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

Abstract

The performance of silver alumina catalysts and silver aluminate was studied in the selective catalytic reduction (SCR) of NO by propene. The use of boehmite during the impregnation step ensured a strong interaction between the silver species and the alumina surface in the final calcined catalyst. Thus, higher silver loadings (5-7 wt % silver) could be used without significant loss in selectivity to N 2 during SCR. The nature of the silver species and the formation of adsorbed surface species during SCR and hydrogen-assisted SCR (H 2 -SCR) was studied by activity measurements and by combined in situ IR and X-absorption spectroscopic measurements. The combination of these techniques in the same reaction cell allowed simultaneous monitoring of the state of silver and the formation of surface species under realistic reaction conditions. The active silver species on alumina support were concluded to be 2-dimensional oxidic Ag n δ+ species. Silver aluminate was ruled out as a possibility for an active phase for the SCR reaction. The oxidic Ag n δ+ species were present under both SCR and H 2 -SCR reaction conditions and even on a prereduced catalyst under the SCR reaction conditions. Hydrogen is proposed to enhance the formation of adsorbed surface species, especially nitrites, but not to change the nature of the active silver sites.

Studies on H2-Assisted Liquefied Petroleum Gas Reduction of NO over Ag/Al2O3 Catalyst

“…The role of each silver site in Ag/Al2O3 catalyst is much discussed topic leading to several articles [10][11][12][13], it is generally accepted that metallic silver has an oxidizing function and is thus responsible for activation of HC and NOx. Small silver clusters and oxidized silver have been proposed to activate the NOx reduction activity by partially oxidizing the reductant and promoting surface nitrate formation.…”

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].…”