Isolation of the Copper Redox Steps in the Standard Selective Catalytic Reduction on Cu-SSZ-13

Operando X-ray absorption experiments and density functional theory (DFT) calculations are reported that elucidate the role of copper redox chemistry in the selective catalytic reduction (SCR) of NO over Cu-exchanged SSZ-13. Catalysts prepared to contain only isolated, exchanged Cu-II ions evidence both Cu-II and Cu-I ions under standard SCR conditions at 473 K. Reactant cutoff experiments show that NO and NH3 together are necessary for Cu-II reduction to Cu-I. DFT calculations show that NO-assisted NH3 dissociation is both energetically favorable and accounts for the observed Cu-II reduction. The calculations predict in situ generation of Bronsted sites proximal to Cu-I upon reduction, which we quantify in separate titration experiments. Both NO and O-2 are necessary for oxidation of Cu-I to Cu-II, which DFT suggests to occur by a NO2 intermediate. Reaction of Cu-bound NO2 with proximal NH4+ completes the catalytic cycle. N-2 is produced in both reduction and oxidation half-cycles.