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/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.

New Insights into the Active Surface Species of Silver Alumina Catalysts in the Selective Catalytic Reduction of NO

“…loading was between 5 and 7 wt % in contrast to literature, 2,8,15,17,20,21,33,34,42,43 where 2 wt % is typically reported as the standard catalyst. It is believed that these high loadings of silver can be obtained by using boehmite as the precursor for γ-alumina, as this ensures a high surface area for the final catalyst (see Table 1) and has been reported 35 to maintain a strong interaction between silver and alumina, preventing silver from sintering during calcination.…”

Adsorption states of typical intermediates on Ag/Al2O3 catalyst employed in the selective catalytic reduction of NOx by ethanol

“…The silver species play the most important role in the HC-SCR process. As widely accepted, different types of Ag species, such as isolated Ag + cations, oxidized silver clusters (Agn δ+ ), and metallic silver clusters (Agn 0 ), are present on the Ag/Al2O3 catalysts before and during the HC-SCR process [31][32][33][34][35]. Among the different species, oxidized silver (Ag + and/or Agn δ+ ) is believed to be the active species for NOx reduction by hydrocarbons [12,[31][32][33][34][35].…”

“…As widely accepted, different types of Ag species, such as isolated Ag + cations, oxidized silver clusters (Agn δ+ ), and metallic silver clusters (Agn 0 ), are present on the Ag/Al2O3 catalysts before and during the HC-SCR process [31][32][33][34][35]. Among the different species, oxidized silver (Ag + and/or Agn δ+ ) is believed to be the active species for NOx reduction by hydrocarbons [12,[31][32][33][34][35]. The optimum silver loading is typically ~2-4 wt%, at which the formation of a Ag-O layer is predominant on the Al2O3 surface [10,11,14,22,24].…”

The role of Ag O Al entities in adsorption of NCO species and reduction of NO

“…It is widely accepted that different kinds of Ag species, such as isolated Ag + cations, oxidized silver clusters (Ag n ı+ ), and metallic silver clusters (Ag n 0 ), are present on Ag/Al 2 O 3 catalysts as prepared and during HC-SCR [14,26,[34][35][36][37][38]. Among them, oxidized silver (Ag + and/or Ag n ı+ ) is believed to be the active species for NO x reaction by hydrocarbons [14,26,34,35,37,38]. At silver loading below 2 wt%, the Ag O shell was predominant [16].…”