WO2001034296A1

WO2001034296A1 - Catalyst for reducing toxic organic compounds and a method for preparing the catalyst

Info

Publication number: WO2001034296A1
Application number: PCT/KR2000/001118
Authority: WO
Inventors: Chul-Hoon Cho; Byung-Chul Shin; Jo-Young Lee; Son-Ki Ihm; Jae-Dong Hwang
Original assignee: Samsung Engineering Co., Ltd.
Priority date: 1999-11-10
Filing date: 2000-10-07
Publication date: 2001-05-17
Also published as: CN1387459A

Abstract

Catalyst for reducing toxic organic compounds and a method for preparing the catalyst are provided. The catalyst includes a titanium dioxide carrier with 2-10% crystallinity, which is doped with vanadium, and palladium or chromium. The catalyst has a high decomposition rate with respect to toxic organic compounds, such as 1,2-dichlorobenzene, and its catalytic activity is kept consistent through the catalytic reaction.

Description

CATALYST FOR REDUCING TOXIC ORGANIC COMPOUNDS AND A METHOD FOR PREPARING THE CATALYST

Technical Field The present invention relates to a catalyst for reducing toxic organic compounds and a method for preparing the catalyst, and more particularly, to a catalyst that can oxidize and decompose toxic organic compounds such as polychlorodibenzo-p-dioxin, polychlorodibenzofuran, polychlorobiphenyl and chlorobenzene produced in the incineration process of urban wastes and medical wastes, and a method for its preparation.

Background Art polychl orodi benzo-p-di oxi n , polych l orodi benzof u ran , polychlorobiphenyl and chlorobenzene contained in incineration exhaust gases are very toxic and contaminate the environment a great deal.

Therefore, there is a growing concern on processing these toxic organic compounds.

The commonest methods used for processing chloro-organic compounds contained in incineration exhaust gases are an adsorption method and a washing method.

The adsorption method is a method where the incineration exhaust gases are passed through an adsorbent such as activated carbon and coke, thereby adsorbing and removing the toxic organic compounds. But this method has a problem that when the adsorbent is reclaimed, there is a possibility that the toxic organic compounds will be eluted. Meanwhile, the washing method is a method of cleansing the incineration exhaust gases with chemicals and has a disadvantage that toxic waste water is generated. Another method by which the toxic organic compounds contained in the incineration exhaust gases can be removed is the oxidation decomposition method. This is a method whereby the incineration exhaust gases are passed through a catalyst at a constant temperature and the toxic organic compounds react with oxygen and decompose. As for the component of the catalyst used in this oxidation decomposition method, the transition metal elements such as titanium, vanadium, tungsten, molybdenum and chromium are heavily used. Precious metal components such as platinum, palladium, ruthenium, and rhodium can be used, but they are expensive.

Meanwhile, U.S. Patent No. 5,260,044 discloses a method of decomposing dioxin using a honeycomb type catalyst in which a titanium dioxide-mullite carrier is doped with platinum. However, in the case of this catalyst, the dispersion property of the metal with which the carrier is doped is not good, and when firing the catalyst the metal component sinters. Due to this reason, the catalyst has a disadvantage that a large space velocity (SV) is required since its decomposition activity is low.

Also, U.S. Patent No. 5,512,259 uses a catalyst that contains 70- 80% by weight of Ti0₂, 0-10% by weight of W0₃, 0.5-3% by weight of V₂0₅ and 0-5% by weight of Mo0₃, by remodeling a catalyst used in the selective reduction reaction of nitrogen. But this catalyst has a small BET surface area of 20-100 m²/g and the sintering of metal particles can occur easily. Thus when used for a long time, its decomposition activity is greatly reduced. Also, there is a disadvantage in that chloro-aromatic compounds do not compose to carbon monoxide, carbon dioxide, water, hydrochloric acid and chlorine gas, and change to organic compounds of other structure.

Disclosure of the Invention It is an objective of the present invention to provide a catalyst in which by improving the degree of dispersion, the sintering of metal particles is prohibited so that the decomposition activity is high and it can be used for a long time without a reduction in its decomposition activity, and which can completely decompose toxic organic compounds into carbon monoxide, carbon dioxide, water, hydrochloric acid, and chlorine gas.

It is another objective of the prevention invention to provide a method for preparing the catalyst.

To accomplish one objective of the present invention, there is provided a catalyst for decomposing toxic organic compounds, characterized in that the titanium dioxide carrier, whose crystallinity measured by X-ray defractometry is 2-10%, is doped with vanadium and palladium or vanadium and chromium

According to a first example, vanadium is preferably doped in the form of vanadium oxide (V₂0₅).

According to a second example, palladium is preferably doped in the form of palladium oxide (PdO).

According to a third example, chromium is preferably doped in the form of chromium oxide (Cr₂0₃).

According to a fourth example, the content of the titanium dioxide is preferably 80-94% by weight based on the total weight of the catalyst. According to a fifth example, the content of the vanadium oxide is preferably 6-12% by weight based on the total weight of the catalyst.

According to a sixth example, the content of the palladium oxide is preferably 0.05-1 % by weight based on the total weight of the catalyst.

According to a seventh example, the content of the chromium oxide is preferably 1 :2-1 :5 in the weight ratio of chromium oxide to vanadium oxide.

To accomplish the other objective of the present invention, there is provided a method for preparing the catalyst for decomposing toxic organic compounds, comprising (a) a step of controlling the firing of titanium dioxide at 400-600°C such that the crystallinity measured by X-ray defractometry is 2-10%;

(b) a step of putting the titanium dioxide prepared in step (a) in a vanadium oxalate aqueous solution;

(c) a step of vacuum drying the resulting material of step (b) for 2-12 hours at 100-300°C;

(d) a step of putting the resulting material of step (c) in a palladium nitrate aqueous solution or chromium nitrate aqueous solution in; and

(e) step of firing the resulting material of step (d) in an air atmosphere at 400-600°C for 5-15 hours.

In general , a metal-doped carrier uses multi-porous oxide as a carrier in order to improve the activity of the carrier by the interaction between the metal and the carrier and to increase the mechanical and thermal stabilities. In other words, a carrier is required to have a large surface area and an excellent mechanical strength and thermal stability. This carrier is doped with the catalyst metal thinly, thereby improving the reaction activity of the entire catalyst.

The decomposition activity of a metal-doped carrier is mainly determined by the degree of dispersion of the active metal, the kind of carrier and the kind of active metal with which the carrier is doped. That is, the decomposition activity is affected by the degree of dispersion, which is how well the metal is dispersed in the carrier. But in general, the degree of dispersion depends on the surface area of a carrier. In addition, the metal- doped carrier can be said to have a high activity when the active metal of a catalyst is not sintered and does not form a crystal on the surface of the carrier so that the active metal exists in a dispersed form. In particular, titanium dioxide as a multi-porous oxide carrier is an oxide that can be reduced in part, and the active component of the catalyst can be dispersed in a homogeneous way. Moreover, it is known to be a material that interacts with the active component of the catalyst. Also, it can be in the oxidation states of +3 or +2, depending on the reduction condition. It has a characteristic that its interaction with metal can be strong after the reduction treatment. In particular, it shows a strong interaction with precious metals such as platinum or palladium. Also, it has an advantage of being strongly resistant to hydrochloric acid that is generated as a byproduct when chloro-aromatic compounds are decomposed. As seen above, the kind of active metal and carrier is an important factor that determines the reaction activity. The present invention uses, as a carrier, titanium dioxide, which has the advantage described above; palladium, which shows a strong interaction with titanium dioxide and facilitates the dechlorination reaction of toxic organic compounds; and vanadium or chromium, which provide a high decomposition activity due to a high activity at low temperature.

In other words, the present invention is characterized in that the degree of dispersion of vanadium and palladium or vanadium and chromium with which the surface of titanium oxide is doped, has been greatly improved by controlling the crystallinity of titanium dioxide and optimizing the surface area. As a result, the catalyst-active metal particles do not sinter, thus maintaining the activity of the catalyst. In the present invention, the crystallinity of the titanium dioxide carrier is controlled by using a firing method to be within 2-10% when the crystallinity is measured using X-ray defractometry. The controlling of the crystallinity of titanium dioxide within the above range makes the BET surface area reach 50-100 rrrVg.

In the catalyst according to the present invention, the amount of titanium dioxide is preferably 80-94% by weight based on the total weight of the catalyst. If the amount is less than 80% by weight, the metal particles cannot be dispersed uniformly on the surface of the titanium dioxide carrier. If the amount exceeds 94% by weight, the metal particles can only be partially dispersed on the surface of the titanium dioxide carrier, not on the entire surface.

According to the first aspect of the present invention, a combination of vanadium oxide and palladium oxide as a catalyst metal is provided. Here, the content of the vanadium oxide is preferably 6-12% by weight based on the total weight of the catalyst, and the content of palladium oxide is preferably 0.05-1 % by weight. More preferably, the content of titanium oxide is 87-94% by weight based on the total weight of the catalyst. According to the second aspect of the present, a combination of vanadium oxide and chromium oxide as a catalyst metal is provided. Here, the weight ratio of the vanadium oxide to the chromium oxide is preferably 2:1-5:1 , and the content of titanium dioxide is more preferably 80-90% by weight.

It has been concluded that the combination of vanadium oxide and palladium oxide according to the first aspect of the invention and the combination of vanadium oxide and chromium oxide according to the second aspect of the invention have an excellent decomposition capability when decomposing particularly chloro-aromatic compounds among toxic organic compounds. In other words, the catalyst according to the invention is excellent in completely decomposing chloro-aromatic compounds into carbon monoxide, carbon dioxide, water, hydrochloric acid and chlorine gas, which are not organic compounds with other structure.

The invention will now be explained in detail by referring to figures and using examples. The examples below are only an illustration of the invention and should not used to limit the scope of the invention.

Brief Description of the Drawings

FIG. 1 is a graph showing the decomposition rate of 1 ,2- dichlorobenzene with respect to the reaction time when the catalyst according to an example of the first aspect of the invention is used.

FIG. 2 is a graph showing the decomposition rate of 1 ,2- dichlorobenzene with respect to the reaction time when the catalyst according to another example of the first aspect of the invention is used.

FIG. 3 is a graph showing the decomposition rate of 1 ,2- dichlorobenzene with respect to the reaction temperature and time when the catalyst according to an example of the second aspect of the invention is used.

Best mode for carrying out the Invention Examples 1 -4 below are examples according to the f i rst aspect of the invention. <Example 1>

50 g of titanium dioxide was fired at 500°C for 10 hours and a titanium dioxide carrier having a crystallinity of 8% measured using X-ray defractometry was prepared. 6.8 g of vanadium precursor NH₄V0₃ was added to oxalic acid of 30% w/w to prepare a vanadium oxalate aqueous solution. The titanium dioxide powder was put in the aqueous solution.

After drying the resulting material in a vacuum at 100°C for 5 hours and putting the material in a palladium nitrate aqueous solution, a titanium dioxide (90.7% by weight)-vanadium oxide (9% by weight)-palladium (0.3% by weight)-doped catalyst was manufactured by firing the material at 500°C for 10 hours in an air atmosphere.

The above catalyst was put in a Pyrex reaction vessel having a diameter of 10 mm, and the decomposition rate of 1 ,2-dichlorobenzene was measured as reaction temperature was varied. The reaction conditions were as follows.

Composition of the reaction gases: 1 ,2-dichlorobenzene 100 ppmv, H₂0 5% by volume, oxygen 11 % by volume.

Space velocity: 24,000 hr^"1

Reaction temperature: 250, 270, 300, 320, 350, 400°C Reaction pressure: 1 atm.

Using a catalyst that is doped with only 9% by weight vanadium, without palladium, the composition rate of 1 ,2-dichlorobenzene was measured in the same way as in Example 1. The results of Example 1 and Comparative example 1 are listed in Table 1 below.

Table 1

From Table 1 , it can be seen that while the decomposition rate of the catalyst of Comparative example 1 is only 64.7% at 300°C, that of Example 1 is above 90% at over 300°C, thus showing a 28.4% increase in the decomposition rate. Moreover, it can be seen that the catalyst of Example

1 has a decomposition rate of 100% at over 320°C.

By using the catalyst of Example 1 , 1 ,2-dichlorobenzene was decomposed at 300°C for 5 hours. The change in the decomposition rate with respect to the passage of reaction time was observed and is shown in FIG. 1. Irregardless of the passage of reaction time, the decomposition rate was almost constant.

< Example 3>

A catalyst is manufactured in the same way as in Example 1 , except that the content of palladium was 1 % by weight and the content of titanium dioxide was 90% by weight. The decomposition rate of 1 ,2-dichlrorbenzene with respect to the reaction temperature was measured and is shown in

Table 2.

Table 2

From Table 2, it can be seen that the catalyst of Example 3 also has a decomposition rate of more than 90% as in the case of Example 1 and an excellent decomposition rate of more than 99.99% at more than 350°C.

< Example 4>

By using the catalyst of Example 3, 1 ,2-dichlorobenzene was decomposed at 300°C for 5 hours. The change in the decomposition rate with respect to the passage of reaction time was observed and is shown in

FIG.2. Irregardless of the passage of reaction time, the decomposition rate was almost constant.

Examples 5-7 relates to the second aspect of the invention. <Example 5>

50 g of titanium dioxide was dried in a vacuum at 100°C for 24 hours, thus removing water completely. By firing the resulting material, a powder -type carrier having a crystallinity of 8% measured using X-ray defractometry was prepared. 6.8 g of vanadium precursor NH₄V0₃ was added to oxalic acid of 36% w/w and then a vanadium oxalate solution was prepared. Then the titanium dioxide carrier was put in the solution. The resulting material was dried at 100°C for 12 hours and then impregnated with 3% by weight of chromium nitrate aqueous solution, which was prepared beforehand.

The resulting material was dried in a vacuum at 100°C for 12 hours and fired at 500°C for 10 hours in an air atmosphere, thus preparing a Ti0₂

(88% by weight)-V₂0₅ (9% by weight)-Cr₂0₃ (3% by weight)-doped catalyst. 0.1 g of the above catalyst was put in a Pyrex reaction vessel having a diameter of 10 mm, and the decomposition rate of 1 ,2-dichlorobenzene with respect to reaction time was measured and is shown in FIG. 3. The reaction conditions are as follows.

Composition of the reaction gases: 1 ,2-dichlorobenzene 100 ppmv, oxygen 11 % by volume. Space velocity: 24,000 hr^"1

Reaction temperature: 300, 320°C

Reaction pressure: 1 atm.

From FIG. 3, it can be seen that the catalyst has a decomposition rate of more than 90% even at 300°C and is a catalyst having a perfect decomposition rate of 100% at 320°C.

< Example 6>

The decomposition rate of the catalyst was measured in the same way as in Example 5, except that the reaction temperature was changed from 270°C to 400°C, and the results are listed in Table 3.

From Table 3, it can be seen that the catalyst of Example 5 has a decomposition rate of more than 90% at over 300°C and a decomposition rate of 100% at over 320°C.

The decomposition rate was measured in the same way as in Example 5, except that 5% v/v of water was added to the reaction gases in order to make the composition similar to that of the exhaust gas of a real incineration furnace, and the results are listed in Table 4.

Table 4

From Table 4, it can be seen that the catalyst of the invention has a decomposition rate of more than 90% at 350°C even under the condition that water is present

Industrial Applicability

As seen above, the surface area of a metal-doped catalyst according to the present invention is optimized by doping a titanium dioxide carrier whose crystallinity is controlled within a appropriate range with vanadium and palladium or chromium oxides. In addition, the degree of dispersion is improved and the sintering of metal particles is prevented. Thus a catalyst having a high decomposition activity with toxic organic compounds, in particular chloro-aromatic compounds, can be provided.

Claims

What is claimed is: ι 1. A catalyst for decomposing toxic organic compounds,

2 characterized in that a titanium dioxide carrier whose crystallinity measured

3 by X-ray defractometry is 2-10% is doped with vanadium and palladium or

4 vanadium and chromium.

ι 2. The catalyst of claim 1 , wherein the vanadium used in doping

2 is in the form of the vanadium oxide (V₂O₅).

ι 3. The catalyst of claim 1 , wherein the palladium used in doping

2 is in the form of the palladium oxide (PdO).

ι 4. The catalyst of claim 1 , wherein the chromium used in doping

2 is in the form of the chromium oxide (Cr₂0₃).

ι 5. The catalyst of claim 1 , wherein the content of titanium dioxide

2 is 80-94% by weight based on the total weight of the catalyst.

ι 6. The catalyst of claim 2, wherein the content of the vanadium

2 oxide is 6-12% by weight based on the total weight of the catalyst.

ι 7. The catalyst of claim 3, wherein the content of the palladium

2 oxide is 0.05-1 % by weight based on the total weight of the catalyst.

ι 8. The catalyst of claim 4, wherein the content of the chromium

2 oxide is 1 :2-1 :5 in the ratio of the chromium oxide to the vanadium oxide.

ι 9. A catalyst preparation method according to any of the

2 preceding claims for decomposing toxic organic compounds, the method

3 comprising: 4 (a) a step of controlling the firing of titanium dioxide at 400-600°C

5 such that the crystallinity measured by X-ray defractometry is 2-10%; e (b) a step of putting the titanium dioxide prepared in step (a) in a

7 vanadium oxalate aqueous solution; s (c) a step of vacuum drying the resulting material of step (b) for 5-12

9 hours at 100-300°C; ιo (d) a step of putting the resulting material of step (c) in a palladium ιι nitrate aqueous solution or chromium nitrate aqueous solution; and

12 (e) step of firing the resulting material of step (d) in an air atmosphere is at 400-600°C for 5-15 hours.