Method for producing high-entropy oxides

Abstract

To provide a method for simply and efficiently producing a high-entropy oxide.SOLUTION: A method for producing a high-entropy oxide according to the present invention includes a step of mixing a base and an oxidizing agent with a solution of a salt of a metal constituting the high-entropy oxide.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for easily and efficiently producing high-entropy oxides. [Background technology]

[0002] High-entropy materials can exhibit new physical properties and characteristics through more complex combinations of elements than conventional alloys and compounds. For example, high-entropy alloys, which were the first to be researched, have a random arrangement of various metal atoms, which gives them excellent mechanical properties, heat resistance, and corrosion resistance. Therefore, they are being researched as high-temperature structural materials for high-temperature gas turbines and engine parts.

[0003] High-entropy oxides (HEOs) are also a type of high-entropy material and were first reported in 2015. HEOs also have a highly disordered atomic structure, and this disorder creates a large number of active sites on the catalyst surface, so they are being studied as ionic conductors, catalysts, superconductors, thermoelectric materials, and more.

[0004] Conventional methods for synthesizing high-entropy oxides (HEOs) often involve solid-phase synthesis or spray pyrolysis, which require high temperatures up to 1000°C and long heating times. Liquid-phase synthesis, on the other hand, offers the advantage of producing homogeneously mixed metal oxides and is a viable alternative (Non-Patent Document 1). More recently, modified HEO nanoparticles have been synthesized using a continuous supercritical water thermal flow process without calcination (Non-Patent Document 2). While the inventions described in Patent Documents 1 to 4 use high-entropy oxides, the synthesis method is not clearly described. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Special Publication No. 2022-502565 [Patent Document 2] Special Publication No. 2024-501159 [Patent Document 3] Special Publication No. 2024-505148 [Patent Document 4] Special Publication No. 2024-513717 [Non-patent literature]

[0006] [Non-Patent Document 1] TANIGUCHI A et al.,Dalton Translations,2024,53(19),8124-8134 [Non-patent document 2] HANABATA S et al., Journal of the American Chemical Society, 2024, 146(1), 181-186 Summary of the Invention [Problem to be solved by the invention]

[0007] As mentioned above, high-entropy oxides have been produced by applying heat and pressure, and therefore a simpler and more efficient production method has been sought. Therefore, an object of the present invention is to provide a method for easily and efficiently producing a high-entropy oxide. [Means for solving the problem]

[0008] The present inventors have conducted extensive research to solve the above problems, and as a result have found that high-entropy oxides can be obtained simply and efficiently by simply mixing a base and an oxidizing agent with a metal salt solution, thereby completing the present invention. The present invention will now be described.

[0009] [1] A method for producing a high-entropy oxide, comprising: A method comprising the step of mixing a base and an oxidizing agent with a solution of a salt of a metal that constitutes the high-entropy oxide. [2] The method according to [1] above, wherein five or more of the metals are used. [3] The method according to [1] or [2] above, wherein the metal comprises a noble metal. [4] The method according to any one of [1] to [3] above, wherein the base is a strong base. [5] The method according to any one of [1] to [3] above, wherein the base is one or more strong bases selected from tetraalkylammonium hydroxide, alkali metal hydroxide, alkaline earth metal hydroxide, and guanidine. [6] The method according to any one of [1] to [5], wherein the oxidizing agent is one or more oxidizing agents selected from hydrogen peroxide and potassium permanganate. [Effects of the Invention]

[0010] The method of the present invention makes it possible to simply and efficiently produce high-entropy oxides, which have conventionally been produced under high-temperature conditions of up to approximately 1000°C or under high-pressure conditions such as in a supercritical water atmosphere. Therefore, the present invention is industrially extremely advantageous as a technology that simply and efficiently produces high-entropy oxides that are expected to be used as ionic conductors, catalysts, superconductors, thermoelectric materials, etc. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a diagram showing the size distribution of colloidal particles of high-entropy oxide produced by the method of the present invention. [Figure 2] FIG. 2 is an X-ray diffraction pattern of the high-entropy oxide produced by the method of the present invention. [Figure 3] Figure 3(a) is a polarization curve of a high-entropy oxide produced by the method of the present invention, Figure 3(b) is a steady-state polarization curve of a high-entropy oxide produced by the method of the present invention, Figure 3(c) is a graph showing the slope of the steady-state polarization curve of a high-entropy oxide produced by the method of the present invention, and Figure 3(d) is a polarization curve of a high-entropy oxide containing a precious metal element produced by the method of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0012] The method for producing a high-entropy oxide according to the present invention will be described below, but the present invention is not limited to the following specific examples.

[0013] In the method for producing a high-entropy oxide according to the present invention, a base and an oxidizing agent are mixed with a solution of a salt of a metal that constitutes the target high-entropy oxide, thereby precipitating the high-entropy oxide.

[0014] The metal constituting the high-entropy oxide may be any desired metal and is not particularly limited. Examples include transition metals such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, W, Ir, Pt, Au, and Hg; Group 2 metals such as Be, Mg, Ca, Sr, and Ba; lanthanides selected from La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu; actinides selected from Ac, Th, Pa, U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No, and Lr; and non-metallic metals such as Al, Ga, In, Sn, Pb, and Bi. They are also classified into catalytically active metals such as Cr, Mn, Fe, Co, Ni, and Cu; passive metals such as Sn, Zn, Y, Zr, In, and Ce; and noble metals selected from Au, Ag, Pt, Pd, Rh, Ru, Os, and Ir. For example, by combining catalytically active metals and passive metals in a balanced manner, high-entropy oxides with high catalytic activity and stability can be obtained.

[0015] The metal salt used as a raw material is not particularly limited as long as it has sufficient solubility in an aqueous solvent, and examples thereof include halide ion salts such as fluorides, chlorides, bromides, and iodides; inorganic acid salts such as sulfates, nitrates, phosphates, and carbonates; organic acid salts such as formates, acetates, and trifluoroacetates; hydroxides; and ammonium hexachlorometalates.

[0016] The greater the number of metals constituting the high-entropy oxide, the higher the constitutive entropy and the more active sites there are, which tends to improve performance, such as by increasing catalytic activity and electronic conductivity. The number is, for example, preferably 5 or more or 6 or more, and more preferably 8 or more or 10 or more. On the other hand, there is no particular upper limit to the number, but if the number is too large, it may be difficult to adjust the desired metal element configuration in the high-entropy oxide, so the number is preferably 35 or less.

[0017] The molar ratio of metals in the metal salts used is generally reflected in the molar ratio of metals constituting the target high-entropy oxide, and therefore may be adjusted according to the molar ratio of metals constituting the target high-entropy oxide. However, according to the experimental findings of the present inventors, when a weak base is used as the base, the Cu concentration in the high-entropy oxide tends to be lower than that of the copper salt used. Furthermore, when the pH of the reaction solution is 10 or higher, the Cr concentration in the reaction solution tends to be lower. 3+ The ions are further divided into Cr 6+ Therefore, it is preferable to use a Cr salt in an amount twice the molar ratio in the desired high-entropy oxide. Furthermore, since the oxidation tendency of noble metal ions is limited and it is difficult to accurately control the concentration of noble metal elements in the high-entropy oxide, it is preferable to determine the amount of noble metal salt used through preliminary experiments, etc.

[0018] An aqueous solvent is preferably used as the solvent for the metal salt solution. The aqueous solvent refers to water, a water-miscible organic solvent, or a mixed solvent of water and a water-miscible organic solvent. The water-miscible organic solvent refers to an organic solvent that is miscible with water without restriction, and examples thereof include alcohol solvents such as methanol, ethanol, and 2-propanol; amide solvents such as diethylformamide and diethylacetamide; and sulfoxide solvents such as dimethyl sulfoxide. A water-miscible organic solvent is used as an auxiliary solvent when the starting metal salt has low solubility in water. The proportion of the water-miscible organic solvent in the mixed solvent is preferably 50% by mass or less, more preferably 20% by mass or less or 10% by mass or less, and even more preferably 5% by mass or less, 2% by mass or less, or 1% by mass or less.

[0019] The concentration of the metal salt in the solution is adjusted appropriately within a range in which the metal salt is sufficiently dissolved. For example, the concentration of each metal salt can be 0.005 mol / L or more and 0.5 mol / L or less. The concentration is preferably 0.01 mol / L or more, more preferably 0.02 mol / L or more, and preferably 0.2 mol / L or less and more preferably 0.1 mol / L or less. The total concentration of the metal salt in the solution can be, for example, 1% by mass or more and 20% by mass or less. The total concentration is preferably 2% by mass or more, more preferably 5% by mass or more, and preferably 15% by mass or less and more preferably 10% by mass or less. As mentioned above, the amounts of Cu salt, Cr salt, and noble metal salt used are preferably determined by preliminary experiments, etc.

[0020] In the method for producing a high-entropy oxide according to the present invention, a base and an oxidizing agent are mixed with a solution of a salt of a metal that constitutes the target high-entropy oxide, thereby precipitating the high-entropy oxide.

[0021] The oxidizing agent is used to oxidize metal ions in the solution to metal oxides. Although the oxidizing agent is not particularly limited, it is preferable not to use acidic oxidizing agents that acidify the reaction solution, such as concentrated sulfuric acid, nitric acid, and sulfur dioxide. Therefore, the oxidizing agent is preferably one or more oxidizing agents selected from hydrogen peroxide and potassium permanganate.

[0022] The amount of oxidizing agent used may be adjusted as appropriate within a range in which the oxidation of metal ions proceeds satisfactorily, but for example, the concentration of the oxidizing agent in a reaction solution containing at least metal ions, a solvent, an oxidizing agent, and a base may be 0.5% by mass or more and 5% by mass or less. The concentration is preferably 1% by mass or more, more preferably 1.5% by mass or more, and is preferably 4% by mass or less, more preferably 3% by mass or less.

[0023] In the present invention, a high-entropy oxide is produced simply and efficiently by using a base in addition to an oxidizing agent. The base is not particularly limited as long as it can precipitate a high-entropy oxide from a metal ion solution in combination with the oxidizing agent, and examples of the base include tetraalkylammonium hydroxide, alkali metal hydroxides, alkaline earth metal hydroxides, organic bases, and ammonia.

[0024] Examples of tetraalkylammonium hydroxide include tetra C hydroxide. 1-6 Alkylammonium compounds are listed. 1-6 An alkyl group refers to a linear or branched monovalent saturated aliphatic hydrocarbon group having 1 to 6 carbon atoms. Examples include methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, s-butyl, t-butyl, n-pentyl, and n-hexyl. 1-6 The alkyl groups may be different or the same. Examples of quaternary ammonium hydroxides include tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetra(N-propyl)ammonium hydroxide, and tetra(N-butyl)ammonium hydroxide.

[0025] Examples of alkali metal hydroxides include lithium hydroxide, sodium hydroxide, and potassium hydroxide, with sodium hydroxide and / or potassium hydroxide being preferred, and sodium hydroxide being more preferred.

[0026] Examples of alkaline earth metal hydroxides include calcium hydroxide, strontium hydroxide, and barium hydroxide, with calcium hydroxide being preferred.

[0027] Examples of organic bases include pyridine and derivatives thereof, such as pyridine, picoline, and lutidine; primary amines, such as methylamine, ethylamine, and propylamine; secondary amines, such as ethylmethylamine; tertiary amines, such as triethylamine and dimethylethylamine; non-nucleophilic bases, such as diazabicycloundecene; basic amino acids, such as histidine; and guanidine and derivatives thereof, such as guanidine, 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD), and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD).

[0028] In the present invention, a strong base is preferably used. A strong base is a base with a large base dissociation constant, for example, a base with a degree of ionization close to 1 in an aqueous solution, which quantitatively generates hydroxide ions, and a base dissociation constant of pK b <0(K b The strong base refers to a base of about >1. Examples of the strong base include one or more bases selected from tetraalkylammonium hydroxide, alkali metal hydroxide, alkaline earth metal hydroxide, and guanidine.

[0029] The amount of base used may be adjusted as appropriate within a range in which oxidation of metal ions proceeds satisfactorily, and may be, for example, 0.5 to 10 times the molar amount of base relative to the total amount of metal ions. The molar ratio is preferably 1 or more or 1.5 or more, more preferably 2 or more, and is preferably 8 or less or 5 or less.

[0030] The reaction conditions may be appropriately adjusted, but the method of the present invention does not require the reaction to be carried out under high pressure, such as in a supercritical fluid, and can be carried out under normal pressure. Furthermore, there is no need to specifically control the reaction temperature, and the reaction can be carried out at normal temperatures, for example, from 5° C. to 35° C.

[0031] When a base and an oxidizing agent are added to a metal salt solution, a high-entropy oxide precipitates from the transparent solution, forming a suspension. After the base and oxidizing agent are added, the reaction solution may be stirred, for example, at room temperature and atmospheric pressure, to ensure that the reaction proceeds sufficiently. The stirring time is not particularly limited, but may be, for example, 10 minutes or more and 50 hours or less.

[0032] After the reaction is complete, a typical post-treatment may be performed. For example, the precipitated high-entropy oxide may be separated from the liquid by filtration or centrifugation, thoroughly washed with a poor solvent such as water, and then dried. The drying method is not particularly limited, and examples thereof include air drying, heat drying, vacuum drying, and combinations thereof.

[0033] According to the method of the present invention, high-entropy oxides can be produced simply and efficiently by simply adding a base and an oxidizing agent to a raw metal salt solution. High-entropy oxides are highly useful as ionic conductors, catalysts, superconductors, thermoelectric materials, and the like. [Example]

[0034] The present invention will be described in more detail below with reference to examples. However, the present invention is not limited to the following examples, and it is possible to carry out the invention by making appropriate modifications within the scope of the above and below-described aims, and all such modifications are included in the technical scope of the present invention.

[0035] Example 1: Preparation of 12-element high-entropy oxide (HEO12) 0.03M manganese(II) chloride tetrahydrate (MnCl2·4H2O), 0.06M chromium(III) chloride hexahydrate (CrCl3·6H2O), 0.03M iron(II) chloride tetrahydrate (FeCl2·4H2O), 0.03M cobalt(II) chloride hexahydrate (CoCl2·6H2O), 0.03M nickel(II) chloride hexahydrate (NiCl2·6H2O), 0.03M copper(II) chloride dihydrate (CuCl2·2H2O), 0.03M zinc nitrate hexahydrate (Z A 10 mL solution of 0.03 M yttrium chloride hexahydrate (YCl3·6H2O), 0.03 M zirconium(IV) chloride (ZrCl4), 0.03 M indium(III) chloride tetrahydrate (InCl3·4H2O), 0.03 M ammonium hexachlorostannate(IV) ((NH4)2[SnCl6]), and 0.03 M cerium(III) chloride heptahydrate (CeCl3·7H2O) was prepared and stirred in air at room temperature. A 20 mL solution of 0.6 M tetramethylammonium hydroxide (TMA-OH) and 3% by weight hydrogen peroxide was added within 5 seconds. The pH of the resulting reaction solution was approximately 11.72. The hydrogen peroxide decomposed instantaneously, generating oxygen gas, and a dark brown suspension was obtained. This suspension was vigorously stirred at room temperature for 3 hours. Thereafter, the suspension was centrifuged, and the obtained solid matter was washed with a large amount of distilled water and then air-dried at room temperature.

[0036] Example 2: Preparation of hexa-element high-entropy oxide (HEO6) A six-element high-entropy oxide was produced in the same manner as in Example 1, except that an aqueous solution (10 mL) of 0.05 M manganese(II) chloride tetrahydrate, 0.1 M chromium(III) chloride hexahydrate, 0.05 M iron(II) chloride tetrahydrate, 0.05 M cobalt(II) chloride hexahydrate, 0.05 M nickel(II) chloride hexahydrate, and 0.05 M copper(II) chloride dihydrate was used instead of the aqueous solution containing the twelve elements.

[0037] Example 3: Preparation of 11-element high-entropy oxide (HEO11) containing noble metal elements High-entropy oxides containing precious metal elements were produced in the same manner as in Example 1, except that an aqueous solution (10 mL) of 0.04 M manganese(II) chloride tetrahydrate, 0.08 M chromium(III) chloride hexahydrate, 0.04 M iron(II) chloride tetrahydrate, 0.04 M cobalt(II) chloride hexahydrate, 0.04 M nickel(II) chloride hexahydrate, 0.04 M copper(II) chloride dihydrate, 0.04 M zirconium chloride, 0.04 M cerium(III) chloride heptahydrate, 0.004 M rhodium(III) chloride trihydrate, 0.004 M hexachloroplatinate(IV) hexahydrate, and 0.004 M iridium acetate was used instead of the aqueous solution containing the 12 elements.

[0038] Test Example 1: Energy Dispersive X-ray Spectroscopy The high-entropy oxides of Examples 1 and 2 were observed with a scanning transmission electron microscope (STEM) and subjected to energy dispersive X-ray spectroscopy (EDS) and inductively coupled plasma (ICP) optical emission spectroscopy, which confirmed that each metal element was uniformly distributed throughout the particles. Table 1 shows the atomic composition percentages based on ICP analysis.

[0039] [Table 1]

[0040] As shown in Table 1, the molar ratio of the metal salt raw materials used and the peak intensity ratio of the ICP analysis were nearly equal, demonstrating that the composition of the target high-entropy oxide can be adjusted by the molar ratio of the metal elements contained in the raw metal salts.

[0041] Furthermore, when NaOH and NH4OH were used as the base instead of TMA-OH, high-entropy oxides were similarly obtained. However, NH4OH is a weaker base than TMA-OH and NaOH, which are strong bases, and when NH4OH was used as the base, the Cu concentration in HEO was somewhat lower. In addition, when the pH is 10 or higher, the Cr in the solution 3+The ions are further converted into Cr by hydrogen peroxide. 6+ Because chromium tends to be oxidized to the metal, the chromium salt must be used in approximately twice the molar amount of the other metal precursor compounds. On the other hand, when a weak base such as NH4OH is used, the ratio of chromium in the chromium salt precursor to the high-entropy oxide is approximately equal. Furthermore, the oxidation tendency of noble metal ions is limited, making it difficult to accurately control the concentration of noble metal elements in high-entropy oxides. As described above, depending on the base used, it may be necessary to adjust the ratio of the raw metal salts according to the metal ratio in the target high-entropy oxide.

[0042] Test Example 2: Colloidal particle size distribution measurement The size distribution of the high-entropy oxide colloid in the post-reaction solution of Examples 1 and 2 was measured by dynamic light scattering, and the results are shown in Figure 1. The size of the 12-element high-entropy oxide colloidal particles (HEO12) was 20-150 nm, and the size of the 6-element high-entropy oxide colloidal particles (HEO6) was 150-700 nm.

[0043] Test Example 3: X-ray diffraction measurement The high-entropy oxides obtained in Examples 1 and 2 were analyzed by X-ray diffraction using an X-ray diffractometer ("RINT 2000" manufactured by Rigaku Corporation) equipped with a monochromatic Cu Kα ray irradiation device (output setting: 40 kV, 40 mA). The results are shown in Figure 2. As shown in FIG. 2, no clear peaks were observed in the XRD patterns of the high-entropy oxides obtained in Examples 1 and 2, indicating that these oxides were amorphous.

[0044] Test Example 4: Electrocatalysis measurement For electrocatalytic measurements at room temperature, an IviumStat electrochemical workstation (IVIUM Technologies BV) equipped with a three-electrode system was used. A graphite rod and an Ag / AgCl (saturated KCl) electrode were used as the counter and reference electrodes, respectively. To prepare the working electrode, 5 mg of the high-entropy oxide powders (Examples 1 to 3) and 12.5 μL of a 5% Nafion solution (Sigma-Aldrich) were added to 500 μL of a dimethylformamide (DMF) solution that had been sonicated in ice water for 30 minutes to form a uniform catalyst ink. An aliquot (10 μL) of the prepared ink was applied to a 1 mm × 5 mm carbon paper (approximately 0.25 mg / cm). 2 The carbon paper was supported on a carbon paper (Fuel Cell Earth, Inc.) and allowed to air dry. The resulting carbon paper was transferred to an electrode holder and used as the working electrode. The electrolyte solution was purged with 99.999% pure oxygen for at least 30 minutes before use. The polarization curve for the oxygen evolution reaction (OER) was measured at 5.0 mVs in 1 M KOH solution. -1 All measured potentials were calibrated to RHE using the following equation: E RHE = E Ag / AgCl + (0.197 + 0.0591 × pH) All data were collected without iR correction (100%). Tafel plots were obtained from chronoamperometry (CA) studies at different conditions to maintain a constant steady state. The long-term stability of the catalyst was determined by chronoamperometry (CA) and chronopotentiometry (CP) analysis. Steady-state polarization curves for HEO were constructed by sampling the OER current density at 350 seconds for chronoamperometry responses obtained at various overpotentials within the catalyst turnover region at regular intervals of 0.05 V. In this experiment, the current density at 350 seconds was measured for each CA response obtained at different potentials. The time required for the interface to reach a steady state is relative and depends on the specific surface and material. However, for electrode-controlled water splitting reactions under extreme pH conditions, this time scale is significantly easier to determine. Under these conditions, there is no diffusion limitation, and the capacitive current becomes negligible after the first few seconds of the experiment. Electrochemical EIS measurements were performed at an operating overpotential of 300 mV, with a 1 × 10 5 The ECSA was performed in potentiometric mode in the frequency range of ~0.1 Hz. dl specific capacitance (Cs, 0.04 mF / cm in 1 M KOH) 2 Based on the LSV, the turnover frequency (TOF) of the catalyst can be calculated as TOF = j × A / (4 × F × m / M), where j is the current density (mA / cm) at a given overpotential. 2 ), A is the surface area of ​​the working electrode, F is Faraday's constant (96485 C / mol), m is the mass loading of the catalyst, and M is the molecular weight of the catalyst with one active center per formula unit. All catalyst-associated metal atoms were assumed to be accessible during the OER catalytic reaction.

[0045] The 12-element high-entropy oxide (HEO12) of Example 1 (η10) was 2The polarization curves at an overpotential of 240 mV (when the potential is reached) are shown in Figure 3(a). HEO12 exhibited a higher current response in the low overpotential (<270 mV) region than the hexa-element high-entropy oxide (HEO6) (265 mV) in Example 2. A significant improvement in current density was observed above 330 mV for HEO12, demonstrating its remarkable practical utility. The improved catalytic performance of HEO12 at high potentials is attributed to its high coordination entropy, resulting in synergistic effects, increased active sites, a tunable electronic structure, and improved resistance to deactivation. Steady-state polarization curves for HEO12 and HEO6 were constructed by sampling the OER current density at 350 seconds from 1.45 to 1.75 V at regular intervals of 0.05 V in the chronoamperometric (CA) response, and are shown in Figure 3(b). The sampled steady-state OER current densities were plotted against their respective overpotentials, and the iR drop (100%) was corrected using uncompensated resistance. To reduce the contribution of the HEO auto-oxidation current, steady-state polarization curves (Tafel plots) constructed using these responses showed slopes of 39 mV / dec and 45 mV / dec, respectively, as shown in Figure 3(c). The measured exchange current densities also showed a similar trend, with HEO6 (0.967 mA / cm) exhibiting a slope of 39 mV / dec and 45 mV / dec, respectively. 2 ) compared to HEO12 (0.984mA / cm 2 ) showed even better performance. The OER kinetics of HEO6 and HEO12 imply that the third electron transfer, involving the formation of a metal-hydroperoxide intermediate, is likely the rate-determining step (RDS). The 11-element high-entropy oxide (HEO11) of Example 3 had a lower number of constituent metals than HEO12, but its catalytic performance was superior to that of HEO12 (Fig. 3(d)). This is thought to be due to the small amount of noble metals contained.

Claims

1. 1. A method for producing a high entropy oxide, comprising: A method comprising the step of mixing a base and an oxidizing agent with a solution of a salt of a metal that constitutes the high-entropy oxide.

2. The method according to claim 1, wherein five or more of said metals are used.

3. The method of claim 1 , wherein the metal comprises a precious metal.

4. The method of claim 1 wherein the base is a strong base.

5. 2. The method of claim 1, wherein the base is one or more strong bases selected from tetraalkylammonium hydroxides, alkali metal hydroxides, alkaline earth metal hydroxides, and guanidine.

6. 2. The method of claim 1, wherein the oxidizing agent is one or more oxidizing agents selected from hydrogen peroxide and potassium permanganate.

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