Abstract
The formation of complexes and disproportionation of nitrogen oxides (NO, N2O) on cationic forms of LTA, FAU, and MOR zeolites was investigated by diffuse-reflectance IR spectroscopy. N2O is adsorbed on the samples under study in the molecular form and the frequencies of the first overtone of the stretching vibrations ν10–2 and the combination bands of the stretching vibrations with other vibrational modes for N2O complexes with cationic sites in zeolites (ν30–1 + ν10–1, ν10–1 + 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 ν10–1 + 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 N2O complex formed, presumably two-point adsorption of N2O on a cation and a neighboring oxygen atom of the framework. Adsorption of CO or CH4 on the samples with preliminarily adsorbed N2O at 20–180 °C does not result in any oxidation of these molecules. NO+ and N2O3 species formed by disproportionation of NO are capable of oxidizing CO and CH4 molecules to CO2, whereas NOx is reduced simultaneously to N2 or N2O. 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.
Highlights
One of the major problems of environmental protection is the problem of purification of exhaust gases of chemical and coke producing industry as well as power plants from nitrogen oxides [1]
Cationic forms of zeolites are the example of the systems that are characterized by the formation of a variety of adsorbed species produced upon adsorption of nitrogen oxides (NOx)
By analogy to the interpretation of the IR spectrum of gaseous Nitrous oxide (N2 O) [32] and in agreement with the assignment made in our previous publication [33], the most intense bands in the region of 2230–2245 cm−1 can be ascribed to the ν3 stretching vibrations of
Summary
One of the major problems of environmental protection is the problem of purification of exhaust gases of chemical and coke producing industry as well as power plants from nitrogen oxides [1]. Zeolite catalysts are considered among the prospective alternatives to the existing honeycomb ceramic catalysts for selective reduction of NOx to N2 by NH3. Other reducing agents such as CO that are present in considerable amounts in exhaust gases together with NOx and CH4 can be used for NOx abatement [2,3]. The transformation of nitrogen oxides accompanying their adsorption have been observed for zeolites containing transition metal ions [8], for instance copper, which are used in selective catalytic reduction of NOx , and reduced metals [9], and for non-transition metal ions [10]
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