Interaction between SO2 and NO in their adsorption and photocatalytic conversion on TiO2

Chemosphere 249 (2020) 126136

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Interaction between SO2 and NO in their adsorption and photocatalytic conversion on TiO2 Haiming Wang a, b, *, Hanzi Liu a, Zhen Chen a, Andrei Veksha b, Grzegorz Lisak b, c, Changfu You a, ** a

Key Laboratory for Thermal Science and Power Engineering of the Ministry of Education, Department of Energy and Power Engineering, Tsinghua University, Beijing, 100084, China Residues and Resource Reclamation Centre, Nanyang Environment and Water Research Institute, Nanyang Technological University, Singapore, 637141, Singapore c School of Civil and Environmental Engineering, Nanyang Technological University, Singapore, 639798, Singapore b

h i g h l i g h t s
Co-existence of NO and SO2 improved adsorption of each other on P25eTiO2 surface.
O2 played a more important role than H2O in the photocatalytic conversion of NO.
The photocatalysis of NO became ineffective in the presence of SO2.
Removal process was controlled by the catalyst deactivation due to sulfur poison.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 21 November 2019 Received in revised form 17 January 2020 Accepted 5 February 2020 Available online xxx

The simultaneous adsorption and photocatalytic conversion of SO2 and NO on P25eTiO2 were studied. In particular, the interaction of SO2 and NO on each other’s adsorption and photocatalytic oxidation was discussed. The adsorption of NO on P25 was negligible when comparing to that of SO2, while with the coexistence of NO and SO2 in flue gas, both the adsorption of SO2 and NO were improved. In the presence of water and oxygen, the photocatalytic oxidation efficiency of NO with an efficiency of >69% was observed on irradiated TiO2 surface, which lasted for at least 1000 min. Oxygen was found to have much more important effect than water on the photocatalytic oxidation of NO. In the presence of SO2 however, the photocatalytic process of NO was largely reshaped. The whole process was controlled by the photocatalytic oxidation of SO2. A dramatic efficiency decease (breakthrough of the catalyst bed) was observed for both NO and SO2 due to the catalyst deactivation caused by the poisoning of SO2 oxidation products. Before the breakthrough, the photocatalytic conversion efficiency of NO increased with increasing the SO2 concentration, which was mainly due to the improved NO adsorption in the presence of SO2. © 2020 Elsevier Ltd. All rights reserved.

Handling Editor: Jun Huang Keywords: Photocatalytic conversion SO2 NO Adsorption Interaction

1. Introduction Nitrogen oxides (NOx) and sulfur dioxide (SO2) are major pollutants originating from fossil fuel combustion, especially coal

* Corresponding author. Residues and Resource Reclamation Centre, Nanyang Environment and Water Research Institute, Nanyang Technological University, Singapore, 637141, Singapore. ** Corresponding author. E-mail addresses: [email protected] (H. Wang), [email protected]. cn (C. You). https://doi.org/10.1016/j.chemosphere.2020.126136 0045-6535/© 2020 Elsevier Ltd. All rights reserved.

combustion (You and Xu, 2010). Due to the detrimental effects on the environment and human health, various techniques were investigated and applied to remove NOx and SO2 to meet the increasingly stringent emission regulations (Sun et al., 2016). Selective catalytic reduction (SCR) and wet flue gas desulfurization (WFGD) are the most widely used techniques in coal fired power plants for NOx and SO2 removal, respectively (Cordoba, 2015; Chen et al., 2018). However, in these two processes, NOx and SO2 are separately handled, which makes the operation cost to be high and requires relatively large land footprint (Xia et al., 2015; Liu et al., 2018). Furthermore, under the great pressure of the “ultra-low

2

H. Wang et al. / Chemosphere 249 (2020) 126136

emission” standard in China, the operation costs for the traditional SCR and WFGD devices would be substantially increased to reach the stringent emission standard (50 mg/Nm3 for SO2 and 30 mg/ Nm3 for NO). Thus there is still a need for the development of novel and cost-effective technique for the deep purification of the flue gas either as standalone device or combined with the existing SCR and WFGD setups. Many researches, therefore, investigated the simultaneous removal of NOx and SO2 in single process to achieve ultralow emissions, such as porous materials adsorption (Abdulrasheed et al., 2018), wet scrubbing absorption (Jin et al., 2006; Hao et al., 2017), and advanced oxidation (Su et al., 2013; Liu et al., 2016; Yang et al., 2019a). Photocatalysis, as an advanced oxidation method, is of great interest in dealing with multiple harmful gases associated with air and water pollution (Fujishima et al., 2008; Di Paola et al., 2012). Simultaneous removal of SO2 and NOx with photocatalysis was widely investigated (Yuan et al., 2012; Su et al., 2013; Liu et al., 2014; Xia et al., 2015; Han et al., 2016). The most used photocatalysts were TiO2-based materials due to their high reactivity, chemical stability, and non-toxicity (Chen et al., 2012). Photocatalytic removal of SO2/NOx on TiO2 is a heterogeneous reaction, in which the adsorption of SO2/NO on catalyst surface plays an important role. However, most of the studies conducting simultaneous photocatalytic conversion of SO2/NO mainly focused on the photocatalysis process, while the adsorption process was seldom discussed. Since the adsorption of SO2/NO on TiO2 surface is the first step prior to the photocatalytic conversion process, it is necessary to investigate their influences on the adsorption of each other. A stronger affinity and higher adsorption capacity of TiO2 surface for SO2 than NO was usually reported since NO is not a highly polar molecule as SO2 is (Li et al., 2016; Yang et al., 2019b). Negligible NO quantities can be adsorbed on pure TiO2 surface (Uddin et al., 2007; Ohko et al., 2009). On the SO2-preadsorbed TiO2 surface, the NO adsorption was found to be promoted (Uddin et al., 2007). Interestingly, the existence of NO also accelerated the adsorption of SO2 over TiO2. In contrast, Ito et al. (2007) reported that the uptake of NO on TiO2/ZrO2 surface decreased in the presence of SO2, while the total adsorption amount of NO and SO2 remained the same, indicating that NO and SO2 competed for the same adsorption sites. In this study however, ZrO2 was used as a support for TiO2, which may have effect on the adsorption of NO and SO2 on TiO2 surface. Thus using pure TiO2 or TiO2 supported on adsorption-inert materials (e.g. glass beads) is necessary to study the SO2/NO adsorption and their mutual effect. For the photocatalytic conversion of SO2 and NO on irradiated TiO2 surface, contradictory results were reported in literature. A positive effect of NO on the photocatalytic removal of SO2 was observed, which was ascribed to the involvement of active radicals, such as O and O3 (Liu et al., 2014). Yet with increasing SO2 concentration, the removal efficiency of NO was found to decrease in that study. The promotion effect of SO2/NO on each other was also reported by Zhao et al. without giving detailed discussion (Zhao et al., 2009). However, in other studies (Ao et al., 2004; Yuan et al., 2012; Liu et al., 2014), the presence of SO2/NO inhibited the photocatalytic conversion of each other. This inhibition effect was described as the result of competition between the sulfate ions and NO for adsorption sites and the oxidative radicals (such as
OH,
2 O ) generated on TiO2. As can been seen from the above studies, the interaction of photocatalytic oxidation of NO and SO2 on TiO2 surface has not been well understood. In addition, only the variation of the final (stable) or average removal efficiency for SO2 (or NO) was shown in the presence of different concentrations of NO (or SO2), which may omit some of important characteristics during the whole photocatalysis process

(Wang and You, 2016). The influences between SO2 and NO during the whole photocatalytic reaction process, i.e. from the adsorption till catalyst deactivation (caused by the poison of photocatalytic products (Wang et al., 2016; Wang et al., 2019)), was seldom discussed in literature. Nevertheless, this is of vital importance for understanding the different photocatalytic reaction mechanisms of SO2 and NO, and making full use of the photocatalytic activity in the simultaneous removal process. As can be seen from the above discussion, several main knowledge gaps can be found for the simultaneous removal of SO2 and NO on TiO2 surface by photoctalysis. i) The co-adsorption and the interaction of SO2 and NO on TiO2 in dark condition lacks of systematic study; ii) contradictory results were obtained for the mutual effects of SO2 and NO during the photocatalytic process, which needs to be revealed; iii) how the catalyst deactivation would affect the simultaneous removal of SO2 and NO remains unclear. Therefore, the objective of the present study is to systematically investigate the interactions of SO2 and NO during their simultaneous adsorption and photocatalytic conversion on TiO2 surface. Both the experimental tests in a fixed bed reactor and the theoretical calculation based on density functional theory (DFT) were conducted to reveal the possible mechanisms. It is worth noting that due to the fast catalyst deactivation caused by the photooxidation products, the photocatalysis technique may not be suitable for the purification of the raw flue gas because of the high SO2 and NOx concentration. Instead, it can be used for the deep purification of the flue gas after the existing SCR and WFGD processes to reach the “ultra-low emission” standard for multiple pollutants at the same time. The current study is based on this application background. 2. Experiments and calculations 2.1. Catalyst samples The photocatalyst used in this study was a commercial TiO2 product (P25, Evonik), which is usually used as a benchmark catalyst for photocatalysis. Inert glass beads with the size of 0.4e0.5 mm were used as a substrate to support the P25 powder. The detailed loading procedure was described in a previous study (Wang et al., 2018). Briefly, 5 g of P25 powder was dispersed in 50 ml deionized water under vigorous stirring to form TiO2 suspension. Hydrofluoric acid etched glass beads were then immersed in the suspension with continuous stirring for 30 min. The catalyst composites were dried at 80  C for 2 h and then calcined at 400  C for another 3 h. The total loading capacity of P25 was ~20 mg/g of glass beads. The morphology and crystal structure changes of the glass beads surface were characterized by SEM and XRD analysis, respectively, which were shown in our previous study (Wang and You, 2016). 2.2. Adsorption tests in dark The adsorption of NO and SO2 on the supported TiO2 in dark condition was tested in a fixed bed reactor with an inner diameter of 4 mm. The simulated flue gas containing NO and/or SO2 was provided by gas cylinders (Air Liquide, Beijing). The concentration of NO and SO2 was maintained at certain levels by several mass flow controllers (Seven Star, Beijing). In each test, simulated flue gas with 0e20 ppm NO, 0e40 ppm SO2, 5 vol% O2, and 2.9 vol% H2O balanced in nitrogen gas passed through the reactor at a flow rate of 100 sccm. The reaction temperature was maintained at 60 ± 1  C. Three grams of catalyst composites were filled in the reactor to create a catalyst bed height of ~16 cm. The residence time was

H. Wang et al. / Chemosphere 249 (2020) 126136

~1.2 s, corresponding to the space velocity of 3000 h1. Before adsorption, the simulated flue gas bypassed the reactor and entered gas analyzer for concentration measurement. SO2 and NOx (NO þ NO2) concentrations were measured online by Model 43i and Model 42i (ThermoFisher Scientific) with a precision of 1% F.S. (full scale), respectively. After the inlet concentration maintained stable for at least 10 min, the flue gas was introduced into the fixed bed reactor for adsorption. The concentration was recorded every minute, unless otherwise specified. 2.3. Photocatalytic conversion measurement The photocatalytic conversion experiments of SO2 and NO were conducted on the same fixed bed reactor as the one used for adsorption. The inner reactor was surrounded by four 20 W UV lamps (Xinyuan Lighting, China) providing 3 mW/cm2 UV intensity at the peak wavelength of 365 nm. In each test, the adsorption of SO2/NO on catalyst surface in dark was firstly conducted until the adsorption breakthrough was reached. After the adsorption equilibrium was reached, UV lamps were turned on to trigger the photocatalytic reactions. The data shown in the following sections for photocatalytic conversion was obtained after the introduction of UV irradiation. Similar to the operation parameters used for the adsorption process, the gas flow rate and reaction temperature were maintained at 100 sccm (standard cubic centimeter per minute) and 60 ± 1  C, respectively. As shown in Table 1, five different experiments were conducted to investigate the adsorption and photocatalytic conversion of SO2/NO on TiO2 surface. Case 1 to 3 studied the photocatalytic conversion of NO in the presence of SO2 on TiO2 with difference water and oxygen content, since the conversion process of NO was totally different with different conditions. Case 4 investigated the effect of 0e40 ppmv SO2 on the adsorption and photocatalytic conversion of NO on TiO2. The effect of 0e40 ppmv NO was studied in case 5. The photocatalytic conversion efficiency for NO and SO2 (h) are defined as:

h ¼ 1  Cout =Cin

(1)

where Cout and Cin are the outlet and inlet concentration of SO2 or NO, respectively. 2.4. DFT calculations To reveal the molecular insights of SO2 and NO co-adsorption process on the surface of P25eTiO2 crystals, the density functional theory (DFT) model calculation for specific facets was performed with Materials Studio package with Dmol3. As indicated by XRD analysis of P25eTiO2 (Wang and You, 2016), the main surface orientations for the crystal phase of P25 are anatase (101) and rutile (110). Thus, the adsorption process of SO2/NO was analyzed on these two crystal facets. The exchange-correlation functional was calculated via Perdew-Burke-Ernzerhoof generalized-gradient approximation (PBE, GGA). Double numerical basis was set using Porbital polarization function for atom figuring and one irreducible k-point (1  2  1) was defined. The periodic structure of TiO2 was

3

employed in this work. For anatase (101) interface, the lattice parameters were a ¼ 10.886 Å, b ¼ 7.552 Å, and c ¼ 15.845 Å to eliminate the interaction between the two adjacent slabs. The vacuum layer contained sixteen titanium atoms and thirty-two oxygen atoms in calculation (Ti16O32). For rutile (110) interface, the lattice parameters of the vacuum layer were a ¼ 5.918 Å, b ¼ 12.994 Å, and c ¼ 11.980 Å. The slab with eight titanium atoms and sixteen oxygen atoms were employed (Ti8O16). The maximum force tolerance was 0.004 Ha/Å. The SCF tolerance was 1.0  106 Ha with an orbital cutoff of 5.2 Å. The maximum displacement tolerance was 0.005 Å. The convergence energy tolerance was 2.0  105 Ha. The parameter tests for DFT calculation was performed and shown in supporting information (Table S1). The adsorption energies of SO2 and NO molecules were calculated as follows: surface Ead ¼ Emolecule  Emolecule opt surface  Eopt

(2)

where Ead was the adsorption energy of each slab, kcal/mol; Emolecule was the total energy after SO2/NO adsorption on each surface interface; the Esurface and Emolecule present the energy of isolated opt opt TiO2 interface and SO2/NO molecule with geometry optimization, respectively. A negative value of Ead indicated a stable adsorption process. Both the physical and chemical adsorption of SO2/NO were considered. When gaseous molecules are getting close to the catalyst surface, they will enter the potential well of physical adsorption without molecule dissociation. The possible sites for further chemical adsorption can therefore be located. Physical adsorption is simulated using the Sorption model to preliminarily screen possible locations for the next chemical adsorption process. 3. Results and discussion 3.1. Adsorption of NO and SO2 on TiO2 surface 3.1.1. Experimental tests The adsorption of SO2 or NO on TiO2 surface under dark condition is shown in Fig. 1. As can be seen in Fig. 1(a), SO2 was adsorbed by TiO2 effectively. The outlet SO2 concentration decreased to zero and maintained for about 50 min before the adsorption breakthrough of the catalyst bed. XPS analysis of the surface speciation of sulfur compound indicates that no oxidation process happened on TiO2 surface during SO2 adsorption in dark since only sulfite species were found (Wang and You, 2016). Our previous kinetic study showed that the adsorption enthalpy was 17.5 kJ/mol indicating the physisorption of SO2 on TiO2 surface (Wang and You, 2018). The uptake of SO2 on TiO2 before breakthrough was calculated as ~10 mg/g-TiO2. Compared to the adsorption of SO2 on TiO2, NO adsorption was much lower as shown in Fig. 1 (b). The outlet NO concentration decreased from about ~19 to ~13 ppmv rapidly, mainly due to the system delay after introducing NO gas flow into the catalyst bed. After the system was stable, the adsorption efficiency of NO was calculated as only ~2%

Table 1 Experimental conditions for the photocatalytic conversion tests. No. Case Case Case Case Case

1 2 3 4 5

[NO] (ppmv)

[SO2] (ppmv)

[O2] (%)

[H2O] (%)

Catalyst usage (g)

Residence time (s)

20

0

1.2

0e40 40

2.9 0 2.9 2.9 2.9

3

20 0e40

5 5 0 5 5

9

3.7

4

H. Wang et al. / Chemosphere 249 (2020) 126136

Fig. 1. Separate adsorption of (a) SO2 and (b) NO on TiO2 surface (Oxygen: 5 vol%, water: 2.9 vol%).

and decreased gradually as the time went on, which was much lower than that of SO2 before the adsorption breakthrough (Fig. 1 (a)). The adsorption of NO under other operation conditions (i.e. different water and oxygen contents) was similar, which is not shown in the figure. The weak adsorption ability of NO on TiO2 surface was also reported by Sivachandiran et al. (2013). The physisorption of SO2/NO is mainly controlled by the van der Waals forces originating from the permanent or transit dipole moments. NO is not a high polar molecule as SO2 is, which results in the relatively week affinity to TiO2 surface. Fig. 2 shows the simultaneous adsorption of NO and SO2 on TiO2 surface. The presence of SO2 promoted the NO adsorption. As the SO2 concentration increased from 0 to 40 ppm, the averaged NO adsorption efficiency within 10 min increased from about 1.2% to 6.1%. Similarly, the addition of 0e40 ppm NO into the flue gas also improved the adsorption ability of SO2 in general. The breakthrough time for SO2 adsorption was prolonged from 125 to 138 min. But the increment slowed down with increasing of NO concentration. The adsorption experiments indicated that the coexistence of NO and SO2 at low concentrations had positive synergistic effect on the simultaneous adsorption of these species on P25eTiO2 surface. Different from the observation of SO2/NO coadsorption on TiO2/ZrO2 surface (Ito et al., 2007), in which SO2 and

NO was reported to compete for similar adsorption active cites on catalyst. In the current study, this adsorption competition was not observed on pure TiO2 surface for SO2 and NO, indicating SO2 and NO could possibly be adsorbed on different types of active sites on TiO2 surface. As a result, the adsorption of SO2 (or NO) was not weakened in the presence of NO (or SO2) as observed in Fig. 2. Instead, slight enhancement was observed with the co-existence of SO2 and NO, which is consistent with the observation reported in (Uddin et al., 2007). It was shown that the presence of SO2 was essential for the adsorption of NO on TiO2 surface (Uddin et al., 2007). The existence of NO was also found to accelerate the adsorption of SO2. However, the detailed mechanism for the enhancement is still unknown. Recently, the adsorption and oxidation of SO2 to SO3 over V2O5-WO3/TiO2 SCR catalyst was found to be promoted with the addition of NO in flue gas (Qing et al., 2019), which was ascribed to the formation of the intermediate sulfur species VOSO4. Even on pure TiO2 nanoparticles with different crystal faces, surface bounded sulfate species (SO2 4 ) was found to increase significantly in the presence of NO, especially on the facet of (101) (Yang et al., 2018), which is the main crystal phase of P25 used in this study. It is most likely that small part of the adsorbed SO2 on TiO2 surface was oxidized to SO3 in the presence of NO possibly through the following reactions (Uddin et al., 2007):

Fig. 2. Effect of SO2 (NO) on the adsorption of NO (SO2) on TiO2 surface under dark. a: effect of SO2 on NO adsorption (NO concentration: 20 ppm); b: effect of NO on SO2 adsorption (SO2 concentration: 40 ppm).

H. Wang et al. / Chemosphere 249 (2020) 126136

TiO2

NO þ SO2 þ O2 / ðNO  O  O  SO2



TiO2 complex

/ NO2 þ SO3 (3)

In the presence of H2O, nitrate and sulfate ions could be formed. With the adsorption of NO2, N2O4 could be formed, which was proposed to be able to oxidize the adsorbed SO2 to sulfate through reactions (4) to (6) (Liu et al., 2012). SO2 þ TieO / TiSO3

(4)

N2O4 þ TiSO3 / TiSO4 þ NOþNO 2

(5)

NOþNO 2 þ TieOeTi / 2TiNO2

(6)

The migration of the formed oxidation products could possibly set some of the active sites free again for SO2/NO adsorption in flue gas and thus improved the adsorption capacity.

3.1.2. DFT calculations The physisorption of SO2 and NO on anatase and rutile surface was calculated with DFT. Since hydroxyl groups (‒OH) and surface defects are ubiquitous on TiO2 surface, while they may significantly affect the SO2/NO adsorption as well, so the existence of eOH and unsaturated O2 ions on TiO2 surface was also calculated for comparison. The calculation results in general agreed with the experimental data that the NO/SO2 adsorption was slightly improved with the coexistence of NO and SO2 on TiO2 surface, especially in the presence of hydroxyl groups. The electron transfers and oxidation reactions may occur when surface defects (unsaturated O2 ions) exists, which could cause the presence of sulfates and nitrates on TiO2 surface as described in Eq (3). The results showed that the average adsorption energies of SO2 and NO caused by van der Waals force on anatase (101) interface are 3.45 and 1.48 kcal/mol, respectively. In the case of rutile (110) facets, the average energies are 3.26 and 1.26 kcal/mol for SO2 and NO, respectively. The adsorption sites are mainly in the form of SeO, NeO and TieO. Therefore, only these three bonding types were further considered. The energies of the adsorption processes are summarized in Table 2 with GGA þ U correlation. The GGA þ U calculation introduces a Hubbard model for electron on-site repulsion to eliminate the self-interaction error. The recommended U values for TiO2 framework were in the range of 4.0e6 eV (Morgan and Watson, 2007; Tilocca and Selloni, 2012; Gong et al., 2014). Here we used the value of 4.2 eV for GGA þ U calculation. For the separate adsorption of NO and SO2, the adsorption energy on TiO2 surface (without surface defects) were 0e9 and 6e20 kcal/mol for NO and SO2, respectively. Nørskov et al. (2015) proposed that the energy of physical adsorption was in the range of 2.31e23.05 kcal/mol, and the energy of chemisorption should be larger than 23.05 kcal/mol. Similar adsorption energy was also observed in the studies conducted by Langhammer et al. (2018). Based on the energy, the adsorption of SO2 and NO on both anatase (101) and rutile (110) were ascribed to physical adsorption. The adsorption energy of SO2

5

was much higher than that of NO indicating the stronger adsorption of SO2 on TiO2 surface, which is consistent with the adsorption result shown in Fig. 1. The adsorption on anatase (101) appeared stronger than that on rutile (110). After conducting co-adsorption, it was found that the bond length of SeO and TieO (adsorption of o

SO2) on rutile (110) facet decreased by 0.752 and 0.716 (0.604) A, respectively, compared to that for the separate adsorption of SO2 (see Fig. 3 (a)). For the co-adsorption of NO and SO2 on anatase (101), similar decrease in the bond length for the adsorbed NO and SO2 was observed as shown in Fig. 3 (b) and (c) for the cases with and without hydroxyl group, respectively. The decrease of bond length indicates the adsorption became more stable on the surface of these TiO2 facets with the coexistence of NO and SO2. The adsorption could therefore be enhanced. This enhancement effect can also be evidenced by the co-adsorption energy shown in Table 2. The overall energy (Ead) for the co-adsorption of SO2 and NO was larger than the sum of the separate adsorption energies of SO2 and NO, revealing the more stable adsorption state for NO and SO2 during the co-adsorption process. In the case of NO and SO2 adsorption on Ti-defect surface, energies about 100 kcal/mol for both separate- and co-adsorption were observed indicating the chemisorption of SO2 and NO. The presence of surface unsaturated O2 ions probably facilitated the electron transfer on S and N atoms and subsequently oxidized the SO2 and NO. The projected density of states (PDOS) of molecular SO2 and NO were performed to reveal the variation of electron energy state of SO2 and NO on the TiO2 anatase (101) facets before and after coadsorption. The results are shown in Figs. 4 and 5. The analysis of p- and s- orbitals were presented since the sum of these two orbitals basically overlapped with total PDOS. For the adsorbed SO2 molecule, the energy of P-state electron decreased significantly near the Fermi level (0 eV) after introducing NO for anatase (101) facets (Fig. 4 (a)). Besides, the energy level splitting occurred and new peaks were generated near the Fermi level for anatase (101) both with and without eOH groups (Fig. 4 (a) and (b)), indicating that the electron transformation of bond occurred between O (from SO2) and surface Ti 2p orbitals. This confirms the enhancement of SO2 adsorption in the presence of NO since the overall structure tends to be more stable with lower energy state and new electronic configuration. Comparatively, the NO adsorption on anatase (101) shows no significant reduction of electron energy after SO2 adsorption, reflecting that NO is less affected by the co-adsorption process. For anatase (101) defect surface (Fig. 4 (c) and Fig. 5 (c)), the new states of both SO2 and NO adsorption appeared at the top of valence band (0.43eV, bonding orbital), suggesting that the coadsorption can stabilize the adsorbed SO2 via the electron transfer between SeO and NeO (O was from TiO2) 2p orbitals and subsequently promote the generation of sulfur oxides and nitric oxides with higher chemical valence (e.g., S6þ and N4þ). Additionally, the band gap for the adsorbed NO decreased by 0.2 eV after coadsorption indicating an enhanced reactivity in the presence of adsorbed SO2 (Peng et al., 2015, 2016; Zhang and Zhong, 2015). The PDOS of specific atoms of NO and SO2 before and after the coadsorption process on the Ti-defect anatase (101) facet are shown in Fig. S1 for comparison.

Table 2 Assignment of energy of SO2 and NO adsorption on periodic TiO2 framework. Eab of interface (kcal/mol)

NO adsorption

SO2 adsorption

Co-adsorption

Rutile (1 1 0) Anatase (1 0 1) Anatase (1 0 1) with eOH Anatase (1 0 1) with Ti defect

0.452 0.263 9.125 101.567

5.796 7.127 19.782 94.349

33.380 11.238 33.399 126.559

6

H. Wang et al. / Chemosphere 249 (2020) 126136

Fig. 3. The co-adsorption process on (a) rutile (1 1 0), (b) anatase (1 0 1) facets and (c) anatase (1 0 1) facets with eOH group, (d) anatase (1 0 1) with defects.

3.2. Photocatalytic conversion of NO The results for the photocatalytic conversion of NO with/ without oxygen and water in the absence of SO2 (case 1 to 3 in Table 1) are shown in Fig. 6. The conversion processes under the three operation conditions appeared to be different in the variation trend indicating the different reaction pathways on the irradiated TiO2 surface. For case-1, Fig. 6 (a) shows the NOx (NO þ NO2) concentration variation after the introducing of UV irradiation with the presence of 5 vol% O2 and 2.9 vol% H2O in flue gas. Initially, the outlet NO concentration decreased to about zero, which gives the photocatalytic conversion efficiency of almost 100%. Within the 40 min after the startup of the photocatalytic reaction, no NO2 was detected at the outlet, and the NOx concentration was zero. After ~40 min, NO concentration increased gradually, while a dramatic increase in NO2 and NOx concentration was observed. At around 900 min, the equilibrium of the total input and outlet NOx was reached with NOx-in (18.6 ppmv) z NOx-out (18.1 ppmv). This indicates almost all the NO that went through the catalyst bed was oxidized into NO2, and the photocatalytic reduction did not take place in the presence of O2 and H2O, which is in line with the observation in other studies (Ishibai et al., 2007; Todorova et al., 2014). Oxygen and water have been well acknowledged to play important roles on the photocatalytic reactions since they are two important oxidative radical sources on the irradiated TiO2 surface (Henderson, 2011). The generated superoxide radicals (
O 2 ) and hydroxyl radicals (
OH) can oxidize NO to NO2 on TiO2 surface due to their strong oxidation abilities. Unlike the weak adsorption of NO (as shown in Fig. 1 (b)), NO2 has a relatively strong affinity to TiO2 surface (Sivachandiran et al., 2013). Thus within the initial 40 min, all the generated NO2/HNO3 was adsorbed on catalyst. After the adsorption approached equilibrium, NO2 concentration increased significantly as observed in Fig. 6 (a). During the whole

photocatalytic test in case one (~1000 min), the outlet NO concentration increased slowly from 0 to ~5.8 ppmv, corresponding to the photocatalytic removal efficiency decreased from 100% to 69%. Due to the adsorption of the oxidation products, the available active sites on catalyst surface were occupied causing the catalyst activity to loss (Wang et al., 2007; Guo et al., 2012), which led to the decrease in the photocatalytic conversion efficiency of NO. There was a sharp increase in the NOx concentration after about 400 min operation, and the value decreased to normal state shortly as can be seen in Fig. 6 (a) and (c). This is most likely the fluctuation caused by the introduction of H2O. When no H2O was applied in the flue gas (case-2), the concentration variation was relatively stable as shown in Fig. 6 (b). After adding H2O into the system, H2O could accumulate and condense on the particle surface after long term operation, which can possibly cause this fluctuation as observed in Fig. 6 (a) and (c). The photocatalytic conversion process of NO in the presence of 5 vol% O2 and 0 vol% H2O was studied in case-2 and the result is shown in Fig. 6 (b). Within the initial 80 min, no NOx was detected at the catalyst outlet indicating the full adsorption of the NO oxidation products. This duration time was about 80 min (comparing to ~40 min in case-1). The existence of H2O may compete with the oxidation product (NO2) for adsorption on TiO2 surface (Guo et al., 2012), which caused the decreased adsorption ability of TiO2 for NO2 in case-1 comparing to case-2. After the catalyst breakthrough (time > 80min in Fig. 6 (b)), the concentration of NO increased remarkably, which is quite different from the observation in case-1, where the outlet NO concentration increased slowly. At the reaction time of 1000 min, the photocatalytic conversion of NO was about 45%, which is much lower than that in case-1 (69%). The only difference in the operation conditions for case-1 and case-2 was the H2O content. Without H2O, HNO3 cannot be formed on catalyst surface in case-2. As stated, the adsorbed HNO3 could block the active sites on catalyst surface causing the

H. Wang et al. / Chemosphere 249 (2020) 126136

7

Fig. 4. PDOS of SO2 on various TiO2 interfaces before and after NO adsorption on (a) anatase (101) facet, (b) anatase (101) facet with eOH groups, and (c) anatase (101) facet with Ti defect.

catalyst poison. However, it was also reported that HNO3 was able to react with NO to generate NO2 on the irradiated TiO2 surface through the following reactions (Ohko et al., 2009):

HNO3 þ hþ / NO3 þ Hþ

(7)

HNO3 þ Oe /NO3 þ OH e

(8)

þ NO 3 þ h /NO3

(9)

e 2 NO 3 þ O /NO3 þ O

(10)

NO3 þ NO / 2NO2

(11)

Therefore, even the generated HNO3 covered the catalyst surface, NO can still be oxidized to NO2 through the consumption of HNO3. Thus in case-1 (in the presence of H2O), NO increased gradually while NO2 increased dramatically after the breakthrough. However, in case-2 (in the absence of H2O), the mainly oxidation product, NO2, occupied the catalyst surface as the reaction went on, which made the NO could not reach the active sites and be oxidized. This is most likely the main reason for the significant increase of NO concentration in case-2. In addition, water as one of

the radical sources for the photocatalytic reaction, its existence can promote the
OH generation and improve the catalyst activity for NO oxidation. For case-3, the photocatalytic conversion of NO in the presence of 0 vol% O2 and 2.9 vol% H2O was studied. The result is shown in Fig. 6 (c). The introduction of UV irradiation caused the initial decrease in NO concentration. But in a short time, NO concentration increased significantly, which indicates a much lower photocatalytic conversion efficiency than that in case-1 and case-2. After about 100 min, only ~12.3% of NO was oxidized into NO2. It can be inferred from Fig. 6 (b) and (c) that O2 played a more important role than H2O in the photocatalytic oxidation of NO. In the photocatalytic oxidation of SO2, we also found the more significant effect of O2 than H2O based on the experimental and kinetic studies (Wang and You, 2018). Two possible reasons caused the significant decrease in the NO conversion efficiency without O2. First, the presence of O2, as the scavenger of the photo-generated electrons, can enhance the separation of the electron-hole pairs thus improve the catalyst activity and the formed
O 2 can take part in the oxidation of NO. The lack of O2 in case-3 would cause the oxidation ability of TiO2 to decrease. Second, another important oxidant radicals originated from H2O,
OH, cannot be produced effectively only in the presence of H2O (Nosaka and Nosaka, 2017). Although H2O is the source for
OH, it is argued that the hydroxyl radicals, as

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Fig. 5. PDOS of NO on various TiO2 interfaces before and after NO adsorption on (a) anatase (101) facet, (b) anatase (101) facet with eOH groups, and (c) anatase (101) facet with Ti defect.

one of the most important oxidants, cannot be formed through the capture of the valence band holes by H2O/OH (Salvador, 2007). The
OH can only be formed through the interaction between H2O and the
O 2 radicals (Salvador, 2007). The generation pathway of
OH through the photo-reduction of adsorbed O by the conduction 2 band electrons was also proposed in our previous study in the photocatalytic oxidation of SO2 (Wang et al., 2018). The relatively high conversion efficiency (26.0%) right after the introduction of UV in Fig. 6 (c) was mainly due to the pre-adsorbed oxygen on catalyst surface, which is inevitable during the sample preparation. By comparing the three cases, one can find that the photocatalytic conversion process of NO could be very different under different operation conditions. Oxygen is of vital importance for the effective photocatalytic oxidation of NO to take place. While in the presence of water, the conversion of NO to NO2 can be further facilitated through reactions (6) to (10). Both water and oxygen are ubiquitous in industrial flue gas. Thus in the following sections, the mutual effects of SO2 and NO on the photocatalytic conversion process were studied in the presence of both H2O and O2. 3.3. Effect of SO2 on the photocatalytic conversion of NO The photocatalytic conversion efficiency of NO in the presence

of SO2 is shown in Fig. 7. The operation conditions were the same as those in case-1 except the SO2 concentration. Comparing the results shown in case-1 (Figs. 6 (a) and 7), it can be found that SO2 has a significant influence on the photocatalytic conversion of NO. Without SO2, a high conversion efficiency of NO was maintained for quite a long time (higher than 69% within 1000 min as shown in Fig. 6 (a)). Even with the addition of 10 ppmv SO2, the NO conversion process under UV irradiation was dramatically changed. The highest conversion efficiency was only about 50%, and decreased gradually as the reaction proceeded. The oxidation process of SO2 and NO consumes the same oxidant radicals, including
OH and
O 2, on the irradiated TiO2 surface (Su et al., 2013). The much stronger adsorption ability of SO2 than NO (see Fig. 1) makes SO2 more competitive in competing with NO for the radicals, since the adsorption is the first step for the heterogeneous photocatalytic reactions (Wang et al., 2016). Thus, the conversion efficiency of NO decreased after adding SO2. After 50 min (Fig. 7 (a)), a significant decrease in the photocatalytic oxidation of both SO2 and NO was observed, which was mainly caused by the deactivation of the catalyst. It was found in our previous study that about 98% of the SO2 oxidation products (SO3/H2SO4) was adsorbed on the catalyst surface causing the rapid deactivation of the catalyst (Wang et al., 2019). In comparison, the

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Fig. 6. Photocatalytic conversion of NO on UV irradiated TiO2 surface with different water and oxygen concentration.

deactivation process caused by the adsorption of NO oxidation products (NO2/HNO3) was much slower as indicated in Fig. 6 (a). In addition, the breakthrough time of the photocatalytic reaction decreased with increasing SO2 concentration as shown in Fig. 7 (a)e(d). It can be seen that the whole photocatalytic oxidation process of NO was controlled by the catalyst deactivation due to the adsorption of SO2 oxidation products. This indicates the photocatalytic conversion of NO using TiO2 could be ineffective in the presence of high SO2 concentrations. Interestingly, even though the reaction breakthrough time of NO was shortened, the highest NO conversion efficiency before the catalyst deactivation increased as the SO2 concentration increased from 10 to 40 ppm. The highest conversion efficiency increased from ~50% at [SO2] ¼ 10 ppm to ~80% at [SO2] ¼ 40 ppm. As aforementioned, reactant adsorption is an important step prior to the oxidation reaction. The weak adsorption of NO on TiO2 was enhanced in the presence of SO2 as indicated by Fig. 2 (a), which mainly caused the photocatalytic conversion of NO to be improved during this stage. As stated in the Introduction section, contradictory results for the effect of SO2 and NO on each other during the photocatalytic conversion were reported. From the results discussed above, these contradictory observations can be explained by the different reaction stages for the photocatalytic processes. Before the catalyst deactivation, the increase of SO2 concentration could improve the photocatalytic oxidation of NO due to the enhanced NO adsorption on TiO2. However, from the perspective of long term operation, the increase in SO2 concentration caused a faster

deactivation of catalyst thus leading to the decreased photocatalytic capacity of NO. Unlike the variation trend of SO2 conversion efficiency, bimodal distribution of NO conversion efficiency was observed in Fig. 7 before the breakthrough of the catalyst bed. The detailed mechanism behind this variation remains unclear, which needs further studies in the future. However, some suggestions may be indicated from the experimental results. The relatively abundant oxidative radicals at the beginning of photocatalytic reaction and the preadsorbed NO on TiO2 surface could possibly lead to the initial high conversion efficiency of NO. Due to the competition from SO2 for the oxidative radicals, the photocatalytic activity decreased, causing the decrease in NO conversion efficiency. At longer reaction time, photocatalytic oxidation product of NO, i.e. HNO3, was formed on the surface. As discussed in section 3.2, the formed HNO3 could react with NO on the irradiated TiO2 surface generating NO2 through reactions (7)e(11). The conversion efficiency of NO could therefore increase slightly. 3.4. Effect of NO on the photocatalytic conversion of SO2 Fig. 8 shows the effect of NO on the photocatalytic oxidation of 40 ppm SO2. With increasing the NO concentration from 0 to 40 ppm, the reaction breakthrough time for photocatalytic oxidation of SO2 decreased from 74 to 54 min (Fig. 8 (a)). The oxidation of NO also consumed the photo-generated oxidant radicals even though NO was less competitive, which decreased the catalytic

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Fig. 7. Effect of SO2 on the photocatalytic conversion of NO. NO concentration: 20 ppm, oxygen: 5 vol%, water: 2.9 vol%.

Fig. 8. Effect of NO on the removal of SO2. (a): Effect of NO on the photocatalytic conversion of SO2; (b): Photocatalytic conversion efficiency of NO in the presence of 40 ppm SO2. SO2 concentration: 40 ppm, oxygen: 5 vol%, water: 2.9%, catalyst usage: 9g.

ability for SO2 oxidation. The simultaneous conversion of NO with different concentrations is shown in Fig. 8 (b). For SO2 oxidation, the conversion efficiency maintained at 100% before the reaction breakthrough either with different NO or SO2 concentrations. By contrast, NO conversion efficiency decreased gradually as NO concentration increased. This difference should mainly be caused

by the different adsorption ability of SO2 and NO on TiO2. The adsorption of NO was weak, so the increase of NO concentration would not change the absorption capacity of NO significantly. Thus, only limited NO in flue gas can be adsorbed on TiO2 surface for further photocatalytic oxidation. The remaining fraction of NO would pass through the catalyst bed without being captured.

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Comparing the results in Figs. 7 and 8, it can be seen that the effect of SO2 on the photocatalytic conversion of NO is much more significant than that of the NO on SO2. Even though the photocatalytic oxidation efficiency of NO in the absence of SO2 was high (as shown in Fig. 6 (a)), the existence of SO2 could easily cause the fast deactivation of the catalyst and lead the conversion of NO to be ineffective. Thus in the future application for the photocatalytic conversion of NO, the catalysts deactivation caused by SO2 products should be taken into consideration. 4. Conclusions In this study, the simultaneous adsorption and photocatalytic conversion of SO2 and NO were studied on P25eTiO2 with a fixed bed reactor. Their mutual effects on each other during the whole process were investigated and discussed. The following conclusions can be drawn based on the obtained data: (1) On pure P25eTiO2 surface, the adsorption of SO2/NO in dark condition was improved with the co-existence of SO2 and NO, which was most likely due to the improved adsorption stability and oxidation reactions. (2) Oxygen was found to play a more important role than H2O for the photocatalytic oxidation of NO to take place. The presence of H2O could further facilitate the conversion of NO to NO2 through reactions between HNO3 and NO on irradiated TiO2 surface. (3) With the addition of SO2, the long term photocatalytic conversion of NO was largely hindered due to fast deactivation of TiO2 caused by the poison of oxidation products of SO2. The photocatalysis of NO became ineffective in the presence of SO2. In the presence of NO, the photocatalytic oxidation of SO2 was also suppressed, but this effect was much smaller than the effect of SO2 on NO. (4) Before the breakthrough of the catalyst bed caused by the poisoning of photocatalysis products, the photocatalytic conversion efficiency of NO increased with increasing the SO2 concentration, which could be explained by the improved NO adsorption ability on TiO2 in the presence of SO2.

Declaration of competing interest The authors have no conflicts of interest to disclose. CRediT authorship contribution statement Haiming Wang: Conceptualization, Methodology, Investigation, Writing - original draft. Hanzi Liu: Software, Investigation. Zhen Chen: Validation, Formal analysis. Andrei Veksha: Writing - review & editing, Formal analysis. Grzegorz Lisak: Writing - review & editing. Changfu You: Supervision, Funding acquisition. Acknowledgement This work was supported by the National Key Research and Development Program of China (No. 2016YFC0204100). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.chemosphere.2020.126136.

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