NH3-SCR reaction for NOx removal over Cu- and Fe-exchanged hydroxyapatite catalysts

Nitrogen oxides (NOx) are known to have a harmful impact on the environment and human health. Selective catalytic reduction with NH3 as the reducing agent (NH3-SCR) is the most valuable technology for NOx emissions abatement1,2. In view of ever more stringent regulations for the emission of NOx, besides the optimization of well-known catalysts (i.e., Cu and Fe zeolites), the development of new catalysts represents an unavoidable challenge. Hydroxyapatites (HAPs, Ca10-x(PO4)6-x(HPO4)x(OH)2-x, with 0<1) are a class of natural minerals, which, similarly to zeolites, are able to accommodate metal ions without altering their own structural and morphological properties. In this work, we explored the potentiality of metal functionalized HAP as novel catalysts for NH3-SCR reaction3. In particular, we investigated the introduction of Cu or Fe on the HAP framework in different amount (metal loading: 1.5 < wt.% < 12), from different salt precursors and by different preparation methods (ion exchange and wet impregnation). The catalytic performances of the functionalized hydroxyapatite materials have been evaluated in NH3-SCR tests carried out in a wide temperature interval (120-500°C) with different NH3/NO ratios and at fixed contact time. XRPD, Uv-vis-DRS, EPR, and Mössbauer analyses provided fundamental details on the metal (Cu or Fe) sitting on HAP surface. As a general trend, Cu/HAP samples resulted more active than Fe-based ones, which worked at higher temperature (Figure). It was found that an optimum Cu-concentration on HAP of ca. 6 Cu wt.% could be associated with the best active and selective SCR catalysts, independent of the used Cu-deposition method. This behaviour indicated that, likely, there is a critical dimension of the formed Cu-species for the obtainment of high activity in the NH3-SCR reaction.