Method for preparing a catalyst composition

Abstract

Method for preparing a catalyst composition comprising indium bismuth alloy which method comprises (a) combining at least an indium containing salt, a bismuth containing salt and an oxalate salt in a liquid to obtain a precursor mixture; (b) combining the precursor mixture with a reducing agent to obtain a suspension comprising the catalyst composition; and (c) separating the catalyst composition from the suspension, wherein a support material is either added to the precursor mixture or to the reducing agent, and to catalyst composition comprising indium bismuth alloy obtainable by this method.

Description

METHOD FOR PREPARING A CATALYST COMPOSITIONField of the invention

[0001] The present invention relates to a method for preparing an indium and bismuth containing catalyst composition and to catalyst compositions obtainable by this method.Description of the background art

[0002] The conversion of carbon dioxide into economically valuable products and intermediates thereof is gaining interest in view of mitigating the emission of carbon dioxide into the atmosphere, resulting in environmental issues such as climate change, ocean acidification and global warming.

[0003] The electrochemical conversion of carbon dioxide is an attractive utilization technique, because of the mild operating conditions such as low temperature and pressure. Furthermore, when powered by renewable energy sources, it can solve the intermittency problem by storing energy in chemical bonds.

[0004] The electrochemical conversion of carbon dioxide is typically a catalyzed electrochemical reaction. A catalyst may influence the reaction rate and / or the selectivity towards the reaction products and / or the Faradaic yield in the electrochemical reaction.

[0005] Formate or formic acid is an example of a valuable carbon dioxide reduction product, because the conversion of carbon dioxide into formate only requires two electrons. Furthermore, formate and formic acid have many applications in the pharmaceutical, textile, and food industry and shows promise as a reactant in liquid fuel cells or as a renewable hydrogen carrier molecule.

[0006] The article by Bai Zhengyu et al. "Electrooxidation of formic acid catalyzed by Pd nanoparticles support on multi-walled carbon nanotubes with sodium oxalate", Ionics, vol. 19, no. 3, 1 March 2013, pages 543-548, XP093214215, ISSN: 0947-7047, DOI: 10.1007 / xll581-012-0779- 8, describes the manufacture of Pd electrocatalysts in the presence of sodium oxalate. It was found that the catalyst contained well-dispersed Pd nanoparticles having a small size which was attributed to coordination interaction between Pd2+and carboxyl anions of sodium oxalate. The electrocatalysts were found to result in better electrocatalytic activity and utilization efficiency in oxidation of formic acid.

[0007] WO2019141827 discloses a method for preparing a catalyst system comprising an indium and bismuth containing catalyst for the electrochemical conversion of carbon dioxide into chemical products such as carboxylates and carboxylic acids, such as formate or formic acid. The method as described in WO2019141827 comprises combining metal salts in a liquid medium, adding a citrate salt, followed by reducing the metal salts to form indium and bismuth containing particles. The indium and bismuth containing particles are then separated from the liquid medium.

[0008] The article by Davide Pavesi et al. "CO2 electroreduction on bimetallic Pd-ln nanoparticles", Catalysis Science & Technology, vol. 10, no. 13, 6 July 2020, pages 4264-4270, XP093214186, ISSN: 2044-4753, DOI: 10.1039 / C0CY00831A, describes the manufacture of Pdln catalysts with the help of citrate. It is taught to use low amounts of In in Pd matrices to hinder or prevent CO poisoning.

[0009] The use of citrate as a ligand for the synthesis of indium and bismuth containing particles is disadvantageous because relatively low metal yields are obtained. The synthesis of metal particles is thus not an efficient process.

[0010] Another disadvantage of the method using citrate was found to be that the synthesisprocess was difficult to reproduce. It was unclear how much of the metal salts must be used in the process to obtain the desired metal loading and the desired ratio between the metals in the final catalyst composition.

[0011] Another disadvantage of the use of citrate as a ligand is that it was observed that the metal particles obtained comprised the respective pure metallic phases instead of alloys depending on the liquid medium used.

[0012] Indium and bismuth containing alloys are believed to be stable active species in the catalyzed electrochemical reduction of CO2 to formate or formic acid. Thus, there is a desire for a method for preparing a catalyst composition comprising indium bismuth alloys, more specifically metal particles that comprise these metallic alloys on a support material.

[0013] It is a further objective to develop a method in which a high portion of the dissolved indium and bismuth salts are incorporated in the catalyst. In other words, a method for preparing indium bismuth alloys containing catalysts with an improved metal yield.

[0014] There is also a desire for a more reproducible method for preparing indium and bismuth alloys containing catalyst compositions.Brief summary of the invention

[0015] According to the present invention, a method for preparing a catalyst composition has been developed which shows an improvement in one or more of the mentioned objectives.

[0016] According to the present invention, it was surprisingly found that the use of oxalate salts in combination with indium and bismuth containing salts allowed to prepare catalyst compositions comprising indium bismuth alloy.

[0017] The use of oxalate salts also allowed for a more reproducible synthesis process for preparing catalyst compositions.

[0018] Furthermore, it was found that the use of oxalate salts allowed to prepare indium bismuth alloys in a high metal yield.

[0019] Thus, the present invention relates to a method for preparing a catalyst composition comprising indium bismuth alloy, said method comprising the steps of (a) combining at least an indium containing salt, a bismuth containing salt and an oxalate salt in a liquid to obtain a precursor mixture; (b) combining the precursor mixture with a reducing agent to obtain a suspension comprising the catalyst composition; and (c) separating the catalyst composition from the suspension; wherein a support material is either added to the precursor mixture or to the reducing agent.

[0020] For the present method, an indium bismuth alloy is homogenous mixture comprising at least indium and bismuth. Determining whether an alloy is present in a catalyst composition is within the general knowledge of the skilled worker. X-ray diffraction (XRD) may be used to determine whether crystalline alloy phases are present in the catalyst composition. Differential scanning calorimetry (DSC) is another way to determine the presence of alloy phases.

[0021] In a preferred embodiment the liquid is water. Furthermore, the method allows to prepare an indium bismuth zinc alloy in which method step (a) further comprises additionally adding a zinc containing salt.

[0022] According to the method according to the invention, the metal salts are combined with the liquid, preferably dissolved in the liquid, in an amount to obtain a precursor mixture having a total metal salt concentration of between 1-20 mM, preferably of between 1-10 mM, more preferably of between 4-6 mM.

[0023] The oxalate salt and the metal salts are preferably combined with the liquid in an oxalate : metal molar ratio of between 1:1 - 20:1, preferably of between 4:1 - 15:1, more preferably of between 6:1 - 12:1. In the present method, metal cation present in salt herein is considered to be metal independent of its actual form.

[0024] The support material is preferably an electrically conductive support, preferably is a support comprising a porous structure of carbon particles. The reducing agent is preferably a borohydride salt. In a preferred embodiment, the reducing agent is added to the precursor mixture at a rate of between 0.1 - 100 mmol / min. Preferably, the precursor mixture is stirred while the reducing agent is added to the precursor mixture.

[0025] The method according to the invention preferably further comprises a step comprising heating the precursor mixture prior to step b) to a temperature between 40-100 °C, preferably 40- 80 °C, more preferably 50-70 °C, even more preferably 55-65 °C.

[0026] In a preferred embodiment of the method according to the invention, the suspension that is obtained in step b) is allowed to react for a duration of at least 30 minutes, preferably at least 1 hour, more preferably at least 2 hours.

[0027] In a preferred embodiment, the catalyst composition that is separated from the suspension in step c) is subjected to a rinsing step with water after step c), preferably with water having a temperature of at least 40 °C, preferably at least 50 °C.

[0028] In another aspect, the invention relates to a catalyst composition comprising indium bismuth alloy obtainable by the method according to the invention.Detailed description of the invention

[0029] In one aspect, the invention relates to a method for preparing a catalyst composition comprising indium bismuth alloy, said method comprising the steps of combining at least two different metal salts and an oxalate salt in a liquid to obtain a precursor mixture; combining the precursor mixture with a reducing agent to obtain a suspension comprising the catalyst composition, wherein a support material is added to either the precursor mixture or to the reducing agent. These steps generally are followed by the step of separating the catalyst composition from the suspension.

[0030] In this method for preparing a catalyst composition, the bismuth and indium cations of the metal salts, optionally in combination with further cations such as zinc and / or tin cations, preferably zinc cations, form metal particles supported by the support material. The resulting catalyst composition comprises support material and metal particles, which catalyst composition can be separated from the liquid. The metal particles comprise metallic alloy, more preferably consist of metallic alloy. The alloy comprises at least indium and bismuth. The alloy preferably consists of indium and bismuth, optionally with zinc and / or tin. Preferably, the alloy consists of indium and bismuth, optionally with zinc. Most preferably, the alloy consists of indium and bismuth.

[0031] A citrate ligand is typically used for the synthesis of bimetallic supported catalysts, such as indium and bismuth containing catalyst. However, it was found that the use of a citrate ligand resulted in a low yield and inefficient use of the metal ions in the precursor mixture. A significant amount of metal ions did not end up in the catalyst composition that was obtained with the method. The inventors surprisingly found that the use of oxalate anions as ligands resulted in a more efficient preparation method wherein higher metal yields of bismuth and bismuth were obtained which moreover were an alloy.

[0032] The inventors also found that the use of oxalate instead of citrate resulted in a more reproducible method. The methods were found to be able to produce catalyst compositions in higher yields and / or more reproducible metal ratios in the obtained catalyst composition.

[0033] The oxalate salt may be chosen from any suitable oxalate salt that allows the at least partial, preferably full, dissolving of the oxalate salt in the liquid. This allows for the oxalate anion to function as a ligand in the synthesis of the catalyst composition. Examples of suitable cations in the oxalate salts include sodium and potassium. Alternatively, the oxalate salt is formed in situ such as by adding oxalic acid and a suitable base such as sodium hydroxide.

[0034] Herein the term "full dissolving" means that at least 80 wt.% of the material to be dissolved in dissolved in the solvent or liquid. Preferably, at least 85 wt.%, more preferably at least 90 wt.%, even more preferably at least 95 wt.% or even 99 wt.% is dissolved in the solvent or liquid.

[0035] Herein the term "oxalate salt" means that the oxalate anion (CjC2-) is combined with a suitable cation, such as sodium or potassium, in a salt. A suitable oxalate salt is a salt that allows the at least partial, preferably full, dissociation of the oxalate anion and cation in the liquid used for the precursor mixture.

[0036] Herein the term "metal salt" means that the metal cation is combined with a suitable anion, such as nitrate (NCh-) or chloride (Cl-), in a salt. A suitable metal salt is a salt that allows the at least partial, preferably full, dissociation of the metal cation and the anion in the liquid used for the precursor mixture.

[0037] In addition to the indium containing salt and the bismuth containing salt, it is possible to additionally add further metal salts such as metal salts containing metals selected from the group consisting of tin, antimony, tellurium, thallium, lead, zinc, copper, nickel, palladium, silver, platinum and gold. In a preferred embodiment, the additional metal salt comprises a metal which is selected from the group consisting of zinc and tin, more preferably is zinc. It can be advantageous to prepare catalyst compositions comprising a metallic alloy comprising three different metals, which metals preferably are selected from the list mentioned before in this paragraph. Most preferably, no additional metal salt is added which means that in step (a) only indium and bismuth containing salts are combined with an oxalate salt in a liquid.

[0038] The step of combining metal salts and an oxalate salt in a liquid may be carried out by adding the metal salts and oxalate salts simultaneously to the liquid or in separate steps. It is also possible, for example, to add a first metal salt to the liquid in a first container and to add a second metal salt to the liquid in a second container after which the contents of the first and second container are combined with the content of third container containing a liquid comprising the oxalate salt to form the precursor mixture. Another example is to combine the two or more metal salts in a liquid and subsequently add an oxalate salt to obtain the precursor mixture.

[0039] The metal salts may be any suitable salt containing a metal cation. Good results were obtained with nitrate salts, preferably InfNOsh and BifNOah, and with chloride salts, preferably InCh and BiCla, in particular with chloride salts.

[0040] The liquid may be any suitable liquid that allows for dissolving the metal salts and the oxalate salts. Examples of suitable liquids include triethylene glycol (TEG) and water. TEG is a known solvent for the preparation of metal particles. Preferably, the liquid is water. Water has a lower toxicity than TEG and the use of water requires a smaller amount of the reducing agent in the method. In addition, the solubility of metal salts is typically better in water compared to TEG, allowing for the method to be carried out at relatively low temperatures. A lower temperaturemay be advantageous as it allows the synthesis to take place below the melting temperature of the alloys that may be formed.

[0041] The metal salts are preferably combined with the liquid in such amount as to obtain a precursor mixture having a total metal salt concentration of between 1-20 mM, preferably of between 1-10 mh / l, more preferably of between 4-6 mM. It was found that dilute solutions result in a more even particle growth. A too high concentration of metal ions in the solution may cause too much interaction between the metal cations which may result in the agglomeration of metal and an uncontrolled growth of the particles. A too low concentration of metal ions in the solution may lead to a low metal loading on the support material.

[0042] The oxalate salt and the metal salts are preferably combined to obtain a precursor mixture having an oxalate : metal molar ratio of between 1:1 - 20:1, preferably of between 4:1 - 15:1. Good results were obtained when the oxalate salt and metal salts were added to the precursor mixture in an oxalate: metal molar ratio of between 6:1 - 12:1. For this, the amount of metal is the molar amount of metal cation.

[0043] The precursor mixture comprising the metal salts and the oxalate salt and / or the precursor mixture comprising the metal salts, oxalate salt and support material may advantageously be heated to a temperature between 40-100 °C prior to the step of combining with the reducing agent, preferably 40-80 °C, more preferably 50-70 °C, even more preferably 55-65 °C. The precursor mixture may also advantageously be stirred. These are measures to enhance the dissolution of the metal salts and / or the oxalate salts. This is advantageous, because the metal ions are more easily reduced to metals than metal ions in undissolved salt. Furthermore, enhancing the dissolution of oxalate salts may lead to a higher concentration of oxalate anions in the precursor mixture that may function as a ligand. A too high temperature may lead to part of the liquid to evaporate, thereby altering the concentration of the salts. A too high temperature may also be disadvantageous during the addition of the reducing agent because some metal alloys may melt at relatively low temperatures.

[0044] A support material is added which is thought to aid the formation of metallic alloy particles with a small size during the synthesis of the catalyst, thereby increasing the overall metallic surface area. Without a support, there is an increased chance that metals will aggregate forming larger particles. In one embodiment, the support material is added to the precursor mixture after which the precursor mixture is combined with the reducing agent. In another embodiment, the support material is added to the reducing agent after which the reducing agent is combined with the precursor mixture. It is also possible to add support material to both the precursor mixture and to the reducing agent.

[0045] The amount of support material that is added may be such that the final catalyst composition has a total metal load between 10-90 wt.% based on the total weight of the metallic alloy and the support material. Advantageously, the total metal load may be the range of 10-60 wt.%, preferably between 25-50 wt.%, for example around 40 wt.% based on the total weight of the metallic alloy and the support material. A too high total metal load on the support material may lead to aggregation of the metals resulting in a relative decrease in active metal surface area. A too low metal load on an electrically conductive support may lead to a low Faradaic yield during catalysis due to a low availability of active metal surface area.

[0046] When applying the catalyst composition obtained by the method according to the invention as a catalyst for catalyzed electrochemical reactions, the use of a support also allows for the catalyst composition to be easily adhered to an electrode in an electrochemical cell. The useof electrically conductive supports is advantageous when applying the catalyst composition in catalyzed electrochemical reactions because an electrically conductive support also enhances the electrical conductivity during reaction.

[0047] A particulate material, in particular carbon particles, may be used as an electrically conductive support. Preferably, the electrically conductive support comprises a porous structure of carbon particles, such as for example carbon black, activated carbon, carbon nanotubes, Vulcan carbon, acetylene black or graphite. These carbon particles tend to be bound together. The use of particles may allow for supports with high surface area. Compared to other known high surface area supports, carbon, while conductive, is not catalytically active under electrochemical conditions. Therefore, a support comprising carbon particles has the advantage that the support material will not interfere with the desired reaction that is catalyzed by the metals.

[0048] Good results in the method according to the invention were obtained with the use of a carbon support material and water as the liquid.

[0049] The carbon particles in the electrically conductive support may be bound together by a binder. The binder is preferably stable under alkaline conditions and / or under reducing conditions, such as a fluorinated binder. The skilled person knows how to select a binder based on the requirements. In an embodiment, the binder is a hydrophobic binder. This is in particular useful when the catalyst system is applied to catalyze electrochemical reactions that take place in an aqueous environment. A hydrophobic binder will less likely disintegrate and / or dissolve in an aqueous medium, allowing for the electrically conductive support and the metals in the catalyst system to stay bound during said electrochemical reactions.

[0050] The reducing agent is preferably a borohydride salt. Herein the term "borohydride salt" means that the borohydride anion (BH4“) is combined with a suitable cation, such as sodium or potassium, in a salt. A suitable borohydride salt is a salt that allows dissociation of the borohydride anion and cation in the liquid used for the precursor mixture. Borohydride has a low toxicity and a low standard reduction potential (-1 .24 E° vs SHE), making borohydride an efficient reducing agent for a variety of metals.

[0051] The reducing agent may be combined with the precursor mixture in any suitable way known to the skilled person. In a preferred method, the reducing agent is added to the precursor mixture at a controlled rate of, for example, between 0.1 - 100 mmol / min. Dosing of the reducing agent allows for a more controlled metal particle formation. In another preferred method, the precursor mixture is added to the reducing agent, preferably at a controlled rate of, for example, between 0.1 - 100 mmol / min.

[0052] Preferably, the precursor mixture comprising the metal salts, oxalate salts, and optionally the support material, is stirred during the step of combining with the reducing agent to the precursor mixture. This aids the mixing of the reducing agent with the metal cations allowing for more controlled particle formation. It may also lead to a more narrow particle size distribution. In case the precursor mixture is added to the reducing agent, optionally comprising the support material, it may be advantageous to stir the reducing agent while adding the precursor mixture.

[0053] In a preferred method according to the invention, the suspension that is obtained after the step of combining the reducing agent with the precursor mixture, is allowed to react for a duration of least 30 minutes, preferably at least one hour, more preferably at least two hours to allow a more full reduction of the metal ions in the precursor mixture and to obtain a higher yield of metal in the catalyst composition.

[0054] After reduction of the metal cations and the formation of particles comprising metallic alloy on the support material, the obtained catalyst composition may be separated from the suspension. This may be done by filtering the catalyst composition from the liquid making use of, for example, a pressure filter.

[0055] Filtration may be followed by a rinsing step with water to allow for removal of remaining salts from the catalyst composition. Preferably, the rinsing step takes place with warm water, specifically water having a temperature of at least 40 °C, preferably at least 50 °C. The use of colder water to rinse may lead salts to crystallize from the solution, thereby ending up in the solid product and thus in the catalyst composition. The use of warm water limits the crystallization of salts and increases the chance that they are removed from the catalyst composition solid product.

[0056] The catalyst compositions obtainable by the method according to the invention comprise two or more different metals and may be characterized by the presence of an alloy comprising at least indium and bismuth, preferably consisting of indium and bismuth. A highly preferred catalyst composition comprises an alloy consisting of indium and bismuth and an electrically conductive support material comprising carbon, more especially activated carbon. These catalyst compositions are very suitable for catalyzing the electrochemical conversion of carbon dioxide into formate or formic acid.

[0057] The use of oxalate in the present method for preparing indium and bismuth containing catalysts tends to produce catalyst compositions comprising InBi and / or InjBi alloys. These alloys are known to be active in catalyzing the carbon dioxide reduction to formate or formic acid. Therefore, in a preferred embodiment, the invention relates to a catalyst composition comprising indium bismuth alloy obtainable by a method according to the invention, more especially InBi and / or InjBi alloys.

[0058] The presence of indium-bismuth alloys can be observed with peaks around 89 °C and 109 °C in Differential Scanning Calorimetry (DSC) measurements.

[0059] Catalyst compositions obtainable by the present method and comprising 5-94 wt.% bismuth and 6-95 wt.% indium, based on the total amount of bismuth and indium, have shown good results as catalyst for the electrochemical reduction of carbon dioxide to formate. Preferably, the catalyst composition comprises 10-90 wt% bismuth based on the total amount of indium and bismuth, more preferably 30-90 wt.%, such as 35-90 wt.% based on the total amount of indium and bismuth. More preferably, the amount of bismuth is in the range of 40-60 wt.% based on the total amount of indium and bismuth, such as 45-55 wt.%. A weight ratio of bismuth to indium of 1:1 is preferred. Any bismuth and indium is considered to be metal independent of their actual form. The amount of metals can be determined with the help of inductively coupled plasma (ICP) analysis.Brief description of the drawings

[0060] The features and advantages of the invention will be appreciated upon reference to the following drawing, in which:

[0061] Figure I is a DSC diagram for catalyst compositions comprising indium and bismuth obtained with different ligands.

[0062] The drawings are intended for illustrative purposes only, and do not serve as restriction of the scope or the protection as specified in the claims.ExamplesEXAMPLE 1: Synthesis of InBi

[0063] Indium and bismuth containing catalysts were prepared using a wet chemical reduction method based on the work of Pavesi et al. as described in the article "CO2 electroreduction on bimetallic Pd-ln nanoparticles" in Catalysis, Science & Technology 10.13 (2020), 4264-4270 and making use of different ligands. Bismuth(lll) chloride (BiCU, 99.999%, Thermo Scientific), indium(lll) chloride (InCU, 99.999%, Thermo Scientific), carbon black (Vulcan XC72R, FuelCellStore), and the ligand salts were added to water and were reduced with sodium borohydride (NaBH4, 98+%, ACROS). Starting materials (metal salts and carbon) were weighed prior to synthesis on an analytical balance. After reduction, the solids were filtered and rinsed thoroughly with warm, ultrapure water. The remaining solids were dried in an oven at 50 °C for 24 hours. When dried, the solids were weighed. Catalyst yield (in wt.%) is calculated as a difference between the final obtained mass and the mass of the starting materials, excluding the metal salt counter anion.

[0064] Table 1 shows that the use of oxalate salts as ligands in the method according to the invention for preparing an InBi catalyst composition results in an improved catalyst yield compared to methods wherein a different ligand has been used. Yields well above 95 wt.% were obtained with oxalate salts as ligand.Table 1: Total yield in synthesis making use of different ligands

[0065] Differential scanning calorimetry (DSC) measurements were performed using the Mettler Toledo DSC 3+ StareSystem. Samples were heated from 25 to 300 °C at 5 °C / min with 50 mL / min N2 flow. Same conditions were applied for the cooling cycle. The cycles were repeated twice.

[0066] Figure 1 shows a DSC diagram for examples 1-7, which all comprise indium and bismuth on a carbon support. Only example 3, obtained with the method according to the invention, was found to comprise indium-bismuth alloy phases, characterized by the peaks around 90 °C (ln2Bi) and 110 °C (InBi).

Claims

CLAIMS1. Method for preparing a catalyst composition comprising indium bismuth alloy, said method comprising the steps of(a) combining at least an indium containing salt, a bismuth containing salt and an oxalate salt in a liquid to obtain a precursor mixture;(b) combining the precursor mixture with a reducing agent to obtain a suspension comprising the catalyst composition;(c) separating the catalyst composition from the suspension; wherein a support material is either added to the precursor mixture or to the reducing agent.

2. Method according to claim 1, wherein the liquid is water.

3. Method according to any of the preceding claims wherein the catalyst composition comprises an indium bismuth zinc alloy and step (a) further comprises adding a zinc containing salt.

4. Method according to any of the preceding claims, wherein the metal salts are combined with the liquid in an amount to obtain a precursor mixture having a total metal salt concentration of between 1-20 mM.

5. Method according to any of the preceding claims, wherein the oxalate salt and the metal salts are combined with the liquid in an oxalate : metal molar ratio of between 1:1 - 20:1.

6. Method according to any of the preceding claims, wherein the reducing agent is a borohydride salt.

7. Method according to any of the preceding claims, further comprising the step of heating the precursor mixture prior to step b) to a temperature between 40-100 °C.

8. Method according to any of the preceding claims, wherein the reducing agent is added to the precursor mixture in step b) at a rate of between 0.1 - 100 mmol / min.

9. Method according to any of the preceding claims, wherein after step b), the obtained suspension is allowed to react for a duration of at least 30 minutes.

10. Method according to any of the preceding claims, further comprising a rinsing step with water after step c), preferably with water having a temperature of at least 40 °C.

11. Method according to any of the preceding claims wherein the catalyst composition consists of metal particles on the support material which metal particles consist of metallic alloy.

12. Method according to any of the preceding claims wherein the metal salts comprise a metal which is selected from the group consisting of indium, bismuth and zinc.

13. Catalyst composition comprising indium bismuth alloy obtainable by a method according to any of claims 1-12.

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