CN1799689B - Close coupled catalyst - Google Patents

Abstract

The invention relates to a highly active and thermally stable close-coupled catalyst for purifying engine cold start exhaust gas. Close-coupling catalysts include at least three of different phases of alumina as catalyst supports, Pd noble metal active components, and include rare earth oxides, Y ₂ O ₃ , La ₂ O ₃ , Nd ₂ O ₃ and Sm ₂ O ₃ At least one of alkaline earth oxides, at least one of SrO, BaO and CaO, and ZrO ₂ .

Description

close-coupled catalyst

1. Technical field

The invention relates to a close-coupled catalyst for effectively reducing the emission of HC pollutants during engine cold start and a preparation method thereof.

2. Background technology

In recent years, the research on catalysts for purifying harmful substances HC, CO and NOx in automobile exhaust has been very active. With the increasingly stringent environmental emission regulations, it is extremely important to address HC emissions during engine cold start.

A cold start is the brief moment when the engine starts from ambient temperature. The cold start process depends on the ambient temperature, engine type, engine control system and how the engine works. A typical cold start process is within the first two minutes after the engine is started from ambient temperature, and the FTP-75 test method considers a cold start to be within 505 seconds after the engine is started from ambient temperature. The FTP-75 test method is currently the main test method used worldwide to evaluate the purification effect of exhaust gas purification catalysts. Among the three phases of the FTP-75 test, the HC emissions generated during the cold start phase, especially during the first 100 seconds after a cold start, accounted for two-thirds of the HC generated during the FTP-75 test. Since the temperature of the exhaust gas discharged from the engine during cold start is low, it does not reach the light-off temperature of the three-way catalyst, causing a large amount of HC to be directly emitted. Close-coupled catalysts, also commonly referred to as "upstream catalysts" or "preheat catalysts," are commonly used to reduce HC emissions during cold starts. Considering the effective reduction of HC emissions during cold start and the long-term stability of the catalyst, the performance of the close-coupled catalyst is as follows: (1) Low-temperature activity, measured by the light-off temperature of the catalyst. The light-off temperature is the temperature at which 50% of a particular component (such as HC) is converted. Catalyst light-off is usually required within 40 seconds or less after cold start; correspondingly, the catalyst should achieve 50% HC conversion at 250°C or less. It is mainly achieved by improving the activity of the catalyst. (2) High temperature stability, the close-coupled catalyst is a part of the entire automobile exhaust purification catalyst, which is closer to the engine than the traditional "chassis catalyst", which enables it to reach the reaction temperature as soon as possible. However, after the engine works stably, due to the close-coupled The catalyst is only a foot or six inches away from the engine or directly in contact with the exhaust manifold, so it is directly exposed to the high temperature exhaust (sometimes over 1100°C). Therefore, close-coupled catalysts must not only have a low light-off temperature, but also must remain stable at high temperatures after the engine has stabilized, which can be solved by improving the high-temperature stability of the catalyst coating. (3) The limited conversion of CO, the strong exotherm of CO oxidation causes the sintering of the catalyst and reduces the life of the close-coupled catalyst. Therefore, limited conversion of CO over close-coupled catalysts should be achieved through catalyst preparation. Provide a large amount of CO to the downstream three-way catalyst to make it reach a higher temperature as soon as possible to improve its efficiency.

In order to achieve the above required performance, Pd is used as the active component of the close-coupling catalyst due to its excellent low-temperature light-off performance, and sometimes Pt, Rh, etc. are also used as the active component together with Pd to improve the performance of the catalyst. The cordierite honeycomb carrier or metal honeycomb carrier is used as the matrix, heat-resistant inorganic oxides (such as Al ₂ O ₃ ) are used as the carrier for dispersing the active components, and together with rare earth and alkaline earth oxides as auxiliary agents and stabilizers, the coating of the catalyst is formed. layer.

In US6254842, γ-Al ₂ O ₃ (160m ² /g) was used to support Pd (100g/ft ³ ), and La, Zr, Sr and Nd were added as additives, and a close-coupling catalyst was prepared with cordierite honeycomb as the substrate . After aging at 950°C for 12 hours in air (10% H ₂ O), the light-off temperatures for HC, CO, and _NOx are 252°C, 228°C, and 213°C, respectively.

In US6602822, closely coupled catalysts with different Pd loads and different coating thicknesses were prepared by using γ-Al ₂ O ₃ to support Pd. It was found that appropriately reducing the coating thickness can reduce the ignition temperature.

In US5878567, it is proposed to use highly loaded Pd or three metals (Pd, Pt and Rh) (100-300 g/ft ³ ) and add La, Ba, Zr, Ce and other auxiliary agents to prepare close-coupled catalysts.

In CN1197686A, a double-coated cold-start catalyst is introduced. The first layer consists of Pd supported on La- _Al2O3 , cerium/zirconium mixed oxide, _zirconia and zeolite ZSM-5. _The second layer consists of Rh supported on La- _Al2O3 , cerium/zirconium mixed oxide and dealuminated Y zeolite. The purpose of adding zeolite is to adsorb HC before the catalyst reaches the light-off temperature, so that it can be desorbed and transformed by the catalyst after the catalyst reaches the light-off temperature. But so far, there is no zeolite that can withstand the high temperature of the exhaust gas after the engine is started, and there are problems with its stability.

Since close-coupled catalysts are exposed to exhaust gases above 1000°C, γ-Al ₂ O ₃ or La-Al ₂ O ₃ are currently used as carriers for close-coupled catalysts, both of which exist at high temperatures, especially in the presence of water vapor. The volume shrinkage caused by the phase transformation at high temperature (about 10% H ₂ O in the tail gas) leads to the loss of the specific surface area of the catalyst, the sintering of the active components and the decrease of the catalytic activity. In this _regard , La- _Al2O3 is somewhat better than γ- _Al2O3 , but the problem _still exists. Therefore, it is desirable to improve the high temperature stability of catalyst supports and catalyst coatings.

3. Contents of the invention

The invention provides a highly stable close-coupled catalyst and a preparation method thereof. Different from the prior art, the present invention does not use a single γ-Al ₂ O ₃ or La-stabilized Al ₂ O ₃ as a carrier, but uses multiple Al ₂ O ₃ in different phases as a carrier. The reason is that Al ₂ O ₃ particles of the same phase have the same phase interface, and the same phase interface is prone to sintering during high temperature aging. resulting in a decrease in the specific surface area. Al ₂ O ₃ particles of different phases have different phase interfaces, and different phase interfaces have better anti-sintering ability. The present invention improves the stability of the catalyst carrier and the catalyst coating by utilizing the excellent anti-sintering ability provided by different phase interfaces between different phases of Al ₂ O ₃ . According to the idea of the present invention, the Al ₂ O ₃ support materials that can be used as different phases of the close-coupled catalyst of the present invention are: γ-Al ₂ O ₃ , θ-Al ₂ O ₃ , δ-Al ₂ O ₃ , La stable γ-Al ₂ O ₃ (denoted as La-γ-Al ₂ O ₃ ), La-stabilized θ-Al ₂ O ₃ (denoted as La-θ-Al ₂ O ₃ ), La-stabilized δ-Al ₂ O ₃ (denoted as La-δ-Al ₂ O ₃ ), Y, Zr stabilized γ-Al ₂ O ₃ (denoted as YSZ-γ-Al ₂ O ₃ ), etc. In the above-mentioned Al ₂ O _3, there are aluminas of different phases and aluminas belonging to the same phase, such as γ-Al ₂ O ₃ , La-γ-Al ₂ O ₃ and YSZ-γ-Al ₂ O ₃ Belong to the same phase, but γ-Al ₂ O ₃ has no additives, La-γ-Al ₂ O ₃ and YSZ-γ-Al ₂ O ₃ have different additives, and their grain size and shape are different of. When mixed together, it has better anti-sintering ability than any one used alone. In order to achieve the effect of the present invention, when using different phases of _Al2O3 as a carrier, at least three of _the above materials are _used at the same time, and the usage amount of each _Al2O3 is no more than 50% of the total usage amount. %, the minimum usage is not less than 10% of the total usage. In principle, the above-mentioned different phases of Al ₂ O ₃ carrier materials can be combined arbitrarily, and all can produce excellent effects. The performance of the different phases _{of Al2O3} used in the present invention is as follows:

γ-Al ₂ O ₃ : pure γ-Al ₂ O ₃ , BET surface area: 180m ² /g.

δ-Al ₂ O ₃ : pure δ-Al ₂ O ₃ , BET surface area: 120 m ² /g.

θ-Al ₂ O ₃ : pure θ-Al ₂ O ₃ , BET surface area: 80m ² /g.

La-γ-Al ₂ O ₃ : γ-Al ₂ O ₃ , stabilized with 2-4% by weight of lanthanum (calculated as lanthanum oxide), BET surface area: 140 m ² /g.

La-θ-Al ₂ O ₃ : θ-Al ₂ O ₃ , stabilized with 2-4% by weight of lanthanum (calculated as lanthanum oxide), BET surface area: 75 m ² /g.

La-δ-Al ₂ O ₃ : δ-Al ₂ O ₃ , stabilized with 2-4% by weight of lanthanum (calculated as lanthanum oxide), BET surface area: 115 m ² /g.

YSZ-γ-Al ₂ O ₃ : γ-Al ₂ O ₃ , stabilized with 2-3% by weight of yttrium and 4-5% by weight of zirconium (both as oxides), BET surface area: 110m ² /g.

The close-coupled catalyst provided by the present invention uses the above-mentioned Al ₂ O ₃ in different phases as a carrier, and the content of the Al ₂ O ₃ carrier in different phases is 70-90% by weight. Pd is an active component, and its content is 1-5% (weight). The promoters are rare earth oxides and alkaline earth oxides, the rare earth oxides are at least one of the oxides of rare earth elements Y, La, Nd and Sm, and the content is 2 to 15% (by weight), and the alkaline earth oxides are Ca, Sr , at least one of the oxides of Ba, the content is 3-15% (weight), and the content of ZrO ₂ is 2-5% (weight). The selected substrate is a cordierite honeycomb ceramic substrate or a metal honeycomb substrate with a pore density of 400cpsi (cells per square inch) and above. The small sample substrate is cut from a large substrate and made into a nearly cylindrical body of φ12×25.

The close-coupled catalyst provided by the invention is prepared by the following method:

1. _The active component Pd is loaded on the _Al2O3 of different phases as the carrier and immobilized;

2. Grinding the carrier loaded with the active component and the precursor of the auxiliary agent together to form a coating solution with a solid weight content of 25-51%;

3. Immerse the substrate in the coating solution;

4. Take the substrate out of the coating liquid and blow off the excess coating liquid in the substrate holes with compressed air;

5. The substrate coated with the coating solution is dried and then roasted to cure the catalyst coating on the substrate to obtain the close-coupled catalyst of the present invention.

In the above preparation method, the carrier loaded with active components is preferably dried at 100-150° C. for 4-8 hours, and then calcined at 400-500° C. for 2-4 hours. The substrate coated with the coating solution is preferably dried at 100-150° C. for 3-5 hours, and then fired at 400-550° C. for 3-5 hours.

The inventors tested the performance of the close-coupled catalyst provided by the present invention. After the close-coupled catalyst was hydrothermally aged at 1050°C, the light-off temperature of the catalyst for HC was below 250°C, showing excellent aging resistance.

Compared with the prior art, the biggest contribution of the present invention is that the Al ₂ O ₃ in different phases is used as the carrier to greatly improve the anti-aging performance of the catalyst. Emissions requirements during cold starts can be better met.

The specific implementation part of the description has a more specific description of the content of the present invention, and other features, details and advantages of the present invention can be understood in more detail by reading this part of the description.

4. Specific implementation

Example 1

Take 112 grams of Al ₂ O ₃ powder in different phases, 112 grams of which γ-Al ₂ O ₃ , La-γ-Al ₂ O ₃ , δ-Al ₂ O ₃ and La-θ-Al ₂ O ₃ each account for 25 %(weight). Impregnate with a palladium nitrate solution containing 2.24 grams of Pd, dry at 120°C for 4 hours, and bake at 550°C for 3 hours to obtain Pd-containing alumina. Pd-containing alumina, strontium nitrate (containing 5 grams of SrO), lanthanum nitrate (containing 7 grams of La ₂ O ₃ ), zirconium nitrate (containing 7 grams of ZrO ₂ ), neodymium nitrate (containing 4 grams of Nd ₂ O ₃ ) and an appropriate amount of water Ball milled to obtain a coating solution with a solid content of 40% by weight. Take a 400cpsi cordierite substrate and immerse it in the coating solution. After taking it out, blow off the excess coating solution with compressed air, dry it at 120°C for 3 hours, and bake it at 500°C for 4 hours to obtain catalyst C1. The Pd content was 2.24 g/L, 112 g/L Al ₂ O ₃ , 5 g/L SrO, 7 g/L La ₂ O ₃ , 8 g/L ZrO ₂ , 4 g/L Nd ₂ O ₃ .

Example 2

The preparation method is the same as in Example 1, except that in different phases of Al ₂ O ₃ , θ-Al ₂ O ₃ , La-γ-Al ₂ O ₃ , YSZ-γ-Al ₂ O ₃ La-δ-Al ₂ O ₃ each accounted for 25%. Catalyst C2 is obtained

Example 3

The preparation method is the same as that in Example 1, except that in different phases of Al ₂ O ₃ , La-δ-Al ₂ O ₃ accounts for 15%, YSZ-γ-Al ₂ O ₃ accounts for 20%, and La-γ-Al ₂ O ₃ accounts for 45%, and γ-Al ₂ O ₃ accounts for 20%. Catalyst C3

Example 4

The preparation method is the same as that in Example 1, except that in different phases of Al ₂ O ₃ , γ-Al ₂ O ₃ accounts for 30%, La-γ-Al ₂ O ₃ accounts for 40%, and La-θ-Al ₂ O ₃ for 30%. Catalyst C4

Example 5

The preparation method is the same as that in Example 1, except that in different phases of Al ₂ O ₃ , θ-Al ₂ O ₃ accounts for 10%, La-γ-Al ₂ O ₃ accounts for 10%, YSZ-γ-Al ₂ O ₃ accounted for 15%, La-δ- _Al2O3 accounted for 15%, γ- _Al2O3 accounted for 10 _% , δ- _{Al2O3 accounted} for 15% and La-θ- _{Al2O3 accounted} for 25 _% . Catalyst C5

Example 6

The preparation method is the same as in Example 1, except that the lanthanum nitrate is replaced by yttrium nitrate, and the strontium nitrate is replaced by barium nitrate; the Pd content is 3.5 g/L. Catalyst C6

Example 7

The preparation method is the same as in Example 1, except that the lanthanum nitrate is replaced by samarium nitrate, and the strontium nitrate is replaced by calcium nitrate; the Pd content is 2.8 g/L. Catalyst C7

Comparative Example 1

The preparation method is the same as in Example 1, except that only γ-Al ₂ O ₃ is used as the carrier. Catalyst C8

Comparative Example 2

The preparation method is the same as in Example 1, except that only La-γ-Al ₂ O ₃ is used as the carrier. Catalyst C9

Comparative Example 3

The preparation method is the same as in Example 1, except that only La-θ-Al ₂ O ₃ is used as the carrier. Catalyst C10 is obtained.

Comparative Example 4

The preparation method is the same as in Example 1, except that only θ-Al ₂ O ₃ is used as the carrier. Catalyst C11 is obtained.

Comparative Example 5

The preparation method is the same as in Example 1, except that only δ-Al ₂ O ₃ is used as the carrier. Catalyst C12 is obtained.

Comparative Example 6

The preparation method is the same as in Example 1, except that only La-δ-Al ₂ O ₃ is used as the carrier. Catalyst C13 is obtained.

Comparative Example 7

The preparation method is the same as in Example 1, except that only YSZ-γ-Al ₂ O ₃ is used as the carrier. Catalyst C14 is obtained.

catalyst aging

The prepared fresh catalyst was placed in a tube furnace, fed with air containing 10% H ₂ O, and aged at 1050° C. for 10 h.

test

The activity evaluation of the catalyst is carried out on the simulation evaluation device of the automobile exhaust gas purification catalyst, and the evaluation device is a fixed bed flow reaction device. The simulated automobile exhaust contains 550ppm HC (half of C ₃ H ₈ and half of C ₃ H ₆ ), 0.6% of CO, 600ppm of NOx and 10% of H ₂ O, and the remainder of N ₂ . The reaction space velocity was controlled at 5.0×10 ⁴ h ^-1 . During the course of the reaction, the oxygen content can be changed.

The gas before and after the reaction was quantitatively analyzed by the FGA4100 five-component exhaust gas analyzer produced by Foshan Fofen Environmental Protection Testing Equipment Co., Ltd.

Since the purpose of the close-coupled catalyst is to treat the HC emission during the cold start of the gasoline vehicle, the light-off temperature (T _{50 %} ) of the catalyst HC after aging is mainly measured.

During the measurement, the catalyst was first activated at 550°C for 2 hours in the exhaust gas state. The measurement results are shown in Table 1.

The light-off temperature (T _50% ) of HC after the aging of table 1

catalyst aging sample C1 245 C2 240 C3 248 C4 240 C5 246 C6 235 C7 241 C8 278 C9 260 C10 288 C12 298 C13 300 C14 276

From the results in Table 1, it can be clearly seen that the light-off temperature of HC after aging of catalysts using different phases of Al ₂ O ₃ as supports is significantly lower than that of single Al ₂ O ₃ as supports. It can well meet the requirements of HC low temperature ignition during cold start.

Claims (5)

1. a stable close coupled catalyst, by the Al of jljl phase not ₂o ₃carrier: γ-Al ₂o ₃, θ-Al ₂o ₃, δ-Al ₂o ₃, γ-Al that La is stable ₂o ₃, θ-Al that La is stable ₂o ₃, δ-Al that La is stable ₂o ₃, γ-Al that Y and Zr are stable ₂o ₃in at least three kinds, auxiliary agent is by rare earth oxide Y ₂o ₃, La ₂o ₃, Nd ₂o ₃and Sm ₂o ₃in at least one, at least one in alkaline-earth oxide SrO, BaO and CaO and ZrO ₂and precious metals pd active component forms.

2. close coupled catalyst according to claim 1, is characterized in that the Al of said not jljl phase ₂o ₃carrier is γ-Al ₂o ₃, θ-Al ₂o ₃, δ-Al ₂o ₃, γ-Al that La is stable ₂o ₃, θ-Al that La is stable ₂o ₃, δ-Al that La is stable ₂o ₃, γ-Al that Y and Zr are stable ₂o ₃in at least three kinds, the content of the alumina support of jljl phase in catalyst is not 70-90% (weight); Rare earth oxide is rare earth element y, La, and at least one in the oxide of Nd and Sm, the content in catalyst is 2-15% (weight); Alkaline-earth oxide is Sr, at least one in the oxide of Ba and Ca, and the content in catalyst is 3-12% (weight); ZrO in catalyst ₂content be 2-7% (weight).

3. close coupled catalyst according to claim 1, is characterized in that the Al of said not jljl phase ₂o ₃any Al in carrier ₂o ₃the highest Al that is no more than of use amount ₂o ₃total use amount 50%, minimum use amount is not less than 10% of total use amount.

4. close coupled catalyst according to claim 1, is characterized in that close coupled catalyst coating load is on matrix, and matrix is cordierite honeycomb ceramic matrix or metal beehive matrix, hole density be 400cpsi and more than.

5. according to the preparation method of the close coupled catalyst described in any one claim in claim 1 to 4, it is characterized in that comprising following processing step:

(1) active component Pd be carried on the alumina support of jljl phase not and carry out immobilization processing;

(2) load there are the carrier of active component and the precursor of auxiliary agent grind in the lump and make the coating solution that solid weight content is 25-51%;

(3) matrix is dipped in coating solution;

(4) matrix takes out with compressed air and blows down unnecessary coating solution in matrix hole from coating solution;

(5) matrix that has applied coating solution is carried out to roasting after drying, catalyst coat is solidificated on matrix.