CN117160448A - Pd single-site catalyst, preparation method of supported palladium catalyst and preparation method of hydrogen peroxide - Google Patents
Pd single-site catalyst, preparation method of supported palladium catalyst and preparation method of hydrogen peroxide Download PDFInfo
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Abstract
The application relates to a Pd single-site catalyst, a preparation method of a supported palladium catalyst and a preparation method of hydrogen peroxide, wherein the Pd single-site catalyst comprises TiO 2 And active Pd supported on TiO in the form of single sites 2 Applying; wherein the mass ratio of the active Pd calculated by Pd element in the Pd single-site catalyst is 0.05% -0.25%. Pd single-site catalyst is adopted as H 2 O 2 The Pd in the catalyst is in atomic fraction, so that the catalyst for direct synthesis has high atomic utilization rate, larger surface energy and better activity; because the Pd single-site catalyst has Pd-Pd metal bond and higher activity compared with the single-site catalyst, H can be dissociated better in the catalytic process 2 And activating O 2 Improve H 2 O 2 Synthetic activity.
Description
Technical Field
The application relates to the field of heterogeneous chemical catalysis, in particular to a Pd single-site catalyst, a preparation method of a supported palladium catalyst and a preparation method of hydrogen peroxide.
Background
Hydrogen peroxide (H) 2 O 2 ) Is used as an indispensable oxidant for the environment protection and is widely applied to a plurality of fields of social production and lifeIn particular to the fields of precision machining, industrial catalysis, environmental protection, medical disinfection and the like. The demand is huge and the demand is gradually increasing due to wide application. Currently, nearly 95% of global hydrogen peroxide production is produced by the anthraquinone process. Currently, the conventional anthraquinone method has some disadvantages, such as: the production and energy consumption are huge, and only large-scale investment or production has certain economic benefit; the generation of three wastes (waste gas mainly comprising heavy aromatic hydrocarbon, waste water mainly comprising waste water and hydrocarbon, waste residue mainly comprising alumina) causes serious pollution to the environment; the noble metal loss of the catalyst is serious, for example, the loss of palladium element in a high-loading catalyst is relatively high; the high-concentration hydrogen peroxide has certain potential safety hazard in transportation and is inconvenient to transport and store; small production and coupling catalysis are not economical. The above-mentioned numerous drawbacks limit the better development of this related field.
Aiming at the defects of anthraquinone process, H is adopted 2 O 2 The direct synthesis method may be an effective synthesis process. The method refers to that H under a certain pressure 2 And O 2 Introducing into the solution containing the catalyst, stirring, etc. to obtain H 2 And O 2 Synthesis of H directly in solution under the action of catalyst 2 O 2 . Through O 2 And H 2 Direct synthesis of H under catalyst agitation 2 O 2 Is a potential method, and has been paid attention to because of high atom utilization rate, convenient preparation, no pollution, simple separation and purification, direct in-situ coupling catalysis and the like.
For the direct synthesis of hydrogen peroxide, the traditional catalyst is usually mainly a supported palladium catalyst, but because water is a product which is easier to form in thermodynamics, the palladium catalyst is favorable for side reactions such as hydrogenation and decomposition of hydrogen peroxide, so that the hydrogen peroxide yield and selectivity are low. In addition, the traditional palladium catalyst has high cost, consumes a large amount of Pd precursor, has low atomic utilization rate of Pd element in the catalysis, and has certain palladium loss in the method, thus leading the H obtained by the method 2 O 2 The yield is affected to some extent.
Now atomically dispersed catalyst pair H 2 O 2 The direct synthesis side reaction of the catalyst is small, the relative loss of palladium element is small, the cost of the noble metal consumed in the preparation is lower, and the catalyst is suitable for industrial mass production in the future. Therefore, it is worth trying to develop a catalyst of single atom or single site with high activity.
Conventional/traditional palladium-based bulk catalysts or nanocatalysts, as described in H 2 O 2 There are high side reactions (hydrogenation and decomposition) in the direct synthesis of (a) and some loss of palladium during the reaction. In addition, these conventional palladium-based catalysts are expensive to prepare and consume large amounts of noble metal palladium. Thus, it hinders H 2 O 2 Direct synthesis has evolved better.
The reason for this is that, for conventional/traditional palladium-based bulk catalysts or nanocatalysts, the catalyst is prepared by reacting H 2 O 2 Is directly synthesized with H 2 And H 2 O 2 And coexist. Thus, the hydrogenation reaction (H 2 +H 2 O 2 →H 2 O) or with H 2 O 2 Is a direct contact decomposition reaction (H) 2 O 2 →H 2 O+O 2 ) Consumption H 2 O 2 Resulting in a decrease in the yield of the final product. In addition, due to the presence of the catalyst in O 2 And H 2 The active site Pd atoms circulate in the continuous oxidation-reduction reaction process to generate soluble Pd, thereby leading to loss of partial Pd element in the catalyst and further affecting the generation of H 2 O 2 Is a product of the above process. Therefore, the side reaction or Pd element loss in the traditional catalyst is too high, which leads to H 2 O 2 Adverse effects in direct synthesis.
For atomically dispersed catalysts, the noble metal content is low due to their high dispersion, use of palladium, and due to metal-support interactions (MSI), the catalyst is dispersed in H 2 O 2 The side reaction is small, the loss of Pd element is relatively less, the consumption of palladium precursor in the synthesis catalyst is less, and the price is low. Although the development of an atomic scale catalyst is advantageous for H in accordance with the advantages described above 2 O 2 But for a single-atom catalyst, since Pd-Pd bonds are not present, for H 2 O 2 Direct synthesis procedure (H) 2 +O 2 →H 2 O 2 ) Catalyst pair H 2 Dissociation and O of (2) 2 Has a limited activation effect, which results in synthesis of H 2 O 2 Has limited effectiveness.
Conventional Pd catalyst in H 2 O 2 The side reaction in the direct synthesis is high, the palladium loss is serious, and H is caused 2 O 2 The yield is lower, the Pd utilization rate is low, the Pd consumption is serious, and the catalyst cost is high. How to overcome the defects of the existing catalyst to obtain a catalyst capable of avoiding side reactions such as hydrogenation, decomposition and the like in the catalytic synthesis process of hydrogen peroxide, and further improving the hydrogen peroxide yield is a technical problem to be solved at present.
Disclosure of Invention
The application provides a Pd single-site catalyst, a preparation method of a supported palladium catalyst and a preparation method of hydrogen peroxide, and aims to further improve the direct synthesis of H by an atomic fraction separation catalyst 2 O 2 Is used for the catalytic performance of the catalyst.
In a first aspect, the present application relates to a Pd on site catalyst comprising TiO 2 And active Pd supported on TiO in the form of single sites 2 Applying; wherein the mass ratio of the active Pd calculated by Pd element in the Pd single-site catalyst is 0.05% -0.25%.
In a second aspect, the present application relates to a process for the preparation of a supported palladium catalyst for hydrogen peroxide synthesis, comprising the steps of:
(1) Making TiO 2 Dispersing in a solvent to obtain a first dispersion;
(2) Dispersing a palladium precursor in the first dispersion liquid, and separating out a solid phase material;
(3) Calcining the solid phase material in a non-reducing atmosphere to obtain the supported palladium catalyst;
wherein the mass ratio of Pd element in the supported palladium catalyst is 0.05-0.25wt%.
In a third aspect, the present application relates to a process for the preparation of hydrogen peroxide comprising reacting H 2 And O 2 In the presence of a catalyst, which is the Pd single-site catalyst of the first aspect or the supported palladium catalyst prepared by the preparation method of the second aspect.
The beneficial effects are that:
(1) Pd single-site catalyst is adopted as H 2 O 2 The Pd in the catalyst is in atomic fraction, so that the catalyst for direct synthesis has high atomic utilization rate (nearly 100%), large surface energy and good activity;
(2) The specific metal-carrier interaction (MSI) ensures that Pd is anchored on the carrier, so that the stability of Pd element in catalysis is increased, and the relative loss of Pd element is reduced;
(3) Because the Pd single-site catalyst has Pd-Pd metal bond and higher activity compared with the single-site catalyst, H can be dissociated better in the catalytic process 2 And activating O 2 Improve H 2 O 2 Synthetic activity;
(4) Pd single-site catalyst, in H 2 O 2 No side reaction or side reaction in the direct synthesis is detected below the detection limit according to the current general method, thereby improving H 2 O 2 Yield of (2);
(5) The cost of the catalyst is reduced (the dosage of Pd precursor is reduced), the relative loss of palladium is less, the consumption of palladium raw material is less, the cost for producing the catalyst with relative quality is low, and the catalyst is cheaper.
Drawings
FIG. 1 is an XRD spectrum of palladium single site catalysts A, B and C prepared in examples 1-3;
FIG. 2 is a bar graph of hydrogen peroxide yields in the synthesis of hydrogen peroxide for the palladium single site catalysts A, B and C prepared in examples 1-3 and the conventional palladium nanoparticle supported catalyst Pd NPs prepared in comparative example 1;
FIG. 3 is a schematic diagram showing the consumption rate of hydrogen peroxide hydrogenation and the decomposition rate of hydrogen peroxide in the hydrogenation reaction of hydrogen peroxide in the presence of the palladium unit catalysts A, B and C prepared in examples 1 to 3 and the conventional palladium nanoparticle supported catalyst Pd NPs prepared in comparative example 1;
FIG. 4 is a high angle annular dark field image (HADDF) photograph of a spherical aberration electron microscope of the palladium unit cell catalyst B prepared in example 2;
FIG. 5 is a Scanning Transmission Electron Microscope (STEM) photograph of the catalyst prepared in comparative example 1;
FIG. 6 shows hydrogen peroxide yields at various times during the synthesis of hydrogen peroxide with the catalyst prepared in example 2, with the catalyst prepared in comparative example 1 and with P25 titanium dioxide as catalyst.
Detailed Description
The application is further described in detail below by means of the figures and examples. The features and advantages of the present application will become more apparent from the description.
The word "exemplary" is used herein to mean "serving as an example, embodiment, or illustration. Any embodiment described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments. Although various aspects of the embodiments are illustrated in the accompanying drawings, the drawings are not necessarily drawn to scale unless specifically indicated.
In addition, the technical features described below in the different embodiments of the present application may be combined with each other as long as they do not collide with each other.
In a first aspect, the present application relates to a Pd on site catalyst comprising TiO 2 And active Pd supported on TiO in the form of single sites 2 Applying;
wherein the mass ratio of the active Pd calculated by Pd element in the Pd single-site catalyst is 0.05% -0.25%.
The active Pd is combined with TiO by strong metal-carrier interaction (SMSI) 2 The carrier coordinates to carry out the loading. The Pd single-site catalyst of the application can be used for directly synthesizing hydrogen peroxide by hydrogen and is an ultra-low load type palladium single-site catalyst. Active Pd is anchored on the surface of the carrier in a single site formCompared with a single-atom catalyst, the catalyst of atomic fraction dispersed single-site catalyst still retains Pd-Pd bond, and breaks hydrogen H-H bond and O in the initial synthesis stage 2 Better promoting effect of activation, thereby being more beneficial to H 2 O 2 Is a synthesis of (a). In addition, the Pd element content is extremely low, and the ultralow-load Pd single-site catalyst can effectively inhibit various side reactions, slow down the relative loss of the Pd element and is more beneficial to the direct synthesis of hydrogen peroxide.
For a single-site catalyst (for example, a diatomic or triatomic Pd catalyst), since Pd atoms with multiple atomic levels are used as active sites and Pd-Pd bonds exist in the catalytic process, the activity of the catalyst is possibly higher than that of the single-site catalyst except that the advantages of small side reaction, relatively less loss of Pd elements, low cost and the like are maintained. Further, corresponding to the use of H 2 O 2 In-situ reactions or series catalytic, coupling catalytic reactions, etc., single-site catalysts may also provide better catalytic activity than single-site catalysts.
In a second aspect, the present application relates to a method for preparing a supported palladium catalyst for hydrogen peroxide synthesis, the method comprising the steps of:
(1) Making TiO 2 Dispersing in a solvent to obtain a first dispersion;
(2) Dispersing a palladium precursor in the first dispersion liquid, and separating out a solid phase material;
(3) Calcining the solid phase material in a non-reducing atmosphere to obtain the supported palladium catalyst;
wherein the mass ratio of Pd element in the supported palladium catalyst is 0.05-0.25wt%.
In the first dispersion obtained in the step (1), tiO 2 Well dispersed in a solvent; dispersing a palladium precursor in the first dispersion liquid through the step (2), and further loading the palladium precursor on TiO 2 Is a surface of (2); calcining in non-reducing atmosphere in the step (3) to convert the palladium precursor into active Pd and anchoring the active Pd to TiO 2 Is a surface of the substrate. The catalyst prepared by the preparation method of the application is Pd single-site catalystAgent, active Pd in TiO 2 The support surface exists in the form of a plurality of active single sites, each of which may comprise 1 to 3 Pd atoms. In the Pd single-site catalyst, single Pd atoms and TiO are present 2 Wherein O forms Pd-O coordination bond, and Pd atoms are similar to Pd to form Pd-Pd metal bond. Pd exists in a monodisperse form (no nanoparticles or clusters are formed) whether it is a Pd-O coordination bond or a Pd-Pd metal bond. Each single site may have Pd-O bond or Pd-Pd bond, and is formed by calcining and decomposing organic matters. The product features a monodisperse catalyst with Pd-Pd bonds (Pd-Pd bonds are present, but not nanoparticles or clusters).
The low-load atomic-level high-dispersion palladium single-site catalyst prepared by the liquid phase method has the advantages of controllability, simplicity in preparation, less consumption of palladium raw materials, low price and convenience in amplification, and is suitable for industrial production.
It should be noted that the supported palladium catalyst for hydrogen peroxide synthesis prepared by the preparation method of the second aspect of the present application may be the Pd unit catalyst according to the first aspect of the present application, or the preparation method of the second aspect of the present application may prepare the Pd unit catalyst according to the first aspect of the present application.
According to a specific embodiment of the preparation method according to the second aspect of the present application, in the step (1), tiO 2 And the dispersion ratio of the solvent is 100mg: (5-15) mL; and/or the number of the groups of groups,
TiO 2 has a particle size of 15-30nm (purchased from Alfa Aesar Co.).
In the preferred embodiment, in the step (1), tiO 2 The dispersion in the solvent may be carried out in a container such as a bottle, the volume of the container being sufficient to allow TiO to be present 2 And the solvent can be well dispersed, and the volume ratio of the container to the solvent can be (2-5): 1.TiO (titanium dioxide) 2 Commercial titanium dioxide P25, i.e., titanium dioxide having an average particle diameter of about 21nm, is preferred.
In the preparation method of the present application, the TiO is controlled in the step (1) as described above 2 And dispersion of the solventRatio, simultaneously controlling TiO 2 Particle size of (2) to make TiO 2 Dispersing in solvent to obtain TiO in the first dispersion 2 Well dispersed in a solvent; then, step (2) is carried out to disperse the palladium precursor in the first dispersion liquid, so that the palladium precursor can be well dispersed and loaded on each TiO 2 The surface of the particles is also coated with TiO because the addition of the palladium precursor is small and the amount is controlled as described above 2 The active site formed on the surface of the carrier can be 1-3 Pd atoms, so that the Pd single-site catalyst is obtained.
According to another embodiment of the preparation method according to the second aspect of the present application, in the step (1), the solvent is selected from one or more of acetone, toluene, N-dimethylacetamide, glacial acetic acid, chloroform, tetrahydrofuran and deionized water.
In the step (1), the solvent is selected to be favorable for TiO 2 Better dispersion is carried out, which is beneficial to better dispersing the palladium precursor in TiO 2 The support surface forms a plurality of active sites, each of which may include 1 to 3 palladium atoms.
According to a specific embodiment of the preparation method according to the second aspect of the present application, in step (2), the palladium precursor is selected from [ Pd (NH) 3 ) 4 ](NO 3 ) 2 、Pd(NO 3 ) 2 、Na 2 PdCl 4 、PdCl 2 、(NH 4 ) 2 PdCl 6 、[Pd(OAc) 2 ] 3 、Pd(acac) 2 And [ Pd (C) 3 H 5 )Cl] 2 A combination of one or more of the following; and/or the number of the groups of groups,
the palladium precursor is dispersed in the first dispersion in the form of a palladium solution.
It should be noted that the palladium precursor is dispersed in the first dispersion liquid in the form of palladium solution, which is favorable for the palladium precursor to be more dispersedly loaded on TiO 2 The surface of the support better forms a single site catalyst.
According to a specific embodiment of the preparation method according to the second aspect of the present application, the step (2) further comprises the following steps before separating the solid phase material:
sequentially heating, mixing, standing and cooling the first dispersion liquid in which the palladium precursor is dispersed;
wherein the temperature of the heating and mixing is 20-100 ℃ and the time is 2-12h.
When the mixed heating is performed, the heating can be performed by using oil bath, water bath, electric heating sleeve heating and other modes, and the mixing can be performed by adopting stirring and other modes; after stopping mixing and heating, standing and cooling to room temperature, and then carrying out subsequent separation to obtain solid phase materials, wherein the standing time can be specifically 10-30min, and the separated solid phase materials can be powder. By controlling the temperature and time of heating as above, the palladium precursor can be better loaded to TiO 2 The Pd single-site catalyst with better catalytic performance can be prepared by the surface of the carrier and then carrying out the subsequent steps.
According to a specific embodiment of the preparation method according to the second aspect of the present application, step (3) further comprises vacuum drying before calcination in the non-reducing atmosphere;
wherein the temperature of the vacuum drying is 25-60 ℃ and the time is 4-8h.
The calcination may be performed in a tube furnace, and after the calcination is completed, the tube furnace is cooled to room temperature, and then solid powder is taken out to obtain the catalyst.
According to a specific embodiment of the preparation method according to the second aspect of the present application, in the step (3), the non-reducing atmosphere is nitrogen; and/or the number of the groups of groups,
the calcining temperature is 200-800 ℃, the calcining time is 2.5-3.5h, and the heating rate is 3-8 ℃/min.
In the production method of the present application, first, tiO is prepared 2 Dispersing in a solvent to obtain a first dispersion, dispersing a palladium precursor in the first dispersion, and loading the palladium precursor to TiO 2 The surface of the carrier is calcined in non-reducing atmosphere to convert the palladium precursor into palladium anchored in TiO 2 A carrier surface; by controlling the temperature and time of calcination in the nitrogen atmosphere as described above, it is possible to obtain a more preferablePd single-site catalyst with high catalytic activity is formed.
In addition, the Pd single site catalyst of the present application may also be prepared in the form of Atomic Layer Deposition (ALD). The Pd single-site catalyst according to the first aspect of the present application or the supported palladium catalyst prepared by the preparation method according to the second aspect of the present application can be used in the fields of hydrogen peroxide in-situ reaction, coupling reaction, tandem reaction, etc. For example: in situ generation of H by single site catalyst 2 O 2 Oxidation with methane light or heat, and propylene light or heat oxidation to produce propylene oxide, cyclohexanol or ketoxime, etc.
In a third aspect, the present application relates to a process for the preparation of hydrogen peroxide comprising reacting H 2 And O 2 In the presence of a catalyst, the catalyst is the Pd single-site catalyst according to the first aspect of the application or the supported palladium catalyst prepared by the preparation method according to the second aspect of the application.
The hydrogen peroxide of the present application is prepared by directly synthesizing hydrogen peroxide from hydrogen and oxygen. The Pd single-site catalyst according to the first aspect of the present application or the supported palladium catalyst prepared by the preparation method according to the second aspect of the present application shows high hydrogen peroxide yield and selectivity and excellent stability in the catalytic process. The preparation method adopted by the application is simple and convenient, has low noble metal loading (the minimum can reach 0.05 wt%) and is suitable for large-scale industrial production and preparation, and the catalyst effect is excellent and can reach 214mol/kg at the maximum cat * Yield of h and long-term use. Wherein, the high-pressure reactor can be a high-pressure reaction kettle.
At H 2 O 2 In the synthesis it is essentially a peroxy bond (O) 2 2- ) In the synthesis process of the catalyst adopting atomic-level dispersion, the active site can be effectively reduced or even inhibited (O-O) due to atomization and dispersion 2- Fracture of (C) to promote H 2 O 2 Is formed by the steps of (a). In addition, the carrier-anchored atomically dispersed active centers effectively prevent the flow of noble metal Pd elements in catalysisThe catalyst has the advantages of prolonging the service life of the catalyst, reducing the use amount of the raw material palladium precursor and lowering the cost of the catalyst. Compared with a single-atom catalyst, the catalyst of atomic fraction dispersed single-site catalyst still retains Pd-Pd bond, and breaks hydrogen H-H bond and O in the initial synthesis stage 2 The activation has better promoting effect, thereby being more beneficial to H 2 O 2 And the content of Pd element is low, belonging to ultra-low load Pd single-site catalyst. Therefore, the atomic-level dispersed palladium single-site catalyst prepared by the application can obtain better effect in industrial production when being applied to the synthesis of hydrogen and oxygen direct hydrogen peroxide.
According to a specific embodiment of the method of preparation according to the third aspect of the application, the contacting comprises:
mixing a catalyst, methanol and water in a reactor;
introducing into the reactor a catalyst comprising H 2 And O 2 Is a mixed gas of (a) and (b).
According to another embodiment of the production method according to the third aspect of the present application, the ratio of the mass of methanol to the total mass of methanol and water is 30% to 90%; and/or the number of the groups of groups,
the mixed gas also comprises N 2 The total pressure of the mixed gas is 0.5-12Mpa; in the mixed gas, H 2 、O 2 And N 2 The ratio of the pressure in the total pressure is 0.5% -5%, 1% -20% and 75% -98.5% respectively; and/or the number of the groups of groups,
mass of the catalyst and H 2 The molar ratio of (1) is (0.275-2.75) mg:1mmol.
Specifically, the catalyst, methanol and water may be added into the autoclave, then the mixed gas containing oxygen, hydrogen and nitrogen is filled, and then the autoclave may be stirred at-10 to 40 ℃ for 5min to 24h to directly obtain hydrogen peroxide. In the preparation of hydrogen peroxide based on the Pd single-site catalyst, H in the mixed gas is controlled as described above 2 、O 2 And N 2 And controlling the amount of Pd single site catalyst as described above, the peroxide can be further improvedHydrogen yield.
The ultra-low-load atomic-level high-dispersion palladium single-site catalyst prepared by the method is less in palladium precursor adopted or loaded in preparation compared with the traditional load type palladium nanoparticle catalyst. For the hydrogen-oxygen direct hydrogen peroxide synthesis process, the anchoring effect of the carrier is relatively stronger, and palladium element is less prone to loss. With lower palladium loading, there is an even higher H up to the traditional supported palladium nanoparticle (loading 1% -5%) 2 O 2 Yield effect.
The present application will be further described in detail by way of examples, which are not intended to limit the scope of the application. The reagents and the like used in the examples are commercially available products except for the specific ones. The titanium dioxide used in the examples below was commercially available P25 titanium dioxide (purchased from Alfa Aesar corporation).
Example 1
To a 30mL vial containing 100mg of titanium dioxide was added 10mL of toluene, followed by 105. Mu.L of a 1mg/mL palladium acetate solution (toluene as the solvent). The vial was placed in an oil bath, kept at 80 ℃ and stirred for 2h. And stopping heating, cooling the solution in the bottle to a greenhouse, centrifuging, collecting solid powder, and drying the solid powder in a vacuum drying oven at normal temperature for 6 hours. Then, the solid powder is ground and then placed in a tube furnace for calcination in the atmosphere of N 2 The temperature was 300℃and maintained for 3h. And collecting the palladium single-site catalyst after the tube furnace is cooled to normal temperature, and marking as a material A. The palladium element content of material A was analyzed to be 0.05wt%.
Example 2
To a 30mL vial containing 100mg of titanium dioxide was added 10mL of toluene, followed by 450. Mu.L of a 1mg/mL palladium acetate solution (toluene as the solvent). The vial was placed in an oil bath, kept at 20 ℃ and stirred for 12h. And stopping heating, cooling the solution in the bottle to a greenhouse, centrifuging, collecting solid powder, and drying the solid powder in a vacuum drying oven at normal temperature for 6 hours. Then, the solid powder is ground and then placed in a tube furnace for calcination in the atmosphere of N 2 The temperature was 300℃and maintained for 3h. Collecting palladium after the tube furnace is cooled to normal temperatureSingle-site catalyst, and labeled material B. The palladium element content of material B was analyzed to be 0.21wt%.
Example 3
To a 30mL vial containing 100mg of titanium dioxide was added 10mL of toluene, followed by 450. Mu.L of a 1mg/mL palladium acetate solution (toluene as the solvent). The vial was placed in an oil bath, kept at 80 ℃ and stirred for 2h. And stopping heating, cooling the solution in the bottle to a greenhouse, centrifuging, collecting solid powder, and drying the solid powder in a vacuum drying oven at normal temperature for 6 hours. Then, the solid powder is ground and then placed in a tube furnace for calcination in the atmosphere of N 2 The temperature was 300℃and maintained for 3h. And collecting the palladium single-site catalyst after the tube furnace is cooled to normal temperature, and marking as a material C. The palladium element content of material C was analyzed to be 0.21wt%.
Comparative example 1
Conventional palladium nanoparticles were prepared by hydrothermal method 100mg of titanium dioxide, 8mL of ethylene glycol, 2mL of N, N-dimethylacetamide (containing 17mg of palladium acetylacetonate) were added to a 30mL vial. The vial was placed in an oil bath, kept at 180 ℃ and stirred for 2h. After that, heating was stopped, and after the solution in the bottle was cooled to a greenhouse, it was centrifuged, and washed several times with an ethanol/acetone mixed solution, and the palladium loading of the obtained sample was about 4%. The solid powder of the mixture was collected and dried in a vacuum oven at 60 ℃ for 6h. Then, grinding the solid powder, and placing the ground solid powder into a tube furnace for calcination, wherein the calcination atmosphere is H 2 The temperature was 300℃and maintained for 3h. And (5) collecting the catalyst after the tube furnace is cooled to normal temperature, and obtaining the traditional palladium nanoparticle supported catalyst.
Test example 1
X-ray diffraction analysis was performed on the catalysts A, B and C prepared in examples 1 to 3, respectively, and XRD patterns obtained are shown in FIG. 1. As can be seen from FIG. 1, the XRD patterns of the catalysts prepared in examples 1 to 3 all show commercial TiO 2 Carrier peaks, except for the absence of any other peak pattern. This phenomenon suggests that single-site Pd catalysts may be successfully prepared and that palladium species are supported on TiO in highly dispersed form 2 And (3) on a carrier. If XRD goes outThe peaks of Pd or PdO are now shown, which indicates that Pd is indeed supported, but supported Pd is present in the form of nanoparticles or in the form of larger sized blocks, not single-site or single-atom Pd catalysts. If XRD does not show peaks of Pd or PdO, the supported palladium may be in a monodisperse form (monoatomic or monodentate), or the amount of supported Pd is small, in a highly dispersed state, and may be small-sized nano-particles or clustered palladium.
XRD does not show peaks of Pd or PdO, indicating that no large particle size Pd or PdO nanoparticles are present in the support. Thus, the above XRD results indicate that the palladium metal element supported in the sample remains in a highly dispersed form after the catalyst is subjected to heat treatment, and that large-sized or large-sized agglomerates, nanoparticles or clusters are not formed to show peaks of palladium element. Therefore, the Pd active site of the catalyst is in a single site form with high probability after calcination, and the catalyst needs to be further characterized.
Test example 2
The palladium single-site catalysts prepared in examples 1 to 3 and the conventional palladium nanoparticle supported catalyst prepared in comparative example 1 were applied to the oxyhydrogen direct hydrogen peroxide synthesis test, respectively, and the obtained test results are shown in fig. 2.
Specifically, 5.0mg of catalyst, 5.54g of methanol, 3.0g of water were added to a 50mL autoclave. The autoclave was filled with a gas having a total pressure of 4MPa (the sum of the pressures of oxygen, hydrogen and nitrogen), which included 0.4MPa of oxygen, 3.6MPa of 5% hydrogen, and the balance nitrogen (3.42 MPa). The autoclave was placed in a thermostat at 2℃and kept stirring at 1200rpm for 15min. Then, acidized cerium sulfate (0.05 mol/L) is taken as a titrant by taking the subtest ferritin as an indicator, and H in the solution is treated 2 O 2 Titration was performed.
Wherein C (Ce 4+ )、V(Ce 4+ ) For titration of the concentration (mol/L) and volume (mL) of cerium sulfate used, m cat For the quality of catalyst use, W Pd For promotingThe mass fraction of the catalyst containing palladium, T is the reaction time, and the obtained result is shown in FIG. 2.
As shown in FIG. 2, when hydrogen peroxide was produced based on the catalyst of example 1, the yield of hydrogen peroxide was 208000mol/kg Pd *h(104mol/kg cat * h) The method comprises the steps of carrying out a first treatment on the surface of the When hydrogen peroxide was prepared based on the catalyst of example 2, the hydrogen peroxide yield was 72144mol/kg Pd *h(154mol/kg cat * h) The method comprises the steps of carrying out a first treatment on the surface of the When hydrogen peroxide was prepared based on the catalyst of example 3, the hydrogen peroxide yield was 99442mol/kg Pd *h(214mol/kg cat * h) A. The application relates to a method for producing a fibre-reinforced plastic composite When hydrogen peroxide was produced based on the catalyst of comparative example 1, the yield of hydrogen peroxide was 1400mol/kg Pd *h(56mol/kg cat *h)。
The catalyst prepared in example 2, the catalyst prepared in comparative example 1 and the P25 titanium dioxide described above were used as catalysts, respectively, and the yields of the resulting hydrogen peroxide (in mol/kg were calculated at different times cat As titanium dioxide does not contain Pd element) as shown in fig. 6. As can be seen from FIG. 6, the single-site catalyst prepared in example 2 can reach 118mol/kg in 1h cat H of (2) 2 O 2 Yield, whereas the conventional catalyst Pd NPs/TiO prepared in comparative example 1 2 Can reach 42mol/kg cat H of (2) 2 O 2 Yield of TiO alone 2 The carrier is only 2-3mol/kg cat H of (2) 2 O 2 Loitering of productivity, H 2 O 2 The yield does not increase over time, so the spent titrant may be consumed only for the purpose of discolouring the indicator.
Test example 3
The rate of consumption of hydrogen peroxide in the hydrogenation reaction and the rate of decomposition of hydrogen peroxide in the decomposition reaction were examined in the presence of the catalysts prepared in examples 1 to 3 and comparative example 1, respectively.
In carrying out the hydrogenation reaction, 5.0mg of catalyst, 5.54g of methanol, 2.0g of water and 1mL of 30% commercial hydrogen peroxide were added to a 50mL autoclave. The autoclave was filled with 5% hydrogen at 3.6 MPa. The autoclave was placed in a thermostat at 2℃and kept stirring at 1200rpm for 30min. Thereafter, byThe subtest ferritin is used as an indicator, acidified cerium sulfate (0.05 mol/L) is used as a titrant, and H in the solution 2 O 2 Titration was performed.
In carrying out the decomposition reaction, 5.0mg of catalyst, 5.54g of methanol, 2.0g of water and 1mL of 30% commercial hydrogen peroxide were added to a 50mL autoclave. The autoclave was filled with 3.6MPa nitrogen. The autoclave was placed in a thermostat at 2℃and kept stirring at 1200rpm for 30min. Then, acidized cerium sulfate (0.05 mol/L) is taken as a titrant by taking the subtest ferritin as an indicator, and H in the solution is treated 2 O 2 Titration was performed.
The hydrogenation rate of hydrogen peroxide in the hydrogenation reaction and the decomposition rate of hydrogen peroxide in the decomposition reaction were calculated, and the results obtained are shown in FIG. 3.
As shown in FIG. 3, the consumption rate of hydrogen peroxide in the side reaction of hydrogenation and decomposition of hydrogen peroxide in the presence of the catalyst of examples 1-3 was substantially 0mol/kg Pd *h(0mol/kg cat * h) (no consumption was detected in 3 experiments, or by current titration). In the presence of the catalyst of comparative example 1, the consumption rate of hydrogen peroxide in the side reactions of hydrogenation and decomposition of hydrogen peroxide was 63832.5mol/kg, respectively Pd *h(2553mol/kg cat * h) And 12500mol/kg Pd *h(500mol/kg cat *h)。
Test example 4
The catalysts prepared in examples 1 to 3 and comparative example 1 were subjected to a spherical aberration electron microscope test, respectively, wherein a high angle annular dark field image (HADDF) photograph of the catalyst of example 2 is shown in fig. 4, and fig. 5 is a Scanning Transmission Electron Microscope (STEM) photograph of the catalyst of comparative example 1, and the results of the high angle annular dark field image (HADDF) and Scanning Transmission Electron Microscope (STEM) tests show that palladium was supported in a unit form in the catalyst prepared in example 2 on TiO 2 Palladium on TiO 2 Formed with a plurality of active sites each containing 1 to 3 palladium atoms and existing in a monodispersed form (the results of examples 1 and 3 are substantially identical to those of example 2), as shown in FIG. 4, wherein yellow circles represent a single palladium atom, and rectangles and triangles represent two or three palladium atomsAnd (5) a seed. In the catalyst prepared in comparative example 1, palladium was supported on TiO in the form of nanoparticles 2 As shown in fig. 5.
The ultra-low load type atomic level high dispersion palladium single-site catalyst prepared by the application has higher selectivity in the direct hydrogen peroxide synthesis of hydrogen and oxygen, and has high selectivity to H 2 O 2 No side effects of hydrogenation and decomposition are generated, and the consumption cost of raw materials and energy sources is greatly reduced.
In the description of the present application, it should be noted that the directions or positional relationships indicated by the terms "upper", "lower", "inner", "outer", "front", "rear", "left", "right", etc. are directions or positional relationships based on the operation state of the present application are merely for convenience of describing the present application and simplifying the description, and do not indicate or imply that the devices or elements to be referred to must have a specific orientation, be configured and operated in a specific orientation, and thus should not be construed as limiting the present application.
In the description of the present application, it should be noted that the terms "mounted," "connected," and "connected" are to be construed broadly, unless otherwise specifically defined and limited. The specific meaning of the above terms in the present application will be understood in specific cases by those of ordinary skill in the art.
The application has been described above in connection with preferred embodiments, which are, however, exemplary only and for illustrative purposes. On this basis, the application can be subjected to various substitutions and improvements, and all fall within the protection scope of the application.
Claims (11)
1. A Pd single-site catalyst, characterized in that the Pd single-site catalyst comprises TiO 2 And active Pd supported on TiO in the form of single sites 2 Applying;
wherein the mass ratio of the active Pd calculated by Pd element in the Pd single-site catalyst is 0.05% -0.25%.
2. The preparation method of the supported palladium catalyst for hydrogen peroxide synthesis is characterized by comprising the following steps:
(1) Making TiO 2 Dispersing in a solvent to obtain a first dispersion;
(2) Dispersing a palladium precursor in the first dispersion liquid, and separating out a solid phase material;
(3) Calcining the solid phase material in a non-reducing atmosphere to obtain the supported palladium catalyst;
wherein the mass ratio of Pd element in the supported palladium catalyst is 0.05-0.25wt%.
3. The method according to claim 2, wherein in the step (1), tiO 2 And the dispersion ratio of the solvent is 100mg: (5-15) mL; and/or the number of the groups of groups,
TiO 2 the particle size of (2) is 15-30nm.
4. The method of claim 2, wherein in step (1), the solvent is selected from the group consisting of acetone, toluene, N-dimethylacetamide, glacial acetic acid, chloroform, tetrahydrofuran, and deionized water.
5. The method of claim 2, wherein in step (2), the palladium precursor is selected from the group consisting of [ Pd (NH) 3 ) 4 ](NO 3 ) 2 、Pd(NO 3 ) 2 、Na 2 PdCl 4 、PdCl 2 、(NH 4 ) 2 PdCl 6 、[Pd(OAc) 2 ] 3 、Pd(acac) 2 And [ Pd (C) 3 H 5 )Cl] 2 A combination of one or more of the following; and/or the number of the groups of groups,
the palladium precursor is dispersed in the first dispersion in the form of a palladium solution.
6. The method of claim 2, wherein step (2) further comprises the steps of, prior to said separating out the solid phase material:
sequentially heating, mixing, standing and cooling the first dispersion liquid in which the palladium precursor is dispersed;
wherein the temperature of the heating and mixing is 20-100 ℃ and the time is 2-12h.
7. The method of claim 2, wherein step (3) further comprises vacuum drying prior to the non-reducing atmosphere calcination;
wherein the temperature of the vacuum drying is 25-60 ℃ and the time is 4-8h.
8. The method according to claim 2, wherein in the step (3), the non-reducing atmosphere is nitrogen; and/or the number of the groups of groups,
the calcination temperature is 200-800 ℃, the time is 2.5-3.5h, and the temperature rising rate is 3-8 ℃/min.
9. A process for producing hydrogen peroxide, comprising reacting H 2 And O 2 Contacting in the presence of a catalyst, said catalyst being the Pd unit catalyst of claim 1 or the supported palladium catalyst prepared by the preparation method of any one of claims 2 to 8.
10. The method of making of claim 9, wherein the contacting comprises:
mixing a catalyst, methanol and water in a reactor;
introducing into the reactor a catalyst comprising H 2 And O 2 Is a mixed gas of (a) and (b).
11. The production method according to claim 10, wherein the mass ratio of methanol to the total mass of methanol and water is 30% to 90%; and/or the number of the groups of groups,
the mixed gas also comprises N 2 The total pressure of the mixed gas is 0.5-12Mpa; in the mixed gas, H 2 、O 2 And N 2 The ratio of the pressure in the total pressure is 0.5% -5%, 1% -20% and 75% -98.5% respectively; and/or the number of the groups of groups,
the saidMass of catalyst and H 2 The molar ratio of (1) is (0.275-2.75) mg:1mmol.
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