Mössbauer spectroscopy and X-ray diffraction studies of Mg–Al–Fe–O mixed spinel sulphur transfer catalysts

Mossbauer spectroscopy with X-ray diffraction technique was used to characterize the chemical valences and coordination environments of iron ions in Mg–Fe–Al–O mixed spinel catalysts under different treatments. It was found that the values of quadrupole splitting gradually altered with the calcining temperature, which was connected with the crystalline size of the catalysts. During the thermal treatment, a probable electron density transfer from the oxygen ion to the iron ion in the Fe–O bond resulted in the values of isomer shift decreasing linearly with calcining temperature. The absorption lines were fitted when the atomic fraction of magnesium increased from 1 to 2 in the Mg–Al–Fe–O sample, indicating that a multiphase coexisted in this sample and iron ions had different chemical valences and different coordinated environments. When a sulphated MgFe0.2Al1.8O4 sample was reduced by hydrogen at 500 °C, various iron species with different chemical valences were identified. Fe3O4 phase with an anion defect, created in when the sulphated sample was reduced, was proposed as an active species during O2 and SO2 simultaneous adsorption on the reduced sulphated sample.