Quaternary metal-loaded titanium dioxide photocatalyst as well as preparation method and application thereof

Abstract

The invention discloses a quaternary metal loaded titanium dioxide photocatalyst and a preparation method and application thereof, the photocatalyst is anatase type titanium dioxide with a (001) crystal face and is loaded with CuxCoyNiyZny, x is 4-7, and y is 1-2. The preparation method comprises the following steps: dropwise adding a metal salt solution containing CuCoNiZn into a suspension solution containing TiO2, dropwise adding a certain amount of sodium hydroxide to adjust the pH value of the solution, continuously introducing Ar in the whole reaction, stirring for a certain time at room temperature to generate hydrotalcite, centrifuging, freeze-drying, and reducing with H2 to obtain the TiO2 loaded CuxCoyNiyZny medium-entropy alloy type photocatalyst with surface active sites. The photocatalyst prepared by the invention is larger in specific surface area and higher in photocatalytic activity, especially the activity of photocatalytic reduction to prepare ammonia is enhanced, and the preparation method of the catalyst reduces the production cost and simplifies the production process, and has great significance that the prepared photocatalyst can be used for photocatalytic reduction of nitrite into NH3.

Description

Technical Field

[0001] The present invention relates to the field of photocatalysis technology, and particularly relates to a titanium dioxide photocatalyst loaded with quaternary metals, a preparation method thereof, and an application thereof. Background Art

[0002] Currently, the photocatalytic reduction of nitrite to ammonia technology, as an important approach for green ammonia synthesis, shows broad prospects in the fields of nitrogen cycle utilization and clean energy development. The core challenge of this technology lies in the development of photocatalysts that combine efficient light absorption, rapid charge carrier separation, and abundant surface active sites. Traditional titanium dioxide (TiO2)-based materials have been widely studied due to their non-toxicity, low cost, and chemical stability. However, their wide bandgap (3.0 - 3.2 eV) results in the ability to utilize only ultraviolet light (accounting for 4% of the solar spectrum), and the photo-generated electron-hole pairs are prone to recombination, severely restricting the activity and selectivity of nitrite (NO2 - ) reduction to ammonia. Although strategies such as noble metal loading (e.g., Pt / TiO2), heterostructure construction (e.g., TiO2 / g-C3N4), or non-metal doping (e.g., N-TiO2) can partially broaden the light response range and inhibit charge carrier recombination, problems such as the high cost of noble metals, low charge transfer efficiency at heterointerfaces, and random distribution of doping sites are still difficult to overcome. For example, although Ag-modified TiO2 nanotubes can shift the light absorption to the visible light region, the metal particles on their surface are prone to agglomeration and lack specific adsorption sites for NO2 - , resulting in an ammonia yield of less than 1.5 μmol·g -1 ·h -1 . In recent years, defect engineering and metal co-catalysis have provided new directions for photocatalyst design. By introducing anion defects such as oxygen vacancies or sulfur vacancies, the energy band structure of the material can be regulated and the adsorption ability of reactants can be enhanced; while the loading of transition metals (such as Fe, Cu) promotes electron transfer through the formation of strong metal-support interactions. For example, the Fe-TiO 2-x catalyst rich in oxygen vacancies selectively adsorbs NO2 - through oxygen sites, and at the same time, Fe nanoparticles accelerate the multi-electron reduction process, increasing the ammonia yield to 8.7 μmol·g -1 ·h -1 . However, single defect types or simple metal loading still have limitations: oxygen vacancies are easily oxidized and filled under light illumination, resulting in a decrease in stability, while isolated metal sites are difficult to synergistically activate the NO2 - reduction and hydrolysis dissociation steps, making it difficult to precisely control the reaction path.

[0003] Through the above analysis, the problems and defects existing in the prior art are as follows:

[0004] (1) The main bottlenecks of the existing photocatalytic nitrite reduction to ammonia technology are narrow light absorption range, high carrier recombination rate, insufficient active sites, uncontrollable reaction pathways, and poor stability; the synthesis method of heterojunction photocatalysts is complex and has no prospect of industrial application.

[0005] (2) Although the performance is partially improved through traditional modification strategies (noble metal loading, heterojunction, single defect engineering), it is limited by cost, stability, and lack of synergy mechanism. In the single defect engineering, the exposure of single types of cationic or anionic defects on the surface still has the problem of a small number of reaction active sites, restricting the improvement of its photocatalytic activity. Summary of the Invention

[0006] In view of the above deficiencies in the existing technology, the purpose of the present invention is to provide a titanium dioxide photocatalyst loaded with quaternary metals, a preparation method thereof, and an application, so as to solve the problems of narrow light absorption range, insufficient active sites, poor stability, and complex synthesis method of heterojunction photocatalysts in the existing technology.

[0007] To solve the above technical problems, the present invention adopts the following technical solutions:

[0008] A titanium dioxide photocatalyst loaded with quaternary metals, wherein the catalyst is anatase titanium dioxide and has a (001) crystal plane, and the catalyst is loaded with Cu x Co y Ni y Zn y , where x is 4 to 7 and y is 1 to 2.

[0009] The present invention provides a preparation method of a titanium dioxide catalyst for preparing the above catalyst, and the specific steps are as follows:

[0010] Step 1: Prepare a titanium dioxide suspension; wherein, the concentration of titanium dioxide is 0.05 to 0.1 mmol / mL;

[0011] Step 2: Prepare metal salt solutions containing Cu, Co, Ni, and Zn respectively; wherein, the molar ratio of Cu, Co, Ni, and Zn is (4 to 7):(1 to 2):(1 to 2):(1 to 2); calculated by mass percentage, the total mass of metal elements accounts for 5 wt% to 15 wt% of the mass of titanium dioxide;

[0012] Step 3: Add the metal salt solutions containing Cu, Co, Ni, and Zn to the titanium dioxide suspension obtained in Step 1 respectively, adjust the pH value to 10 to 12, and stir and react for 20 min to 60 min;

[0013] Step 4: Filter the reaction product of Step 3, collect the solid and dry it, then place the solid in an atmosphere containing H2 and calcine it at 300 °C to 400 °C for 3 h to 5 h to obtain the catalyst.

[0014] Preferably, in Step 1, the titanium dioxide is anatase type and has a (001) crystal plane.

[0015] Preferably, the metal salts include Cu(NO3)2, CoCl2, Zn(NO3)2, NiCl2 and their hydrates.

[0016] Preferably, in Step 1, Ar is continuously introduced into the titanium dioxide suspension during its preparation, the Ar flow rate is 60 mL / min, and the stirring speed is controlled at 100 - 800 r / min.

[0017] Preferably, in Step 3, the pH value is adjusted by a sodium hydroxide solution.

[0018] Preferably, in Step 4, the solid is freeze-dried for 18 h to 24 h.

[0019] Preferably, in Step 4, in the atmosphere containing H2, calculated by volume fraction ratio, the content of H2 is 40%, the content of CO2 is 10%, and the content of Ar is 50%.

[0020] Preferably, in Step 4, the heating rate is 5 °C / min.

[0021] The present invention provides an application of a titanium dioxide catalyst, and the above catalyst or the catalyst prepared by the above preparation method is used for photocatalytic reduction of nitrite to prepare ammonia.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] 1. The present invention optimizes the preparation process, avoids high-temperature treatment through the rapid co-precipitation method, and realizes low-cost synthesis under mild conditions. Compared with the prior art, it does not require complex post-treatment (such as multiple water washes and acid dissolution), and the catalyst has stable performance and can be directly applied to photocatalytic nitrogen cycling, providing a new path for sustainable ammonia synthesis.

[0024] 2. The present invention uses the rapid co-precipitation method to prepare anatase-type titanium dioxide with surface active sites and in-situ load Cu x Co y Ni y Zn yPhotocatalyst. The medium-entropy alloy formed by H2 calcination reduction has a significantly increased specific surface area, exposing more active adsorption sites, thereby enhancing the catalytic activity. The catalyst prepared in the present invention has a lattice distortion effect, which can form a local built-in electric field to drive the directional separation of photo-generated electrons and holes, significantly reducing the probability of electron-hole recombination. In addition, the multi-component synergistic effect of the medium-entropy alloy in the catalyst optimizes the transport path of photo-generated carriers, significantly improving the activity of photocatalytic reduction of nitrite to ammonia. The local reduction environment on the surface of the photocatalyst induces the escape of oxygen atoms from TiO2 to form oxygen vacancies; at the same time, the loading of metal nanoparticles (Cu, Co, Ni, Zn) further introduces lattice defects at the interface, and the two cooperate to inhibit the recombination of photo-generated electrons and holes. Oxygen vacancies, as active centers, can adsorb nitrite molecules and promote their activation, while metal sites enhance light absorption through the surface plasmon resonance effect. Finally, under the irradiation of a xenon lamp simulating sunlight, 10 ppm of nitrite can be converted into 9.969 ppm of NH3 within 90 minutes, greatly improving the photocatalytic performance.

[0025] 3. The present invention uses non-toxic components, avoiding the use of heavy metals or harmful reagents in traditional catalysts, conforming to the principles of green chemistry, reducing the potential risks to the ecological environment and human health, and having good application prospects. Brief Description of the Drawings

[0026] Figure 1 is the flow chart of the preparation method described in the present invention.

[0027] Figure 2 is the XRD spectra of the CuCoNiZn@TNS prepared in the examples of the present invention, Comparative Example 5 (LH-CuCoNiZn@TNS), Example 1 (HH-CuCoNiZn@TNS), Comparative Example 6 (LR-CuCoNiZn@TNS), and the traditional semiconductor TNS photocatalyst materials.

[0028] Figure 3 is the UV-visible absorption spectra of the CuCoNiZn@TNS prepared in the examples of the present invention, Comparative Example 5 (LH-CuCoNiZn@TNS), Example 1 (HH-CuCoNiZn@TNS), Comparative Example 6 (LR-CuCoNiZn@TNS), and the traditional semiconductor TNS photocatalyst materials.

[0029] Figure 4 is the transmission electron microscopy image of the photocatalyst of Example 1 (HH-CuCoNiZn@TNS).

[0030] Figure 5Photocatalytic ammonia production yield diagrams of CuCoNiZn@TNS prepared in the embodiments of the present invention, Comparative Example 5 (LH-CuCoNiZn@TNS), and Example 1 (HH-CuCoNiZn@TNS) photocatalysts.

[0031] Figure 6 Photocatalytic ammonia production yield diagrams of the photocatalysts prepared in Example 2 and Example 3.

[0032] Figure 7 Photocatalytic ammonia production yield diagrams of the photocatalysts prepared in Example 2, Example 4, and Example 5.

[0033] Figure 8 Photocatalytic ammonia production yield diagrams of the photocatalysts prepared in Comparative Examples 2 to 4.

[0034] Figure 9 It is the change process of the solution color in the photocatalytic reactor during the photocatalytic reduction of nitrite to ammonia by the catalyst prepared in Example 1, corresponding to the reaction color changes at 0 min, 15 min, 30 min, 60 min, and 90 min respectively. Detailed implementation manners

[0035] The present invention will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the present invention belong to the scope of protection of the present invention.

[0036] Unless otherwise specified in specific situations in the present invention, the numerical ranges listed herein include the upper and lower limit values, as well as all integers and fractions within the range, rather than the specific values listed when defining the range.

[0037] I. A titanium dioxide photocatalyst loaded with a quaternary metal

[0038] The catalyst is anatase titanium dioxide and has a (001) crystal plane, and Cu x Co y Ni y Zn y are loaded therein, where x is 4 to 7 and y is 1 to 2.

[0039] After studying the prior art, the present invention considered controllably constructing a metal loading and double-defect synergistic active center on the TiO2 substrate to make the obtained catalyst have high activity for photocatalytic reduction of nitrite to ammonia.

[0040] II. Preparation method of a titanium dioxide catalyst

[0041] Step 1: Prepare a titanium dioxide suspension; wherein, the concentration of titanium dioxide is 0.05 - 0.1 mmol / mL;

[0042] Step 2: Prepare metal salt solutions containing Cu, Co, Ni, and Zn respectively; wherein, the molar ratio of Cu, Co, Ni, and Zn is (4 - 7):(1 - 2):(1 - 2):(1 - 2), and the total concentration of the metal salts in the metal salt solution is 0.015 mol / L - 0.02 mol / L; calculated by mass percentage, the total mass percentage of the metal elements in the titanium dioxide mass is 5 wt% - 15 wt%;

[0043] Step 3: Add the metal salt solutions containing Cu, Co, Ni, and Zn into the titanium dioxide suspension obtained in Step 1 respectively, and adjust the pH value to 10 - 12, while stirring and reacting for 20 min - 60 min;

[0044] Step 4: Filter the reaction product of Step 3, collect the solid and dry it, then place the solid in an atmosphere containing H2, and calcine it at 300°C - 400°C for 3 h - 5 h to obtain the catalyst.

[0045] After researching the existing photocatalytic ammonia production catalyst by reducing nitrite, it is found that the current catalyst has problems such as a narrow light absorption range, requiring a specific wavelength of spectrum for catalysis, and insufficient active sites. Based on this, the present invention considers improving the problem of insufficient active sites by loading multiple metal elements. However, the preparation method of the heterojunction photocatalyst is complex and difficult to realize industrial application. Therefore, the present invention considers using a rapid co - precipitation process to prepare the catalyst. The present invention optimizes the dosage of the four metal elements and the preparation process conditions. Subsequently, the present invention unexpectedly discovers that in the quaternary metal system constructed by the present invention, copper plays a leading role as the core element. On the one hand, Cu serves as the active center in the oxidation - reduction reaction, and a relatively high proportion of copper element loading can enhance the electron transfer ability and accelerate the reaction process; on the other hand, in the photocatalytic system, Cu reduces the activation energy of nitrite reduction by adjusting the valence band position of the material, and at the same time promotes the directional migration of electrons from the photocatalyst to the reactants, forming more active sites. Elements such as cobalt (Co), zinc (Zn), and nickel (Ni) strengthen the catalytic effect through an auxiliary mechanism: Co participates in the regulation of the oxidation state (such as Co 2+ / Co 3+The cycle promotes the conversion of intermediate products; Zn maintains the activity by stabilizing the catalyst structure and regulating the surface acidic sites; Ni enhances the conductivity and anti-poisoning ability. The carrier material is anatase titanium dioxide with a (001) crystal plane, which provides abundant loading sites through a high specific surface area, promotes the dispersion of metal nanoparticles and increases the number of active sites, and its conductivity accelerates the transfer of electrons from the catalyst to the reactants. However, the ratio of the four metal elements has a dual effect on the performance of the catalytic system. A high Cu ratio not only broadens the spectral response range of the material and enhances the visible light absorption efficiency, but also prolongs the lifetime of photo-generated carriers and reduces the electron-hole recombination rate by optimizing the lattice structure or defect state distribution; however, an imbalance in the ratio will lead to the coverage or aggregation of active sites, reducing the effective surface area. At the same time, the excess of inactive metals will hinder the reactants from approaching the active sites through competitive adsorption, destroying the synergistic effect between metals.

[0046] In some embodiments of the present invention, the concentration of titanium dioxide is 0.05 mmol / mL to 0.1 mmol / mL. Neither too low nor too high carrier concentration can achieve good catalytic effects. When the loading amount is too small, the metal particles may be unevenly dispersed, or the quantity is insufficient to form enough active centers, thus resulting in limited improvement in the overall catalytic activity; while when the loading amount is too high, the metal particles will agglomerate, forming larger particles that instead reduce the surface area and cover the active sites of titanium dioxide, hindering light absorption and the adsorption of reaction substrates. In addition, too high a loading amount may change the energy band structure of titanium dioxide, affecting the light absorption range and the service life of the catalyst.

[0047] In some embodiments of the present invention, the molar ratio of Cu, Co, Ni, and Zn is controlled between (4 to 7):(1 to 2):(1 to 2):(1 to 2). When the ratio is lower than this range, the content of the dominant Cu element in the catalyst is too low, and the catalytic effect of the catalyst requires long-term light irradiation to start; but when it is higher than this range, especially when the dosage of any one of Co, Ni, and Zn is greater than this range, it will seriously affect the catalytic performance of the catalyst and even cause the catalyst to completely lose its catalytic effect (such as when the dosage of Co is too high). Therefore, the molar ratio of Cu, Co, Ni, and Zn can be 4:1.5:1.5:1.5, 4:2:2:2, 7:1:1:1, etc., as well as all ranges and sub-ranges between the above values. It should be understood that in the embodiments, any of the above ranges can be combined with any other ranges in other embodiments.

[0048] In some embodiments of the present invention, calculated by mass percentage, the total mass of metal elements accounts for 5 wt% to 15 wt% of the mass of titanium dioxide. The percentage of the total mass of the four metal elements in the mass of titanium dioxide will have a certain impact on the catalytic rate. When the loading amount is too low or too high, the catalytic rate will become relatively flat; while when the loading amount is appropriate, it can quickly catalyze nitrite to form a large amount of NH4 + , therefore, by adjusting the percentage of the total mass of the quaternary metal elements in the mass of titanium dioxide, the regulation of the catalytic rate can be achieved. The percentage of the total mass of metal elements in the mass of titanium dioxide can be 5 wt%, 10 wt%, 15 wt%, etc., as well as all ranges and sub-ranges between the above values. It should be understood that in the embodiments, any of the above ranges can be combined with any other range in other embodiments.

[0049] In some embodiments of the present invention, in step 2, the pH value of the reaction system is adjusted by sodium hydroxide solution. The adjustment of the pH value can be carried out in a rhythm of first fast and then slow, and slowly adjusted when the pH is approaching 8 to ensure that the pH value of the reaction system can be controlled within this range, which is beneficial to the formation of flocculent hydrotalcite. Therefore, the pH value can be 10, 11, 11.5, 12, etc., as well as all ranges and sub-ranges between the above values; at the same time, the stirring reaction time is also controlled within 20 min to 60 min. If the reaction time is too short, it is not conducive to the coprecipitation of the quaternary metal elements; if the reaction time is too long, it will cause the quaternary metal elements to agglomerate on the carrier and affect the catalytic effect. Therefore, the reaction time can be 20 min, 30 min, 40 min, 50 min, 60 min, etc., as well as all ranges and sub-ranges between the above values. It should be understood that in the embodiments, any of the above ranges can be combined with any other range in other embodiments.

[0050] In some embodiments of the present invention, in step 4, there is a H2 atmosphere. Calculated by volume fraction ratio, the content of H2 is 40%, the content of CO2 is 10%, and the content of Ar is 50%. The role of carbon dioxide and Ar is to act as protective gases to inhibit oxidation reactions and avoid the formation of oxides; at the same time, H2 is used to calcine and reduce to prepare a medium-entropy alloy.

[0051] In some embodiments of the present invention, in step 4, if the calcination temperature is too low, the amount of the formed medium-entropy alloy will be too small; while if the temperature is too high, the generation of other by-products will affect the catalytic effect. Therefore, the calcination temperature can be 300°C, 350°C, 400°C, etc., as well as all ranges and sub-ranges between the above values; the calcination time ensures that all metal elements are reduced and also ensures that no other substances are formed. The calcination time can be 3h, 4h, 5h, etc., as well as all ranges and sub-ranges between the above values; it should be understood that in the embodiments, any of the above ranges can be combined with any other ranges in other embodiments.

[0052] In some embodiments of the present invention, in step 1, the titanium dioxide is anatase type and has a (001) crystal plane.

[0053] In some embodiments of the present invention, the metal salts include Cu(NO3)2, CoCl2, Zn(NO3)2, and NiCl2 and their hydrates. Specifically, they can be Cu(NO3)2, CoCl2·6H2O, Zn(NO3)2·6H2O, and NiCl2·6H2O.

[0054] In some embodiments of the present invention, in step 1, when preparing the titanium dioxide suspension, Ar is continuously introduced into it at a flow rate of 60 mL / min, and the stirring speed is controlled to be 100 - 800 r / min.

[0055] In some embodiments of the present invention, in step 4, the solid is freeze-dried, and the freeze-drying time is 18h - 24h.

[0056] In some embodiments of the present invention, in step 4, the heating rate is 5°C / min.

[0057] III. Application of a titanium dioxide catalyst

[0058] The catalyst of the present invention or the catalyst prepared by the preparation method is used for photocatalytic reduction of nitrite to prepare ammonia. Specifically: The catalyst of the present invention can carry out the photocatalytic nitrite reduction to ammonia reaction under sunlight conditions, not limited to light of a specific wavelength.

[0059] IV. Examples and comparative examples

[0060] Example 1

[0061] Step 1: After adding 50 ml of deionized water to a 100 ml beaker, dissolve 3 mmol of anatase titanium dioxide, and continuously sonicate it in an ultrasonic instrument to fully disperse it to form a suspension. Then transfer it to a three-necked flask. After that, place the three-necked flask on a magnetic stirrer to fix its position, connect a gas pipe and continuously introduce Ar at a flow rate of 60 ml / min, and continuously stir. The stirring speed should not be too fast, and the rotation speed is 150 r / min.

[0062] Step 2: Prepare a metal salt solution by dissolving compounds containing Cu, Co, Ni, and Zn in 25 ml of deionized water and place it in an ultrasonic instrument to sonicate for 1 min to fully dissolve it; among them, the quaternary metal compounds are Cu(NO3)2, CoCl2·6H2O, Zn(NO3)2·6H2O, and NiCl2·6H2O respectively, and their addition amounts are 37.5 mg, 17.8 mg, 22.3 mg, and 17.8 mg respectively. The metal molar ratio is Cu:Co:Zn:Ni≈4:1.5:1.5:1.5:1.5. The deionized water required to dissolve the metal compounds is 25 ml, so that the total metal ion concentration in the solution system is 0.017 mol / L; calculated by mass percentage, the total mass percentage of metal elements in the mass of titanium dioxide is 10 wt%.

[0063] Step 3: Slowly drop the obtained metal salt solution into the titanium dioxide suspension in the three-necked flask, and under the action of stirring, make the two fully mixed and uniform. Then slowly drop the prepared 0.1 mol / L sodium hydroxide solution into the mixed solution of the two, adjust the pH value of the solution to 11.5, and keep stirring the solution for 30 min under this pH condition to form a flocculent hydrotalcite. Then transfer it to a 100 mL beaker and let it stand for a period of time. Then repeatedly wash and centrifuge it with deionized water, pour the precipitate remaining at the bottom of the centrifuge tube into a glass petri dish and place it in a freeze dryer to freeze-dry at -60 °C for 20 h.

[0064] Step 4: Collect the obtained dried product and place it in a porcelain boat. Use an atmosphere-controlled high-temperature furnace to calcine and reduce it at 350 °C for 4 h in a gas atmosphere of 40% H2, 10% CO2, and 50% Ar to finally obtain a photocatalyst of anatase titanium dioxide in-situ loaded with HH-CuCoNiZn@TNS-10% (the molar ratio of Cu, Co, Ni, and Zn is 4:1.5:1.5:1.5) with surface active sites.

[0065] Example 2

[0066] Based on Example 1, the adjustment is as follows: the molar ratio of Cu, Co, Ni, and Zn is 7:1:1:1. Other steps are exactly the same as those in Example 1, and the photocatalyst CuCoNiZn(7:1:1:1)@TNS-10% is prepared.

[0067] Example 3

[0068] Based on Example 1, the adjustment is as follows: the molar ratio of Cu, Co, Ni, and Zn is 4:2:2:2. Other steps are exactly the same as those in Example 1, and the photocatalyst CuCoNiZn(4:2:2:2)@TNS-10% is prepared.

[0069] Example 4

[0070] Based on Example 2, the adjustment is as follows: the total mass of metal elements accounts for 5% of the mass of titanium dioxide. Other steps are exactly the same as those in Example 3, and the photocatalyst CuCoNiZn(7:1:1:1)@TNS loaded with 5% is prepared.

[0071] Example 5

[0072] Based on Example 2, the adjustment is as follows: the total mass of metal elements accounts for 15% of the mass of titanium dioxide. Other steps are exactly the same as those in Example 3, and the photocatalyst CuCoNiZn(7:1:1:1)@TNS loaded with 15% is prepared.

[0073] Example 6

[0074] Based on Example 1, the adjustment is as follows: in Step 4, the calcination temperature is 250 °C. Other steps are exactly the same as those in Example 1, and LH-CuCoNiZn@TNS is prepared.

[0075] Comparative Example 1

[0076] Based on Example 1, the adjustment is as follows: no metal elements are added. Other steps are exactly the same as those in Example 1, and the TiO2 catalyst is prepared.

[0077] Comparative Example 2

[0078] Based on Example 1, the adjustment is as follows: the molar ratio of Cu, Co, Ni, and Zn is 1.5:4:1.5:1.5. Other steps are exactly the same as those in Example 1, and the photocatalyst CuCoNiZn(1.5:4:1.5:1.5)@TNS-10% is prepared.

[0079] Comparative Example 3

[0080] Based on Example 1, adjustments are made. The difference is that the molar ratio of Cu, Co, Ni, and Zn is 1.5:1.5:4:1.5. Other steps are exactly the same as those in Example 1, and the photocatalyst CuCoNiZn(1.5:1.5:4:1.5)@TNS-10% is prepared.

[0081] Comparative Example 4

[0082] Based on Example 1, adjustments are made. The difference is that the molar ratio of Cu, Co, Ni, and Zn is 1.5:1.5:1.5:4. Other steps are exactly the same as those in Example 1, and the photocatalyst CuCoNiZn(1.5:1.5:1.5:4)@TNS-10% is prepared.

[0083] Comparative Example 5

[0084] Based on Example 1, adjustments are made. The difference is that in Step 4, the calcination temperature is 250 °C. Other steps are exactly the same as those in Example 1, and LH-CuCoNiZn@TNS is prepared.

[0085] Comparative Example 6

[0086] Based on Example 1, adjustments are made. The difference is that in Step 4, instead of calcination, the obtained dried product is collected and subjected to photoreduction under xenon lamp irradiation to obtain LR-CuCoNiZn@TNS.

[0087] V. Performance Comparison

[0088] 1. Microscopic Characterization

[0089] The catalysts prepared in the examples are subjected to XRD detection to obtain XRD patterns, as Figure 2 shown. It can be seen that compared with TNS, the main crystal structure of the product prepared in Example 1 is roughly consistent with that of TNS, and additional diffraction peaks appear in the high-angle region (50° - 70°), indicating that its crystal phase has changed to a certain extent. For Comparative Examples 5 and 6, their main crystal phase structures are somewhat different from those in Example 1, but the main structure changes little. Figure 2

[0090] The transmission electron micrograph of the catalyst prepared in Example 1 is as Figure 4 shown.

[0091] The ultraviolet-visible light absorption spectra of the catalysts prepared in the examples and comparative examples are as Figure 3 shown. Although there is no obvious difference in the microstructure between the examples and comparative examples, there are obvious differences in optical properties:

[0092] ​(1) Example 1 has the highest absorption intensity in the <350nm region and shows a steep attenuation to the visible light region, indicating that the light absorption range of Example 1 is relatively wide. This is due to the formation of a medium-entropy alloy structure by high-temperature calcination (350 °C), which distorts the TiO2 lattice and generates a local built-in electric field, suppressing electron-hole recombination. As a result, the catalyst prepared in Example 1 has high crystallinity and exposed active sites (oxygen vacancies + metal nanoparticles), which is beneficial to improving the catalytic effect.

[0093] (2) The absorption peak intensities of Comparative Examples 5 and 6 are both lower than that of Example 1. Moreover, there is no obvious extension in the long wavelength band for Comparative Example 5, indicating that the disorder of the amorphous structure, the lack of an effective stress dispersion mechanism, and the thermodynamic instability of the metallic glass formed at a relatively low calcination temperature fail to effectively regulate lattice distortion and do not form enough stable active sites. Comparative Example 6 shows an additional absorption peak in the range of 400 - 600 nm, which is due to the surface defects or impurity energy levels retained during the photoreduction process. The lack of calcination results in insufficient structural stability of Comparative Example 6 and a decline in long-term photocatalytic performance.

[0094] 2. Catalytic performance

[0095] The photocatalytic performance of the catalysts prepared in the examples and comparative examples for the reduction of nitrite to ammonia was evaluated.

[0096] The experiment for the photoreduction of nitrite to ammonia was carried out in a photocatalytic reactor with a closed-loop cooling system. First, 40 mg of the sample was dispersed in 95 ml of potassium nitrite (NO2 - concentration: 10 ppm) solution, and then 1 ml of formaldehyde (as a reaction sacrificial agent) was added, and the two were ultrasonically mixed thoroughly. Subsequently, the uniformly mixed solution was poured into the photocatalytic reactor, and Ar was continuously introduced at a flow rate of 100 ml / min for purging for 30 min. After 30 min, the first 1 ml sample was taken and labeled as the 0 min sample. At this time, the light source was turned on, which was a 300 W xenon lamp (MC-PF300C, Beijing Meiruichen Technology Co., Ltd.) with a circular filter with a diameter of 80 mm and a thickness of 5 mm. The Ar flow rate was adjusted to 60 ml / min, and at the same time, the circulating cooling water system was turned on. Starting from the moment the light source was turned on, the sampling times were 15 min, 30 min, 45 min, 60 min, and 90 min, and 1 ml of sample was taken each time. The photocatalytic reduction products of nitrite were analyzed using a Prominence UFLC / XR liquid chromatograph (Shimadzu, Japan), with automatic injection every 10 min. The ammonia product obtained by photocatalysis was used as the evaluation index of photocatalytic activity in ppm.

[0097] Table 1 Performance values of the photocatalysts obtained in Example 1 and Comparative Example 1

[0098] Sample <![CDATA[NO - 2(ppm)]]> <![CDATA[NH3 production (ppm)]]> Example 1 10 9.969 Comparative Example 1 10 3.093

[0099] As can be seen from the table, the yield of converting nitrite to ammonia by the catalyst prepared in Example 1 of the present invention is 99.69%. Compared with Comparative Example 1, Example 1 can obtain 9.969 ppm of NH3 in the reduction of nitrite reaction under photocatalysis for 1.5 h. As time prolongs, the yield will further increase.

[0100] From Figure 5 it can be seen that Example 1 reaches about 9.97 mg / L NH4 within 90 minutes + , indicating that it has the highest catalytic activity; this also proves that the medium-entropy alloy structure formed by high-temperature calcination (350 °C) significantly increases the density of active sites (oxygen vacancies + metal nanoparticles) and inhibits electron-hole recombination. The catalytic performance of Comparative Example 5 and Comparative Example 6 is significantly lower than that of Example 1, because insufficient stable active sites are formed during the low-temperature calcination (250 °C) in Comparative Example 5, resulting in insufficient lattice distortion; although a large number of surface defects are retained in Comparative Example 6 after photoreduction treatment, the lack of high-temperature-induced lattice reconstruction leads to poor stability.

[0101] From Figure 6 it can be seen that on the premise that the total amount of metal elements remains unchanged, the Cu content dominates the redox characteristics of the catalyst. A high Cu ratio (7:1:1:1) can have a higher and faster-starting catalytic effect by enhancing the surface plasmon resonance effect and inhibiting carrier recombination; relatively, although a low Cu ratio also has an appreciable catalytic effect (the yield rapidly increases to about 1.5 ppm in the first 20 minutes), the overall ammonia production of Example 3 is still slightly inferior to that of Example 2.

[0102] Figure 7 shows the performance differences of CuZnNiCo@TNS catalysts with different loadings in the photocatalytic reduction of nitrite to ammonia: The yield of Example 4 is the lowest (<1 ppm) in the first 20 minutes, indicating that a lower metal element loading will lead to insufficient active sites; the yield of Example 2 is the highest (about 3.5 ppm) in the first 60 minutes, indicating that an appropriate metal element loading can optimize the dispersion of metal nanoparticles and improve the initial catalytic efficiency; the overall catalytic performance of Example 5 is comparable to that of Example 4. This shows that although Example 4 exceeds Example 2 in the later stage (eventually reaching 5 ppm), the efficiency lags behind in the initial stage due to particle agglomeration and mass transfer limitations, indicating that Example 2 can maximize the exposure of active sites while avoiding structural collapse caused by high loadings.

[0103] Figure 8It shows that Example 1 has excellent catalytic performance. The high Cu ratio (4:1.5:1.5:1.5) maximizes the exposure of active sites (such as oxygen vacancies) by enhancing the surface plasma resonance effect and suppressing electron-hole recombination, so that the yield reaches 8ppm within 90 minutes, which is significantly better than other proportion samples. Comparative Example 2 has almost no catalytic activity, which indicates that a higher proportion of Co will lead to blocked interfacial charge transfer and reduce the density of effective active sites. Comparative Example 3 contains a higher proportion of Ni. Although it has a certain catalytic effect, the reaction start-up time is longer, and the catalytic effect is significantly lower than that of Example 1, which indicates that a high Ni ratio will introduce inactive phases and lattice defects, reducing the catalytic effect. In Comparative Example 4, an excessively high Zn content will cause excessive distortion of the TiO2 lattice, destroy the metal-carrier synergistic effect, and ultimately lead to a reduction in the catalytic effect. However, by comparing Comparative Example 3 with Comparative Example 4, it can be seen that the respective proportions of Ni and Zn in this system will affect the speed of the photocatalytic reaction start-up.

[0104] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit the technical solution. Those skilled in the art should understand that those modifications or equivalent substitutions of the technical solution of the present invention that do not depart from the purpose and scope of the technical solution should be included in the scope of the claims of the present invention.

Claims

1. A titanium dioxide catalyst loaded with a quaternary metal, characterized in that, The catalyst is anatase titanium dioxide, has a (001) crystal plane, and Cu is loaded in the catalyst x Co y Ni y Zn y , where x is 4 to 7 and y is 1 to 2.

2. A method for preparing a titanium dioxide catalyst, characterized in that, To prepare the catalyst described in claim 1, the specific steps are as follows: Step 1: Prepare a titanium dioxide suspension; wherein, the concentration of titanium dioxide is 0.05 mmol / mL to 0.1 mmol / mL; Step 2: Prepare metal salt solutions containing Cu, Co, Ni, and Zn respectively; wherein, the molar ratio of Cu, Co, Ni, and Zn is (4 - 7):(1 - 2):(1 - 2):(1 - 2), and the total concentration of metal salts in the metal salt solution is 0.015 mol / L to 0.02 mol / L; calculated by mass percentage, the total mass percentage of metal elements accounts for 5 wt% to 15 wt% of the mass of titanium dioxide; Step 3: Add the metal salt solutions containing Cu, Co, Ni, and Zn into the titanium dioxide suspension obtained in Step 1 respectively, and adjust the pH value to 10 - 12, while stirring and reacting for 20 min to 60 min; Step 4: Filter the reaction product of Step 3, collect the solid and dry it, then place the solid in an atmosphere containing H2, and calcine it at 300 °C to 400 °C for 3 h to 5 h to obtain the catalyst.

3. The preparation method according to claim 2, characterized in that, In Step 1, the titanium dioxide is anatase type and has a (001) crystal plane.

4. The preparation method according to claim 2, characterized in that, The metal salts include Cu(NO3)2, CoCl2, Zn(NO3)2, NiCl2 and their hydrates.

5. The preparation method according to claim 2, wherein In Step 1, when preparing the titanium dioxide suspension, Ar is continuously introduced into it, the Ar flow rate is 60 mL / min, and the stirring speed is controlled at 100 - 800 r / min.

6. According to the preparation method described in claim 2, characterized in that, In Step 3, the pH value is adjusted by a sodium hydroxide solution.

7. The preparation method according to claim 2, characterized in that, In Step 4, the solid is freeze-dried, and the freeze-drying time is 18 h to 24 h.

8. The preparation method according to claim 2, wherein In Step 4, in the atmosphere containing H2, calculated by volume fraction ratio, the content of H2 is 40%, the content of CO2 is 10%, and the content of Ar is 50%.

9. The preparation method according to claim 2, wherein, In Step 4, the heating rate is 5 °C / min.

10. Application of a titanium dioxide catalyst, characterized in that, The catalyst described in claim 1 or the catalyst prepared by the preparation method described in any one of claims 2 - 9 is used for photocatalytic reduction of nitrite to prepare ammonia.

Images