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 NOx 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 partial 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 in which NH3 alone reduces mononuclear Cu(II) sites in zeolites, which are undesired side-reactions that occur during steady-state NOx SCR and can unintentionally influence SCR-relevant spectroscopic or titrimetric characterization experiments.