Impact of low temperature sulfur exposure on the aging of small pore Cu-zeolite SCR catalyst

Abstract

This study investigated the impact of low temperature sulfur exposure prior to hydrothermal aging on the catalytic structure and performance of Cu-SSZ-13 catalyst toward NH3-SCR reaction. The sulfur exposure temperature, duration and the sequence of sulfur exposure vs. hydrothermal aging are investigated. All samples hydrothermally aged in presence of SOx (SO3/SOx = 0.7) have showed decreased activity compared to the sample aged hydrothermally only, but the extent of deactivation varies with different sulfur exposure conditions. The sulfur exposure at 200 °C for 100 hours prior to HTA has the most pronounced impact on the population of active site (Cu) and the SCR performance at low temperatures. And the sample of equivalent thermal and sulfur exposure but with the HTA prior to the sulfur exposure, has less decreases in the active Cu sites and less sulfur stored. In contrast, sulfur exposure at high temperature, e.g. 650 °C, has the least impact on the Cu-SSZ-13 catalyst. The molar ratio of stored sulfur to the decrease amount of active Cu sites is close to 1:1 ratio, which implies the deactivation of sulfur exposed catalyst is mainly due to the decrease of active Cu population. Furthermore, DRIFTs demonstrated that both Z2Cu and ZCuOH sites decrease in most cases and ZCuOH is more impacted compared to Z2Cu. With these findings, we concluded that low temperature sulfur exposure prior to HTA treatment leads to accelerated aging. Further characterization reveals that the aging is mainly due to the decrease in the population of active Cu sites. The finding suggests that a hydrothermal pretreatment before sulfur exposure of Cu-SCR catalyst can retard the deactivation of catalyst to a limited extent in the real-world application.