JP2007222843A - CATALYST FOR NOx PURIFICATION, AND NOx PURIFICATION METHOD - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a new catalyst for NOx purification enabling an efficient purification treatment of a diesel exhaust NOx, and a purification method using the catalyst. <P>SOLUTION: The NOx is treated under a coexistence of oxygen and moisture using a catalyst containing hafnium oxide. An oxygen concentration may be on the order of equivalent or more to the NOx. In the case of an exhaust gas of cars, generally a NOx concentration is tens to hundreds of ppm, and since oxygen is contained about tens to thousands times NOx, it is not necessary to supply oxygen particularly. Further, since the moisture is also contained about tens to thousands times NOx, it is not necessary to supply the moisture particularly. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a NOx purification catalyst and a NOx purification method, and provides a novel NOx purification catalyst and a NOx purification method for decomposing NOx in a low temperature region without using a reducing agent in the presence of oxygen and moisture. To do.

Conventionally, purification of NOx emitted from chemical plants, large power plants and the like is usually performed by a so-called ammonia SCR (Selective Catalytic Reduction) method using a transition metal or a platinum group element as a catalyst and ammonia as a reducing agent. Yes. This method has an advantage that NOx can be efficiently purified in a relatively low temperature region of about 100 ° C., but has a problem that expensive ammonia is used as a reducing agent. Conventionally, NOx, carbon monoxide, and hydrocarbons contained in the exhaust gas of gasoline vehicles are usually purified by a three-way catalyst composed of a platinum group element (see Patent Document 1). This method controls the air-fuel ratio, which is the air: fuel weight mixture ratio, to near the stoichiometric air-fuel ratio (= 14.7) (this combustion is called rich burn). Since it can be maintained below, it has the advantage that carbon monoxide and hydrocarbons contained in the exhaust gas can be used as a reducing agent for NOx. However, when the oxygen concentration in the exhaust gas exceeds several percent, the problem of significant oxidative degradation of the catalyst occurs. There is. On the other hand, for exhaust gas treatment of heavy-duty diesel vehicles such as trucks and buses that run on light oil fuel, a so-called urea SCR method using a transition metal compound or a platinum group element as a catalyst and urea water as a reducing agent has been studied (patent) Reference 2). This method has the advantage of being able to efficiently purify NOx from a relatively low temperature region near 100 ° C. to a relatively high temperature region near 600 ° C., but it is necessary to mount expensive urea water as a reducing agent. There is a problem. However, a three-way catalyst cannot be used for the exhaust NOx treatment of small diesel vehicles such as diesel passenger cars. Because the air-fuel ratio is more than several times that of gasoline (combustion of diesel fuel is called lean burn), the oxygen concentration in diesel exhaust gas is usually 5% or more and there is almost no reducing substance. It is. A so-called NOx occlusion reduction catalyst, which is a platinum group catalyst containing a NOx occlusion agent as a catalyst, has been studied for the exhaust NOx treatment of small diesel vehicles (see Patent Document 3). In this method, lean burn and rich burn cycle combustion is performed, lean burn exhaust NOx is absorbed by the NOx storage agent, absorbed NOx is released in a rich burn atmosphere, and the released NOx is reduced in the rich burn exhaust gas ( Carbon monoxide, hydrogen, hydrocarbons, etc. in the rich burn exhaust gas serve as a reducing agent). This method has the advantage that NOx can be purified from about 250 ° C. to about 600 ° C. even in a high-concentration oxygen coexistence atmosphere in which a three-way catalyst used for exhaust gas treatment of gasoline passenger cars cannot be used. There is a problem that NOx purification at temperatures below about 0 ° C. is very difficult and a problem that the NOx occlusion agent deteriorates significantly due to moisture in the exhaust gas and a small amount of SOx. In addition, the catalyst supported by zeolite, which has been studied for purifying exhaust NOx, by the ion exchange method has a problem that the activity is significantly reduced by moisture and oxygen. By the way, in Japan, the temperature of exhaust gas discharged from diesel passenger cars is approximately 120 ° C to 200 ° C during transient driving and approximately 200 ° C to 400 ° C during stable driving, but about 80% of the exhausted NOx is transient driving. Sometimes discharged. Therefore, the catalyst required for the exhaust NOx treatment of diesel passenger cars is desired to be a catalyst having high activity in the low temperature range of 120 ° C. to 200 ° C.
In view of the above situation, an NOx purification catalyst that can efficiently decompose NOx without using a reducing agent even in a low temperature region in the presence of oxygen and moisture is ideal. However, at present, such an ideal catalyst has not been developed.

JP 56-44771 A JP 2005-334681 A JP-A-11-333256

  In view of the above circumstances, an object of the present invention is to provide a NOx purification catalyst and a NOx purification method for purifying NOx in lean burn exhaust gas without supplying a reducing agent. In particular, in order to efficiently purify diesel exhaust NOx, which has been difficult in the past, a novel NOx purifying catalyst capable of decomposing exhaust NOx in a low temperature region where oxygen and moisture coexist without a reducing agent, and the catalyst. It is to provide a NOx purification method.

As a result of intensive studies to achieve the above object, the present inventors have found that hafnium oxide can decompose NOx without using a reducing agent in the presence of oxygen. It came to complete.
That is, the present invention
(1) A catalyst for purifying NOx, wherein the active component of the catalyst is hafnium oxide.
(2) A NOx purification method characterized by decomposing NOx by a catalyst containing hafnium oxide in the presence of oxygen and moisture.

  The hafnium oxide catalyst of the present invention can treat lean burn exhaust NOx, which could not be achieved in the past, extremely efficiently in a low temperature region regardless of the presence or absence of a reducing agent. For example, a three-way catalyst can hardly purify nitric oxide in an atmosphere with an oxygen concentration of 5%. However, the hafnium oxide catalyst of the present invention contains several percent of water and 1% to 20% oxygen in the exhaust gas. 80% or more of nitric oxide can be purified at 80 ° C to 300 ° C with or without reducing agent.

Hereinafter, the present invention will be described in detail.
The present invention is to use hafnium oxide as an active component of the NOx purification catalyst.
Conventionally, a three-way catalyst is known as a catalyst for automobile exhaust gas treatment, but this catalyst is known to have little effect on purification treatment of diesel exhaust NOx. The reason is that palladium and rhodium, which are constituent elements other than platinum, undergo surface oxidation by a low concentration of oxygen. Since the three-way catalyst is composed of platinum-palladium-rhodium, it is easily deactivated when subjected to surface oxidation. The reason for using hafnium oxide in the present invention is that it has been found that hafnium oxide has a catalytic ability to decompose nitrogen monoxide, which is a main component of exhaust NOx, into nitrogen and oxygen in the presence of oxygen and moisture. Conventionally, many catalysts that oxidize nitric oxide to nitrogen dioxide are known, and it is well known that highly oxidizing nitrogen dioxide is easily reduced to nitrogen by a reducing agent on the catalyst surface. Some catalysts that decompose nitrogen monoxide into nitrogen and oxygen under oxygen-free and anhydrous conditions are also known. However, there are few known catalysts that decompose nitrogen monoxide into nitrogen and oxygen in the presence of oxygen and moisture. Surprisingly, it was found for the first time by the present invention that hafnium oxide has such catalytic ability. The mechanism of the special catalytic activity of hafnium oxide has not been fully elucidated at present, but since nitric oxide cannot be purified under conditions without oxygen and moisture, moisture on the catalyst surface cannot be purified. A mechanism for oxidative decomposition of nitric oxide by activating oxygen in the presence and generating oxygen anions or oxygen radicals is conceivable.

Hafnium oxide, which is an active component of the catalyst of the present invention, may be used alone, or may be dissolved in an oxide such as silica, alumina, zirconia, ceria, yttria, niobia, titania, zeolite, magnesia, calcia, cordierite, etc. May be used as a double oxide with rare earth oxides and / or transition metal oxides, or may be used as an oxide having a perovskite structure represented by ABO ₃ , or It can also be used as an oxyacid salt such as a heteropolyacid salt, aluminate, silicate, phosphate, titanate, or ferrate. Further, a metal hafnium or a hafnium compound that is oxidized in an oxidizing atmosphere to become hafnium oxide as a catalyst or a catalyst raw material can be used.

  The NOx purification method of the present invention is characterized in that NOx is decomposed by hafnium oxide in the presence of oxygen and moisture. The oxygen concentration may be about equal to or higher than NOx. In the case of automobile exhaust gas, NOx is usually contained in several tens to several hundred ppm, and oxygen is contained in several tens to several thousand times that of NOx, so that it is not particularly necessary to supply oxygen. Further, the moisture concentration may be about equal to or more than NOx. In the case of automobile exhaust gas, water is usually contained in several tens to several thousand times that of NOx, so it is not necessary to supply water. In addition to NOx, oxygen, moisture, and nitrogen, automobile exhaust gas contains carbon monoxide, hydrocarbons, SOx, etc., but there is almost no decrease in catalytic activity due to these coexisting substances. Carbon monoxide and hydrocarbons in the exhaust gas are simultaneously oxidized by the catalyst of the present invention into carbon dioxide gas and water. The NOx decomposition reaction in the NOx purification method of the present invention usually starts at about room temperature, and the catalytic reaction is accelerated as the temperature increases. Usually, it is possible from about room temperature to less than 2812 ° C., which is the melting point of hafnium oxide, but it is preferable to avoid an excessively high temperature state because the catalytic activity is lowered by sintering of the catalyst particles at high temperature (also called sintering). Since the temperature of automobile exhaust gas is normally about 100 ° C. to 700 ° C., this exhaust gas temperature is a temperature at which NOx purification can be sufficiently performed by the method of the present invention. Further, when the catalyst of the present invention is used for purifying NOx in automobiles, the catalyst of the present invention is usually used by attaching it to the inner wall of the gas flow path of the monolith molded body. The catalyst is attached to the inner wall of the gas flow path by a conventional method of applying a slurry composed of a catalyst component and a binder, and the precursor of hafnium oxide is attached to the inner wall of the gas flow path of the monolith molded body in the form of a thin film, followed by firing. A method for fixing thin film hafnium oxide, or by adhering a mixture of a hafnium oxide precursor and an oxide forming thin film material to the inner wall of the gas flow path of the monolith molded body and then firing. The method can be performed by a method of fixing an oxide thin film doped with hafnium oxide. Examples of the precursor of hafnium oxide include tetramethoxyhafnium, tetraethoxyhafnium, tetra-i-propoxyhafnium, tetra-t-butoxyhafnium, tetrakis (dipivaloylmethanato) hafnium, tetrakis (dimethylamino) hafnium, halogen Volatile compounds such as hafnium halide can be used. The firing temperature of the thin film-like intermediate is usually a temperature from several hundred degrees Celsius to 1000 degrees Celsius.

By adding a co-catalytic component having a different function to the catalyst of the present invention, the catalyst performance can be improved by the synergy effect. Examples of such components include platinum, palladium, rhodium, ruthenium, iridium, rhenium, gold, silver, chromium, manganese, iron, cobalt, nickel, copper, zinc, barium, scandium, yttrium, lanthanum, cerium, neodymium, Samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, titanium, zirconium, niobium, tantalum, molybdenum, tungsten, boron, aluminum, gallium, indium, silicon, germanium, tin, magnesium, calcium, Strontium, barium, and compounds thereof, hafnium boride, hafnium carbide, hafnium nitride, hafnium silicide, and the like can be given. Among them, chromium, iron, cobalt, nickel, NOx occlusion barium oxide, cerium oxide with moderate oxidizing power, manganese sesquioxide, and cobalt trioxide, SOx poison Copper-zinc, iron-chromium, molybdenum oxide, molybdenum sulfide, etc. effective for prevention are preferred. The addition amount of this component is usually about the same mass to about 100 times as the catalyst of the present invention, but may be about 1/10 times to about 1/100 times as necessary.
The NOx purification catalyst of the present invention and the NOx purification method using this catalyst are extremely effective in a wide temperature range of lean burn exhaust NOx of 80 to 700 ° C., preferably 80 to 300 ° C., by being mounted on a diesel vehicle. Can be purified. Moreover, it can also be used for exhaust NOx treatment discharged by lean burn gasoline vehicles.

The present invention will be specifically described below with reference to examples.
As a model gas for automobile exhaust NOx, a mixed gas of nitrogen monoxide and oxygen diluted with helium was used. A reducing gas (ethylene or ammonia) was supplied to the model gas as needed. The treatment rate of nitric oxide was calculated according to the following equation (1) by measuring NOx contained in the treated gas with a reduced pressure chemiluminescence NOx analyzer (manufactured by Nippon Thermo Co., Ltd .: model 42C).

"Comparative Example 1" Synthesis and NOx treatment of a three-way catalyst-like catalyst as a comparative sample
PtCl ₄ · 5H ₂ O (0.215 g), PdCl ₂ · 2H ₂ O (0.106 g), and Rh (NO ₃ ) ₃ · 2H ₂ O (0.162 g) are dissolved in 20 ml of distilled water and placed in an evaporating dish. Put in. 10 g of γ-alumina (manufactured by JGC Chemical Co., Ltd .: fine particles with a particle size of 2 to 3 μm) was added thereto, evaporated to dryness with a steam bath, and then vacuum dried at 100 ° C. for 3 hours. This sample was placed in a quartz tube and reduced at 500 ° C. for 3 hours under a helium-diluted hydrogen gas (10% v / v) stream to synthesize a catalyst having a precious metal content of about 2% by weight. 600 mg of this catalyst was dispersed in 13 g of sea sand, filled in a continuous flow reaction tube made of quartz, and nitrogen monoxide adjusted in concentration with helium was flow-treated. The component molar concentrations of the gas to be treated were 0.1% nitric oxide, 14% oxygen, 10% water vapor, and 0.3% ethylene. The flow rate of the mixed gas introduced into the reaction tube was 100 ml / min, and the treatment temperature was 80 ° C to 300 ° C. The exhaust gas was sampled and the treatment rate of nitric oxide was determined. The results are shown in Table 1.

[Example 1] Synthesis of hafnium oxide catalyst and NOx treatment 1 g of commercially available hafnium oxide powder (manufactured by Kanto Chemical Co., Ltd .: high-purity hafnium oxide) is dispersed in 13 g of sea sand and filled into a quartz continuous-flow reaction tube. Then, nitrogen monoxide whose concentration was adjusted with helium was flow-treated. The component molar concentrations of the gas to be treated were 0.1% nitric oxide, 14% oxygen, 10% water vapor, and 0.3% ethylene. The flow rate of the mixed gas introduced into the reaction tube was 100 ml / min, and the treatment temperature was 80 ° C to 300 ° C. The exhaust gas was sampled and the purification rate of nitric oxide was determined. The results are shown in Table 1.

[Example 2] Synthesis of hafnium oxide catalyst and NOx treatment 1 g of commercially available hafnium oxide powder (manufactured by Kanto Chemical Co., Ltd .: high-purity hafnium oxide) was dispersed in 13 g of sea sand and filled into a quartz continuous-flow reaction tube. Then, nitrogen monoxide whose concentration was adjusted with helium was flow-treated. The component molar concentration of the gas to be treated was 0.1% nitric oxide, 14% oxygen, and 10% water vapor. The flow rate of the mixed gas introduced into the reaction tube was 100 ml / min, and the treatment temperature was 80 ° C to 300 ° C. The exhaust gas was sampled and the purification rate of nitric oxide was determined. The results are shown in Table 1.

Example 3 Synthesis of Monolith Catalyst and NOx Treatment 30 g of distilled water, 24 g of ethanol, 0.3 g of dodecylamine and 0.5 g of sulfuric acid were placed in a beaker having a volume of 100 ml and stirred to prepare a uniform aqueous solution. A mini-molded body (21 cells, diameter 8 mm × length 9 mm) cut from a commercially available cordierite monolith molded body (manufactured by NGK, 400 cells / in ² , diameter 118 mm × length 50 mm, weight 243 g), 10 pieces (weight 0.15 g) were added and allowed to stand for 10 minutes. Place this in a 200 ml heat-resistant glass desiccator containing 2 g of tetra-i-propoxyhafnium in a horizontal position so that it does not touch tetra-i-propoxyhafnium, and place it in a dryer for 1 hour at 200 ° C. Processed. A sample was taken out and baked at 500 ° C. for 5 hours. As a result of observing a section of the obtained sample with a scanning electron microscope, it was confirmed that thin hafnium oxide was formed on the inner wall of the gas channel of the mini-molded body. The thickness of the hafnium oxide was about 200 nm, the specific surface area measured by the BET method was 350 m ² / g, and the pore diameter measured by the BJH method was 3.0 nm. Next, 10 compacts were filled into a continuous flow reaction tube made of quartz, and nitrogen monoxide whose concentration was adjusted with helium was flow-treated. The component molar concentration of the gas to be treated was 0.1% nitric oxide, 14% oxygen, and 10% water vapor. The flow rate of the mixed gas introduced into the reaction tube was 100 ml / min, and the treatment temperature was 80 ° C to 300 ° C. The exhaust gas was sampled and the purification rate of nitric oxide was determined. The results are shown in Table 1.

  From Table 1, it can be seen that the catalyst of the present invention can efficiently purify NOx even in a low temperature region in the presence of high concentration oxygen and moisture regardless of the presence or absence of a reducing agent. Therefore, it turns out that it is suitable for the lean burn exhaust NOx processing of a diesel vehicle and a gasoline vehicle.

  The NOx purification catalyst and purification method of the present invention are useful as a diesel exhaust NOx purification catalyst.

Claims (2)

  A catalyst for purifying NOx, wherein the active component of the catalyst is hafnium oxide. A NOx purification method comprising decomposing NOx with a catalyst containing hafnium oxide in the presence of oxygen and moisture.