378123 Titration of Brønsted Acid Sites in Cu-Zeolites and Their Mechanistic Roles in Standard SCR

Sunday, November 16, 2014: 4:10 PM
308 (Hilton Atlanta)
Shane A. Bates1, Anuj Verma1, W. Nicholas Delgass1, Fabio H. Ribeiro1, Jeffrey T. Miller2 and Rajamani Gounder1, (1)School of Chemical Engineering, Purdue University, West Lafayette, IN, (2)Chemical Science and Engineering Division, Argonne National Laboratory, Argonne, IL

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 [1].

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 [2]. 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 [1]. 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.


[1] Bates, S. A.; Delgass, W. N.; Ribeiro, F. H.; Miller, J. T.; Gounder, R. J. Catal. 2014, 312, 26.

[2] 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.

Extended Abstract: File Not Uploaded
See more of this Session: Future Automotive Catalysis
See more of this Group/Topical: Catalysis and Reaction Engineering Division