Analogous Mechanistic Features of NH3‑SCR over Vanadium Oxide and Copper Zeolite Catalysts

Reduction/oxidation half-cycles of the selective catalytic reduction of NO with NH3 (NH3-SCR) at 200 °C were investigated using in situ and operando spectroscopies to propose a general mechanism for four different catalysts (TiO2-supported and bulk vanadium oxides and Cu-AFX and Cu-CHA zeolites). The reduction half-cycle is initiated by the reaction of NH3 on Lewis acid sites [V­(V) or Cu­(II); L-NH3] and NO, followed by the gradual reaction of NH3 on Brønsted acid sites (B-NH3) and NO; this results in the formation of V­(IV) or Cu­(I) and protons (H+) on the surface, along with N2 and H2O. The UV–vis measurements for the reduction half-cycle indicate that N2 is continuously generated until the surface V­(V) or Cu­(II) species is depleted. The subsequent reoxidation of the reduced catalysts under O2 leads to the regeneration of V­(V) or Cu­(II) and the reaction of surface H+, yielding H2O (oxidation half-cycle). The higher consumption rates of B-NH3 and L-NH3 under NO + O2 than those under NO, which has been previously reported in the literature, were explained based on the continuous reduction/oxidation of V­(V)/V­(IV) or Cu­(II)/Cu­(I) where the regenerated V­(V) or Cu­(II) site is reused in the subsequent (second) reduction half-cycle. Namely, upon the recovery of V­(V) or Cu­(II) via reoxidation, the leftover B-NH3 species react with the supplied NO to yield N2; this suggests that B-NH3 is not a spectator but a reservoir of NH3 to participate in the second reduction half-cycle possibly via the migration of NH3 or HONO species. These results provide comprehensive evidence of the general mechanism of NH3-SCR, which was found to be applicable to both V and Cu catalysts.