Method for producing coating for trapping platinum group metals and mesh made of heat-resistant alloy for trapping platinum group metals

Abstract

FIELD: metal science.SUBSTANCE: group of inventions comprises a method for producing a coating for capturing platinum group metals and a mesh made of a heat-resistant alloy for capturing platinum group metals. Said method includes impregnating the surface of a carrier, which is a mesh made of a heat-resistant alloy, with an aqueous solution of tetraammine palladium (II) chloride [Pd(NH3)4]Cl2 and subsequent drying. After drying, said mesh is subjected to heat treatment in a hydrogen atmosphere at a temperature of 350 to 450 °C for the recovery of palladium. Said mesh comprises a coating obtained by the said method and representing a layer of metallic palladium with a particle size of 5 to 30 nm.EFFECT: obtaining a coating on the surface of a mesh made of a heat-resistant alloy for capturing platinum group metals, which ensures a reduction in the removal of platinum group metals due to the formation of a chemically active palladium coating with increased adhesive strength on the surface of the mesh.13 cl, 1 tbl

Description

[0001] Technical field of the invention group

[0002] The group of inventions relates to the field of chemical technology, in particular to methods of applying functional coatings to metal carriers.

[0003] More specifically, the invention relates to a method for forming a nanodispersed palladium coating on the surface of a mesh made of a heat-resistant alloy and a mesh intended for use as a catcher for reducing the loss of platinum group metals in nitric acid production units.

[0004] Level of technology of the group of inventions

[0005] Modern industrial nitric acid production relies on the high-temperature (850-920°C) catalytic oxidation of ammonia on platinum alloy gauzes, typically alloyed with rhodium and palladium. A key technological challenge in this process is the loss of expensive platinum group metals (PGMs) from the catalyst gauzes' surfaces. This loss occurs due to the formation of volatile oxides, primarily PtO2 and Rh2O3, which are carried away by the process gas flow and deposited on the internal surfaces of the equipment. To minimize these losses, coated gauzes made of heat-resistant alloys are installed directly downstream of the catalyst gauzes. Coated gauzes are primarily designed to capture and retain the volatile PGMs. In particular, methods for applying such coatings to catalyst gauzes are currently being actively developed.

[0006] One such solution is a method for producing a bimetallic platinum-palladium coating for the catalytic conversion of hydrocarbons [CN112007691A, publication date: 01.12.2020], selected as a prototype. This method involves impregnating the surface of a porous composite support with an aqueous solution of tetraamminepalladium (II) chloride [Pd(NH3)4]Cl2, drying, calcining in air, and subsequent reduction in a hydrogen atmosphere at 150-500°C to obtain palladium nanoparticles ranging in size from 1 to 3 nm.

[0007] A drawback of the prototype is its ineffectiveness in creating heat-resistant alloy trapping meshes. This is due to a combination of shortcomings related to the carrier material used, key process stages, and the structure of the resulting coating.

[0008] The first drawback is the incompatibility of the method developed for producing a coating on a porous ceramic support with the physicochemical properties of the metal alloy surface. The prototype relies on the high specific surface area and capillary structure of the ceramic, which ensures uniform absorption of the solution and the fixation of palladium ions within the pores. Applying this approach to a smooth metal surface results in uneven distribution of the solution and, more importantly, the formation of a coating with extremely low adhesion strength. Without a developed porous structure to retain the particles, such a palladium layer will be susceptible to rapid mechanical entrainment (shedding) in the intense gas flow of an industrial unit, leading to a rapid loss of its collecting capacity.

[0009] The second drawback of the prototype is the mandatory high-temperature calcination step in air before reduction. This step, while standard for producing catalysts on oxide supports, becomes ineffective when working with a metal substrate. Its purpose is to decompose the complex salt into palladium oxide (PdO). However, a strong chemical bond is not formed at the metal-oxide interface. As a result, subsequent reduction with hydrogen leads to the formation of metallic palladium, which is mechanically weakly bonded to the substrate through the intermediate layer. This exacerbates the adhesion problem and leads to coating peeling during thermal cycling.

[0010] The third drawback of the prototype lies in the structure of the resulting coating, specifically the size of the formed nanoparticles (1-3 nm), which is a consequence of the above-described steps. Such high dispersion, while advantageous for low-temperature catalysis, poses a disadvantage for the collecting element. This is due to the fact that ultrafine particles have low thermal stability. Consequently, under industrial conditions at temperatures above 850°C, such particles tend to rapidly decrease their collecting capacity. Consequently, the coating cannot ensure long-term and stable operation under the required industrial conditions.

[0011] Thus, there is a technical problem consisting in the absence of a method that allows the formation of a coating on the surface of meshes made of heat-resistant alloys that simultaneously has high chemical activity due to the developed nanodispersed structure, high adhesive strength to the metal substrate and thermomechanical stability under conditions of cyclic thermal shocks and intense gas flow.

[0012] Disclosure of the essence of a group of inventions

[0013] The technical result consists in creating a method for producing a coating on the surface of a mesh made of a heat-resistant alloy for capturing platinum group metals, which ensures a reduction in the removal of platinum group metals by forming a chemically active, as well as thermally and mechanically stable nanodispersed palladium coating on the surface of the mesh.

[0014] An additional technical result is to increase the mechanical stability and adhesive strength of the palladium coating.

[0015] An additional technical result consists in increasing the thermomechanical stability of the collecting coating and its resistance to erosive wear by the gas flow, as well as in further increasing the efficiency of collecting platinum group metals.

[0016] The essence of the first invention from the group of inventions is as follows.

[0017] The method for producing a coating for trapping platinum group metals involves impregnating the surface of the support with an aqueous solution of tetraamminepalladium(II) chloride [Pd(NH3)4]Cl2, followed by drying. Unlike the prototype, the support is a mesh made of a heat-resistant alloy, which, after drying, is heat-treated in a hydrogen atmosphere at a temperature of 350 to 450°C to reduce the palladium.

[0018] The essence of the second invention from the group of inventions is as follows.

[0019] A mesh made of a heat-resistant alloy for capturing platinum group metals, containing a coating obtained by the method of the first invention and representing a layer of metallic palladium with a particle size of 5 to 30 nm.

[0020] The platinum group metals, for the capture of which the developed mesh and coating are intended, within the framework of the group of inventions are understood to mean such metals as platinum, palladium, ruthenium, rhodium, osmium and iridium.

[0021] This group of inventions is aimed at creating a chemically active, thermally and mechanically stable palladium coating on the surface of a heat-resistant alloy mesh. The mesh carrier is a heat-resistant alloy mesh, which provides the necessary mechanical strength and corrosion resistance in the high-temperature oxidizing environment of nitric acid production units. The mesh structure reduces hydrodynamic resistance to gas flow and simplifies operation at high temperatures. Various heat-resistant alloys capable of withstanding harsh operating conditions can be used in the method. In a particular implementation case, a mesh made of a ferritic chromium-aluminum (fechral) alloy of the Kh23U5T grade is used as a heat-resistant alloy mesh. In the case of fechral alloys, the aluminum content in the alloy plays a key role: when heated, it diffuses onto the surface of the carrier, forming a thin layer of Al2O3 oxide.This layer effectively retains palladium particles and prevents coating shedding under the regular cyclic loads of industrial equipment. Other alloys can also be used, such as nickel-chromium alloys of the KhN78T grade and iron-nickel-chromium alloys of the KhN32T grade.

[0022] Formation of a coating on the surface of the mesh involves impregnating it with an aqueous solution of tetraamminepalladium (II) chloride salt [Pd(NH3)4]Cl2. This composition was chosen due to its good solubility in water, which ensures a uniform distribution of palladium ions over the surface of the carrier, as well as its ability to undergo clean thermal decomposition and reduction to the metallic state. Impregnation is preferably carried out until the mass fraction of palladium reaches 0.1 to 2.5% of the mesh weight. With a palladium content of less than 0.1 wt.%, the number of active sites on the surface is insufficient to ensure a high degree of collection under conditions of high volumetric gas flow rates. Exceeding the palladium content above 2.5 wt.% does not lead to a significant increase in efficiency, but unreasonably increases the consumption of expensive metal and can contribute to the coarsening of particles during coating formation.After application of tetraamminepalladium(II) chloride solution, palladium ions are adsorbed on the thin natural oxide film present on the surface of the heat-resistant alloy.

[0023] After impregnation, the mesh is dried to remove water and prevent it from boiling during the subsequent heat treatment. Drying can be carried out in a drying oven or a continuous-flow oven in a stream of air or inert gas at a moderate temperature ranging from 80 to 150°C. Drying time depends on the weight of the product and ranges from 30 minutes to several hours, until the moisture is completely removed.

[0024] After drying, the mesh is heat-treated in a hydrogen atmosphere. This heat treatment first results in dehydration and partial decomposition of the amino complex, followed by chemical reduction of the Pd ions. 2+ to the metallic state of Pd 0Both single atoms and primary clusters of metallic palladium form on the surface, becoming securely fixed on defects in the oxide sublayer. The high thermal conductivity of the metal mesh plays a key role in this process: it ensures rapid and uniform heating of the entire surface, facilitating the simultaneous nucleation of multiple crystallization centers and, consequently, the formation of a uniformly distributed nanodispersed layer. This process is carried out at temperatures ranging from 350 to 450°C, since at temperatures below 350°C, palladium reduction is incomplete, resulting in low activity and poor coating adhesion. At temperatures above 450°C, intense thermal agglomeration (sintering) of the nanoparticles begins, leading to a sharp reduction in the specific surface area and, consequently, a decrease in the collecting capacity.

[0025] In the case of palladium, the formation of a highly dispersed metallic layer is an important factor. Palladium reduced from an amino complex is characterized by an increased tendency to form fine particles uniformly distributed over the surface of the oxide sublayer. Maintaining this dispersion is critical for the stability of the coating under cyclic temperature loads and directly affects the reduction of metal loss during operation.

[0026] Using the described method, we can produce a collection mesh coated with a nanodispersed layer of metallic palladium with particle sizes ranging from 5 to 30 nm. This morphology creates the highest specific surface area, increasing the number of available active palladium sites for heterogeneous chemical interaction with volatile platinum group metal oxides. This, in turn, ensures high efficiency in collecting volatile metal oxides from the gas stream. Particles smaller than 5 nm are extremely unstable to thermal sintering at operating temperatures, leading to rapid agglomeration and coating degradation early in use. At the same time, particle sizes greater than 30 nm significantly reduce the specific surface area of ​​the coating and, consequently, the number of available active sites per unit mass of palladium, making the use of this precious metal ineffective.

[0027] To further enhance the mechanical stability and adhesive strength of the palladium coating, a conditioning step can be performed after heat treatment. This step involves holding the mesh in a flow of inert gas until its mass stabilizes. Conditioning can be carried out at temperatures from 150 to 200°C, which ensures the complete removal of residual volatile products of the complex salt decomposition and promotes the relaxation of residual defects and stresses in the crystal lattice of the palladium nanoparticles. At lower temperatures, complete and rapid removal of residual ammine complex solution is not achieved, while at higher temperatures, partial sintering and unwanted growth of metal particles are possible. Nitrogen can be used as an inert gas in certain applications.The main function of the inert environment at this stage is to prevent oxidation of the highly active surface of the palladium coating, while nitrogen is technologically the most convenient and affordable option compared to other inert gases, such as argon.

[0028] For palladium coating, the conditioning stage in an inert atmosphere is of additional importance. This stage stabilizes the formed Pd particles, removes residual volatile products of the ammine complex decomposition, and ensures a reproducible coating structure before the product is put into service.

[0029] To enhance the thermomechanical stability of the collecting coating and its resistance to erosive wear by the gas flow, a protective oxide layer can be applied to the palladium layer after it has been formed. This oxide layer helps anchor the palladium nanoparticles to the surface and prevent their entrainment without blocking access to the compounds being collected. This process is accomplished by treating the mesh with the pre-formed palladium coating with a sol or solution of oxide precursors. A mixed silicon-zirconium oxide (SiO2-ZrO2), which exhibits high thermal stability, can be used as the protective layer material. The thickness of the protective oxide layer is preferably between 5 and 20 nm. A thinner layer does not provide sufficient mechanical fixation, while a layer thicker than 20 nm can create significant diffusion resistance, reducing the overall collection efficiency.

[0030] The group of inventions is characterized by a previously unknown set of essential features from the prior art, distinguished in that, to form the catching coating, a mesh made of a heat-resistant alloy is used as a carrier, which, after impregnation and drying, is subjected to direct reduction in a hydrogen atmosphere at a temperature of 350 to 450°C, resulting in a product whose coating is a layer of palladium nanoparticles measuring from 5 to 30 nm.

[0031] The use of a heat-resistant alloy mesh as a support (as opposed to porous ceramics) ensures high thermal conductivity of the substrate, which facilitates rapid and uniform heat distribution during synthesis, creating conditions for the simultaneous nucleation of multiple palladium crystallization centers across the entire surface. Furthermore, the metallic nature of the support guarantees high mechanical strength and resistance to thermal shock under operating conditions in industrial nitric acid production units.

[0032] Conducting direct heat treatment in a hydrogen atmosphere (excluding the preliminary high-temperature calcination step in air) enables the formation of metallic palladium directly from the precursor, bypassing the formation of intermediate oxides that weakly adhere to the metal substrate. This enables high adhesion strength of the coating due to the formation of a metal-to-metal chemical bond.

[0033] This method produces a coating with a unique structure—a layer of palladium nanoparticles ranging in size from 5 to 30 nm. This range ensures an optimal balance between a highly developed specific surface area, which is necessary for high chemical activity and capture efficiency, and high thermodynamic stability, which is essential to prevent sintering at operating temperatures of 850-920°C in nitric acid production units.

[0034] The combination of essential features of the invention enables the formation of a highly adhesion coating on the surface of a metal carrier that is resistant to mechanical abrasion, provides an optimal nanodispersed palladium structure (5-30 nm) that is highly reactive with platinum and rhodium oxides, and guarantees the thermal stability of this structure under harsh operating conditions, preventing coating degradation. Thus, the combination of the carrier material, the elimination of the oxidative annealing step, and the reduction within the specified temperature range enables the formation of a chemically active, thermally and mechanically stable nanodispersed palladium coating on the surface of the mesh, ensuring the effective capture of platinum group metals.

[0035] This ensures the achievement of a technical result consisting in the creation of a method for producing a coating on the surface of a mesh made of a heat-resistant alloy for capturing platinum group metals, which ensures a reduction in the removal of platinum group metals by forming a chemically active, as well as thermally and mechanically stable nanodispersed palladium coating on the surface of the mesh.

[0036] The group of inventions has a set of essential features previously unknown in the state of the art, which indicates its compliance with the patentability criterion of “novelty”.

[0037] The group of inventions is unknown in the prior art and does not clearly follow from it. Therefore, the group of inventions meets the patentability criterion of "inventive step."

[0038] The inventions from a group of inventions are interconnected and form a single inventive concept, which indicates that the group of inventions meets the patentability criterion of “unity of invention”.

[0039] Implementation of a group of inventions

[0040] The group of inventions is explained by the following figures.

[0041] Table 1 - Comparative indicators of the efficiency of capturing platinum group metals by mesh samples.

[0042] To illustrate the possibility of implementation and a more complete understanding of the essence of the group of inventions, an embodiment of it is presented below, which can be changed or supplemented in any way, while the present group of inventions is in no way limited to the presented embodiment.

[0043] The developed collecting mesh is designed for installation in nitric acid production units (e.g., UKL-7, AC-72, AK-72, and their equivalents) directly behind a stack of platinum-rhodium alloy catalyst gauzes. During the catalytic oxidation of ammonia on platinum-rhodium gauzes at high temperatures (850-920°C), not only the target reaction occurs but also partial evaporation of platinum group metals in the form of volatile oxides, primarily PtO2 and Rh2O3. The reaction gas stream, containing these oxides, passes through the developed collecting mesh. On the surface of the nanodispersed palladium coating, a heterogeneous chemical-catalytic interaction occurs: palladium reduces platinum and rhodium oxides to a metallic state, forming thermodynamically stable solid solutions or intermetallic phases (Pd-Pt, Pd-Rh) with them.In this way, precious metals are chemically fixed on the surface of the collection mesh, which prevents them from being carried away into subsequent units of the unit or into the atmosphere.

[0044] The invention group works as follows.

[0045] 1. Manufacturing of samples of catching nets.

[0046] The mesh samples for testing were prepared according to the following general procedure.

[0047] Preparing the media.

[0048] A knitted mesh made of Kh23Yu5T fechral with a wire diameter of 0.15 mm was used as the carrier. It should be understood that the method of the invention is also applicable to other heat-resistant alloys capable of functioning effectively under nitric acid production conditions. In particular, other nickel and nickel-chromium alloys (e.g., KhN78T grades), and iron-nickel-chromium alloys (e.g., KhN32T grades) can be used as mesh materials.

[0049] Prior to coating, the mesh was subjected to a standard surface preparation procedure, including degreasing in an organic solvent (e.g., acetone) using an ultrasonic bath, followed by rinsing with distilled water and drying.

[0050] Palladium plating.

[0051] For coating application, an aqueous solution of tetraamminepalladium (II) chloride complex salt [Pd(NH3)4]Cl2 was prepared with a concentration calculated to achieve the target palladium content on the support. The prepared support grid was impregnated with the resulting solution for 30 minutes, then removed and dried in air at a temperature of 100-120°C for 1 hour. The dried grid was placed in a flow reactor and subjected to heat treatment in an atmosphere of pure hydrogen. The temperature was raised to the operating range (350-450°C) at a rate of 5-10°C / min, followed by holding at the target temperature for 2 hours. After complete reduction, the reactor was cooled to room temperature under a stream of inert gas (nitrogen).

[0052] Samples for comparative examples were prepared using a similar method with appropriate modifications.

[0053] 2. Testing method.

[0054] The efficiency of the collection meshes was evaluated using a laboratory setup consisting of a gas mixture feed unit and a tubular furnace with a quartz reactor, arranged in series along the gas flow. The reactor contained a source of volatile platinum oxides heated to 950°C and a test sample of the collection mesh.

[0055] The tests were carried out under the following conditions:

[0056] - Gas mixture composition: air (21% O2, 79% N2);

[0057] - Temperature in the test sample area: 900°C;

[0058] - Gas flow rate: 20000 h -1 ;

[0059] - Test duration: 200 hours.

[0060] Collection efficiency was assessed based on sample weight gain (Δm, mg), which corresponds to the mass of platinum group metals collected. The degree of collection (η, %) was calculated relative to the total mass of platinum removed, which was determined by the mass loss of the source (4). To assess the mechanical stability of the coating after testing, the filter (6) at the reactor outlet was analyzed for palladium content.

[0061] Example 1 (comparative).

[0062] We tested a mesh made of fechral grade X23U5T without a coating, which only went through the surface preparation stage.

[0063] Example 2 (according to the invention, upper limit according to Pd).

[0064] A collection mesh was prepared according to the first invention. A palladium coating was applied to a mesh made of Kh23U5T fechral by impregnation and reduction in hydrogen at 400°C. The palladium content was 2.5% of the mesh weight.

[0065] Example 3 (according to the invention).

[0066] As per Example 2, except that the mass fraction of palladium was 0.1% of the mesh mass.

[0067] Example 4 (comparative).

[0068] As per Example 2, except that the mass fraction of palladium was 0.05% of the mesh mass.

[0069] Example 5 (according to the invention).

[0070] As in Example 2, except that the reduction in hydrogen was carried out at a temperature of 350°C.

[0071] Example 6 (according to the invention).

[0072] As in Example 2, except that the reduction in hydrogen was carried out at a temperature of 450°C.

[0073] Example 7 (comparative).

[0074] As in Example 2, except that the reduction in hydrogen was carried out at a temperature of 300°C.

[0075] Example 8 (comparative).

[0076] As in Example 2, except that the reduction in hydrogen was carried out at a temperature of 500°C.

[0077] Example 9 (according to the invention).

[0078] The sample obtained according to the procedure of Example 2 was additionally subjected to a conditioning step in a nitrogen stream at 180°C until its mass stabilized.

[0079] Example 10 (according to the invention, with a protective layer).

[0080] An additional protective layer of SiO2-ZrO2 with a thickness of 15 nm was applied to the sample obtained according to the method of Example 2.

[0081] The test results are shown in Table 1.

[0082] Analysis of the data presented in Table 1 allows us to draw the following conclusions:

[0083] - Comparison example 1 (uncoated mesh) confirms the need for a special coating for effective collection.

[0084] - Examples 2 and 3, corresponding to the upper and lower limits of the claimed palladium content range (0.1-2.5%), demonstrate high capture efficiency (64-70%). Meanwhile, comparative example 4, where the Pd content is below the claimed limit (0.05%), shows a significant decrease in efficiency to 52%.

[0085] - Examples 5 and 6, corresponding to the lower and upper limits of the claimed reduction temperature range (350-450°C), also demonstrate high efficiency (66-68%). Comparison with comparative Examples 7 (300°C) and 8 (500°C), where the temperature is outside the stated range, clearly demonstrates the need to adhere to the specified temperature range. At 300°C, palladium reduction is incomplete, which leads to low efficiency (38%) and poor coating adhesion. At 500°C, thermal sintering of nanoparticles occurs, which also sharply reduces the efficiency (45%). Analysis of the samples obtained in the optimal temperature range (Examples 2, 3, 5, 6) showed that the palladium particle size is in the range of 5-30 nm, while for the sample according to Example 8, the average particle size exceeded 40-50 nm.

[0086] - Examples 9 and 10 confirm the additional positive effect of introducing the conditioning and coating stages, which results in an increase in the collection efficiency and, more importantly, a sharp reduction in palladium carryover, which affects the durability of the collection mesh.

[0087] Thus, the presented examples clearly confirm that the group of inventions makes it possible to obtain a catching net with the most effective indicators.

[0088] This ensures the achievement of a technical result consisting in the creation of a method for producing a coating on the surface of a mesh made of a heat-resistant alloy for capturing platinum group metals, which ensures a reduction in the removal of platinum group metals by forming a chemically active, as well as thermally and mechanically stable nanodispersed palladium coating on the surface of the mesh.

Claims

1. A method for producing a coating for trapping platinum group metals, comprising impregnating the surface of the carrier with an aqueous solution of tetraamminepalladium (II) chloride [Pd(NH₃)₄]Cl₂ followed by drying, characterized in that a mesh made of a heat-resistant alloy is used as the carrier, which, after drying, is subjected to heat treatment in a hydrogen atmosphere at a temperature of 350 to 450°C to reduce the palladium.

2. The method according to paragraph 1, characterized in that after heat treatment, an additional conditioning stage is carried out in a stream of inert gas until the mass of the mesh is stabilized.

3. The method according to paragraph 2, characterized in that conditioning is carried out at a temperature of 150 to 200°C.

4. The method according to paragraph 2, characterized in that nitrogen is used as the inert gas.

5. The method according to paragraph 1, characterized in that a mesh made of fechral grade Kh23U5T is used as a mesh made of heat-resistant alloy.

6. The method according to paragraph 1, characterized in that the impregnation is carried out until the mass fraction of palladium in the resulting coating reaches an amount of 0.1 to 2.5% of the mass of the mesh.

7. The method according to paragraph 1, characterized in that after the coating is formed, a protective oxide layer is additionally applied to it.

8. The method according to paragraph 7, characterized in that the protective oxide layer is made on the basis of mixed silicon and zirconium oxide.

9. The method according to paragraph 7, characterized in that the thickness of the protective oxide layer is from 5 to 20 nm.

10. A mesh made of a heat-resistant alloy for capturing platinum group metals, containing a coating obtained by the method according to paragraph 1 and representing a layer of metallic palladium with a particle size of 5 to 30 nm.

11. The mesh according to claim 10, characterized in that it additionally contains a protective oxide layer.

12. The mesh according to item 11, characterized in that the protective oxide layer is made on the basis of mixed silicon and zirconium oxide.

13. The mesh according to item 11, characterized in that the thickness of the protective oxide layer is from 5 to 20 nm.

Images