Methane oxidation by N2O over Fe-FER catalysts prepared by different methods: Nature of active iron species, stability of surface oxygen species and selectivity to products

Abstract

Catalysts with different iron species distributions were developed by loading iron ions to ferrierite (FER) zeolite using liquid ion exchange (IE) and solid state ion exchange (SSIE) methods, which in turn facilitates to study the relation between the nature of iron sites with the formation and stability of surface oxygen species, based on the comparable investigation of catalytic activity and selectivity to products over the catalysts, and the characterisation results obtained from a variety of spectroscopic and solid characterization techniques.IR spectra and NH3-TPD profiles demonstrate a greater extent of exchange of protons in bridging OH positions by iron cations with the Fe-FER-SSIE, in comparison to Fe-FER-IE catalyst. The primary catalytic species present in Fe-FER-IE are isolated Fe species in cation exchange positions, as indicated by UV–vis spectra, while isolated and oligomeric extra-framework Fe species were present in Fe-FER-SSIE catalysts, which is consistent with H2-TPR profiles of N2O pre-treated catalysts and IR spectra of NO adsorption on the catalysts. N2O-TPD results demonstrate that the desorption of oxygen molecules (O2) released from surface oxygen species was observed at lower temperature and higher concentration from Fe-FER-IE than Fe-FER-SSIE. Higher ratio of N2O consumption/methane conversion over Fe-FER-IE was observed than Fe-FER-SSIE. With similar methane conversion, over Fe-FER-SSIE a higher selectivity to valuable products methanol, formaldehyde and dimethyl ether was observed than over Fe-FER-IE.