Patent Grant
11547987
The present disclosure relates to a structured catalyst for oxidation for exhaust gas purification, a method for producing the same, an automobile exhaust gas treatment device, a catalytic molding, and a gas purification method.
As the environmental awareness increases in recent years, automobile exhaust gas regulations are getting more severe. Exhaust gases discharged from automobiles include hazardous substances such as carbon monoxide (CO), hydrocarbons (HC), and particulate matter (PM). Environmental standards require that the amounts of these hazardous substances in the exhaust gas be less than or equal to predetermined concentrations. In particular, exhaust gas discharged from diesel engine cars contains large amounts of hazardous substances, so that reduction of the hazardous substances in exhaust gas is strongly demanded.
Therefore, in the related art, exhaust gas treatment devices are installed in automobiles, whereby the amounts of hazardous substances in the exhaust gas are reduced so as to conform to environmental standards. For example, gaseous hazardous substances such as carbon monoxide and hydrocarbons are detoxified by oxidation with an oxidation catalyst. In addition, particulate matter is collected and removed with a filter. However, due to the fact that particulate matter is deposited in the filter over time of use and the pressure loss in the filter increases, the collecting efficiency of the particulate matter is reduced. In addition, particulate matter deposited in the filter causes reduction of engine output and deterioration of fuel consumption. The particulate matter is composed of, for example, solid soot and soluble organic fractions (SOF). Thus, particulate matter collected in the filter is combusted and oxidized to remove the particulate matter, and thus the filter is regenerated.
In order to efficiently perform oxidation of the gaseous hazardous substances and particulate matter described above, oxidation catalysts are typically used. These oxidation catalysts are generally supported on a heat resistant support.
U.S. 2003/0,109,383 proposes a ceramic catalyst body as a structured catalyst for exhaust gas purification for diesel cars, in which a ceramic support is disposed on a ceramic surface of a substrate, and both a main catalyst component and a co-catalyst component are supported on the ceramic support. In this ceramic catalyst body, a large number of pores formed from lattice defects and the like in the crystal lattice are formed in the surface of a ceramic support made of γ-alumina, and a main catalyst component including Ce—Zr, Pt, and the like is directly supported near the surface of the ceramic support.
Journal of the Society of Oil Engineers (1994) vol. 37, No. 5, p. 480 to 491 reports catalytic activity of oxidation catalysts composed of various metals and metal oxides in a CO oxidation reaction. More specifically, it is reported that Ag2O, Co3O4, MnO2, CuO, NiO, Cr2O3, Fe2O3, SnO2, CdO, ZnO, TiO2, V2O5, MoO3, WO3, Bi2O3, MgO, Al2O3, SiO2, Pd, Pt, Au, and others have catalytic activity in a CO oxidation reaction (Table 1).
In a car, a device for removing other hazardous substances in an exhaust gas is normally disposed at the subsequent stage of the exhaust gas treatment device having an oxidation catalyst (downstream in the direction in which exhaust gas flows). For example, a device for reducing nitrogen oxide (NOx) contained in the exhaust gas is disposed at the subsequent stage of the exhaust gas treatment device having an oxidation catalyst. The main flow of this reduction device is one in which the reduction of nitrogen oxides is performed by ammonia (NH3) produced by hydrolysis of urea. When the exhaust gas becomes high temperature, the oxidation catalyst supported in the exhaust gas treatment device may scatter to the reduction device at the subsequent stage. The scattered oxidation catalyst is mixed into the reduction device at the subsequent stage. When a catalyst having high oxidation force for Pt and others is used as the oxidation catalyst, ammonia in the reduction device is oxidized by the oxidation catalyst and turns to a nitrogen oxide. As a result, ammonia in the reduction device cannot function so as to reduce the nitrogen oxide in the exhaust gas, whereby the purification rate for the nitrogen oxide by the reduction device is reduced.
Furthermore, high-temperature exhaust gas is discharged for a long time from an automobile during operation. Therefore, the oxidation catalyst is required to secure stability and reliability during long-term use, and excellent heat resistance. Known oxidation catalysts are supported in a dispersed state on the surface of supports at normal temperature and normal pressure. However, in the temperature region at 300° C. or higher where the oxidation catalyst functions, the oxidation catalyst moves on the support, and the oxidation catalysts cause aggregation and calcination. This reduces the effective surface area of the oxidation catalyst, and thus decreases the catalytic activity. In addition, a typical exhaust gas treatment device requires an overabundant amount of oxidation catalyst, because an excessive amount of the oxidation catalyst is supported on the support on the assumption of reduction in catalytic activity. This leads to cost increase.
Furthermore, the exhaust gas from an automobile may contain components that become catalyst poisons for oxidation catalysts. For example, when palladium (Pd) was used as the oxidation catalyst and sulfur dioxide (SO2) was contained in the exhaust gas, sulfur dioxide strongly and chemically adsorbed to palladium to form a stable adsorption layer on the surface of the oxidation catalyst. As a result, the activity of the oxidation catalyst decreased, and its purification function for target hazardous substances markedly decreased. Therefore, in the related art, reactivation of the oxidation catalyst was performed by heating the oxidation catalyst periodically, thereby subjecting the catalyst poisons adsorbed to the oxidation catalyst to heat desorption. However, there is a problem that the oxidation catalyst can cause aggregation and fusion as described above when subjected to the heat treatment, and its catalytic activity decreases.
An object of the present disclosure is to provide a structured catalyst for oxidation for exhaust gas purification, a method for producing the same, an exhaust gas treatment device for automobiles, a catalytic molding, and a gas purification method, the structured catalyst can realize a long life time by suppressing the decline in catalytic function of the oxidation catalyst and can attempt to save resources without requiring a complicated replacement operation.
As a result of diligent research to achieve the object described above, the present inventors have found that the structured catalyst for oxidation for exhaust gas purification that can suppress the decline in function of the oxidation catalyst (e.g., catalytic function) and that can realize a long life time can be obtained by including:
a support of a porous structure composed of a zeolite-type compound; and
at least one oxidation catalyst present in the support and being selected from the group consisting of metals and metal oxides, in which
the support has channels that communicate with each other, and
the oxidation catalyst is present at least in the channels of the support, and thus completed the present disclosure based on such finding.
The summary configurations of the present disclosure are as follows.
[1] A structured catalyst for oxidation for exhaust gas purification, including:
a support of a porous structure composed of a zeolite-type compound; and
at least one oxidation catalyst present in the support and selected from the group consisting of metals and metal oxides, in which
the support has channels that communicate with each other, and
the oxidation catalyst is present at least in the channels of the support.
[2] The structured catalyst for oxidation for exhaust gas purification according to [1], in which the metal oxides contain at least one perovskite-type oxide.
[3] The structured catalyst for oxidation for exhaust gas purification according to [1], in which the channels have any one of a one-dimensional pore, a two-dimensional pore, and a three-dimensional pore defined by a framework of the zeolite-type compound and an enlarged pore portion that is different from any of the one-dimensional pore, the two-dimensional pore, and the three-dimensional pore, and
the oxidation catalyst is present at least in the enlarged pore portion.
[4] The structured catalyst for oxidation for exhaust gas purification according to [3], in which the enlarged pore portion causes a plurality of pores constituting any one of the one-dimensional pore, the two-dimensional pore, and the three-dimensional pore to communicate with each other.
[5] The structured catalyst for oxidation for exhaust gas purification according to [3], in which the oxidation catalyst is made of at least one type of oxidation catalytic nanoparticles selected from the group consisting of metal nanoparticles and metal oxide nanoparticles, and an average particle size of the oxidation catalytic nanoparticles is greater than an average inner diameter of the channels and is less than or equal to an inner diameter of the enlarged pore portion.
[6] The structured catalyst for oxidation for exhaust gas purification according to [5], in which the oxidation catalytic nanoparticles are made of at least one type of metal selected from the group consisting of cobalt, iron, copper, silver, manganese, nickel, chromium, tin, zinc, titanium, and platinum, and oxides of the metals.
[7] The structured catalyst for oxidation for exhaust gas purification according to [5], in which the average inner diameter of the channels is from 0.1 nm to 1.5 nm, and the inner diameter of the enlarged pore portion is from 0.5 nm to 50 nm.
[8] The structured catalyst for oxidation for exhaust gas purification according to [5], in which the content of the metal element (M) of the oxidation catalytic nanoparticles is from 0.5 to 2.5 mass % with respect to the oxidation catalyst.
[9] The structured catalyst for oxidation for exhaust gas purification according to [5], in which an average particle size of the metal oxide nanoparticles is from 0.1 nm to 50 nm.
[10] The structured catalyst for oxidation for exhaust gas purification according to [9], in which an average particle size of the metal oxide nanoparticles is from 0.5 nm to 14.0 nm.
[11] The structured catalyst for oxidation for exhaust gas purification according to [5], in which the ratio of the average particle size of the metal oxide nanoparticles to the average inner diameter of the channels is from 0.06 to 500.
[12] The structured catalyst for oxidation for exhaust gas purification according to [11], in which the ratio of the average particle size of the metal oxide nanoparticles to the average inner diameter of the channels is from 0.1 to 36.
[13] The structured catalyst for oxidation for exhaust gas purification according to [12], in which the ratio of the average particle size of the metal oxide nanoparticles to the average inner diameter of the channels is from 1.7 to 4.5.
[14] The structured catalyst for oxidation for exhaust gas purification according to [5], in which the average particle size of the metal nanoparticles is from 0.08 nm to 30 nm.
[15] The structured catalyst for oxidation for exhaust gas purification according to [14], in which the average particle size of the metal nanoparticles is from 0.4 nm to 11.0 nm.
[16] The structured catalyst for oxidation for exhaust gas purification according to [5], in which the ratio of the average particle size of the metal nanoparticles to the average inner diameter of the channels is from 0.05 to 300.
[17] The structured catalyst for oxidation for exhaust gas purification according to [16], in which the ratio of the average particle size of the metal nanoparticles to the average inner diameter of the channels is from 0.1 to 30.
[18] The structured catalyst for oxidation for exhaust gas purification according to [17], in which the ratio of the average particle size of the metal nanoparticles to the average inner diameter of the channels is from 1.4 to 3.6.
[19] The structured catalyst for oxidation for exhaust gas purification according to [1], further including at least another one functional substance held on an outer surface of the support.
[20] The structured catalyst for oxidation for exhaust gas purification according to [19], in which the content of the at least one oxidation catalyst present in the support is greater than that of at least the other one functional substance held on the outer surface of the support.
[21] The structured catalyst for oxidation for exhaust gas purification according to [1], in which the zeolite-type compound is a silicate compound.
[22] An automobile exhaust gas treatment device including the structured catalyst for oxidation for exhaust gas purification described in [1].
[23] A catalytic molding including a honeycomb substrate and the structured catalyst for oxidation for exhaust gas purification described in [1] on a surface of the honeycomb substrate.
[24] A method for producing a structured catalyst for oxidation for exhaust gas purification, including:
a calcination step of a precursor material (B) obtained by impregnating a precursor material (A) for obtaining a support of a porous structure composed of zeolite-type compound with a metal-containing solution; and
a hydrothermal treatment step of hydrothermally treating a precursor material (C) obtained by calcining the precursor material (B) to obtain the support and an oxidation catalyst present in the support.
[25] The method for producing a structured catalyst for oxidation for exhaust gas purification according to [24], further including a step of performing reduction treatment on the hydrothermally treated precursor material (C) after the hydrothermal treatment step.
[26] The method for producing a structured catalyst for oxidation for exhaust gas purification according to [24], in which from 50 to 500 mass % of a non-ionic surfactant is added to the precursor material (A) before the calcination step.
[27] The method for producing a structured catalyst for oxidation for exhaust gas purification according to [24], in which the precursor material (A) is impregnated with the metal-containing solution by adding the metal-containing solution to the precursor material (A) in multiple portions before the calcination step.
[28] The method for producing a structured catalyst for oxidation for exhaust gas purification according to [24], in which in impregnating the precursor material (A) with the metal-containing solution before the calcination step, a value obtained by converting the added amount of the metal-containing solution added to the precursor material (A) to a ratio of silicon (Si) constituting the precursor material (A) to a metal element (M) included in the metal-containing solution added to the precursor material (A) (a ratio of number of atoms Si/M) is adjusted to from 10 to 1000.
[29] The method for producing a structured catalyst for oxidation for exhaust gas purification according to [24], in which in the hydrothermal treatment step, the precursor material (C) and a structure directing agent are mixed.
[30] The method for producing a structured catalyst for oxidation for exhaust gas purification according to [25], in which the hydrothermal treatment step is performed in a basic condition.
[31] A gas purification method for purifying a gas containing carbon monoxide and hydrocarbon using a structured catalyst, in which the structured catalyst includes a support of a porous structure composed of a zeolite-type compound and at least one type of oxidation catalyst being present in the support and selected from the group consisting of metals and metal oxides, the support has channels that communicate with each other, and the oxidation catalyst is present in at least the channels of the support.
[32] The gas purification method according to [31], in which the structured catalyst is a structured catalyst for oxidation for exhaust gas purification described in [1].
[33] The gas purification method according to [32], in which the gas is discharged from an internal combustion engine.
[34] A gas purification method for purifying gas discharged from an internal combustion engine with the automobile exhaust gas treatment device described in [22].
Provided is a structured catalyst for oxidation for exhaust gas purification, a method for producing the same, an automobile exhaust gas treatment device, a catalytic molding, and a gas purification method, the structured catalyst for oxidation realizing a long life time by suppressing the decline in catalytic function of the oxidation catalyst and saving resources without requiring a complicated replacement operation can be provided.
Hereinafter, embodiments of the present disclosure will be described in detail with reference to drawings.
Configuration of Structured Catalyst for Oxidation for Exhaust Gas Purification
The structured catalyst for oxidation for exhaust gas purification of one embodiment includes a support of a porous structure composed of a zeolite-type compound, and at least one type of oxidation catalyst that is present in the support and selected from the group consisting of metals and metal oxides, the support having channels that communicate with each other, and the oxidation catalyst being present in at least the channels of the support.
As illustrated in
The oxidation catalyst 20 is a catalytic substance having catalytic function by working alone or in cooperation with the support 10. The support 10 supports a catalytic substance. The oxidation catalyst 20 may have one or a plurality of functions other than a catalytic function, and examples of the function other than the catalytic function include light emission (or fluorescent) functions, light-absorbing functions, and identification functions.
In the structured catalyst for oxidation for exhaust gas purification 1, a plurality of oxidation catalysts 20, 20, . . . are embedded in the porous structure of the support 10. The catalytic substance as an example of the oxidation catalyst 20 is at least one type selected from the group consisting of metals and metal oxides, and the metal oxide may contain at least one perovskite-type oxide. The oxidation catalyst 20 is preferably made of at least one type of oxidation catalytic nanoparticles selected from the group consisting of metal nanoparticles and metal oxide nanoparticles. The metal nanoparticles and metal oxide nanoparticles are described in detail below.
The support 10 is a porous structure, and as illustrated in
With such a configuration, movement of the oxidation catalyst 20 within the support 10 is restricted, and aggregation between the oxidation catalysts 20 and 20 is effectively prevented. As a result, the decrease in effective surface area of the oxidation catalyst 20 can be effectively suppressed, and the function of the oxidation catalyst 20 lasts for a long period of time. In other words, according to the structured catalyst for oxidation for exhaust gas purification 1, the decline in function due to aggregation of the oxidation catalyst 20 can be suppressed, and the life of the structured catalyst for oxidation for exhaust gas purification 1 can be extended. In addition, due to the long life time of the structured catalyst for oxidation for exhaust gas purification 1, the replacement frequency of the structured catalyst for oxidation for exhaust gas purification 1 can be reduced, and the amount of waste of the used structured catalyst for oxidation for exhaust gas purification 1 can be significantly reduced, and thereby can save resources.
Typically, when the structured catalyst for oxidation for exhaust gas purification is used in a fluid (e.g., an exhaust gas containing carbon monoxide (CO), hydrocarbon (HC), particulate matter (PM), etc.), it can be subjected to external forces from the fluid. In this case, if the oxidation catalyst is only held in the state of attachment to the outer surface of the support 10, there is a problem in that it is easy to disengage from the outer surface of the support 10 due to the influence of external force from the fluid. In contrast, in the structured catalyst for oxidation for exhaust gas purification 1, the oxidation catalyst 20 is held at least in the channel 11 of the support 10, and therefore, even if subjected to an external force caused by a fluid, the oxidation catalyst 20 is less likely to detach from the support 10. That is, when the structured catalyst for oxidation for exhaust gas purification 1 is in the fluid, the fluid flows into the channel 11 from the pore 11a of the support 10, so that the speed of the fluid flowing through the channel 11 is slower than the speed of the fluid flowing on the outer surface of the support 10 due to the flow path resistance (frictional force). Due to the influence of such flow path resistance, the pressure experienced by the oxidation catalyst 20 held in the channel 11 from the fluid is lower than the pressure at which the oxidation catalyst is received from the fluid outside of the support 10. As a result, detachment of the oxidation catalyst 20 present in the support 11 can be effectively suppressed, and the function of the oxidation catalyst 20 can be stably maintained over a long period of time. Note that the flow path resistance as described above is thought to be larger so that the channel 11 of the support 10 has a plurality of bends and branches, and the interior of the support 10 becomes a more complex three-dimensional structure.
Preferably, the channel 11 has any one of a one-dimensional pore, a two-dimensional pore, and a three-dimensional pore defined by the framework of the zeolite-type compound and an enlarged pore portion which has a diameter different from that of any of the one-dimensional pore, the two-dimensional pore, and the three-dimensional pore. In this case, the oxidation catalyst 20 is preferably present at least in the enlarged pore portion 12. More preferably, the oxidation catalyst 20 is embedded at least in the enlarged pore portion 12. Here, the “one-dimensional pore” refers to a tunnel-type or cage-type pore forming a one-dimensional channel, or a plurality of tunnel-type or cage-type pores (a plurality of one-dimensional channels) forming a plurality of one-dimensional channels. Also, the “two-dimensional pore” refers to a two-dimensional channel in which a plurality of one-dimensional channels are connected two-dimensionally. The “three-dimensional pore” refers to a three-dimensional channel in which a plurality of one-dimensional channels is connected three-dimensionally. As a result, the movement of the oxidation catalyst 20 within the support 10 is further restricted, and it is possible to further effectively prevent separation of the oxidation catalyst 20 and aggregation between the oxidation catalysts 20, 20. The term “embedding” refers to a state in which the oxidation catalyst 20 is included in the support 10. At this time, the oxidation catalyst 20 and the support 10 need not necessarily be in direct contact with each other, but may be indirectly held by the support 10 with other substance (e.g., a surfactant, etc.) interposed between the oxidation catalyst 20 and the support 10.
Although
Additionally, the channel 11 is formed three-dimensionally by including a branch portion or a merging portion within the support 10, and the enlarged pore portion 12 is preferably provided in the branch portion or the merging portion of the channel 11.
The average inner diameter DF of the channel 11 formed in the support 10 is calculated from the average value of the short diameter and the long diameter of the pore 11a constituting any of the one-dimensional pore, the two-dimensional pore, and the three-dimensional pore. For example, it is from 0.1 nm to 1.5 nm, and preferably from 0.5 nm to 0.8 nm. The inner diameter DE of the enlarged pore portion 12 is from 0.5 nm to 50 nm, for example. The inner diameter DE is preferably from 1.1 nm to 40 nm, and more preferably from 1.1 nm to 3.3 nm. For example, the inner diameter DE of the enlarged pore portion 12 depends on the pore diameter of the precursor material (A) described below and the average particle size DC of the oxidation catalyst 20 present therein. The inner diameter DE of the enlarged pore portion 12 is sized so that it allows the presence of the oxidation catalyst 20.
The support 10 is composed of a zeolite-type compound. Examples of zeolite-type compounds include zeolite analog compounds such as zeolites (alminosilicate salts), cation exchanged zeolites, silicate compounds such as silicalite, alminoborate salts, alminoarsenate salts, and germanate salts; and phosphate-based zeolite analog materials such as molybdenum phosphate. Among these, the zeolite-type compound is preferably a silicate compound.
The framework of the zeolite-type compound is selected from FAU type (Y type or X type), MTW type, MFI type (ZSM-5), FER type (ferrierite), LTA type (A type), MWW type (MCM-22), MOR type (mordenite), LTL type (L type), and BEA type (beta type). Preferably, it is MFI type, and more preferably ZSM-5. A plurality of pores having a pore diameter corresponding to each framework is formed in the zeolite-type compound. For example, the maximum pore diameter of MFI type is 0.636 nm (6.36 Å) and the average pore diameter is 0.560 nm (5.60 Å).
The case where the oxidation catalyst 20 is made of metal nanoparticles and metal oxide nanoparticles will be described below in detail.
When the oxidation catalyst 20 is made of at least one type of oxidation catalytic nanoparticles selected from the group consisting of metal nanoparticles and metal oxide nanoparticles, the oxidation catalytic nanoparticles 20 may be primary particles or secondary particles formed by aggregation of primary particles, but the average particle size DC of the oxidation catalytic nanoparticles 20 is preferably larger than the average inner diameter DF of the channel 11 and not greater than the inner diameter DE of the enlarged pore portion 12 (DF<DC<DE). Such oxidation catalytic nanoparticles 20 are present in the channel 11, preferably embedded in the enlarged pore portion 12, and the movement of the oxidation catalytic nanoparticles 20 within the support 10 is restricted. Thus, even if the oxidation catalytic nanoparticles 20 are subjected to external force from the fluid, movement of the oxidation catalytic nanoparticles 20 within the support 10 is suppressed, and it is possible to effectively prevent the oxidation catalytic nanoparticles 20, 20, . . . embedded in the enlarged pore portions 12, 12, . . . dispersed in the channel 11 of the support 10 from coming into contact with each other.
In addition, when the oxidation catalyst 20 is made of metal nanoparticles, the average particle size DC of the metal nanoparticles 20 is preferably from 0.08 nm to 30 nm, more preferably 0.08 nm or higher and less than 25 nm, and further preferably from 0.4 nm to 11.0 nm, and particularly preferably from 0.8 nm to 2.7 nm for primary particles and second particles. Furthermore, the ratio (DC/DF) of the average particle size DC of the metal nanoparticles 20 to the average inner diameter DF of the channel 11 is preferably from 0.05 to 300, more preferably from 0.1 to 30, even more preferably from 1.1 to 30, and particularly preferably from 1.4 to 3.6. When the oxidation catalyst 20 is made of metal nanoparticles, the metal element (M) of the metal nanoparticles is preferably contained in from 0.5 to 2.5 mass % relative to the structured catalyst for oxidation for exhaust gas purification 1, and more preferably from 0.5 to 1.5 mass % relative to the structured catalyst for oxidation for exhaust gas purification 1. For example, when the metal element (M) is Co, the content of the Co element (mass %) is expressed as ((mass of Co element)/(mass of all elements in the structured catalyst for oxidation for exhaust gas purification 1))×100.
The metal nanoparticles 20 only needs to be constituted by a metal that is not oxidized, and may be constituted by a single metal or a mixture of two or more types of metals, for example. Note that in the present specification, the “metal” constituting the metal nanoparticles (as the raw material) refers to an elemental metal containing one type of metal element (M) and a metal alloy containing two or more types of metal elements (M), and the term is a generic term for a metal containing one or more metal elements (M).
Examples of the metal include complex metals composed mainly of any one or two or more of gold (Au), silver (Ag), rhodium (Rh), iridium (Ir), osmium (Os), platinum (Pt), palladium (Pd), ruthenium (Ru), nickel (Ni), cobalt (Co), molybdenum (Mo), tungsten (W), iron (Fe), chromium (Cr), cerium (Ce), copper (Cu), magnesium (Mg), aluminum (Al), manganese (Mn), tin (Sn), Zinc (Zn), and titanium (Ti).
When the oxidation catalyst 20 is made of metal oxide nanoparticles, the average particle size DC of the metal oxide nanoparticles 20 is preferably from 0.1 nm to 50 nm, more preferably 0.1 nm or higher and less than 30 nm, and further preferably from 0.5 nm to 14.0 nm, and particularly preferably from 1.0 nm to 3.3 nm for primary particles and second particles. Furthermore, the ratio (DC/DF) of the average particle size DC of the metal oxide nanoparticles 20 to the average inner diameter DF of the channel 11 is preferably from 0.06 to 500, more preferably from 0.1 to 36, even more preferably from 1.1 to 36, and particularly preferably from 1.7 to 4.5. When the oxidation catalyst 20 is made of metal oxide nanoparticles, the content of the metal element (M) of the metal oxide nanoparticles in the structured catalyst for oxidation for exhaust gas purification 1 is preferably from 0.5 to 2.5 mass %, and more preferably from 0.5 to 1.5 mass %. For example, when the metal element (M) is Co, the content of the Co element (mass %) is expressed as {(mass of Co element)/(mass of all elements of the structured catalyst for oxidation for exhaust gas purification 1)}×100.
The metal oxide nanoparticles 20 only need to be constituted by a metal oxide. For example, the metal oxide nanoparticles may be constituted by a single metal oxide, or may be constituted by a mixture of two or more types of metal oxides. Note that in the present specification, the “metal oxide” constituting the metal oxide nanoparticles (as the raw material) refers to an oxide containing one type of metal element (M) and a complex oxide containing two or more types of metal elements (M), and the term is a generic term for an oxide containing one or more metal elements (M).
Examples of such metal oxides include cobalt oxide (CoOx), nickel oxide (NiOx), iron oxide (FeOx), copper oxide (CuOx), zirconium oxide (ZrOx), cerium oxide (CeOx), aluminum oxide (AlOx), niobium oxide (NbOx), titanium oxide (TiOx), bismuth oxide (BiOx), molybdenum oxide (MoOx), vanadium oxide (VOx), chromium oxide (CrOx), silver oxide (AgOx), manganese oxide (MnOx), tin oxide (SnOx), and zinc oxide (ZnOx). Preferably, any one of oxides described above is the major component.
The oxidation catalytic nanoparticles are preferably selected from the group consisting of at least one metal selected from cobalt, iron, copper, silver, manganese, nickel, chromium, tin, zinc, titanium, and platinum, and oxides of these metals. As also described in Journal of the Society of Oil Engineers (1994) vol. 37, No. 5, p. 480 to 491, the oxidation catalytic nanoparticles formed of any of the metals or metal oxides described above have excellent oxidation catalytic activity.
Furthermore, the metal oxide nanoparticles may contain at least one perovskite-type oxide as a metal oxide. The perovskite-type oxide is an ABO3-type compound composed of a metal ion (ion A) with a large ion radius (>0.90 Å) such as a rare earth element or an alkaline earth metal, and a metal ion (ion B) with a small ion radius (>0.51 Å). Perovskite-type oxides, which are important as catalysts, are compounds based on a combination of a rare earth element (A-site) and a transition metal (B-site), whose catalytic properties depend primarily on the nature of the B-site transition metal. The perovskite catalyst species to be embedded is not particularly limited as long as it is a compound that satisfies the general formula of perovskite ABO3 (A: at least one element selected from rare earth elements and alkaline earth metals, B: at least one element selected from transition metal elements), and a compound having catalytic activity. Specific examples of the perovskite-type oxide include LaBO3 (B═Mn, Cr, Co, Fe, Al, Pd, or Mg), BaZrO3, La0.8Sr0.2Ga0.8Mg0.2O3 (LSGM), LaFe0.57Co0.38Pd0.05O3, and Ba0.8La0.2Mn0.2O3. The metal oxide nanoparticles 20 may, for example, be composed of a single perovskite-type oxide, or a complex oxide composed of two or more types of perovskite-type oxides, or a complex oxide composed of a perovskite-type oxide and other oxide.
Furthermore, the ratio of silicon (Si) constituting the support 10 to a metal element (M) constituting the oxidation catalytic nanoparticles 20 (the ratio of number of atoms Si/M) is preferably from 10 to 1000, and more preferably from 50 to 200. If the ratio is greater than 1000, the activity is low, so that the action as the oxidation catalyst may not be sufficiently achieved. On the other hand, if the ratio is smaller than 10, the proportion of the oxidation catalytic nanoparticles 20 becomes too large, and the strength of the support 10 tends to decrease. The oxidation catalytic nanoparticles 20 herein refer to the nanoparticles present in the support 10, and do not include nanoparticles adhered to the outer surface of the support 10.
Function of Structured Catalyst for Oxidation for Exhaust Gas Purification
The structured catalyst for oxidation for exhaust gas purification 1 has molecular sieving capability to transmit hazardous substances (e.g., carbon monoxide (CO), hydrocarbons (HC), and particulate matter (PM)) contained in the exhaust gas of an automobile. Specifically, as illustrated in
In addition, in the structured catalyst for oxidation for exhaust gas purification 1, an oxidation catalyst 20 is present in the enlarged pore portion 12 of the channels 11. Thus, hazardous substances that have entered into the pores 11a, or the channels 11 come into contact with the oxidation catalyst 20. When the oxidation catalyst 20 is made of metal nanoparticles or metal oxide nanoparticles, if the average particle size DC of the metal nanoparticles or metal oxide nanoparticles is larger than the average inner diameter DF of the channel 11 and smaller than the inner diameter DE of the enlarged pore portion 12 (DF<DC<DE), a small channel 13 is formed between the metal nanoparticles or metal oxide nanoparticles and the enlarged pore portion 12 (arrow in the figure), and the molecules that have entered into the small channel 13 come into contact with the metal nanoparticles or metal oxide nanoparticles. At this time, the oxidation catalyst 20 is present in the enlarged pore portion 12 and thus its movement is limited, and maintains its contact area with the fluid containing hazardous substances that has entered into the channel 11. When the hazardous substances that have entered into the channel 11 contact the metal nanoparticles or metal oxide nanoparticles as the oxidation catalyst 20, the hazardous substances are detoxified by oxidation reaction. For example, as described below, carbon monoxide and hydrocarbons, which are hazardous substances in the exhaust gas, are efficiently converted to carbon dioxide and water. The oxidation treatment by the catalyst of metal nanoparticles or metal oxide nanoparticles effectively reduces the amount of hazardous substances in the exhaust gas.
CO+1/2O2→CO2
CyHz+(y+z/4)O2→yCO2+z/2H2O
Similarly, particulate matter in the exhaust gas such as solid soot and soluble organic fractions are efficiently converted to carbon dioxide and water, and the like, and detoxified by oxidation reaction of the metal nanoparticles or metal oxide nanoparticles as the oxidation catalysts 20. For example, regeneration of a filter can be performed by converting the particulate matter, which has been trapped in the filter, to other substance through oxidization with the oxidation catalyst 20.
Here, when the exhaust gas is at an elevated temperature, the metal nanoparticles or metal oxide nanoparticles 20 are diffused by heat from the exhaust gas, and tuned to ultrafine nanoparticles by the diffusion, and thus can be detached from the enlarged pore portion 12. However, the phenomenon of diffusion of small metal nanoparticles or metal oxide nanoparticles having a small particle size of approximately 5 nm diffuse as smaller metal nanoparticles or metal oxide nanoparticles is unstable, and the progress of the diffusion requires high activation energy. Therefore, the diffusion as described above is difficult to progress. In addition, even if diffusion has progressed, the metal nanoparticles or metal oxide nanoparticles 20 become finely divided, so the effective surface area as a catalyst after diffusion becomes larger than before diffusion. In addition, although the channel 11 is illustrated in a simplified form in
Automobile Exhaust Gas Treatment Device
In one embodiment, an exhaust gas treatment device for an automobile having a structured catalyst for oxidation for exhaust gas purification may be provided. The exhaust gas treatment device of an automobile may have a structured catalyst for oxidation for exhaust gas purification alone, or the structured catalyst for oxidation may be combined with, for example, other structured catalyst such as a reduction structured catalyst for exhaust gas purification, and a collecting filter for particulate matter. By using the structured catalyst for oxidation for exhaust gas purification in a device having such a configuration, the same effects as those described above can be achieved. In the gas purification method of an embodiment, the gas discharged from the internal combustion engine is preferably purified by an exhaust gas treatment device of an automobile.
Method for Producing Structured Catalyst for Oxidation for Exhaust Gas Purification
The method for producing a structured catalyst for oxidation for exhaust gas purification according to one embodiment includes:
a calcination step of a precursor material (B) obtained by impregnating a precursor material (A) for obtaining a support of a porous structure composed of zeolite-type compound with a metal-containing solution; and
a hydrothermal treatment step of hydrothermally treating a precursor material (C) obtained by calcining the precursor material (B).
Step S1: Preparation Step
As illustrated in
Here, in a case where the zeolite-type compound constituting the structured catalyst for oxidation for exhaust gas purification is a silicate compound, the regular mesopore material is preferably a compound including a Si—O skeletal body in which pores having a pore diameter from 1 to 50 nm are uniformly sized and regularly developed one-dimensionally, two-dimension-ally, or three-dimensionally. While such a regular mesopore material is obtained as a variety of synthetic materials depending on the synthetic conditions. Specific examples of the synthetic material include SBA-1, SBA-15, SBA-16, KIT-6, FSM-16, and MCM-41. Among them, MCM-41 is preferred. Note that the pore diameter of SBA-1 is from 10 to 30 nm, the pore diameter of SBA-15 is from 6 to 10 nm, the pore diameter of SBA-16 is 6 nm, the pore diameter of KIT-6 is 9 nm, the pore diameter of FSM-16 is from 3 to 5 nm, and the pore diameter of MCM-41 is from 1 to 10 nm. Examples of such a regular mesopore material include mesoporous silica, mesoporous aluminosilicate, and mesoporous metallosilicate.
The precursor material (A) may be a commercially available product or a synthetic product. When the precursor material (A) is synthesized, it can be synthesized by a known method for synthesizing a regular mesopore material. For example, a mixed solution including a raw material containing the constituent elements of the precursor material (A) and a molding agent for defining the structure of the precursor material (A) is prepared, and the pH is adjusted as necessary to perform hydrothermal treatment (hydrothermal synthesis). Thereafter, the precipitate (product) obtained by hydrothermal treatment is recovered (e.g., filtered), washed and dried as necessary, and then calcined to obtain a precursor material (A) which is a powdered regular mesopore material. Here, examples of the solvent of the mixed solution that can be used include water, an organic solvent such as alcohol, or a mixed solvent thereof. In addition, the raw material is selected according to the type of the support, but examples include silica agents such as tetraethoxysilane (TEOS), fumed silica, and quartz sand. In addition, various types of surfactants, block copolymers, and the like can be used as the molding agent, and it is preferably selected depending on the type of the synthetic materials of the regular mesopore material. For example, a surfactant such as hexadecyltrimethylammonium bromide is preferable when producing MCM-41. The hydrothermal treatment can be performed at from 0 to 2000 kPa at 80 to 800° C. for 5 hours to 240 hours in a sealed container. For example, the calcination treatment can be performed in air, at 350 to 850° C. for 2 hours to 30 hours.
Step 2: Impregnating Step
The prepared precursor material (A) is then impregnated with the metal-containing solution to obtain the precursor material (B).
The metal-containing solution is a solution containing a metal component (for example, a metal ion) corresponding to the metal element (M) constituting the metal nanoparticles and metal oxide nanoparticles of the structured catalyst for oxidation for exhaust gas purification, and can be prepared, for example, by dissolving a metal salt containing a metal element (M) in a solvent. Examples of such metal salts include metal salts such as chlorides, hydroxides, oxides, sulfates, and nitrates. Of these, nitrates are preferable. Examples of the solvent that can be used include water, an organic solvent such as alcohol, or a mixed solvent thereof.
The method for impregnating the precursor material (A) with the metal-containing solution is not particularly limited; however, for example, the metal-containing solution is preferably added in portions in a plurality of times while mixing the powdered precursor material (A) before the calcination step described below. In addition, the surfactant is preferably added to the precursor material (A) as the additive before adding the metal-containing solution to the precursor material (A) from the perspective of allowing the metal-containing solution to enter the pores of the precursor material (A) more easily. It is believed that such additives serve to cover the outer surface of the precursor material (A) and inhibit the subsequently added metal-containing solution from adhering to the outer surface of the precursor material (A), making it easier for the metal-containing solution to enter the pores of the precursor material (A).
Examples of such additives include non-ionic surfactants such as polyoxyethylene oleyl ether, polyoxyethylene alkyl ether, and polyoxyethylene alkylphenyl ether. It is believed that these surfactants do not adhere to the interior of the pores because their molecular size is large and cannot enter the pores of the precursor material (A), and will not interfere with the penetration of the metal-containing solution into the pores. As the method for adding the non-ionic surfactant, for example, it is preferable to add from 50 to 500 mass % of the non-ionic surfactant to the precursor material (A) before the calcination step described below. In a case where the added amount of the non-ionic surfactant to the precursor material (A) is less than 50 mass %, the aforementioned suppressing action will not easily occur, and when more than 500 mass % of the non-ionic surfactant is added to the precursor material (A), the viscosity is too high, which is not preferable. Thus, the added amount of the non-ionic surfactant to the precursor material (A) is a value within the range described above.
Furthermore, the added amount of the metal-containing solution added to the precursor material (A) is preferably adjusted as appropriate in consideration of the amount of the metal element (M) contained in the metal-containing solution with which the precursor material (A) is impregnated (that is, the amount of the metal element (M) present in the precursor material (B)). For example, before the calcination step described below, the value obtained by converting the added amount of the metal-containing solution added to the precursor material (A) to a ratio of silicon (Si) constituting the precursor material (A) to a metal element (M) included in the metal-containing solution added to the precursor material (A) (the ratio of number of atoms Si/M) is preferably adjusted to from 10 to 1000, and more preferably from 50 to 200. For example, if the surfactant is added to the precursor material (A) as the additive before adding the metal-containing solution to the precursor material (A), when the value obtained by converting the added amount of the metal-containing solution added to the precursor material (A) to the ratio of number of atoms Si/M is from 50 to 200, from 0.5 to 2.5 mass % of the metal element (M) of the metal nanoparticles and metal oxide nanoparticles can be included in the structured catalyst for oxidation for exhaust gas purification. In the state of the precursor material (B), the amount of the metal element (M) present within the pores is generally proportional to the added amount of the metal-containing solution added to the precursor material (A) in a case where the metal concentration of the metal-containing solution, the presence or absence of additives, and other conditions such as temperature, pressure, and the like are the same. The amount of metal element (M) present in the precursor material (B) is also in a proportional relationship to the amount of metal element constituting the metal nanoparticles metal oxide nanoparticles present in the support of the structured catalyst for oxidation for exhaust gas purification. Thus, by controlling the added amount of the metal-containing solution added to the precursor material (A) to the range described above, the pores of the precursor material (A) can be sufficiently impregnated with the metal-containing solution, and thus the amount of the metal nanoparticles and the metal oxide nanoparticles present in the support of the structured catalyst for oxidation for exhaust gas purification can be adjusted.
After impregnating the precursor material (A) with the metal-containing solution, a washing treatment may be performed as necessary. Examples of the solvent of the washing solution that can be used include water, an organic solvent such as alcohol, or a mixed solution thereof. Furthermore, the precursor material (A) is preferably impregnated with the metal-containing solution, and after the washing treatment is performed as necessary, the precursor material (A) is further subjected to drying treatment. Drying treatments include overnight natural drying and high temperature drying at 150° C. or lower. Note that when calcination treatment described below is performed in the state in which there is a large amount of moisture remaining in the metal-containing solution and the wash solution in the precursor material (A), the skeletal structure as the regular mesopore material of the precursor material (A) may be broken, and thus it is preferable to dry them sufficiently.
Step S3: Calcination Step
Next, a precursor material (C) is obtained by calcining the precursor material (B) obtained by impregnating the precursor material (A) for obtaining the support of the porous structure composed of zeolite-type compound with the metal-containing solution.
For example, the calcination treatment is preferably performed in air, at 350 to 850° C. for 2 hours to 30 hours. The metal component that has entered into the pores of the regular mesopore material undergoes crystal growth by such a calcination treatment, and metal nanoparticles or metal oxide nanoparticles are formed in the pores.
Step S4: Hydrothermal Treatment Step
A mixed solution of the precursor material (C) and the structure directing agent is then prepared, and the precursor material (C) obtained by calcining the precursor material (B) is hydrothermally treated to obtain a structured catalyst for oxidation for exhaust gas purification.
The structure directing agent is a molding agent for defining the skeletal structure of the support of the structured catalyst for oxidation for exhaust gas purification, and may be, for example, a surfactant. The structure directing agent is preferably selected according to the skeletal structure of the support of the structured catalyst for oxidation for exhaust gas purification, and for example, a surfactant such as tetramethylammonium bromide (TMABr), tetraethylammonium bromide (TEABr), and tetrapropylammonium bromide (TPABr) is preferable.
The mixing of the precursor material (C) and the structure directing agent may be performed during the hydrothermal treatment step or may be performed before the hydrothermal treatment step. Furthermore, the method for preparing the mixed solution is not particularly limited, and the precursor material (C), the structure directing agent, and the solvent may be mixed simultaneously, or each of the dispersion solutions may be mixed after the precursor material (C) and the structural defining agent are each dispersed in individual solutions. Examples of the solvent that can be used include water, an organic solvent such as alcohol, or a mixed solvent thereof. In addition, it is preferable that the pH of the mixed solution is adjusted using an acid or a base before performing the hydrothermal treatment.
The hydrothermal treatment can be performed by a known method. For example, the hydrothermal treatment can be preferably performed at 0 to 2,000 kPa at 80 to 800° C. for 5 hours to 240 hours in a sealed container. Furthermore, the hydrothermal treatment is preferably performed under a basic condition. Although the reaction mechanism here is not necessarily clear, by performing hydrothermal treatment using the precursor material (C) as a raw material, the skeletal structure of the regular mesopore material of the precursor material (C) becomes increasingly disrupted. However, the action of the structure directing agent forms a new skeletal structure (porous structure) as the support of the structured catalyst for oxidation for exhaust gas purification while maintaining the positions of the metal nanoparticles and the metal oxide nanoparticles within the pores of the precursor material (C). The structured catalyst for oxidation for exhaust gas purification obtained in this way includes the support having a porous structure and at least one oxidation catalyst that is present in the support and selected from the group consisting of metal nanoparticles and metal oxide nanoparticles, the support having channels in which a plurality of pores communicate with each other by the porous structure, and at least a portion of the oxidation catalyst being present in the channel of the support. Furthermore, in the present embodiment, in the hydrothermal treatment step, a mixed solution in which the precursor material (C) and the structural defining agent are mixed is prepared, and the precursor material (C) is subjected to hydrothermal treatment, which is not a limitation. The precursor material (C) may be subjected to hydrothermal treatment without mixing the precursor material (C) and the structure directing agent.
The precipitate obtained after hydrothermal treatment (structured catalyst for oxidation for exhaust gas purification) is preferably washed, dried, and calcined as necessary after recovery (e.g., filtration). Examples of the washing solution that can be used include water, an organic solvent such as alcohol, or a mixed solution thereof. Drying treatments include overnight natural drying and high temperature drying at 150° C. or lower. Note that when calcination treatment is performed in the state in which there is a large amount of moisture remaining in the precipitate, the skeletal structure as a support of the structured catalyst for oxidation for exhaust gas purification may be broken, and thus it is preferable to dry the precipitate sufficiently. The calcination treatment may be performed, for example, in air at 350 to 850° C. for 2 hours to 30 hours. Such calcination treatment burns out the structure directing agent that has been attached to the structured catalyst for oxidation for exhaust gas purification. Furthermore, the structured catalyst for oxidation for exhaust gas purification can be used as-is without subjecting the recovered precipitate to calcination, depending on the intended use. For example, in a case where the environment in which the structured catalyst for oxidation for exhaust gas purification is used is a high temperature environment of an oxidizing atmosphere, exposing the structured catalyst for oxidation for exhaust gas purification to a usage environment for a period of time allows the structure directing agent to be burned out and to obtain an structured catalyst for oxidation for exhaust gas purification similar to that when subjected to calcination treatment. Thus, the obtained structured catalyst for oxidation for exhaust gas purification can be used as is.
The producing method described above is an example in which the metal element (M) contained in the metal-containing solution that impregnates the precursor material (A) is a metal species (e.g., a noble metal) that is resistant to oxidation.
When the metal element (M) contained in the metal-containing solution for impregnating the precursor material (A) is a metal species that is easily oxidized (e.g., Fe, Co, or Cu), and metal nanoparticles are used as the oxidation catalyst, the hydrothermally treated precursor material (C) is preferably subjected to reduction treatment after the hydrothermal treatment step. If the metal element (M) contained in the metal-containing solution is a metal species that is easily oxidized, the metal component is oxidized by the heat treatment in the step (step S3 to 4) after the impregnating step (step S2). Therefore, metal oxide nanoparticles are present in the support formed in the hydrothermal treatment step (step S4). Therefore, in order to obtain a structured catalyst for oxidation for exhaust gas purification in which metal nanoparticles are present in the support, the recovered precipitate is preferably subjected to calcination treatment after the hydrothermal treatment, and further to reduction treatment in a reducing gas atmosphere such as hydrogen gas. Through the reduction treatment, the metal oxide nanoparticles present in the support are reduced, and metal nanoparticles corresponding to the metal element (M) constituting the metal oxide nanoparticles are formed. As a result, obtained is a structured catalyst for oxidation for exhaust gas purification including metal nanoparticles present in the support. The reduction treatment may be performed as necessary. For example, when the environment in which the structured catalyst for oxidation for exhaust gas purification is to be used is a reducing atmosphere, the metal oxide nanoparticles are reduced by being exposed to the usage environment for a certain period of time, and the same structured catalyst for oxidation for exhaust gas purification as that subjected to reduction treatment is obtained. Therefore, the structural body can be used as it is with the metal oxide nanoparticles present in the support.
Modified Example of Structured Catalyst for Oxidation for Exhaust Gas Purification 1
This functional substance 30 is a substance that exhibits one or more functions. The functions of the other functional substance 30 may be the same or different from the catalytic function of the oxidation catalyst 20. A specific example of the function of the other functional substance 30 is the same as that described for the oxidation catalyst 20, and preferably has a catalytic function, and the oxidation catalyst 30 is a catalytic substance. Also, in a case where both of the oxidation catalyst 20 and the functional substance 30 are materials having the same function, the material of the other functional substance 30 may be the same as or different from the material of the oxidation catalyst 20. According to this configuration, the content of the oxidation catalyst present in the structured catalyst for oxidation for exhaust gas purification 2 can be greater than the content of at least another one functional substance held on the outer surface of the support, and the functions of the oxidation catalyst can be further promoted.
In this case, the content of the oxidation catalyst 20 present in the support 10 is preferably greater than that of the other functional substance 30 held on the outer surface 10a of the support 10. As a result, the function of the oxidation catalyst 20 supported in the support 10 becomes dominant, and functions of the functional substance are stably exhibited.
Catalytic Molding
In one embodiment, a catalytic molding having a structured catalyst for oxidation for exhaust gas purification may be provided. The catalytic molding may have a structured catalyst for oxidation for exhaust gas purification alone, or may have other structured catalyst such as a reduction structured catalyst for exhaust gas purification. By using the structured catalyst for oxidation for exhaust gas purification in the catalytic molding having such a configuration, the same effects as those described above can be achieved.
The catalytic molding preferably has a substrate and a structured catalyst for oxidation for exhaust gas purification on the surface of the substrate. The substrate is a member having a tubular channel through which exhaust gas passes in the axial direction. That is, the structure having a tubular channel through which exhaust gas passes increases the contact area of the exhaust gas of the structured catalyst for oxidation for exhaust gas purification. By increasing the contact area between exhaust gas and the structured catalyst for oxidation for exhaust gas purification in this way, the capacity for purifying exhaust gas can be improved. Examples of such substrate include honeycomb substrates. The shape of the honeycomb substrate is not particularly limited, and may be selected from known honeycomb substrates (monolithic supports). Examples of the material of the honeycomb substrate include metals such as stainless steel, and heat resistant ceramics such as cordierite.
Method for Producing Catalytic Molding
In the method for producing a catalytic molding according to one embodiment, the structured catalyst for oxidation for exhaust gas purification is subjected to pulverization treatment or the like, thereby obtaining a structured catalyst for oxidation for exhaust gas purification in a particulate state. Next, the structured catalyst for oxidation for exhaust gas purification in a particulate state is mixed with a medium such as water to obtain a slurry. Thereafter, the slurry is applied to a honeycomb substrate, and dried to produce a catalytic molding having a honeycomb substrate and a structured catalyst for oxidation for exhaust gas purification on a surface of the honeycomb substrate. The method for applying the slurry to the honeycomb substrate is not particularly limited, but a wash coat method is preferable. The drying temperature is preferably from 100 to 300° C., and more preferably from 100 to 200° C. The heating means may be a known heating means such as an electric furnace or a gas furnace.
Gas Purification Method
The gas purification method according to one embodiment is a gas purification method for purifying a gas containing carbon monoxide and a hydrocarbon using a structured catalyst, the structured catalyst including a support of a porous structure composed of a zeolite-type compound, and at least one type of oxidation catalyst that is present in the support and selected from the group consisting of metals and metal oxides, the support having channels that communicate with each other, and the oxidation catalyst being present in at least the channels of the support. The same effects as described above can be exhibited by a gas purification method using a structured catalyst having such a configuration. In one embodiment, a structured catalyst for oxidation for exhaust gas purification can be used as the structured catalyst. The gas to be purified by the gas purification method is preferably a gas discharged from an internal combustion engine.
Hereinbefore, the structured catalyst for oxidation for exhaust gas purification according to the present embodiments has been described, but the present disclosure is not limited to the above embodiments, and various modifications and changes are possible on the basis of the technical concept of the present disclosure.
Synthesis of Precursor Material (A)
A mixed aqueous solution was prepared by mixing a silica agent (tetraethoxysilane (TEOS), manufactured by Wako Pure Chemical Industries, Ltd.) and a surfactant as the molding agent. The pH was adjusted as appropriate, and hydrothermal treatment was performed at 80 to 350° C. for 100 hours in a sealed container. Thereafter, the produced precipitate was filtered out, washed with water and ethanol, and then calcined in air at 600° C. for 24 hours to obtain the precursor material (A) of the type and having the pore diameter shown in Tables 1 to 26. Note that the following surfactant was used depending on the type of the precursor material (A).
Next, a metal-containing aqueous solution was prepared by dissolving a metal salt containing the metal element (M) in water according to the metal element (M) constituting the metal nanoparticles and metal oxide nanoparticles of the type shown in Tables 1 to 26. Note that the metal salt was used in accordance with the type of the metal nanoparticles and metal oxide nanoparticles. In the following, the metal, metal oxide, and metal salt are expressed in the order of “metal oxide nanoparticles: metal salt” or “metal nanoparticles: metal salt”.
LaMnO3: Lz-Mn nitrate salt (La(NO3)3.6H2O (99%) and Mn (NO3)2.9H2O (99%) were used, both manufactured by Wako Pure Chemical Industries, Ltd.
BaMnO3: Ba—Mn nitrate salt (Ba(NO3)2 (99%) and Mn(NO3)2.9H2O (99%) were used, both available from Wako Pure Chemical Industries, Ltd.
LaAlO3: La—Al nitrate salt (La(NO3)3.6H2O (99%) and Al(NO3)3.9H2O (99%) were used, both available from Wako Pure Chemical Industries, Ltd.
LaCoO3: La—Co nitrate salt (La(NO3)3.6H2O (99%) and Co(NO3)2.6H2O (99%) were used, both available from Wako Pure Chemical Industries, Ltd.
Next, a metal-containing aqueous solution was added to the powdered precursor material (A) in portions, and dried at room temperature (20° C.±10° C.) for 12 hours or longer to obtain the precursor material (B).
Note that when the presence or absence of additives shown in Tables 1 to 26 is “yes”, pretreatment in which an aqueous solution of polyoxyethylene (15) oleyl ether (NIKKOL BO-15V, available from Nikko Chemicals Co., Ltd.) was added as the additive to the precursor material (A) before adding the metal-containing aqueous solution, and then the metal-containing aqueous solution was added as described above. Note that when “no” is used in the presence or absence of an additive, pretreatment with an additive such as that described above has not been performed.
Furthermore, the added amount of the metal-containing aqueous solution added to the precursor material (A) was adjusted so that the value obtained by converting to a ratio of silicon (Si) constituting the precursor material (A) to a metal element (M) included in the metal-containing aqueous solution is in Tables 1 to 26.
Next, the precursor material (B) impregnated with the metal-containing aqueous solution obtained as described above was calcined in air at 600° C. for 24 hours to obtain the precursor material (C).
Synthesis of Structured Catalyst for Oxidation for Exhaust Gas Purification
The precursor material (C) obtained as described above and the structure directing agent shown in Tables 1 to 26 were mixed to produce a mixed aqueous solution. Hydrothermal treatment was performed under the conditions of at 80 to 350° C., at pH and time shown in Tables 1 to 26 in a sealed container. Thereafter, the produced precipitate was filtered out, washed with water, dried at 100° C. for 12 hours or longer, and further calcined in air at 600° C. for 24 hours to obtain a structured catalyst for oxidation for exhaust gas purification having the support shown in Tables 1 to 16 and metal oxide nanoparticles as the oxidation catalyst (Examples 1 to 768). The calcined product was then recovered and reduction treatment was performed under the inflow of hydrogen gas at 400° C. for 350 minutes to obtain oxidation catalytic structural bodies for exhaust gas purification including the support shown in Tables 17 to 26 and metal nanoparticles as the oxidation catalyst (Examples 769 to 1248).
In Comparative Example 1, cobalt oxide powder (II, III) having an average particle size of 50 nm or less (available from Sigma-Aldrich Co. LLC.) was mixed with MFI type silicalite, thus obtaining a structured catalyst for oxidation for exhaust gas purification in which cobalt oxide nanoparticles were attached as the oxidation catalyst to the outer surface of the silicalite as the support. MFI type silicalite was synthesized in the similar manner as in Examples 52 to 57 except for a step of adding a metal.
In Comparative Example 2, MFI type silicalite was synthesized in the similar manner as in Comparative Example 1 except that the step of attaching the cobalt oxide nanoparticles was omitted.
In Comparative Example 1, cobalt oxide powder (II, III) having an average particle size of 50 nm or less (available from Sigma-Aldrich Co. LLC.) was mixed with MFI type silicalite, thus obtaining a structured catalyst for oxidation for exhaust gas purification in which cobalt oxide nanoparticles were attached as the oxidation catalyst to the outer surface of the silicalite as the support. MFI type silicalite was synthesized in the similar manner as in Examples 436 to 441 except for the step of adding a metal.
Evaluation
Various characteristic evaluations were performed on the oxidation catalytic structural bodies for exhaust gas purification of the above examples including a support and an oxidation catalyst and comparative examples, and silicalite itself under the conditions described below.
[A] Cross Sectional Observation
Observation samples were made using a pulverization method for the oxidation catalytic structural bodies for exhaust gas purification of the examples including a support and an oxidation catalyst and comparative examples described above and silicalite itself, and the cross section observation was performed using a transmission electron microscope (TEM) (TITAN G2, available from FEI Company).
As a result, it was confirmed that, in the structured catalyst for oxidation for exhaust gas purification of the examples described above, the oxidation catalyst was present and held inside the support made of silicalite or zeolite. On the other hand, in the oxidation catalytic structural bodies for exhaust gas purification of Comparative Examples 1 and 3, the oxidation catalyst was only attached to the outer surface of the support and was not present in the support.
In addition, of the examples described above, the structured catalyst for oxidation for exhaust gas purification in which the oxidation catalyst was iron oxide nanoparticles (FeOx) and the structured catalyst for oxidation for exhaust gas purification in which the oxidation catalyst was iron nanoparticles (Fe) were subjected to FIB (focused ion beam) processing to cut out their cross sections, and the section element analysis was performed using SEM (SU8020, manufactured by Hitachi High-Technologies Corporation) and EDX (X-Max, available from HORIBA, Ltd.). As a result, the Fe element was detected in the support.
It was confirmed that iron oxide nanoparticles or iron nanoparticles were present in the support from the results of the cross-sectional observation using TEM and SEM/EDX.
[B] Average Inner Diameter of the Channel of the Support and Average Particle Size of the Oxidation Catalyst
In the TEM image taken by the cross-sectional observation performed in evaluation [A] above, 500 channels of the support were randomly selected, and the respective major diameter and the minor diameter were measured, and the respective inner diameters were calculated from the average values (N=500), and the average value of the inner diameter was determined to be the average inner diameter DF of the channel of the support. In addition, for the oxidation catalysts, 500 oxidation catalysts were randomly selected from the TEM image, and the respective particle sizes were measured (N=500), and the average value thereof was determined to be the average particle size DC of the oxidation catalyst. The results are shown in Tables 1 to 26.
Also, SAXS (small angle X-ray scattering) was used to analyze the average particle size and dispersion status of the oxidation catalyst. Measurements by SAXS were performed using a Spring-8 beam line BL19B2. The obtained SAXS data was fitted with a spherical model using the Guinier approximation method, and the particle size was calculated. The particle size was measured for the structured catalyst for oxidation for exhaust gas purification including iron oxide nanoparticles as the oxidation catalyst, and the structured catalyst for oxidation for exhaust gas purification including iron nanoparticles as the oxidation catalyst. Furthermore, as a comparative reference, commercially available iron oxide nanoparticles and iron nanoparticles (available from Wako Pure Chemical Industries, Ltd.) were observed and measured on SEM.
As a result, in commercial products, various sizes of iron oxide nanoparticles or iron nanoparticles were randomly present in a range of particle sizes from approximately 50 nm to 400 nm, whereas in the measurement results of SAXS, scattering peaks with particle sizes of 10 nm or less were also detected in the oxidation catalytic structural bodies for exhaust gas purification of each example having an average particle size from 1.2 nm to 2.0 nm determined from the TEM image. From the results of SAXS measurement and the SEM/EDX cross-sectional measurement, it was found that oxidation catalysts having a particle size of 10 nm or less are present in the support in a highly dispersed state with uniform particle sizes.
[C] Relationship Between the Added Amount of the Metal-Containing Solution and the Amount of Metal Embedded in the Support
A structured catalyst for oxidation for exhaust gas purification in which metal oxide nanoparticles were embedded in the support and a structured catalyst for oxidation for exhaust gas purification in which metal nanoparticles were embedded in the support at added amount of the ratio of number of atoms of Si/M=50, 100, 200, 1,000 (M=Co, Ni, Fe, Cu) were produced, and then the amounts of metal (mass %) that was embedded in the supports of the oxidation catalytic structural bodies for exhaust gas purification produced at the above added amount were measured. In the present measurement, the oxidation catalytic structural bodies for exhaust gas purification having the ratio of number of atoms of Si/M=100, 200, and 1000 were produced in the same manner as that for the oxidation catalytic structural bodies for exhaust gas purification having the ratio of number of atoms of Si/M=100, 200, and 1000 among those of Examples 1 to 1248, except for the added amount of the metal-containing solution was adjusted, and the structured catalyst for oxidation for exhaust gas purification having the ratio of number of atoms of Si/M=50 was produced in the same manner as that for the oxidation catalytic structural bodies for exhaust gas purification having the ratio of number of atoms of Si/M=100, 200, and 1000, except for the added amount of the metal-containing solution was changed.
The amount of metal was quantified by ICP (radiofrequency inductively coupled plasma) alone or in combination with ICP and XRF (fluorescence X-ray analysis). XRF (energy dispersive fluorescent x-ray analyzer “SEA1200VX”, available from SII Nanotechnology) was performed under conditions of a vacuum atmosphere, an accelerating voltage 15 kV (using a Cr filter), or an accelerating voltage 50 kV (using a Pb filter).
XRF is a method for calculating the amount of metal present in terms of fluorescence intensity, and XRF alone cannot calculate a quantitative value (in terms of mass %). Therefore, the metal content of the structured catalyst for oxidation for exhaust gas purification to which the metal was added at Si/M=100 was determined by ICP analysis, and the metal content of the structured catalyst for oxidation for exhaust gas purification in which the metal was added at Si/M=50 and less than 100 was calculated based on XRF measurement results and ICPO measurement results.
As a result, it was confirmed that the amount of metal embedded in the structured catalyst for oxidation for exhaust gas purification increased as the added amount of the metal-containing solution increases, at least within a range that the ratio of numbers of atom Si/M was within from 50 to 1000.
[D] Performance Evaluation
The catalytic capacity (catalytic activity and durability) of the metal nanoparticles or metal oxide nanoparticles (oxidation catalytic substance) was evaluated for the structured catalyst for oxidation for exhaust gas purification of the above examples and comparative examples including a support and Mmetal nanoparticles or metal oxide nanoparticles. The results are shown in Tables 1 to 26.
(1) Catalytic Activity of CO Oxidation Reaction
First, 10 mg of the structured catalyst for oxidation for exhaust gas purification was charged into a normal pressure flow reactor, CO (127 μL/minute) and air (21 ml/minute) were supplied thereto, and subjected to CO oxidation reaction while heating at 100 to 600° C. The normal pressure flow reactor used was a single microreactor (available from Frontier Laboratories, Ltd. Rx-3050SR). The generated product was analyzed by gas chromatography mass spectrometry (GC/MS). TRACE 1310GC (available from Thermo Fischer Scientific Inc., detector: thermal conductivity detector) was used as the analysis device for the generated gas. The lowest temperature at which the reaction to generate a CO2-containing gas had occurred was measured, and the oxidation catalytic structural bodies for exhaust gas purification were evaluated by “A”, “B”, “C”, and “D” based on the following evaluation criteria.
(2) Catalytic Activity of Propane Oxidation Reaction
10 mg of the structured catalyst for oxidation for exhaust gas purification was charged into a normal pressure flow reactor, C3H8 (21 μL/minute) and air (21 ml minute) were supplied thereto, and a propane oxidation reaction was performed while heating at 100 to 600° C. The lowest temperature at which the reaction to generate a CO2-containing gas had occurred was measured, and the structured catalyst for oxidation for exhaust gas purification was evaluated by “A”, “B”, “C”, and “D” based on the following evaluation criteria.
(3) Confirmation of Intermediate Product
In recent years, regulations on automobile exhaust gas are getting more severe, and catalysts for exhaust gas purifications are desired to minimize intermediate products (e.g., acetone) during the purification reaction of exhaust gas (e.g., less than 1 ppm). According to the analysis of the gas components after the oxidation reactions of (1) and (2) described above at a reaction temperature of 450° C., the acetone concentration was 0.1 ppm or less.
Evaluation Criteria for Catalytic Activity
(a) CO Oxidation Reaction
“A”: The reaction starting temperature (CO2 generation starting temperature) is 200° C. or lower.
“B”: The reaction starting temperature (CO2 generation starting temperature) is higher than 200° C. and 250° C. or lower.
“C”: The reaction starting temperature (CO2 generation starting temperature) is higher than 250° C. and 300° C. or lower.
“D”: The reaction starting temperature (CO2 generation starting temperature) is higher than 300° C.
(b) Propane Oxidation Reaction
“A”: The reaction starting temperature (CO2 generation starting temperature) is 400° C. or lower.
“B”: The reaction starting temperature (CO2 generation starting temperature) is higher than 400° C. and 500° C. or lower.
“C”: The reaction starting temperature (CO2 generation starting temperature) is higher than 500° C. and 600° C. or lower.
“D”: The reaction starting temperature (CO2 generation starting temperature) is higher than 600° C. and 700° C. or lower.
(4) Durability (Life Time)
The oxidation catalytic structural bodies for exhaust gas purification were subjected to a heat cycle test including ten times of heating the structural bodies at 650° C. for 12 hours, followed by cooling to room temperature and allowed to stand for 30 minutes. Thereafter, the same oxidation reaction test as that in (1) and (2) described above was performed. Durability was measured for both the CO oxidation reaction and the propane oxidation reaction by measuring the lowest temperature at which the reaction to generate a CO2-containing gas had occurred, and the oxidation catalytic structural bodies for exhaust gas purification were evaluated by “A”, “B”, “C”, and “D” based on the following evaluation criteria.
Evaluation Criteria for Durability
(a) CO Oxidation Reaction
“A”: The reaction starting temperature (CO2 generation starting temperature) is 200° C. or lower.
“B”: The reaction starting temperature (CO2 generation starting temperature) is higher than 200° C. and 250° C. or lower.
“C”: The reaction starting temperature (CO2 generation starting temperature) is higher than 250° C. and 300° C. or lower.
“D”: The reaction starting temperature (CO2 generation starting temperature) is higher than 300° C.
(b) Propane Oxidation Reaction
“A”: The reaction starting temperature (CO2 generation starting temperature) is 400° C. or lower.
“B”: The reaction starting temperature (CO2 generation starting temperature) is higher than 400° C. and 500° C. or lower.
“C”: The reaction starting temperature (CO2 generation starting temperature) is higher than 500° C. and 600° C. or lower.
“C”: The reaction starting temperature (CO2 generation starting temperature) is higher than 600° C. and 700° C. or lower.
These evaluation results are shown in Table 1 to Table 26.
As can be seen from Tables 1 to 26, the oxidation catalytic structural bodies for exhaust gas purification (Examples 1 to 1248), in which the presence of the oxidation catalyst held in the support was confirmed by cross-sectional observation, exhibited excellent catalytic activity in both the CO oxidation reaction and propane oxidation reaction, and had excellent durability of catalysts in comparison with the oxidation catalytic structural bodies for exhaust gas purification (Comparative Examples 1 and 3), in which the oxidation catalyst was only adhered to the outer surface of the support.
In addition, the support itself of Comparative Example 2 supporting no oxidation catalyst exhibited little catalytic activity in the CO oxidation reaction and propane oxidation reaction, and both the catalytic activity and the durability were inferior in comparison with the oxidation catalytic structural bodies for exhaust gas purification of Examples 1 to 1248.
On the other hand, the oxidation catalytic structural bodies for exhaust gas purification of Comparative Examples 1 and 3, in which the oxidation catalyst was attached only to the outer surface of the support, exhibited improved catalytic activity in both of the CO oxidation reaction and propane oxidation reaction in comparison with the support of Comparative Example 2 having no oxidation catalyst, but had inferior durability of the catalyst in comparison with the oxidation catalytic structural bodies for exhaust gas purification of Examples 1 to 1248. Note that the structured catalyst for oxidation for exhaust gas purification described above had excellent catalytic activity and excellent durability against oxidation reactions of hazardous substances contained in the exhaust gas from an automobile. Therefore, it was confirmed that the structured catalyst for oxidation for exhaust gas purification described above achieves excellent effects.
[A] A method for using a structured catalyst for exhaust gas purification, including:
a support of a porous structure composed of a zeolite-type compound; and
at least one oxidation catalyst present in the support and selected from the group consisting of metals and metal oxides, in which
the support has channels that communicate with each other, and the oxidation catalyst is present at least in the channels of the support.
[B] The method for using a structured catalyst according to [A], in which the exhaust gas is an exhaust gas discharged from an internal combustion engine.
[C] The method for using a structured catalyst according to [A] or [B], in which the internal combustion engine is an automobile engine.
| Number | Date | Country | Kind |
|---|---|---|---|
| JP2017-108615 | May 2017 | JP | national |
| JP2017-108616 | May 2017 | JP | national |
| JP2017-108617 | May 2017 | JP | national |
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| Number | Date | Country | |
|---|---|---|---|
| 20200114339 A1 | Apr 2020 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP2018/021084 | May 2018 | US |
| Child | 16698602 | US |
