(560gi) Surface Redox-Acid Pair Sites Required for Achieving High Performance for the Selective Catalytic Reduction of NOx with NH3

Selective catalytic reduction of NO_x with NH₃ (NH₃-SCR) is a well-established technology for NO_x pollution control [1]. Different metal oxides, such as CeO₂, Fe₂O₃etc., which have appropriate reduction-oxidation (redox) ability, have attracted much attention for NO_x reduction due to their high NH₃-SCR activity and N₂ selectivity. The NH₃-SCR performance of CeO₂-based and Fe₂O₃-based catalysts can be remarkably enhanced by introducing solid acid components such as tungstate and niobate, which is mainly due to the effective integration of excellent redox ability from CeO_x or FeO_x sites and optimal acidity from WO_x or NbO_x components [2-5]. Previous studies have reported that the local mixed oxide structures, e.g. surface Fe-O-W and Ce-O-Nb bonds or linkages, were beneficial to superior NH₃-SCR activity and N₂ selectivity [2,4,5]. However, the detailed synergetic effect between redox sites and acid sites has not been systematically studied, namely, the effect of surface redox-acid pair sites for NH₃-SCR reaction.

In order to study the critical role of redox-acid pair sites for NH₃-SCR reaction, a series of binary oxide catalysts (CeO₂-WO₃, CeO₂-Nb₂O₅, Fe₂O₃-WO₃ and Fe₂O₃-Nb₂O₅) were prepared by facile impregnation method, in which the redox components were loaded onto acid supports or the acid components were loaded onto reducible oxide supports. The amount of redox-acid pair sites on the catalyst surface were fine-tuned by adjusting the loading of acid components or redox components. The effect of redox-acid pair sites on NH₃-SCR performance was studied in detail. Catalysts were characterized by XRD, Raman spectra, H₂-TPR, NH₃/NO_x-TPD as well as in situ DRIFTS, and the results indicated that the catalysts exhibited the optimal NH₃-SCR activity and N₂ selectivity when the catalysts possessed the most abundant redox-acid pair sites. Based on this research, a comprehensive universal mechanism contributing to high deNO_x performance in NH₃-SCR reaction resulting from highly desired redox-acid pair sites is clearly elucidated, providing theoretical guidance to the design of future highly efficient NH₃-SCR catalytic systems.

References

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[2] R Qu, X Gao, K Cen, et al. Relationship between structure and performance of a novel cerium-niobium binary oxide catalyst for selective catalytic reduction of NO with NH₃. Appl. Catal. B: Environ., 142: 290-297, 2013.

[3] W Shan, F Liu, H He, et al. Novel cerium-tungsten mixed oxide catalyst for the selective catalytic reduction of NO_x with NH₃. Chem. Commun., 47(28): 8046-8048, 2011.

[4] F Liu, W Shan, Z Lian, et al. The smart surface modification of Fe₂O₃ by WO_x for significantly promoting the selective catalytic reduction of NO_x with NH₃. Appl. Catal. B: Environ., 230: 165-176, 2018.

[5] Z Ma, X Wu, Z Si, et al. Impacts of niobia loading on active sites and surface acidity in NbO_x/CeO₂-ZrO₂ NH₃-SCR catalysts. Appl. Catal. B: Environ., 179: 380-394, 2015.