The promoting role of noble metals on NOx storage catalyst and mechanisticstudy of NOx storage under lean-burn conditions

The promoting roles of noble metals (Pt, Rh, and Pd) and NOx storage mechan ism were investigated on alkaline earth metal oxide-based NOx storage catal ysts under lean-burn conditions. It was found that noble metals increased t he 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 w ere found to increase in the order of CaO/Al2O3 < Pd/CaO/Al2O3 < Pt/CaO/A12 O3 < 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 catalyst's were treated by NOx (+O-2) at 300 degreesC. But the adsorbed NO 2 and nitrite species were found to vanish with the formation of strong sur face nitrate species. It was proposed that nitrite and NO2 adspecies acted as intermediates and nitrate mainly came from oxidation of nitrite. NOx sto red on the catalysts was present mainly as nitrate. Introduction of reducin g gas CO/C3H6 led to the reduction of nitrate and formation of surface carb onate species. This process was reversible, i.e., the surface carbonate spe cies were easily replaced by nitrate after NO/O-2 was injected. This explai ns the NOx storage-reduction mechanisms of the lean-rich cyclic reactions o ver the storage catalyst.