Coated bifunctional catalysts for NOx SCR with C3H6: Part I: water-enhanced activity

Bifunctional catalysts comprising metal-exchange zeolites coated by metal oxides have been prepared and shown to be active catalysts for the selective catalytic reduction of NOx with propylene. Characterization of the materials by various means shows that oxide nano-particles adhere to the external surface of the zeolite and create a fine permeable coating which does not block the zeolite pores. When ceria is used, the combination of phases lowers the maximum activity temperature of the deNOx reaction as much as 150 °C relative to the metal-zeolite only. In addition, a unique activity enhancement in the presence of water is observed. These effects are observed in different zeolite systems to various degrees indicating that the nature of the exchanged metal or zeolite framework is not critical. In particular, CeO2/Cu-ZSM-5 displays a maximum of activity at a temperature as low as 250 °C. The benefits of low temperature and water-enhanced activity obtained by the coating technique exceed those displayed by other bifunctional materials combining cerium and zeolites through different approaches.