Mitigated ammonium nitrate inhibition in SCR over Cu-SSZ-13 + Ce/Mn-oxide composite catalysts: insights from temperature-programmed desorption analysis

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 NO_x-SCR reaction rate at low temperatures in NO₂-rich environments (e.g., “fast” SCR, NO₂/NO_x = 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 the 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 confirm 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 the SCR reaction at low temperature, and a key step in the design of SCR catalysts with low N₂O evolution and greenhouse gas impact.