The Catalytic Reduction Of Nitrogen Oxides Over Ag-Rh/CZA Catalysts Under Lean-burn Condition

The application of lean-burn engine is rising now for the more effective burning andmore economy. However, the traditional three-way catalyst would be ineffective under thislean-burn conditions,because of the significant increment of NOBx B(the content of NO>90%)and oxygen among the exhaust of lean-burn engine. Therefore, a new type of catalyst forautomobile exhaust purification under lean-burn condition is urgently needed to be developed.The selective catalytic reduction of NO by hydrocarbons over Ag/AlB2BOB3B catalyst is consideredto be the most effective way to remove NO from the exhaust gas, the Ag/AlB2BOB3B catalyst hashigh catalytic active and good thermal stability, however, it is inactive at low Ttemperature, andits Tactivity window is also TnarrowT.To improve the catalyst activity at low temperature and broaden the activity window, aseries of (Ag)-(Rh)/CZA catalysts were prepared by impregnation methods, usingCeOB2B-ZrOB2B-AlB2BOB3B(CZA) compounds as support due to its high specific surface area andoxygen storage capacity(OSC). The characters of the structure, thermal stability, and the OSCduring the different Ce/Zr ratio CZA compounds were characterized using kinds oftechnologies. By combining the catalytic performances and the characterized methods, thecatalysts adsorption, catalytic activities, effected factors and the reactive mechanism havebeen investigated.The results showed that the CZA which formed by adding the AlB2BOB3B into the CeBxBZrB1-xBOB2Bcompound, have the high thermal stability and OSC, and is more suitable for the lean-burnautomobile exhaust catalyst carrier material.The activities of the catalysts which use the CZA(0.5) as support were higher than theCZA(0.75) catalysts, the Ag-Rh catalysts exhibited higher catalytic performances than thesingle-component Ag or Rh catalysts. The highest NO conversion is reached at 90.3% overthe Ag(0.04)-Rh(0.007)/CZA(0.5) catalyst, accompanied by the lower light-off temperatureand wider activity window (300℃～500℃).According to the in situ DRIFTS results, the process of the NO reduction reaction byCB3BHB6B over Ag-Rh/CZA catalyst is as following: oxidation of CB3BHB6B is the first part of all,formed the carboxylate, NO is oxidized into nitrate, and the oxidation products can react witheach other which produced the crucial intermediate of -CO-NH-, the intermediate furtherreacts with NO and achieve the objective response.In this reaction, the existence of Rh not only favored the adsorption of reactants such asNO and CB3BHB6B on the catalyst surface, but also promoted the formation of crucial intermediate of -CO-NH- by Ag, thus enhancing the NO conversion notably.