CN109847739B

CN109847739B - Method for modifying Pd/gamma-aluminum oxide catalyst

Info

Publication number: CN109847739B
Application number: CN201910232768.4A
Authority: CN
Inventors: 肖益鸿; 陈婷; 李娟娟; 郑勇; 蔡国辉; 钟富兰
Original assignee: Fuzhou University
Current assignee: Fuzhou University
Priority date: 2019-03-26
Filing date: 2019-03-26
Publication date: 2021-06-01
Anticipated expiration: 2039-03-26
Also published as: CN109847739A

Abstract

The invention discloses Pd/gamma-Al₂O₃A method for modifying a catalyst, the method comprising the main steps of: firstly, high-molecular ligand compound is utilized to realize high-efficiency complexation with noble metal particles of the catalyst and carrier surface particles, then the catalyst is treated under the conditions of specific temperature and protective gas, so that a uniform carbon covering layer is constructed on the surface of the catalyst, finally, the catalyst is activated by proper temperature and atmosphere to remove the carbon covering layer in the catalyst, and the catalyst is used for catalytic combustion of methane. The catalyst treated by the method of the invention has obviously improved activity compared with the untreated catalyst.

Description

Method for modifying Pd/gamma-aluminum oxide catalyst

Technical Field

The invention belongs to the technical field of catalytic preparation, and particularly relates to Pd/gamma-Al₂O₃A method for modifying a catalyst.

Background

Methane is a high temperature chamber effect gas. In the coal and oil exploitation process, a large amount of low-concentration reducing gas (the main component of the reducing gas is methane) is associated, the economic benefit of enriching or separating the methane is low, the reducing gas is directly discharged into the atmosphere to cause environmental problems, and the reducing gas needs to be purified; while natural gas is widely used as a clean fuel in automobiles and power stations, the resulting emissions contain a certain amount of methane that is not completely combusted. Currently, the most economical and effective way to purify these low concentrations of methane is by catalytic combustion.

In catalytic combustion of methane, supported noble metal catalysts are the more common catalysts. However, since methane is a stable hydrocarbon with a regular tetrahedral molecular structure (C-H bond energy 413 kJ/mol), and is one of the most difficult gases to activate and oxidize in carbon-containing hydrocarbons, the activation and complete conversion temperatures of methane are usually high even in the presence of a catalyst. The catalyst is easy to deactivate under high temperature condition after long-term high-temperature operation. Therefore, the key to finding a more effective solution to the problem of low methane emission is to develop a methane catalytic combustion catalyst with higher activity and higher stability.

Among them, Pd-based catalysts are widely studied and applied due to their good low-temperature activity and thermal stability. The properties of the Pd-based catalyst are determined by the dispersion of Pd, the nature of the support itself and the interaction between the noble metal particles and the support. For example, patent CN 104014339A discloses Pd @ CeO loaded by anodic alumina nanotube array as carrier₂The core-shell structure unit catalyst is synthesized into Pd @ CeO by adopting a chemical self-assembly method₂Core-shell structure unit, and then Pd @ CeO₂The structural unit is absorbed and adsorbed on the high specific surface area anodic alumina from the solution, and the cerium oxide has valence change and is easily reduced from 4 valence to 3 valence, so that the noble metal Pd and the cerium oxide have strong interaction and Pd @ CeO₂Core-shell structural units in alumina materialDispersed, the catalyst is the catalyst with the best methane combustion catalytic activity. However, the synthesis method is long, the key steps must be carried out in an atmosphere completely isolating oxygen, the synthesis method is high in cost, and commercial popularization and application are difficult at present.

γ-Al₂O₃Has large specific surface area, high porosity, high temperature stability and other advantages over cerium oxide, and is easy to realize high dispersion on the surface of noble metal carrier, so that Pd/gamma-Al₂O₃The catalyst is still the most commonly used commercial methane catalytic combustion catalyst at present. But gamma-Al₂O₃The catalyst is a non-reducing oxide, the electronic defects on the surface of the oxide are less, and the interaction between the oxide and the loaded noble metal Pd particles is weaker, so that the noble metal particles are easy to migrate, aggregate and grow under the high-temperature condition of methane catalytic combustion by adopting the catalyst prepared by the traditional method; meanwhile, phase change occurs when the alumina particles are sintered, and the specific surface area is sharply reduced, so that the catalyst is deactivated.

Disclosure of Invention

The invention aims at Pd/gamma-Al₂O₃The method is used for enhancing the interaction between noble metal Pd and alumina, increasing the surface defect sites of the catalyst, constructing more active site Pd/Al interfaces and further improving the activity of the catalyst.

In order to achieve the purpose, the invention adopts the following technical scheme:

Pd/gamma-Al₂O₃The catalyst modification method specifically comprises the following steps:

step 1, dissolving a saccharide compound in a water-alcohol mixed solvent, and then reacting the saccharide compound with Pd/gamma-Al at low temperature₂O₃Mixing catalysts, adjusting the pH value with specific acid, fully stirring, standing for 3-24 hours, and drying to remove the solvent;

step 2, processing the catalyst sample obtained in the step under the conditions of specific temperature and protective gas, and converting the carbohydrate combined on the surface of the catalyst into a carbon covering layer;

and 3, finally, treating the catalyst at proper temperature and in proper atmosphere and removing a carbon covering layer in the catalyst to obtain the modified catalyst.

Pd/gamma-Al in step 1₂O₃The catalyst is granular, the granularity is 0.1-1000 mu m, and the load of Pd is equal to 0.01-2% of the mass of the carrier.

In the step 1, the carbohydrate is at least one of sucrose, glucose, water-soluble chitosan and Hyaluronic Acid (HA), and the dosage of the carbohydrate is 0.1-5 times of the mass of the catalyst.

The low temperature condition used in step 1 is room temperature to 50 ℃, and the solvent used is a mixed solution of water and alcohol (one of ethanol, isopropanol and glycerol). The mass ratio of alcohol to water in the solvent is (1-10): 100, the using amount of the solvent is 3 to 100 times of the mass of the catalyst.

The acid used for adjusting the acidity in the step 1 is one of hydrochloric acid, acetic acid and nitric acid, and the concentration range of the acid is 0.1-3 mol/L; the pH value of the solvent is adjusted within the range of 2.0-7.0. The drying is preferably carried out in vacuum, and the drying temperature is not more than 80 ℃.

The specific temperature in the step 2 is one of 400, 500, 600, 700 and 800 ℃, the heating rate for reaching the target temperature is controlled to be 2-10 ℃/min, and the processing time at the specific temperature is 2-10 h; the used protective gas is one of nitrogen, argon, helium and carbon dioxide, and the gas flow is that the volume space velocity is 1000-^-1。

The proper temperature in the step 3 is 450-1000 ℃, and the atmosphere is mixed gas 1 (N)₂+O₂) And mixed gas 2 (N)₂+H₂O), the oxygen concentration in the mixed gas 1 is 3-10% by volume; the volume percentage of the water vapor concentration in the mixed gas 2 is 3-10%. The gas flow is 1000-fold 10000 h^-1. And (3) under the condition of target temperature, firstly treating the catalyst with the mixed gas 2 for 10-100min, then switching to the mixed gas 1 for treating for the same time, repeating the treatment for 2-20 times, and then introducing air to remove all carbon on the surface of the catalyst. The total treatment time is controlled to be 2-10h, and the obtained catalyst can be applied to the catalytic combustion of methane.

The invention has the following remarkable advantages:

(1) the preparation method is simple, the conditions are easy to control, and the raw materials are environment-friendly and easy to obtain;

(2) the chitosan and Hyaluronic Acid (HA) adopted by the invention contain a large amount of hydroxyl or amino and can be complexed with noble metal Pd and alumina particles on the surface of the catalyst; the wettability of the surface of the catalyst can be obviously improved by adding proper alcohol into the aqueous solution, so that uniform organic matter coverage on the surface of the catalyst is easy to realize, and meanwhile, the coverage amount of carbon on the surface of the catalyst can be adjusted by adding sucrose as a carbon source;

(3) the surface-covered compound can be converted into carbon under the protection of inert gas at a certain temperature to realize Pd/gamma-Al₂O₃The catalyst is uniformly covered, the generation of lattice oxygen defects on the surface of the alumina of the catalyst can be promoted due to the reduction environment of the carbonized surface, the palladium oxide is converted into amorphous Pd particles, and the migration of the noble metal particles is limited due to the covering of a large number of carbon particles; meanwhile, the catalyst is circularly treated by water vapor and an oxidizing atmosphere at high temperature, so that the surface area loss is avoided while the interface structure rearrangement of partial alumina and Pd particles is promoted, more alumina defects are generated compared with the simple pre-reduction treatment by using reducing gas, so that the noble metal Pd particles are easier to be embedded into the surface alumina crystal lattice, the interaction between Pd = Al is enhanced, and the dispersity and the thermal stability of the noble metal particles are improved. The modified catalyst has obviously raised methane activity.

Drawings

FIG. 1 is a graph comparing the dispersion of the catalyst of the present invention AC-650 and a comparative sample A-650.

Detailed Description

The following is a clear and complete description of the technical solution in the embodiment of the present invention with reference to the embodiment of the present invention:

Pd/γ-Al₂O₃preparation of the catalyst:

selecting commercial gamma-Al₂O₃(specific surface area 230 m²The carrier is Pd (NO) with certain amount₃)₂Impregnation of the solution to gamma-Al₂O₃Placing on a carrier at room temperature for 24 h, drying at 80 ℃, heating to 260 ℃ at a speed of 10 ℃/min, and roasting for 2 h to obtain 1 wt% Pd/gamma-Al₂O₃A catalyst. And screening the catalyst to obtain particles of 120-150 microns for later use. The catalyst was used in the following examples.

The activity of the catalyst was evaluated in a fixed bed quartz reaction apparatus. 200 mg of the catalyst is weighed and placed in a U-shaped quartz microreactor (i.d.,10 mm) for on-line detection. Reaction gas (1 vol% CH)₄，5 vol% O₂，N₂As equilibrium gas) at a flow rate of 120 ml/min through the catalyst bed corresponding to GHSV =36000 ml/g/h: the reaction products were analyzed by an agilent 7820A on-line gas chromatograph equipped with a thermal conductivity detector.

The measurement of catalyst dispersion (percent active metal) was performed on a 2920 chemisorption instrument, a microphone. 500 mg of the catalyst was weighed and placed in a U-shaped quartz microreactor (i.d.,10 mm) for on-line detection. Adsorption gas (5 vol% CO, He as balance gas), carrier gas He, gas flow 50 ml/min, quantitative tube 1 ml, adsorption temperature 50 ℃.

Example 1:

complexing of sample compounds: according to the following steps of 3: 100 g of glycerol and water are respectively taken to prepare 100 g of water-alcohol mixed solvent, 0.5 g of water-soluble chitosan is added into the solvent to be dissolved, and 5 g of standby Pd/gamma-Al is added₂O₃A catalyst. After the pH value is adjusted to be 5 by acetic acid in the full stirring, standing the solution for 24 hours, and drying the solution in vacuum at the temperature of 60 ℃ to obtain a modified catalyst precursor A;

carbonization of the sample: treating about 1 ml of modified catalyst precursor A by heating to 500 ℃ under a nitrogen atmosphere of about 100 ml/min (at a heating rate of 5 ℃/min) for 3 hours;

activation of the sample: the temperature of the sample is controlled at 650 ℃, the gas is switched into the mixed gas, the gas flow is set to be 100 ml/min, and the mixed gas 2 (95% N) is firstly introduced₂+5% H₂O) 30 min later, then mixed gas 1 (95% N)₂+5 % O₂) 30 minutes(ii) a After repeating the above steps for 4 times, air was introduced to remove all carbon on the surface of the catalyst. The total treatment time was controlled to 6 hours. The resulting catalyst was labeled catalyst AC-650.

Directly adopting spare Pd/gamma-Al₂O₃Activating the catalyst: about 1 ml of standby Pd/gamma-Al is taken₂O₃Controlling the temperature of a sample to 650 ℃, switching gas into mixed gas, setting the gas flow to be 100 ml/min, and introducing the mixed gas 2 (95% N)₂+5 %H₂O) 30 min later, then mixed gas 1 (95% N)₂+5% O₂) 30 minutes; after repeating the above steps for 4 times, air was introduced. The total treatment time was controlled to 6 hours. The resulting catalyst was labeled catalyst A-650.

Example 2:

the concrete steps are the same as example 1; the activation temperatures of the different samples were set at 450 ℃ to obtain catalyst samples AC-450 and A-450, respectively.

The samples were subjected to methane activity tests, and Table 1 shows the temperatures at which the methane conversion rates reached 50% and 90%, respectively. The activity of the same catalyst modified by the method of the invention under the same condition is obviously improved compared with the activity of the same catalyst without modification on methane.

FIG. 1 is a graph comparing the dispersion of the catalyst AC-650 of the present invention and the comparative sample A-650, and it can be seen from FIG. 1 that the catalyst AC-650 using the present invention has a higher metal dispersion relative to the comparative sample A-650.

The foregoing is directed to embodiments of the present invention, and not all embodiments of the present invention. All equivalent changes and modifications made according to the claims of the present invention should be covered by the present invention.

Claims

1. Pd/gamma-Al₂O₃The method for modifying the catalyst is characterized by comprising the following main steps of:

(1) at a certain acidity and low temperatureReacting saccharide compound with Pd/gamma-Al under the condition of reagent₂O₃Mixing the catalysts, and drying to remove the solvent;

(2) treating the catalyst with the surface covered with the organic matters obtained in the step (1) under certain temperature and protective gas conditions to convert the carbohydrate combined on the surface of the catalyst into a carbon covering layer;

(3) finally, treating the catalyst at a proper temperature and in a proper atmosphere and removing a carbon covering layer in the catalyst to obtain a modified catalyst;

the proper temperature in the step (3) is 450-1000 ℃; the atmosphere comprises a nitrogen and oxygen mixed gas and a nitrogen and water mixed gas, and the volume percentage of the oxygen concentration range in the mixed gas is 3-25%; the volume percentage of the water vapor concentration range in the mixed gas of nitrogen and water is 3-10%, and the gas flow is the volume space velocity of 1000-^-1The treatment process of the catalyst comprises the steps of firstly treating the catalyst for 10-100min by using a mixed gas of nitrogen and water, then switching to the treatment of the mixed gas of nitrogen and oxygen for the same time, repeating the treatment for 4-20 times, and then introducing air until carbon on the surface of the catalyst is completely removed, wherein the total treatment time is 2-10 h.

2. A Pd/γ -Al according to claim 1₂O₃The modification method of the catalyst is characterized in that the catalyst is Pd/gamma-Al₂O₃The catalyst is granular catalyst with granularity of 0.1-1000 micron and Pd loading amount of 0.01-2 wt% of the carrier.

3. A Pd/γ -Al according to claim 1₂O₃The method for modifying the catalyst is characterized in that the carbohydrate compound in the step (1) is one or more of sucrose, glucose, water-soluble chitosan and hyaluronic acid, and the dosage of the carbohydrate compound is 0.1-5 times of the mass of the catalyst.

4. A Pd/γ -Al according to claim 1₂O₃The catalyst modification method is characterized in that the low temperature condition in the step (1) is room temperature to 50 ℃, the solvent is a mixed solution of water and alcohol, and the mass ratio of the alcohol to the water is (1-10): 100, the alcohol is one of ethanol, isopropanol and glycerol, and the dosage of the solvent is 3-100 times of the mass of the catalyst.

5. A Pd/γ -Al according to claim 1₂O₃The method for modifying the catalyst is characterized in that the acid used under the acidity condition in the step (1) is 1 or 2 of hydrochloric acid, acetic acid and nitric acid, and the concentration of the acid is 0.1-3 mol/L.

6. A Pd/γ -Al according to claim 1₂O₃The method for modifying the catalyst is characterized in that the drying in the step (1) is vacuum drying, and the drying temperature is less than or equal to 80 ℃.

7. A Pd/γ -Al according to claim 1₂O₃The method for modifying the catalyst is characterized in that the treatment temperature in the step (2) is one of 400 ℃, 500 ℃, 600 ℃, 700 ℃ and 800 ℃, the heating rate is 2-10 ℃/min, the treatment time is 2-10h, the protective gas is one of nitrogen, argon, helium and carbon dioxide, the gas flow is volume space velocity of 1000-^-1。