JP2008168228A - Catalyst and method for purifying nitrogen oxides in diesel engine exhaust gas using unburned carbon - Google Patents

Abstract

The present invention provides a catalyst for simultaneously purifying and removing nitrogen oxides and unburned carbon contained in diesel engine exhaust gas in a wide temperature range even in the presence of sulfur oxides.
According to the present invention, according to the present invention,
(A) alumina or (b) alumina supporting at least one transition metal ion and / or oxide selected from the fourth period of the periodic table or (c) at least one selected from the fourth period of the periodic table Provided is a catalyst for catalytic reduction of nitrogen oxides in diesel engine exhaust gas using unburned carbon contained in the exhaust gas of the diesel engine characterized by comprising transition metal aluminate as a reducing agent.
[Selection figure] None

Description

The present invention uses a diesel particulate filter (DPF) supported by a diesel engine and uses unburned carbon in diesel engine exhaust gas captured by the DPF as a reducing agent in a wide temperature range even in the presence of sulfur oxides. The present invention relates to a catalyst and a method for catalytically reducing nitrogen oxides (mainly composed of NO and NO ₂ , hereinafter referred to as NOx) in exhaust gas and simultaneously oxidizing and removing the unburned carbon.

  Exhaust gas discharged from diesel engines contains NOx, particulate matter such as oil, carbon, and sulfuric acid mist, so-called particulates (PM), as well as hydrocarbons and carbon monoxide. So far, various methods for removing and purifying the above components from such diesel engine exhaust gas have been proposed.

  Carbonaceous PM in diesel engine exhaust gas, that is, unburned carbon, is conventionally captured by DPF and removed from the exhaust gas. A DPF is usually a honeycomb structure made of silicon carbide, cordierite, etc. and having a large number of through-holes (cells) partitioned by partition walls in the flow direction of the exhaust gas. The exhaust gas flowing into the DPF from the opening of one through hole on the inlet side of the honeycomb structure has a structure sealed at one end face (see, for example, Patent Documents 1 and 2). It passes through the adjacent through-holes and is discharged from the opening on the outlet side, during which unburned carbon is captured by the partition walls.

  However, according to such a DPF, as the unburned carbon accumulates in the DPF, the pressure loss of the filter increases, which adversely affects the combustion of fuel in the engine. Finally, the function of the DPF itself is lost. So far, when the pressure loss of the DPF reaches a predetermined value, a method of performing rich combustion of the fuel to raise the exhaust gas temperature to about 700 ° C. and burning the captured unburned carbon has been adopted. (See Patent Documents 3 and 4). According to such a method, the DPF can be used while being regenerated. On the other hand, there is a problem that fuel consumption is deteriorated because fuel is consumed for raising the temperature during regeneration.

Therefore, a noble metal oxidation catalyst such as platinum is arranged in front of the DPF to generate NO ₂ , thus promoting combustion of unburned carbon, lowering the regeneration temperature of the filter, or noble metal oxidation to the DPF. There has been proposed a method in which a catalyst is supported and the regeneration temperature of the filter is similarly lowered (see Patent Documents 5 and 6). In such a method, it is said that NOx is somewhat purified by hydrocarbons and carbon monoxide contained in the exhaust gas in the presence of the catalyst.

Thus, by using a combination of DPF and a noble metal oxidation catalyst, NOx in the exhaust gas, among others, although NO ₂ is known to promote the combustion of unburned carbon, however, on the other hand, NO ₂ Is only reduced to NO after participating in the oxidation reaction of unburned carbon, and the oxidation reaction of unburned carbon does not contribute to the reduction of NOx to nitrogen, that is, NOx reduction at all. Are known. Furthermore, in the oxidation reaction of unburned carbon on a noble metal oxidation catalyst such as platinum, the oxidation of unburned carbon proceeds rapidly and the unburned carbon is consumed quickly, so that NOx is temporarily purified. Even so, the purification reaction stops instantaneously, and thus the amount of NOx purified by unburned carbon is very small (see Patent Document 7 and Non-Patent Document 1).

  On the other hand, a method of supporting NOx storage and reduction catalyst on the DPF, storing NOx at the time of lean combustion, and purifying NOx and unburned carbon at the time of rich combustion, so that NOx and unburned carbon can be simultaneously removed. Has been proposed (see Patent Documents 1 and 8). However, in such a method, since unburned carbon is used as part of the NOx reduction, the degree of fuel rich combustion can be somewhat reduced, but it is still necessary to perform rich combustion. It does not essentially improve the deterioration of fuel consumption.

  Further, as described above, as a catalyst for simultaneously removing NOx and unburned carbon, a catalyst in which platinum having a high oxidizing ability is supported on a solid super strong acid has been proposed (see Patent Document 9). Since combustion of carbon monoxide and hydrocarbons proceeds rapidly and the reducing agent disappears rapidly, it seems impossible to purify NOx over a wide temperature range.

  Further, as a catalyst for simultaneously removing NOx and unburned carbon, a composite oxide having a perovskite structure or a spinel structure containing a metal having a high complete oxidation ability and a low electronegativity has been proposed (Patent Document 10 and Non-Patent Document 10). Reference 2). However, this catalyst also has a high complete oxidation ability, and combustion of carbon, carbon monoxide and hydrocarbons proceeds rapidly, and the reducing agent disappears rapidly. Therefore, it is not possible to purify NOx over a wide temperature range. It seems impossible. In addition, since this catalyst contains a metal having a low electronegativity, it also has a problem that its oxidizing ability and reducing ability are lost in the presence of sulfur oxide.

Under these circumstances, in order to regenerate the DPF, the rich combustion of fuel that has been performed so far is not required, and NOx and unburned fuel can be used under normal lean operation conditions even in the presence of sulfur oxides. There is a need for a catalyst and method that can simultaneously remove carbon. Furthermore, it is strongly desired that the simultaneous removal reaction of NOx and unburned carbon proceeds in a wide temperature range so that such a catalyst and method can be applied to a diesel engine. This is because the exhaust gas temperature of a diesel engine operated under various conditions fluctuates greatly, and during the operation of the diesel engine, the simultaneous removal reaction of NOx and unburned carbon proceeds and the DPF is regenerated. This is because the reaction needs to proceed in a wide temperature range.
Japanese Patent Laid-Open No. 09-094434 JP 2001-269585 A Japanese Patent Application Laid-Open No. 08-217565 Japanese Patent Application Laid-Open No. 08-31334 JP 2002-004838 A JP 2002-058924 A Japanese Patent Laid-Open No. 01-318715 International Publication No. 02/096827 JP 2006-289175 A JP 2003-239722 A APPLIED CATALYSIS: B 50 (2004), 185 APPLIED CATALYSIS: B 34 (2004), 29

  The present invention uses unburned carbon contained in diesel engine exhaust gas as a reducing agent to catalytically reduce NOx in exhaust gas in a wide temperature range even in the presence of sulfur oxides. It is an object of the present invention to provide a catalyst and method for catalytic removal.

According to the present invention,
(A) alumina or (b) alumina supporting at least one transition metal ion and / or oxide selected from the fourth period of the periodic table or (c) at least one selected from the fourth period of the periodic table There is provided a catalyst for catalytic reduction of nitrogen oxides in diesel engine exhaust gas using unburned carbon contained in diesel engine exhaust gas as a reducing agent, characterized by comprising alumina carrying a transition metal aluminate.

Further, according to the present invention, the diesel engine exhaust gas is made of (a) alumina or (b) alumina carrying at least one transition metal ion and / or oxide selected from the fourth period of the periodic table, or (c). In contact with a catalyst made of alumina carrying aluminate of at least one transition metal selected from the fourth period of the periodic table, unburned carbon contained in diesel engine exhaust gas is used as a reducing agent in diesel engine exhaust gas. A method for catalytic reduction of nitrogen oxides is provided.

  According to the present invention, harmful unburned carbon contained in diesel engine exhaust gas is used as a reducing agent, and in the presence of sulfur oxides, NOx in exhaust gas is catalytically reduced over a wide temperature range, and Burning carbon can be removed simultaneously in contact. In particular, since the catalyst of the present invention can catalytically remove NOx and unburned carbon in exhaust gas simultaneously over a wide temperature range, NOx and unburned carbon in diesel engine exhaust gas whose temperature fluctuates greatly. Useful for removing at the same time.

  That is, according to the catalyst of the present invention, there is no accumulation of unburned carbon in the DPF, and there is no need to perform rich combustion of fuel that causes a reduction in fuel consumption, as in the DPF carrying the NOx storage catalyst. NOx can be purified. Furthermore, NOx and unburned carbon in diesel engine exhaust gas can be effectively removed catalytically and simultaneously without catalyst deterioration in the presence of sulfur oxides, such as perovskite and composite oxides having a spinel structure. it can.

  Therefore, the diesel engine exhaust gas can be practically purified as a catalytic DPF (CDPF) by supporting the catalyst according to the present invention on the DPF.

The catalyst according to the present invention is a catalyst for catalytic reduction of nitrogen oxides in diesel engine exhaust gas using unburned carbon contained in the diesel engine exhaust gas as a reducing agent,
(A) alumina or (b) alumina supporting at least one transition metal ion and / or oxide selected from the fourth period of the periodic table or (c) at least one selected from the fourth period of the periodic table It consists of alumina carrying a transition metal aluminate.

  As described above, the catalyst according to the present invention catalytically reduces nitrogen oxides in exhaust gas by using unburned carbon in diesel engine exhaust gas as a reducing agent. Removed.

  As the alumina used in the present invention, it is preferable to use a material that oxidizes unburned carbon appropriately and has excellent reaction selectivity of NO-C (carbon) or NO-CO (carbon monoxide). As the alumina exhibiting such characteristics, from the viewpoint of reactivity, the solid acidity is mild, but relatively high alumina is preferable, and the content of alkali metal and / or alkaline earth metal is also preferred. Are preferably 0.5% by weight or less in total. In addition, the alumina used in the present invention is not particularly limited in its crystal structure, but unburned carbon having a particle diameter of many particles of about several nanometers to several tens of nanometers and NOx are easily in contact within the pores of the catalyst. In order to achieve this, a gamma-type alumina having a pore diameter of many nanometers to several tens of nanometers or more and a high surface area is preferable. From such a point of view, for example, PURALOX TH100 and CATALOX HTFa manufactured by SASOL are both examples of alumina that can be preferably used in the present invention because the sodium oxide content is 0.002%. is there.

  According to the present invention, in the purification reaction of diesel engine exhaust gas, the oxidation-reduction activity of the catalyst is increased, and the exhaust gas can be effectively and catalytically purified from the viewpoint of the reaction rate. It is preferable to support at least one transition metal selected from the period, for example, ions and / or oxides such as Cr, Mn, Fe, Co, Ni, Cu, and Zn.

  Here, the transition metal ions and / or oxides of the fourth period are supported on alumina so that the obtained catalyst does not have excessive oxidation activity, that is, to suppress excessive oxidation of unburned carbon. In order to perform this, it is preferable to carry out by a conventionally known ion exchange method in which surface hydrogen ions and metal ions of alumina are ion-exchanged in an aqueous solution. At this time, in order to increase the amount of ion exchange as much as possible, it is preferable to keep the pH of the aqueous solution containing metal ions so high that metal ions do not precipitate as hydroxides. However, since the ion exchange amount of alumina is usually about 1% in terms of the weight of the metal, it is conventionally known to support the transition metal ions and / or oxides in an amount exceeding that amount. In this case, the transition metal is supported on alumina in the form of an oxide.

  For example, when the transition metal ions or oxides are supported on alumina by an ion exchange method, it depends on not only the ion exchange capacity of alumina but also the type of transition metal and the reaction conditions for ion exchange. It is preferable that the transition metal ions or oxides to be supported in the range of 0.5 to 1.5% by weight in terms of metal. Similarly, when the transition metal oxide is supported on alumina by impregnation or evaporation to dryness, the supported amount is preferably in the range of 0.5 to 5% by weight in terms of metal. When the transition metal is supported in an amount exceeding 5% by weight, the flammability of the unburnt carbon of the resulting catalyst is excessively increased, so that the selectivity for NOx reduction is lowered.

  In order to obtain an alumina carrying transition metal ions or oxides, as described above, the transition metal ions are carried on alumina by an ion exchange method, or the transition metal ions are oxidized by an impregnation method or evaporation to dryness method. And then fired at a temperature of about 500 ° C. in air.

  According to the present invention, a particularly preferred catalyst is one in which the fourth period transition metal is supported on alumina in the form of a metal aluminate. In order to obtain such an alumina carrying a transition metal aluminate, after the transition metal is supported on the alumina in the form of an oxide by a conventionally known impregnation method or evaporation to dryness, in the air, 600 What is necessary is just to bake at about 800 degreeC high temperature. However, when obtaining Fe aluminate, it is preferable to bake in a reducing atmosphere.

When the transition metal is supported on alumina in the form of a metal aluminate, the crystal structure of the metal aluminate is a spinel type. That is, if the transition metal in the fourth period is represented by M, the spinel type transition metal aluminate is represented by the general formula MAl ₂ O ₄ . In particular, according to the present invention, among the transition metals in the fourth period, alumina of Cu, Co or Ni having a relatively high electronegativity is preferable.

  Thus, in the catalyst in which the transition metal of the fourth period is supported on alumina in the form of metal aluminate, the supported amount of transition metal aluminate is preferably in the range of 1 to 5% by weight in terms of metal. In particular, according to the present invention, a catalyst loaded with a transition metal having a relatively high electronegativity is superior to a conventionally known transition metal spinel structure having a low electronegativity in terms of NOx purification ability. And has sulfur oxide resistance.

  The catalyst according to the present invention may contain an inorganic component derived from a carrier such as silica or a binder as necessary. In such a case, the component (a), (b) or (c) is used. Preferably at least 75% based on the weight of the catalyst.

  Using the unburned carbon contained in the diesel engine exhaust gas as a reducing agent, the reaction for catalytically reducing the nitrogen oxides in the exhaust gas of the diesel engine in the presence of the above-mentioned catalyst and removing the unburned carbon in a contact manner, Next formula

xC + 2NO _x (adsorbed on the catalyst) → N ₂ + xCO ₂ (gas phase) (1)
C + O (adsorbed on the catalyst) → CO (adsorbed on the catalyst) (2-1)
CO (adsorbed on the catalyst) + NO (adsorbed on the catalyst) →
1 / 2N ₂ + CO ₂ (gas phase) (2-2)
Proceed by.

However, the above reaction is usually performed by the following formula: C + O (adsorbed on the catalyst) → CO (gas phase) (3-1)
C + 2O (adsorbed on the catalyst) → CO ₂ (gas phase) (3-2)
As shown in FIG. 2, since the carbon (C) oxidation reaction that does not participate in the reduction of NOx is accompanied by a secondary reduction, the selective reactivity of the above (1) and (2) is reduced.

  A catalytic diesel particulate filter using a NOx occlusion catalyst that has been proposed so far is a catalyst for the reactions (3-1) and (3-2) described above. It is not a catalyst that selectively proceeds the reaction of (2-1) and (2-2). In addition, according to the catalytic diesel particulate filter, since the temperature range in which unburned carbon burns is narrow, NOx cannot be purified over a wide temperature range using unburned carbon. In addition, the perovskite or spinel structure composite oxide catalyst for simultaneous removal of unburned carbon and NOx proposed so far has high oxidation ability and rapidly burns unburned carbon. There is an important practical problem that the temperature range in which NOx can be removed simultaneously is narrow.

  However, according to the catalyst of the present invention, the reactions according to the above formulas (1), (2-1), and (2-2) proceed selectively, so that unburned carbon contained in diesel engine exhaust gas is used as a reducing agent. It is possible to catalytically reduce nitrogen oxides in diesel engine exhaust gas and simultaneously remove the unburned carbon even in the presence of sulfur oxides.

According to the present invention, the diesel engine exhaust gas is brought into contact with the above-described catalyst, and the unburned carbon contained in the diesel engine exhaust gas is used as a reducing agent to suitably reduce the nitrogen oxides in the diesel engine exhaust gas. The reaction temperature is usually in the range of 350 to 600 ° C., preferably in the range of 400 to 550 ° C., depending on not only the composition of the individual diesel engine exhaust gas but also the physical and chemical characteristics of the unburned carbon. It is. In such a reaction temperature range, the exhaust gas is preferably treated at a space velocity in the range of 5,000 to 100,000 h ⁻¹ .

  EXAMPLES The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples. In the following, all “parts” and “%” are by weight unless otherwise specified.

  Carbon black was used as a substitute for unburned carbon, and NOx purification reaction using this as a reducing agent was carried out by the following two methods.

(1) Exhaust gas purification reaction by temperature rising reaction 0.1 g of catalyst and 0.1 g of carbon black (Tokai Carbon Co., Ltd. # 7350F, average particle size 28 nm, specific surface area 80 m ^{2 /} g) in a 20 mL capacity sample bottle The catalyst / carbon black mixture was prepared by charging and shaking 50 times. A SUS104 mesh with a mesh opening of 0.71 mm is placed on a projection provided on the inner wall of a quartz reaction tube installed vertically, and a ceramic fiber is laid down to a thickness of about 1 mm on it, and the catalyst / carbon black is placed thereon. The mixture was placed, and ceramic fibers were spread over the mixture to a thickness of about 1 mm to prevent scattering.

Nitrogen monoxide (NO) 500 ppm, oxygen 9%, water 3%, hydrogen 500 ppm, sulfur dioxide (SO ₂ ) 5 ppm and the remainder and the balance helium at a rate of 834 mL / min from the inlet of the quartz reaction tube The temperature of the mixture is increased from 30 ° C. to 700 ° C. at a rate of 5 ° C./min, and the composition of the gas from the outlet of the quartz reaction tube is changed to nitrogen monoxide (NO), Nitrogen (N ₂ O), nitrogen dioxide (NO ₂ ), carbon monoxide (CO) and carbon dioxide (CO ₂ ) were analyzed using an FTIR gas analyzer (Gamet CR-2000L manufactured by Temet). The analysis of the gas composition was performed from the time when the temperature of the mixture was 100 ° C. The purification amount of NOx varies depending on the catalyst to be used, but carbon dioxide (CO ₂ ) between the temperature at which the concentration of carbon dioxide (CO ₂ ) in the outlet gas exceeds 0.1% and the temperature below 0.1% is measured by a gas analyzer. As the temperature range of CO ₂ ), that is, the temperature range in which carbon black burns, the NOx purification amount was calculated from the following equation for the temperature range in between.

NOx purification amount (cc) = abd / c
Here, a is (average purified NOx concentration (ppm) in the combustion temperature range of carbon black) × 10 ⁻⁶ , in other words, (NOx concentration at reaction tube inlet−reaction tube in combustion temperature range of carbon black) The average NOx concentration at the outlet (ppm) × 10 ⁻⁶ , b is the combustion temperature range (° C.) of carbon black, c is the heating rate (5 ° C./min), and d is the gas flow rate ( 834 cc / min).

  Moreover, C (carbon) combustion rate (%) was calculated | required by following Formula.

C (carbon) combustion rate (%) = ab / c
Here, a is (average CO concentration (%) + average CO ₂ concentration (%)) / 100 in the temperature range where CO ₂ is generated in the purification reaction, and b is the total gas flow rate in the combustion temperature range of carbon black. It is. That is, b is expressed by the following formula: carbon black combustion temperature range (° C.) / Temperature increase rate (5 ° C./min)×gas flow rate (834 cc / min).

  c is the amount of carbon subjected to the purification reaction (gas basis), that is, (0.1 g (amount of carbon subjected to purification reaction (weight basis) / 12 g) × 22400 cc.

(2) Exhaust gas purification reaction by isothermal reaction A catalyst / carbon black mixture was prepared by lightly mixing 0.4 g of the catalyst and 0.1 g of the same carbon black using a smoked mortar. A SUS104 mesh with a mesh opening of 0.71 mm is placed on a projection provided on the inner wall of a quartz reaction tube installed vertically, and a ceramic fiber is laid down to a thickness of about 1 mm on it, and the catalyst / carbon black is placed thereon. The mixture was placed, and ceramic fibers were spread over the mixture to a thickness of about 1 mm to prevent scattering.

  While the helium gas was supplied from the inlet of the quartz reaction tube at a rate of 500 mL / min, the temperature of the mixture was raised to a predetermined temperature. After reaching constant temperature, the test exhaust gas having the same composition as above was supplied from the inlet of the quartz reaction tube at a rate of 834 mL / min, and the composition of the gas from the outlet of the quartz reaction tube was the same as described above. And analyzed. The NOx purification reaction was carried out for 15 minutes. The NOx purification rate during the 15-minute isothermal reaction was performed based on NOx determined by a blank test.

  Catalysts were prepared according to the following examples and reference examples, and the exhaust gas purification reaction test was performed using each catalyst. The results are shown in Table 2.

Example 1
Alumina (Catalox HTFa manufactured by SASOL, specific surface area 105 m ² / g, average pore diameter 15 nm, Na ₂ O content 0.002%) was pulverized for 1 minute using a smoked mortar, and the pulverized material obtained was used. Then, a purification reaction test of exhaust gas by temperature rising reaction was conducted.

Example 2
5 g of alumina used in Example 1 was added to 20 mL of ion-exchanged water in which 2.50 g of nickel nitrate (Ni (NO ₃ ) ₃ · 6H ₂ O) had been dissolved, and evaporated to dryness at 70 ° C. while stirring on a hot stirrer. The obtained dried product was fired in air at 800 ° C. for 1 hour to obtain 5 wt% Ni-supported nickel aluminate. This was pulverized for 1 minute using a smoked mortar, and the pulverized product thus obtained was used to conduct an exhaust gas purification reaction test by temperature rising reaction and isothermal reaction.

Example 3
5 g of alumina used in Example 1 was added to 20 mL of ion-exchanged water in which 1.25 g of nickel nitrate (Ni (NO ₃ ) ₃ · 6H ₂ O) was dissolved, and the mixture was evaporated to dryness at 70 ° C. with stirring on a hot stirrer. The obtained dried product was fired in air at 800 ° C. for 1 hour to obtain 2.5 wt% Ni-supported nickel aluminate. This was pulverized for 1 minute using a smoked mortar, and the pulverized product thus obtained was used to conduct an exhaust gas purification reaction test by temperature rising reaction and isothermal reaction.

Example 4
5 g of alumina used in Example 1 was added to 20 mL of ion-exchanged water in which 2.45 g of cobalt nitrate (Co (NO ₃ ) ₃ · 6H ₂ O) was dissolved, and the mixture was evaporated to dryness at 70 ° C. with stirring on a hot stirrer. The obtained dried product was fired in air at 800 ° C. for 1 hour to obtain 5 wt% Co-supported cobalt aluminate. This was pulverized for 1 minute using a smoked mortar, and the pulverized product thus obtained was used to conduct an exhaust gas purification reaction test by temperature rising reaction and isothermal reaction.

Example 5
5 g of alumina used in Example 1 is added to 100 mL of ion-exchanged water in which 0.38 g of copper nitrate (Cu (NO ₃ ) ₃ .3H ₂ O) is dissolved, and the evaporation of moisture is prevented while stirring on a hot stirrer. However, ion exchange was performed at 70 ° C. Thereafter, the alumina thus treated was filtered, washed with water, and dried at 80 ° C. overnight. Thereafter, the obtained dried product was calcined in air at 500 ° C. for 1 hour to obtain 1 wt% Cu ion exchange supported alumina. This was pulverized for 1 minute using a smoked mortar, and the obtained pulverized product was used to conduct an exhaust gas purification reaction test using a temperature rising reaction.

Example 6
Alumina 5g was charged using the iron nitrate _{(Fe (NO 3) 3 ·} 9H 2 O) 3.70g exemplary ion exchange water 20mL dissolved Example 1, under stirring on a hot stirrer, evaporated to dryness at 70 ° C. The obtained dried product was fired in a 10% hydrogen stream at 800 ° C. for 1 hour to obtain a 5 wt% Fe-supported iron aluminate. This was pulverized for 1 minute using a smoked mortar, and the obtained pulverized product was used to conduct an exhaust gas purification reaction test using a temperature rising reaction.

Reference example 1
NH ₄ beta zeolite (BEA-25 manufactured by Zude Chemie, silica / alumina ratio = 25, Na content = 0.1%) in 200 mL of ion-exchanged water in which 1.00 g of palladium nitrate aqueous solution (Pd concentration 5% by weight) was dissolved. ) 5 g was charged and ion exchanged at 70 ° C. for 12 hours with stirring to obtain 1 wt% Pd-supported beta zeolite. This was pulverized for 1 minute using a smoked mortar, and the obtained pulverized product was used to conduct an exhaust gas purification reaction test using a temperature rising reaction.

Claims (5)

(A) alumina or (b) alumina supporting at least one transition metal ion and / or oxide selected from the fourth period of the periodic table or (c) at least one selected from the fourth period of the periodic table A catalyst for catalytic reduction of nitrogen oxides in diesel engine exhaust gas using unburned carbon contained in diesel engine exhaust gas as a reducing agent, comprising alumina supporting transition metal aluminate.   The catalyst according to claim 1, wherein the at least one transition metal selected from the fourth period of the periodic table is at least one selected from Cr, Mn, Fe, Co, Ni, Cu and Zn. A catalytic diesel particulate filter obtained by supporting the catalyst according to claim 1 on a diesel particulate filter. Diesel engine exhaust gas is selected from (a) alumina or (b) alumina loaded with ions and / or oxides of at least one transition metal selected from the fourth period of the periodic table, or (c) selected from the fourth period of the periodic table To contact with a catalyst made of alumina supporting at least one transition metal aluminate, and catalytically reduce nitrogen oxides in diesel engine exhaust gas using unburned carbon contained in diesel engine exhaust gas as a reducing agent Method. 5. The method according to claim 4, wherein the at least one transition metal selected from the fourth period of the periodic table is at least one selected from Cr, Mn, Fe, Co, Ni, Cu and Zn.