The promoting role of noble metals on NO_x storage catalyst and mechanistic study of NO_x storage under lean-burn conditions

The promoting roles of noble metals (Pt, Rh, and Pd) and NO_x storage mechanism were investigated on alkaline earth metal oxide-based NO_x storage catalysts under lean-burn conditions. It was found that noble metals increased the reaction rates of NO oxidation to N0₂ and nitrite oxidation to nitrate, resulting in an increase of NO_x adsorption and nitrate formation. The rates of NO oxidation to NO₂, nitrite oxidation to nitrate, and NO_x adsorption were found to increase in the order of CaO/Al₂O₃ < Pd/CaO/Al₂O₃ < Pt/CaO/Al₂O₃ < Rh/CaO/Al₂O₃. In situ FTIR spectra showed that adsorbed NO₂, nitrite (NO₂^-), and nitrate (NO₃^-) ,were formed after the noble metaldoped CaO/Al₂O₃ catalysts were treated by NO_x (+O₂) at 300 °C. But the adsorbed NO₂ and nitrite species were found to vanish with the formation of strong surface nitrate species. It was proposed that nitrite and NO₂ adspecies acted as intermediates and nitrate mainly came from oxidation of nitrite. NO_x stored on the catalysts was present mainly as nitrate. Introduction of reducing gas CO/C₃H₆ led to the reduction of nitrate and formation of surface carbonate species. This process was reversible, i.e., the surface carbonate species were easily replaced by nitrate after NO/O₂ was injected. This explains the NO_x storage-reduction mechanisms of the lean-rich cyclic reactions over the storage catalyst.