WO2017197167A1 - Metal-free catalysts for converting carbon dioxide into hydrocarbons and oxygenates - Google Patents

Abstract

The present invention provides for a method and apparatus for reducing carbon dioxide by exposing the carbon dioxide to a metal-free catalyst, thus converting the carbon dioxide to a reduced product. In some embodiments, the metal-free catalyst includes, without limitation, various nanomaterials, quantum dots, oxides, oxides, and combinations thereof. The metal-free catalyst may further be supported on a substrate to form a complex. The metal-free catalyst may further selectively reduce carbon dioxide without mediating hydrogen evolution reactions.

Description

METAL-FREE CATALYSTS FOR CONVERTING CARBON DIOXIDE INTO

HYDROCARBONS AND OXYGENATES

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to provisional United States Patent Application Serial No. 62/334,825, filed on May 11, 2016, entitled "METAL-FREE CATALYSTS FOR CONVERTING CARBON DIOXIDE INTO HYDROCARBONS AND OXYGENATES" which provisional patent application is commonly assigned to the Assignees of the present invention and is hereby incorporated herein by reference in its entirety for all purposes.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH [0002] None.

BACKGROUND

[0003] Electroreduction of C0₂ into higher-energy liquid fuels and chemicals is a promising but challenging renewable energy conversion technology. Among the electrocatalysts screened so far for C0₂ reduction (including metals, alloys, organometallics, layered materials and carbon nanostructures), only copper (Cu) exhibits selectivity towards formation of hydrocarbons and multi-carbon oxygenates at fairly high efficiencies while the others favor production of carbon monoxide or formate. However, Cu particle catalysts suffer from numerous limitations, including high temperature migration of atomic clusters, agglomeration, and low selectivity towards desired products. Various aspects of the present invention address the aforementioned limitations. SUMMARY

[0004] In some embodiments, the present invention pertains to methods of reducing carbon dioxide by exposing the carbon dioxide to a metal-free catalyst. In some embodiments, the exposure converts the carbon dioxide to a reduced product. In some embodiments, the present invention pertains to apparatus for carbon dioxide reduction, where the apparatus includes a metal-free catalyst of the present invention.

[0005] In some embodiments, the metal-free catalyst includes, without limitation, carbon nanomaterials, doped carbon nanomaterials, pristine carbon nanomaterials, reduced carbon nanomaterials, graphene quantum dots, pristine graphene quantum dots, doped graphene quantum dots, graphene oxides, reduced graphene oxides, doped and reduced graphene oxides, and combinations thereof.

[0006] In some embodiments, the metal-free catalyst is supported on a substrate to form a complex. In some embodiments, the substrate is a metal-based substrate, such as aluminum oxide. In some embodiments, the metal-free catalyst constitutes from about 0.5 wt% to about 20 wt% of the complex. In some embodiments, the metal-free catalyst is dispersed on a surface of the substrate.

[0007] In some embodiments, the reduction of carbon dioxide occurs by electrochemical reduction. In some embodiments, the reduction of carbon dioxide occurs by thermocatalytic reduction. In some embodiments, the reduced product includes, without limitation, hydrocarbons, multi-carbon hydrocarbons, oxygenates, multi-carbon oxygenates, and combinations thereof. In some embodiments, the reduced product includes, without limitation, methane, ethylene, ethanol, propanol, acetate, n-propanol, carbon monoxide, formate, and combinations thereof.

[0008] In some embodiments, the metal-free catalyst selectively reduces carbon dioxide without mediating hydrogen evolution reactions. In some embodiments, the metal-free catalyst selectively reduces carbon dioxide to a reduced product that includes at least one of methane, ethane, ethanol, and combinations thereof. In some embodiments, the metal-free catalyst selectively reduces carbon dioxide to methane. [0009] It is therefore an object of the present invention to provide a method of reducing carbon dioxide, said method comprising exposing the carbon dioxide to a metal-free catalyst, and utilizing the metal-free catalyst to convert the carbon dioxide to a reduced product.

[0010] In general, in one aspect, the invention features a method of reducing carbon dioxide. The method includes the step of exposing the carbon dioxide to a metal-free catalyst. The method further includes the step of utilizing the metal-free catalyst to convert the carbon dioxide to a reduced product.

[0011] Implementations of the invention can include one or more of the following features:

[0012] The step of exposing can occur by mixing the carbon dioxide with the metal-free catalyst.

[0013] The step of exposing can occur by flowing the carbon dioxide through a surface of the metal-free catalyst.

[0014] The metal-free catalyst can be selected from a group consisting of carbon nanomaterials, doped carbon nanomaterials, pristine carbon nanomaterials, reduced carbon nanomaterials, graphene quantum dots, pristine graphene quantum dots, doped graphene quantum dots, graphene oxides, reduced graphene oxides, doped and reduced graphene oxides, and combinations thereof.

[0015] The metal-free catalyst can include doped carbon nanomaterials. The doped carbon nanomaterials can be doped with one or more dopants.

[0016] The one or more dopants can be selected from a group consisting of boron, sulfur, phosphorous, fluorine, nitrogen, and combinations thereof.

[0017] The one or more dopants can include nitrogen.

[0018] The one or more dopants can include pyridinic nitrogen.

[0019] The metal-free catalyst can include nitrogen-doped carbon nanomaterials.

[0020] The metal-free catalyst can include nitrogen-doped graphene quantum dots. [0021] The metal-free catalyst can include nitrogen-doped and reduced graphene oxides.

[0022] The metal-free catalyst can include less than 5 layers.

[0023] The metal-free catalyst can include from about 1 layer to about 3 layers.

[0024] The metal -free catalyst can have diameters of less than about 10 nm.

[0025] The metal -free catalyst can have diameters of between about 1 nm to about 3 nm.

[0026] The metal-free catalyst can be supported on a substrate to form a complex.

[0027] The substrate can be a metal-based substrate.

[0028] The substrate can include aluminum oxide.

[0029] The metal -free catalyst can constitute from about 0.5 wt% to about 20 wt% of the complex.

[0030] The metal-free catalyst can be dispersed on a surface of a substrate.

[0031] The reduction of carbon dioxide can occur by electrochemical reduction.

[0032] The reduction of carbon dioxide can occur by thermocatalytic reduction.

[0033] The reduced product can be selected from a group consisting of hydrocarbons, multi- carbon hydrocarbons, oxygenates, multi-carbon oxygenates, and combinations thereof.

[0034] The reduced product can be selected from the group consisting of methane, ethylene, ethanol, propanol, acetate, n-propanol, carbon monoxide, formate, and combinations thereof.

[0035] The metal-free catalyst can have a Faradaic efficiency of up to about 90%.

[0036] The metal-free catalyst can have a carbon dioxide conversion efficiency of up to about 60%.

[0037] The metal-free catalyst can have a carbon dioxide conversion efficiency of up to about 45%. [0038] The metal-free catalyst can have a carbon dioxide conversion efficiency of up to about 30%.

[0039] The metal-free catalyst can selectively reduce carbon dioxide without mediating hydrogen evolution reactions.

[0040] The metal-free catalyst can selectively reduce carbon dioxide to a reduced product selected from a group consisting of methane, ethane, ethanol, and combinations thereof.

[0041] The metal-free catalyst can selectively reduce carbon dioxide to methane.

[0042] In general, in another aspect, the invention features an apparatus for carbon dioxide reduction. The apparatus includes a metal-free catalyst. The metal-free catalyst is capable of reducing carbon dioxide to a reduced product.

[0043] Implementations of the invention can include one or more of the following features:

[0044] The metal-free catalyst can be selected from a group consisting of carbon nanomaterials, doped carbon nanomaterials, pristine carbon nanomaterials, reduced carbon nanomaterials, graphene quantum dots, pristine graphene quantum dots, doped graphene quantum dots, graphene oxides, reduced graphene oxides, doped and reduced graphene oxides, and combinations thereof.

[0045] The metal-free catalyst can include doped carbon nanomaterials. The doped carbon nanomaterials can be doped with one or more dopants.

[0046] The one or more dopants can be selected from a group consisting of boron, sulfur, phosphorous, fluorine, nitrogen, and combinations thereof.

[0047] The one or more dopants can include nitrogen.

[0048] The one or more dopants can include pyridinic nitrogen.

[0049] The metal-free catalyst can include nitrogen-doped carbon nanomaterials.

[0050] The metal-free catalyst can include nitrogen-doped graphene quantum dots.

[0051] The metal-free catalyst can include nitrogen-doped and reduced graphene oxides. [0052] The metal-free catalyst can include less than 5 layers.

[0053] The metal-free catalyst can include from about 1 layer to about 3 layers.

[0054] The metal -free catalyst can have diameters of less than about 10 nm.

[0055] The metal -free catalyst can have diameters of between about 1 nm to about 3 nm.

[0056] The metal-free catalyst can be supported on a substrate to form a complex.

[0057] The substrate can be a metal-based substrate.

[0058] The substrate can include aluminum oxide.

[0059] The metal -free catalyst can constitute from about 0.5 wt% to about 20 wt% of the complex.

[0060] The metal-free catalyst can be dispersed on a surface of a substrate.

[0061] The reduced product can be selected from a group consisting of hydrocarbons, multi- carbon hydrocarbons, oxygenates, multi-carbon oxygenates, and combinations thereof.

[0062] The reduced product can be selected from a group consisting of methane, ethylene, ethanol, propanol, acetate, n-propanol, carbon monoxide, formate, and combinations thereof.

DESCRIPTION OF THE FIGURES

[0063] FIG. 1 provides a scheme of a method of chemically reducing carbon dioxide.

[0064] FIGS. 2A-2B show an atomic force microscopy (AFM) image of nitrogen-doped graphene quantum dots (NGQDs) (FIG. 2A) and the corresponding thickness profiles (FIG. 2B)

[0065] FIGS. 3A-3D provide data relating to the nanostructure and specific nitrogen dopant configurations of NGQDs. FIG. 3A provides a low-magnification transmission electron microscopy (TEM) image of NGQDs. FIG. 3B shows a high-magnification TEM image of NGQDs. The inset shows a single NGQD containing zigzag edges as circled. FIG. 3C shows Raman spectrum of NGQDs as compared to that of pristine GQDs. FIG. 3D shows high resolution N Is spectrum for NGQDs, deconvoluted into three sub-peaks representing pyridinic, pyrrolic and graphitic N. The value in the parentheses is the corresponding N atomic concentration calculated based on N/(N+C). The inset is a schematic demonstrating the N bonding configuration with respect to pyridinic (dashed-lines), pyrrolic (solid line) and graphitic (solid circles) N. As a comparison, the GQDs do not show the N Is peak.

[0066] FIGS. 4A-4D provide data relating to the morphology and structure of pristine GQDs. FIG. 4A provides an AFM image of pristine GQDs. FIG. 4B provides thickness profiles corresponding to the label in FIG. 4A. FIG. 4C provides low-magnification TEM images of GQDs. FIG. 4D provides high-magnification TEM images of GQDs. The inset is the Fast Fourier Transform pattern of a circled single GQD.

[0067] FIGS. 5A-5C provide various data relating to NGQDs and GQDs, including a survey scan of XPS for NGQDs and GQDs (FIG. 5A), the Cls peak for NGQDs showing a C-N fitting peak (FIG. 5B), and the concentrations of specific N configurations in NGQDs calculated according to N/(C+N) (FIG. 5C).

[0068] FIGS. 6A-6C illustrate an apparatus for C0₂ reduction. FIG. 6A shows a schematic of a flowing cell for C0₂ reduction. FIG. 6B shows a low-magnification NGQD gas diffusion electrode. FIG. 6C shows a high-magnification NGQDs gas diffusion electrode.

[0069] FIGS. 7A-7D show data relating to the electrocatalytic activity of carbon nanostructures towards C0₂ reduction. FIG. 7A shows the Faradaic efficiency of carbon monoxide (CO), methane (CH₄), ethylene (C₂H₄), formate (HCOO-), ethanol (EtOH), acetate (AcO-) and n-propanol (n-PrOH) at various applied cathodic potentials for NGQDs. FIG. 7B shows the Faradaic efficiency of C0₂ reduction products for pristine GQDs. FIG. 7C shows the selectivity to C0₂ reduction products for n-doped reduced graphene oxides (NRGOs). FIG. 7D shows the Tafel plots of partial current density of C0₂ reduction versus applied cathodic potential for three carbon nanostructured catalysts.

[0070] FIGS. 8A-8B provide additional analytical data for NGQDs, including a gas chromatography trace of the gas product (FIG. 8A), and a nuclear magnetic resonance ( MR) spectrum of liquid products for one run at around -1.0 V vs. RHE for NGQDs electrodes (FIG. 8B).

[0071] FIGS. 9A-9C provide a comparison of Faradaic efficiencies of products between NGQDs and commercial Cu nanoparticles (20-40 nm), including carbon monoxide (CO) and formate (HCOO^") (FIG. 9A), hydrocarbons of methane (CH₄) and ethylene (C₂H₄) (FIG. 9B), and multi-carbon oxygenates of ethanol (C₂H₅OH), acetate (AcO ) and n-isopropanol (n- PrOH) (FIG. 9C).

[0072] FIGS. 10A-B provides data relating to post-C0₂ reduction XPS analysis (FIG. 10A), which shows the N configuration concentration change (FIG. 10B).

[0073] FIGS. 11A-11B show scanning electron microscopy (SEM) images of NRGOs, including low-magnification images (FIG. 11 A) and relatively high -magnification images (FIG. 11B)

[0074] FIGS. 12A-12D provide data relating to the XPS analysis of NRGOs. FIG. 12A shows a survey scan. FIG. 12B shows an N Is spectrum and its corresponding fitting. FIG. 12C shows a Cls spectrum and its deconvolution. FIG. 12D shows a comparison of specific N concentration between NRGOs and NGQDs.

[0075] FIGS. 13A-13B show analytical data relating to NGQDs. FIG. 13A shows a schematic of a NGQD with an edge length of 0.98 nm. The balls having dashed linkages represent the N dopants. FIG. 13B shows the dependence of edge sites concentration for possible N doping on the edge length. The calculation is based on an ideal hexagonal NGQD shape. The equation used for calculation is: edge sites% = (2n-l)/n², where n is the number of C6 aromatic rings in one edge of the hexagonal NGQD shapes.

[0076] FIG. 14 shows iR corrected polarization curves for the flowing electrolysis cell incorporating the NGQDs gas diffusion electrode.

[0077] FIGS. 15A-15B show the partial current densities for various products from electrochemical CO² reduction, including NGQDs (FIG. 15A) and graphene quantum dots (GQDs) as catalysts (FIG. 15B). [0078] FIGS. 16A-16B show the production rate of various products from C0₂ reduction catalyzed by NGQDs (FIG. 16A) and GQDs (FIG. 16B).

[0079] FIGS. 17A-17B show the thermal catalytic activity of NGQDs/ A1₂0₃ towards C0₂ hydrogenation. FIG. 17A shows the effect of temperature on C0₂ conversion on NGQDs/Al₂0₃ (3 wt%). FIG. 17B shows the selectivity toward CH4 formation at a variety of temperatures.

[0080] FIGS. 18A-18D depict the dependence of activity and selectivity on N content. FIG. 18A N Is XPS of three NGQDs samples doped by different N precursor solvents, DMF, DMF diluted by IPA/H₂0, and NH₄OH. FIG. 18B shows specific N content estimated from XPS. FIG. 18C C0₂ conversion of three NGQDs/Al₂0₃ samples. FIG. 18D depicts CH₄ selectivity of three NGQDs/ A1₂0₃ samples. The loading of NGQDs for three samples were kept the same at 1 wt.%.

[0081] FIGS. 19A-19D depict thermochemical catalytic activity and selectivity towards C0₂ reduction over NGQDs/Al₂0₃. FIG. 19A depicts dependence of C0₂ conversion on temperature over NGQDs/ A1₂0₃ with three different loadings (0.8 wt%, 1 wt%, and 3 wt%). Results for pristine samples are presented for comparison. FIG. 19B depicts dependence of CO and CH₄ selectivity on temperature over NGQDs/ A1₂0₃ with three different loadings. FIG. 19C depicts FTIR spectra of C0₂ adsorbed onto NGQDs/ A1₂0₃ and GQDs/Al₂0₃ at room temperature. FIG. 19D depicts temperature programmed desorption of C0₂ from NGQDs/Al₂0₃ and GQDs/Al₂0₃.

[0082] FIG. 20 depicts Raman spectra of GQDs and NGQDs doped in different solvents. The ratio of I_D / IG (P band intensity to G band intensity) for NGQDs is much higher than that for the pristine GQDs, due largely to the introduction of additional defects upon N doping.

[0083] FIGS. 21A-21D depict morphology of NGQDs synthesized in NH₄OH. FIG 21A-B depicts TEM images of NGQDs. FIG. 21C depicts AFM images of NGQDs. FIG. 21D depicts thickness profile of NGQDs with respect to the label line in FIG. 21C. [0084] FIGS. 22A-22D depict graphical representations of dependence of the onset temperature, C0₂ conversion, and CH₄ selectivity (at 400°C) on the N content; FIG. 22A Pyridine N; FIG. 22B Pyrrolic N; FIG. 22C Graphitic N; FIG. 22D total N.

[0085] FIG. 23 depicts TOF of CH₄ production for NGQDs samples synthesized with different N precursors. The loading is kept at lwt% for all samples. The TOF is calculated by normalization to the number of total N sites.

DETAILED DESCRIPTION

[0086] It is to be understood that both the foregoing general description and the following detailed description are illustrative and explanatory, and are not restrictive of the subject matter, as claimed. In this application, the use of the singular includes the plural, the word "a" or "an" means "at least one", and the use of "or" means "and/or", unless specifically stated otherwise. Furthermore, the use of the term "including", as well as other forms, such as "includes" and "included", is not limiting. Also, terms such as "element" or "component" encompass both elements or components comprising one unit and elements or components that include more than one unit unless specifically stated otherwise.

[0087] The section headings used herein are for organizational purposes and are not to be construed as limiting the subject matter described. All documents, or portions of documents, cited in this application, including, but not limited to, patents, patent applications, articles, books, and treatises, are hereby expressly incorporated herein by reference in their entirety for any purpose. In the event that one or more of the incorporated literature and similar materials defines a term in a manner that contradicts the definition of that term in this application, this application controls.

[0088] Emissions of greenhouse gases, particularly C0₂, have received broad attention from industries, politicians, and academics due to the associated climate change. Despite many efforts made over the last several decades, an efficient and large-scale C0₂ reduction approach still remains a challenge since the reduction rate of C0₂ is still far away from overall emission levels. According to recent reports, the total annual C0₂ emissions tripled during the past 50 years and this trend is accelerating. One of the solutions that could contribute to a reduction of C0₂ levels relies on the development of efficient and cost effective processes to convert carbon dioxide into chemically useful structures, thereby accomplishing a semi-closed carbon cycle.

[0089] Electro-reduction of carbon dioxide (C0₂) with proton from water into valued commodities offers a potential route towards a carbon-neutral society. However, the development of this technology is limited by the scarcity of active catalysts converting C0₂ into more energy embedded fuels or useful chemicals.

[0090] Among the various catalytic strategies for carbon dioxide reduction, thermochemical catalytic C0₂ hydrogenation is the most scalable option, being highly compatible with industrial plants. The successful implementation of practical thermochemical processes relies on the clever design of catalysts that are able to selectively convert C0₂ into industrially valuable chemicals. The main products obtained after this reduction process are carbon monoxide (CO), methanol (CH₃OH), and methane (CH₄), in proportions that are highly dependent on the choice of catalysts. The direct transformation of C0₂ into CH₃OH is the ideal pathway, but is very limited by the lack of efficient catalysts. Although Cu/ZnO/Al₂0₃ catalyst shows promising selectivity and space-time yield for CH₃OH production, the process requires a pressure of 36 MPa which is likely too high for a commercial operation. An alternative approach is the production of CO via the reverse water-gas shift (RWGS) reaction. CO produced via this pathway could subsequently be used as feedstock for the Fischer- Tropsch synthesis of fuels. Many metal catalysts like Co, Pd and Rh, and carbide (Mo₂C) exhibit satisfactory selectivity (70-90%) towards CO formation in the RWGS reaction, but they suffer from low C0₂ conversion (< 10%). The third pathway is hydrogenation of C0₂ to CH₄, which then can be recycled for use as a fuel or for making chemicals. The most employed catalysts for C0₂ methanation include metals, such as Ni, Ru, and Pd-based systems as well as metal oxides, such as CoO.

[0091] Most metallic catalysts prefer the competitive off-pathway reduction of water to hydrogen while the others such as Au, Ag, Pb and Sn primarily catalyze production of carbon monoxide (CO) or formate (HCOO ) via a two-electron transfer pathway. On the other hand, organometallic, two dimensional materials and nitrogen (N)-doped carbon nanostructured materials in the form of few layers graphene, carbon nanotubes, and carbon fibers have been found only capable of catalyzing C0₂ reduction into CO.

[0092] The reduction of dimension into sub-nanometer scale has been demonstrated in metal catalysts to significantly enhance hydrogenation activity, or even transform non-catalytic bulk counterpart into highly active catalysts toward C0₂ reduction. Additionally, the introduction of defects in carbon nanostructures by heteroatom doping gives rise to activity toward C0₂ activation.

[0093] However, among the numerous amounts of screened materials, only copper or copper oxide derived catalysts have been known to drive the electrolysis of C0₂ into low-carbon hydrocarbons and oxygenates at fairly high Faradaic efficiencies through a multiple-step electron transfer pathway. Therefore, a need exists for more effective catalysts for C0₂ conversion to multi-carbon hydrocarbons and oxygenates.

[0094] In some embodiments, the present invention pertains to methods of reducing carbon dioxide. In some embodiments that are illustrated in FIG. 1, the methods of the present invention include a step of exposing the carbon dioxide to a metal-free catalyst (step 10), where the exposure converts the carbon dioxide to a reduced product (step 12), such as hydrocarbons (step 14), oxygenates (step 16), and combinations thereof. Additional embodiments of the present invention pertain to apparatus for carbon dioxide reduction. In some embodiments, the apparatus includes a metal-free catalyst that is capable of reducing carbon dioxide to a reduced product.

[0095] As set forth in more detail herein, the methods and apparatus of the present invention can have various embodiments. For instance, various metal-free catalysts may be utilized to reduce carbon dioxides. Moreover, carbon dioxides in various phases (such as in gas, liquid, and gas/liquid mixtures) may be reduced into various reduced products. Furthermore, various methods may be utilized to expose carbon dioxides to metal-free catalysts.

Metal-free Catalysts

[0096] The methods and apparatus of the present invention may utilize various types of metal-free catalysts. For instance, in some embodiments, the metal-free catalyst includes, without limitation, carbon nanomaterials, doped carbon nanomaterials, pristine carbon nanomaterials, reduced carbon nanomaterials, graphene quantum dots, pristine graphene quantum dots, doped graphene quantum dots, graphene oxides, reduced graphene oxides, doped and reduced graphene oxides, and combinations thereof.

[0097] In some embodiments, the metal-free catalyst includes doped carbon nanomaterials. In some embodiments, the doped carbon nanomaterials are doped with one or more dopants. In some embodiments, the dopants include, without limitation, boron, sulfur, phosphorous, fluorine, nitrogen, and combinations thereof. In some embodiments, the one or more dopants include nitrogen. In some embodiments, the one or more dopants include pyridinic nitrogen.

[0098] The dopants can be at various locations of carbon nanomaterials. For instance, in some embodiments, the dopants are on the surfaces of the carbon nanomaterials. In some embodiments, the dopants are on the edges of the carbon nanomaterials.

[0099] In some embodiments, the metal-free catalyst includes nitrogen-doped carbon nanomaterials. In some embodiments, the metal-free catalyst includes nitrogen-doped graphene quantum dots. In some embodiments, the metal-free catalyst includes nitrogen- doped and reduced graphene oxides.

[0100] The metal-free catalysts of the present invention can include various layers. For instance, in some embodiments, the metal-free catalysts include less than 5 layers. In some embodiments, the metal-free catalysts include from about 1 layer to about 3 layers.

[0101] The metal-free catalysts of the present invention can also include various diameters. For instance, in some embodiments, the metal-free catalysts include diameters of less than about 10 nm. In some embodiments, the metal-free catalysts include diameters of between about 1 nm to about 3 nm.

[0102] The metal-free catalysts of the present invention may be in various forms. For instance, in some embodiments, the metal-free catalysts of the present invention may be freestanding. In some embodiments, the metal-free catalysts of the present invention may be supported on a substrate to form a complex. In some embodiments, the metal-free catalyst constitutes from about 0.5 wt% to about 20 wt% of the complex. In some embodiments, the metal-free catalyst is dispersed on a surface of the substrate. [0103] The metal-free catalysts of the present invention may be supported on various substrates. For instance, in some embodiments, the substrate is a metal-based substrate. In some embodiments, the substrate includes aluminum oxide (A1₂0₃).

Exposure of Carbon Dioxides to Metal-free Catalysts

[0104] The methods and apparatus of the present invention may be utilized to expose carbon dioxides in various phases (such as gas, liquid, and gas/liquid mixtures) to metal-free catalysts. For instance, in some embodiments, the carbon dioxide may be in a gaseous phase. In some embodiments, the carbon dioxide may be in a liquid phase. In some embodiments, the carbon dioxide may be in a gaseous phase and a liquid phase.

[0105] Various methods may be utilized to expose carbon dioxides to metal-free catalysts. For instance, in some embodiments, the exposure occurs by mixing the carbon dioxide with the metal-free catalyst. In some embodiments, the exposure occurs by flowing the carbon dioxide through a surface of the metal-free catalyst. Additional methods can also be envisioned.

Reduction of Carbon Dioxide to Reduced Products

[0106] The methods and apparatus of the present invention may reduce carbon dioxide into reduced products in various manners. For instance, in some embodiments, the reduction of carbon dioxide occurs by electrochemical reduction. In some embodiments, the reduction of carbon dioxide occurs by thermocatalytic reduction. In some embodiments, the thermocatalytic reduction occurs at temperatures that range from about 100 °C to about 800 °C. In some embodiments, the thermocatalytic reduction occurs at temperatures that range from about 200 °C to about 500 °C.

[0107] The methods and apparatus of the present invention may reduce carbon dioxide into various reduced products. For instance, in some embodiments, the reduced products include, without limitation, hydrocarbons, multi-carbon hydrocarbons, oxygenates, multi-carbon oxygenates, and combinations thereof. In some embodiments, the reduced products include, without limitation, methane (CH₄), ethylene (C₂H₄), methanol (CH₃OH), ethanol (C₂H₅OH), propanol (C₃H₇OH), acetate (CH₃COO ), n-propanol (n-C₃H₇OH), carbon monoxide (CO), formate (HCOO ), and combinations thereof. [0108] Advantageously, the methods and apparatus of the present invention may reduce carbon dioxides into reduced products in a very efficient manner. For instance, in some embodiments, the metal-free catalysts of the present invention have a Faradaic efficiency of up to about 90%. In some embodiments, the metal-free catalysts of the present invention have a Faradaic efficiency of up to about 95%. In some embodiments, the metal-free catalysts of the present invention have a Faradaic efficiency of more than about 75%. In some embodiments, the metal-free catalysts of the present invention have a Faradaic efficiency of more than about 50%.

[0109] In some embodiments, the metal-free catalysts of the present invention have a carbon dioxide conversion efficiency of more than about 30%. In some embodiments, the metal-free catalysts of the present invention have a carbon dioxide conversion efficiency ranging from about 30%) to about 100%. In some embodiments, the metal-free catalysts of the present invention have a carbon dioxide conversion efficiency of up to about 60%. In some embodiments, the metal-free catalysts of the present invention have a carbon dioxide conversion efficiency of up to about 30%. In some embodiments, the metal-free catalysts of the present invention have a carbon dioxide conversion efficiency of up to about 45%.

[0110] In some embodiments, the metal-free catalysts of the present invention selectively reduce carbon dioxide. For instance, in some embodiments, the metal-free catalysts of the present invention selectively reduce carbon dioxide without mediating hydrogen evolution reactions. In some embodiments, the metal-free catalysts of the present invention selectively reduce carbon dioxide to a select number of reduced products (e.g., methane, ethane, ethanol, and combinations thereof). In some embodiments, the metal-free catalyst selectively reduces carbon dioxide to methane.

Additional Embodiments

[0111] Reference will now be made to more specific embodiments of the present invention and experimental results that provide support for such embodiments. However, Applicants note that the disclosure below is for illustrative purposes only and is not intended to limit the scope of the claimed subject matter in any way. Example 1. A Metal-Free Electrocatalyst for Carbon Dioxide Reduction to Multi-

Carbon Hydrocarbons and Oxygenates

[0112] In this Example, applicants demonstrate that metal-free N-doped graphene quantum dots featuring enriched edge sites density and pyridinic N predominance show better performance than conventional copper nanoparticle catalyst for the electroreduction of C0₂. More specifically, applicants demonstrate in this Example that nanometer size N-doped graphene quantum dots (NGQDs) catalyze the electrochemical reduction of C0₂ into multi- carbon hydrocarbons and oxygenates at high Faradaic efficiencies and current densities and low overpotentials. The NGQDs show high total Faradaic efficiency of C0₂ reduction up to 90% with contributions for C₂H₄ and C₂H₅OH conversions reaching 45%. The product distribution and production rate for NGQDs catalyzed C02 reduction is comparable to that for copper (Cu) nanoparticle electrocatalysts.

[0113] In order to expose the edge sites and increase their density in the carbon nanostructure and simultaneously distribute the non-metal heteroatom dopant to the edge location, Applicants intended to synthesize NGQDs through a hydrothermally analogous process of exfoliating and cutting a graphene oxide (GO) precursor, and in-situ N doping in the dimethylformamide (DMF) solvent. The obtained NGQDs have a thickness distribution between 0.7 and 1.8 nm, corresponding to 1 to 3 atomic layers (FIGS. 2A-2B).

[0114] Moreover, the NGQDs possess homogeneous lateral size with a narrow distribution of 1-3 nm (FIGS. 3A-3B). The high resolution transmission electron microscopy images illustrate NGQDs showing the hexagonal like morphology and honeycomb framework with zigzag edges (Inset in FIG. 3B), indicating the preserve of a graphene crystal structure.

[0115] The pristine GQDs without N-doping were also synthesized from the same GO precursor under the identical hydrothermal treatment while a solvent mixture of IPA/H₂0 (1 : 1 by volume) instead of DMF was used. The pristine GQDs show similar morphology as NGQDs regarding the thickness and lateral size (FIGS. 4A-4D). The typical Raman spectrum of NGQDs as presented in FIG. 3C shows characteristic D band at 1350 cm^"1 and G band at 1589 cm^"1, while the 2D band has been quenched compared to pristine GQDs. The ratio of I_D/IG (D band intensity to G band intensity) for NGQDs is much higher than that for the pristine GQDs, due largely to the introduction of additional defects after N doping. The survey scan of X-ray photoelectron spectroscopy clearly shows the presence of N element in the NGQDs while the N Is peak was not observed for the pristine GQDs (FIGS. 5A-5C).

[0116] The specific N configuration in the carbon lattice network was revealed by the fine scan of N Is (FIG. 3D). The high resolution N Is can be deconvoluted into three peaks assigned to pyndinic (398.5 eV), pyrrolic (400.0 eV) and graphitic N (401.2 eV). The total N content in NGQDs based on N/(C+N) is around 6.0 at.%, in which pyndinic N is predominant with -3.9 at.% (FIGS. 3D and 5A-5C).

[0117] The electrocatalytic activity and selectivity of NGQDs towards C02 reduction were evaluated in a flow cell in which NGQDs-based gas diffusion electrode was incorporated into the cathodic compartment (FIG. 6A). The electrolysis was performed in an electrolyte of 1 M KOH under the potentiostatic mode with cathodic potentials between -0.20 and -1.10 V (versus RHE). The total FE of C0₂ reduction increases as a more negative potential is applied (from 18% at - 0.26 V to a maximum of 90% at -0.75 V) and subsequently declines (to 64% at -1.03 V) (FIG. 7A). Only carbon monoxide (CO) and formate (HCOO-) are observed at a low cathode potential of -0.26 V.

[0118] When the applied potential sweeps more negatively, various hydrocarbons of methane (CH₄) and ethylene (C₂H₄) and multi-carbon oxygenates including ethanol (C₂H₅OH), acetate (CH₃COO^") and n-propanol (n-C₃H₇OH) are produced in addition to CO and HCOO^" (FIG. 7A). Applicants have observed that NGQDs favor catalyzing C-C bond formation beyond the potential of -0.61 V, leading to a dominant production of C2 and C3 products (FIGS. 7A and 8A-8B).

[0119] C₂H₄ is the major hydrocarbon product with a maximum FE of 31% at -0.75 V while the CH4 has a maximum FE of only 15% at -0.86 V. In addition to C2 hydrocarbons, another prominent feature of the NGQDs lies in its outstanding selectivity to formation of multi- carbon oxygenate liquid fuels. At -0.78 V, the maximum FE of multi-carbon oxygenates reaches 26%, of which the major component n-C₂H₅OH accounts for 16%. As a comparison, Cu nanoparticles yield trace CH₄ (< 1%), but comparable C₂H₄ (31.2%) and C₂H₅OH (11.8%) at the similar potential of -0.74 V (FIGS. 9A-9C). Overall, the NGQDs exhibit an electrocatalytic behavior similar to Cu nanoparticles catalyst towards C02 reduction in terms of the C2 and C3 products distribution. [0120] In order to verify that the C0₂ reduction reaction was catalyzed by NGQDs, two control experiments were conducted. In one control experiment where argon gas was supplied to the cathode compartment instead of C0₂, hydrogen was exclusively produced at the cathodic potential window of -0.2 ~ -1.0 V. In another experiment where a bare carbon paper substrate served as the cathode while C0₂ gas supplying was maintained, hydrogen formation prevailed at the same potential region. Furthermore, in an experiment feeding isotopically-labelled ¹³C0₂ gas to the NGQDs electrode, the mass spectroscopy showed signals for ¹³CO (m/z = 29), ¹³CH₄ (m/z = 17 ) and ¹³C₂H₄ (m/z = 30) while spin doublet of 1H NMR appeared due to proton coupling to ¹³C in the liquid products, confirming the products are derived from ¹³C0₂ reduction.

[0121] Additionally, inductively coupled plasma optical emission spectroscopy (ICP-OES) measurement revealed that NGQDs sample only contained 1.02 ppm Cu, based on which the Cu loading at the cathode was around 5xl0³ pg cm^"2, which is too low to catalyze C0₂ reduction to hydrocarbons and oxygenates as previously reported. These results confirm the activity toward C0₂ reduction to multi-carbon hydrocarbons and oxygenates originates from the special structure of NGQDs.

[0122] To unveil the origin of the activity of NGQDs, the pristine GQDs were tested for C0₂ reduction under identical experimental conditions. Generally, hydrogen evolution reaction dominates over C0₂ reduction reaction for the GQDs electrode. GQDs not only initialize C0₂ reduction reaction at a more negative potential, but also primarily produce CO and HCOO^", even with lower FEs than NGQDs electrode (FIG. 7B). Compared to NGQDs, the FEs of hydrocarbons such as CH₄ and C₂H₄ on the GQDs electrode are low, exhibiting a maximum value of about 5-6% for both.

[0123] Moreover, only a trace amount of CH₃COO was observed while no C₂H₅OH and n- C₃H₇OH was observed or under the detection limit. Additionally, the Tafel plots derived from the partial current density of C0₂ reduction (jco₂reduction) versus cathode potential show that the GQDs electrode has a larger Tafel slope than the NGQDs electrode (371 mV/dec for GQDs versus 198 mV/dec for NGQDs), indicating that the GQDs electrode exhibits a much slower kinetics than the NGQDs electrode (FIG. 7D). [0124] The aforementioned comparison of performance between the NGQDs and GQDs indicates the significance of N doping defects in determining the activity. The incorporation of N defects to sp² carbon nanostructures to induce charge density was reported to be crucial for actively catalyzing electrochemical reactions such as oxygen reduction reaction (ORR). Among the most popular three N configurations, the pyridinic N is experimentally proven to create the most active site for ORR.

[0125] Similar to ORR, Applicants' previous research on N-doped CNTs and graphene suggests that the leading active site for C0₂ reduction comes from the pyridinic N as well. The acidic C0₂ molecule prefers adsorbing onto the Lewis basic pyridinic N group in carbon nanostructures, which further indicates that pyridinic N is the active site. The NGQDs samples were analyzed by ex-situ post-reaction XPS. After testing for C0₂ reduction, the relative concentration of pyridinic N (398.5 eV) decreases from 65% to 38%, and another N component with a peak of 400.0 eV (pyrrolic or pyridonic N) increases from 20% to 50%, while the percentage of graphitic N (401.2 eV) almost preserves (from 12% to 15%) (FIGS. 10A-10B). The change of N component fraction before and after C0₂ reduction measurement suggests the adsorption of C02 onto the active pyridinic N that causes the upshift of binding energy of pyridinic N to a similar value of pyrrolic N.

[0126] To further explore the active site, the N-doped reduced graphene oxides (NRGOs) were prepared by doping graphene oxides with NH₃ at 800 °C. The NRGOs contain a concentration of each specific N configuration similar to NGQDs but have larger lateral size in micron scale (FIGS. 11A-11B and 12A-12D).

[0127] The C0₂ reduction on NRGOs electrode initiates at a potential identical to that for NGQDs electrode (FIGS. 7A and C). However, the NRGOs electrode primarily catalyzes C0₂ reduction to form CO (FIG. 7C), which is similar to previously reported N-doped graphene and N-doped carbon nanotubes. The formation of noticeable hydrocarbons CH₄ (-6%), C₂H₄ (-5%) and oxygenates C₂H₅OH (-4%) starts at -0.90 V on NRGOs, a more negative potential than that (-0.61 V) on NGQDs. Compared to NRGOs, the pristine RGOs exclusively catalyzes hydrogen evolution reaction, underlining the ability of N doping to transform inertness to highly catalytic activity in carbon nanostructures. The kinetics for C0₂ reduction on NRGOs is also slower than that on NGQDs (e.g., 489 mV/dec for GQDs versus 198 mV/dec for NGQDs) (FIG. 7D).

[0128] The comparison between NGQDs and NRGOs clearly suggests that the morphology also plays a key role in determining the activity of carbon nanostructures towards C0₂ reduction in addition to doping induced defects. When the NRGOs sheets with 1-3 pm diameter are pulverized into quantum dots with 1-3 nm in lateral dimension, the density of exposed edge sites increases by three orders of magnitude, for example 44% for 1 nm NGQD versus 0.05% for 1 pm NRGO sheet (FIGS. 13A-13B).

[0129] Due to the lower formation energy of N doping at the edge sites than the basal planes, especially for pyridinic N, most of the N dopants would locate at the edge sites in NGQDs. In contrast, in the case of NRGOs, N dopants mostly locate in the basal planes because there are not enough edge sites to accommodate the dopants. Although there are similar pyridinic N content in NGQDs and NRGOs, NGQDs tend to have higher density of pyridinic N at edge sites. The pyridinic N at the edge site is suggested being more active to coordinate C-C bond formation than that at the basal plane, which leads to higher yield of C2 and C3 product on NGQDs electrode than on NRGOs electrode.

[0130] In addition to the promising selectivity to multi-carbon hydrocarbons and oxygenates, the NGQDs electrode also exhibits large C0₂ reduction current density with a magnitude of 100 mA cm^"2 at relative low potentials (FIG. 14). The partial current densities of CO, C₂H₄ and C₂H₅OH reach 23, 46 and 21 mA cm^"2 at -0.86 V, respectively (FIG. 15A), which are at the same order of magnitude compared to that for commercial Cu nanoparticles (around 20- 40 nm) under the identical testing condition. Accordingly, the production rate for CO, C₂H₄ and C₂H₅OH could achieve 4.2, 1.4 and 0.7 mol h^"1 m^"2 at -0.86 V, respectively.

[0131] In contrast, the GQDs show at least one order of magnitude lower partial current density or production rate for C₂H₄ at the comparable potential (FIGS. 15B and 16). A metal-free catalyst of NGQDs is discovered to exhibit extraordinary activity toward C0₂ reduction with high current density and low overpotential. More promisingly, NGQDs show predominant selectivity to production of multi-carbon hydrocarbons and oxygenates compared to primary production of CO and HCOO on GQDs and NRGOs. The unique nanostructure in combination of utmost exposure of edge sites and heteroatom N doping grants NGQDs the unprecedented activity and selectivity.

[0132] In sum, Applicants have combined high defect density aspect by tuning the dimension and morphology of carbon nanostructures and N doping to develop an advanced metal-free catalyst for electroreduction of C0₂ to value-added chemicals. The resulting catalyst, namely, N-doped graphene quantum dots (NGQDs) has substantially enriched N-doping defects density at edge sites. Subsequently, NGQDs turn out to be very active toward electrochemical reduction of C0₂ with high reduction current density at low overpotentials, and more importantly, exhibit a high yield of multi-carbon hydrocarbons and oxygenates, especially C2 product of ethylene (C₂H₄) and ethanol (C₂H₅OH) that have Faradaic efficiencies (FEs) comparable to that obtained using Cu nanoparticles catalyst.

Example 1.1. Graphene quantum dots synthesis

[0133] An improved Hummer's method was used to synthesize graphene oxide (GO) from SP1 graphite powder. The procedure involves mixing 3 g of graphite powder with 18 g of KMn0₄ and 3 g NaN0₃ followed by slow addition of 360 mL H₂S0₄. The reaction was then kept under stirring for 12 hours. After that, the solution was poured onto ice (made from 500 mL DI water). Next, 14 mL H₂0₂ was added slowly. After stirring for an hour, the solution was allowed to stand for a day. The yellowish brown material that settled down was collected and the procedure for GO synthesis was again repeated to increase the oxygen functional groups. The material collected after the second oxidation step was then repeatedly washed using DI water, 30% HC1 and ethanol to remove any impurities present. The dried material obtained was GO with additional oxidation.

[0134] The resultant GO was used as the precursor for hydrothermal synthesis of N- doped and pristine graphene quantum dots (GQDs). In the case of N-doped GQDs, typically 300 mg of GO was dispersed in 30 ml dimethylformamide (DMF) and then sonicated in a bath ultrasonicator for 30 minutes. Afterwards, the GO suspension was transferred to a 50 ml PTFE liner. The NGQDs was formed in a hydrothermally analogous process at 200 °C for 10 hours during which GO was exfoliated and cut at the weak sites with oxygen containing groups, and simultaneously doped by N into the carbon lattice with N source from DMF and its derived product of dimethylamine, methylamine and ammonia. [0135] The pristine GQDs were synthesized using the same GO precursors and process except replacing DMF by a mixture of IPA and H₂0 (1 : 1 by volume). The ratio is an optimized one to match the surface energy component of IPA/H₂0 co-solvent to that of GO, so that to maximize the exfoliation and cutting efficiency. The N-doped reduced graphene oxide was prepared in a tube furnace at 800 °C while flowing ammonia for 1 h.

Example 1.2. Materials characterization

The morphology and crystallinity of N-doped and pristine graphene quantum dots (GQDs) was characterized by high resolution field emission gun transmission electron microscope (JEOL 2100 FEG TEM). The TEM samples were prepared by dropping the QDs solution onto the ultrathin carbon film TEM grid followed by vacuum drying at 100 °C. The thickness of QDs was measured by atomic force microscopy (AFM) with tapping mode (Bruker Multimode 8). The AFM samples were prepared by dropping the QDs solution onto the Mica substrate. The morphology of N-doped reduced graphene oxide (NRGOs) was analyzed by scanning electron microscope (FEI Quanta 400 FEG ESEM). The Raman spectra were taken from Renishaw inVia Raman microscope with 514 nm laser excitation.

X-ray photoelectron spectroscopy (XPS) measurements were performed to analyze the element component and oxide state of QDs and NRGOs at ambient temperature using PHI Quantera with Al-Ka X-ray source.

[0136] The Cu content in the NGQDs was determined by using inductively-coupled plasma optical emission spectroscopy (ICP-OES PerkinElmer-Optima 2000DV). 1 mL original liquid sample that is dissolved in DMF was diluted with 5 mL water. Next, 1 drop (-0.05 mL) of concentrated HN0₃ was used to adjust the pH to be less than 7. Two different emission lines (X=224.7000 nm and X=213.5970 nm) were used to detect the Cu element. The detection limit for Cu is 10 ppb.

Example 1.3. Gas diffusion electrode preparation

[0137] The cathodes were prepared using an air-brush method as previously reported. Cathode catalyst inks for QDs were prepared by first mixing QDs solution (10 ml) and Nafion® solution (26 pL, 5 wt%, Fuel Cell Earth), and then sonicating the solution for 5 minutes. The cathode ink for NRGOs were prepared in the same manner except using RGOs powder of 5 mg. Afterwards, the catalyst ink was air-brushed onto a gas diffusion layer (GDL, Sigracet 35 BC, Ion Power) to create a gas diffusion electrode (GDE). The catalyst loading for all cathode GDEs were kept at 0.5 ± 0.1 mg cm^"2. The anodes were prepared by hand-painting of Ir02 catalyst inks onto GDL to reach a loading of about 1.5 mg cm^"2.

Example 1.4. Electrochemical measurements

[0138] An electrochemical flow cell composed of targeted GDE cathode and Ir02 GDE anode as shown in FIGS. 6A-6C was employed to carry out C0₂ reduction at ambient pressure and temperature. The electrolysis was performed under potentiostatic mode with a full cell voltage ranging from - 1.6 V to -3.5 V controlled by a potentiostat (Autolab PGSTAT- 30, EcoChemie). Both the catholyte and anolyte were 1 M KOH (pH = 13.48, as calibrated by a pH meter (Thermo Orion, 9106BNWP)).

[0139] High purity C0₂ was supplied to the cathode at a flowing rate of 7 SCCM monitored by a mass flow controller (MASS- FLO®, MKS instrument). The electrolyte was fed by a syringe pump (PHD 2000, Harvard Apparatus) with a continuing flowing to minimize boundary layer depletion effects and maintain the pH on the electrode surface with a supply of fresh electrolyte.

[0140] The flow rate was set at 0.5 mL min^"1 when applying cell potentials more negative than - 2 V, otherwise using a slower flowing rate of 0.1 mL min^"1 to increase the concentration of the liquid products at a relatively lower current density. For each voltage, after the cell reached steady state, 1 mL of the effluent gas stream was periodically sampled and diverted into a gas chromatograph (Thermo Finnegan Trace GC) equipped with both the thermal conductivity detection (TCD) and flame ionization detector (FID), and a Carboxen 1000 column (Supelco). Helium as the carrier gas flows at a rate of 20 SCCM. Meanwhile, the exit catholyte was collected at each applied voltage followed by identifying and quantifying using 1H MR (nuclear magnetic resonance, UI500 B, Varian). 100 pL of the catholyte was mixed with 100 pL internal standard of 1.25 mM DMSO (99.98%, Calbiochem) in 400 pL D₂0 (99.9% deuterium atom, Sigma-Aldrich). The current reported here was obtained by averaging the span of time (at least 180 s) for each applied voltage. [0141] Individual electrode potentials were recorded using multimeters (AMPROBE 15XP- B) connected to each electrode and a reference electrode (Ag/AgCl; RE-5B, BASi) placed in the electrolyte exit stream. The measured potentials after iR compensation were rescaled to the RHE by the following formula: E (vs. RHE) = E (vs. Ag/AgCl) + 0.209 V + 0.0591 V/pH x pH).

[0142] The onset potential is defined as the lowest cathode potential at which product was detected from either GC or MR. The gas products from ¹³C0₂ were identified by VG 70S double-focusing magnetic sector mass spectrometer.

Example 2. Thermal Catalytic Reduction of CO? to Methane

[0143] This example demonstrates the thermal catalytic reduction of C0₂ to methane. The results are summarized in FIGS. 17A-B. NGQDs were supported onto A1₂0₃ (NGQDS/A1₂0₃) and used as thermal catalysts for C0₂ reduction. The measurement was performed in a fixed bed reactor which was fed by the reaction gas composed of C0₂ (18%), H₂ (72%)) and Ar (10%>) with a total pressure of 10 bar. The composite catalyst with a loading of 3% by weight (NGQDs/ A1₂0₃, 3wt.%>) firstly shows a gradual increase of C0₂ conversion with the increase of temperature, reaching a maximum C0₂ conversion up to 30%> at 280 °C.

[0144] The NGQDs/ A1₂0₃ (3 wt.%) exhibits a high selectivity of CH4 production in addition to CO formation. At 175 °C, the NGQDs/ A1₂0₃ has 30% selectivity towards CH₄ production. With a further increase of temperature, only a slight increase of selectivity towards CH₄ was observed. The maximum selectivity (36%) was reached at 257 °C. The selectivity suddenly increased again beyond 359 °C. As a control, the pure A1₂0₃ does not show any activity toward C0₂ hydrogenation under the studied temperature region (25-400 °C). Moreover, the NGQDs/ A1₂0₃ lost its activity when the loading increased to 20 wt%>. Without being bound by theory, it is envisioned that this occurred due to the aggregation of NGQDs, which decreased the exposure of the edge sites. Example 3. Preparation and Activity of Nitrogen Doped Graphene Quantum Dots

(NGQDs) as Metal Free Catalysts

[0145] The study of catalysts for thermochemical C0₂ hydrogenation has traditionally focused on metal-based materials involving single metals or alloys, and their carbide and oxide phases. Tuning of their structural and electronic properties has not substantially advanced catalytic activity. A broader search of efficient catalysts beyond metals is therefore needed to expand the research horizon. The present example presents an exemplary embodiment of the present invention and how the structure of carbon nanomaterials is modified to improve its thermo-catalytic activity for C0₂ hydrogenation. This activity is governed by the dimension and defect density of the carbon nanomaterials. Reducing the dimension of graphene down to graphene quantum dots (GQD, lateral size < 5nm) with maximal exposure of edges sites and further introduction of nitrogen (N) dopants at those sites greatly promotes the C0₂ hydrogenation activity, similar to our prior discovery on N- doped graphene quantum dots (NGQDs) for its unique selectivity towards multi-carbon products in electrocatalytic C0₂ reduction at room temperature and ambient pressure. The catalytic activity can be tuned with different nitrogen sources to finely adjust the N-C bonding configurations and N content in the NGQDs. The mechanistic influence of N species on the C0₂ catalytic hydrogenation is further investigated with in-situ IR spectroscopy and density functional theory (DFT) calculation. The embodiments of the present invention provides a novel approach to the development of powerful catalyst for thermochemical C0₂ reduction via further exploiting structure-catalytic activity relationship for carbon nanomaterials.

[0146] The NGQDs were synthesized through liquid phase exfoliation and shearing of graphene oxide (GO) precursor in the dimethylformamide (DMF) solvent at 200 °C in a PTFE lined autoclave. The graphene oxide (GO, 300 mg) was dispersed in dimethylformamide (DMF, 60 ml) by sonicating in a bath sonicator for 1 hour. The dispersion was transferred to a PTFE lined autoclave and heat-treated at 200 °C for 10 h. The unreacted GO remained aggregated after reaction. The suspension was vacuum filtrated using a cellulose membrane with 25 nm pore size, resulting in a dispersion of NGQDs in DMF. For other NGQDs with different N contents, the synthesis procedure is the same, except of the use of different solvents (ammonia solution or DMF diluted by isopropanol/water, 1/1 by volume). The pristine graphene quantum dots (GQDs) were prepared in a similar way by using a mixture of isopropanol and water (1 : 1 by volume) as the solvent. The N-doped reduced graphene oxide was prepared by doping graphene oxide in a tube furnace at 800 °C while flowing ammonia for 1 h.

[0147] DMF has a surface tension and ratio of polar/dispersion component matching that of GO, guaranteeing efficient liquid-phase exfoliation of GO and favoring the formation of ultrathin sheets at a high yield. Meanwhile, the cutting of exfoliated sheets takes place preferably along the boundary between the sp² domain and disordered sp³ region with rich oxygen-containing groups. The sp² domains split from ultrathin GO sheets leads to the formation of graphitic NGQDs. As a result, these NGQDs contain predominantly mono- and bi-layers (0.5 to 1.0 nm), as shown in FIG. 2, while they possess lateral sizes of 2-3 nm, as determined by the high resolution transmission electron microscopy (HRTEM) images in FIG. 3A-3B. The N doping occurred simultaneously with the exfoliation, with DMF acting as a nitrogen source as it decomposed at elevated temperature and pressure. The NGQDs have an enhanced density of exposed edge sites compared to GO, which favors the formation of pyridinic N configuration (connecting to two C atoms in a hexagonal ring, illustrative structure shown in inset of FIG. 3D doping. The Raman spectrum of NGQDs demonstrates the characteristic D band at 1351 cm^"1 and the G band at 1584 cm^"1 (FIG. 3C). The large ratio of D/G band intensity (-1.05) mainly originates from the N-doping defects and exposed edge sites. To gain more information about N bonding and content of specific N motif, X-ray photoelectron spectroscopy (XPS) was performed on the NGQDs. The deconvolution of the XPS N Is for NGQDs quantitatively provides information on the content of specific N configurations. The total N content in the NGQDs is around 6.0 at.%, composed of predominant pyridinic N (binding energy 398.5 eV) with a content of -3.9 at.%, pyrrolic N (400.0 eV, -1.2 at.%), and graphitic N (401.2 eV, -0.9 at.%) (see FIGS. 18A and 18B).

[0148] Three samples of NGQDs supported on A1₂0₃ (NGQDs/Al₂0₃) with different loadings (0.8, 1, and 3 wt.%) were prepared by an impregnation method. The NGQDs were loaded onto the γ-Α1₂0₃ (Sigma-Aldrich) support by impregnation method. 0.2 g of A1₂0₃ was impregnated by a certain volume of NGQDs. The mixture was magnetically stirred and heated at -80 °C to evaporate the solvent. After drying, the samples were collected, denoted as NGQDs/y-Al₂0₃. The activity and selectivity of NGQDs/ A1₂0₃ towards hydrogenation of C0₂ were evaluated in the temperature range between 100 - 450 °C and under stoichiometric conditions of C0₂/H₂ (1 :4) at 10 bar. The three NGQDs/ A1₂0₃ samples exhibit similar catalytic behavior. The hydrogenation of C0₂ catalyzed by NGQDs/Al₂0₃ initiates around 170 °C (see FIGS. 19A-19D). The C0₂ conversion generally increases with the increase of temperature, and reaches values over 60% at 400 °C. The two products that were observed were carbon monoxide (CO) and methane (CH₄). The predominant product is CO at lower temperatures, while CH₄ becomes the major product after the temperature increases to 380 °C (see FIG. 19B).The CO selectivity maintains around 60 - 65% in the temperature range of 170 - 255 °C. The CO selectivity rises to a maximum of 85% at a turning point at 300 °C. The initial selectivity of CH₄ is -30% at 170 °C. It gradually decreases to 15% at 300 °C. With further increase of the temperature beyond 300 °C, the selectivity of CH₄ gradually increases to 55% at temperatures above 380 °C. A turning point in the C0₂ conversion and the CO/CH₄ selectivity at around 300 °C is observed for all three NGQD catalysts. A change in the rate-determining step (RDS) is proposed to be responsible for this turning point. The RDS for the C0₂ hydrogenation is supposed to switch from the hydrogenation of *CH₂OH to *CH₂ at 170 °C to the activation of C0₂ into COOH* at 300 °C. The formation of COOH* species is observed experimentally via IR spectroscopy during C0₂ adsorption on the NGQDs (see FIG. 19C). The IR bands around 3480, 1540, and 1392 cm^"1 are associated with the vo-H, vo-c-o, and VC-OH vibrations of adsorbed COOH* species. The rise of COOH* species coincides with the decrease of VC-H vibration at 2857 cm^"1, which suggests that the protons from the C-H bond on the NGQDs are used to activate C0₂ into COOH* species. Other peaks observed in the IR spectra of C0₂ absorbed on NGQDs can be assigned to the vibrations of various carbonate species, including lactones, bi-dentate, and uni-dentate carbonates. Furthermore, the change in the RDS is consistent with the experimental observation that the CH₄ selectivity increases at temperature higher than 300 °C. As the formation of COOH* becomes the RDS at higher temperature, the formation of CH₄ will be promoted as soon as C0₂ is activated, because the rest of the hydrogenation elementary steps leading to CH occur much faster than the RDS. Therefore, the catalytic tests indicate that the catalytic selectivity and reaction kinetics of C0₂ hydrogenation over NGQDs are susceptible to reaction temperature.

[0149] To understand the origin of the catalytic activity in NGQDs, a control experiment for C0₂ hydrogenation was conducted on A1₂0₃ supported, pristine GQDs without N-doping (GQDs/Al₂0₃) under identical reaction conditions. The pristine GQDs were synthesized through the similar hydrothermal method as for NGQDs, except that the solvent was a ∑PA/H₂0 (1 : 1 by volume) mixture. The pristine GQDs possess a morphology similar to NGQDs in terms of thickness, lateral size, and crystalline structure (see FIGS. 4A-4D). However, the GQDs/Al₂0₃ exhibit negligible activity towards the hydrogenation of C0₂ at reaction temperatures of 100 - 400 °C. Only a very small amount of C0₂ conversion below 2% was detected at 400 °C, and the product is exclusively CO. The sharp contrast between catalytic activity of NGQDs/ A1₂0₃ and GQDs/Al₂0₃ strongly suggests a significant role of N doping for C0₂ hydrogenation. Based on C0₂-temperature programmed desorption (TPD) measurements, the NGQDs show great enhancement of C0₂ chemisorption over the GQDs because of the introduction of Lewis base sites via N doping. Such an increase of Lewis base sites by N-doping is consistent with literature reports that showed that doped N atoms are point defects, which can delocalize the π bonds of the graphene framework and lead to the formation of Lewis base sites. The increase in the number of Lewis base sites and point defects is also consistent with our Raman spectra. The NGQDs have a much bigger ratio of D/G peak intensity compared to that of the pristine GQDs, an indicative feature of more defects in the NGQDs (see FIG. 3C). The Lewis base sites can activate C0₂ and form the COOH*, which desorbs at around 300 °C, as seen from the TPD profile of the NGQDs sample. The formation of COOH* on NGQDs is consistent with the IR observations shown in FIG. 19C. However, pristine GQDs lack the Lewis base sites. Therefore, they have a limited amount of COOH* groups, and the corresponding C0₂ desorption peak is far smaller. The desorption profile in the temperature range of 400 - 600 °C can be attributed to desorption of lactone groups. In addition to the decreased capability of C0₂ adsorption, GQDs are inert to activate the adsorbed C0₂ as indicated by the IR spectra where the peak intensity of the VC-H vibration for GQDs remains constant compared to a sharp decrease for NGQDs after C0₂ adsorption. Based on the comparison between the catalytic performance of NGQDs and GQDs in the C0₂ hydrogenation, N-doping strongly promotes the catalytic activity through the introduction of active Lewis base sites.

[0150] To further explore the significance of N doping, a second control experiment was performed using N-doped graphene (NG) as the catalyst. The NG was synthesized by chemical vapor deposition of graphene followed by N doping using g-C₃N₄ as precursor. The NG is in the lateral size range of 10-100 μηι, and therefore possesses far less density of edge C or N atoms compared with NGQDs. The NG/A1₂0₃ shows no activity towards C0₂ hydrogenation, although the NG contains a comparable amount of N. These results suggest that another key aspect, the location of N at the edge, influences the catalytic activity for graphene based materials. We conclude that the N doping and enriched edge sites originating from reduced lateral size together contribute to the excellent catalytic activity of the NGQDs towards C0₂ hydrogenation.

[0151] In a further embodiment of the present invention, NGQDs are synthesized with different contents of N species by changing the N precursors or solvent. By diluting the DMF with IPA/H₂0 (1 : 1), the total N content drops to 3.6 at.%, but pyridinic N is still the dominant N configuration with a content of 2.2 at.% (see FIGS. 18A-18B). The total N content is further reduced to 1.6 at.% and pyridinic N drops to 0.4 at.% when using ammonium hydroxide (NH₄OH) as the N doping solvent. The NGQDs synthesized in NH₄OH exhibit analogous thickness and lateral dimension to these in DMF solvent, minimizing effects of morphological changes on catalytic performance as shown in FIGS. 21A-D. The activity and selectivity of NGQDs towards C0₂ hydrogenation strongly depends on the contents of different N configurations. The onset reaction temperature decreases while both C0₂ conversion and CH₄ selectivity at 400 °C increase with higher doping levels as shown in FIGS. 22A-D. Moreover, the trend of C0₂ conversion and CH₄ selectivity versus pyridinic N content clearly indicates a more linear relationship compared to the pyrrolic and graphitic N content, suggesting that pyridinic N is the most active N site, although the contribution from the other two N configurations cannot be completely ruled out (see FIGS. 22A-D)

[0152] Turnover frequencies (TOFs) are then calculated of NGQDs/ A1₂0₃ normalized to the number of total N defect sites, since the catalytic activity originates from the N-doping defects. The total amount of N defect sites was accounted based on the XPS results. The NGQDs with DMF as N precursor shows the highest TOF for CH₄ production. The TOF of CH₄ production for these NGQDs catalyst is calculated to be -0.03 s^"1 at 177 °C, increases to 0.35 s^"1 at 257 °C and further to 1.50 s^"1 at 400 °C (see FIG. 23). The TOF shown in FIG. 23 is TOF of CH₄ production for NGQDs samples synthesized with different N precursors. The loading is kept at lwt% for all samples. The TOF is calculated by normalization to the number of total N sites. This TOF is comparable to the state-of-the-art metal-based catalysts for C0₂ methanation, such as Co/Si0₂, Ru/Ti0₂ and Ni/Si0₂ (see Table 1), at the similar temperatures but under higher pressure in our reaction.

Table 1

Activity and selectivity of some selected outstanding metal catalysts for

methanation

¾ : C0₂ Temp. Pressure Conversion CH₄ TOF of

Catalyst ratio (°C) (MPa) (%) Selectivity (%) CH₄ (s^"1)

Co/Si0₂ 3 1 250 0.5 N/A 60 0.03

Ru/Ti0₂ 4 1 160 0.1 100 100 0.015

Pd/Si0₂ 4 1 450 0.1 40.8 10.4 N/A

Pd-Mg/Si0₂ 4 1 450 0.1 59.2 95.3 N/A

Ni/Si0₂ 4 1 350 0.1 28.4 88.3 0.067

Ni/Ce_xZn_-x0₂ 4 1 350 0.1 79.7 99.3 0.426

NGQDs/Al₂0₃ ^* 4 1 177 1 2.6 29.7 0.035

NGQDs/Al₂0₃ 4 1 257 1 21.6 35.6 0.35

NGQDs/Al₂0₃ 4 1 359 1 41.8 26.9 0.506

NGQDs/Al₂0₃ 4 1 400 1 61.8 55.2 1.5

The NGQDs (all of the present invention) were synthesized in DMF solvent. The N content is around 6%.

[0153] Materials characterization. To further characterize reaction pathways involved in the C0₂ methanation over the NGQDs/Al₂0₃ catalyst, DFT calculations were carried out using computational hydrogen electrode model on the NGQDs catalyst. The morphology and crystallinity of NGQDs and GQDs were characterized by high resolution field emission gun transmission electron microscope (JEOL 2100 FEG TEM). The TEM samples were prepared by dropping the QDs solution onto the ultrathin carbon film TEM grid followed by vacuum drying at 100 °C. The thickness of QDs was measured by atomic force microscopy (AFM) on tapping mode (Bruker Multimode 8). The AFM samples were prepared by dropping the QDs solution onto the Mica substrate. The Raman spectra were performed in a Renishaw inVia Raman microscope with 514 nm laser excitation. X-ray photoelectron spectroscopy (XPS) measurements were carried out using PHI Quantera with Al-Κα X-ray source. The X- ray absorption spectroscopy (XAS) measurement was conducted at the Advanced Light Source at Lawrence Berkeley National Laboratory. N K-edge spectra were collected at beamline 8.0.1 in total -electron yield (TEY) mode. No evidence of beam damage was observed. [0154] In-situ FTIR characterization. In situ FTIR was conducted using a Bruker IR (model Equinox 55) with a Praying Mantis diffuse reflection accessory. All the in situ IR experiments were conducted at room temperature. NGQDs and GQDs samples are loaded in the Praying Mantis diffuse reflection cell. Background spectra of the samples were collected under ambient conditions. Then, 1% C0₂/He gas at the flow rate of 50 ml/min was permitted to pass through the sample compartment to allow the sample to adsorb C0₂. After 30 minutes of adsorption, the sample compartment was flushed with Ar at the same flow rate to remove gas phase C0₂. IR spectra between 4000-1000 cm^"1 were collected with resolution of 4 cm^"1.

[0155] The C0₂ temperature programmed desorption (TPD). C0₂-TPDs were conducted typically on 0.2 g of sample. The samples were pretreated under flowing Ar (50 ml/min) at 200 °C for 1 h to remove any moisture. Then, the samples were cooled down to room temperature to adsorb C0₂ (1000 ppm in He, with flow rate of 50 ml/min) for 30 min. After C0₂ adsorption, the system was flushed with Ar (50 ml/min) for 30 min to remove any physisorbed C0₂ and gas phase C0₂. Then the C0₂-TPD program was started by ramping up the sample temperature from room temperature to about 600 °C at the rate of 10 °C/min. During the TPD, the effluent gas was monitored by a mass spectrometer (MS). The C0₂ (m/e=44) signal was followed to track the TPD profile. The signal intensity of C0₂ in the MS was calibrated with C0₂ gas at different concentrations.

[0156] Catalytic measurement. All catalytic tests were performed using stoichiometric ratio of C0₂ to H₂ of 4: 1, balanced with Ar (18% C0₂/72% H₂/10% Ar). Argon was added as an inert and internal standard for conversion measurements. The reaction gas stream was analyzed on-line with a Shimadzu Gas Chromatograph (GC 2014) equipped with both TCD and FID detectors. A Shincarbon packed column was used for permanent gas (H₂, Ar, CO, C0₂, and CH₄) separation followed with TCD detector. At the same time, a capillary column (ZB-1 HT) was used for analyzing CH₄ with FID detector. The gas concentrations at each condition was measured with GC at least twice to make sure steady state was reached. The error for the calculated C0₂ conversion is less than 3%, and that of CH₄ selectivity is less than 5%. For each catalytic test, 0.2 g of catalyst was loaded into a stainless steel plug flow reactor. The gas flow rate was controlled by mass flow controllers to maintain a GHSV of 18000 ml-hr^-g^"1. The reaction pressure was regulated with a back pressure valve to be constant at 10 bar for all the tests. [0157] The embodiment of Example 3 thus presents a novel metal -free, carbon-based catalyst for C0₂ hydrogenation at moderate reaction temperatures, and unraveled the underlying factors governing its catalytic activity. The pyridinic N doping at the edge sites of GQDs is responsible for the catalytic activity, with higher nitrogen contents leading to lower onset reaction temperature, higher C0₂ conversion and improved selectivity towards CH₄ formation. The reaction mechanism was also found to be dependent on temperature, with impacts on the selectivity of the catalyst. A selectivity turning point was observed at around 300 °C, resulting from the change of RDS at this temperature, as indicated by DFT calculations. Moreover, the DFT modeling reveals the lower energy pathway to form CH₄ than CH₃OH, in agreement with the experimental results.

[0158] Examples 1, 2 and 3 report a metal-free catalyst of nitrogen doped graphene quantum dots which has high activity towards converting carbon dioxide into hydrocarbons and oxygenates, with a main selectivity to CH₄, C₂H₄ and C₂H₅OH in electrocatalysis, and to CH₄ in thermal catalysis. Applicants have observed that the N doping has a strong effect on the activity. Non nitrogen-doped graphene only has a main product of CO and formic acid/formate rather than CH₄, C₂H₄ and C₂H₅OH.

[0159] In addition, the size of the NGQDs plays an important role in determining the catalytic activity. The carbon nanostructure in the form of one-dimensional N-doped carbon nanotubes and two-dimensional N-doped graphene only shows activity towards reduction of carbon dioxide into CO and formic acid/formate. When the dimension is reduced to zero- dimensional quantum dots with a thickness of 1-3 layers and a lateral size of less than 10 nm (which exposes as much as the edges sites), the resulting carbon nanostructure, namely N- doped graphene quantum dots with a predominant percentage of pyridinic N, can have activity towards hydrocarbons and oxygenates formation from carbon dioxide reduction.

[0160] In addition, the NGQDs show selectivity towards C₂¾ (up to 50%) and C₂H₅OH (up to 20%). The total Faradaic efficiency or conversion for C0₂ reduction are over 90%. Furthermore, N-doped graphene quantum dots show longer stability than copper.

[0161] N-doped graphene quantum dots can be made from graphene oxide. The graphene oxide can be made from graphite in a Hummer's method. The graphene oxide is dispersed in ammonium hydroxide or dimethylformamide and sonicated. The dispersed graphene oxide is hydrothermally treated at 200 °C for 6-10 hours, resulting in the formation of N-doped graphene quantum dots.

[0162] N-doped graphene quantum dots can also be made by preparing the pristine graphene quantum dots from dispersed graphene oxide in a solvent of IPA and water by hydrothermal reactions. Thereafter, N is doped into the pristine graphene quantum dots by NH3 gas precursors in a tube furnace at 800 °C.

[0163] Without further elaboration, it is believed that one skilled in the art can, using the description herein, utilize the present invention to its fullest extent. The embodiments described herein are to be construed as illustrative and not as constraining the remainder of the disclosure in any way whatsoever. While the embodiments have been shown and described, many variations and modifications thereof can be made by one skilled in the art without departing from the spirit and teachings of the invention. Accordingly, the scope of protection is not limited by the description set out above, but is only limited by the claims, including all equivalents of the subject matter of the claims. The disclosures of all patents, patent applications, and publications cited herein are hereby incorporated herein by reference, to the extent that they provide procedural or other details consistent with and supplementary to those set forth herein.

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Claims

WHAT IS CLAIMED IS:

1. A method of reducing carbon dioxide, said method comprising the steps of:

(a) exposing the carbon dioxide to a metal-free catalyst, and

(b) utilizing the metal-free catalyst to convert the carbon dioxide to a reduced product.

2. The method of Claim 1, wherein the step of exposing occurs by mixing the carbon dioxide with the metal-free catalyst.

3. The method of Claim 1, wherein the step of exposing occurs by flowing the carbon dioxide through a surface of the metal-free catalyst.

4. The method of Claim 1, wherein the metal-free catalyst is selected from a group consisting of carbon nanomaterials, doped carbon nanomaterials, pristine carbon nanomaterials, reduced carbon nanomaterials, graphene quantum dots, pristine graphene quantum dots, doped graphene quantum dots, graphene oxides, reduced graphene oxides, doped and reduced graphene oxides, and combinations thereof.

5. The method of Claim 1, wherein the metal -free catalyst comprises doped carbon nanomaterials, wherein the doped carbon nanomaterials are doped with one or more dopants.

6. The method of Claim 5, wherein the one or more dopants are selected from a group consisting of boron, sulfur, phosphorous, fluorine, nitrogen, and combinations thereof.

7. The method of Claim 5, wherein the one or more dopants comprise nitrogen.

8. The method of Claim 5, wherein the one or more dopants comprise pyridinic nitrogen.

9. The method of Claim 1, wherein the metal-free catalyst comprises nitrogen-doped carbon nanomaterials.

10. The method of Claim 1, wherein the metal-free catalyst comprises nitrogen-doped graphene quantum dots.

11. The method of Claim 1, wherein the metal-free catalyst comprises nitrogen-doped and reduced graphene oxides.

12. The method of Claim 1, wherein the metal-free catalyst comprises less than 5 layers.

13. The method of Claim 1, wherein the metal-free catalyst comprises from about 1 layer to about 3 layers.

14. The method of Claim 1, wherein the metal-free catalyst comprises diameters of less than about 10 nm.

15. The method of Claim 1, wherein the metal -free catalyst comprises diameters of between about 1 nm to about 3 nm.

16. The method of Claim 1, wherein the metal-free catalyst is supported on a substrate to form a complex.

17. The method of Claim 16, wherein the substrate is a metal -based substrate.

18. The method of Claim 16, wherein the substrate comprises aluminum oxide.

19. The method of Claim 16, wherein the metal-free catalyst constitutes from about 0.5 wt% to about 20 wt% of the complex.

20. The method of Claim 16, wherein the metal-free catalyst is dispersed on a surface of a substrate.

21. The method of Claim 1, wherein the reduction of carbon dioxide occurs by electrochemical reduction.

22. The method of Claim 1, wherein the reduction of carbon dioxide occurs by thermocatalytic reduction.

23. The method of Claim 1, wherein the reduced product is selected from a group consisting of hydrocarbons, multi-carbon hydrocarbons, oxygenates, multi-carbon oxygenates, and combinations thereof.

24. The method of Claim 1, wherein the reduced product is selected from the group consisting of methane, ethylene, ethanol, propanol, acetate, n-propanol, carbon monoxide, formate, and combinations thereof.

25. The method of Claim 1, wherein the metal-free catalyst has a Faradaic efficiency of up to about 90%.

26. The method of Claim 1, wherein the metal -free catalyst has a carbon dioxide conversion efficiency of up to about 60%.

27. The method of Claim 1, wherein the metal -free catalyst has a carbon dioxide conversion efficiency of up to about 45%.

28. The method of Claim 1, wherein the metal-free catalyst has a carbon dioxide conversion efficiency of up to about 30%.

29. The method of Claim 1, wherein the metal -free catalyst selectively reduces carbon dioxide without mediating hydrogen evolution reactions.

30. The method of Claim 1, wherein the metal-free catalyst selectively reduces carbon dioxide to a reduced product selected from a group consisting of methane, ethane, ethanol, and combinations thereof.

31. The method of Claim 1, wherein the metal -free catalyst selectively reduces carbon dioxide to methane.

32. An apparatus for carbon dioxide reduction, wherein the apparatus comprises a metal- free catalyst, and wherein the metal-free catalyst is capable of reducing carbon dioxide to a reduced product.

33. The apparatus of Claim 32, wherein the metal-free catalyst is selected from a group consisting of carbon nanomaterials, doped carbon nanomaterials, pristine carbon nanomaterials, reduced carbon nanomaterials, graphene quantum dots, pristine graphene quantum dots, doped graphene quantum dots, graphene oxides, reduced graphene oxides, doped and reduced graphene oxides, and combinations thereof.

34. The apparatus of Claim 32, wherein the metal-free catalyst comprises doped carbon nanomaterials, wherein the doped carbon nanomaterials are doped with one or more dopants.

35. The apparatus of Claim 34, wherein the one or more dopants are selected from a group consisting of boron, sulfur, phosphorous, fluorine, nitrogen, and combinations thereof.

36. The apparatus of Claim 34, wherein the one or more dopants comprise nitrogen.

37. The apparatus of Claim 34, wherein the one or more dopants comprise pyridinic nitrogen.

38. The apparatus of Claim 32, wherein the metal-free catalyst comprises nitrogen-doped carbon nanomaterials.

39. The apparatus of Claim 32, wherein the metal-free catalyst comprises nitrogen-doped graphene quantum dots.

40. The apparatus of Claim 32, wherein the metal-free catalyst comprises nitrogen-doped and reduced graphene oxides.

41. The apparatus of Claim 32, wherein the metal-free catalyst comprises less than 5 layers.

42. The apparatus of Claim 32, wherein the metal-free catalyst comprises from about 1 layer to about 3 layers.

43. The apparatus of Claim 32, wherein the metal -free catalyst comprises diameters of less than about 10 nm.

44. The apparatus of Claim 32, wherein the metal-free catalyst comprises diameters of between about 1 nm to about 3 nm.

45. The apparatus of Claim 32, wherein the metal-free catalyst is supported on a substrate to form a complex.

46. The apparatus of Claim 45, wherein the substrate is a metal-based substrate.

47. The apparatus of Claim 45, wherein the substrate comprises aluminum oxide.

48. The apparatus of Claim 45, wherein the metal-free catalyst constitutes from about 0.5 wt% to about 20 wt% of the complex.

49. The apparatus of Claim 45, wherein the metal-free catalyst is dispersed on a surface of a substrate.

50. The apparatus of Claim 32, wherein the reduced product is selected from a group consisting of hydrocarbons, multi-carbon hydrocarbons, oxygenates, multi-carbon oxygenates, and combinations thereof.

51. The apparatus of Claim 32, wherein the reduced product is selected from a group consisting of methane, ethylene, ethanol, propanol, acetate, n-propanol, carbon monoxide, formate, and combinations thereof.