Selective catalytic reduction of nitrogen oxide by ammonia on substituted strontium ferrites

Substituted Sr-ferrites Sr1−xCexMn6−yWyFe4Al2O19 (x = 0; 0.2; y = 0; 0.28; 0.56; 0.84) with the components ratio typical for Sr-hexaferrite obtained by co-precipitation and calcined at 700 °C have been characterized by thermal analysis (TG-DTA), X-ray diffraction (XRD), Fourier transformed infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and adsorption method. Ferrites have been studied in the selective catalytic reduction of NO by ammonia (SCR) using temperature-programmed desorption of ammonia (NH3-TPD) and temperature-programmed reaction of ammonia (NH3-TPR) techniques. It was shown that the substituted Sr-ferrites are multiphase. Its specific surface area is 27–59 m2/g. Surface concentrations of elements in the samples differ from the bulk chemical contents. According to XPS data, the surface of the samples is enriched by strontium. The main components on the surface are in oxidized states: Sr2+, Mn3+, Fe3+, Al3+, Ce4+ and W6+. The surface acidity of the samples determined by NH3-TPD increases from 0.023 to 0.071–0.082 mmol/g when Ce and W ions are introduced in SrMn6Fe4Al2O19. The most active (XNO ≈ 100%) and selective with respect to N2 catalyst is the Sr0.8Ce0.2Mn5.16W0.84Fe4Al2O19 ferrite obtained by the precipitation of the soluble nitrates of Sr, Fe, Mn, Al, Ce using ammonium para-tungstate and ammonium hydrocarbonate as precipitating agents. This sample is characterized by the highest surface acidity, the highest atomic ratio of [Mn]/[Fe] and the lowest oxygen reactivity.

Graphical abstractFigure optionsDownload full-size imageDownload high-quality image (150 K)Download as PowerPoint slideResearch highlights▶ The ferrites calcined at 700 °C are multiphase. ▶ The specific surface area of the samples lies in the range of 27–59 m2/g. ▶ The main components on the surface are in the oxidized states: Mn3+, Fe3+, Ce4+, W6+. ▶ SCR of NO by NH3 was carried out in accordance with light-off test. ▶ Activity of catalysts is determined by atomic ratio of [Mn]/[Fe] and O2 reactivity.