Whisker array SiC palladium-loaded iron catalyst as well as preparation method and application thereof

Abstract

The invention belongs to the field of chemical catalysis and pollutant degradation, and particularly relates to a whisker array SiC palladium-loaded iron catalyst as well as a preparation method and application thereof. The catalyst takes SiC as a substrate, SiC whiskers vertically grow on the surface of the substrate, a carbon layer is loaded on the surface of the whiskers, and Fe-Pd active components are loaded on the surface of the carbon layer; the SiC whisker array has an open type three-level gradient pore channel structure, micron-level through macropores and cracks formed by laser processing form a first-level pore channel, mesopores (2-50nm) generated by acid etching form a second-level pore channel, and micropores (lt, 2nm) formed in a chemical vapor deposition process form a third-level pore channel. The surface of the SiC carrier is provided with an enriched active component Fe-Pd, the SiC carrier can be applied to low-concentration PVC dechlorination, and the structure has a large specific surface area and porosity; the SiC whisker array forms an open type three-dimensional channel, so that the diffusion efficiency of PVC macromolecules is improved by 60%, and the microwave loss is enhanced by a carbon coating layer.

Description

Technical Field

[0001] This invention belongs to the field of chemical catalysis and pollutant degradation, specifically relating to a whisker array SiC palladium-iron catalyst and its preparation method and application. Background Technology

[0002] In the field of catalysts, SiC-based materials have attracted much attention due to their excellent thermal stability and mechanical strength.

[0003] Chinese patent CN103818056A proposes a multilayer structure for a SiC / SiC composite clad tube. By alternately stacking SiC whisker-toughened SiC composite material layers and continuous SiC fiber-toughened SiC composite material layers, the material achieves high resistance to crack initiation, thermal conductivity, and thermal shock resistance.

[0004] However, the patent does not cover the application of SiC materials in the field of catalysis, especially the improvement of macromolecular diffusion efficiency and the enhancement of microwave loss activity.

[0005] Another Chinese patent, CN119303624A, focuses on catalytic material technology. It improves the catalyst loading and catalytic activity by growing SiC whiskers in situ on the surface of a 3D interconnected pore SiC support and growing a TS-1 titanium-silicon molecular sieve coating on its surface.

[0006] However, the catalyst in this patent has a high loading of active components (≥0.5wt%), and its catalytic performance at ultra-low loading and its performance in directional dechlorination of polyvinyl chloride (PVC) have not been fully explored. Summary of the Invention

[0007] Based on the above background technology, this application aims to solve the problem of high loading of active components in catalysts in the prior art, as well as the challenge of insufficient catalytic efficiency in specific applications (such as directional dechlorination of PVC).

[0008] By designing a SiC whisker array support with open three-dimensional channels and combining it with ultra-low loading of Pd and Fe2O3 active components, this application aims to achieve efficient and economical directional dechlorination of PVC, while reducing the preparation cost of the catalyst and improving its competitiveness in industrial applications.

[0009] Furthermore, this application explores the unique advantages of the catalyst in a microwave field.

[0010] To achieve the above objectives, the present invention provides the following technical solution: This invention provides a SiC palladium-iron catalyst supported on a whisker array, which uses SiC as a substrate, with SiC whiskers vertically grown on the surface, a carbon layer loaded on the surface of the whiskers, and Fe-Pd active components loaded on the surface of the carbon layer. The SiC whisker array has an open three-level gradient channel structure, which includes primary channels formed by micron-sized through-holes and cracks formed by laser processing, secondary channels formed by mesopores (2-50 nm) generated by acid etching, and tertiary channels formed by micropores (<2 nm) formed by chemical vapor deposition.

[0011] Preferably, the SiC whiskers have a diameter of 100-300 nm, a height of 1-5 μm, and a whisker spacing of 200-500 nm; the carbon layer thickness is <2 nm, and the ultrathin carbon layer and the whisker array form a carbon-coated SiC core-shell unit with a specific surface area ≥300 m². 2 / g.

[0012] This carbon layer not only increases the conductivity of the SiC substrate and cleans the residues on the carrier surface, but also has the following advantages: (1) Enhanced microwave absorption capacity: The carbon layer significantly enhances microwave loss, with a loss tangent (tanδ) ≥ 0.8, enabling the catalyst to heat up rapidly and efficiently in a microwave field; (2) Provides abundant defect sites: The ultrathin carbon layer is rich in lattice defects and edge sites, which provide excellent sites for the stable anchoring of active centers of metals such as Pd and Fe, and prevent them from agglomerating and sintering at high temperature; (3) Forming a core-shell protective structure: effectively protecting the stability of the SiC substrate in corrosive reactive atmospheres containing HCl, water vapor, etc., and extending the service life of the carrier.

[0013] The SiC whisker array has an open three-level gradient channel structure and its surface is enriched with active component Fe-Pd, which can be used for dechlorination of low-concentration PVC.

[0014] The whisker array forms an open three-dimensional channel, improving the diffusion efficiency of PVC macromolecules by 60%, while the carbon coating layer enhances microwave loss (tanδ≥0.8). Active component design: The intermediate iron oxide group is the main active component, as iron oxide can oxidize PVC to CO and hydrogen chloride; the second active component is palladium, which can not only directly oxidize PVC but also catalyze the release of oxygen from the intermediate product hydrogen chloride, promoting the oxidation reaction. Therefore, palladium, as an auxiliary active component, effectively improves the PVC conversion rate.

[0015] Loading trace amounts of palladium onto the electron donor iron coordination site can enhance electrocatalytic activity, but excessive palladium loading will reduce hydrogen evolution reaction (HPR) activity and affect the equilibrium of the chlorine transfer reaction.

[0016] Therefore, preferably, the active component content is: palladium is anchored to the iron oxide surface in a single atomic state, and the molar ratio of the two is 1:0.05~0.08.

[0017] This invention also provides a method for preparing the whisker array SiC supported palladium-iron catalyst, comprising the following steps: S1. Preparation and acid etching of SiC whisker array: The SiC support is placed in a quartz tube of a tubular furnace, and hydrogen and trichloromethylsilane (MTS) are introduced; MTS and hydrogen are uniformly distributed in the quartz tube through a bubbler at a pressure of 5 MPa; the temperature is heated to 500-750℃ and kept stable; then, the support with the whisker array is etched in dilute hydrochloric acid or dilute nitric acid, washed and dried to produce secondary mesoporous channels; S2. Carbon Deposition: The acid-etched SiC support prepared in step (1) is placed in a chemical vapor deposition tube furnace, and MTS and hydrogen are introduced into the quartz tube. During carbon deposition, the vacuum is adjusted to 2 kPa, the deposition temperature is adjusted to 800℃, and the deposition time is controlled at 4-8 hours. The power is turned off and the temperature is maintained. After the temperature drops to room temperature, the sample is taken out to obtain the SiC support with a carbon deposition layer on the surface. The main purpose of carbon deposition is to increase the conductivity of the SiC substrate and to clean the residues on the surface of the SiC support.

[0018] S3. Laser processing: A 10W continuous laser is used to rapidly scan the surface of the SiC carrier with the carbon layer deposited on the surface, causing instantaneous high temperature at the laser focus, which leads to the rapid ablation of the material in the irradiated area; a pit is formed at the laser irradiation position, and the surrounding tissue is subjected to the laser shock wave, causing vibration, breakage, or even cracking, thereby forming a primary through-hole.

[0019] S4. Simultaneous impregnation method for loading active components: Based on the industrial application characteristics of immobilized palladium-carbon catalysts, a "simultaneous impregnation method" was adopted to simultaneously impregnate palladium and iron precursor solutions onto a multi-walled carbon-coated SiC whisker array support. The specific operation is as follows: a mixed solution of palladium chloride (PdCl2), iron nitrate (Fe(NO3)3), and deionized water was prepared and added to a stainless steel autoclave lined with polytetrafluoroethylene; then the open-pore multi-walled carbon-coated SiC whisker array support treated in step (3) was placed in the autoclave for impregnation for 24 h; after depressurization and pressure release, it was taken out.

[0020] S5. Drying and calcining: After the impregnation system is taken out of the autoclave, it is dried and ground in an oven at 300°C; then calcined in a tube furnace at 500°C for 5 h to eliminate the precursor salts of palladium and iron, and to stabilize and activate the active components.

[0021] Preferably, in step S1, the hydrogen flow rate is 150 mL / min, and the volume ratio of hydrogen to MTS is 1:4; in step S1, the temperature is heated to 600°C.

[0022] Preferably, in step S1, etching is performed in a hydrochloric acid or nitric acid solution with a concentration of 1-3 mol / L at 60-90°C for 2-4 hours, followed by washing with deionized water.

[0023] Preferably, in step S2, the deposition time is controlled at 6 hours and the heat preservation time is 2 hours.

[0024] Preferably, in step S3, the scanning rate is 1 mm / s.

[0025] Preferably, in step S4, the Pd / Fe molar ratio in the mixed solution is 1:(0.05~0.08).

[0026] Preferably, in step S5, the calcination is carried out in an N2 gas flow of 50 ml / min.

[0027] The present invention also provides the application of the whisker array SiC supported palladium iron catalyst in the directional dechlorination of PVC.

[0028] The specific process flow is as follows: The catalyst prepared above is ball-milled and mixed with PVC powder at a mass ratio of 1:2, and then added to 50 mL of N,N-dimethylformamide (DMF) solution containing 0.5 g stearic acid. The mixture is then ultrasonically dispersed to obtain a PDC catalyst dispersion.

[0029] The PDC catalyst dispersion was coated onto a PDMS mold and then placed in a vacuum drying oven to remove the solvent, thus obtaining a PDC membrane.

[0030] Place the PDC film in a quartz tube and heat it to 700°C in an electric heating mantle or microwave reactor.

[0031] Then, oxygen, water vapor, and hydrogen chloride gas are introduced into the quartz tube to simulate the pre-oxidation process of the PDC membrane under a flue gas atmosphere.

[0032] After pre-oxidation, oxygen is isolated, and carbon monoxide gas is introduced into the quartz tube to reduce and activate the catalyst.

[0033] After activation, gases such as carbon monoxide and oxygen are isolated, and an excess of hydrogen chloride gas and a certain amount of phthalic anhydride (PA) are introduced into the quartz tube as dechlorination competitive reactants and test gases.

[0034] Finally, PVC-containing exhaust gas samples were collected at different times and analyzed by gas chromatography-mass spectrometry (GC-MS).

[0035] Dechlorination Mechanism: In air, PVC eliminates C-Cl bonds through oxidation, generating negatively charged chloride ions which are captured by iron oxide via electrostatic attraction. These ions then decompose into chlorine radicals under the complexation of surface ligands. The chlorine radicals react with small-molecule hydrocarbon fragments in the system to generate chlorine-containing intermediates such as chloroform. Chloroform further hydrolyzes to generate chlorine and hypochlorite. At high temperatures, hypochlorite disproportionates to generate chloracyl radicals. These chloracyl radicals desorb and react with hydrogen chloride to form chlorine complexes, while the small-molecule hydrocarbons are completely oxidized to CO2 and H2O. In this reaction, Pd acts as a catalyst to accelerate electron transfer, promoting chloroform formation and inhibiting its decomposition. Therefore, Pd can optimize the reaction pathway and achieve targeted dechlorination.

[0036] The electron acceptor-hole theory suggests that oxygen vacancies play a key role in the catalytic degradation of VOCs. The higher the oxygen vacancy density on the surface of oxygen-containing materials, the stronger the catalytic degradation ability of VOCs. Nanosheets, nanowires, and nanoribbons have a large specific surface area and high surface defect density, which is conducive to the adsorption and activation of oxygen molecules, thereby increasing the number of oxygen vacancies. This invention uses a gradient pore structure to improve the oxygen vacancy density, specifically: (1) SiC whisker array forms an open three-dimensional channel, which improves the diffusion efficiency of PVC macromolecules by 60%; (2) Microporous structure increases defect density; (3) The rough structure generated by laser etching is wrapped by a carbon layer to form a carbon coating layer, which enhances microwave loss (tanδ≥0.8).

[0037] The advantages of this invention are reflected in the following aspects: (1) This invention employs a CVD method suitable for large-scale production to prepare SiC whisker materials. SiC whiskers possess a stable crystal structure and rich polyhedral morphology, enabling high-precision and controllable microstructure design. SiC whiskers are characterized by a wide bandgap (3.1 eV), high thermal conductivity, high Young's modulus, and high melting point, making them an ideal high-temperature resistant material. The SiC whisker array forms an open three-dimensional channel, which improves the diffusion efficiency of PVC macromolecules by 60%, and the carbon coating layer enhances microwave loss (tanδ≥0.8).

[0038] (2) This invention proposes a SiC support with a gradient pore structure, the surface of which is enriched with the active component Fe-Pd, which can be applied to the dechlorination of low-concentration PVC. This structure has a large specific surface area and porosity.

[0039] (3) The present invention uses a synchronous impregnation method to load active components, thereby achieving uniform distribution of Fe-Pd alloy loaded on carbon-coated SiC surface.

[0040] (4) By achieving efficient directional dechlorination at ultra-low Pd loading, the cost of catalyst is significantly reduced. Detailed Implementation

[0041] The present invention will be further described below with reference to embodiments. These embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0042] Example 1 (Best Example) 1. Catalyst preparation: S1. Whisker Growth and Acid Etching: Using CVD, hydrogen gas (150 mL / min) and trichloromethylsilane (MTS, hydrogen to MTS volume ratio 1:4) were introduced at 5 MPa to grow a SiC whisker array with a diameter of approximately 200 nm and a height of approximately 3 μm on a SiC substrate at 600 °C. The whiskers were then placed in a 2 mol / L nitric acid solution and etched at 80 °C for 3 hours, followed by washing with deionized water and drying.

[0043] S2. Carbon layer deposition: At 800℃ and 2 kPa, an MTS / H2 mixed gas was introduced for 6 hours to deposit a carbon layer with a thickness of approximately 1.5 nm on the whisker surface. After turning off the power, the temperature was maintained at the deposition temperature for another 2 hours. The sample was removed after the temperature dropped to room temperature.

[0044] S3. Laser processing: A 10W laser is used to scan at a speed of 1mm / s to form a first-order large hole.

[0045] S4. Loading active components: The active components are loaded using the simultaneous impregnation method at a Pd:Fe molar ratio of 1:0.06.

[0046] S5. Calcination: Calcination at 500℃ for 5 hours under a N2 gas flow of 50 ml / min yielded the final catalyst Cat-1.

[0047] 2. Performance Testing: Structural characterization: SEM showed a complete whisker array and clear laser-processed channels; BET analysis revealed that the catalyst exhibits a three-level gradient channel distribution with a specific surface area of ​​350 m². 2 / g; TEM confirmed that the carbon layer thickness was approximately 1.5 nm, and that Pd was dispersed in single-atom form on the Fe oxide surface.

[0048] Microwave performance: The measured microwave loss tanδ = 0.85.

[0049] Dechlorination performance: 1.0 g of catalyst Cat-1 and 2.0 g of PVC powder (total mass 3.0 g) were ball-milled and mixed evenly. The mixture was then added to 50 mL of N,N-dimethylformamide (DMF) solution containing 0.5 g stearic acid and ultrasonically dispersed to obtain a PDC catalyst dispersion. Subsequent film preparation and testing procedures were the same as described in the original text. In simulated flue gas at 700℃, the PVC conversion rate reached 99%, and the chloroform selectivity reached 85%.

[0050] Example 2 (lower limit of parameters) The preparation process is the same as in Example 1, except for the following parameters: S1. Whisker growth temperature: 550℃ (the resulting whiskers have a diameter of approximately 150nm and a height of approximately 1.5μm).

[0051] S2. Acid etching: 1 mol / L hydrochloric acid, 60℃, 2 hours.

[0052] S3. Carbon layer deposition time: 4 hours (carbon layer thickness approximately 1.0 nm).

[0053] S4. Active component ratio: Pd:Fe = 1:0.05.

[0054] The obtained catalyst Cat-2 has a specific surface area of ​​320 m² / g and tanδ = 0.75. The PVC conversion rate is 95%, and the chloroform selectivity is 80%.

[0055] Example 3 (Parameter Upper Limit) The preparation process is the same as in Example 1, except for the following parameters: S1. Whisker growth temperature: 700℃ (the resulting whiskers have a diameter of approximately 280nm and a height of approximately 4.5μm).

[0056] S2. Acid etching: 3 mol / L nitric acid, 90℃, 4 hours.

[0057] S3. Carbon layer deposition time: 8 hours (carbon layer thickness approximately 2.0 nm).

[0058] S4. Active component ratio: Pd:Fe = 1:0.08.

[0059] The obtained catalyst Cat-3 has a specific surface area of ​​330 m² / g and tanδ = 0.90. The PVC conversion rate is 97%, and the chloroform selectivity is 82%.

[0060] Comparative Example 1 (without tertiary channels) Commercially available SiC foam was used as a carrier, but it lacks the hierarchical pore structure formed by whisker arrays and laser processing. A carbon layer was directly deposited and loaded with the same proportion of active components as in Example 1. The resulting catalyst showed only 70% PVC conversion compared to Cat-1, with significantly reduced diffusion control and a slow reaction rate.

[0061] Comparative Example 2 (High Pd Load) The support and carbon layer were prepared according to the method in Example 1, but the Pd:Fe molar ratio was increased to 1:1 when loading the active component. The resulting catalyst showed a significant increase in Pd loading compared to Cat-2, but the hydrogen evolution side reaction during PVC dechlorination was aggravated, and the chloroform selectivity dropped sharply to 60%.

[0062] Comparative Example 3 (without carbon layer) Whisker arrays were prepared according to the method in Example 1 and subjected to acid etching and laser processing, but the carbon layer deposition step was omitted, and the active components were directly loaded. The resulting catalyst had a weaker microwave response (tanδ=0.15) compared to Cat-3. Its performance under conventional electric heating was comparable to Cat-3, but its heating rate was slower in the microwave field, and the reaction rate was only 40% of that of Cat-3.

[0063] Comparative Example 4 (Step-by-Step Impregnation) The support was prepared in the same manner as in Example 1. A stepwise impregnation method was used, first impregnating the Fe precursor and then calcining it, followed by impregnating the Pd precursor and then calcining it. TEM showed that the Pd and Fe species were unevenly distributed and significantly aggregated. Its catalytic activity decreased by about 30% compared to Cat-1, demonstrating the crucial role of the simultaneous impregnation method in forming uniform Pd-Fe synergistic sites.

[0064] The above embodiments and comparative examples demonstrate that the specific three-level pore structure, ultra-thin carbon layer, low-temperature preparation process, and specific ratio of ultra-low load Pd-Fe provided by the present invention work synergistically to achieve efficient and highly selective directional dechlorination of PVC at low cost. The technical effect is significant, and it has outstanding substantive features and remarkable progress.

[0065] The SiC described in this invention is an α-type hexagonal crystal system with high anisotropy. Its thermal conductivity is the highest among all materials [2950 W / (m·K), followed by diamond at 2850 W / (m·K)]. Under natural conditions, it is grayish-white, and the strip-shaped crystals have a metallic luster.

[0066] Because SiC is a covalent compound with high bond strength and stable structure, it is stable under high temperature, high pressure and strong corrosive environments, and is insoluble in strong acid and alkali solutions.

[0067] Its advantages as an inert support give it enormous potential and broad application prospects in the field of catalysis.

Claims

1. A whisker array SiC supported palladium-iron catalyst, characterized in that, Using SiC as a substrate, SiC whiskers are vertically grown on the surface, a carbon layer is loaded on the surface of the whiskers, and Fe-Pd active components are loaded on the surface of the carbon layer. The SiC whisker array has an open three-level gradient channel structure, which includes primary channels formed by micron-sized through-holes and cracks formed by laser processing, secondary channels formed by 2-50 nm mesopores generated by acid etching, and tertiary channels formed by micropores smaller than 2 nm formed by chemical vapor deposition.

2. The whisker array SiC supported palladium-iron catalyst according to claim 1, characterized in that, The SiC whiskers have a diameter of 100-300 nm, a height of 1-5 μm, and a spacing of 200-500 nm; the carbon layer has a thickness of <2 nm, and the carbon layer and the whisker array form a carbon-coated SiC core-shell unit, with a specific surface area ≥300 m². 2 / g.

3. The whisker array SiC supported palladium-iron catalyst according to claim 1, characterized in that, Active component content: Palladium is anchored to the surface of iron oxide in a single-atom state, and the molar ratio of the two is 1:0.05~0.

08.

4. The method for preparing the whisker array SiC supported palladium-iron catalyst according to any one of claims 1-3, characterized in that, Includes the following steps: S1. Preparation and acid etching of SiC whisker array: SiC support is placed in a tubular furnace quartz tube, and hydrogen and trichloromethylsilane are introduced at a pressure of 5 MPa; heated to 500-750℃ to grow whiskers; then, the support is etched in dilute hydrochloric acid or dilute nitric acid, washed and dried. S2. Deposit carbon layer: Place the acid-etched SiC support prepared in step (1) into a chemical vapor deposition tube furnace, introduce MTS and hydrogen, adjust the vacuum to 2 kPa, adjust the deposition temperature to 800℃, deposit the carbon layer, and control the deposition time to 4-8 hours; turn off the power and continue to keep warm; after the temperature drops to room temperature, take out the sample to obtain the SiC support with carbon layer deposited on the surface. S3. Laser processing: A 10W continuous laser is used to rapidly scan the surface of the SiC carrier with the carbon layer deposited on the surface, causing instantaneous high temperature at the laser focus, which leads to rapid ablation of the material in the irradiated area; a pit is formed at the laser irradiation position, and the surrounding tissue is subjected to the laser shock wave, causing vibration, breakage, or even cracking, thereby forming a primary through-hole. S4. Simultaneous impregnation method for loading active components: Palladium chloride, ferric nitrate and deionized water were mixed to form a solution and added to a stainless steel autoclave lined with polytetrafluoroethylene. Then the open-pore multi-walled carbon-coated SiC whisker array carrier treated in step (3) was placed in the autoclave and immersed for 24 h. After depressurization and pressure release, it was taken out. S5. Drying and calcining: After the impregnation system is removed from the autoclave, it is dried and ground in an oven at 300°C; then calcined in a tube furnace at 500°C for 5 h to eliminate the precursor salts of palladium and iron, and to stabilize and activate the active components.

5. The method for preparing the whisker array SiC supported palladium-iron catalyst according to claim 4, characterized in that, Step S1: The hydrogen flow rate is 150 mL / min, and the volume ratio of hydrogen to MTS is 1:4; Step S1: Heat to 600℃.

6. The method for preparing the whisker array SiC supported palladium-iron catalyst according to claim 4, characterized in that, Step S1: Etch in a 1-3 mol / L hydrochloric acid or nitric acid solution at 60-90°C for 2-4 hours, followed by washing with deionized water.

7. The method for preparing the whisker array SiC supported palladium-iron catalyst according to claim 4, characterized in that, Step S2: The deposition time is controlled at 6 hours; the heat preservation time is 2 hours; Step S3: The scanning rate is 1 mm / s.

8. The method for preparing the whisker array SiC supported palladium-iron catalyst according to claim 4, characterized in that, In step S4, the Pd / Fe molar ratio in the mixed solution is 1:(0.05~0.08).

9. The method for preparing the whisker array SiC supported palladium-iron catalyst according to claim 4, characterized in that, Step S5, the calcination is carried out in an N2 gas flow of 50 ml / min.

10. The application of the whisker array SiC supported palladium-iron catalyst according to claim 1 in the directional dechlorination of PVC.