Deactivation of Cu/ZSM-5 catalysts for lean NO_x reduction: Characterization of changes of Cu state and zeolite support

The dependence of the deactivation rate of Cu/ZSM-5 lean NO_x reduction catalysts on the composition of the feed was studied. No catalyst deactivation was observed in the absence of H₂O at 400°C after 160 h. Catalyst deactivation was much faster if the feed contained C₃H₈, NO, O₂, and H₂O (lean NO_x reduction conditions) than if one of these components was missing. Fresh and deactivated catalysts were characterized by a variety of techniques, including EPR, XRD, BET surface area measurements, IR spectroscopy, and temperature programmed reduction with H₂ (H₂-TPR). With some techniques, the differences between fresh and deactivated catalysts were marginal, but very significant changes were observed with TPR and EPR. No gross destruction of the zeolite framework was detected by XRD or BET, even for a catalyst that was 50% deactivated. Isolated Cu²⁺ ions, [Cu-O-Cu]²⁺ oxocations and CuO particles were identified in fresh Cu/ZSM-5. In deactivated Cu/ZSM-5, the Cu species were redistributed. Besides isolated Cu²⁺ ions (in two different coordination environments), highly dispersed Cu ions in Al₂O₃ and a CuAl₂O⁴ compound were detected. Ion exchange of Cu²⁺ into partially dealuminated H/ZSM-5 yielded TPR and EPR evidence strikingly similar to deactivated Cu/ZSM-5. These results suggested that the formation of proton exchange sites plays a crucial role in catalyst deactivation. In the presence of steam, it induces dealumination of the zeolite, and the copper states are irreversibly changed due to copper interacting with the alumina formed in the dealumination process.