Patent Application
20120175269
The present disclosure generally relates to the field of electrochemical reactions, and more particularly to advanced aromatic amine heterocyclic catalysts for carbon dioxide reduction.
The combustion of fossil fuels in activities such as electricity generation, transportation, and manufacturing produces billions of tons of carbon dioxide annually. Research since the 1970s indicates increasing concentrations of carbon dioxide in the atmosphere may be responsible for altering the Earth's climate, changing the pH of the ocean and other potentially damaging effects. Countries around the world, including the United States, are seeking ways to mitigate emissions of carbon dioxide.
A mechanism for mitigating emissions is to convert carbon dioxide into economically valuable materials such as fuels and industrial chemicals. If the carbon dioxide is converted using energy from renewable sources, both mitigation of carbon dioxide emissions and conversion of renewable energy into a chemical form that can be stored for later use will be possible.
However, the field of electrochemical techniques in carbon dioxide reduction has many limitations, including the stability of systems used in the process, the efficiency of systems, the selectivity of the systems or processes for a desired chemical, the cost of materials used in systems/processes, the ability to control the processes effectively, and the rate at which carbon dioxide is converted. In particular, existing electrochemical and photochemical processes/systems have one or more of the following problems that prevent commercialization on a large scale. Several processes utilize metals, such as ruthenium or gold, that are rare and expensive. In other processes, organic solvents were used that made scaling the process difficult because of the costs and availability of the solvents, such as dimethyl sulfoxide, acetonitrile, and propylene carbonate. Copper, silver and gold have been found to reduce carbon dioxide to various products, however, the electrodes are quickly “poisoned” by undesirable reactions on the electrode and often cease to work in less than an hour. Similarly, gallium-based semiconductors reduce carbon dioxide, but rapidly dissolve in water. Many cathodes produce a mixture of organic products. For instance, copper produces a mixture of gases and liquids including carbon monoxide, methane, formic acid, ethylene, and ethanol. Such mixtures of products make extraction and purification of the products costly and can result in undesirable waste products that must be disposed. Much of the work done to date on carbon dioxide reduction is inefficient because of high electrical potentials utilized, low faradaic yields of desired products, and/or high pressure operation. The energy consumed for reducing carbon dioxide thus becomes prohibitive. Many conventional carbon dioxide reduction techniques have very low rates of reaction. For example, in order to provide economic feasibility, a commercial system currently may require densities in excess of 100 milliamperes per centimeter squared (mA/cm2), while rates achieved in the laboratory are orders of magnitude less.
A method for electrochemical reduction of carbon dioxide may include, but is not limited to, steps (A) to (C). Step (A) may introduce water to a first compartment of an electrochemical cell. Said first compartment may include an anode. Step (B) may introduce carbon dioxide to a second compartment of said electrochemical cell. Said second compartment may include a solution of an electrolyte, a catalyst, and a cathode. Said catalyst may include at least two aromatic amine heterocycles that are at least one of (a) fused or (b) configured to become electronically conjugated upon one electron reduction. Step (C) may apply an electrical potential between said anode and said cathode in said electrochemical cell sufficient for said cathode to reduce said carbon dioxide to a product mixture.
Another method for electrochemical reduction of carbon dioxide may include, but is not limited to, steps (A) to (C). Step (A) may introduce carbon dioxide to a solution of an electrolyte and a heterocyclic catalyst in an electrochemical cell. Said electrochemical cell may include an anode in a first cell compartment and a cathode in a second cell compartment. Said heterocyclic catalyst may include at least one of (a) two or more fused aromatic amines, (b) a substituted 4,4′-bipyridine, (c) a naphthyridine, or (d) an aromatic amine alkylating agent. Step (B) may apply an electrical potential between said anode and said cathode in said electrochemical cell sufficient for said cathode to reduce said carbon dioxide to a product mixture. Step (C) may vary a yield of said product mixture by adjusting at least one of (a) a material of said cathode, (b) a type of said heterocyclic catalyst, (c) and said electrical potential of said cathode.
A system for electrochemical reduction of carbon dioxide may include, but is not limited to, an electrochemical cell including a first cell compartment, an anode positioned within said first cell compartment, a second cell compartment, a separator interposed between said first cell compartment and said second cell compartment, said first cell compartment and said second cell compartment each containing an electrolyte, and a cathode and a heterocyclic catalyst positioned within said second cell compartment. Said heterocyclic catalyst may include at least one of (a) two or more fused aromatic amines, (b) a substituted 4,4′-bipyridine, (c) a naphthyridine, or (d) an aromatic amine alkylating agent. The system may also include a carbon dioxide input, where said carbon dioxide input may be configured to be coupled between a carbon dioxide source and said cathode and may be configured to provide carbon dioxide to said cathode. The system may further include an energy source operably coupled with said anode and said cathode, where said energy source may be configured to provide power to said anode and said cathode to reduce carbon dioxide at said cathode to a product mixture.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not necessarily restrictive of the disclosure as claimed. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the disclosure and together with the general description, serve to explain the principles of the disclosure.
The numerous advantages of the present disclosure may be better understood by those skilled in the art by reference to the accompanying figures in which:
Reference will now be made in detail to the presently preferred embodiments of the present disclosure, examples of which are illustrated in the accompanying drawings.
In accordance with some embodiments of the present disclosure, an electrochemical system is provided that generally allows electrochemical reduction of carbon dioxide utilizing advanced aromatic amine heterocyclic catalysts. The electrocatalysts disclosed herein generally may allow for the formation of carbon-carbon bonded species from carbon dioxide under appropriate electrochemical conditions (e.g., electrode material, electrode potential, cathode material, and the like). Additionally, the electrocatalysts disclosed herein generally may allow for reduction of carbon dioxide to single-carbon products (e.g., methanol, formic acid, formaldehyde, and the like). Product selectivity may be obtained by the matching of electrode material, aromatic amine catalyst, electrode potential, or other electrochemical cell condition.
Industrial synthesis of organic products using current techniques generally requires a large amount of energy, which may come from natural gas. The combustion of natural gas contributes to the concentration of carbon dioxide in the atmosphere and thus, global climate change. In some embodiments of the present disclosure, the energy used by the system may be generated from an alternative energy source to avoid generation of additional carbon dioxide through combustion of fossil fuels. In general, the embodiments for the reduction of carbon dioxide do not require oil or natural gas as feedstocks. Some embodiments of the present invention thus relate to environmentally beneficial methods and systems for reducing carbon dioxide, a major greenhouse gas, in the atmosphere thereby leading to the mitigation of global warming. Moreover, certain processes herein are preferred over existing electrochemical processes due to being stable, efficient, having scalable reaction rates, occurring in water, and having selectivity of products based upon the matching of electrode material, aromatic amine catalyst, and electrode potential.
Advantageously, the carbon dioxide for reduction in systems of the present disclosure may be obtained from any source (e.g., an exhaust stream from fossil-fuel burning power or industrial plants, from geothermal or natural gas wells or the atmosphere itself). Most suitably, the carbon dioxide may be obtained from concentrated point sources of generation prior to being released into the atmosphere. For example, high concentration carbon dioxide sources may frequently accompany natural gas in amounts of 5% to 50%, exist in flue gases of fossil fuel (e.g., coal, natural gas, oil, etc.) burning power plants, and high purity carbon dioxide may be exhausted from cement factories, from fermenters used for industrial fermentation of ethanol, and from the manufacture of fertilizers and refined oil products. Certain geothermal steams may also contain significant amounts of carbon dioxide. The carbon dioxide emissions from varied industries, including geothermal wells, may be captured on-site. Separation of the carbon dioxide from such exhausts is known. Thus, the capture and use of existing atmospheric carbon dioxide in accordance with some embodiments of the present invention generally allow the carbon dioxide to be a renewable and unlimited source of carbon.
Current electrochemical methods may involve a small (<1 liter) glass cell containing electrodes and an aqueous solution with supporting electrolyte in which carbon dioxide is bubbled. In some instances, a solvent other than water may be used. Reduction of the carbon dioxide may occur directly on the cathode or via a dissolved mediator, such as a transition metal complex. Current photoelectrochemical methods may replace one or both of the standard metal electrodes in an electrochemical cell with semiconductor electrodes that convert light energy to electrical energy. In case of photoelectrochemical methods, some or all of the energy for reducing the carbon dioxide comes from light that is incident on the semiconductor surfaces. The reduction of the carbon dioxide for the photoelectrochemical methods may take place on the photovoltaic material, or via a catalyst.
The present disclosure may include use of low-cost heterocyclic amines, such as pyridine, as catalysts for carbon dioxide reduction. The process may provide good selectivity for methanol, with a 30% to 95% faradaic yield for carbon dioxide to methanol, with the remainder evolving hydrogen. The use of alternative cathode materials, alternative aromatic amine electrocatalysts, and alternative mechanisms for improving control over the reaction may provide further benefits.
Before any embodiments of the invention are explained in detail, it is to be understood that the embodiments may not be limited in application per the details of the structure or the function as set forth in the following descriptions or illustrated in the figures of the drawing. Different embodiments may be capable of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of terms such as “including,” “comprising,” or “having” and variations thereof herein are generally meant to encompass the item listed thereafter and equivalents thereof as well as additional items. Further, unless otherwise noted, technical terms may be used according to conventional usage.
For electrochemical reductions, the electrode may be a suitable conductive electrode, such as Al, Au, Ag, C, Cd, Co, Cr, Cu, Cu alloys (e.g., brass and bronze), Ga, Hg, In, Mo, Nb, Ni, Ni alloys, Ni—Fe alloys, Sn, Sn alloys, Ti, V, W, Zn, stainless steel (SS), austenitic steel, ferritic steel, duplex steel, martensitic steel, Nichrome, elgiloy (e.g., Co—Ni—Cr), degenerately doped n-Si, degenerately doped n-Si:As and degenerately doped n-Si:B. Other conductive electrodes may be implemented to meet the criteria of a particular application. For photoelectrochemical reductions, the electrode may be a p-type semiconductor, such as p-GaAs, p-GaP, p-InN, p-InP, p-CdTe, p-GaInP2 and p-Si. Other semiconductor electrodes may be implemented to meet the criteria of a particular application.
The present disclosure may provide for the use of protonated aromatic amines such as pyridine to efficiently reduce CO2 to a variety of chemicals such as methanol. The present method may further include use of substituent groups on the heterocycle, such as methyl groups or hydroxyl groups, which may be used to change the reduction product from methanol to multi-carbon containing products such as propanol.
The reduction of the carbon dioxide may be suitably achieved efficiently in a divided electrochemical or photoelectrochemical cell in which (i) a compartment contains an anode suitable to oxidize or split the water, and (ii) another compartment contains a working cathode electrode and a catalyst. The compartments may be separated by a porous glass frit, microporous separator, ion exchange membrane, or other ion conducting bridge. Both compartments generally contain an aqueous solution of an electrolyte. Carbon dioxide gas may be continuously bubbled through the cathodic electrolyte solution to saturate the solution or the solution may be pre-saturated with carbon dioxide.
Referring to
The cell 102 may be implemented as a divided cell. The divided cell may be a divided electrochemical cell and/or a divided photochemical cell. The cell 102 is generally operational to reduce carbon dioxide (CO2) into single-carbon or multi-carbon products. The reduction generally takes place by bubbling carbon dioxide and an aqueous solution of an electrolyte in the cell 102. A cathode 120 in the cell 102 may reduce the carbon dioxide into a product mixture that may include one or more compounds. For instance, the product mixture may include at least one of butanol, formic acid, methanol, glycolic acid, glyoxal, acetic acid, ethanol, acetone, or isopropanol. In a particular implementation, ethanol may be produced with a yield ranging from approximately 4% to 20%. In other implementation, acetic acid may be produced with an approximately 8% yield, without significant detection of other carbon-containing products.
The cell 102 generally comprises two or more compartments (or chambers) 114a-114b, a separator (or membrane) 116, an anode 118, and a cathode 120. The anode 118 may be disposed in a given compartment (e.g., 114a). The cathode 120 may be disposed in another compartment (e.g., 114b) on an opposite side of the separator 116 as the anode 118. An aqueous solution 122 may fill both compartments 114a-114b. The aqueous solution 122 may include water as a solvent and water soluble salts (e.g., potassium chloride (KCl)). A catalyst 124 may be added to the compartment 114b containing the cathode 120.
The catalyst 124 may include catalysts featuring two or more aromatic amine heterocycles that are either fused or become electronically conjugated upon one electron reduction.
Four distinct classes of heterocyclic compounds for use as catalysts in the reduction of carbon dioxide are shown in
Referring to
Experiments may be performed using 4-azabenzimidazole and 7-azaindole in an H-style electrochemical cell outfit with a glass frit separator. In the anode compartment, a commercial mixed metal oxide anode may be used to oxidize water to oxygen. The anode compartment may be filled with 0.5M KCl (aq). The cathode compartment may incorporate a Pt electrode, SCE reference electrode, and an electrolyte consisting of 0.5M KCl (aq) and saturated with either 4-azabenzimidazole or 7-azaindole. In the case of 4-azabenzimidazole, the catholyte may be adjusted to a pH of 3.1 using hydrochloric acid. A CHI 760 potentiostat may be used to hold the cathode potential at −0.65V vs. SCE. Ethanol may be produced at the cathode with a yield ranging from approximately 4% to 20%. In the case of 7-azaindole, the catholyte was adjusted to pH 4 using hydrochloric acid and the potential was held at −0.70V vs. SCE. Carbon dioxide was observed to reduce to acetic acid with an approximately 8% yield without any significant detection of other carbon containing products.
Utilizing heterocycles with a lone heteroatom or lacking a nearby heteroatom (e.g., lutidines, 4,4′ bipyridine, and the like) as electrochemical catalysts, the potentials required for carbon-carbon bond formation may be on the order of between approximately 0.3V to 1.0V higher than those observed with those observed with 4-azabenzimidazole and 7-azaindole. In addition, the catalysts 202 and 204 show high selectivity for a single product at platinum electrodes.
Referring to
Referring to
Referring now to
Referring again to
The power source 106 may implement a variable voltage source. The power source 106 may be operational to generate an electrical potential between the anode 118 and the cathode 120. The electrical potential may be a DC voltage.
The gas source 108 may implement a carbon dioxide source. The source 108 is generally operational to provide carbon dioxide to the cell 102. In some embodiments, the carbon dioxide is bubbled directly into the compartment 114b containing the cathode 120. For instance, the compartment 114b may include a carbon dioxide input, such as a port 126a configured to be coupled between the gas source 108 and the cathode 120.
The first extractor 110 may implement an organic product and/or inorganic product extractor. The extractor 110 is generally operational to extract (separate) one or more products of the product mixture (e.g., methanol, ethanol, acetone, formic acid, formaldehyde, and/or other single-carbon or multiple-carbon product) from the electrolyte 122. The extracted products may be presented through a port 126b of the system 100 for subsequent storage and/or consumption by other devices and/or processes.
The second extractor 112 may implement an oxygen extractor. The second extractor 112 is generally operational to extract oxygen (e.g., O2) byproducts created by the reduction of the carbon dioxide and/or the oxidation of water. The extracted oxygen may be presented through a port 128 of the system 100 for subsequent storage and/or consumption by other devices and/or processes. Chlorine and/or oxidatively evolved chemicals may also be byproducts in some configurations, such as in an embodiment of processes other than oxygen evolution occurring at the anode 118. Such processes may include chlorine evolution, oxidation of organics to other saleable products, waste water cleanup, and corrosion of a sacrificial anode. Any other excess gases (e.g., hydrogen) created by the reduction of the carbon dioxide and water may be vented from the cell 102 via a port 130.
In the reduction of carbon dioxide to products, water may be oxidized (or split) to protons and oxygen at the anode 118 while the carbon dioxide is reduced to the product mixture at the cathode 120. The electrolyte 122 in the cell 102 may use water as a solvent with any salts that are water soluble, including potassium chloride (KCl) and with a suitable catalyst 124, such as catalysts featuring two or more aromatic amine heterocycles that are either fused or become electronically conjugated upon one electron reduction. Such catalysts are described above, with reference to
An anode material sufficient to oxidize or split water may be used. The overall process may be generally driven by the power source 106. Combinations of cathodes 120, electrolytes 122, and catalysts 124 may be used to control the reaction products of the cell 102.
Product selectivity may be obtained by the matching of electrode material, aromatic amine catalyst, electrode potential, or other electrochemical cell condition. For instance, in an electrochemical system having fixed cathodes (e.g., with stainless steel 2205 cathodes), the electrolyte (such as the catholyte) may be altered to change the product mixture. In another instance, such as with a modular electrochemical system having swappable/interchangeable cathodes, the cathode may be altered to change the product mixture. Additionally, the electrochemical system may incorporate a photoelectrochemical cell where the cathode is a light responsive p-type semiconductor or may incorporate a hybrid photoelectrochemical system where the anode is a light responsive n-type semiconductor and the cathode is a metallic electrode or a p-type light responsive semiconductor.
As described herein, the present disclosure may include catalysts for carbon dioxide reduction featuring two or more aromatic amine heterocyclic that are either fused or become electronically conjugated upon one electron reduction. Additionally the catalysts may provide for improved energy efficiency for carbon dioxide reduction to multi-carbon products and for improved selectivity for carbon dioxide reduction to multi-carbon products.
Referring to
In the step 602, water may be introduced to a first compartment of an electrochemical cell. The first compartment may include an anode. Introducing carbon dioxide to a second compartment of the electrochemical cell may be performed in the step 604. The second compartment may include a solution of an electrolyte, a catalyst, and a cathode. The catalyst may include at least two aromatic amine heterocycles that are at least one of (a) fused or (b) configured to become electronically conjugated upon one electron reduction. In the step 606, an electric potential may be applied between the anode and the cathode in the electrochemical cell sufficient for the cathode to reduce the carbon dioxide to a product mixture.
Referring to
In the step 702, carbon dioxide may be introduced to a solution of an electrolyte and a heterocyclic catalyst in an electrochemical cell. The electrochemical cell may include an anode in a first cell compartment and a cathode in a second cell compartment. The heterocyclic catalyst may include at least one of (a) two or more fused aromatic amines, (b) a substituted 4,4′-bipyridine, (c) a naphthyridine, or (d) an aromatic amine alkylating agent. Applying an electrical potential between the anode and the cathode in the electrochemical cell sufficient for the cathode to reduce the carbon dioxide to a product mixture may be performed in the step 704. In the step 706, a yield of the product mixture may be varied by adjusting at least one of (a) a material of the cathode, (b) a type of the heterocyclic catalyst, (c) and the electrical potential of the cathode.
It is believed that the present disclosure and many of its attendant advantages will be understood by the foregoing description, and it will be apparent that various changes may be made in the form, construction and arrangement of the components thereof without departing from the scope and spirit of the disclosure or without sacrificing all of its material advantages. The form herein before described being merely an explanatory embodiment thereof, it is the intention of the following claims to encompass and include such changes.
The present application claims the benefit under 35 U.S.C. §119(e) of U.S. Patent Application Ser. No. 61/428,528, filed Dec. 30, 2010. The above-listed application is hereby incorporated by reference in its entirety.
This invention was made with government support under Grant CHE-0911114 awarded by the National Science Foundation. The government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 61428528 | Dec 2010 | US |
