Chinese Chemical Letters  2022, Vol. 33 Issue (3): 1259-1262   PDF    
Citation
Wendong Zhang, Yun Wang, Yi Wang, Yi Liang, Fan Dong. Highly efficient photocatalytic NO removal and in situ DRIFTS investigation on SrSn(OH)6[J]. Chin. Chem. Lett., 2022, 33(3): 1259-1262  
Highly efficient photocatalytic NO removal and in situ DRIFTS investigation on SrSn(OH)6
Wendong Zhanga,*, Yun Wanga, Yi Wanga, Yi Lianga, Fan Dongb,c,*     
a Chongqing Key Laboratory of Inorganic Functional Materials, Chongqing Key Laboratory of Green Synthesis and Application, Chongqing Normal University, Chongqing 401331, China;
b Yangtze Delta Region Institute (Huzhou) & Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Huzhou 313001, China;
c State Centre for International Cooperation on Designer Low-carbon and Environmental Materials (CDLCEM), School of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450001, China
Received 23 March 2021, Received in revised form 4 June 2021, Accepted 28 July 2021, Available online 5 August 2021
* Corresponding authors.
E-mail addresses: zhangwendong@cqnu.edu.cn (W. Zhang), dfctbu@126.com (F. Dong).
Abstract: A novel SrSn(OH)6 photocatalyst with large plate and particle size were synthesized via a facile chemical precipitation method. The photocatalytic activity of the SrSn(OH)6 was evaluated by the removal of NO at ppb level under UV light irradiation. Based on the ESR measurements, SrSn(OH)6 photocatalyst was found to have the ability to generate the main active species of O2•−, •OH and 1O2 during the photocatalytic process. Moreover, SrSn(OH)6 photocatalyst not only exhibits high photocatalytic activity for NO removal (79.6%), but also has good stability after five cycles. The in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to investigate the NOx transfer pathway and the intermediate products distribution during the adsorption and photocatalytic NO oxidation process. The present work not only provides an efficient material for air pollutants purification at room temperature but also in-depth understanding of the mechanism involved in the photocatalytic NO removal process.
Keywords: Highly efficient    NO removal    In situ DRIFTS investigation    SrSn(OH)6;    Reaction mechanism    

Air pollution is one of the most serious environmental issues, which results in extremely hazardous effects and risks on public health and ecological security [1, 2]. However, conventional methods including physical, chemical and biological methods are difficult to remove the low-concentration of air pollutants (such as nitrogen oxide (NOx), volatile organic compounds (VOCs), etc.) [3, 4]. Furthermore, most of the above-mentioned methods have disadvantages of requiring complicated techniques, harsh conditions, high costs and time consuming, which greatly limit their practical application. Hence, it is highly desirable to develop a green, stable and efficient method to improve air quality.

Photocatalysis is considered as a promising method for air pollutants purification that needs only photocatalyst, sunlight, O2 and H2O [5, 6]. Over the past few decades, many attempts have been made to develop a large amount of photocatalysts for the field of environmental remediation, including metal-based photocatalysts (e.g., TiO2, Bi4O5Br2, SrTiO3, (BiO)2CO3, Ag3PO4, WS2, Au/TiO2 and Ag/AgCl) [7-10], polymeric photocatalysts (e.g., g-C3N4, poly(diphenylbutadiyne), and polyimide) [11-13], and elemental-based photocatalysts (e.g., Bi, Bi/TiO2, S, and P) [14-16]. However, most of the above-mentioned photocatalysts are still far from satisfactory because of their low sunlight harvesting ability, high charge carriers recombination and poor photocatalytic stability. Therefore, there is an urgent need to develop novel photocatalysts with highly efficient and stable photoactivity.

Very recently, Li et al. prepared SrSn(OH)6 via a facile homogeneous precipitation method, and the as-obtained SrSn(OH)6 samples exhibited excellent UV photocatalytic performance for the degradation of benzene and rhodamine B [17]. However, to the best of our knowledge, application and reaction mechanism of SrSn(OH)6 for photocatalytic removal of air pollutants has not been reported. Herein, SrSn(OH)6 photocatalyst was synthesized by a facile chemical precipitation method in water bath. More importantly, this study focuses on photocatalytic NOx removal over SrSn(OH)6. Interestingly, SrSn(OH)6 exhibited high photocatalytic activity and good stability in the removal of NO under the irradiation of UV light. In addition, on the basis of in situ DRIFTS investigations, the detailed reaction mechanism during the photocatalytic oxidation of NOx was proposed.

Details of SrSn(OH)6 synthesis, characterization method, photocatalytic activity evaluation method, and in situ DRIFTS investigation method (S4) are described in Supporting information.

As shown in Fig. 1a, the photocatalytic removal ratio of NO was up to 79.6% after ultraviolet light irradiation for 30 min with the reactive species, which is more efficient than other Bi-based and g-C3N4 based photocatalysts [15]. Especially, the SrSn(OH)6 shows significantly higher photocatalytic activity than that of BiOBr0.5I0.5/BiOI composite (removal rate of 36.2%) [18], oxygen vacancies-mediated TiO2 nanocrystals (removal rate of 45.0%) [19], Bi spheres/g-C3N4 nanohybrid (removal rate of 59.7%) [20], and defective β-Bi2O3 (removal rate of 52.0%) [21]. However, the concentration of generated NO2 rises as high as 195 ppb. Actually, the NO2 as intermediate shows more toxicity than NO, which could cause secondary pollution of atmosphere. Therefore, highly efficient inhibition of the toxic NO2 generation should be addressed in the future work for practical application of SrSn(OH)6. In addition, SrSn(OH)6 only displays slight loss of photocatalytic activity even after five recycling tests (Fig. 1b). The gradual generation of intermediates and final products may occupy the active sites of SrSn(OH)6, which results in the slight loss of photocatalytic activity. These results indicate that SrSn(OH)6 has excellent activity and good stability during the photocatalytic removal of NO.

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Fig. 1. (a) Photocatalytic activity and (b) recycling tests of SrSn(OH)6 for NO removal under ultraviolet light irradiation (λ = 280 nm).

XRD patterns in Fig. 2a shows that all the diffraction peaks can be indexed to the hexagonal structure of SrSn(OH)6 (JCPDS card No. 22-1442) [17]. As can be seen from Fig. 2b, the as-obtained SrSn(OH)6 demonstrates an stick-like structure formed by stacking of large plates and particles size.

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Fig. 2. (a) XRD patterns and (b) SEM image of SrSn(OH)6.

As shown in Fig. S5a (Supporting information), SrSn(OH)6 featured strong light absorption in the ultraviolet light region. Moreover, the band gap energy (Eg) of SrSn(OH)6 which can be determined through the plots of (αhν)1/2 versus photo energy (Fig. S5b Supporting information) is 3.53 eV. In addition, the conduction band (CB) and valence band (VB) position of SrSn(OH)6 can be calculated from the relationship ECB = X − E0 − 0.5Eg and Eg = EVB − ECB. Thus, the calculated ECB and EVB of SrSn(OH)6 were 0.19 and 3.72 eV, respectively.

As shown in Figs. 3a-c, the ESR signals of O2•−, •OH and 1O2 were increased with prolonged irradiation time, suggesting that O2•−, •OH and 1O2 are the main active species during the photocatalytic reaction. Fig. 3d shows that photogenerated electrons can be rapidly consumed to participate in he generation of abundant active species under the ultraviolet light.

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Fig. 3. ESR spectra of radical adduct trapped by (a) DMPO-O2•−, (b) DMPO-•OH, (c) 4-oxo-TEMP-1O2 and (d) TEMPO-e− over SrSn(OH)6 under ultraviolet irradiation (λ = 280~360 nm).

Based on the mentioned above, the valence band position of SrSn(OH)6 is 3.72 eV, indicating that the hole oxidation potential of SrSn(OH)6 is much higher than those of OH−/•OH (1.99 eV) and H2O/•OH (2.37 eV). Therefore, the •OH radicals could be directly generated from the oxidation of OH− (Eq. 1) or H2O (Eq. 2). And besides, the conduction band position of SrSn(OH)6 is only 0.19 eV, hence, the e− reduction potential of SrSn(OH)6 is obviously more positive than that of O2/O2•− (‒0.33 eV). Thus, the e− reduction potential of SrSn(OH)6 could not directly reduce O2 to O2•− radicals. The main active species can be formed through a series of reactions as follows: Eqs. 1-6. Especially, the 1O2 radicals can be produced by the further oxidation of O2•− radicals with photo-generated holes (Eq. 5) [22].

(1)
(2)
(3)
(4)
(5)

Fig. 4a shows in situ DRIFTS spectra of the adsorption of NO on the surface of SrSn(OH)6. The peaks at 1148 and 1190 cm−1 can be assigned to the original NO. The peaks at 1320, 1345, 1368, 1394 and 1415 cm−1 can be assigned to NO2, which is the preliminary oxidation product during the adsorption process (Eq. 6). The peaks at 1464 cm−1 can be assigned to NO2−, other peaks at 1480, 1515, 1540 and 1559 cm−1 can be attributed to NO3−, which is the subsequent reaction product of NO2 and H2O (Eq. 7). These results indicate that the NO2, NO2− and NO3− yield increased rapidly during the adsorption stage in the NO and O2 environment [23, 24].

(6)
(7)
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Fig. 4. In situ DRIFTS spectra of the adsorption (a) and photocatalytic removal (b) of NO on the surface of SrSn(OH)6.

After the light was turned on Fig. 4b), a new peak appeared rapidly at 1351 cm−1 during the irradiation process, which is regarded as the oxidation of NO2 and NO into NO3− by •OH, O2•− and 1O2− radicals (Eqs. 8-11. In addition, the hole oxidation potential of SrSn(OH)6 is higher than those of EφHNO3/NO (0.94 eV vs. NHE), EφHNO2/NO (0.99 eV vs. NHE), and EφNO2/NO (1.03 eV vs. NHE); hence, the photo-generated holes of SrSn(OH)6 could directly oxidize NO to NO2, NO2− and NO3− (Eq. 12). As the illumination time was prolonged, the intensities of the peaks at 1515, 1540 and 1555 cm−1 gradually decreased. However, the new peak (1411 cm−1) of NO2 gradually became stronger, which reflects the high yield of NO2 owing to the reaction of NO3− and NO (Eq. 13) [25-26].

(8)
(9)
(10)
(11)
(12)
(13)

Based on the ESR and the in situ DRIFTS results, the photocatalytic NO oxidation mechanisms were provided in Fig. 5. The roles of O2•−, •OH and 1O2 in NO oxidation were illustrated.

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Fig. 5. The proposed photocatalytic NO oxidation mechanisms.

In summary, the SrSn(OH)6 photocatalyst with stick-like structure was synthesized by a simple chemical precipitation method in water bath. The as-obtained SrSn(OH)6 exhibited excellent photocatalytic activity and good stability. Interestingly, it was found that SrSn(OH)6 photocatalyst demonstrated the ability to generate the main active species of O2•−, •OH and 1O2 and could oxidize NO into nitrate. To clarify the adsorption and reaction mechanism, the intermediates and final products that distributed on the surface of SrSn(OH)6 were determined and analyzed by in situ DRIFTS. And thus the photocatalytic NO oxidation mechanism on SrSn(OH)6 was proposed. The present work could provide new insights into the SrSn(OH)6 with high activity for photocatalytic NOx removal.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This research is financially supported by the National Natural Science Foundation of China (No. 51708078), the Science and Technology Research Program of Chongqing Municipal Education Commission (No. KJZD-K201900502), and the Natural Science Foundation of Chongqing (No. 2018jcyjA1040), the Innovative Research Team of Chongqing (No. CQYC201903221).

Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2021.07.065.

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