Mechanistic Understanding of Cu-CHA Catalyst as Sensor for Direct NH3-SCR Monitoring: The Role of Cu Mobility

Peirong Chen; Valentina Rizzotto; Abhishek Khetan; Kunpeng Xie; Ralf Moos; Heinz Pitsch; Daiqi Ye; Ulrich Simon

doi:10.1021/acsami.8b22104

Mechanistic Understanding of Cu-CHA Catalyst as Sensor for Direct NH₃-SCR Monitoring: The Role of Cu Mobility

ACS Appl Mater Interfaces. 2019 Feb 27;11(8):8097-8105. doi: 10.1021/acsami.8b22104. Epub 2019 Feb 13.

Authors

Peirong Chen^{1

2

3}, Valentina Rizzotto^{2

3}, Abhishek Khetan^{3

4

5}, Kunpeng Xie^{2

3}, Ralf Moos⁶, Heinz Pitsch^{3

4}, Daiqi Ye¹, Ulrich Simon^{2

3}

Affiliations

¹ Guangdong Provincial Key Laboratory of Atmospheric Environment and Pollution Control, School of Environment and Energy , South China University of Technology , Guangzhou 510006 , China.
² Institute of Inorganic Chemistry , RWTH Aachen University , Landoltweg 1 , Aachen 52074 , Germany.
³ Center for Automotive Catalytic Systems Aachen , RWTH Aachen University , Aachen 52062 , Germany.
⁴ Institute for Combustion Technology , RWTH Aachen University , Templergraben 64 , Aachen 52056 , Germany.
⁵ Department of Mechanical Engineering , Carnegie Mellon University , 5000 Forbes Avenue , Pittsburgh , Pennsylvania 15213 , United States.
⁶ Department of Functional Materials and Bayreuth Engine Research Center (BERC) , University of Bayreuth , Bayreuth 95440 , Germany.

PMID: 30706712
DOI: 10.1021/acsami.8b22104

Abstract

The concept to utilize a catalyst directly as a sensor is fundamentally and technically attractive for a number of catalytic applications, in particular, for the catalytic abatement of automotive emission. Here, we explore the potential of microporous copper-exchanged chabazite (Cu-CHA, including Cu-SSZ-13 and Cu-SAPO-34) zeolite catalysts, which are used commercially in the selective catalytic reduction of automotive nitrogen oxide emission by NH₃ (NH₃-SCR), as impedance sensor elements to monitor directly the NH₃-SCR process. The NH₃-SCR sensing behavior of commercial Cu-SSZ-13 and Cu-SAPO-34 catalysts at typical reaction temperatures (i.e., 200 and 350 °C) was evaluated according to the change of ionic conductivity and was mechanistically investigated by complex impedance-based in situ modulus spectroscopy. Short-range (local) movement of Cu ions within the zeolite structure was found to determine largely the NH₃-SCR sensing behavior of both catalysts. Formation of NH₃-solvated, highly mobile Cu^I species showed a predominant influence on the ionic conductivity of both catalysts and, consequently, hindered NH₃-SCR sensing at 200 °C. Density functional theory calculations over a model Cu-SAPO-34 system revealed that Cu^II reduction to Cu^I by coadsorbed NH₃ and NO weakened significantly the coordination of the Cu site to the CHA framework, enabling high mobility of Cu^I species that influences substantially the NH₃-SCR sensing. The in situ spectroscopic and theoretical investigations not only unveil the mechanisms of Cu-CHA catalyst as sensor elements for direct NH₃-SCR monitoring but also allow us to get insights into the speciation of active Cu sites in NH₃-SCR under different reaction conditions with varied temperatures and gas compositions.

Keywords: Cu redox; Cu-ion movement; DFT calculation; NH3 solvation; in situ impedance spectroscopy; modulus.