(99d) Mechanistic Studies of NH3-Assisted Reduction of Mononuclear Cu(II) Cation Sites in Cu-CHA Zeolites
AIChE Annual Meeting
2022
2022 Annual Meeting
Catalysis and Reaction Engineering Division
Environmental Catalysis II: Fundamental Catalysis
Monday, November 14, 2022 - 1:30pm to 1:50pm
Cu-Exchanged zeolites catalyze various redox reactions including the selective catalytic reduction (SCR) of NOx 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 NOx 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 NOx SCR catalysts.
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