Understanding SO2 Poisoning over Different Copper Species of Cu

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

Understanding SO Poisoning over Different Copper Species of Cu-SAPO-34 Catalyst: A Periodic DFT Study Guangpeng Yang, Xuesen Du, Jingyu Ran, Xiangmin Wang, Yanrong Chen, and Li Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06765 • Publication Date (Web): 28 Aug 2018 Downloaded from http://pubs.acs.org on September 1, 2018

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The Journal of Physical Chemistry

Understanding SO2 Poisoning over Different Copper Species of Cu-SAPO-34 Catalyst: A Periodic DFT Study Guangpeng Yang, a, b Xuesen Du,*, a, b Jingyu Ran, a, b Xiangmin Wang, a, b Yanrong Chen, a, b and Li Zhang a, b a Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Ministry of Education of PRC, Chongqing University, Chongqing, 400044, China b School of Energy and Power Engineering, Chongqing University, Chongqing, 400044, China

Abstract: Cu-SAPO-34 zeolites are excellent SCR catalysts and have been applied for the reduction of NOx in diesel vehicles. However, copper species in these catalysts are sensitive to sulfur oxides and the poisoning mechanism at the molecular level still remains to be elucidated. A periodic density functional theory study using general gradient approximate (GGA) method was applied to compute the poisoning behavior. The calculated GGA energies were corrected by the energies of cluster models supplementing with hybrid B3LYP method. The results indicate isolated Cu2+ in 6MR is not a favorable site for SO2 adsorption, whereas SO2 tends to adsorb on Cu+ and Cu+/H+ sites. The electronic and structural properties of copper species in Cu-SAPO-34 greatly affect the SO2 adsorption. A favorable sulfation process on low oxidation state copper species to form Cu-SO4 complex was found, which we propose will hinder the redox cycle of Cu ion and reduce the amount of isolated Cu2+ species. In addition, SO2 and O2 can easily react with [CuII(OH)]+ species in 8MR to form stable copper sulfate species and sulfate this active site. Under humid condition, H2O and O2 will accelerate the accumulation of copper sulfate species on isolated Cu2+ site in 6MR by reacting with SO2.

1. Introduction Nowadays, diesel vehicles have become the major anthropogenic sources of NOx emission, which is one of the major contributors of air pollution.1 Selective catalysis reduction is regarded as the most effective method to control NOx emission.2, 3 Compared with the traditional catalysts with vanadia supported on TiO2, Cu-SAPO-34 is a promising catalyst applied for the removal of NOx in diesel exhaust, owing to its resistance to hydrothermal aging, excellent NH3-SCR activity and N2 selectivity, especially at low temperature and high space velocity conditions.4-9 Different copper species distributed in the Cu-SAPO-34 catalysts framework have been observed, such as isolated Cu2+, Cu+ and [CuII(OH)]+, based on experiments and density functional theory (DFT) calculations.10-16 X-ray absorption spectroscopy (XAS) experiments also proved that Cu in SAPO-34 shows different valence states (+1 and +2).17 Both Cu2+ and [CuII(OH)]+ are active centers for SCR reaction, which will be affected by sulfur poisoning. Sulfur poisoning is a major problem that critically restricts the application of Cu-SAPO-34 since copper sites are sensitive to sulfur oxides.18-20 Several studies have reported the severe deactivation of copper zeolites when exposed to SO2.21-27 The superior low-temperature activity of copper zeolite catalyst was found to be significantly degraded. 21, 26, 28 The poisoning mechanism of Cu-CHA zeolites has been emphasized for the recent years. Several literatures have concluded the decreasing activity caused by SO2 is attributed to the formation of copper sulfate-like species and ammonia sulfate.23, 25, 29, 30 Wijayanti et al.31 observed an decrease of the BET surface area and simultaneously the pore volume of Cu-SAPO-34 because some pores were blocked by sulfur species. Shen et al.25 and Wijayanti et al.27 found that the

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amount of isolated Cu2+ declines obviously after SO2 exposure according to the EPR spectroscopy results. Furthermore, the H2-TPR results also indicate that less copper in Cu-SAPO-34 are available for participating in the redox cycle after SO2 exposure.25, 31 NO adsorption on copper sites are considerably inhibited due to the presence of sulfate species bonded with copper site, which blocks the generation of nitrite or nitrate.23 It is also reported that gas composition notably affects the formation of sulfate species and poisoning behavior. Jangjou et al.15 observed that ammonium sulfate was generated in Cu-SAPO-34 catalyst only under the standard SCR environment at 210 ⁰C，and NH3 could enhance the adsorption of SO2. They also found ammonia sulfate will decompose over 350 ⁰C, but the decomposition of copper sulfate species requires higher temperature as the activity of poisoned sample can only be partially recovered by heating up to over 600 ⁰C. It is reported that more sulfur species were observed in Cu-SSZ-13 under humid conditions than that under dry environments, and the presence of H2O leads to the formation of more stable copper sulfate.28 Previous studies mainly focused on the experimental methods to investigate the poisoning behavior of sulfur oxides on copper zeolites. Hass and Schneider 32 performed DFT calculations with cluster models and found that copper sulfates, such as ZCu-SO4 or ZCu-SO4-CuZ, were formed in ZSM-5 during SO2 exposure. However, the interactions of SO2 with different copper species such as Cu+, Cu2+ and [CuIIOH]+ in the molecule level remain unclear. As a result, density functional theory (DFT) calculations were implemented to systemically investigate the SO2 adsorption behavior and sulfation process on these copper species. The influence of H2O and O2 on the formation of sulfate species was screened. The mechanism for the interaction of SO2 with O2 and copper species were proposed.

2. Computational details All the supercell DFT calculations were carried out using CASTEP code in Materials Studio.33, 34 The PBE form for generalized gradient approximation (GGA) was used to optimize the periodic structure, and to describe electron exchange and correlation.34-36 We used ultrasoft pseudopotentials and a plane wave cut off energy of 400 eV for core and valence states, respectively. The spin polarized and Van Der Waals (VDW) forces were considered in all calculations. The self-consistent-field (SCF) tolerance energy was converged to 10-6 eV/atom and the configurations was regarded as fully relaxed when the force was less than 0.05 eV/Å. The Brillouin zone was sampled at the Γ point for insulation.37 As was mentioned by previous literatures, the adsorption energies concerning SCR process calculated by HSE06 is more accurate than GGA 38, 39, but cost much more time. In order to improve the energies calculated by GGA, as proposed by Anggara et al.40, we extracted a cluster containing the reaction center from the periodic structure that has been optimized by GGA and use hydrogen atoms to terminate (Figure S1). Then the single-point GGA and B3LYP energies of clusters were evaluated by Gaussian 09 code, with the basic set of 6-311G (d, p) and an integration precision of 10−8. Some 0K binding energies calculated by common PBE, HSE06 and this approach are listed in Table S1. The hybrid DFT supercell energies are estimated according to B3LYP GGA ≈ Esupercell + EB3LYP − EGGA Esupercell cluster cluster

(1) +

All Cu-SAPO-34 periodic structures including different copper species (Cu , Cu , Cu+/H+ and [CuIIOH]+) are built in a 2x1x1 supercell, which are consistent with the previous reports.12, 37,


2+

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41, 42

A certain amount of original Cu+ are reported to exist in the SAPO-34 framework.11, 13 Besides, the oxidation states of isolated Cu ions located in the zeolite were also reported to transform between +1 and +2 under SCR condition.12, 17, 43 One or two P→Si substitutions in the framework determine the oxidation states of Cu ions (Cu+ and Cu2+). According to the DFT and experimental results reported by Mao et al.11, Cu+/H+ pair located in 6MR of Cu-SAPO-34 also exists during standard SCR process when isolated Cu2+ in the six-membered ring (6MR) is reduced by NO and NH3. Thus, an additional Cu species under low oxidation state (Cu+/H+) in 6MR is also studied. [CuII(OH)]+ species close to the eight-membered ring (8MR) was detected in Cu-SAPO-34 with DRIFTS by Jangjou et al.15 In our previous work 44, we identified that the 6MR is the most favorable location for isolated Cu ions, and the 8MR is the most favorable location for [CuIIOH]+. As a result, Cu+, Cu2+ and Cu+/H+ species are located in 6MR and [CuIIOH]+ is located in 8MR, with a lattice Brønsted acid site (Al-O(H)-Si). The Si/Al ratios of the periodic structures including Cu+ and [CuIIOH]+ species are 1/6, and those including Cu2+ and Cu+/H+ are 1/4. The adsorption energy (Eads) is calculated according to

Eads = Emolecular+zeolite − Emolecular − Ezeolite

(2)

Where the Emolecular+zeolite, Ezeolite and Emolecular are the total energy of the zeolite with the adsorbed molecule, the clean zeolite system, and the isolated molecular in the gas phase, respectively.

3. Results and Discussion 3.1. Configurations of different copper species in Cu-SAPO-34 The most stable configurations of Cu-SAPO-34 with Cu species (Cu+, Cu2+, Cu+/H+ and [CuIIOH]+) under different oxidation states and locations are shown in Figure 1. As shown in Figure 1b, Cu2+ ion is bonded with four lattice atoms, with the distance of 1.96 Å, 2.10 Å, 1.92 Å and 2.27 Å respectively, which are in well agreement with previous literatures.12, 38, 45 The relative energy of the lattice Brønsted acid site is slightly lower when H atom is attached to O1, compared with other three different O atoms (O2, O3 and O4). The calculated cell volume is 1668.9 Å3, consistent with previous calculated value (1662.5 Å3) 12 and experimental value (1644.8 Å3) 46.We suggest that sulfur poisoning may have diverse effects on Cu ions with different oxidation states. As shown in Figure 1a, Cu+ ion is bonded with two lattice atoms, with the distance of 1.90 Å and 1.94 Å, respectively. An additional Cu species under low oxidation state (Cu+/H+) in 6MR is shown in Figure 1c. The lengths of Cu-O bonds are 1.92 Å and 1.93 Å, respectively. The valences of Cu in Cu+, Cu2+ and Cu+/H+ species are 0.73 e, 1.20 e and 0.70 e respectively (Figure 2a, b, c), which indicates that the oxidation states of these Cu ions are +1, +2 and +1, respectively. Figure 2d displays the local structure of [CuIIOH]+ in the 8MR and the periodic [CuIIOH]+ structure is shown in Figure 1d. Cu ion in [CuIIOH]+ structure has 3-fold coordination to the O atoms and the lengths of Cu-O bonds are 1.76 Å, 1.98 Å and 2.01 Å, respectively. As shown in Figure 2d, the calculated charge of Cu ion is 1.05 e. These results indicate that the coordination of Cu ion in [CuIIOH]+ is closely saturated.



Figure 1. Optimized periodic structures of Cu-SAPO-34 with (a) Cu+, (b) Cu2+, (c) Cu+/H+ and (d) [CuIIOH]+ species and an additional lattice B-site in 2x1x1 supercells. Different four O atoms are labeled. Green, red, purple, yellow, orange, and white balls represent P, O, Al, Si, Cu, and H atoms, respectively. This assignment will be applied throughout the paper.

Figure 2. Local structures for (a) Cu+, (b) Cu2+ and (c) Cu+/H+ in the 6MR, (d) [CuIIOH]+ in the 8MR. Mulliken partial charges are labeled in the figure. 3.2. SO2 adsorption behaviors SO2 adsorption behaviors on four different Cu species of Cu-SAPO-34 zeolite catalyst were investigated in this section. As we can see from figure 3, SO2 shows stronger interaction and higher adsorption energies on Cu species in the lower oxidation state than those in the higher oxidation states, that is, Cu+ > Cu+/H+ ≈ [CuIIOH]+ > Cu2+. In addition, SO2 tends to interact with Cu species by its O end. The calculation results indicate that the adsorption of SO2 on the Cu+ species is the strongest (exothermic by 0.71 eV). SO2 will slightly draw Cu+ ion out of the 6MR plane and change the coordination of Cu+ ion. The bond length between O (SO2) and Cu+ ion is 1.83 Å, which suggests a close chemical bond has been formed. The S atom can also interact with Cu+ ion and the length of Cu-S bond is 2.15 Å. A slightly weaker interaction between Cu+ and S atom will form, with the adsorption energy of -0.47 eV. The adsorption of SO2 on the Cu2+ ion in 6MR is weakly exothermic. Cu2+ ion shows limited interaction with both the O and S atoms of SO2. The Cu+/H+ intermediate site, however, is relatively beneficial for the adsorption of SO2, as shown in Figure 3ef. SO2 tends to adsorb on Cu+/H+ site by its O end and S atom (-0.50 eV and


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-0.55 eV, respectively), and a close interaction between Cu+/H+ site and sulfur dioxide is formed. Five possible configurations of SO2 adsorption on [CuIIOH]+ site were screened. Three moderately exothermic energies of -0.50 eV, -0.47 eV and -0.46 eV were produced from this adsorption process, as shown in Figure 3hjk. [CuIIOH]+ is also a favored site for SO2 adsorption, for both its Cu and O ions will attract sulfur dioxide. Besides, the lattice Brønsted acid site (Figure S2) will also form a close interaction with SO2, with the O-H bond length of 1.63 Å and adsorption energy of -0.30 eV. The interaction between SO2 and framework O atoms was also examined, and the calculation results demonstrated that SO2 was incapable of reacting with framework O to form sulfate complex, which is consistent with previous experimental results.25 These adsorption profiles suggest that Cu species under low oxidation states are the main sites for SO2 adsorption. The possible reason is that Cu species under high oxidation states are difficult to donate electrons to SO2, which is unfavorable for the adsorption of acid SO2. Though isolated Cu2+ is not the most favorable site for SO2 adsorption, it is the main active site for SCR reaction. Thus, we examined the adsorption energies of other gas molecules (NH3 > H2O > SO2 > O2 > NO) on the isolated Cu2+ site, as shown in Figure S2 in the supplementary materials. These results illustrate that when SO2 are absent, the Cu2+ ions will be occupied by large amount of NH3 and a small quantity of weakly adsorbed NO at the beginning of SCR process, which is in well agreement with previous reports.12, 37, 41 In addition, the co-adsorption of NH3 and SO2, NO and SO2 on these four Cu species are displayed in Figure 4, and the individual adsorption of NH3 and NO on Cu+, Cu+/H+ and [CuIIOH]+ species are shown in Figure S3. NH3 and SO2 can co-adsorb on Cu+ species, but NH3 tends to occupy Cu2+ and Cu+/H+ species preferentially. Thus, SO2 has little effect on the adsorption of NH3 on these Cu species. However, the adsorption of NO on Cu2+ and Cu+/H+ species may be inhibited by SO2, as shown in Figure 4d and f. Besides, only adsorbed NH3 on Cu site in [CuIIOH]+ species can slightly promote the adsorption of SO2 (Figure 4g). In the presence of NH3 and NO, Cu2+ can be reduced to Cu+, and therefore the adsorption of SO2 on Cu+ and Cu+/H+ species becomes important, which is consistent with experimental and computational results reported by Hammershøi et al.47 Weakly bound SO2 on Cu-SAPO-34 and Cu-SSZ-13 was also detected by Zhang et al.23 and Wijayanti et al.28 The inhibition of SCR reaction rate may start with the adsorption of sulfur oxides on these Cu species, and the sulfation mechanism is discussed next.



Figure 3. Local structures and energy profiles of SO2 adsorption on (a, b) Cu+, (c, d) Cu2+, (e, f) Cu+/H+ and (g-k) [CuIIOH]+. Bright yellow ball represent S atom, and this assignment will be applied throughout the paper. All energies and bond lengths in this and the following figures are given in eV and Angstrom, respectively.

Figure 4. Local structures and energy profiles for the co-adsorption of NH3 and SO2, NO and SO2 on (a, b) Cu+, (c, d) Cu2+, (e, f) Cu+/H+ species, and the co-adsorption of NH3 and SO2 on (g-j) [CuIIOH]+ species. The adsorption energies of SO2 on different Cu species are presented. Blue ball represents N atom, and this assignment will be applied throughout the paper. 3.3. Reaction of SO2 with O2 on isolated Cu ions Copper sulfate and ammonium sulfate are commonly considered to be responsible for the poisoning of Cu-CHA catalysts. Researchers have found that less divalent Cu ions still existed in Cu-SAPO-34 catalyst after sulfur poisoning treatment under different condition (with or without NH3), and they contended that this deactivation was caused by the formation of copper sulfate and ammonia sulfate.25, 26, 31 Considering the intense interaction between low oxidation states Cu species and SO2, we suggest that the alternative pathways for the reaction of SO2 and O2 occur with the participation of Cu+, Cu2+ and Cu+/H+ centers, as shown in Figure 5. As a result, we propose that the elementary reactions are SO2* + O2* + Cu+ (Cu2+ or Cu+/H+) →Cu+-SO4* (Cu2+-SO4* or Cu+/H+-SO4*) (3) SO4* indicates one or two O atoms in SO42- bonded with the Cu ion as the copper sulfate like species. We examined the energy barriers and energy changes of the above reactions by starting from with the adsorption of SO2 on these Cu ions, and the relative energy diagrams are shown in Figure 5. The adsorption of SO2 on Cu+ sites is greatly exothermic, but a moderate energy (0.54 eV) needs to be consumed for the following adsorption of O2 on the Cu+ ion. A close chemical bond between Cu+ and O2 is formed, and the length of Cu-O bond is 1.98 Å. O-O bond tends to break and O atoms will closely bond with S atom respectively. The transition state appears after consuming an energy barrier of 1.00 eV. A sulfate like species Cu-SO4 will be produced, and the sulfation process is extremely exothermic (-2.18 eV). The lengths of Cu-O bonds are shown in table 1. The sulfation process on Cu+/H+ species shows the same reaction pathway as that of Cu+ species, as shown in Figure 5c.The energy profile shows that the energy barrier is 1.08 eV and the sulfation process on Cu+/H+ is also greatly exothermic (-2.23 eV). As a result, Cu-SO4 sulfate like


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species can be formed easily on Cu+ and Cu+/H+ sites kinetically and thermodynamically. An immediate species (a SO3 adsorbed on O*) for releasing SO3 from Cu-SO4 complex is shown in Figure 5a. This releasing process is endothermic by 0.87 eV, and the energy barrier is 1.15 eV. The total energy for the escape of SO3 out of Cu-SO4 complex on Cu+ is 2.56 eV. Besides, the release of SO3 on Cu+/H+ also needs to consume remarkably high energies of 1.99 eV. These calculation results suggest that these sulfate like species are hard to decompose. However, as shown in Figure 5b, the energy barrier for the reaction between SO2 and O2 on 2+ Cu (2.17 eV) site is much higher than that on Cu+ site (1.00 eV) and Cu+/H+ site (1.08 eV). Though the adsorption of O2 is slightly exothermic, the distance between Cu2+ ion and O2 is 3.89 Å and the distance between Cu2+ and SO2 is lengthened from 2.29 Å to 2.53 Å. Thus, the calculated results indicate that the interaction between Cu2+ and O2 is considerably limited, which is well in agreement with previous work,12 and the reaction between SO2 and O2 can hardly be activated by Cu2+ ions. These results suggest that the reaction of SO2 and O2 can readily sulfate the Cu+ and Cu+/H+ species, rather than Cu2+ species. These diagrams also indicate that the decomposition of these copper sulfate complexes is a strong endothermic reaction and consume a large amount of energy to release SO3 from these Cu sites. DRIFTS characterization identified the sulfated Cu site was no longer accessible for NO adsorption.15 Besides, the reaction of NO with oxygen is part of the oxidation of Cu+ to Cu2+.39, 41, 48 As a result, the SCR redox cycle would be interrupted by the formation of copper sulfate on Cu+/H+ site and the isolated Cu2+ sites would decrease in the zeolites. It should be noted that SO2 poisoning is more severely under standard SCR condition, because of the existence of ammonia and more generation of Cu species under low oxidation state. NH3 can readily bond with the H site of Cu+/H+ species, which generates Cu-SO4-NH4 complex, and this process is considerably exothermic by -1.87 eV (Figure 5c).



Figure 5. Energy diagrams for the reaction of SO2 with O2 on (a) Cu+, (b) Cu2+, and (c) Cu+/H+ sites. Local structures are inserted. 3.4. Reaction of SO2 and O2 with [CuIIOH]+ species


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Figure 6. Energy diagrams for the reaction of SO2 with the [CuIIOH]+ species starting with the SO2 adsorption on (a) the top of Cu ion by O atom or (b) the parallel adsorption of S=O on CuII-OH site. Local structures are inserted. The reaction of SO2 with [CuIIOH]+ species are examined in this part. Although we have proved the lattice -OH site is a potential site for SO2 adsorption, the interaction between H atom of Cu-OH and SO2 is quite limited (Figure 3g). The energy diagrams shown in Figure 6 suggest that SO2 can easily react with CuII-OH. Meanwhile, the S atom of SO2 will bond with the hydroxyl O atom with or without breaking the bond between Cu ion and the hydroxyl O atom, as demonstrated by the reactions routes in Figure 6 respectively. A stable Cu-HSO3 complex is generated, in which Cu ion is 4-coordinate to O atoms, and the lengths of Cu-O bonds are shown in table 1. In Figure 6a, the energy barrier for the route with breaking the Cu-OH bond (0.54 eV) is slightly higher than that of the other route in which the Cu-OH bond remains (0.34 eV). However, a more stable Cu-HSO3 complex can be produced with the cleavage of Cu-OH bond and this sulfation process is exothermic by -0.48 eV. The release of isolated Cu ion is strongly endothermic by 2.45 eV and 2.70 eV by cutting off HSO3 molecular from these two kinds of Cu-HSO3 species. In figure 6b, another sulfation process starting with parallel adsorption of S=O on Cu-OH site follows the same rule, with slightly higher energy barriers. This indicates that Cu-OH species is considerably difficult to oxidize SO2 to free HSO3 by this process. However, the formation of Cu-HSO3 is kinetically and thermodynamically favorable. As shown in Figure S3, NH3 prefer to adsorb on the Cu ion (-1.01 eV) than on O atom (-0.50 eV) of [CuIIOH]+ species. Then the sulfation process exhibited in Figure S4 with adsorbed NH3 is exothermic by 0.46 eV and the energy barrier is 0.48 eV, which indicates that NH3 adsorption can hardly affect the sulfation process of [CuIIOH]+ species. These calculated results suggest Cu-OH species are ready to capture SO2 and cause the sulfation of active sites, and this kind of M-OH species in V2O5/TiO2 catalyst can also be easily sulfated.49

Figure 7. Local structures and energy diagrams for the reaction of Cu-HSO3 with O2 in the 8MR.



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However, copper sulfate species was reported to exist as CuSO4 in the Cu/CHA catalyst 26, 50, which reveals that this Cu-HSO3 complex is likely to further react with O2 to generate Cu-SO4 complex. The energy diagrams shown in Figure 7 demonstrate that the O2 can adsorb on the Cu or -OH site of Cu-HSO3 complex. In Figure 7a, although the adsorption of O2 on Cu site is exothermic by -0.21 eV, the energy barrier for breaking the O-O bond and generating the CuO-HSO4 complex is extremely high (4.34 eV). Thus in Figure 7b, we consider the reaction started with the adsorption of O2 on the -OH site of Cu-HSO3 complex, which is endothermic by 0.21 eV. Besides, the Cu-HSO3 structure undergoes a slight geometric distortion with the energy change of 0.08 eV and then O2 can interact with both H+ and S atom. This route that generates Cu-SO4 and a free hydroxyl is exothermic by -0.96 eV, and needs to overcome an energy barrier of 0.93 eV, which is energetic favorable. As we can see from Figure 7b, H atom is quite easily to escape from -HSO3 once the distance between O2 and H atom reduces to 1.54 Å. As a result, O2 probably firstly adsorbs on Cu site, and then moves to the -OH site to react with HSO3, leaving the Cu-SO4 complex. The free hydroxyl may further participate in the SCR reaction or generate new [CuIIOH]+ species. The escape of free hydroxyl and the generation of new [CuIIOH]+ species is exothermic by -2.02 eV, as shown in Figure S11. Table 1. Calculated Cu-O Bond Lengths and Mulliken Partial Charges. Species

dCu–O (Å)

Cu+-SO4 Cu2+-SO4 Cu+/H+-SO4 Cu-HSO3 (green, 8MR) Cu-HSO3 (red, 8MR) Cu-HSO3 (6MR) Cu-SO3H (6MR)

1.92, 1.90, 1.84, 1.84 2.08, 2.01, 1.92, 1.90 2.01, 1.94, 1.88, 1.87 2.02, 1.98, 1.96, 1.93 2.00, 1.99, 1.96, 1.96 1.91, 1.96, 2.13 2.22 (Cu-S), 2.00, 1.95

Charge (e) Cu

S

1.33 1.23 1.26 1.15 1.11 1.06 0.84

2.32 2.47 2.33 1.55 1.60 1.60 1.76

3.5. Sulfation process under humid condition Thus far, all the calculations were performed under dry condition. Wijayanti et al. 28 reported that a significantly larger amount of copper sulfates was detected in humid environment, and more stable sulfate species were produced in the presence of H2O and O2. We suggest that H2O may promote the sulfation process of Cu2+ species in 6MR, which results in the further increased amount of sulfated Cu species. On one hand, Our calculation results exhibited in Figure S2 show H2O can adsorb on isolated Cu2+ with moderate adsorption energy (-0.78 eV), which is larger than SO2 adsorption energies (-0.39 eV). On the other hand, the repulsive interaction between H2O and SO2 was observed when they moved to Cu2+ site simultaneously (Figure S6). However, the potential interaction between H2O and the lattice O atom in the 6MR should be noticed. As a result, we suggested that H2O can adsorb on the lattice O atom by its H end. The stabilities of different H2O locations (I−VI, Figure S7 and Table S2) were examined. Locations I, II, V and VI show very close stabilities, among which location I is slightly preferable (-0.32 eV). Thus, the reaction of SO2 with H2O adsorbed on these two kinds of adsorption sites is discussed. As shown in Figure 8a, H2O is ready to dissociate on Cu2+, leaving a hydroxyl bound with Cu


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ion with an energy barrier of 0.66 eV and an endothermic energy of 0.37 eV. Then, the adsorption of SO2 is moderately exothermic (-0.50 eV), and a Cu-HSO3 complex is formed with breaking the Cu-OH bond, which needs to overcome an energy barrier of 0.51 eV. Another possible route to generate Cu-SO3H is shown in Figure 8b. Adsorbed H2O on lattice O atom was capable of promoting the SO2 adsorption on Cu2+ (from -0.18 eV under dry condition to -0.37 eV under humid condition), compared with the adsorption of SO2 displayed in Figure 3d. Energy barrier for this sulfation process is only 0.09 eV, and the process is slightly exothermic by -0.05 eV. The lengths of Cu-O bonds are shown in table 1. The releases of HSO3 complexes are endothermic by 1.54 eV (Figure 8a) and 1.26 eV (Figure 8b), which is considerably lower than the energies needed for releasing SO3 of Cu-SO4 complex in 6MR (2.56 eV and 1.99 eV) and HSO3 complexes in 8MR (2.45 eV and 2.70 eV). However, we suggest that these HSO3 complexes in Figure 8a and 8b tend to react with O2 to generate more stable sulfate species. As shown in Figure S9a, -OH species of Cu-HSO3 can easily capture O2, which is exothermic by -0.21 eV. Then one O atom of O2 will closely bond with S atom, and another one will interact with H atom to generate a free hydroxyl with the breakage of O-H bond of HSO3. This process to generate Cu-SO4 is exothermic by -0.92 eV, and the energy barrier is 0.85 eV, indicating the reaction of Cu-HSO3 with O2 in the 6MR is energetic favorable. In addition, Figure S9b exhibits a similar route for the reaction of Cu-SO3H with O2 to generate stable Cu-SO4 and a free hydroxyl, which is exothermic by -0.31 eV and needs to overcome an energy barrier of 1.38 eV. As we can see from Figure 8c, the adsorption of H2O (-0.40 eV) on Cu+ site is weaker than that on Cu2+ site (-0.78 eV). The energy barrier for H2O dissociation on Cu+ site (1.83 eV) is much higher than that on Cu2+ site (0.66 eV) and the process is endothermic by 0.62 eV. Figure S8 also exhibits that the dissociation of H2O on Cu+/H+ site is endothermic by 0.94 eV and needs to overcome a high energy barrier of 2.18 eV. These high energy barriers and energy consumptions reveal that Cu+ and Cu+/H+ site is not favorable for H2O dissociation. However, as shown in Figure 8c, the reaction of SO2 on Cu+ site with a dissociated -OH only needs to consume an energy barrier of 0.05 eV, and formed HSO3 needs to overcome an extremely high energy barrier of 4.15 eV to escape from this Cu-SO3H complex. These calculated results indicate that H2O and O2 can promote the formation of stable sulfur complex on Cu2+ site, and the second sulfation process (Figure 7b) is kinetically and thermodynamically favorable. However, we could not find a stable Cu-HSO3 product if taking the co-adsorption configuration of H2O and SO2 in Figure S8 as the reactant. As a result, the accumulation of HSO3 complex on Cu+ and Cu+/H+ site with the participation of H2O is difficult. Besides, we propose that the elementary reaction is SO2* + H2O* + Cu2+→ Cu- HSO3* + H* (4)



Figure 8. Reaction of SO2 on isolated Cu ion with H2O adsorbed on (a) Cu2+, (b) lattice O atom beside Cu2+ and (c) Cu+. Local structures are inserted. 3.6 Discussion

Figure 9. Isosurfaces (level 0.02) of charge density differences before and after SO2 adsorption on Cu+ (a), Cu2+ (b) and Cu+/H+ (c) species. Yellow indicates electronic accumulation, and light blue indicates loss. In this study, the interaction of SO2 with various copper species of the Cu-SAPO-34 catalyst were investigated. Cu species under low oxidation state (Cu+) is the most favorable site for SO2 adsorption. The isosurfaces of charge density differences before and after SO2 adsorption on Cu+, Cu2+ and Cu+/H+ species are exhibited in Figure 9. In Figure 9b, the interaction between SO2 molecule and Cu2+ site is weak, since Cu2+ is in its highest oxidation state and hard to further donate electrons to SO2. However, Cu+ and Cu+/H+ species has a strong interaction with SO2 by


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electron accumulation on SO2, and electron depletion on Cu+ and Cu+/H+, as presented in Figure 9a and 9c. The strong interaction between O and Cu+ (or Cu+/H+) ion leads to the high adsorption energy of SO2. SO2 can easily react with O2 to sulfate the Cu+ and Cu+/H+ species with the generation of Cu-SO4 complex. As we discussed in section 3.1, NO-assisted NH3 dissociation and NH2NO decomposition leads to the reduction of Cu2+ to Cu+/H+.39 As shown in Figure S10a, the reaction of NO-assisted NH3 dissociation is endothermic by 0.72 eV and the energy barrier is 1.15 eV. Then O2 and NO will oxide Cu+ back to Cu2+ to complete the reoxidation half-cycle.12, 36 As shown in Figure S10b, the reaction of NO with O2 is slightly endothermic by 0.16 eV and the energy barrier is 1.18 eV. The calculation results indicate that typical sulfation processes are relatively easier to happen than the main SCR reaction, and stable copper sulfate species are generated. The formation of Cu-SO4 on Cu+/H+ site will hinder the SCR reaction cycle, and reduce the amount of isolated Cu2+ ions. In Figure S12, adsorbed NH3 on Cu species can break the bond between Cu+ ion and the lattice O atoms and thus make it migratable. Paolucci et al. reported that NH3-solvated Cu+ ions would travel to adjacent cages to participate in the reoxidation process.51 As shown in Figrue S12e and j, NH3 adsorption on the Cu ion of Cu-SO4 complex slightly lengthens the Cu-O bonds, but these chemical Cu-O bonds remain close, and therefore the migration of Cu ions will obviously be limited after the formation of Cu-SO4 complex. The Brønsted acid site ([CuIIOH]+) in 8MR can enhance the adsorption of SO2 (-0.50 eV) compared with isolated Cu2+ in 6MR (-0.39 eV), and easily form stable -HSO3 complex, in which S remains unoxidized and stays at its original valence state. On one hand, this active site is sulfated. On the other hand, the migration of Cu species and gas molecular through the 8MR CHA windows are blocked. Besides, H2O and O2 can accelerate the accumulation of -HSO3 and -SO4 complex on isolated Cu2+ species. A high energy needs to be consumed for the escape of HSO3 or SO3. This is why Jangjou et al. reported that sulfated Cu-SAPO-34 catalyst exhibited different SCR reaction rate after the regeneration under different temperatures.15

4. Conclusions In this work, computational calculations using DFT theory were performed to comprehensively investigat the SO2 poisoning behaviors of Cu-SAPO-34 catalyst. GGA in CASTEP was used to optimize periodic structures; B3LYP and GGA in GAUSSIAN were applied to improve upon the supercell GGA energies. Different copper species were considered, and the main conclusions are as follows. (1) SO2 tends to adsorb on copper species under low oxidation state. The reaction of SO2 and O2 can easily sulfate Cu+ and Cu+/H+ species. (2) The reoxidation half cycle (Cu+→Cu2+) of SCR reaction for isolated Cu sites will be severely inhibited by SO2 via the formation of copper sulfate on Cu+/H+ site, which can cause the decease of the amount of isolated Cu2+ ion and hinder the SCR reaction rate remarkably. (3) Cu-OH species is ready to capture SO2 and form stable -HSO3 complex, which will be oxidized by O2 to generate Cu-SO4 then. (4) More stable copper sulfate species will form under humid condition with O2 on Cu2+ site, with considerably low energy barriers. The conclusions above are vital to understand the connections between the structural and electronic properties of zeolites, including different copper species and the valence of Cu ions, as well as different reaction conditions, and the sulfur poisoning. These features can be extended to



sulfur poisoning mechanism over other zeolites and contribute to the design of sulfur-resisted catalyst. Supporting Information Figures and tables for the extracted cluster models, calculation method involving the hybrid functional effect, and additional gas molecules (NH3, NO, H2O and O2) adsorption structures, energy diagram for the main SCR reaction and the reaction of Cu-HSO3 with O2 in 6MR.

Author Information Corresponding Author *E-mail: [email protected]. Tel: +86-18183188550. Fax: +86-23-65103101. ORCID Xuesen Du: 0000-0002-3647-1002 Notes The authors declare no competing financial interest.

Acknowledgement We gratefully acknowledge the financial support of National Natural Science Foundation of China (51506015), Chongqing Technology Innovation and Application Demonstration Projects (cstc2018jscx-msyb0999), Fundamental Research Funds for the Central Universities (2018CDQYDL0050, 2018CDJDDL0004) and Open Fund of State Key Laboratory for Clean Energy Utilization of Zhejiang University (ZJUCEU2017016).


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Understanding SO2 Poisoning over Different Copper Species of Cu

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