A single-atom alloy photocatalyst, its preparation method and application

Abstract

The present invention discloses a preparation method of a single-atom alloy photocatalyst, which includes the steps of: preparing a carbon material carrier CN through grinding and calcination, dispersing the carbon material carrier CN in a solution containing Cu element, and removing the solution by rotary evaporation to obtain a carrier Cu@CN loaded with Cu; obtaining a carrier loaded with Cu nanoparticles through hydrogen reduction, dispersing the carrier in a gold precursor solution and mixing, and washing and drying the solid obtained by rotary evaporation to remove the solvent. The present invention dopes metal Au atoms onto the Cu nanoparticles on the surface of the carrier by combining wet mixing with atomic replacement reduction, and the Cu nanoparticles on the surface of the catalyst are small-sized, have a relatively uniform particle size and are evenly distributed. A single-atom metal active center of Au-Cu is constructed on the surface of the photocatalyst to adjust the surface charge of Au and increase its *CO adsorption capacity, so as to achieve a locally high concentration of *CO on the surface of the catalyst during the reaction process, reduce the reaction energy barrier of carbon-carbon coupling, and thus can achieve a highly selective reaction of CO2 reduction to ethanol.

Description

Technical Field

[0001] The present invention relates to the technical field of carbon dioxide reuse, and particularly relates to a single-atom alloy photocatalyst, a preparation method thereof, and an application thereof. Background Art

[0002] Solar-driven CO2 reduction to green and clean fuels is widely regarded as a key approach to alleviating energy crises and climate warming. Significant progress has been made in photocatalytic CO2 reduction in producing C1 products (such as CO, CH4, methanol, etc.). However, selectively converting CO2 to products with more than two carbon atoms (such as ethanol, acetic acid) still faces major challenges because the conversion of photocatalytic CO2 to C2 products involves complex multi-electron transfer and C-C coupling processes, where extremely high reaction energy barriers and slow kinetic processes limit the formation of multi-carbon products. To overcome these challenges, photocatalysts are crucial.

[0003] Plasmonic metal nanoparticles have become ideal materials for enhancing photocatalytic performance due to their unique electronic, optical, and catalytic properties. The local surface plasmon resonance (LSPR) effect enables metal particles to absorb part of the visible light and generate high-energy hot electrons. These hot electrons are captured by the metal sites loaded on the semiconductor, driving surface chemical reactions and forming a high concentration of CO on the catalyst surface to improve the selectivity of C2+ products.

[0004] To achieve high catalytic performance for photocatalytic CO2 reduction to C 2+ products, the designed catalyst needs to simultaneously possess the following capabilities: 1. High photo-generated carrier transfer ability and utilization rate, 2. Efficient adsorption of *CO intermediates and low activation energy for carbon-carbon coupling reactions.

[0005] CN116651459A discloses a tandem photocatalyst of indium oxide loaded with copper single atoms, a preparation method thereof, and an application thereof. The preparation method includes the following steps: (1) Weigh In(NO3)3 and terephthalic acid, dissolve them in DMF, heat and stir, centrifuge, wash, and dry to obtain the precursor In-MOF; (2) Disperse the precursor In-MOF in an ethanol solution, then add a CuCl2 solution and stir for ion exchange to obtain the precursor InCu-MOF; (3) Calcinate the precursor InCu-MOF to obtain a tandem photocatalyst of indium oxide loaded with copper single atoms with a loose and porous microtube morphology, which is the target product. In this catalyst, Cu exists in the form of single atoms, coordinates with the oxygen in In2O3, forms a stable low valence state and low coordination mode, has a wide light absorption range, greatly improves its ability to reduce carbon dioxide (CO2) under visible light, and shows excellent selectivity for reducing CO2 to ethanol. At the same time, in a gas-liquid reaction system, water is oxidized to oxygen synchronously. However, indium, as a rare metal on the earth, is expensive and difficult to be popularized industrially.

[0006] CN118142587A discloses a copper-based MOF photocatalyst, a preparation method and an application thereof, which relates to the synthesis of a constructed copper-based MOF compound and its application in the technical field of photocatalytic direct conversion of carbon dioxide into multi-carbon product acetone, that is, the preparation and application of a copper-based MOF composite material for directly converting carbon dioxide into high-value C3 product acetone under visible light irradiation at room temperature, but the selectivity for ethanol is not high. Summary of the Invention

[0007] The present invention aims at the problems of low efficiency in photocatalytic preparation of C 2+ products such as ethanol and acetaldehyde, especially the low selectivity for ethanol, and provides a preparation method of a single-atom alloy photocatalyst, which overcomes the difficult C-C coupling in the reaction process through the synergistic effect between Cu and Au, thereby realizing the efficient and highly selective conversion of CO2 into ethanol.

[0008] To achieve the above object, the technical solution adopted by the present invention is:

[0009] A preparation method of a single-atom alloy photocatalyst, comprising the steps of:

[0010] Step 1, preparing a carbon material carrier CN by grinding and roasting, dispersing the carbon material carrier CN in a solution containing Cu element, and removing the solution by rotary evaporation to obtain a carrier Cu@CN loaded with Cu;

[0011] Step 2, reducing Cu@CN by hydrogen to obtain a carrier loaded with Cu nanoparticles, dispersing the carrier in a gold precursor solution and mixing, and washing and drying the obtained solid after rotary evaporation to remove the solvent to obtain the single-atom alloy photocatalyst.

[0012] The present invention dopes metal Au atoms onto the Cu nanoparticles on the surface of the carrier by a method combining wet mixing and atomic replacement reduction, and the catalyst surface presents Cu nanoparticles with small size, relatively uniform particle size and uniform distribution. Constructing the Au-Cu single-atom metal active center on the surface of the photocatalyst can adjust the surface charge of Au and increase its *CO adsorption capacity, thereby realizing a locally high concentration of *CO on the catalyst surface during the reaction process, reducing the reaction energy barrier of carbon-carbon coupling, and thus realizing the highly selective reaction of CO2 reduction to ethanol.

[0013] The mass content of Cu in the single-atom alloy photocatalyst is 0.1-20 wt%; preferably, the mass content of Cu is 0.1-10 wt%; more preferably, the mass content of Cu is 1-10 wt%; excessive Cu loading is prone to agglomeration of copper atoms, and the catalytic efficiency will instead decrease.

[0014] The mass content of Au in the single-atom alloy photocatalyst is 0.001-10 wt%. Preferably, the mass content of Au is 0.001-5 wt%, and more preferably the mass content of Au is 0.1-5 wt%.

[0015] The mass ratio of Cu to Au in the single-atom alloy photocatalyst is 100:1 to 3:1. Preferably, the mass ratio of Cu to Au is 50:1 to 5:1, more preferably the mass ratio of Cu to Au is 20:1 to 5:1, and most preferably the mass ratio of Cu to Au is 15:1 to 10:1. At this ratio, the synergistic effect between gold and copper in the catalyst is the strongest, and the selectivity of the catalyst for photocatalytic conversion of carbon dioxide to ethanol is better.

[0016] The temperature of hydrogen reduction in step 2 is 200-300 °C. Further preferably, the temperature of hydrogen reduction is 250-300 °C, and more preferably the temperature of hydrogen reduction is 250-280 °C.

[0017] More preferably, the hydrogen reduction conditions are as follows: keep the temperature at 250 °C for 2 h; the heating rate is set at 2-8 °C / min; the H2 flow rate is set at 100 mL / min; the purity of H2 is 99.999%.

[0018] The carbon material support includes any one or more of graphite flakes, graphene, and carbon nanotubes;

[0019] The carbon material support includes any one or more of graphite flakes, graphene, and carbon nanotubes doped with metal or non-metal elements.

[0020] The grinding includes any one of ball milling, grinding, and stamping; the particle size of the carbon nanocarrier CN is 100-1000 nm.

[0021] The grinding is ball milling. First, ball milling is carried out followed by the first calcination, and then ball milling is carried out again followed by the second calcination;

[0022] The rotation speed of the ball milling is 100-870 rpm / min, and the ball milling time is 4-5 h; the temperature of the first calcination is 200-300 °C, and the calcination time is 2-6 h. The temperature of the second calcination is 500-600 °C, and the calcination time is 4-6 h.

[0023] By grinding and calcination, the particle size of the carbon support is reduced, the surface area is increased, and sufficient pyrolysis and sufficient removal of impurities existing in the support are achieved. The temperature is carried out under the condition of ensuring the stability of the support. If the calcination temperature is too low, the impurities cannot be completely removed, and if it is too high, the support structure may be damaged.

[0024] The solution containing the Cu element includes a copper precursor solution and / or a solution of copper nanoparticles;

[0025] The copper precursor includes any one or more of copper nitrate, oxalate, and halide salts;

[0026] The gold precursor includes any one or more of gold nitrate, oxalate, and halide salts.

[0027] The present invention also provides a single-atom alloy photocatalyst prepared by the described preparation method. The size of the single-atom alloy photocatalyst is 5 - 40 nanometers, wherein the mass content of Cu is 0.1 wt% - 20 wt%; the mass content of Au is 0.001 - 10 wt%.

[0028] Preferably, the content of the noble metal Au in the single-atom alloy photocatalyst is low, only 0.001 - 10 wt%, preferably 0.5 wt%; at the same time, after reduction with H2 in a fixed bed, the Cu nanoparticles are evenly dispersed in the carrier and have a small size. The Cu nanoparticle size of the catalyst is 2 - 10 nanometers.

[0029] The present invention also provides the application of the described single-atom alloy photocatalyst in the photocatalytic carbon dioxide reduction reaction. The high synergy between the noble metal Au and Cu in the catalyst makes it have excellent performance when catalyzing the reduction of carbon dioxide, and it can achieve high-selectivity production of ethanol under the condition of using pure water as a solvent.

[0030] The photocatalytic reaction temperature is 25 - 250 °C, and the reaction pressure is 0.1 - 4.0 Mpa.

[0031] Preferably, the light source for the photocatalytic reaction is a 50 - 300 W xenon lamp (PLS-SXE300 + / UV xenon light source); the purity of carbon dioxide is 90 - 99%.

[0032] In the prior art, for the highly selective preparation of ethanol by photocatalytic carbon dioxide, a certain amount of N,N-dimethylformamide (DMF), sodium sulfite (Na2SO3), etc. need to be added as reaction co-catalysts or sacrificial agents in the reaction solvent to promote the photocatalytic reaction efficiency and achieve the highly selective conversion of carbon dioxide to ethanol. However, the photocatalyst described in the present invention can achieve the highly selective conversion of carbon dioxide to ethanol under pure water conditions. In terms of application, using cheap and abundant H2O as an electron donor should be the direction and goal of photocatalytic CO2 reduction.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] (1) In the present invention, a simple atomic substitution synthesis strategy is adopted to prepare a photocatalyst with a single-atom alloy. By doping Au atoms on the surface of Cu nanoparticles supported on a carrier, the metal-metal synergy in the catalyst is further enhanced. The catalyst achieves excellent ethanol selectivity in the photocatalytic reduction of CO2 under light irradiation conditions.

[0035] (2)The catalyst in the present invention for the carbon dioxide reduction reaction can directly use pure water as the electron donor, that is, high ethanol conversion can be achieved.

[0036] (3)The loading of noble metal Au in the catalyst of the present invention is lower than that of conventional catalysts. The reduction of raw material cost is beneficial to the industrial promotion of the catalyst. Moreover, the preparation process of this catalyst is simple and the raw materials are simple and easy to obtain, having the potential for industrial application. Description of the Drawings

[0037] Figure 1 SEM image of the CN support prepared in Example 1.

[0038] Figure 2 For the preparation of Cu5Au 0.5 @ TEM image of the CN catalyst in Example 1.

[0039] Figure 3 XRD patterns of the support CN, precursor Cu5@CN, and catalyst Cu5Au 0.5 @ CN prepared in Example 1.

[0040] Figure 4 Comparison chart of the catalytic performance of the catalysts with different Au doping amounts prepared in Example 1.

[0041] Figure 5 Comparison chart of the catalytic performance of the catalysts with different reduction temperatures prepared in Example 1 and Example 2.

[0042] Figure 6 Comparison chart of the catalytic performance of the catalysts prepared by different reduction methods in Example 1 and Comparative Example 1. Detailed Embodiments

[0043] In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. Those skilled in the art make modifications or equivalent replacements on the basis of understanding the technical solutions of the present invention, without departing from the spirit and scope of the technical solutions of the present invention, and all should be covered within the protection scope of the present invention.

[0044] The raw materials used in the following detailed embodiments are all purchased from the market. The metal Cu precursor used in Example 3 is CuC2O4 with a molecular weight of 151.56; the metal Cu precursors used in other examples and comparative examples are all Cu(NO3)2·3H2O with a molecular weight of 409.81; the metal Au precursor is HAuCl4·4H2O with a molecular weight of 411.85.

[0045] Example 1 Preparation of Cu5Au with different Au contents by hydrogen reduction method x @CN sample

[0046] (1) Load 2.0 g of nano-graphite powder into a 50 mL ball milling jar, load zirconia balls according to the ball-to-material ratio, place it in an argon (Ar) atmosphere for 12 h, then ball mill for 1 h at 870 rpm / min and continue to fill with argon;

[0047] (2) After taking out the above raw materials, ball mill at 100 - 870 rpm / min for 5 h. The obtained solid is calcined in a muffle furnace at 250 °C for 4 h, and after cooling to room temperature, it is calcined again under the condition of 550 °C for 3 h to remove impurities;

[0048] (3) Transfer the above raw materials to argon and let them stand for 12 h, then ball mill at 870 rpm / min for 5 h to obtain a two-dimensional carbon nanosheet support, denoted as CN.

[0049] (4) Take 200 mg of the CN support and transfer it to a copper nitrate solution containing 5 wt% Cu. Stir for 1 h and let it stand overnight. The obtained solution is rotary evaporated at 40 °C to obtain a precursor of Cu supported on a two-dimensional carbon support, denoted as Cu5@CN.

[0050] (5) Take 85 mg of the above Cu5@CN solid and reduce it with hydrogen at 250 °C in a fixed bed for 2 h. After cooling to room temperature, immediately transfer the obtained solid to chloroauric acid solutions with different mass concentrations (the mass fractions of Au are 0.1 wt%, 0.5 wt%, 1.0 wt%, 2.0 wt%, 3.0 wt% respectively), stir for 4 h and let it stand overnight.

[0051] (6) Rotary evaporate the above solution at 40 °C. The obtained solid is centrifugally washed twice with deionized water and then vacuum dried at 40 °C. The obtained solid is a catalyst with 5 wt% Cu supported on C and different Au loadings, denoted as Cu5Au X @CN, where x is the Au loading, specifically marked as Cu5Au 0.1 @CN, Cu5Au 0.5 @CN, Cu5Au1@CN, Cu5Au2@CN, Cu5Au3@CN.

[0052] Figure 1 is the SEM image of the prepared two-dimensional carbon nanosheet support. It can be seen from the figure that CN shows a flaky stacking pattern.

[0053] Figure 2 is the TEM image of the prepared Cu5Au 0.5 @CN catalyst. From Figure 2It can be seen that Cu nanoparticles are uniformly distributed on the CN support, with sizes ranging from 2 to 40 nm.

[0054] Figure 3 X-ray diffraction patterns of the CN support, the precursor Cu5@CN of Cu supported on two-dimensional carbon support, and the prepared Cu5Au 0.5 @CN catalyst. As can be seen from Figure 3 it, no obvious change in the crystal structure was found after loading Cu and Au on CN.

[0055] Example 2 Cu5Au prepared by hydrogen reduction method under three different temperature conditions 0.5 @CN samples

[0056] (1) Load 2.0 g of nano-graphite powder into a 50 mL ball milling jar, load zirconia balls according to the ball-to-material ratio, and place it in an argon (Ar) atmosphere for 12 h;

[0057] (2) Ball mill the above raw materials at 870 rpm / min for 1 h and then continue to fill with argon;

[0058] (3) After taking out the above raw materials, ball mill them at 870 rpm / min for 5 h. The obtained solid is calcined in a muffle furnace at 250 °C for 4 h, and after cooling to room temperature, it is calcined again under the condition of 550 °C for 3 h. Remove impurities

[0059] (4) Transfer the above raw materials to argon and let them stand for 12 h, then ball mill them at 870 rpm / min for 5 h to obtain the carbon support material, also denoted as CN.

[0060] (5) Transfer 200 mg of the C support to a copper nitrate solution containing 5 wt% Cu, stir for 1 h and then let it stand overnight. The obtained solution is rotary evaporated at 40 °C to obtain the precursor of C-supported Cu, denoted as Cu5@C.

[0061] (6) Take 85 mg of the above solid and reduce it with hydrogen at different temperatures in a fixed bed for 2 h. After cooling to room temperature, immediately transfer the obtained solid to a 0.5 wt% chloroauric acid solution, stir for 4 h and then let it stand overnight.

[0062] (7) Rotary evaporate the above solution at 40 °C. The obtained solid is centrifugally washed twice with deionized water and then vacuum dried at 40 °C. The obtained solid is the catalyst with 5 wt% Cu loaded on C and 0.5 wt% Au loading, denoted as Cu5Au 0.5 @CN-x, where x is the hydrogen reduction temperature, specifically denoted as Cu5Au 0.5 @CN-200, Cu5Au 0.5 @CN-300.

[0063] Comparative Example 1 Cu5Au prepared by different chemical reduction methods 0.5 @CN sample

[0064] (1) Load 2.0 g of nano-graphite powder into a 50 mL ball milling jar, load zirconia balls according to the ball-to-material ratio, and place it in an argon (Ar) atmosphere for 12 h;

[0065] (2) Ball mill the above raw materials at 870 rpm / min for 1 h and then continue to fill with argon;

[0066] (3) After taking out the above raw materials, ball mill them at 870 rpm / min for 5 h. The obtained solid is calcined in a muffle furnace at 250 °C for 4 h, and after cooling to room temperature, it is calcined again under the condition of 550 °C for 3 h. Remove impurities

[0067] (4) Transfer the above raw materials to argon and let them stand for 12 h, then ball mill them at 870 rpm / min for 5 h to obtain the carbon support material, denoted as CN.

[0068] (5) Transfer 200 mg of the CN support to a copper nitrate solution containing 5 wt% Cu, stir for 1 h and then let it stand overnight. The obtained solution is rotary evaporated at 40 °C to obtain the precursor of Cu supported on C, denoted as Cu5@C.

[0069] (6) Take 85 mg of the above solid in a solution, use 800 mg of sodium borohydride, ascorbic acid and hydrazine hydrate as reducing agents for reduction. After the reduction is completed, immediately transfer the obtained solid to a 0.5 wt% chloroauric acid solution at room temperature, stir for 4 h and then let it stand overnight.

[0070] (7) Rotary evaporate the above solution at 40 °C. The obtained solid is centrifugally washed twice with deionized water and then vacuum dried at 40 °C. The obtained solid is the catalyst with 5 wt% Cu loaded on CN and 0.5 wt% Au loading, denoted as Cu5Au 0.5 @CN(z), where z is the catalyst reduced by different reducing agents, and z is specifically A (ascorbic acid), B (sodium borohydride), C (hydrazine hydrate).

[0071] Performance evaluation of the catalyst

[0072] The performance of the catalysts prepared in the examples and comparative examples for photocatalytic carbon dioxide reduction was tested and evaluated as Figures 4 - 6 shown. The catalytic reaction was carried out in a photocatalytic reactor, and the photocatalysis was realized by simulating solar radiation with a xenon lamp.

[0073] Application Example 1

[0074] 1. Weigh 20 mg of the catalysts of Example 1-2 and Comparative Example 1 and 1 magnetic stir bar, and place them at the bottom of a quartz reactor with an inner diameter of 50 mm.

[0075] 2. Take 5 mL of ultrapure water and pour it into a quartz cup. Place the cup on a magnetic stirrer and stir at 870 rpm / min for 10 min to fully disperse the catalyst.

[0076] 3. Place the quartz cup in a photocatalytic reactor, introduce high-purity CO2 to 2.0 MPa, then release the gas, and repeat the operation three times to fully exhaust the residual air in the reactor.

[0077] 4. Introduce high-purity gas to 4.0 MPa, set the reaction temperature and stirring speed, and turn on the xenon lamp.

[0078] 5. After the reaction, introduce the post-reaction gas into a GC to analyze the gas-phase products; centrifuge the post-reaction liquid and take the supernatant to detect the liquid-phase products by H-NMR.

[0079] The data on the photocatalytic preparation of ethanol for the examples and comparative examples are summarized in Table 1. The test results of the photocatalytic reduction of carbon dioxide by the catalysts with different Au element doping amounts in Example 1 are as Figure 4 shown. Under light illumination conditions, without Au doping, no ethanol is produced by the catalyst Cu5@CN; as the amount of Au atoms doped on the surface of Cu nanoparticles on the support continuously increases, the selectivity and activity of ethanol in the products show a trend of first increasing and then decreasing. When the Au doping amount is 0.5 wt%, the catalyst has the highest ethanol activity and selectivity.

[0080] Table 1 Catalytic performance data of the catalysts prepared in the examples and comparative examples

[0081]

[0082] As Figure 5 shown, in Example 2, when comparing the catalytic effects of the catalysts under different hydrogen reduction conditions, when the hydrogen reduction temperature is 250 °C, the Cu5Au 0.5 @CN sample has the best activity. It should be noted that after hydrogen reduction at 300 °C, the activity of Cu5Au 0.5 @CN-300 only increases slightly. Considering economy and operation issues, 250 °C is used as the most preferred condition in the present invention.

[0083] As Figure 6 shown, by comparing different reduction methods, it is found that compared with the chemical reduction methods using ascorbic acid, sodium borohydride, and hydrazine hydrate, Cu5Au0.5@CN shows the best ethanol activity and selectivity when using hydrogen reduction.

Claims

1. A preparation method of a single-atom alloy photocatalyst, characterized in that, Including the steps: Step 1: Prepare the carbon material support CN by grinding and calcination. Disperse the carbon material support CN in a solution containing Cu element, and remove the solution by rotary evaporation to obtain the supported Cu support Cu@CN; Step 2: Reduce Cu@CN with hydrogen to obtain a support loaded with Cu nanoparticles. Disperse the support in a gold precursor solution and mix. The solid obtained by rotary evaporation to remove the solvent is washed and dried to obtain the single-atom alloy photocatalyst; the temperature of hydrogen reduction in Step 2 is 250 - 300 °C; The grinding is ball milling. First, perform ball milling and then the first calcination, and then perform ball milling and the second calcination; the ball milling speed is 100 - 870 rpm, and the ball milling time is 4 - 5 h; the temperature of the first calcination: 200 - 300 °C, the calcination time is 2 - 6 h, the temperature of the second calcination is 500 - 600 °C, and the calcination time is 4 - 6 h; The mass content of Cu in the single-atom alloy photocatalyst is 0.1 - 10 wt%; the mass content of Au in the single-atom alloy photocatalyst is 0.1 - 1 wt%; the mass ratio of Cu to Au in the single-atom alloy photocatalyst is 15:1 to 5:1; The carbon material support includes any one or more of graphite flakes, graphene, and carbon nanotubes; or, the carbon material support includes any one or more of graphite flakes, graphene, and carbon nanotubes doped with metal or non-metal elements.

2. The preparation method of the single-atom alloy photocatalyst according to claim 1, characterized in that, The grinding includes any one of ball milling, grinding, and stamping; the size of the carbon nanosupport CN is 100 - 1000 nm.

3. The preparation method of the single-atom alloy photocatalyst according to claim 1, characterized in that, The solution containing Cu element includes a copper precursor solution and / or a solution of copper nanoparticles; The copper precursor includes any one or more of copper nitrate, oxalate, and halide salts; The gold precursor includes any one or more of gold nitrate, oxalate, and halide salts.

4. The single-atom alloy photocatalyst prepared by the preparation method according to any one of claims 1-3, characterized in that, The size of the single-atom alloy photocatalyst is 5 - 40 nanometers, wherein the mass content of Cu is 0.1 - 10 wt%; the mass content of Au is 0.1 - 1 wt%.

5. Use of the single-atom alloy photocatalyst according to claim 4 in the photocatalytic carbon dioxide reduction reaction.

6. The application according to claim 4, wherein The photocatalytic reaction temperature is 25 - 250 °C, and the reaction pressure is 0.1 - 4.0 Mpa.

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