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

Abstract

The mechanistic cause of the dramatic activity improvement of alumina-supported silver (Ag/Al2O3) by H2 addition for the selective catalytic reduction of NO with propane (C3H8-SCR) was investigated by catalytic and spectroscopic studies. In situ UV−vis, in situ EXAFS, IR, and microcalorimetric experiments show that H2 reduction of Ag+ ions on Ag/Al2O3 at 573 K yields protons on alumina and partially reduced Agnδ+ clusters, which are subsequently aggregated to larger Ag clusters. During H2 + O2 and H2-assisted C3H8-SCR reactions, Ag+ ions and Agnδ+ coexist. Reoxidation with O2 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/Al2O3 of different Ag loadings. The steady-state NO reduction rate correlates fairly well with the amount of Agnδ+ during the H2-assisted C3H8-SCR reaction. It is shown that Agnδ+ 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/Al2O3. ESR has provided evidence, for the first time, for the in situ generation of superoxide ions in H2 + O2 and H2-assisted C3H8-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 O2 activation to superoxide ion.