The alkali resistance of CuNbTi catalyst for selective reduction of NO by NH₃: A comparative investigation with VWTi catalyst

The alkali metal poisoned CuNbTi and VWTi catalysts were prepared by wetness impregnation method. Poisoning effects of various K₂O mass ratios (0.1%–2%) on CuNbTi and VWTi catalysts were studied, respectively. CuNbTi exhibited an excellent alkali metal resistance that 2%K₂O-CuNbTi still remained 80% NO_x conversion efficiency and 98% N₂ selectivity while 2%K₂O-VWTi was completely deactivated. Moreover, different alkali metals (K, Na, Ca, Mg) with the same molar ratio were doped on CuNbTi. The poisoned degree followed the order: K₂O > Na₂O > CaO > MgO. Then 2%K₂O-CuNbTi and 2%K₂O-VWTi were selected for follow-up study. To understand the poisoning mechanism, further investigations were performed by SEM, XRD, N₂-physisorption, XPS, EPR, NH₃-TPD, Py-IR, H₂-TPR, in situ DRIFTS characterizations and DFT calculations. The particle of CuNbTi and VWTi agglomerated and the surface area decreased after 2%K₂O loading. Loss of acid sites and drop of reducibility resulted in the deactivation of 2%K₂O-VWTi. By contrast, experimental and computational results indicated that the alkali resistance of 2%K₂O-CuNbTi was mainly due to the interaction between Ti₂NbO_x support and K atoms that K atoms were preferentially bound to Nb–OH and Nb[dbnd]O with a lower bonding energy of −2.33 eV–−2.83 eV when Cu atoms were coordinated to Ti[dbnd]O with a binding energy of −1.54 eV. This protected the active copper species from linking to K₂O and the weak acid sites were preserved with the increasing isolated Cu²⁺. Ti₂NbO_x weakened the impact of potassium on NH₃ adsorbing over the catalyst while the preserved copper species provided adsorption sites and redox ability for NH₃-SCR reaction. Hence, the synergetic effect of copper and niobium species contributed to the alkali metal resistance. Ti₂NbO_x trapped the potassium and retained active copper species over 2%K₂O-CuNbTi catalyst while potassium deactivated both TiO₂ and active vanadium species on 2%K₂O-VWTi catalyst. Meanwhile, both Eley-Rideal (E-R) and Langmuir-Hinshelwood (H-L) mechanisms with adsorbed NH₃ coordinated to the Lewis acid sites and bidentate nitrate as the dominating intermediate species existed during the NH₃-SCR reaction procedure over 2%K₂O-CuNbTi at 225 °C.