JP5877491B2 - Oxidation catalyst - Google Patents

Description

  The present invention relates to a novel oxidation catalyst. More specifically, the present invention relates to an exhaust gas purifying catalyst that purifies exhaust gas by oxidizing particulates (PM) mainly composed of carbon (C) contained in exhaust gas discharged from an internal combustion engine, for example.

Diesel engines may have thermal efficiency compared to gasoline engines, while having the advantage of low emissions of CO ₂ is greenhouse gas, particulate matter harmful to health (Particulate Matter: PM) to more emissions Have a problem. In this regard, a method has been proposed in which PM generated from a diesel engine is collected by a filter to significantly reduce PM emission. However, PM collected on the filter is partly combusted by exposure to high-temperature exhaust gas, and the amount of deposition is reduced, but normally it does not burn quickly unless it is exposed to exhaust gas of 600 ° C. or higher. . For this reason, in urban driving where the frequency of exhaust gas temperature increase is small, the PM accumulated on the filter increases with the travel distance, and an increase in pressure loss accompanying PM deposition causes a deterioration in fuel consumption. For this reason, it is necessary to improve the fuel consumption deterioration as described above.

  In contrast, many studies on PM oxidation catalysts aimed at lowering the combustion temperature of PM have been reported. By coating the PM oxidation catalyst on the filter, if the combustion temperature of PM deposited on the filter is significantly reduced, PM combustion during normal driving is promoted, and fuel consumption associated with PM deposition is greatly deteriorated. It can be improved.

  For this reason, in order to oxidize PM etc. which are contained in the exhaust gas, it is conceivable to use a composite oxide containing two kinds of metal elements as an oxidation catalyst capable of obtaining a stronger oxidation reaction atmosphere. As such a composite oxide, for example, a cerium-zirconium-bismuth composite oxide (Patent Document 1), a rare earth-manganese composite oxide (Patent Document 2), and the like have been proposed. According to these oxides, the combustion temperature (about 650 ° C.) of normal carbon (C), which is the main component of PM, can be lowered to around 400 ° C., so that the catalyst has high oxidation catalyst characteristics at present. ing.

JP2003-238159 JP2007-216099

  However, recently, there has been a demand for C combustion at a temperature lower by about 50 to 100 ° C. than the combustion temperature of these oxides. Things are not enough. For this reason, development of the catalyst which can purify | clean (oxidize) PM etc. which are contained in exhaust gas at low temperature is anxious.

  Accordingly, a main object of the present invention is to provide an oxidation catalyst capable of completely burning PM and the like.

  As a result of intensive studies in view of the problems of the prior art, the present inventor has found that the above object can be achieved by employing a specific oxide as an oxidation catalyst such as PM, and completes the present invention. It came.

That is, the present invention relates to the following oxidation catalyst.
1. An oxidation catalyst comprising a trivalent thallium oxide selected from Tl ₂ O ₃ , Tl ₂ O ₃ —SiO ₂ glass, or LaTlO ₃ and used to completely burn a carbon-containing material at a temperature of 450 ° C. or lower .
2. 2. An oxidation method comprising completely burning a carbon-containing substance at a temperature of 450 ° C. or lower in the presence of the oxidation catalyst according to item 1 .

According to the oxidation catalyst of the present invention, it is composed of thallium oxide having a basic composition of Tl ₂ O ₃ , which is a trivalent Tl oxide, and therefore a temperature lower than the reaction temperature of the conventional oxidation catalyst. Can completely burn organic components. More specifically, PM or the like can be completely burned at a relatively low temperature and decomposed into carbon dioxide (or carbon dioxide and water).

  For this reason, for example, by coating the catalyst of the present invention on the exhaust gas filter, PM deposited on the filter can be combusted at a relatively low temperature, and therefore, combustion of PM during normal traveling is promoted. Thereby, the deterioration of the fuel consumption accompanying PM accumulation on a filter can also be avoided effectively like the past.

It shows the result of the DSC measurement after mixing the Tl ₂ O ₃ to 5 wt% of carbon (C). The result of having observed explosive C combustion in Test Example 1 is shown. FIG. 3A shows the result of TG-DTA measurement after mixing 2 wt% C with Tl ₂ O ₃ . In FIG.3 (b), the TG-DTA measurement result of C only is shown. The X-ray diffraction results for 30 ° C. and 600 ° C. of Tl ₂ O _3. Showing the crystal structure diagram of Tl ₂ O _3. Is a diagram showing the relationship between the measured temperature lattice constants of Tl ₂ O ₃ (a). A scanning electron micrograph and specific surface area of Tl ₂ O ₃ are shown. 200-600 shows the DSC measurement result after mixing the carbon (C) in 2 wt% _Tl 2 _{O 3} heat-treated at ° C.. The release behavior of the lattice in oxygen Tl ₂ O _3, the oxygen partial pressure on the vertical axis (oxygen molecular weight: 34) is a diagram showing a temperature on the horizontal axis. _(Tl 2 _{O 3)} 35 were mixed C to _{(SiO 2) 75} glass powder at a ratio of 20 to 1, indicating the results of DSC measurement. After mixing C in a weight ratio of 20: 1 to LaTlO ₃ ceramic powder, it shows the result of DSC measurement.

1. Oxidation catalyst The oxidation catalyst of the present invention (the catalyst of the present invention) has a general formula R _x Tl _y O _α (where Tl is trivalent and R represents at least one element other than Tl (III)). x is an integer of 0 or more, y is an integer of 1 or more, and α is an integer of 1 or more).

The catalyst of the present invention has Tl ₂ O ₃ as a basic composition, and the Tl site may be substituted with one or more other elements. By using Tl ₂ O ₃ that is a trivalent Tl oxide as a basic composition, excellent carbon combustion characteristics can be exhibited. That is, carbon dioxide and water can be generated by completely burning carbon at a temperature lower than that of conventional catalysts. In particular, hydrocarbons can be completely burned at a lower temperature than conventional catalysts and converted to carbon dioxide and water.

The catalyst of the present invention may have a composition containing no R (x = 0, that is, Tl (III) ₂ O ₃ ), but also includes a composition containing R as described above. The R is not particularly limited, and examples thereof include one or more of La, Nd, Fe, Nb, Ru, Sb, Tl (I) and the like. Among these, at least one metal element such as La, Nd, and Fe is particularly preferable. Examples of oxides substituted with these metal elements include LaTl (III) O ₃ , NdTl (III) O ₃ , FeTl (III) O ₃ , Nb ₆ Tl (III) ₄ O ₂₁ , Ru ₂ Tl (III). Examples include ₂ O ₇ , SbTl (I) ₄ Tl (III) O _{6 and the} like. Therefore, in the present invention, Tl (III) ₂ O ₃ , LaTl (III) O ₃ , NdTl (III) O ₃ , FeTl (III) O ₃ , Nb ₆ Tl (III) ₄ O ₂₁ , Ru ₂ Tl (III ) At least one of ₂ O ₇ , SbTl (I) ₄ Tl (III) O _{6 and the} like can be suitably used.

  The form of the catalyst of the present invention is not particularly limited, and may be any of powder (granular), molded body and the like, but it is usually preferable to use in the form of powder. When used in powder form, the average particle size is preferably 0.1 to 100 μm, particularly preferably 1 to 10 μm.

The specific surface area of the present invention the catalyst (BET specific surface area) is not particularly limited, it is preferable that the 0.001~100m ² · ^{g -1,} especially 0.01~10m ² · ^{g -1.}

2. Production of Oxidation Catalyst The production method of the oxide catalyst of the present invention is not particularly limited as long as the composition as described above is obtained, and any method of solid phase reaction, liquid phase reaction or gas phase reaction may be adopted. it can. Further, it is possible to use known or even commercial _Tl 2 _{O 3} and the like.

For example, thallium oxide (Tl ₂ O ₃ ) can be produced by adding an alkali to an aqueous solution of thallium salt to form a precipitate.

The thallium oxide containing R can also be synthesized by a known method. For example, after mixing an oxide of R (for example, La ₂ O ₃ when R is La) and Tl ₂ O ₃ , the resulting mixture can be manufactured by heat treatment. The mixing method in this case may be either dry or wet. Further, the mixture may be dried prior to the heat treatment. Drying may be usually 100 ° C. or lower. The heat treatment temperature depends on the type of R and the like, but generally may be about 500 to 1000 ° C. The heat treatment atmosphere is usually an oxidizing atmosphere (for example, in the air).

When an amorphous Tl ₂ O ₃ catalyst is produced, an amorphous oxidation catalyst can be obtained by melting Tl ₂ O ₃ and a glass component and pulverizing the obtained solid. . As the glass component, a known glass component can be used, and examples thereof include SiO ₂ , B ₂ O ₃ , GeO ₂ , P ₂ O ₅ , and PbO ₂ . The ratio between Tl ₂ O ₃ and glass component is not particularly limited, usually as 100% by weight of the total weight of both, _{that Tl} 2 _{O 3} is suitably set to be within a range of 10 to 90 wt% it can.

The thallium oxide containing Tl ₂ O ₃ or R thus obtained is subjected to particle size pulverization, classification, etc. according to a known method so as to obtain a preferable average particle size of the catalyst of the present invention. Can be implemented as appropriate.

3. Oxidation (combustion) with oxidation catalyst
The method of using the catalyst of the present invention can be used in the same manner as a known oxidation catalyst (oxidation catalyst). For example, the carbon-containing material or the organic component may be completely burned by heat treatment in the presence of the catalyst of the present invention.

  Examples of the carbon-containing material include carbon or a mixture containing the same. That is, in addition to carbon (carbon, graphite, etc.), a mixture containing these may be used. Examples of the organic component include organic compounds such as hydrocarbons. The catalyst of the present invention is suitable for complete combustion of a carbon-containing material. Therefore, it can be suitably used for complete combustion of particulate (PM) containing carbon (C) in the exhaust gas.

  In this case, the carbon-containing substance or the organic component (hereinafter, both are collectively referred to as “carbonaceous component”) can be reacted in any of the gas phase, the liquid phase, and the solid phase. It is preferable to use a phase. The gas phase can be reacted by bringing a gas containing a carbon-containing substance or an organic component into contact with the catalyst of the present invention. As said gas, the exhaust gas of an internal combustion engine (automobile etc.) is applicable, for example. In this case, PM in the exhaust gas can be completely combusted, so that the catalyst of the present invention can be suitably used as an exhaust gas purification catalyst. Moreover, when it is a solid phase, it is preferable to heat-process as a mixture which floats a carbonaceous component in gas.

  In addition, the heat treatment temperature (reaction temperature) in the heat treatment may be appropriately set depending on the type of the carbonaceous component as a target, but is generally 450 ° C. or lower, preferably 400 ° C. or lower, and particularly 380 It is more preferable that the temperature be set to ° C. In this case, the lower limit value of the heat treatment temperature is not limited, but it may be about 300 ° C. Moreover, the atmosphere of heat processing will not be restrict | limited if it is the atmosphere which can be combusted, In addition to air | atmosphere, oxygen-containing atmosphere can also be employ | adopted.

  The form of reaction for complete combustion is not particularly limited, and may be any of a batch system, a continuous system, a semi-batch system, and the like. In the case of a continuous type, the catalyst of the present invention can be charged in advance in the reaction system. In addition, the catalyst can be continuously charged into the reaction system together with the carbonaceous component. As an arrangement | positioning form of this invention catalyst, any forms, such as a fixed bed and a fluidized bed, may be sufficient, for example.

  The amount of the catalyst of the present invention is not particularly limited, and can be appropriately set according to the type of catalyst used, the type of carbonaceous component, and the like.

  The features of the present invention will be described more specifically with reference to examples. However, the scope of the present invention is not limited to the examples.

Example 1 ( Preparation of Tl ₂ O ₃ catalyst)
Tl ₂ O ₃ powder having a purity of 99.9% by weight (manufactured by Kojundo Chemical Laboratory Co., Ltd., average particle size of 0.3 μm) was used. This Tl ₂ O ₃ powder was heat-treated at 200 to 600 ° C. for 2 hours in a Pt crucible.

Example 2 (Preparation of (Tl ₂ O ₃ ) ₃₅ (SiO ₂ ) ₇₅ Glass)
As starting materials, the same Tl ₂ O ₃ powder as used in Example 1 and reagent grade SiO ₂ powder (manufactured by Wako Pure Chemical Industries, Ltd.) were used, and the raw materials were weighed and mixed with a total batch amount of 10 g. did. Using an alumina crucible with a lid (manufactured by Nikkato Co., Ltd., SSA-S), it was melted in an electric furnace at 1250 ° C. in an air atmosphere for 1 hour, and the obtained melt was poured onto an iron plate to perform press molding. A sample was prepared by carrying out the process. Some Tl evaporates during melting. Composition analysis of the resultant glass (Tl, Si, analyzed O) was place, actual composition was calculated to _{Tl (I) 0.297 Tl (III} ) 0.008 Si 0.293 O 0.895 . Then, it grind | pulverized using the zirconia mortar and it was set as the powder of an average particle diameter of about 10 micrometers.

Example 3 ( Preparation of LaTlO ₃ ceramics)
As starting materials, La ₂ O ₃ powder (manufactured by Shin-Etsu Chemical Co., Ltd., 99.9% by weight) and the same Tl ₂ O ₃ powder as used in Example 1 were wet-kneaded in a ball mill, LaTlO ₃ ceramics with an average particle size of 1 μm were dried at 100 ° C. and heat-treated in an electric furnace at 700 ° C. for 2 hours in an electric furnace using an alumina crucible (Nikkato Co., Ltd., SSA-S). A powder was obtained.

Test example 1
(1) Measurement of carbon combustion characteristics etc. Carbon combustion characteristics etc. were investigated using the samples obtained in the examples. Measurement items and measurement methods are as follows.

(1-1) Carbon combustion characteristics are obtained by adding 2 to 5% by weight of carbon (C, manufactured by Tokai Carbon Co., Ltd.) to each sample, and measuring the temperature at which the carbon burns by differential scanning calorimetry (DSC) (manufactured by Rigaku Corporation). , DSC8230). The measurement conditions were a test amount of 10 mg and a temperature range of room temperature to 600 ° C. (temperature increase rate of 10 ° C./min) in an air stream of 200 mL / min. The result is shown in FIG.

(1-2) The unheat-treated sample to which 2% by weight of C was added was also measured by a differential thermal balance (TG-DTA). In this measurement, “TG8120” (manufactured by Rigaku Corporation) was used as an apparatus. High temperature X-ray diffraction (XRD) measurement of Tl ₂ O ₃ was performed using “UltimaIV” (manufactured by Rigaku Corporation) as an apparatus and using CuKα rays at 2θ = 20 ° to 70 °, room temperature to 600 ° C. . The specific surface area of each heat-treated Tl ₂ O ₃ sample was measured by a three-point BET method using N ₂ gas adsorption using “NOVA 3200” (manufactured by Quantachrome) as an apparatus. The oxygen release characteristics of Tl ₂ O ₃ were evaluated as follows. In order to clarify the behavior of oxygen in the lattice of Tl ₂ O ₃ , oxygen in the Tl ₂ O ₃ lattice is marked in advance with heavy oxygen ( ¹⁸ O ₂ ), and the release behavior of oxygen in the lattice at the time of temperature rise is monitored. did. First, Tl ₂ O ₃ powder was heat-treated in ¹⁸ O ₂ at 800 ° C. for 1 hour to replace oxygen ( ¹⁶ O ₂ ) in the lattice with ¹⁸ O ₂ . Next, using a mass spectrometer (manufactured by Nippon Vacuum Co., Ltd.), the release behavior of oxygen in the lattice accompanying the temperature increase of Tl ₂ O ₃ was monitored at a mass number of 34. The measurement atmosphere was Ar 80% -O ₂ 20%, and the temperature elevation rate was 15 ° C./min.

(2) Measurement results (2-1) Thermal analysis (C combustion characteristics)
FIG. 1 shows the result of DSC measurement after mixing 5 wt% carbon (C) with Tl ₂ O ₃ . According to FIG. 1, a very sharp DSC exothermic peak accompanying explosive C combustion was observed at around 280 ° C. In the case of C alone (dotted line), a DSC exothermic peak due to C combustion was observed at around 660 ° C. From these results, it can be seen that Tl ₂ O ₃ has the ability to lower the C combustion temperature by about 380 ° C. Moreover, the image at the time of this explosive C combustion is shown in FIG. When thermal analysis measurement is performed under these conditions, the thermal analysis apparatus may be damaged. Therefore, TG-DTA measurement was performed with a C addition amount of 2% by weight. FIG. 3A shows the result of TG-DTA measurement after mixing 2 wt% C with Tl ₂ O ₃ . Weight loss and heat generation due to combustion of C mixed at around 350 ° C. are observed in the DTA results. In addition, it is confirmed from the TG result that the weight reduction due to C combustion started at around 200 ° C. and the weight reduction of 2% by weight ended at around 400 ° C. The weight loss from around 580 ° C. is presumed to accompany evaporation of Tl ₂ O ₃ . FIG. 3 (b) shows the TG-DTA measurement result of C only. As in the DSC measurement result, a DTA exothermic peak associated with C combustion was observed near 660 ° C., and the weight loss associated with C combustion was further reduced. Starting from around 500 ° C., the weight loss of 100% by weight was finished around 660 ° C.

(2-2) High-temperature X-ray diffraction FIG. 4 shows the results of X-ray diffraction of Tl ₂ O ₃ at 30 ° C. and 600 ° C. It can be seen that both maintain the Tl ₂ O ₃ structure (cubic (Ia3), ICDD No. 33-1404). FIG. 5 shows a crystal structure diagram of Tl ₂ O ₃ . Tl atoms occupy two different sites and O atoms occupy one site. From the X-ray diffraction results shown in FIG. 4, the formation of another thallium oxide Tl (I) ₂ O (hexagonal system, ICDD No. 43-1049) different from Tl ₂ O ₃ was not observed.
The Tl ₂ O ₃ raw material powder was subjected to high-temperature X-ray diffraction measurement from room temperature to 600 ° C. and from 600 ° C. to room temperature. The high-temperature X-ray diffraction results, not observed formation of Tl ₂ O is at each measurement temperature was _Tl 2 _{O 3} single phase. The lattice constant (a) change was calculated from the measurement results and summarized in FIG. The lattice constant (a) increased as the measurement temperature was increased and decreased as the measurement temperature was decreased. Looking at the relationship between the measurement temperature and the lattice constant (a), a change in the slope is recognized around 300 ° C. In the vicinity of 300 ° C., the temperature is near the temperature at which C combustion by Tl ₂ O ₃ is promoted based on the thermal analysis results, and the change in the slope of the relationship between the measured temperature and the lattice constant (a) changes from Tl ₂ O ₃ to some O It is thought that this is due to the detachment of.

(2-3) heat treatment temperature and the form, a scanning electron microscope photograph and a specific surface area of the relationship Tl ₂ O ₃ and _Tl 2 _{O 3} heat-treated at 200 to 600 ° C. with the surface area and C combustion characteristics shown in FIG. Since the pretreatment at 200 ° C. is required at the time of measuring the specific surface area, the specific surface area of the Tl ₂ O ₃ raw material powder is not shown. Up to 500 ° C., the sintering reaction of Tl ₂ O ₃ particles progressed gradually, and a large morphological change was observed at 600 ° C. FIG. 8 shows a DSC measurement result after mixing 2% by weight of carbon (C) with Tl ₂ O ₃ heat-treated at 200 to 600 ° C. Regardless of the morphology and specific surface area changes observed in FIG. 6, the DSC exothermic peak accompanying C combustion was observed at around 350 ° C. Therefore, C combustion characteristics (oxidation catalyst characteristics) of _Tl 2 _{O 3} it can be seen that not affected the morphology and specific surface area of _Tl 2 _{O 3.}

(2-4) in order to clarify the release behavior of the lattice in the oxygen release behavior Tl ₂ O ₃ lattice in oxygen _Tl 2 _{O 3,} heavy oxygen ⁽¹⁸ _{O 2)} by in the lattice of _Tl 2 _{O 3} Oxygen was previously marked and the release behavior of oxygen in the lattice at the time of temperature increase was monitored. It is considered that the exchange of oxygen atoms between oxygen existing in the atmosphere and oxygen in the lattice of Tl ₂ O ₃ existing in the vicinity of the surface repeatedly and frequently occurs as the temperature rises. In FIG. 9, the release behavior of oxygen in the lattice of Tl ₂ O ₃ is shown by the oxygen partial pressure (oxygen molecular weight: 34) on the vertical axis and the temperature on the horizontal axis. Similarly, after mixing Tl ₂ O ₃ powder pre-marked with heavy oxygen and C at a weight ratio of 20: 1, the temperature was similarly raised to monitor the C ¹⁸ O (carbon monoxide) release behavior. . The relationship between the C ¹⁸ O partial pressure and temperature is also shown in FIG. It was confirmed that Tl ₂ O ₃ was releasing oxygen in the lattice from a low temperature of 270 ° C. At the same time, the production of C ¹⁸ O in which oxygen and carbon in the lattice were bonded from around 230 ° C. was confirmed from a mixed test of Tl ₂ O ₃ powder and C, which was previously marked with heavy oxygen. It was judged that C, which had been mixed with Tl ₂ O ₃ in advance, reacted by receiving an attack of intra-lattice oxygen (active oxygen) released from within the Tl ₂ O ₃ lattice to produce C ¹⁸ O. . Finally, it was confirmed that C was oxidized to carbon dioxide. It was revealed that Tl ₂ O ₃ liberates oxygen in the lattice from a low temperature of 230 ° C., and oxygen released from the lattice (active oxygen) directly oxidizes C from a low temperature of 230 ° C.

(2-5) C combustion characteristics of thallium (III) compounds (oxidation catalyst characteristics)
FIG. 10 and FIG. 11 show the results of DSC measurement after mixing C in a weight ratio of 20: 1 to (Tl ₂ O ₃ ) ₃₅ (SiO ₂ ) ₇₅ glass powder and LaTlO ₃ ceramic powder, respectively. (Tl ₂ O ₃ ) ₃₅ (SiO ₂ ) ₇₅ glass powder has a DSC exothermic peak associated with C combustion at around 360 ° C. and LaTlO ₃ ceramic powder at around 420 ° C., and even in such thallium (III) compounds, As with the Tl ₂ O ₃ powder, it became clear that it has high C combustion characteristics (oxidation catalyst characteristics).

Test example 2
PM continuous purification characteristics were evaluated using an alumina ceramic foam carrying Tl ₂ O ₃ powder. The foam is installed so that the PM mixed gas passes through the pores of the ceramic foam supporting the Tl ₂ O ₃ powder, the ceramic foam is maintained at 350 ° C., and the PM mixed gas is supplied for 40 hours and then deposited on the ceramic foam. The state of PM was observed. The composition of the PM mixed gas was O ₂ : 7% by volume, NO: 50 ppm, N ₂ balance, and soot generated by arc discharge of a carbon rod was used as the PM component. As supply conditions, the PM mixed gas was passed at 100 L / hour per 1 mL volume of the ceramic foam (catalyst). On the ceramic foam that has not been loaded with Tl ₂ O ₃ powder, about 100 mg of PM was deposited in a 40-hour test, whereas PM was found on the ceramic foam loaded with Tl ₂ O ₃ powder. I couldn't. This is considered to be due to the fact that PM trapped on the ceramic foam was promoted for oxidation by the catalytic effect of Tl ₂ O ₃ and continuously purified at 350 ° C.

Claims (2)

An oxidation catalyst comprising a trivalent thallium oxide selected from Tl ₂ O ₃ , Tl ₂ O ₃ —SiO ₂ glass, or LaTlO ₃ and used to completely burn a carbon-containing material at a temperature of 450 ° C. or lower . An oxidation method comprising completely burning a carbon-containing substance at a temperature of 450 ° C. or lower in the presence of the oxidation catalyst according to claim 1 .