Sulfidation and Sulfur Recovery from SO2 over Ceria - The Journal of

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Sulfidation and Sulfur Recovery from SO2 over Ceria Jolla Kullgren,† Zhansheng Lu,‡ Zongxian Yang,‡ and Kersti Hermansson*,† †

Department of Chemistry, The Ångström Laboratory, Uppsala University, Box 538, 751 21 Uppsala, Sweden College of Physics and Information Engineering, Henan Normal University, Xinxiang, Henan 453007, China

‡

ABSTRACT: Sulfidation and sulfation of ceria(111) and ceria(110) surfaces are studied with the help of density functional theory (DFT) calculations (PBE+U). Under reducing atmosphere, SO2 adsorption leads to the formation of stable surface sulfate species on the (110) surface and sulfides on the (111) surface. A mechanism for sulfur recovery from SO2 is presented where SO2 reacts with a surface sulfide to form a thiosulfite species. This thiosulfite species is subsequently reduced by an oxygen vacancy and desorbed as S2(g).

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1. INTRODUCTION

formation, i.e., that surface sulfides may act as active centers in the formation of sulfur. The results reported in the present theoretical investigation also have implications for the sulfur poisoning phenomena. The deactivation of the three-way catalyst (TWC) involves structural and morphological processes and the formation of inactive compounds.9 Cerium sulfates, sulfides, and oxysulfides4 have been suggested as such inactive compounds. The very same species are considered in the current investigation. In this report, results from our previous studies5,6 are combined with new calculations to provide an understanding of the mechanisms involved in the sulfur poisoning and sulfur recovery reactions on ceria surfaces. We will show in the current investigation that the two most stable ceria surfaces, (111) and (110), surface, display very different behavior toward sulfation, sulfidation, sulfur poisoning and sulfur recovery.

Reduction of SO2 is technically interesting for the recovery of elemental sulfur from SO2-rich gases1,2 and in the context of sulfur poisoning of ceria (CeO2), where sulfidation has been pointed out as a possible source of the problem.3,4 In this study we examine the mechanisms involved in the reduction of SO2 over ceria and in the formation of elemental sulfur under reducing atmosphere. Our method is density functional theory (DFT) calculations. A more extensive review of recent experimental and theoretical studies of the interactions of sulfur-containing molecules with ceria surfaces can be found in refs 5 and 6, and only a brief literature review with focus on the sulfur recovery aspects is given below. The use of ceria-based catalysts in the recovery of elemental sulfur from SO2 has been studied experimentally in quite some detail; see for example refs 1 and 2. Either CO or CH4 is normally used as the reducing agent in such reactions. A Mars− van Krevelen (MvK) mechanism, where oxygen vacancies mediate the reduction of SO2, was proposed by Liu et al.1 This conjecture was supported in a recent publication by Rodriguez et al.,7 where the decomposition of SO2 on the ceria(111) surface was investigated, and it was shown that oxygen vacancies are clearly pivotal in the decomposition mechanism. As we demonstrated in our previous studies,5,6 the intermediate sulfoxy species likely to be involved in such a reduction mechanism are sulfites (SO32−) with S(+IV), thionites (SO2−) with S(+III), (SO2−) with S(0), and sulfides (S2−) with S(−II). In the current study, the MvK mechanism proposed by Liu et al. is compared with another mechanism, based on the observations and reasoning of Overbury et al.8 They found that repeated adsorption and annealing of SO2 on ceria can lead to the formation of S2 which desorb from the surface at temperatures above 850 K. They identified a species that they assigned as SO2 adsorbed on a surface sulfide ion and proposed that this species plays an active role in the process of sulfur © 2014 American Chemical Society

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2. METHOD All results in the present study as well as in refs 5 and 6 were derived from density functional theory calculations. We use the Vienna Ab initio Simulation Package (VASP)10−13 with the projector augmented wave (PAW) method14,15 and the Perdew−Burke−Ernzerhof (PBE) functional.16,17 The core electrons were [Kr]4d10 for Ce, 1s2 for O, and 1s22s22p6 for S, and the reaming electrons (12, 6, and 6) were treated as valence electrons. The DFT+U approach of Dudarev et al.18 was adopted with a U-value of 5 eV to describe the localized Ce4f states appearing in some of the structures. A cutoff energy Received: September 23, 2013 Revised: June 23, 2014 Published: June 26, 2014 17499

dx.doi.org/10.1021/jp4094673 | J. Phys. Chem. C 2014, 118, 17499−17504

The Journal of Physical Chemistry C

Article

Figure 1. Diagram of SO2 reduction over the ceria (111) surface (bottom) and the ceria (110) surface (top). As one moves from the right to the left in the figure, oxygen vacancies are consumed and the energy difference between two steps corresponds to the reaction SOn(a) + VO ⇋ SOn−1(a) + OO. Energies are given in kcal/mol. The oxidation number assigned to sulfur in each of the SOx species is shown in the bottom right corner of the structure subfigures.

of 30 Ry (408 eV) was used in the plane-wave expansion of the Kohn−Sham orbitals. The surfaces were modeled using slabs separated by a 15 Å wide vacuum gaps (partly filled with the adsorbates). The slabs consisted of four O−Ce−O triple layers for the (111) surface and five atomic layers for the (110) surfaces. The two bottom triple layers for the (111) surface and the two bottom atomic layers for the (110) surface were kept fixed at the optimized bulk values at all times, while all other atoms were allowed to relax until the force on each atom was less than 0.02 eV/Å. We use a p(2×2) supercell for both surfaces and a 2 × 2 × 1 kpoint sampling grid for the electronic structure calculations. The SOx species investigated were placed on the “upper” surface of the slab (as displayed in the figures). Bader charges19 were determined from the valence electron density using the code by Henkelman et al.19,20 These charges helped us compare and characterize the different SOx species found in this work. Reaction barriers were calculated with the nudged elastic band (NEB) method21,22 with the same convergence criterion as that used in the geometry optimizations.

and S (S(a)) species on the ceria (111) and (110) surfaces. Because also adsorption at oxygen vacancy sites was included, the structures presented in those papers covered the range from completely oxidized (sulfates) to completely reduced (sulfides) SOx species. The most stable structure that we found for each type of SOn species is schematically illustrated at the top and bottom of Figure 1. All of these structures except one, SO(a) on the ceria (110) surface, were introduced in our previous studies.5,6 In the present study, the SO(a) structure on the (110) surface was found by adsorbing an SO entity to the O−O bridge, which leads to the formation of a bidentate surface sulfite structure with similar S−O bond lengths and atomic Bader charges as the monodentate surface sulfites found from SO2 adsorption on ceria (110) in ref 5. 3.2. Sulfation and Sulfidation through the MvK Mechanism. The various SOx species presented at the top and bottom of Figure 1 can be related to each other with the help of oxygen atoms, and vacancies, in the lattice via the Mars−van Krevelen mechanism:

3. RESULTS AND DISCUSSION 3.1. SOx Species on Ceria. We use the notation SOn(a) to mean a surface SOx species that consists of an SOn entity located “above the stoichiometric surface”; i.e., this SOx species could have been created by adsorption of an SOn molecule on the stoichiometric ceria surface, although the actual origin may in fact be different. In our recent DFT+U studies5,6 we presented adsorption structures, electronic structures, charges, S−O stretching frequencies, and adsorption energies for many SO2 (SO2(a))

where VO denotes an oxygen vacancy and OO a lattice oxygen. Reaction 1 from left to right corresponds reduction and the reverse reaction to oxidation. Reaction 1 above can be used to investigate the stabilities of the various SOx species at reducing conditions characterized by a reservoir of available oxygen vacancies (illustrated by white boxes in Figure 1). The value for the vacancy formation energy (Evac) for the (111) surface with the computational approach used here, is 67 kcal/mol, calculated according to E[CeO2−x(111)] − E[CeO2(111)] − 1/2E[O2(g)]. The

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SOn(a) + VO ⇋ SOn − 1(a) + OO

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(1)



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Figure 2, however, it is clear that, at rather high coverages (2 × 2), the sulfide species, S×O , remains the stable species over the Evac interval considered here. Moreover, S×O is very stable compared to the S(a) species. This latter finding is crucial for the sulfur recovery mechanism presented in section 3.4. 3.3. Sulfation in Oxygen Atmosphere. For completeness we the consider the oxidation and reduction of SOx species in an oxygen-rich atmosphere occurring via adsorbed O species, namely:

corresponding value for the (110) surface is 52 kcal/mol. Using reaction 1 we constructed energy diagrams for SOx reduction all the way from sulfate, S(+VI), to sulfide, S(−II). They are shown for the CeO2(111) and CeO2(110) surfaces in Figure 1. The reduction of SO2(a) to sulfide (S×O ) through the MvK mechanism is seen to be strongly exothermic on the CeO2(111) surface. On the CeO2(110) surface, we note a very different behavior. Here the reduction of the SO2(a) species is endothermic and thus the (110) surface is inactive for sulfur reduction via the MvK mechanism. Hence, our DFT results thus suggest that the stable oxidation number for sulfur under oxygen-lean conditions (reduced surfaces with oxygen vacancies) is S(−II) for the (111) surface and S(+VI) for the (110) surface. In other words, under reducing conditions, the (111) surface is likely to become sulfidated, but the (110) sulfated. Our results for the (111) surface are consistent with the experimental work of Happel et al.23 for CeO2(111), where thionite species and surface sulfides were reported for reduced ceria samples exposed to SO2. Such experiments on the less stable (110) surface are, to the best of our knowledge, lacking. We thus suggest that only the (reduced) ceria (111) surface is active in the reduction of SO2 via the MvK processes and for this surface we also calculated the barriers involved in breaking the covalent bond in each step of the reduction from SO2(a) to S×O , using five-image NEB calculations. The resulting barriers are 16, 14, and 24 kcal/mol for SO2(a) to SO(a), SO(a) to S(a) and S(a) to S×O , respectively. It is interesting to consider how the energy profiles for the reduction of SO2 on a reduced CeO2 surface are affected by a decrease in the vacancy formation energy (Evac). The (110) surface is already inactive and a further decrease would make it even more inactive. Therefore, we only consider the (111) surface in the following exercise, as illustrated in Figure 2. The

SOn(a) + O(a) ⇋ SOn + 1(a)

(2)

where O(a) denotes an oxygen adatom. In agreement with previous studies,25 we find that oxygen adatoms prefer to bind to surface lattice oxygens forming peroxy species (O22−). The adsorption energy for the oxygen adatom, in this configuration, calculated according to E[O/CeO2(111)] − E[CeO2(111)] − 1 /2E[O2(g)], is 5.7 kcal/mol for the (111) surface and (happens to be) 0.0 kcal/mol for the (110) surface with the reference state choosen here. The results of our calculations suggest that SOx species on both the ceria (111) and (110) surface are readily oxidized by oxygen adatoms through reaction 2, with favorable enthalpies that are larger in magnitude than 35 kcal/mol for each step in the oxidation from sulfide (S(−II)) all the way to sulfate (S(+VI)). Our calculations predict that, under oxygen-rich conditions (here characterized by available oxygen adatoms), both surfaces become sulfated. 3.4. A Mechanism for Sulfur Recovery. In this section we will discuss the sulfur recovery mechanism, i.e., the formation of collectable elemental sulfur from SO2(g). This requires a reduction of sulfur from S(+IV) to S(0) and in section 3.2 we found that the CeO2(111) surface has such a capability. CeO2(110), on the other hand, was found above to be inactive for sulfur reduction and will not be further discussed in this section. The energy diagram for the CeO2(111) surface in Figure 1 partly supports the conjecture of Liu et al.,1 namely that the recovery of sulfur from SO2 may involve a MvK mechanism, where oxygen atoms from SO2 are stripped off one by one, until S(0) is formed. However, our calculations suggest that the MvK mechanism continues all the way to S(−II). Even disregarding this fact, the S(0) species in our MvK calculation does not constitute a promising source for S recovery because its desorption energy is very high (60 kcal/mol; Figure 3) and its

Figure 2. Qualitative diagram of SO2 reduction over the ceria (111) surface constructed in the same way as Figure 1. The curve marked (A) is the same as in Figure 1 and corresponds to, from right to left, reaction 1 in the text, using our optimized PBE+U Evac value, which is 67 kcal/mol with respect to O2(g). The curve marked (B) corresponds to an Evac value of 44 kcal/mol (1 eV less), i.e., less reative oxygen vacancies. See text for details.

energy profile marked (A) in Figure 2 corresponds to our Evac value of 67 kcal/mol, mentioned above. For the profile marked (B) we used an Evac value that was 1 eV (∼23 kcal/mol) smaller; i.e., the vacancies here are less potent reducing agents. The comparison of (A) and (B) not only serves to test the sensitivity of our results toward possible errors in the estimated Evac values but also allows us to investigate the effect that, e.g, a dopant-induced lowering of Evac would give rise to. We see that the sulfate species becomes more stable when the vacancy formation energy decreases. This finding is consistent with the DFT results of Liu et al.,24 who found a smaller vacancy formation energy and sulfate species being slightly more stable than sulfite species. In the experimental work by Rodriguez et al.,7 sulfates were reported to be the dominating species after SO2 adsorption on the stoichiometric ceria(111) surface. From

Figure 3. Reaction tree for SO2 on ceria (111) and (110) under reducing conditions. All values are given in kcal/mol with the upper/ lower (green/black) value referring to the (111)/(110) surfaces, respectively. Numbers at the vertical arrows indicate the desorption energy of each species and thus correspond to the reactions SOn(a) → SOn(g). The energies below the framed formulas correspond to the individual steps in the MvK process (formula 1 in the text) and can be obtained from the energy diagram in Figure 1. 17501



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Figure 4. Diagram for the sulfur recovery mechanism. An SO2 molecule is adsorbed atop a surface sulfide and a thiosulfite is formed. The surface thiosulfite reacts with an oxygen vacancy to from an S2O− species, which subsequently desorbs as an S2 molecule. In the frames, top and side views of the structures are shown. Oxygen is drawn with white or red circles, sulfur with yellow circles, and cerium with black small circles. The structures involved in the suggested diffusion mechanism are also shown.

mobility very low,6 thereby preventing the formation of any S2 or other Sn species. On the other hand, we find that also the S(−II) species has a very high desorption energy [S2−→ S(a) → S(g), 92 kcal/mol; Figure 3]. Thus, an alternative mechanism is needed. The article by Overbury et al.8 provides a hint. In their soft X-ray photoemission study, they found that elemental sulfur in the form of S2(g) could be obtained by repeated cycles of SO2 adsorption and annealing and proposed that surface sulfide species may act as active centers in the elemental sulfur formation. In view of this interesting proposition we here suggest and explore a three-step mechanism for the route to

sulfur recovery, starting from an SO2 molecule adsorbing on a surface sulfide created, for example, by the mechanism described in section 3.2. We divide the sulfur recovery into the following stages: (1) thiosulfite (S2O22−) formation, (2) thiosulfite reduction, and (3) S2 desorption. This alternative sulfur recovery mechanism is illustrated in the six numbered panels in the middle of Figure 4. The species in panels 1, 2, and 5 are those suggested by Overbury et al., and the structures in the second stage (panels 3 and 4), linking the experimental observation to a mechanism, are suggested by us. To the best of our knowledge, none of the species in panels 2−4 has been presented in any theoretical investigations elsewhere and we 17502



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3.4.3. S2 Desorption. In the final step of this sulfur recovery mechanism, an S2(a) species is desorbed as S2(g). The desorption requires about 25 kcal/mol, from the S2(a) in panels 4a and 4b, or about 36 kcal/mol from an S2 entity residing in a vacancy (the species discussed in the last paragraph of section 3.4.2). These findings are consistent with experimental observations1,2,8 that desorption of sulfur only occurs at elevated temperatures. Given the very small energy difference between the monoand bidentate S2(a) structures in panels 4a and 4b, there may also be a possibility that S2(a) would tumble across the surface by toggling between these two states and polymerize to larger sulfur chains before desorbing. 3.4.4. Comparison to Experiments by Overbury et al. On the basis of soft X-ray photoemission spectra, Overbury et al.8 reported the existence of sulfur-containing species with S in oxidation states intermediate to those of sulfite (S(+IV)) and sulfide (S(−II)). In their study, elemental sulfur was observed from repeated cycles of SO2 adsorption on ceria and annealing. The most prominent XPS feature close to the sulfide peak was assigned to an altered surface sulfur. They suggested this to be a sulfur atom in the surface, bonded to an adsorbed SO2, e.g., as in panel 2. We assigned the oxidation number −I to this sulfur; see above. They further reported minority features in their spectra, intermediate between peaks originating from the altered sulfur and sulfite. We suggest that these could be signals from some of the intermediate structure presented in panels 4a and 4b, or possibly from the S2− and S22− species when the S2 entity resides in a vacancy. 3.4.5. Competing Reaction. CO is commonly used as a reducing agent in sulfur recovery from SO2-rich gases. In the light of the sulfur recovery mechanisms presented above, the role of CO would be to provide oxygen vacancies that are consumed both in the reduction of SO2 to S2− and in the reduction of thiosulfite (S2O22−). However, there exists a competing side reaction where CO adsorbs on surface sulfide sites. We find that such an adsorption can lead to the formation of a carbonyl sulfide (COS) species and an oxygen vacancy with an energy gain of 3.7 kcal/mol:

will therefore describe them in some detail. These species and all the others in Figure 4 represent optimized structures. 3.4.1. Thiosulfite Formation. In the first step, i.e., panel 1 in Figure 4, an SO2 molecule is placed atop a surface sulfide site, resulting in a thiosulfite species (S2O22−), as shown in the panel labeled 2 (schematically in the upper part of the figure and the optimized structure in the bottom part). Between panels 2 and 3, the thiosulfite sulfur residing in the surface is replaced by one of its own oxygen atoms. This “rotational” motion is indicated by the arrows in panel 2. The thiosulfite species in panel 3 has S−O bond lengths very similar to the sulfite species formed from SO2 adsorption atop a surface oxygen site on the stoichiometric CeO2(111) surface, SO2(a); the difference is less than 0.02 Å. The S−S bond length in the thiosulfite species is 2.0 Å, compared to 1.9 Å for the S2 molecule at the same level of theory (PBE+U). The whole thiosulfite complex has a Bader charge of −1.3 e, compared to −1.4 e for the sulfite in SO2(a). The Bader charge of the sulfur atoms in the thiosulfite are −0.4 e and 2.9 e for the terminal and central sulfur atoms, respectively. Thus, we suggest the assignment S(−I) and S(+III) for these sulfur atoms. Using the NEB method, we calculated the barriers between the structures in panels 1−3 and found them to be smaller than 5 kcal/mol, suggesting a rapid formation of the thiosulfite in panel 3. 3.4.2. Thiosulfite Reduction. In the next step of the mechanism, we let the thiosulfite in panel 3 become reduced by an oxygen vacancy by introducing an oxygen vacancy next to it. This leads to the monodentate S2 adsorbed species, S2(a), shown in panel 4a. We find no barrier for the transformation between panels 3 and 4a and expect that the limiting step here would be the migration of the vacancy itself. The species in panel 4a is similar to the thionite species SO2− that forms when a sulfite species is reduced (denoted SO(a) in this work; see section 3.1). The two carry similar Bader charges (−0.75 e vs −0.79 e) and have slightly contracted bonds compared to their parent structures (the thiosulfite and the sulfite structures). One electron is transferred from the surface to the adsorbate, leaving only one reduced cerium ion as indicated by the single star (*) in panel 4a. Thus, we make the assignment S2O− to this species. The Bader charges are −0.23 and 1.25 e on the sulfur atoms in the S2O− ion, and we make the assignments S(+I) to the central S and S(0) to the terminal S atom. We also find a bidentate S2(a) structure with a very similar energy (within 1 kcal/mol) (panel 4b). This is a bidentate thiosulfite (S2O22−) species and has a net Bader charge of −1.46 e. Both sulfur atoms are assigned the oxidation number +I, consistent with their Bader charges of 1.00 and 1.14 e, respectively. In this case there is no electron transfer between the surface and the adsorbate, hence the two stars (*) in panel 4b. With a good supply of oxygen vacancies one could expect that the S2(a) species in panels a and b of Figure 4 may “fall down” into some vacancy nearby and heal it (process not shown here). Indeed, we find that an S2 entity does bind to a vacancy site. It then retains its identity as an S2 species and is 11 kcal/mol more stable than the species in panels 4a and 4b. In this case we find Bader charges of −0.4 and −0.2 e for the sulfur atoms, suggestive of an S2− species. This assignment is consistent with the fact that one reduced cerium ion remains in the surface. We also find a configuration with very similar energy (2 kcal/mol less stable) where no reduced cerium ion remains in the surface, suggesting an S22− species.

× × CO(a) + SO + 2CeCe → COS(g) + VO + 2Ce′Ce

(3)

This competition between CO and SO2 for the surface sulfides may be the origin of the phenomenon observed in ref 26, namely that COS production increases when excess CO is used.

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4. CONCLUSION The mechanisms of SO2 reduction over CeO2(111) and CeO2(110) were investigated using DFT+U calculations. We find the following: • Sulfides (S2−) are the most stable species under reducing atmosphere on the (111) surface, and surface sulfates are the most stable on the (110) surface (Figure 1). • Sulfates dominate on both surfaces at oxidizing conditions. • The barriers for sequential reduction of SO2 to sulfide on the (111) surface are modest (less than 25 kcal/mol). We conclude that the CeO2(111) surface is active for reduction of SO2 via a Mars−van Krevelen process, and the (110) is inactive for the same process. We propose a new mechanism for sulfur recovery from SO2 over CeO2(111) which proceeds as follows: 17503



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• SO2 is reduced to a surface sulfide via a Mars−van Krevelen mechanism involving oxygen vacancies at the ceria (111) surface (Figure 1). • The formed sulfide species then acts as a reaction center in the subsequent formation of a thiosulfite species (S2O22−) from the adsorption of a second SO2 molecule. • The resulting thiosulfite is reduced to S2O22−, S2O−, S2−, or S22− with the help of oxygen vacancies in a second Mars−van Krevelen process (Figure 4). • Finally, the reduced species is decomposed through the desorption of S2.

(10) Kresse, G.; Hafner, J. Phys. Rev. B 1993, 47, 558−561. (11) Kresse, G.; Hafner, J. Phys. Rev. B 1994, 49, 14251−14269. (12) Kresse, G.; Furthmüller, J. Comput. Mater. Sci. 1996, 6, 15−50. (13) Kresse, G.; Furthmüller, J. Phys. Rev. B 1996, 54, 11169−11186. (14) Blöchl, P. E. Phys. Rev. B 1994, 50, 17953−17979. (15) Kresse, G.; Joubert, D. Phys. Rev. B 1999, 59, 1758−1775. (16) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865−3868. (17) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1997, 78, 1396−1396. (18) Dudarev, S. L.; Botton, G. A.; Savrasov, S. Y.; Humphreys, C. J.; Sutton, A. P. Phys. Rev. B 1998, 57, 1505−1509. (19) Bader, R. F. W. Encyclopedia of Computational Chemistry; John Wiley & Sons, Ltd.: New York, 2002. (20) Henkelman, G.; Arnaldsson, A.; Jónsson, H. Comput. Mater. Sci. 2006, 36, 354−360. (21) Mills, G.; Jónsson, H.; Schenter, G. K. Surf. Sci. 1995, 324, 305− 337. (22) Jonsson, H.; Mills, G.; Jacobsen, K. W. In Classical and Quantum Dynamics in Condensed Phase Simulations; Berne, B. J., Ciccotti, G., Coker, D. F., Eds.; World Scientific: Singapore, 1998. (23) Happel, M.; Lykhach, Y.; Tsud, N.; Skála, T.; Prince, K. C.; Matolín, V.; Libuda, J. J. Phys. Chem. C 2011, 115, 19872−19882. (24) Liu, Y.; Cen, W.; Wu, Z.; Weng, X.; Wang, H. J. Phys. Chem. C 2012, 116, 22930−22937. (25) Huang, M.; Fabris, S. Phys. Rev. B 2007, 75, 081404. (26) Liu, W.; Wadia, C.; Flytzani-Stephanopoulos, M. Catal. Today 1996, 28, 391−403.

Figure 5. Fate of SO2 in contact with the CeO2(111) and CeO2(110) surfaces under reducing conditions.

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AUTHOR INFORMATION

Corresponding Author

*K. Hermansson. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Swedish Research Council (VR). Computer resources provided by the Swedish National Infrastructure for Computing (SNIC) at UPPMAX and NSC are also gratefully acknowledged. We also acknowledge the Swedish National Strategic e-Science program eSSENCE. J.K. acknowledges the Hanse-Wissenschaftskolleg (HWK).

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REFERENCES

(1) Liu, W.; Sarofim, A. F.; Flytzani-Stephanopoulos, M. Appl. Catal. B: Environ. 1994, 4, 167−186. (2) Flytzani-Stephanopoulos, M.; Zhu, T.; Li, Y. Catal. Today 2000, 62, 145−158. (3) Luo, T.; Vohs, J. M.; Gorte, R. J. J. Catal. 2002, 210, 397−404. (4) Karjalainen, H.; Lassi, U.; Rahkamaa-Tolonen, K.; Kröger, V.; Keiski, R. L. Catal. Today 2005, 100, 291−295. (5) Lu, Z.; Müller, C.; Yang, Z.; Hermansson, K.; Kullgren, J. J. Chem. Phys. 2011, 134, 184703. (6) Lu, Z.; Kullgren, J.; Yang, Z.; Hermansson, K. J. Phys. Chem. C 2012, 116, 8417−8425. (7) Rodriguez, J. A.; Liu, P.; Pe?rez, M.; Liu, G.; Hrbek, J. J. Phys. Chem. A 2010, 114, 3802−3810. (8) Overbury, S. H.; Mullins, D. R.; Huntley, D. R.; Kundakovic, L. J. Phys. Chem. B 1999, 103, 11308−11317. (9) Rodriguez, J. A.; Hrbek, J. Acc. Chem. Res. 1999, 32, 719−728. 17504


Sulfidation and Sulfur Recovery from SO2 over Ceria - The Journal of

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