Mechanistic studies of NH 3 -assisted reduction of mononuclear Cu( ii ) cation sites in Cu-CHA zeolites

• Published in: Catalysis science & technology Vol. 11; no. 24; pp. 7932 - 7942
• Main Authors: Wilcox, Laura N., Krishna, Siddarth H., Jones, Casey B., Gounder, Rajamani
• Format: Journal Article
• Language: English
• Published: United Kingdom Royal Society of Chemistry (RSC) 14.12.2021
• ISSN: 2044-4753; 2044-4761
• DOI: 10.1039/D1CY01646F

Cu-Exchanged zeolites catalyze various redox reactions including the selective catalytic reduction (SCR) of NO x with NH 3 and the partial oxidation of hydrocarbons. The reduction of Cu( ii ) cations to Cu( i ) by NH 3 alone has been observed experimentally, yet fundamental details regarding the Cu active site requirements, reaction stoichiometry, and reaction mechanism remain incompletely understood. Here, we synthesized model Cu-exchanged chabazite (Cu-CHA) zeolites with varying Cu ion densities and distributions of mononuclear Cu( ii ) ion site types (Cu 2+ , (CuOH) + ) and studied NH 3 -assisted Cu( ii ) reduction reactions using a combination of spectroscopic, titrimetric, and gas-phase product analysis methods. In situ UV-visible and X-ray absorption spectroscopies were used to monitor and quantify the transient reduction of Cu( ii ) to Cu( i ) during exposure to NH 3 (473 K), in concert with titration methods that use NO and NH 3 co-reductants to fully reduce to the Cu( i ) state any residual Cu( ii ) ions that remained after treatments in NH 3 alone for a given time period. The techniques provide quantitative evidence that both mononuclear Cu( ii ) site types are able to reduce in NH 3 alone, and do so to similar extents as a function of time. NH 3 temperature programmed reduction (TPR) revealed that the reaction stoichiometry of NH 3 -assisted reduction forms approximately one equivalent of N 2 per 6 Cu sites, regardless of Cu speciation or density, consistent with a six-electron reduction process whereby two NH 3 molecules react with six Cu( ii ) species to produce one N 2 molecule and six Cu( i ) species. These findings provide new insights into the reaction pathways and mechanisms by which NH 3 alone reduces mononuclear Cu( ii ) sites in zeolites, which are undesired side-reactions that occur during steady-state NO x SCR and can unintentionally influence SCR-relevant spectroscopic or titrimetric characterization experiments.