CN104437535B - A kind of environmentally friendly manganese cobalt aluminum composite oxide demercuration denitrating catalyst and preparation method thereof - Google Patents

Abstract

An environment-friendly manganese-cobalt-aluminum composite oxide catalyst for mercury and denitrification, the chemical formula of which is Mn _a Co _b Al _c O _x , where a, b and c respectively represent the total molar amounts of Mn, Co and Al in the metal elements in the catalyst The percentage of a, the value range of a is 0.1-5%, the value range of b is 70-75%, the balance is c, x is determined by the chemical balance according to the values of a, b, c and the valence state of the metal element Decision, the present invention also provides the preparation method of described catalyst, the catalyst of the present invention is in the temperature range of 100-400 ℃, and the removal efficiency of gaseous elemental mercury under hydrogen chloride atmosphere can all reach more than 80%; Under SCR atmosphere, 300- In the temperature range of 450°C, the purification efficiency of nitrogen oxides reaches 75.8-90.5%, and the highest synergistic mercury removal efficiency reaches 78%; the composite oxide catalyst in the present invention is environmentally friendly and has no toxic effect on human body and environment; The catalyst synthesis raw material cost of the invention is low.

Description

An environment-friendly manganese-cobalt-aluminum composite oxide mercury denitrification catalyst and preparation method thereof

technical field

The invention belongs to the technical field of environmental protection, and relates to the treatment of mercury and nitrogen oxides in waste gas discharged from industrial furnaces such as thermal power plants, industrial boilers, steel sintering, and garbage incineration, and in particular to an environmentally friendly manganese-cobalt-aluminum composite oxide demercury and denitrification Catalyst and method for its preparation.

Background technique

With the rapid development of my country's economy and the rapid increase of population, people's consumption of fossil energy such as coal has increased rapidly. Coal occupies a very large proportion (about 70%) in my country's primary energy structure. In 2003, my country's coal consumption was about 1.53×1012t, accounting for about 28% of the world's total consumption. It is estimated that China's coal consumption will double to 3.04×1012t by 2020. At present, the mercury emitted by coal combustion has accounted for about 40% of the total anthropogenic mercury emissions in my country. In 1999, the emission of mercury pollution in China was about 536 tons, and it has increased unabated in the past ten years. Such a huge amount of mercury emission not only aggravates mercury pollution in China, but also poses a potential threat to the global environment. On the other hand, nitrogen oxides (NO _x ) produced by coal combustion and discharged into the atmosphere also seriously affect the air quality of our country and restrict the sustainable development of economy and society. The development of mercury and nitrogen oxide control technology adapted to my country's national conditions has become a hot issue in the field of environmental protection.

There are three main types of catalysts for catalytic oxidation of zero-valent mercury in flue gas: SCR catalysts, carbon-based catalysts, metal and metal oxide catalysts. Ammonia Selective Catalytic Reduction (SelectiveCatalyticReductionofNOwithNH ₃ , NH ₃ -SCR) is a mature and stable mainstream stationary source denitrification technology. At present, the common commercial NH ₃ -SCR catalysts are mainly supported vanadium-based catalysts, whose main active component is V ₂ O ₅ , and the operating temperature range is 300-400°C. However, V ₂ O ₅ -WO ₃ /TiO ₂ catalysts have some major defects, such as vanadium is biologically toxic to human body and environment, waste catalyst is difficult to dispose and reuse, itself is a source of pollution, on the other hand, it is harmful to flue gas The catalytic oxidation effect of gaseous zero-valent mercury in the medium is very limited, and it is difficult to realize the control of mercury emissions. These problems can be effectively avoided if non-vanadium denitrification and mercury removal catalysts are used.

In recent years, researchers at home and abroad have conducted a lot of research on non-vanadium system catalysts. Manganese oxide is considered to be the active component of NH ₃ -SCR catalysts below 300°C, but its disadvantage is that the catalyst with high manganese content decreases rapidly with the increase of temperature, and the nitrogen selectivity decreases rapidly, and nitrous oxide (N ₂ O) Generated increased. N ₂ O can damage the ozone layer and is more difficult to remove. This largely restricts the application of manganese oxide catalysts.

Therefore, the development of catalysts suitable for my country's national conditions and having high mercury removal efficiency, denitrification activity and selectivity in a wide operating temperature window has become a key issue for my country's power plants and industrial kiln flue gas denitrification and mercury removal. The solution to this problem is crucial. my country's mercury pollution control and total nitrogen oxide emission control are of great significance.

Contents of the invention

In order to overcome the above-mentioned shortcomings of the prior art, the object of the present invention is to provide an environmentally friendly manganese-cobalt-aluminum composite oxide demercury and denitrification catalyst and its preparation method. Dispersed state, suitable for denitrification and mercury removal in a wide temperature range (300-450°C), has high-efficiency catalytic oxidation of elemental mercury in flue gas in the range of 300-450°C, and has high NO _x conversion In addition, the catalyst of this system is significantly less dependent on the concentration of HCl in the flue gas, and still has a high oxidation efficiency of gaseous elemental mercury in an HCl-free atmosphere. It is non-toxic to humans and the environment, and the price is low.

In order to achieve the above object, the technical scheme adopted in the present invention is:

An environment-friendly manganese-cobalt-aluminum composite oxide catalyst for mercury and denitrification, the chemical formula of which is Mn _a Co _b Al _c O _x , where a, b and c respectively represent the total molar amounts of Mn, Co and Al in the metal elements in the catalyst The percentage of a, the value range of a is 0.1-5%, the value range of b is 70-75%, the balance is c, x is determined by the chemical balance according to the values of a, b, c and the valence state of the metal element Decide.

Preferably, the value range of c may be 29.9-20%.

Preferably, its chemical formula can be Mn _0.005 Co _0.73 Al _0.265 O _x , Mn _0.01 Co _0.72 Al _0.27 O _x or Mn _0.02 Co _0.71 Al _0.27 O _x , the value of x is determined by the chemical equilibrium Decide.

The present invention also provides a preparation method of the environment-friendly manganese-cobalt-aluminum composite oxide demercury and denitrification catalyst, comprising the following steps:

Step 1. Dissolve cobalt nitrate and aluminum nitrate in water at room temperature, stir to form a transparent solution, then add manganese nitrate solution to obtain a mixed salt solution, wherein the manganese content is 0.1%-5% of the total molar weight of the elements cobalt, aluminum and manganese, cobalt The content is 70-75% of the total molar weight of the elements cobalt, aluminum and manganese, and the aluminum content is the balance of the total molar weight of the elements cobalt, aluminum and manganese;

Step 2, drop the sodium hydroxide solution and the mixed salt solution obtained in step 1 into distilled water under the protection of nitrogen, and stir vigorously at a speed of 600 rpm at the same time to form a flocculent precipitate, and adjust the drop of the sodium hydroxide solution in the process Accelerate to stabilize the pH value of the mixed solution at 9±0.1, continue to stir for 30 minutes after the dropwise addition of the mixed salt solution, and then stop stirring to obtain a slurry;

Step 3. Static crystallization of the slurry obtained in Step 2 in a water bath at 65° C. for 24-48 hours under the protection of a nitrogen atmosphere;

Step 4, placing the crystallized slurry in step 3 in a centrifuge for repeated centrifugation until the resulting supernatant is neutral;

Step 5. Dry the washed precipitate in an oven at 80°C for 24 hours, take it out and put it into a muffle furnace and bake it at 450-550°C for 4-6 hours to obtain a hydrotalcite-like precursor that is calcined with MnCoAlO _x Catalyst for Mercury and Denitrification in Spinel Phase.

Preferably, in said step 1, the manganese content is 0.1% of the total molar weight of the elements cobalt, aluminum and manganese, the cobalt content is 75% of the total molar weight of the elements cobalt, aluminum and manganese, and the aluminum content is 0.1% of the total molar weight of the elements cobalt, aluminum and manganese 24.9% of the total molar weight. Alternatively, the manganese content is 5% of the total molar weight of the elements cobalt, aluminum and manganese, the cobalt content is 70% of the total molar weight of the elements cobalt, aluminum and manganese, and the aluminum content is 25% of the total molar weight of the elements cobalt, aluminum and manganese. The amount of water used is 100-200 times of the total molar amount of cobalt nitrate and aluminum nitrate; the manganese nitrate solution is diluted to a concentration of 0.01mol/L at normal temperature with a mass concentration of 50% and then added.

Preferably, in the step 2, the concentration of the sodium hydroxide solution is 1mol/L, and both the sodium hydroxide solution and the mixed salt solution are added dropwise at a speed of 1mL/min to distilled water whose volume is one third of the mixed salt solution .

Preferably, in step four, the centrifuge centrifuges at a speed of 10,000 rpm for 10 minutes, pours out the supernatant, re-adds deionized water, oscillates and mixes, repeats the centrifugation process, and centrifuges and washes several times until the above Serum is neutral.

Compared with the prior art, in the temperature range of 100-400°C, the removal efficiency of gaseous elemental mercury in the hydrogen chloride atmosphere can reach more than 80% in the metal oxide catalyst of the present invention; Inside, the purification efficiency of nitrogen oxides reaches 75.8-90.5%, and the highest synergistic mercury removal efficiency reaches 78%; the composite oxide catalyst in the present invention is environmentally friendly and has no toxic effect on the human body and the environment; The cost of raw materials is low.

Description of drawings

Fig. 1 is a schematic diagram of the mercury oxidation efficiency of the catalyst obtained in the embodiment of the present invention without HCl.

detailed description

The implementation of the present invention will be described in detail below in conjunction with the drawings and examples.

Embodiment one

A manganese-cobalt-aluminum oxide denitrification synergistic demercury catalyst, the ratio of raw material components is: manganese nitrate with a metal element content of 0.5% of the molar percentage of the catalyst metal element, and a content of 73% of the catalyst metal element mole percentage of cobalt nitrate, and the balance is aluminum nitrate.

A method for preparing a manganese-cobalt-aluminum oxide denitrification synergistic demercury catalyst, comprising the following steps:

Step 1. Dissolving cobalt nitrate and aluminum nitrate in water with 100-200 times molar weight at room temperature, stirring to form a transparent solution. Then dilute 50% manganese nitrate solution at room temperature to a solution of 0.01mol/L, accurately measure a certain volume of manganese nitrate solution with a pipette and pour it into the mixed salt solution of cobalt and aluminum, wherein the manganese content is the catalyst metal element mole percent The cobalt content is 0.5% of the catalyst metal element mole percentage, and the balance is aluminum nitrate.

Step 2: Dissolve sodium hydroxide in water to obtain a 1mol/L sodium hydroxide solution, and then add the mixed salt solution and alkali solution obtained in step 1 to 100ml distilled water dropwise at a rate of 1mL/min under nitrogen protection At the same time, stir vigorously at a speed of 600 rpm to form a flocculent precipitate, and at the same time continuously measure the pH value of the solution, and properly adjust the drop rate of the alkali solution to stabilize the pH value of the mixed solution at 9±0.1. Continue to stir for 30 minutes after the dropwise addition of the mixed salt solution, then stop stirring to obtain a slurry;

Step 3. Static crystallization of the slurry obtained in Step 2 in a water bath at 65° C. for 24 hours under the protection of a nitrogen atmosphere;

Step 4. Place the crystallized slurry in step 3 in a centrifuge, centrifuge at a speed of 10,000 rpm for 10 minutes, pour out the supernatant, re-add deionized water, shake and mix well, and repeat the centrifugation process . Centrifuge and wash several times in this way until the supernatant is neutral.

Step 5. Dry the washed precipitate in an oven at 80°C for 24 hours, take it out and put it into a muffle furnace and bake it at 450°C for 4 hours to obtain a spinel-phase crystal obtained by calcining hydrotalcite-like precursors. Mn _0.005 Co _0.73 Al _{0.265 Ox} denitrification synergistic demercury catalyst.

The molar ratio of each metal element in the prepared catalyst is Mn:Co:Al=0.005:0.73:0.265.

The catalyst is under the flue gas conditions of 5ppm hydrogen chloride (HCl), 5% oxygen (O ₂ ), and the balance gas is nitrogen (N ₂ ), the loading amount of the catalyst is 50mg, and the reaction space velocity is 480000h ^-1 , at 100°C, Under the seven temperature conditions of 150°C, 200°C, 250°C, 300°C, 350°C, and 400°C, see Table 1 for the catalytic oxidation efficiency of the catalyst to remove gaseous elemental mercury.

Embodiment two

A manganese-cobalt-aluminum oxide denitrification synergistic demercury catalyst, the raw material component ratio is: manganese nitrate with a metal element content of 1% of the catalyst metal element mole percentage, and a catalyst metal element mole percentage content of 72% of cobalt nitrate, and the balance is aluminum nitrate.

A method for preparing a manganese-cobalt-aluminum oxide denitrification synergistic demercury catalyst, comprising the following steps:

Step 1. Dissolving cobalt nitrate and aluminum nitrate in water with 100-200 times molar weight at room temperature, stirring to form a transparent solution. Then dilute 50% manganese nitrate solution at room temperature to a solution of 0.01mol/L, accurately measure a certain volume of manganese nitrate solution with a pipette and pour it into the mixed salt solution of cobalt and aluminum, wherein the manganese content is the catalyst metal element mole percent content of 1%, the cobalt content is 72% of the catalyst metal element mole percentage, and the balance is aluminum nitrate.

Step 2: Dissolve sodium hydroxide in water to obtain a 1mol/L sodium hydroxide solution, and then add the mixed salt solution and alkali solution obtained in step 1 to 100ml distilled water dropwise at a rate of 1mL/min under nitrogen protection At the same time, stir vigorously at a speed of 600 rpm to form a flocculent precipitate, and at the same time continuously measure the pH value of the solution, and properly adjust the drop rate of the alkali solution to stabilize the pH value of the mixed solution at 9±0.1. Continue to stir for 30 minutes after the dropwise addition of the mixed salt solution, then stop stirring to obtain a slurry;

Step 3. Static crystallization of the slurry obtained in Step 2 in a water bath at 65° C. for 36 hours under the protection of a nitrogen atmosphere;

Step 4. Place the crystallized slurry in step 3 in a centrifuge, centrifuge at a speed of 10,000 rpm for 10 minutes, pour out the supernatant, re-add deionized water, shake and mix well, and repeat the centrifugation process . Centrifuge and wash several times in this way until the supernatant is neutral.

Step 5. Dry the washed precipitate in an oven at 80°C for 24 hours, take it out and put it in a muffle furnace and bake it at 500°C for 5 hours to obtain a spinel-phase crystal obtained by calcining hydrotalcite-like precursors. Mn _0.01 Co _0.72 Al _0.27 Ox catalysts for _synergistic denitrification and mercury removal.

The molar ratio of each metal element in the prepared catalyst is Mn:Co:Al=0.01:0.72:0.27.

The catalyst is under the flue gas conditions of 5ppm hydrogen chloride (HCl), 5% oxygen (O ₂ ), and the balance gas is nitrogen (N ₂ ), the loading amount of the catalyst is 50mg, and the reaction space velocity is 480000h ^-1 , at 100°C, Under the seven temperature conditions of 150°C, 200°C, 250°C, 300°C, 350°C, and 400°C, see Table 1 for the catalytic oxidation efficiency of the catalyst to remove gaseous elemental mercury.

The catalyst is mixed with 500ppm nitric oxide (NO), 500ppm ammonia (NH ₃ ), 5% oxygen (O ₂ ), 5 ppm hydrogen chloride (HCl) and nitrogen (N ₂ ) as the reaction gas. The loading capacity is 200mg, the reaction space velocity is 60000h ^-1 , under the seven temperature conditions of 150°C, 200°C, 250°C, 300°C, 350°C, 400°C, 450°C, the conversion rate and gaseous state of nitrogen oxides reduced by the catalyst The removal rate of elemental mercury is shown in Table 2.

Embodiment three

A manganese-cobalt-aluminum oxide denitrification synergistic demercury catalyst, the proportion of raw material components is: manganese nitrate with a metal element content of 2% of the catalyst metal element molar percentage, and a catalyst metal element molar percentage content of 71% of cobalt nitrate, and the balance is aluminum nitrate.

A method for preparing a manganese-cobalt-aluminum oxide denitrification synergistic demercury catalyst, comprising the following steps:

Step 1. Dissolving cobalt nitrate and aluminum nitrate in water with 100-200 times molar weight at room temperature, stirring to form a transparent solution. Then dilute 50% manganese nitrate solution at room temperature to a solution of 0.01mol/L, accurately measure a certain volume of manganese nitrate solution with a pipette and pour it into the mixed salt solution of cobalt and aluminum, wherein the manganese content is the catalyst metal element mole percent The cobalt content is 71% of the catalyst metal element mole percentage, and the balance is aluminum nitrate.

Step 2: Dissolve sodium hydroxide in water to obtain a 1mol/L sodium hydroxide solution, and then add the mixed salt solution and alkali solution obtained in step 1 to 100ml distilled water dropwise at a rate of 1mL/min under nitrogen protection At the same time, stir vigorously at a speed of 600 rpm to form a flocculent precipitate, and at the same time continuously measure the pH value of the solution, and properly adjust the drop rate of the alkali solution to stabilize the pH value of the mixed solution at 9±0.1. Continue to stir for 30 minutes after the dropwise addition of the mixed salt solution, then stop stirring to obtain a slurry;

Step 3. Static crystallization of the slurry obtained in Step 2 in a water bath at 65° C. for 48 hours under the protection of a nitrogen atmosphere;

Step 4. Place the crystallized slurry in step 3 in a centrifuge, centrifuge at a speed of 10,000 rpm for 10 minutes, pour out the supernatant, re-add deionized water, shake and mix well, and repeat the centrifugation process . Centrifuge and wash several times in this way until the supernatant is neutral.

Step 5. Dry the washed precipitate in an oven at 80°C for 24 hours, take it out and put it into a muffle furnace and bake it at 550°C for 6 hours to obtain a spinel-phase crystal obtained by calcining hydrotalcite-like precursors. Mn _0.02 Co _0.71 Al _0.27 Ox catalysts for _synergistic denitrification and mercury removal.

The molar ratio of each metal element in the prepared catalyst is Mn:Co:Al=0.02:0.71:0.27.

The catalyst is under the flue gas conditions of 5ppm hydrogen chloride (HCl), 5% oxygen (O ₂ ), and the balance gas is nitrogen (N ₂ ), the loading amount of the catalyst is 50mg, and the reaction space velocity is 480000h ^-1 , at 100°C, Under the seven temperature conditions of 150°C, 200°C, 250°C, 300°C, 350°C, and 400°C, see Table 1 for the catalytic oxidation efficiency of the catalyst to remove gaseous elemental mercury.

control group

A vanadium-tungsten-titanium oxide denitration catalyst reported in the prior art, its raw material components are: ammonium metavanadate with a content of 1% of the molar percentage of the catalyst, and ammonium metavanadate with a content of 9% of the molar percentage of the catalyst Ammonium metatungstate, the balance is titanium dioxide.

Its preparation method includes:

Step 1. Dissolve ammonium metavanadate and ammonium metatungstate in 100 times the molar amount of water at room temperature, add 1/10 molar amount of oxalic acid as a cosolvent, and stir to form a transparent solution. Put titanium dioxide powder into the above solution, stir and impregnate for 2 hours, then heat slowly to evaporate the water. Wherein the content of ammonium metavanadate is 1% of the molar percentage of the catalyst, the content of ammonium metatungstate is 9% of the molar percentage of the catalyst, and the balance is titanium dioxide.

Step 2. Dry the solid matter obtained in the above steps in an oven at 110°C for 12 hours, take it out and put it into a muffle furnace and bake it at 500°C for 4 hours to obtain a vanadium-tungsten-titanium oxide denitration catalyst V _0.01 W _0.09 /TiO ₂ .

The molar ratio of each element in the prepared catalyst is V:W:Ti=0.01:0.09:0.90.

The catalyst is mixed with 500ppm nitric oxide (NO), 500ppm ammonia (NH ₃ ), 5% oxygen (O ₂ ), 5 ppm hydrogen chloride (HCl) and nitrogen (N ₂ ) as the reaction gas. The loading capacity is 200mg, the reaction space velocity is 60000h ^-1 , under the seven temperature conditions of 150°C, 200°C, 250°C, 300°C, 350°C, 400°C, 450°C, the conversion rate and gaseous state of nitrogen oxides reduced by the catalyst The removal rate of elemental mercury is shown in Table 2.

It can be seen from the data in Table 2 that, compared with the existing vanadium-tungsten-titanium denitration catalyst, the manganese-cobalt-aluminum oxide catalyst prepared by the present invention has slightly lower denitration efficiency at temperatures higher than 300°C-450°C, but the mercury removal efficiency However, it has significant advantages. It can realize the synergistic removal of nitrogen oxides and mercury, and the highest denitrification efficiency is 90.5%, and the highest mercury removal efficiency is 78%.

Table I

Table II

Under the condition that 5% oxygen (O ₂ ) and 95% nitrogen (N ₂ ) are mixed as the reaction gas, the loading amount of the catalyst is 50 mg, and the reaction space velocity is 480000 h ^-1 , 250°C, 300°C, 350°C, and 400°C under seven temperature conditions, the gaseous elemental mercury removal rate of the catalyst is shown in Figure 1.

It can be seen from Fig. 1 that under the condition of large space velocity (the space velocity is about 480,000), even if there is no HCl in the reaction atmosphere, the catalyst can reach nearly 90% oxidation efficiency of gaseous elemental mercury at 200°C and 250°C. Although it is lower than that in the presence of HCl, the conversion is still very considerable. This indicates that the oxidation of mercury by this catalyst greatly reduces the dependence on HCl in the atmosphere.

Claims (9)

1. An environment-friendly manganese-cobalt-aluminum composite oxide demercury and denitration catalyst is characterized in that its chemical formula is Mn _a Co _b Al _c O _x , Wherein, a, b and c respectively represent the percentages of Mn, Co and Al in the total molar weight of metal elements in the catalyst, the value range of a is 0.1-5%, the value range of b is 70-75%, and the balance is c, x is determined by chemical equilibrium according to the values of a, b, and c and the valence states of metal elements. 2. The environment-friendly manganese-cobalt-aluminum composite oxide demercury and denitrification catalyst according to claim 1, characterized in that the range of c is 29.9-20%. 3. An environment-friendly manganese-cobalt-aluminum composite oxide mercury denitrification catalyst, characterized in that its expression is: Mn _0.005 Co _0.73 Al _0.265 O _x , Mn _0.01 Co _0.72 Al _0.27 O _x or Mn _0.02 Co _0.71 Al _0.27 O _x , the value of x is determined by chemical equilibrium according to the valence state of each metal element. 4. A preparation method for the environment-friendly manganese-cobalt-aluminum composite oxide demercury and denitrification catalyst according to claim 1, characterized in that, comprising the following steps: Step 1. Dissolve cobalt nitrate and aluminum nitrate in water at room temperature, stir to form a transparent solution, then add manganese nitrate solution to obtain a mixed salt solution, wherein the manganese content is 0.1%-5% of the total molar weight of the elements cobalt, aluminum and manganese, cobalt The content is 70-75% of the total molar weight of the elements cobalt, aluminum and manganese, and the aluminum content is the balance of the total molar weight of the elements cobalt, aluminum and manganese; Step 2, drop the sodium hydroxide solution and the mixed salt solution obtained in step 1 into distilled water under the protection of nitrogen, and stir vigorously at a speed of 600 rpm at the same time to form a flocculent precipitate, and adjust the drop of the sodium hydroxide solution in the process Accelerate to stabilize the pH value of the mixed solution at 9±0.1, continue to stir for 30 minutes after the dropwise addition of the mixed salt solution, and then stop stirring to obtain a slurry; Step 3. Static crystallization of the slurry obtained in Step 2 in a water bath at 65° C. for 24-48 hours under the protection of a nitrogen atmosphere; Step 4, placing the crystallized slurry in step 3 in a centrifuge for repeated centrifugation until the resulting supernatant is neutral; Step 5. Dry the washed precipitate in an oven at 80°C for 24 hours, take it out and put it into a muffle furnace and bake it at 450-550°C for 4-6 hours to obtain a hydrotalcite-like precursor that is calcined with Mn _a Co _b Al _c O _x catalyst for mercury and denitrification in spinel phase. 5. according to the preparation method of the described environment-friendly manganese-cobalt-aluminum composite oxide demercuration denitrification catalyst of claim 4, it is characterized in that, in the described step 1, the consumption of water is 100-100-100% of the total molar weight of cobalt nitrate and aluminum nitrate 200 times; the manganese nitrate solution is diluted with a mass concentration of 50% at room temperature to a concentration of 0.01mol/L before adding. 6. according to the preparation method of the described environment-friendly manganese-cobalt-aluminum composite oxide demercury denitrification catalyst of claim 4, it is characterized in that, in described step 2, sodium hydroxide solution concentration is 1mol/L, and sodium hydroxide solution and The mixed salt solution was added dropwise at a rate of 1 mL/min to distilled water whose volume was one third of the mixed salt solution. 7. The preparation method of the environment-friendly manganese-cobalt-aluminum composite oxide demercury and denitrification catalyst according to claim 4, characterized in that, in the step 4, the centrifuge is centrifuged at a speed of 10000 rpm for 10 minutes, and the upper Pour out the supernatant, re-add deionized water, shake and mix, repeat the centrifugation process, and centrifuge and wash several times until the supernatant is neutral. 8. According to the preparation method of the environment-friendly manganese-cobalt-aluminum composite oxide demercury and denitrification catalyst according to claim 4, it is characterized in that, in the step 1, the manganese content is 0.1% of the total molar weight of elements cobalt, aluminum and manganese , the cobalt content is 75% of the total molar weight of the elements cobalt, aluminum and manganese, and the aluminum content is 24.9% of the total molar weight of the elements cobalt, aluminum and manganese. 9. According to the preparation method of the environment-friendly manganese-cobalt-aluminum composite oxide demercury and denitrification catalyst according to claim 4, it is characterized in that, in the step 1, the manganese content is 5% of the total molar weight of elements cobalt, aluminum and manganese , the cobalt content is 70% of the total molar weight of the elements cobalt, aluminum and manganese, and the aluminum content is 25% of the total molar weight of the elements cobalt, aluminum and manganese.