JP6315306B2 - Method for producing exhaust gas purification catalyst - Google Patents

Description

  The present invention relates to a method for producing an exhaust gas purifying catalyst carrying platinum, and an exhaust gas purifying catalyst produced by the method.

  Conventionally, various catalysts have been used to purify components such as carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) contained in gas discharged from internal combustion engines such as automobiles. I came. As such an exhaust gas purifying catalyst, for example, a catalyst in which a noble metal such as platinum (Pt) or rhodium (Rh) is supported on a carrier made of γ-alumina or the like is known. In addition, as a method for producing such an exhaust gas purifying catalyst, a method in which a solution containing a salt of a noble metal and a carrier are brought into contact with each other and calcined to carry the noble metal on the carrier is known (for example, JP-A-5-285385 (Patent Document 1)). However, the activity of the exhaust gas purification catalyst produced by such a method is not always sufficient, and various methods for producing an exhaust gas purification catalyst have been proposed in order to obtain a highly active catalyst.

  For example, in Japanese Patent Application Laid-Open No. 2006-55807 (Patent Document 2), a multinuclear complex composed of a plurality of organic polydentate ligands and a plurality of noble metal atoms is deposited on an oxide support, and then an organic polydentate ligand is formed. A method for producing a noble metal cluster-supported catalyst by removing the catalyst is disclosed. JP 2009-226295 A (Patent Document 3) discloses an organic group containing a functional group capable of bonding to an oxygen atom on the surface of a metal oxide and an atom capable of coordinating to a noble metal atom. A method for producing a catalyst material in which a noble metal atom is supported on a metal oxide support through the organic group by forming a complex of the organic group and a noble metal atom after being introduced onto the surface of the catalyst is disclosed.

  Further, JP 2009-255064 A (Patent Document 4), JP 2011-83765 A (Patent Document 5) and JP 2010-5529 A (Patent Document 6) describe a rhodium carboxylate binuclear complex or A method for producing an exhaust gas purifying catalyst is disclosed in which a ruthenium carboxylate binuclear complex is supported on a support and then calcined to support a rhodium or ruthenium diatomic cluster on the support.

JP-A-5-285385 JP 2006-55807 A JP 2009-226295 A JP 2009-255064 A JP 2011-83765 A JP 2010-5529 A

  However, in the noble metal cluster-supported catalyst manufactured by the method described in Patent Document 2, the noble metal atoms aggregate and the noble metal cluster becomes coarse, so that the catalytic activity is not always sufficient. In addition, when the catalyst material produced by the method described in Patent Document 3 is subjected to heat treatment or the like to remove organic groups in the catalyst material, the noble metal atoms are aggregated and the noble metal particles are coarsened. It was not always enough. Furthermore, since it was difficult to prepare a platinum carboxylate binuclear complex according to the method described in Example 7 or Example 8 of Patent Document 5, platinum diatomic clusters were supported on the carrier using the carboxylate dinuclear complex. It was difficult to do.

  The present invention has been made in view of the above-described problems of the prior art, and it is possible to produce an exhaust gas purifying catalyst that is supported in an atomic state and is hardly coarsened even when subjected to heat treatment and has high catalytic activity. An object is to provide a possible method and an exhaust gas purifying catalyst produced by the method.

  As a result of intensive studies to achieve the above-mentioned object, the present inventors have found that when platinum is supported on a support, it has a Pt-Pt bond and a coordination unsaturated site capable of binding to the support. By using a nuclear complex, it is possible to support a platinum diatomic cluster on a carrier, and such a platinum diatomic cluster is stable even after heat treatment, and coarse platinum particles are not easily formed. As a result, the present invention has been completed.

That is, the method for producing the exhaust gas purifying catalyst of the present invention comprises:
A dissolution step of dissolving a platinum binuclear complex having a Pt-Pt bond and a coordination unsaturated site capable of binding to a carrier in toluene ;
A supporting step in which the solution obtained in the dissolving step is brought into contact with the carrier to adsorb and carry the platinum binuclear complex on the carrier;
Heat-treating the support on which the platinum dinuclear complex is adsorbed and supported to remove the ligand in the platinum dinuclear complex, and obtaining a catalyst in which platinum diatomic clusters are supported on the support;
It is the method characterized by including.

  In the method for producing an exhaust gas purifying catalyst of the present invention, the platinum binuclear complex preferably has a ligand containing a nitrogen atom coordinated to a platinum atom, and the ligand is an amide group. It is more preferable that it has.

  The reason why coarse platinum particles are hardly formed in the exhaust gas purifying catalyst obtained by the production method of the present invention is not necessarily clear, but the present inventors infer as follows. That is, in the conventional method for producing a catalyst in which platinum is supported on a carrier using a platinum salt such as platinum chloride, the platinum salt forms an electrostatic weak bond with the carrier. When the support is calcined (reduced) after the support, the platinum salt is easily separated from the carrier, and the steric hindrance between the platinum salts is small, so the distance between the supported platinum atoms is narrowed, and the platinum atoms are sufficiently aggregated. Could not be suppressed. For this reason, in the conventional catalyst manufacturing method, it is presumed that the platinum particles grow and grow coarse due to aggregation, and sufficient catalytic activity cannot be obtained.

  When platinum is supported on a carrier using a platinum mononuclear complex, the platinum mononuclear complex is supported on the carrier and then subjected to heat treatment to remove the ligand from the complex. Is supported in a single atom state. However, platinum supported in a single atom state is likely to be scattered when heated, and the scattered platinum atoms are likely to aggregate with other platinum atoms. For this reason, it is surmised that platinum supported in a single atom state grows by agglomeration and becomes coarse due to the heat treatment in removing the ligand. Further, even when a catalyst in which platinum is supported in a single atom state is exposed to a high temperature for a long time, it is presumed that platinum grows and coarsens due to aggregation.

  In contrast, in the method for producing an exhaust gas purifying catalyst of the present invention, a platinum binuclear complex having a Pt—Pt bond and a coordination unsaturated site capable of binding to the support is supported on the support. By removing the ligand from the complex by performing heat treatment or the like, platinum is supported while maintaining the form (state) of the nucleus of the binuclear complex, that is, as a two-atom cluster. Since such a platinum diatomic cluster has a mass twice that of a platinum single atom, scattering due to heating is unlikely to occur, and aggregation during heat treatment for removing the ligand is sufficiently suppressed. It is surmised that the exhaust gas purifying catalyst exhibits high catalytic activity. In addition, even when the exhaust gas purifying catalyst of the present invention is exposed to a high temperature for a long time, the platinum diatomic clusters are unlikely to be scattered, so that it is presumed that grain growth due to aggregation is suppressed and coarse platinum particles are not easily formed. The

  According to the present invention, platinum is supported on a carrier as a two-atom cluster, and even when exposed to a high temperature for a long time, a catalyst for exhaust gas purification having high catalytic activity is produced, in which platinum particles are hardly coarsened. It becomes possible to do.

It is a graph which shows the mass spectrum (MALDI-MS) in the various organic solvent of the platinum binuclear complex used in the Example. It is a graph which shows the mass spectrum (MALDI-MS) in toluene of the platinum binuclear complex used in the Example. FIG. 3 is an enlarged view in the vicinity of a mass-to-charge ratio (m / z) = 790 of the mass spectrum shown in FIG. 2. FIG. 3 is an enlarged view in the vicinity of a mass-to-charge ratio (m / z) = 825 in the mass spectrum shown in FIG. 2. Pt ₂ (NHCOC ₄ H ₉₎ is a graph showing a mass spectrum of the ₄ theoretical (calculated). _{Pt 2 Cl (NHCOC 4 H 9} ) is a graph showing a mass spectrum of the ₄ theoretical (calculated). 2 is a scanning transmission electron microscope (STEM) photograph of the dried catalyst powder obtained in Example 1. FIG. 2 is a scanning transmission electron microscope (STEM) photograph of the fired catalyst powder obtained in Example 1. FIG. It is a scanning transmission electron microscope (STEM) photograph which shows the state after the durability test of the catalyst obtained in Example 1. It is a graph which shows the 50% conversion temperature about CO of the catalyst obtained in Example 1 and Comparative Example 1. It is a graph showing a 50% conversion temperature for the C ₃ H ₆ of the catalyst obtained in Example 1 and Comparative Example 1. 2 is a scanning transmission electron microscope (STEM) photograph of the dried catalyst powder obtained in Comparative Example 1. 2 is a scanning transmission electron microscope (STEM) photograph showing a state after a durability test of the catalyst obtained in Comparative Example 1. FIG. 4 is a scanning transmission electron microscope (STEM) photograph of the dried catalyst powder obtained in Comparative Example 3 . 4 is a scanning transmission electron microscope (STEM) photograph of the fired catalyst powder obtained in Comparative Example 3 . 7 is a scanning transmission electron microscope (STEM) photograph of the dried catalyst powder obtained in Comparative Example 6 . 6 is a scanning transmission electron microscope (STEM) photograph of the catalyst powder after firing obtained in Comparative Example 6 .

  Hereinafter, the present invention will be described in detail with reference to preferred embodiments thereof.

First, the manufacturing method of the exhaust gas purifying catalyst of the present invention will be described. The method for producing the exhaust gas purifying catalyst of the present invention comprises:
A dissolution step of dissolving a platinum binuclear complex having a Pt-Pt bond and a coordination unsaturated site capable of binding to a support in an organic solvent;
A supporting step in which the solution obtained in the dissolving step is brought into contact with the carrier to adsorb and carry the platinum binuclear complex on the carrier;
Heat-treating the support on which the platinum dinuclear complex is adsorbed and supported to remove the ligand in the platinum dinuclear complex, and obtaining a catalyst in which platinum diatomic clusters are supported on the support;
It is the method characterized by including.

  The platinum binuclear complex used in the present invention has a Pt—Pt bond and a coordination unsaturated site capable of binding to a carrier. By using a complex having a Pt—Pt bond and a coordination unsaturated site capable of binding to the support, the platinum diatomic cluster according to the present invention can be reliably supported by the support. Aggregation of platinum diatomic clusters during the heat treatment for removing the carbon is sufficiently suppressed, and even when exposed to a high temperature for a long time, grain growth due to aggregation of platinum diatomic clusters is suppressed.

  In the method for producing an exhaust gas purifying catalyst of the present invention, the platinum binuclear complex preferably has a ligand containing a nitrogen atom coordinated to a platinum atom, and the ligand is an amide group. It is more preferable that it has. By using such a ligand containing a nitrogen atom (hereinafter referred to as “nitrogen-containing ligand”), the platinum binuclear complex is not decomposed into ions in an organic solvent, It tends to exist stably while maintaining a coordinated state with the platinum atom. As a result, there is a tendency that the platinum binuclear complex can be reliably adsorbed and supported by the carrier as it is, and then platinum is reliably supported by the carrier as a two-atom cluster by removing the ligand. Tend to be possible.

  Examples of such platinum binuclear complexes include the following formulas (1-1) to (1-3):

(In the formulas (1-1) to (1-3), L represents a nitrogen-containing ligand, and X represents a counter anion.)
The thing represented by is mentioned. In the organic solvent described later, even in the form of the complex represented by the formula (1-1), the counter anion (X) is eliminated from the formula (1-2) or (1-3). The form represented may be sufficient and the state which mixed them may be sufficient. In the platinum binuclear complex according to the present invention, the coordination is such that the site where X is eliminated from the complex represented by the formula (1-1) can be bonded to an atom (for example, oxygen atom) on the support surface described later. It becomes an unsaturated site. As X in the formulas (1-1) and (1-2), the platinum binuclear complex represented by the formula (1-1) is particularly electrically neutral as a whole molecule. There is no limitation, and examples thereof include BF ₄ ⁻ , PO ₄ ⁻ , OH ⁻ , NO ₃ ⁻ , CN ⁻ , ClO ₄ ⁻ , SCN ⁻ , PF ₆ ⁻ , F ⁻ , Cl ⁻ , Br ⁻ and I ^−. Of these, halogen ions are preferable, and Cl ⁻ is more preferable.

  The ligand in the platinum binuclear complex used in the present invention is not particularly limited as long as it can coordinate to a platinum atom forming a Pt-Pt bond. For example, amide group, cyano group, pyridyl And a ligand having a functional group containing a nitrogen atom such as a group or a pyrimidyl group. Among them, from the viewpoint of chelate coordination in a bidentate with respect to the Pt—Pt bond and further stabilizing the platinum dinuclear, the coordination having an amide group {R—CONH— (R represents a hydrocarbon group)}. A ligand is preferred.

  Moreover, as carbon number contained in 1 molecule of ligands in the platinum binuclear complex used for this invention, 1-10 (more preferably 1-6) are preferable. The hydrocarbon group contained in such a ligand may be saturated or unsaturated. Moreover, it may be linear, branched or cyclic, and the cyclic hydrocarbon group may have a substituent. Such a platinum binuclear complex is adsorbed and supported on the support in a highly dispersed state due to its steric hindrance, and then the platinum diatomic clusters are highly dispersed on the support by removing the ligand. A supported exhaust gas-purifying catalyst can be obtained. In particular, since platinum diatomic clusters are unlikely to be scattered by heating, even if heat treatment is performed when removing the ligand, platinum diatomic clusters are unlikely to scatter and aggregation of platinum particles hardly occurs. Furthermore, even when such an exhaust gas purifying catalyst is exposed to a high temperature for a long time, aggregation of platinum particles (platinum two-atom clusters) hardly occurs, and high catalytic activity can be maintained.

  Among such platinum binuclear complexes, the following formulas (2-1) to (2-3):

(In the formulas (2-1) to (2-3), R represents a hydrocarbon group, and X represents a counter anion.)
Is particularly preferred. In the organic solvent described later, even if the complex is represented by the formula (2-1), the counter anion (X) is eliminated from the formula (2-2) or (2-3). The form represented may be sufficient and the state which mixed them may be sufficient. Since this platinum binuclear complex has an amide group as a ligand, it exists stably in an organic solvent and tends to be able to reliably carry platinum diatomic clusters by a carrier. Further, in the amide group, oxygen atoms as well as nitrogen atoms are coordinated to platinum, so that they tend to be adsorbed and supported in a highly dispersed state on the carrier. As a result, by removing the ligand in the platinum binuclear complex, the platinum diatomic clusters tend to be supported in a highly dispersed state by the carrier. In particular, since platinum diatomic clusters are unlikely to be scattered by heating, even if a heat treatment is performed when removing the ligand, the generated platinum diatomic clusters are unlikely to scatter and platinum particles tend not to aggregate. .

  As the hydrocarbon group represented by R in the formulas (2-1) to (2-3), a hydrocarbon group having 1 to 10 carbon atoms is preferable, and a methyl group, an ethyl group, a propyl group, or a butyl group. , A hydrocarbon group having 1 to 6 carbon atoms such as an isobutyl group and a t-butyl group is more preferable. Among these hydrocarbon groups, platinum binuclear complexes are easily formed, and the obtained platinum binuclear complexes are stably present in organic solvents, and the effects of steric hindrance are more reliably exhibited. In view of the tendency to occur, an isobutyl group and a t-butyl group are particularly preferable.

  The organic solvent used in the present invention is not particularly limited as long as it can dissolve the platinum binuclear complex. Toluene, acetonitrile, acetone, ethyl acetate, methanol, THF (tetrahydrofuran), xylene, DMF ( Dimethylformamide), DMSO (dimethyl sulfoxide), chloroform, diethyl ether, dichlorobenzene and the like. Note that “the platinum binuclear complex can be dissolved” means that the platinum binuclear complex can be dissolved even in a small amount in an organic solvent, and the platinum content at room temperature is 0 in terms of the mass of platinum. It is preferable that a complex-containing solution of 0.0001% by mass or more can be obtained. Moreover, such an organic solvent may be used individually by 1 type, or may use 2 or more types together.

  The organic solvent used in the present invention is preferably one having a relative dielectric constant of 10 or less from the viewpoint that the platinum binuclear complex can be adsorbed and supported on the carrier more efficiently. , Toluene (2.4), ethyl acetate (6.4), THF (7.5), xylene (2.3), chloroform (4.8), diethyl ether (4.3), dichlorobenzene (9. 93) and the like (the relative permittivity is in parentheses). Furthermore, among these organic solvents, toluene is particularly preferable from the viewpoint that the amount of platinum supported tends to be increased particularly (or the amount of platinum binuclear complex used is particularly reduced).

  In the present invention, first, in the dissolution step according to the present invention, the platinum binuclear complex is dissolved in an organic solvent to prepare a complex-containing solution. Although there is no restriction | limiting in particular as content rate of platinum in this complex containing solution, 0.01-1.0 mass% is preferable in conversion of the mass of platinum. If the platinum content in the complex-containing solution is less than the lower limit, it tends to be unable to efficiently carry the platinum binuclear complex on the support. On the other hand, if the upper limit is exceeded, both the dispersibility of the complex and the support will decrease. The platinum binuclear complex tends not to be uniformly adsorbed on the support. In addition, there is no restriction | limiting in particular as a preparation method of such a complex containing solution, For example, the well-known method which can melt | dissolve the said platinum binuclear complex in an organic solvent is employable suitably.

  In the present invention, next, in the supporting step according to the present invention, the complex-containing solution and the carrier are brought into contact with each other, and the platinum binuclear complex is adsorbed and supported on the carrier. The method for contacting the complex-containing solution and the carrier is not particularly limited, and a known method capable of adsorbing and supporting a platinum binuclear complex on the carrier can be appropriately employed. For example, the carrier is contained in the complex-containing solution. By soaking, the platinum binuclear complex can be adsorbed and supported on the carrier.

  Moreover, when immersing a support | carrier in a complex containing solution, it is preferable to stir for 0.5 to 100 hours on the conditions of a pressure of 1-10 atm, and the temperature of 0-150 degreeC (preferably room temperature). If the pressure or temperature when immersing the carrier in the complex-containing solution is less than the lower limit, the adsorption / supporting efficiency of the platinum binuclear complex tends to decrease, and if the upper limit is exceeded, excessive stirring is continued. As a result, the manufacturing cost tends to increase.

The carrier according to the present invention is not particularly limited, and a known carrier that can be used as a catalyst for exhaust gas purification can be appropriately used. For example, a carrier made of a carbon material or a metal oxide can be appropriately used. it can. Examples of the carbon material used for such a carrier include activated carbon and graphene. Examples of the metal oxide include activated alumina, alumina-ceria-zirconia, ceria-zirconia, zirconia, lanthanum stabilized activated alumina, and the like. Lanthanum (La), cerium (Ce), praseodymium (Pr), Neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), It consists of oxides of ytterbium (Yb), lutetium (Lu), yttrium (Y), zirconium (Zr), aluminum (Al), magnesium (Mg) and vanadium (V), solid solutions thereof, and composite oxides thereof. At least one selected from the group is preferred. Among such metal oxides, CeO ₂ , ZrO ₂ , Y ₂ O ₃ , TiO ₂ , Al ₂ O ₃ , solid solutions thereof, and composite oxides thereof are obtained from the viewpoint that higher catalytic activity can be obtained. What contains at least 1 type selected from the group which consists of a thing is more preferable.

The shape of such a carrier is not particularly limited, but is preferably a powder from the viewpoint of obtaining a sufficient specific surface area. Further, when such a carrier is in the form of powder, the average primary particle diameter of the carrier is preferably 5 to 200 nm, more preferably 5 to 100 nm, from the viewpoint of obtaining higher catalytic activity. . Further, the specific surface area of such a carrier is not particularly limited, but from the viewpoint of obtaining higher catalytic activity, it is more preferably 30 m ² / g or more, and further preferably 50 to ²⁰⁰ m ² / g. .

  In the present invention, next, in the heat treatment step according to the present invention, the support on which the platinum binuclear complex is adsorbed and supported is subjected to a heat treatment to remove the ligand in the platinum binuclear complex. As a result, an exhaust gas purifying catalyst having a platinum diatomic cluster supported on a carrier can be obtained. As temperature of the said heat processing, 200-600 degreeC is preferable and 300-500 degreeC is more preferable. The heat treatment time is preferably 1 to 5 hours. When the heat treatment temperature or heat treatment time is less than the lower limit, the ligand tends to be not efficiently and sufficiently removed. On the other hand, when the upper limit is exceeded, platinum atoms supported on the support tend to aggregate.

  According to such a method for producing an exhaust gas purifying catalyst of the present invention, it is possible to support platinum in an atomic state on a carrier in a state where at least a part thereof forms a diatomic cluster. Such platinum is supported on the carrier in a sufficiently and highly dispersed state, and the exhaust gas purifying catalyst of the present invention exhibits high catalytic activity.

  Next, the exhaust gas purifying catalyst of the present invention will be described. The exhaust gas purifying catalyst of the present invention is an exhaust gas purifying catalyst obtained by the production method of the present invention, comprising a support and platinum supported in an atomic state on the support, wherein at least a part of the platinum is A two-atom cluster is formed.

  Such a diatomic cluster can be confirmed by a scanning transmission electron microscope (STEM) or the like. In the exhaust gas purifying catalyst of the present invention, platinum is supported in an atomic state, so that the dispersibility of platinum atoms is high, a sufficient amount of active sites are present on the catalyst, and high catalytic activity is obtained. In addition, since the platinum diatomic cluster according to the present invention is heavier than a platinum single atom, scattering due to heating is unlikely to occur, and it is difficult to move on the carrier, so that it is difficult for grains to grow. As a result, even if the exhaust gas purifying catalyst of the present invention is exposed to a high temperature for a long period of time, aggregation of platinum particles hardly occurs and high catalytic activity can be maintained.

  In the exhaust gas purifying catalyst of the present invention, the distance between the centers of two platinum atoms forming such a two-atom cluster is usually more than 0 nm and 0.5 nm or less. If the distance between the centers of platinum atoms exceeds 0.5 nm, even if the Pt-Pt bond distance becomes longer due to the strong interaction between the platinum atoms and the carrier, the two platinum atoms are no longer two-atom clusters. It cannot be said that it forms. From the viewpoint that two platinum atoms surely form a two-atom cluster, the distance between centers of two platinum atoms forming a two-atom cluster is 0.1 to 0.4 nm. Preferably, 0.2 to 0.3 nm is more preferable. In the present invention, the distance between the centers of the platinum atoms can be measured by observing the exhaust gas purifying catalyst of the present invention with an electron microscope.

  Further, in the exhaust gas purifying catalyst of the present invention, 50 at% or more (more preferably 70 at% or more, particularly preferably 80 at% or more) of all platinum on the support forms a diatomic cluster. Is preferred. When the proportion of platinum atoms forming a two-atom cluster is in the above range, platinum is sufficiently dispersed and supported, so that there is a sufficient amount of active sites on the catalyst, and a high catalyst. Activity is obtained. On the other hand, when the proportion of platinum atoms forming a two-atom cluster is less than the lower limit, sufficient catalytic activity tends not to be obtained. In the present invention, the proportion (at%) of platinum atoms forming such a two-atom cluster is determined by observing the catalyst using a scanning transmission electron microscope (Cs-STEM) having a converging lens equipped with a spherical aberration correction device. In the obtained STEM image, a region of 10 nm in length × 10 nm in width on the carrier was randomly extracted, and the total number of platinum atoms present in the region and the number of platinum atoms forming a two-atom cluster Respectively, and the ratio of the number of platinum atoms forming a two-atom cluster to the total number of platinum atoms can be calculated. As the scanning transmission electron microscope (STEM), for example, “JEM-2100F” manufactured by JEOL Ltd. can be used.

  Furthermore, in the exhaust gas purifying catalyst of the present invention, the average distance between adjacent platinum diatomic clusters is preferably 1.0 nm or more, and more preferably 1.5 nm or more. In addition, the average distance between adjacent platinum diatomic clusters is more preferably 1.0 nm (more preferably 1.5 nm) to 3.0 nm, and more preferably 2.0 nm to 3.0 nm. Is particularly preferred. When such an average distance is less than the lower limit, the dispersibility of platinum tends to be lowered, and a sufficiently high catalytic activity tends to be not obtained. On the other hand, when the upper limit is exceeded, the specific surface area of the support is small. Has a tendency that the number of active sites decreases and a sufficiently high catalytic activity cannot be obtained. In the present invention, the average distance between adjacent two-atom clusters is determined by observing the catalyst using Cs-STEM in the same manner as the method for determining the proportion of platinum atoms forming the above-mentioned two-atom clusters. In the image, a region of 10 nm in length × 10 nm in width on the carrier is randomly extracted, and the distance between adjacent two atom clusters for five or more (preferably seven or more) diatomic clusters present in the region. It can calculate by calculating | requiring and averaging these. The “distance between adjacent two-atom clusters” refers to the length of a line segment in which the distance between adjacent two-atom clusters is the shortest when viewed from each two-atom cluster.

  In the exhaust gas purifying catalyst of the present invention, the distance between adjacent platinum diatomic clusters is 1.0 nm or more (more preferably) in 50 to 100% (ratio on the basis of the number) of the total number of platinum diatomic clusters. Is preferably 1.5 nm or more), and from the viewpoint that higher catalytic activity can be obtained, the proportion based on the above-mentioned number is 100% (that is, adjacent platinum 2 in all platinum two-atom clusters). The distance between atomic clusters is preferably 1.0 nm or more (more preferably 1.5 nm or more). The “total number of diatomic clusters” refers to the number of all diatomic clusters confirmed in the STEM image. In addition, when the ratio of platinum diatomic clusters having a distance between adjacent platinum diatomic clusters of 1.5 nm or more is less than 50% of the total number of platinum diatomic clusters, the catalyst is sufficient after being used for a long time under high temperature conditions. There is a tendency that activity cannot be obtained.

  In the exhaust gas purifying catalyst of the present invention, the amount of platinum supported is not particularly limited, but is preferably 0.01 to 1.0 mass%, more preferably 0.05 to 0.5 mass%. If the supported amount of platinum is less than the lower limit, sufficient catalytic activity tends not to be obtained. On the other hand, if the upper limit is exceeded, platinum sintering is likely to occur, and the degree of dispersion of platinum is reduced, resulting in a sufficient catalyst. There is a tendency that activity cannot be obtained.

Further, in the exhaust gas purifying catalyst of the present invention, after performing the following durability test at a set firing temperature of 700 ° C., it is based on the CO adsorption amount and the platinum loading amount measured at 50 ° C. by the CO pulse measurement method described later. And the following formula:
[Platinum dispersity (%)] = ([CO adsorption amount (mol)] / [platinum supported amount (mol)]) × 100
Is preferably 4.5% or more, and more preferably 5.0% or more. When the platinum dispersity is less than the lower limit, sufficient catalytic activity tends not to be obtained.

  In the durability test, first, a catalyst formed by a cold isostatic pressure method is pulverized to a diameter of 0.3 to 0.71 mm to prepare a pellet-shaped catalyst. Next, this pellet catalyst is put in a crucible, and heated from room temperature to a preset firing temperature in 1 hour under atmospheric conditions, then held at the preset firing temperature for 1 hour, and then naturally cooled to room temperature.

In the present invention, the CO adsorption amount can be determined by the following “CO pulse measurement method”. That is, the pellet catalyst sample after the endurance test is weighed at three levels of 0.05 g, 0.10 g, and 0.15 g, and installed in the measuring tube of the gas adsorption amount measuring device. The inside of each measuring tube is made an O ₂ (100% by volume) gas atmosphere, heated to 400 ° C. over 40 minutes, and then held for 15 minutes. Next, the gas atmosphere inside each of the measuring tubes is changed to a gas atmosphere of He (100% by volume) and held at 400 ° C. for 40 minutes. Thereafter, the gas atmosphere inside each of the measuring tubes is changed to a gas atmosphere of H ₂ (100% by volume) and held at 400 ° C. for 15 minutes, and then the gas atmosphere is changed to a gas atmosphere of He (100% by volume). Then, after maintaining at 400 ° C. for 15 minutes, it is naturally cooled to 50 ° C. while maintaining a gas atmosphere of He (100% by volume). Thereafter, in a gas atmosphere of He (100% by volume), while maintaining the temperature at 50 ° C. (constant), pulses of 1.0 μmol / pulse CO are adsorbed to each level amount of the catalyst until the adsorption is saturated. (Adsorption temperature: 50 ° C.) Of the pulsed CO, the amount of CO that was not adsorbed by the catalyst was detected using a thermal conductivity detector (TCD). From the number of pulses and the TCD area when the adsorption was saturated, to each level of catalyst. Measure the amount of CO adsorbed by Then, the “CO adsorption amount” is calculated by averaging the CO adsorption amounts on the three-level amount of catalyst thus obtained. As the gas adsorption amount measuring device, for example, a fully automatic catalytic gas adsorption amount measuring device “R6015” manufactured by Okura Riken Co., Ltd. can be used.

  In the exhaust gas purifying catalyst of the present invention, the average particle size of the platinum particles after the durability test at a set firing temperature of 700 ° C. is preferably 0.5 to 5 nm, and more preferably 1 to 2 nm. When the average particle diameter of the platinum particles after the durability test is less than the lower limit, the interaction between the platinum and the support becomes too strong, and there is a tendency that sufficient catalytic activity is not obtained, whereas, when the upper limit is exceeded, There is a tendency that the number of active sites decreases and a sufficiently high catalytic activity cannot be obtained. In the present invention, the average particle size of the platinum particles is determined by observing the catalyst using the Cs-STEM, and extracting 10 platinum particles at random from the obtained STEM image. It can be determined by measuring and averaging.

  The form of the exhaust gas purifying catalyst of the present invention is not particularly limited, and may be, for example, a honeycomb-shaped monolith catalyst in which the catalyst is supported on a base material, a pellet-shaped pellet catalyst, or the like. The base material used here is not particularly limited, and a particulate filter base material (DPF base material), a monolithic base material, a pellet base material, a plate base material, and the like can be suitably used. Also, the material of such a base material is not particularly limited, but a base material made of a ceramic such as cordierite, silicon carbide, mullite, or a base material made of a metal such as stainless steel including chrome and aluminum is preferably used. can do.

  Further, the method for supporting the catalyst on such a substrate is not particularly limited, and a known method can be appropriately employed. In addition, in such an exhaust gas purification catalyst, other components (for example, NOx occlusion material) that can be used for the exhaust gas purification catalyst as long as the effects of the present invention are not impaired may be supported as appropriate.

  EXAMPLES Hereinafter, although this invention is demonstrated more concretely based on an Example and a comparative example, this invention is not limited to a following example. The amount of platinum supported in the catalyst was measured by the following method.

<Measurement of platinum content>
After 0.2 g of catalyst powder was added to aqua regia ([HNO ₃ ]: [HCl] = 1: 3 (volume ratio)) for decomposition, an aqueous sulfuric acid solution was added to the decomposition solution to completely dissolve the catalyst. did. The obtained solution was subjected to ICP analysis using an ICP (Inductively Coupled Plasma) analyzer (“CIROS 120EOP” manufactured by Rigaku Corporation). That is, the solution was introduced into an argon plasma, the emission spectrum intensity of platinum (measurement wavelength: 214.423 nm) was measured, and the platinum concentration in the solution was determined using a calibration curve prepared in advance. The platinum content in the catalyst was calculated.

(Preparation example)
445 mg of K ₂ PtCl ₄ (manufactured by Aldrich) and 522 mg of K ₂ PtCl ₆ (manufactured by Aldrich) were placed in a flask, and 40 ml of ion-exchanged water was added and dissolved. After adding 6.3 g of ^t Bu-NH ₂ CO (manufactured by Tokyo Chemical Industry Co., Ltd.) to the resulting solution, the temperature of the oil bath was kept at about 90 ° C. and stirred overnight. The step of removing impurities from the reaction, was first washed with hot water to remove excess ^t Bu-NH 2 _CO plus. Next, extraction with acetone, recrystallization, and the following formula:
^{_{Pt 2 (t Bu-NHCO)}} 4 Cl 2
The platinum binuclear complex represented by this was obtained.

(Reference example)
The platinum binuclear complex obtained in the preparation example was dissolved in the following organic solvent (all manufactured by Wako Pure Chemical Industries, Ltd.) so as to have the following concentrations to prepare sample solutions (1) to (6).
<Sample solution>
(1) 0.3 mg / 100 μL-acetonitrile (2) 0.3 mg / 100 μL-acetone (3) 0.3 mg / 100 μL-ethyl acetate (4) 0.3 mg / 100 μL-methanol (5) 0.4 mg / 100 μL- THF (tetrahydrofuran)
(6) 0.4 mg / 100 μL-toluene.

In addition, 2-[(2E) -3- (4-tert-butylphenyl) -2-methyl-2-propenylidene] malononitrile (DCTB, manufactured by Aldrich) was added to the following organic solvents (both of which were sums). Dissolved in Koyo Pharmaceutical Co., Ltd.) to prepare matrix solutions (1) to (6).
<Matrix solution>
(1) 0.9 mg / 100 μL-acetonitrile (2) 0.9 mg / 100 μL-acetone (3) 1.1 mg / 100 μL-ethyl acetate (4) 1.0 mg / 100 μL-methanol (5) 1.1 mg / 100 μL- THF (tetrahydrofuran)
(6) 1.0 mg / 100 μL-toluene.

  Next, 1 μl of the sample solutions (1) to (6) and 10 μl of the matrix solutions (1) to (6) were mixed, and the obtained mixed solutions (1) to (6) were subjected to mass spectrometry ( Using a Bruker Daltonics "Autoflex"), mass spectrometry (MALDI-MS analysis) was performed by a matrix-assisted laser desorption ionization method under the conditions of a laser wavelength: 337 nm (nitrogen gas laser) and an acceleration voltage: 19 kV. The results are shown in FIGS. 2 is an enlarged view of the mass spectrum of (6) toluene in FIG. 1, and FIG. 3 is an enlarged view of the mass spectrum in the vicinity of the mass-to-charge ratio (m / z) = 790 in FIG. 4 is an enlarged view of the mass spectrum in the vicinity of the mass-to-charge ratio (m / z) = 825 in FIG.

In addition, the expected structure and calculated exact mass (exact mass) of the platinum binuclear complex used in the solution are as follows:
<Object (M)>
Pt ₂ Cl ₂ (NHCOC ₄ H ₉ ) ₄ Exact mass: 860.17
<Object [M-Cl] ⁺ >
Pt ₂ Cl (NHCOC ₄ H ₉ ) ₄ Exact mass: 825.20
<Target product [M-2Cl] ²⁺ >
Pt ₂ (NHCOC ₄ H ₉ ) ₄ Exact mass: 790.23
5 shows the theoretical mass spectrum (calculated value) of the target product [M-2Cl] ²⁺ , and FIG. 6 shows the theoretical mass spectrum (calculated value) of the target product [M-Cl] ^+. Each is shown.

  Since the MS pattern of the sample solution shown in FIGS. 1 to 4 is similar to the theoretical isotope MS pattern of a molecule composed of two platinum atoms shown in FIGS. It is thought that two platinum atoms are bonded to each other. That is, it was confirmed that the platinum binuclear complex exists in the state represented by the following formulas (3-1) to (3-3) without being decomposed into ions in the sample solution. . Therefore, it is considered that platinum can be supported on a carrier while maintaining a Pt-Pt bond by using a solution in which the platinum binuclear complex is dissolved in an organic solvent.

Example 1
442 mg of the platinum binuclear complex obtained in the preparation example was dissolved in 2 L of toluene (manufactured by Wako Pure Chemical Industries, Ltd.) in the air. ²⁰ g of γ-alumina (Degussa Co., Ltd., specific surface area: 100 m ² / g, aggregate average particle size (average secondary particle size):> 100 nm, average primary particle size: about 10 nm) is added to the resulting solution. The platinum binuclear complex was adsorbed and supported on γ-alumina by stirring at 30 ° C. for 100 hours using a hot stirrer. Thereafter, the solid content is recovered by filtration, dried in the atmosphere at 85 ° C. for 12 hours, then heated in the atmosphere from room temperature to 300 ° C. in 1 hour, held at that temperature for 1 hour, and calcined. A catalyst powder having platinum supported on alumina was obtained. When this catalyst powder was analyzed by ICP according to the above method, the platinum content was 0.95% by mass as shown in Table 1.

<Observation by scanning transmission electron microscope with spherical aberration corrector>
The dried catalyst powder and the calcined catalyst powder obtained in Example 1 were each used with a scanning transmission electron microscope with a spherical aberration corrector (Cs-STEM, “JEM-2100F” manufactured by JEOL Ltd.). The observation was performed under the following conditions.
[Observation conditions]
Acceleration voltage: 200 kV
Lattice resolution (lattice image): 0.1 nm
Point resolution (particle image): 0.19 nm
STEM resolution: 0.2 nm
Magnification: ~ 150000000 (measured by changing the magnification as appropriate)
Electron gun: Hot cathode field emission X-ray detection solid angle: 0.24 sr (unit sr indicates steradian. Solid angle with respect to the center of the portion on the sphere having an area equal to the square of the sphere radius).

  FIG. 7 shows an STEM photograph of the dried catalyst powder obtained in Example 1, and FIG. 8 shows an STEM photograph of the fired catalyst powder obtained in Example 1. From the results shown in FIG. 7 and FIG. 8, even when the ligand of the platinum binuclear complex is removed by pyrolysis by the baking treatment, most of the platinum atoms are aggregated or scattered. First, it was confirmed that it was dispersed as a two-atom cluster.

<Durability test>
The catalyst powder (2.0 g) obtained in Example 1 was molded by a cold isostatic pressure method (CIP: 1000 kg / cm ² ) for 1 minute, and then pulverized to a diameter of 0.3 to 0.71 mm to obtain a pellet-shaped catalyst. Prepared. Next, the pellet catalyst is placed in a crucible and the temperature is raised from room temperature to each set firing temperature (400 ° C., 500 ° C., 600 ° C., 700 ° C., 800 ° C., 900 ° C., 1000 ° C.) in 1 hour under atmospheric conditions. After warming, each set firing temperature was held for 1 hour, and then naturally cooled to room temperature, and an endurance test at each set firing temperature was performed.

<Observation of catalyst after durability test by Cs-STEM>
The pellet catalyst after the endurance test at a set firing temperature of 700 ° C. was observed under the above conditions using Cs-STEM. FIG. 9 is a STEM photograph showing the state of the catalyst obtained in Example 1 after the durability test. As is apparent from the results shown in FIG. 9, even when the durability test was performed on the catalyst of the present invention obtained in Example 1, most of the platinum atoms were dispersed as two-atom clusters. It was confirmed that aggregation of platinum particles (two atom clusters) hardly occurred.

<Measurement of platinum dispersion after endurance test>
The pellet catalyst after the durability test at each set firing temperature was weighed at three levels of 0.05 g, 0.10 g, and 0.15 g, and a fully automatic catalyst gas adsorption amount measuring device (“R6015” manufactured by Okura Riken Co., Ltd.). ) In the measuring tube. Thereafter, the inside of each measuring tube was changed to a gas atmosphere of O ₂ (100% by volume), heated to 400 ° C. over 40 minutes, and then held for 15 minutes. Next, the gas atmosphere inside each of the measuring tubes was changed to a gas atmosphere of He (100% by volume) and held at 400 ° C. for 40 minutes. Thereafter, the gas atmosphere inside each of the measuring tubes is changed to a gas atmosphere of H ₂ (100% by volume) and held at 400 ° C. for 15 minutes, and then the gas atmosphere is changed to a gas atmosphere of He (100% by volume). Then, after maintaining at 400 ° C. for 15 minutes, it was naturally cooled to 50 ° C. while maintaining a gas atmosphere of He (100% by volume). Thereafter, in a gas atmosphere of He (100% by volume), while maintaining the temperature at 50 ° C., 1.0 μmol / pulse CO was pulsed for each level amount of the catalyst until adsorption was saturated. Of the pulsed CO, the amount of CO that was not adsorbed by the catalyst was detected using a thermal conductivity detector (TCD). From the number of pulses and the TCD area when the adsorption was saturated, to each level of catalyst. The CO adsorption amount of each was measured. The “CO adsorption amount” was calculated by averaging the CO adsorption amounts on the three-level amount of catalyst thus obtained.

From the CO adsorption amount thus obtained and the platinum loading measured by ICP analysis, the following formula:
[Platinum dispersity (%)] = ([CO adsorption amount (mol)] / [platinum supported amount (mol)]) × 100
Was used to calculate the degree of dispersion of platinum. The results are shown in Table 2.

  As is apparent from the results shown in Table 2, the catalyst of the present invention obtained in Example 1 has a high degree of platinum dispersion after the durability test, and the durability test was conducted at a set firing temperature of 700 ° C. Even after this, a high level of platinum dispersion of 6.9% was maintained. From this result, it was confirmed that the catalyst of the present invention hardly aggregates platinum particles even when exposed to high temperatures and maintains a high degree of platinum dispersion.

<Catalyst activity evaluation>
0.5 g of the pellet catalyst after the durability test at each set firing temperature was placed in an atmospheric pressure fixed bed flow type catalyst evaluation device (manufactured by Vest Sokki Co., Ltd.), and CO (0.1000 vol%), CO ₂ ( 100.00% by volume), O ₂ (10.000% by volume), NO (0.0100% by volume), C ₃ H ₆ (0.0500% by volume C1), H ₂ O (10.000% by volume), A stoichiometric gas composed of N ₂ (remaining) was supplied for 10 minutes under the conditions of a temperature of 400 ° C. and a flow rate of 7 L / min. Next, after adjusting the temperature of the gas containing catalyst to 100 ° C., the stoichiometric gas was supplied at a flow rate of 7 L / min while increasing the temperature of the gas containing catalyst to 700 ° C. at 15 ° C./min. At each gas temperature between 700 ° C., the concentrations of carbon monoxide (CO) and propylene (C ₃ H ₆ ) in the catalyst-containing gas and the catalyst output gas were measured to determine the CO and C ₃ H ₆ conversion. Asked. These conversion rates were plotted against the catalyst-containing gas temperature to prepare conversion curves. From the obtained conversion rate curve, a temperature at which the conversion rate of CO and C ₃ H ₆ reached 50% (hereinafter referred to as “50% purification temperature (° C.)”) was determined. FIG. 10 shows the 50% purification temperature for CO, and FIG. 11 shows the 50% purification temperature for C ₃ H ₆ .

As is apparent from the results shown in FIGS. 10 and 11, the catalyst of the present invention obtained in Example 1 has a 50% purification temperature for both carbon monoxide (CO) and propylene (C ₃ H ₆ ). It was confirmed that the catalyst had an excellent catalytic activity even after the durability test at each set firing temperature.

(Comparative Example 1)
4401 mg of platinum nitrate (Pt (NO ₃ ) ₃ ) aqueous solution (Tanaka Kikinzoku Co., Ltd., Pt content: 8.5% by mass) was dissolved in 2 L of ion-exchanged water in the air. 10 g of the γ-alumina was added to the obtained aqueous solution and stirred for 100 hours at 30 ° C. using a hot stirrer, but platinum nitrate was not adsorbed and supported on the γ-alumina. Evaporate to dryness at 100 ° C. The obtained solid content was heated in the atmosphere from room temperature to 300 ° C. over 1 hour, held at that temperature for 1 hour and fired to obtain a catalyst powder having platinum supported on γ-alumina. When this catalyst powder was analyzed by ICP according to the above method, the platinum content was 1.0% by mass as shown in Table 1.

  The dried catalyst powder obtained in Comparative Example 1 was observed by Cs-STEM as in Example 1. FIG. 12 shows a STEM photograph of the dried catalyst powder obtained in Comparative Example 1. From the results shown in FIG. 12, when platinum was supported using a platinum salt, platinum was supported in an aggregated state, and no platinum diatomic cluster was observed on the support.

  Moreover, about the pellet catalyst which performed the said durability test at the setting calcination temperature of 700 degreeC similarly to Example 1 using the catalyst powder obtained by the comparative example 1, it is Cs-STEM after an endurance test similarly to Example 1. FIG. Observation was performed. FIG. 13 is a STEM photograph showing the state of the catalyst obtained in Comparative Example 1 after the durability test. From the results shown in FIG. 13, when platinum is supported using a platinum salt, the platinum is supported in an aggregated state after the durability test, and the platinum particles are aggregated and coarsened at a high temperature. Confirmed to do.

Further, the catalytic activity of the pellet catalyst, which was subjected to the durability test at each set firing temperature in the same manner as in Example 1, using the catalyst powder obtained in Comparative Example 1, was evaluated in the same manner as in Example 1. The obtained results are shown in FIG. 10 (50% purification temperature for CO) and FIG. 11 (50% purification temperature for C ₃ H ₆ ).

As is apparent from the results shown in FIGS. 10 and 11, the catalyst of Comparative Example 1 obtained by supporting platinum using a platinum salt is carbon monoxide (CO) and propylene (C ₃ H ₆ ). It was confirmed that any 50% purification temperature was high and the catalytic activity was inferior.

( Comparative Example 2 )
A catalyst powder was prepared in the same manner as in Example 1 except that 2 L of acetonitrile (manufactured by Wako Pure Chemical Industries, Ltd.) was used instead of toluene. This catalyst powder was subjected to ICP analysis in the same manner as in Example 1 to determine the platinum content. The results are shown in Table 1.

( Comparative Example 3 )
A catalyst powder was prepared in the same manner as in Example 1 except that 2 L of acetone (manufactured by Wako Pure Chemical Industries, Ltd.) was used instead of toluene. This catalyst powder was subjected to ICP analysis in the same manner as in Example 1 to determine the platinum content. The results are shown in Table 1. Further, the dried catalyst powder and the fired catalyst powder obtained in Comparative Example 3 were observed by Cs-STEM in the same manner as in Example 1. FIG. 14 shows an STEM photograph of the dried catalyst powder obtained in Comparative Example 3 , and FIG. 15 shows an STEM photograph of the fired catalyst powder obtained in Comparative Example 3 . From the results shown in FIGS. 14 and 15, even when the ligand of the platinum binuclear complex is removed by pyrolysis by the baking treatment, most of the platinum atoms are aggregated or scattered. First, it was confirmed that it was dispersed as a two-atom cluster.

( Comparative Example 4 )
A catalyst powder was prepared in the same manner as in Example 1 except that 2 L of ethyl acetate (manufactured by Wako Pure Chemical Industries, Ltd.) was used instead of toluene. This catalyst powder was subjected to ICP analysis in the same manner as in Example 1 to determine the platinum content. The results are shown in Table 1.

( Comparative Example 5 )
A catalyst powder was prepared in the same manner as in Example 1 except that 2 L of methanol (manufactured by Wako Pure Chemical Industries, Ltd.) was used instead of toluene. This catalyst powder was subjected to ICP analysis in the same manner as in Example 1 to determine the platinum content. The results are shown in Table 1.

( Comparative Example 6 )
A catalyst powder was prepared in the same manner as in Example 1 except that 2 L of tetrahydrofuran (manufactured by Wako Pure Chemical Industries, Ltd.) was used instead of toluene. This catalyst powder was subjected to ICP analysis in the same manner as in Example 1 to determine the platinum content. The results are shown in Table 1. Further, the dried catalyst powder and the fired catalyst powder obtained in Comparative Example 6 were observed by Cs-STEM in the same manner as in Example 1. 16 shows an STEM photograph of the dried catalyst powder obtained in Comparative Example 6 , and FIG. 17 shows an STEM photograph of the fired catalyst powder obtained in Comparative Example 6 , respectively. From the results shown in FIG. 16 and FIG. 17, even when the ligand of the platinum binuclear complex is thermally decomposed and removed by the firing treatment, most of the platinum atoms are aggregated or scattered. First, it was confirmed that it was dispersed as a two-atom cluster.

  From the above results, in the catalyst of the present invention obtained using a platinum binuclear complex having a Pt—Pt bond and a coordination unsaturated site capable of binding to the support, platinum is supported on the support as a diatomic cluster. It was confirmed that even when exposed to a high temperature for a long time, the platinum particles are not easily coarsened and have high catalytic activity. In particular, when toluene was used as the organic solvent (Example 1), both the amount of platinum supported by ICP and the state of platinum dispersion by STEM image were high, and the amount of platinum binuclear complex used was reduced. It was confirmed that it was possible to maintain high catalytic activity.

  As described above, according to the present invention, it is possible to more reliably obtain an exhaust gas purification catalyst in which platinum in an atomic state is supported on a carrier in a state where at least a part of the platinum forms a two-atom cluster. . Such an exhaust gas purifying catalyst exhibits a high catalytic activity because platinum is supported on a carrier in a sufficiently and highly dispersed state.

  Therefore, the exhaust gas purifying catalyst of the present invention is useful as a catalyst for purifying CO, NO, HC, etc. contained in exhaust gas from automobiles.

Claims (3)

A dissolution step of dissolving a platinum binuclear complex having a Pt-Pt bond and a coordination unsaturated site capable of binding to a carrier in toluene ;
A supporting step in which the solution obtained in the dissolving step is brought into contact with the carrier to adsorb and carry the platinum binuclear complex on the carrier;
Heat-treating the support on which the platinum dinuclear complex is adsorbed and supported to remove the ligand in the platinum dinuclear complex, and obtaining a catalyst in which platinum diatomic clusters are supported on the support;
A method for producing an exhaust gas purifying catalyst, comprising:   The method for producing an exhaust gas purifying catalyst according to claim 1, wherein the platinum binuclear complex has a ligand containing a nitrogen atom coordinated to a platinum atom.   The method for producing an exhaust gas purifying catalyst according to claim 2, wherein the ligand has an amide group.