Revisiting NH3–catalyst interactions in Cu-SSZ-13 SCR catalysts: an in situ spectro-kinetics study

Abstract

Reactor-based NH₃ TPD, and temporally resolved in situ DRIFTS were used to study NH₃–catalyst interactions as a function of fractional saturation (θ). Approximately 10% of the total chemisorbed NH₃ was found to be unexpectedly consumed from the catalyst (∼18 mmol L⁻¹ out of ∼170 mmol L⁻¹) during adsorption. The NH₃ consuming reaction only occurred at the onset of adsorption, where θ was low (below 0.12). In situ DRIFTS experiments provided mechanistic evidence of this previously overlooked parasitic reaction occurring within SCR-relevant zeolite catalysts during NH₃ adsorption, where NH₃ was found to dissociatively adsorb on acid–base pair sites [M–O] (M = Cu or Al), and the dissociated surface intermediates rapidly reacted further to form presumably N₂ and H₂. Moreover, this reaction is found to occur on both Cu-SSZ-13, and H-SSZ-13; albeit much faster on Cu-SSZ-13. Lastly, this reaction was found to occur over CHA and MFI zeolites (H-SSZ-13 and H-ZSM-5; SAR ∼ 15), but the surface –NH₂ and –OH intermediates were not detected over an amorphous AlO_x/SiO₂ sample, indicating the likely involvement of the unique framework-associated zeolitic Lewis acid sites containing acid–base pair moieties, and/or confinement effects unique to the zeolites.