CN115463542B - A method for efficient photocatalytic degradation of hydrocarbon small molecule gases or formaldehyde using zinc oxide nanoparticles modified with metal single atoms - Google Patents

Abstract

The invention relates to a method for efficiently photocatalytic degradation of hydrocarbon micromolecular gas or formaldehyde by utilizing zinc oxide nano particles modified by metal monoatoms, belonging to the technical field of chemical industry. The method comprises the following specific processes: under the illumination condition, under the action of a catalyst, filling dry air into a closed reaction device, injecting hydrocarbon micromolecular gas or formaldehyde, and keeping the whole reaction device in a normal pressure state to perform photocatalytic hydrocarbon micromolecular degradation reaction; the catalyst was used in an amount of 0.3g of catalyst per 100ppm of gas. The method can still be efficiently degraded under the condition that the reaction environment is sunlight, and the reaction environment is mild and simple. The zinc oxide catalyst modified by metal monoatoms is prepared by utilizing the characteristics of the monoatomic catalyst such as increased surface free energy, quantum size effect, unsaturated coordination environment, interaction between metal and carrier and the like, and has high-efficiency catalytic activity on degradation of hydrocarbon micromolecular gas and formaldehyde.

Description

A kind of zinc oxide nanoparticles modified with metal single atoms to efficiently photocatalyze hydrocarbons Methods for degradation of small molecule gases or formaldehyde

Technical field

The invention relates to a method for efficiently photocatalyzing the degradation of hydrocarbon small molecule gases or formaldehyde using zinc oxide nanoparticles modified with metal single atoms, and belongs to the field of chemical engineering technology.

Background technique

Since the middle of the 20th century, with the extensive use of fossil fuels, the development of agriculture and animal husbandry, and the emissions of gas vehicle exhaust, the concentration of small hydrocarbon gases in the atmosphere has continued to rise. On the one hand, some small hydrocarbon molecules are exacerbating global warming. Greenhouse gases and hydrocarbons, on the other hand, may cause photochemical smog atmospheric pollution, causing serious negative impacts on the human living environment. In daily life, the ethylene gas generated during the transportation of agricultural products shortens the shelf life of fruits and vegetables and degrades their quality. Removing ethylene will improve people's quality of life. At the same time, the paints, artificial boards, and decorative materials used in people's house decorations will release large amounts of formaldehyde. Formaldehyde is easily condensed into structurally stable macromolecules such as paraformaldehyde, which are difficult to be oxidized and degraded. Long-term exposure will cause serious harm to people's health.

Photocatalytic technology uses sunlight as direct energy to drive reactions. It can carry out deep reactions without heating and has the natural advantage of being green and energy-saving. Zinc oxide is a common and cheap semiconductor. Its polar structure allows the electron holes generated by light to be quickly separated and transported. Further modification on its surface forms surface defects, which facilitates surface reactions and improves catalyst activity.

Single-Atom Catalysts are a type of supported catalyst containing only isolated single atoms as catalytically active centers. It is characterized by isolated atoms dispersed on the carrier surface, low atomic coordination environment and maximum atomic specific activity. These properties give single-atom catalysts remarkable catalytic activity, stability, and selectivity in many reactions. At the same time, low-content single-atom catalysts can greatly reduce the demand for precious metals, reduce raw material costs, and are conducive to industrial production. However, no single atom catalyst has been used for the photodegradation of small molecular gases such as methane or formaldehyde in the gas-solid phase.

Existing photocatalytic oxidation technology mainly uses semiconductors doped with precious metals. The doping concentration is high. The metals generally gather in the form of clusters on the surface of the carrier. The specific surface area of the active sites is low, and more precious metal raw materials are often used. To achieve the catalytic effect, and some degradation reactions need to be carried out under harsh conditions of high temperature and pressure, which is not conducive to saving energy and protecting the environment.

Contents of the invention

The purpose of the present invention is to overcome the shortcomings of the existing technology and provide a method for efficiently photocatalyzing the degradation of hydrocarbon small molecule gases or formaldehyde using zinc oxide nanoparticles modified with metal single atoms.

In order to achieve the above objects, the present invention adopts the following technical solutions:

A method for efficiently photocatalyzing the degradation of hydrocarbon small molecule gases or formaldehyde using zinc oxide nanoparticles modified with metal single atoms. The specific process is as follows: under light conditions, under the action of a catalyst, in a closed reaction device, Dry air, inject hydrocarbon small molecule gas or formaldehyde, keep the entire reaction device at normal pressure, and carry out photocatalytic hydrocarbon small molecule degradation reaction; the catalyst dosage is 0.3g catalyst per 100ppm gas; the catalyst is Pt, Ru, Rh, Ag, Mg or Pd single atom modified zinc oxide nanoparticles. The catalyst of the invention is composed of active components and a carrier, wherein the active component is Pt, Ru, Rh, Ag, Mg or Pd; the carrier is nano-ZnO; the active component is in a single-atom dispersed state on the carrier.

Further, the hydrocarbon small molecule gas is methane, ethylene, ethane, propylene or propane.

Further, the light source of the illumination is sunlight or xenon lamp.

In one embodiment of the present invention, the catalyst is zinc oxide nanoparticles modified with metal Ru single atoms, the illumination is a 300W xenon lamp, and the photocatalytic degradation reaction times of small hydrocarbon molecules or formaldehyde are as follows: methane is 15min; ethylene is 8min; ethane is 12min; propylene is 10min; propane is 15min; formaldehyde is 150min.

Further, the zinc oxide nanoparticle catalyst modified with a metal single atom is prepared by a two-step synthesis and control process of precipitation method and photoactivation method, including the following steps:

1) Weigh equal molar ratios of zinc nitrate hexahydrate and oxalic acid, dissolve them in deionized water respectively, mix the oxalic acid solution and the zinc nitrate solution, and then centrifuge, wash and dry to obtain zinc oxalate solid powder;

2) Place the zinc oxalate powder obtained in step 1) in a muffle furnace for calcination at 375±25°C for 5-10 hours, then naturally cool to room temperature to obtain zinc oxide nanoparticles;

3) Disperse the zinc oxide powder obtained in step 2) into pure water, drop the salt solution of the required modified metal into the zinc oxide suspension at a certain mass ratio, place it in a dark environment and keep stirring for 60 minutes, and then heat it at more than 100W Light aging under high-power xenon lamp for 120±50 minutes, then centrifuge, wash and dry to remove impurities;

4) Place the dry powder obtained in step 3) into a tube furnace and treat it at a low temperature of 250±50°C for 120±50 minutes in a hydrogen atmosphere to obtain the target product.

Further, in the step 2), during the calcination treatment, the temperature of the high temperature treatment is reached at a heating rate of 5°C/min.

Further, in step 3), the salt solution of the modified metal is a metal chloride solution of Pt, Ru, Rh, Ag, Mg or Pd, and the mass ratio of the modified metal to zinc oxide is 3:10000.

Furthermore, in step 4), during low-temperature treatment, the temperature is reached at a heating rate of 5°C/min.

In one embodiment of the present invention, the preparation method of the metal single atom modified zinc oxide nanoparticle catalyst includes the following steps:

1) Weigh equal molar ratios of zinc nitrate hexahydrate and oxalic acid and dissolve them in deionized water respectively. Add the oxalic acid solution dropwise to the zinc nitrate solution to obtain a white turbid liquid. Filter, wash and dry to obtain zinc oxalate powder;

2) Place the zinc oxalate powder obtained in step 1) in a muffle furnace, heat it to 350°C at a heating rate of 5°C/min in an air atmosphere and keep it warm for 360 minutes, then naturally cool to room temperature to obtain zinc oxide nanoparticles;

3) Disperse the zinc oxide powder obtained in step 2) into pure water, add the modified metal salt solution dropwise to the zinc oxide suspension at a mass ratio of single atoms to zinc oxide of 3:10000, and place in a dark environment Keep stirring for 60 minutes, then irradiate and age with 300W xenon lamp for 120 minutes, wash, filter and dry; the salt solution of the modified metal is a metal chloride solution of Pt, Ru, Rh, Ag, Mg or Pd;

4) Place the dry powder obtained in step 3) into a tube furnace, raise the temperature to 250°C at a heating rate of 5°C/min in a hydrogen atmosphere, keep it warm for 180 minutes, and cool naturally to obtain a single-atom modified zinc oxide photocatalyst.

In the present invention, the anchoring of metal single atoms on the surface of zinc oxide requires two-step treatment of photoaging and annealing at 250±50°C in a hydrogen atmosphere. Step 4) The final product is used as a catalyst in the fields of photocatalytic hydrocarbon small molecule gases and formaldehyde oxidation.

In the present invention, the metal single atom modified zinc oxide nanomaterial of the present invention achieves different photocatalytic activation properties by selecting different metal single atoms and single atom metal content, and has the advantages of simple preparation method, high catalytic activity and stable performance.

Compared with the existing technology, the present invention has the following technical effects:

1) The present invention. For the first time, the present invention uses single-atom catalyst materials in the field of photodegradation of hydrocarbon small molecule gases such as methane or formaldehyde in the gas-solid phase. The prepared series of metal single atom modified zinc oxide nanomaterials are single atom catalysts with more dispersed active site centers and more oxygen defective sites. These metal and oxygen defective active sites can effectively adsorb oxygen and convert it into Convert into free radicals, and the polar structure of zinc oxide is conducive to the breaking of carbon-hydrogen bonds, thereby improving the activation and oxidation efficiency of small molecular hydrocarbons and formaldehyde; the amount of catalyst required is small, the reaction conditions are simple and mild, and it can be used under outdoor sunlight It can be carried out without pressure and heating, and the catalyst performance is stable for a long time. For example, in the Ru single-atom metal-modified zinc oxide photocatalytic material Ru/ZnO in Example 1, the polar structure of zinc oxide builds a built-in electric field that can activate the carbon-hydrogen bonds in methane and promote the breaking of the carbon-hydrogen bonds. Ru can promote Adsorption of oxygen from the air.

2) The method of the present invention can still degrade efficiently under sunlight, and the reaction environment is mild and simple. And it has efficient catalytic activity for hydrocarbon small molecule gases and formaldehyde.

Description of the drawings

Figure 1 shows the X-ray diffraction patterns of the prepared pure zinc oxide and Ru single-atom metal-modified zinc oxide photocatalytic material Ru/ZnO;

Figure 2 is a scanning electron microscope photo of the prepared Ru single atom modified zinc oxide photocatalytic material Ru/ZnO;

Figure 3 is a high-resolution transmission electron microscope photo of the prepared Ru single atom modified zinc oxide photocatalytic material Ru/ZnO;

Figure 4 is the spherical aberration electron microscope image of the prepared Ru single atom modified zinc oxide photocatalytic material Ru/ZnO;

Figure 5 shows the solar photocatalytic oxidation performance test of 100ppm methane and other hydrocarbon gases of the prepared Ru single atom modified zinc oxide photocatalytic material Ru/ZnO;

Figure 6 shows the laboratory photocatalytic oxidation performance test of 100ppm methane and other hydrocarbon gases of the prepared Ru single atom modified zinc oxide photocatalytic material Ru/ZnO;

Figure 7 shows the laboratory photocatalytic oxidation performance test of 100ppm formaldehyde of the prepared Ru single atom modified zinc oxide photocatalytic material Ru/ZnO.

Detailed ways

The present invention will be described in further detail below in conjunction with the accompanying drawings and specific embodiments. The following examples are only used to illustrate the present invention and help those skilled in the relevant fields to further understand, but are not intended to limit the present invention in any form. Those skilled in the relevant fields will refer to it based on Reasonable improvements and adjustments based on the concept of the present invention all fall within the protection scope of the present invention.

Example 1

The steps for preparing Ru single atom modified zinc oxide photocatalyst Ru/ZnO are as follows: Weigh equal molar ratios of zinc nitrate hexahydrate and oxalic acid and dissolve them in deionized water respectively, add the oxalic acid solution dropwise to the zinc nitrate solution to obtain a white turbid liquid, filter and wash , dry to obtain zinc oxalate powder, place it in a muffle furnace, heat it to 350°C at a heating rate of 5°C/min in an air atmosphere and keep it warm for 360 minutes to obtain zinc oxide powder, disperse the zinc oxide into pure water, press Add ruthenium chloride solution dropwise to the suspension at a mass ratio (Ru/ZnO) of 3:10000, place it in a dark environment and keep stirring for 60 minutes, then age it under a 300W xenon lamp for 120 minutes, wash, filter and dry. The dried powder was placed in a tube furnace, heated to 250°C at a heating rate of 5°C/min in a hydrogen atmosphere, kept at the temperature for 180 minutes, and then cooled naturally to obtain Ru single-atom modified zinc oxide photocatalyst Ru/ZnO.

Figure 1 shows the X-ray diffraction patterns of the prepared pure zinc oxide and Ru single-atom metal-modified zinc oxide photocatalytic material Ru/ZnO. It can be seen from Figure 1 that the introduction of low concentration Ru has no obvious effect on the zinc oxide lattice structure.

Figure 2 is a scanning electron microscope photo of the prepared Ru single atom modified zinc oxide photocatalytic material Ru/ZnO. As can be seen from Figure 2: Scanning electron microscopy shows that the prepared zinc oxide is nanoparticles with a size of about 30 nm.

Figure 3 is a high-resolution transmission electron microscope photo of the prepared Ru single atom modified zinc oxide photocatalytic material Ru/ZnO. It can be seen from Figure 3 that high-resolution STEM shows clear characteristic lattice stripes of zinc oxide.

Figure 4 is the spherical aberration electron microscope image of the prepared Ru single atom modified zinc oxide photocatalytic material Ru/ZnO. It can be seen from Figure 4 that the presence of Ru single atoms can be seen with a spherical aberration electron microscope.

Photocatalytic oxidation reaction of small hydrocarbon gases:

1) Experiment under sunlight: In a closed quartz reaction device, put 0.3g of the catalyst prepared in Example 1, fill it with dry air, and inject 100 ppm of methane, formaldehyde, ethylene, ethane, propylene, and propane respectively to make the entire reaction The device is kept at normal pressure and exposed to an open area with direct sunlight for catalytic testing. Samples are taken at fixed intervals to detect the remaining amount of reaction gas and product production through gas chromatography, and the solar power is recorded.

2) Experiment under Xe lamp: The test process is similar to 1) Experiment under sunlight, except that the light source is replaced by the laboratory 300W Xe lamp.

The photocatalytic hydrocarbon small molecule gas oxidation performance test results are shown in Figure 5 (sunlight) and Figure 6 (Xe lamp).

It can be seen from Figure 5 that the catalyst prepared under outdoor sunlight has efficient catalytic activity for methane, ethylene, ethane, propylene, and propane, and the activity is affected by the intensity of sunlight at different times. Among them, 100 ppm ethylene can be used at noon Under sunlight around 12 o'clock, the degradation effect will be completed in 15 minutes.

It can be seen from Figure 6 that under the irradiation of a 300W xenon lamp in the laboratory, the prepared catalyst has a shorter degradation time for gases such as methane and can completely degrade 100ppm methane within 15 minutes.

Under the irradiation of a 300W xenon lamp, the degradation times of various gases of the catalyst prepared in Example 1 are as follows: methane 15 minutes; ethylene 8 minutes; ethane 12 minutes; propylene 10 minutes; propane 15 minutes.

Photocatalytic formaldehyde oxidation reaction: The test process is similar to the photocatalytic hydrocarbon small molecule gas experiment under Xe lamp. The difference is that after the reaction device is filled with dry gas, 100 ppm formaldehyde is injected as the reaction gas.

The photocatalytic formaldehyde oxidation performance test is shown in Figure 7. It can be seen from Figure 7 that under the irradiation of a 300W xenon lamp in the laboratory, the prepared catalyst has relatively efficient catalytic activity for formaldehyde degradation, and the degradation is completed in 150 minutes.

Example 2

The steps for preparing Rh single atom modified zinc oxide photocatalyst Rh/ZnO are as follows: Weigh equal molar ratios of zinc nitrate hexahydrate and oxalic acid and dissolve them in deionized water respectively, add the oxalic acid solution dropwise to the zinc nitrate solution to obtain a white turbid liquid, filter and wash , dry to obtain zinc oxalate powder, place it in a muffle furnace, heat it to 350°C at a heating rate of 5°C/min in an air atmosphere and keep it warm for 360 minutes to obtain zinc oxide powder, disperse the zinc oxide into pure water, press Add rhodium chloride solution dropwise to the suspension at a mass ratio (Rh/ZnO) of 3:10000, place in a dark environment and keep stirring for 60 minutes, then age under 300W xenon lamp for 120 minutes, filter, wash, filter and dry. The dried powder was placed in a tube furnace, heated to 250°C at a heating rate of 5°C/min in a hydrogen atmosphere, kept at the temperature for 180 minutes, and then cooled naturally to obtain Rh single atom modified zinc oxide photocatalyst Rh/ZnO. The photocatalytic hydrocarbon small molecule gas and formaldehyde oxidation performance test conditions are as in Example 1.

Example 3

The steps for preparing Pd single atom modified zinc oxide photocatalyst Pd/ZnO are as follows: Weigh equal molar ratios of zinc nitrate hexahydrate and oxalic acid and dissolve them in deionized water respectively, add the oxalic acid solution dropwise to the zinc nitrate solution to obtain a white turbid liquid, filter and wash , dry to obtain zinc oxalate powder, place it in a muffle furnace, heat it to 350°C at a heating rate of 5°C/min in an air atmosphere and keep it warm for 360 minutes to obtain zinc oxide powder, disperse the zinc oxide into pure water, press Add palladium chloride solution dropwise to the suspension at a mass ratio (Pd/ZnO) of 3:10000, place in a dark environment and keep stirring for 60 minutes, then age under 300W xenon lamp for 120 minutes, filter, wash, filter and dry. The dried powder was placed in a tube furnace, heated to 250°C at a heating rate of 5°C/min in a hydrogen atmosphere, kept at the temperature for 180 minutes, and then cooled naturally to obtain a Pd single atom modified zinc oxide photocatalyst Pd/ZnO. The photocatalytic hydrocarbon small molecule gas and formaldehyde oxidation performance test conditions are as in Example 1.

Example 4

The steps for preparing Pt single atom modified zinc oxide photocatalyst Pt/ZnO are as follows: Weigh equal molar ratios of zinc nitrate hexahydrate and oxalic acid and dissolve them in deionized water respectively, add the oxalic acid solution dropwise to the zinc nitrate solution to obtain a white turbid liquid, filter and wash , dry to obtain zinc oxalate powder, place it in a muffle furnace, heat it to 350°C at a heating rate of 5°C/min in an air atmosphere and keep it warm for 360 minutes to obtain zinc oxide powder, disperse the zinc oxide into pure water, press Add chloroplatinic acid solution dropwise to the suspension at a mass ratio (Pt/ZnO) of 3:10000, place in a dark environment and keep stirring for 60 minutes, then age under 300W xenon light for 120 minutes, filter, wash, filter and dry. The dried powder was placed in a tube furnace, heated to 250°C at a heating rate of 5°C/min in a hydrogen atmosphere, kept at the temperature for 180 minutes, and then cooled naturally to obtain Pt single-atom modified zinc oxide photocatalyst Pt/ZnO. The photocatalytic hydrocarbon small molecule gas and formaldehyde oxidation performance test conditions are as in Example 1.

Example 5

The steps for preparing Ag single atom modified zinc oxide photocatalyst Ag/ZnO are as follows: Weigh equal molar ratios of zinc nitrate hexahydrate and oxalic acid and dissolve them in deionized water respectively, add the oxalic acid solution dropwise to the zinc nitrate solution to obtain a white turbid liquid, filter and wash , dry to obtain zinc oxalate powder, place it in a muffle furnace, heat it to 350°C at a heating rate of 5°C/min in an air atmosphere and keep it warm for 360 minutes to obtain zinc oxide powder, disperse the zinc oxide into pure water, press Add silver chloride solution dropwise to the suspension at a mass ratio (Ag/ZnO) of 3:10000, place in a dark environment and keep stirring for 60 minutes, then age under 300W xenon lamp for 120 minutes, filter, wash, filter and dry. The dried powder was placed in a tube furnace, heated to 250°C at a heating rate of 5°C/min in a hydrogen atmosphere, kept at the temperature for 180 minutes, and then cooled naturally to obtain Ag single-atom modified zinc oxide photocatalyst Ag/ZnO. The photocatalytic hydrocarbon small molecule gas and formaldehyde oxidation performance test conditions are as in Example 1.

Example 6

The steps for preparing Mg single atom modified zinc oxide photocatalyst Mg/ZnO are as follows: Weigh equal molar ratios of zinc nitrate hexahydrate and oxalic acid and dissolve them in deionized water respectively, add the oxalic acid solution dropwise to the zinc nitrate solution to obtain a white turbid liquid, filter and wash , dry to obtain zinc oxalate powder, place it in a muffle furnace, heat it to 350°C at a heating rate of 5°C/min in an air atmosphere and keep it warm for 360 minutes to obtain zinc oxide powder, disperse the zinc oxide into pure water, press Add magnesium chloride solution dropwise to the suspension at a mass ratio (Mg/ZnO) of 3:10000, place in a dark environment and keep stirring for 60 minutes, then age under 300W xenon light for 120 minutes, filter, wash, and dry. The resulting powder was placed in a tube furnace, heated to 250°C at a heating rate of 5°C/min in a hydrogen atmosphere, kept at the temperature for 180 minutes, and then cooled naturally to obtain the Mg single atom modified zinc oxide photocatalyst Mg/ZnO. The photocatalytic hydrocarbon small molecule gas and formaldehyde oxidation performance test conditions are as in Example 1.

The test method is the same and the amount of catalyst is the same, but the photocatalytic performance and efficiency of Ru/ZnO, Rh/ZnO, Pd/ZnO, Mg/ZnO, Pt/ZnO, and Ag/ZnO are ranked from high to low for small molecular hydrocarbon gas compounds. It is: Ru/ZnO>Pd/ZnO>Mg/ZnO>Pt/ZnO>Rh/ZnO>Ag/ZnO.

The above are only examples of the present invention, and are not intended to limit the patent scope of the present invention. Any equivalent transformations made using the contents of the description of the present invention, or directly or indirectly applied in other related technical fields, are equally included in the scope of the present invention. Within the scope of patent protection.

Claims (8)

1. A method for efficiently photocatalyzing the degradation of small hydrocarbon gases or formaldehyde using zinc oxide nanoparticles modified with metal single atoms, which is characterized in that the specific process is as follows: under light conditions, under the action of a catalyst, in a closed In the reaction device, dry air is filled, and hydrocarbon small molecule gas or formaldehyde is injected to keep the entire reaction device at normal pressure to carry out photocatalytic hydrocarbon small molecule degradation reaction; the catalyst dosage is 0.3g of catalyst per 100 ppm of gas; so The catalyst is Pt, Ru, Rh, Ag, Mg or Pd single atom modified zinc oxide nanoparticles; The zinc oxide nanoparticle catalyst modified by a metal single atom is prepared by two-step synthesis and control using a precipitation method and a photoactivation method, including the following steps: 1) Weigh equal molar ratios of zinc nitrate hexahydrate and oxalic acid, dissolve them in deionized water respectively, mix the oxalic acid solution and the zinc nitrate solution, and then centrifuge, wash and dry to obtain zinc oxalate solid powder; 2) Place the zinc oxalate powder obtained in step 1) in a muffle furnace for calcination at 375±25°C for 5-10 hours, then naturally cool to room temperature to obtain zinc oxide nanoparticles; 3) Disperse the zinc oxide powder obtained in step 2) into pure water, drop the salt solution of the required modified metal into the zinc oxide suspension at a certain mass ratio, place it in a dark environment and keep stirring for 60 minutes, and then heat it at more than 100W Light aging under high-power xenon lamp for 120±50 minutes, then centrifuge, wash and dry to remove impurities; 4) Place the dry powder obtained in step 3) into a tube furnace and treat it at a low temperature of 250±50°C for 120±50 minutes in a hydrogen atmosphere to obtain the target product. 2. The method according to claim 1, characterized in that the hydrocarbon small molecule gas is methane, ethylene, ethane, propylene or propane. 3. The method according to claim 1, characterized in that the prepared catalyst structure is a single atom catalyst. 4. The method according to claim 1, characterized in that the light source of the illumination is sunlight or a xenon lamp. 5. The method according to claim 1, characterized in that the catalyst is a zinc oxide nanoparticle modified by a single atom of metal Ru, and the illumination is a 300W xenon lamp, which photocatalyzes the degradation reaction of small hydrocarbon molecules or formaldehyde. The times are as follows: methane is 15min; ethylene is 8min; ethane is 12min; propylene is 10min; propane is 15min; formaldehyde is 150min. 6. The method according to claim 1, characterized in that in step 2), during the calcination treatment, the temperature of the high temperature treatment is reached at a heating rate of 5°C/min. 7. The method according to claim 1, characterized in that, in step 3), the salt solution of the modified metal is a metal chloride solution of Pt, Ru, Rh, Ag, Mg or Pd, and the modified metal and oxidation The mass ratio of zinc is 3:10000. 8. The method according to claim 1, characterized in that in step 4), during low temperature treatment, the temperature is reached at a heating rate of 5°C/min.