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Oxidation of Sulfur Dioxide over VO/TiO Catalyst with Low Vanadium Loading: A Theoretical Study Xuesen Du, Jingyu Xue, Xiangmin Wang, Yanrong Chen, Jingyu Ran, and Li Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00296 • Publication Date (Web): 01 Feb 2018 Downloaded from http://pubs.acs.org on February 2, 2018

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The Journal of Physical Chemistry

Oxidation of Sulfur Dioxide over V2O5/TiO2 Catalyst with Low Vanadium Loading: A Theoretical Study Authors: Xuesen Du a, b, *, Jingyu Xue a, b, Xiangmin Wang a, b, Yanrong Chen a, b, Jingyu Ran a, b, Li Zhang a, b Affiliations:

a

Key Laboratory of Low-grade Energy Utilization Technologies &

Systems of Ministry of Education of China, College of Power Engineering, Chongqing University, Chongqing 400044, China b

Institute of Energy and Environment of Chongqing University, Chongqing, 400044,

China Xuesen Du * (corresponding author), Address: Key Laboratory of Low-grade Energy Utilization Technologies & Systems of Ministry of Education of China, College of Power Engineering, Chongqing University, Chongqing 400044, China. Tel: +86-18183188550 Fax: +86-23-65103101 Email: [email protected]

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Abstract Oxidation of SO2 to SO3 is one of the major drawbacks of the commonly used vanadia based SCR catalyst. DFT calculations were applied to study the interaction between SO2 and the VOx/TiO2 catalyst. Two parallel calculations, one is cluster models implementing with hybrid method and the other is periodical surface model implementing with PBE method, were conducted to compare with each other. The results show that the uncovered TiO2 surface can be easily sulfated by SO2, while it can barely oxidize SO2 to SO3. Supported vanadia site, either vanadia monomer or vanadia dimer, is not a favorable site for SO2 adsorption. However, SO2 can be oxidized by vanadia sites through a sulfation route in which a –V(SO4)- configuration is formed. The terminal oxygen of V=O is found to bond with SO2 to produce SO3. The Brønsted acid site can enhance the SO2 adsorption, while it will be eliminated after interacting with SO2 since the hydrogen ion of Brønsted acid will be deprived by SO2.


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1. Introduction Removal of nitric oxides (NOx) is one of the main issues in air pollution control. Selective catalytic reduction (SCR) with NH3 is regarded as the state-of-the-art technology for the abatement of NOx emitted from vehicles and power plants. The present commercial catalyst for SCR process is V2O5 based catalyst, such as V2O5-WO3/TiO2 and V2O5-MoO3/TiO2 1. During the practical applications, the oxidation of SO2 to SO3 has been found to be a major drawback of the V2O5 based catalysts 2. SO3 is harmful since it can react with water and injected NH3 to produce sulfuric acid and ammonium bisulfate. Sulfuric acid, with strong acidity, will cause severe corrosion to downstream devices. The deposition and accumulation of ammonium bisulfate in air-preheater will result in pressure drop and more severely, clogging. Thus, many studies have been conducted to reduce the oxidation of SO2 on SCR catalysts. Morikawa et al. 3 studied the SO2 oxidation rates over several ternary catalysts V2O5/MxOy/TiO2 (M is Ge, Zn, Mo and W) and found that the addition of Ge can reduce the SO2 oxidation activity of the V2O5/TiO2 catalyst. BaO can also effectively suppress the SO2 oxidation since it lowers the redox ability of V2O5 and the SO2 adsorption capacity on the catalyst surface, as reported by Choo et al.

4

Kobayashi et al. 5 added SiO2 into the V2O5-WO3(MoO3)/TiO2 catalyst and noticeably decrease the SO2 oxidation rate. Though the commercial SCR catalyst has been optimized to be less active for SO2 oxidation based on previous studies, the formation of SO3 on the SCR catalyst is still a major problem for the normal operation of a De-NOx system. Better designing of catalyst with lower SO2 oxidation rate is still necessary, which requires the thorough understanding of the mechanism of SO2 oxidation over vanadia-based catalysts. Several researchers have tried to discover the mechanism of SO2 oxidation over the vanadia based catalysts

2,4-15

. Since the vanadia based catalysts have been the

dominant materials in industrial processes for sulfuric acid production. Lapina et al 11 have reviewed the mechanisms of SO2 oxidation over sulfuric acid production catalysts which contain high amount of V2O5 and M2S2O7 (M=Na, K, Cs). They summarized that dimeric or binuclear vanadia sulfates are formed during SO2 ACS Paragon Plus Environment


oxidation. As we all know, the SCR catalyst contain only small amount of V2O5 (usually lower than 2%) supported on TiO2. The active site of SCR catalysts is essentially different with that of H2SO4 production catalysts. Researchers have also tried to reveal the SO2 oxidation process over SCR catalysts. Kamata et al. 12 searched the active sites for SO2 oxidation and found that V=O and V-OH sites are possibly involved in the adsorption and reaction of SO2. However, Dunn et al. 9,10 declared that bridge Ti-O-V is active for the adsorption of SO2 and subsequent oxidation. Ji et al. 15 have also investigated the relationship between V2O5 coverage and the reactivity of SO2 oxidation. They found that the turnover frequency of SO2 oxidation is almost constant with increasing VOx coverage and V=O was proposed as the key site for SO2 adsorption and oxidation. These results have greatly promoted the study of SO2 oxidation. Nevertheless, only experiments have been employed and the active sites for SO2 oxidation remain in debate. The detailed mechanism has not been reported. In this work, we have employed Density Functional Theory (DFT) calculations to study the mechanism of SO2 over V2O5/TiO2 Catalyst with Low Vanadium Loading. Parallel cluster-based and periodical calculations were conducted to compare with each other. The active sites for SO2 adsorption and oxidation were screened. Mechanisms for interaction of SO2 with the catalyst surface and SO2 oxidation were proposed. 2. Calculation details A catalyst with low loading vanadia supported on anatase TiO2 contains several surface sites including vanadia monomer, vanadia dimer, hydroxy and uncovered anatase (001) surface site. In this study, these different sites were modeled. Employing a proper calculation method is crucial. A calculation with cluster models can employ hybrid methods with higher accuracy, but it suffers from small model size. In small clusters, the calculation results will be influenced by the cut edges. Contrarily, a calculation with periodical boundary does not encounter the problem of model size, but it is significantly expensive to apply hybrid method for this type of calculation. In this study, both calculations with periodical and finite boundary conditions were applied and compared with each other. First, the TiO2 (001) surface was built and optimized ACS Paragon Plus Environment

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using the Materials Studio 8.0 simulation package. The TiO2 anatase crystal structure was optimized using the Castep module at the GGA/PBE level. The cutting energy (400 eV) and k point set (3×3×1) were tested based on energy convergence of the anatase crystal. The parameters of TiO2 anatase were calculated to be a=b=0.380 nm and c=0.966 nm (Fig. S1a), which are identical with those achieved by Arnarson et al

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.

Afterwards, a four layer (001) surface was cut from the optimized (2 ×2) supercell, as shown in Fig. S1b. A vacuum slab of 1.2 nm in the z-direction was applied. The anatase (001) surface was then optimized with only the top layer being relaxed, as shown in Fig. S1b. This model was implemented to conduct the calculations for uncovered TiO2 surface with periodical boundary condition. The VxOy/TiO2 models were constructed with VxOy binding with the TiO2 (001) surface. VO3H and V2O5 were loaded on the TiO2 surface to simulate the vanadia monomers and dimers respectively. The stationary structures are shown in Fig. S2. The stationary structure was validated by the results from literatures. The predicted V=O (1.586 Å) and V-O-V (1.784 Å) bond lengths agree well with the crystal data. 1.576 and 1.778 Å 17 respectively. For the calculations with finite boundaries, clusters were extracted from the above TiO2 (001) surface (Fig. S1b). A two-layer Ti8O24 cluster was extracted to simulate the uncovered TiO2 surface during the cluster-based calculations. The cut bonds were satisfied with hydrogen atoms to maintain charge balance. The outer O-H bond length was fixed at 0.096 nm. The achieved Ti8O24H16 cluster is shown in Fig. S3. For vanadia loaded clusters, only the top layer of TiO2 with a stoichiometry of Ti4O12 was extracted from the above TiO2 surface and VO3H or V2O5 was loaded on this cluster. The cut bonds were satisfied with H atoms and the final formula are VO3HTi4O12H8 and V2O5Ti4O12H8 for vanadia monomer and dimer respectively. Gaussian 03 module

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was used for these cluster-based calculations. All the calculations were performed at the B3LYP/6-31(D,P)

19-21

level. Frequency calculation was performed after each structure

optimization. All the frequencies were scaled by 0.9679 22, 23. The bond lengths of V=O (1.564 Å) and V-O-V (1.773 Å) calculated based on cluster models also agree well with experimental data as shown above. The predicted vibrational frequencies for V=O and –OH are 1091 and 3694 cm-1 respectively, which match well with experimental results ACS Paragon Plus Environment


(1030 24 and 3667 25 cm-1). 3. Results and discussion 3.1. Adsorption of SO2 Adsorption of SO2 on four possible sites of the VOx/TiO2 catalyst was screened in this section. As can be seen from Fig. 1, Cluster-based calculation using hybrid function shows stronger interaction and higher adsorption energy between SO2 and catalyst model than those calculated by PBE method (periodical boundary condition). But these two methods give the same result, i.e. TiO2 > mono-vanadia with Ti-OH > vanadia dimer ≈ mono-vanadia oxo. Both periodical and cluster-based calculations indicate that adsorption of SO2 on the TiO2 (001) surface is extremely favorable. SO2 will parallelly lie on the Ti-O-Ti site with sulfur bonding with titania oxygen and oxygen interacting with titanium atom. In the cluster-based structure, the bond length between S and the surface O is only 1.60 Å, which indicates a close chemical bond has been formed. On the oxo area of mono-vanadia, the adsorption of SO2 is only weakly exothermic. The S atom is weakly bonded to both terminal and bridge oxygens of mono-vanadia site (V-O-Ti). The mono-vanadia site, however, is beneficial for the formation of Brønsted acid 19, as shown in Fig. 1(c). The Ti–OH site will attract SO2 and a close interaction between H atom and sulfuric oxygen is formed. This adsorption process is exothermic by 11.7 kcal/mol based on the cluster-based calculation. The vanadia dimer is also not a favored site for SO2 adsorption. The S atom weakly interact with terminal and bridge oxygens of V-dimer. An exothermic energy of 5.5 kcal/mol is produced from this adsorption process, based on the cluster-based results. These adsorption profiles indicate that loading of vanadia on TiO2 will significantly weaken the adsorption of SO2. This can be attributed to the strengthening of surface acidity by loading vanadia, which is unfavorable for adsorption of acid SO2 on the catalyst surface. The Brønsted acid site brought by loading of vanadia is beneficial for SO2 adsorption.


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Fig. 1. Structure and energy profiles of SO2 adsorption on TiO2 001 surface (a), vanadia monomer (b and c) and vanadia dimer (d), bond lengths are given in Å. 3.2. Reaction of SO2 with anatase (001) surface The reaction of SO2 with the TiO2 (001) surface has been modeled by periodical and cluster-based methods, as shown in Fig. 2. SO2 will first strongly adsorb on Ti-O-Ti structure. The cluster based calculations show that a transition state appears after absorbing an energy of 6.61 kcal/mol. The bridge oxygen of Ti-O-Ti tends to escape from the surface and closely bond with S atom. A symmetric TiO(S=O)OTi structure will be produced and this sulfation process is exothermic by 1.39 kcal/mol (cluster-based energy). The periodical calculation gives the same reaction path as the cluster-based calculation. The energy profile calculated based on periodical models shows that the energy barrier is only 0.21 kcal/mol and the sulfation process is exothermic by 24.21 kcal/mol. Thus, the TiO2 (001) surface can easily be sulfated kinetically and thermodynamically. SO3 is hard to escape from the sulfated TiO2 surface. An incredibly high energy (more than 100 kcal/mol) needs to be consumed for the breaking of SO3 out of the TiO(S=O)OTi structure. These results indicate that SO2 can readily sulfate the TiO2 (001) surface. But the production of SO3 is can hardly happen since the low reducibility of Ti4+ ion.



Fig. 2. Reaction of SO2 with TiO2 (001) surface. All energies in this and the following figures are given in kcal/mol. 3.3. Reaction of SO2 with vanadia monomer The reaction of SO2 with vanadia monomer is discussed in this part. Though the adsorption of SO2 on vanadia oxo sites is only slightly exothermic, the terminal and bridge oxygens of vanadia are possible oxidants for SO2. Fig. 3 shows the structures and energy profiles of the SO2 oxidation by bridge oxygen of V-O-Ti. The extraction of O atom from V-O-Ti by SO2 will consume a large energy. The energy of transition state is close to that of the product containing a reduced vanadia site and a SO3. The energy barrier for SO2 oxidation by bridge V-O-Ti oxygen is 84.72 and 98.64 kcal/mol based on periodical and cluster calculations respectively. The high energy barrier and large energy consumption to produce SO3 indicate the low oxidizing reactivity of bridge V-O-Ti oxygen for SO2.


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Fig. 3. The direct oxidation of SO2 with bridge oxygen of V-O-Ti in vanadia monomer During the modeling of SO2 oxidation by terminal oxygen of V=O, we found the oxidation of SO2 by V=O can process through the sulfation of vanadia site. The periodical surface and cluster based calculations show the same reaction path, as shown in Fig. 4. After adsorbing on -O-V(=O)- site, SO2 will closely interact with both bridge and terminal oxygens and a -V(SO4)- structure is formed. This process is endothermic by 35.63 kcal/mol and the energy barrier is 44.23 kcal/mol, as calculated from the cluster models. The -V(SO4)- structure will further release a SO3 with either a terminal or bridge oxygen detached from the catalyst. The data given in Fig. 4 indicate the extraction of bridge oxygen is harder than that of terminal oxygen during the release of SO3. The energy difference is 22.19 kcal/mol, as calculated by cluster models. Both periodical and cluster-based calculations show that the rate-determining step for SO2 oxidation to SO3 is the sulfation of vanadia monomer. Compared with the direct oxidation route shown in Fig. 3, this sulfation route is much easier to proceed. The energy barrier of sulfation route is 54.41 kcal/mol (cluster-based energy) lower than that of direct oxidation route. Thus, ‘sulfation’ is the preferred route for SO2 oxidation on vanadia monomer.



Fig. 4. The reaction of SO2 with oxo-site of vanadia monomer through sulfation route The Ti-OH adjacent to mono-vanadia site has been proved to be affinitive with SO2. The energy profile shown in Fig. 5 also indicates that the Ti-OH can facilitate the reaction with SO2. The H atom of Ti-OH can easily be captured by SO2 and the exposed O atom will bond with the vanadium atom. Meanwhile, the S atom of SO2 will bond with the bridge O and break the bond between V atom and this bridge O atom, as shown in Fig. 5. A stable intermediate, Ti-OSOOH, is formed. The energy barrier of this process is 13.86 kcal/mol (cluster based). In this Ti-OSOOH structure, the O and Ti atoms are tightly bonded. The break of this Ti-O bond and escape of HSO3 molecule is strongly endothermic by 77.83 and 80.69 kcal/mol based on periodical and cluster-based calculations respectively. This indicates that oxidation of SO2 to HSO3 through this process is difficult. However, the energy profile also reveals that the vandia monomer and the adjacent Ti-OH can readily settle SO2 and cause the sulfation of catalyst surface. It is noticeable that this sulfation process will eliminate the Brønsted Ti-OH acid and form a S-OH site. This is why Kamata et al. 12 found that M-OH was involved in SO2 adsorption.


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Fig. 5. The reaction of SO2 with vanadia monomer and the adjacent -OH site 3.4. Reaction of SO2 with vanadia dimer The reactions of SO2 on vanadia dimer are discussed in this section. The oxidation of SO2 by bridge -V-O-V- oxygen is firstly studied. As shown in Fig. 6, after weakly adsorbing on V-dimer, SO2 can extract bridge oxygen from the -V-O-V- site. But this process needs to climb a remarkably high energy barrier of 100.78 kcal/mol (cluster-based energy). The production of SO3 is endothermic by 88.59 kcal/mol (cluster-based energy). Both kinetic and thermodynamical aspects are unfavorable for this direct oxidation of SO2 by -V-O-V-, which is identical with the reaction profile of SO2 oxidation by -V-O-Ti- (Fig. 3). This result indicates that bridge oxygen atom, either from -V-O-V- or -V-O-Ti- structure, is barely active to oxidize SO2 to produce SO3.



Fig. 6. The reaction of SO2 with bridge V-O-V oxygen of vanadia dimer The vanadia dimer can also oxidize SO2 through the sulfation path, as shown in Fig. 7. After adsorbing on -V(=O)-O-V(=O)- site, SO2 can sulfate the vanadia site after climbing an energy barrier of 49.59 kcal/mol (based on cluster calculations). The S atom of SO2 will bond with a terminal O and a bridge O to form a tetrahedral structure (-V(SO4)-), which is similar as the one formed on vanadia monomer (Fig. 4). It is noticeable that the sulfation of vanadia dimer needs to overcome a higher energy barrier than vanadia monomer. This difference indicates that the sulfation of vanadia dimer is more difficult than that of vanadia monomer. SO3 can also be produced after the sulfation of vanadia dimer. Either terminal or bridge O atom can be captured by SO2 to form SO3. The terminal O atom is easier to detach from the catalyst surface than bridge O, with an energy advantage of 7.4 kcal/mol (cluster-based energy). This is identical with the reaction profile of vanadia monomer. Terminal O atom can easier escape from the catalyst surface with SO2. The release of SO3 from the vanadia dimer with terminal O is endothermic by 33.90 kcal/mol (cluster-based energy), which is lower than the energy needed for releasing SO3 from vanadia monomer (Fig. 4). Both the periodical and cluster-based calculations show that the rate-determining step of SO2 oxidation on vanadia dimer is sulfation, which needs to overcome an energy barrier of 38.85 kcal/mol and 49.59 kcal/mol, as calculated from periodical and cluster models respectively. The sulfation route can reduce the energy barrier by 51.19 kcal/mol, as compared with the direct oxidizing route. The energy barrier for SO2 oxidation on vanadia dimer is about 4-5 kcal/mol higher than that on vanadia monomer through sulfation route. In other words, oxidation of SO2 on vanadia monomer is slightly easier than that on vanadia dimer.


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Fig. 7. The reaction of SO2 with vanadia dimer through sulfation route 3.5. Discussion Our study screened the interactions of SO2 with various sites of the catalyst with low loading vanadia supported on anatase TiO2. The reaction routes are summarized in Fig. 8. The most favorable site for SO2 adsorption is uncovered TiO2 surface. SO2 can easily sulfate the Ti-O-Ti site and a bidentate SO32- is formed on Ti atoms. An incredibly high energy needs to be consumed for the detaching of SO3 from the catalyst surface. Thus, oxidation of SO2 by TiO2 surface to produce SO3 can hardly proceed. Loading of vanadia on the TiO2 support will weaken the adsorption of SO2 since the strong acidity of VxOy sites. However, the vanadia site, either vanadia monomer or dimer, can oxidize SO2 to form gaseous SO3 through the sulfation route. The sulfation of vanadia site accompanies with the oxidation of S ion to higher valence state. As shown in Table 1, the Mulliken charge of S in sulfated TiO2 is only +1.13 e. Settling of SO2 on vanadia site results in oxidation of S to higher valence state (+1.50 and +1.47 on vanadia monomer and dimer respectively). This is due to the higher oxidizing ability of V5+ than Ti4+. V5+=O is a preferred provider of O than V5+-O-M (M = Ti4+ or V5+) during formation of free SO3. The reduced V3+ will be oxidized back to V5+=O by O2. The Brønsted acid site (Ti-OH) adjacent to vanadia oxo, which is produced by vanadia loading, can enhance the adsorption of SO2. Hydrogen ion will be captured by SO2 to form a stable –HSO3, in which S stays at low valence state. The vibration frequencies of fresh and sulfated site of the catalyst are also listed in Table 1. All the sulfated sites



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show at least one S=O stretching band among 1100 to 1300 cm-1, which is corresponding to published experimental results.

Fig. 8. Sulfation and SO2 oxidation on catalyst with low loading vanadia supported on anatase TiO2. V1 and V2 stand for vanadia monomer and dimer respectively. Table 1. Calculated vibration frequencies and charges based on cluster calculations Vibration Frequency (cm-1)

Mulliken Charge (e)

V=O stretch

S=O stretch

O-H stretch

V (Ti)

S

V1

1091

/

3694

+1.40

/

V1-SO4

/

1137, 1312

3743

+1.07

+1.50

V1-OSOOH

1082

1188, 1232

3137

+1.42

+1.18

V2

1081, 1091

/

/

+1.35, +1.37

/

V2-SO4

1062

1098, 1287

/

+1.00, +1.34

+1.47

TiO2

/

/

/

+1.50 (Ti)

/

TiO2-SO3

/

1163

/

+1.49 (Ti)

+1.13

4. Conclusions DFT calculations were performed to study the interaction between SO2 and the


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catalyst with low loading of V2O5 on TiO2. Two series of calculations, one is cluster-based calculations implementing with hybrid method and the other is periodical calculations implementing with PBE method, were conducted. Our computational results imply that these two sets of calculations reveal identical patterns for SO2 adsorption and oxidation. The uncovered TiO2 surface can readily adsorb SO2. A stable -Ti(SO3)Ti- configuration will be generated, in which the S ion is not oxidized by the surface and SO3 can barely break out of the surface. The supported vanadia sites, either vanadia monomer or dimer, can poorly adsorb SO2. However, these vanadia sites can oxidize SO2 to SO3 through a sulfation route. The vanadia site can react with SO2 to form -V(SO4)- in which the terminal V=O oxygen and bridge oxygen of either V-O-V or V-O-Ti are bonded to S ion. SO3 can escape from the -V(SO4)- structure after consuming an energy of 34 - 41 kcal/mol, depending on different vanadia configurations. The Brønsted acid sites, which are generated by the loading of vanadia on TiO2 surface, can enhance the SO2 adsorption on vanadia site. The hydrogen ion of Brønsted acid (M-OH) will transfer to the oxygen ion of SO2 and a strong bond between S and surface O is formed. The Brønsted acid is eliminated after this reaction. Thus, Brønsted acid sites are favorable sites for SO2 adsorption, but they can hardly oxidize SO2 to SO3. Acknowledgements We gratefully acknowledge the financial support of the National Natural Science Foundation of China (51506015), Fundamental & Advanced Research Projects of Chongqing (cstc2015jcyjA20006), Chongqing social undertakings and livelihood security special projects (cstc2015shmszx20004), Fundamental Research Funds for the Central Universities (106112016CDJXY145503, 106112016CDJZR145507). Supporting Information Supporting Information Available: Full description of the calculation models and high-resolution snapshots of the stationary structures. This material is available free of charge via the Internet at http://pubs.acs.org. Reference 1.

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TOC Graphic


TiO2 Catalyst with Low

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