PROCESS FOR PRODUCING AMMONIA

Abstract

A process for producing ammonia includes the step of contacting nitrogen and water with a catalyst containing permanently polarized hydroxyapatite.

Claims

1. A process for producing ammonia comprising the step of: contacting nitrogen and water with a catalyst comprising or consisting of a permanently polarized hydroxyapatite.

2. The process according to claim 1, wherein the permanently polarized hydroxyapatite has at least one of: a crystallinity>65%; a proportion of amorphous calcium phosphate<18%, based on a total weight of the permanently polarized hydroxyapatite; a proportion of ?-tricalcium phosphate<36% based on the total weight of the permanently polarized hydroxyapatite; a bulk resistance from 10.sup.7 ? cm.sup.2 to 10.sup.5 ? cm.sup.2, wherein the bulk resistance increases by only 4% to 73% after 3 months; and a surface capacitance decreasing less than 8%.

3. The process according to claim 1, wherein the permanently polarized hydroxyapatite is obtained or obtainable by a process comprising the steps of: (a) preparing samples comprising or consisting of hydroxyapatite; (b) sintering the samples prepared in step (a) at a temperature between 700? C. and 1200? C.; (c) applying a constant or variable DC voltage between 250 V and 2500 V for at least 1 minute at a temperature between 900? C. and 1200? C., or applying an equivalent electric field between 1.49 kV/cm and 15 kV/cm for at least 1 minute at a temperature between 900? C. and 1200? C., or applying an electrostatic discharge between 2500 V and 1500000 V for less than 10 minutes at a temperature between 900? C. and 1200? C., or applying an equivalent electric field between 148.9 kV/cm and 8928 kV/cm for less than 10 minutes at a temperature between 900? C. and 1200? C.; and (d) cooling the samples obtained in step (c) while maintaining the DC voltage or the equivalent electric field, or cooling the samples obtained in step (c) while maintaining or without maintaining the electrostatic discharge or the equivalent electric field.

4. The process according to claim 1, wherein the permanently polarized hydroxyapatite is obtained or obtainable by a process comprising the steps of: (a) preparing samples comprising or consisting of hydroxyapatite; (b) sintering the samples prepared in step (a) at a temperature of 1000? C.; (c) applying an equivalent electric field of 3 kV/cm at a temperature of 1000? C.; and (d) cooling the samples obtained in step (c) while maintaining the equivalent electric field.

5. The process according to claim 1, wherein the contacting step is carried out with a volumetric ratio of the water to the catalyst of 10000:1 to 0.1:1.

6. The process according to claim 1, wherein the contacting step is carried out under a pressure of 0.01 bar to 20 bar.

7. The process according to claim 1, wherein the contacting step is carried out with a molar ratio of nitrogen to the catalyst of 400 to 20.

8. The process according to claim 1, wherein the contacting step is carried out at a temperature of ?95? C. to 140? C.

9. The process according to claim 1, wherein the contacting step is carried out for 0.0001 h to 120 h.

10. The process according to claim 1, wherein the contacting step is carried out under UV irradiation or UV-Vis irradiation.

11. The process according to claim 10, wherein the UV irradiation and/or visible light irradiation has an irradiance from 0.1 W/m.sup.2 to 200 W/m.sup.2.

12. The process according to claim 10, wherein a surface of the catalyst is exposed to the UV irradiation or UV-Vis irradiation, wherein the surface of the catalyst being exposed to the UV irradiation or UV-Vis irradiation is not covered by the water.

13. The process according to claim 1, wherein the contacting step is carried under an atmosphere which is, apart from the nitrogen, free or basically free of any further gas.

14. The process according to claim 1, wherein the contacting step is carried out by using air, wherein nitrogen is part of the air.

15. (canceled)

16. The process according to claim 6, wherein the pressure is a pressure of nitrogen.

17. The process according to claim 1, wherein the contacting step is carried under an atmosphere which is, apart from nitrogen and water vapor, free or basically free of any further gas.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0149] For a better understanding of what has been disclosed, some figures are attached which schematically or graphically and solely by way of non-limiting example show a practical case of embodiment of the present invention.

[0150] In the figures, the following is schematically displayed:

[0151] FIG. 1. (a) .sup.1H NMR analysis in DMSO-d.sub.6 of the p-HAp catalyst dissolved, after 96 h of reaction, in 15 mL of water with pH adjusted to 2.1?0.2 using 7.6 mM H.sub.2SO.sub.4. The reaction was conducted using 6 bar of N.sub.2, 20 mL of de-ionized water in contact with the surface, 120? C. and UV illumination. The triplet associated to the ammonium is lighted by the orange rectangle, the HDO peak is shown in the green, and the products coming from the degradation of DMSO-d.sub.6 are marked in violet. The products coming from CO.sub.2 adsorbed on the perfluorinated coating are also indicated (labels show the shift in ppm). (b) .sup.1H NMR analyses in DMSO-d.sub.6 of: the supernatant after reaction using the same conditions that in (a) but without p-HAp catalyst (blank; in grey). A total 15 mL of water with pH adjusted to 2.1?0.2 using 7.6 mM H.sub.2SO.sub.4 were previously added to detect the possible formation of NH.sub.4; the p-HAp catalyst dissolved, after reaction, in 15 mL of water with pH adjusted to 2.1?0.2 using 7.6 mM H.sub.2SO.sub.4. The reaction conditions were identical to those described in (a) but without UV illumination (catalyst without UV, in red); and (c) the same catalyst analyzed in (a), as a positive control (catalyst with UV radiation, in blue).

[0152] FIG. 2. (a) 1H NMR spectrum in DMSO-d.sub.6 of the supernatant, to which 15 mL of water with pH adjusted to 2.1?0.2 using 7.6 mM H.sub.2SO.sub.4 were added to visualize the NH.sub.4, and of the p-HAp catalyst dissolved in the same H.sub.2SO.sub.4 solution after 96 h of reaction under UV illumination. The triplet associated to the ammonium formation is marked (light orange rectangle). Complete spectra (including the products coming from the adsorbed CO.sub.2 reaction) are shown in FIG. 7. (b) Yield of NH.sub.4.sup.+ (expressed in ?mol/g of catalyst) in the catalyst and in the supernatant after the reaction with and without UV illumination. For (a) and (b) the reaction was conducted for 96 h at 120? C. using N.sub.2 (6 bar) and H.sub.2O (20 ml).

[0153] FIG. 3. Influence of the (a) N.sub.2 pressure, (b) the temperature, (c) the initial volume of water and the (d) reaction time on the yield of NH.sub.4.sup.+. Results were evaluated by analyzing the 1H NMR spectrum of the p-HAp catalyst dissolved in H.sub.2SO.sub.4 solution with DMSO-d.sub.6 and the 1H NMR spectrum of the supernatant, to which a H.sub.2SO.sub.4 solution with DMSO-d.sub.6 was added to visualize the NH.sub.4.sup.+. The total yield of NH.sub.4.sup.+ (expressed in ?mol/g of catalyst) corresponds to the sum of the yields derived from the catalyst and the supernatant. All reactions were performed in triplicate under UV radiation. Reactions were conducted under the following conditions: (a) reaction time of 48 h, temperature of 120? C. and initial water volume of 20 mL; (b) reaction time of 48 h, N.sub.2 pressure of 6 bar and initial water volume of 20 mL; (c) reaction time of 48 h, N.sub.2 pressure of 6 bar and temperature of 120? C.; and (d) N.sub.2 pressure of 6 bar, temperature of 120? C. and initial water volume of 20 ml.

[0154] FIG. 4. .sup.1H NMR spectrum in DMSO-d.sub.6 of (a) the catalyst and (b) the supernatant after 96 h of reaction of polluted air. The catalyst was dissolved in 15 mL of water with pH adjusted to 2.1?0.2 using 7.6 mM H.sub.2SO.sub.4, while the same volume of solution was added to the supernatant. The reaction was conducted at atmospheric pressure at 120? C. using water (20 mL) and under UV illumination. The triplet associated to the ammonium formation is marked (light orange rectangle).

[0155] FIG. 5. Sketch showing that de-ionized water was not covering the p-HAp catalyst and the migration of the formed ammonia molecular towards the liquid.

[0156] FIG. 6. .sup.1H NMR spectrum of the supernatant (water) in the reaction without catalyst under UV radiation (blank reaction; in grey), the p-HAp/c catalyst dissolved in H.sub.2SO.sub.4 solution with DMSO-d.sub.6 after the reaction without UV radiation (control reaction; in red), and the p-HAp/c catalyst dissolved in H.sub.2SO.sub.4 solution with DMSO-d.sub.6 after the reaction under UV radiation (standard reaction; in blue). In all cases the reaction was conducted for 96 h at 120? C. using N.sub.2 (6 bar) and H.sub.2O (20 ml).

[0157] FIG. 7. .sup.1H NMR spectrum of the supernatant, to which a H.sub.2SO.sub.4 solution with DMSO-d.sub.6 was added to visualize the 1:1:1 triplet from NH.sub.4.sup.+, and of the p-HAp/c catalyst dissolved in H.sub.2SO.sub.4 solution with DMSO-d.sub.6 after the reaction under UV radiation. The reaction was conducted for 96 h at 120? C. using N.sub.2 (6 bar) and H.sub.2O (20 ml).

[0158] FIG. 8. Yield of formic acid, acetone and acetic acid (expressed in ?mol/g of catalyst) in the catalyst (a), in the supernatant (b) and the sum of both (c). The yield of the products coming from the desorption of CO.sub.2 was evaluated for reactions conducted for 48 or 96 h at 120? C. using N.sub.2 (6 bar) and H.sub.2O (20 ml) under UV illumination.

[0159] FIG. 9. A .sup.31P-NMR spectrum of permanently polarized hydroxyapatite.

[0160] FIG. 10. A wide angle x-ray scattering (WAXS) pattern of an inventive catalyst comprising or consisting of permanently polarized hydroxyapatite and brushite and/or brushite-like material.

[0161] FIG. 11. A Raman spectrum of an inventive catalyst comprising or consisting of permanently polarized hydroxyapatite and brushite and/or brushite-like material.

DETAILED DESCRIPTION

Example Section

1. Synthesis of Hydroxyapatite (HAp)

[0162] 15 mL of 0.5 M of (NH.sub.4).sub.2HPO.sub.4 in de-ionized water were added at a rate of 2 mL/min to 25 mL of 0.5 M of Ca(NO.sub.3).sub.2 in ethanol (with pH previously adjusted to 11 using ammonium hydroxide solution) and left aging for 1 h. The whole process was performed under gentle agitation (150 rpm) and at room temperature. Hydrothermal treatment at 150? C. was applied using an autoclave marketed under the trademark DIGESTEC DAB-2? for 24 h. The autoclave was allowed to cool down before opening. The precipitates were separated by centrifugation and washed with water and a 60/40 v/v mixture of ethanol-water (twice). After freeze-drying it for 3 days, the white powder obtained was sintered for 2 h at 1000? C. in air using a furnace marketed under the registered trademark CARBOLITE? ELF11/6B/301.

2. Polarization of Hydroxyapatite (HAp)

[0163] Mechanical consistent discs of ?1.5 mm of thickness and 1.766 mm of diameter were obtained by pressing 150 mg of HAp powder at 620 MPa for 10 min in a mold. Thermal polarization was done placing the HAp discs between two stainless steel (AISI 304) and applying a constant DC voltage conducted by Pt cables of 500 V for 1 h with a GAMMA power supply, while temperature was kept at 1000? C. during such a period using the same laboratory furnace. The discs were allowed to cool down maintaining the applied electric potential for 30 minutes, and finally, all the system was powered off and left to cool overnight.

3. Synthesis

[0164] A high pressure stainless steel reactor was employed to perform the catalytic reactions. The reactor had an inert reaction chamber coated with a perfluorinated polymer (120 mL) where both the catalyst and water were incorporated. The reactor was equipped with an inlet valve for the entrance of gases (i.e. N.sub.2) and an outlet valve to recover the gaseous reaction products. A UV lamp (GPH265T5L/4, 253.7 nm) was also placed in the middle of the reactor to irradiate the catalyst directly, the lamp being protected by a UV transparent quartz tube. All surfaces were coated with a thin film of a perfluorinated polymer in order to avoid any contact between the reaction medium and the reactor surfaces, in this way discarding other catalyst effects.

[0165] Catalyst samples, weighting approximately 150 mg, and de-ionized liquid water were initially incorporated into the reaction chamber. The chamber was extensively purged with N.sub.2 in order to eliminate the initial air content. After this, N.sub.2 gas was introduced to increase the reaction chamber pressure (measured at room temperature) to the target pressure.

[0166] The reaction products were analyzed by 1H NMR spectroscopy. All .sup.1H NMR spectra were acquired with a Bruker Avance-II+ spectrometer operating at 600 MHz. The chemical shift was calibrated using tetramethylsilane (TMS) internal standard. 512 scans were recorded in all cases. In order to remove the reaction products from the catalyst, 10 mg of the reacted catalyst were dissolved in 15 mL of water with pH adjusted to 2.1?0.2 using 7.6 mM H.sub.2SO.sub.4, to promote the conversion of ammonia in NH.sub.4.sup.+, and applying 4 cycles that involved sonication (5 min) and stirring (1 min) steps. Then, for the 1H NMR sample preparation, 500 ?L of the reacted catalyst solution were mixed with 100 ?L of DMSO-d.sub.6 instead of solvents with labile deuterons (i.e. D20) to avoid the formation of ammonium deuterated analogues, not desired for quantitative analysis. The same treatment was applied to the water supernatant.

4. N.SUB.2 .Fixation to Ammonia

[0167] The p-HAp electrocatalyst was prepared as described in previous work (J. Sans, E. Armelin, V. Sanz, J. Puiggali, P. Turon and C. Alem?n, J. Catal., 2020, 389, 646-656; J. Sans, V. Sanz, J. Puiggali and P. Turon, Cryst. Growth Des. 2021, 21, 748-756). In brief, after hydrothermal synthesis of HAp using a recently proposed procedure to control the anisotropic growth, the resulting powder was sintered at 1000? C. Then, discs of ?1.5 mm thickness and 1.766 mm diameter were obtained by pressing in a mold. Then, the discs were polarized applying a DC voltage of 500 V for 1 h at 1000? C.

[0168] In order to investigate the electrocatalytic synthesis of ammonia over p-HAp, a reaction was performed at 120? C. in a stainless steel reactor with an inert reaction chamber (i.e. a chamber coated with a perfluorinated polymer) illuminated with UV light. In order to eliminate the initial air content, the chamber was firstly purged with the N.sub.2 and, subsequently, filled with N.sub.2 (6 bar). A volume of 20 mL of de-ionized water was introduced in the reactor and put in contact with the non-irradiated side of the p-HAp disk, as is sketched in FIG. 5. This represents a significant change with respect to previous carbon-fixation reactions using p-HAp, in which 1 mL of water was put in contact with the catalyst to promote the electro-reduction of CO.sub.2 (M. Rivas, L. J. del Valle, P. Turon, C. Alem?n and J. Puiggali, Green Chem., 2018, 20, 685-693). In the present study, water was expected to act not only as the proton source for the ammonia formation (from water splitting) but also a medium to facilitate the recovery of the formed products (hereafter named supernatant) and, therefore, the volume of liquid was noticeably increased.

[0169] The products generated on the surface of the p-HAp disk after 96 h of reaction were identified adapting a procedure for rapid NH.sub.4.sup.+ analyses using 1H NMR spectroscopy (R. Y. Hodgetts, A. S. Kiryutin, P. Nichols, H.-L. Du, J. M. Bakker, D. R. Macfarlane and A. N. Simonov, ACS Energy Lett., 2020, 5, 736-741). More specifically, 10 mg of the reacted catalyst were dissolved in 15 mL of water with pH adjusted to 2.1?0.2 using 7.6 mM H.sub.2SO.sub.4, to promote the conversion of ammonia in NH.sub.4, and applying 4 cycles that involved sonication (5 min) and stirring (1 min) steps. Then, for the 1H NMR sample preparation, 500 ?L of the reacted catalyst solution were mixed with 100 ?L of DMSO-d.sub.6. As is illustrated in FIG. 1a, in addition of the characteristic DMSO-d.sub.6 and H.sub.2O signals, the 1H NMR collected using this procedure showed in all cases the HDO fingerprint at 3.49 ppm, coming from the proton exchange with deuterium in acid environment (green rectangle in FIG. 1a) (G. Fulmer, A. J. M. Miller, N. H. Sherden, H. E. Gottlieb, A. Nudelman, B. M. Stoltz, J. E. Bercaw, K. I. Goldberg, Organometallics, 2010, 29, 2176-2179). Furthermore, the signals at 1.01 and 3.18 pm (violet rectangle in FIG. 1a), which are systematically detected, have been attributed to degradation products (i.e. .sup..Math.CH.sub.3 and methanesulfonic acid, respectively) from the oxidation of DMSO-do by electroactive species coming from the catalyst (S. Enami, Y. Sakamoto, K. Hara, K. Osada, R. M. Hoffmann and J. A. Colussi, Environ. Sci. Technol., 2016, 50, 1834-1843; S. Mukhopadhyay, M. Zerella and T. A. Bell, Stud. Surf. Sci. Catal., 2004, 147, 523-528). FIG. 1a allows to detect the presence of ammonium at around 7 ppm as a relatively sharp 1:1:1 NH.sub.4.sup.+ triplet due to the spin coupling to 14N. Quantification of NH.sub.4.sup.+ (11.0?1.6 ?mol/g of catalyst) was performed by integrating the 1:1:1 triplet against the signal of a known concentration of NH.sub.4.sup.+ as internal standard.

[0170] Other products coming from CO.sub.2 fixation were also identified in the 1H NMR spectrum: formic acid (8.07 ppm), acetone (2.06 ppm) and acetic acid (1.92 ppm). Although p-HAp was found to catalyze the electroreduction of CO.sub.2, the source of such gas in the reaction chamber was initially uncertain. After different tests aimed at having an exhaustive purge of the reactor chamber, ensuring the elimination of gases other than N.sub.2, and several blank and control reactions, it was concluded that the CO.sub.2 adsorbed by the perfluorinated polymer, which coated all surfaces of the reaction chamber, was the source for the carbon-fixation reaction. Thus, although the yield of NH.sub.4.sup.+ was null in absence of catalyst (blank reaction) and very low (1.3?0.5 ?mol/g) when the p-HAp was not irradiated with UV light (control reaction), weak signals associated to formic acid, acetone and acetic acid were still detectable in the former case while they were not detected in the latter one (FIG. 1b and FIG. 6). Fluorinated polymers are known to be CO.sub.2-philic materials due to the affinity of strong polar CF bonds towards the CO.sub.2 molecule (G. Li, B. Zhang and Z. Wang, Macromolecules, 2016, 49, 2575-2581; Y. F. Zhao, K. X. Yao, B. Y. Teng, T. Zhang and Y. Han, Energy Environ. Sci., 2013, 6, 3684-3692; D. P. Liu, Q. Chen, Y. C. Zhao, L. M. Zhang, A. D. Qi, B. H. Han, ACS Macro Lett., 2013, 2, 522-526; J. J. Reisinger and M. A. Hillmyer, Prog. Polym. Sci., 2002, 27, 971-1005), results suggesting that the desorption of the gas absorbed on the perfluorinated coating under UV irradiation at 120? C.

[0171] An important aspect to be considered is the transfer of the ammonia molecules formed on the surface of the catalyst to the water medium, which is in contact with the p-HAp disk (FIG. 5). In order ascertain the presence of ammonia in the liquid, 15 mL of water with pH adjusted to 2.1?0.2 using 7.6 mM H.sub.2SO.sub.4 were added to the supernatant before to collect the 1H NMR spectrum. Comparison of the .sup.1H spectra recorded for the supernatant and the dissolved catalyst revealed that the ammonium triplet is more intense in the former than in the latter (FIG. 2a). Indeed, the yield was 70% higher in the supernatant than in the dissolved catalyst. This is reflected in FIG. 2b, which compares the yield of ammonium in the dissolved catalyst and in the supernatant with those achieved using control experiments without UV illumination. Without UV light, the effective barrier of the p-HAp is too high, incident UV illumination being required to overcome the activation energy barrier for nitrogen-fixation. These results have important implications not only in extracting the produced ammonia, which can be directly removed from the liquid phase, but also in preventing the catalyst poisoning by accumulation of ammonia. Besides, peaks associated to the CO.sub.2 fixation were also detected in the supernatant, as is evidenced in FIG. 7.

[0172] Analyses of the supernatants have been also used to study the influence of the reaction time in the yield of products coming from desorbed CO.sub.2 fixation. Results are displayed in FIG. 8, which compares the yield of formic acid, acetone and acetic acid in the p-HAp catalyst, in the supernatant and the sum both, after 48 and 96 h of reaction using 6 bar of N.sub.2 at 120? C. and under UV illumination. The increment of the reaction time had a huge impact in the CO.sub.2 fixation reaction, which was particularly noticeable in the formic and acetic acids (i.e. the yield increased by around 97% and 130%, respectively). Thus, the total yield of carbon-based valuable products increased from 55.9?8.5 to 113.2?15.6 ?mol/g of catalyst when the reaction time expanded from 48 to 96 h. Overall, these results show that desorption of the CO.sub.2 molecules from the perfluorinated coating and their subsequent conversion into value organic products occur progressively.

5. Influence of the Pressure, Temperature, Water Volume and Time on the Yield of Ammonia

[0173] The influence of different factors on the yield of ammonia is described in FIG. 3. In all cases reactions were conducted using the p-HAp electrocatalyst and under UV illumination. Firstly, experiments were performed by changing the initial N.sub.2 pressure from 1 to 6 bar while the temperature and the initial content of water were kept at 120? C. and 20 mL, respectively. After 48 h of reaction, it can be seen that the yield of NH.sub.4.sup.+ increased with the N.sub.2 pressure (FIG. 3a). Indeed, the amount of produced NH.sub.4.sup.+ that adsorbed by the catalyst is practically null for reactions with a pressure?2 bar. Above such pressure the total yield increased rapidly, even though the adsorption on the p-HAp catalyst stabilizes at 4 bar.

[0174] Another important factor that deserves consideration is the reaction temperature. This was varied from 95 to 140? C. (FIG. 3b), while the N.sub.2 pressure, the initial content of water and the reaction time were kept at 6 bar, 20 mL and 48 h, respectively. Although the yield of NH.sub.4.sup.+ on the catalyst increased with the temperature, the ammonium collected in the supernatant at 120? C. and 140? C. was practically identical (i.e. 9.8?1.5 and 9.9?1.0 ?mol/g of catalyst, respectively). Consequently, the total yield of synthesized NH.sub.4.sup.+ experienced a drastic enhancement (112%) when the reaction temperature was increased from 95? C. to 120? C., whereas such increment was very low (16% only) when it was changed from 120? C. to 140? C. These results indicate that, although the nitrogen fixation reaction induced by the p-HAp catalyst operates at temperatures higher than the fixation promoted by Nitrogenase in nature (ambient temperature), the required temperature is much lower than that applied in the Haber-Bosch process (375-500? C.).

[0175] The volume of water introduced in the reactor, which is the source of protons for NH.sub.4 production, is a key parameter that deserves consideration. Reactions were conducted considering 0, 10, 20 and 40 mL of water in contact with the p-HAp catalyst (see FIG. 5). In absence of water, the amount of NH.sub.4.sup.+ extracted from the catalyst was practically null (FIG. 3c). In this case, the total yield (2.0?0.3 ?mol/g of catalyst) was attributed to the adsorption of water from the atmosphere on the catalyst during its manipulation. Thus, water contact angle measurements showed that p-HAp is a very hydrophilic material (M. Rivas, L. J. del Valle, E. Armelin, O. Bertran, P. Turon, J. Puiggali and C. Alem?n, ChemPhysChem, 2018, 19, 1746-1755). After that, the yield of NH.sub.4.sup.+ increased with the volume of water, even though this enhancement was less pronounced once the 20 ml threshold was exceeded. Thus, the yield increased by 63% and 34% when the volume of water varies from 10 ml to 20 ml and from 20 ml to 40 ml, respectively.

[0176] The influence of the time on the yield of NH.sub.4.sup.+ was examined considering reactions of 24 h, 48 h and 96 h while the N.sub.2 pressure, the temperature and the initial content of water were kept at 6 bar, 120? C. and 20 ml, respectively (FIG. 3d). The relative yield increment decreased with increasing time. Thus, the total yield increased by 150% when time grew from 24 h to 48 h, while this increment decreased to 76% only when time changed from 48 h to 96 h. The evolution of the yield from the catalyst and the supernatant followed the same behavior.

6. Proof of Concept: Polluted Air

[0177] As a proof of concept, the performance of the catalyst was explored with polluted air at atmospheric pressure. More specifically, air polluted by the combustion of fossil carburant was captured from a road with a large volume of traffic of cars and trucks and transferred to the reaction chamber. In addition of N.sub.2 and O.sub.2, the content of CO.sub.2, CH.sub.4 and other pollutants was significantly higher than the average of the ambient air. Therefore, the valuable products coming from both CO.sub.2-, CH.sub.4- and N.sub.2-fixation were expected to be obtained by exposing the polluted air to the optimized reaction conditions. The reaction was conducted using p-HAp in contact with 20 ml of water at 120? C. and under UV radiation. Representative .sup.1H NMR spectra of the catalyst solution and the supernatant after 96 h reactions are shown in FIG. 4, whereas the average yields for the different reaction products identified for three independent replicas are listed in the following Table 1.

[0178] As it can be seen, NH.sub.4.sup.+ was observed in both the catalyst and the supernatant, even though the amount detected in the last was four times greater than in the first. The total yield of ammonium was of 20.7?4.7 ?mol/g of catalyst. Although this value was lower than the one observed using 6 bar of N.sub.2 and 96 h at 120? C. (27.3?2.8 ?mol/g of catalyst), the difference was less than expected, suggesting that other components and/or pollutants of air, as for example Oz and NO, could affect favorably to nitrogen fixation.

TABLE-US-00001 TABLE 1 Yield of products (in ?mol/g of catalyst) coming from the nitrogen and carbon fixation NH.sub.4.sup.+ as extracted from the catalyst and the supernatant after the reaction of polluted air (see FIG. 4). Catalyst Supernatant Total Product (?mol/g) (?mol/g) (?mol/g) NH.sub.4.sup.+ 4.1 ? 0.8 16.6 ? 3.9 20.7 ? 4.7 HCOOH 20.1 ? 4.1 17.5 ? 2.7 37.6 ? 6.8 CH.sub.3CH.sub.2OH 3.5 ? 0.4 2.8 ? 0.4 6.3 ? 0.8 CH.sub.3COCH.sub.3 7.8 ? 2.1 0.8 ? 0.1 8.6 ? 2.3 CH.sub.3COOH 12.6 ? 1.9 52.8 ? 8.1 65.4 ? 10.0 Total coming 44.0 ? 8.5 73.7 ? 11.3 118.7 ? 19.8 from CO.sub.2 fixation Total of 48.1 ? 8.6 90.3 ? 15.2 138.4 ? 23.8 valuable products

[0179] Valuable products coming from carbon fixation were also detected. In addition of formic acid, acetone and acetic acid, which were previously detected as a consequence of the CO.sub.2 desorption from the perfluorinated coating, ethanol was also identified. This was not a surprising result since previous studies proved that p-HAp catalyzes the formation of ethanol by carbon fixation from mixtures of CO.sub.2 and CH.sub.4, the latter among the common urban volatile organic compound emissions. Overall, these results demonstrate that the p-HAp catalyst cleans polluted air producing valuable compounds using mild reaction condition that can be employed as raw material for manufacturing fertilizers and other chemicals. It is worth noting that the simultaneous fixation of N.sub.2 and CO.sub.2 is a paradox since the CO.sub.2 emitted by conventional production of ammonia using N.sub.2 and H.sub.2 causes massive greenhouse effect. Within this context, the p-HAp appears to be a step in the right direction to fight anthropogenic climate change without detriment in the production of fertilizers.

7. Conclusions

[0180] The electrosynthesis of ammonia from N.sub.2 and water with p-HAp has been demonstrated using mild reaction conditions. The yield of the reaction has been optimized by considering the temperature, the N.sub.2 pressure, the volume of water and the reaction time. The main part of the produced ammonia migrates from the catalyst to the water supernatant, which is in contact with the surface of the catalyst, facilitating its recovery and avoiding the catalyst saturation. On the other hand, this catalyst is also able to convert CO.sub.2 into valuable chemical products, such as formic acid, ethanol and acetone. The coexistence of nitrogen- and carbon-fixation processes and the migration of the products to the liquid phase suggest that p-HAp is particularly suitable for the catalytic cleaning of polluted air. Within this context, the reaction produced using 1 bar of air in polluted by vehicle emissions resulted in the formation of 138.4?23.8 ?mol of valuable chemicals/g of catalyst (i.e. 118.7?19.8 and 20.7?4.7 ?mol/g from carbon- and nitrogen-fixation processes).