RU2352391C1 - Method of obtaining platinum nitrate and based on them platinum catalysts - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: concentrated water solutions of compounds Pt(IV) do not contain halogenide-ions, sulphur compounds, as well as cations of alkaline metals and organic bases. Such solutions can be used as precursors for preparation of highly dispersed platinum catalysts, intended, for instance for process of reforming and different process of complete oxidation, actual, first of all, for purification of discharged gases from industrial stationary sources and automobile engines. According to claimed method stable concentrated solutions of platinum (IV) nitrates are prepared by dissolving hexahydroxoplatinum acid at temperature 50-80°C under impact of nitrogen dioxide in water or under impact of nitrogen dioxide in water solution of nitric acid. Described is use of obtained solutions as precursors for preparation of highly dispersive and ultra dispersive platinum catalysts.

EFFECT: obtaining stable platinum nitrates.

3 cl, 10 ex, 9 dwg, 1 tbl

Description

The invention relates to the production of concentrated aqueous solutions of Pt (IV) compounds not containing halide ions, sulfur compounds, as well as alkali metal cations and organic bases. Such solutions can be used as precursors for the preparation of highly dispersed platinum catalysts intended, for example, for reforming processes and various processes of complete oxidation, relevant, first of all, for the purification of exhaust gases from industrial stationary sources and automobile engines.

Along with other predecessors of platinum catalysts, platinum nitrate solutions attract particular attention and are widely used in the manufacture of catalysts to neutralize gas emissions from industrial stationary sources and vehicles. In US Pat. JP 62197148 A2, B01J 23/63, B01D 53/94, 12.15.87 describes the preparation of a catalyst for the simultaneous removal of hydrocarbons, CO and nitrogen oxides from the exhaust gases of gasoline engines by impregnating the catalytic layer of a catalyst block containing aluminum, cerium and lanthanum oxides with a solution platinum nitrate. In addition, the results of tests of diesel exhaust gas afterburning catalysts prepared using a platinum nitrate solution as a precursor are published, where the authors compare the dispersion of platinum and the efficiency of the prepared catalyst with catalysts based on alternative precursors such as H ₂ PtCl ₆ and [Pt (NH ₃ ) ₄ ] (OH) ₂ [JAA van den Tillaart, J. Leyrer, S. Eckhoff, ES Lox, Appl. Catal. In 10 (1-3) (1996) 53-68].

The first preparation of crystalline platinum nitrate by dissolving H ₂ [Pt (OH) ₆ ] in a large excess of concentrated nitric acid in a water bath followed by evaporation of the solvent was described as early as the 19th century [Nilson LF, Ber., 11 (1878) 882; J. Prakt. Chem., V. 21 {2} (1880) 185]. The resulting product containing 69.4% Pt and 2.5% N was assigned the formula Pt (OH) ₂ (NO ₃ ) ₂ · 3PtO ₂ · 5H ₂ O.

In the patent literature, the first way to synthesize platinum nitrate solutions by dissolving pre-isolated and purified dry Pt (NO ₃ ) ₂ (61.0 wt.% Pt) in water or organic solvents such as acetone and the preparation of catalysts from this precursor was described by Kral and Peter [DE 2233677, B01J 23/42, C01G 55/00, 01.24.74]. Solid platinum nitrate, in turn, was obtained by dissolving H ₂ Pt (OH) ₆ in concentrated nitric acid saturated with nitrogen oxides, followed by evaporation of the excess nitric acid in a water bath at 80 ° C, then drying in a vacuum on a rotary evaporator at a temperature not above 40 ° C to constant weight and purification by recrystallization from organic solvents. Neither the concentration of nitric acid required to dissolve H ₂ Pt (OH) ₆ , nor the maximum possible concentration of platinum in the solutions prepared in this way, nor their stability is specified in the patent. In addition, the authors do not discuss the well-known problem of aging of solid platinum nitrate, which eventually ceases to dissolve in any solvents.

The closest, selected as a prototype, is a direct way to obtain stable solutions of Pt (IV) nitrate by dissolving H ₂ Pt (OH) ₆ with slight heating in concentrated (300 g / l) nitric acid in the absence of nitrogen oxides with a minimum ratio of HNO ₃ : Pt = 8: 1, ensuring the stability of the solution during storage [JP 11092150 A2, B01J 23/42, C01G 55/00, 04/06/1999]. From the indicated acid concentration, it is easy to calculate that the solutions used have a density of 1.155 g / ml and contain 4.81 M HNO ₃ , i.e. in order to achieve an 8-fold excess relative to the stoichiometric ratio per 1 g of platinum (0.0051 mol) contained in 1.54 g of H ₂ Pt (OH) ₆ , it is necessary to add 8 · 0.0051 = 0.041 mol of HNO ₃ contained in 8.52 ml of the desired 4.81 M solution. Thus, the final concentration of the platinum nitrate solution prepared in this way is only 8.8 wt.% Pt, which leads to the need to store large volumes of the precursor solution.

It is known that protonation of the four remaining OH ligands in H ₂ Pt (OH) ₆ with its subsequent dissolution is possible, however, this requires achieving a negative pH [B.I. Nabivanets, L.V. Kalabina, L.N. Kudritskaya. The solubility of hydroxides and the ionic state of palladium (II) and platinum (IV) in perchlorate, chloride and sulfate solutions. ZhNKh, 1971, v.16, issue 12, s.3281-3284]. Upon dissolution of H ₂ Pt (OH) ₆ in concentrated nitric acid, the solution immediately turns dark red, which indicates changes in the coordination sphere of platinum, namely, the formation of oligomeric complexes linked by bridging hydroxyl ligands [Danan Dou; Di-Jia Liu, W. Burton Williamson, Karl C. Kharas, Heinz J. Robota. Structure and chemical properties of Pt nitrate and application in three-way automotive emission catalysts. Applied Catalysis B: Environmental, 30 (2001) 11-24]. The size of these oligomers, however, depends on the concentration of the solutions, pH, and other uncertain factors. Commercial aqueous solutions of dark red platinum nitrate usually contain 14-35 wt.% Pt and a large excess of nitric acid. The manifestation of the Tyndall effect indicates the colloidal nature of these solutions.

Diluted solutions of commercial Pt platinum nitrate with platinum concentrations corresponding to typical impregnating solutions are hydrolyzed to form an orange gel-like precipitate of PtO ₂ · xH ₂ O [Danan Dou; Di-Jia Liu, W. Burton Williamson, Karl C. Kharas, Heinz J. Robota. Structure and chemical properties of Pt nitrate and application in three-way automotive emission catalysts. Applied Catalysis B: Environmental, 30 (2001) 11-24]. Depending on the details of the preparation procedure, which the manufacturers carefully hide, the stratification visible by the eye, followed by precipitation, begins after a period of one hour to one day. The formation of a PtO ₂ · xH ₂ O precipitate appears to be due to further oligomerization or agglomeration of platinum nitrate solutions caused by a decrease in the acidity of the solutions after dilution. Diluted solutions of platinum nitrate become even more unstable when acidity decreases even more after contact with basic alumina, which is the basic carrier in the preparation of the supported catalytic layer for various exhaust gas afterburners. In addition, carrier dissolution occurs with an excess of nitric acid. The addition of traditional promoters — Ba ²⁺ , La ³⁺ , Ce ³⁺ salts — to the impregnation suspension further accelerates the deposition process. This instability of dilute solutions of platinum nitrate calls into question the suitability of platinum as a precursor, since it affects the dispersion and effectiveness of the prepared catalysts and introduces an element of irreproducibility into production [Danan Dou; Di-Jia Liu, W. Burton Williamson, Karl C. Kharas, Heinz J. Robota. Structure and chemical properties of Pt nitrate and application in three-way automotive emission catalysts. Applied Catalysis B: Environmental 30 (2001) 11-24].

Thus, the disadvantages of the known method of preparing a solution of platinum nitrate [JP 11092150] are:

1) insufficiently high concentration of the solution (∼9% Pt), which makes its transportation and storage unprofitable;

2) the instability of these solutions when diluted to concentrations of 10 ^-2 -10 ^-3 M used in the preparation of the catalysts, associated with the dynamic enlargement of colloidal particles of the platinum precursor, amplified upon contact with the carrier, which makes them unsuitable for the preparation of highly dispersed catalysts.

The invention solves the problem of obtaining stable concentrated solutions of platinum (IV) nitrates.

The problem is solved by the method of obtaining concentrated solutions of platinum (IV) nitrates by dissolving hexahydroxoplatinic acid at a temperature of 50-80 ° C under the action of nitrogen dioxide in water or under the action of nitrogen dioxide in an aqueous solution of nitric acid.

The dissolution of hexahydroxoplatinic acid is carried out in an aqueous solution that does not contain or contains no more than 3 mol of HNO ₃ per mole of platinum, while the resulting solutions are stable not only in a concentrated, but also in a diluted state, and when diluted, the size of colloidal particles decreases.

The resulting solutions are used as precursors for the preparation of highly dispersed and ultrafine platinum catalysts.

As starting material for the preparation of platinum nitrate solutions, hexahydroxoplatinic acid can be used, obtained through an intermediate solution of alkali metal hexahydroxoplateate by boiling H ₂ PtCl ₆ · 6H ₂ O with excess alkali for 5-6 hours, followed by precipitation of the product with acetic acid at pH 5 in the form white sediment [B.I. Nabivanets, L.V. Kalabina, L.N. Kudritskaya. The solubility of hydroxides and the ionic state of palladium (II) and platinum (IV) in perchlorate, chloride and sulfate solutions. ZhNKh, 1971, v.16, issue 12, s.3281-3284]. Freshly precipitated H ₂ Pt (OH) _{6 is} separated by centrifugation, washed thoroughly from chlorine ions and alkali metal cations and dried to an air-dry state in air, and then in vacuum at a temperature not exceeding 50 ° C.

A small stream of nitrogen dioxide is passed through an aqueous suspension of hexahydroxoplatinic acid with constant stirring at a temperature of 50-80 ° C and atmospheric pressure until the acid is completely dissolved. Purge stops only after the initial reddish color of the solution changes to dark green. For some enlargement of oligomeric complexes of platinum in the target solution, nitric acid with a concentration of 850 g / l in an amount not exceeding 3 mol per 1 mol of platinum can be added to the initial aqueous suspension of Н ₂ Pt (ОН) ₆ (the amount of water can be correspondingly reduced). Complete dissolution of the acid occurs, usually within half an hour or even faster. After cooling, the solution should be aged 1-2 days until it acquires a final color from yellow to orange, depending on the ratio used.

Pt: HNO ₃ . After that, the state of the target solution of platinum nitrate does not change even after storage for 1 year, and diluted solutions with concentrations of about

10 ^-2 -10 ^-3 M, suitable for impregnation of the carrier during the preparation of supported catalysts, are stable for a month.

A significant advantage of the new platinum-containing precursor is to reduce the size of colloidal particles in the solution when it is diluted, which makes it possible to reproducibly prepare highly dispersed and ultrafine platinum catalysts on traditional oxide supports, for example, on aluminum oxide, and a unimodal size distribution of platinum particles with a uniform and uniform distribution is achieved on the surface of the carrier.

Examples of the preparation of solutions (examples 1-3) and catalysts based on them (examples 6-8) are given below. For comparison, prepared solutions of the precursors by simplified methods [DE 2233677 and JP 11092150], similar in properties to the commercial, and catalysts based on them. The average size of colloidal particles in dilute precursor solutions (0.01 M) 15 minutes after dilution, the dynamics of diluted solutions (small angle X-ray scattering data) and the average particle size of platinum on the surface of alumina in the catalysts prepared from these solutions (transmission electron microscopy) are given in Table.

1-9 illustrate the Table.

Figure 1, 2 and 3 shows a unimodal size distribution with a maximum at ~ 1 nm of colloidal particles of platinum nitrate solutions prepared according to examples 1, 2 and 3, respectively, as a result of dilution with water to a concentration

10 ^-2 M calculated from the processing of data from the MPD.

Figures 4 and 5 illustrate the opposite behavior of platinum nitrate solutions prepared according to examples 4 and 5, respectively, as a result of dilution with water to a similar concentration, with the appearance along with small large particles of about 24 nm in size, the proportion of which increases with time. Micrographs of highly dispersed catalysts of 1 wt.% Pt / γ-Al ₂ O ₃ prepared according to examples 6 and 8, with a uniform distribution of platinum microcrystallites with an average particle size of 0.6 and 1.7 nm are presented in Fig.6 and 7, respectively. The inhomogeneous distribution of platinum particles on the surface of the carrier - micrometer aggregates along with very small particles, typical for samples of catalysts 1 wt.% Pt / γ-Al ₂ O ₃ prepared according to examples 9 and 10 from solutions of precursors with a high content of HNO ₃ shown in Fig and 9, respectively.

Example 1

Through a pale yellow suspension of 1.37 g of H ₂ [Pt (OH) ₆ ] (0.046 mol) in 2.5-3.0 ml of water at 50-70 ° C, a weak stream of nitrogen dioxide is passed with constant stirring, while it is completely absorbed, and from a solution of a little colorless nitric oxide. After complete dissolution of the precipitate, the solution turns reddish, but the resulting solution is not stable enough, so the gas flow is not stopped until the color of the solution turns green. The synthesis time is about 30 minutes. During the day, the color of the solution changes to yellow-orange due to the oxidation of nitrosyl ligands in the platinum complex; Further changes to the solution do not occur. The resulting yellow-orange solution contains 18-20 wt.% Pt (~ 1.3 M), it is stable and can be used to prepare catalysts for at least a year. Diluted solutions with a platinum content of 10 ^-2 -10 ^-3 M pale yellow are stable for a month.

Example 2

To a suspension of 1.37 g of H ₂ [Pt (OH) ₆ ] (0.046 mol) in 2.0-2.5 ml of water, 0.46 ml (0.38 g) of concentrated nitric acid (d 1.37 g / ml, HNO ₃ ratio: Pt = 1: 1). A weak current of nitrogen dioxide is passed through the resulting yellow-orange suspension at 50-70 ° C with constant stirring, while it is completely absorbed, and a little colorless nitric oxide is released from the solution. After complete dissolution of the precipitate, the solution turns reddish, but the gas flow is not stopped until the color of the solution turns into a greenish-brown. The synthesis time is about 20 minutes. During the day, the color of the solution changes to orange. The resulting orange solution contains 18-20 wt.% Pt, it is stable and can be used for preparing catalysts for at least a year. Diluted solutions with a platinum content of 10 ^-2 -10 ^-3 M pale orange are stable for a month.

Example 3

To a suspension of 1.37 g of H ₂ [Pt (OH) ₆ ] (0.046 mol) in 1.12-1.72 ml of water, 1.38 ml (1.15 g) of concentrated nitric acid (d 1.37 g / ml, HNO ₃ ratio: Pt = 3: 1). A weak stream of nitrogen dioxide is passed through the resulting orange suspension at 50-70 ° C with constant stirring, while it is completely absorbed, and a little colorless nitrogen monoxide is released from the solution. After complete dissolution of the precipitate, the solution turns red, but the gas flow does not stop until the color of the solution turns brown. The synthesis time is about 10 minutes. During the day, the color of the solution changes to orange-red. The resulting orange-red solution contains 18-20 wt.% Pt, it is stable and can be used for preparing catalysts for at least a year. Diluted solutions with a platinum content of 10 ^-2 -10 ^-3 M bright orange are stable for a month.

Example 4 (for comparison).

Through a red suspension 1.37 g of H ₂ [Pt (OH) ₆ ] (0.046 mol) in 2.5-3.0 ml of concentrated nitric acid (d 1.37 g / ml, ratio of HNO ₃ : Pt = 5-6: 1) at 50-70 ° C pass a weak stream of nitrogen dioxide with constant stirring. After complete dissolution of the precipitate, the gas flow does not stop for several minutes. The synthesis time is about 10 minutes. During the day, there is some change in the color of the solution. The resulting dark red solution contains 18-20 wt.% Pt, it is stable and can be used to prepare catalysts for at least a year, however, dilute solutions with a platinum content of 10 ^-2 -10 ^-3 M bright red are stable only a few days, after which the precipitation of orange hydrated platinum oxide occurs.

Example 5 (for comparison).

A red suspension of 1.37 g of H ₂ [Pt (OH) ₆ ] (0.046 mol) in 3.6 ml of concentrated nitric acid (d 1.37 g / ml, the ratio of HNO ₃ : Pt = 8: 1, as in the prototype JP 11092150) is stirred at 70 ° C in the absence of nitrogen oxides. Complete dissolution of the precipitate requires several hours; the duration of the synthesis depends on the background of the precursor H ₂ [Pt (OH) ₆ ]. During the day, there is some change in the color of the solution. The resulting dark red solution contains ~ 20 wt.% Pt, it is stable in a concentrated state and can be used to prepare catalysts, but diluted solutions with a platinum content of 10 ^-2 -10 ^-3 M dark red are unstable - they become cloudy after a few minutes, and within an hour, the precipitation of orange hydrated platinum oxide begins.

Example 6. Preparation of a supported catalyst of 1% Pt / γ-Al ₂ O ₃ from a precursor solution prepared according to example 1.

The γ-Al ₂ O ₃ support (Sosol TKA-432, Germany) is preliminarily crushed, dispersed into fractions and calcined at T 500 ° C for 2 hours. To obtain the catalysts, a 0.25-0.50 mm fraction is used (Ssp. ~ 118 m ² / g by BET). The carrier (4.5 g), dehydrated immediately before synthesis at 120 ° C for 4 hours, is soaked in moisture capacity with a solution of Platinum Nitronitrate No. 1 (173 μl of a solution (0.247 g) in 5.6 ml of distilled water); dried to an air-dry state for at least 24 hours, then at 120 ° C for 4 hours and calcined in air at 400 ° C for 4 hours

Example 7. The preparation of the supported catalyst 1% Pt / γ-Al ₂ O ₃ from the precursor solution prepared according to example 2, by the method as in example 6.

Example 8. The preparation of the supported catalyst 1% Pt / γ-Al ₂ O ₃ from the precursor solution prepared according to example 3, by the method as in example 6.

Example 9. The comparison catalyst.

Preparation of supported catalyst 1% Pt / γ-Al ₂ O ₃ from a precursor solution prepared according to example 4, by the method as in example 6.

Example 10. The comparison catalyst.

Preparation of supported catalyst 1% Pt / γ-Al ₂ O ₃ from a precursor solution prepared according to example 5, according to the method as in example 6.

Table A solution of platinum nitrate (example No.) The dynamics of changes in the size of colloidal particles in solution after dilution The average diameter of colloidal particles in 0.01 M diluted solution, nm The catalyst prepared from this precursor solution diluted to 10 ^-2 -10 ^-3 M (example No.) The content of platinum, wt.% The average particle size of Pt on the surface of alumina, nm ¹ one Decrease 0.5 6 0.95 0.6 2 Decrease 0.8 7 0.89 1.3 3 Decrease 1.2 8 0.93 1.7 four Enlargement 4.2 ² 9 0.90 Neodnor. ³ 5 enlargement 5.0 ⁴ 10 0.87 Neodnor. ^{5 1} - For small particles - according to TEM, for large - according to data
² - Bimodal particle size distribution, distribution maximums 1.0 and 24 nm (SANS data)
³ - Bimodal particle distribution, small 1.1 nm and large heterogeneous X-ray amorphous aggregates up to 200 nm in size (TEM, XRD data)
⁴ - Bimodal particle size distribution, distribution maxima of 1 and 24 nm (SANS data)
⁵ - Bimodal particle distribution, small 1-1.5 nm and large heterogeneous X-ray amorphous aggregates up to 1 micron in size (TEM, XRD data)

Claims (3)

1. A method of obtaining stable concentrated solutions of platinum (IV) nitrates by dissolving hexahydroxoplatinic acid at a temperature of 50-80 ° C, characterized in that the dissolution is carried out under the influence of nitrogen dioxide in water or under the action of nitrogen dioxide in an aqueous solution of nitric acid. 2. The method according to claim 1, characterized in that the dissolution of hexahydroxoplatinic acid is carried out in an aqueous solution that does not contain or contains no more than 3 moles of HNO ₃ per mole of platinum, while stable solutions are obtained not only in a concentrated, but also in a diluted state, moreover, upon dilution, a decrease in the size of colloidal particles occurs. 3. The use of solutions obtained according to claim 1 or 2, as precursors for the preparation of highly dispersed and ultrafine platinum catalysts.

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