Insights into the Sm/Zr co-doping effects on N2 selectivity and SO2 resistance of a MnOx-TiO2 catalyst for the NH3-SCR reaction

A series of Sm- and/or Zr-doped MnOx-TiO2 catalysts were prepared, and the catalysts exhibited better N2 selectivity and SO2 resistance than the undoped MnOx-TiO2 catalyst for the selective catalytic reduction of NO by NH3 (NH3-SCR). The reasons for the good N2 selectivity and SO2 resistance of the catalysts were proposed. X-ray photoelectron spectroscopy (XPS) combined with density functional theory (DFT) calculations suggested that electron transfer between the manganese and samarium species by Mn4+ + Sm2+ ↔ Mn3+ + Sm3+ redox cycles occurred in the Sm-containing catalysts. Furthermore, electron transfer from Sm2+ to Mn4+ suppressed electron transfer from NH3 to Mn4+, inhibiting the formation of NH2 or NH. Thus, the pathway for NH generation was removed, and the reaction of 2NH + 4NO → 3N2O + H2O was prevented. Consequently, the N2 selectivity of the NH3-SCR reaction was enhanced. In situ diffused reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) combined with thermogravimetry, differential scanning calorimetry and mass spectrometry (TG-DSC-MS) results revealed that the deposition rate of sulfate species decreases after Sm doping, which was also attributed to the suppressed electron transfer from SO2 to Mn4+, i.e., the oxidation of SO2 to SO3. Thus, the catalysts exhibited better SO2 resistance.