Ir-spectroscopic study of complex formation of nitrogen oxides (No, n2 o) with cationic forms of zeolites and the reactivity of adsorbed species in co and ch4 oxidation

Abstract

The formation of complexes and disproportionation of nitrogen oxides (NO, N2 O) on cationic forms of LTA, FAU, and MOR zeolites was investigated by diffuse-reflectance IR spectroscopy. N2 O is adsorbed on the samples under study in the molecular form and the frequencies of the first overtone of the stretching vibrations ν1 0–2 and the combination bands of the stretching vibrations with other vibrational modes for N2 O complexes with cationic sites in zeolites (ν3 0–1 + ν1 0–1, ν1 0–1 + δ0–2 ) are more significantly influenced by the nature of the zeolite. The presence of several IR bands in the region of 2400–2600 cm−1 (the ν1 0–1 + δ0–2 transitions) for different zeolite types was explained by the availability of different localization sites for cations in these zeolites. The frequencies in this region also depend on the nature of the cation (its charge and radius). The data can be explained by the specific geometry of the N2 O complex formed, presumably two-point adsorption of N2 O on a cation and a neighboring oxygen atom of the framework. Adsorption of CO or CH4 on the samples with preliminarily adsorbed N2 O at 20–180◦ C does not result in any oxidation of these molecules. NO+ and N2 O3 species formed by disproportionation of NO are capable of oxidizing CO and CH4 molecules to CO2, whereas NOx is reduced simultaneously to N2 or N2 O. The peculiarities in the behavior of cationic forms of different zeolites with respect to adsorbed nitrogen oxides determined by different density and localization of cations have been established.