CN113600214B - Core-shell type Fe 2 O 3 @Ti x O y -P z Preparation method and application of photocatalyst - Google Patents

Core-shell type Fe 2 O 3 @Ti x O y -P z Preparation method and application of photocatalyst Download PDF

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CN113600214B
CN113600214B CN202111025871.5A CN202111025871A CN113600214B CN 113600214 B CN113600214 B CN 113600214B CN 202111025871 A CN202111025871 A CN 202111025871A CN 113600214 B CN113600214 B CN 113600214B
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韩冬雪
牛利
张文生
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Guangzhou University
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Abstract

The invention discloses a core-shell Fe 2 O 3 @Ti x O y ‑P z A preparation method and application of a photocatalyst. The method comprises the following steps: preparation of cubic alpha-Fe by hydrothermal method 2 O 3 The method comprises the steps of carrying out a first treatment on the surface of the Preparation of core-shell Fe 2 O 3 @TiO 2 A nanocomposite; core-shell Fe 2 O 3 @TiO 2 Phosphating the nanocomposite at 300 ℃ to obtain the core-shell Fe 2 O 3 @Ti x O y ‑P z A photocatalyst. The invention uses alpha-Fe 2 O 3 Coupled with titanium dioxide with wide band gap can effectively overcome alpha-Fe 2 O 3 Self-defect, strengthen photo-generated electronAnd hole transmission and separation efficiency, and the pNRR activity is improved. The invention carries out phosphating treatment on the composite material, and the composite material is prepared in TiO (titanium dioxide) 2 Surface induced N generation 2 Active site Ti of (2) 3+ Species to enhance N 2 Efficient adsorption and activation of the molecules further enhances pNRR activity.

Description

Core-shell type Fe 2 O 3 @Ti x O y -P z Preparation method and application of photocatalyst
Technical Field
The invention belongs to the technical field of photoelectric energy materials, and particularly relates to a core-shell Fe 2 O 3 @Ti x O y -P z A preparation method and application of a photocatalyst.
Background
Ammonia (NH) 3 ) Is an indispensable chemical substance in modern society, and is the basic component for manufacturing synthetic chemicals such as medicines, fertilizers, resins, dyes, explosives and the like. NH is added to 3 Condensed into liquid, and then condensed with hydrogen (H) 2 ) Compared with the prior art, the energy-saving fuel cell has considerable energy density and transportability, and can be used for power fuel cells in a short period. Up to now, industrial nitrogen fixation synthesis of NH 3 Is realized by a high-temperature and high-pressure Haber-Bosch method; such a process requires a large consumption of resources and energy, requires a complex large-scale infrastructure, and simultaneously emits a large amount of carbon dioxide, which has a great influence on the environment. Therefore, there is an urgent need to develop a new, low-energy, green, sustainable synthesis of NH by nitrogen fixation 3 The method.
Compared with the existing energy-intensive Haber-Bosch method, the photocatalytic nitrogen reduction reaction (photocatactic N 2 reduction reaction, pNRR) with water (H 2 O) is taken as a proton source, and N is realized under the drive of a semiconductor photocatalyst and renewable solar energy 2 To NH 3 The conversion is a research field with a comparison front edge and environmental protection in recent years. From the standpoint of efficient utilization of sunlight, an ideal photocatalyst should absorb visible light because it is a significant proportion (about 44%) of the visible light in the solar spectrum. As is well known, hematite (alpha-Fe 2 O 3 ) Has the advantages of low cost, wide visible light response, high light stability, environmental protection and the like, and is a promising pNRR catalyst. However, in one component alpha-Fe 2 O 3 In the material, the reduction capability of photo-generated electrons is low, and effective pNRR reaction cannot occur. In additionThe high recombination rate of photogenerated electrons and holes prevents its widespread use in pNRR. On the other hand, the N.ident.N bond of nitrogen is very strong (. About.941 kJ mol) -1 ) The dynamics of pNRR is too slow, so that the reaction rate of a common photocatalytic nitrogen fixation ammonia synthesis system is relatively low, and industrial application is difficult to realize.
In the patent document CN03158740.2, a photocatalyst composed of oxide nanoparticles, nonmetallic elements and semiconductor nanoparticles is disclosed, wherein 0.1 to 0.5mol of a compound of a semiconductor nanomaterial is added to a methanol solution of a pre-prepared 0.001 to 0.1M metal salt and nonmetallic salt in 10 minutes to 30 minutes under vigorous stirring, and 0.1ml of 5M HNO is added to the mixed solution every 5 to 10 minutes 3 The aqueous solution is hydrolyzed, stirred for 5 to 12 hours at normal temperature, the semiconductor nano sol is obtained, the semiconductor nano sol is placed for sedimentation and aging for 1 to 10 days, and the photocatalyst is prepared by roasting the solvent at a high temperature of 300 to 900 ℃ after the solvent is dried. However, the result of degrading 2,4, 6-trichlorophenol for 5 times by cycling the photocatalyst shows that the photocatalyst has unstable structure and poor reproducibility: the triclosan is degraded by more than 90% after 240min of the previous three-cycle photoreaction, and the catalytic activity of the catalyst is not reduced basically; however, the fourth and fifth cycle photoreactions 240min, the 2,4, 6-trichlorophenol degradation was only 80% or more.
Disclosure of Invention
To solve the defects and shortcomings of the prior art, the primary aim of the invention is to provide a core-shell type Fe 2 O 3 @Ti x O y -P z A preparation method of a photocatalyst.
Another object of the present invention is to provide a core-shell Fe prepared by the above method 2 O 3 @Ti x O y -P z A photocatalyst.
The invention provides a non-metal phosphorus doped titanium dioxide shell layer-based coated cubic hematite (Fe) 2 O 3 @Ti x O y -P z ) The composite nano material system can realize the reduction of nitrogen at normal temperature and normal pressure to synthesize ammonia by a photocatalysis method, and effectively overcomes the defects of single-component Fe 2 O 3 Photo-generated electricityThe reduction ability of the carriers is low and the recombination rate of the photo-generated carriers is high. In addition, the composite photocatalyst is simple to prepare, low in cost and high in stability, and is favorable for realizing further industrialized development.
It is still another object of the present invention to provide the core-shell Fe as described above 2 O 3 @Ti x O y -P z Use of a photocatalyst.
The invention aims at realizing the following technical scheme:
core-shell type Fe 2 O 3 @Ti x O y -P z The preparation method of the photocatalyst comprises the following steps:
(1) Preparation of cubic alpha-Fe by hydrothermal method 2 O 3 ；
(2) Preparation of core-shell Fe 2 O 3 @TiO 2 A nanocomposite;
(3) Core-shell Fe 2 O 3 @TiO 2 Phosphating the nanocomposite at 300 ℃ to obtain the core-shell Fe 2 O 3 @Ti x O y -P z The photocatalyst (x and y are subscripts of chemical formula, 0 < x < 1,0 < y < 2, z refers to the mass ratio of doped phosphorus element, and the value range is 0 < z < 10%).
Preferably, the specific steps of step (1) are as follows: feCl is added 3 ·6H 2 Adding O into water, stirring to obtain transparent solution, adding 0.1-0.5 mol FeCl into every 10-100 mL water 3 ·6H 2 O; heating the solution to 60-100 ℃, then dropwise adding NaOH solution with the concentration of 5-6 mol/L into the solution, stirring for 5-15 min, and dropwise adding 20-100 mL of NaOH solution into 10-100 mL of water; transferring the obtained mixed solution into an autoclave lined with polytetrafluoroethylene, and reacting for 48-72 h at 80-150 ℃ to obtain alpha-Fe 2 O 3 。
More preferably, the reaction of step (1) is completed further comprising: the autoclave was cooled to room temperature, the precipitate was centrifuged, washed several times with water and ethanol and dried overnight at 60-80 ℃.
Preferably, the specific steps of step (2) are as follows: 0.1 to 0.5 part by massalpha-Fe synthesized in step (1) 2 O 3 Dispersing in 100-500 volume parts of ethanol; then adding 0.5 to 1 part by mass of polyvinylpyrrolidone into the mixed solution, stirring for 30min, adding 0 to 1.0 part by volume of tetrabutyl titanate, slowly adding 50 to 200 parts by volume of ethanol solution, and continuously stirring for 10 to 18h to obtain the core-shell Fe 2 O 3 @TiO 2 A nanocomposite; 1 part by mass: 1 volume = 1g/mL.
More preferably, the reaction of step (2) further comprises, after completion: centrifugally washing with water and ethanol, and drying at 70 ℃ for 2 hours; the ethanol solution is aqueous ethanol, and each 50mL of ethanol solution contains 0-20 mL of water, and each 50mL of ethanol solution preferably contains 5-20 mL of water.
Preferably, the specific steps of step (3) are as follows: 0.2 to 2 parts by mass of NaH 2 PO 2 And 0.1 part by mass of core-shell Fe 2 O 3 @TiO 2 Placing the nanocomposite in a tubular furnace; heating to 300-500 ℃ under the flowing condition of inert gas, and continuing to react for 1-3 h; collecting the reacted product, namely core-shell Fe 2 O 3 @Ti x O y -P z A photocatalyst.
More preferably, in step (3), naH is added 2 PO 2 Placed in the center of a tubular furnace to make core-shell Fe 2 O 3 @TiO 2 The nanocomposite is placed on the downstream side of the tubular furnace, at a distance of about 3-10 cm.
More preferably, in the step (3), the temperature is 1 to 10 ℃ for min -1 Is heated to 300-500 ℃.
More preferably, the inert gas in step (3) is argon or nitrogen.
The invention provides core-shell Fe 2 O 3 @Ti x O y -P z The photocatalyst can be used for synthesizing ammonia by photocatalytic nitrogen fixation.
Compared with the prior art, the invention has the following advantages:
1. single Fe 2 O 3 The catalyst has poor photo-generated electron hole separation efficiency and lower photo-generated electron reduction capability; single TiO 2 The photocatalyst has a relatively low solar energy utilization rate (only absorbs ultraviolet light). The invention uses alpha-Fe 2 O 3 With wide band gap titanium dioxide (TiO 2 ) Coupled together can effectively overcome alpha-Fe 2 O 3 The self defect enhances the transmission and separation efficiency of the photo-generated electrons and holes, and improves the activity of photo-catalytic nitrogen reduction reaction (pNRR).
2. The photocatalytic nitrogen reduction (pNRR) reaction process is primarily limited by its N 2 Effective adsorption process (chemical adsorption), the invention reasonably increases N on the surface of the photocatalyst 2 Adsorption of the active site is an effective measure. The present invention provides for phosphating (pH of) the composite material 3 Annealing the gas at 300 ℃), at TiO 2 Surface induced N generation 2 Active site Ti of (2) 3+ Species to enhance N 2 Efficient adsorption and activation of the molecules further enhances pNRR activity.
Drawings
FIG. 1 shows Fe prepared in example 1 2 O 3 @(0.6)TiO 2 Fe prepared in comparative examples 1 to 5 2 O 3 @(0)TiO 2 、Fe 2 O 3 @(0.2)TiO 2 、Fe 2 O 3 @(0.4)TiO 2 、Fe 2 O 3 @(0.8)TiO 2 、Fe 2 O 3 @(1.0)TiO 2 Photoluminescence spectra (a) and instantaneous photocurrent response (b) of (a-f) respectively correspond to Fe in the figure 2 O 3 @(0)TiO 2 ,Fe 2 O 3 @(0.2)TiO 2 ,Fe 2 O 3 @(0.4)TiO 2 ,Fe 2 O 3 @(0.6)TiO 2 And Fe (Fe) 2 O 3 @(1.0)TiO 2 A complex.
FIG. 2 is a schematic diagram of the apparatus for photocatalytic nitrogen fixation reaction according to the present invention.
FIG. 3 shows the absorbance (a) and absorbance peak fitting curve (b) of mixed samples with different concentration gradients in the method for detecting ammonium ions in aqueous solution, the concentration of the ammonium ions in the arrow shown in a is 0-50 mu mol L from top to bottom -1 。
FIG. 4 is a cube prepared in example 1Bulk alpha-Fe 2 O 3 Core-shell Fe 2 O 3 @TiO 2 Core-shell Fe 2 O 3 @Ti x O y -P z Scanning Electron Microscope (SEM) images (a-e) and core-shell Fe 2 O 3 @Ti x O y -P z An EDS element line scan curve (f).
FIG. 5 is a cubic alpha-Fe prepared in example 1 2 O 3 Core-shell Fe 2 O 3 @TiO 2 Core-shell Fe 2 O 3 @Ti x O y -P z Transmission Electron Microscope (TEM) plots (a, b, c, e, f) and XRD characterization (d), where the scale length of the inset in c is 200nm and the scale length in the upper left plot of f is 1 μm.
FIG. 6 is a cubic alpha-Fe prepared in example 1 2 O 3 Core-shell Fe 2 O 3 @TiO 2 Core-shell Fe 2 O 3 @Ti x O y -P z O1s profile (a), ti 2p XPS profile (b) and electrochemical impedance profile (d), core-shell Fe 2 O 3 @Ti x O y -P z P2P XPS profile (c).
FIG. 7 is a cubic alpha-Fe prepared in example 1 2 O 3 Core-shell Fe 2 O 3 @TiO 2 Core-shell Fe 2 O 3 @Ti x O y -P z Sample UV visible diffuse reflectance spectrum (a), steady state photoluminescence curve (b), transient photocurrent response (c) and N 2 Programmed temperature adsorption and desorption curve (d), wherein a, b and c respectively correspond to cube alpha-Fe 2 O 3 Core-shell Fe 2 O 3 @TiO 2 Core-shell Fe 2 O 3 @Ti x O y -P z 。
FIG. 8 is a cubic alpha-Fe prepared in example 1 2 O 3 Core-shell Fe 2 O 3 @TiO 2 Core-shell Fe 2 O 3 @Ti x O y -P z Fe prepared in comparative example 6 2 O 3 -P z Ti prepared in comparative example 7 x O y -P z The photocatalytic nitrogen fixation performance detection of a is that a is NH synthesized by different catalysts 3 Yield, b is the synthesis of NH by different catalysts 3 Rate, c is Fe 2 O 3 @Ti x O y -P z Synthesis of NH by 6 test experiments 3 Yield, d is Fe 2 O 3 @Ti x O y -P z Is a quantum of apparent quantum efficiency.
FIG. 9 is a core-shell Fe prepared in example 1 2 O 3 @Ti x O y -P z Photocatalytic N 2 XRD patterns before and after the reduction reaction cycle experiment and after 6 times of the cycle reaction.
Detailed Description
The present invention will be described in further detail with reference to examples and drawings, but 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 comprising: ferric trichloride hexahydrate (FeCl3.6H) 2 O) and sodium hypophosphite (NaH) 2 PO 2 ) Purchased from beijing enoKai chemical Co. Potassium hydroxide and absolute ethanol were purchased from national pharmaceutical chemicals limited. Tetrabutyl titanate (TBOT), polyvinylpyrrolidone K-30 (PVP), isotope 15 N 2 Gases, sodium nitroprusside solution and alkaline hypochlorite solution were purchased from Sigma-Aldrich chemical company. Ammonium chloride (NH) 4 Cl) from Shanghai merck chemical technologies. All reagents were analytically pure (AR) in purity and used directly without further purification. The deionized water used throughout the experiment had a resistivity of 18.2 M.OMEGA.cm -1 。
The characterization instruments used in the following examples and comparative examples are as follows: the morphology and elemental composition of the samples were analyzed using a scanning electron microscope (SEM, JEOL JSM-7001F) and a transmission electron microscope (TEM, JEOL 2100F) and their associated energy dispersive X-ray spectroscopy (EDS). The surface electron states were analyzed by X-ray photoelectron spectroscopy (XPS, thermo esclab 250 Xi), all binding energies referenced to the C1s peak of 284.6 eV. UsingHITACHI UV-3900 spectrometer with BaSO 4 For reference, a uv visible diffuse reflectance spectrum (DSR) was recorded. X-ray diffraction (XRD) was performed using a PANalytical X' Pert PRO instrument, using Cu K alpha radiation. N was performed using Micromeritics Auto Chem II with TCD as detector 2 And (5) performing temperature programming adsorption and desorption experiments. The material was tested for a mote-schottky curve using an electrochemical workstation (CHI Instruments CHI 760-1). Steady state and transient Photoluminescence (PL) curves of the catalyst were obtained on FLS1000 fluorescence lifetime spectrophotometer (Edinburgh Instruments, UK) under excitation of a hydrogen flash at a wavelength of 800 nm. The contents of Fe, ti and P in the samples were determined by inductively coupled plasma spectrometry (ICP-OES, perkin Elmer Optima 4300 DV), respectively.
Example 1
Core-shell type Fe 2 O 3 @Ti x O y -P z The preparation method of the photocatalyst comprises the following steps:
(1) Preparation of cubic alpha-Fe by hydrothermal method 2 O 3 : 0.1mol FeCl 3 ·6H 2 O is added into 50mL of water and stirred to a transparent solution; heating the solution to 75 ℃, then dropwise adding 50mL of NaOH solution with the concentration of 5.4mol/L into the solution, stirring for 5min, and reacting for 10min; transferring the obtained mixed solution into an autoclave lined with polytetrafluoroethylene, placing into an oven to react for 56 hours at 100 ℃, cooling the autoclave to room temperature, centrifugally separating precipitate, washing with water and ethanol for several times, and drying overnight at 80 ℃ to obtain cubic alpha-Fe 2 O 3 。
(2) Preparation of core-shell Fe 2 O 3 @TiO 2 Nanocomposite material: 0.1g of the alpha-Fe synthesized in the step (1) is reacted with 2 O 3 Dispersing in 150mL ethanol; next, 0.6g of polyvinylpyrrolidone was added to the mixture and stirred for 30 minutes, 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 12 hours to obtain core-shell Fe 2 O 3 @TiO 2 Nanocomposite (denoted as Fe 2 O 3 @(0.6)TiO 2 ). The mixture was washed with water and ethanol by centrifugation and dried at 70℃for 2h.
(3) Core-shell Fe 2 O 3 @TiO 2 And (3) phosphating the nanocomposite: 1g NaH 2 PO 2 Placed in the center of a tubular furnace, 100mg of core-shell Fe prepared in the step (2) is added 2 O 3 @TiO 2 The nanocomposite was placed on the downstream side of the tubular furnace, at a distance of about 7cm. Under the flowing condition of argon, the temperature is 2 ℃ for min -1 Heating to 300 ℃ at the heating rate, and continuing to react for 1h; collecting the reacted product, namely core-shell Fe 2 O 3 @Ti x O y -P z A photocatalyst.
Comparative example 1
(1) Preparation of cubic alpha-Fe by hydrothermal method 2 O 3 : 0.1mol FeCl 3 ·6H 2 O is added into 50mL of water and stirred to a transparent solution; heating the solution to 75 ℃, then dropwise adding 50mL of NaOH solution with the concentration of 5.4mol/L into the solution, stirring for 5min, and reacting for 10min; transferring the obtained mixed solution into an autoclave lined with polytetrafluoroethylene, placing into an oven to react for 56 hours at 100 ℃, cooling the autoclave to room temperature, centrifugally separating precipitate, washing with water and ethanol for several times, and drying overnight at 80 ℃ to obtain cubic alpha-Fe 2 O 3 。
(2) Preparation of core-shell Fe 2 O 3 @TiO 2 Nanocomposite material: 0.1g of the alpha-Fe synthesized in the step (1) is reacted with 2 O 3 Dispersing in 150mL ethanol; next, 0.6g of polyvinylpyrrolidone was added to the mixture and stirred for 30 minutes, 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 12 hours to obtain core-shell Fe 2 O 3 @TiO 2 Nanocomposite (denoted as Fe 2 O 3 @(0.2)TiO 2 ). The mixture was washed with water and ethanol by centrifugation and dried at 70℃for 2h.
Comparative example 2
Referring to the method of comparative example 1, the procedure was the same as in comparative example 1 except that tetrabutyl titanate added in step (2) was 0 mL. Core-shell Fe prepared in this comparative example 2 O 3 @TiO 2 The nanocomposite is denoted as Fe 2 O 3 @(0)TiO 2 。
Comparative example 3
Referring to the method of comparative example 1, the procedure was the same as in comparative example 1 except that tetrabutyl titanate added in step (2) was 0.4 mL. Core-shell Fe prepared in this comparative example 2 O 3 @TiO 2 The nanocomposite is denoted as Fe 2 O 3 @(0.4)TiO 2 。
Comparative example 4
Referring to the method of comparative example 1, the procedure was the same as in comparative example 1 except that tetrabutyl titanate added in step (2) was 0.8 mL. Core-shell Fe prepared in this comparative example 2 O 3 @TiO 2 The nanocomposite is denoted as Fe 2 O 3 @(0.8)TiO 2 。
Comparative example 5
Referring to the method of comparative example 1, the procedure was the same as in comparative example 1 except that tetrabutyl titanate added in step (2) was 1.0 mL. Core-shell Fe prepared in this comparative example 2 O 3 @TiO 2 The nanocomposite is denoted as Fe 2 O 3 @(1.0)TiO 2 。
Comparative example 6
Preparation of cubic alpha-Fe with reference to step (1) of example 1 2 O 3 Then alpha-Fe 2 O 3 The phosphating treatment was carried out as in step (3) of example 1 as a blank, and the sample obtained was designated Fe 2 O 3 -P z 。
Comparative example 7
Etching Fe with weak acid 2 O 3 @Ti x O y -P z Core Fe 2 O 3 Etching is carried out to obtain a single Ti x O y -P z A defective shell layer. The method comprises the following specific steps: preparation of Fe by the method of reference example 1 2 O 3 @Ti x O y -P z Photocatalyst, 0.2g Fe 2 O 3 @Ti x O y -P z The photocatalyst was mixed with 10mL of hydrochloric acid solution having a concentration of 0.5 mol/L. Then, stirring at room temperature for 10min, and transferring to stainlessA steel Teflon autoclave (15 ml capacity) was reacted at 100℃for 24 hours. Centrifugal washing with deionized water for several times until pH value is close to 7, and oven drying at 80deg.C for 12 hr to obtain Ti x O y -P z And (3) a shell powder sample.
Performance test:
1. evaluation of cubic Fe using photoluminescence spectra (PL) and transient photocurrent response 2 O 3 Loaded TiO 2 The effect of the content of (2) on the photocatalytic activity. In brief, lower PL signal intensity indicates better photo-generated electron and hole separation efficiency of the catalyst, and thus higher photocatalytic activity. As shown in (a) of fig. 1, fe 2 O 3 @(0.6)TiO 2 The composite material can effectively inhibit the recombination rate of photo-generated charges, which indicates that the photo-catalytic activity is higher. Meanwhile, fe 2 O 3 @(0.6)TiO 2 The composite material has the highest photocurrent density, which shows that the photo-electron-hole generation capacity is stronger under the irradiation of visible light, and the photo-catalytic performance is improved (b) in fig. 1.
2. Photocatalytic nitrogen fixation reaction process
The photocatalytic nitrogen fixation ammonia synthesis experiment was performed on a self-assembled photocatalytic reaction platform, as shown in fig. 2. The photocatalytic nitrogen fixation is carried out at the three-phase interface (gas phase N) 2 Liquid phase H 2 O and solid phase catalyst). Using a 300w xenon lamp (full spectrum, 463mW cm) -2 ) The light source is 10cm from the liquid surface. The specific experimental steps are as follows: first, 40mg of Fe prepared in example 1 was added 2 O 3 @Ti x O y -P z The photocatalyst was dispersed in 200mL deionized water and added to a reactor with a circulating water system. Secondly, the mixed solution is filled with high-purity N under the condition of no illumination 2 (200mL min -1 ) Stirring was continued for 30 minutes to give N in the aqueous solution 2 Saturation is reached. Then, the lamp was turned on to give light conditions, and 4.0mL of the reaction solution was taken out every 30min and the content of synthetic ammonia was measured using a 0.22 μm filter.
The method for detecting ammonium ions in the aqueous phase solution comprises the following steps:
light exposure by indoxyl methodAmmonia in the chemical reaction solution is detected, NH with different concentration gradients is known 4 Cl was added to 0.1M KOH and then treated with 0.5. 0.5M H 2 SO 4 Neutralization is performed. 2.0mL of the above mixture was taken, 0.5mL of phenol nitroprusside solution and 0.5mL of alkaline sodium hypochlorite solution were added respectively, and incubated for 30min at room temperature under dark conditions, and absorbance of the non-mixed sample was measured by ultraviolet-visible spectrum (UV-17800, shimadzu). Calibration curves for different ammonium ion concentrations are as follows, 0. Mu. Mol -1 L -1 ，0.01μmol -1 L -1 ，0.05μmol -1 L -1 ，0.1μmol -1 L -1 ，0.5μmol -1 L -1 ，1.0μmol -1 L -1 ，5.0μmol - 1 L -1 ，10μmol -1 L -1 ，50μmol -1 L -1 The absorbance peak fitting curve of each concentration shows linear regression, and the linear correlation coefficient R 2 0.99974 (shown in fig. 3).
3. Cubic alpha-Fe prepared in example 1 was examined by Scanning Electron Microscopy (SEM) 2 O 3 Core-shell Fe 2 O 3 @TiO 2 Core-shell Fe 2 O 3 @Ti x O y -P z Analysis was performed, as shown in fig. 4, to obtain SEM photographs of different materials. A in FIG. 4 is cubic Fe 2 O 3 SEM image, which is uniform in structure and about 547nm in size. B in FIG. 4 is core-shell Fe 2 O 3 @TiO 2 Can be seen from SEM image of the cube alpha-Fe 2 O 3 Surface coated with amorphous TiO 2 The shell layer is wrapped. After phosphating treatment, fe is obtained 2 O 3 @Ti x O y -P z Also a cubic structure, with a size of about 639nm (c in fig. 4). D in FIG. 4 shows Ti x O y -P z The thickness of the layer was about 50nm. Furthermore, fe was further evaluated using EDS elemental line scan curves 2 O 3 @Ti x O y -P z Morphology 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 are first present at both ends of the curveFe element is distributed in the middle; this confirms the designed Fe 2 O 3 @Ti x O y -P z The composite photocatalyst is of a core-shell structure.
4. Further using a Transmission Electron Microscope (TEM) for the cubic alpha-Fe prepared in example 1 2 O 3 Core-shell Fe 2 O 3 @TiO 2 Core-shell Fe 2 O 3 @Ti x O y -P z Characterization was performed. As shown by a in fig. 5, fe 2 O 3 A cubic structure of uniform size. When TBOT is added in Fe 2 O 3 After the surface is hydrolyzed, cubic Fe 2 O 3 TiO with random distribution on surface 2 Amorphous layer (b, fe in FIG. 5 2 O 3 @TiO 2 ). For composite material Fe 2 O 3 @TiO 2 After further phosphating treatment, fe 2 O 3 Surface appearance of dense Ti x O y -P z The layer thickness is about 50nm (c and inset in fig. 5). From e in FIG. 5, fe is clearly seen 2 O 3 @Ti x O y -P z Is 0.35nm, has good crystallinity, and is similar to anatase TiO 2 The (101) crystal planes of (C) are identical.
Next, the composite catalyst Fe was further searched by TEM-element mapping 2 O 3 @Ti x O y -P z Morphology structure. As shown by f in FIG. 5, the elements Fe, O, ti and P are among Fe 2 O 3 @Ti x O y -P z The same cube-like element distribution is presented, which can well outline c in fig. 5.
In addition, for the cubic Fe prepared in example 1 2 O 3 、Fe 2 O 3 @TiO 2 And Fe (Fe) 2 O 3 @Ti x O y -P z XRD characterization was performed on the crystal structure of (c). As shown by d in FIG. 5, first, pure α -Fe is used 2 O 3 Standard PDF cards (PDF 33-0664, ICDD,2004; hematite, syn) and anatase TiO 2 Standard cards (PDF 21-1272, ICDD,2004; anatase, syn) served as controls. Results tableMing, cube Fe prepared 2 O 3 The product is basically matched with a hematite PDF standard card, and has good crystallinity. When coating anatase type TiO 2 After that, fe 2 O 3 @TiO 2 The composite material exhibits obvious anatase type TiO 2 Diffraction peaks at positions of 25.28, 37.80 and 48.05 in terms of 2 theta degree, respectively, correspond to anatase TiO 2 {101}, {004} and {200} crystal planes (d, curve Fe in FIG. 5) 2 O 3 @TiO 2 ). After phosphating treatment, with Fe 2 O 3 @TiO 2 In comparison with the composite material, curve Fe in d in FIG. 5 2 O 3 @Ti x O y -P z The overall diffraction peak signal intensity of (d, curve Fe in FIG. 5) 2 O 3 @Ti x O y -P z ) The method comprises the steps of carrying out a first treatment on the surface of the This is because after calcination annealing, α -Fe 2 O 3 Is stronger, resulting in P-doped TiO 2 Layer diffraction peak signal is submerged in single crystal alpha-Fe 2 O 3 Is present in the diffraction peak.
5. Cube alpha-Fe prepared in example 1 using high resolution O1s and Ti 2p XPS spectra 2 O 3 Core-shell Fe 2 O 3 @TiO 2 Core-shell Fe 2 O 3 @Ti x O y -P z Analysis was performed to evaluate whether the surface thereof was Ti 3+ Species. As in FIG. 6 a (curve α -Fe 2 O 3 And Fe (Fe) 2 O 3 @TiO 2 O1s pattern of (a), signal peaks at 529.6eV,531.4eV, and 532.9eV positions, respectively, correspond to O 2- (Fe-O bond or Ti-O bond), the surface adsorbs oxygen species (OH - ) And a metal hydroxide; three types of oxygen species were demonstrated to be present in these catalysts. From a in FIG. 6 (curve Fe 2 O 3 @Ti x O y -P z ) It can be seen that the O1s signal peak is significantly shifted towards higher binding energies, indicating the presence of Ti 3+ Species and Oxygen Vacancies (OVs). It can be seen in the high resolution XPS spectrum of Ti 2p (b, fe in FIG. 6 2 O 3 @TiO 2 Is a curve of (2) at 458.50eV (Ti 2 p) 3/2 ) And 464.37eV (Ti 2p 1/2 ) Two peaks were found at this point, which is attributed to surface Ti 4+ A species in a state. And Fe (Fe) 2 O 3 @Ti x O y -P z The sample showed a new peak at 460.73eV, which was of Ti 3+ Species, indicating that Ti is formed on the surface after phosphating 3+ Species.
In addition, for Fe 2 O 3 @Ti x O y -P z The high resolution P2P XPS spectrum of the sample doped P element was studied. C in FIG. 6 shows Fe 2 O 3 @Ti x O y -P z The sample has a distinct peak at 132.54eV, confirming that Fe after annealing and phosphating 2 O 3 @Ti x O y -P z P-Ti-O bonds are present in the sample. At the same time, the peak at 133.45eV belongs to the pentavalent oxidation state (P 5 + ). According to previous studies, the doped P atoms may be doped with cations (P 5+ ) Is added in the form of (2) and replaces Ti 4+ Ions lead to the formation of P-O-Ti bonds. Corresponding to Ti-P bonds (P was also found at 128.50eV 3- State) due to P 3- Instead of TiO 2 O atoms in the lattice induce Ti 3+ Active site generation.
Further, as can be seen from the electrochemical impedance spectrum (d in FIG. 6), fe 2 O 3 @Ti x O y -P z Semi-circle radius in high frequency region is smaller than Fe 2 O 3 @TiO 2 And cubic Fe 2 O 3 This indicates Fe 2 O 3 @Ti x O y -P z Has smaller charge transfer resistance and higher charge mobility; this is probably due to Ti x O y -P z Layer and Fe 2 O 3 The close interface contact between the two provides an effective path for charge transfer.
6、Ti 3+ Effect of active site on pNRR process: for the cube alpha-Fe prepared in example 1 2 O 3 Core-shell Fe 2 O 3 @TiO 2 Core-shell Fe 2 O 3 @Ti x O y -P z The samples were tested separately as follows: (a) Use of HITACHI UV-3900 spectrometer with BaSO 4 The ultraviolet visible diffuse reflectance spectra (DSR) of the different samples were recorded for reference. (b) Steady state Photoluminescence (PL) curves were obtained for the different samples on FLS1000 fluorescence lifetime spectrophotometers (Edinburgh Instruments, UK) under excitation with a hydrogen flash at a wavelength of 800 nm. (c) Using an electrochemical workstation (CHI Instruments CHI 760-1), at 0.1M Na 2 SO 4 In the electrolyte, transient photocurrent response curves of different samples are collected under the condition that a light source is switched on/off (turned on/off) every 20 seconds at an open point. (d) N for the different samples using Micromeritics Auto Chem II instrument with TCD as detector 2 Programmed temperature adsorption and desorption (N) 2 TPD) experiment.
UV-vis Diffuse reflectance Spectroscopy (UV-vis DRS) shows that the cube Fe alone 2 O 3 Has better visible light response (a in FIG. 7, curve cube Fe 2 O 3 ). With TiO 2 After recombination, the light response in the visible region is reduced and the ultraviolet response is enhanced (a, curve Fe in fig. 7 2 O 3 @TiO 2 ). Finally, the catalyst Fe after phosphating 2 O 3 @Ti x O y -P z Exhibits a strong absorption capacity in the visible region mainly due to Ti x O y -P z The surface is provided with Ti 3+ Sites (a in FIG. 7, fe 2 O 3 @Ti x O y -P z Curve).
Next, we performed Photoluminescence (PL) tests on the catalysts to further explore the kinetic behavior of the photogenerated carriers. As shown in b of fig. 7, fe is relative to the cube 2 O 3 And Fe (Fe) 2 O 3 @TiO 2 Obvious emission peak of Fe 2 O 3 @Ti x O y -P z Shows a weaker PL peak, indicating Fe 2 O 3 @Ti x O y -P z Has smaller recombination rate of electrons and holes.
Further examined the different catalysts under 300w xenon lamp irradiationInstantaneous photocurrent response, as shown by c in fig. 7. From the figure, fe can be seen 2 O 3 @Ti x O y -P z The photocurrent intensity of (C) is obviously higher than that of cubic Fe 2 O 3 And Fe (Fe) 2 O 3 @TiO 2 Confirm Fe 2 O 3 @Ti x O y -P z The carrier separation efficiency is good; this is mainly due to the fact that Fe is present in the cube 2 O 3 Coupled Ti x O y -P z The defect layer can induce formation of impurity level and carrier trapping center, thereby promoting separation of photogenerated electrons and holes and inhibiting charge recombination. On the other hand, N 2 The efficient chemisorption of the catalyst plays a key role in the photocatalytic nitrogen fixation process, which is generally believed to occur at the catalytically active site, a key step in the pNRR reaction process. Thus we use N 2 Programmed temperature adsorption and desorption (N) 2 -TPD) to characterize photocatalyst pair N 2 Is used for the adsorption capacity of the catalyst. In general, N of the material 2 The higher the TPD peak, the higher the pNRR activity. As shown by d in FIG. 7, fe is in the test range of 250-450 DEG C 2 O 3 @Ti x O y -P z N of (2) 2 Chemical desorption peak ratio cube Fe 2 O 3 And Fe (Fe) 2 O 3 @TiO 2 Much stronger, indicating that Fe 2 O 3 @Ti x O y -P z Ti of (2) 3+ The site can effectively adsorb N 2 A molecule. Due to N 2 The effective adsorption is the photocatalytic synthesis of NH 3 Thus Fe (S) 2 O 3 @Ti x O y -P z Superior N 2 Adsorption favors the overall pNRR process.
7. Detection of photocatalytic nitrogen fixation ammonia synthesis activity of different catalysts
Cube alpha-Fe prepared in example 1 was evaluated using the indoxyl method (specific methods are as described previously) 2 O 3 Core-shell Fe 2 O 3 @TiO 2 Core-shell Fe 2 O 3 @Ti x O y -P z Fe prepared in comparative example 6 2 O 3 -P z Ti prepared in comparative example 7 x O y -P z In the process, full spectrum solar energy is used as driving force, and water molecules are used as solvents for providing protons. As shown in a of fig. 8, in general, N 2 In the absence of illumination, almost no NH was detected 3 . Under the illumination condition, N is introduced 2 Photocatalytic NH for all catalysts under atmosphere 3 The yield increases gradually with time. After 180min of reaction, cubic Fe 2 O 3 Or Fe (Fe) 2 O 3 -P z The catalyst can generate a small amount of NH 3 While Fe 2 O 3 @Ti x O y -P z Catalyst synthesis of NH 3 The yield is obviously improved.
As can be seen from b in FIG. 8, the catalyst Fe 2 O 3 -P z Is NH synthesized by (2) 3 At a rate of 1.87. Mu. Mol g cat. -1 h -1 Only cubic Fe 2 O 3 (1.66μmol g cat. -1 h -1 ) 1.13 times of (2). Catalyst Ti x O y -P z The shell layer has a higher nitrogen fixation synthesis NH 3 Active but still unable to match Fe 2 O 3 @Ti x O y -P z Composite (15.65. Mu. Mol g) cat. -1 h -1 ) In comparison with the prior art. Fe (Fe) 2 O 3 @Ti x O y -P z Nitrogen fixation synthesis NH of composite catalyst 3 The rate is significantly higher than other catalysts, mainly due to coupled TiO 2 Or single defect Ti x O y -P z The layer can improve Fe 2 O 3 Nitrogen fixation synthesis of NH 3 Activity. In addition, fe 2 O 3 @Ti x O y -P z The activity of the compound is basically unchanged after 6 test experiments of cyclic test, the performance of the compound for synthesizing ammonia is not obviously reduced, and 98.1 percent of activity is still maintained (c in fig. 8); and utilize XRD spectrum to study Fe 2 O 3 @Ti x O y -P z The physicochemical properties before and after the cycling experiment, as shown in figure 9,Fe 2 O 3 @Ti x O y -P z the crystal structure did not change significantly before and after the cycling experiment, which confirmed that the structure had excellent photostability.
Using different monochromatic light (365, 420, 470, 535 and 630 nm), for catalyst Fe 2 O 3 @Ti x O y -P z The apparent quantum efficiency of (c) was studied as shown by d in fig. 8. Catalyst Fe 2 O 3 @Ti x O y -P z AQE values corresponding to monochromatic light wavelengths from 365nm to 630nm are 0.032%, 0.026%, 0.029%, 0.031% and 0.016%, respectively, with an acceptable solar energy utilization rate.
8. Mass ratio of titanium dioxide to iron oxide in the catalyst sample
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, respectively, were added to 20mL of a mixture of nitric acid, hydrofluoric acid and hydrochloric acid (volume ratio: 3:1:1) each in a mol/L ratio, and reacted in a microwave oven at 180℃for 20 minutes. After the completion of 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 4300 DV), 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 examples are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above examples, and any other changes, modifications, substitutions, combinations, and simplifications that do not depart from the spirit and principle of the present invention should be made in the equivalent manner, and the embodiments are included in the protection scope of the present invention.

Claims (6)

1. Core-shell type Fe 2 O 3 @Ti x O y -P z The application of the photocatalyst in synthesizing ammonia by photocatalysis and nitrogen fixation,characterized in that the core-shell Fe 2 O 3 @Ti x O y -P z The preparation method of the photocatalyst comprises the following steps:
(1) Preparation of cubic alpha-Fe by hydrothermal method 2 O 3 ；
(2) Preparation of core-shell Fe 2 O 3 @TiO 2 A nanocomposite; 0.1 part by mass of the alpha-Fe synthesized in the step (1) 2 O 3 Dispersing in 100-500 parts by volume of ethanol; adding 0.5-1 part by mass of polyvinylpyrrolidone into the mixed solution, stirring for 30min, adding 0-1.0 part by volume of tetrabutyl titanate, adding 50-200 parts by volume of ethanol solution, and continuously stirring for 10-18 h to obtain core-shell Fe 2 O 3 @TiO 2 A nanocomposite; wherein 1 part by mass: 1 volume = 1 g/mL;
(3) Core-shell Fe 2 O 3 @TiO 2 Phosphating the nanocomposite at 300 ℃ to obtain the core-shell Fe 2 O 3 @Ti x O y -P z A photocatalyst; wherein 0 < x < 1,0 < y < 2,0 < z < 10%;
the specific steps of the step (3) are as follows: 0.2 to 2 parts by mass of NaH 2 PO 2 And 0.1 part by mass of core-shell Fe 2 O 3 @TiO 2 Placing the nanocomposite in a tubular furnace; heating to 300-500 ℃ under the inert gas flowing condition, and continuing to react for 1-3 hours; collecting the reacted product, namely core-shell Fe 2 O 3 @Ti x O y -P z A photocatalyst; the inert gas is argon.
2. The use according to claim 1, characterized in that the specific steps of step (1) are as follows: feCl is added 3 •6H 2 Adding O into water, stirring to obtain a transparent solution, and adding 0.1-0.5 mol FeCl into 10-100 mL water 3 •6H 2 O; heating the solution to 60-100 ℃, then dropwise adding NaOH solution with the concentration of 5-6 mol/L into the solution, stirring for 5-15 min, and dropwise adding 20-100 mL of NaOH solution into 10-100 mL of water; the resulting mixture is then transferred to a polymerizationReacting in an autoclave with a tetrafluoroethylene lining at 80-150 ℃ for 48-72 h to obtain alpha-Fe 2 O 3 。
3. The use according to claim 2, wherein after completion of the reaction of step (1) further comprises: and cooling the autoclave to room temperature, centrifugally separating precipitate, washing the precipitate with water and ethanol for several times, and drying the precipitate at 60-80 ℃ overnight.
4. The use according to claim 1, wherein after completion of the reaction of step (2) further comprises: centrifugal washing with water and ethanol, drying at 70deg.C for 2h; the ethanol solution is aqueous ethanol, and each 50-mL ethanol solution contains 0-20 mL of water.
5. The use according to claim 1, wherein NaH is added in step (3) 2 PO 2 Placed in the center of a tubular furnace to make core-shell Fe 2 O 3 @TiO 2 The nanocomposite is placed on the downstream side of the tubular furnace at a distance of about 3-10 cm.
6. The use according to claim 1, wherein in step (3) the temperature is 1-10 ℃ for a period of time -1 Heating to 300-500 ℃.
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