MXPA06008320A - Synthesis of noble metal sulphilde catalysts in a sulphide ion-free aqueous environment - Google Patents

Abstract

The invention is relative to a catalyst, in particular to a noble metal sulphide electrocatalyst, obtained by reacting the precursor of at least one noble metal with a thionic species in an aqueous environment essentially free of sulphide ions, and to a method for producing the same.

Description

SYNTHESIS OF CATALYSTS BASED ON SÜLFÜROS OF NOBLE METALS IN AN AQUATIC ENVIRONMENT SULFURO ION EXEMPT.

FIELD OF THE INVENTION The invention relates to a catalyst, in particular to an electrocatalyst based on noble metal sulfide, and to a method for producing the latter. BACKGROUND OF THE INVENTION The noble metal chalcogenides are widely known in the field of electrocatalysis; in particular, electrode catalysts based on rhodium sulphides and ruthenium are currently incorporated in gas diffusion electrode structures used as oxygen reduction cathodes in highly aggressive environments, such as depolarized electrolysis of hydrochloric acid. The noble metal sulfides used in the electrocatalysis are prepared by spreading hydrogen sulfide in an aqueous solution of a precursor of the corresponding noble metal, commonly a chloride, for example as described in US 6,149,782 in relation to a rhodium sulfide catalyst. The synthesis of noble metal sulfide catalysts with hydrogen sulfide in aqueous solutions is conveniently carried out in the presence of a conductive support, which in most cases consists of carbon particles. In this manner, the noble metal sulfide precipitates selectively on the surface of the carbon particle, and the resulting product is a catalyst supported on carbon, particularly suitable for incorporation into gas diffusion electrode structures characterized by high efficiency at reduced loads of. noble metal. High surface carbon blacks, such as the Vulcan XC-72 from Cabot Corp. / USA. They are particularly suitable for this purpose. A different process for the preparation of noble metal sulphide catalysts supported on carbon consists in the incipient wet impregnation of the carbon support with a precursor salt of the noble metal, for example a noble metal chloride, followed by the evaporation of the solvent and a gas phase reaction under hydrogen sulphide diluted at room temperature or at high temperature, where the sulfide is formed in a stable phase. This is described, for example, in the concomitant provisional patent application 60 / 473,543, relating to a ruthenium sulfide catalyst. In the case of rhodium, before use, the noble metal sulfide catalysts thus obtained are subjected to a suitable thermal stabilization treatment, at a temperature normally comprised between 300 and 700 ° C. In other cases, a temperature as low as 150 ° C may be sufficient for an adequate heat treatment. Although these catalysts show good performance in terms of activity towards the reduction of oxygen and stability in highly aggressive environments, which makes them practically the only materials admissible for oxygen reduction catalysts in the electrolysis of hydrochloric acid, their production by sulfide is affected by some drawbacks.- First, the use of a highly dangerous species such as hydrogen sulfide, which is a flammable and toxic gas, in its synthesis raises serious environmental and human health problems. The handling of hydrogen sulphide is a very sensitive issue that can only be tackled by resorting to costly safety measures. Second, precipitation in an environment where sulfide ions are present can lead to the formation of compounds of variable stoichiometry, which can impair the reproducibility of the required catalyst, especially with certain noble metals; Sulfur ions are also a toxic species and not healthy for the environment. Other common reagents for the precipitation of sulfides, such as polysulfides, thioacetic acid or thioacetamide, are less dangerous than hydrogen sulfide, but the reaction path in aqueous environment still involves a preionization or hydrolyzing step of these compounds for give undesirable sulfur ions to the free state. An alternative synthetic route for the production of noble metal sulphides for use in oxygen reduction catalysts, in the absence of sulfur ions to the free state and especially the highly flammable and highly toxic hydrogen sulfide, is therefore a strict requirement to successfully scale up the production of noble metal sulfide catalysts, and finally to commercialize potentially large electrochemical processes such as the depolarized electrolysis of hydrochloric acid. SUMMARY OF THE INVENTION It is an object of the present invention to provide a catalyst based on noble metal sulfide, optionally supported on carbon particles, by precipitation in an aqueous environment free of hydrogen sulfide, and essentially free of the species sulfide ion free State. It is another object of the present invention to provide a method for producing catalysts based on noble metal sulfides in an aqueous environment by avoiding the use of highly flammable and highly toxic species. DETAILED DESCRIPTION OF THE INVENTION Under one aspect, the invention relates to a noble metal sulfide catalyst, preferably supported on a carbon black of high surface area, obtained by reaction of a corresponding noble metal precursor, preferably a chloride, with a thionic species in aqueous solution; By carbon black of high surface area is meant a carbon black species whose surface area exceeds 50 m2 / g. A thionic species is any chemical species that contains a thio group, such as thiosulfates, thionic acids and their derivatives. In a preferred embodiment, the reaction is carried out in an aqueous solution essentially free of sulfur ions. The catalyst of the invention may be the sulfide of any noble metal or also a mixed sulfide of at least one noble metal and one or more co-elements; in a preferred embodiment, said noble metal is selected from the group of ruthenium, rhodium, platinum, iridium and palladium. In a more preferred embodiment, the catalyst of the invention is subjected to a heat treatment at a temperature between 150 and 700 ° C before use.The catalyst of the invention is particularly suitable for incorporation into gas diffusion electrode structures produced on conductive fabrics such as carbon fabrics or metal meshes, especially in gas diffusion cathodes for depolarized electrolysis of hydrochloric acid or other consumer cathodes of oxygen in highly aggressive environments. In another aspect, the invention relates to a method for the production of a noble metal sulfide catalyst in the absence of hydrogen sulfide, and essentially in an environment free of sulfur ions to the free state, which comprises reacting a solution of a noble metal precursor, optionally a chloride, with an aqueous solution containing a thionic species, preferably a thiosulfate or tetrathionate solution of sodium or ammonium. The noble metal sulfide catalyst of the invention may comprise the sulfide of a single noble metal, or the mixed sulfide of a noble metal and other metals, noble or non-noble. The noble metal precursor solution can therefore comprise precursors of other metals, noble or non-noble. Alternatively, a mixed sulphide catalyst can be prepared by reacting the precursor solution of a noble metal and a thionic species containing a second, noble or non-noble metal.

It is well known that, in general, the thiosulfate anion can form sulphides by means of a disproportionation reaction, to give as products a sulphide ion and a sulphate: S203"2 + H20? S" 2 + S042"+ 2 H + However, the inventors have discovered that, under certain conditions, the synthesis of noble metal sulphides (for example rhodium, ruthenium, iridium, platinum or palladium) from thiosulfates proceeds without any determinable release of sulfide ions to the free state. Without wishing to limit the present invention to any particular theory, it can be presumed that the process occurs by direct reaction of the metal ion with one of the two sulfur atoms, which results in the separation of the remaining portion. More precisely, in the example reported below, the inventors have observed that the preferred path is that of partial disproportionation, also known as metathesis of the species S203"2 that the two sulfur atoms are not equivalent, according to the following stoichiometry: S203 - »S" SO '3 The inventors have observed in particular that thiosulfates react with some transition metals at a pH between 0.1 and 4.0 to form metal sulphides when the aqueous solution containing the reactant is brought to boiling or at temperatures comprised between 50 and 100 ° C. When thiosulfates are used for the precipitation of sulfides, the order of addition of the reactants is critical to provide the desired sulfur catalyst, in fact, if the thiosulfate was added beforehand to the acid solution in absence of the metals to be precipitated, the following disproportionation reaction would occur: 2H + + S203"2 ~> S ° + S02 + H20 On the contrary, if the metal ions are present in solution before the addition of thiosulfate, the latter is stabilized, thereby delaying the disproportionation and therefore allowing the metathesis to sulfide. The order of addition of the reagents is on the other hand less important as regards other types of thionic species. For example, tetrathionate is very stable in acid solution and does not undergo a disproportionation reaction of the type illustrated above. Precipitation of sulfides from other thionic acid derivatives such as dithionates (S206 ~ 2), trithionates (S306 ~ 2), tetrathionates (S406"2), pentathionates (S506 ~ 2) or heptathusates (S706 ~ 2) is not mentioned in the prior art, and its mechanism is not yet completely clear, however, the inventors have been able to obtain several noble metal chalcogenides from all those species, under conditions similar to those related to thiosulphate precipitation, again without detecting sulfur ions to the free state at some stage of the process. The precipitation of noble metal or mixed sulfides with a tetrathionate species (for example with sodium tetrathionate) is particularly preferred, since tetrathionate sodium is a widespread and cheap commercial product. Also in this case, the reaction with the transition metals occurs in a pH range comprised between 0.1 and 4.0 (more preferably between 1.0 and 4.0), in a temperature range between 50 ° C and the boiling temperature. In a preferred embodiment, the reaction is carried out in the presence of high surface area carbon particles or other inert and preferably conductive particles to obtain a supported noble metal sulfide catalyst. In a preferred embodiment, the ionic reagent solution is added in discrete aliquots, for example from 2 to 10 aliquots added at time intervals ranging from 15 seconds to 10 minutes. In a preferred embodiment, after adding the thionic reagent solution to the noble metal precursor solution, the resulting solution is heated to the boiling temperature to complete the reaction (which may take from 5 minutes to two hours, depending on the chosen precursor and reaction conditions). The reaction is preferably controlled by means of the color variation of the supernatant liquid, so that the termination of the reaction can be easily determined. In a highly preferred embodiment, the method of the invention further comprises subjecting the product thus obtained to a heat treatment at a temperature between 150 and 700 ° C before use. The following examples are intended to better explain the invention, without constituting a limitation of its scope, which is defined exclusively by the appended claims. EXAMPLE 1 A method for precipitating rhodium sulfide on carbon from an aqueous solution free of sulfide ions is described below. Precipitation reactions of other noble metal sulphide catalysts (such as ruthenium, platinum, palladium or iridium sulphides) require only minor adjustments that can be easily derived by those skilled in the art. 7.62 g of RhCl3.H20 were dissolved in 1 liter of deionized water, and the solution was maintained at reflux (preparation of the noble metal precursor solution). To the solution was added 7 g of carbon black of high surface area Vulcan XC72-R from Cabot Corporation, and the mixture was subjected to ultrasound for 1 hour at 0 ° C (preparation of the noble metal precursor solution which also contains carbonaceous particles). 8.64 g of (NH4) 2S203 were diluted in 60 ml of deionized water, to then determine a pH of 7.64 (preparation of the aqueous solution containing a thionic species). The rhodium / Vulcan solution was heated to 70 ° C under stirring and pH control. Upon reaching 70 ° C, the thiosulfate solution was added in four equivalent aliquots (15 ml each), one every 2 minutes. Between each addition, the constancy of pH, temperature and color of the solution were monitored.

After adding the last thiosulfate aliquot, the remaining solution was heated to 100 ° C and the temperature was maintained for 1 hour. The reaction was followed by controlling the color variations: the initial intense pink / orange color, progressively changed to brown as the reaction progressed, finally became colorless upon completion of the reaction, thus indicating a total absorption of the products on the carbon. In this phase, spot tests were also carried out with a lead acetate paper, which confirmed that at no time sulfide ions were present in the free state in the reaction environment. The precipitate was allowed to settle and filtered; the filtrate was washed with 1000 ml of deionized water to remove all excess reagent, then collecting a filter cake that was dried at 110 ° C until the next day. The dried product was finally subjected to heat treatment under argon flow for 1 hour at 650 ° C, resulting in a weight loss of 22.15%. The resulting supported carbon catalyst was first characterized in a corrosion test, to verify its stability in an electrolysis environment of hydrochloric acid. For this purpose, part of the sample was heated to boiling in a solution of HCl saturated with chlorine, under the same conditions described in Example 4 of US 6,149,782. The color of the resulting solution was the characteristic pale pink of the more stable forms of rhodium sulphide. The effective performance in electrolysis of hydrochloric acid of the catalyst prepared according to the method of the invention and incorporated in a gas diffusion electrode structure on a conductive fabric according to the prior art were also verified. A catalyst / binder layer with a noble metal loading of 1 mg / cm2 was obtained on a gas diffuser based on ELAT * carbon fabric produced by De Nora North America / USA; PTFE in aqueous suspension was used as binder. The gas diffusion electrode thus obtained was sintered at 340 ° C under forced ventilation, and then used as an oxygen reduction cathode in a laboratory hydrochloric acid electrolysis cell. A stable voltage was consistently recorded below 1.2 V at 4 kA / m2 during a two week march, which is an indication of excellent electrochemical behavior. EXAMPLE 2 A rhodium sulfide catalyst equivalent to that of the previous example was prepared in a similar manner, with the difference that sodium tetrathionate was used as a thionic form instead of ammonium thiosulfate. 7.62 g of RhCl3.H20 were dissolved in 1 liter of deionized water, and the solution was maintained at reflux (preparation of the noble metal precursor solution). 7 g of Cabot Vulcan XC72-R high surface area carbon black was added to the solution Corporation, and the mixture was subjected to ultrasound by 1 hour at 40 ° C (preparation of the noble metal precursor solution which also contains carbonaceous particles). 17.86 g of Na2S406 * 2H20 were diluted in 100 ml of deionized water, to then determine a pH of 7.72 (preparation of the aqueous solution containing a thionic species). The rhodium / Vulcan solution was heated to 70 ° C under stirring and pH control. Upon reaching 70 ° C, the tetrathionate solution was added in four equivalent aliquots (25 ml each), one every 2 minutes. Between each addition, the constancy of pH, temperature and color of the solution were monitored. After adding the last tetrathionate aliquot, the remaining solution was heated to boiling for 1 hour. The reaction was followed by controlling the color variations: the initial yellow color, progressively changed to brown as the reaction progressed, finally became colorless upon completion of the reaction, thus indicating a total absorption of the products on the carbon. In this phase, spot tests were also carried out with a lead acetate paper, which confirmed that at no time sulfide ions were present in the free state in the reaction environment. The precipitate was allowed to settle and filtered; the filtrate was washed with 1000 ml of deionized water to remove all excess reagent, then collecting a filter cake that was dried at 110 ° C until the next day. The dried product was finally subjected to heat treatment under nitrogen flow for 2 hours at 650 ° C, resulting in a weight loss of 24.65%. The resulting carbon supported catalyst was subjected to the same corrosion and electrochemical tests of the previous example, showing identical results. Equivalent rhodium sulfide catalysts were also obtained using precursors of trithionate, tetrathionate and sodium heptathione prepared above according to known procedures, with minor adjustments easily derivable by those skilled in the art. Analogous corrosion and electrochemical results were also obtained in these cases.

EXAMPLE 3 A rhodium-molybdenum sulfide catalyst was prepared by the following procedure: in a 500 ml beaker, 250 ml of a 3 g / 1 solution of RhCl 3 H0 previously held at reflux (about 0.75) was added. g of Rh, equivalent to 0.0073 moles). 3.37 g of Cabot Vulcan XC72-R high surface area carbon black was added to the solution.

Corporation, and the mixture was subjected to ultrasound for 1 hour at 40 ° C (preparation of the noble metal precursor solution which also contains carbonaceous particles). 1.9 g of tetradiomolybdate (NH4) MoS4 were diluted in 70 ml of deionized water (preparation of the aqueous solution of a thionic species containing a thionate of a second metal, in this case a non-noble metal). The rhodium / Vulcan precursor solution was heated to 70 ° C under agitation and pH control. Upon reaching 70 ° C, the tetrathiomolybdate solution was added in four equivalent aliquots, one every 2 minutes. Between each addition, the constancy of pH, temperature and color of the solution were monitored. After adding the last aliquot of tetrathiomolybdate, the remaining solution was heated to boiling for 1 hour. The reaction was followed by controlling the color variations: the initial yellow color, progressively changed to light yellow as the reaction progressed, finally became colorless upon completion of the reaction, thus indicating a total absorption of the products on the carbon. In this phase, spot tests were also carried out with a lead acetate paper, which confirmed that at no time sulfide ions were present in the free state in the reaction environment. The precipitate was allowed to settle and filtered; the filtrate was washed with 500 ml of warm deionized water (80 ° C) to remove all excess reagent, then collecting a filter cake that was dried at 110 ° C until the next day. EXAMPLE 4 A rhodium ruthenium sulfide catalyst was prepared by the following procedure: in a 500 ml beaker, 100 ml of a 12 g / 1 solution of RuCl3 H20 previously held at reflux (about 1.2 ml) were added. g of Ru + 3) and 100 ml of a solution of 3 g / 1 of RhCl3 H20 previously maintained at reflux (about 0.75 g of Rh), with a consequent weight ratio of about 80% Ru and 20% Rh. The solution was brought to 350 ml with deionized water and 3.5 g of high surface area carbon black Vulcan XC72-R from Cabot Corporation was added. The mixture was subjected to ultrasound for 1 hour at 40 ° C (preparation of the precursor solution of two different noble metals which also contains carbonaceous particles). 4.35 g of (NH4) 2S203 were diluted in 20 ml of deionized water, then determining a pH of 7.64 (preparation of the aqueous solution containing a thionic species). The rhodium-ruthenium / Vulcan solution was heated to 70 ° C under stirring and pH control. Upon reaching 70 ° C, the thiosulfate solution was added in four equivalent aliquots (5 ml each), one every 2 minutes. Between each addition, the constancy of pH, temperature and color of the solution were monitored. After adding the last aliquot of thiosulfate, the resulting solution was heated to 100 ° C and the temperature was maintained for 1 hour. The reaction was followed by controlling the color variations: the initial intense pink / orange color, progressively changed to brown as the reaction progressed, finally became colorless upon completion of the reaction, thus indicating a total absorption of the products on the carbon. In this phase, spot tests were also carried out with a lead acetate paper at different times, which confirmed that at no time sulfide ions were present in the free state in the reaction environment. The precipitate was allowed to settle and filtered; the filtrate was washed with 700 ml of warm deionized water to remove all excess reagent, then collecting a filter cake that was dried at 110 ° C until the next day. The foregoing description will not be understood as limiting the invention, which can be practiced according to different embodiments without departing from its objectives, and whose scope is univocally defined by the appended claims. In the description and claims of the present application, the word "understand" and its variations such as "comprises" and "understood" are not intended to exclude the presence of other accessory elements or components.

Claims (18)

1. NOVELTY OF THE INVENTION Having described the invention as above, property is claimed as contained in the following: CLAIMS 1. A noble metal sulfide catalyst characterized in that it is obtained by reacting the precursor of at least one noble metal with a thionic species in an aqueous environment essentially free of sulfur ions.
2. 2. A noble metal sulphide catalyst supported on carbon characterized in that it is obtained by reacting the precursor of at least one noble metal with a thionic species in an aqueous environment essentially free of sulfur ions, containing carbonaceous particles in suspension.
3. 3. The catalyst of claim 2, characterized in that said carbonaceous particles are carbon black particles with a surface area in excess of 50 m2 / g.
4. 4. The catalyst of any one of claims 1 to 3, characterized in that the pH of said aqueous environment is between 0.1 and 4.
5. The catalyst of any one of claims 1 to 4, characterized in that said precursor of at least one noble metal is a noble metal chloride.
6. The catalyst of any one of claims 1 to 4, characterized in that said at least one noble metal is selected from the group consisting of ruthenium, rhodium, platinum, iridium and palladium.
7. The catalyst of any one of claims 1 to 6, characterized in that said thionic species is selected from the group of noble or non-noble metal thiosulfates, dithionates, trithionates, tetrathionates, pentationates, heptations and thionates.
8. 8. The catalyst of claim 7, characterized in that it is also subjected to a heat treatment at a temperature comprised between 150 and 700 ° C.
9. 9. A gas diffusion electrode characterized in that it comprises the catalyst of any one of the preceding claims on a conductive fabric.
10. A method for producing a noble metal sulfide catalyst, characterized in that it comprises reacting a solution of a precursor of at least one noble metal, optionally a chloride, with an aqueous solution containing a thionic species in an environment essentially free of sulfur ions.
11. 11. The method of claim 10, characterized in that the pH of said solution of a precursor of at least one noble metal and said aqueous solution containing a thionic species is comprised between 0.1 and 4.
12. The method of claim 11, characterized because said solution of a precursor of at least one noble metal also contains carbonaceous particles, optionally a carbon black with a surface area in excess of 50 m2 / g.
13. The method of any one of claims 10 to 12, characterized in that said thionic species is selected from the group of thiosulfates, dithionates, trithionates, tetrathionates, pentathionates, heptatates and thionates of noble or non-noble metals, optionally as salts sodium or ammonium.
14. The method of claim 10, characterized in that said aqueous solution containing a thionic species is added to said solution of a precursor of a noble metal in discrete aliquots, optionally from 2 to 10 equivalent aliquots at time intervals between 15 seconds and ten minutes .
15. The method of any one of claims 10 to 14, characterized in that said aqueous solution containing a thionic species is added to said solution of a precursor of at least one noble metal and the resulting solution is brought to the boiling temperature between 5 and 120 minutes until the completion of the reaction.
16. 16. The method of claim 15, characterized in that said termination of the reaction is determined by a color change.
17. 17. The method of claim 15, characterized in that it further comprises isolating the resulting noble metal sulfide catalyst and subjecting the latter to a heat treatment at a temperature comprised between 150 and 700 ° C. The method of any one of claims 10 to 17, characterized in that said at least one noble metal is selected from the group consisting of ruthenium, rhodium, platinum, iridium and palladium.