The migration of Cu species over Cu–SAPO-34 and its effect on NH3 oxidation at high temperature

Abstract

The influence of Cu species migration, through hydrothermal treatment, on the NH₃ oxidation mechanism and its inhibition on NH₃-SCR activity over Cu–SAPO-34 catalysts were studied. XRD, SEM, EPR, H₂-TPR, CO-DRIFTS were conducted to estimate the Cu species distribution. The results revealed that the CuO species initially existed on the external surface of the fresh impregnated Cu/SAPO-34 catalyst, and converted to isolated Cu²⁺ and nanosized CuO in the cavity of the SAPO-34 support during hydrothermal treatment. The NH₃-SCR activity over the hydrothermally treated Cu/SAPO-34 catalysts improved due to the increase of the isolated Cu²⁺ amount, while the NH₃ oxidation activity declined. Furthermore, the NO amount generated in the NH₃ oxidation reaction over the hydrothermally treated samples was much less than the fresh one, which was also caused by the migration of Cu species. In addition, the kinetic NH₃ oxidation was performed at temperatures from 340 °C to 440 °C, and the results indicated that CuO species were the active sites for the NH₃ oxidation. In conclusion, the Cu species migration caused the decrease of the NH₃ oxidation activity and the great variation in the generated NO amount. Furthermore, it was proposed that the NH₃ oxidation mechanism over the Cu/SAPO-34 catalysts contained two steps: firstly, molecular NH₃ reacted with O₂ at CuO sites; secondly, the generated NO was reduced by NH₃ to N₂ at isolated Cu²⁺ sites.