Effect of supports on formation and reduction rate of stored nitrates on NSR catalysts as investigated by in situ FT/IR

Surface dynamics of nitrates on NOx storage–reduction (NSR) catalysts are examined by means of in situ FT/IR. In the initial state of NOx storage, both nitrite and nitrates are formed on Pt–Ba/Al2O3 surface, and then nitrite become abundant with the increase of storage time. The initial rate of nitrate storage is faster in a flow of NO/O2 mixture than in a flow of NO2 without O2. The presence of oxygen is found to be essential for the conversion of surface nitrite to nitrates. The rate and amount of nitrate formation are strongly affected by type of supports. The storage ability is in the order of MgO > Al2O3 > ZrO2 > SiO2, which is in accordance with that of basicity of the supports. On the other hand, the rate of nitrate reduction by H2 is in the different order: Al2O3 > ZrO2 > SiO2 > MgO. The strong contribution of acid–base property of supports as a controlling factor is suggested.