JP5133287B2 - How to remove poisoning of rhodium catalyst - Google Patents

Description

  The present invention relates to a method for releasing poisoning of a rhodium catalyst. That is, the present invention relates to a method for regenerating the rhodium catalyst used for the reduction and decomposition of nitrous oxide by removing its poisoning.

《Technical background》
Nitrous oxide is used for anesthesia, food, and others, but recently, its reduction along with carbon dioxide has been highlighted as a causative substance of global warming gas and greenhouse gas. In other words, when nitrous oxide is released into the atmosphere, it becomes a causative agent of global warming, and it is said that it has a warming effect about 310 times that of carbon dioxide. The

<Conventional technology>
A typical source of this nitrous oxide is exhaust gas from a chemical factory or the like. For example, nitrous oxide is contained as a by-product in the exhaust gas that has passed through the oxidation step of ammonia.
Such decomposition and removal of nitrous oxide in exhaust gas is currently under investigation and development. Typically, there is a technique (thermal decomposition method) in which this exhaust gas is heated at a high temperature at the thermal decomposition temperature of nitrous oxide, and thus nitrous oxide is reduced to nitrogen and oxygen and thermally decomposed. On the other hand, a technology (catalytic decomposition method) for reducing and decomposing nitrous oxide into nitrogen and oxygen by applying alumina, zeolite, zinc oxide or the like with a catalyst to the exhaust gas has been developed.
However, such thermal decomposition method and catalytic decomposition method are suitable for reduction and decomposition of exhaust gas containing high concentration of nitrous oxide, but when applied to reduction and decomposition of low concentration nitrous oxide, Various problems have been pointed out. That is, equipment costs, fuel costs, etc. are increased, and it is necessary to lower the SV value.
In view of such circumstances, the inventors and applicants of this patent application have invented “a nitrous oxide decomposition catalyst and decomposition and removal method” employing rhodium as a result of research and development. A patent application was filed as -01234.

《About the issue》
By the way, this invention has the following indication. That is, the rhodium catalyst and the patent application “Nitrous oxide decomposition catalyst and decomposition and removal method” solve the above-mentioned problems of the thermal decomposition method and catalytic decomposition method, are excellent in various costs, and have high SV. Based on this value, reduction, decomposition, and removal of nitrous oxide can be realized with certainty.
However, a decrease in decomposition rate due to use was pointed out as a problem. For example, according to test data, the decomposition rate of nitrous oxide of 99% or more at the beginning of use may be reduced to about 40% to 60% by using over time (SV value 10,000 / h, Temperature 500 ° C.).
As a cause of such a decrease in decomposition rate, adsorption of poisonous substances on the rhodium catalyst surface has been estimated. In other words, nitrous oxide to be reduced and decomposed is supplied in exhaust gas, and sulfur compounds such as sulfate ions and bisulfite ions of sulfurous acid are mixed in this type of exhaust gas. There are many cases.
The sulfur compounds in the exhaust gas are adsorbed on the rhodium catalyst surface and become poisonous substances. The rhodium catalyst is poisoned, resulting in hindrance to the reduction and decomposition of nitrous oxide, and the degradation rate is pointed out. It was.

<< About the present invention >>
The rhodium catalyst poisoning release method of the present invention has been made in order to solve the above-described problems of the conventional example in view of such a situation.
In the present invention, firstly, the rhodium catalyst is reliably detoxified, secondly, this is easily realized, and thirdly, participation in reduction and decomposition of nitrous oxide is also conceivable. An object of the present invention is to propose a method for releasing poisoning of a rhodium catalyst.

<About Claim>
The technical means described in the scope of claims of the present invention for solving such a problem is as follows.
That is, in the method for detoxification of a rhodium catalyst according to claim 1, the rhodium catalyst is contained in exhaust gas that has undergone an ammonia oxidation step, and has a low concentration of 50 ppm to 10,000 ppm of nitrous oxide. Is reduced and decomposed into nitrogen and oxygen at a heating temperature of 350 ° C. or higher and lower than 550 ° C. lower than the thermal decomposition temperature. The SV value between the exhaust gas and the rhodium catalyst is 10,000 / h to 30,000 / h.
The poisonous substance is contained in the exhaust gas and is composed of sulfate ions of sulfur compounds and / or hydrogen sulfite ions of sulfurous acid adsorbed on the surface of the rhodium catalyst during the reduction and decomposition of the nitrous oxide. .

Then, the sulfur compound of the poisoning substance is accumulated due to use over time, and the poisoning release of the rhodium catalyst which has become poisoned is performed by supplying hydrogen after stopping the supply of the exhaust gas.
That is, by supplying the hydrogen, nascent atomic hydrogen that is more reducible than the sulfur compound is generated on the surface of the rhodium catalyst and adsorbed on the surface of the rhodium catalyst instead of the sulfur compound. The rhodium catalyst is detoxified.

After such detoxification, the nascent atomic hydrogen adsorbed on the rhodium catalyst surface at a heating temperature lower than the thermal decomposition temperature of the nitrous oxide is separated from the rhodium catalyst surface, and the rhodium. The catalyst surface is cleaned and molecularized into hydrogen.
On the other hand, at the time of supply of the exhaust gas at a heating temperature lower than the thermal decomposition temperature of the nitrous oxide after the release of such poisoning, the nascent atomic hydrogen adsorbed on the rhodium catalyst surface Is separated from the rhodium catalyst surface to clean the rhodium catalyst surface,
It participates in the reduction and decomposition of the nitrous oxide as it is without being molecularized.

<About the action>
Since the present invention comprises such means, the following is achieved.
(1) Exhaust gas containing a low concentration of nitrous oxide is supplied to a rhodium catalyst.
(2) In the rhodium catalyst, two holes in the N-shell 4d orbital have an electron-storing action, and one unpaired electron in the outermost shell O-shell 5s orbital has an electron-donating action.
(3) Therefore, nitrous oxide is reduced, decomposed, and removed into nitrogen and oxygen based on the electron-storing action and electron-donating action of the rhodium catalyst.
(4) The exhaust gas often contains sulfate ions, bisulfite ions of sulfurous acid, and the like. These have an electron donating property, that is, a reducing property, and the holes of the rhodium catalyst are adsorbed on the surface of the rhodium catalyst because they have an electron-storing action and an oxidizing power.
(5) The surface of the rhodium catalyst is poisoned by sulfate ions, hydrogen sulfite ions, etc., making it difficult to reduce, decompose and remove nitrous oxide.
(6) The poisoning is canceled by supplying hydrogen. By supplying hydrogen, atomic hydrogen having a strong reducing property is generated on the surface of the rhodium catalyst, so that sulfate ions, hydrogen sulfite ions and the like are peeled off and adsorbed on the surface of the rhodium catalyst. Sulfuric acid ions, bisulfite ions, and the like are released as sulfuric acid mist, sulfurous acid mist, sulfurous acid gas mist, and the like.
(7) The atomic hydrogen in the nascent stage is separated from the rhodium catalyst surface to be converted into hydrogen molecules.
(8) Although the atomic hydrogen in the nascent stage is separated after the fact, when exhaust gas is supplied, it participates in reduction, decomposition, and removal of nitrous oxide without being molecularized.
(9) Thus, the rhodium catalyst can be easily detoxified by the simple configuration of supplying hydrogen.
(10) The rhodium catalyst poisoning release method of the present invention exhibits the following effects.

<< First effect >>
First, the rhodium catalyst is reliably detoxified. That is, in the rhodium catalyst poisoning elimination method of the present invention, hydrogen is supplied after the supply of exhaust gas is stopped, thereby generating atomic hydrogen in the early stage of strong reduction on the rhodium catalyst surface.
Therefore, the atomic hydrogen in the nascent stage peels off the sulfate ions and bisulfite ions of sulfite that were adsorbed on the rhodium catalyst surface as poisonous substances, and instead adsorbs them to release the poison. It is separated from the rhodium catalyst surface and liberated by subsequent heating.
In this way, the deteriorated rhodium catalyst is detoxified, clean regenerated, the catalytic activity is restored, and the original high decomposition rate is restored, so that nitrous oxide is reliably reduced, decomposed and removed. become.

<< Second effect >>
Second, and this is easily accomplished. That is, the method for detoxifying a rhodium catalyst according to the present invention can achieve the first effect described above easily by the configuration based on supplying hydrogen to the poisoned rhodium catalyst after the exhaust gas supply is stopped. It is also excellent in terms of various costs.

《Third effect》
Third, there is also participation in the reduction and decomposition of nitrous oxide. That is, in the rhodium catalyst poisoning release method of the present invention, nascent atomic hydrogen adsorbed on the rhodium catalyst surface reduces and decomposes nitrous oxide when supplying exhaust gas after the poisoning release. In other words, it participates in the reduction and decomposition of nitrous oxide with a rhodium catalyst.
As described above, the effects exerted by the present invention are remarkably large, such as all the problems existing in this type of conventional example are solved.

《About drawing》
The rhodium catalyst poisoning elimination method of the present invention will be described in detail below based on the best mode for carrying out the invention shown in the drawings.
1 and 2 serve to explain the best mode for carrying out the present invention. FIG. 1 (1) is an explanatory side sectional view of the decomposition processing apparatus and the like, and FIG. 1 (2) is a front sectional view showing an enlarged main part of the decomposition processing apparatus.
FIG. 2 (1) and FIG. 2 (2) are side sectional views of each example. (1) FIG. 2 shows a two-column switching type, and (2) FIG. 2 shows a one-column batch type. FIG. 2 (3) shows a single tower simultaneous mixing type of a reference example not belonging to the present invention. FIG. 3 is a graph of the decomposition rate of nitrous oxide by a rhodium catalyst, (1) FIG. 3 shows before the poisoning release, and (2) FIG. 3 shows after the poisoning release together with the hydrogen concentration.

<Rhodium catalyst 1 etc.>
The present invention relates to a method for releasing poisoning of a rhodium catalyst 1. First, the rhodium catalyst 1 which is the premise of the present invention will be described.
The rhodium catalyst 1 reduces and decomposes low-concentration nitrous oxide contained in the exhaust gas 2 into nitrogen and oxygen at a heating temperature lower than its thermal decomposition temperature.
In other words, the rhodium catalyst 1 uses a low concentration of nitrous oxide contained in the supplied exhaust gas 2 to absorb electrons of holes that are two electron-deficient vacancies in the N-shell 4d orbit of the atomic structure, Based on the electron donating action of one unpaired electron in the outermost shell O shell 5s orbital, it is reduced and decomposed into nitrogen and oxygen at a heating temperature lower than its thermal decomposition temperature.

Such a rhodium catalyst 1 and the like will be further described in detail.
First, nitrous oxide (N ₂ O) is required to be reduced as a causative agent of global warming. In particular, exhaust gas 2 containing nitrous oxide is required to be reduced before being released into the atmosphere. Yes.
The source of nitrous oxide is a chemical production process, and a typical example is the exhaust gas 2 via an ammonia oxidation process, and an exhaust gas 2 containing a low concentration of nitrous oxide is industrially abundant. Have been discharged. For example, in the production process of caprolactam or adipic acid, which is a raw material for nylon, and the production process of nitric acid, nitrous oxide is produced and discharged in large quantities as a by-product.
Further, the exhaust gas 2 discharged from the denitration device that reduces nitrogen oxides using ammonia also contains a large amount of low-concentration nitrous oxide. It contains nitrogen oxides and is often discharged after denitration treatment by a denitration apparatus.
The concentration of nitrous oxide in the exhaust gas 2 is a low concentration of about 50 ppm to 10,000 ppm on a volume basis, but from the viewpoint of stably maintaining the decomposition rate of nitrous oxide, 500 ppm to 2,000 ppm. The degree is preferred.

As described above, nitrous oxide (N ₂ O) contained in the exhaust gas 2 is reduced, decomposed, and removed into nitrogen (N ₂ ) and oxygen (O ₂ ) by the rhodium catalyst 1.
The rhodium (Rh) of the rhodium catalyst 1 has eight counter electrons in the 4d orbit of the N shell inside the outermost shell O shell of the atomic structure. Since up to 10 counter electrons enter, there are two electron vacancies in which electrons are deficient, that is, holes (hole ⁺ ). These two holes transiently take away electrons from the reaction partner to fill the electron deficiency, and transiently promote the oxidation reaction of the partner.
In addition, there is one unpaired electron in the 5s orbit of the outermost O shell of the atomic structure, and this unpaired electron transiently donates an electron to the reaction partner and promotes the other partner's reduction reaction. To do.
The above-described catalytic action and decomposition action of the rhodium catalyst 1 against nitrous oxide are due to such electron-storing action and oxidation promoting ability of holes, and electron donating action and reduction promoting ability of unpaired electrons.

In the example of FIG. 1, the rhodium catalyst 1 is adhered and supported on the porous carrier 4 of the decomposition treatment apparatus 3 that is a catalytic converter. That is, the decomposition treatment apparatus 3 is connected to an exhaust pipe 5 for the exhaust gas 2, and a porous carrier 4 that is a carrier base is inserted into the outer cylinder 6. The porous carrier 5 is composed of, for example, an aggregate of a large number of hollow columnar cell spaces 8 defined by cell walls 7, and a honeycomb core made of ferritic stainless steel, alumina, other metal, or ceramics. Used.
The porous carrier 4 has its axis oriented in the flow direction of the exhaust gas 2, and the rhodium catalyst 1 is adhered and supported on the cell wall 7 by coating or the like. The exhaust gas 2 is supplied to the cell space 8. Pass through.
The exhaust gas 2 is supplied to the decomposition treatment apparatus 4 at a temperature of 350 ° C. or more and less than 550 ° C., which is lower than the thermal decomposition temperature of nitrous oxide, in the blowing unit 9 and the heating unit 10. Of course, when the exhaust gas 2 has such a temperature, the heating unit 10 is unnecessary. The SV value (space velocity) between the exhaust gas 2 and the rhodium catalyst 1 is set to 10,000 / h to 30,000 / h, and the exhaust gas 2 is supplied to the decomposition treatment apparatus 3 at the flow rate and passes therethrough. To do.
The rhodium catalyst 1 and the like are as described above.

About poisoning
Next, poisoning will be described. In the method of detoxifying the rhodium catalyst 1 according to the present invention, the poisoning substance to be detoxified is composed of a sulfur compound adsorbed on the surface of the rhodium catalyst 1.
That is, during the reduction and decomposition of nitrous oxide by the rhodium catalyst 1 described above, the sulfur compound contained in the exhaust gas 2 together with nitrous oxide is subjected to the rhodium catalyst 1 surface due to its transient electron-storing action. It is adsorbed as a poisonous substance and becomes a poisoning source to be poisoned. Typical examples of such poisonous substances are sulfate ions and hydrogensulfite ions of sulfurous acid.

The poisoning will be further described in detail. For example, in the exhaust gas 2 from the chemical production process, which is a source of nitrous oxide, sulfur compounds such as sulfate ions and / or hydrogen sulfite ions of sulfurous acid together with nitrous oxide and moisture Are often contained.
Then, a sulfur compound (SO _x ), typically sulfuric acid (H ₂ SO ₄ ), which is mistified with moisture in the exhaust gas 2 (sulfate ion, SO ₄ ²⁻ ), sulfurous acid gas (SO ² ) ₂ ) The hydrogen sulfite ion (HSO ₃ ⁻ ) of sulfurous acid (H ₂ SO ₃ ) (H ⁺ + HSO ₃ ⁻ ) that has been misted by absorbing moisture in the exhaust gas 2 has its electrons (e ⁻ ) If it is in a deprived situation, it shows electron supply, that is, reduction.
On the other hand, as described above, the rhodium catalyst 1 has two holes (hole ⁺ ), which are electron vacancies, in the N-shell 4d orbit of the surface, and has an electron-storing action and an oxidation promoting power. .
Therefore, sulfate ions and hydrogen sulfite ions of sulfurous acid that flow as mist in the exhaust gas 2 are adsorbed on the surface of the rhodium catalyst 1 and accumulated over time. In the rhodium catalyst 1, holes in the 4d orbit are filled with captured and captured electrons of sulfate ions and hydrogen sulfite ions, making it difficult to perform the catalytic function and being in a poisoned state (described later). (See FIG. 3 (1)).
In this way, the surface of the rhodium catalyst 1 is poisoned by sulfur compound ions such as sulfate ions and hydrogen sulfite ions (see the following structural formulas of Chemical Formula 1 and Chemical Formula 2).

《About poisoning release》
Next, cancellation of poisoning will be described. In the method for detoxifying the rhodium catalyst 1 of the present invention, the poisoning of the rhodium catalyst 1 poisoned as described above is detoxified.
And the poisoning release is carried out by supplying hydrogen. That is, by supplying hydrogen, nascent atomic hydrogen having a strong reducing power as compared with the sulfur compound is generated on the surface of the rhodium catalyst 1 and is adsorbed on the surface of the rhodium catalyst 1 instead of the sulfur compound. 1 is detoxified.

Such poisoning cancellation will be further described in detail. The circulation of hydrogen for detoxication to the surface of the poisoned rhodium catalyst 1 makes the entire surface of the poisoned rhodium catalyst 1 a strong reducing atmosphere.
That is, the supplied hydrogen molecule (H ₂ ) undergoes an atomization reaction on the surface of the rhodium catalyst 1, thereby generating a hydrogen atom, that is, a nascent atomic hydrogen (H ⁺ + e ⁻ ) (reaction of the following chemical formula 3) See formula).
The nascent atomic hydrogen (also referred to as hydrogen radical) has an extremely strong electron donating property, that is, a reducing power. Therefore, it also shows a strong reducing power against holes (hole ⁺ ) that are electron vacancies in the N-shell 4d orbital on the surface of the rhodium catalyst 1 damaged by poisoning.
Therefore, this atomic hydrogen strips away the relatively weakly reducing sulfate ions and bisulfite ions from the surface of the rhodium catalyst 1 that has been adsorbed so far. Then, the holes of the N shell 4d orbit are reduced and adsorbed on the surface of the rhodium catalyst 1 (see the structural formula of the following chemical formula 4).
The stripped sulfate ions and the mist of sulfuric acid by the sulfation reaction on the surface of the rhodium catalyst 1 (see the reaction formula of the following chemical formula 5) are pushed downstream. On the other hand, the removed hydrogen sulfite ion is a sulfite mist or a sulfite gas mist (see the reaction formula of the following chemical formula 7) by the sulfite reaction (see the formula of the chemical formula 6 below) on the rhodium catalyst 1 surface. ) And is pushed downstream.
In this way, adsorption of sulfate ions and hydrogen sulfite ions on the surface of the rhodium catalyst 1 is eliminated.
Thus, the rhodium catalyst 1 is detoxified.

<< Atomic hydrogen in the nascent stage after detoxification is released >>
Next, the atomic hydrogen in the nascent stage after detoxifying the poisoning substance as described above will be described.
In the method of detoxifying the rhodium catalyst 1 according to the present invention, the atomic hydrogen in the nascent stage after detoxifying the rhodium catalyst 1 as described above is separated from the rhodium catalyst 1.
The separation from the rhodium catalyst 1 will be further described in detail. The atomic hydrogen once adsorbed on the surface of the rhodium catalyst 1 as described above is separated from the surface of the rhodium catalyst 1 together with the transient electron donating action of the rhodium catalyst 1 and then molecularized from hydrogen atoms to hydrogen molecules. To do. Such separation of atomic hydrogen is performed at a heating temperature lower than the thermal decomposition temperature of nitrous oxide (350 ° C. or higher and lower than 550 ° C.).
That is, one unpaired electron exists in the 5s orbit of the outermost shell O shell of the rhodium catalyst 1, but when energy is added to the reaction atmosphere, the atoms in which the holes in the N shell 4d orbit were captured. It interacts with the hydrogen of the hydrogen atom, that is, the hydrogen atom in the hybrid orbital field.
As a result, the trapped atomic hydrogen is released, and is molecularized and liberated into hydrogen (see the reaction formula of the following chemical formula 8 and FIG. 3 (2) described later). As a result, the surface of the rhodium catalyst 1 is cleaned (see the following reaction formula 9).

On the other hand, the separation from the rhodium catalyst 1 when the exhaust gas 2 is supplied is as follows.
Even when the exhaust gas 2 is supplied after detoxification is released, the atomic hydrogen in the nascent stage once adsorbed on the surface of the rhodium catalyst 1 is the transient electron donating action of the rhodium catalyst 1 as described above. In combination, after that, the rhodium catalyst 1 is separated from the surface of the rhodium catalyst 1 and the surface of the rhodium catalyst 1 is cleaned.
However, the separated atomic hydrogen is not molecularized into hydrogen as described above when the exhaust gas 2 is supplied (see the reaction formula of Chemical Formula 8 above), that is, generates hydrogen molecules. Instead, it participates in the reduction and decomposition of nitrous oxide as it is (see also FIG. 3, (2)).
Atomic hydrogen participates in the reduction and decomposition reaction of nitrous oxide into nitrogen and oxygen by the rhodium catalyst 1 through the reduction and decomposition reaction of nitrogen and water (see the reaction formula of the following chemical formula 10).
This atomic hydrogen participation reaction that can promote reduction and lower the activation energy supports the reduction and decomposition reactions of the rhodium catalyst 1 under a heating temperature lower than the thermal partial heat of nitrous oxide. It can be said that.
As for the atomic hydrogen in the nascent stage after detoxification is released, it is as above.

<About implementation pattern>
Next, an embodiment pattern of the present invention will be described with reference to FIG. About the implementation pattern of the poisoning cancellation | release method of the rhodium catalyst 1 of this invention, the following two types are typical.
First, as shown in FIG. 2 (1), a two-column switching type is conceivable. That is, two towers of the decomposition treatment apparatus 3 are prepared in advance and are switchable so that one tower is used for reduction and decomposition of nitrous oxide and the other tower that is not used is detoxified. To do. After that, one tower that has been used is switched to non-use to release poisoning, and the other one tower that has not been used is switched to be used.
Second, as shown in FIG. 2 (2), a single-column batch type can be considered. That is, two towers of the decomposition treatment apparatus 3 are prepared in advance and batch type, and one tower is used for reduction and decomposition of nitrous oxide, and the other tower that is not used is detoxified. To do. Then, after the fact, one tower that has been used is removed and the poisoning is canceled, and the other tower that has not been used is attached to be used instead.
Incidentally, as a reference example not belonging to the present invention, there is a one-column simultaneous mixing type as shown in FIG. That is, the decomposition treatment apparatus 3 is prepared for only one tower, and the exhaust gas 2 containing nitrous oxide is also introduced and mixed with hydrogen for detoxication and supplied to one decomposition treatment apparatus 3 at the same time. To do. That is, hydrogen gas is also mixed and supplied to the exhaust gas 2 of the gas to be processed.
In addition, in the two-column switching type and the one-column batch type shown in FIGS. 3A and 3B, the detoxification of the rhodium catalyst 1 by supplying hydrogen is performed on the target decomposition treatment apparatus 3 with exhaust gas. It is carried out after the supply of gas 2 is stopped.
In contrast, in the single-column simultaneous mixing type of the reference example that does not belong to the present invention shown in FIG. 3 (3), the detoxification of the rhodium catalyst 1 by the supply of hydrogen is simultaneously performed while the exhaust gas 2 is being supplied. To be implemented. That is, nitrous oxide reduction, decomposition processing and poisoning release processing are carried out continuously and simultaneously.
The implementation pattern is as described above.

《Action etc.》
The poisoning release method for the rhodium catalyst 1 of the present invention is configured as described above. Therefore, it becomes as follows.
(1) The exhaust gas 2 containing a low concentration of nitrous oxide is supplied to the rhodium catalyst 1 of the decomposition treatment apparatus 3 and passes therethrough.
This type of exhaust gas 2 is typically a gas that has undergone an ammonia oxidation step, but other exhaust gas 2 containing a low concentration of nitrous oxide produced as a by-product is supplied. The rhodium catalyst 1 is attached and supported on the porous carrier 4 of the decomposition treatment apparatus 3 (see FIG. 1).

  (2) In the rhodium catalyst 1, two holes in the N-shell 4d orbit of the atomic structure have a transient electron-storing action, and one unpaired electron in the outermost O-shell 5s orbit of the atomic structure However, it has a transient electron donating action.

(3) Therefore, the nitrous oxide in the exhaust gas 2 is firstly oxidatively promoted based on the transient electron-storing action of the holes of the rhodium catalyst 1, so that nitrogen is liberated and oxygen atoms are released. Adsorbed on the rhodium catalyst 1.
The adsorbed oxygen atoms are transiently promoted to reduce based on the transient electron donating action of the unpaired electrons of the rhodium catalyst 1, and the adsorption is released.
In this way, nitrous oxide is reduced, decomposed and removed into nitrogen and oxygen by the catalytic activity of the rhodium catalyst 1.

(4) Now, the exhaust gas 2 often contains sulfur compounds, typically sulfate ions, bisulfite ions of sulfurous acid, and the like together with such nitrous oxide.
These have an electron donating property, that is, a reducing property. On the other hand, in the rhodium catalyst 1, holes in the N shell 4d orbital have a transient electron-storing action and an oxidizing power. Therefore, sulfate ions, hydrogen sulfite ions, etc. flowing as mist in the exhaust gas 2 are adsorbed on the surface of the rhodium catalyst 1 (see the structural formulas of Chemical Formula 1 and Chemical Formula 2).
To be exact, a part (two) of the outer electrons of sulfate ion (divalent) is discharged, and the sulfate radical has two unpaired electrons, and the sulfate having these two unpaired electrons. The roots attach to the holes of the N-grain d-orbit of the rhodium catalyst 1 and are poisoned. In the case of bisulfite ion (monovalent), a part (one) of the outer shell electrons are discharged, and the bisulfite radical has one unpaired electron, and has this one unpaired electron. The hydrogen sulfite is attached to the holes of the N shell d orbit of the rhodium catalyst 1 and poisoned.
In general, sulfate ions and sulfate radicals are the same, but in this case, sulfate radicals are understood as having some (2) outer electrons removed. The same relationship applies to hydrogen sulfite ions and hydrogen sulfite groups.

(5) In this way, the rhodium catalyst 1 surface is poisoned with poisonous substances such as sulfate ions and hydrogen sulfite ions.
That is, the holes of the rhodium catalyst 1 are filled with the captured electrons, making it difficult to exhibit the catalytic function, and the reduction, decomposition, and removal of nitrous oxide in (3) become difficult (see FIG. 3 described later). (2) See also figure).

(6) There, the poisoning is released by supplying hydrogen. That is, by supplying hydrogen, nascent atomic hydrogen that is much more reducible than sulfate ions, hydrogen sulfite ions, and the like is generated on the surface of the rhodium catalyst 1 (see the reaction formula of Chemical Formula 3).
The generated atomic hydrogen peels sulfate ions, hydrogen sulfite ions, and the like from the holes of the rhodium catalyst 1 and instead adsorbs them, thereby reducing the rhodium catalyst 1 (see the structural formula of Chemical Formula 4). .
The stripped sulfate ions, hydrogen sulfite ions, etc. are separated into sulfuric acid mist, sulfurous acid mist, sulfurous acid gas mist, etc., and are swept away downstream (in the above chemical formulas (5), (6), and (7). (See the reaction formula etc.).
In this way, the rhodium catalyst 1 is reliably released from poisoning.

(7) In addition, the nascent atomic hydrogen adsorbed on the surface of the rhodium catalyst 1 in this way is separated from the surface of the rhodium catalyst 1 by heating or the like, and thus molecularized from hydrogen atoms to hydrogen molecules (see the chemical formula 8). (See also the reaction formula of (2) and FIG. 3 (2) described later).
In this way, the surface of the rhodium catalyst 1 that has been detoxified is further cleaned (see the reaction formula of Chemical Formula 9).

  (8) Although the nascent atomic hydrogen adhering to the surface of the rhodium catalyst 1 is separated after the above, when the exhaust gas 2 is supplied, it is not molecularized as described above. Participates in the reduction, decomposition and removal of nitric oxide. Along with the rhodium catalyst 1, nitrous oxide in the exhaust gas 2 is reduced, decomposed, and removed (see the chemical formula 10 and FIG. 3 (2) described later).

(9) The method of detoxifying the rhodium catalyst 1 according to the present invention is such that by supplying hydrogen to the poisoned rhodium catalyst 1, that is, with a simple configuration, the detoxification of the rhodium catalyst 1 can be easily performed. Realize.
In addition, the two-column switching type and the one-column batch type are also conceivable (see Fig. 2 (1) and (2)), and from this aspect, the poisoning can be easily removed with a simple configuration. Can be implemented.

(10) By the way, as described above, in the present invention, hydrogen is employed for detoxification of the rhodium catalyst 1. That is, by supplying hydrogen and generating nascent atomic hydrogen on the surface of the rhodium catalyst 1, that is, by putting the entire surface of the rhodium catalyst 1 in a strong reducing atmosphere, detoxification is performed. It was.
In view of these, hydrocarbons and ammonia are also worth considering as reducing substances that function similarly to hydrogen. That is, methane (CH ₄ ), ethane (C ₂ H ₆ ), propane (C ₃ H ₈ ), butane (C ₄ H ₁₀ ), ammonia (NH ₃ ), etc. are also used for detoxification of the rhodium catalyst 1. Research and development for adoption as a reducing substance is expected.
The operation of the present invention is as described above.

Hereinafter, an embodiment of the present invention will be described with reference to FIG.
In the field test decomposition processing apparatus 3 of the embodiment, the rhodium catalyst 1 is adhered and supported on the cell wall 7 of the porous carrier 4 made of a ceramic honeycomb core (see FIG. 1). And the exhaust gas 2 containing a low concentration of nitrous oxide was supplied to such a decomposition processing apparatus 3.
Therefore, when the decomposition rate (reduction rate, removal rate) of nitrous oxide and other data were measured, the following results were obtained.

1) First, the model gas of the exhaust gas 2 was supplied to and passed through the rhodium catalyst 1 under the decomposition treatment apparatus 3 under the following conditions. As a result, a nitrous oxide decomposition rate (% by weight) of 100% at a temperature of 500 ° C. and 99.2% at a temperature of 450 ° C. was obtained.
○ Catalyst conditions (size configuration of porous carrier 4 on which catalyst is adhered and supported):
・ Structure: Ceramic honeycomb core ・ Shape: Circular pillar ・ Direct diameter: 25.4mm
・ Length: 50.8mm
-Number of cell spaces: 200 / in ² (31 / cm ² )
-Rh loading: 100 g / ft ³ (3,531 g / m ³ )
○ Model gas conditions (test gas composition):
_{· N 2 O: 1,000volppm (0.1vol} %)
・ O ₂ : 3 vol%
・ H ₂ O: ₂ vol%
・ N ₂ : Balance
○ Test conditions:
-SV value: 10,000 / h (between exhaust gas 2 and rhodium catalyst 1)
・ Temperature: 500,450 ℃

2) On the other hand, separately from the test of 1) above, the exhaust containing the sulfur compound SO _X (typically sulfate ion or hydrogen sulfite ion of sulfurous acid) under the decomposition treatment apparatus 3 under the following conditions: The gas 2 was exposed and supplied to the rhodium catalyst 1 (Rh) as an actual gas. As a result, the nitrous oxide decomposition rate was 98 to 100% on the first day and 8 to 25% on the 15th day.
○ Catalyst conditions (size configuration of porous carrier 4 on which catalyst is adhered and supported):
・ Structure: Ceramic honeycomb core ・ Shape: 4 square pillars ・ Height: 50mm
・ Vertical width: 75mm
-Number of cell spaces: 200 / in ² (31 / cm ² )
-Rh loading: 100 g / ft ³ (3,531 g / m ³ )
○ Actual gas conditions (composition of actual gas):
· N ₂ O: 650~850volppm
・ NO _x : 20-60ppm
· O _2: 3~5vol%
· H ₂ O: 0.4~0.7vol%
· N _2: 93~96vol%
· SO _x: 1~2volppm
○ Test conditions:
-SV value: 9,000 to 19,000 / h
・ Temperature: 480-530 ℃
・ Exposure period: 15 days * Exhaust gas 2 exposed and passed as real gas is converted into nitric acid gas obtained through the oxidation process of ammonia during the production process of caprolactam, which is the raw material of nylon, in sulfuric acid. This is off-gas from the step of absorbing and obtaining nitrosylsulfuric acid, which has been denitrated and discharged. Incidentally, the sulfur compounds SO _X contained, in the are on the order of 1~2volppm but usually is about 1～10Volppm.

3) As described in the above 2), when the exhaust gas 2 made of real gas was continuously supplied to the decomposition treatment apparatus 3 under the above conditions, the decomposition rate of nitrous oxide was significantly reduced. . This is a result of the rhodium catalyst 1 being poisoned with a sulfur compound and thus having a reduced performance. When the poisoned rhodium catalyst 1 surface was analyzed by XRF (fluorescence X-ray), S was detected and confirmed.
Therefore, the rhodium catalyst 1 is taken out to make the decomposition treatment apparatus 3 having the same catalyst conditions as described in 1) above, and the exhaust gas 2 having the same model gas conditions (composition of test gas) as in 1) above. Was supplied and passed again. The test conditions were an SV value of 10,000 / h and a temperature of 500 ° C.
Then, it was verified as shown in FIG. 3 (1), and the decomposition rate of nitrous oxide was about 40%.

4) Therefore, the supply of the exhaust gas 2 was temporarily stopped in order to release the poisoning of the rhodium catalyst 1 thus poisoned. And 5 vol% of hydrogen was added as reducing gas in nitrogen atmosphere, and it supplied to this decomposition processing apparatus 3 for 2 hours at SV value 10,000 / h and the temperature of 500 degreeC. Thus, the rhodium catalyst 1 was detoxified with the generated nascent atomic hydrogen. Thereafter, the decomposition treatment apparatus 3 was cooled and exposed to the atmosphere at room temperature for several days.
5) Then, as shown in FIG. 3 (2), the decomposition treatment apparatus 3 is first heated to a temperature of 500 ° C. (see the left part in FIG. 3 (2)), and then again The exhaust gas 2 containing nitrous oxide at a low concentration was supplied at a temperature of 500 ° C. (see the central portion in FIG. 3 (2)). The model gas conditions of the exhaust gas 2 are the same as those described in 1) and 3), and the test conditions are also the same as those described in 3).
→ Then, the supply of the exhaust gas 2 containing nitrous oxide was stopped, and the gas containing only H ₂ O, N ₂ , etc. without containing nitrous oxide was continuously supplied under heating at a temperature of 500 ° C. ( (See the right part of (2) in FIG. 3).

6) As a result, as shown in FIG. 3B, the following points were confirmed.
a. It was confirmed from the data that the decomposition rate of nitrous oxide was recovered to the 90% level in the time zone in which the exhaust gas 2 containing nitrous oxide N ₂ O at a low concentration (1,000 volppm) was supplied. (Refer to the central portion in FIG. 3 (2)). That is, the removal of poisoning of the rhodium catalyst 1 by supplying hydrogen was supported.
b. It was confirmed in terms of data that hydrogen was continuously produced at a high concentration in the time zone in which the exhaust gas 2 was not supplied (see the left part and the right part in FIG. 3 (2)). That is, it was confirmed that the nascent atomic hydrogen adhering to the rhodium catalyst 1 was converted into hydrogen molecules and separated from the rhodium catalyst 1.
c. On the other hand, during the supply of the exhaust gas 2, the atomic hydrogen in the nascent stage adhering to the rhodium catalyst 1 participated in the reduction of nitrous oxide without being converted into hydrogen molecules. (See the central part in FIG. 3 (2)).
About an Example, it is as above.

The rhodium catalyst poisoning release method according to the present invention will be described in the best mode for carrying out the invention. (1) FIG. 1 is a side sectional view of a decomposition treatment apparatus and the like, and (2) FIG. FIG. 2 is an enlarged front sectional view of a main part of the decomposition processing apparatus. It uses for description of the best form for implementing this invention, and is side sectional explanatory drawing of each example, (1) A figure shows a 2 tower switching type, (2) A figure shows a 1 tower batch type. In addition, FIG. (3) shows a single tower simultaneous mixing type of a reference example not belonging to the present invention. FIG. 4 is a graph showing the decomposition rate of nitrous oxide by a rhodium catalyst, etc., for explanation of the best mode for carrying out the invention, (1) FIG. 1 shows the state before the poisoning release, (2) FIG. After detoxification, with hydrogen concentration.

DESCRIPTION OF SYMBOLS 1 Rhodium catalyst 2 Exhaust gas 3 Decomposition processing apparatus 4 Porous support 5 Exhaust pipe 6 Outer cylinder 7 Cell wall 8 Cell space 9 Blower part 10 Heating part

Claims (1)

A method for detoxifying a rhodium catalyst, wherein the rhodium catalyst contains a low concentration of 50 ppm to 10,000 ppm of nitrous oxide contained in exhaust gas that has undergone an ammonia oxidation step as its heat. The SV value between the exhaust gas and the rhodium catalyst is reduced and decomposed into nitrogen and oxygen at a heating temperature of 350 ° C. to less than 550 ° C., which is less than the decomposition temperature, and the SV value between the exhaust gas and the rhodium catalyst is 10,000 / h to 30,000 / h. And
The poisonous substances are contained in the exhaust gas and are composed of sulfate ions of sulfur compounds and / or hydrogen sulfite ions of sulfurous acid adsorbed on the rhodium catalyst surface during the reduction and decomposition of the nitrous oxide. ,
The sulfur compound of the poisonous substance is accumulated over time, and the poisoning of the rhodium catalyst that has become poisoned is performed by supplying hydrogen after stopping the supply of the exhaust gas,
By supplying the hydrogen, nascent atomic hydrogen that is more reducible than the sulfur compound is generated on the surface of the rhodium catalyst, and adsorbed on the surface of the rhodium catalyst instead of the sulfur compound. The rhodium catalyst is detoxified,
After such detoxification, the nascent atomic hydrogen adsorbed on the rhodium catalyst surface at a heating temperature lower than the thermal decomposition temperature of the nitrous oxide is separated from the rhodium catalyst surface, and the rhodium. While cleaning the catalyst surface, it is molecularized into hydrogen,
On the other hand, at the time of supply of the exhaust gas at a heating temperature lower than the thermal decomposition temperature of the nitrous oxide after the release of such poisoning, the nascent atomic hydrogen adsorbed on the rhodium catalyst surface Is separated from the rhodium catalyst surface to clean the rhodium catalyst surface,
A method for detoxifying a rhodium catalyst, comprising participating in the reduction and decomposition of the nitrous oxide as it is without being molecularized.

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