WO2017212944A1 - 排気ガス浄化触媒 - Google Patents
排気ガス浄化触媒 Download PDFInfo
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- WO2017212944A1 WO2017212944A1 PCT/JP2017/019652 JP2017019652W WO2017212944A1 WO 2017212944 A1 WO2017212944 A1 WO 2017212944A1 JP 2017019652 W JP2017019652 W JP 2017019652W WO 2017212944 A1 WO2017212944 A1 WO 2017212944A1
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- Prior art keywords
- exhaust gas
- cual
- gas purification
- purification catalyst
- catalyst
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Classifications
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
- B01D53/34—Chemical or biological purification of waste gases
- B01D53/92—Chemical or biological purification of waste gases of engine exhaust gases
- B01D53/94—Chemical or biological purification of waste gases of engine exhaust gases by catalytic processes
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
- B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
- B01J23/70—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of the iron group metals or copper
- B01J23/76—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of the iron group metals or copper combined with metals, oxides or hydroxides provided for in groups B01J23/02 - B01J23/36
- B01J23/83—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of the iron group metals or copper combined with metals, oxides or hydroxides provided for in groups B01J23/02 - B01J23/36 with rare earths or actinides
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
- F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
- F01N3/00—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust
- F01N3/08—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous
- F01N3/10—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by thermal or catalytic conversion of noxious components of exhaust
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
- F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
- F01N3/00—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust
- F01N3/08—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous
- F01N3/10—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by thermal or catalytic conversion of noxious components of exhaust
- F01N3/24—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by thermal or catalytic conversion of noxious components of exhaust characterised by constructional aspects of converting apparatus
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N23/00—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00
- G01N23/22—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00 by measuring secondary emission from the material
- G01N23/227—Measuring photoelectric effect, e.g. photoelectron emission microscopy [PEEM]
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02T—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO TRANSPORTATION
- Y02T10/00—Road transport of goods or passengers
- Y02T10/10—Internal combustion engine [ICE] based vehicles
- Y02T10/12—Improving ICE efficiencies
Definitions
- the present invention relates to an exhaust gas purification catalyst that can be used to purify exhaust gas discharged from an internal combustion engine.
- the exhaust gas from gasoline fueled vehicles contains harmful components such as hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NO x ).
- the hydrocarbon (HC) is oxidized to convert it to water and carbon dioxide
- the carbon monoxide (CO) is oxidized to convert it to carbon dioxide
- the nitrogen oxide (NO) As for x 2 ) it is necessary to reduce it to convert it to nitrogen and purify each harmful component with a catalyst.
- a catalyst for treating such exhaust gas hereinafter referred to as "exhaust gas purification catalyst”
- a three-way catalyst (Twc) capable of redoxing CO, HC and NO x is used. ing.
- the three-way catalyst is generally mounted in the form of a converter at an intermediate position between the engine and the muffler of the exhaust pipe.
- a refractory oxide porous body having a high specific surface area such as an alumina porous body having a high specific surface area, such as platinum (Pt), palladium (Pd), rhodium (Rh), etc. It is known to carry a noble metal and to carry it on a substrate, for example a monolithic substrate made of a refractory ceramic or metal honeycomb structure, or on refractory particles. There is.
- Patent Document 2 discloses an exhaust gas purification catalyst obtained by mixing a compound containing Cu and alumina to prepare a mixture, and heat treating the mixture at 850 ° C. or more and less than 1200 ° C. There is.
- Patent Document 3 is a catalyst containing CuAl 2 O 4 having a spinel crystal structure to which a rare earth element is added, wherein the rare earth element is a lanthanoid, and CuO is supported on the surface of the CuAl 2 O 4. It is characterized by having a diffraction peak belonging to CuO and CuAl 2 O 4 and having no diffraction peak belonging to ⁇ -Al 2 O 3 in the X-ray diffraction pattern by the Cu-K ⁇ ray. Discloses a catalyst. According to this patent document 3, CuO is supported on the surface of CuAl 2 O 4 by supporting Cu on alumina and baking it at a temperature to generate CuAl 2 O 4 ([0030]). It is stated that the promoting action of CuAl 2 O 4 can be used while securing the catalytic activity by CuO ([0009]).
- the inventors focused on copper (Cu) as a catalytically active component, and conducted research on an exhaust gas purification catalyst having a configuration in which a Cu element is present on the surface of alumina particles. It was found that the catalyst activity did not increase as expected even if the temperature was increased. Further investigations into the cause have shown that the formation of CuAl 2 O 4 inhibits the catalytic activity of Cu.
- the inventor of the present invention can exhibit excellent catalytic activity and effectively use it as a three-way catalyst with regard to an exhaust gas purification catalyst having a configuration in which Cu elements are present on the surface of alumina particles. It aims to provide a new exhaust gas purification catalyst that can
- the present invention is an exhaust gas purification catalyst having a structure in which Cu elements are present on the surface of alumina particles, and the catalyst is measured by X-ray photoelectron spectroscopy (XPS).
- XPS X-ray photoelectron spectroscopy
- the percentage of the peak area of Cu2p also referred to as "Cu coverage", where the total area of each peak corresponding to the bonding energy of 2p orbital of Cu (Cu2p) and 2p orbital of Al (Al2p) is 100%.
- the exhaust gas purification catalyst is characterized in that 7) to 28%).
- the exhaust gas purification catalyst proposed by the present invention even without supporting a noble metal as a catalytically active component, it can exhibit excellent catalytic activity and can be effectively used as a three-way catalyst.
- An exhaust gas purification catalyst (referred to as “the present exhaust gas purification catalyst”) according to an example of the embodiment of the present invention is an exhaust gas purification catalyst having a configuration in which a Cu element is present on the surface of alumina particles.
- the alumina particles may be particles of Al 2 O 3, it may be particles containing in addition to the other components of the Al 2 O 3.
- other components other than Al 2 O 3 in which the alumina particles may contain, can be, for example, lanthanide and an oxide of barium (Ba).
- the lanthanoid include lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd) and terbium (Tb).
- Dy dysprosium
- Ho holmium
- Er erbium
- Tm thulium
- Yb ytterbium
- Lu lutetium
- the content of the above “other components” from the viewpoint of preventing the decrease of the catalytic activity due to the decrease of the dispersibility of Cu by the reaction of Cu with the “other components”, Al
- the content is preferably 5% by mass or less, and more preferably 3% by mass or less, with respect to 2 O 3 .
- the content of the “other component” is preferably 0.5% by mass or more with respect to Al 2 O 3 .
- Examples of the crystal structure of Al 2 O 3 constituting alumina particles include ⁇ -Al 2 O 3 , ⁇ -Al 2 O 3 , ⁇ -Al 2 O 3 and ⁇ -Al 2 O 3 .
- ⁇ -Al 2 O 3 and ⁇ -Al 2 O 3 are preferable from the viewpoint of the balance between heat resistance and specific surface area, and above all, the dispersibility of Cu is high while heat resistance is maintained.
- ⁇ -Al 2 O 3 is particularly preferred from the viewpoint of
- the alumina particles may be alumina particles in which a plurality of two or more types of Al 2 O 3 having different crystal structures are combined.
- the average particle size (D50) of the alumina particles is preferably 1 ⁇ m to 60 ⁇ m. If the average particle diameter (D50) of the alumina particles is 1 ⁇ m or more, it is preferable because heat resistance can be maintained while maintaining the peeling strength. On the other hand, if the average particle diameter (D50) of the present alumina is 60 ⁇ m or less, it is preferable because the gas contactability can be enhanced while maintaining the peel strength. From this viewpoint, the average particle diameter (D50) of the alumina particles is preferably 1 ⁇ m to 60 ⁇ m, more preferably 3 ⁇ m or more and 50 ⁇ m or less, and particularly preferably 5 ⁇ m or more or 40 ⁇ m or less.
- the Cu 2p orbital (“Cu 2 p” is also measured by X-ray photoelectron spectroscopy (XPS)) with respect to the existing ratio of Cu element present on the surface of alumina particles. Assuming that the total area of each peak corresponding to the binding energy of the 2p orbital (also referred to as “Al2p”) and Al is 100%, the ratio of the Cu2p peak area, that is, the Cu coverage is 7 to 28% Is preferred.
- the ratio of the peak area of Cu2p with respect to the total area of each peak area of Cu2p and Al2p measured by X-ray photoelectron spectroscopy shows the existence rate of Cu element on the surface of alumina particles. Since Cu exposed on the surface acts as an active species, the larger the Cu coverage, the better, but if it is too large, sintering will occur. From this point of view, the Cu coverage is preferably 7 to 28%, and more preferably 20% or less from the viewpoint of preventing sintering of Cu and further improving the NO x purification performance, among which 18 % Or less, particularly preferably 15% or less.
- the presence of the Cu element on the surface of the alumina particle includes the case where the Cu element is present in a solid solution state with alumina as Cu element is CuAl 2 O 4 , for example. It also includes the case of being supported on the particle surface. In addition, “supported” refers to a state in which an active metal such as Cu is immobilized without reacting with an inorganic porous material such as alumina.
- the present exhaust gas purification catalyst contains Cu 0 to 1 and Cu 2 valence, and the Cu 0 to 1 valence contained in the catalyst is larger than the Cu bivalent contained in the catalyst. This is because Cu0 to monovalent metals have higher catalytic activity because they are closer to metal Cu than divalent metals.
- the peak energy of the 925 eV to 940 eV (corresponding to Cu 0 to 2 valence) obtained by measuring the bonding energy of the 2p orbital of Cu by X-ray Photoelectron Spectroscopy (XPS)
- Peak area of 925 eV to 935 eV (corresponding to Cu 0 to 1 valence: Cu 0 valence peak is observed overlappingly in the shoulder portion of Cu monovalent valence, so the ratio of Cu 0 to 1 valence peak including this shoulder portion) (Also referred to as “the area ratio of Cu to monovalent”) is preferably 50% or more.
- the peak area of 925 eV to 940 eV correlates with the content of 0 to 2 divalent Cu
- the peak area of 925 eV to 935 eV correlates to the content of 0 to 1 monovalent Cu.
- the area ratio of 50% or more of Cu is 0 to 1 in Cu 0 to 1 (Cu, Cu 2 O etc.) and Cu 2 (such as CuO, CuAl 2 O 4 ). It means that the one with a high existence rate is preferable. Since the catalytic activity of Cu0 to monovalent is higher than that of Cu, the catalytic activity of the present exhaust gas purification catalyst can be enhanced by increasing the proportion of Cu to monovalent.
- the area ratio of Cu of 0 to 1 is more preferably 55% or more, and more preferably 60% or more.
- the abundance ratio of Cu to monovalent can be higher than that of Cu.
- the method is not limited to this method, and a reducing atmosphere such as hydrogen or carbon monoxide may be used.
- the peak area ratio ((CuAl 2 O 4 / (CuO x + CuAl 2 O 4 )) ⁇ 100) is preferably 50% or less, in particular 45% or less, and in particular 40% or less Is more preferred.
- the baking in a nitrogen atmosphere can suppress the formation of CuAl 2 O 4 while increasing the Cu coating amount.
- the present invention is not limited to such a method, and it is considered that the same effect as firing in a nitrogen atmosphere can be obtained even in a reducing atmosphere such as hydrogen or carbon monoxide as described above.
- the content of CuAl 2 O 4 is preferably 15% by mass or less in the present exhaust gas purification catalyst.
- the catalyst activity is favorably maintained. From this point of view, the content of CuAl 2 O 4 is more preferably 10% by mass or less, and particularly preferably 9% by mass or less.
- the present exhaust gas purification catalyst is prepared, for example, by dissolving copper nitrate in water to prepare an aqueous solution, impregnating this with alumina to impregnate it into a slurry, and drying this slurry, then 600 to 1000 ° C., preferably 600 to 1000 ° C. It can be obtained by firing at 900 ° C. in a nitrogen atmosphere (also referred to as “N 2 firing”). Alternatively, the slurry can be applied to a substrate and sintered at 600 to 1000 ° C., preferably 600 to 900 ° C., for N 2 . However, it is not limited to such a manufacturing method.
- the ratio of Cu to monovalent can be increased by performing N 2 baking. It has been confirmed that the ratio of Cu bivalent is more likely to be higher than Cu bivalent in the case of N 2 firing, while the ratio of Cu bivalent is higher than that of Cu bivalent in the atmospheric baking.
- the present exhaust gas purification catalyst can exhibit exhaust gas purification catalyst performance without supporting a noble metal as a catalytically active species. That is, hydrocarbon (HC) and carbon monoxide (CO) are oxidized and provided with catalytic activity for reducing and purifying nitrogen oxides (NO x ), among which the reduction performance of NO x and the oxidation of CO The performance is particularly good. Therefore, it can be effectively utilized as a three way catalyst capable of oxidizing reduced CO, and HC and NO x. However, it does not prevent supporting the precious metal.
- HC hydrocarbon
- CO carbon monoxide
- NO x nitrogen oxides
- the present exhaust gas purification catalyst may be formed into an appropriate shape such as a pellet, and may be used alone as a catalyst, or may be used as a form supported on a substrate made of a ceramic or a metal material.
- the present exhaust gas purifying catalyst can be on the surface of the substrate exhibiting a honeycomb shape to produce a three-way catalyst by forming a catalyst layer with the NO X storage such as for example a binder or a hydroxide Ba.
- the catalyst layer may have a single layer structure or a multilayer structure of two or more layers.
- the present exhaust gas purifying catalyst, the inorganic porous material as required, OSC materials, NO X absorbent, and the like as a binder mixing water and stirring to form a slurry and the resulting slurry
- a substrate such as a ceramic honeycomb body and fired to form a catalyst layer on the surface of the substrate.
- refractory materials such as ceramics, and a metal material
- the material of the ceramic base material includes refractory ceramic materials such as cordierite, cordierite-alpha alumina, silicon nitride, zircon mullite, spodumene, alumina-silica magnesia, zirconium silicate, sillimanite, magnesium silicate, Mention may be made of petalite, alpha alumina and aluminosilicates.
- the material of the metallic substrate can include refractory metals, such as stainless steel or other suitable corrosion resistant alloys based on iron.
- the shape of the substrate examples include honeycomb, filter, pellet and sphere.
- honeycomb material for example, cordierite materials such as ceramics can be used.
- a honeycomb made of a metal material such as ferritic stainless steel can also be used.
- a honeycomb-shaped substrate for example, a monolithic substrate having a large number of fine gas flow passages parallel to the inside of the substrate, ie, channels, can be used so that the fluid can flow inside the substrate.
- the catalyst composition can be coated on the inner wall surface of each channel of the monolithic substrate by wash coating or the like to form a catalyst layer.
- the inorganic porous material examples include porous materials of compounds selected from the group consisting of silica, alumina and titania compounds, such as alumina, silica, silica-alumina, alumino-silicates, alumina-zirconia, alumina-chromia and A porous body made of a compound selected from alumina-ceria can be mentioned.
- OSC material that is, a cocatalyst having an oxygen storage capacity (OSC)
- OSC material examples include a cerium compound, a zirconium compound, a ceria-zirconia composite oxide, a ceria-zirconia-alumina composite oxide, etc. .
- Examples of the NO x storage agent include alkaline earth metals and alkali metals. Among them, one or more of metals selected from the group consisting of magnesium, barium, boron, thorium, hafnium, silicon, calcium and strontium can be selected. Among them, barium is preferred from the viewpoint of better NO adsorption at low temperatures.
- an organic binder or an inorganic binder for example, an aqueous solution such as zirconia sol or alumina sol can be used.
- Example 1 After adding 1 mass% La-containing ⁇ -Al 2 O 3 powder to a copper nitrate aqueous solution and stirring, a Cu sol-supported alumina slurry was obtained by adding a zirconia sol. ⁇ ⁇ ⁇ ⁇ 40 mm ⁇ L 60 mm (300 cells): 165 g / L of the slurry obtained above is applied to a Koshi Giken stainless steel honeycomb substrate with a carrier volume of 0.0754 L, and after blowing off excess slurry, hot air at 100 ° C. is a slurry It was dried so as to directly hit the coated surface. Next, the catalyst was calcined at 450 ° C.
- Example 1 a catalyst (sample) of Example 1.
- the components in the slurry were 11.3 parts by mass of copper oxide, 80.2 parts by mass of alumina containing 1% by mass of La, and 8.5 parts by mass of zirconia sol.
- Examples 2 to 7 and Comparative Examples 1 to 3 Catalysts of Examples 2 to 7 and Comparative Examples 1 to 3 in the same procedure as Example 1, except that the mass% of copper oxide, the alumina species, the calcination temperature and the calcination atmosphere were changed as shown in Table 1. I got a sample).
- Comparative Example 3 a Cu-supporting alumina slurry was prepared, applied to a stainless steel honeycomb substrate and dried as in Example 1, and fired at 450 ° C. for 1 hour to remove the nitrate root. However, firing was not performed at 600 ° C. for 4 hours in nitrogen.
- the catalyst surface is analyzed under the above conditions using an X-ray photoelectron spectrometer (XPS), and the obtained X-ray photoelectron spectra are obtained.
- XPS X-ray photoelectron spectrometer
- a peak area obtained by detecting a photoelectron of 0 to 2 valence corresponding to the bonding energy of Cu 2 p and a total area of a peak area obtained by detecting a photoelectron of Al oxide corresponding to the binding energy of Al 2 p The ratio of the Cu2p peak area ("Cu coverage" in Table 1) was determined.
- the Cu surface is analyzed under the above conditions using an X-ray photoelectron spectrometer (XPS), and the peak of 925 eV to 940 eV and the peak of 925 eV to 935 eV are waveform separated to a peak area of 925 eV to 940 eV
- XPS X-ray photoelectron spectrometer
- the ratio of the peak area (corresponding to Cu 0 to 1 valence) of 925 eV to 935 eV with respect to 1) (“Cu 0 to 1 valence area ratio (%)” in Table 1) was calculated.
- the X-ray photoelectron spectrometer (XPS) can perform semi-quantitative analysis on elemental components at a depth of about 10 nm from the particle surface.
- the average particle size (D50) of alumina was measured using a laser diffraction / scattering particle size distribution, and is shown in Table 1.
- a sample (powder) is charged into a water-soluble solvent using an automatic sample feeder for laser diffraction particle size distribution measuring apparatus (“Microtorac SDC” manufactured by Nikkiso Co., Ltd.), and an ultrasonic wave of 30 W in a 50% flow rate is 360
- the particle size distribution was measured using a laser diffraction particle size distribution analyzer “MT3000II” manufactured by Nikkiso Co., Ltd., and D50 was measured from the obtained volume-based particle size distribution chart.
- measurement conditions were calculated
- H 2 -TPR Heating the reaction method using H 2 by (H 2-TPR), hydrogen was measured consumption peaks of the catalyst powder obtained in Examples and Comparative Examples (samples). Specifically, H 2 -TPR is measured under normal temperature to 800 ° C. conditions by flowing 2% hydrogen (argon balance) using a flow-through type tubular reactor equipped with a heat conduction detector. went. Note that the hydrogen consumption peak at 100 to 350 ° C. observed by H 2 -TPR measurement is CuO x , and the hydrogen consumption peak appearing at 350 ° C.
- CuAl 2 O 4 with the CuO x and CuAl 2 O 4 peak areas CuO x and CuAl 2 O CuAl 2 O 4 of the peak area ratio to the peak area of 4 ((CuAl 2 O 4 / (CuO x + CuAl 2 O 4)) ⁇ 100, "CuAl 2 O 4 peak area ratio of Table 1 ( In addition, CuAl 2 O 4 was quantified from the obtained peak area (hydrogen consumption) to determine “the amount of CuAl 2 O 4 in the catalyst (mass%)” in Table 1.
- CuAl 2 O 4 peak area ratio (%) 0 in Table 1, it refers to a state where all Cu is supported on alumina as CuO x , and when it is larger than 0, part of Cu is CuO x It means that it is supported on alumina, and a part is present in a state of being solid-dissolved in alumina like CuAl 2 O 4 .
- the content of copper oxide in the catalyst was higher than that of other materials, so the hydrogen consumption peak by H 2 -TPR shows a behavior different from the others, and the accuracy of CuAl 2 O 4 The peak could not be extracted.
- X-ray photoelectron spectroscopy has been made with respect to an exhaust gas purification catalyst having a configuration in which Cu elements are present on the surface of alumina particles.
- XPS X-ray photoelectron spectroscopy
- the peak area of 925 eV to 940 eV obtained by measuring the bonding energy of the 2p orbital of the Cu by X-ray photoelectron spectroscopy from the above-described Examples and Comparative Examples and the test results conducted by the present inventors so far If the ratio of the peak area of 925 eV to 935 eV (corresponding to Cu 0 to 1 valence) to Cu 0 to 2 valence ("Cu 0 to 1 area fraction") is 50% or more, the catalyst activity is further enhanced It turned out that it can do.
Abstract
Description
このような排気ガスを処理するための触媒(以下「排気ガス浄化触媒」と称する)として、CO、HC及びNOxを酸化還元することができる三元触媒(Three way catalysts:TWC)が用いられている。当該三元触媒は、排気パイプのエンジンとマフラーの中間位置にコンバーターの形態で取付けられるのが一般的である。
この特許文献3には、アルミナへCuを担持させて、CuAl2O4の生成させる温度で焼成することにより([0030])、CuOがCuAl2O4の表面に担持された状態で存在することになるから、CuOによる触媒活性を確保しながら、CuAl2O4の助長作用を利用することができる旨が記載されている([0009])。
本発明の実施形態の一例に係る排気ガス浄化触媒(「本排気ガス浄化触媒」と称する)は、アルミナ粒子の表面にCu元素が存在してなる構成を備えた排気ガス浄化触媒である。
上記アルミナ粒子は、Al2O3からなる粒子でもよいし、Al2O3のほかに他の成分を含有する粒子でもよい。
該ランタノイドとしては、例えばランタン(La),セリウム(Ce),プラセオジム(Pr),ネオジム(Nd),プロメチウム(Pm),サマリウム(Sm),ユーロピウム(Eu),ガドリニウム(Gd),テルビウム(Tb),ジスプロジウム(Dy),ホルミウム(Ho),エルビウム(Er),ツリウム(Tm),イッテルビウム(Yb),ルテチウム(Lu)から選ばれる一種または二種以上を挙げることができる。
なお、上記アルミナ粒子は、これら結晶構造の異なる2種類以上のAl2O3が複数組み合わさったアルミナ粒子であってもよい。
アルミナ粒子の平均粒子径(D50)は1μm~60μmであるのが好ましい。
アルミナ粒子の平均粒子径(D50)が1μm以上であれば、剥離強度を維持しつつ、耐熱性を維持できるから好ましい。他方、本アルミナの平均粒子径(D50)が60μm以下であれば、剥離強度を維持しつつガス接触性を高めることができるから好ましい。
かかる観点から、アルミナ粒子の平均粒子径(D50)は1μm~60μmであるのが好ましく、中でも3μmより大きく或いは50μm以下、その中でも特に5μm以上或いは40μm以下であるのが好ましい。
アルミナ粒子の表面に存在するCu元素は、CuOx(0≦x≦1)、CuAl2O4などの状態で存在している場合を包含する。
この際、当該CuAl2O4は、アルミナ粒子の表面において、Cu元素がアルミナに固溶した状態で存在すると推定され、当該CuOx(0≦x≦1)は、アルミナ粒子表面に担持された状態で存在すると推定される。
この際、X線光電子分光法で測定される、Cu2p及びAl2pの各ピーク面積の合計面積に対するCu2pのピーク面積の割合は、アルミナ粒子表面のCu元素の存在割合を示していると言える。表面に露出しているCuが活性種として作用するから、Cu被覆率は大きいほどよいが、大きすぎるとシンタリングしてしまう。
かかる観点から、上記Cu被覆率は7~28%であるのが好ましく、中でもCuのシンタリングを防ぎ、NOx浄化性能をより向上させる観点から20%以下とすることがより好ましく、その中でも18%以下、特に15%以下であるのが好ましい。
なお、「アルミナ粒子の表面にCu元素が存在」とは、例えばCu元素がCuAl2O4となっているように、アルミナに固溶した状態で存在する場合も含むし、またCuOなどとしてアルミナ粒子表面に担持された状態で存在する場合も含む。
また、「担持」とは、Cuなどの活性金属が、アルミナなどの無機多孔質材料と反応せずに固定化されている状態を指す。
かかる観点から、当該Cu0~1価の面積率は、55%以上であるのがより好ましく、中でも60%以上であるのがさらに好ましい。
後述するように、窒素雰囲気下で焼成することにより、Cu2価よりもCu0~1価の存在比率を高めることができる。但し、かかる方法に限定するものではなく、水素や一酸化炭素などの還元雰囲気でも構わない。
かかる観点から、上記ピーク面積率((CuAl2O4/(CuOx+CuAl2O4))×100)は50%以下であるのが好ましく、中でも45%以下、その中でも40%以下であるのがさらに好ましい。
後述するように、窒素雰囲気下で焼成することにより、Cu被覆量を高めつつ、CuAl2O4の生成を抑制することができることが確認されている。但し、かかる方法に限定するものではなく、前述したような水素や一酸化炭素などの還元雰囲気でも、窒素雰囲気での焼成と同様の効果を得ることができると考えられる。
本排気ガス浄化触媒において、CuAl2O4の含有量を15質量%以下とすることで、触媒活性が良好に維持されることになる。
かかる観点から、CuAl2O4の含有量が10質量%以下であるのがより一層好ましく、特に9質量%以下であるのがさらに好ましい。
本排気ガス浄化触媒は、例えば硝酸銅を水に溶解して水溶液を作製し、これにアルミナを入れて含浸させてスラリーとし、このスラリーを乾燥させた後、600~1000℃、好ましくは600~900℃で、窒素雰囲気下で焼成(「N2焼成」とも称する)することにより得ることができる。
また、上記スラリーを基材に塗布し、600~1000℃、好ましくは600~900℃でN2焼成することにより得ることができる。
但し、かかる製法に限定するものではない。
本排気ガス浄化触媒は、触媒活性種としての貴金属を担持することなく、排気ガス浄化触媒性能を発揮することができる。すなわち、炭化水素(HC)及び一酸化炭素(CO)は酸化し、且つ、窒素酸化物(NOx)を還元して浄化する触媒活性を備えており、中でもNOxの還元性能及びCOの酸化性能が特に優れている。よって、CO、HC及びNOxを酸化還元することができる三元触媒として有効利用することができる。もっとも、貴金属を担持することを妨げるものではない。
セラミック製基材の材質としては、耐火性セラミック材料、例えばコージライト、コージライト-アルファアルミナ、窒化ケイ素、ジルコンムライト、スポジュメン、アルミナ-シリカマグネシア、ケイ酸ジルコニウム、シリマナイト(sillimanite)、ケイ酸マグネシウム、ペタライト(petalite)、アルファアルミナおよびアルミノシリケート類などを挙げることができる。
金属製基材の材質としては、耐火性金属、例えばステンレス鋼または鉄を基とする他の適切な耐食性合金などを挙げることができる。
ハニカム材料としては、例えばセラミックス等のコージェライト質のものを用いることができる。また、フェライト系ステンレス等の金属材料からなるハニカムを用いることもできる。
ハニカム形状の基材を用いる場合、例えば基材内部を流体が流通するように、基材内部に平行で微細な気体流通路、すなわちチャンネルを多数有するモノリス型基材を使用することができる。この際、モノリス型基材の各チャンネル内壁表面に、触媒組成物をウォッシュコートなどによってコートして触媒層を形成することができる。
本明細書において「X~Y」(X,Yは任意の数字)と表現する場合、特にことわらない限り「X以上Y以下」の意と共に、「好ましくはXより大きい」或いは「好ましくはYより小さい」の意も包含する。
また、「X以上」(Xは任意の数字)或いは「Y以下」(Yは任意の数字)と表現した場合、「Xより大きいことが好ましい」或いは「Y未満であることが好ましい」旨の意図も包含する。
硝酸銅水溶液にLa1質量%含有θ-Al2O3粉末を加えて撹拌した後、ジルコニアゾルを添加することで、Cu担持アルミナスラリーを得た。
Φ40mm×L60mm(300セル):担体容積0.0754Lの合志技研製ステンレスハニカム基材に、上記で得たスラリーを165g/L塗布し、過剰なスラリーを吹き払った後、100℃の熱風がスラリー塗布面に直接あたるようにして乾燥させた。
次に、450℃で1時間焼成して硝酸根を除去した後、窒素中600℃で4時間焼成して、実施例1の触媒(サンプル)を得た。
なお、スラリー中の各成分は、酸化銅11.3質量部、La1質量%含有アルミナ80.2質量部、ジルコニアゾル8.5質量部であった。
表1に示すように、酸化銅の質量%、アルミナ種、焼成温度、焼成雰囲気を変更した以外、実施例1と同様の手順にて、実施例2~7及び比較例1~3の触媒(サンプル)を得た。
なお、比較例3については、実施例1と同様にCu担持アルミナスラリーを調製及びステンレスハニカム基材への塗布・乾燥を行い、450℃で1時間焼成して硝酸根を除去して作製した。しかし、窒素中600℃で4時間の焼成は行わなかった。
X線光電子分光分析(XPS:X-ray Photoelectron Spectroscopy)により、実施例・比較例で得た触媒(サンプル)表面の分析を行った。
XPSの分析装置としてアルバック・ファイ株式会社製のQuantum2000(ビーム条件:50W、200μm径)を用い、解析ソフトウェアとして「MultiPack ver.6.1」を用いて状態・半定量用ナロー測定を行った。X線源として、Al-Kα線(1486.8eV)を用いて、17kV×0.023Aで操作した。
帯電補正:C1sを284.0eVとして帯電補正を行った。
また、X線光電子分光装置(XPS)を用いて上記条件でCu表面を分析し、925eV~940eVのピーク及び925eV~935eVのピークを波形分離し、925eV~940eVのピーク面積(Cu0~2価に相当)に対する、925eV~935eVのピーク面積(Cu0~1価に相当)の比率(表1の「Cu0~1価の面積率(%)」)を算出した。
なお、X線光電子分光装置(XPS)は、粒子表面から約十nmまでの深さの元素成分について半定量分析を行うことができる。
アルミナの平均粒子径(D50)はレーザー回折・散乱式粒度径分布を用いて測定し、表1に示した。
レーザー回折粒子径分布測定装置用自動試料供給機(日機装株式会社製「Microtorac SDC」)を用い、サンプル(粉体)を水溶性溶媒に投入し、50%の流速中、30Wの超音波を360秒間照射した後、日機装株式会社製レーザー回折粒度分布測定機「MT3000II」を用いて粒度分布を測定し、得られた体積基準粒度分布のチャートからD50を測定した。この際、測定条件は、粒子屈折率1.5、粒子形状真球形、溶媒屈折率1.3、セットゼロ30秒、測定時間30秒、2回測定の平均値として求めた。
H2による昇温反応法(H2-TPR)により、実施例・比較例で得た触媒粉体(サンプル)の水素消費ピークを測定した。
具体的には、熱伝導型検出器を備えた流通式管型反応器を用いて、2%水素(アルゴンバランス)を流通して常温~800℃の条件の下、H2-TPRの測定を行った。なお、H2-TPR測定でみられる100~350℃の水素消費ピークをCuOx、350℃以上に現れる水素消費ピークをCuAl2O4として前記CuOx及びCuAl2O4のピーク面積において、CuOx及びCuAl2O4のピーク面積に対するCuAl2O4のピーク面積率((CuAl2O4/(CuOx+CuAl2O4))×100、表1の「CuAl2O4ピーク面積率(%)」を求めた。また、得られたピーク面積(水素消費量)からCuAl2O4の定量を行い、表1の「触媒中のCuAl2O4量(質量%)」を求めた。
また、実施例6及び比較例2では触媒中の酸化銅の含有量が他の材料よりも多かったため、H2-TPRによる水素消費ピークが他と異なる挙動を示し、CuAl2O4の正確なピークを抽出することができなかった。そのため、「CuAl2O4のピーク面積率」及び「触媒中のCuAl2O4量」は定量不可とした。しかし、XRDから実施例6ではCuの大部分がCuOx、比較例2ではCuの大部分がCuAl2O4として存在することが確認された。
各実施例・比較例で得られた触媒(浄化性能評価サンプル)を、下記組成のモデルガス中のCO、HCおよびNOxそれぞれの50%浄化率に到達する温度(℃)を測定して、各々の触媒の三元浄化性能を評価した。評価条件は下記の通りである。
CO:1.25%
C3H6:1740ppm
NO:2450ppm
O2:0.6%
CO2:14%
H2O:10%
N2:残部
A/F:14.5
ガス流速:25L/min
昇温速度:20℃/min
Claims (5)
- アルミナ粒子の表面にCu元素が存在してなる構成を備えた排気ガス浄化触媒であって、X線光電子分光法(XPS:X-ray Photoelectron Spectroscopy)で測定される、Cuの2p軌道(Cu2p)及びAlの2p軌道(Al2p)の結合エネルギーに対応する各ピークの合計面積を100%としたとき、Cu2pのピーク面積の割合が7~28%であることを特徴とする排気ガス浄化触媒。
- 前記X線光電子分光法(XPS:X-ray Photoelectron Spectroscopy)で測定して得られる、925eV~940eVのピーク面積(Cu0~2価に相当)に対する、925eV~935eVのピーク面積(Cu0~1価に相当)の比率が50%以上であることを特徴とする請求項1に記載の排気ガス浄化触媒。
- CuOx(0≦x≦1)及びCuAl2O4を含み、H2による昇温反応法(H2-TPR)から得られる水素消費ピークにおけるCuOx及びCuAl2O4のピーク面積において、CuOx及びCuAl2O4のピーク面積に対するCuAl2O4のピーク面積率((CuAl2O4/(CuOx+CuAl2O4))×100)が50%以下であることを特徴とする請求項1又は2に記載の排気ガス浄化触媒。
- 触媒に含まれるCuAl2O4の含有量が15質量%以下である請求項1~3の何れかに記載の排気ガス浄化触媒。
- 三元触媒であることを特徴とする請求項1~4の何れかに記載の排気ガス浄化触媒。
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