Environmentally friendly production of carbon quantum dots efficiently from prb coal

Abstract

Embodiments described herein generally relate to processes for making carbon quantum dots (CQDs). A method includes adding a coal product, a H2O2 solution, and water to a reactor. The coal product, the H2O2 solution, and the water form a mixture. The reactor is heated to a temperature of about 60°C to about 100°C to catalyze the carbon quantum dot (CQD) production mechanism within the mixture to form a CQD. The CQD is collected. The CQD includes carbon spheres with a diameter of about 15 nm or less. A controller may be programmed to perform the method.

Description

ENVIRONMENTALLY FRIENDLY PRODUCTION OF CARBON QUANTUM DOTS EFFICIENTLY FROM PRB COALGOVERNMENT RIGHTS

[0001] The invention was made with government support under Award No. DE- FE0031997 awarded by the United States Department of Energy. The government has certain rights in the invention.BACKGROUNDField

[0002] Embodiments described herein generally relate to processes for synthesizing coal- derived carbon quantum dots (CQDs). Embodiments described herein also generally relate to compositions for CQDs.Description of the Related Art

[0003] Coal is a historically important economic resource. However, because conventional utilizations of coal include combustion, coal utilization tends to produce a large amount of CO2 emissions. In recent years, to mitigate the CO2 emission impact of coal utilization, significant technical progress has been made to facilitate the direct use of coal for producing solid carbon materials. Specifically, coal-to-materials production technologies have been diversified to target, for example, the thermal, electrochemical, and mechanical engineering properties of coal materials. Some coal-derived carbon materials have especially high-value. However, in the current state-of-the-art, most coal-to-materials technologies are based on intermediates from coal processing, including coal tar and char. Coal tar and char productions not only generate a lot of pollutants, but also are energy-intensive.

[0004] Carbon quantum dots (CQDs) are an example of a high-value coal-derived material. CQDs are non-toxic, low cost, simple, and easy to control. CQDs also have the capacity for wide absorption of the spectrum (e.g., that of modified CQDs), and are able to efficiently perform charge transfer and storage. CQDs may be broadly as implemented as representativematerials in photocatalysis, photoelectric devices, and detection and sensing fields. Current techniques for CQDs production may be based on electrochemical synthesis, combustion / thermal / hydrothermal / acidic oxidation, supported synthetic procedures, microwave / ultrasonic synthesis, or some combination thereof. While CQD preparation methods represent technological advancements made over many years, there remains shortcomings with techniques as presented in the current state-of-the-art. Specifically, commercialization of CQDs is inhibited on account of high energy costs and high resource expectations.

[0005] Accordingly, there is a need for improved production and utilization of CQDs in the art, as well as technologies targeting high-value coal-derived materials.SUMMARY

[0006] In one embodiment, a method is disclosed. The method includes adding a coal product, a H2O2 solution, and water to a reactor. The coal product, the H2O2 solution, and the water form a mixture. The reactor is heated to a temperature of about 60°C to about 100°C to catalyze the carbon quantum dot (CQD) production mechanism within the mixture to form a CQD. The CQD is collection.

[0007] In another embodiment, a carbon quantum dot (CQD) is disclosed. The CQD includes carbon spheres with a diameter of about 15 nm or less. The carbon sphere is formed from a coal product.

[0008] In yet another embodiment, a system is disclosed. The system includes a controller, and a reactor coupled to the controller. The controller is programmed to perform a method including heating the reactor to a temperature of about 60°C to about 100°C to catalyze the carbon quantum dot (CQD) production mechanism within a mixture to form a CQD. The mixture includes a coal product, a H2O2 solution, and water. The CQD production mechanism is a Fenton reagent driven coal-based CQDs production mechanism. The CQD is collected.BRIEF DESCRIPTION OF THE DRAWINGS

[0009] So that the manner in which the above recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only exemplary embodiments and are therefore not to be considered limiting of its scope, and may admit to other equally effective embodiments.

[0010] Figure 1 is a flow diagram for a method 100 of synthesizing carbon quantum dots, according to embodiments.

[0011] Figure 2 is a schematic diagram of an experimental setup for synthesis of the CQDs from coal and H2O2, according to embodiments.

[0012] Figure 3 A is a graph of the thermogravimetric analysis (TGA) of Powder River Basin (PRB) coal, according to embodiments.

[0013] Figure 3B is a graph of the Fourier-transform infrared spectroscopy (FTIR) analysis of PRB coal, according to embodiments.

[0014] Figure 4A is a graph of the carbon distribution in the final products of gas, solution, and residual resulting from the reactions with different H2O2 concentrations, according to embodiments.

[0015] Figure 4B is an graph of the final products resulting from the reactions with different H2O2 concentrations, according to embodiments.

[0016] Figures 5A-5E are graphs of fluorescence spectra images of CQDs-containing solutions from the reactions at different H2O2 concentrations, according to embodiments.

[0017] Figure 6 is a graph of an example UV-vis absorption spectra resulting from the reactions with different H2O2 concentrations, according to embodiments.

[0018] Figure 7A is a graph of the carbon distribution in the final products of gas, solution, and residual resulting from the reactions at different temperatures, according to embodiments.

[0019] Figure 7B is a graph of the final products resulting from the reactions at different temperatures, according to embodiments.

[0020] Figures 8A-8E are fluorescence spectra images of CQDs-containing solutions from the reactions at different temperatures, according to embodiments.

[0021] Figure 9 is a graph of a UV-vis absorption spectra resulting from the reactions with different temperatures, according to embodiments.

[0022] Figure 10A is a graph of the carbon distribution in the final products of gas, solution, and residual at different reaction times, according to embodiments.

[0023] Figure 10B is a graph of the final products resulting from the reactions at different reaction times, according to embodiments.

[0024] Figures 11A-11F are fluorescence spectra images of CQDs-containing solutions from the reactions at different reaction times, according to embodiments.

[0025] Figure 12 is a graph of a UV-vis absorption spectra resulting from the reactions with different reaction times, according to embodiments.

[0026] Figure 13 A is a graph of the carbon distribution in the final products resulting from the reactions of different raw materials, according to embodiments.

[0027] Figure 13B is a graph of the gas products resulting from the reactions of different raw materials, according to embodiments.

[0028] Figures 14A-14D are fluorescence spectra images of CQDs-containing solutions from the reactions from different materials, according to embodiments.

[0029] Figure 15 is a graph of a UV-vis absorption spectra resulting from the reactions with different raw materials, according to embodiments.

[0030] Figure 16A illustrates an example TEM image of spherical morphology of a monodispersed particle, according to embodiments.

[0031] Figure 16B Figure 16B is a graph of the diameter distributions of the CQDs synthesized from raw coal, according to embodiments.

[0032] Figure 16C is a graph of the FT-IR spectra of the CQDs, according to embodiments.

[0033] Figure 16D is a graph of the XPS spectra of 01 peaks of the CQDs, according to embodiments.

[0034] Figure 16E is a graph of the XPS spectra of the Cl peaks of the CQDs, according to embodiments.

[0035] Figure 16F a graph of statistics for various carbons of prepared CQDs from raw coal, according to embodiments.

[0036] Figure 17 is a diagram of the mechanism of CQDs synthesis from PRB coal and H2O2, according to embodiments.

[0037] To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation.DETAILED DESCRIPTION

[0038] Embodiments described herein generally relate to processes for synthesizing coal- derived carbon quantum dots (CQDs). More specifically, embodiments described herein generally relate to processes for making CQDs that may include treating a mixture comprising coal products, hydrogen peroxide (H2O2), and water, desorption, and / or capture.

[0039] The inventors have found innovative, facile, low-temperature, cost-effective, environmentally friendly technology for producing high-value coal-derived CQDs. To address the present issues with CQD production, embodiments described herein present techniques forproducing green, coal-driven CQDs such that outstanding issues are mitigated. Fluorescent CQDs are synthesized via hydrothermal reactions using inexpensive and earth-abundant coal as raw materials and using simple, low-cost procedures. The CQDs may be synthesized by mixing coal, H2O2, and H2O inside a high-pressure high-temperature reactor.

[0040] In some non-limiting embodiments, the coal products include one of de-ash coal, de-ash coal and 30 parts per million (ppm) ferrous ion (Fe2+), and raw coal.

[0041] In some non-limiting embodiments, the process includes an amount of the H2O2 in the composition is about 2.5 wt% or less based on a total weight of the composition.

[0042] In some non-limiting embodiments, the process includes an amount of the coal product in the composition is about 1 gram or less based on a total weight of the composition.

[0043] In some non-limiting embodiments, the process includes treating a mixture further comprises treating a mixture at a temperature of 60°C or more or at a temperature of 100°C or less.

[0044] In some non-limiting embodiments, the process includes treating a mixture further comprises treating a mixture for a duration of 2 hours or more, such as 12 hours or less.

[0045] A systematic screening procedure is presented herein and may be used to obtain an optimum temperature, reaction time, and H2O2 concentration for the synthesis of the best performing CQDs. The synthesized CQDs may be characterized using TEM, fluorescence and UV spectroscopy, ion chromatography, and elemental analysis. Embodiments described in this disclosure may be applied to the fabrication and characterization of newly developed carbon nanostructures that may be used for various applications such as as solar cells and photocatalysis for the decomposition of organic pollutants.

[0046] As used herein, a “composition” can include component(s) of the composition, reaction product(s) of two or more components of the composition, a remainder balance of remaining starting component(s), or combinations thereof. Compositions of the present disclosure can be prepared by any suitable mixing process.

[0047] The use of headings is for purposes of convenience only and does not limit the scope of the present disclosure. Embodiments described herein can be combined with other embodiments.COMPOSITIONS

[0048] Figure 1 is a flow diagram for a method 100 of synthesizing carbon quantum dots (CQDs). Figure 2 is a schematic diagram of an experimental setup 200 for synthesis of the CQDs from coal and H2O2. The experimental setup 200 includes a high-pressure reactor 201, a gas source 202, a gas flow controller 203, a controller 204, an inlet 205, an outlet 206, a thermocouple 207, a gas chromatography system 208, and a computer 209.

[0049] At operation 102, a pre-determined mass of coal (mo), H2O2 solution, and H2O were added into the high-pressure reactor 201 to form a solution. The high-pressure reactor 201 is coupled to the controller 204. Before each test, the high-pressure reactor was swept with about N2 gas for about 3 min to about 7 min to remove the air remaining in the high-pressure reactor. The N2 was used as the internal standard for the calculation of the gaseous products. The N2 is provided to the high-pressure reactor 201 from the gas source 202. The gas flow controller 203 controls the flow rate of the N2 to the high-pressure reactor 201 via the inlet 205.

[0050] The coal may include de-ashed coal, de-ashed coal with ferrous ion, or raw coal. The de-ashed coal with ferrous ion may include about 25 ppm to about 35 ppm Fe2+. The H2O2 solution is about 1% H2O2 to about 5% H2O2, such as about 1.0% H2O2 to about 2.0% H2O2, about 1.5% H2O2 to about 2.5% H2O2, about 2.0% H2O2 to about 3.0% H2O2, about 2.5% H2O2 to about 3.5% H2O2, about 3.0% H2O2 to about 4.0% H2O2. Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range.

[0051] At operation 104, the high-pressure reactor 201 is heated to catalyze the coal -based CQDs production mechanism and produce CQDs. The high-pressure reactor 201 includes a heat mantle 210 to heat the high-pressure reactor 201. A stirring mechanism 211 agitates the solution within the high-pressure reactor 201. The high-pressure reactor 201 is heated to temperatures from about 50°C to about 110°C, such as about 55°C to about 65°C, such as about65°C to about 75°C, such as about 75°C to about 85°C, such as about 85°C to about 95°C, such as about 95°C to about 105°C. The temperature is held for about 1 to about 15 hours, such as1.5 hours to about 2.5 hours, such as 2.5 hours to about 3.5 hours, such as 3.5 hours to about4.5 hours, such as 4.5 hours to about 5.5 hours, such as 5.5 hours to about 6.5 hours, such as6.5 hours to about 7.5 hours, such as 7.5 hours to about 8.5 hours, such as 8.5 hours to about9.5 hours, such as 9.5 hours to about 10.5 hours, such as 10.5 hours to about 11.5 hours, such as 11.5 hours to about 12.5 hours. The thermocouple measures the temperature of the solution in order to maintain the desired temperature. Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range.

[0052] The reactions of the Fenton reagent driven coal-based CQDs production mechanism are illustrated in R1-R3.Fe2++ H2O2 Fe3++ HO* + OH (Rl)Fe3++ H2O2 Fe2++ HOO* + H+(R2)2H2O2 HO* + HOO* + H2O (R3, or total reaction of R1+R2)

[0053] The first step of Fenton reactions is the interaction between Fe2+existing in coal and H2O2, forming a hydroxyl radical (HO*) and a hydroxide ion (OH ). The Fe3+resulting from Rl may be reduced back to Fe2+by another molecule of H2O2, forming a hydroperoxyl radical (HOO*) and a proton (H+). The net effect is a disproportionation of H2O2 to create two different radicals (HOO* and HO*) and water as a byproduct.

[0054] A clean oxidizer, such as Fenton reagent (H2O2 + Fe2+), is used for CQD production. In some embodiments, only the H2O2 solution is used. The Fe2+catalyst is provided by the coal. The quantity of Fe2+needed may be very low, as it is used as a catalyst instead of a reactant in Rl and R2. Coal generally may contain a small number of water-soluble iron compounds, such as ferrous acetate. Therefore, only H2O2 is needed for the Fenton reagent- driven CQDs production process. The Fenton reaction may involve radicals and proton generations. Accordingly, the reaction is both strong and efficient, and coal may be easily oxidized. Therefore, CO2 and H2O are generated as the byproducts during the CQDs production process independent of potential pollutants. The Fenton reaction is exothermic, andthus heating may not be needed, which can significantly simplify the operation and lower the capital cost as well as increase the safety of the proposed CQD production technology. H2O2 is a less inexpensive oxidizing agent than the frequently used and dangerous oxidizers from the perspective of the cost per electron.

[0055] The oxygen-radical species HO* and HOO* that result from H2O2 decomposition may be catalyzed or initiated by Fe2+(e.g., existing in mineral form in coal) and may quickly oxidize the organic compounds in coal. This may result in the dissociation of organic macromolecules into small carbon spheres with a diameter of about 15 nm or less, such as about 10 nm. Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range.

[0056] At operation 106, a gaseous product is collected. The gaseous products exit the high-pressure reactor 201 via the outlet 206 and are collected using an airbag and analyzed using the gas chromatography system 208. The results of the analysis are provided to the computer 209.

[0057] At operation 108, a liquid mixture is separated from solid materials. The liquid mixture is separated using a glass vacuum filtration distillation apparatus with a filter paper. The filter paper may include a 0.45 pm Teflon filter paper.

[0058] At operation 110, the solid materials are dried. The solid materials (e.g., CQDs, unreacted coal, ash) are dried in an oven for about 24 hours at about 90°C, then weighted and marked as mi. The coal conversion was defined according to the weight loss of coal by Equation 1, as follows:

[0059] where x (%) may be the conversion of the coal, w0may be the initial moisture-free weight of coal, w may be collected moisture-free unreacted coal, d may be the ash content of coal.

[0060] At operation 112, the CQDs are collected from the solid materials.

[0061] The controller 204 is used to control the operation of the high-pressure reactor 201 and implement the method 100 set forth herein. The controller 204 includes a programmable central processing unit that is operable with a memory and support circuits. The support circuits are coupled to the CPU and include cache, clock circuits, input / output subsystems, power supplies, and combinations thereof coupled to the various components of the high- pressure reactor 201, to facilitate control thereof. The CPU is one of any form of general purpose computer processor, such as a programmable logic controller (PLC), for controlling various components and sub-processors of the high-pressure reactor 201. The memory, coupled to the CPU, is non-transitory and is typically one or more of readily available memories such as random access memory (RAM), read only memory (ROM), floppy disk drive, hard disk, or any other form of digital storage, local or remote.

[0062] Typically, the memory is in the form of a computer-readable storage media containing instructions (e.g., non-volatile memory) that, when executed by the CPU, facilitates the operation of the high-pressure reactor 201. The instructions in the memory are in the form of a program product such as a program that implements the methods of the present disclosure. The program code may conform to any one of a number of different programming languages. In one example, the disclosure may be implemented as a program product stored on computer- readable storage media for use with a computer system. The program(s) of the program product define function of the embodiments (including the methods described herein).

[0063] Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range.USES

[0064] Embodiments of the present disclosure generally relate to uses of the compositions described herein. Compositions described herein can also be used for various applications. Illustrative, non-limiting applications include carbon quantum dots (CQDs) for use in photocatalysis, photoelectric devices, and detection and sensing fields.

[0065] The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how to make and use embodiments of thepresent disclosure, and are not intended to limit the scope of embodiments of the present disclosure. Efforts have been made to ensure accuracy with respect to numbers used but some experimental errors and deviations should be accounted for.EXAMPLESTest Methods

[0066] The ultimate analysis, including organic elemental composition (carbon, hydrogen, nitrogen, and sulfur), was characterized on a Vario MACRO Cube Elemental Analyzer (Elementar Analysensysteme GmbH, Germany) according to ASTM D5373-21 - Standard Test Methods for Determination of Carbon, Hydrogen, and Nitrogen in Analysis Samples of Coal and Carbon in Analysis Samples of Coal and Coke.

[0067] Moisture, volatile matter, ash content, the fixed carbon, proximate analysis, and ultimate analysis of PRB coal are measured by ASTM D5142-02 - The Standard Test Methods for Proximate Analysis of the Analysis Sample of Coal and Coke by Instrumental Procedures.

[0068] The functional groups of the CQDs were measured using a Fourier-transform infrared spectroscopy (FT-IR) with a non-destructive attenuated total reflectance (ATR) technique using a Nicolet iS50 ATR from Thermo Scientific, USA.

[0069] The fixed carbon of coal was determined by the weight loss after air oxidation by thermal gravimetric analysis (TGA) using a thermogravimetric analyzer SDT-Q600 from TA instrument, USA by heating to about 800°C with a heating rate of about 10°C / min, followed by a holding time of about 1 hour.

[0070] X-ray photoelectron spectroscopy (XPS) was obtained on a PHI 5800 Spectrometer equipped with the monochromatic AlKa as the X-ray source.

[0071] The morphology of the CQDs was characterized by a high-resolution transmission electron microscopy using a HR-TEM, Joel JEM-2100.

[0072] 3D fluorescence spectroscopy experiments were conducted on a Fluorolog-3 spectrofluorometer (Horiba, USA) at about 25°C.

[0073] The data was collected and analyzed using OriginLab 8.0.

[0074] The experimental setup of Figure 2 includes a Parr 2526 high-pressure reactor with a 2 L container and a 4848 controller. The total volume of the reactants is about 1.5 L in order for the temperature to be accurately controlled. Before each test, the reactor was swept with about 1 L / min N2 gas for about 5 min to remove the air remaining in the reactor.EXPERIMENTALMaterials

[0075] Wyoming Powder River Basin (PRB) coal (i.e., PRB coal) was used as raw materials and carbon resource. The received PRB coal was ground into particles smaller than about 70 pm to 80 pm (under about 200 mesh) and dried overnight at about 80°C. Hydrogen peroxide (H2O2, about 30 wt.%. ACS grade), sulfuric acid (H2SO4, about 98 wt.%, ACS grade), and potassium permanganate (KMnCh, about >99.2 wt.%, ACS grade) were purchased from Fisher Scientific (USA). Nitrogen gas (N2, UHP, about 99.999%) was purchased from United States Wielding, Inc.CODs Synthesis Process

[0076] A pre-determined mass of coal (mo), H2O2, and H2O were added into the about 2 L container of Parr 2526 high-pressure reactor with a 4848 controller. The total volume of the reactants is about 1.5 L in order for the temperature to be accurately controlled. Before each test, the reactor was swept with about 1 L / min N2 gas for about 5 min to remove the air remaining in the reactor. The N2 was used as the internal standard for the calculation of the gaseous products. The reactor is heated to various temperatures from about 60°C to about 90°C, followed by holding the temperature from about 2 to about 12 hours. When the reaction was done, the gaseous products were collected using an airbag and analyzed using gas chromatography. The liquid mixture was separated using an about 47 mm glass vacuum filtration distillation apparatus with an about 0.45 pm Teflon filter paper. The solid materials were dried in an oven for about 24 hours at about 90°C, then weighted and marked as mi. Thus, the coal conversion was defined according to the weight loss of coal by Equation 1.Characterizations

[0077] The fixed carbon of coal was determined by the weight loss after air oxidation by thermal gravimetric analysis (TGA) by heating to about 800°C with a heating rate of about 10°C / min, followed by a holding time of about 1 hour. The morphology of the CQDs was characterized by a high-resolution transmission electron microscopy. The size of the CQDs was collected and counted based on the TEM images. The CDQs were scanned about 64 times at a resolution of about 16 cm’1with a wavenumber range between about 4000 cm’1and about 400 cm’1using a Fourier-transform infrared spectroscopy (FT-IR) with a non-destructive attenuated total reflectance (ATR) technique was used to obtain the functional groups of the CQDs.

[0078] 3D fluorescence spectroscopy experiments were conducted at about 25°C to study the photophysical properties of obtained CQDs. For the steady-state fluorescence excitation and emission measurements, the excitation wavelength range was set from about 200 nm to about 600 nm with an interval of about 4 nm, and the emission wavelength was set from about 296 nm to about 700 nm with an interval of about 2 nm.

[0079] The raw coal was collected from Wyodak coal mine in Wyoming Powder River Basin, Wyoming, the USA by Black Hills Corporation. The raw coal was pulverized into particles smaller than about 70 pm or more to 80 pm or less, such as about 74 pm (under about 200 mesh) and dried overnight at about 80 °C.

[0080] Table 1 is a summary of the Proximate Analysis and Ultimate Analysis of the PRB coal. The moisture of PRB coal was measured as about 5% or more to about 15% or less, such as about 8%, according to the proximate analysis. The volatile carbon of PRB coal was measured as about 35% or more to about 45% or less, such as about 39%. The fixed carbon of PRB coal was measured as about 40% or more to about 50% or less, such as 46%. The ash of PRB coal was measured as about 0.01% or more to about 10% or less, such as about 6%. The elements of PRB coal contained about 65% C to about 75% C, such as about 71% C, about 0.01% H or more to about 10% H or less, such as about 4% H, about 0.01% N or more to about 10% N or less, such as about 2% N, about 15% O or more to about 25% O or less, such as about 22% O, and about 0.01% S or more to about 10% S or less, such as about 0.40% S on aweight basis. Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range.Table 1. Proximate and Ultimate Analyses of PRB coal.

[0081] Table 2 is a summary of ash analysis of the raw PRB coal. The sulfur content of the sub-bituminous PRB coal is about 0.3 wt% or more to about 0.4 wt% or less, such as 0.36 wt.%, which lower than raw coal produced elsewhere. The oxygen content in pitch was about 15% or more to about 20% or less, such as about 19 wt.%, which is relatively high in raw coal. The higher amount of pitch may result in improved reactivity of the coal during its reaction with H2O2 by providing oxygen-containing functional groups on the surface of CQDs. The ash of raw coal was prepared by air oxidation at about 750°C or more to about 850°C or less, such as about 800°C in a high-temperature furnace and the chemical composition was determined by inductively coupled plasma mass spectrometry (ICP-MS). The ash analysis in terms of weight percent, as shown in Table 2, is as follows: Si = 31.42%, Al = 14.58%, Fe = 9.19%, Mg = 5.89%, Ca = 35.88%, K = 0.52%, Na =2.32%, rare earth elements (RRE) = 0.20%. Anyof the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range.Table 2. Ash Analysis of Raw PRB Coal.

[0082] Figure 3 A is a graph of the thermogravimetric analysis (TGA) of Powder River Basin (PRB) coal. The weight percentage of the residual material (e.g., char) generated at a given temperature may be calculated using the weight of the char in the sample pan of the TGA relative to the initial weight of the coal sample. The weight loss of the PRB coal was measured from ambient temperature to about 1200°C under about 100 mL / min N2 flow with a heating rate of about 10°C / min. A peak in mass loss occurred at a temperature of about 420°C or more to about 460°C or less, such as about 440°C for the raw coal. A second smaller peak in mass loss rate may be observed at about 700°C or more to about 740°C or less, such as about 720°C. Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range.

[0083] Figure 3B is a graph of the Fourier-transform infrared spectroscopy (FTIR) analysis of PRB coal. A broad peak at about 3400 cm1is characteristic of water. Two additional peaks occur at about 2850 cm1or more to about 2950The first peak may denotes a hydrogen present in substituted aromatic rings, while the second peak may be indicative of H in CH2 structures (i.e., aliphatic hydrocarbons or branches). The ratio of these two peaks may be defined as Haro / Haii > 1, which may demonstrate the aromatic character of the PRB coal. The strong peak at about 1550 cm1or more to about 1650 cm1or less, such as about 1600 cm \ may also demonstrate the aromatic character of PRB coal. The strong peak may be indicative of C=C aromatic ring stretching vibrations. Due to various degrees of ring substitution, a wide band at about 520 cm1or more to about 560 cm1or less, such as about 540 cm ', indicates that the aromatic H has out of the plane bend at low degrees of substitution and aromatic ring torsions. The presence of aliphatic groups is indicated by the small peak characteristic for CO- O-CO bending at about 1000 cm1or more to about 1050 cm 'or less, such as about 1030 cm The peaks of C-0 stretching at about 1050 cm1or more to about 1250 cm1or less, such as1220 cm1and 1090 cm manifest the existence of the ether and ester in the PRB coal. The presence of a large number of oxygen-containing functional groups including C-O, O-H, and CO-O-CO stretching, may imply that the PRB coal contains a large amount of oxygen providing oxygen-containing functional groups on the surface of the desired CQDs.CQDs synthesis

[0084] CQD synthesis from coal may be performed using a sealed reaction system. In some cases, the consumption of H2O2 may be based on Equation 2 to reduce the consumption of H2O2.Coal + 2H2O2-► CO2+ 2H2O (2)Thus, a complete reaction of about 1 g dry PRB coal may use about 11 mL (e.g., about 12 g) of 30% H2O2 solution.Effect of H2O2 Concentration

[0085] Figure 4A is a graph of the carbon distribution in the final products (gas, solution, and residual) resulting from the reactions with different H2O2 concentrations. Figure 4B is an graph of the final products resulting from the reactions with different H2O2 concentrations. In particular, about 10.0 g dried PRB coal was reacted in a 2000 mL reactor with about 1500 mL of H2O2 solution having about 1.5% H2O2 (e.g., about 65-75 mL H2O2), about 2% H2O2 (e.g., about 85-95 mL H2O2), about 2.5% H2O2 (e.g., about 110 to 120 mL H2O2), about 3% H2O2 (e.g., about 135-145 mL H2O2), and about 3.5% H2O2 (e.g., about 155 to 165 mL H2O2). The H2O2 solutions are held at about 85°C to 95°C for about 12 hours. The carbon in the residual decreased significantly from about 27.6±3.9% to about 14.5±3.2% with increasing concentration of H2O2 from about 1.5% to about 2.5%, then slightly decreased to about 11.6±0.7% with further increasing of FLCh to about 3.5%. The carbon in solution decreased slightly from about 34.6±1.9% to about 27.3±2.9% with increasing concentration of FLChfrom about 1.5% H2O2 to about 3.5% H2O2. The amount of carbon in gas formed in R2 increased dramatically from about 27.4±1.1% at about 1.5% to about 58.6±2.2% at about 2.5% H2O2, then slightly to about 64.0±2.9% at about 3.5% H2O2, indicating that the about 2.5% H2O2 could be the optimal condition for the coal conversion. More CO, CO2, and O2 were formedwith the increasing H2O2 concentration, and the improvement in the oxidation process is indicated by the increase in the production of O2, CO2 gases. This may be due to a stronger transfer of electrons between coal and H2O2.

[0086] Typically, the O2 product increases significantly from about 10-14 mmol to about 230-290 mmol, which may be from the self-decomposition of the H2O2. Thus, when the concentration of H2O2 is over about 2.5%, the H2O2 is excessive and converted into O2, which does not assist in the formation of CQDs. Moreover, the closed system for the CQDs synthesis can reduce the consumption of H2O2, which is more expensive than coal, thus decreasing cost.

[0087] Figure 5A-5E are graphs of fluorescence spectra images of CQDs-containing solutions from the reactions at different H2O2 concentrations. The synthesized CQDs- containing solutions may be synthesized using daylight or UV light (e.g., excitation of about 360 to 370 nm). Figure 5A is a fluorescence spectra images of CQDs-containing solutions synthesized with 1.5% H2O2. Figure 5B is a fluorescence spectra images of CQDs-containing solutions synthesized with 2% H2O2. Figure 5C is a fluorescence spectra images of CQDs- containing solutions synthesized with 2.5% H2O2. Figure 5D is a fluorescence spectra images of CQDs-containing solutions synthesized at with 3% H2O2. Figure 5E is a fluorescence spectra images of CQDs-containing solutions synthesized at with 3.5% H2O2. The color of solutions synthesized in the daylight changed from brown at about 1.5% to yellow at about 2.5% to lighter yellow at about 3.5%, indicating the decomposition of carbon in the solution and the conversion of coal to CQDs. The about 1.5% solution showed a brown solution under UV light, resulting from the dark solution and bad transmission. The increase in H2O2 concentration, particularly after about 2.5%, caused the solutions to emit blue fluorescence. The shift to blue region emission spectra of the CQDs with an increase in H2O2 concentration indicates that there is a transition of particle size range towards smaller particles. In general, an increase in H2O2 concentration increased the photoluminescence effect. The concentrations of CQDs increased with the concentration of H2O2 from about 1.5% to about 3% H2O2, and then decreased under the test conditions which are directly determined by the photoluminescence intensity. This may indicate that the yields of CQDs increased with an increase in H2O2 concentration, and this impact was more pronounced for the case of solution having about 3% H2O2, indicating a higher CQDs concentration at this H2O2 concentration.The lower photoluminescence effect of about 3.5% H2O2 compared to that of 3% might be due to the destruction of some aromatic structures as a result of excessive concentration of H2O2. In some cases, the emission wavelengths of the CQDs generated with above described H2O2 solutions of the non-limiting examples are about 549 nm at about 472 nm excitation, about 450 nm at about 350 nm excitation, about 449 nm at about 348 nm excitation, about 447 nm at about 344 nm excitation, and about 442 nm at about 342 nm excitation for 1.5%, 2%, 2.5%, 3% and 3.5%, respectively.

[0088] Figure 6 is a graph of an example UV-vis absorption spectra resulting from the reactions with different H2O2 concentrations. The size change of synthesized CQDs can be further confirmed by the UV-vis absorption spectra. The absorption band of the CQDs decreased with the increasing H2O2 concentration for the excitation of pi-electrons (jt —> 7t*) of the aromatic 71 system. The absorption bands of the H2O2 solutions were about 369 nm, about 306 nm, about 302 nm, about 300 nm, and about 293 nm for 1.5%, 2%, 2.5%, 3% and 3.5%, respectively, which is consistent with their photoluminescence intensity. The decrease in absorption wavelength with the addition of H2O2 may indicate of an increment in the energy gap between the ground state and excited state of the CQDs molecules. The increment in the energy difference between molecular orbitals may be a result of a reduction in the conjugation of the CQDs molecules, which is a manifestation of a decrease in CQDs particle sizes. Therefore, the results obtained from the example UV tests of Figure 6 are in line with 3-D excitation-emission spectra of Figures 5A-5E, indicating that the size of CQDs decreased as the concentration of H2O2 increased.Effect of Reaction Temperature

[0089] Figure 7A is a graph of the carbon distribution in the final products (gas, solution, and residual) resulting from the reactions at different temperatures. Figure 7B is a graph of the final products resulting from the reactions at different temperatures. About 10.0 g dried PRB coal was separately reacted with about 1500 mL of H2O2 solutions having about 2.5% FFChfor about 12 hours for five reaction temperature iterations ranging from about 50°C or more to about 110°C or less (e.g., about 60°C, about 70°C, about 80°C, about 90°C, and about 100°C). The carbon in the residual decreased significantly from about 75% or less to about 5% or more (e.g., about 63.6±3.8% to about 11.3±3.2%), with increasing reaction temperature from about60°C to about 100°C. The highest coal conversion was achieved at about 100°C, which can be understood from the perspective of the Arrhenius equation that higher temperature led to higher reaction rate constant and thus quicker reactions or coal conversion. The carbon in the solution increased significantly from about 15% or more to about 50% or less (e.g., about 21.7±1.1% to about 45.0±2.8%) with increasing reaction temperature from about 60°C to about 70°C, then slowly decreased to about 20% or more to about 30% or less (e.g., about 27.3±0.84%) with further increase of temperature to about 100°C. The conversion of O2, CO, CO2 may be positively correlated with reaction temperature. This may be due to the increase in temperature, resulting in an increase in the breaking of the chemical bonds in organic compounds of coal undergoing decomposition. When the temperature is over about 90°C, more CO2 and O2 were formed, indicating that elevating the temperature may have led to a higher reaction rate constant and thus quicker conversion of carbon in solution into gas products. Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range.

[0090] Figures 8A-8E are fluorescence spectra images of CQDs-containing solutions from the reactions at different temperatures. In some embodiments, CQD-containing solutions may be synthesized under daylight and UV light (e.g., excitation of about 365 nm). Figure 8A is a fluorescence spectra images of CQDs-containing solutions synthesized at 60°C. Figure 8B is a fluorescence spectra images of CQDs-containing solutions synthesized at 70°C. Figure 8C is a fluorescence spectra images of CQDs-containing solutions synthesized at 80°C. Figure 8D is a fluorescence spectra images of CQDs-containing solutions synthesized at 90°C. Figure 8E is a fluorescence spectra images of CQDs-containing solutions synthesized at 100°C. The color of solutions synthesized in daylight changed from yellow at about 60°C, to darker yellow at about 70°C, to lighter yellow at about 100°C. Using the UV light, CQDs solutions may exhibit a greenish-yellow emission at the lower reaction temperature, which transited to a blue color when the temperature of the reaction was increased. Transition to the blue emission may indicate a reduction in the wavelength region of the electromagnetic spectrum of CQDs, suggesting that the size of synthesized CQDs may be reduced when the temperature of reactions is increased. The intensity decreased from about 60°C to about 70°C, then gradually increased until about 100°C. The mixture obtained at about 100°C exhibited the strongest photoluminescence impact. The greater emission intensity at 100°C may be attributed to thegreater concentration of CQDs synthesized at a higher temperature. Further, the emission wavelengths of the CQDs generated at the temperatures of about 60°C, about 70°C, about 80°C, about 90°C, and about 100°C are about 537 nm at about 471 nm excitation, about 529 nm at about 470 nm excitation, about 517 nm, at about 471 nm excitation, about 449 nm at about 348 nm excitation, and about 405 nm at about 358 nm excitation, respectively. Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range.

[0091] Figure 9 is a graph of a UV-vis absorption spectra resulting from the reactions with different temperatures. The size change of CQDs may be enabled by the wavelengthdependent photoluminescence properties of CQDs. The absorption spectra exhibited a shift in the wavelength spectrum of the UV light absorbed by CQDs from larger to smaller wavelengths, with a change in shoulder position from about 368 to about 260 nm as the temperature of the reaction was increased. The shoulder may be allocated to n-7t* electronic transitions of conjugated C=O, which may be extensively overlapped for CQDs obtained at lower temperatures, and may become less conjugated when the temperature of the reaction was increased. The UV spectroscopy tests of Figure 9 are in line with 3-D excitation-emission spectra of Figures 8A-8E, which indicate that the size of CQDs decreased as the temperature of the reaction increased. Any of the foregoing numbers can be used singly to describe an open- ended range or in combination to describe a close-ended range.Effect of Reaction Time

[0092] Figure 10 is a graph of the carbon distribution in the final products (e.g., gas, solution, and residual) at different reaction times. Figure 10B is a graph of the final products resulting from the reactions at different reaction times. About 10.0 g coal was used to react with about 1500 mL of H2O2 solutions having about 2.5% H2O2 at about 90°C for a predetermined period. Generally, the carbon in the residual decreased significantly from about 50% or less to about 10% or more, (e.g., about 44.6±3.9% to about 14.5±3.2%) with increasing reaction time from about 2 hours to about 12 hours. At the same time, the carbon in the solution increased from about 35% or more to about 50% or less (e.g., about 38.8±1.7% to about 43.5±2.2%) with increasing reaction time from about 2 hours to about 4 hours, then slightly decreased to about 25% to about 35% (e.g., about 30.2±0.9%) with increasing reaction timesto about 12 hours. The carbon in the gas production in the formation of CO and CO2 increased from about 5% or more to about 15% or less (e.g., about 9.7±2.2%) to about 55% or more to about 65% or less (e.g., about 58.67±2.2%) with increasing time. Therefore, the increase of reaction time may lead to more carbon decomposition of coal from the solution into the gas phase. This may be due to the increase in the reaction time inducing more oxidation opportunities by which the carbon atoms of coal gained bonds to more electronegative elements. In some cases, the carbon in the solution may be in the form of both carbons dissolved in the chemical (e.g., Humic acid) and CQDs. Therefore, a reduction in the proportion of carbon in the solution does not necessarily link to the amount of CQDs. Reducing the carbon in the solution due to the increase in the reaction time may lead to a greater amount and superior quality of the CQDs. Moreover, extending the reaction time increased the yield of CO, CO2, and O2 gases, indicating that the reaction was more enhanced. Thus, the reaction time of about 12 hours at a given condition may be the optimal temperature for the CQDs production. Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range.

[0093] Figures 11A-11F are fluorescence spectra images of CQDs-containing solutions from the reactions at different reaction times. The CQDs-containing solutions may be synthesized under daylight and UV light (e.g., excitation of 365 nm). Figure 11A is a fluorescence spectra images of CQDs-containing solutions synthesized at 2 hours. Figure 1 IB is a fluorescence spectra images of CQDs-containing solutions synthesized at 4 hours. Figure 11C is a fluorescence spectra images of CQDs-containing solutions synthesized at 6 hours. Figure 1 ID is a fluorescence spectra images of CQDs-containing solutions synthesized at 8 hours. Figure 1 IE is a fluorescence spectra images of CQDs-containing solutions synthesized at 10 hours. Figures 1 IF is a fluorescence spectra images of CQDs-containing solutions synthesized at 10 hours. The color of solutions in daylight changed from brown at about hours 2 hours to light yellow at about 12 hours. Using the UV light, CQDs solutions exhibited a greenish-yellow emission at a short reaction time, which transitioned to a blue color when the reaction time was increased. This indicates that the synthesized CQDs had different UV absorption properties and fluorescence emission properties. The increase in fluorescence intensities of the CQDs solution may be directly proportional to reaction time. Thus, an increase in the reaction time may result in higher CQDs concentrations, with the strongestphotoluminescence impact obtained after about 12 hours. The greater concentration of CQDs after about 12 hours reaction may be a result of more uniform and ordered aromatic structures. The emission wavelengths for the CQDs were measured to be about 549 nm at about 471 nm excitation, about 534 nm at about 470 nm excitation, about 517 nm at about 471 nm excitation, about 483 nm at about 378 nm excitation, about 455 nm at about 355 nm excitation, and about 449 nm at about 348 nm excitation for about 2 hours, about 4 hours, about 6 hours, about 8 hours, about 10 hours, and about 12 hours, respectively. Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range.

[0094] Figure 12 is a graph of a UV-vis absorption spectra resulting from the reactions with different reaction times. The UV absorption analysis of the CQDs solutions reveals that there may be a shift in the shoulder of CQDs absorption intensity from larger to smaller wavelengths as the time of the reaction was increased, resulting in the absorption band of about 343 nm, about 358 nm, about 333 nm, about 315 nm, about 307 nm, and about 301 nm for about 2 hours, about 4 hours, about 6 hours, about 8 hours, about 10 hours, and about 12 hours, respectively. Therefore, extending the reaction time to about 12 hours may cause a decrease in the CQDs size range. The results obtained from UV tests are in line with the 3-D excitationemission spectra of Figure 11A-11F, confirming that the size of CQDs decreased as the time of reaction increased. Any of the foregoing numbers can be used singly to describe an open- ended range or in combination to describe a close-ended range.In-Situ Fenton Reaction

[0095] Figure 13 A is a graph depicting carbon distribution in the final products resulting from the reactions of different raw materials. Figure 13B is a graph of the final products resulting from the reactions of different raw materials. The different raw materials include deash coal, de-ash coal + about 30 ppm Fe2+, and raw coal at about 90°C with about 2.5% H2O2 for about 12 hrs. The de-ashed coal may have the highest carbon in residual and solution of about 25% or more to about 35% or less (e.g., about 31.6±2.5% and about 36.4±2.7%), which is higher than that of in raw coal of about 10% or more to about 35% or less (e.g., about 14.5±3.2% and about 30.2±0.9%). When about 25-35 ppm Fe2+was added into de-ash coal, more carbon was kept in the residual (e.g., about 30%-40%) and the carbon in solution reduced to about 10% or more to about 20% or less (e.g., about 14.5±0.7%), resulting in more carbonin gaseous products. Typically, the O2 content in de-ash coal plus about 25-35 ppm Fe2+may be increased. The results demonstrate an increased production of O2 in the case of de-ash coal plus about 25-35 ppm Fe2+in comparison with raw coal, resulting from the acceleration in H2O2 decomposition to oxygen and water at the initial stage of the reaction and inefficient consumption of H2O2 due to the existence of abundant Fe2+available for oxidizing the coal. This is also manifested by the lower carbon content obtained from the reaction of de-ash coal plus about 25-35 ppm Fe2+with H2O2. However, for raw coal, the Fe2+was gradually released into solution with the breakdown of raw particles. The slow release of Fe2+from raw coal forms an in-situ catalyst for the Fenton reaction and reduces the consumption of H2O2, which may be confirmed from the results of de-ash coal plus 25-35 ppm Fe2+with H2O2. The high conversion of carbon in raw coal may be described by the enhancement in the effectiveness of thermal decomposition of H2O2 to strong radicals of HOO and HO», due in part to the presence of irons as in-situ catalysts (e.g., Fenton agent) in the coal to promote in-situ Fenton reaction. Therefore, comparing the direct addition of Fe2+and gradual in-situ release of Fe2+from the raw coal, the gradual in-situ release of the ferrous ions in the raw coal can promote the reaction more efficiently and reduce the formation of O2 significantly. Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range.

[0096] Figures 14A-14D are fluorescence spectra images of CQDs-containing solutions from the reactions from different materials. The CQDs-containing solutions may be synthesized under daylight and UV light (e.g., excitation of 365 nm). Figure 14A is the fluorescence spectra image of a depicts a blank image. Figure 14A is the fluorescence spectra image of de-ash coal. Figure 14A is the fluorescence spectra image of de-ash coal plus about 25-35 ppm Fe2+. Figure 14A is the fluorescence spectra image of raw coal. Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range.

[0097] Figure 15 is a graph of a UV-vis absorption spectra resulting from the reactions with different raw materials. The impact of the in-situ Fenton reaction on the CQDs fluorescence properties may be examined through UV light irradiation and fluorescence emission spectroscopy. De-ash coal emitted a yellowish color under UV light, while the addition of ferrous ions to de-ash coal transformed the color fluorescence to green. Theemission of CQDs solution obtained from raw coal was in the blue region. Furthermore, the highest emission intensity was acquired from the synthesized CQDs of raw coal, which may be due to a higher yield of CQDs with raw coal than that of de-ash coal plus about 25-35 ppm Fe2+. The lower CQDs concentration of de-ash coal plus 25-35 ppm Fe2+, compared to that of raw coal, may be due to the increased consumption of H2O2 at the initial stage of reactions in comparison with the gradual in-situ decomposition of raw coal. UV tests also revealed that CQDs absorbed UV light at a lower wavelength when about 25-35 ppm Fe2+was added to deash coal. Raw coal exhibited a smaller wavelength shoulder compared to de-ash coal plus about 25-35 ppm Fe2+. This indicates that the size of CQDs may be decreased by the addition of Fe2+and the use raw coal instead of de-ash coal plus about 25-35 ppm Fe2+. Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range.Characteristics of Prepared CQDs

[0098] Figure 16A is a micrograph of a TEM image. Figure 16B is a graph of the diameter distributions of the CQDs synthesized from raw coal. The CQDs may be spherical and mono- dispersive within the solution. The particle sizes of the CQDs, based on the statistical analysis of their TEM images, may be about 2 nm to about 5 nm. The core of CQDs may be composed of an ^-conjugated structure, and the surface may be amorphous region that connects various oxygen-containing functional groups. Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range.

[0099] Figure 16C is a graph of the FT-IR spectra of the CQDs. The broad bands from about 2,500 cm'13,800 cm'1may result from the O-H stretching vibrations in the molecules in the CQDs associated with the oxygen-containing functional groups in PRB coal. The stretching band at about 1,600 cm'1or more to about 1,650 cm'1or less, such as about 1,620 cm'1corresponds to C=C bending. The band at about 950 cm'1or more to about 1,350 cm'1or less, such as about 1,300 cm'1and about 1,000 cm'1is attributed to the free-CO bond. The presence of the C=C peak may partially indicate the existence of the graphitic structure in the prepared CQDs, whereas the OH, C=O, and C-H bending peaks may suggest the presence of graphite on the surfaces of the CQDs. Compared with the FTIR diagram of raw coal, the stretching vibrations of -OH, C=O, and C-O-C on carbon quantum dots are all enhanced. Insome cases, after H2O2 oxidation, more oxygen-containing functional groups are formed on the surface of the CQDs. This may indicate that the prepared CQDs are rich in oxygen-containing functional groups, including hydroxyl, carboxyl groups on their surfaces, which can improve their hydrophilicity and stability in an aqueous system. Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range.

[0100] Figure 16D is a graph of the XPS spectra of 01 peaks of the CQDs. Figure 16E is a graph of the XPS spectra of the Cl peaks of the CQDs. Figure 16F a graph of statistics for various carbons of prepared CQDs from raw coal. The existence of the elements on the surface of the CQDs, mainly C and O, may be indicated with the peaks at about 280 eV or more to about 290 eV or less, such as about 285 eV, and about 530 eV or more to about 540 eV or less, such as about 533 eV, respectively. Moreover, the functional groups and bonding configurations formed by the introduction of extract may be determined by high-resolution Cis spectra. The five peaks at about 275 eV or more to about 295 eV or less (e.g., about 284.8 eV, about 286.0 eV, about 287.3 eV, about 288.9 eV, and about 290.9 eV) are assigned to C-C / C=C in aromatic rings (e.g., about 15-25%), C-0 in epoxy and alkoxy (e.g., about 45-50%), C=O in carbonyl (e.g., about 15-20), 0=C-0 in ester (about 5-10%), and 7t-7t* (about 5-10%), respectively. The deconvolution of 01 peaks for each CQD revealed that the O atoms may be in two forms corresponding to the C-0 quinone at the peak of about 530 eV or more to about 540 eV or less, such as about 533eV, and C=O at the peak of about 530 eV or more to about 540 eV or less, such as about 531 eV. The XPS results show the CQDs may have a high degree of functionalization, and the result is consistent with that achieved with FT-IR. Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range.Mechanism of CQDs Synthesis

[0101] Figure 17 is a diagram of the mechanism of CQDs synthesis from PRB coal and H2O2. As noted above, H2O2 is a green oxidant; its oxidation products may be generally water and oxygen. Its standard redox potential is about 1 V to about 2 V, and includes has a weak acid. However, H2O2 itself may not be a strong candidate for oxidizing power. Thus, the reaction of H2O2 with de-ash coal may be slow. Compared to raw coal, with the gradual release of Fe2+into the H2O2 solution, the in-situ Fenton reagent may be formed. Most importantly,Fe2+may have activity associated with H2O2, compared with other metals. Therefore, under action of the catalyst Fe2+, H2O2 may decompose to generate free radicals (»OH), which have a strong ability to obtain electrons. In other words, the ability to oxidize with the standard redox potential may be about 2 V to about 4 V. The free radicals (»OH) may oxidize organic matter in coal selectively. In some cases, unsaturated organic matter in coal may be preferentially oxidized, while saturated organic matter may be oxidized through a hydrogen substitution reaction with »OH. Moreover, free radicals (*OH) may attack both the benzene ring and R-H groups, then the R* radicals may be formed after replacing hydrogen, which can initiate other reactions. R* radicals may continue to react with oxygen in coal or organic compounds in solution to generate HCh’ / Ch*-, whereas the HCh’ / Ch*- may promote the conversion of Fe2+and further improve the decomposition of coal. Therefore, the macromolecular polycyclic aromatic in coal may be destroyed and converted to nano-size particles with oxygen-containing functional groups. After the separation, CQDs may be obtained. Any of the foregoing numbers can be used singly to describe an open-ended range or in combination to describe a close-ended range.

[0102] Embodiments described in the present disclosure provide techniques for producing CQDs. According to the non-limiting embodiments described, CQDs may be synthesized using coal as a carbon resource and H2O2 through a facile hydrothermal reaction. The obtained monodispersed and spherical CQDs may an average size of about 2-4 nm. The CQDs may have uniform morphology, quality crystallized, and may generate suitable photoluminescence properties using UV irradiation and fluorescence spectroscopy. In some cases, the Fenton reaction may provide a mechanism for the promotion of coal decomposition, may increase in conversion of the coal, and may increase yield of CQDs. In some cases, the Fenton reaction may induce a reduction in oxygen-related products, consumption of H2O2, and activation energy. Embodiments described herein may facilitate practical applications of the CQDs as carbon-based nanostructures.

[0103] All documents described herein are incorporated by reference herein, including any priority documents and / or testing procedures to the extent they are not inconsistent with this text. As is apparent from the foregoing general description and the specific embodiments, while forms of the embodiments have been illustrated and described, various modifications canbe made without departing from the spirit and scope of the present disclosure. Accordingly, it is not intended that the present disclosure be limited thereby. Likewise, the term “comprising” is considered synonymous with the term “including.” Likewise whenever a composition, an element, a group of elements, or a method is preceded with the transitional phrase “comprising,” it is understood that we also contemplate the same composition, method, or group of elements with transitional phrases “consisting essentially of,” “consisting of,” “selected from the group of consisting of,” or “Is” preceding the recitation of the composition, element, elements, or method, and vice versa, such as the terms “comprising,” “consisting essentially of,” “consisting of’ also include the product of the combinations of elements listed after the term.

[0104] In the foregoing, reference is made to embodiments of the disclosure. However, it should be understood that the disclosure is not limited to specific described embodiments. Instead, any combination of the following features and elements, whether related to different embodiments or not, is contemplated to implement and practice the disclosure. Furthermore, although embodiments of the disclosure may achieve advantages over other possible solutions and / or over the prior art, whether or not a particular advantage is achieved by a given embodiment is not limiting of the disclosure. Thus, the foregoing aspects, features, embodiments and advantages are merely illustrative and are not considered elements or limitations of the appended claims except where explicitly recited in a claim(s). Likewise, reference to “the disclosure” shall not be construed as a generalization of any inventive subject matter disclosed herein and shall not be considered to be an element or limitation of the appended claims except where explicitly recited in a claim(s).

[0105] For purposes of this present disclosure, and unless otherwise specified, all numerical values within the detailed description and the claims herein are modified by “about” or “approximately” the indicated value, and consider experimental error and variations that would be expected by a person having ordinary skill in the art. For the sake of brevity, only certain ranges are explicitly disclosed herein. However, ranges from any lower limit may be combined with any upper limit to recite a range not explicitly recited, as well as, ranges from any lower limit may be combined with any other lower limit to recite a range not explicitly recited, in the same way, ranges from any upper limit may be combined with any other upperlimit to recite a range not explicitly recited. For example, the recitation of the numerical range 1 to 5 includes the subranges 1 to 4, 1.5 to 4.5, 1 to 2, among other subranges. As another example, the recitation of the numerical ranges 1 to 5, such as 2 to 4, includes the subranges 1 to 4 and 2 to 5, among other subranges. Additionally, within a range includes every point or individual value between its end points even though not explicitly recited. For example, the recitation of the numerical range 1 to 5 includes the numbers 1, 1.5, 2, 2.75, 3, 3.80, 4, 5, among other numbers. Thus, every point or individual value may serve as its own lower or upper limit combined with any other point or individual value or any other lower or upper limit, to recite a range not explicitly recited.

[0106] As used herein, the indefinite article “a” or “an” shall mean “at least one” unless specified to the contrary or the context clearly indicates otherwise. For example, embodiments comprising “an amino acid” include embodiments comprising one, two, or more amino acids, unless specified to the contrary or the context clearly indicates only one amino acid is included.

[0107] Certain molecules disclosed herein may contain one or more ionizable groups [groups from which a proton can be removed (e.g., -COOH) or added (e.g., amines) or which can be quaternized (e.g., amines)]. All possible ionic forms of such molecules and salts thereof are intended to be included individually in the disclosure herein.

[0108] While the foregoing is directed to embodiments of the present disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.

Claims

Claims1. A method comprising: adding a coal product, a H2O2 solution, and water to a reactor, wherein the coal product, the H2O2 solution, and the water form a mixture; heating the reactor to a temperature of about 60°C to about 100°C to catalyze the carbon quantum dot (CQD) production mechanism within the mixture to form a CQD; and collecting the CQD.

2. The method of claim 1, wherein the coal product comprises a de-ash coal, a de-ash coal and 30 parts per million (ppm) ferrous ion (Fe2+), a raw coal, or a combination thereof.

3. The method of claim 1, wherein an amount of H2O2 in the H2O2 solution in the mixture is about 1.0 wt% to about 5 wt% of a total weight of the mixture.

4. The method of claim 1, further comprising holding the reactor at the temperature for about 2 hours to about 12 hours.

5. The method of claim 1, wherein an amount of the coal product in the mixture is about 1 gram or less based on a total weight of the mixture.

6. A carbon quantum dot (CQD), comprising: carbon spheres with a diameter of about 15 nm or less, wherein the carbon sphere is formed from a coal product.

7. The CQD of claim 6, wherein the carbon spheres are formed using a Fenton reagent driven coal-based CQDs production mechanism, wherein the Fenton reagent driven coal-based CQDs production mechanism comprises:Fe2++ H2O2 Fe3++ HO* + OHFe3++ H2O2 Fe2++ HOO* + H+.

8. The CQD of claim 7, wherein the Fe2+is provided by the coal product.

9. The CQD of claim 8, wherein the coal product comprises de-ash coal, de-ash coal and 30 parts per million (ppm) ferrous ion (Fe2+), raw coal, or a combination thereof.

10. The CQD of claim 7, wherein an amount of the H2O2 in the H2O2 solution is about 1.0 wt% to about 5 wt% of a total weight of a composition of the coal product, H2O2 solution, and water.

11. A system comprising: a controller; and a reactor coupled to the controller, wherein the controller is programmed to perform a method comprising: heating the reactor to a temperature of about 60°C to about 100°C to catalyze the carbon quantum dot (CQD) production mechanism within a mixture to form a CQD, wherein a coal product, a H2O2 solution, and water form the mixture, and wherein the CQD production mechanism is a Fenton reagent driven coal-based CQDs production mechanism; and collecting the CQD.

12. The system of claim 11, wherein an amount of the H2O2 in the H2O2 solution in the mixture is about 1.0 wt% to about 5 wt% of a total weight of the mixture of the coal product, H2O2 solution, and water.

13. The system of claim 11, wherein the coal product comprises a de-ash coal, a de-ash coal and 30 parts per million (ppm) ferrous ion (Fe2+), a raw coal, or a combination thereof.

14. The system of claim 11, wherein the method further comprises holding the reactor at the temperature for about 2 hours to about 12 hours during the CQD production mechanism.

15. The system of claim 11, wherein the CQD comprises a carbon sphere with a diameter of about 15 nm or less.

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