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 NH 3 (NH 3 -SCR) at 200 °C were investigated using in situ and operando spectroscopies to propose a general mechanism for four different catalysts (TiO 2 -supported and bulk vanadium oxides and Cu-AFX and Cu-CHA zeolites). The reduction half-cycle is initiated by the reaction of NH 3 on Lewis acid sites [V(V) or Cu(II); L-NH 3 ] and NO, followed by the gradual reaction of NH 3 on Brønsted acid sites (B-NH 3 ) and NO; this results in the formation of V(IV) or Cu(I) and protons (H + ) on the surface, along with N 2 and H 2 O. The UV–vis measurements for the reduction half-cycle indicate that N 2 is continuously generated until the surface V(V) or Cu(II) species is depleted. The subsequent reoxidation of the reduced catalysts under O 2 leads to the regeneration of V(V) or Cu(II) and the reaction of surface H +, yielding H 2 O (oxidation half-cycle). The higher consumption rates of B-NH 3 and L-NH 3 under NO + O 2 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-NH 3 species react with the supplied NO to yield N 2; this suggests that B-NH 3 is not a spectator but a reservoir of NH 3 to participate in the second reduction half-cycle possibly via the migration of NH 3 or HONO species. These results provide comprehensive evidence of the general mechanism of NH 3 -SCR, which was found to be applicable to both V and Cu catalysts.