Mechanistic studies of NH3-assisted reduction of mononuclear Cu(ii) cation sites in Cu-CHA zeolites. Issue 24 (8th November 2021)

• Record Type: Journal Article
• Authors: Wilcox, Laura N.; Krishna, Siddarth H.; Jones, Casey B.; Gounder, Rajamani
• Is Part Of: Catalysis science & technology. Volume 11:Issue 24(2021)
• Page Start: 7932
• Page End: 7942
• Publication Date: 2021-11-08
• Subjects: Catalysis -- Periodicals; 541.395
• DOI: 10.1039/d1cy01646f
• Languages: English
• ISSNs: 2044-4753

Abstract

Abstract : Spectroscopic, titrimetric, and gas-phase product analysis methods reveal a six-electron process for NH3 -assisted reduction of mononuclear Cu(ii ) sites to Cu(i ) in Cu-CHA zeolites of different Cu(ii ) site speciation and density. Abstract : Cu-Exchanged zeolites catalyze various redox reactions including the selective catalytic reduction (SCR) of NO x with NH3 and the partial oxidation of hydrocarbons. The reduction of Cu(ii ) cations to Cu(i ) by NH3 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 NH3 -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 NH3 (473 K), in concert with titration methods that use NO and NH3 co-reductants to fully reduce to the Cu(i ) state any residual Cu(ii ) ions that remained after treatments in NH3 alone for a given time period. The techniques provide quantitative evidence that both mononuclear Cu(ii ) site types are able to reduce in NH3 alone, and do so to similar extents as a function of time. NH3 temperature programmed reduction (TPR) revealed that the reaction stoichiometry of NH3 -assisted reduction forms approximately one equivalent of N2 per 6 Cu sites, regardless of Cu speciation or density, consistent with a six-electron reduction process whereby two NH3 molecules react with six Cu(ii ) species to produce one N2 molecule and six Cu(i ) species. These findings provide new insights into the reaction pathways and mechanisms by which NH3 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.