New insights into the various SCR performances of the CeOx-FeOx mixed-oxide catalysts prepared using different iron precursors

This article illustrated the effects of iron precursors on the deNOx performance of the CeOx-FeOx catalysts. With regard to the catalyst prepared using Fe(NO3)3 as the iron precursor (CeFe–N), relatively poor deNOx performance was observed; <80% NOx conversion was obtained at 130–450 °C. While for the samples synthesized using FeCl3 and Fe2(SO4)3 as the iron sources (CeFe–C and CeFe–S), >90% of the NOx was effectively removed from the working gas in the temperature range of 230–350 °C and >270 °C, respectively, signifying the broadened operational temperature range. The superior oxidative ability of metal cations primarily contributed to a low activation energy, which was favorable for the activation of the reactants and the acceleration of the reaction via the "fast SCR" pathway at lower temperatures. Meanwhile, a comparatively low availability of the surface acid sites signified the limited adsorption of NH3, which thus endowed CeFe–N with a low pre-exponential factor and primarily caused the restrained proceeding of the SCR reaction. In contrast, the presence of ample acid sites and the weakened oxidative ability of metal cations successively explained the increased pre-exponential factor and activation energy of CeFe–C and CeFe–S. The synergistic effects of the previously indicated two factors enabled these two catalysts to have a higher or similar low-temperature NOx conversion compared with CeFe–N. With increasing the temperature, the additional generation of NOx was favored over CeFe–N, causing a decline in its NOx conversion efficiency. On the contrary, the inhibited proceeding of NH3 over-oxidation ensured most NH3 to serve as the reductant to reduce NOx on CeFe–C and CeFe–S, which mainly explained their comparatively broad operational temperature window.