The Reduction Half Cycle in the Low-T NH3-SCR of NOx over Cu-CHA Catalysts: Transient Kinetic Analysis and Mechanistic Study

Enrico  Tronconi, Politecnico di Milano

Enrico TRONCONI*¹, Federica GRAMIGNI ¹, Wenshuo HU ², Nicola USBERTI ¹, Nicole D. NASELLO ¹, Roberta VILLAMAINA ¹, Tommaso SELLERI ¹, Shaojun LIU ², Endre FENES ³, Hongfei MA ³, Kumar R. ROUT ⁴, Isabella NOVA ¹, Xiang GAO ², De CHEN ³

1 Laboratory of Catalysis and Catalytic Processes, Energy Dept., Politecnico di Milano, Italy
2 State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou, China
3 Chemical Engineering Dept., Norwegian University of Science and Technology, Norway
4 Kinetic and Catalysis, SINTEF Industry, Trondheim, 7491, Norway

*enrico.tronconi@polimi.it

Cu-CHA systems are currently the state-of-the-art catalysts for the Selective Catalytic Reduction (SCR) of NOx from Diesel vehicles. Great attention has been paid to the low-temperature reaction mechanism under Standard SCR conditions. While there is a general consensus about its redox nature, with the oxidation state of the active Cu cations changing from Cu2+ to Cu+ in the reduction half cycle (RHC) and from Cu+ to Cu2+ in the oxidation half cycle (OHC), the detailed catalytic scheme, as well as the operando speciation of the active sites, are still debated.

In this talk I will present and discuss novel kinetic and mechanistic features of the low-temperature (LT) RHC emerging from our recent fundamental work at Politecnico di Milano, which combines Transient Response methods, Chemical Trapping techniques, UV-vis NIR spectroscopy, CO titration as a probe reaction with Density Functional Theory and transient kinetic modelling [1 – 5]. For industrially representative catalyst formulations, we show that the RHC rates display a quadratic dependence on Cu2+, thus questioning Cu reduction mechanisms based on isolated Cu2+ ions. We propose, instead, a DFT-based Cu2+-pair mediated LT-RHC pathway, in which NO oxidative activation to mobile nitrite-precursor intermediates (HONO) accounts for Cu2+ reduction. These results highlight the role of di-nuclear Cu complexes not only in the oxidation part of the LT-SCR mechanism, as already widely accepted, but also in the RHC reaction cascade.

References

1. R. Villamaina, S. Liu, I. Nova, E. Tronconi, M. P. Ruggeri, J. Collier, A. York, and D. Thompsett, “Speciation of Cu Cations in Cu-CHA Catalysts for NH3 -SCR: Effects of SiO2 /Al2O3 Ratio and Cu-Loading Investigated by Transient Response Methods”, ACS Catalysis, 9 (2019) 8916–8927.
2. R. Villamaina, U. Iacobone, I. Nova, E. Tronconi, M.P. Ruggeri, J. Collier, D. Thompsett, “Low-T CO oxidation over Cu-CHA catalysts in presence of NH3: Probing the mobility of CuII ions and the role of multinuclear Cu species”, ChemCatChem, 12 (2020) 3843–3848.
3. N. Usberti, F. Gramigni, N. D. Nasello, U. Iacobone, T. Selleri, W. Hu, S. Liu, X. Gao, I. Nova, E. Tronconi, “An experimental and modelling study of the reactivity of adsorbed NH3 in the low temperature NH3-SCR reduction half-cycle over a Cu-CHA catalyst”, Appl. Catal. B: Environm., 279 (2020) 119397.
4. W. Hu, T. Selleri, F. Gramigni, E. Fenes, K.R. Rout, S. Liu, I. Nova, De Chen, X. Gao, E. Tronconi, “On the Redox Mechanism Underlying Low-Temperature NH3-SCR over Cu-CHA: A Combined Experimental and Theoretical Study of the Reduction Half Cycle”, Angewandte Chemie Int. Ed., 60 (2021) 7197-7204.
5. F. Gramigni, N.D. Nasello, N. Usberti, U. Iacobone, T. Selleri, W. Hu, S. Liu, X. Gao, I. Nova, E. Tronconi, “Transient kinetic analysis of low-temperature NH3-SCR over Cu-CHA catalysts reveals a quadratic dependence of Cu reduction rates on CuII”, ACS Catalysis, 11 (2021) 4821−4831.

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