Unraveling the C3H6 poisoning mechanism of Z2Cu and ZCuOH sites over Cu-SSZ-13 during NH3-SCR

Abstract

Cu-SSZ-13 catalysts exhibit remarkable activity for NO_x removal by NH₃, but their activity is inhibited under high concentrations of hydrocarbons (HCs). Cu-SSZ-13 catalysts with different Si/Al ratios (SARs) were chosen to investigate the effect of C₃H₆ on Brønsted acid sites (BASs), Z₂Cu species, and ZCuOH species. After HC poisoning treatment, the catalysts achieved 90% NO_x conversion above 260 °C, which was significantly lower than that of the fresh catalysts. The coke formation of condensed aromatic ring species was confirmed by in situ diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS). For H-SSZ-13 zeolites, coke deposition decreased with an increasing SAR. The presence of Cu decreased coke deposition, and this decrease diminished with an increasing copper content. Coke deposited in the zeolite pores and BASs, and covered the active Cu²⁺ species, but Cu species were primarily present in the form of Cu²⁺ species. Density functional theory (DFT) calculations revealed that the energy barrier of C₃H₆ adsorption on Z₂Cu species was −0.62 eV, which was lower than that of BASs and ZCuOH species. C₃H₆ preferentially adsorbed on Z₂Cu sites, thereby inhibiting the NH₃-SCR reaction.