], robust ox-ide ion conducting electrolytes [56], high-temperature solid-stateelectrochemical gas sensors [57,58]. Our phase diagrams for theadsorption and transformation of SO2 on the YSZ/YSZ+O (111) sur-faces can far better provide valuable insight into the structures andcompositions of the surfaces in realistic or technologically relevantenvironments (temperature and pressure), which may be of certainguidance for experiments.

4. Conclusion

Using the ab initio atomistic thermodynamics method whichcombines the DFT result and thermodynamics data, we investi-gate the adsorption and oxidation of SO2 on the YSZ (111) andYSZ+O (111) surfaces. It is found that:

(a) There is the strong interaction between SO2 and the stoichio-metric YSZ (111), as well as the oxygen-enriched YSZ (111)surface (YSZ+O), which primarily leads to the surface SO2−

3

and SO2−4 species (the latter only on the YSZ+O (111) sur-

face);(b) All the formed SOx species (with x = 2–4) are strongly bound

to the surfaces and are of a poisoning nature for the YSZ sur-faces;

(c) The formation of oxidized species and their transformation de-pend strongly on the ambient conditions, such as temperatureand SO2 partial pressure;

(d) At the same ambient conditions, the YSZ+O surface is moresusceptible to sulfur-poison than the YSZ surface.

Through the detecting of oxidized species and analyzing of theirmutual transformation in the ambient conditions for SO2 adsorp-tion on the YSZ (111) surface, we arrive at the conclusion that theformed SOx species are strongly bound to the surface and blockthe active sites for fuel oxidation, which is responsible for the sul-fur poisoning of the YSZ anode of SOFC. However, a more overallunderstanding of the sulfur poisoning mechanism requires infor-mation about how these species interact with other adsorbates,in order to find the effective way for sulfur dioxide destructionfrom the YSZ surface, which will be discussed in the future work.

Acknowledgements

This work is supported by the National Natural Science Foun-dation of China (Grant Nos. 11174070 and 11147006). Parts ofthe simulations are performed on resources provided by the high-performance computing center of College of Physics and Elec-tronic Engineering in Henan Normal University. Z. Lu also acknowl-edges the support from the China Postdoctoral Science Foundationfunded project (Grant No. 2012M521399) and Postdoctoral Re-search sponsorship in Henan Province (Grant No. 2011038), Foun-dation for the Key Young Teachers of Henan Normal University andStart-up Foundation for Doctors of Henan Normal University.

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