A photocatalyst with zinc titanate as the carrier loaded with gold and palladium, and its preparation and application

Abstract

The present invention discloses a photocatalyst with zinc titanate as a carrier loaded with gold and palladium bimetals, and its preparation method and application. It uses tetrabutyl titanate as a precursor to synthesize the ZnTiO3 carrier, and then co-loads Au and Pd on the ZnTiO3 semiconductor carrier with light absorption by the deposition-precipitation method. The obtained Au-Pd / ZnTiO3 photocatalyst in the present invention has a high methane conversion rate for the anaerobic coupling system of methane at room temperature and under light, and its preparation method is simple and easy to implement, having good application prospects in the photocatalytic anaerobic coupling conversion of methane.

Description

Technical Field

[0001] The present invention belongs to the fields of energy utilization and environmental protection, and particularly relates to a photocatalyst with zinc titanate as a carrier loaded with gold-palladium bimetal, a preparation method thereof, and an application in the anaerobic coupling conversion of methane under room temperature and ultraviolet-visible light irradiation. Background Art

[0002] Converting methane into high-grade fuels and high-value chemicals such as hydrogen, methanol, and olefins is a promising chemical and energy supply technology. However, the currently commercialized methane conversion technologies have the problem of high energy consumption. Methane has a symmetric tetrahedral structure and four equivalent C-H bonds, endowing methane with low electron and proton affinity (-1.9 eV), weak acidity, and low polarizability (2.8×10 -40 C 2 m 2 J -1 ), the C-H bond strength of methane is relatively high, and the first dissociation energy is 439 kJ mol -1 . The ionization energy of methane is also very high (12.6 eV), which means that even the most loosely bound electrons are difficult to remove. The energy level of the highest occupied molecular orbital (HOMO) is relatively low, and the energy level of the lowest unoccupied molecular orbital (LUMO) is relatively high, making it difficult to remove electrons from the HOMO for oxidation or contribute electrons to the LUMO for reduction. These structural properties indicate that the activation of the C-H bond usually requires high temperature, which often leads to the rapid accumulation of coke and the sintering of the catalyst, resulting in catalyst deactivation and inherently low selectivity. Methane conversion reactions require both sufficient energy input and a suitable catalyst.

[0003] Solar energy is the most abundant and clean renewable energy source and can be used as an important energy source for driving methane conversion under mild conditions. Photocatalytic anaerobic coupling of methane (NOCM) mainly undergoes the following reaction: 2CH4→C2H6 + H2. The NOCM reaction converts methane into ethane with high added value and clean energy hydrogen. In recent years, metal oxide materials (such as TiO2, ZnO, Ga2O3, etc.) have been widely used in the field of methane conversion due to their strong oxidation ability. TiO2 and ZnO are the most widely used CH4 photo-oxidation catalysts at present. The band gap of TiO2 is 3.2 eV, showing obvious light response in the ultraviolet range. The VB and CB of TiO2 are located at 2.70 and 0.50 V vs NHE respectively. The energy band structure of zinc oxide is basically the same as that of TiO2, but the VB value of zinc oxide is slightly higher than that of TiO2. In addition, photo-generated surface electrons (Zn + cations) and holes (O -The synergistic effect of (anions) can effectively promote the activation of C-H bonds, but currently still faces the problems of low methane conversion rate and low product selectivity. Therefore, improving the activity and product selectivity of photocatalytic low-temperature oxidation of methane and deepening the understanding of the mechanism of methane C-H activation are of great significance for methane conversion. Summary of the Invention

[0004] Aiming at the problems of low activity and low product selectivity of the existing reported catalysts for anaerobic coupling of methane, the present invention provides a supported catalyst with zinc titanate as the carrier and gold-palladium bimetal loaded on it. Its preparation method is simple and rapid, and the prepared Au-Pd / ZnTiO3 catalyst has high activity and product selectivity for photocatalytic anaerobic coupling of methane, and has good application prospects.

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] A photocatalyst with zinc titanate as the carrier and gold-palladium bimetal loaded on it. This photocatalyst is a highly dispersed supported catalyst with ZnTiO3 as the carrier and Au / Pd as the dual active components. Among them, the content of the main active component Au is 0.5 wt%, and the content of Pd is 2 wt% of Au.

[0007] The preparation method of the photocatalyst with zinc titanate as the carrier and gold-palladium bimetal loaded on it is to use tetrabutyl titanate and zinc nitrate hexahydrate as raw materials to prepare a sol-gel precursor, and then obtain the ZnTiO3 carrier through drying, sieving, and calcination. Then, the dual active components Au / Pd are simultaneously loaded on the obtained ZnTiO3 carrier by the deposition-precipitation method. The specific steps are as follows:

[0008] (1) First, dissolve Ti(O-Bu)4 in absolute ethanol to obtain solution A; dissolve zinc nitrate hexahydrate in acetic acid solution to obtain solution B; then drop solution B into solution A and stir for 30 min to obtain a transparent sol. Subsequently, let it stand at room temperature for 24 h to form a gel, then dry it at 105 °C for 12 h and grind and sieve it. Heat the uniformly sized particles obtained by sieving in a muffle furnace to 600 °C at a rate of 2 °C / min and calcine for 3 h to obtain the ZnTiO3 carrier.

[0009] (2) Add the ZnTiO3 carrier obtained in step (1) to the urea solution and continuously stir for 30 min. Subsequently, add the HAuCl4 solution and PdCl2 solution, heat in an oil bath at 80 °C for 3 h, centrifuge, wash with deionized water, dry in an oven at 80 °C overnight, and then heat in a muffle furnace to 400 °C at a rate of 2 °C / min and calcine for 4 h to obtain a gold-palladium bimetal photocatalyst supported on cubic zinc titanate.

[0010] Further, the volume concentration of the solution A obtained in step (1) is 49.3%.

[0011] Further, the volume ratio of water to acetic acid in the acetic acid solution used in step (1) is 1:3.

[0012] Further, the mass concentration of the solution B obtained in step (1) is 51.8%.

[0013] Further, the volume ratio of the solution A and the solution B used in the mixing in step (1) is 23:10.

[0014] Further, in step (2), 100 mL of urea solution is added to each gram of ZnTiO3 support, and the concentration of the urea solution used is 0.42 mol / L.

[0015] Further, the concentration of the HAuCl4 solution used in step (2) is 4.78 mg / mL, and the concentration of the PdCl2 solution used is 0.12 mg / mL.

[0016] The obtained photocatalyst with zinc titanate as the support and loaded with gold-palladium bimetal can be applied to the photocatalytic methane anaerobic coupling system at room temperature and under ultraviolet and visible light.

[0017] The remarkable advantages of the present invention are as follows:

[0018] (1) By loading the bimetal Au-Pd in the present invention, the light response of ZnTiO3 is further broadened, which enables the semiconductor to generate electron-hole pairs under visible light of a certain wavelength. Therefore, the photocatalyst with cubic zinc titanate as the support and loaded with gold-palladium bimetal can realize the efficient photocatalytic coupling conversion of methane to ethane at room temperature through ultraviolet and visible light irradiation, and has good activity and stability, providing a new idea for photocatalytic methane conversion.

[0019] (2) The preparation method and application of the present invention are simple and easy to implement, and are suitable for popularization and application. Description of the Drawings

[0020] Figure 1 XRD pattern of AuPd 2% / ZnTiO3 prepared in Example 1.

[0021] Figure 2 UV-Vis diffuse reflection spectrum of AuPd 2% / ZnTiO3 prepared in Example 1.

[0022] Figure 3 TEM image of AuPd 2% / ZnTiO3 prepared in Example 1.

[0023] Figure 4AuPd prepared for Example 1 2% BET diagram of / ZnTiO3.

[0024] Figure 5 For Au / ZnTiO3 (A) and AuPd prepared in Example 1 2% Performance diagram of / ZnTiO3 (B) for photocatalytic CH4 conversion. Detailed implementation manners

[0025] To make the content of the present invention easier to understand, the technical solutions of the present invention will be further described below in conjunction with specific implementation manners. However, the present invention is not limited thereto.

[0026] Example 1

[0027] (1) Dissolve 22.3117 g of Zn(NO3)2·6H2O in a beaker containing 5 mL of H2O and 15 mL of glacial acetic acid. In another beaker, place 35 mL of absolute ethanol and 34 mL of tetrabutyl titanate. Quickly mix the two solutions in a volume ratio of 10:23 and stir for 30 min. Seal the beaker mouth with plastic wrap and let it stand at room temperature for 24 h to form a gel. Then dry it in an oven at 105 °C for 12 h. After drying, grind the obtained solid and pass it through an 80-mesh sieve. Then, heat the sieved uniform particles in a muffle furnace at a rate of 2 °C / min to 600 °C and calcine for 3 h to obtain the ZnTiO3 support;

[0028] (2) Take 1 g of the above-prepared ZnTiO3 support and dissolve it in 100 mL of 0.42 M urea solution. Stir for 30 min, then add 1.046 mL of 4.78 mg / mL HAuCl4 solution and 83.3 μL of 0.12 mg / mL PdCl2 solution. Stir for 0.5 h, perform an oil bath at 80 °C for 4 h, cool to room temperature, centrifuge, wash with deionized water, dry in vacuum at 80 °C overnight, and then heat in a muffle furnace at a rate of 2 °C / min to 400 °C and calcine for 4 h to obtain the catalyst AuPd 2% / ZnTiO3.

[0029] Figure 1 XRD diagrams of the obtained ZnTiO3 and AuPd 2% / ZnTiO3. It can be seen from the figure that due to the low loading amounts of Au and Pd, no diffraction peaks related to Au and Pd appear in the XRD spectrum of the catalyst, which also indicates that Au / Pd in the catalyst is uniformly dispersed on the support and does not affect the crystal structure of ZnTiO3.

[0030] Figure 2 XRD diagrams of the obtained ZnTiO3 and AuPd 2%UV-Vis diffuse reflectance spectra of Au / Pd / ZnTiO3. It can be seen from the figure that Au / Pd / ZnTiO3 has light absorption in the UV-visible light region. After loading Au / Pd, due to the local surface plasmon resonance effect of Au and Pd, the absorption of the catalyst in the visible light is enhanced, which can better utilize visible light and play its photo-promoting role.

[0031] Figure 3 For the obtained AuPd 2% Transmission electron microscope images of AuPd / ZnTiO3. It can be seen from the figure that no obvious particles are observed, further proving the uniform dispersion of Au / Pd on the support.

[0032] Figure 4 For AuPd 2% BET diagrams of AuPd / ZnTiO3. It can be seen from the figure that both AuPd / ZnTiO3 and ZnTiO3 have relatively small specific surface areas. 2% / ZnTiO3 and ZnTiO3 both have relatively small specific surface areas.

[0033] Performance evaluation of the catalyst in Example 2

[0034] The performance evaluation of the catalyst was carried out in a batch reactor with a volume of 175 mL. The upper part was made of quartz glass, allowing light to irradiate the catalyst surface. Before the reaction, the device was evacuated by a vacuum pump, and the other end was connected to the reaction gas CH4. The reaction conditions were as follows: at room temperature and atmospheric pressure, the mass of the catalyst was 20 mg, a 300 W xenon lamp was used as the light source, and 1 mL of gas was taken from the reaction system every 1 h through a micro-syringe and injected into a gas chromatograph for product detection. The amount of substance of each product was obtained by external standard method, and the selectivity of ethane was calculated by the following formula:

[0035] Selectivity of C2H6 = n(C2H6) / {n(C2H6) + n(CO2) + n(CO)}

[0036] Here, n(C2H6), n(CO), and n(CO2) refer to the amount of substance of C2H6, CO, and CO2 produced, respectively.

[0037] Figure 5 For Au / ZnTiO3 (A) and AuPd 2% / ZnTiO3 (B) catalyst CH4 oxidation activity diagrams. It can be seen from the figure that when using AuPd / ZnTiO3 for catalysis, the average hourly C2H6 yield of 1112 μmol•g can be achieved in the first hour, which is significantly higher than that of Au / ZnTiO3. Moreover, with the extension of the reaction time, the yield of the target product ethane is also gradually increasing, while the loading of single palladium has basically no photocatalytic activity, which proves that this bimetallic catalyst has excellent activity under UV-visible light. 2% / ZnTiO3 for catalysis can achieve an average hourly C2H6 yield of 1112 μmol•g -1 in the first hour, which is significantly higher than that of Au / ZnTiO3, and with the extension of the reaction time, the yield of the target product ethane is also gradually increasing, while the loading of single palladium has basically no photocatalytic activity, which proves that this bimetallic catalyst has excellent activity under UV-visible light.

[0038] The above are only the preferred embodiments of the present invention, and all equivalent changes and modifications made according to the scope of the patent application of the present invention shall fall within the scope of the present invention.

Claims

1. Application of a photocatalyst with zinc titanate as a carrier loaded with gold-palladium bimetal in the anaerobic coupling of methane, characterized in that: The photocatalyst is a highly dispersed supported catalyst composed of synthesizing a ZnTiO3 support using tetrabutyl titanate as a precursor and then simultaneously loading dual active components Au / Pd on the obtained ZnTiO3 support by the deposition-precipitation method; its preparation method includes the following steps: (1) First, dissolve Ti(O-Bu)4 in absolute ethanol to obtain solution A; dissolve zinc nitrate hexahydrate in acetic acid solution to obtain solution B; then dropwise add solution B to solution A and stir for 30 min to obtain a transparent sol, then let it stand at room temperature for 24 h to form a gel, then dry at 105 °C for 12 h, grind and sieve, and heat the uniform particles obtained by sieving in a muffle furnace to 600 °C at a rate of 2 °C / min and calcine for 3 h to obtain the ZnTiO3 support; (2) Add the ZnTiO3 support obtained in step (1) to a urea solution and continuously stir for 30 min, then add HAuCl4 solution and PdCl2 solution, heat in an oil bath at 80 °C for 3 h, centrifuge, wash with deionized water, dry overnight in an oven at 80 °C, and then heat in a muffle furnace to 400 °C at a rate of 2 °C / min and calcine for 4 h to obtain a gold-palladium bimetallic photocatalyst supported on cubic zinc titanate, where the content of Au is 0.5 wt% and the content of Pd is 2 wt% of Au; The photocatalyst can be used for the photocatalytic anaerobic coupling of methane to ethane at room temperature and under ultraviolet and visible light.

2. The application according to claim 1, wherein: The volume concentration of solution A obtained in step (1) is 49.3%; the mass concentration of solution B obtained is 51.8%, where the volume ratio of water to acetic acid in the acetic acid solution used is 1:3; the volume ratio of solution A and solution B used is 23:

10.

3. The application according to claim 1, characterized in that: The concentration of the urea solution used in step (2) is 0.42 mol / L, and its addition amount is 100 mL per gram of ZnTiO3 support; the concentration of the HAuCl4 solution used is 4.78 mg / mL, and the concentration of the PdCl2 solution used is 0.12 mg / mL.

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