WO2016055927A1

WO2016055927A1 - Methods and apparatus for synthetic gas production having controlled hydrogen gas and carbon monoxide ratios in aqueous media

Info

Publication number: WO2016055927A1
Application number: PCT/IB2015/057617
Authority: WO
Inventors: Peng KANG; Zuofeng Chen; Animesh NAYAK; Thomas J. Meyer
Original assignee: The University Of North Carolina At Chapel Hill
Priority date: 2014-10-06
Filing date: 2015-10-05
Publication date: 2016-04-14

Abstract

Provided are methods of producing synthetic gas and an apparatus for preparing the same. A method may include molecular electrocatalytic reduction of carbon dioxide in an aqueous solution with a metal complex compound to prepare a mixture of carbon monoxide and hydrogen in controlled ratios. The reduction may be catalyzed by an electrocatalyst such as a ruthenium polypyridyl complex.

Description

METHODS AND APPARATUS FOR SYNTHETIC GAS PRODUCTION HAVING CONTROLLED HYDROGEN GAS AND CARBON MONOXIDE RATIOS IN AQUEOUS

MEDIA

STATEMENT OF GOVERNMENT SUPPORT

This invention was made with government support under Award Number DE- SC000101 1 awarded by the U.S Department of Energy. The government has certain rights in the invention.

FIELD

The present invention relates generally to the production of synthetic gas and an apparatus for preparing the same. In particular, the present invention relates to molecular electrocatalytic reduction of carbon dioxide and water to prepare a mixture of carbon monoxide and hydrogen in controlled ratios.

BACKGROUND

Synthesis gas (syngas, CO/H2 mixtures) is a critical industrial feedstock for producing bulk chemicals (methanol, dimethyl ether, acetic acid, etc.) and synthetic fuel through industrial processes. See I. Wender, Fuel Process. Techno!., 1996, 48, 189-297; S. C. Tsang, J. B. Claridge and M. L. H. Green, Catal. Today, 1995, 23, 3-15; K. C.

Waugh, Catal. Today, 1992, 15, 51-75; M. E. Dry, Catal. Today, 2002, 71 , 227-241.

Currently, syngas is predominantly derived from non-renewable sources, in particular, from methane and coal. See Wender; Tsang et al. Carbon dioxide is a potential renewable carbon source for fuels and chemicals if recycled reductively in closed energy conversion cycles. See Climate Change: Evidence, Impacts, and Choices: Set of 3 Booklets, The National Academies Press, 2012; E. E. Benson, C. P. Kubiak, A. J.

Sathrum and J. M. Smieja, Chem. Soc. Rev., 2009, 38, 89-99; M. Cokoja, C. Bruckmeier, B. Rieger, W. A. Herrmann and F. E. Kuhn, Angew. Chem., Int. Ed., 201 1 , 50, 8510- 8537; C. Finn, S. Schnittger, L. J. Yellowlees and J. B. Love, Chem. Commun. , 2012, 48, 1392-1399; A. M. Appel, J. E. Bercaw, A. B. Bocarsly, H. Dobbek, D. L. DuBois, M.

Dupuis, J. G. Ferry, E. Fujita, R. Hille, P. J. A. Kenis, C. A. Kerfeld, R. H. Morris, C. H. F. Peden, A. R. Portis, S. W. Ragsdale, T. B. Rauchfuss, J. N. H. Reek, L. C. Seefeldt, R. K. Thauer and G. L. Waldrop, Chem. Rev. , 2013, 1 13, 6621-6658; C. Costentin, M. Robert and J.-M. Saveant, Chem. Soc. Rev., 2013, 2013, 2423-2436; P. Kang, Z. Chen, M.

Brookhart and T. J. Meyer, Top. Catal., 2014, accepted; M. Beller, B. Cornils, C. D.

Frohning and C. W. Kohlpaintner, J. Mol. Catal. A: Chem., 1995, 104, 17-85.

However, achieving co-reduction of C0₂ and water to provide syngas having desired H₂:CO ratios remains a significant challenge. In water with typical catalysts, predominant reduction to H₂ with little to no CO product is common. With the exception of silver and gold, most metal electrodes are selective for hydrogen. See Y. Hori, H.

Wakebe, T. Tsukamoto and O. Koga, Electrochim. Acta, 1994, 39, 1833-1839.

33; C. Delacourt, P. L. Ridgway, J. B. Kerr and J. Newman, J. Electrochem. Soc, 2008,

155, B42-B49. Control of syngas mixtures with adjustable ratios by molecular

electrocatalysis in aqueous media is not yet known.

Applicant has identified deficiencies and problems associated with conventional processes for preparing synthetic gas. Through applied effort, ingenuity, and innovation, certain of these identified problems have been solved by developing solutions that are included in various embodiments of the present invention, which are described in detail below.

BRIEF SUMMARY

Embodiments of the present invention therefore provide a method of preparing synthetic gas and an electrolytic cell for preparing the same.

In one embodiment of the invention, the method of producing syngas via aqueous electrolysis comprises applying a voltage to an electrolytic cell comprising an

electrocatalyst and an aqueous solution, said solution comprising CO2, to obtain a mixture of CO and H2, wherein the electrocatalyst comprises a metal complex compound. In certain embodiments, the method comprises preparing an aqueous solution comprising CO₂ in an electrolytic cell, adding an electrocatalyst to the electrolytic cell, and applying a voltage to the electrolysis cell to obtain a mixture of CO and H₂, wherein the

electrocatalyst comprises a metal complex compound. In another embodiment of the invention, the metal complex compound is a ruthenium polypyridyl complex comprising a monodentate ligand, bidentate ligand, and a tridentate ligand. In a further embodiment, the metal complex compound is selected from the group consisting of [Ru(tpy)(bpy)(L)]²⁺, [Ru(tpy)(Mebim-py)(L)]²⁺, [Ru(tpy)(bpm)(L)]²⁺, [Ru(tpy)(bpz)(L)]²⁺, [Ru(tpy)(Mebim- pz)(L)]²⁺, [Ru(DMAP)(bpy)(L)]²⁺, [Ru(Mebim-py)(bpy)(L)]²⁺, [Ru(Mebim-py)(Mebim- pz)(L)]²⁺, [Ru(Mebim-py)(Mebim-py)(L)]²⁺, and {Ru(Mebim-py)[4,4'- ((HO)20PCH2)2bpy](L)}²⁺, wherein L is a monodentate ligand selected from the group consisting of OH₂ (aqua), NH₃ (ammine), CH₃NH₂ (methylamine), CO (carbonyl), NO (nitrosyl), P (fluoro), CN^" (cyano), CI^" (chloro), Br(bromo), I^" (iodo), N0₂ (nitro), and OH" (hydroxyl). In still yet other embodiments, the metal complex compound comprises Formula A:

Formula A wherein the ligand

is selected from the group consisting of

Mebim-pz Melm-pz For instance, in certain embodiments, the metal complex compound includes Formula B:

2+

Formula B

wherein Ri , R2, R3 can independently be:

H or a C-( -C3o hydrocarbyl radical

^■COOH n=l-30

In certain embodiments of the invention, the method of producing syngas comprises adding an electrocatalyst wherein the electrocatalyst is a metal complex compound comprising a carbene ligand that facilitates CO2 reduction.

In other embodiments of the invention, the method comprises adding a weak acid.

In certain embodiments, the method comprises adding bicarbonate salts such as sodium or potassium bicarbonate. In some embodiments, carbonates, perchlorates, sulfates, phosphates and similar electrolytes of suitable concentrations (e.g., 0.01-10 M) may be added to the electrolytic cell.

In one embodiment of the invention, the method of producing syngas comprises obtaining a mixture of CO and H2 wherein the mixture has a ratio of hydrogen to carbon monoxide from 10: 1 to 1 :10. In a further embodiment, the method comprises controlling a ratio of hydrogen to carbon monoxide by adjusting the applied voltage, adjusting a pH of the solution, or combinations thereof. In some embodiments, the applied voltage is from about 1 to 5 V. In one embodiment, the pH of the solution is from about 5.5 to about 9.5.

In certain embodiments of the invention, the method of producing syngas comprises catalyzing oxidation of water to O2 with the electrocatalyst. In some embodiments, applying a voltage to the electrolytic cell results in oxidation of water to O2.

Aspects of the invention are also directed to an apparatus for preparing synthetic gas. In certain embodiments of the invention, the apparatus for the production of syngas comprises a cathode, an anode, an electrocatalyst, and an aqueous solution comprising CO2, wherein the electrocatalyst catalyzes the reduction of CO2 and water to prepare a mixture of CO and H2 and wherein the electrocatalyst comprises a metal complex compound. In certain embodiments, the aqueous solution comprises a weak acid.

In certain embodiments of the invention, the apparatus comprises a metal complex compound wherein the metal complex compound is a metal polypyridyl complex comprising a monodentate ligand, bidentate ligand, and a tridentate ligand. In some embodiments of the invention, the metal complex compound is selected from the group consisting of [Ru(tpy)(bpy)(L)]²⁺, [Ru(tpy)(Mebim-py)(L)]²⁺, [Ru(tpy)(bpm)(L)]²⁺,

[Ru(tpy)(bpz)(L)]²⁺, [Ru(tpy)(Mebim-pz)(L)] ⁺, [Ru(DMAP)(bpy)(L)]²⁺, [Ru(Mebim- py)(bpy)(L)]²⁺, [Ru(Mebim-py)(Mebim-pz)(L)]²⁺, [Ru(Mebim-py)(Mebim-py)(L)]²⁺, and {Ru(Mebim-py)[4,4'-((HO)₂OPCH2)2bpy](L)} ⁺, wherein L is a monodentate ligand selected from the group consisting of OH₂ (aqua), NH₃ (ammine), CH3NH2 (methylamine), CO (carbonyl), NO (nitrosyl), P (fluoro), CN^" (cyano), CI^" (chloro), Br(bromo), I^" (iodo), N0₂ (nitro), and OH^" (hydroxyl).

In a further embodiment, the apparatus includes a metal complex compound wherein the metal complex compound comprises Formula A

Formula A wherein the ligand

N

is selected from the group consisting of

Mebim-pz Melm-pz For instance, in certain embodiments, the metal complex compound includes

Formula B:

2+

wherein Ri , R₂, R3 can independently be:

hydrocarbyl radical

^■COOH n=l-30

In another embodiment, the metal complex compound comprises a carbene ligand that facilitates CO2 reduction.

In still yet other embodiments, the apparatus comprises a weak acid. In another embodiment, the aqueous solution comprises sodium bicarbonate.

In certain embodiments of the invention, the mixture of CO and H2 has a ratio of hydrogen to carbon monoxide from about 10:1 to about 1 : 10. In one embodiment, the apparatus is configured to allow adjustment of the applied voltage and pH to control the ratio of hydrogen to carbon monoxide.

In another embodiment, water is oxidized to O2 at the anode.

These embodiments of the present invention and other aspects and embodiments of the present invention are described further herein and will become apparent upon review of the following description taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES

Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein:

FIG. 1 (a) is a cyclic voltammogram of an electrocatalyst in accordance with one embodiment of the invention in water under Ar and 1 atm C0₂;

FIG. 1 (b) is a cyclic voltammogram of an electrocatalyst in accordance with one embodiment of the invention in anhydrous THF and in THF with 5% v/v added water under Ar;

FIG. 1 (c) is an enlarged view of wave (3') of FIG. 1 (a) under C0₂;

FIG. 1 (d) is a cyclic voltammogram of an electrocatalyst in accordance with one embodiment of the invention at various scan rates in water under C02; FIG. 1 (e) is a plot of peak current density y_p,_c under C02 vs. the inversed square root of the scan rate (u^'V2 in (V/s)^{" /2});

FIG. 1 (f) is a cyclic voltammogram of an electrocatalyst in accordance with one embodiment of the invention in water under Ar;

FIG. 1 (g) is an enlarged view of FIG. 1 (f);

FIG. 2(a) is a plot of total current densities j and partial current density for CO evolution y^'co and product distributions vs. applied potential in electrolysis using an electrocatalyst of one embodiment of the present invention;

FIG. 2(b) is a plot of total current densities and partial current density for CO evolution ^'co and product distributions vs. solution pH in electrolysis with an

electrocatalyst in accordance with one embodiment of the invention;

FIG. 2(c) is a plot of bulk electrolysis over time using a three-electrode setup with applied potential of -1 .2V vs NHE at the cathode using an electrocatalyst of one embodiment of the present invention;

FIGS. 2(d): A to D are plots of gas chromatography analysis of the cathode headspace after bulk electrolysis with an electrocatalyst in accordance with one embodiment of the invention using a three-electrode setup;

FIG. 3(a) is a cyclic voltammogram of an electrocatalyst in accordance with one embodiment of the invention in water under Ar and 1 atm C02;

FIG. 3(b) is a plot of gas chromotography analysis of the cathode headspace after bulk electrolysis with an electrocatalyst in accordance with one embodiment of the invention using a three-electrode setup;

FIG. 4 is a cyclic voltammograms of an electrocatalyst in accordance with one embodiment of the invention in water under C02;

FIG. 5 is a plot of average cathode and anode potentials (V vs. NHE), and percentage energy efficiency (ε, %) for syngas production vs. applied cell voltage (V_ceii in V) with an electrocatalyst in accordance with one embodiment of the present invention; and

FIG. 6 illustrates an electrolytic cell in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION

The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the inventions are shown. Indeed, these inventions may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal

requirements. Like numbers refer to like elements throughout.

Synthesis gas (syngas, CO/H₂ mixtures) is a critical industrial feedstock for producing bulk chemicals (methanol, dimethyl ether, acetic acid, etc.) and synthetic fuel through industrial processes. The syngas produced by the present invention may be used as feedstock for producing bulk chemicals such as methanol, dimethyl ether, acetic acid, etc. and synthetic fuel.

The present invention provides methods and apparatuses that overcome certain issues related to current syngas production. In current technologies, H₂:CO ratios are adjusted by use of a (reverse) water-gas shift reaction which is a costly process. See I. Wender, Fuel Process. Techno!. , 1996, 48, 189-297. In addition, in water, reduction to hydrogen typically dominates. The preference for H7H₂0 reduction to H₂ relative to CO₂ reduction to CO has a thermodynamic basis with E°^' = -0.41 V vs. NHE for H7H₂0 reduction to H₂ at pH 7 and E°^'= -0.53 V for C0₂ reduction to CO. See E. E. Benson, C. P. Kubiak, A. J. Sathrum and J. M. Smieja, Chem. Soc. Rev., 2009, 38, 89-99. The thermodynamic preference is further exacerbated by the high concentration of water molecules in aqueous media and low solubility of C0₂, ca. 30 mM at ambient temperature and pressure. See J. Schneider, H. Jia, J. T. Muckerman and E. Fujita, Chem. Soc. Rev., 2012, 41 , 2036-2051. However, the present invention utilizes an electrocatalyst that overcomes these issues and provides a reliable method and apparatus for preparing a mixture of carbon monoxide and hydrogen in a controlled ratio.

The desired syngas composition can thereby be reliably made and may avoid further adjustments or requirements for intermediate storage. See Q. Fu, C. Mabilat, M. Zahid, A. Brisse and L. Gautier, Energy Environ. Sci. , 2010, 3, 1382-1397. The present invention provides a method of producing syngas in an apparatus that is simple, robust, and efficient.

In addition, the present invention utilizes electrocatalytic reduction of C0₂ to produce syngas, thus, providing a renewable source for syngas. Using energy from a renewable source to split carbon dioxide and water to produce syngas offers a potential approach to achieving sustainable solutions for fuel and chemical production.

As used herein, the term "syngas" refers to a mixture of carbon monoxide (CO) and hydrogen (H₂). The term covers any appropriate ratio of hydrogen to carbon monoxide ("H₂:CO") and may include other components in the gaseous mixture.

As used herein, the term "electrochemical cell" refers generally to a device capable of generating electrical energy from chemical reactions or of facilitating chemical reactions through the introduction of electrical energy. As used here, the term

"electrolytic cell" refers generally to a device where a redox (reduction-oxidation) reaction is facilitated through the introduction of electrical energy. "Electrolysis" is used generally to refer to the use of electrical energy to drive a chemical reaction. "Reduction" refers to the decrease in oxidation state or the gain in electrons and "oxidation" refers to the increase in oxidation state or the loss of electrons.

As used herein, the term "electrocatalyst" refers to a catalyst used in an electrochemical reaction. In certain embodiments of the invention, an electrocatalyst may be used to catalyze the electrochemical reduction of carbon dioxide and water.

As used herein, the term "ratio of hydrogen to carbon monoxide" or "H₂:CO" refers to the ratio of moles of hydrogen to moles of carbon monoxide. In certain embodiments of the invention, the ratio of hydrogen to carbon monoxide is measured after electrolysis to characterize the composition of the resulting syngas. In certain industrial processes, a specific ratio of hydrogen to carbon monoxide is desired so that the syngas may be used as feedstock for specific downstream processes. For instance, methanol synthesis and Fischer-Tropsch hydrocarbon synthesis use 2: 1 H₂:CO mixtures as the feedstock. See K. C. Waugh, Catal. Today, 1992, 15, 51 -75; M. E. Dry, Catal. Today, 2002, 71 , 227-241. Hydroformylation to form aldehyde requires 1 : 1 H2.CO mixture. See M. Beller, B. Cornils, C. D. Frohning and C. W. Kohlpaintner, J. Mol. Catal. A: Chem., 1995, 104, 17-85.

As used herein, the term "aqueous" refers to solutions with greater than 40% water content by volume, preferably greater than 50%, 60%, 70%, 80%, or 90% water content by volume. Advantageously, the present methods and apparatus utilize aqueous media and adjustment of pH and electropotential to produce the desired syngas products.

As used herein, the term "weak acid" refers to an acid that partially dissociates in water. The term "weak base" refers to a base that does not fully ionize in water. In certain embodiments of the invention, a weak acid may be added to obtain a

concentration of 0.01-5.0 M weak acid, preferably 0.01 -1 M weak acid, or more preferably 0.05-0.5 M weak acid. Useful concentrations may be 0.1 M, 0.15 M. 0.2 M, 0.25 M, 0.3 M, 0.35 M, 0.4 M, and 0.45 M weak acid solutions. In certain embodiments of the invention, a weak base may be added to obtain a concentration of 0.01 -5.0 M weak base, preferably 0.01-1 M weak base, or more preferably 0.05-0.5 M weak base. Useful concentrations may be 0.1 M, 0.15 M. 0.2 M, 0.25 M, 0.3 M, 0.35 M, 0.4 M, and 0.45 M weak base solutions.

The term "controlling" as used herein refers to modulating the conditions described herein under which the syngas is produced to yield the desired ratios of

II. Adjusting the Composition of Synthetic Gas

In certain embodiments, syngas can be produced by electrochemically reducing carbon dioxide in water to carbon monoxide and hydrogen with the disclosed

electrocatalyst. As further described below, the present invention provides a method and apparatus for producing syngas with tunable and controllable adjustment of the ratio of hydrogen to carbon monoxide in the resulting syngas. In certain embodiments, the electrochemical reduction may occur by aqueous electrolysis. For instance, in certain embodiments, the reduction of carbon dioxide and water can proceed in an aqueous solution with the presence of an electrocatalyst, specifically an electrocatalyst disclosed in the present invention such as ruthenium polypyridyl complexes. In some embodiments, the pH and applied voltage may be manipulated to improve and/or control the production of syngas.

In certain embodiments of the present invention, syngas can be formed utilizing a single electrocatalyst to catalyze the formation of both carbon monoxide and hydrogen. In certain embodiments, a single electrocatalyst can be used to catalyze the formation of CO, H2, and O2. By use of the single electrocatalyst, syngas can be produced without the preparation of separate catalysts to catalyze each reduction or oxidation reaction thereby simplifying the method and apparatus as well as lowering the cost of operation.

According to certain embodiments, syngas can be produced in an aqueous environment thereby further simplifying the syngas production. Heretofore, such syngas production with tunable H₂:CO ratios was not thought to have been achievable at efficient levels.

A. Apparatus

In certain embodiments of the invention, the electrocatalytic reduction of carbon dioxide and water may occur in an electrolytic cell. The electrolytic cell may comprise an anode, a cathode, and an electrolyte. Voltage may be applied through a power source. In some embodiments, the voltage from the power source is adjusted to adjust the applied potential to the electrolytic cell. The electrodes can be made of any suitable material so long as the electrodes conduct electricity to the electrolyte and may be located in separate compartments. For instance, the electrode may comprise metal, carbon, semiconductor materials, or combinations thereof. In some embodiments, one or more electrodes may comprise glassy carbon, platinum, silver, or other materials suitable for electrolytic cells. The electrode may be chemically modified to change the properties of the electrode. In some embodiments of the invention, the electrolytic cell may comprise an ion exchange membrane separator, such as a cation or anion exchange membrane, between the anode and cathode. The membrane may allow ion transfer between electrode compartments to balance proton content and charge avoiding slow proton transfer diffusion. Any suitable membrane may be used. In some embodiments, the apparatus allows for additions of acid and/or base before or during electrolysis to adjust the pH of the solution. The electrolyte may comprise a solution of carbon dioxide. In some embodiments of the invention, the electrolyte solution is saturated with carbon dioxide. For instance, the electrolysis may be performed under 1 atm C0₂ to saturate the electrolyte solution with C0₂. In some embodiments, an acid and/or a base may be added to modify the pH of the solution.

In certain embodiments of the invention, an electrocatalyst may be added to the electrolytic cell. In certain embodiments, the electrocatalyst is a metal complex

compound. For instance, in some embodiments, the electrocatalyst may be a metal polypyridyl complex comprising ruthenium (Ru). The complex may comprise one or more _. monodentate ligands, bidentate ligands, tridentate ligands, or combinations thereof. The ligand may be any atom, ion, or molecule that binds to the central metal to produce the complex compound. For instance, in some embodiments, the monodentate ligand may be selected from OH2 (aqua), NH₃ (ammine), CH3NH2 (methylamine), CO (carbonyl), NO (nitrosyl), F^" (fluoro), CN^" (cyano), CI^" (chloro), Br(bromo), I^" (iodo), N0₂ (nitro), and OH^* (hydroxyl). In some embodiments, the bidentate ligand may be selected from bipyridine, phenanthroline, 2-phenylpyridine, bipyrimidine, bipyrazyl, glycinate, acetylacetonate, 2,6- bis(1-methylbenzimidazol-2-yl)pyridine (mebim-py) and ethylenediamine. In some embodiments, the tridentate ligand may be selected from terpyridine, DMAP, and

Mebimpy.

In certain embodiments, the metal complex compound comprises Formula A

Formula A wherein the ligand

is selected from the group consisting of

bpy bprn bpz Mebira-py

Mebiin-pz Melm-pz

In certain embodiments of the invention, the metal complex compound is selected from the group consisting of [Ru(tpy)(bpy)(L)]²⁺, [Ru(tpy)(Mebim-py)(L)]²⁺,

[Ru(tpy)(bpm)(L)]²⁺, [Ru(tpy)(bpz)(L)]²⁺, [Ru(tpy)(Mebim-pz)(L)]²⁺, [Ru(DMAP)(bpy)(L)]²⁺, [Ru(Mebim-py)(bpy)(L)]²⁺, [Ru(Mebim-py)(Mebim-pz)(L)] ⁺, [Ru(Mebim-py)(Mebim- py)(L)]²⁺, and {Ru(Mebim-py)[4,4'-((HO)₂OPCH₂)2bpy](L)}²⁺, where tpy = 2,2':6',2"- terpyridine; bpy = 2,2'-bipyridine; Mebim-py = 3-methyl-1 -pyridyl-benzimidazol-2-ylidene; bprn = 2,2'-bipyrimidine; bpz = 2,2'-bipyraine; Mebim-pz = 3-methyl-1 - pyrazylbenzimidazol-2-ylidene; DMAP = 2,6-bis ((dimethylamino)methyl)pyridine; and L = a monodentate ligand. For instance, in some embodiments of the invention, the electrocatalyst may be [Ru(2,2':6',2"-terpyridine)(3-methyl-1 -pyridyl-benzimidazol-2- ylidene)(L)]²⁺ (hereinafter referred to as [Ru(tpy)( ebim-py)(L)j²⁺), which is shown below as Formula A(1 ) with L = H2O. In certain other embodiments, the electrocatalyst may be [Ru(2,2':6',2"-terpyridine)(2,2'-bipyridine)(L)]²⁺ (hereinafter referred to as

[Ru(tpy)(bpy)(S)]²⁺), which is shown below as Formula A(2) with L = H₂0.

One or more of the ligands of the metal complex may be optionally substituted with one or more substituents. For instance, in some embodiments, one or more of the ligands in the metal complex compound may include one or more substituents such as carboxylic acid, ester, amide, halogen, anhydride, acyl ketone, alkyl ketone, acid chloride, sulfonic acid, phosphonic acid, nitro and nitroso groups.

In certain embodiments, the metal complex compound includes Formula B:

Formula B

wherein Ri , R₂, R3 can independently be:

■PO3H2 ^H2°3P ^ ,

I ⁿ H or a C-|-C₃₀ hydrocarbyl radical

^■COOH n=l-30

In some embodiments, the apparatus allows for modification of the applied voltage and the pH. Acids and/or bases may be added to the apparatus prior to or during electrolysis to adjust the pH of the solution. When the cathode and anode are in separate compartments, acids and/or bases may be added in respective inlets to each

compartment individually or collectively. The modification may thereby allow for control of the syngas composition.

The apparatus may be configured with an outlet for the produced syngas to exit the electrolytic cell. The outlet may be located near the cathode and allow for the removal of produced syngas. Any unreacted CO2 may be separated from H₂/CO using appropriate separation methods, such as cryogenic or nanoporous membrane, etc. The purified syngas may be used as is or compressed and stored in tanks. Separated CO₂ may be fed back to the cell.

FIG. 6 illustrates an electrolytic cell in accordance with one embodiment of the present invention. In the embodiment illustrated in FIG. 6, two electrodes (an anode 20 and a cathode 30) are used in the electrolytic cell 10. Each electrode is located in its respective compartment. In the embodiment of FIG. 6, both of the cathode and anode compartments contain 1 atm saturated CO2 with added NaHCC>3 with I = 0.5 with added Na2S0₄. In this embodiment, the two compartments are separated by an anion exchange membrane 50 (e.g., quaternary ammonium based) to allow for ion (e.g., HCGv) transfer between electrode compartments to balance proton content and charge avoiding slow proton transfer diffusion. In the embodiment illustrated in FIG. 6, [Ru(tpy)(Mebim- py)(OH₂)]²⁺ is the electrocatalyst 60 ("1 mM 1 "). In this embodiment, C0₂ is produced at the anode 20 and consumed at the cathode 30.

B. Methods

As mentioned above, the present invention provides methods of producing syngas by electrolysis. In certain embodiments, syngas is prepared in an aqueous solution comprising C0₂ in an electrolytic cell, adding an electrocatalyst before, during, or after preparing the aqueous solution, and applying a voltage to the electrolytic cell. These steps may be performed in any suitable order and may be performed in conjunction with one another. Without intending to be bound by theory, the application of voltage and the presence of the electrocatalyst initiate the electrochemical reaction. A mixture of CO and H₂ is thereby prepared. In certain embodiments, the CO and H₂ are produced at the cathode. In some embodiments, oxygen is produced at the anode while CO and H₂ are produced at the cathode. The produced gas may be collected and analyzed. The product syngas, if containing unreacted C0₂, may be separated from H₂/CO using appropriate separation methods, such as cryogenic or nanoporous membrane, etc. The purified syngas may be used as is or compressed and stored in tanks. Separated C0₂ may be fed back to the cell.

The present invention additionally provides methods of controlling the composition of the produced syngas, specifically controlling the ratio of hydrogen to carbon monoxide.

Advantageously, by modifying the pH, voltage, and other characteristics of the electrolytic cell, the ratio of hydrogen to carbon monoxide can be controlled. For instance, modifying the amount of added water, pH, and voltage, the ratio of hydrogen to carbon monoxide may be controlled to a ratio from about 10: 1 to 1 :10, such as from about 5: 1 to 1 :5; 4: 1 to 1 :4; or 4: 1 to 1 :2. The ratio may also be about 3: 1 , 2:1 , or 1 : 1. In one embodiment, in an aqueous environment with a pH of 6.7 and electrolysis potential at -1.2 V the ratio of hydrogen to carbon monoxide produced may be about 2.2:1. The syngas can thereby be used in downstream processes as a reliable and renewable feedstock.

In certain embodiments of the invention, an electrocatalyst may be used to catalyze the reduction of carbon dioxide and water. The electrocatalyst may be added to the electrolytic cell by any suitable method such as adding the electrocatalyst to the surface of an electrode, dispersing the catalyst in the organic solvent, and combinations thereof. Preferably, an electrocatalyst is added in amounts ranging from 0.1 to 10 mM, preferably from 0.5 to 5 mM, more preferably from about 1 to 5 mM. Useful amounts of electrocatalyst can be 0.1 , 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1 .0, 1.1 , 1.2, 1.3, 1 .4, 1.5, 1 .6, 1 .7, 1.8, 1.9, 2.0, 2.1 , 2.2, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, 3.1 , 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4.0, 4.1 , 4.2, 4.3, 4.4, 4.5, 4.6, 4.7, 4.8, 4.9, and 5.0 mM. The electrocatalyst is preferably a molecular based catalyst, such as a metal complex compound. In certain embodiments, the electrocatalyst is a ruthenium polypyridyl complex preferably a ruthenium polypyridyl complex with a monodentate ligand. In certain embodiments, a ruthenium polypyridyl complex with a carbene containing ligand may be preferred.

Scheme 1 shown below provides a possible mechanism for the electrolysis of carbon dioxide and water utilizing a ruthenium polypyridyl complex according to one embodiment of the invention.

Scheme 1

As shown in Scheme 1 , in certain embodiments, the mechanism for water/H⁺ reduction to H₂ and C0₂ reduction to CO may include a common intermediate, e.g., [Ru⁰]. Without intending to be bound by theory, the common intermediate may provide the opportunity to exploit reaction conditions to control the syngas H₂:CO ratio in solutions with C0₂.

In some embodiments, the electrocatalyst may be [Ru(tpy)(Mebim-py)(L)]²⁺ where L = a monodentate ligand. For instance, in one embodiment, [Ru(tpy)(Mebim-py)(L)]²⁺ effectively catalyzes the reduction of carbon dioxide and water. Without intending to be bound by theory, the [Ru(tpy)(Mebim-py)(L)]²⁺ catalyst includes a labilizing N-heterocyclic carbene (NHC) ligand that may facilitate ligand loss and reaction of CO2 with twice- reduced [Ru"(tpy)(Mebim-py)(H₂0)]²⁺ to give the metallocarboxylate [Ru"(tpy)(Mebim- py)(COO-)]°.

In some embodiments, an acid and/or a base may be added to improve reduction. Any suitable acid or base may be added. For instance, in some embodiments, sodium bicarbonate may be added as a weak acid. In some embodiments, potassium

bicarbonate, sodium bicarbonate, phosphates such as sodium phosphate monobasic, sodium phosphate dibasic, and phosphoric acid, and combinations thereof can be used as a weak acid. In certain embodiments, a weak acid and/or base may be added to improve or modify the production of carbon monoxide and hydrogen. For instance, in some embodiments, a weak acid may be added to improve the reduction of carbon dioxide to carbon monoxide and water to hydrogen. In some embodiments, a weak acid and/or a base may be added to modify the pH to greater than 5, such as greater than 6 or greater than 7. In certain embodiments, the change in pH may change the preference for production between carbon monoxide and hydrogen.

For instance, at some pH values, carbon monoxide is the preferred product while at other pH values, hydrogen is preferred. In certain embodiments, a more acidic solution will provide a greater amount of hydrogen relative to carbon monoxide. In some embodiments, a more basic solution will provide a greater amount of carbon monoxide relative to hydrogen. By modifying the pH of the solution, the ratio of hydrogen to carbon monoxide may be adjusted and controlled as follows. As shown in the embodiment of FIG. 2(b), a 2.2: 1 ratio of hydrogen to carbon monoxide may be produced at a pH of 6.7. The embodiment evaluated in FIG. 2(b) involved electrolysis of carbon dioxide and water in an aqueous solution of NaHC0₃ with [Ru(tpy)(Mebim-py)(S)]²⁺. In the embodiment of FIG. 2(b), as the pH decreases, the yield of hydrogen relative to carbon monoxide increases. In the same embodiment, as the pH increases, the yield of hydrogen relative to carbon monoxide decreases. Decreasing the amount of NaHCC>3 from 0.5 to 0.05 M in this embodiment resulted in an increase in the H₂:CO ratio from 0.5: 1 to 4:1. See FIG. 2(b). By way of example, the pH of the aqueous solution can be from about 4.5 to about 9.5. For instance, in some embodiments, the pH may be from 4.7 to 9.5, preferably from 6.0 to 7.2, or more preferably from 6.5 to 7.2. Useful pH values are 4.5, 4.6, 4.7, 4.8, 4.9, 5.0, 5.1 , 5.2, 5.3, 5.4, 5.5, 5.6, 5.7, 5.8, 5.9, 6.0, 6.1 , 6.2, 6.3, 6.4, 6.5, 6.6, 6.7, 6.8, 6.9, 7.0, 7.1 , 7.2, 7.3, 7.4, 7.5, 7.6, 7.7, 7.8, 7.9, 8.0, 8.1 , 8.2, 8.3, 8.4, 8.5, 8.6, 8.7, 8.8, 8.9, 9.0, 9.1 , 9.2, 9.3, 9.4, and 9.5.

Equations 1 -3 provide an overview of the electrochemical reduction of carbon dioxide and water to syngas with the addition of a weak acid. HA is a weak acid.

Anode: 3H₂0 + 6A^"— ^ 3/202 + 6HA (1 )

Cathode: CQ₂ + 6HA CO + H₂Q + 2H₂ + 6A^" (2)

Net: 2H₂0 + C0₂ → 2H₂ + CO + 3/20₂ (3)

In certain embodiments of the present invention, the electrocatalytic reduction may proceed according to the following mechanism proposed in Equations 4-8, which is shown with the [Ru(tpy)(Mebim-py)(S)J²⁺ catalyst where S = solvent and with the HC0₃ ^" ion which may be introduced with NaHC03. Electron occupation in the twice-reduced intermediate complex is assumed to be 6 coordination but may be 5 coordination.

[Ru"(tpy)(Mebim-py)(S)}²⁺ + 2e^~→

[Ru"(tpy-)(Mebim-py-)(S)]⁰ (4)

[Ru^ll(tpy-)(Mebim-py-)(S)]°+ HC0₃-→

[Ru"(tpy)(Mebim-py)(H)]⁺ + S + C0₃ ²- (5)

[Ru^H(tpy)(Mebim-py)(H)]⁺ + e^~→

[Ru^H(tpy-)(Mebim-py)(H)]° (6)

[Ru"(tpy-)(Mebim-py)(H)]⁰+ HC0₃-→

[Ru"(tpy-)(Mebim-py)(H₂)]⁺ + C0₃ ²- (7)

[Ru"(tpy-)(Mebim-py)(H₂)]⁺ + S + e^~ →

[Ru"(tpy-)(Mebim-py-)(S)] + H₂ (8)

Added C0₂ controls the pH through the equilibrium in Eq. 9 following proton loss from HC0₃- C0₃ ²- + C0₂ + H₂0→ 2HC0₃- (9)

In certain embodiments, C0₂ reduction may occur by initial, stepwise 2e^" reduction (e.g., to [Ru^M(tpy-)(Mebim-py-)(S)]°) followed by a rate-limiting reaction with C0₂ to give a metallocarboxylate intermediate (e.g., [Ru^H(tpy)(Mebim-py)(COO-)]⁰). The intermediate may undergo further 1 e^~/1 H⁺ reduction to the acid (e.g., [Ruⁿ(tpy-)(Mebim-py)(COOH)]°, see wave (3') in FIG. 1 (a)), with subsequent loss of OH^" to give a CO intermediate. The CO complex is not seen as an intermediate undergoing CO loss following reduction to [Ru^,l(tpy-)(Mebim-py-)(CO)]° and re-entering the catalytic cycle. A possible mechanism for CO2 reduction is summarized in Eqs. 10-14. The mechanism is shown with

[Ru"(tpy)(Mebim-py)(S)]²⁺, but is not so limited.

[Ru"(tpy)(Mebim-py)(S)]²⁺ + 2e^~→

[Ruⁿ(tpy-)(Mebim-py-)(S)]° (10)

[Ru^M(tpy-)(Mebim-py-)(S)]° + C0₂→

[Ru^ll(tpy)(Mebim-py)(C00²-)]° + S (1 1 )

[Ru"(tpy)(Mebim-py)(C00²-)]° + H₂0 + C0₂ + e^" →

[Ru (tpy-)(Mebim-py)(COOH)]° + HCO3- (12)

[Ru^M(tpy-)(Mebim-py)(COOH)]° + C0₂→

[Ru"(tpy-)(Mebim-py)(CO)]⁺ + HC0₃ ^~ (13)

[Ru^M(tpy-)(Mebim-py)(CO)]⁺ + e^" + S →

[Ru^tpy-XMebim-py-XS)]⁰ + CO (14)

In certain embodiments, the applied voltage may also be modified to control the ratio of hydrogen to carbon monoxide. For instance, in some embodiments, hydrogen becomes increasingly favored relative to carbon monoxide as the electrolysis potential decreases. The desired syngas composition can thus be modified by adjusting the applied voltage. In some embodiments, the applied voltage can be from 1 to 5 V, such as from 2 to 4 V, preferably from 2.7-3.3 V, such as 2.7, 2.8, 2.9, 3.0, 3.1 , 3.2, and 3.3 V. In certain embodiments, as the applied voltage is increased, the electrolysis potential at the cathode decreases and hydrogen production increases. For instance, in the embodiment of FIG. 2(a), as the electrolysis potential decreases from -1.2 to -1.5 V (V vs NHE), the ratio of hydrogen to carbon monoxide increases from 0.5: 1 to 2.6: 1. The embodiment evaluated in FIG. 2(a) involved the electrolysis of carbon dioxide and water in an aqueous solution of NaHC0₃ with [Ru(tpy)(Mebim-py)(S)]²⁺.

As described herein, the pH and/or the applied voltage can be adjusted to tune the ratio of H₂:CO in the syngas produced. For instance, in some embodiments, with an electrolysis potential of -1.2 V and a pH of 6.5, a ratio of 4:1 may be achieved. At the same potential and a pH of 6.7, a ratio of 2.2:1 may be achieved. At the same potential and with a pH of 7.0, a ratio of 1.2: 1 may be achieved. At the same potential and with a pH of 7.2, a ratio of 0.5: 1 may be achieved. At a pH of 7.2 and an electrolysis potential of -1 .3 V, a ratio of 1.2:1 may be achieved, while at the same pH and an electrolysis potential of -1.4 V, a ratio of 2.0: 1 may be achieved. Still further, at the same pH (i.e. , 7.2) and with an electrolysis potential of -1.5, a ratio of 2.6:1 may be achieved.

Preferably, the pH is about 6.7 and the electrolysis potential is about -1.2 V or the pH is about 7.2 and the electrolysis potential is about -1.4 V to achieve a syngas composition that may be suitable for certain downstream processes such as methanol synthesis and Fischer-Tropsch hydrocarbon synthesis.

The present invention provides im proved methods of reducing carbon dioxide and water to obtain syngas. In certain em bodiments, an energy efficiency greater than 30% , such as greater than 40% or greater than 50% , 51 % , 52% , 53% , 54% , 55% , 56% , 57% , 58% , 59% , 60% or more can be achieved with an

electrocatalyst of the present invention. The energy efficiency, ε, can be calculated by using Equation 17, in which η₀ο and η_Η2 are the current efficiencies for CO and H₂, respectively, V is the applied cell voltage and ΔΗ⁰ is the enthalpy change per 1 e- for the separate reactions. See C. Delacourt, P. L. Ridgway, J. B. Kerr and J. Newman, J. Electrochem. Soc , 2008, 1 55, B42-B49.

CO2→ CO + 1 /2 0₂ ΔΗ°οο2 = 1 .47 V (1 5)

H20→ H2 + 1 /2 0₂ Δ/-/°Η2θ = 1 .48 V (16)

£ = ncoAhPco + ΠΗΡΑΗ°Η2 (17)

V

For instance, as shown in FIG. 5, electrolysis, according to one embodiment of the invention, achieved about 47% energy efficiency while according to another embodiment of the invention about 53% energy efficiency was achieved. Depending on the conditions of the reaction as described herein (e.g., pH and applied voltage), the energy efficiency of the electrolysis cell can be manipulated to provide a more efficient method of producing syngas at desirable H₂:CO ratios.

In certain embodiments of the invention, carbon dioxide and water are reduced at the cathode. In some embodiments, water is oxidized at the anode to oxygen. Still yet, in some embodiments, carbon dioxide and water are reduced at the cathode to carbon monoxide and hydrogen while at the anode water is oxidized to oxygen. The oxidation of water to oxygen may release protons for water/C0₂ reduction at the cathode. The conditions of the electrolytic cell (e.g., applied voltage and pH) may be modified to control the oxidation of water at the anode and the reduction of carbon dioxide and water at the cathode. The pH may be from 0 to 14, such as from 4.7 to 9.5, preferably from 6.0 to 7.2, or more preferably from 6.5 to 7.2. Useful pH values are 4.5, 4.6, 4.7, 4.8, 4.9, 5.0, 5.1 , 5.2, 5.3, 5.4, 5.5, 5.6, 5.7, 5.8, 5.9, 6.0, 6.1 , 6.2, 6.3, 6.4, 6.5, 6.6, 6.7, 6.8, 6.9, 7.0, 7.1 , 7.2, 7.3, 7.4, 7.5, 7.6, 7.7, 7.8, 7.9, 8.0, 8.1 , 8.2, 8.3, 8.4, 8.5, 8.6, 8.7, 8.8, 8.9, 9.0, 9.1 , 9.2, 9.3, 9.4, and 9.5. In certain embodiments, the pH for water oxidation at the anode is the same as the pH for carbon dioxide/water reduction at the cathode. The applied voltage can be from 1 to 5 V, such as from 2 to 4 V, preferably from 2.7-3.3 V, such as 2.7, 2.8, 2.9, 3.0, 3.1 , 3.2, and 3.3 V. In certain embodiments, the same electrocatalyst used to catalyze the reduction of carbon dioxide and water can be used to catalyze the oxidation of water. For instance, in certain embodiments, water oxidation for related single-site catalysts may occur by O—O bond formation by reaction of Ru^lv=0²⁺ or Ru^v(0)³⁺ with H2O in concert with proton transfer to a water molecule or added base by Atom-Proton Transfer (APT). See J. J. Concepcion, J. W. Jurss, J. L. Templeton and T. J. Meyer, J. Am. Chem. Soc, 2008, 130, 16462-16463; Z. F. Chen, A. K. Vannucci, J. J. Concepcion, J. W. Jurss and T. J. Meyer, P.N.A.S. USA, 201 1 , 108, E1461-E1469; and Y. Tamaki, A. K. Vannucci, C. J. Dares, R. A. Binstead and T. J. Meyer, J. Am. Chem. Soc, 2014, 136, 6854-6857. Under the conditions of an embodiment of the present invention, e.g., in CO2 saturated solutions with added NaHC0₃, HC0₃ ^" may act as the APT acceptor, see Equation 18. Equations 18 and 19, shown below, provide one example of a possible mechanism for water oxidation. Ru^v=0³⁺ + H₂0 + HCO3-→ Ru"'OOH²⁺ + H₂0 + C0₂ (18)

Ru^lllOOH²⁺ + 3HC0₃ ^{: →}

Ru^v=0³⁺ + 0₂ + 3C0₂ + 2H₂0 (rapid) (19)

The separate half-cell and overall reactions, for certain embodiments of the invention, may be summarized in Equations 20-25. Cathode:

3C0₂ + H₂0 + 2e-→ CO + 2HC0₃- (20)

2C0₂ + 2H₂0 + 2e-→ H₂ + 2HC0₃- (21 )

Anode:

4HCO3-→ 0₂ + 2H₂0+ 4C0₂ + 4e- (22)

Trans-membrane:

HCO₃- (cathode)→ HC0₃- (anode) (23)

Overall (pH 7)

C0₂→ CO + ½ 0₂ £°co2 = 1.35 V (24)

H₂0→ H₂ + ½ 0₂ E°H20 = 1.23 V (25) Embodiments of the present invention may be extended to solar cells utilizing solar energy to energize the reaction and produce syngas. For instance, with the ability to control the composition of the syngas produced in the present invention, conditions may be established for using the electrocatalyst in conjunction with a PV/electrolyzer or chromophore-catalyst assembly in a dye sensitized photoelectrosynthesis cell (DSPEC) for the solar production of syngas mixtures.

Further, in some embodiments of the invention, the reduction reactions occur in an aqueous environment, which is often less expensive and easier to work with in operation as compared to other solvents such as organic solvents. Embodiments of the present invention can be extended to industrial processes for producing bulk chemicals and synthetic fuels.

The following examples are offered by way of illustration and not by way of limitation. EXAMPLES

Methods:

Materials and Methods

All chemicals were purchased from commercial sources if not mentioned otherwise. THF was of high performance liquid chromatography (HPLC) grade and further purified by a Pure-Solv Solvent Purification System (Innovative Technology).

Deionized water was further purified by using a Milli-Q Synthesis A10 Water Purification system. Argon was purified by passing through columns of BASF R3-1 1 catalyst

(Chemalog) and 4A molecular sieves. CO2 (National Welders, research grade) was of 99.999% purity with less than 3ppm H₂0 and used as received. D₂0 (Cambridge Isotope) was used as received. Tetrabutylammonium hexafluorophospate (nBu NPF6, Fluka, electrochemical grade) was dried at 60 °C under vacuum for 12 h and stored in the glovebox. ELAT-H carbon fiber cloth (10x 10 cm) was purchased from FuelCellsEtc (College Station, TX). All other reagents are commercially available and were used without further purification. NMR spectra were recorded on Bruker AVANCE-400 NMR spectrometer. Complexes [Ru(tpy)(Mebim-py)(OH₂)](PF₆)2 and [Ru(tpy)(bpy)(OH₂)](PF₆)₂ were synthesized according to previous literature reports. See J. J. Concepcion, J. W. Jurss, M. R. Norris, Z. F. Chen, J. L. Templeton and T. J. Meyer, Inorg. Chem., 2010, 49, 1277-1279; J. J. Concepcion, J. W. Jurss, J. L. Templeton and T. J. Meyer, J. Am. Chem. Soc, 2008, 130, 16462-16463.

Electrochemistry and Product Analyses

Electrochemical experiments were performed using a custom-made CHI 6012D potentiostats (CH Instruments, Inc., TX). A three-electrode setup for aqueous media consisted of a glassy carbon working electrode (BASi, 7.1 mm²), a coiled Pt wire counter electrode, and a SCE reference electrode (0.244 V vs NHE) in an airtight, glass frit- separated two-compartment cell. In THF, the reference electrode was Ag/AgN0₃ reference electrode (BASi, 10 mM AgNC , 0.1 M nBu₄NPF₆ in acetonitrile), and ferrocene was added at the end of the experiment and the potential was converted relative to NHE following a literature protocol. See V. V. Pavlishchuk and A. W. Addison, Inorg. Chim. Acta, 2000, 298, 97-102. Prior to each measurement, the glassy carbon electrode was polished with a 0.05-μιη alumina slurry for 1 min, then sonicated and thoroughly rinsed with Milli-Q water and acetone, and finally dried in an Ar stream. In cyclic voltammetry experiments, the working and counter electrodes were separated from the reference electrode. In controlled potential electrolyses with the three-electrode setup, the reference and working electrodes were separated from the Pt mesh counter electrode with AMI-7001 anion exchange membrane (Membrane International, Inc., Ringwood, NJ). In the two-electrode setup, the cathode compartment was separated from the anode compartment with the same anion exchange membrane, the cathode was connected with the working lead of the potentiostat and the anode was connected with both reference and ancillary leads. Each compartment was added 4 mL electrolyte solution.

Controlled potential electrolysis was performed in 4 mL, 0.5 M NaHCC aqueous solutions in an airtight electrochemical cell under vigorous stirring. The solution was degassed by purging with Ar for 15 min and then saturated with 1 atm of CO2 for 15 min before sealing the cell. Solution resistance was measured and compensated at 85% level in the bulk electrolysis. At the end of electrolysis, gaseous samples (0.6 mL) were drawn from the headspace by a gas-tight syringe (Vici) and injected into the GC (Varian 450-GC). Calibration curves for H₂ and CO were obtained separately. The liquid phase was doped with a known amount of DMF as internal standard and diluted 1 :1 with D₂0 for 1 H-NMR analysis.

Results:

Example 1.

The electrochemistry of an electrocatalyst in accordance with the present invention was investigated in water saturated with C0₂. FIG. 1 (a) is a cyclic

voltammogram of an electrocatalyst in accordance with one embodiment of the invention in water under Ar and 1 atm CO₂. FIG. 1 (b) is a cyclic voltammogram of an

electrocatalyst in accordance with one embodiment of the invention in anhydrous THF and in THF with 5% v/v added water under Ar. FIG. 1 (c) is an enlarged view of wave (3') of FIG. 1 (a) under C0₂;

In this example, the electrocatalyst was [Ru(tpy)(Mebim-py)(OH₂)]²⁺. FIG. 1 (a) and (c) were obtained utilizing a 0.5 M NaHC0₃ aqueous solution (pH of 8.4), 100 mV/s scan rate, glassy carbon working electrode (area 0.072 cm²), and SCE reference electrode. The scan was performed at room temperature. FIG. 1 (b) was obtained utilizing 0.1 M [(ⁿBu₄N)(PF₆)], glassy carbon working electrode (0.072 cm²), 100 mV/s scan rate, and AgN0₃/Ag reference electrode with ferrocene internal standard. The scan was performed at room temperature.

Irreversible reduction waves appear at E_p,_c = -1 .18 V vs NHE for wave (1 ) (E_p,c is the cathodic peak potential), and at £_p,_c = -1.40 V for wave (2). The latter appears on a rising background due to electrocatalysis and H₂ formation.

FIG. 1 (d) is a cyclic voltammogram of an electrocatalyst in accordance with one embodiment of the invention at various scan rates in water under C02. FIG. 1 (d) illustrates the variation of the peak current density (Ja) with variation in the scan rate (varied from 10-200 mV/s). See also FIG. 1 (e). FIG. 1 (e) is a plot of peak current density j_p,c under C02 vs. the inversed square root of the scan rate (ir^{1 2} in (V/s)^{_1 2}) taken in 0.5 M NaHC03 with a glassy carbon electrode (area 0.072 cm²) at 1 atm C02 and room temperature. The peak current density (/_d) for wave 1 varies with the square root of the scan rate (u^1/2), consistent with a diffusional process and the Randles-Sevcik relation⁴⁰ in Equation (26).

j = 0.4463(F³/RT)^{1 2}n_p ^{3 2}DRu^{1 2}[Ru]u^1/2 (26)

In Equation 26, F is Faraday's constant, R the universal gas constant, n_p the number of electrons transferred, Tthe temperature, and DR_u the diffusion coefficient for [Ru(tpy)(Mebim-py)(OH2)]²⁺.

The electrochemistry of an electrocatalyst in accordance with the present invention was further investigated in tetrahydrofuran (THF) with added water. As shown in FIG. 1 (b), in anhydrous THF, [Ru(tpy)(Mebim-py)(OH₂)]²⁺ undergoes sequential, reversible 1 e^~ reductions at Evz = -1.12 V, -1.36 V and -1.87 V vs NHE, respectively. These waves may arise from 1 e^~ reduction of the tpy ligand, [Ru"(tpy)(Mebim-py)(S)]²⁺ + e^~→ [Ru"(tpy^")(Mebim-py)(S)]⁺ (S is solvent), followed by reduction at Mebim-py and a second reduction at tpy. See Z. F. Chen, C. C. Chen, D. R. Weinberg, P. Kang, J. J.

Concepcion, D. P. Harrison, M. S. Brookhart and T. J. Meyer, Chem. Commun., 201 1 , 47, 12607-12609. With 5% v/v added water, the first two waves coalesce. A single 2e^~ reduction wave (1 ') appears at Em = -1.16 V having a peak-to-peak separation of 33 mV, presumably to give the twice-reduced complex. See A. J. Bard and L. R. Faulkner, Electrochemical Methods: Fundamentals and Applications, 2nd edn., Wiley, New York, 2001. The results in FIG. 1 (b) support an initial 2e^~ non-catalytic reduction (previously shown in Equation 4). The reduction may be followed by an additional reduction at £_p,_c = — 1.92 V (wave 4) which appears on a rising current background due to catalytic H₂ evolution. The current enhancement is ~2-fold compared to wave (1 ).

FIG. 1 (a) supports the mechanism illustrated in Equations 4-8. Evidence for this mechanism (e.g., the ruthenium polypyridyl complex with HCC ^" as the acid as proposed in Equations 4-8) comes from the influence of added water on the CV's in THF and the appearance of electrocatalytic reduction to H₂ at wave (2) in FIG. 1 (a). There is evidence for reduced hydride, [Ru"(tpy-)(Mebim-py)(H)]°, and dihydrogen, [Ru"(tpy-)(Mebim- Py)(H2)]⁺, intermediates in reverse CV scans with irreversible waves at E_p,a = -0.8 V (E_p,a, is the anodic peak potential) and E_p,_a = 0.1 V appearing in FIG. 1 (f). FIG. 1 (f) is a cyclic voltammograms of 1 mM [Ru(tpy)(Mebim-py)(OH₂)]²⁺ in accordance with one embodiment of the invention in water under Ar. FIG. 1 (g) is an enlarged view of FIG. 1 (f). The cyclic voltammogram were taken in 0.1 M NaHC0₃ with glassy carbon electrode (area 0.072 cm²) using a scan rate of 100 mV/s and at room temperature. Under C0₂, in this embodiment, HC0₃ ^~ may serve as the initial proton source for H₂ evolution since the concentration of carbonic acid is only -50 μΜ. See D. M. Kern, J. Chem. Educ , 1960, 37, 14. Added C0₂ can buffer the pH through comproportionation with generated COv², Equation 9, allowing C0₂/H₂0 to be the final proton source for H₂ evolution.

As shown in FIG. 1 (a), with added C0₂/HC0₃ ^~ in water, dramatic changes occur in the CV. The initial 2e^_ reduction at wave (1 ) at E_p,_c = -1.18 V is relatively unaffected. Wave (2') at E_p,_c = -1.40 V corresponds to wave (2) under Ar for H₂ evolution, but with added C0₂, its peak current is decreased by ~3.8-fold, consistent with suppression of H₂ evolution. A new wave (3') of comparable peak current appears at E_p,_c = -1.25 V (see FIG. 1 (a) and (c)) arising from catalytic C0₂ reduction. The suppression of H₂ evolution in 0.5 M NaHC0₃ saturated in C0₂ suggests capture of [Ru^ll(tpy-)(Mebim-py-)(S)]° by C0₂ in competition with hydride formation. C0₂ addition may occur through a dissociative mechanism and a 5-coordinate intermediate. See Z. F. Chen, C. C. Chen, D. R.

Weinberg, P. Kang, J. J. Concepcion, D. P. Harrison, M. S. Brookhart and T. J. Meyer, Chem. Commun., 201 1 , 47, 12607-12609. Competitive formation of the hydride

[Ru"(tpy)(Mebim-py)(H)]⁺ may occur by direct protonation of the twice-reduced

intermediate with coordination expansion.

Example 2.

Controlled potential electrolyses of 1 mM solutions of [Ru"(tpy)(Mebim-py)(S)]²⁺ were conducted between -1.2 V and -1.5 V at a carbon cloth electrode (1 .0 cm²) in aqueous solutions saturated in 1 atm C0₂ with various amounts of added NaHCC>3 (I = 0.5 M with added Na₂S0₄). Electrolysis currents were stable during 2-h electrolysis periods (FIG. 2(c)) with current densities ranging from 0.8-2.2 mA/cm² (see FIG. 2(a)-(b)). FIG. 2(c) is a plot of bulk electrolysis over time using a three-electrode setup with applied potential of -1.2V vs NHE at the cathode (electrolysis performed with 1 mM

[Ru"(tpy)(Mebim-py)(S)]²⁺, 0.5 M NaHC03, carbon cloth electrode (area 1.0 cm²), 1 atm C02, and at room temperature). FIG. 2(a) is a plot of total current densities j and partial current density for CO evolution y^'co and product distributions vs. applied potential (1 mM [Ru"(tpy)(Mebim-py)(S)]²⁺ in 0.5 M NaHC0₃). FIG. 2(b) is a plot of solution pH in electrolysis with an electrocatalyst in accordance with one embodiment of the invention (1 mM [Ru"(tpy)(Mebim-py)(S)]²⁺ in 0.05, 0.1 , 0.2, and 0.5 M NaHC0₃).

Headspace gas chromatographic (GC) analysis revealed CO and H₂ as products with combined current efficiencies of up to 99% (FIG. 2(d)). Analyses of the post electrolysis solutions by ¹H NMR failed to reveal appreciable levels of formate or methanol. FIGS. 2(d): A to D are plots of gas chromatography analysis of the cathode headspace after bulk electrolysis with an electrocatalyst in accordance with one embodiment of the invention using three-electrode setup. The applied potential vs NHE for each plot was: (A) -1 .2 V; (B) -1.3 V; (C) -1.4 V; (D) - 1.5 V. The electrolysis was performed with 1 mM [Ru"(tpy)(Mebim-py)(S)]²⁺, 0.5 M NaHC03, carbon cloth electrode, area 1.0 cm², 1 atm C02, and at room temperature.

As shown in FIG. 2(a), the H₂:CO ratio may be dependent on the applied potential and pH with the pH varied by varying the concentration of NaHC0₃ in C0₂ saturated solutions. At pH 7.2 with 0.5 M NaHCOs (FIG. 2(a)), H₂ becomes favored over CO as the electrolysis potential is decreased. At -1.2 V, selectivity for CO was maximized at 66%. Decreasing the potential from -1.2 to -1.5 V increased the H₂:CO ratio from 0.5: 1 to 2.6: 1. The H₂:CO ratio was also pH dependent at a fixed electrolysis potential. At -1.2V, decreasing the pH from 7.2 to 6.5 by decreasing NaHC0₃ from 0.5 to 0.05 M resulted in an increase in the H₂:CO ratio from 0.5: 1 to 4:1 (FIG. 2(b)). At pH 6.7 and with an applied potential of -1.2 V, the H₂:CO ratio is 2.2: 1 with 7.6 pmol of H₂ and 3.5 pmol of CO formed.

As shown in FIG. 2(a) and (b), current densities for C0₂ reduction, y^'co, are relatively constant under the conditions of the electrolysis experiments consistent with its rate limiting dissociative character. A mass transfer or kinetic limitation may exist resulting in a constant y^'co- Under these conditions C0₂ reduction may be a constant background reaction independent of applied potential or pH. H₂ evolution may be sensitive to both variables and may be a key to the control of product selectivity.

With the electrocatalyst as [Ru^N(tpy)(Mebim-py)(S)]²⁺, up to 66% CO was produced.

Example 3.

FIG. 3(a) is a cyclic voltammogram of an electrocatalyst in accordance with one embodiment of the invention in water under Ar and 1 atm C0₂. The cyclic voltammogram was taken using 1 mM [Ru"(tpy)(bpy)(S)]²⁺ in 0.5 M NaHC0₃ with a glassy carbon electrode (area 0.072 cm²) with a scan rate of 100 mV/s and at room temperature. FIG. 3(b) is a plot of gas chromotography analysis of the cathode headspace after bulk electrolysis with an electrocatalyst in accordance with one embodiment of the invention using three-electrode setup. The electrolysis was performed with 1 mM

[Ru"(tpy)(bpy)(S)]²⁺, 0.5 M NaHC0₃, carbon cloth electrode (area 1.0 cm²), applied potential -1.2 V, 1 atm C0₂, and at room temperature. With [Ru"(tpy)(bpy)(S)]²⁺ as the electrocatalyst, a single H₂ evolution wave was observed under either Ar or C0₂ (FIG. 3(a)). H₂ was observed as the dominant product with <1 % CO (Fig 3(b)) and there was no suppression of H₂ evolution by added C0₂. Without intending to be bound by theory, the contrast in reactivity between the [Ru"(tpy)(bpy)(S)]²⁺ catalyst and the

[Ruⁿ(tpy)(Mebim-py)(S)]²⁺ catalyst may be due to the strong trans influence of the labilizing N-heterocyclic carbene (NHC) ligand in [Ru"(tpy)(Mebim-py)(S)]²⁺. The

[Ru(tpy)(Mebim-py)(L)]²⁺ catalyst includes a labilizing N-heterocyclic carbene (NHC) ligand that may facilitate ligand loss and reaction of C0₂ with twice-reduced complex, [Ru"(tpy)(Mebim-py)(H₂0)]° or [Ru"(tpy)(Mebim-py)]°, to give the metallocarboxylate

[Ru"(tpy)(Mebim-py)(COO^")]° in competition with protonation reaction to give the hydride [Ru"(tpy)(Mebim-py)(H)]⁺. There is evidence for capture of the intermediate with added C0₂ by suppression of H₂ formation with added C0₂ in FIG. 1. By contrast, twice reduced [Ru"(tpy)(bpy)(S)]²⁺ is prone to protonation under the same conditions resulting in exclusive H₂ evolution and no redox responses of any captured C0₂ intermediate were observed.

Example 4.

FIG. 4 is a cyclic voltammogram of an electrocatalyst in accordance with one embodiment of the invention in water under C0₂. The cyclic voltammogram was taken using a glassy carbon electrode (area 0.072 cm²) with a scan rate of 100 mV/s and at room temperature. For 1 mM [Ru"(tpy)(Mebim-py)(S)]²⁺ in 0.5 M NaHC0₃ buffer under 1 atm C0₂, CV scans reveal a wave for the Ru^lll-OH²VRu"-OH₂ ⁺ couple at E = 0.92 V, the Ru^lv=0²7Ru'"-OH²⁺ couple at E_p,_a = 1.44 V, and the Ru^v(0)³7Ru^lv=0²⁺ couple. The latter initiates electrocatalytic water oxidation and is masked in the onset of water oxidation catalysis at 1.53 V vs NHE.

Example 5.

FIG. 5 is a plot of average cathode and anode potentials (V vs. NHE), and percentage energy efficiency (ε, %) for syngas production vs. applied cell voltage (Veen in V) with an electrocatalyst in accordance with one embodiment of the present invention. Internal resistances were included in the cell voltage. The electrolysis was performed with 1 mM [Ru"(tpy)(Mebim-py)(S)]²⁺ in 0.5 M NaHC0₃ in 1 atm C0₂ saturated water.

A constant voltage of between 2.7-3.3 V was applied across the cell. During the electrolysis, the current was stable with the pH of both compartments maintained. The separate potentials at each electrode in the operating cell were measured by using a stand-alone SCE electrode. Data are summarized in Figure 5 with the energy efficiency, ε, calculated by using Equation 17.

In the two-electrode electrolysis experiments, a 2:1 H₂:CO syngas ratio was produced under two sets of conditions. With 0.5 M NaHC0₃ buffer at pH 7.2, electrolysis at an applied voltage of 3.1 V with 1 mM catalyst resulted in a current density of 1.3 mA/cm² and with an energy efficiency of 47%. The same product composition was achieved with a cell voltage of 2.8 V at pH 6.7 (0.1 M NaHC0₃) with at a higher energy efficiency (53%) and current density of 1.4 mA/cm².

The term "about" refers to amounts within +/- 10, such as +/- 5 or +/- 2, significant figures of the respective measurement.

Many modifications and other embodiments of the inventions set forth herein will come to mind to one skilled in the art to which these inventions pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the inventions are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

Claims

THAT WHICH IS CLAIMED:

1. A method of producing syngas via aqueous electrolysis comprising:

applying a voltage to an electrolytic cell comprising an electrocatalyst and an aqueous solution, said solution comprising C0₂, to obtain a mixture of CO and H₂,

wherein the electrocatalyst comprises a metal complex compound.

2. The method of producing syngas according to claim 1 , wherein the metal complex compound is a ruthenium polypyridyl complex comprising a monodentate ligand, bidentate ligand, and a tridentate ligand.

3. The method of producing syngas according to claim 1 , wherein the metal complex compound is selected from the group consisting of [Ru(tpy)(bpy)(L)]²⁺,

[Ru(tpy)(Mebim-py)(L)]²⁺, [Ru(tpy)(bpm)(L)]²⁺, [Ru(tpy)(bpz)(L)]²⁺, [Ru(tpy)(Mebim- pz)(L)]²⁺, [Ru(DMAP)(bpy)(L)]²⁺, [Ru(Mebim-py)(bpy)(L)]²⁺, [Ru(Mebim-py)(Mebim- pz)(L)]²⁺, [Ru(Mebim-py)(Mebim-py)(L)3²⁺, and {Ru(Mebim-py)[4,4'- ((HO)₂OPCH₂)₂bpyJ(L)}²⁺,

wherein L is a monodentate ligand selected from the group consisting of OH₂

(aqua), NH₃ (ammine), CHs H₂ (methylamine), CO (carbonyl), NO (nitrosyl), F^" (fluoro), CN- (cyano), CI^" (chloro), Br(bromo), I^" (iodo), N0₂ (nitro), and OH^" (hydroxyl).

4. The method of producing syngas according to claim 1 , wherein the metal complex compound comprises Formula A

Formula A wherein the ligand

5. The method of producing syngas according to claim 1 , wherein the metal complex compound comprises a carbene ligand that facilitates CO2 reduction.

6. The method of producing syngas according to claim 1 , wherein the metal complex compound comprises Formula B: 2+

Formula B

wherein Ri, R₂, R3 are selected from the group consisting of

H or a C-1 -C30 hydrocarbyl radical

^•COOH n=l-30

7. The method of producing syngas according to claim 1 , wherein said aqueous solution further comprises a weak acid.

8. The method of producing syngas according to claim 1 , wherein said aqueous solution further comprises sodium bicarbonate, potassium bicarbonate or combinations thereof.

9. The method of producing syngas according to claim 1 , wherein the mixture of CO and H₂ has a ratio of hydrogen to carbon monoxide from about 10:1 to about 1 :10.

10. The method of producing syngas according to claim 1 further comprising controlling a ratio of hydrogen to carbon monoxide by adjusting the applied voltage, adjusting a pH of the solution, or combinations thereof.

1 1. The method of producing syngas according to claim 1 , wherein the applied voltage is from about 1 to 5 V.

12. The method of producing syngas according to claim 1 , wherein the pH of the solution is from about 5.5 to about 9.5. 3. The method of producing syngas according to claim 1 , wherein applying a voltage results in oxidation of water to O2. 14. An apparatus for the production of syngas comprising:

a cathode,

an anode,

an electrocatalyst, and

an aqueous solution comprising CO2,

wherein the electrocatalyst catalyzes the reduction of CO2 and water to prepare a mixture of CO and H2 and wherein the electrocatalyst comprises a metal complex compound.

15. The apparatus according to claim 14, wherein the metal complex compound is a metal polypyridyl complex comprising a monodentate ligand, bidentate ligand, and a tridentate ligand.

16. The apparatus according to claim 14, wherein the metal complex compound is selected from the group consisting of [Ru(tpy)(bpy)(L)]²⁺, [Ru(tpy)(Mebim- py)(L)]²⁺, [Ru(tpy)(bpm)(L)]²⁺, [Ru(tpy)(bpz)(L)]²⁺, [Ru(tpy)(Mebim-pz)(L)]²⁺,

[Ru(DMAP)(bpy)(L)] ⁺, [Ru(Mebim-py)(bpy)(L)]²⁺, [Ru(Mebim-py)(Mebim-pz)(L)]²⁺,

[Ru(Mebim-py)(Mebim-py)(L)]²⁺, and {Ru(Mebim-py)[4,4'-((HO)₂OPCH₂)₂bpy](L)}²⁺,

wherein L is a monodentate ligand selected from the group consisting of OH₂ (aqua), NH₃ (ammine), CH₃NH₂ (methylamine), CO (carbonyl), NO (nitrosyl), F^" (fluoro), CN- (cyano), CI^" (chloro), Br(bromo), I^" (iodo), N0₂ (nitro), and OH^" (hydroxyl).

17. The apparatus according to claim 14, wherein the metal complex compound comprises Formula A

Formula A wherein the ligand

is selected from the group consisting of

Mebim-pz Melm-pz

18. The apparatus according to claim 14, wherein the metal complex compound comprises a carbene ligand that facilitates CO2 reduction.

19. The apparatus according to claim 14, wherein the metal complex compound comprises Formula B:

Formula B

wherein Ri, R₂, R3 are selected from the group consisting of

H or a C-, -C₃o hydrocarbyl radical

— COOH , n=l-30

20. The apparatus according to claim 14, wherein the aqueous solution further comprises a weak acid.

21 . The apparatus according to claim 14, wherein the aqueous solution further comprises sodium bicarbonate.

22. The apparatus according to claim 14, wherein the mixture of CO and H2 has a ratio of hydrogen to carbon monoxide from about 10: 1 to about 1 : 10.

23. The apparatus according to claim 22, wherein the apparatus is configured to allow adjustment of the applied voltage and pH to control the ratio of hydrogen to carbon monoxide.

24. The apparatus according to claim 14, wherein water is oxidized to 0₂ at