Electrocatalytic process for carbon dioxide conversion

Abstract

An electrocatalytic process for carbon dioxide conversion includes combining a Catalytically Active Element and Helper Catalyst in the presence of carbon dioxide, allowing a reaction to proceed to produce a reaction product, and applying electrical energy to said reaction to achieve electrochemical conversion of said reactant to said reaction product. The Catalytically Active Element can be a metal in the form of supported or unsupported particles or flakes with an average size between 0.6 nm and 100 nm. the reaction products comprise at least one of CO, HCO.sup., H.sub.2CO, (HCO.sub.2).sup., H.sub.2CO.sub.2, CH.sub.3OH, CH.sub.4, C.sub.2H.sub.4, CH.sub.3CH.sub.2OH, CH.sub.3COO.sup., CH.sub.3COOH, C.sub.2H.sub.6, (COOH).sub.2, (COO.sup.).sub.2, and CF.sub.3COOH.

Claims

1. A process for making a chemical reaction product, comprising: combining a Catalytically Active Element and Helper Catalyst in the presence of CO.sub.2, wherein the Catalytically Active Element is a metal in the form of supported or unsupported particles or flakes with an average size between 0.6 nm and 100 nm; allowing a reaction to proceed to produce a reaction product; and applying electrical energy to said reaction to achieve electrochemical conversion of said reactant to said reaction product; wherein a substance S is identifiable as a Helper Catalyst for an electrochemical reaction R that is catalyzed by a Catalytically Active Element M by applying a test comprising: filling a three-electrode electrochemical cell with an electrolyte E suitable for effectuating reaction R, said electrochemical cell comprising a working electrode, a counter electrode and a reference electrode; electrically connecting said Catalytically Active Element M to said working electrode and electrically connecting said working electrode to said counter electrode and to a source of electrical energy, thereby forming an electrical circuit; employing said reference electrode to measure a reversible hydrogen electrode (RHE) potential in said electrolyte E; loading at least one reactant for reaction R into said cell; measuring a cyclic voltammogram for reaction R to identify a potential of a peak associated with reaction R; calculating a difference V1 between RHE and an onset potential of said peak associated with reaction R; calculating a difference V1A between RHE and a maximum potential of said peak associated with reaction R; adding to said electrolyte E between 0.0001% and 99.9999% by weight of said Helper Catalyst; measuring an RHE potential in electrolyte E with said added Helper Catalyst; measuring a cyclic voltammogram of reaction R with said added Helper Catalyst; calculating a difference V2 between RHE and an onset potential of said peak associated with reaction R; calculating a difference V2A between RHE and a maximum potential of said peak associated with reaction R; determining whether V2<V1 or V2A<V1A at any concentration of said Helper Catalyst between 0.0001% and 99.9999%; and identifying substance S as said Helper Catalyst for reaction R at concentrations at which V2<V1 or V2A<V1A.

2. The process of claim 1 wherein said Catalytically Active Element is selected from the group consisting of V, Cr, Mn, Fe, Co, Ni, Cu, Sn, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, Hf, Ta, W, Re, Ir, Pt, Au, Hg, Al, Si, In, Tl, Pb, Bi, Sb, Te, U, Sm, Tb, La, Ce, and Nd.

3. The process of claim 2 wherein said Catalytically Active Element is selected from the group consisting of Pt, Pd, Au, Ag, Cu, Ni, Fe, Co, Ru and Rh.

4. The process of claim 1 wherein said Helper Catalyst comprises at least one of an organic cation and an organic anion.

5. The process of claim 1 wherein said Helper Catalyst is selected from the group consisting of phosphines, imidazoliums, pyridiniums, pyrrolidiniums, phosphoniums, sulfoniums, prolinates, methioninates, cholines, choline chloride, choline bromide, and choline iodide.

6. The process of claim 1 wherein the reaction products comprise at least one of CO, HCO.sup., H.sub.2CO, (HCO.sub.2).sup., H.sub.2CO.sub.2, CH.sub.3OH, CH.sub.4, C.sub.2H.sub.4, CH.sub.3CH.sub.2OH, CH.sub.3COO.sup., CH.sub.3COOH, C.sub.2H.sub.6, (COOH).sub.2, (COO.sup.).sub.2, and CF.sub.3COOH.

7. The process of claim 6 wherein said reaction product is one of formic acid (H.sub.2CO.sub.2) and carbon monoxide (CO).

8. The process of claim 1, wherein the Catalytically Active Element is electrically connected to an electrode of an electrochemical device.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a diagram of a typical electrochemical cell.

(2) FIG. 2 illustrates how the cyclic voltammogram (CV) of electrochemical water reduction varies with the average particle size of the Catalytically Active Element silver on the cathode: (A) bare silver electrode, (B) electrode covered by nominally 200 nm silver particles, (C) electrode covered by nominally 100 nm silver particles, (D) electrode covered by nominally 20-40 nm silver particles, (E) electrode covered by 5 nm silver particles, and (F) electrode covered by 0.67 nm silver particles. The numeric designations 500-505 indicate the positions of the hydrogen peak in the figures. All of the CV's are reported as the current divided by the electrochemical surface area of the particles

(3) FIG. 3 illustrates how the CV changes in FIG. 2 when CO.sub.2 is added to the reaction: (A) bare silver electrode, (B) electrode covered by nominally 200 nm silver particles, (C) electrode covered by nominally 100 nm silver particles, (D) electrode covered by nominally 20-40 nm silver particles, (E) electrode covered by 5 nm silver particles, and (F) electrode covered by 0.67 nm silver particles. The numeric designations 600-605 indicate the CO.sub.2 reduction peak in the figures. All of the CV's are reported as the current divided by the electrochemical surface area of the particles

(4) FIG. 4 illustrates particle size distribution of the (A) nominally 20-40 nm silver particles, (B) nominally 100 nm silver particles, and (C) nominally 200 nm silver particles as measured by dynamic light scattering. The nominally 20-40 nm particles have an average size of 35 nm. The nominally 100 nm particles have an average size of 70 nm and the nominally 200 nm particles have an average size of 190 nm.

DETAILED DESCRIPTION OF EMBODIMENT(S)

(5) The invention disclosed herein is not limited to the particular methodology, protocols, and reagents described herein, as these can vary as persons familiar with the technology involved here will recognize. The terminology employed herein is used for the purpose of describing particular embodiments only, and is not intended to limit the scope of the invention. As used herein and in the appended claims, the singular forms a, an, and the include the plural reference unless the context clearly dictates otherwise. Thus, for example, a reference to a linker is a reference to one or more linkers and equivalents thereof known to persons familiar with the technology involved here.

(6) Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by persons familiar with the technology involved here. The embodiments of the invention and the various features and advantageous details thereof are explained more fully with reference to the non-limiting embodiments and/or illustrated in the accompanying drawings and detailed in the following description. It should be noted that the features illustrated in the drawings are not necessarily drawn to scale, and features of one embodiment can be employed with other embodiments as persons familiar with the technology involved here would recognize, even if not explicitly stated herein.

(7) Any numerical value ranges recited herein include all values from the lower value to the upper value in increments of one unit provided that there is a separation of at least two units between any lower value and any higher value. As an example, if it is stated that the concentration of a component or value of a process variable such as, for example, size, angle size, pressure, time and the like, is, for example, from 1 to 90, specifically from 20 to 80, more specifically from 30 to 70, it is intended that values such as 15 to 85, 22 to 68, 43 to 51, 30 to 32, and so on, are expressly enumerated in this specification. For values which are less than one, one unit is considered to be 0.0001, 0.001, 0.01 or 0.1 as appropriate. These are only examples of what is specifically intended and all possible combinations of numerical values between the lowest value and the highest value are to be treated in a similar manner.

(8) Moreover, provided immediately below is a Definitions section, where certain terms related to the invention are defined specifically. Particular methods, devices, and materials are described, although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the invention. All references referred to herein are incorporated by reference herein in their entirety.

DEFINITIONS

(9) The term electrochemical conversion of CO.sub.2 as used here refers to any electrochemical process where carbon dioxide, carbonate, or bicarbonate is converted into another chemical substance in any step of the process.

(10) The term CV as used here refers to a cyclic voltammogram or cyclic voltammetry.

(11) The term Overpotential as used here refers to the potential (voltage) difference between a reaction's thermodynamically determined reduction or oxidation potential and the potential at which the event is experimentally observed.

(12) The term Cathode Overpotential as used here refers to the overpotential on the cathode of an electrochemical cell.

(13) The term Anode Overpotential as used here refers to the overpotential on the anode of an electrochemical cell.

(14) The term Electron Conversion Efficiency refers to selectivity of an electrochemical reaction. More precisely, it is defined as the fraction of the current that is supplied to the cell that goes to the production of a desired product.

(15) The term Catalytically Active Element as used here refers to any chemical element that can serve as a catalyst for the electrochemical conversion of CO.sub.2.

(16) The term Helper Catalyst refers to any organic molecule or ion, or a mixture of organic molecules and or ions, that does at least one of the following: (a) Speeds up a chemical reaction or (b) Lowers the overpotential of the reaction without being substantially consumed in the process.

(17) The term Active Element, Helper Catalyst Mixture refers to any mixture that includes one or more Catalytically Active Element(s) and at least one Helper Catalyst

(18) The term Ionic Liquid refers to salts or ionic compounds that form stable liquids at temperatures below 200 C.

(19) The term Deep Eutectic Solvent refers to an ionic solvent that includes a mixture which forms a eutectic with a melting point lower than that of the individual components.

(20) The term EMIM refers to 1-ethyl-3-methylimidazolium cations.

(21) The term EMIM-BF4 refers 1-ethyl-3-methylimidazolium tetrafluoroborate.

(22) The term Particle Size refers to the average diameter of the catalytically active element particles in a catalyst or metal powder as determined by any of the following methods: x-ray line broadening or surface area measurements as described in section [0089]. The term QRE refers to a quasi-reference electrode.

(23) The term Ag-QRE refers to a silver quasi reference electrode.

(24) The term potential of the peak refers to the potential where the absolute value of the current is maximized.

(25) Specific Description

(26) The invention relates generally to Active Element, Helper Catalyst Mixtures where the mixture does at least one of the following:

(27) Speeds up a chemical reaction, or

(28) Lowers the overpotential of the reaction,

(29) without being substantially consumed in the process.

(30) For example such mixtures can lower the overpotential for CO.sub.2 conversion to a value less than the overpotentials seen when the same Catalytically Active Element is used without the Helper Catalyst.

(31) U.S. patent application Ser. No. 12/830,338 (published as US 2011/0237830), and Ser. No. 13/174,365 (not yet published), and in International Application No. PCT/US2011/030098 (published as WO 2011/120021) and PCT/US2011/042809 (published as WO 2012/006240) disclose that a catalyst mixture containing a Catalytically Active Metal and a Helper Catalyst could catalyze CO.sub.2 conversions with low overpotential and high electron conversion efficiency. Active Metals included one or more of V, Cr, Mn, Fe, Co, Ni, Cu, Sn, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, Hf, Ta, W, Re, Ir, Pt, Au, Hg, Al, Si, In, Sn, Tl, Pb, Bi, Sb, Te, U, Sm, Tb, La, Ce, and Nd. Examples of Helper Catalysts included one or more of phosphines, imidazoliums, pyridiniums, pyrrolidiniums, phosphoniums, sulfoniums, prolinates, methioninates, cholines, acetylcholines, alanines, aminoacetonitriles, methylammoniums, arginines, aspartic acids, threonines, chloroformamidiniums, thiouroniums, quinoliniums, pyrrolidinols, serinols, benzamidines, sulfamates, acetates, carbamates, triflates, and cyanides.

(32) Of course, the '338 U.S. application, the '365 U.S. application, the '098 international application and the '809 international application disclose that not every substance listed above will act as a Helper Catalyst. In general, one can determine whether a given substance S is a Helper Catalyst for a reaction R catalyzed by an active metal M as follows: Fill a standard 3 electrode electrochemical cell with the electrolyte commonly used for reaction R. Common electrolytes such as 0.1 M sulfuric acid or 0.1 M KOH in water can often be used. Mount the active metal on the working electrode in the 3 electrode electrochemical cell, with an appropriate counter electrode. Run several CV cycles to clean the active metal. Measure the reversible hydrogen electrode (RHE) potential in the electrolyte. Load the reactants for the reaction R into the cell, and measure a CV of the reaction R, noting the potential of the peak associated with the reaction R. Calculate V1=the difference between the onset potential of the peak associated with the reaction R and RHE. Calculate V1A=the difference between the potential of the peak associated with the reaction R and RHE. Add 0.0001 to 99.9999% of the substance S to the electrolyte. Measure RHE in the reaction with Helper Catalyst. Measure the CV of reaction R again, noting the potential of the peak associated with the reaction R. Calculate V2=the difference between the onset potential of the peak associated with the reaction R and RHE. Calculate V2A=the difference between the potential of the peak associated with the reaction R and RHE. If V2<V1 or V2A<V1A at any concentration of the substance S between 0.0001 and 99.9999%, the substance S is a Helper Catalyst for the reaction.

(33) Further, the Helper Catalyst could be in any one of the following forms: (i) a solvent for the reaction; (ii) an electrolyte; (iii) an additive to any component of the system; or (iv) something that is bound to at least one of the catalysts in a system. These examples are meant for illustrative purposes only, and are not meant to limit the scope of the invention.

(34) The catalyst disclosed in '338 U.S. application, the '365 U.S. application, the '098 international application and the '809 international application allowed CO.sub.2 to be converted to useful products at low overpotentials and high electron conversion efficiencies. In particular, an EMIM silver catalyst, with 100 nm silver particles, allowed CO.sub.2 to be converted with an electron conversion efficiency as high as 99%, and an overpotential as small as 0.17 V. Unfortunately, the silver/EMIM catalysts disclosed in the '338 U.S. application, the '365 U.S. application, the '098 international application and the '809 international application had a lower catalytic activity than was desired.

(35) In the foregoing applications, new catalyst formulations are disclosed that maintain the ability to convert CO.sub.2 with a high electron conversion efficiency and low overpotential, but also shows an improved specific catalytic activity. The catalyst was prepared by modifying the structure of the silver so it is more active. The catalyst can be in the form of supported or unsupported metal particles wherein the average particle size is below about 100 nm, preferably between 0.6 nm and 40 nm, more preferably between 0.6 nm and 20 nm, and most preferably between 0.6 nm and 10 nm.

(36) In this context, the particle sizes can be determined by one or more of microscopy, x-ray line broadening, chemisorption, or small x-ray scattering. For example, one might measure the x-ray diffraction pattern of the catalysts, determine the width of the diffraction line at some angle , and then calculate the particle size, D, from the Scherrer equation:

(37) $D=\frac{0.9}{\cos \left(\right)}$

(38) where is the wavelength of the x-ray beam.

(39) Chemisorption can alternately be used to measure the Sg, the surface area of the active element per unit mass, and then calculate the particle size from the formula

(40) $D=\frac{5}{S_g}$

(41) where is the bulk density of the active element.

(42) The invention specifically includes any catalyst with a particle size between 0.6 nm and 100 nm measured by any of x-ray line broadening, chemisorption, or small x-ray scattering.

Example 1

An Illustration of the Effects of Particle Size on CO2 Conversion on a Silver/EMIM Catalyst

(43) Catalysts Tested

(44) This example illustrates the effect of silver particle size on the rate of CO.sub.2 conversion on an EMIM-BF4 (Sigma Aldrich, St Louis Mo.) coated silver catalyst.

(45) Experimental

(46) During the experiments, a silver ink was deposited onto a silver rotating disk electrode (Pine Instruments, Grove City, Pa.). In the first experiment, the silver electrode was polished, and a CV was run as described in paragraph [0099] [0102] below. Subsequent experiments were run by depositing one of i) 200 nm silver (Sigma Aldrich, St Louis, Mo.) ii) 100 nm silver (Sigma Aldrich, St Louis, Mo.) iii) 20-40 nm silver (Sigma Aldrich, St Louis, Mo.) vi) 5 nm (UT Dots, Champaign Ill.) v) 0.67 nm (Purest Colloids, Westampton, N.J.) onto the silver electrode and running the CV as in the fourth paragraph below.

(47) In the case of 500-1000 nm, 100 nm, 20-40 nm silver a silver ink is prepared by mixing 5.6 mg of silver particles with 1 ml deoxygenated Millipore water. The catalyst is applied on the surface of the rotating electrode by adding 60 L of the ink to the surface and allowing the water to evaporate under ambient temperature for 60 minutes. In order to ensure the quality of the measurements, special attention is paid to the material cleaning and solution purity.

(48) The 10 nm arrived suspended in an organic solution, and so this solution was applied to the silver electrode, and subsequently heated in air at 85 C. for 3 hours to remove any residual organics

(49) The 0.6 nm particles arrived suspended in distilled water, so they were used as received.

(50) The counter electrode is made by attaching a 2525 mm platinum mesh (size 52) to a 5 inch platinum wire (99.9%, 0.004 inch diameter). A silver quasi-reference electrode was used. The electrolytes were first loaded into the glass cell and then purged with dry argon (99.99%) for two hours in order to remove oxygen from the electrolytes. Prior to each experiment, a 20-40 linear sweep cyclic voltammogram at 75 mVs.sup.1 is taken with the range between 2 V and 0 vs. Ag-QRE in order to condition the electrodes and remove oxides from the surfaces. Then several cycles were performed at 10 mVs.sup.1 before taking the final cycle to insure that the CV had stabilized (that is, any dirt or other material is removed from the surfaces). Finally, cleaning and stabilizing CV cycles were performed at 10 mVs.sup.1. Then CO.sub.2 was bubbled in the solution at 1 atm pressure for 20 minutes. Cyclic voltammetry was conducted again at a sweep rate of 10 mV/s in the range from 2 V to 0 V vs. Ag-QRE.

(51) Results: Water Electrolysis

(52) FIG. 2 shows how the CV of water varies with particle size. One finds that there is very little variation in the catalytic activity of silver, in agreement with the literature reviewed in a recent paper, M. T. M. Koper, Structural sensitivity and nanostructure effects in electrocatalysis, Nanoscale 3, 2054 (2011) (the Koper Paper)

(53) Results: CO.sub.2

(54) FIG. 3 shows how the CV changes when CO.sub.2 is added to the electrochemical cell. Notice that CO.sub.2 conversion peaks, labeled 100, 101, 102, 103, 104, 105 and 106 are observed. The CO.sub.2 conversion peaks grow as one decreases the particle size from 1000-10 nm, but then we observed the opposite effect, where the CO.sub.2 peak shrinks moving from 10 nm to 0.6 nm. Evidently, there is an optimal particle size somewhere between 0.6 and 100 nm. The optimal size is probably between 0.6 and 40 nm, most probably between 0.6 and 20 nm.

(55) The result above shows that unsupported silver catalysts have an optimal size between 0.6 and 100 nm. Those trained in the art know that if a reaction is structure sensitive on an unsupported catalyst, or single crystal, it will also show structure sensitivity on a supported catalyst. Further, the Korper Paper suggests that if a reaction is structure sensitive on one transition metal, it will also be structure sensitive on most other transition metals. In particular the optimum particle size is very similar on most transition metals. Therefore, we conclude that V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, W, Re, Ir, Pt, Au, Hg, Pb, Bi, electrocatalysts for CO.sub.2 conversion should have an optimal size between 0.6 and 100 nm.

(56) The examples given above are merely illustrative and are not meant to be an exhaustive list of all possible embodiments, applications or modifications of the invention. Thus, various modifications and variations of the described methods and systems of the invention will be apparent to those skilled in the art without departing from the scope and spirit of the invention. Although the invention has been described in connection with specific embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. Indeed, various modifications of the described modes for carrying out the invention which are obvious to those skilled in the chemical arts or in the relevant fields are intended to be within the scope of the appended claims.

(57) The disclosures of all references and publications cited above are expressly incorporated by reference in their entireties to the same extent as if each were incorporated by reference individually.