Mechanistic Pathways of Binuclear Copper Complexes in Cu-SSZ-13 During Low Temperature NOx Selective Catalytic Reduction with Ammonia

Casey  Jones, Purdue University

Mechanistic Pathways of Binuclear Copper Complexes in Cu-SSZ-13 During Low Temperature NOx Selective Catalytic Reduction with Ammonia

 

Ishant Khurana1, Casey B. Jones1, Yujia Wang2, Sichi Li2, Arthur J. Shih3, Krishna Kamasamudram3, Aleksey Yezerets3, W. Nicholas Delgass1, Jeffrey T. Miller1, William F. Schneider2, Fabio H. Ribeiro1*, Rajamani Gounder1*

1Charles D. Davidson School of Chemical Engineering, Purdue University, 480 Stadium Mall Drive, West Lafayette, IN 47907, USA

2Department of Chemical and Biomolecular Engineering, University of Notre Dame, 182 Fitzpatrick Hall, Notre Dame, IN 46556, USA

3Cummins Inc., 1900 McKinley Avenue, MC 50183, Columbus, IN 47201, USA

 

The selective catalytic reduction (SCR) of NOX (x=1,2) with NH3 using Cu-exchanged chabazite (CHA) zeolites at low temperatures (<573 K) proceeds via a CuII / CuI redox cycle. NO and NH3 act as co-reductants of NH3-solvated monomeric CuII complexes to form reduced [CuI(NH3)2]+ species. Diffusion of monomeric [CuI(NH3)2]+ species into adjacent cages in CHA enables formation of dimeric CuII oxo-bridged complexes. In this work, we use steady-state and transient kinetics and X-ray absorption spectroscopy to investigate the reactivity of binuclear CuII-oxo complexes over a wide range of Cu-CHA sample compositions and operating conditions, in order to propose possible reaction pathways and intermediates that can be probed by DFT. In operando X-ray absorption spectroscopy (XAS) was used to monitor the oxidation state of Cu during the reduction of site-isolated CuII species in NO and NH3, and the rates of reduction were found to be independent of sample composition and similar to steady-state SCR turnover rates, consistent with previously proposed mechanisms for CuII reduction. After subsequent oxidation in O2, the reduction of binuclear CuII-oxo complexes in NO and NH3 results in the full reduction to CuI and a consumption stoichiometry of 2 equivalents of NO per oxidized Cu, consistent with the overall standard SCR reaction stoichiometry. Using DFT calculations, 4 possible reaction pathways from the binuclear CuII state were interrogated and all were found to be plausible based on the computed energy landscape. Additional experimental kinetic and spectroscopic data (including in situ IR and in operando XAS) were collected to test these hypothesis regarding the dominant reaction pathways involved in the SCR mechanism.