RU2515727C2 - Method of obtaining nanostructured catalytic coatings on ceramic carriers for neutralisation of waste gasses of internal combustion engines - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to field of catalysis. Described is method of creating efficient platinum-free catalytic coating on ceramic units for neutralisation of waste gases of autotractor diesel engines, which includes formation of substrate with large value of specific surface on ceramic honeycomb carriers.

EFFECT: increase of catalyst activity.

2 cl, 4 dwg, 2 tbl, 1 ex

Description

Heat engines, incl. automotive and transport internal combustion engines (ICE), the total number of which is about 1 billion, emit a large amount of harmful substances into the environment that negatively affect humans, flora and fauna, as well as technical facilities. The sources of harmful substances in internal combustion engines are mainly exhaust gases (exhaust) [1-6]. Their harmful components make up 0.2-5.0% of the exhaust gas volume.

The main harmful substances in exhaust gases are: nitrogen oxides (NO _x ); various sulfur and lead compounds; hydrocarbons of various structures and compositions, including carcinogenic (C _x H _y ); carbon monoxide (CO); soot (C).

Reduce the harmfulness of exhaust gases by:

- improving the internal combustion engine and their work processes,

- use of improved fuels and alternative fuels,

- purification and neutralization of exhaust gas.

Diesel exhaust gas must be cleaned of nitrogen oxides, sulfur dioxide and soot. Standard cleaning and neutralization of engine exhaust gas includes: purification from solid particles and oil by filtration; neutralization by catalytic oxidation of products of incomplete fuel combustion with the simultaneous reduction of nitrogen oxides [4-8]. Catalytic converters differ in catalyst composition and carrier material.

There are simple and complex systems for cleaning and neutralizing exhaust gas of internal combustion engines [3-6]. For example, in the Volkswagen neutralization system, the 1st oxidation catalyst purifies the exhaust gas from CO and CH, and in the 2nd, nitrogen dioxide is formed to oxidize solid particles. To reduce nitrogen oxides, their accumulator was used, periodically cleaned with a rich combustible mixture. The exhaust gas cleaning system for Peugeot diesel engines includes a control unit for its operation with appropriate sensors, a fuel additive dosing system and a particulate filter, which also serves as a catalyst based on silicon carbide of a porous structure. And the European automobile community and KamAZ adopted a program for the development of "SCR technology" - selective recovery of NOx emissions by introducing urea vapor or its equivalent into exhaust gas,

In the Russian Federation, the development of various systems for cleaning and neutralizing exhaust gas from internal combustion engines is carried out by the State Research Center of the Russian Federation FSUE NAMI [3, 4, 6, 7, 10-12], G.K. Boreskova and many other research institutes [13-19]. According to Decree of the Government of the Russian Federation No. 1348-r dated 09.24.2002, all cars in the Russian Federation must ensure compliance with the EURO-1 standards. Domestic diesels, such as YaMZ, at best meet Euro-2 standards, and only with the installation of a Russian-American joint venture CPG is it planned to bring KamAZ diesels to Euro-3 and Euro-4 standards in 2011. The D-240 diesel engine only in 2009 began to satisfy Euro-3 standards. ICE passenger cars in export performance, as a rule, meet Euro-3 standards.

The main way to neutralize the exhaust gas of internal combustion engines remains the catalytic oxidation of products of incomplete combustion and the catalytic reduction of nitrogen oxides [3-7]. One of the factors of catalyst efficiency is the developed structure of its substrate. However, metallic supports made of fechral smooth and corrugated foil, and ceramic ones made of cordierite or mullite ceramics, have an underdeveloped surface. To increase it, fechral carriers are subjected to etching and heat treatment to form γ-alumina with a high specific surface on the surface. A ceramic layer forms a substrate layer with a developed structure and open pores from materials with a high specific surface.

The main factor in the effectiveness of the catalyst is the composition and quality of the working layer. The main component of this layer is used noble metals - platinum, palladium and rhodium. However, with high activity, they have disadvantages - they require clean fuels, because they are vulnerable to catalytic poisons - compounds of sulfur, lead, iron. Resistance to catalytic poisons is also relevant for the purification of gases from incineration plants.

Another disadvantage of noble metal catalysts: if flammable substances get above their norm on their surface, intense burning is possible, which destroys the converter. In addition, noble metals are scarce and expensive. Therefore, research is being carried out on effective free-of-charge catalytic compositions, however, they are not mass-produced.

In general, the effectiveness of the catalyst on the selected medium is determined by the quality of such works [2]:

- the application of a secondary carrier (substrate) with a high specific surface, which determines the rate of intra-diffusion processes and the efficiency of the catalytic component;

- selection and application of the active component that determines the catalysis process, i.e. specific chemical reactions.

The method of applying a substrate according to the patent of Russia [24] includes a 12-15-hour calcination of a honeycomb carrier made of aluminum-containing foil at 850-920 ° C in a stream of air or oxygen, repeatedly applying to its surface a substrate of modified aluminum oxide from an aqueous-alcoholic suspension with nitric acid cerium, and then heat treatment at 500-550 ° C. A method of applying a catalytically active component is not described.

In the method for preparing the catalyst as a whole according to the European patent [21], an alumina layer is formed on a honeycomb carrier with the subsequent introduction of stabilizing and modifying alumina additives and catalytic components. To obtain a high (120-130 m ² / g) specific surface area of the substrate with an Al ₂ O ₃ content _of 7-14 wt.%, The deposition of alumina is repeated, followed by drying. Next, the active phase is applied from platinum group metals with a CeO ₂ content in Al ₂ O _{3 of} 8-15 wt.%.

This method is multi-stage and energy intensive. In addition, incomplete migration of Al atoms to the surface of the metal strip occurs due to insufficient temperature and the formation of an inhomogeneous composition of the oxide layer, which includes mainly iron oxides. This impairs the adhesion of the coating and leads to its peeling. Another disadvantage of this method is the use as a basis of a suspension of aluminum hydroxide with a low specific surface area and adhesion, which requires the addition of a plasticizer - aluminum nitrate.

The known method [SU 1034762 A] preparing a substrate for a CO oxidation catalyst by manufacturing a system of two aluminum strips by folding them and then anodizing them. It is carried out in a 10% solution of oxalic acid at 20 ° C, a current density of 5 A / dm ² for 1.5 hours. In this way, oxide layers of a substrate 100 μm thick are obtained that do not have a sufficient specific surface area.

The known method [SU 733717 A] preparing a substrate for a catalyst with increased activity and mechanical strength. Here, before applying the active layer, the titanium plate is anodized in solutions of hydrochloric and sulfuric acids. However, this method allows to obtain an oxide porous layer only on titanium.

Known [WO 94/09903 A1] is a method of forming a porous oxide layer on an aluminum honeycomb block by oxidizing its surface in an electrolyte. It is conducted for an hour in a solution of sulfuric acid with a concentration of 9 wt.%, At a temperature of 30-40 ° C, a constant current density of 96.2 A / m ² , a voltage of 8-15 V. However, the resulting oxide layer has a small thickness (2- 10 microns), low chemical and thermal resistance, low specific surface area of the oxide layer (30-150 m ² / g).

An aluminum catalyst is also known [JP No. 3-49614 A], in which first a surface layer of aluminum oxide with a specific surface of 100-300 m ² / g is created, then a second similar layer with a specific surface of 10-50 m ² / g is formed, and a catalytic layer is applied with platinum, palladium and / or rhodium. The disadvantages of this catalyst, as well as all with precious metals, are the high cost and complexity of the process.

The surface modification of monolithic ceramic blocks with a small specific surface (0.1-10 m ² / g) and low porosity (30-49%) is also carried out by applying a substrate. So, the proposed method [22], including: heat treatment of the carrier at 800-1050 ° C; applying to it a layer of aluminum hydroxide from an aqueous solution of potassium hydroxide in the presence of a powder of aluminum metal at a temperature of 20-25 ° C; the formation of a layer of aluminum oxide by thermal dehydration of aluminum hydroxide; the introduction of one or more substances, thermostabilizing alumina; application of one or more catalytic substances. The disadvantages of the method are the duration - from 24 to 70 hours, multi-stage and the difficulty of achieving high quality. Long exposure of the carrier, for example, from cordierite in a solution of potassium hydroxide at a temperature of 40-50 ° C leads to swelling of the ceramics and its cracking during heat treatment.

According to US patent [23], the monolithic carrier is repeatedly treated with an alumina suspension in which aluminum oxide powder with cerium oxide is dispersed, the cerium oxide being formed by preliminary impregnation of the aluminum oxide powder with a solution of cerium salt and subsequent calcination. The suspension-treated support is calcined to obtain an alumina coating. The disadvantages of the method is the low adhesion strength of aluminum oxide to a metal surface, resulting in a low catalyst resource due to the gradual peeling of the substrate with the active substances.

In the patent [24], the substrate is applied from the suspension in the following ratio of its components, wt.%: Aluminum hydroxide - 22-32, aluminum nitrate - 2-4, cerium nitrate - 2-5, water-alcohol in a ratio of 1: 1 to 100. For one dive, 7 ... 14 wt.% Alumina is applied to the block with a substrate. If necessary, the steps of immersion of the carrier in the suspension are repeated. Next, an active phase of one or more platinum group metals is applied to the substrate, drying and reduction are carried out. The method of applying the active phase is not described.

One of the latest Russian patents [25] proposed the most advanced method for preparing the catalyst as a whole. A catalytic coating of one or more platinum group metals is applied, after calcining the honeycomb carrier, simultaneously with the application of a modified alumina substrate from a water-alcohol suspension onto its surface, followed by drying and reduction. The suspension contains components in wt.%: Boehmite - 15-30, aluminum nitrate - 1-2, cerium nitrate - 4-8, 25% solution of ammonium hydroxide - 10-20, one or more inorganic salts of platinum group metals, in in terms of metals - 0,020-0,052, water-alcohol in a mass ratio of 1: 5 ... 1: 10 - the rest. In this case, boehmite with an initial specific surface area of at least 300-350 m ² / g is used.

The carrier of corrugated and rolled into a block of steel strip is calcined at a temperature of 1000-1125 ° C, and the ceramic carrier containing cordierite, hematite, rutile, silicon carbide is calcined at a temperature of 500-1000 ° C. This catalyst with a surface of modified alumina and an active phase of one or more platinum group metals has:

- specific coating surface is 100-200 m ² / g,

- the content of Al ₂ About ₃ - 5-13 wt.%,

the content of CeO ₂ is 0.5-1.3 wt.%,

- the active phase of the metals of the platinum group is 0.12-0.26 wt.%.

High catalyst efficiency is achieved by:

- creating on the surface of the aluminum-containing steel strip a layer of aluminum oxides of alpha and gamma modifications,

- removal of adsorbed moisture from the surface of ceramic carriers,

- stabilization of the composition of the silicon carbide carriers containing residual carbon, which increases the adhesion of the coating of aluminum oxide deposited from the suspension;

- introducing aluminum hydroxide successively into the aqueous-alcoholic suspension in the form of its modification AlOOH (boehmite), cerium nitrate (Ce (NO ₃ ) ₂ ), ammonium hydroxide and inorganic salts of noble metals (H ₂ PtCl ₆ , PdCl ₂ or RhCl ₃ ) in the form of solutions directly into the composition of the suspension, which makes it possible to obtain a thermostable strong catalytic coating with a high specific surface area at a time.

- the use of boehmite suspension with a high specific surface, which contributes to the synthesis of ultrafine particles of noble metals, and gamma-alumina stabilized by cerium oxide formed during heat treatment prevents their sintering and conglomeration.

Finally, the carriers are subjected to heat treatment to increase the adhesion of the coatings, stabilize their surface and firmly adhere to the catalytic coating: a metal carrier at 1000-1125 ° С, and a ceramic one at 500-1000 ° С.

When using the suspension of this patent for one dive, 6-13 wt.% Alumina is applied to the substrate, which reduces the complexity of the work. The introduction of a solution of ammonium hydroxide reduces the pH of the solution, which is necessary when processing steel carriers and increases the adhesion of the coating due to the formation of a gel-like aluminum hydroxide during the preparation of the suspension.

The residual suspension is removed by centrifugation for a more uniform coating than when blowing with compressed air, and dried with a stepwise rise in temperature from 50 to 120 ° C, and heat treated at 500-550 ° C followed by reduction of platinum metals, for example, in hydrogen at 350 ° C .

However, the high-performance catalytic converters according to the patent described are expensive, and therefore do not find wide application.

In general, exhaust gas exhaust gas neutralizer with noble metals are quite effective, but they have a high cost and insufficient operating life when using domestic fuels. It is possible to increase the catalyst life by using catalytically active oxides of base metals, for example, metals of variable valency, including Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Mo. But to increase their activity, it is especially necessary to use substrates with a high specific surface. If they are thin, then their surface will not be sufficiently developed to obtain a highly active catalyst, or uneven when cracks appear during heat treatment and partial shedding of the coating.

For the preparation of a free-catalyst catalyst coating, for example, according to the patent of the Russian Federation [26], plasma spraying is carried out. The carrier used foil made of heat-resistant chromium-aluminum steel. A 5 m long tape was spirally wound onto a steel drum 300 mm in diameter and plasma sprayed over the entire surface. Powders were sprayed, including: aluminum 8%, aluminum hydroxide (gibbsite) 33%, boehmite 52% and vanadium carbonates 2%, chromium 2%, nickel 1%, cobalt 0.5%, cerium 0.5%, lanthanum 0.5 %, yttrium 0.5%. The catalytic coating had a thickness of 20 μm of the following chemical composition:

- alumina g-modifications of 92%,

- the sum of the oxides of vanadium, chromium, nickel, cobalt, cerium, yttrium and lanthanum 8%.

The sprayed tape was corrugated with a bending radius of 1.2 mm and rolled up so that longitudinal channels with a surface catalytic layer were formed.

The main method of applying catalytic coatings free of charge consists in preparing solutions of metal salts and a suspension of titanium dioxide with boehmite in them, since these components are not soluble. The carrier with the formed substrate is immersed in a salt solution, later removed, the excess solution is removed by blowing. The coating is repeated. The block is dried in air, then in an oven. The block is then calcined to convert metal salts to oxides.

In [27], microarc oxidation of a non-porous support substrate of a valve metal or its alloy in an alkaline electrolyte with the addition of ultrafine powders of aluminum and / or zirconium oxides and a mixture of transition metal salts: Mn, Cr, Cu, Co, Fe, is used for the same purposes. Salts use cobalt nitrate, potassium or sodium chromate, potassium permanganate, ammonium iron.

The process is conducted for 1200-2400 s in the anode mode at a pulse frequency of 50 Hz, their duration is 50-300 μs, current density 10-120 A / dm ² , voltage 200-520 V. An alkaline electrolyte uses an aqueous solution containing silicates, alkali metal hydroxide . Ultrafine powders of aluminum oxide and / or zirconium and powders of salts of transition metals are taken with a specific surface area of at least 100 m ² / g in the following ratio of components, g / l:

- alkali metal silicate - 20-50;

- alkali metal hydroxide - 1-2;

- ultrafine powder of aluminum and / or zirconium - 20-60;

- salts of transition metals - 0.5-15.

The coating is formed of two layers. The inner layer is formed in the first 300 s of processing. Its initial part is formed only due to the oxidation of the metal with the formation of its oxide, the layer thickness increases to 100-200 μm during the entire processing time of the sample due to the diffusion of oxygen into the metal. This reduces the conductivity of the metal and increases the electric field strength. This creates the prerequisites for the formation of the second part of the inner layer, which is formed from metal oxide and electrolyte components, its thickness is 10-20 microns, time 300-600 s. The formation of this layer leads to an increase in the voltage drop in the oxide, after which an electric breakdown occurs, locally burning the oxide layer. High temperatures in the breakdown zone contribute to the synthesis of the outer coating layer, in which, with metal oxides, the substrates contain in large quantities groups of atoms or particles of powders included in the electrolyte. The thickness of this layer is 50-220 microns, the formation time is 600-1800 s. The content of metal oxides in the coating 3-25% of the mass. The disadvantage of this method is the duration and energy intensity of the process.

Closest to the claimed method is the "Method of preparing a catalyst for purification of exhaust gases of internal combustion engines and the catalyst obtained by this method" according to patent No. 2275962, which is the development of the method according to patent No. 2190470 of the same authors.

The aim of the present invention is to develop a method for producing nanostructured substrates and highly active nanostructured gratuitous catalytic coatings on ceramic blocks for exhaust gas converters of automotive diesel engines.

The proposed method is as follows. Ceramic carriers are calcined for two hours in a muffle furnace at a temperature of 400-600 ° C for purification from organics. After cooling, they are immersed in a 4-5 percent suspension of aluminum oxyhydroxide (boehmite) with a high-temperature binder - aluminum nitrate 1 ... 3%. With the dried coating, the blocks are calcined for 6 hours in a muffle furnace at a temperature of 500-550 ° C.

In a mixture of organic solvents, a solution of metal salts of variable valency is prepared, including copper nitrate 20-23 g / l, cobalt nitrate 20-25 g / l, ammonium iron sulfate 10-15 g / l. A suspension of titanium dioxide and boehmite is made in it. The block with the formed substrate is immersed in a salt solution for a certain time, removed, the excess solution is removed by blowing air or centrifugation. Next, the unit is dried in air and in an oven with a stepwise rise in temperature from 50 to 120 ° C. The application of the catalyst is repeated. After final drying, the block is subjected to heat treatment at a temperature of 600-650 ° C for 240 min to convert metal salts to oxides.

An example of the implementation of the method: Suspensions for the formation of the substrate were prepared with boehmite powders with a specific surface of 78.4 and 170.2 m / g (Table 1).

Table 1 Alumina for the preparation of suspensions No. Production method Phase composition Particle size im The size of the aggregates, microns Specific surface, m ² / g one Hydrothermal synthesis Boehmite 286 1,524 78,4 2 Reprecipitation Boehmite, bayer 236 1,598 170,2 3 Hydrolysis of Organic Compounds γ-alumina 198 1,284 195.2

With the dried coating, the blocks were calcined for 6 hours at a temperature of 500-550 ° C in a muffle furnace.

To select the heat treatment modes during the formation of substrates, the dependences of the specific surface of the powders (Table 1) on the calcination temperature (Fig. 1) are determined. It was revealed that upon heating, all powders increase, and subsequently decrease the specific surface area (Fig. 1). Such a change is determined by the phase composition of the starting powders. The largest increase in specific surface area is observed for the 2nd powder (Table 1). The losses during calcination (Fig. 2) of this powder are greatest and are caused by the release of crystallization water from the structure of bayerite (aluminum trihydroxide). Transformations occur with a decrease in volume, an increase in porosity and specific surface area.

According to the data obtained, the above calcination temperature of 500-550 ° C and powder No. 2 were adopted.

Determination of the specific surface on the Autosorb device showed:

- 3.2 m ² / g for a boehmite-based substrate with S _beats = 78.4 m ² / g,

- 5.4 m ² / g for a boehmite-based substrate with S _beats = 170.2 m ² / g,

- less than 1 m ² / g at the initial surface of the ceramic block.

Figure 2, 3 shows the surface topology of the ceramic carrier without coating and with a substrate, obtained using a Solver Next scanning probe microscope.

According to Fig. 2, the initial carrier has an uneven but underdeveloped surface (Fig. 3), which leads to a small specific surface area. The deposited nanostructured substrate (Fig. 4) using powder No. 2 has a developed surface with pore sizes at the sensitivity boundary of the AUTOSORB-1 instrument used.

The catalytic solution was prepared from salts of iron, cobalt, and copper. A suspension of titanium dioxide and boehmite was made in it. The block with the formed substrate was immersed in the salt solution for the estimated time, removed, the excess solution was removed by blowing air or centrifugation. Next, the block was dried in air and in an oven with a stepwise rise in temperature from 50 to 120 ° C. The application of the catalyst was repeated. After final drying, the block was subjected to heat treatment at a temperature of 600-650 ° C to convert metal salts to oxides.

The boehmite powder (No. 2) with a high specific surface used, the composition of metal salts of variable valency, the temperatures of the heat treatments and the sequence of operations ensured the optimal values of the structural characteristics of the substrates and the high activity of the catalysts. This was confirmed by the tests described below.

To evaluate the catalytic activity, fragments of catalysts with a volume of 5–10 cm ^{3 were} tested in a flow laboratory apparatus in model gas mixtures of CO, O ₂ , N ₂ with the control of an OPTOGAZ 500.1 gas analyzer. The oxidation of CO with oxygen was carried out in the temperature range 25-500 ° C. The activity of the catalyst was evaluated by the degree of conversion (afterburning) of CO depending on the reaction temperature. Test conditions are given in table 2.

table 2 Test conditions for the activity of catalysts for the oxidation of carbon monoxide in a flow unit The composition of the source gas mixture,% vol. 1. CO 0.30 2. O ₂ 0.30 3. N ₂ 99.40 The gas load on the catalyst sample during the test (for all samples), 1 / h 10,000 Values of absolute control errors 1. The content of the components of the gas mixture,% vol. ± 0.001 2. The instability of the flow rate of the components of the gas mixture due to a decrease in pressure in the cylinders at the gas flow rate and inaccuracy of flow control ± 1.5 3. The total flow rate of the gas mixture, nl / h ± 3

Test durations ranged from 3 to 8 hours with a temperature rise from 25 ° C to 450 ° C or to a temperature corresponding to ~ 90% degree of afterburning of CO. The test results are shown in Fig. 4.

It can be seen in Fig. 4 that the oxide of platinum coating with a substrate based on nanostructure boehmite with a specific surface area of 170.2 m ² / g has a high catalytic activity (line 1); it slightly exceeds the platinum coating on the same substrate (line 2). The oxide coating on a boehmite substrate with a specific surface area of 78.4 m ² / g (lines 3 and 4) is more active than the serial one using platinum (line 5). The coating on a ceramic carrier without a substrate (line 6) has insufficient activity and is not suitable for use.

A comparison of curves 1, 4, and 6 of catalytic coatings of the same composition, but on substrates with different specific surfaces, shows that their activity largely depends on the structure of the substrate.

INFORMATION SOURCES

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Claims (2)

1. A method of producing substrates and highly active catalytic coatings on ceramic blocks for exhaust gas converters of automotive diesel engines, including heat treatment of a carrier, surface modification with an emulsion of alumina powders, heat treatment to form a substrate, and then applying a catalytic coating, characterized in that for the purpose for producing free-activity catalytic coatings with high activity, carriers calcined for organics removal are immersed in a 4-5% suspension of ok aluminum hydroxide (boehmite) with a high-temperature binder - aluminum nitrate 1 ... 3%, with a dried coating, the blocks are calcined for 6 hours at a temperature of 500-550 ° C, immersed for a specified time in a solution of metal salts of variable valency in a mixture of organic solvents with a suspension of titanium dioxide and boehmite, removed, dried with a stepwise rise in temperature from 50 to 120 ° C, the catalytic coating is repeated with the calculated number, dried, heat treated at 600-650 ° C for 240 minutes to convert metal salts to oxides. 2. The method according to claim 1, characterized in that in order to prepare a solution for the catalytic coating, salts are used, g / l: nitric acid copper - 20 ... 23, cobalt nitrate - 20 ... 25.

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