CA1122584A

CA1122584A - Process for producing plate-shaped denitrating catalysts

Info

Publication number: CA1122584A
Application number: CA334,129A
Authority: CA
Inventors: Yasumi Kamino; Kenichi Nagai; Hideya Inaba; Kazuo Maeda
Original assignee: Hitachi Zosen Corp
Current assignee: Hitachi Zosen Corp
Priority date: 1978-09-20
Filing date: 1979-08-20
Publication date: 1982-04-27
Also published as: GB2029720B; IT7949657A0; NL7904502A; NL179551C; JPS615772B2; JPS5541881A; BE877183A; FR2436628B1; DE2927253A1; GB2029720A; FR2436628A1; DE2927253C2; IT1118887B; NL179551B

Abstract

ABSTRACT OF THE DISCLOSURE
A plate-shaped denitrating catalyst is produced by the steps of preparing a slurry from hydrated titania and a sol selected from the group consisting of silica sol, alumina sol and titania sol, firing the slurry to obtain a porous material, pulverizing the porous material to a powder, caus-ing a metal not to support the powder thereon with a binder to form a plate-like piece, drying or firing the piece to obtain a porous carrier and de-positing a catalytically active component on the carrier.

Description

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This invention relates to catalysts for use in a reaction in which nitrogen oxides ~N0 ) in exhaust gases are selectively catalytically redueed with NH3.
Since photochemical smog is attributable to N0 released from power plants, sintering or firing ovens, various chemical plants, motor vehicles, etc., it has been desired to provide a method of effectively treating such pollutants. Among the processes heretofore proposed for denitrating exhaust gases, the process for catalytically reducing N0 with NH3 used as a reducing agent is considered advantageous in that the process can be practiced with a relatively small amount of reducing agent because NH3 selectively reacts with N0 even when the exhaust gas contains more than 1 vol. % of oxygen.
Catalysts already known for use in this process comprise a carrier such as aetivated alumina, silica-alumina or zeolite and a heavy metal eom-pound deposited on the carrier. Such catalysts are generally granular and are used chiefly in the form of a fixed bed which is liable to be clogged up with the dust contained in exhaust gases or which involves a great pressure loss, thus giving rise to the necessity of using a blower of large capaeity.
These problems can be overcome to some extent by the use of a catalyst of in-ereased partiele or grain size, but the eores of eatalyst particles will then fail to aet effeetively, resulting in a redueed effieieney. In view of the problems described, it appears favourable to use catalysts of honeycomb structure in avoiding the clogging of the catalyst layer with dust or the in-crease of pressure loss.
Power plants and sintering or firing furnaces usually give off large quantities of exhaust gases whieh require similarly large quantities of catalysts for treatment. Accordingly catalysts of honeycomb structure, , -1- ~

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if useful for this purpose, must be large-si3ed and have sufficient strength so as to be placeable into the treating unit free of any damage. Catalysts of honeycomb structure have already been proposed ~hich compIise a honeycomb support of metal, ceramics or like refractory and an active catalytic com-ponent deposited on the support. However, a metal material, if used for the honeycomb structure, must be rendered porous over the surface through a cum-bersome procedure so as to hold the active component thereon effectively, ~rhereas structures of ceramics must have an increased wall thickness and be fired to sufficient hardness at a high temperature to retain the desired strength. Catalysts of this type therefore require much labor for the pre-paration of the honeycomb structure serving as a support for the active catzly-tic component and become inevitably expensive.
Thus, this invention seeks to provide a plate shaped denitrating catalyst for the reaction of NO with NH3 ~hich has a small thickness, high structured strength and large surface area. It is therefore very suitable for constr~uting honeycomb structures. ~uch catalysts preferably comprise an active component supported on a carrier having a high structural strength, and are produced ~nthout using a firing step. Thus, they retain a high level of porosity and exhibit an enhanced level of activity.
~0 Thus, in a first embodiment this invention provides a process for producing a plate-shaped denitrating catalyst comprising the steps of pre-paring a slurry from hydrated titania and a sol selected from the group con-sisting of silica sol, alumina sol and tit~nia sol, firing the slurry to obtain a porous material, pulveri~ing the porous material to a powder, causing a metal net to support the po~der thereon ~ith a binder to for~l a plate-like piece, drying or firing the piece to obtain a porous carrier and depositing a :

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These and other features of this invention will become more apparent from the following detailed description given by way of example only7 and with reference to the accompanying figures, in which:
Figure l is a perspective view showing a planar plate-shaped catalyst;
Figure 2 is a perspective view showing a folded metal net; and Figure 3 is a perspective view showing a catalyst of honeycomb structure.
E~amples of hydrated titanias useful for the preparation of the slurry of this invention are orthotitanic acid and metatitanic acid. The ratio of the sol to the hydrated titania, which is dependent on the water content of the sol, is 1:10 to 10:1 for example when the sol is silica sol containing 20% of SiO2, alumina sol containing 10% of Al203 or titania sol containing 20% of TiO2.
It is desirable to dry the slurry prior to the slurry firing step.
The slurry is dried preferably at 70 to 120C for 0.5 to 2 hours. The firing operation subjects the sol to dehydration condensation, causing the sol to embracc the titania and forming a three-dimensional reticl~ar structure whicl gives improved strength to the catalyst obtained. ~hile the titania serves as a carrier, the dehydration condensation products of silica sol, alumina sol and titania sol themselves also act to support the active component.
Such condensation products have a reticular structure and will not interfere with the action of the titania serving as a carrier.
The pulverizing step is carried out in a usual manner. The particle size of the resulting powder, although not limitative, is preferably minus s~

100 mesh or smaller.
The plate-like piece having a metal net core is formed usually by preparing a slurry from the powder and a binder and coating the metal net with the slurry. Binders generally used are useful for this purpose. Examples of suitable binders are alumina sol, silical sol, titania sol, phosphoric acid, boric acid and the like which, when dried or fired, undergo dehydration con-densation and form a tough three-dimensional reticular structure. The most suitable of these examples are alumina sol, silica sol and titania sol which act as carriers. Preferably the binder has incorporated therein a substance, such as organic solvent, polymeric emulsion or carbon fiber, which vaporizes, decomposes or burns away when dried or fired. Such substance is effective in permitting the slurry of the powder to dry rapidly and giving higher porosity to the plate~like piece to be obtained. The amount of the binder is dependent on the desired strength of the plate-like piece. When silica sol or alumina sol is used as the binder, the sol is used preferably in an amount, calculated as solids, of 10 to 20~ of the powder.
The metal nets useful in this invention may be made of any of carbon steel, stainless steel, copper~ brass, etc. The wires forming the nets may have such a diameter that the resulting st~lcture shaped to the desired shape will not be deformed during the production of catalysts or during the use of the catalysts obtained. The net is not l;mited in the size of the openings thereof. Satisfactory results can be achieved with openings of usually about 8- to about 100-mesh size. The net may be in the form of a single planar net, an assembly of superposed planar nets, a wavelike, zigzag, pleated or otherwise shaped net formed by bending or folding a planar net, or a honey-comb structure composed of planar nets and such bent or folded nets in com-,` ~ `

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:1~22SB4 bination therewith. Catalysts of honeycomb structure can be fabricated from the combination of a catalyst formed from a bent or folded metal net and another catalyst formed from a planar metal net. The segments forming such a honeycomb structure may be triangular, square, rectangular, hexagonal or otherwise shaped in accordance with the size of dust particles entrained in the exhaust gases and other requirements. Preferably the plate-like piece has a small thickness usually of 0.5 to 2.0 mm.
The plate-like piece is dried or fired under the same conditions as the drying or firing of the starting slurry.
Exemplary of useful catalytically active components to be deposited on the carrier are V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Sn, Sb, Bi, W, Pt, Rh, Pd and like metal compounds. These compounds are used singly or in admixture.
Further, these compounds may be used conjointly with a P compound, B compound, alkaline earth metal compound or the like. Examples of above-mentioned com-pounds are oxides, acid oxide salts, nitrates, sulfates, halides, hydroxides, organic acid salts, organic acid esters, alcoholates, etc. The kind and amount of the active component to be deposited on the carrier are determined in accordance with the temperature, composition and the like of the exhaust gas to be treated. The active component is deposited on the carrier in the usual manner as by immersion.
The process of this invention, which comprises the foregoing steps, gives catalysts in desired sizes and desired shapes including a honeycomb structure. Since particles of titania are firmly held to the metal net by the tough three-dimensional structure resulting from the dehydration conden-sation of a sol, a catalyst can be produced with satisfactory strength with-out the necessity of preparing a fired piece with application of pressure for , :

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reinorcement. This enables the catalyst to retain increased porosity to ex-hibit enhanced activity. The khickness of the catalyst is suitably variable by adjusting the amount of the slurry of pulverized porous material to be applied to the metal net, so that an efficient catalyst of reduced thickness can be produced. As a result, the expensive active component may be used beneficially at low cost.
Examples of this invention are given below in which parts are by weight.
xample 1 Commercial titanyl sulfate (100 parts) was slowly added to 1000 parts of hot water at 80C with stirring, and the metatitanic acid fo~led by the hydrolysis of the titanyl sulfate was sYithdrawn from the mixture, washed with water and dried at 100C. A 100 part portion of the dried product was thoroughly kneaded with 100 parts of commercial silica sol (containing 20% of SiO2) to prepare a slurry, which was dried at 100 C for 1 hour and then fired at 400 C for 3 hours. The fired product was pulverized to a powder up to 88 ~ in particle size. Equal amounts of the powder and silica sol the same as one previously used and serving as a binder were nuL~ed together to obtain a powder-containing slurry. The slurry was applied to both sides of a metal net as shown in Figure 1 and made from wires of steel (SUS 304) 0.25 mm in diameter, the net having 18-mesh openings and measuring 33 mm x 50 mm. The coated net was dried at 100 C for one hour and then baked at 400 for 3 hours.
In this way, a plate-like carrier was obtained which was about o.8 mm in thickness and had the metal net as its core. Subsequently the carrier was immersed in a 2N oxalic acid solution of NH4Y03 (1.0 mole/liter~ at room temperature for 30 minutes, then withdrawn from the solution and thereafter : . :
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dried at 100 C ~or one hour, whereby a plate-shaped catalyst A incorporating V was obtained.
Catalysts B and C were prepared in the same manner as above except that 80 parts and 60 parts of the silica sol were kneaded with the dried pro-duct of metatitanic acid per 100 parts of the latter.
Example 2 Catalysts D, E and F were prepared in the same manner as in Example 1 except that commercial alumina sol (containing 10% of Al203) was used in place of the silica sol kneaded with the dried product of metatitanic acid, the alwnina sol being used in amounts of 200 parts, 160 parts and 120 parts respectively per 100 parts of the dried product.
Example 3 Catalysts G, H and I were prepared in the same manner as in Example 1 except that commercial titania sol (containing 20% of TiO2) ~s used in place of the s~l;ca sol kneaded with the dried product of metatitanic acid, the titania sol being used in amounts of 100 parts, 80 parts, and 60 parts respectively per 100 parts of the dried product.
Comparison Example The dried product o~ metatitanic acid obtained in Example 1 was fired as such at 400 C for 3 hours without being kneaded with silica sol. The same procedure as in Example 1 was thereafter followed to prepare a catalyst J-Activity Test A reactor tub~ of the flow type was prepared which had a rectangular parallelepiped filling portion 50 mm in height and ha~ing 5 ~m x 35 mm open-ings at its opposite ends. The catalyst A was placed into the filling portion, , ~ . :

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and a test exhaust gas of the composition listed in Table 1 was passed through the reactor tube at a temperature of 250 C and at a flow rate of 1 liter/min.

(in standard state).

Table 1 Component of gas Prop~rtion ~vol. ~) 3 0.05 C2 13.0 H20 10.0

2 3.6 S2 0.025 2 Balance The denitration efficiency of the catalyst was calculated from -the difference between the NO concentration at the inlet of the reactor tube and that at the outlet thereof. Similarly the catalyst was tested for denitration efficiency at reaction temperatures of250 C, 300 C and 350 C.
In the same manner as above, the catalysts B to ~ were tested for denitration efficiency at the same temperatures. The resl~ts are given in Table 2, which shows that all the catalysts have e~cellent activity at tem-peratures of 250 C and higher.
Stren~th Test A polyvinyl chloride tape was adhered to the periphery of the catalyst A for the protection of the periphery. The catalyst was then secured to the bottom of a cylindrical screen measuring ~50 mm in diameter and 50 mm in height and made of a 6-mesh net. One hundred milliliters of alumina balls, 5 mm in diameter, were placed into the screen~ The screen was set on an automatic screening device ~amplitude 30 mm, frequency 290/min.~ and oscillated ., ~ , .

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for one hour. The reduction in weight of the catalyst A was measured to determine the amount of the resulting wear. The same procedure as above was repeated for the catalysts B to J. The results are given in Table 2, which reveals that the catalysts A to I of Examples 1 to 3 are more resistant to wear and have higher strength than the catalyst J of Comparison Example.

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Example 4 Seven metal nets (measuring 50 mm x 100 mm) of the same kind as used in Example 1 were folded to a zigzag form as shown in Figure 2. Pieces of catalyst in the form of a folded plate were prepared from the nets in the same manner as in Example 1. Additionally seven planar pieces of catalyst were prepared in the same manner as in Example 1 with the use of planar metal nets (measuring 50 ~m x 50 mm~ of the same kind as used in ~xample 1. The folded pieces of catalyst and planar pieces of catalyst thus formed were alternately surperposed to fabricate a catalyst of cubic honeycomb structure measuring 50 mm in each side and shown in Figure 3.
Activity Test.
In the same manner as above, the honeycomb catalyst was tested for denitration efficiency with use of a reactor tube of the flow type having a portion for accommodating the catalyst. The test exhaust gas was passed through the tube at a rate of 15.5 m3/m per unit geometric area of -the catalyst (in standard state). The results are listed in Table 3.
Table 3 Reaction Denitration temperature efficiency (C~, _ (%~
250 78.o 300 89.5 35 97.3 Table 3 indicates that the honeycomb catalyst has excellent denitra-ting activity.

Claims

THE EMBODIMENT OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVELEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for producing a plate-shaped denitrating catalyst comprising the steps of preparing a slurry from hydrated titania and a sol selected from the group consisting of silica sol, alumina sol and titania sol, firing the slurry to obtain a porous material, pulverizing the porous material to a powder, causing a metal net to support the powder thereon with a binder to form a plate-like piece having the metal net as its core, drying or firing the piece to obtain a porous carrier and depositing a catalytically active component on the carrier.

2. A process as defined in claim 1 wherein the binder is a material selected from the group consisting of alumina sol, silica sol, titania sol, phosphoric acid and boric acid.

3. A process as defined in claim 1 wherein the binder has incorporated therein an organic solvent, polymeric emulsion or carbon fiber.

4. A process as defined in claim 1 wherein the metal net has been bent or folded to a wavelike or zigzag form.

5. A process as defined in claim 1 wherein the metal net has a honeycomb structure.

6. A process as defined in claim 1 wherein the plate-like piece is formed by coating the metal net with a slurry of the powder and the binder.

7. A process as defined in claim 1 wherein the active component is a compound of a metal selected from the group consisting of V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Sn, Sb, Bi, W, Pt, Rh, and Pd.

8. A process as defined in claim 7 wherein the compound is a compound selected from the group consisting of oxides, acid oxide salts, nitrates, sulfates, halides, hydroxides, organic acid salts, organic acid esters and alcoholates.

9. A process as defined in claim 1 wherein the plate-shaped catalyst has a thickness of 0.5 to 2.0 mm.