JP4483348B2 - catalyst - Google Patents

Description

The present invention relates to catalysts.

  In recent years, the presence of harmful gases that can be present in the environment surrounding humans and can affect the human body has been regarded as a problem. For example, aldehydes emitted from the building materials into the air Development of a technology capable of reliably purifying NOx (nitrogen oxide), HC (hydrocarbon) and CO (carbon monoxide), which are harmful components in automobile exhaust gas, is desired.

  Under such a background, various catalysts for purifying harmful gases have been developed. In particular, even at a relatively low temperature, such as when an automobile is started, the necessity for sufficiently and reliably removing harmful gases is extremely high, and various catalysts that can meet such a need have been developed. Yes.

  For example, Patent Document 1 discloses a new exhaust gas purifying catalyst capable of efficiently removing NOx, HC, CO, particularly HC, in exhaust gas discharged from an internal combustion engine of an automobile in a low temperature range of 250 ° C. or lower. It consists of one or more transition metal oxides and one or more precious metals (excluding gold), and is composed of hydrocarbons, carbon monoxide, nitrogen oxides in exhaust gas from internal combustion engines, etc. An exhaust gas removing catalyst for removing substances at a temperature of 250 ° C. or lower is disclosed. In this Patent Document 1, it is described that it is preferable that 0.061 to 12.22 parts by mass of noble metal is contained in the exhaust gas removal catalyst with respect to 100 parts by mass of iron oxide as a metal oxide. ing.

  Further, Patent Document 2 discloses a metal oxidation characterized in that palladium particles are uniformly mixed in metal oxide particles in order to lower the use temperature of the catalyst, and the use temperature of the catalyst is lowered. Catalysts comprising product particles have been proposed. The catalyst disclosed in Patent Document 2 describes that it is preferable that 0.1 to 33.3 parts by mass of noble metal is contained with respect to 100 parts by mass of iron oxide as a metal oxide. .

Furthermore, Patent Document 3 discloses a support made of a CeO2-ZrO2 composite oxide, and a carrier composed of a CeO2-ZrO2 composite oxide for the purpose of suppressing grain growth of supported noble metal particles and suppressing CO poisoning of the noble metal. An exhaust gas purifying catalyst comprising oxide particles made of an oxide of at least one metal selected from supported Al, Ni, and Fe and a noble metal supported on the carrier is disclosed. In Patent Document 3, it is described that the exhaust gas purifying catalyst preferably contains 50 to 250 parts by mass of noble metal with respect to 100 parts by mass of iron oxide which is an oxide of the metal.
JP-A-4-215845 JP-A-7-313877 JP 2003-126694 A

  However, the present inventors have studied in detail the conventional catalysts including those described in Patent Documents 1 to 3 described above. Such conventional catalysts are free from harmful gas removal and production costs. And from the viewpoint of resource protection, the present inventors have found that it is not necessarily suitable for practical use.

  The catalysts described in Patent Documents 1 and 2 attempt to remove harmful gases in a relatively low temperature range by supporting or complexing a noble metal with a transition metal oxide. However, the present inventors have found that in the catalysts described in Patent Documents 1 and 2, the heat resistance of the transition metal oxide contained therein is not sufficient. As a result, in a relatively high temperature region (for example, about 500 ° C.) where the internal combustion engine of the automobile is sufficiently warmed up, the activities of those catalysts are reduced, and it is difficult to function as a catalyst that is sufficiently effective for practical use. Turned out to be.

  In the catalyst described in Patent Document 2, when a metal oxide and a noble metal are used in combination, the noble metal is also present at a site in the catalyst where noxious gases such as NOx, CO or HC cannot diffuse. The present inventors have found that. Therefore, even if such a catalyst is used for removing harmful gases, the precious metal, which is the main active point, and is a relatively expensive and valuable resource, has not been fully utilized effectively, and is sufficient for practical use. It turned out not to be suitable.

  Furthermore, the catalyst described in Patent Document 3 contains a large amount of noble metal, which is the main active site, so that harmful gases can be sufficiently removed. Therefore, it became clear that it was not sufficiently suitable for practical use from the viewpoint of manufacturing cost and resource protection.

  Therefore, the present invention has been made in view of the above circumstances, and can sufficiently remove harmful gases in a sufficiently wide temperature range, and can sufficiently suppress the manufacturing cost and protect resources. It is an object of the present invention to provide a catalyst that can sufficiently contribute to the above and a method for producing the same.

  As a result of intensive studies to achieve the above object, the present inventors have used a catalyst for removing harmful gases with respect to the amount of noble metal used by using iron oxide supported on a heat-resistant carrier and a very small amount of noble metal as a catalyst. The inventors have found that the activity is dramatically improved and have completed the present invention.

That is, the catalyst of the present invention is a catalyst obtained by supporting iron oxide and a noble metal on a carrier containing a metal oxide, and contains 0.0004 to 1% by mass of the noble metal with respect to the total mass of the catalyst. And 0.005 to 15 parts by mass of a noble metal with respect to 100 parts by mass of the iron oxide, cerium oxide and zirconium oxide as metal oxides, and cerium oxide and zirconium oxide being a composite oxide. Further, it is characterized by containing 70 to 99 mol% of zirconium oxide with respect to the total molar amount of the metal oxide and used for exhaust gas purification .

  The catalyst of the present invention contains a very small amount of a precious metal, which is a relatively expensive metal, of 0.0004 to 1% by mass. On the other hand, iron oxide is a relatively inexpensive and abundant resource, and is supported on a metal oxide together with a noble metal to constitute the catalyst of the present invention. Therefore, the production cost of the catalyst of the present invention can be sufficiently suppressed. And it can contribute to resource protection by restraining the content of precious metal, which is a valuable resource, within the above range.

  Further, the catalyst of the present invention is similar to the conventional catalyst supporting a relatively large amount of noble metal, although it contains only a very small amount of noble metal which is considered to be an active species of the conventional catalyst as described above. In addition, harmful gases can be sufficiently effectively removed in a low temperature region (for example, 250 ° C. or lower). The present inventors consider the reason as follows. However, the factors are not limited to these.

  That is, even in the conventional catalyst containing iron oxide as described in Patent Documents 1 and 2, the reaction of iron oxide proceeds by its own oxidation / reduction (redox mechanism). It could show a catalytic action such as CO and HC oxidation. However, since the iron oxide contained in the conventional catalysts including Patent Documents 1 and 2 is one of the carrier or the component to be combined, it can be contacted with the reactant in the iron oxide to be formed, and the reaction Only a portion of the iron oxide contributes to. Further, iron oxide in the catalyst exists in a state of relatively large particles. In the above-mentioned low temperature region, the reaction rate of the catalytic reaction depends on the number of active sites, and therefore the conventional catalyst containing a large amount of iron oxide that hardly acts as an active site is sufficiently extracting its performance. I can't say that.

  In addition, since such a conventional catalyst has a relatively low dispersibility of iron oxide, it is presumed that sintering of iron oxide particles easily occurs in a high temperature region, and its activity is reduced.

  On the other hand, in the catalyst of the present invention, iron oxide is supported on a support containing a metal oxide, and a very small amount of noble metal is also supported on the support. Thus, it is considered that iron oxide is present on the support in a highly dispersed state, and further, noble metal fine particles are present on the iron oxide particles or at the grain boundaries between the support and iron oxide. This noble metal itself exhibits catalytic activity, but in addition to this, it is presumed that it also has a function as a promoter that assists the oxidation / reduction action of iron oxide. Therefore, even in a low temperature range where the reaction rate of the catalytic reaction depends on the number of active sites, iron oxide is likely to act as an active site due to the auxiliary action of the noble metal. Or it is thought that poisoning by the reactive substance of a noble metal active site is relieved by iron oxide. As a result, it is presumed that the catalyst of the present invention exhibits sufficiently high catalytic activity even in a low temperature region and can sufficiently remove harmful gases.

  Furthermore, in the catalyst of the present invention, since iron oxide particles are supported on a metal oxide support, it is estimated that iron oxide sintering hardly occurs even in a high temperature region. As a result, it is considered that the catalyst of the present invention can maintain its catalytic activity in a high state even in a high temperature region.

  As the amount of the precious metal used increases, the number of active sites increases. Therefore, the more the precious metal is supported, the higher the activity of the catalyst. However, since the amount of supported noble metal can be reduced by assisting with the activity of iron oxide, 0.01 to 10 parts by mass of noble metal with respect to 100 parts by mass of iron oxide, or 0.05 to 5 Even when it is contained in an amount of 0.1 to 2 parts by mass, sufficient catalyst performance is exhibited.

  In the catalyst of the present invention, the total amount of iron oxide and noble metal is preferably 2 to 25% by mass. By making the total amount of iron oxide and noble metal in the catalyst within the above range with respect to the total amount of the catalyst, the dispersibility of iron oxide and noble metal can be maintained, and sufficient activity can be ensured. Therefore, such a catalyst can sufficiently exhibit the ability to remove harmful gases while suppressing deterioration in a high temperature region. From this viewpoint, the total amount of iron oxide and noble metal is more preferably 2 to 10% by mass.

Te Catalyst odor of the present invention, acid zirconium is hard to react with the iron oxide, to stably hold the active sites of the iron oxide tends to be improved catalytic activity.

The catalyst of the present invention, as the metal oxide, including cerium oxide and zirconium oxide. As a result, the catalyst of the present invention has both the excellent oxygen storage ability of cerium oxide and the high temperature durability due to zirconium oxide, so that removal of harmful gases such as NOx, CO, and HC can be achieved even in a high temperature environment. It can be done more efficiently.

From the same viewpoint, cerium oxide and zirconium oxide is Ru composite oxide der. Particularly preferably a composite oxide of that forms a solid solution.

  Moreover, the catalyst of this invention will become a catalyst excellent in low-temperature activity when it contains 70-99 mol% of zirconium oxide with respect to the total molar amount of the said metal oxide, and it is more preferable when it contains 85-99 mol%. If the content of zirconium oxide in the metal oxide is less than 70 mol%, the high-temperature durability due to zirconium oxide tends to be insufficient, and if it exceeds 99 mol%, the effect of adding metal oxides other than zirconium oxide can be reduced. There is a tendency that it cannot be fully demonstrated. Therefore, when the content ratio of zirconium oxide in the metal oxide is out of the above numerical range, the catalyst of the present invention tends not to exhibit sufficient low-temperature activity.

  In the catalyst of the present invention, the support preferably has 0.1 mL / g or more of pores having a pore diameter of 3.5 to 150 nm. By adjusting the pore diameter and the pore volume within such ranges, it is possible to secure a loading field for iron oxide and noble metal as active sites and to further increase the diffusion of harmful gases into the pores. In addition, iron oxide and precious metal sintering can be more effectively suppressed. As a result, such a catalyst tends to more efficiently and reliably remove harmful gases in a low temperature region, and the catalyst life tends to be extended. From the same viewpoint, the support preferably has 0.2 mL / g or more of pores having a pore diameter of 3.5 to 150 nm, and more preferably 0.3 mL / g or more.

  In the catalyst of the present invention, the noble metal is preferably one or more metals selected from the group consisting of Pt, Pd and Rh. By supporting these noble metals, the catalyst of the present invention has a tendency that iron oxide is more likely to act as an active site in the low temperature region due to the auxiliary action of the noble metal, and the activity as iron oxide tends to be further improved. In addition, iron oxides relieve the poisoning of these noble metal active sites, and there is a tendency that low temperature activity can be expressed even with a small amount.

The catalyst of the present invention, iron oxide and noble metal described above, NOx reduction, to function very effectively as a catalyst for CO oxidation and HC oxidation, typically Ru used for exhaust gas purification of an automobile or the like comprising such ingredients.

  The method for producing a catalyst of the present invention is characterized by having a noble metal supporting step of supporting a noble metal on a composite formed by supporting iron oxide on a carrier containing a metal oxide. The catalyst obtained by this production method is considered to be in a state where a noble metal is supported on a metal oxide support, on iron oxide particles, or in the vicinity of the grain boundary between the support and iron oxide. The noble metal supported on the metal oxide support itself becomes an active species effective for removing harmful gases. In addition, the noble metal supported on the iron oxide particles and the noble metal supported near the grain boundary between the support and the iron oxide are not only active species themselves, but also have a function of assisting the catalytic action of the iron oxide. Is also considered to have.

  Furthermore, this method for producing a catalyst sufficiently suppresses the loading of a noble metal on a site where harmful gases are difficult to diffuse, such as a site covered with a catalyst carrier and iron oxide. Therefore, the resulting catalyst can more fully demonstrate the catalytic activity of precious metals and the auxiliary functions for iron oxide, and at the same time, it can effectively remove trace amounts of precious metal active sites, so that harmful gases can be removed more efficiently and reliably. It becomes.

  According to the present invention, it is possible to remove harmful gases sufficiently effectively even in a low temperature region, and to sufficiently reduce the manufacturing cost, and a catalyst that can sufficiently contribute to resource protection and its A manufacturing method can be provided.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings as necessary. In the drawings, positional relationships such as up, down, left and right are based on the positional relationships shown in the drawings unless otherwise specified.

  The catalyst according to a preferred embodiment of the present invention is a catalyst in which iron oxide and a noble metal are supported on a support containing a metal oxide, and the noble metal is added to the catalyst in a total mass of 0.0004 to 0.0004. 1 mass% is contained, and 0.005-15 mass parts of noble metals are contained with respect to 100 mass parts of iron oxide.

The metal oxide used as the catalyst carrier of the present embodiment is not particularly limited as long as it can be used as a catalyst carrier and the iron oxide supported thereon can be sufficiently dispersed. Examples of such metal oxides include single metal oxides or those containing two or more thereof. In this specification, “metal oxide” includes silicon oxide (SiO ₂ ) and germanium oxide (GeO ₂ ).

Examples of the single metal oxide include lithium oxide (Li ₂ O), sodium oxide (Na ₂ O), potassium oxide (K ₂ O), cesium oxide (Cs ₂ O), and other alkali metal oxides, magnesium oxide (MgO). , Alkaline earth metal oxides such as calcium oxide (CaO), strontium oxide (SrO) or barium oxide (BaO), boron group metal oxides such as aluminum oxide (Al ₂ O ₃ ) or gallium oxide (Ga ₂ O ₃ ) , Scandium oxide (Sc ₂ O ₃ ), yttrium oxide (Y ₂ O ₃ ), lanthanum oxide (La ₂ O ₃ ), cerium oxide (CeO ₂ ), praseodymium oxide (Pr ₂ O), neodymium oxide (Nd ₂ O) ₃ ) or rare earth oxides such as terbium oxide (Tb ₂ O ₃ ), or titanium oxide (Ti Transition metal oxides other than rare earth such as O ₂ ), zirconium oxide (ZrO ₂ ), vanadium oxide (V ₂ O ₅ ), tin oxide (SnO ₂ ), nickel oxide (NiO) or cobalt oxide (Co ₃ O ₄ ) Etc. Note that the alkali metal oxide and the alkaline earth metal oxide may be in a hydroxide state. In the present specification, the rare earth elements (rare earth elements constituting the rare earth oxide) do not include radioactive elements (Pm and actinoids).

  When two or more metal oxides are contained, they may be contained in any state, for example, a state in which the single metal oxides exemplified above are mixed, a state of a composite oxide, or It may be in a state where an element different from the element constituting the oxide is dissolved in the vicinity of the surface of the single metal oxide (substitution, penetration), or a combination of these states.

Among these, it is preferable to use zirconium oxide (ZrO ₂ ) as a catalyst carrier as a single oxide. Zirconium oxide has poor reactivity with iron oxide and can stably hold the active site of iron oxide.

When two or more metal oxides are contained, the metal oxide preferably contains zirconium oxide, and more preferably contains a rare earth oxide together with zirconium oxide. Since the rare earth oxide has an excellent oxygen storage ability and oxygen releasing ability (hereinafter referred to as “OSC”) due to the oxidation-reduction reaction of rare earth ions, the catalyst is obtained by using the rare earth oxide as a catalyst support. There is a tendency to suppress fluctuations in oxygen concentration in the vicinity. In particular, cerium oxide (CeO ₂ ) has particularly superior oxygen storage ability and oxygen release ability among rare earth oxides, and therefore it is particularly preferable that it be contained in the metal oxide used as a carrier in this embodiment. .

  As the carrier containing cerium oxide, cerium oxide can be used alone, or a composite oxide obtained by dissolving cerium oxide with one or more other metal oxides, or a mixture thereof can be used. .

Therefore, it is preferable to use zirconium oxide (ZrO ₂ ) together with cerium oxide as a catalyst support. When used together, the heat resistance is improved and the decrease in the catalyst activity tends to be suppressed. Further, cerium oxide and zirconium oxide are preferably composite oxides, and particularly preferably used as a solid solution.

  When using a cerium oxide-zirconium oxide composite oxide or a solid solution as a support, from the viewpoint of improving the low-temperature activity of the catalyst, the ratio of cerium and zirconium is expressed as a molar ratio when the content ratio of zirconium is expressed as Zr / (Ce + Zr). Therefore, the range of 0.7 ≦ (Zr / (Ce + Zr)) ≦ 0.99 is preferable, and the range of 0.85 ≦ (Zr / (Ce + Zr)) ≦ 0.99 is more preferable.

  In the catalyst of the present embodiment, the pore distribution of the support containing a metal oxide is preferably 0.1 mL / g or more of pores having a pore diameter of 3.5 to 150 nm, preferably 3.5 to 150 nm. More preferably, the pores having a pore diameter are 0.2 mL / g or more, and more preferably 0.3 mL / g or more. By using such a carrier, gas diffusibility is improved and the heat resistance of the catalyst tends to be excellent, so that the catalytic activity can be maintained high.

  If the support containing the metal oxide shows a pore size distribution smaller than that in the above-mentioned range, it is difficult for the harmful gas to diffuse into the pores where iron oxide or noble metal exists. There is a tendency for harmful gas removal ability to decrease. In addition, when this carrier exhibits a pore size distribution larger than that in the above-mentioned range, the active oxide particles or noble metal particles that are active species present in the pores are bonded by sintering, Since the specific surface area decreases, the harmful gas removal ability of the catalyst tends to decrease.

In the catalyst of the present embodiment, the specific surface area of the support is not particularly limited, preferable to be 20 to 300 m ² / g, more preferably a 50 to 300 m ² / g. When the specific surface area of the support is smaller than 20 m ² / g, the active species particles present on the catalyst surface aggregate or bind to each other, so that the catalyst performance tends to be insufficient. Moreover, when the specific surface area of a support | carrier exceeds 300 m < ² > / g, it exists in the tendency for the heat resistance of a catalyst to fall.

  In addition, the particle diameter (particle diameter) of the primary particles of the carrier containing the metal oxide described above, the content ratio of each oxide, and the like can be analyzed and measured using a conventionally known method. The following can be mentioned. The particle diameter of each primary particle can be examined using XRD (X-ray diffraction method), TEM (transmission electron microscopy), SEM (scanning electron microscopy) or the like. In addition, when employ | adopting SEM, it is preferable after performing metal vapor deposition to oxide powder normally. The specific surface area, average primary particle size or pore distribution of the entire support can be measured using a BET method utilizing nitrogen adsorption, a CI (Cranston and Inkley) method, an air permeation method or a mercury intrusion method.

In the catalyst of this embodiment, iron oxide is supported on the carrier. The content ratio of this iron oxide is preferably 2 to 25% by mass with respect to the total amount of the catalyst as Fe ₂ O ₃ . If the content ratio of iron oxide is less than this numerical range, the active sites tend to be insufficient, and if it exceeds this numerical range, it becomes difficult to support (hold) iron oxide in a highly dispersed state. There is a tendency not to be fully utilized.

  Note that iron oxide has an oxidation / reduction action when removing harmful gases, so the initial oxidation number is not particularly limited.

  As the noble metal supported on the support or iron oxide in the catalyst of the present embodiment, catalyst deterioration due to grain growth or the like tends to hardly occur even under severe conditions, and harmful gases tend to be effectively removed. Further, it is preferable to use a metal of a platinum group element (Ru, Rh, Pd, Os, Ir, Pt), and it is more preferable to use one or more kinds of noble metals selected from the group consisting of Rh, Pt, and Pd.

  Among these, by using Pt as a supported metal, the catalyst of the present embodiment is particularly preferable because the synergistic effect of the carrier, iron oxide and Pt is more exhibited and the catalyst exhibits relatively high activity in a low temperature region. .

In general, Pt grain growth is promoted by aggregation of Pt particles accompanied by thermal contraction of the carrier, diffusion of PtO ₂ (or PtO) into the gas phase in a high-temperature oxidizing atmosphere, and migration to other particles. It is considered. However, when Pt supported on cerium oxide is exposed to a high-temperature oxidizing atmosphere, it strongly interacts with cerium oxide as PtO ₂ (or PtO), so that diffusion into the gas phase is suppressed. It is assumed that grain growth is also suppressed. Further, Pt supported on cerium oxide can be reduced to fine Pt metal when exposed to a stoichiometric or reducing atmosphere, and can exhibit activity. Therefore, it is considered more preferable that Pt is contained as a noble metal and cerium oxide is contained in the support.

  In the catalyst of the present embodiment, the supported amount (content) of the noble metal is 0.005 to 15 parts by mass with respect to 100 parts by mass of iron oxide, and 0.0004 to 1 mass with respect to the total mass of the catalyst. % Is sufficient. Thereby, the catalyst of this embodiment becomes a catalyst using iron oxide which is a comparatively cheap and abundant resource, suppressing the usage-amount of the noble metal which is a comparatively expensive and rare metal as much as possible. Therefore, the catalyst of the present embodiment is sufficiently excellent from the viewpoint of production cost and resource protection.

  In addition, even if only a very small amount of noble metal, which is considered to be an active species of the conventional catalyst as described above, is used, the catalyst of this embodiment can sufficiently remove harmful gases in a low temperature range. .

  In the catalyst of this embodiment, iron oxide is supported on a support containing a metal oxide, and a very small amount of noble metal is also supported on the support. As a result, highly dispersed (large specific surface area) iron oxide is present on the support, and noble metal fine particles are present on the iron oxide particles or at the grain boundaries between the support and iron oxide. It is thought that. Although this noble metal itself exhibits catalytic activity, it is considered that it also has a function as a co-catalyst that assists the oxidation / reduction action of iron oxide. Therefore, even in a low temperature range where the reaction rate of the catalytic reaction depends on the number of active sites, it is considered that the activation energy of iron oxide is reduced and the iron oxide can be made to act as an active site by the auxiliary action of the noble metal. As a result, it is estimated that the catalyst of the present embodiment exhibits sufficiently high catalytic activity even in a low temperature region and can sufficiently remove harmful gases. It is also speculated that iron oxide can reduce the poisoning of trace precious metal reactants.

  Further, in the catalyst of the present embodiment, iron oxide is supported on a metal oxide support, and therefore it is estimated that iron oxide sintering is unlikely to occur even in a high temperature region. As a result, it is considered that the catalyst of this embodiment can maintain its catalytic activity in a high state even after the high temperature durability test.

  From the same viewpoint, in the catalyst of the present embodiment, the supported amount (content) of the noble metal is more preferably 0.005 to 15 parts by mass with respect to 100 parts by mass of iron oxide, and 0.01 to 10 parts by mass. Part is more preferable, 0.05 to 5 parts by mass is particularly preferable, and 0.1 to 2 parts by mass is extremely preferable.

  Moreover, in the catalyst of this embodiment, it is preferable that the content ratio which combined the above-mentioned iron oxide and noble metal is 2-25 mass% with respect to the whole quantity of a catalyst. When the content ratio is less than 2% by mass, it tends to be difficult to effectively act as an active species for removing harmful gases. In addition, when the content ratio exceeds 25% by mass, it is difficult to carry highly dispersed iron oxide mainly, and the harmful gas removal ability tends not to be improved so much, while the amount of noble metal used increases. In addition, the manufacturing cost of the catalyst tends to increase and the contribution to resource saving tends to decrease. From such a viewpoint, in the catalyst of the present embodiment, the content ratio of iron oxide and noble metal is more preferably 2 to 10% by mass with respect to the total amount of the catalyst.

  It is preferable that the supporting site of the iron oxide on the support is as much as possible on the primary particle surface of the support. As a result, high dispersion retention and improved heat resistance are achieved, and iron oxide tends to act as an active species more effectively. Further, when the noble metal is supported on the surface of the iron oxide, the oxidation / reduction reaction of the iron oxide further proceeds in the vicinity thereof. Therefore, in addition to the reaction on the noble metal, the redox mechanism of the iron oxide. The reaction by this also proceeds, and the low temperature activity tends to be improved.

  The catalyst of this embodiment is not particularly limited as its application, but a low-temperature oxidation catalyst, NOx reduction catalyst, methane oxidation catalyst, hydrogen production / purification catalyst, or exhaust gas purification ternary that can remove harmful gases such as CO and HC in a low temperature range. When used as a catalyst, it is preferable because it can effectively exhibit a catalytic action and tends to further exhibit high heat resistance and durability. From the same viewpoint, it is particularly preferable when used as an exhaust gas purification three-way catalyst. The exhaust gas purification three-way catalyst needs to exhibit its function sufficiently under high temperature and harsh environment, but the catalyst of this embodiment has high temperature heat resistance and durability in addition to activity in a low temperature range. Even in such an environment, a decrease in catalyst activity over time is sufficiently suppressed, so that it can be said that the catalyst is sufficiently useful as an exhaust gas purification three-way catalyst.

  The catalyst of the present embodiment is not used as a support, but when Ce, Ti, Y, Zn is further added, the oxidation / reduction action can effectively remove harmful gases or the thermal stability of the catalyst. Tend to improve.

  The catalyst of the present embodiment is not particularly limited in its use shape, and a coating layer is formed from the carrier in the present embodiment on the surface of a substrate such as a honeycomb-shaped monolith substrate, pellet substrate or foam substrate, The coating layer may be configured to carry iron oxide and a noble metal such as Rh, Pt, Pd.

  Next, a method for producing a catalyst according to a preferred embodiment of the present invention will be described.

  The method for producing a catalyst of the present embodiment includes a carrier preparation step for preparing a carrier containing a metal oxide, an iron oxide loading step for loading iron oxide on the carrier to obtain a composite, and a composite on the composite. A noble metal supporting step for supporting the noble metal.

  In the carrier preparation step of the present embodiment, a carrier having a pore volume having a target pore diameter of 0.1 g / L or more is obtained by the following four steps. The four steps are the precursor preparation step, the washing step for washing the precursor precipitate obtained through the precursor preparation step, and the addition of a surfactant that stirs the washed precursor precipitate together with the surfactant in water. It is a firing step for firing the step and the precursor precipitate after stirring.

  Hereinafter, as a specific example of such a carrier preparation step, a carrier preparation step for obtaining a cerium oxide-zirconium oxide composite oxide carrier having Zr / (Zr + Ce) = 0.9 will be described.

  First, in the precursor preparation step, a predetermined amount of zirconium oxynitrate, cerium (III) nitrate and hydrogen peroxide are dissolved in water, and 1.2 times as much ammonia aqueous solution as a precipitant is added as a precipitant. To precipitate (co-precipitate) to form a precursor precipitate.

  Here, the metal salt is not limited to those described above, and any water-soluble compound of the target metal oxide may be used. Further, the precipitating agent is not limited to ammonia, and any precipitant may be used as long as it exhibits alkalinity. Hydrogen peroxide is effective when the cerium salt is trivalent. In this case, trivalent cerium forms tetravalent cerium by forming a complex with hydrogen peroxide. Due to such a change in the oxidation number, a carrier in which the obtained cerium oxide and zirconium oxide are more solid solution tends to be obtained.

  The obtained precursor precipitate is dehydrated in the washing step, then dispersed again in ion-exchanged water, and then dehydrated again to remove by-product salts and unreacted substances. By this operation, it becomes possible to make the effect in the surfactant addition step described later more remarkable.

  The washed precursor precipitate is stirred together with an anionic surfactant corresponding to 20% by mass of the cerium oxide-zirconium oxide composite oxide support to be obtained in the surfactant addition step. This makes it possible to control the pore distribution of the finally obtained cerium oxide-zirconium oxide composite oxide support. Moreover, the effect is recognized even if the surfactant is a cationic or nonionic surfactant other than the anionic surfactant.

  Subsequently, in the firing step, the precursor precipitate that has undergone the above-described steps is first denitrated. This denitration is performed, for example, in air at 150 ° C. for 10 hours, and further fired at 400 ° C. for 5 hours, and then fired at 800 ° C. for 5 hours as main firing. As a result, a carrier having appropriate pores and excellent thermal stability can be obtained.

  In this manner, a cerium oxide-zirconium oxide support having a pore volume of a target pore diameter of 0.1 g / L or more can be prepared. Furthermore, you may grind | pulverize to 75 micrometers or less powder before using this as a catalyst support | carrier.

  Next, in the iron oxide supporting step, iron oxide is supported on the carrier obtained through the precursor adjusting step to obtain a composite.

  When iron oxide is supported on a carrier, a conventionally known method such as an impregnation method can be used. For example, after a carrier powder is mixed with an aqueous solution of iron salt, it is evaporated to dryness and fired. To obtain a complex. The aqueous solution of iron salt is not particularly limited, but iron (III) nitrate nonahydrate, iron (III) citrate n hydrate, iron (II) chloride hydrate, iron (III) chloride hexahydrate , Iron (II) perchlorate hexahydrate, iron (III) perchlorate n hydrate, iron (III) ammonium oxalate trihydrate, iron (II) oxalate dihydrate, etc. An aqueous solution of each hydrate, an aqueous solution of iron (II) chloride anhydride, an aqueous solution of iron (III) chloride, or the like can be used. Among these, it is preferable to use an aqueous solution of iron (III) nitrate nonahydrate or iron (III) citrate n hydrate.

  Further, the firing temperature at this time is preferably 200 to 650 ° C. If the firing temperature is lower than this lower limit, the decomposition of the salt tends to be insufficient, and the firing tends to take a long time. If the firing time is higher than this upper limit, the iron oxide aggregates to form iron oxide. The catalytic activity tends to decrease.

  And the catalyst of this embodiment is obtained by passing through the noble metal carrying | support process which carry | supports a noble metal on the composite_body | complex obtained through the said iron oxide carrying | support process. By performing the noble metal supporting step after the iron oxide supporting step, the resulting catalyst is in a state in which the noble metal is supported on the metal oxide support, on the iron oxide, or in the vicinity of the grain boundary between the support and iron oxide. It is estimated that The noble metal supported on the metal oxide support itself becomes an active species effective for removing harmful gases. In addition, the noble metal supported on the iron oxide and the noble metal supported near the grain boundary between the support and the iron oxide are not only active species themselves, but also have a function of assisting the catalytic action of the iron oxide. I think that.

  Furthermore, this method for producing a catalyst sufficiently suppresses the loading of a noble metal on a site where harmful gases are difficult to diffuse, such as a site covered with a catalyst carrier and iron oxide. Therefore, since the obtained catalyst can more fully exhibit the catalytic activity of the noble metal and the auxiliary function with respect to iron oxide, it is possible to more efficiently and reliably remove harmful gases.

  In the noble metal loading process, a noble metal loading process is performed by a conventionally known method such as impregnation or adsorption. Examples of the noble metal salt aqueous solution used in this case include dinitrodiammine platinum nitric acid solution, tetraammine platinum nitric acid solution, and platinum chloride aqueous solution when platinum (Pt) is employed as the noble metal.

  When rhodium (Rh) is used as the noble metal, examples of the rhodium salt aqueous solution include a rhodium nitrate aqueous solution and a rhodium chloride aqueous solution. Furthermore, when palladium (Pd) is used as the noble metal, examples of the palladium salt aqueous solution include a palladium nitrate aqueous solution and a palladium chloride aqueous solution.

  The firing temperature after impregnating the noble metal is preferably 200 to 650 ° C. from the viewpoint of suppressing decomposition of the salt or aggregation of the noble metal.

  The catalyst thus obtained can be used in the form of powder or pellets in practical use. Furthermore, for example, a cordierite or metal monolith substrate is coated with the above-mentioned carrier carrying iron oxide and noble metal, or the monolith substrate is loaded with this carrier beforehand, and further iron oxide and noble metal are carried. Thus, it can be used as a three-way catalyst for exhaust gas purification. Moreover, even if this catalyst is combined with another catalyst in the form as described above, low-temperature activity can be expressed. The catalyst formed by being supported on the substrate is excellent in heat resistance and also exhibits an effect of reducing pressure loss.

  As mentioned above, although preferred embodiment of the catalyst of this invention and the manufacturing method of a catalyst was described, this invention is not limited to the said embodiment.

  For example, in another embodiment of the method for producing a catalyst of the present invention, the cleaning step may not be provided, or the cleaning step may be provided between the surfactant addition step and the calcination step.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to these Examples.

Example 1
First, in a 3 L beaker, 38.3 g of cerium nitrate aqueous solution (28% by mass as CeO ₂ ), 384.9 g of zirconium oxynitrate aqueous solution (18% by mass as ZrO ₂ ), and 11.3 g of 30% hydrogen peroxide water And added to 113.9 g of 25% aqueous ammonia solution diluted with 1800 g of ion-exchanged water with propeller stirring (600 rpm), and further stirred with a homogenizer for 10 minutes or more to produce metal oxide. A precursor precipitate of the product was obtained (precursor preparation step).

  Next, the resulting precipitate was centrifuged to discard the supernatant, and 4000 g of ion-exchanged water was added to this and stirred, and then again centrifuged to wash the precipitate (washing step).

  Finally, the precipitate after discarding the supernatant was again transferred to a 3 L beaker, and 2000 g of ion-exchanged water was added, followed by stirring using a propeller stirrer and a homogenizer. Thereto was added 16 g of an anionic surfactant (Armoflow, product name, manufactured by Lion Corporation), and the mixture was further stirred for 10 minutes (surfactant addition step). The resulting dispersion was centrifuged.

The cerium oxide-zirconium oxide composite oxide is obtained by baking the surfactant-treated precipitate at 400 ° C. for 5 hours in the air using a degreasing furnace and further baking at 800 ° C. for 5 hours in the air (firing step). (CeO ₂ : ZrO ₂ = 10: 90 (molar ratio)) A carrier was obtained.

  When the pore distribution of the obtained carrier was measured using a mercury porosimeter, it was confirmed that the number of pores having a pore diameter of 3.5 to 150 nm was 0.22 mL / g.

Next, iron nitrate nonahydrate _{(Fe (NO 3) 3 ·} 9H 2 O) was dissolved 10.2g of ion-exchanged water, its said cerium oxide in an aqueous solution - mixing the zirconium oxide composite oxide powder 25g After evaporating and drying the moisture, the composite was obtained by supporting iron oxide on the solid solution by firing at 600 ° C. for 5 hours in a box-type electric furnace (iron oxide supporting step). The amount of iron oxide supported in this composite was 0.1 mol of Fe with respect to 100 g of the carrier.

Then, 25 g of the obtained complex was mixed with ion-exchanged water, and further 0.0022 g of a 4.56 mass% dinitrodiammine platinum nitrate (Pt (NO ₂ ) ₂ (NH ₃ ) ₃ (NO ₃ ) ₃ ) aqueous solution was mixed. did. This was evaporated to dryness and calcined at 600 ° C. for 5 hours in a box-type electric furnace to obtain the catalyst of Example 1 (noble metal supporting step). In the catalyst of Example 1, the supported amount of Pt was 0.0004% by mass with respect to the total amount of the catalyst and 0.0054 parts by mass with respect to 100 parts by mass of Fe ₂ O ₃ . Further, the total amount of Fe ₂ O ₃ and Pt was 7.39% by mass with respect to the total amount of the catalyst.

(Example 2)
In the noble metal supporting step, instead of mixing 0.0022 g of 4.56% by mass dinitrodiammine platinum nitrate aqueous solution, instead of mixing 0.508 g of 4.56% by mass dinitrodiammine platinum nitrate aqueous solution, the same as in Example 1, The catalyst of Example 2 was obtained. In the catalyst of Example 2, the supported amount of Pt was 0.0925% by mass with respect to the total amount of the catalyst, and 1.25 parts by mass with respect to 100 parts by mass of Fe ₂ O ₃ . Further, the total amount of Fe ₂ O ₃ and Pt was 7.48% by mass with respect to the total amount of the catalyst.

(Example 3)
In the noble metal supporting step, instead of mixing 0.0022 g of 4.56 mass% dinitrodiammine platinum nitrate aqueous solution, the same procedure as in Example 1 was conducted except that 4.386 g of 4.56 mass% dinitrodiammine platinum nitrate aqueous solution was mixed. The catalyst of Example 3 was obtained. In the catalyst of Example 3, the supported amount of Pt was 0.79% by mass with respect to the total amount of the catalyst, and 10.81 parts by mass with respect to 100 parts by mass of Fe ₂ O ₃ . Further, the total amount of Fe ₂ O ₃ and Pt was 8.13% by mass with respect to the total amount of the catalyst.

Example 4
In the precious metal loading step, instead of mixing 0.0022 g of 4.56 mass% dinitrodiammine platinum nitrate aqueous solution, 0.343 g of 4.56 mass% dinitrodiammine platinum nitrate aqueous solution and 0.152 g of 2.75 mass% rhodium nitrate aqueous solution were added. A catalyst of Example 4 was obtained in the same manner as Example 1 except for mixing. In the catalyst of Example 4, the supported amount of Pt was 0.0625% by mass with respect to the total amount of the catalyst, 0.85 parts by mass with respect to 100 parts by mass of Fe ₂ O ₃ , and the supported amount of Rh was the total amount of the catalyst. This corresponds to 0.0167% by mass and 0.23 parts by mass with respect to 100 parts by mass of Fe ₂ O ₃ . Further, the total amount of Fe ₂ O ₃ , Pt and Rh was 7.47% by mass with respect to the total amount of the catalyst.

(Example 5)
In the precious metal loading step, instead of mixing 0.0022 g of 4.56 mass% dinitrodiammine platinum nitrate aqueous solution, 0.171 g of 4.56 mass% dinitrodiammine platinum nitrate aqueous solution and 0.075 g of 2.75 mass% rhodium nitrate aqueous solution were added. A catalyst of Example 5 was obtained in the same manner as Example 1 except for mixing. In the catalyst of Example 5, the supported amount of Pt was 0.0312% by mass with respect to the total amount of the catalyst, 0.42 parts by mass with respect to 100 parts by mass of Fe ₂ O ₃ , and the supported amount of Rh was the total amount of the catalyst. It corresponded to 0.183 parts by mass with respect to 0.0083% by mass and 100 parts by mass of Fe ₂ O ₃ . Further, the total amount of Fe ₂ O ₃ , Pt and Rh was 7.43 mass% with respect to the total amount of the catalyst.

(Example 6)
In the precious metal loading step, instead of mixing 0.0022 g of 4.56 mass% dinitrodiammine platinum nitrate aqueous solution, 0.069 g of 4.56 mass% dinitrodiammine platinum nitrate aqueous solution and 0.030 g of 2.75 mass% rhodium nitrate aqueous solution were added. A catalyst of Example 6 was obtained in the same manner as Example 1 except for mixing. In the catalyst of Example 6, the supported amount of Pt is 0.0125% by mass with respect to the total amount of the catalyst, 0.17 parts by mass with respect to 100 parts by mass of Fe ₂ O ₃ , and the supported amount of Rh is the total amount of the catalyst. 0.0033 _mass% against _corresponded to 0.21 parts by weight with respect to Fe ₂ O 3 100 parts by weight. _Further, the total amount of Fe ₂ O 3, Pt and Rh became 7.41 wt% relative to the total catalyst.

(Comparative Example 1)
A catalyst of Comparative Example 1 was obtained by supporting Pt in the same manner as in Example 1 with respect to 25 g of the cerium oxide-zirconium oxide composite oxide obtained in the same manner as in Example 1. In the catalyst of Comparative Example 1, the amount of Pt supported was 0.0004% by mass with respect to the total amount of the catalyst.

(Comparative Example 2)
A catalyst of Comparative Example 2 was obtained by supporting Pt in the same manner as in Example 2 with respect to 25 g of the cerium oxide-zirconium oxide composite oxide obtained in the same manner as in Example 1. In the catalyst of Comparative Example 2, the supported amount of Pt was 0.0999 mass% with respect to the total amount of the catalyst.

(Comparative Example 3)
A catalyst of Comparative Example 3 was obtained by supporting Pt in the same manner as in Example 3 with respect to 25 g of the cerium oxide-zirconium oxide composite oxide obtained in the same manner as in Example 1. In the catalyst of Comparative Example 3, the amount of Pt supported was 0.79% by mass with respect to the total amount of the catalyst.

(Comparative Example 4)
300.1 g of aluminum nitrate nonahydrate was dissolved in 200 g of ion-exchanged water, and this was dissolved in 221.28 g of a cerium (III) nitrate aqueous solution (28 mass% as CeO ₂ ) and a zirconium oxynitrate aqueous solution (18 mass% as ZrO ₂ ). 301.2 g and 30% hydrogen peroxide water 45.33 g were mixed to obtain a raw material mixed aqueous solution.

  Next, 356.4 g of 25% ammonia water is mixed with 500 g of ion-exchanged water, and the above raw material mixed aqueous solution is added while stirring with a propeller (600 rpm). Further, the mixture is stirred for 10 minutes or more together with a homogenizer to form a precipitate. did.

This was calcined in the atmosphere at 400 ° C. for 5 hours using a degreasing furnace, and further calcined in the air at 700 ° C. for 5 hours to obtain an aluminum oxide-cerium oxide-zirconia oxide composite oxide (Al ₂ O ₃ : CeO ₂ : ZrO ₂ = 1: 0.9: 1.1) A support was obtained.

  Next, 21.25 g of the aluminum oxide-cerium oxide-zirconia oxide composite oxide powder thus obtained and 20 g of aluminum oxide powder were mixed in ion-exchanged water, and further 4.56 mass% dinitrodiammine platinum nitrate aqueous solution 8 .22 g and 2.64% by weight of rhodium nitrate aqueous solution 3.64 g were mixed. This was evaporated to dryness and calcined at 500 ° C. for 2 hours in a box-type electric furnace to obtain a catalyst of Comparative Example 4. In the catalyst of Comparative Example 4, the supported amount of Pt was 0.90% by mass with respect to the total amount of the catalyst, and the supported amount of Rh was 0.24% by mass with respect to the total amount of the catalyst.

(Comparative Example 5)
A catalyst of Comparative Example 5 was obtained in the same manner as in Example 1 except that the precious metal supporting step was not passed and Pt was not supported. In the catalyst of Comparative Example 5, the amount of Fe ₂ O ₃ supported was 7.394% by mass with respect to the total amount of the catalyst.

<Catalyst activity evaluation>
In the catalyst layer in which the catalysts of Examples 1 to 6 and Comparative Examples 1 to 5 and 5 are arranged, the stoichiometric model gas (excess air ratio λ = 1) of the theoretical air-fuel ratio (stoichiometric) having the composition shown in Table 1 is used. A gas mixture of CO and H ₂ having a volume ratio of CO / H ₂ = 3/1 and O ₂ gas are appropriately mixed, and a variation model gas whose composition changes in the range of excess air ratio λ = 1 ± 0.02. It was distributed at SV = 90,000 to 100,000 / hour. Next, while flowing the variable model gas, the catalyst layer inlet temperature of the variable model gas was set to 550 ° C., and pretreatment was performed for 10 minutes to obtain an initial catalyst. In addition, in the process so far, about the catalyst of the comparative example 4, it was made to be the same as that of another catalyst except having distribute | circulated the said fluctuation | variation model gas by SV = 480000 / hour.

Next, for the catalysts of Examples 1 to 6 and Comparative Examples 1 to 5, after the catalyst layer inlet temperature of the variable model gas was lowered to 100 ° C., the catalyst layer inlet temperature was changed from 100 ° C. to 550 ° C. at 12 ° C./hour. The temperature was increased at a rate of temperature increase. The changes in the conversion rates of NOx (NO), CO and HC (C ₃ H ₆ ) for the catalysts of Example 2 and Comparative Examples 2 and 5 are shown in FIGS. Moreover, about the catalyst of Examples 1-6 and Comparative Examples 1-5, the catalyst layer inlet_port | entrance temperature (T50; degreeC) of NOx, CO, and HC in each fluctuation | variation model gas when each component of NOx, CO, and HC converted 50% )It was confirmed. The catalyst layer inlet temperature results are shown in Table 2. In Table 2, “greater than 550” means that T50 could not be obtained even at 550 ° C., which is the upper limit temperature at the time of temperature increase.

  From these experimental results, the catalysts of Examples 1 to 3 of Comparative Examples 1 to 3 which do not contain iron oxide, although only a small amount of the precious metal known as the main active site in the past was supported. It was confirmed that the catalyst was activated at a low temperature as compared with the catalyst.

  Further, assuming that the amount of noble metal used per passing amount of the reactant in the catalyst of Comparative Example 4 is 1, the amount of noble metal used in the catalyst of Example is 0.1 in the catalyst of Example 4, and in the catalyst of Example 5. Is equivalent to 0.25 and 0.5 for the catalyst of Example 6. Among these, the catalyst performance of Example 5 was almost the same as that of Comparative Example 4, and it was confirmed that it was effective for reducing the amount of noble metal.

  Furthermore, it was confirmed that the catalyst of Comparative Example 5 in which only iron oxide was supported showed clearly lower activity compared to the catalyst of the present invention. In addition, although not explicitly shown here, a catalyst that supports a larger amount of iron oxide than the catalyst of Comparative Example 5 or a smaller amount of iron oxide than the catalyst of Comparative Example 5 and does not support a noble metal, It was confirmed that the catalyst activity was comparable or lower than that of Example 7.

Further distribution, the catalyst for the examples 4-6, the model gas having a composition shown in Table 3, the O ₂ 5% excess gas and H ₂ 5% excess gas for 5 minutes, alternating Then, the catalyst layer inlet temperature of these gases was set to 600 ° C., 700 ° C., or 800 ° C., and the durability treatment was performed for 5 hours.

The catalyst after the endurance treatment is similar to the evaluation of the activity of the initial catalyst, and after the catalyst layer inlet temperature of the variable model gas is lowered to 100 ° C., the catalyst layer inlet temperature is changed from 100 ° C. to 500 ° C. to 12 ° C. / The temperature was raised at a rate of temperature increase. Then, the catalyst layer inlet temperature (T50; ° C) of the variable model gas when 50% conversion of each component of NOx, CO and HC in the variable model gas was confirmed. The catalyst layer inlet temperature results are shown in Table 4.

4 is a graph showing the results of an activity evaluation test for the catalyst of Example 2. 6 is a graph showing the results of an activity evaluation test for the catalyst of Comparative Example 2. 10 is a graph showing the results of an activity evaluation test for the catalyst of Comparative Example 5.

Claims (5)

A catalyst comprising iron oxide and a noble metal supported on a support containing a metal oxide,
0.0004 to 1% by mass of the noble metal with respect to the total mass of the catalyst, and 0.005 to 15 parts by mass of the noble metal with respect to 100 parts by mass of the iron oxide,
As the metal oxide, containing cerium oxide and zirconium oxide,
The cerium oxide and the zirconium oxide are complex oxides,
Containing 70 to 99 mol% of the zirconium oxide with respect to the total molar amount of the metal oxide ,
A catalyst characterized by being used for exhaust gas purification .   The catalyst according to claim 1, wherein the total amount of the iron oxide and the noble metal is 2 to 10% by mass with respect to the total mass of the catalyst.   The catalyst according to claim 1 or 2, wherein the zirconium oxide is contained in an amount of 85 to 99 mol% with respect to the total molar amount of the metal oxide.   The catalyst according to any one of claims 1 to 3, wherein the support has 0.2 mL / g or more of pores having a pore diameter of 3.5 to 150 nm.   The catalyst according to any one of claims 1 to 4, wherein the noble metal is one or more metals selected from the group consisting of Pt, Pd and Rh.

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