Reaction Mechanism of H2-Promoted Selective Catalytic Reduction of NO with NH3 over Ag/Al2O3

摘要

The rate of the selective catalytic reduction of NO with ammonia (NH3-SCR) on Ag/Al2O3 at 473 K increased by a factor of 630, when H2 (1%) was added to the reaction gas mixture. The mechanistic cause of this dramatic activity improvement was investigated by kinetic and spectroscopic studies. Kinetic studies indicate that H2 addition increases the rate of NH3 reaction in NH3 + O2 below 673 K and decreases the activation energy for NH3 + O2 reaction, indicating that H2 addition is effective for the oxidative activation of NH3. In situ UV-vis results under reaction conditions show that the Ag+ ion and Agn+ cluster (n ≤ 8) coexist during H2-assisted NH3-SCR reaction (H2-NH3-SCR). ESR spectra after exposing H2-NH3-SCR reaction gas mixture at 423 K show that the superoxide ion is formed on the catalyst, and its amount increased with H2 concentration. The steady-state NO reduction rate and relative amount of the cluster during the reaction increased with H2 concentration, and the rate correlates fairly well with the relative amount of the cluster, indicating that the cluster acts as active species in H2-NH3-SCR. It is concluded that Agn+ cluster and protons formed by H2 reduction of Ag+ ions are involved in the reductive activation of molecular oxygen into superoxide ion, which should act as effective oxidant for N-H activation of NH3 to NHx (x ≤ 2) intermediate.