WO2020093517A1

WO2020093517A1 - Photocatalytic material for efficient and selective reduction and removal of nitrate nitrogen in water, and preparation method thereof

Info

Publication number: WO2020093517A1
Application number: PCT/CN2018/120813
Authority: WO
Inventors: 王津南; 候志昂; 吴超越
Original assignee: 南京大学; 南京大学盐城环保技术与工程研究院
Priority date: 2018-11-06
Filing date: 2018-12-13
Publication date: 2020-05-14
Also published as: CN109225215A

Abstract

Disclosed is a method for preparing a photocatalytic material for efficient and selective reduction and removal of nitrate nitrogen in water, comprising the following steps: mixing gadolinium nitrate nonahydrate, chromium trioxide and urea and placing the mixture into an alumina crucible, then adding absolute ethanol and stirring the mixture evenly; placing the crucible into a muffle furnace to conduct calcination; grinding the solid obtained after calcination in a mortar to obtain GdCrO₃ powder; adding the obtained GdCrO₃ powder to a palladium chloride solution, then adding methanoic acid and performing UV light irradiation, whereby palladium chloride is reduced to a palladium simple substance, which is deposited onto the surface of GdCrO₃, affording Pd/GdCrO₃; and finally, performing centrifuging, washing and drying.

Description

Photocatalytic material with high efficiency and selective photocatalytic reduction of nitrate nitrogen in water and preparation method thereof

Technical field

The invention belongs to the field of environmental functional materials, and in particular relates to a photocatalytic material with high efficiency and selective photocatalytic reduction of nitrate nitrogen in water and a preparation method thereof.

Background technique

The rapid development of global agriculture, the extensive use of artificial nitrogen fertilizer, and the discharge of industrial sewage have made the pollution of nitrate nitrogen in water increasingly serious. Nitrate nitrogen in water will not only cause eutrophication of the water body, but also can be converted into nitrite nitrogen, which seriously threatens human health. Many governments and organizations have adopted more and more stringent standards for the control of nitrate nitrogen concentration in water. In this way, the removal of nitrate nitrogen in water has become a hot and difficult point in the field of water treatment. Traditional methods for removing nitrate nitrogen include ion exchange, reverse osmosis, electrolysis, biological denitrification, and chemical reduction. However, different levels of inefficiency, high cost and complex operating conditions limit the wide application of the above methods. The essence of photocatalysis is that under the excitation condition of light, the "electron-hole pair" produced by the photocatalyst and the target substance undergo redox reactions on the valence band and conduction band of the catalyst, respectively. The photocatalyst represented by titanium dioxide has been widely concerned and developed in the field of environmental protection due to its advantages of good stability, low cost, no toxicity and secondary pollution. However, the photocatalyst treatment technology of titanium dioxide still has the following defects: (1) the photocatalytic reduction of pure titanium dioxide has a low efficiency of removing nitrate nitrogen; (2) the photocatalytic reduction of pure titanium dioxide photocatalyst has poor selectivity for nitrate, and it is easy to generate nitrous oxide with higher concentration Nitrogen, ammonia nitrogen and other by-products; (3) The method of controlling the morphology of titanium dioxide and exposing its highly catalytically active crystal planes to prepare nano-titanium dioxide is cumbersome and time-consuming, and still has the problem of more by-products.

The Chinese patent with the patent application number 2006100461728 discloses a method for photocatalytic removal of nitrogen in water. The patent discloses a method of using metal-supported titanium dioxide nanocatalyst or nitrogen-doped titanium dioxide composite catalyst to remove dissolved oxygen by passing nitrogen or argon Under the conditions, the photocatalytic oxidation of ammonia nitrogen and the photocatalytic reduction of oxidation nitrogen are coupled, with nitrogen as the target product, and the nitrogen-containing components are taken out of the water. However, the catalytic material disclosed in this patent has a total nitrogen removal rate of only 38% after loading with precious metal silver (ammonia nitrogen and nitrous nitrogen removal rates are 48% and 27%, respectively). 64% (ammonia nitrogen and nitrous nitrogen are 88% and 43% respectively); In addition, the introduction of ferrous iron ions will produce iron sludge, which may cause secondary pollution of the water body, and the loading of precious metals also leads to an increase in production costs.

The Chinese patent with patent application number 201610891842 discloses a method for photocatalytic reduction of nitrate nitrogen in water. The patent discloses an Ag-Ag ₂ O / TiO ₂ composite photocatalyst agent, which is used as an electron donor in formic acid In the case of a photocatalytic reduction of nitrate nitrogen in water. However, the preparation process of the catalytic material involved in this patent is cumbersome, takes a long time (at least 15 hours or more is required), and has low output, making it difficult to prepare and apply it on a large scale.

The Chinese patent with the patent application number 2015102738202 discloses a precious metal nanoparticle-selectively modified titanium dioxide material and its preparation method and application. The patent discloses a preparation method and application of a precious metal nanoparticle-selectively modified titanium dioxide-based photocatalytic material The reduction and removal of nitrate nitrogen in water. This material first needs to prepare nano- or micro-scale titanium dioxide crystals with excellent surface growth. The reactants need to be kept at 80-240 ° C for 48-96 hours. After that, the product needs to be washed 5-8 times, dried for 10 hours and then ground. After the precious metal modification, the steps of water washing, drying and grinding are required again. The whole preparation process has complicated steps and takes a long time. Although it has a high catalytic conversion rate, it does not necessarily have a high selectivity, and the stability of the catalyst has not been evaluated.

Therefore, on the basis of the above research, overcoming the traditional titanium dioxide requires the use of holes and hydroxyl radicals in the valence band to oxidize hole traps such as formic acid to carbon dioxide radicals, which relies heavily on carbon dioxide radicals to reduce nitrate nitrogen and is easy The production of hydroxyl radicals to oxidize intermediate products leads to the disadvantages of low catalytic efficiency and poor selectivity. The preparation of a catalytic material that improves the efficiency of photocatalytic reduction of nitrate will become a new research direction.

Summary of the invention

The purpose of the present invention is to solve the problems of low efficiency, poor selectivity and difficulty in controlling the removal of nitrate nitrogen in the current photocatalyst of titanium dioxide, and to provide a high-efficiency selective photocatalytic material for reducing nitrate nitrogen in water and a preparation method thereof .

The present invention is achieved through the following technical solutions:

A high-efficiency selective reduction method for preparing photocatalytic material for removing nitrate nitrogen in water includes the following steps:

(1) Mix gadolinium nitrate, chromium trioxide and urea in an alumina crucible, add absolute ethanol and stir well;

(2) Place the crucible in a muffle furnace and calcine it;

(3) Grind the solid obtained after calcination with a mortar to obtain GdCrO ₃ powder;

(4) Place the GdCrO ₃ powder prepared in step (3) in a palladium chloride solution, add formic acid and irradiate with ultraviolet light, palladium chloride is reduced to palladium elemental and deposited on the surface of GdCrO ₃ , and finally centrifuged, washed, After drying, the photocatalytic material Pd / GdCrO _{3 is prepared} .

The preparation method of the photocatalytic material for efficiently and selectively reducing nitrate nitrogen in water according to the present invention, the technical solution to which is further solved is that the gadolinium nitrate nonahydrate, chromium trioxide and urea in the step (1) The molar ratio is 1: (0.5-1): 4.

A method for preparing a photocatalytic material for efficiently and selectively reducing nitrate nitrogen in water according to the present invention, the technical solution for which is further solved is that the volume ratio of anhydrous ethanol to the mixed reactant in the step (1) is 1 :2.

The method for preparing a photocatalytic material for efficiently and selectively reducing nitrate nitrogen in water according to the present invention, the technical solution to which is further solved is that the heating temperature of the muffle furnace in the step (2) is 800 ° C, and the heating rate is It is 5-8 ℃ / min, and the reaction time is 4h.

The method for preparing a photocatalytic material for efficiently and selectively reducing nitrate nitrogen in water according to the present invention, the technical solution for further solving is that the molar ratio of formic acid to palladium chloride in the step (4) is (2- 4): 1.

The method for preparing a photocatalytic material for efficiently and selectively reducing nitrate nitrogen in water according to the present invention has a technical solution for further solving that the mass ratio of palladium to GdCrO ₃ in the step (4) is (0.5-2 ): 100; the ultraviolet light irradiation time is 100-120 min.

The selective efficient photocatalytic material obtained by the above production method and its application, are the scope of the present invention, the photocatalytic material is Pd nanoparticles and composite GdCrO _3, ₃ having an irregular GdCrO nano Plate-like morphology, Pd is attached to the surface of the GdCrO ₃ in the form of elemental nanoparticles, and the average diameter of Pd in the photocatalytic material is 5.7 nm. The photocatalytic material prepared above needs to be used in a liquid environment and can be reduced and removed Nitrate nitrogen in water.

The invention uses chromium trioxide, gadolinium nitrate, urea, absolute ethanol, palladium chloride, formic acid and the like as raw materials and can be prepared by calcining at high temperature for a short time, because the prepared catalyst has a relatively negative conduction band value (-2.02 V vs NHE), the conduction band electron reduction ability is extremely strong, nitrate nitrogen is mainly reduced by electrons in the conduction band, which improves the efficiency of photocatalytic reduction of nitrate, and the load of Pd not only further improves the photocatalytic efficiency of GdCrO ₃ , And it is conducive to the conversion of nitrous nitrogen to nitrogen, which significantly improves the selectivity of photocatalytic reduction of nitrate.

The new GdCrO ₃ photocatalytic material prepared by the present invention has the following advantages compared with the traditional titanium dioxide-based catalyst:

1. The prepared photocatalytic material has high reduction catalytic activity and has a faster reaction rate.

2. The removal rate of nitrate nitrogen and the selectivity of generating nitrogen are high, and the production of by-products such as nitrous nitrogen and ammonia nitrogen is low.

3. The preparation process is simple, time-consuming and high yield.

BRIEF DESCRIPTION

In FIG. 1, (a) is an SEM image of GdCrO ₃ according to the present invention; (b) is a TEM image of de GdCrO ₃ according to the present invention; (c) is a TEM image of Pd / GdCrO ₃ according to the present invention ; (d) Pd nanoparticles of the present invention, the particle size distribution; (e) according to the present invention, Pd / GdCrO HRTEM _{FIG. 3;} (f) of the present invention Pd / GdCrO EDS ₃ of Figure.

FIG. 2 is a GdCrO ₃ electron spectrum after Pd loading and a 3d orbit electron spectrum of Pd according to the present invention.

3 is a schematic diagram of the photocatalytic reaction device of the present invention.

4 is a graph showing the effect of GdCrO ₃ photocatalytic reduction of nitrate nitrogen in water in Comparative Example 1. FIG.

FIG. 5 is an effect diagram of 0.5% wt Pd / GdCrO ₃ photocatalytic reduction of nitrate nitrogen in water in Example 2. FIG.

6 is a graph of the effect of 1% wt Pd / GdCrO ₃ photocatalytic reduction of nitrate nitrogen in water in Example 3. FIG.

7 is a graph of the effect of 2% wt Pd / GdCrO ₃ photocatalytic reduction of nitrate nitrogen in water in Example 4. FIG.

8 is a graph of the effect of recycling 1% wt Pd / GdCrO ₃ photocatalytic reduction of nitrate nitrogen in water in Example 5. FIG.

9 is an XPS diagram of the material in Example 5 before and after the reaction.

10 is an XRD pattern of the material before and after the reaction in Example 5. FIG.

The serial numbers in the picture, 1- reaction vessel, 2- thermometer, 3- sampling tube, 4- high-pressure mercury lamp, 5- magnetic stirrer.

detailed description

The invention content of the present invention will be further described below with reference to the drawings and embodiments.

Example 1

Material preparation process:

(1) Mix gadolinium nitrate, chromium trioxide and urea in the ratio of 1: 0.5: 4 according to the amount of substance and place it in an alumina crucible, add 10mL of absolute ethanol and stir without reagent;

(2) Place the crucible in a muffle furnace, raise it to 800 ° C at a temperature increase rate of 5 ° C / min, and then calcinate at 800 ° C for 4 hours;

(3) Grind the solid obtained after calcination with a mortar to prepare GdCrO ₃ powder;

(4) Add 2g of the GdCrO ₃ powder prepared in step (3) to the palladium chloride solution, add formic acid as an electron donor to reduce palladium chloride to palladium elemental substance by ultraviolet light and deposit it on the surface of GdCrO _{3 to} obtain Pd / GdCrO ₃ , Finally, Pd / GdCrO ₃ photocatalytic material is obtained by centrifugation, washing and drying; the mass ratio of palladium and GdCrO ₃ in palladium chloride is 0.5: 100, the molar ratio of formic acid and palladium chloride is 2: 1, and the light time is 120min.

The SEM, TEM and EDS characterization of the products prepared in each of the above steps can be seen from Figure 1, the prepared material contains elements such as Gd, Cr, O, Pd, etc. GdCrO ₃ has an irregular nano-sheet morphology, Pd The nanoparticles in elemental form are attached to the surface of the GdCrO ₃ and the average diameter of Pd is 5.7 nm.

Example 2

(1) Mix gadolinium nitrate, chromium trioxide and urea in a ratio of 1: 1: 4 in the amount of substance and place it in an alumina crucible, add 10mL of absolute ethanol and stir without reagents;

(2) Place the crucible in a muffle furnace, raise it to 800 ° C at a heating rate of 6 ° C / min, and then calcinate at 800 ° C for 4 hours;

(4) Add 2g of the GdCrO ₃ powder prepared in step (3) to the palladium chloride solution, add formic acid as an electron donor to reduce palladium chloride to palladium elemental substance by ultraviolet light and deposit it on the surface of GdCrO _{3 to} obtain Pd / GdCrO ₃ , Finally, Pd / GdCrO ₃ photocatalytic material is obtained by centrifugation, washing and drying; the mass ratio of palladium and GdCrO ₃ in palladium chloride is 0.5: 100, the molar ratio of formic acid and palladium chloride is 2: 1, and the light time is 120min;

(5) Put the sodium nitrate solution with nitrate concentration of 50mg / L (0.8mmol) in the photoreactor, add 0.5wt% of the Pd / GdCrO3 catalyst prepared in step (4), and the dosage is 0.5g / L. Under the condition of magnetic stirring speed of 350rpm, dark adsorption for 30min, and then add 1mL of formic acid solution, the concentration of the formic acid solution is 1mol / L; turn on the cooling and water bath device to maintain the reaction temperature at 25 ℃, turn on the ultraviolet light source of 350W high pressure mercury lamp for photocatalysis Reducing nitrate nitrogen reaction, the time is 100min.

The denitrification effect is shown in Figure 5. After 100 minutes of photocatalytic reduction reaction, the removal rate of nitrate by GdCrO ₃ loaded with 0.5wt% Pd reached 92.8%, and the ammonia nitrogen produced during the reaction has been maintained at a low level of less than 1 %, The nitrous nitrogen content was also significantly inhibited, and the content was only 5.1%. Due to the selective conversion of nitrous nitrogen to ammonia nitrogen under the catalytic action of Pd, the nitrogen selectivity was greatly increased to 94.1%. It fully shows that the loading of 0.5wt% Pd not only improves the efficiency of photocatalytic reduction of nitrate, but also improves the nitrogen selectivity.

Example 3

(4) Add 2g of the GdCrO ₃ powder prepared in step (3) to the palladium chloride solution, add formic acid as an electron donor to reduce palladium chloride to palladium elemental substance by ultraviolet light and deposit it on the surface of GdCrO _{3 to} obtain Pd / GdCrO ₃ , Finally, Pd / GdCrO ₃ photocatalytic material is obtained by centrifugation, washing and drying; the mass ratio of palladium and GdCrO ₃ in palladium chloride is 1: 100, the molar ratio of formic acid and palladium chloride is 3: 1, and the light time is 120min;

(5) Put the sodium nitrate solution with a nitrate concentration of 50 mg / L (0.8 mmol) in the photoreactor, add 1 wt% of the Pd / GdCrO ₃ catalyst prepared in step (4), and the dosage is 0.5 g / L. Under the condition of magnetic stirring speed of 350rpm, dark adsorption for 30min, and then add 1mL of formic acid solution, the concentration of the formic acid solution is 1mol / L; turn on the cooling and water bath device to maintain the reaction temperature at 25 ℃, turn on the ultraviolet light source of 350W high pressure mercury lamp for photocatalysis Reducing nitrate nitrogen reaction, the time is 100min.

The denitrification effect is shown in Figure 6. After 100 minutes of photocatalytic reduction reaction, the removal rate of nitrate by GdCrO ₃ loaded with 1wt% Pd reached 98.7%, and the ammonia nitrogen generated during the reaction continued to be close to 0, and the nitrous nitrogen content It also dropped from the highest value at 20 minutes to 0, and the nitrogen selectivity reached 100%. It shows that the load of 1wt% Pd is the optimal load.

Example 4

(4) Add 2g of the GdCrO ₃ powder prepared in step (3) to the palladium chloride solution, add formic acid as an electron donor to reduce palladium chloride to palladium elemental substance by ultraviolet light and deposit it on the surface of GdCrO _{3 to} obtain Pd / GdCrO ₃ , Finally, Pd / GdCrO ₃ photocatalytic material is obtained by centrifugation, washing and drying; the mass ratio of palladium and GdCrO ₃ in palladium chloride is 2: 100, the molar ratio of formic acid and palladium chloride is 4: 1, and the light time is 120min;

(5) the nitrate concentration of 50mg / L (0.8mmol) of sodium nitrate was placed photoreactor, 2wt% added in step (4) Preparation of Pd / GdCrO ₃ catalyst, dosage of 0.5g / L, Dark adsorption for 30 min under the condition of magnetic stirring speed of 350 rpm, then add 1 mL of formic acid solution, the concentration of the formic acid solution is 1 mol / L; turn on the cooling and water bath device to maintain the reaction temperature at 25 ℃, turn on the ultraviolet light source of 350 W high-pressure mercury lamp for photocatalysis Reducing nitrate nitrogen reaction, the time is 100min.

The denitrification effect is shown in Figure 7. After 100 minutes of photocatalytic reduction reaction, the removal rate of nitrate by GdCrO ₃ loaded with 2wt% Pd reached 93.1%, and the ammonia nitrogen produced during the reaction process has been maintained at a low level and finally reduced At 0, the nitrous nitrogen content decreased from the highest concentration at 20 minutes to 4.4%, and the nitrogen selectivity was greatly increased to 95.1%. It shows that the loading of 2wt% Pd also improves the efficiency of photocatalytic reduction of nitrate and nitrogen selectivity, but it is not the optimal loading. If the loading of Pd is too much, it may become the recombination center of the electron hole of the photocatalytic material, making The photocatalytic efficiency drops.

Example 5

(4) Add 2g of the GdCrO ₃ powder prepared in step (3) to the palladium chloride solution, add formic acid as an electron donor to reduce palladium chloride to palladium elemental substance by ultraviolet light and deposit it on the surface of GdCrO _{3 to} obtain Pd / GdCrO ₃ , Finally, Pd / GdCrO ₃ photocatalytic material is obtained by centrifugation, washing and drying; the mass ratio of palladium and GdCrO ₃ in palladium chloride is 1: 100, the molar ratio of formic acid and palladium chloride is 3: 1, and the light time is 100min;

(5) Put the sodium nitrate solution with a nitrate concentration of 50 mg / L (0.8 mmol) in the photoreactor, add the 12 wt% Pd / GdCrO ₃ catalyst prepared in step (4), and the dosage is 0.5 g / L. Under the condition of magnetic stirring speed of 350rpm, dark adsorption for 30min, and then add 1mL of formic acid solution, the concentration of the formic acid solution is 1mol / L; turn on the cooling and water bath device to maintain the reaction temperature at 25 ℃, turn on the ultraviolet light source of 350W high pressure mercury lamp for photocatalysis Reducing nitrate nitrogen reaction, the time is 100min.

The effect of photocatalytic reduction of nitrate nitrogen recycling in water is shown in Figure 8. After 6 cycles of recycling, the removal rate of nitrate nitrogen and nitrogen selectivity reached 98.7% and 100%, respectively. Significantly decreased.

The XPS and XRD characteristics of the material before and after the reaction are shown in Figures 9 and 10. It can be seen from the figure that the crystal structure of the material before and after the reaction has not changed significantly, and Pd exists in a simple state, indicating that the material has better stability.

Comparative Example 1

(4) Put the sodium nitrate solution with a nitrate concentration of 50 mg / L (0.8 mmol) in the photoreactor, add 2 g of the GdCrO ₃ powder prepared in step (3) above, and add 0.5 g / L in the magnetic field. Dark adsorption for 30 min at a stirring speed of 350 rpm, followed by the addition of 1 mL of formic acid solution, the concentration of the formic acid solution is 1 mol / L; turn on the cooling and water bath device to maintain the reaction temperature at 25 ° C., and turn on the ultraviolet light source of the 350 W high-pressure mercury lamp for photocatalytic reduction of nitr State nitrogen reaction, the time is 120min.

The denitrification effect is shown in Figure 4. After 100 minutes of photocatalytic reduction reaction, the nitrate removal rate reached 9%. Although the ammonia nitrogen generated during the reaction was maintained at 2.8%, the nitrite nitrogen content reached 11.8%, and the nitrogen selectivity was only 81.4%.

In summary, compared with the traditional photocatalytic materials, the photocatalytic materials prepared by the present invention for high-efficiency selective reduction and removal of nitrate nitrogen in water have simple preparation, high yield, high reduction catalytic activity, and fast reaction rate. The removal rate of nitrate nitrogen and the selectivity of nitrogen reached 98.7% and 100%, respectively, and the effect of 6 cycles of use was not significantly reduced, and it had good catalytic activity and stability.

The above is only the preferred embodiment of the present invention. It should be pointed out that for those of ordinary skill in the art, without departing from the inventive concept of the present invention, several modifications and improvements can be made, which belong to this The scope of protection of the invention.

Claims

A high-efficiency selective reduction method for preparing photocatalytic material for removing nitrate nitrogen in water is characterized in that it includes the following steps:

(1) Mix gadolinium nitrate, chromium trioxide and urea in an alumina crucible, add absolute ethanol and stir well;

(2) Place the crucible in a muffle furnace and calcine it;

(3) Grind the solid obtained after calcination with a mortar to obtain GdCrO 3 powder;

(4) Place the GdCrO 3 powder prepared in step (3) in a palladium chloride solution, add formic acid and irradiate with ultraviolet light, palladium chloride is reduced to palladium elemental and deposited on the surface of GdCrO 3 , and finally centrifuged, washed, After drying, the photocatalytic material Pd / GdCrO 3 is prepared .
The method for preparing a photocatalytic material for efficiently removing nitrate nitrogen in water according to claim 1, wherein the molar ratio of gadolinium nitrate, dichromium trioxide and urea in step (1) is 1 : (0.5-1): 4.
The method for preparing a photocatalytic material for efficiently removing nitrate nitrogen in water according to claim 1, wherein the volume ratio of anhydrous ethanol to the mixed reactant in step (1) is 1: 2.
The method for preparing a photocatalytic material for efficiently removing nitrate nitrogen in water according to claim 1, wherein the heating temperature of the muffle furnace in step (2) is 800 ° C, and the heating rate is 5-8 ℃ / min, the reaction time is 4h.
The method for preparing a photocatalytic material for efficiently removing nitrate nitrogen in water according to claim 1, wherein the molar ratio of formic acid to palladium chloride in step (4) is (2-4): 1 .
The method for preparing a photocatalytic material for efficiently removing nitrate nitrogen in water according to claim 1, wherein in step (4), the mass ratio of palladium to GdCrO 3 is (0.5-2): 100; The ultraviolet light irradiation time is 100-120 min.
A high-efficiency selective reduction photocatalytic material for removing nitrate nitrogen in water, characterized in that it is prepared by the method according to any one of claims 1-6. The photocatalytic material is a composite of Pd nanoparticles and GdCrO 3 , GdCrO 3 has an irregular nano-sheet morphology, and Pd is attached to the surface of the GdCrO 3 in the form of elemental nanoparticles.
The photocatalytic material for efficiently and selectively removing nitrate nitrogen in water according to claim 7, wherein the average diameter of Pd in the photocatalytic material is 5.7 nm.
The photocatalytic material according to claim 7 is used for reducing and removing nitrate nitrogen in water.