Reaction Mechanism of H₂-Promoted Selective Catalytic Reduction of NO with C₃H₈ over Ag−MFI Zeolite

Abstract

The mechanism of the H 2 -assisted selective catalytic reduction of NO with propane (H 2 −C 3 H 8 SCR) over Ag + -exchanged MFI zeolite (Ag−MFI) was investigated by spectroscopic studies. Transient IR experiments at 573 K indicate that H 2 addition increases the rates of C 3 H 8 oxidation to acetate on Ag−MFI and NO oxidation to the weakly adsorbed NO 2 . IR spectra obtained during steady-state C 3 H 8 SCR indicate that H 2 addition increases the coverage of acetate, Ag + −NCO, and Al 3+ −NCO. The reaction of acetate with NO 2 yields, via CH 3 NO 2, NCO species, and their hydrolysis yields NH 3 as a final precursor of N 2 . In situ UV−vis and EXAFS measurements at 573 K shows that, during the H 2 + O 2 and H 2 −C 3 H 8 SCR reactions, Ag + ions and partially reduced Ag 4 2+ clusters coexist. ESR spectra obtained after exposing a H 2 + O 2 gas mixture show the formation of superoxide ions. Combined with the results in our previous studies, the following mechanism is proposed: The Ag 4 2+ 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 an effective oxidant for C 3 H 8 oxidation to oxygenates such as acetate and NO oxidation to NO 2 . Thus, H 2 addition increases the coverage of surface intermediates (NCO species) and the gas-phase concentration of NO 2 and consequently increases the rate of N 2 formation.