Catalytic Conversion of NO and C3H6 in Exhaust Gases Over Silver Catalysts Under Stoichiometric or Excess Oxygen

The role of Ag in simultaneously catalyzing NO reduction and C₃H₆ oxidation was shown to be strongly dependent on the redox properties of its local environment. Under an atmosphere of 1,000 ppm NO, 3,000 ppm C₃H₆, and 1% O₂ and a GHSV of 30,000 h⁻¹, a perovskite La_0.88Ag_0.12FeO₃ prepared by reactive grinding is active giving a complete NO conversion and 92% C₃H₆ conversion at 500 °C. These values are much higher than the NO conversion of 55% and C₃H₆ conversion of 45% obtained over a 3 wt.% Ag/Al₂O₃ catalyst under the same conditions. Under an excess of oxygen (10% O₂) a good SCR performance with a plateau of N₂ yield above 97% over a wide temperature window of 350–500 °C along with C₃H₆ conversion of 90% at 500 °C was observed over Ag/Al₂O₃, while minor N₂ yields (∼10% at 250–350 °C) and high C₃H₆ conversions (reaching ∼100% at 450 °C) were obtained over La_0.88Ag_0.12FeO₃. Abundant molecular oxygen is desorbed from Ag substituted perovskite after 10% O₂ adsorption as verified by O₂- temperature programmed desorption (TPD). This reflects the strongly oxidative properties of La_0.88Ag_0.12FeO₃, which lead to a satisfactory NO reduction at 1% O₂ due to the ease of nitrate formation but to a significant C₃H₆ combustion above that value. The formation of nitrate species over the less oxidizing Ag/Al₂O₃ was accelerated under an excess of oxygen resulting in an excellent lean NO reduction behavior. The redox properties of silver catalysts could be adjusted via mixing perovskite with alumina for an optimal elimination of both NO and C₃H₆ over the whole range of oxygen concentration between 0 to 10%.