WO2017034522A1 - Photochemically converting carbon dioxide into useful reaction products such as ethanol - Google Patents

Abstract

A method comprising irradiating ultraviolet light on at least one solid composite electrode comprising at least one semiconductor material which is in contact with at least one liquid solution comprising carbon dioxide; wherein at least one proton source is also present in the electrode and/or in the liquid solution comprising carbon dioxide; wherein the irradiating step results in conversion of at least some carbon dioxide into at least one reaction product. High efficiency conversion of carbon dioxide to ethanol can be achieved. The liquid solution can be aqueous, the proton source can be hydroquinone, and the semiconductor material can be zinc sulfide. The solid composite electrode can be a working electrode and used in conjunction with a counter electrode in a photoelectrochemical system. The system can be operated in alternating closed and open circuit mode.

Description

PHOTOCHEMICALLY CONVERTING CARBON DIOXIDE INTO USEFUL

REACTION PRODUCTS SUCH AS ETHANOL

BACKGROUND

Carbon dioxide levels are rising in the atmosphere which generates critical environmental issues of global warming. In this context, the reactivity of carbon dioxide is an important subject, as a need exists to reduce atmospheric levels of carbon dioxide. If possible, carbon dioxide should be converted into commercially useful products.

In one reaction of carbon dioxide, the ability to form a carbon-carbon bond from two molecules of CO₂, using ZnS and light was demonstrated by the Eggins lab in the early 1990s (Eggins, B.R., et al., Formation of two-carbon acids from carbon dioxide by photoreduction on cadmium sulphide. J. Chem. Soc, Chem. Commun., 1988(16): p. 1 123-24; Eggins, B.R., et al., Photoreduction of carbon dioxide on zinc sulfide to give four-carbon and two-carbon acids. J. Chem. Soc, Chem. Commun., 1993(4): p. 349-50; collectively, "Eggins"). Mechanistically, upon illumination with UV light, an electron in the ground state of ZnS is believed to be promoted to the first excited state. This electron is transferred to a nearby molecule of CO₂ converting it to the highly reactive free radical anion ^*CO₂ ^", which attacks a nearby molecule of CO₂ to form a carbon-carbon bond. While this was an important advance, the fact that the major product was oxalic acid limited the practical utility of the finding. The production of ethanol was not reported.

Despite these advances, a need exists to develop better methods to react carbon dioxide which are environmentally friendly, produce useful products, and/or are commercially realistic.

SUMMARY

Embodiments described herein include methods of reacting carbon dioxide, systems and apparatuses for carrying out such methods, and components of the systems and apparatuses including electrodes.

A first aspect is a method comprising: irradiating ultraviolet light on at least one solid composite electrode comprising at least one semiconductor material which is in contact with at least one liquid solution comprising carbon dioxide; wherein at least one proton source is also present in the electrode and/or in the liquid solution comprising carbon dioxide; wherein the irradiating step results in conversion of at least some carbon dioxide into at least one reaction product.

A second aspect is a method comprising: irradiating ultraviolet light on at least one solid composite electrode comprising at least one material which is an electron source, wherein the solid composite electrode is in contact with at least one liquid solution comprising carbon dioxide; wherein at least one proton source is also present in the electrode and/or in the liquid solution comprising carbon dioxide;

wherein the irradiating step results in conversion of at least some carbon dioxide into at least one reaction product. In one embodiment, the electron source is a

semiconductor material. In another embodiment, the electron source is not a semiconductor material.

A third aspect is a solid composite electrode comprising at least one semiconductor material in particle form, and at least one proton source.

In a fourth aspect, the electrode can be in colloidal form dispersed in liquid solution comprising carbon dioxide.

In one embodiment, the proton source is a reducing agent.

In one embodiment, the proton source is present in the electrode. In one embodiment, the proton source is present in the electrode and is an organic compound. In one embodiment, the proton source is present in the electrode and is an aromatic organic compound having hydroxyl groups. In one embodiment, the proton source is present in the electrode and is hydroquinone or Raney nickel. In one embodiment, the proton source is present in the electrode and is hydroquinone.

In another embodiment, the proton source is present in the liquid solution comprising carbon dioxide. In one embodiment, the proton source is present in the liquid solution comprising carbon dioxide and is replenished by electrolysis of water. In one embodiment, the proton source is present in the liquid solution comprising carbon dioxide and the proton source also is present in the electrode.

In one embodiment, the liquid solution is an aqueous solution. In one embodiment, the liquid solution is an aqueous solution and further comprises at least one electrolyte.

In one embodiment, the solid composite electrode further comprises at least one binder and/or at least one electronically conductive agent, wherein the binder and the agent may be the same or different. In one embodiment, the solid composite electrode further comprises at least one binder and at least one electronically conductive agent, wherein the binder and the agent are different. In one embodiment, the solid composite electrode further comprises at least one binder which is a hydrophobic polymer. In one embodiment, the solid composite electrode further comprises at least one binder which is a flourinated polymer. In one embodiment, the solid composite electrode further comprises at least one electronically conductive agent which is a carbonaceous electronically conductive agent. In one embodiment, the solid composite electrode further comprises at least one electronically conductive agent which is graphite. In one embodiment, the solid composite electrode further comprises at least one metal. In one embodiment, the solid composite electrode is in the form of a sheet having a thickness of about 1 micron to about 3 mm. In one embodiment, the solid composite electrode is in the form of a sheet which is fused to an electronically conductive substrate. In one embodiment, the semiconductor material is in the form of particles dispersed throughout the solid composite electrode. In one embodiment, the semiconductor material is in the form of particles which have an average particle size of less than one micron and which are dispersed throughout the solid composite electrode. In one embodiment, the semiconductor material is a zinc semiconductor material. In one embodiment, the semiconductor material is a sulfide semiconductor material. In one embodiment, the semiconductor material is zinc sulfide.

In one embodiment, the reaction product includes a two carbon reaction product. In one embodiment, the reaction product includes ethanol and/or acetic acid. In one embodiment, the reaction product includes at least one four carbon reaction product. In one embodiment, the reaction product includes ethanol as the primary reaction product. In one embodiment, the reaction product includes ethanol, and the efficiency of the conversion of carbon dioxide into ethanol is at least 50 mole %. In one embodiment, the reaction product includes acetic acid.

In one embodiment, the solid composite electrode is one component in an electrical circuit which includes a counter electrode also disposed in liquid in conductive communication with the liquid solution comprising carbon dioxide. In one embodiment, the solid composite electrode is one component in an electrical circuit which includes a counter electrode, and wherein the circuit is operated in alternating open- and closed-circuit mode. In one embodiment, the solid composite electrode is one component in an electrical circuit which includes a counter electrode, and water electrolysis occurs at the counter electrode. In one embodiment, the solid composite electrode is one component in an electrical circuit which includes a counter electrode, and wherein the circuit is operated in alternating open- and closed-circuit mode under computer control.

In one embodiment, the ultraviolet light is part of electromagnetic irradiation which includes non-UV light components.

In one embodiment, electrical energy is not applied to the composite solid electrode.

In one embodiment, the irradiating step is carried out at a reaction

temperature of about 15°C to about 50 °C.

In one embodiment, the solid composite electrode is one component in an electrical circuit which includes a counter electrode, and wherein the circuit is operated in alternating open- and closed-circuit mode under computer control, wherein water electrolysis occurs at the counter electrode, wherein the solid composite electrode further comprises at least one binder and at least one electronically conductive agent, wherein the binder and the agent are different.

In one embodiment, the solid composite electrode is one component in an electrical circuit which includes a counter electrode, and wherein the circuit is operated in alternating open- and closed-circuit mode under computer control, wherein water electrolysis occurs at the counter electrode, wherein the

semiconductor material is in the form of particles dispersed throughout the solid composite electrode, wherein the reaction product includes ethanol as the primary reaction product.

In one embodiment, the material which is an electron source is a

semiconducting material.

Another embodiment is an electrochemical system adapted for carrying out the methods described and/or claimed herein, the system comprising the solid composite electrode and a container for the liquid solution comprising carbon dioxide.

In one embodiment, the electrode upon being irradiated with UV light is a source of both protons and electrons.

In one embodiment, the solid composite electrode further comprises at least one binder and/or at least one electronically conductive agent, wherein the binder and the agent may be the same or different. Another embodiment is a process for making a solid composite electrode as described and/or claimed herein, the process comprising mixing the semiconductor material with the at least one proton source.

A variety of advantages can be found for at least one embodiment described herein. For example, the method can produce a commercially useful product such as ethanol with high efficiency where a large fraction of carbon dioxide is converted to ethanol. Also, UV light is used to drive the reaction. Also, in some embodiments, the use of a solid state material enables greater control and flexibility in the design of experiments.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 . A "one electrode" embodiment: a mercury arc lamp is inserted into the middle of the reaction vessel. The black strip lining the inner right is the ZnS-Tf- graphite solid state sheet. CO₂ is delivered through the port at the upper left (Tf is Teflon, or polytetrafluoroethylene).

Figure 2. (A) The ZnS-Tf paste with added HQ (B); and graphite (C); and rolled out (D).

Figure 3. The red dotted lines show the maximum and minimum concentrations of ethanol (top) and acetic acid (bottom) created in the experiments.

Figure 4. CO₂ was bubbled into the system continuously for 200 minutes, but the lamp was turned on only for the first 100 minutes - the difference (upper graph) indicates that 85% of the CO₂ was reduced to products. The experiment was repeated at increasing temperatures but with a UV shield around the lamp. No products were created (lower graph) demonstrating that illumination with UV photons is essential for driving the reactions, but temperature is not.

Figure 5. Colorimetry standard curve: concentration of ethanol vs. Δ in absorbance.

Standard zero contained 5 μΙ_ K₂Cr₂O₇ mixed with 5 μΙ_ of water. Ethanol samples contained 1 , 2, 3, 4 and 5 mM EtOH mixed with 5 μΙ_ K₂Cr₂O₇.

Figure 6. Colorimetry showing that 5 μΙ_ K₂Cr₂O₇ can oxidize ethanol but not ethylene glycol. The original yellow color of the oxidizing agent gets diminished by reacting with ethanol, but stays the same in the presence of ethylene glycol.

Figure 7. Calibration curve for ethanol concentration vs peak area (arbitrary units) for GC/MS. Figure 8. Calibration curve for acetic acid as a function of peak area. The units of peak area are arbitrary.

Figure 9. A "two electrode" embodiment: the photo-electrochemical system is a fabricated ZnS-Teflon-Carbon cathode connected through a potentiostat to a metal strip anode that is either Ti or Pt. A mercury arc lamp is in the flask containing the cathode, which enables running the system in open circuit, closed circuit and oscillating open-closed circuit mode.

Figure 10A. The dark squares are standard solutions of ethanol prepared at concentations of 0.5, 1 , 2, 3, and 5 millimolar and their corresponding areas from GC/MS plotted on the y-axis. Running in open circuit mode with C0₂ bubbled into the system and the mercury arc lamp on, a sample is removed after 6 hours and run through the GC/MS. The horizontal red line is the area under the curve at the characteristic ethanol retention time and the vertical red line shows that the concentration is 4.2 millimolar. This experiment was repeated several times and the results are all very similar.

Figure 10B. The increasing production of ethanol causes a rising pH showing that protons are depleted from the media.

Figure 1 1 A. The system shown in Figure 9 is run in closed circuit mode with a Ti anode. Voltage is plotted in red on the left-side y-axis and current is plotted in green on the right-side y-axis with time plotted on the x-axis. No C0₂ is bubbled into the solution and the mercury arc lamp is turned off. Since no reaction is occurring, both the current and voltage are zero with small oscillations.

Figure 1 1 B. C0₂ is bubbled into the solution but the mercury arc lamp is turned off. Since no reaction is occurring both the current and voltage are zero. While the current has small oscillations the large voltage oscillations are likely the result of C0₂ coming on and off the ZnS electrode.

Figure 1 1 C. C0₂ is bubbled into the solution and the mercury arc lamp is turned on. This striking oscillating current and opposite phase voltage occurs according to our hypothesis that holes created on the ZnS cathode by creating ethanol results in a positive potential that eventually builds up and causes oxidation at the Ti anode. When this point is reached, electrons flow through the wire filling the ZnS holes, reversing the potential and the cycle is repeated. Figure 12. The system is run in open circuit mode for 5 minutes and closed circuit mode for 5 seconds. The measured current density is 130 mA/cm-cm. The current is plotted in green and the voltage is plotted in red.

Figure 13A. The system is run in oscillating open-closed circuit mode switching back and forth every 25 hours. As predicted, the pH rises in open circuit mode as the protons from the media are being depleted to create ethanol and holes on the ZnS cathode. When the system is run in closed circuit mode current is supplied to the Pt anode, which splits water replenishing the media with protons and refilling the ZnS holes with electrons.

Figure 13B. Oxygen bubbles forming on counter electrode (Pt) when the system is run in closed-circuit mode

DETAILED DESCRIPTION

INTRODUCTION

All references cited herein are incorporated herein by reference.

The term "comprising" or "comprises" as used herein can be replaced in other embodiments with the terms "consisting essentially of" and "consisting of" as known in the art. Basic and novel characteristics of the inventions described herein are described and support use of such phrases.

Oxalic acid, which is produced in prior art processes, is a significantly higher energy state than reduction products like ethanol or acetic acid (Table 1 ).

Reactants Products Free Energy

2C0₂ + 2hv +2e^~ HOOC-COOH AG⁰ = 97.9 kJ/mol

HOOC-COOH + 6H⁺ +4e^" CH₃COOH + 2H₂0 AG⁰ = -261.3 kJ/mol

HOOC-COOH + 12H⁺ +10e^~ CH₃CH₂OH + 3H₂0 AG⁰ = -319.8 kJ/mol

Table 1. Creating oxalic acid from carbon dioxide consumes 97.9 kJ/mol of free energy. Transforming oxalic acid into acetic acid and ethanol are substantially negative free energy reactions and thus carbon-carbon bond formation from carbon dioxide in the presence of reducing equivalents is predicted to yield acetic acid and ethanol instead of oxalic acid. In principle, "hitting" C0₂ with electrons and protons simultaneously, would result in reduction products like acetic acid and ethanol instead of oxalic acid, because of the favorable thermodynamics (Benson, E.E., et al., Electrocatalytic and homogeneous approaches to conversion of C0₂ to liquid fuels. Chem Soc Rev, 2009. 38(1 ): p. 89-99). A problem, however, was how to continuously generate a source of electrons and protons. In principle, as soon as the protons and electrons are depleted, the reaction will stop.

Another hypothesis was that the process of creating ethanol by transferring electrons from, for example, ZnS to C0₂ will create holes in the ZnS and thus a positive potential will be created. Hence, a possible by-product of ethanol creation is the harvesting of electric potential energy.

Still another hypothesis was that increasing production of ethanol will cause an increasing population of holes on, for example, the ZnS, which will eventually surpass the electric potential threshold necessary for splitting water. In principle, if this potential can be connected to a Pt-anode, for example, electrolysis of water will occur replenishing the protons and electrons.

A method arose from these and other hypotheses comprising, in a first aspect, irradiating ultraviolet light on at least one solid composite electrode comprising at least one semiconductor material which is in contact with at least one liquid solution comprising carbon dioxide; wherein at least one proton source is also present in the electrode and/or in the liquid solution comprising carbon dioxide;

wherein the irradiating step results in conversion of at least some carbon dioxide into at least one reaction product. The elements and variations of this method are described more hereinafter.

PHOTOELECTROCHEMICAL CELLS AND DEVICES

Photoelectrochemical cells or devices can be used to carry out the methods as known in the art. These are known in the art and include devices with one or more electrodes, such as three electrode devices including working electrode, counter electrode, and reference electrode, as well as the light source and medium for holding the solids, liquids, and gases used in the cell. Observation windows can be built into the cell. Small, medium, and large scale devices and cells can be used. Examples are shown in Figures 1 and 9 and comprise, for example, a lamp, a reference electrode, a container for the working electrode, a container for the counter electrode, a connecting path between the two containers, a proton exchange membrane in the connecting path, and a potentiostat. The devices and cells can be adapted for batch, continuous, or semi-continuous operation.

IRRADIATING UV LIGHT

The step of irradiating an object with UV light, including artificial light, is known in the art. UV lamps, for example, are known in the art. The UV light can be mixed with radiation of other wavelengths including visible wavelengths. Near UV and far UV can be used. The focus of the light which generates a reaction is UV light. The emission spectrum of the UV light can be adapted to match with the absorption profile of the solid composite electrode and the semi-conductor material therein. Factors such as the power, geometry, and wavelength of the light source such as the lamp can be adapted for a particular application in view of the larger system. For purposes herein, the term "light" is used broadly to cover visible light, UV light, and the like. Also, methods known in the art such as filters or monochromators can be used to control the wavelength which impacts the electrode.

The transition band of UV light matches the frequencies for carbon dioxide to take up an electron.

SOLID COMPOSITE ELECTRODE

Solid composite electrodes are known in the art including those used for photochemistry and photoelectrochemistry and those that include semiconductors or semiconducting materials. For example, the solid composite electrode can create holes when it is irradiated by UV light as known in the art. The semiconductor can be selected to have a band gap which allows for the creation of holes and reaction with carbon dioxide.

Semiconductors include group IV semiconductors, group lll-V semiconductors, group ll-VI semiconductors, and the like, as known in the art. Examples of ll-VI semiconductors include sulfide, selenide, and telluride materials including, for example, CdS, CdSe, CdTe, ZnO, ZnS, ZnSe, ZnTe, MoS₂, MoSe₂, MoTe₂, WS₂, WSe₂. Metal dichalcogenides can be used. The semiconductor can be in particle form, including colloidal form, and compounded with other components to form a solid, integral electrode structure.

The semiconducting material can be in particulate form. Average particle size can be, for example, less than about one micron including, for example, from about 5 nm to about 500 nm, or about 10 nm to about 250 nm, or about 25 nm to about 100 nm. The solid composite electrode can be fabricated from colloidal solutions of the semiconducting material, and the colloidal particle structure can be retained in the sold composite electrode.

Multi-layer electrodes can be used. Various geometries can be used. The solid composite electrode can be, for example, disposed on a conductive substrate such as a graphite substrate.

The solid composite electrode can be, for example, a composite comprising at least one semiconductor, at least electronic conductor, and at least one binder. In one embodiment, for example, the solid composite electrode is a composite comprising at least one colloidal ZnS or CdS semiconductor, at least electronic conductor, and at least one binder. In another embodiment, the solid composite electrode is a composite comprising at least one colloidal ZnS semiconductor, at least electronic metallic conductor, and at least one fluoropolymer binder.

The ZnS semiconductor can be used in various forms including zinc blende (band gap is 3.58 eV at 300K), wurtzite (band gap is 3.70 eV at 300K), and combinations thereof.

Electronic conductors for use in electrode formation are well-known in the art including conductive carbon (e.g., graphite) and metal materials including, for example, silver, gold, and copper. Additional examples of electronic conductors include electronically conductive polymers, whether doped or undoped, which can function both as an electronic conductor and a binder.

Binders including insulating or conductive binders for use in electrode formation are well-known in the art. Polymeric materials can be used including polyolefins, carbon backbone polymers, fluorinated polymers, and perfluorinated polymers and copolymers. Poly(tetrafluoroethylene) ("Teflon" or "Tf") can be used. Additional examples of the binder include electronically conductive polymers, whether doped or undoped.

Additives can be used in forming the solid composite electrode. Solvents can be used in forming the electrode to help with dispersion. Compounded materials can be shaped and dried to form solid composite electrodes with proper shape.

The solid composite electrode can be fabricated in different ways and shapes including forming sheets or tubes, or forming structures designed to maximize surface area. LIQUID SOLUTION COMPRISING CARBON DIOXIDE

A liquid solution can be prepared which includes one or more solvents such as water and establishes a semiconductor-liquid interface. The water can be subjected to carbon dioxide mixing so that the carbon dioxide can diffuse in the liquid and participate in the electrochemical reactions at the electrode. As known in the art, carbon dioxide can dissolve in water providing mild acidic pH, forming carbonic acid. Bubbling of carbon dioxide throughout the liquid can be carried out.

Additives can be used in the liquid solution such as, for example, electrolytes or buffers, to control conductivity or pH.

In one embodiment, the solution is an aqueous solution. In one embodiment, no liquid organic co-solvent is used. In one embodiment, if one or more liquid organic co-solvents are used, the amount is 25 wt.% or less, or 10 wt.% or less, or 5 wt.% or less with respect to the total amount of solvent including water. Water can be, for example, at least 80 wt.%, or at least 90 wt.%, or at least 95 wt.% of the solvent system for the liquid solution.

PROTON SOURCE

The proton source can be used in a variety of different embodiments. As used herein, the term "proton source" means that the proton source provides hydrogen atoms in the conversion of carbon dioxide to ethanol in a liquid media via one or more intermediates. As demonstrated herein, reaction of carbon dioxide with both electron and proton sources can result in high levels of acetic acid and/or ethanol compared to less useful products such as oxalic acid.

In one embodiment, the proton source is present in the solid compound electrode. When the solid composite electrode is made, the proton source can be incorporated into the ingredients used to make the composite electrode. One skilled in the art can adapt the amount of the proton source to be included in the solid composite electrode. Hence, the electrode can be a source of both electrons and protons.

In another embodiment, the proton source is present in the liquid solution comprising carbon dioxide.

In another embodiment, the proton source can be present both in the solid composite electrode and in the liquid solution comprising carbon dioxide.

In one embodiment, the proton source is or comprises a reducing agent. In one embodiment, the proton source can be an organic compound. The proton source can be a weakly acidic compound. The organic compound can be, for example, an organic aromatic compound, having one or more hydroxyl moieties such as hydroquinone. The organic compound, such as hydroquinone, optionally can be substituted. As known in the art, the hydroxyl moiety can be converted to a ketone moiety releasing protons. A material such as Raney nickel also can be a proton source.

In one embodiment, the proton source is hydroquinone.

SWITCHING BETWEEN OPEN-CLOSED CIRCUIT MODES

In one embodiment, the solid composite electrode is used in a larger system as a working electrode. The system can include, for example, a potentiostat, a counter electrode, and a reference electrode, as known in the art. In this embodiment, an electrochemical system is provided which allows for switching of the solid composite electrodes between open and closed circuit modes. Potentiostats are known in the art and can be used to control switching. In open circuit mode, the working electrode is not able to have current flow due to the open circuit. Hence, for example, holes can build up in the working electrode when it is irradiated with light. In the closed circuit mode, the working electrode can participate in current flow creating power.

One can control the potentiostat to alternate the working electrode between open and closed circuit modes, producing ethanol.

Time periods can be established for both the open circuit mode and the closed circuit mode. For example, the switching step can comprise alternating steps of open circuit mode and closed circuit mode characterized by a time period of, for example, 5 seconds to 20 minutes, or 30 seconds to 10 minutes, in open circuit mode. In another example, the switching step can comprise alternating steps of open circuit mode and closed circuit mode characterized by a time period of 1 second to 5 minutes, or 1 second to 1 minute, in closed circuit mode. In another example, the switching step can comprise alternating steps of open circuit mode and closed circuit mode characterized by a time period of 1 second to 20 minutes, or 2 seconds to 10 minutes, in both open and closed circuit modes.

One can vary the process to control the amount of power developed. The power built up in the open circuit mode can be, for example, at least 5 mW, or at least 15 mW when switched to close circuit mode. One can vary the time the process is carried whether continuously or intermittently. For example, the method can be carried out for at least 24 hours, at least 48 hours, or at least 72 hours.

In one embodiment, oxygen gas is generated at the counter electrode in the closed circuit mode. In addition, protons can form in the liquid solution comprising carbon dioxide.

REACTION PRODUCTS INCLUDING ETHANOL

The goal is to generate carbon-carbon bond formation and produce useful reaction products such as ethanol and/or acetic acid and/or ethylene glycol. In many cases, complex mixtures of reaction products can form. For example, five or more products, or ten or more products, can be formed. In many cases, one product can be identified as the primary reaction product. In some cases, four carbon reaction products can be formed such as, for example, tartaric acid or succinic acid. Ethanol can be formed in the liquid solution as a primary reaction product and detected by methods known in the art. In addition, the concentration of ethanol can be

measured, and the efficiency of the conversion of carbon dioxide to ethanol can be measured.

Other products can be produced along with ethanol such as acetic acid and/or ethylene glycol.

Reaction conditions can be selected so that elemental carbon is not formed as a substantial reaction product.

OTHER EMBODIMENTS

In some embodiments, to generate high surface area, the composite electrode can be in a colloidal form during irradiating wherein colloidal particles are dispersed in a liquid solution rather than in the form of an integral solid composite electrode. The average particle size in the colloid can be less than one micron. For the electron source, semiconductor materials can be used as described herein but in colloidal form.

WORKING EXAMPLES Additional embodiments are provided in the following non-limiting working examples.

EXAMPLE 1 (COMPARATIVE)

The experimental setup is shown in Figure 1 which can be used for both embodiments according the claimed invention as well as comparative embodiments. In a comparative example, a mercury arc lamp was used to supply the UV light, while the conductive sheet (without proton source such as hydroquinone; hence comparative) was immersed in the solution and CO₂ was bubbled in through a port. The lamp was turned on for 5 minutes. HPLC analysis of the solution indicated that not only was oxalic acid produced, but the solid state electrode was 4-fold more efficient at converting CO₂ into the 2-carbon di-acid than using the colloidal solution. These findings were interpreted to mean that dissolved CO₂ preferentially

congregates in the neighborhood of the electrode and thus there is a more efficient transfer of electrons to create the free radical anions. Just to be clear, this solid state only contained ZnS and so exposure to UV light only causes the release of electrons. This experiment was done to prove that the Eggins' prior art result could be reproduced but using a solid state instead of a colloid and repeated 3 times using 3 independent preparations of the solid phase. See Eggins, B. R et al., J. Chem. Soc, Chem. Commun. 1988, 1123-1124; Eggins, B. R., et al., J. Chem. Soc, Chem. Commun. 1993, 349-350.

EXAMPLE 2

The ZnS-Tf-graphite composite electrode was made as in Example 1 , but this time the proton source hydroquinone (HQ) was also added as shown in Figure 2. The same set of experiments were performed, but this time using the ZnS-Tf- graphite-HQ strip. The hypothesis was that this solid state electrode will transfer both electrons and protons when illuminated with UV - the ZnS will give off electrons and HQ will give off protons becoming benzoquinone. To test if ethanol and acetic acid were produced, two standard GC/MS curves were prepared. A series of aqueous solutions of both acetic acid and ethanol at known concentrations were prepared and run through the GC/MS. The peaks appeared at the known retention times of ethanol and acetic acid and the areas were directly proportional to the concentrations. GC/MS was run on aliquots from the solutions of our experimental runs using the ZnS-Tf-graphite-HQ solid state. Peaks corresponding to the retention times of ethanol and acetic acid appeared confirming that both of these substances were created in this experiment. The GC/MS results, with area of the peak plotted on the y-axis and concentration plotted on the x-axis are shown in Figure 3. The concentrations of ethanol ranged from 2.7-3.5 mM and for acetic acid 1 .1 -1 .6 mM.

EXAMPLE 3

The efficiency of this system was investigated to determine how much of the bubbled in C0₂ was actually converted to reduced products. The setup shown in Figure 1 was modified to include an amine trap.

A set of preliminary experiments was performed to determine how long it took to saturate the solution in the reaction vessel by keeping the lamp turned off while pumping C0₂ into the system until the spill-over saturated the amine trap. The trap with no C0₂ was weighed and weighed again when it was saturated with C0₂.

Restarting with a clean setup, C0₂ was bubbled into the system, but this time with the lamp turned on for 100 minutes and then the lamp was turned off but C0₂ was still bubbled into the system for an additional 100 minutes. The difference in the amount of C0₂ captured in the trap between these two experiments indicates that 85% of the C0₂ is converted into products as shown in the top graph of Figure 4. The solid state ZnS-Tf-graphite-HQ was unexpectedly efficient at transforming C0₂ into reduced products.

EXAMPLE 4

To unequivocally confirm that these reactions were driven by UV photons, the experiment was repeated, but this time with a UV shield around the lamp and at three different temperatures. The bottom graph of Figure 4 summarizes the results of 5 different experiments. The topmost line of this graph was for simply bubbling C0₂ into the system with the UV shield and the lamp off while monitoring the amount of C0₂ filling the amine trap. The red line is this same experiment with the lamp on for the whole 200 minutes - with the UV shield in place there is no difference between the lamp on or off. Finally, the experiments were run for 200 minutes with the lamp on and the vessel temperature maintained at 25, 45, and 65°C. This showed that temperature has no effect on the reaction, which is consistent with a photochemical process.

Materials and Methods

Colloidal ZnS was prepared following the method developed by Eggins (Eggins, B. R. ; et al., J. Chem. Soc, Chem. Commun. 1988, 1123-1124; Eggins, B. R. ; et al., J. Chem. Soc, Chem. Commun. 1993, 349-350). Typically, 0.05 moles of Na₂S were dissolved in 225ml of ultrapure water. The solution was stirred while 225 ml of 0.22 M ZnS0₄ was added drop wise. The addition was completed in about 1 hr. As a phase transfer catalyst is an important component of the reaction, 450 ml of 0.22 M tetra-methyl ammonium chloride (TMACI) was added to the ZnS suspension. After adjusting pH, the completed mixture was charged into a 1 -liter photochemical reactor (Ace Glass product number 7840-185). Carbon dioxide was continuously bubbled through the catalyst suspension via a fritted glass sparger. Illumination came from a 450 W medium-pressure Hg arc lamp placed in a jacketed, quartz immersion well. The water flow through the well's cooling space was set to keep the reaction at a constant 25 °C temperature.

The formation of the catalytic sheet was accomplished as follows. The ZnS suspension (without any TMACI added) was centrifuged and the supernatant discarded. The catalyst was then resuspended in ultrapure water and centrifuged again. The latter process was then repeated twice. Ten milliliters of a 60% PTFE dispersion, 1 mg of hydroquinone and 5 mg of ultrapure graphite were added to the washed precipitate. The mixture was allowed to air-dry overnight. Mineral spirits were added to the catalyst mixture to form a slurry that was spread out onto a glass plate. The catalyst mix was then worked with a spatula until the PTFE contained in it knitted the components together into an amorphous mass. An aluminum roller was then used to calendar the mass into an about 0.5 mm thickness sheet. The membrane was air-dried and then heated to 150°C for 2 hours to drive off any residue from the mineral spirits and the PTFE wetting agent.

Initially, the reaction was run as a batch process. The catalyst suspension was prepared, and the reactor set up and run for a period of time between 8 and 48 hours. After shutting down the reaction, the solution was withdrawn, and centrifuged. The supernatant was then passed through a cation exchange column (Amberlite IR-120H) to remove TMA⁺ ions. Analysis of the reaction products was carried out colorimetrically, by HPLC or by GC/MS. Later, the process was run for a period of 8 to 10 hours with small samples withdrawn for analysis every hour.

In a separate set of experiments, two different reducing agents were added to initial reactants in an attempt to further reduce the photoreaction products. First, a combination of hydroquinone (C₆H₄(OH)₂) and palladium was tested. To the colloidal suspension of ZnS prepared as above, 1 g of hydroquinone and 200 mg of 5% Pd on activated carbon were added. Reaction runs of 8, 18 and 24 hours were performed under the same conditions as for earlier experiments.

As an alternative approach to obtaining reduced forms of initial products, a composite catalyst was produced. This consisted of ZnS formed by the usual procedure combined with Raney nickel (which is a proton source).

The catalyst was formed by adding Raney nickel slurry (50% by volume) to the ZnS colloid and centrifuging the mixture at high speed for 1 hour or more. The supernatant was discarded. The pellet was broken up and dispersed in 900 ml of 0.1 M TMACI. The pH was adjusted and the suspension poured into the photo reactor. The reaction process was the same as above.

HPLC analysis was used to detect organic acids in the product mixture. An Agilent HPLC was fitted with an Aminex HPX-87H column. The mobile phase was 0.008 M H₂S0_4.. Carrier flow was maintained at 0.6 ml/min and the injection volume was 20 μί. The u.v. detector was set to record transmittance of 210nm radiation. Species present in the injected sample were identified by their retention times. A series of standards analyses established the retention times for expected C0₂ reductants oxalic, formic, acetic, glyoxylic, glycolic, tartaric and succinic acids and for species resulting from secondary reduction: ethanol, ethylene glycol and

acetaldehyde. Baselines were also obtained for HCI, as it was present in samples in significant quantity due to the exchange of TMA ions. Concentrations of the primary reductants were estimated by comparing sample peak areas with those of the standard chromatograms. Unfortunately, the secondary reduction products absorb only weakly at 21 Onm. Ethanol and ethylene glycol were difficult to identify and we were unable to quantify their concentrations using HPLC.

A colorimetric test was used to determine the approximate concentration of ethanol present in the reaction product sample. Each sample was cleansed of chloride ions by the addition of an excess of AgN0₃ and subsequent centrifugation and removal of the precipitate. Five μΙ_ of the supernatant was added to 5 μΙ_ of a solution of Κ₂0Γ₂Ο₇ in 25% H₂S0₄ in a standard cuvette. The absorbance of 440 nm wavelength light was measured by a Perkin-Elmer spectrophotometer. The result was compared to a calibration curve obtained by measuring the average absorbance of four samples each of 1 , 2, 3, 4 and 5 mM ethanol in the dichromate solution. The calibration curve used for colorimetric analysis is shown in Figure 5. To confirm that Κ₂0Γ₂Ο₇ is a sensitive test for ethanol, it was also checked that it was unable to oxidize ethylene glycol. K₂Cr₂0₇ is yellow and the reduction of its color is directly proportional to the amount of oxidation it performs on its substrate. Figure 6 shows how the yellow solution goes completely clear in the presence of ethanol, but remains yellow in the presence of ethylene glycol.

To further confirm the presence of EtOH and other species, product samples were also analyzed by GC/MS. An Agilent 5975C system with 7890A GC was used. The machine settings used for this analysis were as follows:

Carrier: Ultra pure He

Flow rate: 1 ml/min

Mode: El

Inlet temp: 230°C

Injection volume: 1 μΙ_

Injection split ratio: 50:1

Ion source temp: 200°C

Temperature profile:

Start: 40°C - hold for 5 minutes

Ramp: 15°C/min to 200°C

End: 200°C - hold for 3 minutes.

Chromatograms and associated mass spectra were analyzed using the Chem Station software supplied with the GC/MS. Species were identified by comparing mass spectral scans (acquired at the rate of 32/minute) with those in the NIST library. For each spectrum, the software presented a list of possible compounds and associated probabilities. Ethanol concentrations were estimated by comparing the peak areas of sample chromatograms with a calibration curve obtained from the analysis of a set of samples containing known concentrations of EtOH in ultrapure water. For the protocol shown above, the ethanol peak appeared at 1 .743 minutes after injection. The peak areas for seven concentrations of ethanol were recorded and plotted to yield the calibration curve shown in Figure 7.

The presence of a considerable amount of acetic acid in the product mixture was noted when a secondary reducing agent was present. For this reason a GC/MS calibration curve was also produced for acetic acid. It is shown in Figure 8.

Experimental Results:

Without the addition of reducing agents to the ZnS photocatalyst, the results were similar to those reported by Eggins (references cited above). The product mixture generally contained measureable concentrations of oxalic, glyoxylic, and tartaric acids. The results of a typical 24-hour run are given in Table 1 . It was found that the concentration of oxalic acid reached a peak after about 8 hr of illumination. This is not unexpected as the output of the Hg lamp used was 3-5 times greater than the lamp used in Eggins' experiments.

The addition of both Raney nickel and hydroquinone to the photocatalyst resulted in the appearance of ethanol in the product mix. Hydroquinone with Pd was somewhat more effective in producing EtOH. The results of the most successful runs for the two are shown in Table 2:

TABLE I

Compound Concentration, μΜ

Oxalic acid 2800

Tartaric acid 1250

Glyoxylic acid 950

Glycolic acid 800

Acetic acid 600

Run # Ethanol Concentration, mM Table 2

Reducing Agent

Hydroquinone + Pd 1 1 .6 Hydroquinone + Pd 2 3.5 Hydroquinone + Pd 3 1 .4 Raney Nickel 1 1 .8 Raney Nickel 2 0.9 Raney Nickel 3 2.6

Acetic acid concentrations for the experiments involving secondary reduction are shown in Table 3.

Table 3

Reducing Agent Run # CH₃COOH Concentration, μΜ

Hydroquinone + Pd 1 950

Hydroquinone + Pd 2 1 120

Hydroquinone + Pd 3 770

Raney Nickel 1 1230

Raney Nickel 2 840

Raney Nickel 3 1540

Additional experiments were carried out with use of a second electrode, an electrochemical circuit, and alternating closed and open circuit modes.

Theory and Construction of the Circuit

As described above, ZnS can be obtained as a colloid. In order to incorporate it into a solid electrode, the oily phase was separated out by ultracentrifugation with decanting of the supernatant. Poly(tetrafluoroethylene) ("Teflon") was added as a stable hydrophobic matrix. This amalgam was rolled out, graphite was added to make the mixture conducting, and a thin slab was melded to a carbon sheet to make the final cathode. No hydroquinone was used as proton source in this example. Rather, protons from the liquid solution were used as a proton source. This sheet was inserted into a flask that comes with a standard mercury arc lamp, and the flask was filled with an acidic solution. In a second flask, a small sheet of titanium or platinum was inserted into an acid solution. The metal was directly connected by a wire through a potentiostat to the ZnS-teflon-graphite cathode, and the circuit was completed with a proton exchange membrane. The potentiostat served as both a volt and current meter and also was programmed to run the system in oscillating open-closed circuit modes. A picture of the setup is shown in Figure 9.

Example 5: Converting CO? Directly to Ethanol

An experiment was carried out to supply CO₂ with electrons and protons and convert it directly to ethanol. The system shown in Figure 9 was set to open circuit mode, the mercury arc lamp was turned on, and a gentle stream of CO₂ was bubbled into the solution. At the 6 hour mark, a sample from the cathode solution was removed and analyzed by quantitative GC/MS. As shown in Figure 10A, the solution contains an ethanol concentration of 4.2 millimolar demonstrating that in fact CO₂ can be converted directly to ethanol using only light energy. This reaction was continued for 48 hours while monitoring the pH. As shown in Figure 10B, the continuing production of ethanol depletes protons from the solution resulting in a steady rise of pH.

Example 6: Harvesting Electric Energy While Converting CO? to Ethanol An experiment was carried out to determine whether light driven C0₂ conversion to ethanol causes a build up of positive potential on the cathode which can be converted to an electric current. The system shown in Figure 9 was set to closed circuit mode with a titanium strip used as the anode. It was unknown whether the continuing creation of ethanol will cause a continuing build-up of holes on the ZnS, which will cause a continuing increase of positive electric potential on the cathode. It was further unknown whether when this voltage exceeded a critical value, oxidation will occur at the titanium anode resulting in a flow of electrons that refills the holes on the ZnS cathode and a reversal of the potential occurs. In Figure 1 1 A, 1 1 B, and 1 1 C, voltage is plotted on the left-side y-axis, current is plotted on the right-side y-axis, and time is plotted on the x-axis. In a control, Figure 1 1 A shows that the current (green) and voltage (red) for the system have very small oscillations around their respective zero-points when no C0₂ was flowed into the system and the mercury arc lamp was turned off. In another control, Figure 1 1 B shows the current and voltage oscillations when C0₂ was flowed into the system with the mercury arc lamp turned off. The current still had very small oscillations around its zero point, but the voltage had much larger oscillations around its zero point possibly reflecting the voltage changes associated with C0₂ jumping on and off the ZnS cathode.

Figure 1 1 C shows the current and voltage as a function of time when C0₂ was flowed into the system and the mercury arc lamp was turned on. This striking data that a buildup of holes results in a positive potential with an associated flow of current, which reverses the potential in a repeating cycle lends strong support to the hypothesis that the photo-induced creation of ethanol is associated with a buildup of positive potential that can be converted to an electric current.

Example 7: Splitting Water to Replenish the Protons and Electrons

An experiment was carried out to determine whether electric current can be harvested from light driven C0₂ conversion to ethanol, which can be used to split water creating protons and electons. The system shown in Figure 1 was set to run in oscillating open-closed circuit mode. It was believed that, possibly, continuing to create ethanol in open circuit mode would cause an increasing population of holes in the ZnS and thus a large current density can be built up in the system.

Figure 12 shows the current and voltage relationships of the system when it was run for approximately 5 minutes in open circuit mode and then run in closed circuit mode for 5 seconds. Running the system in this oscillating open-closed circuit mode results in a current density of 130 mA/cm², which is comparable to the Ballard fuel cell used in electric cars (Laurencelle, F., et al., Characterization of a Ballard MK5-E Proton Exchange Membrane Fuel Cell Stack. Fuel Cells, 2001 . 1 : p. 66-71 ). In the next experiment, the titanium anode was removed and replaced with platinum, because it is known that passing current through Pt will split water into protons, electrons, and molecular oxygen. The possibility was present that when the system was run in open circuit mode to create ethanol, the pH would rise as protons were used up, and when the system was run in closed circuit mode to create ethanol, the pH would fall and oxygen bubbles would be created at the Pt anode as water was split. Figure 13A shows the saw-tooth wave of increasing-decreasing pH running the system in oscillating open-close circuit mode. Figure 13B shows a picture of the Pt anode evolving oxygen bubbles when the system was run in closed circuit mode. These data strongly support the hypothesis that photo-induced creation of ethanol can be coupled to electrolysis of water via harvesting of excess photonic energy that can be converted into a voltage.

Example 8: Efficiency of CO? Conversion into Products

Again, experiments were carried out to determine how much input CO₂ actually gets converted into products. In order to determine this, the system shown in Figure 9 was adapted with an output port that allowed unreacted CO₂ to be captured in an amine trap.

The experiments were started by bubbling CO₂ into the system with the lamp off for 1 hour and the output port NOT placed in the graduated cylinder containing the amine. This was done to completely saturate the system with CO₂ and allow excess CO₂ to freely escape. The CO₂ output port was then placed into the graduated cylinder containing the amine trap and the mercury arc lamp was turned on while the CO₂ was still being bubbled into the reaction vessel. The graduated cylinder containing the amine trap was weighed before the experiment was started, and after the reaction was run for 50 and 100 minutes. At the 100 minute mark, the light was switched off, which stops all conversion of CO₂ into products, and the CO₂ was allowed to flow into the system for another 100 minutes. At the 200 minute mark, the graduated cylinder was weighed again. The differences in the weights of the amine trap between these set of experiments indicates that 85% of the C0₂ is converted into products as shown in Figure 4 (top).

To confirm that all of the C0₂ is accounted for and that none of it escapes by some unknown path the experiment was run by completely bypassing the system and running the C0₂ directly into the amine trap at the same flow rate. After bubbling C0₂ for 100 minutes into the amine trap, the change in the weight of the graduated cylinder was found to be identical to the weight change measured in the experiment shown in Figure 4 (top) between 100 and 200 minutes, demonstrating no unaccounted for loss of C0₂.

As one last control, the experiment summarized in Figure 4 (top) was rerun, but a UV shield was wrapped around the mercury arc lamp, which should preclude any formation of products even though the lamp is turned on. The amine trapped the same amount of C0₂ by weight as the experiment where the C0₂ was run directly into the trap (Figure 4, bottom), thus confirming no unaccounted for losses and that 85% of the gas was converted into products.

Materials and Methods (For Two Electrode System)

Photocathodes were fabricated as follows. Colloidal ZnS was prepared by drop wise addition of 0.05 M anhydrous sodium sulfide (Alfa Aesar) to a stirred solution of 0.05 M zinc perchlorate (Alfa Aesar) stabilized with 0.02 M Si0₂ nanopowder (10-20nm Aldrich). The resulting suspension was centrifuged and resuspended twice. After a final filtration, a paste was formed by adding 5 grams of high purity graphite powder (Fuel Cell Store) and 5 ml of 60% PTFE dispersion (DuPont) to 15 grams of the filtered ZnS and stirring vigorously for several minutes. The mixture was vacuum dried at 250°C overnight. The dried powder was rewetted with odorless mineral spirits, worked with a spatula to knit the PTFE particles into a support network and rolled out to a thickness of about 0.5 mm. The film was dried in vacuo at 150°C. To complete the cathode, two composite films were pressed onto either side of a roughened, perforated titanium collector plate.

The photoelectrochemical cell was constructed from two identical glass vessels each having a provision for an immersion well (see Figure 9). A Nafionl 17 proton exchange membrane separated the two electrolyte chambers. Illumination was provided to the cathode by a 450 W medium-pressure Hg arc lamp in a cathode-side, quartz immersion well. The catholyte was a 0.1 M solution tetramethylammonium chloride adjusted to pH 4. The anolyte was simply 0.1 mM HCI. The counter electrode was smooth Pt. The reference used in some of the potentiostatic measurements was a self-contained H₂ electrode (EDAQ ET-070 Hydroflex). The potentiostat was a NuVant Systems EZStat-Pro.

Identification of product compounds was carried out by first removing small (about 0.5 ml) aliquots of catholyte from the reactor and treating them with a cation exchange resin (Amberlite IR-120H) to remove tetramethylammonium ions.

HPLC analysis was used to detect organic acids in the product mixture. An Agilent HPLC model 1 100 was fitted with an Aminex HPX-87H column. The mobile phase was 0.008 M H₂S0₄.. Carrier flow was maintained at 0.6 ml/min, and the injection volume was 20 ml. The u.v. detector was set to record transmittance of 210nm radiation. Species present in the injected sample were identified by their retention times. A series of standards analyses established the retention times for expected C0₂ reductants such as oxalic, formic, acetic, glyoxylic, glycolic, tartaric and succinic acids and for species resulting from secondary reduction such as ethanol, ethylene glycol, and acetaldehyde. Baselines were also obtained for HCI, as it was present in sample in significant quantity due to the exchange of TMA ions. Concentrations of the primary reductants were estimated by comparing sample peak areas with those of the standard chromatograms. However, the secondary reduction products absorb only weakly at 21 Onm. Ethanol and ethylene glycol were very difficult to identify by HPLC.

To confirm the presence of EtOH and other species, product samples were also analyzed by GC/MS. An Agilent 5975C system with 7890A GC was used. The machine settings used for the analysis were as follows:

Carrier: Ultra pure He

Flow rate: 1 ml/min

Column: Agilent DB-35ms

Mode: El

Inlet temp: 230°C

Injection volume: 1 μΙ Injection split ratio: 50:1 Ion source temp: 200°C

Temperature profile:

Start: 40°C - hold for 5 minutes

Ramp: 15°C/min to 200°C End: 200°C - hold for 3 minutes.

Chromatograms and associated mass spectra were analyzed using the Chem

Station software supplied with the GC/MS. Species were identified by comparing mass spectral scans (acquired at the rate of 32/minute) with those in the NIST library. For each spectrum, the software presented a list of possible compounds and associated probabilities.

Ethanol concentrations were estimated by comparing the peak areas of sample chromatograms with a calibration curve obtained from the analysis of a set of samples containing known concentrations of EtOH in ultrapure water. For the protocol shown above, the ethanol peak appeared at 1 .743 minutes after injection. The peak areas for five concentrations of ethanol were recorded and plotted to yield the calibration curve shown in Figure 10a.

ADDITIONAL REMARKS

In sum, the goal was to design and test a system that can convert CO₂ to useful products, such as ethanol, using light (e.g., UV light) as the only source of energy, CO₂ as the only source of carbon, and protons from, for example, the acidic media as the only source of reducing equivalents. If some aspect of the process had to rely on an energy source derived from burning a fossil fuel, then CO₂ would be created by a process designed to mitigate the gas and thus a priori the system was constrained to use only photo-chemistry. Because the magnitude of global CO₂ emissions is so enormous, it seems clear that it is highly desirable to create carbon- carbon bonds by reacting the gas with itself, thus avoiding the introduction of an exogenous source of carbon that presumably would also be required in approximately equal vast quantities. It was hypothesized that converting CO₂ into ethanol requires supplying the gas with protons and electrons and thus a scalable system must somehow have the ability to replenish these reactants, but use no other source of energy or carbon-based products if the process is to work within the stated constraints. It was reasoned that photo-chemically transferring electrons from a semiconductor to CO₂ must leave holes within the material and that if this substance could be formulated into a conductive solid state that could stably tolerate a large population of holes, then a positive potential sufficient to split water may build up on the electrode. This was the motivation for creating a circuit that could be run in oscillating open-closed circuit mode; in open circuit mode light driven conversion of CO₂ to ethanol would cause a build-up of holes on the cathode and consumption of protons from the media and then switching to closed circuit mode would cause a current to flow causing water splitting at the anode replenishing the electrons and protons. The experimental results clearly demonstrate that supplying protons and electrons to CO₂ induce its conversion into ethanol, that excess photonic energy from the process can be harvested, and that this energy can be converted into electricity used to split water, thus replenishing the protons and electrons in accord with all of the initially applied constraints, making the system scalable.

Claims

WHAT IS CLAIMED IS:

1 . A method comprising:

irradiating ultraviolet light on at least one solid composite electrode comprising at least one semiconductor material which is in contact with at least one liquid solution comprising carbon dioxide; wherein at least one proton source is also present in the electrode and/or in the liquid solution comprising carbon dioxide;

wherein the irradiating step results in conversion of at least some carbon dioxide into at least one reaction product.

2. The method of claim 1 , wherein the proton source is a reducing agent.

3. The method of claims 1 -2, wherein the proton source is present in the electrode.

4. The method of claims 1 -3, wherein the proton source is present in the electrode and is an organic compound.

5. The method of claims 1 -4, wherein the proton source is present in the electrode and is an aromatic organic compound having hydroxyl groups.

6. The method of claims 1 -5, wherein the proton source is present in the electrode and is hydroquinone or Raney nickel.

7. The method of claims 1 -6, wherein the proton source is present in the electrode and is hydroquinone.

8. The method of claims 1 -7, wherein the proton source is present in the liquid solution comprising carbon dioxide.

9. The method of claims 1 -8, wherein the proton source is present in the liquid solution comprising carbon dioxide and is replenished by electrolysis of water.

10. The method of claims 1 -9, wherein the proton source is present in the liquid solution comprising carbon dioxide and the proton source also is present in the electrode.

1 1 . The method of claims 1 -10, wherein the liquid solution is an aqueous solution.

12. The method of claims 1 -1 1 , wherein the liquid solution is an aqueous solution and further comprises at least one electrolyte.

13. The method of claims 1 -12, wherein the solid composite electrode further comprises at least one binder and/or at least one electronically conductive agent, wherein the binder and the agent may be the same or different.

14. The method of claims 1 -13, wherein the solid composite electrode further comprises at least one binder and at least one electronically conductive agent, wherein the binder and the agent are different.

15. The method of claims 1 -14, wherein the solid composite electrode further comprises at least one binder which is a hydrophobic polymer.

16. The method of claims 1 -15, wherein the solid composite electrode further comprises at least one binder which is a flourinated polymer.

17. The method of claims 1 -16, wherein the solid composite electrode further comprises at least one electronically conductive agent which is a carbonaceous electronically conductive agent.

18. The method of claims 1 -17, wherein the solid composite electrode further comprises at least one electronically conductive agent which is graphite.

19. The method of claims 1 -18, wherein the solid composite electrode further comprises at least one metal.

20. The method of claims 1 -19, wherein the solid composite electrode is in the form of a sheet having a thickness of about 1 micron to about 3 mm.

21 . The method of claims 1 -20, wherein the solid composite electrode is in the form of a sheet which is fused to an electronically conductive substrate.

22. The method of claims 1 -21 , wherein the semiconductor material is in the form of particles dispersed throughout the solid composite electrode.

23. The method of claims 1 -22, wherein the semiconductor material is in the form of particles which have an average particle size of less than one micron and which are dispersed throughout the solid composite electrode.

24. The method of claims 1 -23, wherein the semiconductor material is a zinc semiconductor material.

25. The method of claims 1 -24, wherein the semiconductor material is a sulfide semiconductor material.

26. The method of claims 1 -25, wherein the semiconductor material is zinc sulfide.

27. The method of claims 1 -26, wherein the reaction product includes ethanol and/or acetic acid.

28. The method of claims 1 -27, wherein the reaction product includes at least one four carbon reaction product.

29. The method of claims 1 -28, wherein the reaction product includes ethanol as the primary reaction product.

30. The method of claims 1 -29, wherein the reaction product includes ethanol and the efficiency of the conversion of carbon dioxide into ethanol is at least 50 mole %.

31 . The method of claims 1 -30, wherein the reaction product includes acetic acid.

32. The method of claims 1 -31 , wherein the solid composite electrode is one component in an electrical circuit which includes a counter electrode also disposed in liquid in conductive communication with the liquid solution comprising carbon dioxide.

33. The method of claims 1 -32, wherein the solid composite electrode is one component in an electrical circuit which includes a counter electrode, and wherein the circuit is operated in alternating open- and closed-circuit mode.

34. The method of claims 1 -33, wherein the solid composite electrode is one component in an electrical circuit which includes a counter electrode, and water electrolysis occurs at the counter electrode.

35. The method of claims 1 -34, wherein the solid composite electrode is one component in an electrical circuit which includes a counter electrode, and wherein the circuit is operated in alternating open- and closed-circuit mode under computer control.

36. The method of claims 1 -35, wherein the ultraviolet light is part of

electromagnetic irradiation which includes non-UV light components.

37. The method of claims 1 -36, wherein electrical energy is not applied to the composite solid electrode.

38. The method of claims 1 -37, wherein the irradiating step is carried out at a reaction temperature of about 15°C to about 50 °C.

39. The method of claims 1 -38, wherein the solid composite electrode is one component in an electrical circuit which includes a counter electrode, and wherein the circuit is operated in alternating open- and closed-circuit mode under computer control, wherein water electrolysis occurs at the counter electrode, wherein the solid composite electrode further comprises at least one binder and at least one electronically conductive agent, wherein the binder and the agent are different.

40. The method of claims 1 -39, wherein the solid composite electrode is one component in an electrical circuit which includes a counter electrode, and wherein the circuit is operated in alternating open- and closed-circuit mode under computer control, wherein water electrolysis occurs at the counter electrode, wherein the semiconductor material is in the form of particles dispersed throughout the solid composite electrode, wherein the reaction product includes ethanol as the primary reaction product.

41 . A method comprising:

irradiating ultraviolet light on at least one solid composite electrode comprising at least one material which is an electron source, wherein the solid composite electrode is in contact with at least one liquid solution comprising carbon dioxide; wherein at least one proton source is also present in the electrode and/or in the liquid solution comprising carbon dioxide; wherein the irradiating step results in conversion of at least some carbon dioxide into at least one reaction product.

42. The method of claim 41 , wherein the material which is an electron source is a semiconducting material.

43. An electrochemical system adapted for carrying out the methods of claims 1 -42, the system comprising the solid composite electrode and a container for the liquid solution comprising carbon dioxide.

44. A solid composite electrode comprising at least one semiconductor material in particle form, and at least one proton source.

45. The solid composite electrode of claim 44, wherein the electrode upon being irradiated with UV light is a source of both protons and electrons.

46. The solid composite electrode of claims 44-45, wherein the solid composite electrode further comprises at least one binder and/or at least one electronically conductive agent, wherein the binder and the agent may be the same or different.

47. A process for making a solid composite electrode according to claims 44-46, the process comprising mixing the semiconductor material with the at least one proton source.