A nitric acid modified fly ash based scr catalyst, its preparation method and application

Abstract

The present application relates to the technical field of gas-phase inorganic pollutant purification, and specifically provides a nitric acid modified fly ash based SCR catalyst, which takes nitric acid modified fly ash as a carrier and is loaded with manganese and cerium bimetallic oxides. The nitric acid treatment can not only increase the specific surface area of the fly ash and reduce the average pore size, but also effectively increase the proportion of SiO2 in the fly ash and remove some impurities, which is beneficial to the loading of active metals. After loading the manganese and cerium bimetallic oxides, the catalyst can selectively and efficiently catalyze the reduction of nitrogen oxides at low temperature, and the conversion rate of NO X can reach 100% at 150 DEG C, and the conversion rate can be kept above 90% in the range of 140-250 DEG C.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of gas-phase inorganic pollutant purification, in particular to a nitric acid modified fly ash based SCR catalyst and a preparation method and application thereof. BACKGROUND

[0002] The information disclosed in this Background section is only for the purpose of increasing the understanding of the background of the present application and should not be taken as an acknowledgement or any form of suggestion that this information forms prior art with respect to the present application.

[0003] In recent years, nitrogen oxides (NO X ) have become one of the main pollutants causing atmospheric environmental pollution problems, and are one of the important reasons for the formation of haze, acid rain and photochemical smog. The increase of NOx emission will pose a great threat to the environment and human health.

[0004] NH3 selective catalytic reduction of NO X (NH3-SCR) technology has been widely used in the field of NO X emission reduction. V2O5-WO3 / TiO2 catalyst is widely used in commercial due to its excellent SCR activity, thermal stability and resistance to SO2 poisoning. However, V2O5-WO3 / TiO2 catalyst is not suitable for catalytic reduction of flue gas at low temperature below 300℃. At present, the NO X emission control of non-power industries including steel, cement, glass and the like is also relatively strict, and since the flue gas temperature of the above non-power industries is usually lower than 300℃, the commercial V2O5-WO3 / TiO2 catalyst is not suitable. The development of vanadium-free catalysts with low-temperature SCR activity is urgently needed for non-power industries.

[0005] Fly ash is the product after combustion of coal-fired boilers, which has a spherical appearance, smooth surface and pore structure, and the main components are SiO2, Al2O3, Fe2O3 and MgO, etc., which are alkaline. If fly ash is not properly treated, it will produce dust, pollute the atmosphere and water quality, and harm human health. As a porous material, fly ash has good adsorption capacity and can be used as an adsorbent and catalyst carrier. SUMMARY

[0006] In order to overcome the above problems, the present application provides a nitric acid modified fly ash based SCR catalyst and a preparation method and application thereof.

[0007] To achieve the above technical purposes, the present application adopts the following technical solutions:

[0008] The first aspect of the present application provides a nitric acid modified fly ash based SCR catalyst, which is supported by nitric acid modified fly ash and loaded with manganese and cerium bimetallic oxides.

[0009] The second aspect of the present application provides a preparation method of the nitric acid modified fly ash based SCR catalyst of the first aspect, which comprises the following steps:

[0010] (1) soaking dry fly ash in a nitric acid solution, washing to neutral after soaking, and drying to obtain nitric acid modified fly ash;

[0011] (2) adding the nitric acid modified fly ash into a mixed solution of manganese salt and cerium salt, heating and stirring until the water evaporates, and drying to obtain an intermediate;

[0012] (3) oxygen calcining the intermediate to obtain the nitric acid modified fly ash based SCR catalyst.

[0013] The third aspect of the present application provides that the nitric acid modified fly ash based SCR catalyst of the first aspect is applied to selective catalytic reduction of nitrogen oxides at low temperature.

[0014] The present application has the following beneficial effects:

[0015] (1) The present application provides a nitric acid modified fly ash based SCR catalyst, which is supported by nitric acid modified fly ash and loaded with manganese and cerium bimetallic oxides. By nitric acid treatment, the specific surface area of fly ash can be increased, the average pore size can be reduced, the proportion of SiO2 in fly ash can be effectively increased, and some impurities can be removed, which is beneficial to the loading of active metals. After loading manganese and cerium bimetallic oxides, nitrogen oxides can be selectively and efficiently catalyzed at low temperature, and the conversion rate of NO X reaches 100% at 150℃, and remains above 90% in the range of 140-250℃.

[0016] (2) The nitric acid modified fly ash based SCR catalyst provided by the present application has excellent water resistance. Under the condition of 5% water vapor, the conversion rate of NO X of the nitric acid modified fly ash based SCR catalyst only decreases by about 5%, and remains high stability for 6 hours. When the water vapor is removed, the conversion rate of NO X returns to 100%. It is shown that the nitric acid modified fly ash based SCR catalyst is suitable for actual industrial application. BRIEF DESCRIPTION OF DRAWINGS

[0017] The drawings constituting a part of the specification of the present application are used to provide further understanding of the present application, the illustrative embodiments of the present application and the description thereof are used to explain the present application, and do not constitute improper limitation on the present application.

[0018] Figure 1 XRD patterns of the nitric acid modified fly ash based SCR catalyst Mn-Ce / AFA prepared in Example 1, the nitric acid modified fly ash based catalyst Mn / AFA prepared in Comparative Example 1, the nitric acid modified fly ash based catalyst Ce / AFA prepared in Comparative Example 2, and the original fly ash (FA);

[0019] Figure 2 Spectra of Mn 2p XPS;

[0020] Figure 3 Spectra of Ce 3d XPS;

[0021] Figure 4 System used for the fixed bed denitration reaction experiment;

[0022] Figure 5 Catalytic performance of the nitric acid modified fly ash based SCR catalyst Mn-Ce / AFA prepared in Example 1, the nitric acid modified fly ash based catalyst Mn / AFA prepared in Comparative Example 1, the nitric acid modified fly ash based catalyst Mn-Fe / AFA prepared in Comparative Example 3, and the nitric acid modified fly ash based catalyst Fe-Mn-Ce / AFA prepared in Comparative Example 4 at 125-250°C;

[0023] Figure 6 Catalytic performance of the nitric acid modified fly ash based SCR catalyst Mn-Ce / AFA prepared in Example 1, and the catalyst Mn-Ce / Ti prepared in Comparative Example 5 at 125-250°C;

[0024] Figure 7 Catalytic performance of the nitric acid modified fly ash based SCR catalyst Mn-Ce / AFA prepared in Example 1, the nitric acid modified fly ash based catalyst Mn / AFA prepared in Comparative Example 1, the nitric acid modified fly ash based catalyst Mn-Fe / AFA prepared in Comparative Example 3, the nitric acid modified fly ash based catalyst Fe-Mn-Ce / AFA prepared in Comparative Example 4, and the catalyst Mn-Ce / Ti prepared in Comparative Example 5 at 125°C;

[0025] Figure 8 Water resistance performance results of the nitric acid modified fly ash based SCR catalyst Mn-Ce / AFA prepared in Example 1. DETAILED DESCRIPTION

[0026] It should be noted that the following detailed description is merely exemplary and is intended to provide further description of the application. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.

[0027] It is to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments in accordance with the present application. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and / or "comprising," when used in this specification, specify the presence of stated features, steps, operations, elements, components, and / or groups thereof, but do not preclude the presence or addition of one or more other features, steps, operations, elements, components, and / or groups thereof.

[0028] In a first exemplary embodiment of the present application, there is provided a nitric acid modified fly ash based SCR catalyst, which is supported by nitric acid modified fly ash and loaded with manganese and cerium bimetallic oxides.

[0029] In one or more embodiments, the loading amount (mass ratio) of manganese and cerium in the nitric acid modified fly ash based SCR catalyst is 4-6%, preferably 5%.

[0030] In a second exemplary embodiment of the present application, there is provided a preparation method of the nitric acid modified fly ash based SCR catalyst of the first aspect, comprising the following steps:

[0031] (1) soaking dry fly ash in a nitric acid solution, washing to neutral after soaking, and drying to obtain nitric acid modified fly ash;

[0032] (2) adding the nitric acid modified fly ash into a mixed solution of manganese salt and cerium salt, heating and stirring until the water evaporates, and drying to obtain an intermediate;

[0033] (3) oxygen calcining the intermediate to obtain the nitric acid modified fly ash based SCR catalyst.

[0034] In one or more embodiments, in step (1), the particle size of the fly ash is 150-300 mesh, preferably 200 mesh.

[0035] In one or more embodiments, in step (1), the concentration of the nitric acid solution is 4-6 mol / L, preferably 5 mol / L.

[0036] In one or more embodiments, in step (1), the soaking temperature is 80-100°C, preferably 90°C; and the soaking time is 0.5-2 h, preferably 1 h.

[0037] In one or more embodiments, in step (2), the manganese salt is selected from one or more of manganese nitrate, manganese chloride, manganese sulfate or manganese acetate.

[0038] In one or more embodiments, in step (2), the cerium salt is selected from one or more of cerium chloride, cerium nitrate, cerium acetate, cerium ammonium nitrate, preferably cerium nitrate.

[0039] In one or more embodiments, in step (2), the temperature of heating is 55-70 °C, preferably 60 °C.

[0040] In one or more embodiments, in step (3), the temperature of calcination is 450-600 °C, preferably 60 °C; the time of calcination is 2.5-4 h, preferably 3 h.

[0041] In a third typical embodiment of the present application, the nitric acid modified fly ash based SCR catalyst of the first aspect is used for selective catalytic reduction of nitrogen oxides at low temperature.

[0042] In one or more embodiments, the temperature of selective catalytic reduction of nitrogen oxides is 140-250 °C, preferably 150 °C.

[0043] In order to enable a person skilled in the art to more clearly understand the technical solutions of the present application, the technical solutions of the present application will be described in detail below in conjunction with specific examples.

[0044] Example 1

[0045] A fly ash raw material obtained from a certain coal-fired power plant of Huaneng Group was ground to 200 mesh and dried overnight in a drying oven at 105 °C to obtain dried fly ash, which was named FA. Then, the dried fly ash was soaked in a nitric acid (5 mol / L) solution, treated in a water bath at 90 °C for 1 h, and then the mixture of nitric acid and FA was washed with deionized water until the solution was neutral. Then, the FA was dried in a drying oven at 105 °C until the water was completely evaporated to obtain intermediate AFA.

[0046] A certain amount of manganese nitrate solution and cerium nitrate hexahydrate (Ce(NO3)3·6H2O) were added to deionized water and mixed well to form a precursor solution. Then, the intermediate AFA was directly added to the precursor solution, heated in a water bath at 60 °C, and stirred until the water was evaporated. The obtained sample was dried overnight in a drying oven at 105 °C, and then the sample in the drying oven was calcined in a muffle furnace at 500 °C in air atmosphere for 3 h. Finally, the calcined material was naturally cooled to room temperature to obtain the nitric acid modified fly ash based SCR catalyst Mn-Ce / AFA. The loading amount (mass ratio) of manganese and cerium in the nitric acid modified fly ash based SCR catalyst Mn-Ce / AFA was both 5%.

[0047] Comparative Example 1

[0048] A certain amount of manganese nitrate solution was added to deionized water and mixed well, as a precursor solution. Then the intermediate AFA prepared in Example 1 was directly added to the precursor solution, heated in a water bath at 60°C, and stirred until the water evaporated. The obtained sample was dried in a drying oven at 105°C overnight, and the sample in the drying oven was taken out and calcined in a muffle furnace at 500°C in air atmosphere for 3h. Finally, the calcined material was naturally cooled to room temperature to obtain the nitric acid modified fly ash based catalyst Mn / AFA. The loading amount (mass ratio) of manganese in the nitric acid modified fly ash based catalyst Mn / AFA was 5%.

[0049] Comparative Example 2

[0050] A certain amount of manganese nitrate solution and cerium nitrate hexahydrate (Ce(N03)3-6H20) was added to deionized water and mixed well, as a precursor solution. Then the intermediate AFA was directly added to the precursor solution, heated in a water bath at 60°C, and stirred until the water evaporated. The obtained sample was dried in a drying oven at 105°C overnight, and the sample in the drying oven was taken out and calcined in a muffle furnace at 500°C in air atmosphere for 3h. Finally, the calcined material was naturally cooled to room temperature to obtain the nitric acid modified fly ash based catalyst Ce / AFA. The loading amount (mass ratio) of cerium in the nitric acid modified fly ash based catalyst Ce / AFA was 5%.

[0051] Comparative Example 3

[0052] A certain amount of manganese nitrate solution and iron nitrate was added to deionized water and mixed well, as a precursor solution. Then the intermediate AFA was directly added to the precursor solution, heated in a water bath at 60°C, and stirred until the water evaporated. The obtained sample was dried in a drying oven at 105°C overnight, and the sample in the drying oven was taken out and calcined in a muffle furnace at 500°C in air atmosphere for 3h. Finally, the calcined material was naturally cooled to room temperature to obtain the nitric acid modified fly ash based catalyst Mn-Fe / AFA. The loading amount (mass ratio) of manganese and iron in the nitric acid modified fly ash based catalyst Mn-Fe / AFA was 5% respectively.

[0053] Comparative Example 4

[0054] A certain amount of manganese nitrate solution, cerium nitrate hexahydrate (Ce(N03)3-6H20) and iron nitrate was added to deionized water and mixed well, as a precursor solution. Then the intermediate AFA was directly added to the precursor solution, heated in a water bath at 60°C, and stirred until the water evaporated. The obtained sample was dried in a drying oven at 105°C overnight, and the sample in the drying oven was taken out and calcined in a muffle furnace at 500°C in air atmosphere for 3h. Finally, the calcined material was naturally cooled to room temperature to obtain the nitric acid modified fly ash based catalyst Fe-Mn-Ce / AFA. The loading amount (mass ratio) of manganese, cerium and iron in the nitric acid modified fly ash based catalyst Fe-Mn-Ce / AFA was 5% respectively.

[0055] Comparative Example 5

[0056] A certain amount of manganese nitrate solution and cerium nitrate hexahydrate (Ce(NO3)3·6H2O) were added into deionized water and mixed well, as a precursor solution. Then TiO2 (specific particle size is 200 mesh) was directly added into the precursor solution, heated in a water bath at 60°C, and stirred until the water evaporated. The obtained sample was dried in a drying oven at 105°C overnight, and the sample in the drying oven was calcined in a muffle furnace at 500°C in air atmosphere for 3h. Finally, the calcined material was naturally cooled to room temperature to obtain the catalyst Mn-Ce / Ti. The loading amount (mass ratio) of manganese and cerium in the catalyst Mn-Ce / Ti was both 5%.

[0057] Example 2

[0058] Table 1 shows the proportion of each component of the original fly ash (FA) and the nitric acid modified fly ash, i.e. intermediate (AFA). As can be seen from Table 1, the proportion of SiO2 in the fly ash can be effectively improved by nitric acid treatment, and some impurities are removed, which is beneficial to the loading of active metals.

[0059] Table 1 shows the proportion of each component of the original fly ash (FA) and the intermediate (AFA)

[0060]

[0061] Example 3

[0062] 3.1 Table 2 shows the physical structure of the original fly ash (FA) and the nitric acid modified fly ash, i.e. intermediate (AFA), and the catalysts prepared in Comparative Examples 1-4. Compared with FA, the specific surface area of AFA after nitric acid treatment increased from 0.64m 2 / g to 25.50m 2 / g, indicating that acid modification can produce more pores in the fly ash, increase the specific surface area of the fly ash, and enable the active components to be uniformly loaded, thereby improving the activity of the catalyst.

[0063] Table 2 shows the physical structure of the original fly ash (FA) and the intermediate (AFA), and the catalysts prepared in Comparative Examples 1-4

[0064]

[0065] 3.2 Figure 1 The XRD images of the nitric acid modified fly ash based SCR catalyst Mn-Ce / AFA prepared in Example 1, the nitric acid modified fly ash based catalyst Mn / AFA prepared in Comparative Example 1, the nitric acid modified fly ash based catalyst Ce / AFA prepared in Comparative Example 2, and the original fly ash (FA) were obtained from Figure 1It can be seen that the main diffraction peak of all samples is characteristic of the fly ash carrier, i.e. the crystalline phase of quartz and mullite, consistent with the standard PDF card. The diffraction pattern of Ce / AFA shows the diffraction peak of cubic fluorite type CeO2, indicating that CeO2 has poor dispersibility, high crystallinity, and mainly exists in the form of crystals on the surface of the catalyst. For the Mn-Ce / AFA catalyst, XRD cannot detect the crystal form of MnO X and CeOx. The interaction between manganese and cerium in the bimetallic oxide of manganese and cerium can promote the conversion of active components in the catalyst from crystalline phase to amorphous phase, forming Mn-Ce active sites, thereby improving the low-temperature catalytic performance.

[0066] Example 4

[0067] Figure 2 The spectrum of Mn 2p XPS is shown. The proportion of Mn 4+ in the Mn-Ce / AFA catalyst is 32.24%. The presence of Mn 4+ is conducive to accelerating the oxidation of NO to NO2 and promoting the "fast SCR" reaction, thereby improving the catalytic performance of the catalyst at low temperature. In addition, the presence of a large amount of Mn 4+ and Mn 3+ can also provide more acid sites, promote the adsorption and activation of gas molecules, and thus improve the catalytic performance.

[0068] Figure 3 The spectrum of Ce 3d XPS is shown. The spectrum of Ce 3d is fitted to 8 peaks labeled as u, u', u", u'", v, v', v", v'", which belong to Ce 33 d / 3 and Ce 3d 5 / 2 respectively. The peaks labeled as v' and u' belong to Ce 3+ , and the other peaks belong to Ce 4+ . The proportion of Ce 3+ in the Mn-Ce / AFA catalyst is 19.31%, and Ce 3+ will cause charge imbalance, produce more oxygen vacancies and unsaturated chemical bonds, enhance the mobility of oxygen on the surface of the catalyst, make oxygen more easily adsorbed, and promote the low-temperature denitration performance of the catalyst.

[0069] From the above results, it can be known that the bimetallic oxide of manganese and cerium prepared in Example 1 is a mixed oxide composed of multiple oxides. Among them, the valence of manganese is +4, +3 and +2; the valence of cerium is +3 and +4.

[0070] Experimental Example 1

[0071] Figure 4The system used for the fixed bed denitration reaction experiment, and the experimental procedure is as follows: the gases are premixed in proportion to prepare simulated flue gas, the simulated flue gas is introduced into a fixed bed quartz reactor (which is internally filled with 0.1 g of the prepared catalyst) to perform catalytic denitration, and a flue gas analyzer is used to analyze the gas composition.

[0072] The reaction conditions of the experimental process are as follows: the simulated flue gas flow is 100 mL·min -1 [NO X ] = [NH3] = 500 ppm ([NO X ] = [NO] + [NO2]), [O2] = 5 vol%, [H2O] = 5 vol%, N2 is the balance gas, the gas mass space velocity (WHSV) is 60,000 mL / (g·h), and the temperature range of the catalytic test is 75-250°C.

[0073] Figure 5 The catalytic performance of the nitric acid modified fly ash based SCR catalyst Mn-Ce / AFA prepared in Example 1, the nitric acid modified fly ash based catalyst Mn / AFA prepared in Comparative Example 1, the nitric acid modified fly ash based catalyst Mn-Fe / AFA prepared in Comparative Example 3, and the nitric acid modified fly ash based catalyst Fe-Mn-Ce / AFA prepared in Comparative Example 4 at 125-250°C is shown in Table 1. Figure 5 As can be seen from Table 1, loading various metals can improve the NO X conversion rate of the catalyst to a certain extent, but the most obvious improvement is the loading of Mn and Ce on AFA. The denitration performance of Mn-Ce / AFA is better than that of other catalysts, and the NOX conversion rate reaches 100% at 150°C and remains above 90% in the range of 140-250°C.

[0074] Figure 6 The catalytic performance of the nitric acid modified fly ash based SCR catalyst Mn-Ce / AFA prepared in Example 1 and the catalyst Mn-Ce / Ti prepared in Comparative Example 5 at 125-250°C is shown in Table 2. Figure 6 As can be seen from Table 2, the catalytic performance of Mn-Ce / AFA is better than that of the Mn-Ce / Ti catalyst, indicating that the fly ash based catalyst prepared by the present application can effectively improve the activity, and the effect is not inferior to other catalysts.

[0075] Figure 7The catalytic performance at 125°C of the nitric acid-modified fly ash-based SCR catalyst Mn-Ce / AFA prepared in Example 1, the nitric acid-modified fly ash-based catalyst Mn / AFA prepared in Comparative Example 1, the nitric acid-modified fly ash-based catalyst Mn-Fe / AFA prepared in Comparative Example 3, the nitric acid-modified fly ash-based catalyst Fe-Mn-Ce / AFA prepared in Comparative Example 4, and the catalyst Mn-Ce / Ti prepared in Comparative Example 5 was compared. Since increasing temperature increases the average energy of reactant molecules, thereby increasing the collision frequency and collision energy of reactant molecules and accelerating the reaction rate, conducting the test at low temperatures helps to eliminate the influence of diffusion, thus allowing for a more accurate study of the intrinsic kinetics of the reaction. Therefore, the lowest temperature within the measurement range was selected. Figure 7 As can be seen, the reaction rate of the Mn-Ce / AFA catalyst is the fastest at 125℃.

[0076] Figure 8 The results show the water resistance of the nitric acid-modified fly ash-based SCR catalyst Mn-Ce / AFA prepared in Example 1. Figure 8 It can be seen that, under 5% water vapor conditions, the NO content of the nitric acid-modified fly ash-based SCR catalyst is [data missing]. X The conversion rate decreased by only about 5% and remained highly stable for 6 hours. After the water vapor was removed, NO... X The conversion rate recovered to 100%. This indicates that the nitric acid-modified fly ash-based SCR catalyst is suitable for practical industrial applications.

[0077] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a nitric acid-modified fly ash-based SCR catalyst, characterized in that, The nitric acid-modified fly ash-based SCR catalyst uses nitric acid-modified fly ash as a support and is loaded with manganese and cerium bimetallic oxides. The preparation method includes the following steps: (1) Soak dry fly ash in nitric acid solution, wash until neutral, and dry to obtain nitric acid modified fly ash; (2) Nitric acid-modified fly ash is added to a mixed solution of manganese salt and cerium salt, heated and stirred until the water evaporates, and then dried to obtain an intermediate; (3) The intermediate was calcined with oxygen to obtain a nitric acid-modified fly ash-based SCR catalyst; In the nitric acid-modified fly ash-based SCR catalyst, the loading ratio of manganese and cerium is 4-6% by mass.

2. The preparation method according to claim 1, characterized in that, The loading ratio of manganese and cerium is 5% by mass.

3. The preparation method according to claim 1, characterized in that, In step (1), the particle size of fly ash is 150~300 mesh; Alternatively, in step (1), the concentration of the nitric acid solution is 4~6 mol / L.

4. The preparation method according to claim 3, characterized in that, In step (1), the particle size of fly ash is 200 mesh; or, in step (1), the concentration of nitric acid solution is 5 mol / L.

5. The preparation method according to claim 1, characterized in that, In step (1), the soaking temperature is 80~100 ℃; the soaking time is 0.5~2 h.

6. The preparation method according to claim 5, characterized in that, In step (1), the soaking temperature is 90 ℃ and the soaking time is 1 h.

7. The preparation method according to claim 1, characterized in that, In step (2), the manganese salt is selected from one or more of manganese nitrate, manganese chloride, manganese sulfate, or manganese acetate; Alternatively, in step (2), the cerium salt is selected from one or more of cerium chloride, cerium nitrate, cerium acetate, and cerium ammonium nitrate.

8. The preparation method according to claim 7, characterized in that, In step (2), the cerium salt is selected from cerium nitrate.

9. The preparation method according to claim 1, characterized in that, In step (2), the heating temperature is 55~70 ℃.

10. The preparation method according to claim 9, characterized in that, In step (2), the heating temperature is 60 ℃.

11. The preparation method according to claim 1, characterized in that, In step (3), the calcination temperature is 450~600 ℃; the calcination time is 2.5~4 h.

12. The preparation method according to claim 11, characterized in that, In step (3), the calcination temperature is 60 °C and the calcination time is 3 h.

13. The nitric acid-modified fly ash-based SCR catalyst prepared by the method of claim 1 is used for the selective catalytic reduction of nitrogen oxides at low temperatures.

14. The application as described in claim 13, characterized in that, The temperature range for selective catalytic reduction of nitrogen oxides is 140~250 ℃.

15. The application as described in claim 14, characterized in that, The temperature for selective catalytic reduction of nitrogen oxides is 150℃.

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