Technical field

The invention relates to a method for production of platinum nanofibres, platinum nanofibres and a nanofibrous structure.

Background art

From practice as well as studies it is known that chemical, eventually catalytic, acting of solid substances increases with their specific surface. Thank to this, with decreasing size of particles, thus with increasing specific surface of these substances, it is possible to achieve the necessary rate of effect even with markedly lower quantity of the active substance, which in case of rare and precious metals expressively decreases the costs for realizing of chemical processes, at which these metals are utilised.

Based on this knowledge several methods for production of submicron particles formed purely of precious metals with catalytic properties were developed, while most of them focused to production of platinum particles, which is utilised as multi-purpose catalyst, this for both organic as well as anorganic reactions.

These methods mostly use the MOCVD method, possibly the method of plasma spraying, and their result are platinum nanoparticles with characteristic dimensions above 50nm, randomly also short wires with diameter above 20nm and length to 20microns. Nevertheless these methods, thanks to a low production output and high technologic demands, further increase the price of such platinum particles and restrict their applicability. A higher productivity is achieved by grinding, nevertheless this shows lower quality of the resultant product, and thus greater dimensions of particles, thus their smaller specific surface.

Another disadvantage is, that depositing of such prepared platinum particles on a suitable carrier requires further technological step, at which moreover is not guaranteed electrical conductive connection of the whole layer, which reduces their applicability e.g. in electrocatalytic processes, because such layer does not enable transfer of electron.

The goal of the invention is to eliminate disadvantages of background art and to propose a reliable and low-cost method for production of platinum nanofibres, which besides a high specific surface ensures also electrically conductive connection of the whole layer.

Principle of the invention

The goal of the invention has been achieved by a method for production of platinum nanofibres, whose principle consists in that the liquid matrix containing a spinnable polymer material and platinum precursor is prepared, after then this matrix is spun through electrostatic spinning. At the same time the platinum precursor is seized with the polymer material and is incorporated into the structure of nanofibres being produced. In the following step, the polymer material is removed from these nanofibres and the platinum precursor is simultaneously transformed into pure platinum, which copies by its spatial arrangement the structure of primary nanofibres. Through this procedure the nanofibres having diameter within the range of cca 10 - 600nm are prepared, which are in their entire volume formed only of platinum.

In principle any organic polymer may be used as the spinnable polymer material, nevertheless polyvinylpyrrolidone proved to be to the most suitable to date.

In principle, any acid hydrate containing platinum may serve as the platinum precursor, e.g. hexahydrate of hexachloroplatinic acid. In another examples as a platinum precursor an ammonia salt of an acid containing platinum may be used, e.g. of tetrachloroplatinous acid. Usage of ammonia salt ensures, that in the resultant platinum nanofibres no contaminants neither inorganic residuals are present. Preferably, such ammonia salt is ammonium tetrachloroplatinate.

The same or similar results may be achieved also at usage of platinum acetylacetonate.

At the same time the weight ratio of the spinnable polymer material and platinum contained in the precursor lies in the range from 1 :1 to 3:1, which ensures such a structure of platinum nanofibres, that their mechanical fragmentation does not occur.

The highest quality of platinum nanofibres and simultaneously the highest production output is achieved if the matrix containing the polymer material and platinum precursor is transformed into a layer of nanofibres through electrostatic spinning in electrostatic field induced between a collecting electrode and a spinning electrode or spinning elements of a spinning electrode, into which the matrix is supplied on surface of moving spinning electrode or spinning element of the spinning electrode.

In this manner or using another method the prepared polymer nanofibres containing platinum precursor are transformed into platinum nanofibres through calcination, at which all polymer material is removed from them, and the platinum precursor is simultaneously transformed into pure platinum.

The calcination is after then running for a period of 2 to 6 hours at the temperature of 400 to 800 ⁰ C, depending on quantity of polymer material in nanofibres and type of the used platinum precursor.

The result of method according to the invention are the nanofibres having diameter between 10 to 600nm and the length of 30 to 900 microns, which in their entire volume are formed of pure platinum. At usage of common methods of spinning, the nanofibres are usually arranged in a nanofibrous structure which is more suitable for industrial application than separate nanofibres.

This nanofibrous structure is thanks to properties of platinum applicable as a supporting structure for chemical catalysis, possibly as a part of redox catalyst of electrode for fuel cell or of redox agent for a barrier-layer DSC photocell.

For some of these or any other applications it is preferred if the nanofibrous structure is created on electrically conductive, e. g. of metal, surface of the carrier.

Description of the drawing

The drawing represents in the Fig. 1 SEM picture of platinum nanofibres created according to the invention, while from the scale the scope of their dimensions is obvious, and in the Fig. 2 XDR spectrum of such produced nanofibres defining their material composition.

Examples of embodiment

Platinum nanofibres are created through electrostatic spinning of polymer matrix, in which the platinum precursor is dissolved, whereas after spinning from the produced nanofibres in one step the polymer material is removed and the platinum precursor is transformed into a pure metallic platinum, which copies by its spatial arrangement distribution of the precursor in nanofibres, and due to this it obtains the nanofibrous structure.

Due to the fact that the below described matrix for production of platinum nanofibres shows, thanks to its high content of polymer material, quite similar behaviour as the polymer matrices up to date commonly being subject to spinning, for its spinning any to date known device for electrostatic spinning of polymer matrix - solution or melt of polymer, may be used. At the same time the best results and the highest performance is achieved when the device known from WO 2005/024101 or from analogic granted patent CZ 294274 is used, possibly upon utilisation of further knowledge as regards the structure of elements of this device, e.g. according to WO2008028428, WO2006131081 , WO2008011840, etc. Comparable results, though with markedly lower specific output may also be achieved upon use of generally known device, which uses for production of nanofibres a nozzle or a system of nozzles.

The matrix for production of platinum nanofibres is prepared by dissolution of a compound containing platinum (platinum precursor), e.g. of hexahydrate of hexachloroplatinic acid in ethanol under common laboratory conditions, while into the prepared solution next to this as a spinnable component any organic polymer is added, for example polyvinylpyrrolidone (PVP). Matrix created in this manner is after homogenisation and stabilisation subjected to spinning by electrostatic spinning using some of the above mentioned devices. The result of electrostatic spinning is, depending on the specific technology, either the separate layer of polymer nanofibres containing in its structure the platinum precursor, or a layer of such nanofibres deposited on a suitable substrate. It is necessary to select the substrate material not only with respect to utilisation of a layer of platinum nanofibres, but also to a following technological step, which is performed upon an increased temperature. For some of applications of the solution according to the invention it is preferred, if the nanofibrous structure is created on electrically conductive, e.g. metallic surface of a carrier.

The layer of nanofibres, possibly also with the substrate material, is further calcinated in a bisque-firing oven under the temperature in the range of 400-800 ⁰ C, due to which the polymer material is removed from the nanofibres through oxidation (burning), and simultaneously the platinum precursor is by a series of chemical reactions transformed into a pure metallic platinum. Due to impossibility of movement of particles of platinum precursor in solid nanofibres, the platinum holds the same spatial arrangement so that the platinum nanofibres are produced. Their diameter usually in the range of 30 - 300nm, length in the place value of tens to hundreds of micrometers, and the specified specific surface achieves the values about 1 ,3 m ² /g. The layer of nanofibres forms a nanofibrous structure with a high accessible specific surface, which may serve as a supporting structure for catalysis.

In further examples of embodiment the polymer matrix for production of platinum nanofibres may be prepared using the ammonia salt of tetrachloroplatinous acid, at whose utilisation it is ensured, that the ammonium cation oxidises during calcination to nitrogen or nitrogen oxide, thus does not leave any contaminants nor anorganic residuals in the resultant platinum nanofibres. Ammonia salt is then dissolved in mixture of water and ethanol, while their ratio results from solubility of the given salt in water and simultaneously from the requirement to a minimum volume of the water used.

The following examples show in a descriptive manner the respective examples of production of several basic types of matrix for production of platinum nanofibres. These are only illustrative , not the only possible examples of preparation of this matrix. At the same time the quantity of platinum precursor is based on the ratio of in it contained pure platinum and the spinnable polyvinylpyrrolidone PVP, that preferably lies within the range from 1 :1 to 1:3.

Example 1

13,2g of hexahydrate of hexachloroplatinic acid is dissolved in 19Og of denatured, anhydrous ethanol. Into the prepared solution is further added 10g of polyvinylpyrrolidone (PVP) having molecular weight of 1.300.000 g/mol or having viscosity number K80 to K95. After a following dissolution and homogenisation of solution the matrix for electrostatic spinning is prepared. The created layer of nanofibres is after then calcinated in an oven for a period of 2- 4 hours at the temperature of 500 to 700 ⁰ C, while the temperature increase to the maximum value is preferably 50°C/h. The structure of such created nanofibres of platinum is represented in the SEM picture in the Fig. 1 , while their material composition is obvious from the XRD spectrum in the Fig. 2. Example 2

10g of ammonium tetrachloroplatinate is dissolved in 47, 5g of water. After dissolution and homogenisation this solution is mixed with solution of 142, 5g of ethanol and 10g of PVP having molecular weight of 1.300.000 g/mol or viscosity number K80 to K95. After a full dissolution and homogenisation the prepared matrix is subjected to electrostatic spinning. The prepared nanofibres are further calcinated under the same conditions as in Example 1 , while comparable results as to qualitative as well as quantitative parameters of platinum nanofibres are achieved.

Example 3

10, 5g of platinum acetylacetonate is dissolved in 19Og of ethanol and there is further added 10g of PVP having molecular weight of 1.300.000 g/mol or viscosity number K80 to K95. After a full dissolution and homogenisation the prepared matrix is subjected to electrostatic spinning. The prepared nanofibres are further calcinated under the same conditions as in Example 1, while comparable results as to qualitative as well as quantitative parameters of platinum nanofibres are achieved.

In further examples of embodiment the quantity of PVP in a solvent or in a mixture of solvents does not exceed 10% by weight, while the ratio of PVP and pure atomic platinum contained in the precursor lies in the range cca from

1 :1 to 3:1. As a solvent ethanol or mixture of ethanol and water in ratio cca 1:3 may be used.

Industrial applicability

Platinum nanofibres, or the nanofibrous structure formed of platinum nanofibres are applicable in chemical industry as catalyst for number of chemical reactions. The nanofibrous structure formed of platinum nanofibres presents a supporting structure with high accessible specific surface for catalysis. Next to this, the layer of interconnected platinum nanofibres is also available for electro-catalytic processes, where the transfer of electron is important, which this layer, to the contrary of the layer of at randomly arranged nanoparticles, enables. Example of such applicability is e.g. a layer of platinum nanofibres deposited on the ion exchange membrane of fuel cells. Another example of applicability is catalyst of DSC barrier-layer photocells, where platinum plays a role of redox agent/catalyst of an electrolyte on one of electrodes of the cell.