Iron functionalised hydroxyapatites as efficient eco-friendly catalysts for air-quality protection

In the last decades, the ever more growing need of controlling the hazardous gaseous emissions into the air (including NOx, N2O and NH3) moved researchers to the optimization of the current and most performant catalytic processes and related catalytic materials [1]. In this perspective, the central theme of our research is the development of efficient, low cost and eco-friendly catalysts for reactions of air-quality protection. Among all the calcium phosphate materials, calcium hydroxyapatite (HAP, Ca10(PO4)6(OH)2) has emerged through the years as an interesting material in industrial and environmental catalysis due to its peculiar properties (e.g. high chemical and thermal stability, extremely low solubility, low cost, large availability, easy synthesis, modulable number of acid/base sites, and ion exchange capability) [2]. Recent studies have confirmed the possibility to obtain innovative multifunctional materials to be used as catalysts for environmental targeted reactions by functionalisation of HAP with different metal species (e.g. Cu, Fe, Mn). [3-5] Herein we present our results on catalysts obtained from a commercial HAP (kindly supplied from Solvay) functionalised with Fe3+ towards some gaseous pollutant abatement reactions. A series of iron-modified samples (Fe/HAPIE) with different metal loading (2 < wt.% Fe < 13) has been prepared by ionic exchange procedure from iron(III) nitrate as precursor. A detailed study on Fe-speciation, acidity, morphology and structure has been realised through UV-DR and Mӧssbauer spectroscopies, NH3 titration, N2-physisorption and XRPD analyses. In general, these samples possessed highly dispersed Fe3+ centers as predominant species, together with small amount (≤15%) of Fe2O3 nanoclusters (2 < size (nm) < 4), as revealed by UV-DR and Mӧssbauer spectroscopies. The catalytic performances of Fe/HAPIE samples have been evaluated in some environmental reactions: NH3-SCR (catalytic reduction of NOx by NH3), NH3-SCO (catalytic oxidation of NH3) and catalytic N2O decomposition in a large temperature interval (from 120 to 800°C, depending on the reaction type) and at fixed GHSV (ca. 30000 h-1). All Fe/HAPIE samples were active in the studied reactions. As expected, differences in iron loading produced a diversified effect depending on the considered reaction. In particular, SCR activity strongly depended on Fe-amount, while no remarkable differences emerged in SCO and N2O decomposition activity as a function of Fe loading.