DEVELOPMENT OF METAL FUNCTIONALIZED HYDROXYAPATITE CATALYSTS FOR AIR-QUALITY PROTECTION

A captivating challenge for environmental catalysis is nowadays the rational design of efficient catalysts able to take on the new challenges in energy, economy, and environment sustainability of industrial processes. A catalytic structure must be developed so as to be active, stable, resistant, when working in the presence of poisons (e.g., water, sulphur), but also easily to handle and manage, non-toxic, biocompatible and bioavailable in the view of environment sustainability and circular economy. In this scenario, hydroxyapatite (HAP, Ca10(PO4)6(OH)2) has emerged as bio-material characterized by interesting properties (e.g., thermal/chemical stability, low water solubility, modest morphological properties, and intrinsic amphotericity of surface), which make it a promising support for active metal phases (e.g., Cu, Fe, Mn, Sn), giving rise to heterogeneous catalysts for reactions of industrial chemistry and environment protection. The main scope of this Thesis under the joint supervision between Università degli Studi di Milano (Italy) and Université Claude Bernard Lyon 1 (France) is to develop eco-friendly HAP-based catalysts for environmental reactions devoted to the abatement of nitrogen-containing air pollutants (NOx, NH3, and N2O). During three years of work a commercial bare HAP, coming from the NOVOSOL® process (patented procedure by Solvay), has been at first characterized to assess its applicability as support for innovative eco-friendly catalysts for air-quality protection. The used hydroxyapatite was proven to be a mesoporous, crystalline material characterized by good thermal and chemical stability. Considering that HAP possesses a multifunctional surface, whose acid and basic sites may act as anchoring centres for the immobilization of metal species, it has been properly functionalized with selected metal species, in particular Cu(II) and/or Fe(III), from nitrate precursors in different amount (1 < wt.% Me < 13), according to a wet deposition procedure (contact time of 15 min, 24 h, or 48 h, T = 40°C, calcination at 500°C for 1 h), already reported in previous studies. In this way, different series of metal loaded HAP-catalysts (mono- and bi-metallic) have been obtained, varying some parameters (i.e., metal loading, pH of the Fe-precursor solution, and time of contact between the bare HAP support and the aqueous metal solution) during the preparation. Morphology, structure, acidity, metal speciation and sitting at the HAP surface have been studied through targeted physico-chemical techniques (e.g., N2-physisorption, XRPD, NH3-titration, UV-DR, Mӧssbauer, XP, and EXAFS spectroscopies, CO adsorption at -196°C followed by FT-IR, TEM-EDX) to elucidate the main bulk and surface properties. In general, a complex scenario emerged. The copper- and/or iron-introduction onto HAP did not cause evident modifications in the sample morphology, independently on the metal nature and loading. All the samples were described by mesoporosity and only minor microporosity: they were characterized by surface area values in 60-95 m2·g-1 interval, according to the N2 adsorption/desorption isotherms collected at -196°C. Concerning copper-HAP samples, UV-DRS results indicated that the copper-phase was present in highly dispersed and isolated ions/small nanoclusters onto HAP. However, when the Cu-concentration onto HAP was higher than 6 wt.%, copper-species additionally reacted with phosphate groups at the HAP surface, giving rise to a further Cu-containing crystalline phase (libethenite, JCPDS: 00-036-0404), revealed by XRPD and TEM-EDX. Regarding iron-HAP samples, UV-DRS, Mössbauer and XPS results indicated that both isolated and aggregated Fe-species were present on all the catalyst surfaces. Eventually, the bimetallic samples were characterized by a higher dispersion of metal species than the monometallic counterparts, as revealed by NH3-titrations, TEM-EDX, and XPS analyses. Three environmental reactions have been then selected to abate NOx, NH3, and N2O: NH3-SCR, Selective Catalytic Reduction of NOx by NH3, NH3-SCO, Selective Catalytic Oxidation of NH3, and the catalytic N2O-decomposition reaction. Catalytic tests have been performed in continuous reaction lines equipped with flow reactor and inline instruments (FT-IR and µ-GC) for quantitative analysis of fed/vented gaseous species. The catalytic performances have been studied under ideal and quasi-real feeding, also evaluating the time-on-stream stability, reusability and the resistance to some poisoning species (e.g., sulphur dioxide, alkaline species) of selected samples. Copper-HAP samples have been studied in the NH3-SCR and the N2O-decomposition reaction. The observed catalytic performances under ideal- and quasi-real feeding mixtures suggested that the SCR activity was driven by the Cu-dispersion, as reported for Cu-zeolites. However, the studied Cu-HAP samples were less active than conventional Cu-based systems, even if selectivity to the desired N2 higher than 93% was obtained in the whole temperature range studied. Considering that N2O could be obtained as the undesired by-product due to the unselective NH3 oxidation by O2 for temperatures higher than 400°C in the NH3-SCR, Cu-HAP samples have been additionally studied in the N2O decomposition reaction in the view of their potential use for reducing N2O emissions in post-treatment approaches. The obtained results indicated that Cu-HAP samples can be valid alternatives to some conventional catalytic systems because they require ca. 450°C to efficiently decompose N2O. Differently from what observed in the NH3-SCR, here the catalytic activity was governed by the Cu-Cu distance, according to the reaction mechanism known for Cu-zeolites. Indeed, the most active catalyst possessed dispersed copper-species together with small Cu-aggregates, providing the ideal active sites with the optimal Cu-Cu distance. Iron-HAP catalysts have been tested in the NH3-SCR, NH3-SCO, and N2O-decomposition reactions. They showed modest catalytic performances in the studied reactions, but they remained less active than commercial Fe-based systems. However, they have been studied to evaluate their potentialities for the abatement of gaseous pollutants among the worst of our environment. The obtained results indicated that the studied environmental reactions could be also performed in a single cascade process to achieve the desired zero-emissions goal. For the cascade process, the most promising catalyst among those studied is at an average concentration of Fe (about 6–9 wt.%), to guarantee a surface composed of isolated Fe3+ ions or oligonuclear species that ensure good activity with an equally good selectivity. Eventually, bimetallic copper iron-HAP catalysts have been studied in the NH3-SCR reaction. The obtained results indicated that they were active under both dry/wet feeding, even if they remained less active than conventional bimetallic zeolites. Their activity seemed to be governed by the total metal dispersion, even if, when the catalytic performances of the bimetallic samples were compared with those of the monometallic counterparts, no clear trend of activity was identified due to the fact that the metal-phase could experience different environment, if present alone onto HAP or copresent with another one. To cut a long story short, results obtained in this Thesis enlightened the potential use of hydroxyapatite as promising eco-friendly support for environmental reactions devoted to the abatement of nitrogen-containing air pollutants (NOx, NH3, and N2O). Even if the obtained copper and/or iron hydroxyapatite-based catalysts are not as performant as the commercial catalytic systems, it seems that the suitable route to obtain ever more active and effective HAP-based catalysts is the suitable control of the surface metal distribution onto HAP that is not an easy task. In this context, also in accordance with the positive effects of these innovative HAP-based catalysts on the environment, current investigations are ongoing to develop ever more effective sustainable catalysts for air-quality protection.