Low-temperature activation of nitrogen oxide on Cu-ZSM-5 catalysts

Abstract

The adsorption and activation of NO molecules on Cu-ZSM-5 catalysts with different Cu/Al and Si/Al ratios (from 0.05 to 1.4 and from 17 to 45, respectively) subjected to different pretreatment was studied by ultraviolet-visible diffuse reflectance (UV-Vis DR). It was found that the amount of chemisorbed NO and the catalyst activity in NO decomposition increased with an increase in the Cu/Al ratio to 0.35–0.40. The intensity of absorption bands at 18400 and 25600 cm−1 in the UV-Vis DR spectra increased symbatically. It was hypothesized that the adsorption of NO occurs at Cu+ ions localized in chain copper oxide structures with the formation of mono- and dinitrosyl Cu(I) complexes, and this process is accompanied by the Cu2+...Cu+ intervalence transfer band in the region of 18400 cm−1. The low-temperature activation of NO occurs through the conversion of the dinitrosyl Cu(I) complex into the π-radical anion (N2O2)− stabilized at the Cu2+ ion of the chain structure, [Cu2+-cis-(N2O2)−], by electron transfer from the Cu+ ion to the cis dimer (NO)2. This complex corresponds to the L → M charge transfer band in the region of 25600 cm−1. The subsequent destruction of the complex [Cu2+-cis-(N2O2)−] at temperatures of 150–300°C leads to the release of N2O and the formation of the complex [Cu2+O−], which further participates in the formation of the nitrite-nitrate complexes [Cu2+(NO2)−], [Cu2+(NO)(NO2)−], and [Cu2+(NO3)−] and NO decomposition products.