Oxygen defect-induced NO− intermediates promoting NO deep oxidation over Ce doped SnO2 under visible light
Graphical abstract
The adsorbed NO at OVs was activated into NO− intermediates, and then oxidized by the activated oxygen species induced by visible light into NO2− and NO3−.
Introduction
Nowadays, people have paid more attention to environmental issues. Nitric oxide (NO) is the main source of environmental problems such as photochemical smog, increased PM2.5, acid rain, ozone depletion, greenhouse effect (indirect impact) and would cause a series of diseases to humans (chronic pharyngitis, chronic bronchitis, and tooth erosion), even at the concentration of sub-ppm or ppb levels [1,2]. Photocatalysis is an extremely effective technology to solve environmental pollution by using solar energy [3,4]. Photocatalytic NO oxidation is one of the most important technologies to control toxic gas pollution [5,6]. In this reaction, it is necessary not only to increase the removal rate of NO but also to reduce the generation rate of toxic by-product NO2 because NO2 is more toxic than NO [7].
In recent years, metal oxide semiconductors have received widespread attention because of their easy preparation, good photostability and chemical stability, wide source and low cost. [8]. Unfortunately, due to their wide bandgap, metal oxides such as TiO2, CeO2 and ZnO can only absorb a small fraction of sunlight [[9], [10], [11]]. Generally, three strategies have been proposed to improve the utilization of sunlight and reduce the recombination of photoexcited electrons and hole pairs [12]. The first is the method of designing heterogeneous structures. Xu et al. demonstrated that CdS/TiO2 heterojunction photocatalyst could reduce the recombination of surface charges and significantly improve the hydrogen evolution performance of photocatalyst [13]. An alternative way is to doping other heteroatoms into the metal oxide matrixes [14]. Dai et al. reported that the N doped TiO2 showed a great light response and high NO oxidation performance since N could be used as an impurity level to narrow the bandgap and absorb more visible light [15]. Another strategy involves introducing vacancy defects. Dong et al. found that the O-defected Bi2WO6, Ba-defected BaSO4 and N-defected g-C3Nx all showed enhanced photocatalytic activity for NO removal [[16], [17], [18]]. Furthermore, Yu et al. demonstrated that CeO2 nanorods treated under an inert atmosphere could generate more Ce3+ ion and oxygen vacancies (OVs) than those treated under air conditions. These OVs would bring the defect energy levels to reduce the recombination of photo-generated electrons and hole and to promote visible light absorption, thereby enhancing photocatalytic water oxidation [19].
For the reaction of photocatalytic oxidation of NO, the interaction between the reactant and the photocatalyst was crucial [[20], [21], [22]]. Zhang et al. have reported that OVs could greatly promote the adsorption of O2 and NO by DFT calculations [23]. Specifically, O2 and NO were activated by stretching the bond length when they were adsorbed on OVs. NO was mainly converted to NO− intermediate during the adsorption, owing to the localized electrons around the OVs, and O2 would get electrons to form O2- radicals [24].
Tin dioxide (SnO2) is an n-type semiconductor with high stability and excellent electronic and optical properties. SnO2 has a higher positive valence band than TiO2, so it has a stronger oxidizing ability and can more completely oxidize pollutants. Ceria is widely applied in the environmental catalytic field due to its high oxygen storage/release efficiency associated with the formation of oxygen vacancy and excellent reduction behavior resulted from the low redox potential between Ce3+ and Ce4+. So, we constructed Ce-doped SnO2 photocatalysts with different OVs concentrations by treated under air or argon atmosphere. The concentrations of OVs were determined by the experimental results of EPR and XPS. It was proved that Ce doped SnO2 with more OVs showed better activity for NO photocatalytic oxidation removal. To further understand the connection between reaction molecules and photocatalysts, the adsorption modes of O2 and NO over Ce-doped SnO2 was studied by density functional theory calculations (DFT). The adsorption ways, behavior, and intermediates of NO and O2 were finely identified by in-situ DRIFTS. Finally, the mechanism of photocatalytic NO oxidation over Ce doped SnO2 was proposed through a series of characterization.
Section snippets
Synthesis of catalysts
The Ce doped SnO2 samples were obtained by a simple precipitation method [28]. Typically, 8 mmol SnCl4·5H2O was dissolved in 160 mL H2O2 solution in a 250 mL beaker with magnetic stirring. Then, an ammonia solution was introduced to adjust the pH of the solution to 9. Subsequently, 0.4 mmol Ce(NO3)3·6H2O powders were added into the solution to get the suspension, and it was kept in an oil bath for 5 h at a temperature of 95 °C. The resulting yellow precipitate was washed with deionized water to
Structural characteristics
We chose 5 % Ce-SnO2 catalyst calcined in air and argon as the research object in this study because of their best catalytic activity in catalysts with different cerium doping concentrations (Fig. S2b). Fig. 1a showed the XRD results of the as-synthesized samples. The SnO2 could be well indexed as a tetragonal structure corresponding to the JCDPS No. 00-001-0657. The diffraction peaks at 2θ values of 26.67, 33.93, 38.10, 52.23 and 66.23 could be assigned to the (1 1 0), (1 0 1), (2 0 0), (2 1
Conclusions
In summary, we constructed Ce doped SnO2 photocatalyst treated under air or argon atmosphere with abundant OVs for adsorption and activation of O2 and NO, aiming to improve NO deep oxidation removal efficiency under different relative humidity. Based on the above results and analysis, the following conclusions could be drawn:
- (I)
Compared to the pure SnO2, Ce doped SnO2 performed better for NO photocatalytic oxidation. And the Ce-SnO2-Ar samples exhibited excellent NO removal performance under
CRediT authorship contribution statement
Xinjie Song: Conceptualization, Investigation, Formal analysis, Writing - review & editing. Guodong Qin: Investigation. Gang Cheng: Investigation, Formal analysis, Software. Wenjie Jiang: Investigation, Formal analysis. Xun Chen: Resources, Data curation. Wenxin Dai: Conceptualization, Funding acquisition, Formal analysis, Writing - review & editing. Xianzhi Fu: Funding acquisition, Formal analysis, Writing - review & editing.
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (no. 21872030).
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