CN112076777B

CN112076777B - For CO2Reduced photocatalyst and preparation method thereof

Info

Publication number: CN112076777B
Application number: CN202011005997.1A
Authority: CN
Inventors: 罗潇; 李子怡; 陈明; 梁志武
Original assignee: Hunan University
Current assignee: Hunan University
Priority date: 2020-09-23
Filing date: 2020-09-23
Publication date: 2022-02-08
Anticipated expiration: 2040-09-23
Also published as: CN112076777A

Abstract

The invention relates to a method for preparing CO₂The invention relates to a reduced photocatalyst and a preparation method thereof, and the photocatalyst is prepared from g-C₃N₄The composite material is prepared by high-temperature calcination and partial oxidation after being compounded with transition metal carbide, has Z-type carrier transmission characteristics, reduces the hole electron recombination rate of the formed Z-type heterojunction, and separates the oxidation reaction and the reduction reaction on the catalyst, thereby improving the photocatalytic CO₂Reduction efficiency and stability. The photo-thermal effect caused by the larger specific surface area and the stronger light absorption intensity of the catalyst improves the photo-catalytic activity, and products such as CO are easy to desorb on the surface of the catalyst, thereby being beneficial to the reutilization of active sites and enhancing the stability of the photocatalyst. The catalyst of the invention is used for photocatalysis of CO₂The method has the advantages of good application prospect in the reduction field, cheap raw materials, simple preparation method and operation, and suitability for large-scale production.

Description

For CO2Reduced photocatalyst and preparation method thereof

Technical Field

The invention belongs to photocatalytic CO₂The field of utilization, in particular to a method for preparing CO₂Reduced photocatalysts and methods of making the same.

Technical Field

CO₂Capture and utilization are always the hot topics of human research. CO 2₂Is the root of the global greenhouse effect, and can be used as a C source to be used as a raw material for various energy reactions, thereby being resistant to CO₂The fuel material with high potential energy is prepared by carrying out collection and resource utilization, and has quite high research value.

At present, CO₂The trapping mainly comprises three technical schemes, namely pre-combustion trapping, oxygen-enriched combustion and post-combustion trapping, and the three methods are mature at present and CO is generated in the global range₂Capturing items and capturing capabilities are growing rapidly. However, CO₂The technical scheme of capturing has the defects of high cost and profitThe income is greatly influenced by local policy and the like, and the CO is captured₂The degree of utilization of (a) also becomes a necessary consideration for the entire capture project. At present, large scale CO₂The sealing in the ground is still the main treatment method, but the method has the existence of CO₂Secondary leakage and damage to geological structures. In contrast, although CO₂The resource utilization technology is still in the laboratory stage, but has great research value.

CO₂Structurally stable, typically by catalytic hydrogenation, to higher energy species, e.g. CO₂And H₂、H₂O or CH₄And the like, to produce hydrocarbons, alcohols, dimethyl ether, carboxylic acids, and the like. The reaction catalysis mode is also divided into a plurality of modes, including thermal catalysis, electrocatalysis, photocatalysis and the like, wherein the photocatalysis mode has the advantages of environmental protection, low reaction energy consumption, mild conditions and the like, and is favored by researchers. Photocatalyst catalysis of CO₂The reduction activity is high or low, and is related to many factors, such as the type and number of active sites on the surface of the catalyst, the adsorption capacity of the catalyst on reaction substances, the electron hole transfer rate of the catalyst, and the like.

Aiming at the key points capable of improving the catalytic activity of the photocatalyst, transition metal carbide and g-C are combined through reasonable modification design₃N₄Carrying out composite modification and further calcining to prepare the photocatalyst. The catalyst is applied to photocatalysis of CO at room temperature₂And CH₄The reaction shows higher catalytic activity and stability, and the yield of CO after four hours of the reaction is bulk g-C₃N₄More than 4 times, researches show that the transfer type of the photo-generated carriers on the catalyst is Z-type electron transport, which not only improves the transport rate of photo-generated electron holes and prolongs the service life of the photo-generated electron holes, but also enables the electron holes with high reactivity to be reserved. In addition, the catalyst has higher light absorption intensity, and due to the photothermal effect, the temperature of the catalytic surface of the catalyst is increased when the catalyst is irradiated by light, so that reactants on the surface of the catalyst are more easily activated, and the products are generatedAnd is easier to desorb.

Disclosure of Invention

The technical problem solved by the invention is to provide a catalyst for CO by a designed catalyst preparation scheme₂Reduced photocatalysts and methods of making the same. In terms of catalytic activity, the composite catalyst has Z-type carrier transmission characteristics, the transmission rate and the service life of a photon-generated electron hole are improved, and most of electron holes with high reaction activity are reserved; in addition, the catalyst has stronger light absorption intensity and larger specific surface area. The optical performance and the physical and chemical performance are combined, so that the catalyst has higher photocatalytic activity and stability. In the aspect of catalyst preparation, the preparation method is cheap, simple to operate and suitable for large-scale production.

The technical scheme adopted by the invention is as follows:

1. for CO₂A reduced photocatalyst, characterized by:

(1) the photocatalyst consists of transition metal oxide, transition metal carbide and g-C₃N₄Composition is carried out;

(2) the transition metal is one of Ti, Zr, Ta and V;

(3) electrons on the conduction band of the transition metal oxide and g-C₃N₄Holes in the valence band are recombined on the transition metal carbide, so that the Z-type heterojunction formed by the catalyst has Z-type carrier transmission characteristics.

The photocatalyst is prepared by mixing a transition metal carbide with g-C by using a wet chemical method₃N₄The method is characterized by comprising the following steps of:

(1) g to C₃N₄Dispersing the powder into a mixed solution of absolute ethyl alcohol and deionized water, adding the transition metal carbide into the mixed solution, carrying out ultrasonic and stirring treatment to obtain a mixed solution, and evaporating the mixed solution under the stirring condition to obtain a catalyst precursor.

(2) And grinding the catalyst precursor into powder, placing the powder into a reaction furnace for high-temperature calcination, and simultaneously keeping the oxygen atmosphere state in the reaction furnace to finally obtain the photocatalyst.

2. The photocatalyst as claimed in claim 1, wherein the transition metal oxide in the feature (1) is prepared by oxidizing a transition metal carbide under high-temperature calcination conditions, and the degree of oxidation of the transition metal carbide can be adjusted by controlling the calcination temperature and time.

3. The photocatalyst of claim 1, wherein in step (1), the volume ratio of the absolute ethyl alcohol to the deionized water is 0-4 and is not 0.

4. The photocatalyst according to claim 1, wherein in the step (1), the transition metal carbide and g-C₃N₄The mass and dosage ratio of (1): 5-1: 20.

5. the photocatalyst as claimed in claim 1, wherein in the step (1), the total time of the ultrasonic treatment and the stirring is 0.5-2 hours, and the stirring rate is 500-1200 rpm.

6. The photocatalyst as claimed in claim 1, wherein in the step (2), the calcination temperature is 300-600 ℃ and the calcination time is 2-5 hours.

7. The photocatalyst according to claim 1, wherein in the step (2), the oxygen atmosphere is oxygen gas or air, and the volume concentration of the oxygen gas is 0 to 100% and is not 0.

8. The photocatalyst of claim 1 in CO₂The application of the method is characterized in that the reducing agent used in the reduction reaction is one of methane, water and hydrogen.

9. The photocatalyst of claim 1 in CO₂Use in reduction reactions, characterized in that said CO is₂And the volume ratio of the reducing agent is 1-10.

The invention has the following characteristics:

by reacting a transition metal carbide with g-C₃N₄And compounding and further calcining to prepare the photocatalyst. The catalyst has Z-type carrier transmission characteristic and strong light absorption intensityAnd a large specific surface area. The preparation method of the catalyst is cheap, simple to operate and suitable for large-scale production.

The catalyst is applied to the photocatalytic reaction, and has the following advantages:

(1) because the catalyst has Z-type carrier transmission characteristics, the transmission rate of photogenerated electron holes is improved, the service life of the photogenerated electron holes is prolonged, and the electron holes with high reactivity are reserved, so that the photocatalytic activity of the photogenerated electron holes is improved.

(2) The catalyst has higher light absorption intensity, and has higher surface temperature under illumination due to photo-thermal effect, so that reactants on the surface of the catalyst are easy to activate, and products are easy to desorb, thereby improving the photocatalytic activity and stability.

Drawings

FIG. 1 shows 450- (25) TiC-TiO₂/g-C₃N₄The preparation process is shown in the figure.

FIG. 2 shows 450- (25) TiC-TiO₂/g-C₃N₄X-ray single crystal diffraction pattern (XRD).

FIG. 3 shows 450- (25) TiC-TiO₂/g-C₃N₄And g-C₃N₄Ultraviolet-visible absorption spectrum (UV-vis), and a calculated forbidden band width map.

FIG. 4 shows 450- (25) TiC-TiO₂/g-C₃N₄And 450- (20) TNPs/g-C₃N₄Graph of light absorption intensity versus experimental light source wavelength range.

FIG. 5 shows 450- (25) TiC-TiO₂/g-C₃N₄And 450- (20) TNPs/g-C₃N₄The fluorescence spectrum of (a).

FIG. 6 shows 450- (25) TiC-TiO₂/g-C₃N₄The "Z" heterojunction mechanism diagram of (1).

FIG. 7 is a view of a photocatalytic device.

Detailed description of the invention

The present invention will be further described with reference to the following examples.

Example 1: composite catalyst TiC-TiO₂/g-C₃N₄The preparation method of (1). 0.25g g-C₃N₄Dispersing in a beaker filled with 30mL of deionized water, weighing 25mg of TiC, putting into the mixed solution, performing ultrasonic treatment for 10min and stirring for 10min, wherein the stirring speed is 1000 rad/min, transferring the mixed solution into a water bath kettle, and performing water bath evaporation at 80 ℃ and the rotating speed is 500 rad/min. After the solid is evaporated to dryness, taking out the residual solid, grinding the solid, then placing the ground solid in a muffle furnace for calcination, wherein the calcination temperature is 450 ℃, the heating rate is 5 ℃/min, and the calcination time is 3h, so that the TiC part is oxidized into TiO₂The obtained sample is marked as 450- (25) TiC-TiO₂/g-C₃N₄. FIG. 1 shows 450- (25) TiC-TiO₂/g-C₃N₄The preparation process is shown in a brief diagram.

Comparative example 1: 8g of the uniformly ground melamine are placed in a crucible and placed in a tube furnace N₂Calcining for 3h at 550 ℃ in the atmosphere, wherein the heating rate is 5 ℃/min. After calcination, the mixture was ground thoroughly to give a pale yellow powder, and the sample obtained was designated bulk g-C₃N₄。

Comparative example 2: the difference from example 1 is that in the preparation of the catalyst, the reaction is carried out in the direction of g-C₃N₄20mg of TiO was added to the mixture₂The nanoparticles (5-7 nm) were calcined in a muffle furnace for 2h, and the obtained sample was recorded as 450- (20) TNPs/g-C₃N₄。

Example 2: the preparation method is the same as that of example 1, and the difference is that during the preparation of the catalyst, the catalyst is directly added to g-C₃N₄Adding 10mg TiC and 15mg TiO into the mixed solution₂Nanoparticles (5-7 nm) in a tube furnace N₂Calcining for 3h at 550 ℃ in the atmosphere, wherein the heating rate is 5 ℃/min, TiC is not oxidized, and the obtained sample is recorded as 450-TiC/TiO₂/g-C₃N₄。

Example 3: the preparation method is the same as that of example 1, and the difference is that in the preparation process of the catalyst, the calcination temperature in a muffle furnace is 400 ℃, and the obtained sample is marked as 400- (25) TiC-TiO₂/g-C₃N₄。

Example 4: optical characteristics of photocatalystAnd (6) testing. As can be seen from FIG. 2, after the final calcination process, 450- (25) TiC-TiO₂/g-C₃N₄Most of TiC is converted into TiO₂But still a small portion of TiC is not converted. As can be seen from FIG. 3, 450- (25) TiC-TiO₂/g-C₃N₄Has certain response to visible light and is calculated to obtain 450- (25) TiC-TiO₂/g-C₃N₄Has a forbidden band width of 2.66eV relative to g-C₃N₄The forbidden band width is narrowed. As can be seen from FIG. 4, 450- (25) TiC-TiO₂/g-C₃N₄And 450- (20) TNPs/g-C₃N₄The former has higher light absorption intensity under the irradiation of the experimental light source (wavelength range 320-780 nm). FIG. 5 is a fluorescence spectrum clearly showing 450- (25) TiC-TiO₂/g-C₃N₄The electron transfer rate of (A) is higher than that of (A) 450- (20) TNPs/g-C₃N₄Fast, this is benefited from in 450- (25) TiC-TiO₂/g-C₃N₄In the figure, TiC with good conductivity acts as a bridge to accelerate the transfer of photo-generated electron holes and form a Z-type heterojunction electron transport mechanism, and the specific transport mechanism can be seen in figure 6.

Example 5: photocatalytic activity comparative experiment. Photocatalytic CO₂And CH₄The reaction apparatus is shown in figure 7, and the reaction is carried out at room temperature. 50mg of photocatalyst is put into a reactor, the reactor is vacuumized after being filled and sealed, and CO is distributed through three paths of gas distribution₂And CH₄Charging raw material gas with the ratio of 2:1 into a reactor, vacuumizing when the raw material gas is charged to a certain pressure so as to wash the interior of the reactor, then charging 15kPa gas into the reactor, and then starting a circulating pump to circulate the gas in the reactor for 30 min. And then, turning on a light source to react, and sampling and detecting the gas in the reactor every 30min in the reaction process, wherein the total reaction time is 4 h. Wherein the light source of the simulated sunlight is a 300W xenon lamp (320nm-780 nm). During the reaction, CO is the main product, and the reaction results are shown in Table 1.

TABLE 1 comparison of catalytic effects of comparative and examples

Example 6: and testing the stability of the photocatalyst. The experimental method is the same as the comparative experiment of the photocatalytic activity, and the difference is that for 450- (20) TNPs/g-C₃N₄Catalyst, optimized CO₂And CH₄The ratio was 2:1 and the optimized reaction pressure was 30 kPa. For 450- (25) TiC-TiO₂/g- C₃N₄Catalyst, CO₂And CH₄The ratio was 1:1, the reaction pressure was 40kPa, the total reaction time was 12 hours, and the results of comparison of the photocatalytic stability are shown in Table 2.

TABLE 2 comparison of photocatalytic stability results

As is clear from Table 1, 450- (25) TiC-TiO₂/g-C₃N₄Catalyst CO production is bulk g-C₃N₄3.3 times of the catalyst, because of 450- (25) TiC-TiO₂/g-C₃N₄In the process of calcining the catalyst, the TiC surface is oxidized into TiO₂So that the types and the number of the reaction sites on the surface of the catalyst are more than those of bulk g-C₃N₄. In addition, the former catalyst forms a stable heterojunction, which accelerates the mobility rate of photogenerated carriers, increases the survival time of their photogenerated carriers, and thus improves the catalytic activity.

From the aspect of catalyst structure, 450- (25) TiC-TiO₂/g-C₃N₄And 450- (20) TNPs/g-C₃N₄Can be regarded as TiO₂And g-C₃N₄So that their catalytic sites do not differ much. However, the former CO yield was 450- (20) TNPs/g-C₃N₄The catalyst is 1.7 times (Table 1), the catalytic activity and the stability of the catalyst are higher than those of the catalyst of₂/g-C₃N₄Of photo-generated electronsThe hole transport rate is higher than 450- (20) TNPs/g-C₃N₄The "Z" heterojunction electron transport mechanism, which increases the transport rate of photogenerated electron holes, prolongs their lifetime, and retains highly reactive electron holes. In addition, the results of ultraviolet-visible absorption spectroscopy (UV-vis) show that, in the wavelength range of laboratory simulated light source, 450- (25) TiC-TiO₂/g-C₃N₄The absorption intensity of the light is higher than 450- (20) TNPs/g-C₃N₄A catalyst, which has a higher surface temperature under light irradiation, so that reactants on the surface of the catalyst are easily activated and products are easily desorbed, thereby catalyzing CO₂For reduction, the catalyst may be CO₂Provides more effective reaction sites and can timely desorb the generated products: CO and H₂And the like. Therefore, the key points of higher photocatalytic activity and stability are provided.

The above-mentioned embodiments are intended to illustrate the objects, technical solutions and advantages of the present invention in further detail, and it should be understood that the above-mentioned embodiments are only exemplary embodiments of the present invention and are not intended to limit the present invention, and any modifications, equivalents, improvements and the like made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. For CO₂A reduced photocatalyst, characterized by:

(2) the transition metal is one of Ti, Zr, Ta and V;

(3) electrons on the conduction band of the transition metal oxide and g-C₃N₄Holes on the valence band are compounded on the transition metal carbide, so that a Z-shaped heterojunction formed by the catalyst has Z-shaped carrier transmission characteristics;

the photocatalyst is prepared by mixing a transition metal carbide with g-C by using a wet chemical method₃N₄Mixing to prepare a precursorAnd then partially oxidizing the precursor under the high-temperature calcination condition to prepare the catalyst, which is characterized by comprising the following steps:

(1) g to C₃N₄Dispersing the powder into a mixed solution of absolute ethyl alcohol and deionized water, adding a transition metal carbide into the mixed solution, carrying out ultrasonic and stirring treatment to obtain a mixed solution, and evaporating the mixed solution under the stirring condition to obtain a catalyst precursor;