Density Functional Theory Study on the Mechanism of Calcium Sulfate

In this paper, the mechanism of CaSO4 reductive decomposition by CO was studied in the framework of density functional theory (DFT). In the calculatio...
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Density Functional Theory Study on the Mechanism of Calcium Sulfate Reductive Decomposition by Carbon Monoxide Xuemei Zhang,† Xingfu Song,*,† Ze Sun,† Ping Li,† and Jianguo Yu*,† †

National Engineering Research Center for Integrated Utilization of Salt Lake Resources, East China University of Science and Technology, Shanghai, China S Supporting Information *

ABSTRACT: The reductive decomposition of calcium sulfate (CaSO4) to calcium sulfide (CaS) was one of the most important methods for anhydrite resource utilization. When CaSO4 was decomposed reductively by carbon monoxide (CO), usually there were CaS and/or calcium oxide (CaO) in the decomposition products of CaSO4 depending on the reaction temperature and reactant concentrations. In this paper, the mechanism of CaSO4 reductive decomposition by CO was studied in the framework of density functional theory (DFT). In the calculation, the exchange-correlation term was approximated by Perdew−Wang (PW91), a functional within the generalized gradient approximation (GGA) family. To study the interaction of CO and CaSO4, the transition states of CaSO4 decomposition and the minimum energy path (MEP) were analyzed. The results showed that the CaS product could be obtained when CaSO4 was reduced by CO with the 4:1 stoichiometric ratio of CO and CaSO4, and the decomposition of CaSO4 to CaSO3 was the rate-determining step, and activation energy in this step was 191.19 kJ/mol. With the increase of the reaction temperature, the CaO product could be obtained with a 1:1 stoichiometric ratio of CO and CaSO4, and the activation energy is 318.28 kJ/mol during the process. It was found that the CaS product was formatted at a lower reaction temperature and a higher mole ratio of CO and CaSO4, and the CaO product was preferred at a higher reaction temperature and a lower mole ratio of CO and CaSO4.

1. INTRODUCTION Calcium sulfate (CaSO4), being made from the natural sulfates and waste byproducts in chemical and fertilizer industries, was a large potential source of sulfur. Since CaSO4 was very stable, the direct decomposition temperature of CaSO4 was very high. Hence, it was significant to develop new ways of CaSO4 efficient utilization. In fact, various processes had been developed. One of the economical utilizations was the preparation of cement1 and sulfuric acid2 by converting CaSO4 into calcium oxide (CaO) and a form of sulfur, either elemental sulfur (S)3 or sulfur dioxide (SO2). For example, CaSO4 could be reduced by carbon monoxide (CO) to produce CaO and SO2 or calcium sulfide (CaS)4 which can be oxidized by air to produce CaO and SO2. Then SO2 was reduced to produce sulfur.5,6 CaSO4 was also used as a new kind of oxygen carrier in chemical-looping combustion (CLC) due to its relatively higher oxygen capacity and being environmentally friendly.7 In this process, CaSO4 should be reduced to CaS and then oxidated back to CaSO4 by air. Both of the utilizations referred to the reductive decomposition of CaSO4 by reducing gas. The main interaction of CO and CaSO4 was described by two equations8,9 CaSO4 + 4CO → CaS + 4CO2 (1) CaSO4 + CO → CaO + SO2 + CO2

most of the investigations have been carried out in bench-scale or larger reactors by experimental methods. Some reaction mechanisms were proposed for the reductive decomposition of CaSO4. A plausible reaction pathway was postulated by Robbins based on the experimental results.10 It was considered that an unstable intermediate was produced in the initial stage. SO2 and carbon dioxide (CO2) were adsorbed on CaO CaSO4 + CO → CaO·SO2 ·CO2

With desorption of CO2 and SO2, CaO could be obtained through the following reactions CaO· SO2 · CO2 → CaO· SO2 + CO2

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CaO· SO2 → CaO + SO2

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When CaO·SO2 continued to react with CO, CaS could be generated CaO·SO2 + 2CO → CaO·S + 2CO2

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CaO·S + CO → CaS + CO2

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X-ray techniques were used by Hayhurst to study the reaction of CO and CaSO4 at temperatures between 1073 and 1373 K, and calcium sulfite (CaSO3) was detected, but its concentration was less than 1%.11 Schwitzegebel et al.12 calculated the thermodynamics of Ca−SO2−O2 system and

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The reduction of CaSO4 involved the oxidation state transformation of sulfur from S (+VI) in CaSO4 to S (-II) in CaS or to S (+IV) in SO2. These processes involved a series of stepwise reduction reactions coupled to the oxidation of CO, and the reaction mechanism was more complex. Up to now, © 2012 American Chemical Society

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Received: Revised: Accepted: Published: 6563

September 26, 2011 April 7, 2012 April 12, 2012 April 12, 2012 dx.doi.org/10.1021/ie202203b | Ind. Eng. Chem. Res. 2012, 51, 6563−6570

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Table 1. Calculated Parameters of CaSO4, Together with Data Available from the Literature crystal structure

computational data

experimental data

crystal structure

computational data

experimental data

S−O(1) S−O(2) O(1)−O(1′) O(1)−O(2) O(2)−O(2′) O(1)−S−O(1′)

1.506 1.506 2.389 2.487 2.416 106.7

1.475 1.474 2.360 2.427 2.377 107.4

O(1)−S−O(2) O(2)−S−O(2′) Ca−O(1′) Ca−O(1) Ca−O(2) Ca−O(2′)

111.3 105.0 2.519 2.463 2.320 2.513

110.8 106.4 2.561 2.459 2.341 2.510

employed for all calculations to speed up the SCF convergence during optimization. Zero point energies were not included in the calculations for the energies. Surface energies of CaSO4 are calculated, and CaSO4 (010) surface with the lowest energy is adopted as reaction surface in the calculation. A vacuum region of 15 Å is added above the surface to ensure negligible interaction between periodic images normal to the surface. In order to ensure the computational accuracy, all the molecules are allowed to relax freely. Linear/quadratic synchronous transit (LST/QST) algorithm is used to search transition state (TS) structures. First, the LST maximization is performed by an energy minimization in directions conjugate to the reaction pathway. Second, the approximated TS are used to perform QST maximization, with conjugate gradient minimization. The cycle is repeated until a stationary point is located. The nudged elastic band (NEB) method is used to map the reaction pathways and optimize the minimum energy path (MEP) between different minima. Then the calculations of the activation energies for decomposition processes of CaSO4 are done. All optimized structures are considered to be stable with the absence of imaginary frequencies but only one imaginary frequency for TS structures. The activation energy and reaction energy are defined as

thought that CaO would be obtained with the decomposition of CaSO3 when the temperature was about 1273 K CaSO3 → CaO + SO2

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Now, these plausible reasonings have not been verified definitely yet. Namely, the insight into the elementary processes involved in the reductive decomposition of CaSO4 was still not clear. The compositions of the solid and gaseous products depended on various factors. Tian et al.13 investigated the reductive decomposition of CaSO4 using Thermogravimetric Analysis, and the results demonstrated that the mole fraction of product was greatly dependent on the partial pressure of CO in the reduction reaction of CaSO4. Song et al.14 studied the reduction reaction of CaSO4 in a fixed bed reactor. The results showed that the composition of product was affected by temperature, gas flow rate, and sample mass. The reaction of gypsum with carbon was investigated by Van der Merwe et al.15 The results indicated that the reductive decomposition of CaSO4 was a complex reaction, and it was difficult to control the process of the reaction. Therefore, the reaction mechanism was too complex to be modeled entirely by experimental methods. To obtain pure CaS or CaO, theoretical calculation was undertaken to shed light on the decomposition mechanism of CaSO4 on molecular scale. These calculations were carried out using high-level density functional theory (DFT). DFT calculations often lead to a correct estimation of the relative stability of different surface and surface reaction.16 The higher accuracy achievable and anticipated could be achieved by DFT, especially with modern functionals.17 A clear picture of the decomposition of CaSO4 was provided via theoretical studies, and the activation energies and reaction energies of important steps were determined in this study. According to the analysis of the reaction pathway, it was suggested that the product composition was controlled by reaction temperature and reactant concentration.

Ea = ETS − ER E = EP − ER

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where ER, ETS, and EP are the energies of reactants, TSs, and products, respectively. E is the reaction energy of the reaction.

3. RESULTS AND DISCUSSION Selection of Functional for Calculation. Different functionals are applicable for different systems. The reaction of SO3 and CO to form SO2 and CO2 is similar to the reactions studied in this paper, so the activation energy of this reaction is calculated by PW91, BLYP, PBE, and RPBE functionals. The reaction of SO3 and CO can be described by the following equation

2. CALCULATIONS Unless otherwise stated, the DFT calculations are carried out with nonlocal generalized gradient approximation (GGA)18 functional by means of Perdew−Wang (PW91)19 correlation functional in the Dmol3 module of Materials Studio 5.0. Numeric atomic functions are used in Dmol3, which can support exactly solutions to the Kohn−Sham equations for the atoms. Three precisions are provided in the Dmol3 module: coarse, medium, and fine, respectively. Accurate results cannot be obtained by coarse and medium. So ‘fine’ is adopted for all the calculations in the paper. Double numerical plus polarization (DNP) function is utilized in all calculations. The threshold values of convergence criteria for energy, gradient, and displacement convergence are 1 × 10−6 Ha, 0.002 Ha/Å, and 0.005 Å, respectively. For self-consistent-field (SCF) density convergence, the threshold value of is 1 × 10−6 Ha. A small electron thermal smearing value of 0.005 Ha is

SO3 + CO → SO2 + CO2

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The activation energies calculated by PW91, BLYP, PBE, and RPBE functionals are 114.52 kJ/mol, 127.30 kJ/mol, 115.61 kJ/ mol, and 132.42 kJ/mol respectively, and the experimental result for this reaction20 is 108.09 kJ/mol. It is obvious that the result calculated by the PW91 functional is closest to the experimental value. The reaction energies calculated by BLYP, PW91, PBE, and RPBE are −239.35 kJ/mol, −237.69 kJ/mol, −236.89 kJ/mol, and −234.21 kJ/mol, respectively, while the experimental result is −184 kJ/mol. The results show that the reaction energies are underestimated by about 50 kJ/mol. In this paper, all the calculations are done by the PW91 functional. Crystal Parameters and Surface Energies of CaSO4. To test the adequacy of the selected functional further, a series of 6564

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Figure 1. Total electron density and HOMO of CaSO4: (a) CO(HOMO), (b) CO(LUMO), (c) electron density (CaSO4), (d) CaSO4 (HOMO), and (e) CaSO4 (LUMO).

calculations are performed on structures of CaSO4 crystal, CO, and CO2 molecules using the GGA/PW91 functional. The crystal parameters of CaSO4 are obtained and compared with the experimental data21 (see Table 1). The parameter setting is considered acceptable as the error is less than 5%. The optimization of the isolated CO and CO2 molecule is performed in a large cubic box of 10 × 10 × 10 Å. For CO, the equilibrium bond length is 1.140 Å which differs by only 1.1% from the experimental value of 1.128 Å.22 Good agreement with experiment is obtained for the vibrational C−O stretching frequency, the calculated value is 2131 cm−1, and the experimental value is 2143 cm−1. The binding energy for the CO is 11.9 eV, and the experimental value is 11.2 eV. For CO2, the calculated bond length is 1.175 Å, and the experimental data are 1.162 Å.23 The calculated values of bending frequency, symmetric stretching frequency, and asymmetric stretching frequency of CO2 are 641 cm−1, 1332 cm−1, and 2370 cm−1, and the experimental values are 667 cm−1, 1333 cm−1, and 2349 cm−1, respectively. The good agreement of calculated results and the experimental data builds our confidence to proceed with the main focus of this study. The reductive decomposition of CaSO4 in gas can be described by the unreacted-core model.24−26 Hence, crystal surface considered as reaction surface should be selected primarily. Experimental results show that the order of relative stabilities of the CaSO4 crystal faces is (010) > (100) > (001),27 and the cleavage properties of these three low index surfaces are as follows: (010) perfect, (100) nearly perfect, and (001) good to imperfect.21 In addition, (010) and (100) surfaces are considered to be stable, because only one bond is lost per sulfate ion at (010) and (100) surfaces while two bonds are lost with each sulfate ion at the (001) surface.28 The surface energies also show that the three low index faces values are much smaller than those calculated for other faces.29

To compare the stability of various surfaces, the surface energies should be taken into account. For CaSO4, Esurf is calculated as Esurf = [Eslab − Ebulk ]/2A

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Eslab refers to the total energy of the slab, Ebulk is the energy for bulk CaSO4 that has the same number of atoms with the slab, and A is the surface area. With the same algorithm as used for all optimizations, the energy values of (010), (100), and (001) surfaces were calculated. The results are 0.240 J/m−2, 0.304 J/m−2, and 0.340 J/m−2, respectively, which means the stability sequence of these surfaces is (010), (100), and (001). Namely, the (010) surface is more stable and more easily obtained when the CaSO4 crystal is cleaved or in nature. The surface area of the (010) surface is far larger than other surfaces of the CaSO4 crystal, and the collision probability of gas molecules with the (010) surface is also much higher, so the reactions occurring on the most stable surface are more representative. Therefore, all calculations are performed on the (010) surface. Dissociation of CaSO4. In order to determine the initial site of CO on the (010) surface, the electron density of CaSO4 and frontier orbital (including the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO)) of CO and CaSO4 were calculated. The results show that sulfate anion has a tetrahedral molecular symmetry, and the four S−O bands have exactly the similar bond length. From the electric view, SO42‑ possesses zero dipole moment. Both the total electron density and HOMO are nearly spherically symmetrical, as shown in Figure 1. Though S has a high oxidation state (+6), the high symmetry and zero dipole moment make SO42‑ electronically unreactive and difficult to activate. In other words, the high oxidation state of S does not reflect on SO42‑. This indicates that the highly symmetrical sulfate anion is very stable with respect to S−O bond breaking, and certain perturbation of its electronic 6565

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distribution is required for its activation.30 The high transition barrier for the reduction of free sulfate (SO42‑) may explain why the direct uncatalyzed reduction of sulfate has never been observed below 1073 K.31 The HOMO and LUMO of CO are mainly reflected in the C atom, and the HOMO and LUMO of CaSO4 are reflected in SO42‑ and Ca, respectively. The gap of HOMO (CO)-LUMO (CaSO4) is 5.37 eV, and the gap of HOMO (CaSO4)-LUMO (CO) is 3.46 eV. It is more likely that CaSO4 (HOMO) donates electrons to CO (LUMO) rather than CO (HOMO) to CaSO4 (LUMO). Various reaction sites are investigated for CO on CaSO4 (010). In order to form a new bond, a bonding distance of 1.300 Å is adopted for the initial state of CO and CaSO4 when the sum of covalent radius of C and O is 1.50 Å. Different sites of CO and CaSO4 are tested, and it is found that the bonding of C−O is relatively more likely occurring on top site. CaSO3 is obtained at the first step of CaSO4 dissociation CaSO4 + CO → CaSO3 + CO2

Figure 3. LDOS projected on the C atom in the reactant and TS.

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The TS structure with one imaginary frequency of −831.56 cm−1 is determined by the LST/QST method, in which the S− O bond is stretched to 1.881 Å as compared to 1.482 Å in CaSO4, and the C atom has formed a bond with the O atom that is 1.563 Å long. The reaction coordinate, associated with an imaginary frequency of TS, is almost purely a C−O stretch. After the determination of TS, the NEB method is used to calculate MEP and the states of reactants and products are found, as shown in Figure 2. The S−O bond of CaSO4 has

p state. Besides, the peak area of 5σ in TS is smaller than it in reactant, which means the electrons are transferred out from 5σ orbital.32 The formation of the peaks above the Fermi level is due to the contribution of CO 2π* (LUMO) and O p state, and it is obvious that the 2π* orbital is much more delocalized. Decomposition of CaSO4 is described as redox of CO to the S−O bond. This means that the CO molecule provides electronic states near the Fermi level instead of molecule states in the process of reaction. The electrons transfer from the 5σ orbital (CO) to two O p bands and an S s band. The negative charge of O (CaSO4) is increased from −0.485 to −0.599, and for O (CO) it increases from −0.109 to −0.190 (see Table 2). The positive charge of S decreased from 0.930 to 0.856. With the forming of the new chemical bond between C and O, the C−O bond of CO and the S−O bond are activated and stretched. To compensate the charge transfer, partial electrons are reversed back to C 2π* from O p. In other words, the orbital mixing between the CO 2π* and O can be facilitated by the extending of 5σ.33 To assess the role of the CO in breaking the S−O bond, the relevant quantity is the LDOS weighted by the overlap population of C and O atoms. The overlap between the O p and C p orbital illustrates the formation of C−O bond, including bonding and antibonding. Electrons in bonding orbital are supplied by C, and electrons of S and O are increased. Electrons in antibonding are reversed back by O, and electrons of O are decreased. As a result, the electron difference of S and O grows gradually, and this lowers the energy of the TS and finally leads to the dissociation of S− O bond. Finally, the S−O bond is broken, and the charges of S and O are 0.555 and −0.345. The C−O bond of CO is stretched from 1.140 Å to 1.178 Å. Formation of CaS. The interaction between CO with O and S shows the same tendency in the process of S−O bond breaking during the removal of O from CaSO4, but the values have a small difference among them. Since the electrons will transfer from CO to an O atom in the initial stage of reaction, the bonding of C and O is more likely to occur on the site with low charge density. The calculated results show that the speculation is correct, and this makes it easier for the next calculation

Figure 2. Decomposition process of CaSO4 to CaSO3 and structures of reactant, TS, and product.

been elongated to 3.572 Å in products. It is considered that the S−O bond has broken totally, because the sum of ionic radius of S and O is no more than 3.24 Å. In products, C−O bond lengths are 1.178 Å and 1.168 Å, respectively, and these are consistent with the calculated value of CO2 as 1.175 Å. In addition, the angle of O−C−O (178.6°) is in good agreement with experimental value (180°). Therefore, CO2 and CaSO3 are considered to be the final products. From the calculations presented here, the reaction energy of CaSO4 dissociation to CaSO3 is −103.56 kJ/mol, and the activation energy is 191.19 kJ/mol. Interaction between CO and O. The electronic structure of CO is [(1σ)2(2σ)2(3σ)2(4σ)2(1π)4(5σ)2(2π)*(6σ)*]. In order to gain insight into the interaction between CO and CaSO4 surface, the local density of states (LDOS) are calculated. The HOMO and LUMO contributions to the LDOS are shown in Figure 3. It is noticed that 5σ (HOMO) is pushed up in energy, and this is due to the mixing of 5σ and O

CaSO3 + CO → CaSO2 + CO2 6566

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Table 2. Milliken Population of Relevant Atoms Involved in the Interaction of CO and CaSO4 reactant S O C CO−O

transition state

product

charge

s

p

d

charge

s

p

d

charge

s

p

d

0.930 −0.485 0.110 −0.109

0.925 1.849 1.848 1.760

2.393 4.601 1.963 4.306

1.752 0.035 0.079 0.043

0.856 −0.599 0.287 −0.190

1.277 1.887 1.464 1.781

2.511 4.684 2.144 4.366

1.357 0.028 0.105 0.043

0.555 −0.345 0.606 −0.249

1.586 1.783 0.941 1.791

2.818 4.517 2.290 4.409

1.040 0.044 0.163 0.049

dissociation is 26.13 kJ/mol, and the activation energy is 178.14 kJ/mol CaSO2 + CO → CaSO + CO2

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Calculation results show that the C atom tends to bond with the O atom which exposes out of the surface of CaSO2. The TS structure of this reaction has one imaginary frequency of −620.00 cm−1. The S−O bond of TS is stretched from 1.645 Å to 1.903 Å, and the C atom has formed a bond of 1.530 Å with the O atom. The C−O bond of CO is elongated to 1.185 Å. MEP is calculated, and the structures of reactant and product are obtained, as shown in Figure 6. The S−O bond of CaSO2 has been elongated to 4.321 Å in products. It is considered that the S−O bond is disconnected completely, the charge of S declines to −0.598 from 0.061, and the oxidation state of S has changed to 0. Bond lengths of CO2 are 1.168 Å and 1.178 Å, and the angle of O−C−O is 178.7°. The reaction energy of this step is −141.26 kJ/mol, and the activation energy is 90.76 kJ/mol

Figure 4. LDOS projected on C and O atoms in TS.

Top site and bridge site of CaSO3 are both investigated. Due to the symmetry of two O atoms, steric hindrance effect can be ignored. It is noted that C and O prefer to bond on the bridge site where the charge density is lower than that on the top site. The TS structure of this reaction is obtained and shown in Figure 5. It has one imaginary frequency by −570.16 cm−1 which is a pure C−O stretch. The S−O bond is stretched from 1.572 Å to 1.998 Å, and the C−O bond of CO is elongated to 1.190 Å with the formation of a new C−O bond by 1.461 Å. The S−O bond of CaSO3 has been elongated to 4.608 Å in products. It is considered that there is no interaction between S and O, and the charge of S declines from 0.560 to 0.052. Bond lengths of CO2 are 1.167 Å and 1.181 Å, respectively. The bond angle of O−C−O is 175.9°. The reaction energy of CaSO3

CaSO + CO → CaS + CO2

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CaS is generated after the last step of CaSO4 decomposition, and it is found that the stoichiometric ratio of CO and CaSO4 is 4:1. The TS structure has one imaginary frequency by −435.89 cm−1 which is a pure C−O stretch. The S−O bond of TS is stretched from 1.786 Å to 1.807 Å. The bond length of the new C−O bond is 1.643 Å, and the old one is stretched to 1.202 Å. MEP is calculated, and the structures of reactant and product are obtained, as shown in Figure 7. The S−O bond of CaSO has been elongated to 4.594 Å in products. It is considered that the S−O bond has broken

Figure 5. Decomposition process of CaSO3 and structures of reactant, TS, and product. 6567

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Figure 6. Decomposition process of CaSO2 and structures of reactant, TS, and product.

Figure 7. Decomposition process of CaSO and structures of reactant, TS, and product.

totally, and the charge of S declines from −0.572 to −1.422. C−O bond lengths of CO2 are 1.167 Å and 1.180 Å, and the angle of O−C−O is 176.9°. The reaction energy is −172.28 kJ/ mol, and the activation energy is 59.54 kJ/mol. From the kinetic point of view, the step with the highest activation energy has been defined the slowest reaction step. Hence, the dissociation of CaSO4 to CaSO3 is considered to be the rate-limiting step for CaSO4 reductive decomposition. The activation energy of this step is 191.19 kJ/mol, and the experimental result of CaSO4 reductive decomposition by CO is 187 kJ/mol.34 Formation of CaO. CaO is produced with the dissociation of CaSO3 through eq 17 CaSO3 → CaO + SO2

The energy change profiles along the pathways of CaS and CaO formation from CaSO4 and CO are shown in Figure 8. It is obvious that the activation energy of CaO formation is higher than that of CaS. The global reaction energy of CaSO4 reducing by CO to obtain CaS is −390.97 kJ/mol when the experimental result is −174 kJ/mol.13 This is because that the error caused by the PW91 functional is added up in the calculation of the overall reaction.

5. CONCLUSION DFT calculations have been carried out to investigate the mechanism of reductive decomposition of CaSO4 by CO. The activation energy of SO3 reacting with CO, bond length and bonding energy of CO and CO2 molecules, crystal parameters and surface energies of CaSO4, and the interaction between CO and O(CaSO4) have been calculated and discussed. It is clear that the PW91 functional can give reliable results for activation energies, while the error on reaction energies is significantly larger. In the interaction of CaSO4 and CO, electrons transform from 5σ (HOMO) to an O p state and an S s state primarily through bonding orbital. To compensate the charge transfer,

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The calculated results show that the energy is gradually increased in the decomposition process of CaSO3 to CaO and SO2 without the existence of TS. The bond length and bond angle of the obtained SO2 is 1.497 Å and 117.59°, while the experimental values are 1.431 Å and 119.5°.22 The activation energy of this step is 318.28 kJ/mol, and the experimental results are 242 kJ/mol,35 246 kJ/mol,36 or in the range of 290.84−317.34 kJ/mol in pure CO.37 6568

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Figure 8. Energy profiles for proposed mechanism of CaSO4 decomposition: CaSO4 → CaSO3 → CaSO2→ CaSO → CaS and CaSO4 → CaSO3 → CaO.

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partial electrons are reversed back to C 2π*(LUMO) from O p through the antibonding orbital. The C−O (CO) bond and the S−O bond are activated and stretched with the electron transfer, which leads to the dissociation of the S−O bond finally. The most energetically favorable TS structures and activation energies of MEPs have been identified. The CaS product could be obtained first with the reductive decomposition of CaSO4 by CO. The dissociation of CaSO4 to CaSO3 is the rate determining step which has the activation energy of 191.19 kJ/mol, and the stoichiometric ratio of CO and CaSO4 is 4:1. The CaO product could be obtained when CaSO3 decomposes directly. In the process, the activation energy is 318.28 kJ/mol which is higher than the formation of CaS, and the mole ratio of CO and CaSO4 is 1:1. It is evident that CaO will be obtained with the increase of reaction temperature when CO is used up. Therefore, it is reasonable to expect that product selectivity can be increased by controlling reaction temperature and reactant concentration. CaS formation is favored at a lower reaction temperature and a higher ratio of CO and CaSO4. CaO is preferred at a higher reaction temperature and a lower ratio of CO and CaSO4. This has been corroborated.2,35

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This work was supported by National High Technology Research and Development Program of China (2011AA06A107), Program for New Century Excellent Talents in University (NCET-08-0776).

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NOMENCLATURE Ea activation energy, kJ/mol ETS energy of transition state structure, kJ/mol ER energy of reactant structure, kJ/mol E reaction energy, kJ/mol EP energy of product structure, kJ/mol Eslab total energy of slab, J/m−2 Esurf energy of surface, J/m−2 Ebulk energy for bulk CaSO4 that has the same number of atoms with slab, J/m−2 A surface area, m−2 REFERENCES

(1) Mihara, N.; Kuchar, D.; Kojima, Y.; Matsuda, H. Reductive Decomposition of Waste Gypsum with SiO2, Al2O3, and Fe2O3 Additives. J. Mater. Cycles Waste Manage 2007, 9, 21. (2) Oh, J. S.; Wheelock, T. D. Reductive Decomposition of Calcium Sulfate with Carbon Monoxide: Reaction Mechanism. Ind. Eng. Chem. Res. 1990, 29, 544. (3) Kim, B. S.; Sohn, H. Y. A Novel Cyclic Reaction System Involving CaS and CaSO4 for Converting Sulfur Dioxide to Elemental Sulfur without Generating Secondary Pollutants. 3. Kinetics of the Hydrogen Reduction of the Calcium Sulfate Powder to Calcium Sulfide. Ind. Eng. Chem. Res. 2002, 41, 3092. (4) Li, H. J.; Zhuang, Y. H. Catalytic Reduction of Calcium Sulfate to Calcium Sulfide by Carbon Monoxide. Ind. Eng. Chem. Res. 1999, 38, 3333. (5) Paisley, M. A. Method for the Conversion of Gypsum to Elemental Sulfur. US Patent 6,024,932, 2000. (6) Sohn, H. Y.; Kim, B. S. A New Process for Converting SO2 to Sulfur without Generating Secondary Pollutants through Reactions Involving CaS and CaSO4. Environ. Sci. Technol. 2002, 36, 3020.

ASSOCIATED CONTENT

S Supporting Information *

The atomic Cartesian coordinates for CaSO4 (bulk), CaSO4 (100), CaSO4 (001), CaSO4 (010), TS (CaSO4 + CO → CaSO3 + CO2), CaSO3 (010), TS (CaSO3 + CO → CaSO2 + CO2), CaSO2 (010), TS (CaSO2 + CO → CaSO + CO2), CaSO (010), TS (CaSO + CO → CaS + CO2), CaS (010), CaO (010). This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

AUTHOR INFORMATION

Corresponding Author

*Phone: 86-21-64252170. Fax: 86-21-64252826. E-mail: [email protected] (X.S.), [email protected] (J.Y.). Notes

The authors declare no competing financial interest. 6569

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dx.doi.org/10.1021/ie202203b | Ind. Eng. Chem. Res. 2012, 51, 6563−6570