The Effect of CH4 on NH3-SCR Over Metal-Promoted Zeolite Catalysts for Lean-Burn Natural Gas Vehicles

We present a systematic investigation of the deNO(x) activity of two commercial metal exchanged zeolite NH3-SCR catalysts, a Cu-SAPO and a Fe-BEA, in view of their application to the exhaust after-treatment systems of lean-burn natural gas vehicles. The catalytic activity data collected under realistic operating conditions, representative of the after-treatment system of lean-burn vehicles, were compared to those obtained adding methane to the gas feed stream in order to assess the impact of this hydrocarbon, which is usually emitted from natural gas engines, on the NH3-SCR catalytic chemistry. Our results indicate a negligible impact of methane on the SCR activity at all conditions, but in the presence of a large excess of NO2 at T>400 degrees C due to methane oxidation by NO2. The data collected over the two individual metal-promoted zeolites were also compared with those obtained combining both catalysts in sequential arrangements, in order to take advantage of their complementary high activities in different temperature ranges. The Fe-zeolite+Cu-zeolite sequence outperformed the two individual components in terms of both overall deNOx efficiency and N2O selectivity, and was equally insensitive to methane.