Iron functionalized hydroxyapatite : effect of iron speciation on the catalytic performances in the NH3-SCR reaction

Nitrogen oxides (NOx) are known to have a harmful impact on the environment and human health. Selective Catalytic Reduction by ammonia (NH3-SCR) is one among the most performant technologies for NOx emission abatement [1]. In view of ever more stringent regulations for NOx emission, the development of new friendly catalysts represents an unavoidable challenge. Stoichiometric calcium hydroxyapatite (Ca10(PO4)6(OH)2, HAP), is an inorganic material, bioavailable, of low cost, easily modulable for the number of acid-base sites, and capable to exchange the calcium atoms with some other metal ions of catalytic interest and allocate them into its lattice. A preliminary comparative study between several copper- and iron-modified HAP catalysts prepared by ion exchange procedure revealed that both the catalyst series are active and selective in NH3-SCR reaction, even if in different temperature interval [2]. Moreover, iron-based HAP catalysts could have high potentiality, if suitably developed, because of their low cost. In this work, we examined in depth the introduction of iron onto a synthetic HAP framework in different amount (ca. 2-7 wt.%), using iron(III) nitrate as precursor and by three different preparative methods (ionic exchange, deposition-precipitation and impregnation). The catalytic performances of iron functionalized hydroxyapatite catalysts have been evaluated in the NH3-SCR reaction in the 120-500°C interval with different NH3/NO ratios and at fixed contact time. XRPD, Uv-vis-DRS, Mössbauer spectroscopy, ammonia adsorption, and H2-TPR provided fundamental details on catalyst properties and Fe sitting on HAP surface. As a general trend, all Fe/HAP samples were active and selective in the NH3-SCR reaction starting from ca. 350°C. The better performances (Figure) have been observed on catalysts containing ca. 6 wt.% of Fe prepared by deposition-precipitation and impregnation (ca. 70% of NOx conversion and selectivity to N2 higher than 95% at 350°C), where -Fe2O3 and 3D-Fe2O3 nanoclusters were present, as indicated by Mössbauer and UV-vis-DR spectroscopies.