Preparation method and application of an iron-based catalyst

Abstract

The present invention belongs to the field of catalysts, and particularly relates to a preparation method and application of an iron-based catalyst. Activated carbon pretreated with citric acid is used as a carrier to change the types and amounts of functional groups on the catalyst surface. Using ammonia water as a reducing agent, iron is used as the main active substance, and copper is loaded thereon as an auxiliary agent. After drying, an iron-based catalyst is prepared and used in a multiphase catalytic oxidation tower to achieve the degradation of organic substances in the in-situ thermal desorption and extraction gas from an organic pollution site. The preparation process of this method is simple, the cost is low, and it is easy to industrialize. It improves the problem that iron ions are easily lost during the use of traditional iron-carbon catalysts, and avoids the problem that more precipitate substances are easily generated by the dissolved iron ions in an environment with a higher pH, thus avoiding the problems of increasing the difficulty of treating the spray liquid due to the increase of precipitate substances and the caking and passivation of the catalyst, and the decline of the catalytic effect. It prolongs the service life of the catalyst and improves the treatment effect of the multiphase catalytic oxidation tower on organic pollutants.

Description

Technical Field

[0001] The present invention belongs to the field of iron-based catalysts, and particularly relates to a preparation method and application of an iron-based catalyst. Background Art

[0002] The in-situ thermal extraction technology enables the heating wells buried in the soil to be repaired to work by controlling the temperature and heating time through a control system, heating the contaminated soil above the boiling point of the target pollutant, and the pollutant vaporizes and volatilizes after reaching the boiling point. The mobility of the pollutant after being converted into a gas state is increased, and the volatile gaseous products are collected and captured by a collection device or a capture device and purified, so as to separate and remove the target pollutant from the soil particles. The in-situ thermal extraction technology is an effective technology for treating organic contaminated soil. During the treatment process, the pollutants in the soil are not directly degraded, but transferred to the gas phase. The main components of the thermal desorption tail gas include carrier gas, pollutant organic matter, pyrolysis products, soil particles, etc. If these substances are directly discharged into the atmosphere without effective treatment and disposal, they will cause secondary pollution to the atmospheric environment, affecting the surrounding atmospheric environment and the growth of animals and plants. As a direct thermal desorption technology, the in-situ thermal extraction technology has the heat source in direct contact with the soil, high thermal energy utilization rate, and the characteristics of a large amount of tail gas generated and low pollutant concentration in the tail gas. Therefore, it is difficult to recover the organic substances in the tail gas, and the destruction method needs to be used for treatment. Among them, the multiphase catalytic oxidation technology, as a technology that can combine the three ways of absorbing, catalyzing, and oxidizing to destroy pollutants, can complete the degradation of organic waste gas in multiple phases such as gas-gas, gas-solid, gas-liquid, liquid-solid, and liquid-liquid, and effectively treat the waste gas generated by the in-situ thermal extraction technology.

[0003] As the core substance of the multiphase catalytic oxidation technology, the catalyst plays a crucial role in the degradation of pollutants. As the most commonly used catalyst in the multiphase catalytic oxidation technology, the iron-carbon catalyst has a microelectrolysis effect inside, which enhances the synergistic effect existing inside the multiphase catalytic oxidation. However, the iron-carbon catalyst has the problem that iron ions are easily lost, and the dissolved iron ions are prone to generate more precipitated substances (Fe(OH)3) in an environment with a higher pH. For the pollutants in the waste gas, generally, the waste gas is sprayed with a spraying liquid to transfer the pollutants in the gas phase to the liquid phase, and then the clean gas and the liquid containing pollutants are separated. The precipitated substance (Fe(OH)3) will also be transferred to the liquid during the gas-liquid transformation process, increasing the treatment difficulty of the liquid. These precipitated substances are generated on the surface of the porous catalyst, blocking the pores and resulting in a decrease in porosity, or covering the active sites on the surface of the catalyst, causing the problem of catalyst caking and passivation and a decline in the catalytic effect. At present, the modification methods of the iron-carbon catalyst are complex, with many loaded components, and most of the developed catalysts are in powder form, which are easy to lose and not suitable for the treatment of the in-situ thermal extraction waste gas of organic polluted sites by the multiphase catalytic oxidation technology. Summary of the Invention

[0004] Therefore, the technical problem to be solved by the present invention is that in the traditional iron-carbon catalyst used in the multiphase catalytic oxidation technology, iron ions are prone to loss. The dissolved iron ions are likely to produce more precipitate substances in an environment with a higher pH, increasing the difficulty of treating the spray liquid and also causing the catalyst to agglomerate and passivate, resulting in a decline in the catalytic effect.

[0005] To solve the above technical problems, the present invention provides a preparation method of an iron-based catalyst, which includes the following steps:

[0006] (1) First, pretreat the activated carbon particles with citric acid, then wash them in deionized water, and finally filter and dry.

[0007] (2) Mix Cu(NO3)2, Fe(NO3)3 and deionized water together to prepare an impregnation solution;

[0008] (3) Add the activated carbon particles pretreated in step (1) to the impregnation solution obtained in step (2), stir for 2 to 3 hours to obtain a mixture;

[0009] (4) Add ammonia water to the mixture until no precipitate is formed to obtain a mixed solution;

[0010] (5) Perform solid-liquid centrifugal separation on the obtained mixed solution for 1 h, and wash the obtained solid mixture with deionized water to obtain a catalyst precursor;

[0011] (6) Dry the obtained catalyst precursor to obtain an iron-based catalyst.

[0012] Preferably, the activated carbon particles are cylindrical, with a particle size of 4 to 8 mm, a specific surface area of 900 to 1050 m 3 / g, an ash content of <4.5%, and a moisture content of <8%.

[0013] Preferably, the citric acid pretreatment method in step (1): Filter after heating with 5% citric acid in a 75°C constant temperature water bath for 1 h.

[0014] Preferably, the volume ratio concentration of the ammonia water is 1:1.

[0015] Preferably, the mass ratio of Fe and Cu in the iron-based catalyst is 2:1.

[0016] The iron-based catalyst in the present invention is used in a multiphase catalytic oxidation tower to treat the waste gas extracted by in-situ thermal desorption of organic polluted sites.

[0017] The technical solution of the present invention has the following beneficial effects:

[0018] The iron-based catalyst provided by the present invention uses activated carbon pretreated with citric acid as a carrier during the preparation process, which changes the types and amounts of functional groups on the catalyst surface. Using ammonia water as a reducing agent, with iron as the main active substance and copper as a promoter loaded thereon, an iron-based catalyst is prepared after drying and is used in a multiphase catalytic oxidation tower to achieve the degradation of organic substances in the in-situ thermal desorption extraction gas from an organic pollution site. The preparation process of this method is simple, with low cost and easy for industrial production. It improves the problem that iron ions are easily lost during the use of traditional iron-carbon catalysts, and avoids the problem that the dissolved iron ions are prone to produce more precipitated substances in an environment with a higher pH, thus avoiding the problems of increased difficulty in treating the spraying liquid due to the increase of precipitated substances and the passivation of catalyst agglomeration and the decline of catalytic effect, prolonging the service life of the catalyst, and improving the treatment effect of the multiphase catalytic oxidation tower on organic pollutants. Specific embodiments

[0019] The technical solutions of the present invention will be described clearly and completely below. Obviously, the described embodiments are some but not all of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0020] The present invention provides a preparation method for an iron-based catalyst, including the following steps:

[0021] (1) First, pretreat the activated carbon particles with citric acid, then wash them in deionized water to wash away the impurities and unreacted citric acid on the surface of the activated carbon, prevent the impurities from flowing into the subsequent treatment process, and finally filter and dry.

[0022] (2) Mix Cu(NO3)2, Fe(NO3)3 and deionized water together to prepare an impregnation solution, and the mass ratio of Fe and Cu in the iron-based catalyst is 2:1.

[0023] (3) Add the activated carbon particles pretreated in step (1) to the impregnation solution obtained in step (2), stir for 2 to 3 hours to obtain a mixture;

[0024] (4) Add ammonia water to the mixture until no precipitate is formed to obtain a mixed solution, and the volume ratio concentration of the ammonia water is 1:1.

[0025] The chemical reaction process of this step is as follows:

[0026] Cu(NO3)2 + 2NH3·H2O == Cu(OH)2↓ + 2NH4NO3

[0027] Fe(NO3)3 + 3NH3·H2O == Fe(OH)3↓ + 3NH4NO3

[0028] (5) Centrifuge the obtained mixture for 1 h to separate solids from liquids, and wash the obtained solid mixture with deionized water to remove impurities such as unreacted ammonia water, Cu(NO3)2, Fe(NO3)3, and NH4NO3, thus obtaining a catalyst precursor;

[0029] (6) Dry the obtained catalyst precursor to obtain an iron-based catalyst.

[0030] The activated carbon particles are cylindrical, with a particle size of 4 - 8 mm, a specific surface area of 900 - 1050 m 3 / g, an ash content of <4.5%, and a moisture content of <8%.

[0031] The citric acid pretreatment method in step (1): Filter after heating with 5% citric acid in a constant temperature water bath at 75°C for 1 h.

[0032] The iron-based catalyst in the present invention is used in a multiphase catalytic oxidation tower to treat the waste gas extracted by in-situ thermal desorption of organic polluted sites.

[0033] Example 1:

[0034] Soak 500 g of activated carbon particles in 5% citric acid, filter after heating in a water bath at 75°C for 1 h; mix 93.75 g of Cu(NO3)2, 242 g of Fe(NO3)3, and 500 ml of deionized water to prepare an impregnation solution; add the pretreated activated carbon particles to the impregnation solution, stir for 2 - 3 hours to obtain a mixture; add 145 ml of ammonia water to the mixture to obtain a mixed solution, centrifuge the obtained mixed solution for 1 h to separate solids from liquids to obtain a solid mixture, wash it with deionized water to obtain a catalyst precursor; dry the catalyst precursor substance under vacuum conditions at 60°C for 48 h to obtain an iron-based catalyst.

[0035] Example 2:

[0036] Soak 500 g of activated carbon particles in 5% citric acid, filter after heating in a water bath at 75°C for 1 h; mix 93.75 g of Cu(NO3)2, 242 g of Fe(NO3)3, and 500 ml of deionized water to prepare an impregnation solution; add the pretreated activated carbon particles to the impregnation solution, stir for 2 - 3 hours to obtain a mixture; add 145 ml of ammonia water to the mixture to obtain a mixed solution, centrifuge the obtained mixed solution for 1 h to separate solids from liquids to obtain a solid mixture, wash it with deionized water to obtain a catalyst precursor; dry the catalyst precursor substance under vacuum conditions at 80°C for 48 h to obtain an iron-based catalyst.

[0037] Example 3

[0038] Soak 500 g of activated carbon particles in citric acid with a concentration of 5%, filter after heating in a water bath at 75 °C for 1 h; mix 93.75 g of Cu(NO3)2, 242 g of Fe(NO3)3 and 500 ml of deionized water to prepare an impregnation solution; add the pretreated activated carbon particles to the impregnation solution, stir for 2 - 3 hours to obtain a mixture; add 145 ml of ammonia water to the mixture to obtain a mixed solution, perform solid-liquid centrifugal separation on the obtained mixed solution for 1 h to obtain a solid mixture, and after washing with deionized water, obtain a catalyst precursor; dry the catalyst precursor substance under vacuum conditions at 110 °C for 48 h to obtain an iron-based catalyst.

[0039] Comparative Example 1

[0040] Soak 500 g of activated carbon particles in citric acid with a concentration of 5%, filter after heating in a water bath at 75 °C for 1 h; mix 93.75 g of Cu(NO3)2, 242 g of Fe(NO3)3 and 500 ml of deionized water to prepare an impregnation solution; add the pretreated activated carbon particles to the impregnation solution, stir for 2 - 3 hours to obtain a mixture; add 145 ml of ammonia water to the mixture to obtain a mixed solution, perform solid-liquid centrifugal separation on the obtained mixed solution for 1 h to obtain a solid mixture, and after washing with deionized water, obtain a catalyst precursor; dry the catalyst precursor at 200 °C for 6 h and then calcine at 600 °C for 3 h to obtain an iron-based catalyst.

[0041] To compare the catalytic degradation effects of the catalysts prepared by vacuum low-temperature drying and traditional high-temperature calcination in the present invention, the iron-based catalysts prepared in Examples 1 - 3 and Comparative Example 1 were subjected to toluene degradation tests. The specific test method was as follows: Add the prepared iron-based catalysts to an exhaust gas spray tower respectively, and let the toluene to be treated and the oxidant active oxygen (generated by an active oxygen molecule generator, mainly including (·OH, HO2, O2﹢, O, O(1D), O﹣, O2﹣, O2(a1Δg), O3, etc.) enter the spray tower from bottom to top. The spray liquid flows downward and countercurrently to the exhaust gas and the oxidant. The toluene fully contacts with the spray liquid, the catalyst and the oxidant in the spray tower; control the air volume to be 30 L / min, the initial concentration of toluene to be 500 ppm, the residence time to be 6 s, the filling amount of the catalytic packing to be 2.5 L, the injection amount of the oxidant to be 60 mg / L·min, and use 2 L of water with an initial pH of 7 as the spray liquid, and the liquid-gas ratio is 20 L / m 3 , conduct a continuous reaction for 60 min, and then count the toluene degradation rate. The calculation formula is:

[0042]

[0043] C0 —— the concentration of pollutants at the reactor inlet;

[0044] Ct —— Pollutant concentration at the reactor outlet.

[0045] The test results are shown in Table 1

[0046] Table 1 Toluene degradation rate

[0047]

[0048] As can be seen from Table 1, the catalyst prepared by drying under vacuum conditions has better implementation effect in the multiphase catalytic oxidation reaction tower than the catalyst prepared by the traditional calcination method. When the catalyst preparation temperature in Example 3 reaches 110 °C, certain losses of hydroxyl radicals and oxygen vacancies contained in the catalyst will occur. Therefore, the effect is inferior to that of the catalyst prepared by drying at 80 °C under vacuum conditions. Therefore, the drying temperature of the catalyst is more preferably 80 °C.

[0049] Comparative Example 2

[0050] Soak 500 g of activated carbon particles in absolute ethanol, filter after heating in a water bath at 75 °C for 1 h; mix 93.75 Cu(NO3)2, 242 g of Fe(NO3)3 and 500 ml of deionized water to prepare an impregnation solution; add the pretreated activated carbon particles to the impregnation solution, stir for 2 - 3 hours to obtain a mixture; add 145 ml of ammonia water to the mixture to obtain a mixed solution, subject the obtained mixed solution to solid-liquid centrifugal separation for 1 h to obtain a solid mixture, wash with deionized water to obtain a catalyst precursor; dry the catalyst precursor substance under vacuum conditions at 80 °C for 48 h to obtain an iron-based catalyst.

[0051] To verify the catalytic degradation effect of the catalyst prepared by pretreating activated carbon particles with citric acid in the present invention, the toluene degradation test results of the iron-based catalysts prepared in Example 2 and Comparative Example 2 were compared. The specific test method was as follows: Add the prepared different iron-based catalysts to an exhaust gas spray tower respectively. The to-be-treated toluene and the oxidant active oxygen enter the spray tower from bottom to top, and the spray liquid flows downward in a countercurrent manner to the exhaust gas and the oxidant. Toluene fully contacts with the spray liquid, the catalyst and the oxidant in the spray tower; control the air volume to be 30 L / min, the initial concentration of toluene to be 500 ppm, the residence time to be 6 s, the catalytic packing loading to be 2.5 L, the oxidant injection amount to be 60 mg / L·min, and use 2 L of water with an initial pH of 7 as the spray liquid, and the liquid-gas ratio to be 20 L / m 3 , conduct a continuous reaction for 60 min, and then count the toluene degradation rate. The calculation formula is the same as above. The test results are shown in Table 2:

[0052] Table 2

[0053]

[0054] As can be seen from Table 2, the catalytic effect of the activated carbon carrier pretreated with citric acid is better than that of the untreated activated carbon carrier. Modifying the carrier with citric acid can reduce the number of basic functional groups ( Pyrone (cycloketone) and its derivatives), and increase the number of acidic functional groups, which is beneficial to the degradation of organic waste gas.

[0055] Comparative Example 3

[0056] Soak 500 g of activated carbon particles in 5% citric acid, filter after heating in a water bath at 75 °C for 1 h; mix 335.75 g of Fe(NO3)3 and 500 ml of deionized water to prepare an impregnation solution; add the pretreated activated carbon particles to the impregnation solution, stir for 2 - 3 hours to obtain a mixture; add 145 ml of ammonia water to the mixture to obtain a mixed solution, centrifuge the obtained mixed solution for 1 h for solid-liquid separation to obtain a solid mixture, wash it with deionized water to obtain a catalyst precursor; dry the catalyst precursor under vacuum conditions at 80 °C for 48 h to obtain an iron-based catalyst.

[0057] To verify the Cu loading effect of the present invention, the iron-based catalysts prepared in Example 3 and Comparative Example 3 were tested for toluene degradation. The specific test method is as follows: Add the prepared different iron-based catalysts to an exhaust gas spray tower respectively. Let the to-be-treated toluene and the oxidant active oxygen enter the spray tower from bottom to top, and the spray liquid flows downward in reverse with the exhaust gas and the oxidant. Toluene fully contacts with the spray liquid, the catalyst, and the oxidant in the spray tower; control the air volume to be 30 L / min, the initial concentration of toluene to be 500 ppm, the residence time to be 6 s, the filling amount of catalytic packing to be 2.5 L, the injection amount of the oxidant to be 60 mg / L·min, and use 2 L of water with an initial pH of 7 as the spray liquid, and the liquid-gas ratio is 20 L / m 3 , conduct five consecutive reactions of 60 min, and then count the toluene degradation rate. The calculation formula is the same as above. The test results are shown in Table 3:

[0058] Table 3

[0059]

[0060]

[0061] It can be seen from Table 3 that the loading of Cu improves the service life of the catalyst. After five cycles of use, the degradation rate of toluene only decreases by less than 10%. Moreover, in the first use, the catalytic effect of Example 3 is greater than that of Comparative Example 3. The loading of Cu not only increases the crystal growth of the catalyst, increases the metal active sites existing on the catalyst surface, but also has a synergistic effect with iron ions, effectively reducing the loss of iron ions during the catalytic process and extending the service life of the catalyst. The crystal growth of the catalyst can increase the surface area of the catalyst, that is, increase the contact area with pollutants; pollutants are removed by reacting with the active sites on the catalyst surface. The active sites are the decisive factors for the activity of the catalyst. With the increase of active sites, the removal effect of pollutants will surely be better; compared with the individual loading of copper and iron ions on the catalyst surface, the co-loading of copper ions and iron ions can produce iron-copper compounds (such as iron-copper spinel) or can combine with different sites on the catalyst surface to increase the total doping amount, which can affect the energy and structure of the catalyst surface to a greater extent and promote the degradation of pollutants.

[0062] Obviously, the above examples are only for illustration and are not intended to limit the implementation manner. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all implementation manners here. And the obvious changes or modifications derived therefrom are still within the protection scope of the present invention.

Claims

1. A preparation method of an iron-based catalyst, characterized in that, It includes the following steps: (1) First, pretreat the activated carbon particles with citric acid, then wash them in deionized water, and finally filter and dry them; The activated carbon particles are cylindrical, with a particle size of 4 - 8 mm, a specific surface area of 900 - 1050 m / g, an ash content of <4.5%, and a moisture content of <8%; (2) Mix Cu(NO3)2, Fe(NO3)3 and deionized water together to prepare an impregnation solution; (3) Add the activated carbon particles pretreated in step (1) to the impregnation solution obtained in step (2), and stir for 2 - 3 hours to obtain a mixture; (4) Add ammonia water to the mixture until no precipitate is formed to obtain a mixed solution; (5) Perform solid-liquid centrifugal separation on the obtained mixed solution for 1 h, and wash the obtained solid mixture with deionized water to obtain a catalyst precursor; (6) Dry the obtained catalyst precursor to obtain an iron-based catalyst; In the iron-based catalyst, the mass ratio of Fe to Cu is 2:

1.

2. The preparation method of an iron-based catalyst according to claim 1, wherein, The citric acid pretreatment method in step (1): Filter after heating with 5% citric acid in a 75°C constant temperature water bath for 1 h.

3. The preparation method of an iron-based catalyst according to claim 1, characterized in that, The volume ratio concentration of the ammonia water is 1:

1.

4. Application of the iron-based catalyst prepared by the preparation method according to any one of claims 1 - 3, used in a multiphase catalytic oxidation tower to treat the exhaust gas from in-situ thermal desorption extraction of organic polluted sites.

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