CN115970735A - Molecular sieve-multi-element oxide composite denitration catalyst and preparation method thereof - Google Patents

Abstract

The invention discloses a molecular sieve multi-element oxide composite denitration catalyst and a preparation method thereof. The catalyst includes a carrier and an active component; the carrier is a Silicalite-1 molecular sieve with an MFI pore structure, and the active components are copper oxide and cerium The oxide of the element, wherein the cerium element surrounds the carrier, and the copper element is concentrated in the center of the carrier; based on the quality of the Silicalite-1 molecular sieve, the addition of the cerium element is 2wt%-12wt%, and the addition amount of the copper element 0.5wt% - 2.0wt%. The present invention adopts a molecular sieve multi-component oxide composite denitration catalyst with the above structure and its preparation method. The active component grows on the carrier in situ, and the two active components are distributed in different areas on the surface of the carrier, which improves the activity of the active component. The degree of dispersion and binding strength on the surface of the carrier, and the synergy between various metal active components are used to effectively reduce the denitration reaction temperature of the catalyst and widen the temperature window of the denitration reaction of the catalyst.

Description

Molecular sieve-multi-oxide composite denitration catalyst and preparation method thereof

Technical Field

The present invention relates to the technical field of SCR denitration catalysts, and in particular to a molecular sieve-multinary oxide composite denitration catalyst and a preparation method thereof.

Background Art

With the rapid development of China's economy, more and more nitrogen oxides ( _NOx ) are emitted into the atmosphere every year, causing acid rain, haze, and micro-particle pollution in many areas, which are extremely harmful to human body, environment, ecology, and social economy. Selective catalytic reduction (SCR) reaction is an effective method to remove atmospheric pollutants NO. SCR technology refers to the injection of _NH3 , urea or other nitrogen-containing reducing agents into flue gas in the presence of a catalyst, so that it selectively reacts with _NOx to generate _N2 , but does not undergo non-selective oxidation with _O2 , thereby achieving the purpose of reducing _NOx reduction temperature and improving _NOx purification efficiency.

At present, the most effective and widely used industrial SCR catalysts for denitration are V ₂ O ₅ /TiO ₂ and V ₂ O ₅ -WO ₃ /TiO ₂ catalysts, whose main advantages are high activity and high sulfur resistance. However, such catalysts need to be used at higher temperatures (>350°C) to avoid NH ₄ HSO ₄ and (NH ₄ ) ₂ S ₂ O ₇ generated by the reaction of SO ₂ and NH ₃ in flue gas from clogging the pore structure of the catalyst. However, in many cases, high-temperature operation leads to increased energy consumption and operating costs, and low-temperature SCR devices are more suitable for matching with most of the current industrial boilers in China. Therefore, the low temperature of SCR catalysts has attracted widespread attention. At present, many low-temperature denitration catalysts have good denitration efficiency, but they are easily poisoned by SO ₂ and difficult to be used in practice. Moreover, these catalysts mainly use coprecipitation and sol-gel methods, which easily lead to poor dispersion of the active components of the catalyst on the carrier, narrow temperature window, and poor stability. Therefore, it is necessary to develop other low-temperature sulfur-resistant and water-resistant denitration catalysts.

Summary of the invention

The purpose of the present invention is to provide a molecular sieve-multi-oxide composite denitrification catalyst and a preparation method thereof, so as to solve the problems of the above-mentioned denitrification catalyst having high operating temperature, poor sulfur and water resistance, uneven distribution of active components, and narrow temperature window.

To achieve the above-mentioned purpose, the present invention provides a molecular sieve-multi-oxide composite denitrification catalyst and a preparation method thereof, wherein the catalyst comprises a carrier and an active component; the carrier is a Silicalite-1 molecular sieve having an MFI pore structure, and the active component is an oxide of copper and an oxide of cerium, wherein the cerium element surrounds the carrier and the copper element is concentrated in the center of the carrier; based on the mass of the Silicalite-1 molecular sieve, the amount of cerium added is 2wt%-12wt%, and the amount of copper added is 0.5wt%-2.0wt%.

A method for preparing a molecular sieve-multi-oxide composite denitration catalyst comprises the following steps:

(1) Preparation of Silicalite-1 molecular sieve: Ethyl silicate is added dropwise to a tetrapropylammonium hydroxide aqueous solution under stirring conditions, and after hydrolysis, the mixture is transferred to a reaction kettle, and after hydrothermal treatment in an oven, the mixture is centrifuged, washed, and dried to obtain a molecular sieve precursor, and the molecular sieve precursor is transferred to a muffle furnace for calcination to obtain Silicalite-1 molecular sieve;

(2) Preparation of catalyst: First, copper salt and cerium salt are dissolved in deionized water to form a mixed solution, and then Silicalite-1 molecular sieve is immersed in the mixed solution, stirred at room temperature and then dried to obtain a catalyst precursor, and the catalyst precursor is ground and transferred to a muffle furnace for calcination to obtain a Cu _n Ce _m /Silicailite-1 catalyst.

Preferably, in step (1), ethyl silicate is added dropwise into a tetrapropylammonium hydroxide aqueous solution at 40° C. with constant magnetic stirring, and after hydrolysis at 40° C. for 2 hours, the silicon dioxide content in the ethyl silicate is 28%-40%. 28%-40% represents ethyl silicate of different specifications purchased from the market.

Preferably, the hydrothermal temperature in step (1) is 160° C. and the hydrothermal time is 48 hours.

Preferably, in step (1), the temperature is increased to 550° C. at a rate of 2° C./min and calcined for 6 h.

Preferably, in step (2), the temperature is raised to 100° C. to evaporate excess water, and the mixture is transferred to an oven at 80° C. for drying overnight.

Preferably, in step (2), the calcination is carried out at 500° C. for 4 h at a heating rate of 5° C./min in an air atmosphere.

Preferably, the copper salt includes one of copper nitrate, copper sulfate, copper acetate and copper chloride, and the mass of the copper element in the copper salt accounts for 0.5-2.0% of the mass of the Silicalite-1 molecular sieve.

Preferably, the cerium salt includes one of cerium nitrate, cerium chloride and cerium sulfate, and the mass of the cerium element in the cerium salt accounts for 2-12% of the mass of the Silicalite-1 molecular sieve.

The invention discloses an application of a molecular sieve-multi-oxide composite denitration catalyst, wherein the catalyst is applied to sulfur-resistant and water-resistant flue gas denitration at 135-270°C.

Therefore, the present invention adopts a molecular sieve-multi-oxide composite denitration catalyst with the above structure and a preparation method thereof, which has the following beneficial effects:

1. The denitration catalyst of the present invention uses Silicalite-1 molecular sieve with MFI pore structure as a carrier. Compared with the titanium dioxide used as a carrier in the prior art, the specific surface area is greatly increased, which is conducive to the catalyst adsorbing and activating more flue gas and improving the denitration efficiency;

2. The preparation method of the denitration catalyst of the present invention is to synthesize the carrier and the active component in steps, the active component is grown in situ on the carrier, and the two active components are distributed in different areas on the surface of the carrier, which improves the dispersion and bonding strength of the active component on the surface of the carrier, and improves the mechanical strength and thermal stability of the catalyst;

3. Compared with the existing single metal cerium-based or single metal copper-based carriers, the carrier of the present invention utilizes the synergistic effect between multiple metal active components, effectively reduces the denitration reaction temperature of the catalyst, broadens the temperature window of the catalyst denitration reaction, and has a higher denitration efficiency and a wider temperature window;

4. It has good sulfur resistance and water resistance, is environmentally friendly, has low cost and is easy to operate.

The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a SEM image of Silicalite-1 molecular sieve prepared from different silicon sources in Example 1-2;

FIG2 is a nitrogen adsorption-desorption isotherm of the Silicalite-1 molecular sieve prepared in Example 1-2;

FIG3 is a pore size distribution diagram of the Silicalite-1 molecular sieve prepared in Example 1-2;

FIG4 is a graph showing the denitration performance of CeO _x catalysts loaded with different Ce contents in Comparative Example 1;

FIG5 is a denitration performance curve of the CuO _x -CeO _x catalyst prepared in Example 3 in the temperature range of 90-300° C.;

FIG6 is the effect of H ₂ O on the denitration activity of CuO _x -CeO _x catalyst;

Figure 7 shows the effect of SO ₂ on the denitrification activity of CuO _x -CeO _x catalyst;

FIG8 is an XRD spectrum of CeO _x catalysts loaded with different Ce contents;

FIG9 is an XRD spectrum of CuO _x -CeO _x catalyst;

Figure 10 is the N ₂ adsorption/desorption isotherm of the support;

FIG11 is a pore size distribution curve of the carrier;

FIG12 is the N ₂ adsorption/desorption isotherm of CuO _x -CeO _x catalyst;

FIG13 is a pore size distribution curve of CuO _x -CeO _x catalyst;

FIG14 is TEM and HRTEM images of CuO _x -CeO _x catalysts ((a, b) Ce ₁₀ /Silicalite-1; (c, d) Cu _2.0 Ce ₁₀ /Silicalite-1; (e, f) Cu _2.0 /Silicalite-1);

FIG15 is an XPS spectrum of the catalyst ((a) Ce 3d fine spectrum in CuO _x -CeO _x catalyst; (b) Cu 2p fine spectrum in CuO _x -CeO _x catalyst);

FIG16 is a diagram showing the surface redox analysis of CuO _x -CeO _x catalysts;

FIG17 is a diagram of the surface acid sites of CuO _x -CeO _x catalysts;

FIG. 18 is an in-situ infrared adsorption spectrum of NH ₃ on the surfaces of Ce ₁₀ /Silicalite-1 and Cu _2.0 Ce ₁₀ /Silicalite-1 catalysts.

DETAILED DESCRIPTION

The present invention will be further described below. It should be noted that this embodiment is based on the technical solution and provides a detailed implementation method and a specific operation process, but the present invention is not limited to this embodiment.

Example 1

Preparation of Silicalite-1 Molecular Sieve

The preparation was carried out by hydrothermal method, in which tetrapropylammonium hydroxide (TPAOH) was used as a microporous template and ethyl silicate (TEOS, 28%) was used as a hydrolyzed silicon source. First, 12g of TEOS was added dropwise to 12.8g of TPAOH aqueous solution under constant magnetic stirring at 40°C. After hydrolysis at 40°C for 2h, the mixture was transferred to a 50mL reactor and hydrothermally heated in a 160°C oven for 48h. After the reactor cooled to room temperature, the mixture was centrifuged to obtain a white precipitate, which was washed three times with deionized water and ethanol respectively, and dried in an oven at 80°C overnight. Finally, the sample was ground and transferred to a muffle furnace, and calcined at 550°C for 6h at a heating rate of 2°C/min in an air atmosphere. The prepared sample was labeled as Silicalite-1-28%.

Example 2

Preparation of Silicalite-1 Molecular Sieve

The preparation method of Example 2 is different from that of Example 1 in that ethyl silicate (TEOS, 40%) is used as a hydrolyzed silicon source, and the prepared sample is labeled as Silicalite-1-40%.

FIG1 is a SEM image of Silicalite-1 molecular sieves prepared from different silicon sources in Example 1-2. It can be seen from the figure that the Silicalite-1 molecular sieve has an MFI pore size structure, and the particle size of the molecular sieve is about 200 nm. FIG2 is a nitrogen adsorption and desorption isotherm of the Silicalite-1 molecular sieve prepared from Example 1-2. FIG3 is a pore size distribution diagram of the Silicalite-1 molecular sieve prepared from Example 1-2. It can be seen from Table 1 that the specific surface areas of the Silicalite-1 molecular sieves prepared from different silicon sources are not much different. The pore volume of the Silicalite-1 molecular sieve prepared in Example 1 is 0.23 cm ³ /g, and the pore size is 1.97 nm. The pore volume of the Silicalite-1 molecular sieve prepared in Example 2 is 0.48 cm ³ /g, and the pore size is 3.90 nm.

Table 1 Textural data of Silicalite-1-28% and Silicalite-1-40%

Comparative Example 1

Preparation of Ce _m /Silicailite-1 catalyst:

Supported CeO _x catalysts with Ce contents ranging from 2wt% to 12wt% were prepared by impregnation method. First, a certain mass of cerium nitrate was dissolved in 20mL of deionized water, and then the prepared Silicalite-1 molecular sieve powder was immersed in the mixed solution. After magnetic stirring for 2h at room temperature, the temperature was raised to 100°C to evaporate excess water, and then the sample was transferred to an oven at 80°C for overnight drying. Finally, the sample was ground and transferred to a muffle furnace, and calcined at 500°C for 4h at a heating rate of 5°C/min in an air atmosphere. The prepared sample was labeled Ce _m /Silicailite-1 (i.e., CeO _x catalyst), where m represents the mass ratio of Ce content to carrier in the following feed, i.e., Ce:Silicalite-1=m:100. Specifically, m is 2, 4, 6, 8, 10, or 12.

Comparative Example 2

Preparation of Cu _n /Silicailite-1 catalyst:

Similarly, the impregnation method was used to prepare a supported catalyst with Silicalite-1 molecular sieve as the support and only CuO _x as the active component, with the Cu content ranging from 0.5wt% to 2.0wt%, respectively marked as Cu _n /Silicailite-1 (i.e., CuO _x catalyst), where n represents the mass ratio of the Cu content in the feed to the support, i.e., Cu:Silicalite-1=n:100. Specifically, n is 2.0.

Example 3

Preparation of Cu _n Ce _m /Silicailite-1 catalyst: Using the impregnation method, 0.5 wt % to 2.0 wt % of copper element was used to modify the Ce _m /Silicailite-1 catalyst with the best denitrification activity.

First, a certain amount of copper nitrate and cerium nitrate were dissolved in 20 mL of deionized water, and then the prepared Silicalite-1 molecular sieve powder was immersed in the mixed solution. After magnetic stirring at room temperature for 2 hours, the temperature was raised to 100°C to evaporate excess water, and then transferred to an oven at 80°C for overnight drying. Finally, the sample was ground and transferred to a muffle furnace, and calcined at 500°C for 4 hours at a heating rate of 5°C/min in an air atmosphere. The prepared sample was labeled Cu _n Ce _m /Silicailite-1 (i.e., CuO _x -CeO _x catalyst), where m and n represent the mass ratio of Ce and Cu content in the feed to the carrier, i.e., Cu:Ce:Silicalite-1=n:m:100.

Specifically, m is 10, and n is 0.5, 1.0, or 2.0.

Example 4

Catalyst NH ₃ -SCR Activity Test

The NH ₃ -SCR test of the catalyst was carried out on a fixed bed reactor, and the reaction gas composition was 500ppm NO, 500ppm NH ₃ , 3vol% O ₂ , 6vol% H ₂ O and 50ppm SO ₂ (used for water and sulfur resistance reaction), and N ₂ was used as the balance gas. 200mg of catalyst (40-60 mesh) was transferred into a quartz reaction tube with an inner diameter of 6mm, purged with N ₂ at 200℃ for 30min, cooled to room temperature, and cut into the reaction gas for adsorption until saturation. The NH ₃ -SCR reaction was carried out in the range of 90-360℃ and the space velocity was 18000h ^-1 . When the reaction reached stability at the predetermined stability, the concentrations of NO and NO ₂ after the reaction were measured by a portable flue gas analyzer (Testo Pro350). The conversion rate of NO _x was calculated by the following formula:

Where ([NO _x ]=[NO]+[NO ₂ ]).

FIG4 shows the denitration performance of CeO _x catalysts loaded with different Ce contents in Comparative Example 1. It can be seen from the figure that the NO _x conversion efficiency of the CeO _x catalyst increases significantly with the increase of the Ce content in the catalyst. However, when the Ce loading increases from 10wt% to 12wt%, the NO _x conversion efficiency of the catalyst decreases. This indicates that the optimal Ce loading of the catalyst is 10wt%. Too much CeO _x will occupy the active sites on the catalyst surface and hinder the NH ₃ -SCR reaction. However, it can also be seen from the figure that even the most active Ce ₁₀ /Silicalite-1 has less than 50% NO _x conversion efficiency in the low temperature section (<200°C), and can only reach 80% NO _x conversion efficiency in the medium and high temperature section (200-300°C), which is far from meeting the requirements of industrial denitration. Therefore, it is necessary to add other metals to the CeO _x catalyst to form a multi-metal system to improve the activity of the catalyst in the NH ₃ -SCR reaction.

Figure 5 is the denitration performance curve of CuO _x -CeO _x catalyst in the temperature range of 90-300°C. As can be seen from the figure, the denitration activity of Ce ₁₀ /Silicalite-1 catalyst is average, and even at 300°C, it can only reach 80% NO _x conversion efficiency. When a small amount of CuO _x species is added to the catalyst, the NO _x conversion efficiency of the catalyst is significantly improved. Among them, the Cu _2.0 Ce ₁₀ /Silicalite-1 catalyst can maintain more than 80% NO _x denitration efficiency in the temperature range of 135-270°C, and the denitration performance is greatly improved compared with the Ce ₁₀ /Silicalite-1 catalyst. In addition, although the Cu _2.0 /Silicalite-1 catalyst can also achieve 80% NO _x conversion efficiency after 165°C, its reaction activity ignition temperature is 30°C higher than that of the Cu _2.0 Ce ₁₀ /Silicalite-1 catalyst. At the same time, the reaction activity window of Cu _2.0 /Silicalite-1 catalyst is narrow (165-225℃), which makes it difficult to be applied to actual flue gas denitrification. However, with the increase of CuO _x species content in CuO _x -CeO _x catalyst, the NO _x conversion efficiency of the catalyst in the high temperature section decreases rapidly, which is mainly due to the relatively more side reactions caused by the incorporated CuO _x species. In general, the NH ₃ -SCR catalytic activity of CuO _x -CeO _x catalyst doped with a small amount of CuO _x species has been significantly improved, especially in the low temperature stage below 200℃. The Cu _2.0 Ce ₁₀ /Silicalite-1 catalyst with the best denitrification activity has higher low-temperature NO _x conversion efficiency and wider active temperature than the pure CuO _x and pure CeO _x catalysts alone, which can be attributed to the interaction of the binary active metal components.

Since the actual flue gas contains a small amount of H ₂ O and SO ₂ , the effects of H ₂ O and SO ₂ on the denitration activity of CuO _x -CeO _x catalysts were investigated at 210 °C. As can be seen from Figure 6, the effects of H ₂ O on the denitration performance of Ce ₁₀ /Silicalite-1, Cu _2.0 Ce ₁₀ /Silicalite-1 and Cu _2.0 /Silicalite-1 catalysts are similar. When 6 vol% H ₂ O is added to the reaction gas, the NO _x conversion efficiency of the three catalysts decreases by about 10%, but the NO _x conversion efficiency of the Cu _2.0 Ce ₁₀ /Silicalite-1 catalyst can still be maintained at more than 90%. When H ₂ O is removed from the reaction gas, the NO _x conversion efficiency of the catalyst quickly returns to its original level, indicating that the deactivation effect of H ₂ O on the catalyst is reversible, and the main reason for the deactivation is the competitive adsorption of H ₂ O and NH ₃ on the catalyst surface. Figure 7 shows the effect of SO ₂ on the denitration performance of the three catalysts. _When 50ppm SO2 is added to the reaction gas, the _NOx conversion efficiency of the _Ce10 /Silicalite-1 catalyst first increases, then begins to decline, and gradually stabilizes at about 50%. The improvement in the denitrification efficiency of the _Ce10 /Silicalite-1 catalyst is due to the fact that the pretreatment of a small amount of _SO2 helps to improve the surface acidity of the _Ce10 /Silicalite-1 catalyst, thereby improving the denitrification efficiency. However, the continuous introduction of _SO2 will react with _CeO2 to generate sulfate substances on the surface, resulting in irreversible deactivation of the catalyst. Since _CuOx is very easy to react with _SO2 to generate sulfate substances, in the presence of _SO2 , the _NOx conversion efficiency of the _Cu2.0 /Silicalite-1 catalyst is only maintained at about 20%, while the anti- _SO2 performance of the _{Cu2.0 Ce10} /Silicalite-1 catalyst is similar to that of the _Ce10 /Silicalite-1 catalyst, and can maintain more than 50% denitrification activity, which is greatly improved compared to the single _CuOx catalyst.

Example 5

The catalysts prepared in Comparative Examples 1-2 and Example 2 were characterized.

(1) The catalyst prepared in Comparative Example 1 was subjected to ICP test and XRD characterization.

The Ce content of all catalysts was characterized by ICP test, and the characterization results are shown in Table 2. As can be seen from the table, when the Ce content in the catalyst is relatively low (6wt% and below), the Ce content in the catalyst and the feed amount are basically consistent; when the Ce content in the catalyst is high (8wt% and above), the Ce content in the catalyst is about 1-2wt% lower than the feed amount, indicating that the catalyst with a high Ce content has a small loss of active components during the impregnation process, but overall the Ce content of each catalyst basically shows a linear increase trend.

Table 2 Ce content in different Ce _x /Silicailite-1 catalysts

Figure 8 is the powder XRD spectra of CeO _x catalysts loaded with different Ce contents. As can be seen from the figure, all catalysts show obvious characteristic peaks at 7.9°, 8.8°, 23.1°, 23.3° and 23.9°, which can be attributed to the MFI pore structure of the Silicalite-1 molecular sieve, indicating that the carrier still maintains its unique pore structure well during the impregnation process. The characteristic peaks attributed to CeO ₂ are not seen in the spectra of Ce ₂ /Silicalite-1 and Ce ₄ /Silicalite-1, which is mainly because the Ce content in these two catalysts is low. In other catalysts, characteristic peaks attributed to CeO ₂ can be found at 28.6°, 47.5° and 56.3°, indicating that tiny CeO ₂ crystals have been formed on the catalyst surface.

(2) The catalysts prepared in Comparative Examples 1-2 and Example 3 were characterized by ICP test, XRD, TEM, BET, TEM, and EDS.

The content of metal active components in the CuOx _{- CeOx} catalyst prepared in Example 3 was determined by ICP and summarized in Table 3. As can be seen from the table, the Ce content in each catalyst is basically the same, all around 8wt.%, and the loss compared to the feed amount (10wt.%) is basically the same. The Cu content in each catalyst is basically the same as the feed amount, and there is a significant content difference between the catalysts, which is conducive to further exploring their denitration performance.

Table 3 Content of metal components in the CuO _x -CeO _x catalyst prepared in Example 3

Figure 9 is the powder XRD spectrum of the CuOx _{- CeOx} catalyst. As can be seen from the figure, all catalysts show characteristic peaks at 7.9°, 8.8°, 23.1°, 23.3° and 23.9° belonging to the MFI pore structure, indicating that the unique pore structure of the carrier is not affected during the impregnation process. In addition, in the XRD spectrum of the catalyst, characteristic peaks belonging to _CeO2 can be found at 28.6°, 47.5° and 56.3°, but no characteristic peaks belonging to _CuOx are found, mainly because the Cu loading is lower than the minimum detection concentration of the instrument (5wt%).

The N ₂ adsorption/desorption isotherms and pore size distribution curves of the carrier are shown in Figures 10 and 11, and the N ₂ adsorption/desorption isotherms and pore size distribution curves of the CuO _x -CeO x catalysts are shown in Figures 12 and 13. The corresponding texture data of the two are summarized in Table 4. As can be seen from Figure 12, the N ₂ adsorption _/ desorption isotherms and carriers of all CuO _x -CeO _x catalysts are consistent, and all belong to type I isotherms, indicating that all catalysts have microporous structures, which is consistent with the pore size distribution results in Figure 13. All catalysts have obvious absorption peaks at 0.5-0.6nm, which can be attributed to the unique MFI pore structure of the carrier. In addition, the Ce ₁₀ /Silicalite-1 catalyst has a weak absorption peak near 10nm, and the absorption peak moves toward the small pore size direction with the increase of CuO _x doping amount, which can be attributed to the CeO ₂ nanoparticles distributed on the catalyst surface, which is consistent with the analysis of powder XRD. As can be seen from Table 4, the specific surface area and pore volume of Ce ₁₀ /Silicalite-1 catalyst loaded with CeO _x alone are 475.06 ^{m 2} /g and 0.41 cm ³ /g, respectively, which are basically consistent with the catalyst Cu _2.0 /Silicalite-1 loaded with CuO _x alone (478.90 m ² /g and 0.43 cm ³ /g). However, when binary metal oxides are loaded, the specific surface area and pore volume of the catalyst are reduced by 5-10%. However, in general, the specific surface area of the catalysts is above 400 m ² /g, and the pore volume is above 0.35 ^{m 2} /g, so there is little difference in the texture properties between the catalysts.

Table 4 Textural data of supports and CuO _x -CeO _x catalysts

Representative Ce ₁₀ /Silicalite-1, Cu _2.0 Ce ₁₀ /Silicalite-1 and Cu _2.0 /Silicalite-1 were selected as representative catalysts, and the surface morphology of the catalysts was further studied by TEM and EDS characterization. Figure 14 shows the TEM and HRTEM images of the CuO _x -CeO _x catalyst. As can be seen from the figure, in the catalyst loaded with pure CeO _x , there are relatively many dark spots at the edge of the carrier, which belong to CeO _x nanoparticles. In the catalyst loaded with pure CuO _x , the dark spots are mainly concentrated in the center of the carrier, and the size is smaller than that of CeO _x nanoparticles. In the CuO _x -CeO _x catalyst, dark spots are distributed in the center and around the carrier. In order to further confirm the composition of the dark spots on the surface of the carrier, the catalyst was tested and analyzed by EDS mapping. In Figure 14, the interplanar spacing of the dark spots on the edge of the carrier is approximately 0.32nm, corresponding to the (111) crystal plane of CeO ₂ crystals, which is consistent with the results of powder XRD. However, the dark spots in the center of the carrier are small in size and have low crystallinity, so the interplanar spacing cannot be accurately measured, but they can basically be considered as dispersed CuO _x species.

Example 6

The denitration principle analysis was performed on the catalysts of Example 3 and Comparative Examples 1-2.

(1) Surface structure analysis of CuO _x -CeO _x catalyst (XPS)

The atomic concentration and atomic valence of active metals on the surface of Ce ₁₀ /Silicalite-1, Cu _2.0 Ce ₁₀ /Silicalite-1 and Cu _2.0 /Silicalite-1 catalysts were analyzed by XPS test, and the results are shown in Figure 15 and Table 5. Figure 15(a) is the XPS fine spectrum of Ce3d of the catalyst. The orbital peaks marked with μ, μ”, μ”', ν, ν” and ν”' in the figure belong to Ce ⁴⁺ , while the orbital peaks marked with μ' and ν' belong to Ce ³⁺ . The ratio of Ce ³⁺ /(Ce ⁴⁺ +Ce ³⁺ ) is calculated based on the size of the peak area. It can be seen from Table 5 that after the addition of CuO _x species, the proportion of Ce ³⁺ on the catalyst surface is slightly reduced, indicating that there may be an interaction between CeO _x and CuO _x on the catalyst surface. Figure 15(b) is the XPS fine spectrum of the catalyst Cu 2p, where the spectrum of Cu in the Cu _2.0 Ce ₁₀ /Silicalite-1 catalyst can be fitted by five independent orbital peaks (peaks at 932eV, 933eV, 952eV, 953eV and 942eV), which are respectively attributed to Cu 2p _1/2 and Cu 2p _3/2 Cu ⁺ and Cu ²⁺ and satellite peaks. It can be seen from Table 5 that the atomic concentrations of Cu species on the surface of Cu _2.0 Ce ₁₀ /Silicalite-1 and Cu _2.0 /Silicalite-1 catalysts are 0.25% and 0.11%, respectively, indicating that more Cu species are exposed on the surface of the catalyst in Cu _2.0 Ce ₁₀ /Silicalite-1. It can also be found that almost all Cu species in the Cu _2.0 /Silicalite-1 catalyst exist in the form of Cu ²⁺ , while in the Cu _2.0 Ce ₁₀ /Silicalite-1 catalyst, the ratio of Cu ⁺ /(Cu ⁺ +Cu ²⁺ ) is 12.48%. The increase in the Cu ⁺ ratio can be attributed to the redox reaction between Cu and Ce species: Ce ³⁺ +Cu ²⁺ →Ce ⁴⁺ +Cu ⁺ , thereby improving the redox properties of the catalyst surface and helping to improve the reaction activity of low-temperature NH ₃ -SCR.

Table 5 Surface atomic concentration and valence distribution of CuO _x -CeO _x catalyst

(2) Analysis of the surface redox properties of CuO _x -CeO _x catalysts (H ₂ -TPR)

The redox properties of the catalyst are closely related to its catalytic activity in the NH ₃ -SCR reaction. Therefore, the redox properties of the catalyst surface were evaluated using the H ₂ -TPR technique, and the results are shown in Figure 16 and Table 6. As can be seen from the figure, the Ce ₁₀ /Silicalite-1 catalyst has only one reduction peak at 542°C, which can be attributed to the reduction of Ce species, indicating that the redox properties of the Ce ₁₀ /Silicalite-1 catalyst surface are very weak. The Cu _2.0 Ce ₁₀ /Silicalite-1 catalyst has four reduction peaks in the temperature range of 100-600°C, of which the reduction peaks marked as Ⅰ, Ⅱ and Ⅲ are related to the reduction of Cu species (Cu ²⁺ →Cu ⁺ →Cu), and the reduction peak marked as Ⅳ is related to the reduction of Ce species. By comparing the peak values of the reduction peaks of the three catalysts, it can be found that the peak values of the reduction peaks of the Cu _2.0 Ce ₁₀ /Silicalite-1 catalyst all move toward low temperatures, indicating that the Cu and Ce species in the catalyst become more active and easy to reduce. In addition, the reduction peak areas in the figure are integrated and summarized in Table 6. It can be found from the table that the reduction peak area of the Cu _2.0 Ce ₁₀ /Silicalite-1 catalyst is 1023 a.u., which is nearly twice that of the Cu _2.0 /Silicalite-1 catalyst and dozens of times that of the Ce ₁₀ /Silicalite-1 catalyst. This shows that the redox properties of the Cu _2.0 Ce ₁₀ /Silicalite-1 catalyst doped with CuO _x have been greatly improved. Therefore, the Cu _2.0 Ce ₁₀ /Silicalite-1 catalyst can exhibit the best catalytic activity in the low-temperature NH ₃ -SCR reaction.

Table 6 H ₂ reduction peak temperature and consumption of CuO _x -CeO _x catalyst

(3) Analysis of surface acid sites of CuO _x -CeO _x catalyst (NH ₃ -TPD)

The surface acidity of the catalyst determines the adsorption and activation ability of the reaction gas NH ₃ on the catalyst surface, which directly affects its activity in the NH ₃ -SCR reaction. Therefore, the surface acidity of the catalyst was evaluated by NH ₃ -TPD experiment. The desorption curve and integral calculation results of NH ₃ are shown in Figure 17 and Table 7, respectively. As can be seen from the figure, all three catalysts show obvious NH ₃ desorption peaks between 120 and 130 ° C, corresponding to NH ₃ adsorbed on the weakly acidic sites of the carrier. In addition, Cu _2.0 Ce ₁₀ /Silicalite-1 and Cu _2.0 /Silicalite-1 catalysts also have 2 and 1 NH ₃ desorption peaks between 150 and 300 ° C, respectively, which can be attributed to the Lewis acid sites provided by Cu species. Combined with the distribution of surface atomic valence states, the newly added Lewis acidic sites on the surface of Cu _2.0 Ce ₁₀ /Silicalite-1 catalyst can be attributed to Cu ²⁺ and Cu ⁺ , while the Cu _2.0 /Silicalite-1 catalyst only has Cu ²⁺ , and the Ce ₁₀ /Silicalite-1 surface can hardly provide Lewis acidic sites. Therefore, the surface acidity of the Cu _2.0 Ce ₁₀ /Silicalite-1 catalyst is significantly enhanced compared with the pure CeO _x and pure CuO _x catalysts alone, so it exhibits excellent catalytic activity in the low-temperature NH ₃ -SCR reaction.

Table 7 NH ₃ desorption peak temperature and total acid content of CuO _x -CeO _x catalyst

酸位上的NH₄ ⁺物种有关。中心在1395和1325cm^-1的较强吸收峰主要是N₂H₄物种-NH₂基团的摇摆振动。对于图18(b)中的Cu_2.0Ce₁₀/Silicalite-1催化剂，吸附在Lewis酸性位点上的NH₃的吸收峰(1566cm^-1)得到了显著增强，同时在1439cm^-1处出现了归属于NH₄ ⁺的吸收峰，说明Cu_2.0Ce₁₀/Silicalite-1催化剂表面的Lewis酸性和

酸性均得到了增强，这与之前的NH₃-TPD数据是一致的。Cu_2.0Ce₁₀/Silicalite-1催化剂表面酸性的增强有助于NH₃物种的吸附，从而提高催化剂的低温NH₃-SCR活性。In order to further understand the adsorption form of the reactant species on the catalyst surface, in situ infrared studies were conducted on the adsorption of NH ₃ and NO _x species on the catalyst surface. Under the conditions of NH ₃ /NO+O ₂ , infrared spectra of the catalyst surface were collected at different time points until the reactant gas was saturated with adsorption on the catalyst surface. Figure 18 is the in situ infrared adsorption spectrum of NH ₃ on the surfaces of Ce ₁₀ /Silicalite-1 and Cu _2.0 Ce ₁₀ /Silicalite-1 catalysts. For the Ce ₁₀ /Silicalite-1 catalyst in Figure 18 (a), the weak absorption peaks centered at 1570, 1221 and 1081 cm ^-1 can be attributed to the symmetrical and asymmetrical vibrations of the coordinated NH 3 adsorbed on the Lewis acidic sites. The weak absorption peak centered at 1740 cm ^-1 is similar to the symmetrical and asymmetrical vibrations of the coordinated NH ₃ adsorbed on the Lewis acidic sites.

The strong absorption peaks centered at 1395 and 1325 cm ^-1 are mainly due to the swing vibration of the -NH ₂ group of _the N ₂ H ⁴ species. For the _{Cu 2.0} Ce ₁₀ /Silicalite-1 catalyst in Figure 18(b), the absorption peak of NH ₃ adsorbed on the Lewis acidic site (1566 _cm ^-1 ) is significantly enhanced, and an absorption peak attributable to NH ₄ ⁺ appears at 1439 cm ^-1 , indicating that the _Lewis acidity and

The acidity is enhanced, which is consistent with the previous NH ₃ -TPD data. The enhancement of the surface acidity of Cu _2.0 Ce ₁₀ /Silicalite-1 catalyst is conducive to the adsorption of NH ₃ species, thereby improving the low-temperature NH ₃ -SCR activity of the catalyst.

Therefore, the present invention adopts a molecular sieve-multi-oxide composite denitrification catalyst with the above structure and a preparation method thereof, wherein the active components are grown in situ on the carrier, and the two active components are distributed in different areas on the carrier surface, thereby improving the dispersion and bonding strength of the active components on the carrier surface, improving the mechanical strength and thermal stability of the catalyst, utilizing the synergistic effect between multiple metal active components, effectively reducing the denitrification reaction temperature of the catalyst, and broadening the temperature window of the catalyst denitrification reaction.

Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims (10)

1. A molecular sieve-multi-component oxide composite denitration catalyst is characterized in that: the catalyst comprises a carrier and an active component; the carrier is a Silicalite-1 molecular sieve with an MFI pore structure, and the active components are oxides of copper and cerium, wherein the cerium surrounds the carrier, and the copper is concentrated in the central part of the carrier; based on the mass of the Silicalite-1 molecular sieve, the addition amount of the cerium element is 2 to 12 weight percent, and the addition amount of the copper element is 0.5 to 2.0 weight percent.

2. The preparation method of the molecular sieve-multi-component oxide composite denitration catalyst according to claim 1, characterized by comprising the following steps: the method comprises the following steps:

(1) Preparing a Silicalite-1 molecular sieve: dropwise adding ethyl silicate into a tetrapropyl ammonium hydroxide aqueous solution under the stirring condition, transferring the mixture into a reaction kettle after hydrolysis, centrifuging, washing and drying the mixture after hydrothermal in a drying oven to obtain a molecular sieve precursor, and transferring the molecular sieve precursor into a muffle furnace to calcine to obtain a Silicalite-1 molecular sieve;

(2) Preparation of the catalyst: firstly, dissolving copper salt and cerium salt in deionized water to form a mixed solution, then immersing the Silicalite-1 molecular sieve in the mixed solution, stirring at normal temperature, drying to obtain a catalyst precursor, grinding the catalyst precursor, transferring the ground catalyst precursor to a muffle furnace, and calcining to obtain Cu _n Ce _m A silicalite-1 catalyst.

3. The preparation method of the molecular sieve-polyoxide composite denitration catalyst according to claim 2, characterized in that: dropwise adding the ethyl silicate into the tetrapropyl ammonium hydroxide aqueous solution under the condition of constant magnetic stirring at 40 ℃ in the step (1), and hydrolyzing for 2 hours at 40 ℃, wherein the content of silicon dioxide in the ethyl silicate is 28-40%.

4. The preparation method of the molecular sieve-polyoxide composite denitration catalyst according to claim 2, characterized in that: the hydrothermal temperature in the step (1) is 160 ℃, and the hydrothermal time is 48 hours.

5. The preparation method of the molecular sieve-polyoxide composite denitration catalyst according to claim 2, characterized in that: in the step (1), the mixture is roasted for 6h at the temperature rising rate of 2 ℃/min to 550 ℃.

6. The preparation method of the molecular sieve-polyoxide composite denitration catalyst according to claim 2, characterized in that: and (2) raising the temperature to 100 ℃, evaporating excessive water, and transferring to an oven at 80 ℃ for drying overnight.

7. The preparation method of the molecular sieve-polyoxide composite denitration catalyst according to claim 2, characterized in that: in the step (2), roasting for 4h at the temperature rising rate of 5 ℃/min to 500 ℃ in the air atmosphere.

8. The preparation method of the molecular sieve-polyoxide composite denitration catalyst according to claim 2, characterized in that: the copper salt comprises one of copper nitrate, copper sulfate, copper acetate and copper chloride, and the mass of the copper element in the copper salt accounts for 0.5-2.0% of the mass of the Silicalite-1 molecular sieve.

9. The preparation method of the molecular sieve-polyoxide composite denitration catalyst according to claim 2, characterized in that: the cerium salt comprises one of cerium nitrate, cerium chloride and cerium sulfate, and the mass of cerium element in the cerium salt accounts for 2-12% of the mass of the Silicalite-1 molecular sieve.

10. The use of the molecular sieve-polyoxide composite denitration catalyst as claimed in claim 1, wherein: the catalyst is applied to the flue gas denitration of resisting sulfur and water at the temperature of 135-270 ℃.

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