CN109225222B

CN109225222B - A composite photocatalyst and its application

Info

Publication number: CN109225222B
Application number: CN201710562701.8A
Authority: CN
Inventors: 王文中; 孙祥; 张玲
Original assignee: Shanghai Institute of Ceramics of CAS
Current assignee: Jiangsu Institute Of Advanced Inorganic Materials
Priority date: 2017-07-11
Filing date: 2017-07-11
Publication date: 2021-03-16
Anticipated expiration: 2037-07-11
Also published as: CN109225222A

Abstract

The present invention relates to a composite photocatalyst and application thereof. The composite photocatalyst comprises: an N ₂ activating co-catalyst, a photo-generated hole trapping co-catalyst, and a photo-catalyst serving as a carrier and providing photo-generated carriers; the N ₂ activating co-catalyst; The catalyst is an alkaline earth metal oxide; the photogenerated hole trapping cocatalyst is CoO _x or NiO _x , wherein 1≤x≤1.33; the photocatalyst serving as a carrier and providing photogenerated carriers is TiO ₂ or/and g‑ C ₃ N ₄ . The invention has great scientific value and practical significance for preparing high-activity photocatalytic ammonia synthesis material and constructing a mild and low energy consumption photocatalytic ammonia synthesis system.

Description

Composite photocatalyst and application thereof

Technical Field

The invention belongs to the field of artificial nitrogen fixation and ammonia synthesis, and relates to a composite photocatalyst and application thereof, which are suitable for the fields of environment, energy, materials and the like.

Background

Nitrogen fixation is the second most important chemical process in nature, second only to photosynthesis, and it has been desired to obtain a chemical nitrogen fixation system such as nitrogenase, which converts nitrogen and water in the atmosphere into ammonia at normal temperature and pressure. Theoretically, the ammonia synthesis reaction is a reaction which can not be carried out spontaneously in thermodynamics, and the triple bond energy 941 of the nitrogen moleculeKJ mol^-1The first ionization potential is 15.58eV, the stability is very high, the triple bond energy is the largest of all homonuclear diatomic molecules, and the N is difficult to be converted under normal temperature and pressure like nitrogen fixation enzyme₂And H₂O is converted to ammonia. The industrial production of synthetic ammonia is still based on high-temperature and high-pressure reaction conditions at present. The industrially employed process for the synthesis of ammonia is the Haber-Bosch process (N)₂+3H₂→2NH₃) The method uses a multi-component catalyst mainly containing iron and takes place at the temperature of about 500 ℃ and under the pressure of 20-50 MPa. The chemical nitrogen fixation effectively solves the shortage of biological nitrogen fixation, but has strict requirements on equipment and power, and the whole process has large energy consumption. In particular in the production of hydrogen by steam reforming of hydrocarbons, large amounts of CO₂Is discharged into the atmosphere as a by-product, increasing the environmental burden. Second, although ruthenium-based catalysts were developed as the second generation ammonia synthesis catalysts, high temperatures (greater than 300 ℃) were required to lower the energy barrier for activated nitrogen. The high-temperature catalyst can only react with H from the viewpoint of selection of proton source in ammonia synthesis reaction₂And (4) matching. Obviously, compare H₂，H₂The O is used as a proton carrier which is pollution-free, cheap and easy to obtain, and is more suitable for realizing the synthetic ammonia reaction with low energy consumption and no secondary pollution. However, H₂Catalytic dissociation of O molecules and achievement of same N₂The combined proton transfer process remains a bottleneck limiting the implementation of this reaction.

In recent years, some reports have been made on the synthesis of ammonia by a photocatalytic method, but most of the catalysts used are metal-metal oxides or noble metals supported on metal oxides, and the yield of ammonia is low, so that the application of ammonia has not been realized. The defects are mainly reflected in that: 1. the energy band position of the semiconductor photocatalytic material needs to be regulated and controlled, the utilization rate of light and the migration efficiency of photon-generated carriers are improved, and surface electrons/holes migrated to the surface have stronger oxidation-reduction property; 2. n required to construct the surface of the photocatalytic material₂Activating the sites and ensuring the stability of the sites; 3.N₂The formation of the reduction product requires the complexing of H⁺Ion or proton transfer, typically photocatalytic systems, by the selection of an organic sacrificial agent to provide H⁺A donor of an ion or a proton,however, from the application point of view, the sacrificial agent or the solvent increases the reaction cost and brings new problems of discharge and treatment of three wastes and the like.

Disclosure of Invention

In view of the above problems, the present invention aims to provide a high-activity catalyst for photocatalytic synthesis of ammonia and a method for constructing a solar-driven system for photocatalytic synthesis of ammonia at room temperature and normal pressure.

In one aspect, the present invention provides a composite photocatalyst, comprising: n is a radical of₂An activating cocatalyst, a photo-generated hole capturing cocatalyst and a photocatalyst which serves as a carrier and provides photo-generated carriers;

said N is₂The activating cocatalyst is alkaline earth metal oxide;

the photo-generated hole trapping cocatalyst is CoO_xOr NiO_xWherein x is more than or equal to 1 and less than or equal to 1.33;

the photocatalyst used as a carrier and providing photogenerated carriers is TiO₂Or/and g-C₃N₄。

The invention selects alkaline earth metal oxide as N₂Activating cocatalysts, CoO_xOr NiO_xAs photo-generated hole trapping co-catalyst, TiO₂Or/and g-C₃N₄A photocatalyst that acts as a support and provides photogenerated carriers. The invention utilizes inorganic semiconductor catalyst to efficiently catalyze and reduce N₂Can activate N efficiently under the irradiation of ultraviolet-visible light₂And dissociation of H₂O molecule, finally synthesizing NH₃。

Preferably, the alkaline earth metal oxide is CaO or MgO.

Preferably, N is₂The molar ratio of the activating cocatalyst to the photo-generated hole capturing cocatalyst is (0.1-10): 1.

preferably, the mass ratio of the photo-generated hole trapping cocatalyst to the photocatalyst serving as a carrier and providing a photo-generated carrier is (0.001-0.05): 1.

on the other hand, the invention also provides a method for synthesizing ammonia, wherein N is introduced under the irradiation of a light source₂/H₂O mixed gas, and H introduced by using the composite photocatalyst₂O and N₂React to form NH₃。

The invention aims at different reaction courses of a specific catalytic process of synthesizing ammonia by photocatalysis and nitrogen fixation, and comprises the following steps: n is a radical of₂Activation of molecules, separation of photogenerated carriers, capture of photogenerated holes, dissociation of water molecules and NH₃The method designs a composite photocatalyst and a system for catalyzing and synthesizing ammonia based on the composite photocatalyst, and plays the functions of each component to the maximum extent and realizes the capability of concerted catalysis of the components, as shown in figure 4. The invention first uses nanostructured alkaline earth metal oxides such as MgO and the like as N₂The activated catalyst utilizes surface delocalized electrons and N caused by surface defects of MgO material₂Molecular association to N₂ ^-Free radical species, and then the alkaline earth metal oxide is compounded with the photocatalyst, and the photo-generated electrons of the photocatalyst are utilized to further promote N₂Activating, decomposing water by photo-generated holes to obtain protons, and implementing N₂Hydrogenation synthesis of ammonia, and realization of catalytic series reaction.

Preferably, N is₂/H₂Middle N of O mixed gas₂/H₂The molar ratio of O is (10-100): 1, the flow rate is 60-100 ml/min. Preferably, the irradiation power of the light source is 50-500W.

The invention has the following characteristics:

(1) the invention has simple raw materials, wide sources and simple and controllable preparation process, and can realize macro preparation in a short time;

(2) the promoter involved in the invention does not contain noble metal, and is cheap and easy to obtain;

(3) the invention uses H₂O as N₂Reduction of the hydrogenated proton source to avoid the use of H₂Etc., causing environmental load;

(4) the invention has great scientific value and practical significance for preparing the high-activity photocatalytic synthetic ammonia material and constructing a mild and low-energy-consumption photocatalytic synthetic ammonia system.

Drawings

FIG. 1 is MgO-CoO_x/TiO₂X-ray powder diffraction pattern (XRD);

FIG. 2 shows MgO-CoO_x/TiO₂And TiO₂Ultraviolet-visible Diffuse Reflectance Spectrum (DRS);

FIG. 3 shows MgO-CoO_x/TiO₂Transmission Electron Micrographs (TEMs);

FIG. 4 shows MgO-CoO_x/TiO₂Schematic diagram of the photocatalytic synthesis of ammonia;

FIG. 5 shows MgO-CoO_x/TiO₂Yield-time plot of synthetic ammonia photocatalyst;

FIG. 6 shows MgO-CoOx/TiO₂With MgO/TiO₂Photocurrent versus graph of (a);

FIG. 7 shows MgO-CoOx/TiO₂With MgO/TiO₂The performance of the photocatalytic synthetic ammonia is compared;

FIG. 8 shows MgO-CoOx/TiO₂With CoOx/TiO₂The performance of the photocatalytic synthetic ammonia is compared with that of the photocatalytic synthetic ammonia.

Detailed Description

The present invention is further illustrated by the following examples, which are to be understood as merely illustrative and not restrictive.

The composite photocatalyst with high catalytic efficiency and low cost is prepared by activating N₂The catalyst comprises three parts of a cocatalyst for capturing photogenerated holes and a photocatalyst which is used as a carrier and provides photogenerated carriers.

Activation of N as described above₂Catalyst (N)₂The activating co-catalyst) may be an alkaline earth metal oxide, preferably at least one of MgO or CaO. The catalyst for capturing photogenerated holes (photogenerated hole capturing cocatalyst) is one of CoOx or NiOx, preferably CoO_x. The CoO_xOr NiO_xThe value range of x is 1-1.33. The photocatalyst used as a carrier and providing photogenerated carriers is TiO₂Or g-C₃N₄One kind of (1). Said N is₂The molar ratio of the activating cocatalyst to the photo-generated hole capturing cocatalyst can be (0.1-10): 1, preferably (0.6-4): 1. the photo-generated spaceThe mass ratio of the hole-trapping cocatalyst to the photocatalyst serving as the carrier and providing the photogenerated carriers can be (0.001-0.05): 1.

n is as defined above₂The activating cocatalyst, the photo-generated hole capturing cocatalyst and the photocatalyst serving as a carrier and providing photo-generated carriers can be quantum dots, quantum rods, nanowires, nanorods, nanosheets or nano composite structures with other shapes. The morphology and the synthesis mode of the composite photocatalyst are not limited: can be quantum dots, quantum rods, nanowires, nanorods, nanosheets or other semiconductor nanomaterials. The material disclosed by the invention is simple in components, cheap and easy to obtain, high in light utilization rate, suitable for macroscopic preparation and wide in application prospect.

The invention also provides a method for synthesizing ammonia by using the composite photocatalyst mixture, which comprises the following steps: composite photocatalyst, N₂Water and light. The method can efficiently reduce and generate corresponding reduction products under the irradiation of full spectrum or visible light.

The invention provides a catalytic system for synthesizing ammonia by using a composite photocatalyst, which mainly uses H under the condition of illumination by using the composite photocatalyst₂O to N₂Reduction to NH₃. As a photocatalytic reduction of N₂And NH is generated₃The operation steps of the method are as follows: (1) adding the composite photocatalyst into the reactor, and dispersing the composite photocatalyst in the reactor. (2) Introducing N into the reactor₂/H₂And (4) mixing gas O. (3) The photocatalyst in the reactor is irradiated by a light source and reacts with the introduced gas. (4) The obtained product is collected by dilute sulfuric acid aqueous solution, and the ammonia concentration is detected according to a national standard method (a nano-reagent spectrophotometry). The mass of the composite photocatalyst can be 0.1-0.5 g. The flow rate of the gas introduced into the reactor can be 60-100mL/min, and the N is₂/H₂Middle N of O mixed gas₂/H₂The molar ratio of O can be (10-100): 1. in addition, the reaction system can be carried out in a wide temperature range without particular requirements. The light source only needs to provide light for exciting the main catalyst in the composite photocatalyst to generate lightThe light source of the electron hole pair, such as the ultraviolet-visible light, may have an irradiation power of 50 to 500W. Preferably, the light source is an artificial light source or a natural light source.

The characterization means of the composite photocatalyst comprises the following steps: x-ray powder diffraction (XRD), spectroscopy, high resolution electron microscopy (HRTEM), and photoelectrochemical testing, among others.

The present invention will be described in detail by way of examples. It is also to be understood that the following examples are illustrative of the present invention and are not to be construed as limiting the scope of the invention, and that certain insubstantial modifications and adaptations of the invention by those skilled in the art may be made in light of the above teachings. The specific process parameters and the like of the following examples are also only one example of suitable ranges, i.e., those skilled in the art can select the appropriate ranges through the description herein, and are not limited to the specific values exemplified below.

Example 1MgO-CoO_x/TiO₂Preparation of synthetic ammonia photocatalyst

The experimental steps include: 1. first, 0.5g of nano TiO is weighed₂The powder was added to 50ml of water while maintaining stirring. 2. 2mmol of magnesium nitrate was added and the temperature was raised to 100 ℃ with constant stirring until the liquid was completely evaporated. 3. And taking out the dried powder, placing the dried powder in a muffle furnace, heating to 350 ℃, cooling, so as to realize the loading of the MgO cocatalyst 4, dispersing the roasted powder in 0.5mmol of cobalt nitrate aqueous solution, soaking for 3 hours, and filtering to obtain a filter cake. 5. Putting the filtered powder into a muffle furnace again, heating to 300 ℃, and cooling to realize CoO_xAnd (4) carrying the cocatalyst. The resulting sample was designated as MgO-CoO_x/TiO₂. FIG. 1 shows an X-ray powder diffraction pattern (XRD) of the synthesized sample, and as can be seen from FIG. 1, the main phase of the synthesized sample is TiO₂FIG. 3 is a Transmission Electron Micrograph (TEM) of the synthesized sample in which MgO is distributed on TiO₂Edge of grain, CoO_xThen grow on TiO₂The surface of the particles. By analyzing the ultraviolet-visible diffuse reflectance spectrum (fig. 2) of the sample, it can be seen that the loading of MgO and CoOx does not significantly change the TiO₂Focusing lightAbsorption of (2). FIG. 4 is a schematic diagram of the photocatalytic synthesis of ammonia from synthetic samples.

Example 2MgO-CoO_x/g-C₃N₄Preparation of synthetic ammonia photocatalyst

The experimental procedure consists of 1. first weighing 5g of melamine and 0.1g of magnesium nitrate, and mixing the two. 2. And placing the obtained mixture in a muffle furnace, heating to 550 ℃, and cooling. 3. The obtained powder was dispersed in a 1mmol aqueous solution of cobalt nitrate and impregnated, and then filtered to obtain a filter cake. 4. Putting the filtered powder into a muffle furnace again, and preserving heat at 300 ℃ to realize CoO_xAnd (4) carrying the cocatalyst. The resulting sample was designated as MgO-CoO_x/g-C₃N₄。

Example 3 operation method of synthesizing ammonia by composite photocatalyst

The experimental steps include: 1. MgO-CoO prepared in example 1_x/TiO₂The photocatalyst is put into a reactor, and the composite photocatalyst is dispersed in the reactor. 2. Introducing N into the reactor₂/H₂And (4) mixing gas O. 3. The photocatalyst in the reactor was irradiated with 500W xenon lamp to react with the introduced gas. 4. 0.2mmol of aqueous sulfuric acid solution as NH₃The absorption liquid is used for treating NH according to a nano-grade reagent spectrophotometry₃And (5) detecting the concentration. FIG. 5 shows MgO-CoO_x/TiO₂The yield of synthetic ammonia versus time is shown in FIG. 5, and it is understood that the synthetic ammonia efficiency of this sample is stable within 2 hours.

Comparative example 1

Comparative example 1 differs from example 1 in that: removal step 5, i.e. no loading of CoO as a photo-generated hole trapping promoter_xIs marked as MgO/TiO₂The rest of the contents are exactly the same as those described in example 1. The non-loaded CoO is obtained by photoelectrochemical analysis_xThe photocurrent density of the sample is less than that of MgO-CoO_x/TiO₂Sample, from which CoO was known_xThe catalyst can promote separation of photogenerated carriers as a catalyst for trapping photogenerated holes. In combination with the photoelectrochemical results, CoO was also demonstrated_xRecombination of photogenerated electrons and holes can be suppressed, see fig. 6.

Comparative example 2

This comparative example differs from example 3 in that: the photocatalyst used was MgO/TiO as described in comparative example 1₂The rest of the contents are exactly the same as those described in example 3. The CoO is loaded by the performance test analysis_xThe sample shows better performance of photo-catalytic synthesis of ammonia, reaches 340 mu mol/g, compared with MgO/TiO₂Sample, MgO-CoOx/TiO₂The photocatalytic synthesis of ammonia performance improved by 2 times, see figure 7.

Comparative example 3

Comparative example 3 differs from example 1 in that: removing step 3, i.e. not loaded as N₂Activating cocatalyst MgO, noted CoOx/TiO₂The rest of the contents are exactly the same as those described in example 1. This comparative example differs from example 3 in that: the photocatalyst used was CoO as described in comparative example 3_x/TiO₂The rest of the contents are exactly the same as those described in example 3. Through performance test analysis, the sample loaded with MgO shows better performance of photocatalytic synthesis ammonia, reaches 340 mu mol/g, and is compared with CoO_x//TiO₂Sample, MgO-CoO_x/TiO₂The photocatalytic synthesis of ammonia performance improved by 10 times, see figure 8.

Claims

1. a composite photocatalyst, is characterized in that, comprises: N ₂ activation cocatalyst, photogenerated hole capture cocatalyst and the photocatalyst that serves as carrier and provides photogenerated carrier;

_The N activation cocatalyst is an alkaline earth metal oxide;

The photogenerated hole trapping promoter is CoO _x or NiO _x , wherein 1≤x≤1.33;

The photocatalyst serving as a carrier and providing photogenerated carriers is TiO ₂ or/and gC ₃ N ₄ ;

The alkaline earth metal oxide is CaO or MgO;

The molar ratio of the N ₂ activation co-catalyst and the photo-generated hole trapping co-catalyst is (0.1-10): 1;

The mass ratio of the photogenerated hole trapping promoter and the photocatalyst serving as a carrier and providing photogenerated carriers is (0.001-0.05):1.

2. A method for synthesizing ammonia, characterized in that, under the irradiation of a light source, N ₂ /H ₂ O mixed gas is introduced, and the composite photocatalyst according to claim 1 is utilized to make the introduced H ₂ O and N ₂ react _NH3 is formed.

3 . The method according to claim 2 , wherein the molar ratio of N ₂ /H ₂ O in the N ₂ /H ₂ O mixed gas is (10-100):1, and the flow rate is 60-100 mL. 4 . minute.

The method according to claim 2 or 3, wherein the irradiation power of the light source is 50-500W.