A catalyst for dehydrogenation of 12H-N-ethylcarbazole and its preparation method

Abstract

The present application provides a catalyst for dehydrogenation of 12H-N-ethylcarbazole and a preparation method thereof, which includes a carrier and fully exposed active metal clusters disposed on the carrier. The metal coordination number between metals in the fully exposed active metal clusters is 1-6, and the active metal is selected from at least one of palladium, platinum, iridium, ruthenium, iron, nickel, and cobalt. The carrier is selected from at least one of nanodiamond, nanographite sheet, coconut shell carbon, activated carbon, graphitic carbon nitride, hexagonal carbon nitride, graphene oxide, silica, aluminum oxide, and titanium dioxide. Based on the total mass of the catalyst, the loading amount of the active metal in the catalyst is 0.1 wt% - 20 wt%. The catalyst provided by the present application exposes all metal atoms on the surface of the carrier, designs the structure of the catalyst from the atomic size, improves the utilization rate of the active metal, and further improves the catalytic activity of the catalyst. The catalyst for dehydrogenation of 12H-N-ethylcarbazole provided by the present application can achieve a stable dehydrogenation process.

Description

Technical Field

[0001] The present application relates to the technical field of catalytic dehydrogenation, and particularly to a catalyst for dehydrogenation of 12H-N-ethylcarbazole and a preparation method thereof. Background Art

[0002] Hydrogen energy is an important part of the future energy structure, and the wide application of hydrogen energy is restricted by technical limitations such as storage and transportation. The application of liquid hydrogen storage molecules is a strategy with important potential, and its cost and safety are higher than those of traditional cylinder hydrogen storage. In addition, liquid hydrogen storage molecules are convenient for use in wider scenarios of on-site hydrogen production and hydrogen utilization. It is imperative to develop highly efficient dehydrogenation catalysts for liquid hydrogen storage molecules, which has broad application prospects and scientific significance.

[0003] N-ethylcarbazole / 12H-N-ethylcarbazole is a common liquid hydrogen storage system, and the research on its hydrogenation process has been relatively mature, which can be achieved by common commercial palladium-carbon catalysts and ruthenium-carbon catalysts under high hydrogen pressure. However, the dehydrogenation reaction is directly coupled with the hydrogen use process, and more efficient, stable and low-cost catalysts need to be developed. In order to improve the utilization rate of precious metals, supported metal catalysts with atomically dispersed metals have been prepared. However, for the relatively large conjugated molecular structure of N-ethylcarbazole / 12H-N-ethylcarbazole, monodispersed metal atoms cannot achieve the hydrogenation process of the substrate. For metal catalysts with relatively large particle sizes, only the surface metal atoms can be utilized, and the low atomic utilization rate of the metal leads to an increase in cost. Therefore, the exploration of the optimal structure of the catalyst is one of the key issues in the development and application of this series of reactions.

[0004] In addition, in this series of dehydrogenation reactions, the active center of the catalyst is easily strongly adsorbed by intermediate products, resulting in catalyst deactivation. Catalyst deactivation leads to instability of the hydrogen production rate and an increase in the cost of catalyst replacement. Therefore, the preparation of highly active and highly stable dehydrogenation catalysts is also the top priority of the research. Summary of the Invention

[0005] The purpose of the present application is to provide a catalyst for dehydrogenation of 12H-N-ethylcarbazole and a preparation method thereof, so as to obtain a highly active and highly stable catalyst for dehydrogenation of 12H-N-ethylcarbazole. The specific technical solutions are as follows:

[0006] The first aspect of the present application provides a catalyst for dehydrogenation of 12H-N-ethylcarbazole, which comprises a support and fully exposed active metal clusters disposed on the support. The metal coordination number between the active metal clusters is 1-6. The active metal is selected from at least one of palladium, platinum, iridium, ruthenium, iron, nickel and cobalt. The support is selected from at least one of nanodiamond, nanographite sheet, coconut shell carbon, activated carbon, graphitic carbon nitride, hexagonal carbon nitride, graphene oxide, silica, aluminum oxide and titanium dioxide. Based on the mass of the catalyst, the loading amount of the active metal in the catalyst is 0.1 wt%-20 wt%.

[0007] In some embodiments of the present application, the metal coordination number between the fully exposed active metal clusters is 2-4.5.

[0008] In some embodiments of the present application, the cluster size of the fully exposed active metal clusters is 0.2 nm-3 nm.

[0009] The second aspect of the present application provides a preparation method of a catalyst for dehydrogenation of 12H-N-ethylcarbazole, which comprises the following steps:

[0010] Step 1: Disperse the support in deionized water to obtain a uniformly dispersed suspension.

[0011] Step 2: After adjusting the pH to 9-11 by dropping an alkaline solution into the suspension, then drop an active metal precursor solution to obtain a mixed solution. Wherein, the mass ratio of the support to the active metal in the active metal precursor solution is 4:1-999:1.

[0012] Step 3: React the mixed solution at 60°C-100°C for 60 min-180 min, and separate to obtain a catalyst precursor after the reaction ends.

[0013] Step 4: Activate the catalyst precursor at 150°C-450°C for 0.5 h-3 h to obtain the catalyst.

[0014] Wherein, the support is selected from at least one of nanodiamond, nanographite sheet, coconut shell carbon, activated carbon, graphitic carbon nitride, hexagonal carbon nitride, graphene oxide, silica, aluminum oxide and titanium dioxide; the active metal is selected from at least one of palladium, platinum, iridium, ruthenium, iron, nickel and cobalt.

[0015] In some embodiments of the present application, the alkaline solution is selected from at least one of sodium carbonate solution, sodium bicarbonate solution, sodium hydroxide solution, potassium hydroxide solution and ammonia water.

[0016] In some embodiments of the present application, the active metal precursor solution is selected from at least one of a palladium precursor solution, a platinum precursor solution, an iridium precursor solution, a ruthenium precursor solution, an iron precursor solution, a nickel precursor solution, and a cobalt precursor solution.

[0017] In some embodiments of the present application, the palladium precursor solution is selected from at least one of a palladium acetate nitrate solution, a palladium chloride solution, a palladium acetylacetonate solution, a tetraamminepalladium chloride solution, a palladium nitrate solution, and a palladium sulfate solution; the platinum precursor solution is selected from at least one of a chloroplatinic acid solution, a platinum nitrate solution, and a tetraammineplatinum chloride solution; the iridium precursor solution is selected from at least one of an iridium nitrate solution, an iridium chloride solution, and an iridium sulfate solution; the ruthenium precursor solution is selected from at least one of a ruthenium chloride solution and a ruthenium carbonyl solution; the iron precursor solution is selected from at least one of a ferric chloride solution, a ferric nitrate solution, and a ferrous chloride solution; the nickel precursor solution is selected from at least one of a nickel chloride solution, a nickel nitrate solution, and a nickel acetate solution; and the cobalt precursor solution is selected from at least one of a nickel chloride solution, a nickel nitrate solution, and a nickel acetate solution.

[0018] In some embodiments of the present application, the atmosphere of the activation reaction in step four is hydrogen and argon, and the volume ratio of hydrogen to argon is (5 - 100)∶(0 - 95).

[0019] Advantages of the present application:

[0020] The present application provides a catalyst for dehydrogenation of 12H-N-ethylcarbazole and a preparation method thereof, which includes a support and fully exposed active metal clusters disposed on the support. The intermetallic coordination number of the fully exposed active metal clusters is 1 - 6, and the active metal is selected from at least one of palladium, platinum, iridium, ruthenium, iron, nickel, and cobalt. The support is selected from at least one of nanodiamond, nanographite sheet, coconut shell carbon, activated carbon, graphitic carbon nitride, hexagonal carbon nitride, graphene oxide, silica, aluminum oxide, and titanium dioxide. Based on the total mass of the catalyst, the loading amount of the active metal in the catalyst is 0.1 wt% - 20 wt%. The catalyst provided by the present application exposes all metal atoms on the surface of the support, improves the utilization rate of the active metal, and further improves the catalytic activity of the catalyst. The catalyst of the present application is used in the dehydrogenation reaction of 12H-N-ethylcarbazole and can achieve a stable dehydrogenation process.

[0021] Of course, it is not necessary for any product or method implementing the present application to simultaneously achieve all the above-mentioned advantages. Description of the Drawings

[0022] To more clearly illustrate the technical solutions of this application and the prior art, the following briefly introduces the attached drawings required in the embodiments and the prior art. Obviously, the attached drawings in the following description are only some embodiments of this application. For those of ordinary skill in the art, without creative efforts, other embodiments can be obtained based on these attached drawings.

[0023] Figure 1a High-angle annular dark-field (HAADF-STEM) photograph of the scanning transmission electron microscope of the catalyst prepared in Example 1 of this application;

[0024] Figure 1b HAADF-STEM photograph of the catalyst prepared in Example 2 of this application;

[0025] Figure 1c HAADF-STEM photograph of the catalyst prepared in Comparative Example 1 of this application;

[0026] Figure 1d HAADF-STEM photograph of the catalyst prepared in Comparative Example 2 of this application;

[0027] Figure 1e HAADF-STEM photograph of the catalyst prepared in Comparative Example 3 of this application;

[0028] Figure 1f HAADF-STEM photograph of the catalyst prepared in Comparative Example 4 of this application;

[0029] Figure 2 Diffuse reflectance infrared CO adsorption Fourier transform spectroscopy diagram of the catalysts prepared in Examples 1-2 and Comparative Examples 1-4 of this application;

[0030] Figure 3 Coordination and electronic structure energy spectrum diagram of the catalysts prepared in Examples 1-2 and Comparative Examples 1-4 of this application, where Figure 3 a in is the extended X-ray absorption fine structure (EXAFS) spectrum in the R space; Figure 3 b in is the Pd 3d X-ray photoelectron spectroscopy (XPS) spectrum;

[0031] Figure 4 Schematic diagram of the hydrogen storage cycle of 12H-N-ethylcarbazole / N-ethylcarbazole;

[0032] Figure 5a Line graph of the correlation between the metal coordination number and the corrected surface metal atom normalized activity (TOF) of the catalysts prepared in Examples 1-2 and Comparative Examples 1-4 of this application in the dehydrogenation reaction of 12H-N-ethylcarbazole;

[0033] Figure 5bLine graph showing the correlation between the coordination number between metals and the corrected TOF of the catalysts prepared in Examples 1-2 and Comparative Examples 1-4 of the present application in the dehydrogenation reaction of 8H-N-ethylcarbazole;

[0034] Figure 6 XRD spectra of the catalyst prepared in Example 2 of the present application before and after 5 cycles of 12H-N-ethylcarbazole dehydrogenation tests. Among them, Figure 6 a in it is the XRD spectrum of the catalyst prepared in Example 2 of the present application after 5 cycles of 12H-N-ethylcarbazole dehydrogenation tests, Figure 6 b in it is the XRD spectrum of the catalyst prepared in Example 2 of the present application before 5 cycles of 12H-N-ethylcarbazole dehydrogenation tests;

[0035] Figure 7 EXAFS spectra of the catalyst prepared in Example 2 of the present application before and after 5 cycles of 12H-N-ethylcarbazole dehydrogenation tests. Among them, Figure 7 c in it is the EXAFS spectrum of the catalyst prepared in Example 2 of the present application after 5 cycles of 12H-N-ethylcarbazole dehydrogenation tests, Figure 7 d in it is the EXAFS spectrum of the catalyst prepared in Example 2 of the present application before 5 cycles of 12H-N-ethylcarbazole dehydrogenation tests;

[0036] Figure 8 HAADF-STEM photograph of the catalyst prepared in Example 2 of the present application after 5 cycles of 12H-N-ethylcarbazole dehydrogenation tests;

[0037] Figure 9 Bar graph showing the change in the recovery rate of H2 during 20 times of 12H-N-ethylcarbazole dehydrogenation tests of the catalyst prepared in Example 3 of the present application. Detailed implementation manners

[0038] To make the objectives, technical solutions, and advantages of the present application clearer and more understandable, the following further details the present application with reference to the accompanying drawings and examples. Obviously, the described examples are only a part of the examples of the present application, rather than all the examples. All other technical solutions obtained by those of ordinary skill in the art based on the examples in the present application fall within the scope of protection of the present application.

[0039] In view of this, the first aspect of the present application provides a catalyst for dehydrogenation of 12H-N-ethylcarbazole, which includes a support and fully exposed active metal clusters disposed on the support. The metal coordination number between metals in the fully exposed active metal clusters is 1-6, preferably 2-4.5. The active metal is selected from at least one of palladium (Pd), platinum, iridium, ruthenium, iron, nickel, and cobalt, and the support is selected from at least one of nanodiamond, nanographite sheet, coconut shell carbon, activated carbon (VX-72), graphitic carbon nitride (g-C3N4), hexagonal carbon nitride (h-C3N4), graphene oxide, silica, aluminum oxide, and titanium dioxide; based on the total mass of the catalyst, the loading amount of the active metal in the catalyst is 0.1 wt%-20 wt%.

[0040] In the present application, the loading amount refers to the percentage of the mass of the active metal loaded on the support in the total mass of the catalyst. The calculation formula for the loading amount is: loading amount = mass of the active metal loaded on the support / total mass of the catalyst × 100%.

[0041] In the present application, the support (such as nanodiamond) is a type of crystalline sp 2 -C terminated surface sp 3 -C nanoparticles. The sp 2 -C layer contains abundant surface defects and functional groups (such as hydroxyl groups). By selecting this type of support in the synthesis process of the catalyst, these unique surface units serve as nucleation sites for metals, promoting the dispersion of relatively low-loading active metal atoms on the support, which helps to improve the stability of the catalyst structure during the reaction process.

[0042] In the present application, there is no particular limitation on the particle size of the support, as long as the object of the present application can be achieved. For example, the average particle size of the support is 5 nm-200 nm.

[0043] In some embodiments of the present application, the cluster size of the fully exposed active metal clusters is 0.2 nm-3 nm.

[0044] The second aspect of the present application provides a preparation method of a catalyst for dehydrogenation of 12H-N-ethylcarbazole, which includes the following steps:

[0045] Step 1: Disperse the support in deionized water to obtain a uniformly dispersed suspension;

[0046] Step 2: After adjusting the pH of the suspension to 9-11 by dropping an alkaline solution, then dropwise add an active metal precursor solution to obtain a mixed solution; wherein, the mass ratio of the support to the active metal in the active metal precursor solution is 4:1-999:1;

[0047] Step 3: The mixed solution reacts at 60°C - 100°C for 60 min - 180 min, and after the reaction ends, the catalyst precursor is separated out.

[0048] Step 4: The catalyst precursor is subjected to an activation reaction at 150°C - 450°C for 0.5 h - 3 h to obtain the catalyst.

[0049] Among them, the carrier is selected from at least one of nanodiamond, nanographite sheet, coconut shell carbon, activated carbon (VX-72), graphitic carbon nitride (g-C3N4), hexagonal carbon nitride (h-C3N4), graphene oxide, silicon dioxide, aluminum oxide, and titanium dioxide; the active metal is selected from at least one of palladium, platinum, iridium, ruthenium, iron, nickel, and cobalt.

[0050] In some embodiments of the present application, the alkaline solution is selected from at least one of sodium carbonate solution, sodium bicarbonate solution, sodium hydroxide solution, potassium hydroxide solution, and ammonia water.

[0051] In some embodiments of the present application, the active metal precursor solution is selected from at least one of palladium precursor solution, platinum precursor solution, iridium precursor solution, ruthenium precursor solution, iron precursor solution, nickel precursor solution, and cobalt precursor solution.

[0052] In some embodiments of the present application, the palladium precursor solution is selected from at least one of palladium acetate nitrate solution, palladium chloride solution, palladium acetylacetonate solution, tetraamminepalladium chloride solution, palladium nitrate solution, and palladium sulfate solution; the platinum precursor solution is selected from at least one of chloroplatinic acid solution, platinum nitrate solution, and tetraammineplatinum chloride solution; the iridium precursor solution is selected from at least one of iridium nitrate solution, iridium chloride solution, and iridium sulfate solution; the ruthenium precursor solution is selected from at least one of ruthenium chloride solution and ruthenium carbonyl solution; the iron precursor solution is selected from at least one of ferric chloride solution, ferric nitrate solution, and ferrous chloride solution; the nickel precursor solution is selected from at least one of nickel chloride solution, nickel nitrate solution, and nickel acetate solution; the cobalt precursor solution is selected from at least one of nickel chloride solution, nickel nitrate solution, and nickel acetate solution. Exemplarily, palladium nitrate (Pd(NO3)2) solution is used as the metal precursor solution.

[0053] In some embodiments of the present application, the atmosphere of the activation reaction in Step 4 is hydrogen and argon, and the volume ratio of hydrogen to argon is (5 - 100)∶(0 - 95). Exemplarily, the volume ratio of hydrogen to argon is 10∶90.

[0054] In this application, the separation process of obtaining the catalyst precursor after the reaction ends is as follows: First, the solid in the mixed solution is separated by using the commonly used solid-liquid separation methods in the art, and then it is washed several times and dried to obtain the catalyst precursor; among them, the solid-liquid separation methods include, but are not limited to, filtration, suction filtration, centrifugation, sedimentation, etc., and the washing includes, but is not limited to, water washing, alcohol washing, etc. This application does not limit this separation process, as long as the purpose of this application can be achieved. Exemplarily, the following separation process can be adopted: After cooling to room temperature, the mixed solution is suction filtered, then washed 5 times with deionized water, and dried in vacuum at 120 °C for 12 h to obtain the catalyst precursor.

[0055] Hereinafter, examples and comparative examples are given to illustrate the embodiments of this application more specifically. Various tests and evaluations are carried out according to the following methods. In addition, unless otherwise specified, "parts" and "%" are based on mass.

[0056] Test methods and equipment:

[0057] HAADF-STEM images are obtained by operating a scanning transmission electron microscope with spherical aberration correction at 60 kV.

[0058] The average particle size of clusters / particles is obtained by calculating the average value by measuring the sizes of 100 clusters / particles in the scanning transmission electron microscope images of the catalyst at the same magnification.

[0059] The loading amount of the active metal in the catalyst is determined by an inductively coupled plasma atomic emission spectrometer.

[0060] The dispersion of active metal single atoms / clusters / nanoparticles is measured by H2-O2 titration experiment.

[0061] The content of metal atoms dispersed as single atoms in the catalyst is measured by in-situ diffuse reflectance infrared CO adsorption.

[0062] The electronic structure spectrum of the catalyst is tested by an X-ray photoelectron spectrometer (XPS). Before measurement, the catalyst is transferred to a glove box, the catalyst is made into a thin film and sealed in an XPS sample stage, and the catalyst to be tested is activated according to the same activation process as in the example. Subsequently, the activated catalyst is transferred to an ultra-high vacuum chamber without exposing it to air through the sample stage for XPS measurement. After measurement, all spectra are calibrated based on the diamond sp 3 carbon feature (286.8 eV).

[0063] Extended X-ray absorption fine structure (EXAFS) analysis is carried out by X-ray absorption spectroscopy (XAS) to determine the metal-metal coordination number of the catalyst.

[0064] The normalized activity of metal atoms or the normalized activity of surface metal atoms (TOF) in the dehydrogenation reaction is tested through the following process. The normalized activity of Pd atoms includes all Pd atoms on the catalyst, while the normalized activity of surface atoms (TOF) takes into account the Pd metal dispersion measured by the H2-O2 titration experiment. The unit of both is min -1 . Without exposure to air, the activated catalyst is stirred and mixed with a 1.0 mol / L solution of 12H-N-ethylcarbazole (or a 0.5 mol / L solution of 4H-N-ethylcarbazole) in a two-neck flask, and reflux condensation is carried out. The dehydrogenation reaction is carried out at 170 °C by immersing the flask in a heated oil bath. During the dehydrogenation process, argon (Ar) (40 mL / min) is continuously purged into the flask to remove the generated hydrogen (H2) from the system and maintain a stable atmospheric pressure inside the flask. After each dehydrogenation reaction, the liquid phase is separated from the catalyst by centrifugation, and the products are quantitatively analyzed by a gas chromatograph equipped with a capillary column (HP-5) and a flame ionization detector (FID) to obtain the normalized activity of Pd atoms. Then, the normalized activity of surface metal atoms (TOF) of the catalyst is calculated. The calculation formula of TOF is: TOF = TOF Pdn / p ×(1 – F Pd1 ) + TOF Pd1 ×F Pd1

[0065] where, TOF Pdn / p is the normalized activity of Pd atoms of the catalyst, TOF Pd1 is the TOF of the Pd1 / ND catalyst, and F Pd1 is the mass fraction of palladium single atoms dispersed in atoms in the catalyst. TOF Pd1 is determined by in-situ diffuse reflectance infrared CO adsorption.

[0066] Example 1

[0067] Nano-diamond (ND) is used as the carrier, purchased from Beijing Geleshi High-Tech Co., Ltd., with a purity of 99.9% and an average particle size of 5 nm - 10 nm.

[0068] Preparation of catalyst precursor: The catalyst was prepared by the deposition-precipitation method. 200 mg of ND and 50 mL of deionized water were added to a round-bottom flask, and ultrasonicated for 80 min to obtain a uniformly mixed suspension; Na2CO3 solution (molar concentration of 0.5 mol / L) was added dropwise to the above suspension. After adjusting the pH value of the suspension to 10, the flask was immersed in an oil bath at 100 °C and stirred. Then, 5 mL of Pd(NO3)2 solution with a solute mass fraction of 0.021 wt% was added dropwise to the stirred suspension to obtain a mixed solution; this mixed solution was stirred and reacted in an oil bath at 100 °C for 60 min, then the heating was stopped. After cooling to room temperature, the mixed solution was filtered by suction, then washed 5 times with deionized water, and dried under vacuum at 120 °C for 12 hours (h) to obtain the catalyst precursor.

[0069] Activation of catalyst precursor: The catalyst precursor obtained above was reduced in a mixed atmosphere with a volume ratio of hydrogen:argon = 10:90 at 300 °C for 1 h for activation, and then the catalyst, denoted as Pd n1 / ND.

[0070] Example 2

[0071] Except for the Pd(NO3)2 solution with a solute mass fraction of 0.042 wt%, the others were the same as in Example 1.

[0072] Example 3

[0073] Except for the Pd(NO3)2 solution with a solute mass fraction of 0.084 wt%, the others were the same as in Example 1.

[0074] Example 4

[0075] Except for the palladium chloride solution with a solute mass fraction of 0.021%, the others were the same as in Example 1.

[0076] Example 5

[0077] Except for adjusting the pH to 9, the others were the same as in Example 2.

[0078] Example 6

[0079] Except for adjusting the pH to 11, the others were the same as in Example 2.

[0080] Example 7

[0081] Except for the reaction temperature of the mixed solution being 80 °C, the others were the same as in Example 2.

[0082] Example 8

[0083] Except for the reaction temperature of the mixed solution being 60 °C, the others were the same as in Example 2.

[0084] Example 9

[0085] Except that the reaction time of the mixed solution is 60 min, the others are the same as in Example 2.

[0086] Example 10

[0087] Except that the reaction time of the mixed solution is 180 min, the others are the same as in Example 2.

[0088] Example 11

[0089] Except that the activation temperature of the catalyst precursor is 150 °C for 3 h, the others are the same as in Example 2.

[0090] Example 12

[0091] Except that the activation temperature of the catalyst precursor is 450 °C for 0.5 h, the others are the same as in Example 2.

[0092] Example 13

[0093] Except that the support is g-C3N4 and the palladium chloride solution with a solute mass fraction of 3.02 wt% is used, the others are the same as in Example 1.

[0094] Comparative Example 1

[0095] Except that the Pd(NO3)2 solution with a solute mass fraction of 0.042 wt% and the activation temperature is 200 °C, the others are the same as in Example 1.

[0096] Comparative Example 2

[0097] The catalyst was synthesized by the equal-volume impregnation method. In a 10 mL crucible, 200 mg of ND was wetted with 800 μL of a Pd(NO3)2 solution with a solute mass fraction of 0.263 wt%, and the mixture was stirred evenly to obtain a slurry. Then, the slurry was slowly stirred at room temperature for 0.5 h and dried at 60 °C for 12 h to obtain the catalyst precursor.

[0098] Then, after activation using the same activation process as in Example 1, the catalyst was obtained and denoted as Pd p1 / ND.

[0099] Comparative Example 3

[0100] The catalyst was prepared by chemical reduction method. 540 μL of PdCl2-HCl solution (10 mg of PdCl2 was dissolved in 1 mL of 0.4 mol / L HCl) and 0.3 mmol of trisodium citrate were mixed in 80 mL of deionized water. Then, 200 mg of ND was added to the above solution, and then ultrasonicated for 30 min to obtain a uniformly mixed suspension. Subsequently, at 60 °C, 10 mL of NaBH4 solution (molar concentration of 0.1 mol / L) was added dropwise to the above stirring suspension, and stirring was continued at 60 °C for 6 h. Then, heating was stopped. After cooling to room temperature, the mixture was filtered by suction, then washed 3 times with deionized water, and dried in vacuum at 60 °C for 12 h to obtain the catalyst precursor.

[0101] Then, after activation using the same activation process as in Example 1, the catalyst was obtained and denoted as Pd p2 / ND.

[0102] Comparative Example 4

[0103] The catalyst was synthesized by the incipient wetness impregnation method. In a 10 mL crucible, 200 mg of ND was wetted with 800 μL of Pd(NO3)2 solution with a solute mass fraction of 0.263 wt% and 200 mg of citric acid, and mixed uniformly to obtain a slurry. Then, after the slurry was slowly stirred at room temperature for 0.5 h, it was dried at 120 °C for 12 h to obtain the catalyst precursor.

[0104] Then, after activation using the same activation process as in Example 1, the catalyst was obtained and denoted as Pd p3 / ND.

[0105] Table 1 shows the composition and structural characterization parameters of the catalysts prepared in each example and each comparative example.

[0106] Table 1

[0107]

[0108] a : Not detected, content negligible

[0109] b : Content of palladium single atoms dispersed as single atoms

[0110] Compared with Comparative Examples 2-4, Examples 1-13 in Table 1 showed higher dispersion because they were basically composed of fully exposed palladium clusters and single atoms. Comparative Example 1 showed a 100% dispersion because it was entirely composed of single atoms. For the catalysts in Comparative Examples 2-4, their dispersion decreased rapidly from 61.5% to 27.6% because massive palladium particles appeared in these catalysts.

[0111] Figure 1a shows the Pd prepared in Example 1 of the present application n1 / ND catalyst; Figure 1b shows the Pd prepared in Example 2 of the present application n2 / ND catalyst; Figure 1c shows the Pd1 / ND catalyst prepared in Comparative Example 1 of the present application; Figure 1d shows the Pd prepared in Comparative Example 2 of the present application p1 / ND catalyst; Figure 1e shows the Pd prepared in Comparative Example 3 of the present application p2 / ND catalyst; Figure 1f shows the Pd prepared in Comparative Example 4 of the present application p3 / ND catalyst; Figures 1a to 1f Among them, the circular area and the rectangular area highlight palladium single atoms and palladium clusters respectively, and the brighter contrast area is palladium nanoparticles. Among them, the Pd1 / ND catalyst has the smallest palladium loading (0.07 wt%), and it can be seen from Figure 1c that Pd1 / ND mainly shows isolated single atoms on the support; as the loading increases, palladium clusters appear in the Pd n1 / ND catalyst and Pd n2 / ND catalyst. The palladium clusters in the Pd n1 / ND catalyst are just the aggregation of several atoms (see Figure 1a ), while in the Pd n2 / ND catalyst, atomic layer-like clusters of 1 nm - 3 nm are formed (see Figure 1b ). For Pdp1 / ND, Pdp2 / ND and Pdp3 / ND, the catalysts all contain palladium nanoparticles with relatively clear crystal structures. The average size of the palladium nanoparticles gradually increases from 2.7 nm (Pdp1 / ND, see Figure 1d ) to 4 nm (Pdp2 / ND, see Figure 1e ) and 8.2 nm (Pdp3 / ND, see Figure 1f ).

[0112] During the synthesis process of the catalyst of the present application, atomically dispersed palladium single atoms (Pd1) inevitably exist. The present application measured and calculated the Pd1 content in the catalysts prepared in Examples 1 - 13 and Comparative Examples 1 - 4 by diffuse reflectance infrared CO adsorption. The calculation results are shown in Table 1. Among them, the CO adsorption diffuse reflectance infrared Fourier transform spectroscopy diagrams of the catalysts prepared in Examples 1 - 2 and Comparative Examples 1 - 4 are as shown in Figure 2 . The single-atom Pd1 / ND catalyst prepared in Comparative Example 1 of the present application has 100% Pd1. The Pd n1 / ND catalyst and Pd n2The Pd1 in the / ND catalysts are 44.1% and 34.7% respectively. However, for the nanoparticle catalysts prepared in Comparative Examples 1-3 of this application, as the average size of the metal nanoparticles increases, the proportion of Pd1 in the catalyst decreases significantly. In Pd p1 / ND, it is 12.5%, and in Pd p2 / ND, it is only 4.2%, and finally becomes negligible (not detected in Pd p3 / ND).

[0113] Figure 3 Figure a in shows the EXAFS spectra in the R space of the catalysts, palladium foil, and palladium oxide prepared in Examples 1-2 and Comparative Examples 1-4 of this application. This application determined the metal-metal coordination numbers of the catalysts prepared in each example and comparative example through EXAFS fitting calculations, as shown in Table 1. Obviously, as the active metal loading increases, the increasing metal-metal coordination numbers reflect the overall structural evolution of palladium, from mainly isolated single atoms to aggregates of a few atoms (i.e., clusters), and finally to aggregates with a relatively regular structure and larger size (i.e., nanoparticles). As the size of the metal clusters / nanoparticles increases, the metal-metal coordination number increases. Considering the strong correlation between the size of the metal clusters / nanoparticles and the average coordination number, the metal-metal coordination number describes the statistical average local coordination characteristics of the active sites. Compared with the average particle size obtained by sampling in transmission electron microscopy, the metal-metal coordination number is more suitable as a structural description of the metal catalyst.

[0114] Figure 3 Figure b in is the Pd 3d XPS spectrum, from which it can be clearly seen that Pd p2 / ND and Pd p3 / ND have similar peak positions near 335.6 eV, which is attributed to the metallic state of Pd. As the Pd content decreases, the Pd 3d peak gradually shifts to a higher binding energy; it is worth noting that for Pd1 / ND, the position of its peak is at 336.7 eV, and a shift of 1.1 eV from the metallic state of Pd (335.6 eV) is observed. The peak positions of Examples 1 and 2 are 336.2 eV and 336.0 eV respectively, and their valence states are between the positive valence of the Pd1 / ND single atoms and the metallic states of Pd p2 / ND and Pd p3 / ND nanoparticles. Generally speaking, as the Pd-Pd coordination number increases, the electronic structure of Pd changes from a positive valence state to a zero-valence metallic state.

[0115] The catalysts in Examples 1-13 and Comparative Examples 1-4 of this application were applied to the dehydrogenation reaction of 12H-N-ethylcarbazole. The reaction was carried out at 170 °C and 1 atmosphere, and the substrate conversion rate was controlled below 20%. The schematic diagram of the hydrogen storage cycle of 12H-N-ethylcarbazole (DNEC) / N-ethylcarbazole (NEC) is as shown in Figure 4 , and its intermediate reaction in the cycle includes the dehydrogenation reaction of 8H-N-ethylcarbazole (ONEC) / 4H-N-ethylcarbazole (TNEC).

[0116] Table 2 gives the Pd-normalized activity (i.e., the consumption rate of the reactant is normalized on the total Pd loading, showing the catalytic ability of the catalyst based on all the loaded active metals, which is an important economic indicator of noble metal catalysts) and TOF (the normalized activity of surface Pd atoms in the dehydrogenation reaction) of each catalyst in the dehydrogenation reaction of 12H-N-ethylcarbazole and the dehydrogenation reaction of 4H-N-ethylcarbazole.

[0117] Table 2

[0118]

[0119]

[0120] In the dehydrogenation reaction of 12H-N-ethylcarbazole, although the Pd1 / ND single-atom catalyst prepared in Comparative Example 1 has the highest atomic utilization efficiency, it has almost no activity in the reaction, indicating that the dehydrogenation of the five-membered ring requires multi-atom metal aggregates (clusters or nanoparticles) rather than metal single-atom sites. As can be seen from Table 2, the fully exposed cluster catalysts provided in Examples 1-13 of this application all have high activity. It is worth noting that Pd n2 / ND prepared in Example 2 of this application shows the highest activity among all the catalysts, and its TOF is 3-5 times higher than that of the catalysts prepared in Comparative Examples 2-4, indicating that the fully exposed cluster catalysts provided in the examples of this application are the most reactive structures in the reaction.

[0121] Since Pd1 is inactive in the catalytic reaction and does not participate in the catalytic reaction, for the fully exposed cluster catalysts and nanoparticle catalysts, they contain a mixture of active Pd aggregates (clusters or nanoparticles) and inactive Pd1. The presence of Pd1 weakens the normalized activity or TOF of the active Pd aggregates. The inventor obtained the correlation broken line of the Pd-Pd coordination number and the corrected TOF (corrected by deducting the TOF contributed by the atomically dispersed palladium single atoms) by subtracting the contribution of Pd1 to the Pd aggregate (cluster or nanoparticle) catalyst, as shown in Figure 5a . The shape of the broken line of the intermetallic coordination number and the corrected TOF of the catalyst is a volcano-shaped curve, and the intermetallic coordination number corresponding to its highest activity is between 2 and 4.5. Specifically, Pd provided in Example 2 of this applicationn2 The intermetallic coordination number of Pd-Pd in the Pd / ND catalyst is 4.4, and the overall TOF with the highest correction reaches 233.2 min -1 As shown in Table 2, in the present application, the fully exposed Pd cluster catalyst Pd with an intermetallic coordination number of Pd-Pd of 4.4 n2 catalyst has the highest activity. In addition, Pd with a smaller cluster size n1 catalyst (the intermetallic coordination number is 1.9) shows a TOF similar to that of Pd n2 catalyst, indicating that the initial dehydrogenation of 12H-N-ethylcarbazole to 8H-N-ethylcarbazole can be achieved by several consecutive Pd atoms.

[0122] Existing studies have shown that the dehydrogenation of 4H-N-ethylcarbazole to N-ethylcarbazole is the rate-determining step in the whole dehydrogenation process. As shown in Table 2, the dehydrogenation rate of 4H-N-ethylcarbazole is two orders of magnitude lower than that of 12H-N-ethylcarbazole. Despite the huge kinetic differences, a similar trend in catalyst performance was observed, that is, the reactivity of the Pd1 / ND catalyst is very low, and the most active catalyst for the dehydrogenation of 4H-N-ethylcarbazole is Pd n2 / ND catalyst. Similarly, after subtracting the contribution of Pd1 from Pd nx / ND (x = 1, 2) and Pd py / ND (y = 1, 2, 3), a correlation broken line between the intermetallic coordination number of Pd-Pd and the corrected TOF was established, as shown in Figure 5b . It should be noted that although the activity of the Pd n2 / ND catalyst is still several times that of the Pd py / ND catalyst, the TOF of the Pd n1 / ND catalyst drops to half of that of Pd n2 / ND, which is in sharp contrast to the similar reactivity of Pd n1 / ND and Pd n2 / ND in the first dehydrogenation of 12H-N-ethylcarbazole. This result clearly shows the different structural requirements for atomic Pd sites in the first and last dehydrogenation steps of 12H-N-ethylcarbazole. Therefore, in order to ensure the effective dehydrogenation of both 12H-N-ethylcarbazole and 4H-N-ethylcarbazole substrates, a mixed system of fully exposed Pd cluster catalysts can be used. For example, a mixed system of the catalysts provided in Example 1 and Example 2 of the present application can be used; Pd1 / ND provided in Comparative Example 1 is almost inactive in the dehydrogenation reaction of 12H-N-ethylcarbazole and has low catalytic activity; compared with the palladium nanoparticle catalyst, Pd provided in Example 2 of the present application n2The / ND catalyst contains fully exposed atomic-layer palladium clusters with a Pd-Pd intermetallic coordination number of 4.4, having a higher surface density of metal active sites and fewer atoms, and can catalyze all dehydrogenation steps with higher atomic efficiency and intrinsic activity, which is very important for designing noble metal catalysts for industrial applications.

[0123] Figure 6 XRD patterns of the Pd n2 / ND catalyst prepared in Example 2 of this application before and after 5 cycles of 12H-N-ethylcarbazole dehydrogenation tests are presented. Figure 7 XRD patterns of the Pd n2 / ND catalyst prepared in Example 2 of this application before and after 5 cycles of 12H-N-ethylcarbazole dehydrogenation tests are presented. Figure 8 XRD patterns of the Pd n2 / ND catalyst prepared in Example 2 of this application after 5 cycles of 12H-N-ethylcarbazole dehydrogenation tests are presented. HAADF-STEM images from Figure 6 、 Figure 7 、 Figure 8 show that the catalyst provided by this application has good stability, and no obvious structural changes or activity losses are observed for the Pd n2 / ND catalyst after 5 cycles of dehydrogenation tests.

[0124] As Figure 9 shown, the catalyst prepared in Example 3 of this application has an H2 recovery rate lower than 20% in 20 cycles of 12H-N-ethylcarbazole dehydrogenation tests. The dehydrogenation catalyst provided by this application can achieve a stable dehydrogenation reaction for 20 cycles and can realize a stable dehydrogenation process.

[0125] For the catalyst prepared in Example 13 of this application, as shown in Table 1 and Table 2, at a relatively high loading (18.2 wt%), its intermetallic coordination number is 5.2, and the main loading form is fully exposed clusters, which can maintain a high Pd-normalized activity (103.7 min -1 ) and a high TOF (126.3 min -1 ), which is beneficial to obtaining a catalyst with high catalytic activity for 12H-N-ethylcarbazole dehydrogenation.

[0126] The catalyst provided by this application exposes all metal atoms on the surface of the support, designs the structure of the catalyst from the atomic scale, improves the utilization rate of the active metal, and thus improves the catalytic activity of the catalyst. The catalyst provided by this application for 12H-N-ethylcarbazole dehydrogenation can achieve a stable dehydrogenation process.

[0127] The above are only the preferred embodiments of the present application and are not intended to limit the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present application shall be included within the scope of protection of the present application.

Claims

1. A catalyst for dehydrogenation of 12H-N-ethylcarbazole, which comprises a support and fully exposed active metal clusters disposed on the support, the intermetallic coordination number of the fully exposed active metal clusters being 1-6, the active metal being selected from at least one of palladium, platinum, iridium and ruthenium, and the support being selected from at least one of nanodiamond and graphitic carbon nitride; Based on the total mass of the catalyst, the loading amount of the active metal in the catalyst is 0.1 wt%-20 wt%.

2. The catalyst according to claim 1, wherein, The metal coordination number of the fully exposed active metal clusters is 2 - 4.

5.

3. The catalyst according to claim 1, wherein, The cluster size of the fully exposed active metal clusters is 0.2 nm - 3 nm.

4. A method for preparing the catalyst according to claim 1, which comprises the following steps: Step 1: Disperse the support in deionized water to obtain a uniformly dispersed suspension. Step 2: After adjusting the pH of the suspension to 9 - 11 by dropping an alkaline solution, then dropwise add an active metal precursor solution to obtain a mixed solution; wherein, the mass ratio of the support to the active metal in the active metal precursor solution is 4∶1 - 999∶1. Step 3: React the mixed solution at 60 - 100 °C for 60 min - 180 min, and separate to obtain a catalyst precursor after the reaction ends. Step 4: Activate the catalyst precursor at 150 °C - 450 °C for 0.5 h - 3 h to obtain the catalyst. Among them, the support is selected from at least one of nanodiamond and graphitic carbon nitride; the active metal is selected from at least one of palladium, platinum, iridium, and ruthenium.

5. The preparation method according to claim 4, wherein, The alkaline solution is selected from at least one of sodium carbonate solution, sodium bicarbonate solution, sodium hydroxide solution, potassium hydroxide solution, and ammonia water.

6. The preparation method according to claim 4, wherein, The active metal precursor solution is selected from at least one of a palladium precursor solution, a platinum precursor solution, an iridium precursor solution, and a ruthenium precursor solution.

7. The preparation method according to claim 6, wherein, The palladium precursor solution is selected from at least one of palladium chloride solution, palladium acetylacetonate solution, tetraamminepalladium chloride solution, palladium nitrate solution, and palladium sulfate solution; the platinum precursor solution is selected from at least one of chloroplatinic acid solution, platinum nitrate solution, and tetraammineplatinum chloride solution; the iridium precursor solution is selected from at least one of iridium nitrate solution, iridium chloride solution, and iridium sulfate solution; the ruthenium precursor solution is selected from at least one of ruthenium chloride solution and ruthenium carbonyl solution.

8. The preparation method according to claim 4, wherein, The atmosphere of the activation reaction in Step 4 is hydrogen and argon, and the volume ratio of hydrogen to argon is (5 - 100)∶(0 - 95).

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