On the various Cu-redox pathways and O2-mediated Bronsted-to-Lewis adsorbed-NH3 redistribution under SCR half-cycle conditions

Abstract

The Standard SCR reaction on Cu-SSZ-13 is a complex redox process facilitated by a reduction half cycle (RHC) and an oxidation half cycle (OHC). It is generally accepted that RHC requires the simultaneous presence of NO and NH3, while OHC requires both NO and O2; however understanding of how these individual reactants impact the Cu redox cycle is limited. In this study, we provide experimental investigation of NO-only, NH3-only and NO+NH3 RHC, and their relative rates. Simple kinetic models were developed to understand the dependance of NO-only and NH3-only RHC on Cu speciation. Various OHC routes including NO+O2 and O2-only OHC, and a previously unreported H2O-mediated OHC pathway were also investigated. Detailed NH3 temperature-programmed desorption studies showed that RHC selectively consumes NH3 adsorbed on Lewis-acid sites, and that part of the remaining Bronsted-bound NH3 moves to the Lewis-acid sites during subsequent OHC, thereby resulting in a repartitioning of adsorbed NH3.