METHOD FOR PREPARING HIGHLY NITROGEN-DOPED MESOPOROUS CARBON COMPOSITES

Abstract

Some embodiments are directed to a new methodology aimed at preparing highly N-doped mesoporous carbon macroscopic composites, and their use as highly efficient heterogeneous metal-free catalysts in a number of industrially relevant catalytic transformations.

Claims

1. A method of preparing macroscopic composites made of a macroscopic support coated with a thin layer of highly nitrogen-doped mesoporous carbon phase (active phase), said method comprising: (a) providing an aqueous solution of (i) (NH.sub.4).sub.2CO.sub.3; (ii) a carbohydrate as carbon source, selected from aldose monosaccharides and glycosilated forms thereof, disaccharides and oligosaccharides or dextrine deriving from biomass conversion, and (iii) a carboxylic acid source selected from citric acid, and any other mono-, di-, tri-, and poly-carboxylic acid or their ammonium mono-, di-, tri- and poly-basic forms; (b) providing a macroscopic support made of carbon-, silicium- or aluminum-based material, or binary mixtures thereof; wherein the macroscopic support is a single object or an assembly of smaller objects, wherein the overall dimension of the support ranges from 0.1 μm to 100 cm in three orthogonal directions; optionally subjecting the maccroscopic support of step (b) to a passivation process comprising steps of: (a1) providing an aqueous solution of citric acid and a carbohydrate as carbon source, selected from aldose monosaccharides and glycosilated forms thereof, disaccharides and oligosaccharides; (c1) prior to step (c), immerging/soaking or impregnating the macroscopic support of step (b) in the aqueous solution of step (a1) for a suitable amount of time; (d1) optionally removing the immerged macroscopic support from the aqueous solution of step (a1) if an excess aqueous solution is used in step (c1); (e1′) optionally subjecting the resulting macroscopic support to a gentle thermal treatment (drying) under air at low temperatures from 45 to 55° C., preferably 50° C.±3° C.; (e1) subjecting the resulting macroscopic support to a first thermal treatment (drying) under air at moderate temperatures from 110-150° C.±5° C., preferably 130° C.±5° C.; and (f1) subjecting the thermally treated (dried) macroscopic support to a second thermal treatment under inert atmosphere at higher temperatures from 600-800° C.±10° C., preferably 600° C.±5° C.; thereby generating a macroscopic composite coated with a carbon layer; (c) immerging/soaking or impregnating the macroscopic support of step (b), or the passivated macroscopic support obtained in step (f1) when a passivation process is used, in the aqueous solution of step (a) for a suitable amount of time; (d) optionally removing the immerged macroscopic support from the aqueous solution of step (a) if an excess aqueous solution is used in step (c); (e′) optionally subjecting the resulting macroscopic support to a gentle thermal treatment (drying) under air at low temperatures from 45 to 55° C., preferably 50° C.±3° C.; (e) subjecting the resulting macroscopic support to a first thermal treatment (drying) under air at moderate temperatures from 110-150° C.±5° C., preferably 130° C.±5° C.; (f) optionally subjecting the thermally treated (dried) macroscopic support to a second thermal treatment under air at higher temperatures: from 400-500° C.±10° C., preferably 400° C.±5° C. for 1 to 2 hours, or at 300° C.±10° C. for 2 to 4 hours; thereby generating a macroscopic composite composed of a macroscopic support coated with a 20-200 nm thick layer of highly N-doped mesoporous carbonaceous material; wherein the N atom % in the mesoporous carbonaceous material is 25-40%; and (g) optionally subjecting the macroscopic composite obtained in step (f) to a third thermal treatment by heating it to a temperature ranging between 600 to 900° C.±10° C. under inert atmosphere, preferably 700 to 900° C.±10° C.; thereby generating a macroscopic composite composed of a macroscopic support coated with a 10-100 nm thick layer of highly N-doped mesoporous carbonaceous material; wherein the N atom % in the mesoporous carbonaceous material is 2-35%, preferably 5-30%, most preferably about 15% (12-18%); wherein the method comprises at least one of steps (f) or (g).

2. The method of claim 1, wherein steps (c) through (f) are repeated at least once prior to carrying out step (g).

3. The method of claim 1, wherein in the aqueous solution of step (a), (NH.sub.4).sub.2CO.sub.3 is present at a concentration ranging 1 to 8 mol/L preferably from 2 to 5 mol/L; the carbohydrate carbon source is present at a concentration ranging from 1 to 5 mol/L preferably 2 to 4 mol/L; and the carboxylic acid source is present at a concentration ranging from 1 to 3 mol/L, preferably 2 mol/L.

4. The method of any one of claim 1, wherein the macroscopic support is made of a material selected from β-SiC or α-SiC or SiC-based supports, either pure or doped with foreign elements such as TiO.sub.2, SiO.sub.2, Al.sub.2O.sub.3, alumina (α- or β-Al.sub.2O.sub.3 or alumina-based supports, either pure or doped with foreign elements such as TiO.sub.2, SiO.sub.2); or carbon, each of which may be in the form of grains, flakes, rings, pellets, extrudates, beads or foam; or carbon nanotubes, carbon nanofibers, graphene or few-layer graphene.

5. The method of claim 1, wherein the macroscopic support is made of silica (SiO.sub.2), SiC, alumina (Al.sub.2O.sub.3) or titania (TiO.sub.2); preferably silica (SiO.sub.2), SiC or alumina (Al.sub.2O.sub.3).

6. The method of claim 1, wherein the macroscopic support is made of silica (SiO.sub.2), alumina (Al.sub.2O.sub.3) or titania (TiO.sub.2), preferably silica (SiO.sub.2) or alumina (Al.sub.2O.sub.3), the method further comprising the passivation process.

7. The method of claim 1, wherein the immerging/soaking or impregnating step (c) is carried out for 1 to 10 minutes, preferably 2 minutes.

8. The method of claim 1, wherein the first thermal treatment step (e) is carried out for 1 to 10 hours preferably 1 to 2 hours.

9. The method of claim 1, wherein the second thermal treatment step (f) is carried out for 1 to 10 hours preferably 1 to 2 hours.

10. The method of claim 1, wherein the third thermal treatment step (g) is carried out for 1 to 10 hours preferably 2 hours.

11. A macroscopic composite coated with a layer of highly N-doped mesoporous carbonaceous material wherein the N-doped carbonaceous material layer: has an N atom contents of 1-40%, preferably 5-40%, preferably 10-35%, preferably about 15% (e.g., 12-18%); has an average pore size of 2-50 nm, preferably 2-30 nm; most preferably 3-12 nm; and has a thickness of 5 to 200±5 nm, preferably 10 to 100±5 nm.

12. A method of performing a catalytic reaction, comprising: providing a macroscopic composite coated with a layer of highly N-doped mesoporous carbonaceous material, wherein the N-doped carbonaceous material layer: has an N atom contents of 1-40%, preferably 5-40%, preferably 10-35%, preferably about 15% (e.g., 12-18%); has an average pore size of 2-50 nm, preferably 2-30 nm; most preferably 3-12 nm; and has a thickness of 5 to 200±5 nm, preferably 10 to 100±5 nm; and initiating the catalytic reaction with the macroscopic composite.

13. The method according to claim 12, further including performing the catalytic reaction with the macroscopic composite in oxygen reduction reaction, steam-free dehydrogenation of hydrocarbons or partial oxidation of H.sub.2S into elemental sulfur.

14. A method for performing a catalytic reaction, comprising: providing a macroscopic composite coated with a layer of highly N-doped mesoporous carbonaceous material, wherein the N-doped carbonaceous material layer: has an N atom contents of 1-40%, preferably 5-40%, preferably 10-35%, preferably about 15% (e.g., 12-18%); has an average pore size of 2-50 nm, preferably 2-30 nm; most preferably 3-12 nm; and has a thickness of 5 to 200±5 nm, preferably 10 to 100±5 nm; and initiating the catalytic reaction with the macroscopic composite as catalytic support for metal(s) and oxide(s) in at least one of liquid-phase and gas-phase hydrogenation, oxidation of linear alkanes and volatil organic compounds (VOCs), hydrogenation of CO in the Fischer-Tropsch process, and methanization of synthesis gas mixture.

15. A method of manufacturing, comprising: providing a macroscopic composite coated with a layer of highly N-doped mesoporous carbonaceous material, wherein the N-doped carbonaceous material layer: has an N atom contents of 1-40%, preferably 5-40%, preferably 10-35%, preferably about 15% (e.g., 12-18%); has an average pore size of 2-50 nm, preferably 2-30 nm; most preferably 3-12 nm; and has a thickness of 5 to 200±5 nm, preferably 10 to 100±5 nm; and manufacturing metal-free surface heater in inductive heating devices with the macroscopic composite.

16. A method of performing absorption, comprising: providing a macroscopic composite coated with a layer of highly N-doped mesoporous carbonaceous material, wherein the N-doped carbonaceous material layer: has an N atom contents of 1-40%, preferably 5-40%, preferably 10-35%, preferably about 15% (e.g., 12-18%); has an average pore size of 2-50 nm, preferably 2-30 nm; most preferably 3-12 nm; and has a thickness of 5 to 200±5 nm, preferably 10 to 100±5 nm; and performing absorption with the macroscopic composite for volatil organic compounds.

17. A method of performing a catalytic reaction, comprising: providing a macroscopic composite coated with a layer of highly N-doped mesoporous carbonaceous material, wherein the N-doped carbonaceous material layer: has an N atom contents of 1-40%, preferably 5-40%, preferably 10-35%, preferably about 15% (e.g., 12-18%); has an average pore size of 2-50 nm, preferably 2-30 nm; most preferably 3-12 nm; and has a thickness of 5 to 200±5 nm, preferably 10 to 100±5 nm; and initiating the catalytic reaction with the macroscopic composite as a metal-free catalyst in the Advanced Oxidation Processes for water and wastewater treatment.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0100] Hereafter, the macroscopic composite or catalyst code is indicated as follows: (G)N@C/SiC.sub.g.sup.2nd, where (G) indicates that the catalyst has been graphitized under inert atmosphere at high temperature (700-1000° C.) (step (g)); N@C designates the nitrogen doped mesoporous carbon active phase; SiC refers to the nature of the support, e.g., silicon carbide (SiC), alumina (Al.sub.2O.sub.3), silica (SiO.sub.2), etc.; subscript characters designates the macroscopic morphology of the support: (g) for grains, (e) for extrudates, (p) for pellets, (f) for foam; and the last superscript number designates the number of impregnation cycles followed by thermal treatment in air at 450° C. before the graphitization step (step (g)).

[0101] FIG. 1 schematizes an exemplary synthetic procedure for the preparation of macroscopic N-rich carbonaceous composites the invention. (A) Representation of a typical thermal sequence for “dressing” a model silicon-carbide foam support (SiC) with a mesoporous, N-rich catalytically active phase (N@C). (B) Provides the supposed mechanism for the thermal transformation of α,β-D-glucopyranose and ammonium citrate tribasic (pre-catalyst phase) into a N-rich mesoporous-carbon graphene-like network (N@C—catalytically active phase).

[0102] FIG. 2 reports selected chemico-physical and morphological characterizations of a (G)N@C/X.sub.g.sup.2nd active phase prepared according to the method of the invention. (A) BET specific surface area of the as-prepared (G)N@C/X.sub.g.sup.2nd (with X=SiC in the form of powder, grains, extrudates and Al.sub.2O.sub.3 in the form of beads)prepared on supports featured by different chemical composition, shape and size. (B) XPS N is spectra analysis of the nitrogen species present in the N@C catalytically active phase (Thermal treatment: 2h at 450° C. in air (N@C/SiC.sub.p) and then graphitized ((G)N@C/SiC.sub.p) 2h at 900° C. under helium atmosphere) (C) HRTEM micrographs of the N@C layer at the (G)N@C/SiC.sub.g.sup.2nd composite [N-doped composite based on SiC grains as support, after 2 impregnation cycles (c-e steps), thermal treatment at 450° C. (f step) and graphitization at 900° C. under helium atmosphere (g step)]. (D) (Top left) An overview bright field TEM image. (Bottom left) A high-angle annular-dark-field image (STEM-HAADF) which was taken from the area marked by the dashed-line box on the top left image. (Four images on the right side) elemental maps of Silicon, Carbon, Nitrogen and Oxygen respectively taken from the area highlighted by the dashed-line box in the bottom-left image. The colors correspond to the relatively atomic composition ratio indicated by the adjacent vertical bars.

[0103] FIG. 3 illustrates an exemplary catalytic application of a (G)N@C/SiC.sub.g.sup.2nd catalyst prepared according to the invention, in the electrocatalytic Oxygen Reduction Reaction. (A) ORR performance of the (G)N@C/SiC.sub.g.sup.2nd (Average SiC size: 40 μm, 450 μg.Math.cm.sup.2 of N@C active phase), VXC-72 (100 μg.Math.cm.sup.2 of N@C active phase) and Pt-20/XC72 Vulcan (25 μgp.sub.Pt.Math.cm.sup.−2), reaction temperature=25° C., rpm=1600 rpm, KOH 0.1 M). (B) Long-term stability of the (G)N@C/SiC.sub.g.sup.2nd and Pt-20/XC72 catalysts at 25° C. with a RRDE system. The test was carried out for 1,500 cycles between 1.0 and 0.6 mV. (C) Koutecky-Levich plots as recorded at 0.4V for curves registered at different electrode rotation rates (rpm) (D) RRDE measurements on both (G)N@C/SiC.sub.g.sup.2nd and Pt-20/C catalysts under alkaline environment (KOH 0.1M at 25° C.) and respective ring current values (H.sub.2O.sub.2 production) for the calculation of the average number of electron transfer.

[0104] FIG. 4 illustrates an exemplary catalytic application of (G)N@C/SiC.sub.e.sup.1st-composites prepared according to the invention, in the high temperature, oxidation of H.sub.2S residues (desulfurization) from gaseous industrial effluents. (A) Desulfurization performance of the (G)N@C/SiC.sub.e.sup.1st (Entry 3, Table 1) catalyst as a function of the reaction conditions (m.sub.catalyst=1 g, O.sub.2/H.sub.2S molar ratio=2.5, Weight Hourly Space Velocity (WHSV)=0.6 h.sup.−1) (B) H.sub.2S conversion and sulfur selectivity by catalyst (G)N@C/SiC.sub.e.sup.1st as function of the reaction temperature. (Reaction conditions: m.sub.catalyst=1 g; Weight Hourly Space Velocity (WHSV)=0.6 h.sup.−1; O.sub.2/H.sub.2S molar ratio=2.5.) (C) Direct comparison of desulfurization performance of catalysts (G)N@C/SiC.sub.e.sup.1st and Fe.sub.2O.sub.3/SiC (m.sub.catalyst=1 g, O.sub.2/H.sub.2S molar ratio=2.5, Weight Hourly Space Velocity (WHSV)=0.6 h.sup.−1).

[0105] FIG. 5 illustrates an exemplary catalytic application of N@C-composites prepared according to the invention, in the steam-free direct dehydrogenation (DH) of ethylbenzene (EB) into styrene (ST). (A) Ethylbenzene dehydrogenation performance of the (G)N@C/SiC.sub.e.sup.2nd catalyst (Entry 5, Table 1), its graphitized form (G)N@C/SiC.sub.e.sup.2nd (Entry 5, Table 1) and the pristine support. (B) Catalytic performance of the industrial K—Fe-based catalyst and (G)N@C/SiC.sub.e.sup.2nd at 550 and 600° C., respectively. (C) Long-term DH test at 600° C. with the (G)N@C/SiC.sub.e.sup.2nd catalyst (reaction conditions: 300 mg, 600° C.). (D) Benchmarking of various carbon-based industrial catalysts and the (G)N@C/SiC.sub.e.sup.2nd system under steady state (Reaction condition: 550° C. 2.8% EB in helium, 30 mL.Math.min.sup.−1, atmospheric pressure).

[0106] FIG. 6. .sup.1H and .sup.13C{.sup.1H} NMR spectra (D.sub.2O, 298 K) of a model three component mixture obtained by mixing 2 g of glucose, 2.3 g of ammonium carbonate and 3 g of citric acid, after heating at 130° C. for 1 h (C, D) and .sup.1H and NMR spectra (D.sub.2O, 298 K) of the two separate components, D-Glucose (A) and citric acid (B). From a close inspection of the .sup.1H and .sup.13C{.sup.1H} NMR spectra in D.sub.2O, the first thermal treatment at 130° C. of the three-component aqueous solution results into an homogeneous solid mixture of α-D-glucopyranose and β-D-glucopyranose and ammonium citrate tribasic. Almost no traces of either CO.sub.3.sup.2− or HCO.sub.3.sup.− are observed after this phase (D). Higher temperatures (450° C.) foster the progressive construction of a highly N-rich heteroaromatic network where the tribasic ammonium citrate simultaneously acts as N-reservoir (NH.sub.3) and pores forming agent through its complete thermal decomposition in CO.sub.2, H.sub.2O and propene (Scheme 1).

[0107] FIG. 7. (A) Digital photo of different macroscopic materials as supports (host matrices) for the N@C active phase. (B) Digital photos of the pristine SiC, and the N@C decorated macroscopic host structures after thermal treatment in air and graphitization under helium at 900° C. (C-E) SEM images of the (G)N@C/SiC.sub.e.sup.2nd composite showing typically micro- and meso-porous structured surfaces.

[0108] FIG. 8. TEM and EFTEM mapping of a (G)N@C/SiC.sub.e.sup.2nd sample (Table 1, entry 5). (A) TEM bright-field image. (B) Elemental maps, the fack colors correspond to Nitrogen, Carbon and Silicon. (C) Carbon map (D) Nitrogen map (E) Silicon map.

[0109] FIG. 9. TPO-MS analysis of (A) the industrial Fe—K/Al.sub.2O.sub.3 and the (G)N@C/SiC.sub.e.sup.2nd (Entry 5, Table 1) catalysts before and after EB dehydrogenation test illustrating the coke resistance of the (G)N@C/SiC.sub.e.sup.2nd system.

[0110] Scheme 1. Without wishing to be bound by any particular theory, Scheme 1 presents a supposed mechanism for the thermal transformation of α,β-D-glucopyranose and ammonium citrate tribasic (pre-catalyst phase) into a N-rich heteroaromatic graphene-like network (catalytically active phase). The thermal treatment at 450° C. is supposed to start the progressive dehydration of the hexose sugar to give the 5-hydroxymethyl-furfural (5-HMF)(18) this latter being one of the most attractive chemical platforms for the synthesis of more complex chemicals and materials.(20) Although, further insights are required to ascertain the exact nature of the reactions occurring under these thermal conditions, ammonium citrate tribasic, is expected to act as a reservoir of nucleophilic NH.sub.3 that is promptly trapped (20) in the polymerization procedure thus leading to nitrogenated compounds (21-23). The fate of the citric acid core is that of its complete decomposition to the elemental volatiles CO.sub.2, H.sub.2O and propene which reasonably contribute to the ultimate material mesoporosity.

[0111] Table 1 reports different preparation parameters of a model N@C/SiC.sub.e and (G)N@C/SiC.sub.e composite according to the invention and the related nitrogen elemental composition. These composites are employed in the super-Claus H.sub.2S oxidation reaction as shown and detailed in FIG. 4. These composites are also employed for the material characterization and for the electrochemical Oxygen Reduction Reaction (ORR) (See FIG. 3 and related details). These composites are also employed in the steam-free direct dehydrogenation (DH) of ethylbenzene (EB) into styrene (ST) (See FIG. 5 and related details).

DETAILED DESCRIPTION

Equivalents

[0112] The representative examples that follow are intended to help illustrate the invention, and are not intended to, nor should they be construed to, limit the scope of the invention. Indeed, various modifications of the invention and many further embodiments thereof, in addition to those shown and described herein, will become apparent to those with ordinary skill in the art from the full contents of this document, including the examples which follow and the references to the scientific and patent literature cited herein. It should further be appreciated that the contents of those cited references are incorporated herein by reference to help illustrate the state of the art.

[0113] The following examples contain important additional information, exemplification and guidance that can be adapted to the practice of this invention in its various embodiments and the equivalents thereof.

EXEMPLIFICATION

[0114] The method and composites of this invention and their preparation can be understood further by the examples that illustrate some of the processes by which these composites are prepared or used. It will be appreciated, however, that these examples do not limit the invention. Variations of the invention, now known or further developed, are considered to fall within the scope of the present invention as described herein and as hereinafter claimed.

Example 1. General Procedure for a Highly N-Enriched Active Phase

[0115] In a typical procedure, 2 g of dextrose and 3 g of citric acid were added in deionized water (10 mL) at room temperature (r.t.). Then, a fixed amount of ammonium carbonate (i.e. 1, 2 or 3 g) was added in a single portion in the mixture solution at r.t. and an instantaneous effervescence due to CO.sub.2 evolution was observed. The suspension was stirred at r.t. till a clear solution was obtained, that was used as a source mixture for the obtainment of N-doped carbonaceous materials deposited on suitable supports (upon support soaking/impregnation). As for the latter, 2 g of different supports were used, i.e. SiC extrudates (30 m.sup.2 g.sup.−1; SICAT), SiC powder (25 m.sup.2 g.sup.−1; SICAT) SiC foams (30 m.sup.2 g.sup.−1; SICAT), and α-Al.sub.2O.sub.3 beads (6 m.sup.2 g.sup.−1; Sasol). The wet solids were slowly heated in air from room temperature to 130° C. (heating rate of 10° C. min.sup.−1) and keep at this temperature for 1 h. The as obtained dry solids were further impregnated with the same three component solution for several times until the desired loading was achieved. The solids were calcinated in air at 450° C. (heating rate of 2° C. min.sup.−1 from r.t. to 450° C.) for 2h during which the macroscopic host support was coated with a highly N-rich mesoporous carbonaceous phase. N@C composites obtained through 1 single impregnation phase, followed by thermal treatment at 130° C. and carbonization in air at 450° C. are named as N@C/SiC.sub.e.sup.1st. N@C composites obtained through two successive impregnation phases, each of them followed by thermal treatment at 130° C. and a final carbonization phase in air at 450° C. are named as N@C/SiC.sub.e.sup.2nd. The as-prepared solids were further graphitized under inert atmosphere in helium at 900° C. (heating rate of 10° C. min.sup.−1 from r.t. to 900° C.) for 2h in order to increase their final graphitization degree. The latter composites are named as (G)N@C/SiC.sub.e.sup.1st (one impregnation step) and (G)N@C/SiC.sub.e.sup.2nd (two impregnation step), respectively.

Example 2. Materials Characterization

[0116] 1D (.sup.1H and .sup.13C{.sup.1H}) NMR spectra of all samples were obtained on a Bruker Avance DRX-400 spectrometer (400.13 and 100.61 MHz for .sup.1H and .sup.13C, respectively). Chemical shifts (δ) are reported in ppm relative to trimethylsilane (TMS), referenced to the chemical shifts of residual solvent resonances (.sup.1H and .sup.13C).

[0117] Thermal gravimetry analysis (TGA) was performed on a Setaram apparatus with an air flow rate of 25 mL.Math.min.sup.−1 and a heating rate of 10° C..Math.min.sup.−1 from room temperature to 1000° C. The specific surface area of the different samples was measured by the BET method using N.sub.2 as adsorbent at liquid nitrogen temperature (TriStar sorptometer). Before measurement, the samples were outgassed at 200° C. overnight in order to desorb moisture and impurities on its surface. The XPS measurements of the support were performed on a MULTILAB 2000 (THERMO VG) spectrometer equipped with Al Ka anode (hv=1486.6 eV) with 10 min of acquisition. Peak deconvolution has been made with “Avantage” program from Thermoelectron Company. The C1s peak at 284.6 eV was used to correct charging effects. Shirley backgrounds were subtracted from the raw data to obtain the areas of the C1s peak. The gross morphology of the carbon and ceramic materials was observed by scanning electron microscopy (SEM) on a JEOL F-6700 FEG with an accelerating voltage of 10 kV. The transmission electron microscopy (TEM) measurements were performed by a JEOL 2100FX operating at 200 kV, equipped with GATAN Tridiem imaging filter and a prob-aberration corrector. The electron energy loss spectroscopy (EELS) analysis were performed in scanning mode (STEM) with 30 mrad convergent angle and 25 mrad collection angle. The spectral image was acquired in 20×33 pixels with exposure time of 1 s for each pixel. The energy resolution was 1.7 eV. Elemental signals were extracted from Si-L, C—K, N—K and O—K edges respectively. The Energy filtered TEM (EFTEM) measurements were made by three window method with energy slit of 8 eV, 30 eV and 20 eV and acquisition time of 10 s, 30 s and 40 s for Si, C and N respectively. The Raman analysis was carried out using a LabRAM ARAMIS confocal microscope spectrometer equipped with CCD detector. A laser line was used to excite sample, 532 nm/100 mW (YAG) with Laser Quantum MPC600 PSU. All the measurements were performed at room temperature. The Raman spectra were recorded using LabRAM ARAMIS Horiba Raman spectrometer equipment. Spectra were recorded over the range of 500-4000 cm.sup.−1 at a laser excitation wavelength of 532 nm. The sample was deposited on a glass substrate by spin-coating its suspension and carefully dried before measurement.

Example 3. Catalytic Reactions

3.1 Oxygen Reduction Reaction (ORR).

[0118] Electrochemical studies were performed at 25° C. in a three-electrode cell in 0.1 M KOH supporting electrolyte, using Autolab PGSTAT30 (Eco Chemie, The Netherlands) potentiostat equipped with an analogue linear sweep generator at the sweep rate of 10 mV s.sup.−1. Mercury oxide (Hg/HgO) electrode and Pt-wire electrodes were used as reference and counter electrodes, respectively. Unless otherwise stated, all potentials hereinafter are referred to the reversible hydrogen electrode (RHE). The electrochemical impedance spectroscopic (EIS) was used to determine the resistance of electrolyte solution.

[0119] 10.0 mg of the catalyst sample, 5 mL isopropanol, and 50 μL Nafion solution (5 wt %) were ultrasonically mixed to form a homogenous catalyst ink. For the define RRDE test, the working electrode (PINE, AFE6R2GCPT) was prepared by loading 50 μL of catalyst ink onto a pretreated glassy carbon (GC) electrode (5.5 mm diameter and 0.2376 cm.sup.2 geometrical area) and then dried at room temperature. The reference Pt data was recorded with a 20 wt % Pt/VXC-72 (Sigma) catalyst with a loading of 25 μg.sub.pt cm.sup.−2.

[0120] All aqueous solutions were prepared using ultrapure water (18MΩcm, <3 ppb TOC) and supra-pure KOH (Sigma-Aldrich). In O.sub.2-reduction experiments O.sub.2 was constantly bubbled through the solution in order to maintain the saturation level and the ring potential was set at 1.2 V RHE in accordance with previous studies. Collection efficiency (N) was calculated from the experimental data obtained in 10 mM K.sub.3FeCN.sub.6 in 0.1M NaOH at standard measurement conditions (potential sweep rate 10 mV s.sup.−1, 25° C.). The collection efficiency for the Pt(20%)/VXC-72 electrode was found to be 37%, This value was also reported by Wang (Yusheng Andrew Wang, B.A.Sc. The University of British Columbia, 2009) and Chlistunoff et al. (17)

[0121] The four-electron selectivity of catalysts was evaluated based on the H.sub.2O.sub.2 yield, calculated from the following equation:

H.sub.2O.sub.2(%)=200(J.sub.R/N)/(J.sub.R/N−J.sub.D)

[0122] Here, J.sub.D and J.sub.R are the disk and ring currents density, respectively, and N is the ring collection efficiency.

[0123] The electron transfer number can be calculated in two ways. The first is to use the ring current and the disk current n=−4J.sub.D/(J.sub.R/N-J.sub.D). The second way to calculate n is by using the first-order Koutecky-Levich equation:

1/J.sub.D=1/j.sub.k+1/j.sub.d

where j.sub.k is the kinetic current density and j.sub.d is the diffusion-limited current density through the expression j.sub.d=Bω.sup.1/2=0.62 nFγ.sup.−1/6D.sub.O2 .sup.2/3C.sub.O2ω.sup.1/2. Here n is the average electron transfer number; F is the Faraday constant; γ is the kinematic viscosity of the electrolyte; D.sub.O2 is the oxygen diffusion coefficient (1.15×10.sup.−5 cm.sup.2/S); C.sub.O2 is the bulk oxygen concentration in the electrolyte (1.4×10.sup.−6 mol/cm.sup.3); and ω is the angular velocity of the electrode. The kinetic current density (j.sub.k) and the Koutecky-Levich slope (1/B) can be obtained from a plot of 1/j versus 1/ω.sup.1/2.

3.2 Steam Free Dehydrogenation of Ethylbenzene.

[0124] The reaction was carried out in a fixed-bed continuous flow reactor under atmospheric pressure. The catalyst (300 mg) was loaded onto a quartz fritted disk located inside a tubular quartz reactor (id 8×length 800 mm). He gas was fed into the reactor at a flow rate of 30 mL.Math.min.sup.−1 through a mass flow controller (BROOKS MFC) and passed through a glass saturator filled with liquid EB maintained at 40° C. (EB partial pressure of 2922 Pa) using a thermal regulated bath.

[0125] The reaction system was heated to 550° C. and kept for 2h under the He. The reactant flow (2.8 vol. % EB diluted in helium, total flow rate of 30 mL.Math.min.sup.−1) was then fed to the reactor. The reactant and the products (styrene (ST), benzene (BZ) and toluene (TOL) exit from the reactor was analyzed on-line with a PERICHROM (PR 2100) gas chromatography equipped with a flame detector (FID) and CP WAX S2CB column which was previously calibrated. In order to avoid any possible condensation of the reactant or the products all the tube lines were wrapped with a heating wire kept maintaining 110° C.

[0126] The ethylbenzene conversion (X.sub.EB) and styrene selectivity (S.sub.ST) were evaluated using equations: (2) and (3):

[00001]$\begin{array}{cc}{X}_{\mathrm{EB}}=\frac{F_0.Math.C_{\mathrm{EB},\mathrm{inlet}}-{\mathrm{FC}}_{\mathrm{EB},\mathrm{outlet}}}{F_0.Math.C_{\mathrm{EB},\mathrm{inlet}}}\times 100.Math.\%& \left(2\right)\\ S_{\mathrm{ST}}=\frac{C_{\mathrm{ST},\mathrm{outlet}}}{C_{\mathrm{ST},\mathrm{outlet}}+C_{\mathrm{TOL},\mathrm{outlet}}+C_{\mathrm{BZ},\mathrm{outlet}}}\times 100.Math.\%& \left(3\right)\end{array}$

where F and F.sub.0 are the flow rates of the outlet and inlet, respectively, and C.sub.EB, C.sub.ST, C.sub.TOL and C.sub.BZ represent the concentration of ethylbenzene, styrene, toluene and benzene. The carbon balances amounted to >96% in all investigations. The results were obtained after more than 30 h on stream with stable catalytic performance at testing conditions.
3.3 Partial Oxidation of H.sub.2S into Elemental Sulfur

[0127] The catalytic selective oxidation of H.sub.2S by oxygen (Eq. (1)) was carried out in an all glass tubular reactor working isothermally at atmospheric pressure.

H.sub.2S+.sup.1/2O.sub.2.fwdarw.1/n S.sub.n+H.sub.2O ΔH=−222 kJ/mol  (1)

[0128] An amount of catalyst (300 mg) was placed on silica wool in a tubular Pyrex reactor (i.d. 16 mm), which was then placed inside a vertical tubular electrical furnace. The temperature was controlled by a K-type thermocouple and a Minicor regulator. The gas mixture of the reactant feed including H.sub.2S (1 vol %), O.sub.2 (2.5 vol %), H.sub.2O (30 vol %), and He (balance) was passed downward through the catalyst bed. The gases flow rates were monitored by Brooks 5850TR mass flow controllers linked to a control unit. The weight hourly space velocity (WHSV) was fixed at 0.6 h.sup.−1 and the O.sub.2/H.sub.2S molar ratios of 5.

[0129] The reaction was conducted in a continuous mode. The sulfur formed in the reaction was vaporized because of the high partial pressure of sulfur at these reaction temperatures, and it was condensed at the exit of the reactor in a trap maintained at room temperature. The analysis of the inlet and outlet gases was performed online using a Varian CP-3800 gas chromatograph (GC) equipped with a Chrompack CP-SilicaPLOT capillary column and a thermal catharometer detector (TCD), which allowed the detection of O.sub.2, H.sub.2S, H.sub.2O, and SO.sub.2.

Summary of the Results of Examples 1 to 3:

[0130] We described above a new methodology for the obtainment of highly N-doped mesoporous carbons starting from simple, non-toxic raw foodstuff building blocks such as ammonium carbonate, citric acid and D-glucose; this latter being the most abundant sugar unit in biomass and the major product of the acid hydrolysis of lignocellulosic biomass. It is worthy to note that D-glucose could also be replaced by other sugars in the invention. The adopted protocol for the generation of a N-containing active phase includes a fundamental homogeneous aqueous pre-catalytic phase that can be used as an impregnating agent for a virtually infinite variety of macroscopic supports to be soaked. The macroscopic supports can be tuned at will, i.e. grains, pellets, rings, beads or foams, depending on the downstream application. The controlled thermal treatment of the impregnated supports allows for the N-containing mesoporous active phase to grow up as a coating of an ultimate composite material featured by a remarkably high percentage of exposed N-active sites.

[0131] Various composites prepared from different macroscopic host matrices have successfully been scrutinized as metal-free catalysts in three industrially relevant catalytic processes each of them with outstanding outcomes respect to either related metal-free or metal-based catalysts of the state-of-the-art. In particular, the electrochemical oxygen reduction reaction (ORR) (1), the super-Claus H.sub.2S oxidation into elemental sulfur for the purification of the gaseous effluents (4, 15) and the highly selective steam-free dehydrogenation of ethylbenzene into styrene (16) are discussed.

[0132] The N-doped porous carbons, as a catalytic surface coating for different macroscopic host matrixes, were prepared by the impregnation of selected supports (i.e. powders, extrudates and silicon carbide (SiC) foams or α-Al.sub.2O.sub.3 beads) with an homogeneous aqueous solution made of (NH.sub.4).sub.2CO.sub.3, glucose and citric acid, followed by two successive thermal treatments in air; the first one at moderate-temperature (130° C.) where the impregnating solution is slowly dried on the support thus forming a tiny pre-catalytic layer, and the second one at 450° C. (FIG. 6) where a support coating made of highly N-enriched active phase is generated (FIG. 1). The sample was further submitted to a graphitization process under inert gas at 800-900° C., in order to improve the electrical and thermal conductivity of the ultimate catalytic material and also to convert the nitrogen species into more stable and suitable forms for promoting the catalytic process. Depending to the downstream applications the last step is not always necessary.

[0133] D-Glucose and ammonium carbonate constitute the carbon and nitrogen sources, respectively, while the citric acid plays a double role as N-reservoir [in the form of mono-, di- and tri-basic ammonium citrate] and pores forming agent during the higher temperature thermal treatment (FIG. 1, 4 and Scheme 1).

[0134] The thickness of the N-doped carbonaceous coating on the macroscopic support can be improved by repeating the impregnation/drying step (FIG. 1, phase 1) before proceeding with the final high temperature thermal treatment of the composite to obtain the active phase (FIG. 1, phase 2 and FIG. 7).

[0135] The host matrix coating based on a N-rich nanocarbon phase (N@C), prevalently featuring meso- and macroporosity, increases the specific surface area (SSA) of the final composite remarkably (FIG. 2A). Similarly to above, successive impregnation/drying cycles (FIG. 1, phase 1) were exploited to additionally increase the ultimate SSA of the catalytic composite. The high resolution N is XPS analysis of the composites show characteristic profiles featured by two main component at 399 and 401 eV, consisting in pyridinic and pyrrolic nitrogen species, respectively (FIG. 1B). The total nitrogen content spans in the 6%-28 at. % range and it can be controlled by tuning the ammonium carbonate/D-glucose/citric acid molar ratio as well as the number of impregnation cycles (Table 1). The high nitrogen loading and its almost exclusive localization on the topmost surface of the support in the present work is additionally confirmed by high-resolution TEM and TEM-EELS analysis of the N-doped nanomaterial deposited on the SiC host matrix (FIG. 2C,D). These unique material properties makes these composites ideal single-phase, metal-free systems for promoting a high number of industrially relevant catalytic transformations with excellent or even better performance than that of the heterogeneous metal/metal-oxide based counterparts of the state-of-the-art.

[0136] Oxygen Reduction Reaction (ORR) experiments have been performed under alkaline environment (KOH 0.1 M), using the (G)N@C/SiC.sub.g.sup.2nd catalyst (for detail see Table 1) as the most representative catalyst from this series, prepared from SiC powder as support (diameter ranged between 10 to 40 μm). FIG. 3A shows its remarkable electrochemical performance in a direct comparison with that of the commercially available Pt-20/C (Vulcan XC-72 with platinum loading of 20 wt %) and the pristine SiC powder support under identical conditions. The Koutecky-Levich (K-L) plots of the two catalysts show similar trends thus indicating a near four electrons mechanism operating at both systems (FIG. 3C). A more accurate estimation of the number of electrons transferred (n) per mol of O.sub.2 (and the % of H.sub.2O.sub.2 produced by the catalysts in the electrochemical process) is calculated from the Pt-ring current values measured at the rotating-ring-disk electrode (RRDE) (FIG. 3D). The average values of n as calculated from both approaches are equal to 3.6 and 4 for the (G)N@C/SiC.sub.g.sup.2nd and Pt-20/C catalysts, respectively. The onset potential (E.sub.on) of CNT foam sample is close to that of the reference Pt-Vulcan sample (˜1V) but the half potential still lower (around 60 mV less). The E.sub.112 and the intensity at 0.9V show that the activity of the (G)N@C/SiC.sub.g.sup.2nd is slightly less active than the Pt/C.

[0137] Cycling electrochemical tests in the 0.6-1.0 V range (at 100 mVs.sup.−1, 900 rpm in 0.1 M KOH at 25° C.) have been used to check the (G)N@C/SiC.sub.g.sup.2nd vs. Pt-20/C catalyst stability. Notably, the selected metal-free system retains about 90% of its initial ORR activity after 1500 cycles whereas only 70% is maintained by the Pt-20/C catalyst (FIG. 3B). Such a remarkable electrochemical stability (similarly reported for vertically-aligned N-doped carbon nanotubes by other research team (24, 24a, 24b), in combination with the catalysts performance in the ORR, makes these systems a valuable alternative to the existing precious metal-based catalysts.

[0138] The (G)N@C/SiC.sub.e.sup.1st2nd catalyst (for detail see Table 1) has also shown excellent catalytic performance under more severe reaction conditions, once employed for the partial oxidation of H.sub.2S residues (desulfurization) in the gaseous industrial effluents, in agreement with the current legislation constraints. This metal-free system exhibits an relatively high desulfurization performance with a sulfur yield closed to 70% when the process is performed at 210° C. with a Weight Hourly Space Velocity (WHSV) of 0.3 h.sup.−1 (FIG. 4A-C). For the sake of comparison, the desulfurization test has been also carried out using one of the most active and selective desulfurization system of the state-of-the-art like the Fe.sub.2O.sub.3/SiC.sub.p catalyst. This latter show a remarkably lower desulfurization activity compared to that of the (G)N@C/SiC.sub.e.sup.1st catalyst (FIG. 4C) under similar reaction conditions. This latter catalyst also exhibits an extremely high stability as almost no deactivation is observed after hundred hours of reaction (FIG. 4A). XPS analysis carried out on the spent catalyst shows no evidence of nitrogen species modification (Table 2) and confirm the high stability of the catalyst under the selective oxidation reaction. Such a high stability can be attributed to the chemical inclusion of nitrogen atoms in the carbonaceous matrix which significantly prevent leaching effects of the active phase whereas active phase sintering effects are definitively ruled out thanks to the nature of the metal-free system used.

[0139] The (G)N@C/SiC.sub.e.sup.2nd (for detail see Table 1) catalyst was also evaluated in the more drastic steam-free direct dehydrogenation of ethylbenzene (EB) into styrene in a fixed-bed configuration. The effluent was analyzed by online gas chromatography (GC). At 550° C., EB conversion was 22% (specific rate of 300 mmol/g.sub.cat/h) and increased with increasing the reaction temperature to reach 44% at 600° C. (specific rate of 600 mmol/g.sub.cat/h) (FIG. 5B). The catalyst selectivity towards styrene was almost 100% for the whole test at 550° C. while slight selectivity decrease was observed at higher reaction temperature (600° C.), where small amounts of toluene and benzene (<2%) are formed as reaction by-products. Under the same reaction conditions the Fe—K/Al.sub.2O.sub.3 (with 90 wt % of Fe loading) catalyst shows a much lower dehydrogenation activity (FIG. 5A,B). The Fe—K/Al.sub.2O.sub.3 catalyst shows a high DH activity at the beginning of the test followed by a sharp deactivation down to 5% of EB conversion (FIG. 5A,B). The metal-free catalyst exhibits an extremely high stability as almost no deactivation was observed for long term duration tests (FIG. 5C). The blank test carried out on the SiC extrudates shows a relatively low DH activity (FIG. 5A) and confirms the high DH activity of the SiC supported nitrogen-doped porous carbon. The deactivation observed on the Fe—K/Al.sub.2O.sub.3 catalyst was attributed to the progressive coverage of the iron-based catalyst by a layer of coke residues as shown by the temperature-programmed oxidation (TPO) analysis (FIG. 9). On the (G)N@C/SiC.sub.e.sup.2nd catalyst the TPO spectrum only shows a small amount of carbonaceous residue which accounts for a higher stability and durability of the metal-free catalyst (FIG. 9). The specific reaction rate, expressed in terms of molecules of EB converted per gram of active phase per hour, obtained on the different catalysts is presented in FIG. 5A and confirms again the higher DH activity of the (G)N@C/SiC.sub.e.sup.2nd catalyst (26, 27).

[0140] The high catalytic performance of the nitrogen-doped porous carbon studied in the present work could be attributed to the electronic modification of the carbon atoms by the adjacent nitrogen atoms according to the pioneer work by Gong et al. (1) The high activity of the nitrogen-doped catalyst in the ORR and H.sub.2S oxidation could be attributed to the high ability of the doped carbon surface to adsorb oxygen in a dissociative way to produce highly reactive adsorbed oxygen species which will be incorporated in the final product before escaping.

[0141] The spent catalysts, i.e. (G)N@C/SiC.sub.g.sup.2nd grains (<40 μm) for ORR, (G)N@C/SiC.sub.e.sup.2nd extrudates (1×2 mm) for selective oxidation of H.sub.2S and steam-free dehydrogenation of ethylbenzene, are further characterized by mean of the XPS, BET specific surface area measurements and TPO and the results confirm the complete retention of the catalyst characteristics similar to those obtained on the fresh ones. Such results indicate that nitrogen active sites or specific surface area lost are unlikely to occur and highlight again the extremely high stability of these nitrogen-doped mesoporous carbon active phase.

[0142] In summary, a high nitrogen-doped porous carbon can be prepared through a simple chemical reaction involving non-toxic raw materials such as ammonium carbonate, glucose and citric acid mixture at relatively low-temperatures. The method developed also allows one to prepare these nitrogen-carbon composites, not only in a powder form but also with controlled shapes along with high mechanical anchorage, for use as metal-free catalysts in specific gas-phase and liquid-phase processes. The conversion of the nitrogen and carbon sources is extremely high which is not the case of traditional synthetic routes where a large amount of precursors decomposes leading to the formation of high waste amount. Last but not least the overall cost linked with this synthesis process is expected to be much lower than those encountered with traditional nitrogen-doped composites where toxic (or explosive) and high price raw materials, high waste release and high temperature operating are encountered. It is expected that the nitrogen-doped porous carbon prepared in greener conditions than those reported up to date can open the way to the development of new catalytic metal-free platforms featured by higher robustness and low operating costs, in a variety of catalytic processes beyond those exemplified in Example 3, including but not limited to, the liquid-phase and gas-phase hydrogenation, the oxidation of linear alkanes and volatil organic compounds (VOCs), the hydrogenation of CO in the Fischer-Tropsch process, and methanization of synthesis gas mixture, as well as Advanced

[0143] Oxidation Processes for water and wastewater treatment, such as the catalytic ozonation of organic micropollutants (COZ) and/or the catalytic wet air oxidation (CWAO).

[0144] While we have described a number of embodiments of this invention, it is apparent that our basic examples may be altered to provide other embodiments that utilize the catalysts and methods of this invention. Therefore, it will be appreciated that the scope of this invention is to be defined by the appended claims rather than by the specific embodiments that have been represented by way of example.

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