Reductive Activation of O₂ with H₂-Reduced Silver Clusters as a Key Step in the H₂-Promoted Selective Catalytic Reduction of NO with C₃H₈ over Ag/Al₂O₃

The mechanistic cause of the dramatic activity improvement of alumina-supported silver (Ag/Al 2 O 3 ) by H 2 addition for the selective catalytic reduction of NO with propane (C 3 H 8 -SCR) was investigated by catalytic and spectroscopic studies. In situ UV−vis, in situ EXAFS, IR, and microcalorimetric experiments show that H 2 reduction of Ag + ions on Ag/Al 2 O 3 at 573 K yields protons on alumina and partially reduced Ag n δ+ clusters, which are subsequently aggregated to larger Ag clusters. During H 2 + O 2 and H 2 -assisted C 3 H 8 -SCR reactions, Ag + ions and Ag n δ+ coexist. Reoxidation with O 2 results in the redispersion of the cluster to Ag + ion, accompanying a reaction of protons. The relationship between cluster size, redox properties, and catalytic activity is examined using Ag/Al 2 O 3 of different Ag loadings. The steady-state NO reduction rate correlates fairly well with the amount of Ag n δ+ during the H 2 -assisted C 3 H 8 -SCR reaction. It is shown that Ag n δ+ is the active species, whereas monomeric Ag + ion and metallic Ag particles are inactive. With Ag loading, the Ag + reduction rate increases and the rate of cluster reoxidation decreases. A balance between the rate of reduction and reoxidation of Ag species is an important factor that controls the size and oxidation state of the Ag species and consequently the catalytic activity of Ag/Al 2 O 3 . ESR has provided evidence, for the first time, for the in situ generation of superoxide ions in H 2 + O 2 and H 2 -assisted C 3 H 8 -SCR reactions. The comprehensive reason for the hydrogen effect in HC-SCR is discussed, focusing on the role of the cluster and protons on the reductive O 2 activation to superoxide ion.