A Multitechnique Approach for The Investigation of Reaction Mechanisms in Cu-Zeolite Catalysts

Heterogeneous catalysis is involved in the vast majority of industrial chemical processes, with a growing attention to sustainability, energetic and environmental aspects. To develop a sustainable and cleaner technology, based on new or optimized solid catalysts, a comprehension of the working principles of heterogeneous catalysis is a prime requirement. Spectroscopies play a pivotal role in this complex subject, addressing some of the main challenges: i) determine the nature and distribution of the catalytically active sites; ii) unveil the underlying catalytic mechanisms; iii) define structure/properties relationship (activity, selectivity and stability); discriminate between active sites and spectators in reaction conditions [1].<br/>This contribution focuses on the use of <i>in situ</i> and <i>operando</i> vibrational and electronic spectroscopies (infrared, Raman, UV-Vis, X-ray absorption and emission) for the characterization of heterogeneous catalysts for selective redox reactions, <i>i.e.</i> the Selective Catalytic Reduction of NO_x with NH₃ (NH₃-SCR) and the direct conversion of methane to methanol (DMTM) [2,3]. Cu-exchanged zeolites are among the most studied catalysts for both reaction, in relation to the reversible Cu²⁺/Cu⁺ redox cycle and to the ability of Cu⁺ ions to active O₂ in specific conditions. In particular Cu exchanged chabazite (Cu-CHA) is the catalyst of choice for NH₃-SCR, due to high activity and good hydrothermal stability. The high symmetry of the CHA framework coupled to the intrinsic structural definition of Cu counterions implies that this system is also an ideal playground for spectroscopy and computational studies [3,4,5].<br/>Some of the most recent and relevant results obtained on the Cu-CHA catalysts by applying advanced spectroscopic techniques together with complementary computational studies will be discussed, with particular focus on the fundamental knowledge related to the different techniques and the importance of a holistic multitechnique approach. This will include advanced data analysis, such as the wavelet transform (WT) analysis and machine learning (ML)-assisted fitting of the extended X-ray absorption fine structure (EXAFS) spectra [4,6], and experimental methodologies such as Modulation Excitation (ME) spectroscopies [7].<br/><br/>References<br/>[1] C. O. Arean, B. M. Weckhuysen, A. Zecchina, Phys. Chem. Chem. Phys., 2012, 14, 2125–2127<br/>[2] G. Berlier, V. Crocellà, M. Signorile, E. Borfecchia, F. Bonino, S. Bordiga, In: Pérez Pariente, J., Sánchez-Sánchez, M. (eds) Structure and Reactivity of Metals in Zeolite Materials. Structure and Bonding, vol 178. Springer<br/>[3] E. Borfecchia, P. Beato, S. Svelle, U. Olsbye, C. Lamberti, S. Bordiga, Chem. Soc. Rev., 2018,47, 8097-8133<br/>[4] C. Negri, T. Selleri, E. Borfecchia, A. Martini, K. A. Lomachenko, T. V. W. Janssens, M. Cutini, S. Bordiga, G. Berlier J. Am. Chem. Soc. 2020, 142, 15884−15896<br/>[5] L. Chen, T. V. W. Janssens, P. N. R. Vennestrøm, J. Jansson, M. Skoglundh, H. Gronbeck ACS Catal. 2020, 10, 5646−5656<br/>[6] A. Martini, E. Borfecchia, K. A. Lomachenko, I. A. Pankin, C. Negri, G. Berlier, P. Beato, H. Falsig, S. Bordiga, C. Lamberti Chem. Sci., 2017, 8, 6836<br/>[7] A. G. Greenaway, A. Marberger, A. Thetford, I. Lezcano-González, M. Agote-Arán, M. Nachtegaal,c D. Ferri, O. Kröcher, C. R. A. Catlow, A. M. Beale Chem. Sci., 2020,11, 447-455