Desorption study of NO and O2 on Cu-ZSM-5

A detailed temperature-programmed desorption (TPD) study on NO and O-2 saturated Cu-ZSM-5 at different temperatures (300-723 K) has been performed. In the temperature range 373-723 K, the evolution of O-2 and NO2 accompanying the desorption of NO from NO saturated Cu-ZSM-5 suggested the formation of nitrite/nitrate species. The amount of O-2 adsorbed was very much lower than that of NO. The desorption profile of O-2 after contacting Cu-ZSM-5 with O-2 at 623 K showed a low temperature peak (369 K) confirming the spontaneous ability of O-2 desorption from copper zeolite. Moreover, successive saturation cycles of NO followed by O-2 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 O-2 are available together for adsorption on the catalyst sites. After each saturation cycle, a TPD profile was recorded following the evolution of NO, O-2 and other NOx species. The competitive adsorption experiments revealed that, at 623 K, NO was not able to successfully compete with O-2 for the adsorption sites, therefore the adsorption of NO at 623 K on O-2 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. (C) 1997 Elsevier Science B.V.