The Promoting Role of Noble Metals on NOx Storage Catalyst and Mechanistic Study of NOx Storage under Lean-Burn Conditions

Abstract

The promoting roles of noble metals (Pt, Rh, and Pd) and NOx storage mechanism were investigated on alkaline earth metal oxide-based NOx storage catalysts under lean-burn conditions. It was found that noble metals increased the reaction rates of NO oxidation to NO2 and nitrite oxidation to nitrate, resulting in an increase of NOx adsorption and nitrate formation. The rates of NO oxidation to NO2, nitrite oxidation to nitrate, and NOx 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 metal-doped CaO/Al2O3 catalysts were treated by NOx (+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. NOx 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 NOx storage−reduction mechanisms of the lean−rich cyclic reactions over the storage catalyst.