(99d) Mechanistic Studies of NH3-Assisted Reduction of Mononuclear Cu(II) Cation Sites in Cu-CHA Zeolites

Cu-Exchanged zeolites catalyze various redox reactions including the selective catalytic reduction (SCR) of NO_x with NH₃ and the partial oxidation of methane. Reduction of Cu(II) cations to Cu(I) by NH₃ alone has been reported in the literature, yet mechanistic details such as the reaction stoichiometry and Cu site requirements remain incompletely understood. This is an undesired side-reaction that occurs during steady-state NO_x SCR and can unintentionally influence SCR-relevant spectroscopic or titrimetric characterization experiments. Here, we synthesize model Cu-exchanged chabazite (Cu-CHA) zeolites with varying densities of Cu ions and varying distributions of mononuclear Cu(II) site types (Cu²⁺, (CuOH)⁺). We investigate NH₃-assisted Cu(II) reduction reactions through a combination of titrimetric and spectroscopic measurements to quantify changes to Cu oxidation states and the concomitant formation of gas-phase reaction products (Figure 1). The fraction of Cu(II) sites reduced by NH₃ alone in a given time period is quantified through subsequent complete reduction to Cu(I) in an NO + NH₃ gas mixture, corroborated by in situ UV-visible and X-ray absorption spectroscopies. We show that both mononuclear Cu(II) site types are able to reduce in NH₃ alone, and with similar apparent kinetics. NH₃ temperature programmed reduction (TPR) shows that NH₃-assisted reduction of Cu(II) sites forms approximately one N₂ molecule per 6 Cu sites, regardless of Cu-CHA sample composition, implying that two NH₃ molecules react with six Cu(II) species to produce one N₂ molecule and six Cu(I) species. Our findings provide insights into the reaction pathways by which NH₃ alone reduces mononuclear Cu(II) sites in zeolites, which should be considered in both the design and investigation of NO_x SCR catalysts.