Environmental Reactions of Air-Quality Protection on Eco-Friendly Iron-Based Catalysts

: A series of iron functionalized hydroxyapatite (Fe / HAP) samples with di ﬀ erent metal loading (2 < wt.% Fe < 13) was prepared by a ﬂash ionic exchange procedure from iron(III) nitrate as precursor and tested in some environmental air-quality protection reactions such as the catalytic reduction of NO x by NH 3 (NH 3 -SCR), catalytic oxidation of NH 3 (NH 3 -SCO) and catalytic N 2 O decomposition. The catalytic performances of the Fe / HAP catalysts were determined under ﬂow conditions as a function of temperature and using reactant concentrations typical of polluting gaseous emissions from industrial vents. Physico-chemical characterization with various techniques of study (UV-DR and Mössbauer spectroscopies, NH 3 titration, N 2 -physisorption, and XRPD analyses) provided valuable information on Fe-speciation, acidity, morphology, and structure of the samples. In general, highly dispersed Fe 3 + centers were the predominant species, irrespective of Fe-loading, while just low percentage ( ≤ 15%) of Fe x O y nanoclusters (2 < size / nm < 4) was detected on the samples. As expected, the di ﬀ erences in iron concentration produced a diversiﬁed e ﬀ ect of both catalyst properties and catalytic activity, comprising the conversion and selectivity proﬁles, di ﬀ erent for each reaction considered. The obtained results indicate a good potentiality for the eco-friendly Fe-catalysts for some environmental reactions of air protection.


Introduction
The selective catalytic reduction of NO x to harmless N 2 in the presence of NH 3 as reducing agent (NH 3 -SCR) is currently the most effective and popular post-combustion technology for the de-nitration of flue gases [1,2]. The ideal SCR catalyst must satisfy some relevant practical requirements: (i) be effective in selectively converting NO x over a wide temperature range [3]; (ii) ensure high thermal stability and long life in the reaction even under harsh operative conditions [4,5]; (iii) cope with poisoning by some upstream components (e.g., water, sulfur, phosphorus, alkaline metals, etc.) [6][7][8].
At the current state, vanadium-based systems (e.g., V 2 O 5 /TiO 2 ) [9] and Fe-and Cu-zeolites [10] are the most popular commercial NH 3 -SCR catalysts for applications to stationary and mobile sources. Despite their successful and widespread use, these catalytic systems suffer from some relevant limitations and, furthermore, they may not live up to the more stringent regulations in a future scenario calling for zero-emissions to be attained. The need to strike a balance between practical constraints, economic forces, and sustainability principles is thus fueling the demand for a new generation of more aggregation [26]. An innovative flash ion exchange procedure, characterized by an acidic solution containing the Fe-salt and very short contact time with HAP, has demonstrated its effectiveness in highly dispersing and stabilizing iron in the form of isolated or oligonuclear species on the surface. Although Fe/HAP catalysts showed lower activity than Cu/HAP catalysts, the former worked in a broader operating temperature window (275-450 • C) [27]. In addition, the use of iron would result in less expensive and cost-efficient catalysts compared to Cu-based ones.
In this work, we explored in a larger perspective the potential of Fe/HAP catalyst. The preparation of a series of samples with iron loading from 2% to 13% by weight by flash ion exchange was carried out. They were then tested in three distinct air-quality protection reactions: catalytic reduction of NO x by NH 3 (NH 3 -SCR), selective catalytic oxidation of NH 3 (NH 3 -SCO) and catalytic N 2 O decomposition. The latter two reactions, besides being largely used to control the emissions of the NH 3 and N 2 O polluting species from industrial productions (e.g., NH 3 from ammonium phosphate manufacture or N 2 O from adipic acid synthesis), can be also coupled to NH 3 -SCR process in cascade to solve the problem of ammonia slipping and unselective reduction of NO x (to N 2 O). The catalytic behavior of the Fe/HAP catalysts in these reactions will be discussed in relation to the surface acidity and iron dispersion of the catalysts together with other relevant morphological and structural features.

Results
Iron phase was supported on hydroxyapatite with concentration in a wide range (2 < wt.% Fe < 13) according to a flash ion exchange procedure, already presented in the literature [26,27]. In this procedure the operative pH (ca. 3) and contact time (15 min) of HAP and Fe-salt precursor have been properly selected with the intent of depositing highly dispersed iron species on HAP, achieving a compromise between the complex iron speciation in aqueous solution (iron tends to precipitate at pH > 3) and the pH-sensitive interfacial properties of HAP [29]. Indeed, hydrated HAP surfaces generate a complex interphase layer whose thickness, composition, and charge distribution depend mainly on pH [30][31][32].
The properties of as-prepared iron-containing samples (labelled as FeX/HAP IE where X is the iron loading as wt.%) are presented in Table 1. The determination of iron content (by atomic absorption spectroscopy) confirmed that iron was quantitatively immobilized on HAP even at relatively high loading (13 wt.%), despite the very short contact time. The latter should permit preserving, largely, the pristine structural and morphological features of HAP, which is known to suffer from acidic environment. Pristine HAP was a crystalline solid ( Figure S1) with moderate surface area value (44 m 2 ·g −1 ), pore volume of ca. 0.19 cm 3 ·g −1 and mesopores with size distribution centered at around 8.2 nm. The N 2 -adsorption/desorption isotherm showed the hysteresis loop typical of mesoporous materials with broad pore structure [33]. The mesoporosity of HAP was maintained also after the introduction of iron whatever the content (Figure S1b,c). An interesting trend was detected in the surface area values (S.A., Table 1): a small iron amount produced a significant increase (67 m 2 ·g −1 for Fe2/HAP IE ) in S.A. value, which finally almost doubled in the sample containing the highest Fe loading (82 m 2 ·g −1 for Fe13/HAP IE ). These morphological changes can be ascribed to surface rearrangement induced by the iron species distributed on HAP surface as nanosized aggregates or amorphous phases.
X-ray powder diffraction (XRPD) patterns of Fe-loaded HAP samples ( Figure 1) presented exclusively the typical reflections of crystalline HAP and no additional iron-containing crystal phases can be inferred. Consequently, the formation of separate iron oxide aggregates can be ruled out, thus suggesting that iron was highly dispersed as isolated ions/small-sized nanoclusters on HAP surface as an amorphous phase or as nanocrystalline aggregates below the detection size limit of the XRPD. It is noteworthy that the overall intensity of the XRD peaks of Fe/HAP samples ( Figure 1) decreased as the iron concentration increased, and this evidence might suggest that a partial amorphization of the samples occurred.  A further insight on the nature of iron phase can be gained from surface acid site titration by ammonia probe, being Fe species recognized Lewis acid centers. The bare HAP surface possessed a modest number of acid sites (140.5 μmol·g -1 , Table 1), which gradually increased with Fe-loading up to a value of 262.4 μmol·g -1 for the sample with the highest Fe-concentration ( Figure S2). This suggests -FeO(OH), lepidocrocite, JCPDS 00-044-1415, green; Fe 2 O 3 , hematite, JCPDS 00-033-0664, grey) are reported as reference.
A further insight on the nature of iron phase can be gained from surface acid site titration by ammonia probe, being Fe species recognized Lewis acid centers. The bare HAP surface possessed a modest number of acid sites (140.5 µmol·g −1 , Table 1), which gradually increased with Fe-loading up to a value of 262.4 µmol·g −1 for the sample with the highest Fe-concentration ( Figure S2). This suggests that further additions of Fe to HAP surface caused a steady slight increase in the dispersed iron species.

Catalytic Results
The catalytic performances of Fe/HAP samples were evaluated in three reactions of importance for the abatement of air pollutants: NH 3 -SCR, NH 3 -SCO and N 2 O-decomposition. Each reaction has, individually, its worth, but more importantly the three catalytic reactions can be chained together in a cascade configuration with the aim to attain the goal of zero-emission combustion processes. Indeed, to achieve the desired complete conversion of NO x vented from combustion plants, an excess amount of ammonia could be necessary in the NH 3 -SCR post-combustion unit. At the outlet of SCR reactor, unreacted ammonia must not be vented in the atmosphere, therefore, a catalytic process is required to abate the unreacted slipped ammonia. The addition of a selective oxidation (SCO) catalyst in cascade could help in solving the problem of ammonia slipping [34,35]. However, in some cases, and specifically at high temperatures, N 2 O is formed as main undesired by-product in the NH 3 -SCR and SCO processes. N 2 O can be, then, catalytically decomposed to harmless N 2 and O 2 in a third reaction step put in sequence.
In this work, as a preliminary point, the three reactions have been separately studied under typical experimental conditions used in conventionally in the literature to test the performances of Fe/HAP catalysts and individuate the best candidate for a cascade process.

Selective Catalytic Reduction of NO by NH 3 (NH 3 -SCR)
In the standard NH 3 -SCR, NO is reduced by NH 3 with formation of N 2 in the 1:1 stoichiometric ratio, according to the following reaction (Equation (1)): As part of a complete selective process, NH 3 is also oxidized to N 2 . From a kinetic point of view, reaction (1) is in competition with the unselective NH 3 oxidation by O 2 , that produces NO and N 2 O (Equations (2) and (3)): In addition, depending on the gaseous mixture composition, the reaction between NH 3 and NO can proceed through unselective pathways (Equations (4) and (5)), as detailed below: The occurrence of these side-reactions depends on the catalyst, on the temperature and on the feed composition (oxygen concentration, humidity) [36]. In particular, the unselective oxidative pathways are favored at high temperatures (>350 • C). Both activity and selectivity must be met on a high performance SCR catalyst. The SCR activity over the studied Fe/HAP catalysts was investigated in the 120-500 • C temperature interval, by continuously feeding 500 ppm of NO, 500 ppm of NH 3 , 10,000 ppm of O 2 to the reactor at Gas Hourly Space Velocity (GHSV) of 30,000 h −1 . Figure S3a report the curves of NO x (NO + NO 2 ) and of NH 3 conversion and selectivity to N 2 and N 2 O as a function of temperature for all Fe/HAP and bare HAP, respectively. They have been obtained from the experimental profiles of the NO, NO 2 , NH 3 , N 2 and N 2 O outlet concentrations at steady state at the tested temperatures ( Figure S4). Only low activity was observed over bare hydroxyapatite with NO x conversion reaching a maximum of 20% between 300 and 400 • C ( Figure S3a). The addition of iron, even at concentration as low as 2 wt.%, imparted a sensitive enhancement of the catalytic activity with achievement of maximum of around 50% of NO x conversion at 350 • C ( Figure 2a). For higher Fe-concentrations (3-13 wt.%), fine catalytic performances were observed over all the samples with a maximum NO x conversion of about 60-75% at 350 • C, without a clear trend of activity with iron loading. In all cases, NH 3 and NO x conversion curves experienced a monotonic increase, proceeding almost in parallel to each other upon temperature raising from 120 to 300 • C. Consequently, the standard SCR reaction can be assumed to preferentially occur in this temperature window. In the higher temperature interval (300-400 • C), NO x conversion values remained constant around a maximum, while above 400 • C NO x conversion dropped, whereas NH 3 conversion kept increasing up to 100% at 500 • C. This behavior was consistent with the occurrence of the unselective ammonia oxidation to N 2 O and NO, as confirmed by the curve of selectivity to N 2 O, which increased starting from 400 • C, attaining a maximum of 20%. Likely, formation of N 2 O at temperatures above 400 • C is caused also by ammonium nitrate decomposition. Selectivity to N 2 exceeded 90% over all the catalysts in the temperature interval 150-400 • C and decreased down to ca. 80% at temperatures higher than 400 • C.

Selective Catalytic Oxidation of NH 3 (NH 3 -SCO)
Over a suitable catalyst and under certain conditions (e.g., low temperature), NH 3 could be selectively oxidized to N 2 by oxygen, according to the following reaction (Equation (6)): Also in this case, the process selectivity can be affected by the NO and N 2 O formation from parallel oxidation reactions (Equations (2)-(5)).
The catalytic performance of the Fe/HAP catalysts and bare HAP has been investigated under the conditions of 500 ppm NH 3 , 10,000 ppm O 2 at GHSV of 30,000 h −1 .
As reported in Figure S3b, NH 3 conversion over bare HAP was lower than 5% below 375 • C. Over this temperature, NH 3 started to be oxidized and conversion increased up to ca. 45% at 500 • C. Selectivity to N 2 was low (<40%) over the whole temperature interval.
The HAP functionalization with iron produced effective catalysts able to completely convert ammonia below 500 • C. Actually, as shown in the NH 3 conversion profiles of Figure 3a, NH 3 conversion started at 350 • C and regularly increased until reaching 100% at 500 • C. No remarkable differences among catalyst activities emerged with iron loading and NH 3 conversion profiles vs. temperature almost overlapped. Concerning selectivity to N 2 (Figure 3b), although the addition of iron improved the reaction selectivity in comparison with HAP, the evolution of N 2 O and NO x formed from overoxidation side reactions, limited the complete selectivity of the SCO process, leading to relatively low values of N 2 , which did not exceed 70% over all Fe/HAP catalysts.

N 2 O Decomposition
As known, N 2 O is an extremely stable species that can be decomposed to N 2 and O 2 at very high temperature (>800 • C) following the simple reaction (Equation (7)): When the reaction is performed over a suitable catalyst, it can proceed at relatively lower temperature. N 2 O decomposition tests were carried out on all the Fe/HAP samples using low initial N 2 O concentration (ca. 150 ppm) in highly oxidant environment (ca. 6000 ppm) at GHSV = 30,000 h −1 . Figure 4 reports some catalytic activity results in terms of N 2 O conversion at different selected temperatures (400, 500 and 750 • C). As a general trend, all the Fe/HAP samples were active in the N 2 O decomposition, up to 80-100% N 2 O conversion achieved at 750 • C. In particular, the presence of just 2 wt.% of Fe on HAP (Fe2/HAP IE ) conferred an increase of activity in comparison with the almost inactive HAP, which gave rise ca. 35% of N 2 O conversion at 750 • C ( Figure S3c). In any case, the most interesting activities on the Fe/HAP samples were observed at reaction temperatures higher than 500 • C. Low values of N 2 O conversion (up to a maximum of ca. 25%) were detected at temperatures lower than 500 • C, independently from Fe loading (Figure 4). At higher temperatures, the catalytic activity was found to increase with the Fe loading; the presence of high Fe-amount on HAP (ca. 9-13 wt.%) guaranteed almost total N 2 O conversion at 750 • C. In addition, Fe loading influenced also the operating temperature window of Fe/HAP catalysts, since catalysts with higher Fe content (9-13 wt.%) converted N 2 O in a lower temperature interval than those at lower Fe content (2-7 wt.%).

Catalyst Characterization
A critical rationalizing of catalytic results cannot prescind from a targeted investigation on the nuclearity (e.g., isolated centers, dimers, oligomers, clusters, etc.) and nature of the iron phase onto HAP.
According to current knowledge [37,38], iron can be deposited on hydroxyapatite in the form of isolated paramagnetic Fe 3+ ions even at iron loadings as high as 20 wt.%. However, the underlying mechanisms for iron uptake from HAP are widely debated. Indeed, whilst there is overall agreement to consider a pure and stoichiometric exchange process between Ca 2+ and Fe 3+ implausible for charge imbalance reasons, on the other hand the identification of iron species formed after deposition still remains a controversial issue. In particular, Li et al. [37] hypothesized that the exchange between Ca 2+ and Fe 3+ was accompanied by the formation of vacancies on the calcium positions of the HAP lattice for compensating charge imbalance. On the basis of in situ FT-IR measurements, Kandori et al. [39] envisaged that Fe(III) would be incorporated into the HAP framework as hydroxyl ions-bivalent Fe(OH) 2+  species. Subsequent calcination of the catalysts resulted in the formation of isolated paramagnetic Fe 3+ entities accompanied, as shown by InfraRed spectroscopy, by the removal of protons of the OH hydroxyls hosted by the apatite tunnels. These Fe 3+ ions are allocated at the catalyst surface or in its proximity (as suggested by X-ray Photoelectron Spectroscopy, XPS) and occupy two types of sites, which were identified as distorted octahedra or sites with lower coordination. In order to investigate the nature of iron species in the studied Fe/HAP samples, UV-Diffuse Reflectance (UV-DR) spectra were collected at room temperature in the 200-1200 nm range; they are reported in Figure 5    In all cases, spectra decomposition returned several features: an intense band in the 200-300 nm region caused by CT transitions, corresponding to the presence of isolated Fe 3+ species, and a second broader band, in the 460-550 nm interval, typical of d-d transitions from ground state to excited ligand field states ( 6 A 1 → 4 T 1 or 4 T 2 or 4 E). These latter are spin forbidden and generally characterized by low intensity. However, in the obtained spectra they showed a modest intensity, which demonstrated the presence of iron oxide aggregates when their neighboring iron centers possessing magnetic coupling, as reported in [40]. In addition, a further band at ca. 200 nm was observed in all the Fe/HAP spectra, comprised that of bare HAP ( Figure 5, dotted black line): it was assigned to O 2− →Ca 2+ charge transfer transition of the two non-equivalent Ca 2+ sites present in the HAP lattice.
To gain a deeper insight on Fe-coordination at the HAP surface, Mössbauer spectra were collected at room temperature ( Figure S6) and at −260 • C ( Figure S7) and fitted to compute the hyperfine parameters (Tables S2 and S3). Figure 6 reports the spectra obtained in the case of two Fe/HAP samples, presenting the lowest and highest Fe-loading among the prepared samples, (Fe2/HAP IE , Figure 6a and Fe13/HAP IE , Figure 6b).  In general, all the spectra obtained at room temperature ( Figure S6) presented two main peaks, decomposed into two doublets (red and blue lines in Figure S6). However, these measurements did not provide for a unique identification of iron species because at this temperature the doublets could be alternatively associated to the presence of isolated paramagnetic Fe 3+ species or to superparamagnetic iron oxide crystallites. In the former case, the fitting with doublets having distinct quadrupole splitting (∆ , Table S2) could be justified invoking an exchange occurring between isolated Fe 3+ species and the two non-equivalent Ca 2+ sites (Ca(I) and Ca(II)), or a surface complexation by two different HAP functional groups (carbonates, phosphates). On the other hand, in the case of iron oxides the lower values of quadrupole splitting (∆ , Table S2) could be ascribed to iron ions present inside the core of nanoparticles, while the higher values could be typical of iron ions located on the shell (with a highly asymmetric surrounding).
With the final purpose of fully discriminating between the two possible scenarios (isolated Fe 3+ species vs. iron oxides), Mössbauer spectra at −260 • C ( Figure S7) were measured. Then, the best fitting and the hyperfine parameters of the spectra were evaluated (Table S3). The best fitting procedure assuring the lowest errors on calculation of the hyperfine parameters consisted in a combination of two doublets (red and blue lines, Figure S7) with an additional relaxing sextuplet (green line, Figure S7). At this low temperature, the presence of doublets could exclusively be associated to isolated Fe 3+ species, since iron oxides must produce a sextuplet, at least partially blocked. On the other hand, the relaxing sextuplet has hyperfine parameters typical of iron oxides [41,42]. However, the dynamic magnetic behavior, due to the extremely small size of the clusters, hinders determination of presence or not of structural order. For this reason, we refer to them as Fe x O y . Based on previous considerations extensively reported in [26], the size of Fe x O y species could be estimated to be around 2-4 nm. A rough quantification of both isolated Fe 3+ and Fe x O y species could be done. Interestingly, the data shown in Table S3 suggested that the relative abundance of Fe x O y nanoclusters is constant (<15%), independently of the iron loading.

Discussion
An important point concerning the Fe/HAP catalysts is related to Fe sitting on HAP that represents a critical issue. The performed flash ion exchange procedure for depositing Fe-phase on HAP occurs at low pH value (ca. 3); under this condition, HAP gave partial dissolution, favoring the Ca-Fe exchange at the solid-liquid interface. In agreement with that, Mössbauer results indicated that isolated Fe-species could be located in the two inequivalent Ca-sites or dispersed by surface complexation. Actually, XPS data on a sample containing ca. 3 wt.% of Fe showed a Ca/P ratio of about 1.11 and a (Ca + Fe)/P ratio of about 1.27, lower than 1.33, the Ca/P value of bare HAP. This low decrease suggested us that Fe could be exchanged with Ca ions in a certain amount, while the remaining part should be adsorbed at the surface by complexation with the HAP functionalities (OH − , PO 4 3− , etc.). Now, the discussion aims at correlating the catalytic performances of Fe/HAP catalysts in the NH 3 -SCR, NH 3 -SCO and N 2 O-decomposition reactions with their structural and surface properties. The functionalization of hydroxyapatite with iron species produced samples with enhanced acidity as shown by gas-solid phase titration with ammonia probe. The amount of adsorbed ammonia increased along with iron loading in the order: Fe2HAP < Fe3HAP < Fe5HAP < Fe7HAP < Fe9HAP Fe13/HAP (Table 1 and Figure S2). This is also in agreement with presence of Fe-isolated species at the HAP surface.
Actually, Fe/HAP samples had good catalytic performances in the three studied reactions of selective transformation of N-containing gaseous species (NH 3 -SCR, NH 3 -SCO and N 2 O decomposition).
All the Fe/HAP catalysts were active in NO reduction by NH 3 (NH 3 -SCR) in a relatively low temperature range (between 225 • C and 375 • C). Even if the comparison of our results might be specious, it should be remarked that the catalytic performances of the catalysts are comparable or just slightly lower than those of some conventional Fe-zeolites (Fe-BEA, Fe-ZSM-5, Fe-MOR, Fe-FER, Fe-SSZ-13, Fe-SSZ-37) [43]. The best performing catalysts achieved up to 80% NO conversion at 350 • C; unfortunately, the decrease in NO conversion observed above 350 • C limited the catalyst selectivity.
This evidence is congruent with most of previous literature on iron-based SCR catalysts, and in particular on Fe-zeolite catalysts. On Fe-zeolites high de-NO x activity at low temperature has been associated with mononuclear iron species involved as redox sites in the catalytic cycle [44], while dimeric, oligomeric and partially uncoordinated sites at the surface of FeO x particles have been hypothesized to be active species responsible for SCR activity of Fe-zeolites at high temperatures [45,46]. Therefore, the co-presence of isolated and aggregated iron phases in our samples could justify the observed catalytic behavior. Indeed, the evidence that ammonia continued to be converted with increasing temperature until total conversion (>99%) was achieved, suggests that the decrease in NO conversion observed above of 350 • C is probably due to the non-selective conversion of ammonia. From this point of view, the possibility to study separately three interconnected reactions (SCR, SCO and N 2 O decomposition) allowed to find proper correlations. Actually, ammonia oxidation in SCO tests started at 350 • C, a temperature value that coincides with the maximum of NO conversion in SCR investigations. The good agreement between the activity in NH 3 -SCR and NH 3 -SCO tests confirmed the hypothesis that unselective catalytic oxidation of ammonia caused the decrease in NO conversion at high temperatures in the SCR tests.
It is known that on Fe-based catalysts high activity in SCO is associated with monomeric Fe 3+ sites, which simultaneously serve as Lewis acid sites for the adsorption of ammonia and redox centers able to form active oxygen adsorbed species (O 2 δ− with 1 ≤ δ ≤ 2). At high temperature clustered iron species become active in ammonia oxidation, however their high oxidation potential leads to unselective pathways [47]. Considering that Fe/HAP catalysts started to be active only at high temperature (T > 350 • C) and low selectivity to N 2 was observed, we can guess that in this case only aggregated Fe-species are active in the oxidation of ammonia. The low selectivity to N 2 is accompanied with evolution of N 2 O, which could derive either from ammonium nitrate decomposition or from combination between oxygen and adsorbed NH x species, the latter mechanism being predominant at high temperatures [44].
The stability of formed N 2 O species under the conditions of SCR and SCO reactions is supported by catalytic N 2 O decomposition tests, which revealed that N 2 O started to be significantly dissociated into N 2 and O 2 only at temperatures higher than 500 • C. An interesting correlation between the number of acid sites of the Fe/HAP sample and N 2 O decomposition activity has been noted. Figure 7 reports the observed linear trend between N 2 O conversion (evaluated at 750 • C) and Fe/HAP acidity (expressed as mmol NH3,ads ·g -1 ). The trend suggests that isolated Fe 3+ centers could govern the activity of N 2 O decomposition. Actually, previous DFT studies demonstrated that isolated iron cations and oligonuclear iron species, which are also the main species revealed in Fe/HAP samples, are equally active in N 2 O dissociation [48,49]. In the former case an Eley−Rideal mechanism is likely active since two

Materials and Catalyst Preparation
Hydroxyapatite (HAP) used in this work was kindly supplied by Soda Ash & Derivatives (Solvay, Brussels, Belgium): Its preparation procedure, composition and main properties were reported in Ref. [28].
Iron functionalized hydroxyapatite samples with different iron loading (2 < wt.%< 13) were prepared from iron(III)nitrate precursor (Fe(NO 3 ) 3 ·9H 2 O from Sigma Aldrich, St. Louis, MO, USA) by a modified ion exchange (IE) procedure, called flash ionic exchange due to the short contact time of HAP in the Fe-solution [26,27].
For each preparation, 250 mL of iron(III) nitrate solution (with a concentration in the 0.005-0.06 M range) were thermostated at 40 • C. The pH was adjusted to ca. 3 by HNO 3 addition to avoid the hydrolytic polymerisation of iron(III) occurring at higher pH.
HAP powder (typically 6 g, previously dried at 120 • C, overnight) was added to the iron(III) solution and the obtained suspension was vigorously stirred at 40 • C for 15 min. This short contact time assured to preserve both surface and structural properties of HAP, which is sensitive to amorphization or dissolution in an extremely acidic environment [29].
The obtained samples were filtered, thoroughly washed, dried at 120 • C overnight and finally calcined at 500 • C for 1 h under static air at controlled rate (1 • C·min −1 ).
The collected samples were denoted as (FeX/HAP) IE , where X is the Fe loading (in wt.%). Fe loading for all the catalysts was double-checked by atomic absorption spectroscopy (PinAAcle 900T, S/N PTDS11062202, Perkin Elmer, Waltham, MA, USA) both on the solid after mineralization of a small amount (ca. 50 mg) of samples and on the filtration solutions. In any case, the percentage difference between two measurements was lower than 2%.

NH 3 Adsorption
Acidity of bare HAP and Fe/HAP samples was determined by NH 3 adsorption under flowing dynamic experiments. The dried sample (ca. 0.2 g, 45-60 mesh) was activated at 120 • C under flowing air for 30 min in a quartz tubular reactor; then, the NH 3 /N 2 mixture (1.00%, NH 3 concentration of ca. 500 ppm) flowed at 6 NL·h −1 through the reactor and reached a gas cell (path length 2.4 m multiple reflection gas cell) in the beam of an Fourier Transform InfraRed spectrometer, FT-IR (equipped with a DTGS detector, Bio-Rad, Hercules, CA, USA). The NH 3 line (966 cm −1 ) was monitored as a function of time. On all the samples, NH 3 was completely adsorbed, as observed from the shape of the NH 3 -signal profile, which decreased and remained to zero for a given measured time. When saturation of the acid sites by NH 3 was attained, the NH 3 signal was restored at the same value of its starting concentration. From the evaluation of the time during which the NH 3 -signal has remained to zero, the amount of acid sites could be evaluated, according to Equation (8): where [NH 3 ] fed is the flowing NH 3 concentration, in ppm; F is the total flow rate of the NH 3 /N 2 mixture, in NL·h −1 ; t is the time during which NH 3 was completely adsorbed, in min; P is the pressure, in atm; and m sample is the sample mass, in g. Assuming a 1:1 stoichiometry for the NH 3 adsorption on the surface acid site, the amount of acid sites per sample mass (in µmol·g −1 ) was determined. Measurements were replicated and a percent relative uncertainty lower than 2% was obtained.

UV Diffuse Reflectance Spectroscopy (UV-DRS)
Diffuse reflectance spectra (DRS) were collected through a double beam UV-vis-NIR scanning spectrophotometer (UV-3600 plus, Shimadzu, Kyoto, Japan) equipped with a diffuse reflectance accessory (integrating sphere from BIS-603). Powder samples were finely grinded, uniformly pressed in a circular disk (E.D., ca. 4 cm) and included in the sample-holder. The latter was inserted in a quartz cuvette and put on a window of the integrating sphere for the reflectance measurements. Spectra were measured with ultrafine barium sulfate as reference. The measured reflectance profiles (R∞,%) in the 200-1200 nm range were converted to absorbance (Abs) by Equation (9): The as-obtained UV-DR spectra (absorbance vs. wavelength, nm) in the range 200-800 nm were decomposed using the software OriginPro8 (OriginLab Corporation, Northampton, MA, USA, 2018) with a combination of Gaussian functions.

Mössbauer Spectroscopy
Mössbauer spectra were obtained in transmission geometry using a 512-channel constant acceleration spectrometer (WissEl, Starnberg, Germany), provided with a source of 57 Co in Rh matrix of nominally 50 mCi. The ideal thickness of the sample was evaluated considering the weight percentages of the different elements of each catalyst (ca. 100 mg of powder was used in a holder with diameter of ca. 1.8 cm). The reference used for the velocity calibration was a 12-µm-thick α-Fe foil. Spectra were collected at −260 and 25 • C. Measurements at low temperatures were performed working with a closed-cycle cryogenic system (Model DE-202, ARS, Macungie, PA, USA). Each spectrum was folded to minimize geometric effects; the experimental data were fitted using Recoil [50], a commercial program with constraints.

Catalytic Tests
Catalytic performances of all the Fe/HAP samples in NH 3 -SCR, NH 3 -SCO and N 2 O decomposition were evaluated using a continuous reaction line equipped with a set of mass flow controllers (Bronkhorst, Hi-Tech Instruments, Nijverheidsstraat, Netherlands), a tubular vertical electric oven (Controller-Programmer type 818, Eurotherm, Guanzate, Como, Italy), a quartz tubular catalytic micro reactor (5 mm i.d.), and an online FT-IR spectrophotometer with a DTGS detector (Bio-Rad, Hercules, CA, USA) for qualitative and quantitative determination of the fed and vented gaseous species.
In a typical experiment, a fixed amount of catalyst sample (ca. 200 mg) was pressed, crushed, sieved to obtain particles in the range 45-60 mesh (0.35-0.25 mm), and dried at 120 • C overnight. Then, catalyst pretreatment was performed in situ under O 2 /N 2 flow (20% v/v) at 120 • C for 30 min.
The catalytic activities were studied as a function of the temperature in the 120-500 • C interval for both NH 3 -SCR and NH 3 -SCO, and 300-800 • C for N 2 O decomposition, keeping constant the concentration of fed gas mixture and using a total flow rate of 6 NL·h −1 (GHSV of ca. 30.000 h −1 ). Each temperature was maintained for 60 min to guarantee the attainment of the steady-state reaction condition. The temperature was increased at step in aleatory way using a ramp of 10 • C·min −1 .
The fed gas mixtures were prepared by mixing 500 ppm of NH 3 , 500 ppm of NO and 10,000 ppm of O 2 for NH 3 -SCR tests; 500 ppm of NH 3 and 10,000 ppm of O 2 for NH 3 -SCO tests; ca. 150 ppm of N 2 O and 6000 ppm of O 2 for N 2 O decomposition tests. In all cases, nitrogen was used as inert gas.
The effluent gas mixtures from the reactor was monitored by an online FT-IR consisted of a multiple reflection gas cell-with 2.4 m path length; resolution, 2 cm −1 ; sensibility, 1.5, 92 scans per 180 s-for quantifying the unconverted reagents and/or products.
The total absorbance of all the IR active gaseous species (Gram-Schmidt) vented by the reactor was continuously recorded as a function of time and the reaction temperature was changing. NH 3 , NO, N 2 O and NO 2 were quantified considering the peak height of a selected absorbance line and the measured calibration factors. Details on calculations for each reaction are reported in the Supplementary Material (Table S1).

Conclusions
The results reported in this work, where new Fe-based eco-friendly catalysts were tested in three environmental protection reactions, show that it is possible to overcome the use of more conventional ceramic-based catalysts (alumina, silica, zirconium, etc.) and approach the use of more environmentally friendly and eco-sustainable materials, such as hydroxyapatite. Suitably metal-functionalized, hydroxyapatite can give good catalytic performances in the NO x , N 2 O and NH 3 abatement reactions, which can be exploited for the abatement of gaseous pollutants among the worst of our environment.
Three environmental protection reactions studied (NH 3 -SCR, NH 3 -SCO and N 2 O-decomposition) could also be 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 Fe 3+ ions or oligonuclear species that ensure good activity with an equally good selectivity.
Much work still needs to be done to understand the performances of these new catalysts with dispersed iron on hydroxyapatite. In particular, the catalytic performances must be tested under real conditions, i.e., in the presence of water vapor, sulfur, alkali, etc. Part of this work is ongoing and the results will be presented shortly in the literature.

Supplementary Materials:
The following are available online at http://www.mdpi.com/2073-4344/10/12/1415/s1, Figure S1: N 2 adsorption/desorption isotherms at −196 • C of bare HAP calcined at 500 • C (a) and of two Fe/HAP, presenting the lowest and the highest Fe-loading among Fe-loaded samples (Fe2/HAP IE , b, and Fe13/HAP IE , c, respectively). Figure S2: Activity trend (in µmol NH3 /g) of Fe/HAP samples as a function of Fe-concentration (expressed in mmol Fe /g) with indication of HAP acidity (black marker). Figure Figure S5: UV-vis DR spectra (black curves) of Fe/HAP samples (a-e): total calculated curves (red lines) and decomposed curves (dotted black lines) with the related peak centers are also reported. Figure S6 Mössbauer spectra of Fe/HAP samples (a-d) collected at room temperature. Figure S7: Mössbauer spectra of Fe/HAP samples (a-d) collected at −260 • C. Table S1: Symbols and calculations for computing catalytic parameters. Table S2: Mössbauer parameters of all the Fe/HAP samples at room temperature. Table S3