RU2241541C1 - Method of preparing oxide catalysts - Google Patents

Abstract

FIELD: electrochemical deposition processes.

SUBSTANCE: invention relates to electrochemically depositing catalytically active metal oxide-containing layers and can be used in heterogeneous catalysis reactions. Method comprises anodic-sparkle deposition from alkali electrolyte containing bivalent and/or polyvalent metal in amount ensuring formation of colloidal solution. Coating is formed under electroplating conditions at effective current density 0.5 to 40 A/dm² with direct or alternating current, or alternating unipolar current, or under falling-power conditions with electrode voltage up to 600 v.

EFFECT: extended of concentrations of metal oxides introduced into catalytically active layer and increased choice of metal oxides due to enabled possibility of using oxides of bi- and polyvalent metals.

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Description

The invention relates to the preparation of catalysts, and in particular to methods of electrochemical deposition on valve metals and their alloys, in particular titanium and its alloys, aluminum and its alloys, catalytically active layers, as well as catalyst supports containing metal oxides, which can be used in heterogeneous catalysis reactions.

A known method of preparing multicomponent complex catalysts on a substrate of Ti, Fe, Nb, Zr, Al, Sb, Bi by anodic oxidation during spark discharge in an electrolyte containing transition metal compounds [US Pat. GDR No. 248064, publ. 29.07.87]. The obtained catalyst layers with a large specific surface area contain a mixture of substrate oxides and transition metal oxides from an electrolyte, including up to 10% Mn oxides. The disadvantage of this method is the limited number of oxides in the resulting catalyst and their insufficiently high content.

Closest to the claimed is a method of producing oxide catalytically active layers for deep oxidation of organic compounds and carbon monoxide on a substrate made of valve metal or its alloy by anodic-spark deposition in the anode mode at a pulse frequency of 50 Hz, pulse duration of 50-300 μs , current density 10-120 A / dm ² , voltage 200-520V for 1200-2400 s in an alkaline electrolyte with the addition of ultrafine powders of metal oxides (Al, Zr) and transition metal salts selected from the group Mn, Cr, Cu, Co , Fe, or mixtures thereof [US Pat. RF №2152255, publ. 07/10/2000]. The method provides for the preparation of catalytic layers, including 3-25% of oxygen-containing compounds of transition metals from the total weight of the coating.

The disadvantage of this method is the formation of layers with a wide range of concentrations of metal oxides and, as a result, the limited ability to select the optimal content of active oxide and ensure maximum catalytic activity of the obtained oxide layers, as well as a limited number of metals whose oxides can be introduced into the catalytic coating. In addition, the known method requires the preliminary preparation of ultrafine oxide powders of certain metals, which complicates and increases the cost of obtaining oxide coatings.

The technical problem of the proposed method is to increase the range of concentrations of metal oxides introduced into the catalytically active layer, as well as to expand their range by providing the possibility of introducing into the said coating oxides of divalent and / or polyvalent metals, as well as mixtures thereof.

The solution to this problem is provided by the fact that in the method of producing catalytically active layers on a substrate made of valve metal, mainly titanium or its alloy, aluminum and its alloy, by the method of anode-spark deposition in an alkaline electrolyte, the coating is formed in the galvanostatic mode at an effective density of the anode current 0.5-40 A / dm ² at constant or alternating or alternating unipolar current or in the incident power mode with a voltage of formation on the electrodes of not more than 600 V, while The alkaline alkaline electrolyte additionally contains acetate or acetates of divalent and / or polyvalent metals, the oxides of which are part of the catalytically active oxide layer, in an amount that ensures the formation of a colloidal solution at a pH of at least 8.

The method is as follows.

The substrate is immersed in an electrolyte and the treatment is carried out for 30-1200 s in the galvanostatic mode with an effective current density of 0.5-40 A / dm ² for direct current or for alternating current, or for alternating unipolar (pulsed uni-half-wave rectified) frequency of 50 Hz current or in the incident power mode with a formation voltage on the electrodes of 100-600 V in the conditions of the formation of microplasma discharges. On direct and alternating unipolar current, the workpiece is an anode; alloys of nickel, titanium, and stainless steel can be used as a cathode. In addition, the inner surface of the bath, in which an electrolyte made of stainless steel or titanium alloys, can serve as a cathode. On alternating current, either two parts are treated simultaneously, which alternately play the role of the anode, or the inner surface of the bath in which the electrolyte is located serves as the second electrode.

The electrolyte is prepared by separately dissolving each component in distilled water at room temperature and then merging into the total volume. The concentration of the components of the basic alkaline electrolyte, which is prepared by dissolving in distilled water the appropriate amounts of a weak acid salt and a strong base (sodium tetraborate, sodium silicate, etc.), either alkali or a mixture thereof, is selected so that the pH of the working electrolyte formed after the introduction of acetates of the corresponding metals, it was at least 8. Then, a solution of metal acetate or several metal acetates, the oxides of which are part of alitically active layer.

In accordance with the claimed method, acetates of divalent and / or polyvalent metals are used, since their aqueous solution, when added to the base alkaline electrolyte, forms a colloidal solution of the hydroxide of the corresponding metal, which decomposes on the anode to form the oxide of this metal. Monovalent metal acetates do not form a colloidal solution when added to an alkaline base solution, which does not allow to achieve the claimed technical result.

To form a catalytically active layer, including a single metal oxide, the concentration of the corresponding metal acetate is selected so that when it is combined with the base electrolyte, a colloidal solution is formed. When acetates of two or more metals are used, their concentration and ratio can be varied arbitrarily, maintaining only the value of the total concentration, which should ensure the formation of a colloidal solution at a selected pH value of the corresponding base electrolyte. Thus, the method provides the ability to change the content and ratio of metal oxides in the catalytic coating in the desired range.

In electrolytes with a pH value of more than 8, aqueous solutions of acetates of divalent and polyvalent metals, starting from a certain concentration, give insoluble and / or poorly soluble hydroxides of these metals. At lower concentrations of these acetates, they form true solutions.

Colloidal particles that can form at the first moment after the acetate solution is drained with the base solution during the mixing of the electrolyte during the formation of the coating are uniformly distributed over the entire volume of the electrolyte. The temperature of the prepared electrolyte can vary widely (5-80 ° C).

As a result of processing, a porous oxide coating with a thickness of up to 70 μm is formed on the surface of the product, depending on the formation conditions. The presence of colloidal particles of metal hydroxide in the electrolyte contributes to a more efficient coating formation due to deposition on the anode under the influence of electric breakdowns of charged colloidal conglomerates of metal hydroxides and their massive incorporation into the indicated surface. Further, under the influence of high (more than a thousand degrees) local temperatures in the vicinity of electrical breakdowns, thermal transformations of metal hydroxides deposited on the treated surface occur with the formation of the corresponding metal oxides.

Studies have shown that between the substrate metal and the active oxide layer, an anodized metal oxide sublayer forms, which serves as a stable support for the catalytic layer and ensures good adhesion of the catalytically active oxide layer formed from electrolyte components.

Changing the parameters of the formation of oxide layers (current density, voltage, processing time), as well as the concentration of acetate (acetates) in the solution, allows varying the content of catalytically active metal oxides over a wide range (up to 70 wt.%), As well as changing the roughness depending on the need porosity, i.e. active surface layers.

Thus, the inventive method provides for the production of valve metals and their alloys of oxides of divalent and polyvalent metals exhibiting catalytic activity in relation to a number of chemical processes, and allows a wide range to vary the content of these oxides, which is a technical result of the proposed method.

In addition, the inventive method provides the possibility of introducing into the catalytically active layer of oxides of several metals at the same time and allows you to adjust their ratio.

As a result, the inventive method provides the production of catalytically active oxide coatings having optimal efficiency for various specific chemical processes.

Examples of specific implementation of the method

The metal samples are degreased, washed with water and air dried.

Oxide layers are formed in the claimed modes in the conditions of spark and electrical breakdowns.

The process is conducted either in a heat-resistant glass vessel or in a stainless steel bath with constant stirring. With a constant or pulsed unipolar current in the case of using a glass vessel, the cathode is a stainless steel tubular coil cooled by tap water; in the case of using a stainless steel bath or titanium alloys, the inner surface of the bath serves as a cathode. With alternating current, two identical oxidizable samples serve as electrodes, or the inner surface of the bath is one of the electrodes. Treated samples are washed with distilled water.

Example 1

Drain aqueous solutions containing manganese acetate and sodium tetraborate. The resulting electrolyte contains 45 · 10 ^-3 mol / L Mn (CH ₃ COO) ₂ and 0.1 mol / L Na ₂ B ₄ O ₇ . The pH of the solution is 8.5. When merged, a gelatinous white colloid with a pink tint is formed. A sample of VT1-0 titanium alloy (99.4% Ti) with a size of 8 cm ^{2 is} processed in the galvanostatic mode at an effective DC density of i = 2 A / dm ² for 10 minutes. The electrolyte temperature is in the range of 15-30 ° C. The resulting dark brown coating has a thickness of 55 μm. By the method of x-ray phase analysis, the presence of crystalline oxides Mn ₂ O ₃ , Mn ₃ O ₄ in the coating was established. Microprobe analysis of the elemental composition of the surface part of the coating (excitation depth 3-5 microns) showed the presence of the following elements (in mass percent): 63 Mn, 3 Ti.

Example 2

Drain aqueous solutions containing nickel acetate and sodium tetraborate. The resulting electrolyte contains 40 · 10 ^-3 mol / L Ni (CH ₃ COO) ₂ and 0.1 mol / L Na ₂ B ₄ O ₇ . The pH of the electrolyte is 9. A white colloid is formed. A sample of VT1-0 titanium alloy (99.4% Ti) with a size of 8 cm ^{2 is} processed in the incident power mode with an initial voltage of 450 V DC for 10 minutes. The electrolyte temperature is in the range of 17-26 ° C. The resulting black coating has a thickness of 30 μm. Using X-ray phase analysis, the presence of NiO and Ni phases in the coating was established. Microprobe analysis of the elemental composition of the surface part of the coating showed the presence of the following elements (in mass percent): 44 Ni, 10 Ti.

Example 3

Drain the aqueous solutions containing yttrium acetate and sodium tetraborate. The resulting electrolyte contains 29.5 · 10 ^-3 mol / L Y (CH ₃ COO) ₃ and 0.1 mol / L Na ₂ B ₄ O ₇ . The pH of the electrolyte is 8.5. A white colloid is formed. A sample of aluminum alloy AMtsM with a size of 15 cm ^{2 is} processed in the galvanostatic mode at an effective DC density of i = 1.5 A / dm ² for 10 minutes. The temperature of the electrolyte is in the range 18-35 ° C. By the method of x-ray phase analysis, the presence of Al ₂ O ₃ phase in the coating is established. The resulting light gray coating has a thickness of 6 μm. Microprobe analysis of the elemental composition of the surface of the coating showed the presence of the following elements (in mass percent): 1.3 Y, 70 Al.

Example 4

Drain aqueous solutions containing manganese acetate and sodium hydroxide. The resulting electrolyte contains 81.6 · 10 ^-3 mol / L Mn (CH ₃ COO) ₂ and 0.1 mol / L NaOH. The pH of the electrolyte is 8. When merged, a brown colloid is formed. A sample of VT1-0 titanium alloy with a size of 20 cm is processed in the galvanostatic mode at an effective DC density of i = 2 A / dm ² for 5 minutes. The temperature of the electrolyte is in the range of 18-20 ° C. The resulting coating of black with a brown tint has a thickness of 60 μm. The coating is X-ray amorphous. Microprobe analysis of the elemental composition of the surface part of the coating showed the presence of the following elements (in mass percent): 56 Mp, 6 Ti.

Example 5

Drain the aqueous solutions containing copper acetate, nickel acetate and sodium tetraborate. The resulting electrolyte contains 26.3 · 10 ^-3 mol / L Cu (CH ₃ COO) _2, 20 · 10 ^-3 mol / L Ni (CH ₃ COO) ₂ and 0.1 mol / L Na ₂ B ₄ O ₇ . The pH of the electrolyte is 10. When fused, a blue-white colloid is formed. A sample of VT1-0 titanium alloy with a size of 8 cm ^{2 is} treated with alternating current at a frequency of 50 Hz in the galvanostatic mode at an effective current density of i = 2.5 A / dm ² for 10 minutes. The electrolyte temperature is in the range of 20-30 ° C. The resulting black coating has a thickness of 70 μm. By the method of X-ray phase analysis, the presence of NiO and CuO phases in the coating was established. Microprobe analysis of the elemental composition of the surface part of the coating showed the presence of the following elements (in mass percent): 19.6 Ni, 13 Cu, 36 Ti.

Example 6

Drain aqueous solutions containing nickel acetate and sodium silicate. The resulting electrolyte contains 40 · 10 ^-3 mol / L Ni (CH ₃ COO) ₂ and 10.5 · 10 ^-2 mol / L Na ₂ SiO ₃ . The pH of the electrolyte is 11. When merged, a white colloid is formed. A sample of aluminum alloy AMg5 with a size of 8 cm ^{2 is} processed in the galvanostatic mode at a constant current density of i = 0.5 A / dm ² for 10 minutes. The electrolyte temperature is in the range of 15-30 ° C. The resulting dark brown coating has a thickness of 25 μm.

The coating is X-ray amorphous. Microprobe analysis of the elemental composition of the surface part of the coating showed the presence of the following elements (in mass percent): 5 Si, 12 Ni, 33A1.

Example 7

Drain aqueous solutions containing lead acetate and sodium tetraborate. The resulting electrolyte contains 26 · 10 ^-3 mol / L Pb (CH ₃ COO) ₂ and 0.1 mol / L Na ₂ B ₄ O ₇ . The pH value is 10. When fused, a white colloid is formed. A sample of VT1-00 titanium alloy (99.9% Ti) with a size of 6 cm ^{2 is} processed in the potentiodynamic mode (incident power mode) on a thyristor source with a unipolar pulsed current shape at an initial voltage of 500 V on the electrodes for 4 minutes. The temperature of the electrolyte is in the range of 14-16 ° C. The resulting gray coating has a thickness of 15 μm. The coating contains crystalline phases TiO ₂ , rutile and anatase. Microprobe analysis of the elemental composition of the surface part of the coating showed the presence of the following elements (in mass percent): 7.3 Pb and 49.8 Ti.

Example 8

Drain the aqueous solutions containing copper acetate, manganese acetate and sodium tetraborate. The resulting electrolyte contains 26 · 10 ^-3 mol / L Cu (CH ₃ COO) ₂ , 40.5 · 10 ^-3 mol / L Mn (CH ₃ COO) ₂ and 0.1 mol / L Na ₂ B ₄ O ₇ . The pH of the electrolyte is 9.5. When merged, a blue colloid is formed. A sample of VT1-0 titanium alloy with a size of 12 cm ^{2 is} processed on a thyristor source with a unipolar pulsed current waveform in the galvanostatic mode at an effective current density i = 2A / dm ² for 8 minutes. The temperature of the electrolyte is in the range 19-36 ° C. A black coating with a blue tint 12 microns thick is obtained. The coating is X-ray amorphous. Microprobe analysis of the elemental composition of the surface part of the coating showed the presence of the following elements (in mass percent): 22.3 Mn, 24.8 Cu, 8 Ti.

Example 9

Drain aqueous solutions containing zinc acetate and sodium tetraborate. The resulting electrolyte contains 28.4 · 10 ^-3 mol / L Zn (CH ₃ COO) ₂ and 0.1 mol / L Na ₂ B ₄ O ₇ . The pH of the electrolyte is 10.0. When solutions merge, a white colloid is formed. A sample of aluminum grade A7 (99.7% A1) with a size of 10 cm ^{2 is} processed on a thyristor source with a unipolar pulsed current waveform in the galvanostatic mode at an effective current density i = 5 A / dm ² for 6 minutes. A gray coating 6 μm thick was obtained. The coating contains a γ-Al ₂ O ₃ phase. Microprobe analysis of the elemental composition of the surface layer of the coating shows the presence of the following elements (in mass%): 2.6 Zn, 45.0 Al.

Example 10

Drain the aqueous solutions containing copper acetate, nickel acetate and sodium tetraborate. The resulting electrolyte contains 35.6 · 10 ^-3 mol / L Cu (CH ₃ COO) ₂ , 20.1 · 10 ^-3 mol / L Ni (CH ₃ COO) ₂ and 0.1 mol / L Na ₂ B ₄ O ₇ . The pH value is 9.5. When solutions merge, a blue-green colloid is formed. A sample of aluminum grade A7 (99.7% Al) with a size of 25 cm ^{2 is} treated with direct current in the galvanostatic mode at a current density of i = 15 A / dm ² for 5 minutes. A dark gray coating with a thickness of 12 μm was obtained. The coating contains a γ-Al ₂ O ₃ phase. Analysis of the elemental composition of the surface layer of the coating shows the presence of the following elements (in mass%): 0.9 Cu, 1.7 Ni, 47.0 Al.

The coating obtained under the conditions of example 1 was tested for catalytic activity. The studies were carried out in the oxidation reaction of carbon monoxide CO under static conditions. Samples coated in accordance with the inventive method according to example 1, with a total area of 40 cm ² were placed in a quartz reactor with a volume of 0.4 L. CO and O ₂ were introduced into the reactor in a ratio of 1: 2. Samples catalyzed the reaction:

CO + O ₂ → CO ₂

in the temperature range 250-380 ° C. 50% conversion was achieved at a temperature of 340 ° C, which corresponds to the literature data for catalyst samples containing manganese oxides Mn ₂ O ₃ , Mn ₃ O ₄ , which were obtained by the known pyrolytic method.

In a similar manner, the oxide coating obtained under the conditions of Example 8 was tested for catalytic activity. The tested coating samples prepared in accordance with the inventive method catalyzed the above reaction in the temperature range 180-250 ° C. The reaction proceeds completely.

Claims (2)

1. The method of producing oxide catalysts on a substrate made of valve metal or its alloy by the method of anode-spark deposition in an alkaline electrolyte, characterized in that the coating is formed in galvanostatic mode with an effective current density of 0.5-40 A / dm ² at a constant either alternating or alternating unipolar current or in the mode of incident power when the voltage at the electrodes is not more than 600 V, while the electrolyte additionally contains acetate of divalent and / or polyvalent metal in an amount that provides the formation of a colloidal solution at an electrolyte pH of at least 8. 2. The method according to p. 1, characterized in that the valve metal or its alloy mainly use titanium or its alloys or aluminum or its alloys.