Thermal durability of Cu-CHA NH3-SCR catalysts for diesel NOx reduction

Multiple catalytic functions (NOx conversion, NO and NH3 oxidation, NH3 storage) of a commercial Cu-chabazite (CHA) urea/NH3-SCR catalyst were assessed in a laboratory fixed-bed flow reactor system after differing degrees of hydrothermal aging. Catalysts were characterized by using X-ray diffraction (XRD), 27Al solid state nuclear magnetic resonance (NMR) and transmission electron microscopy (TEM)/energy dispersive X-ray (EDX) spectroscopy to develop an understanding of degradation mechanisms during catalyst aging. The catalytic reaction measurements of laboratory-aged catalysts were performed, which allows us to obtain a universal curve for predicting the degree of catalyst performance deterioration as a function of time at each aging temperature. Results show that as the aging temperature becomes higher, the zeolite structure collapses in a shorter period of time after an induction period. The decrease in SCR performance was explained by zeolite structure destruction and/or Cu agglomeration, as detected by XRD/27Al NMR and by TEM/EDX, respectively. Destruction of the zeolite structure and agglomeration of the active phase also results in a decrease in NO/NH3 oxidation activity and NH3 storage capacity of the catalyst. Laboratory aging conditions (16 h at 800 °C) compare well with a 135,000 mile vehicle-aged catalyst, except for the chemically poisoned inlet section of the vehicle-aged catalyst.