DEACTIVATION IN CU-ZSM-5 LEAN-BURN CATALYSTS

Cu-ZSM-5 catalysts that exhibit fair fresh performance for nitric oxide reduction by hydrocarbons under oxidizing conditions also exhibit facile deactivation. Brief periods of catalytic use at temperatures of 600-800-degrees-C result in substantial deactivation. The higher the temperature, the more severe the deactivation. Besides causing performance deactivation, these catalytic treatments result in substantial losses of micropore volume. Under our conditions, these pore volume losses appear not to be caused either by carbon (which could accumulate as ''coke'', conceivably) or by dealumination. Surface decomposition of the zeolite could occur but the data suggest this should be slow (if it occurs at all) with respect to deactivation. Substantial sintering of copper species to CuO, and perhaps also to Cu2O, is observed. Appearance of these phases is accompanied by losses in zeolite crystallinity as probed by X-ray diffraction. We believe CuO crystallites grow primarily in interior surfaces of the zeolite. As these crystallites exceed critical sizes imposed by zeolite structural constraint, copper sintering requires local destruction of the zeolite. This destruction may be the main cause of the loss of micropore volume. Sintering of active copper into inactive phases, such as CuO, together with resultant disruption of zeolitic crystallinity and porosity, appears to be the primary cause of catalyst deactivation under our conditions. Thus, the catalytically active component of ZSM-5, not the supporting zeolitic framework, ages and deactivates.