Use of double wash-coatings of platinum and zeolite catalysts to improve selective reduction of NOx by hydrocarbons

The selective catalytic reduction by hydrocarbons (HC-SCR) of NOx under lea n conditions has been improved by the use of double-layered catalysts with a lower layer Of Pt/SiO2 and an upper layer of a zeolite such as H-, Ce, an d Cu-ferrierite (-FER). H-FER wash-coated over Pt/SiO2 (H-FER//Pt/SiOD2) pe rformed best among the samples examined. The promotional effect was attribu ted to the synergy of the oxidation catalyst (Pt/SiO2) in converting NO int o NO2, which is more reactive to C3H6, and the HC-SCR catalyst (H-FER). Cu- FER//Pt/SiO2 was also effective at widening the temperature window, but wit h this combination the performance was attributed to a simple summation of the activity of two HC-SCR catalysts that were active at different temperat ures.