CATALYTIC POM PARTICLES

POM particles are suitable as photocatalytic or electrocatalytic catalyst in the production of hydrogen and a method of producing such POM particles. The POM particles are produced by subjecting a heteropoly acid with the chemical formula HzXY12O40, or a hydrate thereof, to acidic conditions in the presence of a polyvalent cation, wherein z=3 or 4, X is selected from the group consisting of P, Si, Ge, As, Sb and V, and Y is selected from the group consisting of W, Mo and V.

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Description
TECHNICAL FIELD

The present invention generally relates to polyoxometalate (POM) particles and to a method of producing such POM particles and the use thereof as catalysts in hydrogen gas production.

BACKGROUND

The internationally recognized goal to reduce evolution of greenhouse gases has set focus on the development of efficient fossil-free technologies for energy production. An attractive alternative to fossil fuel-based processes is hydrogen energy, primarily the production of electricity with the aid of fuel cells, but also the use of hydrogen in reduction reactions, such as the recently proclaimed HYBRIT technology for “green synthesis” of steel from iron ore. A common feature of hydrogen energy technologies is the need of high purity hydrogen gas required in large volumes. Its production is possible either via costly purification of hydrogen obtained via the Water Gas-Shift reaction from natural gas or biogas, or via highly energy demanding electrolytic water splitting.

An attractive alternative to electrolysis is the use of photocatalytic or electrocatalytic water decomposition. In these approaches the energy costs for hydrogen gas can be significantly reduced while not compromising its quality. The challenge, however, lies in the need for expensive components used in the making of such catalysts. Typical photocatalysts for water splitting are nanoparticles (NP) of semiconductor oxides or chalcogenides in combination with noble metals. The efficient electrocatalysts applied so far are also commonly containing platinum group metal-based NPs either as oxides (RuO2 or IrO2) or together with platinum NP. Development of noble metal free photo- and especially electrocatalysts is an important and highly addressed research target.

An attractive candidate for photo- and electrocatalyst in water splitting, i.e., oxygen evolution reaction (OER), is pure and chemically doped tungsten oxide along with closely related nanocomposite heterostructures. The challenge in creation of such related nanostructures is the relatively high reactivity and solubility of WO3 and its derivatives in both acidic and basic media. Here in particular, phosphotungstic acid has attracted attention as a possible photo and potentially electro catalyst. However, this compound in its hydrated form is highly soluble in water, which hinders its application as catalyst as it is not possible to form phosphotungstic acid particles.

There is, thus, a general need to provide noble metal free particles that can be used as photo- and/or electrocatalysts for hydrogen gas production.

SUMMARY

It is a general objective to provide a method of producing POM particles.

It is a particular objective to provide such POM particles that are useful as catalysts in hydrogen gas production.

These and other objectives are met by embodiments of the present invention.

An aspect of the invention relates to a method of producing POM particles. The method comprises producing the POM particles by subjecting a heteropoly acid with the chemical formula HzXY12O40, or a hydrate thereof, to acidic conditions in the presence of a polyvalent cation, wherein z=3 or 4, X is selected from the group consisting of P, Si, Ge, As, Sb and V, and Y is selected from the group consisting of W, Mo and V.

Another aspect of the invention relates to POM particles comprising a heteropoly acid with the chemical formula HzXY12O40, or a hydrate thereof, and a polyvalent cation, wherein z=3 or 4, X is selected from the group consisting of P, Si, Ge, As, Sb and V, and Y is selected from the group consisting of W, Mo and V.

A further aspect of the invention relates to an electrocatalytic water-splitting device comprising, a working electrode comprising POM particles according to above, a counter electrode, and a reference electrode.

Yet another aspect of the invention relates to method of producing hydrogen gas. The method comprises producing hydrogen gas by photocatalytic or electrocatalytic water composition in the presence of a photocatalytic or electrocatalytic catalyst comprising POM particles according to above.

The POM particles produced according to the present invention are insoluble particles at acidic and neutral pH. These POM particles further have catalytic properties in hydrogen production for the oxygen evolution reaction (OER). Accordingly, the POM particles are suitable as catalyst for the production of hydrogen gas.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments, together with further objects and advantages thereof, may best be understood by referring to the following description taken together with the accompanying drawings, in which:

FIG. 1. Hierarchical assembly of phosphotungstate spheres by the sol-gel process, illustrated in relation to the La Mer concept graphically (A), and structurally (B). Individual POMs aggregate to form nanoparticles approximately 20 nm in diameter, which in turn assemble into ternary particles up to approximately 2 μm in diameter.

FIG. 2. (A): SEM image of the Ti spheres at 5000× magnification. (B): XRPD pattern of the spheres. (C): Size distribution of the particles in (A) ranges from 0.8 to 2 μm, with an average of 1.4 μm. (D): The nitrogen adsorption/desorption isotherms for PW NPs. (E): EDS spectrum of the spheres. Tungsten and oxygen were most abundant, while traces of potassium and titanium were also detected.

FIG. 3. AFM images of the spheres. Particles in the micrometer range (A-B) can be seen at low magnification and their nano-sized composition at high magnifications (C-D).

FIG. 4. TEM image of the spheres. The large tertiary particle (A) is made up of secondary particles of nano-size (B-C).

FIG. 5. Asymmetric unit (A) and packing (B) of the phosphotungstic acid crystals of acidic lanthanum phosphotungstate, [La(H2O)9](H3O)3[PW12O40]2(H2O)19. Numerous cavities filled with water molecules are seen, as well as close contacts between POMs.

FIG. 6. Electrochemical measurements at pH 0, 3 and 7 where phosphotungstic acid has been evaluated as an electrocatalyst for the OER reaction in water splitting. (A) Linear sweep voltammetry (LSV). (B) Cyclic voltammetry (CV) curves obtained after initial stabilization. (C) pH dependence of overpotential and Tafel slope.

FIG. 7. TGA curve of the spheres. A stepwise loss of water was observed.

FIG. 8. The scheme of hydrogen bonding in the structure of [La(H2O)19](H3O)3[PW12O40]2(H2O)19.

FIG. 9. Electrochemical measurements at pH=0 (0.5 M H2SO4). (A) Linear sweep voltammetry (LSV), (B) Tafel slope −83 mV/dec, (C) Chronoamperometry (CA), (D) Cyclic voltammetry (CV) stabilization during 200 initial cycles.

FIG. 10. Electrochemical measurements at pH=3 (citrate buffer). (A) Linear sweep voltammetry (LSV), (B) Tafel slope −86 mV/dec, (C) Chronoamperometry (CA), (D) Cyclic voltammetry (CV) stabilization during 200 initial cycles.

FIG. 11. Electrochemical measurements at pH=7 (phosphate buffer). (A) Linear sweep voltammetry (LSV), (B) Tafel slope −86 mV/dec, (C) Chronoamperometry (CA), (D) Cyclic voltammetry (CV) stabilization during 200 initial cycles.

FIG. 12. Cyclic voltammetry (CV) at pH 0 (A), 3 (B) and 7 (C) with phosphotungstic acid (catalyst) and carbon black. To demonstrate the OER activity of the catalyst, cyclic voltammetry was performed with catalyst combined with carbon black and pure carbon black deposited on the graphite electrode at different pH values of the electrolytes. We have observed that the carbon black shown no activity towards OER in any of the electrolytes tested, confirming the actual catalyst agent for the OER process is the phosphotungstic acid.

 DETAILED DESCRIPTION

The present invention generally relates to polyoxometalate (POM) particles and to a method of producing such POM particles and the use thereof as catalysts in hydrogen gas production.

Photocatalytic or electrocatalytic water decomposition has emerged as an attractive method for production of hydrogen gas at a comparatively low cost. The challenge is, however, that expensive components are needed to producing catalysts in the form of platinum group metal-based nanoparticles. Noble metal free photo- and electrocatalysts have been proposed based on pure and chemically doped tungsten oxide (WO3). The challenge with such noble metal free photo- and electrocatalysts is their relatively high reactivity and solubility in both acidic and basic media.

The present invention relates to the production of POM particles that are insoluble in acidic and neutral media and have excellent electrochemical properties at such pH. Accordingly, the POM particles of the invention can be used as noble metal free photo- and electrocatalysts in the production of hydrogen.

An aspect of the invention therefore relates to a method of producing POM particles. The method comprises producing the POM particles by subjecting a heteropoly acid with the chemical formula HzXY12O40, or a hydrate thereof, to acidic conditions in the presence of a polyvalent cation. According to the invention, z=3 or 4, X is selected from the group consisting of P, Si, Ge, As, Sb and V, and Y is selected from the group consisting of W, Mo and V.

A polyoxometalate is a polyatomic anion that consists of three or more transition metal oxyanions linked together by shared oxygen atoms to form closed 3-dimensional (3D) frameworks. The metal atoms are, according to the invention, W, Mo or V. The POMs of the invention are preferably of the Keggin type with a central heteratom (X) inside a cage of 12 transition metal atoms (Y) in their highest oxidation state, all connected by oxygen bridges.

Heteropoly acid is the acidic form of a heteropolymetalate and has the chemical formula HzXY12O40, or alternatively (H3O)zXY12O40.

Experimental data as used herein show that when such heteropoly acids of the above-defined chemical formula are subject to acidic conditions in the presence of a polyvalent cation, the Keggin POMs aggregate, typically in the tens of nm size, which then form a tertiary aggregate into particles, typically in the μm range. In more detail, a hierarchical self-assembly process is induced when the heteropoly acids are subject to acidic conditions in the presence of the polyvalent cation, wherein the POM nuclei first make contact at similar 30 distances via hydrogen bonds. Nanospheres made from POMs are then formed and these nanospheres are assembled to form ternary particles.

In an embodiment, the method comprises heating the heteropoly acid, or the hydrate thereof, while exposed to the acidic conditions in the presence of the polyvalent cation. In a particular embodiment, continuous stirring is also applied while heating the heteropoly acid, or the hydrate thereof. Such an optional, but preferable heating and optional stirring, promotes formation of the POM particles. Heating is preferably up to a temperature below 100° C. but above room temperature (20-25° C.), such as to a temperature selected within an interval of from 50° C. up to 98° C., preferably selected within an interval of from 70° C. up to 97° C., and more preferably from 80° C. up to 95° C., such as about 90° C. The heating and stirring are optional as the POM particles can be formed also without any such applied heating.

The POM particles are preferably produced in a sol-gel process by subjecting the heteropoly acid, or the hydrate thereof, to the acidic conditions in the presence of the polyvalent cation causing individual POMs, i.e., POM molecules, to aggregate to form nanoparticles, which assembly into ternary particles.

In an embodiment, the polyvalent cation is selected from the group consisting of Ti(IV), Zr(IV), Ce(IV), La(III), Ce(III), Pr(III), Nd(III), Sm(III), Eu(III), Gd(III), Tb(III), Dy(III), Ho(III), Er(III), Tm(III), Yb(III), Lu(III), Sc(III) and Y(III).

In a preferred embodiment, the polyvalent cation is selected from the group consisting of Ti(IV), Zr(IV), Ce(IV), and La(III). The polyvalent cation is more preferably selected from the group consisting of Ti(IV) and La(III). Hence, in an embodiment, the polyvalent cation is Ti(IV). In another embodiment, the polyvalent cation is La(III).

In an embodiment, a molar ratio of the heteropoly acid, or the hydrate thereof, and the polyvalent cation is selected within an interval of 1:0.25 to 1:2.

In a preferred embodiment, the molar ratio of the heteropoly acid, or the hydrate thereof, and the polyvalent cation is selected within an interval of 1:0.5 to 1:2. In a particular embodiment, the molar ratio of the heteropoly acid, or the hydrate thereof, and the polyvalent cation is selected within an interval of 1:0.75 to 1:1.5, and more preferably selected within an interval 1:0.9 to 1:1.1, and most preferably 1:1.

Thus, the molar ratio of the heteropoly acid, or the hydrate thereof, and the polyvalent cation is preferably at or close to 1:1.

In an embodiment, the method comprises mixing the heteropoly acid, or the hydrate thereof, and an acidic solution comprising the polyvalent cation. Thus, in this embodiment, the heteropoly acid, or the hydrate thereof, is preferably added to and mixed with an acidic solution comprising the polyvalent cation. The method may then comprise the additional step of dissolving or dispersing the polyvalent cation in the acidic solution prior to adding the heteropoly acid, or the hydrate thereof, to the acidic solution, or vice versa, i.e., adding the acidic solution of the heteropoly acid, or the hydrate thereof.

In an embodiment, the method comprises mixing the heteropoly acid, or the hydrate thereof, and an acidic solution selected from the group consisting of potassium titanium oxide oxalate hydrate C4K2O9Ti·xH2O and lanthanum nitrate La(NO3)3·xH2O. Thus, if the polyvalent cation is Ti(IV), a preferred acidic solution is potassium titanium oxide oxalate hydrate. Correspondingly, if the polyvalent cation is La(III), then a preferred acidic solution is lanthanum nitrate.

In an embodiment, the heteropoly acid is selected from the group consisting of H3XW12O40, H3XMo12O40, and H3XV12O40. In a preferred embodiment, the heteropoly acid is selected from the group consisting of H3XW12O40 and H3XMO12O40. In a currently preferred embodiment, the heteropoly acid is H3XW12O40.

According to the invention, X in the chemical formula of the heteropoly acid is selected from the group consisting of P, Si, Ge, As, Sb and V. In a preferred embodiment, X is selected from the group consisting of P and Si. In a currently preferred embodiment, X is P.

In an embodiment, the heteropoly acid is selected from the group consisting of H3PW12O40, H3PMo12O40, and H3PV12O40. In a preferred embodiment, the heteropoly acid is selected from the group consisting of H3PW12O40 and H3PMo12O40. In a currently preferred embodiment, the heteropoly acid is H3PW12O40, i.e., phosphotungstic acid.

In an embodiment, subjecting the heteropoly acid, or the hydrate thereof, to acidic conditions comprises subjecting the heteropoly acid, or the hydrate thereof, to the polyvalent cation at a pH equal to or below 0.5.

In a preferred embodiment, the heteropoly acid, or the hydrate thereof, is subjected to the polyvalent cation in acidic conditions at a pH equal to or below 0.25, preferably equal to or below 0.1, and more preferably equal to or below 0.

In an embodiment, the produced POM particles are isolated, such as by filtration. Also other methods of isolating POM particles, such as by centrifugation, could be used.

The POM particles produced according to the present invention are preferably nanostructured microparticles. For instance, the nanostructured microparticles preferably comprise a plurality of POM nanoparticles having an average diameter selected within an interval of from 1 nm up to 100 nm, preferably selected within an interval of from 5 nm up to 50 nm, more preferably selected within an interval of from 10 nm up to 30 nm, and most preferably selected within an interval of from 15 nm up to 25 nm, such as about 20 nm.

In an embodiment, the POM particles have an average diameter of at least 0.5 μm. In a particular embodiment, the POM particles have an average diameter of at least 0.75 μm, preferably at least 1.0 μm. For instance, the POM particles preferably have an average diameter selected within an interval of from 1.0 μm to 2.0 μm.

Average diameter as used herein means that there is generally a distribution of the diameter of the POM particles cantered around the average diameter. This also means that individual POM particles may have a diameter below the average diameter or above the average diameter. However, the average diameter of a set or batch of POM particles is preferably within the preferred interval of from 1.0 μm to 2.0 μm.

For instance, the POM particles may have a hierarchical composition, i.e., nanostructured microparticles, with tertiary particles with an average diameter in the μm rang, such as 1.0 μm to 2.0 μm, made up of secondary particles in the tens of nanometer in diameter.

Another aspect of the invention relates to POM particles comprising a heteropoly acid with the chemical formula HzXY12O40, or a hydrate thereof, and a polyvalent cation, wherein z=3 or 4, X is selected from the group consisting of P, Si, Ge, As, Sb and V, and Y is selected from the group consisting of W, Mo and V.

In an embodiment, the polyvalent cation is selected from the group consisting of Ti(IV), Zr(IV), Ce(IV), La(III), Ce(III), Pr(III), Nd(III), Sm(III), Eu(III), Gd(III), Tb(III), Dy(III), Ho(III), Er(III), Tm(III), Yb(III), Lu(III), Sc(III) and Y(III). In a particular embodiment, the polyvalent cation is selected from the group consisting of Ti(IV), Zr(IV), Ce(IV), and La(III). In a preferred embodiment, the polyvalent cation is selected from the group consisting of Ti(IV) and La(III).

In an embodiment, the POM particles comprise Z(H2O)mHz[XY12O40]n(H2O)o, wherein Z represents the polyvalent cation and m, n, o are integer numbers independently equal to or larger than one. In a particular embodiment, the POM particles comprise Z(H2O)mHz[XY12O40]2(H2O)o.

In an embodiment, the POM particles comprise a material selected from the group consisting of Z(H2O)mHz[XW12O40]2(H2O)o, Z(H2O)mHz[XMo12O40]2(H2O)o, and Z(H2O)mHz[XV12O40]2(H2O)o. In preferred embodiment, the POM particles comprise a material selected from the group consisting of Z(H2O)mHz[XW12O40]2(H2O)o, and Z(H2O)mHz[XMo12O40]2(H2O)o. In a particular embodiment, the POM particles comprise Z(H2O)mHz[XW12O40]2(H2O)o. As an example, the POM particles could comprise La(H2O)mHz[XW12O40]2(H2O)o or Ti(H2O)mHz[XW12O40]2(H2O)o.

In an embodiment, the POM particles comprise La(H2O)nH3[XW12O40]2(H2O)m or Ti(H2O)xH4[XW12O40]2(H2O)y.

According to the invention, X in the chemical formula is selected from the group consisting of P, Si, Ge, As, Sb and V. In a preferred embodiment, X is selected from the group consisting of P and Si. In a currently preferred embodiment, X is P. In such a preferred embodiment, the heteropoly acid is phosphotungstic acid H3PW12O40.

In a currently preferred embodiment, the POM particles comprise La(H2O)nH3[PW12O40]2(H2O) m or Ti(H2O)xH4[PW12O40]2(H2O)y, or equivalently La(H2O)n(H3O)3[PW12O40]2(H2O) m or Ti(H2O)x(H30)4[PW12O40]2(H2O)y.

The POM particles produced according to the present invention are preferably nanostructured microparticles. For instance, the nanostructured microparticles preferably comprise a plurality of POM nanoparticles having an average diameter selected within an interval of from 1 nm up to 100 nm, preferably selected within an interval of from 5 nm up to 50 nm, more preferably selected within an interval of from 10 nm up to 30 nm, and most preferably selected within an interval of from 15 nm up to 25 nm, such as about 20 nm.

In an embodiment, the POM particles have an average diameter of at least 0.5 μm. In a particular embodiment, the POM particles have an average diameter of at least 0.75 μm, preferably at least 1.0 μm. For instance, the POM particles preferably have an average diameter selected within an interval of from 1.0 μm to 2.0 μm.

For instance, the POM particles may have a hierarchical composition, i.e., nanostructured microparticles, with tertiary particles with an average diameter in the μm rang, such as 1.0 μm to 2.0 μm, made up of secondary particles in the tens of nanometer in diameter.

The POM particles are obtainable by the above-described method of producing POM particles.

The POM particles of the present invention can be used as photo- or electrocatalysts in hydrogen production by water decomposition.

The invention also relates to an electrocatalytic water-splitting device that can be used for hydrogen production by water decomposition. The electrocatalytic water-splitting device comprises a working electrode comprising POM particles according to the invention, a counter electrode and a reference electrode.

In an embodiment, the counter electrode comprises Pt.

In an embodiment, the reference electrode comprises Ag/AgCl.

In an embodiment, the working electrode comprises a mixture of the POM particles according to the invention and a carbon material selected from the group consisting of carbon black and activated carbon.

In an embodiment, the electrocatalytic water-splitting device also comprises an electrolyte at a neutral or acidic pH, preferably an acidic pH. Illustrative, but non-limiting, examples of electrolytes that could be used in the electrocatalytic water-splitting device include H2SO4, such as 0.5 M H2SO4 having a pH of about 0, a citric acid/citrate buffer, such as citric acid/sodium citrate buffer having a pH of about 3 and a phosphate buffer, such as a phosphate buffer having a buffer of about 7.

The invention also relates to a method of producing hydrogen gas. The method comprises producing hydrogen gas by photocatalytic or electrocatalytic water composition in the presence of a photocatalytic or electrocatalytic catalyst comprising POM particles according to the invention.

Examples

Subjecting phosphotungstic acid solutions to low pH in combination with introduction of polyvalent cations led to the formation of nanostructured microspheres of approximately 2 μm in size as shown by scanning electron microscopy (SEM), which were almost insoluble and resistant to degradation at neutral and high pH. These microspheres were composed of secondary nanospheres with diameters around 20 nm as revealed by transmission electron microscopy (TEM) and atomic force microscopy (AFM). Investigations of the crystal structure of a potential intermediate of this process, namely acidic lanthanum phosphotungstate, [La(H2O)9](H3O)3[PW12O40]2(H2O)19, showed a tight network of hydrogen bonding, permitting closer packing of phosphotungstic acid anions, thereby confirming the mechanism of the observed self-assembly process. The new material demonstrated promising electrochemical properties in oxygen evolution reactions with high stability of the obtained electrode material.

Results

Increasing acidity and ionic strength (cation concentration) in solutions of Keggin POMs together with peptides resulted in the formation of compounds with lower peptide-to-POM ratios, where metal cations and, most importantly, oxonium ions became incorporated into the resulting structures (Greijer et al., 2022). Highly charged cations facilitated formation of “acidic” POM derivatives that are usually included in the composition of the product. In the present Example, extremely acidic conditions with pH<0 were used with relatively high concentrations of highly charged cations, such as Ti(IV), Zr(IV), Ce(IV), and La(III), on heating with continuous stirring. This approach resulted in all cases in hierarchical self-assembly of POMs with formation of spherical aggregates several micrometers in size. Analysis of the particles showed a hierarchical structure of three levels of organization (FIG. 1). Firstly, the POM “nuclei” made contact at similar distance via hydrogen bonds. Secondly, nanospheres made from POMs of approximately 20 nm in diameter were formed. Thirdly, assemblies of these nanospheres formed ternary particles of up to 2 μm in diameter.

SEM images revealed spherical particles ranging from 500 nm up to 2 μm in diameter (FIG. 2A). A typical size distribution for Ti(IV) derived material, i.e., nanospheres/particles, is shown in FIG. 2C. Debris of broken spheres were present in the unwashed samples. EDS analysis showed mainly tungsten, with traces of the metal cations present during synthesis, such as titanium and potassium (FIG. 2E, Table 1).

TABLE 1 EDS data of the spheres produced using Ti(IV) species Element Line Type Weight % Weight % Sigma Atomic %* W M series 75.15 0.79 22.88 O K series 20.03 0.75 70.07 K K series 3.35 0.23 4.80 P K series 0.83 0.26 1.51 Ti K series 0.63 0.24 0.74 Total 100.00 100.00 *Tungsten and oxygen are most abundant, while traces of potassium and titanium could be detected. Phosphorus content was in agreement with that required for the Keggin POM composition.

X-ray powder diffraction of freshly obtained materials were consistent with the known pattern of hydrated Keggin type phosphotungstic acid H3PW12O40·21H2O (FIG. 2B). Drying of the samples resulted in broadening of the peaks and weakening of the peak intensity with a fully X-ray amorphous product on longer storage. These transformations were likely caused by the loss of water molecules from the material that became amorphous while preserving its overall morphology at all levels.

The nitrogen adsorption/desorption isotherms for the phosphotungstate (PW) NPs sample are shown in FIG. 2D. The shape of the isotherms corresponds to characteristic type I, typical of microporous solids having relatively small external surfaces, the limiting uptake being governed by the accessible micropore volume rather than by the internal surface area. The Brunauer-Emmett-Teller specific surface area (SBET) was found as 100.6 m2·g−1 and Langmuir Surface area −125.9 m2·g−1 for the PW NPs sample. The Barrett-Joyner-Halenda (BJH) desorption cumulative surface area and volume of pores between 1.7 nm and 300 nm were 23.0 m2·g−1 and 0.019 cm3·g−1 (corresponding to 8.75 vol % of pores) respectively.

AFM investigations showed that the spheres were made up of fairly uniform secondary particles (FIG. 3), although larger than individual POMs, which are primary ones in this sol-gel process, analogues of what in metal-organic sol-gel is called Micelles Templated by Self-Assembly of ligands (MTSALs). This implied, at the first step, aggregation of the Keggin POMs in the tens of nm size, which then formed a tertiary aggregate in the μm range. The secondary particles, as shown from the XRD data, should be originally formed as crystallites of the H3PW12O40·21H2O phase. Their growth, however, was apparently impeded by adsorption of highly charged cations on their surface, which drastically decreased solubility of the material and permits aggregation of these secondary particles.

The hierarchical composition of the spheres was also observed under TEM, where tertiary spheres of approximately 2 μm across (FIG. 4A) were made up of secondary spheres in the tens of nm in diameter (FIGS. 4B-4C), that were presumably composed of individual units of hydrated phosphotungstic acid.

Similar structures, made of potassium phosphotungstate, have been reported by Yan et al., 2018a and Yan et al., 2018b, although these were produced by simple co-precipitation through dropwise addition of KCl solution to a POM solution. The latter were significantly smaller, and singular in composition and structure rather than the hierarchical structure observed in this Example. It can be speculated that they were formed by the action of analogous driving forces with their ground in charge compensation of the POM structural units. The cation content and the solubility, crucial for the application as stable electrocatalysts, were however, drastically lower in the new material reported here.

The thermal stability of the produced material was investigated via both TGA (FIG. 7) and by preparative experiments. The weight loss occurred in several steps in the interval 120° C. to 500° C., being associated initially with loss of the different forms of water content. Elemental analysis of the residues showed that the ratio of W to P did not vary significantly in the samples taken at different temperatures (σ=0.78), and thus no loss of phosphorus content could be observed.

For DLS and zeta potential, three measurements were taken using distilled water as a medium, the mean and standard deviation shown in Table 2. As the particles were fairly large, the standard deviation of the zeta potential was near 5 in all cases.

TABLE 2 DLS intensity and zeta potential mean and standard deviation Mean Intensity (nm) Mean Zeta potential (mV) 741.7 ± 324.6 −52.7 ± 5.58

Single Crystal X-Ray Structure of the Isolated Intermediate

Crystals isolated from synthesis using La(III) nitrate were triclinic centrosymmetric with a P1 space group, containing two phosphotungstate anions, one lanthanum ion and 31 water molecules in an asymmetric unit,

Z=2 (FIG. 5). The composition of the material can, thus, be formulated as [La(H2O)9](H3O)3[PW12O40]2(H2O)19. The large amount of water formed an extensive hydrogen bonding network throughout the crystal. The shortest contacts lied between water at 2.16-2.7 Å. At a bond length of 2.7-2.8 Å, contacts existed between both water molecules and water to bridging POM oxygen. Between 2.8-2.9 Å there were a number of bonds mainly between water, and between water and terminal POM oxygen. In this range there were also contacts between water and bridging POM oxygen, as well as POM-POM contacts. The longest contacts above 2.9 Å, lied between water, POM oxygen and water, or two adjacent POMs.

The extensive contacts between POMs suggested they were protonated at this pH (FIG. 8), allowing for hydrogen bonding between the POMs. Though the protons were not visible in the structure, the bond distances were consistent with H-bonds. The water molecules participated mostly in 4 hydrogen bonds per molecule, which is typical for the structure of liquid water. We could observe two POMs per La ion, necessitating the need for other cations (e.g., protons) to contribute to charge neutralization. This structure can be seen as a “snapshot” of an intermediate in the process by which the spheres form, the next step being accumulation of a larger number of POM units and transfer of the cations to the surface of the hydrated phosphotungstic acid crystallites.

TABLE 3 Details of data collection and refinement for phosphotungstate - lanthanum structure Chemical formula H65LaO113.50P2W24 Formula weight 6494.77 g/mol Temperature 163(2) K Wavelength 0.71073 Å Crystal size 0.160 × 0.200 × 0.320 mm Crystal system triclinic Space group P1 Unit cell dimensions a = 14.008(3) Å α = 88.978(5)° b = 15.140(3) Å β = 89.228(4)° c = 22.386(5) Å γ = 80.885(5)° Volume 4686.7(18) Å3 Z 2 Theta range for data collection 2.26 to 28.00° Index ranges −18 ≤ h ≤ 18, −19 ≤ k ≤ 19, −29 ≤ l ≤ 29 Reflections collected 57188 Independent reflections 22324 [R(int) = 0.0652] Nr. of observed independent 17582 reflections; I > 2σ(I) Final R indices, observed R1 = 0.0596, wR2 = 0.1531 Final R-indices, all data R1 = 0.0787, wR2 = 0.1630

Electrochemistry

Phosphotungstic acid has been used as an electrocatalyst at different pH from pH=0 to pH=7 for the oxygen evolution reaction (OER) in water splitting. The catalyst is highly dependent on the pH of the electrolyte and LSV measurements give the lowest overpotential at acidic conditions, see FIG. 6. At pH 0 (0.5 M H2SO4), the 5 overpotential was as low as 286 mV for formation of O2 (g) bubbles (OER). At pH 3 (citric acid/sodium citrate buffer) the overpotential was 308 mV and at pH 7 (phosphate buffer) the catalyst showed an overpotential of 397 mV. The formation of oxygen bubbles becomes more and more pronounced with increasing potential. The determined Faradaic efficiency was quite high, exceeding 90% and slightly increasing in time (see Tables 4-6).

TABLE 4 Chronopotentiometry at constant current of 10 mA for 30 min, 60 min, 90 and 120 min Time (min) 30 60 90 120 Total gas measured (mL) 5.7 11.7 17.7 24.0 H2 (mL) calculated 3.8 7.8 11.8 16.0 O2 (mL) calculated 1.9 3.9 5.9 8.0 O2 (mg) calculated from measured volume 2.7 5.6 8.42 11.42 (mexpt)

Faraday's law of electrolysis,

m = Q × M F × z

wherein m=mass of oxygen in gram, Q=total charge passed through in Coulomb, F=96500 C/mol, M=molar mass of oxygen in g/mol and z=number of electrons transferred. Applying the Faraday's law, theoretical mass of O2 produced could be calculated.

TABLE 5 Mass of oxygen generated Time (min) 30 60 90 120 O2 (mg) (mcal) 2.98 5.96 8.94 11.92

TABLE 6 Faradic efficiency: mexpt/mcal × 100% Time (min) 30 60 90 120 Faradic efficiency (%) 91 93.5 94.2 95.8

From the Cyclic voltammograms obtained at pH=7, a redox peak was observed at 0.72 V, confirming the W6+/W5+ redox state, while at pH=0, two redox peaks were observed at 0.72 V and 0.50 V, confirming both W6+/W5+ and W5+/W4+ redox states, respectively. The shift in redox peak confirmed the influence of phosphorus to the redox potential of tungsten, as phosphorus incorporation enhanced the acidity.

The Tafel slopes calculated from the LSVs measurements, showed the efficiency of an electrode to produce current in response to change in applied potential. The Tafel slope increases with pH in the tested range: 83 mV/dec at pH 0.86 mV/dec at pH 3 and 96 mV/dec at pH 7. Chronoamperometry was performed to evaluate the stability of the catalyst over time. The current density was observed during 24 hours and found to be stable at an applied potential of 0.4 V (vs RHE) (FIGS. 9-12, Table 4).

Exploiting the factors leading to a decrease in the potential surface charge of phosphotungstic acid nanocrystals allowed development of nanostructured microspheres of this material by sol-gel synthesis. Determination of the crystal structure of the potential intermediate product—an acidic salt of La(III) cations, provided additional clues to how the self-assembly process occurs. Sol-gel produced water-insoluble phosphotungstic acid was demonstrated as a candidate for an electrocatalyst for the OER process at acidic conditions showing a very low overpotential in 0.5 M H2SO4 (pH=0) electrolyte with great stability.

Materials and Methods

All chemicals were purchased from Sigma Aldrich and used without further purification. The particles were produced by the general procedure of using 30 mL of a solution containing 1 mM of the cation Ti(IV) from potassium titanium oxide oxalate hydrate, or La(III) from lanthanum nitrate with pH near 0 (<0.1). To this was added 1 mM of phosphotungstic acid, upon which a precipitate formed. The precipitate was filtered either immediately or after the solution was evaporated to a near volume of 10 mL in a water bath held above 90° C. The precipitate was washed with MILLI-Q® water, and collected by centrifugation. Crystals of La and phosphotungstic acid were prepared by dissolving 0.4 g lanthanum nitrate in 30 mL 2 M HCl and heating the solution in a >95° C. water bath. To this was added 3 g phosphotungstic acid and the solution was evaporated to approximately 12 mL, without stirring, at which point large cube-shaped crystals formed. Upon cooling small, X-ray quality crystals formed. The crystals were stable under the mother liquor, but upon drying degraded into a white powder.

SEM-EDS samples were immobilized on carbon tape and characterized using a Hitachi FlexSEM-1000 II. EDS spectra were analyzed using Oxford Instruments EDS analysis system operated by Aztec software.

For Transmission Electron Microscopy (TEM) observations, dispersions of the sol-gel were deposited on holey carbon grids (Pelco® 50 mesh grids: pitch 508 μm; hole width 425 μm; bar width 83 μm; transmission 70%) and observed using a Philips CM/12 microscope (ThermoFisher Inc.) fitted with LaB6 gun and operated at 100 kV. Negative TEM films were scanned using an Epson Perfection Pro 750 film scanner.

BET specific surface area and pore volume were determined from nitrogen adsorption/desorption isotherms at −196° C. (Micromeritics ASAP 2020 Surface Area and Porosity analyser). The samples were degassed at 120° C. for 12 hours before measurements.

DLS and zeta potential was done by suspending spheres in distilled water and analyzing them on a Malvern Panalytical Zetasizer Nano, equipped with a red (362.8 nm) laser. Data was processed using Zetasizer Ver. 7.12 software.

For AFM, samples were characterized using a Bruker Dimension FastScan Atomic Force Microscope with a Nanoscope V controller in ScanAsyst mode using a Fastscan-B AFM probe (silicon tip, f0: 400 kHz, k: 4 N/m, tip radius: 5 nm nominally) and a scan rate 1-3 Hz. Data was processed using Bruker Nanoscope Analysis.

Preparative thermogravimetric analysis was done using a Nobertherm LE/6/11/P300 furnace and a FA2204B electronic balance. Approximately 160 mg of PW spheres were placed in two crucibles and heated to 120, 170, 220, 270, 320, 400 and 500° C. At each point, the weight of one crucible was recorded and from the other a sample taken for EDS analysis.

Single crystal X-ray diffraction data was collected on a Bruker D8 QUEST ECO instrument and processed using Apex4 software. A total of 2424 frames were collected. The total exposure time was 2.02 hours. The frames were integrated with the Bruker SAINT software package using a narrow-frame algorithm. The integration of the data using a triclinic unit cell yielded a total of 57188 reflections to a maximum θ angle of 28.00° (0.76 Å resolution), of which 22324 were independent (average redundancy 2.562, completeness=98.7%, Rint=6.52%, Rsig=8.17%) and 17582 (78.76%) were greater than 2σ(F2). The final cell constants of a=14.008(3) Å, b=15.140(3) Å, c=22.386(5) Å, α=88.978(5)°, β=89.228(4)°, γ=80.885(5)°, volume=4686.7(18) Å3, are based upon the refinement of the XYZ-centroids of 9879 reflections above 20 σ(l) with 4.512<2θ<71.58. Data were corrected for absorption effects using the Multi-Scan method (SADABS). The ratio of minimum to maximum apparent transmission was 0.307. The calculated minimum and maximum transmission coefficients (based on crystal size) were 0.0370 and 0.0870. The structure was solved and refined using the Bruker SHELXTL Software Package, using the space group P1, with Z=2 for the formula unit, H65LaO113.50P2W24. The structure loses water extremely easily, which likely creates multiple defects reflected in low precision in determination of the electron density, in spite of using low temperature data collection. This generated B-alert for large residual electron density. The B-alerts for isolated oxygen atoms are actually misleading, because these atoms are actually water molecules invoked into a network of hydrogen bonding. The location of hydrogen atoms was impossible to discern because of the challenges in obtaining the correct electron density map. The final anisotropic full-matrix least-squares refinement on F2 with 1268 variables converged at R1=5.96%, for the observed data and wR2=16.30% for all data. The goodness-of-fit was 0.990. The largest peak in the final difference electron density synthesis was 9.579 e−/Å3 and the largest hole was −6.835 e−/Å3 with an RMS deviation of 0.749 e−/Å3. On the basis of the final model, the calculated density was 4.588 g/cm3 and F(000), 5631 e−.

All electrochemical experiments were performed at room temperature. The experiments were performed in a three-electrode system using an SP-50 potentiostat (Biologic). The phosphotungstic material was tested as an electrocatalyst at different pH from pH=0 to pH=7 for the oxygen evolution reaction (OER) in water splitting. The catalyst was then mixed with carbon black to enhance the conductivity and subsequently deposited on graphite felt (loading=0.2 mg/cm2). A three-electrode setup was used to perform the electrocatalysis experiments; a catalyst loaded graphite working electrode, Pt-mesh as counter electrode and Ag/AgCl as reference electrode. Cyclic voltammetry (CV), linear sweep voltammetry (LSV) and chronoamperometry (CA) were performed. OER tests were carried out in a single-compartment electrolytic cell with different electrolytes of 0.5 M H2SO4 (pH=0), citric acid/sodium citrate buffer (pH=3) and phosphate buffer (pH=7). For cyclability, 200 cycles of CVs were performed and the working electrode saturated after 20-30 cycles of activation. The iR drop was directly compensated by the potentiostat (with 82% compensation). The potentials recorded were finally calibrated in relation to the reversible hydrogen electrode (ERHE) by using the equation ERHE=EAg/AgCl+0.059×pH. To minimize the capacitive current, the scan rate for the LSV curve was 10 mV/s. The overpotential (n) of HER was calculated by using the equation: η=ERHE−1.23, after reduction of redox potential of oxygen, EO2/O2−=1.23. The Tafel plots were obtained by transforming the LSV curve into log (j) versus E. All experiments were performed twice to check reproducibility. Faradaic efficiency was evaluated via control of the gas evolution.

The embodiments described above are to be understood as a few illustrative examples of the present invention. It will be understood by those skilled in the art that various modifications, combinations and changes may be made to the embodiments without departing from the scope of the present invention. In particular, different part solutions in the different embodiments can be combined in other configurations, where technically possible.

REFERENCES

    • Greijer, B.; De Donder, T.; Nestor, G.; Eriksson, J. E.; Seisenbaeva, G. A.; Kessler, V. G. Complexes of Keggin POMs [PM12O40]3− (M=Mo, W) with GlyGlyGly and GlyGlyGlyGly Oligopeptides. European Journal of Inorganic Chemistry 2021, 2021(1): 54-61
    • Yan, N.; Zhang, W.; Cui, H.; Feng, X.; Liu, Y.; Shi, J. Potassium Phosphotungstate Spheres as an Anode Material for a Solar Rechargeable Battery. Sustainable Energy Fuels 2018a, 2 (2): 353-356
    • Yan, N.; Zhang, W.; Shi, J.; Liu, Y.; Cui, H. Nano Potassium Phosphotungstate Spheres/Sulfur Composites as Cathode for Li—S Batteries. Materials Letters 2018b, 229:198-201

Claims

1. A method of producing polyoxometalate (POM) particles, the method comprising producing the POM particles by subjecting a heteropoly acid with the chemical formula HzXY12O40, or a hydrate thereof, to acidic conditions in the presence of a polyvalent cation, wherein z=3 or 4, X is selected from the group consisting of P, Si, Ge, As, Sb and V, and Y is selected from the group consisting of W, Mo and V.

2. The method according to claim 1, wherein producing the POM particles comprises heating the heteropoly acid, or the hydrate thereof, while exposed to the acidic conditions in the presence of the polyvalent cation.

3. The method according to claim 1, wherein producing the POM particles comprises producing the POM particles in a sol-gel process by subjecting the heteropoly acid, or the hydrate thereof, to the acidic conditions in the presence of the polyvalent cation causing individual POMs to aggregate to form nanoparticles, which assemble into ternary particles.

4. The method according to claim 1, wherein the polyvalent cation is selected from the group consisting of Ti(IV), Zr(IV), Ce(IV), La(III), Ce(III), Pr(III), Nd(III), Sm(III), Eu(III), Gd(III), Tb(III), Dy(III), Ho(III), Er(III), Tm(III), Yb(III), Lu(III), Sc(III) and Y(III).

5. The method according to claim 4, wherein the polyvalent cation is selected from the group consisting of Ti(IV) and La(III).

6. The method according to claim 1, wherein a molar ratio of the heteropoly acid, or the hydrate thereof, and the polyvalent cation is selected within an interval of 1:0.25 to 1:2.

7. The method according to claim 1, wherein producing the POM particles comprises mixing the heteropoly acid, or the hydrate thereof, and an acidic solution comprising the polyvalent cation.

8. The method according to claim 1, wherein the heteropoly acid is phosphotungstic acid H3PW12O40.

9. The method according to claim 1, wherein the subjecting the heteropoly acid, or the hydrate thereof, to acidic conditions comprises subjecting the heteropoly acid, or the hydrate thereof to the polyvalent cation at a pH equal to or below 0.5.

10. The method according to claim 1, further comprising isolating the POM particles by filtration.

11. The method according to claim 1, wherein the POM particles are nanostructured microparticles.

12. The method according to claim 11, wherein POM nanoparticles have average diameter selected within an interval of from 5 nm up to 50 nm.

13. The method according to claim 1, wherein the POM particles have an average diameter of at least 0.5 μm.

14. Polyoxometallate (POM) particles comprising a heteropoly acid with the chemical formula HzXY12O40, or a hydrate thereof, and a polyvalent cation, wherein z=3 or 4, X is selected from the group consisting of P, Si, Ge, As, Sb and V, and Y is selected from the group consisting of W, Mo and V.

15. The POM particles according to claim 14, wherein the polyvalent cation is selected from the group consisting of Ti(IV), Zr(IV), Ce(IV), La(III), Ce(III), Pr(III), Nd(III), Sm(III), Eu(III), Gd(III), Tb(III), Dy(III), Ho(III), Er(III), Tm(III), Yb(III), Lu(III), Sc(III) and Y(III).

16. The POM particles according to claim 15, wherein the polyvalent cation is selected from the group consisting of Ti(IV) and La(III).

17. The POM particles according to claim 14, wherein the heteropoly acid is phosphotungstic acid H3PW12O40.

18. The POM particles according to claim 14, wherein the POM particles are nanostructured microparticles.

19. The POM particles according to claim 18, wherein POM nanoparticles have average diameter selected within an interval of from 5 nm up to 50 nm.

20. The POM particles according to claim 14, wherein the POM particles have an average diameter of at least 0.5 μm.

21. The POM particles according to claim 14, wherein the POM particles are obtainable by a method comprising producing the POM particles by subjecting a heteropoly acid with the chemical formula HzXY12O40, or a hydrate thereof, to acidic conditions in the presence of a polyvalent cation, wherein z=3 or 4, X is selected from the group consisting of P, Si, Ge, As, Sb and V, and Y is selected from the group consisting of W, Mo and V.

22. An electrocatalytic water-splitting device comprising:

a working electrode comprising POM particles according to claim 14;
a counter electrode; and
a reference electrode.

23. A method of producing hydrogen gas comprising producing hydrogen gas by photocatalytic or electrocatalytic water composition in the presence of a photocatalytic or electrocatalytic catalyst comprising POM particles according to claim 14.

Patent History
Publication number: 20260015744
Type: Application
Filed: Jul 12, 2024
Publication Date: Jan 15, 2026
Inventors: Vadim KESSLER (Uppsala), Gulaim SEISENBAEVA (Uppsala)
Application Number: 18/770,822
Classifications
International Classification: C25B 11/085 (20210101); C25B 1/04 (20210101);