Desorption study of NO and O-2 on Cu-ZSM-5

A detailed temperature-programmed desorption (TPD) study on NO and O2 saturated Cu-ZSM-5 at different temperatures (300-723 K) has been performed. In the temperature range 373-723 K, the evolution of O2 and NO2 accompanying the desorption of NO from NO saturated Cu-ZSM-5 suggested the formation of nitrite/nitrate species. The amount of O2 adsorbed was very much lower than that of NO. The desorption profile of O2 after contacting Cu-ZSM-5 with O2 at 623 K showed a low temperature peak (369 K) confirming the spontaneous ability of O2 desorption from copper zeolite. Moreover, successive saturation cycles of NO followed by O2 and vice versa have been performed at various temperatures (298-623 K) to understand the modifications which the adsorption sites undergo when the two molecules NO and O2 are available together for adsorption on the catalyst sites. After each saturation cycle, a TPD profile was recorded following the evolution of NO, O2 and other NO(x) species. The competitive adsorption experiments revealed that, at 623 K, NO was not able to successfully compete with O2 for the adsorption sites, therefore the adsorption of NO at 623 K on O2 saturated catalyst was not completely restored. On the basis of the experimental work, an overall adsorption reaction scheme of NO on Cu-ZSM-5 was proposed.