Reaction mechanism of H2-promoted selective catalytic reduction of NO with NH3 over Ag/Al2O3

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 H-2 (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 H-2 addition increases the rate of NH3 reaction in NH3 + O-2 below 673 K and decreases the activation energy for NH3 + O-2 reaction, indicating that H-2 addition is effective for the oxidative activation of NH3. In situ UV-vis results under reaction conditions show that the Ag+ ion and Ag-n(delta+) cluster (n <= 8) coexist during H-2-assisted NH3-SCR reaction (H-2-NH3-SCR). ESR spectra after exposing H-2-NH3-SCR reaction gas mixture at 423 K show that the superoxide ion is formed on the catalyst, and its amount increased with H-2 concentration. The steady-state NO reduction rate and relative amount of the cluster during the reaction increased with H-2 concentration, and the rate correlates fairly well with the relative amount of the cluster, indicating that the cluster acts as active species in H-2-NH3-SCR. It is concluded that Ag-n(delta+) cluster and protons formed by H-2 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.