Mechanism and kinetics of the selective NO reduction over Co-ZSM-5 studiedby the SSITKA technique Part 1: NO chi adsorbed species formation

A steady-state isotopic transient analysis of NO adsorption over Co-ZSM-5 c atalyst in the absence or in the presence of oxygen is reported by using do ubly labeled nitric oxide. The kinetics and mechanism of NOX formation as w ell as the number of active sites for NO adsorption and their possible loca tion on the catalyst surface are determined. The formation of NO2delta+ spe cies, active intermediates in deNOX chemistry in the presence of oxygen and hydrocarbons, is shown to proceed according two sequential steps: (i) mole cular oxygen adsorption and (ii) NO reaction with oxidized surface sites. T he frequency factor of that reaction is found to be two orders of magnitude higher than that of mononitrosyls formation via the adsorption of NO on co balt sites. The oxygen exchange within NO2delta+ is considerably slower tha n the formation of that intermediate species, indicating that its two oxyge n atoms are nonequivalent. The concentration of active sites leading to NO2 delta+ formation is also estimated and related to the interface existing be tween cobalt oxide clusters and zeolite framework. (C) 2001 Academic Press.