Cu-exchanged RTH-type zeolites for NH3-selective catalytic reduction of NOx: Cu distribution and hydrothermal stability

Abstract

The Cu-exchanged RTH-type zeolites (Cu-RTH) were applied in ammonia-selective catalytic reduction (NH₃-SCR) of NO_x. The effects of Cu loading and distribution on the NO_x reduction efficiency were investigated. The hydrothermal stabilities of a series of Cu-RTH catalysts were also presented. Excellent NO_x conversion activity was observed for Cu-RTH catalysts with high Cu loadings (2.5–4.4 wt%) in the temperature range from 150 °C to 550 °C. Two possible sites (α and β species) for Cu²⁺ species were proposed based on the analysis of H₂-TPR and DRIFTS results. The α species next to the 8-membered rings showed significantly higher TOF (turnover frequency) of NO conversion than the β species present next to the rth cage. The Cu-RTH catalysts showed lower activation energies (30–40 kJ mol⁻¹) compared with that of the Cu-SSZ-13 catalyst (∼54 kJ mol⁻¹) due to the different zeolite framework structures. After undergoing hydrothermal aging, although the zeolite framework structure stayed stable, the active Cu²⁺ species migrated to inactive sites, which resulted in the loss of NO_x conversion for the NH₃-SCR reaction. The saturated Cu²⁺-exchanged Cu_4.4-RTH catalyst showed the best hydrothermal stability due to the limited Cu²⁺ mobility, which stabilized the active Cu²⁺ species.