High-dispersion rhodium-loaded defect-state black titanium dioxide photocatalyst, preparation and application in preparation of propylene by photocatalytic methane conversion

Abstract

The invention discloses a high-dispersion rhodium-loaded defect-state black titanium dioxide photocatalyst, preparation and application thereof in preparation of propylene through photocatalytic methane conversion, and belongs to the technical field of methane photocatalytic conversion. The preparation method comprises the following steps: firstly, carrying out chemical reduction treatment on titanium dioxide, carrying out high-temperature calcination, and then washing, carrying out suction filtration and drying to obtain black titanium dioxide; carrying out hydrogen peroxide treatment on the titanium dioxide, and further modifying the surface structure of the titanium dioxide to obtain defect-state black titanium dioxide; and finally, mixing with a metal Rh precursor solution, and calcining to obtain the high-dispersion metal Rh loaded defect-state black titanium dioxide photocatalyst. The prepared catalyst has excellent light absorption capacity, energy from visible light to a near-infrared region (accounting for about 96% of solar energy) in sunlight can be fully utilized, efficient and selective conversion from methane to propylene is achieved under the conditions of low temperature and visible light irradiation, the propylene selectivity can reach 70% or above, and reaction activity far exceeding thermodynamic limitation is shown.

Description

Technical Field

[0001] The present invention belongs to the technical field of photocatalytic conversion of methane, and particularly relates to a defect-state black titanium dioxide photocatalyst loaded with highly dispersed rhodium, its preparation, and its application in photocatalytic conversion of methane to prepare propylene. Background Art

[0002] Catalytic dehydrogenation is an important basic reaction in modern chemical industry. However, the development of low-carbon chemical technologies using methane as a raw material is a difficult challenge in catalytic basic science. Due to the high symmetry structure, low polarizability, and extremely stable C-H bonds of methane, its activation energy is relatively high, resulting in the fact that traditional thermal catalytic conversion of methane often relies on harsh conditions such as high temperature (700 - 1100 °C) and high pressure (> 1 MPa), and is accompanied by serious side reactions, coke deposition, and over-oxidation of by-products. In recent years, photocatalytic technology has become a research hotspot for methane activation and selective conversion due to its mild reaction conditions, availability of renewable energy such as solar energy, and environmental friendliness. Photocatalytic conversion of methane provides an attractive alternative. Compared with thermal strategies, the technology of triggering methane dehydrogenation using renewable solar energy can break the thermodynamic limitations and obtain reactions under milder conditions.

[0003] The activation of methane C–H bonds has always been a key challenge in the catalytic conversion of small molecules. Its reaction path often faces two undesirable extreme situations: one is the over-dehydrogenation of methane; the other is that the oxidizing property in the reaction system is too strong, resulting in the direct deep oxidation of methane to carbon monoxide (CO), or even carbon dioxide (CO2), losing the opportunity to construct high-value-added products. In addition, in some systems, the activation process may also stagnate at the methyl (CH3) stage, only generating simple coupling products such as ethane, making the reaction selectivity and product added value limited. To further produce propylene (C3H6), not only is it necessary to efficiently and controllably generate intermediates, but it is also necessary to regulate its ordered coupling path and precisely control the degree and sequence of coupling and dehydrogenation reactions. Achieving such a conversion process usually relies on the design of functional sites with synergistic catalytic capabilities, such as introducing metal-support interface synergistic effects, or guiding the reaction path by regulating the electronic structure, etc., to achieve highly selective conversion of methane to propylene.

[0004] Particularly noteworthy is that propylene, as an important basic chemical raw material, is widely used in fields such as polypropylene plastics, coatings, and synthetic rubbers. Currently, it is mainly obtained from by-products of petroleum cracking. Propylene mainly relies on by-products of petroleum cracking or propane dehydrogenation process (PDH). Not only does the raw material rely on fossil resources, but the production process also has the drawbacks of high energy consumption and high carbon emissions. Therefore, developing a photocatalytic C1→C3 reaction using natural gas (methane) as the raw material and driven by solar energy can not only break the current dependence on the petroleum route but also significantly reduce the carbon emission intensity, realizing the sustainable transformation of the olefin production path. This green conversion strategy has important practical significance and development prospects under the background of "dual carbon". Summary of the Invention

[0005] The purpose of the present invention is to provide a highly dispersed rhodium-loaded defective black titanium dioxide photocatalyst, its preparation method, and its application in photocatalytic methane conversion to prepare propylene.

[0006] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0007] The preparation method of a highly dispersed rhodium-loaded defective black titanium dioxide photocatalyst described in the present invention is as follows:

[0008] (1) First, titanium dioxide is chemically reduced and then calcined at high temperature;

[0009] (2) The product obtained by high-temperature calcination in step (1) is washed multiple times with a solvent, and then filtered by suction and dried to effectively remove residual impurities, obtaining black titanium dioxide;

[0010] (3) Then, the black titanium dioxide after drying in step (2) is treated with hydrogen peroxide for a certain time to achieve surface modification, and then washed, filtered by suction, and dried again to obtain defective black titanium dioxide rich in defects;

[0011] (4) The rhodium precursor solution is uniformly mixed with the defective black titanium dioxide rich in defects obtained in step (3), impregnated and then dried, and then calcined at high temperature to prepare the highly dispersed rhodium-loaded defective black titanium dioxide photocatalyst.

[0012] Furthermore, in step (1), titanium dioxide is chemically reduced using NaBH4, chemically reduced using hydrogen at 200 - 500 °C, or chemically reduced using zinc powder or aluminum powder under acidic or neutral conditions;

[0013] Furthermore, the high-temperature calcination in step (1) means calcining at 200 - 500 °C for 0.1 - 10 hours with a heating rate of 1 - 10 °C / min under atmosphere conditions such as vacuum, argon, helium, nitrogen, etc.;

[0014] Further, in step (2), it is washed 3 to 5 times with deionized water, and suction filtration is carried out with a water pump after each washing;

[0015] Further, in step (3), the concentration of hydrogen peroxide is 0.1 to 10 mol / L, the treatment temperature is 20 to 80 °C, and the treatment duration is 0.1 to 10 h; it is washed 3 to 5 times with deionized water, and suction filtration is carried out with a water pump;

[0016] Further, the rhodium precursor solution in step (4) is an aqueous solution of chlororhodic acid, an aqueous solution of rhodium acetate, rhodium acetylacetonate, or an aqueous solution of rhodium nitrate, and the mass concentration of rhodium ions is 0.1 to 10 mg / mL; based on the mass of the defective black titanium dioxide support, the mass percentage of impregnated rhodium is 0.02 to 1.0%;

[0017] Further, the high-temperature calcination in step (4) means calcining at 200 to 600 °C for 0.1 to 10 h at a heating rate of 1 to 10 °C in an atmosphere of vacuum, argon, nitrogen, hydrogen, oxygen, or air to decompose the rhodium precursor and obtain rhodium single atoms (corresponding to Example 3), rhodium sub-nanoclusters with a particle size of 0.6 nm to 1 nm (corresponding to Example 2), or rhodium nanoparticles with a particle size of 2 to 4 nm (corresponding to Example 4).

[0018] The present invention also relates to a highly dispersed rhodium-loaded defective black titanium dioxide photocatalyst, which is prepared by the above method.

[0019] The present invention also relates to the application of the above highly dispersed rhodium-loaded defective black titanium dioxide photocatalyst in the low-temperature photocatalytic conversion of methane to propylene, and the low-temperature range is 0 to 200 °C, and the further temperature range is 20 to 80 °C.

[0020] The light source for the above photocatalysis is visible light or near-infrared light, and the wavelength range is 400 to 2500 nm.

[0021] The reaction process for the above photocatalytic conversion of methane to propylene is as follows: The reaction temperature is precisely controlled using a thermostatic reaction bath device, high-purity methane gas is introduced into a quartz reactor, the light irradiation duration is 0.01 to 10 h, and the types of reactants are detected by gas chromatography. The photocatalyst is laid on the inner wall of the quartz chamber like a thin film and is in full contact with the cooling system, and this design can remove the heat generated by light irradiation to the greatest extent.

[0022] The advantages of the present invention compared with the prior art are:

[0023] (1) The photocatalyst synthesis method provided by the present invention has a simple process, a short reaction time, and good operability; deionized water is mainly used as a solvent during the synthesis process, which is green, environmentally friendly, and has little pollution; the required equipment has low requirements, the reaction conditions are mild, and it is easy to implement and promote;

[0024] (2) The present invention uses defective black titanium dioxide as a carrier, which can effectively anchor rhodium atoms, thereby improving the stability and reusability of the photocatalyst; the impregnation method used for loading rhodium active components is not only easy to operate, but also conducive to regulating the rhodium loading amount, and has good controllability and repeatability;

[0025] (3) The photocatalyst prepared by the present invention has for the first time achieved the efficient conversion of methane to propylene under photocatalytic conditions. The reaction can be driven under room temperature (about 20 °C) and visible light (λ≥400 nm) irradiation. Methane exhibits excellent light absorption ability and high conversion efficiency, significantly breaking through the thermodynamic limitations of this type of reaction;

[0026] (4) The present invention has for the first time achieved the photocatalytic conversion of methane to propylene under low temperature and visible light irradiation conditions. Photocatalysis effectively avoids side reactions such as excessive dehydrogenation and deep oxidation of methane that are prone to occur under high temperature (≥500 °C) conditions in traditional thermal catalysis, thereby significantly improving the selectivity of propylene formation, which can reach more than 70% at most. Description of the Drawings

[0027] Figure 1 is the XRD pattern of black titanium dioxide (BT), defective black titanium dioxide (BTO) synthesized in Example 1 of the present invention, and commercial titanium dioxide; all samples exhibit anatase titanium dioxide (PDF#21-1272) structure;

[0028] Figure 2 is the ultraviolet-visible absorption spectrum of commercial titanium dioxide raw material (TiO2), black titanium dioxide (BT) synthesized in Example 1, and defective black titanium dioxide (BTO);

[0029] Figure 3 is the scanning transmission high-angle annular dark field photograph (HAADF~STEM) (based on the mass of the defective black titanium dioxide photocatalyst, the mass percentage content of rhodium is 0.2%) and energy dispersive X-ray spectroscopy (EDS) of the high-fraction Rh-loaded defective black titanium dioxide photocatalyst synthesized in Example 2 of the present invention;

[0030] Figure 4 is the extended X-ray absorption fine structure (EXAFS) spectrum of the highly dispersed Rh-loaded defective black titanium dioxide photocatalyst synthesized in Example 2 of the present invention, detecting the electronic structure and coordination of Rh monomers in the 0.2% Rh@BTO catalyst;

[0031] Figure 5 They are the TG-MS spectra of the defective black titanium dioxide (BTO) prepared in Example 1 of the present invention and the highly dispersed Rh-loaded defective black titanium dioxide (0.2% Rh@BTO) synthesized in Example 2; the left figure is the TG spectrum that studies the mass change of BTO and 0.2% Rh@BTO catalysts during the high-temperature vacuum activation process, and the right figure is the MS spectrum that analyzes the gas components desorbed or separated from BTO and 0.2% Rh@BTO catalysts during the high-temperature vacuum activation process;

[0032] Figure 6 They are the high-angle annular dark-field scanning transmission electron microscopy (AC-HAADF-STEM) images of the 0.02% Rh@BTO catalyst prepared in Example 3 of the present invention; it shows that Rh single atoms are dispersed on the support;

[0033] Figure 7 They are the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of the 1% Rh@BTO catalyst prepared in Example 4 of the present invention; it shows that Rh nanoparticles are dispersed on the support;

[0034] Figure 8 They are the standard curves (left figure) for measuring methane with different amounts of substance and the standard curves (right figure) for measuring propylene with different amounts of substance using gas chromatography GC; the ordinate represents the peak areas corresponding to methane (left figure) and propylene (right figure) with different amounts of substance on gas chromatography GC, so as to quantitatively describe the conversion rate of propane and the selectivity of propylene;

[0035] Figure 9 They are the bar chart (left figure) showing the influence of defective black titanium dioxide loaded with different masses of rhodium on the methane conversion rate and the dot-line chart (right figure) showing the influence of different light intensities on the methane conversion rate under the same mass in Examples 6 to 8 of the present invention;

[0036] Figure 10 They are the photocatalytic performance diagrams of the photocatalyst loaded with 0.2% Rh content changing with time in Example 9 of the present invention; the bar chart represents the selectivity of propylene changing with the reaction time during the photocatalytic methane conversion reaction; the dot-line chart represents the conversion rate of methane changing with the reaction time during the photocatalytic methane conversion reaction; it can be calculated that when the reaction time is about 5 - 15 minutes, at this time, both a relatively high methane conversion rate and propylene selectivity are achieved;

[0037] Figure 11It is a comparative curve of methane conversion rates in the photocatalytic system and the thermodynamic system at different temperatures in Example 10 of the present invention; this figure clearly shows that within the same temperature range, the photocatalytic system achieves a methane conversion efficiency far higher than the traditional thermodynamic limit under low-temperature conditions, highlighting the great advantages of photocatalysis in the methane conversion reaction;

[0038] Figure 12 It is a flow-phase catalytic stability diagram of the catalytic methane conversion reaction of a highly dispersed rhodium-loaded defective black titanium dioxide photocatalyst (the mass percentage content of rhodium is 0.2%) in Example 11 of the present invention; this figure shows that the photocatalyst stability of the highly dispersed rhodium-loaded defective black titanium dioxide photocatalyst is very good, and the production of propylene is still maintained after 10 hours of stability testing. Specific embodiments

[0039] Specific embodiments are given below to further describe the present invention in detail. The specific operation processes in the embodiments can enable those skilled in the art to understand the present invention more comprehensively. The following described embodiments are only a part of the embodiments of the present invention, rather than all of them. Therefore, the protection scope of the present invention is not limited to the following embodiments.

[0040] Example 1:

[0041] (1) Weigh 1 g of TiO2 (particle size about 25 nm, P25, molar ratio of anatase to rutile phases is about 10:1) and 2 g of NaBH4 in a mortar, mix and grind them thoroughly at room temperature for 30 min. Transfer the obtained mixture to a quartz boat and calcine it in a vacuum tube furnace at 350 °C for 1 h. After cooling to room temperature, take it out. It can be seen that the color of the mixture changes from white to black. Then disperse the black solid in 500 mL of deionized water, stir and wash for 1 h, then filter by suction. Continue to wash with deionized water multiple times until the residual NaBH4 is completely removed. Finally, dry it under vacuum conditions to obtain a black TiO2 sample (BT);

[0042] (2) Transfer 60 mg of the black titanium dioxide obtained in step (1) to a 250 mL round-bottom flask, then add 10 mL of pre-prepared hydrogen peroxide with a concentration of 5 mol / L. Place the flask on a magnetic stirrer and continuously stir at 1000 revolutions per minute for 1 h. After the stirring ends, wash it 4 times with deionized water. After each washing, filter by suction with a water pump. The obtained product is vacuum dried overnight to obtain defective black titanium dioxide (BTO);

[0043] As Figure 1As shown, X-ray diffraction was tested, and the XRD patterns of BT, BTO, and commercial TiO2 were obtained respectively. The peak positions of the synthesized BT and BTO are consistent with the PDF#21-1272 standard card of commercial TiO2. All samples show anatase titanium dioxide (PDF#21-1272).

[0044] As Figure 2 shown, in this example, black titanium dioxide (BT) and defective black titanium dioxide (BTO) were prepared, and their ultraviolet-visible light absorption properties were tested. Conventional TiO2 only shows absorption in the ultraviolet region (≤400 nm), while BT and defective BTO show obvious absorption characteristics in the whole spectral range.

[0045] Example 2:

[0046] (1) Follow the same steps (1) and (2) of Example 1 to obtain the defective black titanium dioxide photocatalyst (BTO);

[0047] (2) Using deionized water as the solvent, prepare an aqueous rhodium chloride solution with a rhodium ion mass concentration of 2 mg / mL. Pipette 50 μL of the aqueous rhodium chloride solution and impregnate it on the surface of 50 mg of the defective black titanium dioxide photocatalyst. Dry it with an infrared lamp and transfer it to a quartz reactor. Under vacuum, heat it to 240 °C at a heating rate of 6 °C / min and calcine for 1 h to obtain a highly dispersed rhodium-loaded defective black titanium dioxide photocatalyst (0.2% Rh@BTO), where the mass percentage of impregnated rhodium is 0.2% based on the mass of the defective black titanium dioxide photocatalyst.

[0048] As Figure 3 shown, the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of the highly dispersed rhodium-loaded defective black titanium dioxide photocatalyst (0.2% Rh@BTO) prepared by the above method were tested, indicating that each Rh aggregate consists of several adjacent Rh monomers, and several adjacent Rh monomers are isolated Rh monomers that are close to each other but do not directly bond, which is also verified by energy-dispersive X-ray spectroscopy (EDS).

[0049] As Figure 4 shown, extended X-ray absorption fine structure (EXAFS) was used to detect the electronic structure and coordination of Rh monomers in the 0.2% Rh@BTO catalyst. A significant peak appears at 1.38 Å, and the Rh-O shell with a coordination number (CN) of 4.1. No other typical Rh-Rh coordination peaks were detected, only Rh-O bonds. Therefore, isolated Rh monomers that are close to each other but do not directly bond were collected.

[0050] As Figure 5As shown in the figure, TG-MS spectroscopy was used to study the mass changes of BTO and 0.2% Rh@BTO catalysts during high-temperature vacuum activation and to analyze the gas components desorbed or separated. It can be seen from the TG spectrum on the left that the mass of BTO decreases significantly with increasing temperature, and the desorbed substances obtained by MS spectrum are mainly water and oxygen. In the last step of the preparation of BTO, H2O2 is used for oxidation, which will introduce a large number of O–O bonds on its surface. These O–O bonds may exist in the form of peroxides or superoxides, and there may also be some (–OH) present. These oxygen species are more likely to break during heating to form H2O and O2. The addition of Rh clusters "captures" the O–O bonds by forming stable Rh-O-O-Rh bonds with these oxygen species, thereby blocking their decomposition and reducing the generation of water and oxygen. The formation of a large number of Rh-O bonds from Rh-O-O-Rh bonds during high-temperature vacuum activation can further confirm the existence of Rh-O in 0.2% Rh@BTO.

[0051] Example 3

[0052] Same as step (1) of Example 2, the difference is that in step (2), the mass concentration of rhodium is changed from 2 mg / mL to 0.2 mg / mL and the rhodium loading is changed from 0.2% to 0.02%. The specific steps are as follows:

[0053] Using deionized water as the solvent, a rhodium chloride solution with a rhodium ion mass concentration of 0.2 mg / mL was prepared. 50 μL of the rhodium chloride aqueous solution was pipetted and impregnated onto the surface of 50 mg of defective black titanium dioxide photocatalyst. After drying with an infrared lamp, it was transferred to a quartz reactor. Under vacuum, it was heated to 240 °C at a heating rate of 6 °C / min and calcined for 1 h to obtain a highly dispersed rhodium-loaded defective black titanium dioxide photocatalyst (0.02% Rh@BTO), where the mass percentage of impregnated rhodium is 0.02% based on the mass of the defective black titanium dioxide photocatalyst.

[0054] As Figure 6 shown, 0.02% Rh was introduced into BTO by traditional wet impregnation to obtain 0.02% Rh@BTO. The high-angle annular dark-field scanning transmission electron microscopy (AC-HAADF-STEM) image shows that Rh single atoms are dispersed on the support.

[0055] Example 4

[0056] Same as step (1) of Example 2, the difference is that in step (2), the mass concentration of rhodium is 10 mg / mL and the rhodium loading is changed from 0.2% to 1%. The specific steps are as follows:

[0057] Using deionized water as the solvent, a rhodium chloride solution with a rhodium ion mass concentration of 10 mg / mL was prepared. 50 μL of the rhodium chloride aqueous solution was pipetted and impregnated onto the surface of 50 mg of the defective black titanium dioxide photocatalyst. After drying with an infrared lamp, it was transferred to a quartz reactor. Under vacuum, it was heated to 240 °C at a heating rate of 6 °C / min and calcined for 1 h to obtain a highly dispersed rhodium-loaded defective black titanium dioxide photocatalyst (1% Rh@BTO). Based on the mass of the defective black titanium dioxide photocatalyst, the mass percentage of the impregnated rhodium was 1%, and rhodium nanoparticles with a particle size of 2 - 4 nm were obtained.

[0058] As Figure 7 shown, 1% Rh was introduced into BTO by the traditional wet impregnation method to obtain 1% Rh@BTO. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images showed that Rh nanoparticles were dispersed on the support.

[0059] Example 5

[0060] (1) Similar to Example 2, a rhodium-loaded defective black titanium dioxide (0.2% Rh@BTO) catalyst was obtained. After mixing by the impregnation method, it was dried with an infrared lamp and transferred to a quartz reactor. Under vacuum, it was heated to 240 °C at a heating rate of 6 °C / min and calcined for 1 h to obtain a highly dispersed rhodium-loaded defective black titanium dioxide photocatalyst (0.2% Rh@BTO).

[0061] (2) When the quartz reactor cooled to room temperature, 100 μmol of high-purity methane gas was introduced into it. Then the reactor was transferred to a constant-temperature reaction device at 20 °C, and a xenon lamp light source with a 400 nm filter was used for the photocatalytic methane conversion to prepare propylene reaction. The illumination time was 5 min. Gas chromatography was used to quantitatively analyze the reaction products. Combining the standard curves of the amount of substance of methane and propylene in gas chromatography, the conversion rate of methane and the selectivity of producing propylene could be calculated.

[0062] As Figure 8 shown, by recording the corresponding relationship between the peak area and the amount of substance shown by gas chromatography GC, the standard curves of methane and propylene were respectively plotted. The conversion rate of methane and the selectivity of propylene could be calculated by the following formulas:

[0063] Selectivity: Sel(%) = ([3n(C3H6)] out / ([n(CH4)] in - [n(CH4)] out )) x 100

[0064] Conversion rate: Con(%) = ([n(CH4)] in – [n(CH4)] out) / [n(CH4)] in )x100

[0065] wherein, [n(CH4)] in and [n(CH4)] out are respectively the amount of substance of methane introduced before the reaction and the amount of substance of methane remaining after the reaction, and [n(C3H6)] out is the amount of substance of propylene produced after the reaction.

[0066] Example 6:

[0067] The preparation and reaction were carried out by the method of Example 2, the difference being only that 10 mg of defective black titanium dioxide photocatalyst was weighed, 10 μL of an aqueous rhodium chloride solution with a mass concentration of 2 mg / mL was pipetted, dried with an infrared lamp and transferred to a quartz reactor, and under a vacuum state, it was calcined at 240 °C for 1 h at a heating rate of 6 °C / min to obtain 10 mg of defective black titanium dioxide photocatalyst loaded with highly dispersed rhodium.

[0068] Example 7:

[0069] The preparation and reaction were carried out by the method of Example 2, the difference being only that 30 mg of defective black titanium dioxide photocatalyst was weighed, 30 μL of an aqueous rhodium chloride solution with a mass concentration of 2 mg / mL was pipetted, dried with an infrared lamp and transferred to a quartz reactor, and under a vacuum state, it was calcined at 240 °C for 1 h at a heating rate of 6 °C / min to obtain 30 mg of defective black titanium dioxide photocatalyst loaded with highly dispersed rhodium.

[0070] Example 8:

[0071] The preparation and reaction were carried out by the method of Example 2, the difference being only that 80 mg of defective black titanium dioxide photocatalyst was weighed, 80 μL of an aqueous rhodium chloride solution with a mass concentration of 2 mg / mL was pipetted, dried with an infrared lamp and transferred to a quartz reactor, and under a vacuum state, it was calcined at 240 °C for 1 h at a heating rate of 6 °C / min to obtain 80 mg of defective black titanium dioxide photocatalyst loaded with highly dispersed rhodium.

[0072] Analyzing the results and data of the above examples, as Figure 9 shown, it is found from the left figure that the more the catalyst dosage is not necessarily the better the effect. With the increase of the catalyst dosage, the catalytic activity does not show a linear increasing trend, and even the reaction efficiency decreases above a certain loading. This phenomenon can be attributed to the decrease in light absorption efficiency. A high-concentration catalyst may cause light scattering and shielding effects, reducing the utilization rate of photons in the reaction system, thereby affecting the excitation process of the reaction. The right figure shows that the photocatalytic performance is proportional to the light intensity, indicating that under mild conditions, the activation of methane is driven by photo-generated holes.

[0073] Example 9:

[0074] The preparation and reaction were carried out by the method of Example 5, with the only difference being the light irradiation duration in step (1). After the quartz reactor was cooled to room temperature, 100 μmol of high-purity methane gas was introduced into it, and then the reactor was transferred to a constant-temperature reaction device at 20 °C. The photocatalytic methane conversion to propylene reaction was carried out using a xenon light source with a 400 nm filter, and the light irradiation durations were 1 s, 3 min, 5 min, 15 min, and 30 min.

[0075] The results and data of the above examples were analyzed. As Figure 10 shown, the bar chart represents the propylene selectivity during the photocatalytic methane conversion reaction with the change of reaction time. Under the conditions of low temperature and visible light irradiation, this catalytic system can achieve high-selectivity and high-efficiency methane conversion to propylene in a short time. The optimal reaction time is about 5 - 15 minutes, at which time both a relatively high methane conversion rate and propylene selectivity are achieved, avoiding the influence of side reactions on selectivity in the later stage. The dotted line chart represents the methane conversion rate during the photocatalytic methane conversion reaction with the change of reaction time. The content of the figure shows that the conversion rate rapidly rises from nearly 0 at 1 s, reaches about 8% at around 15 minutes, and remains basically flat at 30 minutes, indicating that the activity of the catalyst tends to be stable after 15 minutes or reaches the reaction plateau.

[0076] Example 10:

[0077] The preparation and reaction were carried out by the method of Example 5, with the only difference being the reaction temperature in step (1). After the quartz reactor was cooled to room temperature, 100 μmol of high-purity methane gas was introduced into it, and then the reactor was transferred to reaction devices at 20 °C, 50 °C, and 80 °C. The photocatalytic methane conversion to propylene reaction was carried out using a xenon light source with a 400 nm filter, and the light irradiation duration was 5 min.

[0078] As Figure 11 shown, the comparison of methane conversion rates in the photocatalytic system and the thermodynamic system at different temperatures is presented. It shows the dependence of the methane oxidative coupling reaction on light. The bottom solid line in the figure describes the limitation of using a thermal system for methane oxidative coupling to propylene by a thermal catalyst, which means that no catalyst can convert more methane than this line. In contrast, the methane conversion rate of 0.2% Rh@BT-O under light irradiation at 293.15 K far exceeds the limit of thermal equilibrium.

[0079] Example 11

[0080] The preparation and reaction were carried out by the method of Example 5, with the only difference being that in step (1), a mobile-phase photoreaction device was used, 100% high-purity methane was introduced, the flow rate was 3 mL / min, and the gas hourly space velocity (GHSV) of methane was 3600 mL gcat -1 h -1 (GHSV). The photocatalytic direct dehydrogenation of methane to propylene was carried out using a xenon light source with a 400 nm filter. The reaction products were analyzed online by gas chromatography. Combining the standard curves of methane and propylene in gas chromatography, the conversion rate of methane could be calculated.

[0081] As Figure 12 shown, the stability of 0.2% Rh@BTO was evaluated. 0.2% Rh@BTO showed a stable C3H6 yield within 10 h. This indicates that the Rh clusters have high stability as catalytic active sites.

Claims

1. A preparation method of a highly dispersed rhodium-loaded defective black titanium dioxide photocatalyst, the steps are as follows: (1) First, chemically reduce titanium dioxide and then calcine it at high temperature; (2) Use a solvent to wash the product obtained by high-temperature calcination in step (1) multiple times, and then perform suction filtration and drying to effectively remove residual impurities to obtain black titanium dioxide; (3) Then treat the black titanium dioxide after drying in step (2) with hydrogen peroxide for a certain time to achieve surface modification, and then wash, suction filter and dry again to obtain defective black titanium dioxide rich in defects; (4) Uniformly mix the rhodium precursor solution with the defective black titanium dioxide rich in defects obtained in step (3), dry it after impregnation, and then calcine it at high temperature to prepare the highly dispersed rhodium-loaded defective black titanium dioxide photocatalyst.

2. The preparation method of a highly dispersed rhodium-loaded defective black titanium dioxide photocatalyst according to claim 1, characterized in that: In step (1), titanium dioxide is chemically reduced using NaBH4, chemically reduced using hydrogen at 200-500 °C, or chemically reduced using zinc powder or aluminum powder under acidic or neutral conditions.

3. The preparation method of a highly dispersed rhodium-loaded defective black titanium dioxide photocatalyst according to claim 1, characterized in that: The high-temperature calcination in step (1) means heating to 200-500 °C at a heating rate of 1-10 °C / min and calcining for 0.1-10 hours under an atmosphere condition such as vacuum, argon, helium, or nitrogen.

4. The preparation method of a highly dispersed rhodium-loaded defective black titanium dioxide photocatalyst according to claim 1, characterized in that: In step (2), deionized water is used for washing 3-5 times, and suction filtration is performed with a water pump after each washing.

5. The preparation method of a highly dispersed rhodium-loaded defective black titanium dioxide photocatalyst according to claim 1, characterized in that: In step (3), the concentration of hydrogen peroxide is 0.1-10 mol / L, the treatment temperature is 20-80 °C, and the treatment duration is 0.1-10 h; then it is washed 3-5 times with deionized water and suction filtered with a water pump.

6. The preparation method of a highly dispersed rhodium-loaded defective black titanium dioxide photocatalyst according to claim 1, characterized in that: The rhodium precursor solution in step (4) is an aqueous solution of chlororhodic acid, an aqueous solution of rhodium acetate, rhodium acetylacetonate, or an aqueous solution of rhodium nitrate, and the mass concentration of rhodium ions is 0.1-10 mg / mL; based on the mass of the defective black titanium dioxide support, the mass percentage of impregnated rhodium is 0.02-1.0%.

7. The preparation method of a highly dispersed rhodium-loaded defective black titanium dioxide photocatalyst according to claim 1, characterized in that: The high-temperature calcination in step (4) means heating to 200-600 °C at a heating rate of 1-10 °C in an atmosphere of vacuum, argon, nitrogen, hydrogen, oxygen, or air and calcining for 0.1-10 h to decompose the rhodium precursor to obtain rhodium single atoms, rhodium sub-nanoclusters with a particle size of 0.6 nm-1 nm, or rhodium nanoparticles with a particle size of 2-4 nm.

8. A highly dispersed rhodium-loaded defective black titanium dioxide photocatalyst, characterized in that: It is prepared by the method described in any one of claims 1-7.

9. Application of the highly dispersed rhodium-loaded defective black titanium dioxide photocatalyst described in claim 8 in the low-temperature photocatalytic conversion of methane to prepare propylene.

10. Use of a highly dispersed rhodium-loaded defective black titanium dioxide photocatalyst as described in claim 9 in the low-temperature photocatalytic conversion of methane to prepare propylene, characterized in that: The low-temperature range is 0-200 °C; the light source for photocatalysis is visible light or near-infrared light, and the wavelength range is 400-2500 nm.

Images