Elsevier

Molecular Catalysis

Volume 485, April 2020, 110822
Molecular Catalysis

Core-shell structure effect on CeO2 and TiO2 supported WO3 for the NH3-SCR process

https://doi.org/10.1016/j.mcat.2020.110822Get rights and content

Highlights

  • The synergy between redox property and acidity enhances SCR performance.

  • W/Ti@Ce shows an outstanding NOx→N2 yield of above 95 %.

  • The core-shell catalysts W/Ti@Ce exhibits better resistance to SO2 poisoning.

  • The core-shell structure is conductive to the intimate interaction between Ce and Ti.

Abstract

A series of CeTi samples (CeO2/TiO2, TiO2@CeO2, TiO2/CeO2, and CeO2@TiO2) was prepared and modified by tungsten. The 5 %WO3/TiO2@CeO2 (W/Ti@Ce) catalyst exhibited the best NH3-SCR performance (NO conversion and N2 yield above 95 % at 250–500 ºC) due to the synergistic effect of redox ability and acidity. The techniques of XPS, H2-TPR, and NH3-TPD were introduced to reveal the effect of Ce3+ atom contents, redox properties and acidity on catalytic performance. W/Ti@Ce shows good redox ability, which is conducive to the low-temperature selective catalytic reduction (SCR) reaction. NH3-TPD results show that W/Ti@Ce have a large amount of medium- and strong-strength acids, which are conducive to high-temperature SCR reaction. Therefore, the core-shell catalyst has the optimal synergistic effect of redox property and acidity, which is in favor of the excellent NH3-SCR performance. Furthermore, the W/Ti@Ce with a core-shell structure displayed better SO2 resistance due to the stronger interaction between Ce and Ti.

Introduction

Nitrogen oxide (NOx) emissions have dramatically increased with the development of the industrialization and the application of fossil resources. To date, many de-NOx technologies have been developed to control this atmospheric pollution, such as NOx decomposition, NOx storage reduction (NSR), and selective catalytic reduction (SCR). [1] Among them, the NH3-SCR of NOx has been widely used and developed.

To cater the requirement for high NH3-SCR performance, the regulation of CeO2 and TiO2 with different morphologies and exposed crystal planes has been investigated in recent years. Wang et al. [2] reported that the (001) facets of TiO2 with unique features could enhance the thermal stability of CeO2. Lian et al. [3] studied the NH3-SCR for Nb loaded on CeO2 with different morphologies and found that more chemisorbed oxygens and acid sites, and better redox capability contributed to the NH3-SCR performance. Wang et al. [4] investigated ceria nanotube materials, with superior redox capacity, smooth redox cycle, and abundant acidity, which showed excellent NOx conversion and good tolerance to alkali metals. Chemical promoters have been used to improve the performance of NH3-SCR reaction, such as MoO3 [5], WO3 [6,7], Nb2O5 [8], NiO [9], and MnO2 [10]. Liu et al. [11] added transition metals (Co, Cu and Fe) into Ce-Ti mixed oxide to adjust the Ce3+ ratio and oxygen vacancy. Furthermore, Gao and co-workers [12] studied the effects of CeO2/TiO2 prepared by different methods on NH3-SCR reaction, and they found that high specific surface area and good redox capacity were important for catalytic performance. Although many methods have been used to regulate the performance of modified CeTi catalysts, the excellent catalyst structure can improve the properties of catalyst and promote the cooperative catalytic performance of each component.

In recent years, core-shell structure has been extensively used in NH3-SCR reaction for its special advantages, such as preventing the sintering of active sites at higher temperature and improving the catalytic performance through synergistic effects mainly occurring at the material interface. [13,14] Liu et al. [15] prepared amorphous CeO2@TiO2 catalysts and, compared with CeO2/TiO2, CeO2@TiO2 exhibited larger specific surface area, excellent redox property and acidity, and a strong resistance to H2O and SO2. These properties are conductive to the improvement of NH3-SCR performance. Meantime, Huang et al. [16] also reported that CeO2@TiO2 core-shell nanostructure catalysts presented high activity for larger surface area, more active sites, better NH3 chemisorption ability, and excellent redox activity. Although the above literatures have reported the application of CeTi core-shell catalysts in SCR reaction, the role of CeTi core-shell materials in catalytic reactions and the influence of core-shell structure on the properties of catalysts such as redox performance and acidity have never been well established.

In this study, we synthesized CeO2/TiO2, CeO2@TiO2, TiO2/CeO2, and TiO2@CeO2 supports and modified with 5 wt.% tungsten oxide. And these samples’ efficiency in NH3-SCR reaction was elucidated. The optimal structure of CeTi catalysts has been explored. Moreover, the role of core-shell structure as well as the synergistic behavior between redox property and acidity in NH3-SCR reaction was studied based on the corresponding physicochemical characterization. Furthermore, the resistance towards H2O and SO2 poisoning of 5 %WO3/CeO2/TiO2 and 5 %WO3/TiO2@CeO2 as well as the related mechanism were also investigated.

Section snippets

CeO2@TiO2 (CeO2-core@TiO2-shell) preparation

0.4 g CeO2 (Sinopharm) was dispersed into 380 mL ethanol (AR, Beijing Chemical Works). After the addition of 1.2 mL NH3·H2O (25 %, Sinopharm), ultrasound was performed for 30 min. Then, the suspension was transferred to a three-mouth flask. The flask was placed in a 70 °C water bath. Then, 20 mL ethanol solution containing 6 mL TBOT (36 %, Sigma) was added drop by drop to the three-mouth flask while stirring constantly for about 3 h. The solids were collected by centrifuging, were washed by

XRD analyses

The X-ray diffraction diagram of the CeTi supports (CeO2/TiO2, TiO2@CeO2, TiO2/CeO2, and CeO2@TiO2, Abbreviation: Ce/Ti, Ti@Ce, Ti/Ce, and Ce@Ti) and the W/CeTi catalysts (5 %WO3/CeO2/TiO2, 5 %WO3/TiO2@CeO2, 5 %WO3/TiO2/CeO2, and 5 %WO3/CeO2@TiO2, Abbreviation: W/Ce/Ti, W/Ti@Ce, W/Ti/Ce, and W/Ce@Ti) are shown in Fig. 1. Ce@Ti and CeO2 appeared as pure cubic CeO2 phase, of space group Fm-3 m (JCPDS reference no. 43-1002), while TiO2 presented as pure anatase phase, of space group l41/amd (JCPDS

Conclusion

In this work, Ce/Ti, Ti@Ce, Ti/Ce and Ce@Ti supports were prepared and modified by introducing tungsten. The NH3-SCR performance of these samples was studied and the water and sulfur resistances of W/Ti@Ce core-shell structure and W/Ce/Ti supported catalysts were also compared. The W/Ti@Ce displayed the best activity (NO conversion and N2 yield above 95 % at 250−500 °C) and SO2 resistance (NO conversion > 90 % for 8 h). From XRD results, Ce/Ti and Ti@Ce include anatase TiO2 and cubic CeO2

CRediT authorship contribution statement

Shanshan Liu: Writing - original draft, Data curation, Resources, Investigation, Formal analysis. Hao Wang: Software, Visualization. Ying Wei: Conceptualization, Validation, Methodology. Runduo Zhang: Writing - review & editing, Supervision, Project administration, Funding acquisition.

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.

Acknowledgements

The financial supports of the National Natural Science Foundation of China (No. 21976012 and U1862102), the Fundamental Research Funds for the Central Universities (XK1802-1, JD1903) are gratefully acknowledged.

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