RU2329100C2 - Method of obtaining oxide catalysts on a substrate - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention pertains to the method of obtaining porous substances on a substrate for catalytic applications, to the method of obtaining porous catalysts for decomposition of N₂O and their use in decomposing N₂O, oxidising ammonia and reforming methane with water vapour. Description is given of the method of obtaining porous substances on a substrate for catalytic applications, in which one or more soluble precursor(s) metal of the active phase is added to a suspension, consisting of an insoluble phase of a substrate in water or an organic solvent. The suspension undergoes wet grinding so as to reduce the size of the particles of the substrate phase to less than 50 mcm. The additive is added, which promotes treatment before or after grinding. A pore-forming substance is added and the suspension, viscosity of which is maintained at 100-5000 cP, undergoes spray drying, is pressed and undergoes thermal treatment so as to remove the pore-forming substance, and is then baked. Description is also given of the method of obtaining porous catalysts on a substrate for decomposing N₂O, in which a soluble cobalt precursor is added to a suspension of cerium oxide and an additive, promoting treatment, in water. The suspension is ground to particle size of less than 10 mcm. A pore-forming substance, viscosity of which is regulated to approximately 1000 cP, is added before the suspension undergoes spray drying with subsequent pressing. The pore-forming substance is removed and the product is baked. Description is given of the use of the substances obtained above as catalysts for decomposition of N₂O, oxidation of ammonia and reforming of methane with water vapour.

EFFECT: obtaining catalysts with homogenous distribution of active phases and uniform and regulated porosity for optimisation of characteristics in catalytic applications.

Description

Field of Invention

The present invention relates to a universal method for producing granular catalysts with different active phase distributions and controlled porosity. The requirements for the distribution of active phases and controlled porosity are determined by the characteristics inherent in a given catalytic reaction (application), and may vary in accordance with the proposed method.

Description of the Related Art

The manufacture of the catalyst is carried out in several rather complex stages, depending on a large number of variables that are difficult to control simultaneously.

Traditionally, large-scale catalyst fabrication involves two main approaches. The first is a coprecipitation method. The second involves the impregnation of the previously obtained carrier (substrate) with a precursor of the active phase either by the initial moisture method or by ion exchange method.

In the manufacture of the catalyst by coprecipitation, the components of the catalyst are obtained in the form of a solution and mixed in appropriate ratios and concentrations. The catalyst precursor phase, typically a carbonate, hydroxide or oxide, is obtained by adding a precipitating agent to the catalyst solution. Typically, alkali metal hydroxides or carbonates are used as precipitant. The critical parameters are mixing, pH control and temperature. The catalyst precursor slurry is maintained so that it acquires a more favorable morphology or structure. Flocculants can be added to facilitate filtration or centrifugation, and the solid precursor is separated from the liquid phase.

The precursor is washed to remove unwanted cations or anions. It may be necessary to re-suspend and conduct ion exchange to remove adsorbed alkali metal ions. The precursor is dried and then calcined to obtain an oxide phase. At this stage, the catalyst may be resuspended to precipitate additional phases.

If the granules are to be formed by compression in a mold, the oxide is granulated to obtain a flowing powder, for example, by resuspension and spray drying in either a fluidized bed or a rotary granulator. Additives for processing, for example, lubricants, and then the granulate is formed into granules, and then heat treatment is carried out to give the granules the necessary strength.

The main typical operations for preparing a catalyst by impregnation are as follows.

Impregnation by initial moisture:

A catalyst support is prepared. This process involves many single operations, including either the extrusion and heat treatment of the paste paste, or the granulation, tabletting and heat treatment of the paste paste. The active phase is obtained in the form of a solution. The substrate phase is impregnated with a solution of the active phase. The impregnated granules are dried and heat treated.

The loading of the active metal can be controlled by changing the concentration of the metal precursor in the solution. If a high concentration of the active phase is required or if the solubility of the precursor is low, it may take several cycles of impregnation - heat treatment. In addition, it is difficult to accurately determine the metal content. Several cycles may also be required when it is necessary to obtain bimetallic catalysts. The most important step in this process is the drying process. If it is desired to achieve a uniform distribution of the active phase, rapid drying is required. It is difficult to implement in a large-scale way. Slow drying leads to segregation of the active phase on the outer side of the granule and an uneven distribution of the active phase. This may adversely affect the catalytic performance of this substance.

Ion Exchange:

A catalyst support is prepared. The substrate is immersed in a dilute solution of the active phase. The pH is adjusted such that ion exchange of the active phase occurs with surface groups on the surface of the substrate. The granules are washed to remove active phase ions not adsorbed on the surface of the substrate, and then heat treated. The active phase will tend to be evenly distributed throughout the granule, but its concentration will be low. The maximum concentration of the active phase that can be obtained will be limited by the ion exchange capacity of the substrate phase and / or the saturation solubility of the solution of the active phase.

Other methods for producing oxide catalysts are also known.

US Pat. No. 6,107,238 discloses a method for producing a catalyst with increased abrasion resistance, comprising an oxide-enriched surface layer, comprising preparing a suspension containing catalyst particles, a catalyst precursor or catalyst support (e.g., transition metal oxides), a colloidal oxide sol (e.g., colloidal silica) and a solution of a solvent and a solute in which the solute consists essentially of a precursor of said enriched oxide the surface of a particle size not greater than 5 nm (e.g., aqueous silicic acid or polysilicic acid) and then spray drying the slurry to form porous microspheres of the catalyst which is resistant to abrasion, and calcining the microspheres after spray drying. Such a catalyst is especially useful in oxidation processes in which the oxidation is carried out by means of an oxidized form of the catalyst and the resulting reduced form of the catalyst is separately regenerated (for example, two-stage vapor-phase processes carried out in solid-state recycling reactors, bed-transfer reactors or fluidized-bed circulation reactors, etc. .P.).

US patent No. 6130184 describes a method for producing a cobalt-containing catalyst or catalyst precursor comprising (a) a process of mixing (1) titanium oxide or titanium oxide precursor, (2) a liquid and (3) a cobalt compound that is at least partially insoluble in the amount of liquid used to form a mixture; (b) molding and drying the mixture thus obtained; and (c) calcining the resulting composition. The mixing process may be stirring, grinding or kneading depending on the solids content of the mixture. The molding of the catalyst may include tabletting, granulation, extrusion, spray drying, or a hot oil drip method. This method does not lead to a catalyst with desired properties, in particular, with a controlled distribution of metal centers and porosity. Regulation of these parameters is essential for optimizing performance in catalytic applications.

Object of invention

The object of the invention is to provide a simple method for producing catalysts in general, suitable for industrial use. The next object is to obtain oxide catalysts on a substrate with a certain distribution of the active phase and controlled porosity. Another object is a reproducible and simple method for producing N ₂ O decomposition catalysts. Another object is the use of the obtained catalysts in various catalytic processes.

These and other objects of the invention are implemented by the method described below. In addition, the invention is characterized by the claims.

The invention will be further illustrated by figures 1-9, in which:

FIG. 1 shows a comparison of unit operations in a conventional coprecipitation method for producing oxide catalysts on a substrate and in a spray application method (SDM) described herein.

Figure 2 shows microphotographs obtained by transmission electron microscopy (TEM) of Co / CeO ₂ catalysts obtained by SDM with different metal fineness using (A) a soluble cobalt precursor (high dispersion) or (B) an insoluble cobalt precursor (low dispersion).

Figure 3 represents the porosity of the Co / CeO ₂ catalysts (cylindrical granules 5 mm × 5 mm) obtained by SDM, depending on the starch content (relative to the content of CeO ₂ ) after sintering for six hours at various temperatures.

Figure 4 represents the influence of the porosity of the catalyst on the diffusion coefficient of gas and the radial crushing strength of Co / CeO ₂ catalysts (cylindrical granules 5 mm × 5 mm) obtained by the SDM method.

Figure 5 shows the N ₂ O conversion as a function of temperature in laboratory tests of Co / CeO ₂ catalysts (particles 125-200 μm) obtained by different methods: (a) SDM with a soluble cobalt precursor, (b) SDM with an insoluble cobalt precursor, and (c) the initial moisture method. Experimental conditions: feed = 3 mbar N ₂ O, 20 mbar NO _x , 80 mbar O ₂ , 50 mbar H ₂ O, total pressure = 2 bar, hourly gas space velocity = 140,000 h ^-1 .

6 represents NO selectivity and N ₂ O formation as a function of operating time during the oxidation of ammonia in pilot tests of Co / CeO ₂ catalysts (3 mm × 3 mm cylindrical pellets) prepared by SDM with a soluble cobalt precursor and an insoluble cobalt precursor . Experimental conditions: feed = 10.5 vol.% NH ₃ in air, temperature = 900 ° C, total pressure = 5 bar, hourly gas space velocity = 60,000 h ^-1 .

Fig.7 represents the conversion of N ₂ O depending on the operating time in pilot tests of Co ₂ AlO ₄ / CeO ₂ catalysts (cylindrical granules 5 mm × 5 mm) manufactured by SDM with soluble cobalt precursor with or without added corn starch. Experimental conditions: feed = 6.5 mbar N ₂ O, 50 mbar NO _x , 30 mbar O ₂ , 80 mbar H ₂ O, temperature = 900 ° C, total pressure = 5 bar, hourly gas space velocity = 100000 h ^-1 .

Fig. 8 represents the conversion of N ₂ O as a function of operating time in pilot trials of Co ₂ AlO ₄ / CeO ₂ catalysts (5 mm x 5 mm cylindrical pellets) prepared by SDM with soluble Co precursor and corn starch, as well as coprecipitation. Experimental conditions: feed = 6.5 mbar N ₂ O, 50 mbar NO _x , 30 mbar O ₂ , 80 mbar H ₂ O, temperature = 900 ° C, total pressure = 5 bar, hourly gas space velocity = 100000 h ^-1 .

Fig. 9 shows the activity of Ni / Al ₂ O ₃ catalysts manufactured by SDM in a methane steam reforming reaction in a laboratory test and comparison with a Ni based industrial catalyst (G91-HGS, Sud-Chemie). Tests were carried out on catalyst particles of 300-500 microns. Experimental conditions: temperature = 650 ° C, feed = different ratios of H ₂ O / CO ₂ / CH ₄ , total gas flow = 125 nl h ^-1 , 2 g of catalyst + 10 g of α-Al ₂ O ₃ (diluent), total pressure = 25 bar, mass hourly space velocity = 1000 nl h ^-1 g ^-1 , pre-treatment of the catalyst = pure H ₂ at 600 ° C for 12 hours.

Thus, the invention relates to a method for producing porous substances on a support for catalytic applications, in which one or more soluble active phase precursor (s) are added to a suspension consisting of an insoluble phase of the support in water or in an organic solvent. The suspension is crushed to reduce the particle size of the substrate (carrier) phase to a value of less than 50 μm, processing aids are added before or after grinding, the pore-forming substance is added and the suspension is spray dried, compressed and thermally treated to remove the pore-forming substance. After this, the substance is sintered and preferably subjected to pretreatment with hydrogen, a reducing gas or an inert gas.

Preferably, the suspension is wet milled and the particle size is reduced to 0.1-10 μm, preferably 1 μm. The viscosity of the suspension is maintained at 100-5000 cP, preferably about 1000 cP. The pore-forming substance is preferably cellulose, starch or polymer fibers, is removed at a pressure of 10-20 mbar O ₂ diluted with an inert gas at 300-400 ° C. The heating rate is preferably selected as low as possible, usually 1 ° C · min ^-1 .

The substrate (carrier) phase is one or more components that are selected from Al ₂ O ₃ , SiO ₂ , ZrO ₂ , TiO ₂ , ZnO, MgO, MgAl ₂ O ₄ , CoAl ₂ O ₄ and CeO ₂ . The metal of the active phase is selected from transition metals (e.g., Co, Ni, Cu, Fe), noble metals (e.g., Pt, Rh, Ru), lanthanides (e.g., La, Ce), as well as other elements of the Periodic Table that may act as stabilizers or promoters (alkaline earths, for example, Na, Mg, K, Cs) or other trivalent non-reducing metals (for example, Al, Ga). The active phase precursor is one or more compounds selected from acetates, nitrates, sulfates, oxalates, acetylacetonates, preferably acetates. The cations of the active phase can be complexed with the organic phase before being added to the suspension. It is also possible to carry out partial complexation of cations, which is carried out to prevent the formation of large crystals. Processing aid additives may include dispersants, binders, plasticizers, lubricants, pH modifiers, etc.

The invention also relates to a method for producing porous catalysts on a N ₂ O decomposition substrate, in which a soluble cobalt precursor is added to a suspension of cerium oxide and processing aids in water. After that, the suspension is crushed to a particle size of less than 10 μm, a pore-forming substance is added, the viscosity is adjusted to about 1000 cP before the suspension is spray dried, then pressed, the pore-forming substance is removed and the product is sintered. Zirconia and / or a soluble aluminum compound may be added. It is preferable to use cobalt acetate as a precursor.

The substance obtained according to the method can be used, for example, as a catalyst for the decomposition of N ₂ O, the oxidation of ammonia or the reforming of methane with water vapor.

DETAILED DESCRIPTION OF THE INVENTION

The present invention discloses a new simple, inexpensive, reproducible, universal method that is easy to use on a large scale to produce high-quality porous substances on a substrate for catalytic applications. The authors called it spray application method (SDM). Figure 1 presents a comparison of single operations in the SDM method and the conventional coprecipitation method.

The main novelty of the SDM method is the introduction of one basic operation, i.e. spray drying, for the simultaneous conduct of a large number of operations for the manufacture of the catalyst:

i) applying the metal precursor (s) to the substrate uniformly in one step;

ii) drying the catalyst precursor (avoiding subsequent solid-liquid separation operations, such as filtration, centrifugation);

iii) the introduction of processing aids (dispersants, binders, plasticizers, lubricants, pH modifiers, etc.);

iv) granulating the powder to obtain an easily compressible powder.

After choosing the active form (compounds) and substrate for it, the task arises to build from the precursors of these active forms a catalytic structure with properties and characteristics that will meet the requirements of an industrial customer. It has been shown that the scale of production of N ₂ O decomposition catalysts by the spray application method is estimated at many tons. Methods such as coprecipitation and impregnation / ion exchange lead to many serious technical problems when scaling compared to SDM. Other methods, such as sol-gel, grafting (adhesion), complex heterogenization, and deposition-deposition have so far been applied only on a laboratory scale and only for a limited number of formulations. Catalysts synthesized by these methods are not manufactured on an industrial scale.

The soluble precursor of the active phase is first added to the suspension, consisting of the phase of the substrate in water or another organic solvent in appropriate quantities and concentrations. The suspension is subjected to wet grinding in a mill with a continuously stirred medium or a ball mill to reduce the particle size of the substrate phase, preferably to less than 1 μm, thereby achieving a stable dispersion. To prepare the catalyst for molding, processing aid additives such as dispersants, binders, plasticizers, lubricants and pH modifiers can be added before or after grinding. After grinding, a pore-forming substance such as starch, cellulose or polymer fibers can be added to the suspension. The suspension is spray dried to obtain granules suitable for molding by compression molding, i.e. with a size distribution of from 50 to 400 microns.

The extremely fast drying achieved by spray drying allows one to obtain an active phase precursor deposited on the surface of the substrate phase uniformly throughout the spray dried granule. This stage plays a decisive role in the occurrence of close contact between the active phase and the precursor, which leads to a uniform distribution of metals in the final catalyst. Then the substance is pressed. The removal of pore-forming substances should be carried out in a controlled atmosphere (10-20 mbar O ₂ diluted with an inert gas such as N ₂ , He or Ar). The thermal decomposition of the pore-forming phase in an oxidizing atmosphere can be catalyzed by the metal of the active phase, leading to thermal removal. This occurs not only during the oxidation of starches and cellulose, which proceeds exothermally. Many polymer processing aids that decompose endothermally even when heated in air, such as polymethyl methacrylate (PMMA), decompose exothermically when heated by contact with the catalytic phases. The proposed method of removing pore-forming substances is also considered as a new aspect of the present invention, which is generally applicable in the manufacture of catalysts. After this process, the substance is sintered at high temperature and get the final catalyst. The uniformity of the distribution of the active phase in the granules, dried by spraying, is preserved in the final product - granules of the catalyst.

The spray application method requires 3-5 unit operations, i.e. less than traditional methods, such as coprecipitation and impregnation (see figure 1). Some single operations in the new method are multi-purpose, which makes it very attractive (see above).

The spray application method makes it possible to obtain a wide range of catalysts on a substrate. The only requirement is to have a soluble active phase and an insoluble support. There are many possibilities to find suitable metal precursors. You can use metals of various nature from the Periodic table of the elements, while other methods have great limitations.

Traditional substrates used in catalysis satisfy this requirement (for example, Al ₂ O ₃ , SiO ₂ , ZrO ₂ , TiO ₂ , ZnO, CeO ₂ , MgO, MgAl ₂ O ₄ , CoAl ₂ O ₄ ). The active phase is selected from transition metals (e.g., Co, Ni, Cu, Fe), noble metals (e.g., Pt, Rh, Ru), lanthanides (e.g., La, Ce), as well as other elements of the periodic table, which can act as stabilizers or promoters (alkaline earths, for example, Na, Mg) or Al, Ga. Acetates, nitrates, sulfates, oxalates or acetylacetonates are preferred. Another feature of the spray application method is the ability to obtain polymetallic catalysts with a high dispersion of components. This makes it attractive, for example, for bifunctional catalysis or promoted reactions. Classical co-impregnation or sequential impregnation with two metal salts, as has been shown, is not effective for achieving close interaction of two metals. New methods are also being tested. In all cases, they are looking for a method for producing a monometallic catalyst (i.e., a starting catalyst), which is then modified by the addition of a second metal. This modification is carried out through a selective reaction, which proceeds only on monometallic particles of the initial catalyst. The SDM method offers, through single-tank operation, a simple method to overcome this difficult and labor-intensive manufacturing process, achieving very close contact of the introduced metals.

For the synthesis of bimetallic phases on a substrate, the method of synthesis by spray application can be further improved by binding the active phase to the organic phase complex of cations before adding the substrate to the suspension. This is a modification of the well-known Pechini method for producing mixed metal oxides, in which cations bind to complexes with organic molecules that undergo a polymerization process. This approach leads to the formation of a precursor phase containing a chaotic dispersion of metal ions, which is then applied to the substrate during spray drying. This provides the formation of a phase of mixed oxides at low temperatures. Suitable complexing agents include citric acid, glycine, EDTA and the like.

At a high concentration of the active phase during drying, large crystals of the active phase can form in the granule. This can be avoided by adding a small amount of a monodentate complexing agent, for example triethanolamine (TEM). This amount should be less than the amount sufficient for complete binding to the cation complexes of the active phase, preferably sufficient for the binding of each cation to only one complexing agent molecule. The result of this partial complexation of the cation is that it prevents the formation of large crystals of the active phase precursor due to interruption of the crystal lattice.

The SDM method is universal in that it can be continuous or batch, which contrasts with high requirements to make the coprecipitation method continuous. Zooming from a pilot installation to full-scale production is relatively simple to implement as part of the SDM method, which has been demonstrated. The most important single operations, slurry grinding and spray drying, are standard operations in ceramic processing and are well understood.

The spray application method is well suited to achieve uniform distribution of catalyst components or to create precursors with a specific stoichiometry. The final catalyst consists of small and dispersed mixed crystallites of the components due to good contact of the active phase (for example, in bimetallic systems) and the substrate before and after the spray drying / coating step. Co-precipitation also leads to high dispersion, but the technological requirements here are much higher, as are the difficulties of the quality of the precipitated product and the problems of maintaining a constant quality of the product during the entire precipitation process. The superior characteristics of the spray supported catalyst compared with other methods have been shown to decompose N ₂ O in the presence of Co-based catalysts.

The SDM method also allows you to accurately design the macrostructure of the resulting catalyst (pore size and pore volume) to achieve optimal performance. Strong granules with a porosity of 70% were obtained by this method.

The method of producing catalyst

The following describes a method for producing a catalyst, called "Spray Application Method" and disclosed in the claims. The preparation method is described with reference to the preparation of a N ₂ O decomposition catalyst containing a pore-forming phase. Other catalytic applications are also highlighted. The influence of the pore-forming phase on the properties of the catalyst to gas transfer is also described. Each unit operation in the SDM method described in FIG. 1 for the Co-Al / CeO ₂ catalyst is described in detail below.

Suspension

An aqueous suspension of cerium oxide or another phase of the substrate is prepared by adding to the water a freshly obtained cerium oxide powder with an agglomerate size of d ₅₀ , which is usually in the range of 10-20 μm. Vigorous stirring is used to disperse the suspension and prevent sedimentation. Polyvinyl alcohol (PVA) is added to this suspension in the form of a 5-10 wt.% Aqueous solution. Generally, to dissolve PVA, the PVA solution needs to be heated. After adding PVA to the suspension, it was stirred for 12 hours. In this case, PVA is adsorbed on the surface of cerium oxide. The role of PVA is twofold. In suspension, it acts as a steric stabilizer of cerium oxide, reducing the tendency to sedimentation. At a later stage of manufacture, it acts as a temporary binder for spray dried granules.

With vigorous stirring, cobalt acetate, aluminum hydroxyacetate and zirconium oxide are added to the cerium oxide suspension. For some formulations, nitrate or other salts may be used. The suspension is crushed using a ball mill or a mill with a continuous stirred medium to reduce the size of the cerium oxide agglomerates, so that d _{50 is} reduced to 0.1-10 μm (preferably 1 μm). This allows you to get a suspension that is resistant to sedimentation, which is a practical advantage and can improve the characteristics of the catalyst due to the improved uniformity of the catalyst. The viscosity of the suspension should be maintained at a level of 100-5000 cP, preferably about 1000 cP.

Pore-forming substance

A pore-forming substance is added to the stable suspension. Corn starch with an average particle size of 10-15 microns is used. The pore size can be adjusted by selecting starch. Other starches, such as rice or potato, can be used to produce smaller or larger pores, respectively. Other pore-forming substances other than starch may also be used. In the manufacture of the catalysts by the SDM method, Avicel microcrystalline cellulose was successfully used.

In principle, any substance that is insoluble and thermally decomposes without leaving an inorganic residue can be used. In the past, microcrystalline cellulose was used as an alternative to starch, although this did not give particular advantages. You can also use polymer balls or fibers, but if they melt before thermal decomposition, this can cause difficulties in heat treatment. Starch is added after grinding, so that the structure of the starch grains, which determine the pore size, is maintained. The water content in the suspension is set so that the viscosity of the suspension is 1000 cP.

Spray drying

Spray drying was carried out in a pilot plant using a two-fluid atomizer and centrifugal atomizers. Full-scale spray drying is carried out using both centrifugal sprayers and high-pressure sprayers. In each case, there are no serious problems, and always get a good flowing powder. In full-scale production, it was found that when using a high-pressure spray gun in countercurrent mode, too high an inlet temperature can lead to powder ignition. In this case, inlet temperatures above 400 ° C are used. The counterflow model is not ideal for heat sensitive substances. Therefore, either the inlet temperature should be lower, which will lead to a decrease in the production rate, or it is necessary to use a model of spraying in parallel flows.

In our pilot tests, the inlet temperature was usually from 250 to 180 ° C, depending on the feed rate of the suspension, and the outlet temperature was usually in the range 105-110 ° C. This led to a moisture content of 0.2-0.5% and an average granule size d _{50 of} 60-80 microns.

Pressing

Before pressing, 0.5% aluminum stearate was added to the spray dried powder, and the water content was adjusted to 0.5-2%. This operation was carried out in a Forberg mixer. Water acts as a plasticizer for PVA, and the optimum water content depends on the exact composition of the catalyst. The amount of water required depends on the surface area of cerium oxide and the degree of purity of the PVA. The pellets were pressed by uniaxial pressing on a Pentronix PTX 612 press. For pellets of 5 mm or less, a single-action mold is acceptable, but for the manufacture of larger pellets, for example 9 mm grooved pellets, a double pressing operation is necessary. The compression ratio (the ratio between the filling depth and the height of the pressed pellet) is usually 2.2.

Removal of a pore-forming substance

After pressing, the granules are subjected to heat treatment to remove the pore-forming phase, after which they are sintered for the final preparation of catalyst granules. Heat treatment to remove pore-forming starch is the most critical and critical stage in the manufacture of the catalyst. Heating in air of granules containing starch leads to a rapid rise in temperature in the layer of granules and uncontrolled burning of starch, which negatively affects the integrity of the granules. The heating of the granules in nitrogen, to which a small amount of air is added, regulates the removal of starch. The range of oxygen concentrations in the initial stages is 0.1-0.5 vol.%. At all stages of starch removal, the main starch decomposition products are CO ₂ and water vapor, even with an oxygen content of 0.5%.

After removing the bulk of the starch, as evidenced by the absence of CO ₂ in the exhaust gas, the oxygen content is increased to 20 vol.%. Starch consists of two polymers; one of them decomposes around 170 ° C and the other in the range of 250-300 ° C. The heating rate should be as low as possible, usually 1 ° C · min ^-1 . The granules are loaded into perforated baskets and a nitrogen-oxygen gas mixture is passed through a layer of granules from below. Before blowing through the granules, the gas is heated. The temperature of the granules is controlled by gas temperature and oxygen concentration. During this operation, the temperature of the layer is more sensitive to oxygen concentration than the temperature of the inlet gas, so that it is the means by which the temperature is controlled during decomposition. In the first stage, the granules are heated to 170 ° C in a gas stream containing 0.1 vol.% Oxygen. Once a stable temperature is reached, the oxygen concentration is slowly increased. The development of the reaction can be monitored by the rise in temperature in baskets arranged vertically in series. As soon as the first polymer of starch decomposes, the oxygen concentration is reduced and the temperature is raised to 250 ° C. Upon reaching a stable temperature, the oxygen concentration is again increased until the second decomposition stage is completed in all baskets. The temperature is increased to 300 ° C. and the oxygen concentration is increased to 20% until the reaction stops. The baskets are cooled overnight in a stream of oxygen. The total process time is usually 2.5 days.

After removal from the furnace for burning out the pore-forming substance, the baskets are placed in a vertical tube furnace. They are heated in air at a speed of 1 ° C · min ^-1 to 950-1000 ° C. After 6-12 hours at maximum temperature, the oven is cooled. The sintering process usually takes 2 days. Sintered granules represent the final product.

The invention will now be illustrated by the following examples:

Example 1. Obtaining Co / CeO ₂ catalysts with different distribution of active phases and metal loads by SDM method

Example 1a High dispersion cerium oxide supported catalyst

An aqueous suspension of the phase of the cerium oxide support was prepared by adding 5 kg of freshly obtained cerium oxide powder (Rhodia HSA15) with an agglomerate size d ₅₀ usually in the range of 10-20 μm to 5 l of water containing 40 g of PVA (Rhodoviol 25/140). Vigorous stirring was used to disperse cerium oxide, and then it was stirred for 15 hours to allow PVA to be adsorbed on the surface of cerium oxide.

To the cerium oxide suspension, 289 g of cobalt acetate (Fluka) was added with vigorous stirring. The suspension was ground in a mill with a continuously stirred medium to reduce the size of the cerium oxide agglomerates so that the d ₅₀ is less than 1 μm.

The water content in the suspension was adjusted so that the viscosity of the suspension is 1000 cP. Typically, the moisture content in the suspension is 40-50%.

Spray drying can be carried out in a centrifugal spray dryer. The inlet temperature was usually 180-400 ° C, depending on the injection speed of the suspension, and the outlet temperature was usually 105-110 ° C. As a result, the moisture content was 0.2-0.5 wt.% And the granules had a size d ₅₀ from 60 to 200 microns, depending on the type of atomizer.

Prior to compression, 0.5 wt.% Aluminum stearate was added to the spray dried powder, and the water content was adjusted to 0.5 wt.%. This operation was carried out in a Forberg mixer. Water acts as a plasticizer for PVA, and the optimum water content depends on the exact composition of the catalyst. The amount of water required depends on the surface area of cerium oxide and the degree of purity of the PVA. The granules were pressed by uniaxial pressing on a Pentronix PTX 612 press. Corrugated granules with seven channels were obtained (d _m = 9 mm, d _h = 1.7 mm). Granules of various geometries can be obtained, from simple cylinders to multilayer granules. Pressed granules were sintered in air using a conventional muffle furnace. The granules were heated at a rate of 1 ° C · min ^-1 to 900-1000 ° C and kept at maximum temperature for 6-12 hours. The chemical composition of the final catalysts included 1.9 wt.% Co ₂ O ₃ and 98.1 wt.% CeO ₂ .

Example 1b Low dispersion catalyst for cobalt oxide supported on cerium oxide

A suspension of cerium oxide was prepared as described in Example 1a using 5 kg of cerium oxide (Rhodia HSA15) and 5 L of water containing 40 g of dissolved PVA (Rhodoviol 25/140). To an aqueous suspension of cerium oxide was added 93 g of cobalt oxide (Co ₃ O ₄ , Merck) with vigorous stirring. After 1 hour of stirring, to ensure uniform dispersion of cobalt oxide, the suspension was crushed in a ball mill to reduce the size of cerium oxide agglomerates to a d ₅₀ value of less than 1 μm. The water content in the suspension was adjusted so that the viscosity of the suspension was 1000 cP. Spray drying, adding a lubricant, pressing granules and sintering was carried out as described in example 1A. The chemical composition of the final catalyst included 2.0 wt.% Co ₂ O ₃ and 98.0 wt.% CeO ₂ .

Figure 2 shows the different composition of the catalysts relative to the active phase, depending on the nature of the cobalt precursor used in the preparation. The use of a soluble cobalt precursor (cobalt acetate, Example 1a) leads to a highly dispersed catalyst (FIG. 2A). The micrograph obtained by high resolution transmission electron microscopy does not show cobalt oxide particles, and this shows that cobalt oxide particles have a size of less than 1 nm. When using an insoluble cobalt precursor (Co ₃ O ₄ , Example 1b), large agglomerates of cobalt oxide are observed on a CeO ₂ substrate (FIG. 2B), which indicates a lower dispersion of the active phase in the final catalyst. The different composition of cobalt will undoubtedly affect the catalytic characteristics of the substances, as shown in examples 4 and 5.

Example 2. Obtaining a catalyst Ni / Al ₂ O ₃ method SDM

The nickel oxide catalyst supported on gamma-alumina can be obtained by the method described in example 1A. 40 g of PVA (Rhodoviol 25/140) was dissolved in 5 L of water. With vigorous stirring, 5 kg of gamma-alumina (Sigma-Aldrich) was added. The suspension was stirred for 15 hours and then 3.175 kg of nickel nitrate (Fluka) was added to the suspension of alumina. After stirring for 1 hour, the suspension was crushed using a ball mill to a d ₅₀ value of 1 μm. After grinding, the moisture content in the suspension was 42.9% and the viscosity was 960 cP, which is suitable for spray drying. The suspension was spray dried and granulated according to the procedure described in example 1a. The granules were sintered in a standard muffle furnace in air. The granules were heated at a rate of 1 ° C · min ^-1 to 700 ° C and kept at this temperature for 6 hours. The chemical composition of the final catalysts included 16.6 wt.% NiO and 83.4 wt.% Al ₂ O ₃ .

Example 3. Obtaining catalysts Co / CeO ₂ with different porosity by the SDM method

The SDM method of producing catalysts allows to obtain catalysts with controlled volumes and pore sizes. This is achieved by introducing an unstable pore-forming phase, for example starch, cellulose or polymer, into the catalyst slurry before spray drying. After pressing the spray-dried granules into granules or tablets, the pore-forming phase is removed by controlled heat treatments.

An aqueous suspension of cerium oxide and cobalt acetate, as described in example 1A, was prepared and crushed to the desired particle size. An unstable pore-forming phase, corn starch (Collamyl), was added to the suspension with rapid stirring. The suspension containing the phases of the catalyst and the pore-forming phase of corn starch is spray dried according to the procedure described in example 1b. After adding a lubricant, aluminum stearate, and controlling the moisture content of the spray dried granules, granules or tablets were obtained by uniaxial compression in the form as described in Example 1a.

To create an adjustable porosity while maintaining the mechanical integrity of the granules, the pore-forming phase must be removed in an adjustable manner. This is achieved by thermal oxidation of the pore-forming substance at a low partial pressure of oxygen. The atmosphere consists of air diluted with nitrogen, so that the oxygen concentration is 0.1-2 vol.%. Corn starch consists of two different polymers - amylose and amylopectin. Amylose consists of linear chains of alpha-linked glucose, and amylopectin consists of a branched polymer of alpha-linked glucose. Typically, corn starch consists of 75% amylopectin and 25% amylose. Both polymers decompose at 240 and 380 ° C, respectively, in contact with solids, such as alumina. In the presence of an oxidation catalyst, the decomposition temperature can be lowered by 80-100 ° C. Burning corn starch from a cobalt / cerium oxide catalyst obtained by the SDM method involves slowly heating the surface layer of the granules to 170 ° C. During the heating process, a gas consisting of air diluted with nitrogen is passed through the layer, so that the oxygen concentration is below 1 vol.%. A low oxygen concentration limits the temperature rise in the granule layer due to the oxidation of amylopectin to less than 50 ° C. After the decomposition of amylopectin is completed, as evidenced by the low level of carbon oxides in the exhaust gas, the temperature is slowly increased to 300 ° C and this temperature is maintained until the amylose is completely decomposed. After starch is removed, the granules are heated in air to 900-1000 ° C for sintering.

The starch content in the suspension ranged from 0 to 25 wt.% Based on the weight of cerium oxide. It is possible to produce catalyst granules with a higher pore-forming substance content, but the mechanical properties of the granules after removal of the pore-forming substance are very low.

By varying the amount of pore-forming phase, you can adjust the pore volume. Figure 3 shows the effect of starch content on the porosity of the granules in the Co / CeO _{2 system} (nominal composition: 2 wt.% Co ₂ O ₃ and 98% CeO ₂ ) at various sintering temperatures. As the starch content in the composition exceeds 9%, the porosity of the granules increases. Two properties are critically determined by the porosity of the catalyst: the effective diffusion coefficient inside the granules compared to the diffusion coefficient of gas in free space, and the strength of the granules. These two properties for the Co / CeO _{2 system} are shown in FIG. 4. The increase in porosity leads to an increase in the effective diffusion coefficient in the granule with reduced strength. Therefore, the optimum depends on the specific application.

When producing granules or tablets by compression or extrusion in the absence of a pore-forming phase, porosity in the range of 40-55% is obtained, provided that the sintering temperatures are not very high. The pore size is determined by the size of the primary particles, which depends on the degree of grinding. The more efficient the grinding process, the smaller the pore size between the particles. However, if the particle size is less than one micron, a large proportion of porosity will be less than 0.1-0.2 microns. By introducing a pore-forming phase, both porosity and pore size can be increased.

Example 4. Characteristics of the catalysts Co / CeO ₂ in the direct decomposition of N ₂ O

Catalysts Co / CeO ₂ (nominal composition: 2 wt.% Co ₂ O ₃ and 98% CeO ₂ ) in the form of particles with a size of 125-200 μm, manufactured by different methods, were tested in the decomposition of N ₂ O. The results are shown in figure 5 . A clear correlation is seen between the activity of the catalysts and the dispersion of the cobalt phase. The catalyst obtained by the SDM method using cobalt acetate as a precursor shows the highest activity due to the high degree of dispersion of cobalt in the catalyst. The catalyst obtained by the SDM method from cobalt oxide as a precursor showed significantly lower activity due to the low dispersion of the active phase forming large cobalt oxide agglomerates on a CeO ₂ substrate (see FIG. 2B). The Co / CeO ₂ catalyst obtained by the initial moisture method using cobalt acetate leads to the lowest activity, which correlates with the low dispersion achieved by this method.

Example 5. The influence of the cobalt precursor on the characteristics of the catalyst Co / CeO ₂ in the oxidation of ammonia

Catalysts Co / CeO ₂ in the form of cylindrical granules of 3 mm × 3 mm in size, obtained by the SDM method using soluble and insoluble cobalt precursors, were tested in the oxidation of ammonia (Fig.6). Both catalysts had a chemical composition: 20 wt.% Co ₂ O ₃ and 80 wt.% CeO ₂ . Catalysts showed similar initial NO selectivities (~ 85%). However, the initial formation of N ₂ O on the catalyst obtained using cobalt acetate as a precursor was significantly lower compared to the catalyst obtained using cobalt oxide as a precursor. Moreover, both catalysts vary greatly in stability. The Co / CeO ₂ catalyst obtained using the insoluble cobalt precursor deactivates much faster, which leads to lower NO selectivity and increased formation of N ₂ O.

Example 6. The influence of the porosity of the granules on the characteristics of the catalysts Co ₂ AlO ₄ / CeO ₂ in the decomposition of N ₂ O

CoAl ₂ O ₄ / CeO ₂ catalysts were prepared by the SDM method using cobalt acetate as a precursor with or without corn starch. Both catalysts had a chemical composition: 1 wt.% Co ₂ O ₃ , 0.3 vol.% Al ₂ O ₃ and 98 wt.% CeO ₂ . Catalysts in the form of cylindrical granules of 5 mm × 5 mm were tested in direct decomposition of N ₂ O to evaluate the effect of the porosity of the granules on the activity during decomposition of N ₂ O. The results are shown in Fig.7. The addition of starch has proven to have a beneficial effect on performance; a starch catalyst (granule porosity of 65%) showed a slightly higher N ₂ O conversion than a catalyst with 15 wt.% starch (granule porosity of 50%).

Example 7. The influence of the method of producing catalysts Co ₂ AlO ₄ / CeO ₂ on the decomposition of N ₂ O

The CoAl ₂ O ₄ / CeO ₂ catalyst was obtained by the SDM method using cobalt acetate as a precursor and with the addition of corn starch (15 wt.% Based on the weight of CeO ₂ ). The final catalyst had a chemical composition: 1 wt.% Co ₂ O ₃ , 0.3 vol.% Al ₂ O ₃ and 98 wt.% CeO ₂ . In addition, the CoAl ₂ O ₄ / CeO ₂ catalyst was prepared by coprecipitation from a solution of Co, Al and Ce precursors (soluble) with a solution of sodium hydroxide base and sodium carbonate. The coprecipitated catalyst had a chemical composition: 42 wt.% Co ₂ O ₃ , 13.9 wt.% Al ₂ O ₃ and 42.7 wt.% CeO ₂ . To evaluate the activity in the decomposition of N ₂ O, catalysts in the form of cylindrical granules of 5 mm × 5 mm were used. On Fig a comparison of the results. The catalysts obtained by the SDM method exhibit high initial activity and stability throughout the work. Significant deactivation (~ 20% of the initial activity after 30 days) of the catalyst obtained by coprecipitation is observed. The high dispersion of cobalt in cerium oxide facilitates a strong interaction between the active phase and the substrate, which stabilizes the composition. This cannot be achieved by conventional methods, such as coprecipitation, as shown in the figure.

Example 8. Characteristics of Ni-based catalysts in methane reforming

The activity of the Ni / Al ₂ O ₃ catalyst obtained by the SDM method according to Example 2 in methane reforming at various feed ratios of H ₂ O / CO ₂ / CH _{4 was} compared with the activity of an industrial reforming catalyst (G91-HGS, Süd-Chemie). Both catalysts contained similar amounts of nickel: 16.6 wt.% NiO in Ni / Al ₂ O ₃ and 19 wt.% NiO in industrial catalysts. This catalyst also contains other promoters, such as K. Catalytic tests were carried out on particles of 300-500 microns and received the values of their own activity. The activity of the unpromoted Ni / Al ₂ O ₃ catalyst obtained by the SDM method at 650 ° C, expressed as the molar fraction of the product (on a dry basis), is higher than the activity of a modern industrial Ni-catalyst (Fig. 9). Methane conversions of 26% (SDM) and 23% (industrial) were obtained. The activity of the SDM catalyst decreases with increasing relative CO ₂ content in the feed mixture (dry reforming). The activity of an industrial catalyst in a mixture of H ₂ O / CO ₂ / CH ₄ = 1/1/1 is higher than the catalyst Ni / Al ₂ O obtained by the SDM method. This may be due to the presence of promoters in the industrial catalyst, which reduce the rate of coke formation, while other metals than Ni were not present in the SDM catalyst.

The spray application method allows to obtain catalysts with a uniform distribution of the active phase and uniform and controlled porosity with some control of pore size. The superior performance of the catalysts obtained by the SDM method compared to other conventional methods (coprecipitation or impregnation) has been proven in various applications, including direct decomposition of N ₂ O, oxidation of ammonia and methane reforming with water vapor.

Claims (17)

1. A method of producing porous substances on a substrate for catalytic applications, in which one or more soluble (s) precursor (s) of the active phase metal is added to a suspension consisting of an insoluble phase of the substrate in water or an organic solvent, the suspension is wet milled to reduce size particles of the phase of the substrate to less than 50 microns, add additives that facilitate processing before or after grinding, add a pore-forming substance and a suspension, the viscosity of which is maintained at 100-5000 cP, subjected to Call duration dried, pressed and thermally treated to remove the pore-forming substance and sintered. 2. The method according to claim 1, in which the particle size is reduced to 0.1-10 μm, preferably 1 μm. 3. The method according to claim 1, in which the viscosity of the suspension is maintained at about 1000 cP. 4. The method according to claim 3, in which the pore-forming substance is removed at a pressure of 10-20 mbar O ₂ diluted with an inert gas at 300-400 ° C. 5. The method according to claim 4, in which the heating rate is as low as possible, usually 1 ° C · min ^-1 . 6. The method according to claim 1, in which the phase of the substrate is one or more compounds selected from Al ₂ O ₃ , SiO ₂ , ZrO ₂ , TiO ₂ , ZnO, MgO, MgAl ₂ O ₄ , CoAl ₂ O ₄ and CeO ₂ . 7. The method according to claim 1, in which the metal of the active phase is selected from transition metals, for example Co, Ni, Cu, Fe, noble metals, for example Pt, Rh, Ru, lanthanides, for example La, Ce, as well as other elements of the Periodic table which can act as stabilizers or promoters - alkaline earths, such as Na, Mg, K, Cs, or other trivalent non-reducing metals, such as Al, Ga. 8. The method according to claim 7, in which the precursor of the active phase is one or more compounds selected from acetates, nitrates, sulfates, oxalates, acetylacetonates, preferably acetates. 9. The method according to claim 1, in which the pore-forming substance used is cellulose, starch or polymer fibers. 10. The method according to claim 1, in which the additives that contribute to the processing, are dispersants, binders, plasticizers, lubricants, pH modifiers, etc. 11. A method of obtaining porous catalysts on a substrate for the decomposition of N ₂ O, in which a soluble cobalt precursor is added to a suspension of cerium oxide and processing aids in water, the suspension is ground to a particle size of less than 10 μm, a pore-forming substance is added, the viscosity is adjusted to about 1000 cP before the suspension is spray dried, followed by compression, the pore-forming substance is removed and the product is sintered. 12. The method according to claim 11, in which zirconia and / or a soluble aluminum compound are added. 13. The method according to claim 11, in which cobalt acetate is used as a precursor. 14. The use of the substance obtained according to any one of paragraphs.11-13, as a catalyst for the decomposition of N ₂ O. 15. The use of the substance obtained according to any one of claims 1 to 10, as a catalyst for the decomposition of N ₂ O. 16. The use of the substance obtained according to any one of claims 1 to 10, as a catalyst for the oxidation of ammonia. 17. The use of substances obtained according to any one of claims 1 to 10, as a catalyst for methane reforming with water vapor.

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