CN108404904B

CN108404904B - Mesoporous Ce for low-temperature SCR reactionxW1-xOyProcess for preparing catalyst

Info

Publication number: CN108404904B
Application number: CN201810440013.9A
Authority: CN
Inventors: 肇启东; 王丹; 石勇; 李新勇
Original assignee: Dalian University of Technology
Current assignee: Dalian University of Technology
Priority date: 2018-05-07
Filing date: 2018-05-07
Publication date: 2021-03-26
Anticipated expiration: 2038-05-07
Also published as: CN108404904A

Abstract

The invention provides mesoporous Ce for low-temperature SCR reaction_xW_1‑xO_yA preparation method of a catalyst belongs to the technical field of preparation of catalytic materials and nano materials. Prepared Ce_xW_1‑xO_yCatalyst, at a temperature in the range of 175 ℃ and 400 ℃, catalyst NO_xThe conversion rate reaches 95 percent, and the reported low-temperature limit of the catalyst is broken through. It is characterized by that it uses CTAB as template and uses n-amyl alcohol as cosurfactant to make Ce (NO)₃)₃·6H₂O and N₁₀H₄₀W₁₂O₄₁·xH₂O is condensed and precipitated by triethylamine to prepare Ce_xW_1‑xO_yA catalyst. The material is of a mesoporous structure and has extremely excellent low-temperature catalytic activity. The raw materials used in the invention are cheap and easily available, the preparation method is simple and easy to operate, the conditions are mild, and the equipment requirement is low, so that the preparation method is environment-friendly. Prepared Ce_xW_1‑xO_yThe catalyst can be used in the fields of light, electricity, energy storage, catalysis and the like.

Description

Mesoporous Ce for low-temperature SCR reactionxW1-xOyProcess for preparing catalyst

Technical Field

The invention belongs to the technical field of environmental catalytic materials and environmental protection, and particularly relates to a preparation method of a mesoporous cerium-based catalyst, which is mainly used for automobile exhaust or factory flue gas NO_xAnd (5) low-temperature purification treatment.

Background

With the acceleration of urbanization and industrialization, a series of environmental problems are generated. Especially, the problem of air pollution in recent years is attracting much attention. Air pollution mainly comes from power plants and vehiclesCombustion of medium mineral fuels and other various routes. Among the various air pollutants, Nitrogen Oxides (NO)_x) Are generally considered to be the major contaminants responsible for haze. In addition, it can cause photochemical smog, acid rain, ozone layer holes, greenhouse effect and other environmental problems to further harm the ecological system of human being. Reduction of NO_xAt present, three methods are mainly adopted for discharging: precaution before combustion, control during combustion, and treatment after combustion. Precaution before combustion and control during combustion only controlling a small part of NO_xIn order to meet higher NO_xAnd emission reduction requirements need to adopt a mode of controlling after combustion.

Catalytic reduction process (NH)₃SCR) is one of the most attractive methods among various post-combustion treatment methods because of its advantages such as low cost and high efficiency. Currently, most industrial denitration processes adopt NH₃Of SCR, and WO₃(MoO₃) Modified V₂O₅/TiO₂The catalyst is also NH which is the most widely used in industry at present₃-an SCR catalyst. However, these catalyst systems still suffer from several drawbacks, including: (1) v₂O₅In the process, the material is easy to sublimate and fall off, and secondary pollution and harm to human are easily caused after the material enters the environment; (2) the operating temperature window is narrow, and a large amount of N is formed at high temperature₂O; (3) poor high temperature thermal stability. Furthermore, a typical denitration catalyst needs to be installed downstream of the particulate trap and the flue gas desulfurization, which requires that the catalyst can normally operate at a temperature of less than 200 ℃. In addition, many industrial furnaces have flue gas temperatures much lower than power plants, often also requiring low temperature NH₃-SCR. Thus, an environmentally friendly NH operating at low temperatures was sought₃SCR catalysts are of great interest.

In recent years, cerium oxide has been a hot point of research because of its wide application in the catalytic fields of three-way catalysis, catalytic wet oxidation, oxygen permeable membrane systems, fuel cell processing, photocatalysis, and the like. Cerium oxide has been used as NH due to its good textural properties and ability to interact with other components₃-SCRA cocatalyst or support for the catalyst. W is used as a stabilizer and an accelerant, and can obviously improve the specific surface area and Ce of the catalyst³⁺Ratio, surface acidity and number of active sites. Therefore W-modified Ce-based catalysts have been extensively studied from different perspectives. Zhang et al, (Zhang, et al, Chinese Journal of Catalysis, 2017.38 (10): p.1749-1758.) reported that cerium tungsten prepared by wet impregnation has NO conversion of 88% or more in the range of 200 to 450 ℃. Zhan et al, (Zhan et al, Appl Catal B,2017.203: p.199-209) reported the synthesis of highly ordered mesopores from KIT-6 as a hard template₃(1)-CeO₂(1 represents the molar ratio of W to Ce), and the conversion rate is 100 percent in the range of 225-350 ℃. Although the research and development field of the denitration catalyst is vigorously developed in recent years, the breakthrough is difficult in the aspect of low-temperature catalytic activity.

The invention efficiently prepares Ce by taking CTAB as a template, taking n-amyl alcohol as cosurfactant and taking triethylamine as a condensing agent_xW_1-xO_yCatalyst, so far, no document or patent report adopts a method of efficiently preparing Ce by taking n-amyl alcohol as a cosurfactant and taking triethylamine as a coagulant surfactant for assistance_xW_1-xO_yA catalyst. And prepared Ce_xW_1-xO_yThe conversion rate of the catalyst reaches 95% within the range of 175-400 ℃. Has a great breakthrough in low-temperature activity.

Disclosure of Invention

The invention aims to provide a preparation method of a cerium-tungsten catalyst. The catalyst has good low-temperature catalytic activity, and has the temperature of 175 ℃ and the space velocity of 24000h^-1Under conditions of (1) NO_xThe conversion rate reaches more than 98 percent.

The technical scheme of the invention is as follows:

mesoporous Ce for low-temperature SCR reaction_xW_1-xO_yThe preparation method of the catalyst comprises the following steps:

(1) preparing a template agent aqueous solution by taking CTAB as a template agent; adding n-amyl alcohol as a cosurfactant to the template agent aqueous solution; violently stirring at the temperature of 80-100 ℃ to form a transparent solution;

(2) adding Ce (NO) to the transparent solution obtained in step (1)₃)₃·6H₂O and N₁₀H₄₀W₁₂O₄₁·xH₂Continuously stirring for 1-2 h;

(3) under the stirring state, adding triethylamine into the solution obtained in the step (2), then adding 0.2mol/L NaOH solution, and adjusting the pH value to 8-10; continuously stirring and standing;

(4) aging the layered suspension obtained in the step (3) at the temperature of 90 ℃ for 2-5 h, washing with 60-90 ℃ ionized water for 2-5 times, performing centrifugal separation, drying at 90 ℃ overnight, and calcining at 500 ℃ for 4h to obtain a light yellow catalyst;

wherein CTAB (Ce + W) and the molar ratio of n-pentanol to triethylamine is 3:5:15: 5.

The heating temperature in the step (1) is 90 ℃;

the stirring time of the step (2) is 1 h;

adjusting the pH value to 9, continuously stirring for 2h, and standing for 1 h;

in the step (4), the aging time is 3 hours, and the washing times are 3 times.

Ce prepared by the invention_xW_1-xO_yCatalysts for NH₃-SCR denitration reaction.

The invention has the beneficial effects that: the W species is used as a stabilizer and promoter to significantly improve the catalytic activity of the catalyst. Compared with other preparation methods, the surfactant-assisted method has more excellent low-temperature activity. Ce_0.75W_0.25O_yThe catalyst can reach 95 percent of denitration activity in the temperature range of 175 ℃ and 400 ℃. And the specific surface area increases with the addition of the W species. However, excessive loading of W may block the pore size resulting in a decrease in specific surface area and a decrease in activity. At the same time Ce_xW_1-xO_yRelative to pure CeO_xSo that it possesses higher Ce³⁺Ratio, adsorbed oxygen content and acid sites. The catalyst also has more active species than the catalyst prepared by different preparation methods reported in the literature. Thus, it is possible to provideCe_0.75W_0.25O_yHas better low-temperature catalytic activity.

Drawings

FIG. 1 is a schematic view of an apparatus used in the production method of the present invention.

Fig. 2(a) is a graph comparing the denitration activity of the catalyst prepared by the present invention with that of pure cerium oxide and tungsten oxide.

FIG. 2(b) is a graph comparing denitration activities of examples 1 to 4.

FIG. 3(a) shows N in examples 1 to 3₂The adsorption and desorption curve is shown in the specification,

FIG. 3(b) is a graph showing the distribution of the aperture in examples 1 to 3.

Figure 4 is a XRD characterization chart of the catalyst prepared by the present invention and examples 1-4.

FIG. 5 shows a pure cerium oxide catalyst prepared according to the present invention and NH of example 2₃TPD comparison.

Fig. 6(a) is a Ce3d XPS spectrum of pure cerium oxide prepared according to the present invention.

Fig. 6(b) is an XPS spectrum of the catalyst Ce3d prepared in example 2.

FIG. 6(c) is an O1s XPS spectrum of pure cerium oxide prepared according to the present invention.

FIG. 6(d) is an O1s XPS spectrum of the catalyst prepared in example 2.

Fig. 7 is a high resolution Transmission Electron Microscope (TEM) image of the catalyst prepared in example 2.

In the figure: 1, supplementing gas; 2 NO-NO₂-NO_xAn analyzer; 3, a thermocouple; 4, a catalyst; 5, quartz wool; 6 quartz tube reactor; 7 a gas inlet; 8 gas outlet.

Detailed Description

The following further describes a specific embodiment of the present invention with reference to the drawings and technical solutions.

Example 1

(1) Adding 4.37g CTAB as a template agent into deionized water, then adding 5.3g n-amyl alcohol as a cosurfactant, and violently stirring under the constant temperature heating condition of 90 ℃ to form a transparent solution.

(2) 4.3g of Ce (NO) were weighed out separately₃)₃·6H₂O and 2.47g of N₁₀H₄₀W₁₂O₄₁·xH₂O is added into the transparent solution obtained in the step (1) and stirring is continued for 1 h.

(3) Adding 2g of triethylamine into the solution obtained in the step (2) under the stirring state, then adding 0.2mol/L of NaOH solution, adjusting the pH value to 9, continuously stirring for 2h, and standing for 1 h.

(4) The resulting layered suspension of step (3) was aged in an oven at 90 ℃ for 3h, then washed 3 times with 80 ℃ deionized water, centrifuged, dried at 90 ℃ overnight, and then calcined at 500 ℃ for 4h to give a pale yellow catalyst.

SCR denitration activity was tested as shown in figure 2.

The specific surface area characterization results are shown in table 1 and fig. 3.

The XRD results are characterized in figure 4.

Example 2

(1) Adding 8.7g of CTAB as a template agent into deionized water, then adding 10.6g of n-amyl alcohol as a cosurfactant, and violently stirring under the constant-temperature heating condition of 90 ℃ to form a transparent solution.

(2) 13g of Ce (NO) were weighed out separately₃)₃·6H₂O and 2.47g of N₁₀H₄₀W₁₂O₄₁·xH₂O is added into the transparent solution obtained in the step (1) and stirring is continued for 1 h.

(3) Under the stirring state, 4g of triethylamine is added into the solution obtained in the step (2), then 0.2mol/L of NaOH solution is added, the pH value is adjusted to 9, and after stirring is continued for 2 hours, the solution is kept standing for 1 hour.

SCR denitration activity was tested as shown in figure 2.

The XRD characterization results are shown in FIG. 4.

The XPS characterization results are shown in table 2, fig. 6.

NH₃The TPD characterization results are shown in FIG. 5.

The TEM characterization results are shown in FIG. 7.

Example 3

(1) Adding 13.1g CTAB as a template agent into deionized water, then adding 15.9g n-amyl alcohol as a cosurfactant, and violently stirring under the constant temperature heating condition of 90 ℃ to form a transparent solution.

(2) 21.7g of Ce (NO) was weighed out separately₃)₃·6H₂O and 2.5g of N₁₀H₄₀W₁₂O₄₁·xH₂O is added into the transparent solution obtained in the step (1) and stirring is continued for 1 h.

(3) Under the stirring state, 6g of triethylamine is added into the solution obtained in the step (2), then 0.2mol/L of NaOH solution is added, the pH value is adjusted to 9, and after stirring is continued for 2 hours, the solution is kept standing for 1 hour.

SCR denitration activity was tested as shown in figure 2.

The XRD characterization results are shown in FIG. 4.

Example 4

(2) 4.3g of Ce (NO) were weighed out separately₃)₃·6H₂O and 7.4g of N₁₀H₄₀W₁₂O₄₁·xH₂O is added into the transparent solution obtained in the step (1) and stirring is continued for 1 h.

SCR denitration activity was tested as shown in figure 2.

Simulating the composition of flue gas by distributing gas through a steel gas cylinder, wherein the composition comprises NO and NH₃、O₂Etc. NH₃Is a reducing gas. Simulated flue gas NH₃、NO、O₂And the flow rate of He is controlled by a mass flow meter respectively, the He and the He are mixed uniformly and then enter a fixed bed reactor, and quartz wool is placed below the fixed bed to fix the catalyst. (see attached figure 1) at the beginning of the experiment, 0.2g of the sample prepared in examples 1-4 was weighed and placed in a reactor, a temperature-raising program was set, the data were measured sequentially from low temperature to high temperature, and the initial concentration of NO and the concentration of reaction off-gas were measured using a flue gas analyzer, and the residence time was 40min at each temperature.

The denitration activity of the catalysts prepared in examples 1-4 and the control group was characterized, and the results are shown in FIG. 2

FIG. 2(a) shows pure CeO₂Catalyst NO_xLower conversion, maximum NO_xThe conversion was about 65%. WO₃The activity at low temperature is very weak, the activity reaches 80% at the temperature of 320-400 ℃, the point with the best activity appears at the temperature of 400 ℃, and the activity reaches 90%, which shows that the high-temperature activity can be obviously improved by adding W. Although WO₃The denitration activity of the catalyst has certain limitation but is obviously improved compared with that reported in similar documents published in recent years, which shows that the preparation method has great influence on the catalyst. Relative to pure CeO₂And WO₃Addition of W significantly increases Ce_0.75W_0.25O_xLow temperature activity and temperature window. FIG. 2(b) shows different denitration activities of catalysts of examples 1-4 with different Ce/W ratios. Studies have shown that the best low temperature activity and the best temperature window (175 ℃ C.; 400 ℃ C.; NO) are obtained when the Ce/W ratio is 3:1_xThe conversion rate reaches more than 95 percent), and compared with the reported catalyst of the same type, the activity is obviously increased.

The catalysts prepared in examples 1-3 were characterized by specific surface area and pore structure as shown in table 1 and fig. 3, and by morphology as shown in fig. 7.

As can be seen from table 1, fig. 3 and fig. 7, the catalyst belongs to an ion-stacking mesoporous structure, and the specific surface area is reduced with the increase of W species. It is shown that the specific surface area is not a major factor in determining the amount of catalyst activity, and that the W species are dispersed on the catalyst surface mainly in a free state and an amorphous state, and that an excessive increase in W species may block the pores of the catalyst to cause a decrease in specific surface area, which may be a major cause of the decrease in specific surface area. The larger pore volume and the smaller pore diameter are more beneficial to removing NO_x。Ce_0.75W_0.25O_xRelative to Ce_0.5W_0.5O_xHas larger pore diameter compared with Ce_0.86W_0.14O_xHas smaller pore volume, which may be Ce_0.75W_0.25O_xHas better catalytic activity.

XRD characterization of the catalysts prepared in examples 1-4 and the control was performed, and the results are shown in FIG. 4.

As can be seen from fig. 4, the Ce-containing crystalline phase can be clearly seen in the catalysts with different Ce/W ratios, indicating that the W species are dispersed on the catalyst surface in a dispersed state and an amorphous state. And the crystallinity of the Ce-containing crystalline phase decreases with increasing W species. The addition of W is shown to destroy the crystal structure of Ce, and the reduction of the crystallinity of the catalyst is beneficial to generating more surface defects, so that more adsorption centers and reaction centers are provided for the reaction, and the smooth proceeding of the reaction is promoted.

XPS characterization was performed on the catalysts prepared in example 2 and the control, and the results are shown in table 2 and fig. 6.

As can be seen from table 2 and fig. 6, the peak position of Ce generally shifts toward a high energy level with the addition of W, indicating that Ce interacts with W to form a new species, which is consistent with XRD characterization. And with the addition of W Ce³⁺Ce and O_β/(O_α+O_β) Significantly increased, even surpassed the same type of catalyst reported at present. This may be Ce_0.75W_0.25O_xHave better catalytic activityThus, the method is simple and easy to operate.

NH of the catalysts prepared in example 2 and the control₃TPD characterization, results are shown in FIG. 5.

NH₃TPD characterization can infer the distribution of strong, medium, weak acidic sites and the number of acidic sites on the catalyst surface. The acidic sites are a major factor affecting the efficiency of the catalyst reaction. There were three characteristic ammonia desorption peaks in each catalyst sample. The first peak appears between 100 ℃ and 300 ℃ and belongs to the Lewis acid site, the second peak appears between 400 ℃ and 500 ℃ and the third peak appears in the temperature range of 600 ℃ and 800 ℃ and belongs to the Lewis acid site

An acidic site. As can be seen, with the addition of W,

acid sites are significantly increased, and thus Ce_0.75W_0.25O_xThe catalytic activity of (3) is obviously improved.

Specific surface area characterization results

TABLE 1

XPS characterization results

TABLE 2

Comparison with catalysts of different preparation methods

TABLE 3

Claims

1. Mesoporous Ce for low-temperature SCR reaction_xW_1-xO_yThe preparation method of the catalyst is characterized by comprising the following steps:

(2) adding Ce (NO) to the transparent solution obtained in step (1)₃)₃·6H₂O and N₁₀H₄₀W₁₂O₄₁·zH₂Continuously stirring for 1-2 h;

wherein CTAB (Ce + W), n-pentanol and triethylamine are in a molar ratio of 3:5:15:5, and x is 0.25, 0.5, 0.75 or 0.86.

2. Mesoporous Ce for low-temperature SCR reactions according to claim 1_xW_1-xO_yThe preparation method of the catalyst is characterized in that the heating temperature of the step (1) is 90 ℃.

3. Mesoporous Ce for low-temperature SCR reactions according to claim 1 or 2_xW_1-xO_yThe preparation method of the catalyst is characterized in that the stirring time of the step (2) is 1 h.

4. Mesoporous Ce for low-temperature SCR reactions according to claim 1 or 2_xW_1-xO_yThe preparation method of the catalyst is characterized in that the pH value of the step (3) is adjusted to 9, the continuous stirring time is 2 hours, and the standing time is 1 hour.

5. The method of claim 3 for loweringMesoporous Ce of warm SCR reaction_xW_1-xO_yThe preparation method of the catalyst is characterized in that the pH value of the step (3) is adjusted to 9, the continuous stirring time is 2 hours, and the standing time is 1 hour.

6. Mesoporous Ce for low-temperature SCR reactions according to claim 1, 2 or 5_xW_1-xO_yThe preparation method of the catalyst is characterized in that the aging time in the step (4) is 3 hours, and the washing times are 3 times.

7. Mesoporous Ce for low-temperature SCR reactions according to claim 3_xW_1-xO_yThe preparation method of the catalyst is characterized in that the aging time in the step (4) is 3 hours, and the washing times are 3 times.

8. Mesoporous Ce for low-temperature SCR reactions according to claim 4_xW_1-xO_yThe preparation method of the catalyst is characterized in that the aging time in the step (4) is 3 hours, and the washing times are 3 times.