Sulfidation of Ceria Surfaces from Sulfur and Sulfur Diffusion - The

Article pubs.acs.org/JPCC

Sulfidation of Ceria Surfaces from Sulfur and Sulfur Diffusion Zhansheng Lu,† Jolla Kullgren,‡ Zongxian Yang,*,† and Kersti Hermansson*,‡,§ †

College of Physics and Information Engineering & College of Chemistry and Environmental Science, Henan Normal University, Xinxiang, Henan 453007, China ‡ Department of Chemistry − Ångström Laboratory, Uppsala University, Box 537, SE-75121 Uppsala, Sweden § Department of Theoretical Chemistry, Royal Institute of Technology, Roslagstullsbacken 15, S-106 91 Stockholm, Sweden ABSTRACT: Even very low levels of sulfur contaminants can degrade the catalytic performance of cerium oxide. Here, the interaction of atomic sulfur with the ceria (111) and (110) surfaces has been studied using first-principles methods. Two sulfoxy species are identified: oxido-sulfate(2-) species (SO2−) on both the CeO2(111) and (110) surfaces and hyposulfite (SO22−) on the (110) surface. Sulfide (S2−) is formed when a surface or a subsurface oxygen atoms is replaced by sulfur. These sulfide species are more stable at the surface. Furthermore, sulfite (SO32−) structures are found when sulfur is made to replace one Ce in the ceria (111) and (110) surfaces. The calculated sulfur diffusion barriers are larger than 1.4 eV for both surfaces, and thus sulfur is essentially immobile, providing a possible explanation for the sulfidation phenomena of the ceriabased catalyst. Thus, we find three different species from interaction of S with ceria which are all, due to their strong binding, capable of poisoning the surface, reduced or unreduced. Our results suggest that under reducing conditions sulfur is likely to be found in the (111) surface (replacing oxygen) but on the (110) surface (as SO22−).

1. INTRODUCTION Most of the commercially available fuels hold small amounts of sulfur-containing impurities. These impurities have been found to severely poison CeO2 (ceria) in the current generation of ceria-based three-way catalysts and in solid oxide fuel cells. It has been reported that even parts per million (ppm) levels are enough to severely degrade the catalytic performance of ceria.1 Here, as an aid to our understanding of these sulfur poisoning phenomena, we present a DFT (density functional theory) characterization of different SOx species likely to appear on ceria surfaces under reducing conditions. As we will show, we find SOx species with oxidation numbers from +II to −II and x values < 3, namely, hyposulfite (SO22−), oxido-sulfate(2-) (SO2−), and sulfite (S2−). This work complements our recent DFT study of SOx species likely to form on ceria surfaces when SO2 is adsorbed under nonreducing or weakly reducing conditions.2 There we found, in good agrement with experiment, that SOx species with sulfur oxidation numbers in the range from +III to +VI are formed, namely, thionites (SO2−), sulfites (SO32−), sulfates (SO42−), and possibly physisorbed SO2. Many experimental studies of SO2 adsorption on ceria have been reported in the literature (see the introduction in ref 2). The dominating species reported from such experiments under normal conditions are sulfites (SO32−) and sulfates (SO42−), which as mentioned were closely examined by us in ref 2. However, under reducing conditions, experimental studies using CO (ref 3) or CH4 (ref 4) suggest that ceria can catalyze the reduction of SO2 to elemental sulfur. Liu et al.3 proposed that oxygen vacancies created by CO would react with adsorbed SO2, SO2(a), to form an adsorbed SO group, SO(a), which reacts with yet another vacancy to form elemental sulfur. We already know from our SO2/ceria study that the SO2(a) species likely is a sulfite or sulfate group, while SO(a) likely is a thionite group. © 2012 American Chemical Society

In the current investigation, we focus on the species formed in the final step of the pathway described above, a species that in principle could be examined by adsorbing atomic sulfur on ceria surfaces, which is difficult to achieve experimentally but easy using calculations. Furthermore, Mullins et al.5 found that sulfur could be one of the products formed when H2S decomposes on the ceria (111) surface and that S may replace O in the lattice, i.e., sulfide formation. This sulfur was found to remain at the surface and not diffuse into the bulk until temperatures above 400 K,6 a result which, as we will see, can easily be understood from our calculations in this paper where sulfur is adsorbed into vacancies, both at the very surface and in the surface region. Several authors7−9 have found that H2S levels up to 450 ppm have no effect on the performance of CeO2-based anodes, but at higher H2S concentrations, anode deactivation has been observed and attributed to the reaction of the CeO2 with H2S to form Ce2O2S . Again our simple model of atomic sulfur adsorption, to be explored in the current paper, can shed light on the nature of the SOx species present under these conditions. Theoretical information for the S/ceria system is still very scarce, and to the best of our knowledge, there exist only a few theoretical investigations in the literature where some results for sulfur on ceria are presented, namely, primarily the periodic density functional theory (DFT + U) study of H2S on ceria by Chen et al.10 The conference report by Baranek et al.11 reports results for atomic sulfur adsorbed at the ceria (111), (110), and (100) surfaces, with periodic HF and DFT calculations. In their work, only one type of adsorption species is presented, namely, a SO-like species with S adsorption energies of about 1.3 eV for (111), twice the value for Received: September 26, 2011 Revised: January 31, 2012 Published: April 10, 2012 8417

dx.doi.org/10.1021/jp2092913 | J. Phys. Chem. C 2012, 116, 8417−8425

The Journal of Physical Chemistry C

Article

were kept fixed at the optimized bulk values, while all other atoms were allowed to relax during the structure optimization. To explore possible adsorption sites, a S atom was placed above the top layer atop O, atop Ce, and on O−O, Ce−O, and Ce−Ce bridge positions. As references, calculations for the free gaseous S, SO, SO2, and SO3 molecules were performed using periodic cells of 10 × 10 × 10 Å3 with a 4 × 4 × 4 Monkhorst−Pack grid for the Brillouin-zone integrations and a 30 Ry (408 eV) energy cutoff for the plane wave expansion. Test calculations using supercells of the same size as those used in the S/ceria slab calculations gave only minute changes in energies and geometries. 2.2. Properties. The electronic structure and bonding for each of the optimized S/ceria and S-doped ceria structures were characterized by adsorption energies (Eads), electronic density of states (DOS), atomic charges, electron localization function (ELF), and adsorption-induced electronic charge density differences (CDD, Δρ), which were all calculated. Moreover, diffusion barriers for sulfur on the ceria surfaces are presented. The adsorption energy (Eads) was calculated as

(110) and three times as much as for (100). Here we instead find several different species, and our adsorption energies are very different from ref 11. We will characterize the various structures and relate them to some of the SOx species we found in our previous systematic SO2/ceria investigation.2 The article is organized as follows. Section 2 contains a description of the computational details. We have performed firstprinciples calculations of S adsorbed on the ceria (111) and (110) surfaces and of various structures where sulfur replaces oxygen or cerium at the surface and in the subsurface layers. The results are presented in Section 3, where the nature (geometry, energetics, electronic structures, and sulfur diffusion barriers) of the sulfur− ceria interactions and their implications in catalysis are presented. A systematic test of the DFT + U methodology is also presented in that section (more exactly: the appropriate choice of U-value). Finally, a brief summary is given in Section 4. This study is part of our endeavor to obtain a comprehensive understanding of sulfidation and sulfur poising phenomena on ceria-based catalysts.

2. COMPUTATIONAL DETAILS 2.1. Models and Optimization. We carried out spinpolarized calculations using the Vienna ab initio Simulation Package (VASP)13−15 with the projector augmented wave (PAW) method16,17 and the Perdew−Burke−Ernzerhof (PBE) functional. Altogether 12 electrons were treated as valence electrons for Ce (5s25p66s24f15d1), 6 electrons for O (2p6), and 6 electrons for S (3p6). We use a Hubbard parameter (U) of 5 eV to describe the localized Ce 4f states appearing for the reduced surfaces. Moreover, the dependence of the S/ceria interfacial properties on the value of the Hubbard U-term was tested systematically and will be presented in the paper. The Brillouin zone integrations were performed with a 4 × 4 × 1 Monkhorst−Pack grid18 and a Gaussian smearing parameter σ equal to 0.2 eV. The Kohn−Sham orbitals were expanded using plane waves with a well-converged cutoff energy of 30 Ry (408 eV). The structures were optimized until the force on each atom was less than 0.02 eV /Å. The stoichiometric CeO2(111) and (110) surfaces were modeled using the slab model, i.e., slabs and vacuum gaps in a 3-D periodic construction (Figure 1). For the CeO2(111) systems,

Eads = −[E(S/support) − E(support) − E(S)]

(1)

where E(support), E(S), and E(S/support) are the total energies of the optimized bare ceria support, of a gaseous sulfur atom, and of the optimized S/ceria system. A positive adsorption energy corresponds to a stable adsorption structure. The electron localization function (ELF) can be a useful tool for the characterization of the bonding between S and the ceria surface and provides a measure of bond order. The Δρ maps were calculated according to Δρ(r) = ρ(S/support) − ρ(support) − ρ(S)

(2)

which measures the electron density rearrangement induced by the two entities on each other. Here, by necessity, the supercell parameters and the atomic positions of the S/support, the support, and the S atom must be the same, namely, that of the optimized S/ceria system. We use Bader charge analysis19 applied to the valence electrons with the core charge added back afterward to calculate the charge of the different SOx species. Although these charges will differ from the all-electron Bader charges, they do enable us to determine the oxidation numbers of the different adsorbed S species formed. We will simply denote these charges Bader charges in the following. Moreover, we use Bader spin charges to distinguish Ce3+ (4f1, magnetic) from Ce4+ (4f0, nonmagnetic) species. For each optimized S/ceria structure, harmonic S−O stretching vibrational frequencies, ω(S−O), were calculated from a normal coordinate analysis (at the gamma point in the Brillouin zone) involving the S adatom and all Ce and O ions in the top layer [the top O−Ce−O triple layer in the case of the (111) surface]. S diffusion barriers were calculated with the climbing image nudged elastic band (cNEB) method in the transition-state (TS) search20,21 using the same convergence criterion as in the geometry optimizations.

Figure 1. Slab models for ceria surfaces: (a) CeO2(111) p(2 × 2) and (b) CeO2(110) p(1 × 2). The bottom six [for the (111) surface] or two [for the (110) surface] atomic layers (in the dashed rectangular box) were fixed at their bulk positions. The slabs were repeated in the z-direction (vertical) and separated by 15 Å thick vacuum gaps.

3. RESULTS AND DISCUSSIONS Subsections 3.1−3.5 report the details of the different adsorption species found when a S atom is adsorbed on the ceria (111) and ceria (110) surfaces. We find SO-like structures, SO2-like structures, SO3-like structures, as well as S-like structures.

we used a p(2 × 2) supercell and for the CeO2(110) systems a p(2 × 1) supercell. A vacuum gap of 15 Å was used. The lattice parameter (5.48 Å at the PBE + U level, see below) was used in all calculations. The two lowest layers (see Figure 1) 8418



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Table 1. Selected Data for the Optimized Structures (″Species″) Obtained from S Adsorption or S Substitution at the Ceria (111) and (110) Surfacesa species

surface

Eads (eV)

S−O (Å)

q(SOx) (e)

q(S) (e)

ω1 (cm−1)

ω2 (cm−1)

ω3 (cm−1)

1150 1247 1342

1061 1309

1011

758 780 792 782

741 722

917 977

877 825

ω4 (cm−1)

Gas Phase SO(g) SO2(g) SO3(g)

1.505 1.456 1.447

SO2− SO2− SO22− SO22− S2− S2− S2− S2−

(at (at (at (at

SO32− SO32−

surface) subsurface) surface) subsurface)

(111) (110) (110) (110)

2.60 2.98 3.55 3.00

(111) (111) (110) (110)

1.08 −1.82 1.06 −1.60

(111) (110)

13.13 14.32

+1.6 +3.6 +6.0 Sulfur Adsorbed on Ceria Surfaces 1.697 −1.1 +0.3 1.693 −1.2 −1.2 1.638 −1.5 +2.0 1.643 −1.4 +2.1 Sulfur Replacing O 2.806 −1.0 −1.0 2.565 −0.8 −0.8 2.786 −1.0 −1.0 2.642 −0.9 −0.9 Sulfur Replacing Ce 1.567 −1.4 +4.2 1.591 −1.5 +4.1

758 697

608 646

a Eads is the adsorption energy (positive energy means stable adsorption); S−O is the shortest S−O distance; q(SOx) is the change of the net Bader charge of the identified SOx species; q(S) is the Bader charge of the sulfur atom; ωi are the harmonic S−O frequencies. The calculated gas-phase values are also given.

above the surface and subsurface anions, above the surface cations, as well as the possible bridge locations (O-bridge as in Figure 4a, Ce-bridge as in Figure 4d, and so forth). The site we call ″Ce-bridge″ is actually also an O-bridge, as the figure shows. Two main types of stable structures were found for S adsorbed on ceria (111) and (110) surfaces, namely, SO-like species and SO2-like species. SO-like species were formed when the S adatom was adsorbed on a surface O on the (111) and (110) surfaces (Figure 2a and d). The S/ceria Eads values are large, 2.6−3.0 eV (Table 1).

The different species were identified and characterized by analyzing connectivities and geometries as well as ELF maps, Δρ maps, and Bader charges. Table 1 lists the accompanying data, namely, adsorption energies, selected structural parameters, the Bader charges of the sulfur atom and the SOx species, and harmonic vibrational frequencies. In subsections 3.6 and 3.7, the diffusion barriers and connections to catalysis are discussed, respectively. Subsection 3.8 discusses the choice of Hubbard U parameter. 3.1. Bare Ceria (111) and (110) Surfaces and Gaseous SOx. The relaxed unreduced ceria (111)−p(2 × 2) surface and ceria (110)−p(2 × 1) surface systems are shown in Figure 1a and b, respectively. The CeO2(111) surface relaxation is quite small, while the relaxation of the (110) surface is much larger, and the surface becomes slightly rumpled upon relaxation, as is well established in the literature. Our calculated S−O bond distance is 1.505 Å for the gaseous SO molecule, whose S moiety loses about 1.6 electrons to the O moiety (Bader-type charge analysis), consistent with S(+II). For the gaseous SO2 molecule, we obtain an optimized S−O bond length of 1.456 Å and an O−S−O bond angle of 119.1°. These values agree reasonably well with those obtained in other DFT calculations (1.465 Å, 120.3°)22 and experiment (1.43 Å, 119.3°).23 The S atom in SO2 loses about 3.6 electrons to the O atoms, consistent with S(+IV). Gaseous SO3 is a trigonal planar molecule (D3h symmetry). Our calculated S−O bond length is 1.447 Å (experimental value 1.42 Å24). The S in SO3 loses about six electrons to the oxygens, and in this case the Bader charge happens to be equal to the (nominal) oxidation number of S in SO3, S(+VI). 3.2. Adsorption of S on the Ceria (111) and (110) Surfaces. In search of stable sulfur adsorption structures on the stoichiometric ceria (111) and (110) surfaces, we placed one S atom on one side of the optimized ceria (111) and (110) slabs while simultaneously relaxing the positions of both the surface ions and the S adatom. Many optimizations with different starting geometries were carried out. The symmetry was broken by applying a small distortion to the structures. Various possible adsorption sites of S on the ceria surfaces were explored:

Figure 2. (a) Final, i.e., optimized, structure for sulfur adsorbed at a surface O on the ceria (111) surface. The main figure is a side view and the small inset is a top view. (b) ELF contour map in a vertical plane passing through S and its anchor atom (contour levels are 0.0, 0.1, 0.2, etc.). (c) Δρ maps in the same plane (contour levels are ±0.02 e/Å3, solid lines represent electron excess, dashed lines electron depletion, the zero line is not shown). (d)−(f) The corresponding figures for S adsorbed above a surface oxygen on the CeO2(110) surface.

An analysis of the Bader charges (cf. Table 1) shows that there is electron transfer from the adsorbed S atom primarily to the anchor O ion. The absence of any reduced cerium ions, and 8419



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SO2-like species were only found at the O-bridge and the Cebridge sites of the ceria (110) surface, and the adsorption is very strong (Table 1 and Figure 4). The two SO2-like species are quite similar in character. The formation of SO2-like structures induces strong modifications in the support geometries; namely, two of the surface O ions are pulled out by 0.4 Å from the surface along the z direction, and the corresponding Ce−O bonds in the surface are elongated by 0.23 Å. The ELF maps (Figures 4b and e) reveal the formation of two covalent S−O bonds. The Δρ maps (Figures 4c and f) display a charge redistribution involving an overall substantial loss of electron density from the S atom and a pile-up of electron density between S and O, which is consistent with both the oxidization of the S and the formation of covalent S−O bonds. The charge of the SO2-like species (Table 1) indicates that they are hyposulfite (SO22−) species. The large amount of charge transferred from the S adatom (∼2 e according to Table 1) to the support mainly localizes on the two O atoms binding to S and on the two surface Ce ions neighboring the SO2-like species. The analysis of the Bader charges reveals that each Ce ion receives about 0.3 e from S. A similar change in Bader charge is seen when Ce(+IV) is transformed to Ce(+III) as a result of oxygen vacancy formation in pure CeO2, and we infer that the surface Ce ions in Figures 4a and d also effectively change their oxidation number from +IV to +III. This is consistent with the Bader spin charges and with the yellow isosurface spin charge cloud in Figure 4a. The reduced Ce ions are marked with “#” in Figure 4. We also note that the oxygen bridge site is active in forming other bidentate structures, for example, SO42− and CO32−, from adsorption of SO2 and CO2 at the ceria (110) surface.2,25−27 The (total and partial) DOS for one of the SO22− species (the O-bridge site) are shown in Figure 5. The adsorption process leads to new filled peaks 0.5 eV above the O 2p valence band edge (compare Figures 5a and b) and a number of peaks just below the O 2p band. These peaks originate from the SO2 species and the two reduced cerium ions (Figure 5e). The main peaks from the SO2 species have both oxygen and sulfur character (compare Figures 5c and d), consistent with the formation of covalent S−O bonds. 3.3. Sulfide Ions in Ceria (111) and (110) from S Insertion at an O Vacancy. Oxygen vacancy defects on ceria surfaces are known to be abundant, and here we will investigate whether and how a S atom could adsorb at an O vacancy site. In fact, the degradation of CeO2-based catalysts has been linked with the formation of cerium oxysulfate (Ce2O2S). Moreover, soft X-ray photoelectron spectroscopy studies5 have found evidence of sulfide species, S2−, from H2S interaction with ceria, as mentioned in the Introduction. Sulfide substitutional defects were modeled here by replacing an oxygen ion with sulfur either at the surface or at a subsurface position of ceria (111) and (110) (see Table 1 and Figure 6). The resulting four sulfide species turn out to be quite similar in character and will therefore be scrutinized for one case only, namely, the sulfur replacing oxygen at the surface site in the (111) surface. This structure is shown in Figure 6a. The ELF map (Figure 6b) reveals the ionic character of the Ce−S bonds and so does the Δρ map (Figure 6c). The Ce−S bonds are 0.44 Å longer than the Ce−O bond (2.37 Å) as a consequence of the larger atomic radius of S. This is also reflected in the energy difference between sulfur replacing a surface oxygen and sulfur replacing a subsurface oxygen. This difference is 2.90 eV for the (111) surface and 2.66 eV for the (110)

the similarity between the net charges of the SO species (−1.1 e) and the charge of an oxygen ion in the topmost layer of the bare ceria surface (−1.2 e), suggest that the formed species can be interpreted as an oxido-sulfate(2-) species (SO2−) with a S oxidation number of zero. The ELF maps (Figure 2b and e) suggest that there is covalent bonding (nonvanishing ELF) between the S adatom and the anchor oxygen atom. The Δρ maps in Figure 2c and f display charge redistributions between the S and O atoms consistent with electron transfer from S to O and formation of new covalent S−O bonds. A similar structure and adsorption energy were reported by Chen et al.10 using a smaller ceria (111) supercell [p(√3 × 2)]. However, Baranek et al.11 presented a somewhat different structure for the (111) surface, where the sulfur occupies a bridging position between Ce and O with quite a different adsorption energy (1.36 eV). The (total and partial) DOS curves of the oxido-sulfate(2-) species on ceria (111) are displayed in Figure 3. The electronic

Figure 3. Total and partial DOS diagrams relating to sulfur adsorbed on the (111) surface as detailed in the figures (a) to (e). Hereafter, the vertical line represents the Fermi energy.

states for SO2− on ceria (110) are very similar to the corresponding ceria (111) system, which is therefore not shown. The calculated magnetic moment (2.0 μB) of sulfur is clearly displayed in the DOS for the free S atom (Figure 3a). When S binds to the surface O, we find that its spin moment disappears, and the adsorption process leads to new occupied states located some 0.5 eV above the O 2p band and at the bottom edge of the O 2p band (solid line in Figure 3b). These new states are mainly of S 3p and O 2p character and originate from the newly formed S−O bond. Incidentally, there is no obvious influence on the electronic structure of the other surface O and Ce atoms, which is shown by the PDOS (Figure 3e) for the Ce ions in the S/ceria and ceria system. 8420



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Figure 4. (a) Optimized structure for sulfur adsorbed at an O-bridge site on the (110) surface and spin charge density iso-surface (iso-surface values ±0.02 e/Å3). The reduced Ce ions are marked with “#”. (b) Contour map for the ELF in a plane passing through S and its anchoring atoms. (c) Δρ maps in the same plane. (d)−(f) The corresponding figures for S adsorbed on a Ce-bridge site at the CeO2(110) surface. Same contour levels as in Figure 2.

The final structures of the sulfur at a subsurface position for the (111) surface and at the surface and subsurface positions for the (110) surface are shown in Figure 6d, e, and f, respectively, for comparison. 3.4. SO3-Like Structures from S Insertion at a Ce Vacancy. S insertion was modeled by placing S at a Ce vacancy site on the clean ceria (111) and (110) surfaces and then optimizing the structures (Table 1 and Figure 7). The calculated adsorption (dopant) energies are much larger than those of S on top of the stoichiometric and reduced surfaces, which is consistent with the much lower stability of the surface with Ce vacancies,28 i.e., the high cost to create Ce vacancies. This exercise of S insertion is thus somewhat esoteric; nevertheless, we here briefly report our results. SO3-like structures were found with three slightly longer S−O bonds relative to the calculated bonds length in the free SO2 molecule but shorter than the original Ce−O bonds at the ceria surface (see Figure 7a and d, respectively). The ELF maps (Figure 7b and e) show the formation of covalent S−O bonds, and the Δρ maps (Figure 7c and f) display electron density pileup in the S−O bonds, with an electron transfer from S in SO3-like species to the O in SO3. The Bader charges are 4.1 e for S and ∼ −1.5 e for the SO3-like species. We assign this species to sulfite, SO32−, and also note its strong resemblance to the sulfite arising from SO2 adsorption on the ceria surfaces, as found in ref 2. 3.5. Stretching Frequencies of SOx. Vibrational spectra from quantum-mechanical calculations can help to shed light on the nature of the adsorbed species observed in experiment and of course helps in the assignment of vibrational modes in the experimental spectra. Calculated harmonic frequencies of the S−O vibrational stretching modes for all adsorbate structures reported in the previous sections are given in Table 1 (when discussed below, the numbers are given with a precision of 5 cm−1).

Figure 5. Total and partial DOS diagrams relating to sulfur adsorbed on the O-bridge site at the ceria (110) surface as detailed in the figures (a) to (e). In (e) the selected Ce is a Ce marked with “#” in Figure 4a.

surface, which is consistent with the findings of Overbury et al.6 that S does not diffuse into the bulk until at high temperatures.5 8421



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Figure 6. Top panel shows results for S replacing a surface O atom at the ceria (111) surface: (a) final structure; (b) ELF; (c) Δρ. Bottom panel shows results for S anion doping with one of the O replaced by S: (d) final structure for S replacing a subsurface O atom at the (111) surface; (e) final structure for S replacing a surface O at the (110) surface; (f) final structure for S replacing a subsurface O at the (110) surface.

Figure 7. Top panel shows results for S replacing a surface Ce atom at the ceria (111) surface. S-bound O atoms are shown as pink balls: (a) final structure; (b) ELF; (c) Δρ. Bottom panel shows results for S replacing a surface Ce atom in the ceria (110) surface: (d) final geometrical structure; (e) ELF; (f) Δρ.

at the (110) surface have symmetric and antisymmetric frequencies of 975 and 825 cm−1. The frequency ranges pertaining to SOx species presented both here (SO2−, SO22−, and from doping SO32−) and in our previous work (SO2−, SO32‑, SO42−, and physisorbed SO2) overlap to a large extent, so we suggest that it may be difficult to resolve these species in experimental vibrational spectra. However, we see a trend of decreasing frequency of the antisymmetric S−O stretch for the series SO42−, SO32−, SO22−, and SO2−, while the S−O distance increases monotonically for the same series. 3.6. Diffusion of Atomic S on the CeO2 Surfaces. The mobility of a sulfur atom on ceria surfaces is interesting in the context of the sulfur poisoning mechanism. We have calculated diffusion barriers for the S atom on the different surfaces by sampling energy profiles along selected diffusion pathways

A detailed analysis of the vibrational modes and the surfaceinduced frequency shifts of SO42−, SO32−, and SO2− species were presented in ref 2. The absolute frequencies were found in the range from about 1300 to about 700 cm−1. We find that the S−O stretching frequencies in the current work fall mostly into the lower part of that frequency range. The symmetric and antisymmetric modes of the two SO22− species that we have observed, as well as the S−O stretching modes of the two SO2− species, fall in the range 720−790 cm−1. The sulfite species created by S insertion at the (111) surface give rise to symmetric and antisymmetric frequencies at 915 and 875 cm−1, respectively, which is some 100 cm−1 lower than the corresponding frequencies found for sulfites formed upon SO2 adsorption.2 The sulfite species created by Ce replacement 8422



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O-bridge sites involves several barriers. The barriers from the Ce-bridge site (Figure 10a) to the O-bridge site (Figure 10e) via the atop site of the surface O (Figure 10c) have been calculated. The transition states are found at energies of 2.63 and 2.03 eV with respect to the ground state structure (O-bridge site). Barriers and the corresponding transition states are shown in Figure 10b and d, respectively. 3.7. Implications for Catalysis. Sulfur-containing impurities constitutes a serious poison for ceria-based catalysts. The present study is meant as a step on the way toward elucidating the reasons why. In ref 2, we found strong interaction between SO2 and both stoichiometric and reduced ceria. The adsorption primarily leads to surface S(+VI)O42− and S(+IV)O32− species. Under reducing atmosphere, these might be expected to form S(+II)O22− and S(0)O2− or even surface sulfides(-II), the three main species of the current work (here obtained from atomic S adsorption). These three species are also strongly bound to the ceria surfaces. Hence, all SOx (with x = 0−4) species discussed here and in ref 2 are of a poisoning nature. Furthermore, the channel to direct formation of S2 or S8 from the S(+II)O22− and S(0)O2− species is strongly suppressed by the high S diffusion barriers. As for the sulfides, sulfur adsorbed at the (111) surface readily heals oxygen vacancy sites with a considerable exothermic energy, resulting in the formation of stable sulfide species blocking the oxygen vacancy site. However, on the (110) surface, the corresponding reaction is endothermic. This means that sulfur is likely to be found in the surface (replacing oxygen) for the (111) surface termination and on the surface in the form of S(+II)O22− for the (110) surface termination. 3.8. Dependence of the Properties of the S/Ceria System on the Values of the Hubbard U-Term. The DFT + U approach is suitable for describing localization in reduced ceria. However, an appropriate choice of the U-value cannot always be established, and a strong U-dependence of various properties of small molecules25,29 and metal30 adsorbed on ceria surfaces has been presented. Therefore, to investigate the sensitivity to the U-value for the adsorption energies and other selected properties of the most stable SOx species on the (111) and (110) surfaces, U-values between 0 and 7 eV were used. For the formation of SO2− species at the ceria (111) surface, a process that does not involve any reduction of Ce ions, the Eads value, the S−O bond, and the S charge are all found to only moderately depend on the U parameter (Figure 11); i.e., they undergo only minor changes in the U-range considered here. SO22− formation from S adsorption on the ceria (110) surface does lead to Ce reduction; i.e., occupied Ce 4f states are involved. This leads to a strong U-value dependence of the adsorption energy (Figure 12a). However, changes in S−O bond and S charge remain small up to U = 6 eV (Figure 12b and c). As discussed by Pacchioni,31 without a direct experimental benchmark, it is not possible to conclusively determine if the choice of U-value reproduces the experimental vacancy formation energy. We note that U = 4−7 eV gives welllocalized Ce 4f states, with ∼1.0 μB magnetic moment for the reduced Ce ions (see Figure 12d). However, at U = 7 eV, additional charge is transferred from the sulfur to the bottom of the support. This is probably related to an increased stabilization of the Ce 4f states at U = 7 eV where these states become more stable than the LUMO of the SOx species present at lower U-values. This is quite similar to the findings in our recent tests on the U-value for the Rh/ceria system30 where there is also a trend break at U = 7 eV.

using the climbing image nudged elastic band (cNEB) method transition-state (TS) search. When a S atom is added to the stoichiometric ceria (111) surface, the most stable structure is the oxido-sulfate(2-) species (SO2−) described above. The calculated barrier for sulfur diffusion between two neighboring sites is 1.43 eV. The structure and energy profile are given in Figure 8.

Figure 8. Schematic potential energy profiles for sulfur diffusion on the CeO2(111) surface between two O atop sites. (a) Initial state. (b) Transition state. (c) Final state. Hereafter, the transition states and the corresponding energy barriers are indicated by dashed lines.

We found three stable configurations for a S adatom on the ceria (110) surface, viz., the oxido-sulfate(2-) species (SO2−) at the atop site of the surface O and the hyposulfite (SO22−) species at the Ce-bridge and the O-bridge sites (meta stable). The barriers connecting these three kinds of stable configurations were calculated. The calculated diffusion barrier from one O bridge site to its neighbor O bridge site along the [01̅1] direction on the ceria (110) surface is 2.63 eV. The structures and energy profile are given in Figure 9. The S diffusion along the [001] direction on the ceria (110) surface between two

Figure 9. Schematic potential energy profiles for sulfur diffusion between two O-bridge sites on the CeO2(110) surface along [01̅1] direction. (a) Initial state. (b) Transition state. (c) Final state. 8423



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Figure 10. Schematic potential energy profiles for sulfur diffusion on the CeO2(110) surface along [001] direction. (a) Initial state, Ce-bridge site. (b) Transition state. (c) Intermediate state, O top site. (d) Transition state. (e) Intermediate state, O-bridge site.

Figure 11. (a) Eads plotted versus the Hubbard U-value for S adsorbed on the O top site on the ceria (111) surface. (b) The corresponding S−O bond lengths. (c) The Bader charge for S.

Figure 12. (a) Eads plotted versus the Hubbard U-value for S adsorbed on the O top site on the ceria (110) surface. (b) The corresponding S−O bond lengths. (c) The sulfur Bader charge. (d) The magnetic moments on each of the reduced Ce ions.

It has previously been shown that U-values close to 5 eV give a reasonable description of both reduced and stoichiometric ceria as well as Ce2O3; see for example results and discussion in ref 32. It is therefore reassuring that no qualitative differences are seen in the range U = 4−6 eV for the present systems.

4. CONCLUSION We have discussed the species formed by adsorbing atomic sulfur on stoichiometric or partially reduced ceria (111) and (110) surfaces. This model system also represents SOx species likely to be formed from SO2 (or other sulfur-containing molecules) under 8424



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reducing conditions. Here, the oxidation number of sulfur is −II, 0, and II. We also investigated S insertion by replacing cerium ions with sulfur atoms at the ceria (111) and (110) surface. Our results can be summarized as follows: (a) Oxido-sulfate(2−) species (SO2−) form directly upon sulfur adsorption on both the CeO2(111) and (110) surfaces without any reduction of cerium; (b) Also hyposulfite (SO22−) forms after sulfur adsorption, but only on the (110) surface. This is accompanied by cerium reduction; (c) S2− forms when a surface or subsurface oxygen ion is replaced by sulfur. These sulfide species are more stable at the surface. Furthermore, these surface sulfides are found to be stable with respect to an adsorbed sulfur atom and an oxygen vacancy for the (111) surface, while the opposite is true for the (110) surface. (d) Sulfite (SO32−) structures result if S is made to replace one Ce in the ceria (111) and (110) surfaces; (e) The S−O stretching frequencies of the four SOx− (0 ≤ x ≤ 3) adsorption structures found here fall in the range 760−790 cm−1, which coincides with the lower region of the S−O frequency range we found for the SOx adsorbates created from SO2 adsorption on ceria.6 (f) S atoms adsorbed at the ceria (111) and (110) surfaces are essentially immobile: the calculated diffusion barriers are about 1.4 eV for the (111) surface and 2.0 and 2.6 eV along the [001] and [01̅1] directions on the (110) surface, respectively. In summary, we find three different species resulting from the interaction of a S atom with ceria. These species are all capable of poisoning the surface, reduced or unreduced, due to their strong binding. However, a fuller understanding of the sulfur poisoning mechanism requires information about how these species interact with other adsorbates. We performed test calculations aimed to investigate how selected properties for adsorbed S on the (111) and (110) surfaces depended on the choice of Hubbard U parameter. We find that: (a) The adsorption energy, S−O bond length, and sulfur charge for the (111) surface are only little affected by the U parameter value. This insensitivity is mainly due to the absence of occupied Ce 4f states. (b) On the other hand, properties for the (110) surface are more sensitive to the choice of U-value. This is mainly due to the occupation of Ce 4f states in the adsorption process. (c) For the range of acceptable U-values (U = 4−6 eV) there are no qualitative changes in the properties considered, while there is a trend break at U = 7 eV.

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Troop Construction Projects of Henan Province, China (Grant No. 104200510014), the Swedish Research Council (VR) and the Swedish Research Links Programme funded by VR and the Swedish International Development Cooperation Agency (Sida). Z.L. also acknowledges the LiSUM (Linking SinoEuropean Universities through Mobility) program for a scholarship and travel grant to Sweden. Parts of the simulations were performed on resources provided by the Swedish National Infrastructure for Computing (SNIC) at UPPMAX and NSC and the high-performance computing center of College of Physics and Information Engineering in Henan Normal University. We would also like to acknowledge the National Strategic e-Science program eSSENCE.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Z. Yang); [email protected] (K. Hermansson). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 11174070, No. 11147006, and No. 10947001), the Innovation Scientists and Technicians 8425


Sulfidation of Ceria Surfaces from Sulfur and Sulfur Diffusion - The

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