XAFS Study on the Specific Deoxidation Behavior of Iron Titanate Catalyst for the Selective Catalytic Reduction of NOx with NH3

摘要

The environmentally friendly catalyst iron titanate (FeTiOx) was reported to be very active for the selective catalytic reduction of NOx with NH3 (NH3-SCR), with high N-2 selectivity and H2O/SO2 durability in the medium temperature range, and the specific microstructure of iron titanate crystallites as the active phase was determined. In consideration of the probable existence of a redox cycle between Fe3+ and Fe2+ species in the NH3-SCR reaction, the deoxidation behavior of the FeTiOx catalyst in an H-2 temperature-programmed reduction process was studied extensively by X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine-structure (EXAFS) methods. Owing to the presence of an electronic inductive effect between Fe and Ti species in the unique edge-shared Fe3+(O)(2)Ti4+ structure, the reducibility of Fe3+ species in the FeTiOx catalyst was greatly enhanced compared with that in pristine Fe2O3, leading to the higher oxidation ability of Fe species in FeTiOx. In the H-2 temperature-programmed reduction process, the well-dispersed Fe3+ species in iron titanate crystallites could be totally converted into Fe2+ in the form of ilmenite FeTiO3 below 500 degrees C, whereas pristine Fe2O3 could only be reduced to Fe3O4 up to this temperature point. The typical NH3-SCR reaction is usually conducted below 500 degrees C, and the enhanced oxidation ability of Fe3+ species in FeTiOx catalyst is responsible for its excellent catalytic NOx reduction performance at low temperatures. Based on XANES linear fitting and EXAFS curve-fitting results, the specific deoxidation process of the FeTiOx catalyst was proposed, which can provide useful information for the characterization of the microstructure and redox ability of active sites simultaneously in mixed oxide catalysts for certain catalytic reactions.