Pd-Cu-C (N / Al2O3) catalyst as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of catalyst preparation, and particularly relates to a Pd-Cu-C (N) / Al2O3 catalyst and a preparation method and application thereof. The preparation method comprises the following steps: by taking aluminum nitrate as an aluminum source, dissolving and uniformly mixing, adjusting the pH value of the solution by taking ammonia water as a precipitator to form gel, and drying and roasting to prepare an Al2O3 carrier for later use; chitosan is used as a carbon / nitrogen source and dissolved in an acid solution to obtain chitosan hydrogel, then a palladium salt aqueous solution is added to obtain palladium-chitosan gel, then a copper salt and an Al2O3 carrier are added, and the catalyst is prepared after aging, drying and roasting. According to the catalyst, Pd and Cu species are anchored through specific groups on natural polymer chitosan, the atom utilization rate of the catalyst can be increased, the catalytic synergistic effect between the Pd and Cu species can be effectively regulated and controlled, the catalyst has good catalytic activity and stability, the range of application conditions for achieving CO preferential oxidation is wider, and the performance of resisting H2O and CO2 impurity gases is good.

Description

Technical Field

[0001] The present invention belongs to the technical field of catalyst preparation, and particularly relates to a Pd-Cu-C(N) / Al2O3 catalyst, a preparation method thereof, and an application thereof. Background Art

[0002] A fuel cell is a power generation device that directly and efficiently converts the chemical energy of a fuel and an oxidant into electrical energy through an electrode reaction. Among many fuel cells, proton exchange membrane fuel cells (PEMFCs) have received extensive attention due to their high energy conversion efficiency and environmental friendliness. The hydrogen used in PEMFCs mainly comes from the reforming of hydrocarbons and alcohols. Due to the limitation of the thermodynamic equilibrium, the obtained hydrogen often contains a small amount of impurities, and trace amounts of CO are extremely likely to adsorb on the surface of the Pt catalyst at the anode of the cell, poisoning the electrode and severely reducing the cell performance. Therefore, before the hydrogen-rich gas stream enters the fuel cell, CO purification treatment must be carried out. Considering factors such as the maturity of process technology, H2 recovery rate, and investment cost, compared with physical methods such as pressure swing adsorption and membrane separation, and chemical methods such as methanation and metal hydride separation, CO preferential oxidation (CO-PROX) is considered the most promising method for CO removal in a hydrogen-rich atmosphere (Jing P, Gong X, Liu B, et al., Catal. Sci. Technol., 2020, 10(4): 919-934.; Zhao Y Y, Dai S, Yang K R, et al., Proc. Nat. Acad. Sci., 2023, 120, e2206850120.).

[0003] In recent years, researchers have been committed to finding suitable catalysts for CO preferential oxidation. The catalysts reported currently mainly include two categories: noble metal and non-noble metal oxide catalysts. Common noble metal catalysts, such as Pt, Au, Rh, Ru, and Ir-based catalysts, show excellent catalytic performance in the CO preferential oxidation reaction, and among them, the research on Au-based and Pt-based catalysts is more. Au-based catalysts show high activity at lower temperatures, but as the reaction temperature increases, Au nanoparticles are prone to sintering, resulting in a decrease in CO conversion and catalytic stability (Sun X, Su H, Lin Q, et al. Appl. Catal. A: Gen., 2016, 527: 19-29.). Pt-based catalysts have high catalytic activity and good stability, but the required reaction temperature is relatively high, and a high space velocity must be maintained all the time, otherwise the water gas reverse reaction will occur (Palma S, M, Romero-Sarria F, et al. Mol. Catal., 2022, 517: 112015.). Non-noble metal oxide catalysts have also received extensive attention due to their low price. A typical representative with good catalytic performance is CuO / CeO2. However, the Cu species formed at higher temperatures in this catalyst is favorable for H2 oxidation, resulting in a decrease in CO2 selectivity. In addition, the CuO / CeO2 catalyst has relatively poor resistance to CO2 and H2O (Zhang Z, Chen K, Lu J, et al. Int. J Hydrogen Energy, 2021, 46(43): 22508-22518.).

[0004] Supported Wacker catalysts (Pd-Cu / support) can not only catalyze the selective oxidation of light olefins to aldehydes and ketones, the gas-phase oxidative carbonylation of light alcohols to carbonates, etc., but also are effective catalysts for the low-temperature oxidation of CO. However, there are few reports on the preferential oxidation of CO under hydrogen-rich conditions. Given the excellent low-temperature CO oxidation performance and incomparable water resistance of supported Wacker catalysts, they can be applied to the preferential oxidation of CO under hydrogen-rich conditions. Our research group previously used synthetic hydroxyapatite (HAP) as the support to prepare Pd-Cu-Fe / HAP and investigated its performance in the preferential oxidation of CO. The results showed that CO conversion could be achieved at room temperature. However, most of the Pd species existed in the form of clusters or nanoparticles with poor dispersion, resulting in a decrease in the catalytic cycle efficiency between Pd and Cu species in the catalyst. Especially with the extension of the reaction time, the CO conversion showed a downward trend (Li X, Xing L, Zhao W, et al. Int. J Hydrogen Energy, 2021, 46(58): 29940-29950.); later, the influence of the support material on the performance of supported Wacker catalysts for the room-temperature catalytic preferential oxidation of CO was also explored. It was found that compared with SiO2 and activated carbon, the abundant basic sites on the surface of Al2O3 were beneficial to the formation of more active copper species and had a suitable interaction with Pd species, making it more suitable as the support for Pd-Cu catalysts (Zhao Wanjun, Li Xiao, Dang Hui, et al., Chemical Journal of Chinese Universities, 2022, 43(03): 112-123.). Chitosan is a product obtained by removing part of the acetyl groups from natural polysaccharide chitin. It has many unique properties such as non-toxicity and biodegradability. A large number of C, N, and active hydroxyl groups are distributed on its molecular chain, with a unique electronic structure. At the same time, it is inexpensive and widely available. There is currently no research report on using the natural organic polymer chitosan to simultaneously introduce C and N elements to modify the Pd and Cu active components and regulate the Pd and Cu binary active components of conventional supported Wacker catalysts to prepare a CO preferential oxidation catalyst with good catalytic activity and stability. Summary of the Invention

[0005] In view of this, in order to further improve the low-temperature catalytic activity and stability of the supported Wacker catalyst for the preferential oxidation of CO under hydrogen-rich conditions, the present invention provides a Pd-Cu-C(N) / Al2O3 catalyst, a preparation method thereof, and an application thereof. By significantly reducing the loading amount of noble metal Pd to change its aggregation state, and adding an appropriate amount of chitosan during the preparation process, the properties of the classic Pd and Cu binary active components are regulated by doping non-metals C and N, and the CO preferential oxidation performance of Pd-Cu-C(N) / Al2O3 is significantly improved at low temperatures.

[0006] The technical solution of the present invention is as follows:

[0007] The first aspect of the present invention is a preparation method of a Pd-Cu-C(N) / Al2O3 catalyst, comprising the following steps:

[0008] Step 1: Dropwise add ammonia water to an aluminum nitrate solution and adjust the pH of the solution, stir and react to form a gel, and obtain an Al2O3 support after drying and calcination;

[0009] Step 2: Dissolve chitosan in an acid solution to obtain a chitosan hydrogel; then add an aqueous palladium salt solution to the chitosan hydrogel to obtain a palladium-chitosan gel;

[0010] Step 3: Add a copper salt and an Al2O3 support to the palladium-chitosan gel in sequence, stir evenly, and obtain a Pd-Cu-C(N) / Al2O3 catalyst after aging, drying, and calcination.

[0011] Preferably, in step 1, the concentration of ammonia water is 5% - 25%; the pH is 9 - 11; the reaction temperature is 40 - 90°C, and the time is 0.5 - 3 h.

[0012] Preferably, in step 1, the drying temperature is 60 - 130°C, and the time is 6 - 24 h; the calcination temperature is 300 - 1000°C, and the time is 0.5 - 5 h.

[0013] Preferably, in step 2, the degree of deacetylation of chitosan is one or more of 55% - 70%, 70% - 85%, and 85% - 95%; the concentration of the acid solution is 1% - 10%.

[0014] Preferably, in step 2, the acid solution is one or more of hydrochloric acid, formic acid, acetic acid, and citric acid; the palladium salt is one or more of palladium nitrate, palladium chloride, palladium acetate, and palladium acetylacetonate.

[0015] Preferably, in step 3, the copper salt is one or more of copper nitrate, copper chloride, copper sulfate, copper acetate, and copper carbonate.

[0016] Preferably, in step 3, the mass ratio of the palladium salt, copper salt, chitosan, and alumina is 0.05 - 5:20 - 150:5 - 75:300.

[0017] Preferably, in step 3, the drying temperature is 80 - 150 °C and the time is 3 - 12 h; the calcination atmosphere is N2 / O2 with a volume ratio of 0 - 10:1, the temperature is 200 - 800 °C, and the time is 1 - 4 h.

[0018] The second aspect of the present invention is the Pd - Cu - C(N) / Al2O3 catalyst prepared by the method described in the first aspect.

[0019] The third aspect of the present invention is the application of the Pd - Cu - C(N) / Al2O3 catalyst described in the second aspect in the preferential oxidation reaction of CO under hydrogen - rich conditions. The CO preferential oxidation performance of the catalyst was tested in a fixed - bed continuous - flow micro - reactor (inner diameter 4 mm, length 300 mm) filled with 300 mg of the catalyst. The particle size of the catalyst was 40 - 60 mesh. The feed gas (0.01 - 3 vol.% O2, 0.01 - 3 vol.% CO, 20 - 90 vol.% H2, and the rest N2) was introduced into a bubbler filled with water and then into the micro - reactor, and the water vapor concentration in the feed gas (1 - 10 vol.%) was adjusted by controlling the temperature of the bubbler. In addition, extra CO2 (5 - 15 vol.%) was introduced into the feed gas to test the tolerance of the corresponding catalyst to CO2 impurity gas. Online analysis was carried out using an Agilent 7890B gas chromatograph, and the gas passing through the micro - reactor was sampled and analyzed every 15 min at 10 - 120 °C or 30 °C. Before the test, the catalyst was not pretreated. The Agilent 7890B was equipped with a thermal conductivity detector, a flame ionization detector, and two packed columns (5A molecular sieve, inner diameter 3 mm, length 3 m; carbon molecular sieve, diameter 3 mm, length 2 m). -1 After passing through the bubbler filled with water and then entering the micro - reactor, the water vapor concentration in the feed gas (1 - 10 vol.%) was adjusted by controlling the temperature of the bubbler. In addition, extra CO2 (5 - 15 vol.%) was introduced into the feed gas to test the tolerance of the corresponding catalyst to CO2 impurity gas. Online analysis was carried out using an Agilent 7890B gas chromatograph, and the gas passing through the micro - reactor was sampled and analyzed every 15 min at 10 - 120 °C or 30 °C. Before the test, the catalyst was not pretreated. The Agilent 7890B was equipped with a thermal conductivity detector, a flame ionization detector, and two packed columns (5A molecular sieve, inner diameter 3 mm, length 3 m; carbon molecular sieve, diameter 3 mm, length 2 m).

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0021] (1) Using the natural organic polymer chitosan to simultaneously introduce C and N elements to modify the Pd and Cu active components, regulating the Pd and Cu binary active components of the conventional supported Wacker catalyst, a CO preferential oxidation catalyst with good catalytic activity and stability was prepared. This catalyst has a wider applicable condition range and can effectively catalytically oxidize CO under the conditions of reaction temperature 10 - 120 °C, space velocity 2000 - 20000 h -1 , and the CO content is 10 - 30000 ppm, and the CO2 selectivity is close to 100%.

[0022] (2) The protonated amino groups in chitosan stabilize metal ions by coordinating with Pd and Cu metal ions. Meanwhile, the three-dimensional spatial structure of the Pd / Cu-chitosan hydrogel encapsulates and stabilizes Pd and Cu metal ions, enabling the highly dispersed Pd and Cu active species in the catalyst and preventing their aggregation during the reaction.

[0023] (3) During the actual industrial production process, impurity gases such as H2O and CO2 inevitably exist. This catalyst can achieve CO conversion under the conditions containing H2O and CO2 impurity gases, and the deactivated catalyst can be regenerated.

[0024] (4) The preparation method of this Pd-Cu-C(N) / Al2O3 catalyst has a simple process, is easy to operate, and is suitable for industrial scale production; the amount of precious metals used is low, the cost is lower, and chitosan has a wide source, showing good industrial application prospects. Specific embodiments

[0025] To facilitate the understanding of the present invention, the present invention will be described more comprehensively below. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to more thoroughly and comprehensively understand the disclosure of the present invention.

[0026] Unless otherwise defined, all professional terms used hereinafter have the same meaning as commonly understood by those skilled in the art. The professional terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the protection scope of the present invention.

[0027] Unless otherwise specifically stated, various raw materials, reagents, instruments, and equipment used in the present invention can be obtained through market purchase or can be prepared by existing methods.

[0028] Example 1

[0029] (1) Weigh 36.8 g of aluminum nitrate nonahydrate and dissolve it in 150 mL of distilled water. Place it in a 70 °C constant temperature water bath and gradually add 10% ammonia water solution to adjust the pH to nearly 9.0. During this process, a white sol will be generated. Continue stirring and reacting for 1 h to form an aluminum gel. Then gradually dry the aluminum gel at 70 °C for 24 h, calcine it in an air atmosphere at 300 °C for 3 h, grind and sieve it to obtain an Al2O3 support with a particle size less than 200 mesh.

[0030] (2) First, dissolve 1 g of chitosan (deacetylation degree of 95%) in 3 mL of 5% hydrochloric acid solution to obtain a chitosan hydrogel. Then, gradually add 2.5 mL of PdCl2 solution (10 mg / mL) dropwise to the above gel and mix evenly to obtain a palladium-chitosan gel.

[0031] (3) 1.85 g of CuCl₂·2H₂O and 5 g of Al₂O₃ support were successively added to the palladium-chitosan gel and stirred evenly. After aging for 5 h and drying at 100 °C for 10 h, it was calcined at 360 °C for 2 h in an N₂ / O₂ atmosphere with a volume ratio of 2:1 to obtain the Pd-Cu-C(N) / Al₂O₃ catalyst. After screening, the particles with a particle size between 40 and 60 mesh were taken for standby. The Pd atomic loading in the catalyst was about 0.3 wt.%, and the Cu atomic loading was about 11 wt.%.

[0032] 300 mg of the catalyst was loaded into the reaction tube of a continuous flow micro-reactor, and the feed gas (1 vol.% O₂, 1 vol.% CO, 50 vol.% H₂, 3.3 vol.% water vapor, and the rest was N₂) was introduced for reaction. The space velocity was 4000 h -1 , and after reacting at room temperature for 150 min, the CO conversion rate was close to 80%, and the CO₂ selectivity was 100%.

[0033] Example 2

[0034] The catalyst preparation steps were basically the same as those in Example 1, except that: in step (1), the ammonia water concentration was changed to 15%, and the water bath temperature was changed to 40 °C. The Pd atomic loading in the catalyst was about 0.3 wt.%, and the Cu atomic loading was about 11 wt.%.

[0035] The catalyst evaluation conditions were the same as those in Example 1. After reacting at room temperature for 120 min, the CO conversion rate was close to 75%, and the CO₂ selectivity was 100%.

[0036] Example 3

[0037] The catalyst preparation steps were basically the same as those in Example 1, except that: in step (1), the calcination temperature was changed to 700 °C, and the calcination time was changed to 2 h. The Pd atomic loading in the catalyst was about 0.3 wt.%, and the Cu atomic loading was about 11 wt.%.

[0038] The catalyst evaluation conditions were the same as those in Example 1. After reacting at room temperature for 120 min, the CO conversion rate was close to 40%, and the CO₂ selectivity was 100%.

[0039] Example 4

[0040] The catalyst preparation steps were basically the same as those in Example 1, except that: in step (3), the drying temperature was changed to 140 °C, and the time was changed to 2 h. The Pd atomic loading in the catalyst was about 0.3 wt.%, and the Cu atomic loading was about 11 wt.%.

[0041] The catalyst evaluation conditions were the same as those in Example 1. After reacting at room temperature for 120 min, the CO conversion rate was close to 68%, and the CO₂ selectivity was 100%.

[0042] Example 5

[0043] The catalyst preparation steps are basically the same as those in Example 1, except that: in step (3), the calcination temperature is changed to 400 °C, the calcination atmosphere is changed to an N2 / O2 atmosphere with a volume ratio of 1:1, and the calcination time is changed to 4 h. The Pd atom loading in the catalyst is about 0.3 wt.%, and the Cu atom loading is about 11 wt.%.

[0044] The catalyst evaluation conditions are the same as those in Example 1. After reacting for 150 min at room temperature, the CO conversion rate is close to 55%, and the CO2 selectivity is 100%.

[0045] Example 6

[0046] The catalyst preparation steps are basically the same as those in Example 1, except that: in step (2), the acidic solution of chitosan is changed to a 5% acetic acid aqueous solution, and the dosage is changed to 5 mL. The Pd atom loading in the catalyst is about 0.3 wt.%, and the Cu atom loading is about 11 wt.%.

[0047] The catalyst evaluation conditions are the same as those in Example 1. After reacting for 150 min at room temperature, the CO conversion rate is close to 70%, and the CO2 selectivity is 100%.

[0048] Example 7

[0049] The catalyst preparation steps are basically the same as those in Example 1, except that: in step (2), the deacetylation degree of chitosan in Example 1 is changed to 85%, and the dosage is changed to 0.5 g. The Pd atom loading in the catalyst is about 0.3 wt.%, and the Cu atom loading is about 11 wt.%.

[0050] The catalyst evaluation conditions are the same as those in Example 1. After reacting for 150 min at room temperature, the CO conversion rate is close to 62%, and the CO2 selectivity is 100%.

[0051] Example 8

[0052] The catalyst preparation steps are basically the same as those in Example 1, except that: in step (2), the dosage of the PdCl2 solution is changed to 1 mL. The Pd atom loading in the catalyst is about 0.15 wt.%, and the Cu atom loading is about 11 wt.%.

[0053] The catalyst evaluation conditions are the same as those in Example 1. After reacting for 120 min at room temperature, the CO conversion rate is close to 48%, and the CO2 selectivity is 100%.

[0054] Example 9

[0055] The catalyst preparation steps are basically the same as those in Example 1, except that: in steps (2) and (3), PdCl2 and CuCl2·2H2O are respectively changed to Pd(CH3COO)2 and CuSO4. The Pd atomic loading in the catalyst is about 0.3 wt.%, and the Cu atomic loading is about 11 wt.%.

[0056] The catalyst evaluation conditions are the same as those in Example 1. After reacting for 150 min at room temperature, the CO conversion rate is close to 30%, and the CO2 selectivity is 100%.

[0057] Example 10

[0058] Change the raw gas composition in the evaluation conditions of Example 1 to 2.5 vol.% O2, 1 vol.% CO, 50 vol.% H2, 3.3 vol.% H2O, and the rest is N2. The space velocity is changed to 6000 h -1 , the reaction temperature is 80 °C, and other conditions remain unchanged. After reacting for 120 min, the CO conversion rate is close to 65%, and the CO2 selectivity is 100%.

[0059] Example 11

[0060] Change the raw gas composition in the evaluation conditions of Example 1 to 1 vol.% O2, 2 vol.% CO, 60 vol.% H2, and the rest is N2. The space velocity is changed to 20000 h -1 , other conditions remain unchanged. After reacting for 150 min at room temperature, the CO conversion rate is close to 30%, and the CO2 selectivity is 100%.

[0061] Example 12

[0062] Change the raw gas composition in the evaluation conditions of Example 1 to 1 vol.% O2, 1 vol.% CO, 60 vol.% H2, 5 vol.% H2O, 5 vol.% CO2, and the rest is N2. Other conditions remain unchanged. After reacting for 150 min at room temperature, the CO conversion rate is close to 50%, and the CO2 selectivity is 100%.

[0063] The above are only examples for better explaining the present invention, and are not intended to limit it. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention falls within the scope covered by the present invention.

Claims

1. A method for preparing a Pd-Cu-C(N) / Al2O3 catalyst, characterized in that: The steps include: Step 1: Add ammonia water dropwise to the aluminum nitrate solution and adjust the pH value of the solution, stir to react to form a gel, and then dry and calcine to obtain an Al2O3 carrier; Step 2: dissolving chitosan in an acid solution to obtain a chitosan hydrogel; then adding a palladium salt aqueous solution to the chitosan hydrogel to obtain a palladium-chitosan gel; Step 3: Add copper salt and Al2O3 carrier to palladium-chitosan gel successively, stir evenly, and obtain Pd-Cu-C(N) / Al2O3 catalyst after aging, drying and calcining.

2. The method for preparing a Pd-Cu-C(N) / Al2O3 catalyst according to claim 1, characterized in that: In step 1, the concentration of ammonia water is 5% to 25%; the pH is 9 to 11; the reaction temperature is 40 to 90° C., and the reaction time is 0.5 to 3 hours.

3. The method for preparing a Pd-Cu-C(N) / Al2O3 catalyst according to claim 1, characterized in that: In the step 1, the drying temperature is 60-130° C. and the time is 6-24 hours; the calcination temperature is 300-1000° C. and the time is 0.5-5 hours.

4. The method for preparing a Pd-Cu-C(N) / Al2O3 catalyst according to claim 1, characterized in that: In step 2, the deacetylation degree of chitosan is one or more of 55-70%, 70-85%, 85-95%; and the concentration of the acid solution is 1-10%.

5. The method for preparing a Pd-Cu-C(N) / Al2O3 catalyst according to claim 1, characterized in that: In step 2, the acid solution is one or more of hydrochloric acid, formic acid, acetic acid and citric acid; the palladium salt is one or more of palladium nitrate, palladium chloride, palladium acetate and palladium acetylacetonate.

6. The method for preparing a Pd-Cu-C(N) / Al2O3 catalyst according to claim 1, characterized in that: The copper salt in step 3 is one or more of copper nitrate, copper chloride, copper sulfate, copper acetate and copper carbonate.

7. The method for preparing a Pd-Cu-C(N) / Al2O3 catalyst according to claim 1, characterized in that: In the step 3, the mass ratio of palladium salt, copper salt, chitosan and aluminum oxide is 0.05-5:20-150:5-75:

300.

8. The method for preparing a Pd-Cu-C(N) / Al2O3 catalyst according to claim 1, characterized in that: In step 3, the drying temperature is 80-150° C. and the drying time is 3-12 hours; the calcination atmosphere is N2 / O2 with a volume ratio of 0-10:1, the temperature is 200-800° C. and the time is 1-4 hours.

9. A Pd-Cu-C(N) / Al2O3 catalyst prepared by the method as claimed in any one of claims 1 to 8.

10. Use of the Pd-Cu-C(N) / Al2O3 catalyst as claimed in claim 9 in CO preferential oxidation reaction under hydrogen-rich conditions.