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

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 AlOx/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.