Role of Oxygen Vacancies in Catalytic SO3 Decomposition over

Article pubs.acs.org/JPCC

Role of Oxygen Vacancies in Catalytic SO3 Decomposition over Cu2V2O7 in Solar Thermochemical Water Splitting Cycles Masato Machida,* Takahiro Kawada, Hiroaki Yamashita, and Tonami Tajiri Department of Applied Chemistry and Biochemistry, Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Chuo, Kumamoto 860-8555, Japan S Supporting Information *

ABSTRACT: We report the structure−activity relationship of copper pyrovanadate (Cu2V2O7) as an efficient catalyst for SO3 decomposition in solar thermochemical water splitting cycles. Of the α, β, and γ polymorphs of Cu2V2O7, the α-phase, which has a blossite-type structure, was stable under the catalytic reaction conditions. Spontaneous oxygen desorption accompanied by charge compensation through the reduction of Cu2+ to Cu+ produced an oxygen deficiency corresponding to Cu16V16O55 at 600 °C. Density functional theory calculations based on these results showed that oxygen vacancy formation is more favorable on the Cu−O−V bridging sites than on the V−O− V site in the pyrovanadate unit. The oxygen vacancy formation energy of the (100) surface is considerably less than that of bulk Cu16V16O56. The reaction, Cu16V16O55 + SO3 → Cu16V16O56 + SO2, is exothermic, suggesting that oxygen vacancies play a key role in catalytic SO3 decomposition over a Cu2V2O7 catalyst.

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INTRODUCTION Thermochemical water splitting cycles for producing hydrogen using concentrated solar radiation have been studied extensively.1−4 The decomposition of sulfuric acid is the most promising candidate for the oxygen-generating reaction in these cycles.5,6 A typical example is the sulfur−iodine process,7−10 which is a closed water splitting cycle consisting of the following three reactions: H 2SO4 → H 2O + SO2 + 1/2O2

∼ 900°C (1)

2HI → H 2 + I 2

∼400°C (2)

The combination of the pyrovanadate framework, which resists sulfate formation, and a copper redox center achieves both catalytic activity and stability. The catalytic performance can be enhanced further by supporting the Cu 2V2O7 on 3-D mesoporous SiO2 followed by thermal aging.22 This melts the Cu2V2O7 and allows it to penetrate the mesopores, converting the microstructure from a mesoporous to a macroporous SiO2 framework via the dissolution−reprecipitation mechanism. The resulting macroporous Cu2V2O7/SiO2 catalyst is the first promising substitute for Pt catalysts. In the present study, we have investigated the structure− activity relationship for Cu2V2O7 as a result of our interest in its catalytic SO3 activation. We used temperature programmed desorption (TPD), thermal gravimetric analysis/differential thermal analysis (TG/DTA), and X-ray diffraction (XRD) to study spontaneous oxygen desorption from Cu2V2O7 and the oxygen-deficient structure. A structural model was developed for density functional theory (DFT) calculations. The DFT calculations were performed on a bulk and a surface slab model to determine the formation energy of vacancies in different oxygen sites. Finally, an energy diagram of possible surface SO3 reactions was constructed to elucidate the role of Cu2V2O7 oxygen vacancies in catalytic SO3 decomposition.

SO2 + I 2 + 2H 2O → H 2SO4 + 2HI ∼100°C (3)

In eq 1, the sulfuric acid almost completely dissociates into H2O and SO3 in the gas phase above 350 °C, although the subsequent decomposition of SO3 into SO2 and 1/2O2 requires a temperature of about 900 °C. Active catalysts for SO3 decomposition in an equilibrium-shift reactor are required to reduce the reaction temperature to around 600 °C. This would allow thermochemical water splitting to be achieved with current large-scale solar concentration technology. A number of catalysts have been studied for the decomposition of SO3.7−19 Although Pt is the most active catalyst,7,14 it is deactivated by thermal sintering and sulfate formation of the support materials upon exposure to SO3.20 Various metal oxides and mixed oxides have been proposed as economically viable catalysts. However, their activities are too low to function efficiently around 600 °C. We have recently shown that copper pyrovanadate (Cu2V2O7) is an efficient catalyst for SO3 decomposition at moderate temperatures.21 © 2013 American Chemical Society

Received: October 21, 2013 Revised: November 25, 2013 Published: November 27, 2013 26710

dx.doi.org/10.1021/jp410431a | J. Phys. Chem. C 2013, 117, 26710−26715

The Journal of Physical Chemistry C

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EXPERIMENTAL SECTION Catalyst Preparation and Characterization. Copper pyrovanadate (Cu2V2O7) was prepared by calcining powder mixtures of commercially available CuO and V2O5 (Wako Pure Chemical, Ltd.) at 700 °C for 12 h. XRD measurements were performed using monochromated Cu Kα radiation (30 kV, 20 mA, Multiflex, Rigaku). Structural changes in Cu2V2O7 during a temperature ramp were measured by in situ XRD using Cu Kα radiation (30 kV, 20 mA, Rigaku, RINT-Ultima) equipped with a high-speed 2-D detector (D/teX-25). The sample was placed in a flow of 20% O2/N2 (100 cm3 min−1) in a temperaturecontrolled chamber, which was heated at a constant rate of 10 °C min−1. The XRD pattern was measured every 50 °C with a scan rate of 40° min−1. The high-speed detector enabled each XRD pattern to be obtained within 60 s, which is fast enough to neglect phase changes during data acquisition. The TPD of oxygen was measured in a conventional flow reactor connected to a volumetric vacuum system. Prior to the measurements, the sample was treated under a flow of 20% O2/ N2 at 450 °C for 1 h and then cooled to ambient temperature. After evacuation, the sample was heated from ambient temperature to 700 °C at a constant rate of 10 °C min−1 in a He stream (20 cm3 min−1). The gas mixtures leaving the sample were analyzed by a quadrupole residual gas analyzer mass spectrometer (Pfeiffer, Omnistar). TG (8120, Rigaku) was performed to determine the amount of oxygen vacancies. The sample (∼10 mg) was heated in a flow of N2. DTA (8120, Rigaku) was also performed to detect the phase transformation of Cu2V2O7 during the temperature ramp (10 °C min−1) in a stream of N2 and in 20% O2/N2. DFT Calculation. Oxygen vacancy formation in the bulk and surface of α-Cu2V2O7 was analyzed using periodic DFT calculations. Spin-polarized generalized gradient approximation GGA+U (U = Hubbard U) electronic structure calculations were performed with the Vienna Ab Initio Simulation Package (VASP).23 Calculations were performed using projector augmented-wave PAW potentials for Cu, V, and O atoms. A plane-wave basis set with a cutoff of 520 eV was used. The Perdew-Burke-Ernzerhof GGA was employed for the exchange and correlation functionals.24 Sums over occupied electronic states were performed using the Monkhorst-Pack scheme25 on a 1 × 2 × 2 k-point mesh for the bulk α-Cu2V2O7. A 1 × 1 × 1 set of k-point mesh was used for a (2 × 2) supercell of the (100) surface. The simplified GGA+U method reported by Dudarev et al.26 was utilized. A literature value of U-J (6.52 eV) for the Cu 3d channel was used.27 Unit-cell parameters and atomic coordinates of α-Cu2V2O7 were optimized with a convergence condition of 0.02 eV·Å−1 for an orthorhombic cell (a = 20.680 Ǻ , b = 8.411 Ǻ , c = 6.448 Ǻ ). Initial crystallographic parameters used in the optimization were taken from the literature.28 To calculate the lattice relaxation of the oxygen-deficient bulk structure, all atoms within 3 Ǻ of the oxygen vacancy were allowed to relax until all forces on the atoms were less than 0.02 eV·Å−1. The radius of relaxation region (3 Ǻ ) seems to be reasonable because our preliminary calculation showed that only the vacancy first nearest neighbors move significantly, whereas second nearest neighbors do not. Also, 3 Ǻ is small enough to avoid an artificial interaction between two adjacent vacancies, the shortest distance of which is c = 6.448 Ǻ in the present system. The (100) surface of α-Cu2V2O7 was modeled by a (2 × 2) supercell with seven atomic layers and the top

three layers were allowed to relax under the same convergence condition. An oxygen vacancy was placed on one side of the slab with 30 Å of vacuum separation. Of the possible initial magnetism settings corresponding to ferromagnetic, antiferromagnetic, and nonmagnetic phases, the antiferromagnetic ground state yielded the lowest total energy. The optimized unit-cell parameters and atomic coordinates for the bulk α-Cu2V2O7 showed good agreement with experimental crystallographic data,29 ensuring the validity of the GGA+U and PAW potentials used in this work. The calculation of the oxygen vacancy formation on the (100) surface focused on the antiferromagnetic ground state.

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RESULTS AND DISCUSSION Phase Equilibrium of Cu2V2O7. Cu2V2O7 crystallizes as α-, β-, and γ-phase polymorphs, which have similar crystal structures but different formation kinetics and phase transformation reversibility.30,31 It has previously been reported that the α-phase (blossite, orthorhombic) is stable in the range 610−704 °C, whereas the β-phase (ziesite, monoclinic) exists at lower temperatures.31 The γ-phase (triclinic) appears in the highest temperature range below the melting point (760 °C). These phase equilibria have been studied in air, although no detailed information at low O2 partial pressures has been reported in the literature. It is necessary to determine which crystalline phases are dominant under the catalytic reaction conditions for the first principle calculations of the system described in the following section. Figure 1 shows in situ high-temperature XRD patterns taken from Cu2V2O7 during heating in a stream of 20% O2 in N2. The as-prepared Cu2V2O7 after calcination at 700 °C in air was a mixture of β and γ-phases. The diffraction peak of γ-phase at 2θ = 28.5° became less intense and shifted to lower angles at ≥200 °C. However, it remained up to the temperature of 550 °C together with other diffraction peaks from γ-phase (2θ = 22.5° and 27.2°). The diffraction peaks of β-phase also shifted during the temperature ramp, but their intensity remained unchanged up to the temperature of 550 °C. These β/γ-phases disappeared at ≥600 °C with simultaneous appearance of α-phase. The temperature of the phase transformation was determined by DTA (Figure 2). Heating in a flow of 20 vol % O2 in N2 gave rise to a sharp endothermic peak at 606 °C due to the β/γ→α transformation (Figure 1). The α-phase is stable until the subsequent transformation to the γ-phase, which corresponds to the second endothermic peak at 760 °C. The third endothermic peak at 780 °C corresponds to the melting point. The presence of the high-temperature phase (γ) at ambient temperature (Figure 1) can be explained by the slow kinetics of the γ→α and α→β transformations compared with the γ→β direct transformation during cooling,30 which means this metastable γ-phase remains at lower temperatures. This is consistent with the increase in the fraction of the γ-phase when Cu2V2O7 was prepared by calcination at 700 °C followed by rapid quenching. When the heating was carried out in a stream of N2 (Figure 2b) the β/γ→α transformation remained unchanged, whereas the α→γ transformation was shifted to a lower temperature (∼720 °C). The sequence of the polymorphic transformation of the present system is thus influenced by the presence of O2. When H2SO4 alone was used as a gas feed in the solar thermochemical iodine−sulfur water-splitting process, the equilibrium gas composition at 600 °C under atmospheric pressure was estimated as 47 vol % H2O, 33 vol % SO3, 13 vol 26711



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MO2 → MO + 1/2O2

(5)

According to this simple reaction mechanism, a possible ratedetermining step for SO3 decomposition is the removal of oxygen after SO2 desorption, which becomes particularly important at lower reaction temperatures. Figure 3 compares

Figure 3. Temperature programmed desorption of O2 from Cu2V2O7, CuO, and V2O5 measured in a stream of He. Heating rate: 10 °C min−1.

the temperature programmed desorption of O2 from Cu2V2O7, CuO, and V2O5. V2O5 did not show a noticeable amount of O2 desorption up to its melting point of 675 °C. CuO underwent O2 desorption at around 600 °C because it decomposes to Cu2O, although this was not significant when the temperature was less than 700 °C. In contrast to these end-member compositions, Cu2V2O7 exhibited much greater O2 desorption above 600 °C. Figure 4 shows the XRD patterns for Cu2V2O7 before and after O2 desorption. Although the peaks mainly correspond to β- and γ-Cu2V2O7, oxygen desorption shifted these peaks toward a lower 2θ, consistent with the lattice expansion caused by the elimination of the lattice oxide ions. The XPS spectra of Cu2V2O7 before and after O2 desorption suggested that the charge compensation during oxygen

Figure 1. In situ XRD patterns for Cu2V2O7 during heating in air. Heating rate: 10 °C min−1. The Pt diffraction peak was from the sample holder.

Figure 2. DTA profiles of Cu2V2O7 measured in a stream of a) 20% O2/N2 and b) N2. Heating rate: 10 °C min−1.

% SO2, and 7 vol % O2. The Cu2V2O7 phase is stable even in the presence of high concentrations of SO3/SO2 because the pyrovanadate framework is resistant to sulfate formation. Considering the equilibrium oxygen partial pressure, Cu2V2O7 should be in the α-phase under the catalytic reaction atmosphere. Oxygen Vacancy Formation. Tagawa and Endo proposed that SO3 decomposition over metal oxide catalysts proceeds via a surface intermediate metal sulfate species11 MO + SO3 → (MSO4 ) → MO2 + SO2

Figure 4. XRD patterns of Cu2V2O7 (a) before and (b) after O2-TPD measurements (Figure 3). XRD measurement was at ambient temperature.

(4) 26712



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desorption was accomplished by the reduction of Cu2+ to Cu+ (see Supporting Information). To quantify the oxygen vacancies formed as a result of oxygen desorption, TG of Cu2V2O7 was performed at 500, 600, and 650 °C in a stream of N2 (Figure 5). Very little weight loss

the oxygen deficiency (δ) in Cu2V2O7‑δ was calculated to be 0.121 at 600 °C. This nearly corresponds to one oxygen vacancy in the single unit cell with an eight unit formula, (Cu2V2O7)8 (Figure 6). The resulting defect structure for Cu16V16O55 was modeled for the DFT calculations of the oxygen vacancy formation energy. The inset of Figure 6 shows that the crystal structure of αCu2V2O7 is orthorhombic with four possible oxygen sites: twofold O1 bridging two V atoms; twofold O2 and O4 bridging Cu and V atoms; and threefold O3 bridging two Cu atoms and a V atom. The energy of oxygen vacancy formation (ΔEv) in the bulk and the (100) surface slab of Cu16V16O56 was calculated using ΔEv = E(Cu16V16O55) + 1/2E(O2 ) − E(Cu16V16O56 )

where E(Cu16V16O55) and E(Cu16V16O56) are the total energies of the Cu16V16O56 bulk or surface slab, respectively, with and without an oxygen vacancy at each site, and E(O2) is the ground state of an O2 molecule in the gas phase. The calculated ΔEv values for bulk Cu16V16O56 are listed in Table 2. The

Figure 5. Thermogravimetry of Cu2V2O7 measured in a stream of N2.

Table 2. Calculated Oxygen Vacancy Formation Energy (kJ mol−1) for Different Sites in Bulk and Surface Slab Models of α-(Cu16V16O56)

occurred below 500 °C, whereas larger weight losses were caused by elimination of lattice oxygen at higher temperatures. The oxygen deficiency (δ) in Cu2V2O7‑δ and the average oxidation number of Cu were calculated from the equilibrated weight loss at each temperature (Table 1).

O1 bulk (100) surface

Table 1. Oxygen Deficiency and Cu Oxidation Number Determined from Figure 5 500 °C 600 °C 650 °C

δ in Cu2V2O7−δ

Cu oxidation number

0.025 0.121 0.286

1.974 1.879 1.714

442 −

O2 364 −

O3 a

211 296

O4 355 216

a

Atomic arrangement around the oxygen vacancy was not maintained after optimization.

formation energies of the vacancy on O2 and O4 were 364 and 355 kJ mol−1, respectively, whereas on O1, it was 442 kJ mol−1. Therefore, the vacancies on the oxygen sites bridging V and Cu were energetically preferable, compared with the oxygen site bridging two V atoms. The vacancy on the O3 site should be neglected even though it has the minimum ΔEv value (211 kJ mol−1), because the original atomic configuration surrounding the oxygen vacancy was not preserved after optimization. This means that the oxygen vacancy on the site creates a very unstable crystal structure. Next, the (100) surface was modeled as a slab, periodically repeating in the a direction with a (2 × 2) lateral supercell, the surface of which was terminated by O3 and O4 (Figure 7). The (100) plane was only considered as the surface model because the other low-index planes of α-Cu2V2O7 is not capable of exposing the O4 site without breaking the V−O−V linkage of a pyrovanadate unit (V2O72−). Unlike the bulk structure in Figure 6, the O3 site exposed on the surface was a twofold oxygen, bridging Cu and V. The calculated oxygen vacancy formation

DFT Calculation of Oxygen Vacancy Formation and SO3 Decomposition. Our experimental results show that Cu2V2O7 exists as the oxygen-deficient α-phase under the reaction atmosphere. Because α-Cu2V2O7 has four different oxygen sites (Figure 6), the energies of the hypothetical elimination from each oxygen site in α-Cu2V2O7 were investigated by using DFT calculations. As shown in Table 1,

Figure 7. Slab model of the (100) surface of a 2 × 2 supercell for α(Cu2V2O7)8.

Figure 6. Oxygen sites in bulk α-(Cu2V2O7)8. 26713



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Figure 8. Simple energy diagram for oxygen vacancy formation and subsequent SO3 decomposition over the (100) surface of a 2 × 2 supercell for α(Cu2V2O7)8.

energy for the (100) surface slab was 296 kJ mol−1 for O3 and 216 kJ mol−1 for O4 (Table 2). This suggests again that the twofold bridging O4 on the surface was easier to eliminate than that in the bulk model. Assuming the reaction, Cu16V16O55 + SO3 → Cu16V16O56 + SO2, the reaction energy (ΔEr) was calculated with the following equation:

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CONCLUSION

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ASSOCIATED CONTENT

We have demonstrated that oxygen-deficient α-Cu2V2O7 with a blossite structure is the active phase for catalytic SO 3 decomposition around 600 °C in solar thermochemical water splitting cycles. Spontaneous oxygen desorption accompanied by charge compensation via the reduction of Cu2+ to Cu+ yields oxygen deficiencies corresponding to Cu16V16O55 at 600 °C. The DFT calculation indicates that oxygen vacancy formation is more favorable at Cu−O−V bridging sites than V−O−V sites in the pyrovanadate unit. Furthermore, the oxygen vacancy formation energy of the (100) surface is considerably less than that of bulk Cu16V16O56. The oxygen vacancy creates a downhill pathway for decomposing SO3 via exothermic reaction, Cu16V16O55 + SO3 → Cu16V16O56 + SO2, on the (100) surface.

ΔEr = E(Cu16V16O56 ) + E(SO2 ) − E(Cu16V16O55) − E(SO3)

Here, E(Cu16V16O55) and E(Cu16V16O56), are the total energies of the Cu16V16O56 surface slab with and without oxygen vacancies on O4, respectively. E(SO3) and E(SO2) are the total energy of the ground states of SO3 and SO2 molecules in the gas phase, respectively. The calculated value, ΔEr = −120 kJ mol−1, suggests that the reaction should be exothermic and favorable for converting SO3 to SO2. Based on these calculations, we constructed a simple energy diagram of SO3 decomposition on the (100) surface of α(Cu2V2O7)8 (Figure 8). The first step is the spontaneous desorption of surface oxygen and the creation of an oxygen vacancy at the O4 site bridging Cu and V. This endothermic reaction (ΔEv = 216 kJ mol−1) requires external heat sources of ≥600 °C. The second step is the reaction between the surface oxygen vacancy and SO3 to form SO2, which is exothermic (ΔEr = −120 kJ mol−1). The overall energy, ΔEv + ΔEr = +96 kJ mol −1 , corresponds to the enthalpy change of SO 3 decomposition (SO3 → SO2 + 1/2O2). Thus, the oxygen deficient surface of α-(Cu2V2O7)8 creates a downhill pathway for decomposing SO3, which is a possible reason for the high catalytic activity. These results suggest that redox between Cu2+ and Cu+ accompanied by the oxygen vacancy formed on the surface of Cu 2 V 2 O 7 plays a key role in catalytic SO 3 decomposition.

S Supporting Information *

Cu 2p and V 2p XPS spectra of Cu2V2O7 before and after O2TPD measurement. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel/Fax: +81-96-342-3651. E-mail: [email protected]. jp. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI Grant Number 24246130 and by Energy Carrier Project of JST ALCA (Japan Science and Technology Agency- Advanced Low Carbon Technology Research and Development Program). 26714



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Cycle for Hydrogen Production. Int. J. Hydrogen Energy 2006, 31, 883−890. (20) Ginosar, D. M.; Petkovic, L. M.; Glenn, A. W.; Burch, K. C. Stability of Supported Platinum Sulfuric Acid Decomposition Catalysts for Use in Thermochemical Water Splitting Cycles. Int. J. Hydrogen Energy 2007, 32, 482−488. (21) Machida, M.; Miyazaki, Y.; Matsunaga, Y.; Ikeue, K. Efficient Catalytic Decomposition of Sulfuric Acid with Copper Vanadates as an Oxygen-Generating Reaction for Solar Thermochemical Water Splitting Cycles. Chem. Commun. 2011, 47, 9591−9593. (22) Machida, M.; Kawada, T.; Hebishima, S.; Hinokuma, S.; Takeshima, S. Macroporous Supported Cu-V Oxide as a Promising Substitute of the Pt Catalyst for Sulfuric Acid Decomposition in Solar Thermochemical Hydrogen Production. Chem. Mater. 2012, 24, 557− 561. (23) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758− 1775. (24) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (25) Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-Zone Integrations. Phys. Rev. B 1976, 13, 5188−5192. (26) Dudarev, S. L.; Botton, G. A.; Savrasov, S. Y.; Humphreys, C. J.; Sutton, A. P. Electron-Energy-Loss Spectra and the Structural Stability of Nickel Oxide: An Lsda+U Study. Phys. Rev. B 1998, 57, 1505−1509. (27) Anisimov, V. I.; Zaanen, J.; Andersen, O. K. Band Theory and Mott Insulators: Hubbard U Instead of Stoner I. Phys. Rev. B 1991, 44, 943−954. (28) Mercurio-Lavaud, D.; Frit, B. Structure Cristalline De La Variété Basse Température Du Pyrovanadate De Cuivre: Cu2V2O7. Acta Crystallogr., Sect. B 1973, 29, 2737−2741. (29) Calvo, C.; Faggiani, R. Alpha Cupric Divanadate. Acta Crystallogr., Sect. B 1975, 31, 603−605. (30) Clark, G. M.; Garlick, R. Formation and Properties of Copper(Ii) Divanadate(V). J. Inorg. Nucl. Chem. 1978, 40, 1347−1349. (31) Slobodin, B.; Samigullina, R. Thermoanalytical Study of the Polymorphism and Melting Behavior of Cu2V2O7. Inorg. Mater. 2010, 46, 196−200.

REFERENCES

(1) Brutti, S.; De Maria, G.; Cerri, G.; Giovannelli, A.; Brunetti, B.; Cafarelli, P.; Barbarossa, V.; Ceroli, A. Decomposition of H2SO4 by Direct Solar Radiation. Ind. Eng. Chem. Res. 2007, 46, 6393−6400. (2) Karagiannakis, G.; Agrafiotis, C. C.; Zygogianni, A.; Pagkoura, C.; Konstandopoulos, A. G. Hydrogen Production Via Sulfur-Based Thermochemical Cycles: Part 1: Synthesis and Evaluation of Metal Oxide-Based Candidate Catalyst Powders for the Sulfuric Scid Decomposition Step. Int. J. Hydrogen Energy 2011, 36, 2831−2844. (3) Meier, A.; Ganz, J.; Steinfeld, A. Modeling of a Novel HighTemperature Solar Chemical Reactor. Chem. Eng. Sci. 1996, 51, 3181− 3186. (4) Funk, J. E. Thermochemical Hydrogen Production: Past and Present. Int. J. Hydrogen Energy 2001, 26, 185−190. (5) Schwartz, D.; Gadiou, R.; Brilhac, J. F.; Prado, G.; Martinez, G. A Kinetic Study of the Decomposition of Spent Sulfuric Acids at High Temperature. Ind. Eng. Chem. Res. 2000, 39, 2183−2189. (6) Yilmaz, A.; Hindiyarti, L.; Jensen, A. D.; Glarborg, P. Thermal Dissociation of SO3 at 1000−1400 K. J. Phys. Chem. A 2006, 110, 6654−6659. (7) O’Keefe, D. R.; Norman, J. H.; Williamson, D. G. Catalysis Research in Thermochemical Water-Splitting Processes. Catal. Rev. Sci. Eng. 1980, 22, 325−369. (8) O’Keefe, D.; Allen, C.; Besenbruch, G.; Brown, L.; Norman, J.; Sharp, R.; McCorkle, K. Preliminary Results from Bench-Scale Testing of a Sulfur-Iodine Thermochemical Water-Splitting Cycle. Int. J. Hydrogen Energy 1982, 7, 381−392. (9) Dokiya, M.; Kameyama, T.; Fukuda, K.; Kotera, Y. Thermochemical Hydrogen Preparation-Part V. A Feasibility Study of the Sulfur Iodine Cycle. Bull. Chem. Soc. Jpn. 1977, 50, 2657−2660. (10) Onuki, K.; Kubo, S.; Terada, A.; Sakaba, N.; Hino, R. Thermochemical Water-Splitting Cycle Using Iodine and Sulfur. Energy Environ. Sci. 2009, 2, 491−497. (11) Tagawa, H.; Endo, T. Catalytic Decomposition of Sulfuric Acid Using Metal Oxides as the Oxygen Generating Reaction in Thermochemical Water Splitting Process. Int. J. Hydrogen Energy 1989, 14, 11−17. (12) Kim, T. H.; Gong, G. T.; Lee, B. G.; Lee, K. Y.; Jeon, H. Y.; Shin, C. H.; Kim, H.; Jung, K. D. Catalytic Decomposition of Sulfur Trioxide on the Binary Metal Oxide Catalysts of Fe/Al and Fe/Ti. Appl. Catal. A: Gen. 2006, 305, 39−45. (13) Ishikawa, H.; Ishii, E.; Uehara, I.; Nakane, M. Catalyzed Thermal Decompositon of H2SO4 and Production of HBr by the Reaction of SO2 with Br2 and H2O. Int. J. Hydrogen Energy 1982, 7, 237−246. (14) Rashkeev, S. N.; Ginosar, D. M.; Petkovic, L. M.; Farrell, H. H. Catalytic Activity of Supported Metal Particles for Sulfuric Acid Decomposition Reaction. Catal. Today 2009, 139, 291−298. (15) Ginosar, D. M.; Rollins, H. W.; Petkovic, L. M.; Burch, K. C.; Rush, M. J. High-Temperature Sulfuric Acid Decomposition over Complex Metal Oxide Catalysts. Int. J. Hydrogen Energy 2009, 34, 4065−4073. (16) Banerjee, A. M.; Pai, M. R.; Meena, S. S.; Tripathi, A. K.; Bharadwaj, S. R. Catalytic Activities of Cobalt, Nickel and Copper Ferrospinels for Sulfuric Acid Decomposition: The High Temperature Step in the Sulfur Based Thermochemical Water Splitting Cycles. Int. J. Hydrogen Energy 2011, 36, 4768−4780. (17) Abimanyu, H.; Jung, K. D.; Jun, K. W.; Kim, J.; Yoo, K. S. Preparation and Characterization of Fe/Cu/Al2O3-Composite Granules for SO3 Decomposition to Assist Hydrogen Production. Appl. Catal. A: Gen. 2008, 343, 134−141. (18) Banerjee, A. M.; Pai, M. R.; Bhattacharya, K.; Tripathi, A. K.; Kamble, V. S.; Bharadwaj, S. R.; Kulshreshtha, S. K. Catalytic Decomposition of Sulfuric Acid on Mixed Cr/Fe Oxide Samples and Its Application in Sulfur-Iodine Cycle for Hydrogen Production. Int. J. Hydrogen Energy 2008, 33, 319−326. (19) Barbarossa, V.; Brutti, S.; Diamanti, M.; Sau, S.; De Maria, G. Catalytic Thermal Decomposition of Sulphuric Acid in Sulphur-Iodine 26715


Role of Oxygen Vacancies in Catalytic SO3 Decomposition over

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