Functionalized Iron Hydroxyapatite as Eco-friendly Catalyst for NH3-SCR Reaction : Activity and Role of Iron Speciation on the Surface

Eco-friendly catalysts have been obtained by functionalizing hydroxyapatite (HAP) with iron (Fe/HAP), according to three preparative methods (flash ionic exchange, deposition-precipitation, and impregnation). Fe/HAP samples (ca. 2–7 wt.% Fe) have been tested in the reaction of NOx reduction by ammonia (NH3-SCR) in the 120–500 °C interval with different NH3/NO ratios (0.6–2) at fixed contact time (0.12 s). All Fe/HAP samples were active and selective in the NH3-SCR reaction starting from ca. 350 °C. Better performances have been observed on catalysts prepared by deposition-precipitation and impregnation (about 70 % of NOx conversion and selectivity to N2 higher than 95 % at 350 °C), where α-Fe2O3 and 3D-Fe2O3 nanoclusters were present, as indicated by Mössbauer and UV-Vis-DR spectroscopies. On the opposite, paramagnetic Fe3+ centres were the predominant species on samples prepared by flash ionic exchange. Further characterization techniques (XRPD, N2-physisorption, acidity by NH3 adsorption, and H2-TPR) have concurred to elucidate Fe-sitting HAP and structure-activity relationships.