RU2571099C1 - Method for obtaining catalytically active composite layers on aluminium alloy - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to methods for production of oxide composite catalysts on metal carrier-substrate, which can be applied in reactions of CO to CO₂ conversion, in purification of technological and exhaust gases, in particular, in internal-combustion engines. Method includes formation of intermediate porous layer, which contains aluminium oxide, and layer-by-layer application of metal oxide-based catalytic material, with application being realised by means of water solution, which contains components of said metal oxides in composition of salts. Aluminium oxide-containing porous layer is formed by plasma-electrolytic oxidation of substrate in silicate electrolyte, and oxide catalytically active material is applied by successive impregnation at first with Cu (II) nitrate, then Co (II) nitrate solution and in conclusion with solution of Ce (III) acetate with intermediate drying and burning of applied layers for not less than 4 h at temperature 450-550°C.

EFFECT: increase of mechanical durability of obtained composite catalysts with simultaneous simplification of method due to reduced number of operations.

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Description

The invention relates to the field of technical chemistry, and in particular to methods for the manufacture of oxide composite catalysts on a metal support carrier, which can be used in the reactions of the conversion of CO to CO ₂ taking place in high-temperature processes for the purification of technological and exhaust gases, in particular, in chemical exhaust gases , petrochemicals and internal combustion engines.

Catalysts on metal supports with high thermal conductivity and mechanical strength are subject to less overheating, which can lead to their deactivation and mechanical destruction, while they provide the possibility of obtaining various complex forms, for example, cellular structures with thin walls and a high degree of heat transfer, including for use in microchannel reactors for conducting coupled reactions.

A known method of producing oxide catalytically active layers on a substrate of valve metal or its alloy, mainly aluminum or its alloy (US Pat. RF No. 2152255, publ. 2000.07.10), including the oxidation treatment by microarc oxidation at a current density of 10-120 A / DM ² and a voltage of 200-520 V for 20-40 minutes in an alkaline electrolyte containing alkali metal silicates and hydroxides with the addition of ultrafine powders of aluminum and / or zirconium oxides and transition metal salts from the group comprising Mn, Cr, Cu, Co, Fe as well as them with mixtures, preferably salts, where the metal is part of the oxygen-containing anion. The composition of the catalytic coating obtained in a known manner, which contains, in addition to oxides of aluminum and silicon, oxides of one or two metals, does not provide a sufficiently high catalytic activity.

A known method of producing a metal-based catalyst for the removal of polluting emissions, in particular automobile exhaust gases (US Pat. US No. 7166323, publ. 2007.01.23), including the formation of a porous carrier of metal oxide by physical or chemical vapor / gas vapor deposition (thermal or plasma spraying, electrophoretic deposition, etc.) of Al, Zr, Ti, Si, Mg particles or their mixture with precursors of these metals (salts or organic compounds) on a metal substrate and thermo processing in a vacuum or inert atmosphere at a temperature of 600-1500 ° C followed by thermal oxidation at 400-1200 ° C and applying a catalytically active layer to the formed porous support by immersion. The known method requires strict temperature conditions during the formation of a porous oxide carrier, non-compliance with which leads to a significant deterioration in the quality of the catalyst. When particles of a porous layer are deposited on a metal substrate, an excess of temperature leads to their complete fusion, at low temperatures they do not bind to the substrate, in either case the porous layer does not form; during oxidation at elevated temperatures, the particles are oxidized completely over the entire thickness of the deposited layer, which is separated from the substrate; oxides are not formed when understated.

Closest to the proposed is a method of producing a multicomponent catalyst for conducting gas-phase redox reactions (US Pat. RF No. 2395337, publ. 2010.07.27), including applying to the metal carrier a given configuration of an intermediate layer of aluminum oxide with a high specific surface area by holding the carrier in solution sodium aluminate at 50 ° C, washing, drying and heat treatment at 600 ° C, the subsequent application of a promotion layer of alumina-manganese spinel and layer-by-layer application of catalytic coating events based on a complex metal oxide from an aqueous solution containing the compounds of the mentioned metals and a water-soluble polymer, moreover, to form a promoting layer, the first layer or several first layers of the catalytic coating is applied from an aqueous solution of a compound of manganese and a water-soluble polymer or an aqueous solution of a compound of manganese, lanthanum and a water-soluble polymer however, after applying each of the layers, firing is carried out at 873-1173 K for 0.5-5.0 hours.

The disadvantage of this method is the low mechanical strength of the obtained catalyst, due to insufficient adhesion to the metal substrate of an intermediate layer of aluminum oxide deposited from sodium aluminate, as well as the need for additional formation of a promoting layer of alumina-manganese spinel, associated with double heat treatment and complicating the method.

The objective of the invention is to develop a technologically advanced method of manufacturing on a substrate of aluminum alloy catalytically active composite layers with high mechanical strength.

The technical result of the proposed method is to increase the mechanical strength of the resulting catalytic active composite layers while simplifying the method by reducing the number of operations.

The specified technical result is achieved by the method of producing catalytically active composite layers on an aluminum alloy substrate, including the formation of a porous layer containing aluminum oxide, and layer-by-layer deposition of the catalytically active material based on complex metal oxide from aqueous solutions containing salts of the said metal oxide, which, unlike the known one, a porous layer containing alumina is formed by plasma electrolytic oxidation of the substrate in sili electrolyte, and the catalytically active material is applied by successive impregnation first with a solution of Cu (II) nitrate, then with Co (II) nitrate and finally with a solution of Ce (III) acetate with intermediate drying and calcination for at least 4 hours at a temperature of 450-550 ° C.

The highest mechanical strength of the resulting composite catalyst is ensured by plasma electrolytic oxidation (PEO) of the substrate galvanostatically in the anode mode at an effective current density i = 0.03-0.20 A / cm ² for 10-30 minutes in an electrolyte containing mol / l: 0.05-0.7 Na ₂ SiO ₃ and 0.05-0.07 NaOH.

The method is as follows.

To standardize the surface before coating, the aluminum alloy substrate is subjected to treatment by one of the known methods, cleaned, washed and dried.

The formation of a porous oxide layer on an aluminum alloy substrate is carried out using PEO galvanostatically in the anode mode at an effective current density of i = 0.03-0.20 A / cm ² for 10-30 minutes using an electrolyte containing Na ₂ SiO ₃ and NaOH at 0.05-0.07 mol / L. The temperature of the electrolyte during the PEO process is maintained so that it does not exceed 50 ° C. After processing, the obtained samples are washed and dried in air.

The aluminum alloy substrate coated with a porous oxide layer is sequentially impregnated first with a 2 M solution of Cu (II) nitrate, then with a 2 M solution of Co (II) nitrate, and finally with a 0.5 M solution of Ce (III) acetate. The time of each impregnation is 3-4 minutes. After each impregnation, the sample is dried and calcined at a temperature of 450-550 ° C for at least 4 hours.

The processing of the substrate by the PEO method provides the formation of a porous carrier layer from its own oxide, which has high adhesion to the substrate and contains electrolyte elements, mainly silicon, which provides a solid basis for the applied composite layers. Developed surface, high porosity (Vasilyeva MS, Artemyanov AP, Rudnev BC, Kondrikov NB // Physical chemistry of the surface and protection of materials. 2014. V. 50, No. 4. P. 411) and high moisture absorption (Vasilieva M.S., Rudnev BC, Sklyarenko O.E., Tyrina L.M., Kondrikov NB // Journal of General Chemistry. 2010.V. 80, No. 8. P. 1247) applied PEO coating allows you to easily modify it by impregnation and subsequent annealing.

During sequential impregnation with annealing, the components of each layer interact with each other and with the components of the underlying layer, as well as with the porous carrier and substrate, mainly with the formation of oxides and spinels.

As a result of heat treatment, the release of active elements (Cu, Co) from the deep layers to the surface is initiated, while, unlike copper and cobalt, cerium diffuses into the coating depth rather than onto the surface during annealing, and its concentration in the surface layer decreases. The application of cerium compounds at the final stage of the formation of catalytic layers significantly reduces the consequences of this effect.

Cerium oxide is a supplier of oxygen in catalytic oxidation reactions, and due to the free transition between Ce ⁴⁺ and Ce ^3+, it is able to regulate the amount of oxygen being split off depending on the mass of oxidized gases, in particular, when the intensity of exhaust gas is changed. Of no small importance is its role as a promoter for other catalytically active components.

It has been experimentally established that the sequence of deposition of catalytically active elements (the impregnation sequence), along with the nature of these elements, significantly affects their amount in the surface layer of the composite catalyst.

This effect is explained by differences in the selective adsorption of metal cations by different layers: a porous oxide carrier, layers containing active components.

According to X-ray microprobe analysis, the intermediate porous oxide layer (oxide carrier) obtained by the PEO method, in addition to aluminum and oxygen, contains silicon and sodium - electrolyte elements. As shown by x-ray phase analysis, of the crystalline phases in the composition of this layer there is γ-Al ₂ O ₃ . Since the silicon content in the composition of the coatings is quite high (~ 9 at.%), We can assume the presence of amorphous silica SiO ₂ in the composition of the intermediate porous layer. Thus, the total composition of the formed support can be designated as SiO ₂ + Al ₂ O ₃ / Al.

According to x-ray microprobe analysis (averaged over a layer with a depth of ~ 5 μm), the elemental composition of the obtained composite catalyst on a substrate of aluminum alloy D16 contains, at. %: O 56.6, Al 12.8, Si 5.0, Co 14.9, Cu 8.5, Ce 2.3. Its phase composition includes γ-Al ₂ O ₃ , crystalline CuO, CeO ₂ , Co ₃ O ₄ . For catalysts on substrates from other alloys, the elemental composition is similar and differs only in the presence of an insignificant amount of impurity components of a particular alloy.

According to X-ray electron spectroscopy (total depth of the analyzed layer ~ 6 nm), aluminum and silicon are absent in the composition of the surface and near-surface layers of the obtained composite catalyst. These layers mainly contain oxides of impregnated metals: copper, cobalt and cerium. In the surface layer with a depth of ~ 3 nm, Cu ²⁺ , Co ³⁺ , and Ce ⁴⁺ are present; in the surface layer with a depth of ~ 5-6 nm (after removal of the surface layer), Cu ⁺ , Co ^2+, and Ce ³⁺ are present along with the components listed above . The sum of all active components is ~ 37-50 at. %

Thus, when applying catalytically active oxides in accordance with the proposed method, the optimal composition and maximum total concentration of active elements in the surface layer are ensured.

As shown in FIG. 1 images taken by electron scanning microscopy (ESM), the surface morphology of the porous oxide support formed by the PEO method, when applying catalytically active layers varies slightly. High specific surface area is maintained.

In FIG. 1 shows phase (a) and amplitude (b) ESM images of the surface of a porous SiO ₂ + Al ₂ O ₃ support on an aluminum alloy substrate; in FIG. 1 (c) phase and (d) amplitude images of the surface of the active catalytic layer.

The resulting composite catalysts were tested in a model reaction of CO oxidation in CO ₂ . The temperature of the 50% conversion of CO - T ₅₀ , which was determined based on the dependence of the conversion on temperature (Fig. 2, curve 2), is 161 ° C, while the porous composite support SiO ₂ + Al ₂ O ₃ / Al does not have catalytic activity: at a temperature of 500 ° C, the degree of conversion of CO is only ~ 5% (Fig. 2, curve 1).

In accordance with the literature data (Firsova A.A., Khomenko T.I., Ilyichev A.N., Korchak V.N. // Kinetics and Catalysis. 2008. T. 49. No. 5. C. 713) the high activity of the ternary oxide structures of CoO — CoO — CeO ₂ (in the reaction of selective CO oxidation) is due to the strong interaction of supported copper and cobalt oxides with cerium dioxide with the formation of Cu — Co — Ce — O clusters on the surface. During the adsorption of CO, Co ⁺ –CO and Cu ⁺ –CO complexes are formed on the surface of such catalysts, and the oxidation of the carbonyl group in these complexes by the oxygen of Cu – Co – Ce – O clusters occurs at 140–160 ° C. Found (Spasova I., Velichkova N., Nihtianova D., Kristova M. / Influence of Ce addition on the catalytic behavior of alumina-supported Cu-Co catalysts in NO reduction with CO // J. Colloid and Interface Sci. 2011. V. 354. P. 777-784) that CeO ₂ promotes the formation of active phases forming catalytically active sites on the surface of CeO ₂ + CuO / Al ₂ O ₃ / Al, CeO ₂ + CoO / Al ₂ O ₃ / Al and CeO ₂ + CuCoO _x / Al ₂ O ₃ / Al.

The catalytic properties of the obtained catalyst systems of the type [CeO _x / CoO _x / CuO _x ] / [Al ₂ O ₃ + SiO ₂ ] / Al in the oxidation of CO are reproducible and stable over time, while the coatings exhibit high heat resistance and mechanical strength.

Examples of specific implementation of the method

Anode PEO coatings were formed galvanostatically at an effective current density i = 0.03 A / cm ² for 30 min using a computer-controlled multifunctional current source based on the TER-4 / 460H serial thyristor unit (Russia). The sample served as the anode, the cathode was made in the form of a coil from a hollow tube of corrosion-resistant steel. The temperature of the solution during the process did not exceed 50 ° C. After plasma-electrolytic treatment, the samples were washed first with running water, then distilled, and then dried in air.

To analyze the surface composition, the method of X-ray electron spectroscopy (RES) was applied using an ultrahigh-vacuum installation by Specs (Germany). To compare the composition of the surface (depth ~ 3 nm) and deeper (surface) layers, the surface was etched by bombardment with argon ions with an energy of 5000 eV for 5 min. As a result, the surface layer ~ 3 nm thick was removed.

The phase composition of the oxide layers was determined by X-ray phase analysis (XRD) on a D8 ADVANCE X-ray diffractometer (Germany) in CuK _α radiation.

Data on the averaged over the volume elemental composition of the coatings during scanning to a depth of 5 μm was obtained on a JXA 8100 X-ray microanalyzer (Japan), additionally equipped with an INCA energy-dispersive (X-ray) prefix (England).

For carrying out catalytic tests, a universal flow-through installation type BI-CATflow4.2 (A) was used (IR SB RAS). The initial reaction mixture contained 5% CO + air. The gas flow rate was 50 ml / min. The concentration of CO and CO ₂ at the outlet was determined using a TEM-2 infrared gas analyzer. The range of the studied temperatures is 20-500 ° C. The tested catalytically active samples with a total area of 10 cm ² were heated to a predetermined temperature in increments of 25-50 ° at a speed of 20 ° / min and kept at this temperature until a constant composition of the final reaction mixture was established (15-20 min).

Example 1

An intermediate layer was formed on a substrate of aluminum alloy D16 (wt.%: 3.8-4.9 Cu, 1.2-1.8 Mg, 0.3-0.9 Mn, the rest Al) by PEO at a current density of 0 05 A / cm ² in an electrolyte containing mol / L: 0.05 Na ₂ SiO ₃ and 0.05 NaOH for 30 minutes

The impregnation was carried out with a 2 M solution of Cu (II) nitrate for 4 minutes, dried in air, and calcined for 4 hours at 550 ° C. Then it was impregnated with a 2 M solution of Co (II) nitrate for 4 min, dried in air and calcined for 4 hours at 550 ° C. Then it was impregnated with a 0.5 M solution of Ce (III) acetate for 4 min, dried in air, and calcined for 4 hours at 500 ° C.

Phase composition: γ-Al ₂ O ₃ , crystalline oxides CuO, CeO ₂ , Co ₃ O ₄ . The temperature of the semi-conversion of CO is 161 ° C.

Example 2

AMg5 aluminum alloy substrate (wt.%: 4.8-5.8 Mg, 0.5-0.8 Mn, 0.02-0.1 Ti, 0.0002-0.005 Be, up to 0.5 Fe, up to 0.5 Si, up to 0.2 Zn, up to 0.1 Cu, 91.9-94.68 Al) was processed under the conditions of example 1, while PEO was carried out with a current density of 0.2 A / cm ² in an electrolyte containing mol / L: 0.07 Na ₂ SiO ₃ and 0.07 NaOH for 10 minutes, impregnation at each stage was carried out for 3 minutes, calcination for 4 and a half hours at 450 ° C.

Averaged elemental composition of a layer 5 microns thick, at. %: O 54.9, Al 13.6, Si 4.9, Co 15.0, Cu 8.4, Ce 2.1, Mg 1.1.

The remaining results are similar to those obtained in example 1.

Claims (2)

1. A method of producing a catalytically active oxide composite layers on an aluminum alloy substrate, comprising the formation of an intermediate porous layer containing aluminum oxide, and layer-by-layer deposition of a catalytic material based on metal oxides, the application being carried out using an aqueous solution containing the components of these metal oxides in the composition salts, characterized in that the porous layer containing alumina is formed by plasma-electrolytic oxidation of the substrate in silicate e electrolyte, and the oxide catalytically active material is deposited by sequentially impregnating with a solution of Cu (II) nitrate, then Co (II) nitrate and finally with a Ce (III) acetate solution with intermediate drying and calcination of the deposited layers for at least 4 hours at a temperature of 450 -550 ° C. 2. The method according to p. 1, characterized in that the plasma-electrolytic oxidation of the substrate is carried out galvanostatically in the anode mode at an effective current density i = 0.03-0.20 A / cm ² for 10-30 minutes in an electrolyte containing mol / L: 0.05-0.7 Na ₂ SiO ₃ and 0.05-0.07 NaOH.

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