(617p) On the Stability of Cu-Chabazite SCR Catalysts

Zeolites exchanged with different transition metal cations, primarily Cu or Fe, are being used as catalyst for removing NOx from the effluent of internal combustion engines operating in an excess of oxygen, i.e. Diesel or lean-burn gasoline engines. This is accomplished via Selective Catalytic Reduction (SCR) with NH₃. The last generation of catalysts involves the use of zeolites with the Chabazite structure, such as SSZ-13 or SAPO-34. These materials have high NOx reduction activity, good selectivity towards N₂ and high hydrothermal stability. The last parameter is usually determined via accelerated testing protocols that show that the chabazite structure is robust at high temperatures.

Although much work has centered upon the stability of the zeolite framework, less work has been reported upon the stability and structural changes undergone by the metal centers responsible for the activity of zeolite catalysts. We are interested in understanding what are these structures and how they evolve as the catalysts age. To do so, we perform isothermal reaction experiments under realistic SCR conditions and analyze the transients of operation as a function of water presence. We follow the metal species in-situ using UV-Vis spectroscopy, and ex-situ using X-ray diffraction, MAS-NMR, H₂-TPR, and EPR in the case of Cu.

In this work we report studies done under isothermal SCR aimed at understanding how the structure and siting of Cu cations in different zeolites influences NH₃-SCR operation, and to determine their stability in the absence of framework modifications. We present results using chabazites, both natural and synthetic (SSZ-13), and compare them with ZSM-5 and Mordenite. We also present deactivation models for the different zeolites that correlate changes in activity with those in cation-siting experienced as a function of time-on-stream.