DE102019201153A1 - Process for the energy-efficient production of CO - Google Patents

Abstract

The present invention relates to a method for producing CO from CO, and an apparatus for performing the method.

Description

The present invention relates to a method for producing CO from CO ₂ and an apparatus for carrying out the method.

State of the art

Approximately 80% of the world's energy requirements are currently covered by the combustion of fossil fuels, the combustion processes of which cause around 34,000 million tons of carbon dioxide to be emitted into the atmosphere each year. As a result of this release into the atmosphere, most of the carbon dioxide is disposed of, which can be up to 50,000 tons per day, for example in a brown coal power plant. Carbon dioxide is one of the so-called greenhouse gases, the negative effects on the atmosphere and the climate are discussed. It is a technical challenge to produce valuable products from CO ₂ . Since carbon dioxide is thermodynamically very low, it is difficult to reduce it to recyclable products, which has so far left the actual recycling of carbon dioxide in theory or in the academic world.

Natural carbon dioxide degradation takes place, for example, through photosynthesis. In this process, carbon dioxide is converted into carbohydrates in a process that is broken down into many sub-steps in terms of time and at the molecular level. This process is not easily adaptable on an industrial scale. A copy of the natural photosynthesis process with large-scale photo catalysis has not been sufficiently efficient so far.

An alternative is the electrochemical reduction of carbon dioxide. Systematic studies of the electrochemical reduction of carbon dioxide are still a relatively young field of development. Only a few years ago there have been efforts to develop an electrochemical system that can reduce an acceptable amount of carbon dioxide. Laboratory-scale research has shown that metals are preferred as catalysts for the electrolysis of carbon dioxide. From the publication Electrochemical CO ₂ reduction on metal electrodes by Y. Hori, published in: C. Vayenas, et al. (Eds.), Modern Aspects of Electrochemistry, Springer, New York, 2008, pp. 89-189, Faraday efficiencies can be found on different metal cathodes, see Table 1, which is taken from it. Table 1: Faraday efficiencies for CO ₂ on different electrodes electrode CH ₄ C ₂ H ₄ C ₂ H ₅ OH C ₃ H ₇ OH CO HCOO ^- H ₂ Total Cu 33.3 25.5 5.7 3.0 1.3 9.4 20.5 103.5 Au 0.0 0.0 0.0 0.0 87.1 0.7 10.2 98.0 Ag 0.0 0.0 0.0 0.0 81.5 0.8 12.4 94.6 Zn 0.0 0.0 0.0 0.0 79.4 6.1 9.9 95.4 Pd 2.9 0.0 0.0 0.0 28.3 2.8 26.2 60.2 Ga 0.0 0.0 0.0 0.0 23.2 0.0 79.0 102.0 Pb 0.0 0.0 0.0 0.0 0.0 97.4 5.0 102.4 Ed 0.0 0.0 0.0 0.0 0.0 99.5 0.0 99.5 In 0.0 0.0 0.0 0.0 2.1 94.9 3.3 100.3 Sn 0.0 0.0 0.0 0.0 7.1 88.4 4.6 100.1 CD 1.3 0.0 0.0 0.0 13.9 78.4 9.4 103.0 Tl 0.0 0.0 0.0 0.0 0.0 95.1 6.2 101.3 Ni 1.8 0.1 0.0 0.0 0.0 1.4 88.9 92.4 Fe 0.0 0.0 0.0 0.0 0.0 0.0 94.8 94.8 Pt 0.0 0.0 0.0 0.0 0.0 0.1 95.7 95.8 Ti 0.0 0.0 0.0 0.0 0.0 0.0 99.7 99.7

In Table 1, Faraday efficiencies FE [%] of products are shown that result from the reduction of carbon dioxide on various metal electrodes. The values given apply to a 0.1 M potassium bicarbonate solution as the electrolyte.

If, for example, carbon dioxide is almost exclusively reduced to carbon monoxide on silver, gold, zinc, palladium and gallium cathodes, a large number of hydrocarbons are formed as reaction products on a copper cathode.

• Kathode: 2 CO₂ + 4 e^- + 4H⁺ → 2 CO + 2 H₂O
• Anode: 2 H₂O → O₂ + 4 H⁺ + 4 e^-

For example, carbon monoxide and little hydrogen would predominantly be produced on a silver cathode. The reactions at the anode (with water) and cathode can be represented with the following reaction equations:

• Cathode: 2 CO ₂ + 4 e ^- + 4H ⁺ → 2 CO + 2 H ₂ O
• Anode: 2 H ₂ O → O ₂ + 4 H ⁺ + 4 e ^-

For example, the electrochemical production of carbon monoxide, ethylene or alcohols is of particular economic interest.

• Kohlenmonoxid: CO₂ + 2 e^- + H₂O ➙ CO + 2 OH^-
• Ethylen: 2 CO₂ + 12 e^- + 8 H₂O ➙ C₂H₄ + 12 OH^-
• Methan: CO₂ + 8 e^- + 6 H₂O ➙ CH₄ + 8 OH^-
• Ethanol: 2 CO₂ + 12 e^- + 9 H₂O ➙ C₂H₅OH + 12 OH^-
• Monoethylenglykol: 2 CO₂ + 10 e^- + 8 H₂O ➙ HOC₂H₄OH + 10 OH^-

Examples:

• Carbon monoxide: CO ₂ + 2 e ^- + H ₂ O ➙ CO + 2 OH ^-
• Ethylene: 2 CO ₂ + 12 e ^- + 8 H ₂ O ➙ C ₂ H ₄ + 12 OH ^-
• Methane: CO ₂ + 8 e ^- + 6 H ₂ O ➙ CH ₄ + 8 OH ^-
• Ethanol: 2 CO ₂ + 12 e ^- + 9 H ₂ O ➙ C ₂ H ₅ OH + 12 OH ^-
• Monoethylene glycol: 2 CO ₂ + 10 e ^- + 8 H ₂ O ➙ HOC ₂ H ₄ OH + 10 OH ^-

The energy efficiency of the one-step electrochemical reduction of CO ₂ depends not only on the overvoltages on the electrodes, ohmic losses in the electrolyte and / or on the membranes, but also on the selected mode of operation of the electrolysis cell (stack), since the electrolysis cell is operated asymmetrically. For example, CO _{2 is} reduced at the cathode and H ₂ O is oxidized at the anode. To ensure continuous continuous operation, the side reactions or buffer reactions of the reaction by-products (hydroxide, proton, hydrogen carbonate formation) must also be considered.

The minimum voltage of a cell can be derived from the enthalpy of combustion from CO to CO ₂ , since electrolysis is purely formally the reversal of the combustion. Analogously, these considerations naturally also apply to other possible reaction products for electrochemical CO ₂ reduction, such as ethylene, ethanol, methane, acetate, formic acid, formate, etc.

This is considered theoretically below.

Since electrolysers are not in thermal equilibrium with their environment and are therefore operated adiabatically, reaction enthalpies ΔH are used for these theoretical considerations.

In principle, a calculation with free reaction enthalpies ΔG gives the same result. CO ₂ → CO + ½ O ₂ ΔH = 283, 16 kJ / mol CO

Dividing this number by the Faraday constant (96485.33289 C / mol) and the number of electrons transferred (z = 2) gives the minimum voltage for the reaction of 1.47 V.

For modes of operation as described in "Technical photosynthesis involving CO ₂ electrolysis and fermentation", Thomas Haas, Ralf Krause, Rainer Weber, Martin Demler and Guenter Schmid, Nature Catalysis, Vol 1, JAN 2018 | 32-39, https://doi.org/10.1038/s41929-017-0005-1 are presented, the minimum voltages of the single cell are slightly above 2 V if the IV characteristics of current densities are in the range of several 100 mA / cm ² extrapolated to a current of 0 mA / cm ² .

With very small current densities in the range far below 1 mA / cm ² , the same extrapolation results in voltages in the range of 1.5V as from Rosen, BA; Salehi-Khojin, A .; Thorson, MR; Zhu, W .; Whipple, DT; Kenis, PJ; Masel, RI, "Ionic liquid-mediated selective conversion of CO ₂ to CO at low overpotentials", Science 2011, 334 (6056), 643-4. The behavior is explained by a cocatalytic effect of the ionic liquid used as the electrolyte.

However, this could not be confirmed in our own experiments at current densities above 50-100 mA / cm ² , as in Neubauer, SS; Schmid, B .; Reller, C .; Guldi, DM; Schmid, G., "Alkalinity Initiated Decomposition of Mediating Imidazolium Ions in High Current Density CO ₂ Electrolysis", Chemelectrochem 2017, 4 (1), 160-167 reports.

An explanation of this observation can be provided by considering the half-cell potentials including the aqueous electrolyte solution.

The water oxidation with the half-cell potential derived below is always considered as an anode reaction. 2 H ₂ O - 4e- → O ₂ + 4H ⁺ ΔH = 571.7 kJ

With z = 4 one obtains U _A = 1.48 V.

The cathode reaction to produce carbon monoxide is: CO ₂ + H ₂ O + 2e- → CO + 2OH ^- ΔH = 108.8 kJ

With z = 2 one obtains U _K = 0.56 V.

The cell voltage results from U _Z = U _A + U _K = 2.04 V.

The representation used does not correspond to the convention, since ΔH = -z · F · E normally applies. However, the minus sign is omitted for clarity. The voltage obtained therefore has the same sign as ΔH, so that the reasoning is easier to understand.

If the sum of the cathode and anode reactions is formed and the stoichiometry is correct, the cell voltage for the reaction 2 CO ₂ → 2 CO + O ₂ in aqueous media is 2.04 V, in contrast to 1.47 V in the gas phase analysis. This means that when CO _{2 is} reduced to CO in aqueous media, purely thermodynamically speaking, more electrical energy must be used than would be required for the actual reduction.

This energy is generated in the cathode compartment, as in "Technical photosynthesis involving CO ₂ electrolysis and fermentation", Thomas Haas, Ralf Krause, Rainer Weber, Martin Demler and Guenter Schmid, Nature Catalysis, Vol 1, JAN 2018 | 32-39 described as thermal energy released by the absorption of CO ₂ : OH ^- + CO ₂ → HCO ₃ ^- , ΔH = 66, 37 kJ. With z = 1 this can be assigned to a potential of 0.69 V.

Some of this thermal energy can be used to decompose CO ₂ in the anode compartment from hydrogen carbonate: H ⁺ + HCO ₃ ^- → CO ₂ + H ₂ O ΔH = 10.57 kJ. With z = 1 this can be assigned to a potential of 0.11 V. This means that - if you summarize the thermal energy balance in the cathode and anode compartment - at least the neutralization energy must be applied as electrical energy in addition to the thermodynamic decomposition voltage if the reduction of CO ₂ to CO is to be carried out in aqueous media. H ⁺ + OH ^- → H ₂ O, ΔH = - 55.86 kJ

With z = 1 this can be assigned to a potential of -0.58 V.

In other words: electrical energy is unintentionally converted into thermal waste heat. Unfortunately, this waste heat is at the difficult-to-use temperature level below 100 ° C and can usually be regarded as an inevitable loss.

In addition to electrolysis in aqueous media, there is also a high-temperature variant that is followed by the Haldor Topsoe company. Since the main charge carrier O ^2- is transported directly from cathode to anode here, the thermal energy loss does not occur here. Because the technology on fragile ceramic Based on materials, it seems difficult to scale into the megawatt range. In addition, the high operating temperatures create additional thermal stress, which complicates the long-term stability of the operation.
Cathode ^reaction : CO ₂ + 2e ^- → CO + O ^2-- ΔH = 1249, 47 kJ

With z = 2 one obtains U _K = 6.48 V.
Anode reaction: 2 O ^2- - 4e ^- → O ₂ ΔH = -1933.01 kJ

With z = 4 one obtains U _A = -5.01 V.

As expected, the cell voltage is 1.47 V.

The object of the present application for the invention is to largely avoid converting valuable electrical energy into thermal waste heat by means of a suitable sequence of electrochemical and thermal processes, without going back to a complex process which is highly complex and difficult to scale.

Summary of the invention

The inventors have found that the waste heat and energy loss can be reduced or even avoided if the generation of the CO is decoupled from the cathode reaction.

In a first aspect, the present invention relates to a method for producing CO from CO ₂ comprising electrolytic conversion of a starting material comprising CO ₂ to a mixture comprising formic acid and / or formate in a first electrolytic cell,
Removing the mixture comprising formic acid and / or formate,
Introducing the mixture comprising formic acid and / or formate into a first reactor, and
Releasing a product comprising CO from the mixture comprising formic acid and / or formate in the first reactor.

In addition, the invention relates to a process for the electrolytic conversion of a starting material comprising CO to a mixture comprising at least one hydrocarbon, the reaction taking place in the presence of a basic electrolyte, preferably the anode comprising Fe, Ni and / or NiFe.

In addition, a device for producing CO from CO _{2 is} disclosed, comprising
a first electrolytic cell comprising comprising CO ₂ comprising CO ₂ comprising implementing a first cathode and a first anode for the electrolytic reaction of a reactant to a mixture comprising formic acid and / or formate, which is formed to be a starting material to a mixture of formic acid and / or formate;
a first supply device for an educt comprising CO ₂ to the first electrolytic cell, which is designed with the first electrolytic cell to supply an educt comprising CO _{2 to} the first cathode;
a first discharge device for a mixture comprising formic acid and / or formate, which is connected to the first electrolytic cell in a region between the first cathode and the first anode and which is designed to discharge a mixture comprising formic acid and / or formate from the first electrolytic cell ;
a first reactor for releasing a product comprising CO from the mixture comprising formic acid and / or formate, which is designed to release a product comprising CO from the mixture comprising formic acid and / or formate;
a second feed device for the mixture comprising formic acid and / or formate, which is connected to the first reactor and is designed to feed the mixture comprising formic acid and / or formate to the first reactor; and
a second discharge device for discharging the product comprising CO from the first reactor, which is designed to discharge the product comprising CO from the first reactor.

Further aspects of the present invention can be found in the dependent claims and the detailed description.

Figure list

• 1 bis 4 zeigen schematisch theoretische Stufen der elektrochemischen Umsetzung von CO₂ zu CO.
• 5 bis 8 zeigen schematisch theoretische Stufen der elektrochemischen Umsetzung von CO₂ im Sauren.
• In 9 bis 12 sind schematisch energetische Betrachtungen für die Umsetzung von CO₂ zu CO über verschiedene elektrochemische Wege gezeigt.
• 13 zeigt schematisch eine beispielhafte erfindungsgemäße Vorrichtung zum Herstellen von CO aus CO₂.
• Den 14 bis 16 können Ergebnisse aus Referenzmessungen in den erfindungsgemäßen Beispielen entnommen werden.

The accompanying drawings are intended to illustrate and provide further understanding of embodiments of the present invention. In connection with the description they serve the Explanation of concepts and principles of the invention. Other embodiments and many of the advantages mentioned result from the drawings. The elements of the drawings are not necessarily drawn to scale with respect to one another. Identical, functionally identical and identically acting elements, features and components are provided with the same reference symbols in the figures of the drawings, unless stated otherwise.

• 1 to 4th schematically show theoretical stages of the electrochemical conversion of CO ₂ to CO.
• 5 to 8th schematically show theoretical stages of the electrochemical conversion of CO ₂ in acid.
• In 9 to 12th energetic considerations for the conversion of CO ₂ to CO via different electrochemical routes are shown schematically.
• 13 schematically shows an exemplary device according to the invention for producing CO from CO ₂ .
• The 14 to 16 results can be taken from reference measurements in the examples according to the invention.

Detailed Description of the Invention Definitions

Unless otherwise defined, technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art.

Quantities in the context of the present invention relate to% by weight, unless stated otherwise or can be seen from the context.

In the context of the present invention, hydrophobic is understood to be water-repellent. Hydrophobic pores and / or channels are therefore those that repel water. In particular, according to the invention, hydrophobic properties are associated with substances or molecules with non-polar groups.

In contrast, the ability to interact with water and other polar substances is understood to be hydrophilic.

Gas diffusion electrodes (GDE) in general are electrodes in which there are liquid, solid and gaseous phases, and in particular where a conductive catalyst can catalyze an electrochemical reaction between the liquid and the gaseous phase.

The design can be of various types, for example as a porous “full material catalyst” with possibly auxiliary layers to adjust the hydrophobicity; or as a conductive porous support onto which a catalyst can be applied in a thin layer.

For the purposes of this invention, a gas diffusion electrode (GDE) is in particular a porous electrode, inside which gases can move through diffusion. For example, it can be designed to separate a gas and an electrolyte space from one another. According to certain embodiments, it can be distinguished in that product gases can escape from these gas diffusion electrodes on the side facing away from the counterelectrode, that is to say, for example, into a gas space to which a gas is supplied for conversion. Liquid and / or dissolved products and / or by-products of the electrochemical conversion, in particular the charge carriers generated in the process, can be released, for example, from the electrode into the electrolyte. A GDE can be constructed in one or more layers with hydrophilic and / or hydrophobic regions, with at least one hydrophobic layer being advantageous, for example, for good contacting of a gas comprising CO ₂ , while a hydrophilic layer can be advantageous for good contacting of an aqueous electrolyte. GDEs are in particular electrodes in which there are liquid, solid and gaseous phases and where the conductive catalyst catalyzes the electrochemical reaction between the liquid and the gaseous phase.

The normal pressure is 101325 Pa = 1.01325 bar.

• Unter Elektro-Osmose versteht man ein elektrodynamisches Phänomen, bei dem auf in Lösung befindliche Teilchen mit einem positiven Zeta-Potential eine Kraft hin zur Kathode und auf alle Teilchen mit negativem Zeta-Potential eine Kraft zur Anode wirkt. Findet an den Elektroden ein Umsatz statt, d.h. fließt ein galvanischer Strom, so kommt es auch zu einem Stoffstrom der Teilchen mit positivem Zeta-Potential zur Kathode, unabhängig davon, ob die Spezies an der Umsetzung beteiligt ist oder nicht. Entsprechendes gilt für ein negatives Zeta-Potential und die Anode. Ist die Kathode porös, wird das Medium auch durch die Elektrode hindurch gepumpt. Man spricht auch von einer Elektro-Osmotischen-Pumpe.

Electro-osmosis:

• Electro-osmosis is an electrodynamic phenomenon in which particles in solution with a positive zeta potential have a force towards the cathode and all particles with a negative zeta potential have a force towards the anode. If there is a conversion at the electrodes, ie if a galvanic current flows, there is also a material flow of the particles with positive zeta potential to the cathode, regardless of whether the species is involved in the conversion or not. The same applies to a negative zeta potential and the anode. If the cathode is porous, the medium is also pumped through the electrode. One also speaks of an electro-osmotic pump.

The material flows caused by electro-osmosis can also flow in the opposite direction to concentration gradients. Diffusion-related currents that compensate for the concentration gradients can thus be overcompensated. In the present case, this can relate, for example, to a stream of carbonate and / or hydrogen carbonate ions in the first electrolytic cell.

A separator is a barrier, for example a layer, which in an electrolysis cell has a spatial and at least partially also material separation between different spaces of the electrolysis cell, e.g. Anode space, electrolyte space, salt bridge space, cathode space, etc. can accomplish as well as an electrical separation between anode and cathode, but allows ion transport between the different spaces. In particular, a separator has no permanently assigned potential, like an electrode. A separator can be, for example, a flat barrier with the same surface area. In particular, membranes and diaphragms are to be regarded as special examples of separators.

In a first aspect, the present invention relates to a method for producing CO from CO ₂ comprising electrolytic conversion of a starting material comprising CO ₂ to a mixture comprising formic acid and / or formate in a first electrolytic cell,
Removing the mixture comprising formic acid and / or formate,
Introducing the mixture comprising formic acid and / or formate into a first reactor, and
Releasing a product comprising CO from the mixture comprising formic acid and / or formate in the first reactor.

The sequence of the steps in the process is in the order mentioned.

The product comprising CO is also suitable for building up pressure. In particular, the product comprising CO, according to certain embodiments, is also low in H2 because it can be produced in a separate container, a first reactor.

The starting material comprising CO ₂ is not particularly limited and can be present, for example, as a gas mixture which, for example, can also be moistened for better contacting an electrode - here, for example, a first cathode, or a solution. In addition to CO ₂ , other gas components can also be present, but preferably preferably essentially CO _{2 is present} in the starting material, for example more than 50, 90, 95 or 99% by volume, and up to 100% by volume.

The electrolytic conversion is also not particularly restricted, in particular the anode reaction is not particularly restricted and can be, for example, the conversion of water. Accordingly, the first electrolytic cell is not particularly limited if it has a first cathode and a first anode. The two electrodes, that is, the first cathode and the first anode, are not particularly limited. According to certain embodiments, the first cathode is a gas diffusion electrode. Here, an educt comprising CO _{2 is introduced} into the electrolysis cell and reduced on the cathode side using the GDE, a liquid, preferably aqueous, electrolyte then being able to be present on the other side. According to certain embodiments, the first anode is a gas diffusion electrode.

A mixture comprising formic acid and / or formate is formed in the electrolytic reaction, the mixture may also contain other components such as other products of the electrolytic reaction, but preferably essentially formic acid and / or formate are formed in the reaction, preferably more than 50, 90, 95 or even 99% by weight, based on the CO ₂ that is reacted. According to certain embodiments, a mixture comprising formic acid is formed. According to certain embodiments, a mixture comprising formate is formed. This can depend, for example, on an electrolyte during the electrolytic conversion.

This mixture comprising formic acid and / or formate is removed in a suitable manner from the first electrolytic cell, which is not particularly limited, the removal being, for example, in the Co-current or counter-current to supply the starting material comprising CO ₂ can take place, for example via a suitable discharge device.

After removal, the mixture comprising formic acid, if appropriate after passing through a container for temporary storage, is introduced into a first reactor in which a product comprising CO is released from the mixture, the release not being restricted here and also being able to depend on it, whether it is essentially formic acid or formate.

For the method according to the invention for producing CO from CO ₂ , the above theoretical considerations regarding cell voltage are now further discussed, which now form the basis of the present invention. As stated above, thermal energy is generated as an unusable energy loss in the electrolytic conversion of CO ₂ to CO in aqueous media.

In aqueous media, energy loss could be avoided if work could be done in acidic media.
Cathode reaction: CO ₂ + 2 H ⁺ + 2e ^- → CO + H ₂ O ΔH = -2.87 kJ

• Anodenreaktion: 2 H₂O - 4e- → O₂ + 4H⁺    ΔH = 571,7 kJ

With z = 2 one obtains rounded U _K = -0.01 V:

• Anode reaction: 2 H ₂ O - 4e- → O ₂ + 4H ⁺ ΔH = 571.7 kJ

With z = 4 you get rounded U _A = 1.48 V.

The cell voltage is again rounded to 1.47 V.

Unfortunately, the overvoltage of hydrogen at the usual electrodes described for converting CO ₂ to CO (e.g. Au, Ag) in acidic media is low, so that the CO ₂ reduction is greatly reduced compared to the H ₂ O reduction and thus almost only hydrogen is produced and this variant without intensive catalyst development is not a solution to the problem.

Accordingly, this is not a practical solution, so that the theoretical considerations are now considered in more detail.

The CO ₂ reduction process takes place over several stages. A chemical intuitive idea is summarized in the following reaction equations and reaction states 1 to 4th represented schematically in several steps.

In 1 shows the first step of CO ₂ adsorption on a surface, here an exemplary metal M. Then it is done in one step 2nd the first reduction in electrochemical implementation, which in 2nd is shown. Subsequently, in a third step, a first protonation takes place in aqueous media 3rd is shown. Then takes place in one step 4th which in 4th is shown, a second reduction, wherein an output channel for format F (format type exit channel) is also possible, whereupon CO desorption or a further reduction follows in a fifth step (not shown).

• Zunächst wird das CO₂ im ersten Schritt an der Oberfläche des Metalls M absorbiert und ein Elektron im Schritt 2 übertragen, wodurch sich die Oxidationszahl des Kohlenstoffs formal um 1 erhöht. Metallcarboxylate sind basisch und können durch Wasser relativ leicht protoniert werden, wodurch das erste Hydroxid-Ion im Schritt 3 freigesetzt wird. Man kann sich vorstellen, dass aufgrund der Stabilität des Metallcarboxylats hierzu relativ wenig Energie notwendig ist. Im Anschluss daran erfolgt die zweite Reduktionsstufe im Schritt 4, wodurch ein „Strukturisomeres“ des Formiats entsteht.

In detail, this takes place as follows:

• First, the CO _{2 is} absorbed on the surface of the metal M in the first step and an electron in the step 2nd transferred, which formally increases the oxidation number of carbon by 1. Metal carboxylates are basic and can be protonated relatively easily by water, creating the first hydroxide ion in the crotch 3rd is released. One can imagine that due to the stability of the metal carboxylate, relatively little energy is required for this. This is followed by the second reduction step 4th , which creates a “structural isomer” of the formate.

1. a. Das Pseudoformiat zerfällt in ein Hydroxid-Ion und Kohlenmonoxid, wie in 4 gezeigt.
2. b. Das Pseudoformiat lagert sich in Formiat um und wird anschließend desorbiert, wie durch den Pfad zu Formiat F dargelegt.

This creates two more paths.

1. a. The pseudoformate breaks down into a hydroxide ion and carbon monoxide, as in 4th shown.
2. b. The pseudoformate rearranges into formate and is then desorbed, as shown by the path to formate F.

All of these steps must overcome certain energy barriers for them to take place. This energy can be supplied either electrically or thermally. The thermodynamic considerations above show that for the CO formation via step a. considerable amounts of energy must be supplied in the form of electrical energy.

Thermodynamic considerations are path independent i.e. in principle it is not important whether the energy is supplied in the form of electrical or thermal energy. However, the usable energy share of the different forms of energy (exergy) is significantly different. Electricity is a form of energy with 100% exergy. In the case of heat, the usable energy is limited by Carnot efficiencies.

It is therefore important to consider process sequences where the electrical energy used has the most benefit and is not necessarily lost in the form of heat.

The following table 2 summarizes the thermodynamic considerations. The anode reaction, which is not particularly limited - here, for example, water oxidation - is the same for both process sequences. Table 2: Thermodynamic considerations for the reaction of CO ₂ to CO or format / formic acid, with regard to the thermodynamic limit of this reaction of 1.47 V Thermodynamic limit CO ₂ → CO + ½ O ₂ 1.47 V CO reduction U _{half cell} Format response U _{half cell} cathode CO ₂ + H ₂ O + 2e ^- -> CO + 2OH ^- 0.56 V CO ₂ + H ₂ O + 2e ^- → HCOO ^- + OH ^- 0.12 V. anode H ₂ O - 2e ^- → ½ O ₂ + 2 H ⁺ 1.48 V H ₂ O - 2e ^- → ½ O ₂ + 2 H ⁺ 1.48 V total CO ₂ → CO + ½ O ₂ 2.04 V. CO ₂ + H ₂ O -> HCOOH + ½ O ₂ 1.60 V HCOOH -> CO + H ₂ O (eff. 0.15 V)

It was explained in detail above that the minimum cell voltage for the production of CO from CO _{2 is} 2.04 V and then at least the neutralization energy in the cathode compartment is released again in the form of undesired heat. The reference point is always the total reaction CO ₂ → CO + ½ O ₂ with 1.47 V. In carbon monoxide and formate / formic acid, the carbon has the same oxidation state of +2.

For the purposes of the invention, it is therefore advantageous first of all to generate formate and / or formic acid on the cathode. CO ₂ + H ₂ O + 2e ^- → HCOO ^- + OH ^- ΔH = 23.88 kJ

With z = 2 one obtains U _K = 0.12 V.

In these considerations, the anode reaction provides the protons necessary for neutralization to formic acid. The minimum total voltage in aqueous media is 1.60 V. Formic acid can easily be thermally decomposed into CO and H ₂ O (ΔH = -29.06 kJ, with z = 2 this would correspond formally to U = 0.15 V). This energy can possibly be applied from the waste heat of the electrolyzer.

In retrospect, the formation of the second hydroxide ion is very energy-intensive.

1. 1. Herstellung von Formiat an der Kathode, das dann mit Protonen der Anode zu Ameisensäure neutralisiert werden kann.
2. 2. Thermische Zersetzung der Ameisensäure zu CO und Wasser (bzw. Formiat zu CO, beispielsweise mit Säurezugabe).

This results in the inventive process sequence based on the electrical energy to be used, which can be provided, for example, from renewable energies:

1. 1. Production of formate on the cathode, which can then be neutralized with formons of the anode to formic acid.
2. 2. Thermal decomposition of formic acid to CO and water (or formate to CO, for example with the addition of acid).

While in a direct synthesis of CO with a basic cathode reaction minimal (2.04 V - 1.47 V) / 2.04 V = 28% electrical energy in the form of heat is lost, in the case of the sequence discussed here (with the theoretical Release voltage of CO) (1.75 V - 1.47 V) / 1.75 V = 16%. This energy can also be used, at least in part, from the waste heat of the electrolyzer for the CO release.

If the formate production is carried out in acidic electrolytes, the following half-cell voltage results: CO ₂ + 2 H ⁺ + 2e ^- → HCOO ^- ΔH = -31.92 kJ With z = 2 one obtains U _K = -0.17 V and thus a rounded cell voltage of 1.32 V.

A chemical intuitive idea for this “acid-catalyzed path” can be seen in the following reaction equations and reaction states 5 to 8th represented schematically in several steps.

In 5 is again appropriate 1 the first step of CO ₂ adsorption on a surface, here an exemplary metal M, is shown. Then, in a second step - according to the consideration in 2nd - The first reduction in electrochemical implementation, which in 6 is shown. Subsequently, in contrast to the above scheme, protonation from the acidic medium takes place in a third step 7 is shown. Then follows in a fourth step, which in 8th is shown, a surface rearrangement which here enables an output channel for format F (format type exit channel), whereupon a thermal CO release can follow in a fifth step (not shown).

The corresponding reactions in various states and media are also shown schematically in terms of their energy balances 9 to 12th shown.

9 shows the theoretical voltage of 1.47 V, which has to be used for the gas phase conversion from CO ₂ to CO and, for example, from renewable energy 1 can originate. As stated above, the same voltage would also result from a direct conversion of CO ₂ to CO in acidic medium (half-cell voltage rounded at the anode during water electrolysis - 1.48 V, rounded at the cathode 0.02 V, so that the cell is rounded -1.47 V), which has not been technically possible due to the low hydrogen overvoltage, e.g. on silver.

In 10th it is shown that when implemented in a non-acidic environment, additional thermal energy is used 3rd must be used as energy 2nd , for example, renewable energy must also be used. This is a result of the cathode half-cell voltage of -0.56V, so that for the same anode half-cell, a total voltage of rounded -2.04 V results.

In 11 the route via formic acid is shown in basic form, with a voltage of -0.12 V for the same anode half cell for the cathode half cell (formation of formate). For the total energy to be used 4th this results in a thermal loss 5 , which corresponds to a voltage of 0.13 V when rounded.

In 12th the energy balance of the formation of formic acid on the cathode side is shown (CO ₂ + H ⁺ + 2 e ^- → HCOO ^- ; U _{half cell} = -0.17 V), whereby in comparison to the gas phase reduction in 9 the energy to be used 6 reduced and heat loss 7 can be recycled (for water conversion as an anode reaction).

It should be noted that for 9 to 12th the convention ΔH = -z · F · E was used.

It should be noted in the above considerations that the anode reaction is irrelevant in the case of aqueous electrolytes, since it brings the same energy for all half cells into the overall balance and there are only differences with regard to an acidic, neutral or basic electrolyte.

The CO ₂ / HCO ₃ ^- / CO ₃ ^2- / OH ^- equal wight is thus the main problem for the energy efficient use of renewable energy in the direct one-step electrochemical synthesis of CO ₂ to CO and / or also to hydrocarbons.

In accordance with the theoretical considerations above, there are therefore various configurations of the method according to the invention.

In principle, two configurations are possible. One possibility is via formate, the other via formic acid itself. Formation with formic acid is preferred in accordance with the considerations above.

According to certain embodiments, the mixture comprising formic acid and / or formate is removed in solution, preferably in aqueous solution. This is possible, for example, when using an aqueous electrolyte, at least in the space between the first anode and the first cathode.

A less preferred embodiment of the invention is that a formate salt such as potassium formate is generated in a, for example neutral, electrolyte (for example with KHCO ₃ , K ₂ SO ₄ , etc.) on a suitable first cathode, for example a Pb or Sn-containing electrode , and this is isolated by crystallization. In this case, the mixture comprising formic acid and / or formate essentially comprises a formate salt or even only a formate salt. This can then be decomposed in the first reactor. For example, potassium formate decomposes in the range around its melting point of 260-300 ° C. However, this configuration is less preferred due to the high temperatures.

Another alternative is the use of an acid which is not particularly limited and which can be added to the mixture comprising formic acid and / or formate in the first electrolytic cell and / or outside, for example in a container for temporary storage and / or a first reactor. Exemplary preferred acids are sulfuric acid and / or hydrochloric acid. The formate obtained can be neutralized and decomposed by the acid, for example sulfuric acid.

For example, the following equation is possible: 2 KOOCH + H ₂ SO ₄ → K ₂ SO ₄ + CO + H ₂ O.

Sulfuric acid is a preferred acid here because it is a common commodity product. When using a potassium salt-containing electrolyte, a potassium sulfate obtained can also be used as a fertilizer and / or electrolyte additive.

A third possibility is to generate formic acid directly electrochemically, as described, for example, in “Electrochemical conversion of CO ₂ to formic acid utilizing Sustainion ™ membranes”, Hongzhou Yang, Jerry J. Kaczur, Syed Dawar Sajjad, Richard I. Masel, Journal of CO ₂ Utilization 20 (2017) 208-217, which is referred to for the generation of formic acid. Of course, other electrochemical methods for generating formic acid are also possible.

With its simple thermal decomposition into CO and H ₂ O, the electrochemical generation of formic acid becomes a process that generates CO more energy-efficiently, based on the use of electrical energy, than the electrochemical direct reduction of CO ₂ .

Formic acid conducts poorly in principle, so that to increase the conductivity in "Electrochemical conversion of CO ₂ to formic acid utilizing Sustainion ™ membranes", Hongzhou Yang, Jerry J. Kaczu, Syed Dawar Sajjad, Richard I. Masel, Journal of CO ₂ Utilization 20 (2017) 208-217, solid electrolytes are added, the conductivity of which is also limited.

The poor electrical conductivity σ of formic acid in water is shown in Table 3 below. Table 3: Electrical conductivity of formic acid at 25 ° C in a mixture with water Formic acid [% by weight] σ [mS / cm] 5 6.22 10th 8.26 15 9.86 20 11, 1 25th 11.4 30th 11.8 40 11.1 50 9.78 60 7.92 70 5.92 80 3.92 90 1.95 100 0.32

Accordingly, the concentration of formic acid in the mixture comprising formate and / or formic acid should not be too high, but also not too low, in order to ensure sufficient conversion. According to certain embodiments, the content of formic acid and / or formate in the mixture comprising formic acid and / or formate after the electrolytic reaction of the starting material comprising CO _{2 is} in a range from 0.5% by weight or more and 100% by weight or less, preferably 1% by weight or more and 95% by weight or less, more preferably 5% by weight or more and 90% by weight or less.

Since the formic acid does not have to be isolated as such in the proposed process, non-volatile soluble additives for increasing the conductivity, for example in the electrolyte, can also be added to the mixture comprising formic acid and / or formate. These can also support the decomposition to CO and H ₂ O at the same time.

According to certain embodiments, an aqueous electrolyte is present in the first electrolytic cell at least between the first cathode and the first anode.

According to certain embodiments, a, preferably aqueous, first electrolyte for the electrolytic conversion of the starting material comprising CO ₂ to a mixture comprising formic acid and / or formate comprises an acid and / or a salt in order to increase the conductivity.

According to certain embodiments, a, preferably aqueous, first electrolyte for the electrolytic conversion of the starting material comprising CO ₂ to a mixture comprising formic acid and / or formate comprises an acid and / or a formate salt and / or a sulfate salt, more preferably a liquid acid, for example sulfuric acid.

This can significantly increase the conductivity in view of the fact that the formic acid itself does not conduct enough.

For example, salts as additives can make this possible, preferably formates and / or sulfates, for example preferably alkali metal formate, preferably potassium formate, or else alkali metal sulfate, for example potassium sulfate. These additives in particular do not interfere with the subsequent decomposition of the formic acid, but increase the conductivity of the solution up to a factor 10th .

The addition of, for example, sulfuric acid can increase the conductivity above 100 mS / cm due to the availability of free protons.

An excessively high acid concentration, for example sulfuric acid concentration, should however be avoided, since otherwise the formic acid can already decompose in the space or gap between the first cathode and the first anode, which results in a CO, CO ₂ , H ₂ separation task. The decomposition can also be controlled well or additionally via a temperature control. According to certain embodiments, the concentration of the acid in the electrolyte of the first electrolytic cell is set in a range from 0.01 to 50% by weight, for example 0.1 to 40% by weight, in particular 1 to 35% by weight. A concentration of a salt in the electrolyte of the first electrolytic cell can likewise be set in a range from 0.01 to 50% by weight, for example 0.1 to 40% by weight, in particular 1 to 35% by weight. For sulfuric acid, for example, a maximum concentration can also be up to and including 20% by weight, for example in order to enable a sufficient anode reaction. However, it is in accordance certain embodiments also make it possible to use an acid with a concentration of 50% by weight or more, for example more than 50% by weight, in the electrolyte, but this is not preferred, since here the acid, for example sulfuric acid, dehydrates the formic acid can work.

An acid addition, in particular sulfuric acid addition, to the (first) electrolyte in the first electrolytic cell is possible in particular if an anion exchange membrane is present on the side of the first cathode, which additionally blocks the diffusion of protons, in particular at higher current densities. The anion exchange membrane is not particularly limited here and can be separate from the first cathode or bound as in a membrane electrode assembly (MEA).

According to certain embodiments, at least one anion exchange membrane, preferably in a region of the first cathode, is present between a first cathode and a first anode of the first electrolytic cell. This is particularly advantageous when using a basic electrolyte. Depending on the cathode used, it may not be present, for example if the formation of formate and / or formic acid on the first cathode is good, e.g. with a cathode comprising Pb. According to certain embodiments, the first electrolytic cell at least has no anion exchange membrane in a region of the cathode or even has no anion exchange membrane.

According to certain embodiments, the first electrolytic cell has a cation exchange membrane or a bipolar membrane between a first cathode and a first anode in a region of the first anode. The cation exchange membrane or the bipolar membrane are not particularly limited and can in turn be present separately or as an MEA together with the anode. In addition to controlling the ion transport, they can preferably prevent formate from migrating to the anode and being decomposed there.

According to certain embodiments, a - first - cathode of the first electrolytic cell comprises or contains a metal which is selected from Cu, Bi, Pb, Hg, In, Sn, Cd, Tl and / or compounds, in particular chalcogenidic compounds, and / or alloys and / or mixtures thereof, preferably Pb, Bi, Hg, In, Sn, Cd, Tl and / or compounds, in particular chalcogenide compounds, and / or alloys and / or mixtures thereof, further preferably Pb, Bi, in particular Pb. These are particularly capable of formate and / or formic acid formation. According to certain embodiments, the first cathode consists of a metal which is selected from Cu, Bi, Pb, Hg, In, Sn, Cd, Tl and / or compounds, in particular chalcogenidic compounds, and / or alloys and / or mixtures thereof, preferably Pb, Bi, Hg, In, Sn, Cd, Tl and / or compounds, in particular chalcogenide compounds, and / or alloys and / or mixtures thereof, more preferably Pb, Bi, in particular Pb.

With the method according to the invention, the electrolytic conversion of the starting material comprising CO ₂ to a mixture comprising formic acid and / or formate can also be carried out at a practical cell voltage which is much lower than usual voltages during the reduction of CO ₂ to CO, for example at 2.5 - 2.7 V with common overvoltages.

After the mixture comprising formic acid and / or formate has been prepared, it is suitably removed from the first electrolytic cell, for example via a tube, a hose, a line, etc. Before the mixture comprising formic acid and / or formate is introduced into the first reactor they are temporarily stored in a first container, for example for storage, setting a suitable concentration of formic acid and / or formate (for example by evaporation), etc. If temporarily stored in a first container, it can also be delivered to a further distant first reactor, so that A temporal and / or spatial separation between the first electrolytic cell and the first reactor is also possible.

According to certain embodiments, the mixture comprising formic acid and / or formate is temporarily stored in a first container after it has been removed from the first electrolytic cell and before being introduced into the first reactor.

If present and the mixture comprising formic acid and / or formate cannot be introduced directly into the first reactor, for example via a pipe, a hose, a line and / or another suitable connecting device, the mixture comprising formic acid and / or or formate can be introduced into the first reactor in a suitable manner, for example via a pipe, a hose, a line, etc.

The release of CO from formic acid and / or formate can then take place in a suitable manner in the first reactor, for example by setting a suitable temperature, acid concentration and / or presence of a suitable catalyst, etc., the measures being not particularly limited and common measures for formic acid decomposition can correspond. According to certain embodiments, CO is released from the mixture comprising formic acid and / or formate in the first reactor by dehydration, preferably by heating and / or heterocatalytic dehydration. A suitable temperature is not particularly limited here, and can also depend, for example, on the pressure and / or a catalyst, etc., and can be, for example, 10-130 K, preferably 10-80 K, above the electrolyte temperature and / or the mixture comprising formic acid and / or formate that is introduced into the first reactor are, for example, in a range from 70 ° C. to 190 ° C., for example 70 ° C. to 150 ° C. Formic acid usually boils at normal pressure with decomposition at 101 ° C. The decomposition can also take place slowly at room temperature.

The use of at least one acid in the release of CO is particularly advantageous. A solution of acid / formic acid is liquid, ie virtually any pressure can be built up during the decomposition. Pressures above 1 bar (1 * 10 ⁵ Pa) are preferred, particularly preferred pressures between 30 bar (30 * 10 ⁵ Pa) and 100 bar (100 * 10 ⁵ Pa), as they occur, for example, in the chemical process industry. This also saves compressors.

Of particular importance here is not the decomposition to CO, but the synergy with regard to the cell voltage during the electrolytic conversion in the first electrolytic cell, which allows a lowering of the cell voltage compared to the direct CO generation. Accordingly, CO is formed effectively. This CO can then be used in a suitable manner.

It is particularly advantageous that it can ultimately be used to efficiently produce hydrocarbons from CO ₂ . In contrast to CO ₂ , CO can also be efficiently converted into hydrocarbons in a basic electrolyte. This means that common anodes can also be used here, in contrast to the usual production of hydrocarbons in a single electrolytic cell. By decoupling the CO production from the electrolytic conversion of CO ₂ , an advantageous further conversion is also possible.

According to certain embodiments, the CO produced in the process according to the invention set out above is thus further converted in a second electrolytic cell to at least one hydrocarbon, the reaction preferably taking place in the presence of a second, basic electrolyte. As stated, the anode can include common metals such as Fe, Ni and / or NiFe, and the cathode material is not particularly limited either, as will be discussed further below.

Another aspect of the present invention relates to a process for the electrolytic conversion of a starting material comprising CO to a mixture comprising at least one hydrocarbon, the reaction taking place in the presence of a basic electrolyte, in particular anolyte, preferably wherein the anode comprises Fe, Ni and / or NiFe . Here, the material of the cathode is not particularly limited and can include any materials that enable CO conversion, for example a metal that is selected from Cu, Ag, Pd, Pt and / or Au, preferably Cu, and / or alloys and / or compounds thereof, and / or mixtures thereof.

From the CO ₂ / HCO ₃ ^- / CO ₃ ^2- / OH ^- equal wight is not only affected the reduction product CO, but any other reduction products such as ethylene, ethanol, propanol, acetate or the trace products of allyl alcohol, ethylene glycol, etc.

This is explained using the example of ethylene. The overall reaction serves as a reference: 2 CO ₂ + 2 H ₂ O → C ₂ H ₄ + 3 O ₂ ΔH = 1411.07 kJ

With z = 12 one obtains U _{cell, min} = 1.22 V.

In aqueous medium there are 2 CO ₂ + 8 H ₂ O + 12e ^- = C ₂ H ₄ + 12 OH ^- , ΔH = 365.76 kJ

With z = 12 one obtains U _K = 0.32 V.

With an acidic anode reaction, the minimum cell voltage is 1.80 V. The difference again corresponds to the neutralization energy / voltage: 55.08 kJ / z = 1 / 0.58 V.

As in “Reactivity of Copper Electrodes towards Functional Groups and Small Molecules in the Context of CO ₂ Electro-Reductions”, Bernhard Schmid, Christian Reller, Sebastian S. Neubauer, Maximilian Fleischer, Romano Dorta and Guenter Schmid; Catalysts 2017, 7, 161; doi: 10.3390 / cata17050161 shown, the reduction of CO ₂ and CO result in the almost identical product range in the hydrocarbon sector, which identifies CO as the most important intermediate.

The reduction of CO to hydrocarbons has a decisive advantage over that of CO ₂ when considering what has been presented so far. Basic electrolytes are highly advantageous for both reactions, where in the case of CO _2, the CO ₂ / HCO ₃ ^- must be equal wight taken into account and in the case of CO not ^- / CO ₃ ^2- / OH.

This means that in the case of CO, the electrolyte (anolyte and catholyte) as a whole can be strongly basic and e.g. a KOH solution in water can be chosen. However, the basic electrolyte is not particularly limited.

• Kathodenreaktion: 2 CO + 6 H₂O + 8e^- → C₂H₄ + 8 OH^-    ΔH = 148, 27 kJ

The following applies:

• Cathode reaction: 2 CO + 6 H ₂ O + 8e ^- → C ₂ H ₄ + 8 OH ^- ΔH = 148, 27 kJ

With z = 8 one obtains U _K = 0.19 V,
Anode reaction: 4 OH ^- - 4 e ^- → O ₂ + 2 H ₂ O ΔH = 348, 44 kJ

With z = 4 one obtains U _A = 0.90 V.

The minimum cell voltage is therefore: U _cell = 1.09 V. This voltage is identical to that calculated from the sum reaction: 2 CO + 2 H ₂ O → C ₂ H ₄ + 2 O ₂ ΔH = 845, 14 kJ

With z = 8 one obtains U _cell = 1.09 V.

The possibility to work in the basic also means that no iridium is necessary as an anode material and the classic catalysts (Fe, Ni, NiFe etc.) of the alkali electrolysers can be used. This also circumvents the disadvantage of Iridium's resource limitation.

From a broader perspective, this also means that it can make sense to use CO from fossil sources (coal power gasifier, etc.) in order to then process it with renewable electricity, for example to produce ethylene, in order to obtain a partially green product. CO can also come from the decomposition of formic acid, which in turn is also accessible to CO ₂ and H ₂ through homogeneous catalysis.

In addition, the present invention relates to a device for producing CO from CO ₂ comprising
a first Elektrolyezelle comprising comprising CO ₂ comprising CO ₂ comprising implementing a first cathode and a first anode for the electrolytic reaction of a reactant to a mixture comprising formic acid and / or formate, which is formed to be a starting material to a mixture of formic acid and / or formate;
a first supply device for an educt comprising CO ₂ to the first electrolytic cell, which is designed with the first electrolytic cell to supply an educt comprising CO _{2 to} the first cathode;
a first discharge device for a mixture comprising formic acid and / or formate, which is connected to the first electrolytic cell in a region between the first cathode and the first anode and which is designed to discharge a mixture comprising formic acid and / or formate from the first electrolytic cell ;
a first reactor for releasing a product comprising CO from the mixture comprising formic acid and / or formate, which is designed to release a product comprising CO from the mixture comprising formic acid and / or formate;
a second feed device for the mixture comprising formic acid and / or formate, which is connected to the first reactor and is designed to feed the mixture comprising formic acid and / or formate to the first reactor; and
a second discharge device for discharging the product comprising CO from the first reactor, which is designed to discharge the product comprising CO from the first reactor.

With the device according to the invention, in particular the method according to the invention for producing CO from CO ₂ can be carried out. Correspondingly, certain aspects which are described with regard to embodiments of the method according to the invention can also be used in the device according to the invention.

In the device, the first electrolytic cell is not particularly limited, provided that it comprises a first cathode and a first anode. The material of the cathode is not particularly limited. According to certain embodiments, the first cathode of the first electrolytic cell comprises or contains a metal which is selected from Cu, Bi, Pb, Hg, In, Sn, Cd, Tl and / or compounds, in particular chalcogenide compounds, and / or alloys and / or Mixtures thereof, preferably Pb, Bi, Hg, In, Sn, Cd, Tl and / or compounds, in particular chalcogenide compounds, and / or alloys and / or mixtures thereof, more preferably Pb, Bi, in particular Pb. According to certain embodiments, the first cathode consists of a metal which is selected from Cu, Bi, Pb, Hg, In, Sn, Cd, Tl and / or compounds, in particular chalcogenidic compounds, and / or alloys and / or mixtures thereof, preferably Pb, Bi, Hg, In, Sn, Cd, Tl and / or compounds, in particular chalcogenide compounds, and / or alloys and / or mixtures thereof, more preferably Pb, Bi, in particular Pb.

The first anode is not particularly limited and can be adapted to the anode reaction, and the anode material can also differ depending on whether a cation exchange membrane or a bipolar membrane is present or not.

The first feed device is not particularly limited and can be, for example, a pipe, a hose, a line, etc. Likewise, the first discharge device is not particularly limited and can be, for example, a pipe, a hose, a line, etc. A further discharge device for unreacted educt comprising CO ₂ and / or a further feed device for electrolyte and / or educt for the anode and / or further feed and / or discharge devices for the first electrolytic cell can also be present, which can also be used, for example, with other feed devices can be connected, for example if unreacted CO _{2 is} recycled. In addition, there may be heating and / or cooling devices for the first electrolytic cell.

In addition, the second feed device and the second discharge device are not particularly limited and can be, for example, a tube, a hose, a line, etc. The second feed device may or may not be connected to the first discharge device and, for example, is not connected directly to it if a first container for the intermediate storage of the mixture comprising formic acid and / or formate is present.

The first reactor is also not particularly limited. According to certain embodiments, the first reactor comprises at least one heater, which is not particularly limited to favor thermal decomposition of formic acid, e.g. at least one heat pump to use the waste heat from the electrolysis. Corresponding heat exchangers can also be provided when the electrolyte is returned.

According to certain embodiments, the first electrolytic cell has at least one anion exchange membrane between the first anode and the first cathode, preferably in a region of the first cathode. It can be designed as an MEA or separately from the cathode.

According to certain embodiments, the first electrolytic cell has a cation exchange membrane or a bipolar membrane between the first cathode and the first anode in a region of the first anode. These can also be provided as MEAs or separately.

According to certain embodiments, the first cathode of the first electrolytic cell comprises a metal which is selected from Cu, Bi, Pb, Hg, In, Sn, Cd, Tl and / or alloys and / or mixtures thereof, preferably Pb, Bi, Hg, In , Sn, Cd, Tl and / or alloys and / or mixtures thereof, more preferably Pb, Bi, in particular Pb.

According to certain embodiments, the device according to the invention further comprises a first container for temporarily storing the mixture comprising formic acid and / or formate, which is arranged between the first discharge device and the first supply device and which is designed to temporarily store the mixture comprising formic acid and / or formate. The first container is not particularly limited. It can comprise a third discharge device, which is designed to discharge the mixture comprising formic acid and / or formate after the intermediate storage. The third discharge device may or may not be connected to the second feed device.

The first container can also have further discharge devices, for example for unreacted CO ₂ and / or decay products of formic acid and / or water, which can be evaporated to concentrate the formic acid and / or the formate. Correspondingly, the first container can also comprise at least one heating device.

The above embodiments, refinements and developments can, if appropriate, be combined with one another as desired. Further possible refinements, developments and implementations of the invention also include combinations of features of the invention described above or below with reference to the exemplary embodiments that are not explicitly mentioned. In particular, the person skilled in the art will also add individual aspects as improvements or additions to the respective basic form of the present invention.

The invention is explained in more detail below with reference to various examples thereof. However, the invention is not limited to these examples.

Examples

example 1

An exemplary method according to the invention and an exemplary device according to the invention are shown schematically in FIG 13 shown schematically.

The formic acid becomes electrochemical in the first electrolytic cell 10th on the first cathode 11 manufactured, being on the first anode 12th Water is electrolyzed as an electrolyte. At the first anode 12th there is a cation exchange membrane, and on the first cathode 11 an anion exchange membrane AEM. The educt comprising CO ₂ is via the first feed device 13 supplied, and the mixture comprising formic acid via the first discharge device by means of the line 14 to a first container 20 headed. There is also an outlet for oxygen on the anode side.

In the container 20 the mixture comprising formic acid is temporarily stored, which preferably serves at the same time for the removal of further gas such as excess CO ₂ or the by-products H ₂ or CH ₄ . This vessel can, but does not have to, serve as an electrolyte reservoir and / or formic acid store. Electrolyte can, for example, via the line 15 to the first electrolytic cell 10th be recycled, as well as unreacted CO ₂ via the line 16 .

The process chain can also be spatially and temporally separated at this point. The formic acid content can be, for example, between 1 and 100%, and can be adjusted in the course of the electrolysis to a preferred limit in the range of 5 to 90%. After transferring the mixture comprising formic acid via the line 21 becomes pure CO in the first reactor in the second process step 30th released by dehydration of the formic acid, whereby the formic acid content drops again. The decomposition is achieved, for example, by heating or heterocatalytic dehydration, which are not shown in detail. Via line 22 can not decompose formic acid to the first container 20 be returned.

In the first container 20 For example, a suitable temperature of, for example, 60 ° C. can be set for storage, the electrolyte here also possibly being buffered. The temperature in the first reactor can then be raised to a suitable temperature of, for example, 10 to 80 K above the electrolyte temperature and / or compression to facilitate the release of CO. Depending on the decomposition - for example with a suitable catalyst - low CO ₂ or even CO ₂ free CO can be obtained.

Example 2

The structure corresponds to that of Example 1, with a bipolar membrane being used instead of a CEM.

Example 3

The structure corresponds to that of Example 1, but there is no AEM on the cathode side and in particular a Pb-containing cathode is used. For example, the overvoltage on lead electrodes for hydrogen is very high, so that it may also be possible to work in acidic electrolytes (see also Example 5). As described above, lead electrodes provide formate in CO ₂ electrolysis. The local pH of the electrode becomes more basic, the higher the current density at which it is operated, ie it can be expected that the formate yield will be even higher at higher current densities than at lower ones.

Example 4

In the structure of Example 1, water with a salt such as K ₂ SO _{4 is} used as the electrolyte instead of water.

Example 5

In the structure of Example 1, water with an acid such as sulfuric acid is used as the electrolyte instead of water. In this example in particular, the AEM can also be dispensed with, in particular if the first cathode 11 Pb includes. It is advantageous here that the Pb has a high overvoltage against hydrogen of -0.71 V.

Reference example 1

Conductivity measurements were carried out for various mixtures of formic acid as a possible product, water as a possible electrolyte and / or sulfuric acid or potassium sulfate as a possible electrolyte additive.

The blends in 14 and 15 were adjusted appropriately by weighing. For 16 the initial mixtures were adjusted by weighing, after which sulfuric acid was gradually added by weighing. The conductivity was determined using a conventional conductivity sensor.

The results of the measurements are in 14 to 16 shown.

In 14 the measurements of mixtures of formic acid with K ₂ SO _{4 are} shown, the electrical conductivity being plotted against the mass fraction of K ₂ SO ₄ in% by weight.

For the measurement results in 15 Mixtures of formic acid and sulfuric acid were used, the x-axis here representing the mass fraction of sulfuric acid in% by weight.

For the measurements in 16 Various mixtures of water and formic acid were produced, the mixing ratios [m / m] being given for the individual graphs. 100% water was also applied as a reference.

The mixtures and the water were mixed with sulfuric acid and the mass fraction of sulfuric acid in% by weight with respect to the mixtures was plotted on the x-axis.

In the 14 to 16 it can be seen that the addition of salt or acid can increase the poor conductivity of formic acid - a possible intermediate in the present process for the production of CO - so that it can also assume technically relevant values.

CO ₂ can be reduced electrochemically to CO in one step. However, it is disadvantageous that the hydroxide ions formed form with excess CO ₂ hydrogen carbonate. This reaction is the cause of an energy (better-exergy) loss of the electrical energy invested. Here, 28% of the electrical energy used is converted into heat.

1. a. Die elektrochemische Herstellung von Ameisensäure und/oder Formiat aus CO₂
2. b. Die thermochemische, beispielsweise säurekatalysierte, Spaltung von Ameisensäure in das Zielprodukt CO und H₂O.

This loss can be synergistically reduced to approximately 16% (280 mV) if an electrochemical and a thermochemical step are combined.

1. a. The electrochemical production of formic acid and / or formate from CO ₂
2. b. The thermochemical, for example acid-catalyzed, splitting of formic acid into the target product CO and H ₂ O.

Preferred embodiments result because the two processes can be combined without the formic acid having to be presented in pure form. The electrolyte volume also represents a storage volume for formic acid or CO. Acid or salt-like additives both increase the conductivity and are control variables for the thermal release of the CO.

Claims (15)

Process for producing CO from CO ₂ comprising electrolytic conversion of a starting material comprising CO ₂ to a mixture comprising formic acid and / or formate in a first electrolysis cell, removing the mixture comprising formic acid and / or formate, introducing the mixture comprising formic acid and / or formate into a first reactor, and releasing a product comprising CO from the mixture comprising formic acid and / or formate in the first reactor. Procedure according to Claim 1 , wherein a, preferably aqueous, first electrolyte of the electrolytic conversion of the starting material comprising CO ₂ to a mixture comprising formic acid and / or formate comprises an acid and / or a formate salt and / or a sulfate salt, more preferably a liquid acid. Procedure according to Claim 2 , wherein at least one anion exchange membrane, preferably in a region of the first cathode, is present between a first cathode and a first anode of the first electrolytic cell. Method according to one of the preceding claims, wherein the first electrolytic cell between a first cathode and a first anode in a region of the first anode has a cation exchange membrane or a bipolar membrane. Method according to one of the preceding claims, wherein a cathode of the first electrolytic cell comprises a metal which is selected from Cu, Bi, Pb, Hg, In, Sn, Cd, Tl, preferably Pb, Bi, Hg, In, Sn, Cd, Tl, more preferably Pb, Bi. Method according to one of the preceding claims, wherein the mixture comprising formic acid and / or formate is temporarily stored in a first container after removal from the first electrolysis cell and before introduction into the first reactor. Method according to one of the preceding claims, wherein the content of formic acid and / or formate in the mixture comprising formic acid and / or formate after the electrolytic reaction of the educt comprising CO ₂ in a range of 0.5 wt.% Or more and 100 wt. % or less, preferably 1% or more and 95% or less, more preferably 5% or more and 90% or less. Method according to one of the preceding claims, wherein the release of CO from the mixture comprising formic acid and / or formate in the first reactor is carried out by dehydration, preferably by heating and / or heterocatalytic dehydration. Method according to one of the preceding claims, wherein the CO is further converted in a second electrolytic cell to at least one hydrocarbon, the reaction preferably taking place in the presence of a second, basic electrolyte. Process for the electrolytic conversion of a starting material comprising CO to a mixture comprising at least one hydrocarbon, the reaction taking place in the presence of a basic electrolyte, preferably the anode comprising Fe, Ni and / or NiFe. Apparatus for producing CO from CO ₂ comprising a first electrolytic cell comprising a first cathode and a first anode for the electrolytic conversion of a starting material comprising CO ₂ to a mixture comprising formic acid and / or formate, which is designed to form a starting material comprising CO ₂ to one To implement a mixture comprising formic acid and / or formate; a first supply device for an educt comprising CO ₂ to the first electrolytic cell, which is designed with the first electrolytic cell to supply an educt comprising CO _{2 to} the first cathode; a first discharge device for a mixture comprising formic acid and / or formate, which is connected to the first electrolytic cell in a region between the first cathode and the first anode and which is designed to discharge a mixture comprising formic acid and / or formate from the first electrolytic cell ; a first reactor for releasing a product comprising CO from the mixture comprising formic acid and / or formate, which is designed to release a product comprising CO from the mixture comprising formic acid and / or formate; a second feed device for the mixture comprising formic acid and / or formate, which is connected to the first reactor and is designed to feed the mixture comprising formic acid and / or formate to the first reactor; and a second discharge device for discharging the product comprising CO from the first reactor, which is designed to discharge the product comprising CO from the first reactor. Device after Claim 11 , wherein the first electrolytic cell has at least one anion exchange membrane between the first anode and the first cathode, preferably in a region of the first cathode. Device after Claim 11 or 12th , wherein the first electrolytic cell between the first cathode and the first anode in a region of the first anode has a cation exchange membrane or a bipolar membrane Device according to one of the Claims 11 to 13 , wherein the first cathode of the first electrolytic cell comprises a metal which is selected from Cu, Bi, Pb, Hg, In, Sn, Cd, Tl, preferably Pb, Bi, Hg, In, Sn, Cd, Tl, more preferably Pb . Device according to one of the Claims 11 to 14 , further comprising a first container for temporarily storing the mixture comprising formic acid and / or formate, which is arranged between the first discharge device and the first supply device and which is designed to temporarily store the mixture comprising formic acid and / or formate.

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