Zeolite-driven Ag species during redox treatments and catalytic implications for SCO of NH3

Abstract

[EN] Supported silver species are among the most promising catalysts for the depletion of ammonia emission by selective catalytic oxidation (NH3-SCO). Here, an investigation on the influence of small pore CHA and RHO zeolite structures on the silver species formed and their catalytic activity for the NH3-SCO reaction has been conducted. To this end, AgRHO(4) and AgCHA(4) zeolites with similar Si/Al molar ratios (approximate to 4) and silver content (similar to 15 wt%, Ag/Al approximate to 0.6), and AgCHA(2) with Si/Al approximate to 2 and higher silver loading, while maintaining the Ag/Al ratio (similar to 26 wt%, Ag/Al approximate to 0.6), have been submitted to different treatments and characterized by using a large variety of techniques (XRD, UV-Vis, Ag-109 NMR, in situ XAS and operando FT-IR). The reduction of the AgCHA and AgRHO zeolites at low temperature (100-200 degrees C) produces silver clusters, which remain in the AgRHO zeolite when the temperature is increased to 400 degrees C. However, the silver species in the AgCHA zeolites evolve to nanoparticles (NPs) at 400 degrees C under H-2. The catalytic tests show that metal particles are the active sites while silver clusters are inactive for the NH3-SCO reaction. Also, there are important differences in the stability of the reduced Ag species under oxidizing or under reaction conditions at 400 degrees C. Metal NPs are partially redispersed and oxidized to (Ag)(n)(+), while silver clusters are completely oxidized to Ag+. Our results indicate that silver clusters are stabilized in the RHO-type and not in the CHA-type zeolite, and thus they display very different catalytic activities for the NH3-SCO reaction.