Exhaust gas cleaner and method of cleaning exhaust gas

BACKGROUND OF THE INVENTION
The present invention relates to an exhaust gas cleaner and a method of cleaning an exhaust gas by using such an exhaust gas cleaner, and more particularly to an exhaust gas cleaner capable of efficiently removing nitrogen oxides and fine carbonaceous particles simultaneously from exhaust gases of diesel engines, etc., and a method of cleaning an exhaust gas by using such an exhaust gas cleaner.
Recently, fine carbonaceous particles (hereinafter referred to simply as "particulate matter") and nitrogen oxides (hereinafter referred to simply as "NOx") contained in exhaust gases of diesel engines, etc. are causing environmental problems. In particular, the particulate matter having an average particle size of 0.1-1 .mu.m is likely to float in the air and be inhaled by breathing. As a result of recent clinical tests, it is confirmed that the particulate matter contains carcinogenic substances.
As a method for removing the particulate matter, there are two candidates:
One method comprises the steps of trapping the particulate matter in heat-resistant filters by filtrating exhaust gases, and burning the trapped particulate matter by a burner, an electrical heater, etc. when a pressure drop increases due to the particulate matter accumulated, to regenerate filters. The heat resistant filters may be honeycomb-type ceramic filters, foam-type ceramic filters having three-dimensional network structures, steel wool, wire mesh, etc. The other method comprises the step of trapping and self-burning the particulate matter by the action of catalysts supported by the above filters.
In the former method, as the efficiency for removing the particulate matter increases, the pressure drop increases more quickly, meaning that the filters are required to be regenerated more frequently with a high reliability, leading to an economical disadvantage.
In contrast, the latter method is considered to be excellent as long as a proper catalyst exists, which is capable of maintaining a catalytic activity under the conditions of the exhaust gases of diesel engines with which the catalyst is brought into contact (gas composition, gas temperature, etc).
However, since a diesel oil is used as a fuel in diesel engines, exhaust gases contain a large amount of SO.sub.2. The oxygen concentration in exhaust gases varies in a wide range of 2-20%, depending upon the operation conditions of diesel engines. Under these conditions, there has been no established method of well igniting and burning fine carbon particles accumulated without causing secondary pollution.
For instance, as catalysts for removing particulate matter from exhaust gases, which have been proposed so far, there are precious metal catalysts and base metal catalysts. The precious metal catalysts are durable and function efficiently to oxidize CO and unburned hydrocarbons (hereinafter referred to simply as "HC"), but it is likely to convert SO.sub.2 existing in exhaust gases to SO.sub.3, leading to secondary pollution. Besides, there are problems such that ignition activity of soot in the particulate matter is lowered. On the other hand, the base metal catalysts are effective for removing the particulate matter, but their durability is poor.
Most of the catalysts for exhaust gas cleaners, which have been proposed hitherto, mainly function to lower the ignition temperature of particulate matter, leaving unsolved the problems of removing NOx from exhaust gases of diesel engines having a large oxygen concentration and/or a considerably variable oxygen concentration.
Japanese Patent Laid-Open No. 3-47539 discloses an exhaust gas cleaner comprising a heat-resistant, porous filter, a first catalyst supported by the filter in the inlet region, and a second catalyst supported by the filter in the outlet region, the first catalyst consisting essentially of (a) one or more alkali metal elements, (b) one or more elements in Groups IB, IIA, IIB, transition metal elements of the Periodic Table and Sn, and (c) one or more rare earth elements, and the second catalyst consisting essentially of at least one platinum-group element. By this exhaust gas cleaner, NOx and particulate matter are removed by the first catalyst in the inlet region by using mainly particulate matter and HC as reducing agents, and HC, CO, and other toxic gases are removed by the second catalyst in the outlet region. However, according to their research, the catalytic activity of this catalyst is not satisfactory even when any one of the transition metal elements is selected.
OBJECT AND SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide an exhaust gas cleaner capable of efficiently removing not only particulate matter but also nitrogen oxides from exhaust gas having a considerably variable oxygen concentration, particularly from exhaust gas containing large amounts of nitrogen oxides, particulate matter and unburned hydrocarbons.
Another object of the present invention is to provide an exhaust gas cleaner capable of efficiently removing particulate matter, nitrogen oxides, HC, CO, etc. from exhaust gas having a considerably variable oxygen concentration at a relatively low temperature.
A further object of the present invention is to provide a method of cleaning an exhaust gas by using the above exhaust gas cleaner.
As a result of intense research in view of the above objects, the inventors have found that by using an exhaust gas cleaner comprising a heat-resistant, porous filter with a porous ceramic powder which carries a catalyst consisting essentially of at least one alkali metal element, particular transition elements and at least one rare earth metal element, the particulate matter and the nitrogen oxides can be efficiently removed simultaneously from the exhaust gas having a considerably variable oxygen concentration, and the cleaning-efficiency of the exhaust gas cleaner can be kept high for a long period of time. The inventors also have found that when the exhaust gas contains large amounts of unburned hydrocarbons, the amount of the catalyst should rather be limited in order to increase the efficiency of removing NOx at a lower regeneration temperature. The inventors further have found that by combining a first catalyst consisting of an alkali metal, a particular transition metal and a rare earth metal in the inlet region of the filter and a second catalyst of a Pt-group metal in the outlet region of the filter, such toxic gases as NOx, HC, CO, etc. as well as particulate matter can be efficiently removed for a long period of time. The present invention is based on these findings.
Thus, the first exhaust gas cleaner according to the present invention comprises a heat-resistant, porous filter, a porous ceramic powder layer formed on the filter, and a catalyst supported by the ceramic powder layer, the catalyst consisting essentially of:
(a) at least one of alkali metal elements;
(b) cobalt and/or manganese;
(c) vanadium; and
(d) at least one of rare earth elements.
The first method of cleaning an exhaust gas containing nitrogen oxides and particulate matter according to the present invention comprises using the above first exhaust gas cleaner, whereby the nitrogen oxides are reduced by the particulate matter in the exhaust gas serving as a reducing agent.
The second exhaust gas cleaner according to the present invention comprises a heat-resistant, porous filter, and a catalytic layer formed on the filter, the catalytic layer comprising a uniform mixture of a ceramic powder carrier, and a catalyst consisting essentially of:
(a) at least one of alkali metal elements;
(b) cobalt and/or manganese;
(c) vanadium; and
(d) at least one of rare earth elements, the catalyst being 1-20 parts by weight and the ceramic powder carrier being 1-10 parts by weight per 100 parts by weight of the filter.
The second method of cleaning an exhaust gas containing nitrogen oxides and particulate matter according to the present invention comprises using the above second exhaust gas cleaner, whereby the nitrogen oxides are reduced by the particulate matter in the exhaust gas serving as a reducing agent.
The third exhaust gas cleaner according to the present invention comprises a heat-resistant, porous filter, a porous ceramic powder layer formed on the filter, and a catalyst supported by the ceramic powder layer, the catalyst consisting essentially of:
(a) at least one of alkali metal elements;
(b) at least one element selected from the group consisting of copper, cobalt, manganese and vanadium; and
(c) at least one of rare earth elements, the porous ceramic powder layer being 3-15 parts by weight per 100 parts by weight of the filter, and the catalyst being 2.0-7.0 weight % based on the porous ceramic powder layer.
The third method of cleaning an exhaust gas containing nitrogen oxides, particulate matter and unburned hydrocarbons according to the present invention comprises using the third exhaust gas cleaner, whereby the nitrogen oxides are reduced by the particulate matter and the unburned hydrocarbons in the exhaust gas serving as reducing agents.
The fourth exhaust gas cleaner according to the present invention comprises a heat-resistant, porous filter, a first catalyst supported by the filter in the inlet region, and a second catalyst supported by the filter in the outlet region, the first catalyst consisting essentially of:
(a) at least one of alkali metal elements;
(b) at least one of elements selected from the group consisting of copper, cobalt, manganese and molybdenum;
(c) vanadium; and
(d) at least one of rare earth elements; and the second catalyst consisting essentially of at least one platinum-group element.
The fourth method of cleaning an exhaust gas containing nitrogen oxides, particulate matter, unburned hydrocarbons and CO according to the present invention comprises using the fourth exhaust gas cleaner, whereby the nitrogen oxides are reduced by particulate matter and unburned hydrocarbons existing as reducing agents in the exhaust gas, and the unburned hydrocarbons and CO remaining in the exhaust gas flowing into the outlet region are oxidized by the second catalyst.
DETAILED DESCRIPTION OF THE INVENTION
The present invention will be explained in detail below.
[1] First exhaust gas cleaner
[A] Heat-resistant, porous filter
Since the heat-resistant, porous filter according to the present invention is exposed to a high-temperature exhaust gas, it is required to have an excellent heat resistance, and particularly an excellent thermal shock resistance. It is also required to have a necessary particulate matter-capturing capacity while causing pressure drop only within the permitted range. Such materials for the heat-resistant, porous filter include ceramics such as alumina, silica, titania, zirconia and their composites such as silica-alumina, alumina-zirconia, alumina-titania, silica-titania, silica-zirconia, titania-zirconia, mullite, cordierite, etc. The filter may be in the form of a honeycomb filter or a foam filter, which is already known.
The shape and the size of the filter may vary depending upon its purpose. When the filter is a cylindrical, ceramic foam-type, it preferably has a diameter of 30-400 mm and a length of 5-30 mm. The ceramic foam-type filter preferably has a porosity of 40-80% and an average pore diameter of about 200-400 .mu.m. If necessary, a plurality of filters may be stacked or arranged in tandem.
[B] Porous ceramic powder layer
The catalyst is supported by the above heat-resistant, porous filter via a more porous ceramic powder layer formed on the porous filter.
The porous ceramic powder layer may be made of a porous ceramic material having a large surface area such as titania, alumina, zirconia, silica, titania-alumina, alumina-zirconia, alumina-silica, titania-silica, titania-zirconia, etc. Preferably, the porous ceramic powder layer is made of titania (TiO.sub.2).
The amount of the ceramic carrier powder may be 3-15 parts by weight per 100 parts by weight of the filter. When the amount of ceramic carrier powder is less than 3 parts by weight, the ceramic carrier cannot support a sufficient amount of the catalyst. On the other hand, when the amount of the ceramic powder exceeds 15 parts by weight, the pressure drop in the exhaust gas cleaner becomes too high. The preferred amount of ceramic powder is 5-12 parts by weight.
The porous ceramic powder layer is formed on the filter by a wash-coating method, a sol-gel method, etc.
In the wash-coating method, the filter is immersed in a slurry of the above porous ceramic carrier material and dried so that a carrier powder layer is formed on the filter.
The sol-gel method comprises hydrolyzing organic salts (for instance, alkoxides) of the ceramic carrier-constituting metals; applying the resulting sol to the filter; bringing the coated filter into contact with water vapor, etc. to form a layer composed of colloidal particles; and drying and burning it to convert it to a carrier layer for the catalyst. For instance, when catalytic metals are to be supported by a titania (TiO.sub.2) carrier layer, a solution of Ti alkoxide (for instance, Ti(O-isoC.sub.3 H.sub.7).sub.4) in alcohol is mixed with an acid such as CH.sub.3 COOH, HNO.sub.3, HCl, etc. to prepare a coating solution, and the filter is immersed in the coating solution. After removing the filter from the coating solution, it is reacted with vapor or water to cause gelation. After drying and burning, a thin titania coating is formed on a porous surface of the filter. In the sol-gel method, the acid serves as a hydrolysis catalyst in the course of gelation. However, alkalis may be added in place of the acids to conduct the hydrolysis reaction.
Although the above explanation has been made with respect to the case of using titania as a ceramic carrier material, any other ceramics may be similarly used to support the catalyst by the sol-gel method. For instance, in the case of supporting the catalytic components by alumina, the same methods as above may be used except for using alkoxides of Al. Other porous carriers may be used in the same manner as above.
[C] Catalyst
(1) Composition
The catalyst supported by the above filter via the ceramic powder layer consists essentially of (a) at least one of alkali metal elements; (b) Co and/or Mn, (c) V; and (d) at least one of rare earth elements.
(i) Alkali metal elements
the alkali metal element (a) is preferably selected from Na, K and Cs. Particularly when Cs is used, any unburned hydrocarbons including saturated hydrocarbons such as propane, etc. can be reacted with the NOx, so that the NOx is efficiently removed from the exhaust gas. This is due to the fact that the presence of Cs serves to increase the selectivity of the reaction of the hydrocarbons with NOx, thereby reducing the reaction between the hydrocarbons and oxygen in the exhaust gas.
The amount of the component (a), measured as an alkali metal itself (active species), is 10-50 weight % based on the total weight of the catalyst (on a metal basis). When the amount of the alkali metal element is smaller than 10 weight % or larger than 50 weight %, the efficiency of the catalyst to remove particulate matter and NOx simultaneously is low. The preferred amount of the alkali metal element is 15-30 weight %.
(ii) Transition metal elements
The transition metal elements consist of (b) Co and/or Mn, and (c) V.
The amount of the component (b), measured as a metal itself (active species), is 15-65 weight % based on the total weight of the catalyst (on a metal basis). When the amount of Co and/or Mn is smaller than 15 weight %, the ability of the catalyst to ignite particulate matter is low. On the other hand, when the amount of Co and/or Mn is larger than 65 weight %, the function of HC to reduce the amount of NOx is reduced. The preferred amount of Co and/or Mn is 20-50 weight %.
The amount of the component (c), measured as a metal itself (active species), is 15-65% based on the total weight of the catalyst (on a metal basis). When the amount of V is smaller than 15 weight %, the catalyst is less resistant to sulfur. On the other hand, when the amount of V is larger than 65 weight %, the efficiency of the catalyst to remove particulate matter and NOx simultaneously is low. The preferred amount of V is 20-50 weight %.
The total amount of the components (b) and (c) is 30-80 weight %, preferably 40-80 weight %.
(iii) Rare earth elements
The rare earth elements (d) are preferably Ce, La, Nd or Sm, etc., and Misch metal which is a mixture of rare earth elements may be used as the rare earth elements.
The amount of the component (d), measured as a metal itself (active species), is 10-50 weight % based on the total weight of the catalyst (on a metal basis). When the amount of the rare earth element is smaller than 10 weight % or larger than 50 weight %, the efficiency of the catalyst to remove particulate matter and NOx simultaneously is low. The preferred amount of the rare earth element is 15-30 weight %.
The total amount of (a)+(b)+(c)+(d) as metallic active components is generally 1-40 weight %, preferably 5-30 weight % based on the ceramic powder layer.
(2) Carrying catalyst by ceramic powder layer
After forming a porous ceramic carrier layer on the filter by the wash-coating method or the sol-gel method, etc., the carrier layer-coated filter is immersed in aqueous solutions of carbonates, nitrates, acetates, hydroxides, etc. of catalytic components, dried and then burned to obtain the filter supporting the catalyst via the ceramic layer. The catalytic metal salts may be of any type, as long as they are soluble in water, including carbonates, nitrates, acetates, hydroxides, etc. With respect to V, a solution of NH.sub.4 VO.sub.3 and oxalic acid may be used. Alkali metals and V may simultaneously be applied by using a solution of alkali vanadate.
(3) Function of catalyst
By using the catalyst consisting essentially of the above components, the particulate matter in the exhaust gas can be ignited and burned at a relatively low temperature, and NOx can be efficiently removed with the particulate matter serving as a reducing agent. Specifically, when the particulate matter in the exhaust gas is brought into contact with the above catalyst in the presence of oxygen, the ignition temperature of the particulate matter is lowered. As a result, the particulate matter is burned (oxidized) at a temperature lower than about 450.degree. C. At the same time, NOx is reduced to N.sub.2 by the particulate matter serving as a reducing agent, whereby the exhaust gas can efficiently be cleaned. The reason why the reduction of the NOx can efficiently be carried out at a relatively low temperature is that the components (a), (b), (c) and (d) in the catalyst show synergistic effects. By using Co and/or Mn and V as the components (b) and (c), the durability of the catalyst is so high that the catalyst can stably remove NOx for a long period of time.
The exhaust gas of diesel engines, etc. usually contains about 100-500 ppm of the unburned hydrocarbons (HC) and about 200-4000 ppm of the NOx.
Typical examples of the HC may be propane, propylene, ethylene, acetylene, etc. When the unburned HC is hydrocarbons having unsaturated bonds such as propylene, acetylene, etc., previously proposed catalysts containing Cu alone as a catalytic component are effective to some extent to reduce NOx by using the unburned HC as reducing agents (see, for instance, Japanese Patent Laid-Open No. 63-100919). However, when saturated hydrocarbons such as propane, etc. are used as reducing agents, the reduction reaction of NOx does not efficiently take place. This is due to the fact the reactivity of carbon-carbon bonds is in the following order: triple bonds (acetylene, etc.)>double bonds (ethylene, propylene, etc.)> single bonds (propane, etc.), and that when saturated hydrocarbons having poor reactivity such as propane are used, the reduction reaction of the NOx does not take place sufficiently in the presence of the conventional catalysts.
However, when the catalyst of the present invention is used, the NOx can be efficiently removed even when saturated hydrocarbons such as propane, etc. are used as reducing agents. Accordingly, the removal of NOx is remarkably improved with the catalyst of the present invention as compared to the conventional catalysts.
[D] Method of cleaning exhaust gas
With the first exhaust gas cleaner, the exhaust gas is cleaned. Since the HC in the exhaust gas is mainly composed of methane, ethylene and acetylene, etc., the reduction reaction temperature of the NOx is 200.degree.-500.degree. C., preferably 250.degree.-450.degree. C. When the reaction temperature is too high, the burning of the HC itself takes place, making it ineffective as a reducing agent for the NOx.
When the amount of the HC serving as a reducing agent in the exhaust gas is too small, the HC such as propane, propylene, etc. may be introduced into the exhaust gas in an amount necessary for the reduction of the NOx, before the exhaust gas enters into the exhaust gas cleaner.
[2] Second exhaust gas cleaner
[A] Heat-resistant, porous filter
The heat-resistant, porous filter in the second exhaust gas cleaner may be the same as in the first exhaust gas cleaner.
[B] Catalytic layer
The catalytic layer is composed of a uniform mixture of a catalyst and a ceramic powder carrier.
(1) Catalyst
The catalyst itself may be the same as in the first exhaust gas cleaner. The amount of the catalyst is 1-20 parts by weight per 100 parts by weight of the filter. When the amount of the catalyst is less than 1 parts by weight, it is difficult to remove nitrogen oxides and particulate matter simultaneously from the exhaust gas. On the other hand, even if the amount of the catalyst exceeds 20 parts by weight, the ability of the exhaust gas cleaner to remove nitrogen oxides and particulate matter would not increase accordingly. Therefore, the upper limit of the amount of the catalyst is set at 20 parts by weight. The preferred amount of the catalyst is 5-15 parts by weight.
(2) Ceramic powder carrier
The ceramic powder carrier itself may be the same as in the first exhaust gas cleaner. The amount of the ceramic carrier powder is 1-10 parts by weight per. 0 100 parts by weight of the filter. When the amount of the ceramic carrier powder is less than 1 parts by weight, the ceramic carrier cannot support a sufficient amount of the catalyst. On the other hand, when the amount of the ceramic carrier powder exceeds 10 parts by weight, the pressure drop in the exhaust gas cleaner becomes too high. The preferred amount of ceramic carrier powder is 2-6 parts by weight.
(3) Formation of catalytic layer
The catalytic layer may be formed on the filter by a wash-coating method, a sol-gel method, etc.
In the wash-coating method, the ceramic carrier powder such as titania is impregnated with solutions of the catalyst components, and the filter is then immersed in a slurry of the ceramic carrier powder carrying the catalyst.
In the sol-gel method, a coating solution is first prepared by mixing a solution of the ceramic carrier material with solutions of the catalyst components, and the filter is then immersed in the coating solution. For instance, a solution of Ti alkoxide in alcohol is mixed with an acid such as CH.sub.3 COOH, HNO.sub.3, HCl, etc. and aqueous solutions of catalytic component metal salts to prepare the coating solution, and the ceramic filter is immersed in the coating solution. After removing the ceramic filter from the coating solution, it is reacted with vapor or water to prepare a thin layer of colloidal particles (sol) on the filter, which is then converted to a gel. The gel is dried and then burned to provide a catalyst-supporting ceramic carrier layer. In the sol-gel method, the acid serves as a hydrolysis catalyst in the course of gelation. However, alkalis may be added in place of the acids to conduct the hydrolysis reaction.
By using this sol-gel method, the catalyst can be extremely uniformly dispersed in the ceramic filter, leading to an increase in catalytic activity.
Incidentally, any other ceramics than TiO.sub.2 may be similarly used as ceramic carrier materials to support the catalyst by the sol-gel method, etc.
In the second exhaust gas cleaner having the catalytic layer comprising a uniform mixture of the catalyst and the ceramic powder carrier, a relatively large amount of the catalyst can be carried even when the amount of the ceramic carrier powder is small, thereby lowering a pressure drop. This means that a large amount of the catalyst can be applied to the filter, even in the case of using ceramic powder in an amount small enough to prevent a pressure drop.
[C] Method of cleaning exhaust gas
With the second exhaust gas cleaner, the exhaust gas is cleaned under the same conditions as in the first exhaust gas cleaner.
[3] Third exhaust gas cleaner
[A] Heat-resistant, porous filter
The heat-resistant, porous filter in the third exhaust gas cleaner may be the same as in the first exhaust gas cleaner.
[B] Porous ceramic powder layer
The catalyst is supported by the above heat-resistant, porous filter via a more porous ceramic powder layer formed on the porous filter.
The porous ceramic powder layer may be the same as in the first exhaust gas cleaner. In order to increase the reactivity of NOx with the particulate and HC, the porous ceramic powder layer is preferably titania, zirconia, alumina, titania-zirconia, alumina-zirconia, etc.
The amount of the ceramic carrier powder is 3-15 parts by weight, preferably 5-12 parts by weight per 100 parts by weight of the filter.
[C] Catalyst
(1) Composition
The catalyst supported by the above filter via the ceramic powder layer consists essentially of (a) at least one of alkali metal elements (Na, K, Cs, etc.); (b) at least one of Cu, Co, Mn and V; and (c) at least one of rare earth elements (Ce, La, Nd, Sm, etc.).
(i) Alkali metal elements
The amount of the component (a), measured as an alkali metal itself (active species), is 5-50 weight % based on the total weight of the catalyst (on a metal basis). When the amount of the alkali metal element is smaller than 5 weight % or larger than 50 weight %, the efficiency of the catalyst to remove particulate matter and NOx simultaneously is low. The preferred amount of the alkali metal element is 10-30 weight %.
(ii) Transition metal elements
It is preferable to use a combination of V and at least one of Cu, Co and Mn. By this combination, the catalytic activity stably lasts long.
The amount of the component (b), measured as a metal itself (active species), is 30-80 weight % based on the total weight of the catalyst (on a metal basis). When the amount of the component (b) is smaller than 30 weight %, the ability of the catalyst to ignite particulate matter is low. On the other hand, when the amount of the component (b) is larger than 80 weight %, the function of HC to reduce the amount of NOx is reduced. The preferred amount of the component (b) is 40-70 weight %.
In the case of a combination of V and at least one of Cu, Co and Mn, a weight ratio of at least one of Cu, Co and Mn to V is preferably about 5/1-1/15. When the weight ratio is larger than 5/1, the resistance of the catalyst to sulfur is low. On the other hand, when the weight ratio is smaller than 1/15, the efficiency of the catalyst to remove particulate matter and NOx simultaneously is low.
(iii) Rare earth elements
The amount of the component (c), measured as a metal itself (active species), is 5-50% based on the total weight of the catalyst (on a metal basis). When the amount of the rare earth element is smaller than 5 weight % or larger than 50 weight %, the efficiency of the catalyst to remove particulate matter and NOx simultaneously is low. The preferred amount of the rare earth element is 10-30 weight %.
The amount of the catalyst (total amount of (a)+(b)+(c)) as metallic active components is determined as 2.0-7.0 weight % based on the ceramic powder layer, judging from the fact that high burning efficiency of particulate matter is obtained in the case of large amounts of catalyst components, while high reactivity of HC with NOx is obtained in the case of small amounts of catalyst components.
When the amount of the catalyst is smaller than 2.0 weight %, sufficient catalytic activity cannot be obtained. On the other hand, when the amount of the catalyst is larger than 7.0 weight %, the efficiency of the catalyst to remove particulate matter and NOx simultaneously is rather low. The preferred amount of the catalyst is 2.0-5.0 weight %.
(2) Carrying catalyst by ceramic powder layer
The same procedures as in the first exhaust gas cleaner may be used.
(3) Function of catalyst
By using the catalyst consisting essentially of the above components, NOx can be efficiently removed with the HC and the particulate matter serving as reducing agents.
[D] Method of cleaning exhaust gas
With the third exhaust gas cleaner, the exhaust gas is cleaned. Since the HC in the exhaust gas is mainly composed of methane, ethylene and acetylene, etc., the reduction reaction temperature of the NOx is 200.degree.-500.degree. C., preferably 250.degree.-450.degree. C. When the reaction temperature is too high, the burning of the HC itself takes place, making it ineffective as a reducing agent for the NOx.
When the amount of the HC serving as a reducing agent in the exhaust gas is too small, the HC such as propane, propylene, etc. may be introduced into the exhaust gas in an amount necessary for the reduction of the NOx, before the exhaust gas enters into the exhaust gas cleaner.
[4] Fourth exhaust gas cleaner
[A] Heat-resistant, porous filter
The heat-resistant, porous filter in the fourth exhaust gas cleaner may be the same as in the first exhaust gas cleaner.
A volume ratio of the inlet region to the outlet region in the filter is preferably 1/4-8/1. Incidentally, the inlet region and the outlet region may be formed in a single filter, or a filter carrying the first catalyst and a filter carrying the second catalyst may be combined at the above volume ratio.
[B] Porous ceramic powder layer
The catalyst may be supported by the above heat-resistant, porous filter via a more porous ceramic powder layer formed on the porous filter.
The porous ceramic powder layer may be the same as in the first exhaust gas cleaner. In order to increase the reactivity of NOx with the particulate and HC, the porous ceramic powder layer is preferably titania, zirconia, alumina, titania-zirconia, alumina-zirconia, etc.
The amount of the ceramic carrier powder may be 5-20 parts by weight per 100 parts by weight of the filter both in the inlet and outlet regions.
In order to reduce pressure drop, the porous ceramic powder layer is formed on the filter by a wash-coating method, a sol-gel method, etc. The wash-coating method and the sol-gel method themselves may be the same as in the first exhaust gas cleaner.
[C] Catalyst
(1) First catalyst
The catalyst supported by the inlet region of the filter consists essentially of (a) at least one of alkali metal elements; (b) at least one of Cu, Co Mn and Mo, (c) V; and (d) at least one of rare earth elements.
(i) Alkali metal elements
The amount of the component (a), measured as an alkali metal itself (active species), is 10-50 weight % based on the total weight of the catalyst (on a metal basis). The preferred amount of the alkali metal element is 15-30 weight %.
(ii) Transition metal elements
The amount of the component (b), measured as a metal itself (active species), is 15-65 weight % based on the total weight of the catalyst (on a metal basis). The preferred amount of the component (b) is 20-50 weight %.
The amount of the component (c), measured as a metal itself (active species), is 15-65% based on the total weight of the catalyst (on a metal basis). The preferred amount of V is 20-50 weight %.
(iii) Rare earth elements
The rare earth elements (d) are preferably Ce, La, Nd or Sm, etc., and Misch metal which is a mixture of rare earth elements may be used as the rare earth elements.
The amount of the component (d), measured as a metal itself (active species), is 10-50 weight % based on the total weight of the catalyst (on a metal basis). The preferred amount of the rare earth element is 15-30 weight %.
The total amount of (a)+(b)+(c)+(d) as metallic active components is preferably 0.05-6 weight % based on the ceramic powder layer.
(2) Second catalyst
The second catalyst used together with the first catalyst is composed of a Pt-group metal having a high oxidation capability. The Pt-group metal is preferably Pt, Pd a mixture of Pt and Pd, or a mixture of Pt, Pd and Rh. The second catalyst may contain Au and/or Ag to increase its ability to clean the exhaust gas.
The second catalyst is disposed in the outlet region of the filter to remove the HC and CO remaining in the exhaust gas which has passed through the first catalyst region and entered into the second catalyst region.
The amount of the second catalyst is 0.1-1 weight % based on the ceramic powder layer. When Au and/or Ag is contained, the amount of Au and/or Ag is 50 parts by weight or less, preferably 10-20 parts by weight, per 100 parts by weight of the Pt-group metal.
(3) Carrying catalyst by ceramic powder layer
The first and second catalysts may be applied to a single filter. Alternatively, they may be applied to separate filters which are then combined such that the first catalyst is positioned in the inlet region and the second catalyst is positioned in the outlet region.
In the latter case, the first catalyst may be applied to the filter in the same manner as described above. The second catalyst may be applied to the filter by immersing the filter in an aqueous solution of chloride, etc. of the Pt-group metal and if necessary, an aqueous solution of chloride, etc. of Au and/or Ag, and then drying and burning.
Further, by conducting light irradiation to the filter impregnated with an aqueous solution of platinum-group element compounds such as chlorides of Pt, Pd, Rh, etc., catalyst-carrying efficiency is extremely increased. The irradiation can be conducted by means of a mercury vapor lamp having a power output of 500 W or so. Alternatively, it is possible to deposit the platinum-group element catalyst in the carrier powder by light irradiation and then to coat the catalyst-supporting carrier powder on the filter. By this light irradiation method, the catalyst can be supported by the carrier powder in high dispersion, and the coating layer of the filter can be made thin, whereby pressure drop can be minimized in the high-density, thin-layer portion.
(4) Function of catalyst
By using the first and second catalysts, NOx can be efficiently removed with the particulate matter and HC serving as reducing agents. Specifically, when the particulate matter in the exhaust gas is brought into contact with the first catalyst region in the presence of oxygen, the ignition temperature of the particulate matter is lowered. As a result, the particulate matter is burned (oxidized) at a temperature lower than about 400.degree. C. At the same time, NOx is reduced to N.sub.2 by the particulate matter and HC serving as reducing agents, whereby the exhaust gas can efficiently be cleaned. By using a combination of at least one of Cu, Co, Mn and Mo and V, the durability of the catalyst is so high that the catalyst can stably remove NOx for a long period of time. Incidentally, in the first catalyst region, the generation of SO.sub.3 can be suppressed because the the first catalyst is composed of base metal compounds.
From the exhaust gas which enters into the second catalyst region, the remaining HC and CO, etc. are removed by the second catalyst.
[D] Method of cleaning exhaust gas
Since the HC in the exhaust gas is mainly composed of methane, ethylene and acetylene, etc., the reduction reaction temperature of the NOx is 200.degree.-500.degree. C., preferably 250.degree.-450.degree. C. When the reaction temperature is too high, the burning of the HC itself takes place, making it ineffective as a reducing agent for the NOx.
When the amount of the HC serving as a reducing agent in the exhaust gas is too small, the HC such as propane, propylene, etc. may be introduced into the exhaust gas in an amount necessary for the reduction of the NOx, before the exhaust gas enters into the exhaust gas cleaner.
Incidentally, in the above explanation, catalyst components are expressed as metal elements. However, the base metal catalysts usually exist in the form of oxides. K, for instance, is actually in the form of K.sub.2 O. Accordingly, please note that the base metal catalysts may be in the form of oxides.
It is also to be noted that the detailed description of the second to fourth exhaust gas cleaners are omitted for simplicity as long as they are already given with respect to the first exhaust gas cleaner. Accordingly, please refer to the corresponding sections in the part of the the first exhaust gas cleaner for detailed explanations.
The present invention will be described in further detail by way of the following Examples. In each Example and Comparative Example, catalytic components are described simply by metal elements for simplicity, and the amount of each catalyst component is expressed by a weight of a metal component in the catalyst component. For instance, the amount of Cs.sub.2 O is expressed by a weight of Cs.

EXAMPLE 1
A ceramic foam-type filter made of cordierite (apparent volume: 2 l, density: 0.65 g/ml) was coated with TiO.sub.2 powder in an amount of 10 weight % based on the filter, by a wash-coating method.
The coated filter was impregnated with 2.5 weight % of Co by using an aqueous solution of CoCl.sub.2, 2.5 weight % of La by using an aqueous solution of La(NO.sub.3).sub.3, and 2.5 weight % of Cs by using an aqueous solution of CsNO.sub.3, each based on TiO.sub.2. After drying the impregnated filter, it was burned at 700.degree. C. Next, the burned filter was impregnated with 2.5 weight % of V by using an aqueous solution of NH.sub.4 VO.sub.3 and oxalic acid. After drying the impregnated filter, it was burned again at 700.degree. C. for 3 hours to produce an exhaust gas cleaner having the following composition:
Cs/Co/V/La (TiO.sub.2)
With respect to this exhaust gas cleaner, a regeneration temperature of the filter (expressed by a temperature at which pressure drop started to decrease by burning particulate matter) and a conversion rate of NOx to N.sub.2 at the regeneration temperature were measured by using a diesel engine having a displacement of 510 cc. The regeneration temperature and the conversion rate of NOx to N.sub.2 were evaluated at two points, i.e., at a time when decrease in pressure drop was observed for the first time in an initial stage of the operation, and at a time when decrease in pressure drop was observed after the lapse of 10 hours from the initiation of the operation. The diesel engine was operated at 1500 rpm under a load of 90%. Under these conditions, the exhaust gas comprised 90 ppm of HC (total of hydrocarbons), 460 ppm of CO, 480 ppm of NOx, 10% of O.sub.2 and 200 ppm of SO.sub.2. The results are shown in Table 1.
EXAMPLE 2
In the same manner as in Example 1, a ceramic foam-type filter made of cordierite (apparent volume: 2 l, density: 0.65 g/ml) was coated with TiO.sub.2 in an amount of 10 weight % based on the filter. The coated filter was impregnated with 2.5 weight % of Co, 2.5 weight % of Ce, and 2.5 weight % of Cs, each based on the TiO.sub.2 carrier layer, by using aqueous solutions of CoCl.sub.2, Ce(NO.sub.3).sub.3 and CsNO.sub.3, respectively. The impregnated filter was dried and burned in the same manner as in Example 1. Next, the burned filter was impregnated with 2.5 weight % of V in the same manner as in Example 1 to produce an exhaust gas cleaner having the following composition:
Cs/Co/V/Ce (TiO.sub.2)
EXAMPLE 3
In the same manner as in Example 2, an exhaust gas cleaner supporting 2.5 weight % of Co, 2.5 weight % of La, 2.5 weight % of K and 2.5 weight % of V, each based on TiO.sub.2 carrier layer, was produced by using aqueous solutions of CoCl.sub.2, La(NO.sub.3).sub.3, KCl and NH.sub.4 VO.sub.3, respectively. The resulting exhaust gas cleaner had the following composition:
K/Co/V/La (TiO.sub.2)
EXAMPLE 4
In the same manner as in Example 2, an exhaust gas cleaner supporting 2.5 weight % of Co, 2.5 weight % of Ce, 2.5 weight % of K and 2.5 weight % of V, each based on the TiO.sub.2 carrier layer, was produced by using aqueous solutions of CoCl.sub.2, Ce(NO.sub.3).sub.3, KCl and NH.sub.4 VO.sub.3, respectively. The resulting exhaust gas cleaner had the following composition:
K/Co/V/Ce (TiO.sub.2)
EXAMPLES 5 AND 6
In the same manner as in Example 1, catalyst components were applied to the filter except for using manganese acetate in place of CoCl.sub.2.
______________________________________Cs/Mn/V/La (TiO.sub.2): (Example 5)Cs/Mn/V/Ce (TiO.sub.2): (Example 6)______________________________________
EXAMPLES 7 AND 8
In the same manner as in Example 2, catalyst components were applied to the filter except for using manganese acetate in place of CoCl.sub.2.
______________________________________K/Mn/V/La (TiO.sub.2): (Example 7)K/Mn/V/Ce (TiO.sub.2): (Example 8)______________________________________
With respect to the resulting exhaust gas cleaners of Examples 2-8, a regeneration temperature and a conversion rate of NOx to N.sub.2 at the regeneration temperature were measured in the same manner as in Example 2. The results are shown in Table 1.
COMPARATIVE EXAMPLES 1-8
In the same manner as in Example 1, each of eight ceramic foam-type filters made of cordierite (apparent volume: 2 l, density: 0.65 g/ml) was coated with TiO.sub.2 in an amount of 10 weight % based on the filter, by using a wash-coating method.
Each coated filter was immersed in such a combination of aqueous solutions of La(NO.sub.3).sub.3, Ce(NO.sub.3).sub.3, CoCl.sub.2, manganese acetate, CsNO.sub.3 and KCl as to produce an exhaust gas cleaner supporting the following catalytic metals each in an amount of 2.5 weight % on a metal basis, dried and burned in the same manner as in Example 1.
______________________________________Cs/Co/La (TiO.sub.2): (Comparative Example 1)Cs/Co/Ce (TiO.sub.2): (Comparative Example 2)K/Co/La (TiO.sub.2): (Comparative Example 3)K/Co/Ce (TiO.sub.2): (Comparative Example 4)Cs/Mn/La (TiO.sub.2): (Comparative Example 5)Cs/Mn/Ce (TiO.sub.2): (Comparative Example 6)K/Mn/La (TiO.sub.2): (Comparative Example 7)K/Mn/Ce (TiO.sub.2): (Comparative Example 8)______________________________________
With respect to each of these exhaust gas cleaners, a regeneration temperature and a conversion rate of NOx to N.sub.2 at the regeneration temperature were measured in the same manner as in Example 1. The results are shown in Table 1.
TABLE 1______________________________________ Initial Stage.sup.(1) After 10 Hours.sup.(2) Regener- Conversion Regener- Conversion ation Rate of ation Rate of Temp. NOx to N.sub.2.sup.(3) Temp. NOx to N.sub.2.sup.(3)No. (.degree.C.) (%) (.degree.C.) (%)______________________________________Example 1 390 15 390 18Example 2 395 12 395 15Example 3 410 10 410 14Example 4 410 10 410 14Example 5 410 14 410 18Example 6 415 12 415 16Example 7 420 10 420 14Example 8 420 10 420 14Comparative 340 25 420 12Example 1Comparative 360 20 430 12Example 2Comparative 370 18 440 10Example 3Comparative 370 18 450 10Example 4Comparative 370 24 420 12Example 5Comparative 375 20 430 12Example 6Comparative 380 18 440 10Example 7Comparative 380 18 450 10Example 8______________________________________ Note .sup.(1) Measured at a time when decrease in pressure drop was first appreciated. .sup.(2) Measured at a time when decrease in pressure drop was first appreciated after 10 hours from the initiation of passing exhaust gas through the filter. .sup.(3) Calculated from the amount (Xa) of NOx in exhaust gas before entering into the filter, and the amount of (Xb) of NOx in exhaust gas after passage through the filter, by using the formula: (Xa - Xb)/Xa.
As is clear from Table 1, the exhaust gas cleaners of Examples 1-8 show high conversion rates of NOx to N.sub.2 than those of Comparative Examples 1-8, when 10 hours passed after the operation had started. Also, at a time after the lapse of 10 hours, the regeneration temperature of the filter at which particulate matter is ignited and burned is lower in Examples than in Comparative Examples, meaning that exhaust gas is efficiently cleaned for a long period of time in the present invention.
EXAMPLE 9
To prepare a coating solution, a solution of Ti alkoxide (Ti(O-isoC.sub.3 H.sub.7).sub.4) in alcohol was mixed with acetic acid, and then mixed with an aqueous solution of CoCl.sub.2, an aqueous solution of La(NO.sub.3).sub.3, an aqueous solution of CsNO.sub.3, and a mixture of an aqueous solution of NH.sub.4 VO.sub.3 and oxalic acid.
A ceramic foam-type filter made of cordierite (apparent volume: 0.25 l, density: 0.65 g/ml) was immersed in the resulting coating solution. After removing the filter from the coating solution, the impregnated filter was brought into contact with water vapor to cause gelation. After drying, it was burned again at 600.degree. C. for 3 hours to produce a filter element provided with a catalytic layer. 4 filter elements were stacked to produce an exhaust gas cleaner having an apparent volume of 1 l.
The amount of TiO.sub.2 was 3 weight % based on the filter, and the amounts of Co, La, Cs and V on a metal basis were all 1.5 weight % based on the filter. The composition of the exhaust gas cleaner was as follows:
Cs/Co/V/La (TiO.sub.2)
With respect to this exhaust gas cleaner, a pressure drop due to the capturing of particulate matter, a regeneration temperature of the filter (expressed by a temperature at which pressure drop started to decrease by burning particulate matter) and a conversion rate of NOx to N.sub.2 at the regeneration temperature were measured by using a diesel engine having a displacement of 500 cc. The regeneration temperature and the conversion rate of NOx to N.sub.2 were evaluated at two points, i.e., at a time when decrease in pressure drop was observed for the first time in an initial stage of the operation, and at a time when decrease in pressure drop was observed after the lapse of 10 hours from the initiation of the operation. The diesel engine was operated at 2500 rpm under a load of 80%. Under these conditions, the exhaust gas comprised 90 ppm of HC (total of hydrocarbons), 460 ppm of CO, 480 ppm of NOx, 10% of O.sub.2 and 200 ppm of SO.sub.2. The results are shown in Table 2.
EXAMPLE 10
Example 9 was repeated except for using manganese acetate in place of CoCl.sub.2. The composition of the exhaust gas cleaner was as follows:
Cs/Mn/V/La (TiO.sub.2)
EXAMPLE 11
Example 9 was repeated except for using Ce(NO.sub.3).sub.3 in place of La(NO.sub.3).sub.3, and changing the amount of ceramic carrier component. The composition of the exhaust gas cleaner was as follows:
Cs/Co/V/Ce (TiO.sub.2)
The amount of TiO.sub.2 was 4 weight % based on the filter, and the amounts of Co, Ce, Cs and V on a metal basis were all 1.5 weight % based on the filter.
EXAMPLE 12
Example 10 was repeated except for using Ce(NO.sub.3).sub.3 in place of La(NO.sub.3).sub.3, and changing the amount of ceramic carrier component. The composition of the exhaust gas cleaner was as follows:
Cs/Mn/V/Ce (TiO.sub.2)
The amount of TiO.sub.2 was 4 weight % based on the filter, and the amounts of Mn, Ce, Cs and V on a metal basis were all 1.5 weight % based on the filter.
EXAMPLE 13
A slurry of potassium titanate, La(NO.sub.3).sub.3, Co(NO.sub.3).sub.2, and a mixture of an aqueous solution of NH.sub.4 VO.sub.3 and oxalic acid was prepared.
The same filter as in Example 9 was immersed in the resulting slurry. After removing the filter from the slurry and drying, it was burned at 700.degree. C. for 3 hours to produce a filter element provided with a catalytic layer. 4 filter elements were stacked to produce an exhaust gas cleaner having an apparent volume of 1 l.
The amount of TiO.sub.2 was 3 weight % based on the filter, the amount of K on a metal basis was 3 weight % based on the filter, and the amounts of Co, La and V on a metal basis were all 2 weight % based on the filter. The composition of the exhaust gas cleaner was as follows:
K/Co/V/La (TiO.sub.2)
EXAMPLE 14
Example 13 was repeated except for using manganese acetate in place of Co(NO.sub.3).sub.2. The composition of the exhaust gas cleaner was as follows:
K/Mn/V/La (TiO.sub.2)
EXAMPLE 15
Example 13 was repeated except for using Ce(NO.sub.3).sub.3 in place of La(NO.sub.3).sub.3. The composition of the exhaust gas cleaner was as follows:
K/Co/V/Ce (TiO.sub.2)
EXAMPLE 16
Example 14 was repeated except for using Ce(NO.sub.3).sub.3 in place of La(NO.sub.3).sub.3. The composition of the exhaust gas cleaner was as follows:
K/Mn/V/Ce (TiO.sub.2)
With respect to the resulting exhaust gas cleaners of Examples 10-16, the pressure drop, the regeneration temperature and the conversion rate of NOx to N.sub.2 at the regeneration temperature were measured in the same manner as in Example 9. The results are shown in Table 2.
COMPARATIVE EXAMPLES 9-16
In the same manner as in Example 9, each of eight ceramic foam-type filters made of cordierite was formed with a catalytic layer of TiO.sub.2 containing catalyst components described below.
The amount of the catalytic layer of TiO.sub.2 +catalyst components was 10 weight % based on the filter, and the amount of each metal component on a metal basis was 2.5 weight % based on the filter.
______________________________________Cs/Co/La (TiO.sub.2): (Comparative Example 9)Cs/Mn/La (TiO.sub.2): (Comparative Example 10)Cs/Co/Ce (TiO.sub.2): (Comparative Example 11)Cs/Mn/Ce (TiO.sub.2): (Comparative Example 12)K/Co/La (TiO.sub.2): (Comparative Example 13)K/Mn/La (TiO.sub.2): (Comparative Example 14)K/Co/Ce (TiO.sub.2): (Comparative Example 15)K/Mn/Ce (TiO.sub.2): (Comparative Example 16)______________________________________
With respect to each of the resulting exhaust gas cleaners of Comparative Examples 9-16, the pressure drop, the regeneration temperature and the conversion rate of NOx to N.sub.2 at the regeneration temperature were measured in the same manner as in Example 9. The results are shown in Table 2.
TABLE 2__________________________________________________________________________ Initial Stage.sup.(1) After 10 Hours.sup.(4) Pressure drop Regeneration Conversion Rate Regeneration Conversion RateNo..sup.(2) (mm Hg) Temp (.degree.C.) of NOx to N.sub.2.sup.(3) Temp (.degree.C.) of NOx to N.sub.2.sup.(3)__________________________________________________________________________Ex. 9 50 390 18 390 18Ex. 10 52 400 17 390 17Ex. 11 70 400 13 400 13Ex. 12 70 405 14 405 14Ex. 13 68 405 12 405 12Ex. 14 67 405 12 405 12Ex. 15 70 410 12 410 12Ex. 16 71 410 10 410 10Com. Ex. 9 95 390 18 430 15Com. Ex. 10 96 395 16 430 12Com. Ex. 11 100 410 15 435 12Com. Ex. 12 102 412 14 435 10Com. Ex. 13 100 415 13 440 10Com. Ex. 14 98 415 12 445 10Com. Ex. 15 95 415 13 445 10Com. Ex. 16 96 415 13 445 10__________________________________________________________________________ Note .sup.(1) Measured at a time when decrease in pressure drop was first appreciated. .sup.(2) Ex.: Example, and Com. Ex.: Comparative Example. .sup.(3) Calculated from the amount (Xa) of NOx in exhaust gas before entering into the filter, and the amount of (Xb) of NOx in exhaust gas after passage through the filter, by using the formula: (Xa - Xb)/Xa. .sup.(4) Measured at a time when decrease in pressure drop was first appreciated after 10 hours from the initiation of passing exhaust gas through the filter.
As is clear from Table 2, the exhaust gas cleaners of Examples 9-16 show smaller pressure drop than those of Comparative Examples 9-16, and even after the lapse of 10 hours, they show high conversion rates of NOx to N.sub.2 than those of Comparative Examples 9-16. Also, at that time, the regeneration temperature of the filter at which particulate matter is ignited and burned is lower in Examples than in Comparative Examples, meaning that exhaust gas is efficiently cleaned for a long period of time in the present invention.
EXAMPLE 17
A ceramic foam-type filter made of cordierite (apparent volume: 2 l, density: 0.65 g/ml) was coated with TiO.sub.2 powder in an amount of 10 weight % based on the filter, by a wash-coating method.
The coated filter was impregnated with 1 weight % of Cu by using an aqueous solution of CuCl.sub.2, 1 weight % of La by using an aqueous solution of La(NO.sub.3).sub.3, and 1 weight % of Cs by using an aqueous solution of CsNO.sub.3, each based on TiO.sub.2. After drying the impregnated filter, it was burned at 700.degree. C. In the resulting exhaust gas cleaner, metal components of Cu, La and Cs existed as metal oxides. The composition of the exhaust gas cleaner was as follows:
Cs/Cu/La (TiO.sub.2)
With respect to this exhaust gas cleaner, a regeneration temperature of the filter (expressed by a temperature at which pressure drop started to decrease by burning particulate matter) and a conversion rate of NOx to N.sub.2 at the regeneration temperature were measured by using a diesel engine having a displacement of 510 cc. The diesel engine was operated at 1500 rpm under a load of 90%. Under these conditions, the exhaust gas comprised 150 ppm of HC (total of hydrocarbons), 460 ppm of CO, 480 ppm of NOx, 10% of O.sub.2 and 200 ppm of SO.sub.2. The results are shown in Table 3.
EXAMPLE 18
In the same manner as in Example 17, a ceramic foam-type filter made of cordierite was coated with TiO.sub.2 in an amount of 10 weight % based on the filter. The coated filter was impregnated with 1 weight % of Cu, 1 weight % of Ce, and 1 weight % of Cs, each based on the TiO.sub.2 carrier layer, by using aqueous solutions of CuCl.sub.2, Ce(NO.sub.3).sub.3 and CsNO.sub.3, respectively. The impregnated filter was dried and burned in the same manner as in Example 17 to produce an exhaust gas cleaner having the following composition:
Cs/Cu/Ce (TiO.sub.2)
EXAMPLE 19
In the same manner as in Example 18, an exhaust gas cleaner supporting 1 weight % of Cu, 1 weight % of La, and 1 weight % of K, each based on TiO.sub.2 carrier layer, was produced by using aqueous solutions of CuCl.sub.2, La(NO.sub.3).sub.3, and KCl, respectively. The composition of the exhaust gas cleaner was as follows:
K/Cu/La (TiO.sub.2)
EXAMPLE 20
In the same manner as in Example 18, an exhaust gas cleaner supporting 1 weight % of Cu, 1 weight % of Ce, and 1 weight % of K, each based on the TiO.sub.2 carrier layer, was produced by using aqueous solutions of CuCl.sub.2, Ce(NO.sub.3).sub.3, and KCl, respectively. The composition of the exhaust gas cleaner was as follows:
K/Cu/Ce (TiO.sub.2)
EXAMPLES 21-24
Examples 17-20 were repeated except for using CoCl.sub.2 in place of CuCl.sub.2, and changing the types of ceramic carrier powder.
______________________________________Cs/Co/La (Al.sub.2 O.sub.3): (Example 21)Cs/Co/Ce (Al.sub.2 O.sub.3 -ZrO.sub.2): (Example 22)K/Co/La (ZrO.sub.2): (Example 23)K/Co/Ce (TiO.sub.2 -ZrO.sub.2): (Example 24)______________________________________
EXAMPLES 25-28
Examples 17-20 were repeated except for using manganese acetate in place of CuCl.sub.2.
______________________________________Cs/Mn/La (TiO.sub.2): (Example 25)Cs/Mn/Ce (TiO.sub.2): (Example 26)K/Mn/La (TiO.sub.2): (Example 27)K/Mn/Ce (TiO.sub.2): (Example 28)______________________________________
With respect to the resulting exhaust gas cleaners of Examples 18-28, a regeneration temperature and a conversion rate of NOx to N.sub.2 at the regeneration temperature were measured in the same manner as in Example 17. The results are shown in Table 3.
COMPARATIVE EXAMPLE 17
In the same manner as in Example 17, a ceramic foam-type filter made of cordierite was coated with TiO.sub.2 in an amount of 10 weight % based on the filter, by using a wash-coating method. The coated filter was impregnated with Cu in an amount of 1 weight % based on the filter by using an aqueous solution of CuCl.sub.2, dried and burned in the same manner as in Example 17.
With respect to the resulting exhausts gas cleaner, a regeneration temperature and a conversion rate of NOx to N.sub.2 at the regeneration temperature were measured in the same manner as in Example 17. The results are shown in Table 3.
TABLE 3______________________________________ Regeneration Conversion Rate ofNo. Temp. (.degree.C.) NOx to N.sub.2.sup.(1) (%)______________________________________Example 17 340 50Example 18 360 40Example 19 370 35Example 20 370 35Example 21 360 45Example 22 365 35Example 23 380 30Example 24 380 30Example 25 385 40Example 26 390 35Example 27 395 30Example 28 395 30Comparative 450 20Example 17______________________________________ Note .sup.(1) Calculated from the amount (Xa) of NOx in exhaust gas before entering into the filter, and the amount of (Xb) of NOx in exhaust gas after passage through the filter, by using the formula: (Xa - Xb)/Xa.
As is clear from Table 3, the exhaust gas cleaners of Examples 17-28 show as high conversion rates of NOx to N.sub.2 as 30% or more. Also, at the regeneration temperature of the filter at which particulate matter is ignited and burned, exhaust gas is efficiently cleaned in the present invention.
EXAMPLE 29
A ceramic foam-type filter made of cordierite (apparent volume: 2 l, density: 0.65 g/ml) was coated with TiO.sub.2 powder in an amount of 10 weight % based on the filter, by a wash-coating method.
The coated filter was impregnated with 1 weight % of Cu by using an aqueous solution of CuCl.sub.2, 1 weight % of La by using an aqueous solution of La(NO.sub.3).sub.3, and 1 weight % of Cs by using an aqueous solution of CsNO.sub.3, each based on TiO.sub.2. After drying the impregnated filter, it was burned at 700.degree. C. Next, the burned filter was impregnated with 1 weight % of V by using an aqueous solution of NH.sub.4 VO.sub.3 and oxalic acid. After drying the impregnated filter, it was burned again at 700.degree. C. for 3 hours to produce an exhaust gas cleaner.
Composition: Cs/Cu/La/V (TiO.sub.2)
With respect to this exhaust gas cleaner, a regeneration temperature of the filter and a conversion rate of NOx to N.sub.2 at the regeneration temperature were measured by using a diesel engine having a displacement of 510 cc. The regeneration temperature and the conversion rate of NOx to N.sub.2 were evaluated at two points, i.e., at a time when decrease in pressure drop was observed for the first time in an initial stage of the operation, and at a time when decrease in pressure drop was observed after the lapse of 10 hours from the initiation of the operation. The diesel engine was operated at 1500 rpm under a load of 90%. Under these conditions, the exhaust gas comprised 150 ppm of HC (total of hydrocarbons), 460 ppm of CO, 480 ppm of NOx, 10% of O.sub.2 and 200 ppm of SO.sub.2. The results are shown in Table 4.
EXAMPLE 30
Example 29 was repeated except for using Al.sub.2 O.sub.3 in an amount of 10 weight % based on the filter as a ceramic carrier powder, and impregnating 1 weight %, based on the Al.sub.2 O.sub.3 layer, of Ce by using Ce(NO.sub.3).sub.3 in place of La(NO.sub.3).sub.3.
______________________________________Composition: Cs/Cu/Ce/V (Al.sub.2 O.sub.3)______________________________________
EXAMPLE 31
Example 29 was repeated except for using Al.sub.2 O.sub.3 -ZrO.sub.2 in an amount of 10 weight % based on the filter as a ceramic carrier powder, and impregnating 1 weight %, based on the Al.sub.2 O.sub.3 -ZrO.sub.2 layer, of K by using KCl in place of CsNO.sub.3.
______________________________________Composition: K/Cu/La/V (Al.sub.2 O.sub.3 -ZrO.sub.2)______________________________________
EXAMPLE 32
Example 30 was repeated except for using ZrO.sub.2 in an amount of 10 weight % based on the filter as a ceramic carrier powder, and impregnating 1 weight %, based on the ZrO.sub.2 layer, of K by using KCl in place of CsNO.sub.3.
______________________________________Composition: K/Cu/Ce/V (ZrO.sub.2)______________________________________
EXAMPLES 33-36
Examples 29-32 were repeated except for using CoCl.sub.2 in place of CuCl.sub.2, and changing the types of ceramic carrier powder.
______________________________________Cs/Co/La/V (TiO.sub.2 -ZrO.sub.2): (Example 33)Cs/Co/Ce/V (TiO.sub.2): (Example 34)K/Co/La/V (TiO.sub.2): (Example 35)K/Co/Ce/V (TiO.sub.2): (Example 36)______________________________________
EXAMPLES 37-40
Examples 29-32 were repeated except for using manganese acetate in place of CuCl.sub.2, and changing the types of ceramic carrier powder.
______________________________________Cs/Mn/La/V (TiO.sub.2): (Example 37)Cs/Mn/Ce/V (TiO.sub.2): (Example 38)K/Mn/La/V (TiO.sub.2): (Example 39)K/Mn/Ce/V (TiO.sub.2): (Example 40)______________________________________
COMPARATIVE EXAMPLE 18
In the same manner as in Example 29, a ceramic foam-type filter made of cordierite was coated with TiO.sub.2 in an amount of 10 weight % based on the filter, by using a wash-coating method. The coated filter was impregnated with Cu in an amount of 1 weight % based on the filter by using an aqueous solution of CuCl.sub.2, dried and burned in the same manner as in Example 29.
With respect to the resulting exhaust gas cleaners of Examples 30-40 and Comparative Example 18, a regeneration temperature and a conversion rate of NOx to N.sub.2 at the regeneration temperature were measured in the same manner as in Example 29. The results are shown in Table 4.
TABLE 4______________________________________ Initial Stage.sup.(1) After 10 Hours.sup.(2) Regener- Conversion Regener- Conversion ation Rate of ation Rate of Temp. NOx to N.sub.2.sup.(3) Temp. NOx to N.sub.2.sup.(3)No. (.degree.C.) (%) (.degree.C.) (%)______________________________________Example 29 380 40 382 38Example 30 385 35 385 32Example 31 400 25 403 22Example 32 400 25 405 22Example 33 385 30 390 28Example 34 390 25 395 23Example 35 405 20 408 19Example 36 405 20 408 18Example 37 400 30 410 28Example 38 405 25 415 24Example 39 410 20 420 18Example 40 410 20 420 18Comparative 450 20 500 10Example 18______________________________________ Note .sup.(1) Measured at a time when decrease in pressure drop was first appreciated. .sup.(2) Measured at a time when decrease in pressure drop was first appreciated after 10 hours from the initiation of passing exhaust gas through the filter. .sup.(3) Calculated from the amount (Xa) of NOx in exhaust gas before entering into the filter, and the amount of (Xb) of NOx in exhaust gas after passage through the filter, by using the formula: (Xa - Xb)/Xa.
As is clear from Table 4, the exhaust gas cleaners of Examples 29-40 show high conversion rates of NOx to N.sub.2 than those of Comparative Example 18, when 10 hours passed after the operation had started. Also, at a time after the lapse of 10 hours, the regeneration temperature of the filter at which particulate matter is ignited and burned is lower in Examples than in Comparative Example, meaning that exhaust gas is efficiently cleaned for a long period of time in the present invention.
EXAMPLE 41
A ceramic foam-type filter made of cordierite (apparent volume: 1 l, density: 0.65 g/ml) was coated with TiO.sub.2 powder in an amount of 10 weight % based on the filter, by a wash-coating method. The coated filter was impregnated with 1 weight % of Cs by using an aqueous solution of CsNO.sub.3, 2 weight % of Cu by using an aqueous solution of CuCl.sub.2, 1 weight % of La by using an aqueous solution of La(NO.sub.3).sub.3, each based on TiO.sub.2. Next, the impregnated filter was further impregnated with 1 weight % of V by using an aqueous solution of NH.sub.4 VO.sub.3 and oxalic acid. After drying the impregnated filter, it was burned at 700.degree. C. for 3 hours to prepare a first filter.
The same type of ceramic foam-type filter as above (apparent volume: 0.2 l, density: 0.65 g/ml) was coated with .gamma.-Al.sub.2 O.sub.3 powder in an amount of 10 weight % based on the filter, by a wash-coating method. The coated filter was impregnated with 0.1 weight %, based on .gamma.-Al.sub.2 O.sub.3, of Pt by using an aqueous solution of H.sub.2 PtCl.sub.6. After drying the impregnated filter, it was burned at 700.degree. C. to prepare a second filter.
The first and second filters were stacked such that the first filter was positioned on the inlet side, while the second filter was positioned on the outlet side, to produce an exhaust gas cleaner having the following composition:
Cs/Cu/V/La/TiO.sub.2 -Pt/Al.sub.2 O.sub.3
With respect to this exhaust gas cleaner, a regeneration temperature of the filter and a conversion rate of NOx to N.sub.2 at the regeneration temperature were measured by using a diesel engine having a displacement of 510 cc. The regeneration temperature and the conversion rate of NOx to N.sub.2 were evaluated at two points, i.e., at a time when decrease in pressure drop was observed for the first time in an initial stage of the operation, and at a time when decrease in pressure drop was observed after the lapse of 10 hours from the initiation of the operation. The diesel engine was operated at 2500 rpm under a load of 80%. Under these conditions, the exhaust gas comprised 90 ppm of HC (total of hydrocarbons), 460 ppm of CO, 480 ppm of NOx, 8% of O.sub.2 and 100 ppm of SO.sub.2. The results are shown in Table 5.
EXAMPLE 42
In the same manner as in Example 41, a first filter was prepared by coating TiO.sub.2 in an amount of 10 weight % based on the filter and impregnating it with 1 weight % of Cs by using an aqueous solution of CsNO.sub.3, 2 weight % of Cu by using an aqueous solution of CuCl.sub.2, 1 weight % of Ce by using an aqueous solution of Ce(NO.sub.3).sub.3, each based on TiO.sub.2, and then impregnating the filter with 1 weight % of V.
This first filter was stacked with the second filter obtained in Example 41 to produce an exhaust gas cleaner having the following composition:
Cs/Cu/V/Ce/TiO.sub.2 -Pt/Al.sub.2 O.sub.3
EXAMPLE 43
In the same manner as in Example 41, a first filter was prepared by coating TiO.sub.2 in an amount of 10 weight % based on the filter and impregnating it with 1 weight % of K by using an aqueous solution of K.sub.2 CO.sub.3, 2 weight % of Cu by using an aqueous solution of Cu(NO.sub.3).sub.2, and 1 weight % of La by using an aqueous solution of La(NO.sub.3).sub.3, each based on TiO.sub.2, and then impregnating the filter with 1 weight % of V.
This first filter was stacked with the second filter obtained in Example 41 to produce an exhaust gas cleaner having the following composition:
K/Cu/V/La/TiO.sub.2 -Pt/Al.sub.2 O.sub.3
EXAMPLE 44
In the same manner as in Example 41, a first filter was prepared by coating TiO.sub.2 in an amount of 10 weight % based on the filter and impregnating it with 1 weight % of K by using an aqueous solution of K.sub.2 CO.sub.3, 2 weight % of Cu by using an aqueous solution of Cu(NO.sub.3).sub.2, and 1 weight % of Ce by using an aqueous solution of Ce(NO.sub.3).sub.3, each based on TiO.sub.2, and then impregnating the filter with 1 weight % of V.
This first filter was stacked with the second filter obtained in Example 41 to produce an exhaust gas cleaner having the following composition:
K/Cu/V/Ce/TiO.sub.2 -Pt/Al.sub.2 O.sub.3
With respect to the resulting exhaust gas cleaners of Examples 42-44, a regeneration temperature and a conversion rate of NOx to N.sub.2 at the regeneration temperature were measured in the same manner as in Example 41. The results are shown in Table 5.
EXAMPLES 45-47
The same first filters as in Examples 42-44 were prepared.
On the other hand, TiO.sub.2 was dispersed in an aqueous solution of PdCl.sub.2 (Example 45), in an aqueous solution of H.sub.2 PtCl.sub.6 (Example 46), and in an aqueous solution of H.sub.2 PtCl.sub.6 and PdCl.sub.2 (Example 47), to impregnate it with 0.1 weight % of Pd (Example 45), 0.1 weight % of Pt (Example 46), and 0.1 weight % of Pt+0.1 weight % of Pd (Example 47). Each ceramic foam-type filter made of cordierite (apparent volume: 0.2 l, density: 0.65 g/ml) was coated with the above TiO.sub.2 carrying the Pt-group metal to prepare a second filter.
The first and second filters were stacked in the same manner as in Example 41, to produce an exhaust gas cleaner having the following composition:
______________________________________Cs/Cu/V/La/TiO.sub.2 -Pd/TiO.sub.2 (Example 45)Cs/Cu/V/Ce/TiO.sub.2 -Pt/TiO.sub.2 (Example 46)K/Cu/V/La/TiO.sub.2 -Pt, Pd/TiO.sub.2 (Example 47)______________________________________
With respect to the resulting exhaust gas cleaners of Examples 45-47, a regeneration temperature and a conversion rate of NOx to N.sub.2 at the regeneration temperature were measured in the same manner as in Example 41. The results are shown in Table 5.
COMPARATIVE EXAMPLE 19
Example 41 was repeated except for using only Cu in the catalyst applied to the first filter. The resulting exhaust gas cleaner had the following composition:
Cu/TiO.sub.2 -Pt/Al.sub.2 O.sub.3
A regeneration temperature and a conversion rate of NOx to N.sub.2 at the regeneration temperature were measured in the same manner as in Example 41. The results are shown in Table 5.
TABLE 5__________________________________________________________________________ Initial Stage.sup.(1) After 10 Hours.sup.(4) Conversion Rate Conversion Rate of NOx to N.sub.2.sup.(3) of NOx to N.sub.2.sup.(3)No..sup.(2) Regen. Temp (.degree.C.) NOx CO HC Regen. Temp (.degree.C.) NOx CO HC__________________________________________________________________________Ex. 41 380 10 55 40 390 8 50 38Ex. 42 385 12 55 40 395 10 50 38Ex. 43 400 8 57 39 400 8 55 38Ex. 44 410 7 58 39 405 7 55 38Ex. 45 385 10 50 30 390 8 45 28Ex. 46 385 12 60 50 390 10 58 48Ex. 47 405 15 70 60 400 12 65 58Com. Ex. 19 450 5 30 20 530 2 15 10__________________________________________________________________________ Note .sup.(1) Measured at a time when decrease in pressure drop was first appreciated. .sup.(2) Ex.: Example, and Com. Ex.: Comparative Example. .sup.(3) Calculated from the amount (Xa) of NOx in exhaust gas before entering into the filter, and the amount of (Xb) of NOx in exhaust gas after passage through the filter, by using the formula: (Xa - Xb)/Xa. .sup.(4) Measured at a time when decrease in pressure drop was first appreciated after 10 hours from the initiation of passing exhaust gas through the filter.
As is clear from Table 5, the exhaust gas cleaners of Examples 41-47 show higher cleaning efficiency than those of Comparative Example 19 even after the lapse of 10 hours. Also, at that time, the regeneration temperature of the filter at which particulate matter is ignited and burned is lower in Examples than in Comparative Example.
As described above in detail, by using the exhaust gas cleaner according to the present invention, both particulate matter and NOx can efficiently be removed from the exhaust gas. Such exhaust gas cleaners are effective for cleaning exhaust gases even at a low temperature, such as those of diesel engines. Also, since the catalyst contains Co and/or Mn and V as transition metals, the efficiency of the catalyst is kept high for a long period of time even when it is in contact with an exhaust gas containing a high concentration of SO.sub.2.
In the first exhaust gas cleaner of the present invention, since the catalyst is supported by the porous ceramic carrier powder layer formed on the filter by a wash-coating method, a sol-gel method, etc., the catalyst can be uniformly supported on the filter in a high concentration, leading to a good catalytic activity.
In the second exhaust gas cleaner of the present invention, since a uniform mixture of the catalyst with the porous ceramic carrier powder is supported by the filter, it suffers from only little pressure drop, thereby effecting the removal of both particulate matter and NOx efficiently.
In the third exhaust gas cleaner of the present invention, since the catalyst having a particular composition is supported by the filter in an extremely small amount, high reactivity of HC with particulate matter and unburned hydrocarbons can be maintained for a long period of time.
In the fourth exhaust gas cleaner of the present invention, since the first and second catalyst are combined, NOx and particulate matter, HC, CO, etc. are efficiently removed from the exhaust gas for a long period of time.
The exhaust cleaner and the exhaust gas-cleaning method according to the present invention are highly useful for cleaning the exhaust gas of diesel engines, etc.

Number	Date	Country
3-155206	May 1991	JPX
3-155212	May 1991	JPX
3-162004	Jun 1991	JPX
3-308439	Oct 1991	JPX

Number	Name	Date
3896048	Mehalchick et al.	Jul 1975
4485191	Sekido et al.	Nov 1984
5041270	Fujitani et al.	Aug 1991
5059575	Jorgensen et al.	Oct 1991
5075274	Kiyohide et al.	Dec 1991
5108977	Yoshida et al.	Apr 1992
5141714	Obuchi et al.	Aug 1992

Number	Date	Country
4008371	Sep 1990	DEX
63-100919	May 1988	JPX
3047539	Feb 1991	JPX

Exhaust gas cleaner and method of cleaning exhaust gas

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