A ZnO photocatalyst rich in interstitial zinc

Abstract

The present invention discloses a ZnO photocatalyst rich in interstitial zinc, its preparation method, and its application, belonging to the technical field of photocatalyst preparation. The ZnO photocatalyst rich in interstitial zinc is prepared by thermally decomposing ZnO2. The catalyst has readily available raw materials, a simple and easy preparation method, and is suitable for large-scale production. It exhibits high activity, high selectivity, and high stability in the photocatalytic oxidation of CH4 to liquid oxygen-containing compounds, and has promising application prospects in the photocatalytic oxidation of CH4.

Description

Technical Field

[0001] The present invention belongs to the technical field of photocatalyst preparation, and specifically relates to a ZnO photocatalyst rich in interstitial zinc, a preparation method thereof, and an application thereof in photocatalytic oxidation of CH4 to liquid oxygen-containing compounds. Background Art

[0002] Methane (CH4) plays a vital role in industrial production, but is often burned as fuel, resulting in low utilization rates. Therefore, upgrading CH4 to higher-value liquid oxygenates not only improves CH4 utilization but also provides essential raw material intermediates for the chemical industry. Traditionally, CH4 is converted to syngas, which is then further synthesized into CH3OH or other liquid chemical products via the Fischer-Tropsch reaction. However, this indirect conversion process requires high-temperature thermal reactions (>700°C) and carries significant environmental pollution and operational risks, prompting the search for alternative routes for CH4 conversion under mild conditions. Photocatalysis, an advanced green technology, utilizes light energy rather than heat to initiate chemical reactions, offering a method for directly converting CH4 to liquid oxygenates. However, the inherent high bond energy and low polarizability of CH4 bonds severely hinder CH4 activation. Furthermore, liquid oxygenates are more susceptible to overoxidation to CO2 than CH4. Therefore, the development of photocatalysts with high selectivity for liquid oxygenates is crucial for CH4 conversion.

[0003] The photocatalytic aerobic conversion of CH4 into liquid oxygen-containing compounds is a very promising route. Compared with the photocatalytic anaerobic conversion of CH4, the photocatalytic aerobic conversion of CH4 can make full use of photogenerated holes and electrons to react with H2O and O2 to generate reactive oxygen species. On the one hand, O2 has a strong electron capture ability to promote the separation of photogenerated charges, thereby increasing the availability of holes and oxidizing H2O to ·OH active species. On the other hand, O2 can be quickly reduced to ·OOH species through a single electron, and further combined with ·CH3 to form relatively stable CH3OOH, which can effectively prevent the ·CH3 intermediate from being over-oxidized. However, O2 can also be reduced to ·OH through a two-electron process (O2+2e - +2H + → 2·OH), and excessive ·OH in the liquid phase can easily lead to serious overoxidation.

[0004] ZnO, a traditional semiconductor material, typically exhibits a highly positive valence band potential (3.0 V vs. NHE), demonstrating its strong oxidizing ability. Currently, ZnO modified with noble metals (Pt, Au, Ag, and Pd) has been developed for the oxidation of CH4 to liquid oxygenates in an O2 atmosphere. However, O2 is typically adsorbed on the noble metals in a lateral configuration, which hinders the release of the generated ·OOH. This unstable ·OOH rapidly decomposes into excess ·OH in the liquid phase, where it undergoes further peroxidation. Conversely, if O2 is adsorbed on the active sites in a terminal configuration, this facilitates the desorption of ·OOH from the photocatalyst surface. However, to control the conversion of O2 to ·OOH, it is necessary to construct active sites on the photocatalyst surface with high charge density and favorable terminal O2 adsorption. To address these challenges, the present invention utilizes a photocatalyst modification method to introduce a high concentration of interstitial zinc sites on ZnO, achieving efficient conversion of O2 to ·OOH and improving the yield and selectivity of liquid oxygenates. Summary of the Invention

[0005] To address the current challenges of low product yield and selectivity in the photocatalytic conversion of CH4 to liquid oxygenates, this invention prepares interstitial zinc-rich ZnO through the thermal decomposition of ZnO2. The interstitial zinc centers effectively enhance O2 adsorption, promoting its conversion to ·OOH species rather than ·OH, thereby controlling the selectivity and yield of the photocatalytic conversion of CH4 to liquid oxygenates. The raw materials for this photocatalyst are readily available, the preparation is simple, and it is suitable for large-scale production. Compared to ZnO with a low interstitial zinc concentration, the photocatalytic performance of ZnO enriched with interstitial zinc is significantly enhanced, suggesting promising application prospects.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] A method for preparing a ZnO photocatalyst material rich in interstitial zinc comprises the following steps:

[0008] (1) Dissolve Zn(NO3)2·6H2O and NaOH in 100 mL of deionized water. After complete dissolution, slowly add the NaOH solution dropwise to the Zn(NO3)2·6H2O solution. Stir for 2 h and centrifuge the solution five times with deionized water to collect the white precipitate Zn(OH)2. The amount of Zn(NO3)2·6H2O is 5.94 g, and the amount of NaOH is 0.8 g.

[0009] (2) Disperse the prepared Zn(OH)2 in 100 mL of H2O2 solution, transfer the solution to a 250 mL beaker, seal the beaker with plastic film, and heat and stir at 70°C for 2 h. After the reaction, separate the yellow precipitate by centrifugation and dry it at 60°C for 8 h to obtain ZnO2. The H2O2 solution ratio is: deionized water to 30 wt% H2O2 by volume = 9:1.

[0010] (3) The ZnO2 obtained above was transferred to a porcelain boat, heated to 350°C in a muffle furnace at a rate of 5°C / min, and calcined for 2 h to obtain ZnO rich in interstitial zinc.

[0011] The prepared ZnO photocatalyst rich in interstitial zinc is used in the photocatalytic oxidation of CH4 to generate liquid oxygen-containing compounds.

[0012] The beneficial effect of the present invention is that the present invention obtains ZnO with a high interstitial zinc concentration by pyrolyzing ZnO2. The density of Zn atoms in the ZnO2 crystal structure is significantly lower than that of ZnO. During the thermal decomposition of ZnO2 into ZnO, lattice and volume contraction occurs, which may cause zinc atoms to occupy the lattice gaps to form interstitial zinc atoms, thereby obtaining ZnO rich in interstitial zinc. The photocatalytic oxidation of CH4 to liquid oxygen-containing compounds is carried out in a high-pressure reactor. In addition to having good photogenerated carrier separation efficiency, more importantly, interstitial zinc-rich ZnO can effectively convert O2 into ·OOH instead of ·OH as an active site, thereby avoiding over-oxidation of liquid oxygen-containing compounds and facilitating the acquisition of high liquid oxygen-containing compound yield and selectivity. This provides an important idea for the photocatalytic conversion of methane into liquid oxygen-containing compounds. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 This is the thermogravimetric curve of the decomposition of ZnO2 into ZnO by the synthesis method in Example 1.

[0014] Figure 2 The XRD spectra of the products obtained by the synthesis methods of Comparative Example 1, Comparative Example 2 and Example 1 are shown.

[0015] Figure 3 This is a TEM image of the product obtained by the synthesis method in Example 1.

[0016] Figure 4 The EPR spectra of the products obtained by the synthetic methods of Comparative Example 1, Comparative Example 2 and Example 1 are shown.

[0017] Figure 5 This is a graph showing the methane oxidation performance of the products obtained by the synthetic methods of Comparative Example 1, Comparative Example 2 and Example 1.

[0018] Figure 6The photocurrent spectra of the products prepared by the synthesis methods of Comparative Example 1, Comparative Example 2 and Example 1 are shown.

[0019] Figure 7 The EPR spectra of the products obtained by the synthetic methods of Comparative Example 1, Comparative Example 2 and Example 1 are shown. DETAILED DESCRIPTION

[0020] The present invention is further described below with reference to the following examples, which are only used to illustrate the present invention and are not intended to limit the present invention.

[0021] Comparative Example 1

[0022] The preparation of ordinary ZnO photocatalyst, the specific steps are as follows:

[0023] Dissolve Zn(NO₃)₂·6H₂O and NaOH separately in 100 mL of deionized water. Once completely dissolved, slowly add the NaOH solution dropwise to the Zn(NO₃)₂·6H₂O solution. Stir for 2 hours and centrifuge five times with deionized water. Collect the white precipitate, Zn(OH)₂, dry it at 60°C for 8 hours, and then calcine it in air at 350°C for 2 hours. The resulting sample is named ZnO. The Zn(NO₃)₂·6H₂O content is 5.94 g, and the NaOH content is 0.8 g.

[0024] Comparative Example 2

[0025] The preparation of ZnO photocatalyst with low concentration of interstitial zinc is as follows:

[0026] Zn(NO3)2·6H2O and NaOH were dissolved in 100 mL of deionized water respectively. After complete dissolution, the NaOH solution was slowly added dropwise to the Zn(NO3)2·6H2O solution. After stirring for 2 hours, the mixture was centrifuged 5 times with deionized water to collect the white precipitate Zn(OH)2. The mixture was then dispersed in 100 mL of H2O2 solution (the volume ratio of deionized water to 30wt% H2O2 was 9:1). After the cup was sealed and stirred at 70°C for 2 hours, the product was centrifuged and dried at 60°C for 8 hours to obtain ZnO2. Subsequently, the obtained ZnO2 powder was calcined in air at 550°C for 2 hours. The obtained sample was named ZnO(Zn i )-550. Among them, Zn(NO3)2·6H2O is 5.94 g and NaOH is 0.8 g.

[0027] Example 1

[0028] The preparation of ZnO photocatalyst with high concentration of interstitial zinc is as follows:

[0029] Zn(NO3)2·6H2O and NaOH were dissolved in 100 mL of deionized water respectively. After complete dissolution, the NaOH solution was slowly added dropwise to the Zn(NO3)2·6H2O solution. After stirring for 2 hours, the mixture was centrifuged 5 times with deionized water to collect the white precipitate Zn(OH)2. The mixture was then dispersed in 100 mL of H2O2 solution (the volume ratio of deionized water to 30wt% H2O2 was 9:1). After the cup was sealed and stirred at 70°C for 2 hours, the product was centrifuged and dried at 60°C for 8 hours to obtain ZnO2. Subsequently, the obtained ZnO2 powder was calcined in air at 350°C for 2 hours. The obtained sample was named ZnO(Zn i )-350. Among them, Zn(NO3)2·6H2O is 5.94 g and NaOH is 0.8 g.

[0030] Figure 2 1 and 2 are XRD spectra of the products obtained by the synthetic methods of Comparative Example 1, Comparative Example 2 and Example 1; the characteristic peaks all correspond to hexagonal wurtzite (JCPDS No. 36-1451).

[0031] Figure 3 This is a TEM image of the product obtained by the synthesis method in Example 1; its morphology is uniform nanoparticles with a size of 23.5 nm.

[0032] Figure 4 The EPR spectra of the products obtained by the synthesis methods of Comparative Example 1, Comparative Example 2 and Example 1 are shown; ZnO (Zn i The interstitial zinc signal (g=1.96) of )-350 is much stronger than that of Comparative Example 1 and Comparative Example 2.

[0033] Figure 5 The methane oxidation performance diagram of the products prepared by the synthesis methods of Comparative Example 1, Comparative Example 2 and Example 1; ZnO (Zn i )-350 showed the best liquid oxygenate yield and liquid oxygenate selectivity of 98.6%.

[0034] Figure 6 The photocurrent spectra of the products prepared by the synthesis methods of Comparative Example 1, Comparative Example 2 and Example 1 are shown; ZnO (Zn i )-350 showed the strongest photocurrent signal, indicating the best photoelectric separation efficiency.

[0035] Figure 7 The EPR spectra of the products obtained by the synthesis methods of Comparative Example 1, Comparative Example 2 and Example 1 are shown; ZnO (Zn i )-350 showed the strongest superoxide radical (·OOH) signal.

[0036] Application Comparative Example / Example:

[0037] The specific steps of the experiment of photocatalytic oxidation of methane into liquid oxygen-containing compounds are as follows:

[0038] Photocatalytic CH₄ oxidation was tested in a 120 mL stainless steel high-pressure reactor equipped with a quartz window. 10 mg of the photocatalyst was dispersed in 50 mL of H₂O and sonicated for 5 minutes. The photocatalyst suspension was then transferred to a polytetrafluoroethylene (PTFE) reaction cup and placed in the high-pressure reactor. After sealing, the reactor was purged with O₂ (99.99% purity) for 40 minutes to completely remove air. The reactor was then pressurized with 0.2 MPa of O₂ and 1.9 MPa of CH₄ (99.99% purity). Irradiation was performed with a 300 W xenon lamp at 350 rpm / min for 2 hours. The temperature was maintained at 30 ± 2°C during the reaction. After the reaction, the reactor was cooled to below 10°C in an ice-water bath to minimize the volatilization of liquid oxygen-containing compounds. A gas bag was used to collect the post-reaction gases.

[0039] Analysis Process

[0040] pass 1 The CH3OH, CH3OOH, and HCOOH contents of the reaction solution were analyzed by H NMR (AVANCE NEO 600 MHz). 0.4 mL of the liquid product was mixed with 0.1 mL of D2O, and 0.1 mL of dimethyl sulfoxide (DMSO) solution (containing 0.03 μL) was added as an internal standard. The DMSO proton peak area was compared with the product proton peak area to calculate the product content. The HCHO concentration was quantified using the acetylacetone colorimetric method. Gaseous products such as CO and CO2 were quantified using GC (Ar carrier gas, Panolog) equipped with a flame ionization detector.

[0041]

[0042] The above data can show that compared with the comparative examples ZnO and ZnO (Zn i )-550, the ZnO photocatalyst material with high concentration of interstitial zinc in this embodiment [ZnO(Zn i )-350)] showed more efficient photocatalytic oxidation of CH4 to liquid oxygen-containing compounds.

[0043] It should be understood that the above embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention. In addition, it should be understood that after reading the content described in the present invention, those skilled in the art may make various changes or modifications to the present invention, and these equivalent forms also fall within the scope defined by the present invention.

Claims

1. An application of a ZnO photocatalyst rich in interstitial zinc in the photocatalytic oxidation of CH4 to generate liquid oxygen-containing compounds, characterized by: The ZnO photocatalyst rich in interstitial zinc is generated by thermal decomposition of ZnO2.

2. The use according to claim 1, characterized in that: The preparation method of the ZnO photocatalyst rich in interstitial zinc comprises the following steps: (1) Dissolve Zn(NO3)2·6H2O and NaOH in deionized water respectively. Slowly add the NaOH solution dropwise into the Zn(NO3)2·6H2O solution while stirring. After the reaction is complete, centrifuge and wash to obtain Zn(OH)2 precipitate. (2) Dispersing the Zn(OH)2 precipitate in a mixture of deionized water and H2O2, sealing and heating, centrifuging to separate the ZnO2, and drying; (3) calcining ZnO2 in air to obtain the interstitial zinc-rich ZnO.

3. The use according to claim 1, characterized in that: In step (1), the mass ratio of Zn(NO3)2·6H2O to NaOH is 7.4:1, and the reaction time is 1-3 h.

4. The use according to claim 1, characterized in that: In step (2), the volume ratio of deionized water to H2O2 is 9:1; the heating temperature is 60-80°C, and the time is 1-3 hours; the drying temperature is 50-70°C, and the time is 7-9 hours.

5. The use according to claim 1, characterized in that: In step (3), the heating rate is 2-6 °C / min, the calcination temperature is 350-550 °C, and the calcination time is 1-3 h.

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