Catalyst manufacturing method with low levels of waste products

Abstract

A method for manufacturing a supported catalyst. The method involves the steps of: (ia) sequential deposition of an aqueous solution comprising a platinum group metal (PGM) salt followed by depositio

Description

Field The invention relates to a process for producing a supported catalyst comprising a platinum group metal and a transition metal. Background to the Invention The production or bimetallic or mutimetallic catalysts sometimes involves a step of dealloying in order to remove base metals from the surface of the alloy particles. Typically in this process a pre-catalyst is treated with a solution of acid in order to selectively remove one or more of the base metals. A step of dealloying is commonly used in the production of fuel cell catalysts containing a platinum group metal (usually platinum) and a transition metal. For example, the article “Record activity and stability of dealloyed bimetallic catalysts for proton exchange membrane fuel cells" in Energy Environ. Sci., 2015, 8, 258-266 describes the dealloying of PtNis catalyst precursors supported on high surface area carbon using nitric acid or sulfuric acid. An acid dealloying step produces a solution containing the leached metal(s). Whilst this is not problematic for lab scale syntheses, the logistics and costs of disposing of the leach solution can be significant at commercially relevant scales. To the present inventors’ best knowledge, such leach solutions have previously been treated as waste and disposed of. It would be advantageous if the production of a leach solution could be avoided and / or if the costs associated with disposing of the leach solution could be reduced. The present invention addresses this problem. Summary of the Invention Instead of viewing the leach solution as a waste stream which needs to be disposed of, the present inventors have recognised that the leach solution is a valuable source of metals which can be used in the production of the catalyst. Therefore the invention provides a process comprising the steps of: (i-a) sequentially depositing an aqueous solution comprising a platinum group metal salt onto a catalyst support, followed by depositing an aqueous solution comprising a transition metal salt onto the catalyst support; or (i-b) sequentially depositing an aqueous solution comprising a transition metal salt onto a catalyst support, followed by depositing an aqueous solution comprising a platinum group metal salt onto the catalyst support; or (i-c) depositing an aqueous solution comprising a transition metal salt and a platinum group metal salt onto a catalyst support; (ii) treating the product of step (i) to form alloy particles comprising the platinum group metal and the transition metal; (iii) treating the product of step (ii) with acid in order to leach a portion of the transition metal and produce a dealloyed catalyst and a leach solution comprising the transition metal; (iv) adjusting the concentration and / or composition of the leach solution, including a step of replenishing the transition metal by adding a transition metal hydroxide, a transition metal carbonate, a transition metal bicarbonate, a transition metal hydroxycarbonate, or a solution thereof, to the leach solution, to produce a neutralised and replenished leach solution; (v-i-a) sequentially depositing an aqueous solution comprising a platinum group metal salt onto a catalyst support, followed by depositing the neutralised and replenished leach solution onto the catalyst support; or (v-i-b) sequentially depositing the neutralised and replenished leach solution onto a catalyst support, followed by depositing an aqueous solution comprising a platinum group metal salt onto the catalyst support; or (v-i-c) depositing an aqueous solution comprising a transition metal salt and a platinum group metal salt, prepared from the neutralised and replenished leach solution, onto a catalyst support; (v-ii) treating the product of step (v-i) to form alloy particles comprising the platinum group metal and the transition metal; (v-iii) treating the product of step (v-ii) with acid in order to leach a portion of the transition metal and produce a dealloyed catalyst and a leach solution comprising the transition metal. In this way it is possible to reduce or entirely avoid the formation of a by-product solution which needs to be sent for disposal. This not only reduces or eliminates the costs of disposing of the leach solution, but also reduces the material costs for the transition metal salt used in the synthesis. Description of the Figures Figure 1 shows the XRD spectra of PtCo / C catalysts prepared by a method without recycling the leach solution (Examples 1 and 2), and by a method which has used a recycled leach solution (Example 3). Detailed Description Sub-headings are provided for convenience only but are not intended to limit the disclosure. Deposition step (i) In step (i) the platinum group metal and transition metal are deposited onto the support. In a first option, referred to as step (i-a), the metals are deposited by sequentially depositing an aqueous solution comprising a platinum group metal salt onto a catalyst support, followed by depositing an aqueous solution comprising a transition metal salt onto the catalyst support. Optionally a step of drying may be carried out after depositing the platinum group metal salt and before depositing the transition metal salt. In a second option, referred to as step (i-b), the metals are deposited by sequentially depositing an aqueous solution comprising a transition metal salt onto a catalyst support, followed by depositing an aqueous solution comprising a platinum group metal salt onto the catalyst support. Optionally a step of drying may be carried out after depositing the transition metal salt and before depositing the platinum group metal salt. In a third option, referred to as step (i-c), the metals are deposited together by depositing an aqueous solution comprising a transition metal salt and a platinum group metal salt onto a catalyst support. Any suitable deposition technique using aqueous solutions may be used. In some embodiments the deposition technique may be an impregnation techniques, such as incipient wetness impregnation. The method is applicable to the preparation of catalysts containing any transition metal which can be acid leached from the catalyst. Particularly suitable transition metals are those selected from the group consisting of cobalt, nickel, chromium, iron, copper and zinc. The transition metal salt is therefore preferably a cobalt salt, a nickel salt, a chromium salt, an iron salt, a copper salt or a zinc salt. It is particularly preferred that the transition metal is selected from the group consisting of cobalt, nickel, chromium and zinc, most preferably from the group consisting of cobalt or nickel. The method is applicable to making alloy catalysts with any platinum group metal which is resistant to acid leaching from the catalyst. As used herein “platinum group metal” refers to the metals ruthenium, rhodium, palladium, osmium, iridium and platinum, and “platinum group metal salt” is understood accordingly. Ruthenium and platinum are particularly preferred platinum group metals for use in the invention because of their catalytic activity and their resistance to acid leaching. Platinum is particularly preferred. The platinum group metal salt is therefore preferably a ruthenium salt or a platinum salt, preferably a platinum salt. The process is particularly applicable to the manufacture of catalysts for use in fuel cells, and in this duty it is particularly preferred that the transition metal salt is a cobalt salt or a nickel salt. It is especially preferred that the transition metal salt is a cobalt salt or a nickel salt, and that the platinum group metal salt is a platinum salt. The support may be any which is resistant to the acid leach step (iii). Carbon supports are preferred because they are widely used in catalysis, especially in fuel cell catalysts. Alloy formation step (ii) After the transition metal salt and platinum group metal salt have been deposited onto the support the resulting material is treated to form alloy particles comprising the platinum group metal and the transition metal. This may be achieved by any method known to those skilled in the art. For instance, an alloy may be formed through heat treatment, typically carried out under an inert atmosphere such as N2, or a reducing atmosphere such as dilute H2 in N2. Alternatively, an alloy may be formed by a chemical reduction method, such as by chemical reduction using ethylene glycol or sodium borohydride. Acid leach step (Hi) After step (ii) the material is treated with an acid in order to leach a portion of the transition metal and produce a dealloyed catalyst and a leach solution comprising the transition metal. The acid is chosen so as to selectively leach the transition metal instead of the platinum group metal. The skilled person will be able to select suitable acids depending on the desired combination of platinum group metal and transition metal. Preferred acids are sulfuric acid, nitric acid, phosphoric acid and formic acid. These acids are particularly preferred in the case where the platinum group metal is platinum, especially when the platinum group metal is platinum and the transition metal is cobalt, nickel, chromium or zinc. It is preferred that the acid used in step (iii) corresponds to the acid of the anion in the transition metal salt used in step (v-i). This would be the case for instance if cobalt sulfate were used in step (v-i) and sulfuric acid were used in step (iii), or if cobalt nitrate were used in step (v-i) and nitric acid were used in step (iii), etc. This has the benefit of simplifying the species which are present in the solution used in step (v-i). Therefore, in a preferred embodiment the transition metal salt used in step (v-i) is a sulfate salt and the acid used in step (iii) is sulfuric acid. In another preferred embodiment the transition metal salt used in step (v-i) is a nitrate salt and the acid used in step (iii) is nitric acid. In another preferred embodiment the transition metal salt used in step (v-i) is a phosphate salt and the acid used in step (iii) is phosphoric acid. In a less preferred embodiment the acid used in step (iii) does not correspond to the acid of the anion in the transition metal salt used in step (v-i). This would be the case for instance if cobalt nitrate were used in step (v-i) and sulfuric acid were used in step (iii). Whilst this embodiment produces a more complex deposition solution compared to when the transition metal salt and acid are matched, this is not thought to negatively impact the properties of the catalyst produced. Replenishment and neutralisation step (iv) The leach solution generated in step (iii) will generally be too acidic to be used directly a deposition step. Furthermore, because typically not all of the transition metal is leached in step (iii), the leach solution will require replenishment with transition metal Therefore, a transition metal hydroxide, a transition metal carbonate, a transition metal bicarbonate, a transition metal hydroxycarbonate, or a solution thereof, is added to the leach solution. A mixture of these salts or a solution comprising a mixture of these salts may be used, but for simplicity it is preferred that a single salt is used. The use of such basic transition metal salts serves the dual purpose of replenishing the transition metal content of the leach solution to make it useable in step (v-i), and also of neutralising some or all of the acid used during the leach step. For the avoidance of doubt, the term “neutralised” as used herein should be understood as meaning that the solution formed at the end of step (iv) has a higher pH than the leach solution generated in step (iii). The term “neutralised” does not require the solution to have a pH of 7. For the avoidance of doubt, the transition metal used in steps (i) and (iv) is the same. For example, if the transition metal salt used in step (i) is a cobalt salt, then cobalt hydroxide, cobalt carbonate, cobalt bicarbonate, cobalt hydroxycarbonate, or a solution thereof, is added to the leach solution in step (iv). The leach solution produced in acid leach step (iii) may contain unwanted particulate matter. Therefore, it is preferred that step (iv) includes a step of filtering the leach solution. The leach solution will typically have a concentration of transition metal which is less concentrated than the solution used in step (i). Therefore, it is preferred that step (iv) includes a step of concentrating the leach solution. In one embodiment concentrating the leach solution may be achieved by evaporation, but this is a less preferred option because it is relatively energy intensive. In a preferred embodiment concentrating the leach solution may be achieved using a filtration membrane. Step (v) Step (v) includes three distinct steps: step (v-i) {(v-i-a) or (v-i-b) or (v-i-c)}, step (v-ii) and step (v-iii). Step (v-i) In step (v-i) the neutralised and replenished leach solution from step (iv) is used as the transition metal salt-containing solution in step (v-i-a) or step (v-i-b), or to prepare the transition metal salt-containing solution in step (v-i-c). When used to prepare the solution in step (v-i-c) a platinum group metal salt or a solution of platinum group metal salt may be added to the neutralised and replenished leach solution, or vice versa. The platinum group metal of the salt(s) used in steps (i) and (v-i) is the same. It is preferred that the same platinum group metal salt is used in steps (i) and (v-i). Features described as being preferred in connection with step (i) are also preferred in connection with step (v-i). Step (v-ii) Features described as being preferred in connection with step (ii) are also preferred in connection with step (v-ii). Step (v-iii) Features described as being preferred in connection with step (iii) are also preferred in connection with step (v-iii). It will be appreciated that at the end of step (v-iii) two batches of supported catalyst have been prepared. A first batch from step (iii), prepared using a fresh transition metal salt solution in step (i), and a second batch from step (v-iii), prepared using a neutralised and replenished leach solution in step (v-i). The leach solution prepared in step (v-iii) may be used to repeat the cycle of steps (iv), (v-i), (v-ii) and (v-iii). Therefore, in a preferred embodiment the method includes a step (v-iv) of adjusting the concentration and / or composition of the leach solution prepared in step (v-iii), including a step of replenishing the transition metal by adding a transition metal hydroxide, a transition metal carbonate, a transition metal bicarbonate, a transition metal hydroxycarbonate, or a solution thereof, to the leach solution, to produce a neutralised and replenished leach solution, and then repeating steps (v-i), (v-ii) and (v-iii). This sequence may be repeated as many times as is desired. Examples Catalysts comprising Pt and Co supported on carbon were prepared by the following methods. The loadings were chosen so as to achieve a Pt: Co molar ratio of 1 : 1 in the annealed material (prior to the leach), with the weight ratio of Pt: carbon being 40 : 60. Example 1 (Comparative) Carbon black (124.1 g, 10.3 mol) was dispersed in water using a shear mixer and transferred to a reaction vessel. Solid NaHCOs (330.7 g, 3.94 mol) was added, corresponding to a 20% molar excess compared to the combined amount of chloride (from I^PtCle) and nitrate (from Co(NO3)2-6H2O) added in the subsequent steps. Subsequently, a solution of H2PtCle (0.41 mol, equivalent to 80 g of Pt, in 2000 ml demineralised water) followed by a solution of Co(NO3)2-6H2O (119.3 g, 0.41 mol, in 1500 ml of demineralised water). When deposition of the metals was complete the material was recovered by filtration and washed on the filter bed with demineralised water to a conductivity of 20 pS. The material was dried for 24 hours at 105 °C and then annealed in a reducing atmosphere. A portion of the annealed material (160 g) was dispersed in 20ml / g of 0.5M H2SO4 and heated to 80 °C for 24 hours. After cooling the solid was isolated from the cobalt containing acid leachate (solution 1) and washed with demineralised water to 50 pS. The process was repeated and washed to a conductivity of 20 pS following which the solid product was isolated and dried. Example 2 (Comparative) The procedure of Example 1 was followed except that a solution of COSO4 7H2O (115.3 g, 0.41 mol, in 1500 ml of demineralised water) was used in place of the solution of Co(NO3)2-6H2O. Example 3 (According to the invention) Co(OH)2 (23.4 g, 0.25 mol) was dissolved in 1500 ml of solution 1 from Example 1 above. Carbon black (93.06 g, 7.75 mol) was dispersed in water using a shear mixer and transferred to a reaction vessel. Solid NaHCOs (289.35 g, 3.44 mol) was added. Subsequently, a solution of H2PtCle (0.31 mol, equivalent to 60 g of Pt, in 1500 ml demineralised water) followed by the mixture of Co(OH)2 / solution 1 (1500 ml). When deposition of the metals was complete the material was recovered by filtration and washed on the filter bed with demineralised water to a conductivity of 20 pS. The material was dried for 24 hours at 105 °C and then annealed in a reducing atmosphere. The solid product was filtered, washed, dried and annealed following the same procedure 5 reported in Example 1. The XRD spectra of the catalysts prepared in Examples 1-3 are shown in Figure 1. The XRD spectra of the catalysts prepared in Examples 1 and 2 appeared identical irrespective of the choice of cobalt salt used in step (i). A catalyst which was prepared by recycling the leach solution (Example 3) also appeared to have identical properties to those of Examples 10 land 2.

Claims

1. A process for manufacturing a supported catalyst, comprising the steps of:(i-a) sequentially depositing an aqueous solution comprising a platinum group metal salt onto a catalyst support, followed by depositing an aqueous solution comprising a transition metal salt onto the catalyst support; or(i-b) sequentially depositing an aqueous solution comprising a transition metal salt onto a catalyst support, followed by depositing an aqueous solution comprising a platinum group metal salt onto the catalyst support; or(i-c) depositing an aqueous solution comprising a transition metal salt and a platinum group metal salt onto a catalyst support;(ii) treating the product of step (i) to form alloy particles comprising the platinum group metal and the transition metal;(iii) treating the product of step (ii) with acid in order to leach a portion of the transition metal and produce a dealloyed catalyst and a leach solution comprising the transition metal;(iv) adjusting the concentration and / or composition of the leach solution, including a step of replenishing the transition metal by adding a transition metal hydroxide, a transition metal carbonate, a transition metal bicarbonate, a transition metal hydroxycarbonate, or a solution thereof, to the leach solution, to produce a neutralised and replenished leach solution;(v-i-a) sequentially depositing an aqueous solution comprising a platinum group metal salt onto a catalyst support, followed by depositing the neutralised and replenished leach solution onto the catalyst support; or(v-i-b) sequentially depositing the neutralised and replenished leach solution onto a catalyst support, followed by depositing an aqueous solution comprising a platinum group metal salt onto the catalyst support; or(v-i-c) depositing an aqueous solution comprising a transition metal salt and a platinum group metal salt, prepared from the neutralised and replenished leach solution, onto a catalyst support;(v-ii) treating the product of step (v-i) to form alloy particles comprising the platinum group metal and the transition metal;(v-iii) treating the product of step (v-ii) with acid in order to leach a portion of the transition metal and produce a dealloyed catalyst and a leach solution comprising the transition metal.

2. A process according to claim 1, wherein the transition metal salt used in steps (i-a) and (v-i-a), or steps (i-b) and (v-i-b) or steps (i-c) and (v-i-c) is selected from the group consisting of: a cobalt salt, a nickel salt, a chromium salt or a zinc salt.

3. A process according to claim 2, wherein the transition metal salt is a cobalt salt.

4. A process according to claim 2, wherein the transition metal salt is a nickel salt.

5. A process according to any of claims 1 to 4, wherein the platinum group metal saltis a platinum salt.

6. A process according to any of claims 1 to 5, wherein the support is a carbon support.

7. A process according to any of claims 1 to 6, wherein the acid used in step (iii) is selected from the group consisting of: sulfuric acid, nitric acid, phosphoric acid or formic acid.

8. A process according to any of claims 1 to 7, wherein the acid used in step (v-iii) is selected from the group consisting of: sulfuric acid, nitric acid, phosphoric acid or formic acid.

9. A process according to any of claims 1 to 8, wherein the acid used in step (iii) corresponds to the acid of the anion in the transition metal salt used in step (v-i).

10. A process according to any of claims 1 to 9, wherein step (iv) includes a step of filtering the leach solution.

11. A process according to any of claims 1 to 10, wherein step (iv) includes a step of concentrating the leach solution.

12. A process according to claim 11, wherein concentrating the leach solution is achieved by evaporation.

13. A process according to claim 11, wherein concentrating the leach solution is achieved by a filtration membrane.

14. A process according to claim 1, wherein:the platinum group metal salt used in steps (i) and (v-i) is a platinum salt;the transition metal salt used in steps (i) and (v-i) is a cobalt salt; andstep (iv) includes a step of replenishing the cobalt by adding cobalt hydroxide, cobalt carbonate, cobalt bicarbonate or cobalt hydroxycarbonate, to the leach solution.

15. A process according to claim 1, wherein:the platinum group metal salt used in steps (i) and (v-i) is a platinum salt;the transition metal salt used in steps (i) and (v-i) is a nickel salt; andstep (iv) includes a step of replenishing the nickel by adding nickel hydroxide, nickel carbonate, nickel bicarbonate or nickel hydroxycarbonate, to the leach solution.

16. A process according to claim 1, wherein:the platinum group metal salt used in steps (i) and (v-i) is a platinum salt;the transition metal salt used in steps (i) and (v-i) is a chromium salt; andstep (iv) includes a step of replenishing the chromium by adding chromium hydroxide, chromium carbonate, chromium bicarbonate or chromium hydroxycarbonate, to the leach solution.

17. A process according to claim 1, wherein:the platinum group metal salt used in steps (i) and (v-i) is a platinum salt;the transition metal salt used in steps (i) and (v-i) is a zinc salt; andstep (iv) includes a step of replenishing the zinc by adding zinc hydroxide, zinc carbonate, zinc bicarbonate or zinc hydroxycarbonate, to the leach solution.

18. A process according to any of claims 14 to 17, wherein the acid used in steps (iii) and (v-iii) is selected from the group consisting of: sulfuric acid, nitric acid, phosphoric acid or formic acid.

19. A process according to any of claims 13 to 17, wherein the support is a carbon support.

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