AU2011282767C1 - Reducing carbon dioxide to products - Google Patents

Abstract

A method for reducing carbon dioxide to one or more products is disclosed. The method may include steps (A) to (C). Step (A) may bubble the carbon dioxide into a solution of an electrolyte and a catalyst in a divided electrochemical cell. The divided electrochemical cell may include an anode in a first cell compartment and a cathode in a second cell compartment. The cathode generally reduces the carbon dioxide into the products. Step {B} may vary at least one of (i) which of the products is produced and (ii) a faradaic yield of the products by adjusting one or more of (a) a cathode material and (b) a surface morphology of the cathode. Step (C) may separate the products from the solution.

Description

WO 2012/015905 PCT/US2011/045515 REDUCING CARBON DIOXIDE TO PRODUCTS 5 10 Field of the Invention The present invention relates to chemical reduction generally and, more particularly, to a method and/or apparatus for implementing reducing carbon dioxide to products. 15 Background of the Invention The combustion of fossil fuels in activities such as the electricity generation, transportation, and manufacturing produces billions of tons of carbon dioxide annually. Research since the 20 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. 25 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 30 that can be stored for later use will be possible. Electrochemical 1 WO 2012/015905 PCT/US2011/045515 and photochemical pathways are means for the carbon dioxide conversion. Previous work in the field has many limitations, including the stability of systems used in the process, the 5 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. No commercially available solutions for converting carbon dioxide to 10 economically valuable fuels or industrial chemicals currently exist. Laboratories around the world have attempted for many years to use electrochemistry and/or photochemistry to convert carbon dioxide to economically valuable products. Hundreds of publications exist on the subject, starting with work in the 19th 15 century. Much of the work done prior to 1999 is summarized in "Greenhouse Gas Carbon Dioxide Mitigation Science and Technology", by Halmann and Steinberg. A more recent overview of work on electrochemical means of reducing carbon dioxide is "Electrochemical Carbon Dioxide Reduction - Fundamental and Applied 20 Topics (Review)", by Maria Jitaru in Journal of the University of Chemical Technology and Metallurgy, 2007, pages 333-344. Laboratory electrochemical methods usually involve a small (i.e., <1 liter) glass cell containing electrodes and an aqueous solution with supporting electrolyte in which carbon 25 dioxide is bubbled, though a solvent other than water can be used. Reduction of the carbon dioxide takes place directly on the cathode or via a mediator in the solution that is either a transition metal or a transition metal complex. Photoelectrochemical methods also incorporate aqueous solutions with supporting electrolyte in which 30 carbon dioxide is bubbled. The main difference is that some or all 2 WO 2012/015905 PCT/US2011/045515 of the energy for reducing the carbon dioxide comes from sunlight. The reduction of the carbon dioxide takes place on a photovoltaic material or on a catalyst photosensitized by a dye. All systems developed to date have failed to make commercial systems for the 5 reasons outlined above. The systems developed in laboratories could not be scaled to commercial or industrial size because of various performance limitations. Existing electrochemical and photochemical processes/ systems have one or more of the following problems that prevent 10 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 15 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 20 water. Many cathodes make a mix of organic products. For instance, copper produces a mix of gases and liquids including carbon monoxide, methane, formic acid, ethylene and ethanol. A mix of products makes extraction and purification of the products costly and can result in undesirable waste products that must be 25 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 30 reduction techniques have very low rates of reaction. For example, 3 some commercial systems have current densities in excess of 100 milliamperes per centimeter squared (mA/cm 2 ), while rates achieved in the laboratory are orders of magnitude less. A reference herein to a patent document or other matter 5 which is given as prior art is not to be taken as an admission that that document or matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims. 10 Summary of the Invention The present invention concerns a method for reducing carbon dioxide to one or more products. The method may include steps (A) to (C) . Step (A) may bubble the carbon dioxide into a solution of an electrolyte and a catalyst in a divided 15 electrochemical cell. The divided electrochemical cell may include an anode in a first cell compartment and a cathode in a second cell compartment. The cathode generally reduces the carbon dioxide into the products. Step (B) may vary at least one of (i) which of the products is produced and (ii) a 20 faradaic yield of the products by adjusting one or more of (a) a cathode material and (b) a surface morphology of the cathode. Step (C) may separate the products from the solution. The aspects, features and advantages of the present invention include providing a method and/or apparatus for 25 implementing reducing carbon dioxide to products that may (i) catalytically reduce carbon dioxide using steel cathodes or other low cost cathode materials, (ii) produce high faradaic yields (e.g., >20%), (iii) produce organic products with steel and nickel alloy cathodes at ambient temperature and pressure, 30 (iv) provide stabile long-term reduction of carbon dioxide using copper-based alloy electrodes and/or (v) provide for commercialization of electrochemical carbon dioxide reduction. 4 In one aspect, the present invention provides a method for reducing carbon dioxide to formic acid, comprising the steps of: (A) bubbling said carbon dioxide into a solution of an electrolyte and a catalyst in a divided electrochemical cell, 5 said catalyst selected from the group consisting of imidazole, substituted imidazole, thiazole and adenine, wherein (i) said divided electrochemical cell comprises an anode in a first cell compartment and a cathode in a second cell compartment and (ii) said cathode reducing said carbon dioxide into formic acid, 10 said cathode comprising a material selected from Nb, Co, Ni, Sn, and stainless steel; (B) varying a faradaic yield of said formic acid by (B1) adjusting one or more of (a) a cathode material and (b) a surface morphology of said cathode, or (B2) adjusting one or more of (a) said electrolyte and (b) a manner 15 in which carbon dioxide is bubbled, or (B3) adjusting one or more of (a) a pH level of said solution and (b) an electrical potential; and (C) separating said formic acid from said solution. 4a Brief Description of the Drawings These and other aspects, features and advantages of the present invention will be apparent from the following detailed description and the appended claims and drawings in which: 5 FIG. 1 is a block diagram of a system in accordance with a preferred embodiment of the present invention; FIGS. 2A-2C are tables illustrating relative product yields for different cathode material, catalyst, electrolyte and pH level combinations; 10 FIG. 3 is a formula of an aromatic heterocyclic amine catalyst; FIGS. 4-6 are formulae of substituted or unsubstituted aromatic 5-member heterocyclic amines or 6-member heterocyclic amines; 15 FIG. 7 is a flow diagram of an example method used in electrochemical examples; and FIG. 8 is a flow diagram of an example method used in photochemical examples. 20 Detailed Description of the Preferred Embodiments In accordance with some embodiments of the present invention, an electro-catalytic system is provided that generally allows carbon dioxide to be converted at modest overpotentials to highly reduced species in an aqueous 25 solution. Some embodiments generally relate to simple, efficient and economical conversion of carbon dioxide to reduced organic products, such as methanol, formic acid and formaldehyde. Inorganic products such as polymers may also be formed. Carbon-carbon bonds and/or carbon-hydrogen bonds may 30 be formed in the aqueous solution under mild conditions 5 WO 2012/015905 PCT/US2011/045515 utilizing a minimum of energy. In some embodiments, the energy used by the system may be generated from an alternative energy source or directly using visible light, depending on how the system is implemented. 5 The reduction of carbon dioxide may be suitably catalyzed by aromatic heterocyclic amines (e.g., pyridine, imidazole and substituted derivatives) . Simple organic compounds have been found to be effective and stable homogenous electrocatalysts and photoelectrocatalysts for the aqueous multiple electron, multiple 10 proton reduction of carbon dioxide to organic products, such as formic acid, formaldehyde and methanol. For production of methanol, the reduction of carbon dioxide may proceed along a 6 electron (e-) transfer pathway. High faradaic yields for the reduced products have generally been found in both electrochemical 15 and photoelectrochemical systems at low reaction overpotentials. Metal-derived multi-electron transfer was previously thought to achieve highly reduced products such as methanol. Currently, simple aromatic heterocyclic amine molecules may be capable of producing many different chemical species on route to 20 methanol through multiple electron transfers, instead of metal-based multi-electron transfers. Some embodiments of the present invention thus relate to environmentally beneficial methods for reducing carbon dioxide. The methods generally include electrochemically and/or 25 photoelectrochemically reducing the carbon dioxide in an aqueous, electrolyte-supported divided electrochemical cell that includes an anode (e.g., an inert conductive counter electrode) in a cell compartment and a conductive or p-type semiconductor working cathode electrode in another cell compartment. A catalyst may be 30 included to produce a reduced product. Carbon dioxide may be 6 WO 2012/015905 PCT/US2011/045515 continuously bubbled through the cathode electrolyte solution to saturate the solution. For electrochemical reductions, the electrode may be a suitable conductive electrode, such as Al, Au, Ag, C, Cd, Co, Cr, 5 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 p-Si, degenerately doped p-Si:As and degenerately doped p-Si:B. Other 10 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-GaInP 2 and p-Si. Other semiconductor electrodes may be implemented to meet the criteria of a particular 15 application. The catalyst for conversion of carbon dioxide electrochemically or photoelectrochemically may be a substituted or unsubstituted aromatic heterocyclic amine. Suitable amines are generally heterocycles which may include, but are not limited to, 20 heterocyclic compounds that are 5-member or 6-member rings with at least one ring nitrogen. For example, pyridines, imidazoles and related species with at least one five-member ring, bipyridines (e.g., two connected pyridines) and substituted derivatives were generally found suitable as catalysts for the electrochemical 25 reduction and/or the photoelectrochemical reduction. Amines that have sulfur or oxygen in the rings may also be suitable for the reductions. Amines with sulfur or oxygen may include thiazoles or oxazoles. Other aromatic amines (e.g., quinolines, adenine, azoles, indoles, benzimidazole and 1,10-phenanthroline) may also be 30 effective electrocatalysts. 7 WO 2012/015905 PCT/US2011/045515 Carbon dioxide may be photochemically or electrochemically reduced to formic acid with formaldehyde and methanol being formed in smaller amounts. Catalytic hydrogenation of carbon dioxide using heterogeneous catalysts generally provides 5 methanol together with water as well as formic acid and formaldehyde. The reduction of carbon dioxide to methanol with complex metal hydrides, such as lithium aluminum hydrides, may be costly and therefore problematic for bulk production of methanol. Current reduction processes are generally highly energy-consuming 10 and thus are not efficient ways for a high yield, economical conversion of carbon dioxide to various products. On the other hand, the use of processes for converting carbon dioxide to reduced organic and/or inorganic products in accordance with some embodiments of the invention generally has the 15 potential to lead to a significant reduction of carbon dioxide, a major greenhouse gas, in the atmosphere and thus to the mitigation of global warming. Moreover, some embodiments may advantageously produce methanol and related products without adding extra reactants, such as a hydrogen source. The resultant product 20 mixture may use little in the way of further treatment. For example, a resultant 1 molar (M) methanol solution may be used directly in a fuel cell. For other uses, simple removal of the electrolyte salt and water may be readily accomplished. Before any embodiments of the invention are explained in 25 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, 30 it is to be understood that the phraseology and terminology used 8 WO 2012/015905 PCT/US2011/045515 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 5 well as additional items. Further, unless otherwise noted, technical terms may be used according to conventional usage. In the following description of methods, process steps may be carried out over a range of temperatures (e.g., approximately 10'C (Celsius) to 50'C) and a range of pressures 10 (e.g., approximately 1 to 10 atmospheres) unless otherwise specified. Numerical ranges recited herein generally include all values from the lower value to the upper value (e.g., all possible combinations of numerical values between the lowest value and the highest value enumerated are considered expressly stated) . For 15 example, if a concentration range or beneficial effect range is stated as 1% to 50%, it is intended that values such as 2% to 40%, 10% to 30%, or 1% to 3%, etc., are expressly enumerated. The above may be simple examples of what is specifically intended. A use of electrochemical or photoelectrochemical 20 reduction of carbon dioxide, tailored with certain .electrocatalysts, may produce methanol and related products in a high yield of about 60% to about 100%, based on the amount of carbon dioxide, suitably about 75% to 90%, and more suitably about 85% to 95%. At an electric potential of about -0.50 to -2 volts 25 (V) with respect to a saturated calomel electrode (SCE), methanol may be produced with good faradaic efficiency at the cathode. An example of an overall reaction for the reduction of carbon dioxide may be represented as follows:

CO

2 + 2 H 2 0 - CH 3 0H + 3/2 02 9 WO 2012/015905 PCT/US2011/045515 For a 6 e- reduction, the reactions at the cathode and anode may be represented as follows:

CO

2 + 6 H+ + 6 e- - CH 3 0H + H 2 0 (cathode) 3 H 2 0 - 3/2 02 + 6 H + 6 e- (anode) 5 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 that is an inert counter electrode and (ii) another compartment contains a working cathode electrode and a catalyst. 10 The compartments may be separated by a porous glass frit 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. 15 In the working electrode compartment, carbon dioxide may be continuously bubbled through the solution. In some embodiments, if the working electrode is a conductor, an external bias may be impressed across the cell such that the potential of the working electrode is held constant. In other embodiments, if the working 20 electrode is a p-type semiconductor, the electrode may be suitably illuminated with light. An energy of the light may be matching or greater than a bandgap of the semiconductor during the electrolysis. Furthermore, either no external source of electrical energy may be used or a modest bias (e.g., about 500 millivolts) 25 may be applied. The working electrode potential is generally held constant relative to the SCE. The electrical energy for the electrochemical reduction of carbon dioxide may come from a normal energy source, including nuclear and alternatives (e.g., hydroelectric, wind, solar power, geothermal, etc.), from a solar 30 cell or other nonfossil fuel source of electricity, provided that 10 the electrical source supply at least 1.6 volts across the cell. Other voltage values may be adjusted depending on the internal resistance of the cell employed. Advantageously, the carbon dioxide may be obtained from 5 any sources (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 10 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 nearly pure carbon dioxide may be exhausted from cement factories and from fermenters used for 15 industrial fermentation of ethanol. 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 has been disclosed. Thus, 20 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. For electrochemical conversions, the carbon dioxide may be 25 readily reduced in an aqueous medium with a conductive electrode. Faradaic efficiencies have been found high, some reaching about 100%. For photoelectrochemical conversions, the carbon dioxide may be readily reduced with a p-type semiconductor electrode, such as p-GaP, p-GaAs, p-InP, p-InN, 30 p-WSe 2 , p-CdTe, p-GaInP 2 and p-Si. 11 WO 2012/015905 PCT/US2011/045515 The electrochemical/photoelectrochemical reduction of the carbon dioxide generally utilizes one or more catalysts in the aqueous solution. Aromatic heterocyclic amines may include, but are not limited to, unsubstituted and substituted pyridines and 5 imidazoles. Substituted pyridines and imidazoles may include, but are not limited to mono and disubstituted pyridines and imidazoles. For example, suitable catalysts may include straight chain or branched chain lower alkyl (e.g., Cl-C10) mono and disubstituted compounds such as 2-methylpyridine, 4-tertbutyl pyridine, 2,6 10 dimethylpyridine (2,6-lutidine); bipyridines, such as 4,4'-bipyridine; amino-substituted pyridines, such as 4 dimethylamino pyridine; and hydroxyl-substituted pyridines (e.g., 4-hydroxy-pyridine) and substituted or unsubstituted quinoline or isoquinolines. The catalysts may also suitably include substituted 15 or unsubstituted dinitrogen heterocyclic amines, such as pyrazine, pyridazine and pyrimidine. Other catalysts generally include azoles, imidazoles, indoles, oxazoles, thiazoles, substituted species and complex multi-ring amines such as adenine, pterin, pteridine, benzimidazole, phenonthroline and the like. 20 Referring to FIG. 1, a block diagram of a system 100 is shown in accordance with a preferred embodiment of the present invention. The system (or apparatus) 100 generally comprises a cell (or container) 102, a liquid source 104, a power source 106, a gas source 108, an extractor 110 and an extractor 112. A product 25 may be presented from the extractor 110. An output gas may be presented from the extractor 112. Another output gas may be presented from the cell 102. The cell 102 may be implemented as a divided cell. The divided cell may be a divided electrochemical cell and/or a divided 30 photochemical cell. The cell 102 is generally operational to 12 WO 2012/015905 PCT/US2011/045515 reduce carbon dioxide (C0 2 ) and protons into one or more organic products and/or inorganic products. The reduction generally takes place by bubbling carbon dioxide into an aqueous solution of an electrolyte in the cell 102. A cathode in the cell 102 may reduce 5 the carbon dioxide into one or more compounds. 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 10 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. A catalyst 124 may be added to the compartment 114b containing the cathode 120. The liquid source 104 may implement a water source. The 15 liquid source 104 may be operational to provide pure water to the cell 102. The power source 106 may implement a variable voltage source. The source 106 may be operational to generate an electrical potential between the anode 118 and the cathode 120. 20 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 25 120. The extractor 110 may implement an organic product and/or inorganic product extractor. The extractor 110 is generally operational to extract (separate) products (e.g., formic acid, acetone, glyoxal, isopropanol, formaldehyde, methanol, polymers and 30 the like) from the electrolyte 122. The extracted products may be 13 WO 2012/015905 PCT/US2011/045515 presented through a port 126 of the system 100 for subsequent storage and/or consumption by other devices and/or processes. The extractor 112 may implement an oxygen extractor. The extractor 112 is generally operational to extract oxygen (e.g., 02) 5 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 10 configurations. The organic pollutants may be rendered harmless by oxidization. Any other excess gases (e.g., hydrogen) created by the reduction of the carbon dioxide may be vented from the cell 102 via a port 130. In the process described, water may be oxidized (or 15 split) to protons and oxygen at the anode 118 while the carbon dioxide is reduced to organic products at the cathode 120. The electrolyte 122 in the cell 102 may use water as a solvent with any salts that are water soluble and with a pyridine or pyridine-derived catalyst 124. The catalysts 124 may include, but 20 are not limited to, nitrogen, sulfur and oxygen containing heterocycles. Examples of the heterocyclic compounds may be pyridine, imidazole, pyrrole, thiazole, furan, thiophene and the substituted heterocycles such as amino-thiazole and benzimidazole. Cathode materials generally include any conductor. Any anode 25 material may be used. The overall process is generally driven by the power source 106. Combinations of cathodes 120, electrolytes 122, catalysts 124, introduction of carbon dioxide to the cell 102, pH levels and electric potential from the power source 106 may be used to control the reaction products of the cell 102. Organic 30 products and inorganic products resulting from the reaction may 14 WO 2012/015905 PCT/US2011/045515 include, but are not limited to, acetaldehyde, acetone, carbon, carbon monoxide, carbonates, ethanol, ethylene, formaldehyde, formic acid, glyoxal, glyoxylic acid, graphite, isopropanol, methane, methanol, oxalate, oxalic acid and/or carbon dioxide 5 containing polymers. In some nonaqueous embodiments, the solvent may include methanol, acetonitrile, and/or other nonaqueous solvents. The electrolytes 122 generally include tetraalkyl ammonium salts and a heterocyclic catalyst. A primary product may be oxalate in a 10 completely nonaqueous system. In a system containing a nonaqueous catholyte and an aqueous anolyte, the products generally include all of the products seen in aqueous systems with higher yields. Experiments were conducted in one, two and three compartment electrochemical cells 102 with an SCE as the reference 15 electrode. The experiments were generally conducted at ambient temperature and pressure. Current densities were observed to increase with increased temperature, but the experiments were generally operated at ambient temperature for best efficiency. Carbon dioxide was bubbled into the cells during the experiments. 20 A potentiostat or DC power supply 106 provided the electrical energy to drive the process. Cell potentials ranged from 2 volts to 4 volts, depending on the cathode material. Half cell potentials at the cathode ranged from -0.7 volts to -2 volts relative to the SCE, depending on the cathode material used. 25 Products from the experiments were analyzed using gas chromatography and a spectrometer. The process is generally controlled to get a desired product by using combinations of specific cathode materials, catalysts, electrolytes, surface morphology of the electrodes, 30 introduction of reactants relative to the cathode, adjusting pH 15 WO 2012/015905 PCT/US2011/045515 levels and/or adjusting electrical potentials. Faradaic yields for the products generally range from less than 1% to more than 90% with the remainder being hydrogen, though methane, carbon monoxide and/or ethylene may also be produced as gaseous byproducts. 5 Referring to FIGS. 2A-2C, tables illustrating relative product yields for different cathode material, catalyst, electrolyte, pH level and cathode potential combinations are shown. The combinations listed in the tables generally are not the only combinations providing a given product. The combinations 10 illustrated may demonstrate high yields of the products at the lowest potential. The cathodes tested generally include all conductive elements on the periodic table, steels, nickel alloys, copper alloys such as brass and bronze and elgiloy. Most of the conductors may be used with heterocyclic catalysts 124 to reduce 15 the carbon dioxide. The products created may vary based on which cathode material is used. For instance, a W cathode 120 with pyridine catalyst 124 may give acetone as a product whereas a Sn cathode 120 with pyridine may primarily give formic acid and methanol as products. A product yield may also be changed by the 20 manner in which the carbon dioxide was bubbled into the cell 102. For instance, with a stainless steel 2205 cathode 120 in a KCl electrolyte 122, if the carbon dioxide bubbles directly hit the cathode 120, the product mix may switch to methanol and isopropanol, rather than formic acid and acetone when the carbon 25 dioxide bubbles miss the cathode 120. Cell design and cathode treatment (e.g., surface morphology or surface texture) may affect both product yields and current density at the cathode. For instance, a divided cell 102 with a stainless steel 2205 cathode 120 in a KC1 electrolyte 122 30 generally has higher yields with a heavily scratched (rough) 16 cathode 120 than an unscratched (smooth) cathode 120. Matte tin generally performs different than bright tin. Maintaining carbon dioxide bubbling only on the cathode side of the divided cell 102 (e.g., in compartment 114b) may also increase yields. 5 Raising or lowering the cathode potential may also alter the reduced products. For instance, ethanol is generally evolved at lower potentials between -0.8 volts and -1 volt using the duplex steel/pyridine/KC1, while methanol is favored beyond -1 volt. 10 Faradaic yields for the products may be improved by controlling the electrical potential of the reaction. By maintaining a constant potential at the cathode 120, hydrogen evolution is generally reduced and faradaic yields of the products increased. Addition of hydrogen inhibitors, such as 15 acetonitrile, certain heterocycles, alcohols, and other chemicals may also increase yields of the products. With some embodiments, stability may be improved with cathode materials disclosed to poison rapidly when reducing carbon dioxide. Copper and copper-alloy electrodes commonly 20 poison in less than an hour of electrochemically reducing carbon dioxide. However, when used with a heterocyclic amine catalyst, copper-based alloys were operated for many hours without any observed degradation in effectiveness. The effects were particularly enhanced by using sulfur containing 25 heterocycles. For instance, a system with a copper cathode and 2-amino thiazole catalyst showed very high stability for the reduction of carbon dioxide to carbon monoxide and formic acid. Heterocycles other than pyridine may catalytically reduce carbon dioxide in the electrochemical process using many 30 aforementioned cathode materials, including tin, steels, nickel alloys and copper alloys. Nitrogen-containing heterocyclic amines 17 WO 2012/015905 PCT/US2011/045515 shown to be effective include azoles, indoles, 4, 4'-bipyridines, picolines (methyl pyridines), lutidines (dimethyl pyridines), hydroxy pyridines, imidazole, benzimidazole, methyl imidazole, pyrazine, pyrimidine, pyridazine, pyridazineimidazole, nicotinic 5 acid, quinoline, adenine and 1,10-phenanthroline. Sulfur containing heterocycles include thiazole, aminothiazoles, thiophene. Oxygen containing heterocycles include furan and oxazole. As with pyridine, the combination of catalyst, cathode material and electrolyte may be used to control product mix. 10 Some process embodiments of the present invention for making/converting hydrocarbons generally consume a small amount of water (e.g., approximately I to 3 moles of water) per mole of carbon. Therefore, the processes may be a few thousand times more water efficient than existing production techniques. 15 Referring to FIG. 3, a formula of an aromatic heterocyclic amine catalyst is shown. The ring structure may be an aromatic 5-member heterocyclic ring or 6-member heterocyclic ring with at least one ring nitrogen and is optionally substituted at one or more ring positions other than nitrogen with R. L may be C 20 or N. R1 may be H. R2 may be H if L is N or R2 is R if L is C. R is an optional substitutent on any ring carbon and may be independently selected from H, a straight chain or branched chain lower alkyl, hydroxyl, amino, pyridyl, or two R's taken together with the ring carbons bonded thereto are a fused six-member aryl 25 ring and n = 0 to 4. Referring to FIGS. 4-6, formulae of substituted or unsubstituted aromatic 5-member heterocyclic amines or 6-member heterocyclic amines are shown. Referring to FIG. 4, R3 may be H. R4, R5, R7 and R8 are generally independently H, straight chain or 30 branched chain lower alkyl, hydroxyl, amino, or taken together are 18 WO 2012/015905 PCT/US2011/045515 a fused six-member aryl ring. R6 may be H, straight chain or branched chain lower alkyl, hydroxyl, amino or pyridyl. Referring to FIG. 5, one of L1, L2 and L3 may be N, while the other L's may be C. R9 may be H. If Li is N, R10 may be H. 5 If L2 is N, R11 may be H. If L3 is N, R12 may be H. If L1, L2 or L3 is C, then R10, R11, R12, R13 and R14 may be independently selected from straight chain or branched chain lower alkyl, hydroxyl, amino, or pyridyl. Referring to FIG. 6, R15 and R16 may be H. R17, R18 and 10 R19 are generally independently selected from straight chain or branched chain lower alkyl, hydroxyl, amino, or pyridyl. Suitably, the concentration of aromatic heterocyclic amine catalysts is about 1 millimolar (mM) to 1 M. The electrolyte may be suitably a salt, such as KCi, NaNO 3 , Na 2

SO

4 , NaCl, NaF, 15 NaClO 4 , KClO 4 , K 2 SiO 3 , or CaCl 2 at a concentration of about 0.5 M. Other electrolytes may include, but are not limited to, all group 1 cations (e.g., H, Li, Na, K, Rb and Cs) except Francium (Fr), Ca, ammonium cations, alkylammonium cations and alkyl amines. Additional electrolytes may include, but are not limited to, all 20 group 17 anions (e.g., F, Cl, Br, I and At), borates, carbonates, nitrates, nitrites, perchlorates, phosphates, polyphosphates, silicates and sulfates. Na generally performs as well as K with regard to best practices, so NaCl may be exchanged with KC1. NaF may perform about as well as NaCl, so NaF may be exchanged for NaCl 25 or KCT in many cases. Larger anions tend to change the chemistry and favor different products. For instance, sulfate may favor polymer or methanol production while Cl may favor products such as acetone. The pH of the solution is generally maintained at about pH 3 to 8, suitably about 4.7 to 5.6. 19 WO 2012/015905 PCT/US2011/045515 At conductive electrodes, formic acid and formaldehyde were found to be intermediate products along the pathway to the 6 e- reduced product of methanol, with an aromatic amine radical (e.g., the pyridinium radical, playing a role in the reduction of 5 both intermediate products). The intermediate products have generally been found to also be the final products of the reduction of carbon dioxide at conductive electrodes or p-type semiconductor electrodes, depending on the particular catalyst used. Other C-C couple products may also be possible. For example, reduction of 10 carbon dioxide may suitably yield formaldehyde, formic acid, glyoxal, methanol, isopropanol, or ethanol, depending on the particular aromatic heterocyclic amine used as the catalyst. The products of the reduction of carbon dioxide are generally substitution-sensitive. As such, the products may be selectively 15 produced. For example, use of 4,4'-bipyridine as the catalyst may produce methanol and/or 2-propanol. Lutidines and amino substituted pyridines may produce 2-propanol. Hydroxy-pyridine may produce formic acid. The effective electrochemical/photoelectrochemical 20 reduction of carbon dioxide disclosed herein may provide new methods of producing methanol and other related products in an improved, efficient, and environmentally beneficial way, while mitigating carbon dioxide-caused climate change (e.g., global warming) . Moreover, the methanol product of reduction of carbon 25 dioxide may be advantageously used as (1) a convenient energy storage medium, which allows convenient and safe storage and handling, (2) a readily transported and dispensed fuel, including for methanol fuel cells and (3) a feedstock for synthetic hydrocarbons and corresponding products currently obtained from oil 30 and gas resources, including polymers, biopolymers and even 20 WO 2012/015905 PCT/US2011/045515 proteins, that may be used for animal feed or human consumption. Importantly, the use of methanol as an energy storage and transportation material generally eliminates many difficulties of using hydrogen for such purposes. The safety and versatility of 5 methanol generally makes the disclosed reduction of carbon dioxide further desirable. Some embodiments of the present invention may be further explained by the following examples, which should not be construed by way of limiting the scope of the invention. 10 Example 1: General Electrochemical Methods. Chemicals and materials. All chemicals used were > 98% purity and used as received from the vendor (e.g., Aldrich), without further purification. Either deionized or high purity water (Nanopure, Barnstead) was used to prepare the aqueous 15 electrolyte solutions. Electrochemical system. The electrochemical system was composed of a standard two-compartment electrolysis cell 102 to separate the anode 118 and cathode 120 reactions. The compartments were separated by a porous glass frit or other ion conducting 20 bridge 116. The electrolytes 122 were used at concentrations of 0.1 M to 1 M, with 0.5 M being a typical concentration. A concentration of between about 1 mM to 1 M of the catalysts 124 were used. The particular electrolyte 122 and particular catalyst 124 of each given test were generally selected based upon what 25 product or products were being created. Referring to FIG. 7, a flow diagram of an example method 140 used in the electrochemical examples is shown. The method (or process) 140 generally comprises a step (or block) 142, a step (or block) 144, a step (or block) 146, a step (or block) 148 and a step 21 WO 2012/015905 PCT/US2011/045515 (or block) 150. The method 140 may be implemented using the system 100. In the step 142, the electrodes 118 and 120 may be activated where appropriate. Bubbling of the carbon dioxide into 5 the cell 102 may be performed in the step 144. Electrolysis of the carbon dioxide into organic and/or inorganic products may occur during step 146. In the step 148, the products may be separated from the electrolyte. Analysis of the reduction products may be performed in the step 150. 10 The working electrode was of a known area. All potentials were measured with respect to a saturated calomel reference electrode (Accumet) . Before and during all electrolysis, carbon dioxide (Airgas) was continuously bubbled through the electrolyte to saturate the solution. The resulting pH of the 15 solution was maintained at about pH 3 to pH 8 with a suitable range depending on what product or products were being made. For example, under constant carbon dioxide bubbling, the pH levels of 10 mM solutions of 4-hydroxy pyridine, pyridine and 4-tertbutyl pyridine were 4.7, 5.28 and 5.55, respectively. For Nuclear 20 Magnetic Resonance (NMR) experiments, isotopically enriched 3 3 C NaHCO 3 (99%) was obtained from Cambridge Isotope Laboratories, Inc. Example 2: General Photoelectrochemical Methods. Chemicals and materials. All chemicals used were analytical grade or higher. Either deionized or high purity water 25 (Nanopure, Barnstead) was used to prepare the aqueous electrolyte solutions. Photoelectrochemical system. The photoelectrochemical system was composed of a Pyrex three-necked flask containing 0.5 M KCl as supporting electrolyte and a 1 mM to 1 M catalyst (e.g., 10 30 mM pyridine or pyridine derivative). The photocathode was a single 22 WO 2012/015905 PCT/US2011/045515 crystal p-type semiconductor etched for approximately 1 to 2 minutes in a bath of concentrated HNO 3 :HCl, 2:1 v/v prior to use. An ohmic contact was made to the back of the freshly etched crystal using an indium/zinc (2 wt. % Zn) solder. The contact was 5 connected to an external lead with conducting silver epoxy (Epoxy Technology H31) covered in glass. tubing and insulated using an epoxy cement (Loctite 0151 Hysol) to expose only the front face of the semiconductor to solution. All potentials were referenced against a saturated calomel electrode (Accumet) . The three 10 electrode assembly was completed with a carbon rod counter electrode to minimize the reoxidation of reduced carbon dioxide products. During all electrolysis, carbon dioxide gas (Airgas) was continuously bubbled through the electrolyte to saturate the solution. The resulting pH of the solution was maintained at about 15 pH 3 to 8 (e.g., pH 5.2). Referring to FIG. 8, a flow diagram of an example method 160 used in the photochemical examples is shown. The method (or process) 160 generally comprises a step (or block) 162, a step (or block) 164, a step (or block) 166, a step (or block) 168 and a step 20 (or block) 170. The method 160 may be implemented using the system 100. In the step 162, the photoelectrode may be activated. Bubbling of the carbon dioxide into the cell 102 may be performed in the step 164. Electrolysis of the carbon dioxide into the 25 products may occur during step 166. In the step 168, the products may be separated from the electrolyte. Analysis of the reduction products may be performed in the step 170. Light sources. Four different light sources were used for the illumination of the p-type semiconductor electrode. For 30 initial electrolysis experiments, a Hg-Xe arc lamp (USHIO UXM 200H) 23 WO 2012/015905 PCT/US2011/045515 was used in a lamp housing (PTI Model A-1010) and powered by a PTI LTS-200 power supply. Similarly, a Xe arc lamp (USHIC UXL 151H) was used in the same housing in conjunction with a PTI monochromator to illuminate the electrode at various specific 5 wavelengths. A fiber optic spectrometer (Ocean Optics S2000) or a silicon photodetector (Newport 818-SL silicon detector) was used to measure the relative resulting power emitted through the monochromator. The flatband potential was obtained by measurements 10 of the open circuit photovoltage during various irradiation intensities using the 200 watt (W) Hg-Xe lamp (3 W/cm2 - 23 W/cm2 The photovoltage was observed to saturate at intensities above approximately 6 W/cm 2 . For quantum yield determinations, electrolysis was 15 performed under illumination by two different light-emitting diodes (LEDs). A blue LED (Luxeon V Dental Blue, Future Electronics) with a luminous output of 500 milliwatt (mW) +/- 50 mW at 465 nanometers (nm) and a 20 n full width at half maximum (FWHM) was driven at to a maximum rated current of 700 mA using a Xitanium Driver (Advance 20 Transformer Company). A Fraen collimating lens (Future Electronics) was used to direct the output light. The resultant power density that reached the window of the photoelectrochemical cell was determined to be 42 mW/cm2, measured using a Scientech 364 thermopile power meter and silicon photodetector. The measured 25 power density was assumed to be greater than the actual power density observed at the semiconductor face due to luminous intensity loss through the solution layer between the wall of the photoelectrochemical cell and the electrode. 24 WO 2012/015905 PCT/US2011/045515 Example 3: Analysis of Products of Electrolysis. Electrochemical experiments were generally performed using a CH Instruments potentiostat or a DC power supply with current logger to run bulk electrolysis experiments. The CH 5 Instruments potentiostat was generally used for cyclic voltammetry. Electrolysis was run under potentiostatic conditions from approximately 6 hours to 30 hours until a relatively similar amount of charge was passed for each run. Gas Chromatography. The electrolysis samples were 10 analyzed using a gas chromatograph (HP 5890 GC) equipped with a FID detector. Removal of the supporting electrolyte salt was first achieved with an Amberlite IRN-150 ion exchange resin (cleaned prior to use to ensure no organic artifacts by stirring in a 0.1% v/v aqueous solution of Triton X-100, reduced (Aldrich), filtered 15 and rinsed with a copious amount of water, and vacuum dried below the maximum temperature of the resin (approximately 600C) before the sample was directly injected into the GC which housed a DB-Wax column (Agilent Technologies, 60 m, 1 micrometer (pm) film thickness) . Approximately 1 gram of resin was used to remove the 20 salt from 1 milliliter (mL) of the sample. The injector temperature was held at 2000C, the oven temperature maintained at 1200C, and the detector temperature at 2000C. Spectrophotometry. The presence of formaldehyde and formic acid was also determined by the chromotropic acid assay. 25 Briefly, a solution of 0.3 g of 4,5-dihydroxynaphthalene 2,7-disulfonic acid, disodium salt dihydrate (Aldrich) was dissolved in 10 mL deionized water before diluting to 100 mL with concentrated sulfuric acid. For formaldehyde, an aliquot of 1.5 mL was then added to 0.5 mL of the sample. The presence of 30 formaldehyde (absorbency at 577 nm) was detected against a standard 25 WO 2012/015905 PCT/US2011/045515 curve using an HP 8453 UV-Vis spectrometer. For formic acid, a 0.5 mL aliquot of sample was first reduced with an approximately 100 mg piece of Mg wire and 0.5 mL concentrated hydrochloric acid (added slowly in aliquots over a 10 minute period) to convert to 5 formaldehyde before following the chromotropic acid assay as described above. Mass spectrometry. Mass spectral data was also collected to identify all organic compounds. In a typical experiment, the sample was directly leaked into an ultrahigh vacuum chamber and 10 analyzed by an attached SRS Residual Gas Analyzer (with the ionizer operating at 70 electron-volts and an emission current of 1 mA). Samples were analyzed against standard methanol spectra obtained at the same settings to ensure comparable fragmentation patterns. Mass spectral data confirmed the presence of methanol and proved 15 that the initial solution before electrolysis contained no reduced

CO

2 species. Control experiments also showed that after over 24 hours under illumination the epoxy used to insulate the backside of the electrode did not leach any organic material that would give false results for the reduction of C0 2 . NMR spectra of electrolyte 20 volumes after illumination were obtained using an automated Bruker UltrashieldTM 500 Plus spectrometer with an excitation sculpting pulse technique for water suppression. Data processing was achieved using MestReNova software. For methanol standards and electrolyte samples, the representative signal for methanol was 25 observed between 3.18 to 3.30 parts per million (ppm). Nuclear Magnetic Resonance. NMR spectra of electrolyte volumes after bulk electrolysis were also obtained using an automated Bruker UltrashieldTM 500 Plus spectrometer with an excitation sculpting pulse technique for water suppression. Data 30 processing was achieved using MestReNova software. The 26 WO 2012/015905 PCT/US2011/045515 concentrations of formate and methanol present after bulk electrolysis were determined using acetone as the internal standard. Carbon dioxide may be efficiently converted to value 5 added products, using either a minimum of electricity (that may be generated from an alternate energy source) or directly using visible light. Some processes described above may generate high energy density fuels that are not fossil-based as well as being chemical feedstock that are not fossil or biologically based. 10 Moreover, the catalysts for the processes may be substituents-sensitive and provide for selectivity of the value added products. By way of example, a fixed cathode (e.g., stainless steel 2205) may be used in an electrochemical system where the 15 electrolyte and/or catalyst are altered to change the product mix. In a modular electrochemical system, the cathodes may be swapped out with different materials to change the product mix. In a hybrid photoelectrochemical system, the anode may use different photovoltaic materials to change the product mix. 20 Some embodiments of the present invention generally provide for new cathode materials, new electrolyte materials and new sulfur and oxygen-containing heterocyclic catalysts. Specific combinations of cathode materials, electrolytes, catalysts, pH levels and/or electrical potentials may be used to get a desired 25 product. The organic products may include, but are not limited to, acetaldehyde, acetone, carbon, carbon monoxide, carbonates, ethanol, ethylene, formaldehyde, formic acid, glyoxal, glyoxylic acid, graphite, isopropanol, methane, methanol, oxalate, oxalic acid. Inorganic products may include, but are not limited to, 30 polymers containing carbon dioxide. Specific process conditions 27 WO 2012/015905 PCT/US2011/045515 may be established that maximize the carbon dioxide conversion to specific chemicals beyond methanol. Cell parameters may be selected to minimize unproductive side reactions like H2 evolution from water electrolysis. Choice 5 of specific configurations of heterocyclic amine pyridine catalysts with engineered functional groups may be utilized in the system 100 to achieve high faradaic rates. Process conditions described above may facilitate long life (e.g., improved stability), electrode and cell cycling and product recovery. The organic products created 10 may include methanol, formaldehyde, formic acid, glyoxal, acetone, and isopropanol using the same pyridine catalyst with different combinations of electrolytes, cathode materials, bubbling techniques and cell potentials. Heterocyclic amines related to pyridine may be used to improve reaction rates, product yields, 15 cell voltages and/or other aspects of the reaction. Heterocyclic catalysts that contain sulfur or oxygen may also be utilized in the carbon dioxide reduction. Some embodiments of the present invention may provide cathode and electrolyte combinations for reducing carbon dioxide to 20 products in commercial quantities. Catalytic reduction of carbon dioxide may be achieved using steel or other low cost cathodes. High faradaic yields (e.g., >20%) of organic products with steel and nickel alloy cathodes at ambient temperature and pressure may also be achieved. Copper-based alloys used at the electrodes may 25 remain stabile for long-term reduction of carbon dioxide. The relative low cost and abundance of the combinations described above generally opens the possibility of commercialization of electrochemical carbon dioxide reduction. Various process conditions disclosed above, including 30 cathode materials, cathode surface morphology, electrolyte choice, 28 WO 2012/015905 PCT/US2011/045515 catalyst choice, cell voltage, pH level and manner in which the carbon dioxide is bubbled, generally improve control of the reaction so that different products or product mixes may be made. Greater control over the reaction generally opens the possibility 5 for commercial systems that are modular and adaptable to make different products. The new materials and process conditions combinations generally have high faradaic efficiency and relatively low cell potentials, which allows an energy efficient cell to be constructed. 10 While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the scope of the invention. 29

Claims (15)

1. A method for reducing carbon dioxide to formic acid, comprising the steps of: 5 (A) bubbling said carbon dioxide into a solution of an electrolyte and a catalyst in a divided electrochemical cell, said catalyst selected from the group consisting of imidazole, substituted imidazole, thiazole and adenine, wherein (i) said divided electrochemical cell comprises an anode in a first cell 10 compartment and a cathode in a second cell compartment and (ii) said cathode reducing said carbon dioxide into formic acid, said cathode comprising a material selected from Nb, Co, Ni, Sn, and stainless steel; (B) varying a faradaic yield of said formic acid by 15 (B1) adjusting one or more of (a) a cathode material and (b) a surface morphology of said cathode, or (B2) adjusting one or more of (a) said electrolyte and (b) a manner in which carbon dioxide is bubbled, or (B3) adjusting one or more of (a) a pH level of said 20 solution and (b) an electrical potential; and (C) separating said formic acid from said solution.

2. The method according to claim 1, wherein said step (B) comprises step (B1) and said surface morphology of said 25 cathode comprises an unscratched surface.

3. The method according to claim 1, wherein said step (B) comprises step (B1) and said surface morphology of said cathode comprises a heavily scratched surface. 30

4. The method according to claim 1, wherein said step (B) comprises step (B2) and said electrolyte is at least one of Na 2 SO 4 , KC1, NaNO 3 , NaCl, NaF, NaC10 4 , KClO 4 , K 2 SiO 3 , CaC1 2 , a H cation, a Li cation, a Na cation, a K cation, a Rb cation/ a Cs 30 cation, a Ca cation, an ammonium cation, an alkylammonium cation, a F anion, a Cl anion, a Br anion an I anion, an At anion, an alkyl amine, borates, carbonates, nitrites, nitrates, phosphates, polyphosphates, perchlorates, silicates, sulfates, 5 and a tetraalkyl ammonium salt.

5. The method according to claim 1 or claim 4, wherein said step (B) comprises step (B2) and said bubbling comprises the sub-step of: 10 bubbling said carbon dioxide to hit said cathode.

6. The method according to claim 1 or claim 4, wherein said step (B) comprises step (B2) and said bubbling comprises the sub-step of: 15 bubbling said carbon dioxide to miss said cathode.

7. The method according to any one of claims 1 and 4 to 6, wherein said step (B) comprises step (B2) and said faradaic yield is at least 20 percent. 20

8. The method according to claim 1, wherein said adjusting comprises both (B1) adjusting one or more of (a) a cathode material and (b) a surface morphology of said cathode, and 25 (B2) adjusting one or more of (a) said electrolyte and (b) a manner in which said carbon dioxide is bubbled.

9. The method according to claim 1, wherein said step (B) comprises step (B3) and said pH level ranges from 30 approximately 3 to approximately 8.

10. The method according to claim 1 or claim 9, wherein said step (B) comprises step (B3) and said electrical potential ranges from approximately -0.7 volts to -2 volts. 31

11. The method according to any one of claims 1, 9 and 10, wherein step (B) comprises step (B3) and further comprises the step of: 5 adding to said solution one or more of (i) a hydrogen inhibitor, (ii) a heterocyclic compound and (iii) an alcohol.

12. The method according to any one of claims 1 and 9 to 11, wherein step (B) comprises step (B3) and further comprises 10 the step of: performing said reducing at ambient temperature and ambient pressure.

13. The method according to claim 1, wherein said 15 adjusting comprises both (B1) adjusting one or more of (a) a cathode material and (b) a surface morphology of said cathode, and (B3) adjusting one or more of (a) a pH level of said solution and (b) an electrical potential. 20

14. The method of any one of claims 1 to 13, substantially as hereinbefore described with reference to any one of the Examples and/or Accompanying Figures. 25

15. Formic acid produced by the method of any one of claims 1 to 14. 32