Modified ZSM-5 zeolite as DENOX catalyst

For a better understanding of the first steps of the reaction of NO over Cu-ZSM-5 zeolite prepared by conventional and solid-state ion-exchange, a NO/Cu ratio less than 0.3 was applied and the following measurements were performed: (i) the products of the gas phase interactions were followed by mass spectrometry, (ii) the adsorbed species by IR spectroscopy, and (iii) the valence state and coordination of transition metal ions in zeolites by ESR spectroscopy. Catalysts were prepared both by conventional and solid-state ion-exchange methods and pretreated in vacuum, in oxidative and in reductive atmosphere. The conventional ion-exchanged samples are more active in NO decomposition than the solid-state exchanged ones. Over the reduced catalysts the first step consists in N2O formation and the oxidation of Cu+ --> Cu2+ followed by N2O reduction to N-2 (in these conditions O-2 release was not detected). These results are consistent with a simple redox mechanism in which NO adsorption and decomposition on active (reduced) sites leads to the formation of gaseous nitrogen and adsorbed oxygen. Over oxidized samples NO transformation is slower than over reduced ones, and the first step is to be the reduction of metal ions (Cu2+) With NO as reducing agent.