Mechanistic studies of NH3-assisted reduction of mononuclear Cu(ii) cation sites in Cu-CHA zeolites

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 (Cu²⁺, (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.