JP5285459B2 - Exhaust gas purification catalyst and exhaust gas purification method - Google Patents

Description

  The present invention purifies nitrogen oxides contained in combustion exhaust gas using ammonia as a reducing agent, oxidizes ammonia to nitrogen, and oxidizes and removes flammable components such as carbon monoxide and hydrocarbons. The present invention relates to an exhaust gas purification catalyst and an exhaust gas purification method.

  Nitrogen oxides (NOx: x = 1 or 2, ie, nitric oxide and nitrogen dioxide) contained in combustion exhaust gas and nitric acid production process exhaust gases cause acid rain, photochemical smog, respiratory disease, etc. It is considered harmful and its emissions are regulated. To purify nitrogen oxides in exhaust gas containing excessive oxygen, such as boiler and diesel engine exhaust gas, ammonia is added as a reducing agent, and the catalyst is made of titanium oxide-vanadium oxide or titanium oxide-vanadium oxide-tungsten oxide. An ammonia selective reduction method is widely used in which exhaust gas is brought into contact with nitrogen to convert nitrogen oxides into harmless nitrogen. For example, Japanese Patent Laid-Open No. 50-51966 (Patent Document 1) discloses a catalytic reduction of nitrogen oxide in which nitrogen oxide is contacted with a catalyst in which vanadium oxide is supported on titanium oxide or zirconium oxide in the presence of ammonia. A decomposition method is disclosed.

  The ammonia selective reduction method is characterized in that a denitration rate close to 100% can be obtained if the molar ratio of ammonia to nitrogen oxide in exhaust gas is strictly controlled. However, in some combustion exhaust gases, etc., the nitrogen oxide concentration in the exhaust gas fluctuates over time, so it is extremely difficult to control the amount of ammonia injected, and ammonia remains in the process gas when the ammonia becomes excessive. There is a problem with leaking ammonia. In order to avoid this problem, there is a method in which the amount of ammonia added is slightly smaller than the amount of nitrogen oxide, but this method involves a reduction in the denitration rate.

As another method, a method has been proposed in which a small amount of noble metal is added to the ammonia denitration catalyst described above to decompose excess ammonia. For example, in JP-A-5-146634 (Patent Document 2), a composition containing an oxide of one or more elements selected from titanium, vanadium, tungsten, and molybdenum is used as a first component, and platinum, palladium, and rhodium are used. It is composed of a composition containing the noble metal previously supported on a porous body such as a noble metal salt selected from zeolite, alumina, silica, etc. as a second component, and simultaneously reduces and simultaneously reduces nitrogen oxides. An ammonia decomposition catalyst having a denitration function characterized by decomposing unreacted ammonia injected as an agent is disclosed. This document discloses that when the ammonia decomposition catalyst is used, in addition to the reaction between ammonia and nitrogen oxides, the decomposition reaction of ammonia occurs at the same time, so that even if the ammonia is excessive, the leaked ammonia can be kept low. Has been. JP-A-8-290062 (Patent Document 3) discloses a composition containing titanium oxide and an oxide of one or more elements selected from molybdenum, tungsten and vanadium, or a zeolite carrying copper or iron. The composition containing the first component, comprising at least one metal selected from iridium, palladium, rhodium, and ruthenium and platinum, wherein the weight ratio of the metal to platinum exceeds 0 and is 5 or less. An exhaust gas purification catalyst having an ammonia reduction function of nitrogen oxides and an oxidative decomposition function of ammonia, which is composed of a supported second component, is disclosed, and it is disclosed that the same effects as described above are achieved. Furthermore, in JP 2002-336699 A (Patent Document 4), in a method of decomposing and removing ammonia by contacting a gas containing ammonia with an ammonia decomposition catalyst, γ-Al ₂ O ₃ , A method for decomposing and removing ammonia using a catalyst in which ruthenium is supported as an active metal on a support containing at least one porous material selected from θ-Al ₂ O ₃ , ZrO _{2 and the} like is disclosed. In this document, when ammonia is decomposed and removed using a catalyst having ruthenium supported on a carrier having an acidic point, nitrogen is generated without generating nitrogen oxides, so excessive ammonia after ammonia denitration reaction is decomposed. It is described that it is useful to do.

  Titanium oxide-vanadium oxide-tungsten oxide catalysts are widely used in thermal power plants and garbage incinerators, but they are used in applications where the catalyst is pulverized or scattered by vibration, such as in automobile exhaust gas purification. Some point out that attention is needed. In response to this problem, Japanese Patent Application Laid-Open No. 2005-238195 (Patent Document 5) discloses a composite oxide composed of two or more oxides selected from silica, alumina, titania, zirconia, and tungsten oxide, and a rare earth element. A catalyst for denitration containing a metal or a transition metal (except Cu, Co, Ni, Mn, Cr, V) as an active ingredient is disclosed. In addition, Schmieg and Lee, SAE Technical Paper Series, 2005-01-3881 (2005) (Non-patent Document 1) include zeolite catalysts supporting iron and copper. Is disclosed.

  The exhaust gas from the internal combustion engine contains a small amount of carbon monoxide and hydrocarbons in addition to nitrogen oxides. Since carbon monoxide is also harmful, it is advantageous if it can also be removed. However, the carbon monoxide removal performance of the titanium oxide-vanadium oxide-tungsten oxide catalyst is low. JP-A-1-266668 (Patent Document 6) contains titanium oxide as a main component, and contains at least one of molybdenum, tungsten, vanadium, cerium, nickel, cobalt, and manganese as a second component, Furthermore, a catalyst containing at least one or more selected from platinum, palladium, rhodium and ruthenium and / or at least one selected from iron, chromium and copper oxides as a third component is a reducing agent for ammonia. It is disclosed that it is effective for the reduction of nitrogen oxides and simultaneous purification of carbon monoxide. However, this catalyst has a problem that the temperature range in which the reduction of nitrogen oxides and the simultaneous purification of carbon monoxide are limited. Japanese Patent Application Laid-Open No. 2003-305338 (Patent Document 7) discloses an iron-substituted zeolite as a first component, a noble metal-supported zeolite or the like as a second component, and the noble metal content is the total mass of the first component and the second component. It is disclosed that a catalyst of 0 to 100 ppm is effective for reducing nitrogen oxides using ammonia as a reducing agent and simultaneously purifying carbon monoxide. However, zeolite has a problem in durability because the performance of the zeolite may drop under the condition where water vapor coexists, and aluminum atoms in the lattice fall off or the crystal lattice collapses.

  Hydrocarbons are also known to inhibit the reduction reaction of nitrogen oxides using ammonia as a reducing agent (for example, Non-Patent Document 1). Hydrocarbons are also a causative agent of photochemical smog, so removal is still desired, but the titanium oxide-vanadium oxide-tungsten oxide catalyst is not very effective in removing hydrocarbons. JP-A-2006-68663 (Patent Document 8) discloses a catalyst in which manganese oxide is supported on a carrier mainly composed of titanium oxide, a reduction reaction of nitrogen oxides using ammonia as a reducing agent, and an odor component (acetaldehyde). It is described that it is effective for removing oxidants. However, this catalyst does not have sufficient carbon monoxide removal performance. Therefore, in the exhaust gas treatment of an internal combustion engine, it is common to use a nitrogen oxide reduction catalyst using ammonia as a reducing agent and an oxidation catalyst carrying a platinum group metal in combination. For example, Japanese Patent Laying-Open No. 2005-48733 (Patent Document 9) discloses a cogeneration apparatus including an exhaust gas purification device for purifying exhaust gas from a reciprocating engine, wherein the exhaust gas purification device is the engine. Disclosed is a combined heat and power supply device arranged in the order of an oxidation unit including an oxidation catalyst, a recovery unit including an exhaust heat recovery device, and a denitration unit including a denitration catalyst from the upstream side to the downstream side of exhaust gas discharged from ing. This document describes that the oxidation catalyst has a noble metal element supported on a refractory inorganic carrier, and that the noble metal element is preferably rhodium, iridium, palladium, or platinum. However, this apparatus requires two kinds of catalysts, and the problem is that it is expensive.

JP-A-50-51966 (Claims) Japanese Patent Laid-Open No. 5-146634 (Claims) JP-A-8-290062 (Claims) JP 2002-336699 A (Claims) JP 2005-238195 A (Claims) Japanese Patent Laid-Open No. 1-266849 (Claims) JP 2003-305338 A (Claims) JP 2006-68663 A (Claims) JP-A-2005-48733 (Claims)

Schmieg and Lee, SAE Technical Paper Series, 2005-01-3881 (2005)

  An object of the present invention is to achieve both a reduction reaction of nitrogen oxides using ammonia as a reducing agent and removal of combustible components (for example, carbon monoxide and hydrocarbons) in a single catalyst in an oxygen atmosphere. A catalyst (exhaust gas purification catalyst) and an exhaust gas purification method using the catalyst are provided.

  Another object of the present invention is to provide a catalyst capable of obtaining a high denitration rate while suppressing leakage of ammonia even under conditions in which the concentration of nitrogen oxides varies, and an exhaust gas purification method using this catalyst.

  Still another object of the present invention is to provide a catalyst capable of achieving both an oxidation reaction for oxidizing ammonia into nitrogen and removal of combustible components (for example, carbon monoxide and hydrocarbons) in a single catalyst in an oxygen atmosphere. (Exhaust gas purification catalyst) and an exhaust gas purification method using this catalyst.

  As a result of intensive studies to achieve the above-mentioned problems, the present inventors have found that the support containing titanium oxide, iron and tungsten is at least one selected from palladium (Pd), iridium (Ir) and rhodium (Rh). (I) In an oxygen-excess atmosphere, the catalyst supporting platinum group metal simultaneously performs the reduction reaction of nitrogen oxide using ammonia as a reducing agent and the removal of combustible components such as carbon monoxide and hydrocarbons. And / or (ii) oxidize ammonia to nitrogen in an oxygen-rich atmosphere even when ammonia is excessively present relative to nitrogen oxides, and ammonia can be decomposed into nitrogen and leakage of ammonia can be suppressed. Discovered that reaction and removal of combustible components such as carbon monoxide and hydrocarbons can be performed simultaneously, completing the present invention It was.

  That is, the catalyst of the present invention is a catalyst in which at least one platinum group metal selected from palladium, iridium and rhodium is supported on a support containing titanium oxide, iron and tungsten. Such a catalyst, for example, reduces nitrogen oxides in the presence of ammonia in an oxygen atmosphere (oxygen-rich atmosphere) and oxidizes combustible components (for example, carbon monoxide and / or hydrocarbons). It can be used as a catalyst for removal. In addition, such a catalyst oxidizes (oxidizes and removes) ammonia into nitrogen in an oxygen atmosphere (oxygen-excess atmosphere), and removes a combustible component (for example, carbon monoxide and / or hydrocarbon). It can also be used as a catalyst for oxidative removal.

  In the catalyst, the platinum group metal may contain at least palladium, and the proportion of palladium may be about 0.005 to 0.025% by mass with respect to the support. In particular, the platinum group metal may contain palladium and at least one selected from iridium and rhodium, and the ratio of at least one selected from iridium and rhodium is in a mass ratio with respect to palladium. It may be about 0.5 to 5 times. In addition, the catalyst (or the support) may further contain a sulfuric acid component.

  The present invention also includes a method for reducing nitrogen oxides in the presence of ammonia in an oxygen atmosphere and oxidizing and removing combustible components using the catalyst. Such a method includes, for example, bringing flue gas containing nitrogen oxides, ammonia, and combustible components (such as carbon monoxide and hydrocarbons) into contact with the catalyst, reducing and removing nitrogen oxides, and carbon monoxide. And an exhaust gas purification method that oxidizes and removes combustible components such as hydrocarbons. Such a purification method is typically a purification method of exhaust gas containing oxygen (specifically, exhaust gas containing oxygen, nitrogen oxides, and combustible components), and the exhaust gas contains ammonia and / or its After adding a precursor (sometimes collectively referred to as an ammonia component), the exhaust gas (the exhaust gas to which the ammonia component has been added) is added to the catalyst (that is, a carrier containing titanium oxide, iron and tungsten), palladium, iridium and It is passed through (or brought into contact with) a catalyst supporting at least one platinum group metal selected from rhodium to purify nitrogen oxides and combustible components in the exhaust gas (specifically, reducing nitrogen oxides) And a method of purifying the flammable component by oxidizing and removing it). In such a method, the exhaust gas may be, for example, a combustion exhaust gas of an internal combustion engine. In this method, the ammonia and / or its precursor (for example, urea, ammonium carbonate, etc.) is added in a molar ratio of ammonia of about 0.75 to 1.25 times with respect to nitrogen oxides in the exhaust gas. Also good.

  The present invention also provides a method for purifying exhaust gas containing oxygen (specifically, exhaust gas containing oxygen, ammonia, and a combustible component), wherein the exhaust gas is circulated (or brought into contact with) a catalyst, And a method for purifying combustible components while oxidizing ammonia into nitrogen.

  The catalyst of the present invention can achieve both the reduction reaction of nitrogen oxides using ammonia as a reducing agent and the removal of combustible components (carbon monoxide, hydrocarbons, etc.) in a single catalyst in an oxygen atmosphere. Furthermore, it has high nitrogen oxide reducing ability and also has the ability to convert excess ammonia into nitrogen. Therefore, when nitrogen oxide treatment is performed using this catalyst, a high NOx removal rate can be obtained while suppressing leakage of ammonia even under conditions where the nitrogen oxide concentration varies. Such a catalyst of the present invention is also effective in removing combustible components such as carbon monoxide and hydrocarbons, so that exhaust gas purification can be comprehensively performed with a single catalyst. Further, the catalyst of the present invention can achieve both an oxidation reaction for oxidizing ammonia into nitrogen and removal of combustible components (for example, carbon monoxide and hydrocarbons) in a single catalyst in an oxygen atmosphere.

  The catalyst of the present invention comprises at least one selected from palladium (Pd), iridium (Ir) and rhodium (Rh) on a carrier containing titanium oxide, iron and tungsten (titanium oxide carrier containing iron and tungsten). A platinum group metal is supported.

  As long as the carrier contains titanium oxide, iron (iron component), and tungsten (tungsten component), the form of inclusion thereof is not limited, but usually contains titanium and iron and tungsten as a main component. Typically, the support is often in a form in which iron and tungsten are supported on titanium oxide (that is, the support is a support in which iron and tungsten are supported on titanium oxide as a support). For example, the proportion of titanium oxide in the carrier (or the catalyst) is 40 to 99%, preferably 50 to 98% (for example, 55 to 97%), more preferably 60 to 96% (for example, 70) by mass ratio. (About -95%).

  In the support or the catalyst, the ratio of iron to titanium oxide (the ratio in terms of metal atom when included as an oxide, hereinafter the same for tungsten, Pd, Ir, and Rh) is 0.3 by mass ratio. -20% (for example, 0.4-15%), Preferably it is 0.5-10% (for example, 0.7-7%), More preferably, about 1-5% may be sufficient. Moreover, the ratio of tungsten with respect to titanium oxide is 0.3-30% (for example, 0.5-25%) by mass ratio, Preferably it is 1-20% (for example, 3-18%), More preferably, it is 5-5%. It may be about 15%.

  Furthermore, the ratio of tungsten is 0.03 to 20 times (for example, 0.05 to 15 times), preferably 0.1 to 10 times (for example, 0.5 to 8 times), for example, by mass ratio with respect to iron. Times), more preferably about 1 to 7 times (for example, 1.5 to 5 times).

  In addition, the form of iron and tungsten contained in the carrier is not particularly limited, and may be contained in the carrier in the form of an oxide, salt (such as an inorganic salt such as sulfate), or the like. Typically, the carrier often supports (disperses and supports) iron and tungsten as oxide particles (fine particles) on titanium oxide particles.

  The catalyst of the present invention is a catalyst in which at least one platinum group metal selected from Pd, Ir and Rh is supported on the carrier as described above. The catalyst of the present invention may be a catalyst in which an iron component (for example, iron oxide), a tungsten component (for example, tungsten oxide), and the platinum group metal are supported on titanium oxide as a support. In such a catalyst, the platinum group metal may be used in any combination of Pd, Ir and Rh. However, in order to ensure the removal performance of the combustible component, it is preferable that the platinum group metal contains at least Pd. In particular, it is preferably composed of Pd and at least one selected from Ir and Rh.

  In the catalyst, the ratio of the platinum group metal to the support (or titanium oxide) is, for example, 0.001 to 0.1% (for example, 0.002 to 0.08%), preferably 0.00. It may be about 005 to 0.07% (for example, 0.007 to 0.06%), more preferably about 0.008 to 0.05% (for example, 0.01 to 0.04%).

  Particularly, in the catalyst, the ratio of Pd to the support (or titanium oxide) is, for example, 0.001 to 0.035% (for example, 0.002 to 0.03%), preferably 0.00. It may be about 005 to 0.025% (for example, 0.006 to 0.02%), more preferably about 0.007 to 0.018% (for example, 0.008 to 0.015%). When the content of Pd is in such a range, it is easy to improve the removal performance of the combustible component, and it is easy to improve the nitrogen oxide purification performance by appropriately promoting the oxidation of ammonia. In addition, the ratio of at least one selected from Ir and Rh is, for example, 0.1 to 10 times (for example, 0.3 to 7 times), preferably 0.5 to, mass ratio with respect to Pd. It may be about 5 times (for example, 0.7 to 4 times), more preferably about 1 to 4 times (for example, 1.5 to 3 times), and usually about 1 to 3 times. Furthermore, the ratio (mass ratio) of Ir and Rh can be selected from the range of the former / the latter = 0/100 to 100/0, for example, 10/90 to 90/10, preferably 20/80 to 80/20. More preferably, it may be about 30/70 to 70/30.

  The catalyst (or the support) may contain a trace amount of metals other than iron and tungsten (for example, manganese, nickel, etc.) as long as the effects of the present invention are not impaired. In many cases, vanadium is usually not included (substantially free).

  The catalyst (or the carrier) may contain (support) a sulfuric acid component. For example, the carrier has a sulfuric acid component (or sulfate group) based on sulfur (S) atoms, for example, 0.1 to 10% by mass (for example, 0.2 to 8% by mass), preferably 0.3 to 5%. It may contain about 0.5% by mass, more preferably about 0.5-3% by mass. Such a sulfuric acid component is preferable in terms of catalytic activity.

The specific surface area of the catalyst of the present invention is usually about 10 to 100 m ² / g, preferably 20 to 80 m ² / g, more preferably about 30 to 70 m ² / g.

  The method for producing the catalyst of the present invention is not particularly limited. For example, the catalyst may be produced by supporting the platinum group metal on a carrier containing (i) titanium oxide, iron and tungsten, and (ii) titanium oxide. After the platinum group metal is supported on a carrier containing, iron and tungsten may be contained. Usually, the catalyst of the present invention is often produced by the above method (i).

  In the method (i), the support containing titanium oxide, iron and tungsten can be produced, for example, by allowing titanium oxide to contain iron and tungsten. Iron and tungsten may be contained in titanium oxide at the same time. After one component (tungsten or iron) is contained (or supported) in titanium oxide, the other component (iron or tungsten) is contained (supported). May be. In particular, it is often the case that iron is contained after containing (or supporting) tungsten in titanium oxide. In addition, a commercial item may be used for titanium oxide, and what was synthesize | combined by the usual method (For example, the method of condensing titanium tetraalkoxide, titanium tetrahalide, etc.) may be used. Moreover, titanium oxide (for example, titanium oxide-tungsten oxide catalyst) containing one component (tungsten or iron) may be used as a commercial product, and the other component (for example, iron) may be contained in this titanium oxide.

  As a method of containing (supporting), a known method such as an impregnation method or a kneading method can be used. In any method, inclusion (support) usually requires a firing step in many cases. For example, when tungsten is contained in titanium oxide (or titanium oxide containing iron), a mixture of titanium oxide (or titanium oxide containing iron) and a tungsten compound (tungsten source) is, for example, 400 ° C. or higher (for example, 400 to 650 ° C.), preferably 420 to 600 ° C., more preferably 450 to 550 ° C. Further, when iron is contained in titanium oxide (or titanium oxide containing tungsten), a mixture of titanium oxide (or titanium oxide containing tungsten) and an iron compound (iron source) is, for example, 400 ° C. or more (for example, 400 to 650 ° C.), preferably 420 to 600 ° C., more preferably 450 to 550 ° C. Firing may be performed in an oxygen atmosphere (for example, in air). In many cases, the iron compound or the tungsten compound is usually used in the form of a solution (particularly an aqueous solution) (or in the presence of a soluble solvent) in both the impregnation method and the kneading method. For example, the mixture can be obtained by immersing the other component (titanium oxide, titanium oxide containing tungsten, etc.) in a solution containing one component (for example, an iron compound). In such a case, usually, the solvent (particularly water) is often removed from the mixture by evaporation (evaporation to dryness) or the like, and then calcined.

  Examples of the iron compound include inorganic acid iron [iron sulfate (iron sulfate (II), iron sulfate (III), etc.), iron nitrate, etc.], iron halide (iron chloride, iron bromide, etc.) and the like. These iron compounds are usually water-soluble and can be suitably used. These iron compounds can be used alone or in combination of two or more. Of these iron compounds, inorganic acid iron is often used. In particular, when iron sulfate is used, the catalyst (or support) can easily contain a sulfuric acid component.

  Examples of the tungsten compound include tungstic acid (such as paratungstic acid and metatungstic acid) and ammonium tungstate (such as ammonium tungstate and parapentahydrate). These tungsten compounds are also usually water-soluble and can be suitably used. These tungsten compounds can be used alone or in combination of two or more.

  As the method for supporting the platinum group metal (palladium, iridium, rhodium) on the carrier (or titanium oxide), a known method such as an impregnation method can be applied as described above. In any method, the inclusion (support) often requires a firing step. When two or more platinum group metals are combined, these platinum group metals may be contained (supported) at the same time or may be contained (supported) separately (stepwise). For example, a mixture of the carrier (or titanium oxide) and a platinum group metal compound (platinum group metal source) is, for example, 400 ° C. or higher (for example, 400 to 650 ° C.), preferably 420 to 600 ° C., more preferably 450 It may be fired at a temperature of about 550 ° C, usually about 400-600 ° C. Firing may be performed in an oxygen atmosphere (for example, in air). The platinum group metal compound is usually used in the form of a solution (particularly an aqueous solution) (or in the presence of a soluble solvent) in both the impregnation method and the kneading method. In such a case, usually, the solvent (particularly water) is often removed from the mixture by evaporation (evaporation to dryness) or the like, and then calcined.

  Platinum group metal compounds include salts such as inorganic acid salts (sulfates, nitrates (palladium nitrate, iridium nitrate, rhodium nitrate, etc.), complex salts (tetraammine palladium nitrate, pentaammine aquadium nitrate, hexaammine iridium hydroxide, etc.) ], Halides (palladium chloride, iridium chloride, rhodium chloride, etc.) These platinum group metal compounds are usually water-soluble and can be suitably used. A metal compound can be used individually or in combination of 2 or more types.

  In addition, as mentioned above, when the sulfuric acid component is contained, the sulfuric acid component may be contained by using it in the form of a metal compound (such as iron sulfate). In the step of containing iron and / or tungsten, a compound containing a sulfate group (for example, ammonium sulfate) may be used, or a combination thereof may be contained.

  The catalyst of the present invention can be used as a granule, a pellet molded body or a honeycomb molded body regardless of its shape, but is preferably a honeycomb molded body from the viewpoint of reducing pressure loss. There are two methods for forming into a honeycomb: a method in which a catalyst is coated on a refractory ceramic and a method in which a binder is added if necessary, and extrusion is performed into a honeycomb shape (further firing if necessary). From the latter, the latter is better.

  In the case of forming a honeycomb molded body, the molding process can be performed at any stage. For example, (i) after forming titanium oxide, tungsten, iron, and platinum group metals are sequentially supported (for example, impregnated). (Ii) Titanium oxide supporting tungsten is first prepared and molded, and then iron or platinum group metal is supported (for example, impregnated) on the molded body. It may be a body catalyst, or (iii) a catalyst supporting all components may be molded to form a molded body catalyst.

  Such a catalyst of the present invention is effective for purification of exhaust gas. Therefore, the present invention also includes a method for purifying exhaust gas using the above catalyst. Such a purification method includes, for example, (i) adding ammonia and / or a precursor thereof (ammonia component) to exhaust gas (treated gas), then circulating the exhaust gas through the catalyst, and adding nitrogen in the exhaust gas. Includes a method for purifying oxides and combustible components, and (ii) a method for purifying combustible components while oxidizing exhaust gas (treated gas) through the catalyst to oxidize ammonia in the exhaust gas to nitrogen. .

  Although the object of the gas to be processed (processed gas) is not limited, the gas to be processed (exhaust gas) usually contains oxygen (for example, about 2 to 15% oxygen). Further, in the purification method (i), the gas to be treated may contain nitrogen oxide at a concentration of about 10 to 3000 ppm (molar or volume ratio, the same in gas ratio). Further, the gas to be treated may contain 2 to 20% water vapor and 1 to 10% carbon dioxide. Although combustion exhaust gas often contains sulfur oxides, the catalyst of the present invention can also be applied to gases containing about 0.1 to 100 ppm of sulfur oxides.

  Exhaust gas (combustion exhaust gas, etc.) usually contains a small amount of carbon monoxide as a combustible component, but its concentration is as high as about 1000 ppm in terms of THC (total hydrocarbons) [for example, 1000 ppm or more (for example, 1000 to 5000 ppm, In particular, the nitrogen oxide reduction performance of the catalyst of the present invention is not greatly affected. Further, exhaust gas (combustion exhaust gas, etc.) often contains combustible components such as hydrocarbons and oxygen-containing compounds. The concentration of such combustible components (for example, hydrocarbons) is 200 in terms of THC (total hydrocarbons). Even if it is about -2000 ppm (for example, 300-1800 ppm, especially 500-1500 ppm), the nitrogen oxide reduction performance of the catalyst of the present invention is not greatly affected.

  In the exhaust gas purification method (i) of the present invention, exhaust gas (nitrogen oxide-containing gas) containing ammonia (ammonia component) is communicated (circulated or brought into contact) with the catalyst. The exhaust gas containing ammonia can be usually obtained by adding ammonia to exhaust gas (exhaust gas containing combustible components such as oxygen, nitrogen oxides, carbon monoxide, and hydrocarbons). A known method can be applied. For example, gaseous ammonia may be added using liquefied ammonia, or an aqueous solution of a compound (ammonia precursor) that generates ammonia by thermal decomposition such as urea or ammonium carbonate may be sprayed. For example, the ammonia component may be either ammonia, an ammonia precursor (a compound that generates ammonia), or both. If the amount of ammonia (ammonia component) added is too small, a sufficient denitration rate will not be obtained, and if it is too large, the amount of ammonia leak may increase, and the economy will also deteriorate, so the molar ratio of ammonia to nitrogen oxides In general, it may be about 0.75 to 1.25 times, preferably about 0.8 to 1.2 times, and more preferably about 0.9 to 1.1 times.

  The use temperature of the catalyst (temperature when contacting with exhaust gas, contact temperature, circulation temperature) may be appropriately selected depending on the amount of iron, tungsten and platinum group metal components supported, and the type of hydrocarbons to be removed. It is preferable to use at about 300-500 degreeC, More preferably, it is used at about 300-450 degreeC, More preferably, it is used at about 350-450 degreeC. Aldehydes such as formaldehyde can be oxidized and removed relatively easily, but ethylene and acetic acid have relatively low reactivity, so it is desirable to use a catalyst at a higher temperature. If the catalyst operating temperature (temperature when contacting with exhaust gas, contact temperature, circulation temperature) is too low, the catalyst performance will be insufficient, the denitration rate and the removal rate of combustible components will decrease, and ammonia There is a risk that the amount of leakage increases. On the other hand, if it is too high, the denitration rate is lowered and the durability of the catalyst is also deteriorated.

If the amount of the catalyst used is too large, the economic efficiency deteriorates, and if it is too small, the performance may be insufficient. Accordingly, a space velocity per gas hourly (GHSV) is, 2,000～100,000H ^-1, a preferably 3,000～50,000H ^-1, more preferably 5,000~30,000h about ^-1 Also good. Within such a range, both economic efficiency and denitration performance are suitable. In addition, when a high conversion rate is required for a combustible component having a relatively low reactivity, the range may be 2,000 to 15,000 h ⁻¹ .

  Hereinafter, the present invention will be described in more detail based on examples, but the present invention is not limited to these examples.

Example 1
A commercially available titanium oxide-tungsten oxide denitration catalyst (manufactured by Sakai Chemical Industry Co., Ltd., SCN-515) was crushed and sieved to a particle size of 1 to 2 mm. 18.7 g of the obtained powdery substance was immersed in an aqueous solution in which 3.9 g of iron (III) sulfate hydrate (containing 24.3 mass% as Fe) was dissolved in 20 g of pure water for 15 hours. Then, after evaporating and drying on a 120 degreeC hotplate, it dried with the 120 degreeC dryer for 1 hour, and also baked at 500 degreeC for 6 hours, and obtained iron tungsten titania.

Tetraamminepalladium nitrate (Pd (NH ₃ ) ₄ (NO ₃ ) ₂ ) 0.0028 g and hexaammineiridium hydroxide (Ir (NH ₃ ) ₆ (OH) ₃ ) aqueous solution (containing 5% by mass as Ir) 0.04 g Were diluted with 9 ml of pure water. In the obtained aqueous solution, 10 g of the iron tungsten titania is immersed, dried on a hot plate at 100 ° C., and further fired at 500 ° C. for 4 hours to obtain 0.01% Pd−0.02% Ir / iron tungsten titania. A catalyst (meaning a catalyst containing 0.01% by mass of Pd and 0.02% by mass of Ir with respect to iron tungsten titania, the same applies hereinafter) was obtained.

4.5 ml of this catalyst was weighed and filled in a quartz reaction tube (inner diameter 20 mm). The catalyst layer temperature was kept at a predetermined temperature in a heating furnace, and a predetermined gas was circulated to evaluate the catalyst activity. The composition (mol%) of the gas used in the test is: 300 ppm nitrogen monoxide, 10% oxygen, 10% water vapor, 500 ppm carbon monoxide, 200 ppm ethylene, 0.5 ppm sulfur dioxide, and 225,300,375,450 ppm ammonia. The balance is helium in (corresponding to [NH ₃ ] / [NO] = 0.75, 1.0, 1.25, 1.5 in the table below, respectively).

First, the temperature of the catalyst layer is 300 to 500 ° C. (300 ° C., 350 ° C., 400 ° C., 450 ° C., and 500 ° C.), [NH ₃ ] / [NO] = 0.75, 1.0 , 1.25 and 1.5 in order, and the initial activity of the catalyst was measured. Next, the gas of [NH ₃ ] / [NO] = 1.0 was passed for 12 hours while maintaining the temperature of the catalyst layer at 500 ° C., and subsequently the temperature of the catalyst layer was changed to [NH at each temperature of 300 to 500 ° C. ₃ ] / [NO] = 0.75, 1.0, 1.25, 1.5. The catalytic activity after the endurance treatment was measured. The gas flow rate was 1.2 liters per minute (volume at 0 ° C., 1 atm) under any conditions (GHSV 16,000 h ⁻¹ ). The nitrogen oxide concentration of the gas before and after the reaction was measured with a chemiluminescent NOx analyzer, and the ethylene and carbon monoxide concentrations were measured with a gas chromatograph, and the NOx, CO, and HC conversion rates were calculated.

Each conversion rate is calculated by the following formula.
NOx conversion rate (%) = 100 × {1- (post-reaction NOx concentration) / (pre-reaction NOx concentration)}
CO conversion rate (%) = 100 × {1- (post-reaction CO concentration) / (pre-reaction CO concentration)}
HC conversion (%) = 100 × {1- (ethylene concentration after reaction) / (ethylene concentration before reaction)}
Here, “before the reaction” means that the measurement is performed upstream of the catalyst with respect to the gas flow direction, and “after the reaction” means that the measurement is performed on the downstream side of the catalyst with respect to the gas distribution direction.

The results are shown in Table 1. As shown in Table 1, under the condition of NH ₃ /NO=1.0, a NOx conversion rate of 80% or more is obtained at a temperature of 450 ° C. or lower, while a CO and HC conversion rate of 90% is obtained at a temperature of 350 ° C. or higher. %. Therefore, in the range of 350 ° C. to 450 ° C., the reduction of nitrogen oxides and the oxidative removal of CO and hydrocarbons can be performed simultaneously.

Further, the ammonia concentration after the reaction was 20 ppm or less under the conditions of NH ₃ /NO=1.25 and 1.5 at 400 ° C. That is, according to the catalyst obtained in Example 1, even when NH ₃ becomes excessive with respect to NOx, leaked ammonia can be suppressed to the minimum.

(Comparative Example 1)
The catalytic activity of iron tungsten titania before supporting palladium and iridium prepared in Example 1 was evaluated in the same manner as in Example 1. The results are shown in Table 2. As shown in Table 2, the catalyst exhibits high activity for reducing nitrogen oxides, but has low CO and hydrocarbon oxidation removal activity. The reason why the CO conversion rate becomes negative is thought to be that CO was generated by incomplete oxidation of ethylene.

(Example 2)
In Example 1, the gas composition (mol%) was: oxygen 10%, water vapor 10%, carbon monoxide 500 ppm, ethylene 200 ppm, sulfur dioxide 0.5 ppm, ammonia 300 ppm, the balance being helium (no nitrogen monoxide) Except for the above, catalytic activity (CO and HC conversion rate and N ₂ and NOx production rate due to ammonia decomposition) was measured in the same manner as in Example 1.

CO and HC are calculated by the above equations, and the N ₂ production rate and the NOx production rate are calculated by the following equations.
N ₂ production rate (%) = 200 × {(post-reaction nitrogen concentration) / (pre-reaction NH ₃ concentration)}
NOx production rate (%) = 100 × {(post-reaction nitrogen oxide concentration) / (pre-reaction NH ₃ concentration)}
The results are shown in Table 3. As shown in Table 3, the catalyst obtained in Example 1 has high ammonia decomposition activity, does not produce a large amount of nitrogen oxides, and is effective for oxidizing combustible components.

(Comparative Example 2)
For the catalyst of Comparative Example 1, the CO and HC conversion rates and the N ₂ and NOx production rates due to ammonia decomposition were measured in the same manner as in Example 2. The results are shown in Table 4. As shown in Table 4, the catalyst obtained in Comparative Example 1 has low ammonia decomposition activity and low oxidation activity of combustible components.

(Comparative Example 3)
60 g of commercially available titanium oxide (MC-90 manufactured by Ishihara Sangyo Co., Ltd.) was immersed in a solution of 15 g of vanadyl sulfate (VOSO ₄ · nH ₂ O; 62% contained as VOSO ₄ ) dissolved in 90 g of pure water for 15 hours. Water was removed with an evaporator, dried in a dryer at 120 ° C., and then calcined in air at 480 ° C. to obtain a titania-vanadia catalyst (BET specific surface area 43 m ² / g). The activity of this catalyst was evaluated in the same manner as in Example 1. However, the temperature at which the gas of [NH ₃ ] / [NO] = 1.0 was circulated for 12 hours was 450 ° C., and the activities at 450 ° C., 400 ° C., 350 ° C., 300 ° C. and 275 ° C. were measured. The results are shown in Table 5. As shown in Table 5, this catalyst had a low CO conversion rate and a negative conversion rate depending on the temperature. This is presumably because CO is generated by incomplete oxidation of ethylene.

(Comparative Example 4)
The titania-vanadia catalyst was loaded with 0.02% (mass basis) of Pt using chloroplatinic acid (H ₂ PtCl ₆ ) to obtain a 0.02% Pt / titania-vanadia catalyst, and its activity Was evaluated in the same manner as in Comparative Example 3. The results are shown in Table 6. As shown in Table 6, the CO removal performance is improved to some extent, but the CO conversion is only about 40% even at 450 ° C.

  INDUSTRIAL APPLICABILITY The present invention can be used to purify combustible components such as nitrogen oxides, carbon monoxide, and hydrocarbons contained in combustion exhaust gas discharged from combustion equipment such as a prime mover or chemical process exhaust gas. Even when an excessive amount of ammonia is added to the nitrogen oxide, the amount of ammonia leaking out is suppressed to a low level, which is particularly useful for the treatment of combustion exhaust gas and process exhaust gas having a large time fluctuation of the nitrogen oxide concentration. In addition, since carbon monoxide and hydrocarbons can be removed by oxidation, harmful components can be simultaneously purified without separately providing an oxidation catalyst, which is excellent in terms of environment and economy.

  In addition, the catalyst of the present invention is effective for removing ammonia and combustible components in the exhaust gas, and can be applied to purification of exhaust gas containing these components. The catalyst of the present invention can be used in combination with a known denitration catalyst. For example, when the exhaust gas is configured to first pass through a known catalyst such as a titanium oxide-vanadium oxide-tungsten oxide catalyst and then to the catalyst of the present invention, the following effects are obtained. First, when the denitration reaction is not completed with the catalyst of the previous stage, the gas leading to the catalyst of the present invention contains nitrogen oxides, ammonia and combustible components. All of that is reduced. When the denitration reaction is completed with the catalyst in the previous stage, but leaked ammonia remains, the gas leading to the catalyst of the present invention contains ammonia and combustible components. According to the catalyst of the present invention, Ammonia and combustible components are reduced and nitrogen oxides are not increased.

Claims (8)

  A catalyst for reducing nitrogen oxides in the presence of ammonia and oxidizing and removing flammable components in an oxygen atmosphere, comprising a support containing titanium oxide, iron and tungsten, from palladium, iridium and rhodium A catalyst supporting at least one selected platinum group metal.   A catalyst for oxidizing ammonia to nitrogen and oxidizing and removing combustible components in an oxygen atmosphere, and a carrier containing titanium oxide, iron and tungsten, and at least one selected from palladium, iridium and rhodium A catalyst carrying a platinum group metal.   The catalyst according to claim 1 or 2, wherein the platinum group metal contains at least palladium, and the proportion of palladium is 0.005 to 0.025% by mass with respect to the support.   The platinum group metal contains palladium and at least one selected from iridium and rhodium, and the ratio of at least one selected from iridium and rhodium is 0.5 to 5 times by mass with respect to palladium. The catalyst according to any one of claims 1 to 3.   Furthermore, the catalyst in any one of Claims 1-4 containing a sulfuric acid component. A method for purifying exhaust gas containing oxygen, in the exhaust gas, after the addition of ammonia and / or a precursor thereof, the flue gas was passed through to claim 1 Symbol placement of the catalyst, the nitrogen oxides in the flue gas and the combustible A method of purifying sexual components.   The method according to claim 6, wherein ammonia and / or a precursor thereof is added 0.75 to 1.25 times in molar ratio of ammonia to nitrogen oxide in the exhaust gas. A method for purifying exhaust gas containing oxygen, the flue gas was passed through to claim 2 Symbol placement of the catalyst, method of ammonia in the exhaust gas purifying combustible components as well as oxidation to nitrogen.