Irreversible Silver(I) Interconversion in Ag:ZSM-5 and Ag:SAPO-5 by Propene and Hydrogen

Hydrogen reportedly enhances the activity of silver catalytic systems for the selective catalytic reduction (SCR) of nitrogen oxides by hydrocarbons. This can be ascribed to the formation of silver nanoclusters, which act as active sites for removing NOx. Two bifunctional catalysts were prepared by initial conventional and hydrothermal silver(I) ion exchange of two microporous systems to form Ag:ZSM-5 and Ag:SAPO-5. The redox behavior of the metal for the SCR deNOx process was monitored using either propene or hydrogen as the reducing agents. Reversibility in NO was also investigated with the emphasis of identifying the reoxidized silver species. XAS was used to establish reversibility between ion-exchanged silver(I) and silver metallic particles formed during the process. It is evident that silver(I) is reduced by both hydrogen and propene to metallic silver nanoclusters in both the Ag:ZSM-5 and the Ag:SAPO-5 systems. A promoting effect of H 2 for the SCR-HC deNOx process is believed to be the reduction of silver to form Ag n + species, but these studies also reveal the formation of metallic silver nanoparticles by propene. Subsequent treatment in NO disperses the metal clusters via oxidation to silver(I) in both systems. EXAFS reveals changes in the local silver(I) coordination of the oxygen shell between the initial and final state. The results are consistent with the process being reversible in terms of valence state, but not necessarily coordination. Silver(I) oxide was not formed during any stages of the redox process.