WO2018020356A1

WO2018020356A1 - Nitrogen rich carbon nitride materials with a three dimensional cubic mesoporosity from diaminotetrazine

Info

Publication number: WO2018020356A1
Application number: PCT/IB2017/054290
Authority: WO
Inventors: Siddulu Naidu TALAPANENI; Ajayan Vinu; Ugo RAVON; Khalid Al-Bahily
Original assignee: Sabic Global Technologies B.V.
Priority date: 2016-07-28
Filing date: 2017-07-14
Publication date: 2018-02-01

Abstract

Embodiments are directed to carbon nitride materials that provide appropriate characteristics for photocatalytic water splitting. The carbon nitride materials have a three dimensional CN material with a high nitrogen content that is stable under photocatalytic conditions. A method of producing a photocatalytic CN material comprises (a) contacting a mesoporous template having a selected porosity with an aminoguanidine hydrochloride solution, forming a template reactant mixture; and (b) heating the template reactant mixture, forming a carbon nitride material/template composite; (c) heating the CN/template composite in an inert gas atmosphere to about 350 °C to 450 °C, forming a cubic mesoporous carbon nitride material/template complex; and (d) removing the template from the mesoporous carbon nitride material/ template complex. The carbon nitrogen matrix is a based on C3N6diamino-s-tetrazine.

Description

NITROGEN RICH CARBON NITRIDE MATERIALS WITH A THREE DIMENSIONAL CUBIC MESOPOROSITY FROM DIAMINOTETRAZINE

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority of U.S. Provisional Patent Application No. 62/367,843 filed July 28, 2016, which is hereby incorporated by reference in its entirety.

BACKGROUND

1. Field of the Invention

[0002] The invention generally concerns a photocatalytic carbon nitride (CN) material. In particular, the invention concerns a three-dimensional mesoporous carbon nitride matrix having an atomic nitrogen to atomic carbon (N:C) ratio of 1.8 to 2.1 and a band gap of 1.8 to 3 eV.

2. Description of Related Art

[0003] Scientific interest in carbon nitride (CN) materials has increased this last decade because of their unique semi-conductor behavior, basic sites, electronic properties, and other unique characteristics. One use of CN materials is to use them as catalysts for the water splitting reaction. Hydrogen (H₂) production via water splitting is one of the most important and challenging reactions, and despite decades of research, a catalyst appropriate for use on an industrial scale is yet to be developed. The three main requirements to achieve a good photocatalyst for this reaction are: (i) an oxidative active site for the oxygen evolution, (ii) a reductive site for the hydrogen generation, and (iii) a good semi-conductor for the photon absorption.

[0004] In the continued search for more effective and practical applications of CN materials, several researchers devoted their research efforts to improve the optical and electronic properties of these materials by simply altering the synthetic pathway with an ultimate aim of extending the insufficient light absorption of graphitic C₃N₄ (g-C₃N₄) (band gap of 2.7 eV, which corresponds to wavelengths shorter than 460 nm) towards the maximum of the solar spectrum (Guo et al. Chem. Commun., 2010, 46:7325; Yan et al., Dalton Trans., 2010, 39: 1488; Xiang et al, J. Phys. Chem. 2011, 115:7355; Yan and Huang, Chem. Commun., 2011, 47:4168; Zhou e/ a/., Chem. Commun., 2011, 47: 10323; Zhang et al., Angew. Chem. Int. Ed., 2012, 51 :3183; Cui et al., Angew. Chem. Int. Ed., 2012, 51 : 11814). In this context, researchers tried to decrease the band gap of polymeric C₃N₄ by doping with different elements such as sulfur and phosphorus with the aim of designing unique electronic structures that display an increased valence bandwidth in combination with an elevated conduction band minimum and a slightly reduced absorbance (Liu et al., J. Am. Chem. Soc, 2010, 132: 1 1642 and Zhang et al, J. Am. Chem. Soc, 2010, 132:6294).

[0005] Recently, Talpaneni et al., (ChemSusChem, 2012, 5 :700) have reported diamino-s- tetrazine based nitrogen enriched mesoporous carbon nitride (MCN-4) through the self- condensation of aminoguanidine hydrochloride followed by polymerization reaction in the inside of the mesoporous channels of 2D hexagonal SBA-15 template. However, these materials possess a uni-dimensional porous structure consisting of a two dimensional morphology that registered low specific surface area and pore volume, which limits their potential use in different applications. Vinu et al., reported the preparation of polyaniline based three-dimensional nitrogen doped mesoporous carbon with body centered cubic Ia3d structure using 3D mesoporous silica KIT-6 as a hard template with an aniline molecule as a precursor by using nanocasting for use in heterogeneous catalysis and sensing applications (Vinu et al., Micropor. Mesopor. Mater., 2008, 109:398; Talapaneni et al., J. Mater. Chem. , 2012, 22:9831 ; Mane et al, J. Mater. Chem. A, 2013, 1 :2913). Lee et al. has also reported the synthesis of highly ordered 3D body-centered mesoporous graphitic C₃N₄ with a high specific surface area and employed them for detecting trace amounts of metal ions from aqueous solutions (Lee et al., Angew. Chem., Int. Ed., 2010, 49:9706). ).

[0006] Many semiconductors have been tested for utilization as a photocatalyst and not found to meet the requirements of (i) a band gap from 1.23 eV to 2.5 eV in order to maximize the photon adsorption emitted by the solar spectrum and (ii) stability under photocatalytic conditions. There remains a need for additional catalyst materials for use in photocatalytic water splitting.

SUMMARY

[0007] A discovery has been made that provides a solution for photocatalytic water splitting. The solution is premised on a three dimensional photocatalytic CN material with a high nitrogen content and having a band gap of 1.8 to 3 eV. Notably, this material is stable under photocatalytic conditions. The methods and catalysts of the present invention provide an elegant way to tune the band gap of a carbon nitride material to allow optimization of the photon absorption relative to photon energy. [0008] In one aspect of the present invention, photocatalytic nitrogen rich mesoporous three dimensional (3-D) CN materials are described. The photocatalytic 3-D material can include a three dimensional mesoporous CN matrix having an atomic nitrogen to atomic carbon (N:C) ratio of 1.8 to 2.1 and a band gap of 1.8 to 3 eV. The CN matrix can be a C₃N₆ diamino-s-tetrazine based material. In certain aspects, the mesoporous material can have an elemental N:C (C) ratio of 1.8 to 2.1. The mesoporous material can have a pore volume of at least, at most, or about 0.2 to 1.5 cm³g^_1. In certain aspects, the mesoporous material is in the form of a powder. In a further aspect, the mesoporous material has a pore size of about 2 nm to 50 nm, or preferably 2 nm to 10 nm. In other aspects, photocatalytic CN material of the present invention can have a specific surface area of 140 to 500 m²g^_1. The photocatalytic CN material can further include a co-catalyst. The co-catalyst can be a transition metal (e.g., platinum, titanium, nickel, palladium, rhodium, ruthenium, tungsten, molybdenum, gold, silver, copper, or combinations thereof, or alloys thereof). In a preferred embodiment, the co- catalyst is platinum. Co-catalysts can be added in amounts of 0.01 to 2.0 wt.% of the mesoporous material. In certain aspects, the co-catalyst is incorporated on or in the surface and/or pores of the carbon nitride materials.

[0009] Certain embodiments are directed to a method of producing a nitrogen rich mesoporous material of the present invention. Methods to prepare a photocatalytic CN material of the present invention can include one or more of the following steps: (a) contacting a mesoporous KIT-6 template having a selected porosity with an aminoguanidine hydrochloride solution forming, a template reactant mixture; and (b) heating the template reactant mixture, forming a carbon nitride material/KIT-6 composite (c) heating the template reactant mixture, forming a carbon nitride material/KIT-6 composite; and (e) removing the KIT-6 template from the mesoporous carbon nitride material/KIT-6 complex. Step (b) heating can include (i) heating the template reactant mixture at a predetermined first temperature for a first time interval; and (ii) raising the temperature to a second temperature for a second time interval. In certain aspects, the first temperature is 90 to 1 10 °C. In one particular aspect, the first temperature is about 100 °C. The first time interval can be 4 to 8 hours. In certain aspects, the first time interval can be about 6 hours. The second temperature can be 150 to 170 °C. In certain aspects, the second temperature is about 160 °C. The second time interval can be 4 to 8 hours. In certain aspects, the second time interval is about 6 hours. The CN/KIT-6 composite can be heated under nitrogen flow to temperature at a rate of about 3 °C per minute. The nitrogen flow can be at about 50 to 150 ml per minute. The CN/KIT-6 composite can be heated in step (c) at about 400 C°. In certain aspects, the CN/KIT-6 composite is heated under an inert conditions (e.g., a nitrogen flow) at the desired temperature for about 5 hours. The KIT-6 template can be removed by contacting the CN/KIT-6 composite hydrofluoric acid or similar solvents. The method can further include collecting the 3-D mesoporous CN material by filtration. In a further aspect, the filtered material can be ground to a powder.

[0010] Certain embodiments are directed to methods of producing the KIT-6 template by (a) mixing an amphiphilic triblock copolymer dispersed in an aqueous hydrogen chloride solution with 1-butanol and tetraethyl orthosilicate (TEOS) to form a polymerization mixture; (b) heating the polymerization mixture at a predetermined synthesis temperature to form a KIT-6 template, where the predetermined temperature determines the pore size of the KIT-6 template; and (c) calcining the KIT-6 template by heating the KIT-6 template to about 540 °C. The polymerization mixture can be heated at a temperature of about 90 °C to 200 °C. In certain aspects, the polymerization mixture can be heated at a temperature of about 100 °C, 130 °C, or 150 °C. In certain aspects, the polymerization mixture is heated at a temperature of about 150 °C.

[0011] Certain embodiments are directed to photocatalytic processes that include using the photocatalytic CN material of the present invention in a water splitting process. The process can include contacting the photocatalytic mesoporous 3-D CN material of the present invention with water under conditions sufficient to produce hydrogen from the water. In some embodiments, photocatalytic mesoporous 3-D CN material includes a platinum co- catalyst.

[0012] Other embodiments of the invention are discussed throughout this application. Any embodiment discussed with respect to one aspect of the invention applies to other aspects of the invention as well and vice versa. Each embodiment described herein is understood to be embodiments of the invention that are applicable to all aspects of the invention. It is contemplated that any embodiment discussed herein can be implemented with respect to any method or composition of the invention, and vice versa. Furthermore, compositions of the invention can be used to achieve methods of the invention. [0013] The following includes definitions of various terms and phrases used throughout this specification. [0014] The phrase "nitrogen rich" refers to carbon nitrides having more nitrogen atoms than graphitic carbon nitrides having the general formula of C₃N₄.

[0015] The terms "about" or "approximately" are defined as being close to as understood by one of ordinary skill in the art. In one non-limiting embodiment, the terms are defined to be within 10%, preferably within 5%, more preferably within 1%, and most preferably within 0.5%.

[0016] The terms "wt.%", "vol.%", or "mol.%" refers to a weight percentage of a component, a volume percentage of a component, or molar percentage of a component, respectively, based on the total weight, the total volume of material, or total moles, that includes the component. In a non-limiting example, 10 grams of component in 100 grams of the material is 10 wt.% of component.

[0017] The term "substantially" and its variations are defined to include ranges within 10%, within 5%, within 1%, or within 0.5%.

[0018] The terms "inhibiting" or "reducing" or "preventing" or "avoiding" or any variation of these terms, when used in the claims and/or the specification includes any measurable decrease or complete inhibition to achieve a desired result.

[0019] The term "effective," as that term is used in the specification and/or claims, means adequate to accomplish a desired, expected, or intended result.

[0020] The use of the words "a" or "an" when used in conjunction with any of the terms "comprising," "including," "containing," or "having" in the claims, or the specification, may mean "one," but it is also consistent with the meaning of "one or more," "at least one," and "one or more than one."

[0021] The words "comprising" (and any form of comprising, such as "comprise" and "comprises"), "having" (and any form of having, such as "have" and "has"), "including" (and any form of including, such as "includes" and "include") or "containing" (and any form of containing, such as "contains" and "contain") are inclusive or open-ended and do not exclude additional, unrecited elements or method steps.

[0022] The catalysts of the present invention can "comprise," "consist essentially of," or "consist of particular ingredients, components, compositions, etc. disclosed throughout the specification. With respect to the transitional phase "consisting essentially of," in one non- limiting aspect, a basic and novel characteristic of the catalysts of the present invention are their abilities to catalyze water-splitting reactions.

[0023] Other objects, features and advantages of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and the specific examples, while indicating specific embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

DESCRIPTION OF THE DRAWINGS [0024] The following drawings form part of the present specification and are included to further demonstrate certain aspects of the present invention. The invention may be better understood by reference to one or more of these drawings in combination with the detailed description of the specification embodiments presented herein. The drawings may not be to scale. [0025] FIG. 1. Schematic representation for the preparation of diamino-s-tetrazine based 3D cubic mesoporous carbon nitride (c-MCN) of the present invention using KIT-6.

[0026] FIG. 2. Lower angle and wide angle (insert) powder X-ray diffraction (XRD) patterns of (a) c-MCN -100 (b) c-MCN-130 and (c) c-MCN-150 of the present invention prepared by using different pore diameters of KIT-6 silica templates. [0027] FIG. 3. Nitrogen adsorption-desorption isotherms (top) and BJH pore-size distributions (bottom) of c-MCN materials of the present invention with different textural parameters synthesized by employing 3D KIT-6 having various pore diameters (open symbols: desorption, closed symbols: adsorption); c-MCN-100 (circles), c-MCN-130 (diamonds) and c-MCN-150 (squares). [0028] FIG. 4. Nitrogen adsorption-desorption isotherms (top) and BJH pore-size distributions (bottom) of c-MCN-100 (circles) of the present invention and KIT-6- 100 (diamonds) silica template.

[0029] FIG. 5. High resolution transmission electron microscopy (HRTEM) images of (a, b) c-MCN-100, (c, d) c-MCN-130 and (e, f) c-MCN-150 of the present invention at 50 nm and 20 nm resolution, respectively. [0030] FIG. 6. High resolution scanning electron microscopy (HRSEM) images of (a, b) c-MCN-100, (c, d) c-MCN-130 and (e, f) c-MCN-150 samples of the present invention at 3 and 1 micron magnifications, respectively.

[0031] FIG. 7. Electron energy loss spectra (EELS) of diamino-s-tetrazine based 3D cubic mesoporous carbon nitride (c-MCN) materials of the present invention with various textural parameters prepared from KIT-6-X templates: (a) c-MCN-100, (b) c-MCN-130, and (c) c-MCN-150.

[0032] FIG. 8. Energy dispersive X-ray spectroscopy (EDX) images of (a) c-MCN-100, (b) c-MCN-130, and (c) c-MCN-150 of the present invention. [0033] FIG. 9. (A) X-ray photon spectroscopy (XPS) survey spectra, (B) CI s XPS survey spectra, and (C) Nls XPS survey spectra of c-MCN-150 of the present invention.

[0034] FIG. 10. Fourier transform infrared (FTIR) spectra from (a) c-MCN-100, (b) c- MCN-130, and (c) c-MCN-150 of the present invention.

[0035] FIG. 11. Band gap data and ultraviolet and visible (UV-Vis) spectra (inset) of (a) c-MCN-100, (b) c-MCN-130 and (c) c-MCN-150 of the present invention.

[0036] FIG. 12 Elemental mappings of (a) c-MCN- 100, (b) c-MCN- 130 and (c) c-MCN- 150 of the present invention.

[0037] FIG. 13. Thermal gravimetric (TG) curve of c-MCN-150 of the present invention.

[0038] FIG. 14. Graphs of H₂ gas evolution using c-MCN photocatalyst of the present invention (circles) as a function of time, with reference to non-porous CN (squares).

DETAILED DESCRIPTION

[0039] Described herein are 3-D mesoporous carbon nitride materials that provide the appropriate characteristics for photocatalytic water-splitting. By using a doping approach or adjusting the nitrogen to carbon (N/C) ratio, the band gap of the CN material can be tuned in order to optimize the photon absorption relative to photon energy. In certain aspects, tuning a CN is accomplished by controlling the pore size and other dimensions of the CN material.

A. Mesoporous Carbon Nitride Materials

[0040] Certain embodiments are directed to a nitrogen rich mesoporous material. Such a material can have a 3-D body-centered cubic structure and have a general formula of C₃N₆, which is designated as c-MCN throughout the specification. The c-MCN can be a 3-D matrix structure based on diamino-s-tetrazine having a pore size of 2, 5, 10, or 20 to 10, 20, 30, 40, or 50 nm. In certain aspects, the mesoporous material can have an atomic N:C ratio of 1.8 to 2.1, or 1.8, 1.9, 2.0, or 2.1, or any range or value there between. The pore volume of the mesoporous material can range from 0.1 to 1, or 0.3 to 0.8, or 0.3 to 0.65 cm³g^_1, or any value or range there between (e.g., 0.3, 0.31, 0.32, 0.33, 0.34, 0.35, 0.36, 0.37, 0.38, 0.39, 0.40, 0.41, 0.42, 0.43, 0.5, 0.51, 0.52, 0.53, 0.54, 0.55, 0.56, 0.57, 0.58, 0.59, 0.60, 0.61, 0.62, 0.63, 0.64, 0.65, 0.67, 0.68, 0.69, 0.70, 0.71, 0.72, 0.73, 0.74, 0.75, 0.76, 0.77, 0.78, 0.79, 0.80, 0.81, 0.82, 0.83, 0.84, 0.85, 0.86, 0.87, 0.88, 0.89, 0.9, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99, or 1.0 cm³g^-1)- The specific surface area of the c-MCN can be from 140 to 500 m²g^_1, 150 to 400

200 to 350 m²g^_1, or greater than, equal to, or between any two of 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200, 205, 210, 215, 220, 225, 230, 235, 240, 245, 250, 255, 260, 265, 270, 275, 280, 285, 290, 295, 300, 305, 310, 315, 320, 330, 340, 350, 360, 370, 380, 390, 400, 450, 500. m²g^"\ In certain aspects, the c-MCN material is tuned to a band gap of 1.8 to 3 eV, or 2.0 to 2.8 eV, 2.2 to 2.5 eV, or 1.8, 1.9, 2.0, 2.2, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, or 3.0 eV.

[0041] The photocatalytic reaction of c-MCN materials of the present invention can be increased by the addition of metals, which can serve as a co-catalyst in a water splitting reaction. In certain aspects, the co-catalyst is or comprises titanium, nickel, palladium, platinum, rhodium, ruthenium, tungsten, molybdenum, gold, silver, or copper metal. The c- MCN material can include 0.01, 0.10, 0.15 to 2.0 wt.% of the co-catalyst. In certain aspects, the co-catalyst is incorporated on the surface of or embedded in the carbon nitride matrix. The photon energy necessary to split water is greater than 1.23 eV, thus tuning the band gap of the mesoporous CN material can allow for more water splitting than recombination. Without wishing to be bound by theory, it is believed that tuning the CN band gap reduces the likelihood that an excited electron will spontaneously revert to its non-excited state (i.e., the electron-hole recombination rate can be reduced or suppressed). When the c-MCN material is irradiating with light (i.e., visible light) an electron can move from a given valence band (VB) to a given conduction (CB) (e.g., excitation through absorption of light), the electron will be restrained from spontaneously moving back to the VB, as the spontaneous emission of a photon that is typically associated with such a move from the CB to the VB would be at a frequency that is restricted due to the material's photonic band gap. The electron can remain in the CB for a longer period of time, which can result in use of said electron to split water rather than moving back to its VB (i.e., the electron-hole pair remains in existence for a longer period of time). This, coupled with the electrically conductive material (co-catalyst) deposited on the photoactive material, provides for a more efficient use of the excited electrons in water-splitting applications. The co-catalyst can be an electron sink and/or promote H₂ production from water instead of electron-hole (e^"-h⁺) recombination events during the photocatalytic water-splitting reaction.

B. Method of Making

[0042] The c-MCN material can be formed by nanocasting methodology using a template. Nanocasting is a technique to form periodic mesoporous framework using a hard template to produce a negative replica of the hard template structure. A molecular precursor can be infiltrated into the pores of the hard template and subsequently polymerized within the pores of the hard template at elevated temperatures. Then the hard template can be removed by a suitable method. This nanocasting route is advantageous because no cooperative assembly processes between the template and the precursors are needed. A hard template can be a mesoporous silica. [0043] FIG. 1 is a schematic representation of one embodiment of a method for producing a c-MCN (C₃N₆) material by using a hard templating approach, also called a replica approach, as described herein. Template 10 (e.g., calcined KIT-6) can include canal 12 and pores 14. Canal 12 is representative of the pore volume of template 10. Pores 14 can be filled corresponding carbon nitride precursor material 16 to form a tempi ate/carb on nitride precursor material. By way of example, an aqueous solution of an amine salt (e.g., aminoguanidine hydrochloride) can be added to a KIT-6 having a desired pore size. The tempi ate/carb on nitride precursor material can undergo a thermal treatment to polymerize the precursor inside the pore of the material to form template/CN composite 16 having canal 12 and polymerized CN material 18. Template/CN composite 16 can be subjected to conditions sufficient to dissolve the template 10 (e.g., KIT-6), and form the mesoporous carbon nitride material 20 of the present invention. By way of example, the template 10 can be dissolved using an HF treatment, a very high alkaline solution, or any other dissolution agent capable of removing the template and not dissolving the CN framework. The kind of template and the CN precursor used influence the characteristics of the final material. By way of example, various KIT-6 with various pore diameters can be used as templates. In certain aspects, the pore size of the KIT-6 template can be tuned and a diamino-s-tetrazine CN precursor (e.g., aminoguanidine hydrochloride) can be used to produce a high nitrogen content. [0044] In step one of a non-limiting method of preparing the nitrogen rich mesoporous material of the present invention can include contacting a calcined mesoporous KIT-6 template having a selected porosity with an CN precursor material (e.g., aqueous aminoguanidine hydrochloride) solution to form a template reactant mixture. In some embodiments, the template reactant mixture is a gel. In step 2 of the method, the template reactant mixture can be heated to a first temperature of 90 to 1 10 °C or 95 to 105 °C, or 100 °C for a desired amount of time (e.g., 4 to 8 hours or 5 to 7 hours, or 4, 5, 6, 7, or 8 hours). In step 3, the temperature of the templating reactant mixture can be increased to a second temperature (e.g., 150 to 170 °C, or 155 to 165 °C, or 150 °C, 155 °C, 160 °C, 165 °C, or 170 °C) and held (incubated) at the second temperature for a desired amount of time (e.g., 4 to 8 hours or 5 to 7 hours, or 4, 5, 6, 7, or 8 hours) to form an s-tetrazine/KIT-6 composite. The incremental heating can facilitate filling of pores of the KIT-6 material by the CN precursor gel to form a CN/KIT-6 composite. Step 4 of the method can include polymerization of the s-tetrazine/KIT-6 composite. The s-tetrazine /KIT-6 composite can be heated in a nitrogen flow to about 350 to 450 °C, or 375 °C to 425 °C , or 400 °C for about 3 to 7 hours, 3, 4, 5, 6, to 7 hours, or until polymerization of the tetrazine is complete, forming a cubic mesoporous carbon nitride material/KIT-6 complex. In some aspects, the CN/KIT-6 composite can be heated under an inert gaseous (e.g., nitrogen, argon, helium) atmosphere. By way of example, CN/KIT-6 composite can be heated under a nitrogen flow to temperature at a rate of about 1, 2, 3, 4, 5, or 6 °C per minute. The nitrogen flow can be at about 50, 60, or 70 to 100, 120, or 150 mL per minute, including all values and ranges there between while the CN/KIT-6 composite is heated at about 400 C°. The KIT-6 can be removed by dissolving the KIT-6 template from the cubic mesoporous carbon nitride material/KIT-6 complex to form the c-MCN of the present invention having a desired pore size. In some aspects, hydrofluoric acid or other suitable solvent or treatment can be used that dissolves the KIT-6 without dissolving the CN framework. The method can further comprise collecting the cubic mesoporous carbon nitride material of the present invention by filtration. In a further aspect, the filtered material can be ground to a powder.

[0045] In some aspects, the c-MCN compound includes a metal or metal alloy as a co- catalyst. The metal or metal alloys can be obtained from a variety of commercial sources in a variety of forms (e.g., particles, rods, films, etc.) and sizes (e.g., Nano scale or Micro scale). By way of example, each of Sigma- Aldrich® Co. LLC and Alfa Aesar GmbH & Co KG offer such products. Alternatively, the metal containing c-MCN can be prepared using co- precipitation or deposition-precipitation methods. The metal can be deposited on the c-MCN material prior to or during a photochemical reactions. By way of example, a metal precursor (e.g., a metal nitrate or metal halide) can be added to an aqueous solution containing the c- MCN material and a sacrificial agent. The metal salt can absorb on the surface of the c-MCN material. Upon irradiation, the metal ions can be converted to the active metal species (i.e., zero valance).

[0046] A KIT-6 template can be produced by (a) mixing an amphiphilic triblock copolymer dispersed in an aqueous hydrogen chloride solution with 1-butanol and tetraethyl orthosilicate (TEOS) to form a polymerization mixture; (b) heating the polymerization mixture at a predetermined synthesis temperature to form a KIT-6 template, where the predetermined temperature determines the pore size of the KIT-6 template; and (c) calcining the KIT-6 template by heating the KIT-6 template to about 480, 500, 520, 540, 560, or 580 °C. The polymerization mixture can be heated at a temperature of about 75 °C, 100 °C, or 125 °C to 150 °C, 175 °C, or 200 °C or any value or range there between. In certain aspects, the polymerization mixture can be incubated at a synthesis temperature of about 100 °C, 130 °C, or 150 °C or any value or range there between.

C. Use of the Mesoporous Carbon Nitride Materials in Water-Splitting Reactions

[0047] The 3-D diamino-s-tetrazine based mesoporous carbon nitride matrix material of the present invention with or without a co-catalyst can be used as a photocatalyst in water- splitting reactions. By way of example, an aqueous reactant mixture that includes a photocatalytic 3-D mesoporous CN material of the present invention that includes platinum can be irradiated with light (e.g., visible light) to form a product stream that includes hydrogen and oxygen. The hydrogen product can be collected.

[0048] In some embodiments, a sacrificial agent can be added to the reactant mixture. The presence of the sacrificial agent can increase the efficiency of the photosystem by further reducing the likelihood of hole/electron recombination via oxidation of the sacrificial agent by the hole rather than recombination with the excited electron and/or assist in photodeposition of the co-catalyst on the c-MCN surface. Non-limiting examples of sacrificial agents that can be used in the methods of the present invention include ethanolamines, alcohols, diols, polyols, dioic acids, or any combination thereof. A non- limiting example of particular sacrificial agent includes triethanolamine. EXAMPLES

[0049] The following examples as well as the figures are included to demonstrate preferred embodiments of the invention. It should be appreciated by those of skill in the art that the techniques disclosed in the examples or figures represent techniques discovered by the inventors to function well in the practice of the invention, and thus can be considered to constitute preferred modes for its practice. However, those of skill in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments which are disclosed and still obtain a like or similar result without departing from the spirit and scope of the invention.

[0050] Materials. Tetraethyl orthosilicate (TEOS), aminoguanidine hydrochloride and triblock copolymer poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (Pluronic PI 23, molecular weight 5800 g mol^"1, EO₂₀PO₇₀EO₂₀), which were obtained from Sigma-Aldrich® (U.S. A). Ethanol and hydrofluoric acid (HF) were purchased from Wako Pure Chemical Industries (U.S.A.). All the chemicals were used without further purification. Doubly deionized water has been used throughout the synthesis process.

EXAMPLE 1

(Preparation of mesoporous 3D KIT-6 silica template with different pore diameters)

[0051] KIT-6 having different pore diameters was synthesized by using a P123 and n- butanol mixture as the structure directing agent at different synthesis temperatures. In a typical synthesis, P123 (4.0 g) was dispersed in a water (144 g) and HC1 solution (7.9 g), and stirred for 4 h to obtain an aqueous P123 homogeneous solution. 1-butanol (4.0 g) and TEOS (8.6 g) were added at once to the aqueous P123 homogeneous solution under stirring, and stirring was continued at 35 °C for 24 hours to produce a reaction mixture. Subsequently, the reaction mixture was aged at 100 °C for 24 h under static conditions. At these conditions a white solid product was formed. The white solid product was filtered without washing under hot conditions and dried at 100 °C for 24 hours in an air oven. Finally, the product was calcined at 540 °C in air to remove the template. KIT-6 silica template materials with different pore diameters were synthesized at the synthesis temperatures of 100, 130, and 150 °C. The samples were labeled KIT-6-X, for which X denotes the synthesis temperature.

EXAMPLE 2

(Synthesis of 3-D mesoporous CN materials with different pore diameters) c-MCN materials having a three dimensional body centered cubic porous structure and various textural parameters were prepared by using 3D mesoporous silica KIT-6-X having various pore diameters as templates. An aqueous solution of aminoguanidine hydrochloride (4.0 g H₂N HC HNH₂.HC1 in 3.0 g of water) was added to the appropriate calcined KIT-6- X (1.0 g) prepared in Example 1. The resultant gel was heated at 100 °C for 6 hours, and further heated at 160 °C for another 6 hours in a programmed oven to form a CN/KIT-6-X composite. The CN/KIT-6-X composite was heated in a nitrogen flow of 100 mL per minute to 400 °C with a heating rate of 3 °C min^"1 and kept under these conditions for 5 hours for polymerization. The c-MCN-X material (e.g., cMCN-100, cMCN-130 and cMCN-150), where X represents the temperature used for formation of the KIT-6 material was recovered after dissolution of the silica framework in 5 wt.% hydrofluoric acid by filtration, washed several times with ethanol, and dried at 100 °C.

EXAMPLE 3

(Comparative Sample)

[0052] Nonporous CN was prepared using the above conditions except for the addition of the mesoporous silica template in the synthesis mixture. EXAMPLE 4

(Characterization of cMCN-X of the present invention)

[0053] XRD: Powder XRD patterns of the Example 2 catalysts were recorded on Rigaku

Ultima+ (JAPAN) diffractometer using CuKa (λ = 1.5408 A) radiation. Low angle powder x- ray diffractograms were recorded in the 2Θ range of 0.6-6° with a 2Θ step size of 0.0017 and a step time of 1 sec. In case of wide angle X-ray diffraction, the patterns were obtained in the

2Θ range of 10-80° with a step size of 0.0083 and a step time of 1 sec. FIG. 2 shows lower angle and wide (inset) angle powder XRD patterns of (a) c-MCN -100 (b) c-MCN-130 and

(c) c-MCN-150.

[0054] Textural parameters: Textural parameters and mesoscale ordering (d₍₂₁₁> spacing, unit cell size, surface area, pore volume and pore diameter) of the KIT-6-X compounds of Example 1 and the Example 2 catalysts was determined from nitrogen adsorption-desorption isotherms using a Quantachrome Instruments (U.S.A.) sorption analyzer at -196 °C. All samples were out-gassed for 12 hrs at high temperatures under vacuum (p<l >< 10-5 h.Pa) in the degas port of the adsorption analyzer. The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method. The pore size distributions were obtained from either adsorption or desorption branches of the isotherms using Barrett- Joy ner-Halenda (BJH) method. [0055] Chemical analysis: Elementary analysis of the KIT-6-X compounds of Example 1 and the Example 2 catalysts was carried out by using a Yanaco MT-5 CHN elemental analyzer (Yanaco Bunseki Kogyo Co., JAPAN ). Table 1 lists the values for the textural parameters and chemical analysis of c-MCN-100, c-MCN-130, and c-MCN-150. Table 2 lists for the textural parameters and chemical analysis of KIT-6-100, KIT-6-130, and KIT-6- 150. FIG. 3 shows the nitrogen adsorption-desorption isotherms and shows the BJH pore- size distributions for c-MCN-100 (circles), c-MCN-130 (diamonds) and c-MCN-150 (squares). FIG. 4 shows nitrogen adsorption-desorption isotherms and BJH pore-size distributions of c-MCN-100 and KIT-6-100 silica template.

Table 1

Table 2

[0056] HRTEM and EELS: HRTEM and EEL images of the Example 2 catalysts were obtained using a high-resolution transmission electron microscope JEOL-3100FEF, (JOEL, U.S.A.) equipped with a Gatan-766 electron energy-loss spectrometer (EELS). The preparation of the samples for HRTEM analysis involved sonication in ethanol for 5 min and deposition on a copper grid. The accelerating voltage of the electron beam was 200 kV. FIG. 5. are HRTEM images of (a,b) c-MCN-100, (c,d) c-MCN-130 and (e,f) c-MCN-150 of the present invention at 50 and 20 nm magnifications, respectively FIG. 6 are HRSEM images of (a,b) c-MCN-100, (c,d) c-MCN-130 and (e,f) c-MCN-150 at 3 and 2 micron magnifications, respectively. FIG. 7 shows EEL spectra of Example 2 catalysts: (a) c-MCN-100, (b) c-MCN- 130, and (c) c-MCN-150.

[0057] FESEM and EDX: Morphology of the Example 2 catalysts was observed on a Hitachi S-4800 (U.S.A.) field emission scanning electron microscope (FE-SEM). The machine is equipped with energy dispersive X-ray (EMAX) elemental analyzer. Prior to observation, all the samples were sputtered with Pt for 20 sec by using ion coater. Samples were measured under the accelerating voltage of 5-10 kV, emission current around 10 mA and condensed lens of 5 Megapixel. For SEM, objective aperture 2 was used with a working distance around 8 mm while during elemental analysis (EDX), aperture number 1 with working distance around 15 mm was used. EDX along with elemental mapping were recorded on the same machine using accelerating voltage of 15 kV. EDX spectra of the Example 2 catalysts were obtained. FIG. 8 shown energy dispersive X-ray spectroscopy (EDX) images of (a) c-MCN-100, (b) c-MCN-130, and (c) c-MCN-150 of the present invention. [0058] XPS: X-ray spectroscopy measurements of the Example 2 catalysts were carried out using PHI Quantera SXM (UL VAC-PHI, JAPAN) instrument with a 20 kV, Al Ka probe beam (E=1486.6eV). Prior to the analysis, the samples were evacuated at high vacuum (4>< 10-7Pa), and then introduced into the analysis chamber. For narrow scans, analyzer pass energy of 55 eV with a step of 0.1 eV was applied. To account for the charging effect, all the spectra were referred to the Cls peak at 284.5 eV. Survey and multiregion spectra were recorded at Cls and Nls photoelectron peaks. Each spectral region of photoelectron interest was scanned several times to obtain a good signal-to-noise ratio. FIG. 9 shows (top) X-ray photoelectron spectroscopy (XPS) survey spectra, (middle) Cls XPS survey spectra, and (bottom) Nls XPS survey spectra of c-MCN-150. Table 3 presents a description of the peaks obtained from XPS.

Table 3

[0059] UV-VIS: UV-Vis absorption spectra of the Example 2 catalysts were recorded by using LAMBDA 750 UV/VIS/NIR spectrophotometer (190 nm-3300 nm) from Perkin Elmer (U.S.A.). Instrument is equipped with a diffuse reflectance integrating sphere coated with BaS04, which serve as a standard. Thickness of the quartz optical cell was 5 mm. The band gap of the materials were calculated using Tauc Plot method. Band gap data was determined using UV-Vis spectra of the Example 2 catalysts was obtained. FIG. 10 shows the UV spectra (insert) and the band gap data for (a) c-MCN-100, (b) c-MCN-130 and (c) c-MCN- [0060] FTIR: FTIR spectra of the Example 2 catalysts were recorded by using Perkin Elmer (U.S.A.) spectrum 100 series, bench top model equipped with the optical system that gives the data collection over the range of 7800 to 370 cm-1. The spectra were recorded by averaging 200 scans with a resolution of 2 cm-1, measuring in transmission mode using the KBr self-supported pellet technique. The spectrometer chamber was continuously purged with dry air to remove water vapor. FIG. 11 shows spectra of (a) c-MCN-100, (b) c-MCN- 130, and (c) c-MCN-150 samples.

[0061] Elemental Mapping. Elemental mapping of the Example 2 catalysts was obtained during EDX analysis. FIG. 12 shows the results of elemental mappings of carbon (C) and nitrogen (N) of the (a) c-MCN-100, (b) c-MCN-130, and (c) c-MCN-150 samples.

[0062] TGA: Thermo-gravimetric analysis (TGA) of the c-MCN-150 sample was performed on an SII Nano Technology TG/DTA 6200 instrument (Hitachi High Tech Science Corporation, JAPAN) using pure nitrogen as a carrier gas in with different heating rates. TG analysis was performed. FIG. 13 shows the results of thermogravimetric (TG) analysis of the c-MCN-150 sample.

EXAMPLE 5

(Photocatalytic reactions using the catalysts of the present invention)

[0063] Photocatalytic reactions were carried out in a Pyrex top irradiation reaction vessel connected to a glass closed gas circulation system. Hydrogen (H₂) production was performed by dispersing 0.1 g well ground catalyst powder in an aqueous solution (100 mL) containing triethanolamine (10 vol.%) as sacrificial electron donor. Platinum (Pt) was photodeposited on the catalysts using H₂PtCl₆ dissolved in the reactant solution. The reactant solution was evacuated several times to remove air completely prior to irradiation under a 300 W Xe lamp and a water cooling filter. The wavelength of the incident light was controlled by using an appropriate long pass cut-off filter. The temperature of the reactant solution was maintained at room temperature by a flow of cooling water during the reaction. The evolved gases were analyzed by gas chromatography equipped with a thermal conductive detector. FIG. 14 shows the time course of H₂ gas evolution using c-MCN photocatalyst of Examples 2 referenced to non-porous CN. [0064] The production of H₂ increases steadily with prolonged time of light irradiation. After four consecutive runs (12 hours), a total of 0.5 mmol H₂ gas (11.2 mL) was produced, and no deactivation of the photocatalysts was detected, suggesting stability of c-MCN as an organic photocatalyst for solar H₂ generation. Catalytic stability is needed for a photocatalyst in solar energy application. There is no N₂ evolution observed, indicating the strong binding of N within the covalent structure, even in the mesoporous environment. No reaction occurred when the system was illuminated in the absence of c-MCN photocatalyst of the present invention, which confirms that the reaction is indeed driven by light absorption on the catalyst.

[0065] In summary, a highly ordered three dimensional diamino-s-tetrazine based mesoporous carbon nitride materials (c-MCN) with high specific surface area, large pore volume, tunable pore diameters and a cubic type mesoporous structure have been prepared through the self-condensation followed by polymerization of nitrogen enriched aminoguanidine hydrochloride (H₂NNHCNHNH₂.HC1) as a single molecular precursor inside the mesopore channels of three dimensional body centered cubic mesoporous silica KIT-6 having various pore diameters. The c-MCN shows moderate activity for photochemical reduction of water with visible light in the presence of Pt as a co-catalyst and electron donors. This material is stable under experimental conditions.

Claims

1. A photocatalytic carbon nitride (CN) material comprising a three dimensional mesoporous CN matrix having an atomic nitrogen to atomic carbon (N:C) ratio of 1.8 to 2.1 and a band gap of 1.8 to 3 eV.

2. The photocatalytic CN material of any one of claims 1 to 2, having a pore volume of 0.2 to 1.5 cnrg^"1 _.

3. The photocatalytic CN material of any one of claims 1 to 3, wherein the CN matrix has an average pore size of 2 to 50 nm,

4. The photocatalytic CN material of claim 4, wherein the average pore size is 2 to 10 nm.

5. The photocatalytic CN material of claim 4, wherein the average pore size is 3 to 5 nm.

6. The photocatalytic CN material of any one of claims 1 to 5, having a specific surface area of 140 to 500 m²g^_1-

7. The photocatalytic CN material of any one of claims 1 to 6, further comprising a co- catalyst.

8. The photocatalytic CN material of claim 7, wherein the co-catalyst comprises platinum, titanium, nickel, palladium, rhodium, ruthenium, tungsten, molybdenum, gold, silver, copper, or combinations thereof or alloys thereof.

9. The photocatalytic CN material of claim 7, wherein the co-catalyst is platinum.

10. The photocatalytic CN material of any one of claims 1 to 9, wherein the carbon nitrogen matrix is a based on C₃N₆ diamino-s-tetrazine.

11. The photocatalytic CN material of any one of claims 1 to 9, wherein the material is stable under photocatalytic conditions.

12. A method of producing a photocatalytic CN material of any one of claims 1 to 11, the method comprising

(a) contacting a mesoporous template having a selected porosity with an aminoguanidine hydrochloride solution, forming a template reactant mixture; and

(b) heating the template reactant mixture, forming a carbon nitride material/template composite; (c) heating the CN/template composite in an inert gas atmosphere to about 350 °C to 450 °C, forming a cubic mesoporous carbon nitride material/template complex; and

(d) removing the template from the mesoporous carbon nitride material/ template complex.

13. The method of claim 12, wherein step (b) heating comprises:

(i) heating the template reactant mixture at a predetermined first temperature for a first time interval; and

(ii) raising the temperature to a second temperature for a second time interval.

14. The method of claim 13, wherein the first temperature is 90 °C to 110 °C, or about 100 °C.

15. The method of claim 13, wherein the second temperature is 150 °C to 170 °C, or about 160 °C.

16. The method of any one of claims 12 to 15, wherein the CN/KIT-6 composite is heated to 350 °C to 450 °C at a rate of 3 °C per minute.

17. The method of any one of claims 12 to 16, wherein the step (c) temperature is about 400 °C.

18. The method of any one of claims 12 to 17, wherein the template is removed contacting the cubic mesoporous carbon nitride material/template complex with hydrofluoric acid.

19. A photocatalytic process comprising:

(a) contacting the photocatalytic CN material of claims 1 to 8 with water forming a reactant mixture;

(b) exposing the reactant mixture to light under conditions sufficient to produce a product stream comprising hydrogen; and

(c) collecting the hydrogen product.

20. The photocatalytic process of claim 19, wherein the carbon nitride material comprises a noble metal, preferably platinum.