Adsorption-Oxidation Reaction Mechanism of NO on Na-ZSM-5 Molecular Sieves with a High Si/Al Ratio at Ambient Temperature

Abstract

The oxidation reaction mechanism of NO by O 2 over Na-ZSM-5 molecular sieves with a high SiO 2 /Al 2 O 3 ratio was studied at ambient temperature by temperature-programmed surface reaction (TPSR) and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The results show that the NO catalytic oxidation process is accompanied by a significant adsorption of NO 2 and it exhibits a dynamic equilibrium between NO 2 adsorption and the reaction of NO. TPSR and in situ DRIFTS characterization of the adsorbed NO x species revealed that adsorbed NO 3 formed on the surface of the Na-ZSM-5 molecular sieve as a result of the reaction between adsorbed NO and gaseous O 2 . The oxidation of NO 3 with NO produced weakly-adsorbing NO 2 and N 2 O 4 which were released once the adsorption reached saturation. Therefore, the strongly-adsorbing NO 3 was an intermediate of the NO oxidation reaction and its presence promoted NO adsorption. ：采用程序升温表面反应 (TPSR) 和原位漫反射红外光谱 (DRIFTS) 等手段研究了常温下 NO 和 O 2 在高硅 Na-ZSM-5 分子筛上吸附-氧化反应机理. 结果表明, Na-ZSM-5 分子筛上 NO 的催化氧化过程中伴随着显著的 NO 2 物理吸附, 表现为 NO 氧化和 NO 2 吸附间的动态平衡. Na-ZSM-5 分子筛表面 NO x 吸附物种的 TPSR 和原位 DRIFTS 表征表明, 化学吸附的 NO 和气相中的 O 2 在 Na-ZSM-5 表面反应生成吸附态的 NO 3 , 并继续与 NO 作用生成弱吸附的 NO 2 和 N 2 O 4 , 它们吸附饱和后释放出来; 其中, 强吸附的 NO 3 在 NO 氧化过程中起到了反应中间体的作用, 同时也促进了 NO 的吸附. A dynamic equilibrium exists for the physical adsorption of NO 2 and for NO oxidation on Na-ZSM-5. Strongly adsorbed NO 3 (ONOO) is formed by adsorbed NO and gaseous O 2 , is an intermediate for NO oxidation, and accelerates the reaction.