1. A supported palladium-cerium based catalyst, which is characterized in that the specific surface area is 100-200m²The cerium dioxide is used as carrier to load metal palladium, and the load of palladium in the catalyst is 0.5-4 wt%; in the supported palladium-cerium-based catalyst, palladium atoms and cerium dioxide form Pd-O-Ce bonds through interaction, so that the palladium atoms are dispersed on the surface of the cerium dioxide carrier in a sub-nanometer or atomic scale.

2. An application of a supported palladium-cerium based catalyst in low-temperature storage reaction of nitrogen oxides in the cold start stage of an automobile.

3. Use according to claim 2, wherein the low temperature is 80-200 ℃.

4. Use according to claim 2, wherein, in storing nitrogen oxides at low temperatures, the nitrogen oxides are stored as nitrite species on the Pd-O-Ce species on the surface of the supported palladium-cerium based catalyst, and the stored nitrite species are completely desorbed from the surface of the catalyst below 500 ℃.

5. The use as claimed in claim 2, wherein the storage amount of the noble metal palladium-supported ceria-based catalyst after high temperature aging at 800 ℃ of 700-_cat。

6. Use according to claim 2, characterized in that the reaction atmosphere during the cold start phase of the vehicle is NO_x、O₂、H₂O、CO₂Mixed gas of CO and Ar, the reaction pressure is normal pressure, the reaction temperature is 80-200 ℃, and NO is in the reaction atmosphere_xThe concentration is 100-300ppm, the CO concentration is 400-600ppm, and O₂、H₂O and CO₂The flow ratio of (1) is 10:0-2:5, and the volume space velocity is 150000-.

Background

In recent years, environmental protection is more and more strongly demanded, and industrially developed countries such as the united states, the japanese, the korean, and the europe have recently made and executed new emission standards, and the emission limit of NO is becoming strict. Therefore, how to control nitrogen oxides has become a research hotspot at home and abroad. Commercial NO_xThe emission reduction technology is NH₃Selective catalytic reduction of NO_x(NH₃-SCR) and NO_xStorage Reduction (NSR), however, the catalyst operating temperature of these methods is generally 250-400 ℃ and the denitration system has not been raised to its operating temperature below 200 ℃, so most of the NO is_xThe emissions are generated during the cold start phase. Since environmental regulations increasingly limit regulated nitrogen oxide emission concentrations, PNA (Passive NO) was introduced during the cold start phase of automobiles_xAdsorber) technique to control NO during cold start phase of a lean-burn engine_xAnd (4) discharging. The goal of PNA technology is to store nitrogen oxides during cold start of a vehicle when NO is present_xReleasing NO when the reduction catalyst reaches its operating temperature_xPNA technology with NH₃Selective Catalytic Reduction (SCR) technology is used in combination.

The core catalyst of PNA technology is mainly divided into zeolite molecular sieve and oxide catalyst, wherein Pd ion exchange molecular sieve catalyst has atomic level dispersed Pd ion generally regarded as NO_xAnd at the storage site of (3), and in the NO_xThe storage site being capable of carrying out NO in the temperature range of the cold start reaction_xWhen the loading amount of Pd exceeds 1 wt%, it is difficult to achieve high dispersion of noble metal Pd by ion exchange method so as to obtain high NO_xPd ion storage sites of storage capacity. Usually, Pd molecular sieve catalyst adsorbs NO directly in the form of nitroso at low temperature, so that the Pd molecular sieve catalyst has higher NO_xStorage capacity, but has the disadvantage that Pd migrates out of the zeolite molecular sieve framework and sinters, causing it to gradually deactivate.

In general, the storage process of oxide PNA material is influenced by the dispersion degree of storage component, and the synergistic effect between the storage component and carrier is also on NO_xAdsorption was affected. Cerium oxide has a wide application in the field of automobile exhaust treatment due to its low price and excellent redox ability resulting from the reversible conversion of tri-and tetra-valent cerium oxide. In one aspect, CeO₂The nitrogen oxide adsorption performance of the method enables the nitrogen oxide to have good application prospect in the low-temperature PNA technology; on the other hand, the cerium oxide can be used as a carrier to be combined with loaded active metal, so as to provide sites for loading the noble metal, stabilize and disperse the noble metal, and simultaneously, the electronic interaction between the cerium oxide and the metal can be enhanced, and the electronic density of the metal can be improved. Currently NO_xApplication of low-temperature storage technology to lean burn engine exhaust treatment still has NO_xThe problem of low storage efficiency and the insufficient abundance of catalyst storage sites are key problems leading to low storage efficiency of PNA oxide-based catalysts. How to develop high-activity low-temperature NO_xStorage catalyst, NO at low temperature_xThe storage site of the storage process, the influence of the interaction of the noble metal and the carrier on the storage process of the PNA technology and the like are still unclear key problems in the application of the PNA technology, so that the research and development of high-performance low-temperature NO_xStorage catalyst and investigation of Low temperature Effect of NO_xThe factors of the stored procedure are all of great significance.

In addition, the high temperature resistance of the catalyst is also an important property in the process of treating the automobile exhaust in practical application. The change of the catalyst structure in the high-temperature aging process comprises the serious reduction of the specific surface area of the carrier or the sintering of the noble metal is a main factor which causes the oxide PNA material to completely or partially lose the nitrogen oxide storage capacity, so the high-temperature sintering problem of the oxide catalyst is another key problem to be solved urgently in practical application at present.

Disclosure of Invention

The invention aims to provide a supported palladium cerium oxide based catalyst applied to low-temperature nitrogen oxide storage. The catalyst is applied to low-temperature nitrogen oxide storage reaction in the cold start stage of an automobile, the cerium dioxide carrier with high specific surface area has high storage capacity for nitrogen oxide due to the stable and high dispersion effect of the cerium dioxide carrier on palladium, and the stored nitrogen oxide is easy to desorb at low temperature, so that the catalyst is preferably used as a catalyst for cold start reaction.

To achieve the purpose, the invention provides a supported palladium-cerium based catalyst with the specific surface area of 100-200m²The cerium dioxide is used as carrier to load metal palladium, and the load of palladium in the catalyst is 0.5-4 wt%; in the supported palladium-cerium-based catalyst, palladium atoms and cerium dioxide form Pd-O-Ce bonds through interaction, so that the palladium atoms are dispersed on the surface of the cerium dioxide carrier in a sub-nanometer or atomic scale.

The invention also provides application of the supported palladium-cerium based catalyst in low-temperature storage reaction of nitrogen oxides at the cold start stage of an automobile.

Further, in the above technical scheme, the low temperature is 80-200 ℃.

Further, in the above technical scheme, when storing nitrogen oxide at low temperature, the nitrogen oxide is stored on Pd-O-Ce species on the surface of the supported palladium-cerium based catalyst in the form of nitrite species, and the stored nitrite species can be completely desorbed from the surface of the catalyst at a temperature below 500 ℃.

Further, in the above technical scheme, the storage amount of the cerium dioxide based catalyst loaded with noble metal palladium is not significantly reduced after high temperature aging at 800 ℃ of 700-_cat。

Further, in the above technical scheme, the reaction atmosphere at the cold start stage of the automobile is NO_x、O₂、H₂O、CO₂Mixed gas of CO and Ar, the reaction pressure is normal pressure, the reaction temperature is 80-200 ℃, and NO is in the reaction atmosphere_xThe concentration is 100-300ppm, the CO concentration is 400-600ppm, and O₂、H₂O and CO₂The flow ratio of (1) is 10:0-2:5, and the volume space velocity is 150000-.

Advantageous effects of the invention

1. The method of the invention uses the impregnation method to prepare the cerium dioxide base load noble metal palladium catalyst, has simple process and preparesThe catalyst has higher storage performance in low-temperature nitrogen oxide storage reaction. The experimental results show that 2 wt% Pd/CeO is adopted when the storage temperature is 100 ℃ and the volume space velocity is 200000/h₂Nitrogen oxide storage of the-HSA catalyst up to 193. mu. mol NO_x/g_cat。

2. In the cerium dioxide-based supported noble metal palladium catalyst prepared by the method, the high specific surface area of the carrier promotes the noble metal palladium to have higher dispersity and utilization rate, and 1mol of NO can be achieved_xThe method effectively solves the problem of low utilization rate of easily agglomerated noble metals by 1mol of Pd, and has wide application prospect in the field of storing nitrogen oxides at low temperature.

3. The cerium dioxide-based catalyst loaded with noble metal palladium prepared by the method has good thermal stability in the reaction of storing nitrogen oxides at low temperature, and solves the problem that the storage capacity of the nitrogen oxides is seriously reduced due to high-temperature sintering of the oxide catalyst in the process of treating automobile exhaust.

Drawings

FIG. 1 is a graph of 2 wt% Pd/CeO prepared in comparative example 1 and example 1 of example 4₂HSA and 2 wt% Pd/CeO₂Catalyst for LSA and two CeO species₂Vector raman spectrum.

FIG. 2 is a graph of the nitroxides of example 5 at 2 wt% Pd/CeO prepared in comparative example 1 and example 1₂HSA and 2 wt% Pd/CeO₂Catalyst for LSA and CeO with high specific surface area₂And (3) an in-situ diffuse reflection infrared spectrogram adsorbed on the surface of the carrier.

Detailed Description

The following non-limiting examples are presented to enable those of ordinary skill in the art to more fully understand the present invention and are not intended to limit the invention in any way.

The following describes specific embodiments of the present invention in detail with reference to the technical solutions.

Comparative example 12 wt% Pd/CeO₂Preparation of LSA and Activity evaluation

(1) Preparation of

The 2 wt% Pd/CeO is prepared by an isovolumetric impregnation method₂-LSA catalyst. Weighing 1g of commercial CeO₂Carrier (S)_BET＝51m²(g) and a palladium nitrate solution (palladium nitrate mass: 0.05g) corresponding to the pore volume of the CeO₂Soaking the carrier in a palladium nitrate solution, standing the soaked product overnight, drying in a vacuum drying oven at 60 ℃ for 12h, and roasting in a muffle furnace at 500 ℃ for 3h to obtain 2 wt% Pd/CeO₂-a LSA catalyst;

(2) activity evaluation test

The low-temperature nitrogen oxide storage reaction is carried out in a quartz tube fixed bed reactor with the inner diameter of 6 mm. The flow of each path of gas required by the experiment is regulated and controlled by a mass flow meter, and the gas flows into the reactor after being mixed. 130mg of 2 wt% Pd/CeO were weighed out₂LSA (20-40 mesh) catalyst in quartz tubes at 20% O₂The catalyst was pretreated with the mixed gas/Ar at 500 ℃ for 1h, and then activity evaluation was carried out under the following conditions: the reaction atmosphere adopts 200ppm NO_x/500ppm CO/10％O₂/2％H₂O/5％CO₂Ar, the gas space velocity is 200000/h, and the reaction temperature is 100 ℃.

2wt％Pd/CeO₂NO of LSA catalyst_xThe storage amount and desorption efficiency are shown in table 1.

Example 12 wt% Pd/CeO₂Preparation of-HSA and evaluation of Activity

The 2 wt% Pd/CeO is prepared by an isovolumetric impregnation method₂-a HSA catalyst. Weighing 1g of CeO with high specific surface area₂Carrier (S)_BET＝146m²(g) and a palladium nitrate solution (palladium nitrate mass: 0.05g) corresponding to the pore volume of the CeO₂Soaking the carrier in a palladium nitrate solution, standing the soaked product overnight, drying in a vacuum drying oven at 60 ℃ for 12h, and roasting in a muffle furnace at 500 ℃ for 3h to obtain 2 wt% Pd/CeO₂-a HSA catalyst;

evaluation of catalyst Activity the experimental conditions were the same as in comparative example 1. 2 wt% Pd/CeO₂NO of-HSA catalyst_xThe storage amount and desorption efficiency are shown in table 1.

TABLE 12 wt% Pd/CeO prepared in comparative example 1₂LSA catalyst with 2 wt% Pd/CeO prepared in example 1₂-HSA catalyst NO_xComparison of storage capacity and desorption efficiency

As can be seen from Table 1, 2 wt% Pd/CeO₂Lower dispersion of catalyst Pd for LSA leads to a lower utilization of catalyst Pd, resulting in 2 wt% Pd/CeO₂The nitrogen oxide storage capacity of the LSA catalyst is markedly reduced, according to the NSC mol/Pd mol result, 2 wt% Pd/CeO₂Pd utilization on LSA catalyst as low as 52%, NO at 20min storage time_xThe storage amount is 97 mu mol NO_x/g_cat。

According to the NSC mol/Pd mol results in Table 1, Pd and CeO₂The Pd-O-Ce bonding formed by the unique interaction between the Pd and the CeO facilitates the high dispersion of Pd species in CeO with larger surface area₂Surface of the support capable of storing 1mol of NO per mole of Pd_xDescription of Pd/CeO₂The dispersion degree of Pd on the-HSA catalyst reaches 100 percent, so that higher NO can be achieved_xStorage amount, NO at 20min storage time_xThe storage capacity is as high as 193 mu mol/g_catAnd NO_xCompletely desorbed from the catalyst surface below 500 ℃.

EXAMPLE 22 wt% Pd/CeO₂Preparation and activity evaluation of-HSA-800 ℃/10h

This example is 2 wt% Pd/CeO₂Thermal stability test of HSA catalyst, the previous preparation of the catalyst in the process being identical to that of example 1, except that the catalyst was calcined in a muffle furnace at 800 ℃ for 10h to obtain 2 wt% Pd/CeO₂HSA-800 ℃/10h catalyst, the activity evaluation experimental conditions of the catalyst are the same as those of example 1. 2 wt% Pd/CeO₂NO of HSA-800 ℃/10h catalyst_xThe storage amount and desorption efficiency are shown in table 2.

TABLE 2 preparation of example 1 of 2 wt% Pd/CeO₂HSA catalyst with 2 wt% Pd/CeO prepared in example 2₂Catalyst NO at 800 ℃/10h_xMemory comparison

According to the results in Table 2, there is a very strong interaction between Pd atoms and the carrier, which can prevent the Pd atoms from agglomerating and sintering, and thus keep stable, so that the catalyst NO can be obtained after high-temperature roasting_xAlmost NO decrease in storage capacity, NO after aging at high temperature_xThe storage capacity can still reach 175 mu mol/g_cat。

Example 3 Raman Spectroscopy

The raman spectra of the catalysts were taken on an inVia Qontor model raman spectrometer. Using a 532nm light source, the resolution is 1cm^-1The scanning range is 50-4000cm^-1。

Using Raman spectroscopy on Pd/CeO₂The Pd species present on the series of catalysts were characterized. The results in FIG. 1 show Pd/CeO₂LSA at 646cm^-1The peak is attributed to the square plane [ PdO ] in the palladium oxide structure₄]B of subunit_1gVibration mode, described in Pd/CeO₂The predominant form of Pd present on LSA catalysts is PdO. Pd/CeO₂HSA at 188cm^-1The peak at (A) is attributed to the formation of Pd-O-Ce bonds. At the same time, at 838cm^-1The peak is also Pd and CeO₂The result of the interaction. The Raman results show that Pd/CeO₂Pd species on HSA catalyst with CeO₂The carrier generates strong interaction, so that Pd atoms are dispersed on the surface of the cerium dioxide carrier in a sub-nanometer scale or atomic scale.

Example 4 in situ Diffuse reflectance Infrared Spectroscopy

In situ infrared diffuse reflectance spectroscopy was performed on a Bruker model 27 fourier transform infrared spectrometer with MCT as the detector. Resolution and scanning range were 4cm each^-1And 4000-^-1All spectra were accumulated for 60 scan recordings. Preparing the catalyst with CaF₂In-situ reaction pool of window piece at 500 deg.C and flow rate of 100mL min^-1The method comprises the steps of pretreating for 1h under He, collecting background spectra under He atmosphere at 100 ℃, and mainly researching the nitrogen oxide adsorption mechanism through in-situ infrared diffuse reflection spectroscopy. After the purging step is finished, converting He gas into NO and O₂Mixing gases andadsorbing for 30 min.

Pd/CeO by using in-situ infrared diffuse reflection spectrum₂The nitrogen oxide storage forms on the series of catalysts were characterized. The results in FIG. 2 show that CeO₂The nitrogen oxide on the surface of the carrier is mainly stored in the form of nitrate, and is stored in Pd/CeO₂Assignment of Nitrogen oxide storage species as nitrite on HSA catalyst, indicating Pd/CeO₂The adsorption sites of the nitrogen oxides of the HSA catalyst are highly dispersed Pd species, Pd/CeO₂Initial storage of nitrogen oxides on LSA catalyst as CeO₂A carrier on which nitrogen oxides are stored as nitrite species on the Pd sites as the nitrogen oxide adsorption time increases. The result of the in-situ infrared diffuse reflection spectrum shows that Pd/CeO₂Pd-O-Ce on-HSA catalyst is NO_xStorage sites due to Pd/CeO₂Agglomerated Pd sites on the LSA catalyst, with the initial adsorbed species assigned CeO₂The nitrate species adsorbed on the surface further proves different existing forms of Pd species, Pd/CeO, on the surface of the two catalysts from in-situ infrared diffuse reflection spectrum results₂The highly dispersed Pd species on the surface of the HSA catalyst is more favorable for the storage of nitrogen oxides than Pd/CeO₂LSA catalysts have higher nitrogen oxide storage efficiency.