Preparation method of two-dimensional inorganic symbiotic PN bulk phase heterojunction and application of the two-dimensional inorganic symbiotic PN bulk phase heterojunction in catalyzing CO2 methanation

Abstract

The application discloses a preparation method of two-dimensional inorganic symbiotic PN bulk phase heterojunction and application of the two-dimensional inorganic symbiotic PN bulk phase heterojunction in catalyzing CO2 methanation. The two-dimensional inorganic symbiotic PN bulk phase heterojunction is prepared by twice topological pyrolysis with a hydrotalcite as a precursor, and then is used for catalyzing and reducing CO2 to generate methane under light. Experiment proves that the prepared PN bulk phase heterojunction has high methane conversion capacity. The application has simple process, low cost and can improve the selectivity of CO2 reduction to generate methane to 80-90%.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of heterojunction material preparation, and particularly relates to a preparation method of a two-dimensional inorganic symbiotic PN bulk phase heterojunction and application of the two-dimensional inorganic symbiotic PN bulk phase heterojunction in catalyzing CO2 methanation. BACKGROUND

[0002] At present, energy problems and environmental problems are two big problems that need to be solved by global scientific research circles. In order to solve the above problems, light-driven CO2 methanation can be carried out to store clean renewable solar energy in high energy density methane. Due to the difficulty of breaking the bond of CO2 linear molecules for reaction, and the difficulty of coupling CO2 reduction with H2O oxidation in the process of photocatalysis, the photocatalytic process is hindered.

[0003] In photocatalysis, a heterojunction catalyst is constructed to improve the migration rate of photo-generated electrons in the three steps of photocatalytic reaction, because in the actual reaction process, photo-generated carriers may be recombined and return to the ground state before reaching the surface of the catalyst. The built-in electric field in the PN heterojunction greatly reduces the recombination of bulk photo-generated carriers. In order to minimize the number of carriers that relax to the ground state, the interface between the heterojunctions must be as close as possible (tens of nanometers). Considering this, the traditional plane heterojunction (PH) uses two continuously deposited semiconductors to construct, but due to the limitations of solid-solid reactions, the heterojunction interface contact is not tight or there are structural defects, such as growth dislocations. Such "trap" defects can become new surface recombination centers for photo-generated carriers.

[0004] Layered double hydroxides (LDHs) are typical two-dimensional materials, and their chemical formula can be represented as [M 2+ 1-x M 3+ x (OH)2] x+ (A n- ) x / n ·yH2O(x=M 3+ / (M 2+ +M 3+ ),0.1<x<0.5) represents, wherein M 2+ is a divalent metal cation (Mg 2+ , Cu 2+ , Ni 2+ , Zn 2+ , etc.), M 3+ is a trivalent metal cation (Al 3+ , Fe 3+ , Cr 3+ , etc.), and An- for alternative interlayer anions (Cl - , NO3 - , CO3 2- , etc.), and y is the number of interlayer water. LDHs materials have a unique structural memory effect (SME). SME is briefly that the structure of LDHs will lose interlayer water, anions and hydroxyl at a certain calcination temperature, forming a mixed metal oxide (MMO), and the layered LDHs structure can be restored by exposing the MMO to an anion environment. It is worth noting that not all LDHs have a structural memory effect because of the difference in the tendency of coordination of different metal ions. SUMMARY

[0005] The present application overcomes the shortcomings in the prior art heterostructure technology, and provides a preparation method of two-dimensional inorganic symbiotic PN bulk phase heterojunction and application thereof in catalyzing CO2 methanation. The present application uses hydrotalcite as a precursor to prepare two-dimensional inorganic symbiotic PN bulk phase heterojunction through secondary topological pyrolysis, and then reduces CO2 under visible light. Experiments prove that the prepared PN bulk phase heterojunction has high methane conversion capacity. The present application has simple process, low cost, and can improve the selectivity of CO2 reduction to methane up to 80-90%.

[0006] The preparation method of the two-dimensional inorganic symbiotic PN bulk phase heterojunction comprises the following steps:

[0007] (1) calcining CuNiAl hydrotalcite at 350-600℃ for 5-8h to obtain a mixed metal oxide;

[0008] (2) dispersing the mixed metal oxide in a Na2CO3 solution with a pH of 8-8.5, stirring at 50-65℃ for 6-8h, filtering, and washing the precipitate with deionized water and ethanol in sequence, and drying;

[0009] (3) calcining the product of step (2) at 350-600℃ for 5-8h to obtain a two-dimensional inorganic symbiotic PN bulk phase heterojunction.

[0010] The ratio of the sum of the moles of Cu and Ni to the moles of Al in the CuNiAl hydrotalcite is 2-4, and the ratio of the moles of Cu to the moles of Ni is 0.45-1.

[0011] The two-dimensional inorganic symbiotic PN bulk phase heterojunction prepared above is applied to catalyze CO2 methanation reaction. The method for catalyzing CO2 methanation reaction comprises the following steps: dispersing the two-dimensional inorganic symbiotic PN bulk phase heterojunction in deionized water under ultrasonic; then introducing CO2 gas, and catalyzing the reduction of CO2 to generate methane under light.

[0012] Compared with the prior art, the present application has the beneficial effects that:

[0013] 1、The present application uses the structural memory effect difference (the migration ability of ions on the LDHs layer and the difference of coordination stability) of different LDHs, constructs two-dimensional inorganic intergrowth bulk heterojunction (2D-IIBH) NiO(Al) / CuO(Ni,Al) with LDHs as a precursor, breaks through the inherent defects of the traditional planar PN heterojunction, that is, the heterojunction interface contact is not close, becomes the light-generated carrier recombination center similar to "trap", and solves the problem of the separation efficiency of excitons. The two-dimensional intergrowth PN bulk heterojunction makes the N-type semiconductor and the P-type semiconductor naturally transition in a way of co-crystal lattice growth, achieves the interpenetration state and shortens the diffusion distance of the carrier. The bulk heterojunction greatly increases the interface quantity, thereby increasing the active sites for CO2 reduction. NiO(Al) / CuO(Ni,Al) has a dense interface and a high-efficiency component synergistic effect, makes the charge in the heterojunction composite material quickly transfer, thereby making the photo-generated electrons and holes effectively separate through the interface, and thus excellent photocatalytic reduction CO2 activity is obtained.

[0014] 2、In the present application, the layered double hydroxide is used as a precursor, and the two-dimensional inorganic intergrowth PN bulk heterojunction not only inherits the advantages of high dispersion of metal ions and adjustable metal proportion in the hydrotalcite layer, but also has the directional movement of the photo-generated carrier at the bulk heterojunction interface. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1 It is a schematic diagram of the secondary topology pyrolysis process of the present application.

[0016] Figure 2 It is an XRD graph of the two-dimensional inorganic intergrowth PN bulk heterojunction prepared in Example 1.

[0017] Figure 3 It is an SEM and HRTEM graph of the morphology change in the secondary topology pyrolysis process in Example 1.

[0018] Figure 4 It is an XAF (A) and EXAFS (B) graph of the change of the coordination number of metal ions in the secondary topology pyrolysis process in Example 1.

[0019] Figure 5 It is a Mott-Schottky curve graph of the two-dimensional inorganic intergrowth PN bulk heterojunction prepared in Example 1.

[0020] Figure 6The XRD patterns of two-dimensional inorganic symbiotic PN phase heterojunctions constructed with CuNiAl-LDHs precursors of different proportions in the embodiments are shown.

[0021] Figure 7 The XRD patterns of two-dimensional inorganic symbiotic PN phase heterojunctions constructed at different primary and secondary calcination temperatures in the examples are shown.

[0022] Figure 8 The diagram shows the methanation selectivity (A) and cyclic catalysis (B) of the two-dimensional inorganic symbiotic PN bulk heterojunction in Application Example 1. Detailed Implementation

[0023] Example 1

[0024] Prepare a 50 ml mixed salt solution by adding 10 mmol Cu(NO3)2·6H2O, 10 mmol Ni(NO3)2·6H2O and 5 mmol Al(NO3)2·9H2O to deionized water; prepare a 50 ml alkaline solution by adding 50 mmol NaOH and 10 mmol Na2CO3; add the mixed salt solution and alkaline solution dropwise to the reaction flask simultaneously, controlling the pH of the mixture in the reaction flask to be 8-8.5. After the addition is complete, crystallize in a water bath at 65℃ for 6 h, collect the product, wash three times with deionized water and ethanol respectively, and dry to obtain CuNiAl-LDHs. Then, it was placed in a muffle furnace and heated to 350℃ for 8 hours at a heating rate of 10 min / ℃. It was then dispersed in a Na2CO3 solution with a pH of 8-8.5 and bathed in a 65℃ water bath for 6 hours. It was washed three times by centrifugation with deionized water and ethanol, respectively. After drying, it was heated to 350℃ for 8 hours again at a heating rate of 10 min / ℃ to obtain a two-dimensional inorganic symbiotic PN bulk heterojunction NiO(Al) / CuO(Ni,Al).

[0025] Characterization and analysis of two-dimensional inorganic symbiotic PN bulk heterojunction NiO(Al) / CuO(Ni,Al):

[0026] like Figure 2 As shown in the figure, CuNiAl-LDHs exhibit typical layered structure diffraction peaks, namely 00l (l = 3, 6, 9), confirming the successful formation of LDHs. After the first calcination at 350℃, the Bragg peak of 00l disappeared, leaving only three peaks at 37.4°, 43.7°, and 36.3°. This may be due to the lack of a clear crystalline phase in the solid solution CuNiAl-MMO, resulting in a NiO phase with a large angular shift. When CuNiAl-MMO was placed in Na2CO3 solution, the series of 00l peaks reappeared, indicating that the LDHs structure was restored. 2+The Jahn-Teller effect of CuO(Ni, Al) coexists with NiAl-LDHs. After the second calcination, CuO(JPCDS #48-1548) and NiO(JPCDS #44-1159) diffraction peaks appear at the same time. It can be said that the two-dimensional inorganic symbiotic PN bulk phase heterojunction NiO(Al) / CuO(Ni, Al) is successfully prepared by the two-step topological pyrolysis method.

[0027] As shown in Figure 3 , the morphology of (A) CuNiAl-LDHs, (B) CuNiAl-MMO, (C) CuO(Ni, Al)@NiAl-LDHs, and (D) NiO(Al) / CuO(Ni, Al) does not collapse and agglomerate obviously during the whole synthesis process, indicating that the synthesis process is a topological transformation of the structure. And from the HRTEM image of NiO(Al) / CuO(Ni, Al) (D), the transition interface of eutectic lattice growth and the high dispersion of Ni, Cu, and Al can be clearly observed.

[0028] As shown in Figure 4 , the spectrum shows that pure CuO has a wide front peak at ~8985 eV, which is due to the four-coordinated Cu 2+ of CuO(Ni, Al) appears continuously with the topological pyrolysis process, indicating that the geometric configuration of Cu 2+ gradually changes from the six-coordinated structure of CuNiAl-LDHs to the four-coordinated structure of NiO(Al) / CuO(Ni, Al).

[0029] As shown in Figure 5 , it can be clearly seen from the figure that CuO shows a typical positive slope of N-type semiconductor, the negative slope of NiO is a P-type semiconductor, and the Mott-Schottky graph of NiO(Al) / CuO(Ni, Al) has two linear profiles (“inverted V” type), and the curve has a tendency to shrink inward, which is due to the flow of electrons from the N-CuO region with a higher Fermi energy (Ef) to the P-NiO region with a lower Ef until the same value is reached.

[0030] Example 2

[0031] The molar ratio of Cu(NO3)2·6H2O, Ni(NO3)2·6H2O, and Al(NO3)2·9H2O in Example 1 is replaced by 1:1:1, 1:2:1, and 1:3:1, and Al(NO3)2·9H2O is 5 mmol, and the remaining conditions are the same, to obtain a two-dimensional inorganic symbiotic PN bulk phase heterojunction NiO(Al) / CuO(Ni, Al).

[0032] Due to the Cu 2+The ratio of the sum of the moles of Cu and Ni to the moles of Al is 2-4, and the ratio of the moles of Cu to the moles of Ni is 0.45-1, so that pure LDHs crystal form can be prepared.

[0033] Example 3

[0034] In Example 1, the calcination temperature of the first and second times is changed to 400℃, 500℃, 600℃, and the rest of the conditions are the same, and two-dimensional inorganic symbiotic PN bulk phase heterojunction NiO(Al) / CuO(Ni,Al) is obtained. The structure of LDHs cannot be restored when the first calcination temperature is 800℃, and spinel is generated when the second calcination temperature is 800℃, which reduces the photocatalytic efficiency.

[0035] Application Example 1

[0036] 30mg of NiO(Al) / CuO(Ni,Al) two-dimensional inorganic symbiotic PN bulk phase heterojunction is ultrasonically dispersed in 35ml of deionized water, poured into a 100ml quartz glass top light reactor, and high-purity CO2 gas is introduced into the closed reaction tank at a rate of 10min / ml, and the circulation is continued for 30min until saturation. A xenon lamp with a 420nm filter is used as visible light to simulate sunlight, and the photocatalytic reduction of CO2 to produce methane is carried out. The photocatalytic reduction of CO2 reaction is carried out continuously for 5h, and 1ml of gas product is collected every 1h, and the product is analyzed by gas chromatography.

[0037] In the above CO2 reduction to CH4 reaction process (CO2 * →COOH * →CHO * →CH2O * →CH3 * →CH4), the adsorption site of the intermediate product is controlled on the surface of the N-type semiconductor CuO(Ni,Al); and the adsorption energy of CO2 on the surface is increased by doping Ni. The two-dimensional inorganic symbiotic PN bulk phase heterojunction is beneficial to the directional movement of photo-generated carriers, and at the same time shortens the diffusion distance of the carriers, so that the photo-generated electrons accumulate on the surface of CuO(Ni,Al) and then rapidly undergo methanation.

[0038] As Figure 8 shown in the figure, it can be seen directly that the selectivity of NiO(Al) / CuO(Ni,Al) for CO2 methanation is as high as 80% or more, and the catalytic efficiency remains at about 90% after 3 cycles of experiments.

[0039] By Figure 8As can be seen from Table 1, the CH4 yield of the secondary pyrolysis product pn-IIBH NiO(Al) / CuO(Al) reached 86.59 mol / g / h, which was 4.2 times that of CuNiAl-LDHs and 1.35 times that of the primary pyrolysis product CuNiAl-MMO. At the same time, the generation of O2 product was detected, and the oxygen production of NiO(Al) / CuO(Al) was about 150.84 μmol / g / h, and the molar ratio of CH4 to O2 was close to 1:2. This means that the reduction of CO2 and the oxidation of O2 occur at the same time.

[0040] Table 1. Photocatalytic reduction of CO2 results of photocatalysts under UV-visible light irradiation for 5 hours

[0041]

Claims

1. A method for preparing a two-dimensional inorganic coexisting PN bulk phase heterojunction for catalyzing CO2 methanation reaction, characterized in that, The specific steps of the preparation method are: (1) CuNiAl hydrotalcite is calcined at 350-600 ℃ for 5-8 h to obtain a mixed metal oxide; (2) the mixed metal oxide is dispersed in a Na2CO3 solution with a pH of 8-8.5, stirred at 50-65 ℃ for 6-8 h, filtered, and the precipitate is washed with deionized water and ethanol in sequence, and dried; (3) the product of step (2) is calcined at 350-600 ℃ for 5-8 h to obtain a two-dimensional inorganic symbiotic PN bulk phase heterojunction; The CuNiAl hydrotalcite has a ratio of the sum of the moles of Cu and Ni to the moles of Al of 2-4, and a ratio of the moles of Cu to the moles of Ni of 0.45-1.

2. Application of the two-dimensional inorganic symbiotic PN bulk phase heterojunction obtained by the preparation method of claim 1 in catalyzing a CO2 methanation reaction.

3. Use according to claim 2, characterized in that, The method for catalyzing the CO2 methanation reaction is: the two-dimensional inorganic symbiotic PN bulk phase heterojunction is dispersed in deionized water by ultrasonic; then CO2 gas is introduced, and the CO2 is reduced to generate methane under light irradiation.

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