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% NOx conversion efficiency and 98% N-2 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, N-2-physisorption, XPS, EPR, NH3-TPD, Py-IR, H-2-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 Ti2NbOx support and K atoms that K atoms were preferentially bound to Nb OH and Nb=O with a lower bonding energy of -2.33 eV- -2.83 eV when Cu atoms were coordinated to Ti=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+. Ti2NbOx 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. Ti2NbOx 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 degrees C.