JP3556695B2 - Exhaust gas purification catalyst - Google Patents

Description

【００４１】

【００４２】
【発明の効果】
以上のように本発明によれば、熱的安定性が高く、熱水蒸気による性能低下の極めて少ない、また耐硫黄性の高い排ガス浄化用触媒を得ることができる。[0001]
[Industrial applications]
The present invention relates to a catalyst for purifying exhaust gas, and more particularly to a catalyst used for purifying exhaust gas containing harmful and odorous components such as carbon monoxide and hydrocarbons discharged from automobiles and industrial facilities.
[0002]
[Prior art]
Purification of carbon monoxide and unburned hydrocarbons in automobile exhaust gas, complete combustible harmful and odorous components such as CO, hydrocarbons, organic acids, alcohols, esters and aldehydes in exhaust gas discharged from various industrial facilities In recent years, demand for a catalyst for purifying by oxidation (= combustion) has been increasing. In general, a combustion catalyst in which a noble metal such as platinum, palladium, and rhodium is dispersed and supported as a catalytically active component on an inorganic carrier such as alumina or silica having a high specific surface area has been used as an exhaust gas purifying catalyst.
[0003]
However, such catalysts have an activity due to thermal degradation of the support and, consequently, sintering of the active ingredient metal under use conditions such as an oxidation reaction that generates a large amount of heat of reaction or prolonged exposure to a high-temperature atmosphere. It decreases significantly. In order to suppress this thermal degradation, a high-melting point oxide such as lanthanum or barium is added to alumina to form a composite oxide, and a catalyst using the composite oxide as a carrier has been proposed and put into practical use (Japanese Patent Laid-Open No. 60-22929, 61-28453). It is said that LaAlO ₃ and BaAl ₂ O ₄ are generated in these carriers, which inhibit the growth of alumina particles by sintering due to heat and, as a result, sintering of the active ingredient metal.
[0004]
[Problems to be solved by the invention]
The catalyst using the above-mentioned composite oxide as a carrier exhibits durability when used at high temperatures, but when exposed to hot steam for a long time (hydrogen-containing fuels such as hydrocarbons generate steam during combustion), Since hydration and dehydration are repeated, and LaAlO ₃ and BaAl ₂ O ₄ are slightly water-soluble, aggregation of carrier particles occurs and the active site aggregates due to a change in the surface composition of the carrier, There was a problem that the activity was reduced.
Further, the catalyst using the above-mentioned composite oxide as a carrier has a problem that its activity is reduced even when a sulfur compound is present in exhaust gas. One of the causes of the catalyst deterioration is sulfation of the alumina carrier. It was considered that the porosity and surface properties of the carrier were changed by the sulfation, resulting in a change and collapse of the active site structure, resulting in a decrease in activity.
[0005]
Therefore, a catalyst using TiO ₂ —SiO ₂ and TiO ₂ as a sulfur-resistant carrier has been proposed (Japanese Patent Publication No. 60-6695, 61-47568). However, the TiO ₂ —SiO ₂ and TiO ₂ -supported noble metal catalysts have low high-temperature heat resistance and are said to work effectively in a temperature range of at most 500 ° C. Therefore, in general, when the catalyst is used in the combustion temperature range for exhaust gas purification of 600 to 900 ° C., the catalyst is deteriorated and the activity is reduced. It is considered that this is because the carrier undergoes crystallization or crystal transition at about 500 ° C., and at a high temperature, the specific surface area is remarkably reduced, and sintering of the active ingredient proceeds.
[0006]
On the other hand, a composite oxide carrier obtained by adding P ₂ O ₅ to TiO ₂ —SiO ₂ is also known (Japanese Patent Publication No. 60-6995), but it is considered that the heat resistance is equivalent to that of TiO ₂ —SiO ₂ . Furthermore, the effect of hot steam on catalysts using these supports is not specified at all.
Further, Pt as an active component has a higher oxidizing activity and a higher sulfur resistance against combustible components such as CO and hydrocarbons than Pd, but also has a problem in that heat resistance is inferior to Pd.
SUMMARY OF THE INVENTION An object of the present invention is to provide an exhaust gas purifying catalyst which has high durability for use at high temperatures and has a small activity decrease due to sulfur oxides and water vapor in exhaust gas.
[0007]
[Means for Solving the Problems]
The above object is achieved by the following constitutions claimed in the present application.
1. In an exhaust gas purifying catalyst in which a catalytically active component is supported on a catalyst carrier component, a composite oxide formed from three components of silica (SiO ₂ ), titania (TiO ₂ ) and ceria (CeO ₂ ) is used as a carrier component, An exhaust gas purifying catalyst characterized in that 1% by weight or more of palladium (Pd) or palladium and platinum (Pt) are dispersed and supported as a catalytically active component.
[0008]
2. 1. The exhaust gas purifying catalyst according to 1, wherein the composition of the composite oxide is 10 to 50 wt% as SiO ₂ , 30 to 80 wt% as TiO ₂ , and 10 to 50 wt% as CeO ₂ .
[0009]
3. 1. The exhaust gas purifying catalyst according to 1 or 2 above, wherein the starting material of SiO ₂ and TiO ₂ is an oxide colloid solution, and the starting material of CeO ₂ is cerium nitrate or cerium acetate.
[0010]
4. 2. The exhaust gas purifying catalyst according to the above item 1, wherein the amount of palladium or palladium and platinum, which is a catalytically active component, is 1.0 to 10 wt% as a metal relative to the weight of the carrier.
[0011]
[Action]
In the present invention, a ternary composite oxide (hereinafter abbreviated as SiO ₂ —TiO ₂ —CeO ₂ ) formed of silica (SiO ₂ ), titania (TiO ₂ ) and ceria (CeO ₂ ) is used as a carrier. To significantly reduce the deterioration of catalytic performance due to high temperature, hot steam and sulfur compounds, and to improve the heat resistance of Pt by using a binary metal formed from Pd and Pt as an active component. It is.
Among the above-mentioned drawbacks of the prior art, some of the carriers are altered by hot steam treatment (exposure to high-temperature steam) due to the dissolution and movement of the carrier components. Therefore, insoluble compounds (TiO ₂ , SiO ₂ , CeO _2, etc.) are used. It can be prevented by using it as a constituent. In addition, in order to suppress aggregation of active sites due to a change in the surface structure of the carrier (pore collapse) due to high-temperature treatment or hydrothermal treatment, a compound having high thermal stability that suppresses sintering of the carrier ( This can be achieved by using CeO ₂ ) as a carrier component. Further, deterioration of the carrier due to a sulfur compound can be prevented by using a metal oxide (TiO ₂ , SiO ₂ , CeO ₂ or the like) having high acid resistance and low reactivity with a sulfur atom as a carrier component.
[0012]
Most of these metal oxides alone as carriers have a small specific surface area, so they are often unsuitable as catalyst carriers by themselves.However, it is necessary to increase the specific surface area by combining multiple metal oxides. Is possible. Generally, a binary or ternary composite oxide is prepared by coprecipitating a mixed oxide (hydroxide) from a mixture of two or three types of metal salt solutions, drying and calcining the mixed oxide. If the raw materials are not mixed uniformly, a uniform composite oxide will not be obtained, and as a result, this will not lead to an increase in specific surface area or improvement in thermal degradation. In order to prevent this, uniform mixing of the raw material components can be achieved by using a colloid solution of each component.
[0013]
On the other hand, Pt as an active ingredient has excellent low-temperature activity and sulfur resistance, but has low heat resistance. On the other hand, Pd has lower temperature activity than Pt, but has heat resistance. By paying attention to this fact, by allowing both to be present in an appropriate ratio, a catalyst having excellent heat resistance, high sulfur resistance and high low-temperature activity can be obtained.
[0014]
【Example】
The reason why SiO ₂ —TiO ₂ —CeO _2, which is a catalyst carrier in the present invention, is particularly effective is not clear yet, but the heat resistance of the carrier is affected by the structure and acidity of the ternary composite oxide. It is believed that they would be higher or less interact with water, water vapor and sulfur compounds. The ternary composite oxide prepared at a temperature of 800-1000 ° C., which is a preferred processing condition according to the present invention, has a low crystallinity. For example, some CeO _{2 is} contained in the ternary composite oxide prepared at 1000 ° C. and in addition admit rutile TiO _2, could not have detected the specific compound. This means that SiO ₂ , TiO ₂ , and CeO ₂ do not exist alone, but mostly exist as a ternary composite oxide, and the activity when a palladium-platinum is supported as a carrier and used as a catalyst is as follows. , Very expensive. Therefore, these three kinds of oxides are complexed and are mostly amorphous or in the state of microcrystals and can maintain a high specific surface area, which is considered to be the reason for exhibiting high activity. The reason that each colloidal solution of silica and titania is selected as a raw material when preparing the composite oxide is that the size of the oxide colloidal particles is usually several hundreds of millimeters, so that both are very fine particles. This is because they can be mixed uniformly. The reason why the water-soluble cerium salt is selected as the ceria raw material is that, in the case of a cerium aqueous solution, cerium is present as ions, so that it can be sufficiently diffused into the colloid solution and uniformly mixed. As water-soluble salts, nitrates and acetates are preferred because of their high solubility and low decomposition temperature. Since ultrafine particles and ions uniformly mixed are apt to be combined by heat, a composite oxide is easily generated by firing.
[0015]
The colloidal oxide solution is commercially available, and its concentration is usually 10 to 30% by weight, and may be used. Although a colloid solution having a concentration outside this range can be prepared, it lacks stability and cannot withstand long-term storage, so there is a problem from the practical viewpoint of catalyst production.
The composite oxide does not need to be 100% complex or 100% amorphous. Due to the reaction between CeO ₂ and TiO ₂ , cerium titanate is formed as a thin film on the surface of the TiO ₂ particles (it cannot be detected by X-ray diffraction because it is so small) that the growth and sintering of the TiO ₂ particles are suppressed, It is considered that the SiO ₂ and CeO ₂ particles are interposed in the TiO ₂ particle gap, and the effect of suppressing sintering is further increased. It is also conceivable that CeO ₂ has an effect of suppressing sintering of SiO ₂ particles by intervening between the particles.
[0016]
As a result of a number of experiments, the present inventors have found that the carrier used in the present invention is composed of ₁₀ to 50 wt% of SiO ₂ , 80 to 30 wt% of TiO _{2, and 10} to 50 wt% of CeO _2. It has been found that it is suitable for exhibiting a specific effect. Although it can be used in other ranges, the heat resistance and heat-resistant steam resistance are lower, the specific surface area is smaller, and the catalytic activity is lower than those in the above range.
The use of palladium as the metal of the catalytically active component is preferred when the main combustible component in the exhaust gas is methane, and because it has excellent heat resistance, the exhaust gas can be treated at a high temperature. When a binary metal of palladium and platinum is used, both combustible components of hydrocarbon and CO can be efficiently burned. When sulfur compounds are contained in the exhaust gas with only palladium, the catalyst is poisoned by the sulfur compounds, but the sulfur resistance of the catalyst is improved by using a binary metal with platinum that is less poisoned by the sulfur compounds. . Further, there is an effect that the disadvantage that the heat resistance is low only with platinum can be overcome by using the above-mentioned binary metal. The mixing ratio of this binary system is preferably Pd / Pt = 3/2 to 3/1 by weight. If it is smaller than 3/2, the effect of improving heat resistance is reduced, and if it is larger than 3/1, the effect of improving sulfur resistance is reduced.
[0017]
The loading amount of the catalytically active component is preferably 1.0 to 10% by weight based on the weight of the carrier. Less than 1.0 wt% had little effect. If it exceeds 10% by weight, the catalyst becomes expensive, which is economically disadvantageous, and a corresponding increase in the effect cannot be expected. When the active ingredient was only palladium, the sulfur resistance was low, but the heat resistance and the heat-resistant steam resistance showed the same performance as that of the binary metal.
The catalyst of the present invention is effective when used in the form of granules, spheres, columns, honeycombs, etc., and particularly, those obtained by supporting the catalyst of the present invention on a heat-resistant three-dimensional structure are mechanical. Obviously, it is excellent in strength and economically advantageous.
[0018]
Hereinafter, the present invention will be described with reference to specific examples and comparative examples.
Example 1
100 g of a composite oxide carrier composed of 50 wt% of TiO ₂ , 20 wt% of SiO _{2 and} 30 wt% of CeO ₂ was supported on a cordierite honeycomb (volume: 1 liter). The preparation method is as follows. An aqueous cerium nitrate solution is added to the aqueous colloid mixture containing predetermined amounts of the TiO ₂ and SiO ₂ components so as to contain the predetermined amount of the CeO ₂ component, and mixed uniformly. This mixed solution is dip-coated on a cordierite honeycomb of 200 cells / in ² , dried at 110 ° C. for 2 hours, and baked at 900 ° C. for 2 hours. An aqueous solution of palladium nitrate containing 5 wt% of palladium corresponding to the amount of the carrier was supported on the honeycomb body by a suction impregnation method, dried at 110 ° C. for 2 hours, and calcined at 600 ° C. for 2 hours to obtain a honeycomb-shaped integrated catalyst. Got.
[0019]
The performance of this catalyst was evaluated using an atmospheric pressure fixed bed flow tube reactor. A reaction tube was filled with a honeycomb-shaped catalyst cut into a size of 20 mm × 20 mm × 20 mm, and a propane combustion reaction and a CO combustion reaction were carried out under the following conditions.
Space velocity: 30000 h ⁻¹ , catalyst layer inlet temperature: 350 ° C.
Propane concentration: 1000 ppm, CO concentration: 600 ppm (containing 200 ppm of SO ₂ ) After measuring the air-based initial activity, the catalyst was heat-treated at 800 ° C. for 200 hours in air, and 40% steam at 600 ° C. for 200 hours. Air containing was circulated to produce a product subjected to steam treatment. Each of them was again measured for propane combustion activity under the above conditions. The CO burning activity was continuously performed, and the activity after 200 hours was compared. The results are shown in Table 1 together with the results of the catalysts of the other examples.
[0020]
Example 2
Using a mixed aqueous solution of palladium nitrate containing 3 wt% of palladium and dinitrodiammine platinum containing 2 wt% of platinum instead of the supported palladium in Example 1, a catalyst prepared in the same manner as in Example 1 was used. Obtained.
[0021]
Example 3
In the same manner as in Example 2, a mixed aqueous solution of palladium nitrate containing 3 wt% of palladium and dinitrodiammine platinum containing 1.5 wt% of platinum was used instead of the supported palladium and platinum in Example 2. The prepared catalyst was obtained.
[0022]
Example 4
In the same manner as in Example 2, a mixed aqueous solution of palladium nitrate containing 3 wt% of palladium and dinitrodiammine platinum containing 1.0 wt% of platinum was used instead of the supported palladium and platinum in Example 2. The prepared catalyst was obtained.
[0023]
Example 5
A catalyst prepared in the same manner as in Example 3 except that the composite oxide carrier used in Example 3 was replaced with a composite oxide carrier consisting of 50 wt% of TiO ₂ , 40 wt% of SiO ₂ , and 10 wt% of CeO ₂ Got.
[0024]
Example 6
A catalyst prepared in the same manner as in Example 3 except that the composite oxide carrier used in Example 3 was replaced with a composite oxide carrier composed of 30 wt% of TiO ₂ , 20 wt% of SiO ₂ , and 50 wt% of CeO ₂ Got.
[0025]
Example 7
Instead of the composite oxide support used in Example 3, TiO ₂ is 80 wt%, SiO ₂ is 10 wt%, catalyst CeO ₂ by using a composite oxide support consisting of 10 wt%, were prepared in the same manner as in Example 3 Got.
[0026]
Example 8
A catalyst prepared in the same manner as in Example 3 except that the composite oxide carrier used in Example 3 was replaced with a composite oxide carrier consisting of 40 wt% of TiO ₂ , 50 wt% of SiO ₂ , and 10 wt% of CeO ₂ Got.
[0027]
Example 9
A catalyst prepared in the same manner as in Example 1 was obtained using an aqueous solution of palladium nitrate containing 1% by weight of palladium instead of the palladium supported in Example 1.
[0028]
Example 10
In the same manner as in Example 1, a mixed aqueous solution of palladium nitrate containing 0.7% by weight of palladium and dinitrodiammine platinum containing 0.3% by weight of platinum was used instead of the supported palladium in Example 1. The prepared catalyst was obtained.
[0029]
Comparative Example 1
A catalyst prepared in the same manner as in Example 3 was obtained using a composite oxide carrier composed of 60 wt% of TiO _{2 and} 40 wt% of SiO ₂ instead of the composite oxide carrier used in Example 3. The results of the same measurement as in Example 15 performed on this catalyst are shown in Table 2 together with the results of the other comparative catalysts.
[0030]
Comparative Example 2
A catalyst prepared in the same manner as in Example 3 was obtained using a composite oxide carrier composed of 60 wt% of TiO _{2 and} 40 wt% of CeO ₂ instead of the composite oxide carrier used in Example 3.
[0031]
Comparative Example 3
A catalyst prepared in the same manner as in Example 3 was obtained using a composite oxide carrier composed of 60 wt% of SiO _{2 and} 40 wt% of CeO ₂ instead of the composite oxide carrier used in Example 3.
[0032]
Comparative Example 4
Catalyst prepared in the same manner as in Example 3 except that the composite oxide carrier used in Example 3 was replaced with a composite oxide carrier composed of 60 wt% of TiO ₂ , 35 wt% of SiO ₂ , and 5 wt% of CeO ₂ Got.
[0033]
Example 11
A catalyst prepared in the same manner as in Example 3 except that the composite oxide carrier used in Example 3 was replaced with a composite oxide carrier composed of 20 wt% of TiO ₂ , 50 wt% of SiO ₂ , and 30 wt% of CeO ₂ Got.
[0034]
Example 12
A catalyst prepared in the same manner as in Example 3 except that the composite oxide carrier used in Example 3 was replaced with a composite oxide carrier consisting of 65 wt% of TiO ₂ , 5 wt% of SiO _{2 and} 30 wt% of CeO ₂ Got.
[0035]
Comparative Example 5
A catalyst prepared in the same manner as in Example 1 was obtained using a dinitrodiammine platinum aqueous solution containing 3 wt% of platinum instead of the supported palladium in Example 1.
[0036]
Example 13
Instead of the supported palladium and platinum in Example 2, a mixed aqueous solution of palladium nitrate containing 0.8 wt% of palladium and dinitrodiammine platinum containing 0.2 wt% of platinum was used. A similarly prepared catalyst was obtained.
[0037]
Example 14
Instead of the supported palladium and platinum in Example 2, a mixed aqueous solution of palladium nitrate containing 0.5 wt% of palladium and dinitrodiammine platinum containing 0.5 wt% of platinum was used. A similarly prepared catalyst was obtained.
[0038]
In Table 1, it can be seen that all of the catalysts of Examples 1 to 8 have excellent heat resistance at 800 ° C. and heat resistance to steam, and are excellent catalysts. The catalysts of Examples 9, 10, 13, and 14 having an active ingredient loading of 1 wt% have lower initial activity than others, but have good heat resistance and heat resistant steam resistance. As can be seen by comparing Examples 1, 9, and 10, when the active ingredient is a binary system of Pd and Pt, the sulfur resistance is significantly increased. However, even when the active ingredient carrying amount is 1 wt%, the heat resistance is reduced when the Pd / Pt ratio is 1 (Example 14).
[0039]
When the Pd / Pt ratio is 4 (Example 13), the sulfur resistance decreases.
It can be seen that the catalysts without CeO ₂ or with a small amount of addition (Comparative Examples 1 and 4) are inferior in heat resistance and heat-resistant steam resistance, and particularly inferior in heat-resistant steam resistance. This means that the addition of the CeO ₂ component plays a very important role. When the carrier is titania-ceria or silica-ceria alone, the heat resistance and the heat-resistant steam resistance are extremely low (Comparative Examples 2 and 3). Further, those having less than 30 wt% of TiO ₂ (Example 11) and those having less than 10 wt% of SiO ₂ (Example 12) have reduced heat resistance. A catalyst containing only Pt as an active component (Comparative Example 5) has poor heat resistance. The heat resistance is improved by increasing the supported amount of Pt, but the price of Pt is usually several times that of Pd. Therefore, increasing Pt only further increases the cost of the catalyst undesirably. If the amount of Pd carried on the catalyst of Example 9 is 9 wt%, it is apparent that the catalyst has sulfur resistance and heat resistance higher than those of the catalyst of Example 1.
[0040]

[0041]

[0042]
【The invention's effect】
As described above, according to the present invention, it is possible to obtain an exhaust gas purifying catalyst that has high thermal stability, has extremely low performance degradation due to hot steam, and has high sulfur resistance.

Claims (4)

In an exhaust gas purifying catalyst in which a catalytically active component is carried on a catalyst carrier component, a composite oxide formed of three components of silica (SiO ₂ ), titania (TiO ₂ ) and ceria (CeO ₂ ) is used as a carrier component, An exhaust gas purifying catalyst characterized in that 1% by weight or more of palladium (Pd) or palladium and platinum (Pt) are dispersed and supported as a catalytically active component. In claim 1, the composition of the composite oxide, range of 10 to 50 wt%, 30 to 80 wt%, an exhaust gas purifying catalyst, which is a range of 10 to 50 wt% as CeO ₂ as TiO ₂ as SiO _2. 3. The exhaust gas purifying catalyst according to claim 1, wherein the starting material of SiO ₂ and TiO ₂ is an oxide colloid solution, and the starting material of CeO ₂ is cerium nitrate or cerium acetate. 2. The exhaust gas purifying catalyst according to claim 1, wherein the supported amount of palladium or palladium and platinum as a catalytically active component is 1.0 to 10 wt% as a metal relative to the weight of the carrier.