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

Abstract

The rate of the selective catalytic reduction of NO with ammonia (NH 3 -SCR) on Ag/Al 2 O 3 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 NH 3 reaction in NH 3 + O 2 below 673 K and decreases the activation energy for NH 3 + O 2 reaction, indicating that H 2 addition is effective for the oxidative activation of NH 3 . In situ UV-vis results under reaction conditions show that the Ag + ion and Ag n δ+ cluster (n ≤ 8) coexist during H 2 -assisted NH 3 -SCR reaction (H 2 -NH 3 -SCR). ESR spectra after exposing H 2 -NH 3 -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 -NH 3 -SCR. It is concluded that Ag n δ+ 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 NH 3 to NH X (x ≤ 2) intermediate.