Insights into hydrothermal aging of phosphorus-poisoned Cu-SSZ-13 for NH3-SCR

Artikel i vetenskaplig tidskrift, 2019

Automotive catalysts are continuously exposed to different poisons and to high temperature, which result in hydrothermal aging. Phosphorus is one of the poisons that the catalyst could be exposed to and it originates from the lubricant oil. In this work, we study the influence of phosphorus poisoning on the catalytic performance and the deactivation mechanism in the hydrothermal aging of Cu-SSZ-13 catalysts. Phosphorus poisoning was performed by impregnating the Cu-SSZ-13 catalysts with (NH4)2HPO4solution using the incipient wetness impregnation method, which allowed us to precisely control the amount of phosphorus poison on the catalyst. XPS results show that phosphorus oxide (P2O5), metaphosphate (PO3–) and phosphate (PO43–) are formed on the surface of phosphorus-poisoned catalysts, and metaphosphate is the main compound among those species. Phosphorus tends to interact with copper sites and strongly impacts the electronic and reduction property of Cu, as disclosed using EDX mapping, UV–vis DRS and H2-TPR. Results of catalytic activities show that phosphorus poisoning is more severe on ammonia and NO oxidation reactions than on standard ammonia-SCR. This is possibly due to the formation of copper phosphates in the large cages, as revealed using H2-TPR, which reduces the oxidation capacity. Moreover, results of XRD and27Al and31P solid-state NMR reveal that phosphorus not only interacts with copper by forming a P–O–Cu linkage but also partially inserts into the zeolite framework by forming a localized AlPO4phase. Both non-poisoned and P-poisoned catalysts lose their active sites and chabazite structures due to dealumination after successively aging in hydrothermal conditions (8 vol.% O2, 5 vol.% H2O in Ar) at different temperatures (800, 850, and 900 °C), which results in a large decrease in SCR performance. Specifically, the P-poisoned catalysts degrade faster than the non-poisoned catalysts. Results demonstrate that phosphorus induces the extensive formation of AlPO4that accelerates the dealumination during hydrothermal aging. This work provides new insight into understanding the deactivation mechanism in Cu-SSZ-13 from the perspectives of phosphorus-poisoning and hydrothermal aging, and it clearly shows that it is crucial to examine poisoning and hydrothermal aging simultaneously.