378123 Titration of Brønsted Acid Sites in Cu-Zeolites and Their Mechanistic Roles in Standard SCR
Small-pore, bifunctional, Cu-exchanged zeolites with the chabazite (CHA) topology are used in commercial deNOX applications using selective catalytic reduction (SCR) with NH3. Here, we discuss methods that enable using NH3 as a stoichiometric chemical titrant for H+ sites in ZSM-5 (MFI) and SSZ-13 (CHA) zeolites in their H- and partially-Cu-exchanged forms, and discuss how these methods provide new insight into the mechanistic roles of H+ sites in standard SCR.
In H-ZSM-5 and Cu-ZSM-5 samples with varying aluminum content (Si/Al = 17-90), H+ sites were titrated selectively and counted consistently using three different NH3 titration procedures and one reactive n-propylamine titration method. The number of H+ sites on ZSM-5 zeolites measured by direct chemical titration was often less than the number of framework Al (Alf) atoms determined from 27Al MAS NMR spectra (H+:Alf = 0.7–1.0), providing yet another demonstration that Alf structures can be imprecise surrogates for active H+ sites on zeolites. In contrast with ZSM-5, the number of H+ sites accessible to NH3 on H-SSZ-13 (Si:Al = 4.5) and Cu-SSZ-13 (Cu:Al = 0-0.20) was higher by at least a factor of four than the number of H+ sites accessible to n-propylamine under the saturation conditions studied. The number of H+ sites on these Cu-SSZ-13 samples decreased with increasing Cu loading with a 2:1 stoichiometry, consistent with the exchange of isolated Cu2+ ions for two H+ sites associated with proximal Alf atoms. We conclude that NH3 is more appropriately suited to titrate all accessible H+ sites in small-pore Cu-zeolites, because residual OH groups in SSZ-13 were not detected in infrared spectra after NH3 exposure, and because NH3 is the reductant in NOX SCR reactions .
Standard SCR rates were measured under differential conditions and normalized by the number of isolated Cu2+ sites, quantified by combining elemental analysis with ambient UV-Visible and X-ray absorption spectral data . SCR turnover rates (473 K, per Cu2+) show a zero-order dependence on the concentration of residual H+ sites that remain after Cu2+ exchange on Cu-SSZ-13 . The primary function of these residual H+ sites appears to be for NH3 storage under the conditions studied. We also show how NH3 titrations performed under different gaseous environments can probe the nature of active H+ sites that participate in standard SCR turnovers.
 Bates, S. A.; Delgass, W. N.; Ribeiro, F. H.; Miller, J. T.; Gounder, R. J. Catal. 2014, 312, 26.
 Bates, S. A.; Verma, A. A.; Paolucci, C. P.; Parekh, A. A.; Anggara, T.; Yezerets, A.; Schneider, W. F.; Miller, J. T.; Delgass, W. N.; Ribeiro, F. H. J. Catal. 2014, 312, 87.