Influence of sulfation on iron titanate catalyst for the selective catalytic reduction of NOx with NH3

Iron titanate catalyst (FeTiOx) is a potential candidate for the substitution of conventional V2O5–WO3 (MoO3)/TiO2 and Fe/Cu-zeolite catalysts for the selective catalytic reduction (SCR) of NOx with NH3 because of its high SCR activity and N2 selectivity in the medium temperature range. Due to the presence of small amount of SO2 in typical diesel exhaust derived from combustion of sulfur-containing fuels, it is very important to investigate the influence of sulfation on SCR activity, catalyst structure and reaction mechanism. After sulfation under the SCR condition, the surface area and pore volume of FeTiOx catalyst decreased to a certain extent due to the formation of sulfate species. According to the characterizations of FeTiOx catalyst using X-ray diffraction, X-ray absorption fine structure spectroscopy, and in situ diffuse reflectance infrared Fourier transform spectroscopy of SO2 + O2 treatment, the sulfate species mainly formed on iron sites in a chelating bidentate conformation, resulting in the enhancement of Brønsted acidity and Lewis acid strength simultaneously. NH3 adsorption was greatly enhanced in the high temperature range, while NOx adsorption was severely inhibited due to the stronger acidity of sulfate species. The operation temperature window of the sulfated catalyst shifted ca. 50 °C towards high temperature range accordingly. The reaction mechanism study shows that the Langmuir–Hinshelwood reaction pathway was cut off by the sulfation process, resulting in the activity loss at low temperatures; only Eley–Rideal reaction pathway between adsorbed NH3 species and gaseous or weakly adsorbed NO dominated in the SCR reaction, which made this catalyst resistant to SO2 poisoning at relatively high temperatures.

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► Structure change of sulfated FeTiOx was the main reason for its activity variation.
► The sulfate was mainly formed on iron sites in a chelating bidentate conformation.
► The L–H pathway was cut off due to the inhibition of NOx adsorption by sulfation.
► The E–R pathway could proceed at high temperatures, leading to high SO2 durability.
► FeTiOx can be used without SO2 or with SO2 if reaction temperature exceeds 250 °C.