CN1321742C - Visible light response photocatalyst and application thereof - Google Patents

Abstract

The present invention provides highly active photocatalysts for efficiently adsorbing middle ultraviolet light and visual light in sunlight, and a method for using the photocatalysts to decompose harmful chemical substances and decompose water to prepare hydrogen. The photocatalysts are composite oxide semiconductors formed from indium, an element in 5A of the periodic table of elements and a transition metal element M. Oxide semiconductors represented by InAO4(A is an element in 5A), and composite oxide semiconductors expressed by the formula of In<1-x>MxAO4 derived by displacing part of indium with the transitional metal M, wherein the total mole number of indium to M equals to the mole number of A. Under the irradiation of sunlight containing ultraviolet light and visual light, the photocatalysts can decompose harmful chemical substances and decompose water to prepare hydrogen.

Description

Visible light-responsive photocatalysts and their applications

1. Technical field:

The invention relates to photocatalysts responsive to ultraviolet and visible light and their applications, in particular to highly active photocatalysts composed of indium compound oxide semiconductors that can efficiently absorb ultraviolet and visible light in sunlight, and to use it to decompose harmful chemicals Matter and applications for making hydrogen.

2. Technical Background

At present, due to the constraints of fossil resources and the profound environmental and energy problems caused by their massive consumption, such as global warming and environmental degradation, etc., which directly affect the survival of human beings and the improvement of quality of life, human beings urgently need to develop green, safe and of new energy. As a source of new energy, although nuclear power generation has been put into use, there have been problems in safety and waste disposal.

One of the ways to solve the problem is to use solar energy. In one year, the amount of solar energy reaching the earth's surface is enormous, which is equivalent to 10,000 times the annual energy consumption of human beings. One of the purposes of research on visible light semiconductor photocatalysts is to simulate the photosynthesis of plants and develop artificial photosynthesis technology, so as to use the inexhaustible sunlight and water to directly produce hydrogen and oxygen as green fuels. The decomposition reaction of water, as shown in (1), is an energy accumulation type reaction. Considering that light is a necessary light reaction condition for oxygen generation, the photosynthetic reaction is the same as it, and it is also such a decomposition reaction:

H ₂ O＝H ₂ +1/2O ₂ formula (1)

Photocatalysts generate holes and electrons after absorbing photons with energies above their bandgap. These holes and electrons undergo oxidation and reduction reactions, respectively, to produce oxygen and hydrogen. Considering the practicability of photocatalysts, utilizing sunlight as a light source is indispensable. Among the sunlight irradiated to the surface, the intensity of visible light with a wavelength near 500nm is the highest, and the energy in the visible light region with a wavelength of 400nm-750nm is about 43% of the total energy of sunlight. On the other hand, ultraviolet rays with a wavelength of 440 nm or less are less than 5%. Therefore, in order to efficiently utilize the solar spectrum, people hope to find photocatalysts with catalytic activity under visible light.

However, although conventional semiconductor photocatalysts can generate hydrogen and oxygen under high-energy ultraviolet irradiation, there are very limited examples of hydrogen production using visible light-responsive semiconductor photocatalysts, and their activity is also low. Making full use of visible light is the basic premise for efficient use of sunlight.

In recent years, research on the application of photocatalysts to decompose harmful chemicals has attracted attention. Although titanium dioxide has been used to decompose organic substances such as pesticides and odorous substances in water and the atmosphere, and to apply it on solid surfaces for self-cleaning, titanium dioxide can hardly use visible light. If visible-light photocatalysts are available, the efficiency of the above applications can be greatly improved. The holes in the valence band in oxide semiconductors have a very strong oxidation ability, which can oxidize electron donors such as water and most organic substances, thereby releasing oxygen. The conduction band electrons have the ability to reduce water to generate hydrogen. That is to say, the conduction band energy level is higher than the oxidation potential of oxygen, and the valence band energy level is lower than the reduction potential of hydrogen, which is an indispensable condition for water splitting. Photocatalysts capable of splitting water to generate hydrogen and oxygen have strong oxidizing and reducing abilities, and thus can be expected to be used in the above-mentioned fields.

3. Contents of the invention

The object of the present invention is to provide a highly active photocatalyst composed of an indium-based composite oxide semiconductor that efficiently absorbs ultraviolet and visible light in sunlight, and a method for decomposing harmful chemical substances and producing hydrogen using it.

The purpose of the present invention is achieved like this:

1. A photocatalyst responsive to visible light, a compound oxide semiconductor formed by indium and A elements, and a divalent transition metal element M: In _1-x M _X AO ₄ , A element is at least one selected from Nb, Ta, V , the divalent transition metal element M is at least one selected from Cr, Mn, Fe, Co, Ni, Cu, Zn elements, 0<x<1, and these composite oxide semiconductors are used as photocatalysts.

2. An oxide semiconductor composed of indium and element A in the periodic table represented by InAO ₄ and an oxide semiconductor represented by the chemical formula In _1-x M _X AO ₄ after a part of the indium is substituted with a transition metal M has wolfranite A compound oxide semiconductor with crystal structure of type is the photocatalyst recorded in claim 1.

3. The oxide semiconductor that responds as visible light is the photocatalyst recorded in 1-3 above.

4. Use the composite oxide semiconductor represented by the chemical formula In _1-x Ni _x TaO ₄ or In _1-x Ni _x NbO ₄ as the photocatalyst recorded in 1-4 above.

5. A photocatalyst of the co-catalyst described in 1-5 above.

6. A method for decomposing harmful chemical substances by irradiating the photocatalyst in the above 1-6 with light rays containing ultraviolet rays and visible light.

7. A method of producing hydrogen by irradiating the photocatalyst in the above 1-6 with light rays including ultraviolet rays and visible light.

8. A method for producing hydrogen by completely decomposing water into hydrogen and oxygen by using the photocatalyst in the above 1-6 by irradiation with light containing ultraviolet light or visible light.

In the present invention, the photocatalyst used is an oxide with the expression In _1-x M _X AO ₄ , wherein M is a 2-valent metal element, which is obtained from Cr, Mn, Fe, Co, Ni, Cu, Zn Among other elements, A is a 5-valent element, and it is selected from V, Nb, Ta and other elements. The oxygen in the chemical formula uses 4 atoms in form, and the result actually obtained is less than 4 due to the existence of defects such as oxygen vacancies. indivual. It can be considered that if the number of divalent M increases, the number of oxygen will become less than four. X is a number that is greater than 0 and less than 1. The basic molecular formula is expressed as the general formula B ^3- A ^5- O ₄ , which has a wolfranite crystal structure, and x in the compound only needs to be within the stable range of the crystal structure. The composite oxide semiconductor of the present invention can be synthesized by a general solid-state reaction method, that is, by mixing oxides each having each metal component according to a stoichiometric ratio, and calcining it under normal air pressure. For ingredients that sublime easily, a little more is needed in the serving size. In addition, various methods such as various sol-gel methods and citric acid complex methods using metal alkoxides or metal salts as raw materials can also be used.

In order to utilize light more effectively, it is desirable that the shape of the photocatalyst in the present invention is a fine particle to obtain a larger surface area.

The oxide prepared by the solid state reaction method has larger particles and a smaller surface area, but the particle diameter can be reduced by pulverizing means such as a ball mill. The general particle size is about 10 μm-200 μm, preferably less than 50 μm. In addition, fine particles may be pressed into a plate shape and used. For the method of solid phase reaction, please refer to related inventions 92107282 and 98111231 of the applicant. Inventor: Xin Xinquan et al.

Common co-catalysts such as noble metals such as Pt and Pd, transition metals such as Ni and Co, oxides such as NiO and IrO2, _NiOx and _RuO2 can be modified on the surface of the semiconductor of the present invention as a photocatalyst with a microstructure. The preparation method can be impregnation method or electrodeposition method and the like. The impregnation method is to immerse the semiconductor oxide in an aqueous solution of compounds such as chlorides and nitrates of the active species of the oxide, and then dry it at 100-200°C for about 2-5 hours. -500°C), calcined in reducing gas and/or oxidizing gas for 2-5 hours. The amount of co-catalyst used is 0.01-10wt%, preferably 0.1-5wt%.

In addition, the reaction solution used for the complete decomposition reaction of water is not limited to pure water. In general, water splitting reactions can also be made to occur in brines where carbonates or bicarbonates, iodine salts, and bromide salts are usually dissolved.

The photocatalyst of this invention is added to the said aqueous solution. The amount of catalyst added should basically be the amount that ensures that the incident light can be efficiently absorbed. When the irradiation area is 25cm ² , the amount of photocatalyst added is 0.05-10g, preferably 0.2-3g. By irradiating the aqueous solution to which the photodecomposition catalyst is added in this way, water can be decomposed and oxygen can be generated. For example, the photocatalyst of the present invention can be added to pure water, and the water can be decomposed by irradiating visible light to generate oxygen and hydrogen in a stoichiometric ratio of 2:1. The spectrum of light used to illuminate the water must contain wavelengths that can be absorbed by the semiconductor. In the present invention, sunlight may also be irradiated.

The specific reaction method can be to suspend the catalyst in an aqueous solution containing organic matter for light irradiation, or to fix the catalyst on the base, and the liquid flows slowly on its surface, or to decompose the malodorous substance in the gasification chamber bed. gas phase reaction.

The photocatalyst of the present invention can not only be used for the complete decomposition of water, but also can be applied to various photocatalytic reactions. For example, when decomposing complex organic molecules, molecules of pollutants such as alcohol or pesticides, dyes, intermediates, and odors generally act as electron donors, and they are oxidized by holes to be decomposed, and at the same time, or reduced by electrons Hydrogen is produced, or oxygen is reduced by electrons. The device for the specific reaction can be that the catalyst is suspended in an aqueous solution containing organic matter for light irradiation, or that the catalyst can be fixed on a base, or it can be a gas phase reaction when decomposing malodorous substances.

The effect of the invention also includes: Regarding the photocatalyst that directly utilizes solar energy to completely decompose water, currently used photocatalysts are only active under ultraviolet rays, and cannot effectively utilize visible light that accounts for the vast majority of solar energy. Visible light can be used to split water into hydrogen and oxygen through the present invention. In the future, it may be possible to cover artificial pools with photocatalysts to efficiently mass-produce hydrogen from endless solar energy, which provides a promising way to solve energy problems.

4. Specific implementation

Example 1

In the chemical formula In _1-x M _x AO ₄ of the present invention, In _1-x M _x AO ₄ is synthesized using indium, elements M and A. Depending on the stoichiometric ratio, oxides of various compositions were synthesized by a solid-state method. The catalyst is adjusted by stoichiometric ratio (1-x) mol In ₂ O ₃ x=(0-1), x mol NiO (x=0-1) and 1 mol Ta ₂ O ₅ (Table 1). For example, when x=0.2, In _0.8 Ni _0.2 TaO ₄ weighs 3.201 g of In ₂ O ₈ , 0.431 g of NiO, and 6.368 g of Ta ₂ O ₅ . Put these raw materials into an alumina crucible, and after pre-sintering in an electric furnace at 900°C for 24 hours in air under normal pressure, repeat 3 times of calcination at 1200°C for 50 hours. After the calcination, the calcined product is ground into a powder with a size of 10 μm or less in a mortar. The chemical composition and crystal structure of the catalyst before and after the photoreaction were investigated using XRD and SEM-EDS. According to Rietveld's analysis, this kind of oxide has a monoclinic crystal structure, the space group is P2/C, and the crystal configuration is a layered Wolframite structure. Semiconductors with a Wolframite-type crystal structure allow electrons to move relatively easily. Through the measurement of ultraviolet light absorption spectrum, it can be obtained that its energy gap is below 2.5Ev, so it has the responsiveness of visible light.

The method of doping 1.0wt% NiOx co-catalyst into the above oxide semiconductor is achieved by impregnating Ni(NO ₃ ) ₂ aqueous solution, drying at 200°C for 5 hours, hydrogen reduction at 500°C, and oxidation at 200°C.

Suspend 0.5 g of NiOx/In _1-x Ni _x TaO ₄ in 250 ml of pure water to cause a photolysis reaction of water. Visible light is irradiated from the outside while performing magnetic stirring using a closed-cycle catalytic reaction device. A 300W xenon lamp was used as a light source, and a container made of borosilicate glass was used as a reaction tank. Light of a long wavelength is obtained by passing the light through a chopper filter (wavelength>420 nm), and then irradiates the photocatalyst. Detection and quantification of generated hydrogen and oxygen were performed by gas chromatography.

The experimental results show that the stoichiometric ratio of hydrogen and oxygen is 2:1, which proves that the complete decomposition of water is realized by relying on visible light. Hydrogen and the rate at which it occurs are shown in Table 1. This is very different from the performance when a part of In is replaced by Ni. In such semiconductors, the activity is maximum at x=0.1.

In In _1-x M _x AO ₄ , similar to the above examples, by replacing NiO with oxides such as IrO ₂ , CoO, NiO _x , RuO ₂ , CuO, and ZnO, similar composite oxides can be obtained with similar effects. .

Similarly, replacing Ta ₂ O ₅ with V ₂ O ₅ and Nb ₂ O ₅ , similar to the above-mentioned embodiment, a similar composite oxide can be obtained, and the effect is also similar.

Example 2

_RuO2 was used instead of NiOx in Example 1 as a cocatalyst. In1 _{- xNixTaO4 semiconductors} doped with 1.0wt% _RuO2 were achieved by impregnation with _RuCl4 aqueous solution, drying at 200 °C for 5 h, and calcination at 500 °C under oxidizing gas for 2 h. The results are shown in Table 1. Also at this time, it can be seen that water is completely decomposed under visible light. The embodiment of IrO2 is the same as above.

The electrodeposition method, that is, the electrolysis process, deposits Pt noble metals and Co transition metals on the In _1-x Ni _x TaO ₄ semiconductor to obtain a photocatalyst with a microstructure. The results are similar to Table 1. It is also possible to use Pt carbon aggregated on In _1-x Ni _x TaO ₄ and similar semiconductors.

Example 3

In order to confirm that the decomposition of organic matter can be carried out relatively efficiently under visible light, the decomposition of methanol was carried out in an aqueous solution. Pt (0.1 wt%) was used as a co-catalyst for In _0.9 Ni _0.1 TaO ₄ . 0.5 g of the photocatalyst was suspended in a mixed solution of 240 ml of pure water and 10 ml of methanol to cause a photolysis reaction. Visible light is irradiated from the outside while performing magnetic stirring using a closed-cycle catalytic reaction device. A 300W xenon lamp was used as a light source, and a container made of borosilicate glass was used as a reaction tank. The light from the light source passes through a chopper filter (wavelength>420nm) to obtain long-wavelength light, and then irradiates the sample (photocatalyst). Detection and quantification of generated hydrogen and oxygen were performed by gas chromatography. The experimental results showed that hydrogen was produced stably at a rate of 146 μmol/h, and no oxygen was produced. This indicates that under the irradiation of visible light, the following reactions occurred: the holes oxidatively decomposed methanol, and the electrons reduced water to generate hydrogen.

Decomposition of organic pollutants: It is used to remove complex chemical substances such as phenols, acids, ketones, and aldehydes in wastewater. You can refer to the treatment method with titanium dioxide: Southeast University CN01238388 "Photocatalytic Reactor for Treating Wastewater and Waste Gas.

This embodiment is used for the treatment of phenol-containing wastewater (production wastewater of terephthalic acid). With the above conditions, 0.5g of photocatalyst is put into 250ml of wastewater, and the xenon lamp is directly irradiated for 2 hours, and the COD removal rate reaches more than 80%.

This embodiment is used for the treatment of gulonic acid wastewater: put 0.5g of photocatalyst into 250ml of wastewater and directly irradiate it with a xenon lamp for 2 hours, and the decolorization effect is obvious by visual observation.

Example 4

In Example 3, Pt (1 wt %) was used as a co-catalyst for In _0.8 Cu _0.2 TaO ₄ , and Pt (1 wt %) was used as a co-catalyst for In _0.8 Fe _0.2 TaO ₄ . Put 0.5g of photocatalyst into the mixed solution of 240ml of pure water and 10ml of methanol and suspend it to cause the photolysis reaction of water. Visible light is irradiated from the outside while performing magnetic stirring using a closed-cycle catalytic reaction device. A 400W high-pressure mercury lamp was used as a light source, and a container made of borosilicate glass was used as a reaction tank to irradiate visible light and ultraviolet rays. Detection and quantification of generated hydrogen and oxygen were performed by gas chromatography. The experimental results show that hydrogen is stably produced at a rate of 100 μmol/h and 80 μmol/h, and no oxygen is produced. This indicates that the following reactions occurred: the holes oxidatively decompose methanol, and the electrons reduce water to produce hydrogen.

Comparative example 1

In Example 1, the activity of NiOx/InTaO ₄ and RuO2/InTaO ₄ which did not replace Ni was evaluated, but the activity of NiOx/In _lx Ni _x TaO ₄ and RuO2/In _1-x Ni _x TaO ₄ was evaluated. 1 is less active.

Comparative example 2

In the representative photocatalyst Pt-TiO2, there is no reaction only under visible light irradiation.

Table 1: Photocatalyst activity

example semiconductor Co-catalyst reaction Light source   Gas generation rate (μmol/h) H 0 Example 1 Example 2 In _1-x Ni _x TaO ₄ (x=0.05)(x=0.1)(x=0.15)In _1-x Ni _x TaO ₄ (x=0.05)(x=0.1) NiOxRuO2 water splitting water splitting Xe lamp (>420nm)Xe lamp (>420nm) 4.217.08.32.08.74.8 2.18.14.11.04.32.3 Comparative example 1 InTaO4TiO2 NiOxRuO2Pt Decomposition of water Xe lamp (>420nm)Xe lamp (>420nm) 3.20.8tr 1.10.40

Claims (9)

1. A photocatalyst responding to visible light, which is characterized by a compound oxide semiconductor formed by indium, element A, and a divalent transition metal element M: In _1-x M _x AO ₄ , the element A is selected from Nb, Ta, and V At least one of the divalent transition metal elements M is at least one selected from Cr, Mn, Fe, Co, Ni, Cu, Zn elements, 0<x<1, these composite oxide semiconductors are used as photocatalysts . 2. The photocatalyst responding to visible light according to claim 1, characterized in that the cocatalyst used for surface modification of the composite oxide semiconductor is Pt, Pd, Ni, Co, NiO, IrO ₂ , NiO _x or RuO ₂ . 3. The photocatalyst responding to visible light according to claim 2, characterized in that the amount of the cocatalyst is 0.01-10wt%. 4. The photocatalyst responding to visible light according to claim 3, characterized in that the amount of the cocatalyst is 0.1-5wt%. 5. The photocatalyst responding to visible light according to claim 1, characterized in that it adopts the chemical formula In _1-x Ni _x TaO ₄ or In _1-x Ni _x NbO ₄ . 6. The use of the visible light-responsive photocatalyst according to claim 1 or 2, characterized in that the catalyst is irradiated with light including ultraviolet light and visible light to decompose harmful chemical substances. 7. The use of the visible light-responsive photocatalyst according to claim 6: characterized in that: the reaction method is to suspend the catalyst in an aqueous solution containing organic matter for light irradiation, or to fix the catalyst on the base, and the liquid in it The surface flows slowly, or it is used for the gas phase reaction when the gasification chamber decomposes the malodorous substance. 8. Use of the photocatalyst responsive to visible light according to claim 1 or 2, characterized in that the catalyst is irradiated with light including ultraviolet light and visible light to produce hydrogen. 9. The use of the visible light-responsive photocatalyst according to claim 8, characterized in that the reaction method is to suspend the catalyst in an aqueous solution containing organic matter for light irradiation, or to fix the catalyst on the base, and the water slows down on the surface. Slow flow.