Abstract:
The present invention provides an oxidation catalyst for cleaning exhaust gas, capable of achieving an excellent catalytic activity at a lower temperature for particulates and high boiling point hydrocarbons in exhaust gas from internal-combustion engines. The oxidation catalyst for cleaning exhaust gas according to the present invention is a composite metal oxide represented by the general formula: Ln y Mn 1-x A x O 3 , wherein Ln is a metal selected from the group consisting of Sc, Y, Ho, Er, Tm, Yb, and Lu; A is a metal selected from the group consisting of Ti, Nb, Ta, and Ru; 0.005≦x≦0.2; and 0.9≦y≦1. Ln is Y. The composite metal oxide has a hexagonal structure.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an oxidation catalyst for cleaning exhaust gas, oxidizing particulates and hydrocarbons contained in exhaust gas from internal-combustion engines to clean the gas. 
     2. Description of the Related Art 
     For oxidizing particulates and hydrocarbons contained in exhaust gas from internal-combustion engines to clean the gas, oxidation catalysts comprising perovskite-type composite metal oxides have been previously known. 
     As the perovskite-type composite metal oxide used as the above-described oxidation catalyst, there is known, for example, a composite metal oxide represented by the general formula: AB 1-x C x O 3 , wherein A is at least one metal selected from the group consisting of La, Sr, Ce, Ba, and Ca; B is at least one metal selected from the group consisting of Co, Fe, Ni, Cr, Mn, and Mg; and C is one of Pt and Pd (see Japanese Patent Laid-Open No. 07-116519). 
     As the perovskite-type composite metal oxide used as the above-described oxidation catalyst, there is also known, for example, a composite metal oxide represented by the general formula: Ce x M 1-x ZrO 3 , wherein M is at least one metal selected from the group consisting of La, Sm, Nd, Gd, Sc, and Y; and 0.1≦x≦20 (see, for example, Japanese Patent Laid-Open No. 2003-334443). 
     However, the above-described conventional perovskite-type composite metal oxides have drawbacks that they have high oxidation temperatures for particulates and high boiling point hydrocarbons and cannot achieve sufficient catalytic activities. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to eliminate such drawbacks and to provide an oxidation catalyst for cleaning exhaust gas, capable of achieving an excellent catalytic activity at a lower temperature for particulates and high boiling point hydrocarbons in exhaust gas from internal-combustion engines. 
     For accomplishing this object, the oxidation catalyst for cleaning exhaust gas according to the present invention is an oxidation catalyst oxidizing contents in exhaust gas from internal-combustion engines to clean the gas, comprising a composite metal oxide represented by the general formula: Ln y Mn 1-x A x O 3 , wherein Ln is a metal selected from the group consisting of Sc, Y, Ho, Er, Tm, Yb, and Lu; A is a metal selected from the group consisting of Ti, Nb, Ta, and Ru; 0.005≦x≦0.2; and 0.9≦y≦1. 
     The oxidation catalyst for cleaning exhaust gas according to the present invention is one in which metal A as a third metal component is added to a composite metal oxide represented by the general formula: LnMnO 3  to produce a distortion in the crystal lattice thereof, or the metal component A is added thereto to produce a defect in a portion of the crystal lattice as well as the distortion in the crystal lattice, thereby increasing the catalytic activity and reducing the bond energy of oxygen in the crystal lattice. As a result, the oxidation catalyst for cleaning exhaust gas according to the present invention can oxidize contents such as particulates and high boiling point hydrocarbons contained in exhaust gas from internal-combustion engines at a lower temperature and also cause a faster oxidation than an oxidation catalyst comprising a compound represented by the general formula: LnMnO 3 . 
     The oxidation catalyst for cleaning exhaust gas according to the present invention produces a lower bond energy of oxygen in the crystal lattice of the composite metal oxide represented by the general formula: LnMnO 3 , first when y=1, by metal A as a third metal component being added to the oxide to cause a distortion in the crystal lattice. As a result, the oxidation catalyst for cleaning exhaust gas according to the present invention can oxidize the particulates, high boiling point hydrocarbons, and the like at a lower temperature than the oxidation catalyst comprising the composite metal oxide represented by the general formula: LnMnO 3 . Here, x less than 0.005 renders insufficient the effect of producing a distortion in the crystal lattice; x more than 0.2 makes it difficult to maintain the crystal lattice. 
     Then, the oxidation catalyst for cleaning exhaust gas according to the present invention produces a lower bond energy of oxygen in the crystal lattice of the composite metal oxide represented by the general formula: LnMnO 3 , when 0.9≦y&lt;1, by metal A as a third metal component being added to the oxide to cause a defect in a portion of the Ln site constituting the crystal lattice as well as a distortion in the crystal lattice. As a result, the oxidation catalyst for cleaning exhaust gas according to the present invention can oxidize the particulates, high boiling point hydrocarbons, and the like at a lower temperature and also cause a faster oxidation than the oxidation catalyst comprising the composite metal oxide represented by the general formula: LnMnO 3 . 
     Here, y less than 0.9 produces an excessive defect to make it difficult to maintain the crystal lattice; y at 1 cannot produce a defect in the crystal lattice. In addition, x can be in the above-described range to balance the positive and negative electric charges of the constituent atoms in the composite metal oxide. 
     In the composite metal oxide, Ln may be a metal selected from the group consisting of Sc, Y, Ho, Er, Tm, Yb, and Lu, but is preferably Y. In addition, the composite metal oxide preferably has a hexagonal structure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a graph showing the effects of catalysts for cleaning exhaust gas in accordance with the present invention; 
         FIG. 2  is a graph showing the effect of a catalyst for cleaning exhaust gas in accordance with the present invention; 
         FIG. 3  is a graph showing the effects of catalysts for cleaning exhaust gas in accordance with the present invention; 
         FIG. 4  is a graph showing the effects of catalysts for cleaning exhaust gas in accordance with the present invention; 
         FIG. 5  is a graph showing the effect of a catalyst for cleaning exhaust gas in accordance with the present invention; and 
         FIG. 6  is a graph showing the effect of a catalyst for cleaning exhaust gas in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of the present invention will now be described in further detail with reference to the accompanying drawings. 
     A catalyst for cleaning exhaust gas according to a first aspect of the present embodiment comprises a composite metal oxide represented by the general formula: YMn 1-x A x O 3 , wherein A is a metal selected from the group consisting of Ti, Nb, Ta, and Ru; and 0.005≦x≦0.2. The composite metal oxide produces a lower bond energy of oxygen in the crystal lattice of YMnO 3  by a portion of Mn being the metal A to cause a distortion in the crystal lattice. As a result, the metal oxide can have an increased catalytic activity compared to YMnO 3  and can oxidize contents such as particulates and high boiling point hydrocarbons contained in the exhaust gas at a lower temperature. 
     A catalyst for cleaning exhaust gas according to a second aspect of the present embodiment is a composite metal oxide similar to the catalyst for cleaning exhaust gas according to the first aspect, but different from it only in that the former is represented by the general formula: Y y Mn 1-x A x O 3  except with 0.9≦y&lt;1. The composite metal oxide produces a lower bond energy of oxygen in the crystal lattice of YMnO 3  by a defect arising in a portion of the Y site constituting the crystal lattice as well as by a portion of Mn being the metal A to cause a distortion in the crystal lattice. As a result, the metal oxide can have an increased catalytic activity compared to YMnO 3 , can oxidize contents such as particulates and high boiling point hydrocarbons contained in the exhaust gas at a lower temperature, and even can promote the oxidation. 
     Here, the above-described x and y are set so as to balance the positive and negative electric charges of the constituent atoms in the composite metal oxide. 
     By way of example, Y and Mn are positive trivalent, and O is negative bivalent. Accordingly, when the metal A is one of Ti and Ru and positive tetravalent, setting y=1−x/3 and x=0.15 leads to the general formula: Y y Mn 1-x A x O 3  being equal to Y 0.95 Mn 0.85 A 0.15 O 3 . 
     Here, the sum of positive charges is:
 
(3×0.95)+(3×0.85)+(4×0.15)=2.85+2.55+0.60=+6.00; and
 
the sum of negative charges is:
 
(−2)×3=−6.
 
     Therefore, it follows that (+6.00)+(−6)=0, the positive and negative charges being balanced. 
     When the metal A is one of Nb and Ta and positive pentavalent, setting y=1−2x/3 and x=0.05 leads to the general formula: Y y Mn 1-x A x O 3  being equal to Y 0.95 Mn 0.925 A 0.075 O 3 . 
     Here, the sum of positive charges is:
 
(3×0.95)+(3×0.925)+(5×0.075)=2.85+2.775+0.375=+6.00; and
 
the sum of negative charges is:
 
(−2)×3=−6.
 
     Therefore, it follows that (+6.00)+(−6)=0, the positive and negative charges being balanced. 
     When the metal A is one of Nb and Ta and positive pentavalent, y=1−2x/3 and x=0.15 may be also set. 
     This setting leads to the general formula: Y y Mn 1−x A x O 3  being equal to Y 0.9 Mn 0.85 A 0.15 O 3 . 
     Here, the sum of positive charges is:
 
(3×0.9)+(3×0.85)+(5×0.075)=2.7+2.55+0.75=+6.00; and
 
the sum of negative charges is:
 
(−2)×3=−6.
 
     Therefore, it follows that (+6.00)+(−6)=0, the positive and negative charges being balanced. 
     Examples and Comparative Example of the present invention will now be given. 
     EXAMPLE 1 
     In this Example, first, yttrium acetate, manganese nitrate, and anatase type titanium oxide were used in such amounts that a molar ratio thereof of 1:0.95:0.05 is obtained, and mixed in a ball mill for 5 hours, followed by primary firing at 250° C. for 30 minutes, at 300° C. for 30 minutes, and at 350° C. for one hour. Ethanol was then added to the resultant material from the primary firing process, which was then subjected to wet grinding using a ball mill before drying, followed by secondary firing at 1,000° C. for one hour to provide a powder of the composite metal oxide represented by YMn 0.95 Ti 0.05 O 3 . 
     The composite metal oxide powder obtained in this Example was then subjected to differential thermal analysis (DTA) for the activity evaluation thereof. The differential thermal analysis was performed by using the composite metal oxide powder obtained in this Example as a catalyst for cleaning exhaust gas to mix 2.5 mg of carbon black with 50 mg of the catalyst, followed by heating the mixture at a rate of temperature rise of 10° C./minute under an atmosphere of an air stream of 15 ml/minute to determine a relationship between heat flow and temperature. 
     The above-described carbon black corresponds to particulates or a high boiling point hydrocarbon contained in the exhaust gas. In the heat flow, the peak thereof indicates the burning temperature of the carbon black; a higher peak shows that the burning is more promoted. The result is shown in  FIG. 1 . 
     EXAMPLE 2 
     In this Example, the composite metal oxide represented by YMn 0.95 Nb 0.05 O 3  was obtained just as described in Example 1 except for the use of niobium oxide in place of anatase type titanium oxide. 
     A relationship between heat flow and temperature was then determined just as described in Example 1 except for the use of the composite metal oxide obtained in this Example as a catalyst for cleaning exhaust gas. The result is shown in  FIG. 1 . 
     EXAMPLE 3 
     In this Example, the composite metal oxide represented by YMn 0.95 Ta 0.05 O 3  was obtained just as described in Example 1 except for the use of tantalum oxide in place of anatase type titanium oxide. 
     A relationship between heat flow and temperature was then determined just as described in Example 1 except for the use of the composite metal oxide obtained in this Example as a catalyst for cleaning exhaust gas. The result is shown in  FIG. 1 . 
     EXAMPLE 4 
     In this Example, the composite metal oxide represented by YMn 0.95 Ru 0.05 O 3  was obtained just as described in Example 1 except for the use of ruthenium oxide in place of anatase type titanium oxide. 
     A relationship between heat flow and temperature was then determined just as described in Example 1 except for the use of the composite metal oxide obtained in this Example as a catalyst for cleaning exhaust gas. The result is shown in  FIG. 1 . 
     COMPARATIVE EXAMPLE 1 
     In this Comparative Example, the composite metal oxide represented by YMnO 3  was obtained just as described in Example 1 except for no use of anatase type titanium oxide. 
     A relationship between heat flow and temperature was then determined just as described in Example 1 except for the use of the composite metal oxide obtained in this Comparative Example as a catalyst for cleaning exhaust gas. The result is shown in  FIG. 1 . 
     It is apparent from  FIG. 1  that the catalysts for cleaning exhaust gas of Examples 1 to 4 can oxidize (burn) the above-described carbon black at low temperature compared to the catalyst for cleaning exhaust gas of Comparative Example 1, which comprises the composite metal oxide represented by YMnO 3 . 
     EXAMPLE 5 
     In this Example, the composite metal oxide represented by Y 0.95 Mn 0.85 Ti 0.15 O 3  was obtained just as described in Example 1 except for the use of yttrium acetate, manganese nitrate, and anatase type titanium oxide in such amounts that a molar ratio thereof of 0.95:0.85:0.15 is obtained. 
     A relationship between heat flow and temperature was then determined just as described in Example 1 except for the use of the composite metal oxide obtained in this Example as a catalyst for cleaning exhaust gas. The result is shown together with the result of Comparative Example 1 in  FIG. 2 . 
     It is apparent from  FIG. 2  that the catalyst for cleaning exhaust gas of Example 5 can oxidize (burn) the above-described carbon black at low temperature and can achieve the effect of further promoting the oxidation, compared to the catalyst for cleaning exhaust gas of Comparative Example 1, which comprises the composite metal oxide represented by YMnO 3 . 
     EXAMPLE 6 
     In this Example, the composite metal oxide represented by Y 0.95 Mn 0.925 Nb 0.075 O 3  was obtained just as described in Example 1 except for the use of yttrium acetate, manganese nitrate, and niobium oxide in such amounts that a molar ratio thereof of 0.95:0.925:0.075 is obtained. 
     A relationship between heat flow and temperature was then determined just as described in Example 1 except for the use of the composite metal oxide obtained in this Example as a catalyst for cleaning exhaust gas. The result is shown together with the result of Comparative Example 1 in  FIG. 3 . 
     EXAMPLE 7 
     In this Example, the composite metal oxide represented by Y 0.9 Mn 0.85 Nb 0.15 O 3  was obtained just as described in Example 1 except for the use of yttrium acetate, manganese nitrate, and niobium oxide in such amounts that a molar ratio thereof of 0.9:0.85:0.15 is obtained. 
     A relationship between heat flow and temperature was then determined just as described in Example 1 except for the use of the composite metal oxide obtained in this Example as a catalyst for cleaning exhaust gas. The result is shown together with the result of Comparative Example 1 in  FIG. 3 . 
     It is apparent from  FIG. 3  that the catalysts for cleaning exhaust gas of Examples 6 and 7 can oxidize (burn) the above-described carbon black at low temperature and can achieve the effect of further promoting the oxidation, compared to the catalyst for cleaning exhaust gas of Comparative Example 1, which comprises the composite metal oxide represented by YMnO 3 . 
     EXAMPLE 8 
     In this Example, the composite metal oxide represented by Y 0.95 Mn 0.925 Ta 0.075 O 3  was obtained just as described in Example 1 except for the use of yttrium acetate, manganese nitrate, and tantalum oxide in such amounts that a molar ratio thereof of 0.95:0.925:0.075 is obtained. 
     A relationship between heat flow and temperature was then determined just as described in Example 1 except for the use of the composite metal oxide obtained in this Example as a catalyst for cleaning exhaust gas. The result is shown together with the result of Comparative Example 1 in  FIG. 4 . 
     EXAMPLE 9 
     In this Example, the composite metal oxide represented by Y 0.9 Mn 0.85 Ta 0.15 O 3  was obtained just as described in Example 1 except for the use of yttrium acetate, manganese nitrate, and tantalum oxide in such amounts that a molar ratio thereof of 0.9:0.85:0.15 is obtained. 
     A relationship between heat flow and temperature was then determined just as described in Example 1 except for the use of the composite metal oxide obtained in this Example as a catalyst for cleaning exhaust gas. The result is shown together with the result of Comparative Example 1 in  FIG. 4 . 
     It is apparent from  FIG. 4  that the catalysts for cleaning exhaust gas of Examples 8 and 9 can oxidize (burn) the above-described carbon black at low temperature and can achieve the effect of further promoting the oxidation, compared to the catalyst for cleaning exhaust gas of Comparative Example 1, which comprises the composite metal oxide represented by YMnO 3 . 
     EXAMPLE 10 
     In this Example, the composite metal oxide represented by Y 0.95 Mn 0.85 Ru 0.15 O 3  was obtained just as described in Example 1 except for the use of yttrium acetate, manganese nitrate, and ruthenium oxide in such amounts that a molar ratio thereof of 0.95:0.85:0.15 is obtained. 
     A relationship between heat flow and temperature was then determined just as described in Example 1 except for the use of the composite metal oxide obtained in this Example as a catalyst for cleaning exhaust gas. The result is shown together with the result of Comparative Example 1 in  FIG. 5 . 
     It is apparent from  FIG. 5  that the catalyst for cleaning exhaust gas of Example 10 can oxidize (burn) the above-described carbon black at low temperature and can achieve the effect of further promoting the oxidation, compared to the catalyst for cleaning exhaust gas of Comparative Example 1, which comprises the composite metal oxide represented by YMnO 3 . 
     EXAMPLE 11 
     In this Example, the composite metal oxide represented by YMn 0.995 Ru 0.005 O 3  was obtained just as described in Example 1 except for the use of yttrium acetate, manganese nitrate, and ruthenium oxide in such amounts that a molar ratio thereof of 1:0.995:0.005 is obtained. 
     A relationship between heat flow and temperature was then determined just as described in Example 1 except for the use of the composite metal oxide obtained in this Example as a catalyst for cleaning exhaust gas. The result is shown together with the result of Comparative Example 1 in  FIG. 6 . 
     It is apparent from  FIG. 6  that the catalyst for cleaning exhaust gas of Example 11 can oxidize (burn) the above-described carbon black at low temperature and can achieve the effect of further promoting the oxidation, compared to the catalyst for cleaning exhaust gas of Comparative Example 1, which comprises the composite metal oxide represented by YMnO 3 .