Potential synergic effect between MOR and BEA zeolites in NOx SCR with methane: A dual bed design approach

Abstract

The selective catalytic reduction of NOx with methane (NOx CH4–SCR) under lean conditions was investigated with catalysts based on two different zeolite structures (MOR and BEA) containing Pd and Ce. The catalytic performance for NO oxidation to NO2 reaction, considered an important first key step in the NOx CH4–SCR mechanism, was also assessed.Pd(0.3)Ce(2)–HBEA was found to be very active for NO oxidation but exhibits poor activity for NOx CH4–SCR. Conversely, Pd(0.3)Ce(2)–HMOR presents modest activity for NO oxidation, compared to Pd(0.3)Ce(2)–HBEA, but exhibits mild activity for NOx CH4–SCR reaction. Characterisation by H2–TPR, DRS UV–vis, TEM/EDS and FTIR-CO allowed the identification of palladium stabilised as Pd2+ ions in exchange positions in both monometallic and bimetallic MOR based catalysts, whereas, in BEA catalysts, it is presented as PdO clusters. Cerium is stabilised in Pd(0.3)Ce(2)–HMOR as small CeO2 particles, whereas, in Pd(0.3)Ce(2)–HBEA, it is present as large clusters. Catalysts were also tested in dual bed configuration, in which Pd(0.3)Ce(2)–HBEA was placed as first layer and Pd(0.3)Ce(2)–HMOR as second layer in the catalytic bed. The catalytic performance was significantly improved (higher NOx conversion into N2 and higher CH4 selectivity to SCR reaction), when compared to the catalytic performance of each catalyst individually, suggesting the existence of a synergic effect. This synergy is explained by the complementary roles that each catalyst play in HC–SCR mechanism.