Elsevier

Catalysis Communications

Volume 135, February 2020, 105751
Catalysis Communications

Short communication
A multiple-active-site Cu/SSZ-13 for NH3-SCO: Influence of Si/Al ratio on the catalytic performance

https://doi.org/10.1016/j.catcom.2019.105751Get rights and content

Highlights

  • Acidic property and Cu2+ species of Cu/SSZ-13 are systemically tuned by varying Si/Al ratio.

  • Cu/SSZ-13 (Si/Al-20) shows excellent NH3-SCO activity due to the proper ratio of CuO and Cu2+ ions.

  • Cu/SSZ-13 with CuO and isolated Cu2+ ions acting as multiple active sites follows i-SCR mechanism.

Abstract

The Cu/SSZ-13 catalyst can provide multiple active sites for the ammonia selective catalytic oxidation (NH3-SCO), the influence of which can be tuned by varying the Si/Al ratio. The latter parameter was found to influence the surface acidic property and the Cu species distribution, which are critical for NH3 adsorption and activation. It was found that both CuO and isolated Cu2+ ions coexist in Cu/SSZ-13 catalyst. The selected Si/Al-20 sample with the highest content of CuO species and abundant acidity exhibited the best NH3-SCO performance.

Introduction

The NH3-slip in upstream NH3-SCR systems will lead to secondary atmospheric pollution. Therefore, it is necessary to eliminate surplus NH3 and selective catalytic oxidation (SCO) reaction over effective catalysts would be preferable. Currently, Pt/Al2O3 has been used as ammonia-slip catalyst in heavy-loaded diesel automobiles. Unfortunately, the low N2-selectivity led to the formation of undesired products such as N2O and NO [1,2]. To address this problem, bi-functional catalysts were designed by adding transition metal, including Cu and Fe. These bi-functional catalysts, such as Pt/Cu/Al2O3 [3,4], followed the internal selective catalytic reduction (i-SCR) mechanism. The noble or rare earth metal (e.g. Pt, Ag) over oxidized NH3 to form NOx. Then, the generated NOx can be selectively reduced with surplus NH3 on transition metal (e.g. Cu, Fe) [5,6]. However, new problems such as the strong oxidizing ability and high cost of noble metals remain. Therefore, it is intensively desirable to design bi-functional catalysts with single low-cost transition metal. Copper-based catalysts were reported to be relatively excellent NH3-SCO catalysts compared with other transition metal catalysts. In particular, the Cu-loaded small-pore chabazite (CHA) zeolites (i.e. Cu/SSZ-13) exhibited outstanding NH3-SCO activity and N2-selectivity among copper-based catalysts [7]. However, the mechanism of NH3-SCO reaction over Cu/CHA catalysts remains unclear and needs to be investigated in detail.

Cu-exchanged CHA zeolite has been investigated extensively as NH3-SCR catalysts. The Si/Al ratio would impact the surface acidic property and Cu species of Cu/CHA, which further affect their NH3-SCR performance. For example, Gao et al. [8] proposed that the location of Cu2+ ions and their redox property can be adjusted by varying the Si/Al ratio of Cu/SSZ-13 catalysts. Yu et al. [9] tuned the Si content of Cu/SAPO-34 and evaluated the effect of acidity on NH3-SCR activity. Besides, it was reported that the acidic property and type of Cu species were also of vital importance to the NH3-SCO reaction [10]. However, few works focused so far on the relationship between the Cu/CHA catalyst structure and NH3-SCO performance. Especially, the influence of Si/Al ratio on the Cu/SSZ-13 catalyst for NH3-SCO reaction has not been reported yet.

In this work, the dispersion of Cu species, surface acidic and redox properties over Cu/SSZ-13 catalyst were well tuned by varying the Si/Al ratio. The synthesized novel Cu/SSZ-13 catalyst (especially Si/Al-20) exhibits excellent NH3-SCO activity and high N2-selectivity in a wide temperature window. The structure-activity relationship is systematically discussed. Additionally, the respective function of different types of Cu species are distinguished to explain the reaction route during the NH3-SCO process. The Cu/SSZ-13 catalyst would be a potential catalyst in the practical application of ammonia oxidation process.

Section snippets

Catalysts preparation

Cu/SSZ-13 catalysts with different Si/Al ratio were prepared by the wet impregnation method, where “Si/Al ratio” represents the n(SiO2)/n(Al2O3) molar ratio. The H/SSZ-13 supports were purchased from Zhenjiang Spring Breeze Environmental Material Company. The precursor of Cu was Cu(NO3)2·3H2O and the Cu loading was 10 wt%. After impregnation and drying (100 °C, 12 h) procedures, the samples were calcined at 550 °C for 6 h in air. According to their Si/Al ratio (shown in Table S1), the three

Results and discussion

The characteristic powder X-ray diffraction peaks of CuO (2θ = 35.6° and 38.7°) were observed as shown in Fig. S1. The typical CHA crystal structure in all three samples remained intact after the wet-impregnation process. Besides, the N2 sorption isotherms (Fig. S2) of both catalysts and supports with different Si/Al ratio are all type-I, indicating the characteristics of microporous materials. This means that the porous structure seems that barely changed. The BET specific surface area, listed

Conclusions

Cu/SSZ-13 catalyst possesses multiple active sites (CuO and isolated Cu2+ ions), which attributed to its excellent NH3-SCO activity and N2-selectivity. The acidic and redox properties of the solid can be tuned by adjusting the Si/Al ratio. Moreover, the i-SCR mechanism of NH3-SCO reaction has been confirmed on Cu/SSZ-13 catalyst for the first time to our knowledge. According to this mechanism, CuO and isolated Cu2+ ions are active sites in the oxidation step and SCR step, respectively. Current

Acknowledgements

This study was financially supported by the National Key Research and Development Program of China (Grant No. 2016YFE0126600 and 2016YFC0209203).

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