Reaction Mechanism and Microkinetic Model for the Binary Catalyst Combination of In/ZSM-5 and Pt/Al2O3 for NOx Reduction by Methane under Lean Conditions

A significantly enhanced activity in NO x reduction by methane was observed when a small amount (5 wt %) of Pt/Al 2 O 3 or Mn 3 O 4 was mechanically mixed with In/ZSM-5 in both dry and wet lean conditions. The highest NO conversions to nitrogen were over 98% with these novel multifunctional catalysts at 500 °C in dry conditions. The same activity was not reached with bimetallic Pt−In/ZSM-5, which showed the benefit of macroscale separation of active compounds (In separated from Pt or Mn 3 O 4 ) in CH 4 -SCR. Neither Pd/Al 2 O 3 nor Ir/Al 2 O 3 was able to promote NO x reduction as a mechanical mixture with In/ZSM-5. Based on the surface intermediate (FTIR) and kinetic studies, a reaction mechanism on the binary combination of In/ZSM-5 and Pt/Al 2 O 3 was defined. The presence of NO 2 enhanced the formation of partially oxidized, nitrogen-free, and nitrogen-containing methane derivates. In the key reaction, the adsorbed NH 2 (amine) and NO form N 2 on In/ZSM-5. Pt/Al 2 O 3 enhanced NO 2 formation, methane oxidation, CO oxidation, and surprisingly the reduction step to nitrogen. The increased formation of N 2 O in two-component reactors revealed that N 2 was formed on In/ZSM-5 by HC-SCR reactions but also on Pt/Al 2 O 3 by NO decomposition. The microkinetic model, based on defined surface reactions on separated indium and platinum sites on different supports, was able to predict all observed reactant and product concentrations in steady-state CH 4 -SCR.