CN113600214A

CN113600214A - Core-shell type Fe2O3@TixOy-PzPreparation method and application of photocatalyst

Info

Publication number: CN113600214A
Application number: CN202111025871.5A
Authority: CN
Inventors: 韩冬雪; 牛利; 张文生
Original assignee: Guangzhou University
Current assignee: Guangzhou University
Priority date: 2021-09-02
Filing date: 2021-09-02
Publication date: 2021-11-05
Anticipated expiration: 2041-09-02
Also published as: CN113600214B

Abstract

The invention discloses core-shell Fe₂O₃@Ti_xO_y‑P_zA preparation method and application of the photocatalyst. The method comprises the following steps: preparation of cubic alpha-Fe by hydrothermal method₂O₃(ii) a Preparation of core-Shell Fe₂O₃@TiO₂A nanocomposite; mixing core-shell type Fe₂O₃@TiO₂The core-shell type Fe is obtained by carrying out phosphating treatment on the nano composite material at 300 DEG C₂O₃@Ti_xO_y‑P_zA photocatalyst. The invention uses alpha-Fe₂O₃Coupled with titanium dioxide with wide band gap can effectively overcome alpha-Fe₂O₃The self defects enhance the transmission and separation efficiency of photoproduction electrons and holes and improve the pNRR activity. The invention carries out phosphating treatment on the composite material on TiO₂Surface induced generation of N₂Active site Ti of (1)³⁺Species to enhance N₂Efficient adsorption and activation of the molecule further enhances pNRR activity.

Description

A kind ofCore-shell type Fe2O3@TixOy-PzPreparation method and application of photocatalyst

Technical Field

The invention belongs to the technical field of photoelectric energy materials, and particularly relates to core-shell Fe₂O₃@Ti_xO_y-P_zA preparation method and application of the photocatalyst.

Background

Ammonia (NH)₃) Is an indispensable chemical substance in modern society and is a basic component for manufacturing synthetic chemicals such as medicines, fertilizers, resins, dyes, explosives and the like. Reacting NH₃When condensed into liquid, with hydrogen (H)₂) In contrast, it has considerable energy density and transportability and can be used for power fuel cells in a short period of time. Up to now, industrial nitrogen fixation for the synthesis of NH₃Is realized by a high-temperature and high-pressure Haber-Bosch method; such a process requires the consumption of large amounts of resources and energy, requires a complex large-scale infrastructure, and also emits large amounts of carbon dioxide, which has a great environmental impact. Therefore, the development of a new low-energy-consumption, green and sustainable nitrogen fixation for synthesizing NH is urgently needed₃A method.

Photocatalytic nitrogen reduction (photocatalytic N) compared to the existing energy intensive Haber-Bosch process₂reduction reaction, pNRR) with water (H)₂O) is a proton source, and N is realized under the drive of a semiconductor photocatalyst and renewable solar energy₂To NH₃The method becomes a research field which is relatively leading-edge and green environmental protection in recent years. From the standpoint of efficient use of sunlight, an ideal photocatalyst should be capable of absorbing visible light, since visible light is a large proportion (about 44%) of the solar spectrum. It is well known that hematite (alpha-Fe)₂O₃) The catalyst has the advantages of low cost, wide visible light response, high light stability, environmental friendliness and the like, and is a promising pNRR catalyst. However, in a single component of alpha-Fe₂O₃In the material, the reduction capability of photo-generated electrons is low, and the pNRR reaction cannot be effectively carried out. In addition, the high recombination rate of photogenerated electrons and holes prevents the photogenerated electrons and holes from being used in the fieldWide application in pNRR. On the other hand, the N.ident.N bond of nitrogen gas is very strong (. about.941 kJ mol)^-1) And the kinetics of pNRR is too slow, so that the reaction rate of a common photocatalytic nitrogen fixation ammonia synthesis system is low, and industrial application is difficult to realize.

Patent document CN03158740.2 discloses a photocatalyst consisting of oxide nanoparticles, non-metal elements and semiconductor nanoparticles, wherein 0.1 to 0.5mol of a compound of a semiconductor nanomaterial is added to a pre-prepared methanol solution of 0.001 to 0.1M of a metal salt and a non-metal salt under vigorous stirring for 10 minutes to 30 minutes, and 0.1ml of 5M HNO is added to the mixed solution every 5 to 10 minutes₃Hydrolyzing the aqueous solution, stirring for 5-12 hours at normal temperature to obtain semiconductor nano sol, standing, settling, aging for 1-10 days, drying the solvent, and roasting at high temperature of 300-900 ℃ to obtain the photocatalyst. However, the result of 5 times of cycle degradation of 2,4, 6-trichlorophenol by the photocatalyst shows that the structure is unstable and the reproducibility is poor: the first three times of cyclic photoreaction are carried out for 240min, the degradation of the trichlorophenol can reach more than 90 percent, and the catalytic activity of the catalyst is not reduced basically; however, the fourth and fifth cycles of photoreaction are 240min, and the degradation of 2,4, 6-trichlorophenol is only 80% or more.

Disclosure of Invention

In order to overcome the defects and shortcomings of the prior art, the invention aims to provide core-shell Fe₂O₃@Ti_xO_y-P_zA preparation method of the photocatalyst.

Another object of the present invention is to provide a core-shell type Fe obtained by the above method₂O₃@Ti_xO_y-P_zA photocatalyst.

The invention provides cubic hematite (Fe) coated on the basis of a nonmetallic phosphorus-doped titanium dioxide shell layer₂O₃@Ti_xO_y-P_z) The composite nano material system can realize the reduction of nitrogen to synthesize ammonia at normal temperature and normal pressure by a photocatalysis method, and effectively overcomes the defect of single-component Fe₂O₃Low reduction of photogenerated electrons and high recombination of photogenerated carriersThe problem of rate. In addition, the composite photocatalyst is simple to prepare, low in cost and high in stability, and is beneficial to realizing further industrial development.

It is still another object of the present invention to provide the above core-shell type Fe₂O₃@Ti_xO_y-P_zApplication of photocatalyst is provided.

The purpose of the invention is realized by the following technical scheme:

core-shell type Fe₂O₃@Ti_xO_y-P_zThe preparation method of the photocatalyst comprises the following steps:

(1) preparation of cubic alpha-Fe by hydrothermal method₂O₃；

(2) Preparation of core-Shell Fe₂O₃@TiO₂A nanocomposite;

(3) mixing core-shell type Fe₂O₃@TiO₂The core-shell type Fe is obtained by carrying out phosphating treatment on the nano composite material at 300 DEG C₂O₃@Ti_xO_y-P_zThe photocatalyst (x and y are subscripts of chemical formula, 0 < x < 1, 0 < y < 2, and z is mass ratio of doped phosphorus element, and the value range of 0 < z < 10%).

Preferably, the specific steps of step (1) are as follows: FeCl is added₃·6H₂Adding O into water, stirring to obtain a transparent solution, and adding 0.1-0.5 mol of FeCl into every 10-100 mL of water₃·6H₂O; heating the solution to 60-100 ℃, then dropwise adding a NaOH solution with the concentration of 5-6 mol/L into the solution, stirring the solution for 5-15 min, and dropwise adding 20-100 mL of NaOH solution into every 10-100 mL of water; then transferring the obtained mixed solution into an autoclave with a polytetrafluoroethylene lining, and reacting for 48-72 h at 80-150 ℃ to obtain alpha-Fe₂O₃。

More preferably, the reaction of step (1) is completed and further comprises: and cooling the autoclave to room temperature, centrifugally separating precipitates, washing the precipitates with water and ethanol for several times, and drying the precipitates at the temperature of 60-80 ℃ overnight.

Preferably, the specific steps of step (2) are as follows: 0.1-0.5 part by mass of alpha-Fe synthesized in the step (1)₂O₃DispersingIn 100-500 parts by volume of ethanol; then, 0.5-1 part by mass of polyvinylpyrrolidone is added into the mixed solution and stirred for 30min, then 0-1.0 part by volume of tetrabutyl titanate is added, then 50-200 parts by volume of ethanol solution is slowly added, and stirring is continued for 10-18 h, so that core-shell type Fe is prepared₂O₃@TiO₂A nanocomposite; 1 part by mass: 1 part by volume is 1 g/mL.

More preferably, the reaction of step (2) is completed and further comprises: centrifugally washing with water and ethanol, and drying at 70 deg.C for 2 h; the ethanol solution is hydrous ethanol, each 50mL of the ethanol solution contains 0-20 mL of water, and each 50mL of the ethanol solution preferably contains 5-20 mL of water.

Preferably, the specific steps of step (3) are as follows: adding 0.2-2 parts by mass of NaH₂PO₂And 0.1 part by mass of core-shell Fe₂O₃@TiO₂Placing the nanocomposite material in a tubular furnace; under the condition of inert gas flow, heating to 300-500 ℃, and continuing to react for 1-3 h; collecting the product after reaction, namely the core-shell type Fe₂O₃@Ti_xO_y-P_zA photocatalyst.

More preferably, NaH is added in step (3)₂PO₂Placing in the center of a tubular furnace, and adding core-shell Fe₂O₃@TiO₂The nanocomposite is placed on the downstream side of the tubular furnace at a distance of about 3-10 cm.

More preferably, the temperature in the step (3) is 1-10 ℃ per minute^-1Heating to 300-500 ℃ at a heating rate.

More preferably, the inert gas in step (3) is argon or nitrogen.

The invention provides core-shell Fe₂O₃@Ti_xO_y-P_zThe photocatalyst can be used for photocatalytic nitrogen fixation to synthesize ammonia.

Compared with the prior art, the invention has the following advantages and beneficial effects:

1. fe alone₂O₃The catalyst has poor photoproduction electron hole separation efficiency and lower photoproduction electron reduction capability; single TiO 2₂The photocatalyst has low solar energy utilization rate (onlyAbsorb ultraviolet light). The invention uses alpha-Fe₂O₃With titanium dioxide (TiO) having a wide band gap₂) Coupled together to effectively overcome alpha-Fe₂O₃The self defects enhance the transmission and separation efficiency of photoproduction electrons and holes and improve the activity of the photocatalytic nitrogen reduction reaction (pNRR).

2. The photocatalytic nitrogen reduction (pNRR) reaction process is mainly limited by its N₂Effective adsorption process (chemical adsorption), the invention reasonably increases N on the surface of the photocatalyst₂Adsorption of active sites is an effective measure. The composite material is subjected to phosphating treatment (by using PH)₃Gas annealing at 300 deg.C) in TiO₂Surface induced generation of N₂Active site Ti of (1)³⁺Species to enhance N₂Efficient adsorption and activation of the molecule further enhances pNRR activity.

Drawings

FIG. 1 shows Fe prepared in example 1₂O₃@(0.6)TiO₂Fe prepared in comparative examples 1 to 5₂O₃@(0)TiO₂、Fe₂O₃@(0.2)TiO₂、Fe₂O₃@(0.4)TiO₂、Fe₂O₃@(0.8)TiO₂、Fe₂O₃@(1.0)TiO₂The photoluminescence spectrum (a) and the transient photocurrent response (b) of (a-f) in the figure correspond to Fe, respectively₂O₃@(0)TiO₂,Fe₂O₃@(0.2)TiO₂,Fe₂O₃@(0.4)TiO₂,Fe₂O₃@(0.6)TiO₂And Fe₂O₃@(1.0)TiO₂And (c) a complex.

FIG. 2 is a diagram of a device for a photocatalytic nitrogen fixation reaction process according to the present invention.

FIG. 3 is a graph showing the absorbance (a) of a mixed sample having different concentration gradients and the peak-fitted curve (b) of the absorbance at each concentration in the method for detecting ammonium ions in an aqueous solution, in which the concentration of ammonium ions is 0 to 50. mu. mol L from the top to the bottom of the arrow shown in a^-1。

FIG. 4 is a cubic α -Fe prepared in example 1₂O₃Core-shell type Fe₂O₃@TiO₂Core-shell type Fe₂O₃@Ti_xO_y-P_zScanning Electron Microscope (SEM) pictures (a-e) and core-shell Fe₂O₃@Ti_xO_y-P_zEDS element line scan curve (f).

FIG. 5 is a cubic α -Fe prepared in example 1₂O₃Core-shell type Fe₂O₃@TiO₂Core-shell type Fe₂O₃@Ti_xO_y-P_zTransmission Electron Microscope (TEM) images (a, b, c, e, f) and XRD characterization (d), where the scale length in the inset in c is 200nm and the scale length in the upper left panel of f is 1 μm.

FIG. 6 is a cubic α -Fe prepared in example 1₂O₃Core-shell type Fe₂O₃@TiO₂Core-shell type Fe₂O₃@Ti_xO_y-P_zO1s spectrum (a), Ti 2p XPS spectrum (b) and electrochemical impedance spectrum (d), and core-shell Fe₂O₃@Ti_xO_y-P_zP2P XPS spectrum (c).

FIG. 7 is a cubic α -Fe prepared in example 1₂O₃Core-shell type Fe₂O₃@TiO₂Core-shell type Fe₂O₃@Ti_xO_y-P_zSample ultraviolet visible diffuse reflectance spectrum (a), steady state photoluminescence curve (b), transient photocurrent response (c) and N₂Temperature programmed adsorption and desorption curve (d), wherein a, b and c respectively correspond to cube alpha-Fe₂O₃Core-shell type Fe₂O₃@TiO₂Core-shell type Fe₂O₃@Ti_xO_y-P_z。

FIG. 8 is a cubic α -Fe prepared in example 1₂O₃Core-shell type Fe₂O₃@TiO₂Core-shell type Fe₂O₃@Ti_xO_y-P_zFe prepared in comparative example 6₂O₃-P_zTi prepared in comparative example 7_xO_y-P_zThe photocatalytic nitrogen fixation performance is detected, and a is differentCatalyst synthesis of NH₃Yield, b Synthesis of NH for different catalysts₃Rate, c is Fe₂O₃@Ti_xO_y-P_zNH synthesized by 6 test experiments in cycle use₃Yield d is Fe₂O₃@Ti_xO_y-P_zThe apparent quantum efficiency of (a).

FIG. 9 is core-shell Fe prepared in example 1₂O₃@Ti_xO_y-P_zPhotocatalytic N₂XRD spectrograms before the reduction reaction cycle experiment and after 6 times of cycle reaction.

Detailed Description

The present invention will be described in further detail with reference to examples and drawings, but the embodiments of the present invention are not limited thereto. The raw materials related to the invention can be directly purchased from the market. For process parameters not specifically noted, reference may be made to conventional techniques.

The following examples and comparative examples relate to materials and pharmaceutical products including: ferric chloride hexahydrate (FeCl3 & 6H)₂O) and sodium hypophosphite (NaH)₂PO₂) Purchased from chemical reagents, Inc. of Yinakai, Beijing. Potassium hydroxide and anhydrous ethanol were purchased from national Chemicals, Inc. Tetrabutyl titanate (TBOT), polyvinylpyrrolidone K-30(PVP), isotope¹⁵N₂Gases, phenol sodium nitroprusside solution and basic hypochlorite solution were purchased from Sigma-Aldrich chemical company. Ammonium chloride (NH)₄Cl) was obtained from Shanghai Merck chemical technology, Inc. All reagents were of analytical purity (AR) and were used as received without further purification. The resistivity of the deionized water used throughout the experiment was 18.2 M.OMEGA.cm^-1。

The following examples and comparative examples use the following characterization instruments: the morphology and the elemental composition of the sample are analyzed by a scanning electron microscope (SEM, JEOL JSM-7001F) and a transmission electron microscope (TEM, JEOL 2100F) and energy dispersive X-ray spectroscopy (EDS) matched with the scanning electron microscope and the transmission electron microscope. The surface electronic states were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250Xi), and all binding energies were referenced to the C1s peak at 284.6 eV. Using HITACHI UV-3900 spectrometer with BaSO₄For referenceThe ultraviolet visible diffuse reflectance spectrum (DSR) is recorded. X-ray diffraction (XRD) using PANalytical X' Pert PRO instrument, Cu K α radiation was used. N Using Micromeritics Auto Chem II with TCD as detector₂Temperature programmed adsorption and desorption experiments. The material was tested for the mott-schottky curve using an electrochemical workstation (CHI Instruments CHI 760-1). Steady and transient Photoluminescence (PL) curves of the catalyst were obtained on an FLS1000 fluorescence lifetime spectrophotometer (Edinburgh Instruments, UK) under excitation of a hydrogen flash lamp at a wavelength of 800 nm. The contents of Fe, Ti and P in the samples were measured by inductively coupled plasma spectrometry (ICP-OES, Perkin Elmer Optima 4300DV), respectively.

Example 1

(1) preparation of cubic alpha-Fe by hydrothermal method₂O₃: 0.1mol of FeCl₃·6H₂Adding O into 50mL of water, and stirring until a transparent solution is obtained; heating the solution to 75 ℃, then dropwise adding 50mL of 5.4mol/L NaOH solution into the solution, stirring for 5min, and reacting for 10 min; transferring the obtained mixed solution into an autoclave with a polytetrafluoroethylene lining, putting the autoclave into an oven to react for 56 hours at the temperature of 100 ℃, cooling the autoclave to room temperature, centrifugally separating precipitates, washing the precipitates with water and ethanol for a plurality of times, and drying the precipitates at the temperature of 80 ℃ overnight to obtain cubic alpha-Fe₂O₃。

(2) Preparation of core-Shell Fe₂O₃@TiO₂Nano composite material: 0.1g of alpha-Fe synthesized in the step (1)₂O₃Dispersing in 150mL of ethanol; then, 0.6g of polyvinylpyrrolidone was added to the mixed solution and stirred for 30min, then 0.6mL of tetrabutyl titanate was added, then 50mL of ethanol solution (containing 5mL of water) was slowly added, and stirring was continued for 12h to obtain core-shell Fe₂O₃@TiO₂Nanocomposite (noted as Fe)₂O₃@(0.6)TiO₂). Washing with water and ethanol by centrifugation, and drying at 70 deg.C for 2 h.

(3) Mixing core-shell type Fe₂O₃@TiO₂And (3) carrying out phosphating treatment on the nano composite material: 1g of NaH₂PO₂Placed in the center of a tube furnace, and 100mg of the core-shell type Fe obtained in step (2) was charged₂O₃@TiO₂The nanocomposite was placed on the downstream side of the tube furnace at a distance of about-7 cm. Under the flowing condition of argon, at the temperature of 2 ℃ for min^-1Heating to 300 ℃ at the heating rate, and continuing to react for 1 h; collecting the product after reaction, namely the core-shell type Fe₂O₃@Ti_xO_y-P_zA photocatalyst.

Comparative example 1

(2) Preparation of core-Shell Fe₂O₃@TiO₂Nano composite material: 0.1g of alpha-Fe synthesized in the step (1)₂O₃Dispersing in 150mL of ethanol; then, 0.6g of polyvinylpyrrolidone was added to the mixed solution and stirred for 30min, then 0.2mL of tetrabutyl titanate was added, then 50mL of ethanol solution (containing 5mL of water) was slowly added, and stirring was continued for 12h to obtain core-shell Fe₂O₃@TiO₂Nanocomposite (noted as Fe)₂O₃@(0.2)TiO₂). Washing with water and ethanol by centrifugation, and drying at 70 deg.C for 2 h.

Comparative example 2

Referring to the process of comparative example 1, the same procedure as in comparative example 1 was followed except that 0mL of tetrabutyl titanate was added in step (2). Core-shell Fe was obtained in this comparative example₂O₃@TiO₂Nanocomposite noted Fe₂O₃@(0)TiO₂。

Comparative example 3

Referring to the process of comparative example 1, the same procedure as in comparative example 1 was followed except that 0.4mL of tetrabutyl titanate was added in step (2). Core-shell Fe was obtained in this comparative example₂O₃@TiO₂Nanocomposite noted Fe₂O₃@(0.4)TiO₂。

Comparative example 4

Referring to the process of comparative example 1, the same procedure as in comparative example 1 was followed except that 0.8mL of tetrabutyl titanate was added in step (2). Core-shell Fe was obtained in this comparative example₂O₃@TiO₂Nanocomposite noted Fe₂O₃@(0.8)TiO₂。

Comparative example 5

Referring to the process of comparative example 1, the same procedure as in comparative example 1 was followed except that 1.0mL of tetrabutyl titanate was added in step (2). Core-shell Fe was obtained in this comparative example₂O₃@TiO₂Nanocomposite noted Fe₂O₃@(1.0)TiO₂。

Comparative example 6

Cubic alpha-Fe was prepared with reference to step (1) of example 1₂O₃Then adding alpha-Fe₂O₃Phosphating treatment as a blank test according to step (3) of example 1, the sample obtained was designated as Fe₂O₃-P_z。

Comparative example 7

Weak acid etching method for Fe₂O₃@Ti_xO_y-P_zCore Fe₂O₃Etching is carried out to obtain the titanium alloy with single Ti_xO_y-P_zAnd (4) a defect shell layer. The method comprises the following specific steps: fe was prepared according to the method of example 1₂O₃@Ti_xO_y-P_zPhotocatalyst, 0.2g Fe₂O₃@Ti_xO_y-P_zThe photocatalyst was mixed with 10mL of a hydrochloric acid solution having a concentration of 0.5 mol/L. Then, the mixture was stirred at room temperature for 10min, and the mixture was further transferred to a stainless Teflon autoclave (15 ml capacity) and stripped at 100 ℃The reaction was carried out under the stirring for 24 hours. Centrifugally washing with deionized water for several times until the pH value is close to 7, and drying at 80 ℃ for 12h to obtain Ti_xO_y-P_zShell powder samples.

And (3) performance testing:

1. evaluation of cubic Fe by photoluminescence Spectroscopy (PL) and transient photocurrent response₂O₃Supported TiO₂The influence of the content of (b) on the photocatalytic activity. In short, a lower PL signal intensity indicates a better separation efficiency of photo-generated electrons and holes of the catalyst, and thus a higher photocatalytic activity. As shown in FIG. 1 (a), Fe₂O₃@(0.6)TiO₂The composite material can effectively inhibit the recombination rate of photo-generated charges, which shows that the photocatalytic activity is higher. At the same time, Fe₂O₃@(0.6)TiO₂The composite material has the highest photocurrent density, which shows that the composite material has stronger capability of generating electron holes under the irradiation of visible light, and is beneficial to improving the photocatalytic performance (figure 1 (b)).

2. Photocatalytic nitrogen fixation reaction process

The photocatalytic nitrogen fixation ammonia synthesis experiment is carried out on a self-assembled photocatalytic reaction platform, as shown in figure 2. The photocatalytic nitrogen fixation is carried out on a three-phase interface (gas phase N) at room temperature and normal pressure₂Liquid phase H₂O and a solid phase catalyst). The light source used was a 300w xenon lamp (full spectrum, 463 mW. cm)^-2) The light source is 10cm from the liquid level. The specific experimental steps are as follows: first, 40mg of Fe prepared in example 1 was mixed₂O₃@Ti_xO_y-P_zThe photocatalyst was dispersed in 200mL of deionized water and added to the reactor with a circulating water system. Secondly, introducing high-purity N into the mixed solution under the condition of no illumination₂(200mL min^-1) Stirring continuously for 30 minutes to ensure that N in the aqueous solution₂Saturation is reached. Then, the lamp was turned on to apply light, and 4.0mL of the reaction solution was taken out at intervals of 30min and the content of the synthetic ammonia was measured after using a 0.22 μm filter.

The detection method of ammonium ions in the aqueous phase solution comprises the following steps:

the ammonia in the photocatalytic reaction solution is detected by an indoxyl method and is not known to beNH of the same concentration gradient₄Cl was added to 0.1M KOH, then 0.5M H₂SO₄And (4) neutralizing. Taking 2.0mL of the above mixed solution, adding 0.5mL of phenol nitroprusside solution and 0.5mL of alkaline sodium hypochlorite solution respectively, incubating for 30min under the dark condition at room temperature, and testing the absorbance of the mixed sample by using ultraviolet-visible spectrum (UV-17800, Shimadzu). The calibration curve for different ammonium ion concentrations is as follows, 0 mu mol^-1L^-1，0.01μmol^-1L^-1，0.05μmol^-1L^-1，0.1μmol^-1L^-1，0.5μmol^-1L^-1，1.0μmol^-1L^-1，5.0μmol^- ¹L^-1，10μmol^-1L^-1，50μmol^-1L^-1The absorbance peak fitting curve of each concentration presents linear regression, and the linear correlation coefficient R²0.99974 (as shown in fig. 3).

3. Cubic alpha-Fe prepared in example 1 was subjected to Scanning Electron Microscopy (SEM)₂O₃Core-shell type Fe₂O₃@TiO₂Core-shell type Fe₂O₃@Ti_xO_y-P_zAnalysis was performed and SEM pictures of the different materials were obtained as shown in figure 4. In FIG. 4, a is a cubic Fe₂O₃SEM picture, its structure is homogeneous, the size is about 547 nm. B in FIG. 4 is core-shell Fe₂O₃@TiO₂SEM image of (g), cubic alpha-Fe can be seen₂O₃Surface-coated amorphous TiO₂The shell layer is wrapped. After phosphating treatment, the obtained Fe₂O₃@Ti_xO_y-P_zAgain, a cubic structure with a size of about 639nm (c in fig. 4). D in FIG. 4 represents Ti_xO_y-P_zThe thickness of the layer is about 50 nm. In addition, the EDS element line scan curve was used to further evaluate Fe₂O₃@Ti_xO_y-P_zMorphology and structure of (a). As can be seen from e and f in fig. 4, the contents of P and Ti elements are smaller than those of Fe and O elements, and P, Ti and O elements appear at the two ends of the curve first, while Fe elements are distributed in the middle; this confirms the designed Fe₂O₃@Ti_xO_y-P_zThe composite photocatalyst is of a core-shell structure.

4. The cubic α -Fe prepared in example 1 was further subjected to Transmission Electron Microscopy (TEM)₂O₃Core-shell type Fe₂O₃@TiO₂Core-shell type Fe₂O₃@Ti_xO_y-P_zAnd (6) performing characterization. As shown by a in FIG. 5, Fe₂O₃A cubic structure of uniform size. In Fe when adding TBOT₂O₃After surface hydrolysis, cubic Fe₂O₃Surface of the material is randomly distributed TiO₂Amorphous layer (b, Fe in FIG. 5)₂O₃@TiO₂). For composite material Fe₂O₃@TiO₂After further phosphating, Fe₂O₃The surface of the titanium alloy is dense Ti_xO_y-P_zLayer with a thickness of about 50nm (c and inset in fig. 5). Fe can be clearly seen from e in FIG. 5₂O₃@Ti_xO_y-P_zHas a lattice spacing of 0.35nm and good crystallinity, which is comparable to anatase TiO₂The (101) crystal planes of the crystal planes are identical.

Next, the composite catalyst Fe was further explored by TEM-element mapping₂O₃@Ti_xO_y-P_zAnd (5) morphology structure. As shown by f in FIG. 5, Fe, O, Ti and P are in Fe₂O₃@Ti_xO_y-P_zThe element distribution of the same cube shape is presented, and the outline of c in fig. 5 can be well drawn.

In addition, cubic Fe prepared in example 1₂O₃、Fe₂O₃@TiO₂And Fe₂O₃@Ti_xO_y-P_zThe crystal structure of (3) was subjected to XRD characterization. As shown by d in FIG. 5, first, pure α -Fe is used₂O₃Standard PDF cards (PDF33-0664, ICDD, 2004; hematite, syn) and anatase TiO₂Standard cards (PDF21-1272, ICDD, 2004; Anatase, syn) were used as controls. The results show that the cubic Fe prepared₂O₃Basically matched with hematite PDF standard cards, and has better crystallinity. When coating anatase type TiO₂Then, Fe₂O₃@TiO₂The composite material shows obvious anatase type TiO₂Diffraction peaks at positions of 25.28, 37.80 and 48.05 degrees 2 theta respectively corresponding to anatase type TiO₂{101}, {004} and {200} crystal planes (d in FIG. 5, curve Fe₂O₃@TiO₂). After phosphating with Fe₂O₃@TiO₂Composite material comparison, curve Fe in d in FIG. 5₂O₃@Ti_xO_y-P_zThe overall diffraction peak signal intensity of (1) is reduced (d in FIG. 5, curve Fe₂O₃@Ti_xO_y-P_z) (ii) a This is because after the calcination annealing, α -Fe₂O₃Has stronger diffraction peak, resulting in P-doped TiO₂The diffraction peak signal of the layer is submerged in the single crystal alpha-Fe₂O₃In the strong diffraction peak of (2).

5. Cubic alpha-Fe prepared in example 1 using high resolution O1s and Ti 2p XPS spectra₂O₃Core-shell type Fe₂O₃@TiO₂Core-shell type Fe₂O₃@Ti_xO_y-P_zAnalysis was performed to evaluate whether its surface was Ti³⁺Species of the species. As in a in FIG. 6 (curve α -Fe)₂O₃And Fe₂O₃@TiO₂O1s spectrum) shows that the signal peaks at positions 529.6eV,531.4eV and 532.9eV correspond to O^2-(Fe-O bond or Ti-O bond), surface adsorption of oxygen species (OH)^-) And metal hydroxides; three types of oxygen species are demonstrated to be present in these catalysts. From a in FIG. 6 (curve Fe)₂O₃@Ti_xO_y-P_z) It can be seen that the O1s signal peak is clearly shifted towards higher binding energies, indicating the presence of Ti³⁺Species and Oxygen Vacancies (OVs). It can be seen in the high resolution XPS spectrum of Ti 2p (b, Fe in FIG. 6)₂O₃@TiO₂Curve (d) at 458.50eV (Ti 2 p)_3/2) And 464.37eV (Ti 2 p)_1/2) Two peaks were found, which are ascribed to the tableFace Ti⁴⁺A specie. And Fe₂O₃@Ti_xO_y-P_zThe sample shows a new peak at 460.73eV, which belongs to Ti³⁺Species indicating the formation of Ti on the surface after phosphating³⁺Species of the species.

In addition, for Fe₂O₃@Ti_xO_y-P_zThe high-resolution P2P XPS spectrum of the doped P element of the sample is studied. C in FIG. 6 shows Fe₂O₃@Ti_xO_y-P_zThe sample has a distinct peak at 132.54eV, which confirms Fe after annealing phosphating₂O₃@Ti_xO_y-P_zP-Ti-O bonds are present in the sample. Meanwhile, the peak at 133.45eV belongs to the pentavalent oxidation state (P)⁵ ⁺). According to previous studies, it was reported that the doped P atom may be cationic (P)⁵⁺) Is added in the form of, and replaces Ti⁴⁺The ions cause the formation of P-O-Ti bonds. A bond corresponding to Ti-P (P) was also found at 128.50eV^3-State) due to P^3-Substituted for TiO₂O atoms in the crystal lattice induce Ti³⁺Generation of active sites.

In addition, Fe is known from the electrochemical impedance spectrum (d in FIG. 6)₂O₃@Ti_xO_y-P_zSemicircular radius in high frequency region smaller than Fe₂O₃@TiO₂And cubic Fe₂O₃This indicates Fe₂O₃@Ti_xO_y-P_zHas smaller charge transfer resistance and higher charge mobility; this is probably attributable to Ti_xO_y-P_zLayer and Fe₂O₃The close interface contact between the two provides an effective path for charge transfer.

6、Ti³⁺Effect of active site on pNRR process: cubic alpha-Fe prepared in example 1₂O₃Core-shell type Fe₂O₃@TiO₂Core-shell type Fe₂O₃@Ti_xO_y-P_zThe samples were subjected to the following tests, respectively: (a) use ofHITACHI UV-3900 spectrometer with BaSO₄The ultraviolet-visible diffuse reflectance spectra (DSR) of the different samples were recorded for reference. (b) Steady state Photoluminescence (PL) curves for the different samples were obtained on an FLS1000 fluorescence lifetime spectrophotometer (Edinburgh Instruments, UK) under excitation of a hydrogen flash lamp at a wavelength of 800 nm. (c) Using an electrochemical workstation (CHI Instruments CHI760-1), at 0.1M Na₂SO₄In the electrolyte, under the condition of switching on and off the light source (turning off or on the light) at intervals of 20 seconds at open-circuit points, transient photocurrent response curves of different samples are collected. (d) N-performance of different samples using a Micromeritics Auto Chem II instrument with TCD as detector₂Temperature programmed desorption (N)₂TPD).

UV-vis diffuse reflectance spectroscopy (UV-vis DRS) indicated that Fe alone was cubic₂O₃Has better visible light response (a in figure 7, curve cube Fe₂O₃). With TiO₂After recombination, the light response is reduced in the visible region and the ultraviolet response is enhanced (a in FIG. 7, curve Fe₂O₃@TiO₂). Finally, the catalyst Fe is subjected to phosphating treatment₂O₃@Ti_xO_y-P_zExhibit a strong absorption capacity in the visible light region, mainly because of Ti_xO_y-P_zHaving Ti on the surface³⁺Site (a, Fe in FIG. 7)₂O₃@Ti_xO_y-P_zCurve) is shown.

Next, we performed a Photoluminescence (PL) test on the catalyst to further explore the kinetic behavior of the photogenerated carriers. As shown in b of fig. 7, with respect to the cube Fe₂O₃And Fe₂O₃@TiO₂Of a significant emission peak, Fe₂O₃@Ti_xO_y-P_zShows a weaker PL peak, indicating Fe₂O₃@Ti_xO_y-P_zHas smaller recombination rate of electrons and holes.

The transient photocurrent response of the different catalysts was further examined under 300w xenon lamp illumination, as shown in fig. 7 c. From the figureIt can be seen that Fe₂O₃@Ti_xO_y-P_zThe intensity of the photocurrent is obviously higher than that of the cubic Fe₂O₃And Fe₂O₃@TiO₂Prove Fe₂O₃@Ti_xO_y-P_zThe carrier separation efficiency is good; this is mainly due to the presence of Fe in the cube₂O₃Coupled Ti_xO_y-P_zThe defect layer may induce formation of impurity levels and carrier trapping centers, thereby promoting separation of photo-generated electrons and holes and suppressing charge recombination. On the other hand, N₂The effective chemisorption of the catalyst plays a key role in the photocatalytic energy nitrogen fixation process, which is generally considered to occur at a catalytically active site and is a key step in the pNRR reaction process. Therefore, we use N₂Temperature programmed desorption (N)₂TPD) to characterize the photocatalyst pair N₂The adsorption capacity of (1). In general, N of the material₂The higher the peak TPD, the higher the pNRR activity. As shown in d of FIG. 7, Fe in the test range of 250-450 deg.C₂O₃@Ti_xO_y-P_zN of (A)₂Chemical desorption peak ratio cube Fe₂O₃And Fe₂O₃@TiO₂Much stronger, indicating Fe₂O₃@Ti_xO_y-P_zTi of (A)³⁺The site can effectively adsorb N₂A molecule. Due to N₂The effective adsorption is the photocatalytic synthesis of NH₃Of Fe, thus Fe₂O₃@Ti_xO_y-P_zUpper good N₂Adsorption favors the entire pNRR process.

7. Detection of activity of photocatalytic nitrogen fixation and ammonia synthesis of different catalysts

The indoxyl method (specific methods as described above) was used to evaluate the cubic alpha-Fe prepared in example 1₂O₃Core-shell type Fe₂O₃@TiO₂Core-shell type Fe₂O₃@Ti_xO_y-P_zFe prepared in comparative example 6₂O₃-P_zComparative example 7 prepared Ti_xO_y-P_zThe full spectrum solar energy is used as a driving force in the process, and water molecules are used as a solvent for providing protons. As shown in a of fig. 8, in the general formula N₂In the absence of light, almost no NH was detected₃. Under the condition of illumination, introducing N₂Photocatalytic NH of all catalysts under atmosphere₃The yield gradually increased with time. After 180min of reaction, cubic Fe₂O₃Or Fe₂O₃-P_zThe catalyst can generate small amount of NH₃And is of Fe₂O₃@Ti_xO_y-P_zCatalyst synthesis of NH₃The yield is obviously improved.

From b in FIG. 8, it can be seen that the catalyst Fe₂O₃-P_zSynthesis of NH₃The rate was 1.87. mu. mol g_cat. ^-1h^-1Of only cubic Fe₂O₃(1.66μmol g_cat. ^-1h^-1) 1.13 times of. Catalyst Ti_xO_y-P_zThe shell layer has a higher nitrogen fixation for synthesizing NH₃Active, but still not capable of reacting with Fe₂O₃@Ti_xO_y-P_zComposite material (15.65. mu. mol g)_cat. ^-1h^-1) And (4) comparing. Fe₂O₃@Ti_xO_y-P_zNitrogen fixation synthesis of NH by composite catalyst₃The rate is significantly higher than other catalysts, mainly benefiting from coupled TiO₂Or single defect Ti_xO_y-P_zThe layer can increase Fe₂O₃Nitrogen fixation to synthesize NH₃And (4) activity. Further, Fe₂O₃@Ti_xO_y-P_zThe product has excellent light stability, the activity of the product is basically unchanged after 6 times of test experiments of cycle test, the performance of the synthetic ammonia is not obviously reduced, and the 98.1 percent activity (c in figure 8) is still maintained; and the XRD spectrogram is used for researching Fe₂O₃@Ti_xO_y-P_zPhysicochemical Properties before and after the cycling experiment, Fe, as shown in FIG. 9₂O₃@Ti_xO_y-P_zThe crystal structure did not change significantly before and after the cycling experiment, confirming that the structure has excellent photostability.

Using different monochromatic lights (365, 420, 470, 535 and 630nm) for the catalyst Fe₂O₃@Ti_xO_y-P_zThe apparent quantum efficiency of (a) was investigated as shown by d in fig. 8. Catalyst Fe₂O₃@Ti_xO_y-P_zThe AQE values corresponding to the wavelengths of monochromatic light from 365nm to 630nm are respectively 0.032%, 0.026%, 0.029%, 0.031% and 0.016%, and the solar energy utilization rate is acceptable.

8. Mass ratio of titanium dioxide to iron oxide in catalyst samples

About 10mg of the intermediate product and the catalyst sample prepared in example 1 and the catalyst samples prepared in comparative examples 1 to 7 were added to 20mL of a mixture of 6mol/L nitric acid, hydrofluoric acid and hydrochloric acid (in a volume ratio of 3:1:1), respectively, and reacted in a microwave oven at 180 ℃ for 20 min. After the reaction, the contents of Fe, Ti and P in the reaction solution were measured by inductively coupled plasma emission spectroscopy (ICP-OES, Perkin Elmer Optima 4300DV), and the results are shown in Table 1.

TABLE 1 determination of the mass ratio of titanium dioxide to iron oxide by ICP-OES method

The above embodiments are preferred embodiments of the present invention, but the present invention is not limited to the above embodiments, and any other changes, modifications, substitutions, combinations, and simplifications which do not depart from the spirit and principle of the present invention should be construed as equivalents thereof, and all such changes, modifications, substitutions, combinations, and simplifications are intended to be included in the scope of the present invention.

Claims

1. Core-shell type Fe₂O₃@Ti_xO_y-P_zThe preparation method of the photocatalyst is characterized by comprising the following steps:

(1) passing through waterThermal method for preparing cubic alpha-Fe₂O₃；

(2) Preparation of core-Shell Fe₂O₃@TiO₂A nanocomposite;

(3) mixing core-shell type Fe₂O₃@TiO₂The core-shell type Fe is obtained by carrying out phosphating treatment on the nano composite material at 300 DEG C₂O₃@Ti_xO_y-P_zA photocatalyst; wherein 0 < x < 1, 0 < y < 2, and 0 < z < 10%.

2. The core-shell Fe of claim 1₂O₃@Ti_xO_y-P_zThe preparation method of the photocatalyst is characterized in that the specific steps of the step (1) are as follows: FeCl is added₃·6H₂Adding O into water, stirring to obtain a transparent solution, and adding 0.1-0.5 mol of FeCl into every 10-100 mL of water₃·6H₂O; heating the solution to 60-100 ℃, then dropwise adding a NaOH solution with the concentration of 5-6 mol/L into the solution, stirring the solution for 5-15 min, and dropwise adding 20-100 mL of NaOH solution into every 10-100 mL of water; then transferring the obtained mixed solution into an autoclave with a polytetrafluoroethylene lining, and reacting for 48-72 h at 80-150 ℃ to obtain alpha-Fe₂O₃。

3. The core-shell Fe of claim 2₂O₃@Ti_xO_y-P_zThe preparation method of the photocatalyst is characterized by further comprising the following steps after the reaction in the step (1) is completed: and cooling the autoclave to room temperature, centrifugally separating precipitates, washing the precipitates with water and ethanol for several times, and drying the precipitates at the temperature of 60-80 ℃ overnight.

4. The core-shell Fe of claim 1₂O₃@Ti_xO_y-P_zThe preparation method of the photocatalyst is characterized in that the specific steps in the step (2) are as follows: 0.1 part by mass of alpha-Fe synthesized in the step (1)₂O₃Dispersing in 100-500 parts by volume of ethanol; adding 0.5-1 part by mass of polyvinylpyrrolidone into the mixed solution and stirringAfter 30min, adding 0-1.0 volume part of tetrabutyl titanate, adding 50-200 volume parts of ethanol solution, and continuously stirring for 10-18 h to obtain core-shell Fe₂O₃@TiO₂A nanocomposite; wherein 1 part by mass: 1 part by volume is 1 g/mL.

5. The core-shell Fe of claim 4₂O₃@Ti_xO_y-P_zThe preparation method of the photocatalyst is characterized by further comprising the following steps after the reaction in the step (2) is completed: centrifugally washing with water and ethanol, and drying at 70 deg.C for 2 h; the ethanol solution is hydrous ethanol, and each 50mL of the ethanol solution contains 0-20 mL of water.

6. The core-shell Fe of claim 1₂O₃@Ti_xO_y-P_zThe preparation method of the photocatalyst is characterized in that the specific steps in the step (3) are as follows: adding 0.2-2 parts by mass of NaH₂PO₂And 0.1 part by mass of core-shell Fe₂O₃@TiO₂Placing the nanocomposite material in a tubular furnace; under the condition of inert gas flow, heating to 300-500 ℃, and continuing to react for 1-3 h; collecting the product after reaction, namely the core-shell type Fe₂O₃@Ti_xO_y-P_zA photocatalyst.

7. The core-shell Fe of claim 6₂O₃@Ti_xO_y-P_zThe preparation method of the photocatalyst is characterized in that NaH is added in the step (3)₂PO₂Placing in the center of a tubular furnace, and adding core-shell Fe₂O₃@TiO₂The nanocomposite is placed on the downstream side of the tubular furnace at a distance of about 3-10 cm.

8. The core-shell Fe of claim 1₂O₃@Ti_xO_y-P_zThe preparation method of the photocatalyst is characterized in that in the step (3), the temperature is 1-10 ℃ per minute^-1Rate of temperature rise ofHeating to 300-500 ℃;

and (4) the inert gas in the step (3) is argon.

9. Core-shell Fe obtainable by a process according to any one of claims 1 to 8₂O₃@Ti_xO_y-P_zA photocatalyst.

10. The core-shell Fe according to claim 9₂O₃@Ti_xO_y-P_zThe application of the photocatalyst in synthesizing ammonia by photocatalysis and nitrogen fixation.