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

Abstract

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 (≈ 4) and silver content (∼15 wt%, Ag/Al ≈ 0.6), and AgCHA(2) with Si/Al ≈ 2 and higher silver loading, while maintaining the Ag/Al ratio (∼26 wt%, Ag/Al ≈ 0.6), have been submitted to different treatments and characterized by using a large variety of techniques (XRD, UV-Vis, ¹⁰⁹Ag NMR, in situ XAS and operando FT-IR). The reduction of the AgCHA and AgRHO zeolites at low temperature (100–200 °C) produces silver clusters, which remain in the AgRHO zeolite when the temperature is increased to 400 °C. However, the silver species in the AgCHA zeolites evolve to nanoparticles (NPs) at 400 °C under H₂. 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 °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.