Mineral-derived catalysts optimized for selective catalytic reduction of NOx with NH3

Abstract A type of novel mineral-derived catalyst (MDC) composed of vanadium–titanium magnetite and sintering ore was prepared via calcination or acid modification for selective catalytic reduction of nitrogen oxides (NOx) with ammonia (NH3) to reduce the production cost and simplify the preparation process. Characterizations by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) mapping, X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction of hydrogen (H2–TPR) characterizations were performed to observe the evolution of the physicochemical properties of the mineral catalyst during modification, which were corelated to their catalytic activity. The in-situ DRIFTS was also investigated to discuss the reaction mechanism for different catalysts. The results show that the mineral catalysts with sulfuric acid modification exhibit high NOx conversion (above 90% at 350 °C) and sulfur resistance due to the formation of ferric sulfate species and the presence of Fe3+/Fe2+ and V5+/V4+ couples in the catalysts that results in abundant Bronsted acid sites and promotes the redox properties of catalysts. The in-situ DRIFTS results indicate that the catalysts modified by calcination-pretreatment primarily follow the Langmuir-Hinshelwood (L–H) mechanism where gaseous NO is more easily adsorbed on α-Fe2O3 to form stable nitrates and react with a small number of NH3 species adsorbed on α-Fe2O3, leading to decreased activity. On the other hand, the catalysts modified with acid mainly follow the Eley-Rideal (E–R) mechanism in which adsorbed nitrogen species show less thermal stability and a large amount of adsorbed NH3 on the ferric sulfate species directly react with gaseous NO, resulting in the higher activity.