PNNL

Abstract

This work reports synergistic effects within SCR (selective catalytic reduction) composite catalysts, consisting of Ce/Mn-oxide and Cu-SSZ-13. These effects reduce NH4NO3 inhibition on the NOx-SCR reaction rate at low temperatures in NO2-rich environments (e.g., “fast” SCR, NO2/NOx = 0.5). Catalytic performance and kinetics indicate strong influence of Ce/Mn-oxide on both the quantity and nature of NH4NO3 deposits. Temperature-programmed desorption/decomposition analyses of NH4NO3–laden composite catalysts, through in-situ (via pre-exposure to “fast” SCR atmosphere) and ex-situ (via physical mixture with NH4NO3 solid) deposition techniques, reveal (i) reduced deposits formed on the composite catalysts at low temperatures, and (ii) comparatively facile NH4NO3 decomposition on the composite catalysts pre-exposed to fast-SCR that is remarkably similar to physically mixed NH4NO3 solids. This suggests that during catalytic fast SCR reaction over the composite catalyst, lower buildup of NH4NO3 occurs in the zeolite phase and is deposited in a form that is less stable in the zeolite (i.e., ‘destabilized’). Both observations are believed to be the result of influence by nitrite intermediates generated by Ce/Mn-oxide, the same species responsible for the synergistic effect in standard SCR (i.e., absence of NO2), and confirms the close contact requirement for the synergistic effects of Ce/Mn-oxide on Cu-SSZ-13. This is, to our knowledge, the first reported effect of SCR composite catalysts reducing the quantity and altering the nature of NH4NO3 deposits formed during SCR reaction at low temperature, and a key step in the design of SCR catalysts with low N2O evolution and greenhouse gas impact.