PNNL

Abstract

Hydrothermal aging (HTA) and chemical poisoning are key factors behind the real-world aging of Cu-SSZ-13 SCR catalysts. Investigating field-returned samples provides valuable insights into how catalyst performance deactivates due to chemical deposits, focusing on sulfur, and loss of active Cu sites. However, the simultaneous presence of both deactivation routes complicates the isolation of each pathway's individual effects from such post-mortem analyses. In this study, we separately prepared hydrothermal-aged and sulfur-aged model Cu-SSZ-13 SCR catalysts and utilized various characterization techniques to elucidate the role of each aging method in catalyst deactivation in comparison to real-world field-aged catalysts. Our findings show that hydrothermal aging at 650 °C for 100 hours caused minor dealumination of the zeolite framework but no significant CuOx cluster formation. In contrast, sulfur aging (via sulfur exposure, calcination at 550 °C, and desulfation up to 750 °C) led to CuOx formation without dealumination. On model catalysts, sulfur poisoning was found to reduce Cu mobility and the amount of active Cu sites thus degrading catalyst activity. Although some activity was regained upon desulfation, a portion of initial catalyst activity remained lost due to CuOx formation. We demonstrate that this occurs because sulfated species impede the ability of multi-nuclear Cu species (e.g., Cu dimers) to split back into their isolated form, leading to CuSO4-clusters that oxidatively desulfate to CuOx species. This degradation pathway explains the significant reduction in activity of field-aged samples, where substantial CuSO4-cluster accumulation leads to reduced active Cu and subsequent conversion to CuOx. Our characterization techniques and kinetic tests lend confidence that conclusions from model catalysts apply directly to field-aged commercial samples, elucidating the decline in activity and chemical properties during field deployment.