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.
展开▼
