Elsevier

Applied Catalysis B: Environmental

Volume 252, 5 September 2019, Pages 230-239
Applied Catalysis B: Environmental

Phosphorus modification to improve the hydrothermal stability of a Cu-SSZ-13 catalyst for selective reduction of NOx with NH3

https://doi.org/10.1016/j.apcatb.2019.04.037Get rights and content

Highlights

  • Phosphate loading enhances the hydrothermal stability of Cu-SSZ-13 with low Si/Al ratio.

  • The silicoaluminophosphate interface retard further degradation of the CHA structure.

  • Phosphate protects the isolated Cu2+ with retaining TFAl in the framework.

Abstract

Phosphorus is introduced to modify the Cu-SSZ-13 catalyst via incipient wetness impregnation, with P/Cu-SSZ-13 = 1 and 2 wt.%, and Si/Al = 4, for selective catalytic reduction of NOx with NH3. N2 physisorption and XRD results show the incorporation of phosphorus as phosphate acid enhanced the hydrothermal stability of Cu-SSZ-13 significantly, during hydrothermal aging in 10 vol.% H2O/air at 750 °C for 16 h. NMR and Raman results suggest that phosphate ions migrate and coordinate with the framework-bonded Al species, forming a framework silicoaluminophosphate interface, thus impeding further dealumination and structure collapse. Before the hydrothermal aging, the isolated Cu2+ ions partly interact with the phosphate ions, forming Cu-phosphate species and reducing the SCR performance. Nevertheless, the appropriate content of phosphate ions can prevent the structure collapse caused by the hydrothermal aging, remaining the isolated Cu2+ ions as well as excellent SCR performance.

Introduction

Various catalytic technologies have been developed to eliminate the emission of engines [[1], [2], [3]]. For diesel vehicles, the emission of hydrocarbons and carbon monoxide is controlled by a diesel oxidation catalyst, whereas the particulate matter is removed with a diesel particulate filter (DPF) [4]. Subsequently, nitrogen oxides (NOx) can be removed with either a lean NOx trap for light-duty vehicles, or a NH3 selective catalytic reduction (NH3-SCR) unit for heavy-duty ones [5,6]. Typically, the DPF requires regeneration in hot vapor at above 650 °C, inducing hydrothermal condition to the SCR catalysts in the downstream [[7], [8], [9]]. Therefore, a high hydrothermal durability is required for the SCR catalyst to achieve effective NOx emission control [[9], [10], [11], [12], [13]].

Cu−CHabazite (Cu−CHA) zeolites, including Cu-SSZ-13 and Cu-SAPO-34, have been successfully commercialized as catalysts for NH3-SCR reaction, to meet the stringent standards for diesel NOx emission in both North America and Europe, marking a significant breakthrough of catalytic technology in recent years [[14], [15], [16]]. Better hydrothermal stability has been shown with the Cu−CHA catalysts, as compared to the other zeolite-based catalysts, e.g., Cu-ZSM-5, Cu-Beta and Cu-Y [17]. One well-accepted explanation is that the unique topology and the small pore size in Cu−CHA zeolites prevent the detached Al(OH)3 moieties from escaping the CHA cavity [4,11].

Cu-SSZ-13 has been found inferior to Cu-SAPO-34 in the hydrothermal stability [18,19]. For the next generation Cu-SSZ-13 catalyst, stronger hydrothermal stability should be achieved to prolong the lifetime and lower the cost [17,[20], [21], [22], [23]]. One approach is to maintain the content of two adjacent framework Al sites (Al pairs), such as Al-O-Si-O-Al or Al-O-(Si-O)2-Al, stabilizing the isolated Cu2+ active sites in the six-membered rings (6MR) in the catalyst under hydrothermal conditions [[24], [25], [26], [27], [28]]. High content of the Al pairs requires a low Si/Al ratio, typically below 6 [22,29,30]. Nevertheless, the framework of the zeolite can be collapsed more easily with the low Si/Al ratio, caused the hydrothermal condition [31].

Phosphorus (P) is well known for improving the hydrothermal stability of H-ZSM-5 as well as the other zeolites with similar multidimensional 10 MR structures, such as ZSM-11, MCM-22, ITQ-13 and IM-5 [[32], [33], [34], [35], [36]]. Meanwhile, phosphorus in SAPO-34 has been proposed critical to stabilize the CHA type framework in hydrothermal condition [37]. It is therefore interesting to investigate the effect of P on the hydrothermal stability of the other CHA type catalyst, i.e. Cu-SSZ-13 with a low Si/Al ratio.

In this work, phosphorous is incorporated to modify the Cu-SSZ-13 catalyst with Si/Al ratio = 4. The effect of phosphorus on the framework structure, the cupric sites and the SCR performance will be discussed.

Section snippets

Chemicals

Copper(II) sulfate (CuSO4 < 99 wt.%), sodium aluminate (NaAlO2 < 98 wt.%) and phosphoric acid (80 wt.%) were purchased from Guangfu (Tianjin). Tetraethylenepentamine (TEPA < 98 wt.%,) and sodium hydroxide (NaOH < 98 wt.%) were purchased from Aladdin Industrial Corporation (Shanghai). Ammonium nitrate (NH4NO3 < 99 wt.%) was purchased from Yuanli (Tianjin). Colloidal silica (JN-30, SiO2 = 30 wt.%) was purchased from Haiyang (Qingdao). All the aqueous solutions were prepared using ultra-pure water

Effect of phosphorus on the SCR performance

The activity of the as-prepared and doped Cu-SSZ-13 was compared in Fig. 1a. The NOx conversion was around 20% with the as-prepared Cu-SSZ-13 at 150 °C, and increased rapidly to 78% at 200 °C. The conversion remained above 90% from 250 to 450 °C, then decreased with the further increase of the temperature, to approximately 77% NOx conversion at 550 °C. The P loading caused decrease of the activity at low temperatures, with the NOx conversion of only 67% and 56% at 200 °C for the P1Cu and P2Cu

Hydrothermal aging effect on the structure

The hydrothermal aging at 750 °C for 16 h destroyed the Cu-SSZ-13 structure completely for the sample without P loading (the sample Cu-H), according to the XRD pattern in Fig. 2a. The surface area and the pore volume of the sample lost almost completely as shown in Table 3.

It has been well documented that Brønsted acid sites (i.e., -Si-(OH)-Al-) in zeolites are the most vulnerable to H2O attack during hydrothermal aging, resulting in the collapse of the zeolite structure with dealumination [45

Conclusion

Doping with P as phosphate acid detach a part of TFAl to EFAl in the Cu-SSZ-13. Nevertheless, the formation of a framework silicoaluminophosphate interface retards the further dealumination of the Cu-SSZ-13 sample with phosphorus doping, retaining the CHA structure intact.

For the cupric sites, the isolated Cu2+ ions react partly with the phosphate, forming the Cu-phosphate species in the PxCu samples. The Cu-phosphate species may be inactive for NH3-SCR reaction due to the very high redox

Acknowledgments

This work was supported in part by the Program of Introducing Talents to the University Disciplines under file number B06006, and the Program for Changjiang Scholars and Innovative Research Teams in Universities under file number IRT 0641.

References (69)

  • H. Zhao et al.

    Catal. Today

    (2017)
  • J.H. Kwak et al.

    J. Catal.

    (2012)
  • X. Dong et al.

    Catal. Today

    (2015)
  • X. Chen et al.

    SAE Int. J. Eng.

    (2013)
  • L. Xie et al.

    Appl. Catal. B: Environ.

    (2015)
  • J.H. Kwak et al.

    J. Catal.

    (2010)
  • D.W. Fickel et al.

    Appl. Catal. B: Environ.

    (2011)
  • Y. Shan et al.

    Catal. Today

    (2019)
  • L. Ma et al.

    Appl. Catal. B: Environ.

    (2014)
  • J. Wang et al.

    Appl. Catal. B: Environ.

    (2017)
  • L. Ma et al.

    Chem. Eng. J.

    (2013)
  • D. Wang et al.

    Appl. Catal. B: Environ.

    (2015)
  • F. Gao et al.

    J. Catal.

    (2015)
  • Y.J. Kim et al.

    J. Catal.

    (2014)
  • S. Prodinger et al.

    Appl. Catal. B

    (2017)
  • J. Luo et al.

    Catal. Today

    (2016)
  • S. Han et al.

    Appl. Surf. Sci.

    (2017)
  • Z. Zhao et al.

    Appl. Catal. B: Environ.

    (2017)
  • P. Zeng et al.

    J. Energy Chem.

    (2014)
  • X. Wang et al.

    Microporous Mesoporous Mater.

    (2012)
  • A. Corma et al.

    Appl. Catal. A Gen.

    (2013)
  • J. Zhuang et al.

    J. Catal.

    (2004)
  • A. Buchholz et al.

    Microporous Mesoporous Mater.

    (2002)
  • N. Yamanaka et al.

    Microporous Mesoporous Mater.

    (2012)
  • K. Xie et al.

    Appl. Catal. B: Environ.

    (2019)
  • I. Lezcano-Gonzalez et al.

    Appl. Catal. B: Environ.

    (2014)
  • K. Damodaran et al.

    Microporous Mesoporous Mater.

    (2006)
  • N. Xue et al.

    J. Catal.

    (2007)
  • G. Lischke et al.

    J. Catal.

    (1991)
  • A.G. Stepanov
    (2016)
  • Z. Yan et al.

    J. Mol. Catal. A Chem.

    (2003)
  • T. Blasco et al.

    J. Catal.

    (2006)
  • L. Wondraczek et al.

    J. Non-Cryst. Solids

    (2013)
  • F. Gao et al.

    J. Catal.

    (2013)
  • Cited by (0)

    View full text