IR-Spectroscopic Study of Complex Formation of Nitrogen Oxides (NO, N(2)O) with Cationic Forms of Zeolites and the Reactivity of Adsorbed Species in CO and CH(4) Oxidation

The formation of complexes and disproportionation of nitrogen oxides (NO, N(2)O) on cationic forms of LTA, FAU, and MOR zeolites was investigated by diffuse-reflectance IR spectroscopy. N(2)O is adsorbed on the samples under study in the molecular form and the frequencies of the first overtone of the stretching vibrations ν(1)(0–2) and the combination bands of the stretching vibrations with other vibrational modes for N(2)O complexes with cationic sites in zeolites (ν(3)(0–1) + ν(1)(0–1), ν(1)(0–1) + δ(0–2)) 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 ν(1)(0–1) + δ(0–2) 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 N(2)O complex formed, presumably two-point adsorption of N(2)O on a cation and a neighboring oxygen atom of the framework. Adsorption of CO or CH(4) on the samples with preliminarily adsorbed N(2)O at 20–180 °C does not result in any oxidation of these molecules. NO(+) and N(2)O(3) species formed by disproportionation of NO are capable of oxidizing CO and CH(4) molecules to CO(2), whereas NO(x) is reduced simultaneously to N(2) or N(2)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.