Cadmium and / or zinc chalcogenide nanoparticles as catalyst material for co2 reduction

Abstract

The present invention relates to an electrode for the reduction of carbon dioxide CO2, for example to carbon monoxide CO and / or synthesis gas CO / H2, the electrode containing, as a catalyst, cadmium and / or zinc chalcogenide nanoparticles, to an electrochemical process for reducing carbon dioxide CO2, for example to carbon monoxide CO and / or synthesis gas CO / H2, by means of an electrode which contains cadmium and / or zinc chalcogenide nanoparticles, and to the use of cadmium and / or zinc chalcogenide nanoparticles as a catalyst in an electrode for the electrochemical reduction of reduced species, for example carbon dioxide CO2 to carbon monoxide CO and / or synthesis gas CO / H2.

Description

[0001]Cadmium and / or zinc chalcogenide nanoparticles as catalyst material for CO2 reduction The present invention relates to an electrode for the reduction of carbon dioxide CO2, for example to carbon monoxide CO and / or synthesis gas CO / H2, which contains cadmium and / or zinc chalcogenide nanoparticles as catalyst, an electrochemical process for the reduction of carbon dioxide CO2, for example to carbon monoxide CO and / or synthesis gas CO / H2 using an electrode containing cadmium and / or zinc chalcogenide nanoparticles, and the use of cadmium and / or zinc chalcogenide nanoparticles as catalyst in an electrode for the electrochemical reduction of reduced species, such as carbon dioxide CO2 to carbon monoxide CO and / or synthesis gas CO / H2. Technical background The reduction of CO2 emissions, in particular from established and only slightly avoidable industrial plants, e.g.from waste incineration and cement plants, as well as dependence on fossil fuels, make a comprehensive defossilization of industry and society necessary with increased use of a closed CO2 cycle. While converting processes with the aim of producing less or no CO2 at all is desirable, it is lengthy and sometimes not feasible. In order to ensure the time horizon of current climate targets, a technology is therefore needed that can be retrofitted to existing plants. This technology extracts CO2 directly from the exhaust stream, upgrades it and makes it available again for subsequent processes. In this way, both social benefits through resource conservation and CO2 reduction and economic benefits through the production of energy-rich carbon derivatives (e.g. a cheap synthesis gas) and increasing the sustainability of existing plants can be achieved.The conversion of CO2 into a reactive form is fundamentally possible through electrocatalytic conversion. This process is established. However, the catalysts used in this process, e.g. nanoparticulate silver, are extremely sensitive to foreign substances. For example, sulfur-containing compounds absorb on the surface of these catalysts and severely impair the efficiency of CO2 conversion. This negative effect is referred to as catalyst poisoning. Therefore, with the established methods, the conversion of CO2 from exhaust gas streams is only possible after prior purification. However, the purification process is complicated and cost-intensive and is therefore not used industrially. Conversion of CO2 in the exhaust stream based on established catalysts is therefore not carried out. The present invention allows the direct electrocatalytic conversion of CO2 from an unpurified exhaust stream. The CO2 is thus converted to CO orSynthesis gas for synthetic, defossilized fuels and other carbon-based basic chemicals. The present invention describes cadmium and / or zinc chalcogenide nanoparticles as active material for cathode catalysts in electrodes, in particular in gas diffusion electrodes, which are robust against catalyst poisoning because they do not have a high chemical affinity for the contaminants in the exhaust gas stream. With the nanoparticles described in this invention, a high efficiency of the electrocatalytic CO2 conversion could surprisingly be demonstrated even in the presence of typical catalyst poisons. Summary of the Invention In a first aspect, the present invention relates to an electrode for the reduction of carbon dioxide CO2 to carbon monoxide CO and / or synthesis gas CO / H2, which contains cadmium and / or zinc chalcogenide nanoparticles as catalyst.In a second aspect, the present invention relates to a method for reducing carbon dioxide CO2, comprising the following steps: ^ Providing an electrochemical cell with an electrode as described herein; ^ Applying current to the electrode; ^ Contacting a carbon dioxide-containing gas stream with the cadmium and / or zinc chalcogenide nanoparticles in the electrode, such that the carbon dioxide is reduced, preferably to carbon monoxide CO and / or synthesis gas CO / H2. In a third aspect, the present invention relates to the use of cadmium and / or zinc chalcogenide nanoparticles in an electrode as described herein as a catalyst for the electrochemical reduction of carbon dioxide CO2 to reduced species, such as carbon monoxide CO and / or synthesis gas CO / H2.Brief description of the figures Figure 1 shows a comparison of the Faradaic efficiencies of H2 and CO measured with gas diffusion electrodes in a liquid electrolyte flow cell after 1 hour (left column) and 2 hours (right column) of chronopotentiometry at 50 mA cm. -2 (top) and 200 mA cm -2(below). In the gas diffusion electrodes, cadmium chalcogenide nanoparticles in the form of spherical CdSe particles, spherical particles with a CdSe core and a CdS shell, spherical CdS particles, coin-shaped particles with a CdSe core and a CdS shell, rod-shaped particles with a CdSe core and a CdS shell, and rod-shaped CdS particles were used as catalysts. These particles were compared with silver-containing nanoparticles as a reference. Figure 2 shows a comparison of the Faradaic efficiencies of H2 and CO measured with untreated and poisoned gas diffusion electrodes containing rod-shaped CdS nanoparticles in a liquid electrolyte flow cell after 1 hour (left column) and 2 hours (right column) of chronopotentiometry at 50 mA cm -2 (top) and 200 (bottom) mA cm -2were determined. NaS, H2S, SO2, and NO2 were used as catalyst poisons. Figure 3 shows a comparison of the Faradaic efficiencies of H2 and CO measured with gas diffusion electrodes in a zero-gap electrolysis cell with a solid polymer electrolyte after chronopotentiometry at 200 mA cm -2were determined. In the gas diffusion electrodes, cadmium chalcogenide nanoparticles in the form of spherical particles with a CdSe core and a CdS shell, spherical CdS particles, rod-shaped particles with a CdSe core and a CdS shell, and rod-shaped CdS particles were compared as catalysts. Description of the invention The present invention relates to an electrode for the reduction of carbon dioxide CO2 to carbon monoxide CO and / or synthesis gas CO / H2, which contains cadmium and / or zinc chalcogenide nanoparticles as catalyst. The electrode is typically a porous electrode, which allows large-area contact between the catalyst material and the carbon dioxide CO2 to be reduced. The carbon dioxide CO2 is usually dissolved in a liquid electrolyte, such as a carbonate solution, with the electrode, preferably the porous electrode.brought into contact. Alternatively, the carbon dioxide CO2 can be moistened with a non- or weakly ion-conductive liquid, for example water, and brought into contact with the electrode, preferably the porous electrode. In this embodiment, the electrolyte is usually a solid electrolyte, for example with an ion-conducting polymer (ionomer). The production of porous electrodes is generally known, for example, from the field of Li-ion batteries, fuel cells, and electrolyzers. Preferably, the electrode is a gas diffusion electrode. Gas diffusion electrodes are generally known as electrodes in which the three media - electrolyte, catalyst, and reactant - are in contact with each other and the solid,An electron-conducting catalyst catalyzes an electrochemical reaction between the liquid and gaseous phases. A design with a solid polymer electrolyte instead of a liquid electrolyte is also conceivable. The catalyst is preferably mixed with a hydrophobic plastic, preferably polytetrafluoroethylene PTFE, and formed as an electrode. Designs with an ion-conducting polymer (ionomer) are also conceivable to optimize the conductivity of ionic species in the catalyst layer. The mixtures can be an aqueous dispersion containing water, (ion-conducting) plastic, and catalyst, or a dry mixture containing plastic powder and catalyst powder. Both variants are well known. The mixture can contain additional substances, such as binders, substances to increase wettability,conductive materials and support materials. Materials commonly used for these applications are commonly used. One example is carbon black, which both increases the conductivity of the gas diffusion electrode and can serve as an additional support material. The production of a gas diffusion electrode is well known. The gas diffusion electrode can be used in an electrochemical cell as a cathode for the reduction of carbon dioxide CO2 to carbon monoxide CO and / or synthesis gas CO / H2. The electrochemical cell generally also contains an anode and one or more electrolytes. Any commonly used material can be used as the anode and electrolyte. This also depends on the type of electrochemical cell. The electrochemical cell can be a liquid electrolyte cell, such as a liquid electrolyte flow cell or a solid electrolyte cell.such as a solid electrolyte flow cell or zero-gap electrolysis cell. A suitable example of a liquid electrolyte flow cell contains as electrolyte a salt dissolved in a solvent such as water, such as KHCO3 / K2SO4, a metal electrode, for example metal- or metal-oxide-coated titanium, preferably IrO2-coated titanium or Pt-coated titanium, as anode, and a suitable ion exchange membrane in addition to the gas diffusion electrode. A suitable example of a solid electrolyte flow cell or zero-gap electrolysis cell contains as electrolyte an anion exchange membrane, which is conditioned with an anode substrate such as KOH and KHCO3 prior to measurement, and a metal electrode, for example IrO2-coated titanium, as anode, in addition to the gas diffusion electrode. The structure of these electrochemical cells,and suitable materials are generally known. The electrode contains cadmium and / or zinc chalcogenide nanoparticles as a catalyst for the reduction of carbon dioxide CO2 to carbon monoxide CO and / or synthesis gas CO / H2. The cadmium and / or zinc chalcogenide nanoparticles preferably contain cadmium sulfide CdS, cadmium selenide CdSe, zinc sulfide ZnS, zinc selenide ZnSe and mixtures of two or more compounds of cadmium sulfide CdS, cadmium selenide CdSe, zinc sulfide ZnS and zinc selenide ZnSe. The cadmium and / or zinc chalcogenide nanoparticles can contain mixtures of two or more compounds of cadmium sulfide CdS, cadmium selenide CdSe, zinc sulfide ZnS and zinc selenide ZnSe in the form of layers or shells of different individual compounds or in the form of an alloy of two or more compounds. In one embodiment, the cadmium and / or zinc chalcogenide nanoparticles contain only cadmium chalcogenide compounds,preferably cadmium sulfide CdS, cadmium selenide CdSe and mixtures of cadmium sulfide CdS and cadmium selenide CdSe in the form of layers or shells of the individual compounds or in the form of an alloy. In this embodiment, the cadmium and / or zinc chalcogenide nanoparticles are cadmium chalcogenide nanoparticles and do not contain any zinc chalcogenide compounds. It is preferred that the cadmium and / or zinc chalcogenide nanoparticles preferably contain the cadmium chalcogenide nanoparticles, cadmium selenide CdSe. Preferably, the cadmium and / or zinc chalcogenide nanoparticles, preferably the cadmium chalcogenide nanoparticles, cadmium selenide CdSe or mixtures of cadmium sulfide CdS and cadmium selenide CdSe,Particularly preferred are mixtures of cadmium sulfide (CdS) and cadmium selenide (CdSe). Mixtures of cadmium sulfide (CdS) and cadmium selenide (CdSe) can be present in the form of layers or shells of the individual compounds or in the form of an alloy. Preferably, the cadmium and / or zinc chalcogenide nanoparticles contain no further semiconductor materials other than cadmium sulfide CdS, cadmium selenide CdSe, zinc sulfide ZnS, zinc selenide ZnSe, and mixtures of two or more compounds of cadmium sulfide CdS, cadmium selenide CdSe, zinc sulfide ZnS, zinc selenide ZnSe, preferably other than cadmium sulfide CdS, cadmium selenide CdSe, and mixtures of cadmium sulfide CdS and cadmium selenide CdSe, more preferably other than cadmium selenide CdSe and mixtures of cadmium sulfide CdS and cadmium selenide CdSe, and most preferably other than mixtures of cadmium sulfide CdS and cadmium selenide CdSe. The cadmium and / or zinc chalcogenide nanoparticles preferably have a molar ratio of selenium Se to sulfur S Se : S of 0.0.01:99.999 to 50.0:50.0, preferably 0.01:99.99 to 25:75, more preferably 0.1:99.9 to 10.0:90.0. In addition to the semiconductor materials of the cadmium and / or zinc chalcogenide group, the cadmium chalcogenide nanoparticles preferably contain one or more types of organic ligands on their surface. This coating with organic ligands is typically used during electrode production as an aid to keep the nanoparticles dispersed in solvent. To coat the electrode with the catalyst layer, the organic ligand-coated nanoparticles are typically exposed to high temperatures (e.g., hot-pressing the catalyst layer onto the substrate at 200-300°C).in which the organic ligands are at least partially or completely pyrolyzed. Therefore, the organic ligands are usually no longer detectable or only detectable in a small proportion in the finished electrode. Suitable organic ligands are, for example, selected from aliphatic carboxylic acids and their derivatives, such as saturated and unsaturated fatty acids with 16 to 20 carbon atoms, aliphatic amines and their derivatives, such as oleylamine, heterocyclic compounds, such as tetrazole, aliphatic mercapto compounds and their derivatives, such as octanethiol, alkylphosphoric acid derivatives, such as tri-n-hexyl phosphate, alkylphosphonic acid derivatives, such as hexylphosphonic acid, trioctylphosphine, trioctylphosphine oxide and / or octadecylphosphonic acid, and mixtures thereof. The cadmium and / or zinc chalcogenide nanoparticles preferably have the shape of spherical particles,the shape of coins or the shape of rods, more preferably the shape of spherical particles or the shape of rods, most preferably the shape of spherical particles. The cadmium and / or zinc chalcogenide nanoparticles preferably have an average particle diameter in the smallest direction of extension of 1.5 to 45.0 nm, preferably 3.0 to 30.0 nm, more preferably 4.0 to 25.0 nm. In addition, the cadmium and / or zinc chalcogenide nanoparticles preferably have an average particle diameter in the largest direction of extension of 1.5 to 200.0 nm, preferably 5.0 to 125.0 nm, more preferably 8.0 to 75.0 nm. Cadmium and / or zinc chalcogenide nanoparticles in the form of spherical particles preferably have an average particle diameter of 1.5 to 5.0 nm, preferably 5.0 to 30.0 nm, more preferably 8.0 to 25.0 nm.0 nm. Cadmium and / or zinc chalcogenide nanoparticles in the form of coins preferably have an average particle diameter in the smallest direction of extension of 3.0 to 20.0 nm, preferably 5.0 to 17.5 nm, more preferably 7.0 to 15.0 nm. In addition, the cadmium and / or zinc chalcogenide nanoparticles in the form of coins preferably have an average particle diameter in the largest direction of extension of 10.0 to 50.0 nm, preferably 15.0 to 40.0 nm, more preferably 20.0 to 30.0 nm. Cadmium and / or zinc chalcogenide nanoparticles in the form of rods preferably have an average particle diameter in the smallest direction of extension of 2 to 15.0 nm, preferably 3.0 to 10.0 nm, more preferably 4.0 to 7.0 nm. In addition, the cadmium and / or zinc chalcogenide nanoparticles in the form of rods preferably have an average particle diameter in the largest dimension direction of 3.0 to 200.0 nm, preferably 5.0 to 125.0 nm, more preferably 8.0 to 75.0 nm.0 nm. The cadmium and / or zinc chalcogenide nanoparticles preferably have a surface area of ​​1.0 to 100.0 m² / 100 mg, preferably 3.0 to 50.0 m² / 100 mg, more preferably 4.5 to 30.0 m² / 100 mg. The surface area refers to the total surface area of ​​100 mg of nanoparticles. Furthermore, the cadmium and / or zinc chalcogenide nanoparticles preferably have a particle volume (volume of an individual particle) of 2 to 50,000 nm³, preferably 14 to 15,000 nm³, more preferably 50 to 10,000 nm³. It is preferred that the cadmium and / or zinc chalcogenide nanoparticles contain a core and a shell, preferably a cadmium selenide CdSe core and a cadmium sulfide CdS shell. Preferably, the cadmium and / or zinc chalcogenide nanoparticles have a ratio of the mean diameter of the core to the mean diameter of the total particle of 1.0:7.0 to 1.0:1.0, preferably 1.0:5.0 to 1.0:2, more preferably 1.0:4.5 to 1.0:3,0. Particularly preferred are cadmium chalcogenide nanoparticles in the form of spherical particles with a cadmium selenide CdSe core and a cadmium sulfide CdS shell. The ratio of the average diameter of the core to the average diameter of the total particle is preferably in a range of 1.0:7.0 to 1.0:1.0, preferably 1.0:5.0 to 1.0:2, more preferably 1.0:4.5 to 1.0:3. The average particle diameter is preferably in the range of 1.5 to 40.0 nm, preferably 3.0 to 30.0 nm, more preferably 5.0 to 25.0 nm. The surface area is preferably in the range of 1 to 100 m² / 100 mg, preferably 3 to 50.0 m² / 100 mg, more preferably 5.0 to 30.0 m² / 100 mg. The particle volume (volume of a single particle) is preferably in the range of 2 to 50,000 nm³, preferably 14 to 15,000 nm³, more preferably 50 to 10,000 nm³. The organic ligands on the surface are preferably fatty acid derivatives,such as saturated and unsaturated fatty acids with 16 to 20 carbon atoms, preferably oleic acid. Information on the nanoparticles, such as average particle diameter, shell composition, shell diameter, surface area, and volume, is usually obtained by imaging methods such as transmission electron microscopy (TEM), scanning electron microscopy (SEM), or X-ray diffraction, or gravimetric methods such as thermogravimetric analysis (TGA). In a further aspect, the present invention relates to a method for reducing carbon dioxide CO2, comprising the following steps: ^ Providing an electrochemical cell with an electrode as described herein; ^ Applying current to the electrode; ^ Contacting a carbon dioxide-containing gas stream with the cadmium and / or zinc chalcogenide nanoparticles in the electrode so that the carbon dioxide is reduced,preferably to carbon monoxide CO and / or synthesis gas CO / H2. In this case, all aspects and embodiments of the electrode, the cadmium and / or zinc chalcogenide nanoparticles and the electrochemical cell as described herein are preferably used in the process according to the invention. The current density is preferably in the range of 25 to 1000 mA / cm², preferably 35 to 750 mA / cm², more preferably 45 to 500 mA / cm². The carbon dioxide-containing gas stream is preferably an off-gas stream, preferably an unpurified off-gas stream, more preferably an off-gas stream that additionally contains sulfur-containing compounds. The off-gas stream is preferably obtained from industrial plants, such as waste incineration plants or cement works. The species reduced from carbon dioxide, usually carbon monoxide or synthesis gas,are preferably separated from the gas stream. In general, the species reduced from carbon dioxide can contain other species such as methane CH4 or formic acid HCOOH in addition to carbon monoxide or synthesis gas. Furthermore, the species reduced from carbon dioxide, usually reduced carbon monoxide or synthesis gas, is preferably used to produce carbon derivatives such as methane, olefins, organic acids and their derivatives and alcohols. In the process according to the invention, when using a liquid electrolyte flow cell, Faradaic efficiencies for carbon monoxide formation in the range of 60 to 90%, preferably 65 to 85% at 50 mA / cm² are preferably achieved. Furthermore, in the process according to the invention, when using a liquid electrolyte flow cell, Faradaic efficiencies for carbon monoxide formation in the range of 1 to 65% are preferably achieved.preferably 20 to 50% at 200 mA / cm². In addition, when using a liquid electrolyte flow cell, Faradaic efficiencies for hydrogen formation in the range of 1 to 30%, preferably 5 to 20% at 50 mA / cm² are preferably achieved in the process according to the invention. In addition, when using a liquid electrolyte flow cell, Faradaic efficiencies for hydrogen formation in the range of 1 to 65%, preferably 20 to 50% at 200 mA / cm² are preferably achieved in the process according to the invention. The overall Faradaic efficiencies for carbon monoxide and hydrogen formation are preferably in the range of 80 to 99%, preferably 90 to 98% at 50 mA / cm² when using a liquid electrolyte flow cell. The overall Faradaic efficiencies for carbon monoxide and hydrogen formation are preferably in the range of 85 to 99% when using a liquid electrolyte flow cell.preferably 90 to 98% at 200 mA / cm². In the process according to the invention, when using a zero-gap electrolysis cell, Faradaic efficiencies for carbon monoxide formation in the range of 10 to 85%, preferably 30 to 80% at 50 mA / cm² are preferably achieved. In addition, when using a zero-gap electrolysis cell, Faradaic efficiencies for hydrogen formation in the range of 1 to 60%, preferably 2 to 20% at 50 mA / cm² are preferably achieved. The overall Faradaic efficiencies for carbon monoxide and hydrogen formation are preferably in the range of 50 to 90%, preferably 60 to 85% at 50 mA / cm² when using a zero-gap electrolysis cell. After poisoning the gas diffusion electrode with catalyst poisons such as Na2S, H2S, SO2 and NO2, the process according to the invention using a liquid electrolyte flow cell preferably achieves Faradaic efficiencies for hydrogen formation in the range of 1 to 30%.preferably 5 to 25% at 50 mA / cm². Furthermore, after poisoning the electrode with catalyst poisons such as Na2S, H2S, SO2 and NO2 in the process according to the invention, when using a liquid electrolyte flow cell, Faradaic efficiencies for carbon monoxide formation in the range of 30 to 85%, preferably 40 to 80% at 50 mA / cm² are preferably achieved. The total Faradaic efficiencies for carbon monoxide and hydrogen formation when using a liquid electrolyte flow cell after poisoning the electrode with catalyst poisons such as Na2S, H2S, SO2 and NO2 are preferably in the range of 70 to 95%, preferably 75 to 90% at 50 mA / cm². It can be seen that the electrode according to the invention reacts only slightly sensitively to catalyst poisons such as Na2S, H2S, SO2 and NO2. Thus, the process according to the invention can also be used for the reduction of carbon dioxide from unpurified exhaust gas streams, preferably exhaust gas streams containing typical catalyst poisons,such as sulfur-containing compounds. Thus, a high efficiency of the electrocatalytic CO2 conversion could be demonstrated even in the presence of typical catalyst poisons. This allows efficient and cost-effective use of the method according to the invention and the electrode according to the invention in industrial plants for the reduction of carbon dioxide. In a further aspect, the present invention relates to the use of cadmium and / or zinc chalcogenide nanoparticles in an electrode as described herein as a catalyst for the electrochemical reduction of carbon dioxide CO2 to reduced species, such as carbon monoxide CO and / or synthesis gas CO / H2. Preferably, all aspects and embodiments of the electrode, the cadmium and / or zinc chalcogenide nanoparticles,of the electrochemical cell and the method according to the invention as described herein. Examples The invention is illustrated below by non-limiting examples. 1. Measurement Methods X-ray Diffraction The diffractogram of the pure nanoparticles was recorded using a Philips X'Pert Pro MPD (Cu-Kα1 / 2 radiation, 45 kV, 40 mA, 30 min). Transmission Electron Microscopy (TEM) To determine size and shape as well as the size distribution, TEM images were recorded using a Jeol JEM-1011 or a Jeol JEM-2800 (depending on availability) at 100 kV. Nanoparticle solutions were concentrated to an optical density close to 0.1 and dropped onto a copper grid covered with a carbon film. Quantum-functionalized carbon black was dispersed in water and also dropped onto a copper grid covered with a carbon film. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy: SEM images were acquired using a Zeiss Merlin / Gemini II FE-SEM with a thermal Schottky field emitter. Secondary electrons were detected using an Everhardt-Thornley detector.An Oxford Instruments N-Max silicon detector was used for EDX analyses. An acceleration range of 20 kV and a current density of 500 pA were selected for the EDX analyses. All samples were sputtered with gold. Thermogravimetric analysis (TGA): The analyses were performed by combusting at least 3 mg of dried material from each sample. A rate of 10 degrees per minute was used for the measurement, from 20 to 650 °C. The measurements were performed using a Netzsch TGA 209 f1 iris. The residual mass (inorganic mass) was evaluated using Proteus 4.8.4 software. 2. Synthesis of Cadmium Chalcogenide Nanoparticles: Spherical CdSe quantum dots were prepared according to the protocol of Talapin et al. (Talapin et al., 2001). Spherical CdSe / CdS core / shell and pure CdS quantum dots were prepared according to the protocol of Chen et al. (Chen et al.,2008). Coin-shaped CdSe / CdS quantum dots were synthesized using a modified protocol for CdSe / CdS core / shell quantum dots, but with a higher sulfur flow rate. CdSe / CdS quantum dots in quantum rods were synthesized in a flow facility using the procedure described by Jochum et al. (Jochum et al., 2016). The CdS materials were prepared using the described method without the CdSe cores. Talapin, DV, Rogach, AL, Kornowski, A., Haase, M. and Weller, H. (2001) 'Highly Luminescent Monodisperse CdSe and CdSe / ZnS Nanocrystals Synthesized in a Hexadecylamine-Trioctylphosphine Oxide-Trioctylphospine Mixture', Nano Letters, vol.1, no.4, pp.207-211. Chen, Y., Vela, J., Htoon, H., Casson, JL, Werder, DJ, Bussian, DA, Klimov, VI and Hollingsworth, JA (2008) '"Giant" multishell CdSe nanocrystal quantum dots with suppressed blinking', Journal of the American Chemical Society, vol.130, no.15, pp.5026-5027. Jochum, T.,Niehaus, J. and Weller, H. (2016) '27-5L: Late-News Paper : Elongated semiconductor nanorods - Emitter of polarized light in red and green', SID Symposium Digest of Technical Papers, vol.47, no.1, pp.347-349. The following cadmium chalcogenide nanoparticles were obtained: ^ spherical CdSe particles (CdSe spheres) with a mixture of trioctylphosphine (TOP), trioctylphosphine oxide (TOPO), hexylphosphonic acid (HPA) and octadecylphosphonic acid (ODPA) as organic ligands ^ spherical CdS particles (CdS spheres) with oleic acid (OA) as organic ligand ^ spherical particles with CdSe core and CdS shell (CdSe / CdS spheres) with oleic acid (OA) as organic ligand ^ coin-shaped particles with CdSe core and CdS shell (CdSe / CdS coins) with oleic acid (OA) as organic ligand ^ rod-shaped particles with CdSe core and CdS shell (CdSe / CdS rods) with a mixture of trioctylphosphine (TOP), trioctylphosphine oxide (TOPO),Hexylphosphonic acid (HPA) and octadecylphosphonic acid (ODPA) as organic ligands ^ CdS particles in rod shape (CdS rods) with a mixture of trioctylphosphine (TOP), trioctylphosphine oxide (TOPO), hexylphosphonic acid (HPA) and octadecylphosphonic acid (ODPA) as organic ligands ^ As reference: spherical Ag particles (Ag NP) with oleic acid (OA) as organic ligand The properties of the nanoparticles are listed in Table 1: Table 1: Properties of the nanoparticles Nano- Diameter Size Se : S Volume Surface area Content particle (dcore / dshell) distribution ratio [nm³] [m² / 100 mg] Ligand [nm] [nm] [mol / mol] [wt.%] CdSe (2.8 / 0.0) < ±0.5 100.0:0.0 11.5 36.8 23 Spheres CdSe / Cd (3.5 / 7.8) ±1.0 8.3:91.7 248.5 15.7 15 S Spheres CdS (0.0 / 14.7) ±3.2 0.0:100.0 1663.2 8.5 34 Spheres CdSe / Cd (3.5 / 21.4) x ±3.3 0.5:99.5 4424.1 7.2 11 S Coins 12.3 CdSe / Cd (3.5 / 5.3) x ±0.5 (width) 2.1:97.9 970.7 16.5 26 S Rods 44 ±3.7 (length) CdS Rods (0.0 / 6.5) x ±1.3 (width) 0.0:100,0 3169.0 13.2 22.8 95.5 ± 51.7 (length) Ag NP (30) ±10 - 14137.2 1.9 50 3. Electrode preparation and CO2 electrolysis in the liquid electrolyte flow cell To prepare the catalyst powder mixture, carbon black (ENSACO 250G) was dispersed in n-hexane (Merck, HPLC grade) and the CdSe / CdS particle dispersions were added dropwise with vigorous stirring (25 mg of inorganic catalyst mass per g of CB). Silver (Alfa Aesar, 20-40 nm, 99.9%) as a reference electrode was prepared analogously by adding 50 wt% oleic acid (Carl Roth, ≥99%) to the particles and the dispersion using an IKA T18 digital ULTRA-TURRAX for 30 s at 8000 rpm. The hexane was evaporated overnight with stirring. PTFE (3M TF1750, 15 wt% of the total powder mixture) was added, and the powder mixture was homogenized in an IKA M20 blade mill for 4 x 15 s. The catalyst layer (2 mm thick powder layer, corresponding to ~8 mg cm³) -2was applied to carbon fabric (Fuel Cell Store, W1S1010) by hot pressing at 260°C and 3.6 kN cm -2 for 10 min (Servitec Polystat 300S). The electrodes were coated with 2.5 mg cm -2 Sustainion XA-9 was drop-coated and conditioned for 72 h in 1 M KOH (Carl Roth, 85%). For the poisoning experiments, the electrodes were stored under SO2, NO2, or 1:1 H2S:N2 atmospheres for 72 h prior to coating with Sustainion. Electrochemical measurements in a liquid electrolyte flow cell were performed using 1.5 M KHCO3 / 0.1 M K2SO4 (Alfa Aesar, 99% and Thermo Scientific, 99+%) dissolved in ultrapure water as the electrolyte, a Fumatech Fumasep 1120 PK cation exchange membrane, a Gaskatel Mini Hydroflex RHE as the reference electrode, and a Pt-coated Ti wire as the counter electrode. For the Na2S poisoning experiment, 1.5 M KHCO3 / 0.1 M Na2S was used as the electrolyte. The CO2 flow was 25 mL min -1and a system overpressure of 80 mbar was applied. The product gases were detected by GC-TCD / FID on an Agilent Technologies 7820 GC equipped with an HP-PLOT Q and an HP-Molsieve 5 Å column. CO and CO2 were converted in a methanizer for detection by FID. 4. Electrode Preparation and CO2 Electrolysis in the Zero-Gap Electrolysis Cell For application in the zero-gap electrolysis cell, particle dispersions with a loading of 1 mg cm -2 onto the carbon cloth substrate. A Versogen PiperION-A40 membrane was used as the electrolyte, and an IrO2-coated Ti felt (2-GDL40, Bekaert) was used as the anode. Before measurement, the membrane was conditioned in 1 M KOH for 1 hour. A 0.1 M KHCO3 solution was used as the anode substrate. The electrochemical cell was heated to 60 °C, and the gas supply (22.5 mL min -1 CO2 and 2.5 mL min -1Ar as internal standard with a back pressure of 40 mbar) was humidified by passing it over water at 40 °C. The electrodes were heated for 20 minutes at 50 mA cm -2 conditioned and the CO2 electrolysis was carried out for 2 hours at 200 mA cm -2 The products were detected using a Shimadzu GC-2010 Plus / GCMS-QP2020 equipped with a Supelco Carboxen 1010 PLOT column. 5. Electrochemical Screening in a Liquid Electrolyte Flow Cell. Gas diffusion electrodes were fabricated by mixing the CdSe / CdS particles with carbon black as an additional conductive and porous support and then hot-pressing the PTFE-blended powder mixture onto a carbon cloth substrate. Prior to electrochemical measurements, the electrodes were coated with Sustainion XA-9 to improve surface wettability and reduce proton availability at the catalyst. Galvanostatic measurements were performed at 50 and 200 mA cm. -2were carried out in a liquid electrolyte flow cell, with the gas phase products being analyzed after 1 and 2 h. H2 and CO were detected as the main reaction products in different ratios depending on the catalyst type and current density used. In addition, traces of CH4(FE CH4 < 0.5%) was found. For benchmarking, electrodes with commercially available silver nanoparticles were fabricated and measured. Figure 1 shows the Faradaic efficiencies (FEs) for H2 and CO using the different particle types as cathode catalysts at 50 (top) and 200 (bottom) mA cm -2 after 1 and 2 hours. At 50 mA cm 2 Most nanoparticle types show comparable selectivity for CO formation and hydrogen formation, with the selectivity changing only slightly during electrolysis. The commercially available silver nanoparticles exhibit 78 ± 1 % FECO and 22 ± 5 % FE after 1 h. H2and after 2 h 76 ± 2 % FE CO and 23 ± 8 % FE H2 The spherical CdSe / CdS particles show a comparable selectivity of 73 ± 4 % FECO and 15 ± 1 % FEH2 after 1 h and 70.7 ± 0.1 % FECO and 14.6 ± 0.3 % FEH2 after 2 h. The spherical CdS particles without a CdSe core show a similar performance with an FE CO of 72 ± 6 % after 1 h and 73 ± 3 after 2 h and an FE H2 of 19 ± 4 % after 1 h and 15 ± 4 after 2 h. The CdSe / CdS coins also show similar performance (FE CO of 75 ± 3%, FE H2 of 10 ± 2 % after 1 h and FE CO of 73 ± 4%, FE H2 of 9 ± 3 % after 2 h). However, the rod-shaped particle types show a slightly increased FECO and a lower FE H2The CdSe / CdS core-shell rods show a selectivity towards CO of 82.3 ± 0.6 % FE after 1 h and 76.7 ± 0.6 % FE after 2 h and 5 ± 0.2 % FE towards H2 after 1 h and 8 ± 1 % FE after 2 h, respectively. The pure CdS rods also show a similar selectivity with FE CO of 81.1 ± 0.4%, FEH2 of 5.3 ± 0.6% after 1 h and FECO of 80 ± 2%, FEH2 of 6.0 ± 0.7% after 2 h. In contrast to the measurement at 50 mA cm -2 shifts at 200 mA cm -2The selectivity of the reaction with each measurement towards a more pronounced hydrogen formation and greater differences between the particle types used become apparent. The commercial Ag NPs reach 70 ± 13 % FECO (39 ± 14 % FEH2 ) after 1 h and 52 ± 22 % FECO (54 ± 22 % FEH2 ) after 2 h. The highest selectivity for CO among the CdS-based particles is still shown by electrodes based on spherical CdS particles, with FECO of 56 ± 5 % after 1 h and 25 ± 11 % after 2 h and FEH2 of 32 ± 4 % after 1 h and 66 ± 18 % after 2 h. The CdSe / CdS rods still show an FE CO of 44 ± 6 % after 1 h and 19 ± 10 % after 2 h (FEH243 ± 7 % after 1 h and 79 ± 7 % after 2 h). The spherical CdSe / CdS particles perform comparable to FE COof 41 ± 10 % and FEH2 of 48 ± 8 % after 1 h and FECO of 31 ± 4 % and FEH2 of 57 ± 3 % after 2 h. The CdS rods still produce CO with a selectivity of 33 ± 3 % after 1 h and 7 ± 1 % after 2 h with corresponding FE H2 of 56 ± 2 % after 1 h and 90 ± 5 % after 2 h. A significantly lower selectivity towards CO2R is again obtained for the spherical CdSe particles with 18 ± 4 % FE CO after 1 h and 16 ± 5 % after 2 h and 78 ± 7 % FEH2 after 1 h and 78 ± 6 % after 2 h. In contrast to the measurements at 50 mA cm -2 However, the CdSe / CdSe coins show a significantly lower selectivity for the CO2R at 200 mA cm -2compared to the other materials. The FECO is only 14 ± 2% after 1 h and 16 ± 6% after 2 h, while H2 is generated with FEs of 76 ± 2% after 1 h and 76 ± 7% after 2 h, respectively. 6. Catalyst poisoning. Sulfide compounds are assumed to have increased stability towards catalyst poisons. To test this assumption, the CdS electrodes were exposed to an H2S, SO2, and NO2 atmosphere as well as to a Na2S-containing electrolyte, and the performance of the poisoned electrodes was compared with their untreated analogues (Figure 2). In particular, the SO2-pretreated electrodes show comparable performance to the untreated ones (FECO of 79.2 ± 0.9% after 1 h and 79.3 ± 0.2% after 2 h, FE H2of 5.3 ± 0.3% after 1 h and 4.2 ± 0.1% after 2 h). Treatment with H2S also shows a negligible effect in the form of a decrease in FECO to a stable 74 ± 2%, accompanied by an FEH2 of 7.8 ± 0.6% after 1 h and 7 ± 3% after 2 h. However, treatment of the electrodes with NO2 causes a greater loss of CO2 R-selectivity to FECO of 70 ± 1% and FEH2 of 11 ± 3% after 1 h (FECO of 56 ± 1% and FE H2 of 22 ± 5 % after 2 h). In the presence of S 2-In the electrolyte, the electrodes show a significantly reduced selectivity for CO. After 1 hour, the FECO drops to 54 ± 2%, while FEH232 reaches ± 3%. However, it must be noted that the catalyst is not the only part of the electrode exposed to potential poisoning. Since it has previously been reported that H2S can also poison Sustainion membranes, it is likely that the ionomer coating was also damaged by the sulfide-containing electrolyte [Liu, Z., White, E., Resch, M. and Masel, R. (2021) 'Poison Effects on Alkaline CO2 Electrolyzer Using Sustainion® Membrane', ECS Meeting Abstracts, MA2021-02, no. 26, p. 827]. 7. CO2 electrolysis in a zero-gap electrolysis cell The particle suspensions of spherical CdS and CdSe / CdS particles and CdS and CdSe / CdS rods were drop-cast directly onto the carbon cloth GDL without any ionomer or additional binder.The particle layer obtained with this technique was more homogeneous for the spherical particles than for the rod-shaped particles. The results of the 2-hour electrolysis are shown in Figure 3. The spherical CdSe / CdS particles reached [context unclear] ... -2 a FE CO of 62 ± 2 %, which remains stable for 2 h (58 ± 3 %). The spherical CdS particles also show a relatively stable FE CO from an initial 57 ± 2%, which drops to 45 ± 4% during electrolysis. The FECO achieved with the CdSe / CdS rods fluctuates considerably. From an initial 38.4 ± 0.2%, it drops after 60 to 80 minutes at 200 mA cm -2 initially to < 10%, rising again after 2 h to 25 ± 2%. The CdS rods yield a relatively low but stable FECO of 19 ± 2% initially and 16 ± 1% after 2 h.

Claims

Claims 1. An electrode for the reduction of carbon dioxide CO2 to carbon monoxide CO and / or synthesis gas CO / H2, which contains cadmium and / or zinc chalcogenide nanoparticles as catalyst.

2. The electrode according to claim 1, wherein the cadmium and / or zinc chalcogenide nanoparticles have the shape of spherical particles, the shape of coins, or the shape of rods. 3.The electrode according to claim 1 or 2, wherein the cadmium and / or zinc chalcogenide nanoparticles contain cadmium sulfide CdS, cadmium selenide CdSe, zinc sulfide ZnS, zinc selenide ZnSe and mixtures of two or more compounds of cadmium sulfide CdS, cadmium selenide CdSe, zinc sulfide ZnS, zinc selenide ZnSe in the form of layers or shells of respectively different individual compounds or in the form of an alloy, preferably cadmium sulfide CdS, cadmium selenide CdSe and mixtures of cadmium sulfide CdS and cadmium selenide CdSe in the form of layers or shells of the individual compounds or in the form of an alloy.

4. The electrode according to one of the preceding claims, wherein the cadmium and / or zinc chalcogenide nanoparticles contain cadmium selenide (CdSe).

5. The electrode according to one of the preceding claims, wherein the cadmium and / or zinc chalcogenide nanoparticles contain a core and a shell, preferably a cadmium selenide (CdSe) core and a cadmium sulfide (CdS) shell. 6.The electrode according to any one of the preceding claims, wherein the cadmium and / or zinc chalcogenide nanoparticles have a molar ratio of selenium Se. to sulfur S Se : S of 0.001 : 99.999 to 50.0 : 50.0, preferably 0.01 : 99.99 to 25 : 75, more preferably 0.1 : 99.9 to 10.0 : 90.

0.

7. The electrode according to one of the preceding claims, wherein the cadmium and / or zinc chalcogenide nanoparticles have a ratio of the average diameter of the core to the average diameter of the total particle of 1.0 : 7.0 to 1.0 : 1.0, preferably 1.0 : 5.0 to 1.0 : 2, more preferably 1.0 : 4.5 to 1.0 : 3.

0.

8. The electrode according to one of the preceding claims, wherein the cadmium and / or zinc chalcogenide nanoparticles have an average particle diameter in the smallest dimension of 1.5 to 45.0 nm, preferably 3.0 to 30.0 nm, more preferably 4.0 to 25.0 nm, and / or an average particle diameter in the largest dimension of 1.5 to 200.0 nm, preferably 5.0 to 125.0 nm, more preferably 8.0 to 75.0 nm. 9.The electrode according to one of the preceding claims, wherein the cadmium and / or zinc chalcogenide nanoparticles have a surface area of ​​1.0 to 100.0 m² / 100 mg, preferably 3.0 to 50.0 m² / 100 mg, more preferably 4.5 to 30.0 m² / 100 mg.

10. The electrode according to one of the preceding claims, wherein the cadmium and / or zinc chalcogenide nanoparticles have a particle volume of 2 to 50,000 nm³, preferably 14 to 15,000 nm³, more preferably 50 to 10,000 nm³.

11. The electrode according to one of the preceding claims, wherein the electrode is a porous electrode, preferably a gas diffusion electrode.

12. A process for the reduction of carbon dioxide CO2, comprising the following steps: ^ Providing an electrochemical cell with an electrode according to any one of the preceding claims; ^ Applying current to the electrode; ^ Contacting a carbon dioxide-containing gas stream with the cadmium and / or zinc chalcogenide nanoparticles in the electrode, such that the carbon dioxide is reduced, preferably to carbon monoxide CO and / or synthesis gas CO / H2.

13. The process according to claim 12, wherein the carbon dioxide-containing gas stream is an exhaust gas stream, preferably an unpurified exhaust gas stream, more preferably an exhaust gas stream additionally containing sulfur-containing compounds, from industrial plants, such as waste incineration plants or cement works. 14.The process according to claim 12 or 13, wherein the carbon monoxide or synthesis gas reduced from carbon dioxide is separated from the gas stream and used to produce carbon derivatives, such as methane, olefins, organic acids and their derivatives, and alcohols.

15. Use of cadmium and / or zinc chalcogenide nanoparticles in an electrode according to any one of claims 1 to 11 as a catalyst for the electrochemical reduction of carbon dioxide CO2 to reduced species, such as carbon monoxide CO and / or synthesis gas CO / H2.

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