Methods for NH_3 titration of Br?nsted acid sites in Cu-zeolites that catalyze the selective catalytic reduction of NO_x with NH_3

摘要

Three methods were developed to use ammonia as a selective titrant of residual Br?nsted acid sites in partially Cu-exchanged MFI (ZSM-5) and CHA (SSZ-13) zeolites that mediate the selective catalytic reduction (SCR) of NO with NH_3. These methods involved saturation with gas-phase NH_3 at 433 K followed by purging in flowing helium, saturation with gas-phase NH3 at 323 K followed by washing with deionized water, or saturation with aqueous NH_4NO_3 solution (353 K) followed by washing with deionized water. H-ZSM-5 (Si:Al_(tot) = 17–89) and Cu-ZSM-5 (Cu:Al_(tot) = 0–0.27 at Si:Al_(tot) = 17) samples saturated with NH3 using these three methods, and with n-propylamine titrants that react selectively with Br?nsted acid sites to form C_3H_6 and NH_3, evolved identical amounts of NH3 in temperature-programmed desorption experiments. The consistency among these four methods indicates that NH_3 can be used as a selective titrant of H~+ sites in the presence of Lewis acid sites on Cu-zeolites. The number of H~+ sites on MFI and CHA zeolites measured by direct chemical titration was often less than the number of framework Al (Alf) atoms determined from ~(27)Al MAS NMR spectra (H~+:Alf = 0.7–1.0), yet another reminder that Al_f structures can be imprecise surrogates for active H+ sites on zeolites. Although n-propylamine (at 323 K) titrated all H~+ sites in Cu-ZSM-5, it titrated in Cu-SSZ-13 (Cu:Altot = 0–0.20 at Si:Altot = 4.5) only a small fraction (0–0.26) of the H+ sites that were accessible to NH3, which is the reductant in NO SCR reactions with NH3 (standard SCR). Standard SCR turnover rates on Cu-SSZ-13 samples, after normalization by the number of active Cu(II) sites, showed an apparent zero-order dependence on the number of residual H~+ sites measured by NH_3 titration. These data suggest that reactive NH_4~~+intermediates are present in excess, relative to metal-bound NOx species, under the SCR conditions studied. The methods reported herein, which titrate Br?nsted acid sites selectively in the presence of Lewis acid sites, are useful in investigating the mechanistic role of H~+ sites in standard SCR on small-pore Cu-zeolites and in other reactions catalyzed by metal-exchanged zeolites.