Analysis of the adsorption state and desorption kinetics of NO2 over Fe–zeolitecatalyst by FT-IR and temperature-programmed desorption

Abstract

States of NO₂ adsorption and kinetics of NO₂ desorption over a Fe-loaded ZSM-5 type zeolite were investigated using Fourier transform infrared (FT-IR) spectroscopy and temperature-programmed desorption (TPD). The FT-IR spectra in NO₂/N₂ flows showed that several adsorption species (NO₂, nitrite, nitrate, and NO⁺) existed; except for NO₂, these were considered to be formed via NO₂ dimerization and disproportionation reactions. The TPD spectra showed two distinct peaks, a low-temperature (LT) peak that can be assigned to weakly adsorbed NO_x in the zeolite channel and a high-temperature (HT) peak that can be assigned to chemisorbed NO_x bonded to ion-exchanged Fe sites. By varying flow rates and heating rates in TPD measurements, the peak maximum temperatures in the both peaks were found to be constant with the former, but shifted to higher temperatures with the latter; this suggests that desorption is not controlled by an adsorption/desorption equilibrium, i.e., in the no-readsorption limit. Furthermore, it was found that desorption at both LT and HT peaks proceeds at second order; this implies that the reverse reaction of NO₂ dimerization and disproportionation and/or some sort of lateral interaction between NO₂ molecules might be occurring. The desorption energies and the pre-exponential factors were estimated to be 67 ± 1 kJ mol⁻¹ and 10^5.5±0.2 s⁻¹ for the LT peak and 138 ± 4 kJ mol⁻¹ and 10^9.8±0.3 s⁻¹ for the HT peak. These values show that interaction strengths between adsorbed NO_x and Fe sites are relatively large.