Preparation method of carbon-based noble metal catalyst

Abstract

The invention discloses a preparation method of a carbon-based noble metal catalyst with high dispersion and small nanometer size. The catalyst is prepared by taking a water-soluble organic matter with a polymerization function as a carbon precursor and a water-soluble noble metal compound as an active component, and the specific preparation process is as follows: A, mixing; b, precipitating; c, sol-gel preparation; d, drying to obtain dry gel; e, cracking; and F, reduction: after the temperature is constant, H2 is introduced into inert shielding gas, and reduction treatment is carried out. The catalyst has the advantages that the catalyst is prepared by taking a water-soluble organic matter with a polymerization function as a carbon precursor and a water-soluble noble metal compound as an active component, a specific preparation process is adopted in the preparation process, and meanwhile, corresponding parameters are strictly controlled; the prepared catalyst has the advantages that the content of noble metal is high, crystal grains are small, active components are uniformly distributed, and microcosmic pore structure parameters of the catalyst can be adjusted according to a xerogel cracking process and specific application.

Description

Technical Field

[0001] This invention relates to a method for preparing a noble metal catalyst, and more particularly to a method for preparing a carbon-based noble metal catalyst with high dispersion and small nanoscale size. Background Technology

[0002] Noble metal catalysts are widely used in industrial production and the treatment of toxic gases due to their excellent catalytic activity and chemical stability. Depending on the application, the supports for noble metal catalysts include porous materials such as activated carbon, activated alumina, titanium dioxide, and molecular sieves. Among these, carbonaceous supports, represented by activated carbon, are one of the most important supports for preparing high-performance noble metal catalysts due to their highly developed pore structure and excellent surface chemical properties.

[0003] The most traditional method for preparing precious metal catalysts is the impregnation method, which uses a shaped porous material as a carrier and impregnates it in a precious metal solution to load the precious metal components onto the carrier. Then, it is prepared by aging, drying, inert component protection calcination, reduction and other steps. For example, in 2003, a Chinese invention patent with publication number CN1436595A, entitled "A Method for Preparing a Palladium / Carbon Supported Noble Metal Catalyst," was published. This patent specifically discloses a method for directly impregnating an activated carbon support by pre-adding hydrogen peroxide or a mixture of hydrogen peroxide and sodium hypochlorite to a palladium salt solution as an oxidant impregnation solution. After impregnation, basic sodium carbonate, bicarbonate, or alkaline earth metal oxides are added as precipitants to deposit the noble metal component. Finally, formaldehyde, hydrazine hydrate, glucose, or hydrogen are added as reducing agents to reduce the catalyst. After washing and drying, an eggshell-shaped noble metal catalyst is prepared. Another example is another Chinese invention patent published in 2003 with publication number CN1425499A, entitled "A Supported Metal Catalyst and Its Preparation Method," which specifically discloses a method for preparing a metal catalyst with a metal particle size of 0.5-6 nm by dispersing the active component, alkali metal or alkaline earth metal hydroxide or carbonate, and the support in a solvent, mixing them in a specific ratio, adding a settling agent, and drying the mixture.

[0004] Impregnation is more suitable for the preparation of catalysts with low content of precious metal components (generally less than 1.0 wt.%). When the loading is large, it is easy to cause the accumulation of precious metal components in the confined space of the support, which is not conducive to the dispersion of active components. After high-temperature calcination and reduction treatment in the later stage, the size of precious metal grains is large and it is easy to cause blockage of the support pores, resulting in poor catalytic activity.

[0005] For the preparation of noble metal catalysts using sol-gel methods, the addition of a noble metal source during the support synthesis process allows the noble metal particles to be highly dispersed on the inner surface of the support under a high loading. For example, in 2011, a method for preparing a sol-gel catalyst for deep removal of trace amounts of hydrogen from industrial CO gas was published (CN 102284286A). This method specifically discloses using palladium chloride as the palladium source, adding aluminum isopropoxide, water, and nitric acid in a certain proportion, forming a sol-gel at a specific temperature, and then drying and calcining to obtain an alumina-based palladium catalyst. However, due to the underdeveloped pore structure and low specific surface area of ​​alumina, the content of the noble metal active component in the prepared catalyst is still insufficient, and the distribution of the active component is uneven. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a method for preparing a carbon-based noble metal catalyst with high noble metal content, small crystal size, uniform distribution of active components, and adjustable micropore structure parameters of the catalyst according to specific applications.

[0007] To address the aforementioned technical problems, the present invention provides a method for preparing a carbon-based noble metal catalyst, using a water-soluble organic compound with polymerization function as a carbon precursor and a water-soluble noble metal compound as the active component. The specific preparation process is as follows:

[0008] A. Mixing: Dissolve the water-soluble organic compound with polymerization function in deionized water in proportion, stir and mix evenly, and then add water-soluble precious metal compound and surfactant.

[0009] B. Precipitation: Under stirring, an alkaline precipitant is added dropwise to the above mixture, so that the noble metal compound forms nano-sized hydroxide colloids suspended in the mixed system. After the noble metal compound is completely precipitated, the precipitant is added dropwise until the pH value of the system reaches 8-10.

[0010] C. Sol-gel: The above suspension is transferred to a glass bottle and sealed. It is then placed in a constant temperature water bath for aging treatment for 8-24 hours to obtain a hydrogel containing precious metal elements. The aging temperature is 50-90℃.

[0011] D. Drying: Place the hydrogel containing precious metal elements in an oven at 40-60℃ and dry for 6-8 hours until the water in the gel network structure is removed to obtain a dry gel.

[0012] E. Pyrolysis: Under the protection of inert gas, the crushed dry gel is heated to 300-600℃ in a tube furnace and kept at the temperature for 1-10 hours to carry out the pyrolysis reaction. The escape of small molecules causes the material to form a well-developed porous structure at the microscopic level.

[0013] F. Reduction: After the pyrolysis reaction is completed, the material is cooled to 100-300℃ under the protection of inert gas. After the temperature is constant, H2 is introduced into the inert protective gas for reduction treatment.

[0014] After the reduction reaction is completed, the H2 supply is stopped, the material is cooled to room temperature under inert gas protection, and then vacuum sealed for storage, thus obtaining the carbon-based noble metal catalyst.

[0015] Furthermore, the water-soluble organic compound with polymerization function mentioned above is a mixture of organic aldehydes and organic phenols.

[0016] Furthermore, the organic aldehydes mentioned are formaldehyde, propionaldehyde, butyraldehyde, glyoxal, or glutaraldehyde, and the organic phenols are phenol, resorcinol, hydroquinone, or hydroxyl.

[0017] Furthermore, the molar ratio of the organic aldehydes and organic phenols is 1:1 to 1:4.

[0018] Furthermore, the precious metal elements in the water-soluble precious metal compounds mentioned are platinum and palladium.

[0019] Furthermore, the soluble palladium noble metal compounds mentioned are palladium nitrate, palladium chloride, palladium acetate, palladium acetylacetonate, tetraaminopalladium nitrate, sodium tetrachloropalladate, potassium tetrachloropalladate, and potassium hexachloropalladate, and the soluble platinum noble metal compounds mentioned are platinum chloride, chloroplatinic acid, ammonium chloroplatinate, potassium chloroplatinate, and platinum acetylacetonate.

[0020] Furthermore, the surfactant mentioned is one or more of polyvinyl alcohol (PVA), Span 20 (SP20), Span 60 (SP60), and Span 80 (SP80).

[0021] Furthermore, the alkaline precipitant mentioned is one of sodium carbonate, sodium bicarbonate, sodium hydroxide, potassium carbonate, potassium bicarbonate, and potassium hydroxide.

[0022] The advantages of this invention are:

[0023] (1) The catalyst is prepared by using water-soluble organic matter with polymerization function as carbon precursor and water-soluble noble metal compound as active component. In the preparation process, a specific preparation process is adopted and the corresponding parameters are strictly controlled. The catalyst prepared has advantages such as high noble metal content, small crystal size, uniform distribution of active components, and the ability to adjust the micropore structure parameters of the catalyst according to the dry gel pyrolysis process (pyrolysis temperature, pyrolysis time) and specific application.

[0024] (2) Through a specific reduction process, compared with other reducing agents, firstly, hydrogen can achieve a highly efficient reduction reaction at a relatively low temperature, which helps to reduce energy consumption and equipment costs; secondly, controllable reduction process conditions (such as temperature, pressure, gas flow rate, etc.) can adjust the rate and extent of the reduction reaction and suppress the clustering of active components; finally, almost no impurities are introduced, the catalyst is purer and has better activity. Attached Figure Description

[0025] Figure 1 A comparison diagram of the specific surface area of ​​the noble metal catalysts prepared in each embodiment;

[0026] Figure 2 A comparison chart of the noble metal content of the noble metal catalysts prepared in each embodiment;

[0027] Figure 3 This is a TEM image of the noble metal catalyst prepared in Example 3 of the present invention;

[0028] Figure 4 This is a TEM image of the noble metal catalyst prepared in Example 4 of the present invention. Detailed Implementation

[0029] The noble metal catalyst involved in this invention is prepared by a method comprising the following steps:

[0030] (1) Organic phenols and organic aldehydes are mixed in deionized water at a molar ratio of 1:2 to 1:4, and then noble metal compounds and surfactants are added to form a homogeneous system. The amount of surfactant added is 0.5-2.0 wt.%. Among them, organic aldehydes can be formaldehyde, propionaldehyde, butyraldehyde, glyoxal, glutaraldehyde, and organic phenols can be phenol, resorcinol, hydroquinone, and hydroxyl. Noble metal compounds include palladium nitrate, palladium chloride, palladium acetate, palladium acetylacetonate, tetraaminopalladium nitrate, sodium tetrachloropalladium, potassium tetrachloropalladium, potassium hexachloropalladium, platinum chloride, chloroplatinic acid, ammonium chloroplatinate, potassium chloroplatinate, and platinum acetylacetonate.

[0031] (2) Under vigorous stirring, a precipitant (concentration of 0.1-1.0 wt.%) is slowly added dropwise to the homogeneous system. The formed noble metal hydroxide colloid is suspended in the solution as nano-sized particles under the protection of the surfactant. The amount of precipitant added is increased until the pH of the solution stabilizes at 8-10. The precipitant can be any one of sodium carbonate, sodium bicarbonate, sodium hydroxide, potassium carbonate, potassium bicarbonate, or potassium hydroxide. The surfactant can be one or more of PVA, SP20, SP60, and SP80.

[0032] (3) The suspension was transferred to a glass bottle and sealed, and placed in a constant temperature water bath at 50-90℃ for aging treatment for 8-24 hours to obtain a hydrogel containing noble metal particles. In order to alleviate the collapse of the network structure of the hydrogel during the drying process, the dehydration treatment was carried out at a lower temperature (40-60℃) for 5-15 hours to obtain a dry gel without water molecules.

[0033] (4) The dry gel is pyrolyzed at high temperature to remove small organic molecules and elements such as H and O from the precursor, forming a carbon-rich structure. The pyrolysis temperature is 300-600℃ and the pyrolysis time is 1-10 hours. During the formation of the carbon-rich structure, the escape of small molecules causes the catalyst to form a porous structure.

[0034] (5) The sample after high-temperature pyrolysis consists of amorphous carbon and noble metal oxides or chlorides. The formation of a porous structure facilitates the entry of H2 and the removal of reaction products during the reduction process. The reduction temperature is 100-300℃, and the reduction time is adjusted according to the amount of material loaded and the amount of H2 supplied. The reaction continues until the noble metal compound is completely reduced to its elemental form. After the reduction reaction is completed, the H2 supply is stopped, the material is cooled to room temperature under inert gas protection, and then vacuum-sealed for storage, thus obtaining the carbon-based noble metal catalyst.

[0035] To illustrate the effects of the present invention, three embodiments are provided below for further explanation. The purpose of these embodiments is to better verify the effects of the present invention rather than to limit the scope of the present invention.

[0036] Example 1

[0037] Phenol, formaldehyde, and palladium chloride were mixed in a molar ratio of 1:3:0.01 to prepare a 200 ml solution. Then, surfactant PVA was added to achieve a PVA concentration of 0.2 wt.%. Under stirring, a 3 wt.% sodium hydroxide solution was slowly added dropwise until the pH of the mixed solution reached 9 ± 0.2. The suspension was transferred to a glass bottle, sealed, and placed in a 75°C constant temperature water bath for 12 hours to obtain a solid hydrogel. The hydrogel was then removed from the sealed glass bottle and slowly dried in a 50°C oven for 12 hours to obtain an anhydrous dry gel. After the dry gel was broken into the required size, it was heated to 450℃ at a heating rate of 1℃ / min under inert gas protection and held at the temperature for 5h for pyrolysis reaction. After the reaction was completed, it was cooled to 150℃ under inert gas protection and H2 was introduced into the inert gas to make the volume concentration of H2 5%. After holding at the temperature for 4h, the H2 supply was turned off and the sample was cooled to room temperature under inert gas protection to prepare the Pd / C catalyst.

[0038] Example 2

[0039] Phenol, formaldehyde, and palladium acetate were mixed in a molar ratio of 1:3:0.02 to prepare a 200 ml solution. Then, surfactant SP60 was added to achieve a concentration of 0.25 wt.% SP60 in the system. Under stirring, a 5 wt.% potassium carbonate solution was slowly added dropwise until the pH of the mixed solution reached 9 ± 0.2. The suspension was transferred to a glass bottle, sealed, and placed in a 60°C constant temperature water bath for 12 h to obtain a solid hydrogel. The hydrogel was then removed from the sealed glass bottle and slowly dried in a 45°C oven for 12 h to obtain an anhydrous dry gel. After the dry gel was broken into the required size, it was heated to 450℃ at a heating rate of 1℃ / min under inert gas protection and held at the temperature for 5h for pyrolysis reaction. After the reaction was completed, it was cooled to 150℃ under inert gas protection and H2 was introduced into the inert gas to make the volume concentration of H2 5%. After holding at the temperature for 4h, the H2 supply was turned off and the sample was cooled to room temperature under inert gas protection to prepare the Pd / C catalyst.

[0040] Example 3

[0041] Resorcinol, propionaldehyde, and palladium acetate were mixed in a molar ratio of 1:2:0.03 to prepare a 200 ml solution. Then, surfactant SP60 was added to achieve a concentration of 0.25 wt.% SP60 in the system. Under stirring, a 5 wt.% sodium carbonate solution was slowly added dropwise until the pH of the mixed solution reached 9 ± 0.2. The suspension was transferred to a glass bottle, sealed, and placed in a 60°C constant temperature water bath for 12 h to obtain a solid hydrogel. The hydrogel was then removed from the sealed glass bottle and slowly dried in a 45°C oven for 12 h to obtain an anhydrous dry gel. After the dry gel was broken into the required size, it was heated to 450℃ at a heating rate of 1℃ / min under inert gas protection and held at the temperature for 5h for pyrolysis reaction. After the reaction was completed, it was cooled to 150℃ under inert gas protection and H2 was introduced into the inert gas to make the volume concentration of H2 5%. After holding at the temperature for 4h, the H2 supply was turned off and the sample was cooled to room temperature under inert gas protection to prepare the Pd / C catalyst.

[0042] Example 4

[0043] Resorcinol, formaldehyde, and platinum acetylacetone were mixed in a molar ratio of 1:3:0.04 to prepare a 200 ml solution. Then, surfactant SP80 was added to achieve a concentration of 0.15 wt.% SP80 in the system. Under stirring, a 5 wt.% sodium carbonate solution was slowly added dropwise until the pH of the mixed solution reached 9 ± 0.2. The suspension was transferred to a glass bottle, sealed, and placed in a 60°C constant temperature water bath for 12 h to obtain a solid hydrogel. The hydrogel was then removed from the sealed glass bottle and slowly dried in a 45°C oven for 12 h to obtain an anhydrous dry gel. After the dry gel was broken into the required size, it was heated to 450℃ at a heating rate of 1℃ / min under inert gas protection and held at the temperature for 5h for pyrolysis reaction. After the reaction was completed, it was cooled to 150℃ under inert gas protection and H2 was introduced into the inert gas to make the volume concentration of H2 5%. After holding at the temperature for 4h, the H2 supply was turned off and the sample was cooled to room temperature under inert gas protection to prepare the Pt / C catalyst.

[0044] The experimental results are as follows:

[0045]

[0046] in, Figure 1 The medium specific surface area was measured by volumetric method with N2 as adsorbate at liquid nitrogen temperature (77K) and determined by BET method. Figure 2 The content of precious metals was determined using inductively coupled plasma atomic emission spectrometry (ICP). Figure 3 , Figure 4 In this study, the nanoscale dimensions of precious metals were observed using a transmission electron microscope.

[0047] The test results show that the Pd / C catalyst prepared in Example 3 has a specific surface area of ​​200 m². 2 The Pt / C catalyst prepared in Example 4 has a specific surface area of ​​240 m² / g and a Pd content of 2.6 wt.%. TEM analysis showed that the maximum grain size was only 4.2 nm, with no grain clusters observed, indicating good dispersion of the active component. 2 / g, the precious metal Pt content reaches 3.08wt.%, TEM shows that the maximum grain size is only 4.5nm, no grain clusters were found, and it also shows good dispersion of active components.

[0048] Of course, the above description is not intended to limit the present invention, and the present invention is not limited to the examples given above. Any changes, modifications, additions or substitutions made by those skilled in the art within the scope of the present invention should also fall within the protection scope of the present invention.

Claims

1. A method for preparing a carbon-based noble metal catalyst, comprising the following steps: A. mixing: dissolving water-soluble organic compounds with polymerization function in deionized water in proportion, and then adding water-soluble noble metal compounds and surfactants after stirring and mixing uniformly; B. precipitating: adding an alkaline precipitant to the mixture under vigorous stirring to make the noble metal compounds form nanoscale hydroxide colloid suspended in the mixture, and then continuously adding the precipitant until the pH value of the system reaches 8-10 after the noble metal compounds are completely precipitated; C. sol-gel: moving the suspension to a glass bottle and sealing, and then placing in a constant-temperature water bath for aging treatment for 8-24 hours to obtain a hydrogel containing noble metal elements, and the aging temperature is 50-90°C; D. drying: placing the hydrogel containing noble metal elements in an oven at 40-60°C for drying for 6-8 hours to remove water in the gel network structure to obtain a xerogel; E. cracking: under the protection of inert gas, heating the broken xerogel to 300-600°C in a tube furnace and keeping constant temperature for 1-10 hours to perform a cracking reaction, and the escape of small molecules makes the material form a developed pore structure in microcosm; F. reducing: after the cracking reaction is completed, the material is cooled to 100-300°C under the protection of inert gas, and then H2 is introduced in the inert protective gas to perform a reduction treatment, and after the reduction reaction is completed, the H2 supply is stopped, the material is cooled to room temperature under the protection of inert gas, and then vacuum sealed for preservation, thereby obtaining the carbon-based noble metal catalyst. The water-soluble organic compounds with polymerization function are a mixture of organic aldehyde and organic phenol. The organic aldehyde is formaldehyde, propyl aldehyde, butyl aldehyde, glyoxal or pentanedial, and the organic phenol is phenol, m-dihydroxybenzene, p-dihydroxybenzene or quinol. The molar ratio of the organic aldehyde to the organic phenol is 1:1-1:

4. The noble metal elements in the water-soluble noble metal compounds are platinum and palladium. The soluble palladium noble metal compound is palladium nitrate, palladium chloride, palladium acetate, palladium acetylacetone, tetraamino palladium nitrate, sodium tetrachloropalladate, potassium tetrachloropalladate, and the soluble platinum noble metal compound is platinum chloride, chloroplatinic acid, ammonium chloroplatinate, potassium chloroplatinate and platinum acetylacetone. The surfactant is one or more of polyvinyl alcohol (PVA), Span 20 (SP20), Span 60 (SP60) and Span 80 (SP80).

2. The method for preparing a carbon-based noble metal catalyst according to claim 1, characterized by: The alkaline precipitant is one of sodium carbonate, sodium bicarbonate, sodium hydroxide, potassium carbonate, potassium bicarbonate and potassium hydroxide.

3. The method for preparing a carbon-based noble metal catalyst according to claim 2, characterized by: ​ 4. The method for producing a carbon-based noble metal catalyst according to claim 2 or 3, characterized by: ​ 5. The method for preparing a carbon-based noble metal catalyst according to claim 4, characterized by: ​ 6. The method for preparing a carbon-based noble metal catalyst according to claim 5, characterized by: ​ 7. The method of making a carbon-based noble metal catalyst according to claim 1, characterized in that: ​ 8. The method of making a carbon-based noble metal catalyst according to claim 1, characterized in that: ​

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