Cu-Exchanged zeolites catalyze a variety of redox reactions including the selective catalytic reduction (SCR) of NOx with NH3 and partial oxidation of methane. The reduction of Cu(II) cations to Cu(I) by NH3 alone has been observed in the literature, yet fundamental details regarding the reaction stoichiometry, Cu site requirements, and mechanism remain incompletely understood. In this work, we synthesize model Cu-exchanged chabazite (Cu-CHA) zeolites with varying Cu ion densities and distributions of mononuclear Cu(II) ion site types (Cu2+, (CuOH)+). We investigated NH3-assisted Cu(II) reduction reactions using a combination of site titrations, spectroscopy, and analysis of gas-phase reaction products (Figure 1). In situ X-ray absorption and UV-visible spectroscopies quantify the transient reduction of Cu(II) to Cu(I) upon exposure to NH3 (473 K), corroborating titration of residual Cu(II) sites via complete reduction to Cu(I) in an NO + NH3 gas mixture following treatments in NH3 alone for a given time period. This set of experiments reveals 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) shows that NH3-assisted reduction of Cu(II) sites forms approximately one N2 molecule per 6 Cu sites, regardless of Cu speciation or site density, implying that two NH3 molecules react with six Cu(II) species to produce one N2 molecule and six Cu(I) species. These 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 NOx SCR catalysts; reduction of Cu(II) by NH3 alone is an undesired side-reaction that occurs during steady-state NOx SCR and can unintentionally influence SCR-relevant spectroscopic or titrimetric characterization experiments.

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

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