Patent Application
20080196396
The present invention relates to a catalyst which is to be supported on a diesel particulate filter (DPF) and which is used not only for catalytically reducing nitrogen oxides, which are mainly comprised of NO and NO2 and are hereinafter referred to as NOx, contained in diesel engine exhaust gas, using as a reducing agent unburnt carbon contained in the diesel engine exhaust gas and captured by the DPF, in a wide temperature range even in the presence of sulfur oxides; at the same time for catalytically oxidizing the unburnt carbon to remove it. The invention also relates to a method for catalytically reducing NOx using unburnt carbon as a reducing agent and catalytically oxidizing the unburnt carbon to remove it in such a manner as mentioned above.
Exhaust gas emitted from diesel engines contains NOx, and particulate matter, so-called particulates (PM), such as oily particulate matter, carbonaceous particulate matter and sulfuric acid mist, and moreover, hydrocarbons, carbon monoxide, etc. Various methods for purification of such diesel engine exhaust gas by removing those components therefrom have been proposed.
Conventionally, carbonaceous PM contained in diesel engine exhaust gas, i.e., unburnt carbon, is captured by a DPF, thereby being removed from the exhaust gas. As disclosed in JP-A 09-094434 and JP-A 2001-269585, a DPF is normally a honeycomb structure which is made of silicon carbide, cordierite or the like and which has many through-holes (cells) divided by partition walls along the flow direction of exhaust gas, and at both ends of which adjacent through-holes are alternately closed at one end. Exhaust gas which has flown into the DPF through the opening of one through-hole at the inlet side of the honeycomb structure passes a partition wall and is emitted through the opening at the outlet side via the adjacent through-hole. During this process, unburnt carbon is captured by the partition wall.
However, when such a DPF is used, as unburnt carbon is accumulated in the DPF, the pressure loss of the filter increases to have adverse effect on combustion of fuel in an engine, and finally the function of the DPF will be lost. As disclosed in JP-A 08-217565 and JP-A 08-312334, a method has been used in which when the pressure loss of a DPF reaches a predetermined value, rich combustion of fuel is performed to increase the exhaust gas temperature to about 700° C. and thereby unburnt carbon captured is burnt. According to such a method, a DPF can be used while it is reproduced. However, there is a problem that fuel efficiency is decreased because fuel is consumed in order to increase the temperature of DPF when it is reproduced.
In JP-A 2002-004838 and JP-A 2002-058924 proposed is a method in which such a noble metal oxidation catalyst as platinum is arranged in the preceding region of a DPF to produce NO2, thereby promoting combustion of unburnt carbon to lower the reproduction temperature of the filter, or a noble metal oxidation catalyst is supported on a DPF, thereby to similarly lower the reproduction temperature of the filter. It is believed that, in such a method, NOx is also purified to some extent by hydrocarbons and carbon monoxide contained in exhaust gas in the presence of that catalyst.
It is known that by use of a DPF and a noble metal oxidation catalyst in combination, NOx in exhaust gas, especially NO2, promotes combustion of unburnt carbon. On the other hand, however, as disclosed in JP-A 01-318715 and APPLIED CATALYSIS: B50 (2004), 185, it has already been known also that NO2 is only reduced to NO after contributing to the oxidation reaction of unburnt carbon and that the oxidation reaction of unburnt carbon has no contribution to reduction of NOx to nitrogen, namely, reduction in the amount of NOx. Moreover, in the oxidation reaction of unburnt carbon on a noble metal oxidation catalyst such as platinum, oxidation of unburnt carbon proceeds rapidly and unburnt carbon is exhausted promptly. Therefore, even if NOx is purified, the purification reaction will stop instantly and, therefore, the amount of NOx purified by unburnt carbon is very small.
On the other hand, in JP-A 09-094434 and WO02/096827, there is proposed a method in which NOx and unburnt carbon can be removed simultaneously by using a DPF which supports a NOx occlusion reduction catalyst thereon, thereby occluding NOx during lean combustion of fuel and purifying NOx and unburnt carbon during rich combustion of fuel. In such a method, the degree of rich combustion of fuel can be reduced to some extent because unburnt carbon is used for a part of NO2 reduction. It, however, does not fundamentally improve the deterioration of fuel efficiency because it is still necessary to perform rich combustion of fuel.
A catalyst is also proposed for removing NOx and unburnt carbon simultaneously as mentioned above in JP-A 2006-289175. The catalyst comprises a solid superacid and platinum or the like having high oxidizing ability supported thereon. It, however, seems impossible to purify NOx over a wide temperature range because combustion of carbon, carbon monoxide and hydrocarbons proceeds rapidly and, as a result, a reducing agent disappears rapidly.
Moreover, a composite oxide having perovskite structure or spinel structure containing a metal having a high perfectly oxidizing ability and a low electronegativity is proposed as a catalyst for removing NOx and unburnt carbon simultaneously in JP-A 2003-239722 and APPLIED CATALYSIS: B 34 (2004), 29. It, however, seems impossible to purify NOx over a wide temperature range because this catalyst also has a high perfectly oxidizing ability and, therefore, combustion of carbon, carbon monoxide and hydrocarbons proceeds rapidly and, as a result, a reducing agent disappears rapidly. In addition, this catalyst has a problem that the oxidizing ability and reducing ability thereof will be lost in the presence of sulfur oxides because the catalyst contains a metal having a low electronegativity.
Under these circumstances, a catalyst and a method are awaited which can remove NOx and unburnt carbon simultaneously under normal lean driving conditions even in the presence of sulfur oxides without needing rich combustion of fuel to reproduce a DPF, which has been performed. Moreover, in order that such a catalyst and method may be applied to diesel engines, it is strongly desired that a reaction to remove NOx and unburnt carbon simultaneously proceeds over a wide temperature range. The reason for this is that since the exhaust gas temperature of a diesel engine driven under various conditions varies greatly, in order to cause a reaction of simultaneous removal of NOx and unburnt carbon to proceed and, at the same time, to reproduce a DPF, the reaction must proceed over a wide temperature range.
It is, therefore, an object of the invention to provide a catalyst and a method not only for catalytically reducing NOx in diesel engine exhaust gas using unburnt carbon contained therein as a reducing agent in a wide temperature range even in the presence of sulfur oxides; at the same time for catalytically removing the unburnt carbon.
The invention provides a catalyst for catalytically reducing nitrogen oxides in diesel engine exhaust gas by use of unburnt carbon contained in the diesel engine exhaust gas as a reducing agent, the catalyst comprising:
(a) alumina,
(b) alumina supporting an ion and/or an oxide of at least one transition metal selected from the elements of Period 4 of the Periodic Table, or
(c) alumina supporting an aluminate of at least one transition metal selected from the elements of Period 4 of the Periodic Table.
The invention further provides a method for catalytically reducing nitrogen oxides in diesel engine exhaust gas by use of unburnt carbon contained in the diesel engine exhaust gas as a reducing agent, wherein the method comprises bringing the diesel engine exhaust gas into contact with a catalyst comprising:
(a) alumina,
(b) alumina supporting an ion and/or an oxide of at least one transition metal selected from the elements of Period 4 of the Periodic Table, or
(c) alumina supporting an aluminate of at least one transition metal selected from the elements of Period 4 of the Periodic Table.
The catalyst according to the invention is a catalyst for catalytically reducing nitrogen oxides in diesel engine exhaust gas by use of unburnt carbon contained in the diesel engine exhaust gas as a reducing agent. The catalyst comprises:
(a) alumina,
(b) alumina supporting an ion and/or an oxide of at least one transition metal selected from the elements of Period 4 of the Periodic Table, or
(c) alumina supporting an aluminate of at least one transition metal selected from the elements of Period 4 of the Periodic Table.
Since the catalyst according to the invention catalytically reduces nitrogen oxides in diesel engine exhaust gas by use of unburnt carbon in the exhaust gas as a reducing agent as mentioned above, the unburnt carbon in the exhaust gas is also catalytically removed at the same time.
The alumina used in the invention is preferably such that it oxidizes unburnt carbon moderately and exhibits superior reaction selectivity for reactions of NO—C (carbon) or NO—CO (carbon monoxide). As an alumina which exhibits such properties, one having a solid acidity which is mild but is relatively high among the alumina is preferred from the viewpoint of reactivity, and moreover, one having a total content of alkali metal and/or alkaline earth metal of 0.5% by weight or less is preferred. Further, the alumina used in the invention is not particularly restricted with respect to its crystal structure, however, a gamma-form alumina is preferred, which has micropores, many of which have pore diameter of from several nanometers to several tens nanometers or more so that unburnt carbon, many particles of which have a particle diameter of from several nanometers to several tens nanometers, and NOx can readily come into contact together in the micropores of the catalyst and has a high surface area. From such a viewpoint, both PURALOX TH100 and CATALOX HTFa produced by SASOL have a sodium oxide content of 0.002% and therefore are examples of alumina which can be used preferably in the invention.
According to the invention, it is preferable that an ion and/or an oxide of at least one transition metal selected from the elements of Period 4 of the Periodic Table, e.g., Cr, Mn, Fe, Co, Ni, Cu and Zn is supported on alumina so that the resulting alumina catalyst has an increased oxidation-reduction activity and thereby it can catalytically purify exhaust gas effectively from the viewpoint of reaction rate in a purification reaction of diesel engine exhaust gas.
In order to control a resulting catalyst so as not to have an excessive oxidation activity, i.e., in order to control superfluous oxidation of unburnt carbon, it is preferable to support an ion and/or an oxide of the above-mentioned transition metals of Period 4 of the Periodic Table on alumina by any known ion exchange process in which a hydrogen ion on the surface of alumina is exchanged for a metal ion in an aqueous solution. In order to make the amount of ion exchange as great as possible, it is preferable to keep the pH of the aqueous solution containing metal ions high enough so that the metal ions do not precipitate in the form of hydroxide. However, the ion exchange amount of alumina is usually about 1%, expressed in the weight of metal. Therefore, in order to support the transition metal ion and/or oxide on alumina more than that amount, it must be carried thereon by a conventionally known process, such as an impregnation process or an evaporation to dryness process, etc., which have been known. In such cases, the transition metal is carried in the form of oxide on alumina.
For example, when an ion or an oxide of a transition metal is supported on alumina by the ion exchange process, it is usually preferable that the amount of the ion or oxide of the transition metal supported on alumina is within the range of from 0.5 to 1.5% by weight, expressed in the weight of the metal, though it depends not only on the ion exchange capacity of alumina but also on the kind of the transition metal and reaction conditions of the ion-exchange. Similarly, when an oxide of a transition metal is carried on alumina by impregnation or evaporation to dryness, the amount of the oxide carried is preferably within the range of from 0.5 to 5% by weight, expressed in the weight of the metal. If more than 5% by weight of transition metal is carried, the resulting catalyst has an excessively high ability to burn unburnt carbon and therefore the selectivity of NOx reduction is lowered.
An alumina supporting an ion or oxide of a transition metal can be obtained, as previously mentioned, by supporting an ion of a transition metal on alumina by ion exchange, or by carrying a transition metal in the form of oxide on alumina by impregnation or evaporation to dryness, and then calcining it in the air at a temperature of about 500° C.
According to the invention, a particularly preferable catalyst is one in which a transition metal of Period 4 of the Periodic Table is supported on alumina in the form of metal aluminate. Such an alumina that carries a transition metal aluminate can be obtained by supporting a transition metal in the form of oxide on alumina by the known impregnation process or evaporation to dryness process, and then calcining it in the air at a temperature of about 600 to 800° C. In obtaining iron aluminate, calcination in a reductive atmosphere is preferred.
When a transition metal is supported on alumina in the form of metal aluminate, the crystal structure of the metal aluminate is a spinel form. Namely, when a transition metal of Period 4 of the Periodic Table is represented by M, the spinel-form transition metal aluminate is represented by a general formula MAl204. In particular, according to the invention, aluminates of Cu, Co or Ni, each having a relatively high electronegativity among the transition metals of Period 4 of the Periodic Table are preferred.
In such catalysts in which a transition metal of Period 4 of the Periodic Table is carried on alumina in the form of metal aluminate, the amount of transition metal aluminate carried is preferably within the range of from 1 to 5% by weight, expressed in the weight of the metal. In particular, according to the invention, catalysts carrying transition metals having a relatively high electronegativity have more superior NOx purification ability and sulfur oxidation resistance in comparison with conventionally known spinel structures of transition metals having a low electronegativity.
The catalyst according to the invention may contain a support such as silica or inorganic ingredients derived from a binder, if necessary. In this case, it is preferable that the catalyst contains the component (a), (b) or (c) in an amount of at least 75% based on the weight of the catalyst.
The reaction in which nitrogen oxide in diesel engine exhaust gas is catalytically reduced in the presence of the catalyst by using unburnt carbon contained in the diesel engine exhaust gas as a reducing agent and at the same time the unburnt carbon is catalytically removed proceeds as shown by the following formulas:
xC+2NOx (adsorbed on catalyst)→N2+xCO2 (gas phase) (1)
C+O (adsorbed on catalyst)→CO (adsorbed on catalyst) (2-1)
CO (adsorbed on catalyst)+NO (adsorbed on catalyst)→½N2+CO2 (gas phase) (2-2)
The above reaction, however, is accompanied auxiliarily by an oxidation reaction of carbon (C) which has no contribution to the reduction of NOx as shown by the following formulas:
C+O (adsorbed on catalyst)→CO (gas phase) (3-1)
C+2O (adsorbed on catalyst)→C02 (gas phase) (3-2)
As a result, the selective reactivity of the reactions (1) and (2) above mentioned are reduced.
Catalytic diesel particulate filters using NOx occlusion catalysts which have heretofore been proposed are catalysts not for selectively promoting the reaction (1) and the reactions (2-1) and (2-2), but for the reactions (3-1) and (3-2). Moreover, such catalytic diesel particulate filters are not able to purify NOx by use of unburnt carbon in a wide temperature range because the temperature range where unburnt carbon can be burnt is narrow. Composite oxide catalysts having perovskite or spinel structure for simultaneous removal of unburnt carbon and NOx which have been heretofore proposed have a serious problem in practical use that the temperature range where unburnt carbon and NOx can be removed simultaneously is narrow because the catalysts have high oxidizing abilities and therefore they burn unburnt carbon rapidly.
However, the use of the catalyst of the invention makes it possible not only to catalytically reduce nitrogen oxides in diesel engine exhaust gas by using the unburnt carbon contained in the diesel engine exhaust gas as a reducing agent even in the presence of sulfur oxides because the catalyst causes the reaction shown by the formulas (1), (2-1) and (2-2) to proceed selectively; at the same time to remove unburnt carbon.
The reaction temperature suitable for catalytically reducing nitrogen oxides in diesel engine exhaust gas by use of unburnt carbon contained in the diesel engine exhaust gas as a reducing agent by bringing the diesel engine exhaust gas into contact with the catalyst of the invention, which depends not only upon the composition of individual diesel engine exhaust gas but also upon the physical and chemical properties of unburnt carbon, is typically within the range of from 350 to 600° C., and preferably within the range of from 400 to 550° C. In such reaction temperature ranges, exhaust gas is preferably treated at a space velocity within the range of from 5,000 to 100,000 h−1.
According to the invention, nitrogen oxides in diesel engine exhaust gas can be catalytically reduced by using harmful unburnt carbon contained in the diesel engine exhaust gas as a reducing agent, and at the same time the unburnt carbon can be catalytically removed, in a wide temperature range even in the presence of sulfur oxides. In particular, the catalyst of the invention is useful for simultaneous catalytic removal of NOx and unburnt carbon in diesel engine exhaust gas the temperature of which varies greatly, because the catalyst can catalytically remove NOx and unburnt carbon in exhaust gas simultaneously in a wide temperature range.
In other words, the use of the catalyst of the invention makes it possible to purify NOx without accumulation of unburnt carbon in a DPF and without needing to perform rich combustion of fuel, which will lead to decrease in fuel efficiency, as in DPFs supporting NOx occlusion catalysts. Furthermore, the use of the catalyst of the invention makes it possible to catalytically remove NOx and unburnt carbon in diesel engine exhaust gas simultaneously and effectively without being accompanied by catalyst degradation in the presence of sulfur oxides as in complex oxides having perovskite or spinel structure.
Therefore, a catalytic DPF (i.e., CDPF) prepared by supporting the catalyst of the invention on a DPF makes it possible to purify diesel engine exhaust gas in a practical manner.
The invention is described in more detail below with reference to Examples, but the invention is not limited thereto. Hereinafter, all “part” and “%” are on weight basis unless otherwise specified.
By use of carbon black instead of unburnt carbon, a purification reaction of NOx using carbon black as a reducing agent was conducted in the following two procedures.
0.1 g of a catalyst and 0.1 g of carbon black (#7350F produced by Tokai Carbon Co., Ltd., average particle diameter=28 nm, specific surface area=80 m2/g) were placed in a 20-mL sample tube, followed by 50-stroke shaking, thereby preparing a catalyst/carbon black mixture. A SUS 104 mesh having an opening of 0.71 mm was placed on projects on the inner wall of a perpendicularly-arranged reaction tube made of quartz. Ceramic fiber was spread in a thickness of about 1 mm on the mesh and the catalyst/carbon mixture was placed on the fiber. Ceramic fiber was then spread in a thickness of about 1 mm on the mixture in order to prevent the mixture from scattering.
The temperature of the mixture was raised from 30° C. to 700° C. at a rate of 5° C./min while exhaust gas for test composed of 500 ppm of nitrogen monoxide (NO), 9% of oxygen, 3% of water, 500 ppm of hydrogen, 5 ppm of sulfur dioxide (SO2) and the remainder helium was fed at a rate for 834 mL/min into the quartz reaction tube from the entrance thereof. The composition of the gas emitted from an outlet of the quartz reaction tube was then analyzed about nitrogen monoxide (NO), dinitrogen oxide (N2O), nitrogen dioxide (NO2), carbon monoxide (CO) and carbon dioxide (CO2) with an FTIR gas analyzer (Gasmet CR-2000L produced by Temet). The gas composition analysis was started when the temperature of the mixture was at 100° C.
A temperature range between a temperature at which the concentration of carbon dioxide (C02) in the outlet gas determined by the gas analyzer exceeds 0.1% and a temperature at which the concentration becomes lower than 0.1% is defined as a temperature range of carbon dioxide (C02) generation, that is, a temperature range of carbon black combustion. The amount of NOx purified, which may vary depending upon the catalyst used, was calculated using the following formula for the temperature range of carbon black combustion:
Amount (cc) of NOx purified=abd/c
In this formula, a represents (average concentration (ppm) of purified NOx in a temperature range of carbon black combustion)×10−6, that is, (NOx concentration at the reaction tube inlet—average NOx concentration at the reaction tube outlet in the temperature range of carbon black combustion (ppm))×10−6; b represents a temperature range (° C.) of carbon black combustion; c represents a temperature raising rate (5° C./min); and d represents a gas flow rate (834 cc/min).
The combustion ratio (%) of C (carbon) was calculated using the following formula:
Combustion ratio (%) of C (carbon)=ab/c
In this formula, a represents (average CO concentration (%)+average CO2 concentration (%))/100 in a temperature range where CO2 was generated in a purification reaction; and b represents the total amount of gas flow in the temperature range of carbon black combustion. That is, b is represented using a formula: temperature range of carbon black combustion (° C.)/temperature raising rate (5° C./min)×amount of gas flow (834 cc/min).
c represents the amount of carbon used in the purification reaction (on gas basis), that is, (0.1 g (amount of carbon used in the purification reaction test (on weight basis)/12 g)×22400 cc.
0.4 g of a catalyst and 0.1 g of the same carbon black as that mentioned above were lightly mixed using an agate mortar to prepare a catalyst/carbon black mixture. A SUS 104 mesh having an opening of 0.71 mm was placed on projects on the inner wall of a perpendicularly-arranged reaction tube made of quartz. Ceramic fiber was spread in a thickness of about 1 mm on the mesh and the catalyst/carbon mixture was placed on the fiber. Ceramic fiber was then spread in a thickness of about 1 mm on the mixture in order to prevent the mixture from scattering.
The temperature of the mixture was increased to a predetermined temperature with helium gas being supplied into the quartz reaction tube at a rate of 500 mL/min from the inlet thereof. After arrival at a constant temperature, exhaust gas for test with the same composition as that mentioned hereinbefore was supplied into the quartz reaction tube at a rate of 834 mL/min from the inlet thereof, and simultaneously, the composition of gas emitted from the outlet of the quartz reaction tube was analyzed in the same manner as mentioned hereinbefore. The purification reaction of NOx was performed for 15 minutes. The NOx purification percentage during the 15-minute isothermal reaction was determined on the basis of the NOx determined in a blank test.
Alumina (CATALOX HTFa produced by SASOL, specific surface area=105 m2/g, average pore diameter=15 nm, Na2O content=0.002%) was ground for one minute with an agate mortar. Using the resultant ground matter, a purification reaction test of exhaust gas using a temperature raising reaction was conducted.
To 20 mL of ion exchange water containing 2.50 g of nickel nitrate (Ni(NO3)3.6H20) dissolved, 5 g of alumina used in Example 1 was added, followed by evaporation to dryness at 70° C. under stirring on a hot stirrer. The resultant dried matter was calcined at 800° C. in the air to yield alumina carrying nickel aluminate in an amount of 5% by weight expressed in weight of the metal. This was ground for one minute with an agate mortar. Using the resultant ground matter, purification reaction tests of exhaust gas using a temperature raising reaction and an isothermal reaction were conducted.
To 20 mL of ion exchange water containing 1.25 g of nickel nitrate (Ni(NO3)3.6H2O) dissolved, 5 g of alumina used in Example 1 was added, followed by evaporation to dryness at 70° C. under stirring on a hot stirrer. The resultant dried matter was calcined at 800° C. in the air to yield alumina carrying nickel aluminate in an amount of 2.5% by weight expressed in weight of the metal. This was ground for one minute with an agate mortar. Using the resultant ground matter, purification reaction tests of exhaust gas using a temperature raising reaction and an isothermal reaction were conducted.
To 20 mL of ion exchange water containing 2.45 g of cobalt nitrate (Co(NO3)3.6H2O) dissolved, 5 g of alumina used in Example 1 was added, followed by evaporation to dryness at 70° C. under stirring on a hot stirrer. The resultant dried matter was calcined at 800° C. in the air to yield alumina carrying cobalt aluminate in an amount of 5% by weight expressed in weight of the metal. This was ground for one minute with an agate mortar. Using the resultant ground matter, purification reaction tests of exhaust gas using a temperature raising reaction and an isothermal reaction were conducted.
To 100 mL of ion exchange water containing 0.38 g of copper nitrate (Cu(NO3)3.3H2O) dissolved, 5 g of alumina used in Example 1 was added, followed by ion exchange at 70° C. under stirring on a hot stirrer while preventing evaporation of water. Then, the treated alumina was collected by filtration, washed with water, and dried at 80° C. overnight. Thereafter, the resultant dried matter was calcined at 500° C. for one hour in the air to yield 1% by weight Cu ion-exchanged alumina. This was ground for one minute with an agate mortar. Using the resultant ground matter, a purification reaction test of exhaust gas using a temperature programmed reaction was conducted.
To 20 mL of ion exchange water containing 3.70 g of ferric nitrate (Fe(NO3)3.9H2O) dissolved, 5 g of alumina used in Example 1 was added, followed by evaporation to dryness at 70° C. under stirring on a hot stirrer. The resultant dried matter was calcined at 800° C. in an air flow containing 10% of hydrogen to yield alumina carrying iron aluminate in an amount of 5% by weight expressed in weight of the metal. This was ground for one minute with an agate mortar. Using the resultant ground matter, a purification reaction test of exhaust gas using a temperature raising reaction was conducted.
To 200 mL of ion exchange water containing 1.00 g of aqueous palladium nitrate solution (Pd content=5% by weight) dissolved, 5 g of NH4-beta zeolite (BEA-25 produced by Sud-Chemie AG, silica/alumina ratio=25, Na content=0.1%) was added, followed by ion exchange at 70° C. for 12 hours under stirring. Thus, 1% by weight Pd-exchanged beta zeolite was obtained. This was ground for one minute with an agate mortar. Using the resultant ground matter, a purification reaction test of exhaust gas using a temperature raising reaction was conducted.
For each of the catalysts obtained in Examples 1 to 6 and Referential Example 1, the result of the purification reaction of exhaust gas using the temperature raising reaction is shown in Table 1. For each of the catalysts obtained in Examples 2, 4 and 7 and Referential Example 1, the result of the purification reaction of exhaust gas using the isothermal reaction is shown in Table 2.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2007-004949 | Jan 2007 | JP | national |
