Deactivation of Cu-SSZ-13 SCR catalysts by vapor-phase phosphorus exposure

Abstract

Phosphorus in vehicle exhaust is one of the typical derivatives from fuels and lubricant oils that cause irreversible deactivation of automotive catalysts. In this work, we investigate the deactivation of Cu-SSZ-13 by vapor-phase phosphorus poisoning (100 ppm H3PO4) under NH3-free lean conditions and NH3-SCR operating conditions. The poisoned monolith catalysts were characterized with XPS, ICP-SFMS, SEM-EDX mapping, and H2-TPR. The influence of phosphorus on catalytic performance (i.e. standard NH3-SCR, NH3 oxidation, NO oxidation, NH3 storage, and NO storage) was evaluated. Phosphorus is mainly stored in the form of metaphosphate in poisoned catalysts, and it possesses axial and radial gradients on catalyst washcoats. Phosphorus strongly affects the copper reduction property, as revealed by the shifts in copper reduction to higher temperature in H2-TPR experiments. Variations in phosphorus poisoning conditions are found to mainly impact the amounts of phosphorus captured and stored in the monolith catalysts. In order to elucidate the deactivation mechanism, the deactivation is correlated with the P/Cu ratio. The temperature shift for copper reduction significantly increases with the P/Cu ratio and levels at high Cu/P ratios. The deactivation degrees of NH3 oxidation and NO oxidation as a function of the P/Cu ratio follow the same trend as the copper reduction temperature. This is possibly due to the formation of copper phosphates in the large cages, which deactivates the copper sites for NH3 and NO oxidation. The deactivation for standard NH3-SCR, NH3 storage, and NO storage is proportional to the P/Cu ratio. The impact of phosphorus on NO storage is significantly stronger than on NH3 storage, indicating that greater contribution to deactivation is brought about by the decreased capacity of NO chemisorption.