Patent Grant
11298674
The present invention relates to an NOx storage material capable of efficiently storing nitrogen oxide (hereinafter, referred to as NOx) in the low temperature region, and an exhaust gas purification method using the NOx storage material.
Recent increasing demand for fuel-efficient vehicles is expected to promote lowering of the exhaust gas temperature. In turn, the challenge is to promote purification of harmful gas components at low temperatures. Among others, NOx emission control at a temperature lower than 200° C. that is a temperature at which the exhaust gas purification catalyst develops its activity is becoming a large challenge.
The temperature region in which the conventional exhaust gas purification catalyst functions is 250° C. or more, and it is therefore difficult to purify NOx generated before achieving sufficient warmup, for example, at the time of engine starting. Accordingly, as a means for controlling NOx emission from when sufficient warmup is not achieved until reaching a temperature region in which the catalyst functions, studies are being made on, for example, a method of disposing an NOx storage material in a first half of the catalyst, holding NOx in the NOx storage material in a low temperature region where the catalyst does not function, desorbing NOx from the NOx storage material in a temperature region where the catalyst exhibits activity, and purifying the desorbed NOR.
As such a material for storing NOx in a low temperature region, Ce1-xZrxO2 is known. In this material, desorption of surface oxygen accompanied by reduction of Ce cation participates in oxidation of NOR, and NOx is held as a nitrate (nitrite) salt having Ce as the cation. The NOx storage properties are further enhanced by making a noble metal including Pt be supported on such a material.
In addition, it has been recently proposed that a material obtained by ion exchanging Pd on to a zeolite compound such as chabazite exhibits high NOx adsorption properties (see, Low Temperature NO Storage of Zeolite Supported Pd for Low Temperature Diesel Engine Emission Control, Catal Lett (2016) 146:1706-1711). In this material, NO is molecularly adsorbed to the ion-exchanged Pd cation site, and high low-temperature NOx adsorption properties are thereby exerted.
On the other hand, studies are being made on an NOx storage material using no platinum group catalyst metal but using various nonmetals and include, for example, a study of Mn having high NO oxidation activity, and it was confirmed that a composite oxide composed of MnOx and CeO2 manufactured by the coprecipitation method exhibits high NOx storage performance in a temperature region of 200° C. or less.
However, even the material described in Non-Paten Document 1 is not yet satisfied in terms of NOx storage capacity at low temperatures. Furthermore, in the composite oxide composed of MnOx and CeO2, an essential requirement for exerting the performance is to contain a transition metal such as manganese, but since harmful effects of such a transition metal itself are pointed out, a material which uses an inexpensive metal without employing a transition metal and temporarily holds NOx in a low temperature region of 200° C. or less, is demanded. An object of the present invention is to solve these problems and provide an NOx storage material having sufficient NOx storage capacity even in a low temperature region without using a transition metal.
The present invention achieves the object above by the following means.
<1> An NOx storage material including a composite oxide of silver and gallium.
<2> The NOx storage material according to <1> above, wherein the composite oxide further contains aluminum.
<3> The NOx storage material according to <1> or <2> above, wherein the composite oxide is a delafossite-type composite oxide.
<4> The NOx storage material according to any one of <1> to <3> above, wherein the concentration of the composite oxide is 1.7 mmol/g or more.
<5> A method for producing the NOx storage material according to any one of <1> to <4> above, including dissolving salts of metals constituting the composite oxide in a solvent, and baking the solution.
<6> The method according to <5> above, wherein the molar ratio of silver:gallium or silver:(gallium+aluminum) in the composite oxide is from 2:8 to 7:3.
<7> An NOx storage device having the NOx storage material according to any one of <1> to <4> above.
<8> An exhaust gas purification method including causing an exhaust gas discharged from an internal combustion engine to flow through the NOx storage device according to <7> above and then flow through an exhaust gas purification device having a catalyst metal.
The NOx storage material of the present invention can develop high NOx storage capacity at low temperatures, particularly, at a low temperature of 200° C. or less.
The mode for carrying out the present invention is descried n detail below. However, the present invention is not limited to the following embodiments and can be conducted by making various modifications thereto within the scope of the gist of the present invention.
<NOx Storage Material>
The NOx storage material of the present invention contains a composite oxide of silver and gallium. The composite oxide may further contain aluminum. Aluminum substitutes for part of gallium in the composite oxide of silver and gallium.
As illustrated in
In the composite oxide of silver and gallium contained in the NOx storage material of the present invention, the molar ratio between silver and gallium may be from 3:7 to 7:3 or from 4:6 to 6:4, and in the composite oxide of silver, gallium and aluminum, the molar ratio between silver and (gallium+aluminum) may be from 2:8 to 7:3. The composite oxide is preferably a delafossite-type composite oxide.
In the NOx storage material of the present invention, the concentration of the composite oxide of silver and gallium may be 1.0 mmol/g or more and is preferably 1.7 mmol/g or more, or 1.8 mmol/g or more. In addition, the concentration may be 2.4 mmol/g or less.
The NOx storage material of the present invention exhibits higher NOx storage capacity than a compound in which palladium is ion exchanged on to a zeolite compound such as chabazite conventionally used as the NOx storage material, in a low temperature region, particularly in a low temperature region of 200° C. or less, furthermore 100° C. or less.
<Production Method of NOx Storage Material>
The NOx storage material of the present invention can be produced by any method commonly employed as the production method of a composite oxide and in particular, can be produced by the following method of the present invention.
The method for producing the NOx storage material of the present invention includes dissolving salts of metals constituting the composite oxide, i.e., a silver salt, a gallium salt and, if desired, an aluminum salt, in a solvent, and baking the solution. As the silver salt, gallium salt and aluminum salt, a nitrate, a carbonate, etc. employed in a general production method of a composite oxide can be used. As the solvent, a solvent capable of dissolving these salts, for example, water, can be used. The silver salt and gallium salt are preferably mixed in a ratio such that the molar ratio of silver and gallium Ag:Ga becomes from 2:8 to 7:3, from 3:7 to 7:3, or from 4:6 to 6:4, and here, part of the gallium, i.e., from 10 to 90 mol % or from 10 to 50 mol %, may be substituted by aluminum.
The baking temperature is usually 250° C. or more, preferably 300° C. or more, and is 600° C. or less, preferably 400° C. or less. The baking may be performed in air or in oxygen but is preferably performed in oxygen. The baking time can be appropriately determined according to the balance with the baking temperature and is usually from 1 to 10 hours.
The baking is preferably performed in the presence of a fuel. As the fuel, glycine, glucose, sucrose, urea, citric acid, hydrazine, carbohydrazine, oxalyl dihydrazine, acetylacetone, hexamethylenetetramine, etc. can be used.
<NOx Storage Device>
The NOx storage device of the present invention has the above-described NOx storage material. In the NOx storage device of the present invention, the NOx storage material may be configured to be housed in a case, or the NOx storage material may be supported on a support particle and then housed.
The support particle includes particles of a metal oxide such as alumina (Al2O3), zirconia (ZrO2), ceria (CeO2), silica (SiO2), magnesia (MgO) and titanium oxide (titania: TiO2), or a composite oxide thereof (for example, a ceria-zirconia (CeO2—ZrO2) composite oxide). Among these, particles of alumina, zirconia, and a ceria-zirconia composite oxide are preferred.
<Exhaust Gas Purification Method>
The exhaust gas purification method of the present invention includes causing an exhaust gas discharged from an internal combustion engine to flow through the NOx storage device above and then flow through an exhaust gas purification device having a catalyst metal.
An exhaust gas discharged from an internal combustion engine is first flowed through the NOx storage device and put into contact with the NOx storage material, and NOx in the exhaust gas can thereby be stored at a low temperature as in starting of the internal combustion engine, particularly, at a low temperature of 100° C. Then, when the exhaust gas reaches a high temperature, NOx stored is desorbed from the NOx storage material and reduced by flowing through an exhaust gas purification device having a catalyst metal and consequently, the harmful component in the exhaust gas is purified.
As the catalyst metal in the exhaust gas purification device, one or a plurality of metals such as platinum (Pt), rhodium (Rh), palladium (Pd), ruthenium (Ru), iridium (Ir) and osmium (Os) can be used.
Although not particularly limited, the content of the catalyst metal relative to the total volume of the catalyst in the exhaust gas purification device is, usually, appropriately from 0.5 to 10 g/L and is preferably, for example, from 0.5 to 7.0 g/L. If the amount of the catalyst metal supported is too small, the catalytic activity (in particular, oxidation catalytic activity) by virtue of the catalyst metal is insufficient, and on the other hand, if the amount of the catalyst metal supported is too large, the catalyst metal is likely to undergo grain growth and at the same time, a cost disadvantage occurs.
The catalyst in the exhaust gas purification device can be used as a pellet by itself or may be used by disposing it on a substrate. As the substrate, the same substrate as employed in the conventionally known exhaust gas purification catalyst can be used. For example, the substrate is preferably a substrate composed of a heat-resistant material having a porous structure. The heat-resistant material includes a heat-resistant metal such as cordierite, silicon carbide (SiC), aluminum titanate, silicon nitride and stainless steel, an alloy thereof, etc. In addition, the substrate preferably has a honeycomb structure, a foam shape, a pellet shape, etc. Incidentally, as for the outer shape of the substrate as a whole, a cylindrical shape, an elliptic cylindrical shape, a polygonal cylindrical shape, etc. can be employed.
2.55 g (15 mmol) of silver nitrate (AgNO3), 14.0 g (35 mmol) of gallium nitrate n-hydrate (Ga(NO3)3.nH2O) and 1.41 g (19 mmol) of glycine were put in a 200 mL beaker and dissolved by adding 10 mL of distilled water. The resulting solution was placed in a muffle furnace, heated to 350° C. at 5.4° C./min and further burned in air at 350° C. for 1 hour. The obtained solid was put in a mortar and pulverized, and the obtained powder was then fired at 500° C. for 2 hours. The powder was compact-molded at 2 t/cm2 to obtain a pellet having a diameter (ϕ) of 1.0 to 1.7 mm. The composite oxide thus produced at a charging ratio of silver and gallium of 3:7 is referred to as Ag3Ga7 composite oxide in the present description.
A pellet was produced in the same manner as in Example 1 using 4.25 g (25 mmol) of silver nitrate (AgNO3), 10.0 g (25 mmol) of gallium nitrate n-hydrate (Ga(NO3)3.nH2O) and 1.41 g (19 mmol) of glycine.
A pellet was produced in the same manner as in Example 1 using 5.95 g (35 mmol) of silver nitrate (AgNO3), 6.0 g (15 mmol) of gallium nitrate n-hydrate (Ga(NO3)3.nH2O) and 1.41 g (19 mmol) of glycine.
A pellet was produced in the same manner as in Example 1 using 0.85 g (5 mmol) of silver nitrate (AgNO3), 18.0 g (45 mmol) of gallium nitrate n-hydrate (Ga(NO3)3.nH2O) and 1.41 g (19 mmol) of glycine.
A pellet was produced in the same manner as in Example 1 using 4.25 g (25 mmol) of silver nitrate (AgNO3), 7.0 g (17.5 mmol) of gallium nitrate n-hydrate (Ga(NO3)3.nH2O), 2.81 g (7.5 mmol) of aluminum nitrate nonahydrate (Al(NO3)3.9H2O) and 0.62 g (8.3 mmol) of glycine.
A pellet was produced in the same manner as in Example 1 using 4.25 g (25 mmol) of silver nitrate (AgNO3), 5.0 g (12.5 mmol) of gallium nitrate n-hydrate (Ga(NO3)3.nH2O), 4.69 g (12.5 mmol) of aluminum nitrate nonahydrate (Al(NO3)3.9H2O) and 0.62 g (8.3 mmol) of glycine.
A pellet was produced in the same manner as in Example 1 using 4.25 g (25 mmol) of silver nitrate (AgNO3), 10.0 g (25.0 mmol) of gallium nitrate n-hydrate (Ga(NO3)3.nH2O) and 0.62 g (8.3 mmol) of glycine.
A pellet was produced in the same manner as in Example 1 using 4.25 g (25 mmol) of silver nitrate (AgNO3), 9.0 g (22.5 mmol) of gallium nitrate n-hydrate (Ga(NO3)3.nH2O), 0.94 g (2.5 mmol) of aluminum nitrate nonahydrate (Al(NO3)3.9H2O) and 0.62 g (8.3 mmol) of glycine.
A pellet was produced in the same manner as in Example 1 using 22.0 g (50 mmol) of gallium nitrate n-hydrate (Ga(NO3)3.nH2O) and 1.41 g (19 mmol) of glycine.
A silver powder produced by Wako Pure Chemical Industries, Ltd. was used.
A material obtained by loading 1 wt % of palladium on chabazite (SiO2/Al2O3=13, BET surface area: 504 m2/g) was used.
<Measurement Method and Evaluation>
With respect to the samples obtained, the properties were measured using the following methods and evaluated.
(XRD)
The measurement was performed using RINT2000 manufactured by Rigaku Corporation with an X-ray source of CuKα (λ=1.5418 nm) under the conditions of a range from 10 to 90 deg., a step width of 0.02 deg., a tube voltage of 50 kV and a tube current of 300 mA. The diffraction peak positions were compared with known data of JCPDS data file. The crystallite diameter was determined using the Scherrer equation.
(Specific Surface Area Analysis)
The measurement was performed at 77 K by the nitrogen adsorption/desorption method with use of BELSORP-max-12-N-VP manufactured by MicrotracBEL Corp. The measurement sample was subjected to vacuum pretreatment for 3 hours at 250° C. The specific surface area was calculated from the straight portion f of BET curve.
(STEM-EDS Mapping Analysis)
Observation was performed under a transmission scanning microscope (STEM) by using JEM-1000 (accelerating voltage: 200 kV) manufactured by JEOL Ltd. The sample powder was dispersed in ethanol, dropped on a copper grid, then dried and measured.
(XAFS Analysis)
The analysis was performed at BL07 Experimental Station of The Kyushu Synchrotron Light Research Center. The spectrum was measured by a transmission method using an ion chamber by means of an Si (220) double crystal spectrometer.
(O2-TPD Analysis)
In the measurement, Belcat A manufactured by MicrotracBEL Corp. was used. 200 mg of the sample powder was accurately weighed and put in a sample tube, 20 Vol % O2/He was flowed at 30 mL/min, and after the temperature was raised to 500° C. and this temperature was maintained for 10 minutes, the system was cooled to 50° C. in the same atmosphere. The gas in the tube was substituted by He gas at 50° C., He was then flowed at 30 mL/mi, the temperature was raised to 500° C. while heating the sample at 10° C./min, and the O2 desorption amount was analyzed by TCD.
(IR Analysis)
In the measurement, Jasco FT/IR-6200 infrared spectrophotometer manufactured by JASCO Corp. was used. A sample and KBr were mixed at a weight ratio of 3:7 in a mortar, 12 mg was accurately weighed and compact-molded into a disk of 8 mm in diameter ϕ, and the disk was used as the sample. As for the window material, CaF was used. As a pretreatment operation, an operation of flowing a mixed gas of O2 3%/Ar balance at a total flow rate of 200 mL/min at 350° C. for 5 minutes and subsequently introducing a mixed gas of H2 5%/Ar balance for 10 minutes was repeatedly performed. The mixed gas of O2 3%/Ar balance at the same flow rate was cooled to the measurement temperature, background measurement was then performed, a mixed gas of NO 400 ppm+O2 9%/Ar balance was flowed, and the spectra after 2, 5, 10, 15, 20, 30, 40, 50 and 60 minutes were measured. The measurement temperature was set to be 100° C., and in the measurement range of 1,000 to 4,000 cm−1, 128 co-added scans were collected at 4 cm−1 resolution.
(NOx-TPD Analysis)
In a flow reactor, 1 mL of a pellet catalyst was placed, and each gas at 180° C., 140° C. and 100° C. was analyzed by an FT-IR analyzer (SESAM-HL, BEST Instruments Co., Ltd.). After evaluating the below-described NOx storage properties, the sample was heated at 20° C./min under N2 flow, and the desorbed NOx species was analyzed.
(Evaluation of NOx Storage Properties)
In a flow reactor, 1 mL of a pellet sample produced in each of Examples and Comparative Examples was placed, and the conversion behavior of each gas at 180° C., 140° C. and 100° C. was analyzed by an FT-IR analyzer (SESAM-HL, BEST Instruments Co., Ltd.) and a magnetic-pressure analyzer (Bex, BEST Instruments Co., Ltd.). After performing a reduction pretreatment at 500° C. for 5 minutes under H2 flow, the gas was switched to a O2 9%+CO2 10%+water 10%/N2 gas, and a pretreatment was performed at the same temperature for 5 minutes. In the same oxidizing atmosphere, the gas was cooled to the evaluation temperature, the lean gas shown in Table 1 below was introduced at 1 L/min for 15 minutes, and the NOx storage amount was calculated based on the difference from the NOx profile in a blank test where the catalyst was not placed.
<Measurement Results>
(Specific Surface Area)
The composition ratio and specific surface area of each of the samples obtained in Examples and Comparative Examples are shown in Table 2 below.
(Results of Structural Analysis by XRD)
An XRD diffraction pattern of each sample is illustrated in
(Results of Structural Analysis by STEM)
(Results of Structural Analysis by XAFS)
As illustrated in
(O2-TPD Analysis Results)
(Analysis Results of NOx Adsorption Species by IR)
(NOx Storage Test Results)
(NOx-TPD Analysis Results)
These results indicate that when part of Ga is substituted by Al, a large amount of nitrate salt is produced at 180° C., and it is presumed that the reason therefor is because the reactivity in oxide or of surface oxygen was increased and the conversion rate from nitrite salt with low thermal stability to nitrate salt with high thermal stability was thereby more accelerated than the desorption rate of nitrous acid.
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