Cu-zeolites with the chabazite (CHA) topology (SSZ-13, SAPO-34) have been adopted in commercial deNOX applications using selective catalytic reduction (SCR) reactions with NH3. While considerable effort has focused on determining the structural and electronic properties of the Cu species that mediate SCR reactions, far less effort has focused on determining such properties of the Br�nsted acid sites in partially-Cu-exchanged CHA zeolites. Here, we discuss methods that use NH3 and n-propylamine as stoichiometric chemical titrants for accessible H+ sites in ZSM-5 (MFI) and SSZ-13 (CHA) zeolites in their H- and partially-Cu-exchanged forms. In H-ZSM-5 and Cu-ZSM-5 samples with varying silicon to aluminum ratio (Si/Al = 17-90), H+ sites were titrated selectively and counted consistently using three different NH3 titration procedures and one n-propylamine titration method. Yet, in H-SSZ-13 (Si/Al = 4.5), the number of H+ sites accessible to NH3 was four times higher than the number accessible to n-propylamine under the saturation conditions we chose. We propose that NH3 is more appropriately suited to titrate accessible H+ sites in small-pore zeolites active for NOXSCR reactions with NH3, as it is also a reactant in such turnovers. The normalization of standard SCR turnover rates by the number of Cu2+ sites, when determined from the stoichiometry required by the number of residual H+ sites after Cu2+-exchange, lead to the correct conclusion that rates are zero-order in the concentration of H+ sites for Cu-SSZ-13 (Si/Al = 4.5, Cu/Al = 0 to 0.2) when measured by NH3, but to an incorrect conclusion that rates are negative-order in H+ concentration when measured by n-propylamine. These findings establish a gas-phase titration method that is selective for H+ sites on bifunctional solid acids using NH3, and has specific implications for the mechanistic role of H+ sites on Cu-SSZ-13 in standard SCR.

Methods for Br�nsted acid site titration in Cu-zeolites that mediate the selective catalytic reduction of NO<sub>X</sub> with NH<sub>3</sub>

247th National Meeting (2014)

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Methods for Br�nsted acid site titration in Cu-zeolites that mediate the selective catalytic reduction of NOX with NH3

Methods for Br�nsted acid site titration in Cu-zeolites that mediate the selective catalytic reduction of NO_X with NH₃