A three-way catalyst of a noble metal supported on a composite oxide modified with a transition metal oxide and a method for producing the same

Abstract

The application discloses a kind of transition metal oxide modified composite oxide supported noble metal three-effect catalyst and its preparation method, belong to catalyst field.The transition metal oxide modified composite oxide supported noble metal three-effect catalyst uses transition metal oxide and noble metal as double active sites, and catalytic activity is significantly improved, and thermal stability is excellent.In the three-effect catalyst described in the application, the loading of noble metal is low, and the modification of transition metal oxide promotes the generation of oxygen vacancies on the surface of the carrier, thereby improving the catalyst activity.The three-effect catalyst is prepared by using the molten salt method, so that the catalyst has better dispersibility, the preparation process is simplified, the preparation period is shorter, and it is suitable for large-scale industrial production.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of catalysts, in particular to a transition metal oxide modified composite oxide supported noble metal three-way catalyst and a preparation method thereof. BACKGROUND

[0002] The increasingly serious air pollution poses a great threat to our health, causing widespread fear and concern in society. As one of the main pollution sources, the emission of automobile exhaust must be strictly controlled. Although previous research has made great achievements, the current three-way catalysts still have the problems of high light-off temperature and poor hydrothermal stability, which cannot match the increasingly powerful engine. Therefore, it is imminent to develop new and efficient three-way catalysts.

[0003] As a commonly used rare earth material, CeO2 is widely studied and applied in three-way catalytic reactions due to its good oxygen storage capacity, rich oxygen vacancies and excellent redox performance. However, due to the low specific surface area and poor thermal stability of pure CeO2, its practical application has been greatly hindered. Previous studies have shown that the formation of composite oxide structure between doped foreign metals (such as Zr, Ti, Mg, Mn, etc.) and CeO2 can effectively improve the interface interaction of the catalyst, increase the specific surface area, and promote the generation of oxygen vacancies, thereby improving the thermal stability and enhancing the catalytic performance, but the improvement of catalytic performance still has limitations.

[0004] Existing noble metal supported three-way catalysts usually only have noble metals as active sites for catalysts, which often requires high loading of noble metals, is high in cost and energy consumption, and is difficult to achieve large-scale effective preparation.

[0005] In addition, the traditional preparation methods of three-way catalysts include co-precipitation, sol-gel method, hydrothermal synthesis method, etc. However, for these wet chemical methods operating in liquid phase, the close contact of precursors is difficult to control due to the influence of inherent solubility product (K sp ) and other complex factors (pH, concentration, temperature, aging time, drying method). Moreover, leaching of soluble substances is often unavoidable during filtration and washing, which may bring some environmental burden. Therefore, it is of great application value and scientific significance to research and develop new three-way catalysts to improve the poor catalytic performance and poor thermal stability of existing three-way catalysts, and design a simple synthesis strategy to prepare new three-way catalysts with high activity, high selectivity and high stability. SUMMARY

[0006] Therefore, the present application aims to provide a three-way catalyst with transition metal oxide modified composite oxide and noble metal supported on the surface of the transition metal oxide modified composite oxide and a preparation method thereof.

[0007] To achieve the above object, the technical solution adopted by the present application is as follows:

[0008] The present application provides a three-way catalyst, which is composed of transition metal oxide modified CeSn composite oxide and noble metal supported on the surface of the transition metal oxide modified CeSn composite oxide.

[0009] The formation of the CeSn composite oxide is specifically preferred to be doping SnO2 on the structure of CeO2 lattice, which effectively improves the redox behavior of CeO2, increases the migration rate of lattice oxygen, and enhances the interface interaction between SnO2 and CeO2.

[0010] Preferably, the transition metal oxide is selected from one or more of copper oxide, iron oxide, manganese oxide, and cobalt oxide.

[0011] After the transition metal oxide modifies the CeSn composite oxide, the generation of oxygen vacancies on the surface of the carrier is promoted, the double active sites of the catalyst formed by the transition metal and the active component noble metal are promoted, the reaction activity and thermal stability of the three-way catalyst are improved, and thus the oxidation of carbon monoxide and the reduction of nitrogen-containing oxides in the three-way reaction are promoted.

[0012] Preferably, the transition metal oxide is selected from cuprous oxide and / or copper oxide.

[0013] Preferably, the noble metal is selected from one or more of ruthenium, rhodium, palladium, iridium, and platinum; more preferably, rhodium or palladium.

[0014] The noble metal and the transition metal have a certain synergistic effect. The addition of the transition metal can significantly reduce the binding energy of the noble metal, increase the electron cloud density of the noble metal, make the noble metal maintain a lower oxidation state, and thus make the three-way catalyst have better catalytic effect.

[0015] Meanwhile, the addition of the noble metal can improve the dispersibility of the transition metal in the three-way catalyst, which is more conducive to the three-way catalytic reaction.

[0016] In the three-way catalyst, the loading amount of the noble metal is low, and the transition metal also has a synergistic effect with the noble metal to form a double active center.

[0017] Preferably, the noble metal content is 0.2wt% to 1.0wt% of the transition metal oxide modified CeSn composite oxide; more preferably, 0.5wt% to 0.8wt%. In some embodiments of the present application, the noble metal is rhodium, and the content of the rhodium is 0.5wt% of the transition metal oxide modified CeSn composite oxide.

[0018] The present application also provides a preparation method of a transition metal oxide modified composite oxide supported noble metal three-way catalyst, comprising the following steps:

[0019] The cerium source, the tin source, the transition metal source and the molten salt are mixed and calcined, and then the calcined mixture is mixed with a noble metal precursor solution, and a transition metal oxide modified CeSn composite oxide supported noble metal three-way catalyst is obtained by heating and evaporating to dryness and calcination.

[0020] The above preparation method directly synthesizes a transition metal oxide modified composite oxide carrier by a molten salt method, and then a noble metal is loaded on the carrier by an impregnation method to obtain a transition metal oxide modified composite oxide supported noble metal three-way catalyst.

[0021] Since the liquid molten salt is always present between the generated particles in the method, the synthesized three-way catalyst has good dispersity.

[0022] The molten salt method for preparing the three-way catalyst simplifies the preparation steps, shortens the preparation period, and is conducive to industrial production.

[0023] Preferably, the cerium source is selected from one or more of cerium nitrate hexahydrate, cerium chloride and cerium acetate; more preferably, cerium nitrate hexahydrate.

[0024] Preferably, the tin source is selected from tin tetrachloride pentahydrate or stannous chloride dihydrate; more preferably, tin tetrachloride pentahydrate.

[0025] Preferably, the transition metal source is selected from a nitrate or a chloride of any one of copper, iron, manganese and cobalt; more preferably, copper nitrate trihydrate.

[0026] The molten salt in step 1) of the above preparation method can be a single salt or a mixed salt, including but not limited to one or more of sodium chloride, potassium chloride, sodium nitrate and potassium nitrate.

[0027] In some embodiments of the present application, sodium nitrate is preferred.

[0028] The above preparation method further comprises washing before loading the noble metal, so as to clean the molten salt and avoid contaminating the sample.

[0029] Preferably, the noble metal precursor is selected from one or more of chloroplatinic acid, chloropalladic acid, rhodium chloride, ruthenium chloride, chloroiridic acid; more preferably rhodium chloride.

[0030] Preferably, the molar ratio of the cerium source and the tin source is (2-4):1; more preferably 4:1.

[0031] Preferably, the molar ratio of the transition metal source and the tin source is (0.2-0.8):1; more preferably 0.25:1.

[0032] In the above preparation method, the temperature for heating and evaporating is preferably 60-100°C, and the time is preferably 6-12h.

[0033] The preparation method of the present application selects different melting points of the molten salt and different calcination temperatures.

[0034] In some embodiments of the present application, the calcination temperature for preparing the carrier transition metal oxide modified CeSn composite oxide is 400°C, and the calcination temperature for preparing the three-way catalyst is 500°C.

[0035] In the present application, the unique noble metal-transition metal dual site three-way catalyst can significantly improve the three-way catalytic reaction activity, achieve high reaction activity at a relatively low temperature, and has a stable structure and good durability after a long time of reaction.

[0036] Compared with the prior art, the three-way catalyst of the present application, in which the transition metal oxide modified composite oxide is loaded with noble metal, uses transition metal oxide and noble metal as dual active sites, and has significantly improved catalytic activity and excellent thermal stability. In the three-way catalyst, the loading amount of noble metal is low, and the modification of the transition metal oxide promotes the generation of oxygen vacancies on the surface of the carrier, thereby improving the catalyst activity. The molten salt method is used to prepare the three-way catalyst, which makes the catalyst have better dispersibility, the preparation process is simpler, the preparation period is shorter, and the method is suitable for large-scale industrial production. BRIEF DESCRIPTION OF DRAWINGS

[0037] Figure 1 The performance comparison chart of the catalysts prepared in Example 1 and Comparative Examples 1-3 for CO oxidation is shown in Figure 1;

[0038] Figure 2 The performance comparison chart of the catalysts prepared in Example 1 and Comparative Examples 1-3 for NO reduction is shown in Figure 2;

[0039] Figure 3 The performance comparison chart of the catalysts prepared in Example 1 and Comparative Examples 1-3 for C3H6 oxidation is shown in Figure 3;

[0040] Figure 4Figure of catalytic performance of Rh / Cu-CeSn catalyst prepared in Example 1 for CO oxidation, NO reduction, and C3H6 oxidation;

[0041] Figure 5 Figure of catalytic stability of Rh / Cu-CeSn catalyst prepared in Example 1 for CO oxidation, NO reduction, and C3H6 oxidation;

[0042] Figure 6 Figure of performance comparison of catalysts prepared in Example 2 and Comparative Examples 1, 3, and 4 for CO oxidation;

[0043] Figure 7 Figure of performance comparison of catalysts prepared in Example 2 and Comparative Examples 1, 3, and 4 for NO reduction;

[0044] Figure 8 Figure of performance comparison of catalysts prepared in Example 2 and Comparative Examples 1, 3, and 4 for C3H6 oxidation. DETAILED DESCRIPTION

[0045] In order to further illustrate the present application, the three-way catalyst of transition metal oxide modified composite oxide supported noble metal and the preparation method thereof provided by the present application are described in detail below in conjunction with examples.

[0046] Example 1

[0047] 1) 8 mmol of cerium nitrate hexahydrate, 2 mmol of tin tetrachloride pentahydrate, and 0.5 mmol of copper nitrate trihydrate were weighed, mixed, and ground in a mortar to uniformity to obtain a mixed powder;

[0048] 2) 20 mmol of sodium nitrate was mixed and ground with the mixed powder of step 1), and then the obtained mixed powder was subjected to calcination treatment, the calcination temperature was 400°C, and the calcination time was 4 hours, to obtain a calcined mixed powder;

[0049] 3) The mixed powder obtained by calcination in step 2) was washed with deionized water and anhydrous ethanol for multiple times and then vacuum dried;

[0050] 4) The sample obtained in step 3) was weighed and dispersed in deionized water, and ultrasonic treatment was performed for 10 min;

[0051] 5) A certain proportion of rhodium chloride solution, which is a noble metal salt precursor, was added to the solution obtained in step 4), and after stirring for 60 min, the solution was transferred to an 80°C oil bath for 10 h, and subjected to evaporation treatment;

[0052] 6) The sample obtained in step 5) is subjected to calcination treatment, the calcination temperature is 500°C, and the calcination time is 4h, to obtain a three-way catalyst of rhodium supported on copper oxide modified composite oxide (CeO2 and SnO2) (Rh / Cu-CeSn), wherein the content of the noble metal rhodium is controlled to be 0.5wt% of the copper oxide modified CeSn composite oxide.

[0053] Example 2

[0054] The same as the conditions of Example 1, except that "0.5mmol copper nitrate trihydrate" is replaced by "0.5mmol manganese nitrate tetrahydrate", finally obtaining a three-way catalyst of rhodium supported on manganese oxide modified composite oxide (CeO2 and SnO2) (Rh / Mn-CeSn), wherein the content of the noble metal rhodium is controlled to be 0.5wt% of the manganese oxide modified CeSn composite oxide.

[0055] The difference from Examples 1 and 2 is that no copper nitrate trihydrate or manganese nitrate tetrahydrate is added as a transition metal source in step 1), and the others are the same, finally preparing a three-way catalyst of rhodium supported on CeSn composite oxide (Rh / CeSn), wherein the content of the noble metal rhodium is controlled to be 0.5wt% of the CeSn composite oxide.

[0056] Comparative Example 2

[0057] 1) 8mmol of cerium nitrate hexahydrate, 2mmol of tin tetrachloride pentahydrate, and 0.5mmol of copper nitrate trihydrate are weighed, mixed and thoroughly ground in a mortar to obtain a mixed powder;

[0058] 2) 20mmol of sodium nitrate is thoroughly mixed and ground with the mixed powder of step 1), and then the obtained mixed powder is subjected to calcination treatment, the calcination temperature is 400°C, and the calcination time is 4h, to obtain a calcined mixed powder;

[0059] 3) The mixed powder obtained by calcination in step 2) is washed with deionized water and anhydrous ethanol for several times and then vacuum dried to obtain copper oxide modified composite oxide (CeO2 and SnO2), denoted as Cu-CeSn.

[0060] 1) 8mmol of cerium nitrate hexahydrate and 2mmol of tin tetrachloride pentahydrate are weighed, mixed and thoroughly ground in a mortar to obtain a mixed powder;

[0061] 2) 20mmol of sodium nitrate is thoroughly mixed and ground with the mixed powder of step 1), and then the obtained mixed powder is subjected to calcination treatment, the calcination temperature is 400°C, and the calcination time is 4h, to obtain a calcined mixed powder;

[0062] 3) The resulting mixed powder from step 2) was washed with deionized water and anhydrous ethanol for several times and then vacuum dried to obtain the composite metal oxide (CeO2 and SnO2), denoted as CeSn.

[0063] Comparative Example 4

[0064] The same conditions as Comparative Example 2, except that "0.5 mmol copper nitrate trihydrate" was replaced by "0.5 mmol manganese nitrate tetrahydrate", and finally the manganese oxide modified composite oxide (CeO2 and SnO2) was obtained, denoted as Mn-CeSn.

[0065] Performance test process:

[0066] 60 mg of catalyst was weighed and mixed with 60 mg of quartz sand, and then ground thoroughly. The mixture was placed in a quartz tube (the catalyst was fixed at both ends with quartz wool with a diameter of 3-5 μm to prevent flow), and then placed in a temperature rising furnace for air flow reaction test. The catalyst layer of the temperature rising furnace was gradually heated from room temperature to 300°C at a constant rate of 10°C / min. During the process, a point was taken every 30°C, and the temperature was kept for 10 min at that temperature, i.e. the time interval between different test points was 13 min. At the same time, the reaction gas mixture was continuously introduced at a flow rate of 200,000 mL / (g·h), and the catalytic activity of the catalyst was evaluated according to the display results. -1

[0067] The composition of the gas phase sample was identified, analyzed and conversion rate was calculated by using Nicolet iS-50 FTIR spectrometer and connected gas cell. First, the reaction gas was introduced at room temperature, and after the spectrometer characterization showed that the reaction gas flow was stable, the intensity peak displayed after the room temperature was stable was taken as the initial value, and the temperature was gradually increased for testing. During the process, the characteristic peak wave number of different gases was used to identify the gas composition.

[0068] The quartz tube type: inner diameter 7 mm, outer diameter 10 mm, length 50 cm, recessed at 25 cm, both ends unobstructed;

[0069] The test space velocity was 200,000 ml / (g·h);

[0070] The reaction gas ratio: 1.995% CO, 1.75% O2, 21.081 ppm C3H6, 5.1506 ppm NO and balanced Ar gas;

[0071] The Nicolet iS-50 FTIR spectrometer was from Thermo Scientific Company;

[0072] The gas cell was from ThermoFisher Scientific Company (10M Gas Cell KBR).​

[0073] For example, the conversion rate of CO at 250℃ is:

[0074] CO conversion rate (%) = [CO (初始值) -CO (250℃) ] / CO (初始值) ×100% Performance test results:

[0075] Figure 1 The performance comparison chart of the catalysts prepared in Example 1 and Comparative Examples 1-3 for CO oxidation.

[0076] Figure 2 The performance comparison chart of the catalysts prepared in Example 1 and Comparative Examples 1-3 for NO reduction.

[0077] Figure 3 The performance comparison chart of the catalysts prepared in Example 1 and Comparative Examples 1-3 for C3H6 oxidation.

[0078] According to the results of Figures 1 to 3 , it is shown that the catalytic performance of the Rh / Cu-CeSn three-way catalyst prepared in Example 1 for CO oxidation, NO reduction and C3H6 oxidation is significantly better than that of the Rh / CeSn, Cu-CeSn and CeSn catalysts of Comparative Examples 1-3.

[0079] Figure 4 The catalytic performance chart of the Rh / Cu-CeSn catalyst prepared in Example 1 for CO oxidation, NO reduction and C3H6 oxidation.

[0080] Figure 5 The catalytic stability chart of the Rh / Cu-CeSn three-way catalyst prepared in Example 1 for CO oxidation, NO reduction and C3H6 oxidation. The results show that after 40 hours of testing at 300℃, the catalytic conversion rates of the Rh / Cu-CeSn three-way catalyst for CO oxidation, NO reduction and C3H6 oxidation all decrease by less than 5%, thus the catalyst has good stability.

[0081] In addition, through XPS (X-ray photoelectron spectroscopy) analysis, it is shown that by comparing the Rh / Cu-CeSn prepared in Comparative Example 1 with the Rh / CeSn prepared in Comparative Example 1, the addition of Cu significantly increases the ratio of Rh 0 (Rh 0 / Rh 0 +Rh 3+ +Rh 4+ ), while Rh 0More beneficial to three-way catalytic reaction, so the Rh / Cu-CeSn three-way catalyst, the modification of Cu oxide significantly improves the catalytic performance of the three-way catalyst;

[0082] Figure 6 The performance comparison chart of the catalysts prepared in Example 2 and Comparative Examples 1, 3 and 4 for CO oxidation is shown in the following figure. Figure 7 The performance comparison chart of the catalysts prepared in Example 2 and Comparative Examples 1, 3 and 4 for NO reduction is shown in the following figure. Figure 8 The performance comparison chart of the catalysts prepared in Example 2 and Comparative Examples 1, 3 and 4 for C3H6 oxidation is shown in the following figure.

[0083] According to the results Figures 6 to 8 The results show that the Rh / Mn-CeSn three-way catalyst prepared in Example 2 has significantly better catalytic performance for CO oxidation, NO reduction and C3H6 oxidation than the Rh / CeSn, CeSn and Mn-CeSn catalysts prepared in Comparative Examples 1, 3 and 4. By comparing the Rh / Cu-CeSn prepared in Comparative Example 1 with the Cu-CeSn prepared in Comparative Example 1, and comparing the Rh / Mn-CeSn prepared in Example 2 with the Mn-CeSn prepared in Comparative Example 4, it is shown that Rh can significantly promote the dispersion of transition metal oxides (copper or manganese oxides), increase the active sites and effectively improve the catalytic performance.

[0084] In summary, the three-way catalyst of the present application has the transition metal oxide modified composite oxide supported noble metal as the dual active sites of the catalyst, and the two synergistically improve the catalytic performance of the three-way catalyst.

[0085] The above examples are only used to help understand the method of the present application and its core idea. It should be noted that for those skilled in the art, without departing from the principles of the present application, the present application can be improved and modified in several ways, and these improvements and modifications also fall within the scope of protection of the claims of the present application.

Claims

1. A three-way catalyst, characterized in that, It consists of CeSn composite oxide modified by transition metal oxide and noble metal supported on the surface of CeSn composite oxide modified by transition metal oxide; The transition metal oxide is selected from cuprous oxide and / or copper oxide; The precious metal is selected from one or more of ruthenium, rhodium, palladium, iridium, and platinum; The three-way catalyst is prepared according to the following method: A three-way catalyst with noble metal supported on CeSn composite oxide modified by transition metal oxide was obtained by calcining a mixture of cerium source, tin source, transition metal source and molten salt, then mixing the calcined mixture with a noble metal precursor solution, heating to dryness and calcining. The molten salt includes one or more of sodium chloride, potassium chloride, sodium nitrate, and potassium nitrate.

2. The three-way catalyst according to claim 1, characterized in that, The noble metal content is 0.2 wt% to 1.0 wt% of CeSn composite oxide modified with transition metal oxide.

3. The method for preparing the three-way catalyst of noble metal supported on a composite oxide modified by a transition metal oxide according to any one of claims 1 to 2, characterized in that, Includes the following steps: A three-way catalyst with CeSn composite oxide supported on noble metals and modified by transition metal oxide is obtained by mixing cerium source, tin source, transition metal source and molten salt and then calcining. The calcined mixture is then mixed with a noble metal precursor solution and heated to dryness and calcined again.

4. The preparation method according to claim 3, characterized in that, The cerium source is selected from one or more of cerium nitrate hexahydrate, cerium chloride, and cerium acetate. The tin source is selected from tin tetrachloride pentahydrate or stannous chloride dihydrate; The transition metal source is selected from copper nitrates or chlorides.

5. The preparation method according to claim 3, characterized in that, The noble metal precursor is selected from one or more of chloroplatinic acid, chloropalladiumic acid, rhodium chloride, ruthenium chloride, and chloroiridium acid.

6. The preparation method according to claim 3, characterized in that, The molar ratio of the cerium source to the tin source is (2~4):

1.

7. The preparation method according to claim 3, characterized in that, The molar ratio of the transition metal source to the tin source is (0.2~0.8):1.

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