Eco-friendly catalysts based on hydroxyapatite for air-quality protection reactions

Nowadays, intense research efforts are needed to comply with increasingly stringent legislative constraints regarding hazardous pollutants (NOx, N2O, NH3, etc.) into the air. Among the many methods that can be used to contain harmful emissions, environmental catalytic reactions (NH3-SCR, NH3-SCO, N2O decomposition, de-VOCs, etc.) represent the best solution to try to achieve the desired zero emissions. In this context, the development of non-toxic, low-cost, ecological, and highly effective catalysts for pollution control reactions is a central theme and represents a great challenge. Among all the calcium phosphates, calcium hydroxyapatite (Ca10(PO4)6(OH)2, HAP) has proven through the years to be a versatile material, playing an important role in different fields of the applied science, from medical engineering to pollution remediation and catalysis [1]. HAP is a promising material for catalytic applications because of its easy functionalization [2] and compositional flexibility thanks to tuneable Ca/P ratio. It can be functionalized with different metallic species of catalytic interest (Cu, Fe, Co, Mn, and others) assuring uniform dispersion of the metallic centres. The obtained catalytic materials show a double functionality: the amphoteric properties, typical of bare HAP, are summed to the electron transfer ability of the metal centers and catalytic activity can benefit of such properties. There is still no extensive use of HAP-based catalysts for environmental protection reactions, with the exception of interesting series of materials containing metallic species (i.e., Pd, Co, Mn) used for the total combustion of volatile organic compounds (de-VOCs) [3]. Thanks to the many interesting properties of HAP, we have started studies on this bio-material [4,5], which should find a more important role in environmental catalysis and could lead to the development of biocompatible catalyst families. We present our recent results on catalysts prepared from a synthetic HAP functionalized with Cu2+ and/or Fe3+ towards some gaseous pollutant abatement reactions. The monometallic Cu/HAP, Fe/HAP and bimetallic Cu-Fe/HAP series (with total metal loading from 1% to 10% by weight) have been prepared by conventional ion exchange or wet impregnation and tested in several reactions, like: NH3-SCR, NH3-SCO, N2O-decomposition. All the reactions were carried out in a continuous reaction line comprising a tubular micro-reactor and a FT-IR connected on line, as analytical apparatus. Catalysts were tested in a large temperature interval, from 150° to 800°C, depending on the reaction type, with and without water among the reactants (5% volume) at variable GHSV from 30,000 to 120,000 h-1. In parallel, bulk and surface characterization studies of the catalysts were performed by employing various techniques, like: N2 adsorption-desorption, XRPD, XPS, TEM-EDX, UV-vis-DRS, and Mossbauer spectroscopy with the aim to understand the accommodation and dispersion of Cu/Fe species on HAP surface. Some interesting results have been summarized in Figure 1. In general, it was possible to obtain catalysts with higher activity by functionalizing HAP with the metallic species by means of ion exchange procedure instead of impregnation. In short, it was possible to observe that Cu/HAP can work at much lower temperature than Fe/HAP catalysts. This can be associated with the Lewis higher acid feature of Cu2+ than Fe3+ species, so favouring strong bond with ammonia in the reactions in which NH3 acts as a reagent (i.e., NH3-SCR and NH3-SCO). When N2O is concerned, once again its higher adsorption with Cu2+ than Fe3+ species can explain the activity of Cu/HAP at lower temperatures. In particular, the role of surface acidity and metal dispersion of the HAP-based catalysts will be discussed in relation to the catalytic performance of the samples. In conclusion, Cu- and Fe-functionalized HAP samples have demonstrated promising performances in some interesting reactions of environmental protection and deserve further investigation.