WO2021022593A1 - 一种纳米铈锆复合氧化物及其在催化nox还原反应中的应用 - Google Patents

一种纳米铈锆复合氧化物及其在催化nox还原反应中的应用 Download PDF

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WO2021022593A1
WO2021022593A1 PCT/CN2019/102759 CN2019102759W WO2021022593A1 WO 2021022593 A1 WO2021022593 A1 WO 2021022593A1 CN 2019102759 W CN2019102759 W CN 2019102759W WO 2021022593 A1 WO2021022593 A1 WO 2021022593A1
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oxide
cerium
composite oxide
zirconium
nano
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French (fr)
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宋锡滨
张兵
张曦
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山东国瓷功能材料股份有限公司
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/74—General processes for purification of waste gases; Apparatus or devices specially adapted therefor
    • B01D53/86—Catalytic processes
    • B01D53/8621—Removing nitrogen compounds
    • B01D53/8625—Nitrogen oxides
    • B01D53/8628—Processes characterised by a specific catalyst
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/92—Chemical or biological purification of waste gases of engine exhaust gases
    • B01D53/94—Chemical or biological purification of waste gases of engine exhaust gases by catalytic processes
    • B01D53/9404—Removing only nitrogen compounds
    • B01D53/9409—Nitrogen oxides
    • B01D53/9413—Processes characterised by a specific catalyst
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
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    • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
    • B01J23/38—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals
    • B01J23/54—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals combined with metals, oxides or hydroxides provided for in groups B01J23/02 - B01J23/36
    • B01J23/56—Platinum group metals
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    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/02—Impregnation, coating or precipitation
    • B01J37/0201—Impregnation
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/02—Impregnation, coating or precipitation
    • B01J37/03—Precipitation; Co-precipitation
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    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/08—Heat treatment
    • B01J37/10—Heat treatment in the presence of water, e.g. steam
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y30/00—Nanotechnology for materials or surface science, e.g. nanocomposites
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y40/00—Manufacture or treatment of nanostructures
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    • C01G25/00—Compounds of zirconium
    • C01G25/006—Compounds containing, besides zirconium, two or more other elements, with the exception of oxygen or hydrogen
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2258/00—Sources of waste gases
    • B01D2258/01—Engine exhaust gases
    • B01D2258/012—Diesel engines and lean burn gasoline engines
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
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    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2004/00—Particle morphology
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    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2004/00—Particle morphology
    • C01P2004/60—Particles characterised by their size
    • C01P2004/64—Nanometer sized, i.e. from 1-100 nanometer
    • C—CHEMISTRY; METALLURGY
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    • C01P2004/80—Particles consisting of a mixture of two or more inorganic phases
    • C01P2004/82—Particles consisting of a mixture of two or more inorganic phases two phases having the same anion, e.g. both oxidic phases

Definitions

  • the invention relates to the technical field of mobile source tail gas purification catalysis, in particular to a nano-cerium-zirconium composite oxide and its application in catalyzing NO X reduction reactions.
  • Mobile source is the abbreviation of mobile air pollution source, which can produce a lot of pollutants.
  • the cold start phase of the engine can emit a large amount of nitrogen-containing compounds, such as nitrogen monoxide (NO), nitrogen dioxide (NO 2 ) and other nitrogen oxides (NO). X ) and ammonia (NH 3 ) and so on.
  • the main method to solve the problem of cold-start exhaust is to install a purifier with a built-in catalyst in the vehicle exhaust.
  • Cerium-zirconium composite oxide is widely used in the field of automobile exhaust catalysis. In addition to participating in the catalytic reaction, the cerium-zirconium composite oxide also functions as a carrier, which not only supports and disperses the catalytically active metal, but also provides a suitable place for the catalytic reaction of the reactant molecules.
  • cerium-zirconium solid solution greatly depend on its own microstructure, such as morphology, size, specific surface area, and pore structure.
  • the size is one of the important factors affecting the physical and chemical indicators and functional indicators of the cerium-zirconium solid solution.
  • the cerium-zirconium solid solution with nanoporous structure can not only meet the material's high-efficiency adsorption requirements for pollutants, but also because nano-structured materials have higher surface energy characteristics, the adsorbed pollutants are more easily activated, thereby reducing the pollution Light-off temperature.
  • the nano-structured cerium-zirconium solid solution can also increase the specific surface area of the material, increase the surface active sites of the carrier, and improve the catalytic efficiency of pollutants.
  • the solid solution size should be appropriate. If the size is too small, the surface activity is too high, and it is easy to agglomerate, which will cause the specific surface to drop sharply after high temperature aging; if the size is too large, the initial specific surface is too low, and there are fewer active sites. At the same time, the surface activity is low.
  • the above two factors Will affect the conversion efficiency of the catalyst to pollutants. Therefore, how to prepare and obtain a cerium-zirconium solid solution with a suitable size structure is particularly important.
  • the present invention will particle diameter of ceria-zirconia solid solution as the starting point to carry out a series of studies to determine the relationship with the particle diameter of ceria-zirconia solid solution of the NO X catalyst.
  • the present invention aims to provide a nano-sized cerium-zirconium composite oxide with appropriate particle size, which has a higher oxygen storage capacity and also exhibits excellent low-temperature catalytic activity And high temperature stability, especially suitable for catalytic purification of nitrogen oxides.
  • the present invention provides a nano-cerium-zirconium composite oxide, comprising cerium oxide, zirconium oxide and at least one oxide selected from rare earth metal elements other than cerium, wherein:
  • the ratio of the mass fraction of cerium oxide to the mass fraction of zirconium oxide contained in the composite oxide is less than 1; the composite oxide has a particle size of 5-20 nm after being heat-treated at 750°C for 4-8 hours, And at least 1.10mmol[O]/g oxygen storage.
  • the heat treatment may be calcination, or is called calcination or sintering.
  • the ratio of the mass fraction of cerium oxide to the mass fraction of zirconium oxide is greater than 0.15, and more preferably, the ratio is between 0.20-0.90.
  • the composite oxide has a particle size of 8-15 nm, preferably 9-13 nm after being heat-treated at 750°C for 4-8 hours. More preferably, the particle size of the composite oxide after heat treatment at 750° C. for 4-8 hours may be 9.3 nm, 11.5 nm, and 12.7 nm.
  • the composite oxide has an oxygen storage capacity of at least 1.12 mmol[O]/g after being heat-treated at 750° C. for 4-8 hours, preferably at least 1.15 mmol[O]/g.
  • the crystal grain size of the nano-cerium-zirconium composite oxide is mainly controlled by adjusting the oxide concentration, but the cerium-zirconium composite oxide of the appropriate particle size described in this application is not limited by the preparation method. It can also be obtained by adjusting other process parameters or using other methods, such as controlling the hydrothermal time, hydrothermal temperature, the pH or viscosity of the hydrothermal system, and adding mineralizers, etc., all of which can affect the hydrothermally obtained cerium-zirconium composite oxide The grain size is adjusted.
  • the at least one oxide of rare earth metal elements other than cerium is selected from one or more of lanthanum oxide, yttrium oxide, and praseodymium oxide; preferably, it includes lanthanum oxide and yttrium oxide.
  • the content of cerium oxide in the composite oxide is 20-40 wt%
  • the content of zirconia is 45-80 wt%
  • the content of lanthanum oxide is 2-10 wt%
  • the content of yttrium oxide is 2-10 wt%.
  • the content of cerium oxide is 28 wt%
  • the content of zirconium oxide is 62 wt%
  • the content of lanthanum oxide is 5 wt%
  • the content of yttrium oxide is 5 wt%.
  • the composite oxide after heat treatment at 750°C for 4-8h, and then aged at 1100°C for 4-8h, has a particle size of 15-24nm, preferably 17-23nm; the oxygen storage capacity is at least 1.08mmol [O] /g, preferably at least 1.10mmol[O]/g.
  • the loading amount of the noble metal is 1-2 wt% of the composite oxide, preferably 1.5 wt%; the noble metal is selected from platinum, rhodium, and palladium One or more.
  • the present invention also provides the application of the aforementioned nano-cerium-zirconium composite oxide in the catalytic reduction of NO X ; preferably, the NO X includes NO.
  • the present invention also provides a method for preparing the above-mentioned nano-cerium-zirconium composite oxide.
  • the method includes the step of hydrothermally dissolving and mixing the solution of the raw materials, and the mixed oxide concentration is 40-140g/L, preferably 80-120g/L.
  • the above preparation method adopts a hydrothermal method, which specifically includes the following steps: separately dissolving the salts of cerium, zirconium and rare earth metals and mixing them, the oxide concentration after constant volume mixing is 40-140g/L, and the pH is adjusted to acidity, React for a period of time; then adjust the pH to alkaline, react for a period of time, and calcinate at high temperature for a period of time.
  • the acidic pH is 1-3, preferably 1.5-2
  • the reaction temperature is 100-300°C, preferably 120-220°C, preferably 130-180°C, more preferably 150°C
  • the reaction time is 5-30h.
  • the above alkaline pH is 8-11, preferably 9-10
  • the reaction temperature is 100-300°C, preferably 120-220°C, preferably 130-180°C, more preferably 150°C
  • the reaction time is 5-30h.
  • the above-mentioned high-temperature calcination conditions are calcination at 500-900°C for 2-10 hours, preferably at 750°C for 4-8 hours.
  • the salts of cerium and zirconium are nitrates.
  • the raw materials are zirconium nitrate, cerium ammonium nitrate, lanthanum nitrate, and yttrium nitrate, wherein yttrium nitrate is prepared by dissolving yttrium oxide in concentrated nitric acid.
  • the cerium salt may also be cerium nitrate, cerium chloride, cerium sulfate, or cerium carbonate;
  • the zirconium salt may also be zirconium carbonate, zirconium oxychloride, zirconium sulfate, or zirconium acetate.
  • the above preparation method includes:
  • S1 dissolve the salts of cerium, zirconium, and rare earth metals separately, and the total oxide concentration is 40-140g/L;
  • the alkaline precipitation agent is one or more of ammonia, sodium hydroxide, and amines, preferably ammonia, and adjust the pH of the solution to 1-3 ;
  • the precursor slurry obtained in S4 is introduced into the autoclave, and the hydrothermal reaction is carried out at 120°C-220°C for 6-10h;
  • cerium-zirconium composite oxide is loaded with precious metal components by an equal volume immersion method.
  • precious metal palladium the specific steps of using the isometric impregnation method to load precious metal components are as follows:
  • chloropalladium acid solution H 2 PdCl 4
  • the loaded cerium-zirconium composite oxide slurry is dried in a rotary evaporator, and then placed in a drum at 110°C Dry it in an air drying oven for 3 hours, and then place it in a calcining furnace at 500°C for 3 hours in an air atmosphere.
  • the present invention also provides a method for catalytic reduction of NO X using nano-cerium-zirconium composite oxide at low temperature.
  • the method includes using nano-cerium-zirconium composite oxide to catalyze NO X- containing gas at low temperature.
  • the low temperature is a temperature not higher than 250°C.
  • the nano-cerium-zirconium composite oxide may be heat treated at 750 °C -1100 °C catalytic NO X after 4-8h. More preferably, it can be used to catalyze the NO X in the calcined for 4 hours before 750 deg.] C.
  • the temperature T 50 when the NO X conversion efficiency is 50% can be as low as 170° C., and the lowest can be as low as 147° C. ;
  • the temperature T 90 when the NO X conversion efficiency is 90% can be as low as 230 °C or less, and the lowest can be as low as 199 °C.
  • NO X can be as low as the T 50 190 deg.] C or less, the lowest possible To 167°C; T 90 can be as low as 250°C and the lowest can be as low as 218°C.
  • the nano-cerium-zirconium composite oxide obtained by the present invention has an appropriate micro-nano particle size and a relatively high oxygen storage capacity, and also shows excellent low-temperature catalytic activity and high-temperature stability, especially for nitrogen oxides. Excellent catalytic advantages, and can still maintain good catalytic effect after high temperature aging.
  • the nano cerium-zirconium composite oxide of the present invention provides, after calcination at 750 deg.] C 4 hours T 50, T 90 are about 150 °C and 200 °C, 1100 °C high temperature aging after 4 hours T 50 and T 90 At about 170°C and 220°C, respectively, it is of great significance to the research of mobile source exhaust gas treatment purifiers containing nano-cerium-zirconium composite oxide.
  • Figure 1 is a transmission electron micrograph of the nano-cerium-zirconium composite oxide prepared in Example 1 (calcined at 750°C for 4 hours);
  • Example 2 is a transmission electron micrograph of the nano-cerium-zirconium composite oxide prepared in Example 2 (calcined at 750°C for 4h);
  • Figure 3 is a transmission electron micrograph of the nano-cerium-zirconium composite oxide prepared in Example 3 (calcined at 750°C for 4h);
  • Example 4 is a transmission electron micrograph of the nano-cerium-zirconium composite oxide prepared in Example 4 (calcined at 750°C for 4h);
  • Figure 5 is a transmission electron micrograph of the nano-cerium-zirconium composite oxide prepared in Example 5 (calcined at 750°C for 4h);
  • Example 6 is a transmission electron micrograph of the nano-cerium-zirconium composite oxide prepared in Example 6 (calcined at 750°C for 4 hours).
  • the raw materials used to prepare the cerium-zirconium composite oxide can be purchased commercially;
  • the vessel used for the hydrothermal reaction is the capacity specification provided by Yantai Yifang Special Chemical Equipment Co., Ltd. of 10L
  • the PTFE-lined pressure-melting bomb and a titanium high-pressure reactor with a capacity specification of 10L; the structure characterization and particle size determination of the obtained composite oxide were performed using a transmission electron microscope model HT7800 provided by Hitachi.
  • the ChemBET-3000 instrument of Tajikistan is used to analyze the oxygen storage of cerium-zirconium solid solution; the catalytic activity test uses the infrared flue gas analyzer model HN-CK21 provided by Taiyuan Hainachenke Instrument and Meter Co., Ltd.
  • the embodiment of the present invention provides a method for preparing nano-cerium-zirconium composite oxide, and the specific steps include:
  • S1 Dissolve the salts of cerium, zirconium, lanthanum, and praseodymium or yttrium, and dilute to a total concentration of 40-140g/L, stir until clear, and continue to stir at low speed for 0.5h;
  • the alkaline precipitant is one or more of ammonia, sodium hydroxide, and amines, preferably ammonia, and then adjust the pH of the solution to 1.5- 2;
  • the obtained filter cake is dried for 10 hours, and then calcined at 700° C.-800° C. for 4-8 hours to obtain nano-cerium-zirconium composite oxide.
  • Example 1 provides a composition based on cerium oxide, zirconium oxide, lanthanum oxide, and yttrium oxide.
  • the proportion of the composition by weight of the oxide is CeO 2 28%, ZrO 2 62%, and La 2 O 3 5%. Y 2 O 3 5%.
  • the equipment used in this example is a 10L polytetrafluoroethylene lined autoclave, the solution filling degree is 80%, and the total oxide concentration is 40g/l.
  • the preparation method is as follows:
  • the slurry B was transferred to a titanium material kettle, and hydrothermally reacted at 150° C. for 8 hours at a rotation speed of 200 r/min.
  • the hydrothermally heated slurry is press filtered, and then washed 3 times with 50L deionized water and 2 times with 40L of 5g/L lauric acid alkali solution to obtain the final filter cake, which is calcined and degummed at high temperature.
  • the calcining conditions are: The temperature is increased to 750°C in the way of programmed heating, the heating rate is 2°C/min, and the furnace gas flow is controlled at 10-20L (air)/min/kg (oxide). After calcination for 4 hours, the material will pass 200-250 mesh. Sieve to obtain nano-cerium-zirconium composite oxide.
  • Example 2 to Example 6 are the same as that of Example 1, except that the concentration of the precursor oxide in each example is different, and a series of cerium-zirconium solid solutions with different grain sizes are obtained.
  • the specific examples are The concentration of the precursor oxide is shown in Table 1.
  • Example Precursor oxide concentration (g/l) Example 1 40 Example 2 60 Example 3 80 Example 4 100 Example 5 120 Example 6 140
  • the nano-cerium-zirconium composite oxides obtained in Examples 1-6 were freshly prepared (calcined at 750°C for 4h) and after high-temperature aging (aging at 1100°C for 4h). Determination of diameter.
  • the particle size measurement of the nanostructure is obtained by randomly measuring 50 crystal grains according to the equipment ruler and taking the average.
  • the oxygen storage capacity of the samples of each example before and after aging was measured.
  • the method is as follows: take 0.2g sample and keep it in high purity oxygen at 600°C for 1h, and then put it in 5% hydrogen-argon flow (100sccm) Heated from 100°C to 1000°C at a heating rate of 10°C/min, continuously measured the consumed hydrogen with a quadrupole mass spectrometer to obtain a temperature-water vapor volume curve, and determine the oxygen release amount based on the curve and its area, that is, the sample’s Oxygen storage.
  • the catalytically active component palladium is loaded on the above-prepared nano-cerium-zirconium composite oxide by an equal volume impregnation method.
  • the specific method is as follows:
  • the nano-cerium-zirconium composite oxide was impregnated and loaded with a loading amount of 1.5wt%; the loaded slurry was dried in a rotary evaporator and then placed at 110°C It was dried in a blast drying oven for 3 hours, and then placed in a calcining furnace at 500°C for 3 hours in an air atmosphere.
  • the examples 1-6 loaded with 1.5wt% of palladium are respectively denoted as CZ1-CZ6, and the above examples are loaded when freshly prepared (calcined at 750°C for 4h) and loaded after high temperature aging (calcined at 1100°C for 4h).
  • Catalytic activity test to evaluate the catalytic effect of the obtained catalyst on NO X reduction. The test method is as follows:
  • the composition of the mixed gas is NH 3 (520ppm), NO (520ppm), H 2 O (6%) and O 2 (14%), with pure N 2 as the mixed balance gas, passing through 0.1 at a flow rate of 35000mL ⁇ h -1 g sample, react at 150-400°C (5°C/min).
  • the gas concentration detector is used to detect the composition of the gas after passing through the catalyst, and the conversion rate of the catalyst to NO X is calculated by analyzing the change of the gas concentration before and after.
  • T 50 and T 90 in the table mean the temperature required for the reduction rate of NO X at 50% and 90%, respectively.
  • T 50 may be as low as 190 deg.] C or less, as low as 167 °C; T 90 can be as low as below 250°C and as low as 218°C.
  • the difference between before and after aging is very small, showing good anti-high temperature aging activity.
  • the above-mentioned nano-cerium-zirconium composite oxide has a relatively high oxygen storage capacity, and also shows excellent low-temperature catalytic activity and high-temperature stability for nitrogen oxides, especially when the grain size is 9-13nm At this time, its oxygen storage performance, catalytic purification activity and anti-aging performance are all optimal.

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Abstract

一种纳米铈锆复合氧化物，包含氧化铈、氧化锆以及至少一种选自铈以外的稀土金属元素的氧化物，所述复合氧化物中含有的铈氧化物的质量分数与锆氧化物的质量分数的比值小于1；所述复合氧化物在750℃下热处理4-8小时后，具有5-20nm的粒径，以及至少1.10mmol[O]/g的储氧量。获得的纳米铈锆复合氧化物，具有适当的微观纳米粒径，以及较高的储氧量。实验表明，纳米铈锆复合氧化物在750℃煅烧4小时后的T 50、T 90分别为150℃和200℃左右，1100℃高温老化4小时后的T 50和T 90分别在170℃和220℃左右，显示出了优异的低温催化活性和抗老化活性。

Description

一种纳米铈锆复合氧化物及其在催化NO X还原反应中的应用 技术领域
本发明涉及移动源尾气净化催化技术领域，尤其涉及一种纳米铈锆复合氧化物及其在催化NO X还原反应中的应用。
背景技术
移动源是移动式空气污染源的简称，可以产生大量污染物，例如，发动机的冷启动阶段可排放大量含氮化合物，如一氧化氮(NO)、二氧化氮(NO 2)等氮氧化物(NO X)以及氨气(NH 3)等。目前解决冷启动排气问题的主要手段是在汽车尾气排放处安装内置有催化剂的净化器。
铈锆复合氧化物被广泛应用于汽车尾气催化领域。除参与催化反应外，铈锆复合氧化物还具有载体的功能，不但起到了对催化活性金属的支撑分散作用，还为反应物分子的催化反应提供了合适的场所。
研究发现，铈锆固溶体的物理化学性能极大地依赖于其自身的微观结构，例如形貌、尺寸、比表面、孔道结构等。而尺寸大小是影响铈锆固溶体物化指标及功能性指标的重要因素之一。具有纳米多孔结构的铈锆固溶体，不仅能够满足材料对污染物高效吸附的要求，同时由于纳米结构材料具有较高表面能的特点，被其吸附的污染物更容易被活化，从而降低污染物的起燃温度。
纳米结构的铈锆固溶体还可以增加材料的比表面积，增加载体的表面活性位，提升污染物催化效率。但在实际应用中，固溶体尺寸应该大小合适。尺寸过小、表面活性过高，容易团聚，会导致高温老化后的比 表面急剧下降；尺寸过大，初始比表面过低，活性位较少，同时，表面活性能较低，以上两个因素都会影响催化剂的对污染物的转化效率。因此，如何制备并获得具有合适尺寸结构的铈锆固溶体就显得尤为重要。
本发明将以铈锆固溶体的粒径为切入点开展一系列研究工作，确定铈锆固溶体粒径与NO X催化的关系。
发明内容
为了解决上述问题，本发明旨在提供一种具有适当粒径的纳米级铈锆复合氧化物，该纳米铈锆复合氧化物具有较高的储氧量，同时还显示出了优异的低温催化活性和高温稳定性，特别适用于催化净化氮氧化物。
一方面，本发明提供了一种纳米铈锆复合氧化物，包含氧化铈、氧化锆以及至少一种选自铈以外的稀土金属元素的氧化物，其中：
所述复合氧化物中含有的铈氧化物的质量分数与锆氧化物的质量分数的比值小于1；所述复合氧化物在750℃下热处理4-8小时后，具有5-20nm的粒径，以及至少1.10mmol[O]/g的储氧量。
作为一种优选地实施方式，所述热处理可以是煅烧，或称之为焙烧、烧结。作为另一种优选地实施方式，铈氧化物的质量分数与锆氧化物的质量分数的比值大于0.15，更优选的，所述比值在0.20-0.90之间。
进一步地，所述复合氧化物在750℃下热处理4-8小时后，具有8-15nm的粒径，优选9-13nm。更优选的，所述复合氧化物在750℃下热处理4-8小时后的粒径可以是9.3nm、11.5nm、12.7nm。
进一步地，所述复合氧化物在750℃下热处理4-8小时后，具有至少1.12mmol[O]/g的储氧量，优选的，至少1.15mmol[O]/g。
具有上述粒径和/或储氧量的纳米铈锆复合氧化物，在用于催化还原NO X时，具有更低的起燃温度和完全转化温度，表现出了更好的低温催 化活性，同时在高温老化后仍具有很好的净化催化效果，老化前后的催化效果相差较小，表现出了更具优势的抗高温老化效果。
在本申请中，所述纳米铈锆复合氧化物的晶粒尺寸主要是通过调节氧化物浓度来控制的，但本申请所述适当粒径的铈锆复合氧化物并不受制备方法的限制，还可以通过调节其他工艺参数或使用其他方法获得，例如控制水热时间、水热温度、水热体系的pH或粘度以及加入矿化剂等均可以对水热后所得的铈锆复合氧化物的晶粒大小进行调节。
进一步地，所述至少一种选自铈以外的稀土金属元素的氧化物选自氧化镧、氧化钇、氧化镨中的一种或多种；优选包括氧化镧和氧化钇。
进一步地，所述复合氧化物中氧化铈的含量为20-40wt％，氧化锆的含量为45-80wt％，氧化镧的含量为2-10wt％，氧化钇的含量为2-10wt％。
优选的，所述氧化铈的含量为28wt％，氧化锆的含量为62wt％，氧化镧的含量为5wt％，氧化钇的含量为5wt％。
进一步地，所述经750℃热处理4-8h后的复合氧化物，再经1100℃老化4-8h后的粒径为15-24nm，优选17-23nm；储氧量至少为1.08mmol[O]/g，优选至少1.10mmol[O]/g。
进一步地，还包括负载在所述复合氧化物上的贵金属，所述贵金属的负载量为所述复合氧化物1-2wt％，优选1.5wt％；所述贵金属选自铂、铑、钯中的一种或多种。
另一方面，本发明还提供了上述纳米铈锆复合物氧化物在催化还原NO X中的应用；优选，所述NO X包括NO。
另一方面，本发明还提供了一种制备上述纳米铈锆复合物氧化物的方法，所述方法包括将各原料溶解混合后的溶液进行水热的步骤，所述混合后的氧化物浓度为40-140g/L，优选80-120g/L。
优选地，上述制备方法采用水热法，具体包括如下步骤：将铈、锆 以及稀土金属的盐分别溶解后混合，定容混合后的氧化物浓度为40-140g/L，调pH至酸性，反应一段时间；再将pH调至碱性，反应一段时间，于高温煅烧一段时间即可得到。
进一步，上述酸性pH为1-3，优选1.5-2，反应温度为100-300℃，优选120-220℃，优选130-180℃，更优选150℃，反应时间为5-30h。
进一步，上述碱性pH为8-11，优选9-10，反应温度为100-300℃，优选120-220℃，优选130-180℃，更优选150℃，反应时间为5-30h。
进一步，上述高温煅烧条件为500-900℃下煅烧2-10h，优选，750℃下煅烧4-8h。
进一步地，上述方法中，铈、锆的盐为硝酸盐。优选的，所述原料为硝酸锆、硝酸铈铵、硝酸镧、硝酸钇，其中硝酸钇由氧化钇溶解于浓硝酸制得。优选地，上述铈盐还可以是硝酸铈、氯化铈、硫酸铈、碳酸铈；上述锆盐还可以是碳酸锆、氧氯化锆、硫酸锆、醋酸锆。
作为优选的实施方式，上述制备方法包括：
S1，将铈、锆、稀土金属的盐分别溶解，总氧化物浓度为40-140g/L；
S2，向上述溶液中滴加碱性沉淀剂，所述碱性沉淀剂为氨水、氢氧化钠、胺类中的一种或多种，优选以氨水为主，将溶液pH调至1-3；
S3，将上述溶液引入高压反应釜中，于120℃-220℃下进行高温水解反应，时间10-20h；
S4，向S3得到的前驱体浆料滴加碱性沉淀剂调pH至8-11；
S5，将S4获得的前驱体浆料引入至高压釜中，于120℃-220℃下进行水热反应，时间6-10h；
S6，抽滤后打浆洗涤，滤饼干燥10h后，500℃-900℃下煅烧4-8h。
进一步地，利用等体积浸渍法向铈锆复合氧化物负载贵金属成分。以贵金属钯为例，上述利用等体积浸渍法负载贵金属组分的具体步骤如 下：
以氯钯酸溶液(H 2PdCl 4)为前驱体浸渍液，其中钯的负载量为1.5wt％；负载后的铈锆复合氧化物浆液在旋转蒸发仪中干燥，然后置于110℃的鼓风干燥箱中干燥3h，再置于煅烧炉中在空气气氛中500℃煅烧3小时。
另一方面，本发明还提供了一种在低温下利用纳米铈锆复合氧化物催化还原NO X的方法，所述方法包括在低温下利用纳米铈锆复合氧化物对含NO X的气体进行催化的步骤，所述低温为不高于250℃的温度。
优选的，纳米铈锆复合氧化物可以在750℃-1100℃热处理4-8h后进行NO X的催化。更优选的，可以在750℃煅烧4小时后再用于NO X的催化。
本发明所述的纳米铈锆复合氧化物经750℃煅烧4小时后，对NO X进行催化时：NO X转化效率为50％时的温度T 50可以低至170℃以下，最低可以到147℃；NO X转化效率为90％时的温度T 90可以低至230℃以下，最低可以到199℃。
此外，本发明所述的纳米铈锆复合氧化物经750℃煅烧4小时后，再经1100℃老化4小时，对NO X进行催化时：NO X的T 50可以低至190℃以下，最低可以到167℃；T 90可以低至250℃以下，最低可以到218℃。
本发明的有益效果在于：
本发明获得的纳米铈锆复合氧化物，具有适当的微观纳米粒径，以及较高的储氧量，同时还显示出了优异的低温催化活性和高温稳定性，特别是对氮氧化物表现出优异的催化优势，并且在高温老化后仍可以保持很好的催化效果。实验表明，本发明所提供的纳米铈锆复合氧化物，在750℃煅烧4小时后的T 50、T 90分别为150℃和200℃左右，1100℃高温老化4小时后的T 50和T 90分别在170℃和220℃左右，对含有纳米铈锆复合氧化物的移动源尾气处理净化剂的研究具有重大意义。
附图说明
此处所说明的附图用来提供对本申请的进一步理解，构成本申请的一部分，本申请的示意性实施例及其说明用于解释本申请，并不构成对本申请的不当限定。在附图中：
图1为实施例1制备的纳米铈锆复合氧化物(750℃煅烧4h)透射电镜图；
图2为实施例2制备的纳米铈锆复合氧化物(750℃煅烧4h)透射电镜图；
图3为实施例3制备的纳米铈锆复合氧化物(750℃煅烧4h)透射电镜图；
图4为实施例4制备的纳米铈锆复合氧化物(750℃煅烧4h)透射电镜图；
图5为实施例5制备的纳米铈锆复合氧化物(750℃煅烧4h)透射电镜图；
图6为实施例6制备的纳米铈锆复合氧化物(750℃煅烧4h透射电镜图。
具体实施方式
为了更清楚的阐释本申请的整体构思，下面以实施例的方式进行详细说明。在下文的描述中，给出了大量具体的细节以便提供对本申请更为彻底的理解。然而，对于本领域技术人员来说是显而易见的，本申请可以无需一个或多个这些细节而得以实施。在其他的例子中，为了避免与本申请发生混淆，对于本领域公知的一些技术特征未进行描述。
如未特殊说明，以下各实施例中，制备铈锆复合氧化物所用的原料 均可通过商业途径购得；其中水热反应所使用的容器为烟台一方特种化工设备有限公司提供的容量规格为10L的聚四氟乙烯内衬的压力溶弹和容量规格为10L的钛材质高压反应釜；所得复合氧化物的结构表征以及粒径的测定采用日立公司提供的型号为HT7800的透射电子显微镜，利用康塔公司的ChemBET-3000仪器进行铈锆固溶体储氧量的分析；催化活性测试使用太原海纳辰科仪器仪表有限公司提供的型号为HN-CK21的红外烟气分析仪。
本发明的实施例提供了一种纳米铈锆复合氧化物的制备方法，具体步骤包括：
S1，将铈、锆、镧及镨或钇的盐分别溶解，定容至总浓度为40-140g/L，搅拌至澄清，并继续低速搅拌0.5h；
S2，向上述溶液中滴加碱性沉淀剂，所述碱性沉淀剂为氨水、氢氧化钠、胺类中的一种或多种，优选以氨水为主，再将溶液pH调至1.5-2；
S3，将上述溶液引入高压反应釜中，于150℃下水热水解10-20h；
S4，向S3得到的前驱体浆料滴加碱性沉淀剂调pH至8-10；
S5，将S4获得的前驱体浆料引入至高压釜中，于150℃下水热反应6-10h；
S6，抽滤后打浆洗涤，所得滤饼干燥10h后，700℃-800℃下煅烧4-8h得到纳米铈锆复合氧化物。
如未特殊说明，以下各实施例均采用上述方法制备。
实施例1
实施例1提供了一种基于氧化铈、氧化锆、氧化镧、氧化钇的组合物，该组合物按氧化物重量计比例为CeO 228％，ZrO 262％，La 2O 35％，Y 2O 35％。
本实施例所用设备为10L聚四氟内衬高压反应釜，溶液填充度为80％， 氧化物总浓度为40g/l，其制备方法如下：
取硝酸氧锆574g，硝酸钇54.2g,用2000ml去离子水溶解至澄清得溶液A；
取硝酸铈铵278.5g、硝酸镧42.7g加入至溶液A中，搅拌至澄清，于50℃水浴条件下用氨水将溶液A的pH调至1.5-1.6，定容至8000ml，并引入至聚四氟乙烯内衬压力溶弹中，于150℃水热水解反应20h，降至常温下用氨水调pH至9.5-9.7，得到浆料B；
将浆料B转移至钛材釜中，在转速为200r/min条件下于150℃水热反应8h。水热后的浆料压滤，再用50L去离子水洗涤3遍，40L浓度为5g/L月桂酸碱液洗涤2遍得到最终的滤饼，于高温下煅烧排胶，煅烧条件为：采用程序升温的方式将温度升至750℃，升温速率为2℃/min，炉体气流量控制在10-20L(air)/min/kg(氧化物)，煅烧4h后，物料过200-250目筛，即获得纳米铈锆复合氧化物。
实施例2至实施例6与实施例1的制备方法相同，区别仅在于各实施例的前驱体氧化物的浓度不同，并获得了一系列不同晶粒尺寸的铈锆固溶体，具体各实施例的前驱体氧化物的浓度如表1所示。
表1实施例1-6的前驱体氧化物浓度
示例 前驱体氧化物浓度(g/l)
实施例1 40
实施例2 60
实施例3 80
实施例4 100
实施例5 120
实施例6 140
性能评价
一、结构表征
利用透射电镜(TEM)对实施例1-6所得的纳米铈锆复合氧化物分 别在新鲜制得(750℃煅烧4h)时和高温老化(1100℃老化4h)后，进行纳米结构的表征以及粒径的测定。其中纳米结构的粒度测定是根据设备标尺，随机量取50个晶粒，取平均数得到。
与此同时，还对各实施例样品在老化前后的储氧量进行测定，方法如下：取0.2g样品在高纯氧气中于600℃保持1h，再在5％氢气-氩气流(100sccm)中以10℃/分钟的加热速率从100℃加热至1000℃，用四极质谱仪连续测量所消耗的氢气，得到温度-水蒸气量曲线，根据曲线及其面积测定氧的释放量，即样品的储氧量。
上述表征结果中，实施例1-6新鲜样品的TEM表征结果见图1-6，测得的粒径和储氧量结果见表2。
表2各示例在新鲜制得时和高温老化后的粒径和储氧量
Figure PCTCN2019102759-appb-000001
结合图1-6以及表2的结果可知，本申请通过调整制备方法中的前驱体氧化物浓度，可以制备出具有不同晶粒尺寸的铈锆复合氧化物，并且不仅实施例1-6所得铈锆复合氧化物之间的晶粒尺寸差异较大，其对应的储氧性能方面也差别明显。其中，由表2的数据可得，实施例3-5的纳米铈锆复合氧化物表现出了更优的储氧能力。
二、催化活性测试
利用等体积浸渍法将催化活性组分钯负载于上述制得的纳米铈锆复合氧化物上，具体方法如下：
以氯钯酸溶液(H 2PdCl 4)为前驱体浸渍液，对纳米铈锆复合氧化物浸渍负载，负载量为1.5wt％；负载后的浆液在旋转蒸发仪中干燥，然后置于110℃的鼓风干燥箱中干燥3h，再置于煅烧炉中在空气气氛中500℃煅烧3小时。
其中，负载钯1.5wt％后的实施例1-6分别记为CZ1-CZ6，并对上述各示例分别进行新鲜制得(750℃煅烧4h)时负载以及高温老化(1100℃煅烧4h)后负载的催化活性测试，以评价所得催化剂对NO X的还原催化效果，其中测试方法如下：
采用配有U型石英反应管自制的小样评价反应装置，在U型石英反应管一侧底部塞一小团石英棉，再放入秤好的催化剂样品，通入混合气进行升温测定。其中，混合气组成为NH 3(520ppm)、NO(520ppm)、H 2O(6％)和O 2(14％)，以纯N 2作为混合平衡气，以35000mL·h -1的流速经过0.1g样品，在150-400℃(5℃/min)进行反应。用气体浓度检测器检测通过催化剂后气体的组成，并通过分析前后气体浓度的变化计算催化剂对NO X的转化率。
具体各示例的催化效果见表3，表中的T 50和T 90分别意为NO X的还原率在50％和90％时所需的温度，该温度越低说明在相同的催化效率下所需的温度越低，催化效果越好，其中T 50被称为起燃温度，T 90被称为完全脱除温度。
表3各示例的NO X催化还原效果
Figure PCTCN2019102759-appb-000002
由表3可知，不同晶粒粒径尺寸下的铈锆复合氧化物，对氮氧化物的催化还原效果存在较大差异，并且随着样品粒径的减小，其低温还原效果呈先升高后降低的趋势。特别是实施例3-5显示出了更显著的氮氧化物低温催化效果，新鲜制得(经750℃煅烧4h)的纳米铈锆复合氧化物负载贵金属后，其T 50低至147℃，T 90低至199℃。
与此同时，各实施例纳米铈锆复合氧化物在高温老化(1100℃老化4h)后负载贵金属仍具有很好的氮氧化物低温催化效果，T 50可低至190℃以下，最低可达167℃；T 90可低至250℃以下，最低可达218℃，老化前后差异很小，显示出了很好的抗高温老化活性。
综合上述可得，上述纳米铈锆复合氧化物，具有较高的储氧量，对氮氧化物还显示出了优异的低温催化活性和高温稳定性，特别是当其晶粒尺寸为9-13nm时，其储氧性能、催化净化活性以及抗老化性能均达到最优。
以上所述仅为本申请的实施例而已，并不用于限制本申请。对于本领域技术人员来说，本申请可以有各种更改和变化。凡在本申请的精神和原理之内所作的任何修改、等同替换、改进等，均应包含在本申请的权利要求范围之内。

Claims (10)

  1. 一种纳米铈锆复合氧化物，所述氧化物包含铈氧化物、锆氧化物以及至少一种选自铈以外的稀土金属元素的氧化物，其特征在于，
    所述复合氧化物中含有的铈氧化物的质量分数与锆氧化物的质量分数的比值小于1；
    所述复合氧化物在750℃下热处理4-8小时后，具有5-20nm的粒径，以及至少1.10mmol[O]/g的储氧量。
  2. 根据权利要求1所述的复合氧化物，其特征在于，所述复合氧化物在750℃下热处理4-8小时后，具有8-15nm的粒径，优选9-13nm。
  3. 根据权利要求1所述的复合氧化物，其特征在于，所述复合氧化物在750℃下热处理4-8小时后，具有至少1.12mmol[O]/g的储氧量，优选的，至少1.15mmol[O]/g。
  4. 根据权利要求1所述的复合氧化物，其特征在于，所述至少一种选自铈以外的稀土金属元素的氧化物选自氧化镧、氧化钇、氧化镨中的一种或多种；优选包括氧化镧和氧化钇。
  5. 根据权利要求1-4任一所述的复合氧化物，其特征在于，所述复合氧化物中氧化铈的含量为20-40wt％，氧化锆的含量为45-80wt％，氧化镧的含量为2-10wt％，氧化钇的含量为2-10wt％；优选，所述氧化铈的含量为28wt％，氧化锆的含量为62wt％，氧化镧的含量为5wt％，氧化钇的含量为5wt％。
  6. 根据权利要求1所述的复合氧化物，其特征在于，所述经750℃热处理4-8h后的复合氧化物，再经1100℃老化4-8h后的粒径为15-24nm，储氧量至少为1.08mmol[O]/g。
  7. 根据权利要求1所述的复合氧化物，其特征在于，还包括负载在 所述复合氧化物上的贵金属，所述贵金属的负载量为所述复合氧化物1-2wt％，优选1.5wt％；所述贵金属选自铂、铑、钯中的一种或多种。
  8. 权利要求1-7任一所述的纳米铈锆复合物氧化物在催化还原NO X中的应用；优选，所述NO X包括NO。
  9. 一种制备如权利要求1-7任一所述的纳米铈锆复合物氧化物的方法，其特征在于，所述方法包括将各原料溶解混合后的溶液进行水热的步骤，所述混合后的氧化物浓度为40-140g/L，优选80-120g/L。
  10. 一种低温下催化还原NO X的方法，其特征在于，所述方法包括在低温下利用权利要求1-7任一所述的纳米铈锆复合氧化物对含NO X的气体进行催化的步骤，所述低温为不高于250℃的温度。
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