New insights into the properties of and mechanistic roles of Bronsted acid sites in Cu-zeolites that
Rajamani Gounder, Purdue University
Cu-exchanged zeolites with the chabazite (CHA) topology (SSZ-13, SAPO-34), which contain both Lewis acidic Cu sites and BrÃ¸nsted acidic H+ sites, have recently been adopted in commercial deNOX applications using selective catalytic reduction (SCR) reactions with NH3. Recent efforts have focused predominantly on understanding the structural and electronic properties of Cu species in NOX SCR, while less effort has focused on understanding such properties of residual BrÃ¸nsted acid sites and their potential mechanistic role in this chemistry. This talk will discuss the literature detailing the complexities inherent in using NH3 as a probe for BrÃ¸nsted acid sites in zeolites, and our recent work that provides evidence that NH3 can be used as selective titrant of residual H+ sites in partially Cu-exchanged Cu-ZSM-5 and Cu-SSZ-13 zeolites. The number of H+ sites on ZSM-5 and SSZ-13 samples measured by direct chemical titration are not always equivalent to the number of framework Al atoms determined from 27Al MAS NMR spectra, providing another example for the imprecision with which structural surrogates in aluminosilicate solids estimate the number of active H+ sites that participate in catalytic turnovers. We present kinetic data on Cu-SSZ-13 samples in the form of standard SCR turnover rates, normalized by the number of Cu(II) sites measured spectroscopically under ambient conditions, that show an apparent zero-order dependence on the number of residual H+ sites determined by direct NH3 titration. These findings provide new insight into the possible mechanistic roles of residual H+ sites in Cu-SSZ-13 zeolites for standard SCR, establish a gas-phase titration method using NH3 that is selective for H+ sites on bifunctional solid acids, and describe a methodology useful in future studies of the fundamental kinetic and mechanistic details of NOX SCR with NH3.