CN115585039B - Application of cerium-zirconium oxide carrier material containing pyrochlore structure in diesel vehicle - Google Patents

Abstract

The invention discloses application of a cerium-zirconium oxide carrier material containing a pyrochlore structure in a diesel vehicle. Experiments show that k-CeZrO _x The material has excellent oxidation-reduction performance, NO oxidation activity and rich oxygen vacancies, and has excellent NH ₃ SCR Activity, for NO _x Has strong adsorptivity, rich nitrate species on the surface, and can prolong NO in cold start stage of diesel vehicle _x Is improved by "adsorption" + "reduction _x The purification efficiency of the diesel engine is greatly reduced in the cold start stage NO _x Is arranged in the air. k-CeZrO containing pyrochlore structure _x The carrier material is loaded on the acid component to form NH ₃ The SCR catalyst has higher SCR activity under the low-temperature condition and can reduce NO of a diesel engine under the low-temperature condition _x Is arranged in the air.

Description

Application of cerium-zirconium oxide carrier materials containing pyrochlore structure in diesel vehicles

Technical field

The invention belongs to the technical field of NH ₃ -SCR for diesel vehicle exhaust gas treatment, and specifically relates to the application of CeZrO _x carrier materials containing pyrochlore structure k-Ce ₂ Zr ₂ O ₈ in NH ₃ -SCR.

Background technique

NH ₃ selective catalytic reduction of NO _x (NH ₃ -SCR) is considered to be the most promising technology for NO _x purification in oxygen-rich diesel engines. In the diesel vehicle exhaust after-treatment system, NH ₃ injected from urea decomposition (CO(NH ₂ ) ₂ +H ₂ O=2NH ₃ +CO ₂ ) is the reducing agent for the NH ₃ -SCR reaction. However, the NH ₃ -SCR catalyst is located at the end of the diesel exhaust aftertreatment system. During cold start, the temperature at the SCR catalyst is very low, even lower than 180°C. At this time, urea cannot decompose into NH3 efficiently. If the system injects urea at this time, firstly, the incomplete decomposition of urea will lead to a lower concentration of the reducing agent _NH3 at low temperature, which may inhibit the effective reduction of _NOx ; secondly, the incomplete decomposition of urea will lead to by-products such as Melamine and incompletely decomposed urea are deposited on the surface of the catalyst, which may poison and deactivate the catalyst. Therefore, urea is generally injected in the SCR system when the temperature is higher than 180°C, and the NO _x emitted during cold start (< 180°C) is directly leaked. The _NOx emitted by diesel vehicles is the main source of mobile source NOx, of which _NOx emissions during the cold start process account for more than 90%. It is crucial to reduce _NOx emissions during the cold start phase of diesel vehicles. Due to the limitation of the urea injection temperature threshold (< 180°C), the sole use of NH3-SCR catalyst cannot meet the requirements for purifying NO _x at low temperatures. Therefore, it is urgent to improve the low-temperature or even ultra-low-temperature denitration activity of catalysts to meet strict emission regulations.

Contents of the invention

The purpose of the present invention is to address the shortcomings of the existing technology and provide a cerium-zirconium oxide carrier material containing a pyrochlore structure for use in diesel vehicles to improve the low-temperature activity of the NH ₃ -SCR catalyst and obtain a cerium-zirconium oxide carrier material under low-temperature conditions. Catalysts with high NH ₃ -SCR activity can reduce NO _x emissions from diesel engines at low temperatures during the cold start phase.

The cerium-zirconium oxide (k-CeZrO _x ) carrier material containing pyrochlore structure (k-Ce ₂ Zr ₂ O ₈ ) provided by the invention improves the low-temperature NOx adsorption-storage and NH3-SCR activity of the NH ₃ -SCR catalyst. applications in.

The k-Ce ₂ Zr ₂ O ₈ pyrochlore structure has an orderly arrangement of cations. 1/8 of the oxygen anions are very easy to remove. The utilization rate of cerium can be as high as 100%, showing excellent oxygen storage capacity and redox performance. This is conducive to the progress of heterogeneous catalytic reactions, and at the same time, oxygen vacancies are easily formed on the surface, which is conducive to the activation of reactants and the oxidation of NO to NO ₂ and promotes the progress of the SCR reaction. Experiments show that k-CeZrO _x material (CeZrO _x support material containing pyrochlore structure k-Ce ₂ Zr ₂ O ₈ ) has excellent redox properties, NO oxidation activity and abundant oxygen vacancies, with excellent NH ₃ - SCR activity can extend _the complete storage time _{of NO}

Further, the k-CeZrO _x carrier material containing pyrochlore structure (k-Ce ₂ Zr ₂ O ₈ ) is prepared by the following method: the CeZrO _x material is roasted at a high temperature of 900-1500ºC in an H ₂ atmosphere for 1 to 10 h, obtain k-CeZrO _x support material containing pyrochlore structure k-Ce ₂ Zr ₂ O ₈ .

Further, the molar ratio of Ce:Zr in the CeZrO _x is (0.2~0.95): (0.05~0.8).

The present invention also provides an NH ₃ -SCR catalyst with high low-temperature activity. The catalyst is a k-CeZrO _x carrier material containing a pyrochlore structure k-Ce ₂ Zr ₂ O ₈ .

The invention also provides an NH ₃ -SCR catalyst with high low-temperature activity. The catalyst is composed of a k-CeZrO _x carrier material containing a pyrochlore structure k-CeZrO _x and an acidic component loaded on the k-CeZrO _x carrier material. The acidic component is composed of at least one of WO ₃ , MoO ₃ , Nb ₂ O ₅ , SnO ₂ and HPAs.

Preferably, the NH ₃ -SCR catalyst with high low-temperature activity is one of WO ₃ /k-CeZrO _x , MoO ₃ /k-CeZrO _x , and WO ₃ -MoO ₃ /k-CeZrO _x

The present invention also provides that the above-mentioned NH ₃ -SCR catalyst with high low-temperature activity can be prepared by a conventional impregnation method.

Compared with the prior art, the present invention has the following beneficial effects:

In the present invention _, k-CeZrO _x material containing pyrochlore structure is used as an NH ₃ -SCR catalyst carrier material to purify diesel vehicle exhaust _NO The carrier material of the exhaust gas purification catalyst can adsorb and reduce _NOx produced by diesel vehicles during the cold start stage, greatly reducing _NOx emissions during the cold start stage, and solving the problem that the existing NH ₃ -SCR catalyst can no longer meet the requirements for low-temperature purification of _NOx. The requirements cause the problem of _NOx leakage emissions and can meet strict emission regulations.

Description of the drawings

Figure 1 shows the Raman spectrum (a) and XRD diffraction pattern (b) of the materials prepared in Examples and Comparative Examples;

Figure 2 shows the EPR spectrum (a) and H ₂ -TPR curve (b) prepared in Examples and Comparative Examples;

Figure 3 NO _x storage performance of the catalyst (a) and NH ₃ -SCR activity of the catalyst (b) during simulated cold start;

Detailed ways

The present invention will be further described below through examples. It is necessary to point out that the following examples are only used to further illustrate the present invention and cannot be understood as limiting the protection scope of the present invention. Those skilled in the art can make some non-essential improvements and adjustments to the present invention based on the above invention content for specific implementation. It still falls within the scope of invention protection.

Example 1

The CeZrO _x support (the molar ratio of Ce:Zr is 0.68:0.32) is treated at 950 ° C for 1-10 h in a H ₂ atmosphere to obtain a k-CeZrO _x material containing a k-Ce ₂ Zr ₂ O _pyrochlore structure.

Example 2

The CeZrO _x support (the molar ratio of Ce:Zr is 0.68:0.32) is treated at 950°C for 1-10 h in a H ₂ atmosphere to obtain a k-CeZrO _x material containing a k-Ce ₂ Zr _{2 O} pyrochlore structure, and then 1.1071g ammonium metatungstate was impregnated into 10g k-CeZrO _x . The sample was dried at 80°C for 10h, and then calcined at 550°C for 3h in an air atmosphere to obtain a WO ₃ /k-CeZrO _x catalyst.

Comparative example 1

Treat the CeZrO _x support (the molar ratio of Ce:Zr is 0.68:0.32) in an air atmosphere at 950°C for 1-10 h to obtain the A-CeZrO _x material.

Comparative example 2

The CeZrO _x support (the molar ratio of Ce:Zr is 0.68:0.32) is treated in an air atmosphere at 950°C for 1-10 h to obtain the A-CeZrO _x material. Then 1.1071g of ammonium metatungstate is impregnated into 10g of CeZrO _x . Dry at 80°C for 10 hours, and then calcined at 550°C for 3 hours in an air atmosphere to obtain a WO ₃ /A-CeZrO _x catalyst.

Each catalyst prepared as outlined above was tested for structure-activity performance.

(1) Vis-Raman

The Raman spectra of the samples were collected using a LabRAM HR laser Raman spectrometer (Horiba Jobin Yvon, France). A diode-pumped YAG laser is used, with an excitation wavelength of 532 nm, a power of 73.5 mw, and a detection range of 50-2000 cm ^-1 .

(2) X-ray diffraction (XRD)

Powder X-ray diffraction experiments were conducted using a Rigaku DX-2500 diffractometer (Rigaku, Japan) with Cu Kα (λ = 0.15406 nm) as the radiation source. The tube voltage is 40 kV and the current is 25 mA. XRD diffraction patterns were recorded in the range 10-80º at intervals of 0.2º/s.

(3) Electron paramagnetic spin resonance (ERP)

An electron paramagnetic resonance spectrometer (JEOL JES-FA200, Japan) was used to detect the concentration of oxygen vacancies on the catalyst at a temperature of 77 K.

(4) Hydrogen temperature programmed reduction (H ₂ -TPR)

The H ₂ temperature programmed reduction experiment was tested on a TP-5076TPD/TPR dynamic adsorber (Xianquan, Tianjin) using TCD as the detector. The sample mass in the quartz tube microreactor was 100 mg. The sample was pretreated at 450 °C for 1 h in a He atmosphere and then cooled to 30 °C. The temperature was increased from 30 °C to 800 °C at a rate of 10 °C/min in an atmosphere of 5 vol.%H ₂ -95 vol.%N ₂ and the curve was recorded. CuO was used as a standard sample to calculate the _H consumption of the catalyst.

(5) Catalyst NO _x purification performance evaluation

The AdSCR performance of the catalyst was evaluated in a self-contained fixed-bed quartz tube flow reactor. When the temperature is lower than 180 °C, NO _x emissions during the cold start phase are simulated in the reactor to evaluate the NO _x adsorption and storage performance of the monolithic catalyst. The simulated reaction atmosphere is: 500 ppm NO, 5 vol.%O ₂ and balance gas N ₂ , starting from 50 °C and heating at a rate of 20 °C/min, monitored and recorded by FT-IR Antaris IGS (Nicolet, USA) _NOx concentration as a function of reaction temperature and time. When the temperature is higher than 180 °C, 550 ppm NH ₃ is introduced into the reactor to continuously perform the NH ₃ -SCR reaction, and the concentration changes of N _x O _y and NH ₃ in the exhaust gas are monitored by FT-IR AntarisIGS.

The above test results are shown in Figures 1 to 3:

The Raman spectrum of the sample is shown in Figure 1a. The Raman vibration peak at 475 cm ^-1 is attributed to the F _2g symmetric vibration peak of the cubic fluorite structure O-Ce-O; the vibration peak at 615 cm ^-1 is attributed to defects. Structure; the Raman peaks at 432 cm ^-1 and 527 cm ^-1 are attributed to the characteristic vibration peaks of the pyrochlore structure k -Ce ₂ Zr ₂ O ₈ ^[226-228] . O-Ce-O and defect structures were detected in all samples, while characteristic peaks belonging to the pyrochlore structure were only observed in the Raman spectra of k-CeZrO _x and WO ₃ /k-CeZrO _x .

The XRD pattern of the sample is shown in Figure 1b. In the A-CeZrO _x sample obtained by air treatment at 950 °C, only the Ce _0.6 Zr _{0.4 O 2} solid solution with cubic fluorite structure was detected. However, the , in addition to the presence of Ce _0.6 Zr _0.4 O ₂ solid solution, the characteristic diffraction peak of pyrochlore structure k -Ce ₂ Zr ₂ O ₈ was observed at 2q = 14°. In addition, the loading of WO ₃ and calcination at 550 °C for 3 h in air atmosphere will not affect the structure of the corresponding support. The diffraction peaks of the cubic fluorite structure Ce _0.6 Zr _0.4 O ₂ solid solution can be detected on all catalysts, and the pyrochlore k The -Ce ₂ Zr ₂ O ₈ structure still exists in the W/k-CeZrO _x catalyst, which is consistent with the Raman results. Therefore, CeZrO _x will not form a pyrochlore structure k-Ce ₂ Zr ₂ O ₈ when treated in an air atmosphere at high temperature. Treatment of CeZrO _x in H ₂ atmosphere at high temperature can promote the formation of pyrochlore structure k-Ce ₂ Zr ₂ O ₈ and has good stability.

The low-temperature electron paramagnetic resonance (EPR) results are shown in Figure 2a. The symmetrical peak at g=2.003 is attributed to the characteristic peak of unpaired electrons in oxygen vacancies, and its intensity is closely related to the concentration of oxygen vacancies. The peak intensity of k-CeZrOx and WO ₃ /k-CeZrO _x catalysts with k -Ce ₂ Zr ₂ O ₈ structure is significantly stronger than that of A-CeZrO _x and WO ₃ /A-CeZrO _x , indicating that the existence of pyrochlore structure is indeed The concentration of oxygen vacancies on the surface of k-CeZrO _x and WO ₃ /k-CeZrO _x is increased. Therefore, the existence of the k -Ce ₂ Zr ₂ O ₈ structure promotes the generation of oxygen vacancies on the catalyst, which may be because the k- Ce ₂ Zr ₂ O ₈ structure has a lower average oxygen vacancy formation energy.

The H ₂ -TPR results are shown in Figure 2b. A-CeZrO _x presents a single-peak curve at 543 °C; while the peak temperature of k-CeZrO _x is located at 566 °C, but there is a shoulder peak at 486 °C. Although the reduction peak temperature of A-CeZrO _x is lower than that of k -CZ-6, importantly, the reduction peak intensity of A-CeZrO _x is significantly lower than that of k-CeZrO _x . The hydrogen consumption of k-CeZrO _x catalyst is 1208 μmol/g, which is at least twice that of A-CeZrO _x (549 μmol/g). Obviously, k-CeZrO _x has stronger redox performance than A-CeZrO _x . After loading WO ₃ , the reduction peak of the catalyst moves to high temperature. WO ₃ /A-CeZrO _x and WO ₃ /k-CeZrO _x have similar reduction peak shapes; the peaks below 600 °C are attributed to surface and subsurface oxygen. reduction, while the peak above 600 °C is attributed to the reduction of bulk oxygen. Although the reduction peak temperature of WO ₃ /k- _{CeZrO x} (706 _° C) is 32 ºC higher than that of _{WO 3 /} A-CeZrO The reduction peak intensity is also much lower than that of WO ₃ /k-CeZrO _x , and the hydrogen consumption of WO ₃ /k-CeZrO _x is 1354 mmol/g, which is twice that of WO ₃ /A-CeZrO _x (641 mmol/g) , indicating that WO ₃ /k-CeZrO _x has more reducible oxygen species than W/A-CeZrO _x . The presence of k -Ce ₂ Zr ₂ O ₈ promotes the reduction of oxygen species on the surface of the W/k-CeZrO _x catalyst, enhances the redox performance of the catalyst, and thereby improves the adsorption-selective catalytic reduction performance of the catalyst. This is mainly because the presence of pyrochlore structure improves the utilization rate and diffusivity of oxygen species, thereby improving the redox performance of the catalyst.

Figure 3a shows the adsorption and storage performance of each _catalyst on _NO The _NOx concentration at the outlet is used to measure the complete storage time of _NOx on the catalyst, that is, the duration for which _NOx achieves zero emissions in the _NOx storage reaction. The complete NO _x storage time of k-CeZrO _x is 115 s, that is _, NO _x is completely adsorbed on the surface of the sample during this period; the complete NO _x storage time of A- _CeZrO 115s. After loading the acidic promoter WO ₃ , the complete _NOx storage time of the catalyst is shortened, which is mainly due to the introduction of the acid promoter which is not conducive to the adsorption of acidic gas _NOx . However, the complete NO _x storage time (75 s) of WO ₃ /k-CeZrO _x is still 1.5 times that of the WO ₃ /A-CeZrO _x catalyst. Therefore, the presence of pyrochlore structure k- Ce ₂ Zr ₂ O ₈ significantly improves the NO _x adsorption and storage performance of k-CeZrO _x and WO ₃ /k-CeZrO _x catalysts.

_{Above 180 °} C, _{NO The NO x conversion rate of _ _ _} Therefore, the existence of the k- Ce ₂ Zr ₂ O ₈ structure can not only promote the adsorption storage of NO _x and extend the complete storage time of NO _x on the catalyst, but also improve the medium and low temperature activity of the NH ₃ -SCR reaction.

The above results show that: k-CeZrO _x support material containing k-Ce ₂ Zr ₂ O ₈ has excellent redox performance, NO oxidation performance and abundant oxygen vacancies; k-CeZrO _x containing k-Ce ₂ Zr ₂ O ₈ The carrier material has excellent NH ₃ -SCR activity, especially at medium and low temperatures; the k-CeZrO _x material containing k-Ce ₂ Zr ₂ O ₈ has strong NO _x adsorption performance, and abundant nitrate species are generated on its surface, which prolongs the The complete storage time of _NOx during cold start is determined. Therefore, k-CeZrO _x support material containing pyrochlore structure k-Ce ₂ Zr ₂ O ₈ can improve the NO _x adsorption-storage and reduction performance of the catalyst.

Claims (5)

1. Cerium zirconium oxide support material containing pyrochlore structure for improving NH ₃ Low temperature NO of SCR catalyst _x Adsorption-storage and NH ₃ Use of the pyrochlore-containing structure kappa-Ce in SCR Activity ₂ Zr ₂ O ₈ kappa-CeZrO of (C) _x The carrier material is prepared by mixing CeZrO _x Material at H ₂ Roasting at 900-1500 ℃ for 1-10 h under atmosphere to obtain the product; the CeZrO _x The molar ratio of Ce to Zr in the material is (0.2-0.95): 0.05-0.8.

2. Cerium-zirconium oxide carrier material containing pyrochlore structure as NH in purifying nitrogen oxides in cold start stage of diesel vehicle ₃ Use of a support material for SCR catalysts, said support material containing pyrochlore structures kappa-Ce ₂ Zr ₂ O ₈ kappa-CeZrO of (C) _x The carrier material is prepared by mixing CeZrO _x Material at H ₂ Roasting at 900-1500 ℃ for 1-10 h under atmosphere to obtain the product; the CeZrO _x The molar ratio of Ce to Zr in the material is (0.2-0.95): 0.05-0.8.

3. NH with high low temperature activity ₃ -SCR catalyst, characterized in that the catalyst is a pyrochlore structure containing kappa-Ce ₂ Zr ₂ O ₈ kappa-CeZrO of (C) _x A carrier material; the pyrochlore structure-containing kappa-Ce ₂ Zr ₂ O ₈ kappa-CeZrO of (C) _x The carrier material is prepared by mixing CeZrO _x Material at H ₂ Roasting at 900-1500 ℃ for 1-10 h under atmosphere to obtain the product; the CeZrO _x The molar ratio of Ce to Zr in the material is (0.2-0.95): 0.05-0.8.

4. NH with high low temperature activity ₃ SCR catalyst, characterized in that the catalyst consists of a pyrochlore-containing structure kappa-Ce ₂ Zr ₂ O ₈ kappa-CeZrO of (C) _x Support material and support on kappa-CeZrO _x An acidic component on a support material, said acidic component being WO ₃ 、MoO ₃ 、Nb ₂ O ₅ 、SnO ₂ At least one of HPAs; the pyrochlore structure-containing kappa-Ce ₂ Zr ₂ O ₈ kappa-CeZrO of (C) _x The carrier material is prepared by mixing CeZrO _x Material at H ₂ Roasting at 900-1500 ℃ for 1-10 h under atmosphere to obtain the product; the CeZrO _x The molar ratio of Ce to Zr in the material is (0.2-0.95): 0.05-0.8.

5. NH having high Low temperature Activity according to claim 4 ₃ -SCR catalyst, characterized in that the catalyst is WO ₃ /κ-CeZrO _x 、MoO ₃ /κ-CeZrO _x 、WO ₃ -MoO ₃ /κ-CeZrO _x At least one of them.