RU2745741C1 - Method for producing catalyst and ammonia oxidation catalyst itself - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: method for preparing a catalyst on a metal support made of iron-chromium-aluminum alloys includes oxidative annealing of the support, applying a protective and catalytic coating to its surface, and the catalytic coating is formed by impregnating the active alumina support with a hydrosol containing dispersed particles of platinum metals or alloys, and subsequent drying. In this case, the mesh is annealed at temperatures of 1050-1100 °C for 1-2 hours in air, then an aqueous sol and (or) suspension is applied containing 0.5-1.0 mass% of aluminum hydroxide and 0.1-0.2 wt.% surfactant, dried and calcined, and the hydrosol of catalytically active particles is obtained by arc electric pulse dispersion of platinum metals and alloys in water. Describes a catalyst for the oxidation of ammonia, obtained according to the above method, made on a substrate of woven metal mesh, containing 0.1-0.3 wt.% active alumina, 0.005-0.1 wt.% platinum or platinum alloy. The catalyst can also be used for other heterogeneous catalytic processes with activation by platinum metals and alloys.

EFFECT: technical result is a reduction in the time of technological processes, the absence of harmful emissions during production, a low content of platinum group metals, due to which the cost of the catalyst is significantly reduced.

2 cl, 1 tbl, 4 ex

Description

The invention relates to the field of obtaining heterogeneous catalysts on supports coated with active alumina containing catalytically active platinum group metals (PGM). Such catalysts can be used mainly for the oxidation of ammonia in the production of nitric acid, hydroxylamine sulfate, hydrocyanic acid, as well as for the neutralization of industrial waste gases, exhaust gases of internal combustion engines, etc.

The catalytic oxidation of ammonia for the production of nitric acid is a large-scale production, during which nitrogen oxide (NO) as the first intermediate product is obtained at a temperature of 800-950 ° C in a gas stream of an ammonia-air mixture using catalysts in the form of wire woven or knitted nets made of alloys based on platinum with PGM (usually Rh, Pd and Ru are used as alloying elements). The use of such a technological scheme, known as the Ostwald process, provides a sufficiently high productivity and continuous operation of the equipment. However, at high temperatures, significant losses of PGMs occur in the reaction gas flow associated with physicochemical effects on the surface and internal structure of the wire (formation and entrainment of gaseous oxides, recrystallization of the wire surface, grain growth, accumulation of impurities in alloys, etc.), leading to a decrease in the efficiency of the catalyst in the reaction zone, including due to mechanical destruction of the nets. These losses of PGM can be more than 50% of the weight of the installed catalyst and significantly affect the cost of the final product.

One of the ways to increase the efficiency of the catalytic oxidation of ammonia by reducing the investment of PGMs and their losses is to replace at least some of the gauze catalysts made of platinum-based alloys with cheaper catalysts.

Known is the so-called two-stage ammonia oxidation process, in which the first stage is a network of platinum-based alloys, the second is catalysts based on oxides of base metals such as cobalt, iron, chromium, etc. Catalysts of the second stage can be used in the form of a layer of granules or regular honeycomb structures. Nitric acid production process with conversion at atmospheric pressure using two-stage ammonia oxidation provided a 40% reduction in the investment and loss of PGMs.

In apparatuses for the conversion of medium and high pressure ammonia, operating at higher temperatures, the efficiency of the second stage oxide catalysts was not so significant, and they did not find wide industrial application. The reason for this is the instability of the properties of the catalysts, a narrow temperature operating range, and insufficient selectivity (the yield of bivalent nitrogen oxide NO).

It is now generally accepted that platinum and its alloys with PGM in the catalytic oxidation of ammonia to NO are significantly superior in activity and selectivity to other catalysts. One of the ways to reduce the investment of PGMs and, at the same time, losses of PGMs is the development and use of catalysts on supports, in which, due to the distribution of dispersed particles of the active component on the developed surface of the support, conditions are created for a highly efficient catalytic process.

Promising bases (substrates) for carriers of catalysts for the oxidation of ammonia with dispersed particles of PGM are network structures of precision heat-resistant alloys of the iron-chromium-aluminum system with modifying additives Ti, Y, Zr (Fechral, Fecralloy, Megapyr, Kanthal, etc.). It was found that grids made of these alloys under the operating conditions of ammonia conversion units do not affect the course of the target conversion of NH ₃ to NO and, at the same time, are able to trap a part of the PGM lost by grid catalysts made of platinum alloys (Karavaev MM et al. Catalytic oxidation of ammonia. - M .: Chemistry, 1983, p. 106). At present, fechral nets, due to their high heat resistance, satisfactory heat resistance, and relative cheapness, are widely used in ammonia oxidation units, for example, as separating nets in palladium-based PGM collection systems and as part of supporting devices.

The main task in the development of catalysts on a metal substrate is to create on its surface a heat-resistant porous coating of a support made of a material inert to PGM. The presence of a high specific surface area of the support ensures the stability of the active components - PGMs, which determines their high catalytic activity and durability.

Known catalyst for the oxidation of ammonia on a mesh metal support made of heat-resistant steels of the composition: Fe - (23-25)% Cr - 5% Al - 1% Ti or Ni (hereinafter, unless otherwise indicated - wt.%), To the surface which deposited a nanocrystalline layer of platinum in the amount of 0.20-0.40% of the mass of the components of the carrier (Patent RF No. 2 638 927 IPC B01J 23/42, B01J 23/881, 2017). The patent does not specify what the term "nanocrystalline layer" means and how it can be applied to the surface of the meshes. Apparently, we mean the initial structure of the active coating, which under the conditions of the catalytic oxidation of ammonia against the background of usual losses of platinum will undergo recrystallization changes and contamination with iron from the composition of the support, leading to a rapid degradation of the initial properties of the catalyst. The claimed long-term activity of the catalyst is maintained by the known mechanism for the deposition of platinum on the surface of the metal carrier used. Thus, the disadvantages of this catalyst are the low lifetime and high platinum content.

Known multilayer catalyst on a metal or metal alloy carrier (US Pat. No. 3 907 708, 1975). The specified support is coated with a layer of aluminide of nickel, cobalt, iron, chromium and their mixtures, the layer of metal aluminide is coated with a layer of alpha-alumina at least 10 microns thick and the layer of alpha-alumina (α-Al ₂ O ₃ ) is coated with a gamma - or eta-alumina (γ- or η-Al ₂ O ₃ ), bearing a catalytic surface layer of a metal selected from the PGM family or mixtures thereof. In this case, a metal or metal alloy carrier is selected from the group consisting of iron, steel, ferronickel, nickel, chromium-nickel, ferrite, austenite, stainless steel, cobalt-chromium and cobalt-nickel-chromium alloys, and alumina coatings are obtained from alkaline aluminates, which requires additional cleaning operations for the coatings. The described method of coating the first layer of aluminide is technologically complex and cannot be used in the case of using metal nets made of thin wire as the catalyst base.

There is also known a catalyst for the decomposition of nitrous oxide in the Ostwald process, one of the variants of which includes applying an active surface shell of γ-Al ₂ O _{3 to a} FeCrAl-alloy mesh in a known manner, drying and firing at a high temperature (950 ° C), in order to carry out transformation into α-Al ₂ O ₃ , and then applying also according to the known method of coatings Rh or Pd-Rh, or Rh _x O _y (RF patent No. 2304465 IPC B01J 23/56, B01J 21/04, 2007). However, this catalyst is intended for an auxiliary function in the process of ammonia oxidation - to reduce the content of harmful nitrous oxide emissions, is located under the catalyst and capture systems made of PGM and practically does not participate in the target oxidation of ammonia to NO.

There is a known method of producing a catalyst (RF patent No. 2 005 538 IPC B01J 37/03, B01J 23/46, 1992), including applying to a support made of steel (an iron-based alloy with a chromium content of 15-23% and aluminum 4.5-5 , 1%) layer of aluminum oxide, followed by its impregnation with aqueous solutions of salts of cerium, platinum, rhodium, drying and reduction, and the application of the layer of aluminum oxide is carried out in an aqueous solution of sodium hydroxide with a concentration of 0.7-1.5% with direct dissolution of aluminum with subsequent drying and calcination, and the reduction of platinum and rhodium is carried out in a stream of hydrogen. In this case, the support is preliminarily subjected to oxidative annealing at 900-950 ° C for 20 h, and the duration of subsequent operations of applying a layer of aluminum oxide, drying, impregnation and reduction of PGM is several tens of hours (more than 60 h). The disadvantages of the described method include the fact that due to the large thickness of the aluminum oxide layer (10-20%), the coating has low heat resistance and adhesion strength to the metal surface, and the resulting oxide coating will obviously be contaminated with sodium, which usually reduces the properties of the catalyst ... In addition, the long duration of the technological process determines the high cost of the catalyst.

There is also known a method for preparing a catalyst, including pretreating an inert carrier, applying on its surface an intermediate coating of aluminum oxide and one or more catalytically active platinum group metals (RF patent No. 2 169 614 IPC B01J 37/025, B01J 23/63, 2000) characterized in that at first the carrier, which is a block of Al-containing foil, is calcined at a temperature of 850-920 ° C in a stream of air or oxygen for 12-15 hours, and then an intermediate coating is applied to it - aluminum oxide from a hydroxide suspension aluminum in a solution of aluminum and cerium nitrates, and the final content of aluminum oxide in the catalyst is 7-14%. The disadvantage of this method is the low heat resistance and adhesion strength of the coating due to its large thickness, as well as the long duration of heat treatment (calcination) to form an Al ₂ O _3- enriched oxide layer on the alloy surface.

There is a known method (RF patent No. 2 378 051, IPC B01J 37/025, 2008) for preparing a catalyst, which includes preliminary heat treatment of an inert carrier (in the description, these are stainless steel meshes, to which the authors mistakenly classify precision alloy X23Yu5T) in a stream of air or oxygen, and obtaining an intermediate coating of aluminum oxide is carried out by applying a gel obtained by the known reaction of interaction of aluminum nitrate with an aqueous solution of ammonia with the formation of aluminum hydroxide and a solution of ammonium nitrate. Moreover, based on the stoichiometry of the reaction, the stated initial concentrations of the reagents in the patent under consideration are such that both ammonia and aluminum nitrate can be in excess of the reaction (in this case, a mixture of salt solutions is formed). The patent does not contain information about the technology of applying the obtained reagents to the mesh of fundamentally different consistencies (gel and solution), but it is reported that when the gel is applied, a coating of aluminum oxide is obtained, constituting from 0.5 to 5.5% of the mass of the carrier, while the range of stated concentrations of the starting aluminum salt changes only 3.3 times. The disadvantages of this method are long preliminary heat treatment of the metal carrier (900 ° C, 12 h), the presence of harmful emissions during the formation of an oxide coating (during the decomposition of ammonium nitrate, aluminum nitrate) and the application of PGMs, a low specific surface area of the resulting intermediate coating of aluminum oxide (20 m ² / g).

A catalyst preparation method similar to the previous one (RF patent No. 2 703 560, IPC B01J 37/025, 2019) differs in that in this case the active reagents are selected in proportion to an excess of aluminum nitrate, and a gelling additive is additionally introduced into the composition for applying aluminum oxide. The disadvantages of the method are the same as those given above according to the technical solution described in the patent of the Russian Federation No. 2 378 051.

It should be noted that all the above-described production methods include the formation of an active layer of catalysts by applying solutions of PGM compounds to the surface or porous structures of aluminum oxide, followed by drying operations and various options for reducing salts to metal. Moreover, for example, for the thermal reduction of platinum, in order to avoid large losses of the latter, a long multistage annealing is required. Using this technology, it is practically impossible to obtain catalysts from PGM alloys due to the fact that salts are reduced at different temperatures and, accordingly, at least initially, only a mechanical mixture of PGMs on a support can be obtained. At high operating temperatures, coalescence and coagulation of deposited PGM particles with the formation of alloys is possible, which leads to a decrease in their active surface and a change in the properties of the catalyst.

The closest in technical essence and the achieved effect is a catalyst and a method for producing a catalyst (prototype) described in US Pat. USA No. 4 096 095, 1979, where a substrate made of Fecralloy based on iron containing aluminum and chromium, in the form of a powder, a sheet honeycomb structure or a mesh, is subjected to preliminary heat treatment in air at 1000 ° C, 2-12 hours in order to form on it the surface of an oxide film enriched with alumina, on which a "mixed" hydrosol is applied containing colloidal particles of Al ₂ O ₃ obtained by condensation from the vapor phase, and a soluble Cr salt, thermally decomposed to Cr ₂ O ₃ . After drying and firing, a protective coating of Al ₂ O ₃ in combination with Cr ₂ O _{3 is} obtained, and then another aqueous dispersion with colloidal Al ₂ O ₃ and dissolved PGM compounds is applied to the resulting coating, which are subsequently reduced to a metallic state. The disadvantage of this method is the technologically complex stage of obtaining colloidal aluminum oxide. In addition, the used sols with a high content of aluminum oxide (9-13.5 wt.%) Lead to the formation of thick coatings with low thermal resistance and, accordingly, a limited resource during high-temperature operation.

The problem solved by the invention is to accelerate and simplify the technology of obtaining a catalyst for the oxidation of ammonia to nitric oxide (II) and other processes activated by platinum group metals.

The technical result is achieved due to: 1) preliminary high-temperature treatment of a substrate made of an alloy based on the iron-chromium-aluminum system in an oxidizing atmosphere; 2) the use of an aqueous sol and (or) a suspension of aluminum hydroxide of a certain composition to obtain a coating of active aluminum oxide (η-Al ₂ O ₃ , γ-Al ₂ O ₃ ), firmly adhered to the surface of the alloy and having high adsorption properties; 3) the use of a hydrosol of platinum metals or their alloys for impregnation of the outer layer of aluminum oxide and obtaining a catalytically active catalyst surface; 4) a hydrosol of platinum metals or their alloys is obtained by pulsed electric arc (electric pulse, electroerosive, electric spark) dispersion of the corresponding metal or alloy in water.

The task is solved in the following way.

Fechral is used as a catalyst support, for example, alloy Kh23Yu5T (precision alloy according to GOST 10994-74, containing 22-24% chromium, 5.0-5.8% aluminum, 0.2-0.5% titanium, the rest is iron) or similar alloys based on iron with 21-28% chromium, 4-6% aluminum and possible modifying additives of titanium, zirconium, yttrium. This alloy is preferably used in the form of square-mesh woven wire meshes that provide a large catalyst surface and low resistance to gas flow. After cleaning from surface contaminants, the grids are subjected to oxidative annealing at a temperature of 1050-1100 ° C in air for 1-2 hours, which leads to the stabilization of the internal structure of the alloy, which, obviously, will not undergo significant changes during operation (at operating temperatures up to 950 ° C), and on the surface of the wire - to the formation of an aluminum-enriched oxide layer of a mixed composition. This increases the surface roughness, which has a positive effect on the adhesive properties of the wire from which the substrate is made. The boundaries of the temperature and time intervals are selected within the specified limits in order to convert the formed alumina into the stable modification of α-Al ₂ O ₃ and to obtain a strong oxide layer on the wire surface, providing good adhesion to both the metal base and the next layer of alumina coating ... Higher temperatures and times of heat treatment are inappropriate due to an increase in the thickness of the oxide layer on the surface and deterioration of its adhesion to the wire metal, as well as due to an increase in time and energy costs.

The mesh substrate treated in this way is straightened and applied to it (for example, by immersion) an aqueous sol and (or) a suspension of aluminum hydroxide, then - a sol, with the addition of a surfactant (surfactant). After that, the mesh is placed horizontally, dried in air at room temperature and calcined in air in a chamber electric furnace while heating it to 500-700 ° C. To prevent or minimize the interaction of the catalytically active particles IPY when operating with high temperature oxide layer formed on the surface of the substrate grid fehrali the oxidative annealing, coating operation active Al ₂ O ₃ (aluminum hydroxide sol coating, drying, calcination) is preferably repeated. One can assume that during the second coating covers most part through the open porosity that formed during the first deposition, and thus formed simultaneously dense protective and adsorptive coating of the active Al ₂ O ₃ which allows to localize the placement of active PGM particles in a thin surface layer of the carrier - oxide aluminum.

The sol for deposition is obtained by dispersing (for example, using ultrasound) in water a gel of aluminum hydroxide Al (OH) ₃ or AlOOH, followed by the addition of a surfactant. Gel Al (OH) _{3 is} obtained by the known technology of neutralization of solutions of aluminum salts (Al (NO ₃ ) ₃ , Al ₂ (SO ₄ ) ₃ , AlCl ₃ , etc.) with an aqueous solution of ammonia, followed by purification of the precipitate of aluminum hydroxide from ammonium salts. It is also possible to use other known methods for producing aluminum hydroxide: autoclave interaction of aluminum with water, hydrolysis of aluminum alkoxides, hydrolysis of aluminates in an acidic medium, etc.

The composition of the sol for application to the surface of the substrate: 0.5-1.0% aluminum hydroxide; 0.1-0.2% surfactant; up to 100% - water.

The use of the proposed sol allows, in one cycle, including the application of the sol, drying and calcination, to apply on the substrate (a mesh of wire with a diameter of 0.25 mm and a cell size in the light of 0.5 mm) - up to 0.15% of active Al ₂ O ₃ s developed specific surface area and high adhesion to the oxidized wire surface. It was experimentally found that higher than those indicated above, the concentration of aluminum hydroxide in the ash lead to flaking and destruction of the coating already at the drying stage. A decrease in concentration is impractical due to a decrease in the specific surface area of the coating. The addition of a surfactant makes it possible to distribute the sol more evenly over the surface of the substrate; moreover, at a surfactant concentration in the ash of less than 0.1%, its efficiency is low, and at a concentration of more than 0.2%, foaming increases, as a result of which areas with an increased concentration of aluminum hydroxide are formed after drying and low adhesion to the surface of the mesh backing.

After obtaining a coating of Al ₂ O _3, it is impregnated (for example, by immersion or by brushing) with PGM hydrosol or their alloys. Thereafter, the catalyst gauze is placed horizontally and dried in air at room temperature. For a more uniform distribution of PGM particles or their alloys on the catalyst, it is preferable to repeat the impregnation and subsequent drying operations.

Hydrosol of PGM or their alloys is obtained by the method of pulsed electric arc (electric pulse, electric spark, electroerosive) dispersion of the corresponding metals or alloys in water. In this case, in the process of dispersion, part of the metal in the arc discharge zone passes into the vapor phase, from which, after cooling and condensation in water, a stable sol (colloidal solution) of ultradispersed metal particles with a size of less than 40 A is formed, which at room temperature can be prolonged (at least several tens of hours) to store, dilute or concentrate. The maximum concentration of metal particles in aqueous ash, which is obtained after the dispersion process, is about 0.2%, which allows, due to the high dispersion of particles, to obtain a highly active and durable catalyst with a low PGM content.

The invention is illustrated by the following examples:

Example 1.

For the manufacture of catalysts for the oxidation of ammonia at atmospheric pressure in a unit consisting of two contact devices, we used a woven metal mesh made of a precision alloy Kh23Yu5T, made of wire 0.25 mm in diameter and a cell side in the light of 0.5 mm. In a chamber electric furnace, the grid was subjected to oxidative annealing at a temperature of 1100 ° C in air for 1 h. After cooling, the grid was straightened in order to prevent the sol from flowing over the structure, and immersed in a sol of the following composition: aluminum hydroxide - 0.9%; Surfactant (wetting agent OP-10) - 0.16%; water - up to 100%. Then the mesh was removed from the sol and fixed in the horizontal plane. After 1.5-2.0 h of drying at room temperature, the mesh substrate was calcined in air in a chamber electric furnace while heating to 700 ° C. The rate of temperature rise was about 300 ° C / h. The alumina coated substrate was unloaded from the oven and cooled to room temperature. Then the operations of applying the sol, drying and calcining were repeated. The weight of the supported alumina was determined by weighing, which was 0.27%.

Platinum hydrosol was obtained by the method of pulsed electric arc (electric pulse, electroerosive) dispersion of electrically conductive materials in a liquid medium.

For this, the platinum ingot was rolled to a thickness of 1.5 mm, and two electrodes and granules with a size of 10 × 10 × 1.5 mm were made from the rolled stock. The electrodes were placed at the opposite walls of the reactor made of dielectric material, and pellets were loaded into the space between the electrodes. Then the reactor was immersed in a container with distilled water, and the electrodes were connected to a pulse generator. The process of electropulse dispersion was carried out at a single pulse energy of ~ 6.5 J and a frequency of 500 Hz. After dispersing for 1.5 hours, about 6 kg of a hydrosol (colloidal solution) containing 0.2% platinum was obtained. The resulting sol was diluted with distilled water to a concentration of 0.12-0.13%.

After straightening, the grid support with the support was immersed in the prepared platinum hydrosol and then fixed in the horizontal plane for natural drying at room temperature. The impregnation and drying operations were repeated. The thus obtained blanks of the deposited mesh catalyst (SCN) had in their composition ~ 0.07% platinum.

Further, two catalysts with a diameter of 2900 mm each and samples for testing were made from the blanks.

Samples of the obtained KSN were tested on a pilot plant for the oxidation of ammonia, including mechanisms that regulate the supply of air and ammonia, a mixer, a reactor made of a quartz tube with an inner diameter of -32 mm, and pipelines.

Process parameters:

- working size of catalyst gauzes - 31.5 mm;

- the number of grids - 3;

- pressure - 0.1 MPa;

- ammonia consumption - 0.2 nm ³ / h;

- the temperature on the upper grid is 800-820 ° С;

- the concentration of ammonia in the ammonia-air mixture - 10-12 vol. %.

During the tests, the resource of the catalyst and the level of its losses were evaluated.

The total test duration was more than 100 hours, with any

no changes in the operation and appearance of the catalysts were noted. The reaction of autocatalytic oxidation of ammonia was started using a heater located on the outer diameter of the quartz reactor, and the process continued continuously for 7-9 hours until the ammonia supply was turned off. The total loss of catalyst during the tests was 0.015%.

Two KCH catalysts with a diameter of 2900 mm were installed in contact devices of an industrial ammonia oxidation unit operating at atmospheric pressure. Scheme of loading the catalysts from the supply side of the ammonia-air mixture:

- knitted mesh made of Pt-5% Rh alloy from wire with a diameter of 85 microns;

- the grid of the SPC;

- granules of oxide iron-aluminum catalyst.

Contact devices operation parameters:

- ammonia consumption - 2300 nm ³ / h;

- temperature of nitrous gases 800-820 ° С;

- the concentration of ammonia in the ammonia-air mixture - 10.5-11 vol. %;

- the degree of conversion of ammonia - 96-98%;

- service life -7200 h.

A similar catalytic system, but without a CCH grid, under the same conditions showed the degree of conversion of ammonia to nitric oxide 91.4-94.3% and, due to the low conversion, was dismantled after ~ 600 h of operation.

After the first operational campaign, the KSN meshes worked out one more service life (7300 h) together with woven meshes made of Pt-7.5% Rh alloy. It is noted that such a system, in comparison with the version without SPV, reaches the operating mode twice as fast (10-15 days versus 20-30 days).

It was also noted that a part of the platinum metals, carried away from the catalysts of the first stage by the reaction gas flow, is deposited on the SCN grids.

Example 2.

To obtain a set of KSN, consisting of two grids with a diameter of 2900 mm, a grid was used as a substrate similar to that used in Example 1. Mode of oxidative annealing of grids: air atmosphere, 1050 ° C, 2 h. Sol for deposition of γ-Al ₂ O ₃ (wt%): aluminum hydroxide - 1.0; Surfactant (wetting agent OP-10) - 0.12; water - up to 100. Operations and modes of drying, calcining and obtaining a catalytically active coating layer are the same as in Example 1, but a hydrosol of Pt-5% Rh alloy with a PGM content of 0.1% was used to impregnate the alumina coating. The mass of the aluminum oxide coating was ~ 0.3 mass. %, the content of the Pt-5% Rh alloy in the SCN is 0.05 wt. %.

A set of KSN with a diameter of 2900 mm was installed in the contact devices of an industrial ammonia oxidation unit operating at atmospheric pressure. Scheme of loading the catalysts from the supply side of the ammonia-air mixture:

- knitted mesh made of composite material based on Pt-5% Rh alloy, wire diameter - 80 microns;

- the grid of the SPC;

- granules of iron-chromium oxide catalyst (KN-2P).

Contact devices operation parameters:

- average consumption of ammonia - 1850 nm / h;

- temperature of nitrous gases 800-820 ° С;

- the concentration of ammonia in the ammonia-air mixture - 11-12 vol. %;

- the degree of conversion of ammonia - 97-99%;

- specific losses of PGM - 17 mg / t 100% HNO ₃ ;

- service life - 9500 h;

The KSN nets after the first operational campaign were installed to work with new knitted nets.

Under similar conditions, two-stage systems without the use of SPG grids require an investment of PGMs by 12% more, and, accordingly, the specific losses of PGMs increase.

Example 3.

To test the SCN meshes under the conditions of ammonia oxidation units at elevated pressure, 12 samples with a size of 50 × 100 mm were made, which were fixed between a catalyst package consisting of six knitted meshes made of Pt-5% Rh alloy and a catching package consisting of woven meshes made of Pd-5% Ni alloy and Fechrali separating meshes. A grid similar to that used in Example 1. The mode of oxidative annealing of the grids: air atmosphere, 1100 ° C, 1 h. Sol for deposition of Al ₂ O ₃ : aluminum hydroxide - 0.6%; wetting agent OP-10 - 0.1%; water - up to 100%. Sol application operations, drying and calcination modes to obtain a coating are the same as in Example 1; for the impregnation of the alumina coating, a Pt-5% Rh alloy sol was used. The content of the aluminum oxide coating in the SCN was ~ 0.2%, the Pt-5% Rh alloy - 0.06%.

The catalytic system (catalyst and catching packs) with CCH samples were installed in the contact apparatus of an industrial ammonia oxidation unit operating at a pressure of 0.71 MPa.

Contact devices operation parameters:

- average consumption of ammonia - 6 200 nm ³ / h;

- effective diameter - 1880 mm;

- temperature of nitrous gases 880-910 ° С;

- the concentration of ammonia in the ammonia-air mixture - 10.5-11.0 vol. %;

- the degree of conversion of ammonia - 92-94%;

- service life of the catalytic system - 5000 h.

After the completion of the operation of the catalytic system, the SPC samples were removed and half of them (6 pcs.) Were installed in the same way in the composition of the new catalytic system. Thus, after the completion of the operation of the second catalytic system, KSN samples were obtained, which were kept in contact devices for the oxidation of ammonia for ~ 5000 and ~ 10000 h.

It has been established that all samples of SSC under conditions of prolonged exposure under the operating conditions of an industrial ammonia conversion unit practically retained their mechanical properties (strength, plasticity) and catalytic activity.

The catalytic activity was evaluated on a pilot plant for the oxidation of ammonia (see Example 1), by pairwise loading into the reactor KCH samples in the form of a semicircle after 5000 and 10000 hours of operation and measuring the temperature of the mesh surface with a pyrometer at various ratios of ammonia and air in the reaction mixture. The temperature of the samples during the autocatalytic oxidation of ammonia was found to be identical.

Microscopic examination of the samples showed that a metallized platinum alloy coating strongly adhered to the substrate is formed on the surface of the SCN from the side facing the gas flow. On the reverse side, the structure of the SPC surface is similar to the original one. It was also noted that the entire surface of the SPC after operation is covered with individual crystals of platinum alloy with a size of 3-5 micrometers; analysis of the KSN samples showed that the content of platinum metals in them increased by 10-12 times.

Example 4 (determination of the range of PGM content in the catalyst for the oxidation of ammonia).

Samples of meshes coated with alumina (0.27%) as in Example 1 were impregnated with sol with different contents of platinum and Pt-5% Rh alloy and, after drying, tested in a pilot ammonia oxidizer. In this case, the activity of the catalysts was evaluated by comparing the temperature of the mesh surface (measurements were carried out using a pyrometer) during the autocatalytic process of KSN samples with different PGM compositions loaded in pairs into the reactor. It has been established that, under the conditions of a pilot plant, the catalytic activity of the SSC with an equal content of platinum and platinum-rhodium alloy is practically the same. The data obtained on samples with different contents of the active phase of the Pt-5% Rh alloy are shown in the table below. As a reference sample, we chose KSN with 0.08% platinum-rhodium alloy, the surface temperature of which was 840 ± 5 ° C was controlled by the ammonia-air ratio in the reaction mixture.

From the data presented, it can be seen that within the PGM content from 0.1 to 0.005%, the inventive catalyst activates the catalytic process of ammonia oxidation in almost the same way. A decrease in the platinum alloy content below 0.005% reduces the activity of the KCH catalyst, and an increase in the PGM concentration above 0.1 is impractical due to an increase in the cost of the catalyst.

Thus, the claimed method makes it possible to obtain efficient and economical catalysts, in particular for the oxidation of ammonia to divalent nitrogen oxide, due to the reduction in the duration of technological stages, the absence of harmful emissions and a relatively low content of highly dispersed active particles of platinum or platinum alloy as an activator of the catalytic process.

Claims (8)

1. A method of obtaining a catalyst on a support made on the basis of iron-chromium-aluminum alloys, including oxidative annealing of the support, the deposition of protective and catalytic coatings containing platinum group metals on its surface, characterized in that the oxidative annealing of the support is carried out at 1050-1100 ° C for 1-2 hours, to obtain a protective coating of active alumina use a sol and (or) a suspension containing, wt%: aluminum hydroxide - 0.5 - 1.0; Surfactant - 0.1-0.2; water - up to 100, and the catalytic coating is formed by impregnating the coating of aluminum oxide with a hydrosol obtained by electric-pulse arc dispersion of platinum metals and alloys in water, and subsequent drying. 2. Catalyst for the oxidation of ammonia, obtained according to claim 1, consisting of a substrate in the form of a woven wire mesh, the surface of which is coated with active alumina with dispersed particles of platinum or platinum alloy, characterized in that it contains, wt%: active aluminum oxide - 0.1-0.3; platinum or platinum alloy - 0.005-0.1.